team with images generously made available by the bibliothèque nationale de france (bnf/gallica) at http://gallica.bnf.fr the future of astronomy by professor edward c. pickering reprinted from the popular science monthly, august, . the future of astronomy[ ] by professor edward c. pickering harvard college observatory it is claimed by astronomers that their science is not only the oldest, but that it is the most highly developed of the sciences. indeed it should be so, since no other science has ever received such support from royalty, from the state and from the private individual. however this may be, there is no doubt that in recent years astronomers have had granted to them greater opportunities for carrying on large pieces of work than have been entrusted to men in any other department of pure science. one might expect that the practical results of a science like physics would appeal to the man who has made a vast fortune through some of its applications. the telephone, the electric transmission of power, wireless telegraphy and the submarine cable are instances of immense financial returns derived from the most abstruse principles of physics. yet there are scarcely any physical laboratories devoted to research, or endowed with independent funds for this object, except those supported by the government. the endowment of astronomical observatories devoted to research, and not including that given for teaching, is estimated to amount to half a million dollars annually. several of the larger observatories have an annual income of fifty thousand dollars. [ ] commencement address at case school of applied science, cleveland, may , . i once asked the wisest man i know, what was the reason for this difference. he said that it was probably because astronomy appealed to the imagination. a practical man, who has spent all his life in his counting room or mill, is sometimes deeply impressed with the vast distances and grandeur of the problems of astronomy, and the very remoteness and difficulty of studying the stars attract him. my object in calling your attention to this matter is the hope that what i have to say of the organization of astronomy may prove of use to those interested in other branches of science, and that it may lead to placing them on the footing they should hold. my arguments apply with almost equal force to physics, to chemistry, and in fact to almost every branch of physical or natural science, in which knowledge may be advanced by observation or experiment. the practical value of astronomy in the past is easily established. without it, international commerce on a large scale would have been impossible. without the aid of astronomy, accurate boundaries of large tracts of land could not have been defined and standard time would have been impossible. the work of the early astronomers was eminently practical, and appealed at once to every one. this work has now been finished. we can compute the positions of the stars for years, almost for centuries, with all the accuracy needed for navigation, for determining time or for approximate boundaries of countries. the investigations now in progress at the greatest observatories have little, if any, value in dollars and cents. they appeal, however, to the far higher sense, the desire of the intellectual human being to determine the laws of nature, the construction of the material universe, and the properties of the heavenly bodies of which those known to exist far outnumber those that can be seen. three great advances have been made in astronomy. first, the invention of the telescope, with which we commonly associate the name of galileo, from the wonderful results he obtained with it. at that time there was practically no science in america, and for more than two centuries we failed to add materially to this invention. half a century ago the genius of the members of one family, alvan clark and his two sons, placed america in the front rank not only in the construction, but in the possession, of the largest and most perfect telescopes ever made. it is not easy to secure the world's record in any subject. the clarks constructed successively, the -inch lens for chicago, the -inch for washington, the -inch for pulkowa, the -inch for lick and the -inch for yerkes. each in turn was the largest yet made, and each time the clarks were called upon to surpass the world's record, which they themselves had already established. have we at length reached the limit in size? if we include reflectors, no, since we have mirrors of inches aperture at mt. wilson and cambridge, and a still larger one of inches has been undertaken. it is more than doubtful, however, whether a further increase in size is a great advantage. much more depends on other conditions, especially those of climate, the kind of work to be done and, more than all, the man behind the gun. the case is not unlike that of a battleship. would a ship a thousand feet long always sink one of five hundred feet? it seems as if we had nearly reached the limit of size of telescopes, and as if we must hope for the next improvement in some other direction. the second great advance in astronomy originated in america, and was in an entirely different direction, the application of photography to the study of the stars. the first photographic image of a star was obtained in , by george p. bond, with the assistance of mr. j.a. whipple, at the harvard college observatory. a daguerreotype plate was placed at the focus of the -inch equatorial, at that time one of the two largest refracting telescopes in the world. an image of [greek: alpha] lyræ was thus obtained, and for this mr. bond received a gold medal at the first international exhibition, that at the crystal palace, in london, in . in , mr. bond, then professor bond, director of the harvard observatory, again took up the matter with collodion wet plates, and in three masterly papers showed the advantages of photography in many ways. the lack of sensitiveness of the wet plate was perhaps the only reason why its use progressed but slowly. quarter of a century later, with the introduction of the dry plate and the gelatine film, a new start was made. these photographic plates were very sensitive, were easily handled, and indefinitely long exposures could be made with them. as a result, photography has superseded visual observations, in many departments of astronomy, and is now carrying them far beyond the limits that would have been deemed possible a few years ago. the third great advance in astronomy is in photographing the spectra of the stars. the first photograph showing the lines in a stellar spectrum was obtained by dr. henry draper, of new york, in . sir william huggins in had obtained an image of the spectrum of sirius, on a photographic plate, but no lines were visible in it. in he again took up the subject, and, by an early publication, preceded dr. draper. when we consider the attention the photography of stellar spectra is receiving at the present time, in nearly all the great observatories in the world, it may well be regarded as the third great advance in astronomy. what will be the fourth advance, and how will it be brought about? to answer this question we must consider the various ways in which astronomy, and for that matter any other science, may be advanced. first, by educating astronomers. there are many observatories where excellent instruction in astronomy is given, either to the general student or to one who wishes to make it his profession. at almost any active observatory a student would be received as a volunteer assistant. unfortunately, few young men can afford to accept an unpaid position, and the establishment of a number of fellowships each offering a small salary sufficient to support the student would enable him to acquire the necessary knowledge to fill a permanent position. the number of these scholarships should not be large, lest more students should undertake the work than would be required to fill the permanent paying positions in astronomy, as they become vacant. in europe, a favorite method of aiding science is to offer a prize for the best memoir on a specified subject. on theoretical grounds this is extremely objectionable. since the papers presented are anonymous and confidential, no one but the judges know how great is the effort wasted in duplication. the larger the prize, the greater the injury to science, since the greater will be the energy diverted from untried fields. it would be much wiser to invite applications, select the man most likely to produce a useful memoir, and award the prize to him if he achieved success. the award of a medal, if of great intrinsic value, would be an unwise expenditure. the victoria cross is an example of a successful foundation, highly prized, but of small intrinsic value. if made of gold, it would carry no greater honor, and would be more liable to be stolen, melted down or pawned. honorary membership in a famous society, or honorary degrees, have great value if wisely awarded. both are highly prized, form an excellent stimulus to continued work, and as they are both priceless, and without price, they in no way diminish the capacity for work. i recently had occasion to compare the progress in various sciences of different countries, and found that the number of persons elected as foreign associates of the seven great national societies of the world was an excellent test. eighty-seven persons were members of two or more of these societies. only six are residents of the united states, while an equal number come from saxony, which has only a twentieth of the population. of the six residents here, only three were born in the united states. not a single mathematician, or doctor, from this country appears on the list. only in astronomy are we well represented. out of a total of ten astronomers, four come from england, and three from the united states. comparing the results for the last one hundred and fifty years, we find an extraordinary growth for the german races, an equally surprising diminution for the french and other latin races, while the proportion of englishmen has remained unchanged. a popular method of expending money, both by countries and by individuals, is in sending expeditions to observe solar eclipses. these appeal both to donors and recipients. the former believe that they are making a great contribution to science, while the latter enjoy a long voyage to a distant country, and in case of clouds they are not expected to make any scientific return. if the sky is clear at the time of the eclipse, the newspapers of the next day report that great results have been secured, and after that nothing further is ever heard. exceptions should be made of the english eclipse committee and the lick observatory, which, by long continued study and observation, are gradually solving the difficult problems which can be reached in this way only. the gift of a large telescope to a university is of very doubtful value, unless it is accompanied, first, by a sum much greater than its cost, necessary to keep it employed in useful work, and secondly, to require that it shall be erected, not on the university grounds, but in some region, probably mountainous or desert, where results of real value can be obtained. having thus considered, among others, some of the ways in which astronomy is not likely to be much advanced, we proceed to those which will secure the greatest scientific return for the outlay. one of the best of these is to create a fund to be used in advancing research, subject only to the condition that results of the greatest possible value to science shall be secured. one advantage of this method is that excellent results may be obtained at once from a sum, either large or small. whatever is at first given may later be increased indefinitely, if the results justify it. one of the wisest as well as the greatest of donors has said: "find the particular man," but unfortunately, this plan has been actually tried only with some of the smaller funds. any one who will read the list of researches aided by the rumford fund, the elizabeth thompson fund or the bruce fund of will see that the returns are out of all proportion to the money expended. the trustees of such a fund as is here proposed should not regard themselves as patrons conferring a favor on those to whom grants are made, but as men seeking for the means of securing large scientific returns for the money entrusted to them. an astronomer who would aid them in this work, by properly expending a grant, would confer rather than receive a favor. they should search for astronomical bargains, and should try to purchase results where the money could be expended to the best advantage. they should make it their business to learn of the work of every astronomer engaged in original research. a young man who presented a paper of unusual importance at a scientific meeting, or published it in an astronomical journal, would receive a letter inviting him to submit plans to the trustees, if he desired aid in extending his work. in many cases, it would be found that, after working for years under most unfavorable conditions, he had developed a method of great value and had applied it to a few stars, but must now stop for want of means. a small appropriation would enable him to employ an assistant who, in a short time, could do equally good work. the application of this method to a hundred or a thousand stars would then be only a matter of time and money. the american astronomical society met last august at a summer resort on lake erie. about thirty astronomers read papers, and in a large portion of the cases the appropriation of a few hundred dollars would have permitted a great extension in these researches. a sad case is that of a brilliant student who may graduate at a college, take a doctor's degree in astronomy, and perhaps pass a year or two in study at a foreign observatory. he then returns to this country, enthusiastic and full of ideas, and considers himself fortunate in securing a position as astronomer in a little country college. he now finds himself overwhelmed with work as a teacher, without time or appliances for original work. what is worse, no one sympathizes with him in his aspirations, and after a few years he abandons hope and settles down to the dull routine of lectures, recitations and examinations. a little encouragement at the right time, aid by offering to pay for an assistant, for a suitable instrument, or for publishing results, and perhaps a word to the president of his college if the man showed real genius, might make a great astronomer, instead of a poor teacher. for several years, a small fund, yielding a few hundred dollars annually, has been disbursed at harvard in this way, with very encouraging results. a second method of aiding astronomy is through the large observatories. these institutions, if properly managed, have after years of careful study and trial developed elaborate systems of solving the great problems of the celestial universe. they are like great factories, which by taking elaborate precautions to save waste at every point, and by improving in every detail both processes and products, are at length obtaining results on a large scale with a perfection and economy far greater than is possible by individuals, or smaller institutions. the expenses of such an observatory are very large, and it has no pecuniary return, since astronomical products are not salable. a great portion of the original endowment has been spent on the plant, expensive buildings and instruments. current expenditures, like library expenses, heating, lighting, etc., are independent of the output. it is like a man swimming up stream. he may struggle desperately, and yet make no progress. any gain in power effects a real advance. this is the condition of nearly all the larger observatories. their income is mainly used for current expenses, which would be nearly the same whatever their output. a relatively small increase in income can thus be spent to great advantage. the principal instruments are rarely used to their full capacities, and the methods employed could be greatly extended without any addition to the executive or other similar expenses. a man superintending the work of several assistants can often have their number doubled, and his output increased in nearly the same proportion, with no additional expense except the moderate one of their salaries. a single observatory could thus easily do double the work that could be accomplished if its resources were divided between two of half the size. a third, and perhaps the best, method of making a real advance in astronomy is by securing the united work of the leading astronomers of the world. the best example of this is the work undertaken in by the astronomische gesellschaft, the great astronomical society of the world. the sky was divided into zones, and astronomers were invited to measure the positions of all the stars in these zones. the observation of two of the northern and two of the southern zones were undertaken by american observatories. the zone from + ° to + ° was undertaken by the chicago observatory, but was abandoned owing to the great fire of , and the work was assumed and carried to completion by the dudley observatory at albany. the zone from + ° to + ° was undertaken by harvard. an observer and corps of assistants worked on this problem for a quarter of a century. the completed results now fill seven quarto volumes of our annals. of the southern zones, that from - ° to - ° was undertaken by the naval observatory at washington, and is now finished. the zone from - ° to - ° was undertaken at harvard, and a second observer and corps of assistants have been working on it for twenty years. it is now nearly completed, and we hope to begin its publication this year. the other zones were taken by european astronomers. as a result of the whole, we have the precise positions of nearly a hundred and fifty thousand stars, which serve as a basis for the places of all the objects in the sky. another example of cooperative work is a plan proposed by the writer in , at the celebration of the two-hundredth anniversary of the birth of franklin. it was proposed, first to find the best place in the world for an astronomical observatory, which would probably be in south africa, to erect there a telescope of the largest size, a reflector of seven feet aperture. this instrument should be kept at work throughout every clear night, taking photographs according to a plan recommended by an international committee of astronomers. the resulting plates should not be regarded as belonging to a single institution, but should be at the service of whoever could make the best use of them. copies of any, or all, would be furnished at cost to any one who wished for them. as an example of their use, suppose that an astronomer at a little german university should discover a law regulating the stars in clusters. perhaps he has only a small telescope, near the smoke and haze of a large city, and has no means of securing the photographs he needs. he would apply to the committee, and they would vote that ten photographs of twenty clusters, each with an exposure of an hour, should be taken with the large telescope. this would occupy about a tenth part of the time of the telescope for a year. after making copies, the photographs would be sent to the astronomer who would perhaps spend ten years in studying and measuring them. the committee would have funds at their disposal to furnish him, if necessary, with suitable measuring instruments, assistants for reducing the results, and means for publication. they would thus obtain the services of the most skilful living astronomers, each in his own special line of work, and the latter would obtain in their own homes material for study, the best that the world could supply. undoubtedly, by such a combination if properly organized, results could be obtained far better than is now possible by the best individual work, and at a relatively small expense. many years of preparation will evidently be needed to carry out such a plan, and to save time we have taken the first step and have sent a skilful and experienced observer to south africa to study its climate and compare it with the experience he has gained during the last twenty years from a similar study of the climate of south america and the western portion of the united states. the next question to be considered is in what direction we may expect the greatest advance in astronomy will be made. fortunate indeed would be the astronomer who could answer this question correctly. when ptolemy made the first catalogue of the stars, he little expected that his observations would have any value nearly two thousand years later. the alchemists had no reason to doubt that their results were as important as those of the chemists. the astrologers were respected as much as the astronomers. although there is a certain amount of fashion in astronomy, yet perhaps the best test is the judgment of those who have devoted their lives to that science. thirty years ago the field was narrow. it was the era of big telescopes. every astronomer wanted a larger telescope than his neighbors, with which to measure double stars. if he could not get such an instrument, he measured the positions of the stars with a transit circle. then came astrophysics, including photography, spectroscopy and photometry. the study of the motion of the stars along the line of sight, by means of photographs of their spectra, is now the favorite investigation at nearly all the great observatories of the world. the study of the surfaces of the planets, while the favorite subject with the public, next to the destruction of the earth by a comet, does not seem to appeal to astronomers. undoubtedly, the only way to advance our knowledge in this direction is by the most powerful instruments, mounted in the best possible locations. great astronomers are very conservative, and any sensational story in the newspapers is likely to have but little support from them. instead of aiding, it greatly injures real progress in science. there is no doubt that, during the next half century, much time and energy will be devoted to the study of the fixed stars. the study of their motions as indicated by their change in position was pursued with great care by the older astronomers. the apparent motions were so small that a long series of years was required and, in general, for want of early observations of the precise positions of the faint stars, this work was confined mainly to the bright stars. photography is yearly adding a vast amount of material available for this study, but the minuteness of the quantities to be measured renders an accurate determination of their laws very difficult. moreover, we can thus only determine the motions at right angles to the line of sight, the motion towards us or from us being entirely insensible in this way. then came the discovery of the change in the spectrum when a body was in motion, but still this change was so small that visual observations of it proved of but little value. attaching a carefully constructed spectroscope to one of the great telescopes of the world, photographing the spectrum of a star, and measuring it with the greatest care, provided a tool of wonderful efficiency. the motion, which sometimes amounts to several hundreds of miles a second could thus be measured to within a fraction of a mile. the discovery that the motion was variable, owing to the star's revolving around a great dark planet sometimes larger than the star, added greatly not only to the interest of these researches, but also to the labor involved. instead of a single measure for each star, in the case of the so-called spectroscopic binaries, we must make enough measures to determine the dimensions of the orbit, its form and the period of revolution. what has been said of the motions of the stars applies also, in general, to the determination of their distances. a vast amount of labor has been expended on this problem. when at length the distance of a single star was finally determined, the quantity to be measured was so small as to be nearly concealed by the unavoidable errors of measurement. the parallax, or one half of the change in the apparent position of the stars as the earth moves around the sun, has its largest value for the nearest stars. no case has yet been found in which this quantity is as large as a foot rule seen at a distance of fifty miles, and for comparatively few stars is it certainly appreciable. an extraordinary degree of precision has been attained in recent measures of this quantity, but for a really satisfactory solution of this problem, we must probably devise some new method, like the use of the spectroscope for determining motions. two or three illustrations of the kind of methods which might be used to solve this problem may be of interest. there are certain indications of the presence of a selective absorbing medium in space. that is, a medium like red glass, for instance, which would cut off the blue light more than the red light. such a medium would render the blue end of the spectrum of a distant star much fainter, as compared with the red end, than in the case of a near star. a measure of the relative intensity of the two rays would servo to measure the distance, or thickness of the absorbing medium. the effect would be the same for all stars of the same class of spectrum. it could be tested by the stars forming a cluster, like the pleiades, which are doubtless all at nearly the same distance from us. the spectra of stars of the tenth magnitude, or fainter, can be photographed well enough to be measured in this way, so that the relative distances of nearly a million stars could be thus determined. another method which would have a more limited application, would depend on the velocity of light. it has been maintained that the velocity of light in space is not the same for different colors. certain stars, called algol stars, vary in light at regular intervals when partially eclipsed by the interposition of a large dark satellite. recent observations of these eclipses, through glass of different colors, show variations in the time of obscuration. apparently, some of the rays reach the earth sooner than others, although all leave the star at the same time. as the entire time may amount to several centuries, an excessively small difference in velocity would be recognizable. a more delicate test would be to measure the intensity of different portions of the spectrum at a time when the light is changing most rapidly. the effect should be opposite according as the light is increasing or diminishing. it should also show itself in the measures of all spectroscopic binaries. a third method of great promise depends on a remarkable investigation carried on in the physical laboratory of the case school of applied science. according to the undulatory theory of light, all space is filled with a medium called ether, like air, but as much more tenuous than air as air is more tenuous than the densest metals. as the earth is moving through space at the rate of several miles a second, we should expect to feel a breeze as we rush through the ether, like that of the air when in an automobile we are moving with but one thousandth part of this velocity. the problem is one of the greatest delicacy, but a former officer of the case school, one of the most eminent of living physicists, devised a method of solving it. the extraordinary result was reached that no breeze was perceptible. this result appeared to be so improbable that it has been tested again and again, but every time, the more delicate the instrument employed, the more certainly is the law established. if we could determine our motion with reference to the ether, we should have a fixed line of reference to which all other motions could be referred. this would give us a line of ever-increasing length from which to measure stellar distances. still another method depends on the motion of the sun in space. there is some evidence that this motion is not straight, but along a curved line. we see the stars, not as they are now, but as they were when the light left them. in the case of the distant stars this may have occurred centuries ago. accordingly, if we measure the motion of the sun from them, and from near stars, a comparison with its actual motion will give us a clue to their distances. unfortunately, all the stars appear to have large motions whose law we do not know, and therefore we have no definite starting point unless we can refer all to the ether which may be assumed to be at rest. if the views expressed to you this morning are correct, we may expect that the future of astronomy will take the following form: there will be at least one very large observatory employing one or two hundred assistants, and maintaining three stations. two of these will be observing stations, one in the western part of the united states, not far from latitude + °, the other similarly situated in the southern hemisphere, probably in south africa, in latitude - °. the locations will be selected wholly from their climatic conditions. they will be moderately high, from five to ten thousand feet, and in desert regions. the altitude will prevent extreme heat, and clouds or rain will be rare. the range of temperature and unsteadiness of the air will be diminished by placing them on hills a few hundred feet above the surrounding country. the equipment and work of the two stations will be substantially the same. each will have telescopes and other instruments of the largest size, which will be kept at work throughout the whole of every clear night. the observers will do but little work in the daytime, except perhaps on the sun, and will not undertake much of the computation or reductions. this last work will be carried on at a third station, which will be near a large city where the cost of living and of intellectual labor is low. the photographs will be measured and stored at this station, and all the results will be prepared for publication, and printed there. the work of all three stations will be carefully organized so as to obtain the greatest result for a given expenditure. every inducement will be offered to visiting astronomers who wish to do serious work at either of the stations and also to students who intend to make astronomy their profession. in the case of photographic investigations it will be best to send the photographs so that astronomers desiring them can work at home. the work of the young astronomers throughout the world will be watched carefully and large appropriations made to them if it appears that they can spend them to advantage. similar aid will be rendered to astronomers engaged in teaching, and to any one, professional or amateur, capable of doing work of the highest grade. as a fundamental condition for success, no restrictions will be made that will interfere with the greatest scientific efficiency, and no personal or local prejudices that will restrict the work. these plans may seem to you visionary, and too utopian for the twentieth century. but they may be nearer fulfilment than we anticipate. the true astronomer of to-day is eminently a practical man. he does not accept plans of a sensational character. the same qualities are needed in directing a great observatory successfully, as in managing a railroad, or factory. any one can propose a gigantic expenditure, but to prove to a shrewd man of affairs that it is feasible and advisable is a very different matter. it is much more difficult to give away money wisely than to earn it. many men have made great fortunes, but few have learned how to expend money wisely in advancing science, or to give it away judiciously. many persons have given large sums to astronomy, and some day we shall find the man with broad views who will decide to have the advice and aid of the astronomers of the world, in his plans for promoting science, and who will thus expend his money, as he made it, taking the greatest care that not one dollar is wasted. again, let us consider the next great advance, which perhaps will be a method of determining the distances of the stars. many of us are working on this problem, the solution of which may come to some one any day. the present field is a wide one, the prospects are now very bright, and we may look forward to as great an advance in the twentieth century, as in the nineteenth. may a portion of this come to the case school and, with your support, may its enviable record, in the past, be surpassed by its future achievements. [page ii] [illustration: the constellations of orion and taurus. notes.--star a in taurus is red, has eight metals; moves east (page ). at o above tip of right horn is the crab nebula (page ). in orion, a is variable, has five metals; recedes miles per second. b, d, e, x, r, etc., are double stars, the component parts of various colors and magnitudes (page , note). l and i are triple; s, octuple; th, multiple, surrounded by a fine nebula (page ).] [page iii] recreations in astronomy with _directions for practical experiments and telescopic work_ by henry white warren, d.d. author of "sights and insights; or, knowledge by travel," etc. with eighty-three illustrations and maps of stars [page v] [greek: taei psuchaei taei agapaetaei astrapousaei kai isaggedoi] [page vii] preface. all sciences are making an advance, but astronomy is moving at the double-quick. since the principles of this science were settled by copernicus, four hundred years ago, it has never had to beat a retreat. it is rewritten not to correct material errors, but to incorporate new discoveries. once astronomy treated mostly of tides, seasons, and telescopic aspects of the planets; now these are only primary matters. once it considered stars as mere fixed points of light; now it studies them as suns, determines their age, size, color, movements, chemical constitution, and the revolution of their planets. once it considered space as empty; now it knows that every cubic inch of it quivers with greater intensity of force than that which is visible in niagara. every inch of surface that can be conceived of between suns is more wave-tossed than the ocean in a storm. the invention of the telescope constituted one era in astronomy; its perfection in our day, another; and the discoveries of the spectroscope a third--no less important than either of the others. while nearly all men are prevented from practical experimentation in these high realms of knowledge, few [page viii] have so little leisure as to be debarred from intelligently enjoying the results of the investigations of others. this book has been written not only to reveal some of the highest achievements of the human mind, but also to let the heavens declare the glory of the divine mind. in the author's judgment, there is no gulf that separates science and religion, nor any conflict where they stand together. and it is fervently hoped that anyone who comes to a better knowledge of god's works through reading this book, may thereby come to a more intimate knowledge of the worker. i take great pleasure in acknowledging my indebtedness to j. m. van vleck, ll.d., of the u.s. nautical almanac staff, and professor of astronomy at the wesleyan university, for inspecting some of the more important chapters; to dr. s. s. white, of philadelphia, for telescopic advantages; to professor henry draper, for furnishing, in advance of publication, a photograph of the sun's corona in ; and to the excellent work on "popular astronomy," by professor simon newcomb, ll.d., professor u. s. naval observatory, for some of the most recent information, and for the use of the unequalled engravings of jupiter, saturn, and the great nebula of orion. [page ix] contents. chap. i. creative processes ii. creative progress constitution of light chemistry of suns revealed by light creative force of light iii. astronomical instruments the telescope the reflecting telescope the spectroscope iv. celestial measurements celestial movements how to measure v. the sun what the sun does for us vi. the planets, as seen from space the outlook from the earth vii. shooting-stars, meteors, and comets aerolites comets famous comets of what do comets consist? will comets strike the earth? viii. the planets as individuals vulcan mercury venus the earth the aurora borealis [page x] the delicate balance of forces tides the moon telescopic appearance eclipses mars satellites of mars asteroids jupiter satellites of jupiter saturn rings of saturn satellites of saturn uranus neptune ix. the nebular hypothesis. x. the stellar system the open page of the heavens equatorial constellations characteristics of the stars number double and multiple stars colored stars clusters of stars nebulæ variable stars temporary, new, and lost stars movements of stars xi. the worlds and the word xii. the ultimate force summary of latest discoveries and conclusions some elements of the solar system explanation of astronomical symbols signs of the zodiac other abbreviations used in the almanac greek alphabet used indicating the stars chautauqua outline for students glossary of astronomical terms and index [page xi] illustrations fig. the constellations of orion and taurus . an orbit resulting from attraction and projection . the moon's orbit about the earth . changes of orbit by mutual attraction . velocity of light measured by jupiter's satellites . velocity of light measured by fizeau's toothed wheel . white light resolved into colors . showing amount of light received by different planets . measuring intensities of lights . reflection and diffusion of light . manifold reflections . refraction by water . atmospherical reflection . refracting telescope . reflecting telescope . the cambridge equatorial refractor . the new reflecting telescope at paris . spectroscope, with battery of prisms . spectra of glowing hydrogen and of the sun . illustrating arcs and angles . measuring objects by observing angles . mural circle . scale to measure hundredths of an inch . spider-lines to determine star transits . illustrating triangulation [page xii] . measuring distance to an inaccessible object . measuring elevation of an inaccessible object . illustrating parallax . illustrating stellar parallax . mode of ascertaining longitude . relative size of sun, as seen from different planets . zodiacal light . corona of the sun in --brazil . corona of the sun in --colorado . solar prominences of flaming hydrogen . changes in solar cavities during rotation . solar spot . holding telescope to see the sun-spots . orbits and comparative sizes of the planets . orbit of earth, illustrating seasons . inclination of planes of planetary orbits . inclination of orbits of earth and venus . showing the sun's movement among the stars . passage of the sun by star regulus . apparent path of jupiter among the stars . illustrating position of planets . apparent movements of an inferior planet . apparent movements of a superior planet _a_. a swarm of meteors meeting the earth . explosion of a bolide . flight of bolides . the santa rosa aerolite . orbit of november meteors and the comet of . aspects of remarkable comets . phases and apparent dimensions of venus . the earth and moon in space . aurora as waving curtains . tide resulting from centrifugal motion . lunar landscape [page xiii] . telescopic view of the moon . illumination of lunar craters and peaks . lunar crater "copernicus" . eclipses: shadows of earth and moon . apparent sizes of mars, seen from the earth . jupiter . various positions of jupiter's satellites . view of saturn and his rings . perturbations of uranus . map: circumpolar constellations . map of constellations on the meridian in december . map of constellations on the meridian in january . map of constellations on the meridian in april . map of constellations on the meridian in june . map of constellations on the meridian in september . map of constellations on the meridian in november . southern circumpolar constellations . aspects of double stars . sprayed star cluster below ae in hercules . globular star cluster in the centaur . great nebula about th orionis . the crab nebula above z tauri . the ring nebula in lyra . showing place of ring nebula . the horizontal pendulum colored plate representing various specta maps to find the stars [page ] i. creative processes. "in the beginning god created the heaven and the earth. and the earth was without form, and void; and darkness was upon the face of the deep."--_genesis_ i. , . [page ] "not to the domes, where crumbling arch and column attest the feebleness of mortal hand, but to that fane, most catholic and solemn, which god hath planned,-- to that cathedral, boundless as our wonder, whose quenchless lamps the sun and stars supply; its choir the winds and waves, its organ thunder, its dome the sky." h. w. longfellow. "the heavens are a point from the pen of his perfection; the world is a rose-bud from the bower of his beauty; the sun is a spark from the light of his wisdom; and the sky a bubble on the sea of his power." sir w. jones. [page ] recreations in astronomy. * * * * * i. _creative processes._ during all the ages there has been one bright and glittering page of loftiest wisdom unrolled before the eye of man. that this page may be read in every part, man's whole world turns him before it. this motion apparently changes the eternally stable stars into a moving panorama, but it is only so in appearance. the sky is a vast, immovable dial-plate of "that clock whose pendulum ticks ages instead of seconds," and whose time is eternity. the moon moves among the illuminated figures, traversing the dial quickly, like a second-hand, once a month. the sun, like a minute-hand, goes over the dial once a year. various planets stand for hour-hands, moving over the dial in various periods reaching up to one hundred and sixty-four years; while the earth, like a ship of exploration, sails the infinite azure, bearing the observers to different points where they may investigate the infinite problems of this mighty machinery. this dial not only shows present movements, but it keeps the history of uncounted ages past ready to be [page ] read backward in proper order; and it has glorious volumes of prophecy, revealing the far-off future to any man who is able to look thereon, break the seals, and read the record. glowing stars are the alphabet of this lofty page. they combine to form words. meteors, rainbows, auroras, shifting groups of stars, make pictures vast and significant as the armies, angels, and falling stars in the revelation of st. john--changing and progressive pictures of infinite wisdom and power. men have not yet advanced as far as those who saw the pictures john describes, and hence the panorama is not understood. that continuous speech that day after day uttereth is not heard; the knowledge that night after night showeth is not seen; and the invisible things of god from the creation of the world, even his eternal power and godhead, clearly discoverable from things that are made, are not apprehended. the greatest triumphs of men's minds have been in astronomy--and ever must be. we have not learned its alphabet yet. we read only easy lessons, with as many mistakes as happy guesses. but in time we shall know all the letters, become familiar with the combinations, be apt at their interpretation, and will read with facility the lessons of wisdom and power that are written on the earth, blazoned in the skies, and pictured by the flowers below and the rainbows above. in order to know how worlds move and develop, we must create them; we must go back to their beginning, give their endowment of forces, and study the laws of their unfolding. this we can easily do by that faculty wherein man is likest his father, a creative imagination. god creates and embodies; we create, but [page ] it remains in thought only. but the creation is as bright, strong, clear, enduring, and real, as if it were embodied. every one of us would make worlds enough to crush us, if we could embody as well as create. our ambition would outrun our wisdom. let us come into the high and ecstatic frame of mind which shakspeare calls frenzy, in the exigencies of his verse, when "the poet's eye, in a fine frenzy rolling, doth glance from heaven to earth, from earth to heaven; and, as imagination bodies forth the forms of things unknown, the poet's pen turns them to shapes, and gives to airy nothing a local habitation and a name." in the supremacy of our creative imagination let us make empty space, in order that we may therein build up a new universe. let us wave the wand of our power, so that all created things disappear. there is no world under our feet, no radiant clouds, no blazing sun, no silver moon, nor twinkling stars. we look up, there is no light; down, through immeasurable abysses, there is no form; all about, and there is no sound or sign of being--nothing save utter silence, utter darkness. it cannot be endured. creation is a necessity of mind--even of the divine mind. we will now, by imagination, create a monster world, every atom of which shall be dowered with the single power of attraction. every particle shall reach out its friendly hand, and there shall be a drawing together of every particle in existence. the laws governing this attraction shall be two. when these particles are associated together, the attraction shall be in proportion to the mass. a given mass will pull twice [page ] as much as one of half the size, because there is twice as much to pull. and a given mass will be pulled twice as much as one half as large, because there is twice as much to be pulled. a man who weighed one hundred and fifty pounds on the earth might weigh a ton and a half on a body as large as the sun. that shall be one law of attraction; and the other shall be that masses attract inversely as the square of distances between them. absence shall affect friendships that have a material basis. if a body like the earth pulls a man one hundred and fifty pounds at the surface, or four thousand miles from the centre, it will pull the same man one-fourth as much at twice the distance, one-sixteenth as much at four times the distance. that is, he will weigh by a spring balance thirty-seven and a half pounds at eight thousand miles from the centre, and nine pounds six ounces at sixteen thousand miles from the centre, and he will weigh or be pulled by the earth / of a pound at the distance of the moon. but the moon would be large enough and near enough to pull twenty-four pounds on the same man, so the earth could not draw him away. thus the two laws of attraction of gravitation are-- , _gravity is proportioned to the quantity of matter_; and , _the force of gravity varies inversely as the square of the distance from the centre of the attracting body_. the original form of matter is gas. almost as i write comes the announcement that mr. lockyer has proved that all the so-called primary elements of matter are only so many different sized molecules of one original substance--hydrogen. whether that is true or not, let us now create all the hydrogen we can [page ] imagine, either in differently sized masses or in combination with other substances. there it is! we cannot measure its bulk; we cannot fly around it in any recordable eons of time. it has boundaries, to be sure, for we are finite, but we cannot measure them. let it alone, now; leave it to itself. what follows? it is dowered simply with attraction. the vast mass begins to shrink, the outer portions are drawn inward. they rush and swirl in vast cyclones, thousands of miles in extent. the centre grows compact, heat is evolved by impact, as will be explained in chapter ii. dull red light begins to look like coming dawn. centuries go by; contraction goes on; light blazes in insufferable brightness; tornadoes, whirlpools, and tempests scarcely signify anything as applied to such tumultuous tossing. there hangs the only world in existence; it hangs in empty space. it has no tendency to rise; none to fall; none to move at all in any direction. it seethes and, flames, and holds itself together by attractive power, and that is all the force with which we have endowed it. leave it there alone, and withdraw millions of miles into space: it looks smaller and smaller. we lose sight of those distinctive spires of flame, those terrible movements. it only gives an even effulgence, a steady unflickering light. turn one quarter round. still we see our world, but it is at one side. now in front, in the utter darkness, suddenly create another world of the same size, and at the same distance from you. there they stand--two huge, lone bodies, in empty space. but we created them dowered with attraction. each instantly feels the drawing influence of the other. they are mutually attractive, and begin to [page ] move toward each other. they hasten along an undeviating straight line. their speed quickens at every mile. the attraction increases every moment. they fly swift as thought. they dash their flaming, seething foreheads together. and now we have one world again. it is twice as large as before, that is all the difference. there is no variety, neither any motion; just simple flame, and nothing to be warmed thereby. are our creative powers exhausted by this effort? [illustration: fig. .--orbit a d, resulting from attraction, a c, and projectile force, a b.] no, we will create another world, and add another power to it that shall keep them apart. that power shall be what is called the force of inertia, which is literally no power at all; it is an inability to originate or change motion. if a body is at rest, inertia is that quality by which it will forever remain so, unless acted upon by some force from without; and if a body is in motion, it will continue on at the same speed, in a straight line, forever, unless it is quickened, retarded, or turned from its path by some other force. suppose our newly created sun is , miles in diameter. go away , , miles and create an earth eight thousand miles in diameter. it instantly feels the attractive power of the sun drawing it to itself sixty-eight [page ] miles a second. now, just as it starts, give this earth a push in a line at right angles with line of fall to the sun, that shall send it one hundred and eighty-nine miles a second. it obeys both forces. the result is that the world moves constantly forward at the same speed by its inertia from that first push, and attraction momentarily draws it from its straight line, so that the new world circles round the other to the starting-point. continuing under the operation of both forces, the worlds can never come together or fly apart. they circle about each other as long as these forces endure; for the first world does not stand still and the second do all the going; both revolve around the centre of gravity common to both. in case the worlds are equal in mass, they will both take the same orbit around a central stationary point, midway between the two. in case their mass be as one to eighty-one, as in the case of the earth and the moon, the centre of gravity around which both turn will be / of the distance from the earth's centre to the moon's centre. this brings the central point around which both worlds swing just inside the surface of the earth. it is like an apple attached by a string, and swung around the hand; the hand moves a little, the apple very much. thus the problem of two revolving bodies is readily comprehended. the two bodies lie in easy beds, and swing obedient to constant forces. when another body, however, is introduced, with its varying attraction, first on one and then on the other, complications are introduced that only the most masterly minds can follow. introduce a dozen or a million bodies, and complications arise that only omniscience can unravel. [page ] [illustration: fig. .] let the hand swing an apple by an elastic cord. when the apple falls toward the earth it feels another force besides that derived from the hand, which greatly lengthens the elastic cord. to tear it away from the earth's attraction, and make it rise, requires additional force, and hence the string is lengthened; but when it passes over the hand the earth attracts it downward, and the string is very much shortened: so the moon, held by an elastic cord, swings around the earth. from its extreme distance from the earth, at a, fig. , it rushes with increasing speed nearly a quarter of a million of miles toward the sun, feeling its attraction increase with every mile until it reaches b; then it is retarded in its speed, by the same attraction, as it climbs back its quarter of a million of miles away from the sun, in defiance of its power, to c. all the while the invisible elastic force of the earth is unweariedly maintained; and though the moon's distances vary over a range of , miles, the moon is always in a determinable place. a simple revolution of one world about another in a circular orbit would be a problem of easy solution. it would always be at the same distance from its centre, and going with the same velocity. but there are over sixty causes that interfere with such a simple orbit in the case of the moon, all of which causes and their disturbances must be considered in calculating such a simple matter as an eclipse, or predicting the moon's place as the sailors guide. one of the most puzzling of the irregularities [page ] of our night-wandering orb has just been explained by professor hansen, of gotha, as a curious result of the attraction of venus. [illustration: fig. .--changes of orbit by mutual attraction.] take a single instance of the perturbations of jupiter and saturn which can be rendered evident. the times of orbital revolution of saturn and jupiter are nearly as five to two. suppose the orbits of the planets to be, as in fig. , both ellipses, but not necessarily equally distant in all parts. the planets are as near as possible at , . drawn toward each other by mutual attraction, jupiter's orbit bends outward, and saturn's becomes more nearly straight, as shown by the dotted lines. a partial correction of this difficulty immediately follows. as jupiter moves on ahead of saturn it is held back--retarded in its orbit by that body; and saturn is hastened in its orbit by the attraction of jupiter. now greater speed means a straighter orbit. a rifle-ball flies nearer in a straight line than a thrown stone. a greater velocity given to a whirled ball pulls the elastic cord far enough to give the ball a larger orbit. hence, being hastened, saturn stretches out nearer its proper orbit, and, retarded, jupiter approaches the smaller curve that is its true orbit. but if they were always to meet at this point, as they would if jupiter made two revolutions to saturn's one, it would be disastrous. in reality, when saturn has gone around two-thirds of its orbit to , jupiter will have gone once and two-thirds around and overtaken [page ] saturn; and they will be near again, be drawn together, hastened, and retarded, as before; their next conjunction would be at , , etc. now, if they always made their conjunction at points equally distant, or at thirds of their orbits, it would cause a series of increasing deviations; for jupiter would be constantly swelling his orbit at three points, and saturn increasingly contracting his orbit at the same points. disaster would be easily foretold. but as their times of orbital revolutions are not exactly in the ratio of five and two, their points of conjunction slowly travel around the orbit, till, in a period of nine hundred years, the starting-point is again reached, and the perturbations have mutually corrected one another. for example, the total attractive effect of one planet on the other for years is to quicken its speed. the effect for the next years is to retard. the place of saturn, when all the retardations have accumulated for years, is one degree behind what it is computed if they are not considered; and years later it will be one degree before its computed place--a perturbation of two degrees. when a bullet is a little heavier or ragged on one side, it will constantly swerve in that direction. the spiral groove in the rifle, of one turn in forty-five feet, turns the disturbing weight or raggedness from side to side--makes one error correct another, and so the ball flies straight to the bull's-eye. so the place of jupiter and saturn, though further complicated by four moons in the case of jupiter, and eight in the case of saturn, and also by perturbations caused by other planets, can be calculated with exceeding nicety. the difficulties would be greatly increased if the orbits [page ] of saturn and jupiter, instead of being , , miles apart, were interlaced. yet there are the orbits of one hundred and ninety-two asteroids so interlaced that, if they were made of wire, no one could be lifted without raising the whole net-work of them. nevertheless, all these swift chariots of the sky race along the course of their intermingling tracks as securely as if they were each guided by an intelligent mind. _they are guided by an intelligent mind and an almighty arm._ still more complicated is the question of the mutual attractions of all the planets. lagrange has been able to show, by a mathematical genius that seems little short of omniscience in his single department of knowledge, that there is a discovered system of oscillations, affecting the entire planetary system, the periods of which are immensely long. the number of these oscillations is equal to that of all the planets, and their periods range from , to , , years, looking into the open page of the starry heavens we see double stars, the constituent parts of which must revolve around a centre common to them both, or rush to a common ruin. eagerly we look to see if they revolve, and beholding them in the very act, we conclude, not groundlessly, that the same great law of gravitation holds good in distant stellar spaces, and that there the same sufficient mind plans, and the same sufficient power directs and controls all movements in harmony and security. when we come to the perturbations caused by the mutual attractions of the sun, nine planets, twenty moons, one hundred and ninety-two asteroids, millions [page ] of comets, and innumerable meteoric bodies swarming in space, and when we add to all these, that belong to one solar system, the attractions of all the systems of the other suns that sparkle on a brilliant winter night, we are compelled to say, "as high as the heavens are above the earth, so high above our thoughts and ways must be the thoughts and ways of him who comprehends and directs them all." [page ] ii. creative progress. "and god said, let there be light, and there was light."--_genesis_ i., . "god is light."-- _john_, i. . [page ] "hail! holy light, offspring of heaven first born, or of the eternal, co-eternal beam, may i express thee unblamed? since god is light, and never but in unapproached light dwelt from eternity, dwelt then in thee, bright effluence of bright essence increate." milton. "a million torches lighted by thy hand wander unwearied through the blue abyss: they own thy power, accomplish thy command, all gay with life, all eloquent with bliss. what shall we call them? piles of crystal light-- a glorious company of golden streams-- lamps of celestial ether burning bright-- suns lighting systems with their joyous beams? but 'thou to these art as the noon to night." derzhavin, trans. by bowring. [page ] ii. _creative progress._ worlds would be very imperfect and useless when simply endowed with attraction and inertia, if no time were allowed for these forces to work out their legitimate results. we want something more than swirling seas of attracted gases, something more than compacted rocks. we look for soil, verdure, a paradise of beauty, animal life, and immortal minds. let us go on with the process. light is the child of force, and the child, like its father, is full of power. we dowered our created world with but a single quality--a force of attraction. it not only had attraction for its own material substance, but sent out an all-pervasive attraction into space. by the force of condensation it flamed like a sun, and not only lighted its own substance, but it filled all space with the luminous outgoings of its power. a world may be limited, but its influence cannot; its body may have bounds, but its soul is infinite. everywhere is its manifestation as real, power as effective, presence as actual, as at the central point. he that studies ponderable bodies alone is not studying the universe, only its skeleton. skeletons are somewhat interesting in themselves, but far more so when covered with flesh, flushed with beauty, and inspired with soul. the universe [page ] has bones, flesh, beauty, soul, and all is one. it can be understood only by a study of all its parts, and by tracing effect to cause. but how can condensation cause light? power cannot be quiet. the mighty locomotive trembles with its own energy. a smitten piece of iron has all its infinitesimal atoms set in vehement commotion; they surge back and forth among themselves, like the waves of a storm-blown lake. heat is a mode of motion. a heated body commences a vigorous vibration among its particles, and communicates these vibrations to the surrounding air and ether. when these vibrations reach , , , , per second, the human eye, fitted to be affected by that number, discerns the emitted undulations, and the object seems to glow with a dull red light; becoming hotter, the vibrations increase in rapidity. when they reach , , , , per second the color becomes violet, and the eye can observe them no farther. between these numbers are those of different rapidities, which affect the eye--as orange, yellow, green, blue, indigo, in an almost infinite number of shades--according to the sensitiveness of the eye. we now see how our dark immensity of attractive atoms can become luminous. a force of compression results in vibrations within, communicated to the ether, discerned by the eye. illustrations are numerous. if we suddenly push a piston into a cylinder of brass, the force produces heat enough to set fire to an inflammable substance within. strike a half-inch cube of iron a moderate blow and it becomes warm; a sufficient blow, and its vibrations become quick enough to be seen--it is red-hot. attach a thermometer to an extended [page ] arm of a whirling wheel; drive it against the air five hundred feet per second, the mercury rises °. the earth goes , feet per second, or one thousand miles a minute. if it come to an aerolite or mass of metallic rock, or even a cloudlet of gas, standing still in space, its contact with our air evolves , ° of heat. and when the meteor comes toward the world twenty-six miles a second, the heat would become proportionally greater if the meteor could abide it, and not be consumed in fervent heat. it vanishes almost as soon as seen. if there were meteoric masses enough lying in our path, our sky would blaze with myriads of flashes of light. enough have been seen to enable a person to read by them at night. if a sufficient number were present, we should miss their individual flashes as they blend their separate fires in one sea of insufferable glory. the sun is , , times as large as our planet; its attraction proportionally greater; the aerolites more numerous; and hence an infinite hail of stones, small masses and little worlds, makes ceaseless trails of light, whose individuality is lost in one dazzling sea of glory. on the st day of september, , two astronomers, independently of each other, saw a sudden brightening on the surface of the sun. probably two large meteoric masses were travelling side by side at two or three hundred miles per second, and striking the sun's atmosphere, suddenly blazed into light bright enough to be seen on the intolerable light of the photosphere as a background. the earth responded to this new cause of brilliance and heat in the sun. vivid auroras appeared, not only at the north and south poles, but even where such spectacles are seldom seen. the electro-magnetic [page ] disturbances were more distinctly marked. "in many places the telegraphic wires struck work. in washington and philadelphia the electric signalmen received severe electric shocks; at a station in norway the telegraphic apparatus was set fire to; and at boston a flame of fire followed the pen of bain's electric telegraph." there is the best of reason for believing that a continuous succession of such bodies might have gone far toward rendering the earth uncomfortable as a place of residence. of course, the same result of heat and light would follow from compression, if a body had the power of contraction in itself. we endowed every particle of our gas, myriads of miles in extent, with an attraction for every other particle. it immediately compressed itself into a light-giving body, which flamed out through the interstellar spaces, flushing all the celestial regions with exuberant light. but heat exerts a repellent force among particles, and soon an equilibrium is reached, for there comes a time when the contracting body can contract no farther. but heat and light radiate away into cold space, then contraction goes on evolving more light, and so the suns flame on through the millions of years unquenched. it is estimated that the contraction of our sun, from filling immensity of space to its present size, could not afford heat enough to last more than , , years, and that its contraction from its present density (that of a swamp) to such rock as that of which our earth is composed, could supply heat enough for , , years longer. but the far-seeing mind of man knows a time must come when the present force of attraction [page ] shall have produced all the heat it can, and a new force of attraction must be added, or the sun itself will become cold as a cinder, dead as a burned-out char. since light and heat are the product of such enormous cosmic forces, they must partake of their nature, and be force. so they are. the sun has long arms, and they are full of unconquerable strength ninety-two millions, or any other number of millions, of miles away. all this light and heat comes through space that is ° below zero, through utter darkness, and appears only on the earth. so the gas is darkness in the underground pipes, but light at the burner. so the electric power is unfelt by the cable in the bosom of the deep, but is expressive of thought and feeling at the end. having found the cause of light, we will commence a study of its qualities and powers. light is the astronomer's necessity. when the sublime word was uttered, "let there be light!" the study of astronomy was made possible. man can gather but little of it with his eye; so he takes a lens twenty-six inches in diameter, and bends all the light that passes through it to a focus, then magnifies the image and takes it into his eye. or he takes a mirror, six feet in diameter, so hollowed in the middle as to reflect all the rays falling upon it to one point, and makes this larger eye fill his own with light. by this larger light-gathering he discerns things for which the light falling on his pupil one-fifth of an inch in diameter would not be sufficient. we never have seen any sun or stars; we have only seen the light that left them fifty minutes or years ago, more or less. light is the aërial sprite that carries our measuring-rods across the infinite [page ] spaces; light spreads out the history of that far-off beginning; brings us the measure of stars a thousand times brighter than our sun; takes up into itself evidences of the very constitutional elements of the far-off suns, and spreads them at our feet. it is of such capacity that the divine nature, looking for an expression of its own omnipotence, omniscience, and power of revelation, was content to say, "god is light." we shall need all our delicacy of analysis and measurement when we seek to determine the activities of matter so fine and near to spirit as light. [illustration: fig. .--velocity of light measured by eclipses of jupiter's moons.] we first seek the velocity of light. in fig. the earth is , , miles from the sun at e; jupiter is , , miles from the sun at j. it has four moons: the inner one goes around the central body in forty-two hours, and is eclipsed at every revolution. the light that went out from the sun to m ceases to be reflected back to the earth by the intervention of the planet jupiter. we know to a second when these eclipses take place, and they can be seen with a small telescope. but when the earth is on the opposite side of the sun [page ] from jupiter, at e', these eclipses at j' take place sixteen and a half minutes too late. what is the reason? is the celestial chronometry getting deranged? no, indeed; these great worlds swing never an inch out of place, nor a second out of time. by going to the other side of the sun the earth is , , miles farther from jupiter, and the light that brings the intelligence of that eclipse consumes the extra time in going over the extra distance. divide one by the other and we get the velocity, , miles per second. that is probably correct to within a thousand miles. methods of measurement by the toothed wheel of fizeau confirm this result. suppose the wheel, fig. , to have one thousand teeth, making five revolutions to the second. five thousand flashes of light each second will dart out. let each flash travel nine miles to a mirror and return. if it goes that distance in / of a second, or at the rate of , miles a second, the next tooth will have arrived before the eye, and each returning ray be cut off. hasten the revolutions a little, and the next notch will then admit the ray, on its return, that went out of each previous notch: the eighteen miles having been traversed meanwhile. the method of measuring by means of a revolving mirror, used by faucault, is held to be even more accurate. [illustration: fig. .--measuring the velocity of light.] when we take instantaneous photographs by the exposure [page ] of the sensitive plate / part of a second, a stream of light nine miles long dashes in upon the plate in that very brief period of time. the highest velocity we can give a rifle-ball is feet a second, the next second it is only feet, and soon it comes to rest. we cannot compact force enough behind a bit of lead to keep it flying. but light flies unweariedly and without diminution of speed. when it has come from the sun in eight minutes, alpha centauri in three years, polaris in forty-five years, other stars in one thousand, its wings are in nowise fatigued, nor is the rapidity of its flight slackened in the least. it is not the transactions of to-day that we read in the heavens, but it is history, some of it older than the time of adam. those stars may have been smitten out of existence decades of centuries ago, but their poured-out light is yet flooding the heavens. it goes both ways at once in the same place, without interference. we see the light reflected from the new moon to the earth; reflected back from the house-tops, fields, and waters of earth, to the moon again, and from the moon to us once more--three times in opposite directions, in the same place, without interference, and thus we see "the old moon in the arms of the new." _constitution of light._ [illustration: fig. .--white light resolved into colors.] light was once supposed to be corpuscular, or consisting of transmitted particles. it is now known to be the result of undulations in ether. reference has been made to the minuteness of these undulations. their velocity is equally wonderful. put a prism of glass into a ray of light coming into a dark room, and it is [page ] instantly turned out of its course, some parts more and some less, according to the number of vibrations, and appears as the seven colors on different parts of the screen. fig. shows the arrangement of colors, and the number of millions of millions of vibrations per second of each. but the different divisions we call colors are not colors in themselves at all, but simply a different number of vibrations. color is all in the eye. violet has in different places from to , , , , of vibrations per second; red has, in different places, from to , , , , vibrations per second. none of these in any sense are color, but affect the eye differently, and we call these different effects color. they are simply various velocities of vibration. an object, like one kind of stripe in our flag, which absorbs all kinds of vibrations except those between and , , , , , and reflects those, appears red to us. the field for the stars absorbs and destroys all but those vibrations numbering about , , , , of [page ] vibrations per second. a color is a constant creation. light makes momentary color in the flag. drake might have written, in the continuous present as well as in the past, "freedom mingles with its gorgeous dyes the milky baldrick of the skies, and stripes its pore celestial white with streakings of the morning light." every little pansy, tender as fancy, pearled with evanescent dew, fresh as a new creation of sunbeams, has power to suppress in one part of its petals all vibrations we call red, in another those we call yellow, and purple, and reflect each of these in other parts of the same tender petal. "pansies are for thoughts," even more thoughts than poor ophelia knew. an evening cloud that is dense enough to absorb all the faster and weaker vibrations, leaving only the stronger to come through, will be said to be red; because the vibrations that produce the impression we have so named are the only ones that have vigor enough to get through. it is like an army charging upon a fortress. under the deadly fire and fearful obstructions six-sevenths go down, but one-seventh comes through with the glory of victory upon its face. light comes in undulations to the eye, as tones of sound to the ear. must not light also sing? the lowest tone we can hear is made by . vibrations of air per second; the highest, so shrill and "fine that nothing lives 'twixt it and silence," is made by , vibrations per second. between these extremes lie eleven octaves; c of the g clef having - / vibrations to the second, and its octave above - / . not that sound vibrations cease [page ] at , , but our organs are not fitted to hear beyond those limitations. if our ears were delicate enough, we could hear even up to the almost infinite vibrations of light. in one of those semi-inspirations we find in shakspeare's works, he says-- "there's not the smallest orb which thou beholdest, but in his motion like an angel sings, still quiring to the young-eyed cherubim. such harmony is in immortal souls; but, whilst this muddy vesture of decay doth grossly close it in, we cannot hear it." and that older poetry which is always highest truth says, "the morning stars sing together." we misconstrued another passage which we could not understand, and did not dare translate as it was written, till science crept up to a perception of the truth that had been standing there for ages, waiting a mind that could take it in. now we read as it is written--"thou makest the out-goings of the morning and evening to sing." were our senses fine enough, we could hear the separate keynote of every individual star. stars differ in glory and in power, and so in the volume and pitch of their song. were our hearing sensitive enough, we could hear not only the separate key-notes but the infinite swelling harmony of these myriad stars of the sky, as they pour their mighty tide of united anthems in the ear of god: "in reason's ear they all rejoice, and utter forth a glorious voice. forever singing, as they shine, the hand that made us is divine." this music is not monotonous. stars draw near each other, and make a light that is unapproachable by mortals; [page ] then the music swells beyond our ability to endure. they recede far away, making a light so dim that the music dies away, so near to silence that only spirits can perceive it. no wonder god rejoices in his works. they pour into his ear one ceaseless tide of rapturous song. our senses are limited--we have only five, but there is room for many more. some time we shall be taken out of "this muddy vesture of decay," no longer see the universe through crevices of our prison-house, but shall range through wider fields, explore deeper mysteries, and discover new worlds, hints of which have never yet been blown across the wide atlantic that rolls between them and men abiding in the flesh. _chemistry of suns revealed by light._ when we examine the assemblage of colors spread from the white ray of sunlight, we do not find red simple red, yellow yellow, etc., but there is a vast number of fine microscopic lines of various lengths, parallel--here near together, there far apart, always the same number and the same relative distance, when the same light and prism are used. what new alphabets to new realms of knowledge are these! remember, that what we call colors are only various numbers of vibrations of ether. remember, that every little group in the infinite variety of these vibrations may be affected differently from every other group. one number of these is bent by the prism to where we see what we call the violet, another number to the place we call red. all of the vibrations are destroyed when they strike a surface we call black. a part of them are destroyed when [page ] they strike a substance we call colored. the rest are reflected, and give the impression of color. in one place on the flag of our nation all vibrations are destroyed except the red; in another, all but the blue. perhaps on that other gorgeous flag, not of our country but of our sun, the flag we call the solar spectrum, all vibrations are destroyed where these dark lines appear. perhaps this effect is not produced by the surface upon which the rays fall, but by some specific substance in the sun. this is just the truth. light passing through vapor of sodium has the vibrations that would fall on two narrow lines in the yellow utterly destroyed, leaving two black spaces. light passing through vapor of burning iron has some four hundred numbers or kinds of vibrations destroyed, leaving that number of black lines; but if the salt or iron be glowing gas, in the source of the light itself the same lines are bright instead of dark. thus we have brought to our doors a readable record of the very substances composing every world hot enough to shine by its own light. thus, while our flag means all we have of liberty, free as the winds that kiss it, and bright as the stars that shine in it, the flag of the sun means all that it is in constituent elements, all that it is in condition. we find in our sun many substances known to exist in the earth, and some that we had not discovered when the sun wrote their names, or rather made their mark, in the spectrum. thus, also, we find that betelguese and algol are without any perceivable indications of hydrogen, and sirius has it in abundance. what a sense of acquaintanceship it gives us to look up and recognize [page ] the stars whose very substance we know! if we were transported thither, or beyond, we should not be altogether strangers in an unknown realm. but the stars differ in their constituent elements; every ray that flashes from them bears in its very being proofs of what they are. hence the eye of omniscience, seeing a ray of light anywhere in the universe, though gone from its source a thousand years, would be able to tell from what orb it originally came. _creative force of light._ just above the color vibrations of the unbraided sunbeam, above the violet, which is the highest number our eyes can detect, is a chemical force; it works the changes on the glass plate in photography; it transfigures the dark, cold soil into woody fibre, green leaf, downy rose petals, luscious fruit, and far pervasive odor; it flushes the wide acres of the prairie with grass and flowers, fills the valleys with trees, and covers the hills with corn, a single blade of which all the power of man could not make. this power is also fit and able to survive. the engineer stephenson once asked dr. buckland, "what is the power that drives that train?" pointing to one thundering by. "well, i suppose it is one of your big engines." "but what drives the engine?" "oh, very likely a canny newcastle driver." "no, sir," said the engineer, "it is sunshine." the doctor was too dull to take it in. let us see if we can trace such an evident effect to that distant cause. ages ago the warm sunshine, falling on the scarcely lifted hills of pennsylvania, caused the reedy vegetation to grow along the banks of [page ] shallow seas, accumulated vast amounts of this vegetation, sunk it beneath the sea, roofed it over with sand, compacted the sand into rock, and changed this vegetable matter--the products of the sunshine--into coal; and when it was ready, lifted it once more, all garnered for the use of men, roofed over with mighty mountains. we mine the coal, bring out the heat, raise the steam, drive the train, so that in the ultimate analyses it is sunshine that drives the train. these great beds of coal are nothing but condensed sunshine--the sun's great force, through ages gone, preserved for our use to-day. and it is so full of force that a piece of coal that will weigh three pounds (as big as a large pair of fists) has as much power in it as the average man puts into a day's work. three tons of coal will pump as much water or shovel as much sand as the average man will pump or shovel in a lifetime; so that if a man proposes to do nothing but work with his muscles, he had better dig three tons of coal and set that to do his work and then die, because his work will be better done, and without any cost for the maintenance of the doer. come down below the color vibrations, and we shall find that those which are too infrequent to be visible, manifest as heat. naturally there will be as many different kinds of heat as tints of color, because there is as great a range of numbers of vibration. it is our privilege to sift them apart and sort them over, and find what kinds are best adapted to our various uses. take an electric lamp, giving a strong beam of light and heat, and with a plano-convex lens gather it into a single beam and direct it upon a thermometer, twenty feet away, that is made of glass and filled with air. the [page ] expansion or contraction of this air will indicate the varying amounts of heat. watch your air-thermometer, on which the beam of heat is pouring, for the result. there is none. and yet there is a strong current of heat there. put another kind of test of heat beyond it and it appears; coat the air-thermometer with a bit of black cloth, and that will absorb heat and reveal it. but why not at first? because the glass lens stops all the heat that can affect glass. the twenty feet of air absorbs all the heat that affects air, and no kind of heat is left to affect an instrument made of glass and air; but there are kinds of heat enough to affect instruments made of other things. a very strong current of heat may be sent right through the heart of a block of ice without melting the ice at all or cooling off the heat in the least. it is done in this way: send the beam of heat through water in a glass trough, and this absorbs all the heat that can affect water or ice, getting itself hot, and leaving all other kinds of heat to go through the ice beyond; and appropriate tests show that as much heat comes out on the other side as goes in on this side, and it does not melt the ice at all. gunpowder may be exploded by heat sent through ice. dr. kane, years ago, made this experiment. he was coming down from the north, and fell in with some esquimaux, whom he was anxious to conciliate. he said to the old wizard of the tribe, "i am a wizard; i can bring the sun down out of the heavens with a piece of ice." that was a good, deal to say in a country where there was so little sun. "so," he writes, "i took my hatchet, chipped a small piece of ice into the form of a double-convex lens, [page ] smoothed it with my warm hands, held it up to the sun, and, as the old man was blind, i kindly burned a blister on the back of his hand to show him i could do it." these are simple illustrations of the various kinds of heat. the best furnace or stove ever invented consumes fifteen times as much fuel to produce a given amount of heat as the furnace in our bodies consumes to produce a similar amount. we lay in our supplies of carbon at the breakfast, dinner, and supper table, and keep ourselves warm by economically burning it with the oxygen we breathe. heat associated with light has very different qualities from that which is not. sunlight melts ice in the middle, bottom, and top at once. ice in the spring-time is honey-combed throughout. a piece of ice set in the summer sunshine crumbles into separate crystals. dark heat only melts the surface. nearly all the heat of the sun passes through glass without hinderance; but take heat from white-hot platinum and only seventy-six per cent. of it goes through glass, twenty-four per cent. being so constituted that it cannot pass with facility. of heat from copper at ° only six per cent. can go through glass, the other ninety-four per cent. being absorbed by it. the heat of the sun beam goes through glass without [page ] any hinderance whatever. it streams into the room as freely as if there were no glass there. but what if the furnace or stove heat went through glass with equal facility? we might as well try to heat our rooms with the window-panes all out, and the blast of winter sweeping through them. the heat of the sun, by its intense vibrations, comes to the earth dowered with a power which pierces the miles of our atmosphere, but if our air were as pervious to the heat of the earth, this heat would flyaway every night, and our temperature would go down to ° below zero. this heat comes with the light, and then, dissociated from it, the number of its vibrations lessened, it is robbed of its power to get away, and remains to work its beneficent ends for our good. worlds that are so distant as to receive only / of the heat we enjoy, may have atmospheres that retain it all. indeed it is probable that mars, that receives but one-quarter as much heat as the earth, has a temperature as high as ours. the poet drew on his imagination when he wrote: "who there inhabit must have other powers, juices, and veins, and sense and life than ours; one moment's cold like theirs would pierce the bone, freeze the heart's-blood, and turn us all to stone." the power that journeys along the celestial spaces in the flashing sunshine is beyond our comprehension. it accomplishes with ease what man strives in vain to do with all his strength. at west point there are some links of a chain that was stretched across the river to prevent british ships from ascending; these links were made of two-and-a-quarter-inch iron. a powerful locomotive might tug in vain at one of them and not stretch it the thousandth part of an inch. but the heat of a single gas-burner, that glows with the preserved sunlight of other ages, when suitably applied to the link, stretches it with ease; such enormous power has a little heat. there is a certain iron bridge across the thames at london, resting on arches. the warm sunshine, acting [page ] upon the iron, stations its particles farther and farther apart. since the bottom cannot give way the arches must rise in the middle. as they become longer they lift the whole bridge, and all the thundering locomotives and miles of goods-trains cannot bring that bridge down again until the power of the sunshine has been withdrawn. there is bunker hill monument, thirty-two feet square at the base, with an elevation of two hundred and twenty feet. the sunshine of every summer's day takes hold of that mighty pile of granite with its aërial fingers, lengthens the side affected, and bends the whole great mass as easily as one would bend a whipstock. a few years ago we hung a plummet from the top of this monument to the bottom. at a.m. it began to move toward the west; at noon it swung round toward the north; in the afternoon it went east of where it first was, and in the night it settled back to its original place. the sunshine says to the sea, held in the grasp of gravitation, "rise from your bed! let millions of tons of water fly on the wings of the viewless air, hundreds of miles to the distant mountains, and pour there those millions of tons that shall refresh a whole continent, and shall gather in rivers fitted to bear the commerce and the navies of nations." gravitation says, "i will hold every particle of this ocean as near the centre of the earth as i can." sunshine speaks with its word of power, and says, "up and away!" and in the wreathing mists of morning these myriads of tons rise in the air, flyaway hundreds of miles, and supply all the niagaras, mississippis and amazons of earth. the sun says to the earth, wrapped in the mantle of winter, [page ] "bloom again;" and the snows melt, the ice retires, and vegetation breaks forth, birds sing, and spring is about us. thus it is evident that every force is constitutionally arranged to be overcome by a higher, and all by the highest. gravitation of earth naturally and legitimately yields to the power of the sun's heat, and then the waters fly into the clouds. it as naturally and legitimately yields to the power of mind, and the waters of the red sea are divided and stand "upright as an heap." water naturally bursts into flame when a bit of potassium is thrown into it, and as naturally when elijah calls the right kind of fire from above. what seems a miracle, and in contravention of law, is only the constitutional exercise of higher force over forces organized to be swayed. if law were perfectly rigid, there could be but one force; but many grades exist from cohesion to mind and spirit. the highest forces are meant to have victory, and thus give the highest order and perfectness. across the astronomic spaces reach all these powers, making creation a perpetual process rather than a single act. it almost seems as if light, in its varied capacities, were the embodiment of god's creative power; as if, having said, "let there be light," he need do nothing else, but allow it to carry forward the creative processes to the end of time. it was newton, one of the earliest and most acute investigators in this study of light, who said, "i seem to have wandered on the shore of truth's great ocean, and to have gathered a few pebbles more beautiful than common; but the vast ocean itself rolls before me undiscovered and unexplored." [page ] experiments with light. a light set in a room is seen from every place; hence light streams in every possible direction. if put in the centre of a hollow sphere, every point of the surface will be equally illumined. if put in a sphere of twice the diameter, the same light will fall on all the larger surface. the surfaces of spheres are as the squares of their diameters; hence, in the larger sphere the surface is illumined only one-quarter as much as the smaller. the same is true of large and small rooms. in fig. it is apparent that the light that falls on the first square is spread, at twice the distance, over the second square, which is four times as large, and at three times the distance over nine times the surface. the varying amount of light received by each planet is also shown in fractions above each world, the amount received by the earth being . [illustration: fig. .] [illustration: fig. .--measuring intensities of light.] the intensity of light is easily measured. let two lights of different brightness, as in fig. , cast shadows on the same screen. arrange them as to distance so that both shadows shall be equally dark. let them fall side by side, and study them carefully. measure the respective distances. suppose one is twenty inches, the other forty. light varies as the square [page ] of the distance: the square of is , of is . divide by , and the result is that one light is four times as bright as the other. [illustration: fig. .--reflection and diffusion of light.] light can be handled, directed, and bent, as well as iron bars. darken a room and admit a beam of sunlight through a shutter, or a ray of lamp-light through the key-hole. if there is dust in the room it will be observed that light goes in straight lines. because of this men are able to arrange houses and trees in rows, the hunter aims his rifle correctly, and the astronomer projects straight lines to infinity. take a hand-mirror, or better, a piece of glass coated on one side with black varnish, and you can send your ray anywhere. by using two mirrors, or having an assistant and using several, you can cause a ray of light to turn as many corners as you please. i once saw mr. tyndall send a ray into a glass jar filled with smoke (fig. ). admitting a slender ray through a small hole in a card over the mouth, one ray appeared; removing the cover, the whole jar was luminous; as the smoke disappeared in spots cavities of darkness appeared. turn the same ray into a tumbler of water, [page ] it becomes faintly visible; stir into it a teaspoonful of milk, then turn in the ray of sunlight, and it glows like a lamp, illuminating the whole room. these experiments show how the straight rays of the sun are diffused in every direction over the earth. set a small light near one edge of a mirror; then, by putting the eye near the opposite edge, you see almost as many flames as you please from the multiplied reflections. how can this be accounted for? into your beam of sunlight, admitted through a half-inch hole, put the mirror at an oblique angle; you can arrange it so as to throw half a dozen bright spots on the opposite wall. [illustration: fig. .--manifold reflections.] in fig. the sunbeam enters at a, and, striking the mirror _m_ at _a_, is partly reflected to on the wall, and partly enters the glass, passes through to the silvered back at b, and is totally reflected to _b_, where it again divides, some of it going to the wall at , and the rest, continuing to make the same reflections and divisions, causes spots , , , etc. the brightest spot is at no. , because the silvered glass at b is the best reflector and has the most light. when the discovery of the moons of mars was announced in , it was also widely published that they could be seen by a mirror. of course this is impossible. the point of light mistaken for the moon in this secondary reflection was caused by holding the mirror in an oblique position. take a small piece of mirror, say an inch in surface, and putting under it three little pellets of wax, putty, or clay, set it on the wrist, with one of the pellets on the pulse. hold the mirror steadily in the beam of light, and the frequency and prominence of each pulse-beat will be indicated by the tossing spot of light on the wall. if the operator becomes excited the fact will be evident to all observers. [illustration: fig. .] place a coin in a basin (fig. ), and set it so that the rim will conceal the coin from the eye. pour in water, and the coin will [page ] appear to rise into sight. when light passes from a medium of one density to a medium of another, its direction is changed. thus a stick in water seems bent. ships below the horizon are sometimes seen above, because of the different density of the layers of air. thus light coming from the interstellar spaces, and entering our atmosphere, is bent down more and more by its increasing density. the effect is greatest when the sun or star is near the horizon, none at all in the zenith. this brings the object into view before it is risen. allowance for this displacement is made in all delicate astronomical observations. [illustration: fig. .--atmospherical refraction.] notice on the floor the shadow of the window-frames. the glass of almost every window is so bent as to turn the sunlight aside enough to obliterate some of the shadows or increase their thickness. decomposition of light. admit the sunbeam through a slit one inch long and one-twentieth of an inch wide. pass it through a prism. either purchase one or make it of three plain pieces of glass one and a half inch wide by six inches long, fastened together in triangular shape--fasten the edges with hot wax and fill it with water; then on a screen or wall you will have the colors of the rainbow, not merely seven but seventy, if your eyes are sharp enough. take a bit of red paper that matches the red color of the spectrum. move it along the line of colors toward the violet. in the orange it is dark, in the yellow darker, in the green and all beyond, black. that is because there are no more red rays to be reflected by it. so a green object is true to its color only in the green rays, and black elsewhere. all these colors may be recombined by a second prism into white light. [page ] iii. astronomical instruments. "the eyes of the lord are in every place."--_proverbs_ xv. . [page ] "man, having one kind of an eye given him by his maker, proceeds to construct two other kinds. he makes one that magnifies invisible objects thousands of times, so that a dull razor-edge appears as thick as three fingers, until the amazing beauty of color and form in infinitesimal objects is entrancingly apparent, and he knows that god's care of least things is infinite. then he makes the other kind four or six feet in diameter, and penetrates the immensities of space thousands of times beyond where his natural eye can pierce, until he sees that god's immensities of worlds are infinite also."--bishop foster. [page ] iii. _the telescope._ frequent allusion has been made in the previous chapter to discovered results. it is necessary to understand more clearly the process by which such results have been obtained. some astronomical instruments are of the simplest character, some most delicate and complex. when a man smokes a piece of glass, in order to see an eclipse of the sun, he makes a simple instrument. ferguson, lying on his back and slipping beads on a string at a certain distance above his eye, measured the relative distances of the stars. the use of more complex instruments commenced when galileo applied the telescope to the heavens. he cannot be said to have invented the telescope, but he certainly constructed his own without a pattern, and used it to good purpose. it consists of a lens, o b (fig. ), which acts as a multiple prism to bend all the rays to one point at r. place the eye there, and it receives as much light as if it were as large as the lens o b. the rays, however, are convergent, and the point difficult to [page ] find. hence there is placed at r a concave lens, passing through which the rays emerge in parallel lines, and are received by the eye. opera-glasses are made upon precisely this principle to-day, because they can be made conveniently short. [illustration: fig. .--refracting telescope.] if, instead of a concave lens at r, converting the converging rays into parallel ones, we place a convex or magnifying lens, the minute image is enlarged as much as an object seems diminished when the telescope is reversed. this is the grand principle of the refracting telescope. difficulties innumerable arise as we attempt to enlarge the instruments. these have been overcome, one after another, until it is now felt that the best modern telescope, with an object lens of twenty-six inches, has fully reached the limit of optical power. _the reflecting telescope_. this is the only kind of instrument differing radically from the refracting one already described. it receives the light in a concave mirror, m (fig. ), which reflects it to the focus f, producing the same result as the lens of the refracting telescope. here a mirror may be placed obliquely, reflecting the image at right angles to the eye, outside the tube, in which case it is called the newtonian telescope; or a mirror at r may be placed perpendicularly, and send the rays through [page ] an opening in the mirror at m. this form is called the gregorian telescope. or the mirror m may be slightly inclined to the coming rays, so as to bring the point f entirely outside the tube, in which case it is called the herschelian telescope. in either case the image may be magnified, as in the refracting telescope. [illustration: fig. .--reflecting telescope.] reflecting telescopes are made of all sizes, up to the cyclopean eye of the one constructed by lord rosse, which is six feet in diameter. the form of instrument to be preferred depends on the use to which it is to be put. the loss of light in passing through glass lenses is about two-tenths. the loss by reflection is often one-half. in view of this peculiarity and many others, it is held that a twenty-six-inch refractor is fully equal to any six-foot reflector. the mounting of large telescopes demands the highest engineering ability. the whole instrument, with its vast weight of a twenty-six-inch glass lens, with its accompanying tube and appurtenances, must be pointed as nicely as a rifle, and held as steadily as the axis of the globe. to give it the required steadiness, the foundation on which it is placed is sunk deep in the earth, far from rail or other roads, and no part of the observatory is allowed to touch this support. when a star is once found, the earth swiftly rotates the telescope away from it, and it passes out of the field. to avoid this, clock-work is so arranged that the great telescope follows the star by the hour, if required. it will take a star at its eastern rising, and hold it constantly in view while it climbs to the meridian and sinks in the west (fig. ). the reflector demands still more difficult engineering. that of lord rosse has a metallic mirror [page ] weighing six tons, a tube forty feet long, which, with its appurtenances, weighs seven tons more. it moves between two walls only ° east and west. the new paris reflector (fig. ) has a much wider range of movement. [illustration: fig. .--cambridge equatorial.] [illustration: fig. .--new paris reflector.] _the spectroscope._ a spectrum is a collection of the colors which are dispersed by a prism from any given light. if it is sunlight, it is a solar spectrum; if the source of light is a [page ] star, candle, glowing metal, or gas, it is the spectrum of a star, candle, glowing metal, or gas. an instrument to see these spectra is called a spectroscope. considering the infinite variety of light, and its easy modification and absorption, we should expect an immense number of spectra. a mere prism disperses the light so imperfectly that different orders of vibrations, perceived as colors, are mingled. no eye can tell where one commences or ends. such a spectrum is said to be impure. what we want is that each point in the spectrum should be made of rays of the same number of vibrations. as we can let only a small beam of light pass through the prism, in studying celestial objects with a telescope and spectroscope we must, in every instance, contract the aperture of the instrument until we get only a small beam of light. in order to have the colors thoroughly dispersed, the best instruments pass the beam of light through a series of prisms called a battery, each one spreading farther the colors which the previous ones had spread. in fig. the ray is seen entering through the telescope a, which renders the rays parallel, and passing [page ] through the prisms out to telescope b, where the spectrum can be examined on the retina of the eye for a screen. in order to still farther disperse the rays, some batteries receive the ray from the last prism at o upon an oblique mirror, send it up a little to another, which delivers it again to the prism to make its journey back again through them all, and come out to be examined just above where it entered the first prism. [illustration: fig. .--spectroscope, with battery of prisms.] attached to the examining telescope is a diamond-ruled scale of glass, enabling us to fix the position of any line with great exactness. [illustration: fig. .--spectra of glowing hydrogen and the sun.] in fig. is seen, in the lower part, a spectrum of the sun, with about a score of its thousands of lines made evident. in the upper part is seen the spectrum of bright lines given by glowing hydrogen gas. these lines are given by no other known gas; they are its autograph. it is readily observed that they precisely correspond with certain dark lines in the solar spectrum. hence we easily know that a glowing gas gives the same bright lines that it absorbs from the light of another source passing through it--that is, glowing gas gives out the same rays of light that it absorbs when it is not glowing. the subject becomes clearer by a study of the chromolithic plate. no. represents the solar spectrum, with a few of its lines on an accurately graduated scale. [page ] no. shows the bright line of glowing sodium, and, corresponding to a dark line in the solar spectrum, shows the presence of salt in that body. no. shows that potassium has some violet rays, but not all; and there being no dark line to correspond in the solar spectrum, we infer its absence from the sun. no. shows the numerous lines and bands of barium--several red, orange, yellow, and four are very bright green ones. the lines given by any volatilized substances are always in the same place on the scale. a patient study of these signs of substances reveals, richer results than a study of the cuniform characters engraved on assyrian slabs; for one is the handwriting of men, the other the handwriting of god. one of the most difficult and delicate problems solved by the spectroscope is the approach or departure of a light-giving body in the line of sight. stand before a locomotive a mile away, you cannot tell whether it approaches or recedes, yet it will dash by in a minute. how can the movements of the stars be comprehended when they are at such an immeasurable distance? it can best be illustrated by music. the note c of the g clef is made by two hundred and fifty-seven vibrations of air per second. twice as many vibrations per second would give us the note c an octave above. sound travels at the rate of three hundred and sixty-four yards per second. if the source of these two hundred and fifty-seven vibrations could approach us at three hundred and sixty-four yards per second, it is obvious that twice as many waves would be put into a given space, and we should hear the upper c when only waves enough were made for the lower c. the same [page ] result would appear if we carried our ear toward the sound fast enough to take up twice as many valves as though we stood still. this is apparent to every observer in a railway train. the whistle of an approaching locomotive gives one tone; it passes, and we instantly detect another. let two trains, running at a speed of thirty-six yards a second, approach each other. let the whistle of one sound the note e, three hundred and twenty-three vibrations per second. it will be heard on the other as the note g, three hundred and eighty-eight vibrations per second; for the speed of each train crowds the vibrations into one-tenth less room, adding + vibrations per second, making three hundred and eighty-eight in all. the trains pass. the vibrations are put into one-tenth more space by the whistle making them, and the other train allows only nine-tenths of what there are to overtake the ear. each subtracts + vibrations from three hundred and twenty-three, leaving only two hundred and fifty-eight, which is the note c. yet the note e was constantly uttered. [illustration: . solar spectrum. . spectrum of potassium. . spectrum of sodium. . spectrum of strontium. . spectrum of calcium. . spectrum of barium.] if a source of light approach or depart, it will have a similar effect on the light waves. how shall we detect it? if a star approach us, it puts a greater number of waves into an inch, and shortens their length. if it recedes, it increases the length of the wave--puts a less number into an inch. if a body giving only the number of vibrations we call green were to approach sufficiently fast, it would crowd in vibrations enough to appear what we call blue, indigo, or even violet, according to its speed. if it receded sufficiently fast, it would leave behind it only vibrations enough to fill up [page ] the space with what we call yellow, orange, or red, according to its speed; yet it would be green, and green only, all the time. but how detect the change? if red waves are shortened they become orange in color; and from below the red other rays, too far apart to be seen by the eye, being shortened, become visible as red, and we cannot know that anything has taken place. so, if a star recedes fast enough, violet vibrations being lengthened become indigo; and from above the violet other rays, too short to be seen, become lengthened into visible violet, and we can detect no movement of the colors. the dark lines of the spectrum are the cutting out of rays of definite wave-lengths. if the color spectrum moves away, they move with it, and away from their proper place in the ordinary spectrum. if, then, we find them toward the red end, the star is receding; if toward the violet end, it is approaching. turn the instrument on the centre of the sun. the dark lines take their appropriate place, and are recognized on the ruled scale. turn it on one edge, that is approaching us one and a quarter miles a second by the revolution of the sun on its axis, the spectral lines move toward the violet end; turn the spectroscope toward the other edge of the sun, it is receding from us one and a quarter miles a second by reason of the axial revolution, and the spectral lines move toward the red end. turn it near the spots, and it reveals the mighty up-rush in one place and the down-rush in another of one hundred miles a second. we speak of it as an easy matter, but it is a problem of the greatest delicacy, almost defying the mind of man to read the movements of matter. it should be recognized that professor young, of [page ] princeton, is the most successful operator in this recent realm of science. he already proposes to correct the former estimate of the sun's axial revolutions, derived from observing its spots, by the surer process of observing accelerated and retarded light. within a very few years this wonderful instrument, the spectroscope, has made amazing discoveries. in chemistry it reveals substances never known before; in analysis it is delicate to the detection of the millionth of a grain. it is the most deft handmaid of chemistry, the arts, of medical science, and astronomy. it tells the chemical constitution of the sun, the movements taking place, the nature of comets, and nebulæ. by the spectroscope we know that the atmospheres of venus and mars are like our own; that those of jupiter and saturn are very unlike; it tells us which stars approach and which recede, and just how one star differeth from another in glory and substance. in the near future we shall have the brilliant and diversely colored flowers of the sky as well classified into orders and species as are the flowers of the earth. [page ] iv. celestial measurements. "who hath measured the waters in the hollow of his hand, and meted out heaven with the span? mine hand also hath laid the foundation of the earth, and my right hand hath spanned the heavens."--_isa._ xl. ; xlviii. . [page ] "go to yon tower, where busy science plies her vast antennæ, feeling thro' the skies; that little vernier, on whose slender lines the midnight taper trembles as it shines, a silent index, tracks the planets' march in all their wanderings thro' the ethereal arch, tells through the mist where dazzled mercury burns, and marks the spot where uranus returns. "so, till by wrong or negligence effaced, the living index which thy maker traced repeats the line each starry virtue draws through the wide circuit of creation's laws; still tracks unchanged the everlasting ray where the dark shadows of temptation stray; but, once defaced, forgets the orbs of light, and leaves thee wandering o'er the expanse of night." oliver wendell holmes. [page ] iv. _celestial measurements._ we know that astronomy has what are called practical uses. if a ship had been driven by euroclydon ten times fourteen days and nights without sun or star appearing, a moment's glance into the heavens from the heaving deck, by a very slightly educated sailor, would tell within one hundred yards where he was, and determine the distance and way to the nearest port. we know that, in all final and exact surveying, positions must be fixed by the stars. earth's landmarks are uncertain and easily removed; those which we get from the heavens are stable and exact. in the united states steam-ship _enterprise_ was sent to survey the amazon. every night a "star party" went ashore to fix the exact latitude and longitude by observations of the stars. our real landmarks are not the pillars we rear, but the stars millions of miles away. all our standards of time are taken from the stars; every railway train runs by their time to avoid collision; by them all factories start and stop. indeed, we are ruled by the stars even more than the old astrologers imagined. man's finest mechanism, highest thought, and broadest exercise of the creative faculty have been inspired by astronomy. no other instruments approximate in delicacy those which explore the heavens; no other [page ] system of thought can draw such vast and certain conclusions from its premises. "too low they build who build beneath the stars;" we should lay our foundations in the skies, and then build upward. we have been placed on the outside of this earth, instead of the inside, in order that we may look abroad. we are carried about, through unappreciable distance, at the inconceivable velocity of one thousand miles a minute, to give us different points of vision. the earth, on its softly-spinning axle, never jars enough to unnest a bird or wake a child; hence the foundations of our observatories are firm, and our measurements exact. whoever studies astronomy, under proper guidance and in the right spirit, grows in thought and feeling, and becomes more appreciative of the creator. _celestial movements._ let it not be supposed that a mastery of mathematics and a finished education are necessary to understand the results of astronomical research. it took at first the highest power of mind to make the discoveries that are now laid at the feet of the lowliest. it took sublime faith, courage, and the results of ages of experience in navigation, to enable columbus to discover that path to the new world which now any little boat can follow. ages of experience and genius are stored up in a locomotive, but quite an unlettered man can drive it. it is the work of genius to render difficult matters plain, abstruse thoughts clear. [illustration: fig. .] a brief explanation of a few terms will make the principles of world inspection easily understood. imagine a perfect circle thirty feet in diameter--that is, create [page ] one (fig. ). draw through it a diameter horizontally, another perpendicularly. the angles made by the intersecting lines are each said to be ninety degrees, marked thus °. the arc of a circle included between any two of the lines is also °. every circle, great or small, is divided into these °. if the sun rose in the east and came to the zenith at noon, it would have passed °. when it set in the west it would have traversed half the circle, or °. in fig. the angle of the lines measured on the graduated arc is °. the mountain is ° high, the world ° in diameter, the comet moves ° a day, the stars are ° apart. the height of the mountain, the diameter of the world, the velocity of the comet, and the distance between the stars, depend on the distance of each from the point of sight. every degree is divided into minutes (marked '), and every minute into seconds (marked "). [illustration: fig. .--illustration of angles.] imagine yourself inside a perfect sphere one hundred feet in diameter, with the interior surface above, around, and below studded with fixed bright points like stars. the familiar constellations of night might be blazoned there in due proportion. if this star-sprent sphere were made to revolve once in twenty-four hours, all the stars would successively [page ] pass in review. how easily we could measure distances between stars, from a certain fixed meridian, or the equator! how easily we could tell when any particular star would culminate! it is as easy to take all these measurements when our earthly observatory is steadily revolved within the sphere of circumambient stars. stars can be mapped as readily as the streets of a great city. looking down on it in the night, one could trace the lines of lighted streets, and judge something of its extent and regularity. but the few lamps of evening would suggest little of the greatness of the public buildings, the magnificent enterprise and commerce of its citizens, or the intelligence of its scholars. looking up to the lamps of the celestial city, one can judge something of its extent and regularity; but they suggest little of the magnificence of the many mansions. stars are reckoned as so many degrees, minutes, and seconds from each other, from the zenith, or from a given meridian, or from the equator. thus the stars called the pointers, in the great bear, are ° apart; the nearest one is ° from the pole star, which is ° ' " above the horizon at philadelphia. in going to england you creep up toward the north end of the earth, till the pole star is ° high. it stays near its place among the stars continually, "of whose true-fixed and resting quality there is no fellow in the firmament." _how to measure._ suppose a telescope, fixed to a mural circle, to revolve on an axis, as in fig. ; point it horizontally at a star; [page ] turn it up perpendicular to another star. of course the two stars are ° apart, and the graduated scale, which is attached to the outer edge of the circle, shows a revolution of a quarter circle, or °, but a perfect accuracy of measurement must be sought; for to mistake the breadth of a hair, seen at the distance of one hundred and twenty-five feet, would cause an error of , , miles at the distance of the sun, and immensely more at the distance of the stars. the correction of an inaccuracy of no greater magnitude than that has reduced our estimate of the distance of our sun , , miles. [illustration: fig. .--mural circle.] consider the nicety of the work. suppose the graduated scale to be thirty feet in circumference. divided into °, each would be one inch long. divide each degree into ', each one is / of an inch long. it takes good eyesight to discern it. but each minute must be [page ] divided into ", and these must not only be noted, but even tenths and hundredths of seconds must be discerned. of course they are not seen by the naked eye; some mechanical contrivance must be called in to assist. a watch loses two minutes a week, and hence is unreliable. it is taken to a watch-maker that every single second may be quickened / part of itself. now / part of a second would be a small interval of time to measure, but it must be under control. if the temperature of a summer morning rises ten or twenty degrees we scarcely notice it; but the magnetic tastimeter measures / of a degree. come to earthly matters. in , after nearly twenty-eight years' work, the state of massachusetts opened a tunnel nearly five miles long through the hoosac mountains. in the early part of the work the engineers sunk a shaft near the middle feet deep. then the question to be settled was where to go so as to meet the approaching excavations from the east and west. a compass could not be relied on under a mountain. the line must be mechanically fixed. a little divergence at the starting-point would become so great, miles away, that the excavations might pass each other without meeting; the grade must also rise toward the central shaft, and fall in working away from it; but the lines were fixed with such infinitesimal accuracy that, when the one going west from the eastern portal and the one going east from the shaft met in the heart of the mountain, the western line was only one-eighth of an inch too high, and three-sixteenths of an inch too far north. to reach this perfect result they had to triangulate from the eastern portal to distant [page ] mountain peaks, and thence down the valley to the central shaft, and thus fix the direction of the proposed line across the mouth of the shaft. plumb-lines were then dropped one thousand and twenty-eight feet, and thus the line at the bottom was fixed. three attempts were made--in , , and --to fix the exact time-distance between greenwich and washington. these three separate efforts do not differ one-tenth of a second. such demonstrable results on earth greatly increase our confidence in similar measurements in the skies. [illustration: fig. .] a scale is frequently affixed to a pocket-rule, by which we can easily measure one-hundredth of an inch (fig. ). the upper and lower line is divided into tenths of an inch. observe the slanting line at the right hand. it leans from the perpendicular one-tenth of an inch, as shown by noticing where it reaches the top line. when it reaches the second horizontal line it has left the perpendicular one-tenth of that tenth--that is, one-hundredth. the intersection marks / of an inch from one end, and one-hundredth from the other. when division-lines, on measures of great nicety, get too fine to be read by the eye, we use the microscope. by its means we are able to count , lines ruled on a glass plate within an inch. the smallest object that can be seen by a keen eye makes an angle of ", but by putting six microscopes on the scale of the telescope on the mural circle, we are able to reach an exactness of ". , or / of an inch. this instrument is used to measure the declination of stars, or angular [page ] distance north or south of the equator. thus a star's place in two directions is exactly fixed. when the telescope is mounted on two pillars instead of the face of a wall, it is called a transit instrument. this is used to determine the time of transit of a star over the meridian, and if the transit instrument is provided with a graduated circle it can also be used for the same purposes as the mural circle. man's capacity to measure exactly is indicated in his ascertainment of the length of waves of light. it is easy to measure the three hundred feet distance between the crests of storm-waves in the wide atlantic; easy to measure the different wave-lengths of the different tones of musical sounds. so men measure the lengths of the undulations of light. the shortest is of the violet light, . ten-millionths of an inch. by the horizontal pendulum professor root has made / of an inch apparent. the next elements of accuracy must be perfect time and perfect notation of time. as has been said, we get our time from the stars. thus the infinite and heavenly dominates the finite and earthly. clocks are set to the invariable sidereal time. sidereal noon is when we have turned ourselves under the point where the sun crosses the equator in march, called the vernal equinox. sidereal clocks are figured to indicate twenty-four hours in a day: they tick exact seconds. to map stars we wish to know the exact second when they cross the meridian, or the north and south line in the celestial dome above us. the telescope (fig. , p. ) swings exactly north and south. in its focus a set of fine threads of spider-lines is placed (fig. ). the telescope is set just high enough, so that by the rolling over of the earth [page ] the star will come into the field just above the horizontal thread. the observer notes the exact second and tenth of a second when the star reaches each vertical thread in the instrument, adds together the times and divides by five to get the average, and the exact time is reached. [illustration: fig. .--transit of a star noted.] but man is not reliable enough to observe and record with sufficient accuracy. some, in their excitement, anticipate its positive passage, and some cannot get their slow mental machinery in motion till after it has made the transit. moreover, men fall into a habit of estimating some numbers of tenths of a second oftener than others. it will be found that a given observer will say three tenths or seven tenths oftener than four or eight. he is falling into ruts, and not trustworthy. general o. m. mitchel, who had been director of the cincinnati observatory, once told one of his staff-officers that he was late at an appointment. "only a few minutes," said the officer, apologetically. "sir," said the general, "where i have been accustomed to work, hundredths of a second are too important to be neglected." and it is to the rare genius of this astronomer, and to others, that we owe the mechanical accuracy that we now attain. the clock is made to mark its seconds on paper wrapped around a revolving cylinder. under the observer's fingers is an electric key. this he can touch at the instant of the transit of the star [page ] over each wire, and thus put his observation on the same line between the seconds dotted by the clock. of course these distances can be measured to minute fractional parts of a second. but it has been found that it takes an appreciable time for every observer to get a thing into his head and out of his finger-ends, and it takes some observers longer than others. a dozen men, seeing an electric spark, are liable to bring down their recording marks in a dozen different places on the revolving paper. hence the time that it takes for each man to get a thing into his head and out of his fingers is ascertained. this time is called his personal equation, and is subtracted from all of his observations in order to get at the true time; so willing are men to be exact about material matters. can it be thought that moral and spiritual matters have no precision? thus distances east or west from any given star or meridian are secured; those north and south from the equator or the zenith are as easily fixed, and thus we make such accurate maps of the heavens that any movements in the far-off stars--so far that it may take centuries to render the swiftest movements appreciable--may at length be recognized and accounted for. [illustration: fig. .] we now come to a little study of the modes of measuring distances. create a perfect square (fig. ); draw a diagonal line. the square angles are °, the divided angles give two of ° each. now the base a b is equal to the perpendicular a c. now any point--c, where a perpendicular, a c, and a diagonal, b c, meet--will be [page ] as far from a as b is. it makes no difference if a river flows between a and c, and we cannot go over it; we can measure its distance as easily as if we could. set a table four feet by eight out-doors (fig. ); so arrange it that, looking along one end, the line of sight just strikes a tree the other side of the river. go to the other end, and, looking toward the tree, you find the line of sight to the tree falls an inch from the end of the table on the farther side. the lines, therefore, approach each other one inch in every four feet, and will come together at a tree three hundred and eighty-four feet away. [illustration: fig. .--measuring distances.] [illustration: fig. .--measuring elevations.] the next process is to measure the height or magnitude of objects at an ascertained distance. put two pins in a stick half an inch apart (fig. ). hold it up two feet from the eye, and let the upper pin fall in line with your eye and the top of a distant church steeple, and the lower pin in line with the bottom of the church and your eye. if the church is three-fourths of a mile away, it must be eighty-two feet high; if a mile away, it must be one hundred and ten feet high. for if two lines spread [page ] one-half an inch going two feet, in going four feet they will spread an inch, and in going a mile, or five thousand two hundred and eighty feet, they will spread out one-fourth as many inches, viz., thirteen hundred and twenty--that is, one hundred and ten feet. of course these are not exact methods of measurement, and would not be correct to a hair at one hundred and twenty-five feet, but they perfectly illustrate the true methods of measurement. imagine a base line ten inches long. at each end erect a perpendicular line. if they are carried to infinity they will never meet: will be forever ten inches apart. but at the distance of a foot from the base line incline one line toward the other / of an inch, and the lines will come together at a distance of three hundred miles. that new angle differs from the former right angle almost infinitesimally, but it may be measured. its value is about three-tenths of a second. if we lengthen the base line from ten inches to all the miles we can command, of course the point of meeting will be proportionally more distant. the angle made by the lines where they come together will be obviously the same as the angle of divergence from a right angle at this end. that angle is called the parallax of any body, and is the angle that would be made by two lines coming from that body to the two ends of any conventional base, as the semi-diameter of the earth. that that angle would vary according to the various distances is easily seen by fig. . [illustration: fig. .] let o p be the base. this would subtend a greater angle seen from star a than from star b. let b be far enough away, and o p would become invisible, and b [page ] would have no parallax for that base. thus the moon has a parallax of " with the semi-equatorial diameter of the earth for a base. and the sun has a parallax ". on the same base. it is not necessary to confine ourselves to right angles in these measurements, for the same principles hold true in any angles. now, suppose two observers on the equator should look at the moon at the same instant. one is on the top of cotopaxi, on the west coast of south america, and one on the west coast of africa. they are ° apart--half the earth's diameter between them. the one on cotopaxi sees it exactly overhead, at an angle of ° with the earth's diameter. the one on the coast of africa sees its angle with the same line to be ° ' "--that is, its parallax is ". try the same experiment on the sun farther away, as is seen in fig. , and its smaller parallax is found to be only ". . it is not necessary for two observers to actually station themselves at two distant parts of the earth in order to determine a parallax. if an observer could go from one end of the base-line to the other, he could determine both angles. every observer is actually carried along through space by two motions: one is that of the earth's revolution of one thousand miles an hour around the axis; and the other is the movement of the earth around the sun of one thousand miles in a minute. hence we can have the diameter not only of [page ] the earth (eight thousand miles) for a base-line, but the diameter of the earth's orbit ( , , miles), or any part of it, for such a base. two observers at the ends of the earth's diameter, looking at a star at the same instant, would find that it made the same angle at both ends; it has no parallax on so short a base. we must seek a longer one. observe a certain star on the st of march; then let us traverse the realms of space for six months, at one thousand miles a minute. we come round in our orbit to a point opposite where we were six months ago, with , , of miles between the points. now, with this for a base-line, measure the angles of the same stars: it is the same angle. sitting in my study here, i glance out of the window and discern separate bricks, in houses five hundred feet away, with my unaided eye; they subtend a discernible angle. but one thousand feet away i cannot distinguish individual bricks; their width, being only two inches, does not subtend an angle apprehensible to my vision. so at these distant stars the earth's enormous orbit, if lying like a blazing ring in space, with the world set on its edge like a pearl, and the sun blazing like a diamond in the centre, would all shrink to a mere point. not quite to a point from the nearest stars, or we should never be able to measure the distance of any of them. professor airy says that our orbit, seen from the nearest star, would be the same as a circle six-tenths of an inch in diameter seen at the distance of a mile: it would all be hidden by a thread one-twenty-fifth of an inch in diameter, held six hundred and fifty feet from the eye. if a straight line could be drawn from a star, sirius in the east to the star vega in the west, touching our [page ] earth's orbit on one side, as t r a (fig. ), and a line were to be drawn six months later from the same stars, touching our earth's orbit on the other side, as r b t, such a line would not diverge sufficiently from a straight line for us to detect its divergence. numerous vain attempts had been made, up to the year , to detect and measure the angle of parallax by which we could rescue some one or more of the stars from the inconceivable depths of space, and ascertain their distance from us. we are ever impelled to triumph over what is declared to be unconquerable. there are peaks in the alps no man has ever climbed. they are assaulted every year by men zealous of more worlds to conquer. so these greater heights of the heavens have been assaulted, till some ambitious spirits have outsoared even imagination by the certainties of mathematics. [illustration: fig. .] it is obvious that if one star were three times as far from us as another, the nearer one would seem to be displaced by our movement in our orbit three times as much as the other; so, by comparing one star with another, we reach a ground of judgment. the ascertainment of longitude at sea by means of the moon affords a good illustration. along the track where the moon sails, nine bright stars, four planets, and the sun have been selected. the nautical almanacs give the distance of the moon from these successive stars every hour in the night for three years in advance. the sailor can measure the distance at any time by his sextant. looking from the world at d (fig. ), the distance of the moon and [page ] star is a e, which is given in the almanac. looking from c, the distance is only b e, which enables even the uneducated sailor to find the distance, c d, on the earth, or his distance from greenwich. [illustration: fig. .--mode of ascertaining longitude.] so, by comparisons of the near and far stars, the approximate distance of a few of them has been determined. the nearest one is the brightest star in the centaur, never visible in our northern latitudes, which has a parallax of about one second. the next nearest is no. in the swan, or cygni, having a parallax of ". . approximate measurements have been made on sirius, capella, the pole star, etc., about eighteen in all. the distances are immense: only the swiftest agents can traverse them. if our earth were suddenly to dissolve its allegiance to the king of day, and attempt a flight to the north star, and should maintain its flight of one thousand miles a minute, it would flyaway toward polaris for thousands upon thousands of years, till a million years had passed away, before it reached that northern dome of the distant sky, and gave its new allegiance to another sun. the sun it had left behind it would gradually diminish till it was small as arcturus, then small as could be discerned by the naked eye, until at last it would finally fade out in utter darkness long before the new sun was reached. light can traverse the distance around our earth eight times in one second. it comes in eight minutes from the sun, but it takes three and a quarter years to come from alpha [page ] centauri, seven and a quarter years from cygni, and forty-five years from the polar star. sometimes it happens that men steer along a lee shore, dependent for direction on polaris, that light-house in the sky. sometimes it has happened that men have traversed great swamps by night when that star was the light-housse of freedom. in either case the exigency of life and liberty was provided for forty-five years before by a providence that is divine. we do not attempt to name in miles these enormous distances; we must seek another yard-stick. our astronomical unit and standard of measurement is the distance of the earth from the sun-- , , miles. this is the golden reed with which we measure the celestial city. thus, by laying down our astronomical unit , times, we measure to alpha centauri, more than twenty millions of millions of miles. doubtless other suns are as far from alpha centauri and each other as that is from ours. stars are not near or far according to their brightness. cygni is a telescopic star, while sirius, the brightest star in the heavens, is twice as far away from us. one star differs from another star in intrinsic glory. the highest testimonies to the accuracy of these celestial observations are found in the perfect predictions of eclipses, transits of planets over the sun, occultation of stars by the moon, and those statements of the nautical almanac that enable the sailor to know exactly where he is on the pathless ocean by the telling of the stars: "on the trackless ocean this book is the mariner's trusted friend and counsellor; daily and nightly its revelations bring safety to ships in all parts of the [page ] world. it is something more than a mere book; it is an ever-present manifestation of the order and harmony of the universe." another example of this wonderful accuracy is found in tracing the asteroids. within , , or , , miles from the sun, the one hundred and ninety-two minute bodies that have been already discovered move in paths very nearly the same--indeed two of them traverse the same orbit, being one hundred and eighty degrees apart;--they look alike, yet the eye of man in a few observations so determines the curve of each orbit, that one is never mistaken for another. but astronomy has higher uses than fixing time, establishing landmarks, and guiding the sailor. it greatly quickens and enlarges thought, excites a desire to know, leads to the utmost exactness, and ministers to adoration and love of the maker of the innumerable suns. [page ] v. the sun. "and god made two great lights; the greater light to rule the day, and the lesser light to rule the night: he made the stars also."--_gen._ i. . [page ] "it is perceived that the sun of the world, with all its essence, which is heat and light, flows into every tree, and into every shrub and flower, and into every stone, mean as well as precious; and that every object takes its portion from this common influx, and that the sun does not divide its light and heat, and dispense a part to this and a part to that. it is similar with the sun of heaven, from which the divine love proceeds as heat, and the divine wisdom as light; these two flow into human minds, as the heat and light of the sun of the world into bodies, and vivify them according to the quality of the minds, each of which takes from the common influx as much as is necessary."--swedenborg. [page ] v. _the sun._ suppose we had stood on the dome of boston statehouse november th, , on the night of the great conflagration, and seen the fire break out; seen the engines dash through the streets, tracking their path by their sparks; seen the fire encompass a whole block, leap the streets on every side, surge like the billows of a storm-swept sea; seen great masses of inflammable gas rise like dark clouds from an explosion, then take fire in the air, and, cut off from the fire below, float like argosies of flame in space. suppose we had felt the wind that came surging from all points of the compass to fan that conflagration till it was light enough a mile away to see to read the finest print, hot enough to decompose the torrents of water that were dashed on it, making new fuel to feed the flame. suppose we had seen this spreading fire seize on the whole city, extend to its environs, and, feeding itself on the very soil, lick up worcester with its tongues of flame--albany, new york, chicago, st. louis, cincinnati--and crossing the plains swifter than a prairie fire, making each peak of the rocky mountains hold up aloft a separate torch of flame, and the sierras whiter with heat than they ever were with snow, the waters of the pacific resolve into their constituent elements of oxygen and hydrogen, and [page ] burn with unquenchable fire! we withdraw into the air, and see below a world on fire. all the prisoned powers have burst into intensest activity. quiet breezes have become furious tempests. look around this flaming globe--on fire above, below, around--there is nothing but fire. let it roll beneath us till boston comes round again. no ember has yet cooled, no spire of flame has shortened, no surging cloud has been quieted. not only are the mountains still in flame, but other ranges burst up out of the seething sea. there is no place of rest, no place not tossing with raging flame! yet all this is only a feeble figure of the great burning sun. it is but the merest hint, a million times too insignificant. the sun appears small and quiet to us because we are so far away. seen from the various planets, the relative size of the sun appears as in fig. . looked for from some of the stars about us, the sun could not be seen at all. indeed, seen from the earth, it is not always the same size, because the distance is not always the same. if we represent the size of the sun by one thousand on the d of september or st of march, it would be represented by nine hundred and sixty-seven on the st of july, and by one thousand and thirty-four on the st of january. [illustration: fig. .--relative size of sun as seen from different planets.] we sometimes speak of the sun as having a diameter of , miles. we mean that that is the extent of the body as soon by the eye. but that is a small part of its real diameter. so we say the earth has an equatorial diameter of - / miles, and a polar one of . but the air is as much a part of the earth as the rocks are. the electric currents are as much a part of the [page ] earth as the ores and mountains they traverse. what the diameter of the earth is, including these, no man can tell. we used to say the air extended forty-five miles, but we now know that it reaches vastly farther. so of the sun, we might almost say that its diameter is infinite, for its light and heat reach beyond our measurement. its living, throbbing heart sends out pulsations, keeping all space full of its tides of living light. [page ] [illustration: fig. .--zodiacal light.] we might say with evident truth that the far-off planets are a part of the sun, since the space they traverse is filled with the power of that controlling king; not only with light, but also with gravitating power. but come to more ponderable matters. if we look [page ] into our western sky soon after sunset, on a clear, moonless night in march or april, we shall see a dim, soft light, somewhat like the milky-way, often reaching, well defined, to the pleiades. it is wedge-shaped, inclined to the south, and the smallest star can easily be seen through it. mairan and cassini affirm that they have seen sudden sparkles and movements of light in it. all our best tests show the spectrum of this light to be continuous, and therefore reflected; which indicates that it is a ring of small masses of meteoric matter surrounding the sun, revolving with it and reflecting its light. one bit of stone as large as the end of one's thumb, in a cubic mile, would be enough to reflect what light we see looking through millions of miles of it. perhaps an eye sufficiently keen and far away would see the sun surrounded by a luminous disk, as saturn is with his rings. as it extends beyond the earth's orbit, if this be measured as a part of the sun, its diameter would be about , , miles. come closer. when the sun is covered by the disk of the moon at the instant of total eclipse, observers are startled by strange swaying luminous banners, ghostly and weird, shooting in changeful play about the central darkness (fig. ). these form the corona. men have usually been too much moved to describe them, and have always been incapable of drawing them in the short minute or two of their continuance. but in men travelled eight thousand miles, coming and returning, in order that they might note the three minutes of total eclipse in colorado. each man had his work assigned to him, and he was drilled to attend to that and nothing else. improved instruments were put into his [page ] hands, so that the sun was made to do his own drawing and give his own picture at consecutive instants. fig. is a copy of a photograph of the corona of , by mr. henry draper. it showed much less changeability that year than common, it being very near the time of least sun-spot. the previous picture was taken near the time of maximum sun-spot. [illustration: fig. .--the corona in , brazil.] it was then settled that the corona consists of reflected light, sent to us from dust particles or meteoroids swirling in the vast seas, giving new densities and [page ] rarities, and hence this changeful light. whether they are there by constant projection, and fall again to the sun, or are held by electric influence, or by force of orbital revolution, we do not know. that the corona cannot be in any sense an atmosphere of any continuous gas, is seen from the fact that the comet of , passing within , miles of the body of the sun, was not burned out of existence as a comet, nor in any perceptible degree retarded in its motion. if the sun's diameter is to include the corona, it will be from , , to , , miles. [illustration: fig. .--the corolla in , colorado.] [page ] come closer still. at the instant of the totality of the eclipse red flames of most fantastic shape play along the edge of the moon's disk. they can be seen at any time by the use of a proper telescope with a spectroscope attached. i have seen them with great distinctness and brilliancy with the excellent eleven-inch telescope of the wesleyan university. a description of their appearance is best given in the language of professor young, of princeton college, who has made these flames the object of most successful study. on september th, , he was observing a large hydrogen cloud by the sun's edge. this cloud was about , miles long, and its upper side was some , miles above the sun's surface, the lower side some , miles. the whole had the appearance of being supported on pillars of fire, these seeming pillars being in reality hydrogen jets brighter and more active than the substance of the cloud. at half-past twelve, when professor young chanced to be called away from his observatory, there were no indications of any approaching change, except that one of the connecting stems of the southern extremity of the cloud had grown considerably brighter and more curiously bent to one side; and near the base of another, at the northern end, a little brilliant lump had developed itself, shaped much like a summer thunderhead. [illustration: fig. .--solar prominences of flaming hydrogen.] but when professor young returned, about half an hour later, he found that a very wonderful change had taken place, and that a very remarkable process was actually in progress. "the whole thing had been literally blown to shreds," he says, "by some inconceivable uprush from beneath. in place of the quiet cloud i had [page ] left, the air--if i may use the expression--was filled with the flying _débris_, a mass of detached vertical fusi-form fragments, each from ten to thirty seconds (_i. e._, from four thousand five hundred to thirteen thousand five hundred miles) long, by two or three seconds (nine hundred to thirteen hundred and fifty miles) wide--brighter, and closer together where the pillars had formerly stood, and rapidly ascending. when i looked, some of them had already reached a height of nearly four minutes ( , miles); and while i watched them they arose with a motion almost perceptible to the eye, until, in ten minutes, the uppermost were more than , miles above the solar surface. this was ascertained by careful measurements, the mean of three closely accordant determinations giving , miles as the extreme altitude attained. i am particular in the statement, because, so far as i know, chromatospheric matter (red hydrogen in this case) has never before been observed at any altitude exceeding five minutes, or , miles. the velocity of ascent, also--one hundred and sixty-seven miles per second--is considerably greater than anything hitherto recorded. * * * as the filaments arose, they gradually faded away like a dissolving cloud, and at a quarter past one only a few filmy wisps, with some brighter streamers low down near the chromatosphere, remained to mark the place. but in the mean while the little 'thunder-head' before alluded to had grown and developed wonderfully into a mass of rolling and ever-changing flame, to speak according to appearances. first, it was crowded down, as it were, along the solar surface; later, it arose almost pyramidally , miles in height; then [page ] its summit was drawn down into long filaments and threads, which were most curiously rolled backward and forward, like the volutes of an ionic capital, and finally faded away, and by half-past two had vanished like the other. the whole phenomenon suggested most forcibly the idea of an explosion under the great prominence, acting mainly upward, but also in all directions outward; and then, after an interval, followed by a corresponding in-rush." no language can convey nor mind conceive an idea of the fierce commotion we here contemplate. if we call these movements hurricanes, we must remember that what we use as a figure moves but one hundred miles an hour, while these move one hundred miles a second. such storms of fire on earth, "coming down upon us from the north, would, in thirty seconds after they had crossed the st. lawrence, be in the gulf of mexico, carrying with them the whole surface of the continent in a mass not simply of ruins but of glowing vapor, in which the vapors arising from the dissolution of the materials composing the cities of boston, new york, and chicago would be mixed in a single indistinguishable cloud." in the presence of these evident visions of an actual body in furious flame, we need hesitate no longer in accepting as true the words of st. peter of the time "in which the [atmospheric] heavens shall pass away with a great noise, and the elements shall melt with fervent heat; the earth also, and the works that are therein, shall be burned up." this region of discontinuous flame below the corona is called the chromosphere. hydrogen is the principal material of its upper part; iron, magnesium, and other [page ] metals, some of them as yet unknown on earth, but having a record in the spectrum, in the denser parts below. if these fierce fires are a part of the sun, as they assuredly are, its diameter would be from , , to , , miles. let us approach even nearer. we see a clearly recognized even disk, of equal dimensions in every direction. this is the photosphere. we here reach some definitely measurable data for estimating its visible size. we already know its distance. its disk subtends an angle of ' ". , or a little more than half a degree. three hundred and sixty such suns, laid side by side, would span the celestial arch from east to west with a half circle of light. two lines drawn from our earth at the angle mentioned would be , miles apart at the distance of , , miles. this, then, is the diameter of the visible and measurable part of the sun. it would require one hundred and eight globes like the earth in a line to measure the sun's diameter, and three hundred and thirty-nine, to be strung like the beads of a necklace, to encircle his waist. the sun has a volume equal to , , earths, but being only one-quarter as dense, it has a mass of only , earths. it has seven hundred times the mass of all the planets, asteroids, and satellites put together. thus it is able to control them all by its greater power of attraction. concerning the condition of the surface of the sun many opinions are held. that it is hot beyond all estimate is indubitable. whether solid or gaseous we are not sure. opinions differ: some incline to the first theory, others to the second; some deem the sun composed of solid particles, floating in gas so condensed [page ] by pressure and attraction as to shine like a solid. it has no sensible changes of general level, but has prodigious activity in spots. these spots have been the objects of earnest and almost hourly study on the part of such men as secchi, lockyer, faye, young, and others, for years. but it is a long way off to study an object. no telescope brings it nearer than , miles. theory after theory has been advanced, each one satisfactory in some points, none in all. the facts about the spots are these: they are most abundant on the two sides of the equator. they are gregarious, depressed below the surface, of vast extent, black in the centre, usually surrounded by a region of partial darkness, beyond which is excessive light. they have motion of their own over the surface--motion rotating about an axis, upward and downward about the edges. they change their apparent shape as the sun carries them across its disk by axial revolution, being narrow as they present their edges to us, and rounder as we look perpendicularly into them (fig. ). [illustration: fig. .--change in spots as rotated across the disk, showing cavities.] these spots are also very variable in number, sometimes there being none for nearly two hundred days, and again whole years during which the sun is never without them. the period from minimum to maximum [page ] of spots is about eleven years. we might look for them again and again in vain this year ( ). they will be most numerous in and . the cause of this periodicity was inferred to be the near approach of the enormous planet jupiter, causing disturbance by its attraction. but the periods do not correspond, and the cause is the result of some law of solar action to us as yet unknown. these spots may be seen with almost any telescope, the eye being protected by deeply colored glasses. until within one hundred years they were supposed to be islands of scoriæ floating in the sea of molten matter. but they were depressed below the surface, and showed a notch when on the edge. wilson originated and herschel developed the theory that the sun's real body was dark, cool, and habitable, and that the photosphere was a luminous stratum at a distance from the real body, with openings showing the dark spots below. such a sun would have cooled off in a week, but would previously have annihilated all life below. the solar spots being most abundant on the two sides of the equator, indicates their cyclonic character; the centre of a cyclone is rarefied, and therefore colder, and cold on the sun is darkness. m. faye says: "like our cyclones, they are descending, as i have proved by a special study of these terrestrial phenomena. they carry down into the depths of the solar mass the cooler materials of the upper layers, formed principally of hydrogen, and thus produce in their centre a decided extinction of light and heat as long as the gyratory movement continues. finally, the hydrogen set free at the base of the whirlpool becomes reheated at this [page ] great depth, and rises up tumultuously around the whirlpool, forming irregular jets, which appear above the chromosphere. these jets constitute the protuberances. the whirlpools of the sun, like those on the earth, are of all dimensions, from the scarcely visible pores to the enormous spots which we see from time to time. they have, like those of the earth, a marked tendency, first to increase and then to break up, and thus form a row of spots extending along the same parallel." [illustration: fig. .--solar spot, by langley.] a spot of , miles diameter is quite small; there was one , miles across, visible to the naked eye for a week in . this particular sun-spot somewhat [page ] helped the millerites. on the day of the eclipse, in , a spot over , miles in extent was clearly seen. in such vast tempests, if there were ships built as large as the whole earth, they would be tossed like autumn leaves in an ocean storm. the revolution of the sun carries a spot across its face in about fourteen days. after a lapse of as much more time, they often reappear on the other side, changed but recognizable. they often break ont or disappear under the eye of the observer. they divide like a piece of ice dropped on a frozen pond, the pieces sliding off in every direction, or combine like separate floes driven together into a pack. sometimes a spot will last for more than two hundred days, recognizable through six or eight revolutions. sometimes a spot will last only half an hour. the velocities indicated by these movements are incredible. an up-rush and down-rush at the sides has been measured of twenty miles a second; a side-rush or whirl, of one hundred and twenty miles a second. these tempests rage from a few days to half a year, traversing regions so wide that our indian ocean, the realm of storms, is too small to be used for comparison; then, as they cease, the advancing sides of the spots approach each other at the rate of , miles an hour; they strike together, and the rising spray of fire leaps thousands of miles into space. it falls again into the incandescent surge, rolls over mountains as the sea over pebbles, and all this for eon after eon without sign of exhaustion or diminution. all these swift succeeding himalayas of fire, where one hundred worlds could be buried, do not usually prevent the sun's appearing to our far-off eyes as a perfect sphere. [page ] _what the sun does for us._ to what end does this enormous power, this central source of power, exist? that it could keep all these gigantic forces within itself could not be expected. it is in a system where every atom is made to affect every other atom, and every world to influence every other. the author of all lives only to do good, to send rain on the just and unjust, to cause his sun to rise on the evil and the good, and to give his spirit, like a perpetually widening river, to every man to profit withal. the sun reaches his unrelaxing hand of gravitation to every other world at every instant. the tendency of every world is to fly off in a straight line. this tendency must be momentarily curbed, and the planet held in its true curve about the sun. these giant worlds must be perfectly handled. their speed, amounting to seventy times as fast as that of a rifle-ball, must be managed. each and every world may be said to be lifted momentarily and swung perpetually at arm's-length by the power of the sun. the sun warms us. it would convey but a small idea of the truth to state how many hundreds of millions of cubic miles of ice could be hailed at the sun every second without affecting its heat; but, if any one has any curiosity to know, it is , , cubic miles of ice per second. we journey through space which has a temperature of ° below zero; but we live, as it were, in a conservatory, in the midst of perpetual winter. we are roofed over by the air that treasures the heat, floored under by strata both absorptive and retentive of heat, [page ] and between the earth and air violets grow and grains ripen. the sun has a strange chemical power. it kisses the cold earth, and it blushes with flowers and matures the fruit and grain. we are feeble creatures, and the sun gives us force. by it the light winds move one-eighth of a mile an hour, the storm fifty miles, the hurricane one hundred. the force is as the square of the velocity. it is by means of the sun that the merchant's white-sailed ships are blown safely home. so the sun carries off the miasma of the marsh, the pollution of cities, and then sends the winds to wash and cleanse themselves in the sea-spray. the water-falls of the earth turn machinery, and make lowells and manchesters possible, because the sun lifted all that water to the hills. intermingled with these currents of air are the currents of electric power, all derived from the sun. these have shown their swiftness and willingness to serve man. the sun's constant force displayed on the earth is equal to , , , engines of -horse power each, working day and night; and yet the earth receives only / part of the whole force of the sun. besides all this, the sun, with provident care, has made and given to us coal. this omnipotent worker has stored away in past ages an inexhaustible reservoir of his power which man may easily mine and direct, thus releasing himself from absorbing toil. experiments. any one may see the spots on the sun who has a spy-glass. darken the room and put the glass through an opening toward the sun, as shown in fig. . the eye-piece should be drawn out about half an inch beyond [page ] its usual focusing for distant objects. the farther it is drawn, the nearer must we hold the screen for a perfect image. by holding a paper near the eye-piece, the proper direction of the instrument may be discovered without injury to the eyes. by this means the sun can be studied from day to day, and its spots or the transits of mercury and venus shown to any number of spectators. [illustration: fig. .--holding telescope to see the sun's spots.] first covering the eyes with very dark or smoked glasses, erect a disk of pasteboard four inches in diameter between you and the sun; close one eye; stand near it, and the whole sun is obscured. withdraw from it till the sun's rays just shoot over the edge of the disk on every side. measure the distance from the eye to the disk. you will be able to determine the distance of the sun by the rule of three: thus, as four inches is to , miles, so is distance from eye to disk to distance from disk to the sun. take such measurements at sunrise, noon, and sunset, and see the apparently differing sizes due to refraction. [page ] vi. the planets, as seen from space. "he hangeth the earth upon nothing."--_job_ xxvi. . [page ] "let a power be delegated to a finite spirit equal to the projection of the most ponderous planet in its orbit, and, from an exhaustless magazine, let this spirit select his grand central orb. let him with puissant arm locate it in space, and, obedient to his mandate, there let it remain forever fixed. he proceeds to select his planetary globes, which he is now required to marshal in their appropriate order of distance from the sun. heed well this distribution; for should a single globe be misplaced, the divine harmony is destroyed forever. let us admit that finite intelligence may at length determine the order of combination; the mighty host is arrayed in order. these worlds, like fiery coursers, stand waiting the command to fly. but, mighty spirit, heed well the grand step, ponder well the direction in which thou wilt launch each wailing world; weigh well the mighty impulse soon to be given, for out of the myriads of directions, and the myriads of impulsive forces, there comes but a single combination that will secure the perpetuity of your complex scheme. in vain does the bewildered finite spirit attempt to fathom this mighty depth. in vain does it seek to resolve the stupendous problem. it turns away, and while endued with omnipotent power, exclaims, 'give to me infinite wisdom, or relieve me from the impossible task!'"- . m. mitchel, ll. d. [page ] vi. _the planets, as seen from space_ if we were to go out into space a few millions of miles from either pole of the sun, and were endowed with wonderful keenness of vision, we should perceive certain facts, viz: that space is frightfully dark except when we look directly at some luminous body. there is no air to bend the light out of its course, no clouds or other objects to reflect it in a thousand directions. every star is a brilliant point, even in perpetual sunshine. the cold is frightful beyond the endurance of our bodies. there is no sound of voice in the absence of air, and conversation by means of vocal organs being impossible, it must be carried on by means of mind communication. we see below an unrevolving point on the sun that marks its pole. ranged round in order are the various planets, each with its axis pointing in very nearly the same direction. all planets, except possibly venus, and all moons except those of uranus and neptune, present their equators to the sun. the direction of orbital and axial revolution seen from above the north pole would be opposite to that of the hands of a watch. [illustration: fig. .--orbits and comparative sizes of the planets.] the speed of this orbital revolution must be proportioned to the distance from the sun. the attraction of the sun varies inversely as the square of the distance. [page ] it holds a planet with a certain power; one twice as far off, with one-fourth that power. this attraction must be counterbalanced by centrifugal force; great force from great speed when attraction is great, and small from less [page ] speed when attractive power is diminished by distance. hence mercury must go . miles per second--seventy times as fast as a rifle-ball that goes two-fifths of a mile in a second--or be drawn into the sun; while neptune, seventy-five times as far off, and hence attracted only / as much, must be slowed down to . miles a second to prevent its flying away from the feebler attraction of the sun. the orbital velocity of the various planets in miles per second is as follows: mercury . | jupiter . venus . | saturn . earth . | uranus . mars . | neptune . hence, while the earth makes one revolution in its year, mercury has made over four revolutions, or passed through four years; the slower neptune has made only / of one revolution. the time of axial revolution which determines the length of the day varies with different planets. the periods of the four planets nearest the sun vary only half an hour from that of the earth, while the enormous bodies of jupiter and saturn revolve in ten and ten and a quarter hours respectively. this high rate of speed, and its resultant, centrifugal force, has aided in preventing these bodies from becoming as dense as they would otherwise be--jupiter being only . as dense as the earth, and saturn only . . this extremely rapid revolution produces a great flattening at the poles. if jupiter should rotate four times more rapidly than it does, it could not be held together compactly. as it is, the polar diameter is five thousand miles less than the equatorial: the difference in diameters produced by the [page ] same cause on the earth, owing to the slower motion and smaller mass, being only twenty-six miles. the effect of this will be more specifically treated hereafter. the difference in the size of the planets is very noticeable. if we represent the sun by a gilded globe two feet in diameter, we must represent vulcan and mercury by mustard-seeds; venus, by a pea; earth, by another; mars, by one-half the size; asteroids, by the motes in a sunbeam; jupiter, by a small-sized orange; saturn, by a smaller one; uranus, by a cherry; and neptune, by one a little larger. apply the principle that attraction is in proportion to the mass, and a man who weighs one hundred and fifty pounds on the earth weighs three hundred and ninety-six on jupiter, and only fifty-eight on mars; while on the asteroids he could play with bowlders for marbles, hurl hills like milton's angels, leap into the fifth-story windows with ease, tumble over precipices without harm, and go around the little worlds in seven jumps. [illustration: fig. .--orbit of earth, showing parallelism of axis and seasons.] the seasons of a planet are caused by the inclination of its axis to the plane of its orbit. in fig. the rotating earth is seen at a, with its northern pole turning in constant sunlight, and its southern pole in constant darkness; everywhere south of the equator is more darkness than day, and hence winter. passing on to b, the world is seen illuminated equally on each side of the equator. every place has its twelve hours' darkness and light at each revolution. but at c--the axis of the earth always preserving the same direction--the northern pole is shrouded in continual gloom. every place [page ] north of the equator gets more darkness than light, and hence winter. the varying inclination of the axes of the different planets gives a wonderful variety to their seasons. the sun is always nearly over the equator of jupiter, and every place has nearly its five hours day and five hours night. the seasons of earth, mars, and saturn are so much alike, except in length, that no comment is necessary. the ice-fields at either pole of mars are observed to enlarge and contract, according as it is winter or summer there. saturn's seasons are each seven and a half years long. the alternate darkness and light at the poles is fifteen years long. but the seasons of venus present the greatest anomaly, if its assigned inclination of axis ( °) can be relied on as correct, which is doubtful. its tropic zone extends nearly to the pole, and at the same time the winter at the other pole reaches the equator. the short period of this planet causes it to present the south pole to the sun only one hundred and twelve days after it has been scorching the one at the north. this gives two winters, springs, summers, and autumns to the equator in two hundred and twenty-five days. if each whirling world should leave behind it a trail of light to mark its orbit, and our perceptions of form were sufficiently acute, we should see that these curves of light are not exact circles, but a little flattened into an ellipse, with the sun always in one of the foci. hence each planet is nearer to the sun at one part of its orbit than another; that point is called the perihelion, and the farthest point aphelion. this eccentricity of orbit, or distance of the sun from the centre, is very small. [page ] in the case of venus it is only . of the whole, and in no instance is it more than . , viz., that of mercury. this makes the sun appear twice as large, bright, and hot as seen and felt on mercury at its perihelion than at its aphelion. the earth is , , miles nearer to the sun in our winter than summer. hence the summer in the southern hemisphere is more intolerable than in the northern. but this eccentricity is steadily diminishing at a uniform rate, by reason of the perturbing influence of the other planets. in the case of some other planets it is steadily increasing, and, if it were to go on a sufficient time, might cause frightful extremes of temperature; but lalande has shown that there are limits at which it is said, "thus far shalt thou go, and no farther." then a compensative diminution will follow. conceive a large globe, to represent the sun, floating in a round pond. the axis will be inclined - / ° to the surface of the water, one side of the equator be - / ° below the surface, and the other side the same distance above. let the half-submerged earth sail around the sun in an appropriate orbit. the surface of the water will be the plane of the orbit, and the water that reaches out to the shore, where the stars would be set, will be the plane of the ecliptic. it is the plane of the earth's orbit extended to the stars. the orbits of all the planets do not lie in the same plane, but are differently inclined to the plane of the ecliptic, or the plane of the earth's orbit. going out from the sun's equator, so as to see all the orbits of the planets on the edge, we should see them inclined to that of the earth, as in fig. . [illustration: fig. .--inclination of the planes of orbits.] if the earth, and saturn, and pallas were lying in [page ] the same direction from the sun, and the outer bodies were to start in a direct line for the sun, they would not collide with the earth on their way; but saturn would pass , , and pallas , , miles over our heads. from this same cause we do not see venus and mercury make a transit across the disk of the sun at every revolution. [illustration: fig. .--inclination of orbits of venus and earth. nodal line, d b.] fig. shows a view of the orbits of the earth and venus seen not from the edge but from a position somewhat above. the point e, where venus crosses the plane of the earth's orbit, is called the ascending node. if the earth were at b when venus is at e, venus would be seen on the disk of the sun, making a transit. the same would be true if the earth were at d, and venus at the descending node f. this general view of the flying spheres is full of interest. [page ] while quivering themselves with thunderous noises, all is silent about them; earthquakes may be struggling on their surfaces, but there is no hint of contention in the quiet of space. they are too distant from one another to exchange signals, except, perhaps, the fleet of asteroids that sail the azure between mars and jupiter. some of these come near together, continuing to fill each other's sky for days with brightness, then one gradually draws ahead. they have all phases for each other--crescent, half, full, and gibbous. these hundreds of bodies fill the realm where they are with inexhaustible variety. beyond are vast spaces--cold, dark, void of matter, but full of power. occasionally a little spark of light looms up rapidly into a world so huge that a thousand of our earths could not occupy its vast bulk. it swings its four or eight moons with perfect skill and infinite strength; but they go by and leave the silence unbroken, the darkness unlighted for years. nevertheless, every part of space is full of power. nowhere in its wide orbit can a world find a place; at no time in its eons of flight can it find an instant when the sun does not hold it in safety and life. _the outlook from the earth._ if we come in from our wanderings in space and take an outlook from the earth, we shall observe certain movements, easily interpreted now that we know the system, but nearly inexplicable to men who naturally supposed that the earth was the largest, most stable, and central body in the universe. we see, first of all, sun, moon, and stars rise in the east, mount the heavens, and set in the west. as i [page ] revolve in my pivoted study-chair, and see all sides of the room--library, maps, photographs, telescope, and windows--i have no suspicion that it is the room that whirls; but looking out of a car-window in a depot at another car, one cannot tell which is moving, whether it be his car or the other. in regard to the world, we have come to feel its whirl. we have noticed the pyramids of egypt lifted to hide the sun; the mountains of hymettus hurled down, so as to disclose the moon that was behind them to the watchers on the acropolis; and the mighty mountains of moab removed to reveal the stars of the east. train the telescope on any star; it must be moved frequently, or the world will roll the instrument away from the object. suspend a cannon-ball by a fine wire at the equator; set it vibrating north and south, and it swings all day in precisely the same direction. but suspend it directly over the north pole, and set it swinging toward washington; in six hours after it is swinging toward rome, in italy; in twelve hours, toward siam, in asia; in nineteen hours, toward the sandwich islands; and in twenty-four, toward washington again, not because it has changed the plane of its vibration, but because the earth has whirled beneath it, and the torsion of the wire has not been sufficient to compel the plane of the original direction to change with the turning of the earth. the law of inertia keeps it moving in the same direction. the same experimental proof of revolution is shown in a proportional degree at any point between the pole and the equator. but the watchers on the acropolis do not get turned over so as to see the moon at the same time every night. [page ] we turn down our eastern horizon, but we do not find fair luna at the same moment we did the night before. we are obliged to roll on for some thirty to fifty minutes longer before we find the moon. it must be going in the same direction, and it takes us longer to get round to it than if if it were always in the same spot; so we notice a star near the moon one night--it is ° west of the moon the next night. the moon is going around the earth from west to east, and if it goes ° in one day, it will take a little more than twenty-seven days to go the entire circle of °. [illustration: fig. .--showing the sun's movement among the stars.] [page ] in our outlook we soon observe that we do not by our revolution come to see the same stars rise at the same hour every night. orion and the pleiades, our familiar friends in the winter heavens, are gone from the summer sky. have they fled, or are we turned from them? this is easily understood from fig. . when the observer on the earth at a looks into the midnight sky he sees the stars at e; but as the earth passes on to b, he sees those stars at e three minutes sooner every night; and at midnight the stars at f are over his head. thus in a year, by going around the sun, we have every star of the celestial dome in our midnight sky. we see also how the sun appears among the successive constellations. when we are at a, we see the sun among the stars at g; but as we move toward b, the sun appears to move toward h. if we had observed the sun rise on the th of august, , we should have seen it rise a little before regulus, and a little south of it, in such a relation as circle is to the star in fig. . by sunset the earth had moved enough to make the sun appear to be at circle , and by the next morning at circle , at which time regulus would rise before the sun. thus the earth's motion seems to make the sun traverse a regular circle among the stars once a year: but it is not the sun that moves. [illustration: fig. .] there are certain stars that have such irregular, uncertain, vagarious ways that they were called vagabonds, or planets, by the early astronomers. here is the path of jupiter in the year (fig. ). these bodies go forward for awhile, then stop, start aside, then retrograde, [page ] and go on again. some are never seen far from the sun, and others in all parts of the ecliptic. [illustration: fig. .] first see them as they stand to-day, as in fig. . the observer stands on the earth at a. it has rolled over so far that he cannot see the sun; it has set. but venus is still in sight; jupiter is ° behind venus, and saturn is seen ° farther east. when a has rolled a little farther, if he is awake, he will see mars before he sees the sun; or, in common language, venus will set after, and mars rise before the sun. all these bodies at near and far distances seem set in the starry dome, as the different stars seem in fig. , p. . [illustration: fig. . showing position of planets.] the mysterious movements of advance and retreat are rendered intelligible by fig. . the planet mercury is at a, and, seen from the earth, b is located at _a_, [page ] on the background of the stars it seems to be among. it remains apparently stationary at _a_ for some time, because approaching the earth in nearly a straight line. passing d to c, it appears to retrograde among the stars to _c_; remains apparently stationary for some time, then, in passing from c to e and a, appears to pass back among the stars to _a_. the progress of the earth, meanwhile, although it greatly retards the apparent motion from a to c, greatly hastens it from c to a. [illustration: fig. .--apparent movements of an inferior planet.] it is also apparent that mercury and venus, seen from the earth, can never appear far from the sun. they must be just behind the sun as evening stars, or just before it as heralds of the morning. venus is never more than ° from the sun, and mercury never more than °; indeed, it keeps so near the sun that very few people have ever seen the brilliant sparkler. observe how much larger the planet appears near the earth in conjunction at d than in opposition at e. observe also what phases it must present, and how transits sometimes take place. [page ] the movement of a superior planet, one whose orbit is exterior to the earth, is clear from fig. . when the earth is at a and mars at b, it will appear among the stars at c. when the earth is at d, mars having moved more slowly to e, will have retrograded to f. it remains there while the earth passes on, in a line nearly straight, from mars to g; then, as the earth begins to curve around the sun, mars will appear to retraverse the distance from f to c, and beyond. the farther the superior planet is from the earth the less will be the retrograde movement. [illustration: fig. .--illustrating movements of a superior planet.] the reader should draw the orbits in proportion, and, remembering the relative speed of each planet, note the movement of each in different parts of their orbits. to account for these most simple movements, the earlier astronomers invented the most complex and impossible machinery. they thought the earth the centre, and that the sun, moon, and stars were carried about it, as stoves around a person to warm him. they thought these strange movements of the planets were accomplished by mounting them on subsidiary eccentric wheels in the revolving crystal sphere. all that was [page ] needed to give them a right conception was a sinking of their world and themselves to an appropriate proportion, and an enlargement of their vision, to take in from an exalted stand-point a view of the simplicity of the perfect plan. experiments. fix a rod, or tube, or telescope pointing at a star in the cast or west, and the earth's revolution will be apparent in a moment, turning the tube away from the star. point it at stars about the north pole, and those on one side will be found going in an opposite direction from those on the other, and very much slower than those about the equator. anyone can try the pendulum experiment who has access to some lofty place from which to suspend the ball. it was tried in bunker hill monument a few years ago, and is to be tried in paris, in the summer of , with a seven-hundred-pound pendulum and a suspending wire seventy yards long. the advance and retrograde movements of planets can be illustrated by two persons walking around a centre and noticing the place where the person appears projected on the wall beyond. * * * * * procession of stars and souls. "i stood upon the open casement, and looked upon the night, and saw the westward-going stars pass slowly out of sight. "slowly the bright procession went down the gleaming arch, and my soul discerned the music of the long triumphal march; "till the great celestial army, stretching far beyond the poles, became the eternal symbol of the mighty march of souls. [page ] "onward, forever onward, red mars led on his clan; and the moon, like a mailèd maiden, was riding in the van. "and some were bright in beauty, and some were faint and small, but these might be, in their great heights, the noblest of them all. "downward, forever downward, behind earth's dusky shore, they passed into the unknown night-- they passed, and were no more. "no more! oh, say not so! and downward is not just; for the sight is weak and the sense is dim that looks through heated dust. "the stars and the mailèd moon, though they seem to fall and die, still sweep in their embattled lines an endless reach of sky. "and though the hills of death may hide the bright array, the marshalled brotherhood of souls still keeps its onward way. "upward, forever upward, i see their march sublime, and hear the glorious music of the conquerors of time. "and long let me remember that the palest fainting one may to diviner vision be a bright and blazing sun." thomas buchanan read. [page ] vii. shooting-stars, meteors, and comets. "the lord cast down great stones from heaven upon them unto azekah, and they died."--_joshua_ x. ii. [page ] [illustration: a swarm of meteors meeting the earth. their orbits are all parallel. those coming in direct line to the eye appear as stars, having no motion. those on one side of this line are seen in foreshortened perspective. those furthest from the centre, other things being equal, appear longest. the centre, called the radiant point, of these november meteors is situated in leo; that of the august meteors in perseus. over fifty such radiant points have been discovered. over , meteors have been visible in an hour.] [page ] vii. _shooting-stars, meteors, and comets._ before particularly considering the larger aggregations of matter called planets or worlds as individuals, it is best to investigate a part of the solar system consisting of smaller collections of matter scattered everywhere through space. they are of various densities, from a cloudlet of rarest gas to solid rock; of various sizes, from a grain's weight to little worlds; of various relations to each other, from independent individuality to related streams millions of miles long. when they become visible they are called shooting-stars, which are evanescent star-points darting through the upper air, leaving for an instant a brilliant train; meteors, sudden lights, having a discernible diameter, passing over a large extent of country, often exploding with violence (fig. ), and throwing down upon the earth aerolites; and comets, vast extents of ghostly light, that come we know not whence and go we know not whither. all these forms of matter are governed by the same laws as the worlds, and are an integral part of the solar system--a part of the unity of the universe. [illustration: fig. .--explosion of a bolide.] everyone has seen the so-called shooting-stars. they break out with a sudden brilliancy, shoot a few degrees with quiet speed, and are gone before we can say, "see there!" the cause of their appearance, the [page ] conversion of force into heat by their contact with our atmosphere, has been already explained. other facts remain to be studied. they are found to appear about seventy-three miles above the earth, and to disappear about twenty miles nearer the surface. their average velocity, thirty-five, sometimes rises to one hundred miles a second. they exhibit different colors, according to their different chemical substances, which are consumed. the number of them to be seen on different nights is exceedingly variable; sometimes not more [page ] than five or six an hour, and sometimes so many that a man cannot count those appearing in a small section of sky. this variability is found to be periodic. there are everywhere in space little meteoric masses of matter, from the weight of a grain to a ton, and from the density of gas to rock. the earth meets , , little bodies every day--there is collision--the little meteoroid gives out its lightning sign of extinction, and, consumed in fervent heat, drops to the earth as gas or dust. if we add the number light enough to be seen by a telescope, they cannot be less than , , a day. everywhere we go, in a space as large as that occupied by the earth and its atmosphere, there must be at least , bodies--one in , , cubic miles--large enough to make a light visible to the naked eye, and forty times that number capable of revealing themselves to telescopic vision. professor peirce is about to publish, as the startling result of his investigations, "that the heat which the earth receives directly from meteors is the same in amount which it receives from the sun by radiation, and that the sun receives five-sixths of its heat from the meteors that fall upon it." [illustration: fig. .--bolides.] [page ] in dr. schmidt was fortunate enough to have a telescopic view of a system of bodies which had turned into meteors. these were two larger bodies followed by several smaller ones, going in parallel lines till they were extinguished. they probably had been revolving about each other as worlds and satellites before entering our atmosphere. it is more than probable that the earth has many such bodies, too small to be visible, revolving around it as moons. [illustration: fig. .--santa rosa aerolite.] _aerolites._ sometimes the bodies are large enough to bear the heat, and the unconsumed centre comes to the earth. [page ] their velocity has been lessened by the resisting air, and the excessive heat diminished. still, if found soon after their descent, they are too hot to be handled. these are called aerolites or air-stones. there was a fall in iowa, in february, , from which fragments amounting to five hundred pounds weight were secured. on the evening of december st, , a meteor of unusual size and brilliancy passed over the states of kansas, missouri, illinois, indiana, and ohio. it was first seen in the western part of kansas, at an altitude of about sixty miles. in crossing the state of missouri it began to explode, and this breaking up continued while passing illinois, indiana, and ohio, till it consisted of a large flock of brilliant balls chasing each other across the sky, the number being variously estimated at from twenty to one hundred. it was accompanied by terrific explosions, and was seen along a path of not less than a thousand miles. when first seen in kansas, it is said to have appeared as large as the full moon, and with a train from twenty-five to one hundred feet long. another, very similar in appearance and behavior, passed over a part of the same course in february, . at laigle, france, on april th, , about one o'clock in the day, from two to three thousand fell. the largest did not exceed seventeen pounds weight. one fell in weston, connecticut, in , weighing two hundred pounds. a very destructive shower is mentioned in the book of joshua, chap. x. ver. . these bodies are not evenly distributed through space. in some places they are gathered into systems which circle round the sun in orbits as certain as those of the [page ] planets. the chain of asteroids is an illustration of meteoric bodies on a large scale. they are hundreds in number--meteors are millions. they have their region of travel, and the sun holds them and the giant jupiter by the same power. the power that cares for a world cares for a sparrow. if their orbit so lies that a planet passes through it, and the planet and the meteors are at the point of intersection at the same time, there must be collisions, and the lightning signs of extinction proportioned to the number of little bodies in a given space. it is demonstrated that the earth encounters more than one hundred such systems of meteoric bodies in a single year. it passes through one on the th of august, another on the th of november. in a certain part of the first there is an agglomeration of bodies sufficient to become visible as it approaches the sun, and this is known as the comet of ; in the second is a similar agglomeration, known as temple's comet. it is repeating the same thing to say that meteoroids follow in the train of the comets. the probable orbit of the november meteors and the comet of is an exceedingly elongated ellipse, embracing the orbit of the earth at one end and a portion of the orbit of uranus at the other (fig. ). that of the august meteors and the comet of embraces the orbit of the earth at one end, and thirty per cent. of the other end is beyond the orbit of neptune. [illustration: fig. .--orbit of the november meteors and the comet or .] in january, , biela's comet was observed to be divided. at its next return, in , the parts were , , miles apart. they could not be found on their periodic returns in , , and ; but it [page ] should have crossed the earth's orbit early in september, . the earth itself would arrive at the point of crossing two or three months later. if the law of revolution held, we might still expect to find some of the trailing meteoroids of the comet not gone by on our arrival. it was shown that the point of the earth that would strike them would be toward a certain place in the constellation of andromeda, if the remains of the diluted comet were still there. the prediction was verified in every respect. at the appointed time, place, [page ] and direction, the streaming lights were in our sky. that these little bodies belonged to the original comet none can doubt. by the perturbations of planetary attraction, or by different original velocities, a comet may be lengthened into an invisible stream, or an invisible stream agglomerated till it is visible as a comet. _comets._ comets will be most easily understood by the foregoing considerations. they are often treated as if they were no part of the solar system; but they are under the control of the same laws, and owe their existence, motion, and continuance to the same causes as jupiter and the rest of the planets. they are really planets of wider wandering, greater ellipticity, and less density. they have periodic times less than the earth, and fifty times as great as neptune. they are little clouds of gas or meteoric matter, or both, darting into the solar system from every side, at every angle with the plane of the ecliptic, becoming luminous with reflected light, passing the sun, and returning again to outer darkness. sometimes they have no tail, having a nucleus surrounded by nebulosity like a dim sun with zodiacal light; sometimes one tail, sometimes half a dozen. these follow the comet to perihelion, and precede it afterward (fig. ). the orbits of some comets are enormously elongated; one end may lie inside the earth's orbit, and the other end be as far beyond neptune as that is from the sun. of course only a small part of such a curve can be studied by us: the comet is visible only when near the sun. the same curve around the sun may be an orbit that will bring it back again, [page ] or one that will carry it off into infinite space, never to return. one rate of speed on the curve indicates an elliptical orbit that returns; a greater rate of speed indicates that it will take a parabolic orbit, which never returns. the exact rate of speed is exceedingly difficult to determine; hence it cannot be confidently asserted that any comet ever visible will not return. they may all belong to the solar system; but some will certainly be gone thousands of years before their fiery forms will greet the watchful eyes of dwellers on the earth. a comet that has an elliptic orbit may have it changed to [page ] parabolic by the accelerations of its speed, by attracting planets; or a parabolic comet may become elliptic, and so permanently attracted to the system by the retardations of attracting bodies. a comet of long period may be changed to one of short period by such attraction, or _vice versa_. [illustration: fig. .--aspects of remarkable comets.] the number of comets, like that of meteor streams, is exceedingly large. five hundred have been visible to the naked eye since the christian era. two hundred have been seen by telescopes invented since their invention. some authorities estimate the number belonging to our solar system by millions; professor peirce says more than five thousand millions. _famous comets._ the comet of is perhaps the one that appeared in a.d. , soon after the death of julius cæsar, also in the reign of justinian, a.d. , and in . this is not determined by any recognizable resemblance. it had a tail ° long; it was not all arisen when its head reached the meridian. it is possible, from the shape of its orbit, that it has a periodic time of nine thousand years, or that it may have a parabolic orbit, and never return. observations taken two hundred years ago have not the exactness necessary to determine so delicate a point. on august th, , halley discovered a comet which he soon declared to be one seen by kepler in . looking back still farther, he found that a comet was seen in having the same orbit. still farther, by the same exact period of seventy-five years, he found that it was the same comet that had disturbed [page ] the equanimity of pope calixtus in . calculations were undertaken as to the result of all the accelerations and retardations by the attractions of all the planets for the next seventy-five years. there was not time to finish all the work; but a retardation of six hundred and eighteen days was determined, with a possible error of thirty days. the comet actually came to time within thirty-three days, on march th, . again its return was calculated with more laborious care. it came to time and passed the sun within three days of the predicted time, on the th of november, . it passed from sight of the most powerful telescopes the following may, and has never since been seen by human eye. but the eye of science sees it as having passed its aphelion beyond the orbit of neptune in , and is already hastening back to the warmth and light of the sun. it will be looked for in ; and there is good hope of predicting, long before it is seen, the time of its perihelion within a day. _biela's lost comet._--this was a comet with a periodic time of six years and eight months. it was observed in january, , to have separated into two parts of unequal brightness. the lesser part grew for a month until it equalled the other, then became smaller and disappeared, while the other was visible a month longer. at disappearance the parts were , miles asunder. on its next return, in , the parts were , , miles apart; sometimes one was brighter and sometimes the other; which was the fragment and which was the main body could not be recognized. they vanished in september, , and have never been seen since. three revolutions have been made since that time, but no [page ] trace of it could be discovered. probably the same influence that separated it into parts, separated the particles till too thin and tenuous to be seen. there is ground for believing that the earth passed through a part of it, as before stated under the head of meteors. _the great comet of_ passed nearer the sun than any known body. it almost grazed the sun. if it ever returns, it will be in a.d. . _donati's comet of_ .--this was one of the most magnificent of modern times. during the first three months it showed no tail, but from august to october it had developed one forty degrees in length. its period is about two thousand years. every reader remembers the comet of the summer of . _encke's comet._--this comet has become famous for its supposed confirmation of the theory that space was filled with a substance infinitely tenuous, which resisted the passage of this gaseous body in an appreciable degree, and in long ages would so retard the motion of all the planets that gravitation would draw them all one by one into the sun. we must not be misled by the term retardation to suppose it means behind time, for a retarded body is before time. if its velocity is diminished, the attraction of the sun causes it to take a smaller orbit, and smaller orbits mean increased speed--hence the supposed retardation would shorten its periodic time. this comet was thought to be retarded two and a half hours at each revolution. if it was, it would not prove the existence of the resisting medium. other causes, unknown to us, might account for it. subsequent and more exact calculations fail to find any retardations in at least two revolutions between and [page ] . indications point to a retardation of one and a half hours both before and since. but such discrepancy of result proves nothing concerning a resisting medium, but rather is an argument against its existence. besides, faye's comet, in four revolutions of seven years each, shows no sign of retardation. the truth may be this, that a kind of atmosphere exists around the sun, perhaps revealed by the zodiacal light, that reaches beyond where encke's comet dips inside the orbit of mercury, and thus retards this body, but does not reach beyond the orbit of mars, where faye's comet wheels and withdraws. _of what do comets consist?_ the unsolved problems pertaining to comets are very numerous and exceedingly delicate. whence come they? why did they not contract to centres of nebulæ? are there regions where attractions are balanced, and matter is left to contract on itself, till the movements of suns and planets adds or diminishes attractive force on one side, and so allows them to be drawn slowly toward one planet, and its sun, or another? there is ground for thinking that the comet of and its train of meteors, visible to us in november, was thus drawn into our system by the planet uranus. indeed, leverrier has conjecturally fixed upon the date of a.d. as the time when it occurred; but another and closer observation of its next return, in , will be needed to give confirmation to the opinion. our sun's authority extends at least half-way to the nearest fixed star, one hundred thousand times farther than the orbit of the earth. meteoric and cometary matter lying [page ] there, in a spherical shell about the solar system, balanced between the attraction of different suns, finally feels the power that determines its destiny toward our sun. it would take , , years to come thence to our system. the conditions of matter with which we are acquainted do not cover all the ground presented by these mysterious visitors. we know a gas sixteen times as light as air, but hydrogen is vastly too heavy and dense; for we see the faintest star through thousands of miles of cometary matter; we know that water may become cloudy vapor, but a little of it obscures the vision. into what more ethereal, and we might almost say spiritual, forms matter may be changed we cannot tell. but if we conceive comets to be only gas, it would expand indefinitely in the realms of space, where there is no force of compression but its own. we might say that comets are composed of small separate masses of matter, hundreds of miles apart; and, looking through thousands of miles of them, we see light enough reflected from them all to seem continuous. doubtless that is sometimes the case. but the spectroscope shows another state of things: it reveals in some of these comets an incandescent gas--usually some of the combinations of carbon. the conclusion, then, naturally is that there are both gas and small masses of matter, each with an orbit of its own nearly parallel to those of all the others, and that they afford some attraction to hold the mass of intermingled and confluent gas together. our best judgment, then, is that the nucleus is composed of separate bodies, or matter in a liquid condition, capable of being vaporized by the heat of the sun, and driven off, [page ] as steam from a locomotive, into a tail. indications of this are found in the fact that tails grow smaller at successive returns, as the matter capable of such vaporization becomes condensed. in some instances, as in that of the comet of , the head was diminished by the manufacture of a tail. on the other hand, professor peirce showed that the nucleus of the comets of , , and must have had a tenacity equal to steel, to prevent being pulled apart by the tidal forces caused by its terrible perihelion sweep around the sun. it is likely that there are great varieties of condition in different comets, and in the same comet at times. we see them but a few days out of the possible millions of their periodic time; we see them only close to the sun, under the spur of its tremendous attraction and terrible heat. this gives us ample knowledge of the path of their orbit and time of their revolution, but little ground for judgment of their condition, when they slowly round the uttermost cape of their far-voyaging, in the terrible cold and darkness, to commence their homeward flight. the unsolved problems are not all in the distant sun and more distant stars, but one of them is carried by us, sometimes near, sometimes far off; but our acquaintance with the possible forms and conditions of matter is too limited to enable us to master the difficulties. _will comets strike the earth?_ very likely, since one or two have done so within a recent period. what will be the effect? that depends on circumstances. there is good reason to suppose we passed through the tail of a comet in , and the only [page ] observable effect was a peculiar phosphorescent mist. if the comet were composed of small meteoric masses a brilliant shower would be the result. but if we fairly encountered a nucleus of any considerable mass and solidity, the result would be far more serious. the mass of donati's comet has been estimated by m. faye to be / of that of the earth. if this amount of matter were dense as water, it would make a globe five hundred miles in diameter; and if as dense as professor peirce proved the nucleus of this comet to be, its impact with the earth would develop heat enough to melt and vaporize the hardest rocks. happily there is little fear of this: as professor newcomb says, "so small is the earth in comparison with celestial space, that if one were to shut his eyes and fire at random in the air, the chance of bringing down a bird would be better than that of a comet of any kind striking the earth." besides, we are not living under a government of chance, but under that of an almighty father, who upholdeth all things by the word of his power; and no world can come to ruin till he sees that it is best. [page ] viii. the planets as individuals. "through faith we understand that the worlds [plural] were framed by the word of god, so that things which were seen were not made of things which do appear."--_heb._ xi. . [page ] "o rich and various man! thou palace of sight and sound, carrying in thy senses the morning, and the night, and the unfathomable galaxy; in thy brain the geometry of the city of god; in thy heart the power of love, and the realms of right and wrong. an individual man is a fruit which it costs all the foregoing ages to form and ripen. he is strong, not to do but to live; not in his arms, but in his heart; not as an agent, but as a fact."--emerson. [page ] vii. _the planets as individuals._ how many bodies there may be revolving about the sun we have no means to determine or arithmetic to express. when the new star of the american republic appeared, there were but six planets discovered. since then three regions of the solar system have been explored with wonderful success. the outlying realms beyond saturn yielded the planet uranus in , and neptune in . the middle region between jupiter and mars yielded the little planetoid ceres in , pallas in , and one hundred and ninety others since. the inner region between mercury and the sun is of necessity full of small meteoric bodies; the question is, are there any bodies large enough to be seen? the same great genius of leverrier that gave us neptune from the observed perturbations of uranus, pointed out perturbations in mercury that necessitated either a planet or a group of planetoids between mercury and the sun. theoretical astronomers, aided by the fact that no planet had certainly been seen, and that all asserted discoveries of one had been by inexperienced observers, inclined to the belief in a group, or that the disturbance was caused by the matter reflecting the zodiacal light. when the total eclipse of the sun occurred in , [page ] astronomers were determined that the question of the existence of an intra-mercurial planet should be settled. maps of all the stars in the region of the sun were carefully studied, sections of the sky about the sun were assigned to different observers, who should attend to nothing but to look for a possible planet. it is now conceded that professor watson, of ann arbor, actually saw the sought-for body. vulcan. the god of fire; its sign [symbol], his hammer. distance from the sun, , , miles. orbital revolution, about days. mercury. the swift messenger of the gods; sign [symbol], his caduceus. distance from the sun, , , miles. diameter, miles. orbital revolution, . days. orbital velocity, miles per minute. axial revolution, h. m. mercury shines with a white light nearly as bright as sirius; is always near the horizon. when nearly between us and the sun, as at d (fig. , p. ), its illuminated side nearly opposite to us, we, looking from e, see only a thin crescent of its light. when it is at its greatest angular distance from the sun, as a or c, we see it illuminated like the half-moon. when it is beyond the sun, as at e, we see its whole illuminated face like the full-moon. the variation of its apparent size from the varying distance is very striking. at its extreme distance from the earth it subtends an angle of only five seconds; nearest to us, an angle of twelve seconds. its distance from the earth varies nearly as one to three, and its apparent size in the inverse ratio. [page ] when mercury comes between the earth and the sun, near the line where the planes of their orbits cut each other by reason of their inclination, the dark body of mercury will be seen on the bright surface of the sun. this is called a transit. if it goes across the centre of the sun it may consume eight hours. it goes , miles an hour, and has , miles of disk to cross. the transit of occupied seven and a half hours. the transits for the remainder of the century will occur: november th | november th may th | november th venus. goddess of beauty; its sign [symbol], a mirror. distance from the sun, , , miles. diameter, miles. orbital velocity, miles per minute. axial revolution, h. m. orbital revolution, . days. this brilliant planet is often visible in the daytime. i was once delighted by seeing venus looking down, a little after mid-day through the open space in the dome of the pantheon at rome. it has never since seemed to me as if the home of all the gods was deserted. phoebus, diana, venus and the rest, thronged through that open upper door at noon of night or day. arago relates that bonaparte, upon repairing to luxemburg when the directory was about to give him a _fête_, was much surprised at seeing the multitude paying more attention to the heavens above the palace than to him or his brilliant staff. upon inquiry, he learned that these curious persons were observing with astonishment a star which they supposed to be that of the conqueror of italy. the emperor himself was not indifferent when [page ] his piercing eye caught the clear lustre of venus smiling upon him at mid-day. this unusual brightness occurs when venus is about five weeks before or after her inferior conjunction, and also nearest overhead by being north of the sun. this last circumstance occurs once in eight years, and came on february th, . venus may be as near the earth as , , miles, and as far away as , , . this variation of its distances from the earth is obviously much greater than that of mercury, and its consequent apparent size much more changeable. its greatest and least apparent sizes are as ten and sixty-five (fig. ). [illustration: fig. .--phases of venus, and varions apparent dimensions.] when copernicus announced the true theory of the solar system, he said that if the inferior planets could be clearly seen they would show phases like the moon. when galileo turned the little telescope he had made on venus, he confirmed the prophecy of copernicus. desiring to take time for more extended observation, and still be able to assert the priority of his discovery, he published the following anagram, in which his discovery was contained: [page ] "hæc immatura a me jam frustra leguntur o. y." (these unripe things are now vainly gathered by me.) he first saw venus as gibbous; a few months revealed it as crescent, and then he transposed his anagram into: "cynthiæ figuras æmulatur mater amorum." (the mother of loves imitates the phases of cynthia.) many things that were once supposed to be known concerning venus are not confirmed by later and better observations. venus is surrounded by an atmosphere so dense with clouds that it is conceded that her time of rotation and the inclination of her axis cannot be determined. she revealed one of the grandest secrets of the universe to the first seeker; showed her highest beauty to her first ardent lover, and has veiled herself from the prying eyes of later comers. florence has built a kind of shrine for the telescope of galileo. by it he discovered the phases of venus, the spots on the sun, the mountains of the moon, the satellites of jupiter, and some irregularities of shape in saturn, caused by its rings. galileo subsequently became blind, but he had used his eyes to the best purpose of any man in his generation. the earth. its sign [symbol]. distance from the sun, , , miles. diameter, polar, miles; equatorial, - / miles. axial revolution, h. m. . s.; orbital, . . orbital velocity per minute, . miles. let us lift ourselves up a thousand miles from the earth. we see it as a ball hung upon nothing in empty space. as the drop of falling water gathers itself [page ] into a sphere by its own inherent attraction, so the earth gathers itself into a ball. noticing closely, we see forms of continents outlined in bright relief, and oceanic forms in darker surfaces. we see that its axis of revolution is nearly perpendicular to the line of light from the sun. one-half is always dark. the sunrise greets a new thousand miles every hour; the glories of [page ] the sunset follow over an equal space, ° behind. we are glad that the darkness never overtakes the morning. [illustration: fig. .--earth and moon in space.] _the aurora borealis._ while east and west are gorgeous with sunrise and sunset, the north is often more glorious with its aurora borealis. we remember that all worlds have weird and inexplicable appendages. they are not limited to their solid surfaces or their circumambient air. the sun has its fiery flames, corona, zodiacal light, and perhaps a finer kind of atmosphere than we know. the earth is [page ] not without its inexplicable surroundings. it has not only its gorgeous eastern sunrise, its glorious western sunset, high above its surface in the clouds, but it also has its more glorious northern dawn far above its clouds and air. the realm of this royal splendor is as yet an unconquered world waiting for its alexander. there are certain observable facts, viz., it prevails mostly near the arctic circle rather than the pole; it takes on various forms--cloud-like, arched, straight; it streams like banners, waves like curtains in the wind, is inconstant; is either the cause or result of electric disturbance; it is often from four hundred to six hundred miles above the earth, while our air cannot be over one hundred miles. it almost seems like a revelation to human eyes of those vast, changeable, panoramic pictures by which the inhabitants of heaven are taught. [illustration: fig. .--the aurora as waving curtains.] investigation has discovered far more mysteries than it has explained. it is possible that the same cause that produces sun-spots produces aurora in all space, visible in all worlds. if so, we shall see more abundant auroras at the next maximum of sun-spot, between - . _the delicate balance of forces._ a soap-bubble in the wind could hardly be more flexible in form and sensitive to influence than is the earth. on the morning of may th, , the earth's crust at peru gave a few great throbs upward, by the action of expansive gases within. the sea fled, and returned in great waves as the land rose and fell. then these waves fled away over the great mobile surface, and in less than five hours they had covered a space equal to half of europe. the waves ran out to the sandwich islands, six [page ] thousand miles, at the rate of five hundred miles an hour, and arrived there thirty feet high. they not only sped on in straight radial lines, but, having run up the coast to california, were deflected away into the former series of waves, making the most complex undulations. similar beats of the great heart of the earth have sent its pulses as widely and rapidly on previous occasions. the figure of the earth, even on the ocean, is irregular, in consequence of the greater preponderance of land--and hence greater density--in the northern hemisphere. these irregularities are often very perplexing in making exact geodetic measurements. the tendency of matter to fly from the centre by reason of revolution causes the equatorial diameter to be twenty-six, miles longer than the polar one. by this force the mississippi river is enabled to run up a hill nearly three miles high at a very rapid rate. its mouth is that distance farther from the centre of the earth than its source, when but for this rotation both points would be equally distant. if the water became more dense, or if the world were to revolve faster, the oceans would rush to the equator, burying the tallest mountains and leaving polar regions bare. if the water should become lighter in an infinitesimal degree, or the world rotate more slowly, the poles would be submerged and the equator become an arid waste. no balance, turning to / of a grain, is more delicate than the poise of forces on the world. laplace has given us proof that the period of the earth's axial rotation has not changed / of a second of time in two thousand years. [page ] _tides._ but there is an outside influence that is constantly acting upon the earth, and to which it constantly responds. two hundred and forty thousand miles from the earth is the moon, having / the mass of the world. its attractive influence on the earth causes the movable and nearer portions to hurry away from the more stable and distant, and heap themselves up on that part of the earth nearest the moon. gravitation is inversely as the square of the distance; hence the water on the surface of the earth is attracted more than the body of the earth, some parts of which are eight thousand miles farther off; hence the water rises on the side next the moon. but the earth, as a whole, is nearer the moon than the water on the opposite side, and being drawn more strongly, is taken away from the water, leaving it heaped up also on the side opposite to the moon. a subsidiary cause of tides is found in the revolution of the earth and moon about their common centre of gravity. revolution about an axis through the centre of a sphere enlarges the equator by centrifugal force. revolution about an axis touching the surface of a flexible globe converts it into an egg-shaped body, with the longer axis perpendicular to the axis of revolution. in fig. the point of revolution is seen at the centre of gravity at g; hence, in the revolution of earth and moon as one, a strong centrifugal force is caused at d, and a less one at c. this gives greater height to the tides than the attraction of the moon alone could produce. [page ] [illustration: fig. .] if the earth had no axial revolution, the attractive point where the tide rises would be carried around the earth once in twenty-seven days by the moon's revolution about the earth. but since the earth revolves on its axis, it presents a new section to the moon's attraction every hour. if the moon were stationary, that would bring two high tides in exactly twenty-four hours; but as the moon goes forward, we need nearly twenty-five hours for two tides. the attractive influence of the sun also gives us a tide four-tenths as great as that of the moon. when these two influences of the sun and moon combine, as they do, in conjunction--when both bodies are on one side of the earth; or in opposition, sun and moon being on opposite sides of the earth--we have spring or increased tides. when the moon is in its first or third quarter, _i. e._, when a line from the moon to the earth makes a right angle with one from the sun to the earth, these influences antagonize one another, and we have the neap or low tides. it is easy to see that if, when the moon was drawing its usual tide, the sun drew four-tenths of the water in a tide at right angles with it, the moon's tide must be by so much lower. because of the inertia of the water [page ] it does not yield instantly to the moon's influence, and the crest of the tide is some hours behind the advancing moon. the amount of tide in various places is affected by almost innumerable influences, as distance of moon at its apogee or perigee; its position north, south, or at the equator; distance of earth from sun at perihelion and aphelion; the position of islands; the trend of continents, etc. all eastern shores have far greater tides than western. as the earth rolls to the east it leaves the tide-crest under the moon to impinge on eastern shores, hence the tides of from seventy-five to one hundred feet in the bay of fundy. lakes and most seas are too small to have perceptible tides. the spring-tides in the mediterranean sea are only about three inches. this constant ebb and flow of the great sea is a grand provision for its purification. even the wind is sent to the sea to be cleansed. the sea washes every shore, purifies every cove, bay, and river twice every twenty-four hours. all putrescible matter liable to breed a pestilence is carried far from shore and sunk under fathoms of the never-stagnant sea. the distant moon lends its mighty power to carry the burdens of commerce. she takes all the loads that can be floated on her flowing tides, and cheerfully carries them in opposite directions in successive journeys. it must be conceded that the profoundest study has not mastered the whole philosophy of tides. there are certain facts which are apparent, but for an explanation of their true theory such men as laplace, newton, and airy have labored in vain. there are plenty of other worlds still to conquer. [page ] [illustration: fig. .--lunar day.] [page ] the moon. new moon, [symbol]; first quarter, [symbol]; full moon, [symbol]; last quarter, [symbol]. extreme distance from the earth, , miles; least, , miles; mean, , miles. diameter, . miles [ , lockyer]. revolution about the earth, - / days. axial revolution, same time. when the astronomer herschel was observing the southern sky from the cape of good hope, the most clever hoax was perpetrated that ever was palmed upon a credulous public. some new and wonderful instruments were carefully described as having been used by that astronomer, whereby he was enabled to bring the moon so close that he could see thereon trees, houses, animals, and men-like human beings. he could even discern their movements, and gestures that indicated a peaceful race. the extent of the hoax will be perceived when it is stated that no telescope that we are now able to make reveals the moon more clearly than it would appear to the naked eye if it was one hundred or one hundred and fifty miles away. the distance at which a man can be seen by the unaided eye varies according to circumstances of position, background, light, and eye, but it is much inside of five miles. since, however, the moon is our nearest neighbor, a member of our own family in fact, it is a most interesting object of study. a glance at its familiar face reveals its unequal illumination. all ages and races have seen a man in the moon. all lovers have sworn by its constancy, and only part of them have kept their oaths. every twenty-nine or thirty days we see a silver crescent in the west, and are glad if it comes over the right shoulder--so [page ] much tribute does habit pay to superstition. the next night it is thirteen degrees farther east from the sun. we note the stars it occults, or passes by, and leaves behind as it broadens its disk, till it rises full-orbed in the east when the sun sinks in the west. it is easy to see that the moon goes around the earth from west to east. afterward it rises later and smaller each night, till at length, lost from sight, it rises about the same time as the sun, and soon becomes the welcome crescent new moon again. the same peculiarities are always evident in the visible face of the moon; hence we know that it always presents the same side to the earth. obviously it must make just one axial to one orbital revolution. hold any body before you at arm's-length, revolve it one-quarter around you until exactly overhead. if it has not revolved on an axis between the hands, another quarter of the surface is visible; but if in going up it is turned a quarter over, by the hands holding it steady, the same side is visible. three causes enable us to see a little more than half the moon's surface: . the speed with which it traverses the ellipse of its orbit is variable. it sometimes gets ahead of us, sometimes behind, and we see farther around the front or back part. . the axis is a little inclined to the plane of its orbit, and its orbit a little inclined to ours; hence we see a little over its north pole, and then again over the south pole. . the earth being larger, its inhabitants see a little more than half-way around a smaller body. these causes combined enable us to see / of the moon's surface. our eyes will never see the other side of the moon. if, now, being solid, her axial revolution could [page ] be increased enough to make one more revolution in two or three years, that difference between her axial and orbital revolution would give the future inhabitants of the earth a view of the entire circumference of the moon. yet if the moon were once in a fluid state, or had oceans on the surface, the enormous tide caused by the earth would produce friction enough, as they moved over the surface, to gradually retard the axial revolution till the two tidal elevations remained fixed toward and opposite the earth, and then the axial and orbital revolutions would correspond, as at present. in fact, we can prove that the form of the moon is protuberant toward the earth. its centre of gravity is thirty-three miles beyond its centre of magnitude, which is the same in effect as if a mountain of that enormous height rose on the earth side. hence any fluid, as water or air, would flow round to the other side. the moon's day, caused by the sun's light, is - / times as long as ours. the sun shines unintermittingly for fifteen days, raising a temperature as fervid as boiling water. then darkness and frightful cold for the same time succeed, except on that half where the earth acts as a moon. the earth presents the same phases--crescent, full, and gibbous--to the moon as the moon does to us, and for the same causes. lord rosse has been enabled, by his six-foot reflector, to measure the difference of heat on the moon under the full blaze of its noonday and midnight. he finds it to be no less than five hundred degrees. people not enjoying extremes of temperature should shun a lunar residence. the moon gives us only / as much light as the sun. a sky full of moons would scarcely make daylight. [page ] [illustration: fig. .--view of the moon near the third quarter. from a photograph by professor henry draper.] there are no indications of air or water on the moon. when it occults a star it instantly shuts off the light and as instantly reveals it again. an atmosphere would gradually diminish and reveal the light, and by refraction [page ] cause the star to be hidden in much less time than the solid body of the moon would need to pass over it. if the moon ever had air and water, as it probably did, they are now absorbed in the porous lava of its substance. _telescopic appearance._ [illustration: fig. .--illumination of craters and peaks.] probably no one ever saw the moon by means of a good telescope without a feeling of admiration and awe. except at full-moon, we can see where the daylight struggles with the dark along the line of the moon's sunrise or sunset. this line is called the terminator. it is broken in the extreme, because the surface is as rough as possible. in consequence of the small gravitation of the moon, utter absence of the expansive power of ice shivering the cliffs, or the levelling power of rains, precipices can stand in perpendicularity, mountains shoot up like needles, and cavities three miles deep remain unfilled. the light of the sun falling on the rough body of the moon, shown in section (fig. ), illuminates the whole cavity at _a_, part of the one at _b_, casts a long shadow from the mountain at _c_, and touches the tip of the one at _d_, which appears to a distant observer as a point of light beyond the terminator, as the moon revolves the conical cavity, _a_ is illuminated on the forward side only; the light creeps down the backward side of cavity _b_ to the bottom; mountain _c_. comes directly under the sun and casts no shadow, and mountain _d_ casts its long shadow over the plain. knowing the time of revolution, and observing the change of [page ] illumination, we can easily measure the height of mountain and depth of crater. an apple, with excavations and added prominences, revolved on its axis toward the light of a candle, admirably illustrates the crescent light that fills either side of the cavities and the shadows of the mountains on the plain. notice in fig. the crescent forms to the right, showing cavities in abundance. [illustration: fig. .--lunar crater "copernicus," after secchi.] the selenography of one side of the moon is much better known to us than the geography of the earth. our maps of the moon are far more perfect than those of the earth; and the photographs of lunar objects by messrs. draper and de la rue are wonderfully perfect, [page ] and the drawings of padre secchi equally so (fig. ). the least change recognizable from the earth must be speedily detected. there are frequently reports of discoveries of volcanoes on the moon, but they prove to be illusions. the moon will probably look the same to observers a thousand years hence as it does to-day. this little orb, that is only / of the mass of the earth, has twenty-eight mountains that are higher than mont blanc, that "monarch of mountains," in europe. _eclipses._ [illustration: fig. .--eclipses; shadows of earth and moon.] it is evident that if the plane of the moon's orbit were to correspond with that of the earth, as they all lie in the plane of the page (fig. ), the moon must pass between the centres of the earth and sun, and exactly behind the earth at every revolution. such successive and total darkenings would greatly derange all affairs dependent on light. it is easily avoided. venus does [page ] not cross the disk of the sun at every revolution, because of the inclination of the plane of its orbit to that of the earth (see fig. , p. ). so the plane of the orbit of the moon is inclined to the orbit of the earth ° ' "; hence the full-moon is often above or below the earth's shadow, and the earth is below or above the moon's shadow at new moon. it is as if the moon's orbit were pulled up one-quarter of an inch from the page behind the earth, and depressed as much below it between the earth and the sun. the point where the orbit of the moon penetrates the plane of the ecliptic is called a node. if a new moon occur when the line of intersection of the planes of orbits points to the sun, the sun must be eclipsed; if the full-moon occur, the moon must be eclipsed. in any other position the sun or moon will only be partially hidden, or no eclipse will occur. if the new moon be near the earth it will completely obscure the sun. a dime covers it if held close to the eye. it may be so far from the earth as to only partially hide the sun; and, if it cover the centre, leave a ring of sunlight on every side. this is called an annular eclipse. two such eclipses will occur this year ( ). if the full-moon passes near the earth, or is at perigee, it finds the cone of shadow cast by the earth larger, and hence the eclipse is greater; if it is far from the earth, or near apogee, the earth's shadow is smaller, and the eclipse less, or is escaped altogether. there is a certain periodicity in eclipses. whenever the sun, moon, and earth are in a line, as in the total eclipse of july th, , they will be in the same position after the earth has made about eighteen revolutions, [page ] and the moon two hundred and sixteen--that is, eighteen years after. this period, however, is disregarded by astronomers, and each eclipse calculated by itself to the accuracy of a second. how terrible is the fear of ignorance and superstition when the sun or moon appear to be in the process of destruction! how delightful are the joys of knowledge when its prophesies in regard to the heavenly bodies are being fulfilled! mars. the god or war; its sign [symbol], spear and shield. mean distance from the sun, , , miles. diameter, miles. revolution, axial, h. m. . s.; orbital, . days. velocity per minute, miles. satellites, two. [illustration: fig. .--apparent size of mars at mean and extreme distances.] at intervals, on an average of two years one month and nineteen days, we find rising, as the sun goes down, the reddest star in the heavens. its brightness is exceedingly variable; sometimes it scintillates, and sometimes it shines with a steady light. its marked peculiarities demand a close study. we find it to be mars, the fiery god of war. its orbit is far from circular. at perihelion it is , , miles from the sun, and at aphelion , , ; hence its mean distance is about , , . so great a change in its distance from the sun easily accounts for the change in its brilliancy. now, if mars and the earth revolved in circular orbits, the one , , miles from the sun, and the other , , , they would approach at conjunction within , , miles of each other, and at opposition be , , miles apart. but mars at perihelion may be only , , miles from the sun, and earth at [page ] aphelion may be , , miles from the sun. they are, then, but , , miles apart. this favorable opportunity occurs about once in seventy-nine years. at its last occurrence, in , mars introduced to us his two satellites, that had never before been seen by man. in consequence of this greatly varying distance, the apparent size of mars differs very much (fig. ). take a favorable time when the planet is near, also as near overhead as it ever comes, so as to have as little atmosphere as possible to penetrate, and study the planet. the first thing that strikes the observer is a dazzling spot of white near the pole which happens to be toward him, or at both poles when the planet is so situated that they can be seen. when the north pole is turned toward the sun the size of the spot sensibly diminishes, and the spot at the south pole enlarges, and _vice versa_. clearly they are ice-fields. hence mars has water, and air to carry it, and heat to melt ice. it is winter at the south pole when mars is farthest from the sun; therefore the ice-fields are larger than at the north pole. it is summer at the south pole when mars is nearest the sun. hence its ice-fields grow smaller [page ] than those of the north pole in its summer. this carrying of water from pole to pole, and melting of ice over such large areas, might give rise to uncomfortable currents in ocean and air; but very likely an inhabitant of earth might be transported to the surface of mars, and be no more surprised at what he observed there than if he went to some point of the earth to him unknown. day and night would be nearly of the same length; winter would linger longer in the lap of spring; summer would be one hundred and eighty-one days long; but as the seas are more intermingled with the land, and the divisions of land have less of continental magnitude, it may be conjectured that mars might be a comfortable place of residence to beings like men. perhaps the greatest surprise to the earthly visitor would be to find himself weighing only four-tenths as much as usual, able to leap twice as high, and lift considerable bowlders. _satellites of mars._ the night of august th, , is famous in modern astronomy. mars has been a special object of study in all ages; but on that evening professor hall, of washington, discovered a satellite of mars. on the th it was seen again, and its orbital motion followed. on the following night it was hidden behind the body of the planet when the observation began, but at the calculated time--at four o'clock in the morning--it emerged, and established its character as a true moon, and not a fixed star or asteroid. blessings, however, never come singly, for another object soon emerged which proved to be an inner satellite. this is extraordinarily near [page ] the planet--only four thousand miles from the surface--and its revolution is exceedingly rapid. the shortest period hitherto known is that of the inner satellite of saturn, h. m. the inner satellite of mars makes its revolution in h. m.--a rapidity so much surpassing the axial revolution of the planet itself, that it rises in the west and sets in the east, showing all phases of our moon in one night. the outer satellite is , miles from mars, and makes its revolution in h. m. its diameter is six and a quarter miles; that of the inner one is seven and a half miles. this can be estimated only by the amount of light given. asteroids. already discovered ( ), . distances from the sun, from , , to , , miles. diameters, from to miles. mass of all, less than one-quarter of the earth. the sense of infinite variety among the countless number of celestial orbs has been growing rapidly upon us for half a century, and doubtless will grow much more in half a century to come. just as we paused in the consideration of planets to consider meteors and comets, at first thought so different, so must we now pause to consider a ring of bodies, some of which are as small in comparison to jupiter, the next planet, as aerolites are compared to the earth. in an association of astronomers, suspecting that a planet might be found in the great distance between mars and jupiter, divided the zodiac into twenty-four parts, and assigned one part to each astronomer for a thorough search; but, before their organization could commence work, piazzi, an italian astronomer of palermo, [page ] found in taurus a star behaving like a planet. in six weeks it was lost in the rays of the sun. it was rediscovered on its emergence, and named ceres. in march, , a second planet was discovered by olbers in the same gap between mars and jupiter, and named pallas. here was an embarrassment of richness. olbers suggested that an original planet had exploded, and that more pieces could be found. more were found, but the theory is exploded into more pieces than a planet could possibly be. up to one hundred and ninety-two have been discovered, with a prospect of more. between - forty-five were discovered, showing that they are sought for with great skill. in the discovery of these bodies, our american astronomers, professors watson and peters, are without peers. between mars and jupiter is a distance of some , , miles. subtract , , miles next to mars and , , miles next to jupiter, and there is left a zone , , miles wide outside of which the asteroids never wander. if any ever did, the attraction of mars or jupiter may have prevented their return. since the orbits of mars and jupiter show no sign of being affected by these bodies for a century past, it is probable that their number is limited, or at least that their combined mass does not approximate the size of a planet. professor newcomb estimates that if all that are now discovered were put into one planet, it would not be over four hundred miles in diameter; and if a thousand more should exist, of the average size of those discovered since , their addition would not increase the diameter to more than five hundred miles. [page ] that all these bodies, which differ from each other in no respect except in brilliancy, can be noted and fixed so as not to be mistaken one for another, and instantly recognized though not seen for a dozen years, is one of the highest exemplifications of the accuracy of astronomical observation. jupiter. the king of the gods; sign [symbol], the bird of jove. distance from the sun, perihelion, , , miles; aphelion, , , miles. diameter, equatorial, , miles; polar, , miles. volume, earths. mass, earths. axial revolution, h. m s. orbital revolution, years days. velocity, . miles per minute. [illustration: fig. .--jupiter as seen by the great washington telescope. drawn by mr. holden.] jupiter rightly wears the name of the "giant planet." his orbit is more nearly circular than most smaller planets. he could not turn short corners with facility. we know little of his surface. his spots and belts are [page ] changeable as clouds, which they probably are. some spots may be slightly self-luminous, but not the part of the planet we see. it is covered with an enormous depth of atmosphere. since the markings in the belts move about one hundred miles a day, the jovian tempests are probably not violent. it is, however, a singular and unaccountable fact, as remarked by arago, that its trade-winds move in an opposite direction from ours. jupiter receives only one twenty-seventh as much light and heat from the sun as the earth receives. its lighter density, being about that of water, indicates that it still has internal heat of its own. indeed, it is likely that this planet has not yet cooled so as to have any solid crust, and if its dense vapors could be deposited on the surface, its appearance might be more suggestive of the sun than of the earth. _satellites of jupiter._ in one respect jupiter seems like a minor sun--he is royally attended by a group of planets: we call them moons. this system is a favorite object of study to everyone possessing a telescope. indeed, i have known a man who could see these moons with the naked eye, and give their various positions without mistake. galileo first revealed them to ordinary men. we see their orbits so nearly on the edge that the moons seem to be sliding back and forth across and behind the disk, and to varying distances on either side. fig. is the representation of their appearance at successive observations in november, . their motion is so swift, and the means of comparison by one another and the planet so excellent, that they can be seen to change their places, [page ] be occulted, emerge from shadow, and eclipse the planet, in an hour's watching. [illustration: fig. .--_a._ various positions of jupiter's moons; _b._ greatest elongation of each satellite.] elements of jupiter's satellites. +-------------------------------------------------------------+ | | mean distance | | | | | from jupiter. | sidereal period. | diameter.| | |---------------+------------------+----------| | | miles. | days hrs. min. | miles. | | i. io | , | | , | | ii. europa | , | | , | | iii. ganymede | , | | , | | iv. callisto | , , | | , | +-------------------------------------------------------------+ it is seen by the above table that all these moons are larger than ours, one larger than mercury, and the asteroids are hardly large enough to make respectable moons for them. they differ in color: i. and ii. have a bluish tinge; iii. a yellow; and iv. is red. the amount of light given by these satellites varies in the most sudden and inexplicable manner. perhaps it may be owing to the different distributions of land and water on them. the mass of all of them is . of jupiter. [page ] if the jovian system were the only one in existence, it would be a surprising object of wonder and study. a monster planet, , miles in diameter, hung on nothing, revolving its equatorial surface forty-five miles a minute, holding four other worlds in steady orbits, some of them at a speed of seven hundred miles a minute, and the whole system carried through space at five hundred miles a minute. yet the discovery of all this display of power, skill, and stability is only reading the easiest syllables of the vast literature of wisdom and power. saturn. the god or time; sign [symbol], his scythe. mean distance from the sun, , , miles. diameter, polar, , miles; equatorial, , miles. axial revolution, h. m. periodic time, t years. moons, eight. the human mind has used saturn and the two known planets beyond for the last years as a gymnasium. it has exercised itself in comprehending their enormous distances in order to clear those greater spaces, to where the stars are set; it has exercised its ingenuity at interpreting appearances which signify something other than they seem, in order that it may no longer be deluded by any sunrises into a belief that the heavenly dome goes round the earth. that a wandering point of light should develop into such amazing grandeurs under the telescope, is as unexpected as that every tiny seed should show peculiar markings and colors under the microscope. [illustration: fig. .--view of saturn and his rings.] there are certain things that are easy to determine, such as size, density, periodic time, velocity, etc.; but other things are exceedingly difficult to determine. it requires long sight to read when the book is held [page ] , , miles away. only very few, if more than two, opportunities have been found to determine the time of saturn's rotation. on the evening of december th, , professor hall observed a brilliant white spot suddenly show itself on the body of this planet. it was as if an eruption of white hot matter burst up from the interior. it spread eastward, and remained bright till january, when it faded. no such opportunity for getting a basis on which to found a calculation of the time of the rotation of saturn has occurred since sir william herschel's observations; and, very singularly, the two times deduced wonderfully coincide--that of herschel being h. m., that of mr. hall being h. m. [page ] the density of saturn is less than that of water, and its velocity of rotation so great that centrifugal force antagonizes gravitation to such an extent that bodies weigh on it about the same as on the earth. all the fine fancies of the habitability of this vaporous world, all the calculations of the number of people that could live on the square miles of the planet and its enormous rings, are only fancy. nothing could live there with more brains than a fish, at most. it is a world in formative processes. we cannot hear the voice of the creator there, but we can see matter responsive to the voice, and moulded by his word. _rings of saturn._ the eye and mind of man have worked out a problem of marvellous difficulty in finding a true solution of the strange appearance of the rings. galileo has the immortal honor of first having seen something peculiar about this planet. he wrote to the duke of tuscany, "when i view saturn it seems _tricorps_. the central body seems the largest. the two others, situated, the one on the east, and the other on the west, seem to touch it. they are like two supporters, who help old saturn on his way, and always remain at his side." looking a few years later, the rings having turned from view, he said, "it is possible that some demon mocked me;" and he refused to look any more. huyghens, in march, , solved the problem of the triform appearance of saturn. he saw them as handles on the two sides. in a year they had disappeared, and the planet was as round as it seemed to galileo in . he did not, however, despair; and in october, [page ] , he was rewarded by seeing them appear again. he wrote of saturn, "it is girdled by a thin plain ring, nowhere touching, inclined to the ecliptic." since that time discoveries have succeeded one another rapidly. "we have seen by degrees a ring evolved out of a triform planet, and the great division of the ring and the irregularities on it brought to light. enceladus, and coy mimas, faintest of twinklers, are caught by herschel's giant mirrors. and he, too, first of men, realizes the wonderful tenuity of the ring, along which he saw those satellites travelling like pearls strung on a silver thread. then bond comes on the field, and furnishes evidence to show that we must multiply the number of separate rings we know not how many fold. and here we reach the golden age of saturnian discovery, when bond, with the giant refractor of cambridge, and dawes, with his - / -inch munich glass, first beheld that wonderful dark semi-transparent ring, which still remains one of the wonders of our system. but the end is not yet: on the southern surface of the ring, ere summer fades into autumn, otto struve in turn comes upon the field, detects, as dawes had previously done, a division even in the dark ring, and measures it, while it is invisible to lassell's mirror--a proof, if one were needed, of the enormous superiority possessed by refractors in such inquiries. then we approach , when the ring plane again passes through the earth, and struve and wray observe curious nebulous appearances."[*] [footnote *: lockyer.] our opportunities for seeing saturn vary greatly. as the earth at one part of its orbit presents its south pole [page ] to the sun, then its equator, then the north pole, so saturn; and we, in the direction of the sun, see the south side of the rings inclined at an angle of °; next the edge of the rings, like a fine thread of light; then the north side at a similar inclination. on february th, , saturn was between aquarius and pisces, with the edge of the ring to the sun. in , the planet being in taurus, the south side of the rings will be seen at the greatest advantage. from till all circumstances will combine to give most favorable studies of saturn. meanwhile study the picture of it. the outer ring is narrow, dark, showing hints of another division, sometimes more evident than at others, as if it were in a state of flux. the inner, or second, ring is much brighter, especially on the outer edge, and shading off to the dusky edge next to the planet. there is no sign of division into a third dusky innermost ring, as was plainly seen by bond. this, too, may be in a state of flux. the markings of the planet are delicate, difficult of detection, and are not like those stark zebra stripes that are so often represented. the distance between the planet and the second ring seems to be diminished one-half since , and this ring has doubled its breadth in the same time. some of this difference may be owing to our greater telescopic power, enabling us to see the ring closer to the planet; but in all probability the ring is closing in upon the central body, and will touch it by a.d. . thus the whole ring must ultimately fall upon the planet, instead of making a satellite. we are anxious to learn the nature of such a ring. [page ] laplace mathematically demonstrated that it cannot be uniform and solid, and survive. professor peirce showed it could not be fluid, and continue. then professor maxwell showed that it must be formed of clouds of satellites too small to be seen individually, and too near together for the spaces to be discerned, unless, perhaps, we may except the inner dark ring, where they are not near enough to make it positively luminous. indeed, there is some evidence that the meteoroids are far enough apart to make the ring partially transparent. we look forward to the opportunities for observation in with the brightest hope that these difficult questions will be solved. _satellites of saturn._ the first discovered satellite of saturn seen by huyghens was in , and the last by the bonds, father and son, of cambridge, in . these are eight in number, and are named: distant from saturn's centre. i. mimas , miles. ii. enceladus , " iii. tethys , " iv. dione , " v. rhea , " vi. titan , " vii. hyperion , " viii. japetus , , " titan can be seen by almost any telescope; i., ii., and iii., only by the most powerful instrument. all except japetus revolve nearly in the plane of the ring. like the moons of jupiter, they present remarkable and unaccountable variations of brilliancy. an inspection [page ] of the table reveals either an expectation that another moon will be discovered between v. and vi., and about three more between vii. and viii., or that these gaps may be filled with groups of invisible asteroids, as the gap between mars and jupiter. this will become more evident by drawing saturn, the rings, and orbits of the moons all as circles, on a scale of , miles to the inch. saturn will be in the centre, , miles in diameter; then a gap, decreasing twenty-nine miles a year to the first ring, of, say, , miles; a dark ring miles wide; next the brightest ring , miles wide; then a gap of miles; then the outer ring , miles wide; then the orbits of the satellites in order. if the scenery of jupiter is magnificent, that of saturn must be sublime. if one could exist there, he might wander from the illuminated side of the rings, under their magnificent arches, to the darkened side, see the swift whirling moons; one of them presenting ten times the disk of the earth's moon, and so very near as to enable him to watch the advancing line of light that marks the lunar morning journeying round that orb. uranus. sign [symbol]; the initial of herschel, and sign of the world. distance from the sun, , , , miles. diameter, , miles. axial revolution unknown. orbital, years. velocity per minute, miles. moons, four. uranus was presented to the knowledge of man as an unexpected reward for honest work. it was first mistaken by its discoverer for a comet, a mere cloud of vapor; but it proved to be a world, and extended the [page ] boundaries of our solar system, in the moment of its discovery, as much as all investigation had done in all previous ages. sir william herschel was engaged in mapping stars in , when he first observed its sea-green disk. he proposed to call it _georgium sidus_, in honor of his king; but there were too many names of the gods in the sky to allow a mortal name to be placed among them. it was therefore called uranus, since, being the most distant body of our system, as was supposed, it might appropriately bear the name of the oldest god. finding anything in god's realms of infinite riches ought not to lead men to regard that as final, but as a promise of more to follow. this planet had been seen five times by flamsteed before its character was determined--once nearly a century before--and eight times by le monnier. these names, which might easily have been associated with a grand discovery, are associated with careless observation. eyes were made not only to be kept open, but to have minds behind them to interpret their visions. herschel thought he discovered six moons belonging to uranus, but subsequent investigation has limited the number to four. two of these are seen with great difficulty by the most powerful telescopes. if the plane of our moon's orbit were tipped up to a greater inclination, revolving it on the line of nodes as an axis until it was turned °, the moon, still continuing on its orbit in that plane, would go over the poles instead of about the equator, and would go back to its old path when the plane was revolved °; but its revolution would now be from east to west, or [page ] retrograde. the plane of the moons of uranus has been thus inclined till it has passed ° beyond the pole, and the moons' motions are retrograde as regards other known celestial movements. how uranus itself revolves is not known. there are more worlds to conquer. neptune. god of the sea; sign [symbol], his trident. distance from the sun, , , , miles. diameter, , miles. velocity per minute, . miles. axial revolution unknown. orbital, . years. one moon. men sought for neptune as the heroes sought the golden fleece. the place of uranus had been mapped for nearly one hundred years by these accidental observations. on applying the law of universal gravitation, a slight discrepancy was found between its computed place and its observed place. this discrepancy was exceedingly slight. in it was only "; in , "; in , '. two stars that were ' apart would appear as one to the keenest unaided eye, but such an error must not exist in astronomy. years of work were given to its correction. mr. john c. adams, of cambridge, england, finding that the attraction of a planet exterior to uranus would account for its irregularities, computed the place of such a hypothetical body with singular exactness in october, ; but neither he nor the royal astronomer airy looked for it. another opportunity for immortality was heedlessly neglected. meanwhile, m. leverrier, of paris, was working at the same problem. in the summer of leverrier announced the place of the exterior planet. the conclusion was in striking coincidence with that of mr. [page ] clark. mr. challis commenced to search for the planet near the indicated place, and actually saw and mapped the star august th, , but did not recognize its planetary character. dr. galle, of berlin, on the d of september, , found an object with a planetary disk not plotted on the map of stars. it was the sought-for world. it would seem easy to find a world seventy-six times as large as the earth, and easy to recognize it when seen. the fact that it could be discovered only by such care conveys an overwhelming idea of the distance where it moves. [illustration: fig. .--perturbation of uranus.] the effect of these perturbations by an exterior planet is understood from fig. . uranus and neptune were in conjunction, as shown, in . but in it had been found that uranus was too far from the sun, and too much accelerated. since , neptune, in his orbit from f to e, had been hastening uranus in his orbit d from c to b, and also drawing it farther from the sun. after , neptune, in passing from e to d, had been retarding uranus in his orbit from b to a. we have seen it is easy to miss immortality. there is still another instance. lalande saw neptune on may th and th, , noted that it had moved a little, and that the observations did not agree; but, supposing the first was wrong, carelessly missed the glory of once more doubling the bounds of the empire of the sun. [page ] it is time to pause and review our knowledge of this system. the first view reveals a moon and earth endowed with a force of inertia going on in space in straight lines; but an invisible elastic cord of attraction holds them together, just counterbalancing this tendency to fly apart, and hence they circle round their centre of gravity. the revolving earth turns every part of its surface to the moon in each twenty-four hours. by an axial revolution in the same time that the moon goes round the earth, the moon holds the same point of its surface constantly toward the earth. if we were to add one, two, four, eight moons at appropriate distances, the result would be the same. there is, however, another attractive influence--that of the sun. the sun attracts both earth and moon, but their nearer affection for each other keeps them from going apart. they both, revolving on their axes and around their centre of gravity, sweep in a vastly wider curve around the sun. add as many moons as has jupiter or saturn, the result is the same--an orderly carrying of worlds through space. there lies the unsupported sun in the centre, nearer to infinity in all its capacities and intensities of force than our minds can measure, filling the whole dome to where the stars are set with light, heat, and power. it holds five small worlds--vulcan, mercury, venus, earth, and mars--within a space whose radius it would require a locomotive half a thousand years to traverse. it next holds some indeterminate number of asteroids, and the great jupiter, equal in volume to , earths. it holds saturn, uranus, and neptune, and all their variously related satellites and rings. the two thoughts that overwhelm us are distance and power. the period of [page ] man's whole history is not sufficient for an express train to traverse half the distance to neptune. thought wearies and fails in seeking to grasp such distances; it can scarcely comprehend one million miles, and here are thousands of them. even the wings of imagination grow weary and droop. when we stand on that outermost of planets, the very last sentinel of the outposts of the king, the very sun grown dim and small in the distance, we have taken only one step of the infinite distance to the stars. they have not changed their relative position--they have not grown brighter by our approach. neptune carries us round a vast circle about the centre of the dome of stars, but we seem no nearer its sides. in visiting planets, we have been only visiting next-door neighbors in the streets of a seaport town. we know that there are similar neighbors about sirius and arcturus, but a vast sea rolls between. as we said, we stand with the outermost sentinel; but into the great void beyond the king of day sends his comets as scouts, and they fly thousands of years without for one instant missing the steady grasp of the power of the sun. it is nearer almightiness than we are able to think. if we cannot solve the problems of the present existence of worlds, how little can we expect to fathom the unsoundable depths of their creation and development through ages measureless to man! yet the very difficulty provokes the most ambitious thought. we toil at the problem because it has been hitherto unsolvable. every error we make, and discover to be such, helps toward the final solution. every earnest thinker who climbs the shining worlds as steps to a higher thought is trying to solve the problem god has given us to do. [page ] ix. the nebular hypothesis. "and the earth was without form, and void; and darkness was upon the face of the deep."--_genesis_ i. . [page ] "a dark illimitable ocean, without bound, without dimension, where length, breadth, and height, and time, and place are lost."--milton. "it is certain that matter is somehow directed, controlled, and arranged; while no material forces or properties are known to be capable of discharging such functions."--lionel beale. "the laws of nature do not account for their own origin."--john stuart mill. [page ] ix. _the nebular hypothesis._ the method by which the solar system came into its present form was sketched in vast outline by moses. he gave us the fundamental idea of what is called the nebular hypothesis. swedenborg, that prodigal dreamer of vagaries, in threw out some conjectures of the way in which the outlines were to be filled up; buffon followed him closely in ; kant sought to give it an ideal philosophical completeness; as he said, "not as the result of observation and computation," but as evolved out of his own consciousness; and laplace sought to settle it on a mathematical basis. it has been modified greatly by later writers, and must receive still greater modifications before it can be accepted by the best scientists of to-day. it has been called "the grandest generalization of the human mind;" and if it shall finally be so modified as to pass from a tentative hypothesis to an accepted philosophy, declaring the modes of a divine worker rather than the necessities of blind force, it will still be worthy of that high distinction. let it be clearly noted that it never proposes to do more than to trace a portion of the mode of working which brought the universe from one stage to another. it only goes back to a definite point, never to absolute beginning, nor to nothingness. it takes matter from [page ] the hand of the unseen power behind, and merely notes the progress of its development. it finds the clay in the hands of an intelligent potter, and sees it whirl in the process of formation into a vessel. it is not in any sense necessarily atheistic, any more than it is to affirm that a tree grows by vital processes in the sun and dew, instead of being arbitrarily and instantly created. the conclusion reached depends on the spirit of the observer. newton could say, "this most beautiful system of the sun, planets, and comets could only proceed from the counsel and dominion of an intelligent and powerful being!" still it is well to recognize that some of its most ardent defenders have advocated it as materialistic. and laplace said of it to napoleon, "i have no need of the hypothesis of a god." the materialistic statement of the theory is this: that matter is at first assumed to exist as an infinite cloud of fire-mist, dowered with power latent therein to grow of itself into every possibility of world, flower, animal, man, mind, and affection, without any interference or help from without. but it requires far more of the divine worker than any other theory. he must fill matter with capabilities to take care of itself, and this would tax the abilities of the infinite one far more than a constant supervision and occasional interference. instead of making the vase in perfect form, and coloring it with exquisite beauty by an ever-present skill, he must endow the clay with power to make itself in perfect form, adorn itself with delicate beauty, and create other vases. the nebular hypothesis is briefly this: all the matter composing all the bodies of the sun, planets, and satellites once existed in an exceedingly diffused state; [page ] rarer than any gas with which we are acquainted, filling a space larger than the orbit of neptune. gravitation gradually contracted this matter into a condensing globe of immense extent. some parts would naturally be denser than others, and in the course of contraction a rotary motion, it is affirmed, would be engendered. rotation would flatten the globe somewhat in the line of its axis. contracting still more, the rarer gases, aided by centrifugal force, would be left behind as a ring that would ultimately be separated, like saturn's ring, from the retreating body. there would naturally be some places in this ring denser than others; these would gradually absorb all the ring into a planet, and still revolve about the central mass, and still rotate on its own axis, throwing off rings from itself. thus the planet neptune would be left behind in the first sun-ring, to make its one moon; the planet uranus left in the next sun-ring, to make its four moons from four successive planet-rings; saturn, with its eight moons and three rings not made into moons, is left in the third sun-ring; and so on down to vulcan. the outer planets would cool off first, become inhabitable, and, as the sun contracted and they radiated their own heat, become refrigerated and left behind by the retreating sun. of course the outer planets would move slowly; but as that portion of the sun which gave them their motion drew in toward the centre, keeping its absolute speed, and revolving in the lessening circles of a contracting body, it would give the faster motion necessary to be imparted to earth, mercury, and vulcan. the four great classes of facts confirmatory of this hypothesis are as follows: st. all the planets move [page ] in the same direction, and nearly in the same plane, as if thrown off from one equator; d. the motions of the satellites about their primaries are mostly in the same direction as that of their primaries about the sun; d. the rotation of most of these bodies on their axes, and also of the sun, is in the same direction as the motion of the planets about the sun; th. the orbits of the planets, excluding asteroids, and their satellites, have but a comparatively small eccentricity; th. certain nebulæ are observable in the heavens which are not yet condensed into solids, but are still bright gas. the materialistic evolutionist takes up the idea of a universe of material world-stuff without form, and void, but so endowed as to develop itself into orderly worlds, and adds to it this exceeding advance, that when soil, sun, and chemical laws found themselves properly related, a force in matter, latent for a million eons in the original cloud, comes forward, and dead matter becomes alive in the lowest order of vegetable life; there takes place, as herbert spencer says, "a change from an indefinite, incoherent homogeneity, into a definite, coherent heterogeneity, through continuous differentiation and integration." the dead becomes alive; matter passes from unconsciousness to consciousness; passes up from plant to animal, from animal to man; takes on power to think, reason, love, and adore. the theistic evolutionist may think that the same process is gone through, but that an ever-present and working god superintends, guides, and occasionally bestows a new endowment of power that successively gives life, consciousness, mental, affectional, and spiritual capacity. is this world-theory true? and if so, is either of the [page ] evolution theories true also? if the first evolution theory is true, the evolved man will hardly know which to adore most, the being that could so endow matter, or the matter capable of such endowment. there are some difficulties in the way of the acceptance of the nebular hypothesis that compel many of the most thorough scientists of the day to withhold their assent to its entirety. the latest, and one of the most competent writers on the subject, professor newcomb, who is a mathematical astronomer, and not an easy theorist, evolving the system of the universe from the depth of his own consciousness, says: "should any one be sceptical as to the sufficiency of these laws to account for the present state of things, science can furnish no evidence strong enough to overthrow his doubts until the sun shall be found to be growing smaller by actual measurement, or the nebulæ be actually seen to condense into stars and systems." in one of the most elaborate defences of the theory, it is argued that the hypothesis explains why only one of the four planets nearest the sun can have a moon, and why there can be no planet inside of mercury. the discovery of the two satellites to mars and of the planet vulcan makes it all the worse for these facts. some of the objections to the theory should be known by every thinker. laplace must have the cloud "diffused in consequence of excessive heat," etc. helmholtz, in order to account for the heat of the contracting sun, must have the cloud relatively cold. how he and his followers diffused the cloud without heat is not stated. the next difficulty is that of rotation. the laws [page ] of science compel a contraction into one non-rotating body--a central sun, indeed, but no planets about it. laplace cleverly evades the difficulty by not taking from the hand of the creator diffused gas, but a sun with an atmosphere filling space to the orbit of neptune, and _already in revolution_. he says: "it is four millions to one that all motions of the planets, rotations and revolutions, were at once imparted by an original common cause, of which we know neither the nature nor the epoch." helmholtz says of rotation, "the existence of which must be assumed." professor newcomb says that the planets would not be arranged as now, each one twice as far from the sun as the next interior one, and the outer ones made first, but that all would be made into planets at once, and the small inner ones quite likely to cool off more rapidly. it is a very serious difficulty that at least one satellite does not revolve in the right direction. how neptune or uranus could throw their moons backward from its equator is not easily accounted for. it is at least one parthian arrow at the system, not necessarily fatal, but certainly dangerous. a greater difficulty is presented by the recently discovered satellites of mars. the inner one goes round the planet in one-third part of the time of the latter's revolution. how mars could impart three times the speed to a body flying off its surface that it has itself, has caused several defenders of the hypothesis to rush forward with explanations, but none with anything more than mere imaginary collisions with some comet. it is to be noticed that accounting for three times the speed is not enough; for as mars shrunk away from the [page ] ring that formed that satellite, it ought itself to attain more speed, as the sun revolves faster than its planets, and the earth faster than its moon. in defending the hypothesis, mitchel said: "suppose we had discovered that it required more time for saturn or jupiter to rotate on their axes than for their nearest moon to revolve round them in its orbit; this would have falsified the theory." it is also asserted that the newly discovered planet vulcan makes an orbital in less time than the sun makes an axial revolution. in regard to one martial moon, professor kirkwood, on whom proctor conferred the highest title that could be conferred, "the modern kepler," says: "unless some explanation can be given, the short period of the inner satellite will be doubtless regarded as a conclusive argument against the nebular hypothesis." if gravitation be sufficient to account for the various motions of the heavenly bodies, we have a perplexing problem in the star known as groombridge, now in the hunting dogs of bootes. it is thought to have a speed of two hundred miles per second--a velocity that all the known matter in the universe could not give to the star by all its combined attraction. neither could all that attraction stop the motion of the star, or bend it into an orbit. its motion must be accounted for on some hypothesis other than the nebular. the nebulæ which we are able to observe are not altogether confirmatory of the hypothesis under consideration. they have the most fantastic shapes, as if they had no relation to rotating suns in the formative stages. there are vast gaps in the middle, where they ought to be densest. mr. plumer, in the _natural science review_, [page ] says, in regard to the results of the spectroscopic revelations: "we are furnished with distinct proof that the gases so examined are not only of nearly equal density, but that they exist in a low state of _tension. this fact is fatal to the nebular theory._" in the autumn of a star blazed out in cygnus, which promised to throw a flood of light on the question of world-making. its spectrum was like some of the fixed stars. it probably blazed ont by condensation from some previously invisible nebula. but its brilliancy diminished swiftly, when it ought to have taken millions of years to cool. if the theory was true, it ought to have behaved very differently. it should have regularly condensed from gas to a solid sun by slow process. but, worst of all, after being a star awhile, it showed unmistakable proofs of turning into a cloud-mist--a star into a nebula, instead of _vice versa_. a possible explanation will be considered under variable stars. such are a few of the many difficulties in the way of accepting the nebular hypothesis, as at present explained, as being the true mode of development of the solar system. doubtless it has come from a hot and diffused condition into its present state; but when such men as proctor, newcomb, and kirkwood see difficulties that cannot be explained, contradictions that cannot be reconciled by the principles of this theory, surely lesser men are obliged to suspend judgment, and render the scotch verdict of "not proven." whatever truth there may be in the theory will survive, and be incorporated into the final solution of the problem; which solution will be a much grander generalization of the human mind than the nebular hypothesis. [page ] of some things we feel very sure: that matter was once without form and void, and darkness rested on the face of the mighty deeps; that, instead of chaos, we have now cosmos and beauty; and that there is some process by which matter has been brought from one state to the other. whether, however, the nebular hypothesis lays down the road travelled to this transfiguration, we are not sure. some of it seems like solid rock, and some like shifting quicksand. doubtless there is a road from that chaos to this fair cosmos. the nebular hypothesis has surveyed, worked, and perfected many long reaches of this road, but the rivers are not bridged, the chasms not filled, nor the mountains tunnelled. when men attempt to roll the hypothesis of evolution along the road of the nebular hypothesis of worlds, and even beyond to the production of vegetable and animal life, mind and affection, the gaps in the road become evident, and disastrous. a soul that has reached an adoration for the supreme father cares not how he has made him. doubtless the way god chose was the best. it is as agreeable to have been thought of and provided for in the beginning, to have had a myriad ages of care, and to have come from the highest existent life at last, as to have been made at once, by a single act, out of dust. the one who is made is not to say to the maker, "why hast thou formed me in this or that manner?" we only wish the question answered in what manner we were really made. evolution, without constant superintendence and occasional new inspiration of power, finds some tremendous chasms in the road it travels. these must be spanned by the power of a present god or the airy imagination [page ] of man. dr. mccosh has happily enumerated some of these tremendous gaps over which mere force cannot go. given, then, matter with mechanical power only, what are the gaps between it and spirituality? " . chemical action cannot be produced by mechanical power. " . life, even in the lowest forms, cannot be produced from unorganized matter. " . protoplasm can be produced only by living matter. " . organized matter is made up of cells, and can be produced only by cells. whence the first cell? " . a living being can be produced only from a seed or germ. whence the first vegetable seed? " . an animal cannot be produced from a plant. whence the first animal? " . sensation cannot be produced in insentient matter. " . the genesis of a new species of plant or animal has never come under the cognizance of man, either in pre-human or post-human ages, either in pre-scientific or scientific times. darwin acknowledges this, and says that, should a new species suddenly arise, we have no means of knowing that it is such. " . consciousness--that is, a knowledge of self and its operations--cannot be produced out of mere matter or sensation. " . we have no knowledge of man being generated out of the lower animals. " . all human beings, even savages, are capable of forming certain high ideas, such as those of god and duty. the brute creatures cannot be made to entertain these by any training. [page ] "with such tremendous gaps in the process, the theory which would derive all things out of matter by development is seen to be a very precarious one. the truth, according to the best judgment to be formed in the present state of knowledge, would seem to be about this: the nebular hypothesis is correct in all the main facts on which it is based; but that neither the present forces of matter, nor any other forces conceivable to the mind of man, with which it can possibly be endowed, can account for all the facts already observed. there is a demand for a personal volition, for an exercise of intelligence, for the following of a divine plan that embraces a final perfection through various and changeful processes. the five great classes of facts that sustain the nebular hypothesis seem set before us to show the regular order of working. the several facts that will not, so far as at present known, accord with that plan, seem to be set before us to declare the presence of a divine will and power working his good pleasure according to the exigencies of time and place. [page ] x. the stellar system. "the heavens number out the glory of the strong god."--david. [page ] richter says that "an angel once took a man and stripped him of his flesh, and lifted him up into space to show him the glory of the universe. when the flesh was taken away the man ceased to be cowardly, and was ready to fly with the angel past galaxy after galaxy, and infinity after infinity, and so man and angel passed on, viewing the universe, until the sun was out of sight--until our solar system appeared as a speck of light against the black empyrean, and there was only darkness. and they looked onward, and in the infinities of light before, a speck of light appeared, and suddenly they were in the midst of rushing worlds. but they passed beyond that system, and beyond system after system, and infinity after infinity, until the human heart sank, and the man cried out: 'end is there none of the universe of god?' the angel strengthened the man by words of counsel and courage, and they flew on again until worlds left behind them were out of sight, and specks of light in advance were transformed, as they approached them, into rushing systems; they moved over architraves of eternities, over pillars of immensities, over architecture of galaxies, unspeakable in dimensions and duration, and the human heart sank again and called ont: 'end is there none of the universe of god?' and all the stars echoed the question with amazement: 'end is there none of the universe of god?' and this echo found no answer. they moved on again past immensities of immensities, and eternities of eternities, until in the dizziness of uncounted galaxies the human heart sank for the last time, and called out: 'end is there none of the universe of god?' and again all the stars repeated the question, and the angel answered: 'end is there none of the universe of god. lo, also, there is no beginning.'" [page ] x. _the open page of the heavens._ the greeks set their mythological deities in the skies, and read the revolving pictures as a starry poem. not that they were the first to set the blazonry of the stars as monuments of their thought; we read certain allusions to stars and asterisms as far back as the time of job. and the pleiades, arcturus, and orion are some of the names used by him who "calleth all the stars by their names, in the greatness of his power." homer and hesiod, b.c., allude to a few stars and groups. the arabians very early speak of the great bear; but the greeks completely nationalized the heavens. they colonized the earth widely, but the heavens completely; and nightly over them marched the grand procession of their apotheosized divinities. there hercules perpetually wrought his mighty labors for the good of man; there flashed and faded the changeful star algol, as an eye in the head of the snaky-haired medusa; over them flew pegasus, the winged horse of the poet, careering among the stars; there the ship argo, which had explored all strange seas of earth, nightly sailed in the infinite realms of heaven; there perseus perpetually killed the sea-monster by celestial aid, and perpetually won the chained andromeda for his bride. very evident was their recognition of divine help: equally evident was [page ] their assertion of human ability and dominion. they gathered the illimitable stars, and put uncountable suns into the shape of the great bear--the most colossal form of animal ferocity and strength--across whose broad forehead imagination grows weary in flying; but they did not fail to put behind him a representative of themselves, who forever drives him around a sky that never sets--a perpetual type that man's ambition and expectation correspond to that which has always been revealed as the divine. the heavens signify much higher power and wisdom to us; we retain the old pictures and groupings for the convenience of finding individual stars. it is enough for the astronomer that we speak of a star as situated right ascension ' ", declination ° '. but for most people, if not all, it is better to call it polaris. so we might speak of a lake in latitude ° ', longitude ° ', but it would be clearer to most persons to say chatauqua. for exact location of a star, right ascension and declination must be given; but for general indication its name or place in a constellation is sufficiently exact. the heaven is rather indeterminably laid out in irregular tracts, and the mythological names are preserved. the brightest stars are then indicated in order by the letters of the greek alphabet--alpha (a), beta (b), gamma (g), etc. after these are exhausted, the roman alphabet is used in the same manner, and then numbers are resorted to; so that the famous star cygni is the th star in brightness in that one constellation. an acquaintance with the names, peculiarities, and movements of the stars visible at different seasons of the year is an unceasing source of pleasure. it [page ] is not vision alone that is gratified, for one fine enough may hear the morning stars sing together, and understand the speech that day uttereth unto day, and the knowledge that night showeth unto night. one never can be alone if he is familiarly acquainted with the stars. he rises early in the summer morning, that he may see his winter friends; in winter, that he may gladden himself with a sight of the summer stars. he hails their successive rising as he does the coming of his personal friends from beyond the sea. on the wide ocean he is commercing with the skies, his rapt soul sitting in his eyes. under the clear skies of the east he hears god's voice speaking to him, as to abraham, and saying, "look now toward the heavens, and tell the number of the stars, if thou be able to number them." a general acquaintance with the stars will be first attempted; a more particular knowledge afterward. fig. (page ) is a map of the circumpolar region, which is in full view every clear night. it revolves daily round polaris, its central point. toward this star, the two end stars of the great dipper ever point, and are in consequence called "the pointers." the map may be held toward the northern sky in such a position as the stars may happen to be. the great bear, or dipper, will be seen at nine o'clock in the evening above the pole in april and may; west of the pole, the pointers downward, in july and august; close to the north horizon in october and november; and east of the pole the pointers highest, in january and february. the names of such constantly visible stars should be familiar. in order, from the end of the tail of the great bear, we have benetnasch ae, mizar z, little alcor close to it, [page ] alioth, e megrez, d at the junction, has been growing dimmer for a century, phad, g dubhe and merak. it is best to get some facility at estimating distances in degrees. dubhe and merak, "the pointers," are five degrees apart. eighteen degrees forward of dubhe is the bear's nose; and three pairs of stars, fifteen degrees apart, show the position of the bear's three feet. follow "the pointers" twenty-nine degrees from dubhe, and we come to the pole-star. this star is double, made of two suns, both appearing as one to the naked eye. it is a test of an excellent three-inch telescope to resolve it into two. three stars beside it make the curved-up handle of the little dipper of ursa minor. between the two bears, thirteen degrees from megrez, and eleven degrees from mizar, are two stars in the tail of the dragon, which curves about to appropriate all the stars not otherwise assigned. follow a curve of fifteen stars, doubling back to a quadrangle from five to three degrees on a side, and thirty-five degrees from the pole, for his head. his tongue runs out to a star four degrees in front. we shall find, hereafter, that the foot of hercules stands on this head. this is the dragon slain by cadmus, and whose teeth produced such a crop of sanguinary men. the star thuban was once the pole-star. in the year b.c. it was ten times nearer the pole than polaris is now. in the year a.d. the pole will be within ' of polaris; in a.d. , it will be at a of cepheus; in a.d. , , within ° of vega; in a.d. , , at the star in the tongue of draco; in a.d. , , at thuban; in a.d. , , back to polaris. this indicates no change in the position of the dome [page ] of stars, but a change in the direction of the axis of the earth pointing to these various places as the cycles pass. as the earth goes round its orbit, the axis, maintaining nearly the same direction, really points to every part of a circle near the north star as large as the earth's orbit, that is, , , miles in diameter. but, as already shown, that circle is too small to be discernible at our distance. the wide circle of the pole through the ages is really made up of the interlaced curves of the annual curves continued through , years. the stem of the spinning top wavers, describes a circle, and finally falls; the axis of the spinning earth wavers, describes a circle of nearly , years, and never falls. the star g draconis, also called etanin, is famous in modern astronomy, because observations on this star led to the discovery of the _aberration of light_. if we held a glass tube perpendicularly out of the window of a car at rest, when the rain was falling straight down, we could see the drops pass directly through. put the car in motion, and the drops would seem to start toward us, and the top of the tube must be bent forward, or the drops entering would strike on the backside of the tube carried toward them. so our telescopes are bent forward on the moving earth, to enable the entered light to reach the eye-piece. hence the star does not appear just where it is. as the earth moves faster in some parts of its orbit than others, this aberration is sometimes greater than at others. it is fortunate that light moves with a uniform velocity, or this difficult, problem would be still further complicated. the displacement of a star from this course is about ". . [page ] on the side of polaris, opposite to ursa major, is king cepheus, made of a few dim stars in the form of the letter k. near by is his brilliant wife cassiopeia, sitting on her throne of state. they were the graceless parents who chained their daughter to a rock for the sea-monster to devour; but perseus, swift with the winged sandals of mercury, terrible with his avenging sword, and invincible with the severed head of medusa, whose horrid aspect of snaky hair and scaly body turned to stone every beholder, rescues the maiden from chains, and leads her away by the bands of love. nothing could be more poetical than the life of perseus. when he went to destroy the dreadful gorgon, medusa, pluto lent him his helmet, which would make him invisible at will; minerva loaned her buckler, impenetrable, and polished like a mirror; mercury gave him a dagger of diamonds, and his winged sandals, which would carry him through the air. coming to the loathsome thing, he would not look upon her, lest he, too, be turned to stone; but, guided by the reflection in the buckler, smote off her head, carried it high over libya, the dropping blood turning to serpents, which have infested those deserts ever since. [illustration: fig. .--circumpolar constellations. always visible. in this position.--january th, at o'clock; february th, at o'clock; and february th, at o'clock.] the human mind has always been ready to deify and throne in the skies the heroes that labor for others. both perseus and hercules are divine by one parent, and human by the other. they go up and down the earth, giving deliverance to captives, and breaking every yoke. they also seek to purge away all evil; they slay dragons, gorgons, devouring monsters, cleanse the foul places of earth, and one of them so wrestles with death as to win a victim from his grasp. finally, by [page ] an ascension in light, they go up to be in light forever. they are not ideally perfect. they right wrong by slaying wrong-doers, rather than by being crucified themselves; they are just murderers; but that only plucks the fruit from the tree of evil. they never attempted to infuse a holy life. they punished rather than regenerated. it must be confessed, also, that they were not sinless. but they were the best saviors the race could imagine, and are examples of that perpetual effort of the human mind to incarnate a divine helper who shall labor and die for the good of men. [page ] [illustration: fig. .--algol is on the meridian, ° south of pole.--at o'clock, december th; o'clock, december d; o'clock, january th.] _equatorial constellations._ if we turn our backs on polaris on the th of november, at o'clock in the evening, and look directly overhead, we shall see the beautiful constellation of andromeda. together with the square of pegasus, it makes another enormous dipper. the star a alpheratz is in her face, the three at the left cross her breast. b and the two above mark the girdle of her loins, and g is in the foot. perseus is near enough for help; and cetus, the sea-monster, is far enough away to do no harm. below, and east of andromeda, is the ram of the golden fleece, recognizable by the three stars in an acute triangle. the brightest is called arietis, or hamel. east of this are the pleiades, and the v-shaped hyades in taurus, or the bull. the pleiades rise about o'clock on the evening of the th of september, and at o'clock a.m. on june th. [page ] [illustration: fig. .--capella ( ° from the pole) and rigel ( °) are on the meridian at o'clock february th, o'clock january d, and at o'clock january th.] fig. extends east and south of our last map. it is the most gorgeous section of our heavens. (see the notes to the frontispiece.) note the triangle, ° on a side, made by betelguese, sirius, and procyon. a line from procyon to pollux leads quite near to polaris. orion is the mighty hunter. under his feet is a hare, behind him are two dogs, and before him is the rushing bull. the curve of stars to the right of bellatrix, g, represents his shield of the nemean lion's hide. the three stars of his belt make a measure ° long; the upper one, mintaker, is less than ' south of the equinoctial. the ecliptic passes between aldebaran and the pleiades. sirius rises about o'clock p.m. on the st of december, and about o'clock a.m. on the th of august. procyon rises about half an hour earlier. [page ] [illustration: fig. --regulus comes on the meridian, ° south from the pole, at o'clock march d, o'clock april th, and at o'clock april d.] fig. continues eastward. note the sickle in the head and neck of the lion. the star b is denebola, in his tail. arcturus appears by the word bootes, at the edge of the map. these two stars make a triangle with spica, about ° on a side. the geometric head of hydra is easily discernible east of procyon: the star g in the virgin is double, with a period of years. z is just above the equinoctial. there is a fine nebula two-thirds of the way from d to ae, and a little above the line connecting the two. coma berenices is a beautiful cluster of faint stars. spica rises at o'clock on the th of february, at o'clock a.m. on the th of november. [page ] [illustration: fig. l.--arcturus comes to the meridian, ° from the pole, at o'clock may th, o'clock june th, and at o'clock june th.] fig. represents the sky to the eastward and northward of the last. a line drawn from polaris and benetnasch comes east of arcturus to the little triangle called his sons. bootes drives the great bear round the pole. arcturus and denebola make a triangle with a, also called cor coroli, in the hunting dogs. this triangle, and the one having the same base, with spica for its apex, is called the "diamond of the virgin." hercules appears head down--a in the face, b, g, d; in his shoulders, p; and ae; in the loins, t in the knee, the foot being bent to the stars at the right. the serpent's head, making an x, is just at the right of the g of hercules, and the partial circle of the northern crown above. the head of draco is seen at b on the left of the map. arcturus rises at o'clock about the th of february, and at a.m. on the d of october; regulus h. m. earlier. [page ] [illustration: fig. .--altair comes to the meridian, ° from the pole, at o'clock p.m. august th, at o'clock september d, and at o'clock september th.] fig. portrays the stars eastward and southward. scorpio is one of the most brilliant and easily traced constellations. antares, a, in the heart, is double. in sagittarius is the little milk-dipper, and west of it the bended bow. vega is at the top of the map. near it observe z, a double, and e, a quadruple star. the point to which the solar system is tending is marked by the sign of the earth below p; herculis. the serpent, west of hercules, and coiled round nearly to aquila, is very traceable. in the right-hand lower corner is the centaur. below, and always out of our sight, is the famous a centauri. the diamond form of the dolphin is sometimes called "job's coffin." the ecliptic passes close [page ] to b of scorpio, which star is in the head. antares, in scorpio, rises at o'clock p.m. on may th, and at o'clock a.m. on january th. [illustration: fig. .--fomalhaut comes to the meridian, only ° from the horizon, at o'clock november th.] in fig. we recognize the familiar stars of pegasus, which tell us we have gone quite round the heavens. note the beautiful cross in the swan. b in the bill is named albireo, and is a beautiful double to almost any glass. its yellow and blue colors are very distinct. the place of the famous double star cygni is seen. the first magnitude star in the lower left-hand corner is fomalhaut, in the southern fish. a pegasi is in the diagonal corner from alpharetz, in andromeda. the star below altair is b aquilæ, and is called alschain; the one above is g aquilæ, named tarazed. this is not a brilliant section of the sky. altair rises at o'clock on the th of may, and at o'clock a.m. on the th of january. [page ] [illustration: fig. .--southern circumpolar constellations invisible north of the equator.] fig. gives the stars that are never seen by persons north of the earth's equator. in the ship is brilliant canopus, and the remarkable variable ae. below it is the beautiful southern cross, near the pole of the southern heavens. just below are the two first magnitude stars bungala, a, and achernar, b, of the centaur. such a number of unusually brilliant stars give the southern sky an unequalled splendor. in the midst of them, as if for contrast, is the dark hole, called by the sailors the "coal-sack," where even the telescope reveals no sign of light. here, also, are the two magellanic clouds, both easily discernible by the naked eye; the larger two hundred times the apparent size of the moon, lying between the pole and canopus, and the other between achernar and the pole. the smaller cloud is only one-fourth the size of the other. both are mostly resolvable into groups of stars from the fifth to the fifteenth magnitude. [page ] for easy out-door finding of the stars above the horizon at any time, see star-maps at end of the book. _characteristics of the stars._ such a superficial examination of stars as we have made scarcely touches the subject. it is as the study of the baptismal register, where the names were anciently recorded, without any knowledge of individuals. the heavens signify much more to us than to the greeks. we revolve under a dome that investigation has infinitely enlarged from their estimate. their little lights were turned by clumsy machinery, held together by material connections. our vast worlds are connected by a force so fine that it seems to pass out of the realm of the material into that of the spiritual. animal ferocity or a human hercules could image their idea of power. ours finds no symbol, but rises to the almighty. their heavens were full of fighting orions, wild bulls, chained andromedas, and devouring monsters. our heavens are significant of harmony and unity; all worlds carried by one force, and all harmonized into perfect music. all their voices blend their various significations into a personal speaking, which says, "hast thou not heard that the everlasting god, the lord, the creator of the ends of the earth, fainteth not, neither is weary?" there is no searching of his understanding. lift up your eyes on high, and behold who hath created all these things, that brought out their host by number, that calleth them all by their names in the greatness of his power; for that he is strong in power not one faileth. [page ] _number._ we find about five thousand stars visible to the naked eye in the whole heavens, both north and south. of these twenty are of the first magnitude, sixty-five of the second, two hundred of the third, four hundred of the fourth, eleven hundred of the fifth, and three thousand two hundred of the sixth. we think we can easily number the stars; but train a six-inch telescope on a little section of the twins, where six faint stars are visible, and over three thousand luminous points appear. the seventh magnitude has , stars; the eighth, , ; the ninth, , . there are , , stars in the zone called the milky way. when our eyes are not sensitive enough to be affected by the light of far-off stars the tastimetre feels their heat, and tells us the word of their maker is true--"they are innumerable."[*] [footnote *: _telescopic work._--look at the hyades and pleiades in taurus. notice the different colors of stars in them both. find the cluster præsepe in fig. , just a trifle above a point midway between procyon and regulus. it is equally distant from procyon and a point a little below pollux. sweep along the milky way almost anywhere, and observe the distribution of stars; in some places perfect crowds, in others more sparsely scattered. find with the naked eye the rich cluster in perseus. draw a line from algol to a of perseus (fig. ); turn at right angles to the right, at a distance of once and four-tenths the first line a brightness will be seen. the telescope reveals a gorgeous cluster.] _double and multiple stars._ if we look up during the summer months nearly overhead at the star e lyra, east of vega (fig. ), we shall see with the naked eye that the star appears a little [page ] elongated. turn your opera-glass upon it, and two stars appear. turn a larger telescope on this double star, and each of the components separate into two. it is a double double star. we know that if two stars are near in reality, and not simply apparently so by being in the same line of sight, they must revolve around a common centre of gravity, or rush to a common ruin. eagerly we watch to see if they revolve. a few years suffice to show them in actual revolution. nay, the movement of revolution has been decided before the companion star was discovered. sirius has long been known to have a proper motion, such as it would have if another sun were revolving about it. even the direction of the unseen body could always be indicated. in february, , alvan clark, artist, poet, and maker of telescopes (which requires even greater genius than to be both poet and artist), discovered the companion of sirius just in its predicted place. as a matter of fact, one of mr. clark's sons saw it first; but their fame is one. the time of revolution of this pair is fifty years. but one companion does not meet the conditions of the movements. here must also be one or more planets too small or dark to be seen. the double star x in the great bear (see fig. ) makes a revolution in fifty-eight years. procyon moves in an orbit which requires the presence of a companion star, but it has as yet eluded our search. castor is a double star; but a third star or planet, as yet undiscovered, is required to account for its perturbations. men who discovered neptune by the perturbations of uranus are capable of judging the cause of the perturbations of suns. we have spoken of [page ] the whole orbit of the earth being invisible from the stars. the nearest star in our northern hemisphere, cygni, is a telescopic double star; the constituent parts of it are forty-five times as far from each other as the earth is from the sun, yet it takes a large telescope to show any distance between the stars.[*] [footnote *: _telescopic work._--only such work will be laid out here as can be done by small telescopes of from two to four inch object-glasses. the numbers in fig. correspond to those of the table. -------------------------------------------------------------------- | | | |dist. of|magni-| | |no.| name. | fig. | parts. |tudes.| remarks. | |---|------------|-------------|--------|------|---------------------| | .| e lyræ | | ' " | |quadruple. | | .| z lyræ | | | & |topaz and green. | | .| b cygni | | - / | & |yellow and blue. | | .| cygni | | | & |nearest star but one.| | .| mizar | | | & |both white. | | .| polaris | | - / | & |test object of eye | | | | | | | and glass. | | .| r orionis |frontispiece.| | & |yellow and blue. | | .| b orionis | " | | & | rigel. | | .| d " | " | | & | red and white. | | .| th " | " | | |septuple. | | .| l " | " | | |white and violet. | | .| s " | " a, b.| | & |octuple. | | .| castor | | - / | & |white. | | .| pollux | | |triple|orange, gray, lilac. | | .| g virginis | | | & |both yellow. | -------------------------------------------------------------------- ] when g virginis was observed in by bradley, the component parts were " asunder. he incidentally remarked in his note-book that the line of their connection was parallel to the line of the two stars spica, or a and d virginis. by they were not more than " apart, and the line of their connection greatly changed. the appearance of the star is given in fig. ( ), commencing at the left, for the years ' ' ' ' ' ' and ' . also a conjectural [page ] orbit, placed obliquely, and the position of the stars at the times mentioned, commencing at the top. the time of its complete revolution is one hundred and fifty years. [illustration: fig. .--aspects and revolution of double stars.] the meaning of these double stars is that two or more suns revolve about their centre of gravity, as the moon and earth about their centre. if they have planets, as doubtless they have, the movement is no more complicated than the planets we call satellites of saturn revolving about their central body, and also about the sun. kindle saturn and jupiter to a blaze, or let out their possible light, and our system would appear a triple star in the distance. doubtless, in the far past, before these giant planets were cooled, it so appeared. we find some stars double, others triple, quadruple, octuple, and multiple. it is an extension of the same principles that govern our system. some of these suns are so far asunder that they can swing their neptunes between them, with less perturbation than uranus and neptune have in ours. light all our planets, and there would be a multiple star with more or less suns seen, [page ] according to the power of the instrument. perhaps the octuple star s in orion differs in no respect from our system, except in the size and distance of its separate bodies, and less cooling, either from being younger, or from the larger bodies cooling more slowly. suns are of all ages. infinite variety fills the sky. it is as preposterous to expect that every system or world should have analogous circumstances to ours at the present time, as to insist that every member of a family should be of the same age, and in the same state of development. there are worlds that have not yet reached the conditions of habitability by men, and worlds that have passed these conditions long since. let them go. there are enough left, and an infinite number in the course of preparation. some are fine and lasting enough to be eternal mansions. _colored stars._ in the cloudy morning we get only red light, but the sun is white. so aldebaran and betelguese may be girt by vapors, that only the strong red rays can pass. again, an iron moderately heated gives out dull red light; becoming hotter, it emits white light. sirius, regulus, vega, and spica may be white from greater intensity of vibration. procyon, capella, and polaris are yellow from less intensity of vibration. again, burn salt in a white flame, and it turns to yellow; mix alcohol and boracic acid, ignite them, and a beautiful green flame results; alcohol and nitrate of strontia give red flame; alcohol and nitrate of barytes give yellow flame. so the composition of a sun, or the special development of anyone substance thereof at any time, may determine the color of a star. [page ] the special glory of color in the stars is seen in the marked contrasts presented in the double and multiple stars. the larger star is usually white, still in the intensity of heat and vibration; the others, smaller, are somewhat cooled off, and hence present colors lower down the scale of vibration, as green, yellow, orange, and even red. that stars should change color is most natural. many causes would produce this effect. the ancients said sirius was red. it is now white. the change that would most naturally follow mere age and cooling would be from white, through various colors, to red. we are charmed with the variegated flowers of our gardens of earth, but he who makes the fields blush with flowers under the warm kisses of the sun has planted his wider gardens of space with colored stars. "the rainbow flowers of the footstool, and the starry flowers of the throne," proclaim one being as the author of them all. _clusters of stars._ from double and multiple we naturally come to groups and clusters. allusion has been made to the hyades, pleiades, etc. everyone has noticed the milky way. it seems like two irregular streams of compacted stars. it is not supposed that they are necessarily nearer together than the stars in the sparse regions about the pole. but the , , suns belonging to our system are arranged within a space represented by a flattened disk. if one hundred lights, three inches apart, are arranged on a hoop ten feet in diameter, they would be in a circle. add a thousand or two more the same distance apart, filling up the centre, and [page ] extending a few inches on each side of the inner plane of the hoop: an eye in the centre, looking out toward the edge, would see a milky way of lights; looking out toward the sides or poles, would see comparatively few. it would seem as if this oblate spheroidal arrangement was the result of a revolution of all the suns composing the system. jupiter and earth are flattened at the poles for the same reason. [illustration: fig. .--sprayed cluster below ae in hercules.] [illustration: fig. .--globular cluster.] in various parts of the heavens there are small globular well-defined clusters, and clusters very irregular in form, marked with sprays of stars. there is a cluster of this latter class in hercules, just under the s, in fig. . "probably no one ever saw it with a good telescope without a shout of wonder." here is a cluster of the former class represented in fig. . "the noble globular cluster, o centauri is beyond all comparison the richest and largest object of the kind in the heavens. its stars are literally innumerable; and as their total light, when received by the naked eye, affects it hardly more than a star of the fifth to fourth [page ] magnitude, the minuteness of each star may be imagined." there are two possibilities of thought concerning these clusters. either that they belong to our stellar system, and hence the stars must be small and young, or they are another universe of millions of suns, so far way that the inconceivable distances between the stars are shrunken to a hand's-breadth, and their unbearable splendor of innumerable suns can only make a gray haze at the distance at which we behold them. the latter is the older and grander thought; the former the newer and better substantiated. _nebulæ._ the gorgeous clusters we have been considering appear to the eye or the small telescope as little cloudlets of hazy light. one after another were resolved into stars; and the natural conclusion was, that all would yield and reveal themselves to be clustered suns, when we had telescopes of sufficient power. but the spectroscope, seeing not merely form but substance also, shows that some of them are not stars in any sense, but masses of glowing gas. two of these nebulæ are visible to the naked eye: one in andromeda (see fig. ), and one around the middle star of the sword of orion, shown in fig. . a three-inch telescope resolves th orionis into the famous trapezium, and a nine-inch instrument sees two stars more. the shape of the nebula is changeable, and is hardly suggestive of the moulding influence of gravitation. it is probably composed of glowing nitrogen and hydrogen gases. nebulæ are of all conceivable shapes--circular, annular, oval, lenticular, [page ] conical, spiral, snake-like, looped, and nameless. compare the sprays of the crab nebulæ above z tauri, seen in fig. , and the ring nebula, fig. . this last possibly consists of stars, and is situated, as shown in fig. , midway between b and g lyræ. [illustration: fig. .--the great nebula about the multiple star th orionis. (see frontispiece.)] when herschel was sweeping the heavens with his telescope, and saw but few stars, he often said to his assistant, "prepare to write; the nebulæ are coming." they are most abundant where the stars are least so. a zone about the heavens ° wide, with the milky way in the centre, would include one-fourth of the celestial sphere; but instead of one-fourth, we find nine-tenths [page ] of the stars in this zone, and but one-tenth of the nebulæ. these immense masses of unorganized matter are noticed to change their forms, vary their light greatly, but not quickly; they change through the ages. "god works slowly." he takes a thousand years to lift his hand off. [illustration: fig. .--crab nebula, near z tauri. (see frontispiece.)] there are many unsolved problems connected with these strange bodies. whether they belong to our system, or are beyond it, is not settled; the weight of evidence leans to the first view. [page ] _variable stars._ [illustration: fig. .--the ring nebula.] our sun gives a variable amount of light, changing through a period of eleven years. probably every star, if examined by methods sufficiently delicate and exact, would be found to be variable. the variations of some [page ] stars are so marked as to challenge investigation. b lyræ (fig. ) has two maxima and minima of light. in three days it rises from magnitude - / to - / ; in a week falls to , and rises to - / ; and in three days more drops to - / : it makes all these changes in thirteen days; but this period is constantly increasing. the variations of one hundred and forty-three stars have been well ascertained. [illustration: fig. .--constellation lyra, showing place of the ring nebula.] mira, or the wonderful, in the whale (fig. ), is easily found when visible. align from capella to the pleiades, and as much farther, and four stars will be seen, situated thus: * * * * the right-hand one is mira. for half a month it shines as a star of the second magnitude. then for three months it fades away, and lost to sight; going down even to the eleventh magnitude. but after five months its resurrection morning mes; and in three months more--eleven months in all--our wonderful is in its full glory in the heavens. it its period and brilliancy are also variable. the star megrez, d in the great bear, has been growing dim [page ] for a century. in betelguese was exceedingly variable, and continued so till , when the changes became much less conspicuous. algol (fig. ) has been already referred to. this slowly winking eye is of the second magnitude during d. h. then it dozes off toward sleep for h. m., when it is nearly invisible. it wakes up during the same time; so that its period from maximum brilliancy to the same state again is d. h. m. its recognizable changes are within five or six hours. as i write, march th, , algol gives its minimum light at h. m. p.m. it passes fifteen minima in d. m. there will therefore be another minimum may th, at h. m. its future periods are easy to estimate. perhaps it has some dark body revolving about it at frightful speed, in a period of less than three days. the period of its variability is growing shorter at an increasing rate. if its variability is caused by a dark body revolving about it, the orbit of that body is contracting, and the huge satellite will soon, as celestial periods are reckoned, commence to graze the surface of the sun itself, rebound again and again, and at length plunge itself into the central fire. such an event would evolve heat enough to make algol flame up into a star of the first magnitude, and perhaps out-blaze sirius or capella in our winter sky. none of the causes for these changes we have been able to conjecture seem very satisfactory. the stars may have opaque planets revolving about them, shutting off their light; they may rotate, and have unequally illuminated sides; they may revolve in very elliptical orbits, so as to greatly alter their distance from us; they may be so situated in regard to zones of meteorites as [page ] to call down periodically vast showers; but none or all of these suppositions apply to all cases, if they do to any. _temporary, new, and lost stars._ besides regular movements to right and left, up and down, to and from us--changes in the intensity of illumination by changes of distance--besides variations occurring at regular and ascertainable intervals, there are stars called _temporary_, shining awhile and then disappearing; _new_, coming to a definite brightness, and so remaining; and _lost_, those whose first appearance was not observed, but which have utterly disappeared. in november, , a new star blazed out in cassiopeia. its place is shown in fig. , ch g being the stars d * g ch in the seat of the chair, and d being the first one in the back. this star was visible at noonday, and was brighter than any other star in the heavens. in january, , it was less bright than jupiter; in april it was below the second magnitude, and the last of may it utterly disappeared. it was as variable in color as in brilliancy. during its first two months, the period of greatest brightness, it was dazzling white, then became yellow, and finally as red as mars or aldebaran, and so expired. a bright star was seen very near to the place of the _pilgrim_, as the star of was called, in a.d. and . a star of the tenth magnitude is now seen brightening slowly almost exactly in the same place. it is possible that this is a variable star of a period of about three hundred and ten years, and will blaze out again about . but we have had, within a few years, fine opportunities [page ] to study, with improved instruments, two new stars; on the evening of may th, , a star of the second magnitude was observed in the northern crown, where no star above the fifth magnitude had been twenty-four hours before. in argelander's chart a star of the tenth magnitude occupies the place. may th it had declined to the third magnitude, may th to the fourth, may th to the fifth, may th to the seventh, may st to the ninth, and has since diminished to the tenth. the spectroscope showed it to be a star in the usual condition; but through the usual colored spectrum, crossed with bright lines, shone four bright lines, two of which indicated glowing hydrogen. here was plenty of proof that an unusual amount of this gas had given this sun its sudden flame. as the hydrogen burned out the star grew dim. two theories immediately presented themselves: first, that vast volumes had been liberated from within the orb by some sudden breaking up of the doors of its great deeps; or, second, this star had precipitated upon itself, by attraction, some other sun or planet, the force of whose impact had been changed into heat. though we see the liberated hydrogen of our sun burst up with sudden flame, it can hardly be supposed that enough could be liberated at once to increase the light and heat one hundred-fold. in regard to the second theory, it is capable of proof that two suns half as large as ours, moving at a velocity of four hundred and seventy-six miles per second, would evolve heat enough to supply the radiation of our sun for fifty million years. how could it be possible for a sun like this newly blazing orb to cool off to such a [page ] degree in a month? besides, there would not be one chance in a thousand for two orbs to come directly together. they would revolve about each other till a kind of grazing contact of grinding worlds would slowly kindle the ultimate heat. it is far more likely that this star encountered an enormous stream of meteoric bodies, or perhaps absorbed a whole comet, that laid its million leagues of tail as fuel on the central fire. only let it be remembered that the fuel is far more force than substance. allusion has already been made to the sudden brightening of our sun on the first day of september, . that was caused, no doubt, by the fall of large meteors, following in the train of the comet of , or some other comet. what the effect would have been, had the whole mass of the comet been absorbed, cannot be imagined. another new star lately appeared in cygnus, near the famous star --the first star in the northern hemisphere whose distance was determined. it was first seen november th, , as a third magnitude star of a yellow color. by december d it had sunk to the fourth magnitude, and changed to a greenish color. it had then three bright hydrogen lines, the strong double sodium line, and others, which made, it strongly resemble the spectrum of the chromosphere of our sun. an entirely different result appeared in the fading of these two stars. in the case of the star in the crown, the extraordinary light was the first to fade, leaving the usual stellar spectrum. in the case of the star in cygnus, the part of the spectrum belonging to stellar light was the first to fade, leaving the bright lines; that is, the gas of one gave way to regular starlight, and the starlight [page ] of the other having faded, the regular light of the glowing gas continued. by some strange oversight, no one studied the star again for six months. in september and november, , the light of this star was found to be blue, and not to be starlight at all. it had no rainbow spectrum, only one kind of rays, and hence only one color. its sole spectroscopic line is believed to be that of glowing nitrogen gas. we have then, probably, in the star of , a body shining by a feeble and undiscernible light, surrounded by a discernible immensity of light of nitrogen gas. this is its usual condition; but if a flight of meteors should raise the heat of the central body so as to outshine the nebulous envelope, we should have the conditions we discovered in november, . but a rapid cooling dissipates the observable light of all colors, and leaves only the glowing gas of one color. _movements of stars._ we call the stars _fixed_, but motion and life are necessary to all things. besides the motion in the line of sight described already, there is motion in every other conceivable direction. we knew sirius moved before we had found the cause. we know that our sun moves back and forth in his easy bed one-half his vast diameter, as the larger planets combine their influence on one side or the other. the sun has another movement. we find the stars in hercules gradually spreading from each other. hercules's brawny limbs grow brawnier every century. there can be but one cause: we are approaching that quarter of the heavens. (see [symbol], fig. .) we are even [page ] able to compute the velocity of our approach; it is four miles a second. the stars in the opposite quarter of the heavens in argo are drawing nearer together. this movement would have no effect on the apparent place of the stars at either pole, if they were all equally distant; but it must greatly extend or contract the apparent space between them, since they are situated at various distances. independent of this, the stars themselves are all in motion, but so vast is the distance from which we observe them that it has taken an accumulation of centuries before they could be made measurable. a train going forty miles an hour, seen from a distance of two miles, almost seems to stand still. arcturus moves through space three times as fast as the earth, but it takes a century to appear to move the eighth part of the diameter of the moon. there is a star in the hunting dogs, known as groombridge, which has a velocity beyond what all the attraction of the matter of the known universe could give it. by the year it may be in berenice's hair. some stars have a common movement, being evidently related together. a large proportion of the brighter stars between aldebaran and the pleiades have a common motion eastward of about ten seconds a century. all the angles marked by a, b, g, ch orionis will be altered in different directions; l is moving toward g. l and e will appear as a double star. in a.d. , procyon will be nearer ch orionis than rigel now is, and sirius will be in line with a and ch orionis. all the stars of the great dipper, except benetnasch and dubhe, have a common motion somewhat in the direction [page ] of thuban (fig. ), while the two named have a motion nearly opposite. in , years the end of the dipper will have fallen out so that it will hold no water, and the handle will be broken square off at mizar. "the southern cross," says humboldt, "will not always keep its characteristic form, for its four stars travel in different directions with unequal velocities. at the present time it is not known how many myriads of years must elapse before its entire dislocation." these movements are not in fortuitous or chaotic ways, but are doubtless in accordance with some perfect plan. we have climbed up from revolving earth and moon to revolving planets and sun, in order to understand how two or ten suns can revolve about a common centre. let us now leap to the grander idea that all the innumerable stars of a winter night not only loan, but must revolve about some centre of gravity. men have been looking for a central sun of suns, and have not found it. none is needed. two suns can balance about a point; all suns can swing about a common centre. that one unmoving centre may be that city more gorgeous than eastern imagination ever conceived, whose pavement is transparent gold, whose walls are precious stones, whose light is life, and where no dark planetary bodies ever cast shadows. there reigns the king and lord of all, and ranged about are the far-off provinces of his material systems. they all move in his sight, and receive power from a mind that never wearies. [page ] xi. the worlds and the word. "the worlds were framed by the word of god."--_heb._ xi., . [page ] "mysterious night! when our first parent knew thee from report divine, and heard thy name, did he not tremble for this lovely frame, this glorious canopy of light and blue? yet, 'neath a curtain of translucent dew, bathed in the rays of the great setting flame, hesperus, with all the host of heaven, came, and lo! creation widened in man's view. who could have thought such darkness lay concealed within thy beams, o sun! oh who could find, whilst fruit and leaf and insect stood revealed, that to such countless worlds thou mad'st us blind! why do we then shun death with anxious strife? if light conceal so much, wherefore not life?" blanco white. [page ] xi. _the worlds and the word._ men have found the various worlds to be far richer than they originally thought. they have opened door after door in their vast treasuries, have ascended throne after throne of power, and ruled realms of increasing extent. we have no doubt that unfoldings in the future will amaze even those whose expectations have been quickened by the revealings of the past. what if it be found that the word is equally inexhaustible? after ages of thought and discovery we have come out of the darkness and misconceptions of men. we believe in no serpent, turtle, or elephant supporting the world; no atlas holding up the heavens; no crystal domes, "with cycles and epicycles scribbled o'er." what if it be found that one book, written by ignorant men, never fell into these mistakes of the wisest! nay, more, what if some of the greatest triumphs of modern science are to be found plainly stated in a book older than the writings of homer? if suns, planets, and satellites, with all their possibilities of life, changes of flora and fauna, could be all provided for, as some scientists tell us, in the fiery star-dust of a cloud, why may not the same author provide a perpetually widening river of life in his word? as we believe he is perpetually present in his worlds, we know he has [page ] promised to be perpetually present in his word, making it alive with spirit and life. the wise men of the past could not avoid alluding to ideas the falsity of which subsequent discovery has revealed; but the writers of the bible did avoid such erroneous allusion. of course they referred to some things, as sunrise and sunset, according to appearance; but our most scientific books do the same to-day. that the bible could avoid teaching the opposite of scientific truth proclaims that a higher than human wisdom was in its teaching. that negative argument is strong, but the affirmative argument is much stronger. the bible declares scientific truth far in advance of its discovery, far in advance of man's ability to understand its plain declarations. take a few conspicuous illustrations: the bible asserted from the first that the present order of things had a beginning. after ages of investigation, after researches in the realms of physics, arguments in metaphysics, and conclusions by the necessities of resistless logic, science has reached the same result. the bible asserted from the first that creation of matter preceded arrangement. it was chaos--void--without form--darkness; arrangement was a subsequent work. the world was not created in the form it was to have; it was to be moulded, shaped, stratified, coaled, mountained, valleyed, subsequently. all of which science utters ages afterward. the bible did not hesitate to affirm that light existed before the sun, though men did not believe it, and used it as a weapon against inspiration. now we praise men for having demonstrated the oldest record. [page ] it is a recently discovered truth of science that the trata of the earth were formed by the action of water, and the mountains were once under the ocean. it is an idea long familiar to bible readers: "thou coverest the earth with the deep as with a garment. the waters stood above the mountains. at thy rebuke they fled; at the voice of thy thunder they hasted away. the mountains ascend; the valleys descend into the place thou hast founded for them." here is a whole volume of geology in a paragraph. the thunder of continental convulsions is god's voice; the mountains rise by god's power; the waters haste away unto the place god prepared for them. our slowness of geological discovery is perfectly accounted for by peter. "for of this they are _willingly ignorant_, that by the word of god there were heavens of old, and land framed out of water, and by means of water, whereby the world that then was, being overflowed by water, perished." we recognize these geological subsidences, but we read them from the testimony of the rocks more willingly than from the testimony of the word. science exults in having discovered what it is pleased to call an order of development on earth--tender grass, herb, tree; moving creatures that have life in the waters; bird, reptile, beast, cattle, man. the bible gives the same order ages before, and calls it god's successive creations. during ages on ages man's wisdom held the earth to be flat. meanwhile, god was saying, century after century, of himself, "he sitteth upon the sphere of the earth" (gesenius). men racked their feeble wits for expedients to uphold [page ] the earth, and the best they could devise were serpents, elephants, and turtles; beyond that no one had ever gone to see what supported them. meanwhile, god was perpetually telling men that he had hung the earth upon nothing. men were ever trying to number the stars. hipparchus counted one thousand and twenty-two; ptolemy one thousand and twenty-six; and it is easy to number those visible to the naked eye. but the bible said, when there were no telescopes to make it known, that they were as the sands of the sea, "innumerable." science has appliances of enumeration unknown to other ages, but the space-penetrating telescopes and tastimeters reveal more worlds--eighteen millions in a single system, and systems beyond count--till men acknowledge that the stars are innumerable to man. it is god's prerogative "to number all the stars; he also calleth them all by their names." torricelli's discovery that the air had weight was received with incredulity. for ages the air had propelled ships, thrust itself against the bodies of men, and overturned their works. but no man ever dreamed that weight was necessary to give momentum. during all the centuries it had stood in the bible, waiting for man's comprehension: "he gave to the air its weight" (job xxviii. ). the pet science of to-day is meteorology. the fluctuations and variations of the weather have hitherto baffled all attempts at unravelling them. it has seemed that there was no law in their fickle changes. but at length perseverance and skill have triumphed, and a single man in one place predicts the weather and winds [page ] for a continent. but the bible has always insisted that the whole department was under law; nay, it laid down that law so clearly, that if men had been willing to learn from it they might have reached this wisdom ages ago. the whole moral law is not more clearly crystallized in "thou shalt love the lord thy god with all thy heart, and thy neighbor as thyself," than all the fundamentals of the science of meteorology are crystallized in these words: "the wind goeth toward the south (equator), and turneth about (up) unto the north; it whirleth about continually, and the wind returneth again according to his circuits (established routes). all the rivers run into the sea; yet the sea is not full: unto the place from whence the rivers come, thither they return again" (eccles. i. , ). those scientific queries which god propounded to job were unanswerable then; most of them are so now. "whereon are the sockets of the earth made to sink?" job never knew the earth turned in sockets; much less could he tell where they were fixed. god answered this question elsewhere. "he stretcheth the north (one socket) over the empty place, and hangeth the earth upon nothing." speaking of the day-spring, god says the earth is _turned_ to it, as clay to the seal. the earth's axial revolution is clearly recognized. copernicus declared it early; god earlier. no man yet understands the balancing of the clouds, nor the suspension of the frozen masses of hail, any more than job did. had god asked if he had perceived the _length_ of the earth, many a man to-day could have answered yes. but the eternal ice keeps us from perceiving the _breadth_ [page ] of the earth, and shows the discriminating wisdom of the question. the statement that the sun's going is from the end of the heaven, and his circuit to the ends of it, has given edge to many a sneer at its supposed assertion that the sun went round the earth. it teaches a higher truth--that the sun itself obeys the law it enforces on the planets, and flies in an orbit of its own, from one end of heaven in argo to the other in hercules. so eminent an astronomer and so true a christian as general mitchell, who understood the voices in which the heavens declare the glory of god, who read with delight the word of god em bodied in worlds, and who fed upon the written word of god as his daily bread, declared, "we find an aptness and propriety in all these astronomical illustrations, which are not weakened, but amazingly strengthened, when viewed in the clear light of our present knowledge." herschel says, "all human discoveries seem to be made only for the purpose of confirming more strongly the truths that come from on high, and are contained in the sacred writings." the common authorship of the worlds and the word becomes apparent; their common unexplorable wealth is a necessary conclusion. since the opening revelations of the past show an unsearchable wisdom in the word, has that word any prophecy concerning mysteries not yet understood, and events yet in the future? there are certain problems as yet insolvable. we have grasped many clews, and followed them far into labyrinths of darkness, but not yet through into light. we ask in vain, "what is matter?" no man can [page ] answer. we trace it up through the worlds, till its increasing fineness, its growing power, and possible identity of substance, seem as if the next step would reveal its spirit origin. what we but hesitatingly stammer, the word boldly asserts. we ask, "what is force?" no man can answer. we recognize its various grades, each subordinate to the higher--cohesion dissolvable by heat; the affinity of oxygen and hydrogen in water overcome by the piercing intensity of electric fire; rivers seeking the sea by gravitation carried back by the sun; rock turned to soil, soil to flowers; and all the forces in nature measurably subservient to mind. hence we partly understand what the word has always taught us, that all lower forces must be subject to that which is highest. how easily can seas be divided, iron made to swim, water to burn, and a dead body to live again, if the highest force exert itself over forces made to be mastered. when we have followed force to its highest place, we always find ourselves considering the forces of mind and spirit, and say, in the words of the scriptures, "god is spirit." we ask in vain what is the end of the present condition of things. we have read the history of our globe with great difficulty--its prophecy is still more difficult. we have asked whether the stars form a system, and if so, whether that system is permanent. we are not able to answer yet. we have said that the sun would in time become as icy cold and dead as the moon, and then the earth would wander darkling in the voids of space. but the end of the earth, as prophesied in the word, is different: "the heavens will pass away with [page ] a rushing noise, and the elements will be dissolved with burning heat, and the earth and the works therein will be burned up." the latest conclusions of science point the same way. the great zones of uncondensed matter about the sun seem to constitute a resisting medium as far as they reach. encke's comet, whose orbit comes near the sun, is delayed. this gives gravitation an overwhelming power, and hence the orbit is lessened and a revolution accomplished more quickly. faye's comet, which wheels beyond the track of mars, is not retarded. if the earth moves through a resisting substance, its ultimate fall into the sun is certain. whether in that far future the sun shall have cooled off, or will be still as hot as to-day, peter's description would admirably portray the result of the impact. peters description, however, seems rather to indicate an interference of divine power at an appropriate time before a running down of the system at present in existence, and a re-endowment of matter with new capabilities. after thousands of years, science discovered the true way to knowledge. it is the baconian way of experiment, of trial, of examining the actual, instead of imagining the ideal. it is the acceptance of the scriptural plan. "if a man wills to do god's will, he shall know." oh taste and see! in science men try hypotheses, think the best they can, plan broadly as possible, and then see if facts sustain the theory. they have adopted the scriptural idea of accepting a plan, and then working in faith, in order to acquire knowledge. fortunately, in the work of salvation the plan is always perfect. but, in order to make the trial under the most favorable circumstances, there must be faith. the faith of [page ] science is amazing; its assertions of the supersensual are astounding. it affirms a thousand things that cannot be physically demonstrated: that the flight of a rifle-ball is parabolic; that the earth has poles; that gages are made of particles; that there are atoms; that an electric light gives ten times as many rays as are visible; that there are sounds to which we are deaf, sights to which we are blind; that a thousand objects and activities are about us, for the perception of which we need a hundred senses instead of five. these faiths have nearly all led to sight; they have been rewarded, and the world's wealth of knowledge is the result. the word has ever asserted the supersensuous, solicited man's faith, and ever uplifted every true faith into sight. lowell is partly right when he sings: "science was faith once; faith were science now, would she but lay her bow and arrows by, and aim her with the weapons of the time." faith laid her bow and arrows by before men in pursuit of worldly knowledge discovered theirs. what becomes of the force of the sun that is being spent to-day? it is one of the firmest rocks of science that there can be no absolute destruction of force. it is all conserved somehow. but how? the sun contracts, light results, and leaps swiftly into all encircling space. it can never be returned. heat from stars invisible by the largest telescope enters the tastimeter, and declares that that force has journeyed from its source through incalculable years. there is no encircling dome to reflect all this force back upon its sources. is it lost? science, in defence of its own dogma, should [page ] assign light a work as it flies in the space which we have learned cannot be empty. there ought to be a realm where light's inconceivable energy is utilized in building a grander universe, where there is no night. christ said, as he went out of the seen into the unseen, "i go to prepare a place for you;" and when john saw it in vision the sun had disappeared, the moon was gone, but the light still continued. science finds matter to be capable of unknown refinement; water becomes steam full of amazing capabilities: we add more heat, superheat the steam, and it takes on new aptitudes and uncontrollable energy. zinc burned in acid becomes electricity, which enters iron as a kind of soul, to fill all that body with life. all matter is capable of transformation, if not transfiguration, till it shines by the light of an indwelling spirit. scripture readers know that bodies and even garments can be transfigured, be made astrapton (luke xxiv. ), shining with an inner light. they also look for new heavens and a new earth endowed with higher powers, fit for perfect beings. when god made matter, so far as our thought permits us to know, he simply made force stationary and unconscious. thereafter he moves through it with his own will. he can at any time change these forces, making air solid, water and rock gaseous, a world a cloud, or a fire-mist a stone. he may at some time restore all force to consciousness again, and make every part of the universe thrill with responsive joy. "then shall the mountains and the hills break forth before you into singing, and all the trees of the field clap their hands." one of these changes is to come to the earth. [page ] amidst great noise the heaven shall flee, the earth be burned up, and all their forces be changed to new forms. perhaps it will not then be visible to mortal eyes. perhaps force will then be made conscious, and the flowers thereafter return our love as much as lower creatures do now. a river and tree of life may be consciously alive, as well as give life. poets that are nearest to god are constantly hearing the sweet voices of responsive feeling in nature. "for his gayer hours she has a voice of gladness and a smile, and eloquence of beauty; and she glides into his darker musings with a mild and gentle sympathy, that steals away their sharpness ere he is aware." prophets who utter god's voice of truth say, "the wilderness and the solitary place shall be glad for holy men, and the desert shall rejoice and blossom as the rose. it shall blossom abundantly and rejoice, even with joy and singing." distinguish clearly between certainty and surmise. the certainty is that the world will pass through catastrophic changes to a perfect world. the grave of uniformitarianism is already covered with grass. he that creates promises to complete. the invisible, imponderable, inaudible ether is beyond our apprehension; it transmits impressions , miles a second; it is millions of times more capable and energetic than air. what may be the bounds of its possibility none can imagine, for law is not abrogated nor designs disregarded as we ascend into higher realms. law works out more beautiful designs with more absolute certainty. why [page ] should there not be a finer universe than this, and disconnected from this world altogether--a fit home for immortal souls? it is a necessity. god filleth all in all, is everywhere omnipotent and wise. why should there be great vacuities, barren of power and its creative outgoings? god has fixed the stars as proofs of his agency at some points in space. but is it in points only? science is proud of its discovery that what men once thought to be empty space is more intensely active than the coarser forms of matter can be. but in the long times which are past job glanced at earth, seas, clouds, pillars of heaven, stars, day, night, all visible things, and then added: "lo! these are only the outlying borders of his works. what a whisper of a word we hear of _him!_ the thunder of his power who can comprehend?" science discovers that man is adapted for mastery in this world. he is of the highest order of visible creatures. neither is it possible to imagine an order of beings generically higher to be connected with the conditions of the material world. this whole secret was known to the author of the oldest writing. "and god blessed them, and god said unto them: be fruitful, and multiply, and replenish the earth, and subdue it: and have dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moveth upon the earth." the idea is never lost sight of in the sacred writings. and while every man knows he must fail in one great contest, and yield himself to death, the later portions of the divine word offer him victory even here. the typical man is commissioned to destroy even death, and make man a sharer in the victory. [page ] science babbles at this great truth of man's position like a little child; scripture treats it with a breadth of perfect wisdom we are only beginning to grasp. science tells us that each type is prophetic of a higher one. the whale has bones prophetic of a human hand. has man reached perfection? is there no prophecy in him? not in his body, perhaps; but how his whole soul yearns for greater beauty. as soon as he has found food, the savage begins to carve his paddle, and make himself gorgeous with feathers. how man yearns for strength, subduing animal and cosmic forces to his will! how he fights against darkness and death, and strives for perfection and holiness! these prophecies compel us to believe there is a world where powers like those of electricity and luminiferous ether are ever at hand; where its waters are rivers of life, and its trees full of perfect healing, and from which all unholiness is forever kept. what we infer, scripture affirms. science tells us there has been a survival of the fittest. doubtless this is so. so in the future there will be a survival of the fittest. what is it? wisdom, gentleness, meekness, brotherly kindness, and charity. over those who have these traits death hath no permanent power. the caterpillar has no fear as he weaves his own shroud; for there is life within fit to survive, and ere long it spreads its gorgeous wings, and flies in the air above where once it crawled. man has had two states of being already. one confined, dark, peculiarly nourished, slightly conscious; then he was born into another--wide, differently nourished, and intensely [page ] conscious. he knows he may be born again into a life wider yet, differently nourished, and even yet more intensely conscious. science has no hint how a long ascending series of developments crowned by man may advance another step, and make man isaggelos--equal to angels. but the simplest teaching of scripture points out a way so clear that a child need not miss the glorious consummation. when uranus hastened in one part of its orbit, and then retarded, and swung too wide, men said there must be another attracting world beyond; and, looking there, neptune was found. so, when individual men are so strong that nations or armies cannot break down their wills; so brave, that lions have no terrors; so holy, that temptation cannot lure nor sin defile them; so grand in thought, that men cannot follow; so pure in walk, that god walks with them--let us infer an attracting world, high and pure and strong as heaven. the eleventh chapter of hebrews is a roll-call of heroes of whom this world was not worthy. they were tortured, not accepting deliverance, that they might obtain a better resurrection. the world to come influenced, as it were, the orbits of their souls, and when their bodies fell off, earth having no hold on them, they sped on to their celestial home. the tendency of such souls necessitates such a world. the worlds and the word speak but one language, teach but one set of truths. how was it possible that the writers of the earlier scriptures described physical phenomena with wonderful sublimity, and with such penetrative truth? they gazed upon the same heaven that those men saw who ages afterward led the world in knowledge. these latter were near-sighted, and absorbed [page ] in the pictures on the first veil of matter; the former were far-sighted, and penetrated a hundred strata of thickest material, and saw the immaterial power behind. the one class studied the present, and made the gravest mistakes; the other pierced the uncounted ages of the past, and uttered the profoundest wisdom. there is but one explanation. he that planned and made the worlds inspired the word. science and religion are not two separate departments, they are not even two phases of the same truth. science has a broader realm in the unseen than in the seen, in the source of power than in the outcomes of power, in the sublime laws of spirit than in the laws of matter; and religion sheds its beautiful light over all stages of life, till, whether we eat or whether we drink, or whatsoever we do, we may do all for the glory of god. science and religion make common confession that the great object of life is to learn and to grow. both will come to see the best possible means, for the attainment of this end is a personal relation to a teacher who is the way, the truth, and the life. [page ] xii. the ultimate force. "in the beginning was the word, and the word was with god, and the word was god. the same was in the beginning with god. all things became by him, and without him was not anything made that was made * * * and by him all things stand together." [page ] "o thou eternal one; whose presence blight all space doth occupy--all motion guide-- thou from primeval nothingness didst call first chaos, then existence. lord, on thee eternity had its foundation: all sprung forth from thee--of light, joy, harmony, sole origin: all life, all beauty thine. thy word created all, and doth create; thy splendor fills all space with rays divine; thou art and wert, and shalt be glorious, great; life-giving, life-sustaining potentate, thy chains the unmeasured universe surround-- upheld by thee, by thee inspired with breath." derzhavin. [page ] xii. _the ultimate force._ the universe is god's name writ large. thought goes up the shining suns as golden stairs, and reads the consecutive syllables--all might, and wisdom, and beauty; and if the heart be fine enough and pure enough, it also reads everywhere the mystic name of love. let us learn to read the hieroglyphics, and then turn to the blazonry of the infinite page. that is the key-note; the heavens and the earth declaring the glory of god, and men with souls attuned listening. to what voices shall we listen first? stand on the shore of a lake set like an azure gem among the bosses of green hills. the patter of rain means an annual fall of four cubic feet of water on every square foot of it. it weighs two hundred and forty pounds to the cubic foot, one hundred million tons on the surface of a little sheet of water twenty miles long by three wide. now, all that weight of falling rain had to be lifted, a work compared to which taking up mountains and casting them into the sea is pastime. all that water had to be taken up before it could be cast down, and carried hundreds of miles before it could be there. you have heard niagara's thunder; have stood beneath the falling immensity; seen it ceaselessly poured from an infinite hand; felt that you would be ground to atoms if you fell into that resistless flood. well, all that infinity of [page ] water had to be lifted by main force, had to be taken up out of the far pacific, brought over the rocky mountains; and the mississippi keeps bearing its wide miles of water to the gulf, and niagara keeps thundering age after age, because there is power somewhere to carry the immeasurable floods all the time the other way in the upper air. but this is only the alpha of power. professor clark, of amherst, massachusetts, found that such a soft and pulpy thing as a squash had so great a power of growth that it lifted three thousand pounds, and held it day and night for months. it toiled and grew under the growing weight, compacting its substance like oak to do the work. all over the earth this tremendous power and push of life goes on--in the little star-eyed flowers that look up to god only on the alpine heights, in every tuft of grass, in every acre of wheat, in every mile of prairie, and in every lofty tree that wrestles with the tempests of one hundred winters. but this is only the b in the alphabet of power. rise above the earth, and you find the worlds tossed like playthings, and hurled seventy times as fast as a rifle-ball, never an inch out of place or a second out of time. but this is only the c in the alphabet of power. rise to the sun. it is a quenchless reservoir of high-class energy. our tornadoes move sixty miles an hour, those of the sun twenty thousand miles an hour. a forest on fire sends its spires of flame one hundred feet in air, the sun sends its spires of flame two hundred thousand miles. all our fires exhaust the fuel and burn out. if the sun were pure coal, it would burn out in five thousand years; and yet this sea of unquenchable [page ] flame seethes and burns, and rolls and vivifies a dozen worlds, and flashes life along the starry spaces for a million years without any apparent diminution. it sends out its power to every planet, in the vast circle in which it lies. it fills with light not merely a whole circle, but a dome; not merely a dome above, but one below, and on every side. at our distance of ninety-two and a half millions of miles, the great earth feels that power in gravitation, tides, rains, winds, and all possible life--every part is full of power. fill the earth's orbit with a circle of such receptive worlds--seventy thousand instead of one--everyone would be as fully supplied with power from this central source. more. fill the whole dome, the entire extent of the surrounding sphere, bottom, sides, top, a sphere one hundred and eighty-five million miles in diameter, and everyone of these uncountable worlds would be touched with the same power as one; each would thrill with life. this is only the d of the alphabet of power. and glancing up to the other suns, one hundred, five hundred, twelve hundred times as large, double, triple, septuple, multiple suns, we shall find power enough to go through the whole alphabet in geometrical ratio; and then in the clustered suns, galaxies, and nebulæ, power enough still unrepresented by single letters to require all combinations of the alphabet of power. what is the significance of this single element of power? the answer of science to-day is "correlation," the constant evolution of one force from another. heat is a mode of motion, motion a result of heat. so far so good. but are we mere reasoners in a circle? then we would be lost men, treading our round of death in a limitless forest. what is the ultimate? reason [page ] out in a straight line. no definition of matter allows it to originate force; only mind can do that. hence the ultimate force is always mind. carry your correlation as far as you please--through planets, suns, nebulæ, concretionary vortices, and revolving fire-mist--there must always be mind and will beyond. some of that willpower that works without exhaustion must take its own force and render it static, apparent. it may do this in such correlated relation that that force shall go on year after year to a thousand changing forms; but that force must originate in mind. go out in the falling rain, stand under the thunderous niagara, feel the immeasurable rush of life, see the hanging worlds, and trace all this--the carried rain, the terrific thunder with god's bow of peace upon it, and the unfailing planets hung upon nothing--trace all this to the orb of day blazing in perpetual strength, but stop not there. who _made_ the sun? contrivance fills all thought. _who_ made the sun? nature says there is a mind, and that mind is almighty. then you have read the first syllables, viz., being and power. what is the continuous relation of the universe to the mind from which it derived its power? some say that it is the relation of a wound-up watch to the winder. it was dowered with sufficient power to revolve its ceaseless changes, and its maker is henceforth an absentee god. is it? let us have courage to see. for twenty years one devotes ten seconds every night to putting a little force into a watch. it is so arranged that it distributes that force over twenty-four hours. in that twenty years more power has been put into that watch than a horse could exert at once. but suppose [page ] one had tried to put all that force into the watch at once: it would have pulverized it to atoms. but supposing the universe had been dowered with power at first to run its enormous rounds for twenty millions of years. it is inconceivable; steel would be as friable as sand, and strengthless as smoke, in such strain. we have discovered some of the laws of the force we call gravitation. but what do we know of its essence? how it appears to act we know a little, what it is we are profoundly ignorant. few men ever discuss this question. all theories are sublimely ridiculous, and fail to pass the most primary tests. how matter can act where it is not, and on that with which it has no connection, is inconceivable. newton said that anyone who has in philosophical matters a competent faculty of thinking, could not admit for a moment the possibility of a sun reaching through millions of miles, and exercising there an attractive power. a watch may run if wound up, but how the watch-spring in one pocket can run the watch in another is hard to see. a watch is a contrivance for distributing a force outside of itself, and if the universe runs at all on that principle, it distributes some force outside of itself. le sage's theory of gravitation by the infinitive hail of atoms cannot stand a minute, hence we come back as a necessity of thought to herschel's statement. "it is but reasonable to regard gravity as a result of a consciousness and a will existent somewhere." where? i read an old book speaking of these matters, and it says of god, he hangeth the earth upon nothing; he upholdeth constantly all things by the word of his power. [page ] by him all things consist or hold together. it teaches an imminent mind; an almighty, constantly exerted power. proof of this starts up on every side. there is a recognized tendency in all high-class energy to deteriorate to a lower class. there is steam in the boiler, but it wastes without fuel. there is electricity in the jar, but every particle of air steals away a little, unless our conscious force is exerted to regather it. there is light in the sun, but infinite space waits to receive it, and takes it swift as light can leap. we said that if the sun were pure coal, it would burn out in five thousand years, but it blazes undimmed by the million. how can it? there have been various theories: chemical combustion, it has failed; meteoric impact, it is insufficient; condensation, it is not proved; and if it were, it is an intermediate step back to the original cause of condensation. the far-seeing eyes see in the sun the present active power of him who first said, "let there be light," and who at any moment can meet a saul in the way to damascus with a light above the brightness of the sun--another noon arisen on mid-day; and of whom it shall be said in the eternal state of unclouded brightness, where sun and moon are no more, "the glory of the lord shall lighten it, and the lamb is the light thereof." but suppose matter could be dowered, that worlds could have a gravitation, one of two things must follow: it must have conscious knowledge of the position, exact weight, and distance of every atom, mass, and world, in order to proportion the exact amount of gravity, or it must fill infinity with an omnipresent attractive power, pulling in myriads of places at nothing; in [page ] a few places at worlds. every world must exert an infinitely extended power, but myriads of infinities cannot be in the same space. the solution is, one infinite power and conscious will. to see the impossibility of every other solution, join in the long and microscopic hunt for the ultimate particle, the atom; and if found, or if not found, to a consideration of its remarkable powers. bring telescopes and microscopes, use all strategy, for that atom is difficult to catch. make the first search with the microscope: we can count , lines ruled on a glass plate inside of an inch. but we are here looking at mountain ridges and valleys, not atoms. gold can be beaten to the / of an inch. it can be drawn as the coating of a wire a thousand times thinner, to the / of an inch. but the atoms are still heaped one upon another. take some of the infusorial animals. alonzo gray says millions of them would not equal in bulk a grain of sand. yet each of them performs the functions of respiration, circulation, digestion, and locomotion. some of our blood-vessels are not a millionth of our size. what must be the size of the ultimate particles that freely move about to nourish an animal whose totality is too small to estimate? a grain of musk gives off atoms enough to scent every part of the air of a room. you detect it above, below, on every side. then let the zephyrs of summer and the blasts of winter sweep through that room for forty years, bearing out into the wide world miles on miles of air, all perfumed from the atoms of that grain of musk, and at the end of the forty years the weight of musk has not appreciably diminished. [page ] yet uncountable myriads on myriads of atoms have gone. our atom is not found yet. many are the ways of searching for it which we cannot stop to consider. we will pass in review the properties with which materialists preposterously endow it. it is impenetrable and indivisible, though some atoms are a hundred times larger than others. each has definite shape; some one shape, and some another. they differ in weight, in quantity of combining power, in quality of combining power. they combine with different substances, in certain exact assignable quantities. thus one atom of hydrogen combines with eighty of bromine, one hundred and sixty of mercury, two hundred and forty of boron, three hundred and twenty of silicon, etc. hence our atom of hydrogen must have power to count, or at least to measure, or be cognizant of bulk. again, atoms are of different sorts, as positive or negative to electric currents. they have power to take different shapes with different atoms in crystallization; that is, there is a power in them, conscious or otherwise, that the same bricks shall make themselves into stables or palaces, sewers or pavements, according as the mortar varies. "no, no," you cry out; "it is only according as the builder varies his plan." there is no need to rehearse these powers much further; though not one-tenth of the supposed innate properties of this infinitesimal infinite have been recited--properties which are expressed by the words atomicity, quantivilence, monad, dryad, univalent, perissad, quadrivalent, and twenty other terms, each expressing some endowment of power in this in visible atom. refer to one more presumed ability, an ability [page ] to keep themselves in exact relation of distance and power to each other, without touching. it is well known that water does not fill the space it occupies. we can put eight or ten similar bulks of different substances into a glass of water without greatly increasing its bulk, some actually diminishing it. a philosopher has said that the atoms of oxygen and hydrogen are probably not nearer to each other in water than one hundred and fifty men would be if scattered over the surface of england, one man to four hundred square miles. the atoms of the luminiferous ether are infinitely more diffused, and yet its interactive atoms can give four hundred millions of light-waves a second. and now, more preposterous than all, each atom has an attractive power for every other atom of the universe. the little mote, visible only in a sunbeam streaming through a dark room, and the atom, infinitely smaller, has a grasp upon the whole world, the far-off sun, and the stars that people infinite space. the sage of concord advises you to hitch your wagon to a star. but this is hitching all stars to an infinitesimal part of a wagon. such an atom, so dowered, so infinite, so conscious, is an impossible conception. but if matter could be so dowered as to produce such results by mechanism, could it be dowered to produce the results of intelligence? could it be dowered with power of choice without becoming mind? if oxygen and hydrogen could be made able to combine into water, could the same unformed matter produce in one case a plant, in another a bird, in a third a man; and in each of these put bone, brain, blood, and nerve in [page ] proper relations? matter must be mind, or subject to a present working mind, to do this. there must be a present intelligence directing the process, laying the dead bricks, marble, and wood in an intelligent order for a living temple. if we do put god behind a single veil in dead matter, in all living things he must be apparent and at work. if, then, such a thing as an infinite atom is impossible, shall we not best understand matter by saying it is a visible representation of god's personal will and power, of his personal force, and perhaps knowledge, set aside a little from himself, still possessed somewhat of his personal attributes, still responsive to his will. what we call matter may be best understood as god's force, will, knowledge, rendered apparent, static, and unweariably operative. unless matter is eternal, which is unthinkable, there was nothing out of which the world could be made, but god himself; and, reverently be it said, matter seems to retain fit capabilities for such source. is not this the teaching of the bible? i come to the old book. i come to that man who was taken up into the arcana of the third heaven, the holy of holies, and heard things impossible to word. i find he makes a clear, unequivocal statement of this truth as god's revelation to him. "by faith," says the author of hebrews, "we understand the worlds were framed by the word of god, so that things which are seen were not made of things which do appear." in corinthians, paul says--but to us there is but one god, the father, of whom [as a source] are all things; and one lord jesus christ, by whom [as a creative worker] are all things. so in romans he says--"for out of him, and through him, and to him are all things, to whom be glory forever. amen." [page ] god's intimate relation to matter is explained. no wonder the forces respond to his will; no wonder pantheism--the idea that matter is god--has had such a hold upon the minds of men. matter, derived from him, bears marks of its parentage, is sustained by him, and when the divine will shall draw it nearer to himself the new power and capabilities of a new creation shall appear. let us pay a higher respect to the attractions and affinities; to the plan and power of growth; to the wisdom of the ant; the geometry of the bee; the migrating instinct that rises and stretches its wings toward a provided south--for it is all god's present wisdom and power. let us come to that true insight of the old prophets, who are fittingly called seers; whose eyes pierced the veil of matter, and saw god clothing the grass of the field, feeding the sparrows, giving snow like wool and scattering hoar-frost like ashes, and ever standing on the bow of our wide-sailing world, and ever saying to all tumultuous forces, "peace, be still." let us, with more reverent step, walk the leafy solitudes, and say: "father, thy hand hath reared these venerable columns: thou did'st weave this verdant roof. thou did'st look down upon the naked earth, and forthwise rose all these fair ranks of trees. they in thy sun budded, and shook their green leaves in thy breeze. "that delicate forest flower, with scented breath and looks so like a smile, seems, as it issues from the shapeless mould, an emanation of the indwelling life, a visible token of the unfolding love that are the soul of this wide universe."--bryant. [page ] philosophy has seen the vast machine of the universe, wheel within wheel, in countless numbers and hopeless intricacy. but it has not had the spiritual insight of ezekiel to see that they were everyone of them full of eyes--god's own emblem of the omniscient supervision. what if there are some sounds that do not seem to be musically rhythmic. i have seen where an avalanche broke from the mountain side and buried a hapless city; have seen the face of a cliff shattered to fragments by the weight of its superincumbent mass, or pierced by the fingers of the frost and torn away. all these thunder down the valley and are pulverized to sand. is this music? no, but it is a tuning of instruments. the rootlets seize the sand and turn it to soil, to woody fibre, leafy verdure, blooming flowers, and delicious fruit. this asks life to come, partake, and be made strong. the grass gives itself to all flesh, the insect grows to feed the bird, the bird to nourish the animal, the animal to develop the man. notwithstanding the tendency of all high-class energy to deteriorate, to find equilibrium, and so be strengthless and dead, there is, somehow, in nature a tremendous push upward. ask any philosopher, and he will tell you that the tendency of all endowed forces is to find their equilibrium and be at rest--that is, dead. he draws a dismal picture of the time when the sun shall be burned out, and the world float like a charnel ship through the dark, cold voids of space--the sun a burned-out char, a dead cinder, and the world one dismal silence, cold beyond measure, and dead beyond consciousness. the philosopher has wailed a dirge without [page ] hope, a requiem without grandeur, over the world's future. but nature herself, to all ears attuned, sings pæans, and shouts to men that the highest energy, that of life, does not deteriorate. mere nature may deteriorate. the endowments of force must spend themselves. wound-up watches and worlds must run down. but nature sustained by unexpendable forces must abide. nature filled with unexpendable forces continues in form. nature impelled by a magnificent push of life must ever rise. study her history in the past. sulphurous realms of deadly gases become solid worlds; surplus sunlight becomes coal, which is reserved power; surplus carbon becomes diamonds; sediments settle until the heavens are azure, the air pure, the water translucent. if that is the progress of the past, why should it deteriorate in the future? there is a system of laws in the universe in which the higher have mastery over the lower. lower powers are constitutionally arranged to be overcome; higher powers are constitutionally arranged for mastery. at one time the water lies in even layers near the ocean's bed, in obedience to the law or power of gravitation. at another time it is heaved into mountain billows by the shoulders of the wind. again it flies aloft in the rising mists of the morning, transfigured by a thousand rain bows by the higher powers of the sun. again it develops the enormous force of steam by the power of heat. again it divides into two light flying airs by electricity. again it stands upright as a heap by the power of some law in the spirit realm, whose mode of working we are not yet large enough [page ] to comprehend. the water is solid, liquid, gaseous on earth, and in air according to the grade of power operating upon it. the constant invention of man finds higher and higher powers. once he throttled his game, and often perished in the desperate struggle; then he trapped it; then pierced it with the javelin; then shot it with an arrow, or set the springy gases to hurl a rifle-ball at it. sometime he may point at it an electric spark, and it shall be his. once he wearily trudged his twenty miles a day, then he took the horse into service and made sixty; invoked the winds, and rode on their steady wings two hundred and forty; tamed the steam, and made almost one thousand; and if he cannot yet send his body, he can his mind, one thousand miles a second. it all depends upon the grade of power he uses. now, hear the grand truth of nature: as the years progress the higher grades of power increase. either by discovery or creation, there are still higher class forces to be made available. once there was no air, no usable electricity. there is no lack of those higher powers now. the higher we go the more of them we find. mr. lockyer says that the past ten years have been years of revelation concerning the sun. a man could not read in ten years the library of books created in that time concerning the sun. but though we have solved certain problems and mysteries, the mysteries have increased tenfold. we do not know that any new and higher forces have been added to matter since man's acquaintance with it. but it would be easy to add any number of them, or change any lower into higher. that is the [page ] meaning of the falling granite that becomes soil, of the pulverized lava that decks the volcano's trembling sides with flowers; that is the meaning of the grass becoming flesh, and of all high forces constitutionally arranged for mastery over lower. take the ore from the mountain. it is loose, friable, worthless in itself. raise it in capacity to cast-iron, wrought-iron, steel, it becomes a highway for the commerce of nations, over the mountains and under them. it becomes bones, muscles, body for the inspiring soul of steam. it holds up the airy bridge over the deep chasm. it is obedient in your hand as blade, hammer, bar, or spring. it is inspirable by electricity, and bears human hopes, fears, and loves in its own bosom. it has been raised from valueless ore. change it again to something as far above steel as that is above ore. change all earthly ores to highest possibility; string them to finest tissues, and the new result may fit god's hand as tools, and thrill with his wisdom and creative processes, a body fitted for god's spirit as well as the steel is fitted to your hand. from this world take opacity, gravity, darkness, bring in more mind, love, and god, and then we will have heaven. an immanent god makes a plastic world. when man shall have mastered the forces that now exist, the original creator and sustainer will say, "behold, i create all things new." nature shall be called nearer to god, be more full of his power. to the long-wandering Æneas, his divine mother sometimes came to cheer his heart and to direct his steps. but the goddess only showed herself divine by her departure; only when he stood in desolation did the hero know he had [page ] stood face to face with divine power, beauty, and love. not so the christian scholars, the wanderers in nature's bowers to-day. in the first dawn of discovery, we see her full of beauty and strength; in closer communion, we find her full of wisdom; to our perfect knowledge, she reveals an indwelling god in her; to our ardent love, she reveals an indwelling god in us. but the evidence of the progressive refinements of habitation is no more clear than that of progressive refinement of the inhabitant: there must be some one to use these finer things. an empty house is not god's ideal nor man's. the child may handle a toy, but a man must mount a locomotive; and before there can be new jerusalems with golden streets, there must be men more avaricious of knowledge than of gold, or they would dig them up; more zealous for love than jewels, or they would unhang the pearly gates. the uplifting refinement of the material world has been kept back until there should appear masterful spirits able to handle the higher forces. doors have opened on every side to new realms of power, when men have been able to wield them. if men lose that ability they close again, and shut out the knowledge and light. then ages, dark and feeble, follow. some explore prophecy for the date of the grand transformation of matter by the coming of the son of man, for a new creation. a little study of nature would show that the date cannot be fixed. a little study of peter would show the same thing. he says, "what manner of persons ought ye to be, in all holy conversation and godliness, looking for and hastening the coming [page ] of the day of god, wherein the heavens being on fire shall be dissolved, and the elements shall melt with fervent heat? nevertheless we, according to his promise, look for a new heaven and a new earth." the idea is, that the grand transformation of matter waits the readiness of man. the kingdom waits the king. the scattered cantons of italy were only prostrate provinces till victor emanuel came, then they were developed into united italy. the prostrate provinces of matter are not developed until the man is victor, able to rule there a realm equal to ten cities here. every good man hastens the coming of the day of god and nature's renovation. not only does inference teach that there must be finer men, but fact affirms that transformation has already taken place. life is meant to have power over chemical forces. it separates carbon from its compounds and builds a tree, separates the elements and builds the body, holds them separate until life withdraws. more life means higher being. certainly men can be refined and recapacitated as well as ore. in ovid's "metamorphoses" he represents the lion in process of formation from earth, hind quarters still clay, but fore quarters, head, erect mane, and blazing eye--live lion--and pawing to get free. we have seen winged spirits yet linked to forms of clay, but beating the celestial air, endeavoring to be free; and we have seen them, dowered with new sight, filled with new love, break loose and rise to higher being. in this grand apotheosis of man which nature teaches, progress lias already been made. man has already outgrown his harmony with the environment of mere matter. he has given his hand to science, and been lifted up above the earth into the voids of infinite space. he [page ] has gone on and on, till thought, wearied amidst the infinities of velocity and distance, has ceased to note them. but he is not content; all his faculties are not filled. he feels that his future self is in danger of not being satisfied with space, and worlds, and all mental delights, even as his manhood fails to be satisfied with the materiel toys of his babyhood. he asks for an author and maker of things, infinitely above them. he has seen wisdom unsearchable, power illimitable; but he asks for personal sympathy and love. paul expresses his feeling: every creature--not the whole creation--groaneth and travaileth in pain together until now, waiting for the adoption--the uplifting from orphanage to parentage--a translation out of darkness into the kingdom of god's dear son. he hears that a man in christ is a new creation: old things pass away, all things become new. there is then a possibility of finding the author of nature, and the father of man. he begins his studies anew. now he sees that all lines of knowledge converge as they go out toward the infinite mystery; sees that these converging lines are the reins of government in this world; sees the converging lines grasped by an almighty hand; sees a loving face and form behind; sees that these lines of knowledge and power are his personal nerves, along which flashes his will, and every force in the universe answers like a perfect muscle. then he asks if this personality is as full of love as of power. he is told of a tenderness too deep for tears, a love that has the cross for its symbol, and a dying cry for its expression: seeking it, he is a new creation. he sees more wondrous things in the word than in the [page ] world. he comes to know god with his heart, better than he knows god's works by his mind. every song closes with the key-note with which it began, and the brief cadence at the close hints the realms of sound through which it has tried its wings. the brief cadence at the close is this: all force runs back into mind for its source, constant support, and uplifts into higher grades. mr. grove says, "causation is the will, creation is the act, of god." creation is planned and inspired for the attainment of constantly rising results. the order is chaos, light, worlds, vegetable forms, animal life, then man. there is no reason to pause here. this is not perfection, not even perpetuity. original plans are not accomplished, nor original force exhausted. in another world, free from sickness, sorrow, pain, and death, perfection of abode is offered. perfection of inhabitant is necessary; and as the creative power is everywhere present for the various uplifts and refinements of matter, it is everywhere present with appropriate power for the uplifting and refinement of mind and spirit. [page ] summary of latest discoveries and conclusions. _movements on the sun._--the discovery and measurement of the up-rush, down-rush, and whirl of currents about the sunspots, also of the determination of the velocity of rotation by means of the spectroscope, as described (page ), is one of the most delicate and difficult achievements of modern science. _movement of stars in line of sight_ (page ).--the following table shows this movement of stars, so far as at present known: --------------------------------------------------------------- | aproaching. || receding. | |------------------------------||-------------------------------| | map. | name. | rate || map. | name. | rate | | | | per sec. || | | per sec. | |-------|-----------|----------||--------|-----------|----------| |fig. |arcturus | miles ||fig. |sirius | miles | | " |vega | " ||fr'piece|betelguese | " | | " |a cygni | " || " |rigel | " | | " |pollux | " ||fig. |castor | " | | " |dubhe | " || " |regulus | " | --------------------------------------------------------------- _sun's appearance._--this was formerly supposed to be an even, regular, dazzling brightness, except where the spots appeared. but the sun's surface is now known to be mottled with what are called rice grains or willow leaves. but the rice grains are as large as the continent of america. the spaces between are called pores. they constitute an innumerable number of small spots. this appearance of the general surface is well portrayed in the cut on page . _close relation between sun and earth._-men always knew that the earth received light from the sun. they subsequently discovered that the earth was momentarily held by the power [page ] of gravitation. but it is a recent discovery that the light is one of the principal agents in chemical changes, in molecular grouping and world-building, thus making all kinds of life possible (p. - ). the close connection of the sun and the earth will be still farther shown in the relation of sun-spots and auroras. one of the most significant instances is related on page , when the earth felt the fall of bolides upon the sun. members of the body no more answer to the heart than the planets do to the sun. _hydrogen flames._--it has been demonstrated that the sun flames , miles high are hydrogen in a state of flaming incandescence (page ). _sun's distance._--the former estimate, , , miles, has been reduced by nearly one-thirtieth. lockyer has stated it as low as , , miles, and proctor, in "encyclopædia britannica," at , , miles, but discovered errors show that these estimates are too small. newcomb gives , , as within , miles of the correct distance. the data for a new determination of this distance, obtained from the transit of venus, december th, , have not yet been deciphered; a fact that shows the difficulty and laboriousness of the work. meanwhile it begins to be evident that observations of the transit of venus do not afford the best basis for the most perfect determination of the sun's distance. since the earth's distance is our astronomical unit of measure, it follows that all other distances will be changed, when expressed in miles, by this ascertained change of the value of the standard. _oxygen in the sun._--in professor draper announced the discovery of oxygen lines in the spectrum of the sun. the discovery was doubted, and the methods used were criticised by lockyer and others, but later and more delicate experiments substantiate professor draper's claim to the discovery. the elements known to exist in the sun are salt, iron, hydrogen, [page ] magnesium, barium, copper, zinc, cromium, and nickel. some elements in the sun are scarcely, if at all, discoverable on the earth, and some on the earth not yet discernible in the sun. _substance of stars._--aldebaran (_frontispiece_) shows salt, magnesium, hydrogen, calcium, iron, bismuth, tellurium, antimony, and mercury. some of the sun's metals do not appear. stars differ in their very substance, and will, no doubt, introduce new elements to us unknown before. the theory that all nebulæ are very distant clusters of stars is utterly disproved by the clearest proof that some of them are only incandescent gases of one or two kinds. _discoveries of new bodies._--vulcan, the planet nearest the sun (page ). the two satellites of mars were discovered by mr. hall, u. s. naval observatory, august th, (page ). "the outer one is called diemas; the inner, phobus. sir william herschel thought he discovered six satellites of uranus. the existence of four of them has been disproved by the researches of men with larger telescopes. two new ones, however, were discovered by mr. lassell in . _saturn's rings_ are proved to be in a state of fluidity and contraction (page ). _meteors and comets._--the orbits of over one hundred swarms of meteoric bodies are fixed: their relation to, and in some cases indentity with, comets determined. some comets are proved to be masses of great weight and solidity (page ). _aerolites._-some have a texture like our lowest strata of rocks. there is a geology of stars and meteors as well as of the earth. m. meunier has just received the lalande medal from the paris academy for his treatise showing that, so far as our present knowledge can determine, some of these meteors once belonged to a globe developed in true geological epochs, and which has been separated into fragments by agencies with which we are not acquainted. [illustration: fig. .--horizontal pendulum.] _the horizontal pendulum._--this delicate instrument is [page ] represented in fig. . it consists of an upright standard, strongly braced; a weight, _m_, suspended by the hair-spring of a watch, b d, and held in a horizontal position by another watch-spring, a c. the weight is deflected from side to side by the slightest influence. the least change in the level of a base thirty-nine inches long that could be detected by a spirit-level is ". of an arc--equal to raising one end / of an inch. but the pendulum detects a raising of one end / of an inch. to observe the movements of the pendulum, it is kept in a dark room, and a ray of light is directed to the mirror, _m_, and thence reflected upon a screen. thus the least movement may be enormously magnified, and read and measured by the moving spot on the screen. it has been discovered that when the sun rises it has sufficient attraction to incline this instrument to the east; when it sets, to incline it to the west. the same is true of the moon. when either is exactly overhead or underfoot, of course there is no deflection. the mean deflection caused by the moon at rising or setting is ". ; by the sun, ". . great results are expected from this instrument hardly known as yet: among others, whether gravitation acts instantly or consumes time in coming from the sun. this will be shown by the time of the change of the pendulum from east to west when the sun reaches the zenith, and _vice versa_ when it crosses the nadir. the sun will be best studied without light, in the quiet and darkness of some deep mine. [page ] _light of unseen stars._--from careful examination, it appears that three-fourths of the light on a fine starlight night comes from stars that cannot be discerned by the naked eye. the whole amount of star light is about one-eightieth of that of the full moon. _lateral movements of stars_, page - . _future discoveries_--_a trans-neptunian planet._--professor asaph hall says: "it is known to me that at least two american astronomers, armed with powerful telescopes, have been searching quite recently for a trans-neptunian planet. these searches have been caused by the fact that professor newcomb's tables of uranus and neptune already begin to differ from observation. but are we to infer from these errors of the planetary tables the existence of a trans-neptunian planet? it is possible that such a planet may exist, but the probability is, i think, that the differences are caused by errors in the theories of these planets. * * * a few years ago the remark was frequently made that the labors of astronomers on the solar system were finished, and that henceforth they could turn their whole attention to sidereal astronomy. but to-day we have the lunar theory in a very discouraging condition, and the theories of mercury, jupiter, saturn, uranus, and neptune all in need of revision; unless, indeed, leverrier's theories of the last two planets shall stand the test of observation. but, after all, such a condition of things is only the natural result of long and accurate series of observations, which make evident the small inequalities in the motions, and bring to light the errors of theory." future discoveries will mostly reveal the laws and conditions of the higher and finer forces. already professor loomis telegraphs twenty miles without wire, by the electric currents between mountains. we begin to use electricity for light, and feel after it for a motor. comets and auroras show its presence between worlds, and in the interstellar spaces. let another newton arise. [page ] some elements of the solar system ------------------------------------------------------------------------ | | | | mean dist. | | | | | | | from sun. | | | | | | |-------------------| mean |density.| | | | | earth's| |diameter |[earth] | | name. | sign. | masses. | dist. | millions |in miles.| = . | | | | | as . | of miles.| | | |-----------|--------|------------|--------|----------|---------|--------| | sun |[symbol]| unity | | | , | . | | mercury |[symbol]| / (?)| . | - / | , | . | | venus |[symbol]| / | . | - / | , | . | | earth |[symbol]| / | . | - / | , | . | | mars |[symbol]| / | . | | , | . | | asteroids | (no.) | | | | | | | jupiter |[symbol]| / | . | | , | . | | saturn |[symbol]| / | . | | , | . | | uranus |[symbol]| / | . | | , | . | | neptune |[symbol]| / | . | | , | . | ------------------------------------------------------------------------ ------------------------------------------------------------- | | | gravity | | | | | axial | at | | orbital | | | revolu- | surface. | periodic | velocity | | name. | tion | [earth] | time. | in miles | | | | = | | per sec. | |-----------|---------------|----------|-----------|----------| | sun | to d | . | | | | mercury | h m(?) | . | . d | . | | venus | h m(?) | . | . d | . | | earth | h m s | . | . d | . | | mars | h m . s | . | . d | . | | asteroids | | | | | | jupiter | h m s | . | . yrs | . | | saturn | h m | . | . yrs | . | | uranus | unknown. | . | . yrs | . | | neptune | unknown. | . | . yrs | . | ------------------------------------------------------------- [page ] explanation of astronomical symbols. signs of the zodiac . [symbol] aries ° | vi. [symbol] libra ° i. [symbol] taurus | vii. [symbol] scorpio ii. [symbol] gemini | viii. [symbol] sagittarius iii. [symbol] cancer | ix. [symbol] capricornus iv. [symbol] leo | x. [symbol] aquarius v. [symbol] virgo | xi. [symbol] pisces * * * * * [symbol] conjunction. | s. seconds of time. [symbol] quadrature. | ° degrees. [symbol] opposition. | ' minutes of arc. [symbol] ascending node. | " seconds of arc. [symbol] descending node. | r. a. right ascension. h. hours. | decl. or d. declination. m. minutes of time. | n. p. d. dist. from north pole. other abbreviations used in the almanac. s., south, _i.e._, crosses the meridian; m., morning; a, afternoon; gr. h. l. n., greatest heliocentric latitude north, _i.e._, greatest distance north of the ecliptic, as seen from the sun. [symbols] inf., inferior conjunction; sup., superior conjunction. greek alphabet used indicating the stars. a, alpha. | ae, eta. | n, nu. | t, tau. b, beta. | th, theta. | x, xi. | u, upsilon. g, gamma. | i, iota. | o, omicron. | ph, phi. d, delta. | k, kappa. | p, pi. | ch, chi. e, epsilon. | l, lambda. | r, rho. | ps, psi. z, zeta. | m, mu. | s, sigma. | o, omega. [page ] chautauqua outline for students. as an aid to comprehension, every student should draw illustrative figures of the various circles, planes, and situations described. (for example, see fig. , page .) as an aid to memory, the portion of this outline referring to each chapter should be examined at the close of the reading, and this mere sketch filled up to a perfect picture from recollection. i. _creative processes._--the dial-plate of the sky. cause or different weights--on sun, moon. two laws of gravity. inertia. fall of earth to sun per second. forward motion. elastic attraction. perturbation of moon; of jupiter and saturn. oscillations of planets. ii. _light._--from condensation. number of vibrations of red; violet. thermometer against air. aerolite against earth. two bolides against the sun. large eye. velocity of light. prism. color means different vibrations. music of light. light reports substance of stars. force of; bridge, rain, dispersion, intensities, reflection, refraction, decomposition. iii. _astronomical instruments._--refracting telescope. reflecting; largest. spectroscope. spectra of sun, hydrogen, sodium, etc. e made g by approach; c by departure. stars approach and recede. iv. _celestial measurements._-place and time by stars. degrees, minutes, seconds. mapping stars. mural circle. slow watch. hoosac tunnel. fine measurements. sidereal time. spider-lines. personal equation. measure distance--height. ten-inch base line. parallax of sun, stars. longitude at sea. distance of polaris, a centauri, cygni. orbits of asteroids. v. _the sun._--world on fire. apparent size from planets. zodiacal light. corona. hydrogen--how high? size. how many earths? spots: . motion; . edges; . variable; . periodic; . cyclonic; . size; . velocities. what the sun does. experiments. vi. _the planets from space._--north pole. speed. sizes. axial revolution. man's weight on. seasons. parallelism of axis. earth near [page ] sun in winter. plane of ecliptic. orbits inclined to. earth rotates. proof. sun's path among stars. position of planets. motion--direct, retrograde. experiments. vii. _meteors._--size; number; cause of; above earth; velocity; colors; number in space; telescopic view of. aerolites: systems of; how many known. comets: orbits; number of comets; halley's; biela's lost; encke's. resisting medium. whence come comets? composed of what? amount of matter in. [symbol]. viii. _the planets._--how many? uranus discovered? neptune? asteroids? vulcan? distance from sun. periodic time. mercury: elements; shapes, as seen from earth; transits. venus: elements; seen by day; how near earth? how far from? phases; galileo. earth: elements; in space; aurora; balance of forces. tides: main and subsidiary causes; eastern shores; mediterranean sea. moon: elements; hoax; moves east; see one side; three causes help to see more than half. revolution: why twenty-nine and a half days: heat--cold; how much light? craters and peaks lighted; measured. eclipses--why not every new and full moon? periodicity. mars: elements; how near earth? how far from? apparent size; ice-fields; which end most? satellites--asteroids: how found? when? by whom? how many? jupiter: elements; trade-winds; how much light received? own heat. satellites: how many? colors. saturn: elements; habitability; rings; flux; satellites. uranus: elements; discoverer; seen by; moon's motion. neptune: elements; discovered by; how? review system. ix. _the nebular hypothesis._--state it; facts confirmatory. objections-- . heat; . rotation; . retrograde; . martial moons; . star of . evolution: gaps in; conclusion. x. _the stellar system._-motto. man among stars; open page; starry poem; stars located; named. thuban. etanin. constellations: know them; number of stars; double; e lyræ, sirius, procyon, castor, cygni, g virginis. colored stars; change color. clusters: two theories. nebulæ: two visible; composed of; shapes; where? variable stars. sun. b lyræ, mira, betelguese, algol; cause. temporary; . new star of : two theories. star of . movements of stars; sirius; sun; groombridge. stars near pleiades: orion, great dipper, southern cross. centre of gravity. xi. _the worlds and the word._--rich. number. erroneous allusions. truth before discovery: . a beginning; . creation before arrangement; . light before sun; . mountains under water; . order of development; [page ] . sphere of earth; . how upheld; . number of stars; . weight of air; . meteorology; . queries to job; . sun to end of heaven; . view of mitchell; . herschel. what is matter? force? end of earth. way to knowledge. work of light. transfiguration of matter. uniformitarianism. a whisper of him. man for mastery. each a type of higher. survival of fittest. uranus. worlds and word one language. xii. _the ultimate force._--universe shows power: . rain; niagara; . vegetable growth; . worlds carried; . sun; fill dome with worlds; . double suns; . galaxies. correlation. what ultimate? mind and will. what continuous relation? watch. theories of gravitation: newton's, le sage's, bible's. high-class energy deteriorates. search for atoms: . microscope; . gold; . infusoria; . musk. properties of atoms: . impenetrable; . indivisible; . shape; . quality; . crystallization; . not touch each other; . active; . attractive; . intelligent. whose? relation of matter to god; rock to soil. push upward. highest has mastery. man advances by highest. matter recapacitated. refined habitations. inhabitants. all force leads back to mind. personal and infinite. [page ] glossary of astronomical terms and index. abbreviations used in astronomies, . aberration of light (_a wandering away_), an apparent displacement of a star, owing to the progressive motion of light combined with that of the earth and its orbit, . aerolite (_air-stone_), . air, refraction of the, . algol, the variable star, . almanac, nautical, ; explanation of signs used, . alphabet, greek, . altitude, angular elevation of a body above the horizon. angle, difference in directions of two straight lines that meet. annular (_ring-shaped_) eclipses, ; nebulæ, , . aphelion, the point in an orbit farthest from the sun. apogee, the point of an orbit which is farthest from the earth. apsis, plural _apsides_, the line joining the aphelion and perihelion points; or the major axis of elliptical orbits. arc, a part of a circle. ascension, right, the angular distance of a heavenly body from the first point of aries, measured on the equator. asteroids (_star-like_), ; orbits of interlaced, . astronomical instruments, . astronomy, use of, . atom, size of, ; power of, . aurora borealis, . axis, the line about which a body rotates. azimuth, the angular distance of any point or body in the horizon from the north or south points. bailey's beads, dots of light on the edge of the moon seen in a solar eclipse, caused by the moon's inequalities of surface. base line, . biela's comet, . binary system, a double star, the component parts of which revolve around their centre of gravity. bode's law of planetary distances is no law at all, but a study of coincidences. bolides, small masses of matter in space. they are usually called meteors when luminous by contact with air, . [page ] celestial sphere, the apparent dome in which the heavenly bodies seem to be set; appears to revolve, . centre of gravity, the point on which a body, or two or more related bodies, balances. centrifugal force (_centre fleeing_). chromolithic plate of spectra of metals, to face . circumpolar stars, map of north, . colors of stars, . colures, the four principal meridians of the celestial sphere passing from the pole, one through each equinox, and one through each solstice. comets, ; halley's, ; biela's lost, ; encke's, ; constitution of, ; will they strike the earth? . conjunction. two or more bodies are in conjunction when they are in a straight line (disregarding inclination of orbit) with the sun. planets nearer the sun than the earth are in inferior conjunction when they are between the earth and the sun; superior conjunction when they are beyond the sun. constellation, a group of stars supposed to represent some figure: circumpolar, ; equatorial, for december, ; for january, ; april, ; june, ; september, ; november, ; southern circumpolar, . culmination, the passage of a heavenly body across the meridian or south point of a place; it is the highest point reached in its path. cusp, the extremities of the crescent form of the moon or an interior planet. declination, the angular distance of a celestial body north or south from the celestial equator. degree, the / part of a circle. direct motion, a motion from west to east among stars. disk, the visible surface of sun, moon, or planets. distance of stars, . double stars, . earth, revolution of, ; in space, ; irregular figure, . eccentricity of an ellipse, the distance of either focus from centre divided by half the major axis. eclipse (_a disappearance_), . ecliptic, the apparent annual path of the sun among the stars; plane of, . egress, the passing of one body off the disk of another. elements, the quantities which determine the motion of a planet: data for predicting astronomical phenomena; table of solar, . elements, chemical, present in the sun, . elongation, the angular distance of a planet from the sun. emersion, the reappearance of a body after it has been eclipsed or occulted by another. [page ] equator, terrestrial, the great circle half-way between the poles of the earth. when the plane of this is extended to the heavens, the line of contact is called the celestial equator. equinox, either of the points in which the sun, in its apparent annual course among the stars, crosses the equator, making days and nights of equal length. evolution, materialistic, ; insufficient, . fizeau determines the velocity of light, . forces, delicate balance of, . galileo, construction of his telescope, . geocentric, a position of a heavenly body as seen or measured from the earth's centre. geodesy, the art of measuring the earth without reference to the heavenly bodies. god, relation of, to the universe, . gravitation, laws of, ; extends to the stars, ; theories of, . gravity on different bodies, , . helical, rising or setting of a star, as near to sunrise or sunset as it can be seen. heliocentric, as seen from the centre of the sun. hoosac tunnel, example of accuracy, . horizontal pendulum, . immersion, the disappearance of one body behind another, or in its shadow. inclination of an orbit, the angle between its plane and the plane of the ecliptic. inferior conjunction, when an interior planet is between the earth and the sun. jupiter, apparent path of, in , ; elements of, ; satellites of, ; positions of satellites, ; elements of satellites, ; the jovian system, . kepler's laws-- st, that the orbits of planets are ellipses, having the sun or central body in one of the foci; d, the radius-vector passes over equal spaces in equal times; d, the squares of the periodic times of the planets are in proportion to the cubes of their mean distances from the sun. latitude, the angular distance of a heavenly body from the ecliptic. light, the child of force, ; number of vibrations of, , ; velocity of, ; undulatory and musical, ; chemical force of, ; experiments with, ; approach and departure of a light-giving body measured, ; aberration of, . limb, the edge of the disk of the moon, sun, or a planet. longitude. if a perpendicular be dropped from a body to the ecliptic, its celestial longitude is the distance of the foot of the perpendicular from the vertical equinox, counted toward the east; mode of ascertaining terrestrial, . magellanic clouds, . [page ] mars, ; snow spots of, ; satellites of, . mass, the quantity of matter a body contains. mean distance of a planet, half the sum of the aphelion and perihelion distances. measurements, celestial, . mercury, . meridian, terrestrial, of a place, a great circle of the heavens passing through the poles, the zenith, and the north and south points of the horizon; celestial, any great circle passing from one pole to the other. meteors, ; swarm of, meeting the earth, ; explosion of, ; systems of, ; relation of, to comets, . micrometer, any instrument for the accurate measurement of very small distances or angles. mind, origin of force, ; continuous relation of, to the universe, . milky way, , . mira, the wonderful, . moon, the, ; greatest and least distance from the earth, ; telescopic appearance of, . mural circle, . nadir, the point in the celestial sphere directly beneath our feet, opposite to zenith. nebulÆ, . nebular hypothesis, not atheistic, ; stated, ; confirmatory facts, ; objections to, . neptune, elements of, . node, the point in which an orbit intersects the ecliptic, or other plane of reference; ascending, descending, line of, . occultation, the hiding of a star, planet, or satellite by the interposition of a nearer body of greater angular magnitude. opposition. a superior planet is in opposition when the sun, earth, and the planet are in a line, the earth being in the middle. orbit, the path of a planet, comet, or meteor around the sun, or of a satellite around a primary; inclination of, ; earth's, seen from the stars, . outline for students, . parallax, the difference of direction of a heavenly body as seen from two points, as the centre of the earth and some point of its surface, . parallels, imaginary circles on the earth or in the heavens parallel to the equator, having the poles for their centre. perigee, nearest the earth; said of a point in an orbit. perihelion, the point of an orbit nearest the sun. periodic time, time of a planet's, comet's, or satellite's revolution. personal equation, . perturbation, the effect of the attractions of the planets or other [page ] bodies upon each other, disturbing their regular motion; of saturn and jupiter, ; of asteroids, ; of uranus and neptune, . phases, the portions of the illuminated half of the moon or interior planet, as seen from the earth, called crescent, full, and gibbous. photosphere of the sun, . planet (_a wanderer_), as seen from space, ; speed of, ; size of, ; movements retrograde and direct, . pointers, the, . pole, north, movement of, . poles, the extremities of an imaginary line on which a celestial body rotates. quadrant, the fourth part of the circumference of a circle, or °. quadrature, a position of the moon or other body when ° from the sun. radiant point, that point of the heavens from which meteors seem to diverge, . radius-vector, an imaginary line joining the sun and a planet or comet in any part of its orbit. rain, weight of, . reflecting telescope, . refracting telescope, . refraction, a bending of light by passing through any medium, as air, water, prism. retrograde motion, the apparent movement of a planet from east to west among the stars. revolution, the movement of bodies about their centre of gravity. rotation, the motion of a body around its axis. satellites, smaller bodies revolving around planets and stars. saturn, elements of, ; revolution of, ; rings of, ; decreasing, ; nature of, ; satellites of, . seasons, of the earth, ; of other planets, . selenography (_lunography_), a description of the moon's surface. signs of the zodiac, the twelve equal parts, of ° each, into which the zodiac is divided. solar system, view of, , . solstices, those points of the ecliptic which are most distant from the equator. the sun passes one about june st, and the other about december st, giving the longest days and nights. spectroscope, . spectrum of sun and metals, . stars, chemistry of, ; distance of, - ; mode of naming, ; number of, ; double and multiple, ; colored, ; clusters of, ; variable, ; temporary, new, and lost, ; movements of lateral, ; in line of sight, . stationary points, places in a planet's orbit at which it has no motion among the stars. [page ] stellar system, the, . summary of recent discoveries, . sun, fall of two meteoric bodies into, ; light from contraction of, ; as seen from planets, ; corona, ; hydrogen flames of, ; condition of, ; spots, ; experiments, ; apparent path among the stars, ; power of, . symbols used in astronomy, . telescope, refracting, ; reflecting, ; cambridge equatorial, . telescopic work, clusters, ; double stars, . temporary stars, . terminator, the boundary-line between light and darkness on the moon or a planet. tides, . transit, the passage of an object across some fixed line, as the meridian, or between the eye of an observer and an apparently larger object, as that of mercury or venus over the disk of the sun, and the satellites of jupiter over its disk; of a star, . ultimate force, the, . uranus, elements of, ; moons of, retrograde, ; perturbed by neptune, . variable stars, . venus, . vernier, a scale to measure very minute distances. vertical circle, one that passes through the zenith and nadir of the celestial sphere. the prime vertical circle passes through the east and west points of the horizon. vulcan, discovery of, . worlds, the, and the word, teach the same truth, - . year, the, length of, on any planet, is determined by the periodic time. zenith, the point in the celestial sphere directly overhead. zodiac, a belt ° wide encircling the heavens, the ecliptic being the middle. in this belt the larger planets always appear. in the older astronomy it was divided into twelve parts of ° each, called signs of the zodiac. zodiacal light, . to find the stars in the sky. detach any of the following maps, appropriate to the time of year, hold it between you and a lantern out-of-doors, and you have an exact miniature of the sky. or, better, cut squares of suitable sizes from the four sides of a box; put a map over each aperture; provide for ventilation, and turn the box over a lamp or candle out-of-doors. use an opera glass to find the smaller stars, if one is accessible. [illustration: circumpolar constellations. always visible. in this position.--january th, at o'clock; february th, at o'clock; and february th, at o'clock.] [illustration: algol is on the meridian, ° south of pole.--at o'clock, december th; o'clock, december d; o'clock, january th.] [illustration: capella ( ° from pole) and rigel ( °) are on the meridian at o'clock february th, o'clock january d, and at o'clock january th.] [illustration: regulus comes on the meridian, ° south from the pole, at o'clock march d, o'clock april th, and at o'clock april d.] [illustration: arcturus comes to the meridian, ° from the pole, at o'clock may th, o'clock june th, and at o'clock june th.] [illustration: altair comes to the meridian, ° from the pole, at o'clock p.m. august th, at o'clock september d, and at o'clock september th.] [illustration: fomalhaut comes to the meridian, only ° from the horizon, at o'clock november th.] making of america collection of the digital & multimedia center, michigan state university libraries (http://digital.lib.msu.edu/) note: images of the original pages are available through the digital & multimedia center, michigan state university libraries. see http://www.hti.umich.edu/cgi/t/text/text-idx?c=moa;idno=aan . . the uses of astronomy. an oration delivered at albany, on the th of july, by edward everett, on the occasion of the inauguration of the dudley astronomical observatory, with a condensed report of the proceedings, and an account of the dedication of new york state geological hall. new york: published by ross & tousey, nassau street. . a note explanatory. the undersigned ventures to put forth this report of mr. everett's oration, in connection with a condensed account of the inauguration of the dudley observatory, and the dedication of the new state geological hall, at albany,--in the hope that the demand which has exhausted the newspaper editions, may exhaust this as speedily as possible; not that he is particularly tenacious of a reward for his own slight labors, but because he believes that the extensive circulation of the record of the two events so interesting and important to the cause of science will exercise a beneficial influence upon the public mind. the effort of the distinguished statesman who has invested astronomy with new beauties, is the latest and one of the most brilliant of his compositions, and is already wholly out of print, though scarcely a month has elapsed since the date of its delivery. the account of the proceedings at albany during the ceremonies of inauguration is necessarily brief, but accurate, and is respectfully submitted to the consideration of the reader. a. maverick. new york, _october , ._ two new institutions of science; and the scenes which attended their christening. in the month of august last, two events took place in the city of albany, which have more than an ephemeral interest. they occurred in close connection with the proceedings of a scientific convention, and the memory of them deserves to be cherished as a recollection of the easy way in which science may be popularized and be rendered so generally acceptable that the people will cry, like oliver twist, for more. it is the purpose of this small publication to embody, in a form more durable than that of the daily newspaper, the record of proceedings which have so near a relation to the progress of scientific research. a marked feature in the ceremonies was the magnificent oration of the hon. edward everett, inaugurating the dudley observatory of albany; and it is believed that the reissue of that speech in its present form will be acceptable to the admirers of that distinguished gentleman, not less than to the lovers of science, who hung with delight upon his words. the dedication of the geological hall. on wednesday, august , , the state geological hall of new york was dedicated with appropriate ceremonies. for the purpose of affording accommodation to the immense crowds of people who, it was confidently anticipated, would throng to this demonstration and that of the succeeding day, at which mr. everett spoke, a capacious tent was arranged with care in the center of academy park, on capitol hill; and under its shelter the ceremonies of the inauguration of both institutions were conducted without accident or confusion; attended on the first day by fully three thousand persons, and on the second by a number which may be safely computed at from five to seven thousand. the announcement that hon. wm. h. seward would be present at the dedication of the geological hall, excited great interest among the citizens; but the hope of his appearance proved fallacious. his place was occupied by seven picked men of the american association for the advancement of science, one of whom (prof. henry) declared his inability to compute the problem why seven men of science were to be considered equal to one statesman. the result justified the selections of the committee, and although the senator was not present, the seven commoners of science made the occasion a most notable one by the flow of wit, elegance of phrase, solidity and cogency of argument, and rare discernment of natural truths, with which their discourse was garnished. the members of the american association marched in procession to the tent, from their place of meeting in the state capitol. on the stage were assembled many distinguished gentlemen, and in the audience were hundreds of ladies. gov. clark and ex-governors hunt and seymour, of new york, sir wm. logan, of canada, hon. george bancroft, and others as well known as these, were among the number present. the tent was profusely decorated. small banners in tri-color were distributed over the entire area covered by the stage, and adorned the wings. the following inscriptions were placed over the front of the rostrum,--that in honor of "_the press_" occupying a central position: geology. the press. meteorology. mineralogy. metallurgy. ethnology. astronomy. the following were arranged in various positions on the right and left: chemistry. telegraph. physiology. letters. conchology. hydrology. palÆontology. zoology. microscopy. ichthyology. art. manufactures. steam. agriculture. commerce. physics. science. anatomy. navigation. botany. the proceedings of the day were opened with prayer by rev. geo. w. bethune, d.d., of brooklyn. hon. garrit y. lansing, of albany, then introduced professor louis agassiz, of cambridge, mass., who was the first of the "seven men of science" to entertain his audience, always with the aid of the inevitable black-board, without which the excellent professor would be as much at a loss as a chemist without a laboratory. professor agassiz spoke for an hour, giving his views of a new theory of animal development. he began by saying:-- we are here to inaugurate the geological hall, which has grown out of the geological survey of the state. to make the occasion memorable, a distinguished statesman of your own state, and mr. frank c. gray, were expected to be present and address you. the pressure of public duties has detained mr. seward, and severe sickness has detained mr. gray. i deeply lament that the occasion is lost to you to hear my friend mr. gray, who is a devotee to science, and as warm-hearted a friend as ever i knew. night before last i was requested to assist in taking their place--i, who am the most unfit of men for the post. i never made a speech. i have addressed learned bodies, but i lack that liberty of speech--the ability to present in finished style, and with that rich imagery which characterize the words of the orator, the thoughts fitting to such an occasion as this. he would limit himself, he continued, to presenting some motives why the community should patronize science, and foster such institutions as this. we scientific men regard this as an occasion of the highest interest, and thus do not hesitate to give the sanction of the highest learned body of the country as an indorsement of the liberality of this state. the geological survey of new york has given to the world a new nomenclature. no geologist can, hereafter, describe the several strata of the earth without referring to it. its results, as recorded in your published volumes, are treasured in the most valuable libraries of the world. they have made this city famous; and now, when the scientific geologist lands on your shore, his first question is, "which is the way to albany? i want to see your fossils." but paleontology is only one branch of the subject, and many others your survey has equally fostered. he next proceeded to show that organized beings were organized with reference to a plan, which the relations between different animals, and between different plants, and between animals and plants, everywhere exhibit;--drew sections of the body of a fish, and of the bird, and of man, and pointed out that in each there was the same central back-bone, the cavity above and the ribbed cavity below the flesh on each side, and the skin over all--showing that the maker of each possessed the same thought--followed the same plan of structure. and upon that plan he had made all the kinds of quadrupeds, , in number, all the kinds of birds, , in number, all of the reptiles, , to , in number, all the fish, , to , in number. all their forms may be derived as different expressions of the same formula. there are only four of these great types; or, said he, may i not call them the four tunes on which divinity has played the harmonies that have peopled, in living and beautiful reality, the whole world? professor hitchcock on reminiscences. erastus c. benedict, esq. of new york, introduced prof. hitchcock, of amherst, as a gentleman whose name was very familiar, who had laid aside, voluntarily, the charge of one of the largest colleges in new england, but who could never lay aside the honors he had earned in the literature and science of geology. after a few introductory observations, prof. hitchcock said:-- this, i believe, is the first example in which a state government in our country has erected a museum for the exhibition of its natural resources, its mineral and rock, its plants and animals, living and fossil. and this seems to me the most appropriate spot in the country for placing the first geological hall erected by the government; for the county of albany was the district where the first geological survey was undertaken, on this side of the atlantic, and, perhaps, the world. this was in , and ordered by that eminent philanthropist, stephen van rensselaer, who, three years later, appointed prof. eaton to survey, in like manner, the whole region traversed by the erie canal. this was the commencement of a work, which, during the last thirty years, has had a wonderful expansion, reaching a large part of the states of the union, as well as canada, nova scotia, and new brunswick, and, i might add, several european countries, where the magnificent surveys now in progress did not commence till after the survey of albany and rensselaer counties. how glad are we, therefore, to find on this spot the first museum of economical geology on this side of the atlantic! nay, embracing as it does all the department of natural history, i see in it more than a european museum of economical geology, splendid though they are. i fancy, rather, that i see here the germ of a cis-atlantic british museum, or garden of plants. north carolina was the first state that ordered a geological survey; and i have the pleasure of seeing before me the gentleman who executed it, and in - published a report of pages. i refer to professor olmstead, who, though he has since won brighter laurels in another department of science, will always be honored as the first commissioned state geologist in our land. of the new york state survey he said:-- this survey has developed the older fossiliferous rocks, with a fullness and distinctness unknown elsewhere. hence european savans study the new york reports with eagerness. in , as i entered the woodwardian museum, in the university of cambridge, in england, i found professor mccoy busy with a collection of silurian fossils before him, which he was studying with hall's first volume of paleontology as his guide; and in the splendid volumes, entitled _british paleozoric rocks and fossils_, which appeared last year as the result of those researches, i find professor hall denominated the great american paleontologist. i tell you, sir, that this survey has given new york a reputation throughout the learned world, of which she may well be proud. am i told that it will, probably, cost half a million? very well. the larger the sum, the higher will be the reputation of new york for liberality; and what other half million expended in our country, has developed so many new facts or thrown so much light upon the history of the globe, or won so world-wide and enviable a reputation? and of geological surveys in general:-- in regard to this matter of geological surveys, i can hardly avoid making a suggestion here. so large a portion of our country has now been examined, more or less thoroughly, by the several state governments, that it does seem to me the time has come when the national government should order a survey--geological, zoological, and botanical--of the whole country, on such a liberal and thorough plan as the surveys in great britain are now conducted; in the latter country it being understood that at least thirty years will be occupied in the work. could not the distinguished new york statesman who was to have addressed us to-day be induced, when the present great struggle in which he is engaged shall have been brought to a close, by a merciful providence, to introduce this subject, and urge it upon congress? and would it not be appropriate for the american association for the advancement of science to throw a petition before the government for such an object? or might it not, with the consent of the eminent gentleman who has charge of the coast survey, be connected therewith, as it is with the ordnance survey in great britain. the history of the american association was then given:-- prof. mather, i believe, through prof. emmons, first suggested to the new-york board of geologists in november, , in a letter proposing a number of points for their consideration. i quote from him the following paragraph relating to the meeting. as to the credit he has here given me of having personally suggested the subject, i can say only that i had been in the habit for several years of making this meeting of scientific men a sort of hobby in my correspondence with such. whether others did the same, i did not then, and do not now know. were this the proper place, i could go more into detail on this point; but i will merely quote prof. mather's language to the board:-- * * * * "would it not be well to suggest the propriety of a meeting of geologists and other scientific men of our country at some central point next fall,--say at new-york or philadelphia? there are many questions in our geology that will receive new light from friendly discussion and the combined observations of various individuals who have noted them in different parts of our country. such a meeting has been suggested by prof. hitchcock; and to me it seems desirable. it would undoubtedly be an advantage not only to science but to the several surveys that are now in progress and that may in future be authorized. it would tend to make known our scientific men to each other personally, give them more confidence in each other, and cause them to concentrate their observation on those questions that are of interest in either a scientific or economical point of view. more questions may be satisfactorily settled in a day by oral discussion in such a body, than a year by writing and publication."[a] [footnote a: in the letter alluded to, on examination, we discover another passage bearing on the point, which, owing to the professor's modesty we suspect, he did not read. prof. mather adds. "you, so far as i know, first suggested the matter of such an association. i laid the matter before the board of geologists of new-york, specifying some of the advantages that might be expected to result; and prof. vanuxem probably made the motion before the board in regard to it."] though the board adopted the plan of a meeting, various causes delayed the first over till april, , when we assembled in philadelphia, and spent a week in most profitable and pleasant discussion, and the presentation of papers. our number that year was only , because confined almost exclusively to the state geologists; but the next year, when we met again in philadelphia, and a more extended invitation was given, about eighty were present; and the members have been increasing to the present time. but, in fact, those first two meetings proved the type, in all things essential, of all that have followed. the principal changes have been those of expansion and the consequent introduction of many other branches of science with their eminent cultivators. in , we changed the name to that of the association of american geologists and naturalists; and in , to that of the american association for the advancement of science. i trust it has not yet reached its fullest development, as our country and its scientific men multiply, and new fields of discovery open. prof. h. said of this particular occasion:-- we may be quite sure that this hall will be a center of deep interest to coming generations. long after we shall have passed away will the men of new-york, as they survey these monuments, feel stimulated to engage in other noble enterprises by this work of their progenitors, and from many a distant part of the civilized world will men come here to solve their scientific questions, and to bring far-off regions into comparison with this. new-york, then, by her liberal patronage, has not only acquired an honorable name among those living in all civilized lands, but has secured the voice of history to transmit her fame to far-off generations. sir william logan asks "the way to albany." sir william e. logan, of canada, in a brief speech acknowledged the services rendered by the new-york survey to canada. he should manifest ingratitude if he declined to unite in the joyful occasion of inaugurating the museum which was to hold forever the evidence of the truth of its published results. the survey of canada had been ordered, and the commission of five years twice renewed; and the last time, the provision for it was more than doubled. it happened to him, as mr. agassiz had said: after crossing the ocean first, the first thing he asked was, "which is the way to albany?" and when he arrived here, he found that with the aid of prof. hall's discoveries, he had only to take up the different formations as he had left them on the boundary line, and follow them into canada. it was both a convenience and a necessity to adopt the new-york nomenclature, which was thus extended over an area six times as large as new-york. in paris he heard de vernier using the words trenton and niagara, as if they were household words. he was delighted to witness the impatience with which barron inquired when the remaining volumes of the paleontology of new-york would be published. your paleontological reputation, said he, has made new-york known, even among men not scientific, all over europe. i hope you will not stop here, but will go on and give us in equally thorough, full, and magnificent style, the character of the durassic and cretaceous formations. professor henry on dutchmen. professor henry was at a loss to know by what process they had arrived at the conclusion that seven men of science must be substituted to fill the place of one distinguished statesman whom they had expected to hear. he prided himself on his albany nativity. he was proud of the old dutch character, that was the substratum of the city. the dutch are hard to be moved, but when they do start their momentum is not as other men's in proportion to the velocity, but as the square of the velocity. so when the dutchman goes three times as fast, he has nine times the force of another man. the dutchman has an immense potentia agency, but it wants a small spark of yankee enterprise to touch it off. in this strain the professor continued, making his audience very merry, and giving them a fine chance to express themselves with repeated explosions of laughter. professor davies on the practical nature of science. prof. charles davies was introduced by ex-governor seymour, and spoke briefly, but humorously and very much to the point, in defense of the practical character of scientific researches. he said that to one accustomed to speak only on the abstract quantities of number and space, this was an unusual occasion, and this an unusual audience; and inquired how he could discuss the abstract forms of geometry, when he saw before him, in such profusion, the most beautiful real forms that providence has vouchsafed to the life of man. he proposed to introduce and develop but a single train of thought--the unchangeable connection between what in common language is called the theoretical and practical, but in more technical phraseology, the ideal and the actual. the actual, or true practical, consists in the uses of the forces of nature, according to the laws of nature; and here we must distinguish between it and the empirical, which uses, or attempts to use, those forces, without a knowledge of the laws. the true practical, therefore, is the result, or actual, of an antecedent ideal. the ideal, full and complete, must exist in the mind before the actual can be brought forth according to the laws of science. who, then, are the truly practical men of our age? are they not those who are engaged most laboriously and successfully in investigating the great laws? are they not those who are pressing out the boundaries of knowledge, and conducting the mind into new and unexplored regions, where there may yet be discovered a california of undeveloped thought? is not the gentleman from massachusetts (professor agassiz) the most practical man in our country in the department of natural history, not because he has collected the greatest number of specimens, but because he has laid open to us all the laws of the animal kingdom? are the formulas written on the black-board by the gentleman from cambridge (prof. pierce) of no practical value, because they cannot be read by the uninstructed eye? a single line may contain the elements of the motions of all the heavenly bodies; and the eye of science, taking its stand-point at the center of gravity of the system, will see in the equation the harmonious revolutions of all the bodies which circle the heavens. it is such labors and such generalizations that have rendered his name illustrious in the history of mathematical science. is it of no practical value that the chief of the coast survey (prof. bache), by a few characters written upon paper, at washington, has determined the exact time of high and low tide in the harbor of boston, and can determine, by a similar process, the exact times of high and low water at every point on the surface of the globe? are not these results, the highest efforts of science, also of the greatest practical utility? and may we not, then, conclude that _there is nothing truly practical which is not the consequence of an antecedent ideal_? science is to art what the great fly-wheel and governor of a steam-engine are to the working part of the machinery--it guides, regulates, and controls the whole. science and art are inseparably connected; like the siamese twins, they cannot be separated without producing the death of both. how, then, are we to regard the superb specimens of natural history, which the liberality, the munificence; and the wisdom of our state have collected at the capitol? they are the elements from which we can here determine all that belongs to the natural history of our state; and may we not indulge the hope, that science and genius will come here, and, striking them with a magic wand, cause the true practical to spring into immortal life? remarks were also uttered by prof. chester dewey, president anderson, and rev. dr. cox. and thus ended the inauguration of the state geological hall. we turn to the observatory, in regular order of succession. inauguration of dudley observatory. the inauguration of the dudley observatory took place under the same tent which was appropriated to the dedication of the geological hall, and on the day following that event. an immense audience was assembled, drawn by the announcement of mr. everett's oration. at a little past three o'clock the procession of _savans_ arrived from the assembly chamber, escorted by the burgesses corps. directly in front of the speaker's stand sat mrs. dudley, the venerable lady to whose munificence the world is indebted for this observatory. she was dressed in an antique, olive-colored silk, with a figure of a lighter color, a heavy, red broché shawl, and her bonnet, cap, &c., after the strictest style of the old school. her presence added a new point of interest. prayer having been uttered by rev. dr. sprague, of albany, thomas w. olcott, esq., introduced to the audience ex-governor washington hunt, who spoke briefly in honor of the memory of charles e. dudley, whose widow has founded and in part endowed this observatory with a liberality so remarkable. remarks were offered by dr. b. a. gould and prof. a. d. bache, and judge harris read the following letter from mrs. dudley, announcing another munificent donation in aid of the new observatory--$ , , in addition to the $ , which had been already expended in the construction of the building. the letter was received with shouts of applause, prof. agassiz rising and leading the vast assemblage in three vehement cheers in honor of mrs. dudley! albany, thursday, aug. , . _to the trustees of the dudley observatory:_ gentlemen,--i scarcely need refer in a letter to you to the modest beginning and gradual growth of the institution over which you preside, and of which you are the responsible guardians. but we have arrived at a period in its history when its inauguration gives to it and to you some degree of prominence, and which must stamp our past efforts with weakness and inconsideration, or exalt those of the future to the measure of liberality necessary to certain success. you have a building erected and instruments engaged of unrivaled excellence; and it now remains to carry out the suggestion of the astronomer royal of england in giving permanency to the establishment. the very distinguished professors bache, pierce, and gould, state in a letter, which i have been permitted to see, that to expand this institution to the wants of american science and the honors of a national character, will require an investment which will yield annually not less than $ , ; and these gentlemen say, in the letter referred to,-- "if the greatness of your giving can rise to this occasion, as it has to all our previous suggestions, with such unflinching magnanimity, we promise you our earnest and hearty coöperation, and stake our reputation that the scientific success shall fill up the measure of your hopes and anticipations." for the attainment of an object so rich in scientific reward and national glory, guaranteed by men with reputations as exalted and enduring as the skies upon which they are written, contributions should be general, and not confined to an individual or a place. for myself, i offer, as my part of the required endowment, the sum of $ , in addition to the advances which i have already made; and, trusting that the name which you have given to the observatory may not be regarded as an undeserved compliment, and that it will not diminish the public regard by giving to the institution a seemingly individual character, i remain, gentlemen, your obedient servant, blandina dudley. judge harris then introduced the orator of the occasion, hon. edward everett, whose speech is given verbatim in these pages. the instruments of the dudley observatory. during the sessions of the american association, the new astronomical instruments of dudley observatory were described in detail by dr. b. a. gould, who is the astronomer in charge. we condense his statements:-- the meridian circle and transit instrument were ordered from pistor & martins, the celebrated manufacturers of berlin, by whom the new instrument at ann arbor was made. a number of improvements have been introduced in the albany instruments, not perhaps all absolutely new, but an eclectic combination of late adaptations with new improvements. dr. gould made a distinction of modern astronomical instruments into two classes, the english and the german. the english is the massive type; the german, light and airy. the english instrument is the instrument of the engineer; the german, the instrument of the artist. in ordering the instruments for the albany observatory, the doctor preferred the german type and discarded the heavier english. he instanced, as a specimen of the latter, the new instrument at greenwich, recently erected under the superintendence of the astronomer royal. that instrument registers observations in single seconds; the dudley instrument will register to tenths of seconds. that has six or eight microscopes; this has four. that has a gas lamp, by the light of which the graduations are read off; the albany instrument has no lamp, and the doctor considered the lamp a hazardous experiment, affecting the integrity of the experiment, not only by its radiant heat but by the currents of heated air which it produces. the diameter of the object-glass of the albany instrument is - / french inches clear aperture, or english inches, and the length of the tube feet. he would have preferred an instrument in which the facilities of manipulation would have been greater, but was hampered by one proviso, upon which the trustees of the institution insisted--that this should be the biggest instrument of its kind; and the instruction was obeyed. the glass was made by chance, and ground by pistor himself. the eye-piece is fitted with two micrometers, for vertical and horizontal observations. another apparatus provides for the detection and measurement of the flexure of the tube. much trouble was experienced in securing a good casting for the steel axis of the instrument. three were found imperfect under the lathe, and the fourth was chosen; but even then the pivots were made in separate pieces, which were set in very deeply and welded. dr. gould said he had been requested by the gentlemen who had this enterprise in charge to suggest, as a mark of respect to a gentleman of albany who was a munificent patron of science, that this instrument be known as the olcott meridian circle. what the dudley observatory is. it stands a mile from the capitol, in the city of albany, upon the crest of a hill, so difficult of approach, as to be in reality a hill of science. there are two ways of getting to it. in both cases there are rail fences to be clambered over, and long grass to wade through, settlements to explore, and a clayey road to travel; but these are minor troubles. the elevation of the hill above tide-water is, perhaps, feet; its distance from the capitol about a mile and a half. the view for miles is unimpeded; and the observatory is belted about with woods and verdant lawns. there could not be a finer location or a purer air. the plateau contains some fifteen acres. the observatory is constructed in the form of a latin cross. its eastern arm is an apartment by feet, in which the meridian circle is to be placed. the western arm is a room of the same dimensions, intended for the transit instrument. from the north and south faces of both rooms are semi-circular apsides, projecting feet inches, containing the collimator piers and the vertical openings for observation. the entire length of each room is, therefore, feet. in the northern arm are placed the library, feet by feet; two computing rooms, feet by feet each; side entrance halls, staircases, &c. the southern arm contains the principal entrance, consisting of an arched colonnade of four tuscan columns, surrounded by a pediment. a broad flight of stone steps leads to this colonnade; and through the entrance door beneath it to the main central hall, feet square, in which are placed (in niches) the very beautiful electric clock and pendulum presented by erastus corning, esq. the center of this hall is occupied by a massive pier of stone, feet square, passing from the basement into the dome above, and intended for the support of the great heliometer. directly opposite the entrance door is a large niche, in which it is proposed to place the bust of the late mr. dudley. immediately above this hall is the equatorial room, a circular apartment, feet inches in diameter, and feet high, covered by a low conical roof, in which and in the walls are the usual observing slits. the drum, or cylindrical portion, of this room is divided into two parts--the lower one fixed, the upper, revolving on cast-iron balls moving in grooved metal plates, can command the entire horizon. the building is in two stories--the upper of brick, with freestone quoins, impost and window and door dressings, rests upon a rusticated basement of freestone, six feet high. the style adopted is the modern italian, of which it is a very excellent specimen. the building has been completed some time; but, in consequence of the size of the instruments now procured being greater than that originally contemplated, sundry alterations were required in the transit and meridian circle rooms. these consist of the semi-circular projections already mentioned, and which, by varying the outlines of the building, will add greatly to its beauty and picturesqueness. the piers for the meridian circle and transit have, after careful investigation, been procured from the lockport quarries. the great density and uniformity of the structure of the stone, and the facility with which such large masses as are required for this purpose can be procured there, have induced the selection of these quarries. the stones will weigh from six and a half to eight tons each. the main building was erected from the drawings of messrs. woollett and ogden, architects, albany; the additions and the machinery have been designed by mr. w. hodgins, civil engineer; and the latter is now being constructed under his superintendence, in a very superior manner, at the iron works of messrs. pruyn and lansing, albany. the entire building is a tasteful and elegant structure, much superior in architectural character to any other in america devoted to a similar purpose. oration. fellow citizens of albany:-- assembled as we are, under your auspices, in this ancient and hospitable city, for an object indicative of a highly-advanced stage of scientific culture, it is natural, in the first place, to cast a historical glance at the past. it seems almost to surpass belief, though an unquestioned fact, that more than a century should have passed away, after cabot had discovered the coast of north america for england, before any knowledge was gained of the noble river on which your city stands, and which was destined by providence to determine, in after times, the position of the commercial metropolis of the continent. it is true that verazzano, a bold and sagacious florentine navigator, in the service of france, had entered the narrows in , which he describes as a very large river, deep at its mouth, which forced its way through steep hills to the sea; but though he, like all the naval adventurers of that age, was sailing westward in search of a shorter passage to india, he left this part of the coast without any attempt to ascend the river; nor can it be gathered from his narrative that he believed it to penetrate far into the interior. voyage of hendrick hudson. near a hundred years elapsed before that great thought acquired substance and form. in the spring of , the heroic but unfortunate hudson, one of the brightest names in the history of english maritime adventure, but then in the employment of the dutch east india company, in a vessel of eighty tons, bearing the very astronomical name of the _half moon_, having been stopped by the ice in the polar sea, in the attempt to reach the east by the way of nova zembla, struck over to the coast of america in a high northern latitude. he then stretched down southwardly to the entrance of chesapeake bay (of which he had gained a knowledge from the charts and descriptions of his friend, captain smith), thence returning to the north, entered delaware bay, standing out again to sea, arrived on the second of september in sight of the "high hills" of neversink, pronouncing it "a good land to fall in with, and a pleasant land to see;" and, on the following morning, sending his boat before him to sound the way, passed sandy hook, and there came to anchor on the third of september, ; two hundred and forty-seven years ago next wednesday. what an event, my friends, in the history of american population, enterprise, commerce, intelligence, and power--the dropping of that anchor at sandy hook! discovery of the hudson river. here he lingered a week, in friendly intercourse with the natives of new jersey, while a boat's company explored the waters up to newark bay. and now the great question. shall he turn back, like verazzano, or ascend the stream? hudson was of a race not prone to turn back, by sea or by land. on the eleventh of september he raised the anchor of the _half moon_, passed through the narrows, beholding on both sides "as beautiful a land as one can tread on;" and floated cautiously and slowly up the noble stream--the first ship that ever rested on its bosom. he passed the palisades, nature's dark basaltic malakoff, forced the iron gateway of the highlands, anchored, on the fourteenth, near west point; swept onward and upward, the following day, by grassy meadows and tangled slopes, hereafter to be covered with smiling villages;--by elevated banks and woody heights, the destined site of towns and cities--of newburg, poughkeepsie, catskill;--on the evening of the fifteenth arrived opposite "the mountains which lie from the river side," where he found "a very loving people and very old men;" and the day following sailed by the spot hereafter to be honored by his own illustrious name. one more day wafts him up between schodac and castleton; and here he landed and passed a day with the natives,--greeted with all sorts of barbarous hospitality,--the land "the finest for cultivation he ever set foot on," the natives so kind and gentle, that when they found he would not remain with them over night, and feared that he left them--poor children of nature!--because he was afraid of their weapons,--he, whose quarter-deck was heavy with ordnance,--they "broke their arrows in pieces, and threw them in the fire." on the following morning, with the early flood-tide, on the th of september, , the _half moon_ "ran higher up, two leagues above the shoals," and came to anchor in deep water, near the site of the present city of albany. happy if he could have closed his gallant career on the banks of the stream which so justly bears his name, and thus have escaped the sorrowful and mysterious catastrophe which awaited him the next year! champlain's voyage and the growth of colonies. but the discovery of your great river and of the site of your ancient city, is not the only event which renders the year memorable in the annals of america and the world. it was one of those years in which a sort of sympathetic movement toward great results unconsciously pervades the races and the minds of men. while hudson discovered this mighty river and this vast region for the dutch east india company, champlain, in the same year, carried the lilies of france to the beautiful lake which bears his name on your northern limits; the languishing establishments of england in virginia were strengthened by the second charter granted to that colony; the little church of robinson removed from amsterdam to leyden, from which, in a few years, they went forth, to lay the foundations of new england on plymouth rock; the seven united provinces of the netherlands, after that terrific struggle of forty years (the commencement of which has just been embalmed in a record worthy of the great event by an american historian) wrested from spain the virtual acknowledgment of their independence, in the twelve years' truce; and james the first, in the same year, granted to the british east india company their first permanent charter,--corner-stone of an empire destined in two centuries to overshadow the east. galileo's discoveries one more incident is wanting to complete the list of the memorable occurrences which signalize the year , and one most worthy to be remembered by us on this occasion. cotemporaneously with the events which i have enumerated--eras of history, dates of empire, the starting-point in some of the greatest political, social, and moral revolutions in our annals, an italian astronomer, who had heard of the magnifying glasses which had been made in holland, by which distant objects could be brought seemingly near, caught at the idea, constructed a telescope, and pointed it to the heavens. yes, my friends, in the same year in which hudson discovered your river and the site of your ancient town, in which robinson made his melancholy hegira from amsterdam to leyden, galileo galilei, with a telescope, the work of his own hands, discovered the phases of venus and the satellites of jupiter; and now, after the lapse of less than two centuries and a half, on a spot then embosomed in the wilderness--the covert of the least civilized of all the races of men--we are assembled--descendants of the hollanders, descendants of the pilgrims, in this ancient and prosperous city, to inaugurate the establishment of a first-class astronomical observatory. early days of albany. one more glance at your early history. three years after the landing of the pilgrims at plymouth, fort orange was erected, in the center of what is now the business part of the city of albany; and, a few years later, the little hamlet of beverswyck began to nestle under its walls. two centuries ago, my albanian friends, this very year, and i believe this very month of august, your forefathers assembled, not to inaugurate an observatory, but to lay the foundations of a new church, in the place of the rude cabin which had hitherto served them in that capacity. it was built at the intersection of yonker's and handelaar's, better known to you as state and market streets. public and private liberality coöperated in the important work. the authorities at the fort gave fifteen hundred guilders; the patroon of that early day, with the liberality coëval with the name and the race, contributed a thousand; while the inhabitants, for whose benefit it was erected, whose numbers were small and their resources smaller, contributed twenty beavers "for the purchase of an oaken pulpit in holland." whether the largest part of this subscription was bestowed by some liberal benefactress, tradition has not informed us. new amsterdam nor is the year memorable in the annals of albany alone. in that same year your imperial metropolis, then numbering about three hundred inhabitants, was first laid out as a city, by the name of new amsterdam.[a] in eight years more, new netherland becomes new york; fort orange and its dependent hamlet assumes the name of albany. a century of various fortune succeeds; the scourge of french and indian war is rarely absent from the land; every shock of european policy vibrates with electric rapidity across the atlantic; but the year finds a population of , in your growing province. albany, however, may still be regarded almost as a frontier settlement. of the twelve counties into which the province was divided a hundred years ago, the county of albany comprehended all that lay north and west of the city; and the city itself contained but about three hundred and fifty houses. [footnote a: these historical notices are, for the most part, abridged from mr. brodhead's excellent history of new york.] two hundred years. one more century; another act in the great drama of empire; another french and indian war beneath the banners of england; a successful revolution, of which some of the most momentous events occurred within your limits; a union of states; a constitution of federal government; your population carried to the st. lawrence and the great lakes, and their waters poured into the hudson; your territory covered with a net-work of canals and railroads, filled with life and action, and power, with all the works of peaceful art and prosperous enterprise with all the institutions which constitute and advance the civilization of the age; its population exceeding that of the union at the date of the revolution; your own numbers twice as large as those of the largest city of that day, you have met together, my friends, just two hundred years since the erection of the little church of beverswyck, to dedicate a noble temple of science and to take a becoming public notice of the establishment of an institution, destined, as we trust, to exert a beneficial influence on the progress of useful knowledge at home and abroad, and through that on the general cause of civilization. scientific progress. you will observe that i am careful to say the progress of science "at home and abroad;" for the study of astronomy in this country has long since, i am happy to add, passed that point where it is content to repeat the observations and verify the results of european research. it has boldly and successfully entered the field of original investigation, discovery, and speculation; and there is not now a single department of the science in which the names of american observers and mathematicians are not cited by our brethren across the water, side by side with the most eminent of their european contemporaries. this state of things is certainly recent. during the colonial period and in the first generation after the revolution, no department of science was, for obvious causes, very extensively cultivated in america--astronomy perhaps as much as the kindred branches. the improvement in the quadrant, commonly known as hadley's, had already been made at philadelphia by godfrey, in the early part of the last century; and the beautiful invention of the collimating telescope was made at a later period by rittenhouse, an astronomer of distinguished repute. the transits of venus of and were observed, and orreries were constructed in different parts of the country; and some respectable scientific essays are contained and valuable observations are recorded in the early volumes of the transactions of the philosophical society, at philadelphia, and the american academy of arts and sciences at boston and cambridge. but in the absence of a numerous class of men of science to encourage and aid each other, without observatories and without valuable instruments, little of importance could be expected in the higher walks of astronomical life. american observations. the greater the credit due for the achievement of an enterprise commenced in the early part of the present century, and which would reflect honor on the science of any country and any age; i mean the translation and commentary on laplace's _mécanique celeste_, by bowditch; a work of whose merit i am myself wholly unable to form an opinion, but which i suppose places the learned translator and commentator on a level with the ablest astronomers and geometers of the day. this work may be considered as opening a new era in the history of american science. the country was still almost wholly deficient in instrumental power; but the want was generally felt by men of science, and the public mind in various parts of the country began to be turned towards the means of supplying it. in , president john quincy adams brought the subject of a national observatory before congress. political considerations prevented its being favorably entertained at that time; and it was not till , and as an incident of the exploring expedition, that an appropriation was made for a dépôt for the charts and instruments of the navy. on this modest basis has been reared the national observatory at washington; an institution which has already taken and fully sustains an honorable position among the scientific establishments of the age. besides the institution at washington, fifteen or twenty observatories have within the last few years, been established in different parts of the country, some of them on a modest scale, for the gratification of the scientific taste and zeal of individuals, others on a broad foundation of expense and usefulness. in these establishments, public and private, the means are provided for the highest order of astronomical observation, research, and instruction. there is already in the country an amount of instrumental power (to which addition is constantly making), and of mathematical skill on the part of our men of science, adequate to a manly competition with their european contemporaries. the fruits are already before the world, in the triangulation of several of the states, in the great work of the coast survey, in the numerous scientific surveys of the interior of the continent, in the astronomical department of the exploring expedition, in the scientific expedition to chili, in the brilliant hydrographical labors of the observatory at washington, in the published observations of washington and cambridge, in the journal conducted by the nestor of american science, now in its eighth lustrum; in the _sidereal messenger_, the _astronomical journal_, and the _national ephemeris_; in the great chronometrical expeditions to determine the longitude of cambridge, better ascertained than that of paris was till within the last year; in the prompt rectification of the errors in the predicted elements of neptune; in its identification with lalande's missing star, and in the calculation of its ephemeris; in the discovery of the satellite of neptune, of the eighth satellite of saturn, and of the innermost of its rings; in the establishment, both by observation and theory, of the non-solid character of saturn's rings; in the separation and measurement of many double and triple stars, amenable only to superior instrumental power, in the immense labor already performed in preparing star catalogues, and in numerous accurate observations of standard stars; in the diligent and successful observation of the meteoric showers; in an extensive series of magnetic observations; in the discovery of an asteroid and ten or twelve telescopic comets; in the resolution of nebulæ which had defied every thing in europe but lord rosse's great reflector; in the application of electricity to the measurement of differences in longitude; in the ascertainment of the velocity of the electro-magnetic fluid, and its truly wonderful uses in recording astronomical observations. these are but a portion of the achievements of american astronomical science within fifteen or twenty years, and fully justify the most sanguine anticipations of its further progress. how far our astronomers may be able to pursue their researches, will depend upon the resources of our public institutions, and the liberality of wealthy individuals in furnishing the requisite means. with the exception of the observatories at washington and west point, little can be done, or be expected to be done, by the government of the union or the states; but in this, as in every other department of liberal art and science, the great dependence,--and may i not add, the safe dependence?--as it ever has been, must continue to be upon the bounty of enlightened, liberal, and public-spirited individuals. the dudley observatory. it is by a signal exercise of this bounty, my friends, that we are called together to-day. the munificence of several citizens of this ancient city, among whom the first place is due to the generous lady whose name has with great propriety been given to the institution, has furnished the means for the foundation of the dudley observatory at albany. on a commanding elevation on the northern edge of the city, liberally given for that purpose by the head of a family in which the patronage of science is hereditary, a building of ample dimensions has been erected, upon a plan which combines all the requisites of solidity, convenience, and taste. a large portion of the expense of the structure has been defrayed by mrs. blandina dudley; to whose generosity, and that of several other public-spirited individuals, the institution is also indebted for the provision which has been made for an adequate supply of first-class instruments, to be executed by the most eminent makers in europe and america; and which, it is confidently expected, will yield to none of their class in any observatory in the world.[a] [footnote a: prof. loomis, in _harper's magazine_ for june, p. .] with a liberal supply of instrumental power; established in a community to whose intelligence and generosity its support may be safely confided, and whose educational institutions are rapidly realizing the conception of a university; countenanced by the gentleman who conducts the united states coast survey with such scientific skill and administrative energy; committed to the immediate supervision of an astronomer to whose distinguished talent had been added the advantage of a thorough scientific education in the most renowned universities of europe, and who, as the editor of the _american astronomical journal_, has shown himself to be fully qualified for the high trust;--under these favorable circumstances, the dudley observatory at albany takes its place among the scientific foundations of the country and the world. wonders of astronomy. it is no affected modesty which leads me to express the regret that this interesting occasion could not have taken place under somewhat different auspices. i feel that the duty of addressing this great and enlightened assembly, comprising so much of the intelligence of the community and of the science of the country, ought to have been elsewhere assigned; that it should have devolved upon some one of the eminent persons, many of whom i see before me, to whom you have been listening the past week, who, as observers and geometers, could have treated the subject with a master's power; astronomers, whose telescopes have penetrated the depths of the heavens, or mathematicians, whose analysis unthreads the maze of their wondrous mechanism. if, instead of commanding, as you easily could have done, qualifications of this kind, your choice has rather fallen on one making no pretensions to the honorable name of a man of science,--but whose delight it has always been to turn aside from the dusty paths of active life, for an interval of recreation in the green fields of sacred nature in all her kingdoms,--it is, i presume, because you have desired on an occasion of this kind, necessarily of a popular character, that those views of the subject should be presented which address themselves to the general intelligence of the community, and not to its select scientific circles. there is, perhaps, no branch of science which to the same extent as astronomy exhibits phenomena which, while they task the highest powers of philosophical research, are also well adapted to arrest the attention of minds barely tinctured with scientific culture, and even to teach the sensibilities of the wholly uninstructed observer. the profound investigations of the chemist into the ultimate constitution of material nature, the minute researches of the physiologist into the secrets of animal life, the transcendental logic of the geometer, clothed in a notation, the very sight of which terrifies the uninitiated,--are lost on the common understanding. but the unspeakable glories of the rising and the setting sun; the serene majesty of the moon, as she walks in full-orbed brightness through the heavens; the soft witchery of the morning and the evening star; the imperial splendors of the firmament on a bright, unclouded night; the comet, whose streaming banner floats over half the sky,--these are objects which charm and astonish alike the philosopher and the peasant, the mathematician who weighs the masses and defines the orbits of the heavenly bodies, and the untutored observer who sees nothing beyond the images painted upon the eye. what is an astronomical observatory? an astronomical observatory, in the general acceptation of the word, is a building erected for the reception and appropriate use of astronomical instruments, and the accommodation of the men of science employed in making and reducing observations of the heavenly bodies. these instruments are mainly of three classes, to which i believe all others of a strictly astronomical character may be referred. . the instruments by which the heavens are inspected, with a view to discover the existence of those celestial bodies which are not visible to the naked eye (beyond all comparison more numerous than those which are), and the magnitude, shapes, and other sensible qualities, both of those which are and those which are not thus visible to the unaided sight. the instruments of this class are designated by the general name of telescope, and are of two kinds,--the refracting telescope, which derives its magnifying power from a system of convex lenses; and the reflecting telescope, which receives the image of the heavenly body upon a concave mirror. d. the second class of instruments consists of those which are designed principally to measure the angular distances of the heavenly bodies from each other, and their time of passing the meridian. the transit instrument, the meridian circle, the mural circle, the heliometer, and the sextant, belong to this class. the brilliant discoveries of astronomy are, for the most part, made with the first class of instruments; its practical results wrought out by the second. d. the third class contains the clock, with its subsidiary apparatus, for measuring the time and making its subdivisions with the greatest possible accuracy; indispensable auxiliary of all the instruments, by which the positions and motions of the heavenly bodies are observed, and measured, and recorded. the telescope. the telescope may be likened to a wondrous cyclopean eye, endued with superhuman power, by which the astronomer extends the reach of his vision to the further heavens, and surveys galaxies and universes compared with which the solar system is but an atom floating in the air. the transit may be compared to the measuring rod which he lays from planet to planet, and from star to star, to ascertain and mark off the heavenly spaces, and transfer them to his note-book; the clock is that marvelous apparatus by which he equalizes and divides into nicely measured parts a portion of that unconceived infinity of duration, without beginning and without end, in which all existence floats as on a shoreless and bottomless sea. in the contrivance and the execution of these instruments, the utmost stretch of inventive skill and mechanical ingenuity has been put forth. to such perfection have they been carried, that a single second of magnitude or space is rendered a distinctly visible and appreciable quantity. "the arc of a circle," says sir j. herschell, "subtended by one second, is less than the , th part of the radius, so that on a circle of six feet in diameter, it would occupy no greater linear extent than - part of an inch, a quantity requiring a powerful microscope to be discerned at all."[a] the largest body in our system, the sun, whose real diameter is , miles, subtends, at a distance of , , miles, but an angle of little more than ; while so admirably are the best instruments constructed, that both in europe and america a satellite of neptune, an object of comparatively inconsiderable diameter, has been discovered at a distance of , millions of miles. [footnote a: _outlines_, § .] utility of astronomical observations. the object of an observatory, erected and supplied with instruments of this admirable construction, and at proportionate expense, is, as i have already intimated, to provide for an accurate and systematic survey of the heavenly bodies, with a view to a more correct and extensive acquaintance with those already known, and as instrumental power and skill in using it increase, to the discovery of bodies hitherto invisible, and in both classes to the determination of their distances, their relations to each other, and the laws which govern their movements. why should we wish to obtain this knowledge? what inducement is there to expend large sums of money in the erection of observatories, and in furnishing them with costly instruments, and in the support of the men of science employed in making, discussing, and recording, for successive generations, those minute observations of the heavenly bodies? in an exclusively scientific treatment of this subject, an inquiry into its utilitarian relations would be superfluous--even wearisome. but on an occasion like the present, you will not, perhaps, think it out of place if i briefly answer the question, what is the use of an observatory, and what benefit may be expected from the operations of such an establishment in a community like ours? . in the first place, then, we derive from the observations of the heavenly bodies which are made at an observatory, our only adequate measures of time, and our only means of comparing the time of one place with the time of another. our artificial time-keepers--clocks, watches, and chronometers--however ingeniously contrived and admirably fabricated, are but a transcript, so to say, of the celestial motions, and would be of no value without the means of regulating them by observation. it is impossible for them, under any circumstances, to escape the imperfection of all machinery the work of human hands; and the moment we remove with our time-keeper east or west, it fails us. it will keep home time alone, like the fond traveler who leaves his heart behind him. the artificial instrument is of incalculable utility, but must itself be regulated by the eternal clock-work of the skies. relations between natural phenomena and daily life. this single consideration is sufficient to show how completely the daily business of life is affected and controlled by the heavenly bodies. it is they--and not our main-springs, our expansion balances, and our compensation pendulums--which give us our time. to reverse the line of pope: "'tis with our watches as our judgments;--none go just alike, but each believes his own." but for all the kindreds and tribes and tongues of men--each upon their own meridian--from the arctic pole to the equator, from the equator to the antarctic pole, the eternal sun strikes twelve at noon, and the glorious constellations, far up in the everlasting belfries of the skies, chime twelve at midnight;--twelve for the pale student over his flickering lamp; twelve amid the flaming glories of orion's belt, if he crosses the meridian at that fated hour; twelve by the weary couch of languishing humanity; twelve in the star-paved courts of the empyrean; twelve for the heaving tides of the ocean; twelve for the weary arm of labor; twelve for the toiling brain; twelve for the watching, waking, broken heart; twelve for the meteor which blazes for a moment and expires; twelve for the comet whose period is measured by centuries; twelve for every substantial, for every imaginary thing, which exists in the sense, the intellect, or the fancy, and which the speech or thought of man, at the given meridian, refers to the lapse of time. not only do we resort to the observation of the heavenly bodies for the means of regulating and rectifying our clocks, but the great divisions of day and month and year are derived from the same source. by the constitution of our nature, the elements of our existence are closely connected with celestial times. partly by his physical organization, partly by the experience of the race from the dawn of creation, man as he is, and the times and seasons of the heavenly bodies, are part and parcel of one system. the first great division of time, the day-night (nychthemerum), for which we have no precise synonym in our language, with its primal alternation of waking and sleeping, of labor and rest, is a vital condition of the existence of such a creature as man. the revolution of the year, with its various incidents of summer and winter, and seed-time and harvest, is not less involved in our social, material, and moral progress. it is true that at the poles, and on the equator, the effects of these revolutions are variously modified or wholly disappear; but as the necessary consequence, human life is extinguished at the poles, and on the equator attains only a languid or feverish development. those latitudes only in which the great motions and cardinal positions of the earth exert a mean influence, exhibit man in the harmonious expansion of his powers. the lunar period, which lies at the foundation of the _month_, is less vitally connected with human existence and development; but is proved by the experience of every age and race to be eminently conducive to the progress of civilization and culture. but indispensable as are these heavenly measures of time to our life and progress, and obvious as are the phenomena on which they rest, yet owing to the circumstance that, in the economy of nature, the day, the month, and the year are not exactly commensurable, some of the most difficult questions in practical astronomy are those by which an accurate division of time, applicable to the various uses of life, is derived from the observation of the heavenly bodies. i have no doubt that, to the supreme intelligence which created and rules the universe, there is a harmony hidden to us in the numerical relation to each other of days, months, and years; but in our ignorance of that harmony, their practical adjustment to each other is a work of difficulty. the great embarrassment which attended the reformation of the calendar, after the error of the julian period had, in the lapse of centuries, reached ten (or rather twelve) days, sufficiently illustrates this remark. it is most true that scientific difficulties did not form the chief obstacle. having been proposed under the auspices of the roman pontiff, the protestant world, for a century and more, rejected the new style. it was in various places the subject of controversy, collision, and bloodshed.[a] it was not adopted in england till nearly two centuries after its introduction at rome; and in the country of struve and the pulkova equatorial, they persist at the present day in adding eleven minutes and twelve seconds to the length of the tropical year. [footnote a: stern's "_himmelskunde_," p. .] geographical science. . the second great practical use of an astronomical observatory is connected with the science of geography. the first page of the history of our continent declares this truth. profound meditation on the sphericity of the earth was one of the main reasons which led columbus to undertake his momentous voyage; and his thorough acquaintance with the astronomical science of that day was, in his own judgment, what enabled him to overcome the almost innumerable obstacles which attended its prosecution.[a] in return, i find that copernicus in the very commencement of his immortal work _de revolutionibus orbium coelestium_, fol. , appeals to the discovery of america as completing the demonstration of the sphericity of the earth. much of our knowledge of the figure, size, density, and position of the earth, as a member of the solar system, is derived from this science; and it furnishes us the means of performing the most important operations of practical geography. latitude and longitude, which lie at the basis of all descriptive geography, are determined by observation. no map deserves the name, on which the position of important points has not been astronomically determined. some even of our most important political and administrative arrangements depend upon the coöperation of this science. among these i may mention the land system of the united states, and the determination of the boundaries of the country. i believe that till it was done by the federal government, a uniform system of mathematical survey had never in any country been applied to an extensive territory. large grants and sales of public land took place before the revolution, and in the interval between the peace and the adoption of the constitution; but the limits of these grants and sales were ascertained by sensible objects, by trees, streams, rocks, hills, and by reference to adjacent portions of territory, previously surveyed. the uncertainty of boundaries thus defined, was a never-failing source of litigation. large tracts of land in the western country, granted by virginia under this old system of special and local survey, were covered with conflicting claims; and the controversies to which they gave rise formed no small part of the business of the federal court after its organization. but the adoption of the present land-system brought order out of chaos. the entire public domain is now scientifically surveyed before it is offered for sale; it is laid off into ranges, townships, sections, and smaller divisions, with unerring accuracy, resting on the foundation of base and meridian lines; and i have been informed that under this system, scarce a case of contested location and boundary has ever presented itself in court. the general land office contains maps and plans, in which every quarter-section of the public land is laid down with mathematical precision. the superficies of half a continent is thus transferred in miniature to the bureaus of washington; while the local land offices contain transcripts of these plans, copies of which are furnished to the individual purchaser. when we consider the tide of population annually flowing into the public domain, and the immense importance of its efficient and economical administration, the utility of this application of astronomy will be duly estimated. [footnote a: humboldt, _histotre de la geographie_, &c., tom. , page .] i will here venture to repeat an anecdote, which i heard lately from a son of the late hon. timothy pickering. mr. octavius pickering, on behalf of his father, had applied to mr. david putnam of marietta, to act as his legal adviser, with respect to certain land claims in the virginia military district, in the state of ohio. mr. putnam declined the agency. he had had much to do with business of that kind, and found it beset with endless litigation. "i have never," he added, "succeeded but in a single case, and that was a location and survey made by general washington before the revolution; and i am not acquainted with any surveys, except those made by him, but what have been litigated." at this moment, a most important survey of the coast of the united states is in progress, an operation of the utmost consequence, in reference to the commerce, navigation, and hydrography of the country. the entire work, i need scarce say, is one of practical astronomy. the scientific establishment which we this day inaugurate is looked to for important coöperation in this great undertaking, and will no doubt contribute efficiently to its prosecution. astronomical observation furnishes by far the best means of defining the boundaries of states, especially when the lines are of great length and run through unsettled countries. natural indications, like rivers and mountains, however indistinct in appearance, are in practice subject to unavoidable error. by the treaty of , a boundary was established between the united states and great britain, depending chiefly on the course of rivers and highlands dividing the waters which flow into the atlantic ocean from those which flow into the st. lawrence. it took twenty years to find out which river was the true st. croix, that being the starting point. england then having made the extraordinary discovery that the bay of fundy is not a part of the atlantic ocean, forty years more were passed in the unsuccessful attempt to re-create the highlands which this strange theory had annihilated; and just as the two countries were on the verge of a war, the controversy was settled by compromise. had the boundary been accurately described by lines of latitude and longitude, no dispute could have arisen. no dispute arose as to the boundary between the united states and spain, and her successor, mexico, where it runs through untrodden deserts and over pathless mountains along the d degree of latitude. the identity of rivers may be disputed, as in the case of the st. croix; the course of mountain chains is too broad for a dividing line; the division of streams, as experience has shown, is uncertain; but a degree of latitude is written on the heavenly sphere, and nothing but an observation is required to read the record. questions of boundary. but scientific elements, like sharp instruments, must be handled with scientific accuracy. a part of our boundary between the british provinces ran upon the forty-fifth degree of latitude; and about forty years ago, an expensive fortress was commenced by the government of the united states, at rouse's point, on lake champlain, on a spot intended to be just within our limits. when a line came to be more carefully surveyed, the fortress turned out to be on the wrong side of the line; we had been building an expensive fortification for our neighbor. but in the general compromises of the treaty of washington by the webster and ashburton treaty in , the fortification was left within our limits.[a] [footnote a: webster's works. vol. v., , .] errors still more serious had nearly resulted, a few years since, in a war with mexico. by the treaty of guadalupe hidalgo, in , the boundary line between the united states and that country was in part described by reference to the town of el paso, as laid down on a specified map of the united states, of which a copy was appended to the treaty. this boundary was to be surveyed and run by a joint commission of men of science. it soon appeared that errors of two or three degrees existed in the projection of the map. its lines of latitude and longitude did not conform to the topography of the region; so that it became impossible to execute the text of the treaty. the famous mesilla valley was a part of the debatable ground; and the sum of $ , , , paid to the mexican government for that and for an additional strip of territory on the southwest, was the smart-money which expiated the inaccuracy of the map--the necessary result, perhaps, of the want of good materials for its construction. it became my official duty in london, a few years ago, to apply to the british government for an authentic statement of their claim to jurisdiction over new zealand. the official _gazette_ for the d of october, , was sent me from the foreign office, as affording the desired information. this number of the _gazette_ contained the proclamations issued by the lieutenant governor of new zealand, "in pursuance of the instructions he received from the marquis of normanby, one of her majesty's principal secretaries of state," asserting the jurisdiction of his government over the islands of new zealand, and declaring them to extend "from ° ' north to ° ' south latitude." it is scarcely necessary to say that south latitude was intended in both instances. this error of ° of latitude, which would have extended the claim of british jurisdiction over the whole breadth of the pacific, had, apparently, escaped the notice of that government. commerce and navigation. it would be easy to multiply illustrations in proof of the great practical importance of accurate scientific designations, drawn from astronomical observations, in various relations connected with boundaries, surveys, and other geographical purposes; but i must hasten to . a third important department, in which the services rendered by astronomy are equally conspicuous. i refer to commerce and navigation. it is mainly owing to the results of astronomical observation, that modern commerce has attained such a vast expansion, compared with that of the ancient world. i have already reminded you that accurate ideas in this respect contributed materially to the conception in the mind of columbus of his immortal enterprise, and to the practical success with which it was conducted. it was mainly his skill in the use of astronomical instruments--imperfect as they were--which enabled him, in spite of the bewildering variation of the compass, to find his way across the ocean. with the progress of the true system of the universe toward general adoption, the problem of finding the longitude at sea presented itself. this was the avowed object of the foundation of the observatory at greenwich;[a] and no one subject has received more of the attention of astronomers, than those investigations of the lunar theory on which the requisite tables of the navigator are founded. the pathways of the ocean are marked out in the sky above. the eternal lights of the heavens are the only pharos whose beams never fail, which no tempest can shake from its foundation. within my recollection, it was deemed a necessary qualification for the master and the mate of a merchant-ship, and even for a prime hand, to be able to "work a lunar," as it was called. the improvements in the chronometer have in practice, to a great extent, superseded this laborious operation; but observation remains, and unquestionably will for ever remain, the only dependence for ascertaining the ship's time and deducting the longitude from the comparison of that time with the chronometer. [footnote a: grant's _physical astronomy_, p. .] it may, perhaps, be thought that astronomical science is brought already to such a state of perfection that nothing more is to be desired, or at least that nothing more is attainable, in reference to such practicable applications as i have described. this, however, is an idea which generous minds will reject, in this, as in every other department of human knowledge. in astronomy, as in every thing else, the discoveries already made, theoretical or practical, instead of exhausting the science, or putting a limit to its advancement, do but furnish the means and instruments of further progress. i have no doubt we live on the verge of discoveries and inventions, in every department, as brilliant as any that have ever been made; that there are new truths, new facts, ready to start into recognition on every side; and it seems to me there never was an age, since the dawn of time, when men ought to be less disposed to rest satisfied with the progress already made, than the age in which we live; for there never was an age more distinguished for ingenious research, for novel result, and bold generalization. that no further improvement is desirable in the means and methods of ascertaining the ship's place at sea, no one i think will from experience be disposed to assert. the last time i crossed the atlantic, i walked the quarter-deck with the officer in charge of the noble vessel, on one occasion, when we were driving along before a leading breeze and under a head of steam, beneath a starless sky at midnight, at the rate certainly of ten or eleven miles an hour. there is something sublime, but approaching the terrible, in such a scene;--the rayless gloom, the midnight chill,--the awful swell of the deep,--the dismal moan of the wind through the rigging, the all but volcanic fires within the hold of the ship. i scarce know an occasion in ordinary life in which a reflecting mind feels more keenly its hopeless dependence on irrational forces beyond its own control. i asked my companion how nearly he could determine his ship's place at sea under favorable circumstances. theoretically, he answered, i think, within a mile;--practically and usually within three or four. my next question was, how near do you think we may be to cape race;--that dangerous headland which pushes its iron-bound unlighted bastions from the shore of newfoundland far into the atlantic,--first landfall to the homeward-bound american vessel. we must, said he, by our last observations and reckoning, be within three or four miles of cape race. a comparison of these two remarks, under the circumstances in which we were placed at the moment, brought my mind to the conclusion, that it is greatly to be wished that the means should be discovered of finding the ship's place more accurately, or that navigators would give cape race a little wider berth. but i do not remember that one of the steam packets between england and america was ever lost on that formidable point. it appears to me by no means unlikely that, with the improvement of instrumental power, and of the means of ascertaining the ship's time with exactness, as great an advance beyond the present state of art and science in finding a ship's place at sea may take place, as was effected by the invention of the reflecting quadrant, the calculation of lunar tables, and the improved construction of chronometers. babbage's difference machine. in the wonderful versatility of the human mind, the improvement, when made, will very probably be made by paths where it is least expected. the great inducement to mr. babbage to attempt the construction of an engine by which astronomical tables could be calculated, and even printed, by mechanical means and with entire accuracy, was the errors in the requisite tables. nineteen such errors, in point of fact, were discovered in an edition of taylor's logarithms printed in ; some of which might have led to the most dangerous results in calculating a ship's place. these nineteen errors, (of which one only was an error of the press), were pointed out in the _nautical almanac_ for . in one of these _errata_ the seat of the error was stated to be in cosine of ° ' ". subsequent examination showed that there was an error of one second in this correction; and, accordingly, in the _nautical almanac_ of the next year a new correction was necessary. but in making the new correction of one second, a new error was committed of ten degrees. instead of cosine ° ' " the correction was printed cosine ° ' " making it still necessary, in some future edition of the _nautical almanac_, to insert an _erratum_ in an _erratum_ of the _errata_ in taylor's logarithms.[a] [footnote a: edinburgh review, vol. lix., .] in the hope of obviating the possibility of such errors, mr. babbage projected his calculating, or, as he prefers to call it, his difference machine. although this extraordinary undertaking has been arrested, in consequence of the enormous expense attending its execution, enough has been achieved to show the mechanical possibility of constructing an engine of this kind, and even one of far higher powers, of which mr. babbage has matured the conception, devised the notation, and executed the drawings--themselves an imperishable monument of the genius of the author. i happened on one occasion to be in company with this highly distinguished man of science, whose social qualities are as pleasing as his constructive talent is marvelous, when another eminent _savant_, count strzelecki, just returned from his oriental and australian tour, observed that he found among the chinese, a great desire to know something more of mr. babbage's calculating machine, and especially whether, like their own _swampan_, it could be made to go into the pocket. mr. babbage good-humouredly observed that, thus far, he had been very much out of pocket with it. increased command of instrumental power. whatever advances may be made in astronomical science, theoretical or applied, i am strongly inclined to think that they will be made in connection with an increased command of instrumental power. the natural order in which the human mind proceeds in the acquisition of astronomical knowledge is minute and accurate observation of the phenomena of the heavens, the skillful discussion and analysis of these observations, and sound philosophy in generalizing the results. in pursuing this course, however, a difficulty presented itself, which for ages proved insuperable--and which to the same extent has existed in no other science, viz.: that all the leading phenomena are in their appearance delusive. it is indeed true that in all sciences superficial observation can only lead, except by chance, to superficial knowledge; but i know of no branch in which, to the same degree as in astronomy, the great leading phenomena are the reverse of true; while they yet appeal so strongly to the senses, that men who could foretell eclipses, and who discovered the precession of the equinoxes, still believed that the earth was at rest in the center of the universe, and that all the host of heaven performed a daily revolution about it as a center. it usually happens in scientific progress, that when a great fact is at length discovered, it approves itself at once to all competent judges. it furnishes a solution to so many problems, and harmonizes with so many other facts,--that all the other _data_ as it were crystallize at once about it. in modern times, we have often witnessed such an impatience, so to say, of great truths, to be discovered, that it has frequently happened that they have been found out simultaneously by more than one individual; and a disputed question of priority is an event of very common occurrence. not so with the true theory of the heavens. so complete is the deception practiced on the senses, that it failed more than once to yield to the suggestion of the truth; and it was only when the visual organs were armed with an almost preternatural instrumental power, that the great fact found admission to the human mind. the copernican system. it is supposed that in the very dawn of science, pythagoras or his disciples explained the apparent motion of the heavenly bodies about the earth by the diurnal revolution of the earth on its axis. but this theory, though bearing so deeply impressed upon it the great seal of truth, _simplicity_, was in such glaring contrast with the evidence of the senses, that it failed of acceptance in antiquity or the middle ages. it found no favor with minds like those of aristotle, archimedes, hipparchus, ptolemy, or any of the acute and learned arabian or mediæval astronomers. all their ingenuity and all their mathematical skill were exhausted in the development of a wonderfully complicated and ingenious, but erroneous history. the great master truth, rejected for its simplicity, lay disregarded at their feet. at the second dawn of science, the great fact again beamed into the mind of copernicus. now, at least, in that glorious age which witnessed the invention of printing, the great mechanical engine of intellectual progress, and the discovery of america, we may expect that this long-hidden revelation, a second time proclaimed, will command the assent of mankind. but the sensible phenomena were still too strong for the theory; the glorious delusion of the rising and the setting sun could not be overcome. tycho de brahe furnished his observatory with instruments superior in number and quality to all that had been collected before; but the great instrument of discovery, which, by augmenting the optic power of the eye, enables it to penetrate beyond the apparent phenomena, and to discern the true constitution of the heavenly bodies, was wanting at uranienburg. the observations of tycho as discussed by kepler, conducted that most fervid, powerful, and sagacious mind to the discovery of some of the most important laws of the celestial motions; but it was not till galileo, at florence, had pointed his telescope to the sky, that the copernican system could be said to be firmly established in the scientific world. the home of galileo. on this great name, my friends, assembled as we are to dedicate a temple to instrumental astronomy, we may well pause for a moment. there is much, in every way, in the city of florence to excite the curiosity, to kindle the imagination, and to gratify the taste. sheltered on the north by the vine-clad hills of fiesoli, whose cyclopean walls carry back the antiquary to ages before the roman, before the etruscan power, the flowery city (fiorenza) covers the sunny banks of the arno with its stately palaces. dark and frowning piles of mediæval structure; a majestic dome, the prototype of st. peter's; basilicas which enshrine the ashes of some of the mightiest of the dead; the stone where dante stood to gaze on the campanile; the house of michael angelo, still occupied by a descendant of his lineage and name, his hammer, his chisel, his dividers, his manuscript poems, all as if he had left them but yesterday; airy bridges, which seem not so much to rest on the earth as to hover over the waters they span; the loveliest creations of ancient art, rescued from the grave of ages again to enchant the world; the breathing marbles of michael angelo, the glowing canvas of raphael and titian, museums filled with medals and coins of every age from cyrus the younger, and gems and amulets and vases from the sepulchers of egyptian pharaohs coëval with joseph, and etruscan lucumons that swayed italy before the romans,--libraries stored with the choicest texts of ancient literature,--gardens of rose and orange, and pomegranate, and myrtle,--the very air you breathe languid with music and perfume;--such is florence. but among all its fascinations, addressed to the sense, the memory, and the heart, there was none to which i more frequently gave a meditative hour during a year's residence, than to the spot where galileo galilei sleeps beneath the marble door of santa croce; no building on which i gazed with greater reverence, than i did upon the modest mansion at arcetri, villa at once and prison, in which that venerable sage, by command of the inquisition, passed the sad closing years of his life. the beloved daughter on whom he had depended to smooth his passage to the grave, laid there before him; the eyes with which he had discovered worlds before unknown, quenched in blindness: ahime! quegli occhi si son fatti oscuri, che vider più di tutti i tempi antichi, e luce fur dei secoli futuri. that was the house, "where," says milton (another of those of whom the world was not worthy), "i found and visited the famous galileo, grown old--a prisoner to the inquisition, for thinking on astronomy otherwise than as the dominican and franciscan licensers thought."[a] great heavens! what a tribunal, what a culprit, what a crime! let us thank god, my friends, that we live in the nineteenth century. of all the wonders of ancient and modern art, statues and paintings, and jewels and manuscripts,--the admiration and the delight of ages,--there was nothing which i beheld with more affectionate awe than that poor, rough tube, a few feet in length,--the work of his own hands,--that very "optic glass," through which the "tuscan artist" viewed the moon, "at evening, from the top of fesolé, or in valdarno, to descry new lands, rivers, or mountains, in her spotty globe." that poor little spy-glass (for it is scarcely more) through which the human eye first distinctly beheld the surface of the moon--first discovered the phases of venus, the satellites of jupiter, and the seeming handles of saturn--first penetrated the dusky depths of the heavens--first pierced the clouds of visual error, which, from the creation of the world, involved the system of the universe. [footnote a: prose works, vol. , p. .] there are occasions in life in which a great mind lives years of rapt enjoyment in a moment. i can fancy the emotions of galileo, when, first raising the newly-constructed telescope to the heavens, he saw fulfilled the grand prophecy of copernicus, and beheld the planet venus crescent like the moon. it was such another moment as that when the immortal printers of mentz and strasburg received the first copy of the bible into their hands, the work of their divine art; like that when columbus, through the gray dawn of the th of october, (copernicus, at the age of eighteen, was then a student at cracow), beheld the shores of san salvador; like that when the law of gravitation first revealed itself to the intellect of newton; like that when franklin saw by the stiffening fibers of the hempen cord of his kite, that he held the lightning in his grasp; like that when leverrier received back from berlin the tidings that the predicted planet was found. yes, noble galileo, thou art right, _e pur si muove._ "it does move." bigots may make thee recant it; but it moves, nevertheless. yes, the earth moves, and the planets move, and the mighty waters move, and the great sweeping tides of air move, and the empires of men move, and the world of thought moves, ever onward and upward to higher facts and bolder theories. the inquisition may seal thy lips, but they can no more stop the progress of the great truth propounded by copernicus, and demonstrated by thee, than they can stop the revolving earth. close now, venerable sage, that sightless, tearful eye; it has seen what man never before saw--it has seen enough. hang up that poor little spy-glass--it has done its work. not herschell nor rosse have, comparatively, done more. franciscans and dominicans deride thy discoveries now; but the time will come when, from two hundred observatories in europe and america, the glorious artillery of science shall nightly assault the skies, but they shall gain no conquests in those glittering fields before which thine shall be forgotten. rest in peace, great columbus of the heavens--like him scorned, persecuted, broken-hearted!--in other ages, in distant hemispheres, when the votaries of science, with solemn acts of consecration, shall dedicate their stately edifices to the cause of knowledge and truth, thy name shall be mentioned with honor. new periods in astronomical science. it is not my intention, in dwelling with such emphasis upon the invention of the telescope, to ascribe undue importance, in promoting the advancement of science, to the increase of instrumental power. too much, indeed, cannot be said of the service rendered by its first application in confirming and bringing into general repute the copernican system; but for a considerable time, little more was effected by the wondrous instrument than the gratification of curiosity and taste, by the inspection of the planetary phases, and the addition of the rings and satellites of saturn to the solar family. newton, prematurely despairing of any further improvement in the refracting telescope, applied the principle of reflection; and the nicer observations now made, no doubt, hastened the maturity of his great discovery of the law of gravitation; but that discovery was the work of his transcendent genius and consummate skill. with bradley, in , a new period commenced in instrumental astronomy, not so much of discovery as of measurement. the superior accuracy and minuteness with which the motions and distances of the heavenly bodies were now observed, resulted in the accumulation of a mass of new materials, both for tabular comparison and theoretical speculation. these materials formed the enlarged basis of astronomical science between newton and sir william herschell. his gigantic reflectors introduced the astronomer to regions of space before unvisited--extended beyond all previous conception the range of the observed phenomena, and with it proportionably enlarged the range of constructive theory. the discovery of a new primary planet and its attendant satellites was but the first step of his progress into the labyrinth of the heavens. cotemporaneously with his observations, the french astronomers, and especially la place, with a geometrical skill scarcely, if at all, inferior to that of its great author, resumed the whole system of newton, and brought every phenomenon observed since his time within his laws. difficulties of fact, with which he struggled in vain, gave way to more accurate observations; and problems that defied the power of his analysis, yielded to the modern improvements of the calculus. herschell's nebular theory. but there is no _ultima thule_ in the progress of science. with the recent augmentations of telescopic power, the details of the nebular theory, proposed by sir w. herschell with such courage and ingenuity, have been drawn in question. many--most--of those milky patches in which he beheld what he regarded as cosmical matter, as yet in an unformed state,--the rudimental material of worlds not yet condensed,--have been resolved into stars, as bright and distinct as any in the firmament. i well recall the glow of satisfaction with which, on the d of september, , being then connected with the university at cambridge, i received a letter from the venerable director of the observatory there, beginning with these memorable words:--"you will rejoice with me that the great nebula in orion has yielded to the powers of our incomparable telescope! * * * it should be borne in mind that this nebula, and that of andromeda [which has been also resolved at cambridge], are the last strongholds of the nebular theory."[a] [footnote a: _annals of the observatory of harvard college_, p. .] but if some of the adventurous speculations built by sir william herschell on the bewildering revelations of his telescope have been since questioned, the vast progress which has been made in sidereal astronomy, to which, as i understand, the dudley observatory will be particularly devoted, the discovery of the parallax of the fixed stars, the investigation of the interior relations of binary and triple systems of stars, the theories for the explanation of the extraordinary, not to say fantastic, shapes discerned in some of the nebulous systems--whirls and spirals radiating through spaces as vast as the orbit of neptune;[a] the glimpses at systems beyond that to which our sun belongs;--these are all splendid results, which may fairly be attributed to the school of herschell, and will for ever insure no secondary place to that name in the annals of science. [footnote a: see the remarkable memoir of professor alexander, "on the origin of the forms and the present condition of some of the clusters of stars, and several of the nebulæ," (gould's _astronomical journal_, vol. iii, p. .)] relationship of the liberal arts. in the remarks which i have hitherto made, i have had mainly in view the direct connection of astronomical science with the uses of life and the service of man. but a generous philosophy contemplates the subject in higher relations. it is a remark as old, at least, as plato, and is repeated from him more than once by cicero, that all the liberal arts have a common bond and relationship.[a] the different sciences contemplate as their immediate object the different departments of animate and inanimate nature; but this great system itself is but one, and its parts are so interwoven with each other, that the most extraordinary relations and unexpected analogies are constantly presenting themselves; and arts and sciences seemingly the least connected, render to each other the most effective assistance. [footnote a: archias, i.; de oratore, iii., .] the history of electricity, galvanism, and magnetism, furnishes the most striking illustration of this remark. commencing with the meteorological phenomena of our own atmosphere, and terminating with the observation of the remotest heavens, it may well be adduced, on an occasion like the present. franklin demonstrated the identity of lightning and the electric fluid. this discovery gave a great impulse to electrical research, with little else in view but the means of protection from the thunder-cloud. a purely accidental circumstance led the physician galvani, at bologna, to trace the mysterious element, under conditions entirely novel, both of development and application. in this new form it became, in the hands of davy, the instrument of the most extraordinary chemical operations; and earths and alkalis, touched by the creative wire, started up into metals that float on water, and kindle in the air. at a later period, the closest affinities are observed between electricity and magnetism, on the one hand; while, on the other, the relations of polarity are detected between acids and alkalis. plating and gilding henceforth become electrical processes. in the last applications of the same subtle medium, it has become the messenger of intelligence across the land and beneath the sea; and is now employed by the astronomer to ascertain the difference of longitudes, to transfer the beats of the clock from one station to another, and to record the moment of his observations with automatic accuracy. how large a share has been borne by america in these magnificent discoveries and applications, among the most brilliant achievements of modern science, will sufficiently appear from the repetition of the names of franklin, henry, morse, walker, mitchell, lock, and bond. versatility of genius. it has sometimes happened, whether from the harmonious relations to each other of every department of science, or from rare felicity of individual genius, that the most extraordinary intellectual versatility has been manifested by the same person. although newton's transcendent talent did not blaze out in childhood, yet as a boy he discovered great aptitude for mechanical contrivance. his water-clock, self-moving vehicle, and mill, were the wonder of the village; the latter propelled by a living mouse. sir david brewster represents the accounts as differing, whether the mouse was made to advance "by a string attached to its tail," or by "its unavailing attempts to reach a portion of corn placed above the wheel." it seems more reasonable to conclude that the youthful discoverer of the law of gravitation intended by the combination of these opposite attractions to produce a balanced movement. it is consoling to the average mediocrity of the race to perceive in these sportive assays, that the mind of newton passed through the stage of boyhood. but emerging from boyhood, what a bound it made, as from earth to heaven! hardly commencing bachelor of arts, at the age of twenty-four, he untwisted the golden and silver threads of the solar spectrum, simultaneously or soon after conceived the method of fluxions, and arrived at the elemental idea of universal gravity before he had passed to his master's degree. master of arts indeed! that degree, if no other, was well bestowed. universities are unjustly accused of fixing science in stereotype. that diploma is enough of itself to redeem the honors of academical parchment from centuries of learned dullness and scholastic dogmatism. but the great object of all knowledge is to enlarge and purify the soul, to fill the mind with noble contemplations, to furnish a refined pleasure, and to lead our feeble reason from the works of nature up to its great author and sustainer. considering this as the ultimate end of science, no branch of it can surely claim precedence of astronomy. no other science furnishes such a palpable embodiment of the abstractions which lie at the foundation of our intellectual system; the great ideas of time, and space, and extension, and magnitude, and number, and motion, and power. how grand the conception of the ages on ages required for several of the secular equations of the solar system; of distances from which the light of a fixed star would not reach us in twenty millions of years, of magnitudes compared with which the earth is but a foot-ball; of starry hosts--suns like our own--numberless as the sands on the shore; of worlds and systems shooting through the infinite spaces, with a velocity compared with which the cannon-ball is a way-worn, heavy-paced traveler![a] [footnote a: nichol's _architecture of the heavens_, p. .] the spectacle of the heavens. much, however, as we are indebted to our observatories for elevating our conceptions of the heavenly bodies, they present, even to the unaided sight, scenes of glory which words are too feeble to describe. i had occasion, a few weeks since, to take the early train from providence to boston; and for this purpose rose at o'clock in the morning. every thing around was wrapped in darkness and hushed in silence, broken only by what seemed at that hour the unearthly clank and rush of the train. it was a mild, serene midsummer's night; the sky was without a cloud--the winds were whist. the moon, then in the last quarter, had just risen, and the stars shone with a spectral luster but little affected by her presence; jupiter, two hours high, was the herald of the day; the pleiades, just above the horizon, shed their sweet influence in the east; lyra sparkled near the zenith; andromeda veiled her newly discovered glories from the naked eye in the south; the steady pointers, far beneath the pole, looked meekly up from the depths of the north to their sovereign. such was the glorious spectacle as i entered the train. as we proceeded, the timid approach of twilight became more perceptible; the intense blue of the sky began to soften, the smaller stars, like little children, went first to rest; the sister-beams of the pleiades soon melted together; but the bright constellations of the west and north remained unchanged. steadily the wondrous transfiguration went on. hands of angels hidden from mortal eyes shifted the scenery of the heavens; the glories of night dissolved into the glories of the dawn. the blue sky now turned more softly gray; the great watch-stars shut up their holy eyes; the east began to kindle. faint streaks of purple soon blushed along the sky; the whole celestial concave was filled with the inflowing tides of the morning light, which came pouring down from above in one great ocean of radiance; till at length, as we reached the blue hills, a flash of purple fire blazed out from above the horizon, and turned the dewy teardrops of flower and leaf into rubies and diamonds. in a few seconds the everlasting gates of the morning were thrown wide open, and the lord of day, arrayed in glories too severe for the gaze of man, began his course. i do not wonder at the superstition of the ancient magians, who in the morning of the world went up to the hill-tops of central asia, and ignorant of the true god, adored the most glorious work of his hand. but i am filled with amazement, when i am told that in this enlightened age, and in the heart of the christian world, there are persons who can witness this daily manifestation of the power and wisdom of the creator, and yet say in their hearts, "there is no god." undiscovered bodies. numerous as are the heavenly bodies visible to the naked eye, and glorious as are their manifestations, it is probable that in our own system there are great numbers as yet undiscovered. just two hundred years ago this year, huyghens announced the discovery of one satellite of saturn, and expressed the opinion that the six planets and six satellites then known, and making up the perfect number of _twelve_, composed the whole of our planetary system. in an astronomical writer expressed the opinion that there might be other bodies in our system, but that the limit of telescopic power had been reached, and no further discoveries were likely to be made.[a] the orbit of one comet only had been definitively calculated. since that time the power of the telescope has been indefinitely increased; two primary planets of the first class, ten satellites, and forty-three small planets revolving between mars and jupiter, have been discovered, the orbits of six or seven hundred comets, some of brief period, have been ascertained;--and it has been computed, that hundreds of thousands of these mysterious bodies wander through our system. there is no reason to think that all the primary planets, which revolve about the sun, have been discovered. an indefinite increase in the number of asteroids may be anticipated; while outside of neptune, between our sun and the nearest fixed star, supposing the attraction of the sun to prevail through half the distance, there is room for ten more primary planets succeeding each other at distances increasing in a geometrical ratio. the first of these will, unquestionably, be discovered as soon as the perturbations of neptune shall have been accurately observed; and with maps of the heavens, on which the smallest telescopic stars are laid down, it may be discovered much sooner. [footnote a: _memoirs of a.a.s._, vol. iii, .] the vastness of creation. but it is when we turn our observation and our thoughts from our own system, to the systems which lie beyond it in the heavenly spaces, that we approach a more adequate conception of the vastness of creation. all analogy teaches us that the sun which gives light to us is but one of those countless stellar fires which deck the firmament, and that every glittering star in that shining host is the center of a system as vast and as full of subordinate luminaries as our own. of these suns--centers of planetary systems--thousands are visible to the naked eye, millions are discovered by the telescope. sir john herschell, in the account of his operations at the cape of good hope (p. ) calculates that about five and a half millions of stars are visible enough to be _distinctly counted_ in a twenty-foot reflector, in both hemispheres. he adds, that "the actual number is much greater, there can be little doubt." his illustrious father, estimated on one occasion that , stars passed through the field of his forty foot reflector in a quarter of an hour. this would give , , for the entire circuit of the heavens, in a single telescopic zone; and this estimate was made under the assumption that the nebulæ were masses of luminous matter not yet condensed into suns. these stupendous calculations, however, form but the first column of the inventory of the universe. faint white specks are visible, even to the naked eye of a practiced observer in different parts of the heavens. under high magnifying powers, several thousands of such spots are visible,--no longer however, faint, white specks, but many of them resolved by powerful telescopes into vast aggregations of stars, each of which may, with propriety, be compared with the milky way. many of these nebulæ, however, resisted the power of sir wm. herschell's great reflector, and were, accordingly, still regarded by him as masses of unformed matter, not yet condensed into suns. this, till a few years since, was, perhaps, the prevailing opinion; and the nebular theory filled a large space in modern astronomical science. but with the increase of instrumental power, especially under the mighty grasp of lord rosse's gigantic reflector, and the great refractors at pulkova and cambridge, the most irresolvable of these nebulæ have given way; and the better opinion now is, that every one of them is a galaxy, like our own milky way, composed of millions of suns. in other words, we are brought to the bewildering conclusion that thousands of these misty specks, the greater part of them too faint to be seen with the naked eye, are, not each a universe like our solar system, but each a "swarm" of universes of unappreciable magnitude.[a] the mind sinks, overpowered by the contemplation. we repeat the words, but they no longer convey distinct ideas to the understanding. [footnote a: humboldt's _cosmos_, iii. .] conceptions of the universe. but these conclusions, however vast their comprehension, carry us but another step forward in the realms of sidereal astronomy. a proper motion in space of our sun, and of the fixed stars as we call them, has long been believed to exist. their vast distances only prevent its being more apparent. the great improvement of instruments of measurement within the last generation has not only established the existence of this motion, but has pointed to the region in the starry vault around which our whole solar and stellar system, with its myriad of attendant planetary worlds, appears to be performing a mighty revolution. if, then, we assume that outside of the system to which we belong and in which our sun is but a star like aldebaran or sirius, the different nebulæ of which we have spoken,--thousands of which spot the heavens--constitute a distinct family of universes, we must, following the guide of analogy, attribute to each of them also, beyond all the revolutions of their individual attendant planetary systems, a great revolution, comprehending the whole; while the same course of analogical reasoning would lead us still further onward, and in the last analysis, require us to assume a transcendental connection between all these mighty systems--a universe of universes, circling round in the infinity of space, and preserving its equilibrium by the same laws of mutual attraction which bind the lower worlds together. it may be thought that conceptions like these are calculated rather to depress than to elevate us in the scale of being; that, banished as he is by these contemplations to a corner of creation, and there reduced to an atom, man sinks to nothingness in this infinity of worlds. but a second thought corrects the impression. these vast contemplations are well calculated to inspire awe, but not abasement. mind and matter are incommensurable. an immortal soul, even while clothed in "this muddy vesture of decay," is in the eye of god and reason, a purer essence than the brightest sun that lights the depths of heaven. the organized human eye, instinct with life and soul, which, gazing through the telescope, travels up to the cloudy speck in the handle of orion's sword, and bids it blaze forth into a galaxy as vast as ours, stands higher in the order of being than all that host of luminaries. the intellect of newton which discovered the law that holds the revolving worlds together, is a nobler work of god than a universe of universes of unthinking matter. if, still treading the loftiest paths of analogy, we adopt the supposition,--to me i own the grateful supposition,--that the countless planetary worlds which attend these countless suns, are the abodes of rational beings like man, instead of bringing back from this exalted conception a feeling of insignificance, as if the individuals of our race were but poor atoms in the infinity of being, i regard it, on the contrary, as a glory of our human nature, that it belongs to a family which no man can number of rational natures like itself. in the order of being they may stand beneath us, or they may stand above us; _he_ may well be content with his place, who is made "a little lower than the angels." contemplation of the heavens. finally, my friends, i believe there is no contemplation better adapted to awaken devout ideas than that of the heavenly bodies,--no branch of natural science which bears clearer testimony to the power and wisdom of god than that to which you this day consecrate a temple. the heart of the ancient world, with all the prevailing ignorance of the true nature and motions of the heavenly orbs, was religiously impressed by their survey. there is a passage in one of those admirable philosophical treatises of cicero composed in the decline of life, as a solace under domestic bereavement and patriotic concern at the impending convulsions of the state, in which, quoting from some lost work of aristotle, he treats the topic in a manner which almost puts to shame the teachings of christian wisdom. "præclare ergo aristoteles, 'si essent,' inquit, 'qui sub terra semper habitavissent, bonis et illustribus domiciliis quæ essent ornata signis atque picturis, instructaque rebus iis omnibus quibus abundant ii qui beati putantur, nec tamen exissent unquam supra terram; accepissent autem fama et auditione, esse quoddam numen et vim deorum,--deinde aliquo tempore patefactis terræ faucibus ex illis abditis sedibus evadere in hæc loca quæ nos incolimus, atque exire potuissent; cum repente terram et maria coelumque, vidissent; nubium magnitudinem ventorumque vim, cognovissent; aspexissentque solem, ejusque tum magnitudinem, pulchritudinemque; tum etiam efficientiam cognovissent, quod is diem efficeret, toto coelo luce diffusa; cum autem terras nox opacasset, tum coelum totum cernerent astris distinctum et ornatum, lunæque luminum varietatem tum crescentis tum senescentis, corumque omnium ortus et occasus atque in æternitate ratos immutabilesque cursus;--hæc cum viderent, profecto et esse deos, et hæc tanta opera deorum esse, arbitrarentur."[a] there is much by day to engage the attention of the observatory; the sun, his apparent motions, his dimensions, the spots on his disc (to us the faint indications of movements of unimagined grandeur in his luminous atmosphere), a solar eclipse, a transit of the inferior planets, the mysteries of the spectrum;--all phenomena of vast importance and interest. but night is the astronomer's accepted time; he goes to his delightful labors when the busy world goes to its rest. a dark pall spreads over the resorts of active life; terrestrial objects, hill and valley, and rock and stream, and the abodes of men disappear; but the curtain is drawn up which concealed the heavenly hosts. there they shine and there they move, as they moved and shone to the eyes of newton and galileo, of kepler and copernicus, of ptolemy and hipparchus; yes, as they moved and shone when the morning stars sang together, and all the sons of god shouted for joy. all has changed on earth; but the glorious heavens remain unchanged. the plow passes over the site of mighty cities,--the homes of powerful nations are desolate, the languages they spoke are forgotten; but the stars that shone for them are shining for us; the same eclipses run their steady cycle; the same equinoxes call out the flowers of spring, and send the husbandman to the harvest; the sun pauses at either tropic as he did when his course began; and sun and moon, and planet and satellite, and star and constellation and galaxy, still bear witness to the power, the wisdom, and the love, which placed them in the heavens and uphold them there. [footnote a: "nobly does aristotle observe, that if there were beings who had always lived under ground, in convenient, nay, in magnificent dwellings, adorned with statues and pictures, and every thing which belongs to prosperous life, but who had never come above ground; who had heard, however, by fame and report, of the being and power of the gods; if, at a certain time, the portals of the earth being thrown open, they had been able to emerge from those hidden abodes to the regions inhabited by us; when suddenly they had seen the earth, the sea, and the sky; had perceived the vastness of the clouds and the force of the winds; had contemplated the sun, his magnitude and his beauty, and still more his effectual power, that it is he who makes the day, by the diffusion of his light through the whole sky; and, when night had darkened the earth, should then behold the whole heavens studded and adorned with stars, and the various lights of the waxing and waning moon, the risings and the settings of all these heavenly bodies, and the courses fixed and immutable in all eternity; when, i say, they should see these things, truly they would believe that there were gods, and these so great things are their works."--cicero, _de natura deorum_ lib. ii., § .] the reminiscences of an astronomer by simon newcomb preface the earlier chapters of this collection are so much in the nature of an autobiography that the author has long shrunk from the idea of allowing them to see the light during his lifetime. his repugnance has been overcome by very warm expressions on the subject uttered by valued friends to whom they were shown, and by a desire that some at least who knew him in youth should be able to read what he has written. the author trusts that neither critic nor reader will object because he has, in some cases, strayed outside the limits of his purely personal experience, in order to give a more complete view of a situation, or to bring out matters that might be of historic interest. if some of the chapters are scrappy, it is because he has tried to collect those experiences which have afforded him most food for thought, have been most influential in shaping his views, or are recalled with most pleasure. contents i the world of cold and darkness ancestry.--squire thomas prince.--parentage.--early education.-- books read. ii dr. foshay a long journey on foot.--a wonderful doctor.--the botanic system of medicine.--phrenology.--a launch into the world.--a disillusion.-- life in maryland.--acquaintance with professor henry.--removal to cambridge. iii the world of sweetness and light the american astronomical ephemeris.--the men who made it.-- harvard in the middle of the century.--a librarian of the time.-- professor peirce.--dr. gould, the "astronomical journal," and the dudley observatory.--w. p. g. bartlett.--john d. runkle and the "mathematical monthly."--a mathematical politician.--a trip to manitoba and a voyage up the saskatchewan.--a wonderful star. iv life and work at an observatory a professor, united states navy.--the naval observatory in .-- captain gilliss and his plans.--admiral davis.--a new instrument and a new departure.--astronomical activity.--the question of observatory administration.--visit from the emperor of brazil.-- admiral john rodgers.--efforts to improve the work of the observatory. v great telescopes and their work curious origin of the great washington telescope.--congress is induced to act.--a case of astronomical fallibility.-- the discovery of the satellites of mars.--the great telescope of the pulkova observatory.--alvan clark and his sons.--a sad astronomical accident. vi the transits of venus old transits of venus.--an astronomical expedition in the th century.--father hell and his observations.--a suspected forger vindicated.--the american commission on the transit of venus.-- the photographic method to be applied.--garfield and the appropriation committee.--weather uncertainties.--voyage to the cape of good hope.--the transit of .--our failure to publish our observations. vii the lick observatory james lick and his ideas.--mr. d. o. mills.--plans for the lick observatory.--edward e. barnard.--professor holden.--wonderful success of the observatory. viii the author's scientific work the orbits of the asteroids.--the problems of mathematical astronomy.--the motion of the moon and its perplexing inequalities.--a visit to the paris observatory to search for forgotten observations.--wonderful success in finding them.-- the paris commune.--the history of the moon's motion carried back a century.--the harvard observatory.--the "nautical almanac" office and its work.--mr. george w. hill and his work.--a wonderful algebraist.--the meridian conference of , and the question of universal time.--tables of the planets completed.-- the astronomical constants.--work unfinished. ix scientific washington professor henry and the smithsonian institution.-- alumni associations.--the scientific club.--general sherman.-- mr. hugh mcculloch.--a forgotten scientist.--the national academy of sciences.--the geological survey of the territories.--the government forestry system.--professor o. c. marsh.--scientific humbugs.-- life on the plains. x scientific england my first trip to europe.--mr. thomas hughes.--mr. john stuart mill. --mr. gladstone and the royal society dinner.--other eminent englishmen.--professors cayley and adams.--professor airy and the greenwich observatory.--a visit to edinburgh. xi men and things in europe a voyage to gibraltar with professor tyndall.--the great fortress. --"whispering boanerges."--a winter voyage in the mediterranean.-- malta and messina.--advantage of not understanding a language.-- german astronomers.--the pulkova observatory.--a meeting which might have been embarrassing.--from germany to paris at the close of the war.--experiences at paris during the commune.--the greatest astronomer of france.--the paris observatory. xii the old and the new washington washington during the civil war.--secretary stanton.-- the raid of general early.--a presidential levee in .-- the fall of richmond.--the assassination of president lincoln.-- negro traits and education.--senator sumner.--an ambitious academy. --president garfield and his assassination.--cooling the white house during his illness.--the shepherd régime in washington. xiii miscellanea the great star-catalogue case.--professor peters and the almagest of ptolemy.--scientific cranks.--the degrees of the french universities.--a virginia country school.--political economy and education.--exact science in america before the johns hopkins university.--professor ely and economics.--spiritualism and psychic research.--the georgia magnetic girl. the reminiscences of an astronomer i the world of cold and darkness i date my birth into the world of sweetness and light on one frosty morning in january, , when i took my seat between two well-known mathematicians, before a blazing fire in the office of the "nautical almanac" at cambridge, mass. i had come on from washington, armed with letters from professor henry and mr. hilgard, to seek a trial as an astronomical computer. the men beside me were professor joseph winlock, the superintendent, and mr. john d. runkle, the senior assistant in the office. i talked of my unsuccessful attempt to master the "mécanique céleste" of laplace without other preparation than that afforded by the most meagre text-books of elementary mathematics of that period. runkle spoke of the translator as "the captain." so familiar a designation of the great bowditch--ll. d. and a member of the royal societies of london, edinburgh, and dublin--quite shocked me. i was then in my twenty-second year, but it was the first time i had ever seen any one who was familiar with the "mécanique céleste." i looked with awe upon the assistants who filed in and out as upon men who had all the mysteries of gravitation and the celestial motions at their fingers' ends. i should not have been surprised to learn that even the hibernian who fed the fire had imbibed so much of the spirit of the place as to admire the genius of laplace and lagrange. my own rank was scarcely up to that of a tyro; but i was a few weeks later employed on trial as computer at a salary of thirty dollars a month. how could an incident so simple and an employment so humble be in itself an epoch in one's life--an entrance into a new world? to answer this question some account of my early life is necessary. the interest now taken in questions of heredity and in the study of the growing mind of the child may excuse a word about my ancestry and early training. though born in nova scotia, i am of almost pure new england descent. the first simon newcomb, from whom i am of the sixth generation, was born in massachusetts or maine about , and died at lebanon, conn., in . his descendants had a fancy for naming their eldest sons after him, and but for the chance of my father being a younger son, i should have been the sixth simon in unbroken lineal descent. [ ] among my paternal ancestors none, so far as i know, with the exception of elder brewster, were what we should now call educated men. nor did any other of them acquire great wealth, hold a high official position, or do anything to make his name live in history. on my mother's side are found new england clergymen and an english nonconformist preacher, named prince, who is said to have studied at oxford towards the end of the seventeenth century, but did not take a degree. i do not know of any college graduate in the list. until i was four years old i lived in the house of my paternal grandfather, about two miles from the pretty little village of wallace, at the mouth of the river of that name. he was, i believe, a stonecutter by trade and owner of a quarry which has since become important; but tradition credits him with unusual learning and with having at some time taught school. my maternal grandfather was "squire" thomas prince, a native of maine, who had moved to moncton, n. b., early in his life, and lived there the rest of his days. he was an upright magistrate, a puritan in principle, and a pillar of the baptist church, highly respected throughout the province. he came from a long-lived family, and one so prolific that it is said most of the princes of new england are descended from it. i have heard a story of him which may illustrate the freedom of the time in matters of legal proceedings before a magistrate's court. at that time a party in a suit could not be a witness. in the terse language of the common people, "no man could swear money into his own pocket." the plaintiff in the case advised the magistrate in advance that he had no legal proof of the debt, but that defendant freely acknowledged it in private conversation. "well," said the magistrate, "bring him in here and get him to talk about it while i am absent." the time came. "if you had n't sued me i would have paid you," said the defendant. on the moment the magistrate stepped from behind a door with the remark,-- "i think you will pay him now, whether or no." my father was the most rational and the most dispassionate of men. the conduct of his life was guided by a philosophy based on combe's "constitution of man," and i used to feel that the law of the land was a potent instrument in shaping his paternal affections. his method of seeking a wife was so far unique that it may not be devoid of interest, even at this date. from careful study he had learned that the age at which a man should marry was twenty-five. a healthy and well-endowed offspring should be one of the main objects in view in entering the marriage state, and this required a mentally gifted wife. she must be of different temperament from his own and an economical housekeeper. so when he found the age of twenty-five approaching, he began to look about. there was no one in wallace who satisfied the requirements. he therefore set out afoot to discover his ideal. in those days and regions the professional tramp and mendicant were unknown, and every farmhouse dispensed its hospitality with an arcadian simplicity little known in our times. wherever he stopped overnight he made a critical investigation of the housekeeping, perhaps rising before the family for this purpose. he searched in vain until his road carried him out of the province. one young woman spoiled any possible chance she might have had by a lack of economy in the making of bread. she was asked what she did with an unnecessarily large remnant of dough which she left sticking to the sides of the pan. she replied that she fed it to the horses. her case received no further consideration. the search had extended nearly a hundred miles when, early one evening, he reached what was then the small village of moncton. he was attracted by the strains of music from a church, went into it, and found a religious meeting in progress. his eye was at once arrested by the face and head of a young woman playing on a melodeon, who was leading the singing. he sat in such a position that he could carefully scan her face and movements. as he continued this study the conviction grew upon him that here was the object of his search. that such should have occurred before there was any opportunity to inspect the doughpan may lead the reader to conclusions of his own. he inquired her name--emily prince. he cultivated her acquaintance, paid his addresses, and was accepted. he was fond of astronomy, and during the months of his engagement one of his favorite occupations was to take her out of an evening and show her the constellations. it is even said that, among the daydreams in which they indulged, one was that their firstborn might be an astronomer. probably this was only a passing fancy, as i heard nothing of it during my childhood. the marriage was in all respects a happy one, so far as congeniality of nature and mutual regard could go. although the wife died at the early age of thirty-seven, the husband never ceased to cherish her memory, and, so far as i am aware, never again thought of marrying. my mother was the most profoundly and sincerely religious woman with whom i was ever intimately acquainted, and my father always entertained and expressed the highest admiration for her mental gifts, to which he attributed whatever talents his children might have possessed. the unfitness of her environment to her constitution is the saddest memory of my childhood. more i do not trust myself to say to the public, nor will the reader expect more of me. my father followed, during most of his life, the precarious occupation of a country school teacher. it was then, as it still is in many thinly settled parts of the country, an almost nomadic profession, a teacher seldom remaining more than one or two years in the same place. thus it happened that, during the first fifteen years of my life, movings were frequent. my father tried his fortune in a number of places, both in nova scotia and prince edward island. our lot was made harder by the fact that his ideas of education did not coincide with those prevalent in the communities where he taught. he was a disciple and admirer of william cobbett, and though he did not run so far counter to the ideas of his patrons as to teach cobbett's grammar at school, he always recommended it to me as the one by which alone i could learn to write good english. the learning of anything, especially of arithmetic and grammar, by the glib repetition of rules was a system that he held in contempt. with the public, ability to recite the rules of such subjects as those went farther than any actual demonstration of the power to cipher correctly or write grammatically. so far as the economic condition of society and the general mode of living and thinking were concerned, i might claim to have lived in the time of the american revolution. a railway was something read or heard about with wonder; a steamer had never ploughed the waters of wallace bay. nearly everything necessary for the daily life of the people had to be made on the spot, and even at home. the work of the men and boys was "from sun to sun,"--i might almost say from daylight to darkness,--as they tilled the ground, mended the fences, or cut lumber, wood, and stone for export to more favored climes. the spinning wheel and the loom were almost a necessary part of the furniture of any well-ordered house; the exceptions were among people rich enough to buy their own clothes, or so poor and miserable that they had to wear the cast-off rags of their more fortunate neighbors. the women and girls sheared the sheep, carded the wool, spun the yarn, wove the homespun cloth, and made the clothes. in the haying season they amused themselves by joining in the raking of hay, in which they had to be particularly active if rain was threatened; but any man would have lost caste who allowed wife or daughter to engage in heavy work outside the house. the contrast between the social conditions and those which surround even the poorest classes at the present day have had a profound influence upon my views of economic subjects. the conception which the masses of the present time have of how their ancestors lived in the early years of the century are so vague and shadowy as not to influence their conduct at the present time. what we now call school training, the pursuit of fixed studies at stated hours under the constant guidance of a teacher, i could scarcely be said to have enjoyed. for the most part, when i attended my father's school at all, i came and went with entire freedom, and this for causes which, as we shall see, he had reasons for deeming good. it would seem that i was rather precocious. i was taught the alphabet by my aunts before i was four years old, and i was reading the bible in class and beginning geography when i was six. one curious feature of my reading i do not remember to have seen noticed in the case of children. the printed words, for the most part, brought no well-defined images to my mind; none at least that were retained in their connection. i remember one instance of this. we were at bedeque, prince edward island. during the absence of my father, the school was kept for a time by mr. bacon. the class in reading had that chapter in the new testament in which the treason of judas is described. it was then examined on the subject. to the question what judas did, no one could return an answer until it came my turn. i had a vague impression of some one hanging himself, and so i said quite at random that he hanged himself. it was with a qualm of conscience that i went to the head of the class. arithmetic was commenced at the age of five, my father drawing me to school day by day on a little sled during the winter. just what progress i made at that time i do not recall. long years afterward, my father, at my request, wrote me a letter describing my early education, extracts from which i shall ask permission to reproduce, instead of attempting to treat the matter myself. the letter, covering twelve closely written foolscap pages, was probably dashed off at a sitting without supposing any eye but my own would ever see it:-- june th, ' . i will now proceed to write, according to your request, about your early life. while in your fifth year, your mother spoke several times of the propriety of teaching you the first rudiments of book-learning; but i insisted that you should not be taught the first letter until you became five. [ ] i think, though, that at about four, or four and a half i taught you to count, as far, perhaps, as . when a little over four and a half, one evening, as i came home from school, you ran to me, and asked, "father, is not and and and , ?" "yes, how did you find it out?" you showed me the counterpane which was napped. the spot of four rows each way was the one you had counted up. after this, for a week or two, you spent a considerable number of hours every day, making calculations in addition and multiplication. the rows of naps being crossed and complexed in various ways, your greatest delight was to clear them out, find how many small ones were equal to one large one, and such like. after a space of two or three weeks we became afraid you would calculate yourself "out of your head," and laid away the counterpane. winter came, and passed along, and your birthday came; on that day, having a light hand-sled prepared, i fixed you on it, and away we went a mile and a half to school. according to my belief in educational matters "that the slate should be put into the child's hands as soon as the book is," you of course had your slate, and commenced making figures and letters the first day. in all cases, after you had read and spelled a lesson, and made some figures, and worked a sum, suppose one hour's study, i sent you out, telling you to run about and play a "good spell." to the best of my judgment you studied, during the five months that this school lasted, nearly four hours a day, two being at figures. * * * * * during the year that i taught at bedeque, you studied about five hours a day in school; and i used to exercise you about an hour a day besides, either morning or evening. this would make six hours per day, nearly or quite two and a half hours of that time at numbers either at your slate or mentally. when my school ended here, you were six and a half years of age, and pretty well through the arithmetic. you had studied, i think, all the rules preceding including the cube root. . . . i had frequently heard, during my boyhood, of a supposed mental breakdown about this period, and had asked my father for a description of it in the letter from which i am quoting. on this subject the letter continues:-- you had lost all relish for reading, study, play, or talk. sat most of the day flat on the floor or hearth. when sent of an errand, you would half the time forget what you went for. i have seen you come back from cale schurman's crying, [ ] and after asking you several times you would make out to answer, you had not been all the way over because you forgot what you went for. you would frequently jump up from the corner, and ask some peculiar question. i remember three you asked me. st. father, does form mean shape? yes. has everything some shape? yes. can it be possible for anything to be made that would not have any shape? i answered no; and then showed you several things, explaining that they all had some shape or form. you now brightened up like a lawyer who had led on a witness with easy questions to a certain point, and who had cautiously reserved a thunderbolt question, to floor the witness at a proper time; proceeded with, "well, then, how could the world be without form when god made it?" * * * * * d. does cale schurman's big ram know that he has such big crooked horns on him? does he know it himself, i mean? does he know himself that he has such horns on him? you were taken down suddenly i think about two or three days from the first symptoms until you were fairly in the corner. your rise was also rapid, i think about a week (or perhaps two weeks) from your first at recovery, until you seemed to show nothing unusual. from the time you were taken down until you commenced recovery was about a month. we returned to prince edward island, and after a few weeks i began to examine you in figures, and found you had forgotten nearly all you had ever learned. * * * * * while at new london i got an old work on astronomy; you were wonderfully taken with it, and read it with avidity. while here you read considerable in "goldsmith's history of england." we lived two years in new london; i think you attended school nearly one year there. i usually asked you questions on the road going to school, in the morning, upon the history you had read, or something you had studied the day previous. while there, you made a dozen or two of the folks raise a terrible laugh. i one evening lectured on astronomy at home; the house was pretty well filled, i suppose about twenty were present. you were not quite ten years old and small at that. almost as soon as i was done you said: "father, i think you were wrong in one thing." such a roar of laughter almost shook the house. you were an uncommon child for _truth_. i never knew you to deviate from it in one single instance, either in infancy or youth. from your infancy you showed great physical courage in going along the woods or in places in the dark among cattle, and i am surprised at what you say about your fears of a stove-pipe and trees. perhaps i should have said "mental" instead of physical courage, for in one respect you were uncommonly deficient in that sort of courage necessary to perform bodily labor. until nine or ten years of age you made a most pitiful attempt at any sort of bodily or rather "handy" work. * * * * * an extraordinary peculiarity in you was never to leap past a word you could not make out. i certainly never gave you any particular instructions about this, or the fact itself would not at the time have appeared so strange to me. i will name one case. after a return to wallace (you were eleven) i, one day, on going from home for an hour or so, gave you a borrowed newspaper, telling you there was a fine piece; to read it, and tell me its contents when i returned. on my return you were near the house chopping wood. "well, simon, did you read the piece?" "no, sir." "why not?" "i came to a word i did not know." this word was just about four lines from the commencement. at thirteen you read phrenology. i now often impressed upon you the necessity of bodily labor; that you might attain a strong and healthy physical system, so as to be able to stand long hours of study when you came to manhood, for it was evident to me that you would not labor with the hands for a business. on this account, as much as on account of poverty, i hired you out for a large portion of the three years that we lived at clements. at fifteen you studied euclid, and were enraptured with it. it is a little singular that all this time you never showed any self-esteem; or spoke of getting into employment at some future day, among the learned. the pleasure of intellectual exercise in demonstrating or analyzing a geometrical problem, or solving an algebraic equation, seemed to be your only object. no junior, seignour or sophomore class, with annual honors, was ever, i suppose, presented to your mind. your almost intuitive knowledge of geography, navigation, and nautical matters in general caused me to think most ardently of writing to the admiral at halifax, to know if he would give you a place among the midshipmen of the navy; but my hope of seeing you a leading lawyer, and finally a judge on the bench, together with the possibility that your mother would not consent, and the possibility that you would not wish to go, deterred me: although i think i commenced a letter. among the books which profoundly influenced my mode of life and thought during the period embraced in the foregoing extracts were fowler's "phrenology" and combe's "constitution of man." it may appear strange to the reader if a system so completely exploded as that of phrenology should have any value as a mental discipline. its real value consisted, not in what it taught about the position of the "organs," but in presenting a study of human nature which, if not scientific in form, was truly so in spirit. i acquired the habit of looking on the characters and capabilities of men as the result of their organism. a hot and impulsive temper was checked by the reflection that it was beneath the dignity of human nature to allow a rush of blood to the organs of "combativeness" and "destructiveness" to upset one's mental equilibrium. that i have gotten along in life almost without making (so far as i am aware) a personal enemy may be attributed to this early discipline, which led me into the habit of dealing with antagonism and personal opposition as i would deal with any physical opposition--evade it, avoid it, or overcome it. it goes without saying, however, that no discipline of this sort will avail to keep the passions of a youth always in check, and my own were no exception. when about fifteen i once made a great scandal by taking out my knife in prayer meeting and assaulting a young man who, while i was kneeling down during the prayer, stood above me and squeezed my neck. he escaped with a couple of severe though not serious cuts in his hand. he announced his intention of thrashing me when we should meet again; so for several days thereafter i tried, so far as possible, in going afield to keep a pitchfork within reach, determined that if he tried the job and i failed to kill him, it would be because i was unable to do so. fortunately for both of us he never made the attempt. i read combe's "constitution of man" when between ten and twelve years of age. though based on the ideas of phrenology and not, i believe, of high repute as a system of philosophy, it was as good a moral tonic as i can imagine to be placed in the hands of a youth, however fallacious may have been its general doctrines. so far as i can recall, it taught that all individual and social ills were due to men's disregard of the laws of nature, which were classified as physical and moral. obey the laws of health and we and our posterity will all reach the age of one hundred years. obey the moral law and social evils will disappear. its reading was accompanied by some qualms of conscience, arising from the non-accordance of many of its tenets with those of the "catechism" and the "new england primer." the combination of the two, however, led to the optimistic feeling that all wrongs would be righted, every act of injustice punished, and truth and righteousness eventually triumph through the regular processes of nature and society. i have been led to abandon this doctrine only by much experience, some of which will be found in the following pages. in the direction of mathematical and physical science and reading generally, i may add something to what i have quoted from my father. my grandfather simon had a small collection of books in the family. among those purely literary were several volumes of "the spectator" and "roderick random." of the former i read a good deal. the latter was a story which a boy who had scarcely read any other would naturally follow with interest. two circumstances connected with the reading, one negative and the other positive, i recall. looking into the book after attaining years of maturity, i found it to contain many incidents of a character that would not be admitted into a modern work. yet i read it through without ever noticing or retaining any impression of the indelicate side of the story. the other impression was a feeling of horror that a man fighting a duel and finding himself, as he supposed, mortally wounded by his opponent, should occupy his mind with avenging his own death instead of making his peace with heaven. three mathematical books were in the collection, hammond's algebra, simpson's euclid, and moore's navigator, the latter the predecessor of bowditch. the first was a miserable book, and i think its methods, which were crude in the extreme, though not incorrect, were rather more harmful than beneficial. the queer diagrams in euclid had in my early years so little attraction for me that my curiosity never led me to examine its text. i at length did so in consequence of a passage in the algebra which referred to the th proposition of the first book. it occurred to me to look into the book and see what this was. it was the first conception of mathematical proof that i had ever met with. i saw that the demonstration referred to a previous proposition, went back to that, and so on to the beginning. a new world of thought seemed to be opened. that principles so profound should be reached by methods so simple was astonishing. i was so enraptured that i explained to my brother thomas while walking out of doors one day how the pythagorean proposition, as it is now called, could be proved from first principles, drawing the necessary diagrams with a pencil on a piece of wood. i thought that even cattle might understand geometry could they only be communicated with and made to pay attention to it. some one at school had a copy of mrs. marcet's "conversations on natural philosophy." with this book i was equally enraptured. meagre and even erroneous though it was, it presented in a pleasing manner the first principles of physical science. i used to steal into the schoolhouse after hours to read a copy of the book, which belonged to one of the scholars, and literally devoured it in a few evenings. my first undertaking in the way of scientific experiment was in the field of economics and psychology. when about fourteen i spent the winter in the house of an old farmer named jefferson. he and his wife were a very kindly couple and took much interest in me. he was fond of his pipe, as most old farmers are. i questioned whether anything else would not do just as well as tobacco to smoke, and whether he was not wasting his money by buying that article when a cheap substitute could be found. so one day i took his pipe, removed the remains of the tobacco ashes, and stuffed the pipe with tea leaves that had been steeped, and which in color and general appearance looked much like tobacco. i took care to be around when he should again smoke. he lit the pipe as usual and smoked it with, seemingly, as much satisfaction as ever, only essaying the remark, "this tobacco tastes like tea." my conscience pricked me, but i could say nothing. my father bought a copy of lardner's "popular lectures on science and art." in this i first read of electricity. i recall an incident growing out of it. in lardner's description of a leyden jar, water is the only internal conductor. the wonders of the newly invented telegraph were then explained to the people in out of the way places by traveling lecturers. one of these came to clements, where we then lived, with a lot of apparatus, amongst which was what i recognized as a leyden jar. it was coated with tin-foil on the outside, but i did not see the inner coating, or anything which could serve as the necessary conductor. so with great diffidence i asked the lecturer while he was arranging his things, if he was not going to put water into the jar. "no, my lad," was his reply, "i put lightning into it." i wondered how the "lightning" was going to be conveyed to the interior surface of the glass without any conductor, such as water, but was too much abashed to ask the question. moore's "navigator" taught not only a very crude sort of trigonometry, but a good deal about the warship of his time. to a boy living on the seacoast, who naturally thought a ship of war one of the greatest works of man, the book was of much interest. notwithstanding the intellectual pleasure which i have described, my boyhood was on the whole one of sadness. occasionally my love of books brought a word of commendation from some visitor, perhaps a methodist minister, who patted me on the head with a word of praise. otherwise it caused only exclamations of wonder which were distasteful. "you would n't believe what larnin' that boy has got. he has more larnin' than all the people around here put together," i heard one farmer say to another, looking at me, in my own view of the case, as if i were some monster misshapen in the womb. instead of feeling that my bookish taste was something to be valued, i looked upon myself as a _lusus naturæ_ whom nature had cruelly formed to suffer from an abnormal constitution, and lamented that somehow i never could be like other boys. the maladroitness described by my father, of which i was fully conscious, added to the feeling of my unfitness for the world around me. the skill required on a farm was above my reach, where efficiency in driving oxen was one of the most valued of accomplishments. i keenly felt my inability to acquire even respectable mediocrity in this branch of the agricultural profession. it was mortifying to watch the dexterous motions of the whip and listen to the torrent of imperatives with which a young farmer would set a team of these stolid animals in motion after they had failed to respond to my gentle requests, though conveyed in the best of ox language. i had indeed gradually formed, from reading, a vague conception of a different kind of world,--a world of light,--where dwelt men who wrote books and people who knew the men who wrote books,--where lived boys who went to college and devoted themselves to learning, instead of driving oxen. i longed much to get into this world, but no possibility of doing so presented itself. i had no idea that it would be imbued with sympathy for a boy outside of it who wanted to learn. true, i had once read in some story, perhaps fictitious, how a nobleman had found a boy reading newton's "principia," and not only expressed his pleased surprise at the performance, but actually got the boy educated. but there was no nobleman in sight of the backwoods of nova scotia. i read in the autobiography of franklin how he had made his way in life. but he was surrounded with opportunities from which i was cut off. it does seem a little singular that, well known as my tastes were to those around me, we never met a soul to say, "that boy ought to be educated." so far as i know, my father's idea of making me a lawyer met with nothing but ridicule from the neighbors. did not a lawyer have to know latin and have money to pursue his studies? in my own daydreams i was a farmer driving his own team; in my mother's a preacher, though she had regretfully to admit that i might never be good enough for this profession. [ ] the actual sixth was my late excellent and esteemed cousin, judge simon bolivar newcomb, of new mexico. [ ] he had evidently forgotten the home instruction from my aunts, received more than a year previous to the date he mentions. [ ] the grandfather of president schurman of cornell university. i retain a dreamy impression of two half-grown or nearly grown boys, perhaps between fourteen and eighteen years of age, one of whom became, i believe, the father of the president. ii doctor foshay in the summer of , when i had passed the age of sixteen, we lived in a little school district a mile or two from the town of yarmouth, n. s. late in the summer we had a visit from a maternal uncle and aunt. as i had not seen moncton since i was six years old, and as i wanted very much to visit my grandfather prince once more, it was arranged that i should accompany them on their return home. an additional reason for this was that my mother's health had quite failed; there was no prospect of my doing anything where i was, and it was hoped that something might turn up at moncton. there was but one difficulty; the visitors had driven to st. john in their own little carriage, which would hold only two people; so they could not take me back. i must therefore find my own way from st. john to moncton. we crossed the bay of fundy in a little sailing vessel. among the passengers was an english ship captain who had just been wrecked off the coast of newfoundland, and had the saved remnant of his crew with him. on the morning of our departure the weather was stormy, so that our vessel did not put to sea--a precaution for which the captain passenger expressed great contempt. he did not understand how a vessel should delay going to see on account of a little storm. the walk of one hundred miles from st. john to moncton was for me, at that time, a much less formidable undertaking than it would appear in our times and latitude. a thirty-mile tramp was a bagatelle, and houses of entertainment--farmhouses where a traveler could rest or eat for a few pennies--were scattered along the road. but there was one great difficulty at the start. my instructions had been to follow the telegraph wires. i soon found that the line of telegraph came into the town from one direction, passed through it, and then left, not in the opposite direction, but perhaps at right angles to it. in which direction was the line to be followed? it was difficult to make known what i wanted. "why, my boy, you can't walk to moncton," was one answer. in a shop the clerks thought i wanted to ride on the telegraph, and, with much chuckling, directed me to the telegraph office where the man in charge would send me on. i tried in one direction which i thought could not be right, then i started off in the opposite one; but it soon became evident that that branch led up the river to frederickton. so i had to retrace my steps and take the original line, which proved to be the right one. the very first night i found that my grandfather's name was one to conjure with. i passed it with a hearty old farmer who, on learning who i was, entertained me with tales of mr. prince. the quality which most impressed the host was his enormous physical strength. he was rather below the usual stature and, as i remember him, very slightly built. yet he could shoulder a barrel of flour and lift a hogshead of molasses on its end, feats of strength which only the most powerful men in the region were equal to. on reaching my destination, i was not many days in learning that my grandfather was a believer in the maxims of "poor richard's almanac," and disapproved of the aimless way in which i had been bred. he began to suggest the desirableness of my learning to do something to make a living. i thought of certain mechanical tastes which had moved me in former years to whittle and to make a reel on which to wind yarn, and to mend things generally. so i replied that i thought the trade of a carpenter was the one i could most easily learn. he approved of the idea, and expressed the intention of finding a carpenter who would want my services; but before he did so, i was started in a new and entirely different direction. on her last visit to her birthplace, my mother brought back glowing reports of a wonderful physician who lived near moncton and effected cures of the sick who had been given up by other doctors. i need hardly remark that physicians of wonderful proficiency--diomeds of the medical profession, before whose shafts all forms of disease had to fall--were then very generally supposed to be realities. the point which specially commended dr. foshay to us was that he practiced the botanic system of medicine, which threw mineral and all other poisons out of the materia medica and depended upon the healing powers of plants alone. people had seen so much of the evil effects of calomel, this being the favorite alternative of the profession, that they were quite ready to accept the new system. among the remarkable cures which had given dr. foshay his great reputation was one of a young man with dyspepsia. he was reduced to a shadow, and the regular doctors had given him up as incurable. the new doctor took him to his home. the patient was addicted to two practices, both of which had been condemned by his former medical advisers. one was that of eating fat pork, which he would do at any hour of the day or night. the new doctor allowed him to eat all he wanted. another was getting up in the night and practicing an ablution of the stomach by a method too heroic to be described in anything but a medical treatise. [ ] he was now allowed to practice it to his heart's content. the outcome of the whole proceeding was that he was well in a few months, and, when i saw him, was as lusty a youth as one could desire to meet. before mr. prince could see a carpenter, he was taken ill. i was intensely interested to learn that his physician was the great doctor i had heard of, who lived in the village of salisbury, fifteen miles on the road to st. john. one of my aunts had an impression that the doctor wanted a pupil or assistant of some kind, and suggested that a possible opening might here be offered me. she promised to present me to the doctor on his next visit, after she had broached the subject to him. the time for which i waited impatiently at length arrived. never before had i met so charming a man. he was decidedly what we should now call magnetic. there was an intellectual flavor in his talk which was quite new to me. what fascinated me most of all was his speaking of the difficulties he encountered in supplying himself with sufficient "reading matter." he said it as if mental food was as much a necessity as his daily bread. he was evidently a denizen of that world of light which i had so long wished to see. he said that my aunt was quite right in her impression, and our interview terminated in the following liberal proposition on his part:-- s. n. to live with the doctor, rendering him all the assistance in his power in preparing medicines, attending to business, and doing generally whatever might be required of him in the way of help. the doctor, on his part, to supply s. n.'s bodily needs in food and clothing, and teach him medical botany and the botanic system of medicine. the contract to terminate when the other party should attain the age of twenty-one. after mentioning the teaching clause, he corrected himself a moment, and added: "at least all i know about it." all he knows about it! what more could heart desire or brain hold? the brilliancy of the offer was dimmed by only a single consideration; i had never felt the slightest taste for studying medicine or caring for the sick. that my attainments in the line could ever equal those of my preceptor seemed a result too hopeless to expect. but, after all, something must be done, and this was better than being a carpenter. before entering upon the new arrangement, a ratification was required on both sides. the doctor had to make the necessary household arrangements, and secure the consent of his wife. i had to ask the approval of my father, which i did by letter. like general grant and many great men, he was a man of exceptional sagacity in matters outside the range of his daily concerns. he threw much cold water on the scheme, but consented to my accepting the arrangement temporarily, as there was nothing better to be done. i awaited the doctor's next visit with glowing anticipation. in due course of time i stepped with him into his gig for the long drive, expecting nothing less on the journey than a complete outline of the botanic system of medicine and a programme of my future studies. but scarcely had we started when a chilling process commenced. the man erstwhile so effusive was silent, cold, impassive,--a marble statue of his former self. i scarcely got three sentences out of him during the journey, and these were of the most commonplace kind. could it be the same man? there was something almost frightful in being alongside a man who knew so much. when we reached our destination the horse had to be put away in the stable. i jumped up to the haymow to throw down the provender. it was a very peculiar feeling to do so under the eye of a man who, as he watched me, knew every muscle that i was setting in operation. a new chill came on when we entered the house and i was presented to its mistress. "so you 're the boy that's come to work for the doctor, are you?" "i have come to study with him, ma'am"' was my interior reply, but i was too diffident to say it aloud. naturally the remark made me very uncomfortable. the doctor did not correct her, and evidently must have told her something different from what he told me. her tone was even more depressing than her words; it breathed a coldness, not to say harshness, to which i had not been accustomed in a woman. there was nothing in her appearance to lessen the unpleasant impression. small in stature, with florid complexion, wide cheek bones that gave her face a triangular form, she had the eye and look of a well-trained vixen. as if fate were determined to see how rapid my downfall should be before the close of the day, it continued to pursue me. i was left alone for a few minutes. a child some four years old entered and made a very critical inspection of my person. the result was clearly unfavorable, for she soon asked me to go away. finding me indisposed to obey the order, she proceeded to the use of force and tried to expel me with a few strong pushes. when i had had enough of this, i stepped aside as she was making a push. she fell to the floor, then picked herself up and ran off crying, "mamma." the latter soon appeared with added ire infused into her countenance. "what did you hit the child for?" "i did n't hit her. what should i want to strike a child like that for?" "but she says you hit her and knocked her down." "i did n't, though--she was trying to push me and fell and hurt herself." a long piercing look of doubt and incredulity followed. "strange, very strange. i never knew that child to tell a lie, and she says you struck her." it was a new experience--the first time i had ever known my word to be questioned. during the day one thought dominated all others: where are those treasures of literature which, rich though they are, fail to satisfy their owner's voracious intellectual appetite? as houses were then built, the living and sleeping rooms were all on one main floor. here they comprised a kitchen, dining room, medicine room, a little parlor, and two small sleeping rooms, one for the doctor and one for myself. before many hours i had managed to see the interior of every one except the doctor's bedroom, and there was not a sign of a book unless such common ones as a dictionary or a bible. what could it all mean? next day the darkness was illuminated, at least temporarily, by a ray of light. the doctor had been absent most of the day before on a visit to some distant patient. now he came to me and told me he wanted to show me how to make bilious powders. several trays of dried herbs had been drying under the kitchen stove until their leaves were quite brittle. he took these and i followed him to the narrow stairway, which we slowly ascended, he going ahead. as i mounted i looked for a solution of the difficulty. here upstairs must be where the doctor kept his books. at each step i peered eagerly ahead until my head was on a level with the floor. rafters and a window at the other end had successively come into view and now the whole interior was visible. nothing was there but a loft, at the further end of which was a bed for the housemaid. the floor was strewn with dried plants. nothing else was visible. the disillusion seemed complete. my heart sank within me. on one side of the stairway at a level with the floor was screwed a large coffee mill. the doctor spread a sheet of paper out on the floor on the other side, and laid a line sieve upon it. then he showed me how to grind the dry and brittle leaves in the coffee mill, put them into the sieve, and sift them on the paper. this work had a scientific and professional look which infused a glimmer of light into the cimmerian darkness. the bilious powders were made of the leaves of four plants familiarly known as spearmint, sunflower, smartweed, and yarrow. in his practice a heaping teaspoonful of the pulverized leaves was stirred in a cup of warm water and the grosser parts were allowed to settle, while the patient took the finer parts with the infusion. this was one of dr. foshay's staple remedies. another was a pill of which the principal active ingredient was aloes. the art of making these pills seemed yet more scientific than the other, and i was much pleased to find how soon i could master it. beside these a number of minor remedies were kept in the medicine room. among them were tinctures of lobelia, myrrh, and capsicum. there was also a pill box containing a substance which, from its narcotic odor, i correctly inferred to be opium. this drug being prohibited by the botanic school i could not but feel that dr. foshay's orthodoxy was painfully open to question. determined to fathom the mystery in which the doctor's plans for my improvement were involved, i announced my readiness to commence the study of the botanic system. he disappeared in the direction of his bedroom, and soon returned with--could my eyes believe it?--a big book. it was one which, at the time of its publication, some thirty or forty years before, was well known to the profession,--miner and tully on the "fevers of the connecticut valley." he explained bringing me this book. "before beginning the regular study of the botanic system, you must understand something of the old system. you can do so by reading this book." a duller book i never read. there was every sort of detail about different forms of fever, which needed different treatment; yet calomel and, i think, opium were its main prescriptions. in due time i got through it and reported to my preceptor. "well, what do you think of the book?" "it praises calomel and opium too much. but i infer from reading it that there are so many kinds of fever and other diseases that an immense amount of study will be required to distinguish and treat them." "oh, you will find that all these minute distinctions are not necessary when we treat the sick on the botanic system." "what is the next thing for me? can i not now go on with the study of the botanic system?" "you are not quite ready for it yet. you must first understand something about phrenology. one great difference between us and doctors of the old school is that they take no account of difference of temperament, but treat the lymphatic and bilious in the same way. but we treat according to the temperament of the patient and must therefore be expert in distinguishing temperaments." "but i studied phrenology long ago and think i understand it quite well." he was evidently surprised at this statement, but after a little consideration said it was very necessary to be expert in the subject, and thought i had better learn it more thoroughly. he returned to his bedroom and brought a copy of fowler's "phrenology," the very book so familiar to me. i had to go over it again, and did so very carefully, paying special attention to the study of the four temperaments,--nervous, bilious, lymphatic, and sanguine. before many days i again reported progress. the doctor seemed a little impatient, but asked me some questions about the position of the organs and other matters pertaining to the subject, which i answered promptly and correctly by putting my fingers on them on my own head. but though satisfied with the answers, it was easy to see that he was not satisfied with me. he had, on one or two previous occasions, intimated that i was not wise and prudent in worldly matters. now he expressed himself more plainly. "this world is all a humbug, and the biggest humbug is the best man. that 's the yankee doctrine, and that 's the reason the yankees get along so well. you have no organ of secretiveness. you have a window in your breast that every one can look into and see what you are thinking about. you must shut that window up, like i do. no one can tell from my talk or looks what i am thinking about." it may seem incredible to the reader that i marveled much at the hidden meaning of this allegorical speech, and never for one moment supposed it to mean: "i, dr. foshay, with my botanic system of medicine, am the biggest humbug in these parts, and if you are going to succeed with me you must be another." but i had already recognized the truth of his last sentence. probably neither of us had heard of talleyrand, but from this time i saw that his hearty laugh and lively talk were those of a manikin. his demeanor toward me now became one of complete gravity, formality, and silence. he was always kindly, but never said an unnecessary word, and avoided all reference to reading or study. the mystery which enveloped him became deeper month after month. in his presence i felt a certain awe which prevented my asking any questions as to his intentions toward me. it must, of course, be a matter of lifelong regret that two years so important in one's education should have been passed in such a way,--still, they were not wholly misspent. from a teacher named monroe, [ ] who then lived near salisbury, i borrowed draper's chemistry, little thinking that i would one day count the author among my friends. a book peddler going his rounds offered a collection of miscellaneous books at auction. i bought, among others, a latin and a greek grammar, and assiduously commenced their study. with the first i was as successful as could be expected under the circumstances, but failed with the greek, owing to the unfamiliarity of the alphabet, which seemed to be an obstacle to memory of the words and forms. but perhaps the greatest event of my stay was the advent of a botanic druggist of boston, who passed through the region with a large wagonload of medicines and some books. he was a pleasant, elderly gentleman, and seemed much interested on learning that i was a student of the botanic system. he had a botanic medical college in or near boston, and strongly urged me to go thither as soon as i could get ready to complete my studies. from him the doctor, willing to do me a favor, bought some books, among them the "eclectic medical dispensary," published in cincinnati. of this book the doctor spoke approvingly, as founded on the true system which he himself practiced, and though i never saw him read it, he was very ready to accept the knowledge which i derived from it. the result was quite an enlargement of his materia medica, both in the direction of native plants and medicines purchased from his druggist. on one occasion this advance came near having serious consequences. i had compounded some pills containing a minute quantity of elaterium. the doctor gave them to a neighboring youth affected with a slight indisposition in which some such remedy was indicated. the directions were very explicit,--one pill every hour until the desired effect was produced. "pshaw," said the patient's brother, "there's nothin' but weeds in them pills, and a dozen of them won't hurt you." the idea of taking weed pills one at a time seemed too ridiculous, and so the whole number were swallowed at a dose. the result was, happily, not fatal, though impressive enough to greatly increase the respect of the young man's family for our medicines. the intellectual life was not wholly wanting in the village. a lodge of a temperance organization, having its headquarters in maine, was formed at a neighboring village. it was modeled somewhat after the fashion of the sons of temperance. the presiding officer, with a high sounding title, was my mother's cousin, tommy nixon. he was the most popular young man of the neighborhood. the rudiments of a classical education gained at a reputable academy in sackville had not detracted from his qualities as a healthy, rollicking young farmer. the lodge had an imposing ritual of which i well remember one feature. at stated intervals a password which admitted a member of any one lodge to a meeting of any other was received from the central authority--in maine, i believe. it was never to be pronounced except to secure admission, and was communicated to the members by being written on a piece of paper in letters so large that all could read. after being held up to view for a few moments, the paper was held in the flame of a candle with these words: "this paper containing our secret password i commit to the devouring element in token that it no longer exists save in the minds of the faithful brethren." the fine sonorous voice of the speaker and his manly front, seen in the lurid light of the burning paper, made the whole scene very impressive. there was also a society for the discussion of scientific questions, of which the founder and leading spirit was a youth named isaac steves, who was beginning the study of medicine. the president was a "worthy archon." our discussions strayed into the field of physiological mysteries, and got us into such bad odor with mrs. foshay and, perhaps, other ladies of the community, that the meetings were abandoned. a soil like that of the provinces at this time was fertile in odd characters including, possibly, here and there, a "heart pregnant with celestial fire." one case quite out of the common line was that of two or three brothers employed in a sawmill somewhere up the river petticodiac. according to common report they had invented a new language in order to enable them to talk together without their companions knowing what they were saying. i knew one of them well and, after some time, ventured to inquire about this supposed tongue. he was quite ready to explain it. the words were constructed out of english by the very simple process of reversing the syllables or the spelling. everything was pronounced backward. those who heard it, and knew the key, had no difficulty in construing the words; to those who did not, the words were quite foreign. the family of the neighborhood in which i was most intimate was that of a scotch farmer named parkin. father, mother, and children were very attractive, both socially and intellectually, and in later years i wondered whether any of them were still living. fifty years later i had one of the greatest and most agreeable surprises of my life in suddenly meeting the little boy of the family in the person of dr. george r. parkin, the well-known promoter of imperial federation in australia and the agent in arranging for the rhodes scholarships at oxford which are assigned to america. my duties were of the most varied character. i composed a little couplet designating my professions as those of physician, apothecary, chemist, and druggist, girl about house and boy in the barn. i cared for the horse, cut wood for the fire, searched field and forest for medicinal herbs, ordered other medicines from a druggist [ ] in st. john, kept the doctor's accounts, made his pills, and mixed his powders. this left little time for reading and study, and such exercises were still farther limited by the necessity of pursuing them out of sight of the housewife. as time passed on, the consciousness that i was wasting my growing years increased. i long cherished a vague hope that the doctor could and would do something to promote my growth into a physician, especially by taking me out to see his patients. this was the recognized method of commencing the study of medicine. but he never proposed such a course to me, and never told me how he expected me to become a physician. every month showed my prospects in a less hopeful light. i had rushed into my position in blind confidence in the man, and without any appreciation of the requirements of a medical practitioner. but these requirements now presented themselves to my mind with constantly increasing force. foremost among them was a knowledge of anatomy, and how could that be acquired except at a medical school? it was every day more evident that if i continued in my position i should reach my majority without being trained for any life but that of a quack. while in this state of perplexity, an event happened which suggested a way out. one day the neighborhood was stirred by the news that tommy nixon had run away--left his home without the consent of his parents, and sailed for the gold fields of australia. i was struck by the absence of any word of reprobation for his act. the young men at least seemed to admire the enterprising spirit he had displayed. a few weeks after his departure a letter which he wrote from london, detailing his adventures in the great metropolis, was read in my presence to a circle of admiring friends with expressions of wonder and surprise. this little circumstance made it clear to me that the easiest way out of my difficulty was to out the gordian knot, run away from dr. foshay, and join my father in new england. no doubt the uppermost question in the mind of the reader will be: why did you wait so long without having a clear understanding with the doctor? why not ask him to his face how he expected you to remain with him when he had failed in his pledges, and demand that he should either keep them or let you go? one answer, perhaps the first, must be lack of moral courage to face him with such a demand. i have already spoken of the mystery which seemed to enshroud his personality, and of the fascination which, through it, he seemed to exercise over me. but behind this was the conviction that he could not do anything for me were he ever so well disposed. that he was himself uneducated in many essentials of his profession had gradually become plain enough; but what he knew or possibly might know remained a mystery. i had heard occasional allusions, perhaps from mrs. foshay rather than from himself, to an institution supposed to be in maine, where he had studied medicine, but its name and exact location were never mentioned. altogether, if i told him of my intention, it could not possibly do any good, and he might be able to prevent my carrying it out, or in some other way to do much harm. and so i kept silent. tuesday, september , , was the day on which i fixed for the execution of my plan. the day previous i was so abstracted as to excite remarks both from mrs. foshay and her girl help, the latter more than once declaring me crazy when i made some queer blunder. the fact is i was oppressed by the feeling that the step about to be taken was the most momentous of my life. i packed a few books and clothes, including some mementoes of my mother, and took the box to the stage and post-office in the evening, to be forwarded to an assumed name in st. john the next afternoon. this box i never saw again; it was probably stopped by foshay before being dispatched. my plan was to start early in the morning, walk as far as i could during the day, and, in the evening, take the mail stage when it should overtake me. this course was necessitated by the fact that the little money that i had in my pocket was insufficient to pay my way to boston, even when traveling in the cheapest way. i thought it only right that the doctor should be made acquainted with my proceeding and my reason for taking it, so i indited a short letter, which i tried to reproduce from memory ten years later with the following result:-- dear doctor,--i write this to let you know of the step i am about to take. when i came to live with you, it was agreed that you should make a physician of me. this agreement you have never shown the slightest intention of fulfilling since the first month i was with you. you have never taken me to see a patient, you have never given me any instruction or advice whatever. beside this, you must know that your wife treats me in a manner that is no longer bearable. i therefore consider the agreement annulled from your failure to fulfill your part of it, and i am going off to make my own way in the world. when you read this, i shall be far away, and it is not likely that we shall ever meet again. if my memory serves me right, the doctor was absent on a visit to some distant patient on the night in question, and i did not think it likely that he would return until at least noon on the following day. by this time my box would have been safely off in the stage, and i would be far out of reach. to delay his receiving the letter as much as possible, i did not leave it about the house, but put it in the window of a shop across the way, which served the neighbors as a little branch post-office. but he must have returned sooner than i expected, for, to my great regret, i never again saw or heard of the box, which contained, not only the entire outfit for my journey, but all the books of my childhood which i had, as well as the little mementoes of my mother. the postmaster who took charge of the goods was a mr. pitman. when i again passed through salisbury, as i did ten years later, he had moved away, no one could tell me exactly where. i was on the road before daybreak, and walked till late at night, occasionally stopping to bathe my feet in a brook, or to rest for a few minutes in the shadow of a tree. the possibility of my being pursued by the doctor was ever present to my mind, and led me to keep a sharp lookout for coming vehicles. toward sunset a horse and buggy appeared, coming over a hill, and very soon the resemblance of vehicle and driver to the turnout of the doctor became so striking that i concealed myself in the shrubbery by the wayside until the sound of the wheels told me he was well past. the probability that my pursuer was in front of me was an added source of discomfort which led me to avoid the road and walk in the woods wherever the former was not visible to some distance ahead. but i neither saw nor heard anything more of the supposed pursuer, though, from what i afterward learned, there can be little doubt that it was actually foshay himself. the advent of darkness soon relieved me of the threatened danger, but added new causes of solicitude. the evening advanced, and the lights in the windows of the houses were becoming fewer and fewer, and yet the stage had not appeared. i slackened my pace, and made many stops, beginning to doubt whether i might not as well give up the stage and look for an inn. it was, i think, after ten o'clock when the rattling of wheels announced its approach. it was on a descending grade, and passed me like a meteor, in the darkness, quite heedless of my calls and gesticulations. fortunately a house was in sight where i was hospitably entertained, and i was very soon sound asleep, as became one who had walked fifty miles or more since daylight. thus ended a day to which i have always looked back as the most memorable of my life. i felt its importance at the time. as i walked and walked, the question in my mind was, what am i doing and whither am i going? am i doing right or wrong? am i going forward to success in life, or to failure and degradation? vainly, vainly, i tried to peer into the thick darkness of the future. no definite idea of what success might mean could find a place in my mind. i had sometimes indulged in daydreams, but these come not to a mind occupied as mine on that day. and if they had, and if fancy had been allowed its wildest flight in portraying a future, it is safe to say that the figure of an honorary academician of france, seated in the chair of newton and franklin in the palace of the institute, would not have been found in the picture. as years passed away i have formed the habit of looking back upon that former self as upon another person, the remembrance of whose emotions has been a solace in adversity and added zest to the enjoyment of prosperity. if depressed by trial, i think how light would this have appeared to that boy had a sight of the future been opened up to him. when, in the halls of learning, i have gone through the ceremonies which made me a citizen of yet another commonwealth in the world of letters, my thoughts have gone back to that day; and i have wished that the inexorable law of nature could then have been suspended, if only for one moment, to show the scene that providence held in reserve. next morning i was on my way betimes, having still more than thirty miles before me. and the miles seemed much longer than they did the day before, for my feet were sore and my limbs stiff. quite welcome, therefore, was a lift offered by a young farmer, who, driving a cart, overtook me early in the forenoon. he was very sociable, and we soon got into an interesting conversation. i knew that dr. foshay hailed from somewhere in this region, where his father still lived, so i asked my companion whether he knew a family of that name. he knew them quite well. "do you know anything of one of the sons who is a doctor?" "yes indeed; i know all about him, but he ain't no doctor. he tried to set up for one in salisbury, but the people there must a' found him out before this, and i don't know where he is now." "but i thought he studied medicine in fredericton or maine or somewhere on the border." "oh, he went off to the states and pretended to study, but he never did it. i tell you he ain't no more a doctor nor i am. he ain't smart enough to be a doctor." i fell into a fit of musing long enough to hear, in my mind's ear, with startling distinctness, the words of two years before: "this world is all a humbug, and the biggest humbug is the best man. . . . you have a window in your breast and you must close that window before you can succeed in life." now i grasped their full meaning. ten years later i went through the province by rail on my wedding journey. at dorchester, the next village beyond moncton, i was shown a place where insolvent debtors were kept "on the limits." "by stopping there," said my informant, "you can see dr. foshay." i suggested the question whether it was worth while to break our journey for the sake of seeing him. the reply of my informant deterred me. "it can hardly be worth while to do so. he will be a painful object to see,--a bloated sot, drinking himself to death as fast as he can." the next i heard of him was that he had succeeded. i reached st. john on the evening that a great celebration of the commencement of work on the first railway in the province was in progress. when things are undecided, small matters turn the scale. the choice of my day for starting out on my adventurous journey was partly fixed by the desire to reach st. john and see something of the celebration. darkness came on when i was yet a mile or two from the city; then the first rocket i had ever beheld rose before me in the sky. two of what seemed like unfortunate incidents at the time were most fortunate. subsequent and disappointing experience showed that had i succeeded in getting the ride i wished in the stage, the resulting depletion of my purse would have been almost fatal to my reaching my journey's end. arriving at the city, i naturally found all the hotels filled. at length a kindly landlady said that, although she had no bed to give me, i was quite welcome to lie on a soft carpeted floor, in the midst of people who could not find any other sleeping place. no charge was made for this accommodation. my hope of finding something to do which would enable me to earn a little money in st. john over and above the cost of a bed and a daily loaf of bread was disappointed. the efforts of the next week are so painful to recall that i will not harrow the feelings of the reader by describing them. suffice it to say that the adventure was wound up by an interview at calais, a town on the maine border, a few miles from eastport, with the captain of a small sailing vessel, hardly more than a boat. he was bound for salem. i asked him the price of a passage. "how much money have you?" he replied. i told him; whether it was one or two dollars i do not recall. "i will take you for that if you will help us on the voyage." the offer was gladly accepted. the little craft was about as near the opposite of a clipper ship as one can imagine, never intended to run in any but fair winds, and even with that her progress was very slow. there was a constant succession of west winds, and the result was that we were about three weeks reaching salem. here i met my father, who, after the death of my mother, had come to seek his fortune in the "states." he had reached the conclusion, on what grounds i do not know, that the eastern part of maryland was a most desirable region, both in the character of its people and in the advantages which it offered us. the result was that, at the beginning of , i found myself teacher of a country school at a place called massey's cross roads in kent county. after teaching here one year, i got a somewhat better school at the pleasant little village of sudlersville, a few miles away. of my abilities as a manager and teacher of youth the reader can judge. suffice it to say that, looking back at those two years, i am deeply impressed with the good nature of the people in tolerating me at all. my most pleasant recollection is that of two of my best pupils of sudlersville, nearly my own age. one was arthur e. sudler, for whose special benefit some chemical apparatus was obtained from philadelphia. he afterwards studied medicine at the university of pennsylvania and delighted me by writing that what i had taught him placed him among the best in his class in chemistry. the other was b. s. elliott, who afterward became an engineer or surveyor. one of my most vivid recollections at massey's relates to a subject which by no means forms a part of one's intellectual development, and yet is at the bottom of all human progress, that of digestion. the staple food of the inhabitants of a southern farming region was much heartier than any to which i had been accustomed. "pork and pone" were the staples, the latter being a rather coarse cake with little or no seasoning, baked from cornmeal. this was varied by a compound called "shortcake," a mixture of flour and lard, rapidly baked in a pan, and eaten hot. though not distasteful, i thought it as villainous a compound as a civilized man would put into his stomach. quite near my school lived a young bachelor farmer who might be designated as william bowler, esq., though he was better known as billy bowler. he had been educated partly at delaware college, newark, and was therefore an interesting young man to know. in describing his experiences at the college, he once informed me that they were all very pleasant except in a single point; that was the miserably poor food that the students got to eat. he could not, he declared, get along without good eating. this naturally suggested that my friend was something of a gourmand. great, therefore, was my delight when, a few weeks later, he expressed a desire to have me board with him. i accepted the offer as soon as possible. much to my disappointment, shortcake was on the table at the first meal and again at the second. it proved to be the principal dish twice, and i am not sure but three times a day. the other staple was fried meat. on the whole this was worse than pork and pone, which, if not toothsome, was at least wholesome. as the days grew into weeks, i wondered what delaware college could give its students to eat. to increase the perplexity, there were plenty of chickens in the yard and vegetables in the garden. i asked the cook if she could not boil some vegetables and bring them on the table. "mas'er bowler don't like wegetable." then i found that the chickens were being consumed in the kitchen and asked for one. "mas'er bowler don't like chicken," was the reply, with an added intimation that the chickens belonged to the denizens of the kitchen. the mystery was now so dark and deep that i determined to fathom it. i drew mr. bowler into conversation once more about delaware college, and asked him what the students had to eat when there. he had evidently forgotten his former remark and described what seemed to me a fairly well provided students' table. now i came down on him with my crusher. "you told me once that the table was miserably poor, so that you could hardly stand it. what fault had you to find with it?" he reflected a moment, apparently recalling his impression, then replied: "oh, they had no shortcake there!" in i availed myself of my summer vacation to pay my first visit to the national capital, little dreaming that it would ever be my home. i went as far as the gate of the observatory, and looked wistfully in, but feared to enter, as i did not know what the rules might be regarding visitors. i speculated upon the possible object of a queer red sandstone building, which seemed so different from anything else, and heard for the first time of the smithsonian institution. on the very beginning of my work at massey's the improvement in my position was so remarkable that i felt my rash step of a few months before fully justified. i wrote in triumph to my favorite aunt, rebecca prince, that leaving dr. foshay was the best thing i had ever done. i was no longer "that boy," but a respectable young man with a handle to my name. just what object i should pursue in life was still doubtful; the avenues of the preferment i would have liked seemed to be closed through my not being a college graduate. i had no one to advise me as to the subjects i should pursue or the books i should study. on such books as i could get, i passed every spare hour. my father sent me cobbett's english grammar, which i found amusing and interesting, especially the criticisms upon the grammar found here and there in royal addresses to parliament and other state papers. on the whole i am not sure but that the book justified my father's good opinion, although i cannot but think that it was rather hypercritical. i had been taught the rudiments of french in wallace when quite a child by a mr. oldright, of whose methods and pronunciation my memory gives me a most favorable impression. i now got cobbett's french grammar, probably a much less commendable book than his english one. i had never yet fathomed the mysteries of analytic geometry or the calculus, and so got davies' books on those subjects. that on the calculus was perhaps the worst that could be put into the hands of a person situated as i was. two volumes of bezout's mathematics, in french, about a century old, were, i think, rather better. say's political economy was the first book i read on that subject, and it was quite a delight to see human affairs treated by scientific methods. i finally reached the conclusion that mathematics was the study i was best fitted to follow, though i did not clearly see in what way i should turn the subject to account. i knew that newton's "principia" was a celebrated book, so i got a copy of the english translation. the path through it was rather thorny, but i at least caught the spirit here and there. no teacher at the present time would think of using it as a text-book, yet as a mental discipline, and for the purpose of enabling one to form a mental image of the subject, its methods at least are excellent. i got a copy of the "american journal of science," hoping it might enlighten me, but was frightened by its big words, and found nothing that i could understand. during the year at sudlersville i made several efforts which, though they were insignificant so far as immediate results were concerned, were in some respects of importance for my future work. with no knowledge of algebra except what was derived from the meagre text-books i could pick up,--not having heard even the name of abel, or knowing what view of the subject was taken by professional mathematicians,--i made my first attempt at a scientific article, "a new demonstration of the binomial theorem." this i sent to professor henry, secretary of the smithsonian institution, to see if he deemed it suitable for publication. he promptly replied in the negative, but offered to submit it to a professional mathematician for an opinion of its merits. i gladly accepted this proposal, which was just what i wanted. in due course a copy of the report was sent me. one part of the work was praised for its elegance, but a lack of completeness and rigor was pointed out. it was accompanied by a pleasant note from professor henry remarking that, while not so favorable as i might have expected, it was sufficiently so to encourage me in persevering. the other effort to which i refer was of quite a different character. a copy of the "national intelligencer," intended for some subscriber who had left sudlersville, came to the post-office for several months, and, there being no claimant, i frequently had an opportunity to read it. one of its features was frequent letters from volunteer writers on scientific subjects. among these was a long letter from one g. w. eveleth, the object of which was to refute the accepted theory of the universe, especially the view of copernicus. for aught i knew mr. eveleth held as high a position as any one else in the world of science and letters, so i read his article carefully. it was evidently wholly fallacious, yet so plausible that i feared the belief of the world in the doctrine of copernicus might suffer a severe shock, and hastened to the rescue by writing a letter over my own name, pointing out the fallacies. this was published in the "national intelligencer"--if my memory serves me right--in . my full name, printed in large capitals, in a newspaper, at the bottom of a letter, filled me with a sense of my temerity in appearing so prominently in print, as if i were intruding into company where i might not be wanted. my letter had two most unexpected and gratifying results. one was a presentation of a copy of lee's "tables and formulæ," which came to me a few days later through the mail with the compliments of colonel abert. not long afterward came a letter from professor j. lawrence smith, afterward a member of the national academy of sciences, transmitting a copy of a pamphlet by him on the theory that meteorites were masses thrown up from the volcanoes of the moon, and asking my opinion on the subject. i had not yet gotten into the world of light. but i felt as one who, standing outside, could knock against the wall and hear an answering knock from within. the beginning of found me teaching in the family of a planter named bryan, residing in prince george county, md., some fifteen or twenty miles from washington. this opened up new opportunities. i could ride into washington whenever i wished, leave my horse at a livery stable, and see whatever sights the city offered. the smithsonian library was one of the greatest attractions. sometime in may, , i got permission from the attendant in charge to climb into the gallery and see the mathematical books. here i was delighted to find the greatest treasure that my imagination had ever pictured,--a work that i had thought of almost as belonging to fairyland. and here it was right before my eyes--four enormous volumes,--"mécanique céleste, by the marquis de laplace, peer of france; translated by nathaniel bowditch, ll. d., member of the royal societies of london, edinburg, and dublin." i inquired as to the possibility of my borrowing the first volume, and was told that this could be done only by special authority of professor henry. i soon got the necessary authority through mr. rhees, the chief clerk, whose kindness in the matter deeply impressed me, signed a promise to return it within one month, and carried it in triumph to my little schoolhouse. i dipped into it here and there, but at every step was met by formulæ and methods quite beyond the power of one who knew so little of mathematics. in due time i brought the book back as promised. up to this time i think i had never looked upon a real live professor; certainly not upon one of eminence in the scientific world. i wondered whether there was any possibility of my making the acquaintance of so great a man as professor henry. some time previous a little incident had occurred which caused me some uneasiness on the subject. i had started out very early on a visit to washington, or possibly i had stayed there all night. at any rate, i reached the smithsonian building quite early, opened the main door, stepped cautiously into the vestibule, and looked around. here i was met by a short, stout, and exceedingly gruff sort of a man, who looked upon my entrance with evident displeasure. he said scarcely a word, but motioned me out of the door, and showed me a paper or something in the entrance which intimated that the institution would be open at nine o'clock. it was some three minutes before that hour so i was an intruder. the man looked so respectable and so commanding in his appearance that i wondered if he could be professor henry, yet sincerely hoped he was not. i afterward found that he was only "old peake," the janitor. [ ] when i found the real professor henry he received me with characteristic urbanity, told me something of his own studies, and suggested that i might find something to do in the coast survey, but took no further steps at that time. the question whether i was fitted for any such employment now became of great interest. the principal question was whether one must know celestial mechanics in order to secure such a position, so, after leaving professor henry, i made my way to the coast survey office, and was shown to the chief clerk, as the authority for the information. i modestly asked him whether a knowledge of physical astronomy was necessary to a position in that office. instead of frankly telling me that he did not know what physical astronomy was, he answered in the affirmative. so i left with the impression that i must master the "mécanique céleste" or some similar treatise before finding any opening there. i could not, of course, be satisfied with a single visit to such a man, and so called several times during the year. one thing i wondered about was whether he would remember me when he again saw me. on one occasion i presented him with a plan for improving the cavendish method of determining the density of the earth, which he took very kindly. i subsequently learned that he was much interested in this problem. on another occasion he gave me a letter to mr. j. e. hilgard, assistant in charge of the coast survey office. my reception by the latter was as delightful as that by professor henry. i found from my first interview with him that the denizens of the world of light were up to the most sanguine conceptions i ever could have formed. at this time, or probably some time before, i bought a copy of the "american ephemeris" for , and amused myself by computing on a slate the occultations visible at san francisco during the first few months of the year. at this time i had learned nothing definite from mr. hilgard as to employment in his office. but about december, , i received a note from him stating that he had been talking about me to professor winlock, superintendent of the "nautical almanac," and that i might possibly get employment on that work. when i saw him again i told him that i had not yet acquired such a knowledge of physical astronomy as would be necessary for the calculations in question; but he assured me that this was no drawback, as formulæ for all the computations would be supplied me. i was far from satisfied at the prospect of doing nothing more than making routine calculations with formulæ prepared by others; indeed, it was almost a disappointment to find that i was considered qualified for such a place. i could only console myself by the reflection that the ease of the work would not hinder me from working my way up. shortly afterward i understood that it was at least worth while to present myself at cambridge, and so started out on a journey thither about the last day of the year . at that time even a railroad journey was quite different from what it is now. the cars were drawn through baltimore by horses. at havre de grace the train had to stop and the passengers were taken across the river in a ferryboat to another train. at philadelphia the city had to be traversed by transfer coaches. looking around for this conveyance, i met a man who said he had it. he shoved me into it and drove off. i remarked with suspicion that no other coaches were accompanying us. after a pretty long drive the speed of the horses gradually began to slacken. at length it came to a complete stop in front of a large building, and i got out. but it was only a freight station, locked up and dark throughout. the driver mumbled something about his fare, then rolled back on his seat, seemingly dead drunk. the nearest sign of life was at a tavern a block or two away. there i found that i was only a short distance from the station of departure, and reached my train barely in time. landing in new york at the first glimmer of dawn, near the end of the line of passengers i was momentarily alarmed to see a man pick up what seemed to be a leather purse from right between my feet. it was brown and, so far as i could see, just like my own. i immediately felt the breast pocket of my coat and found that my own was quite safe. the man who picked up the purse inquired in the politest tone possible if it was mine, to which i replied in the negative. he retreated a short distance and then a bystander came up and chided me in a whisper for my folly in not claiming the purse. the only reply he got was, "oh, i'm up to all your tricks." on a repetition of this assurance the pair sneaked away. arriving at cambridge, i sought out professor winlock and was informed that no immediate employment was open at his office. it would be necessary for him to get authority from washington. after this was obtained some hope might be held out, so i appeared in the office from time to time as a visitor, my first visit being that described in the opening chapter. [ ] i may remark, for the benefit of any medical reader, that it involved the use of two pails, one full of water, the other empty. when he got through the ablution, one pail was empty, and the other full. my authority for the actuality of this remarkable proceeding was some inmate of the house at the time, and i give credence to the story because it was not one likely to be invented. [ ] rev. alexander h. monroe, who, i have understood, afterward lived in montreal. i have often wished to find a trace of him, but do not know whether he is still living. [ ] our druggist was mr. s. l. tilley, afterward sir leonard tilley, the well-known canadian minister of finance. [ ] peake, notwithstanding his official title, would seem to have been more than an ordinary janitor, as he was the author of a guide to the smithsonian institution. iii the world of sweetness and light the term "nautical almanac" is an unfortunate misnomer for what is, properly speaking, the "astronomical ephemeris." it is quite a large volume, from which the world draws all its knowledge of times and seasons, the motions of the heavenly bodies, the past and future positions of the stars and planets, eclipses, and celestial phenomena generally which admit of prediction. it is the basis on which the family almanac is to rest. it also contains the special data needed to enable the astronomer and navigator to determine their position on land or sea. the first british publication of the sort, prepared by maskelyne, astronomer royal, a century ago, was intended especially for the use of navigators; hence the familiar appellation, which i call unfortunate because it leads to the impression that the work is simply an enlargement and improvement of the household almanac. the leading nations publish ephemerides of this sort. the introductions and explanations are, of course, in the languages of the respective countries; but the contents of the volume are now so much alike that the duplication of work involved in preparing them seems quite unnecessary. yet national pride and emulation will probably continue it for some time to come. the first appropriation for an american ephemeris and nautical almanac was made by congress in . lieutenant charles henry davis, as a leader and moving spirit in securing the appropriation, was naturally made the first superintendent of the work. at that time astronomical science in our country was so far from being reduced to a system that it seemed necessary to have the work prepared at some seat of learning. so, instead of founding the office in washington, it was established at cambridge, the seat of harvard university, where it could have the benefit of the technical knowledge of experts, and especially of professor benjamin peirce, who was recognized as the leading mathematician of america. here it remained until , when conditions had so far changed that the office was removed to washington, where it has since remained. to this work i was especially attracted because its preparation seemed to me to embody the highest intellectual power to which man had ever attained. the matter used to present itself to my mind somewhat in this way: supply any man with the fundamental data of astronomy, the times at which stars and planets cross the meridian of a place, and other matters of this kind. he is informed that each of these bodies whose observations he is to use is attracted by all the others with a force which varies as the inverse square of their distance apart. from these data he is to weigh the bodies, predict their motion in all future time, compute their orbits, determine what changes of form and position these orbits will undergo through thousands of ages, and make maps showing exactly over what cities and towns on the surface of the earth an eclipse of the sun will pass fifty years hence, or over what regions it did pass thousands of years ago. a more hopeless problem than this could not be presented to the ordinary human intellect. there are tens of thousands of men who could be successful in all the ordinary walks of life, hundreds who could wield empires, thousands who could gain wealth, for one who could take up this astronomical problem with any hope of success. the men who have done it are therefore in intellect the select few of the human race,--an aristocracy ranking above all others in the scale of being. the astronomical ephemeris is the last practical outcome of their productive genius. on the question whether the world generally reasoned in this way, i do not remember having any distinct idea. this was certainly not because i was indifferent to the question, but because it never strongly presented itself to my mind. from my point of view it would not have been an important one, because i had already formed the conviction that one should choose that sphere in life to which he was most strongly attracted, or for which his faculties best fitted him. a few months previous to my advent commander davis had been detached from the superintendency and ordered to command the sloop st. mary's. he was succeeded by professor joseph winlock, who afterward succeeded george p. bond as director of the harvard observatory. most companionable in the society of his friends, winlock was as silent as general grant with the ordinary run of men. withal, he had a way of putting his words into exact official form. the following anecdote of him used to be current. while he was attached to the naval academy, he was introduced one evening at a reception to a visiting lady. he looked at the lady for a decorous length of time, and she looked at him; then they parted without saying a word. his introducer watched the scene, and asked him, "why did you not talk to that lady?" "i had no statement to make to her," was the reply. dr. gould told me this story was founded on fact, but when, after winlock's death, it was put off on me with some alterations, i felt less sure. the following i believe to be authentic. it occurred several years later. hilgard, in charge of the coast survey office, was struck by the official terseness of the communications he occasionally received from winlock, and resolved to be his rival. they were expecting additions to their families about the same time, and had doubtless spoken of the subject. when hilgard's arrived, he addressed a communication to winlock in these terms:-- "mine's a boy. what's yours?" in due course of time the following letter was received in reply:-- dear hilgard:-- _boy._ yours, etc., j. winlock. when some time afterward i spoke to winlock on the subject, and told him what hilgard's motive was, he replied, "it was not fair in hilgard to try and take me unawares in that way. had i known what he was driving at, i might have made my letter still shorter." i did not ask him how he would have done it. it is of interest that the "boy" afterward became one of the assistant secretaries of the smithsonian institution. one of the most remarkable features of the history of the "nautical almanac" is the number of its early assistants who have gained prominence or distinction in the various walks of life. it would be difficult to find so modest a public work to exceed it in this respect. john d. runkle, who lived till , was, as i have said, the senior and leading assistant in the office. he afterward became a professor in the institute of technology, and succeeded rogers as its president. in he started the school of manual training, which has since been one of the great features of the institute. he afterward resigned the presidency, but remained its principal professor of mathematics. he was the editor and founder of the "mathematical monthly," of which i shall presently have more to say. the most wonderful genius in the office, and the one who would have been the most interesting subject of study to a psychologist, was truman henry safford. in early childhood he had excited attention by his precocity as what is now sometimes called a "lightning calculator." a committee of the american academy of arts and science was appointed to examine him. it very justly and wisely reported that his arithmetical powers were not in themselves equal to those of some others on record, especially zerah colburn, but that they seemed to be the outcome of a remarkable development of the reasoning power. when nine years old, he computed almanacs, and some of his work at this age is still preserved in the harvard university library. he graduated at harvard in , and was soon afterward taken into the nautical almanac office, while he also worked from time to time at the cambridge observatory. it was found, however, that the power of continuous work was no greater in him than in others, nor did he succeed in doing more than others in the course of a year. the mental process by which certain gifted arithmetical computers reach almost in an instant the results of the most complicated calculations is a psychological problem of great interest, which has never been investigated. no more promising subject for the investigation could ever have been found than safford, and i greatly regret having lost all opportunities to solve the problem. what was of interest in safford's case was the connection of this faculty with other remarkable mental powers of an analogous but yet different kind. he had a remarkable faculty for acquiring, using, and reading languages, and would have been an accomplished linguist had he turned his attention in that direction. he was a walking bibliography of astronomy, which one had only to consult in order to learn in a moment what great astronomers of recent times had written on almost any subject, where their work was published, and on what shelf of the harvard library the book could be found. but the faculty most closely connected with calculation was a quickness and apprehension of vision, of which the following is an example:-- about he visited the naval observatory in washington for the first time in his life. we wanted a certain catalogue of stars and went together into the library. the required catalogue was on one of a tier of shelves containing altogether a hundred, or perhaps several hundred volumes. "i do not know whether we have the book," said i, "but if we have, it is on one of these shelves." i began to go through the slow process of glancing at the books one by one until my eyes should strike the right title. he stood back six or eight feet and took in all the shelves seemingly at one glance, then stepped forward and said, "here it is." i might have supposed this an accident, but that he subsequently did practically the same thing in my office, selecting in a moment a book we wanted to see, after throwing a rapid glance over shelves containing perhaps a hundred volumes. an example of his apprehension and memory for numbers was narrated by mr. alvan clark. when the latter had completed one of his great telescopes for the university of chicago, safford had been named as director, and accompanied the three members of the firm to the city when they carried the object glass thither. on leaving the train all four took their seats in a hotel omnibus, safford near the door. then they found that they had forgotten to give their baggage checks to the expressman; so the other three men passed their checks to safford, who added his own and handed all four to the conductor of the omnibus. when it was time for the baggage to come to the hotel, there was such a crowd of new arrivals that the attendants could not find it. the hotel clerk remarked on inquiry, "if i only knew the numbers of your checks, i would have no difficulty in tracing your trunks." safford at once told off the four numbers, which he had read as he was passing the checks to the conductor. the great fire practically put an end to the activity of the chicago observatory and forced its director to pursue his work in other fields. that he failed to attain that commanding position due to his genius is to be ascribed to a cause prevalent among us during all the middle part of the century; perhaps that from which most brilliant intellects fail to reach eminence: lack of the power of continuous work necessary to bring important researches to a completion. another great intellect of the office was chauncey wright. if wright had systematically applied his powers, he might have preceded or supplanted herbert spencer as the great exponent of the theory of evolution. he had graduated at harvard in , and was a profound student of philosophy from that time forward, though i am not aware that he was a writer. when in sir william hamilton's "lectures on metaphysics" appeared, he took to them with avidity. in appeared darwin's "origin of species," and a series of meetings was held by the american academy, the special order of which was the discussion of this book. wright and myself, not yet members, were invited to be present. to judge of the interest it is only necessary to remark that agassiz and gray were the two leading disputants, the first taking ground against darwin, the other in his favor. wright was a darwinist from the very beginning, explaining the theory in private conversation from a master's point of view, and soon writing upon it in the "north american review" and in other publications. of one of his articles darwin has been quoted as saying that it was the best exposition of his theory that had then appeared. after his untimely death in , wright's papers were collected and published under the title of "philosophical discussions." [ ] their style is clear-cut and faultless in logical form, yet requiring such close attention to every word as to be less attractive to the general reader of to-day than that of spencer. in a more leisurely age, when men wanted to think profoundly as they went along in a book, and had little to disturb the current of their thoughts, it would have commanded wide attention among thinking men. a singular peculiarity which i have sometimes noticed among men of intelligence is that those who are best informed on the subject may be most reckless as regards the laws of health. wright did all of his office work in two or three months of the year. during those months he worked at his computations far into the hours of the morning, stimulating his strength with cigars, and dropping his work only to take it up when he had had the necessary sleep. a strong constitution might stand this for a few years, as his did. but the ultimate result hardly needs to be told. besides the volume i have mentioned, wright's letters were collected and printed after his death by the subscription of his friends. in these his philosophic views are from time to time brought out in a light, easy way, much more charming than the style of his elaborate discussions. it was in one of his letters that i first found the apothegm, "men are born either platonists or aristotelians," a happy drawing of the line which separates the hard-headed scientific thinker of to-day from the thinkers of all other classes. william ferrell, a much older man than myself, entered the office about the same time as i did. he published papers on the motions of fluids on the earth's surface in the "mathematical monthly," and became one of the great authorities on dynamic meteorology, including the mathematical theory of winds and tides. he was, i believe, the first to publish a correct theory of the retardation produced in the rotation of the earth by the action of the tides, and the consequent slow lengthening of the day. james edward oliver might have been one of the great mathematicians of his time had he not been absolutely wanting in the power of continuous work. it was scarcely possible to get even his year's office work out of him. yet when i once wrote him a question on certain mathematical forms which arise in the theory of "least squares," he replied in a letter which, with some developments and change of form, would have made a worthy memoir in any mathematical journal. as a matter of fact, the same thoughts did appear some years after, in an elaborate paper by professor j. w. l. glaisher, of england, published by the royal astronomical society. oliver, who afterward became professor of higher mathematics at cornell university, was noted for what i think should be considered the valuable quality of absent-mindedness. it was said of him that he was once walking on the seashore with a small but valuable gold watch loose in his pocket. while deep in thought he started a kind of distraction by picking up flat stones and skipping them on the water. taking his watch from his pocket he skipped it as a stone. when i became well acquainted with him i took the liberty of asking him as to the correctness of this story. he could not positively say whether it was true or not. the facts were simply that he had the watch, that he had walked on the seashore, had skipped stones, missed the watch at some subsequent time, and never saw it again. more definite was an observation made on his movements one afternoon by a looker-out from a window of the nautical almanac office. across the way the road was bounded by no fence, simply passing along the side of an open field. as oliver got near the office, his chin on his breast, deep in thought, he was seen gradually to deviate from the sidewalk, and direct his steps along the field. he continued on this erratic course until he ran almost against the fence at the other end. this awoke him from his reverie, and he started up, looked around, and made his way back to the road. i have spoken only of the men who were employed at the office at the time i entered. previous to my time were several who left to accept professorships in various parts of the country. among them were professors van vleck, of middletown, and hedrick and kerr, of north carolina. not desiring to leave upon the mind of the reader the impression that all of whom i have not spoken remained in obscurity, i will remark that mr. isaac bradford rose to the position of mayor of the city of cambridge, and that fugitive pieces in prose and poetry by mr. e. j. loomis were collected in a volume. [ ] the discipline of the public service was less rigid in the office at that time than at any government institution i ever heard of. in theory there was an understanding that each assistant was "expected" to be in the office five hours a day. the hours might be selected by himself, and they generally extended from nine until two, the latter being at that time the college and family dinner hour. as a matter of fact, however, the work was done pretty much where and when the assistant chose, all that was really necessary being to have it done on time. it will be seen that the excellent opportunities offered by this system were well improved by those who enjoyed them--improved in a way that i fear would not be possible in any other surroundings. i took advantage of them by enrolling myself as a student of mathematics in the lawrence scientific school. on this occasion i well remember my pleasant reception by charles w. eliot, tutor in mathematics, and e. n. horsford, professor of chemistry, and, i believe, dean of the school. as a newcomer into the world of light, it was pleasant to feel the spirit with which they welcomed me. the departments of chemistry and engineering were about the only ones which, at that time, had any distinct organization. as a student of mathematics it could hardly be said that anything was required of me either in the way of attendance on lectures or examinations until i came up for the degree of bachelor of science. i was supposed, however, to pursue my studies under the direction of professor peirce. so slight a connection with the university does not warrant me in assuming an authoritative position as an observer of its men or its workings. yet there are many features associated with it which i have not seen in print, which have probably disappeared with the progress of the age, and to which, therefore, allusion may be made. one, as it presents itself to my memory, is the great variety and picturesqueness of character which the university then presented. i would like to know whether the changes in men which one fancies he sees during his passage from youth to age are real, or only relative to his point of view. if my impressions are correct, our educational planing mill cuts down all the knots of genius, and reduces the best of the men who go through it to much the same standard. does not the harvard professor of to-day always dine in a dress coat? is he not free from every eccentricity? do the students ever call him "benny" or "tobie"? is any "old soph" [ ] now ambulant on the college green? is not the administration of the library a combination of liberality and correctness? is such a librarian as john langdon sibley possible? mr. sibley, under a rough exterior, was one of the best-hearted and most admirable of men, with whom i ultimately formed an intimate friendship. but our first acquaintance was of a very unfavorable kind. it came about in this way: not many days after being taken into the nautical almanac office i wanted a book from the university library, and asked a not over-bright old gentleman in the office what formalities were necessary in order to borrow it. "just go over and tell them you want it for the nautical almanac." "but they don't know me at the library, and surely will not give a book to any stray caller because he says he wants it for the nautical almanac." "you have only to say 'nautical almanac' and you will get the book." i argued the matter as stoutly as courtesy admitted, but at length, concluding that i was new to the rules and regulations of the place, accepted the supposedly superior knowledge of my informer and went over to the library with a due measure of assurance. the first attendant whom i addressed referred me to the assistant librarian, and he again to the librarian. after these formalities, conducted with impressive gravity, my assurance wilted when i was ushered into the august presence of the chief librarian. as the mental picture of the ensuing scene has shaped itself through more than forty years it shows a personage of imposing presence, gigantic features, and forbidding countenance, standing on a dais behind a desk, expounding the law governing the borrowing of books from the library of harvard college to an abashed youth standing before him. i left without the book, but with a valuable addition to my knowledge of library management. we both remembered this interview, and exchanged impressions about it long years after. "i thought you the most crusty and disobliging old man i had ever seen." "and i thought _you_ the most presumptuous youth that had ever appeared in the library." one of mr. sibley's professional doctrines was that at least one copy of everything printed was worth preserving. i strove to refute him, but long failed. half in derision, i offered the library the stub of my wash-book. instead of throwing it into the wastebasket he kept it, with the remark that the wash-book of a nineteenth century student would at some future time be of interest to the antiquarian. in due time i received a finely engraved acknowledgment of the gift. but i forced him from his position at last. he had to admit that copies of the theatre posters need not all be preserved. it would suffice to keep a few specimens. professor peirce was much more than a mathematician. like many men of the time, he was a warm lover and a cordial hater. it could not always be guessed which side of a disputed question he would take; but one might be fairly sure that he would be at one extreme or the other. as a speaker and lecturer he was very pleasing, neither impressive nor eloquent, and yet interesting from his earnestness and vivacity. for this reason it is said that he was once chosen to enforce the views of the university professors at a town meeting, where some subject of interest to them was coming up for discussion. several of the professors attended the meeting, and peirce made his speech. then a townsman rose and took the opposite side, expressing the hope that the meeting would not allow itself to be dictated to by these nabobs of harvard college. when he sat down, peirce remained in placid silence, making no reply. when the meeting broke up, some one asked peirce why he had not replied to the man. "why! did you not hear what he called us? he said we were nabobs! i so enjoyed sitting up there and seeing all that crowd look up to me as a nabob that i could not say one word against the fellow." the first of the leading astronomers whose acquaintance i made was dr. benjamin apthorp gould. knowing his eminence, i was quite surprised by his youthful vivacity. his history, had i time to recount it, might be made to serve well the purpose of a grave lesson upon the conditions required, even by the educated public, of a scientific investigator, capable of doing the highest and best work in his branch. the soul of generosity and the pink of honor, ever ready to lend a hand to a struggling youth whom he found deserving of help, enthusiastically devoted to his favorite science, pursuing it in the most exalted spirit, animated by not a single mean motive, it might have been supposed that all the facilities the world could offer would have been open to him in his career. if such was not the case to the extent one might have wished, i do not mean to intimate that his life can be regarded as a failure. in whatever respect the results may have fallen off from his high ideal, it is more to be regretted on the score of science than on his own. scorning pretense and charlatanry of all kinds, believing that only the best were to be encouraged, he was far from being a man of the people. only a select few enjoyed his favor, but these few well deserved it. that no others would have deserved it i should be far from intimating. the undisguised way in which he expressed his sentiments for any one, no matter how influential, who did not come up to the high standard he set, was not adapted to secure the favor even of the most educated community. of worldly wisdom in this matter he seemed, at least in his early days, to know nothing. he graduated at harvard in , in one of the very distinguished classes. being fond of astronomy, he was struck with the backward condition of that science in our country. he resolved to devote his life to building up the science in america. he went to germany, then the only country in which astronomy was pursued in its most advanced form, studied under gauss and argelander, and took his degree at göttingen in . soon after his return he founded the "astronomical journal," and also took a position as chief of the longitude department in the coast survey. the great misfortune of his life, and temporarily at least, a severe blow to american astronomy, were associated with his directorship of the dudley observatory at albany. this institution was founded by the munificence of a wealthy widow of albany. the men to whom she intrusted the administration of her gift were among the most prominent and highly respected citizens of the place. the trustees went wisely to work. they began by forming an advisory scientific council, consisting of bache, henry, and peirce. under the direction of this council the observatory was built and equipped with instruments. when ready for active work in , gould moved thither and took personal charge. very soon rumors of dissension were heard. the affair gradually grew into a contest between the director and the trustees, exceeding in bitterness any i have ever known in the world of learning or even of politics. it doubtless had its origin in very small beginnings. the policy of the director recognized no end but scientific efficiency. the trustees, as the responsible administrators of the trust, felt that they had certain rights in the matter, especially that of introducing visitors to inspect the institution and look through the telescope. how fatal the granting of such courtesies is to continuous work with an instrument only astronomers know; and one of the most embarrassing difficulties the director of such an institution meets with is to effect a prudent compromise between the scientific efficiency of his institution and the wishes of the public. but gould knew no such word as compromise. it was humiliating to one in the position of a trustee to send some visitor with a permit to see the observatory, and have the visitor return with the report that he had not been received with the most distinguished courtesy, and, perhaps, had not seen the director at all, but had only been informed by an assistant of the rules of the place and the impossibility of securing admission. this spark was enough to kindle a fire. when the fire gathered strength, the director, instead of yielding, called on the scientific council for aid. it is quite likely that, had these wise and prudent men been consulted at each step, and their advice been followed, he would have emphasized his protest by resigning. but before they were called in, the affair had gone so far that, believing the director to be technically right in the ground he had taken and the work he had done, the council felt bound to defend him. the result was a war in which the shots were pamphlets containing charges, defenses, and rejoinders. the animosity excited may be shown by the fact that the attacks were not confined to gould and his administration, but extended to every institution with which he and the president of the council were supposed to be connected. bache's administration of the coast survey was held up to scorn and ridicule. it was supposed that gould, as a cambridge astronomer, was, as a matter of course, connected with the nautical almanac office, and paid a high salary. this being assumed, the office was included in the scope of attack, and with such success that the item for its support for the year , on motion of mr. dawes, was stricken out of the naval bill. how far the fire spread may be judged by the fact that a whole edition of the "astronomical journal," supposed to have some mention of the affair in the same cover, was duly sent off from the observatory, but never reached its destination through the mails. gould knew nothing of this fact until, some weeks later, i expressed my surprise to him at not receiving no. . how or by whom it was intercepted, i do not know that he ever seriously attempted to inquire. the outcome of the matter was that the trustees asserted their right by taking forcible possession of the observatory. during my first year at cambridge i made the acquaintance of a senior in the college whose untimely death seven years later i have never ceased to deplore. this was william p. g. bartlett, son of a highly esteemed boston physician, dr. george bartlett. the latter was a brother of sidney bartlett, long the leader of the boston bar. bartlett was my junior in years, but his nature and the surrounding circumstances were such that he exercised a powerful influence upon me. his virile and aggressive honesty could not be exceeded. his mathematical abilities were of a high order, and he had no ambition except to become a mathematician. had he entered public life at washington, and any one had told me that he was guilty of a dishonest act, i should have replied, "you might as well tell me that he picked up the capitol last night and carried it off on his back." the fact that one could say so much of any man, i have always looked upon as illustrating one of the greatest advantages of having a youth go through college. the really important results i should look for are not culture or training alone, but include the acquaintance of a body of men, many of whom are to take leading positions in the world, of a completeness and intimacy that can never be acquired under other circumstances. the student sees his fellow students through and through as he can never see through a man in future years. it was, and i suppose still is, the custom for the members of a graduating class at harvard to add to their class biographies a motto expressing their aspirations or views of life. bartlett's was, "i love mathematics and hate humbug." what the latter clause would have led to in his case, had he gone out into the world, one can hardly guess. "i have had a long talk with my uncle sidney," he said to me one day. "he wants me to study law, maintaining that the wealth one can thereby acquire, and the prominence he may assume, will give him a higher position in society and public esteem than mere learning ever can. but i told him that if i could stand high in the esteem of twenty such men as cayley, sylvester, and peirce, i cared nothing to be prominent in the eyes of the rest of the world." such an expression from an eminent member of the boston bar, himself a harvard graduate, was the first striking evidence i met with that my views of the exalted nature of astronomical investigation were not shared by society at large. one of the greatest advantages i enjoyed through bartlett was an intimate acquaintance with a cultured and refined boston family. in mr. runkle founded the "mathematical monthly," having secured, in advance, the coöperation of the leading professors of the subject in the country. the journal was continued, under many difficulties, for three years. as a vehicle for publishing researches in advanced mathematics, it could not be of a high order, owing to the necessity of a subscription list. its design was therefore to interest students and professors in the subject, and thus prepare the way for the future growth of mathematical study among us. its principal feature was the offer of prize problems to students as well as prizes for essays on mathematical subjects. the first to win a prize for an essay was george w. hill, a graduate of rutgers just out of college, who presented a memoir in which the hand of the future master was evident throughout. in the general conduct of the journal bartlett and myself, though not ostensibly associate editors, were at least assistants. simple though the affair was, some of our experiences were of an interesting and, perhaps, instructive nature. soon after the first number appeared, a contribution was offered by a professor in a distant state. an important part of the article was found to be copied bodily from walton's "problems in mechanics," an english book which, it might be supposed, was not much known in this country. runkle did not want to run the risk of injuring his subscription list by offending one occupying an influential position if he could help it with honor to the journal. of course it was not a question of publishing the paper, but only of letting the author know why he did not do so,--"letting him down easy." bartlett's advice was characteristic. "just write to the fellow that we don't publish stolen articles. that's all you need say." i suggested that we might inflict on him all necessary humiliation by letting him know in the gentlest manner possible that we saw the fraud. of course runkle preferred this course, and wrote him, calling his attention to a similarity between his treatment of the subject and that of walton, which materially detracted from the novelty of the former. i think it was suggested that he get the book, if possible, and assure himself on the subject. a vigorous answer came by return of mail. he was a possessor of walton's book, knew all about the similar treatment of the subject by walton, and did not see that that should be any bar to the publication of the article. i think it was he who wound up his letter with the statement that, while he admitted the right of the editor to publish what he pleased, he, the writer, was too busy to spend his time in writing rejected articles. an eminent would-be contributor was a prominent pennsylvania politician, who had read a long and elaborate article, before some teachers' association, on an arithmetical problem about oxen eating grass, the power to solve which was taken as the highest mark of mathematical ability, among school teachers during the first half of the century. the association referred the paper to the editor of the "mathematical monthly," by whom it was, i believe, consigned to the wastebasket. the result was a good deal of correspondence, such a proceeding being rather humiliating to a man of eminence who had addressed so distinguished an assembly. the outcome of the matter was that the paper, which was much more in the nature of a legal document than of a mathematical investigation, was greatly reduced in length by its author, and then still further shorn by the editor, until it would fill only two or three pages of the journal; thus reduced, it was published. the time was not yet ripe for the growth of mathematical science among us, and any development that might have taken place in that direction was rudely stopped by the civil war. perhaps this may account for the curious fact that, so far as i have ever remarked, none of the student contributors to the journal, hill excepted, has made himself known as a mathematical investigator. not only the state of mathematical learning, but the conditions of success at that time in a mathematical text-book, are strikingly illustrated by one of our experiences. one of the leading publishing houses of educational text-books in the country issued a very complete and advanced series, from the pen of a former teacher of the subject. they were being extensively introduced, and were sent to the "mathematical monthly" for review. they were distinguished by quite apt illustrations, well fitted, perhaps, to start the poorly equipped student in the lower branches of the work, but the advanced works, at least, were simply ridiculous. a notice appeared in which the character of the books was pointed out. the evidence of the worthlessness of the entire series was so strong that the publishers had it entirely rewritten by more competent authors. now came the oddest part of the whole affair. the new series was issued under the name of the same author as the old one, just as if the acknowledgment of his total failure did not detract from the value of his name as an author. in a total eclipse of the sun was visible in british america. the shadow of the moon, starting from near vancouver's island, crossed the continent in a northeast direction, passed through the central part of the hudson bay region, crossed hudson bay itself and greenland, then inclining southward, swept over the atlantic to spain. as this was the first eclipse of the kind which had recently been visible, much interest was taken in its observation. on the part of the nautical almanac office i computed the path of the shadow and the times of crossing certain points in it. the results were laid down on a map which was published by the office. one party, fitted out in connection with the american association for the advancement of science, was sent to greenland. admiral davis desired to send another, on behalf of his own office, into the central regions of the continent. as members of this party mr. ferrel and myself were chosen. at the request of professor agassiz one of the assistants in the museum of comparative zoölogy, mr. samuel h. scudder, accompanied us. more than twenty years later mr. scudder published a little book describing some of our adventures, which was illustrated with sketches showing the experiences of a party in the wild west at that time. our course lay from st. paul across minnesota to the red river of the north, thence north to fort garry near the southern end of lake winnipeg, then over the lake and some distance up the saskatchewan river. at st. paul we paid our respects to governor ramsey, afterward senator from minnesota and secretary of war. we were much surprised at the extraordinary deference paid by the community to a mr. burbank, a leading citizen of the town, and owner of the stages which we had to engage for our journey across the country. he seemed to be a man whom every one was afraid to offend. even the local newspapers were careful what they printed about matters in which he was interested. the two or three days which we passed in getting things ready to start were rather dull. the morning after our arrival i saw, during a morning walk, on a hill just outside the town, a large new building, on which the word "athenæum" was conspicuously shown. the boston athenæum had a very fine library; is it not possible that this may have a beginning of something of the same sort? animated by this hope, i went up the hill and entered the building, which seemed to be entirely vacant. the first words that met my eyes were "bar room" painted over a door. it was simply a theatre, and i left it much disappointed. here we were joined by a young methodist clergyman,--edward eggleston,--and the four of us, with our instruments and appliances, set out on our journey of five days over the plains. on the first day we followed partly the line of a projected railway, of which the embankments had been completed, but on which work had, for some reason, been stopped to await a more prosperous season. here was our first experience of towns on paper. from the tone in which the drivers talked of the places where we were to stop over night one might have supposed that villages, if not cities, were plentiful along our track. one example of a town at that time will be enough. the principal place on our route, judging from the talk, was breckenridge. we would reach it at the end of the fourth day, where we anticipated a pleasant change after camping out in our tent for three nights. it was after dark before we arrived, and we looked eagerly for signs of the town we were approaching. the team at length stopped in front of an object which, on careful examination in the darkness, appeared to be the most primitive structure imaginable. it had no foundations, and if it had a wall at all, it was not more than two or three feet in height. imagine the roof taken off a house forty feet long and twenty feet wide and laid down on the ground, and you have the hotel and only building, unless perhaps a stable, in breckenridge at that time. the entrance was at one end. going in, a chimney was seen in the middle of the building. the floor was little more than the bare ground. on each side of the door, by the flickering light of a fire, we saw what looked like two immense boxes. a second glance showed that these boxes seemed to be filled with human heads and legs. they were, in fact, the beds of the inhabitants of breckenridge. beds for the arriving travelers, if they existed at all, which i do not distinctly remember, were in the back of the house. i think the other members of the party occupied that portion. i simply spread my blanket out on the hearth in front of the fire, wrapped up, and slept as soundly as if the bed was the softest of a regal palace. at fort garry we were received by governor mctavish, with whom captain davis had had some correspondence on the subject of our expedition, and who gave us letters to the "factors" of the hudson bay company scattered along our route. we found that the rest of our journey would have to be made in a birch bark canoe. one of the finest craft of this class was loaned us by the governor. it had been, at some former time, the special yacht of himself or some visiting notable. it was manned by eight half-breeds, men whose physical endurance i have never seen equaled. it took three or four days to get everything ready, and this interval was, of course, utilized by scudder in making his collections. he let the fishermen of the region know that he wanted specimens of every kind of fish that could be found in the lake. a very small reward stirred them into activity, and, in due time, the fish were brought to the naturalist,--but lo! all nicely dressed and fit for cooking. they were much surprised when told that all their pains in dressing their catch had spoiled it for the purposes of the visiting naturalist, who wanted everything just as it was taken from the water. slow indeed was progress through the lake. a canoe can be paddled only in almost smooth water, and we were frequently stormbound on some desolate island or point of land for two or three days at a time. when, after many adventures, some of which looked like hairbreadth escapes, we reached the saskatchewan river, the eclipse was only three or four days ahead, and it became doubtful whether we should reach our station in time for the observation. it was to come off on the morning of july , and, by dint of paddling for twenty-four hours at a stretch, our men brought us to the place on the evening before. now a new difficulty occurred. in the wet season the saskatchewan inundates the low flat region through which it flows, much like the nile. the country was practically under water. we found the most elevated spot we could, took out our instruments, mounted them on boxes or anything else in the shallow puddles of water, and slept in the canoe. next morning the weather was hopelessly cloudy. we saw the darkness of the eclipse and nothing more. astronomers are greatly disappointed when, having traveled halfway around the world to see an eclipse, clouds prevent a sight of it; and yet a sense of relief accompanies the disappointment. you are not responsible for the mishap; perhaps something would have broken down when you were making your observations, so that they would have failed in the best of weather; but now you are relieved from all responsibility. it was much easier to go back and tell of the clouds than it would have been to say that the telescope got disarranged at the critical moment so that the observations failed. on our return across minnesota we had an experience which i have always remembered as illustrative of the fallacy of all human testimony about ghosts, rappings, and other phenomena of that character. we spent two nights and a day at fort snelling. some of the officers were greatly surprised by a celestial phenomenon of a very extraordinary character which had been observed for several nights past. a star had been seen, night after night, rising in the east as usual, and starting on its course toward the south. but instead of continuing that course across the meridian, as stars invariably had done from the remotest antiquity, it took a turn toward the north, sunk toward the horizon, and finally set near the north point of the horizon. of course an explanation was wanted. my assurance that there must be some mistake in the observation could not be accepted, because this erratic course of the heavenly body had been seen by all of them so plainly that no doubt could exist on the subject. the men who saw it were not of the ordinary untrained kind, but graduates of west point, who, if any one, ought to be free from optical deceptions. i was confidently invited to look out that night and see for myself. we all watched with the greatest interest. in due time the planet mars was seen in the east making its way toward the south. "there it is!" was the exclamation. "yes, there it is," said i. "now that planet is going to keep right on its course toward the south." "no, it is not," said they; "you will see it turn around and go down towards the north." hour after hour passed, and as the planet went on its regular course, the other watchers began to get a little nervous. it showed no signs of deviating from its course. we went out from time to time to look at the sky. "there it is," said one of the observers at length, pointing to capella, which was now just rising a little to the east of north; "there is the star setting." "no, it is n't," said i; "there is the star we have been looking at, now quite inconspicuous near the meridian, and that star which you think is setting is really rising and will soon be higher up." a very little additional watching showed that no deviation of the general laws of nature had occurred, but that the observers of previous nights had jumped at the conclusion that two objects, widely apart in the heavens, were the same. i passed more than four years in such life, surroundings, and activities as i have described. in i received the degree of d. s. from the lawrence scientific school, and thereafter remained on the rolls of the university as a resident graduate. life in the new atmosphere was in such pleasant and striking contrast to that of my former world that i intensely enjoyed it. i had no very well marked object in view beyond continuing studies and researches in mathematical astronomy. not long after my arrival in cambridge some one, in speaking of professor peirce, remarked to me that he had a european reputation as a mathematician. it seemed to me that this was one of the most exalted positions that a man could attain, and i intensely longed for it. yet there was no hurry. reputation would come to him who deserved it by his works; works of the first class were the result of careful thought and study, and not of hurry. a suggestion had been made to me looking toward a professorship in some western college, but after due consideration, i declined to consider the matter. yet the necessity of being on the alert for some opening must have seemed quite strong, because in i became a serious candidate for the professorship of physics in the newly founded washington university at st. louis. i was invited to visit the university, and did so on my way to observe the eclipse of . my competitor was lieutenant j. m. schofield of the united states army, then an instructor at west point. it will not surprise the reader to know that the man who was afterward to command the army of the united states received the preference, so i patiently waited more than another year. [ ] henry holt & co.: new york, . [ ] _wayside sketches_, by e. j. loomis. roberts: boston [ ] evangelinus apostolides sophocles, a native greek and a learned professor of the literature of his country. iv life and work at an observatory in august, , while i was passing my vacation on cape ann, i received a letter from dr. gould, then in washington, informing me that a vacancy was to be filled in the corps of professors of mathematics attached to the naval observatory, and suggesting that i might like the place. i was at first indisposed to consider the proposition. cambridge was to me the focus of the science and learning of our country. i feared that, so far as the world of learning was concerned, i should be burying myself by moving to washington. the drudgery of night work at the observatory would also interfere with carrying on any regular investigation. but, on second thought, having nothing in view at the time, and the position being one from which i could escape should it prove uncongenial, i decided to try, and indited the following letter:-- nautical almanac office, cambridge, mass., august , . sir,--i have the honor to apply to you for my appointment to the office of professor of mathematics in the united states navy. i would respectfully refer you to commander charles henry davis, u. s. n., professor benjamin peirce, of harvard university, dr. benjamin a. gould, of cambridge, and professor joseph henry, secretary of the smithsonian institution, for any information respecting me which will enable you to judge of the propriety of my appointment. with high respect, your obedient servant, simon newcomb, assistant, nautical almanac. hon. gideon welles, secretary of the navy, washington, d. c. i also wrote to captain davis, who was then on duty in the navy department, telling him what i had done, but made no further effort. great was my surprise when, a month later, i found in the post-office, without the slightest premonition, a very large official envelope, containing my commission duly signed by abraham lincoln, president of the united states. the confidence in the valor, abilities, etc., of the appointee, expressed in the commission, was very assuring. accompanying it was a letter from the secretary of the navy directing me to report to the bureau of ordnance and hydrography, in washington, for such duty as it might assign me. i arrived on october , and immediately called on professor j. s. hubbard, who was the leading astronomer of the observatory. on the day following i reported as directed, and was sent to captain gilliss, the recently appointed superintendent of the naval observatory, before whom i stood with much trepidation. in reply to his questions i had to confess my entire inexperience in observatory work or the making of astronomical observations. a coast survey observer had once let me look through his transit instrument and try to observe the passage of a star. on the eclipse expedition mentioned in the last chapter i had used a sextant. this was about all the experience in practical astronomy which i could claim. in fact i had never been inside of an observatory, except on two or three occasions at cambridge as a visitor. the captain reassured me by saying that no great experience was expected of a newcomer, and told me that i should go to work on the transit instrument under professor yarnall, to whose care i was then confided. as the existence of a corps of professors of mathematics is peculiar to our navy, as well as an apparent, perhaps a real, anomaly, some account of it may be of interest. early in the century--one hardly knows when the practice began--the secretary of the navy, in virtue of his general powers, used to appoint men as professors of mathematics in the navy, to go to sea and teach the midshipmen the art of navigation. in , when work at the observatory was about to begin, no provision for astronomers was made by congress. the most convenient way of supplying this want was to have the secretary appoint professors of mathematics, and send them to the observatory on duty. a few years later the naval academy was founded at annapolis, and a similar course was pursued to provide it with a corps of instructors. up to this time the professors had no form of appointment except a warrant from the secretary of the navy. early in the history of the academy the midshipmen burned a professor in effigy. they were brought before a court-martial on the charge of disrespect to a superior officer, but pleaded that the professor, not holding a commission, was not their superior officer, and on this plea were acquitted. congress thereupon took the matter up, provided that the number of professors should not exceed twelve, and that they should be commissioned by the president by and with the advice and consent of the senate. this raised their rank to that of a commissioned corps in the navy. they were to perform such duty as the secretary of the navy might direct, and were, for the most part, divided between the naval academy and the observatory. during the civil war some complaint was made that the midshipmen coming from the academy were not well trained in the duties of a seagoing officer; and it was supposed that this was due to too much of their time being given to scientific studies. this was attributed to the professors, with the result that nearly all those attached to the academy were detached during the four years following the close of the civil war and ordered elsewhere, mostly to the observatory. their places were taken by line officers who, in the intervals between their turns of sea duty, were made heads of departments and teachers of the midshipmen in nearly every branch. this state of things led to the enactment of a law (in , i think), "that hereafter no vacancy in the grade of professors of mathematics in the navy shall be filled." in this provision was annulled by a law, again providing for a corps of twelve professors, three of whom should have the relative rank of captain, four of commander, and the remainder of lieutenant-commander or lieutenant. up to the secretary of the navy was placed under no restrictions as to his choice of a professor. he could appoint any citizen whom he supposed to possess the necessary qualifications. then it was enacted that, before appointment, a candidate should pass a medical and a professional examination. i have said that the main cause of hesitation in making my application arose from my aversion to very late night work. it soon became evident that there was less ground than i had supposed for apprehension on this point. there was a free and easy way of carrying on work which was surprising to one who had supposed it all arranged on strict plans, and done according to rule and discipline. professor yarnall, whose assistant i was, was an extremely pleasant gentleman to be associated with. although one of the most industrious workers at the observatory, there was nothing of the martinet about him. he showed me how to handle the instrument and record my observations. there was a nautical almanac and a catalogue of stars. out of these each of us could select what he thought best to observe. the custom was that one of us should come on every clear evening, make observations as long as he chose, and then go home. the transit instrument was at one end of the building and the mural circle, in charge of professor hubbard, at the other. he was weak in health, and unable to do much continuous work of any kind, especially the hard work of observing. he and i arranged to observe on the same nights; but i soon found that there was no concerted plan between the two sets of observers. the instruments were old-fashioned ones, of which mine could determine only the right ascension of a star and his only its declination; hence to completely determine the position of a celestial body, observations must be made on the same object with both instruments. but i soon found that there was no concert of action of this kind. hubbard, on the mural circle, had his plan of work; yarnall and myself, on the transit, had ours. when either hubbard or myself got tired, we could "vote it cloudy" and go out for a plate of oysters at a neighboring restaurant. in justice to captain gilliss it must be said that he was not in any way responsible for this lack of system. it grew out of the origin and history of the establishment and the inaction of congress. the desirableness of our having a national observatory of the same rank as those of other countries was pointed out from time to time by eminent statesmen from the first quarter of the century. john quincy adams had, both while he filled the presidential office and afterward, made active efforts in this direction; but there were grave doubts whether congress had any constitutional authority to erect such an institution, and the project got mixed up with parties and politics. so strong was the feeling on the subject that, when the coast survey was organized, it was expressly provided that it should not establish an astronomical observatory. the outcome of the matter was that, in , when congress at length decided that we should have our national observatory, it was not called such, but was designated as a "house" to serve as a depot for charts and instruments for the navy. but every one knew that an observatory was meant. gilliss was charged with its erection, and paid a visit to europe to consult with astronomers there on its design, and to order the necessary instruments. when he got through with this work and reported it as completed he was relieved, and lieutenant matthew f. maury was appointed superintendent of the new institution. maury, although (as he wrote a few years later) quite without experience in the use of astronomical instruments, went at his work with great energy and efficiency, so that, for two or three years, the institution bade fair to take a high place in science. then he branched off into what was, from a practical standpoint, the vastly more important work of studying the winds and currents of the ocean. the epoch-making character of his investigations in this line, and their importance to navigation when ships depended on sails for their motive power, were soon acknowledged by all maritime nations, and the fame which he acquired in pursuing them added greatly to the standing of the institution at which the work was done, though in reality an astronomical outfit was in no way necessary to it. the new work was so absorbing that he seemed to have lost interest in the astronomical side of the establishment, which he left to his assistants. the results were that on this side things fell into the condition i have described, and stayed there until maury resigned his commission and cast his fortunes with the confederacy. then gilliss took charge and had to see what could be done under the circumstances. it soon became evident to him that no system of work of the first order of importance could be initiated until the instrumental equipment was greatly improved. the clocks, perfection in which is almost at the bottom of good work, were quite unfit for use. the astronomical clock with which yarnall and i made our observations kept worse time than a high-class pocket watch does to-day. the instruments were antiquated and defective in several particulars. before real work could be commenced new ones must be procured. but the civil war was in progress, and the times were not favorable to immediately securing them. that the work of the observatory was kept up was due to a feeling of pride on the part of our authorities in continuing it without interruption through the conflict. the personnel was as insufficient as the instruments. on it devolved not only the making of the astronomical observations, but the issue of charts and chronometers to the temporarily immense navy. in fact the observatory was still a depot of charts for the naval service, and continued to be such until the hydrographic office was established in . in gilliss obtained authority to have the most pressing wants supplied by the construction of a great transit circle by pistor and martins in berlin. he had a comprehensive plan of work with this instrument when it should arrive, but deferred putting any such plan in operation until its actual reception. somehow the work of editing, explaining, and preparing for the press the new series of observations made by yarnall and myself with our old transit instrument devolved on me. to do this in the most satisfactory way, it was necessary to make a careful study of the methods and system at the leading observatories of other countries in the line we were pursuing, especially greenwich. here i was struck by the superiority of their system to ours. everything was there done on an exact and uniform plan, and one which seemed to me better adapted to get the best results than ours was. for the non-astronomical reader it may be remarked that after an astronomer has made and recorded his observations, a large amount of calculation is necessary to obtain the result to which they lead. making such calculations is called "reducing" the observations. now in the previous history of the observatory, the astronomers fell into the habit of every one not only making his observations in his own way, but reducing them for himself. thus it happened that yarnall had been making and reducing his observations in his own way, and i, on alternate nights, had been making and reducing mine in my way, which was modeled after the greenwich fashion, and therefore quite different from his. now i suddenly found myself face to face with the problem of putting these two heterogeneous things together so as to make them look like a homogeneous whole. i was extremely mortified to see how poor a showing would be made in the eyes of foreign astronomers. but i could do nothing more than to describe the work and methods in such a way as to keep in the background the want of system that characterized them. notwithstanding all these drawbacks of the present, the prospect of future success seemed brilliant. gilliss had the unlimited confidence of the secretary of the navy, had a family very popular in washington society, was enthusiastically devoted to building up the work of the observatory, and was drawing around him the best young men that could be found to do that work. he made it a point that his relations with his scientific subordinates should be not only official, but of the most friendly social character. all were constantly invited to his charming family circle. it was from the occasional talks thus arising that i learned the details of his plan of work with the coming instrument. in gilliss had the working force increased by the appointment of four "aides," as they were then called,--a number that was afterwards reduced to three. this was the beginning of the corps of three assistant astronomers, which is still maintained. it will be of interest to know that the first aide was asaph hall; but before his appointment was made, an impediment, which for a time looked serious, had to be overcome. gilliss desired that the aide should hold a good social and family position. the salary being only $ , this required that he should not be married. hall being married, with a growing family, his appointment was long objected to, and it was only through much persuasion on the part of hubbard and myself that gilliss was at length induced to withdraw his objections. among other early appointees were william harkness and john a. eastman, whose subsequent careers in connection with the observatory are well known. the death of professor hubbard in led to my taking his place, in charge of the mural circle, early in september of that year. this gave me an opportunity of attempting a little improvement in the arrangements. i soon became conscious of the fact, which no one had previously taken much account of, that upon the plan of each man reducing his own observations, not only was there an entire lack of homogeneity in the work, but the more work one did at night the more he had to do by day. it was with some trepidation that i presented the case to gilliss, who speedily saw that work done with the instruments should be regarded as that of the observatory, and reduced on a uniform plan, instead of being considered as the property of the individual who happened to make it. thus was introduced the first step toward a proper official system. in february, , the observatory sustained the greatest loss it had ever suffered, in the sudden death of its superintendent. what it would have grown to had he lived it is useless to guess, but there is little doubt that its history would have been quite different from what it is. soon afterward admiral davis left his position as chief of the bureau of navigation to take the subordinate one of superintendent of the observatory. this step was very gratifying to me, davis had not only a great interest in scientific work, especially astronomy, but a genuine admiration of scientific men which i have never seen exceeded, accompanied with a corresponding love of association with them and their work. in october, , occurred what was, in my eyes, the greatest event in the history of the observatory. the new transit circle arrived from berlin in its boxes. now for the first time in its history, the observatory would have a meridian instrument worthy of it, and would, it was hoped, be able to do the finest work in at least one branch of astronomy. to my great delight, davis placed me in charge of it. the last three months of the year were taken up with mounting it in position and making those investigations of its peculiarities which are necessary before an instrument of the kind is put into regular use. on the st day of january, , this was all done, and we were ready to begin operations. an opportunity thus arose of seeing what we could do in the way of a regular and well-planned piece of work. in the greater clearness of our sky, and the more southern latitude of our observatory, we had two great advantages over greenwich. looking back at his first two or three years of work at the observatory, maury wrote to a friend, "we have beaten greenwich hollow." it may be that i felt like trying to do the same thing over again. at any rate, i mapped out a plan of work the execution of which would require four years. it was a piece of what, in astronomy, is called "fundamental work," in which results are to be obtained independent of any previously obtained by other observers. it had become evident to me from our own observations, as well as from a study of those made at european observatories, that an error in the right ascension of stars, so that stars in opposite quarters of the heavens would not agree, might very possibly have crept into nearly all the modern observations at greenwich, paris, and washington. the determination of this error was no easy matter. it was necessary that, whenever possible, observations should be continued through the greater part of the twenty-four hours. one observer must be at work with comparative steadiness from nine o'clock in the morning until midnight or even dawn of the morning following. this requirement was, however, less exacting than might appear when stated. one half the nights would, as a general rule, be cloudy, and an observer was not expected to work on sunday. hence no one of the four observers would probably have to do such a day's work as this more than thirty or forty times in a year. all this was hard work enough in itself, but conditions existed which made it yet harder. no houses were then provided for astronomers, and the observatory itself was situated in one of the most unhealthy parts of the city. on two sides it was bounded by the potomac, then pregnant with malaria, and on the other two, for nearly half a mile, was found little but frame buildings filled with quartermaster's stores, with here and there a few negro huts. most of the observers lived a mile or more from the observatory; during most of the time i was two miles away. it was not considered safe to take even an hour's sleep at the observatory. the result was that, if it happened to clear off after a cloudy evening, i frequently arose from my bed at any hour of the night or morning and walked two miles to the observatory to make some observation included in the programme. this was certainly a new departure from the free and easy way in which we had been proceeding, and it was one which might be unwelcome to any but a zealous astronomer. as i should get the lion's share of credit for its results, whether i wanted to or not, my interest in the work was natural. but it was unreasonable to expect my assistants, one or two of whom had been raised to the rank of professor, to feel the same interest, and it is very creditable to their zeal that we pursued it for some time as well as we did. if there was any serious dissatisfaction with the duty, i was not informed of that fact. during the second year of this work admiral davis was detached and ordered to sea. the question of a successor interested many besides ourselves. secretary welles considered the question what policy should be pursued in the appointment. professor henry took part in the matter by writing the secretary a letter, in which he urged the appointment of an astronomer as head of the institution. his position prevented his supporting any particular candidate; so he submitted a list of four names, any one of which would be satisfactory. these were: professor william chauvenet, dr. b. a. gould, professor j. h. c. coffin, u. s. n., and mr. james ferguson. the latter held a civil position at the observatory, under the title of "assistant astronomer," and was at the time the longest in service of any of its force. a different view was urged upon the secretary in terms substantially these: "professors so able as those of the observatory require no one to direct their work. all that the observatory really needs is an administrative head who shall preserve order, look after its business generally, and see that everything goes smoothly." such a head the navy can easily supply. the secretary allowed it to be given out that he would be glad to hear from the professors upon the subject. i thereupon went to him and expressed my preference for professor coffin. he asked me, "how would it do to have a purely administrative head?" i replied that we might get along for a time if he did not interfere with our work. "no," said the secretary, "he shall not interfere. that shall be understood." as i left him there was, to my inexperienced mind, something very odd in this function, or absence of function, of the head of an establishment; but of course i had to bow to superior wisdom and could say nothing. the policy of commodore (afterward rear-admiral) sands, the incoming superintendent, toward the professors was liberal in the last degree. each was to receive due credit for what he did, and was in every way stimulated to do his best at any piece of scientific work he might undertake with the approval of the superintendent. whether he wanted to observe an eclipse, determine the longitude of a town or interior station, or undertake some abstruse investigation, every facility for doing it and every encouragement to go on with it was granted him. under this policy the observatory soon reached the zenith of its fame and popularity. whenever a total eclipse of the sun was visible in an accessible region parties were sent out to observe it. in three professors, i being one, were sent to des moines, iowa, to observe the solar eclipse which passed across the country in june of that year. as a part of this work, i prepared and the observatory issued a detailed set of instructions to observers in towns at each edge of the shadow-path to note the short duration of totality. the object was to determine the exact point to which the shadow extended. at this same eclipse professor harkness shared with professor young of princeton the honor of discovering the brightest line in the spectrum of the sun's corona. the year following parties were sent to the mediterranean to observe an eclipse which occurred in december, . i went to gibraltar, although the observation of the eclipse was to me only a minor object. some incidents connected with this european trip will be described in a subsequent chapter. the reports of the eclipse parties not only described the scientific observations in great detail, but also the travels and experiences, and were sometimes marked by a piquancy not common in official documents. these reports, others pertaining to longitude, and investigations of various kinds were published in full and distributed with great liberality. all this activity grew out of the stimulating power and careful attention to business of the head of the observatory and the ability of the young professors of his staff. it was very pleasant to the latter to wear the brilliant uniform of their rank, enjoy the protection of the navy department, and be looked upon, one and all, as able official astronomers. the voice of one of our scientific men who returned from a visit abroad declaring that one of our eclipse reports was the laughing-stock of europe was drowned in the general applause. in the latter part of i had carried forward the work with the transit circle as far as it could be profitably pursued under existing conditions. on working up my observations, the error which i had suspected in the adopted positions of the stars was proved to be real. but the discovery of this error was due more to the system of observation, especially the pursuit of the latter through the day and night, than it was to any excellence of the instrument. the latter proved to have serious defects which were exaggerated by the unstable character of the clayey soil of the hill on which the observatory was situated. other defects also existed, which seemed to preclude the likelihood that the future work of the instrument would be of a high class. i had also found that very difficult mathematical investigations were urgently needed to unravel one of the greatest mysteries of astronomy, that of the moon's motion. this was a much more important work than making observations, and i wished to try my hand at it. so in the autumn i made a formal application to the secretary of the navy to be transferred from the observatory to the nautical almanac office for the purpose of engaging in researches on the motion of the moon. on handing this application to the superintendent he suggested that the work in question might just as well be done at the observatory. i replied that i thought that the business of the observatory was to make and reduce astronomical observations with its instruments, and that the making of investigations of the kind i had in view had always been considered to belong to the nautical almanac office. he replied that he deemed it equally appropriate for the observatory to undertake it. as my objection was founded altogether on a principle which he refused to accept, and as by doing the work at the observatory i should have ready access to its library, i consented to the arrangement he proposed. accordingly, in forwarding my application, he asked that my order should be so worded as not to detach me from the observatory, but to add the duty i asked for to that which i was already performing. so far as i was personally concerned, this change was fortunate rather than otherwise. as things go in washington, the man who does his work in a fine public building can gain consideration for it much more readily than if he does it in a hired office like that which the "nautical almanac" then occupied. my continued presence on the observatory staff led to my taking part in two of the great movements of the next ten years, the construction and inauguration of the great telescope and the observations of the transit of venus. but for the time being my connection with the regular work of the observatory ceased. on the retirement of admiral sands in , admiral davis returned to the observatory, and continued in charge until his death in february, . the principal event of this second administration was the dispatch of parties to observe the transit of venus. of this i shall speak in full in a subsequent chapter. one incident, although of no public importance, was of some interest at the time. this was a visit of the only emperor who, i believe, had ever set foot on our shores,--dom pedro of brazil. he had chosen the occasion of our centennial for a visit to this country, and excited great interest during his stay, not only by throwing off all imperial reserve during his travels, but by the curiosity and vigor with which he went from place to place examining and studying everything he could find, and by the singular extent of his knowledge on almost every subject of a scientific or technical character. a philadelphia engineer with whom he talked was quoted as saying that his knowledge of engineering was not merely of the ordinary kind to be expected in an intelligent man, but extended to the minutest details and latest improvements in the building of bridges, which was the specialty of the engineer in question. almost as soon as he arrived in washington i received the following letter by a messenger from the arlington hotel:-- mr.: en arrivant à washington j'ai tout-de-suite songé à votre observatoire, où vous avez acquis tant de droit à l'estime de tout ceux qui achèvent la science. je m'y rendrai donc aujourd'hui à heures du soir, et je compte vous y trouver, surtout pour vous remercier de votre beau mémoire que j'ai reçu peu avant mon départ de mon pays, et que je n'ai pas pu, par conséquent, apprécier autant que je l'aurais voulu. en me plaisant de l'espoir de vous connaître personnellement je vous prie de me compter parmi vos affectionnés. d. pedro d'alcantara. mai, . like other notes which i subsequently received from him, it was in his own autograph throughout: if he brought any secretary with him on his travels i never heard of it. the letter placed me in an embarrassing position, because its being addressed to me was in contravention of all official propriety. of course i lost no time in calling on him and trying to explain the situation. i told him that admiral davis, whom he well knew from his being in command of the brazilian station a few years before, was the head of the observatory, and hinted as plainly as i could that a notification of the coming of such a visitor as he should be sent to the head of the institution. but he refused to take the hint, and indicated that he expected me to arrange the whole matter for him. this i did by going to the observatory and frankly explaining the matter to admiral davis. happily the latter was not a stickler for official forms, and was cast in too large a mould to take offense where none was intended. at his invitation i acted as one of the receiving party. the carriage drove up at the appointed hour, and its occupant was welcomed by the admiral at the door with courtly dignity. the visitor had no time to spend in preliminaries; he wished to look through the establishment immediately. the first object to meet his view was a large marble-cased clock which, thirty years before, had acquired some celebrity from being supposed to embody the first attempt to apply electricity to the recording of astronomical observations. it was said to have cost a large sum, paid partly as a reward to its inventor. its only drawbacks were that it would not keep time and had never, so far as i am aware, served any purpose but that of an ornament. the first surprise came when the visitor got down on his hands and knees in front of the clock, reached his hands under it, and proceeded to examine its supports. we all wondered what it could mean. when he arose, it was explained. he did not see how a clock supported in this way could keep the exact time necessary in the work of an astronomer. so we had to tell him that the clock was not used for this purpose, and that he must wait until we visited the observing rooms to see our clocks properly supported. the only evidence of the imperial will came out when he reached the great telescope. the moon, near first quarter, was then shining, but the night was more than half cloudy, and there was no hope of obtaining more than a chance glimpse at it through the clouds. but he wished to see the moon through the telescope. i replied that the sky was now covered, and it was very doubtful whether we should get a view of the moon. but he required that the telescope should be at once pointed at it. this was done, and at that moment a clear space appeared between the clouds. i remarked upon the fact, but he seemed to take it as a matter of course that the cloud would get out of the way when he wanted to look. i made some remark about the "vernier" of one of the circles on the telescope. "why do you call it a vernier?" said he. "its proper term is a nonius, because nonius was its inventor and vernier took the idea from him." in this the national spirit showed itself. nonius, a portuguese, had invented something on a similar principle and yet essentially different from the modern vernier, invented by a frenchman of that name. accompanying the party was a little girl, ten or twelve years old, who, though an interested spectator, modestly kept in the background and said nothing. on her arrival home, however, she broke her silence by running upstairs with the exclamation,-- "oh, mamma, he's the funniest emperor you ever did see!" my connection with the observatory ceased september , , when i was placed in charge of the nautical almanac office. it may not, however, be out of place to summarize the measures which have since been taken both by the navy department and by eminent officers of the service to place the work of the institution on a sound basis. one great difficulty in doing this arises from the fact that neither congress nor the navy department has ever stated the object which the government had in view in erecting the observatory, or assigned to it any well-defined public functions. the superintendent and his staff have therefore been left to solve the question what to do from time to time as best they could. in the spring of rear-admiral john rodgers became the superintendent of the observatory. as a cool and determined fighter during the civil war he was scarcely second even to farragut, and he was at the same time one of the ablest officers and most estimable men that our navy ever included in its ranks. "i would rather be john rodgers dead than any other man i know living," was said by one of the observatory assistants after his death. not many months after his accession he began to consider the question whether the wide liberty which had been allowed the professors in choosing their work was adapted to attain success. the navy department also desired to obtain some expressions of opinion on the subject. the result was a discussion and an official paper, not emanating from the admiral, however, in which the duty of the head of the observatory was defined in the following terms:-- "the superintendent of the observatory should be a line officer of the navy, of high rank, who should attend to the business affairs of the institution, thus leaving the professors leisure for their proper work." although he did not entirely commit himself to this view, he was under the impression that to get the best work out of the professors their hearts must be in it; and this would not be the case if any serious restraint was placed upon them as to the work they should undertake. after rodgers's death vice-admiral rowan was appointed superintendent. about this time it would seem that the department was again disposed to inquire into the results of the liberal policy heretofore pursued. commander (since rear-admiral) william t. sampson was ordered to the observatory, not as its head, but as assistant to the superintendent. he was one of the most proficient men in practical physics that the navy has ever produced. i believe that one reason for choosing so able and energetic an officer for the place was to see if any improvement could be made on the system. as i was absent at the cape of good hope to observe the transit of venus during the most eventful occasion of his administration, i have very little personal knowledge of it. it seems, however, that newspaper attacks were made on him, in which he was charged with taking possession of all the instruments of the observatory but two, and placing them in charge of naval officers who were not proficient in astronomical science. in reply he wrote an elaborate defense of his action to the "new york herald," which appeared in the number for february , . the following extract is all that need find a place in the present connection. when i came here on duty a little more than a year since, i found these instruments disused. the transit instrument had not been used since , and then only at intervals for several years previous; the mural circle had not been used since ; the prime vertical had not been used since . these instruments had been shamefully neglected and much injured thereby. . . . the small equatorial and comet seeker were in the same disgraceful condition, and were unfit for any real work. admiral franklin was made superintendent sometime in , i believe, and issued an order providing that the work of the observatory should be planned by a board consisting of the superintendent, the senior line officer, and the senior professor. professors or officers in charge of instruments were required to prepare a programme for their proposed work each year in advance, which programme would be examined by the board. of the work of this board or its proceedings, no clear knowledge can be gleaned from the published reports, nor do i know how long it continued. in secretary whitney referred to the national academy of sciences the question of the advisability of proceeding promptly with the erection of a new naval observatory upon the site purchased in . the report of the academy was in the affirmative, but it was added that the observatory should be erected and named as a national one, and placed under civilian administration. the year following congress made the preliminary appropriation for the commencement of the new building, but no notice was taken of the recommendation of the academy. in the new buildings were approaching completion, and secretary tracy entered upon the question of the proper administration of the observatory. he discussed the subject quite fully in his annual report for that year, stating his conclusion in the following terms:-- i therefore recommend the adoption of legislation which shall instruct the president to appoint, at a sufficient salary, without restriction, from persons either within or outside the naval service, the ablest and most accomplished astronomer who can be found for the position of superintendent. at the following session of congress senator hale introduced an amendment to the naval appropriation bill, providing for the expenses of a commission to be appointed by the secretary of the navy, to consider and report upon the organization of the observatory. the house non-concurred in this amendment, and it was dropped from the bill. at the same session, all the leading astronomers of the country united in a petition to congress, asking that the recommendation of the secretary of the navy should be carried into effect. after a very patient hearing of arguments on the subject by professor boss and others, the house naval committee reported unanimously against the measure, claiming that the navy had plenty of officers able to administer the observatory in a satisfactory way, and that there was therefore no necessity for a civilian head. two years later, senator morrill offered an amendment to the legislative appropriation bill, providing that the superintendent of the observatory should be selected from civil life, and be learned in the science of astronomy. he supported his amendment by letters from a number of leading astronomers of the country in reply to questions which he had addressed to them. this amendment, after being approved by the senate naval committee, was referred by the committee on appropriations to the secretary of the navy. he recommended a modification of the measure so as to provide for the appointment of a "director of astronomy," to have charge of the astronomical work of the observatory, which should, however, remain under a naval officer as superintendent. this arrangement was severely criticised in the house by mr. thomas b. reed, of maine, and the whole measure was defeated in conference. in , when the new observatory was being occupied, the superintendent promulgated regulations for its work. these set forth in great detail what the observatory should do. its work was divided into nine departments, each with its chief, besides which there was a chief astronomical assistant and a chief nautical assistant to the superintendent, making eleven chiefs in all. the duties of each chief were comprehensively described. as the entire scientific force of the observatory numbered some ten or twelve naval officers, professors, and assistant astronomers, with six computers, it may be feared that some of the nine departments were short-handed. in september, , new regulations were established by the secretary of the navy, which provided for an "astronomical director," who was to "have charge of and to be responsible for the direction, scope, character, and preparation for publication of all work purely astronomical, which is performed at the naval observatory." as there was no law for this office, it was filled first by the detail of professor harkness, who served until his retirement in , then by the detail of professor brown, who served until march, . in the secretary of the navy appointed a board of visitors to the observatory, comprising senator chandler, of new hampshire, hon. a. g. dayton, house of representatives, and professors pickering, comstock, and hale. this board, "in order to obviate a criticism that the astronomical work of the observatory has not been prosecuted with that vigor and continuity of purpose which should be shown in a national observatory," recommended that the astronomical director and the director of the nautical almanac should be civil officers, with sufficient salaries. a bill to this effect was introduced into each house of congress at the next session, and referred to the respective naval committees, but never reported. in congress, in an amendment to the naval appropriation bill, provided a permanent board of visitors to the observatory, in whom were vested full powers to report upon its condition and expenditures, and to prescribe its plan of work. it was also provided in the same law that the superintendent of the observatory should, until further legislation by congress, be a line officer of the navy of a rank not below that of captain. in the first annual report of this board is the following clause:-- "we wish to record our deliberate and unanimous judgment that the law should be changed so as to provide that the official head of the observatory--perhaps styled simply the director--should be an eminent astronomer appointed by the president by and with the consent of the senate." although the board still has a legal existence, congress, in , practically suspended its functions by declining to make any appropriation for its expenses. moreover, since the detachment of professor brown, astronomical director, no one has been appointed to fill the vacancy thus arising. at the time of the present writing, therefore, the entire responsibility for planning and directing the work of the observatory is officially vested in the naval superintendent, as it was at the old observatory. v great telescopes and their work one hardly knows where, in the history of science, to look for an important movement that had its effective start in so pure and simple an accident as that which led to the building of the great washington telescope, and went on to the discovery of the satellites of mars. very different might have been a chapter of astronomical history, but for the accident of mr. cyrus field, of atlantic cable fame, having a small dinner party at the arlington hotel, washington, in the winter of . among the guests were senators hamlin and casserly, mr. j. e. hilgard of the coast survey, and a young son of mr. field, who had spent the day in seeing the sights of washington. being called upon for a recital of his experiences, the youth described his visit to the observatory, and expressed his surprise at finding no large telescope. the only instrument they could show him was much smaller and more antiquated than that of mr. rutherfurd in new york. the guests listened to this statement with incredulity, and applied to mr. hilgard to know whether the visitor was not mistaken, through a failure to find the great telescope of the observatory. mr. hilgard replied that the statement was quite correct, the observatory having been equipped at a time when the construction of great refracting telescopes had not been commenced, and even their possibility was doubted. "this ought not to be," said one of the senators. "why is it so?" mr. hilgard mentioned the reluctance of congress to appropriate money for a telescope. "it must be done," replied the senator. "you have the case properly represented to congress, and we will see that an appropriation goes through the senate at least." it chanced that this suggestion had an official basis which was not known to the guests. although mr. alvan clark had already risen into prominence as a maker of telescopes, his genius in this direction had not been recognized outside of a limited scientific circle. the civil war had commenced just as he had completed the largest refracting telescope ever made, and the excitement of the contest, as well as the absorbing character of the questions growing out of the reconstruction of the union, did not leave our public men much time to think about the making of telescopes. mr. clark had, however, been engaged by captain gilliss only a year or two after the latter had taken charge of the observatory, to come to washington, inspect our instruments, and regrind their glasses. the result of his work was so striking to the observers using the instruments before and after his work on them, that no doubt of his ability could be felt. accordingly, in preparing items for the annual reports of the observatory for the years and , i submitted one to the superintendent setting forth the great deficiency of the observatory in respect to the power of its telescope, and the ability of mr. clark to make good that deficiency. these were embodied in the reports. it was recommended that authority be given to order a telescope of the largest size from mr. clark. it happened, however, that secretary welles had announced in his annual reports as his policy that he would recommend no estimates for the enlargement and improvement of public works in his department, but would leave all matters of this kind to be acted on by congress as the latter might deem best. as the telescope was thrown out of the regular estimates by this rule, this subject had failed to be considered by congress. now, however, the fact of the recommendation appearing in the annual report, furnished a basis of action. mr. hilgard did not lose a day in setting the ball in motion. he called upon me immediately, and i told him of the recommendations in the last two reports of the superintendent of the observatory. together we went to see admiral sands, who of course took the warmest interest in the movement, and earnestly promoted it on the official side. mr. hilgard telegraphed immediately to some leading men of science, who authorized their signatures to a petition. in this paper attention was called to the wants of the observatory, as set forth by the superintendent, and to the eminent ability of the celebrated firm of the clarks to supply them. the petition was printed and put into the hands of senator hamlin for presentation to the senate only three or four days after the dinner party. the appropriation measure was formally considered by the committee on naval affairs and that on appropriations, and was adopted in the senate as an amendment to the naval appropriation bill without opposition. the question then was to get the amendment concurred in by the house of representatives. the session was near its close, and there was no time to do much work. several members of the house committee on appropriations were consulted, and the general feeling seemed to be favorable to the amendment. great, therefore, was our surprise to find the committee recommending that the amendment be not concurred in. to prevent a possible misapprehension, i may remark that the present system of non-concurring in all amendments to an appropriation bill, in order to bring the whole subject into conference, had not then been introduced, so that this action showed a real opposition to the movement. one of the most curious features of the case is that the leader in the opposition was said to be mr. washburn, the chairman of the committee, who, not many years later, founded the washburn observatory of the university of wisconsin. there is, i believe, no doubt that his munificence in this direction arose from what he learned about astronomy and telescopes in the present case. it happened, most fortunately, that the joint committee of conference included drake of the senate and niblack of the house, both earnestly in favor of the measure. the committee recommended concurrence, and the clause authorizing the construction became a law. the price was limited to $ , , and a sum of $ , was appropriated for the first payment. no sooner were the clarks consulted than difficulties were found which, for a time, threatened to complicate matters, and perhaps delay the construction. in the first place, our currency was then still on a paper basis. gold was at a premium of some ten or fifteen per cent., and the clarks were unwilling to take the contract on any but a gold basis. this, of course, the government could not do. but the difficulty was obviated through the action of a second one, which equally threatened delay. mr. l. j. mccormick, of reaping-machine fame, had conceived the idea of getting the largest telescope that could be made. he had commenced negotiations with the firm of alvan clark & sons before we had moved, and entered into a contract while the appropriation was still pending in congress. if the making of one great telescope was a tedious job, requiring many years for its completion, how could two be made? i was charged with the duty of negotiating the government contract with the clarks. i found that the fact of mr. mccormick's contract being on a gold basis made them willing to accept one from the government on a currency basis; still they considered that mr. mccormick had the right of way in the matter of construction, and refused to give precedence to our instrument. on mature consideration, however, the firm reached the conclusion that two instruments could be made almost simultaneously, and mr. mccormick very generously waived any right he might have had to precedence in the matter. the question how large an instrument they would undertake was, of course, one of the first to arise. progress in the size of telescopes had to be made step by step, because it could never be foreseen how soon the limit might be met; and if an attempt were made to exceed it, the result would be not only failure for the instrument, but loss of labor and money by the constructors. the largest refracting telescope which the clarks had yet constructed was one for the university of mississippi, which, on the outbreak of the civil war, had come into the possession of the astronomical society of chicago. this would have been the last step, beyond which the firm would not have been willing to go to any great extent, had it not happened that, at this very time, a great telescope had been mounted in england. this was made by thomas cooke & sons of york, for mr. r. s. newall of gateshead on tyne, england. the clarks could not, of course, allow themselves to be surpassed or even equaled by a foreign constructor; yet they were averse to going much beyond the cooke telescope in size. twenty-six inches aperture was the largest they would undertake. i contended as strongly as i could for a larger telescope than mr. mccormick's, but they would agree to nothing of the sort,--the supposed right of that gentleman to an instrument of equal size being guarded as completely as if he had been a party to the negotiations. so the contract was duly made for a telescope of twenty-six inches clear aperture. at that time cooke and clark were the only two men who had ever succeeded in making refracting telescopes of the largest size. but in order to exercise their skill, an art equally rare and difficult had to be perfected, that of the glassmaker. ordinary glass, even ordinary optical glass, would not answer the purpose at all. the two disks, one of crown glass and the other of flint, must be not only of perfect transparency, but absolutely homogeneous through and through, to avoid inequality of refraction, and thus cause all rays passing through them to meet in the same focus. it was only about the beginning of the century that flint disks of more than two or three inches diameter could be made. even after that, the art was supposed to be a secret in the hands of a swiss named guinand, and his family. looking over the field, the clarks concluded that the only firm that could be relied on to furnish the glass was that of chance & co., of birmingham, england. so, as soon as the contracts were completed, one of the clark firm visited england and arranged with chance & co. to supply the glass for the two telescopes. the firm failed in a number of trials, but by repeated efforts finally reached success at the end of a year. the glasses were received in december, , and tested in the following month. a year and a half more was required to get the object glasses into perfect shape; then, in the spring or summer of , i visited cambridge for the purpose of testing the glasses. they were mounted in the yard of the clark establishment in a temporary tube, so arranged that the glass could be directed to any part of the heavens. i have had few duties which interested me more than this. the astronomer, in pursuing his work, is not often filled with those emotions which the layman feels when he hears of the wonderful power of the telescope. not to say anything so harsh as that "familiarity breeds contempt," we must admit that when an operation of any sort becomes a matter of daily business, the sentiments associated with it necessarily become dulled. now, however, i was filled with the consciousness that i was looking at the stars through the most powerful telescope that had ever been pointed at the heavens, and wondered what mysteries might be unfolded. the night was of the finest, and i remember, sweeping at random, i ran upon what seemed to be a little cluster of stars, so small and faint that it could scarcely have been seen in a smaller instrument, yet so distant that the individual stars eluded even the power of this instrument. what cluster it might have been it was impossible to determine, because the telescope had not the circles and other appliances necessary for fixing the exact location of an object. i could not help the vain longing which one must sometimes feel under such circumstances, to know what beings might live on planets belonging to what, from an earthly point of view, seemed to be a little colony on the border of creation itself. in his report dated october , , admiral sands reported the telescope as "nearly completed." the volume of washington observations showed that the first serious observations made with it, those on the satellites of neptune, were commenced on november of the same year. thus, scarcely more than a month elapsed from the time that the telescope was reported still incomplete in the shop of its makers until it was in regular nightly use. associated with the early history of the instrument is a chapter of astronomical history which may not only instruct and amuse the public, but relieve the embarrassment of some astronomer of a future generation who, reading the published records, will wonder what became of an important discovery. if the faith of the public in the absolute certainty of all astronomical investigation is thereby impaired, what i have to say will be in the interest of truth; and i have no fear that our science will not stand the shock of the revelation. of our leading astronomical observers of the present day--of such men as burnham and barnard--it may be safely said that when they see a thing it is there. but this cannot always be said of every eminent observer, and here is a most striking example of this fact. when the telescope was approaching completion i wrote to the head of one of the greatest european observatories, possessing one of the best telescopes of the time, that the first thing i should attempt with the telescope would be the discovery of the companion of procyon. this first magnitude star, which may be well seen in the winter evenings above orion, had been found to move in an exceedingly small orbit, one too small to be detected except through the most refined observations of modern precision. the same thing had been found in the case of sirius, and had been traced to the action of a minute companion revolving around it, which was discovered by the clarks a dozen years before. there could be no doubt that the motion of procyon was due to the same cause, but no one had ever seen the planet that produced it, though its direction from the star at any time could be estimated. now, it happened that my european friend, as was very natural, had frequently looked for this object without seeing it. whether my letter set him to looking again, or whether he did not receive it until a later day, i do not know. what is certain is that, in the course of the summer, he published the discovery of the long-looked-for companion, supplemented by an excellent series of observations upon it, made in march and april. of course i was a little disappointed that the honor of first finding this object did not belong to our own telescope. still i was naturally very curious to see it. so, on the very first night on which the telescope could be used, i sat up until midnight to take a look at procyon, not doubting that, with the greater power of our telescope, it would be seen at the first glance. to my great concern, nothing of the sort was visible. but the night was far from good, the air being somewhat thick with moisture, which gave objects seen through it a blurred appearance; so i had to await a better night and more favorable conditions. better nights came and passed, and still not a trace of the object could be seen. supposing that the light of the bright star might be too dazzling, i cut it off with a piece of green glass in the focus. still no companion showed itself. could it be that our instrument, in a more favorable location, would fail to show what had been seen with one so much smaller? this question i could not answer, but wrote to my european friend of my unavailing attempts. he replied expressing his perplexity and surprise at the occurrence, which was all the greater that the object had again been seen and measured in april, . a fine-looking series of observations was published, similar to those of the preceding year. what made the matter all the more certain was that there was a change in the direction of the object which corresponded very closely to the motion as it had been predicted by auwers. the latter published a revision of his work, based on the new observations. a year later, the parties that had been observing the transit of venus returned home. the head of one of them, professor c. h. f. peters of clinton, stopped a day or two at washington. it happened that a letter from my european friend arrived at the same time. i found that peters was somewhat skeptical as to the reality of the object. sitting before the fire in my room at the observatory, i read to him and some others extracts from the letter, which cited much new evidence to show the reality of the discovery. not only had several of his own observers seen the object, but it had been seen and measured on several different nights by a certain professor blank, with a telescope only ten or twelve inches aperture. "what," said peters, "has blank seen it?" "yes, so the letter says." "then it is n't there!" and it really was not there. the maker of the discovery took it all back, and explained how he had been deceived. he found that the telescope through which the observations were made seemed to show a little companion of the same sort alongside of every very bright star. everything was explained by this discovery. even the seeming motion of the imaginary star during the twelve months was accounted for by the fact that in procyon was much nearer the horizon when the observations were made than it was the year following. [ ] there is a sequel to the history, which may cause its revision by some astronomer not many years hence. when the great telescope was mounted at the lick observatory, it is understood that burnham and barnard, whose eyes are of the keenest, looked in vain for the companion of procyon. yet, in , it was found with the same instrument by schaeberle, and has since been observed with the great yerkes telescope, as well as by the observers at mount hamilton, so that the reality of the discovery is beyond a doubt. the explanation of the failure of burnham and barnard to see it is very simple: the object moves in an eccentric orbit, so that it is nearer the planet at some points of its orbit than at others. it was therefore lost in the rays of the bright star during the years - . is it possible that it could have been far enough away to be visible in - ? i need scarcely add that this question must be answered in the negative, yet it may be worthy of consideration, when the exact orbit of the body is worked out twenty or thirty years hence. in my work with the telescope i had a more definite end in view than merely the possession of a great instrument. the work of reconstructing the tables of the planets, which i had long before mapped out as the greatest one in which i should engage, required as exact a knowledge as could be obtained of the masses of all the planets. in the case of uranus and neptune, the two outer planets, this knowledge could best be obtained by observations on their satellites. to the latter my attention was therefore directed. in the case of neptune, which has only one satellite yet revealed to human vision, and that one so close to the planet that the observations are necessarily affected by some uncertainty, it was very desirable that a more distant one should be found if it existed. i therefore during the summer and autumn of made most careful search under the most favorable conditions. but no second satellite was found. i was not surprised to learn that the observers with the great lick telescope were equally unsuccessful. my observations with the instrument during two years were worked up and published, and i turned the instrument over to professor hall in . the discovery of the satellites of mars was made two years later, in august, . as no statement that i took any interest in the discovery has ever been made in any official publication, i venture, with the discoverer's permission, to mention the part that i took in verifying it. one morning professor hall confidentially showed me his first observations of an object near mars, and asked me what i thought of them. i remarked, "why, that looks very much like a satellite." yet he seemed very incredulous on the subject; so incredulous that i feared he might make no further attempt to see the object. i afterward learned, however, that this was entirely a misapprehension on my part. he had been making a careful search for some time, and had no intention of abandoning it until the matter was cleared up one way or the other. the possibility of the object being an asteroid suggested itself. i volunteered to test this question by looking at the ephemerides of all the small planets in the neighborhood of mars. a very little searching disproved the possibility of the object belonging to this class. one such object was in the neighborhood, but its motion was incompatible with the measures. then i remarked that, if the object were really a satellite, the measures already made upon it, and the approximately known mass of the planet, would enable the motion of the satellite to be determined for a day or two. thus i found that on that night the satellite would be hidden in the early evening by the planet, but would emerge after midnight. i therefore suggested to professor hall that, if it was not seen in the early evening, he should wait until after midnight. the result was in accordance with the prediction,--the satellite was not visible in the early evening, but came out after midnight. no further doubt was possible, and the discovery was published. the labor of searching and observing was so exhausting that professor hall let me compute the preliminary orbit of the satellites from his early observations. my calculations and suggestions lost an importance they might otherwise have claimed, for the reason that several clear nights followed. had cloudy weather intervened, a knowledge of when to look for the object might have greatly facilitated its recognition. it is still an open question, perhaps, whether a great refracting telescope will last unimpaired for an indefinite length of time. i am not aware that the twin instruments of harvard and pulkowa, mounted in , have suffered from age, nor am i aware that any of alvan clark's instruments are less perfect to-day than when they left the hands of their makers. but not long after the discovery of the satellites of mars, doubts began to spread in some quarters as to whether the great washington telescope had not suffered deterioration. these doubts were strengthened in the following way: when hundreds of curious objects were being discovered in the heavens here and there, observers with small instruments naturally sought to find them. the result was several discoveries belonging to the same class as that of the satellite of procyon. they were found with very insignificant instruments, but could not be seen in the large ones. professor hall published a letter in a european journal, remarking upon the curious fact that several objects were being discovered with very small instruments, which were invisible in the washington telescope. this met the eye of professor wolf, a professor at the sorbonne in paris, as well as astronomer at the paris observatory. in a public lecture, which he delivered shortly afterward, he lamented the fact that the deterioration of the washington telescope had gone so far as that, and quoted professor hall as his authority. the success of the washington telescope excited such interest the world over as to give a new impetus to the construction of such instruments. its glass showed not the slightest drawbacks from its great size. it had been feared that, after a certain limit, the slight bending of the glass under its own weight would be injurious to its performance. nothing of the kind being seen, the clarks were quite ready to undertake much larger instruments. a -inch telescope for the pulkova observatory in russia, the -inch telescope of the lick observatory in california, and, finally, the -inch of the yerkes observatory in chicago, were the outcome of the movement. of most interest to us in the present connection is the history of the -inch telescope of the pulkova observatory, the object glass of which was made by alvan clark & sons. it was, i think, sometime in that i received a letter from otto struve, [ ] director of the pulkova observatory, stating that he was arranging with his government for a grant of money to build one of the largest refracting telescopes. in answering him i called his attention to the ability of alvan clark & sons to make at least the object glass, the most delicate and difficult part of the instrument. the result was that, after fruitless negotiations with european artists, struve himself came to america in the summer of to see what the american firm could do. he first went to washington and carefully examined the telescope there. then he proceeded to cambridge and visited the workshop of the clarks. he expressed some surprise at its modest dimensions and fittings generally, but was so well pleased with what he saw that he decided to award them the contract for making the object glass. he was the guest of the pickerings at the cambridge observatory, and invited me thither from where i was summering on the coast of massachusetts to assist in negotiating the contract. he requested that, for simplicity in conference, the preliminary terms should be made with but a single member of the firm to talk with. george b. clark, the eldest member, was sent up to represent the firm. i was asked to take part in the negotiations as a mutual friend of both parties, and suggested the main conditions of the contract. a summary of these will be found in the publication to which i have already referred. there was one provision the outcome of which was characteristic of alvan clark & sons. struve, in testing some object glasses which they had constructed and placed in their temporary tube, found so great physical exertion necessary in pointing so rough an instrument at any heavenly body with sufficient exactness, that he could not form a satisfactory opinion of the object glass. as he was to come over again when the glass was done, in order to test it preliminary to acceptance, he was determined that no such difficulty should arise. he therefore made a special provision that $ extra, to be repaid by him, should be expended in making a rough equatorial mounting in which he could test the instrument. george clark demurred to this, on the ground that such a mounting as was necessary for this purpose could not possibly cost so much money. but struve persistently maintained that one to cost $ should be made. the other party had to consent, but failed to carry out this provision. the tube was, indeed, made large enough to test not only struve's glass but the larger one of the lick observatory, which, though not yet commenced, was expected to be ready not long afterward. yet, notwithstanding this increase of size, i think the extra cost turned out to be much less than $ , and the mounting was so rough that when struve came over in to test the glass, he suffered much physical inconvenience and met, if my memory serves me aright, with a slight accident, in his efforts to use the rough instrument. in points like this i do not believe that another such business firm as that of the clarks ever existed in this country or any other. here is an example. shortly before the time of struve's visit, i had arranged with them for the construction of a refined and complicated piece of apparatus to measure the velocity of light. as this apparatus was quite new in nearly all its details, it was impossible to estimate in advance what it might cost; so, of course, they desired that payment for it should be arranged on actual cost after the work was done. i assured them that the government would not enter into a contract on such terms. there must be some maximum or fixed price. this they fixed at $ . i then arranged with them that this should be taken as a maximum and that, if it was found to cost less, they should accept actual cost. the contract was arranged on this basis. there were several extras, including two most delicate reflecting mirrors which would look flat to the eye, but were surfaces of a sphere of perhaps four miles diameter. the entire cost of the apparatus, as figured up by them after it was done, with these additions, was less than $ , or about forty per cent. below the contract limit. no set of men were ever so averse to advertising themselves. if anybody, in any part of the world, wanted them to make a telescope, he must write to them to know the price, etc. they could never be induced to prepare anything in the form of a price catalogue of the instruments they were prepared to furnish. the history of their early efforts and the indifference of our scientific public to their skill forms a mortifying chapter in our history of the middle of the century. when mr. clark had finished his first telescope, a small one of four inches aperture, which was, i have no reason to doubt, the best that human art could make, he took it to the cambridge observatory to be tested by one of the astronomers. the latter called his attention to a little tail which the glass showed as an appendage of a star, and which was, of course, non-existent. it was attributed to a defect in the glass, which was therefore considered a failure. mr. clark was quite sure that the tail was not shown when he had previously used the glass, but he could not account for it at the time. he afterwards traced it to the warm air collecting in the upper part of the tube and producing an irregular refraction of the light. when this cause was corrected the defect disappeared. but he got no further encouragement at home to pursue his work. the first recognition of his genius came from england, the agent being rev. w. r. dawes, an enthusiastic observer of double stars, who was greatly interested in having the best of telescopes. mr. clark wrote him a letter describing a number of objects which he had seen with telescopes of his own make. from this description mr. dawes saw that the instruments must be of great excellence, and the outcome of the matter was that he ordered one or more telescopes from the american maker. not until then were the abilities of the latter recognized in his own country. i have often speculated as to what the result might have been had mr. clark been a more enterprising man. if, when he first found himself able to make a large telescope, he had come to washington, got permission to mount his instrument in the grounds of the capitol, showed it to members of congress, and asked for legislation to promote this new industry, and, when he got it, advertised himself and his work in every way he could, would the firm which he founded have been so little known after the death of its members, as it now unhappily is? this is, perhaps, a rather academic question, yet not an unprofitable one to consider. in recent years the firm was engaged only to make object glasses of telescopes, because the only mountings they could be induced to make were too rude to satisfy astronomers. the palm in this branch of the work went to the firm of warner & swasey, whose mounting of the great yerkes telescope of the university of chicago is the last word of art in this direction. during the period when the reputation of the cambridge family was at its zenith, i was slow to believe that any other artist could come up to their standard. my impression was strengthened by a curious circumstance. during a visit to the strasburg observatory in i was given permission to look through its great telescope, which was made by a renowned german artist. i was surprised to find the object glass affected by so serious a defect that it could not be expected to do any work of the first class. one could only wonder that european art was so backward. but, several years afterward, the astronomers discovered that, in putting the glasses together after being cleaned, somebody had placed one of them in the wrong position, the surface which should have been turned toward the star being now turned toward the observer. when the glass was simply turned over so as to have the right face outward, the defect disappeared. [ ] in justice to mr. blank, i must say that there seems to have been some misunderstanding as to his observations. what he had really seen and observed was a star long well known, much more distant from procyon than the companion in question. [ ] otto struve was a brother of the very popular russian minister to washington during the years - . he retired from the direction of the pulkowa observatory about . the official history of his negotiations and other proceedings for the construction of the telescope will be found in a work published in in honor of the jubilee of the observatory. vi the transits of venus it was long supposed that transits of venus over the sun's disk afforded the only accurate method of determining the distance of the sun, one of the fundamental data of astronomy. unfortunately, these phenomena are of the rarest. they come in pairs, with an interval of eight years between the transits of a pair. a pair occurred in and , and again in and . now the whole of the twentieth century will pass without another recurrence of the phenomenon. not until the years and will our posterity have the opportunity of witnessing it. much interesting history is associated with the adventures of the astronomers who took part in the expeditions to observe the transits of and . in the almost chronic warfare which used to rage between france and england during that period, neither side was willing to regard as neutral even a scientific expedition sent out by the other. the french sent one of their astronomers, le gentil, to observe the transit at pondicherry in the east indies. as he was nearing his station, the presence of the enemy prevented him from making port, and he was still at sea on the day of the transit. when he at length landed, he determined to remain until the transit of , and observe that. we must not suppose, however, that he was guilty of the eccentricity of doing this with no other object in view than that of making the observation. he found the field open for profitable mercantile enterprise, as well as interesting for scientific observations and inquiries. the eight long years passed away, and the morning of june , , found him in readiness for his work. the season had been exceptionally fine. on the morning of the transit the sun shone in a cloudless sky, as it had done for several days previous. but, alas for all human hopes! just before venus reached the sun, the clouds gathered, and a storm burst upon the place. it lasted until the transit was over, and then cleared away again as if with the express object of showing the unfortunate astronomer how helpless he was in the hands of the elements. the royal society of england procured a grant of £ from king george ii. for expeditions to observe the transit of . [ ] with this grant the society sent the rev. nevil maskelyne to the island of st. helena, and, receiving another grant, it was used to dispatch messrs. mason and dixon (those of our celebrated "line") to bencoolen. the admiralty also supplied a ship for conveying the observers to their respective destinations. maskelyne, however, would not avail himself of this conveyance, but made his voyage on a private vessel. cloudy weather prevented his observations of the transit, but this did not prevent his expedition from leaving for posterity an interesting statement of the necessaries of an astronomer of that time. his itemized account of personal expenses was as follows:-- one year's board at st. helena . . £ s. d. liquors at s. per day . . . . washing at d. per day . . . . other expenses . . . . . . liquors on board ship for six months --- --- --- £ s. d. seven hundred dollars was the total cost of liquors during the eighteen months of his absence. admiral smyth concludes that maskelyne "was not quite what is now ycleped a teetotaler." he was subsequently astronomer royal of england for nearly half a century, but his published observations give no indication of the cost of the drinks necessary to their production. mason and dixon's expedition met with a mishap at the start. they had only got fairly into the english channel when their ship fell in with a french frigate of superior force. an action ensued in which the english crew lost eleven killed and thirty-eight wounded. the frenchman was driven off, but the victorious vessel had to return to plymouth for repairs. this kind of a scientific expedition was more than the astronomers had bargained for, and they wrote from plymouth to the royal society, describing their misfortune and resigning their mission. but the council of the society speedily let them know that they were unmoved by the misfortunes of their scientific missionaries, and pointed out to them in caustic terms that, having solemnly undertaken the expedition, and received money on account of it, their failure to proceed on the voyage would be a reproach to the nation in general, and to the royal society in particular. it would also bring an indelible scandal upon their character, and probably end in their utter ruin. they were assured that if they persisted in the refusal, they would be treated with the most inflexible resentment, and prosecuted with the utmost severity of the law. under such threats the unfortunate men could do nothing but accept the situation and sail again after their frigate had been refitted. when they got as far as the cape of good hope, it was found very doubtful whether they would reach their destination in time for the transit; so, to make sure of some result from their mission, they made their observations at the cape. one of the interesting scraps of history connected with the transit of concerns the observations of father maximilian hell, s. j., the leading astronomer of vienna. he observed the transit at wardhus, a point near the northern extremity of norway, where the sun did not set at the season of the transit. owing to the peculiar circumstances under which the transit was observed,--the ingress of the planet occurring two or three hours before the sun approached the northern horizon, and the end of the transit about as long afterward,--this station was the most favorable one on the globe. hell, with two or three companions, one of them named sajnovics, went on his mission to this isolated place under the auspices of the king of denmark. the day was cloudless and the observations were made with entire success. he returned to copenhagen, where he passed several months in preparing for the press a complete account of his expedition and the astronomical observations made at the station. astronomers were impatient to have the results for the distance of the sun worked out as soon as possible. owing to the importance of hell's observations, they were eagerly looked for. but he at first refused to make them known, on the ground that, having been made under the auspices of the king of denmark, they ought not to be made known in advance of their official publication by the danish academy of sciences. this reason, however, did not commend itself to the impatient astronomers; and suspicions were aroused that something besides official formalities was behind the delay. it was hinted that hell was waiting for the observations made at other stations in order that he might so manipulate his own that they would fit in with those made elsewhere. reports were even circulated that he had not seen the transit at all, owing to cloudy weather, and that he was manufacturing observations in copenhagen. the book was, however, sent to the printer quite promptly, and the insinuations against its author remained a mere suspicion for more than sixty years. then, about , a little book was published on the subject by littrow, director of the vienna observatory, which excited much attention. father hell's original journal had been conveyed to vienna on his return, and was still on deposit at the austrian national observatory. littrow examined it and found, as he supposed, that the suspicions of alterations in observations were well founded; more especially that the originals of the all-important figures which recorded the critical moment of "contact" had been scraped out of the paper, and new ones inserted in their places. the same was said to be the case with many other important observations in the journal, and the conclusion to which his seemingly careful examination led was that no reliance could be placed on the genuineness of hell's work. the doubts thus raised were not dispelled until another half-century had elapsed. in i paid a visit to vienna for the purpose of examining the great telescope which had just been mounted in the observatory there by grubb, of dublin. the weather was so unfavorable that it was necessary to remain two weeks, waiting for an opportunity to see the stars. one evening i visited the theatre to see edwin booth, in his celebrated tour over the continent, play king lear to the applauding viennese. but evening amusements cannot be utilized to kill time during the day. among the tasks i had projected was that of rediscussing all the observations made on the transits of venus which had occurred in and , by the light of modern science. as i have already remarked, hell's observations were among the most important made, if they were only genuine. so, during my almost daily visits to the observatory, i asked permission of director weiss to study hell's manuscript. at first the task of discovering anything which would lead to a positive decision on one side or the other seemed hopeless. to a cursory glance, the descriptions given by littrow seemed to cover the ground so completely that no future student could turn his doubt into certainty. but when one looks leisurely at an interesting object, day after day, he continually sees more and more. thus it was in the present case. one of the first things to strike me as curious was that many of the alleged alterations had been made before the ink got dry. when the writer made a mistake, he had rubbed it out with his finger, and made a new entry. the all-important point was a certain suspicious record which littrow affirmed had been scraped out so that the new insertion could be made. as i studied these doubtful figures, day by day, light continually increased. evidently the heavily written figures, which were legible, had been written over some other figures which were concealed beneath them, and were, of course, completely illegible, though portions of them protruded here and there outside of the heavy figures. then i began to doubt whether the paper had been scraped at all. to settle the question, i found a darkened room, into which the sun's rays could be admitted through an opening in the shutter, and held the paper in the sunlight in such a way that the only light which fell on it barely grazed the surface of the paper. examining the sheet with a magnifying glass, i was able to see the original texture of the surface with all its hills and hollows. a single glance sufficed to show conclusively that no eraser had ever passed over the surface, which had remained untouched. the true state of the case seemed to me almost beyond doubt. it frequently happened that the ink did not run freely from the pen, so that the words had sometimes to be written over again. when hell first wrote down the little figures on which, as he might well suppose, future generations would have to base a very important astronomical element, he saw that they were not written with a distinctness corresponding to their importance. so he wrote them over again with the hand, and in the spirit of a man who was determined to leave no doubt on the subject, little weening that the act would give rise to a doubt which would endure for a century. this, although the most important case of supposed alteration, was by no means the only one. yet, to my eyes, all the seeming corrections in the journal were of the most innocent and commonplace kind,--such as any one may make in writing. then i began to compare the manuscript, page after page, with littrow's printed description. it struck me as very curious that where the manuscript had been merely retouched with ink which was obviously the same as that used in the original writing, but looked a little darker than the original, littrow described the ink as of a different color. in contrast with this, there was an important interlineation, which was evidently made with a different kind of ink, one that had almost a blue tinge by comparison; but in the description he makes no mention of this plain difference. i thought this so curious that i wrote in my notes as follows:-- "that littrow, in arraying his proofs of hell's forgery, should have failed to dwell upon the obvious difference between this ink and that with which the alterations were made leads me to suspect a defect in his sense of color." then it occurred to me to inquire whether, perhaps, such could have been the case. so i asked director weiss whether anything was known as to the normal character of littrow's power of distinguishing colors. his answer was prompt and decisive. "oh, yes, littrow was color blind to red. he could not distinguish between the color of aldebaran and that of the whitest star." no further research was necessary. for half a century the astronomical world had based an impression on the innocent but mistaken evidence of a color-blind man respecting the tints of ink in a manuscript. about the middle of the nineteenth century other methods of measuring the sun's distance began to be developed which, it was quite possible, might prove as good as the observation in question. but the relative value of these methods and of transits of venus was a subject on which little light could be thrown; and the rarity of the latter phenomena naturally excited universal interest, both among the astronomers and among the public. for the purpose in question it was necessary to send expeditions to different and distant parts of the globe, because the result had to depend upon the times of the phases, as seen from widely separated stations. in the question what stations should be occupied and what observations should be made was becoming the subject of discussion in europe, and especially in england. but our country was still silent on the subject. the result of continued silence was not hard to foresee. congress would, at the last moment, make a munificent appropriation for sending out parties to observe the transit. the plans and instruments would be made in a hurry, and the parties packed off without any well-considered ideas of what they were to do; and the whole thing would end in failure so far as results of any great scientific value were concerned. i commenced the discussion by a little paper on the subject in the "american journal of science," but there was no one to follow it up. so, at the spring meeting of the national academy of sciences, in , i introduced a resolution for the appointment of a committee to consider the subject and report upon the observations which should be made. this resolution was adopted, and a few days afterward professor henry invited me to call at his office in the evening to discuss with himself and professor peirce, then superintendent of the coast survey, the composition of the committee. at the conference i began by suggesting professor peirce himself for chairman. naturally this met with no opposition; then i waited for the others to go on. but they seemed determined to throw the whole onus of the matter on me. this was the more embarrassing, because i believe that, in parliamentary law and custom, the mover of a resolution of this sort has a prescribed right to be chairman of the committee which he proposes shall be appointed. if not chairman, it would seem that he ought at any rate to be a member. but i was determined not to suggest myself in any way, so i went on and suggested admiral davis. this nomination was, of course, accepted without hesitation. then i remarked that the statutes of the academy permitted of persons who were not members being invited to serve on a committee, and as the naval observatory would naturally take a leading part in such observations as were to be made, i suggested that its superintendent, admiral sands, should be invited to serve as a member of the committee. "there," said peirce, "we now have three names. committees of three are always the most efficient. why go farther?" i suggested that the committee should have on it some one practiced in astronomical observation, but he deemed this entirely unnecessary, and so the committee of three was formed. i did not deem it advisable to make any opposition at the time, because it was easy to foresee what the result would be. during the summer nothing was heard of the committee, and in the autumn i made my first trip to europe. on my return, in may, , i found that the committee had never even held a meeting, and that it had been enlarged by the addition of a number of astronomers, among them myself. but, before it went seriously to work, it was superseded by another organization, to be described presently. at that time astronomical photography was in its infancy. enough had been done by rutherfurd to show that it might be made a valuable adjunct to astronomical investigation. might we not then photograph venus on the sun's disk, and by measurements of the plates obtain the desired result, perhaps better than it could be obtained by any kind of eye observation? this question had already suggested itself to professor winlock, who, at the cambridge observatory, had designed an instrument for taking the photographs. it consisted of a fixed horizontal telescope, into which the rays of the sun were to be thrown by a reflector. this kind of an instrument had its origin in france, but it was first practically applied to photographing the sun in this country. as whatever observations were to be made would have to be done at governmental expense, an appropriation of two thousand dollars was obtained from congress for the expense of some preliminary instruments and investigations. admiral sands, superintendent of the observatory, now took an active part in the official preparations. it was suggested to him, on the part of the academy committee, that it would be well to join hands with other organizations, so as to have the whole affair carried on with unity and harmony. to this he assented. the result was a provision that these and all other preparations for observing the transit of venus should be made under the direction of a commission to be composed of the superintendent of the naval observatory, the superintendent of the united states coast survey, the president of the national academy of sciences, and two professors of mathematics attached to the naval observatory. under this provision the commission was constituted as follows: commodore b. f. sands, u. s. n., professor benjamin peirce, professor joseph henry, professor simon newcomb, professor william harkness. the academy committee now surrendered its functions to the commission, and the preparations were left entirely in the hands of the latter. so far as scientific operations were concerned, the views of the commission were harmonious through the whole of their deliberations. it was agreed from the beginning that the photographic method offered the greatest promise of success. but how, with what sort of instruments, and on what plan, must the photographs be taken? europeans had already begun to consider this question, and for the most part had decided on using photographic telescopes having no distinctive feature specially designed for the transit. in fact, one might almost say that the usual observations with the eye were to be made on the photograph instead of on the actual sun. the american commissioners were of opinion that this would lead to nothing but failure, and that some new system must be devised. the result was a series of experiments and trials with professor winlock's instrument at the cambridge observatory. the outcome of the matter was the adoption of his plan, with three most important additions, which i shall mention, because they may possibly yet be adopted with success in other branches of exact astronomy if this telescope is used, as it seems likely it may be. the first feature was that the photographic telescope should be mounted exactly in the meridian, and that its direction should be tested by having the transit instrument mounted in front of it, in the same line with it. in this way the axis of the telescope was a horizontal north and south line. the next feature was that, immediately in front of the photographic plate, in fact as nearly in contact with it as possible without touching it, a plumb line of which the thread was a very fine silver wire should be suspended, the bob of which passed down below, and was immersed in a vessel of water to prevent vibration. in this way the direction of the north and south line on the plate admitted of being calculated with the greatest exactness, and the plumb line being photographed across the disk of the sun, the position angle could be measured with the same precision that any other measure could be made. the third feature was that the distance between the photographic plate and the object glass of the telescope should be measured by a long iron rod which was kept in position above the line of sight of the telescope itself. this afforded the means of determining to what angle a given measure on the plate would correspond. the whole arrangement would enable the position of the centre of venus with respect to the centre of the sun to be determined by purely geometric methods. one reason for relying entirely on this was that the diameter of the sun, as photographed, would be greater the greater the intensity of the photographic impression, so that no reliance could be placed upon its uniformity. ours were the only parties whose photographic apparatus was fitted up in this way. the french used a similar system, but without the essentials of the plumb line and the measurement of the length of the telescope. the english and germans used ordinary telescopes for the purpose. one of the earliest works of the commission was the preparation and publication of several papers, which were published under the general title, "papers relating to the transit of venus in ." the first of these papers was a discussion of our proposed plan of photographing, in which the difficulties of the problem, and the best way of surmounting them, were set forth. the next, called part ii., related to the circumstances of the transit, and was therefore entirely technical. part iii. related to the corrections of hansen's table of the moon, and was published as a paper relating to the transit of venus, because these corrections were essential in determining the longitudes of the stations by observations of the moon. in england the preparations were left mostly in the hands of professor airy, astronomer royal, and, i believe, captain tupman, who at least took a leading part in the observations and their subsequent reduction. in france, germany, and russia, commissions were appointed to take charge of the work and plan the observations. as coöperation among the parties from different countries would be generally helpful, i accepted an invitation to attend a meeting of the german commission, to be held at hanover in august, . hansen was president of the commission, while auwers was its executive officer. one of my main objects was to point out the impossibility of obtaining any valuable result by the system of photographing which had been proposed, but i was informed, in reply, that the preparations had advanced too far to admit of starting on a new plan and putting it in operation. from the beginning of our preparations it began to be a question of getting from congress the large appropriations necessary for sending out the expeditions and fitting them up with instruments. the sum of $ , was wanted for instruments and outfit. hon. james a. garfield was then chairman of the committee on appropriations. his principles and methods of arranging appropriations for the government were, in some features, so different from those generally in vogue that it will be of interest to describe them. first of all, garfield was rigidly economical in grants of money. this characteristic of a chairman of a committee on appropriations was almost a necessary one. but he possessed it in a different way from any other chairman before or since. the method of the "watch dogs of the treasury" who sometimes held this position was to grant most of the objects asked for, but to cut down the estimated amounts by one fourth or one third. this was a very easy method, and one well fitted to impress the public, but it was one that the executive officers of the government found no difficulty in evading, by the very simple process of increasing their estimate so as to allow for the prospective reduction. [ ] garfield compared this system to ordering cloth for a coat, but economizing by reducing the quantity put into it. if a new proposition came before him, the question was whether it was advisable for the government to entertain it at all. he had to be thoroughly convinced before this would be done. if the question was decided favorably all the funds necessary for the project were voted. when the proposition for the transit of venus came before him, he proceeded in a manner which i never heard of the chairman of an appropriation committee adopting before or since. instead of calling upon those who made the proposition to appear formally before the committee, he asked me to dinner with his family, where we could talk the matter over. one other guest was present, judge black of pennsylvania. he was a dyed-in-the-wool democrat, wielding as caustic a pen as was ever dipped into ink, but was, withal, a firm personal friend and admirer of garfield. as may readily be supposed, the transit of venus did not occupy much time at the table. i should not have been an enthusiastic advocate of the case against opposition, in any case, because my hopes of measuring the sun's distance satisfactorily by that method were not at all sanguine. my main interest lay in the fact that, apart from this, the transit would afford valuable astronomical data for the life work which i had mainly in view. so the main basis of my argument was that other nations were going to send out parties; that we should undoubtedly do the same, and that they must be equipped and organized in the best way. it appears that judge black was an absent-minded man, as any man engaged in thought on very great subjects, whether of science, jurisprudence, or politics, has the right to be. garfield asked him whether it was true that, on one occasion, when preparing an argument, and walking up and down the room, his hat chanced to drop on the floor at one end of the room, and was persistently used as a cuspidor until the argument was completed. mr. black neither affirmed nor denied the story, but told another which he said was true. while on his circuit as judge he had, on one occasion, tried a case of theft in which the principal evidence against the accused was the finding of the stolen article in his possession. he charged the jury that this fact was _prima facie_ evidence that the man was actually the thief. when through his business and about to leave for home, he went into a jeweler's shop to purchase some little trinket for his wife. the jeweler showed him a number of little articles, but finding none to suit him, he stepped into his carriage and drove off. in the course of the day he called on a street urchin to water his horse. reaching into his pocket for a reward, the first thing he got hold of was a diamond ring which must have been taken from the shop of the jeweler when he left that morning. "i wondered," said the judge, "how i should have come out had i been tried under my own law." the outcome of the matter was that the appropriations were duly made; first, in , $ , for instruments, then, the year following, $ , for the expeditions. in , $ , more was appropriated to complete the work and return the parties to their homes. the date of the great event was december - , . to have the parties thoroughly drilled in their work, they were brought together at washington in the preceding spring for practice and rehearsal. in order that the observations to be made by the eye should not be wholly new, an apparatus representing the transit was mounted on the top of winder's building, near the war department, about two thirds of a mile from the observatory. when this was observed through the telescope from the roof of the observatory, an artificial black venus was seen impinging upon an artificial sun, and entering upon its disk in the same way that the actual venus would be seen. this was observed over and over until, as was supposed, the observers had gotten into good practice. in order to insure the full understanding of the photographic apparatus, the instruments were mounted and the parties practiced setting them up and going through the processes of photographing the sun. to carry out this arrangement with success, it was advisable to have an expert in astronomical photography to take charge of the work. dr. henry draper of new york was invited for this purpose, and gave his services to the commission for several weeks. this transit was not visible in the united states. it did not begin until after the sun had set in san francisco, and it was over before the rising sun next morning had reached western europe. all the parties had therefore to be sent to the other side of the globe. three northern stations were occupied,--in china, japan, and siberia; and five southern ones, at various points on the islands of the pacific and indian oceans. this unequal division was suggested by the fact that the chances of fair weather were much less in the southern hemisphere than in the northern. the southern parties were taken to their destinations in the u. s. s. swatara, captain ralph chandler, u. s. n., commanding. in astronomical observations all work is at the mercy of the elements. clear weather was, of course, a necessity to success at any station. in the present case the weather was on the whole unpropitious. while there was not a complete failure at any one station, the number or value of the observations was more or less impaired at all. where the sky was nearly cloudless, the air was thick and hazy. this was especially the case at nagasaki and pekin, where from meteorological observations which the commission had collected through our consuls, the best of weather was confidently expected. what made this result more tantalizing was that the very pains we had taken to collect the data proved, by chance, to have made the choice worse. for some time it was deliberated whether the japanese station should be in nagasaki or yokohama. consultation with the best authorities and a study of the records showed that, while yokohama was a favorable spot, the chances were somewhat better at nagasaki. so to nagasaki the party was sent. but when the transit came, while the sky was of the best at yokohama, it was far from being so at nagasaki. something of the same sort occurred at the most stormy of all the southern stations, that at kerguelen island. the british expeditions had, in the beginning, selected a station on this island known as christmas harbor. we learned that a firm of new london, conn., had a whaling station on the island. it was therefore applied to to know what the weather chances were at various points in the island. information was obtained from their men, and it was thus found that molloy point, bad though the weather there was, afforded better chances than christmas harbor; so it was chosen. but this was not all; the british parties, either in consequence of the information we had acquired, or through what was learned from the voyage of the challenger, established their principal station near ours. but it happened that the day at christmas harbor was excellent, while the observations were greatly interfered with by passing clouds at molloy point. after the return of the parties sent out by the various nations, it did not take long for the astronomers to find that the result was disappointing, so far, at least, as the determination of the sun's distance was concerned. it became quite clear that this important element could be better measured by determining the velocity of light and the time which it took to reach us from the sun than it could by any transit of venus. it was therefore a question whether parties should be sent out to observe the transit of . on this subject the astronomers of the country at large were consulted. as might have been expected, there was a large majority in favor of the proposition. the negative voices were only two in number, those of pickering and myself. i took the ground that we should make ample provisions for observing it at various stations in our own country, where it would now be visible, but that, in view of the certain failure to get a valuable result for the distance of the sun by this method, it was not worth while for us to send parties to distant parts of the world. i supposed the committee on appropriations might make careful inquiry into the subject before making the appropriation, but a representation of the case was all they asked for, and $ , was voted for improving the instruments and $ , for sending out parties. expeditions being thus decided upon, i volunteered to take charge of that to the cape of good hope. the scientific personnel of my party comprised an officer of the army engineers, one of the navy, and a photographer. the former were lieutenant thomas l. casey, jr., corps of engineers, u. s. a., and lieutenant j. h. l. holcombe, u. s. n. we took a cunard steamer for liverpool about the middle of september, , and transported our instruments by rail to southampton, there to have them put on the cape steamship. at liverpool i was guilty of a remissness which might have caused much trouble. our apparatus and supplies, in a large number of boxes, were all gathered and piled in one place. i sent one of my assistants to the point to see that it was so collected that there should be no possibility of mistake in getting it into the freight car designed to carry it to southampton, but did not require him to stay there and see that all was put on board. when the cases reached southampton it was found that one was missing. it was one of the heaviest of the lot, containing the cast-iron pier on which the photoheliograph was to be mounted. while it was possible to replace this by something else, such a course would have been inconvenient and perhaps prejudicial. the steamer was about to sail, but would touch at plymouth next day. only one resource was possible. i telegraphed the mistake to liverpool and asked that the missing box be sent immediately by express to plymouth. we had the satisfaction of seeing it come on board with the mail just as the steamer was about to set sail. we touched first at madeira, and then at ascension island, the latter during the night. one of the odd things in nomenclature is that this island, a british naval station, was not called such officially, but was a "tender to her majesty's ship flora," i believe. it had become astronomically famous a few years before by gill's observations of the position of mars to determine the solar parallax. we touched six hours at st. helena, enough to see the place, but scarcely enough to make a visit to the residence of napoleon, even had we desired to see it. the little town is beautifully situated, and the rocks around are very imposing. my most vivid recollection is, however, of running down from the top of a rock some six hundred or eight hundred feet high, by a steep flight of steps, without stopping, or rather of the consequences of this imprudent gymnastic performance. i could scarcely move for the next three days. cape town was then suffering from an epidemic of smallpox, mostly confined to the malay population, but causing some disagreeable results to travelers. our line of ships did not terminate their voyage at the cape, but proceeded thence to other african ports east of the cape. here a rigid quarantine had been established, and it was necessary that the ships touching at the cape of good hope should have had no communication with the shore. thus it happened that we found, lying in the harbor, the ship of our line which had preceded us, waiting to get supplies from us, in order that it might proceed on its voyage. looking at a row-boat after we had cast anchor, we were delighted to see two faces which i well knew: those of david gill, astronomer of the cape observatory, and dr. w. l. elkin, now director of the yale observatory. the latter had gone to the cape as a volunteer observer with gill, their work being directed mostly to parallaxes of stars too far south to be well observed in our latitude. our friends were not, however, even allowed to approach the ship, for fear of the smallpox, the idea appearing to be that the latter might be communicated by a sort of electric conduction, if the boat and the ship were allowed to come into contact, so we had to be put ashore without their aid. we selected as our station the little town of wellington, some forty miles northeast of cape town. the weather chances were excellent anywhere, but here they were even better than at the cape. the most interesting feature of the place was what we might call an american young ladies' school. the dutch inhabitants of south africa are imbued with admiration of our institutions, and one of their dreams is said to be a united states of south africa modeled after our own republic. desiring to give their daughters the best education possible, they secured the services of miss ferguson, a well-known new england teacher, to found a school on the american model. we established our station in the grounds of this school. the sky on the day of the transit was simply perfect. notwithstanding the intensity of the sun's rays, the atmosphere was so steady that i have never seen the sun to better advantage. so all our observations were successful. on our departure we left two iron pillars, on which our apparatus for photographing the sun was mounted, firmly imbedded in the ground, as we had used them. whether they will remain there until the transit of , i do not know, but cannot help entertaining a sentimental wish that, when the time of that transit arrives, the phenomenon will be observed from the same station, and the pillars be found in such a condition that they can again be used. all the governments, except our own, which observed the two transits of venus on a large scale long ago completed the work of reduction, and published the observations in full. on our own part we have published a preliminary discussion of some observations of the transit of . of that of nothing has, i believe, been published except some brief statements of results of the photographs, which appeared in an annual report of the naval observatory. having need in my tables of the planets of the best value of the solar parallax that could be obtained by every method, i worked up all the observations of contacts made by the parties of every country, but, of course, did not publish our own observations. up to the present time, twenty-eight years after the first of the transits, and twenty years after the second, our observations have never been officially published except to the extent i have stated. the importance of the matter may be judged by the fact that the government expended $ , on these observations, not counting the salaries of its officers engaged in the work, or the cost of sailing a naval ship. as i was a member of the commission charged with the work, and must therefore bear my full share of the responsibility for this failure, i think it proper to state briefly how it happened, hoping thereby to enforce the urgent need of a better organization of some of our scientific work. the work of reducing such observations, editing and preparing them for the press, involved much computation to be done by assistants, and i, being secretary of the commission, was charged with the execution of this part of the work. the appropriations made by congress for the observations were considered available for the reduction also. there was a small balance left over, and i estimated that $ more would suffice to complete the work. this was obtained from congress in the winter of . about the end of i was surprised to receive from the treasury department a notification that the appropriation for the transit of venus was almost exhausted, when according to my accounts, more than $ still remained. on inquiry it was found that the sum appropriated about two years before had never been placed to the credit of the transit of venus commission, having been, in fact, inserted in a different appropriation bill from that which contained the former grant. i, as secretary of the commission, made an application to the treasury department to have the sum, late though it was, placed to our credit. but the money had been expended and nothing could be now done in the matter. [ ] the computers had therefore to be discharged and the work stopped until a new appropriation could be obtained from congress. during the session of - , $ was therefore asked for for the reduction of the observations. it was refused by the house committee on appropriations. i explained the matter to mr. julius h. seelye, formerly president of amherst college, who was serving a term in congress. he took much interest in the subject, and moved the insertion of the item when the appropriation bill came up before the house. mr. atkins, chairman of the appropriations committee, opposed the motion, maintaining that the navy department had under its orders plenty of officers who could do the work, so there was no need of employing the help of computers. but the house took a different view, and inserted the item over the heads of the appropriations committee. now difficulties incident to the divided responsibility of the commission were met with. during the interim between the death of admiral davis, in february, , and the coming of admiral john rodgers as his successor, a legal question arose as to the power of the commission over its members. the work had to stop until it was settled, and i had to discharge my computers a second time. after it was again started i discovered that i did not have complete control of the funds appropriated for reducing the observations. the result was that the computers had to be discharged and the work stopped for the third time. this occurred not long before i started out to observe the transit in . for me the third hair was the one that broke the camel's back. i turned the papers and work over to professor harkness, by whom the subject was continued until he was made astronomical director of the naval observatory in . i do not know that the commission was ever formally dissolved. practically, however, its functions may be said to have terminated in the year , when a provision of law was enacted by which all its property was turned over to the secretary of the navy. what the present condition of the work may be, and how much of it is ready for the press, i cannot say. my impression is that it is in that condition known in household language as "all done but finishing." whether it will ever appear is a question for the future. all the men who took part in it or who understood its details are either dead or on the retired list, and it is difficult for one not familiar with it from the beginning to carry it to completion. [ ] for the incidents connected with the english observations of this transit, the author is indebted to vice-admiral w. h. smyth's curious and rare book, _speculum hartwellianum_, london, . it and other works of the same author may be described as queer and interesting jumbles of astronomical and other information, thrown into an interesting form; and, in the case of the present work, spread through a finely illustrated quarto volume of nearly five hundred pages. [ ] "the war department got ahead of us in the matter of furniture," said an officer of the navy department to me long afterwards, when the furniture for the new department building was being obtained. "they knew enough to ask for a third more than they wanted; we reduced our estimate to the lowest point. both estimates were reduced one third by the appropriations committee. the result is that they have all the furniture they want, while we are greatly pinched." [ ] as this result would not be possible under our present system, which was introduced by the first cleveland administration, i might remark that it resulted from a practice on the part of the treasury of lumping appropriations on its books in order to simplify the keeping of the accounts. vii the lick observatory in the wonderful development of astronomical research in our country during the past twenty years, no feature is more remarkable than the rise on an isolated mountain in california of an institution which, within that brief period, has become one of the foremost observatories of the world. as everything connected with the early history of such an institution must be of interest, it may not be amiss if i devote a few pages to it. in the announcement reached the public eye that james lick, an eccentric and wealthy californian, had given his entire fortune to a board of trustees to be used for certain public purposes, one of which was the procuring of the greatest and most powerful telescope that had ever been made. there was nothing in the previous history of the donor that could explain his interest in a great telescope. i am sure he had never looked through a telescope in his life, and that if he had, and had been acquainted with the difficulties of an observation with it, it is quite likely the lick observatory would never have existed. from his point of view, as, indeed, from that of the public very generally, the question of telescopic vision is merely one of magnifying power. by making an instrument large and powerful enough we may hope even to discover rational beings on other planets. the president of the first board of trustees was mr. d. o. mills, the well-known capitalist, who had been president of the bank of california. mr. mills visited washington in the summer or autumn of , and conferred with the astronomers there, among others myself, on the question of the proposed telescope. i do not think that an observatory properly so called was, at first, in mr. lick's mind; all he wanted was an immense telescope. the question was complicated by the result of some correspondence between mr. lick and the firm of alvan clark & sons. the latter had been approached to know the cost of constructing the desired telescope. without making any exact estimate, or deciding upon the size of the greatest telescope that could be constructed, they named a very large sum, $ , i believe, as the amount that could be put into the largest telescope it was possible to make. mr. lick deemed this estimate exorbitant, and refused to have anything more to do with the firm. the question now was whether any one else besides the clarks could make what was wanted. i suggested to mr. mills that this question was a difficult one to answer, as no european maker was known to rival the clerks in skill in the desired direction. it was impossible to learn what could be done in europe except by a personal visit to the great optical workshops and a few observatories where great telescopes had been mounted. i also suggested that a director of the new establishment should be chosen in advance of beginning active work, so that everything should be done under his supervision. as such director i suggested that very likely professor holden, then my assistant on the great equatorial, might be well qualified. at least i could not, at the moment, name any one i thought would be decidedly preferable to him. i suggested another man as possibly available, but remarked that he had been unfortunate. "i don't want to have anything to do with unfortunate men," was the reply. the necessity of choosing a director was not, however, evident, but communication was opened with professor holden as well as myself to an extent that i did not become aware of until long afterward. the outcome of mr. mills's visit was that in december, , i was invited to visit the european workshops as an agent of the lick trustees, with a view of determining whether there was any chance of getting the telescope made abroad. the most difficult and delicate question arose in the beginning; shall the telescope be a reflector or a refractor? the largest and most powerful one that could be made would be, undoubtedly, a reflector. and yet reflecting telescopes had not, as a rule, been successful in permanent practical work. the world's work in astronomy was done mainly with refracting telescopes. this was not due to any inherent superiority in the latter, but to the mechanical difficulties incident to so supporting the great mirror of a reflecting telescope that it should retain its figure in all positions. assuming that the choice must fall upon a refractor, unless proper guarantees for one of the other kind should be offered, one of my first visits was to the glass firm of chance & co. in birmingham, who had cast the glass disks for the washington telescope. this firm and feil of paris were the only two successful makers of great optical disks in the world. chance & co. offered the best guarantees, while feil had more enthusiasm than capital, although his skill was of the highest. another paris firm was quite willing to undertake the completion of the telescope, but it was also evident that its price was suggested by the supposed liberality of an eccentric california millionaire. i returned their first proposal with the assurance that it would be useless to submit it. a second was still too high to offer any inducement over the american firm. besides, there was no guarantee of the skill necessary to success. in germany the case was still worse. the most renowned firm there, the successors of fraunhofer, were not anxious to undertake such a contract. the outcome of the matter was that howard grubb, of dublin, was the only man abroad with whom negotiations could be opened with any chance of success. he was evidently a genius who meant business. yet he had not produced a work which would justify unlimited confidence in his ability to meet mr. lick's requirements. the great vienna telescope which he afterward constructed was then only being projected. not long after my return with this not very encouraging report, mr. lick suddenly revoked his gift, through some dissatisfaction with the proceedings of his trustees, and appointed a new board to carry out his plans. this introduced legal complications, which were soon settled by a friendly suit on the part of the old trustees, asking authority to transfer their trust. the president of the new board was mr. richard s. floyd, a member of the well-known virginia family of that name, and a graduate, or at least a former cadet, of the united states naval academy. i received a visit from him on his first trip to the east in his official capacity, early in , i believe. some correspondence with mr. lick's home representative ensued, of which the most interesting feature was the donor's idea of a telescope. he did not see why so elaborate and expensive a mounting as that proposed was necessary, and thought that the object glass might be mounted on the simplest kind of a pole or tower which would admit of its having the requisite motions in connection with the eyepiece. whether i succeeded in convincing him of the impracticability of his scheme, i do not know, as he died before the matter was settled. this left the trustees at liberty to build and organize the institution as they deemed best. it was speedily determined that the object glass should be shaped by the clarks, who should also be responsible for getting the rough disks. this proved to be a very difficult task. chance & co. were unwilling to undertake the work and feil had gone out of business, leaving the manufacture in the hands of his son. the latter also failed, and the father had to return. ultimately the establishment was purchased by mantois, whose success was remarkable. he soon showed himself able to make disks not only of much larger size than had ever before been produced, but of a purity and transparency which none before him had ever approached. he died in or , and it is to be hoped that his successor will prove to be his equal. the original plan of mr. lick had been to found the observatory on the borders of lake tahoe, but he grew dissatisfied with this site and, shortly before his death, made provisional arrangements for placing it on mount hamilton. in preparations had so far advanced that it became necessary to decide whether this was really a suitable location. i had grave doubts on the subject. a mountain side is liable to be heated by the rays of the sun during the day, and a current of warm air which would be fatal to the delicacy of astronomical vision is liable to rise up the sides and envelope the top of the mountain. i had even been informed that, on a summer evening, a piece of paper let loose on the mountain top would be carried up into the air by the current. but, after all, the proof of the pudding is in the eating, and holden united with me in advising that an experienced astronomer with a telescope should be stationed for a few weeks on the mountain in order to determine, by actual trial, what the conditions of seeing were. the one best man for this duty was s. w. burnham of chicago, who had already attained a high position in the astronomical world by the remarkable skill shown in his observations of double stars. so, in august, , huts were built on the mountain, and burnham was transported thither with his telescope. i followed personally in september. we passed three nights on the mountain with captain floyd, studying the skies by night and prospecting around in the daytime to see whether the mountain top or some point in the neighboring plateau offered the best location for the observatory. so far as the atmospheric conditions were concerned, the results were beyond our most sanguine expectations. what the astronomer wants is not merely a transparent atmosphere, but one of such steadiness that the image of a star, as seen in a telescope, may not be disturbed by movements of the air which are invisible to the naked eye. burnham found that there were forty-two first-class nights during his stay, and only seven which would be classed as low as medium. in the east the number of nights which he would call first-class are but few in a year, and even the medium night is by no means to be counted on. no further doubt could remain that the top of the mountain was one of the finest locations in the world for an astronomical observatory, and it was definitely selected without further delay. sometime after my return mr. floyd sent me a topographical sketch of the mountain, with a request to prepare preliminary plans for the observatory. as i had always looked on professor holden as probably the coming director, i took him into consultation, and the plans were made under our joint direction in my office. the position and general arrangement of the buildings remain, so far as i am aware, much as then planned; the principal change being the omission of a long colonnade extending over the whole length of the main front in order to secure an artistic and imposing aspect from the direction of san josé. in the summer of , as i was in new york in order to sail next day to europe, i was surprised by a visit from judge hagar, a prominent citizen of san francisco, a member of the board of regents of the university of california, and an active politician, who soon afterward became collector of the port, to consult me on the question of choosing professor holden as president of the university. this was not to interfere with his becoming director of the lick observatory whenever that institution should be organized, but was simply a temporary arrangement to bridge over a difficulty. in the autumn of i received an invitation from mr. floyd to go with him to cleveland, in order to inspect the telescope, which was now nearly ready for delivery. it was mounted in the year following, and then holden stepped from the presidency of the university into the directorship of the observatory. the institution made its mark almost from the beginning. i know of no example in the world in which young men, most of whom were beginners, attained such success as did those whom holden collected around him. the names of barnard, campbell, and schaeberle immediately became well known in astronomy, owing to the excellence of their work. burnham was, of course, no beginner, being already well known, nor was keeler, who was also on the staff. in a few years commenced the epoch-making work of campbell, in the most refined and difficult problem of observational astronomy,--that of the measurement of the motion of stars to or from us. through the application of photography and minute attention to details, this work of the lick observatory almost immediately gained a position of preëminence, which it maintains to the present time. if any rival is to appear, it will probably be the yerkes observatory. the friendly competition which we are likely to see between these two establishments affords an excellent example of the spirit of the astronomy of the future. notwithstanding their rivalry, each has done and will do all it can to promote the work of the other. the smiles of fortune have been bestowed even upon efforts that seemed most unpromising. after work was well organized, mr. crossley, of england, presented the observatory with a reflecting telescope of large size, but which had never gained a commanding reputation. no member of the staff at first seemed ambitious to get hold of such an instrument, but, in time, keeler gave it a trial in photographing nebulæ. then it was found that a new field lay open. the newly acquired reflector proved far superior to other instruments for this purpose, the photographic plates showing countless nebulæ in every part of the sky, which the human eye was incapable of discerning in the most powerful of telescopes. in , only four years after the mounting of the telescope, came the surprising announcement that the work of galileo on jupiter had been continued by the discovery of a fifth satellite to that planet. this is the most difficult object in the solar system, only one or two observers besides barnard having commanded the means of seeing it. the incident of my first acquaintance with the discoverer is not flattering to my pride, but may be worth recalling. in i was president of the american association for the advancement of science at the meeting held in nashville. there i was told of a young man a little over twenty years of age, a photographer by profession, who was interested in astronomy, and who desired to see me. i was, of course, very glad to make his acquaintance. i found that with his scanty earnings he had managed either to purchase or to get together the materials for making a small telescope. he was desirous of doing something with it that might be useful in astronomy, and wished to know what suggestions i could make in that line. i did not for a moment suppose that there was a reasonable probability of the young man doing anything better than amuse himself. at the same time, feeling it a duty to encourage him, i suggested that there was only one thing open to an astronomical observer situated as he was, and that was the discovery of comets. i had never even looked for a comet myself, and knew little about the methods of exploring the heavens for one, except what had been told me by h. p. tuttle. but i gave him the best directions i could, and we parted. it is now rather humiliating that i did not inquire more thoroughly into the case. it would have taken more prescience than i was gifted with to expect that i should live to see the bashful youth awarded the gold medal of the royal astronomical society for his work. the term of holden's administration extended through some ten years. to me its most singular feature was the constantly growing unpopularity of the director. i call it singular because, if we confine ourselves to the record, it would be difficult to assign any obvious reason for it. one fact is indisputable, and that is the wonderful success of the director in selecting young men who were to make the institution famous by their abilities and industry. if the highest problem of administration is to select the right men, the new director certainly mastered it. so far as liberty of research and publication went, the administration had the appearance of being liberal in the extreme. doubtless there was another side to the question. nothing happens spontaneously, and the singular phenomenon of one who had done all this becoming a much hated man must have an adequate cause. i have several times, from pure curiosity, inquired about the matter of well-informed men. on one occasion an instance of maladroitness was cited in reply. "true," said i, "it was not exactly the thing to do, but, after all, that is an exceedingly small matter." "yes," was the answer, "that was a small thing, but put a thousand small things like that together, and you have a big thing." a powerful factor in the case may have been his proceeding, within a year of his appointment, to file an astounding claim for the sum of $ , on account of services rendered to the observatory in the capacity of general adviser before his appointment as director. these services extended from the beginning of preparations in up to the completion of the work. the trustees in replying to the claim maintained that i had been their principal adviser in preparing the plans. however true this may have been, it was quite evident, from holden's statement, that they had been consulting him on a much larger scale than i had been aware of. this, however, was none of my concern. i ventured to express the opinion that the movement was made merely to place on record a statement of the director's services; and that no serious intention of forcing the matter to a legal decision was entertained. this surmise proved to be correct, as nothing more was heard of the claim. much has been said of the effect of the comparative isolation of such a community, which is apt to be provocative of internal dissension. but this cause has not operated in the case of holden's successors. keeler became the second director in , and administered his office with, so far as i know, universal satisfaction till his lamented death in . it would not be a gross overstatement to say that his successor was named by the practically unanimous voice of a number of the leading astronomers of the world who were consulted on the subject, and who cannot but be pleased to see how completely their advice has been justified by the result of campbell's administration. viii the author's scientific work perhaps an apology is due to the reader for my venturing to devote a chapter to my own efforts in the scientific line. if so, i scarcely know what apology to make, unless it is that one naturally feels interested in matters relating to his own work, and hopes to share that interest with his readers, and that it is easier for one to write such an account for himself than for any one else to do it for him. having determined to devote my life to the prosecution of exact astronomy, the first important problem which i took up, while at cambridge, was that of the zone of minor planets, frequently called asteroids, revolving between the orbits of mars and jupiter. it was formerly supposed that these small bodies might be fragments of a large planet which had been shattered by a collision or explosion. if such were the case, the orbits would, for a time at least, all pass through the point at which the explosion occurred. when only three or four were known, it was supposed that they did pass nearly through the same point. when this was found not to be the case, the theory of an explosion was in no way weakened, because, owing to the gradual changes in the form and position of the orbits, produced by the attraction of the larger planets, these orbits would all move away from the point of intersection, and, in the course of thousands of years, be so mixed up that no connection could be seen between them. this result was that nothing could be said upon the subject except that, if the catastrophe ever did occur, it must have been many thousand years ago. the fact did not in any way militate against the theory because, in view of the age of the universe, the explosion might as well have occurred hundreds of thousands or even millions of years ago as yesterday. to settle the question, general formulæ must be found by which the positions of these orbits could be determined at any time in the past, even hundreds of thousands of years back. the general methods of doing this were known, but no one had applied them to the especial case of these little planets. here, then, was an opportunity of tracing back the changes in these orbits through thousands of centuries in order to find whether, at a certain epoch in the past, so great a cataclysm had occurred as the explosion of a world. were such the case, it would be possible almost to set the day of the occurrence. how great a feat would it be to bring such an event at such a time to light! i soon found that the problem, in the form in which it had been attacked by previous mathematicians, involved no serious difficulty. at the springfield meeting of the american association for the advancement of science, in , i read a paper explaining the method, and showed by a curve on the blackboard the changes in the orbit of one of the asteroids for a period, i think, of several hundred thousand years,--"beyond the memory of the oldest inhabitants"--said one of the local newspapers. a month later it was extended to three other asteroids, and the result published in the "astronomical journal." in the following spring, , the final results of the completed work were communicated to the american academy of arts and sciences in a paper "on the secular variations and mutual relations of the orbits of the asteroids." the question of the possible variations in the orbits and the various relations amongst them were here fully discussed. one conclusion was that, so far as our present theory could show, the orbits had never passed through any common point of intersection. the whole trend of thought and research since that time has been toward the conclusion that no such cataclysm as that looked for ever occurred, and that the group of small planets has been composed of separate bodies since the solar system came into existence. it was, of course, a great disappointment not to discover the cataclysm, but next best to finding a thing is showing that it is not there. this, it may be remarked, was the first of my papers to attract especial notice in foreign scientific journals, though i had already published several short notes on various subjects in the "astronomical journal." at this point i may say something of the problems of mathematical astronomy in the middle of the last century. it is well known that we shall at least come very near the truth when we say that the planets revolve around the sun, and the satellites around their primaries according to the law of gravitation. we may regard all these bodies as projected into space, and thus moving according to laws similar to that which governs the motion of a stone thrown from the hand. if two bodies alone were concerned, say the sun and a planet, the orbit of the lesser around the greater would be an ellipse, which would never change its form, size, or position. that the orbits of the planets and asteroids do change, and that they are not exact ellipses, is due to their attraction upon each other. the question is, do these mutual attractions completely explain all the motions down to the last degree of refinement? does any world move otherwise than as it is attracted by other worlds? two different lines of research must be brought to bear on the question thus presented. we must first know by the most exact and refined observations that the astronomer can make exactly how a heavenly body does move. its position, or, as we cannot directly measure distance, its direction from us, must be determined as precisely as possible from time to time. its course has been mapped out for it in advance by tables which are published in the "astronomical ephemeris," and we may express its position by its deviation from these tables. then comes in the mathematical problem how it ought to move under the attraction of all other heavenly bodies that can influence its motion. the results must then be compared, in order to see to what conclusion we may be led. this mathematical side of the question is of a complexity beyond the powers of ordinary conception. i well remember that when, familiar only with equations of algebra, i first looked into a book on mechanics, i was struck by the complexity of the formulæ. but this was nothing to what one finds when he looks into a work on celestial mechanics, where a single formula may fill a whole chapter. the great difficulty arises from the fact that the constant action upon a planet exerted at every moment of time through days and years by another planet affects its motion in all subsequent time. the action of jupiter upon our earth this morning changes its motion forever, just as a touch upon a ball thrown by a pitcher will change the direction of the ball through its whole flight. the wondrous perfection of mathematical research is shown by the fact that we can now add up, as it were, all these momentary effects through years and centuries, with a view of determining the combined result at any one moment. it is true that this can be done only in an imperfect way, and at the expense of enormous labor; but, by putting more and more work into it, investigating deeper and deeper, taking into account smaller and smaller terms of our formulæ, and searching for the minutest effects, we may gradually approach, though we may never reach, absolute exactness. here we see the first difficulty in reaching a definite conclusion. one cannot be quite sure that a deviation is not due to some imperfection in mathematical method until he and his fellows have exhausted the subject so thoroughly as to show that no error is possible. this is hard indeed to do. taking up the question on the observational side, a source of difficulty and confusion at once presented itself. the motions of a heavenly body from day to day and year to year are mapped out by comparative observations on it and on the stars. the question of the exact positions of the stars thus comes in. in determining these positions with the highest degree of precision, a great variety of data have to be used. the astronomer cannot reach a result by a single step, nor by a hundred steps. he is like a sculptor chiseling all the time, trying to get nearer and nearer the ideal form of his statue, and finding that with every new feature he chisels out, a defect is brought to light in other features. the astronomer, when he aims at the highest mathematical precision in his results, finds nature warring with him at every step, just as if she wanted to make his task as difficult as possible. she alters his personal equation when he gets tired, makes him see a small star differently from a bright one, gives his instrument minute twists with heat and cold, sends currents of warm or cold air over his locality, which refract the rays of light, asks him to keep the temperature in which he works the same as that outside, in order to avoid refraction when the air enters his observing room, and still will not let him do it, because the walls and everything inside the room, being warmed up during the day, make the air warmer than it is outside. with all these obstacles which she throws in his way he must simply fight the best he can, exerting untiring industry to eliminate their effects by repeated observations under a variety of conditions. a necessary conclusion from all this is that the work of all observing astronomers, so far as it could be used, must be combined into a single whole. but here again difficulties are met at every step. there has been, in times past, little or no concert of action among astronomers at different observatories. the astronomers of each nation, perhaps of each observatory, to a large extent, have gone to work in their own way, using discordant data, perhaps not always rigidly consistent, even in the data used in a single establishment. how combine all the astronomical observations, found scattered through hundreds of volumes, into a homogeneous whole? what is the value of such an attempt? certainly if we measure value by the actual expenditure of nations and institutions upon the work, it must be very great. every civilized nation expends a large annual sum on a national observatory, while a still greater number of such institutions are supported at corporate expense. considering that the highest value can be derived from their labors only by such a combination as i have described, we may say the result is worth an important fraction of what all the observatories of the world have cost during the past century. such was, in a general way, the great problem of exact astronomy forty or fifty years ago. its solution required extended coöperation, and i do not wish to give the impression that i at once attacked it, or even considered it as a whole. i could only determine to do my part in carrying forward the work associated with it. perhaps the most interesting and important branch of the problem concerned the motion of the moon. this had been, ever since the foundation of the greenwich observatory, in , a specialty of that institution. it is a curious fact, however, that while that observatory supplied all the observations of the moon, the investigations based upon these observations were made almost entirely by foreigners, who also constructed the tables by which the moon's motion was mapped out in advance. the most perfect tables made were those of hansen, the greatest master of mathematical astronomy during the middle of the century, whose tables of the moon were published by the british government in . they were based on a few of the greenwich observations from to . the period began with , because that was the earliest at which observations of any exactness were made. only a few observations were used, because hansen, with the limited computing force at his command,--only a single assistant, i believe,--was not able to utilize a great number of the observations. the rapid motion of the moon, a circuit being completed in less than a month, made numerous observations necessary, while the very large deviations in the motion produced by the attraction of the sun made the problem of the mathematical theory of that motion the most complicated in astronomy. thus it happened that, when i commenced work at the naval observatory in , the question whether the moon exactly followed the course laid out for her by hansen's tables was becoming of great importance. the same question arose in the case of the planets. so from a survey of the whole field, i made observations of the sun, moon, and planets my specialty at the observatory. if the astronomical reader has before him the volume of observations for , he will, by looking at pages - , be able to infer with nearly astronomical precision the date when i reported for duty. for a year or two our observations showed that the moon seemed to be falling a little behind her predicted motion. but this soon ceased, and she gradually forged ahead in a much more remarkable way. in five or six years it was evident that this was becoming permanent; she was a little farther ahead every year. what could it mean? to consider this question, i may add a word to what i have already said on the subject. in comparing the observed and predicted motion of the moon, mathematicians and astronomers, beginning with laplace, have been perplexed by what are called "inequalities of long period." for a number of years, perhaps half a century, the moon would seem to be running ahead, and then she would gradually relax her speed and fall behind. laplace suggested possible causes, but could not prove them. hansen, it was supposed, had straightened out the tangle by showing that the action of venus produced a swinging of this sort in the moon; for one hundred and thirty years she would be running ahead and then for one hundred and thirty years more falling back again, like a pendulum. two motions of this sort were combined together. they were claimed to explain the whole difficulty. the moon, having followed hansen's theory for one hundred years, would not be likely to deviate from it. now, it was deviating. what could it mean? taking it for granted, on hansen's authority, that his tables represented the motions of the moon perfectly since , was there no possibility of learning anything from observations before that date? as i have already said, the published observations with the usual instruments were not of that refined character which would decide a question like this. but there is another class of observations which might possibly be available for the purpose. millions of stars, visible with large telescopes, are scattered over the heavens; tens of thousands are bright enough to be seen with small instruments, and several thousand are visible to any ordinary eye. the moon, in her monthly course around the heavens, often passes over a star, and of course hides it from view during the time required for the passage. the great majority of stars are so small that their light is obscured by the effulgence of the moon as the latter approaches them. but quite frequently the star passed over is so bright that the exact moment when the moon reaches it can be observed with the utmost precision. the star then disappears from view in an instant, as if its light were suddenly and absolutely extinguished. this is called an occultation. if the moment at which the disappearance takes place is observed, we know that at that instant the apparent angle between the centre of the moon and the star is equal to the moon's semi-diameter. by the aid of a number of such observations, the path of the moon in the heavens, and the time at which she arrives at each point of the path, can be determined. in order that the determination may be of sufficient scientific precision, the time of the occultation must be known within one or two seconds; otherwise, we shall be in doubt how much of the discrepancy may be due to the error of the observation, and how much to the error of the tables. occultations of some bright stars, such as aldebaran and antares, can be observed by the naked eye; and yet more easily can those of the planets be seen. it is therefore a curious historic fact that there is no certain record of an actual observation of this sort having been made until after the commencement of the seventeenth century. even then the observations were of little or no use, because astronomers could not determine their time with sufficient precision. it was not till after the middle of the century, when the telescope had been made part of astronomical instruments for finding the altitude of a heavenly body, and after the pendulum clock had been invented by huyghens, that the time of an occultation could be fixed with the required exactness. thus it happens that from to somewhat coarse observations of the kind are available, and after the latter epoch those made by the french astronomers become almost equal to the modern ones in precision. the question that occurred to me was: is it not possible that such observations were made by astronomers long before ? searching the published memoirs of the french academy of sciences and the philosophical transactions, i found that a few such observations were actually made between and . i computed and reduced a few of them, finding with surprise that hansen's tables were evidently much in error at that time. but neither the cause, amount, or nature of the error could be well determined without more observations than these. was it not possible that these astronomers had made more than they published? the hope that material of this sort existed was encouraged by the discovery at the pulkowa observatory of an old manuscript by the french astronomer delisle, containing some observations of this kind. i therefore planned a thorough search of the old records in europe to see what could be learned. the execution of this plan was facilitated by the occurrence, in december, , of an eclipse of the sun in spain and along the mediterranean. a number of parties were going out from this country to observe it, two of which were fitted out at the naval observatory. i was placed in charge of one of these, consisting, practically, of myself. the results of my observation would be of importance in the question of the moon's motion, but, although the eclipse was ostensibly the main object, the proposed search of the records was what i really had most in view. in paris was to be found the most promising mine; but the franco-prussian war was then going on, and i had to wait for its termination. then i made a visit to paris, which will be described in a later chapter. at the observatory the old records i wished to consult were placed at my disposal, with full liberty not only to copy, but to publish anything of value i could find in them. the mine proved rich beyond the most sanguine expectation. after a little prospecting, i found that the very observations i wanted had been made in great numbers by the paris astronomers, both at the observatory and at other points in the city. and how, the reader may ask, did it happen that these observations were not published by the astronomers who made them? why should they have lain unused and forgotten for two hundred years? the answer to these questions is made plain enough by an examination of the records. the astronomers had no idea of the possible usefulness and value of what they were recording. so far as we can infer from their work, they made the observations merely because an occultation was an interesting thing to see; and they were men of sufficient scientific experience and training to have acquired the excellent habit of noting the time at which a phenomenon was observed. but they were generally satisfied with simply putting down the clock time. how they could have expected their successors to make any use of such a record, or whether they had any expectations on the subject, we cannot say with confidence. it will be readily understood that no clocks of the present time (much less those of two hundred years ago) run with such precision that the moment read from the clock is exact within one or two seconds. the modern astronomer does not pretend to keep his clock correct within less than a minute; he determines by observation how far it is wrong, on each date of observation, and adds so much to the time given by the clock, or subtracts it, as the case may be, in order to get the correct moment of true time. in the case of the french astronomers, the clock would frequently be fifteen minutes or more in error, for the reason that they used apparent time, instead of mean time as we do. thus when, as was often the case, the only record found was that, at a certain hour, minute, and second, by a certain clock, _une étoile se cache par la lune_, a number of very difficult problems were presented to the astronomer who was to make use of the observations two centuries afterward. first of all, he must find out what the error of the clock was at the designated hour, minute, and second; and for this purpose he must reduce the observations made by the observer in order to determine the error. but it was very clear that the observer did not expect any successor to take this trouble, and therefore did not supply him with any facilities for so doing. he did not even describe the particular instrument with which the observations were made, but only wrote down certain figures and symbols, of a more or less hieroglyphic character. it needed much comparison and examination to find out what sort of an instrument was used, how the observations were made, and how they should be utilized for the required purpose. generally the star which the moon hid was mentioned, but not in all cases. if it was not, the identification of the star was a puzzling problem. the only way to proceed was to calculate the apparent position of the centre of the moon as seen by an observer at the paris observatory, at the particular hour and minute of the observation. a star map was then taken; the points of a pair of dividers were separated by the length of the moon's radius, as it would appear on the scale of the map; one point of the dividers was put into the position of the moon's centre on the map, and with the other a circle was drawn. this circle represented the outline of the moon, as it appeared to the observer at the paris observatory, at the hour and minute in question, on a certain day in the seventeenth century. the star should be found very near the circumference of the circle, and in nearly all cases a star was there. of course all this could not be done on the spot. what had to be done was to find the observations, study their relations and the method of making them, and copy everything that seemed necessary for working them up. this took some six weeks, but the material i carried away proved the greatest find i ever made. three or four years were spent in making all the calculations i have described. then it was found that seventy-five years were added, at a single step, to the period during which the history of the moon's motion could be written. previously this history was supposed to commence with the observations of bradley, at greenwich, about ; now it was extended back to , and with a less degree of accuracy thirty years farther still. hansen's tables were found to deviate from the truth, in and subsequent years, to a surprising extent; but the cause of the deviation is not entirely unfolded even now. during the time i was doing this work, paris was under the reign of the commune and besieged by the national forces. the studies had to be made within hearing of the besieging guns; and i could sometimes go to a window and see flashes of artillery from one of the fortifications to the south. nearly every day i took a walk through the town, occasionally as far as the arc. as my observations during these walks have no scientific value, i shall postpone an account of what i saw to another chapter. one curious result of this work is that the longitude of the moon may now be said to be known with greater accuracy through the last quarter of the seventeenth century than during the ninety years from to . the reason is that, for this more modern period, no effective comparison has been made between observations and hansen's tables. just as this work was approaching completion i was called upon to decide a question which would materially influence all my future activity. the lamented death of professor winlock in left vacant the directorship of the harvard observatory. a month or two later i was quite taken by surprise to receive a letter from president eliot tendering me this position. i thus had to choose between two courses. one led immediately to a professorship in harvard university, with all the distinction and worldly advantages associated with it, including complete freedom of action, an independent position, and the opportunity of doing such work as i deemed best with the limited resources at the disposal of the observatory. on the other hand was a position to which the official world attached no importance, and which brought with it no worldly advantages whatever. i first consulted mr. secretary robeson on the matter. the force with which he expressed himself took me quite by surprise. "by all means accept the place; don't remain in the government service a day longer than you have to. a scientific man here has no future before him, and the quicker he can get away the better." then he began to descant on our miserable "politics" which brought about such a state of things. such words, coming from a sagacious head of a department who, one might suppose, would have been sorry to part with a coadjutor of sufficient importance to be needed by harvard university, seemed to me very suggestive. and yet i finally declined the place, perhaps unwisely for myself, though no one who knows what the cambridge observatory has become under professor pickering can feel that harvard has any cause to regret my decision. an apology for it on my own behalf will seem more appropriate. on the cambridge side it must be remembered that the harvard observatory was then almost nothing compared with what it is now. it was poor in means, meagre in instrumental outfit, and wanting in working assistants; i think the latter did not number more than three or four, with perhaps a few other temporary employees. there seemed little prospect of doing much. on the washington side was the fact that i was bound to washington by family ties, and that, if harvard needed my services, surely the government needed them much more. true, this argument was, for the time, annulled by the energetic assurance of secretary robeson, showing that the government felt no want of any one in its service able to command a university professorship. but i was still pervaded by the optimism of youth in everything that concerned the future of our government, and did not believe that, with the growth of intelligence in our country, an absence of touch between the scientific and literary classes on the one side, and "politics" on the other, could continue. in addition to this was the general feeling by which i have been actuated from youth--that one ought to choose that line of activity for which nature had best fitted him, trusting that the operation of moral causes would, in the end, right every wrong, rather than look out for place and preferment. i felt that the conduct of government astronomy was that line of activity for which i was best fitted, and that, in the absence of strong reason to the contrary, it had better not be changed. in addition to these general considerations was the special point that, in the course of a couple of years, the directorship of the nautical almanac would become vacant, and here would be an unequaled opportunity for carrying on the work in mathematical astronomy i had most at heart. yet, could i have foreseen that the want of touch which i have already referred to would not be cured, that i should be unable to complete the work i had mapped out before my retirement, or to secure active public interest in its continuance, my decision would perhaps have been different. on september , , i took charge of the nautical almanac office. the change was one of the happiest of my life. i was now in a position of recognized responsibility, where my recommendations met with the respect due to that responsibility, where i could make plans with the assurance of being able to carry them out, and where the countless annoyances of being looked upon as an important factor in work where there was no chance of my being such would no longer exist. practically i had complete control of the work of the office, and was thus, metaphorically speaking, able to work with untied hands. it may seem almost puerile to say this to men of business experience, but there is a current notion, spread among all classes, that because the naval observatory has able and learned professors, therefore they must be able to do good and satisfactory work, which may be worth correcting. i found my new office in a rather dilapidated old dwelling-house, about half a mile or less from the observatory, in one of those doubtful regions on the border line between a slum and the lowest order of respectability. if i remember aright, the only occupants of the place were the superintendent, my old friend mr. loomis, senior assistant, who looked after current business, a proof-reader and a messenger. all the computers, including even one copyist, did their work at their homes. a couple of changes had to be made in the interest of efficiency. the view taken of one of these may not only interest the reader, but give him an idea of what people used to think of government service before the era of civil service reform. the proof-reader was excellent in every respect except that of ability to perform his duty. he occupied a high position, i believe, in the grand army of the republic, and thus wielded a good deal of influence. when his case was appealed to the secretary of the navy, apellant was referred to me. i stated the trouble to counsel,--he did not appear to see figures, or be able to distinguish whether they were right or wrong, and therefore was useless as a proof-reader. "it is not his fault," was the reply; "he nearly lost his eyesight in the civil war, and it is hard for him to see at all." in the view of counsel that explanation ought to have settled the case in his favor. it did not, however, but "influence" had no difficulty in making itself more successful in another field. among my first steps was that of getting a new office in the top of the corcoran building, then just completed. it was large and roomy enough to allow quite a number of assistants around me. much of the work was then, as now, done by the piece, or annual job, the computers on it very generally working at their homes. this offers many advantages for such work; the government is not burdened with an officer who must be paid his regular monthly salary whether he supplies his work or not, and whom it is unpleasant and difficult to get rid of in case of sickness or breakdown of any sort. the work is paid for when furnished, and the main trouble of administration saved. it is only necessary to have a brief report from time to time, showing that the work is actually going on. i began with a careful examination of the relation of prices to work, making an estimate of the time probably necessary to do each job. among the performers of the annual work were several able and eminent professors at various universities and schools. i found that they were being paid at pretty high professional prices. i recall with great satisfaction that i was able to reduce the prices and, step by step, concentrate all the work in washington, without detriment to the pleasant relations i sustained with these men, some of them old and intimate friends. these economies went on increasing year by year, and every dollar that was saved went into the work of making the tables necessary for the future use of the ephemeris. the programme of work which i mapped out, involved, as one branch of it, a discussion of all the observations of value on the positions of the sun, moon, and planets, and incidentally, on the bright fixed stars, made at the leading observatories of the world since . one might almost say it involved repeating, in a space of ten or fifteen years, an important part of the world's work in astronomy for more than a century past. of course, this was impossible to carry out in all its completeness. in most cases what i was obliged practically to confine myself to was a correction of the reductions already made and published. still, the job was one with which i do not think any astronomical one ever before attempted by a single person could compare in extent. the number of meridian observations on the sun, mercury, venus, and mars alone numbered , . they were made at the observatories of greenwich, paris, königsberg, pulkowa, cape of good hope,--but i need not go over the entire list, which numbers thirteen. the other branches of the work were such as i have already described,--the computation of the formulæ for the perturbation of the various planets by each other. as i am writing for the general reader, i need not go into any further technical description of this work than i have already done. something about my assistants may, however, be of interest. they were too numerous to be all recalled individually. in fact, when the work was at its height, the office was, in the number of its scientific employees, nearly on an equality with the three or four greatest observatories of the world. one of my experiences has affected my judgment on the general morale of the educated young men of our country. in not a single case did i ever have an assistant who tried to shirk his duty to the government, nor do i think there was more than a single case in which one tried to contest my judgment of his own merits, or those of his work. i adopted the principle that promotion should be by merit rather than by seniority, and my decisions on that matter were always accepted without complaint. i recall two men who voluntarily resigned when they found that, through failure of health or strength, they were unable to properly go on with their work. in frankness i must admit that there was one case in which i had a very disagreeable contest in getting rid of a learned gentleman whose practical powers were so far inferior to his theoretical knowledge that he was almost useless in the office. he made the fiercest and most determined fight in which i was ever engaged, but i must, in justice to all concerned, say that his defect was not in will to do his work but in the requisite power. officially i was not without fault, because, in the press of matters requiring my attention, i had entrusted too much to him, and did not discover his deficiencies until some mischief had been done. perhaps the most eminent and interesting man associated with me during this period was mr. george w. hill, who will easily rank as the greatest master of mathematical astronomy during the last quarter of the nineteenth century. the only defect of his make-up of which i have reason to complain is the lack of the teaching faculty. had this been developed in him, i could have learned very much from him that would have been to my advantage. in saying this i have one especial point in mind. in beginning my studies in celestial mechanics, i lacked the guidance of some one conversant with the subject on its practical side. two systems of computing planetary perturbations had been used, one by leverrier, while the other was invented by hansen. the former method was, in principle, of great simplicity, while the latter seemed to be very complex and even clumsy. i naturally supposed that the man who computed the direction of the planet neptune before its existence was known, must be a master of the whole subject, and followed the lines he indicated. i gradually discovered the contrary, and introduced modified methods, but did not entirely break away from the old trammels. hill had never been bound by them, and used hansen's method from the beginning. had he given me a few demonstrations of its advantages, i should have been saved a great deal of time and labor. the part assigned to hill was about the most difficult in the whole work,--the theory of jupiter and saturn. owing to the great mass of these "giant planets," the inequalities of their motion, especially in the case of saturn, affected by the attraction of jupiter, is greater than in the case of the other planets. leverrier failed to attain the necessary exactness in his investigation of their motion. hill had done some work on the subject at his home in nyack turnpike before i took charge of the office. he now moved to washington, and seriously began the complicated numerical calculations which his task involved. i urged that he should accept the assistance of less skilled computers; but he declined it from a desire to do the entire work himself. computers to make the duplicate computations necessary to guard against accidental numerical errors on his part were all that he required. he labored almost incessantly for about ten years, when he handed in the manuscript of what now forms volume iv. of the "astronomical papers." a pleasant incident occurred in , when the office was honored by a visit from professor john c. adams of england, the man who, independently of leverrier, had computed the place of neptune, but failed to receive the lion's share of the honor because it happened to be the computations of the frenchman and not his which led immediately to the discovery of the planet. it was of the greatest interest to me to bring two such congenial spirits as adams and hill together. it would be difficult to find a more impressive example than that afforded by hill's career, of the difficulty of getting the public to form and act upon sane judgments in such cases as his. the world has the highest admiration for astronomical research, and in this sentiment our countrymen are foremost. they spend hundreds of thousands of dollars to promote it. they pay good salaries to professors who chance to get a certain official position where they may do good work. and here was perhaps the greatest living master in the highest and most difficult field of astronomy, winning world-wide recognition for his country in the science, and receiving the salary of a department clerk. i never wrestled harder with a superior than i did with hon. r. w. thompson, secretary of the navy, about , to induce him to raise mr. hill's salary from $ to $ . it goes without saying that hill took even less interest in the matter than i did. he did not work for pay, but for the love of science. his little farm at nyack turnpike sufficed for his home, and supplied his necessities so long as he lived there, and all he asked in washington was the means of going on with his work. the deplorable feature of the situation is, that this devotion to his science, instead of commanding due recognition on the public and official side, rather tended to create an inadequate impression of the importance of what he was doing. that i could not secure for him at least the highest official consideration is among the regretful memories of my official life. although, so far as the amount of labor is concerned, mr. hill's work upon jupiter and saturn is the most massive he ever undertook, his really great scientific merit consists in the development of a radically new method of computing the inequalities of the moon's motion, which is now being developed and applied by professor e. w. brown. his most marked intellectual characteristic is the eminently practical character of his researches. he does not aim so much at elegant mathematical formulæ, as to determine with the greatest precision the actual quantities of which mathematical astronomy stands in need. in this direction he has left every investigator of recent or present time far in the rear. after the computations on jupiter and saturn were made, it was necessary to correct their orbits and make tables of their motions. this work i left entirely in mr. hill's hands, the only requirement being that the masses of the planets and other data which he adopted should be uniform with those i used in the rest of the work. his tables were practically completed in manuscript at the beginning of . when they were through, doubtless feeling, as well he might, that he had done his whole duty to science and the government, mr. hill resigned his office and returned to his home. during the summer he paid a visit to europe, and visiting the cambridge university, was honored with the degree of doctor of laws, along with a distinguished company, headed by the duke of edinburgh. one of the pleasant things to recall was that, during the fifteen years of our connection, there was never the slightest dissension or friction between us. i may add that the computations which he made on the theory of jupiter and saturn are all preserved complete and in perfect form at the nautical almanac office, so that, in case any question should arise respecting them in future generations, the point can be cleared up by an inspection. in , three years before i left the observatory, i was informed by dr. henry draper that he had a mechanical assistant who showed great fondness for and proficiency in some work in mathematical astronomy. i asked to see what he was doing, and received a collection of papers of a remarkable kind. they consisted mainly of some of the complicated developments of celestial mechanics. in returning them i wrote to draper that, when i was ready to begin my work on the planetary theories, i must have his man,--could he possibly be spared? but he came to me before the time, while i was carrying on some investigations with aid afforded by the smithsonian institution. of course, when i took charge of the nautical almanac office, he was speedily given employment on its work. his name was john meier, a swiss by birth, evidently from the peasant class, but who had nevertheless been a pupil of professor rudolph wolf at zurich. emigrating to this country, he was, during the civil war, an engineer's mate or something of that grade in the navy. he was the most perfect example of a mathematical machine that i ever had at command. of original power,--the faculty of developing new methods and discovering new problems, he had not a particle. happily for his peace of mind, he was totally devoid of worldly ambition. i had only to prepare the fundamental data for him, explain what was wanted, write down the matters he was to start with, and he ground out day after day the most complicated algebraic and trigonometrical computations with untiring diligence and almost unerring accuracy. but a dark side of the picture showed itself very suddenly and unexpectedly in a few years. for the most selfish reasons, if for no others, i desired that his peace of mind should be undisturbed. the result was that i was from time to time appealed to as an arbitrator of family dissensions, in which it was impossible to say which side was right and which wrong. then, as a prophylactic against malaria, his wife administered doses of whiskey. the rest of the history need not be told. it illustrates the maxim that "blood will tell," which i fear is as true in scientific work as in any other field of human activity. a man of totally different blood, the best in fact, entered the office shortly before meier broke down. this was mr. cleveland keith, son of professor reuel keith, who was one of the professors at the observatory when it was started. his patience and ability led to his gradually taking the place of a foreman in supervising the work pertaining to the reduction of the observations, and the construction of the tables of the planets. without his help, i fear i should never have brought the tables to a conclusion. he died in , just as the final results of the work were being put together. high among the troublesome problems with which i had to deal while in charge of the nautical almanac, was that of universal time. all but the youngest of my readers will remember the period when every railway had its own meridian, by the time of which its trains were run, which had to be changed here and there in the case of the great trunk lines, and which seldom agreed with the local time of a place. in the pennsylvania station at pittsburg were three different times; one that of philadelphia, one of some point farther west, and the third the local pittsburg time. the traveler was constantly liable to miss a train, a connection, or an engagement by the doubt and confusion thus arising. this was remedied in by the adoption of our present system of standard times of four different meridians, the introduction of which was one of the great reforms of our generation. when this change was made, i was in favor of using washington time as the standard, instead of going across the ocean to greenwich for a meridian. but those who were pressing the measure wanted to have a system for the whole world, and for this purpose the meridian of greenwich was the natural one. practically our purpose was served as well by the greenwich meridian as it would have been by that of washington. the year following this change an international meridian conference was held at washington, on the invitation of our government, to agree upon a single prime meridian to be adopted by the whole world in measuring longitudes and indicating time. of course the meridian of greenwich was the only one that would answer the purpose. this had already been adopted by several leading maritime nations, including ourselves as well as great britain. it was merely a question of getting the others to fall into line. no conference was really necessary for this purpose, because the dissentients caused much more inconvenience to themselves than to any one else by their divergent practice. the french held out against the adoption of the greenwich meridian, and proposed one passing through behring strait. i was not a member of the conference, but was invited to submit my views, which i did orally. i ventured to point out to the frenchmen that the meridian of greenwich also belonged to france, passing near havre and intersecting their country from north to south. it was therefore as much a french as an english meridian, and could be adopted without any sacrifice of national position. but they were not convinced, and will probably hold out until england adopts the metric system, on which occasion it is said that they will be prepared to adopt the greenwich meridian. one proceeding of the conference illustrates a general characteristic of reformers. almost without debate, certainly without adequate consideration, the conference adopted a recommendation that astronomers and navigators should change their system of reckoning time. both these classes have, from time immemorial, begun the day at noon, because this system was most natural and convenient, when the question was not that of a measure of time for daily life, but simply to indicate with mathematical precision the moment of an event. navigators had begun the day at noon, because the observations of the sun, on which the latitude of a ship depends, are necessarily made at noon, and the run of the ship is worked up immediately afterward. the proposed change would have produced unending confusion in astronomical nomenclature, owing to the difficulty of knowing in all cases which system of time was used in any given treatise or record of observations. i therefore felt compelled, in the general interest of science and public convenience, to oppose the project with all my power, suggesting that, if the new system must be put into operation, we should wait until the beginning of a new century. "i hope you will succeed in having its adoption postponed until ," wrote airy to me, "and when comes, i hope you will further succeed in having it again postponed until the year ." the german official astronomers, and indeed most of the official ones everywhere, opposed the change, but the efforts on the other side were vigorously continued. the british admiralty was strongly urged to introduce the change into the nautical almanac, and the question of doing this was warmly discussed in various scientific journals. one result of this movement was that, in , rear-admiral george h. belknap, superintendent of the naval observatory, and myself were directed to report on the question. i drew up a very elaborate report, discussing the subject especially in its relations to navigation, pointing out in the strongest terms i could the danger of placing in the hands of navigators an almanac in which the numbers were given in a form so different from that to which they were accustomed. if they chanced to forget the change, the results of their computations might be out to any extent, to the great danger and confusion of their reckoning, while not a solitary advantage would be gained by it. there is some reason to suppose that this document found its way to the british admiralty, but i never heard a word further on the subject except that it ceased to be discussed in london. a few years later some unavailing efforts were made to revive the discussion, but the twentieth century is started without this confusing change being introduced into the astronomical ephemerides and nautical almanacs of the world, and navigators are still at liberty to practice the system they find most convenient. in i had succeeded in bringing so much of the work as pertained to the reduction of the observations and the determination of the elements of the planets to a conclusion. so far as the larger planets were concerned, it only remained to construct the necessary tables, which, however, would be a work of several years. with the year came what was perhaps the most important event in my whole plan. i have already remarked upon the confusion which pervaded the whole system of exact astronomy, arising from the diversity of the fundamental data made use of by the astronomers of foreign countries and various institutions in their work. it was, i think, rather exceptional that any astronomical result was based on entirely homogeneous and consistent data. to remedy this state of things and start the exact astronomy of the twentieth century on one basis for the whole world, was one of the objects which i had mapped out from the beginning. dr. a. m. w. downing, superintendent of the british nautical almanac, was struck by the same consideration and animated by the same motive. he had especially in view to avoid the duplication of work which arose from the same computations being made in different countries for the same result, whereby much unnecessary labor was expended. the field of astronomy is so vast, and the quantity of work urgently required to be done so far beyond the power of any one nation, that a combination to avoid all such waste was extremely desirable. when, in , my preliminary results were published, he took the initiative in a project for putting the idea into effect, by proposing an international conference of the directors of the four leading ephemerides, to agree upon a uniform system of data for all computations pertaining to the fixed stars. this conference was held in paris in may, . after several days of discussion, it resolved that, beginning with , a certain set of constants should be used in all the ephemerides, substantially the same as those i had worked out, but without certain ulterior, though practically unimportant, modifications which i had applied for the sake of symmetry. my determination of the positions and motions of the bright fixed stars, which i had not yet completed, was adopted in advance for the same purpose, i agreeing to complete it if possible in time for use in . i also agreed to make a new determination of the constant of precession, that which i had used in my previous work not being quite satisfactory. all this by no means filled the field of exact astronomy, yet what was left outside of it was of comparatively little importance for the special object in view. more than a year after the conference i was taken quite by surprise by a vigorous attack on its work and conclusions on the part of professor lewis boss, director of the dudley observatory, warmly seconded by mr. s. c. chandler of cambridge, the editor of the "astronomical journal." the main grounds of attack were two in number. the time was not ripe for concluding upon a system of permanent astronomical standards. besides this, the astronomers of the country should have been consulted before a decision was reached. ultimately the attack led to a result which may appear curious to the future astronomer. he will find the foreign ephemerides using uniform data worked out in the office of the "american ephemeris and nautical almanac" at washington for the years beginning with . he will find that these same data, after being partially adopted in the ephemeris for , were thrown out in , and the antiquated ones reintroduced in the main body of the ephemeris. the new ones appear simply in an appendix. as, under the operation of law, i should be retired from active service in the march following the conference, it became a serious question whether i should be able to finish the work that had been mapped out, as well as the planetary tables. mr. secretary herbert, on his own motion so far as i know, sent for me to inquire into the subject. the result of the conference was a movement on his part to secure an appropriation somewhat less than the highest salary of a professor, to compensate me for the completion of the work after my retirement. the house committee on appropriations, ever mindful of economy in any new item, reduced the amount to a clerical salary. the committee of conference compromised on a mean between the two. it happened that the work on the stars was not specified in the law,--only the tables of the planets. in consequence i had no legal right to go on with the former, although the ephemerides of europe were waiting for the results. after much trouble an arrangement was effected under which the computers on the work were not to be prohibited from consulting me in its prosecution. astronomical work is never really done and finished. the questions growing out of the agreement or non-agreement of the tables with observations still remain to be studied, and require an immense amount of computation. in what country and by whom these computations will be made no one can now tell. the work which i most regretted to leave unfinished was that on the motion of the moon. as i have already said, this work is complete to . the computations for carrying it on from to the present time were perhaps three fourths done when i had to lay them aside. in , when the carnegie institution was organized, it made a grant for supplying me with the computing assistance and other facilities necessary for the work, and the secretary of the navy allowed me the use of the old computations. under such auspices the work was recommenced in march, . so far as i can recall, i never asked anything from the government which would in any way promote my personal interests. the only exception, if such it is, is that during the civil war i joined with other professors in asking that we be put on the same footing with other staff corps of the navy as regarded pay and rank. so far as my views were concerned, the rank was merely a _pro forma_ matter, as i never could see any sound reason for a man pursuing astronomical duties caring to have military rank. in conducting my office also, the utmost economy was always studied. the increase in the annual appropriations for which i asked was so small that, when i left the office in , they were just about the same as they were back in the fifties, when it was first established. the necessary funds were saved by economical administration. all this was done with a feeling that, after my retirement, the satisfaction with which one could look back on such a policy would be enhanced by a feeling on the part of the representatives of the public that the work i had done must be worthy of having some pains taken to secure its continuance in the same spirit. i do not believe that the men who conduct our own government are a whit behind the foremost of other countries in the desire to promote science. if after my retirement no special measures were deemed necessary to secure the continuance of the work in which i had been engaged, i prefer to attribute it to adventitious circumstances rather than to any undervaluation of scientific research by our authorities. ix scientific washington it is sometimes said that no man, in passing away, leaves a place which cannot be equally well filled by another. this is doubtless true in all ordinary cases. but scientific research, and scientific affairs generally at the national capital, form an exception to many of the rules drawn from experience in other fields. professor joseph henry, first secretary of the smithsonian institution, was a man of whom it may be said, without any reflection on men of our generation, that he held a place which has never been filled. i do not mean his official place, but his position as the recognized leader and exponent of scientific interests at the national capital. a world-wide reputation as a scientific investigator, exalted character and inspiring presence, broad views of men and things, the love and esteem of all, combined to make him the man to whom all who knew him looked for counsel and guidance in matters affecting the interests of science. whether any one could since have assumed this position, i will not venture to say; but the fact seems to be that no one has been at the same time able and willing to assume it. on coming to washington i soon became very intimate with professor henry, and i do not think there was any one here to whom he set forth his personal wishes and convictions respecting the policy of the smithsonian institution and its relations to the government more freely than he did to me. as every point connected with the history and policy of this establishment is of world-wide interest, and as professor henry used to put some things in a different light from that shed upon the subject by current publications, i shall mention a few points that might otherwise be overlooked. it has always seemed to me that a deep mystery enshrouded the act of smithson in devising his fortune as he did. that an englishman, whose connections and associations were entirely with the intellectual classes,--who had never, so far as is known, a single american connection, or the slightest inclination toward democracy,--should, in the intellectual condition of our country during the early years of the century, have chosen its government as his trustee for the foundation of a scientific institution, does of itself seem singular enough. what seems yet more singular is that no instructions whatever were given in his will or found in his papers beyond the comprehensive one "to found an institution at washington to be called the smithsonian institution for the increase and diffusion of knowledge among men." no plan of the institution, no scrap of paper which might assist in the interpretation of the mandate, was ever discovered. not a word respecting his intention was ever known to have been uttered. only a single remark was ever recorded which indicated that he had anything unusual in view. he did at one time say, "my name shall live in the memory of men when the titles of the northumberlands and the percys are extinct and forgotten." one result of this failure to indicate a plan for the institution was that, when the government received the money, congress was at a loss what to do with it. some ten years were spent in discussing schemes of various kinds, among them that of declining the gift altogether. then it was decided that the institution should be governed by a board of regents, who should elect a secretary as their executive officer and the administrator of the institution. the latter was to include a library, a museum, and a gallery of art. the plans for the fine structure, so well known to every visitor to the capital, were prepared, the building was started, the regents organized, and professor henry made secretary. we might almost say that henry was opposed to every special function assigned to the institution by the organic law. he did not agree with me as to any mystery surrounding the intentions of the founder. to him they were perfectly clear. smithson was a scientific investigator; and the increase and diffusion of knowledge among men could be best promoted on the lines that he desired, by scientific investigation and the publication of scientific researches. for this purpose a great building was not necessary, and he regretted all the money spent on it. the library, museum, and gallery of art would be of only local advantage, whereas "diffusion among men" implied all men, whether they could visit washington or not. it was clearly the business of the government to supply purely local facilities for study and research, and the endowment of smithson should not be used for such a purpose. his opposition to the building tinged the whole course of his thought. i doubt whether he was ever called upon by founders of institutions of any sort for counsel without his warning them to beware of spending their money in bricks and mortar. the building being already started before he took charge, and the three other objects being sanctioned by law, he was, of course, hampered in carrying out his views. but he did his utmost to reduce to a minimum the amount of the fund that should be devoted to the objects specified. this policy brought on the most animated contest in the history of the institution. it was essential that his most influential assistants should share his views or at least not thwart them. this, he found, was not the case. the librarian, mr. c. c. jewett, an able and accomplished man in the line of his profession, was desirous of collecting one of the finest scientific libraries. a contest arose, to which professor henry put an end by the bold course of removing the librarian from office. mr. jewett denied his power to do this, and the question came before the board of regents. the majority of these voted that the secretary had the power to remove his assistants. among the minority was rufus choate, who was so strongly opposed to the action that he emphasized his protest against it by resigning from the board. a question of legal interpretation came in to make the situation yet more difficult. the regents had resolved that, after the completion of the building, one half the income should be devoted to those objects which professor henry considered most appropriate. meanwhile there was no limit to the amount that might be appropriated to these objects, but mr. jewett and other heads of departments wished to apply the rule from the beginning. henry refused to do so, and looked with entire satisfaction on the slowness of completion of what was, in his eyes, an undesirable building. it must be admitted that there was one point which professor henry either failed to appreciate, or perhaps thought unworthy of consideration. this is, the strong hold on the minds of men which an institution is able to secure through the agency of an imposing building. saying nothing of the artistic and educational value of a beautiful piece of architecture, it would seem that such a structure has a peculiar power of impressing the minds of men with the importance of the object to which it is devoted, or of the work going on within it. had professor henry been allowed to perform all the functions of the smithsonian institution in a moderate-sized hired house, as he felt himself abundantly able to do, i have very serious doubts whether it would have acquired its present celebrity and gained its present high place in the estimation of the public. in the winter of the institution suffered an irreparable loss by a conflagration which destroyed the central portion of the building. at that time the gallery of art had been confined to a collection of portraits of indians by stanley. this collection was entirely destroyed. the library, being at one end, remained intact. the lecture room, where courses of scientific lectures had been delivered by eminent men of science, was also destroyed. this event gave professor henry an opportunity of taking a long step in the direction he desired. he induced congress to take the smithsonian library on deposit as a part of its own, and thus relieve the institution of the cost of supporting this branch. the corcoran art gallery had been founded in the mean time, and relieved the institution of all necessity for supporting a gallery of art. he would gladly have seen the national museum made a separate institution, and the smithsonian building purchased by the government for its use, but he found no chance of carrying this out. after the death of professor henry the institution grew rapidly into a position in which it might almost claim to be a scientific department of the government. the national museum, remaining under its administration, was greatly enlarged, and one of its ramifications was extended into the national zoölogical park. the studies of indian ethnology, begun by major j. w. powell, grew into the bureau of ethnology. the astrophysical observatory was established, in which professor langley has continued his epoch-making work on the sun's radiant heat with his wonderful bolometer, an instrument of his own invention. before he was appointed to succeed professor henry, professor baird was serving as united states fish commissioner, and continued to fill this office, without other salary than that paid by the smithsonian institution. the economic importance of the work done and still carried on by this commission is too well known to need a statement. about the time of baird's death, the work of the commission was separated from that of the institution by providing a salary for the commissioner. we have here a great extension of the idea of an institution for scientific publications and research. i recall once suggesting to professor baird the question whether the utilization of the institution founded by smithson for carrying on and promoting such government work as that of the national museum was really the right thing to do. he replied, "it is not a case of using the smithsonian fund for government work, but of the government making appropriations for the work of the smithsonian institution." between the two sides of the question thus presented,--one emphasizing the honor done to smithson by expanding the institution which bears his name, and the other aiming solely at the best administration of the fund which we hold in trust for him,--i do not pretend to decide. on the academic side of social life in washington, the numerous associations of alumni of colleges and universities hold a prominent place. one of the earliest of these was that of yale, which has held an annual banquet every year, at least since , when i first became a member. its membership at this time included mr. w. m. evarts, then secretary of state, chief justice waite, senator dawes, and a number of other men prominent in political life. the most attractive speaker was mr. evarts, and the fact that his views of education were somewhat conservative added much to the interest of his speeches. he generally had something to say in favor of the system of a prescribed curriculum in liberal education, which was then considered as quite antiquated. when president dwight, shortly after his accession to office, visited the capital to explain the modernizing of the yale educational system, he told the alumni that the college now offered ninety-five courses to undergraduates. evarts congratulated the coming students on sitting at a banquet table where they had their choice of ninety-five courses of intellectual aliment. perhaps the strongest testimonial of the interest attached to these reunions was unconsciously given by president hayes. he had received an honorary degree from yale, and i chanced to be on the committee which called to invite him to the next banquet. he pleaded, as i suppose presidents always do, the multiplicity of his engagements, but finally said,-- "well, gentlemen, i will come, but it must be on two well-understood conditions. in the first place, i must not be called to my feet. you must not expect a speech of me. the second condition is, i must be allowed to leave punctually at ten o'clock." "we regret your conditions, mr. president," was the reply, "but must, of course, accede to them, if you insist." he came to the banquet, he made a speech,--a very good, and not a very short one,--and he remained, an interested hearer, until nearly two o'clock in the morning. in recent years i cannot avoid a feeling that a change has come over the spirit of such associations. one might gather the impression that the apothegm of sir william hamilton needed a slight amendment. on earth is nothing great but man, in man is nothing great but mind. strike out the last word, and insert "muscle." the reader will please not misinterpret this remark. i admire the physically perfect man, loving everything out of doors, and animated by the spirit that takes him through polar snows and over mountain tops. but i do not feel that mere muscular practice during a few years of college life really fosters this spirit. among the former institutions of washington of which the memory is worth preserving, was the scientific club. this was one of those small groups, more common in other cities than in washington, of men interested in some field of thought, who meet at brief intervals at one another's houses, perhaps listen to a paper, and wind up with a supper. when or how the washington club originated, i do not know, but it was probably sometime during the fifties. its membership seems to have been rather ill defined, for, although i have always been regarded as a member, and am mentioned in mcculloch's book as such, [ ] i do not think i ever received any formal notice of election. the club was not exclusively scientific, but included in its list the leading men who were supposed to be interested in scientific matters, and whose company was pleasant to the others. mr. mcculloch himself, general sherman, and chief justice chase are examples of the members of the club who were of this class. it was at the club meetings that i made the acquaintance of general sherman. his strong characteristics were as clearly seen at these evening gatherings as in a military campaign. his restlessness was such that he found it hard to sit still, especially in his own house, two minutes at a time. his terse sentences, leaving no doubt in the mind of the hearer as to what he meant, always had the same snap. one of his military letters is worth reviving. when he was carrying on his campaign in georgia against hood, the latter was anxious that the war should damage general commercial interests as little as possible; so he sent general sherman a letter setting forth the terms and conditions on which he, hood, would refrain from burning the cotton in his line of march, but leave it behind,--at as great length and with as much detail as if it were a treaty of peace between two nations. sherman's reply was couched in a single sentence: "i hope you will burn all the cotton you can, for all you don't burn i will." when he introduced two people, he did not simply mention their names, but told who each one was. in introducing the adjutant-general to another officer who had just come into washington, he added, "you know his signature." mr. mcculloch, who succeeded mr. chase as secretary of the treasury, was my beau idéal of an administrator. in his personal make-up, he was as completely the opposite of general sherman as a man well could be. deliberate, impassive, heavy of build, slow in physical movement, he would have been supposed, at first sight, a man who would take life easy, and concern himself as little as possible about public affairs. but, after all, there is a quality in the head of a great department which is quite distinct from sprightliness, and that is wisdom. this he possessed in the highest degree. the impress which he made on our fiscal system was not the product of what looked like energetic personal action, but of a careful study of the prevailing conditions of public opinion, and of the means at his disposal for keeping the movement of things in the right direction. his policy was what is sometimes claimed, and correctly, i believe, to embody the highest administrative wisdom: that of doing nothing himself that he could get others to do for him. in this way all his energies could be devoted to his proper work, that of getting the best men in office, and of devising measures from time to time calculated to carry the government along the lines which he judged to be best for the public interests. the name of another attendant at the meetings of the club has from time to time excited interest because of its connection with a fundamental principle of evolutionary astronomy. this principle, which looks paradoxical enough, is that up to a certain stage, as a star loses heat by radiation into space, its temperature becomes higher. it is now known as lane's law. some curiosity as to its origin, as well as the personality of its author, has sometimes been expressed. as the story has never been printed, i ask leave to tell it. among the attendants at the meetings of the scientific club was an odd-looking and odd-mannered little man, rather intellectual in appearance, who listened attentively to what others said, but who, so far as i noticed, never said a word himself. up to the time of which i am speaking, i did not even know his name, as there was nothing but his oddity to excite any interest in him. one evening about the year , the club met, as it not infrequently did, at the home of mr. mcculloch. after the meeting mr. w. b. taylor, afterward connected with the smithsonian institution in an editorial capacity, accompanied by the little man, set out to walk to his home, which i believe was somewhere near the smithsonian grounds. at any rate, i joined them in their walk, which led through these grounds. a few days previous there had appeared in the "reader," an english weekly periodical having a scientific character, an article describing a new theory of the sun. the view maintained was that the sun was not a molten liquid, as had generally been supposed up to that time, but a mass of incandescent gas, perhaps condensed at its outer surface, so as to form a sort of immense bubble. i had never before heard of the theory, but it was so plausible that there could be no difficulty in accepting it. so, as we wended our way through the smithsonian grounds, i explained the theory to my companions in that _ex cathedra_ style which one is apt to assume in setting forth a new idea to people who know little or nothing of the subject. my talk was mainly designed for mr. taylor, because i did not suppose the little man would take any interest in it. i was, therefore, much astonished when, at a certain point, he challenged, in quite a decisive tone, the correctness of one of my propositions. in a rather more modest way, i tried to maintain my ground, but was quite silenced by the little man informing us that he had investigated the whole subject, and found so and so--different from what i had been laying down. i immediately stepped down from the pontifical chair, and asked the little man to occupy it and tell us more about the matter, which he did. whether the theorem to which i have alluded was included in his statement, i do not recall. if it was not, he told me about it subsequently, and spoke of a paper he had published, or was about to publish, in the "american journal of science." i find that this paper appeared in volume l. in . naturally i cultivated the acquaintance of such a man. his name was j. homer lane. he was quite alone in the world, having neither family nor near relative, so far as any one knew. he had formerly been an examiner or something similar in the patent office, but under the system which prevailed in those days, a man with no more political influence than he had was very liable to lose his position, as he actually did. he lived in a good deal such a habitation and surroundings as men like johnson and goldsmith lived in in their time. if his home was not exactly a garret, it came as near it as a lodging of the present day ever does. after the paper in question appeared, i called mr. lane's attention to the fact that i did not find any statement of the theorem which he had mentioned to me to be contained in it. he admitted that it was contained in it only impliedly, and proceeded to give me a very brief and simple demonstration. so the matter stood, until the centennial year, , when sir william thomson paid a visit to this country. i passed a very pleasant evening with him at the smithsonian institution, engaged in a discussion, some points of which he afterwards mentioned in an address to the british association. among other matters, i mentioned this law, originating with mr. j. homer lane. he did not think it could be well founded, and when i attempted to reproduce mr. lane's verbal demonstration, i found myself unable to do so. i told him i felt quite sure about the matter, and would write to him on the subject. when i again met mr. lane, i told him of my difficulty and asked him to repeat the demonstration. he did so at once, and i sent it off to sir william. the latter immediately accepted the result, and published a paper on the subject, in which the theorem was made public for the first time. it is very singular that a man of such acuteness never achieved anything else of significance. he was at my station on one occasion when a total eclipse of the sun was to be observed, and made a report on what he saw. at the same time he called my attention to a slight source of error with which photographs of the transit of venus might be affected. the idea was a very ingenious one, and was published in due course. altogether, the picture of his life and death remains in my memory as a sad one, the brightest gleam being the fact that he was elected a member of the national academy of sciences, which must have been to him a very grateful recognition of his work on the part of his scientific associates. when he died, his funeral was attended only by a few of his fellow members of the academy. altogether, i feel it eminently appropriate that his name should be perpetuated by the theorem of which i have spoken. if the national academy of sciences has not proved as influential a body as such an academy should, it has still taken such a place in science, and rendered services of such importance to the government, that the circumstances connected with its origin are of permanent historic interest. as the writer was not a charter member, he cannot claim to have been "in at the birth," though he became, from time to time, a repository of desultory information on the subject. there is abundant internal and circumstantial evidence that dr. b. a. gould, although his name has, so far as i am aware, never been mentioned in this connection, was a leading spirit in the first organization. on the other hand, curiously enough, professor henry was not. i was quite satisfied that bache took an active part, but henry assured me that he could not believe this, because he was so intimate with bache that, had the latter known anything of the matter, he would surely have consulted him. some recent light is thrown on the subject by letters of rear-admiral charles h. davis, found in his "life," as published by his son. everything was carried on in the greatest secrecy, until the bill chartering the body was introduced by senator henry wilson of massachusetts. fifty charter members were named, and this number was fixed as the permanent limit to the membership. the list did not include either george p. bond, director of the harvard observatory, perhaps the foremost american astronomer of the time in charge of an observatory, nor dr. john w. draper. yet the total membership in the section of astronomy and kindred sciences was very large. a story to which i give credence was that the original list, as handed to senator wilson, did not include the name of william b. rogers, who was then founding the institute of technology. the senator made it a condition that room for rogers should be found, and his wish was acceded to. it is of interest that the man thus added to the academy by a senator afterward became its president, and proved as able and popular a presiding officer as it ever had. the governmental importance of the academy arose from the fact that its charter made it the scientific adviser of the government, by providing that it should "investigate, examine, experiment, and report upon any subject of science or art" whenever called upon by any department of the government. in this respect it was intended to perform the same valuable functions for the government that are expected of the national scientific academies or societies of foreign countries. the academy was empowered to make its own constitution. that first adopted was sufficiently rigid and complex. following the example of european bodies of the same sort, it was divided into two classes, one of mathematical and physical, the other of natural science. each of these classes was divided into sections. a very elaborate system of procedure for the choice of new members was provided. any member absent from four consecutive stated meetings of the academy had his name stricken from the roll unless he communicated a valid reason for his absence. notwithstanding this requirement, the academy had no funds to defray the traveling expenses of members, nor did the government ever appropriate money for this purpose. for seven years it became increasingly doubtful whether the organization would not be abandoned. several of the most eminent members took no interest whatever in the academy,--did not attend the meetings, but did tender their resignations, which, however, were not accepted. this went on at such a rate that, in , to avoid a threatened dissolution, a radical change was made in the constitution. congress was asked to remove the restriction upon the number of members, which it promptly did. classes and sections were entirely abandoned. the members formed but a single body. the method of election was simplified,--too much simplified, in fact. the election of new members is, perhaps, the most difficult and delicate function of such an organization. it is one which cannot be performed to public satisfaction, nor without making many mistakes; and the avoidance of the latter is vastly more difficult when the members are so widely separated and have little opportunity to discuss in advance the merits of the men from whom a selection is to be made. an ideal selection cannot be made until after a man is dead, so that his work can be summed up; but i think it may fairly be said that, on the whole, the selections have been as good as could be expected under the conditions. notwithstanding the indifference of the government to the possible benefits that the academy might render it, it has--in addition to numerous reports on minor subjects--made two of capital importance to the public welfare. one of these was the planning of the united states geological survey, the other the organization of a forestry system for the united states. during the years - , besides several temporary surveys or expeditions which had from time to time been conducted under the auspices of the government, there were growing up two permanent surveys of the territories. one of these was the geographical survey of territories west of the th meridian, under the chief of engineers of the army; the other was the geological survey of the territories under the interior department, of which the chief was professor f. v. hayden. the methods adopted by the two chiefs to gain the approval of the public and the favoring smiles of congress were certainly very different. wheeler's efforts were made altogether by official methods and through official channels. hayden considered it his duty to give the public every possible opportunity to see what he was doing and to judge his work. his efforts were chronicled at length in the public prints. his summers were spent in the field, and his winters were devoted to working up results and making every effort to secure influence. an attractive personality and extreme readiness to show every visitor all that there was to be seen in his collections, facilitated his success. one day a friend introduced a number of children with an expression of doubt as to the little visitors being welcome. "oh, i always like to have the children come here," he replied, "they influence their parents." he was so successful in his efforts that his organization grew apace, and soon developed into the geological survey of the territories. ostensibly the objects of the two organizations were different. one had military requirements mainly in view, especially the mapping of routes. hayden's survey was mainly in the interests of geology. practically, however, the two covered the same field in all points. the military survey extended its scope by including everything necessary for a complete geographical and geological atlas. the geological survey was necessarily a complete topographical and geological survey from the beginning. between and , both were engaged in making an atlas of colorado, on the maps of which were given the same topographical features and the same lines of communication. parties of the two surveys mounted their theodolites on the same mountains, and triangulated the same regions. the hayden survey published a complete atlas of colorado, probably more finely gotten up than any atlas of a state in the union, while the wheeler survey was vigorously engaged in issuing maps of the same territory. no effort to prevent this duplication of work by making an arrangement between the two organizations led to any result. neither had any official knowledge of the work of the other. unofficially, the one was dissatisfied with the political methods of the other, and claimed that the maps which it produced were not fit for military purposes. hayden retorted with unofficial reflections on the geological expertness of the engineers, and maintained that their work was not of the best. he got up by far the best maps; wheeler, in the interests of economy, was willing to sacrifice artistic appearance to economy of production. we thus had the curious spectacle of the government supporting two independent surveys of the same region. various compromises were attempted, but they all came to nothing. the state of things was clear enough to congress, but the repugnance of our national legislature to the adoption of decisive measures of any sort for the settlement of a disputed administrative question prevented any effective action. infant bureaus may quarrel with each other and eat up the paternal substance, but the parent cannot make up his mind to starve them outright, or even to chastise them into a spirit of conciliation. unable to decide between them, congress for some years pursued the policy of supporting both surveys. the credit for introducing a measure which would certainly lead to unification is due to mr. a. s. hewitt, of new york, then a member of the committee on appropriations. he proposed to refer the whole subject to the national academy of sciences. his committee accepted his view, and a clause was inserted in the sundry civil bill of june , , requiring the academy at its next meeting to take the matter into consideration and report to congress "as soon thereafter as may be practicable, a plan for surveying and mapping the territory of the united states on such general system as will, in their judgment, secure the best results at the least possible cost." several of the older and more conservative members of the academy objected that this question was not one of science or art, with which alone the academy was competent to deal, but was a purely administrative question which congress should settle for itself. they feared that the academy would be drawn into the arena of political discussion to an extent detrimental to its future and welfare and usefulness. whether the exception was or was not well taken, it was felt that the academy, the creature of congress, could not join issue with the latter as to its functions, nor should an opportunity of rendering a great service to the government be lost for such a reason as this. the plan reported by the academy was radical and comprehensive. it proposed to abolish all the existing surveys of the territories except those which, being temporary, were completing their work, and to substitute for them a single organization which would include the surveys of the public lands in its scope. the interior work of the coast and geodetic survey was included in the plan, it being proposed to transfer this bureau to the interior department, with its functions so extended as to include the entire work of triangulation. when the proposition came up in congress at the following session, it was vigorously fought by the chief of engineers of the army, and by the general land office, of which the surveying functions were practically abolished. the land office carried its point, and was eliminated from the scheme. general humphreys, the chief of engineers, was a member of the academy, but resigned on the ground that he could not properly remain a member while contesting the recommendations of the body. but the academy refused to accept the resignation, on the very proper ground that no obligation was imposed on the members to support the views of the academy, besides which, the work of the latter in the whole matter was terminated when its report was presented to congress. although this was true of the academy, it was not true of the individual members who had taken part in constructing the scheme. they were naturally desirous of seeing the plan made a success, and, in the face of such vigorous opposition, this required constant attention. a dexterous movement was that of getting the measure transferred from one appropriation bill to another when it passed over to the senate. the measure at length became a law, and thus was established the geological survey of the united states, which was to be governed by a director, appointed by the president, by and with the advice and consent of the senate. then, on march , , an important question arose. the right man must be placed at the head of the new bureau. who is he? at first there seemed to be but one voice on the subject, professor hayden had taken the greatest pains to make known the work of his survey, not only to congress, but to every scientific society, small and great, the world over. many of these had bestowed their approbation upon it by electing its director to honorary membership. it has been said, i do not know how truly, that the number of these testimonials exceeded that received by any other scientific man in america. if this were so, they would have to be counted, not weighed. it was, therefore, not surprising that two thirds of the members of congress were said to have sent a recommendation to the president for the appointment of so able and successful a man to the new position. the powerful backing of so respectable a citizen as hon. j. d. cox, formerly secretary of the interior, was also heartily proffered. to these forces were added that of a certain number of geologists, though few or none of them were leaders in the science. had it not been for a private intimation conveyed to secretary schurz that the scientific men interested might have something to say on the subject, hayden might have been appointed at the very moment the bill was signed by the president. notwithstanding all of hayden's merits as the energetic head of a survey, the leaders in the movement considered that mr. clarence king was the better qualified for the duties of the new position. it is not unlikely that a preference for a different method of influencing congress than that which i have described, was one of the reasons in favor of mr. king. he was a man of charming personality and great literary ability. some one said of him that he could make a more interesting story out of what he saw during a ride in a street car than most men could with the best material at their disposal. his "mountaineering in the sierra nevadas" was as interesting an account of western exploration as has ever been published. i understand it was suppressed by the author because some of the characters described in it were much hurt by finding themselves painted in the book. hopeless though the contest might have seemed, an effort was made by three or four of the men most interested to secure mr. king's appointment. if i wanted to show the fallacy of the common impression that scientific men are not fitted for practical politics, i could not do it better than by giving the internal history of the movement. this i shall attempt only in the briefest way. the movers in the matter divided up the work, did what they could in the daytime, and met at night at wormley's hotel to compare notes, ascertain the effect of every shot, and decide where the next one should be fired. as all the parties concerned in the matter have now passed off the stage, i shall venture to mention one of these shots. one eminent geologist, whose support was known to be available, had not been called in, because an impression had been formed that president hayes would not be willing to consider favorably what he might say. after the matter had been discussed at one or two meetings, one of the party proposed to sound the president on the subject at his next interview. so, when the occasion arose, he gently introduced the name of the gentleman. "what view does he take?" inquired the president. "i think he will be favorable to mr. king," was the reply; "but would you give great weight to his opinion?" "i would give great weight to it, very great weight, indeed," was the reply. this expression was too decided in its tone to leave any doubt, and the geologist in question was on his way to washington as soon as electricity could tell him that he was wanted. when the time finally came for a decision, the president asked secretary schurz for his opinion. both agreed that king was the man, and he was duly appointed. the new administration was eminently successful. but king was not fond of administrative work, and resigned the position at the end of a year or so. he was succeeded by john w. powell, under whom the survey grew with a rapidity which no one had anticipated. as originally organized, the survey was one of the territories only, but the question whether it should not be extended to the states as well, and prepare a topographical atlas of the whole country, was soon mooted, and decided by congress in the affirmative. for this extension, however, the original organizers of the survey were in no way responsible. it was the act of congress, pure and simple. if the success of an organization is to be measured by the public support which it has commanded, by the extension of its work and influence, and by the gradual dying out of all opposition, it must be admitted that the plan of the academy was a brilliant success. it is true that a serious crisis had once to be met. while mr. cleveland was governor of new york, his experience with the survey of that state had led him to distrust the methods on which the surveys of the united states were being conducted. this distrust seems to have pervaded the various heads of the departments under his administration, and led to serious charges against the conduct of both the coast and geological surveys. an unfavorable report upon the administration of the former was made by a committee especially appointed by the secretary of the treasury, and led to the resignation of its superintendent. but, in the case of the geological survey, the attacks were mostly conducted by the newspapers. at length, director powell asked permission of secretary lamar to write him a letter in reply. his answers were so sweeping, and so conclusive on every point, that nothing more was heard of the criticisms. the second great work of the academy for the government was that of devising a forestry system for the united states. the immediate occasion for action in this direction was stated by secretary hoke smith to be the "inadequacy and confusion of existing laws relating to the public timber lands and consequent absence of an intelligent policy in their administration, resulting in such conditions as may, if not speedily stopped, prevent the proper development of a large part of our country." even more than in the case of the geological survey might this work seem to be one of administration rather than of science. but granting that such was the case, the academy commanded great advantages in taking up the subject. the commission which it formed devoted more than a year to the study, not only of the conditions in our own country, but of the various policies adopted by foreign countries, especially germany, and their results. as in the case of the geological survey, a radically new and very complete system of forestry administration was proposed. interests having other objects than the public good were as completely ignored as they had been before. the soundness of the conclusions reached by the academy commission were challenged by men wielding great political power in their respective states. for a time it was feared that the academy would suffer rather than gain in public opinion by the report it had made. but the moral force behind it was such that, in the long run, some of the severest critics saw their error, and a plan was adopted which, though differing in many details from that proposed, was, in the main, based on the conclusion of the commission. the interior department, the geological survey, and the department of agriculture all have their part in the work. notwithstanding these signal demonstrations of the valuable service which the academy may render to the government, the latter has done nothing for it. the immediate influence of the leading scientific men in public affairs has perhaps been diminished as much in one direction as it has been increased in another by the official character of the organization. the very fact that the members of the academy belong to a body which is, officially, the scientific adviser of the government, prevents them from coming forward to exercise that individual influence which they might exercise were no such body in existence. the academy has not even a place of meeting, nor is a repository for its property and records provided for it. although it holds in trust large sums which have been bequeathed from time to time by its members for promoting scientific investigation, and is, in this way, rendering an important service to the progress of knowledge, it has practically no income of its own except the contributions of its own members, nearly all of whom are in the position described by the elder agassiz, of having "no time to make money." among the men who have filled the office of president of the academy, professor o. c. marsh was perhaps the one whose activity covered the widest field. though long well known in scientific circles, he first came into public prominence by his exposure of the frauds practiced by contractors in furnishing supplies for the indians. this business had fallen into the hands of a small ring of contractors known as the "indian ring," who knew the ropes so well that they could bid below any competitor and yet manage things so as to gain a handsome profit out of the contracts. in the course of his explorations marsh took pains to investigate the whole matter, and published his conclusions first in the new york "tribune," and then more fully in pamphlet form, taking care to have public attention called to the subject so widely that the authorities would have to notice it. in doing so, mr. delano, secretary of the interior, spoke of them as charges made by "a mr. marsh." this method of designating such a man was made effective use of by mr. delano's opponents in the case. although the investigation which followed did not elicit all the facts, it had the result of calling the attention of succeeding secretaries of the interior to the necessity of keeping the best outlook on the administration of indian affairs. what i believe to have been the final downfall of the ring was not brought about until cleveland's first administration. then it happened in this way. mr. lamar, the secretary of the interior, was sharply on the lookout for frauds of every kind. as usual, the lowest bid for a certain kind of blanket had been accepted, and the secretary was determined to see whether the articles furnished actually corresponded with the requirements of the contract. it chanced that he had as his appointment clerk mr. j. j. s. hassler, a former manufacturer of woolen goods. mr. hassler was put on the board to inspect the supplies, and found that the blankets, although to all ordinary appearance of the kind and quality required, were really of a much inferior and cheaper material. the result was the enforced failure of the contractor, and, i believe, the end of the indian ring. marsh's explorations in search of fossil remains of the animals which once roamed over the western parts of our continent were attended by adventures of great interest, which he long had the intention of collecting and publishing in book form. unfortunately, he never did it, nor, so far as i am aware, has any connected narrative of his adventures ever appeared in print. this is more to be regretted, because they belong to a state of things which is rapidly passing away, leaving few records of that lifelike sort which make the most impressive picture. his guide during his early explorations was a character who has since become celebrated in america and europe by the vivid representations of the "wild west" with which he has amused and instructed the dwellers on two continents. marsh was on his way to explore the region in the rocky mountains where he was to find the fossils which have since made his work most celebrated. the guide was burning with curiosity as to the object of the expedition. one night over the campfire he drew his chief into a conversation on the subject. the latter told him that there was once a time when the rocky mountains did not exist, and that part of the continent was a level plain. in the course of long ages mountains rose, and animals ran over them. then the mountains split open; the animals died and left their bones in the clefts. the object of his expedition was now to search for some of these bones. the bones were duly discovered, and it was not many years thereafter before the wild west exhibition was seen in the principal eastern cities. when it visited new haven, its conductor naturally renewed the acquaintance of his former patron and supporter. "do you remember, professor," said he, "our talk as we were going on your expedition to the rockies,--how you told me about the mountains rising up and being split open and the bones of animals being lost in there, and how you were going to get them?" "oh, yes," said the other, "i remember it very well." "well, professor, do you know, when you told me all that i r'ally thought you was puttin' up a job on me." the result was a friendship between the two men, which continued during marsh's whole life. when the one felt that he ought no longer to spend all the money he earned, he consulted marsh on the subject of "salting it down," and doubtless got good advice. as an exposer of humbugs marsh took a prominent place. one of these related to the so-called "cardiff giant." sometime in the newspapers announced the discovery in northern new york, near the canadian border, of an extraordinary fossil man, or colossal statue, people were not sure which, eight or ten feet high. it was found several feet below the ground while digging a well. men of some scientific repute, including even one so eminent as professor james hall, had endorsed the genuineness of the find, and, on the strength of this, it was taken around to show the public. in the course of a journey through new york state, marsh happened to pass through the town where the object was on exhibition. his train stopped forty minutes for dinner, which would give him time to drive to the place and back, and leave a margin of about fifteen minutes for an examination of the statue. hardly more than a glance was necessary to show its fraudulent character. inside the ears the marks of a chisel were still plainly visible, showing that the statue had been newly cut. one of the most curious features was that the stone had not been large enough to make the complete statue, so that the surface was, in one place, still in the rough. the object had been found in wet ground. its material was sulphate of lime, the slight solubility of which would have been sufficient to make it dissolve entirely away in the course of centuries. the absence of any degradation showed that the thing was comparatively new. on the strength of this, marsh promptly denounced the affair as a humbug. only a feeble defense was made for it, and, a year or two later, the whole story came out. it had been designed and executed somewhere in the northwest, transported to the place where discovered, and buried, to be afterward dug up and reported as a prehistoric wonder. only a few years ago the writer had an opportunity of seeing with what wonderful ease intelligent men can be imposed upon by these artificial antiquities. the would-be exhibitor of a fossil woman, found i know not where, appeared in washington. he had not discovered the fossil himself, but had purchased it for some such sum as $ , on the assurance of its genuine character. he seems, however, to have had some misgivings on the subject, and, being an honest fellow, invited some washington scientific men to examine it in advance of a public exhibition. the first feature to strike the critical observer was that the arms of the fossil were crossed over the breast in the most approved undertaker's fashion, showing that if the woman had ever existed, she had devoted her dying moments to arranging a pose for the approval of posterity. little more than a glance was necessary to show that the fossil was simply baked clay. yet the limbs were hard and stiff. one of the spectators therefore asked permission of the owner to bore with an auger into the leg and see what was inside. a few moments' work showed that the bone of the leg was a bar of iron, around which clay had been moulded and baked. i must do the crestfallen owner the justice to say that his anxiety to convince the spectators of his own good faith in the matter far exceeded his regret at the pecuniary loss which he had suffered. another amusing experience that marsh had with a would-be fossil arose out of the discovery here and there in connecticut of the fossil footprints of birds. shortly after a find of this kind had been announced, a farmer drove his wagon up in front of the peabody museum, called on the professor, and told him he had dug up something curious on his farm, and he wished the professor would tell him what it was. he thought it looked like the footprints of a bird in a stone, but he was not quite sure. marsh went out and looked at the stone. a single glance was enough. "oh, i see what they are. they are the footprints of the domestic turkey. and the oddest part of it is, they are all made with the right foot." the simple-minded countryman, in making the prints with the turkey's foot, had overlooked the difference between the right and left foot, and the consequent necessity of having the tracks which pertained to the two feet alternate. washington is naturally a centre of information on all subjects relating to the aboriginal tribes of america and to life on the plains generally. besides the geological survey, the bureau of ethnology has been an active factor in this line. an official report cannot properly illustrate life in all its aspects, and therefore should be supplemented by the experiences of leading explorers. this is all the more necessary if, as seems to be the case, the peculiar characteristics of the life in question are being replaced by those more appropriate to civilization. yet the researches of the bureau in question are not carried on in any narrow spirit, and will supply the future student of humanity with valuable pictures of the most heroic of all races, and yet doomed, apparently, to ultimate extinction. i do not think i ever saw a more impressive human figure and face than those of chief joseph as he stood tall, erect, and impassive, at a president's reception in the winter of . he was attired in all the brilliancy of his official costume; but not a muscle of his strongly marked face betrayed the sentiments with which he must have gazed on the shining uniforms passing before him. [ ] _men and measures of half a century_, by hugh mcculloch. new york: chas. scribner's sons, . x scientific england my first trip to europe, mentioned in the last chapter, was made with my wife, when the oldest transatlantic line was still the fashionable one. the passenger on a cunarder felt himself amply compensated for poor attendance, coarse food, and bad coffee by learning from the officers on the promenade deck how far the ships of their line were superior to all others in strength of hull, ability of captain, and discipline of crew. things have changed on both sides since then. although the cunard line has completed its half century without having lost a passenger, other lines are also carefully navigated, and the cunard passenger, so far as i know, fares as well as any other. captain mcmickan was as perfect a type of the old-fashioned captain of the best class as i ever saw. his face looked as if the gentlest zephyr that had ever fanned it was an atlantic hurricane, and yet beamed with hibernian good humor and friendliness. he read prayers so well on sunday that a passenger assured him he was born to be a bishop. one day a ship of the north german lloyd line was seen in the offing slowly gaining on us. a passenger called the captain's attention to the fact that we were being left behind. "oh, they're very lightly built, them german ships; built to carry german dolls and such like cargo." in london one of the first men we met was thomas hughes, of rugby fame, who made us feel how worthy he was of the love and esteem bestowed upon him by americans. he was able to make our visit pleasant in more ways than one. among the men i wanted to see was mr. john stuart mill, to whom i was attracted not only by his fame as a philosopher and the interest with which i had read his books, but also because he was the author of an excellent pamphlet on the union side during our civil war. on my expressing a desire to make mr. mill's acquaintance, mr. hughes immediately offered to give me a note of introduction. mill lived at blackheath, which, though in an easterly direction down the thames, is one of the prettiest suburbs of the great metropolis. his dwelling was a very modest one, entered through a passage of trellis-work in a little garden. he was by no means the grave and distinguished-looking man i had expected to see. he was small in stature and rather spare, and did not seem to have markedly intellectual features. the cordiality of his greeting was more than i could have expected; and he was much pleased to know that his work in moulding english sentiment in our favor at the commencement of the civil war was so well remembered and so highly appreciated across the atlantic. as a philosopher, it must be conceded that mr. mill lived at an unfortunate time. while his vigor and independence of thought led him to break loose from the trammels of the traditional philosophy, modern scientific generalization had not yet reached a stage favorable to his becoming a leader in developing the new philosophy. still, whatever may be the merits of his philosophic theories, i believe that up to a quite recent time no work on scientific method appeared worthy to displace his "system of logic." a feature of london life that must strongly impress the scientific student from our country is the closeness of touch, socially as well as officially, between the literary and scientific classes on the one side and the governing classes on the other. mr. hughes invited us to make an evening call with him at the house of a cabinet minister,--i think it was mr. goschen,--where we should find a number of persons worth seeing. among those gathered in this casual way were mr. gladstone, dean stanley, and our general burnside, then grown quite gray. i had never before met general burnside, but his published portraits were so characteristic that the man could scarcely have been mistaken. the only change was in the color of his beard. then and later i found that a pleasant feature of these informal "at homes," so universal in london, is that one meets so many people he wants to see, and so few he does not want to see. congress had made a very liberal appropriation for observations of the solar eclipse,--the making of which was one object of my visit,--to be expended under the direction of professor peirce, superintendent of the coast survey. peirce went over in person to take charge of the arrangements. he arrived in london with several members of his party a few days before we did, and about the same time came an independent party of my fellow astronomers from the naval observatory, consisting of professors hall, harkness, and eastman. the invasion of their country by such an army of american astronomers quite stirred up our english colleagues, who sorrowfully contrasted the liberality of our government with the parsimony of their own, which had, they said, declined to make any provision for the observations of the eclipse. considering that it was visible on their own side of the atlantic, they thought their government might take a lesson from ours. of course we could not help them directly; and yet i suspect that our coming, or at least the coming of peirce, really did help them a great deal. at any rate, it was a curious coincidence that no sooner did the american invasion occur than it was semi-officially discovered that no application of which her majesty's government could take cognizance had been made by the scientific authorities for a grant of money with which to make preparations for observing the eclipse. that the scientific authorities were not long in catching so broad a hint as this goes without saying. a little more of the story came out a few days later in a very unexpected way. in scientific england, the great social event of the year is the annual banquet of the royal society, held on st. andrew's day, the date of the annual meeting of the society, and of the award of its medals for distinguished work in science. at the banquet the scientific outlook is discussed not only by members of the society, but by men high in political and social life. the medalists are toasted, if they are present; and their praises are sung, if, as is apt to be the case with foreigners, they are absent. first in rank is the copley medal, founded by sir godfrey copley, a contemporary of newton. this medal has been awarded annually since , and is now considered the highest honor that scientific england has to bestow. the recipient is selected with entire impartiality as to country, not for any special work published during the year, but in view of the general merit of all that he has done. five times in its history the medal has crossed the atlantic. it was awarded to franklin in , agassiz in , dana in , and j. willard gibbs in . the long time that elapsed between the first and the second of these awards affords an illustration of the backwardness of scientific research in america during the greater part of the first century of our independence. the year of my visit the medal was awarded to mr. joule, the english physicist, for his work on the relation of heat and energy. i was a guest at the banquet, which was the most brilliant function i had witnessed up to that time. the leaders in english science and learning sat around the table. her majesty's government was represented by mr. gladstone, the premier, and mr. lowe, afterward viscount sherbrooke, chancellor of the exchequer. both replied to toasts. mr. lowe as a speaker was perhaps a little dull, but not so mr. gladstone. there was a charm about the way in which his talk seemed to display the inner man. it could not be said that he had either the dry humor of mr. evarts or the wit of mr. depew; but these qualities were well replaced by the vivacity of his manner and the intellectuality of his face. he looked as if he had something interesting he wanted to tell you; and he proceeded to tell it in a very felicitous way as regarded both manner and language, but without anything that savored of eloquence. he was like carl schurz in talking as if he wanted to inform you, and not because he wanted you to see what a fine speaker he was. with this he impressed one as having a perfect command of his subject in all its bearings. i did not for a moment suppose that the premier of england could have taken any personal interest in the matter of the eclipse. great, therefore, was my surprise when, in speaking of the relations of the government to science, he began to talk about the coming event. i quote a passage from memory, after twenty-seven years: "i had the pleasure of a visit, a few days since, from a very distinguished american professor, professor peirce of harvard. in the course of the interview, the learned gentleman expressed his regret that her majesty's government had declined to take any measures to promote observations of the coming eclipse of the sun by british astronomers. i replied that i was not aware that the government had declined to take such measures. indeed, i went further, and assured him that any application from our astronomers for aid in making these observations would receive respectful consideration." i felt that there might be room for some suspicion that this visit of professor peirce was a not unimportant factor in the changed position of affairs as regarded british observations of the eclipse. not only the scene i have described, but subsequent experience, has impressed me with the high appreciation in which the best scientific work is held by the leading countries of europe, especially england and france, as if the prosecution were something of national importance which men of the highest rank thought it an honor to take part in. the marquis of salisbury, in an interval between two terms of service as premier of england, presided over the british association for the advancement of science, and delivered an address showing a wide and careful study of the generalizations of modern science. in france, also, one great glory of the nation is felt to be the works of its scientific and learned men of the past and present. membership of one of the five academies of the institute of france is counted among the highest honors to which a frenchman can aspire. most remarkable, too, is the extent to which other considerations than that of merit are set aside in selecting candidates for this honor. quite recently a man was elected a member of the academy of sciences who was without either university or official position, and earned a modest subsistence as a collaborator of the "revue des deux mondes." but he had found time to make investigations in mathematical astronomy of such merit that he was considered to have fairly earned this distinction, and the modesty of his social position did not lie in his way. at the time of this visit lister was an eminent member of the medical profession, but had not, so far as i am aware, been recognized as one who was to render incalculable service to suffering humanity. from a professional point of view there are no two walks in life having fewer points of contact than those of the surgeon and the astronomer. it is therefore a remarkable example of the closeness of touch among eminent englishmen in every walk of life, that, in subsequent visits, i was repeatedly thrown into contact with one who may fairly be recommended as among the greatest benefactors of the human race that the nineteenth century has given us. this was partly, but not wholly, due to his being, for several years, the president of the royal society. i would willingly say much more, but i am unable to write authoritatively upon the life and work of such a man, and must leave gossip to the daily press. for the visiting astronomer at london scarcely a place in london has more attractions than the modest little observatory and dwelling house on upper tulse hill, in which sir william huggins has done so much to develop the spectroscopy of the fixed stars. the owner of this charming place was a pioneer in the application of the spectroscope to the analysis of the light of the heavenly bodies, and after nearly forty years of work in this field, is still pursuing his researches. the charm of sentiment is added to the cold atmosphere of science by the collaboration of lady huggins. almost at the beginning of his work mr. huggins, analyzing the light of the great nebula of orion, showed that it must proceed from a mass of gas, and not from solid matter, thus making the greatest step possible in our knowledge of these objects. he was also the first to make actual measures of the motions of bright stars to or from our system by observing the wave length of the rays of light which they absorbed. quite recently an illustrated account of his observatory and its work has appeared in a splendid folio volume, in which the rigor of science is tempered with a gentle infusion of art which tempts even the non-scientific reader to linger over its pages. in england, the career of professor cayley affords an example of the spirit that impels a scientific worker of the highest class, and of the extent to which an enlightened community may honor him for what he is doing. one of the creators of modern mathematics, he never had any ambition beyond the prosecution of his favorite science. i first met him at a dinner of the astronomical society club. as the guests were taking off their wraps and assembling in the anteroom, i noticed, with some surprise, that one whom i supposed to be an attendant was talking with them on easy terms. a moment later the supposed attendant was introduced as professor cayley. his garb set off the seeming haggardness of his keen features so effectively that i thought him either broken down in health or just recovering from some protracted illness. the unspoken words on my lips were, "why, professor cayley, what has happened to you?" being now in the confessional, i must own that i did not, at the moment, recognize the marked intellectuality of a very striking face. as a representation of a mathematician in the throes of thought, i know nothing to equal his portrait by dickenson, which now hangs in the hall of trinity college, cambridge, and is reproduced in the sixth volume of cayley's collected works. his life was that of a man moved to investigation by an uncontrollable impulse; the only sort of man whose work is destined to be imperishable. until forty years of age he was by profession a conveyancer. his ability was such that he might have gained a fortune by practicing the highest branch of english law, if his energies had not been diverted in another direction. the spirit in which he pursued his work may be judged from an anecdote related by his friend and co-worker, sylvester, who, in speaking of cayley's even and placid temper, told me that he had never seen him ruffled but once. entering his office one morning, intent on some new mathematical thought which he was discussing with sylvester, he opened the letter-box in his door and found a bundle of papers relating to a law case which he was asked to take up. the interruption was too much. he flung the papers on the table with remarks more forcible than complimentary concerning the person who had distracted his attention at such an inopportune moment. in he was made a professor at cambridge, where, no longer troubled with the intricacies of land tenure, he published one investigation after another with ceaseless activity, to the end of his life. among my most interesting callers was professor john c. adams, of whom i have spoken as sharing with leverrier the honor of having computed the position of the planet neptune before its existence was otherwise known. the work of the two men was prosecuted at almost the same time, but adopting the principle that priority of publication should be the sole basis of credit, arago had declared that no other name than that of leverrier should even be mentioned in connection with the work. if repute was correct, leverrier was not distinguished for those amiable qualities that commonly mark the man of science and learning. his attitude toward adams had always been hostile. under these conditions chance afforded the latter a splendid opportunity of showing his superiority to all personal feeling. he was president of the royal astronomical society when its annual medal was awarded to his french rival for his work in constructing new tables of the sun and planets. it thus became his duty to deliver the address setting forth the reasons for the award. he did this with a warmth of praise for leverrier's works which could not have been exceeded had the two men been bosom friends. adams's intellect was one of the keenest i ever knew. the most difficult problems of mathematical astronomy and the most recondite principles that underlie the theory of the celestial motions were to him but child's play. his works place him among the first mathematical astronomers of the age, and yet they do not seem to do his ability entire justice. indeed, for fifteen years previous to the time of my visit his published writings had been rather meagre. but i believe he was justly credited with an elaborate witticism to the following effect: "in view of the fact that the only human being ever known to have been killed by a meteorite was a monk, we may concede that after four hundred years the pope's bull against the comet has been justified by the discovery that comets are made up of meteorites." those readers who know on what imperfect data men's impressions are sometimes founded will not be surprised to learn of my impression that an englishman's politics could be inferred from his mental and social make-up. if all men are born either aristotelians or platonists, then it may be supposed that all englishmen are born conservatives or liberals. the utterances of english journalists of the conservative party about american affairs during and after our civil war had not impressed me with the idea that one so unfortunate as to be born in that party would either take much interest in meeting an american or be capable of taking an appreciative view of scientific progress. so confident was i of my theory that i remarked to a friend with whom i had become somewhat intimate, that no one who knew mr. adams could have much doubt that he was a liberal in politics. an embarrassed smile spread over the friend's features. "you would not make that conclusion known to mr. adams, i hope," said he. "but is he not a liberal?" "he is not only a conservative, but declares himself 'a tory of the tories.'" i afterward found that he fully justified his own description. at the university, he was one of the leading opponents of those measures which freed the academic degrees from religious tests. he was said to have been among those who objected to sylvester, a jew, receiving a degree. i had decided to observe the eclipse at gibraltar. in order that my results, if i obtained any, might be utilized in the best way, it was necessary that the longitude of the station should be determined by telegraph. this had never been done for gibraltar. how great the error of the supposed longitude might have been may be inferred from the fact that a few years later, captain f. green of the united states navy found the longitude of lisbon on the admiralty charts to be two miles in error. the first arrangements i had to make in england were directed to this end. considering the relation of the world's great fortress to british maritime supremacy, it does seem as if there were something presumptuous in the coolness with which i went among the authorities to make arrangements for the enterprise. nevertheless, the authorities permitted the work, with a cordiality which was of itself quite sufficient to remove any such impression, had it been entertained. the astronomers did, indeed, profess to feel it humiliating that the longitude of such a place as gibraltar should have to be determined from greenwich by an american. they did not say "by a foreigner," because they always protested against americans looking upon themselves as such. still, it would not be an english enterprise if an american carried it out. i suspect, however, that my proceedings were not looked upon with entire dissatisfaction even by the astronomers. they might prove as good a stimulant to their government in showing a little more enterprise in that direction as the arrival of our eclipse party did. the longitude work naturally took me to the royal observatory which has made the little town of greenwich so famous. it is situated some eight miles east from charing cross, on a hill in greenwich park, with a pleasant outlook toward the thames. from my youth up i had been working with its observations, and there was no institution in the world which i had approached, or could approach, with the interest i felt in ascending the little hill on which it is situated. when the calabria was once free from her wharf in new york harbor, and on her way down the narrows, the foremost thought was, "off for europe; we shall see greenwich!" the day of my arrival in london i had written to professor airy, and received an answer the same evening, inviting us to visit the observatory and spend an afternoon with him a day or two later. i was shown around the observatory by an assistant, while my wife was entertained by mrs. airy and the daughters inside the dwelling. the family dined as soon as the day's work was over, about the middle of the afternoon. after the meal, we sat over a blazing fire and discussed our impressions of london. "what place in london interested you most?" said airy to my wife. "the first place i went to see was cavendish square." "what was there in cavendish square to interest you?" "when i was a little girl, my mother once gave me, as a birthday present, a small volume of poems. the first verse in the book was:-- "'little ann and her mother were walking one day through london's wide city so fair; and business obliged them to go by the way that led them through cavendish square.'" to our astonishment the astronomer royal at once took up the thread:-- "'and as they passed by the great house of a lord, a beautiful chariot there came, to take some most elegant ladies abroad, who straightway got into the same,'" and went on to the end. i do not know which of the two was more surprised: airy, to find an american woman who was interested in his favorite ballad, or she to find that he could repeat it by heart. the incident was the commencement of a family friendship which has outlived both the heads of the airy family. we may look back on airy as the most commanding figure in the astronomy of our time. he owes this position not only to his early works in mathematical astronomy, but also to his ability as an organizer. before his time the working force of an observatory generally consisted of individual observers, each of whom worked to a greater or less extent in his own way. it is true that organization was not unknown in such institutions. nominally, at least, the assistants in a national observatory were supposed to follow the instructions of a directing head. this was especially the case at greenwich. still, great dependence was placed upon the judgment and ability of the observer himself, who was generally expected to be a man well trained in his specialty, and able to carry on good work without much help. from airy's point of view, it was seen that a large part of the work necessary to the attainment of the traditional end of the royal observatory was of a kind that almost any bright schoolboy could learn to do in a few weeks, and that in most of the remaining part plodding industry, properly directed, was more important than scientific training. he could himself work out all the mathematical formulæ and write all the instructions required to keep a small army of observers and computers employed, and could then train in his methods a few able lieutenants, who would see that all the details were properly executed. under these lieutenants was a grade comprising men of sufficient technical education to enable them to learn how to point the telescope, record a transit, and perform the other technical operations necessary in an astronomical observation. a third grade was that of computers: ingenious youth, quick at figures, ready to work for a compensation which an american laborer would despise, yet well enough schooled to make simple calculations. under the new system they needed to understand only the four rules of arithmetic; indeed, so far as possible airy arranged his calculations in such a way that subtraction and division were rarely required. his boys had little more to do than add and multiply. thus, so far as the doing of work was concerned, he introduced the same sort of improvement that our times have witnessed in great manufacturing establishments, where labor is so organized that unskilled men bring about results that formerly demanded a high grade of technical ability. he introduced production on a large scale into astronomy. at the time of my visit, it was much the fashion among astronomers elsewhere to speak slightingly of the greenwich system. the objections to it were, in substance, the same that have been made to the minute subdivision of labor. the intellect of the individual was stunted for the benefit of the work. the astronomer became a mere operative. yet it must be admitted that the astronomical work done at greenwich during the sixty years since airy introduced his system has a value and an importance in its specialty that none done elsewhere can exceed. all future conclusions as to the laws of motion of the heavenly bodies must depend largely upon it. the organization of his little army necessarily involved a corresponding change in the instruments they were to use. before his time the trained astronomer worked with instruments of very delicate construction, so that skill in handling them was one of the requisites of an observer. airy made them in the likeness of heavy machinery, which could suffer no injury from a blow of the head of a careless observer. strong and simple, they rarely got out of order. it is said that an assistant who showed a visiting astronomer the transit circle some times hit it a good slap to show how solid it was; but this was not done on the present occasion. the little army had its weekly marching orders and made daily reports of progress to its commander, who was thus enabled to control the minutest detail of every movement. in the course of the evening airy gave me a lesson in method, which was equally instructive and entertaining. in order to determine the longitude of gibraltar, it was necessary that time signals should be sent by telegraph from the royal observatory. our conversation naturally led us into a discussion of the general subject of such operations. i told him of the difficulties we had experienced in determining a telegraphic longitude,--that of the harvard observatory from washington, for example,--because it was only after a great deal of talking and arranging on the evening of the observation that the various telegraph stations between the two points could have their connections successfully made at the same moment. at the appointed hour the washington operator would be talking with the others, to know if they were ready, and so a general discussion about the arrangements might go on for half an hour before the connections were all reported good. if we had such trouble in a land line, how should we get a connection from london to the gibraltar cable through lines in constant use? "but," said airy, "i never allow an operator who can speak with the instruments to take part in determining a telegraphic longitude." "then how can you get the connections all made from one end of the line to the other, at the same moment, if your operators cannot talk to one another?" "nothing is simpler. i fix in advance a moment, say eight o'clock greenwich mean time, at which signals are to commence. every intermediate office through which the signals are to pass is instructed to have its wires connected in both directions exactly at the given hour, and to leave them so connected for ten minutes, without asking any further instructions. at the end of the line the instruments must be prepared at the appointed hour to receive the signals. all i have to do here is to place my clock in the circuit and send on the signals for ten minutes, commencing at eight o'clock. they are recorded at the other end of the line without further trouble." "but have you never met with a failure to understand the instructions?" "no; they are too simple to be mistaken, once it is understood that no one has anything to do but make his connections at the designated moment, without asking whether any one else is ready." airy was noted not less for his ability as an organizer than for his methodical habits. the care with which he preserved every record led sir william rowan hamilton to say that when airy wiped his pen on a blotter, he fancied him as always taking a press copy of the mark. his machinery seemed to work perfectly, whether it was constructed of flesh or of brass. he could prepare instructions for the most complicated piece of work with such effective provision against every accident and such completeness in every detail that the work would go on for years without further serious attention from him. the instruments which he designed half a century ago are mostly in use to this day, with scarcely an alteration. yet there is some reason to fear that airy carried method a little too far to get the best results. of late years his system has been greatly changed, even at greenwich. it was always questionable whether so rigid a military routine could accomplish the best that was possible in astronomy; and airy himself, during his later years, modified his plan by trying to secure trained scientific men as his assistants, giving them liberty to combine independent research, on their own account, with the work of the establishment. his successor has gone farther in the same direction, and is now gathering around him a corps of young university men, from whose ability much may be expected. observations with the spectroscope have been pursued, and the observatory has taken a prominent part in the international work of making a photographic map of the heavens. of special importance are the regular discussions of photographs of the sun, taken in order to determine the law of the variation of the spots. the advantage of the regular system which has been followed for more than fifty years is seen in the meteorological observations; these disprove some theories of the relation between the sun and the weather, in a way that no other set of meteorological records has done. while delicate determinations of the highest precision, such as those made at pulkova, are not yet undertaken to any great extent, a regular even if slow improvement is going on in the general character of the observations and researches, which must bear fruit in due time. one of the curious facts we learned at greenwich was that astronomy was still supposed to be astrology by many in england. that a belief in astrology should survive was perhaps not remarkable, though i do not remember to have seen any evidence of it in this country. but applications received at the royal observatory, from time to time, showed a widespread belief among the masses that one of the functions of the astronomer royal was the casting of horoscopes. we went to edinburgh. our first visit was to the observatory, then under the direction of professor c. piazzi smyth, who was also an egyptologist of repute, having made careful measurements of the pyramids, and brought out some new facts regarding their construction. he was thus led to the conclusion that they bore marks of having been built by a people of more advanced civilization than was generally supposed,--so advanced, indeed, that we had not yet caught up to them in scientific investigation. these views were set forth with great fullness in his work on "the antiquity of intellectual man," as well as in other volumes describing his researches. he maintained that the builders of the pyramids knew the distance of the sun rather better than we did, and that the height of the great pyramid had been so arranged that if it was multiplied by a thousand millions we should get this distance more exactly than we could measure it in these degenerate days. with him, to believe in the pyramid was to believe this, and a great deal more about the civilization which it proved. so, when he asked me whether i believed in the pyramid, i told him that i did not think i would depend wholly upon the pyramid for the distance of the sun to be used in astronomy, but should want its indications at least confirmed by modern researches. the hint was sufficient, and i was not further pressed for views on this subject. he introduced us to lady hamilton, widow of the celebrated philosopher, who still held court at edinburgh. the daughter of the family was in repute as a metaphysician. this was interesting, because i had never before heard of a female metaphysician, although there were several cases of female mathematicians recorded in history. first among them was donna maria agnesi, who wrote one of the best eighteenth-century books on the calculus, and had a special dispensation from the pope to teach mathematics at bologna. we were therefore very glad to accept an invitation from lady hamilton to spend an evening with a few of her friends. her rooms were fairly filled with books, the legacy of one of whom it was said that "scarcely a thought has come down to us through the ages which he has not mastered and made his own." the few guests were mostly university people and philosophers. the most interesting of them was professor blackie, the grecian scholar, who was the liveliest little man of sixty i ever saw; amusing us by singing german songs, and dancing about the room like a sprightly child among its playmates. i talked with miss hamilton about mill, whose "examination of sir william hamilton's philosophy" was still fresh in men's minds. of course she did not believe in this book, and said that mill could not understand her father's philosophy. with all her intellect, she was a fine healthy-looking young lady, and it was a sad surprise, a few years later, to hear of her death. madame sophie kovalevsky afterward appeared on the stage as the first female mathematician of our time, but it may be feared that the woman philosopher died with miss hamilton. a large party of english astronomers were going to algeria to observe the eclipse. the government had fitted up a naval transport for their use, and as i was arranging for a passage on a ship of the peninsular and oriental line we received an invitation to become the guests of the english party. among those on board were professor tyndall; mr. huggins, the spectroscopist; sir erastus ommaney, a retired english admiral, and a fellow of the royal society; father perry, s. j., a well-known astronomer; and lieutenant wharton, who afterward became hydrographer to the admiralty. the sprightliest man on board was professor tyndall. he made up for the absence of mountains by climbing to every part of the ship he could reach. one day he climbed the shrouds to the maintop, and stood surveying the scene as if looking out from the top of the matterhorn. a sailor followed him, and drew a chalk-line around his feet. i assume the reader knows what this means; if he does not, he can learn by straying into the sailors' quarters the first time he is on board an ocean steamer. but the professor absolutely refused to take the hint. we had a rather rough passage, from which father perry was the greatest sufferer. one day he heard a laugh from the only lady on board, who was in the adjoining stateroom. "who can laugh at such a time as this!" he exclaimed. he made a vow that he would never go on the ocean again, even if the sun and moon fought for a month. but the vows of a seasick passenger are forgotten sooner than any others i know of; and it was only four years later that father perry made a voyage to kerguelen island, in the stormiest ocean on the globe, to observe a transit of venus. off the coast of spain, the leading chains of the rudder got loose, during a gale in the middle of the night, and the steering apparatus had to be disconnected in order to tighten them. the ship veered round into the trough of the sea, and rolled so heavily that a table, twenty or thirty feet long, in the saloon, broke from its fastenings, and began to dance around the cabin with such a racket that some of the passengers feared for the safety of the ship. just how much of a storm there was i cannot say, believing that it is never worth while for a passenger to leave his berth, if there is any danger of a ship foundering in a gale. but in professor tyndall's opinion we had a narrow escape. on arriving at gibraltar, he wrote a glowing account of the storm to the london times, in which he described the feelings of a philosopher while standing on the stern of a rolling ship in an ocean storm, without quite knowing whether she was going to sink or swim. the letter was anonymous, which gave admiral ommaney an excellent opportunity to write as caustic a reply as he chose, under the signature of "a naval officer." he said that sailor was fortunate who could arrange with the clerk of the weather never to have a worse storm in crossing the bay of biscay than the one we had experienced. we touched at cadiz, and anchored for a few hours, but did not go ashore. the brooklyn, an american man-of-war, was in the harbor, but there was no opportunity to communicate with her, though i knew a friend of mine was on board. gibraltar is the greatest babel in the world, or, at least, the greatest i know. i wrote home: "the principal languages spoken at this hotel are english, spanish, moorish, french, italian, german, and danish. i do not know what languages they speak at the other hotels." moorish and spanish are the local tongues, and of course english is the official one; but the traders and commercial travelers speak nearly every language one ever heard. i hired a moor--who bore some title which indicated that he was a descendant of the caliphs, and by which he had to be addressed--to do chores and act as general assistant. one of the first things i did, the morning after my arrival, was to choose a convenient point on one of the stone parapets for "taking the sun," in order to test the running of my chronometer. i had some suspicion as to the result, but was willing to be amused. a sentinel speedily informed me that no sights were allowed to be taken on the fortification. i told him i was taking sights on the sun, not on the fortification. but he was inexorable; the rule was that no sights of any sort could be taken without a permit. i soon learned from mr. sprague, the american consul, who the proper officer was to issue the permit, which i was assured would be granted without the slightest difficulty. the consul presented me to the military governor of the place, general sir fenwick williams of kars. i did not know till long afterward that he was born very near where i was. he was a man whom it was very interesting to meet. his heroic defense of the town whose name was added to his own as a part of his title was still fresh in men's minds. it had won him the order of the bath in england, the grand cross of the legion of honor and a sword from napoleon iii., and the usual number of lesser distinctions. the military governor, the sole authority and viceroy of the queen in the fortress, is treated with the deference due to an exalted personage; but this deference so strengthens the dignity of the position that the holder may be frank and hearty at his own pleasure, without danger of impairing it. certainly, we found sir fenwick a most genial and charming gentleman. the alabama claims were then in their acute stage, and he expressed the earnest hope that the two nations would not proceed to cutting each other's throats over them. there was no need of troubling the governor with such a detail as that of a permit to take sights; but the consul ventured to relate my experience of the morning. he took the information in a way which showed that england, in making him a general, had lost a good diplomatist. instead of treating the matter seriously, which would have implied that we did not fully understand the situation, he professed to be greatly amused, and said it reminded him of the case of an old lady in "punch" who had to pass a surveyor in the street, behind a theodolite. "please, sir, don't shoot till i get past," she begged. before leaving england, i had made very elaborate arrangements, both with the astronomer royal and with the telegraph companies, to determine the longitude of gibraltar by telegraphic signals. the most difficult part of the operation was the transfer of the signals from the end of the land line into the cable, which had to be done by hand, because the cable companies were not willing to trust to an automatic action of any sort between the land line and the cable. it was therefore necessary to show the operator at the point of junction how signals were to be transmitted. this required a journey to port curno, at the very end of the land's end, several miles beyond the terminus of the railway. it was the most old-time place i ever saw; one might have imagined himself thrown back into the days of the lancasters. the thatched inn had a hard stone floor, with a layer of loose sand scattered over it as a carpet in the bedroom. my linguistic qualities were put to a severe test in talking with the landlady. but the cable operators were pleasing and intelligent young gentlemen, and i had no difficulty in making them understand how the work was to be done. the manager of the cable was sir james anderson, who had formerly commanded a cunard steamship from boston, and was well known to the harvard professors, with whom he was a favorite. i had met him, or at least seen him, at a meeting of the american academy ten years before, where he was introduced by one of his harvard friends. after commanding the ship that laid the first atlantic cable, he was made manager of the cable line from england to gibraltar. he gave me a letter to the head operator at gibraltar, the celebrated de sauty. i say "the celebrated," but may it not be that this appellation can only suggest the vanity of all human greatness? it just occurs to me that many of the present generation may not even have heard of the-- whispering boanerges, son of silent thunder, holding talk with nations, immortalized by holmes in one of his humorously scientific poems. during the two short weeks that the first atlantic cable transmitted its signals, his fame spread over the land, for the moment obscuring by its brilliancy that of thomson, field, and all others who had taken part in designing and laying the cable. on the breaking down of the cable he lapsed into his former obscurity. i asked him if he had ever seen holmes's production. he replied that he had received a copy of "the atlantic monthly" containing it from the poet himself, accompanied by a note saying that he might find in it something of interest. he had been overwhelmed with invitations to continue his journey from newfoundland to the united states and lecture on the cable, but was sensible enough to decline them. the rest of the story of the telegraphic longitude is short. the first news which de sauty had to give me was that the cable was broken,--just where, he did not know, and would not be able soon to discover. after the break was located, an unknown period would be required to raise the cable, find the place, and repair the breach. the weather, on the day of the eclipse, was more than half cloudy, so that i did not succeed in making observations of such value as would justify my waiting indefinitely for the repair of the cable, and the project of determining the longitude had to be abandoned. xi men and things in europe we went from gibraltar to berlin in january by way of italy. the mediterranean is a charming sea in summer, but in winter is a good deal like the atlantic. the cause of the blueness of its water is not completely settled; but its sharing this color with lake geneva, which is tinged with detritus from the shore, might lead one to ascribe it to substances held in solution. the color is noticeable even in the harbor of malta, to which we had a pleasant though not very smooth passage of five days. here was our first experience of an italian town of a generation ago. i had no sooner started to take a walk than a so-called guide, who spoke what he thought was english, got on my track, and insisted on showing me everything. if i started toward a shop, he ran in before me, invited me in, asked what i would like to buy, and told the shopman to show the gentleman something. i could not get rid of him till i returned to the hotel, and then he had the audacity to want a fee for his services. i do not think he got it. everything of interest was easily seen, and we only stopped to take the first italian steamer to messina. we touched at syracuse and catania, but did not land. Ætna, from the sea, is one of the grandest sights i ever saw. its snow-covered cone seems to rise on all sides out of the sea or the plain, and to penetrate the blue sky. in this it gives an impression like that of the weisshorn seen from randa, but gains by its isolation. at messina, of course, our steamer was visited by a commissionnaire, who asked me in good english whether i wanted a hotel. i told him that i had already decided upon a hotel, and therefore did not need his services. but it turned out that he belonged to the very hotel i was going to, and was withal an american, a native-born yankee, in fact, and so obviously honest that i placed myself unreservedly in his hands,--something which i never did with one of his profession before or since. he said the first thing was to get our baggage through the custom-house, which he could do without any trouble, at the cost of a franc. he was as good as his word. the italian custom-house was marked by primitive rigor, and baggage was commonly subjected to a very thorough search. but my man was evidently well known and fully trusted. i was asked to raise the lid of one trunk, which i did; the official looked at it, with his hands in his pockets, gave a nod, and the affair was over. my yankee friend collected one franc for that part of the business. he told us all about the place, changed our money so as to take advantage of the premium on gold, and altogether looked out for our interests in a way to do honor to his tribe. i thought there might be some curious story of the way in which a new englander of such qualities could have dropped into such a place, but it will have to be left to imagination. we reached the bay of naples in the morning twilight, after making an unsuccessful attempt to locate scylla and charybdis. if they ever existed, they must have disappeared. vesuvius was now and then lighting up the clouds with its intermittent flame. but we had passed a most uncomfortable night, and the morning was wet and chilly. a view requires something more than the objective to make it appreciated, and the effect of a rough voyage and bad weather was such as to deprive of all its beauty what is considered one of the finest views in the world. moreover, the experience made me so ill-natured that i was determined that the custom-house officer at the landing should have no fee from me. the only article that could have been subject to duty was on top of everything in the trunk, except a single covering of some loose garment, so that only a touch was necessary to find it. when it came to the examination, the officer threw the top till contemptuously aside, and devoted himself to a thorough search of the bottom. the only unusual object he stumbled upon was a spyglass inclosed in a shield of morocco. perhaps a gesture and a remark on my part aroused his suspicions. he opened the glass, tried to take it to pieces, inspected it inside and out, and was so disgusted with his failure to find anything contraband in it that he returned everything to the trunk, and let us off. it is commonly and quite justly supposed that the more familiar the traveler is with the language of the place he visits, the better he will get along. it is a common experience to find that even when you can pronounce the language, you cannot understand what is said. but there are exceptions to all rules, and circumstances now and then occur in which one thus afflicted has an advantage over the native. you can talk to him, while he cannot talk to you. there was an amusing case of this kind at munich. the only train that would take us to berlin before nightfall of the same day left at eight o'clock in the morning, by a certain route. there was at munich what we call a union station. i stopped at the first ticket-office where i saw the word "berlin" on the glass, asked for a ticket good in the train that was going to leave at eight o'clock the next morning for berlin, and took what the seller gave me. he was a stupid-looking fellow, so when i got to my hotel i showed the ticket to a friend. "that is not the ticket that you want at all," said he; "it will take you by a circuitous route in a train that does not leave until after nine, and you will not reach berlin until long after dark." i went directly back to the station and showed my ticket to the agent. "i--asked--you--for--a--ticket--good--in--the--train--which-- leaves--at--eight--o'--clock. this--ticket--is--not--good-- in--that--train. sie--haben--mich--betrügen. i--want--you-- to--take--the--ticket--back--and--return--me--the--money. what--you--say--can--i--not--understand." he expostulated, gesticulated, and fumed, but i kept up the bombardment until he had to surrender. he motioned to me to step round into the office, where he took the ticket and returned the money. i mention the matter because taking back a ticket is said to be quite unusual on a german railway. at berlin, the leading astronomers then, as now, were förster, director of the observatory, and auwers, permanent secretary of the academy of sciences. i was especially interested in the latter, as we had started in life nearly at the same time, and had done much work on similar lines. it was several days before i made his acquaintance, as i did not know that the rule on the continent is that the visitor must make the first call, or at least make it known by direct communication that he would be pleased to see the resident; otherwise it is presumed that he does not wish to see callers. this is certainly the more logical system, but it is not so agreeable to the visiting stranger as ours is. the art of making the latter feel at home is not brought to such perfection on the continent as in england; perhaps the french understand it less than any other people. but none can be pleasanter than the germans, when you once make their acquaintance; and we shall always remember with pleasure the winter we passed in berlin. to-day, auwers stands at the head of german astronomy. in him is seen the highest type of the scientific investigator of our time, one perhaps better developed in germany than in any other country. the work of men of this type is marked by minute and careful research, untiring industry in the accumulation of facts, caution in propounding new theories or explanations, and, above all, the absence of effort to gain recognition by being the first to make a discovery. when men are ambitious to figure as newtons of some great principle, there is a constant temptation to publish unverified speculations which are likely rather to impede than to promote the advance of knowledge. the result of auwers's conscientiousness is that, notwithstanding his eminence in his science, there are few astronomers of note whose works are less fitted for popular exposition than his. his specialty has been the treatment of all questions concerning the positions and motions of the stars. this work has required accurate observations of position, with elaborate and careful investigations of a kind that offer no feature to attract public attention, and only in exceptional cases lead to conclusions that would interest the general reader. he considers no work as ready for publication until it is completed in every detail. the old astronomical observations of which i was in quest might well have been made by other astronomers than those of paris, so while awaiting the end of the war i tried to make a thorough search of the writings of the mediæval astronomers in the royal library. if one knew exactly what books he wanted, and had plenty of time at his disposal, he would find no difficulty in consulting them in any of the great continental libraries. but at the time of my visit, notwithstanding the cordiality with which all the officials, from professor lepsius down, were disposed to second my efforts, the process of getting any required book was very elaborate. although one could obtain a book on the same day he ordered it, if he went in good time, it was advisable to leave the order the day before, if possible. when, as in the present case, one book only suggests another, this a third, and so on, in an endless chain, the carrying on of an extended research is very tedious. one feature of the library strongly impressed me with the comparatively backward state of mathematical science in our own country. as is usual in the great european libraries, those books which are most consulted are placed in the general reading-room, where any one can have access to them, at any moment. it was surprising to see amongst these books a set of crelle's "journal of mathematics," and to find it well worn by constant use. at that time, so far as i could learn, there were not more than two or three sets of the journal in the united states; and these were almost unused. even the library of congress did not contain a set. there has been a great change since that time,--a change in which the johns hopkins university took the lead, by inviting sylvester to this country, and starting a mathematical school of the highest grade. other universities followed its example to such an extent that, to-day, an american student need not leave his own country to hear a master in any branch of mathematics. i believe it was dr. b. a. gould who called the pulkova observatory the astronomical capital of the world. this institution was founded in by the emperor nicholas, on the initiative of his greatest astronomer. it is situated some twelve miles south of st. petersburg, not far from the railway between that city and berlin, and gets its name from a peasant village in the neighborhood. from its foundation it has taken the lead in exact measurements relating to the motion of the earth and the positions of the principal stars. an important part of its equipment is an astronomical library, which is perhaps the most complete in existence. this, added to all its other attractions, induced me to pay a visit to pulkova. otto struve, the director, had been kind enough to send me a message, expressing the hope that i would pay him a visit, and giving directions about telegraphing in advance, so as to insure the delivery of the dispatch. the time from berlin to st. petersburg is about forty-eight hours, the only through train leaving and arriving in the evening. on the morning of the day that the train was due i sent the dispatch. early in the afternoon, as the train was stopping at a way station, i saw an official running hastily from one car to another, looking into each with some concern. when he came to my door, he asked if i had sent a telegram to estafetta. i told him i had. he then informed me that estafetta had not received it. but the train was already beginning to move, so there was no further chance to get information. the comical part of the matter was that "estafetta" merely means a post or postman, and that the directions, as struve had given them, were to have the dispatch sent by postman from the station to pulkova. it was late in the evening when the train reached zarsko-selo, the railway station for pulkova, which is about five miles away. the station-master told me that no carriage from pulkova was waiting for me, which tended to confirm the fear that the dispatch had not been received. after making known my plight, i took a seat in the station and awaited the course of events, in some doubt what to do. only a few minutes had elapsed when a good-looking peasant, well wrapped in a fur overcoat, with a whip in his hand, looked in at the door, and pronounced very distinctly the words, "observatorio pulkova." ah! this is struve's driver at last, thought i, and i followed the man to the door. but when i looked at the conveyance, doubt once more supervened. it was scarcely more than a sledge, and was drawn by a single horse, evidently more familiar with hard work than good feeding. this did not seem exactly the vehicle that the great russian observatory would send out to meet a visitor; yet it was a far country, and i was not acquainted with its customs. the way in which my doubt was dispelled shows that there is one subject besides love on which difference of language is no bar to the communication of ideas. this is the desire of the uncivilized man for a little coin of the realm. in south africa, zulu chiefs, who do not know one other word of english, can say "shilling" with unmistakable distinctness. my russian driver did not know even this little english word, but he knew enough of the universal language. when we had made a good start on the snow-covered prairie, he stopped his horse for a moment, looked round at me inquiringly, raised his hand, and stretched out two fingers so that i could see them against the starlit sky. i nodded assent. then he drew his overcoat tightly around him with a gesture of shivering from the cold, beat his hands upon his breast as if to warm it, and again looked inquiringly at me. i nodded again. the bargain was complete. he was to have two rubles for the drive, and a little something to warm up his shivering breast. so he could not be struve's man. there is no welcome warmer than a russian one, and none in any country warmer than that which the visiting astronomer receives at an observatory. great is the contrast between the winter sky of a clear moonless night and the interior of a dining-room, forty feet square, with a big blazing fire at one end and a table loaded with eatables in the middle. the fact that the visitor had never before met one of his hosts detracted nothing from the warmth of his reception. the organizer of the observatory, and its first director, was wilhelm struve, father of the one who received me, and equally great as man and astronomer. like many other good russians, he was the father of a large family. one of his sons was for ten years the russian minister at washington, and as popular a diplomatist as ever lived among us. the instruments which struve designed sixty years ago still do as fine work as any in the world; but one may suspect this to be due more to the astronomers who handle them than to the instruments themselves. the air is remarkably clear; the entrance to st. petersburg, ten or twelve miles north, is distinctly visible, and struve told me that during the crimean war he could see, through the great telescope, the men on the decks of the british ships besieging kronstadt, thirty miles away. one drawback from which the astronomers suffer is the isolation of the place. the village at the foot of the little hill is inhabited only by peasants, and the astronomers and employees have nearly all to be housed in the observatory buildings. there is no society but their own nearer than the capital. at the time of my visit the scientific staff was almost entirely german or swedish, by birth or language. in the state, two opposing parties are the russian, which desires the ascendency of the native muscovites, and the german, which appreciates the fact that the best and most valuable of the tsar's subjects are of german or other foreign descent. during the past twenty years the russian party has gradually got the upper hand; and the result of this ascendency at pulkova will be looked for with much solicitude by astronomers everywhere. once a year the lonely life of the astronomers is enlivened by a grand feast--that of the russian new year. one object of the great dining-room which i have mentioned, the largest room, i believe, in the whole establishment, was to make this feast possible. my visit took place early in march, so that i did not see the celebration; but from what i have heard, the little colony does what it can to make up for a year of ennui. every twenty-five years it celebrates a jubilee; the second came off in . there is much to interest the visitor in a russian peasant village, and that of pulkova has features some of which i have never seen described. above the door of each log hut is the name of the occupant, and below the name is a rude picture of a bucket, hook, or some other piece of apparatus used in extinguishing fire. inside, the furniture is certainly meagre enough, yet one could not see why the occupants should be otherwise than comfortable. i know of no good reason why ignorance should imply unhappiness; altogether, there is some good room for believing that the less civilized races can enjoy themselves, in their own way, about as well as we can. what impressed me as the one serious hardship of the peasantry was their hours of labor. just how many hours of the twenty-four these beings find for sleep was not clear to the visitor; they seemed to be at work all day, and at midnight many of them had to start on their way to st. petersburg with a cartload for the market. a church ornamented with tinsel is a feature of every russian village; so also are the priests. the only two i saw were sitting on a fence, wearing garments that did not give evidence of having known water since they were made. one great drawback to the growth of manufactures in russia is the number of feast days, on which the native operators must one and all abandon their work, regardless of consequences. the astronomical observations made at pulkova are not published annually, as are those made at most of the other national observatories; but a volume relating to one subject is issued whenever the work is done. when i was there, the volumes containing the earlier meridian observations were in press. struve and his chief assistant, dr. wagner, used to pore nightly over the proof sheets, bestowing on every word and detail a minute attention which less patient astronomers would have found extremely irksome. dr. wagner was a son-in-law of hansen, the astronomer of the little ducal observatory at gotha, as was also our bayard taylor. my first meeting with hansen, which occurred after my return to berlin, was accompanied with some trepidation. modest as was the public position that he held, he may now fairly be considered the greatest master of celestial mechanics since laplace. in what order leverrier, delaunay, adams, and hill should follow him, it is not necessary to decide. to many readers it will seem singular to place any name ahead of that of the master who pointed out the position of neptune before a human eye had ever recognized it. but this achievement, great as it was, was more remarkable for its boldness and brilliancy than for its inherent difficulty. if the work had to be done over again to-day, there are a number of young men who would be as successful as leverrier; but there are none who would attempt to reinvent the methods of hansen, or even to improve radically upon them. their main feature is the devising of new and refined methods of computing the variations in the motions of a planet produced by the attraction of all the other planets. as laplace left this subject, the general character of these variations could be determined without difficulty, but the computations could not be made with mathematical exactness. hansen's methods led to results so precise that, if they were fully carried out, it is doubtful whether any deviation between the predicted and the observed motions of a planet could be detected by the most refined observation. at the time of my visit mrs. wagner was suffering from a severe illness, of which the crisis passed while i was at pulkova, and left her, as was supposed, on the road to recovery. i was, of course, very desirous of meeting so famous a man as hansen. he was expected to preside at a session of the german commission on the transit of venus, which was to be held in berlin about the time of my return thither from pulkova. the opportunity was therefore open of bringing a message of good news from his daughter. apart from this, the prospect of the meeting might have been embarrassing. the fact is that i was at odds with him on a scientific question, and he was a man who did not take a charitable view of those who differed from him in opinion. he was the author of a theory, current thirty or forty years ago, that the farther side of the moon is composed of denser materials than the side turned toward us. as a result of this, the centre of gravity of the moon was supposed to be farther from us than the actual centre of her globe. it followed that, although neither atmosphere nor water existed on our side of the moon, the other side might have both. here was a very tempting field into which astronomical speculators stepped, to clothe the invisible hemisphere of the moon with a beautiful terrestrial landscape, and people it as densely as they pleased with beings like ourselves. if these beings should ever attempt to explore the other half of their own globe, they would find themselves ascending to a height completely above the limits of their atmosphere. hansen himself never countenanced such speculations as these, but confined his claims to the simple facts he supposed proven. in i had published a little paper showing what i thought a fatal defect, a vicious circle in fact, in hansen's reasoning on this subject. not long before my visit, delaunay had made this paper the basis of a communication to the french academy of sciences, in which he not only indorsed my views, but sought to show the extreme improbability of hansen's theory on other grounds. when i first reached germany, on my way from italy, i noticed copies of a blue pamphlet lying on the tables of the astronomers. apparently, the paper had been plentifully distributed; but it was not until i reached berlin that i found it was hansen's defense against my strictures,--a defense in which mathematics were not unmixed with seething sarcasm at the expense of both delaunay and myself. the case brought to mind a warm discussion between hansen and encke, in the pages of a scientific journal, some fifteen years before. at the time it had seemed intensely comical to see two enraged combatants--for so i amused myself by fancying them--hurling algebraic formulæ, of frightful complexity, at each other's heads. i did not then dream that i should live to be an object of the same sort of attack, and that from hansen himself. to be revised, pulled to pieces, or superseded, as science advances, is the common fate of most astronomical work, even the best. it does not follow that it has been done in vain; if good, it forms a foundation on which others will build. but not every great investigator can look on with philosophic calm when he sees his work thus treated, and hansen was among the last who could. under these circumstances, it was a serious question what sort of reception hansen would accord to a reviser of his conclusions who should venture to approach him. i determined to assume an attitude that would show no consciousness of offense, and was quite successful. our meeting was not attended by any explosion; i gave him the pleasant message with which i was charged from his daughter, and, a few days later, sat by his side at a dinner of the german commission on the coming transit of venus. as hansen was germany's greatest master in mathematical astronomy, so was the venerable argelander in the observational side of the science. he was of the same age as the newly crowned emperor, and the two were playmates at the time germany was being overrun by the armies of napoleon. he was held in love and respect by the entire generation of young astronomers, both germans and foreigners, many of whom were proud to have had him as their preceptor. among these was dr. b. a. gould, who frequently related a story of the astronomer's wit. when with him as a student, gould was beardless, but had a good head of hair. returning some years later, he had become bald, but had made up for it by having a full, long beard. he entered argelander's study unannounced. at first the astronomer did not recognize him. "do you not know me, herr professor?" the astronomer looked more closely. "mine gott! it is gould mit his hair struck through!" argelander was more than any one else the founder of that branch of his science which treats of variable stars. his methods have been followed by his successors to the present time. it was his policy to make the best use he could of the instruments at his disposal, rather than to invent new ones that might prove of doubtful utility. the results of his work seem to justify this policy. we passed the last month of the winter in berlin waiting for the war to close, so that we could visit paris. poor france had at length to succumb, and in the latter part of march, we took almost the first train that passed the lines. delaunay was then director of the paris observatory, having succeeded leverrier when the emperor petulantly removed the latter from his position. i had for some time kept up an occasional correspondence with delaunay, and while in england, the autumn before, had forwarded a message to him, through the prussian lines, by the good offices of the london legation and mr. washburn. he was therefore quite prepared for our arrival. the evacuation of a country by a hostile army is rather a slow process, so that the german troops were met everywhere on the road, even in france. they had left paris just before we arrived; but the french national army was not there, the communists having taken possession of the city as fast as the germans withdrew. as we passed out of the station, the first object to strike our eyes was a flaming poster addressed to "citoyens," and containing one of the manifestoes which the communist government was continually issuing. of course we made an early call on mr. washburn. his career in paris was one of the triumphs of diplomacy; he had cared for the interests of german subjects in paris in such a way as to earn the warm recognition both of the emperor and of bismarck, and at the same time had kept on such good terms with the french as to be not less esteemed by them. he was surprised that we had chosen such a time to visit paris; but i told him the situation, the necessity of my early return home, and my desire to make a careful search in the records of the paris observatory for observations made two centuries ago. he advised us to take up our quarters as near to the observatory as convenient, in order that we might not have to pass through the portions of the city which were likely to be the scenes of disturbance. we were received at the observatory with a warmth of welcome that might be expected to accompany the greeting of the first foreign visitor, after a siege of six months. yet a tinge of sadness in the meeting was unavoidable. delaunay immediately began lamenting the condition of his poor ruined country, despoiled of two of its provinces by a foreign foe, condemned to pay an enormous subsidy in addition, and now the scene of an internal conflict the end of which no one could foresee. while i was mousing among the old records of the paris observatory, the city was under the reign of the commune and besieged by the national forces. the studies had to be made within hearing of the besieging guns; and i could sometimes go to a window and see flashes of artillery from one of the fortifications to the south. nearly every day i took a walk through the town, occasionally as far as the arc de triomphe. the story of the commune has been so often written that i cannot hope to add anything to it, so far as the main course of events is concerned. looking back on a sojourn at so interesting a period, one cannot but feel that a golden opportunity to make observations of historic value was lost. the fact is, however, that i was prevented from making such observations not only by my complete absorption in my work, but by the consideration that, being in what might be described as a semi-official capacity, i did not want to get into any difficulty that would have compromised the position of an official visitor. i should not deem what we saw worthy of special mention, were it not that it materially modifies the impressions commonly given by writers on the history of the commune. what an historian says may be quite true, so far as it goes, and yet may be so far from the whole truth as to give the reader an incorrect impression of the actual course of events. the violence and disease which prevail in the most civilized country in the world may be described in such terms as to give the impression of a barbarous community. the murder of the archbishop of paris and of the hostages show how desperate were the men who had seized power, yet the acts of these men constitute but a small part of the history of paris during that critical period. what one writes at the time is free from the suspicion that may attach to statements not recorded till many years after the events to which they relate. the following extract from a letter which i wrote to a friend, the day after my arrival, may therefore be taken to show how things actually looked to a spectator:-- dear charlie,--here we are, on this slumbering volcano. perhaps you will hear of the burst-up long before you get this. we have seen historic objects which fall not to the lot of every generation, the barricades of the paris streets. as we were walking out this morning, the pavement along one side of the street was torn up for some distance, and used to build a temporary fort. said fort would be quite strong against musketry or the bayonet; but with heavy shot against it, i should think it would be far worse than nothing, for the flying stones would kill more than the balls. the streets are placarded at every turn with all sorts of inflammatory appeals, and general orders of the comité central or of the commune. one of the first things i saw last night was a large placard beginning "citoyens!" among the orders is one forbidding any one from placarding any orders of the versailles government under the severest penalties; and another threatening with instant dismissal any official who shall recognize any order issuing from the said government. i must do all hands the justice to say that they are all very well behaved. there is nothing like a mob anywhere, so far as i can find. i consulted my map this morning, right alongside the barricade and in full view of the builders, without being molested, and wife and i walked through the insurrectionary districts without being troubled or seeing the slightest symptoms of disturbance. the stores are all open, and every one seems to be buying and selling as usual. in all the cafés i have seen, the habitués seem to be drinking their wine just as coolly as if they had nothing unusual on their minds. from this date to that of our departure i saw nothing suggestive of violence within the limited range of my daily walks, which were mostly within the region including the arc de triomphe, the hôtel de ville, and the observatory; the latter being about half a mile south of the luxembourg. the nearest approach to a mob that i ever noticed was a drill of young recruits of the national guard, or a crowd in the court of the louvre being harangued by an orator. with due allowance for the excitability of the french nature, the crowd was comparatively as peaceable as that which we may see surrounding a gospel wagon in one of our own cities. a drill-ground for the recruits happened to be selected opposite our first lodgings, beside the gates of the luxembourg. this was so disagreeable that we were glad to accept an invitation from delaunay to be his guests at the observatory, during the remainder of our stay. we had not been there long before the spacious yard of the observatory was also used as a drill-ground; and yet later, two or three men were given _billets de logement_ upon the observatory; but i should not have known of the latter occurrence, had not delaunay told me. i believe he bought the men off, much as one pays an organ-grinder to move on. in one of our walks we entered the barricade around the hôtel de ville, and were beginning to make a close examination of a mitrailleuse, when a soldier (beg his pardon, _un citoyen membre de la garde nationale_) warned us away from the weapon. the densest crowd of communists was along the rue de rivoli and in the region of the colonne vendôme, where some of the principal barricades were being erected. but even here, not only were the stores open as usual, but the military were doing their work in the midst of piles of trinkets exposed for sale on the pavement by the shopwomen. the order to destroy the column was issued before we left, but not executed until later. i have no reason to suppose that the shopwomen were any more concerned while the column was being undermined than they were before. to complete the picture, not a policeman did we see in paris; in fact, i was told that one of the first acts of the commune had been to drive the police away, so that not one dared to show himself. an interesting feature of the sad spectacle was the stream of proclamations poured forth by the communist authorities. they comprised not only decrees, but sensational stories of victories over the versailles troops, denunciations of the versailles government, and even elaborate legal arguments, including a not intemperate discussion of the ethical question whether citizens who were not adherents of the commune should be entitled to the right of suffrage. the conclusion was that they should not. the lack of humor on the part of the authorities was shown by their commencing one of a rapid succession of battle stories with the words, "citoyens! vous avez soif de la vérité!" the most amusing decree i noticed ran thus:-- "article i. all conscription is abolished. "article ii. no troops shall hereafter be allowed in paris, except the national guard. "article iii. every citizen is a member of the national guard." we were in daily expectation and hope of the capture of the city, little imagining by what scenes it would be accompanied. it did not seem to my unmilitary eye that two or three batteries of artillery could have any trouble in demolishing all the defenses, since a wall of paving-stones, four or five feet high, could hardly resist solid shot, or prove anything but a source of destruction to those behind it if attacked by artillery. but the capture was not so easy a matter as i had supposed. we took leave of our friend and host on may , three weeks before the final catastrophe, of which he wrote me a graphic description. as the barricades were stormed by macmahon, the communist line of retreat was through the region of the observatory. the walls of the building and of the yard were so massive that the place was occupied as a fort by the retreating forces, so that the situation of the few non-combatants who remained was extremely critical. they were exposed to the fire of their friends, the national troops, from without, while enraged men were threatening their lives within. so hot was the fusillade that, going into the great dome after the battle, the astronomer could imagine all the constellations of the sky depicted by the bullet-holes. when retreat became inevitable, the communists tried to set the building on fire, but did not succeed. then, in their desperation, arrangements were made for blowing it up; but the most violent man among them was killed by a providential bullet, as he was on the point of doing his work. the remainder fled, the place was speedily occupied by the national troops, and the observatory with its precious contents was saved. the academy of sciences had met regularly through the entire prussian siege. the legal quorum being three, this did not imply a large attendance. the reason humorously assigned for this number was that, on opening a session, the presiding officer must say, _messieurs, la séance est ouverte_, and he cannot say _messieurs_ unless there are at least two to address. at the time of my visit a score of members were in the city. among them were elie de beaumont, the geologist; milne-edwards, the zoölogist; and chevreul, the chemist. i was surprised to learn that the latter was in his eighty-fifth year; he seemed a man of seventy or less, mentally and physically. yet we little thought that he would be the longest-lived man of equal eminence that our age has known. when he died, in , he was nearly one hundred and three years old. born in , he had lived through the whole french revolution, and was seven years old at the time of the terror. his scientific activity, from beginning to end, extended over some eighty years. when i saw him, he was still very indignant at a bombardment of the jardin des plantes by the german besiegers. he had made a formal statement of this outrage to the academy of sciences, in order that posterity might know what kind of men were besieging paris. i suggested that the shells might have fallen in the place by accident; but he maintained that it was not the case, and that the bombardment was intentional. the most execrated man in the scientific circle at this time was leverrier. he had left paris before the prussian siege began, and had not returned. delaunay assured me that this was a wise precaution on his part; for had he ventured into the city he would have been mobbed, or the communists would have killed him as soon as caught. just why the mob should have been so incensed against one whose life was spent in the serenest fields of astronomical science was not fully explained. the fact that he had been a senator, and was politically obnoxious, was looked on as an all-sufficient indictment. even members of the academy could not suppress their detestation of him. their language seemed not to have words that would fully express their sense of his despicable meanness, not to say turpitude. four years later i was again in paris, and attended a meeting of the academy of sciences. in the course of the session a rustle of attention spread over the room, as all eyes were turned upon a member who was entering rather late. looking toward the door, i saw a man of sixty, a decided blond, with light chestnut hair turning gray, slender form, shaven face, rather pale and thin, but very attractive, and extremely intellectual features. as he passed to his seat hands were stretched out on all sides to greet him, and not until he sat down did the bustle caused by his entrance subside. he was evidently a notable. "who is that?" i said to my neighbor. "leverrier." delaunay was one of the most kindly and attractive men i ever met. we spent our evenings walking in the grounds of the observatory, discussing french science in all its aspects. his investigation of the moon's motion is one of the most extraordinary pieces of mathematical work ever turned out by a single person. it fills two quarto volumes, and the reader who attempts to go through any part of the calculations will wonder how one man could do the work in a lifetime. his habit was to commence early in the morning, and work with but little interruption until noon. he never worked in the evening, and generally retired at nine. i felt some qualms of conscience at the frequency with which i kept him up till nearly ten. i found it hopeless to expect that he would ever visit america, because he assured me that he did not dare to venture on the ocean. the only voyage he had ever made was across the channel, to receive the gold medal of the royal astronomical society for his work. two of his relatives--his father and, i believe, his brother--had been drowned, and this fact gave him a horror of the water. he seemed to feel somewhat like the clients of the astrologists, who, having been told from what agencies they were to die, took every precaution to avoid them. i remember, as a boy, reading a history of astrology, in which a great many cases of this sort were described; the peculiarity being that the very measures which the victim took to avoid the decree of fate became the engines that executed it. the death of delaunay was not exactly a case of this kind, yet it could not but bring it to mind. he was at cherbourg in the autumn of . as he was walking on the beach with a relative, a couple of boatmen invited them to take a sail. through what inducement delaunay was led to forget his fears will never be known. all we know is that he and his friend entered the boat, that it was struck by a sudden squall when at some distance from the land, and that the whole party were drowned. there was no opposition to the reappointment of leverrier to his old place. in fact, at the time of my visit, delaunay said that president thiers was on terms of intimate friendship with the former director, and he thought it not at all unlikely that the latter would succeed in being restored. he kept the position with general approval till his death in . the only occasion on which i met leverrier was after the incident i have mentioned, in the academy of sciences. i had been told that he was incensed against me on account of an unfortunate remark i had made in speaking of his work which led to the discovery of neptune. i had heard this in germany as well as in france, yet the matter was so insignificant that i could hardly conceive of a man of philosophic mind taking any notice of it. i determined to meet him, as i had met hansen, with entire unconsciousness of offense. so i called on him at the observatory, and was received with courtesy, but no particular warmth. i suggested to him that now, as he had nearly completed his work on the tables of the planets, the question of the moon's motion would be the next object worthy of his attention. he replied that it was too large a subject for him to take up. to leverrier belongs the credit of having been the real organizer of the paris observatory. his work there was not dissimilar to that of airy at greenwich; but he had a much more difficult task before him, and was less fitted to grapple with it. when founded by louis xiv. the establishment was simply a place where astronomers of the academy of sciences could go to make their observations. there was no titular director, every man working on his own account and in his own way. cassini, an italian by birth, was the best known of the astronomers, and, in consequence, posterity has very generally supposed he was the director. that he failed to secure that honor was not from any want of astuteness. it is related that the monarch once visited the observatory to see a newly discovered comet through the telescope. he inquired in what direction the comet was going to move. this was a question it was impossible to answer at the moment, because both observations and computations would be necessary before the orbit could be worked out. but cassini reflected that the king would not look at the comet again, and would very soon forget what was told him; so he described its future path in the heavens quite at random, with entire confidence that any deviation of the actual motion from his prediction would never be noted by his royal patron. one of the results of this lack of organization has been that the paris observatory does not hold an historic rank correspondent to the magnificence of the establishment. the go-as-you-please system works no better in a national observatory than it would in a business institution. up to the end of the last century, the observations made there were too irregular to be of any special importance. to remedy this state of things, arago was appointed director early in the present century; but he was more eminent in experimental physics than in astronomy, and had no great astronomical problem to solve. the result was that while he did much to promote the reputation of the observatory in the direction of physical investigation, he did not organize any well-planned system of regular astronomical work. when leverrier succeeded arago, in , he had an extremely difficult problem before him. by a custom extending through two centuries, each astronomer was to a large extent the master of his own work. leverrier undertook to change all this in a twinkling, and, if reports are true, without much regard to the feelings of the astronomers. those who refused to fall into line either resigned or were driven away, and their places were filled with men willing to work under the direction of their chief. yet his methods were not up to the times; and the work of the paris observatory, so far as observations of precision go, falls markedly behind that of greenwich and pulkova. in recent times the institution has been marked by an energy and a progressiveness that go far to atone for its former deficiencies. the successors of leverrier have known where to draw the line between routine, on the one side, and initiative on the part of the assistants, on the other. probably no other observatory in the world has so many able and well-trained young men, who work partly on their own account, and partly in a regular routine. in the direction of physical astronomy the observatory is especially active, and it may be expected in the future to justify its historic reputation. xii the old and the new washington a few features of washington as it appeared during the civil war are indelibly fixed in my memory. an endless train of army wagons ploughed its streets with their heavy wheels. almost the entire southwestern region, between the war department and the potomac, extending west on the river to the neighborhood of the observatory, was occupied by the quartermaster's and subsistence departments for storehouses. among these the astronomers had to walk by day and night, in going to and from their work. after a rain, especially during winter and spring, some of the streets were much like shallow canals. under the attrition of the iron-bound wheels the water and clay were ground into mud, which was at first almost liquid. it grew thicker as it dried up, until perhaps another rainstorm reduced it once more to a liquid condition. in trying first one street and then another to see which offered the fewest obstacles to his passage, the wayfarer was reminded of the assurance given by a bright boy to a traveler who wanted to know the best road to a certain place: "whichever road you take, before you get halfway there you'll wish you had taken t' other." by night swarms of rats, of a size proportional to their ample food supply, disputed the right of way with the pedestrian. across the potomac, arlington heights were whitened by the tents of soldiers, from which the discharges of artillery or the sound of the fife and drum became so familiar that the dweller almost ceased to notice it. the city was defended by a row of earthworks, generally not far inside the boundary line of the district of columbia, say five or six miles from the central portions of the city. one of the circumstances connected with their plans strikingly illustrates the exactness which the science or art of military engineering had reached. of course the erection of fortifications was one of the first tasks to be undertaken by the war department. plans showing the proposed location and arrangements of the several forts were drawn up by a board of army engineers, at whose head, then or afterward, stood general john g. barnard. when the plans were complete, it was thought advisable to test them by calling in the advice of professor d. h. mahan of the military academy at west point. he came to washington, made a careful study of the maps and plans, and was then driven around the region of the lines to be defended to supplement his knowledge by personal inspection. then he laid down his ideas as to the location of the forts. there were but two variations from the plans proposed by the board of engineers, and these were not of fundamental importance. willard's hotel, then the only considerable one in the neighborhood of the executive offices, was a sort of headquarters for arriving army officers, as well as for the thousands of civilians who had business with the government, and for gossip generally. inside its crowded entrance one could hear every sort of story, of victory or disaster, generally the latter, though very little truth was ever to be gleaned. the newsboy flourished. he was a bright fellow too, and may have developed into a man of business, a reporter, or even an editor. "another great battle!" was his constant cry. but the purchaser of his paper would commonly read of nothing but a skirmish or some fresh account of a battle fought several days before--perhaps not even this. on one occasion an officer in uniform, finding nothing in his paper to justify the cry, turned upon the boy with the remark,-- "look here, boy, i don't see any battle here." "no," was the reply, "nor you won't see one as long as you hang around washington. if you want to see a battle you must go to the front." the officer thought it unprofitable to continue the conversation, and beat a retreat amid the smiles of the bystanders. this story, i may remark, is quite authentic, which is more than one can say of the report that a stick thrown by a boy at a dog in front of willard's hotel struck twelve brigadier generals during its flight. the presiding genius of the whole was mr. edwin m. stanton, secretary of war. before the actual outbreak of the conflict he had been, i believe, at least a democrat, and, perhaps, to a certain extent, a southern sympathizer so far as the slavery question was concerned. but when it came to blows, he espoused the side of the union, and after being made secretary of war he conducted military operations with a tireless energy, which made him seem the impersonation of the god of war. ordinarily his character seemed almost savage when he was dealing with military matters. he had no mercy on inefficiency or lukewarmness. but his sympathetic attention, when a case called for it, is strikingly shown in the following letter, of which i became possessed by mere accident. at the beginning of the war mr. charles ellet, an eminent engineer, then resident near washington, tendered his services to the government, and equipped a fleet of small river steamers on the mississippi under the war department. in the battle of june , , he received a wound from which he died some two weeks later. his widow sold or leased his house on georgetown heights, and i boarded in it shortly afterward. amongst some loose rubbish and old papers lying around in one of the rooms i picked up the letter which follows. war department, washington city, d. c., june , . dear madam,--i understand from mr. ellet's dispatch to you that as he will be unfit for duty for some time it will be agreeable to him for you to visit him, traveling slowly so as not to expose your own health. with this view i will afford you every facility within the control of the department, by way of pittsburg and cincinnati to cairo, where he will probably meet you. yours truly, edwin m. stanton, _secretary of war._ the interesting feature of this letter is that it is entirely in the writer's autograph, and bears no mark of having been press copied. i infer that it was written out of office hours, after all the clerks had left the department, perhaps late at night, while the secretary was taking advantage of the stillness of the hour to examine papers and plans. only once did i come into personal contact with mr. stanton. a portrait of ferdinand r. hassler, first superintendent of the coast survey, had been painted about by captain williams of the corps of engineers, u. s. a., a son-in-law of mr. g. w. p. custis, and therefore a brother-in-law of general lee. the picture at the arlington house was given to mrs. colonel abert, who loaned it to mr. custis. when the civil war began she verbally donated it to my wife, who was mr. hassler's grand-daughter, and was therefore considered the most appropriate depositary of it, asking her to get it if she could. but before she got actual possession of it, the arlington house was occupied by our troops and mr. stanton ordered the picture to be presented to professor agassiz for the national academy of sciences. on hearing of this, i ventured to mention the matter to mr. stanton, with a brief statement of our claims upon the picture. "sir," said he, "that picture was found in the house of a rebel in arms [general robert e. lee], and was justly a prize of war. i therefore made what i considered the most appropriate disposition of it, by presenting it to the national academy of sciences." the expression "house of a rebel in arms" was uttered with such emphasis that i almost felt like one under suspicion of relations with the enemy in pretending to claim the object in question. it was clearly useless to pursue the matter any further at that time. some years later, when the laws were no longer silent, the national academy decided that whoever might be the legal owner of the picture, the academy could have no claim upon it, and therefore suffered it to pass into the possession of the only claimant. among the notable episodes of the civil war was the so-called raid of the confederate general, early, in july, . he had entered maryland and defeated general lew wallace. this left nothing but the well-designed earthworks around washington between his army and our capital. some have thought that, had he immediately made a rapid dash, the city might have fallen into his hands. all in the service of the war and navy departments who were supposed capable of rendering efficient help, were ordered out to take part in the defense of the city, among them the younger professors of the observatory. by order of captain gilliss i became a member of a naval brigade, organized in the most hurried manner by admiral goldsborough, and including in it several officers of high and low rank. the rank and file was formed of the workmen in the navy yard, most of whom were said to have seen military service of one kind or another. the brigade formed at the navy yard about the middle of the afternoon, and was ordered to march out to fort lincoln, a strong earthwork built on a prominent hill, half a mile southwest of the station now known as rives. the reform school of the district of columbia now stands on the site of the fort. the position certainly looked very strong. on the right the fort was flanked by a deep intrenchment running along the brow of the hill, and the whole line would include in the sweep of its fire the region which an army would have to cross in order to enter the city. the naval brigade occupied the trench, while the army force, which seemed very small in numbers, manned the front. i was not assigned to any particular duty, and simply walked round the place in readiness to act whenever called upon. i supposed the first thing to be done was to have the men in the trench go through some sort of drill, in order to assure their directing the most effective fire on the enemy should he appear. the trench was perhaps six feet deep; along its bottom ran a little ledge on which the men had to step in order to deliver their fire, stepping back into the lower depth to load again. along the edge was a sort of rail fence, the bottom rail of which rested on the ground. in order to fire on an enemy coming up the hill, it would be necessary to rest the weapon on this bottom rail. it was quite evident to me that a man not above the usual height, standing on the ledge, would have to stand on tiptoe in order to get the muzzle of his gun properly directed down the slope. if he were at all flurried he would be likely to fire over the head of the enemy. i called attention to this state of things, but did not seem to make any impression on the officers, who replied that the men had seen service and knew what to do. we bivouacked that night, and remained all the next day and the night following awaiting the attack of the enemy, who was supposed to be approaching fort stevens on the seventh street road. at the critical moment, general h. g. wright arrived from fort monroe with his army corps. he and general a. mcd. mccook both took their stations at fort lincoln, which it was supposed would be the point of attack. a quarter or half a mile down the hill was the mansion of the rives family, which a passenger on the baltimore and ohio railway can readily see at the station of that name. a squad of men was detailed to go to this house and destroy it, in case the enemy should appear. the attack was expected at daybreak, but general early, doubtless hearing of the arrival of reinforcements, abandoned any project he might have entertained and had beat a retreat the day before. whether the supposition that he could have taken the city with great celerity has any foundation, i cannot say; i should certainly greatly doubt it, remembering the large loss of life generally suffered during the civil war by troops trying to storm intrenchments or defenses of any sort, even with greatly superior force. i was surprised to find how quickly one could acquire the stolidity of the soldier. during the march from the navy yard to the fort i felt extremely depressed, as one can well imagine, in view of the suddenness with which i had to take leave of my family and the uncertainty of the situation, as well as its extreme gravity. but this depression wore off the next day, and i do not think i ever had a sounder night's sleep in my life than when i lay down on the grass, with only a blanket between myself and the sky, with the expectation of being awakened by the rattle of musketry at daybreak. i remember well how kindly we were treated by the army. the acquaintance of generals wright and mccook, made under such circumstances, was productive of a feeling which has never worn off. it has always been a matter of sorrow to me that the washington of to-day does not show a more lively consciousness of what it owes to these men. one of the entertainments of washington during the early years of the civil war was offered by president lincoln's public receptions. we used to go there simply to see the people and the costumes, the latter being of a variety which i do not think was ever known on such occasions before or since. well-dressed and refined ladies and gentlemen, men in their working clothes, women arrayed in costumes fanciful in cut and brilliant in color, mixed together in a way that suggested a convention of the human race. just where the oddly dressed people came from, or what notion took them at this particular time to don an attire like that of a fancy-dress ball, no one seemed to know. among the never-to-be-forgotten scenes was that following the news of the fall of richmond. if i described it from memory, a question would perhaps arise in the reader's mind as to how much fancy might have added to the picture in the course of nearly forty years. i shall therefore quote a letter written to chauncey wright immediately afterwards, of which i preserved a press copy. observatory, april , . dear wright,--yours of the th just received. i heartily reciprocate your congratulations on the fall of richmond and the prospective disappearance of the s. c. alias c. s. you ought to have been here monday. the observatory is half a mile to a mile from the thickly settled part of the city. at a. m. we were put upon the qui vive by an unprecedented commotion in the city. from the barracks near us rose a continuous stream of cheers, and in the city was a hubbub such as we had never before heard. we thought it must be petersburg or richmond, but hardly dared to hope which. miss gilliss sent us word that it was really richmond. i went down to the city. all the bedlams in creation broken loose could not have made such a scene. the stores were half closed, the clerks given a holiday, the streets crowded, every other man drunk, and drums were beating and men shouting and flags waving in every direction. i never felt prouder of my country than then, as i compared our present position with our position in the numerous dark days of the contest, and was almost ashamed to think that i had ever said that any act of the government was not the best possible. not many days after this outburst, the city was pervaded by an equally intense and yet deeper feeling of an opposite kind. probably no event in its history caused such a wave of sadness and sympathy as the assassination of president lincoln, especially during the few days while bands of men were scouring the country in search of the assassin. one could not walk the streets without seeing evidence of this at every turn. the slightest bustle, perhaps even the running away of a dog, caused a tremor. i paid one short visit to the military court which was trying the conspirators. the court itself was listening with silence and gravity to the reading of the testimony taken on the day previous. general wallace produced on the spectators an impression a little different from the other members, by exhibiting an artistic propensity, which subsequently took a different direction in "ben hur." the most impressive sight was that of the conspirators, all heavily manacled; even mrs. surratt, who kept her irons partly concealed in the folds of her gown. payne, the would-be assassin of seward, was a powerful-looking man, with a face that showed him ready for anything; but the other two conspirators were such simple-minded, mild-looking youths, that it seemed hardly possible they could have been active agents in such a crime, or capable of any proceeding requiring physical or mental force. the impression which i gained at the time from the evidence and all the circumstances, was that the purpose of the original plot was not the assassination of the president, but his abduction and transportation to richmond or some other point within the confederate lines. while booth himself may have meditated assassination from the beginning, it does not seem likely that he made this purpose known to his fellows until they were ready to act. then payne alone had the courage to attempt the execution of the programme. two facts show that a military court, sitting under such circumstances, must not be expected to reach exactly the verdict that a jury would after the public excitement had died away. among the prisoners was the man whose business it was to assist in arranging the scenery on the stage of the theatre where the assassination occurred. the only evidence against him was that he had not taken advantage of his opportunity to arrest booth as the latter was leaving, and for this he was sentenced to twenty years penal servitude. he was pardoned out before a great while. the other circumstance was the arrest of surratt, who was supposed to stand next to booth in the conspiracy, but who escaped from the country and was not discovered until a year or so later, when he was found to have enlisted in the papal guards at rome. he was brought home and tried twice. on the first trial, notwithstanding the adverse rulings and charge of the judge, only a minority of the jury were convinced of his guilt. on the second trial he was, i think, acquitted. one aftermath of the civil war was the influx of crowds of the newly freed slaves to washington, in search of food and shelter. with a little training they made fair servants if only their pilfering propensities could be restrained. but religious fervor did not ensure obedience to the eighth commandment. "the good lord ain't goin' to be hard on a poor darky just for takin' a chicken now and then," said a wench to a preacher who had asked her how she could reconcile her religion with her indifference as to the ownership of poultry. in the seventies i had an eight-year-old boy as help in my family. he had that beauty of face very common in young negroes who have an admixture of white blood, added to which were eyes of such depth and clearness that, but for his color, he would have made a first-class angel for a mediæval painter. one evening my little daughters had a children's party, and zeke was placed as attendant in charge of the room in which the little company met. here he was for some time left alone. next morning a gold pen was missing from its case in a drawer. suspicion rested on zeke as the only person who could possibly have taken it, but there was no positive proof. i thought so small and innocent-looking a boy could be easily cowed into confessing his guilt; so next morning i said to him very solemnly,-- "zeke, come upstairs with me." he obeyed with alacrity, following me up to the room. "zeke, come into this room." he did so. "now, zeke," i said sternly, "look here and see what i do." i opened the drawer, took out the empty case, opened it, and showed it to him. "zeke, look into my eyes!" he neither blinked nor showed the slightest abashment or hesitation as his soft eyes looked steadily into mine with all the innocence of an angel. "zeke, where is the pen out of that case?" "missr newcomb," he said quietly, "i don't know nothin' about it." i repeated the question, looking into his face as sternly as i could. as he repeated the answer with the innocence of childhood, "deed, missr newcomb, i don't know what was in it," i felt almost like a brute in pressing him with such severity. threats were of no avail, and i had to give the matter up as a failure. on coming home in the afternoon, the first news was that the pen had been found by zeke's mother hidden in one corner of her room at home, where the little thief had taken it. she, being an honest woman, and suspecting where it had come from, had brought it back. there was a vigorous movement, having its origin in new england, for the education of the freedmen. this movement was animated by the most philanthropic views. here were several millions of blacks of all ages, suddenly made citizens, or eligible to citizenship, and yet savage so far as any education was concerned. a small army of teachers, many, perhaps most of them, young women, were sent south to organize schools for the blacks. it may be feared that there was little adaptation of the teaching to the circumstances of the case. but one method of instruction widely adopted was, so far as i can learn, quite unique. it was the "loud method" of teaching reading and spelling. the whole school spelled in unison. the passer-by on the street would hear in chorus from the inside of the building, "b-r-e-a-d--bread!" all at the top of the voice of the speakers. schools in which this method was adopted were known as "loud schools." a queer result of this movement once fell under my notice. i called at a friend's house in georgetown. in the course of the conversation, it came out that the sable youngster who opened the door for me filled the double office of scullion to the household and tutor in latin to the little boy of the family. probably the senate of the united states never had a member more conscientious in the discharge of his duties than charles sumner. he went little into society outside the circles of the diplomatic corps, with which his position as chairman of the foreign affairs committee placed him in intimate relations. my acquaintance with him arose from the accident of his living for some time almost opposite me. i was making a study of some historic subject, pertaining to the feeling in south carolina before the civil war, and called at his rooms to see if he would favor me with the loan of a book, which i was sure he possessed. he received me so pleasantly that i was, for some time, an occasional visitor. he kept bachelor quarters on a second floor, lived quite alone, and was accessible to all comers without the slightest ceremony. one day, while i was talking with him, shortly after the surrender of lee, a young man in the garb of a soldier, evidently fresh from the field, was shown into the room by the housemaid, unannounced, as usual. very naturally, he was timid and diffident in approaching so great a man, and the latter showed no disposition to say anything that would reassure him. he ventured to tell the senator that he had come to see if he could recommend him for some public employment. i shall never forget the tone of the reply. "but _i_ do not know _you_." the poor fellow was completely dumfounded, and tried to make some excuses, but the only reply he got was, "i cannot do it; i do not know you at all." the visitor had nothing to do but turn round and leave. at the time i felt some sympathy with the poor fellow. he had probably come, thinking that the great philanthropist was quite ready to become a friend to a union soldier without much inquiry into his personality and antecedents, and now he met with a stinging rebuff. but it must be confessed that subsequent experience has diminished my sympathy for him, and probably it would be better for the country if the innovation were introduced of having every senator of the united states dispose of such callers in the same way. foreign men of letters, with whom sumner's acquaintance was very wide, were always among his most valued guests. a story is told of thackeray's visit to washington, which i distrust only for the reason that my ideas of sumner's make-up do not assign him the special kind of humor which the story brings out. he was, however, quoted as saying, "thackeray is one of the most perfect gentlemen i ever knew. i had a striking illustration of that this morning. we went out for a walk together and, thoughtlessly, i took him through lafayette square. shortly after we entered it, i realized with alarm that we were going directly toward the jackson statue. it was too late to retrace our steps, and i wondered what thackeray would say when he saw the object. but he passed straight by without seeming to see it at all, and did not say one word about it." sumner was the one man in the senate whose seat was scarcely ever vacant during a session. he gave the closest attention to every subject as it arose. one instance of this is quite in the line of the present book. about , an association was organized in washington under the name of the "american union academy of literature, science, and art." its projectors were known to few, or none, but themselves. a number of prominent citizens in various walks of life had been asked to join it, and several consented without knowing much about the association. it soon became evident that the academy was desirous of securing as much publicity as possible through the newspapers and elsewhere. it was reported that the secretary of the treasury had asked its opinion on some instrument or appliance connected with the work of his department. congress was applied to for an act of incorporation, recognizing it as a scientific adviser of the government by providing that it should report on subjects submitted to it by the governmental departments, the intent evidently being that it should supplant the national academy of sciences. the application to congress satisfied the two requirements most essential to favorable consideration. these are that several respectable citizens want something done, and that there is no one to come forward and say that he does not want it done. such being the case, the act passed the house of representatives without opposition, came to the senate, and was referred to the appropriate committee, that on education, i believe. it was favorably reported from the committee and placed on its passage. up to this point no objection seems to have been made to it in any quarter. now, it was challenged by mr. sumner. the ground taken by the massachusetts senator was comprehensive and simple, though possibly somewhat novel. it was, in substance, that an academy of literature, science, and art, national in its character, and incorporated by special act of congress, ought to be composed of men eminent in the branches to which the academy related. he thought a body of men consisting very largely of local lawyers, with scarcely a man of prominence in either of the three branches to which the academy was devoted, was not the one that should receive such sanction from the national legislature. mr. j. w. patterson, of new hampshire, was the principal advocate of the measure. he claimed that the proposed incorporators were not all unscientific men, and cited as a single example the name of o. m. poe, which appeared among them. this man, he said, was a very distinguished meteorologist. this example was rather unfortunate. the fact is, the name in question was that of a well-known officer of engineers in the army, then on duty at washington, who had been invited to join the academy, and had consented out of good nature without, it seems, much if any inquiry. it happened that senator patterson had, some time during the winter, made the acquaintance of a west indian meteorologist named poey, who chanced to be spending some time in washington, and got him mixed up with the officer of engineers. the senator also intimated that the gentleman from massachusetts had been approached on the subject and was acting under the influence of others. this suggestion mr. sumner repelled, stating that no one had spoken to him on the subject, that he knew nothing of it until he saw the bill before them, which seemed to him to be objectionable for the very reasons set forth. on his motion the bill was laid on the table, and thus disposed of for good. the academy held meetings for some time after this failure, but soon disappeared from view, and was never again heard of. in the year , a fine-looking young general from the west became a boarder in the house where i lived, and sat opposite me at table. his name was james a. garfield. i believe he had come to washington as a member of the court in the case of general fitz john porter. he left after a short time and had, i supposed, quite forgotten me. but, after his election to congress, he one evening visited the observatory, stepped into my room, and recalled our former acquaintance. i soon found him to be a man of classical culture, refined tastes, and unsurpassed eloquence,--altogether, one of the most attractive of men. on one occasion he told me one of his experiences in the state legislature of ohio, of which he was a member before the civil war. a bill was before the house enacting certain provisions respecting a depository. he moved, as an amendment, to strike out the word "depository" and insert "depositary." supposing the amendment to be merely one of spelling, there was a general laugh over the house, with a cry of "here comes the schoolmaster!" but he insisted on his point, and sent for a copy of webster's dictionary in order that the two words might be compared. when the definitions were read, the importance of right spelling became evident, and the laughing stopped. it has always seemed to me that a rank injustice was done to garfield on the occasion of the credit mobilier scandal of , which came near costing him his position in public life. the evidence was of so indefinite and flimsy a nature that the credence given to the conclusion from it can only illustrate how little a subject or a document is exposed to searching analysis outside the precincts of a law court. when he was nominated for the presidency this scandal was naturally raked up and much made of it. i was so strongly impressed with the injustice as to write for a new york newspaper, anonymously of course, a careful analysis of the evidence, with a demonstration of its total weakness. whether the article was widely circulated, or whether garfield ever heard of it, i do not know; but it was amusing, a few days after it appeared, to see a paragraph in an opposition paper claiming that its contemporary had gone to the trouble of hiring a lawyer to defend garfield. no man better qualified as a legislator ever occupied a seat in congress. a man cast in the largest mould, and incapable of a petty sentiment, his grasp of public affairs was rarely equaled, and his insight into the effects of legislation was of the deepest. but on what the author of the autocrat calls the arithmetical side,--in the power of judging particular men and not general principles; in deciding who were the good men and who were not, he fell short of the ideal suggested by his legislative career. the brief months during which he administered the highest of offices were stormy enough, perhaps stormier than any president before him had ever experienced, and they would probably have been outdone by the years following, had he lived. but i believe that, had he remained in the senate, his name would have gone into history among those of the greatest of legislators. sixteen years after the death of lincoln public feeling was again moved to its depth by the assassination of garfield. the cry seemed to pass from mouth to mouth through the streets faster than a messenger could carry the news, "the president has been shot." it chanced to reach me just as i was entering my office. i at once summoned my messenger and directed him to go over to the white house, and see if anything unusual had happened, but gave him no intimation of my fears. he promptly returned with the confirmation of the report. the following are extracts from my journal at the time:-- "july , saturday: at . this morning president garfield was shot by a miserable fellow named guiteau, as he was passing through the baltimore and potomac r. r. station to leave washington. one ball went through the upper arm, making a flesh wound, the other entered the right side on the back and cannot be found; supposed to have lodged in the liver. in the course of the day president rapidly weakened, and supposed to be dying from hemorrhage." "sunday morning: president still living and rallied during the day. small chance of recovery. at night alarming symptoms of inflammation were exhibited, and at midnight his case seemed almost hopeless." "monday: president slightly better this morning, improving throughout the day." "july . this p. m. sought an interview with dr. woodward at the white house, to talk of an apparatus for locating the ball by its action in retarding a rapidly revolving el. magnet. i hardly think the plan more than theoretically practical, owing to the minuteness of the action." "the president still improving, but great dangers are yet to come, and nothing has been found of the ball, which is supposed to have stayed in the liver because, were it anywhere else, symptoms of irritation by its presence would have been shown." "july . this is saturday evening. met major powell at the cosmos club, who told me that they would like to have me look at the air-cooling projects at the white house. published statement that the physicians desired some way to cool the air of the president's room had brought a crowd of projects and machines of all kinds. among other things, a mr. dorsey had got from new york an air compressor such as is used in the virginia mines for transferring power, and was erecting machinery enough for a steamship at the east end of the house in order to run it." dr. woodward was a surgeon of the army, who had been on duty at washington since the civil war, in charge of the army medical museum. among his varied works here, that in micro-photography, in which he was a pioneer, gave him a wide reputation. his high standing led to his being selected as one of the president's physicians. to him i wrote a note, offering to be of any use i could in the matter of cooling the air of the president's chamber. he promptly replied with a request to visit the place, and see what was being done and what suggestions i could make. mr. dorsey's engine at the east end was dispensed with after a long discussion, owing to the noise it would make and the amount of work necessary to its final installation and operation. among the problems with which the surgeons had to wrestle was that of locating the ball. the question occurred to me whether it was not possible to do so by the influence produced by the action of a metallic conductor in retarding the motion of a rapidly revolving magnet, but the effect would be so small, and the apparatus to be made so delicate, that i was very doubtful about the matter. if there was any one able to take hold of the project successfully, i knew it would be alexander graham bell, the inventor of the telephone. when i approached him on the subject, he suggested that the idea of locating the ball had also occurred to him, and that he thought the best apparatus for the purpose was a telephonic one which had been recently developed by mr. hughes. as there could be no doubt of the superiority of his project, i dropped mine, and he went forward with his. in a few days an opportunity was given him for actually trying it. the result, though rather doubtful, seemed to be that the ball was located where the surgeons supposed it to be. when the autopsy showed that their judgment had been at fault, mr. bell admitted his error to dr. woodward, adding some suggestion as to its cause. "expectant attention," was woodward's reply. i found in the basement of the house an apparatus which had been brought over by a mr. jennings from baltimore, which was designed to cool the air of dairies or apartments. it consisted of an iron box, two or three feet square, and some five feet long. in this box were suspended cloths, kept cool and damp by the water from melting ice contained in a compartment on top of the box. the air was driven through the box by a blower, and cooled by contact with the wet cloths. but no effect was being produced on the temperature of the room. one conversant with physics will see one fatal defect in this appliance. the cold of the ice, if i may use so unscientific an expression, went pretty much to waste. the air was in contact, not with the ice, as it should have been, but with ice-water, which had already absorbed the latent heat of melting. evidently the air should be passed over the unmelted ice. the question was how much ice would be required to produce the necessary cooling? to settle this, i instituted an experiment. a block of ice was placed in an adjoining room in a current of air with such an arrangement that, as it melted, the water would trickle into a vessel below. after a certain number of minutes the melted water was measured, then a simple computation led to a knowledge of how much heat was absorbed from the air per minute by a square foot of the surface of the ice. from this it was easy to calculate from the known thermal capacity of air, and the quantity of the latter necessary per minute, how many feet of cooling surface must be exposed. i was quite surprised at the result. a case of ice nearly as long as an ordinary room, and large enough for men to walk about in it, must be provided. this was speedily done, supports were erected for the blocks of ice, the case was placed at the end of mr. jennings's box, and everything gotten in readiness for directing the air current through the receptacle, and into the room through tubes which had already been prepared. it happened that mr. jennings's box was on the line along which the air was being conducted, and i was going to get it out of the way. the owner implored that it should be allowed to remain, suggesting that the air might just as well as not continue to pass through it. the surroundings were those in which one may be excused for not being harsh. such an outpouring of sympathy on the part of the public had never been seen in washington since the assassination of lincoln. those in charge were overwhelmed with every sort of contrivance for relieving the sufferings of the illustrious patient. such disinterested efforts in behalf of a public and patriotic object had never been seen. mr. jennings had gone to the trouble and expense of bringing his apparatus all the way from baltimore to washington in order to do what in him lay toward the end for which all were striving. to leave his box in place could not do the slightest harm, and would be a gratification to him. so i let it stand, and the air continued to pass through it on its way to the ice chest. while these arrangements were in progress three officers of engineers of the navy reported under orders at the white house, to do what they could toward the cooling of the air. they were messrs. william l. baillie, richard inch, and w. s. moore. all four of us coöperated in the work in a most friendly way, and when we got through we made our reports to the navy department. a few weeks later these reports were printed in a pamphlet, partly to correct a wrong impression about the jennings cold-box. regular statements had appeared in the local evening paper that the air was being cooled by this useless contrivance. their significance first came out several months later, on the occasion of an exhibition of mechanical or industrial implements at boston. among these was mr. jennings's cold-box, which was exhibited as the instrument that had cooled the air of president garfield's chamber. more light yet was thrown on the case when the question of rewarding those who had taken part in treating the president, or alleviating his sufferings in any way, came before congress. mr. jennings was, i believe, among the claimants. congress found the task of making the proper awards to each individual to be quite beyond its power at the time, so a lump sum was appropriated, to be divided by the treasury department according to its findings in each particular case. before the work of making the awards was completed, i left on the expedition to the cape of good hope to observe the transit of venus, and never learned what had been done with the claims of mr. jennings. it might naturally be supposed that when an official report to the navy department showed that he had no claims whatever except those of a patriotic citizen who had done his best, which was just nothing at all, to promote the common end, the claim would have received little attention. possibly this may have been the case. but i do not know what the outcome of the matter was. shortly after the death of the president, i had a visit from an inventor who had patented a method of cooling the air of a room by ice. he claimed that our work at the executive mansion was an infringement on his patent. i replied that i could not see how any infringement was possible, because we had gone to work in the most natural way, without consulting any previous process whatever, or even knowing of the existence of a patent. surely the operation of passing air over ice to cool it could not be patentable. he invited me to read over the statement of his claims. i found that although this process was not patented in terms, it was practically patented by claiming about every possible way in which ice could be arranged for cooling purposes. placing the ice on supports was one of his claims; this we had undoubtedly done, because otherwise the process could not have been carried out. in a word, the impression i got was that the only sure way of avoiding an infringement would have been to blindfold the men who put the ice in the box, and ask them to throw it in pellmell. every method of using judgment in arranging the blocks of ice he had patented. i had to acknowledge that his claim of infringement might have some foundation, and inquired what he proposed to do in the case. he replied that he did not wish to do more than have his priority recognized in the matter. i replied that i had no objection to his doing this in any way he could, and he took his leave. nothing more, so far as i am aware, was done in his case. but i was much impressed by this as by other examples i have had of the same kind, of the loose way in which our patent office sometimes grants patents. i do not think the history of any modern municipality can show an episode more extraordinary or, taken in connection with its results, more instructive than what is known as the "shepherd régime" in washington. what is especially interesting about it is the opposite views that can be taken of the same facts. as to the latter there is no dispute. yet, from one point of view, shepherd made one of the most disastrous failures on record in attempting to carry out great works, while, from another point of view, he is the author of the beautiful washington of to-day, and entitled to a public statue in recognition of his services. as i was a resident of the city and lived in my own house, i was greatly interested in the proposed improvements, especially of the particular street on which i lived. i was also an eye-witness to so much of the whole history as the public was cognizant of. the essential facts of the case, from the two, opposing points of view, are exceedingly simple. one fact is the discreditable condition of the streets of washington during and after the civil war. the care of these was left entirely to the local municipality. congress, so far as i know, gave no aid except by paying its share of street improvements in front of the public buildings. it was quite out of the power of the residents, who had but few men of wealth among them, to make the city what it ought to be. congress showed no disposition to come to the help of the citizens in this task. in , however, some public-spirited citizens took the matter in hand and succeeded in having a new government established, which was modeled after that of the territories of the united states. there was a governor, a legislature, and a board of public works. the latter was charged with the improvements of the streets, and the governor was _ex officio_ its president. the first governor was henry d. cooke, the banker, and mr. shepherd was vice-president of the board of public works and its leading member. mr. cooke resigned after a short term, and mr. shepherd was promoted to his place. he was a plumber and gas-fitter by trade, and managed the leading business in his line in washington. through the two or three years of his administration the city directory still contained the entry-- shepherd, alex. r. & co., plumbers and gas-fitters, pa. ave. n. w. in recent years he had added to his plumbing business that of erecting houses for sale. he had had no experience in the conduct of public business, and, of course, was neither an engineer nor a financier. but such was the energy of his character and his personal influence, that he soon became practically the whole government, which he ran in his own way, as if it were simply his own business enlarged. of the conditions which the law imposes on contracts, of the numerous and complicated problems of engineering involved in the drainage and street systems of a great city, of the precautions to be taken in preparing plans for so immense a work, and of the legal restraints under which it should be conducted, he had no special knowledge. but he had in the highest degree a quality which will bear different designations according to the point of view. his opponents would call it unparalleled recklessness; his supporters, boldness and enterprise. such were the preliminaries. three years later the results of his efforts were made known by an investigating committee of congress, with senator allison, a political friend, at its head. it was found that with authority to expend $ , , in the improvement of the streets, there was an actual or supposed expenditure of more than $ , , , and a crowd of additional claims which no man could estimate, based on the work of more than one thousand principal contractors and an unknown number of purchasers and sub-contractors. chaos reigned supreme. some streets were still torn up and impassable; others completely paved, but done so badly that the pavements were beginning to rot almost before being pressed by a carriage. a debt had been incurred which it was impossible for the local municipality to carry and which was still piling up. for all this congress was responsible, and manfully shouldered its responsibility. mr. shepherd was legislated out of office as an act of extreme necessity, by the organization of a government at the head of which were three commissioners. the feeling on the subject may be inferred from the result when president grant, who had given shepherd his powerful support all through, nominated him as one of the three commissioners. the senate rejected the nomination, with only some half dozen favorable votes. the three commissioners took up the work and carried it on in a conservative way. congress came to the help of the municipality by bearing one half the taxation of the district, on the very sound basis that, as it owned about one half of the property, it should pay one half the taxes. the spirit of the time is illustrated by two little episodes. the reservation on which the public library founded by mr. carnegie is now built, was then occupied by the northern liberties market, one of the three principal markets of the city. being a public reservation, it had no right to remain there except during the pleasure of the authorities. due notice was given to the marketmen to remove the structures. the owners were dilatory in doing so, and probably could not see why they should be removed when the ground was not wanted for any other purpose, and before they had time to find a new location. it was understood that, if an attempt was made to remove the buildings, the marketmen would apply to the courts for an injunction. to prevent this, an arrangement was made by which the destruction of the buildings was to commence at dinner-time. at the same time, according to current report, it was specially arranged that all the judges to whom an application could be made should be invited out to dinner. however this may have been, a large body of men appeared upon the scene in the course of the evening and spent the night in destroying the buildings. with such energy was the work carried on that one marketman was killed and another either wounded or seriously injured in trying to save their wares from destruction. the indignation against shepherd was such that his life was threatened, and it was even said that a body-guard of soldiers had to be supplied by the war department for his protection. the other event was as comical as this was tragic. it occurred while the investigating committee of congress was at its work. the principal actors in the case were mr. harrington, secretary of the local government and one of mr. shepherd's assistants, the chief of police, and a burglar. harrington produced an anonymous letter, warning him that an attempt would be made in the course of a certain night to purloin from the safe in which they were kept, certain government papers, which the prosecutors of the case against shepherd were anxious to get hold of. he showed this letter to the chief of police, who was disposed to make light of the matter. but on harrington's urgent insistence the two men kept watch about the premises on the night in question. they were in the room adjoining that in which the records were kept, and through which the robber would have to pass. in due time the latter appeared, passed through the room and proceeded to break into the safe. the chief wanted to arrest him immediately, but harrington asked him to wait, in order that they might see what the man was after, and especially what he did with the books. so they left and took their stations outside the door. the burglar left the building with the books in a satchel, and, stepping outside, was confronted by the two men. i believe every burglar of whom history or fiction has kept any record, whether before or after this eventful night, when he broke open a safe and, emerging with his booty, found himself confronted by a policeman, took to his heels. not so this burglar. he walked up to the two men, and with the utmost unconcern asked if they could tell him where mr. columbus alexander lived. mr. alexander, it should be said, was the head man in the prosecution. the desired information being conveyed to the burglar, he went on his way to mr. alexander's house, followed by the two agents of the law. arriving there, he rang the bell. in the ordinary course of events, mr. alexander or some member of his family would have come to the door and been informed that the caller had a bundle for him. a man just awakened from a sound sleep and coming downstairs rubbing his eyes, would not be likely to ask any questions of such a messenger, but would accept the bundle and lock the door again. then what a mess the prosecution would have been in! its principal promoter detected in collusion with a burglar in order to get possession of the documents necessary to carry on his case! it happened, however, that mr. alexander and the members of his household all slept the sleep of the just and did not hear the bell. the patience of the policeman was exhausted and the burglar was arrested and lodged in jail, where he was kept for several months. public curiosity to hear the burglar's story was brought to a high pitch, but never gratified. before the case came to trial the prisoner was released on straw bail and never again found. i do not think the bottom facts, especially those connected with the anonymous letter, were ever brought to light. so every one was left to form his own theory of what has since been known as the "safe burglary conspiracy." what seems at present the fashionable way of looking at the facts is this: shepherd was the man who planned the beautiful washington of to-day, and who carried out his project with unexampled energy until he was stopped through the clamor of citizens who did not want to see things go ahead so fast. other people took the work up, but they only carried out shepherd's ideas. the latter, therefore, should have all the credit due to the founder of the new washington. the story has always seemed to me most interesting as an example of the way in which public judgment of men and things is likely to be influenced. public sentiment during the thirty years which have since elapsed has undergone such a revolution in favor of shepherd that a very likely outcome will be a monument to commemorate his work. but it is worth while to notice the mental processes by which the public now reaches this conclusion. it is the familiar and ordinarily correct method of putting this and that together. _this_ is one of the most beautiful cities in the united states, of which americans generally are proud when they pay it a visit. _that_ is the recollection of the man who commenced the work of transforming an unsightly, straggling, primitive town into the present washington, and was condemned for what he did. these two considerations form the basis of the conclusion, all intermediate details dropping out of sight and memory. the reckless maladministration of the epoch, making it absolutely necessary to introduce a new system, has no place in the picture. there is also a moral to the story, which is more instructive than pleasant. the actors in the case no doubt believed that if they set about their work in a conservative and law-abiding way, spending only as much money as could be raised, congress would never come to their help. so they determined to force the game, by creating a situation which would speedily lead to the correct solution of the problem. i do not think any observant person will contest the proposition that had shepherd gone about his work and carried it to a successful conclusion in a peaceable and law-abiding way,--had he done nothing to excite public attention except wisely and successfully to administer a great public work,--his name would now have been as little remembered in connection with what he did as we remember those of ketchem, phelps, and the other men who repaired the wreck he left and made the city what it is to-day. in my mind one question dominates all others growing out of the case: what will be the moral effect on our children of holding up for their imitation such methods as i have described? xiii miscellanea if the "great star-catalogue case" is not surrounded with such mystery as would entitle it to a place among _causes célèbres_, it may well be so classed on account of the novelty of the questions at issue. it affords an instructive example of the possibility of cases in which strict justice cannot be done through the established forms of legal procedure. it is also of scientific interest because, although the question was a novel one to come before a court, it belongs to a class which every leader in scientific investigation must constantly encounter in meting out due credit to his assistants. the plaintiff, christian h. f. peters, was a dane by birth, and graduated at the university of berlin in . during the earlier years of his manhood he was engaged in the trigonometrical survey of the kingdom of naples, where, for a time, he had charge of an observatory or some other astronomical station. it is said that, like many other able european youth of the period, he was implicated in the revolution of , and had to flee the kingdom in consequence. five years later, he came to the united states. here his first patron was dr. b. a. gould, who procured for him first a position on the coast survey, and then one as his assistant at the dudley observatory in albany. he was soon afterward appointed professor of astronomy and director of the litchfield observatory at hamilton college, where he spent the remaining thirty years of his life. he was a man of great learning, not only in subjects pertaining to astronomy, but in ancient and modern languages. the means at his disposal were naturally of the slenderest kind; but he was the discoverer of some forty asteroids, and devoted himself to various astronomical works and researches with great ability. of his personality it may be said that it was extremely agreeable so long as no important differences arose. what it would be in such a case can be judged by what follows. those traits of character which in men like him may be smoothed down to a greater or less extent by marital discipline were, in the absence of any such agency, maintained in all their strength to his latest years. the defendant, charles a. borst, was a graduate of the college and had been a favorite pupil of peters. he was a man of extraordinary energy and working capacity, ready to take hold in a business-like way of any problem presented to him, but not an adept at making problems for himself. his power of assimilating learning was unusually developed; and this, combined with orderly business habits, made him a most effective and valuable assistant. the terms of his employment were of the first importance in the case. mr. litchfield of new york was the patron of the observatory; he had given the trustees of hamilton college a capital for its support, which sufficed to pay the small salary of the director and some current expenses, and he also, when the latter needed an assistant, made provision for his employment. it appears that, in the case of borst, peters frequently paid his salary for considerable periods at a time, which sums were afterward reimbursed to him by mr. litchfield. i shall endeavor to state the most essential facts involved as they appear from a combination of the sometimes widely different claims of the two parties, with the hope of showing fairly what they were, but without expecting to satisfy a partisan of either side. where an important difference of statement is irreconcilable, i shall point it out. in his observations of asteroids peters was continually obliged to search through the pages of astronomical literature to find whether the stars he was using in observation had ever been catalogued. he long thought that it would be a good piece of work to search all the astronomical journals and miscellaneous collections of observations with a view of making a complete catalogue of the positions of the thousands of stars which they contained, and publishing it in a single volume for the use of astronomers situated as he was. the work of doing this was little more than one of routine search and calculation, which any well-trained youth could take up; but it was naturally quite without the power of peters to carry it through with his own hand. he had employed at least one former assistant on the work, professor john g. porter, but very little progress was made. now, however, he had a man with the persistence and working capacity necessary to carry out the plan. there was an irreconcilable difference between the two parties as to the terms on which borst went to work. according to the latter, peters suggested to him the credit which a young man would gain as one of the motives for taking up the job. but plaintiff denied that he had done anything more than order him to do it. he did not, however, make it clear why an assistant at the litchfield observatory should be officially ordered to do a piece of work for the use of astronomy generally, and having no special connection with the litchfield observatory. however this may be, borst went vigorously to work, repeating all the calculations which had been made by peters and former assistants, with a view of detecting errors, and took the work home with him in order that his sisters might make a great mass of supplementary calculations which, though not involved in the original plan, would be very conducive to the usefulness of the result. one or two of these bright young ladies worked for about a year at the job. how far peters was privy to what they did was not clear; according to his claim he did not authorize their employment to do anything but copy the catalogue. by the joint efforts of the assistant and his two sisters, working mostly or entirely at their own home, the work was brought substantially to a conclusion about the beginning of . borst then reported the completion to his chief and submitted a proposed title-page, which represented that the work was performed by charles a. borst under the direction of christian h. f. peters, professor of astronomy, etc. according to borst's account, peters tore up the paper, opened the stove door, put the fragments into the fire, and then turned on the assistant with the simple order, "bring me the catalogue!" this was refused, and a suit in replevin was immediately instituted by peters. the ablest counsel were engaged on both sides. that of the plaintiff was mr. elihu root, of new york, afterward secretary of war, one of the leading members of the new york bar, and well known as an active member of the reform branch of the republican party of that city. for the defendant was the law firm of an ex-senator of the united states, the messrs. kernan of utica. i think the taking of evidence and the hearing of arguments occupied more than a week. one claim of the defendant would, if accepted, have brought the suit to a speedy end. peters was an employee of the corporation of hamilton college, and by the terms of his appointment all his work at the litchfield observatory belonged to that institution. borst was summoned into the case as an official employee of the litchfield observatory. therefore the corporation of the college was the only authority which had power to bring the suit. but this point was disposed of by a decision of the judge that it was not reasonable, in view of the low salary received by the plaintiff, to deprive him of the right to the creations of his own talent. he did not, however, apply this principle of legal interpretation to the case of the defendant, and not only found for the plaintiff, but awarded damages based on the supposed value of the work, including, if i understand the case aright, the value of the work done by the young ladies. it would seem, however, that in officially perfecting the details of his decision he left it a little indefinite as to what papers the plaintiff was entitled to, it being very difficult to describe in detail papers many of which he had never seen. altogether it may be feared that the decision treated the catalogue much as the infant was treated by the decision of solomon. however this might he, the decision completely denied any right of the defendant in the work. this feature of it i thought very unjust, and published in a utica paper a review of the case in terms not quite so judicial as i ought to have chosen. i should have thought such a criticism quite a breach of propriety, and therefore would never have ventured upon it but for an eminent example then fresh in my mind. shortly after the supreme court of the united states uttered its celebrated decision upholding the constitutionality of the legal tender act, i happened to be conversing at an afternoon reception with one of the judges, gray, who had sustained the decision. mr. george bancroft, the historian, stepped up, and quite surprised me by expressing to the judge in quite vigorous language his strong dissent from the decision. he soon afterward published a pamphlet reviewing it adversely. i supposed that what mr. bancroft might do with a decision of the supreme court of the united states, a humbler individual might be allowed to do with the decision of a local new york judge. the defense appealed the case to a higher court of three judges, where the finding of the lower court was sustained by a majority of two to one. it was then carried to the court of appeals, the highest in the state. here the decision was set aside on what seemed to me the common sense ground that the court had ignored the rights of the defendant in the case, who certainly had some, and it must therefore be remanded for a new trial. meantime peters had died; and it is painful to think that his death may have been accelerated by the annoyances growing out of the suit. one morning, in the summer of , he was found dead on the steps of his little dwelling, having apparently fallen in a fit of apoplexy or heart failure as he was on his way to the observatory the night before. his heirs had no possible object in pushing the suit; probably his entire little fortune was absorbed in the attendant expenses. when the difference with borst was first heard of it was, i think, proposed to peters by several of his friends, including myself, that the matter should be submitted to an arbitration of astronomers. but he would listen to nothing of the sort. he was determined to enforce his legal rights by legal measures. a court of law was, in such a case, at an enormous disadvantage, as compared with an astronomical board of arbitration. to the latter all the circumstances would have been familiar and simple, while the voluminous evidence, elucidated as it was by the arguments of counsel on the two sides, failed to completely enlighten the court on the points at issue. one circumstance will illustrate this. some allusion was made during the trial to peters's work while he was abroad, in investigating the various manuscripts of the almagest of ptolemy and preparing a commentary and revised edition of ptolemy's catalogue of stars. this would have been an extremely important and original work, most valuable in the history of ancient astronomy. but the judge got it mixed up in his mind with the work before the court, and actually supposed that peters spent his time in europe in searching ancient manuscripts to get material for the catalogue in question. he also attributed great importance to the conception of the catalogue, forgetting that, to use the simile of a writer in the "new york evening post," such a conception was of no more value than the conception of a railroad from one town to another by a man who had no capital to build it. no original investigation was required on one side or the other. it was simply a huge piece of work done by a young man with help from his sisters, suggested by peters, and now and then revised by him in its details. it seemed to me that the solution offered by borst was eminently proper, and i was willing to say so, probably at the expense of peters's friendship, on which i set a high value. i have always regarded the work on ptolemy's catalogue of stars, to which allusion has just been made, as the most important peters ever undertook. it comprised a critical examination and comparison of all the manuscripts of the almagest in the libraries of europe, or elsewhere, whether in arabic or other languages, with a view of learning what light might be thrown on the doubtful questions growing out of ptolemy's work. at the litchfield observatory i had an opportunity of examining the work, especially the extended commentaries on special points, and was so impressed by the learning shown in the research as to express a desire for its speedy completion and publication. in fact, peters had already made one or more communications to the national academy of sciences on the subject, which were supposed to be equivalent to presenting the work to the academy for publication. but before the academy put in any claim for the manuscript, mr. e. b. knobel of london, a well-known member of the royal astronomical society, wrote to peters's executors, stating that he was a collaborator with peters in preparing the work, and as such had a claim to it, and wished to complete it. he therefore asked that the papers should be sent to him. this was done, but during the twelve years which have since elapsed, nothing more has been heard of the work. no one, so far as i know, ever heard of peters's making any allusion to mr. knobel or any other collaborator. he seems to have always spoken of the work as exclusively his own. among the psychological phenomena i have witnessed, none has appeared to me more curious than a susceptibility of certain minds to become imbued with a violent antipathy to the theory of gravitation. the anti-gravitation crank, as he is commonly called, is a regular part of the astronomer's experience. he is, however, only one of a large and varied class who occupy themselves with what an architect might consider the drawing up of plans and specifications for a universe. this is, no doubt, quite a harmless occupation; but the queer part of it is the seeming belief of the architects that the actual universe has been built on their plans, and runs according to the laws which they prescribe for it. ether, atoms, and nebulæ are the raw material of their trade. men of otherwise sound intellect, even college graduates and lawyers, sometimes engage in this business. i have often wondered whether any of these men proved that, in all the common schools of new york, the power which conjugates the verbs comes, through some invisible conduit in the earth, from the falls of niagara. this would be quite like many of the theories propounded. babbage's "budget of paradoxes" is a goodly volume descriptive of efforts of this sort. it was supplemented a year or two ago by a most excellent and readable article on eccentric literature, by mr. john fiske, which appeared in the "atlantic monthly." here the author discussed the subject so well that i do not feel like saying much about it, beyond giving a little of my own experience. naturally the smithsonian institution was, and i presume still is, the great authority to which these men send their productions. it was generally a rule of professor henry always to notice these communications and try to convince the correspondents of their fallacies. many of the papers were referred to me; but a little experience showed that it was absolutely useless to explain anything to these "paradoxers." generally their first communication was exceedingly modest in style, being evidently designed to lead on the unwary person to whom it was addressed. moved to sympathy with so well-meaning but erring an inquirer, i would point out wherein his reasoning was deficient or his facts at fault. back would come a thunderbolt demonstrating my incapacity to deal with the subject in terms so strong that i could not have another word to say. the american association for the advancement of science was another attraction for such men. about thirty years ago there appeared at one of its meetings a man from new jersey who was as much incensed against the theory of gravitation as if it had been the source of all human woe. he got admission to the meetings, as almost any one can, but the paper he proposed to read was refused by the committee. he watched his chance, however, and when discussion on some paper was invited, he got up and began with the words, "it seems to me that the astronomers of the present day have gravitation on the brain." this was the beginning of an impassioned oration which went on in an unbroken torrent until he was put down by a call for the next paper. but he got his chance at last. a meeting of section q was called; what this section was the older members will recall and the reader may be left to guess. a programme of papers had been prepared, and on it appeared mr. joseph treat, on gravitation. mr. treat got up with great alacrity, and, amid the astonishment and laughter of all proceeded to read his paper with the utmost seriousness. i remember a visit from one of these men with great satisfaction, because, apparently, he was an exception to the rule in being amenable to reason. i was sitting in my office one morning when a modest-looking gentleman opened the door and looked in. "i would like to see professor newcomb." "well, here he is." "you professor newcomb?" "yes." "professor, i have called to tell you that i don't believe in sir isaac newton's theory of gravitation!" "don't believe in gravitation! suppose you jump out of that window and see whether there is any gravitation or not." "but i don't mean that. i mean"-- "but that is all there is in the theory of gravitation; if you jump out of the window you'll fall to the ground." "i don't mean that. what i mean is i don't believe in the newtonian theory that gravitation goes up to the moon. it does n't extend above the air." "have you ever been up there to see?" there was an embarrassing pause, during which the visitor began to look a little sheepish. "n-no-o," he at length replied. "well, i have n't been there either, and until one of us can get up there to try the experiment, i don't believe we shall ever agree on the subject." he took his leave without another word. the idea that the facts of nature are to be brought out by observation is one which is singularly foreign not only to people of this class, but even to many sensible men. when the great comet of was discovered in the neighborhood of the sun, the fact was telegraphed that it might be seen with the naked eye, even in the sun's neighborhood. a news reporter came to my office with this statement, and wanted to know if it was really true that a comet could be seen with the naked eye right alongside the sun. "i don't know," i replied; "suppose you go out and look for yourself; that is the best way to settle the question." the idea seemed to him to be equally amusing and strange, and on the basis of that and a few other insipid remarks, he got up an interview for the "national republican" of about a column in length. i think there still exists somewhere in the northwest a communistic society presided over by a genius whose official name is koresh, and of which the religious creed has quite a scientific turn. its fundamental doctrine is that the surface of the earth on which we live is the inside of a hollow sphere, and therefore concave, instead of convex, as generally supposed. the oddest feature of the doctrine is that koresh professes to have proved it by a method which, so far as the geometry of it goes, is more rigorous than any other that science has ever applied. the usual argument by which we prove to our children the earth's rotundity is not purely geometric. when, standing on the seashore, we see the sails of a ship on the sea horizon, her hull being hidden because it is below, the inference that this is due to the convexity of the surface is based on the idea that light moves in a straight line. if a ray of light is curved toward the surface, we should have the same appearance, although the earth might be perfectly flat. so the koresh people professed to have determined the figure of the earth's surface by the purely geometric method of taking long, broad planks, perfectly squared at the two ends, and using them as a geodicist uses his base apparatus. they were mounted on wooden supports and placed end to end, so as to join perfectly. then, geometrically, the two would be in a straight line. then the first plank was picked up, carried forward, and its end so placed against that of the second as to fit perfectly; thus the continuation of a straight line was assured. so the operation was repeated by continually alternating the planks. recognizing the fact that the ends might not be perfectly square, the planks were turned upside down in alternate settings, so that any defect of this sort would be neutralized. the result was that, after they had measured along a mile or two, the plank was found to be gradually approaching the sea sand until it touched the ground. this quasi-geometric proof was to the mind of koresh positive. a horizontal straight line continued does not leave the earth's surface, but gradually approaches it. it does not seem that the measurers were psychologists enough to guard against the effect of preconceived notions in the process of applying their method. it is rather odd that pure geometry has its full share of paradoxers. runkle's "mathematical monthly" received a very fine octavo volume, the printing of which must have been expensive, by mr. james smith, a respectable merchant of liverpool. this gentleman maintained that the circumference of a circle was exactly / times its diameter. he had pestered the british association with his theory, and come into collision with an eminent mathematician whose name he did not give, but who was very likely professor demorgan. the latter undertook the desperate task of explaining to mr. smith his error, but the other evaded him at every point, much as a supple lad might avoid the blows of a prize-fighter. as in many cases of this kind, the reasoning was enveloped in a mass of verbiage which it was very difficult to strip off so as to see the real framework of the logic. when this was done, the syllogism would be found to take this very simple form:-- the ratio of the circumference to the diameter is the same in all circles. now, take a diameter of and draw round it a circumference of / . in that circle the ratio is / ; therefore, by the major premise, that is the ratio for all circles. the three famous problems of antiquity, the duplication of the cube, the quadrature of the circle, and the trisection of the angle, have all been proved by modern mathematics to be insoluble by the rule and compass, which are the instruments assumed in the postulates of euclid. yet the problem of the trisection is frequently attacked by men of some mathematical education. i think it was about that i received from professor henry a communication coming from some institution of learning in louisiana or texas. the writer was sure he had solved the problem, and asked that it might receive the prize supposed to be awarded by governments for the solution. the construction was very complicated, and i went over the whole demonstration without being able at first to detect any error. so it was necessary to examine it yet more completely and take it up point by point. at length i found the fallacy to be that three lines which, as drawn, intersected in what was to the eye the same point on the paper, were assumed to intersect mathematically in one and the same point. except for the complexity of the work, the supposed construction would have been worthy of preservation. some years later i received, from a teacher, i think, a supposed construction, with the statement that he had gone over it very carefully and could find no error. he therefore requested me to examine it and see whether there was anything wrong. i told him in reply that his work showed that he was quite capable of appreciating a geometric demonstration; that there was surely something wrong in it, because the problem was known to be insoluble, and i would like him to try again to see if he could not find his error. as i never again heard from him, i suppose he succeeded. one of the most curious of these cases was that of a student, i am not sure but a graduate, of the university of virginia, who claimed that geometers were in error in assuming that a line had no thickness. he published a school geometry based on his views, which received the endorsement of a well-known new york school official and, on the basis of this, was actually endorsed, or came very near being endorsed, as a text-book in the public schools of new york. from my correspondence, i judge that every civilized country has its share of these paradoxers. i am almost constantly in receipt of letters not only from america, but from europe and asia, setting forth their views. the following are a few of these productions which arrived in the course of a single season. baltimore, sept. , . collington ave. prof. simon newcomb: _dear sir_,--though a stranger to you, sir, i take the liberty to enlist your interest in a cause,--so grand, so beautiful, as to eclipse anything ever presented to the highest tribunal of human intellect and intuition. trusting you to be of liberal mind, sir, i have mailed you specimen copy of the "banner of light," which will prove somewhat explanatory of my previous remarks. being a student of nature and her wonderful laws, as they operate in that subtle realm of human life,--the soul, for some years, i feel well prepared to answer inquiries pertaining to this almost unknown field of scientific research, and would do so with much pleasure, as i am desirous to contribute my mite to the enlightenment of mankind upon this most important of all subjects. yours very truly, ------ ------ p. s.--would be pleased to hear from you, sir. mexico, oct. . dear sir,--i beg to inform you that i have forwarded by to days mail to your adress a copy of my th century planetary spectacle with a clipping of a german newspaper here. thirty hours for years is to day better accepted than it was years ago when i wrote it, although it called even then for some newspaper comment, especially after president cleveland's election, whose likeness has been recognized on the back cover, so has been my comet, which was duly anounced by an italian astronomer hours before said election. a hint of jupiters fifth satelite and mars satelites is also to be found in my planetary spectacle but the most striking feature of such a profetic play is undoubtedly the allegory of the paris fire my entire mercury scene and next to it is the mars scene with the wholesale retreat of the greecs that is just now puzzling some advanced minds. of cours the musical satelites represent at the same time the european concert with the disgusted halfuroons face in one corner and egypt next to it and there can be no doubt that the world is now about getting ready to applaud such a grand realistic play on the stage after even the school children of chicago adopted a great part of my moral scuol-club (act ii) as i see from the times herald oct. d. and they did certainly better than the mars fools did in n. y. years ago with that dire play, a trip to mars. the only question now is to find an enterprising scientist to not only recomend my play but put some $ up for to stage it at once perhaps you would be able to do so. yours truly g. a. kastelic, hotel buenavista. in the following dr. diaforus of the _malade imaginaire_ seems to have a formidable rival. chicago, oct. , . mr. newcombe: _dear sir_,--i forwarded you photographs of several designs which demonstrate by illustrations in physics, metaphysics, phrenology, mechanics, theology, law magnetism astronomy etc--the only true form and principles of universal government, and the greatest life sustaining forces in this universe, i would like to explain to you and to some of the expert government detectives every thing in connection with those illustrations since ; i have traveled over this continent; for many years i have been persecuted. my object in sending you those illustrations is to see if you could influence some journalist in this city, or in washington to illustrate and write up the interpretation of those designs, and present them to the public through the press. you know that very few men can grasp or comprehend in what relation a plumb line stands to the sciences, or to the nations of this earth, at the present time, by giving the correct interpretation of christian, hebrew, & mohammedian prophesy, this work presents a system of international law which is destined to create harmony peace and prosperity. sincerely yours ------ ------ monadnock bld chicago ill c/o l. l. smith. p. s. the very law that moulds a tear; and bids it trickel from its source; that law preserves this earth a sphere, and guides the planets in their course. ord neb nove , . professor simon newcomb washington d c _dear sir_,--as your labors have enabled me to protect my honor and prove the copernican newton keplar and gallileo theories false i solicit transportation to your department so that i can come and explain the whole of nature and so enable you to obtain the true value of the moon from both latitudes at the same instant. my method of working does not accord with yours hence will require more time to comprehend i have asked professor james e keeler to examine the work and forward his report with this application for transportation yours truly ------ ------ one day in july, , i was perplexed by the receipt of a cable dispatch from paris in the following terms:-- will you act? consult gould. furber. the dispatch was accompanied by the statement that an immediate answer was requested and prepaid. dr. gould being in cambridge, and i in washington, it was not possible to consult him immediately as to what was meant. after consultation with an official of the coast survey, i reached the conclusion that the request had something to do with the international metric commission, of which dr. gould was a member, and that i was desired to act on some committee. as there could be no doubt of my willingness to do this, i returned an affirmative answer, and wrote to dr. gould to know exactly what was required. great was my surprise to receive an answer stating that he knew nothing of the subject, and could not imagine what was meant. the mystery was dispelled a few days later by a visit from dr. e. r. l. gould, the well-known professor of economics, who soon after extended his activities into the more practical line of the presidency of the suburban homes and improvement company of new york. he had just arrived from paris, where a movement was on foot to induce the french government to make such modifications in the regulations governing the instruction and the degrees at the french universities as would make them more attractive to american students, who had hitherto frequented the german universities to the almost entire exclusion of those of france. it was desired by the movers in the affair to organize an american committee to act with one already formed at paris; and it was desired that i should undertake this work. i at first demurred on two grounds. i could not see how, with propriety, americans could appear as petitioners to the french government to modify its educational system for their benefit. moreover, i did not want to take any position which would involve me in an effort to draw american students from the german universities. he replied that neither objection could be urged in the case. the american committee would act only as an adviser to the french committee, and its sole purpose was to make known to the latter what arrangements as regarded studies, examinations, and degrees would be best adapted to meet the views and satisfy the needs of american students. there was, moreover, no desire to draw american students from the german universities; it was only desired to give them greater facilities in paris. the case was fortified by a letter from m. michel bréal, member of the institute of france, and head of the franco-american committee, as it was called in paris, expressing a very flattering desire that i should act. i soon gave my consent, and wrote to the presidents of eight or ten of our leading universities and several washington officials interested in education, to secure their adhesion. with a single exception, the responses were unanimous in the affirmative, and i think the exception was due to a misapprehension of the objects of the movement. the views of all the adhering americans were then requested, and a formal meeting was held, at which they were put into shape. it is quite foreign to my present object to go into details, as everything of interest in connection with the matter will be found in educational journals. one point may, however, be mentioned. the french committee was assured that whatever system of instruction and of degrees was offered, it must be one in which no distinction was made between french and foreigners. american students would not strive for a degree which was especially arranged for them alone. i soon found that the movement was a much more complex one than it appeared at first sight, and that all the parties interested in paris did not belong to one and the same committee. not long after we had put our suggestions into shape, i was gratified by a visit from dom de la tremblay, prior of the benedictine convent of santa maria, in paris, a most philanthropic and attractive gentleman, who desired to promote the object by establishing a home for the american students when they should come. knowing the temptations to which visiting youth would be exposed, he was desirous of founding an establishment where they could live in the best and most attractive surroundings. he confidently hoped to receive the active support of men of wealth in this country in carrying out his object. it was a somewhat difficult and delicate matter to explain to the philanthropic gentleman that american students were not likely to collect in a home specially provided for them, but would prefer to find their own home in their own way. i tried to do it with as little throwing of cold water as was possible, but, i fear, succeeded only gradually. but after two or three visits to new york and washington, it became evident to him that the funds necessary for his plan could not be raised. the inception of the affair was still not clear to me. i learned it in paris the year following. then i found that the movement was started by mr. furber, the sender of the telegram, a citizen of chicago, who had scarcely attained the prime of life, but was gifted with that indomitable spirit of enterprise which characterizes the metropolis of the west. what he saw of the educational institutions of paris imbued him with a high sense of their value, and he was desirous that his fellow-countrymen should share in the advantages which they offered. to induce them to do this, it was only necessary that some changes should be made in the degrees and in the examinations, the latter being too numerous and the degrees bearing no resemblance to those of germany and the united states. he therefore addressed a memorial to the minister of public instruction, who was much impressed by the view of the case presented to him, and actively favored the formation of a franco-american committee to carry out the object. everything was gotten ready for action, and it only remained that the prime mover should submit evidence that educators in america desired the proposed change, and make known what was wanted. why i should have been selected to do this i do not know, but suppose it may have been because i had just been elected a foreign associate of the institute, and was free from trammels which might have hindered the action of men who held official positions in the government or at the heads of universities. the final outcome of the affair was the establishment in the universities of france of the degree of doctor of the university, which might be given either in letters or in science, and which was expected to correspond as nearly as possible to the degree of doctor of philosophy in germany and america. one feature of the case was brought out which may be worthy of attention from educators. in a general way it may be said that our bachelor's degree does not correspond to any well-defined stage of education, implying, as it does, something more than that foundation of a general liberal education which the degree implies in europe, and not quite so much as the doctor's degree. i found it very difficult, if not impossible, to make our french friends understand that our american bachelor's degree was something materially higher than the baccalaureate of the french lycée, which is conferred at the end of a course midway between our high school and our college. from education at the sorbonne i pass to the other extreme. during a stay in harper's ferry in the autumn of , i had an object lesson in the state of primary education in the mountain regions of the south. accompanied by a lady friend, who, like myself, was fond of climbing the hills, i walked over the loudon heights into a sequestered valley, out of direct communication with the great world. after visiting one or two of the farmhouses, we came across a school by the roadside. it was the hour of recess, and the teacher was taking an active part in promoting the games in which the children were engaged. it was suggested by one of us that it would be of interest to see the methods of this school; so we approached the teacher on the subject, who very kindly offered to call his pupils together and show us his teaching. first, however, we began to question him as to the subjects of instruction. the curriculum seemed rather meagre, as he went over it. i do not think it went beyond the three r's. "but do you not teach grammar as well as reading?" i asked. "no, i am sorry to say, i do not. i did want to teach grammar, but the people all said that they had not been taught grammar, and had got along very well without it, and did not see why the time of the children should be taken up by it." "if you do not teach grammar from the book, you could at least teach it by practice in composition. do you not exercise them in writing compositions?" "i did try that once, and let me tell you how it turned out. they got up a story that i was teaching the children to write love letters, and made such a clamor about it that i had to stop." he then kindly offered to show us what he did teach. the school was called together and words to spell were given out from a dictionary. they had got as far as "patrimony," and went on from that word to a dozen or so that followed it. the words were spelled by the children in turn, but nothing was said about the definition or meaning of the word. he did not explain whether, in the opinion of the parents, it was feared that disastrous events might follow if the children knew what a "patrimony" was, but it seems that no objections were raised to their knowing how to spell it. we thanked him and took our leave, feeling that we were well repaid for our visit, however it might have been with the teacher and his school. i have never been able to confine my attention to astronomy with that exclusiveness which is commonly considered necessary to the highest success in any profession. the lawyer finds almost every branch of human knowledge to be not only of interest, but of actual professional value, but one can hardly imagine why an astronomer should concern himself with things mundane, and especially with sociological subjects. but there is very high precedent for such a practice. quite recently the fact has been brought to light that the great founder of modern astronomy once prepared for the government of his native land a very remarkable paper on the habit of debasing the currency, which was so prevalent during the middle ages. [ ] the paper of copernicus is, i believe, one of the strongest expositions of the evil of a debased currency that had ever appeared. its tenor may be judged by the opening sentence, of which the following is a free translation:-- innumerable though the evils are with which kingdoms, principalities, and republics are troubled, there are four which in my opinion outweigh all others,--war, death, famine, and debasement of money. the three first are so evident that no one denies them, but it is not thus with the fourth. a certain interest in political economy dates with me from the age of nineteen, when i read say's work on the subject, which was at that time in very wide circulation. the question of protection and free trade was then, as always, an attractive one. i inclined towards the free trade view, but still felt that there might be another side to the question which i found myself unable fully to grasp. i remember thinking it quite possible that smith's "wealth of nations" might be supplemented by a similar work on the strength of nations, in which not merely wealth, but everything that conduces to national power should be considered, and that the result of the inquiry might lead to practical conclusions different from those of smith. very able writers, among them henry c. carey, had espoused the side of protection, but for some years i had not time to read their works, and therefore reserved my judgment until more light should appear. thus the matter stood until an accident impelled me to look into the subject. about or president thomas hill, of harvard university, paid a visit to washington. i held him in very high esteem. he was a mathematician, and had been the favorite student of professor benjamin peirce; but i did not know that he had interested himself in political economy until, on the occasion in question, i passed an evening with him at the house where he was a guest. here he told me that in a public lecture at philadelphia, a few evenings before, he had informed his hearers that they had amongst them one of the greatest philosophers of the time, henry c. carey. he spoke of his works in such enthusiastic terms, describing especially his law of the tendency of mankind to be attracted towards the great capitals or other centres of population, that i lost no time in carefully reading carey's "principles of social science." the result was much like a slap in the face. with every possible predisposition to look favorably on its teachings, i was unable to find anything in them but the prejudiced judgments of a one-sided thinker, fond of brilliant general propositions which really had nothing serious to rest upon either in fact or reason. the following parody on his method occurred to me:-- the physicians say that quinine tends to cure intermittent fever. if this be the case, then where people use most quinine, they will have least intermittent fever. but the facts are exactly the opposite. along the borders of the lower mississippi, where people take most quinine, they suffer most from fever; therefore the effect of quinine is the opposite of that alleged. i earnestly wished for an opportunity to discuss the matter further with mr. hill, but it was never offered. during the early years of the civil war, when the country was flooded with an irredeemable currency, i was so much disturbed by what seemed to me the unwisdom of our financial policy, that i positively envied the people who thought it all right, and therefore were free from mental perturbation on the subject. i at length felt that i could keep silent no longer, and as the civil war was closing, i devoted much time to writing a little book, "critical examination of our financial policy during the southern rebellion." i got this published by the appletons, but had to pay for the production. it never yielded enough to pay the cost of printing, as is very apt to be the case with such a hook when it is on the unpopular side and by an unknown author. it had, however, the pleasant result of bringing me into friendly relations with two of the most eminent financiers of the country, mr. hugh mcculloch and mr. george s. coe, the latter president of one of the principal banks of new york. the compliments which these men paid to the book were the only compensation i got for the time and money expended upon it. in the "north american review" published a centennial number devoted to articles upon our national progress during the first century of our existence. i contributed the discussion of our work in exact science. natural science had been cultivated among us with great success, but i was obliged to point out our backward condition in every branch of exact science, which was more marked the more mathematical the character of the scientific work. in pure mathematics we seemed hopelessly behind in the race. i suppose that every writer who discusses a subject with a view of influencing the thought of the public, must be more or less discouraged by the small amount of attention the best he can say is likely to receive from his fellow-men. no matter what his own opinion of the importance of the matters he discusses, and the results that might grow out of them if men would only give them due attention, they are lost in the cataract of utterances poured forth from the daily, weekly, and monthly press. i was therefore much pleased, soon after the article appeared, to be honored with a visit from president gilman, who had been impressed with my views, and wished to discuss the practicability of the johns hopkins university, which was now being organized, doing something to promote the higher forms of investigation among us. one of the most remarkable mathematicians of the age, professor j. j. sylvester, had recently severed his connection with the royal military academy at woolich, and it had been decided to invite him to the chair of mathematics at the new university. it was considered desirable to have men of similar world-wide eminence in charge of the other departments in science. but this was found to be impracticable, and the policy adopted was to find young men whose reputation was yet to be made, and who would be the leading men of the future, instead of belonging to the past. all my experience would lead me to say that the selection of the coming man in science is almost as difficult as the selection of youth who are to become senators of the united states. the success of the university in finding the young men it wanted, has been one of the most remarkable features in the history of the johns hopkins university. of this the lamented rowland affords the most striking, but by no means the only instance. few could have anticipated that the modest and scarcely known youth selected for the chair of physics would not only become the leading man of his profession in our country, but one of the chief promoters of scientific research among us. mathematical study and research of the highest order now commenced, not only at baltimore, but at harvard, columbia, and other centres of learning, until, to-day, we are scarcely behind any nation in our contributions to the subject. the development of economic study in our country during the last quarter of the last century is hardly less remarkable than that of mathematical science. a great impulse in this direction was given by professor r. t. ely, who, when the johns hopkins university was organized, became its leading teacher in economics. he had recently come from germany, where he had imbibed what was supposed to be a new gospel in economics, and he now appeared as the evangelist of what was termed the historical school. my own studies were of course too far removed from this school to be a factor in it. but, so far as i was able, i fought the idea of there being two schools, or of any necessary antagonism between the results of the two methods. it was true that there was a marked difference in form between them. some men preferred to reach conclusions by careful analysis of human nature and study of the acts to which men were led in seeking to carry out their own ends. this was called the old-school method. others preferred to study the problem on a large scale, especially as shown in the economic development of the country. but there could be no necessary difference between the conclusions thus reached. one curious fact, which has always been overlooked in the history of economics in our country, shows how purely partisan was the idea of a separation of the two schools. the fact is that the founder of the historic school among us, the man who first introduced the idea, was not ely, but david a. wells. up to the outbreak of the civil war, mr. wells had been a writer on scientific subjects without any special known leaning toward economies; but after it broke out he published a most noteworthy pamphlet, setting forth the resources of our country for carrying on war and paying a debt, in terms so strong as to command more attention than any similar utterance at the time. this led to his appointment as special commissioner of revenue, with the duty of collecting information devising the best methods of raising revenue. his studies in this line were very exhaustive, and were carried on by the methods of the historic school of economics. i was almost annoyed to find that, if any economic question was presented to him, he rushed off to the experience of some particular people or nation--it might be sweden or australia--instead of going down to fundamental principles. but i could never get him interested in this kind of analysis. one of professor ely's early movements resulted in the organization of the american economic association. his original plan was that this society should have something like a creed to which its members were expected to subscribe. a discussion of the whole subject appeared in the pages of "science," a number of the leading economists of the country being contributors to it. the outcome of the whole matter has been a triumph for what most men will now consider reason and good sense. the economic association was scarcely more than organized when it broke loose from all creeds and admitted into its ranks investigators of the subject belonging to every class. i think the last discussion on the question of two schools occurred at the new york meeting, about , after which the whole matter was dropped and the association worked together as a unit. as professor ely is still a leader on the stage, i desire to do him justice in one point. i am able to do so because of what i have always regarded as one of the best features of the johns hopkins university--the unity of action which pervaded its work. there is a tendency in such institutions to be divided up into departments, not only independent of each other, but with little mutual help or sympathy. of course every department has the best wishes of every other, and its coöperation when necessary, but the tendency is to have nothing more than this. in , after the resignation of professor sylvester, i was invited by president gilman to act as head of the department of mathematics. i could not figure as the successor of sylvester, and therefore suggested that my title should be professor of mathematics and astronomy. the examinations of students for the degree of doctor of philosophy were then, as now, all conducted by a single "board of university studies," in which all had equal powers, although of course no member of the board took an active part in cases which lay entirely outside of his field. but the general idea was that of mutual coöperation and criticism all through. each professor was a factor in the department of another in a helpful and not an antagonistic way, and all held counsel on subjects where the knowledge of all was helpful to each. i cannot but think that the wonderful success of the johns hopkins university is largely due to this feature of its activity, which tended to broaden both professors and students alike. in pursuance of this system i for several years took part in the examinations of students of economics for their degrees. i found that professor ely's men were always well grounded in those principles of economic theory which seemed to me essential to a comprehension of the subject on its scientific side. being sometimes looked upon as an economist, i deem it not improper to disclaim any part in the economic research of to-day. what i have done has been prompted by the conviction that the greatest social want of the age is the introduction of sound thinking on economic subjects among the masses, not only of our own, but of every other country. this kind of thinking i have tried to promote in our own country by such books as "a plain man's talk on the labor question," and "principles of political economy." my talks with professor henry used to cover a wide field in scientific philosophy. adherence to the presbyterian church did not prevent his being as uncompromising an upholder of modern scientific views of the universe as i ever knew. he was especially severe on the delusions of spiritualism. to a friend who once told him that he had seen a "medium" waft himself through a window, he replied, "judge, you never saw that; and if you think you did, you are in a dangerous mental condition and need the utmost care of your family and your physician." among the experiences which i heard him relate more than once, i think, was one with a noted medium. henry was quite intimate with president lincoln, who, though not a believer in spiritualism, was from time to time deeply impressed by the extraordinary feats of spiritualistic performers, and naturally looked to professor henry for his views and advice on the subject. quite early in his administration one of these men showed his wonderful powers to the president, who asked him to show professor henry his feats. although the latter generally avoided all contact with such men, he consented to receive him at the smithsonian institution. among the acts proposed was that of making sounds in various quarters of the room. this was something which the keen senses and ready experimental faculty of the professor were well qualified to investigate. he turned his head in various positions while the sounds were being emitted. he then turned toward the man with the utmost firmness and said, "i do not know how you make the sounds, but this i perceive very clearly: they do not come from the room but from your person." it was in vain that the operator protested that they did not, and that he had no knowledge how they were produced. the keen ear of his examiner could not be deceived. sometime afterward the professor was traveling in the east, and took a seat in a railway car beside a young man who, finding who his companion was, entered into conversation with him, and informed him that he was a maker of telegraph and electrical instruments. his advances were received in so friendly a manner that he went further yet, and confided to henry that his ingenuity had been called into requisition by spiritual mediums, to whom he furnished the apparatus necessary for the manifestations. henry asked him by what mediums he had been engaged, and was surprised to find that among them was the very man he had met at the smithsonian. the sounds which the medium had emitted were then described to the young man, who in reply explained the structure of the apparatus by which they were produced, which apparatus had been constructed by himself. it was fastened around the muscular part of the upper arm, and was so arranged that clicks would be produced by a simple contraction of the muscle, unaccompanied by any motion of the joints of the arm, and entirely invisible to a bystander. during the philadelphia meeting of the american association for the advancement of science, held in , a few members were invited by one of the foreign visitors, professor fitzgerald of dublin, i think, to a conference on the subject of psychical research. the english society on this subject had been organized a few years before, and the question now was whether there was interest enough among us to lead to the organization of an american society for psychical research. this was decided in the affirmative; the society was soon after formed, with headquarters in boston, and i was elected its first president, a choice which powell, of washington, declared to be ridiculous in the highest degree. on accepting this position, my first duty was to make a careful study of the publications of the parent society in england, with a view of learning their discoveries. the result was far from hopeful. i found that the phenomena brought out lacked that coherence and definiteness which is characteristic of scientific truths. remarkable effects had been witnessed; but it was impossible to say, do so and so, and you will get such an effect. the best that could be said was, perhaps you will get an effect, but more likely you will not. i could not feel any assurance that the society, with all its diligence, had done more than add to the mass of mistakes, misapprehensions of fact, exaggerations, illusions, tricks, and coincidences, of which human experience is full. in the course of a year or two i delivered a presidential address, in which i pointed out the difficulties of the case and the inconclusiveness of the supposed facts gathered. i suggested further experimentation, and called upon the english society to learn, by trials, whether the mental influences which they had observed to pass from mind to mind under specially arranged conditions, would still pass when a curtain or a door separated the parties. fifteen years have since elapsed, and neither they nor any one else has settled this most elementary of all the questions involved. the only conclusion seems to be that only in exceptional cases does any effect pass at all; and when it does, it is just as likely to be felt halfway round the world as behind a curtain in the same room. shortly after the conference in philadelphia i had a long wished-for opportunity to witness and investigate what, from the descriptions, was a wonder as great as anything recorded in the history of psychic research or spiritualism. early in a tall and well-built young woman named lulu hurst, also known as the "georgia magnetic girl," gave exhibitions in the eastern cities which equaled or exceeded the greatest feats of the spiritualists. on her arrival in washington invitations were sent to a number of our prominent scientific men to witness a private exhibition which she gave in advance of her public appearance. i was not present, but some who attended were so struck by her performance that they arranged to have another exhibition in dr. graham bell's laboratory. i can give the best idea of the case if i begin with an account of the performance as given by the eye-witnesses at the first trial. we must remember that this was not the account of mere wonder-seekers, but of trained scientific men. their account was in substance this:-- a light rod was firmly held in the hands of the tallest and most muscular of the spectators. miss lulu had only to touch the rod with her fingers when it would begin to go through the most extraordinary manoeuvres. it jerked the holder around the room with a power he was unable to resist, and finally threw him down into a corner completely discomfited. another spectator was then asked to take hold of the rod, and miss lulu extended her arms and touched each end with the tip of her finger. immediately the rod began to whirl around on its central axis with such force that the skin was nearly taken off the holder's hands in his efforts to stop it. a heavy man being seated in a chair, man and chair were lifted up by the fair performer placing her hands against the sides. to substantiate the claim that she herself exerted no force, chair and man were lifted without her touching the chair at all. the sitter was asked to put his hands under the chair; the performer put her hands around and under his in such a way that it was impossible for her to exert any force on the chair except through his hands. the chair at once lifted him up without her exerting any pressure other than the touch upon his hands. several men were then invited to hold the chair still. the performer then began to deftly touch it with her finger, when the chair again began to jump about in spite of the efforts of three or four men to hold it down. a straw hat being laid upon a table crown downwards, she laid her extended hands over it. it was lifted up by what seemed an attractive force similar to that of a magnet upon an armature, and was in danger of being torn to pieces in the effort of any one holding it to keep it down, though she could not possibly have had any hold upon the object. among the spectators were physicians, one or more of whom grasped miss lulu's arms while the motions were going on, without finding any symptoms of strong muscular action. her pulse remained normal throughout. the objects which she touched seemed endowed with a force which was wholly new to science. so much for the story. now for the reality. the party appeared at the volta laboratory, according to arrangement. those having the matter in charge were not professional mystifiers of the public, and showed no desire to conceal anything. there was no darkening of rooms, no putting of hands under tables, no fear that spirits would refuse to act because of the presence of some skeptic, no trickery of any sort. we got up such arrangements as we could for a scientific investigation of the movements. one of these was a rolling platform on which miss lulu was requested to stand while the forces were exerted. another device was to seat her on a platform scale while the chair was lifting itself. these several experiments were tried in the order in which i have mentioned them. i took the wonderful staff in my hands, and miss lulu placed the palms of her hands and extended them against the staff near the ends, while i firmly grasped it with my two hands in the middle. of course this gave her a great advantage in the leverage. i was then asked to resist the staff with all my force, with the added assurance from mrs. hurst, the mother, that the resistance would be in vain. although the performer began with a delicate touch of the staff, i noticed that she changed the position of her hands every moment, sometimes seizing the staff with a firm grip, and that it never moved in any direction unless her hands pressed it in that direction. as nearly as i could estimate, the force which she exerted might have been equal to forty pounds, and this exerted first in one way and then in another was enough to upset the equilibrium of any ordinary man, especially when the jerks were so sudden and unexpected that it was impossible for one to brace himself against them. after a scene of rather undignified contortion i was finally compelled to retire in defeat, but without the slightest evidence of any other force than that exerted by a strong, muscular young woman. i asked that the rod might be made to whirl in my hands in the manner which has been described, but there was clearly some mistake in this whirl, for miss lulu knew nothing on the subject. then we proceeded to the chair performance, which was repeated a number of times. i noticed that although, at the beginning, the sitter held his fingers between the chair and the fingers of the performer, the chair would not move until miss lulu had the ball of her hand firmly in connection with it. even then it did not actually lift the sitter from the ground, but was merely raised up behind, the front legs resting on the ground, whereupon the sitter was compelled to get out. this performance was repeated a number of times without anything but what was commonplace. in order to see whether, as claimed, no force was exerted on the chair, the performer was invited to stand on the platform of the scales while making the chair move. the weights had been so adjusted as to balance a weight of forty pounds above her own. the result was that after some general attempts to make the chair move the lever clicked, showing that a lifting force exceeding forty pounds was being exerted by the young woman on the platform. the click seemed to demoralize the operator, who became unable to continue her efforts. the experiment of raising a hat turned out equally simple, and the result of all the trials was only to increase my skepticism as to the whole doctrine of unknown forces and media of communication between one mind and another. i am now likely to remain a skeptic as to every branch of "occult science" until i find some manifestation of its reality more conclusive than any i have yet been able to find. [ ] prowe: nicolaus copernicus, bd. ii. (berlin, ), p. . index absence of mind, examples of, , . academy of science, a would-be, . academy of sciences, paris, . adams, prof. john c., ; intellectual capacity, ; politics, . agnesi, donna maria, . agassiz, louis, discusses origin of species, . airy, sir george b., observations of transit of venus, ; hospitality, ; poetic taste, ; executive ability, ; methods of works, . alexander, columbus, . anderson, sir james, . angle, trisection of, . argelander, prof., master of observational astronomy, , . atlantic cable, the first, . auwers, the great astronomer, . bacon, mr., teacher at bedeque, . baillie, william, u. s. engineer, . baird, spencer f., . bancroft, george, reviews judicial decision of star catalogue case, . barnard, e. e., . barnard, gen. john g., . bartlett, william p. g., . belknap, admiral g. h., . bell, alexander graham, tries to locate ball in garfield's body, . black, jeremiah, , . blackie, prof. j. s., . bond, george p., . booth, edwin, . borst, charles a., . boss, prof. lewis, , . bowditch, nathaniel, . bradford, isaac, . brewster, elder, . brown, prof. s. j., . burnham, s. w., . campbell, william w., . carey, henry c., . cassey, thomas l., jr., . casserly, eugene, . cassini, astronomer, of paris observatory, . cayley, prof. arthur, . chandler, captain ralph, u. s. n., . chandler, w. e., . chauvenet, william, . chevreul, m., his remarkable age, . circle, quadrature of, . clark, alvan, , . clark, alvan, & sons, character of the firm, . cleveland, keith, . cobbett, william, , . coe, george s., financier, . coffin, j. h. c., . combe, george, , . commune of paris, - . comstock, g. c., . cooke, thomas, & sons, . cox, jacob d., . crank, the anti-gravitation, ; a reasonable, . cranks, specimen letters from, . darwin's "origin of species," discussion of, . dawes, henry l., . dawes, rev. w. r., . davis, charles h., ; becomes superintendent at naval observatory, . dayton, a. g., . delaunay, charles, indorses prof. newcomb, ; director of paris observatory, ; attractive personality, , . draper, dr. henry, expert in astronomical photography, , . draper, dr. john w., . dudley observatory troubles, . early, gen. jubal a., raid of, . eastman, john r., , . eclipse, solar, of , journey to observe, . economics, studies in, ; alleged schools of, . education in mountain regions of south, . eggleston, edward, . eliot, charles w., . elkin, dr. w. l., . elliot, benjamin s., . ely, prof. r. t., as economist, ; organizes american economic association, ; merits as a teacher, . evarts, william m., . eveleth, g. w., . feil, maker of optical discs, . ferguson, james, . ferrell, william, , . field, cyrus w., . fiske, john, on eccentric literature, . fixed stars, paris conference regarding, . floyd, richard s., . france, universities of, . franklin, admiral, . furber, mr., starts movement for admission of american students in french universities, . garfield, james a., first acquaintance with, ; his early life, ; injustice done him, ; his intellectual gifts, ; assassination of, . geological survey, circumstances leading to origin of, - ; attacks on, . gibraltar, determination of the longitude of, , . gill, sir david, . gillis, capt. j. m., superintendent of naval observatory, ; obtains new transit circle, . gilman, daniel c., . gladstone, william ewart, meeting with, , . glaisher, j. w. l., . goldsborough, admiral, . gould, benjamin a., personality, ; dudley observatory directorship, ; candidate for naval observatory director, . gould, dr. e. r. l., . gravitation, detestable to some minds, . green, capt. f. m., . greenwich observatory, situation, ; value of observations at, . grubb, sir howard j., , . hagar, judge, . hale, eugene p., . hale, george e., . hall, asaph, ; discovers satellites of mars, . hamlin, hannibal, . harkness, william, appointed to naval observatory, ; shares honor of discovering brightest line in spectrum of sun's corona, ; director of observatory, . harrington, attorney, . harvard observatory, prof. newcomb called to directorship of, ; pickering's directorship, . hassler, j. j. s., . hansen, prof., greatest master of celestial mechanics, , . hayden, prof. f. v., . hayes, rutherford b., , . hedrick, prof., . hell, father maximilian, his alleged forgery, . henry, prof. joseph, prof. newcomb's relations with, , , , ; characteristics, - ; on spiritualism, . herbert, hilary a., . hewitt, a. s., . hilgard, j. e., , ; in charge of coast survey, , . hill, george w., , , . hill, thomas prescott, . holcombe, lieut. j. h. l., . holden, prof. e. s., - . horsford, e. n., . hubbard, prof. j. s., head astronomer of naval observatory, ; in charge of mural circle, . huggins, sir william, . hughes, thomas, . humphreys, gen., chief of engineers, . hurst, lulu, the "georgia magnetic girl," exhibitions of, - . illusion, an astronomical, . inch, richard, united states engineer, . jennings, mr., cooling device of, . jewett, c. c., . keeler, james e., . kelvin, lord, . kerr, prof., . king, clarence, , . knobel, e. b., . koresh, his theory, . lamar, judge lucius, . langley, prof. samuel p., . language, advantage of not knowing a, . laplace, the "mécanique céleste" of, . lardner's "popular lectures on science and art," . lawrence, prof. smith j., . lee, gen. robert e., . lee's "tables and formulæ," . leverrier, m., two views of, ; meeting with, ; his merits, . leverrier and hansen's systems of planetary computation, . lick, james, . lick observatory, origin of, ; location discussed, ; telescope at, ; holden's administration, ; keeler's administration, ; campbell's administration, . lincoln, pres., his war-time receptions, ; assassination of, ; trial of assassins, . lister, lord, . litchfield observatory, founder of, . loomis, e. j., . lowe, mr. (viscount sherbrooke), . mahan, prof. d. h., . mars, discovery of the satellites of, . marsh, prof. o. c., exposure of indian ring, ; relation to "wild west," ; exposure of cardiff giant, ; his modern fossil, . maskelyne, rev. nevil, . "mathematical monthly," foundation of, . mathematics and exact sciences, state of, in america, . maury, matthew f., work of, . mccook, gen. a. d., . mccormick, l. j., . mcculloch, hugh, , . mcmickan, captain, of cunard line, . mctavish, governor, . "mécanique céleste," first sight of, . meier, john, . meridian conference of , . mill, john stuart, . mills, d. o., . miner and tully's "fevers of the connecticut valley," . monroe, rev. alexander h., n. moore, capt. w. s., . moore's navigator, . morrill, justin s., . national academy of science, early proceedings, ; report of geological survey, ; report of forestry system, . "national intelligencer," letter in, . natural philosophy, mrs. marcet's conversations on, . nautical almanac, assistants on, ; in charge of, . naval observatory, early history of, ; work at, ; conditions at, ; civilian head proposed, ; views of administration in regard to, ; reports of eclipse of , ; visit of emperor dom pedro, ; efforts to improve, ; board of visitors appointed, ; telescope of, ; congressional action regarding new telescope, ; observations of satellites of neptune, , ; search for companion of procyon, . negro, characteristics of, ; education of, . neptune, observation of the satellites of, , . newall, r. s., . newcomb, john, father of simon, characteristics and marriage, . newcomb, simon, the first, . newcomb, judge simon b., . newcomb, prof. simon, ancestry, , ; parentage, ; early education at bedeque, ; begins study of arithmetic, ; influence of books, - ; winter spent with farmer jefferson, ; residence at yarmouth, ; ancestral home, ; begins study of medicine, ; manufacture of botanic medicine under dr. foshay, , ; joins temperance lodge, ; intimacy with parkin family, ; first sight of smithsonian, ; reading in political economy, ; study of newton's "principia," ; first attempt at mathematical paper, ; letter in "national intelligencer," ; colonel abert sends lee's "tables and formulæ," ; letter from prof. l. j. smith, ; teaching in a planter's family, ; first sight of "mécanique céleste," ; assistant on staff of nautical almanac, ; discussion of darwin's "origin of species," ; student in lawrence scientific school, ; acquaintance with dr. b. a. gould, ; friendship with william p. g. bartlett, ; journey in to observe solar eclipse, ; meets governor ramsey and edward eggleston, ; received by governor mctavish, ; saskatchewan journey, ; candidate for professorship in washington university, ; application for professorship in naval observatory, ; early experience at observatory, ; edits yarnall's observations, ; in charge of mural circle, ; journey to observe eclipse, ; new transit circle, ; investigation of moon's motion, ; visit of dom pedro to observatory, ; assumes charge of nautical almanac office, ; verification of satellites of mars, ; transit of venus expedition to europe, ; expedition to cape of good hope, ; agent of lick observatory trustees, ; first meeting with schaeberle, ; study of orbits of asteroids, ; problems of astronomy, ; motion of moon, ; occultations of stars, ; offered harvard observatory directorship, ; head of nautical almanac office, ; policy of office, , ; computations for planet tables, ; assistants, ; suggestions to meridian conference, ; computations regarding fixed stars, ; member yale alumni association, ; member washington scientific club, ; first trip to europe, ; meets thomas hughes, ; john stuart mill, ; william ewart gladstone, ; general burnside, ; attends banquet of royal society, ; visit to lord lister, ; meets prof. cayley, ; prof. j. c. adams calls, ; determination of gibraltar longitude, ; visits greenwich, ; friendship with sir george airy, - ; visits edinburgh, ; meets prof. blackie, ; joins party of english astronomers bound for algeria, ; stormy voyage, ; at gibraltar, ; sir james anderson, an old acquaintance, ; mediterranean trip, - ; wilhelm förster, a berlin acquaintance, ; meets great astronomer auwers, ; visits pulkova observatory, ; winter ride in russia, ; first meeting with hansen, ; arrives in paris during german evacuation, ; visits paris observatory, ; meets leverrier, ; washington during civil war and after, - ; two days military service, ; assassination of lincoln, ; attends trial of conspirators, ; acquaintance with sumner, ; with president garfield, ; asked to device means for cooling his sick chamber, ; suggestions for location of bullet, ; experience with eccentric theorists, - ; assists in obtaining entrance of american students to french universities, ; object lesson in regard to education in mountain regions of south, ; studies in economics, ; publishes "critical examination of our financial policy during the southern rebellion," ; contribution to "north american review," ; conference with prof. daniel c. gilman, ; contributions to economic literature: "a plain man's talk on the labor question," "principles of political economy," ; "psychical research," - . nixon, thomas, , . occultism, . old peake, janitor of the smithsonian, . oldright, mr., . oliver, james e., . ommaney, sir erastus, . paine, thomas, . paradoxers, experience with, . paris conference, conclusions of, ; attacked by prof. boss and s. c. chandler, . paris observatory, , . parkin, george r., . patent claim, a curious, . patterson, j. w., . peirce, benjamin professor of mathematics, ; personality, , ; chairman of committee on methods of observing transit of venus, ; director of solar eclipse expedition, ; presence in england valuable to british astronomers, . peters, c. h. f., heads transit of venus expedition, ; star catalogue case, ; work on ptolemy's catalogue, . photoheliograph, horizontal . phrenology, study of, , . pickering, e. c., . pistor and martin's transit circle, . poe, gen. o. m., . powell, john w., ; during garfield's illness, . "principia," newton's, . procyon, search for companion of, ; at lick observatory, . professors in navy, origin of corps of, . "psychical research," . ptolemy's star catalogue, peter's work on, . pulkova observatory, object glass made by alvan clark & sons, , ; foundation and situation, - . reed, thomas b., . rhodes scholarships, . rodgers, admiral john, . rogers, william b., . royal society, banquet of, . runkle, john d., , . safe burglary conspiracy, . safford, truman h., . sampson, admiral w. t., . sands, admiral, superintendent of naval observatory, ; retirement, ; assists in obtaining new telescope, . sauty, de, cable operator at gibraltar, . schaeberle, assistant to prof. holden, . schofield, j. m., . schurman, caleb, . schurman, jacob gould, n. scientific club, . scudder, samuel h., . shepherd, alexander h., career, - . sherman, gen. w. t., . sibley, j. langdon, . smith, james, circle squarer, . smithson, james, . smithsonian institution, policy of, , ; difficulties in administration, ; expansion of scope, . smyth, prof. c. piazzi, . smyth, admiral, w. h., . sophocles, evangelinus apostolides, . standard time, adoption of, , . stanton, edwin m., ; his tireless energy, ; his law of war, . star catalogue case, the great, . steeves, isaac, . struve, otto, , . struve, wilhelm, . struve, russian minister at washington, . sudler, dr. arthur e., . sumner, charles, characteristics, , ; kills an incipient "academy," . sylvester, prof. j. j., . telescope, horizontal, planned by prof. winlock, . thomson, sir william, . tilley, sir leonard, . tracy, benjamin, . transit of venus, early observations of, ; observed by mason and dixon, ; hell's alleged forgeries, ; preparation for observation of, ; committee of national academy of sciences to consider subject, ; transit commission, ; appropriation for observation station, , , ; value of observations, ; observations at cape town, ; publication of observations, . tremblay, dom de la, . tuttle, h. p., . tyndall, prof., . van vleck, prof., . wagner, dr., . wallace, gen. lew, . washburn, mr., minister to paris, . washington, during the civil war, ; newsboys of, ; early's raid on, ; after the fall of richmond, ; shepherd régime, ; the new city, . weiss, director of vienna observatory, . welles, gideon, . wells, david a., . white house, incidents at, during garfield's illness, . whitney, william c., . williams, sir fenwick, . wilson, henry, . winlock, prof. joseph, superintendent nautical almanac, , ; personality, ; constructs instrument for astronomical photography, . wolf, prof. charles, . woodward, dr. j. j., . wright, chauncey, . wright, gen. h. g., . yale alumni association, . yarnall, prof. m., characteristics, ; observations of, . [illustration: fig. . the constellation of orion (hubble). photographed with a small camera lens of inch aperture and inches focal length. the three bright stars in the centre of the picture form the belt of orion. just below, in the sword handle, is an irregular white patch about one-eighth of an inch in diameter. this is a small-scale image of the great nebula in orion, shown on a larger scale in fig. .] the new heavens by george ellery hale director of the mount wilson observatory of the carnegie institution of washington with numerous illustrations new york charles scribner's sons to my wife preface fourteen years ago, in a book entitled "the study of stellar evolution" (university of chicago press, ), i attempted to give in untechnical language an account of some modern methods of astrophysical research. this book is now out of print, and the rapid progress of science has left it completely out of date. as i have found no opportunity to prepare a new edition, or to write another book of similar purpose, i have adopted the simpler expedient of contributing occasional articles on recent developments to _scribner's magazine_, three of which are included in the present volume. i am chiefly indebted, for the illustrations, to the mount wilson observatory and the present and former members of its staff whose names appear in the captions. special thanks are due to mr. ferdinand ellerman, who made all of the photographs of the observatory buildings and instruments, and prepared all material for reproduction. the cut of the original cavendish apparatus is copied from the _philosophical transactions for _ with the kind permission of the royal society, and i am also indebted to the royal society and to professor fowler and father cortie for the privilege of reproducing from the _proceedings_ two illustrations of their spectroscopic results. g. e. h. january, . contents chapter i. the new heavens ii. giant stars iii. cosmic crucibles illustrations fig. . the constellation of orion (hubble) . the great nebula in orion (pease) . model by ellerman of summit of mount wilson, showing the observatory buildings among the trees and bushes . the -inch hooker telescope . erecting the polar axis of the -inch telescope . lowest section of tube of -inch telescope, ready to leave pasadena for mount wilson . section of a steel girder for dome covering the -inch telescope, on its way up mount wilson . erecting the steel building and revolving dome that cover the hooker telescope . building and revolving dome, feet in diameter, covering the -inch hooker telescope . one-hundred-inch mirror, just silvered, rising out of the silvering-room in pier before attachment to lower end of telescope tube. (seen above) . the driving-clock and worm-gear that cause the -inch hooker telescope to follow the stars . large irregular nebula and star cluster in sagittarius (duncan) . faint spiral nebula in the constellation of the hunting dogs (pease) . spiral nebula in andromeda, seen edge on (ritchey) . photograph of the moon made on september , , with the -inch hooker telescope (pease) . photograph of the moon made on september , , with the -inch hooker telescope (pease) . hubble's variable nebula. one of the few nebulæ known to vary in brightness and form . ring nebula in lyra, photographed with the -inch (ritchey) and -inch (duncan) telescopes . gaseous prominence at the sun's limb, , miles high (ellerman) . the sun, , miles in diameter, from a direct photograph showing many sun-spots (whitney) . great sun-spot group, august , (whitney) . photograph of the hydrogen atmosphere of the sun (ellerman) . diagram showing outline of the -inch hooker telescope, and path of the two pencils of light from a star when under observation with the -foot michelson interferometer . twenty-foot michelson interferometer for measuring star diameters, attached to upper end of the skeleton tube of the -inch hooker telescope . the giant betelgeuse (within the circle), familiar as the conspicuous red star in the right shoulder of orion (hubble) . arcturus (within the white circle), known to the arabs as the "lance bearer," and to the chinese as the "great horn" or the "palace of the emperors" (hubble) . the giant star antares (within the white circle), notable for its red color in the constellation scorpio, and named by the greeks "a rival of mars" (hubble) . diameters of the sun, arcturus, betelgeuse, and antares compared with the orbit of mars . aldebaran, the "leader" (of the pleiades), was also known to the arabs as "the eye of the bull," "the heart of the bull," and "the great camel" (hubble) . solar prominences, photographed with the spectroheliograph without an eclipse (ellerman) . the -foot tower telescope of the mount wilson observatory . pasadena laboratory of the mount wilson observatory . sun-spot vortex in the upper hydrogen atmosphere (benioff) . splitting of spectrum lines by a magnetic field (bacock) . electric furnace in the pasadena laboratory of the mount wilson observatory . titanium oxide in red stars . titanium oxide in sun-spots . the cavendish experiment . the trifid nebula in sagittarius (ritchey) . spiral nebula in ursa major (ritchey) . mount san antonio as seen from mount wilson chapter i the new heavens go out under the open sky, on a clear and moon-less night, and try to count the stars. if your station lies well beyond the glare of cities, which is often strong enough to conceal all but the brighter objects, you will find the task a difficult one. ranging through the six magnitudes of the greek astronomers, from the brilliant sirius to the faintest perceptible points of light, the stars are scattered in great profusion over the celestial vault. their number seems limitless, yet actual count will show that the eye has been deceived. in a survey of the entire heavens, from pole to pole, it would not be possible to detect more than from six to seven thousand stars with the naked eye. from a single viewpoint, even with the keenest vision, only two or three thousand can be seen. so many of these are at the limit of visibility that ptolemy's "almagest," a catalogue of all the stars whose places were measured with the simple instruments of the greek astronomers, contains only , stars. back of ptolemy, through the speculations of the greek philosophers, the mysteries of the egyptian sun-god, and the observations of the ancient chaldeans, the rich and varied traditions of astronomy stretch far away into a shadowy past. all peoples, in the first stirrings of their intellectual youth, drawn by the nightly splendor of the skies and the ceaseless motions of the planets, have set up some system of the heavens, in which the sense of wonder and the desire for knowledge were no less concerned than the practical necessities of life. the measurement of time and the needs of navigation have always stimulated astronomical research, but the intellectual demand has been keen from the first. hipparchus and the greek astronomers of the alexandrian school, shaking off the vagaries of magic and divination, placed astronomy on a scientific basis, though the reaction of the middle ages caused even such a great astronomer as tycho brahe himself to revert for a time to the practice of astrology. early instruments the transparent sky of egypt, rarely obscured by clouds, greatly favored ptolemy's observations. here was prepared his great star catalogue, based upon the earlier observations of hipparchus, and destined to remain alone in its field for more than twelve centuries, until ulugh bey, prince of samarcand, repeated the work of his greek predecessor. throughout this period the stars were looked upon mainly as points of reference for the observation of planetary motions, and the instruments of observation underwent little change. the astrolabe, which consists of a circle divided into degrees, with a rotating diametral arm for sighting purposes, embodies their essential principle. in its simple form, the astrolabe was suspended in a vertical plane, and the stars were observed by bringing the sights on the movable diameter to bear upon them. their altitude was then read off on the circle. ultimately, the circle of the astrolabe, mounted with one of its diameters parallel to the earth's axis, became the armillary sphere, the precursor of our modern equatorial telescope. great stone quadrants fixed in the meridian were also employed from very early times. out of such furnishings, little modified by the lapse of centuries, was provided the elaborate instrumental equipment of uranibourg, the great observatory built by tycho brahe on the danish island of huen in . in this "city of the heavens," still dependent solely upon the unaided eye as a collector of starlight, tycho made those invaluable observations that enabled kepler to deduce the true laws of planetary motion. but after all these centuries the sidereal world embraced no objects, barring an occasional comet or temporary star, that lay beyond the vision of the earliest astronomers. the conceptions of the stellar universe, except those that ignored the solid ground of observation, were limited by the small aperture of the human eye. but the dawn of another age was at hand. [illustration: fig. . the great nebula in orion (pease). photographed with the -inch telescope. this short-exposure photograph shows only the bright central part of the nebula. a longer exposure reveals a vast outlying region.] the dominance of the sun as the central body of the solar system, recognized by aristarchus of samos nearly three centuries before the christian era, but subsequently denied under the authority of ptolemy and the teachings of the church, was reaffirmed by the polish monk copernicus in . kepler's laws of the motions of the planets, showing them to revolve in ellipses instead of circles, removed the last defect of the copernican system, and left no room for its rejection. but both the world and the church clung to tradition, and some visible demonstration was urgently needed. this was supplied by galileo through his invention of the telescope. [illustration: fig. . model by ellerman of summit of mount wilson, showing the observatory buildings among the trees and bushes. the -foot tower on the extreme left, which is at the edge of a precipitous cañon , feet deep, is the vertical telescope of the smithsonian astrophysical observatory. above it are the "monastery" and other buildings used as quarters by the astronomers of the mount wilson observatory while at work on the mountain. (the offices, computing-rooms, laboratories, and shops are in pasadena.) following the ridge, we come successively to the dome of the -inch photographic telescope, the power-house, laboratory, snow horizontal telescope, -foot-tower telescope, and -foot-tower telescope, these last three used for the study of the sun. the dome of the -inch reflecting telescope is just below the -foot tower, while that of the -inch telescope is farther to the right. the altitude of mount wilson is about , feet.] the crystalline lens of the human eye, limited by the iris to a maximum opening about one-quarter of an inch in diameter, was the only collector of starlight available to the greek and arabian astronomers. galileo's telescope, which in suddenly pushed out the boundaries of the known stellar universe and brought many thousands of stars into range, had a lens about - / inches in diameter. the area of this lens, proportional to the square of its diameter, was about eighty-one times that of the pupil of the eye. this great increase in the amount of light collected should bring to view stars down to magnitude . , of which nearly half a million are known to exist. it is not too much to say that galileo's telescope revolutionized human thought. turned to the moon, it revealed mountains, plains, and valleys, while the sun, previously supposed immaculate in its perfection, was seen to be blemished with dark spots changing from day to day. jupiter, shown to be accompanied by four encircling satellites, afforded a picture in miniature of the solar system, and strongly supported the copernican view of its organization, which was conclusively demonstrated by galileo's discovery of the changing phases of venus and the variation of its apparent diameter during its revolution about the sun. galileo's proof of the copernican theory marked the downfall of mediævalism and established astronomy on a firm foundation. but while his telescope multiplied a hundredfold the number of visible stars, more than a century elapsed before the true possibilities of sidereal astronomy were perceived. [illustration: fig. . the -inch hooker telescope.] structure of the universe sir william herschel was the first astronomer to make a serious attack upon the problem of the structure of the stellar universe. in his first memoir on the "construction of the heavens," read before the royal society in , he wrote as follows: "hitherto the sidereal heavens have, not inadequately for the purpose designed, been represented by the concave surface of a sphere in the centre of which the eye of an observer might be supposed to be placed.... in future we shall look upon those regions into which we may now penetrate by means of such large telescopes, as a naturalist regards a rich extent of ground or chain of mountains containing strata variously inclined and directed as well as consisting of very different materials." on turning his -inch reflecting telescope to a part of the milky way in orion, he found its whitish appearance to be completely resolved into small stars, not separately seen with his former telescopes. "the glorious multitude of stars of all possible sizes that presented themselves here to my view are truly astonishing; but as the dazzling brightness of glittering stars may easily mislead us so far as to estimate their number greater than it really is, i endeavored to ascertain this point by counting many fields, and computing from a mean of them, what a certain given portion of the milky way might contain." by this means, applied not only to the milky way but to all parts of the heavens, herschel determined the approximate number and distribution of all the stars within reach of his instrument. by comparing many hundred gauges or counts of stars visible in a field of about one-quarter of the area of the moon, herschel found that the average number of stars increased toward the great circle which most nearly conforms with the course of the milky way. ninety degrees from this plane, at the pole of the milky way, only four stars, on the average, were seen in the field of the telescope. in approaching the milky way this number increased slowly at first, and then more and more rapidly, until it rose to an average of stars per field. [illustration: fig. . erecting the polar axis of the -inch telescope.] these observations were made in the northern hemisphere, and subsequently sir john herschel, using his father's telescope at the cape of good hope, found an almost exactly similar increase of apparent star density for the southern hemisphere. according to his estimates, the total number of stars in both hemispheres that could be seen distinctly enough to be counted in this telescope would probably be about five and one-half millions. the herschels concluded that "the stars of our firmament, instead of being scattered in all directions indifferently through space, form a stratum of which the thickness is small, in comparison with its length and breadth; and in which the earth occupies a place somewhere about the middle of its thickness, between the point where it subdivides into two principal laminæ inclined at a small angle to each other." this view does not differ essentially from our modern conception of the form of the galaxy; but as the herschels were unable to see stars fainter than the fifteenth magnitude, it is evident that their conclusions apply only to a restricted region surrounding the solar system, in the midst of the enormously extended sidereal universe which modern instruments have brought within our range. modern methods the remarkable progress of modern astronomy is mainly due to two great instrumental advances: the rise and development of the photographic telescope, and the application of the spectroscope to the study of celestial objects. these new and powerful instruments, supplemented by many accessories which have completely revolutionized observatory equipment, have not only revealed a vastly greater number of stars and nebulæ: they have also rendered feasible observations of a type formerly regarded as impossible. the chemical analysis of a faint star is now so easy that it can be accomplished in a very short time--as quickly, in fact, as an equally complex substance can be analyzed in the laboratory. the spectroscope also measures a star's velocity, the pressure at different levels in its atmosphere, its approximate temperature, and now, by a new and ingenious method, its distance from the earth. it determines the velocity of rotation of the sun and of nebulæ, the existence and periods of orbital revolution of binary stars too close to be separated by any telescope, the presence of magnetic fields in sunspots, and the fact that the entire sun, like the earth, is a magnet. [illustration: fig. . lowest section of tube of -inch telescope, ready to leave pasadena for mount wilson.] such new possibilities, with many others resulting from the application of physical methods of the most diverse character, have greatly enlarged the astronomer's outlook. he may now attack two great problems: ( ) the structure of the universe and the motions of its constituent bodies, and ( ) the evolution of the stars: their nature, origin, growth, and decline. these two problems are intimately related and must be studied as one.[*] [footnote *: a third great problem open to the astronomer, the study of the constitution of matter, is described in chapter iii.] if space permitted, it would be interesting to survey the progress already accomplished by modern methods of astronomical research. hundreds of millions of stars have been photographed, and the boundaries of the stellar universe have been pushed far into space, but have not been attained. globular star clusters, containing tens of thousands of stars, are on so great a scale (according to shapley) that light, travelling at the rate of , miles per second, may take years to cross one of them, while the most distant of these objects may be more than , light-years from the earth. the spiral nebulæ, more than a million in number, are vast whirling masses in process of development, but we are not yet certain whether they should be regarded as "island universes" or as subordinate to the stellar system which includes our minute group of sun and planets, the great star clouds of the milky way, and the distant globular star clusters. [illustration: fig. . section of a steel girder for dome covering the -inch telescope, on its way up mount wilson.] these few particulars may give a slight conception of the scale of the known universe, but a word must be added regarding some of its most striking phenomena. the great majority of the stars whose motions have been determined belong to one or the other of two great star streams, but the part played by these streams in the sidereal system as a whole is still obscure. the stars have been grouped in classes, presumably in the order of their evolutional development, as they pass from the early state of gaseous masses, of low density, through the successive stages resulting from loss of heat by radiation and increased density due to shrinkage. strangely enough, their velocities in space show a corresponding change, increasing as they grow older or perhaps depending upon their mass. it is impossible within these limits to do more than to give some indication of the scope of the new astronomy. enough has been said, however, to assist in appreciating the increased opportunity for investigation, and the nature of the heavy demands made upon the modern observatory. but before passing on to describe one of the latest additions to the astronomer's instrumental equipment, a word should be added regarding the chief classes of telescopes. refractors and reflectors astronomical telescopes are of two types: refractors and reflectors. a refracting telescope consists of an object-glass composed of two or more lenses, mounted at the upper end of a tube, which is pointed at the celestial object. the light, after passing through the lenses, is brought to a focus at the lower end of the tube, where the image is examined visually with an eyepiece, or photographed upon a sensitive plate. the largest instruments of this type are the -inch lick telescope and the -inch refractor of the yerkes observatory. [illustration: fig. . erecting the steel building and revolving dome that cover the hooker telescope.] reflecting telescopes, which are particularly adapted for photographic work, though also excellent for visual observations, are very differently constructed. no lens is used. the telescope tube is usually built in skeleton form, open at its upper end, and with a large concave mirror supported at its base. this mirror serves in place of a lens. its upper surface is paraboloidal in shape, as a spherical surface will not unite in a sharp focus the rays coming from a distant object. the light passes through no glass--a great advantage, especially for photography, as the absorption in lenses cuts out much of the blue and violet light, to which photographic plates are most sensitive. the reflection occurs on the _upper_ surface of the mirror, which is covered with a coat of pure silver, renewed several times a year and always kept highly burnished. silvered glass is better than metals or other substances for telescope mirrors, chiefly because of the perfection with which glass can be ground and polished, and the ease of renewing its silvered surface when tarnished. the great reflectors of herschel and lord rosse, which were provided with mirrors of speculum metal, were far inferior to much smaller telescopes of the present day. with these instruments the star images were watched as they were carried through the field of view by the earth's rotation, or kept roughly in place by moving the telescope with ropes or chains. photographic plates, which reveal invisible stars and nebulæ when exposed for hours in modern instruments, were not then available. in any case they could not have been used, in the absence of the perfect mechanism required to keep the star images accurately fixed in place upon the sensitive film. [illustration: fig. . building and revolving dome, feet in diameter, covering the -inch hooker telescope. photographed from the summit of the -foot-tower telescope.] it would be interesting to trace the long contest for supremacy between refracting and reflecting telescopes, each of which, at certain stages in its development, appeared to be unrivalled. in modern observatories both types are used, each for the purpose for which it is best adapted. for the photography of nebulæ and the study of the fainter stars, the reflector has special advantages, illustrated by the work of such instruments as the crossley and mills reflectors of the lick observatory; the great -inch reflector, recently brought into effective service at the dominion observatory in canada; and the -inch and -inch reflectors of the mount wilson observatory. the unaided eye, with an available area of one-twentieth of a square inch, permits us to see stars of the sixth magnitude. herschel's -inch reflector, with an area , times as great, rendered visible stars of the fifteenth magnitude. the -inch reflector, with an area , times that of the eye, reveals stars of the eighteenth magnitude, while to reach stars of about the twentieth magnitude, photographic exposures of four or five hours suffice with this instrument. every gain of a magnitude means a great gain in the number of stars rendered visible. stars of the second magnitude are . times as numerous as those of the first, those of the eighth magnitude are three times as numerous as those of the seventh, while the sixteenth magnitude stars are only . as numerous as those of the fifteenth magnitude. this steadily decreasing ratio is probably due to an actual thinning out of the stars toward the boundaries of the stellar universe, as the most exhaustive tests have failed to give any evidence of absorption of light in its passage through space. but in spite of this decrease, the gain of a single additional magnitude may mean the addition of many millions of stars to the total of those already shown by the -inch reflector. here is one of the chief sources of interest in the possibilities of a -inch reflecting telescope. -inch telescope [illustration: fig. . one-hundred-inch mirror, just silvered, rising out of the silvering-room in pier before attachment to lower end of telescope tube. (seen above.)] in the late john d. hooker, of los angeles, gave the carnegie institution of washington a sum sufficient to construct a telescope mirror inches in diameter, and thus large enough to collect , times the light received by the eye. (fig. .) the casting and annealing of a suitable glass disk, inches in diameter and inches thick, weighing four and one-half tons, was a most difficult operation, finally accomplished by a great french glass company at their factory in the forest of st. gobain. a special optical laboratory was erected at the pasadena headquarters of the mount wilson observatory, and here the long task of grinding, figuring, and testing the mirror was successfully carried out by the observatory opticians. this operation, which is one of great delicacy, required years for its completion. meanwhile the building, dome, and mounting for the telescope were designed by members of the observatory staff, and the working drawings were prepared. an opportune addition by mr. carnegie to the endowment of the carnegie institution of washington, of which the observatory is a branch, permitted the necessary appropriations to be made for the completion and erection of the telescope. though delayed by the war, during which the mechanical and optical facilities of the observatory shops were utilized for military and naval purposes, the telescope is now in regular use on mount wilson. the instrument is mounted on a massive pier of reinforced concrete, feet high and feet in diameter at the top. a solid wall extends south from this pier a distance of feet, on the west side of which a very powerful spectrograph, for photographing the spectra of the brightest stars, will be mounted. within the pier are a photographic dark room, a room for silvering the large mirror (which can be lowered into the pier), and the clock-room, where stands the powerful driving-clock, with which the telescope is caused to follow the apparent motion of the stars. (fig. .) [illustration: fig. . the driving-clock and worm-gear that cause the -inch hooker telescope to follow the stars.] the telescope mounting is of the english type, in which the telescope tube is supported by the declination trunnions between the arms of the polar axis, built in the form of a rectangular yoke carried by bearings on massive pedestals to the north and south. these bearings must be aligned exactly parallel to the axis of the earth, and must support the polar axis so freely that it can be rotated with perfect precision by the driving-clock, which turns a worm-wheel feet in diameter, clamped to the lower end of the axis. as this motion must be sufficiently uniform to counteract exactly the rotation of the earth on its axis, and thus to maintain the star images accurately in position in the field of view, the greatest care had to be taken in the construction of the driving-clock and in the spacing and cutting of the teeth in the large worm-wheel. here, as in the case of all of the more refined parts of the instrument, the work was done by skilled machinists in the observatory shops in pasadena or on mount wilson after the assembling of the telescope. the massive sections of the instrument, some of which weigh as much as ten tons each, were constructed at quincy, mass., where machinery sufficiently large to build battleships was available. they were then shipped to california, and transported to the summit of mount wilson over a road built for this purpose by the construction division of the observatory, which also built the pier on which the telescope stands, and erected the steel building and dome that cover it. [illustration: fig. . large irregular nebula and star cluster in sagittarius (duncan). photographed with the -inch telescope.] [illustration: fig. . faint spiral nebula in the constellation of the hunting dogs (pease). photographed with the -inch telescope.] the parts of the telescope which are moved by the driving-clock weigh about tons, and it was necessary to provide means of reducing the great friction on the bearings of the polar axis. to accomplish this, large hollow steel cylinders, floating in mercury held in cast-iron tanks, were provided at the upper and lower ends of the polar axis. almost the entire weight of the instrument is thus floated in mercury, and in this way the friction is so greatly reduced that the driving-clock moves the instrument with perfect ease and smoothness. the -inch mirror rests at the bottom of the telescope tube on a special support system, so designed as to prevent any bending of the glass under its own weight. electric motors, forty in number, are provided to move the telescope rapidly or slowly in right ascension (east or west) and in declination (north or south), for focussing the mirrors, and for many other purposes. they are also used for rotating the dome, feet in diameter, under which the telescope is mounted, and for opening the shutter, feet wide, through which the observations are made. a telescope of this kind can be used in several different ways. the -inch mirror has a focal length of about feet, and in one of the arrangements of the instrument, the photographic plate is mounted at the centre of the telescope tube near its upper end, where it receives directly the image formed by the large mirror. in another arrangement, a silvered glass mirror, with plane surface, is supported near the upper end of the tube at an angle of °, so as to form the image at the side of the tube, where the photographic plate can be placed. in this case, the observer stands on a platform, which is moved up and down by electric motors in front of the opening in the dome through which the observations are made. [illustration: fig. . spiral nebula in andromeda, seen edge on (ritchey). photographed with the -inch telescope.] other arrangements of the telescope, for which auxiliary convex mirrors carried near the upper end of the tube are required, permit the image to be photographed at the side of the tube near its lower end, either with or without a spectrograph; or with a very powerful spectrograph mounted within a constant-temperature chamber south of the telescope pier. in this last case, the light of a star is so reflected by auxiliary mirrors that it passes down through a hole in the south end of the polar axis and brings the star to a focus on the slit of the fixed spectrograph. atmospheric limitations the huge dimensions of such a powerful engine of research as the hooker telescope are not in themselves a source of satisfaction to the astronomer, for they involve a decided increase in the labor of observation and entail very heavy expense, justifiable only in case important results, beyond the reach of other instruments, can be secured. the construction of a telescope of these dimensions was necessarily an experiment, for it was by no means certain, after the optical and mechanical difficulties had been overcome, that even the favorable atmosphere of california would be sufficiently tranquil to permit sharply defined celestial images to be obtained with so large an aperture. it is therefore important to learn what the telescope will actually accomplish under customary observing conditions. fortunately we are able to measure the performance of the instrument with certainty. close beside it on mount wilson stands the -inch reflector, of similar type, erected in . the two telescopes can thus be rigorously compared under identical atmospheric conditions. the large mirror of the -inch telescope has an area about . times that of the -inch, and therefore receives nearly three times as much light from a star. under atmospheric conditions perfect enough to allow all of this light to be concentrated in a point, it should be capable of recording on a photographic plate, with a given exposure, stars about one magnitude fainter than the faintest stars within reach of the -inch. the increased focal length, permitting such objects as the moon to be photographed on a larger scale, should also reveal smaller details of structure and render possible higher accuracy of measurement. finally, the greater theoretical resolving power of the larger aperture, providing it can be utilized, should permit the separation of the members of close double stars beyond the range of the smaller instrument. critical tests the many tests already made indicate that the advantages expected of the new telescope will be realized in practice. the increased light-gathering power will mean the addition of many millions of stars to those already known. spectroscopic observations now in regular progress have carried the range of these investigations far beyond the possibilities of the -inch telescope. a great class of red stars, for example, almost all the members of which were inaccessible to the -inch, are now being made the subject of special study. and in other fields of research equal advantages have been gained. the increase in the scale of the images over those given by the -inch telescope is illustrated by two photographs of the ring nebula in lyra, reproduced in fig. . the great nebula in orion, photographed with the -inch telescope with a comparatively short exposure, sufficient to bring out the brighter regions, is reproduced in fig. . it is interesting to compare this picture with the small-scale image of the same nebula shown in fig. . [illustration: fig. . photograph of the moon made on september , , with the -inch hooker telescope (pease). the ring-like formations are the so-called craters, most of them far larger than anything similar on the earth. that in the lower left corner with an isolated mountain in the centre is albategnius, sixty-four miles in diameter. peaks in the ring rise to a height of fifteen thousand feet above the central plain. note the long sunset shadows cast by the mountains on the left. the level region below on the right is an extensive plain, the mare nubium.] [illustration: fig. . photograph of the moon made on september , , with the -inch hooker telescope (pease). the mountains above and to the left are the lunar apennines; those on the left just below the centre are the alps. both ranges include peaks from fifteen thousand to twenty thousand feet in height. in the upper right corner is copernicus, about fifty miles in diameter. the largest of the conspicuous group of three just below the apennines is archimedes and at the lower end of the alps is plato. note the long sunset shadows cast by the isolated peaks on the left. the central portion of the picture is a vast plain, the mare imbrium.] the sharpness of the images given by the new telescope may be illustrated by some recent photographs of the moon, obtained with an equivalent focal length of feet. in fig. is shown a rugged region of the moon, containing many ring-like mountains or craters. fig. shows the great arc of the lunar apennines (above) and the alps (below), to the left of the broad plain of the mare imbrium. the starlike points along the moon's terminator, which separates the dark area from the region upon which the sun (on the right) shines, are the mountain peaks, about to disappear at sunset. the long shadows cast by the mountains just within the illuminated area are plainly seen. some of the peaks of the lunar apennines attain a height of , feet. in less powerful telescopes the stars at the centre of the great globular clusters are so closely crowded together that they cannot be studied separately with the spectrograph. moreover, most of them are much too faint for examination with this instrument. at the -foot focus the -inch telescope gives a large-scale image of such clusters, and permits the spectra of stars as faint as the fifteenth magnitude to be separately photographed. [illustration: fig. . hubble's variable nebula. one of the few nebulæ known to vary in brightness and form. photographed with the -inch telescope (hubble).] close double stars a remarkable use of the -inch telescope, which permits its full theoretical resolving power to be not merely attained but to be doubled, has been made possible by the first application of michelson's interference method to the measurement of very close double stars. when employing this, the -inch mirror is completely covered, except for two slits. beams of light from a star, entering by the slits, unite at the focus of the telescope, where the image is examined by an eyepiece magnifying about five thousand diameters. across the enlarged star image a series of fine, sharp fringes is seen, even when the atmospheric conditions are poor. if the star is single the fringes remain visible, whatever the distance between the slits. but in the case of a star like capella, previously inferred to be double from the periodic displacement of the lines in its spectrum, but with components too close together to be distinguished separately, the fringes behave differently. as the slits are moved apart a point is reached where the fringes completely disappear, only to reappear as the separation is continued. this effect is obtained when the slits are at right angles to the line joining the two stars of the pair, found by this method to be . of a second of arc apart (on december , ). subsequent measures, of far greater precision than those obtainable by other methods in the case of easily separated double stars, show the rapid orbital motion of the components of the system. this device will be applied to other close binaries, hitherto beyond the reach of measurement. [illustration: fig. . ring nebula in lyra, photographed with the -inch (ritchey) and -inch (duncan) telescopes. showing the increased scale of the images given by the larger instrument.] without entering into further details of the tests, it is evident that the new telescope will afford boundless possibilities for the study of the stellar universe.[*] the structure and extent of the galactic system, and the motions of the stars comprising it; the distribution, distances, and dimensions of the spiral nebulæ, their motions, rotation, and mode of development; the origin of the stars and the successive stages in their life history: these are some of the great questions which the new telescope must help to answer. in such an embarrassment of riches the chief difficulty is to withstand the temptation toward scattering of effort, and to form an observing programme directed toward the solution of crucial problems rather than the accumulation of vast stores of miscellaneous data. this programme will be supplemented by an extensive study of the sun, the only star near enough the earth to be examined in detail, and by a series of laboratory investigations involving the experimental imitation of solar and stellar conditions, thus aiding in the interpretation of celestial phenomena. [footnote *: it is not adapted for work on the sun, as the mirrors would be distorted by its heat. three other telescopes, especially designed for solar observations, are in use on mount wilson.] chapter ii giant stars our ancestral sun, as pictured by laplace, originally extended in a state of luminous vapor beyond the boundaries of the solar system. rotating upon its axis, it slowly contracted through loss of heat by radiation, leaving behind it portions of its mass, which condensed to form the planets. still gaseous, though now denser than water, it continues to pour out the heat on which our existence depends, as it shrinks imperceptibly toward its ultimate condition of a cold and darkened globe. laplace's hypothesis has been subjected in recent years to much criticism, and there is good reason to doubt whether his description of the mode of evolution of our solar system is correct in every particular. all critics agree, however, that the sun was once enormously larger than it now is, and that the planets originally formed part of its distended mass. even in its present diminished state, the sun is huge beyond easy conception. our own earth, though so minute a fragment of the primeval sun, is nevertheless so large that some parts of its surface have not yet been explored. seen beside the sun, by an observer on one of the planets, the earth would appear as an insignificant speck, which could be swallowed with ease by the whirling vortex of a sun-spot. if the sun were hollow, with the earth at its centre, the moon, though , miles from us, would have room and to spare in which to describe its orbit, for the sun is , miles in diameter, so that its volume is more than a million times that of the earth. [illustration: fig. . gaseous prominence at the sun's limb, , miles high (ellerman). photographed with the spectroheliograph, using the light emitted by glowing calcium vapor. the comparative size of the earth is indicated by the white circle.] but what of the stars, proved by the spectroscope to be self-luminous, intensely hot, and formed of the same chemical elements that constitute the sun and the earth? are they comparable in size with the sun? do they occur in all stages of development, from infancy to old age? and if such stages can be detected, do they afford indications of the gradual diminution in volume which laplace imagined the sun to experience? [illustration: fig. . the sun, , miles in diameter, from a direct photograph showing many sun-spots (whitney) the small black disk in the centre represents the comparative size of the earth, while the circle surrounding it corresponds in diameter to the orbit of the moon.] star images prior to the application of the powerful new engine of research described in this article we have had no means of measuring the diameters of the stars. we have measured their distances and their motions, determined their chemical composition, and obtained undeniable evidence of progressive development, but even in the most powerful telescopes their images are so minute that they appear as points rather than as disks. in fact, the larger the telescope and the more perfect the atmospheric conditions at the observer's command, the smaller do these images appear. on the photographic plate, it is true, the stars are recorded as measurable disks, but these are due to the spreading of the light from their bright point-like images, and their diameters increase as the exposure time is prolonged. from the images of the brighter stars rays of light project in straight lines, but these also are instrumental phenomena, due to diffraction of light by the steel bars that support the small mirror in the tube of reflecting telescopes. in a word, the stars are so remote that the largest and most perfect telescopes show them only as extremely minute needle-points of light, without any trace of their true disks. [illustration: fig. . great sun-spot group, august , (whitney). the disk in the corner represents the comparative size of the earth.] how, then, may we hope to measure their diameters? by using, as the man of science must so often do, indirect means when the direct attack fails. most of the remarkable progress of astronomy during the last quarter-century has resulted from the application of new and ingenious devices borrowed from the physicist. these have multiplied to such a degree that some of our observatories are literally physical laboratories, in which the sun and stars are examined by powerful spectroscopes and other optical instruments that have recently advanced our knowledge of physics by leaps and bounds. in the present case we are indebted for our star-measuring device to the distinguished physicist professor albert a. michelson, who has contributed a long array of novel apparatus and methods to physics and astronomy. the interferometer the instrument in question, known as the interferometer, had previously yielded a remarkable series of results when applied in its various forms to the solution of fundamental problems. to mention only a few of those that have helped to establish michelson's fame, we may recall that our exact knowledge of the length of the international metre at sevres, the world's standard of measurement, was obtained by him with an interferometer in terms of the invariable length of light-waves. a different form of interferometer has more recently enabled him to measure the minute tides within the solid body of the earth--not the great tides of the ocean, but the slight deformations of the earth's body, which is as rigid as steel, that are caused by the varying attractions of the sun and moon. finally, to mention only one more case, it was the michelson-morley experiment, made years ago with still another form of interferometer, that yielded the basic idea from which the theory of relativity was developed by lorentz and einstein. [illustration: fig. . photograph of the hydrogen atmosphere of the sun (ellerman). made with the spectroheliograph, showing the immense vortices, or whirling storms like tornadoes, that centre in sun-spots. the comparative size of the earth is shown by the white circle traced on the largest sun-spot.] the history of the method of measuring star diameters is a very curious one, showing how the most promising opportunities for scientific progress may lie unused for decades. the fundamental principle of the device was first suggested by the great french physicist fizeau in . in the theory was developed by the french astronomer stéphan, who observed interference fringes given by a large number of stars, and rightly concluded that their angular diameters must be much smaller than . of a second of arc, the smallest measurable with his instrument. in michelson, unaware of the earlier work, published in the _philosophical magazine_ a complete description of an interferometer capable of determining with surprising accuracy the distance between the components of double stars so close together that no telescope can separate them. he also showed how the same principle could be applied to the measurement of star diameters if a sufficiently large interferometer could be built for this purpose, and developed the theory much more completely than stéphan had done. a year later he measured the diameters of jupiter's satellites by this means at the lick observatory. but nearly thirty years elapsed before the next step was taken. two causes have doubtless contributed to this delay. both theory and experiment have demonstrated the extreme sensitiveness of the "interference fringes," on the observation of which the method depends, and it was generally supposed by astronomers that disturbances in the earth's atmosphere would prevent them from being clearly seen with large telescopes. furthermore, a very large interferometer, too large to be carried by any existing telescope, was required for the star-diameter work, though close double stars could have been easily studied by this device with several of the large telescopes of the early nineties. but whatever the reasons, a powerful method of research lay unused. the approaching completion of the -inch telescope of the mount wilson observatory led me to suggest to professor michelson, before the united states entered the war, that the method be thoroughly tested under the favorable atmospheric conditions of southern california. he was at that time at work on a special form of interferometer, designed to determine whether atmospheric disturbances could be disregarded in planning large-scale experiments. but the war intervened, and all of our efforts were concentrated for two years on the solution of war problems.[*] in , as soon as the -inch telescope had been completed and tested, the work was resumed on mount wilson. [footnote *: professor michelson's most important contribution during the war period was a new and very efficient form of range-finder, adopted for use by the u. s. navy.] a laboratory experiment the principle of the method can be most readily seen by the aid of an experiment which any one can easily perform for himself with simple apparatus. make a narrow slit, a few thousandths of an inch in width, in a sheet of black paper, and support it vertically before a brilliant source of light. observe this from a distance of or feet with a small telescope magnifying about diameters. the object-glass of the telescope should be covered with an opaque cap, pierced by two circular holes about one-eighth of an inch in diameter and half an inch apart. the holes should be on opposite sides of the centre of the object-glass and equidistant from it, and the line joining the holes should be horizontal. when this cap is removed the slit appears as a narrow vertical band with much fainter bands on both sides of it. with the cap in place, the central bright band appears to be ruled with narrow vertical lines or fringes produced by the "interference"[*] of the two pencils of light coming through different parts of the object-glass from the distant slit. cover one of the holes, and the fringes instantly disappear. their production requires the joint effect of the two light-pencils. [footnote *: for an explanation of the phenomena of interference, see any encyclopæedia or book on physics.] now suppose the two holes over the object-glass to be in movable plates, so that their distance apart can be varied. as they are gradually separated the narrow vertical fringes become less and less distinct, and finally vanish completely. measure the distance between the holes and divide this by the wavelength of light, which we may call / of an inch. the result is the angular width of the distant slit. knowing the distance of the slit, we can at once calculate its linear width. if for the slit we substitute a minute circular hole, the method of measurement remains the same, but the angular diameter as calculated above must be multiplied by . .[*] [footnote *: more complete details may be found in michelson's lowell lectures on "light-waves and their uses," university of chicago press, .] to measure the diameter of a star we proceed in a similar way, but, as the angle it subtends is so small, we must use a very large telescope, for the smaller the angle the farther apart must be the two holes over the object-glass (or the mirror, in case a reflecting telescope is employed). in fact, when the holes are moved apart to the full aperture of the -inch hooker telescope, the interference fringes are still visible even with the star betelgeuse, though its angular diameter is perhaps as great as that of any other star. thus, we must build an attachment for the telescope, so arranged as to permit us to move the openings still farther apart. [illustration: fig. . diagram showing outline of the -inch hooker telescope, and path of the two pencils of light from a star when under observation with the -foot michelson interferometer. a photograph of the interferometer is shown in fig. .] the -foot instrument the -foot interferometer designed by messrs. michelson and pease, and constructed in the mount wilson observatory instrument-shop, is shown in the diagram (fig. ) and in a photograph of the upper end of the skeleton tube of the telescope (fig. ). the light from the star is received by two flat mirrors (ml, m ) which project beyond the tube and can be moved apart along the supporting arm. these take the place of the two holes over the object-glass in our experiment. from these mirrors the light is reflected to a second pair of flat mirrors (m , m ), which send it toward the -inch concave mirror (m ) at the bottom of the telescope tube. after this the course of the light is exactly as it would be if the mirrors m , m were replaced by two holes over the -inch mirror. it is reflected to the convex mirror (m ), then back in a less rapidly convergent beam toward the large mirror. before reaching it the light is caught by the plane mirror (m ) and reflected through an opening at the side of the telescope tube to the eye-piece e. here the fringes are observed with a magnification ranging from , to , diameters. [illustration: fig. . twenty-foot michelson interferometer for measuring star diameters, attached to upper end of the skeleton tube of the -inch hooker telescope. the path of the two pencils of light from the star is shown in fig. . for a photograph of the entire telescope, see fig. .] in the practical application of this method to the measurement of star diameters, the chief problem was whether the atmosphere would be quiet enough to permit sharp interference fringes to be produced with light-pencils more than inches apart. after successful preliminary tests with the -inch refracting telescope of the yerkes observatory, professor michelson made the first attempt to see the fringes with the -inch and -inch reflectors on mount wilson in september, . he was surprised and delighted to find that the fringes were perfectly sharp and distinct with the full aperture of both these instruments. doctor anderson, of the observatory staff, then devised a special form of interferometer for the measurement of close double stars, and applied it with the -inch telescope to the measurement of the orbital motion of the close components of capella, with results of extraordinary accuracy, far beyond anything attainable by previous methods. the success of this work strongly encouraged the more ambitious project of measuring the diameter of a star, and the -foot interferometer was built for this purpose. the difficult and delicate problem of adjusting the mirrors of this instrument with the necessary extreme accuracy was solved by professor michelson during his visit to mount wilson in the summer of , and with the assistance of mr. pease, of the observatory staff, interference fringes were observed in the case of certain stars when the mirrors were as much as feet apart. all was thus in readiness for a decisive test as soon as a suitable star presented itself. the giant betelgeuse russell, shapley, and eddington had pointed out betelgeuse (arabic for "the giant's shoulder"), the bright red star in the constellation of orion (fig. ), as the most favorable of all stars for measurement, and the last-named had given its angular diameter as . of a second of arc. this deduction from theory appeared in his recent presidential address before the british association for the advancement of science, in which professor eddington remarked: "probably the greatest need of stellar astronomy at the present day, in order to make sure that our theoretical deductions are starting on the right lines, is some means of measuring the apparent angular diameter of stars." he then referred to the work already in progress on mount wilson, but anticipated "that atmospheric disturbance will ultimately set the limit to what can be accomplished." [illustration: fig. . the giant betelgeuse (within the circle), familiar as the conspicuous red star in the right shoulder of orion (hubble). measures with the interferometer show its angular diameter to be . of a second of arc, corresponding to a linear diameter of , , miles, if the best available determination of its distance can be relied upon. this determination shows betelgeuse to be light-years from the earth. light travels at the rate of , miles per second, and yet spends years on its journey to us from this star.] on december , , mr. pease successfully measured the diameter of betelgeuse with the -foot interferometer. as the outer mirrors were separated the interference fringes gradually became less distinct, as theory requires, and as doctor merrill had previously seen when observing betelgeuse with the interferometer used for capella. at a separation of feet the fringes disappeared completely, giving the data required for calculating the diameter of the star. to test the perfection of the adjustment, the telescope was turned to other stars, of smaller angular diameter, which showed the fringes with perfect clearness. turning back to betelgeuse, they were seen beyond doubt to be absent. assuming the mean wave-length of the light of this star to be / of a millimetre, its angular diameter comes out . of a second of arc, thus falling between the values-- . and . of a second--predicted by eddington and russell from slightly different assumptions. subsequent corrections and repeated measurement will change mr. pease's result somewhat, but it is almost certainly within or per cent of the truth. we may therefore conclude that the angular diameter of betelgeuse is very nearly the same as that of a ball one inch in diameter, seen at a distance of seventy miles. [illustration: fig. . arcturus (within the white circle), known to the arabs as the "lance bearer," and to the chinese as the "great horn" or the "palace of the emperors" (hubble). its angular diameter, measured at mount wilson by pease with the -foot michelson interferometer on april , , is . of a second, in close agreement with russell's predicted value of . of a second. the mean parallax of arcturus, based upon several determinations, is . of a second, corresponding to a distance of light-years. the linear diameter, computed from pease's measure and this value of the distance is about million miles.] but this represents only the angle subtended by the star's disk. to learn its linear diameter, we must know its distance. four determinations of the parallax, which determines the distance, have been made. elkin, with the yale heliometer, obtained . of a second of arc. schlesinger, from photographs taken with the -inch allegheny refractor, derived . . adams, by his spectroscopic method applied with the -inch mount wilson reflector, obtained . . lee's recent value, secured photographically with the -inch yerkes refractor, is . . the heliometer parallax is doubtless less reliable than the photographic ones, and doctor adams states that the spectral type and luminosity of betelgeuse make his value less certain than in the case of most other stars. if we take a (weighted) mean value of . of a second, we shall probably not be far from the truth. this parallax represents the angle subtended by the radius of the earth's orbit ( , , miles) at the distance of betelgeuse. by comparing it with . , the angular diameter of the star, we see that the linear diameter is about two and one-third times as great as the distance from the earth to the sun, or approximately , , miles. thus, if this measure of its distance is not considerably in error, betelgeuse would nearly fill the orbit of mars. all methods of determining the distances of the stars are subject to uncertainty, however, and subsequent measures may reduce this figure very appreciably. but there can be no doubt that the diameter of betelgeuse exceeds , , miles, and it is probably much greater. the extremely small angle subtended by this enormous disk is explained by the great distance of the star, which is about light-years. that is to say, light travelling at the rate of , miles per second spends years in crossing the space that lies between us and betelgeuse, whose tremendous proportions therefore seem so minute even in the most powerful telescopes. stellar evolution this actual measure of the diameter of betelgeuse supplies a new and striking test of russell's and hertzsprung's theory of dwarf and giant stars. just before the war russell showed that our old methods of classifying the stars according to their spectra must be radically changed. stars in an early stage of their life history may be regarded as diffuse gaseous masses, enormously larger than our sun, and at a much lower temperature. their density must be very low, and their state that of a perfect gas. these are the "giants." in the slow process of time they contract through constant loss of heat by radiation. but, despite this loss, the heat produced by contraction and from other sources (see p. ) causes their temperature to rise, while their color changes from red to bluish white. the process of shrinkage and rise of temperature goes on so long as they remain in the state of a perfect gas. but as soon as contraction has increased the density of the gas beyond a certain point the cycle reverses and the temperature begins to fall. the bluish-white light of the star turns yellowish, and we enter the dwarf stage, of which our own sun is a representative. the density increases, surpassing that of water in the case of the sun, and going far beyond this point in later stages. in the lapse of millions of years a reddish hue appears, finally turning to deep red. the falling temperature permits the chemical elements, existing in a gaseous state in the outer atmosphere of the star, to unite into compounds, which are rendered conspicuous by their characteristic bands in the spectrum. finally comes extinction of light, as the star approaches its ultimate state of a cold and solid globe. [illustration: fig. . the giant star antares (within the white circle), notable for its red color in the constellation scorpio, and named by the greeks "a rival of mars" (hubble). the distance of antares, though not very accurately known, is probably not far from light-years. its angular diameter of . of a second would thus correspond to a linear diameter of about million miles.] we may thus form a new picture of the two branches of the temperature curve, long since suggested by lockyer, on very different grounds, as the outline of stellar life. on the ascending side are the giants, of vast dimensions and more diffuse than the air we breathe. there are good reasons for believing that the mass of betelgeuse cannot be more than ten times that of the sun, while its volume is at least a million times as great and may exceed eight million times the sun's volume. therefore, its average density must be like that of an attenuated gas in an electric vacuum tube. three-quarters of the naked-eye stars are in the giant stage, which comprises such familiar objects as betelgeuse, antares, and aldebaran, but most of them are much denser than these greatly inflated bodies. the pinnacle is reached in the intensely hot white stars of the helium class, in whose spectra the lines of this gas are very conspicuous. the density of these stars is perhaps one-tenth that of the sun. sirius, also very hot, is nearly twice as dense. then comes the cooling stage, characterized, as already remarked, by increasing density, and also by increasing chemical complexity resulting from falling temperature. this life cycle is probably not followed by all stars, but it may hold true for millions of them. the existence of giant and dwarf stars has been fully proved by the remarkable work of adams and his associates on mount wilson, where his method of determining a star's distance and intrinsic luminosity by spectroscopic observations has already been applied to , stars. discussion of the results leads at once to the recognition of the two great classes of giants and dwarfs. now comes the work of michelson and pease to cap the climax, giving us the actual diameter of a typical giant star, in close agreement with predictions based upon theory. from this diameter we may conclude that the density of betelgeuse is extremely low, in harmony with russell's theory, which is further supported by spectroscopic analysis of the star's light, revealing evidence of the comparatively low temperature called for by the theory at this early stage of stellar existence. two other giants the diameter of arcturus was successfully measured by mr. pease at mount wilson on april . as the mirrors of the interferometer were moved apart, the fringes gradually decreased in visibility until they finally disappeared at a mirror separation of . feet. adopting a mean wave-length of / of a millimetre for the light of arcturus, this gives a value of . of a second of arc for the angular diameter of the star. if we use a mean value of . of a second for the parallax, the corresponding linear diameter comes out , , miles. the angular diameter, as in the case of betelgeuse, is in remarkably close agreement with the diameter predicted from theory. antares, the third star measured by mr. pease, is the largest of all. if it is actually a member of the scorpius-centaurus group, as we have strong reason to believe, it is fully light-years from the earth, and its diameter is about , , miles. [illustration: fig. . diameters of the sun, arcturus, betelgeuse, and antares compared with the orbit of mars. sun, diameter, , miles. arcturus, diameter, , , miles. betelgeuse, diameter, , , miles. antares, diameter, , , miles.] it now remains to make further measures of betelgeuse, especially because its marked changes in brightness suggest possible variations in diameter. we must also apply the interferometer method to stars of the various spectral types, in order to afford a sure basis for future studies of stellar evolution. unfortunately, only a few giant stars are certain to fall within the range of our present instrument. an interferometer of -feet aperture would be needed to measure sirius accurately, and one of twice this size to deal with less brilliant white stars. a -foot instrument, if feasible to build, would permit objects representing most of the chief stages of stellar development to be measured, thus contributing in the highest degree to the progress of our knowledge of the life history of the stars. fortunately, though the mechanical difficulties are great, the optical problem is insignificant, and the cost of the entire apparatus, though necessarily high, would be only a small fraction of that of a telescope of corresponding aperture, if such could be built. a -foot interferometer might be designed in many different forms, and one of these may ultimately be found to be within the range of possibility. meanwhile the -foot interferometer has been improved so materially that it now promises to yield approximate measures of stars at first supposed to be beyond its capacity. [illustration: fig. . aldebaran, the "leader" (of the pleiades), was also known to the arabs as "the eye of the bull," "the heart of the bull," and "the great camel" (hubble). like betelgeuse and antares, it is notable for its red color, which accounts for the fact that its image on this photograph is hardly more conspicuous than the images of stars which are actually much fainter but contain a larger proportion of blue light, to which the photographic plates here employed are more sensitive than to red or yellow. aldebaran is about light-years from the earth. interferometer measures, now in progress on mount wilson, indicate that its angular diameter is about . of a second.] while the theory of dwarf and giant stars and the measurements just described afford no direct evidence bearing on laplace's explanation of the formation of planets, they show that stars exist which are comparable in diameter with our solar system, and suggest that the sun must have shrunk from vast dimensions. the mode of formation of systems like our own, and of other systems numerously illustrated in the heavens, is one of the most fascinating problems of astronomy. much light has been thrown on it by recent investigations, rendered possible by the development of new and powerful instruments and by advances in physics of the most fundamental character. all the evidence confirms the existence of dwarf and giant stars, but much work must be done before the entire course of stellar evolution can be explained. chapter iii cosmic crucibles "shelter during raids," marking the entrance to underground passages, was a sign of common occurrence and sinister suggestion throughout london during the war. with characteristic ingenuity and craftiness, ostensibly for purposes of peace but with bomb-carrying capacity as a prime specification, the zeppelin had been developed by the germans to a point where it seriously threatened both london and paris. searchlights, range-finders, and anti-aircraft guns, surpassed by the daring ventures of british and french airmen, would have served but little against the night invader except for its one fatal defect--the inflammable nature of the hydrogen gas that kept it aloft. a single explosive bullet served to transform a zeppelin into a heap of scorched and twisted metal. this characteristic of hydrogen caused the failure of the zeppelin raids. had the war lasted a few months longer, however, the work of american scientists would have made our counter-attack in the air a formidable one. at the signing of the armistice hundreds of cylinders of compressed helium lay at the docks ready for shipment abroad. extracted from the natural gas of texas wells by new and ingenious processes, this substitute for hydrogen, almost as light and absolutely uninflammable, produced in quantities of millions of cubic feet, would have made the dirigibles of the allies masters of the air. the special properties of this remarkable gas, previously obtainable only in minute quantities, would have sufficed to reverse the situation. solar helium helium, as its name implies, is of solar origin. in , when lockyer first directed his spectroscope to the great flames or prominences that rise thousands of miles, sometimes hundreds of thousands, above the surface of the sun, he instantly identified the characteristic red and blue radiations of hydrogen. in the yellow, close to the position of the well-known double line of sodium, but not quite coincident with it, he detected a new line, of great brilliancy, extending to the highest levels. its similarity in this respect with the lines of hydrogen led him to recognize the existence of a new and very light gas, unknown to terrestrial chemistry. many years passed before any chemical laboratory on earth was able to match this product of the great laboratory of the sun. in ramsay at last succeeded in separating helium, recognized by the same yellow line in its spectrum, in minute quantities from the mineral uraninite. once available for study under electrical excitation in vacuum tubes, helium was found to have many other lines in its spectrum, which have been identified in the spectra of solar prominences, gaseous nebulæ, and hot stars. indeed, there is a stellar class known as helium stars, because of the dominance of this gas in their atmospheres. [illustration: fig. . solar prominences, photographed with the spectroheliograph without an eclipse (ellerman). in these luminous gaseous clouds, which sometimes rise to elevations exceeding half the sun's diameter, the new gas helium was discovered by lockyer in . helium was not found on the earth until . since then it has been shown to be a prominent constituent of nebulæ and hot stars.] the chief importance of helium lies in the clue it has afforded to the constitution of matter and the transmutation of the elements. radium and other radioactive substances, such as uranium, spontaneously emit negatively charged particles of extremely small mass (electrons), and also positively charged particles of much greater mass, known as alpha particles. rutherford and geiger actually succeeded in counting the number of alpha particles emitted per second by a known mass of radium, and showed that these were charged helium atoms. to discuss more at length the extraordinary characteristics of helium, which plays so large a part in celestial affairs, would take us too far afield. let us therefore pass to another case in which a fundamental discovery, this time in physics, was first foreshadowed by astronomical observation. sun-spots as magnets no archæologist, whether young or champollion deciphering the rosetta stone, or rawlinson copying the cuneiform inscription on the cliff of behistun, was ever faced by a more fascinating problem than that which confronts the solar physicist engaged in the interpretation of the hieroglyphic lines of sun-spot spectra. the colossal whirling storms that constitute sun-spots, so vast that the earth would make but a moment's scant mouthful for them, differ materially from the general light of the sun when examined with the spectroscope. observing them visually many years ago, the late professor young, of princeton, found among their complex features a number of double lines which he naturally attributed, in harmony with the physical knowledge of the time, to the effect of "reversal" by superposed layers of vapors of different density and temperature. what he actually saw, however, as was proved at the mount wilson observatory in , was the effect of a powerful magnetic field on radiation, now known as the zeeman effect. [illustration: fig. . the -foot tower telescope of the mount wilson observatory. an image of the sun about inches in diameter is formed in the laboratory at the base of the tower. below this, in a well extending feet into the earth, is the powerful spectroscope with which the magnetic fields in sun-spots and the general magnetic field of the sun are studied.] faraday was the first to detect the influence of magnetism on light. between the poles of a large electromagnet, powerful for those days ( ), he placed a block of very dense glass. the plane of polarization of a beam of light, which passed unaffected through the glass before the switch was closed, was seen to rotate when the magnetic field was produced by the flow of the current. a similar rotation is now familiar in the well-known tests of sugars--lævulose and dextrose--which rotate plane-polarized light to left and right, respectively. but in this first discovery of a relationship between light and magnetism faraday had not taken the more important step that he coveted--to determine whether the vibration period of a light-emitting particle is subject to change in a magnetic field. he attempted this in --the last experiment of his life. a sodium flame was placed between the poles of a magnet, and the yellow lines were watched in a spectroscope when the magnet was excited. no change could be detected, and none was found by subsequent investigators until zeeman, of leiden, with more powerful instruments made his famous discovery, the twenty-fifth anniversary of which has recently been celebrated. [illustration: fig. . pasadena laboratory of the mount wilson observatory. showing the large magnet (on the left) and the spectroscopes used for the study of the effect of magnetism on radiation. a single line in the spectrum is split by the magnetic field into from three to twenty-one components, as illustrated in fig. . the corresponding lines in the spectra of sun-spots are split up in precisely the same way, thus indicating the presence of powerful magnetic fields in the sun.] his method of procedure was similar to faraday's, but his magnet and spectroscope were much more powerful, and a theory due to lorentz, predicting the nature of the change to be expected, was available as a check on his results. when the current was applied the lines were seen to widen. in a still more powerful magnetic field each of them split into two components (when the observation was made along the lines of force), and the light of the components of each line was found to be circularly polarized in opposite directions. strictly in harmony with lorentz's theory, this splitting and polarization proved the presence in the luminous vapor of exactly such negatively charged electrons as had been indicated there previously by very different experimental methods. in great cyclonic storms, or vortices, were discovered at the mount wilson observatory centring in sun-spots. such whirling masses of hot vapors, inferred from sir joseph thomson's results to contain electrically charged particles, should give rise to a magnetic field. this hypothesis at once suggested that the double lines observed by young might really represent the zeeman effect. the test was made, and all the characteristic phenomena of radiation in a magnetic field were found. thus a great physical experiment is constantly being performed for us in the sun. every large sunspot contains a magnetic field covering many thousands of square miles, within which the spectrum lines of iron, manganese, chromium, titanium, vanadium, calcium, and other metallic vapors are so powerfully affected that their widening and splitting can be seen with telescopes and spectroscopes of moderate size. the tower telescope both of these illustrations show how the physicist and chemist, when adequately armed for astronomical attack, can take advantage in their studies of the stupendous processes visible in cosmic crucibles, heated to high temperatures and influenced, as in the case of sun-spots, by intense magnetic fields. certain modern instruments, like the -foot and -foot tower telescopes on mount wilson, are especially designed for observing the course of these experiments. the second of these telescopes produces at a fixed point in a laboratory an image of the sun about inches in diameter, thus enlarging the sun-spots to such a scale that the magnetic phenomena of their various parts can be separately studied. this analysis is accomplished with a spectroscope feet in length, mounted in a subterranean chamber beneath the tower. the varied results of such investigations cannot be described here. only one of them may be mentioned--the discovery that the entire sun, rotating on its axis, is a great magnet. hence we may reasonably infer that every star, and probably every planet, is also a magnet, as the earth has been known to be since the days of gilbert's "de magnete." here lies one of the best clues for the physicist who seeks the cause of magnetism, and attempts to produce it, as barnett has recently succeeded in doing, by rapidly whirling masses of metal in the laboratory. [illustration: fig. . sun-spot vortex in the upper hydrogen atmosphere. (benioff). photographed with the spectroheliograph. the electric vortex that causes the magnetic field of the spot lies at a lower level, and is not shown by such photographs.] perhaps a word of caution should be interpolated at this point. solar magnetism in no wise accounts for the sun's gravitational power. indeed, its attraction cannot be felt by the most delicate instruments at the distance of the earth, and would still be unknown were it not for the influence of magnetism on light. auroras, magnetic storms, and such electric currents as those that recently deranged several atlantic cables are due, not to the magnetism of the sun or its spots, but probably to streams of electrons, shot out from highly disturbed areas of the solar surface surrounding great sun-spots, traversing ninety-three million miles of the ether of space, and penetrating deep into the earth's atmosphere. these striking phenomena lead us into another chapter of physics, which limitations of space forbid us to pursue. stellar chemistry let us turn again to chemistry, and see where experiments performed in cosmic laboratories can serve as a guide to the investigator. a spinning solar tornado, incomparably greater in scale than the devastating whirlwinds that so often cut narrow paths of destruction through town and country in the middle west, gradually gives rise to a sun-spot. the expansion produced by the centrifugal force at the centre of the storm cools the intensely hot gases of the solar atmosphere to a point where chemical union can occur. titanium and oxygen, too hot to combine in most regions of the sun, join to form the vapor of titanium oxide, characterized in the sunspot spectrum by fluted bands, made up of hundreds of regularly spaced lines. similarly magnesium and hydrogen combine as magnesium hydride and calcium and hydrogen form calcium hydride. none of these compounds, stable at the high temperatures of sun-spots, has been much studied in the laboratory. the regions in which they exist, though cooler than the general atmosphere of the sun, are at temperatures of several thousand degrees, attained in our laboratories only with the aid of such devices as powerful electric furnaces. [illustration: fig. . splitting of spectrum lines by a magnetic field (babcock). the upper and lower strips show lines in the spectrum of chromium, observed without a magnetic field. when subjected to the influence of magnetism, these single lines are split into several components. thus the first line on the right is resolved by the field into three components, one of which (plane polarized) appears in the second strip, while the other two, which are polarized in a plane at right angles to that of the middle component, are shown on the third strip. the next line is split by the magnetic field into twelve components, four of which appear in the second strip and eight in the third. the magnetic fields in sun-spots affect these lines in precisely the same way.] it is interesting to follow our line of reasoning to the stars, which differ widely in temperature at various stages in their life-cycle.[*] a sun-spot is a solar tornado, wherein the intensely hot solar vapors are cooled by expansion, giving rise to the compounds already named. a red star, in russell's scheme of stellar evolution, is a cooler sun, vast in volume and far more tenuous than atmospheric air when in the initial period of the "giant" stage, but compressed and denser than water in the "dwarf" stage, into which our sun has already entered as it gradually approaches the last phases of its existence. therefore we should find, throughout the entire atmosphere of such stars, some of the same compounds that are produced within the comparatively small limits of a sun-spot. this, of course, on the correct assumption that sun and stars are made of the same substances. fowler has already identified the bands of titanium oxide in such red stars as the giant betelgeuse, and in others of its class. it is safe to predict that an interesting chapter in the chemistry of the future will be based upon the study of such compounds, both in the laboratory and under the progressive temperature conditions afforded by the countless stellar "giants" and "dwarfs" that precede and follow the solar state. [footnote *: see chapter ii.] [illustration: fig. . electric furnace in the pasadena laboratory of the mount wilson observatory. with which the chemical phenomena observed in sun-spots and red stars are experimentally imitated.] astrophysical laboratories it is precisely in this long sequence of physical and chemical changes that the astrophysicist and the astrochemist can find the means of pushing home their attack. it is true, of course, that the laboratory investigator has a great advantage in his ability to control his experiments, and to vary their progress at will. but by judicious use of the transcendental temperatures, far out ranging those of his furnaces, and extreme conditions, which he can only partially imitate, afforded by the sun, stars, and nebulæ, he may greatly widen the range of his inquiries. the sequence of phenomena seen during the growth of a sun-spot, or the observation of spots of different sizes, and the long series of successive steps that mark the rise and decay of stellar life, resemble the changes that the experimenter brings about as he increases and diminishes the current in the coils of his magnet or raises and lowers the temperature of his electric furnace, examining from time to time the spectrum of the glowing vapors, and noting the changes shown by the varying appearance of their lines. [illustration: fig. . titanium oxide in red stars. the upper spectrum is that of titanium in the flame of the electric arc, where its combination with oxygen gives rise to the bands of titanium oxide (fowler). the lower strip shows the spectrum of the red star mira (omicron ceti), as drawn by cortie at stonyhurst. the bands of titanium oxide are clearly present in the star.] [illustration: fig. . titanium oxide in sun-spots. the upper strip shows a portion of the spectrum of a sun-spot (ellerman); the lower one the corresponding region of the spectrum of titanium oxide (king). the fluted bands of the oxide spectrum are easily identified in the spot, where they indicate that titanium and oxygen, too hot to combine in the solar atmosphere, unite in the spot because of the cooling produced by expansion in the vortex.] astronomical observations of this character, it should be noted, are most effective when constantly tested and interpreted by laboratory experiment. indeed, a modern astrophysical observatory should be equipped like a great physical laboratory, provided on the one hand with telescopes and accessory apparatus of the greatest attainable power, and on the other with every device known to the investigator of radiation and the related physical and chemical phenomena. its telescopes, especially designed with the aims of the physicist and chemist in view, bring images of sun, stars, nebulæ, and other heavenly bodies within the reach of powerful spectroscopes, sensitive bolometers and thermopiles, and the long array of other appliances available for the measurement and analysis of radiation. its electric furnaces, arcs, sparks, and vacuum tubes, its apparatus for increasing and decreasing pressure, varying chemical conditions, and subjecting luminous gases and vapors to the influence of electric and magnetic fields, provide the means of imitating celestial phenomena, and of repeating and interpreting the experiments observed at the telescope. and the advantage thus derived, as we have seen, is not confined to the astronomer, who has often been able, by making fundamental physical and chemical discoveries, to repay his debt to the physicist and chemist for the apparatus and methods which he owes to them. newton and einstein take, for another example, the greatest law of physics--newton's law of gravitation. huge balls of lead, as used by cavendish, produce by their gravitational effect a minute rotation of a delicately suspended bar, carrying smaller balls at its extremities. but no such feeble means sufficed for newton's purpose. to prove the law of gravitation he had recourse to the tremendous pull on the moon of the entire mass of the earth, and then extended his researches to the mutual attractions of all the bodies of the solar system. later herschel applied this law to the suns which constitute double stars, and to-day adams observes from mount wilson stars falling with great velocity toward the centre of the galactic system under the combined pull of the millions of objects that compose it. thus full advantage has been taken of the possibility of utilizing the great masses of the heavenly bodies for the discovery and application of a law of physics and its reciprocal use in explaining celestial motions. [illustration: fig. . the cavendish experiment. two lead balls, each two inches in diameter, are attached to the ends of a torsion rod six feet long, which is suspended by a fine wire. the experiment consists in measuring the rotation of the suspended system, caused by the gravitational attraction of two lead spheres, each twelve inches in diameter, acting on the two small lead balls.] or consider the einstein theory of relativity, the truth or falsity of which is no less fundamental to physics. its inception sprang from the michelson-morley experiment, made in a laboratory in cleveland, which showed that motion of the earth through the ether of space could not be detected. all of the three chief tests of einstein's general theory are astronomical--because of the great masses required to produce the minute effects predicted: the motion of the perihelion of mercury, the deflection of the light of a star by the attraction of the sun, and the shift of the lines of the solar spectrum toward the red--questions not yet completely answered. but it is in the study of the constitution of matter and the evolution of the elements, the deepest and most critical problem of physics and chemistry, that the extremes of pressure and temperature in the heavenly bodies, and the prevalence of other physical conditions not yet successfully imitated on earth, promise the greatest progress. it fortunately happens that astrophysical research is now at the very apex of its development, founded as it is upon many centuries of astronomical investigation, rejuvenated by the introduction into the observatory of all the modern devices of the physicist, and strengthened with instruments of truly extraordinary range and power. these instruments bring within reach experiments that are in progress on some minute region of the sun's disk, or in some star too distant even to be glimpsed with ordinary telescopes. indeed, the huge astronomical lenses and mirrors now available serve for these remote light-sources exactly the purpose of the lens or mirror employed by the physicist to project upon the slit of his spectroscope the image of a spark or arc or vacuum tube within which atoms and molecules are exposed to the influence of the electric discharge. the physicist has the advantage of complete control over the experimental conditions, while the astrophysicist must observe and interpret the experiments performed for him in remote laboratories. in actual practice, the two classes of work must be done in the closest conjunction, if adequate utilization is to be made of either. and this is only natural, for the trend of recent research has made clear the fact that one of the three greatest problems of modern astronomy and astrophysics, ranking with the structure of the universe and the evolution of celestial bodies, is the constitution of matter. let us see why this is so. transmutation of the elements the dream of the alchemist was to transmute one element into another, with the prime object of producing gold. such transmutation has been actually accomplished within the last few years, but the process is invariably one of disintegration--the more complex elements being broken up into simpler constituents. much remains to be done in this same direction; and here the stars and nebulæ, which show the spectra of the elements under a great variety of conditions, should help to point the way. the progressive changes in spectra, from the exclusive indications of the simple elements hydrogen, helium, nitrogen, possibly carbon, and the terrestrially unknown gas nebulium in the gaseous nebulæ, to the long list of familiar substances, including several chemical compounds, in the red stars, may prove to be fundamentally significant when adequately studied from the standpoint of the investigator of atomic structure. the existing evidence seems to favor the view, recently expressed by saha, that many of these differences are due to varying degrees of ionization, the outer electrons of the atoms being split off by high temperature or electrical excitation. it is even possible that cosmic crucibles, unrivalled by terrestrial ones, may help materially to reveal the secret of the formation of complex elements from simpler ones. physicists now believe that all of the elements are compounded of hydrogen atoms, bound together by negative electrons. thus helium is made up of four hydrogen atoms, yet the atomic weight of helium ( ) is less than four times that of hydrogen ( . ). the difference may represent the mass of the electrical energy released when the transmutation occurred. [illustration: fig. . the trifid nebula in sagittarius (ritchey). the gas "nebulium," not yet found on the earth, is the most characteristic constituent of irregular nebulæ. nebulium is recognized by two green lines in its spectrum, which cause the green color of nebulæ of the gaseous type.] eddington has speculated in a most interesting way on this possible source of stellar heat in his recent presidential address before the british association for the advancement of science (see _nature_, september , ). he points out that the old contraction hypothesis, according to which the source of solar and stellar heat was supposed to reside in the slow condensation of a radiating mass of gas under the action of gravity, is wholly inadequate to explain the observed phenomena. if the old view were correct, the earlier history of a star, from the giant stage of a cool and diaphanous gas to the period of highest temperature, would be run through within eighty thousand years, whereas we have the best of evidence that many thousands of centuries would not suffice. some other source of energy is imperatively needed. if per cent of a star's mass consists originally of hydrogen atoms, which gradually combine in the slow process of time to form more complex elements, the total heat thus liberated would more than suffice to account for all demands, and it would be unnecessary to assume the existence of any other source of heat. [illustration: fig. . spiral nebula in ursa major (ritchey). luminous matter, in every variety of physical and chemical state, is available for study in the most diverse celestial objects, from the spiral and irregular nebulæ through all the types of stars. doctor van maanen's measures of the mount wilson photographs indicate outward motion along the arms of spiral nebulæ, while the spectroscope shows them to be whirling at enormous velocities.] cosmic pressures this, it may fairly be said, is very speculative, but the fact remains that celestial bodies appear to be the only places in which the complex elements may be in actual process of formation from their known source--hydrogen. at least we may see what a vast variety of physical conditions these cosmic crucibles afford. at one end of the scale we have the excessively tenuous nebulæ, the luminosity of which, mysterious in its origin, resembles the electric glow in our vacuum tubes. here we can detect only the lightest and simplest of the elements. in the giant stars, also extremely tenuous (the density of betelgeuse can hardly exceed one-thousandth of an atmosphere) we observe the spectra of iron, manganese, titanium, calcium, chromium, magnesium, vanadium, and sodium, in addition to titanium oxide. the outer part of these bodies, from which light reaches us, must therefore be at a temperature of only a few thousand degrees, but vastly higher temperatures must prevail at their centres. in passing up the temperature curve more and more elements appear, the surface temperature rises, and the internal temperature may reach millions of degrees. at the same time the pressure within must also rise, reaching enormous figures in the last stages of stellar life. cook has calculated that the pressure at the centre of the earth is between , and , tons per square inch, and this must be only a very small fraction of that attained within larger celestial bodies. jeans has computed the pressure at the centre of two colliding stars as they strike and flatten, and finds it may be of the order of , , , tons per square inch--sufficient, if their diameter be equal to that of the sun--to vaporize them , times over. compare these pressures with the highest that can be produced on earth. if the german gun that bombarded paris were loaded with a solid steel projectile of suitable dimensions, a muzzle velocity of , feet per second could be reached. suppose this to be fired into a tapered hole in a great block of steel. the instantaneous pressure, according to cook, would be about , tons per square inch, only / of that possible through the collision of the largest stars. [illustration: fig. . mount san antonio as seen from mount wilson. michelson is measuring the velocity of light between stations on mount wilson and mount san antonio. astronomical observations afford the best means, however, of detecting any possible difference between the velocities of light of different colors. from studies of variable stars in the cluster messier shapley concludes that if there is any difference between the velocities of blue and yellow light in free space it cannot exceed two inches in one second, the time in which light travels , miles.] finally, we may compare the effects of light pressure on the earth and stars. twenty years ago nichols and hull succeeded, with the aid of the most sensitive apparatus, in measuring the minute displacements produced by the pressure of light. the effect is so slight, even with the brightest light-sources available, that great experimental skill is required to measure it. yet in the case of some of the larger stars eddington calculates that one-half of their mass is supported by radiation pressure, and this against their enormous gravitational attraction. in fact, if their mass were as great as ten times that of the sun, the radiation pressure would so nearly overcome the pull of gravitation that they would be likely to break up. but enough has been said to illustrate the wide variety of experimental devices that stand at our service in the laboratories of the heavens. here the physicist and chemist of the future will more and more frequently supplement their terrestrial apparatus, and find new clues to the complex problems which the amazing progress of recent years has already done so much to solve. practical value of researches on the constitution of matter the layman has no difficulty in recognizing the practical value of researches directed toward the improvement of the incandescent lamp or the increased efficiency of the telephone. he can see the results in the greatly decreased cost of electric illumination and the rapid extension of the range of the human voice. but the very men who have made these advances, those who have succeeded beyond all expectation in accomplishing the economic purposes in view, are most emphatic in their insistence upon the importance of research of a more fundamental character. thus vice-president j. j. carty, of the american telephone and telegraph company, who directs its great department of development and research, and doctor w. j. whitney, director of the research laboratory of the general electric company, have repeatedly expressed their indebtedness to the investigations of the physicist, made with no thought of immediate practical return. faraday, studying the laws of electricity, discovered the principle which rendered the dynamo possible. maxwell, henry, and hertz, equally unconcerned with material advantage, made wireless telegraphy practicable. in fact, all truly great advances are thus derived from fundamental science, and the future progress of the world will be largely dependent upon the provision made for scientific research, especially in the fields of physics and chemistry, which underlie all branches of engineering. the constitution of matter, therefore, instead of appealing as a subject to research only to the natural philosopher or to the general student of science, is a question of the greatest practical concern. already the by-products of investigations directed toward its elucidation have been numerous and useful in the highest degree. helium has been already cited; x-rays hardly require mention; radium, which has so materially aided sufferers from cancer, is still better known. wireless telephony and transcontinental telephony with wires were both rendered possible by studies of the nature of the electric discharge in vacuum tubes. thus the "practical man," with his distrust of "pure" science, need not resent investments made for the purpose of advancing our knowledge of such fundamental subjects as physics and chemistry. on the contrary, if true to his name, he should help to multiply them many fold in the interest of economic and commercial development. note: project gutenberg also has an html version of this file which includes the original illustrations. see -h.htm or -h.zip: (http://www.gutenberg.net/dirs/ / / / / / -h/ -h.htm) or (http://www.gutenberg.net/dirs/ / / / / / -h.zip) a popular history of astronomy during the nineteenth century * * * * * by the same author problems in astrophysics. demy vo., cloth. containing over illustrations. price s. net. the system of the stars. second edition. thoroughly revised and largely rewritten. containing numerous and new illustrations. demy vo., cloth. price s. net. modern cosmogonies. crown vo., cloth. price s. d. net. a. and c. black, soho square, london, w. * * * * * [illustration: the great nebula in orion, _see p. _] a popular history of astronomy during the nineteenth century by agnes m. clerke [illustration: jupiter saturn ] london adam and charles black first edition, post vo., published second edition, post vo., published third edition, demy vo., published fourth edition, demy vo., published fourth edition, post vo., reprinted february, preface to the fourth edition since the third edition of the present work issued from the press, the nineteenth century has run its course and finished its record. a new era has dawned, not by chronological prescription alone, but to the vital sense of humanity. novel thoughts are rife; fresh impulses stir the nations; the soughing of the wind of progress strikes every ear. "the old order changeth" more and more swiftly as mental activity becomes intensified. already many of the scientific doctrines implicitly accepted fifteen years ago begin to wear a superannuated aspect. dalton's atoms are in process of disintegration; kirchhoff's theorem visibly needs to be modified; clerk maxwell's medium no longer figures as an indispensable factotum; "absolute zero" is known to be situated on an asymptote to the curve of cold. ideas, in short, have all at once become plastic, and none more completely so than those relating to astronomy. the physics of the heavenly bodies, indeed, finds its best opportunities in unlooked-for disclosures; for it deals with transcendental conditions, and what is strange to terrestrial experience may serve admirably to expound what is normal in the skies. in celestial science especially, facts that appear subversive are often the most illuminative, and the prospect of its advance widens and brightens with each divagation enforced or permitted from the strait paths of rigid theory. this readiness for innovation has undoubtedly its dangers and drawbacks. to the historian, above all, it presents frequent occasions of embarrassment. the writing of history is a strongly selective operation, the outcome being valuable just in so far as the choice what to reject and what to include has been judicious; and the task is no light one of discriminating between barren speculations and ideas pregnant with coming truth. to the possession of such prescience of the future as would be needed to do this effectually i can lay no claim; but diligence and sobriety of thought are ordinarily within reach, and these i shall have exercised to good purpose if i have succeeded in rendering the fourth edition of _a popular history of astronomy during the nineteenth century_ not wholly unworthy of a place in the scientific literature of the twentieth century. my thanks are due to sir david gill for the use of his photograph of the great comet of , which i have added to my list of illustrations, and to the council of the royal astronomical society for the loan of glass positives needed for the reproduction of those included in the third edition. london, _july_, . preface to the first edition the progress of astronomy during the last hundred years has been rapid and extraordinary. in its distinctive features, moreover, the nature of that progress has been such as to lend itself with facility to untechnical treatment. to this circumstance the present volume owes its origin. it embodies an attempt to enable the ordinary reader to follow, with intelligent interest, the course of modern astronomical inquiries, and to realize (so far as it can at present be realized) the full effect of the comprehensive change in the whole aspect, purposes, and methods of celestial science introduced by the momentous discovery of spectrum analysis. since professor grant's invaluable work on the _history of physical astronomy_ was published, a third of a century has elapsed. during the interval a so-called "new astronomy" has grown up by the side of the old. one effect of its advent has been to render the science of the heavenly bodies more popular, both in its needs and in its nature, than formerly. more popular in its needs, since its progress now primarily depends upon the interest in, and consequent efforts towards its advancement of the general public; more popular in its nature, because the kind of knowledge it now chiefly tends to accumulate is more easily intelligible--less remote from ordinary experience--than that evolved by the aid of the calculus from materials collected by the use of the transit-instrument and chronograph. it has thus become practicable to describe in simple language the most essential parts of recent astronomical discoveries, and, being practicable, it could not be otherwise than desirable to do so. the service to astronomy itself would be not inconsiderable of enlisting wider sympathies on its behalf, while to help one single mind towards a fuller understanding of the manifold works which have in all ages irresistibly spoken to man of the glory of god might well be an object of no ignoble ambition. the present volume does not profess to be a complete or exhaustive history of astronomy during the period covered by it. its design is to present a view of the progress of celestial science, on its most characteristic side, since the time of herschel. abstruse mathematical theories, unless in some of their more striking results, are excluded from consideration. these, during the eighteenth century, constituted the sum and substance of astronomy, and their fundamental importance can never be diminished, and should never be ignored. but as the outcome of the enormous development given to the powers of the telescope in recent times, together with the swift advance of physical science, and the inclusion, by means of the spectroscope, of the heavenly bodies within the domain of its inquiries, much knowledge has been acquired regarding the nature and condition of those bodies, forming, it might be said, a science apart, and disembarrassed from immediate dependence upon intricate, and, except to the initiated, unintelligible formulæ. this kind of knowledge forms the main subject of the book now offered to the public. there are many reasons for preferring a history to a formal treatise on astronomy. in a treatise, _what_ we know is set forth. a history tells us, in addition, _how_ we came to know it. it thus places facts before us in the natural order of their ascertainment, and narrates instead of enumerating. the story to be told leaves the marvels of imagination far behind, and requires no embellishment from literary art or high-flown phrases. its best ornament is unvarnished truthfulness, and this, at least, may confidently be claimed to be bestowed upon it in the ensuing pages. in them unity of treatment is sought to be combined with a due regard to chronological sequence by grouping in separate chapters the various events relating to the several departments of descriptive astronomy. the whole is divided into two parts, the line between which is roughly drawn at the middle of the present century. herschel's inquiries into the construction of the heavens strike the keynote of the first part; the discoveries of sun-spot and magnetic periodicity and of spectrum analysis determine the character of the second. where the nature of the subject required it, however, this arrangement has been disregarded. clearness and consistency should obviously take precedence of method. thus, in treating of the telescopic scrutiny of the various planets, the whole of the related facts have been collected into an uninterrupted narrative. a division elsewhere natural and helpful would here have been purely artificial, and therefore confusing. the interests of students have been consulted by a full and authentic system of references to the sources of information relied upon. materials have been derived, as a rule with very few exceptions, from the original authorities. the system adopted has been to take as little as possible at second-hand. much pains have been taken to trace the origin of ideas, often obscurely enunciated long before they came to resound through the scientific world, and to give to each individual discoverer, strictly and impartially, his due. prominence has also been assigned to the biographical element, as underlying and determining the whole course of human endeavour. the advance of knowledge may be called a vital process. the lives of men are absorbed into and assimilated by it. inquiries into the kind and mode of the surrender in each separate case must always possess a strong interest, whether for study or for example. the acknowledgments of the writer are due to professor edward s. holden, director of the washburn observatory, wisconsin, and to dr. copeland, chief astronomer of lord crawford's observatory at dunecht, for many valuable communications. london, _september_, . contents _introduction_ page three kinds of astronomy--progress of the science during the eighteenth century--popularity and rapid advance during the nineteenth century part i _progress of astronomy during the first half of the nineteenth century_ chapter i foundation of sidereal astronomy state of knowledge regarding the stars in the eighteenth century-- career of sir william herschel--constitution of the stellar system-- double stars--herschel's discovery of their revolutions-- his method of star-gauging--discoveries of nebulæ--theory of their condensation into stars--summary of results chapter ii progress of sidereal astronomy exact astronomy in germany--career of bessel--his _fundamenta astronomiæ_--career of fraunhofer--parallaxes of fixed stars--translation of the solar system--astronomy of the invisible--struve's researches in double stars--sir john herschel's exploration of the heavens--fifty years' progress chapter iii progress of knowledge regarding the sun early views as to the nature of sun-spots--wilson's observations and reasonings--sir william herschel's theory of the solar constitution--sir john herschel's trade-wind hypothesis--baily's beads--total solar eclipse of --corona and prominences--eclipse of chapter iv planetary discoveries bode's law--search for a missing planet--its discovery by piazzi-- further discoveries of minor planets--unexplained disturbance of uranus--discovery of neptune--its satellite--an eighth saturnian moon--saturn's dusky ring--the uranian system chapter v comets predicted return of halley's comet--career of olbers--acceleration of encke's comet--biela's comet--its duplication--faye's comet--comet of --electrical theory of cometary emanations--the earth in a comet's tail--second return of halley's comet--great comet of --results to knowledge chapter vi instrumental advances two principles of telescopic construction--early reflectors--three varieties--herschel's specula--high magnifying powers--invention of the achromatic lens--guinand's optical glass--the great rosse reflector--its disclosures--mounting of telescopes--astronomical circles--personal equation part ii _recent progress of astronomy_ chapter i foundation of astronomical physics schwabe's discovery of a decennial sun-spot period--coincidence with period of magnetic disturbance--sun-spots and weather--spectrum analysis--preliminary inquiries--fraunhofer lines--kirchhoff's principle--anticipations--elementary principles of spectrum analysis--unity of nature chapter ii solar observations and theories black openings in spots--carrington's observations--rotation of the sun--kirchhoff's theory of the solar constitution--faye's views--solar photography--kew observations--spectroscopic method--cyclonic theory of sun-spots--volcanic hypothesis--a solar outburst--sun-spot periodicity--planetary influence--structure of the photosphere chapter iii recent solar eclipses expeditions to spain--great indian eclipse--new method of viewing prominences--total eclipse visible in north america--spectrum of the corona--eclipse of --young's reversing layer--eclipse of --corona of --varying coronal types--egyptian eclipse--daylight coronal photography--observations at caroline island--photographs of corona in and --eclipses of , , , and --mechanical theory of corona--electro-magnetic theories--nature of corona chapter iv solar spectroscopy chemistry of prominences--study of their forms--two classes--photographs and spectrographs of prominences--their distribution--structure of the chromosphere--spectroscopic measurement of radial movements--spectroscopic determination of solar rotation--velocities of transport in the sun--lockyer's theory of dissociation--solar constituents--oxygen absorption in solar spectrum chapter v temperature of the sun thermal power of the sun--radiation and temperature--estimates of solar temperature--rosetti's and wilson's results--zöllner's method --langley's experiment at pittsburg--the sun's atmosphere--langley's bolometric researches--selective absorption by our air--the solar constant chapter vi the sun's distance difficulty of the problem--oppositions of mars--transits of venus--lunar disturbance--velocity of light--transit of --inconclusive result--opposition of mars in --measurements of minor planets--transit of --newcomb's determination of the velocity of light--combined result chapter vii planets and satellites schröter's life and work--luminous appearances during transits of mercury--mountains of mercury--intra-mercurian planets--schiaparelli's results for the rotation of mercury and venus--illusory satellite--mountains and atmosphere of venus--ashen light--solidity of the earth--variation of latitude--secular changes of climate--figure of the globe--study of the moon's surface--lunar atmosphere--new craters--thermal energy of moonlight--tidal friction chapter viii planets and satellites--(_continued_) analogy between mars and the earth--martian snowcaps, seas, and continents--climate and atmosphere--schiaparelli's canals--discovery of two martian satellites--photographic detection of minor planets--orbit of eros--distribution of the minor planets--their collective mass and estimated diameters--condition of jupiter--his spectrum--transits of his satellites--discovery of a fifth satellite--the great red spot--constitution of saturn's rings--period of rotation of the planet--variability of japetus--equatorial markings on uranus--his spectrum--rotation of neptune--trans-neptunian planets chapter ix theories of planetary evolution origin of the world according to kant--laplace's nebular hypothesis--maintenance of the sun's heat--meteoric hypothesis--radiation as an effect of contraction--regenerative theory--faye's scheme of planetary development--origin of the moon--effects of tidal friction chapter x recent comets donati's comet--the earth again involved in a comet's tail--comets of the august and november meteors--star showers--comets and meteors--biela's comet and the andromedes--holmes's comet--deflection of the leonids--orbits of meteorites--meteors with stationary radiants--spectroscopic analysis of cometary light--comet of --coggia's comet chapter xi recent comets--(_continued_) forms of comets' tails--electrical repulsion--brédikhine's three types--great southern comet--supposed previous appearances--tebbutt's comet and the comet of --successful photographs--schaeberle's comet--comet wells--sodium blaze in spectrum--great comet of --transit across the sun--relation to comets of and --cometary systems--spectral changes in comet of --brooks's comet of --swift's comet of --origin of comets chapter xii stars and nebulÆ stellar chemistry--four orders of stars--their relative ages--gaseous stars--spectroscopic star-catalogues--stellar chemistry--hydrogen spectrum in stars--the draper catalogue--velocities of stars in line of sight--spectroscopic binaries--eclipses of algol--catalogues of variables--new stars--outbursts in nebulæ--nova aurigæ--nova persei--gaseous nebulæ--variable nebulæ--movements of nebulæ--stellar and nebular photography--nebulæ in the pleiades--photographic star-charting--stellar parallax--double stars--stellar photometry--status of nebulæ--photographs and drawings of the milky way--star drift chapter xiii methods of research development of telescopic power--silvered glass reflectors--giant refractors--comparison with reflectors--the yerkes telescope--atmospheric disturbance--the lick observatory--mechanical difficulties--the equatoreal _coudé_--the photographic camera--retrospect and conclusion appendix chronology, - --chemical elements in the sun (rowland, )--epochs of sun-spot maximum and minimum from to --movements of sun and stars--list of great telescopes--list of observatories employed in the construction of the photographic chart and catalogue of the heavens index list of illustrations photograph of the great nebula in orion, _frontispiece_ photographs of jupiter, , and of saturn, _vignette_ plate i. photographs of the solar chromosphere and prominences _to face p. _ plate ii. photograph of the great comet of may, (taken at the royal observatory, cape of good hope) " plate iii. the great comet of september, (photographed at the cape of good hope) " plate iv. photographs of swift's comet, " plate v. photographic and visual spectrum of nova aurigæ " plate vi. photograph of the milky way in sagittarius " history of astronomy during the nineteenth century _introduction_ we can distinguish three kinds of astronomy, each with a different origin and history, but all mutually dependent, and composing, in their fundamental unity, one science. first in order of time came the art of observing the returns, and measuring the places, of the heavenly bodies. this was the sole astronomy of the chinese and chaldeans; but to it the vigorous greek mind added a highly complex geometrical plan of their movements, for which copernicus substituted a more harmonious system, without as yet any idea of a compelling cause. the planets revolved in circles because it was their nature to do so, just as laudanum sets to sleep because it possesses a _virtus dormitiva_. this first and oldest branch is known as "observational," or "practical astronomy." its business is to note facts as accurately as possible; and it is essentially unconcerned with schemes for connecting those facts in a manner satisfactory to the reason. the second kind of astronomy was founded by newton. its nature is best indicated by the term "gravitational"; but it is also called "theoretical astronomy."[ ] it is based on the idea of cause; and the whole of its elaborate structure is reared according to the dictates of a single law, simple in itself, but the tangled web of whose consequences can be unravelled only by the subtle agency of an elaborate calculus. the third and last division of celestial science may properly be termed "physical and descriptive astronomy." it seeks to know what the heavenly bodies are in themselves, leaving the how? and the wherefore? of their movements to be otherwise answered. now, such inquiries became possible only through the invention of the telescope, so that galileo was, in point of fact, their originator. but herschel first gave them a prominence which the whole progress of science during the nineteenth century served to confirm and render more exclusive. inquisitions begun with the telescope have been extended and made effective in unhoped-for directions by the aid of the spectroscope and photographic camera; and a large part of our attention in the present volume will be occupied with the brilliant results thus achieved. the unexpected development of this new physical-celestial science is the leading fact in recent astronomical history. it was out of the regular course of events. in the degree in which it has actually occurred it could certainly not have been foreseen. it was a seizing of the prize by a competitor who had hardly been thought qualified to enter the lists. orthodox astronomers of the old school looked with a certain contempt upon observers who spent their nights in scrutinising the faces of the moon and planets rather than in timing their transits, or devoted daylight energies, not to reductions and computations, but to counting and measuring spots on the sun. they were regarded as irregular practitioners, to be tolerated perhaps, but certainly not encouraged. the advance of astronomy in the eighteenth century ran in general an even and logical course. the age succeeding newton's had for its special task to demonstrate the universal validity, and trace the complex results, of the law of gravitation. the accomplishment of that task occupied just one hundred years. it was virtually brought to a close when laplace explained to the french academy, november , , the cause of the moon's accelerated motion. as a mere machine, the solar system, so far as it was then known, was found to be complete and intelligible in all its parts; and in the _mécanique céleste_ its mechanical perfections were displayed under a form of majestic unity which fitly commemorated the successive triumphs of analytical genius over problems amongst the most arduous ever dealt with by the mind of man. theory, however, demands a practical test. all its data are derived from observation; and their insecurity becomes less tolerable as it advances nearer to perfection. observation, on the other hand, is the pitiless critic of theory; it detects weak points, and provokes reforms which may be the beginnings of discovery. thus, theory and observation mutually act and react, each alternately taking the lead in the endless race of improvement. now, while in france lagrange and laplace were bringing the gravitational theory of the solar system to completion, work of a very different kind, yet not less indispensable to the future welfare of astronomy, was being done in england. the royal observatory at greenwich is one of the few useful institutions which date their origin from the reign of charles ii. the leading position which it still occupies in the science of celestial observation was, for near a century and a half after its foundation, an exclusive one. delambre remarked that, had all other materials of the kind been destroyed, the greenwich records alone would suffice for the restoration of astronomy. the establishment was indeed absolutely without a rival.[ ] systematic observations of sun, moon, stars, and planets were during the whole of the eighteenth century made only at greenwich. here materials were accumulated for the secure correction of theory, and here refinements were introduced by which the exquisite accuracy of modern practice in astronomy was eventually attained. the chief promoter of these improvements was james bradley. few men have possessed in an equal degree with him the power of seeing accurately, and reasoning on what they see. he let nothing pass. the slightest inconsistency between what appeared and what was to be expected roused his keenest attention; and he never relaxed his mental grip of a subject until it had yielded to his persistent inquisition. it was to these qualities that he owed his discoveries of the aberration of light and the nutation of the earth's axis. the first was announced in . what is meant by it is that, owing to the circumstance of light not being instantaneously transmitted, the heavenly bodies appear shifted from their true places by an amount depending upon the ratio which the velocity of light bears to the speed of the earth in its orbit. because light travels with enormous rapidity, the shifting is very slight; and each star returns to its original position at the end of a year. bradley's second great discovery was finally ascertained in . nutation is a real "nodding" of the terrestrial axis produced by the dragging of the moon at the terrestrial equatorial protuberance. from it results an _apparent_ displacement of the stars, each of them describing a little ellipse about its true or "mean" position, in a period of nearly nineteen years. now, an acquaintance with the fact and the laws of each of these minute irregularities is vital to the progress of observational astronomy; for without it the places of the heavenly bodies could never be accurately known or compared. so that bradley, by their detection, at once raised the science to a higher grade of precision. nor was this the whole of his work. appointed astronomer-royal in , he executed during the years - a series of observations which formed the real beginning of exact astronomy. part of their superiority must, indeed, be attributed to the co-operation of john bird, who provided bradley in with a measuring instrument of till then unequalled excellence. for not only was the art of observing in the eighteenth century a peculiarly english art, but the means of observing were furnished almost exclusively by british artists. john dollond, the son of a spitalfields weaver, invented the achromatic lens in , removing thereby the chief obstacle to the development of the powers of refracting telescopes; james short, of edinburgh, was without a rival in the construction of reflectors; the sectors, quadrants, and circles of graham, bird, ramsden, and cary were inimitable by continental workmanship. thus practical and theoretical astronomy advanced on parallel lines in england and france respectively, the improvement of their several tools--the telescope and the quadrant on the one side, and the calculus on the other--keeping pace. the whole future of the science seemed to be theirs. the cessation of interest through a too speedy attainment of the perfection towards which each spurred the other, appeared to be the only danger it held in store for them. when all at once, a rival stood by their side--not, indeed, menacing their progress, but threatening to absorb their popularity. the rise of herschel was the one conspicuous anomaly in the astronomical history of the eighteenth century. it proved decisive of the course of events in the nineteenth. it was unexplained by anything that had gone before; yet all that came after hinged upon it. it gave a new direction to effort; it lent a fresh impulse to thought. it opened a channel for the widespread public interest which was gathering towards astronomical subjects to flow in. much of this interest was due to the occurrence of events calculated to arrest the attention and excite the wonder of the uninitiated. the predicted return of halley's comet in verified, after an unprecedented fashion, the computations of astronomers. it deprived such bodies for ever of their portentous character; it ranked them as denizens of the solar system. again, the transits of venus in and were the first occurrences of the kind since the awakening of science to their consequence. imposing preparations, journeys to remote and hardly accessible regions, official expeditions, international communications, all for the purpose of observing them to the best advantage, brought their high significance vividly to the public consciousness; a result aided by the facile pen of lalande, in rendering intelligible the means by which these elaborate arrangements were to issue in an accurate knowledge of the sun's distance. lastly, herschel's discovery of uranus, march , , had the surprising effect of utter novelty. since the human race had become acquainted with the company of the planets, no addition had been made to their number. the event thus broke with immemorial traditions, and seemed to show astronomy as still young and full of unlooked-for possibilities. further popularity accrued to the science from the sequel of a career so strikingly opened. herschel's huge telescopes, his detection by their means of two saturnian and as many uranian moons, his piercing scrutiny of the sun, picturesque theory of its constitution, and sagacious indication of the route pursued by it through space; his discovery of stellar revolving systems, his bold soundings of the universe, his grandiose ideas, and the elevated yet simple language in which they were conveyed--formed a combination powerfully effective to those least susceptible of new impressions. nor was the evoked enthusiasm limited to the british isles. in germany, schröter followed--_longo intervallo_--in herschel's track. von zach set on foot from gotha that general communication of ideas which gives life to a forward movement. bode wrote much and well for unlearned readers. lalande, by his popular lectures and treatises, helped to form an audience which laplace himself did not disdain to address in the _exposition du système du monde_. this great accession of public interest gave the impulse to the extraordinarily rapid progress of astronomy in the nineteenth century. official patronage combined with individual zeal sufficed for the elder branches of the science. a few well-endowed institutions could accumulate the materials needed by a few isolated thinkers for the construction of theories of wonderful beauty and elaboration, yet precluded, by their abstract nature, from winning general applause. but the new physical astronomy depends for its prosperity upon the favour of the multitude whom its striking results are well fitted to attract. it is, in a special manner, the science of amateurs. it welcomes the most unpretending co-operation. there is no one "with a true eye and a faithful hand" but can do good work in watching the heavens. and not unfrequently, prizes of discovery which the most perfect appliances failed to grasp, have fallen to the share of ignorant or ill-provided assiduity. observers, accordingly, have multiplied; observatories have been founded in all parts of the world; associations have been constituted for mutual help and counsel. a formal astronomical congress met in at gotha--then, under duke ernest ii. and von zach, the focus of german astronomy--and instituted a combined search for the planet suspected to revolve undiscovered between the orbits of mars and jupiter. the astronomical society of london was established in , and the similar german institution in . both have been highly influential in promoting the interests, local and general, of the science they are devoted to forward; while functions corresponding to theirs have been discharged elsewhere by older or less specially constituted bodies, and new ones of a more popular character are springing up on all sides. modern facilities of communication have helped to impress more deeply upon modern astronomy its associative character. the electric telegraph gives a certain ubiquity which is invaluable to an observer of the skies. with the help of a wire, a battery, and a code of signals, he sees whatever is visible from any portion of our globe, depending, however, upon other eyes than his own, and so entering as a unit into a widespread organisation of intelligence. the press, again, has been a potent agent of co-operation. it has mainly contributed to unite astronomers all over the world into a body animated by the single aim of collecting "particulars" in their special branch for what bacon termed a history of nature, eventually to be interpreted according to the sagacious insight of some one among them gifted above his fellows. the first really effective astronomical periodical was the _monatliche correspondenz_, started by von zach in the year . it was followed in by the _astronomische nachrichten_, later by the _memoirs_ and _monthly notices_ of the astronomical society, and by the host of varied publications which now, in every civilised country, communicate the discoveries made in astronomy to divers classes of readers, and so incalculably quicken the current of its onward flow. public favour brings in its train material resources. it is represented by individual enterprise, and finds expression in an ample liberality. the first regular observatory in the southern hemisphere was founded at paramatta by sir thomas makdougall brisbane in . the royal observatory at the cape of good hope was completed in . similar establishments were set to work by the east india company at madras, bombay, and st. helena, during the first third of the nineteenth century. the organisation of astronomy in the united states of america was due to a strong wave of popular enthusiasm. in john quincy adams vainly urged upon congress the foundation of a national observatory; but in the lectures on celestial phenomena of ormsby macknight mitchel stirred an impressionable audience to the pitch of providing him with the means of erecting at cincinnati the first astronomical establishment worthy the name in that great country. on the st of january, , no less than one hundred and forty-four were active within its boundaries. the apparition of the great comet of gave an additional fillip to the movement. to the excitement caused by it the harvard college observatory--called the "american pulkowa"--directly owed its origin; and the example was not ineffective elsewhere. the united states naval observatory was built in , lieutenant maury being its first director. corporations, universities, municipalities, vied with each other in the creation of such institutions; private subscriptions poured in; emissaries were sent to europe to purchase instruments and to procure instruction in their use. in a few years the young republic was, in point of astronomical efficiency, at least on a level with countries where the science had been fostered since the dawn of civilisation. a vast widening of the scope of astronomy has accompanied, and in part occasioned, the great extension of its area of cultivation which our age has witnessed. in the last century its purview was a comparatively narrow one. problems lying beyond the range of the solar system were almost unheeded, because they seemed inscrutable. herschel first showed the sidereal universe as accessible to investigation, and thereby offered to science new worlds--majestic, manifold, "infinitely infinite" to our apprehension in number, variety, and extent--for future conquest. their gradual appropriation has absorbed, and will long continue to absorb, the powers which it has served to develop. but this is not the only direction in which astronomy has enlarged, or rather has levelled, its boundaries. the unification of the physical sciences is perhaps the greatest intellectual feat of recent times. the process has included astronomy; so that, like bacon, she may now be said to have "taken all knowledge" (of that kind) "for her province." in return, she proffers potent aid for its increase. every comet that approaches the sun is the scene of experiments in the electrical illumination of rarefied matter, performed on a huge scale for our benefit. the sun, stars, and nebulæ form so many celestial laboratories, where the nature and mutual relations of the chemical "elements" may be tried by more stringent tests than sublunary conditions afford. the laws of terrestrial magnetism can be completely investigated only with the aid of a concurrent study of the face of the sun. the solar spectrum will perhaps one day, by its recurrent modifications, tell us something of impending droughts, famines, and cyclones. astronomy generalises the results of the other sciences. she exhibits the laws of nature working over a wider area, and under more varied conditions, than ordinary experience presents. ordinary experience, on the other hand, has become indispensable to her progress. she takes in at one view the indefinitely great and the indefinitely little. the mutual revolutions of the stellar multitude during tracts of time which seem to lengthen out to eternity as the mind attempts to traverse them, she does not admit to be beyond her ken; nor is she indifferent to the constitution of the minutest atom of matter that thrills the ether into light. how she entered upon this vastly expanded inheritance, and how, so far, she has dealt with it, is attempted to be set forth in the ensuing chapters. footnotes: [footnote : the denomination "physical astronomy," first used by kepler, and long appropriated to this branch of the science, has of late been otherwise applied.] [footnote : _histoire de l'astronomie au xviii^e siècle_, p. .] part i progress of astronomy during the first half of the nineteenth century chapter i _foundation of sidereal astronomy_ until nearly a hundred years ago the stars were regarded by practical astronomers mainly as a number of convenient fixed points by which the motions of the various members of the solar system could be determined and compared. their recognised function, in fact, was that of milestones on the great celestial highway traversed by the planets, as well as on the byways of space occasionally pursued by comets. not that curiosity as to their nature, and even conjecture as to their origin, were at any period absent. both were from time to time powerfully stimulated by the appearance of startling novelties in a region described by philosophers as "incorruptible," or exempt from change. the catalogue of hipparchus probably, and certainly that of tycho brahe, some seventeen centuries later, owed each its origin to the temporary blaze of a new star. the general aspect of the skies was thus (however imperfectly) recorded from age to age, and with improved appliances the enumeration was rendered more and more accurate and complete; but the secrets of the stellar sphere remained inviolate. in a qualified though very real sense, sir william herschel may be called the founder of sidereal astronomy. before his time some curious facts had been noted, and some ingenious speculations hazarded, regarding the condition of the stars, but not even the rudiments of systematic knowledge had been acquired. the facts ascertained can be summed up in a very few sentences. giordano bruno was the first to set the suns of space in motion; but in imagination only. his daring surmise was, however, confirmed in , when halley announced[ ] that sirius, aldebaran, betelgeux, and arcturus had unmistakably shifted their quarters in the sky since ptolemy assigned their places in his catalogue. a similar conclusion was reached by j. cassini in , from a comparison of his own observations with those made at cayenne by richer in ; and tobias mayer drew up in a list showing the direction and amount of about fifty-seven proper motions,[ ] founded on star-places determined by olaus römer fifty years previously. thus the stars were no longer regarded as "fixed," but the question remained whether the movements perceived were real or only apparent; and this it was not yet found possible to answer. already, in the previous century, the ingenious robert hooke had suggested an "alteration of the very system of the sun,"[ ] to account for certain suspected changes in stellar positions; bradley in , and lambert in , pointed out that such apparent displacements (by that time well ascertained) were in all probability a combined effect of motions both of sun and stars; and mayer actually attempted the analysis, but without result. on the th of august, , david fabricius, an unprofessional astronomer in east friesland, saw in the neck of the whale a star of the third magnitude, which by october had disappeared. it was, nevertheless, visible in , when bayer marked it in his catalogue with the greek letter omicron, and was watched, in - , through its phases of brightening and apparent extinction by a dutch professor named holwarda.[ ] from hevelius this first-known periodical star received the name of "mira," or the wonderful, and boulliaud in fixed the length of its cycle of change at days. it was not a solitary instance. a star in the swan was perceived by janson in to show fluctuations of light, and montanari found in that algol in perseus shared the same peculiarity to a marked degree. altogether the class embraced in half-a-dozen members. when it is added that a few star-couples had been noted in singularly, but it was supposed accidentally, close juxtaposition, and that the failure of repeated attempts to measure stellar parallaxes pointed to distances _at least_ , times that of the earth from the sun,[ ] the picture of sidereal science, when the last quarter of the eighteenth century began, is practically complete. it included three items of information: that the stars have motions, real or apparent; that they are immeasurably remote; and that a few shine with a periodically variable light. nor were these scantily collected facts ordered into any promise of further development. they lay at once isolated and confused before the inquirer. they needed to be both multiplied and marshalled, and it seemed as if centuries of patient toil must elapse before any reliable conclusions could be derived from them. the sidereal world was thus the recognised domain of far-reaching speculations, which remained wholly uncramped by systematic research until herschel entered upon his career as an observer of the heavens. the greatest of modern astronomers was born at hanover, november , . he was the fourth child of isaac herschel, a hautboy-player in the band of the hanoverian guard, and was early trained to follow his father's profession. on the termination, however, of the disastrous campaign of , his parents removed him from the regiment, there is reason to believe, in a somewhat unceremonious manner. technically, indeed, he incurred the penalties of desertion, remitted--according to the duke of sussex's statement to sir george airy--by a formal pardon handed to him personally by george iii. on his presentation in .[ ] at the age of nineteen, then, his military service having lasted four years, he came to england to seek his fortune. of the life of struggle and privation which ensued little is known beyond the circumstances that in he was engaged in training the regimental band of the durham militia, and that in he was appointed organist at halifax. in the following year he removed to bath as oboist in linley's orchestra, and in october was promoted to the post of organist in the octagon chapel. the tide of prosperity now began to flow for him. the most brilliant and modish society in england was at that time to be met at bath, and the young hanoverian quickly found himself a favourite and the fashion in it. engagements multiplied upon him. he became director of the public concerts; he conducted oratorios, engaged singers, organised rehearsals, composed anthems, chants, choral services, besides undertaking private tuitions, at times amounting to thirty-five or even thirty-eight lessons a week. he in fact personified the musical activity of a place then eminently and energetically musical. but these multifarious avocations did not take up the whole of his thoughts. his education, notwithstanding the poverty of his family, had not been neglected, and he had always greedily assimilated every kind of knowledge that came in his way. now that he was a busy and a prosperous man, it might have been expected that he would run on in the deep professional groove laid down for him. on the contrary, his passion for learning seemed to increase with the diminution of the time available for its gratification. he studied italian, greek, mathematics; maclaurin's fluxions served to "unbend his mind"; smith's harmonics and optics and ferguson's astronomy were the nightly companions of his pillow. what he read stimulated without satisfying his intellect. he desired not only to know, but to discover. in he hired a small telescope, and through it caught a preliminary glimpse of the rich and varied fields in which for so many years he was to expatiate. henceforward the purpose of his life was fixed: it was to obtain "a knowledge of the construction of the heavens";[ ] and this sublime ambition he cherished to the end. a more powerful instrument was the first desideratum; and here his mechanical genius came to his aid. having purchased the apparatus of a quaker optician, he set about the manufacture of specula with a zeal which seemed to anticipate the wonders they were to disclose to him. it was not until fifteen years later that his grinding and polishing machines were invented, so the work had at that time to be entirely done by hand. during this tedious and laborious process (which could not be interrupted without injury, and lasted on one occasion sixteen hours), his strength was supported by morsels of food put into his mouth by his sister,[ ] and his mind amused by her reading aloud to him the arabian nights, don quixote, or other light works. at length, after repeated failures, he found himself provided with a reflecting telescope--a - / -foot gregorian--of his own construction. a copy of his first observation with it, on the great nebula in orion--an object of continual amazement and assiduous inquiry to him--is preserved by the royal society. it bears the date march , .[ ] in the following year he executed his first "review of the heavens," memorable chiefly as an evidence of the grand and novel conceptions which already inspired him, and of the enthusiasm with which he delivered himself up to their guidance. overwhelmed with professional engagements, he still contrived to snatch some moments for the stars; and between the acts at the theatre was often seen running from the harpsichord to his telescope, no doubt with that "uncommon precipitancy which accompanied all his actions."[ ] he now rapidly increased the power and perfection of his telescopes. mirrors of seven, ten, even twenty feet focal length, were successively completed, and unprecedented magnifying powers employed. his energy was unceasing, his perseverance indomitable. in the course of twenty-one years no less than parabolic specula left his hands. he had entered upon his forty-second year when he sent his first paper to the _philosophical transactions_; yet during the ensuing thirty-nine years his contributions--many of them elaborate treatises--numbered sixty-nine, forming a series of extraordinary importance to the history of astronomy. as a mere explorer of the heavens his labours were prodigious. he discovered , nebulæ, double stars, passed the whole firmament in review four several times, counted the stars in , "gauge-fields," and executed a photometric classification of the principal stars, founded on an elaborate (and the first systematically conducted) investigation of their relative brightness. he was as careful and patient as he was rapid; spared no time and omitted no precaution to secure accuracy in his observations; yet in one night he would examine, singly and attentively, up to separate objects. the discovery of uranus was a mere incident of the scheme he had marked out for himself--a fruit, gathered as it were by the way. it formed, nevertheless, the turning-point in his career. from a star-gazing musician he was at once transformed into an eminent astronomer; he was relieved from the drudgery of a toilsome profession, and installed as royal astronomer, with a modest salary of £ a year; funds were provided for the construction of the forty-foot reflector, from the great space-penetrating power of which he expected unheard-of revelations; in fine, his future work was not only rendered possible, but it was stamped as authoritative.[ ] on whit-sunday , william and caroline herschel played and sang in public for the last time in st. margaret's chapel, bath; in august of the same year the household was moved to datchet, near windsor, and on april , , to slough. here happiness and honours crowded on the fortunate discoverer. in he married mary, only child of james baldwin, a merchant of the city of london, and widow of mr. john pitt--a lady whose domestic virtues were enhanced by the possession of a large jointure. the fruit of their union was one son, of whose work--the worthy sequel of his father's--we shall have to speak further on. herschel was created a knight of the hanoverian guelphic order in , and in he became the first president of the royal astronomical society, his son being its first foreign secretary. but his health had now for some years been failing, and on august , , he died at slough, in the eighty-fourth year of his age, and was buried in upton churchyard. his epitaph claims for him the lofty praise of having "burst the barriers of heaven." let us see in what sense this is true. the first to form any definite idea as to the constitution of the stellar system was thomas wright, the son of a carpenter living at byer's green, near durham. with him originated what has been called the "grindstone theory" of the universe, which regarded the milky way as the projection on the sphere of a stratum or disc of stars (our sun occupying a position near the centre), similar in magnitude and distribution to the lucid orbs of the constellations.[ ] he was followed by kant,[ ] who transcended the views of his predecessor by assigning to nebulæ the position they long continued to occupy, rather on imaginative than scientific grounds, of "island universes," external to, and co-equal with, the galaxy. johann heinrich lambert,[ ] a tailor's apprentice from mühlhausen, followed, but independently. the conceptions of this remarkable man were grandiose, his intuitions bold, his views on some points a singular anticipation of subsequent discoveries. the sidereal world presented itself to him as a hierarchy of systems, starting from the planetary scheme, rising to throngs of suns within the circuit of the milky way--the "ecliptic of the stars," as he phrased it--expanding to include groups of many milky ways; these again combining to form the unit of a higher order of assemblage, and so onwards and upwards until the mind reels and sinks before the immensity of the contemplated creations. "thus everything revolves--the earth round the sun; the sun round the centre of his system; this system round a centre common to it with other systems; this group, this assemblage of systems, round a centre which is common to it with other groups of the same kind; and where shall we have done?"[ ] the stupendous problem thus speculatively attempted, herschel undertook to grapple with experimentally. the upshot of this memorable inquiry was the inclusion, for the first time, within the sphere of human knowledge, of a connected body of facts, and inferences from facts, regarding the sidereal universe; in other words, the foundation of what may properly be called a science of the stars. tobias mayer had illustrated the perspective effects which must ensue in the stellar sphere from a translation of the solar system, by comparing them to the separating in front and closing up behind of trees in a forest to the eye of an advancing spectator;[ ] but the appearances which he thus correctly described he was unable to detect. by a more searching analysis of a smaller collection of proper motions, herschel succeeded in rendering apparent the very consequences foreseen by mayer. he showed, for example, that arcturus and vega did, in fact, appear to recede from, and sirius and aldebaran to approach, each other by very minute amounts; and, with a striking effort of divinatory genius, placed the "apex," or point of direction of the sun's motion, close to the star lambda in the constellation hercules,[ ] within a few degrees of the spot indicated by later and indefinitely more refined methods of research. he resumed the subject in ,[ ] but though employing a more rigorous method, was scarcely so happy in his result. in ,[ ] he made a preliminary attempt to ascertain the speed of the sun's journey, fixing it, by doubtless much too low an estimate, at about three miles a second. yet the validity of his general conclusion as to the line of solar travel, though long doubted, has been triumphantly confirmed. the question as to the "secular parallax" of the fixed stars was in effect answered. with their _annual_ parallax, however, the case was very different. the search for it had already led bradley to the important discoveries of the aberration of light and the nutation of the earth's axis; it was now about to lead herschel to a discovery of a different, but even more elevated character. yet in neither case was the object primarily sought attained. from the very first promulgation of the copernician theory the seeming immobility of the stars had been urged as an argument against its truth; for if the earth really travelled in a vast orbit round the sun, objects in surrounding space should appear to change their positions, unless their distances were on a scale which, to the narrow ideas of the universe then prevailing, seemed altogether extravagant.[ ] the existence of such apparent or "parallactic" displacements was accordingly regarded as the touchstone of the new views, and their detection became an object of earnest desire to those interested in maintaining them. copernicus himself made the attempt; but with his "triquetrum," a jointed wooden rule with the divisions marked in ink, constructed by himself,[ ] he was hardly able to measure angles of ten minutes, far less fractions of a second. galileo, a more impassioned defender of the system, strained his ears, as it were, from arcetri, in his blind and sorrowful old age, for news of a discovery which two more centuries had still to wait for. hooke believed he had found a parallax for the bright star in the head of the dragon; but was deceived. bradley convinced himself that such effects were too minute for his instruments to measure. herschel made a fresh attempt by a practically untried method. it is a matter of daily experience that two objects situated at different distances seem to a beholder in motion to move relatively to each other. this principle galileo, in the third of his dialogues on the systems of the world,[ ] proposed to employ for the determination of stellar parallax; for two stars, lying apparently close together, but in reality separated by a great gulf of space, must shift their mutual positions when observed from opposite points of the earth's orbit; or rather, the remoter forms a virtually fixed point, to which the movements of the other can be conveniently referred. by this means complications were abolished more numerous and perplexing than galileo himself was aware of, and the problem was reduced to one of simple micrometrical measurement. the "double-star method" was also suggested by james gregory in , and again by wallis in ;[ ] huygens first, and afterwards dr. long of cambridge (about ), made futile experiments with it; and it eventually led, in the hands of bessel, to the successful determination of the parallax of cygni. its advantages were not lost upon herschel. his attempt to assign definite distances to the nearest stars was no isolated effort, but part of the settled plan upon which his observations were conducted. he proposed to sound the heavens, and the first requisite was a knowledge of the length of his sounding-line. thus it came about that his special attention was early directed to double stars. "i resolved," he writes,[ ] "to examine every star in the heavens with the utmost attention and a very high power, that i might collect such materials for this research as would enable me to fix my observations upon those that would best answer my end. the subject has already proved so extensive, and still promises so rich a harvest to those who are inclined to be diligent in the pursuit, that i cannot help inviting every lover of astronomy to join with me in observations that must inevitably lead to new discoveries." the first result of these inquiries was a classed catalogue of double stars presented to the royal society in , followed, after three years, by an additional list of . in both these collections the distances separating the individuals of each pair were carefully measured, and (with a few exceptions) the angles made with the hour-circle by the lines joining their centres (technically called "angles of position") were determined with the aid of a "revolving-wire micrometer," specially devised for the purpose. moreover, an important novelty was introduced by the observation of the various colours visible in the star-couples, the singular and vivid contrasts of which were now for the first time described. double stars were at that time supposed to be a purely optical phenomenon. their components, it was thought, while in reality indefinitely remote from each other, were brought into fortuitous contiguity by the chance of lying nearly in the same line of sight from the earth. yet bradley had noticed a change of °, between and , in the position-angle of the two stars forming castor, and was thus within a hair's breadth of the discovery of their physical connection.[ ] while the rev. john michell, arguing by the doctrine of probabilities, wrote as follows in :--"it is highly probable in particular, and next to a certainty in general, that such double stars as appear to consist of two or more stars placed very near together, do really consist of stars placed near together, and under the influence of some general law."[ ] and in :[ ] "it is not improbable that a few years may inform us that some of the great number of double, triple stars, etc., which have been observed by mr. herschel, are systems of bodies revolving about each other." this remarkable speculative anticipation had a practical counterpart in germany. father christian mayer, a jesuit astronomer at mannheim, set himself, in january , to collect examples of stellar pairs, and shortly after published the supposed discovery of "satellites" to many of the principal stars.[ ] but his observations were neither exact nor prolonged enough to lead to useful results in such an inquiry. his disclosures were derided; his planet-stars treated as results of hallucination. _on n'a point cru à des choses aussi extraordinaires_, wrote lalande[ ] within one year of a better-grounded announcement to the same effect. herschel at first shared the general opinion as to the merely optical connection of double stars. of this the purpose for which he made his collection is in itself sufficient evidence, since what may be called the _differential_ method of parallaxes depends, as we have seen, for its efficacy upon disparity of distance. it was "much too soon," he declared in ,[ ] "to form any theories of small stars revolving round large ones;" while in the year following,[ ] he remarked that the identical proper motions of the two stars forming, to the naked eye, the single bright orb of castor could only be explained as both equally due to the "systematic parallax" caused by the sun's movement in space. plainly showing that the notion of a physical tie, compelling the two bodies to travel together, had not as yet entered into his speculations. but he was eminently open to conviction, and had, moreover, by observations unparalleled in amount as well as in kind, prepared ample materials for convincing himself and others. in he was able to announce the fact of his discovery, and in the two ensuing years, to lay in detail before the royal society proofs, gathered from the labours of a quarter of a century, of orbital revolution in the case of as many as fifty double stars, henceforth, he declared, to be held as real binary combinations, "intimately held together by the bond of mutual attraction."[ ] the fortunate preservation in dr. maskelyne's note-book of a remark made by bradley about , to the effect that the line joining the components of castor was an exact prolongation of that joining castor with pollux, added eighteen years to the time during which the pair were under scrutiny, and confirmed the evidence of change afforded by more recent observations. approximate periods were fixed for many of the revolving suns--for castor years; for gamma leonis, , delta serpentis, , eta bootis, years; eta lyræ was noted as a "double-double-star," a change of relative situation having been detected in each of the two pairs composing the group; and the occultation was described of one star by another in the course of their mutual revolutions, as exemplified in by the rapidly circulating system of zeta herculis. thus, by the sagacity and perseverance of a single observer, a firm basis was at last provided upon which to raise the edifice of sidereal science. the analogy long presumed to exist between the mighty star of our system and the bright points of light spangling the firmament was shown to be no fiction of the imagination, but a physical reality; the fundamental quality of attractive power was proved to be common to matter so far as the telescope was capable of exploring, and law, subordination, and regularity to give testimony of supreme and intelligent design no less in those limitless regions of space than in our narrow terrestrial home. the discovery was emphatically (in arago's phrase) "one with a future," since it introduced the element of precise knowledge where more or less probable conjecture had previously held almost undivided sway; and precise knowledge tends to propagate itself and advance from point to point. we have now to speak of herschel's pioneering work in the skies. to explore with line and plummet the shining zone of the milky way, to delineate its form, measure its dimensions, and search out the intricacies of its construction, was the primary task of his life, which he never lost sight of, and to which all his other investigations were subordinate. he was absolutely alone in this bold endeavour. unaided, he had to devise methods, accumulate materials, and sift out results. yet it may safely be asserted that all the knowledge we possess on this sublime subject was prepared, and the greater part of it anticipated, by him. the ingenious method of "star-gauging," and its issue in the delineation of the sidereal system as an irregular stratum of evenly-scattered suns, is the best-known part of his work. but it was, in truth, only a first rude approximation, the principle of which maintained its credit in the literature of astronomy a full half-century after its abandonment by its author. this principle was the general equality of star distribution. if equal portions of space really held equal numbers of stars, it is obvious that the number of stars visible in any particular direction would be strictly proportional to the range of the system in that direction, apparent accumulation being produced by real extent. the process of "gauging the heavens," accordingly, consisted in counting the stars in successive telescopic fields, and calculating thence the depths of space necessary to contain them. the result of , such operations was the plan of the galaxy familiar to every reader of an astronomical text-book. widely-varying evidence was, as might have been expected, derived from an examination of different portions of the sky. some fields of view were almost blank, while others (in or near the milky way) blazed with the radiance of many hundred stars compressed into an area about one-fourth that of the full-moon. in the most crowded parts , were stated to have been passed in review within a quarter of an hour. here the "length of his sounding-line" was estimated by herschel at about times the distance of sirius--in other words, the bounding orb, or farthest sun of the system in that direction, so far as could be seen with the -foot reflector, was thus inconceivably remote. but since the distance of sirius, no less than of every other fixed star, was as yet an unknown quantity, the dimensions inferred for the galaxy were of course purely relative; a knowledge of its form and structure might (admitting the truth of the fundamental hypothesis) be obtained, but its real or absolute size remained altogether undetermined. even as early as , however, herschel perceived traces of a tendency which completely invalidated the supposition of any approach to an average uniformity of distribution. this was the action of what he called a "clustering power" in the milky way. "many gathering clusters"[ ] were already discernible to him even while he endeavoured to obtain a "true _mean_ result" on the assumption that each star in space was separated from its neighbours as widely as the sun from sirius. "it appears," he wrote in , "that the heavens consist of regions where suns are gathered into separate systems"; and in certain assemblages he was able to trace "a course or tide of stars setting towards a centre," denoting, not doubtfully, the presence of attractive forces.[ ] thirteen years later, he described our sun and his constellated companions as surrounded by "a magnificent collection of innumerable stars, called the milky way, which must occasion a very powerful balance of opposite attractions to hold the intermediate stars at rest. for though our sun, and all the stars we see, may truly be said to be in the plane of the milky way, yet i am now convinced, by a long inspection and continued examination of it, that the milky way itself consists of stars very differently scattered from those which are immediately about us." "this immense aggregation," he added, "is by no means uniform. its component stars show evident signs of clustering together into many separate allotments."[ ] the following sentences, written in , contain a definite retractation of the view frequently attributed to him:-- "i must freely confess," he says, "that by continuing my sweeps of the heavens my opinion of the arrangement of the stars and their magnitudes, and of some other particulars, has undergone a gradual change; and indeed, when the novelty of the subject is considered, we cannot be surprised that many things formerly taken for granted should on examination prove to be different from what they were generally but incautiously supposed to be. for instance, an equal scattering of the stars may be admitted in certain calculations; but when we examine the milky way, or the closely compressed clusters of stars of which my catalogues have recorded so many instances, this supposed equality of scattering must be given up."[ ] another assumption, the fallacy of which he had not the means of detecting since become available, was retained by him to the end of his life. it was that the brightness of a star afforded an approximate measure of its distance. upon this principle he founded in his method of "limiting apertures,"[ ] by which two stars, brought into view in two precisely similar telescopes, were "equalised" by covering a certain portion of the object-glass collecting the more brilliant rays. the distances of the orbs compared were then taken to be in the ratio of the reduced to the original apertures of the instruments with which they were examined. if indeed the absolute lustre of each were the same, the result might be accepted with confidence; but since we have no warrant for assuming a "standard star" to facilitate our computations, but much reason to suppose an indefinite range, not only of size but of intrinsic brilliancy, in the suns of our firmament, conclusions drawn from such a comparison are entirely worthless. in another branch of sidereal science besides that of stellar aggregation, herschel may justly be styled a pioneer. he was the first to bestow serious study on the enigmatical objects known as "nebulæ." the history of the acquaintance of our race with them is comparatively short. the only one recognised before the invention of the telescope was that in the girdle of andromeda, certainly familiar in the middle of the tenth century to the persian astronomer abdurrahman al-sûfi; and marked with dots on spanish and dutch constellation-charts of the fourteenth and fifteenth centuries.[ ] yet so little was it noticed that it might practically be said--as far as europe is concerned--to have been discovered in by simon marius (mayer of genzenhausen), who aptly described its appearance as that of a "candle shining through horn." the first mention of the great orion nebula is by a swiss jesuit named cysatus, who succeeded father scheiner in the chair of mathematics at ingolstadt. he used it, apparently without any suspicion of its novelty, as a term of comparison for the comet of december .[ ] a novelty, nevertheless, to astronomers it still remained in , when huygens discerned, "as it were, an hiatus in the sky, affording a glimpse of a more luminous region beyond."[ ] halley in knew of six nebulæ, which he believed to be composed of a "lucid medium" diffused through the ether of space.[ ] he appears, however, to have been unacquainted with some previously noticed by hevelius. lacaille brought back with him from the cape a list of forty-two--the first-fruits of observation in southern skies--arranged in three numerically equal classes;[ ] and messier (nicknamed by louis xv. the "ferret of comets"), finding such objects a source of extreme perplexity in the pursuit of his chosen game, attempted to eliminate by methodising them, and drew up a catalogue comprising, in , entries.[ ] these preliminary attempts shrank into insignificance when herschel began to "sweep the heavens" with his giant telescopes. in he presented to the royal society a descriptive catalogue of , nebulæ and clusters, followed, three years later, by a second of as many more; to which he added in a further gleaning of . on the subject of their nature his views underwent a remarkable change. finding that his potent instruments resolved into stars many nebulous patches in which no signs of such a structure had previously been discernible, he naturally concluded that "resolvability" was merely a question of distance and telescopic power. he was (as he said himself) led on by almost imperceptible degrees from evident clusters, such as the pleiades, to spots without a trace of stellar formation, the gradations being so well connected as to leave no doubt that all these phenomena were equally stellar. the singular variety of their appearance was thus described by him:-- "i have seen," he says, "double and treble nebulæ variously arranged; large ones with small, seeming attendants; narrow, but much extended lucid nebulæ or bright dashes; some of the shape of a fan, resembling an electric brush, issuing from a lucid point; others of the cometic shape, with a seeming nucleus in the centre, or like cloudy stars surrounded with a nebulous atmosphere; a different sort, again, contain a nebulosity of the milky kind, like that wonderful, inexplicable phenomenon about theta orionis; while others shine with a fainter, mottled kind of light, which denotes their being resolvable into stars."[ ] "these curious objects" he considered to be "no less than whole sidereal systems,"[ ] some of which might "well outvie our milky way in grandeur." he admitted, however, a wide diversity in condition as well as compass. the system to which our sun belongs he described as "a very extensive branching congeries of many millions of stars, which probably owes its origin to many remarkably large as well as pretty closely scattered small stars, that may have drawn together the rest."[ ] but the continued action of this same "clustering power" would, he supposed, eventually lead to the breaking-up of the original majestic galaxy into two or three hundred separate groups, already visibly gathering. such minor nebulæ, due to the "decay" of other "branching nebulæ" similar to our own, he recognised by the score, lying, as it were, stratified in certain quarters of the sky. "one of these nebulous beds," he informs us, "is so rich that in passing through a section of it, in the time of only thirty-six minutes, i detected no less than thirty-one nebulæ, all distinctly visible upon a fine blue sky." the stratum of coma berenices he judged to be the nearest to our system of such layers; nor did the marked aggregation of nebulæ towards both poles of the circle of the milky way escape his notice. by a continuation of the same process of reasoning, he was enabled (as he thought) to trace the life-history of nebulæ from a primitive loose and extended formation, through clusters of gradually increasing compression, down to the kind named by him "planetary" because of the defined and uniform discs which they present. these he regarded as "very aged, and drawing on towards a period of change or dissolution."[ ] "this method of viewing the heavens," he concluded, "seems to throw them into a new kind of light. they now are seen to resemble a luxuriant garden which contains the greatest variety of productions in different flourishing beds; and one advantage we may at least reap from it is, that we can, as it were, extend the range of our experience to an immense duration. for, to continue the simile which i have borrowed from the vegetable kingdom, is it not almost the same thing whether we live successively to witness the germination, blooming, foliage, fecundity, fading, withering, and corruption of a plant, or whether a vast number of specimens, selected from every stage through which the plant passes in the course of its existence, be brought at once to our view?"[ ] but already this supposed continuity was broken. after mature deliberation on the phenomena presented by nebulous stars, herschel was induced, in , to modify essentially his original opinion. "when i pursued these researches," he says, "i was in the situation of a natural philosopher who follows the various species of animals and insects from the height of their perfection down to the lowest ebb of life; when, arriving at the vegetable kingdom, he can scarcely point out to us the precise boundary where the animal ceases and the plant begins; and may even go so far as to suspect them not to be essentially different. but, recollecting himself, he compares, for instance, one of the human species to a tree, and all doubt upon the subject vanishes before him. in the same manner we pass through gentle steps from a coarse cluster of stars, such as the pleiades ... till we find ourselves brought to an object such as the nebula in orion, where we are still inclined to remain in the once adopted idea of stars exceedingly remote and inconceivably crowded, as being the occasion of that remarkable appearance. it seems, therefore, to require a more dissimilar object to set us right again. a glance like that of the naturalist, who casts his eye from the perfect animal to the perfect vegetable, is wanting to remove the veil from the mind of the astronomer. the object i have mentioned above is the phenomenon that was wanting for this purpose. view, for instance, the th cluster of my th class, and afterwards cast your eye on this cloudy star, and the result will be no less decisive than that of the naturalist we have alluded to. our judgment, i may venture to say, will be, that _the nebulosity about the star is not of a starry nature_."[ ] the conviction thus arrived at of the existence in space of a widely diffused "shining fluid" (a conviction long afterwards fully justified by the spectroscope) led him into a field of endless speculation. what was its nature? should it "be compared to the coruscation of the electric fluid in the aurora borealis? or to the more magnificent cone of the zodiacal light?" above all, what was its function in the cosmos? and on this point he already gave a hint of the direction in which his mind was moving by the remark that this self-luminous matter seemed "more fit to produce a star by its condensation, than to depend on the star for its existence."[ ] this was not a novel idea. tycho brahe had tried to explain the blaze of the star of as due to a sudden concentration of nebulous material in the milky way, even pointing out the space left dark and void by the withdrawal of the luminous stuff; and kepler, theorising on a similar stellar apparition in , followed nearly in the same track. but under herschel's treatment the nebular origin of stars first acquired the consistency of a formal theory. he meditated upon it long and earnestly, and in two elaborate treatises, published respectively in and , he at length set forth the arguments in its favour. these rested entirely upon the "principle of continuity." between the successive classes of his assortment of developing objects there was, as he said, "perhaps not so much difference as would be in an annual description of the human figure, were it given from the birth of a child till he comes to be a man in his prime."[ ] from diffused nebulosity, barely visible in the most powerful light-gathering instruments, but which he estimated to cover nearly square degrees of the heavens,[ ] to planetary nebulæ, supposed to be already centrally solid, instances were alleged of every stage and phase of condensation. the validity of his reasoning, however, was evidently impaired by his confessed inability to distinguish between the dim rays of remote clusters and the milky light of true gaseous nebulæ. it may be said that such speculations are futile in themselves, and necessarily barren of results. but they gratify an inherent tendency of the human mind, and, if pursued in a becoming spirit, should be neither reproved nor disdained. herschel's theory still holds the field, the testimony of recent discoveries with regard to it having proved strongly confirmatory of its principle, although not of its details. strangely enough, it seems to have been propounded in complete independence of laplace's nebular hypothesis as to the origin of the solar system. indeed, it dated, as we have seen, in its first inception, from , while the french geometrician's view was not advanced until . we may now briefly sum up the chief results of herschel's long years of "watching the heavens." the apparent motions of the stars had been disentangled; one portion being clearly shown to be due to a translation towards a point in the constellation hercules of the sun and his attendant planets; while a large balance of displacement was left to be accounted for by real movements, various in extent and direction, of the stars themselves. by the action of a central force similar to, if not identical with, gravity, suns of every degree of size and splendour, and sometimes brilliantly contrasted in colour, were seen to be held together in systems, consisting of two, three, four, even six members, whose revolutions exhibited a wide range of variety both in period and in orbital form. a new department of physical astronomy was thus created,[ ] and rigid calculation for the first time made possible within the astral region. the vast problem of the arrangement and relations of the millions of stars forming the milky way was shown to be capable of experimental treatment, and of at least partial solution, notwithstanding the variety and complexity seen to prevail, to an extent previously undreamt of, in the arrangement of that majestic system. the existence of a luminous fluid, diffused through enormous tracts of space, and intimately associated with stellar bodies, was virtually demonstrated, and its place and use in creation attempted to be divined by a bold but plausible conjecture. change on a stupendous scale was inferred or observed to be everywhere in progress. periodical stars shone out and again decayed; progressive ebbings or flowings of light were indicated as probable in many stars under no formal suspicion of variability; forces were everywhere perceived to be at work, by which the very structure of the heavens themselves must be slowly but fundamentally modified. in all directions groups were seen to be formed or forming; tides and streams of suns to be setting towards powerful centres of attraction; new systems to be in process of formation, while effete ones hastened to decay or regeneration when the course appointed for them by infinite wisdom was run. and thus, to quote the words of the observer who "had looked farther into space than ever human being did before him,"[ ] the state into which the incessant action of the clustering power has brought the milky way at present, is a kind of chronometer that may be used to measure the time of its past and future existence; and although we do not know the rate of going of this mysterious chronometer, it is nevertheless certain that, since the breaking-up of the parts of the milky way affords a proof that it cannot last for ever, it equally bears witness that its past duration cannot be admitted to be infinite.[ ] footnotes: [footnote : _phil. trans._, vol. xxx., p. .] [footnote : out of eighty stars compared, fifty-seven were found to have changed their places by more than ". lesser discrepancies were at that time regarded as falling within the limits of observational error. _tobiæ mayeri op. inedita_, t. i., pp. , , and herschel in _phil. trans._, vol. lxxiii., pp. - .] [footnote : _posthumous works_, p. .] [footnote : arago in _annuaire du bureau des longitudes_, , p. .] [footnote : bradley to halley, _phil. trans._, vol. xxxv. ( ), p. . his observations were directly applicable to only two stars, gamma draconis and eta ursæ majoris, but some lesser ones were included in the same result.] [footnote : holden, _sir william herschel, his life and works_, p. .] [footnote : _phil. trans._, vol. ci., p. .] [footnote : caroline lucretia herschel, born at hanover, march , , died in the same place, january , . she came to england in , and was her brother's devoted assistant, first in his musical undertakings, and afterwards, down to the end of his life, in his astronomical labours.] [footnote : holden, _op. cit._, p. .] [footnote : _memoir of caroline herschel_, p. .] [footnote : see holden's _sir william herschel_, p. .] [footnote : _an original theory or new hypothesis of the universe_, london, . see also de morgan's summary of his views in _philosophical magazine_, april, .] [footnote : _allgemeine naturgeschichte und theorie des himmels_, .] [footnote : _cosmologische briefe_, augsburg, .] [footnote : _the system of the world_, p. , london, (a translation of _cosmologische briefe_). lambert regarded nebulæ as composed of stars crowded together, but _not_ as external universes. in the case of the orion nebula, indeed, he throws out such a conjecture, but afterwards suggests that it may form a centre for that one of the subordinate systems composing the milky way to which our sun belongs.] [footnote : _opera inedita_, t. i., p. .] [footnote : _phil. trans._, vol. lxxiii. ( ), p. . pierre prévost's similar investigation, communicated to the berlin academy of sciences four months later, july , , was inserted in the _memoirs_ of that body for , and thus _seems_ to claim a priority not its due. georg simon klügel at halle gave about the same time an analytical demonstration of herschel's result. wolf, _gesch. der astronomie_, p. .] [footnote : _phil. trans._, vol. xcv., p. .] [footnote : _ibid._, vol. xcvi., p. .] [footnote : "ingens bolus devorandus est," kepler admitted to herwart in may, .] [footnote : described in "præfatio editoris" to _de revolutionibus_, p. xix. (ed. ).] [footnote : _opere_, t. i., p. .] [footnote : _phil. trans._, vol. xvii., p. .] [footnote : _ibid._, vol. lxxii., p. .] [footnote : doberck, _observatory_, vol. ii., p. .] [footnote : _phil. trans._, vol. lvii., p. .] [footnote : _ibid._, vol. lxxiv., p. .] [footnote : _beobachtungen von fixsterntrabanten_, ; and _de novis in coelo sidereo phænomenis_, .] [footnote : _bibliographie_, p. .] [footnote : _phil. trans._, vol. lxxii., p. .] [footnote : _ibid._, vol. lxxiii., p. .] [footnote : _ibid._, vol. xciii., p. .] [footnote : _phil. trans._, vol. lxxv., p. .] [footnote : _ibid._, vol. lxxix., pp. , .] [footnote : _ibid._, vol. xcii., pp. , .] [footnote : _phil. trans._, vol. ci., p. .] [footnote : _ibid._, vol. cvii., p. .] [footnote : bullialdus, _de nebulosâ stellâ in cingulo andromedæ_ ( ); see also g. p. bond, _mém. am. ac._, vol. iii., p. , holden's monograph on the orion nebula, _washington observations_, vol. xxv., (pub. ), and lady huggins's drawing, _atlas of spectra_, p. .] [footnote : _mathemata astronomica_, p. .] [footnote : _systema saturnium_, p. .] [footnote : _phil. trans._, vol. xxix., p. .] [footnote : _mém. ac. des sciences_, .] [footnote : _conn. des temps_, (pub. ), p. . a previous list of forty-five had appeared in _mém. ac. des sciences_, .] [footnote : _phil. trans._, vol. lxxiv., p. .] [footnote : _ibid._, vol. lxxix., p. .] [footnote : _ibid._, vol. lxxv., p. .] [footnote : _ibid._, vol. lxxix., p. .] [footnote : _phil. trans._, vol. lxxix., p. .] [footnote : _ibid._, vol. lxxxi., p. .] [footnote : _ibid._, p. .] [footnote : _phil. trans._, vol. ci., p. .] [footnote : _ibid._, p. .] [footnote : j. herschel, _phil. trans._, vol. cxvi., part iii., p. .] [footnote : his own words to the poet campbell cited by holden, _life and works_, p. .] [footnote : _phil. trans._, vol. civ., p. .] chapter ii _progress of sidereal astronomy_ we have now to consider labours of a totally different character from those of sir william herschel. exploration and discovery do not constitute the whole business of astronomy; the less adventurous, though not less arduous, task of gaining a more and more complete mastery over the problems immemorially presented to her, may, on the contrary, be said to form her primary duty. a knowledge of the movements of the heavenly bodies has, from the earliest times, been demanded by the urgent needs of mankind; and science finds its advantage, as in many cases it has taken its origin, in condescension to practical claims. indeed, to bring such knowledge as near as possible to absolute precision has been defined by no mean authority[ ] as the true end of astronomy. several causes concurred about the beginning of the last century to give a fresh and powerful impulse to investigations having this end in view. the rapid progress of theory almost compelled a corresponding advance in observation; instrumental improvements rendered such an advance possible; herschel's discoveries quickened public interest in celestial inquiries; royal, imperial, and grand-ducal patronage widened the scope of individual effort. the heart of the new movement was in germany. hitherto the observatory of flamsteed and bradley had been the acknowledged centre of practical astronomy; greenwich observations were the standard of reference all over europe; and the art of observing prospered in direct proportion to the fidelity with which greenwich methods were imitated. dr. maskelyne, who held the post of astronomer royal during forty-six years (from to ), was no unworthy successor to the eminent men who had gone before him. his foundation of the _nautical almanac_ (in ) alone constitutes a valid title to fame; he introduced at the observatory the important innovation of the systematic publication of results; and the careful and prolonged series of observations executed by him formed the basis of the improved theories, and corrected tables of the celestial movements, which were rapidly being brought to completion abroad. his catalogue of thirty-six "fundamental" stars was besides excellent in its way, and most serviceable. yet he was devoid of bradley's instinct for divining the needs of the future. he was fitted rather to continue a tradition than to found a school. the old ways were dear to him; and, indefatigable as he was, a definite purpose was wanting to compel him, by its exigencies, along the path of progress. thus, for almost fifty years after bradley's death, the acquisition of a small achromatic[ ] was the only notable change made in the instrumental equipment of the observatory. the transit, the zenith sector, and the mural quadrant, with which bradley had done his incomparable work, retained their places long after they had become deteriorated by time and obsolete by the progress of invention; and it was not until the very close of his career that maskelyne, compelled by pond's detection of serious errors, ordered a troughton's circle, which he did not live to employ. meanwhile, the heavy national disasters with which germany was overwhelmed in the early part of the nineteenth century seemed to stimulate rather than impede the intellectual revival already for some years in progress there. astronomy was amongst the first of the sciences to feel the new impulse. by the efforts of bode, olbers, schröter, and von zach, just and elevated ideas on the subject were propagated, intelligence was diffused, and a firm ground prepared for common action in mutual sympathy and disinterested zeal. they received powerful aid through the foundation, in , by a young artillery officer named von reichenbach, of an optical and mechanical institute at munich. here the work of english instrumental artists was for the first time rivalled, and that of english opticians--when fraunhofer entered the new establishment--far surpassed. the development given to the refracting telescope by this extraordinary man was indispensable to the progress of that fundamental part of astronomy which consists in the exact determination of the places of the heavenly bodies. reflectors are brilliant engines of discovery, but they lend themselves with difficulty to the prosaic work of measuring right ascensions and polar distances. a signal improvement in the art of making and working flint-glass thus most opportunely coincided with the rise of a german school of scientific mechanicians, to furnish the instrumental means needed for the reform which was at hand. of the leader of that reform it is now time to speak. friedrich wilhelm bessel was born at minden, in westphalia, july , . a certain taste for figures, coupled with a still stronger distaste for the latin accidence, directed his inclination and his father's choice towards a mercantile career. in his fifteenth year, accordingly, he entered the house of kuhlenkamp and sons, in bremen, as an apprenticed clerk. he was now thrown completely upon his own resources. from his father, a struggling government official, heavily weighted with a large family, he was well aware that he had nothing to expect; his dormant faculties were roused by the necessity for self-dependence, and he set himself to push manfully forward along the path that lay before him. the post of supercargo on one of the trading expeditions sent out from the hanseatic towns to china and the east indies was the aim of his boyish ambition, for the attainment of which he sought to qualify himself by the industrious acquisition of suitable and useful knowledge. he learned english in two or three months; picked up spanish with the casual aid of a gunsmith's apprentice; studied the geography of the distant lands which he hoped to visit; collected information as to their climates, inhabitants, products, and the courses of trade. he desired to add some acquaintance with the art (then much neglected) of taking observations at sea; and thus, led on from navigation to astronomy, and from astronomy to mathematics, he groped his way into a new world. it was characteristic of him that the practical problems of science should have attracted him before his mind was as yet sufficiently matured to feel the charm of its abstract beauties. his first attempt at observation was made with a sextant, rudely constructed under his own directions, and a common clock. its object was the determination of the longitude of bremen, and its success, he tells us himself,[ ] filled him with a rapture of delight, which, by confirming his tastes, decided his destiny. he now eagerly studied bode's _jahrbuch_ and von zach's _monatliche correspondenz_, overcoming each difficulty as it arose with the aid of lalande's _traité d'astronomie_, and supplying, with amazing rapidity, his early deficiency in mathematical training. in two years he was able to attack a problem which would have tasked the patience, if not the skill, of the most experienced astronomer. amongst the earl of egremont's papers von zach had discovered harriot's observations on halley's comet at its appearance in , and published them as a supplement to bode's annual. with an elaborate care inspired by his youthful ardour, though hardly merited by their loose nature, bessel deduced from them an orbit for that celebrated body, and presented the work to olbers, whose reputation in cometary researches gave a special fitness to the proffered homage. the benevolent physician-astronomer of bremen welcomed with surprised delight such a performance emanating from such a source. fifteen years previously, the french academy had crowned a similar work; now its equal was produced by a youth of twenty, busily engaged in commercial pursuits, self-taught, and obliged to snatch from sleep the hours devoted to study. the paper was immediately sent to von zach for publication, with a note from olbers explaining the circumstances of its author; and the name of bessel became the common property of learned europe. he had, however, as yet no intention of adopting astronomy as his profession. for two years he continued to work in the counting-house by day, and to pore over the _mécanique céleste_ and the differential calculus by night. but the post of assistant in schröter's observatory at lilienthal having become vacant by the removal of harding to göttingen in , olbers procured for him the offer of it. it was not without a struggle that he resolved to exchange the desk for the telescope. his reputation with his employers was of the highest; he had thoroughly mastered the details of the business, which his keen practical intelligence followed with lively interest; his years of apprenticeship were on the point of expiring, and an immediate, and not unwelcome prospect of comparative affluence lay before him. the love of science, however, prevailed; he chose poverty and the stars, and went to lilienthal with a salary of a hundred thalers yearly. looking back over his life's work, olbers long afterwards declared that the greatest service which he had rendered to astronomy was that of having discerned, directed, and promoted the genius of bessel.[ ] for four years he continued in schröter's employment. at the end of that time the prussian government chose him to superintend the erection of a new observatory at königsberg, which after many vexatious delays, caused by the prostrate condition of the country, was finished towards the end of . königsberg was the first really efficient german observatory. it became, moreover, a centre of improvement, not for germany alone, but for the whole astronomical world. during two-and-thirty years it was the scene of bessel's labours, and bessel's labours had for their aim the reconstruction, on an amended and uniform plan, of the entire science of observation. a knowledge of the places of the stars is the foundation of astronomy.[ ] their configuration lends to the skies their distinctive features, and marks out the shifting tracks of more mobile objects with relatively fixed, and generally unvarying points of light. a more detailed and accurate acquaintance with the stellar multitude, regarded from a purely uranographical point of view, has accordingly formed at all times a primary object of celestial science, and was, during the last century, cultivated with a zeal and success by which all previous efforts were dwarfed into insignificance. in lalande's _histoire céleste_, published in , the places of no less than , stars were given, but in the rough, as it were, and consequently needing laborious processes of calculation to render them available for exact purposes. piazzi set an example of improved methods of observation, resulting in the publication, in and , of two catalogues of about , stars--the second being a revision and enlargement of the first--which for their time were models of what such works should be.[ ] stephen groombridge at blackheath was similarly and most beneficially active. but something more was needed than the diligence of individual observers. a systematic reform was called for; and it was this which bessel undertook and carried through. direct observation furnishes only what has been called the "raw material" of the positions of the heavenly bodies.[ ] a number of highly complex corrections have to be applied before their _mean_ can be disengaged from their _apparent_ places on the sphere. of these, the most considerable and familiar is atmospheric refraction, by which objects seem to stand higher in the sky than they in reality do, the effect being evanescent at the zenith, and attaining, by gradations varying with conditions of pressure and temperature, a maximum at the horizon. moreover, the points to which measurements are referred are themselves in motion, either continually in one direction, or periodically to and fro. the _precession_ of the equinoxes is slowly progressive, or rather retrogressive; the _nutation_ of the pole oscillatory in a period of about eighteen years. added to which, the non-instantaneous transmission of light, combined with the movement of the earth in its orbit, causes a small annual displacement known as _aberration_. now it is easy to see that any uncertainty in the application of these corrections saps the very foundations of exact astronomy. extremely minute quantities, it is true, are concerned; but the life and progress of modern celestial science depends upon the sure recognition of extremely minute quantities. in the early years of the nineteenth century, however, no uniform system of "reduction" (so the complete correction of observational results is termed) had been established. much was left to the individual caprice of observers, who selected for the several "elements" of reduction such values as seemed best to themselves. hence arose much hurtful confusion, tending to hinder united action and mar the usefulness of laborious researches. for this state of things, bessel, by the exercise of consummate diligence, sagacity, and patience, provided an entirely satisfactory remedy. his first step was an elaborate investigation of the precious series of observations made by bradley at greenwich from until his death in . the catalogue of , stars which he extracted from them gave the earliest example of the systematic reduction on a uniform plan of such a body of work. it is difficult, without entering into details out of place in a volume like the present, to convey an idea of the arduous nature of this task. it involved the formation of a theory of the errors of each of bradley's instruments, and a difficult and delicate inquiry into the true value of each correction to be applied, before the entries in the greenwich journals could be developed into a finished and authentic catalogue. although completed in , it was not until five years later that the results appeared with the proud, but not inappropriate title of _fundamenta astronomiæ_. the eminent value of the work consisted in this, that by providing a mass of entirely reliable information as to the state of the heavens at the epoch , it threw back the beginning of _exact_ astronomy almost half a century. by comparison with piazzi's catalogues the amount of precession was more accurately determined, the proper motions of a considerable number of stars became known with certainty, and definite prediction--the certificate of initiation into the secrets of nature--at last became possible as regards the places of the stars. bessel's final improvements in the methods of reduction were published in in his _tabulæ regiomontanæ_. they not only constituted an advance in accuracy, but afforded a vast increase of facility in application, and were at once and everywhere adopted. thus astronomy became a truly universal science; uncertainties and disparities were banished, and observations made at all times and places rendered mutually comparable.[ ] more, however, yet remained to be done. in order to verify with greater strictness the results drawn from the bradley and piazzi catalogues, a third term of comparison was wanted, and this bessel undertook to supply. by a course of , observations, executed during the years - , with the utmost nicety of care, the number of accurately known stars was brought up to above , , and an ample store of trustworthy facts laid up for the use of future astronomers. in this department argelander, whom he attracted from finance to astronomy, and trained in his own methods, was his assistant and successor. the great "bonn durchmusterung,"[ ] in which , stars visible in the northern hemisphere are enumerated, and the corresponding "atlas" published in - , constituting a picture of our sidereal surroundings of heretofore unapproached completeness, may be justly said to owe their origin to bessel's initiative, and to form a sequel to what he commenced. but his activity was not solely occupied with the promotion of a comprehensive reform in astronomy; it embraced special problems as well. the long-baffled search for a parallax of the fixed stars was resumed with fresh zeal as each mechanical or optical improvement held out fresh hopes of a successful issue. illusory results abounded. piazza in perceived, as he supposed, considerable annual displacements in vega, aldebaran, sirius, and procyon; the truth being that his instruments were worn out with constant use, and could no longer be depended upon.[ ] his countryman, calandrelli, was similarly deluded. the celebrated controversy between the astronomer royal and dr. brinkley, director of the dublin college observatory, turned on the same subject. brinkley, who was in possession of a first-rate meridian-circle, believed himself to have discovered relatively large parallaxes for four of the brightest stars; pond, relying on the testimony of the greenwich instruments, asserted their nullity. the dispute, protracted for fourteen years, from until , was brought to no definite conclusion; but the strong presumption on the negative side was abundantly justified in the event. there was good reason for incredulity in the matter of parallaxes. announcements of their detection had become so frequent as to be discredited before they were disproved; and struve, who investigated the subject at dorpat in - , had clearly shown that the quantities concerned were too small to come within the reliable measuring powers of any instrument then in use. already, however, the means were being prepared of giving to those powers a large increase. on the st july, , two old houses in an alley of munich tumbled down, burying in their ruins the occupants, of whom one alone was extricated alive, though seriously injured. this was an orphan lad of fourteen named joseph fraunhofer. the elector maximilian joseph was witness of the scene, became interested in the survivor, and consoled his misfortune with a present of eighteen ducats. seldom was money better bestowed. part of it went to buy books and a glass-polishing machine, with the help of which young fraunhofer studied mathematics and optics, and secretly exercised himself in the shaping and finishing of lenses; the remainder purchased his release from the tyranny of one weichselberger, a looking-glass maker by trade, to whom he had been bound apprentice on the death of his parents. a period of struggle and privation followed, during which, however, he rapidly extended his acquirements; and was thus eminently fitted for the task awaiting him, when, in , he entered the optical department of the establishment founded two years previously by von reichenbach and utzschneider. he now zealously devoted himself to the improvement of the achromatic telescope; and, after a prolonged study of the theory of lenses, and many toilsome experiments in the manufacture of flint-glass, he succeeded in perfecting, december , , an object-glass of exquisite quality and finish, - / inches in diameter, and of feet focal length. this (as it was then considered) gigantic lens was secured by struve for the russian government, and the "great dorpat refractor"--the first of the large achromatics which have played such an important part in modern astronomy--was, late in , set up in the place which it still occupies. by ingenious improvements in mounting and fitting, it was adapted to the finest micrometrical work, and thus offered unprecedented facilities both for the examination of double stars (in which struve chiefly employed it), and for such subtle measurements as might serve to reveal or disprove the existence of a sensible stellar parallax. fraunhofer, moreover, constructed for the observatory at königsberg the first really available heliometer. the principle of this instrument (termed with more propriety a "divided object-glass micrometer") is the separation, by a strictly measurable amount, of two distinct images of the same object. if a double star, for instance, be under examination, the two half-lenses into which the object-glass is divided are shifted until the upper star (say) in one image is brought into coincidence with the lower star in the other, when their distance apart becomes known by the amount of motion employed.[ ] this virtually new engine of research was delivered and mounted in , three years after the termination of the life of its deviser. the dorpat lens had brought to fraunhofer a title of nobility and the sole management of the munich optical institute (completely separated since from the mechanical department). what he had achieved, however, was but a small part of what he meant to achieve. he saw before him the possibility of nearly quadrupling the light-gathering capacity of the great achromatic acquired by struve; he meditated improvements in reflectors as important as those he had already effected in refractors; and was besides eagerly occupied with investigations into the nature of light, the momentous character of which we shall by-and-by have an opportunity of estimating. but his health was impaired, it is said, from the weakening effects of his early accident, combined with excessive and unwholesome toil, and, still hoping for its restoration from a projected journey to italy, he died of consumption, june , , aged thirty-nine years. his tomb in munich bears the concise eulogy, _approximavit sidera_. bessel had no sooner made himself acquainted with the exquisite defining powers of the königsberg heliometer, than he resolved to employ them in an attack upon the now secular problem of star-distances. but it was not until that he found leisure to pursue the inquiry. in choosing his test-star he adopted a new principle. it had hitherto been assumed that our nearest neighbours in space must be found among the brightest ornaments of our skies. the knowledge of stellar proper motions afforded by the critical comparison of recent with earlier star-places, suggested a different criterion of distance. it is impossible to escape from the conclusion that the apparently swiftest-moving stars are, _on the whole_, also the nearest to us, however numerous the individual exceptions to the rule. now, as early as ,[ ] piazzi had noted as an indication of relative vicinity to the earth, the unusually large proper motion ( · " annually) of a double star of the fifth magnitude in the constellation of the swan. still more emphatically in [ ] bessel drew the attention of astronomers to the fact, and cygni became known as the "flying star." the _seeming_ rate of its flight, indeed, is of so leisurely a kind, that in a thousand years it will have shifted its place by less than - / lunar diameters, and that a quarter of a million would be required to carry it round the entire circuit of the visible heavens. nevertheless, it has few rivals in rapidity of movement, the apparent displacement of the vast majority of stars being, by comparison, almost insensible. this interesting, though inconspicuous object, then, was chosen by bessel to be put to the question with his heliometer, while struve made a similar and somewhat earlier trial with the bright gem of the lyre, whose arabic title of the "falling eagle" survives as a time-worn remnant in "vega." both astronomers agreed to use the "differential" method, for which their instruments and the vicinity to their selected stars of minute, physically detached companions offered special facilities. in the last month of bessel made known the result of one year's observations, showing for cygni a parallax of about a third of a second ( · ").[ ] he then had his heliometer taken down and repaired, after which he resumed the inquiry, and finally terminated a series of measures in march .[ ] the resulting parallax of · " (corresponding to a distance about , times that of the earth from the sun), seemed to be ascertained beyond the possibility of cavil, and is memorable as the first _published_ instance of the fathom-line, so industriously thrown into celestial space, having really and indubitably _touched bottom_. it was confirmed in - with curious exactness by c. a. f. peters at pulkowa; but later researches showed that it required increase to nearly half a second.[ ] struve's measurements inspired less confidence. they extended over three years ( - ), but were comparatively few, and were frequently interrupted. the parallax, accordingly, of about a quarter of a second ( · ") which he derived from them for alpha lyræ, and announced in ,[ ] has proved considerably too large.[ ] meanwhile a result of the same kind, but of a more striking character than either bessel's or struve's, had been obtained, one might almost say casually, by a different method and in a distant region. thomas henderson, originally an attorney's clerk in his native town of dundee, had become known for his astronomical attainments, and was appointed in to direct the recently completed observatory at the cape of good hope. he began observing in april, , and, the serious shortcomings of his instrument notwithstanding, executed during the thirteen months of his tenure of office a surprising amount of first-rate work. with a view to correcting the declination of the lustrous double star alpha centauri (which ranks after sirius and canopus as the third brightest orb in the heavens), he effected a number of successive determinations of its position, and on being informed of its very considerable proper motion ( · " annually), he resolved to examine the observations already made for possible traces of parallactic displacement. this was done on his return to scotland, where he filled the office of astronomer royal from until his premature death in . the result justified his expectations. from the declination measurements made at the cape and duly reduced, a parallax of about one second of arc clearly emerged (diminished by gill's and elkin's observations, - , to o· "); but, by perhaps an excess of caution, was withheld from publication until fuller certainty was afforded by the concurrent testimony of lieutenant meadows's determinations of the same star's right ascension.[ ] when at last, january , , henderson communicated his discovery to the astronomical society, he could no longer claim the priority which was his due. bessel had anticipated him with the parallax of cygni by just two months. thus from three different quarters, three successful and almost simultaneous assaults were delivered upon a long-beleaguered citadel of celestial secrets. the same work has since been steadily pursued, with the general result of showing that, as regards their overwhelming majority, the stars are far too remote to show even the slightest trace of optical shifting from the revolution of the earth in its orbit. in nearly a hundred cases, however, small parallaxes have been determined, some certainly (that is, within moderate limits of error), others more or less precariously. the list is an instructive one, in its omissions no less than in its contents. it includes stars of many degrees of brightness, from sirius down to a nameless telescopic star in the great bear;[ ] yet the vicinity to the earth of this minute object is so much greater than that of the brilliant vega, that the latter transported to its place would increase in lustre thirty-eight times. moreover, many of the brightest stars are found to have no sensible parallax, while the majority of those ascertained to be nearest to the earth are of fifth, sixth, even ninth magnitudes. the obvious conclusions follow that the range of variety in the sidereal system is enormously greater than had been supposed, and that estimates of distance based upon apparent magnitude must be wholly futile. thus, the splendid canopus, betelgeux, and rigel can be inferred, from their indefinite remoteness, to exceed our sun thousands of times in size and lustre; while many inconspicuous objects, which prove to be in our relative vicinity, must be notably his inferiors. the limits of real stellar magnitude are then set very widely apart. at the same time, the so-called "optical" and "geometrical" methods of relatively estimating star-distances are both seen to have a foundation of fact, although so disguised by complicated relations as to be of very doubtful individual application. on the whole, the chances are in favour of the superior vicinity of a bright star over a faint one; and, on the whole, the stars in swiftest _apparent_ motion are amongst those whose _actual_ remoteness is least. indeed, there is no escape from either conclusion, unless on the supposition of special arrangements in themselves highly improbable, and, we may confidently say, non-existent. the distances even of the few stars found to have measurable parallaxes are on a scale entirely beyond the powers of the human mind to conceive. in the attempt both to realize them distinctly, and to express them conveniently, a new unit of length, itself of bewildering magnitude, has originated. this is what we may call the _light-journey_ of one year. the subtle vibrations of the ether, propagated on all sides from the surface of luminous bodies, travel at the rate of , miles a second, or (in round numbers) six billions of miles a year. four and a third such measures are needed to span the abyss that separates us from the nearest fixed star. in other words, light takes four years and four months to reach the earth from alpha centauri; yet alpha centauri lies some ten billions of miles nearer to us (so far as is yet known) than any other member of the sidereal system! the determination of parallax leads, in the case of stars revolving in known orbits, to the determination of mass; for the distance from the earth of the two bodies forming a binary system being ascertained, the seconds of arc apparently separating them from each other can be translated into millions of miles; and we only need to add a knowledge of their period to enable us, by an easy sum in proportion, to find their combined mass in terms of that of the sun. thus, since--according to dr. doberck's elements--the components of alpha centauri revolve round their common centre of gravity at a mean distance nearly times the radius of the earth's orbit, in a period of years, the attractive force of the two together must be just twice the solar. we may gather some idea of their relations by placing in imagination a second luminary like our sun in circulation between the orbits of neptune and uranus. but systems of still more majestic proportions are reduced by extreme remoteness to apparent insignificance. a double star of the fourth magnitude in cassiopeia (eta), to which a small parallax is ascribed on the authority of o. struve, appears to be above eight times as massive as the central orb of our world; while a much less conspicuous pair-- pegasi--exerts, if the available data can be depended upon, no less than thirteen times the solar gravitating power. further, the actual rate of proper motions, so far as regards that part of them which is projected upon the sphere, can be ascertained for stars at known distance. the annual journey, for instance, of cygni _across the line of sight_ amounts to , , and that of alpha centauri to millions of miles. a small star, numbered , in groombridge's circumpolar catalogue, "devours the way" at the rate of at least miles a second--a speed, in newcomb's opinion, beyond the gravitating power of the entire sidereal system to control; and mu cassiopeiæ possesses above two-thirds of that surprising velocity; while for both objects, radial movements of just sixty miles a second were disclosed by professor campbell's spectroscopic measurements. herschel's conclusion as to the advance of the sun among the stars was not admitted as valid by the most eminent of his successors. bessel maintained that there was absolutely no preponderating evidence in favour of its supposed direction towards a point in the constellation hercules.[ ] biot, burckhardt, even herschel's own son, shared his incredulity. but the appearance of argelander's prize-essay in [ ] changed the aspect of the question. herschel's first memorable solution in was based upon the motions of thirteen stars, imperfectly known; his second, in , upon those of no more than six. argelander now obtained an entirely concordant result from the large number of , determined with the scrupulous accuracy characteristic of bessel's work and his own. the reality of the fact thus persistently disclosed could no longer be doubted; it was confirmed five years later by the younger struve, and still more strikingly in [ ] by galloway's investigations, founded exclusively on the apparent displacements of southern stars. in and , sir george airy and mr. dunkin ( - ),[ ] employing all the resources of modern science, and commanding the wealth of material furnished by , proper motions carefully determined by mr. main, reached conclusions closely similar to that indicated nearly eighty years previously by the first great sidereal astronomer; which mr. plummer's reinvestigation of the subject in [ ] served but slightly to modify. yet astronomers were not satisfied. dr. auwers of berlin completed in a splendid piece of work, for which he received in the gold medal of the royal astronomical society. it consisted in reducing afresh, with the aid of the most refined modern data, bradley's original stars, and comparing their places thus obtained for the year with those assigned to them from observations made at greenwich after the lapse of ninety years. in the interval, as was to be anticipated, most of them were found to have travelled over some small span of the heavens, and there resulted a stock of nearly three thousand highly authentic proper motions. these ample materials were turned to account by m. ludwig struve[ ] for a discussion of the sun's motion, of which the upshot was to shift its point of aim to the bordering region of the constellations hercules and lyra. and the more easterly position of the solar apex was fully confirmed by the experiments, with variously assorted lists of stars, of lewis boss of albany,[ ] and oscar stumpe of bonn.[ ] fresh precautions of refinement were introduced into the treatment of the subject by ristenpart of karlsruhe,[ ] by kapteyn of groningen,[ ] by newcomb[ ] and porter[ ] in america, who ably availed themselves of the copious materials accumulated before the close of the century. their results, although not more closely accordant than those of their predecessors, combined to show that the journey of our system is directed towards a point within a circle about ten degrees in radius, having the brilliant vega for its centre. to determine its rate was a still more arduous problem. it involved the assumption, very much at discretion, of an average parallax for the stars investigated; and otto struve's estimate of million miles as the span yearly traversed was hence wholly unreliable. fortunately, however, as will be seen further on, a method of determining the sun's velocity independently of any knowledge of star-distances, has now become available. as might have been expected, speculation has not been idle regarding the purpose and goal of the strange voyage of discovery through space upon which our system is embarked; but altogether fruitlessly. the variety of the conjectures hazarded in the matter is in itself a measure of their futility. long ago, before the construction of the heavens had as yet been made the subject of methodical inquiry, kant was disposed to regard sirius as the "central sun" of the milky way; while lambert surmised that the vast orion nebula might serve as the regulating power of a subordinate group including our sun. herschel threw out the hint that the great cluster in hercules might prove to be the supreme seat of attractive force;[ ] argelander placed his central body in the constellation perseus;[ ] fomalhaut, the brilliant of the southern fish, was set in the post of honour by boguslawski of breslau. mädler (who succeeded struve at dorpat in ) concluded from a more formal inquiry that the ruling power in the sidereal system resided, not in any single prepondering mass, but in the centre of gravity of the self-controlled revolving multitude.[ ] in the former case (as we know from the example of the planetary scheme), the stellar motions would be most rapid near the centre; in the latter, they would become accelerated with remoteness from it.[ ] mädler showed that no part of the heavens could be indicated as a region of exceptionally swift movements, such as would result from the presence of a gigantic (though possibly obscure) ruling body; but that a community of extremely sluggish movements undoubtedly existed in and near the group of the pleiades, where, accordingly, he placed the centre of gravity of the milky way.[ ] the bright star alcyone thus became the "central sun," but in a purely passive sense, its headship being determined by its situation at the point of neutralisation of opposing tendencies, and of consequent rest. by an avowedly conjectural method, the solar period of revolution round this point was fixed at , , years. the scheme of sidereal government framed by the dorpat astronomer was, it may be observed, of the most approved constitutional type; deprivation, rather than increase of influence accompanying the office of chief dignitary. but while we are still ignorant, and shall perhaps ever remain so, of the fundamental plan upon which the galaxy is organised, recent investigations tend more and more to exhibit it, not as monarchical (so to speak), but as federative. the community of proper motions detected by mädler in the vicinity of the pleiades may accordingly possess a significance altogether different from what he imagined. bessel's so-called "foundation of an astronomy of the invisible" now claims attention.[ ] his prediction regarding the planet neptune does not belong to the present division of our subject; a strictly analogous discovery in the sidereal system was, however, also very clearly foreshadowed by him. his earliest suspicions of non-uniformity in the proper motion of sirius dated from ; they extended to procyon in ; and after a series of refined measurements with the new repsold circle, he announced in his conclusion that these irregularities were due to the presence of obscure bodies round which the two bright dog-stars revolved as they pursued their way across the sphere.[ ] he even assigned to each an approximate period of half a century. "i adhere to the conviction," he wrote later to humboldt, "that procyon and sirius form real binary systems, consisting of a visible and an invisible star. there is no reason to suppose luminosity an essential quality of cosmical bodies. the visibility of countless stars is no argument against the invisibility of countless others."[ ] an inference so contradictory to received ideas obtained little credit, until peters found, in ,[ ] that the apparent anomalies in the movements of sirius could be completely explained by an orbital revolution in a period of fifty years. bessel's prevision was destined to be still more triumphantly vindicated. on the st of january, , while in the act of trying a new -inch refractor, mr. alvan g. clark (one of the celebrated firm of american opticians) actually discovered the hypothetical sirian companion in the precise position required by theory. it has now been watched through nearly an entire revolution (period · years), and proves to be very slightly luminous in proportion to its mass. its attractive power, in fact, is nearly half that of its primary, while it emits only / th of its light. sirius itself, on the other hand, possesses a far higher radiative intensity than our sun. it gravitates--admitting sir david gill's parallax of · " to be exact--like two suns, but shines like twenty. possibly it is much distended by heat, and undoubtedly its atmosphere intercepts a very much smaller proportion of its light than in stars of the solar class. as regards procyon, visual verification was awaited until november , , when professor schaeberle, with the great lick refractor, detected the long-sought object in the guise of a thirteenth-magnitude star. dr. see's calculations[ ] showed it to possess one-fifth the mass of its primary, or rather more than half that of our sun.[ ] yet it gives barely / th of the sun's light, so that it is still nearer to total obscurity than the dusky satellite of sirius. the period of forty years assigned to the system by auwers in [ ] appears to be singularly exact. but bessel was not destined to witness the recognition of "the invisible" as a legitimate and profitable field for astronomical research. he died march , , just six months before the discovery of neptune, of an obscure disease, eventually found to be occasioned by an extensive fungus-growth in the stomach. the place which he left vacant was not one easy to fill. his life's work might be truly described as "epoch-making." rarely indeed shall we find one who reconciled with the same success the claims of theoretical and practical astronomy, or surveyed the science which he had made his own with a glance equally comprehensive, practical, and profound. the career of friedrich georg wilhelm struve illustrates the maxim that science _differentiates_ as it develops. he was, while much besides, a specialist in double stars. his earliest recorded use of the telescope was to verify herschel's conclusion as to the revolving movement of castor, and he never varied from the predilection which this first observation at once indicated and determined. he was born at altona, of a respectable yeoman family, april , , and in took a degree in philology at the new russian university of dorpat. he then turned to science, was appointed in to a professorship of astronomy and mathematics, and began regular work in the dorpat observatory just erected by parrot for alexander i. it was not, however, until that the acquisition of a -foot refractor by troughton enabled him to take the position-angles of double stars with regularity and tolerable precision. the resulting catalogue of stellar systems gave the signal for a general resumption of the herschelian labours in this branch. his success, so far, and the extraordinary facilities for observation afforded by the fraunhofer achromatic encouraged him to undertake, february , , a review of the entire heavens down to ° south of the celestial equator, which occupied more than two years, and yielded, from an examination of above , stars, a harvest of about , previously unnoticed composite objects. the ensuing ten years were devoted to delicate and patient measurements, the results of which were embodied in _mensuræ micrometricæ_, published at st. petersburg in . this monumental work gives the places, angles of position, distances, colours, and relative brightness of , double and multiple stars, all determined with the utmost skill and care. the record is one which gains in value with the process of time, and will for ages serve as a standard of reference by which to detect change or confirm discovery. it appears from struve's researches that about one in forty of all stars down to the ninth magnitude is composite, but that the proportion is doubled in the brighter orders.[ ] this he attributed to the difficulty of detecting the faint companions of very remote orbs. it was also noticed, both by him and bessel, that double stars are in general remarkable for large proper motions. struve's catalogue included no star of which the components were more than " apart, because beyond that distance the chances of merely optical juxtaposition become considerable; but the immense preponderance of extremely close over (as it were) loosely yoked bodies is such as to demonstrate their physical connection, even if no other proof were forthcoming. many stars previously believed to be single divided under the scrutiny of the dorpat refractor; while in some cases, one member of a supposed binary system revealed itself as double, thus placing the surprised observer in the unexpected presence of a triple group of suns. five instances were noted of two pairs lying so close together as to induce a conviction of their mutual dependence;[ ] besides which, examples occurred of triple, quadruple, and multiple combinations, the reality of which was open to no reasonable doubt.[ ] it was first pointed out by bessel that the fact of stars exhibiting a common proper motion might serve as an unfailing test of their real association into systems. this was, accordingly, one of the chief criteria employed by struve to distinguish true binaries from merely optical couples. on this ground alone, cygni was admitted to be a genuine double star; and it was shown that, although its components appeared to follow almost strictly rectilinear paths, yet the probability of their forming a connected pair is actually greater than that of the sun rising to-morrow morning.[ ] moreover, this tie of an identical movement was discovered to unite bodies[ ] far beyond the range of distance ordinarily separating the members of binary systems, and to prevail so extensively as to lead to the conclusion that single do not outnumber conjoined stars more than twice or thrice.[ ] in struve was summoned by the emperor nicholas to superintend the erection of a new observatory at pulkowa, near st. petersburg, destined for the special cultivation of sidereal astronomy. boundless resources were placed at his disposal, and the institution created by him was acknowledged to surpass all others of its kind in splendour, efficiency, and completeness. its chief instrumental glory was a refractor of fifteen inches aperture by merz and mahler (fraunhofer's successors), which left the famous dorpat telescope far behind, and remained long without a rival. on the completion of this model establishment, august , , struve was installed as its director, and continued to fulfil the important duties of the post with his accustomed vigour until , when illness compelled his virtual resignation in favour of his son otto struve, born at dorpat in . he died november , . an inquiry into the laws of stellar distribution, undertaken during the early years of his residence at pulkowa, led struve to confirm in the main the inferences arrived at by herschel as to the construction of the heavens. according to his view, the appearance known as the milky way is produced by a collection of irregularly condensed star-clusters, within which the sun is somewhat eccentrically placed. the nebulous ring which thus integrates the light of countless worlds was supposed by him to be made up of stars scattered over a bent or "broken plane," or to lie in two planes slightly inclined to each other, our system occupying a position near their intersection.[ ] he further attempted to show that the limits of this vast assemblage must remain for ever shrouded from human discernment, owing to the gradual extinction of light in its passage through space,[ ] and sought to confer upon this celebrated hypothesis a definiteness and certainty far beyond the aspirations of its earlier advocates, chéseaux and olbers; but arbitrary assumptions vitiated his reasonings on this, as well as on some other points.[ ] in his special line as a celestial explorer of the most comprehensive type, sir william herschel had but one legitimate successor, and that successor was his son. john frederick william herschel was born at slough, march , , graduated with the highest honours from st. john's college, cambridge, in , and entered upon legal studies with a view to being called to the bar. but his share in an early compact with peacock and babbage, "to do their best to leave the world wiser than they found it," was not thus to be fulfilled. the acquaintance of dr. wollaston decided his scientific vocation. already, in , we find him reviewing some of his father's double stars; and he completed in the -inch speculum which was to be the chief instrument of his investigations. soon afterwards, he undertook, in conjunction with mr. (later sir james) south, a series of observations, issuing in the presentation to the royal society of a paper[ ] containing micrometrical measurements of binary stars, by which the elder herschel's inferences of orbital motion were, in many cases, strikingly confirmed. a star in the northern crown, for instance (eta coronæ), had completed more than one entire circuit since its first discovery; another, tau ophiuchi, had _closed up_ into apparent singleness; while the motion of a third, xi ursæ majoris, in an obviously eccentric orbit, was so rapid as to admit of being traced and measured from month to month. it was from the first confidently believed that the force retaining double stars in curvilinear paths was identical with that governing the planetary revolutions. but that identity was not ascertained until savary of paris showed, in ,[ ] that the movements of the above-named binary in the great bear could be represented with all attainable accuracy by an ellipse calculated on orthodox gravitational principles with a period of - / years. encke followed at berlin with a still more elegant method; and sir john herschel, pointing out the uselessness of analytical refinements where the data were necessarily so imperfect, described in a graphical process by which "the aid of the eye and hand" was brought in "to guide the judgment in a case where judgment only, and not calculation, could be of any avail."[ ] improved methods of the same kind were published by dr. see in ,[ ] and by mr. burnham in ;[ ] and our acquaintance with stellar orbits is steadily gaining precision, certainty, and extent. in herschel undertook, and executed with great assiduity during the ensuing eight years, a general survey of the northern heavens, directed chiefly towards the verification of his father's nebular discoveries. the outcome was a catalogue of , nebulæ and clusters, of which were observed for the first time, besides , double stars discovered almost incidentally.[ ] "strongly invited," as he tells us himself, "by the peculiar interest of the subject, and the wonderful nature of the objects which presented themselves," he resolved to attempt the completion of the survey in the southern hemisphere. with this noble object in view, he embarked his family and instruments on board the _mount stewart elphinstone_, and, after a prosperous voyage, landed at cape town on the th of january, . choosing as the scene of his observations a rural spot under the shelter of table mountain, he began regular "sweeping" on the th of march. the site of his great reflector is now marked with an obelisk, and the name of feldhausen has become memorable in the history of science; for the four years' work done there may truly be said to open the chapter of our knowledge as regards the southern skies. the full results of herschel's journey to the cape were not made public until , when a splendid volume[ ] embodying them was brought out at the expense of the duke of northumberland. they form a sequel to his father's labours such as the investigations of one man have rarely received from those of another. what the elder observer did for the northern heavens, the younger did for the southern, and with generally concordant results. reviving the paternal method of "star-gauging," he showed, from a count of , fields, that the milky way surrounds the solar system as a complete annulus of minute stars; not, however, quite symmetrically, since the sun was thought to lie somewhat nearer to those portions visible in the southern hemisphere, which display a brighter lustre and a more complicated structure than the northern branches. the singular cosmical agglomerations known as the "magellanic clouds" were now, for the first time, submitted to a detailed, though admittedly incomplete, examination, the almost inconceivable richness and variety of their contents being such that a lifetime might with great profit be devoted to their study. in the greater nubecula, within a compass of forty-two square degrees, herschel reckoned distinct nebulæ and clusters, besides fifty or sixty outliers, and a large number of stars intermixed with diffused nebulosity--in all, catalogued objects, and, for the lesser cloud, . yet this was only the most conspicuous part of what his twenty-foot revealed. such an extraordinary concentration of bodies so various led him to the inevitable conclusion that "the nubeculæ are to be regarded as systems _sui generis_, and which have no analogues in our hemisphere."[ ] he noted also the blankness of surrounding space, especially in the case of nubecula minor, "the access to which on all sides," he remarked, "is through a desert;" as if the cosmical material in the neighbourhood had been swept up and garnered in these mighty groups.[ ] of southern double stars, he discovered and gave careful measurements of , , and described , nebulæ, of which at least were new. the list was illustrated with a number of drawings, some of them extremely beautiful and elaborate. sir john herschel's views as to the nature of nebulæ were considerably modified by lord rosse's success in "resolving" with his great reflectors a crowd of these objects into stars. his former somewhat hesitating belief in the existence of phosphorescent matter, "disseminated through extensive regions of space in the manner of a cloud or fog,"[ ] was changed into a conviction that no valid distinction could be established between the faintest wisp of cosmical vapour just discernible in a powerful telescope, and the most brilliant and obvious cluster. he admitted, however, an immense range of possible variety in the size and mode of aggregation of the stellar constituents of various nebulæ. some might appear nebulous from the closeness of their parts; some from their smallness. others, he suggested, might be formed of "discrete luminous bodies floating in a non-luminous medium;"[ ] while the annular kind probably consisted of "hollow shells of stars."[ ] that a physical, and not merely an optical, connection unites nebulæ with the _embroidery_ (so to speak) of small stars with which they are in many instances profusely decorated, was evident to him, as it must be to all who look as closely and see as clearly as he did. his description of no. , in his northern catalogue as "a network or tracery of nebula following the lines of a similar network of stars,"[ ] would alone suffice to dispel the idea of accidental scattering; and many other examples of a like import might be quoted. the remarkably frequent occurrence of one or more minute stars in the close vicinity of "planetary" nebulæ led him to infer their dependent condition; and he advised the maintenance of a strict watch for evidences of circulatory movements, not only over these supposed stellar satellites, but also over the numerous "double nebulæ," in which, as he pointed out, "all the varieties of double stars as to distance, position, and relative brightness, have their counterparts." he, moreover, investigated the subject of nebular distribution by the simple and effectual method of graphic delineation or "charting," and succeeded in showing that while a much greater uniformity of scattering prevails in the southern than in the northern heavens, a condensation is nevertheless perceptible about the constellations pisces and cetus, roughly corresponding to the "nebular region" in virgo by its vicinity (within ° or °) to the opposite pole of the milky way. he concluded "that the nebulous system is distinct from the sidereal, though involving, and perhaps to a certain extent intermixed with, the latter."[ ] towards the close of his residence at feldhausen, herschel was fortunate enough to witness one of those singular changes in the aspect of the firmament which occasionally challenge the attention even of the incurious, and excite the deepest wonder of the philosophical observer. immersed apparently in the argo nebula is a star denominated eta carinæ. when halley visited st. helena in , it seemed of the fourth magnitude; but lacaille in the middle of the following century, and others after him, classed it as of the second. in the traveller burchell, being then at st. paul, near rio janeiro, remarked that it had unexpectedly assumed the first rank--a circumstance the more surprising to him because he had frequently, when in africa during the years to , noted it as of only fourth magnitude. this observation, however, did not become generally known until later. herschel, on his arrival at feldhausen, registered the star as a bright second, and had no suspicion of its unusual character until december , , when he suddenly perceived its light to be almost tripled. it then far outshone rigel in orion, and on the nd of january following it very nearly matched alpha centauri. from that date it declined; but a second and even brighter maximum occurred in april, , when maclear, then director of the cape observatory, saw it blaze out with a splendour approaching that of sirius. its waxings and wanings were marked by curious "trepidations" of brightness extremely perplexing to theory. in it had sunk below the fifth magnitude, and in was barely visible to the naked eye; yet it was not until eighteen years later that it touched a minimum of · magnitude. soon afterwards a recovery of brightness set in, but was not carried very far; and the star now shines steadily as of the seventh magnitude, its reddish light contrasting effectively with the silvery rays of the surrounding nebula. an attempt to include its fluctuations within a cycle of seventy years[ ] has signally failed; the extent and character of the vicissitudes to which it is subject stamping it rather as a species of connecting link between periodic and temporary stars.[ ] among the numerous topics which engaged herschel's attention at the cape was that of relative stellar brightness. having contrived an "astrometer" in which an "artificial star," formed by the total reflection of moonlight from the base of a prism, served as a standard of comparison, he was able to estimate the lustre of the _natural_ stars examined by the distances at which the artificial object appeared equal respectively to each. he thus constructed a table of of the principal stars,[ ] both in the northern and southern hemispheres, setting forth the numerical values of their apparent brightness relatively to that of alpha centauri, which he selected as a unit of measurement. further, the light of the full moon being found by him to exceed that of his standard star , times, and dr. wollaston having shown that the light of the full moon is to that of the sun as : , [ ] (zöllner made the ratio : , ), it became possible to compare stellar with solar radiance. hence was derived, in the case of the few stars at ascertained distances, a knowledge of real lustre. alpha centauri, for example, emits less than twice, capella one hundred times as much light as our sun; while arcturus, at its enormous distance, must display the splendour of , such luminaries. herschel returned to england in the spring of , bringing with him a wealth of observation and discovery such as had perhaps never before been amassed in so short a time. deserved honours awaited him. he was created a baronet on the occasion of the queen's coronation (he had been knighted in ); universities and learned societies vied with each other in showering distinctions upon him; and the success of an enterprise in which scientific zeal was tinctured with an attractive flavour of adventurous romance, was justly regarded as a matter of national pride. his career as an observing astronomer was now virtually closed, and he devoted his leisure to the collection and arrangement of the abundant trophies of his father's and his own activity. the resulting great catalogue of , nebulæ (including all then certainly known), published in the _philosophical transactions_ for , is, and will probably long remain, the fundamental source of information on the subject;[ ] but he unfortunately did not live to finish the companion work on double stars, for which he had accumulated a vast store of materials.[ ] he died at collingwood in kent, may , , in the eightieth year of his age, and was buried in westminster abbey, close beside the grave of sir isaac newton. the consideration of sir john herschel's cape observations brings us to the close of the period we are just now engaged in studying. they were given to the world, as already stated, three years before the middle of the century, and accurately represent the condition of sidereal science at that date. looking back over the fifty years traversed, we can see at a glance how great was the stride made in the interval. not alone was acquaintance with individual members of the cosmos vastly extended, but their mutual relations, the laws governing their movements, their distances from the earth, masses, and intrinsic lustre, had begun to be successfully investigated. _begun to be_; for only regarding a scarcely perceptible minority had even approximate conclusions been arrived at. nevertheless the whole progress of the future lay in that beginning; it was the thin end of the wedge of exact knowledge. the principle of measurement had been substituted for that of probability; a basis had been found large and strong enough to enable calculation to ascend from it to the sidereal heavens; and refinements had been introduced, fruitful in performance, but still more in promise. thus, rather the kind than the amount of information collected was significant for the time to come--rather the methods employed than the results actually secured rendered the first half of the nineteenth century of epochal importance in the history of our knowledge of the stars. footnotes: [footnote : bessel, _populäre vorlesungen_, pp. , .] [footnote : fitted to the old transit instrument, july , .] [footnote : _briefwechsel mit olbers_, p. xvi.] [footnote : r. wolf, _gesch. der astron._, p. .] [footnote : bessel, _pop. vorl._, p. .] [footnote : a new reduction of the observations upon which they were founded was undertaken in by herman s. davis, of the u.s. coast survey.] [footnote : bessel, _pop. vorl._, p. .] [footnote : durège, _bessel's leben und wirken_, p. .] [footnote : _bonner beobachtungen_, bd. iii.-v., - .] [footnote : bessel, _pop. vorl._, p. .] [footnote : the heads of the screws applied to move the halves of the object-glass in the königsberg heliometer are of so considerable a size that a thousandth part of a revolution, equivalent to / of a second of arc, can be measured with the utmost accuracy. main, _r. a. s. mem._, vol. xii., p. .] [footnote : _specola astronomica di palermo_, lib. vi., p. , _note_.] [footnote : _monatliche correspondenz_, vol. xxvi., p. .] [footnote : _astronomische nachrichten_, nos. - . it should be explained that what is called the "annual parallax" of a star is only half its apparent displacement. in other words, it is the angle subtended at the distance of that particular star by the _radius_ of the earth's orbit.] [footnote : _astr. nach._, nos. - .] [footnote : sir r. ball's measurements at dunsink gave to cygni a parallax of · "; professor pritchard obtained, by photographic determinations, one of · ".] [footnote : _additamentum in mensuras micrometricas_, p. .] [footnote : elkin's corrected result (in ) for the parallax of vega is · ".] [footnote : _mem. roy. astr. soc._, vol. xi., p. .] [footnote : that numbered , in lalande's _hist. cél._, found by argelander to have a proper motion of · ", and by winnecke a parallax of o· ". _month. not._, vol. xviii., p. .] [footnote : _fund. astr._, p. .] [footnote : _mém. prés. à l'ac. de st. pétersb._, t. iii.] [footnote : _phil. trans._, vol. cxxxvii., p. .] [footnote : _mem. roy. astr. soc._, vols. xxviii. and xxxii.] [footnote : _ibid._, vol. xlvii., p. .] [footnote : _mémoires de st. pétersbourg_, t. xxxv., no. , ; revised in _astr. nach._, nos. , - , .] [footnote : _astronomical journal_, nos. , .] [footnote : _astr. nach._, nos. , , , .] [footnote : _veröffentlichungen der grossh. sternwarte zu karlsruhe_, bd. iv., .] [footnote : _proceedings amsterdam acad. of sciences_, jan. , .] [footnote : _astr. jour._, no. .] [footnote : _ibid._, nos. , .] [footnote : _phil. trans._, vol. xcvi., p. .] [footnote : _mém. prés. à l'ac. de st. pétersbourg_, t. iii., p. (read feb. , ).] [footnote : _die centralsonne, astr. nach._, nos. - , .] [footnote : sir j. herschel, note to _treatise on astronomy_, and _phil. trans._, vol. cxxiii., part ii., p. .] [footnote : the position is (as sir j. herschel pointed out, _outlines of astronomy_, p. , th ed.) placed beyond the range of reasonable probability by its remoteness (fully °) from the galactic plane.] [footnote : mädler in _westermann's jahrbuch_, , p. .] [footnote : letter from bessel to sir j. herschel, _month. not._, vol. vi., p. .] [footnote : wolf, _gesch. d. astr._, p. , _note_.] [footnote : _astr. nach._, nos. - .] [footnote : _astr. jour._, no. .] [footnote : adopting elkin's revised parallax for procyon of · ".] [footnote : _astr. nach._, nos. - .] [footnote : _ueber die doppelsterne_, bericht, , p. .] [footnote : _ueber die doppelsterne_, bericht, , p. .] [footnote : _mensuræ micr._, p. xcix.] [footnote : _stellarum fixarum imprimis duplicium et multiplicum positiones mediæ_, pp. cxc., cciii.] [footnote : for instance, the southern stars, a ophiuchi (itself double) and scorpii, which are ' " apart. _ibid._, p. cciii.] [footnote : _stellarum fixarum_, etc., p. ccliii.] [footnote : _Études d'astronomie stellaire_, , p. .] [footnote : _ibid._, p. .] [footnote : see encke's criticism in _astr. nach._, no. .] [footnote : _phil. trans._, vol. cxiv., part iii., .] [footnote : _conn. d. temps_, .] [footnote : _r. a. s. mem._, vol. v., p. , .] [footnote : _astr. and astrophysics_, vol. xii., p. .] [footnote : _popular astr._, vol. i., p. .] [footnote : _phil. trans._, vol. cxxiii., and _results_, etc., introd.] [footnote : _results of astronomical observations made during the years - at the cape of good hope._] [footnote : _results_, etc., p. .] [footnote : see proctor's _universe of stars_, p. .] [footnote : _a treatise on astronomy_, , p. .] [footnote : _results_, etc., p. .] [footnote : _ibid._, pp. , .] [footnote : _phil. trans._, vol. cxxiii., p. .] [footnote : _results_, etc., p. .] [footnote : loomis, _month. not._, vol. xxix., p. .] [footnote : see the author's _system of the stars_, pp. - .] [footnote : _outlines of astr._, app. i.] [footnote : _phil. trans._, vol. cxix., p. .] [footnote : dr. dreyer's new general catalogue, published in as vol. xlix. of the royal astronomical society's _memoirs_, is an enlargement of herschel's work. it includes , entries, and was supplemented, in , by an "index catalogue" of , nebulæ discovered to . _mem. r. a. s._, vol. li.] [footnote : a list of , composite stars was drawn out by him in order of right ascension, and has been published in vol. xl. of _mem. r. a. s._; but the data requisite for their formation into a catalogue were not forthcoming. see main's and pritchard's _preface_ to above, and dunkin's _obituary notices_, p. .] chapter iii _progress of knowledge regarding the sun_ the discovery of sun-spots in by fabricius and galileo first opened a way for inquiry into the solar constitution; but it was long before that way was followed with system or profit. the seeming irregularity of the phenomena discouraged continuous attention; casual observations were made the basis of arbitrary conjectures, and real knowledge received little or no increase. in we find jean tarde, canon of sarlat, arguing that because the sun is "the eye of the world," and the eye of the world _cannot suffer from ophthalmia_, therefore the appearances in question must be due, not to actual specks or stains on the bright solar disc, but to the transits of a number of small planets across it! to this new group of heavenly bodies he gave the name of "borbonia sidera," and they were claimed in for the house of hapsburg, under the title of "austriaca sidera" by father malapertius, a belgian jesuit.[ ] a similar view was temporarily maintained against galileo by the justly celebrated father scheiner of ingolstadt, and later by william gascoigne, the inventor of the micrometer; but most of those who were capable of thinking at all on such subjects (and they were but few) adhered either to the _cloud theory_ or to the _slag theory_ of sun-spots. the first was championed by galileo, the second by simon marius, "astronomer and physician" to the brother margraves of brandenburg. the latter opinion received a further notable development from the fact that in , a year remarkable for the appearance of three bright comets, the sun was almost free from spots; whence it was inferred that the cindery refuse from the great solar conflagration, which usually appeared as dark blotches on its surface, was occasionally thrown off in the form of comets, leaving the sun, like a snuffed taper, to blaze with renewed brilliancy.[ ] in the following century, derham gathered from observations carried on during the years - , "that the spots on the sun are caused by the eruption of some new volcano therein, which at first pouring out a prodigious quantity of smoke and other opacous matter, causeth the spots; and as that fuliginous matter decayeth and spendeth itself, and the volcano at last becomes more torrid and flaming, so the spots decay, and grow to umbræ, and at last to faculæ."[ ] the view, confidently upheld by lalande,[ ] that spots were rocky elevations uncovered by the casual ebbing of a luminous ocean, the surrounding penumbræ representing shoals or sandbanks, had even less to recommend it than derham's volcanic theory. both were, however, significant of a growing tendency to bring solar phenomena within the compass of terrestrial analogies. for years, then, after galileo first levelled his telescope at the setting sun, next to nothing was learned as to its nature; and the facts immediately ascertained, of its rotation on an axis nearly erect to the plane of the ecliptic, in a period of between twenty-five and twenty-six days, and of the virtual limitation of the spots to a so-called "royal" zone extending some thirty degrees north and south of the solar equator, gained little either in precision or development from five generations of astronomers. but in november, , a spot of extraordinary size engaged the attention of alexander wilson, professor of astronomy in the university of glasgow. he watched it day by day, and to good purpose. as the great globe slowly revolved, carrying the spot towards its western edge, he was struck with the gradual contraction and final disappearance of the penumbra _on the side next the centre of the disc_; and when on the th of december the same spot re-emerged on the eastern limb, he perceived, as he had anticipated, that the shady zone was now deficient _on the opposite side_, and resumed its original completeness as it returned to a central position. in other spots subsequently examined by him, similar perspective effects were visible, and he proved in ,[ ] by strict geometrical reasoning, that they could only arise in vast photospheric excavations. it was not, indeed, the first time that such a view had been suggested. father scheiner's later observations plainly foreshadowed it;[ ] a conjecture to the same effect was emitted by leonard rost of nuremburg early in the eighteenth century;[ ] both by lahire in and by j. cassini in spots had been seen as notches on the solar limb; while in pastor schülen of essingen, from the careful study of phenomena similar to those noted by wilson, concluded their depressed nature.[ ] modern observations, nevertheless, prove those phenomena to be by no means universally present. wilson's general theory of the sun was avowedly tentative. it took the modest form of an interrogatory. "is it not reasonable to think," he asks, "that the great and stupendous body of the sun is made up of two kinds of matter, very different in their qualities; that by far the greater part is solid and dark, and that this immense and dark globe is encompassed with a thin covering of that resplendent substance from which the sun would seem to derive the whole of his vivifying heat and energy?"[ ] he further suggests that the excavations or spots may be occasioned "by the working of some sort of elastic vapour which is generated within the dark globe," and that the luminous matter, being in some degree fluid, and being acted upon by gravity, tends to flow down and cover the nucleus. from these hints, supplemented by his own diligent observations and sagacious reasonings, herschel elaborated a scheme of solar constitution which held its ground until the physics of the sun were revolutionised by the spectroscope. a cool, dark, solid globe, its surface diversified with mountains and valleys, clothed in luxuriant vegetation, and "richly stored with inhabitants," protected by a heavy cloud-canopy from the intolerable glare of the upper luminous region, where the dazzling coruscations of a solar aurora some thousands of miles in depth evolved the stores of light and heat which vivify our world--such was the central luminary which herschel constructed with his wonted ingenuity, and described with his wonted eloquence. "this way of considering the sun and its atmosphere," he says,[ ] "removes the great dissimilarity we have hitherto been used to find between its condition and that of the rest of the great bodies of the solar system. the sun, viewed in this light, appears to be nothing else than a very eminent, large, and lucid planet, evidently the first, or, in strictness of speaking, the only primary one of our system; all others being truly secondary to it. its similarity to the other globes of the solar system with regard to its solidity, its atmosphere, and its diversified surface, the rotation upon its axis, and the fall of heavy bodies, leads us on to suppose that it is most probably also inhabited, like the rest of the planets, by beings whose organs are adapted to the peculiar circumstances of that vast globe." we smile at conclusions which our present knowledge condemns as extravagant and impossible, but such incidental flights of fancy in no way derogate from the high value of herschel's contributions to solar science. the cloud-like character which he attributed to the radiant shell of the sun (first named by schröter the "photosphere") is borne out by all recent investigations; he observed its mottled or corrugated aspect, resembling, as he described it, the roughness on the rind of an orange; showed that "faculæ" are elevations or heaped-up ridges of the disturbed photospheric matter; and threw out the idea that spots may ensue from an excess of the ordinary luminous emissions. a certain "empyreal" gas was, he supposed (very much as wilson had done), generated in the body of the sun, and rising everywhere by reason of its lightness, made for itself, when in moderate quantities, small openings or "pores,"[ ] abundantly visible as dark points on the solar disc. but should an uncommon quantity be formed, "it will," he maintained, "burst through the planetary[ ] regions of clouds, and thus will produce great openings; then, spreading itself above them, it will occasion large shallows (penumbræ), and mixing afterwards gradually with other superior gases, it will promote the increase, and assist in the maintenance, of the general luminous phenomena."[ ] this partial anticipation of the modern view that the solar radiations are maintained by some process of circulation within the solar mass, was reached by herschel through prolonged study of the phenomena in question. the novel and important idea contained in it, however, it was at that time premature to attempt to develop. but though many of the subtler suggestions of herschel's genius passed unnoticed by his contemporaries, the main result of his solar researches was an unmistakable one. it was nothing less than the definitive introduction into astronomy of the paradoxical conception of the central fire and hearth of our system as a cold, dark, terrestrial mass, wrapt in a mantle of innocuous radiance--an earth, so to speak, within--a sun without. let us pause for a moment to consider the value of this remarkable innovation. it certainly was not a step in the direction of truth. on the contrary, the crude notions of anaxagoras and xeno approached more nearly to what we now know of the sun, than the complicated structure devised for the happiness of a nobler race of beings than our own by the benevolence of eighteenth-century astronomers. and yet it undoubtedly constituted a very important advance in science. it was the first earnest attempt to bring solar phenomena within the compass of a rational system; to put together into a consistent whole the facts ascertained; to fabricate, in short, a solar machine that would in some fashion work. it is true that the materials were inadequate and the design faulty. the resulting construction has not proved strong enough to stand the wear and tear of time and discovery, but has had to be taken to pieces and remodelled on a totally different plan. but the work was not therefore done in vain. none of bacon's aphorisms show a clearer insight into the relations between the human mind and the external world than that which declares "truth to emerge sooner from error than from confusion."[ ] a definite theory (even if a false one) gives holding-ground to thought. facts acquire a meaning with reference to it. it affords a motive for accumulating them and a means of co-ordinating them; it provides a framework for their arrangement, and a receptacle for their preservation, until they become too strong and numerous to be any longer included within arbitrary limits, and shatter the vessel originally framed to contain them. such was the purpose subserved by herschel's theory of the sun. it helped to _clarify_ ideas on the subject. the turbid sense of groping and viewless ignorance gave place to the lucidity of a possible scheme. the persuasion of knowledge is a keen incentive to its increase. few men care to investigate what they are obliged to admit themselves entirely ignorant of; but once started on the road of knowledge, real or supposed, they are eager to pursue it. by the promulgation of a confident and consistent view regarding the nature of the sun, accordingly, research was encouraged, because it was rendered hopeful, and inquirers were shown a path leading indefinitely onwards where an impassable thicket had before seemed to bar the way. we have called the "terrestrial" theory of the sun's nature an innovation, and so, as far as its general acceptance is concerned, it may justly be termed; but, like all successful innovations, it was a long time brewing. it is extremely curious to find that herschel had a predecessor in its advocacy who never looked through a telescope (nor, indeed, imagined the possibility of such an instrument), who knew nothing of sun-spots, was still (mistaken assertions to the contrary notwithstanding) in the bondage of the geocentric system, and regarded nature from the lofty standpoint of an idealist philosophy. this was the learned and enlightened cardinal cusa, a fisherman's son from the banks of the moselle, whose distinguished career in the church and in literature extended over a considerable part of the fifteenth century ( - ). in his singular treatise _de doctâ ignorantiâ_, one of the most notable literary monuments of the early renaissance, the following passage occurs:--"to a spectator on the surface of the sun, the splendour which appears to us would be invisible, since it contains, as it were, an earth for its central mass, with a circumferential envelope of light and heat, and between the two an atmosphere of water and clouds and translucent air." the luminary of herschel's fancy could scarcely be more clearly portrayed; some added words, however, betray the origin of the cardinal's idea. "the earth also," he says, "would appear as a shining star to any one outside the fiery element." it was, in fact, an extension to the sun of the ancient elemental doctrine; but an extension remarkable at that period, as premonitory of the tendency, so powerfully developed by subsequent discoveries, to assimilate the orbs of heaven to the model of our insignificant planet, and to extend the brotherhood of our system and our species to the farthest limit of the visible or imaginable universe. in later times we find flamsteed communicating to newton, march , , his opinion "that the substance of the sun is terrestrial matter, his light but the liquid menstruum encompassing him."[ ] bode in arrived independently at the conclusion that "the sun is neither burning nor glowing, but in its essence a dark planetary body, composed like our earth of land and water, varied by mountains and valleys, and enveloped in a vaporous atmosphere";[ ] and the learned in general applauded and acquiesced. the view, however, was in still so far from popular, that the holding of it was alleged as a proof of insanity in dr. elliot when accused of a murderous assault on miss boydell. his friend dr. simmons stated on his behalf that he had received from him in the preceding january a letter giving evidence of a deranged mind, wherein he asserted "that the sun is not a body of fire, as hath been hitherto supposed, but that its light proceeds from a dense and universal aurora, which may afford ample light to the inhabitants of the surface beneath, and yet be at such a distance aloft as not to annoy them. no objection, he saith, ariseth to that great luminary's being inhabited; vegetation may obtain there as well as with us. there may be water and dry land, hills and dales, rain and fair weather; and as the light, so the season must be eternal, consequently it may easily be conceived to be by far the most blissful habitation of the whole system!" the recorder, nevertheless, objected that if an extravagant hypothesis were to be adduced as proof of insanity, the same might hold good with regard to some other speculators, and desired dr. simmons to tell the court what he thought of the theories of burnet and buffon.[ ] eight years later, this same "extravagant hypothesis," backed by the powerful recommendation of sir william herschel, obtained admittance to the venerable halls of science, there to abide undisturbed for nearly seven decades. individual objectors, it is true, made themselves heard, but their arguments had little effect on the general body of opinion. ruder blows were required to shatter an hypothesis flattering to human pride of invention in its completeness, in the plausible detail of observations by which it seemed to be supported, and in its condescension to the natural pleasure in discovering resemblance under all but total dissimilarity. sir john herschel included among the results of his multifarious labours at the cape of good hope a careful study of the sun-spots conspicuously visible towards the end of the year and in the early part of . they were remarkable, he tells us, for their forms and arrangement, as well as for their number and size; one group, measured on the th of march in the latter year, covering (apart from what may be called its outlying dependencies) the vast area of five square minutes or , million square miles.[ ] we have at present to consider, however, not so much these observations in themselves, as the chain of theoretical suggestions by which they were connected. the distribution of spots, it was pointed out, on two zones parallel to the equator, showed plainly their intimate connection with the solar rotation, and indicated as their cause fluid circulations analogous to those producing the terrestrial trade and anti-trade winds. "the spots, in this view of the subject," he went on to say,[ ] "would come to be assimilated to those regions on the earth's surface where, for the moment, hurricanes and tornadoes prevail; the upper stratum being temporarily carried downwards, displacing by its impetus the two strata of luminous matter beneath, the upper of course to a greater extent than the lower, and thus wholly or partially denuding the opaque surface of the sun below. such processes cannot be unaccompanied by vorticose motions, which, left to themselves, die away by degrees and dissipate, with the peculiarity that their lower portions come to rest more speedily than their upper, by reason of the greater resistance below, as well as the remoteness from the point of action, which lies in a higher region, so that their centres (as seen in our waterspouts, which are nothing but small tornadoes) appear to retreat upwards. now this agrees perfectly with what is observed during the obliteration of the solar spots, which appear as if filled in by the collapse of their sides, the penumbra closing in upon the spot and disappearing after it." but when it comes to be asked whether a cause can be found by which a diversity of solar temperature might be produced corresponding with that which sets the currents of the terrestrial atmosphere in motion, we are forced to reply that we know of no such cause. for sir john herschel's hypothesis of an increased retention of heat at the sun's equator, due to the slightly spheroidal or bulging form of its outer atmospheric envelope, assuredly gives no sufficient account of such circulatory movements as he supposed to exist. nevertheless, the view that the sun's rotation is intimately connected with the formation of spots is so obviously correct, that we can only wonder it was not thought of sooner, while we are even now unable to explain with any certainty _how_ it is so connected. mere scrutiny of the solar surface, however, is not the only means of solar observation. we have a satellite, and that satellite from time to time acts most opportunely as a screen, cutting off a part or the whole of those dazzling rays in which the master-orb of our system veils himself from over-curious regards. the importance of eclipses to the study of the solar surroundings is of comparatively recent recognition; nevertheless, much of what we know concerning them has been snatched, as it were, by surprise under favour of the moon. in former times, the sole astronomical use of such incidents was the correction of the received theories of the solar and lunar movements; the precise time of their occurrence was the main fact to be noted, and subsidiary phenomena received but casual attention. now, their significance as a geometrical test of tabular accuracy is altogether overshadowed by the interest attaching to the physical observations for which they afford propitious occasions. this change may be said to date, in its pronounced form, from the great eclipse of . although a necessary consequence of the general direction taken by scientific progress, it remains associated in a special manner with the name of francis baily. the "philosopher of newbury" was by profession a london stockbroker, and a highly successful one. nevertheless, his services to science were numerous and invaluable, though not of the brilliant kind which attract popular notice. born at newbury in berkshire, april , , and placed in the city at the age of fourteen, he derived from the acquaintance of dr. priestley a love of science which never afterwards left him. it was, however, no passion such as flames up in the brain of the destined discoverer, but a regulated inclination, kept well within the bounds of an actively pursued commercial career. after travelling for a year or two in what were then the wilds of north america, he went on the stock exchange in , and earned during twenty-four years of assiduous application to affairs a high reputation for integrity and ability, to which corresponded an ample fortune. in the meantime the astronomical society (largely through his co-operation) had been founded; he had for three years acted as its secretary, and he now felt entitled to devote himself exclusively to a subject which had long occupied his leisure hours. he accordingly in retired from business, purchased a house in tavistock place, and fitted up there a small observatory. he was, however, by preference a computator rather than an observer. what sir john herschel calls the "archæology of practical astronomy" found in him an especially zealous student. he re-edited the star-catalogues of ptolemy, ulugh beigh, tycho brahe, hevelius, halley, flamsteed, lacaille, and mayer; calculated the eclipse of thales and the eclipse of agathocles, and vindicated the memory of the first astronomer royal. but he was no less active in meeting present needs than in revising past performances. the subject of the reduction of observations, then, as we have already explained,[ ] in a state of deplorable confusion, attracted his most earnest attention, and he was close on the track of bessel when made acquainted with the method of simplification devised at königsberg. anticipated as an inventor, he could still be of eminent use as a promoter of these valuable improvements; and, carrying them out on a large scale in the star-catalogue of the astronomical society (published in ), "he put" (in the words of herschel) "the astronomical world in possession of a power which may be said, without exaggeration, to have changed the face of sidereal astronomy."[ ] his reputation was still further enhanced by his renewal, with vastly improved apparatus, of the method, first used by henry cavendish in - , for determining the density of the earth. from a series of no less than , delicate and difficult experiments, conducted at tavistock place during the years - , he concluded our planet to weigh · as much as a globe of water of the same bulk; and this result slightly corrected is still accepted as a very close approximation of the truth. what we have thus glanced at is but a fragment of the truly surprising mass of work accomplished by baily in the course of a variously occupied life. a rare combination of qualities fitted him for his task. unvarying health, undisturbed equanimity, methodical habits, the power of directed and sustained thought, combined to form in him an intellectual toiler of the surest, though not perhaps of the highest quality. he was in harness almost to the end. he was destined scarcely to know the miseries of enforced idleness or of consciously failing powers. in he completed the laborious reduction of lalande's great catalogue, undertaken at the request of the british association, and was still engaged in seeing it through the press when he was attacked with what proved his last, as it was probably his first serious illness. he, however, recovered sufficiently to attend the oxford commemoration of july , , where an honorary degree of d.c.l. was conferred upon him in company with airy and struve; but sank rapidly after the effort, and died on the th of august following, at the age of seventy, lamented and esteemed by all who knew him. it is now time to consider his share in the promotion of solar research. eclipses of the sun, both ancient and modern, were a speciality with him, and he was fortunate in those which came under his observation. such phenomena are of three kinds--partial, annular, and total. in a partial eclipse, the moon, instead of passing directly between us and the sun, slips by, as it were, a little on one side, thus cutting off from our sight only a portion of his surface. an annular eclipse, on the other hand, takes place when the moon is indeed centrally interposed, but falls short of the apparent size required for the entire concealment of the solar disc, which consequently remains visible as a bright ring or annulus, even when the obscuration is at its height. in a total eclipse, on the contrary, the sun completely disappears behind the dark body of the moon. the difference of the two latter varieties is due to the fact that the apparent diameter of the sun and moon are so nearly equal as to gain alternate preponderance one over the other through the slight periodical changes in their respective distances from the earth. now, on the th of may, , an annular eclipse was visible in the northern parts of great britain, and was observed by baily at inch bonney, near jedburgh. it was here that he saw the phenomenon which obtained the name of "baily's beads," from the notoriety conferred upon it by his vivid description. "when the cusps of the sun," he writes, "were about ° asunder, a row of lucid points, like a string of bright beads, irregular in size and distance from each other, _suddenly_ formed round that part of the circumference of the moon that was about to enter on the sun's disc. its formation, indeed, was so rapid that it presented the appearance of having been caused by the ignition of a fine train of gunpowder. finally, as the moon pursued her course, the dark intervening spaces (which, at their origin, had the appearance of lunar mountains in high relief, and which still continued attached to the sun's border) were stretched out into long, black, thick, parallel lines, joining the limbs of the sun and moon; when all at once they _suddenly_ gave way, and left the circumference of the sun and moon in those points, as in the rest, comparatively smooth and circular, and the moon perceptibly advanced on the face of the sun."[ ] these curious appearances were not an absolute novelty. weber in , and von zach in , had seen the "beads"; van swinden had described the "belts" or "threads."[ ] these last were, moreover (as baily clearly perceived), completely analogous to the "black ligament" which formed so troublesome a feature in the transits of venus in and , and which, to the regret and confusion, though no longer to the surprise of observers, was renewed in that of . the phenomenon is largely an effect of what is called _irradiation_, by which a bright object seems to encroach upon a dark one; but under good atmospheric and instrumental conditions it becomes inconspicuous. the "beads" must always appear when the projected lunar edge is serrated with mountains. in baily's observation, they were exaggerated and distorted by an irradiative _clinging together_ of the limbs of sun and moon. the immediate result, however, was powerfully to stimulate attention to solar eclipses in their _physical_ aspect. never before had an occurrence of the kind been expected so eagerly or prepared for so actively as that which was total over central and southern europe on the th of july, . astronomers hastened from all quarters to the favoured region. the astronomer royal (airy) repaired to turin; baily to pavia; otto struve threw aside his work amidst the stars at pulkowa, and went south as far as lipeszk; schumacher travelled from altona to vienna; arago from paris to perpignan. nor did their trouble go unrewarded. the expectations of the most sanguine were outdone by the wonders disclosed. baily (to whose narrative we again have recourse) had set up his dollond's achromatic in an upper room of the university of pavia, and was eagerly engaged in noting a partial repetition of the singular appearances seen by him in , when he was "astounded by a tremendous burst of applause from the streets below, and at the same moment was electrified at the sight of one of the most brilliant and splendid phenomena that can well be imagined. for at that instant the dark body of the moon was suddenly surrounded with a corona, or kind of bright glory similar in shape and relative magnitude to that which painters draw round the heads of saints, and which by the french is designated an _auréole_. pavia contains many thousand inhabitants, the major part of whom were, at this early hour, walking about the streets and squares or looking out of windows, in order to witness this long-talked-of phenomenon; and when the total obscuration took place, which was _instantaneous_, there was a universal shout from every observer, which 'made the welkin ring,' and, for the moment, withdrew my attention from the object with which i was immediately occupied. i had indeed anticipated the appearance of a luminous circle round the moon during the time of total obscurity; but i did not expect, from any of the accounts of preceding eclipses that i had read, to witness so magnificent an exhibition as that which took place.... the breadth of the corona, measured from the circumference of the moon, appeared to me to be nearly equal to half the moon's diameter. it had the appearance of brilliant rays. the light was most dense close to the border of the moon, and became gradually and uniformly more attenuate as its distance therefrom increased, assuming the form of diverging rays in a rectilinear line, which at the extremity were more divided, and of an unequal length; so that in no part of the corona could i discover the regular and well-defined shape of a ring at its _outer_ margin. it appeared to me to have the sun for its centre, but i had no means of taking any accurate measures for determining this point. its colour was quite white, not pearl-colour, nor yellow, nor red, and the rays had a vivid and flickering appearance, somewhat like that which a gaslight illumination might be supposed to assume if formed into a similar shape.... splendid and astonishing, however, as this remarkable phenomenon really was, and although it could not fail to call forth the admiration and applause of every beholder, yet i must confess that there was at the same time something in its singular and wonderful appearance that was appalling; and i can readily imagine that uncivilised nations may occasionally have become alarmed and terrified at such an object, more especially at times when the true cause of the occurrence may have been but faintly understood, and the phenomenon itself wholly unexpected. "but the most remarkable circumstance attending the phenomenon was the appearance of _three large protuberances_ apparently emanating from the circumference of the moon, but evidently forming a portion of the corona. they had the appearance of mountains of a prodigious elevation; their colour was red, tinged with lilac or purple; perhaps the colour of the peach-blossom would more nearly represent it. they somewhat resembled the snowy tops of the alpine mountains when coloured by the rising or setting sun. they resembled the alpine mountains also in another respect, inasmuch as their light was perfectly steady, and had none of that flickering or sparkling motion so visible in other parts of the corona. all the three projections were of the same roseate cast of colour, and very different from the brilliant vivid white light that formed the corona; but they differed from each other in magnitude.... the whole of these three protuberances were visible even to the last moment of total obscuration; at least, i never lost sight of them when looking in that direction; and when the first ray of light was admitted from the sun, they vanished, with the corona, altogether, and daylight was instantaneously restored."[ ] notwithstanding unfavourable weather, the "red flames" were perceived with little less clearness and no less amazement from the superga than at pavia, and were even discerned by mr. airy with the naked eye. "their form" (the astronomer royal wrote) "was nearly that of saw-teeth in the position proper for a circular saw turned round in the same direction in which the hands of a watch turn.... their colour was a full lake-red, and their brilliancy greater than that of any other part of the ring."[ ] the height of these extraordinary objects was estimated by arago at two minutes of arc, representing, at the sun's distance, an actual elevation of , miles. when carefully watched, the rose-flush of their illumination was perceived to fade through violet to white as the light returned, the same changes in a reversed order having accompanied their first appearance. their forms, however, during about three minutes of visibility, showed no change, although of so apparently unstable a character as to suggest to arago "mountains on the point of crumbling into ruins" through topheaviness.[ ] the corona, both as to figure and extent, presented very different appearances at different stations. this was no doubt due to varieties in atmospheric conditions. at the superga, for instance, all details of structure seem to have been effaced by the murky air, only a comparatively feeble ring of light being seen to encircle the moon. elsewhere, a brilliant radiated formation was conspicuous, spreading at four opposite points into four vast luminous expansions, compared to feather-plumes or _aigrettes_.[ ] arago at perpignan noticed considerable irregularities in the divergent rays. some appeared curved and twisted, a few lay _across_ the others, in a direction almost tangential to the moon's limb, the general effect being described as that of a "hank of thread in disorder."[ ] at lipeszk, where the sun stood much higher above the horizon than in italy or france, the corona showed with surprising splendour. its apparent extent was judged by struve to be no less than twenty-five minutes (more than six times airy's estimate), while the great plumes spread their radiance to three or four degrees from the dark lunar edge. so dazzling was the light that many well-instructed persons denied the totality of the eclipse. nor was the error without precedent, although the appearances attending respectively a total and an annular eclipse are in reality wholly dissimilar. in the latter case, the surviving ring of sunlight becomes so much enlarged by irradiation, that the interposed dark lunar body is reduced to comparative insignificance, or even invisibility. maclaurin tells us[ ] that during an eclipse of this character which he observed at edinburgh in , "gentlemen by no means shortsighted declared themselves unable to discern the moon upon the sun without the aid of a smoked glass;" and baily (who, however, _was_ shortsighted) could distinguish, in , with the naked eye, no trace of "the globe of purple velvet" which the telescope revealed as projected upon the face of the sun.[ ] moreover, the diminution of light is described by him as "little more than might be caused by a temporary cloud passing over the sun"; the birds continued in full song, and "one cock in particular was crowing with all his might while the annulus was forming." very different were the effects of the eclipse of , as to which some interesting particulars were collected by arago.[ ] beasts of burthen, he tells us, paused in their labour, and could by no amount of punishment be induced to move until the sun reappeared. birds and beasts abandoned their food; linnets were found dead in their cages; even ants suspended their toil. diligence-horses, on the other hand, seemed as insensible to the phenomenon as locomotives. the convolvulus and some other plants closed their leaves, but those of the mimosa remained open. the little light that remained was of a livid hue. one observer described the general coloration as resembling the lees of wine, but human faces showed pale olive or greenish. we may, then, rest assured that none of the remarkable obscurations recorded in history were due to eclipses of the annular kind. the existence of the corona is no modern discovery. indeed, it is too conspicuous an apparition to escape notice from the least attentive or least practised observer of a total eclipse. nevertheless, explicit references to it are rare in early times. plutarch, however, speaks of a "certain splendour" compassing round the hidden edge of the sun, as a regular feature of total eclipses;[ ] and the corona is expressly mentioned in a description of an eclipse visible at corfu in a.d.[ ] the first to take the phenomenon into scientific consideration was kepler. he showed, from the orbital positions at the time of the sun and moon, that an eclipse observed by clavius at rome in could not have been annular,[ ] as the dazzling coronal radiance visible during the obscuration had caused it to be believed. although he himself never witnessed a total eclipse of the sun, he carefully collected and compared the remarks of those more fortunate, and concluded that the ring of "flame-like splendour" seen on such occasions was caused by the reflection of the solar rays from matter condensed in the neighbourhood either of the sun or moon.[ ] to the solar explanation he gave his own decided preference; but, with one of those curious flashes of half-prophetic insight characteristic of his genius, declared that "it should be laid by ready for use, not brought into immediate requisition."[ ] so literally was his advice acted upon, that the theory, which we now know to be (broadly speaking) the correct one, only emerged from the repository of anticipated truths after years of almost complete retirement, and even then timorously and with hesitation. the first eclipse of which the attendant phenomena were observed with tolerable exactness was that which was central in the south of france, may , . cassini then put forward the view that the "crown of pale light" seen round the lunar disc was caused by the illumination of the zodiacal light;[ ] but it failed to receive the attention which, as a step in the right direction, it undoubtedly merited. nine years later we meet with halley's comments on a similar event, the first which had occurred in london since march , . by nine in the morning of may , , the obscuration, he tells us, "was about ten digits,[ ] when the face and colour of the sky began to change from perfect serene azure blue to a more dusky livid colour, having an eye of purple intermixt.... a few seconds before the sun was all hid, there discovered itself round the moon a luminous ring, about a digit or perhaps a tenth part of the moon's diameter in breadth. it was of a pale whiteness, or rather pearl colour, seeming to be a little tinged with the colours of the iris, and to be concentric with the moon, whence i concluded it the moon's atmosphere. but the great height thereof, far exceeding our earth's atmosphere, and the observation of some, who found the breadth of the ring to increase on the west side of the moon as emersion approached, together with the contrary sentiments of those whose judgment i shall always revere" (newton is most probably referred to), "makes me less confident, especially in a matter whereto i confess i gave not all the attention requisite." he concludes by declining to decide whether the "enlightened atmosphere," which the appearance "in all respects resembled," "belonged to sun or moon."[ ] a french academician, who happened to be in london at the time, was less guarded in expressing an opinion. the chevalier de louville declared emphatically for the lunar atmospheric theory of the corona,[ ] and his authority carried great weight. it was, however, much discredited by an observation made by maraldi in , to the effect that the luminous ring, instead of travelling _with_ the moon, was traversed _by_ it.[ ] this was in reality decisive, though, as usual, belief lagged far behind demonstration. in a novel explanation had been offered by delisle and lahire,[ ] supported by experiments regarded at the time as perfectly satisfactory. the aureola round the eclipsed sun, they argued, is simply a result of the _diffraction_, or apparent bending of the sunbeams that graze the surface of the lunar globe--an effect of the same kind as the coloured fringes of shadows. and this view prevailed amongst men of science until (and even after) brewster showed, with clear and simple decisiveness, that such an effect could by no possibility be appreciable at our distance from the moon.[ ] don josé joaquim de ferrer, however, who observed a total eclipse of the sun at kinderhook, in the state of new york, on june , , ignoring this refined optical _rationale_, considered two alternative explanations of the phenomenon as alone possible. the bright ring round the moon must be due to the illumination either of a lunar or of a solar atmosphere. if the former, he calculated that it should have a height fifty times that of the earth's gaseous envelope. "such an atmosphere," he rightly concluded, "cannot belong to the moon, but must without any doubt belong to the sun."[ ] but he stood alone in this unhesitating assertion. the importance of the problem was first brought fully home to astronomers by the eclipse of . the brilliant and complex appearance which on that occasion challenged the attention of so many observers, demanded and received, no longer the casual attention hitherto bestowed upon it, but the most earnest study of those interested in the progress of science. nevertheless, it was only by degrees, and through a process of "exclusions" (to use a baconian phrase) that the corona was put in its right place as a solar appendage. as every other available explanation proved inadmissible and dropped out of sight, the broad presentation of fact remained, which, though of sufficiently obvious interpretation, was long and persistently misconstrued. nor was it until that absolutely decisive evidence on the subject was forthcoming, as we shall see further on. sir john herschel, writing to his venerable aunt, relates that when the brilliant red flames burst into view behind the dark moon on the morning of the th of july, , the populace of milan, with the usual inconsequence of a crowd, raised the shout, "_es leben die astronomen!_"[ ] in reality, none were less prepared for their apparition than the class to whom the applause due to the magnificent spectacle was thus adjudged. and in some measure through their own fault, for many partial hints and some distinct statements from earlier observers had given unheeded notice that some such phenomenon might be expected to attend a solar eclipse. what we now call the "chromosphere" is an envelope of glowing gases, by which the sun is completely covered, and from which the "prominences" are emanations, eruptive or flame-like. now, continual indications of the presence of this fire-ocean had been detected during eclipses in the eighteenth and nineteenth centuries. captain stannyan, describing in a letter to flamsteed an occurrence of the kind witnessed by him at berne on may (o.s.), , says that the sun's "getting out of the eclipse was preceded by a blood-red streak of light from its left limb."[ ] a precisely similar appearance was noted by both halley and de louville in ; during annular eclipses by lord aberdour in ,[ ] and by short in ,[ ] the tint of the ruby border being, however, subdued to "brown" or "dusky red" by the surviving sunlight; while observations identical in character were made at amsterdam in ,[ ] at edinburgh by henderson in , and at new york in .[ ] "flames" or "prominences," if more conspicuous, are less constant in their presence than the glowing stratum from which they spring. the first to describe them was a swedish professor named vassenius, who observed a total eclipse at gothenburg, may (o.s.), .[ ] his astonishment equalled his admiration when he perceived, just outside the edge of the lunar disc, and suspended, as it seemed, in the coronal atmosphere, three or four reddish spots or clouds, one of which was so large as to be detected with the naked eye. as to their nature, he did not even offer a speculation, further than by tacitly referring them to the moon. the observation was repeated in by a spanish admiral, but with no better success in directing efficacious attention to the phenomenon. don antonio ulloa was on board his ship the _espagne_ in passage from the azores to cape st. vincent on the th of june in that year, when a total eclipse of the sun occurred, of which he has left a valuable description. his notices of the corona are full of interest; but what just now concerns us is the appearance of "a red luminous point" "near the edge of the moon," which gradually increased in size as the moon moved away from it, and was visible during about a minute and a quarter.[ ] he was satisfied that it belonged to the sun because of its fiery colour and growth in magnitude, and supposed that it was occasioned by some crevice or inequality in the moon's limb, through which the solar light penetrated. allusions less precise, both prior and subsequent, which it is now easy to refer to similar objects (such as the "slender columns of smoke" seen by ferrer)[ ] might be detailed; but the evidence already adduced suffices to show that the prominences viewed with such amazement in were no unprecedented or even unusual phenomenon. it was more important, however, to decide what was their nature than whether their appearance might have been anticipated. they were generally, and not very incorrectly, set down as solar clouds. arago believed them to shine by reflected light,[ ] but the abbé peytal rightly considered them to be self-luminous. writing in a montpellier paper of july , , he declared that we had now become assured of the existence of a third or outer solar envelope, composed of a glowing substance of a bright rose tint, forming mountains of prodigious elevation, analogous in character to the clouds piled above our horizons.[ ] this first distinct recognition of a very important feature of our great luminary was probably founded on an observation made by bérard at toulon during the then recent eclipse, "of a very fine red band, irregularly dentelated, or, as it were, crevassed here and there,"[ ] encircling a large arc of the moon's circumference. it can hardly, however, be said to have attracted general notice until july , . on that day a total eclipse took place, which was observed with considerable success in various parts of sweden and norway by a number of english astronomers. mr. hind saw, on the south limb of the moon, "a long range of rose-coloured flames,"[ ] described by dawes as "a low ridge of red prominences, resembling in outline the tops of a very irregular range of hills."[ ] airy termed the portion of this "rugged lines of projections" visible to him the _sierra_, and was struck with its brilliant light and "nearly scarlet" colour.[ ] its true character of a continuous solar envelope was inferred from these data by grant, swan, and littrow, and was by father secchi, after the great eclipse of ,[ ] formally accepted as established. several prominences of remarkable forms, especially one variously compared to a turkish scimitar, a sickle, and a boomerang, were seen in . in connection with them two highly significant circumstances were pointed out. first, that of the approximate coincidence between their positions and those of sun-spots previously observed.[ ] next, that "the moon passed over them, leaving them behind, and revealing successive portions as she advanced."[ ] this latter perfectly well-attested fact was justly considered by the astronomer royal and others as affording absolute certainty of the solar dependence of these singular objects. nevertheless sceptics were still found. m. faye, of the french academy, inclined to a lunar origin for them;[ ] feilitsch of greifswald published in a treatise for the express purpose of proving all the luminous phenomena attendant on solar eclipses--corona, prominences and "sierra"--to be purely optical appearances.[ ] happily, however, the unanswerable arguments of the photographic camera were soon to be made available against such hardy incredulity. thus, the virtual discovery of the solar appendages, both coronal and chromospheric, may be said to have been begun in , and completed in . the current herschelian theory of the solar constitution remained, however, for the time, intact. difficulties, indeed, were thickening around it; but their discussion was perhaps felt to be premature, and they were permitted to accumulate without debate, until fortified by fresh testimony into unexpected and overwhelming preponderance. footnotes: [footnote : kosmos, bd. iii., p. ; lalande, _bibliographie astronomique_, pp. , .] [footnote : r. wolf, _die sonne und ihre flecken_, p. . marius himself, however, seems to have held the aristotelian terrestrial-exhalation theory of cometary origin. see his curious little tract, _astronomische und astrologische beschreibung der cometen_, nürnberg, .] [footnote : _phil. trans._, vol. xxvii., p. . _umbræ_ (now called _penumbræ_) are spaces of half-shadow which usually encircle spots. _faculæ_ ("little torches," so named by scheiner) are bright streaks or patches closely associated with spots.] [footnote : _mém. ac. sc._, (pub. ), p. . d. cassini, however, first put forward about the hypothesis alluded to in the text. see delambre, _hist. de l'astr. mod._, t. ii., p. ; and _kosmos_, bd. iii., p. .] [footnote : _phil. trans._, vol. lxiv., part i., pp. - .] [footnote : _rosa ursina_, lib. iv., p. .] [footnote : r. wolf, _die sonne und ihre flecken_, p. .] [footnote : schellen, _die spectralanalyse_, bd. ii., p. ( rd ed.).] [footnote : _phil. trans._, vol. lxiv., p. .] [footnote : _ibid._, vol. lxxxv., , p. .] [footnote : _phil. trans._, vol. xci., , p. .] [footnote : the supposed opaque or protective stratum beneath the photosphere was named by him "planetary," from the analogy of terrestrial clouds.] [footnote : _ibid._, p. .] [footnote : _novum organum_, lib. ii. aph. .] [footnote : brewster's _life of newton_, vol. ii., p. .] [footnote : _beschäftigungen d. berl. ges. naturforschender freunde_, bd. ii., p. .] [footnote : _gentleman's magazine_, , vol. ii., p. .] [footnote : _results_, etc., p. .] [footnote : _ibid._, p. .] [footnote : see _ante_, p. .] [footnote : _memoir of francis baily, mem. r. a. s._, vol. xv., p. .] [footnote : _mem. r. a. s._, vol. x., pp. - .] [footnote : _ibid._, pp. - .] [footnote : _mem. r. a. s._, vol. xv., pp. - .] [footnote : _ibid._, p. .] [footnote : _annuaire_, , p. .] [footnote : _ibid._, p. .] [footnote : _ibid._, p. .] [footnote : _phil. trans._, vol. xl., p. .] [footnote : _mem. r. a. s._, vol. x., p. .] [footnote : _ann. du bureau des long._, , p. .] [footnote : _de facie in orbe lunæ_, xix., . cf. grant, _astr. nach._, no. . as to the phenomenon mentioned by philostratus in his _life of apollonius_ (viii. ), see w. t. lynn, _observatory_, vol. ix., p. .] [footnote : schmidt, _astr. nach._, no. .] [footnote : _astronomiæ pars optica, op. omnia_, t. ii., p. .] [footnote : _de stellâ novâ, op._, t. ii., pp. , .] [footnote : _astr. pars op._, p. .] [footnote : _mém. de l'ac. des sciences_, , p. .] [footnote : a digit = / of the solar diameter.] [footnote : _phil. trans._, vol. xxix., pp. - .] [footnote : _mém. de l'ac. des sciences_, ; _histoire_, p. ; _mémoires_, pp. - .] [footnote : _ibid._, , p. .] [footnote : _mém. de l'ac. des sciences_, , pp. , - .] [footnote : _ed. ency._, art. _astronomy_, p. .] [footnote : _trans. am. phil. soc._, vol. vi., p. .] [footnote : _memoir of caroline herschel_, p. .] [footnote : _phil. trans._, vol. xxv., p. .] [footnote : _ibid._, vol. xl., p. .] [footnote : _ibid._, vol. xlv., p. .] [footnote : _mem. r. a. s._, vol. i., pp. , .] [footnote : _american journal of science_, vol. xlii., p. .] [footnote : _phil. trans._, vol. xxxviii., p. . father secchi, however, adverted to a distinct mention of a prominence observed in a.d. a description of a total eclipse of that date includes the remark, "et quoddam foramen erat ignitum in circulo solis ex parte inferiore" (muratori, _rer. it. scriptores_, t. xiv., col. ). the "circulus solis" of course signifies the corona.] [footnote : _phil. trans._, vol. lxix., p. .] [footnote : _trans. am. phil. soc._, vol. vi., , p. .] [footnote : _annuaire_, , p. .] [footnote : _ibid._, p. , _note_.] [footnote : _ibid._, p. .] [footnote : _mem. r. a. s._, vol. xxi., p. .] [footnote : _ibid._, p. .] [footnote : _ibid._, pp. , .] [footnote : _le soleil_, t. i., p. .] [footnote : by williams and stanistreet, _mem. r. a. s._, vol. xxi., pp. , . santini had made a similar observation at padua in . grant, _hist. astr._, p. .] [footnote : lassell in _month. not._, vol. xii., p. .] [footnote : _comptes rendus_, t. xxxiv., p. .] [footnote : _optische untersuchungen_, and _zeitschrift für populäre mittheilungen_, bd. i., , p. .] chapter iv _planetary discoveries_ in the course of his early gropings towards a law of the planetary distances, kepler tried the experiment of setting a planet, invisible by reason of its smallness, to revolve in the vast region of seemingly desert space separating mars from jupiter.[ ] the disproportionate magnitude of the same interval was explained by kant as due to the overweening size of jupiter. the zone in which each planet moved was, according to the philosopher of königsberg, to be regarded as the empty storehouse from which its materials had been derived. a definite relation should thus exist between the planetary masses and the planetary intervals.[ ] lambert, on the other hand, sportively suggested that the body or bodies (for it is noticeable that he speaks of them in the plural) which once bridged this portentous gap in the solar system, might, in some remote age, have been swept away by a great comet, and forced to attend its wanderings through space.[ ] these speculations were destined before long to assume a more definite form. johann daniel titius, a professor at wittenberg (where he died in ), pointed out in , in a note to a translation of bonnet's _contemplation de la nature_,[ ] the existence of a remarkable symmetry in the disposition of the bodies constituting the solar system. by a certain series of numbers, increasing in regular progression,[ ] he showed that the distances of the six known planets from the sun might be represented with a close approach to accuracy. but with one striking interruption. the term of the series succeeding that which corresponded to the orbit of mars was without a celestial representative. the orderly flow of the sequence was thus singularly broken. the space where a planet should--in fulfilment of the "law"--have revolved, was, it appeared, untenanted. johann elert bode, then just about to begin his long career as leader of astronomical thought and work at berlin, marked at once the anomaly, and filled the vacant interval with a hypothetical planet. the discovery of uranus, at a distance falling but slightly short of perfect conformity with the law of titius, lent weight to a seemingly hazardous prediction, and von zach was actually at the pains, in , to calculate what he termed "analogical" elements[ ] for this unseen and (by any effect or influence) _unfelt_ body. the search for it, through confessedly scarcely less chimerical than that of alchemists for the philosopher's stone, he kept steadily in view for fifteen years, and at length (september , ) succeeded in organising, in combination with five other german astronomers assembled at lilienthal, a force of what he jocularly termed celestial police, for the express purpose of tracking and intercepting the fugitive subject of the sun. the zodiac was accordingly divided for purposes of scrutiny into twenty-four zones; their apportionment to separate observers was in part effected, and the association was rapidly getting into working order, when news arrived that the missing planet had been found, through no systematic plan of search, but by the diligent, though otherwise directed labours of a distant watcher of the skies. giuseppe piazzi was born at ponte in the valtelline, july , . he studied at various places and times under tiraboschi, beccaria, jacquier, and le sueur; and having entered the theatine order of monks at the age of eighteen, he taught philosophy, science, and theology in several of the italian cities, as well as in malta, until , when the chair of mathematics in the university of palermo was offered to and accepted by him. prince caramanico, then viceroy of sicily, had scientific leanings, and was easily won over to the project of building an observatory, a commodious foundation for which was afforded by one of the towers of the viceregal palace. this architecturally incongruous addition to an ancient saracenic edifice--once the abode of kelbite and zirite emirs--was completed in february, . piazzi, meanwhile, had devoted nearly three years to the assiduous study of his new profession, acquiring a practical knowledge of lalande's methods at the École militaire, and of maskelyne's at the royal observatory; and returned to palermo in , bringing with him, in the great five-foot circle which he had prevailed upon ramsden to construct, the most perfect measuring instrument hitherto employed by an astronomer. he had been above nine years at work on his star-catalogue, and was still profoundly unconscious that a place amongst the lilienthal band[ ] of astronomical detectives was being held in reserve for him, when, on the first evening of the nineteenth century, january , , he noticed the position of an eighth-magnitude star in a part of the constellation taurus to which an error of wollaston's had directed his special attention. reobserving, according to his custom, the same set of fifty stars on four consecutive nights, it seemed to him, on the nd, that the one in question had slightly shifted its position to the west; on the rd he assured himself of the fact, and believed that he had chanced upon a new kind of comet without tail or coma. the wandering body, whatever its nature, exchanged retrograde for direct motion on january ,[ ] and was carefully watched by piazzi until february , when a dangerous illness interrupted his observations. he had, however, not omitted to give notice of his discovery; but so precarious were communications in those unpeaceful times, that his letter to oriani of january did not reach milan until april , while a missive of one day later addressed to bode came to hand at berlin, march . the delay just afforded time for the publication, by a young philosopher of jena named hegel, of a "dissertation" showing, by the clearest light of reason, that the number of the planets could not exceed seven, and exposing the folly of certain devotees of induction who sought a new celestial body merely to fill a gap in a numerical series.[ ] unabashed by speculative scorn, bode had scarcely read piazzi's letter when he concluded that it referred to the precise body in question. the news spread rapidly, and created a profound sensation, not unmixed with alarm lest this latest addition to the solar family should have been found only to be again lost. for by that time piazzi's moving star was too near the sun to be any longer visible, and in order to rediscover it after conjunction a tolerably accurate knowledge of its path was indispensable. but a planetary orbit had never before been calculated from such scanty data as piazzi's observation afforded;[ ] and the attempts made by nearly every astronomer of note in germany to compass the problem were manifestly inadequate, failing even to account for the positions in which the body had been actually seen, and _à fortiori_ serving only to mislead as to the places where, from september, , it ought once more to have become discernible. it was in this extremity that the celebrated mathematician gauss came to the rescue. he was then in his twenty-fifth year, and was earning his bread by tuition at brunswick, with many possibilities, but no settled career before him. the news from palermo may be said to have converted him from an arithmetician into an astronomer. he was already in possession of a new and more general method of computing elliptical orbits; and the system of "least squares," which he had devised though not published, enabled him to extract the most probable result from a given set of observations. armed with these novel powers, he set to work; and the communication in november of his elements and ephemeris for the lost object revived the drooping hopes of the little band of eager searchers. their patience, however, was to be still further tried. clouds, mist, and sleet seemed to have conspired to cover the retreat of the fugitive; but on the last night of the year the sky cleared unexpectedly with the setting in of a hard frost, and there, in the north-western part of virgo, nearly in the position assigned by gauss to the runaway planet, a strange star was discerned by von zach[ ] at gotha, and on a subsequent evening--the anniversary of the original discovery--by olbers at bremen. the name of ceres (as the tutelary goddess of sicily) was, by piazzi's request, bestowed upon this first known of the numerous, and probably all but innumerable family of the minor planets. the recognition of the second followed as the immediate consequence of the detection of the first. olbers had made himself so familiar with the positions of the small stars along the track of the long-missing body, that he was at once struck (march , ) with the presence of an intruder near the spot where he had recently identified ceres. he at first believed the new-comer to be a variable star usually inconspicuous, but just then at its maximum of brightness; but within two hours he had convinced himself that it was no _fixed_ star, but a rapidly moving object. the aid of gauss was again invoked, and his prompt calculations showed that this fresh celestial acquaintance (named "pallas" by olbers), revolved round the sun at nearly the same mean distance as ceres, and was beyond question of a strictly analogous character. this result was perplexing in the extreme. the symmetry and simplicity of the planetary scheme appeared fatally compromised by the admission of many, where room could, according to old-fashioned rules, only be found for one. a daring hypothesis of olbers's invention provided an exit from the difficulty. he supposed that both ceres and pallas were fragments of a primitive trans-martian planet, blown to pieces in the remote past, either by the action of internal forces or by the impact of a comet; and predicted that many more such fragments would be found to circulate in the same region. he, moreover, pointed out that these numerous orbits, however much they might differ in other respects, must all have a common line of intersection,[ ] and that the bodies moving in them must consequently pass, at each revolution, through two opposite points of the heavens, one situated in the whale, the other in the constellation of the virgin, where already pallas had been found and ceres recaptured. the intimation that fresh discoveries might be expected in those particular regions was singularly justified by the detection of two bodies now known respectively as juno and vesta. the first was found near the predicted spot in cetus by harding, schröter's assistant at lilienthal, september , ; the second by olbers himself in virgo, after three years of persistent scrutiny, march , . the theory of an exploded planet now seemed to have everything in its favour. it required that the mean or average distances of the newly-discovered bodies should be nearly the same, but admitted a wide range of variety in the shapes and positions of their orbits, provided always that they preserved common points of intersection. these conditions were fulfilled with a striking approach to exactness. three of the four "asteroids" (a designation introduced by sir. w. herschel[ ]) conformed with very approximate precision to "bode's law" of distances; they all traversed, in their circuits round the sun, nearly the same parts of cetus and virgo; while the eccentricities and inclinations of their paths departed widely from the planetary type--that of pallas, to take an extreme instance, making with the ecliptic an angle of nearly °. the minuteness of these bodies appeared further to strengthen the imputation of a fragmentary character. herschel estimated the diameter of ceres at , that of pallas at miles.[ ] but these values are now known to be considerably too small. a suspected variability of brightness in some of the asteroids, somewhat hazardously explained as due to the irregularities of figure to be expected in cosmical _potsherds_ (so to speak), was added to the confirmatory evidence.[ ] the strong point of the theory, however, lay not in what it explained, but in what it had predicted. it had been twice confirmed by actual exploration of the skies, and had produced, in the recognition of vesta, the first recorded instance of the _premeditated_ discovery of a heavenly body. the view not only commended itself to the facile imagination of the unlearned, but received the sanction of the highest scientific authority. the great lagrange bestowed upon it his analytical _imprimatur_, showing that the explosive forces required to produce the supposed catastrophe came well within the bounds of possibility; since a velocity of less than twenty times that of a cannon-ball leaving the gun's mouth would have sufficed, according to his calculation, to launch the asteroidal fragments on their respective paths. indeed, he was disposed to regard the hypothesis of disruption as more generally available than its author had designed it to be, and proposed to supplement with it, as explanatory of the eccentric orbits of comets, the nebular theory of laplace, thereby obtaining, as he said, "a complete view of the origin of the planetary system more conformable to nature and mechanical laws than any yet proposed."[ ] nevertheless the hypothesis of olbers has not held its ground. it seemed as if all the evidence available for its support had been produced at once and spontaneously, while the unfavourable items were elicited slowly, and, as it were, by cross-examination. a more extended acquaintance with the group of bodies whose peculiarities it was framed to explain has shown them, after all, as recalcitrant to any such explanation. coincidences at the first view significant and striking have been swamped by contrary examples; and a hasty general conclusion has, by a not uncommon destiny, at last perished under the accumulation of particulars. moreover, as has been remarked by professor newcomb,[ ] mutual perturbations would rapidly efface all traces of a common disruptive origin, and the catastrophe, to be perceptible in its effects, should have been comparatively recent. a new generation of astronomers had arisen before any additions were made to the little family of the minor planets. piazzi died in , harding in , olbers in ; all those who had prepared or participated in the first discoveries passed away without witnessing their resumption. in , however, a certain hencke, ex-postmaster in the prussian town of driessen, set himself to watch for new planets, and after fifteen long years his patience was rewarded. the asteroid found by him, december , , received the name of astræa, and his further prosecution of the search resulted, july , , in the discovery of hebe. a few weeks later (august ), john russell hind ( - ), after many months' exploration from mr. bishop's observatory in the regent's park, picked up iris, and october , flora.[ ] the next on the list was metis, found by mr. graham, april , , at markree, in ireland.[ ] at the close of the period to which our attention is at present limited, the number of these small bodies known to astronomy was thirteen; and the course of discovery has since proceeded far more rapidly and with less interruption. both in itself and in its consequences the recognition of the minor planets was of the highest importance to science. the traditional ideas regarding the constitution of the solar system were enlarged by the admission of a new class of bodies, strongly contrasted, yet strictly co-ordinate with the old-established planetary order; the profusion of resource, so conspicuous in the living kingdoms of nature, was seen to prevail no less in the celestial spaces; and some faint preliminary notion was afforded of the indefinite complexity of relations underlying the apparent simplicity of the majestic scheme to which our world belongs. both theoretical and practical astronomy derived profit from the admission of these apparently insignificant strangers to the rights of citizenship of the solar system. the disturbance of their motions by their giant neighbours afforded a more accurate knowledge of the jovian mass, which laplace had taken about / too small; the anomalous character of their orbits presented geometers with highly stimulating problems in the theory of perturbation; while the exigencies of the first discovery had produced the _theoria motus_, and won gauss over to the ranks of calculating astronomy. moreover, the sure prospect of further detections powerfully incited to the exploration of the skies; observers became more numerous and more zealous in view of the prizes held out to them; star-maps were diligently constructed, and the sidereal multitude strewn along the great zodiacal belt acquired a fresh interest when it was perceived that its least conspicuous member might be a planetary shred or projectile in the dignified disguise of a distant sun. harding's "celestial atlas," designed for the special purpose of facilitating asteroidal research, was the first systematic attempt to represent to the eye the _telescopic_ aspect of the heavens. it was while engaged on its construction that the lilienthal observer successfully intercepted juno on her passage through the whale in ; whereupon promoted to göttingen, he there completed, in , the arduous task so opportunely entered upon a score of years previously. still more important were the great star-maps of the berlin academy, undertaken at bessel's suggestion, with the same object of distinguishing errant from fixed stars, and executed, under encke's supervision, during the years - . they have played a noteworthy part in the history of planetary discovery, nor of the minor kind alone. we have now to recount an event unique in scientific history. the discovery of neptune has been characterised as the result of a "movement of the age,"[ ] and with some justice. it had become necessary to the integrity of planetary theory. until it was accomplished, the phantom of an unexplained anomaly in the orderly movements of the solar system must have continued to haunt astronomical consciousness. moreover, it was prepared by many, suggested as possible by not a few, and actually achieved, simultaneously, independently, and completely, by two investigators. the position of the planet uranus was recorded as that of a fixed star no less than twenty times between and the epoch of its final detection by herschel. but these early observations, far from affording the expected facilities for the calculation of its orbit, proved a source of grievous perplexity. the utmost ingenuity of geometers failed to combine them satisfactorily with the later uranian places, and it became evident, either that they were widely erroneous, or that the revolving body was wandering from its ancient track. the simplest course was to reject them altogether, and this was done in the new tables published in by alexis bouvard, the indefatigable computating partner of laplace. but the trouble was not thus to be got rid of. after a few years fresh irregularities began to appear, and continued to increase until absolutely "intolerable." it may be stated as illustrative of the perfection to which astronomy had been brought, that divergencies regarded as menacing the very foundation of its theories never entered the range of unaided vision. in other words, if the theoretical and the real uranus had been placed side by side in the sky, they would have seemed, to the sharpest eye, to form a single body.[ ] the idea that these enigmatical disturbances were due to the attraction of an unknown exterior body was a tolerably obvious one; and we accordingly find it suggested in many different quarters. bouvard himself was perhaps the first to conceive it. he kept the possibility continually in view, and bequeathed to his nephew's diligence the inquiry into its reality when he felt that his own span was drawing to a close; but before any progress had been made with it, he had already (june , ) "ceased to breathe and to calculate." the rev. t. j. hussey actually entertained in the notion, but found his powers inadequate to the task, of assigning an approximate place to the disturbing body; and bessel, in , laid his plans for an assault in form upon the uranian difficulty, the triumphant exit from which fatal illness frustrated his hopes of effecting or even witnessing. the problem was practically untouched when, in , an undergraduate of st. john's college, cambridge, formed the resolution of grappling with it. the projected task was an arduous one. there were no guiding precedents for its conduct. analytical obstacles had to be encountered so formidable as to appear invincible even to such a mathematician as airy. john couch adams, however, had no sooner taken his degree, which he did as senior wrangler in january, , than he set resolutely to work, and on october , , was able to communicate to the astronomer royal numerical estimates of the elements and mass of the unknown planet, together with an indication of its actual place in the heavens. these results, it has been well said,[ ] gave "the final and inexorable proof" of the validity of newton's law. the date october , , "may therefore be regarded as marking a distinct epoch in the history of gravitational astronomy." sir george biddell airy had begun in his long and energetic administration of the royal observatory, and was already in possession of data vitally important to the momentous inquiry then on foot. at his suggestion, and under his superintendence, the reduction of all the planetary observations made at greenwich from onwards had been undertaken in . the results, published in , constituted a permanent and universal stock of materials for the correction of planetary theory. but in the meantime, investigators, both native and foreign, were freely supplied with the "places and errors," which, clearly exhibiting the discrepancies between observation and calculation--between what _was_ and what was _expected_--formed the very groundwork of future improvements. mr. adams had no reason to complain of official discourtesy. his labours received due and indispensable aid; but their purpose was regarded as chimerical. "i have always," sir george airy wrote,[ ] "considered the correctness of a distant mathematical result to be a subject rather of moral than of mathematical evidence." and that actually before him seemed, from its very novelty, to incur a suspicion of unlikelihood. no problem in planetary disturbance had heretofore been attacked, so to speak, from the rear. the inverse method was untried, and might well be deemed impracticable. for the difficulty of determining the perturbations produced by a given planet is small compared with the difficulty of finding a planet by its resulting perturbations. laplace might have quailed before it; yet it was now grappled with as a first essay in celestial dynamics. moreover, adams unaccountably neglected to answer until too late a question regarded by airy in the light of an _experimentum crucis_ as to the soundness of the new theory. nor did he himself take any steps to obtain a publicity which he was more anxious to merit than to secure. the investigation consequently remained buried in obscurity. it is now known that had a search been instituted in the autumn of for the remote body whose existence had been so marvellously foretold, it would have been found within _three and a half lunar diameters_ ( ° ') of the spot assigned to it by adams. a competitor, however, equally daring and more fortunate--_audax fortunâ adjutus_, as gauss said of him--was even then entering the field. urbain jean joseph leverrier, the son of a small government _employé_ in normandy, was born at saint-lô, march , . he studied with brilliant success at the École polytechnique, accepted the post of astronomical teacher there in , and, "docile to circumstance," immediately concentrated the whole of his vast, though as yet undeveloped powers upon the formidable problems, of celestial mechanics. he lost no time in proving to the mathematical world that the race of giants was not extinct. two papers on the stability of the solar system, presented to the academy of sciences, september and october , , showed him to be the worthy successor of lagrange and laplace, and encouraged hopes destined to be abundantly realised. his attention was directed by arago to the uranian difficulty in , when he cheerfully put aside certain intricate cometary researches upon which he happened to be engaged, in order to obey with dutiful promptitude the summons of the astronomical chief of france. in his first memoir on the subject (communicated to the academy, november , ), he proved the inadequacy of all known causes of disturbance to account for the vagaries of uranus; in a second (june , ), he demonstrated that only an exterior body, occupying at a certain date a determinate position in the zodiac, could produce the observed effects; in a third (august , ), he assigned the orbit of the disturbing body, and announced its visibility as an object with a sensible disc about as bright as a star of the eighth magnitude. the question was now visibly approaching an issue. on september , sir john herschel declared to the british association respecting the hypothetical new planet: "we see it as columbus saw america from the coast of spain. its movements have been felt, trembling along the far-reaching line of our analysis with a certainty hardly inferior to that of ocular demonstration." less than a fortnight later, september , professor galle, of the berlin observatory, received a letter from leverrier requesting his aid in the telescopic part of the inquiry already analytically completed. he directed his refractor to the heavens that same night, and perceived, within less than a degree of the spot indicated, an object with a measurable disc nearly three seconds in diameter. its absence from bremiker's recently-completed map of that region of the sky showed it to be no star, and its movement in the predicted direction confirmed without delay the strong persuasion of its planetary nature.[ ] in this remarkable manner the existence of the remote member of our system known as "neptune" was ascertained. but the discovery, which faithfully reflected the duplicate character of the investigation which led to it, had been already secured at cambridge before it was announced from berlin. sir george airy's incredulity vanished in the face of the striking coincidence between the position assigned by leverrier to the unknown planet in june, and that laid down by adams in the previous october; and on the th of july he wrote to professor challis, director of the cambridge observatory, recommending a search with the great northumberland equatoreal. had a good star-map been at hand, the process would have been a simple one; but of bremiker's "hora xxi." no news had yet reached england, and there was no other sufficiently comprehensive to be available for an inquiry which, in the absence of such aid, promised to be both long and laborious. as the event proved, it might have been neither. "after four days of observing," challis wrote, october , , to airy, "the planet was in my grasp if only i had examined or mapped the observations."[ ] had he done so, the first honours in the discovery, both theoretical and optical, would have fallen to the university of cambridge. but professor challis had other astronomical avocations to attend to, and, moreover, his faith in the precision of the indications furnished to him was, by his own confession, a very feeble one. for both reasons he postponed to a later stage of the proceedings the discussion and comparison of the data nightly furnished to him by his telescope, and thus allowed to lie, as it were, latent in his observations the momentous result which his diligence had insured, but which his delay suffered to be anticipated.[ ] nevertheless, it should not be forgotten that the berlin astronomer had two circumstances in his favour apart from which his swift success could hardly have been achieved. the first was the possession of a good star-map; the second was the clear and confident nature of leverrier's instructions. "look where i tell you," he seemed authoritatively to say, "and you will see an object such as i describe."[ ] and in fact, not only galle on the rd of september, but also challis on the th, immediately after reading the french geometer's lucid and impressive treatise, picked out from among the stellar points strewing the zodiac, a small planetary disc, which eventually proved to be that of the precise body he had been in search of during two months. the controversy that ensued had its ignominious side; but it was entered into by neither of the parties principally concerned. adams bore the disappointment, which the dilatory proceedings at greenwich and cambridge had inflicted upon him, with quiet heroism. his silence on the subject of what another man would have called his wrongs remained unbroken to the end of his life;[ ] and he took every opportunity of testifying his admiration for the genius of leverrier. personal questions, however, vanish in the magnitude of the event they relate to. by it the last lingering doubts as to the absolute exactness of the newtonian law were dissipated. recondite analytical methods received a confirmation brilliant and intelligible even to the minds of the vulgar, and emerged from the patient solitude of the study to enjoy an hour of clamorous triumph. for ever invisible to the unaided eye of man, a sister-globe to our earth was shown to circulate, in perpetual frozen exile, at thirty times its distance from the sun. nay, the possibility was made apparent that the limits of our system were not even thus reached, but that yet profounder abysses of space might shelter obedient, though little favoured, members of the solar family, by future astronomers to be recognised through the sympathetic thrillings of neptune, even as neptune himself was recognised through the tell-tale deviations of uranus. it is curious to find that the fruit of adams's and leverrier's laborious investigations had been accidentally all but snatched half a century before it was ripe to be gathered. on the th, and again on the th of may, , lalande noted the position of neptune as that of a fixed star, but perceiving that the two observations did not agree, he suppressed the first as erroneous, and pursued the inquiry no further. an immortality which he would have been the last to despise hung in the balance; the feather-weight of his carelessness, however, kicked the beam, and the discovery was reserved to be more hardly won by later comers. bode's law did good service in the quest for a trans-uranian planet by affording ground for a probable assumption as to its distance. a starting-point for approximation was provided by it; but it was soon found to be considerably at fault. even uranus is about millions of miles nearer to the sun than the order of progression requires; and neptune's vast distance of , million should be increased by no less than million miles, and its period of lengthened out to years,[ ] in order to bring it into conformity with the curious and unexplained rule which planetary discoveries have alternately tended to confirm and to invalidate. within seventeen days of its identification with the berlin achromatic, neptune was found to be attended by a satellite. this discovery was the first notable performance of the celebrated two-foot reflector[ ] erected by mr. lassell at his suggestively named residence of starfield, near liverpool. william lassell was a brewer by profession, but by inclination an astronomer. born at bolton in lancashire, june , , he closed a life of eminent usefulness to science, october , , thus spanning with his well-spent years four-fifths of the momentous period which we have undertaken to traverse. at the age of twenty-one, being without the means to purchase, he undertook to construct telescopes, and naturally turned his attention to the reflecting sort, as favouring amateur efforts by the comparative simplicity of its structure. his native ingenuity was remarkable, and was developed by the hourly exigencies of his successive enterprises. their uniform success encouraged him to enlarge his aims, and in he visited birr castle for the purpose of inspecting the machine used in polishing the giant speculum of parsonstown. in the construction of his new instrument, however, he eventually discarded the model there obtained, and worked on a method of his own, assisted by the supreme mechanical skill of james nasmyth. the result was a newtonian of exquisite definition, with an aperture of two, and a focal length of twenty feet, provided by a novel artifice with the equatoreal mounting, previously regarded as available only for refractors. this beautiful instrument afforded to its maker, october , , a cursory view of a neptunian attendant. but the planet was then approaching the sun, and it was not until the following july that the observation could be verified, which it was completely, first by lassell himself, and somewhat later by otto stuve and bond of cambridge (u.s.). when it is considered that this remote object shines by reflecting sunlight reduced by distance to / th of the intensity with which it illuminates our moon, the fact of its visibility, even in the most perfect telescopes, is a somewhat surprising one. it can only, indeed, be accounted for by attributing to it dimensions very considerable for a body of the secondary order. it shares with the moons of uranus the peculiarity of retrograde motion; that is to say, its revolutions, running counter to the grand current of movement in the solar system, are performed from east to west, in a plane inclined at an angle of ° to that of the ecliptic. their swiftness serves to measure the mass of the globe round which they are performed. for while our moon takes twenty-seven days and nearly eight hours to complete its circuit of the earth, the satellite of neptune, at a distance not greatly inferior, sweeps round its primary in five days and twenty-one hours, showing (according to a very simple principle of computation) that it is urged by a force seventeen times greater than the terrestrial pull upon the lunar orb. combining this result with those of professor barnard's[ ] and dr. see's[ ] recent measurements of the small telescopic disc of this farthest known planet, it is found that while in _mass_ neptune equals seventeen, in _bulk_ it is equivalent to forty-nine earths. this is as much as to say that it is composed of relatively very light materials, or more probably of materials distended by internal heat, as yet unwasted by radiation into space, to about five times the volume they would occupy in the interior of our globe. the fact, at any rate, is fairly well ascertained, that the average density of neptune is about twice that of water. we must now turn from this late-recognised member of our system to bestow some brief attention upon the still fruitful field of discovery offered by one of the immemorial five. the family of saturn, unlike that of its brilliant neighbour, has been gradually introduced to the notice of astronomers. titan, the sixth saturnian moon in order of distance, led the way, being detected by huygens, march , ; cassini made the acquaintance of four more between and ; while mimas and enceladus, the two innermost, were caught by herschel in , as they threaded their lucid way along the edge of the almost vanished ring. in the distances of these seven revolving bodies from their primary, an order of progression analogous to that pointed out by titius in the planetary intervals was found to prevail; but with one conspicuous interruption, similar to that which had first suggested the search for new members of the solar system. between titan and japetus--the sixth and seventh reckoning outwards--there was obviously room for another satellite. it was discovered on both sides of the atlantic simultaneously, on the th of september, . mr. w. c. bond, employing the splendid -inch refractor of the harvard observatory, noticed, september , a minute star situated in the plane of saturn's rings. the same object was discerned by mr. lassell on the th. on the following evening, both observers perceived that the problematical speck of light kept up with, instead of being left behind by the planet as it moved, and hence inferred its true character.[ ] hyperion, the seventh by distance and eighth by recognition of saturn's attendant train, is of so insignificant a size when compared with some of its fellow-moons (titan is but little inferior to the planet mars), as to have suggested to sir john herschel[ ] the idea that it might be only one of several bodies revolving very close together--in fact, an _asteroidal satellite_; but the conjecture has, so far, not been verified. the coincidence of its duplicate discovery was singularly paralleled two years later. galileo's amazement when his "optic glass" revealed to him the "triple" form of saturn--_planeta tergeminus_--has proved to be, like the laughter of the gods, "inextinguishable." it must revive in every one who contemplates anew the unique arrangements of that world apart known to us as the saturnian system. the resolution of the so-called _ansæ_, or "handles," into one encircling ring by huygens in , the discovery by cassini in of the division of that ring into two concentric ones, together with laplace's investigation of the conditions of stability of such a formation, constituted, with some minor observations, the sum of the knowledge obtained, up to the middle of the last century, on the subject of this remarkable formation. the first place in the discovery now about to be related belongs to an american astronomer. william cranch bond, born in at portland, in the state of maine, was a watchmaker, whom the solar eclipse of attracted to study the wonders of the heavens. when, in , the erection of an observatory in connection with harvard college, cambridge, was first contemplated, he undertook a mission to england for the purpose of studying the working of similar institutions there, and on his return erected a private observatory at dorchester, where he worked diligently for many years. then at last, in , the long-postponed design of the harvard authorities was resumed, and on the completion of the new establishment, bond, who had been from officially connected with the college and had carried on his scientific labours within its precincts, was offered and accepted the post of its director. placed in in possession of one of the finest instruments in the world--a masterpiece of merz and mahler--he headed the now long list of distinguished transatlantic observers. like the elder struve, he left an heir to his office and to his eminence, but george bond unfortunately died in , at the early age of thirty-nine, having survived his father but six years. on the night of november , --the air, remarkably enough, being so hazy that only the brightest stars could be perceived with the naked eye--william bond discerned a dusky ring, extending about halfway between the inner brighter one and the globe of saturn. a fortnight later, but before the observation had been announced in england, the same appearance was seen by the rev. w. r. dawes with the comparatively small refractor of his observatory at wateringbury, and on december was described by mr. lassell (then on a visit to him) as "something like a crape veil covering a part of the sky within the inner ring."[ ] next morning the _times_ containing the report of bond's discovery reached wateringbury. the most surprising circumstance in the matter was that the novel appendage had remained so long unrecognised. as the rings opened out to their full extent, it became obvious with very moderate optical assistance; yet some of the most acute observers who have ever lived, using instruments of vast power, had heretofore failed to detect its presence. it soon appeared, however, that galle of berlin[ ] had noticed, june , , a veil-like extension of the lucid ring across half the dark space separating it from the planet; but the observation, although communicated at the time to the berlin academy of sciences, had remained barren. traces of the dark ring, moreover, were found in drawings executed by campani in [ ] and by hooke in ;[ ] while picard (june , ),[ ] hadley (spring of ),[ ] and herschel,[ ] had all undoubtedly seen it under the aspect of a dark bar or belt crossing the saturnian globe. it was, then, of no recent origin; but there seemed reason to think that it had lately gained considerably in brightness. the full meaning of this suspected change it was reserved for later investigations to develop. what we may, in a certain sense, call the closing result of the race for discovery, in which several observers seemed at that time to be engaged, was the establishment, on a satisfactory footing, of our acquaintance with the dependent system of uranus. sir william herschel, whose researches formed, in so many distinct lines of astronomical inquiry, the starting-points of future knowledge, detected, january , ,[ ] two uranian moons, since called oberon and titania, and ascertained the curious circumstance of their motion in a plane almost at right angles to the ecliptic, in a direction contrary to that of all previously known denizens (other than cometary) of the solar kingdom. he believed that he caught occasional glimpses of four more, but never succeeded in assuring himself of their substantial existence. even the two first remained unseen save by himself until , when his son re-observed them with a -foot reflector, similar to that with which they had been originally discovered. thenceforward they were kept fairly within view, but their four questionable companions, in spite of some false alarms of detection, remained in the dubious condition in which herschel had left them. at last, on october , ,[ ] after some years of fruitless watching, lassell espied "ariel" and "umbriel," two uranian attendants, interior to oberon and titania, and of about half their brightness; so that their disclosure is still reckoned amongst the very highest proofs of instrumental power and perfection. in all probability they were then for the first time seen; for although professor holden[ ] made out a plausible case in favour of the fitful visibility to herschel of each of them in turn, lassell's argument[ ] that the glare of the planet in herschel's great specula must have rendered almost impossible the perception of objects so minute and so close to its disc, appears tolerably decisive to the contrary. uranus is thus attended by four moons, and, so far as present knowledge extends, by no more. among the most important of the "negative results"[ ] secured by lassell's observations at malta during the years - and - , were the convincing evidence afforded by them that, without great increase of optical power, no further neptunian or uranian satellites can be perceived, and the consequent relegation of herschel's baffling quartette, notwithstanding the unquestioned place long assigned to them in astronomical text-books, to the nirvana of non-existence. footnotes: [footnote : _op._, t. i., p. . he interposed, but tentatively only, another similar body between mercury and venus.] [footnote : _allgemeine naturgeschichte_ (ed. ), pp. , .] [footnote : _cosmologische briefe_, no. (quoted by von zach, _monat. corr._, vol. iii., p. ).] [footnote : second ed., p. . see bode, _von dem neuen hauptplaneten_, p. , _note_.] [footnote : the representative numbers are obtained by adding to the following series (irregular, it will be observed, in its first member, which should be / instead of ); , , , , , , etc. the formula is a purely empirical one, and is, moreover, completely at fault as regards the distance of neptune.] [footnote : _monat. corr._, vol. iii., p. .] [footnote : wolf, _geschichte der astronomie_, p. .] [footnote : such reversals of direction in the apparent movements of the planets are a consequence of the earth's revolution in its orbit.] [footnote : _dissertatio philosophica de orbitis planetarum_, . see wolf, _gesch. d. astr._, p. .] [footnote : observations on uranus, as a supposed fixed star, went back to .] [footnote : he had caught a glimpse of it on december , but was prevented by bad weather from verifying his suspicion. _monat. corr._, vol. v., p. .] [footnote : planetary fragments, hurled _in any direction_, and _with any velocity_ short of that which would for ever release them from the solar sway, would continue to describe elliptic orbits round the sun, all passing through the scene of the explosion, and thus possessing a common line of intersection.] [footnote : _phil. trans._, vol. xcii., part ii., p. .] [footnote : _ibid._, p. . in a letter to von zach of june , , he speaks of pallas as "almost incredibly small," and makes it only seventy english miles in diameter. _monat. corr._, vol. vi., pp. , .] [footnote : olbers, _monat. corr._, vol. vi., p. .] [footnote : _conn. d. tems_ for , p. .] [footnote : _popular astronomy_, p. .] [footnote : _month. not._, vol. vii., p. ; vol. viii., p. .] [footnote : _ibid._, p. .] [footnote : airy, _mem. r. a. s._, vol. xvi., p. .] [footnote : see newcomb's _pop. astr._, p. . the error of uranus amounted, in , to '; but even the tailor of breslau, whose extraordinary powers of vision humboldt commemorates (_kosmos_, bd. ii., p. ), could only see jupiter's first satellite at its greatest elongation, ' ". he might, however, possibly have distinguished two objects of _equal_ lustre at a lesser interval.] [footnote : j. w. l. glaisher, _observatory_, vol. xv., p. .] [footnote : _mem. r. a. s._, vol. xvi., p. .] [footnote : for an account of d'arrest's share in the detection see _copernicus_, vol. ii., pp. , .] [footnote : _mem. r. a. s._, vol. xvi., p. .] [footnote : he had recorded the places of , stars (three of which were different positions of the planet), and was preparing to map them, when, october , news of the discovery arrived from berlin. prof. challis's _report_, quoted in obituary notice, _month. not._, feb., , p. .] [footnote : see airy in _mem. r. a. s._, vol. xvi., p. .] [footnote : he died january , , in his st year.] [footnote : ledger, _the sun, its planets and their satellites_, p. .] [footnote : presented by the misses lassell, after their father's death, to the greenwich observatory.] [footnote : _astr. jour._, no. .] [footnote : _report of u.s. naval observatory for _, p. .] [footnote : grant, _hist. of astr._, p. .] [footnote : _month. not._, vol. ix., p. .] [footnote : _month. not._, vol. xi., p. .] [footnote : _astr. nach._, no. (may , ).] [footnote : _phil. trans._, vol. i., p. . see h. t. vivian, _engl. mech._, april , .] [footnote : secchi, _month. not._, vol. xiii., p. .] [footnote : hind, _ibid._, vol. xv., p. .] [footnote : lynn, _observatory_, oct. , ; hadley, _phil. trans._, vol. xxxii., p. .] [footnote : proctor, _saturn and its system_, p. .] [footnote : _phil. trans._, vol. lxxvii., p. .] [footnote : _month. not._, vol. xi., p. .] [footnote : _ibid._, vol. xxxv., pp. - .] [footnote : _ibid._, p. .] [footnote : _ibid._, vol. xli., p. .] chapter v _comets_ newton showed that the bodies known as "comets," or _hirsute_ stars, obey the law of gravitation; but it was by no means certain that the individual of the species observed by him in formed a permanent member of the solar system. the velocity, in fact, of its rush round the sun was quite possibly sufficient to carry it off for ever into the depths of space, there to wander, a celestial casual, from star to star. with another comet, however, which appeared two years later, the case was different. edmund halley, who afterwards succeeded flamsteed as astronomer royal, calculated the elements of its orbit on newton's principles, and found them to resemble so closely those similarly arrived at for comets observed by peter apian in , and by kepler in , as almost to compel the inference that all three were apparitions of a single body. this implied its revolution in a period of about seventy-six years, and halley accordingly fixed its return for - . so fully alive was he to the importance of the announcement that he appealed to a "candid posterity," in the event of its verification, to acknowledge that the discovery was due to an englishman. the prediction was one of the test-questions put by science to nature, on the replies to which largely depend both the development of knowledge and the conviction of its reality. in the present instance, the answer afforded may be said to have laid the foundation of this branch of astronomy. halley's comet punctually reappeared on christmas day, , and effected its perihelion passage on the th of march following, thus proving beyond dispute that some at least of these erratic bodies are domesticated within our system, and strictly conform, if not to its unwritten customs (so to speak), at any rate to its fundamental laws. their movements, in short, were demonstrated by the most unanswerable of all arguments--that of verified calculation--to be _calculable_, and their investigation was erected into a legitimate department of astronomical science. this notable advance was the chief _result_ obtained in the field of inquiry just now under consideration during the eighteenth century. but before it closed, its cultivation had received a powerful stimulus through the invention of an improved _method_. the name of olbers has already been brought prominently before our readers in connection with asteroidal discoveries; these, however, were but chance excursions from the path of cometary research which he steadily pursued through life. an early predilection for the heavens was fixed in this particular direction by one of the happy inspirations of genius. as he was watching, one night in the year , by the sick-bed of a fellow-student in medicine at göttingen, an important simplification in the mode of computing the paths of comets occurred to him. although not made public until , "olbers's method" was then universally adopted, and is still regarded as the most expeditious and convenient in cases where absolute rigour is not required. by its introduction, not only many a toilsome and thankless hour was spared, but workers were multiplied, and encouraged in the prosecution of labours more useful than attractive. the career of heinrich olbers is a brilliant example of what may be done by an amateur in astronomy. he at no time did regular work in an observatory; he was never the possessor of a transit or any other fixed instrument; moreover, all the best years of his life were absorbed in the assiduous exercise of a toilsome profession. born in at the village of arbergen, where his father was pastor, he settled in as a physician in the neighbouring town of bremen, and continued in active practice there for over forty years. it was thus only the hours which his robust constitution enabled him to spare from sleep that were available for his intellectual pleasures. yet his recreation was, as von zach remarked,[ ] no less prolific of useful results than the severest work of other men. the upper part of his house in the sandgasse was fitted up with such instruments and appliances as restrictions of space permitted, and there, night after night during half a century and upwards, he discovered, calculated, or observed the cometary visitants of northern skies. almost as effective in promoting the interests of science as the valuable work actually done by him, was the influence of his genial personality. he engaged confidence by his ready and discerning sympathy; he inspired affection by his benevolent disinterestedness; he quickened thought and awakened zeal by the suggestions of a lively and inventive spirit, animated with the warmest enthusiasm for the advancement of knowledge. nearly every astronomer in germany enjoyed the benefits of a frequently active correspondence with him, and his communications to the scientific periodicals of the time were numerous and striking. the motive power of his mind was thus widely felt and continually in action. nor did it wholly cease to be exerted even when the advance of age and the progress of infirmity rendered him incapable of active occupation. he was, in fact, _alive_ even to the last day of his long life of eighty-one years; and his death, which occurred march , , left vacant a position which a rare combination of moral and intellectual qualities had conspired to render unique. amongst the many younger men who were attracted and stimulated by intercourse with him was johann franz encke. but while olbers became a mathematician because he was an astronomer, encke became an astronomer because he was a mathematician. a born geometer, he was naturally sent to göttingen and placed under the tuition of gauss. but geometers are men; and the contagion of patriotic fervour which swept over germany after the battle of leipsic did not spare gauss's promising pupil. he took up arms in the hanseatic legion, and marched and fought until the oppressor of his country was safely ensconced behind the ocean-walls of st. helena. in the course of his campaigning he met lindenau, the militant director of the seeberg observatory, and by his influence was appointed his assistant, and eventually, in , became his successor. thence he was promoted in to berlin, where he superintended the building of the new observatory, so actively promoted by humboldt, and remained at its head until within some eighteen months of his death in august, . on the th of november, , pons of marseilles discovered a comet, whose inconspicuous appearance gave little promise of its becoming one of the most interesting objects in our system. encke at once took the calculation of its elements in hand, and brought out the unexpected result that it revolved round the sun in a period of about - / years.[ ] he, moreover, detected its identity with comets seen by méchain in , by caroline herschel in , by pons, huth, and bouvard in , and after six laborious weeks of research into the disturbances experienced by it from the planets during the entire interval since its first ascertained appearance, he fixed may , , as the date of its next return to perihelion. although on that occasion, owing to the position of the earth, invisible in the northern hemisphere, sir thomas brisbane's observatory at paramatta was fortunately ready equipped for its recapture, which rümker effected quite close to the spot indicated by encke's ephemeris. the importance of this event can be better understood when it is remembered that it was only the second instance of the recognised return of a comet (that of halley's, sixty-three years previously, having, as already stated, been the first); and that it, moreover, established the existence of a new class of celestial objects, somewhat loosely distinguished as "comets of short period." these bodies (of which about thirty have been found to circulate within the orbit of saturn) are remarkable as showing certain planetary affinities in the manners of their motions not at all perceptible in the wider travelling members of their order. they revolve, without exception, in the same direction as the planets--from west to east; they exhibit a marked tendency to conform to the zodiacal track which limits planetary excursions north and south; and their paths round the sun, although much more eccentric than the approximately circular planetary orbits, are far less so than the extravagantly long ellipses in which comets comparatively untrained (as it were) in the habits of the solar system ordinarily perform their revolutions. no _great_ comet is of the "planetary" kind. these are, indeed, only by exception visible to the naked eye; they possess extremely feeble tail-producing powers, and give small signs of central condensation. thin wisps of cosmical cloud, they flit across the telescopic field of view without sensibly obscuring the smallest star. their appearance, in short, suggests--what some notable facts in their history will presently be shown to confirm--that they are bodies already effete, and verging towards dissolution. if it be asked what possible connection can be shown to exist between the shortness of period by which they are essentially characterised, and what we may call their _superannuated_ condition, we are not altogether at a loss for an answer. kepler's remark,[ ] that comets are consumed by their own emissions, has undoubtedly a measure of truth in it. the substance ejected into the tail must, in overwhelmingly large proportion, be for ever lost to the central mass from which it issues. true, it is of a nature inconceivably tenuous; but unrepaired waste, however small in amount, cannot be persisted in with impunity. the incitement to such self-spoliation proceeds from the sun; it accordingly progresses more rapidly the more numerous are the returns to the solar vicinity. comets of short period may thus reasonably be expected to _wear out_ quickly. they are, moreover, subject to many adventures and vicissitudes. their aphelia--or the farthest points of their orbits from the sun--are usually, if not invariably, situated so near to the path either of jupiter or of saturn, as to permit these giant planets to act as secondary rulers of their destinies. by their influence they were, in all likelihood, originally fixed in their present tracks; and by their influence, exerted in an opposite sense, they may, in some cases, be eventually ejected from them. careers so varied, as can easily be imagined, are apt to prove instructive, and astronomers have not been backward in extracting from them the lessons they are fitted to convey. encke's comet, above all, has served as an index to much curious information, and it may be hoped that its function in that respect is by no means at an end. the great extent of the solar system traversed by its eccentric path makes it peculiarly useful for the determination of the planetary masses. at perihelion it penetrates within the orbit of mercury; it considerably transcends at aphelion the farthest excursion of pallas. its vicinity to the former planet in august, , offered the first convenient opportunity of placing that body in the astronomical balance. its weight or mass had previously been assumed, not ascertained; and the comparatively slight deviation from its regular course impressed upon the comet by its attractive power showed that it had been assumed nearly twice too great.[ ] that fundamental datum of planetary astronomy--the mass of jupiter--was corrected by similar means; and it was reassuring to find the correction in satisfactory accord with that already introduced from observations of the asteroidal movements. the fact that comets contract in approaching the sun had been noticed by hevelius; pingré admitted it with hesitating perplexity;[ ] the example of encke's comet rendered it conspicuous and undeniable. on the th of october, , the diameter of the nebulous matter composing this body was estimated at , miles. it was then about one and a half times further from the sun than the earth is at the time of the equinox. on the th of december following, its distance being reduced by nearly two-thirds, it was found to be only , miles across.[ ] that is to say, it had shrunk during those two months of approach to / th part of its original volume! yet it had still seventeen days' journey to make before reaching perihelion. the same curious circumstance was even more markedly apparent at its return in . its bulk, or the actual space occupied by it, appeared to be reduced, as it drew near the hearth of our system, in the enormous proportion of , to . a corresponding expansion accompanied on each occasion its retirement from the sphere of observation. similar changes of volume, though rarely to the same astounding extent, have been perceived in other comets. they still remain unexplained; but it can scarcely be doubted that they are due to the action of the same energetic internal forces which reveal themselves in so many splendid and surprising cometary phenomena. another question of singular interest was raised by encke's acute inquiries into the movements and disturbances of the first known "comet of short period." he found from the first that its revolutions were subject to some influence besides that of gravity. after every possible allowance had been made for the pulls, now backward, now forward, exerted upon it by the several planets, there was still a surplus of acceleration left unaccounted for. each return to perihelion took place about two and a half hours sooner than received theories warranted. here, then, was a "residual phenomenon" of the utmost promise for the disclosure of novel truths. encke (in accordance with the opinion of olbers) explained it as due to the presence in space of some such "subtle matter" as was long ago invoked by euler[ ] to be the agent of eventual destruction for the fair scheme of planetary creation. the apparent anomaly of accounting for an accelerative effect by a retarding cause disappears when it is considered that any check to the motion of bodies revolving round a centre of attraction causes them to draw closer to it, thus shortening their periods and quickening their circulation. if space were filled with a resisting medium capable of impeding, even in the most infinitesimal degree, the swift course of the planets, their orbits should necessarily be, not ellipses, but very close elliptical spirals along which they would slowly, but inevitably, descend into the burning lap of the sun. the circumstance that no such tendency can be traced in their revolutions by no means sets the question at rest. for it might well be that an effect totally imperceptible until after the lapse of countless ages, as regards the solid orbs of our system, might be obvious in the movements of bodies like comets of small mass and great bulk; just as a feather or a gauze veil at once yields its motion to the resistance of the air, while a cannon-ball cuts its way through with comparatively slight loss of velocity. it will thus be seen that issues of the most momentous character hang on the _time-keeping_ of comets; for plainly all must in some degree suffer the same kind of hindrance as encke's, if the cause of that hindrance be the one suggested. none of its congeners, however, show any trace of similar symptoms. true, the late professor oppolzer announced,[ ] in , that a comet, first seen by pons in , and rediscovered by winnecke in , having a period of , days ( · years), was accelerated at each revolution precisely in the manner required by encke's theory. but m. von haerdtl's subsequent investigation, the materials for which included numerous observations of the body in question at its return to the sun in , decisively negatived the presence of any such effect.[ ] moreover, the researches of von asten and backlund[ ] into the movements of encke's comet revealed a perplexing circumstance. they confirmed encke's results for the period covered by them, but exhibited the acceleration as having _suddenly diminished_ by nearly one-half in . the reality and permanence of this change were fully established by observations of the ensuing return in march, . some physical alteration of the retarded body seems indicated; but visual evidence countenances no such assumption. in aspect the comet is no less thin and diffuse than in or in . the character of the supposed resistance in inter-planetary space has, it may be remarked, been often misapprehended. what encke stipulated for was not a medium equally diffused throughout the visible universe, such as the ethereal vehicle of the vibrations of light, but a rare fluid, rapidly increasing in density towards the sun.[ ] this cannot be a solar atmosphere, since it is mathematically certain, as laplace has shown,[ ] that no envelope partaking of the sun's axial rotation can extend farther from his surface than nine-tenths of the mean distance of mercury; while physical evidence assures us that the _actual_ depth of the solar atmosphere bears a very minute proportion to the _possible_ depth theoretically assigned to it. that matter, however, not atmospheric in its nature--that is, neither forming one body with the sun nor altogether aëriform--exists in its neighbourhood, can admit of no reasonable doubt. the great lens-shaped mass of the zodiacal light, stretching out at times far beyond the earth's orbit, may indeed be regarded as an extension of the corona, the streamers of which themselves mark the wide diffusion, all round the solar globe, of granular or gaseous materials. yet comets have been known to penetrate the sphere occupied by them without perceptible loss of velocity. the hypothesis, then, of a resisting medium receives at present no countenance from the movements of comets, whether of short or of long periods. although encke's comet has made thirty-five complete rounds of its orbit since its first detection in , it shows no certain signs of decay. variations in its brightness are, it is true, conspicuous, but they do not proceed continuously.[ ] the history of the next known planet-like comet has proved of even more curious interest than that of the first. it was discovered by an austrian officer named wilhelm von biela at josephstadt in bohemia, february , , and ten days later by the french astronomer gambart at marseilles. both observers computed its orbit, showed its remarkable similarity to that traversed by comets visible in and , and connected them together as previous appearances of the body just detected by assigning to its revolutions a period of between six and seven years. the two brief letters conveying these strikingly similar inferences were printed side by side in the same number of the _astronomische nachrichten_ (no. ); but biela's priority in the discovery of the comet was justly recognised by the bestowal upon it of his name. the object in question was at no time, subsequently to , visible to the naked eye. its aspect in sir john herschel's great reflector on the rd of september, , was described by him as that of a "conspicuous nebula," nearly minutes in diameter. no trace of a tail was discernible. while he was engaged in watching it, a small knot of minute stars was directly traversed by it, "and when on the cluster," he tells us,[ ] it "presented the appearance of a nebula resolvable and partly resolved into stars, the stars of the cluster being visible through the comet." yet the depth of cometary matter through which such faint stellar rays penetrated undimmed, was, near the central parts of the globe, not less than , miles. it is curious to find that this seemingly harmless, and we may perhaps add effete body, gave occasion to the first (and not the last) cometary "scare" of an enlightened century. its orbit, at the descending node, may be said to have intersected that of the earth; since, according as it _bulged in or out_ under the disturbing influence of the planets, the passage of the comet was affected _inside_ or _outside_ the terrestrial track. now, certain calculations published by olbers in [ ] showed that, on october , , a considerable portion of its nebulous surroundings would actually sweep over the spot which, a month later, would be occupied by our planet. it needed no more to set the popular imagination in a ferment. astronomers, after all, could not, by an alarmed public, be held to be infallible. their computations, it was averred, which a trifling oversight would suffice to vitiate, exhibited clearly enough the danger, but afforded no guarantee of safety from a collision, with all the terrific consequences frigidly enumerated by laplace. nor did the panic subside until arago formally demonstrated that the earth and the comet could by no possibility approach within less than fifty millions of miles.[ ] the return of the same body in - was marked by an extraordinary circumstance. when first seen, november , it wore its usual aspect of a faint round patch of cosmical fog; but on december , mr. hind noticed that it had become distorted somewhat into the form of a pear; and ten days later, it had divided into two separate objects. this singular duplication was first perceived at new haven in america, december ,[ ] by messrs. herrick and bradley, and by lieutenant maury at washington, january , . the earliest british observer of the phenomenon (noticed by wichmann the same evening at königsberg) was professor challis. "i see _two_ comets!" he exclaimed, putting his eye to the great equatoreal of the cambridge observatory on the night of january ; then, distrustful of what his senses had told him, he called in his judgment to correct their improbable report by resolving one of the dubious objects into a hazy star.[ ] on the rd, however, both were again seen by him in unmistakable cometary shape, and until far on in march (otto struve caught a final glimpse of the pair on the th of april),[ ] continued to be watched with equal curiosity and amazement by astronomers in every part of the northern hemisphere. what seneca reproved ephorus for supposing to have taken place in b.c.--what pingré blamed kepler for conjecturing in --had then actually occurred under the attentive eyes of science in the middle of the nineteenth century! at a distance from each other of about two-thirds the distance of the moon from the earth, the twin comets meantime moved on tranquilly, so far, at least, as their course through the heaven was concerned. their extreme _lightness_, or the small amount of matter contained in each, could not have received a more signal illustration than by the fact that their revolutions round the sun were performed independently; that is to say, they travelled side by side without experiencing any appreciable mutual disturbance, thus plainly showing that at an interval of only , miles their attractive power was virtually inoperative. signs of internal agitation, however, were not wanting. each fragment threw out a short tail in a direction perpendicular to the line joining their centres, and each developed a bright nucleus, although the original comet had exhibited neither of these signs of cometary vitality. a singular interchange of brilliancy was, besides, observed to take place between the coupled objects, each of which alternately outshone and was outshone by the other, while an arc of light, apparently proceeding from the more lustrous, at times bridged the intervening space. obviously, the gravitational tie, rendered powerless by exiguity of matter, was here replaced by some other form of mutual action, the nature of which can as yet be dealt with only by conjecture. once more, in august, , the double comet returned to the neighbourhood of the sun, but under circumstances not the most advantageous for observation. indeed, the companion was not detected until september , when father secchi at rome perceived it to have increased its distance from the originating body to a million and a quarter of miles, or about eight times the average interval at the former appearance. both vanished shortly afterwards, and have never since been seen, notwithstanding the eager watch kept for objects of such singular interest, and the accurate knowledge of their track supplied by santini's investigations. a dangerously near approach to jupiter in is believed to have occasioned their disruption, and the disaggregating process thus started was likely to continue. we can scarcely doubt that the fate has overtaken them which newton assigned as the end of all cometary existence. _diffundi tandem et spargi per coelos universos._[ ] biela's is not the only vanished comet. brorsen's, discovered at kiel in , and observed at four subsequent returns, failed unaccountably to become visible in .[ ] yet numerous sentinels were on the alert to surprise its approach along a well-ascertained track, traversed in five and a half years. the object presented from the first a somewhat time-worn aspect. it was devoid of tail, or any other kind of appendage; and the rapid loss of the light acquired during perihelion passage was accompanied by inordinate expansion of an already tenuous globular mass. another lost or mislaid comet is one found by de vico at rome, august , . it was expected to return early in , but did not, and has never since been seen; unless its re-appearance as e. swift's comet of should be ratified by closer inquiry.[ ] a telescopic comet with a period of - / years, discovered november , , by m. faye of the paris observatory, formed the subject of a characteristically patient and profound inquiry on the part of leverrier, designed to test its suggested identity with lexell's comet of . the result was decisive against the hypothesis of valz, the divergences between the orbits of the two bodies being found to increase instead of to diminish, as the history of the new-comer was traced backward into the previous century.[ ] faye's comet pursues the most nearly circular path of any similar known object; even at its nearest approach to the sun it remains farther off than mars when he is most distant from it; and it was proved by the admirable researches of professor axel möller,[ ] director of the swedish observatory of lund, to exhibit no trace of the action of a resisting medium. periodical comets are evidently bodies which have each lived through a chapter of accidents, and a significant hint as to the nature of their adventures can be gathered from the fact that their aphelia are pretty closely grouped about the tracks of the major planets. halley's, and five other comets are thus related to neptune; three connect themselves with uranus, two with saturn, above a score with jupiter. some form of dependence is plainly indicated, and the researches of tisserand,[ ] callandreau,[ ] and newton[ ] of yale college, leave scarcely a doubt that the "capture-theory" represents the essential truth in the matter. the original parabolic paths of these comets were then changed into ellipses by the backward pull of a planet, whose sphere of attraction they chanced to enter when approaching the sun from outer space. moreover, since a body thus affected should necessarily return at each revolution to the scene of encounter, the same process of retardation may, in some cases, have been repeated many times, until the more restricted cometary orbits were reduced to their present dimensions. the prevalence, too, among periodical comets, of direct motion, is shown to be inevitable by m. callandreau's demonstration that those travelling in a retrograde direction would, by planetary action, be thrown outside the probable range of terrestrial observation. the scarcity of hyperbolic comets can be similarly explained. they would be created whenever the attractive influence of the disturbing planet was exerted in a forward or accelerative sense, but could come only by a rare exception to our notice. the inner planets, including the earth, have also unquestionably played their parts in modifying cometary orbits; and mr. plummer suggests, with some show of reason, that the capture of encke's comet may be a feat due to mercury.[ ] no _great_ comet appeared between the "star" which presided at the birth of napoleon and the "vintage" comet of . the latter was first described by flaugergues at viviers, march , ; wisniewski, at neu-tscherkask in southern russia, caught a final glimpse of it, august , . two disappearances in the solar rays as the earth moved round in its orbit, and two reappearances after conjunction, were included in this unprecedentedly long period of visibility of days. this relative permanence (so far as the inhabitants of europe were concerned) was due to the high northern latitude attained near perihelion, combined with a certain leisureliness of movement along a path everywhere external to that of the earth. the magnificent luminous train of this body, on october , the day of its nearest terrestrial approach, covered an arc of the heavens - / degrees in length, corresponding to a real extension of one hundred millions of miles. its form was described by sir william herschel as that of "an inverted hollow cone," and its colour as yellowish, strongly contrasted with the bluish-green tint of the "head," round which it was flung like a transparent veil. the planetary disc of the head, , miles across, appeared to be composed of strongly-condensed nebulous matter; but somewhat eccentrically situated within it was a star-like nucleus of a reddish tinge, which herschel presumed to be solid, and ascertained, with his usual care, to have a diameter of miles. from the total absence of phases, as well as from the vivacity of its radiance, he confidently inferred that its light was not borrowed, but inherent.[ ] this remarkable apparition formed the subject of a memoir by olbers,[ ] the striking yet steadily reasoned out suggestions contained in which there was at that time no means of following up with profit. only of late has the "electrical theory," of which zöllner[ ] regarded olbers as the founder, assumed a definite and measurable form, capable of being tested by the touchstone of fact, as knowledge makes its slow inroads on the fundamental mystery of the physical universe. the paraboloidal shape of the bright envelope separated by a dark interval from the head of the great comet of , and constituting, as it were, the _root_ of its tail, seemed to the astronomer of bremen to reveal the presence of a double repulsion; the expelled vapours accumulating where the two forces, solar and cometary, balanced each other, and being then swept backwards in a huge train. he accordingly distinguished three classes of these bodies:--first, comets which develop _no_ matter subject to solar repulsion. these have no tails, and are probably mere nebulosities, without solid nuclei. secondly, comets which are acted upon by solar repulsion _only_, and consequently throw out no emanations _towards_ the sun. of this kind was a bright comet visible in .[ ] thirdly, comets like that of , giving evidence of action of both kinds. these are distinguished by a dark _hoop_ encompassing the head and dividing it from the luminous envelope, as well as by an obscure caudal axis, resulting from the hollow, cone-like structure of the tail. again, the ingenious view subsequently propounded by m. bredikhin as to the connection between the _form_ of these appendages and the _kind_ of matter composing them, was very clearly anticipated by olbers. the amount of tail-curvature, he pointed out, depends in each case upon the proportion borne by the velocity of the ascending particles to that of the comet in its orbit; the swifter the outrush, the straighter the resulting tail. but the velocity of the ascending particles varies with the energy of their repulsion by the sun, and this again, it may be presumed, with their quality. thus multiple tails are developed when the same comet throws off, as it approaches perihelion, specifically distinct substances. the long, straight ray which proceeded from the comet of , for example, was doubtless made up of particles subject to a much more vigorous solar repulsion than those formed into the shorter curved emanation issuing from it nearly in the same direction. in the comet of he calculated that the particles expelled from the head travelled to the remote extremity of the tail in eleven minutes, indicating by this enormous rapidity of movement (comparable to that of the transmission of light) the action of a force much more powerful than the opposing one of gravity. the not uncommon phenomena of multiple envelopes, on the other hand, he explained as due to the varying amounts of repulsion exercised by the nucleus itself on the different kinds of matter developed from it. the movements and perturbations of the comet of were no less profoundly studied by argelander than its physical constitution by olbers. the orbit which he assigned to it is of such vast dimensions as to require no less that , years for the completion of its circuit; and to carry the body describing it at each revolution to fourteen times the distance from the sun of the frigid neptune. thus, when it last visited our neighbourhood, achilles may have gazed on its imposing train as he lay on the sands all night bewailing the loss of patroclus; and when it returns, it will perhaps be to shine upon the ruins of empires and civilizations still deep buried among the secrets of the coming time.[ ] on the th of june, , while the head of a comet passed across the face of the sun, the earth was in all probability involved in its tail. but of this remarkable double event nothing was known until more than a month later, when the fact of its past occurrence emerged from the calculations of olbers.[ ] nor had the comet itself been generally visible previous to the first days of july. several observers, however, on the publication of these results, brought forward accounts of singular spots perceived by them upon the sun at the time of the transit, and an original drawing of one of them, by pastorff of buchholtz, has been preserved. this undoubtedly authentic delineation[ ] represents a round nebulous object with a _bright_ spot in the centre, of decidedly cometary aspect, and not in the least like an ordinary solar "macula." mr. hind,[ ] nevertheless, showed its position on the sun to be irreconcilable with that which the comet must have occupied; and mr. ranyard's discovery of a similar smaller drawing by the same author, dated may , ,[ ] reduces to evanescence the probability of its connection with that body. indeed, recent experience renders very doubtful the possibility of such an observation. the return of halley's comet in was looked forward to as an opportunity for testing the truth of floating cometary theories, and did not altogether disappoint expectation. as early as , its movements and disturbances since were proposed by the turin academy of sciences as the subject of a prize ultimately awarded to baron damoiseau. pontécoulant was adjudged a similar distinction by the paris academy in ; while rosenberger's calculations were rewarded with the gold medal of the royal astronomical society.[ ] they were verified by the detection at rome, august , , of a nearly circular misty object not far from the predicted place of the comet. it was not, however, until the middle of september that it began to throw out a tail, which by the th of october had attained a length of about degrees (on the th, at madras, it extended to fully ),[ ] the head showing to the naked eye as a reddish star rather brighter than aldebaran or antares.[ ] some curious phenomena accompanied the process of tail-formation. an outrush of luminous matter, resembling in shape a partially opened fan, issued from the nucleus _towards_ the sun, and at a certain point, like smoke driven before a high wind, was vehemently swept backwards in a prolonged train. the appearance of the comet at this time was compared by bessel,[ ] who watched it with minute attention, to that of a blazing rocket. he made the singular observation that this fan of light, which seemed the source of supply for the tail, oscillated like a pendulum to and fro across a line joining the sun and nucleus, in a period of - / days; and he was unable to escape from the conclusion[ ] that a repulsive force, about twice as powerful as the attractive force of gravity, was concerned in the production of these remarkable effects. nor did he hesitate to recur to the analogy of magnetic polarity, or to declare, still more emphatically than olbers, "the emission of the tail to be a purely electrical phenomenon."[ ] the transformations undergone by this body were almost as strange and complete as those which affected the brigands in dante's _inferno_. when first seen, it wore the aspect of a nebula; later it put on the distinctive garb of a comet; it next appeared as a star; finally, it dilated, first in a spherical, then in a paraboloidal form, until may , , when it vanished from herschel's observation at feldhausen as if by melting into adjacent space from the excessive diffusion of its light. a very uncommon circumstance in its development was that it lost all trace of tail _previous_ to its arrival at perihelion on the th of november. nor did it begin to recover its elongated shape for more than two months afterwards. on the rd of january, boguslawski perceived it as a star of the sixth magnitude, _without measurable disc_.[ ] only two nights later, maclear, director of the cape observatory, found the head to be seconds across.[ ] and so rapidly did the augmentation of size progress, that sir john herschel estimated the actual bulk of this singular object to have increased forty-fold in the ensuing week. "i can hardly doubt," he remarks, "that the comet was fairly evaporated in perihelio by the heat, and resolved into transparent vapour, and is now in process of rapid condensation and re-precipitation on the nucleus."[ ] a plausible, but no longer admissible, interpretation of this still unexplained phenomenon. the next return of this body, which will be considerably accelerated by jupiter's influence, is expected to take place in .[ ] by means of an instrument devised to test the quality of light, arago obtained decisive evidence that some at least of the radiance proceeding from halley's comet was derived by reflection from the sun.[ ] indications of the same kind had been afforded[ ] by the comet which suddenly appeared above the north-western horizon of paris, july , , after having enveloped (as already stated) our terrestrial abode in its filmy appendages; but the "polariscope" had not then reached the perfection subsequently given to it, and its testimony was accordingly far less reliable than in . such experiments, however, are in reality more beautiful and ingenious than instructive, since ignited as well as obscure bodies possess the power of throwing back light incident upon them, and will consequently transmit to us from the neighbourhood of the sun rays partly direct, partly reflected, of which a certain proportion will exhibit the peculiarity known as polarisation. the most brilliant comets of the century were suddenly rivalled if not surpassed by the extraordinary object which blazed out beside the sun, february , . it was simultaneously perceived in mexico and the united states, in southern europe, and at sea off the cape of good hope, where the passengers on board the _owen glendower_ were amazed by the sight of a "short, dagger-like object," closely following the sun towards the western horizon.[ ] at florence, amici found its distance from the sun's centre at noon to be only ° '; and spectators at parma were able, when sheltered from the direct glare of mid-day, to trace the tail to a length of four or five degrees. the full dimensions of this astonishing appurtenance began to be disclosed a few days later. on the rd of march it measured °, and on the th, at calcutta, mr. clerihew observed a second streamer, nearly twice as long as the first, and making an angle with it of °, to have been emitted in a single day. this rapidity of projection, sir john herschel remarked, "conveys an astounding impression of the intensity of the forces at work." "it is clear," he continued, "that _if we have to deal here with matter, such as we conceive it_--viz., _possessing inertia--at all_, it must be under the dominion of forces incomparably more energetic than gravitation, and quite of a different nature."[ ] on the th of march a silvery ray, some ° long and slightly curved at its extremity, shone out above the sunset clouds in this country. no previous intimation had been received of the possibility of such an apparition, and even astronomers--no lightning messages across the seas being as yet possible--were perplexed. the nature of the phenomenon, indeed, soon became evident, but the wonder of it did not diminish with the study of its attendant circumstances. never before, within astronomical memory, had our system been traversed by a body pursuing such an adventurous career. the closest analogy was offered by the great comet of (newton's), which rushed past the sun at a distance of only , miles; but even this--on the cosmical scale--scarcely perceptible interval was reduced nearly one-half in the case we are now concerned with. the centre of the comet of approached the formidable luminary within , miles, leaving, it is estimated, a clear space of not more than , between the surfaces of the bodies brought into such perilous proximity. the escape of the wanderer was, however, secured by the extraordinary rapidity of its flight. it swept past perihelion at a rate-- miles a second--which, if continued, would have carried it right round the sun in _two hours_; and in only eleven minutes more than that short period it actually described half the _curvature_ of its orbit--an arc of °--although in travelling over the remaining half many hundreds of sluggish years will doubtless be consumed. the behaviour of this comet may be regarded as an _experimentum crucis_ as to the nature of tails. for clearly no fixed appendage many millions of miles in length could be whirled like a brandished sabre from one side of the sun to the other in minutes. cometary trains are then, as olbers rightly conceived them to be, emanations, not appendages--inconceivably rapid outflows of highly rarefied matter, the greater part, if not all, of which becomes permanently detached from the nucleus. that of the comet of reached, about the time that it became visible in this country, the extravagant length of millions of miles.[ ] it was narrow, and bounded by nearly parallel and nearly rectilinear lines, resembling--to borrow a comparison of aristotle's--a "road" through the constellations; and after the rd of march showed no trace of hollowness, the axis being, in fact, rather brighter than the edges. distinctly perceptible in it were those singular aurora-like coruscations which gave to the "tresses" of charles v.'s comet the appearance--as cardan described it--of "a torch agitated by the wind," and have not unfrequently been observed to characterise other similar objects. a consideration first adverted to by olbers proves these to originate in our own atmosphere. for owing to the great difference in the distances from the earth of the origin and extremity of such vast effluxes, the light proceeding from their various parts is transmitted to our eyes in notably different intervals of time. consequently a luminous undulation, even though propagated instantaneously from end to end of a comet's tail, would appear to us to occupy many minutes in its progress. but the coruscations in question pass as swiftly as a falling star. they are, then, of terrestrial production. periods of the utmost variety were by different computators assigned to the body, which arrived at perihelion, february , , at . p.m. professor hubbard of washington found that it required years to complete a revolution; mm. laugier and mauvais of paris considered the true term to be ;[ ] clausen looked for its return at the end of between six and seven. a recent discussion[ ] by professor kreutz of all the available data gives a probable period of years for this body, and precludes its hypothetical identity with the comet of , known as the "spina" of cassini. it may now be asked, what were the conclusions regarding the nature of comets drawn by astronomers from the considerable amount of novel experience accumulated during the first half of this century? the first and best assured was that the matter composing them is in a state of extreme tenuity. numerous and trustworthy observations showed that the feeblest rays of light might traverse some hundreds of thousands of miles of their substance, even where it was apparently most condensed, without being perceptibly weakened. nay, instances were recorded in which stars were said to have gained in brightness from the process![ ] on the th of june, , olbers[ ] saw the comet then visible all but obliterated by the central passage of a star too small to be distinguished with the naked eye, its own light remaining wholly unchanged. a similar effect was noted december , , when the great comet of that year approached so close to altair, the _lucida_ of the eagle, that the star seemed to be transformed into the nucleus of the comet.[ ] even the central blaze of halley's comet in was powerless to impede the passage of stellar rays. struve[ ] observed at dorpat, on september , an all but central occultation; glaisher[ ] one (so far as he could ascertain) absolutely so eight days later at cambridge. in neither case was there any appreciable diminution of the star's light. again, on the th of october, , mr. dawes,[ ] an exceptionally keen observer, distinctly saw a star of the tenth magnitude through the exact centre of a comet discovered on the first of that month by maria mitchell of nantucket. examples, on the other hand, are not wanting of the diminution of stellar light under similar circumstances;[ ] and we meet two alleged instances of the vanishing of a star behind a comet. wartmann of geneva observed the first, november , ;[ ] but his instrument was defective, and the eclipsing body, encke's comet, has shown itself otherwise perfectly translucent. the second case of occultation occurred september , , when an eleventh magnitude star was stated to have completely disappeared during the transit over it of denning's comet.[ ] from the failure to detect any effects of refraction in the light of stars occulted by comets, it was inferred (though, as we know now, erroneously) that their composition is rather that of dust than that of vapour; that they consist not of any continuous substance, but of discrete solid particles, very finely divided and widely scattered. in conformity with this view was the known smallness of their masses. laplace had shown that if the amount of matter forming lexell's comet had been as much as / of that contained in our globe, the effect of its attraction, on the occasion of its approach within , , miles of the earth, july , , must have been apparent in the lengthening of the year. and that some comets, at any rate, possess masses immeasurably below this maximum value was clearly proved by the undisturbed parallel march of the two fragments of biela's in . but the discovery in this branch most distinctive of the period under review is that of "short period" comets, of which four[ ] were known in . these, by the character of their movements, serve as a link between the planetary and cometary worlds, and by the nature of their construction, seem to mark a stage in cometary decay. for that comets are rather transitory agglomerations, than permanent products of cosmical manufacture, appeared to be demonstrated by the division and disappearance of one amongst their number, as well as by the singular and rapid changes in appearance undergone by many, and the seemingly irrevocable diffusion of their substance visible in nearly all. they might then be defined, according to the ideas respecting them prevalent fifty years ago, as bodies unconnected by origin with the solar system, but encountered, and to some extent appropriated, by it in its progress through space, owing their visibility in great part, if not altogether, to light reflected from the sun, and their singular and striking forms to the action of repulsive forces emanating from him, the penalty of their evanescent splendour being paid in gradual waste and final dissipation and extinction. footnotes: [footnote : _allgemeine geographische ephemeriden_, vol. iv., p. .] [footnote : _astr. jahrbuch_, , p. . the period ( , days) of this body is considerably shorter than that of any other known comet.] [footnote : "sicut bombyces filo fundendo, sic cometas cauda exspiranda consumi et denique mori."--_de cometis_, op., vol. vii., p. .] [footnote : considerable uncertainty, however, still prevails on the point. the inverse relation assumed by lagrange to exist between distance from the sun and density brought out the mercurian mass / that of the sun (laplace, _exposition du syst. du monde_, t. ii., p. , ed. ). von asten deduced from the movements of encke's comet, - , a value of / ; while backlund from its seven returns, - , derived / (_comptes rendus_, oct. , ).] [footnote : arago, _annuaire_ ( ), p. .] [footnote : hind, _the comets_, p. .] [footnote : _phil. trans._, vol. xlvi., p. .] [footnote : _astr. nach._, no. , .] [footnote : _comptes rendus_, t. cvii., p. .] [footnote : _mém. de st. pétersbourg_, t. xxxii., no. , ; _astr. nach._, no. , .] [footnote : _month. not._, vol. xix., p. .] [footnote : _mécanique céleste_, t. ii., p. .] [footnote : see berberich, _astr. nach._, nos. , - , , ; deichmüller, _ibid._, no. , .] [footnote : _month. not._, vol. ii., p. .] [footnote : _astr. nach._, no. .] [footnote : _annuaire_, , p. .] [footnote : _am. journ. of science_, vol. i. ( nd series), p. . prof. hubbard's calculations indicated a probability that the definitive separation of the two nuclei occurred as early as september , . _astronomical journal_ (gould's), vol. iv., p. . see also, on the subject of this comet, w. t. lynn, _intellectual observer_, vol. xi., p. ; e. ledger, _observatory_, august, , p. ; and h. a. newton, _am. journ. of science_, vol. xxxi., p. , february, .] [footnote : _month. not._, vol. vii., p. .] [footnote : _bulletin ac. imp. de st. pétersbourg_, t. vi., col. . the latest observation of the parent nucleus was that of argelander, april , at bonn.] [footnote : d'arrest, _astr. nach._, no. , .] [footnote : _der brorsen'sche comet._ von dr. e. lamp, kiel, ; plummer, _knowledge_, vol. xix., p. .] [footnote : schulhof, _astr. nach._, no. , ; _observatory_, vol. xviii., p. ; f. h. seares, _astr. nach._, nos. , - ; plummer, _knowledge_, vol. xix., p. .] [footnote : _comptes rendus_, t. xxv., p. .] [footnote : _month. not._, vol. xii., p. .] [footnote : _bull. astr._, t. vi., pp. , .] [footnote : _Étude sur la théorie des comètes périodiques. annales de l'observatoíre_, t. xx., paris, .] [footnote : _amer. journ. of science_, vol. xlii., pp. , , .] [footnote : _observatory_, vol. xiv., p. .] [footnote : _phil. trans._, vol. cii., pp. - .] [footnote : _ueber den schweif des grossen cometen von , monat. corr._, vol. xxv., pp. - . reprinted by zöllner. _ueber die natur der cometen_, pp. - .] [footnote : _natur der cometen_, p. .] [footnote : the subject of a classical memoir by bessel, published in .] [footnote : a fresh investigation of its orbit has been published by n. herz of vienna. see _bull. astr._, t. ix., p. .] [footnote : _astr. jahrbuch_ (bode's), , p. .] [footnote : reproduced in webb's _celestial objects_, th ed.] [footnote : _month. not._, vol. xxxvi., p. .] [footnote : _celestial objects_, p. , note.] [footnote : see airy's address, _mem. r. a. s._, vol. x., p. . rosenberger calculated no more, though he lived until . w. t. lynn, _observatory_, vol. xiii., p. .] [footnote : hind, _the comets_, p. .] [footnote : arago, _annuaire_, , p. .] [footnote : _astr. nach._, no. .] [footnote : it deserves to be recorded that robert hooke drew a very similar inference from his observations of the comets of and . _month. not._, vol. xiv., pp. - .] [footnote : _briefwechsel zwischen olbers und bessel_, bd. ii., p. .] [footnote : herschel, _results_, p. .] [footnote : _mem. r. a. s._, vol. x., p. ,] [footnote : _results_, p. .] [footnote : pontécoulant, _comptes rendus_, t. lviii., p. .] [footnote : _annuaire_, , p. .] [footnote : _cosmos_, vol. i., p. , _note_ (otté's trans.).] [footnote : herschel, _outlines of astronomy_, p. , th ed.] [footnote : _outlines_, p. .] [footnote : boguslawski calculated that it extended on the st of march to millions.--_report. brit. ass._, , p. .] [footnote : _comptes rendus_, t. xvi., p. .] [footnote : _observatory_, vol. xxiv., p. ; astr. nach., no. , .] [footnote : piazzi noticed a considerable increase of lustre in a very faint star of the twelfth magnitude viewed through a comet. mädler, _reden_, etc., p. , _note_.] [footnote : _astr. jahrbuch_, , p. .] [footnote : mädler, _gesch. d. astr._, bd. ii., p. .] [footnote : _recueil de l'ac. imp. de st. pétersbourg_, , p. .] [footnote : guillemin's _world of comets_, trans, by j. glaisher, p. , _note_.] [footnote : _month. not._, vol. viii., p. .] [footnote : a real, though only partial stoppage of light seems indicated by herschel's observations on the comet of . stars seen through the tail, october , lost much of their lustre. one near the head was only faintly visible by glimpses. _phil. trans._, vol. xcvii., p. .] [footnote : arago, _annuaire_, , p. .] [footnote : _ibid._, , p. .] [footnote : viz., encke's, biela's, faye's, and brorsen's.] chapter vi _instrumental advances_ it is impossible to follow with intelligent interest the course of astronomical discovery without feeling some curiosity as to the means by which such surpassing results have been secured. indeed, the bare acquaintance with _what_ has been achieved, without any corresponding knowledge of _how_ it has been achieved, supplies food for barren wonder rather than for fruitful and profitable thought. ideas advance most readily along the solid ground of practical reality, and often find true sublimity while laying aside empty marvels. progress is the result, not so much of sudden flights of genius, as of sustained, patient, often commonplace endeavour; and the true lesson of scientific history lies in the close connection which it discloses between the most brilliant developments of knowledge and the faithful accomplishment of his daily task by each individual thinker and worker. it would be easy to fill a volume with the detailed account of the long succession of optical and mechanical improvements by means of which the observation of the heavens has been brought to its present degree of perfection; but we must here content ourselves with a summary sketch of the chief amongst them. the first place in our consideration is naturally claimed by the telescope. this marvellous instrument, we need hardly remind our readers, is of two distinct kinds--that in which light is gathered together into a focus by _refraction_, and that in which the same end is attained by _reflection_. the image formed is in each case viewed through a magnifying lens, or combination of lenses, called the eye-piece. not for above a century after the "optic glasses" invented or stumbled upon by the spectacle-maker of middelburg ( ) had become diffused over europe, did the reflecting telescope come, even in england, the place of its birth, into general use. its principle (a sufficiently obvious one) had indeed been suggested by mersenne as early as ;[ ] james gregory in [ ] described in detail a mode of embodying that principle in a practical shape; and newton, adopting an original system of construction, actually produced in a tiny speculum, one inch across, by means of which the apparent distance of objects was reduced thirty-nine times. nevertheless, the exorbitantly long tubeless refractors, introduced by huygens, maintained their reputation until hadley exhibited to the royal society, january , ,[ ] a reflector of six inches aperture, and sixty-two in focal length, which rivalled in performance, and of course indefinitely surpassed in manageability, one of the "aerial" kind of feet. the concave-mirror system now gained a decided ascendant, and was brought to unexampled perfection by james short of edinburgh during the years - . its resources were, however, first fully developed by william herschel. the energy and inventiveness of this extraordinary man marked an epoch wherever they were applied. his ardent desire to measure and gauge the stupendous array of worlds which his specula revealed to him, made him continually intent upon adding to their "space-penetrating power" by increasing their light-gathering surface. these, as he was the first to explain,[ ] are in a constant proportion one to the other. for a telescope with twice the linear aperture of another will collect four times as much light, and will consequently disclose an object four times as faint as could be seen with the first, or, what comes to the same, an object equally bright at twice the distance. in other words, it will possess double the space-penetrating power of the smaller instrument. herschel's great mirrors--the first examples of the giant telescopes of modern times--were then primarily engines for extending the bounds of the visible universe; and from the sublimity of this "final cause" was derived the vivid enthusiasm which animated his efforts to success. it seems probable that the seven-foot telescope constructed by him in --that is within little more than a year after his experiments in shaping and polishing metal had begun--already exceeded in effective power any work by an earlier optician; and both his skill and his ambition rapidly developed. his efforts culminated, after mirrors of ten, twenty, and thirty feet focal length had successively left his hands, in the gigantic forty-foot, completed august , . it was the first reflector in which only a single mirror was employed. in the "gregorian" form, the focussed rays are, by a second reflection from a small concave[ ] mirror, thrown _straight back_ through a central aperture in the larger one, behind which the eye-piece is fixed. the object under examination is thus seen in the natural direction. the "newtonian," on the other hand, shows the object in a line of sight at right angles to the true one, the light collected by the speculum being diverted to one side of the tube by the interposition of a small plane mirror, situated at an angle of ° to the axis of the instrument. upon these two systems herschel worked until , when, becoming convinced of the supreme importance of economising light (necessarily wasted by the second reflection), he laid aside the small mirror of his forty-foot then in course of construction, and turned it into a "front-view" reflector. this was done--according to the plan proposed by lemaire in --by slightly inclining the speculum so as to enable the image formed by it to be viewed with an eye-glass fixed at the upper margin of the tube. the observer thus stood with his back turned to the object he was engaged in scrutinising. the advantages of the increased brilliancy afforded by this modification were strikingly illustrated by the discovery, august and september , , of the two saturnian satellites nearest the ring. nevertheless, the monster telescope of slough cannot be said to have realised the sanguine expectations of its constructor. the occasions on which it could be usefully employed were found to be extremely rare. it was injuriously affected by every change of temperature. the great weight ( cwt.) of a speculum four feet in diameter rendered it peculiarly liable to distortion. with all imaginable care, the delicate lustre of its surface could not be preserved longer than two years,[ ] when the difficult process of repolishing had to be undertaken. it was accordingly never used after , when, having _gone blind_ from damp, it lapsed by degrees into the condition of a museum inmate. the exceedingly high magnifying powers employed by herschel constituted a novelty in optical astronomy, to which he attached great importance. the work of ordinary observation would, however, be hindered rather than helped by them. the attempt to increase in this manner the efficacy of the telescope is speedily checked by atmospheric, to say nothing of other difficulties. precisely in the same proportion as an object is magnified, the disturbances of the medium through which it is seen are magnified also. even on the clearest and most tranquil nights, the air is never for a moment really still. the rays of light traversing it are continually broken by minute fluctuations of refractive power caused by changes of temperature and pressure, and the currents which these engender. with such luminous quiverings and waverings the astronomer has always more or less to reckon; their absence is simply a question of degree; if sufficiently magnified, they are at all times capable of rendering observation impossible. thus, such powers as , , , , , , even , ,[ ] which herschel now and again applied to his great telescopes, must, save on the rarest occasions, prove an impediment rather than an aid to vision. they were, however, used by him only for special purposes, experimentally, not systematically, and with the clearest discrimination of their advantages and drawbacks. it is obvious that perfectly different ends are subserved by increasing the _aperture_ and by increasing the _power_ of a telescope. in the one case, a larger quantity of light is captured and concentrated; in the other, the same amount is distributed over a wider area. a diminution of brilliancy in the image accordingly attends, _coeteris paribus_, upon each augmentation of its apparent size. for this reason, such faint objects as nebulæ are most successfully observed with moderate powers applied to instruments of a great capacity for light, the details of their structure actually disappearing when highly magnified. with stellar groups the reverse is the case. stars cannot be magnified, simply because they are too remote to have any sensible dimensions; but the space between them can. it was thus for the purpose of dividing very close double stars that herschel increased to such an unprecedented extent the magnifying capabilities of his instruments; and to this improvement incidentally the discovery of uranus, march , ,[ ] was due. for by the examination with strong lenses of an object which, even with a power of , presented a suspicious appearance, he was able at once to pronounce its disc to be real, not merely "spurious," and so to distinguish it unerringly from the crowd of stars amidst which it was moving. while the reflecting telescope was astonishing the world by its rapid development in the hands of herschel, its unpretending rival was slowly making its way towards the position which the future had in store for it. the great obstacle which long stood in the way of the improvement of refractors was the defect known as "chromatic aberration." this is due to no other cause than that which produces the rainbow and the spectrum--the separation, or "dispersion" in their passage through a refracting medium, of the variously coloured rays composing a beam of white light. in an ordinary lens there is no common point of concentration; each colour has its own separate focus; and the resulting image, formed by the superposition of as many images as there are hues in the spectrum, is indefinitely terminated with a tinted border, eminently baffling to exactness of observation. the extravagantly long telescopes of the seventeenth century were designed to _avoid_ this evil (as well as another source of indistinct vision in the spherical shape of lenses); but no attempt to _remedy_ it was made until an essex gentleman succeeded, in , in so combining lenses of flint and crown glass as to produce refraction without colour.[ ] mr. chester more hall was, however, equally indifferent to fame and profit, and took no pains to make his invention public. the _effective_ discovery of the achromatic telescope was, accordingly, reserved for john dollond, whose method of correcting at the same time chromatic and spherical aberration was laid before the royal society in . modern astronomy may be said to have been thereby rendered possible. refractors have always been found better suited than reflectors to the ordinary work of observatories. they are, so to speak, of a more robust, as well as of a more plastic nature. they suffer less from vicissitudes of temperature and climate. they retain their efficiency with fewer precautions and under more trying circumstances. above all, they co-operate more readily with mechanical appliances, and lend themselves with far greater facility to purposes of exact measurement. a practical difficulty, however, impeded the realisation of the brilliant prospects held out by dollond's invention. it was found impossible to procure flint-glass, such as was needed for optical use--that is, of perfectly homogeneous quality--except in fragments of insignificant size. discs of more than two or three inches in diameter were of extreme rarity; and the crushing excise duty imposed upon the article by the financial unwisdom of the government, both limited its production, and, by rendering experiments too costly for repetition, barred its improvement. up to this time, great britain had left foreign competitors far behind in the instrumental department of astronomy. the quadrants and circles of bird, cary and ramsden were unapproached abroad. the reflecting telescope came into existence and reached maturity on british soil. the refracting telescope was cured of its inherent vices by british ingenuity. but with the opening of the nineteenth century, the almost unbroken monopoly of skill and contrivance which our countrymen had succeeded in establishing was invaded, and british workmen had to be content to exchange a position of supremacy for one of at least partial temporary inferiority. somewhat about the time that herschel set about polishing his first speculum, pierre louis guinand, a swiss artisan, living near chaux-de-fonds, in the canton of neuchâtel, began to grind spectacles for his own use, and was thence led on to the rude construction of telescopes by fixing lenses in pasteboard tubes. the sight of an england achromatic stirred a higher ambition, and he took the first opportunity of procuring some flint glass from england (then the only source of supply), with the design of imitating an instrument the full capabilities of which he was destined to be the humble means of developing. the english glass proving of inferior quality, he conceived the possibility, unaided and ignorant of the art as he was, of himself making better, and spent seven years ( - ) in fruitless experiments directed to that end. failure only stimulated him to enlarge their scale. he bought some land near les brenets, constructed upon it a furnace capable of melting two quintals of glass, and reducing himself and his family to the barest necessaries of life, he poured his earnings (he at this time made bells for repeaters) unstintingly into his crucibles.[ ] his undaunted resolution triumphed. in he carried to paris and there showed to lalande several discs of flawless crystal four to six inches in diameter. lalande advised him to keep his secret, but in he was induced to remove to munich, where he became the instructor of the immortal fraunhofer. his return to les brenets in was signalised by the discovery of an ingenious mode of removing striated portions of glass by breaking and re-soldering the product of each melting, and he eventually attained to the manufacture of perfect discs up to inches in diameter. an object-glass for which he had furnished the material to cauchoix, procured him, in , a royal invitation to settle in paris; but he was no longer equal to the change, and died at the scene of his labours, february following. this same lens ( inches across) was afterwards purchased by sir james south, and the first observation made with it, february , , disclosed to sir john herschel the sixth minute star in the central group of the orion nebula, known as the "trapezium."[ ] bequeathed by south to trinity college, dublin, it was employed at the dunsink observatory by brünnow and ball in their investigations of stellar parallax. a still larger objective (of nearly inches) made of guinand's glass was secured in paris, about the same time, by mr. edward cooper of markree castle, ireland. the peculiarity of the method discovered at les brenets resided in the manipulation, not in the quality of the ingredients; the secret, that is to say, was not chemical, but mechanical.[ ] it was communicated by henry guinand (a son of the inventor) to bontemps, one of the directors of the glassworks at choisy-le-roi, and by him transmitted to messrs. chance of birmingham, with whom he entered into partnership when the revolutionary troubles of obliged him to quit his native country. the celebrated american opticians, alvan clark & sons, derived from the birmingham firm the materials for some of their early telescopes, notably the -inch chicago and -inch washington equatoreals; but the discs for the great lick refractor, and others shaped by them in recent years, have been supplied by feil of paris. two distinguished amateurs, meanwhile, were preparing to reassert on behalf of reflecting instruments their claim to the place of honour in the van of astronomical discovery. of mr. lassell's specula something has already been said.[ ] they were composed of an alloy of copper and tin, with a minute proportion of arsenic (after the example of newton[ ]), and were remarkable for perfection of figure and brilliancy of surface. the capabilities of the newtonian plan were developed still more fully--it might almost be said to the uttermost--by the enterprise of an irish nobleman. william parsons, known as lord oxmantown until , when, on his father's death, he succeeded to the title of earl of rosse, was born at york, june , . his public duties began before his education was completed. he was returned to parliament as member for king's county while still an undergraduate at oxford, and continued to represent the same constituency for thirteen years ( - ). from until his death, which took place, october , , he sat, silent but assiduous, in the house of lords as an irish representative peer; he held the not unlaborious post of president of the royal society from to ; presided over the meeting of the british association at cork in , and was elected vice-chancellor of dublin university in . in addition to these extensive demands upon his time and thoughts, were those derived from his position as practically the feudal chief of a large body of tenantry in times of great and anxious responsibility, to say nothing of the more genial claims of an unstinted hospitality. yet, while neglecting no public or private duty, this model nobleman found leisure to render to science services so conspicuous as to entitle his name to a lasting place in its annals. he early formed the design of reaching the limits of the attainable in enlarging the powers of the telescope, and the qualities of his mind conspired with the circumstances of his fortune to render the design a feasible one. from refractors it was obvious that no such vast and rapid advance could be expected. english glass-manufacture was still in a backward state. so late as , simms (successor to the distinguished instrumentalist edward troughton) reported a specimen of crystal scarcely - / inches in diameter, and perfect only over six, to be unique in the history of english glass-making.[ ] yet at that time the fifteen-inch achromatic of pulkowa had already left the workshop of fraunhofer's successors at munich. it was not indeed until , when the impost which had so long hampered their efforts was removed, that the optical artists of these islands were able to compete on equal terms with their rivals on the continent. in the case of reflectors, however, there seemed no insurmountable obstacle to an almost unlimited increase of light-gathering capacity; and it was here, after some unproductive experiments with fluid lenses, that lord oxmantown concentrated his energies. he had to rely entirely on his own invention, and to earn his own experience. james short had solved the problem of giving to metallic surfaces a perfect parabolic figure (the only one by which parallel incident rays can be brought to an exact focus); but so jealous was he of his secret, that he caused all his tools to be burnt before his death;[ ] nor was anything known of the processes by which herschel had achieved his astonishing results. moreover, lord oxmantown had no skilled workmen to assist him. his implements, both animate and inanimate, had to be formed by himself. peasants taken from the plough were educated by him into efficient mechanics and engineers. the delicate and complex machinery needed in operations of such hairbreadth nicety as his enterprise involved, the steam-engine which was to set it in motion, at times the very crucibles in which his specula were cast, issued from his own workshops. in experiments on the composition of speculum-metal were set on foot, and the first polishing-machine ever driven by steam-power was contrived in . but twelve arduous years of struggle with recurring difficulties passed before success began to dawn. a material less tractable than the alloy selected, of four chemical equivalents of copper to one of tin,[ ] can scarcely be conceived. it is harder than steel, yet brittle as glass, crumbling into fragments with the slightest inadvertence of handling or treatment;[ ] and the precision of figure requisite to secure good definition is almost beyond the power of language to convey. the quantities involved are so small as not alone to elude sight, but to confound imagination. sir john herschel tells us that "the _total_ thickness to be abraded from the edge of a spherical speculum inches in diameter and feet focus, to convert it into a paraboloid, is only / of an inch;"[ ] yet upon this minute difference of form depends the clearness of the image, and, as a consequence, the entire efficiency of the instrument. "almost infinite," indeed (in the phrase of the late dr. robinson), must be the exactitude of the operation adapted to bring about so delicate a result. at length, in , two specula, each three feet in diameter, were turned out in such perfection as to prompt a still bolder experiment. the various processes needed to insure success were now ascertained and under control; all that was necessary was to repeat them on a larger scale. a gigantic mirror, six feet across and fifty-four in focal length, was accordingly cast on the th of april, ; in two months it was ground down to figure by abrasion with emery and water, and daintily polished with rouge; and by the month of february, , the "leviathan of parsonstown" was available for the examination of the heavens. the suitable mounting of this vast machine was a problem scarcely less difficult than its construction. the shape of a speculum needs to be maintained with an elaborate care equal to that used in imparting it. in fact, one of the most formidable obstacles to increasing the size of such reflecting surfaces consists in their liability to bend under their own weight. that of the great rosse speculum was no less than four tons. yet, although six inches in thickness, and composed of a material only a degree inferior in rigidity to wrought iron, the strong pressure of a man's hand at its back produced sufficient flexure to distort perceptibly the image of a star reflected in it.[ ] thus the delicacy of its form was perishable equally by the stress of its own gravity, and by the slightest irregularity in the means taken to counteract that stress. the problem of affording a perfectly equable support in all possible positions was solved by resting the speculum upon twenty-seven platforms of cast iron, felt-covered, and carefully fitted to the shape of the areas they were to carry, which platforms were themselves borne by a complex system of triangles and levers, ingeniously adapted to distribute the weight with complete uniformity.[ ] a tube which resembled, when erect, one of the ancient round towers of ireland,[ ] served as the habitation of the great mirror. it was constructed of deal staves bound together with iron hoops, was fifty-eight feet long (including the speculum-box), and seven in diameter. a reasonably tall man may walk through it (as dean peacock once did) with umbrella uplifted. two piers of solid masonry, about fifty feet high, seventy long, and twenty-three apart, flanked the huge engine on either side. its lower extremity rested on a universal joint of cast iron; above, it was slung in chains, and even in a gale of wind remained perfectly steady. the weight of the entire, although amounting to fifteen tons, was so skilfully counterpoised, that the tube could with ease be raised or depressed by two men working a windlass. its horizontal range was limited by the lofty walls erected for its support to about ten degrees on each side of the meridian; but it moved vertically from near the horizon through the zenith as far as the pole. its construction was of the newtonian kind, the observer looking into the side of the tube near its upper end, which a series of galleries and sliding stages enabled him to reach in any position. it has also, though rarely, been used without a second mirror, as a "herschelian" reflector. the splendour of the celestial objects as viewed with this vast "light-grasper" surpassed all expectation. "never in my life," exclaimed sir james south, "did i see such glorious sidereal pictures."[ ] the orb of jupiter produced an effect compared to that of the introduction of a coach-lamp into the telescope;[ ] and certain star-clusters exhibited an appearance (we again quote sir james south) "such as man before had never seen, and which for its magnificence baffles all description." but it was in the examination of the nebulæ that the superiority of the new instrument was most strikingly displayed. a large number of these misty objects, which the utmost powers of herschel's specula had failed to resolve into stars, yielded at once to the parsonstown reflector; while many others showed under entirely changed forms through the disclosure of previously unseen details of structure. one extremely curious result of the increase of light was the abolition of any sharp distinction between the two classes of "annular" and "planetary" nebulæ. up to that time, only four ring-shaped systems--two in the northern and two in the southern hemisphere--were known to astronomers; they were now reinforced by five of the planetary kind, the discs of which were observed to be centrally perforated; while the definite margins visible in weaker instruments were replaced by ragged edges or filamentous fringes. still more striking was the discovery of an entirely new and most remarkable species of nebulæ. these were termed "spiral," from the more or less regular convolutions, resembling the whorls of a shell, in which the matter composing them appeared to be distributed. the first and most conspicuous specimen of this class was met with in april, ; it is situated in canes venatici, close to the tail of the great bear, and wore, in sir j. herschel's instruments, the aspect of a split ring encompassing a bright nucleus, thus presenting, as he supposed, a complete analogue to the system of the milky way. in the rosse mirror it shone out as a vast whirlpool of light--a stupendous witness to the presence of cosmical activities on the grandest scale, yet regulated by laws as to the nature of which we are profoundly ignorant. professor stephen alexander of new jersey, however, concluded, from an investigation (necessarily founded on highly precarious data) of the mechanical condition of these extraordinary agglomerations, that we see in them "the partially scattered fragments of enormous masses once rotating in a state of dynamical equilibrium." he further suggested "that the separation of these fragments may still be in progress,"[ ] and traced back their origin to the disruption, through its own continually accelerated rotation, of a "primitive spheroid" of inconceivably vast dimensions. such also, it was added (the curvilinear form of certain outliers of the milky way giving evidence of a spiral structure), is probably the history of our own cluster; the stars composing which, no longer held together in a delicately adjusted system like that of the sun and planets, are advancing through a period of seeming confusion towards an appointed goal of higher order and more perfect and harmonious adaptation.[ ] the class of spiral nebulæ included, in , fourteen members, besides several in which the characteristic arrangement seemed partial or dubious.[ ] a tendency in the exterior stars of other clusters to gather into curved branches (as in our galaxy) was likewise noted; and the existence of unsuspected analogies was proclaimed by the significant combination in the "owl" nebula (a large planetary in ursa major)[ ] of the twisted forms of a spiral with the perforated effect distinctive of an annular nebula. once more, by the achievements of the parsonstown reflector, the supposition of a "shining fluid" filling vast regions of space was brought into (as it has since proved) undeserved discredit. although lord rosse himself rejected the inference, that because many nebulæ had been resolved, all were resolvable, very few imitated his truly scientific caution; and the results of bond's investigations[ ] with the harvard college refractor quickened and strengthened the current of prevalent opinion. it is now certain that the evidence furnished on both sides of the atlantic as to the stellar composition of some conspicuous objects of this class (notably the orion and "dumb-bell" nebulæ) was delusive; but the spectroscope alone was capable of meeting it with a categorical denial. meanwhile there seemed good ground for the persuasion, which now, for the last time, gained the upper hand, that nebulæ are, without exception, true "island-universes," or assemblages of distant suns. lord rosse's telescope possesses a nominal power of , --that is, it shows the moon as if viewed with the naked eye at a distance of forty miles. but this seeming advantage is neutralised by the weakening of the available light through excessive diffusion, as well as by the troubles of the surging sea of air through which the observation must necessarily be made. professor newcomb, in fact, doubts whether with _any_ telescope our satellite has ever been seen to such advantage as it would be if brought within miles of the unarmed eye.[ ] the french opticians' rule of doubling the number of millimetres contained in the aperture of an instrument to find the highest magnifying power usually applicable to it, would give , as the maximum for the leviathan of birr castle; but in a climate like that of ireland the occasions must be rare when even that limit can be reached. indeed, the experience acquired by its use plainly shows that atmospheric rather than mechanical difficulties impede a still further increase of telescopic power. its construction may accordingly be said to mark the _ne plus ultra_ of effort in one direction, and the beginning of its conversion towards another. it became thenceforward more and more obvious that the conditions of observation must be ameliorated before any added efficacy could be given to it. the full effect of an uncertain climate in nullifying optical improvements was recognised, and the attention of astronomers began to be turned towards the advantages offered by more tranquil and more translucent skies. scarcely less important for the practical uses of astronomy than the optical qualities of the telescope is the manner of its mounting. the most admirable performance of the optician can render but unsatisfactory service if its mechanical accessories are ill-arranged or inconvenient. thus the astronomer is ultimately dependent upon the mechanician; and so excellently have his needs been served, that the history of the ingenious contrivances by which discoveries have been prepared would supply a subject (here barely glanced at) not far inferior in extent and instruction to the history of those discoveries themselves. there are two chief modes of using the telescope, to which all others may be considered subordinate.[ ] either it may be invariably directed towards the south, with no motion save in the plane of the meridian, so as to intercept the heavenly bodies at the moment of transit across that plain; or it may be arranged so as to follow the daily revolution of the sky, thus keeping the object viewed permanently in sight instead of simply noting the instant of its flitting across the telescopic field. the first plan is that of the "transit instrument," the second that of the "equatoreal." both were, by a remarkable coincidence, introduced about [ ] by olaus römer, the brilliant danish astronomer who first measured the velocity of light. the uses of each are entirely different. with the transit, the really fundamental task of astronomy--the determination of the movements of the heavenly bodies--is mainly accomplished; while the investigation of their nature and peculiarities is best conducted with the equatoreal. one is the instrument of mathematical, the other of descriptive astronomy. one furnishes the materials with which theories are constructed and the tests by which they are corrected; the other registers new facts, takes note of new appearances, sounds the depths and peers into every nook of the heavens. the great improvement of giving to a telescope equatoreally mounted an automatic movement by connecting it with clockwork, was proposed in by robert hooke. bradley in actually observed mars with a telescope "moved by a machine that made it keep pace with the stars;"[ ] and von zach relates[ ] that he had once followed sirius for twelve hours with a "heliostat" of ramsden's construction. but these eighteenth-century attempts were of no practical effect. movement by clockwork was virtually a complete novelty when it was adopted by fraunhofer in to the dorpat refractor. by simply giving to an axis unvaryingly directed towards the celestial pole an equable rotation with a period of twenty-four hours, a telescope attached to it, and pointed in _any_ direction, will trace out on the sky a parallel of declination, thus necessarily accompanying the movement of any star upon which it may be fixed. it accordingly forms part of the large sum of fraunhofer's merits to have secured this inestimable advantage to observers. sir john herschel considered that lassell's application of equatoreal mounting to a nine-inch newtonian in made an epoch in the history of "that eminently british instrument, the reflecting telescope."[ ] nearly a century earlier,[ ] it is true, short had fitted one of his gregorians to a complicated system of circles in such a manner that, by moving a handle, it could be made to follow the revolution of the sky; but the arrangement did not obtain, nor did it deserve, general adoption. lassell's plan was a totally different one; he employed the crossed axes of the true equatoreal, and his success removed, to a great extent, the fatal objection of inconvenience in use, until then unanswerably urged against reflectors. the very largest of these can now be mounted equatoreally; even the rosse, within its limited range, has been for some years provided with a movement by clockwork along declination-parallels. the art of accurately dividing circular arcs into the minute equal parts which serve as the units of astronomical measurement, remained, during the whole of the eighteenth century, almost exclusively in english hands. it was brought to a high degree of perfection by graham, bird and ramsden, all of whom, however, gave the preference to the old-fashioned mural quadrant and zenith-sector over the entire circle, which römer had already found the advantage of employing. the five-foot vertical circle, which piazzi with some difficulty induced ramsden to complete for him in , was the first divided instrument constructed in what may be called the modern style. it was provided with magnifiers for reading off the divisions (one of the neglected improvements of römer), and was set up above a smaller horizontal circle, forming an "altitude and azimuth" combination (again römer's invention), by which both the elevation of a celestial object above the horizon and its position as referred to the horizon could be measured. in the same year, borda invented the "repeating circle" (the principle of which had been suggested by tobias mayer in [ ]), a device for exterminating, so far as possible, errors of graduation by _repeating_ an observation with different parts of the limb. this was perhaps the earliest systematic effort to correct the imperfections of instruments by the manner of their use. the manufacture of astronomical circles was brought to a very refined state of excellence early in the nineteenth century by reichenbach at munich, and after by repsold at hamburg. bessel states[ ] that the "reading-off" on an instrument of the kind by the latter artist was accurate to about / th of a human hair. meanwhile the traditional reputation of the english school was fully sustained; and sir george airy did not hesitate to express his opinion that the new method of graduating circles, published by troughton in ,[ ] was the "greatest improvement ever made in the art of instrument-making."[ ] but a more secure road to improvement than that of mere mechanical exactness was pointed out by bessel. his introduction of a regular theory of instrumental errors might almost be said to have created a new art of observation. every instrument, he declared in memorable words,[ ] must be twice made--once by the artist, and again by the observer. knowledge is power. defects that are ascertained and can be allowed for are as good as non-existent. thus the truism that the best instrument is worthless in the hands of a careless or clumsy observer, became supplemented by the converse maxim, that defective appliances may, through skilful use, be made to yield valuable results. the königsberg observations--of which the first instalment was published in --set the example of regular "reduction" for instrumental errors. since then, it has become an elementary part of an astronomer's duty to study the _idiosyncrasy_ of each one of the mechanical contrivances at his disposal, in order that its inevitable, but now certified deviations from ideal accuracy may be included amongst the numerous corrections by which the pure essence of even approximate truth is distilled from the rude impressions of sense. nor is this enough; for the casual circumstances attending each observation have to be taken into account with no less care than the inherent or _constitutional_ peculiarities of the instrument with which it is made. there is no "once for all" in astronomy. vigilance can never sleep; patience can never tire. variable as well as constant sources of error must be anxiously heeded; one infinitesimal inaccuracy must be weighed against another; all the forces and vicissitudes of nature--frosts, dews, winds, the interchanges of heat, the disturbing effects of gravity, the shiverings of the air, the tremors of the earth, the weight and vital warmth of the observer's own body, nay, the rate at which his brain receives and transmits its impressions, must all enter into his calculations, and be sifted out from his results. it was in that bessel drew attention to discrepancies in the times of transits given by different astronomers.[ ] the quantities involved were far from insignificant. he was himself nearly a second in advance of all his contemporaries, argelander lagging behind him as much as a second and a quarter. each individual, in fact, was found to have a certain definite _rate of perception_, which, under the name of "personal equation," now forms so important an element in the correction of observations that a special instrument for accurately determining its amount in each case is in actual use at greenwich. such are the refinements upon which modern astronomy depends for its progress. it is a science of hairbreadths and fractions of a second. it exists only by the rigid enforcement of arduous accuracy and unwearying diligence. whatever secrets the universe still has in store for man will only be communicated on these terms. they are, it must be acknowledged, difficult to comply with. they involve an unceasing struggle against the infirmities of his nature and the instabilities of his position. but the end is not unworthy the sacrifices demanded. one additional ray of light thrown on the marvels of creation--a single, minutest encroachment upon the strongholds of ignorance--is recompense enough for a lifetime of toil. or rather, the toil is its own reward, if pursued in the lofty spirit which alone becomes it. for it leads through the abysses of space and the unending vistas of time to the very threshold of that infinity and eternity of which the disclosure is reserved for a life to come. footnotes: [footnote : grant, _hist. astr._, p. .] [footnote : _optica promota_, p. .] [footnote : _phil. trans._, vol. xxxii., p. .] [footnote : _ibid._, vol. xc., p. .] [footnote : cassegrain, a frenchman, substituted in a _convex_ for a _concave_ secondary speculum. the tube was thereby enabled to be shortened by twice the focal length of the mirror in question. the great melbourne reflector (four feet aperture, by grubb) is constructed upon this plan.] [footnote : _phil. trans._, vol. civ., p. , _note_.] [footnote : _phil. trans._, vol. xc., p. . with the forty-foot, however, only very moderate powers seemed to have been employed, whence dr. robinson argued a deficiency of defining power. _proc. roy. irish ac._, vol. ii., p. .] [footnote : _phil. trans._, vol. lxxi., p. .] [footnote : it is remarkable that, as early as , the possibility of an achromatic combination was inferred by david gregory from the structure of the human eye. see his _catoptricæ et dioptricæ sphericæ elementa_, p. .] [footnote : wolf, _biographien_, bd. ii., p. .] [footnote : _month. not._, vol. i., p. . _note_.] [footnote : henrivaux, _encyclopédie chimique_, t. v., fasc. , p. .] [footnote : see _ante_, p. .] [footnote : _phil. trans._, vol. vii., p. .] [footnote : j. herschel, _the telescope_, p. .] [footnote : _month. not._, vol. xxix., p. .] [footnote : a slight excess of copper renders the metal easier to work, but liable to tarnish. robinson, _proc. roy. irish ac._, vol. ii., p. .] [footnote : _brit. ass._, , dr. robinson's closing address. _athenæum_, sept. , p. .] [footnote : _the telescope_, p. .] [footnote : lord rosse in _phil. trans._, vol. cxl., p. .] [footnote : this method is the same in principle with that applied by grubb in to a -inch speculum for the observatory of armagh. _phil. trans._, vol. clix., p. .] [footnote : robinson, _proc. roy. ir. ac._, vol. iii., p. .] [footnote : _astr. nach._, no. .] [footnote : airy, _month. not._, vol. ix., p. .] [footnote : _astronomical journal_ (gould's), vol. ii., p. .] [footnote : _ibid._, p. .] [footnote : lord rosse in _phil. trans._, vol. cxl., p. .] [footnote : no. of herschel's ( ) catalogue. before a star was visible in each of the two larger openings by which it is pierced; since then, one only. webb, _celestial objects_ ( th ed.), p. .] [footnote : _mem. am. ac._, vol. iii., p. ; _astr. nach._, no. .] [footnote : _pop. astr._, p. .] [footnote : this statement must be taken in the most general sense. supplementary observations of great value are now made at greenwich with the altitude and azimuth instrument, which likewise served piazzi to determine the places of his stars; while a "prime vertical instrument" is prominent at pulkowa.] [footnote : as early as , according to r. wolf (_ges. der astr._, p. ), father scheiner made the experiment of connecting a telescope with an axis directed to the pole, while chinese "equatoreal armillæ," dating from the thirteenth century, existed at pekin until , when they were carried off as "loot" to berlin. j. l. e. dreyer, _copernicus_, vol. i., p. .] [footnote : _miscellaneous works_, p. .] [footnote : _astr. jahrbuch_, (published ), p. .] [footnote : _month. not._, vol. xli., p. .] [footnote : _phil. trans._, vol. xlvi., p. .] [footnote : grant, _hist. of astr._, p. .] [footnote : _pop. vorl._, p. .] [footnote : _phil. trans._, vol. xcix., p. .] [footnote : _report brit. ass._, , p. .] [footnote : _pop. vorl._, p. .] [footnote : c. t. anger, _grundzüge der neucren astronomischen beobachtungs-kunst_, p. .] part ii recent progress of astronomy chapter i _foundation of astronomical physics_ in the year , heinrich schwabe of dessau, elated with the hope of speedily delivering himself from the hereditary incubus of an apothecary's shop,[ ] obtained from munich a small telescope and began to observe the sun. his choice of an object for his researches was instigated by his friend harding of göttingen. it was a peculiarly happy one. the changes visible in the solar surface were then generally regarded as no less capricious than the changes in the skies of our temperate regions. consequently, the reckoning and registering of sun-spots was a task hardly more inviting to an astronomer than the reckoning and registering of summer clouds. cassini, keill, lemonnier, lalande, were unanimous in declaring that no trace of regularity could be detected in their appearances or effacements.[ ] delambre pronounced them "more curious than really useful."[ ] even herschel, profoundly as he studied them, and intimately as he was convinced of their importance as symptoms of solar activity, saw no reason to suspect that their abundance and scarcity were subject to orderly alternation. one man alone in the eighteenth century, christian horrebow of copenhagen, divined their periodical character, and foresaw the time when the effects of the sun's vicissitudes upon the globes revolving round him might be investigated with success; but this prophetic utterance was of the nature of a soliloquy rather than of a communication, and remained hidden away in an unpublished journal until , when it was brought to light in a general ransacking of archives.[ ] indeed, schwabe himself was far from anticipating the discovery which fell to his share. he compared his fortune to that of saul, who, seeking his father's asses, found a kingdom.[ ] for the hope which inspired his early resolution lay in quite another direction. his patient ambush was laid for a possible intramercurial planet, which, he thought, must sooner or later betray its existence in crossing the face of the sun. he took, however, the most effectual measures to secure whatever new knowledge might be accessible. during forty-three years his "imperturbable telescope"[ ] never failed, weather and health permitting, to bring in its daily report as to how many, or if any, spots were visible on the sun's disc, the information obtained being day by day recorded on a simple and unvarying system. in he made his first announcement of a probable decennial period,[ ] but it met with no general attention; although julius schmidt of bonn (afterwards director of the athens observatory) and gautier of geneva were impressed with his figures, and littrow had himself, in ,[ ] hinted at the likelihood of some kind of regular recurrence. schwabe, however, worked on, gathering each year fresh evidence of a law such as he had indicated; and when humboldt published in , in the third volume of his _kosmos_,[ ] a table of the sun-spot statistics collected by him from downwards, the strength of his case was perceived with, so to speak, a start of surprise; the reality and importance of the discovery were simultaneously recognised, and the persevering hofrath of dessau found himself famous among astronomers. his merit--recognised by the bestowal of the astronomical society's gold medal in --consisted in his choice of an original and appropriate line of work, and in the admirable tenacity of purpose with which he pursued it. his resources and acquirements were those of an ordinary amateur; he was distinguished solely by the unfortunately rare power of turning both to the best account. he died where he was born and had lived, april , , at the ripe age of eighty-six. meanwhile an investigation of a totally different character, and conducted by totally different means, had been prosecuted to a very similar conclusion. two years after schwabe began his solitary observations, humboldt gave the first impulse, at the scientific congress of berlin in , to a great international movement for attacking simultaneously, in various parts of the globe, the complex problem of terrestrial magnetism. through the genius and energy of gauss, göttingen became its centre. thence new apparatus, and a new system for its employment, issued; there, in , the first regular magnetic observatory was founded, whilst at göttingen was fixed the universal time-standard for magnetic observations. a letter addressed by humboldt in april, , to the duke of sussex as president of the royal society, enlisted the co-operation of england. a network of magnetic stations was spread all over the british dominions, from canada to van diemen's land; measures were concerted with foreign authorities, and an expedition was fitted out, under the able command of captain (afterwards sir james) clark ross, for the special purpose of bringing intelligence on the subject from the dismal neighbourhood of the south pole. in , the elaborate organisation created by the disinterested efforts of scientific "agitators" was complete; gauss's "magnetometers" were vibrating under the view of attentive observers in five continents, and simultaneous results began to be recorded. ten years later, in september, , dr. john lamont, the scotch director of the munich observatory, in reviewing the magnetic observations made at göttingen and munich from to , perceived with some surprise that they gave unmistakable indications of a period which he estimated at - / years.[ ] the manner in which this periodicity manifested itself requires a word of explanation. the observations in question referred to what is called the "declination" of the magnetic needle--that is, to the position assumed by it with reference to the points of the compass when moving freely in a horizontal plane. now this position--as was discovered by graham in --is subject to a small daily fluctuation, attaining its maximum towards the east about a.m., and its maximum towards the west shortly before p.m. in other words, the direction of the needle approaches (in these countries at the present time) nearest to the true north some four hours before noon, and departs farthest from it between one and two hours after noon. it was the _range_ of this daily variation that lamont found to increase and diminish once in every - / years. in the following winter, sir edward sabine, ignorant as yet of lamont's conclusion, undertook to examine a totally different set of observations. the materials in his hands had been collected at the british colonial stations of toronto and hobarton from to , and had reference, not to the regular diurnal swing of the needle, but to those curious spasmodic vibrations, the inquiry into the laws of which was the primary object of the vast organisation set on foot by humboldt and gauss. yet the upshot was practically the same. once in about ten years, magnetic disturbances (termed by humboldt "storms") were perceived to reach a maximum of violence and frequency. sabine was the first to note the coincidence between this unlooked-for result and schwabe's sun-spot period. he showed that, so far as observation had yet gone, the two cycles of change agreed perfectly both in duration and phase, maximum corresponding to maximum, minimum to minimum. what the nature of the connection could be that bound together by a common law effects so dissimilar as the rents in the luminous garment of the sun, and the swayings to and fro of the magnetic needle, was and still remains beyond the reach of well-founded theory; but the fact was from the first undeniable. the memoir containing this remarkable disclosure was presented to the royal society, march , and read may , .[ ] on the st of july following, rudolf wolf at berne,[ ] and on the th of august, alfred gautier at sion,[ ] announced, separately and independently, perfectly similar conclusions. this triple event is perhaps the most striking instance of the successful employment of the baconian method of co-operation in discovery, by which "particulars" are amassed by one set of investigators--corresponding to the "depredators" and "inoculators" of solomon's house--while inductions are drawn from them by another and a higher class--the "interpreters of nature." yet even here the convergence of two distinct lines of research was wholly fortuitous, and skilful combination owed the most brilliant part of its success to the unsought bounty of what we call fortune. the exactness of the coincidence thus brought to light was fully confirmed by further inquiries. a diligent search through the scattered records of sun-spot observations, from the time of galileo and scheiner onwards, put wolf[ ] in possession of materials by which he was enabled to correct schwabe's loosely-indicated decennial period to one of slightly over eleven ( . ) years; and he further showed that this fell in with the ebb and flow of magnetic change even better than lamont's - / year cycle. the analogy was also pointed out between the "light-curve," or zig-zagged line representing on paper the varying intensity in the lustre of certain stars, and the similar delineation of spot-frequency; the ascent from minimum to maximum being, in both cases, usually steeper than the descent from maximum to minimum; while an additional point of resemblance was furnished by the irregularities in height of the various maxima. in other words, both the number of spots on the sun and the brightness of variable stars increase, as a rule, more rapidly than they decrease; nor does the amount of that increase, in either instance, show any approach to uniformity. the endeavour, suggested by the very nature of the phenomenon, to connect sun-spots with weather was less successful. the first attempt of the kind was made by sir william herschel in , and a very notable one it was. meteorological statistics, save of the scantiest and most casual kind, did not then exist; but the price of corn from year to year was on record, and this, with full recognition of its inadequacy, he adopted as his criterion. nor was he much better off for information respecting the solar condition. what little he could obtain, however, served, as he believed, to confirm his surmise that a copious emission of light and heat accompanies an abundant formation of "openings" in the dazzling substance whence our supply of those indispensable commodities is derived.[ ] he gathered, in short, from his inquiries very much what he had expected to gather, namely, that the price of wheat was high when the sun showed an unsullied surface, and that food and spots became plentiful together.[ ] yet this plausible inference was scarcely borne out by a more exact collocation of facts. schwabe failed to detect any reflection of the sun-spot period in his meteorological register. gautier[ ] reached a provisional conclusion the reverse--though not markedly the reverse--of herschel's. wolf, in , derived from an examination of vogel's collection of zürich chronicles ( - a.d.) evidence showing (as he thought) that minimum years were usually wet and stormy, maximum years dry and genial;[ ] but a subsequent review of the subject in convinced him that no relation of any kind between the two kinds of effects was traceable.[ ] with the singular affection of our atmosphere known as the aurora borealis (more properly aurora polaris) the case was different. here the zürich chronicles set wolf on the right track in leading him to associate such luminous manifestations with a disturbed condition of the sun; since subsequent detailed observation has exhibited the curve of auroral frequency as following with such fidelity the jagged lines figuring to the eye the fluctuations of solar and magnetic activity, as to leave no reasonable doubt that all three rise and sink together under the influence of a common cause. as long ago as ,[ ] halley had conjectured that the northern lights were due to magnetic "effluvia," but there was no evidence on the subject forthcoming until hiorter observed at upsala in their agitating influence upon the magnetic needle. that the effect was no casual one was made superabundantly clear by arago's researches in and subsequent years. now both were perceived to be swayed by the same obscure power of cosmical disturbance. the sun is not the only one of the heavenly bodies by which the magnetism of the earth is affected. proofs of a similar kind of lunar action were laid by kreil in before the bohemian society of sciences, and with minor corrections were fully substantiated by sabine's more extended researches. it was thus ascertained that each lunar day, or the interval of twenty-four hours and about fifty-four minutes between two successive meridian passages of our satellite, is marked by a perceptible, though very small, double oscillation of the needle--two progressive movements from east to west, and two returns from west to east.[ ] moreover, the lunar, like the solar influence (as was proved in each case by sabine's analysis of the hobarton and toronto observations), extends to all three "magnetic elements," affecting not only the position of the horizontal or _declination_ needle, but also the dip and intensity. it seems not unreasonable to attribute some portion of the same subtle power to the planets and even to the stars, though with effects rendered imperceptible by distance. we have now to speak of the discovery and application to the heavenly bodies of a totally new method of investigation. spectrum analysis may be shortly described as a mode of distinguishing the various species of matter by the kind of light proceeding from each. this definition at once explains how it is that, unlike every other system of chemical analysis, it has proved available in astronomy. light, so far as _quality_ is concerned, ignores distance. no intrinsic change, that we yet know of, is produced in it by a journey from the farthest bounds of the visible universe; so that, provided only that in _quantity_ it remain sufficient for the purpose, its peculiarities can be equally well studied whether the source of its vibrations be one foot or a hundred billion miles distant. now the most obvious distinction between one kind of light and another resides in colour. but of this distinction the eye takes cognisance in an æsthetic, not in a scientific sense. it finds gladness in the "thousand tints" of nature, but can neither analyse nor define them. here the refracting prism--or the combination of prisms known as the "spectroscope"--comes to its aid, teaching it to measure as well as to perceive. it furnishes, in a word, an accurate scale of colour. the various rays which, entering the eye together in a confused crowd, produce a compound impression made up of undistinguishable elements, are, by the mere passage through a triangular piece of glass, separated one from the other, and ranged side by side in orderly succession, so that it becomes possible to tell at a glance what kinds of light are present, and what absent. thus, if we could only be assured that the various chemical substances when made to glow by heat, emit characteristic rays--rays, that is, occupying a place in the spectrum reserved for them, and for them _only_--we should at once be in possession of a mode of identifying such substances with the utmost readiness and certainty. this assurance, which forms the solid basis of spectrum analysis, was obtained slowly and with difficulty. the first to employ the prism in the examination of various flames (for it is only in a state of vapour that matter emits distinctive light) was a young scotchman named thomas melvill, who died in , at the age of twenty-seven. he studied the spectrum of burning spirits, into which were successively introduced sal ammoniac, potash, alum, nitre, and sea-salt, and observed the singular predominance, under almost all circumstances, of a particular shade of yellow light, perfectly definite in its degree of refrangibility[ ]--in other words, taking up a perfectly definite position in the spectrum. his experiments were repeated by morgan,[ ] wollaston, and--with far superior precision and diligence--by fraunhofer.[ ] the great munich optician, whose work was completely original, rediscovered melvill's deep yellow ray and measured its place in the colour-scale. it has since become well known as the "sodium line," and has played a very important part in the history of spectrum analysis. nevertheless, its ubiquity and conspicuousness long impeded progress. it was elicited by the combustion of a surprising variety of substances--sulphur, alcohol, ivory, wood, paper; its persistent visibility suggesting the accomplishment of some universal process of nature rather than the presence of one individual kind of matter. but if spectrum analysis were to exist as a science at all, it could only be by attaining certainty as to the unvarying association of one special substance with each special quality of light. thus perplexed, fox talbot[ ] hesitated in to enounce this fundamental principle. he was inclined to believe that the presence in the spectrum of any individual ray told unerringly of the volatilisation in the flame under scrutiny of some body as whose badge or distinctive symbol that ray might be regarded; but the continual prominence of the yellow beam staggered him. it appeared, indeed, without fail where sodium _was_; but it also appeared where it might be thought only reasonable to conclude that sodium _was not_. nor was it until thirty years later that william swan,[ ] by pointing out the extreme delicacy of the spectral test, and the singularly wide dispersion of sodium, made it appear probable (but even then only probable) that the questionable yellow line was really due invariably to that substance. common salt (chloride of sodium) is, in fact, the most diffusive of solids. it floats in the air; it flows with water; every grain of dust has its attendant particle; its absolute exclusion approaches the impossible. and withal, the light that it gives in burning is so intense and concentrated, that if a single grain be divided into million parts, and one alone of such inconceivably minute fragments be present in a source of light, the spectroscope will show unmistakably its characteristic beam. amongst the pioneers of knowledge in this direction were sir john herschel[ ]--who, however, applied himself to the subject in the interests of optics, not of chemistry--w. a. miller,[ ] and wheatstone. the last especially made a notable advance when, in the course of his studies on the "prismatic decomposition" of the electric light, he reached the significant conclusion that the rays visible in its spectrum were different for each kind of metal employed as "electrodes."[ ] thus indications of a wider principle were to be found in several quarters, but no positive certainty on any single point was obtained, until, in , gustav kirchhoff, professor of physics in the university of heidelberg, and his colleague, the eminent chemist robert bunsen, took the matter in hand. by them the general question as to the necessary and invariable connection of certain rays in the spectrum with certain kinds of matter, was first resolutely confronted, and first definitely answered. it was answered affirmatively--else there could have been no science of spectrum analysis--as the result of experiments more numerous, more stringent, and more precise than had previously been undertaken.[ ] and the assurance of their conclusion was rendered doubly sure by the discovery, through the peculiarities of their light alone, of two new metals, named from the blue and red rays by which they were respectively distinguished, "cæsium," and "rubidium."[ ] both were immediately afterwards actually obtained in small quantities by evaporation of the durckheim mineral waters. the link connecting this important result with astronomy may now be indicated. in the year it occurred to william hyde wollaston to substitute for the round hole used by newton and his successors for the admittance of light to be examined with the prism, an elongated "crevice" / th of an inch in width. he thereupon perceived that the spectrum, thus formed of light, as it were, _purified_ by the abolition of overlapping images, was traversed by seven dark lines. these he took to be natural boundaries of the various colours,[ ] and satisfied with this quasi-explanation, allowed the subject to drop. it was independently taken up after twelve years by a man of higher genius. in the course of experiments on light, directed towards the perfecting of his achromatic lenses, fraunhofer, by means of a slit and a telescope, made the surprising discovery that the solar spectrum is crossed, not by seven, but by thousands of obscure transverse streaks.[ ] of these he counted some , and carefully mapped , while a few of the most conspicuous he set up (if we may be permitted the expression) as landmarks, measuring their distances apart with a theodolite, and affixing to them the letters of the alphabet, by which they are still universally known. nor did he stop here. the same system of examination applied to the rest of the heavenly bodies showed the mild effulgence of the moon and planets to be deficient in precisely the same rays as sunlight; while in the stars it disclosed the differences in likeness which are always an earnest of increased knowledge. the spectra of sirius and castor, instead of being delicately ruled crosswise throughout, like that of the sun, were seen to be interrupted by three massive bars of darkness--two in the blue and one in the green;[ ] the light of pollux, on the other hand, seemed precisely similar to sunlight attenuated by distance or reflection, and that of capella, betelgeux, and procyon to share some of its peculiarities. one solar line especially--that marked in his map with the letter d--proved common to all the four last-mentioned stars; and it was remarkable that it exactly coincided in position with the conspicuous yellow beam (afterwards, as we have said, identified with the light of glowing sodium) which he had already found to accompany most kinds of combustion. moreover, both the _dark_ solar and the _bright_ terrestrial "d lines" were displayed by the refined munich appliances as double. in this striking correspondence, discovered by fraunhofer in , was contained the very essence of solar chemistry; but its true significance did not become apparent until long afterwards. fraunhofer was by profession, not a physicist, but a practical optician. time pressed; he could not and would not deviate from his appointed track; all that was possible to him was to indicate the road to discovery, and exhort others to follow it.[ ] partially and inconclusively at first this was done. the "fixed lines" (as they were called) of the solar spectrum took up the position of a standing problem, to the solution of which no approach seemed possible. conjectures as to their origin were indeed rife. an explanation put forward by zantedeschi[ ] and others, and dubiously favoured by sir david brewster and dr. j. h. gladstone,[ ] was that they resulted from "interference"--that is, a destruction of the motion producing in our eyes the sensation of light, by the superposition of two light-waves in such a manner that the crests of one exactly fill up the hollows of the other. this effect was supposed to be brought about by imperfections in the optical apparatus employed. a more plausible view was that the atmosphere of the earth was the agent by which sunlight was deprived of its missing beams. for a few of them this is actually the case. brewster found in that certain dark lines, which were invisible when the sun stood high in the heavens, became increasingly conspicuous as he approached the horizon.[ ] these are the well-known "atmospheric lines;" but the immense majority of their companions in the spectrum remain quite unaffected by the thickness of the stratum of air traversed by the sunlight containing them. they are then obviously due to another cause. there remained the true interpretation--absorption in the _sun's_ atmosphere; and this, too, was extensively canvassed. but a remarkable observation made by professor forbes of edinburgh[ ] on the occasion of the annular eclipse of may , , appeared to throw discredit upon it. if the problematical dark lines were really occasioned by the stoppage of certain rays through the action of a vaporous envelope surrounding the sun, they ought, it seemed, to be strongest in light proceeding from his edges, which, cutting that envelope obliquely, passed through a much greater depth of it. but the circle of light left by the interposing moon, and of course derived entirely from the rim of the solar disc, yielded to forbes's examination precisely the same spectrum as light coming from its central parts. this circumstance helped to baffle inquirers, already sufficiently perplexed. it still remains an anomaly, of which no satisfactory explanation has been offered. convincing evidence as to the true nature of the solar lines was however at length, in the autumn of , brought forward at heidelburg. kirchhoff's _experimentum crucis_ in the matter was a very simple one. he threw bright sunshine across a space occupied by vapour of sodium, and perceived with astonishment that the dark fraunhofer line d, instead of being effaced by flame giving a luminous ray of the same refrangibility, was deepened and thickened by the superposition. he tried the same experiment, substituting for sunbeams light from a drummond lamp, and with similar result. a dark furrow, corresponding in every respect to the solar d-line, was instantly seen to interrupt the otherwise unbroken radiance of its spectrum. the inference was irresistible, that the effect thus produced artificially was brought about naturally in the same way, and that sodium formed an ingredient in the glowing atmosphere of the sun.[ ] this first discovery was quickly followed up by the identification of numerous bright rays in the spectra of other metallic bodies with others of the hitherto mysterious fraunhofer lines. kirchhoff was thus led to the conclusion that (besides sodium) iron, magnesium, calcium, and chromium, are certainly solar constituents, and that copper, zinc, barium, and nickel are also present, though in smaller quantities.[ ] as to cobalt, he hesitated to pronounce, but its existence in the sun has since been established. these memorable results were founded upon a general principle first enunciated by kirchhoff in a communication to the berlin academy, december , , and afterwards more fully developed by him.[ ] it may be expressed as follows: substances of every kind are opaque to the precise rays which they emit at the same temperature; that is to say, they stop the kinds of light or heat which they are then actually in a condition to radiate. but it does not follow that _cool_ bodies absorb the rays which they would give out if sufficiently heated. hydrogen at ordinary temperatures, for instance, is almost perfectly transparent, but if raised to the glowing point--as by the passage of electricity--it _then_ becomes capable of arresting, and at the same time of displaying in its own spectrum light of four distinct colours. this principle is fundamental to solar chemistry. it gives the key to the hieroglyphics of the fraunhofer lines. the identical characters which are written _bright_ in terrestrial spectra are written _dark_ in the unrolled sheaf of sun-rays; the meaning remains unchanged. it must, however, be remembered that they are only _relatively_ dark. the substances stopping those particular tints in the neighbourhood of the sun are at the same time vividly glowing with the very same. remove the dazzling solar background, by contrast with which they show as obscure, and they will be seen, and, at critical moments, actually have been seen, in all their native splendour. it is because the atmosphere of the sun is cooler than the globe it envelops that the different kinds of vapour constituting that atmosphere take more than they give, absorb more light than they are capable of emitting; raise them to the same temperature as the sun itself, and their powers of emission and absorption being brought exactly to the same level, the thousands of dusky rays in the solar spectrum will be at once obliterated. the establishment of the terrestrial science of spectrum analysis was due, as we have seen, equally to kirchhoff and bunsen, but its celestial application to kirchhoff alone. he effected this object of the aspirations, more or less dim, of many other thinkers and workers, by the union of two separate, though closely related lines of research--the study of the different kinds of light _emitted_ by various bodies, and the study of the different kinds of light _absorbed_ by them. the latter branch appears to have been first entered upon by dr. thomas young in ;[ ] it was pursued by the younger herschel,[ ] by william allen miller, brewster, and gladstone. brewster indeed made, in ,[ ] a formal attempt to found what might be called an inverse system of analysis with the prism based upon absorption; and his efforts were repeated, just a quarter of a century later, by gladstone.[ ] but no general point of view was attained; nor, it may be added, was it by this path attainable. kirchhoff's map of the solar spectrum, drawn to scale with exquisite accuracy, and printed in three shades of ink to convey the graduated obscurity of the lines, was published in the transactions of the berlin academy for and .[ ] representations of the principal lines belonging to various elementary bodies formed, as it were, a series of marginal notes accompanying the great solar scroll, enabling the veriest tiro in the new science to decipher its meaning at a glance. where the dark solar and bright metallic rays agreed in position, it might safely be inferred that the metal emitting them was a solar constituent; and such coincidences were numerous. in the case of iron alone, no less than sixty occurred in one-half of the spectral area, rendering the chances[ ] absolutely overwhelming against mere casual conjunction. the preparation of this elaborate picture proved so trying to the eyes that kirchhoff was compelled by failing vision to resign the latter half of the task to his pupil hofmann. the complete map measured nearly eight feet in length. the conclusions reached by kirchhoff were no sooner announced than they took their place, with scarcely a dissenting voice, among the established truths of science. the broad result, that the dark lines in the spectrum of the sun afford an index to its chemical composition no less reliable than any of the tests used in the laboratory, was equally captivating to the imagination of the vulgar, and authentic in the judgment of the learned; and, like all genuine advances in the knowledge of nature, it stimulated curiosity far more than it gratified it. now the history of how discoveries were missed is often quite as instructive as the history of how they were made; it may then be worth while to expend a few words on the thoughts and trials by which, in the present case, the actual event was heralded. three times it seemed on the verge of being anticipated. the experiment, which in kirchhoff's hands proved decisive, of passing sunlight through glowing vapours and examining the superposed spectra, was performed by professor w. a. miller of king's college in .[ ] nay, more, it was performed with express reference to the question, then already (as has been noted) in debate, of the possible production of fraunhofer's lines by absorption in a solar atmosphere. yet it led to nothing. again, at paris in , with a view to testing the asserted coincidence between the solar d-line and the bright yellow beam in the spectrum of the electric arc (really due to the unsuspected presence of sodium), léon foucault threw a ray of sunshine across the arc and observed its spectrum.[ ] he was surprised to see that the d-line was rendered more intensely dark by the combination of lights. to assure himself still further, he substituted a reflected image of one of the white-hot carbon-points for the sunbeam, with an identical result. _the same ray was missing._ it needed but another step to have generalised this result, and thus laid hold of a natural truth of the highest importance; but that step was not taken. foucault, keen and brilliant though he was, rested satisfied with the information that the _voltaic arc_ had the power of stopping the kind of light emitted by it; he asked no further question, and was consequently the bearer of no further intelligence on the subject. the truth conveyed by this remarkable experiment was, however, divined by one eminent man. professor stokes of cambridge stated to sir william thomson (now lord kelvin), shortly after it had been made, his conviction that an absorbing atmosphere of sodium surrounded the sun. and so forcibly was his hearer impressed with the weight of the argument based upon the absolute agreement of the d-line in the solar spectrum with the yellow ray of burning sodium (then freshly certified by w. h. miller), combined with foucault's "reversal" of that ray, that he regularly inculcated, in his public lectures on natural philosophy at glasgow, five or six years before kirchhoff's discovery, not only the _fact_ of the presence of sodium in the solar neighbourhood, but also the _principle_ of the study of solar and stellar chemistry in the spectra of flames.[ ] yet it does not appear to have occurred to either of these two distinguished professors--themselves among the foremost of their time in the successful search for new truths--to verify practically a sagacious conjecture in which was contained the possibility of a scientific revolution. it is just to add, that kirchhoff was unacquainted, when he undertook his investigation, either with the experiment of foucault or the speculation of stokes. for c. j. Ångström, on the other hand, perhaps somewhat too much has been claimed in the way of anticipation. his _optical researches_ appeared at upsala in , and in their english garb two years later.[ ] they were undoubtedly pregnant with suggestion, yet made no epoch in discovery. the old perplexities continued to prevail after, as before their publication. to Ångström, indeed, belongs the great merit of having revived euler's principle of the equivalence of emission and absorption; but he revived it in its original crude form, and without the qualifying proviso which alone gave it value as a clue to new truths. according to his statement, a body absorbs all the series of vibrations it is, under any circumstances, capable of emitting, as well as those connected with them by simple harmonic relations. this is far too wide. to render it either true or useful, it had to be reduced to the cautious terms employed by kirchhoff. radiation strictly and necessarily corresponds with absorption only _when the temperature is the same_. in point of fact, Ångström was still, in , divided between adsorption and interference as the mode of origin of the fraunhofer dark rays. very important, however, was his demonstration of the compound nature of the spark-spectrum, which he showed to be made up of the spectrum of the metallic electrodes superposed upon that of the gas or gases across which the discharge passed. it may here be useful--since without some clear ideas on the subject no proper understanding of recent astronomical progress is possible--to take a cursory view of the elementary principles of spectrum analysis. to many of our readers they are doubtless already familiar; but it is better to appear trite to some than obscure even to a few. the spectrum, then, of a body is simply the light proceeding from it _spread out_ by refraction[ ] into a brilliant variegated band, passing from brownish-red through crimson, orange, yellow, green, and azure into dusky violet. the reason of this spreading-out or "dispersion" is that the various colours have different wave-lengths, and consequently meet with different degrees of retardation in traversing the denser medium of the prism. the shortest and quickest vibrations (producing the sensation we call "violet") are thrown farthest away from their original path--in other words, suffer the widest "deviation;" the longest and slowest (the red) travel much nearer to it. thus the sheaf of rays which would otherwise combine into a patch of white light are separated through the divergence of their tracks after refraction by a prism, so as to form a tinted riband. this _visible_ spectrum is prolonged _invisibly_ at both ends by a long range of vibrations, either too rapid or too sluggish to affect the eye as light, but recognisable through their chemical and heating effects. now all incandescent solid or liquid substances, and even gases ignited under great pressure, give what is called a "continuous spectrum;" that is to say, the light derived from them is of every conceivable hue. sorted out with the prism, its tints merge imperceptibly one into the other, uninterrupted by any dark spaces. no colours, in short, are missing. but gases and vapours rendered luminous by heat emit rays of only a few tints, which accordingly form an interrupted spectrum, usually designated as one of lines or bands. and since these rays are perfectly definite and characteristic--not being the same for any two substances--it is easy to tell what kind of matter is concerned in producing them. we may suppose that the inconceivably minute particles which by their rapid thrilling agitate the ethereal medium so as to produce light, are free to give out their peculiar tone of vibration only when floating apart from each other in gaseous form; but when crowded together into a condensed mass, the clear ring of the distinctive note is drowned, so to speak, in a universal molecular clang. thus prismatic analysis has no power to identify individual kinds of matter, except when they present themselves as glowing vapours. a spectrum is said to be "reversed" when lines previously seen bright on a dark background appear dark on a bright background. in this form it is equally characteristic of chemical composition with the "direct" spectrum, being due to _absorption_, as the latter is to _emission_. and absorption and emission are, by kirchhoff's law, strictly correlative. this is easily understood by the analogy of sound. for just as a tuning-fork responds to sound-waves of its own pitch, but remains indifferent to those of any other, so those particles of matter whose nature it is, when set swinging by heat, to vibrate a certain number of times in a second, thus giving rise to light of a particular shade of colour, appropriate those same vibrations, and those only, when transmitted past them,--or, phrasing it otherwise, are opaque to them, and transparent to all others. it should further be explained that the _shape_ of the bright or dark spaces in the spectrum has nothing whatever to do with the nature of the phenomena. the "lines" and "bands" so frequently spoken of are seen as such for no other reason than because the light forming them is admitted through a narrow, straight opening. change that opening into a fine crescent or a sinuous curve, and the "lines" will at once appear as crescents or curves. resuming in a sentence what has been already explained, we find that the prismatic analysis of the heavenly bodies was founded upon three classes of facts: first, the unmistakable character of the light given by each different kind of glowing vapour; secondly, the identity of the light absorbed with the light emitted by each; thirdly, the coincidence observed between rays missing from the solar spectrum and rays absorbed by various terrestrial substances. thus, a realm of knowledge, pronounced by morinus[ ] in the seventeenth century, and no less dogmatically by auguste comte[ ] in the nineteenth, hopelessly out of reach of the human intellect, was thrown freely open, and the chemistry of the sun and stars took at once a leading place among the experimental sciences. the immediate increase of knowledge was not the chief result of kirchhoff's labours; still more important was the change in the scope and methods of astronomy, which, set on foot in by the detection of a common period affecting at once the spots on the sun and the magnetism of the earth, was extended and accelerated by the discovery of spectrum analysis. the nature of that change is concisely indicated by the heading of the present chapter; we would now ask our readers to endeavour to realise somewhat distinctly what is implied by the "foundation of astronomical physics." just three centuries ago, kepler drew a forecast of what he called a "physical astronomy"--a science treating of the efficient causes of planetary motion, and holding the "key to the inner astronomy."[ ] what kepler dreamed of and groped after, newton realized. he showed the beautiful and symmetrical revolutions of the solar system to be governed by a uniformly acting cause, and that cause no other than the familiar force of gravity, which gives stability to all our terrestrial surroundings. the world under our feet was thus for the first time brought into physical connection with the worlds peopling space, and a very tangible relationship was demonstrated as existing between what used to be called the "corruptible" matter of the earth and the "incorruptible" matter of the heavens. this process of unification of the cosmos--this levelling of the celestial with the sublunary--was carried no farther until the fact unexpectedly emerged from a vast and complicated mass of observations, that the magnetism of the earth is subject to subtle influences, emanating, certainly from some, and presumably from all of the heavenly bodies; the inference being thus rendered at least plausible, that a force not less universal than gravity itself, but with whose modes of action we are as yet unacquainted, pervades the universe, and forms, it might be said, an intangible bond of sympathy between its parts. now for the investigation of this influence two roads are open. it may be pursued by observation either of the bodies from which it proceeds, or of the effects which it produces--that is to say, either by the astronomer or by the physicist, or, better still, by both concurrently. their acquisitions are mutually profitable; nor can either be considered as independent of the other. any important accession to knowledge respecting the sun, for example, may be expected to cast a reflected light on the still obscure subject of terrestrial magnetism; while discoveries in magnetism or its _alter ego_ electricity must profoundly affect solar inquiries. the establishment of the new method of spectrum analysis drew far closer this alliance between celestial and terrestrial science. indeed, they have come to merge so intimately one into the other, that it is no easier to trace their respective boundaries than it is to draw a clear dividing-line between the animal and vegetable kingdoms. yet up to the middle of the last century, astronomy, while maintaining her strict union with mathematics, looked with indifference on the rest of the sciences; it was enough that she possessed the telescope and the calculus. now the materials for her inductions are supplied by the chemist, the electrician, the inquirer into the most recondite mysteries of light and the molecular constitution of matter. she is concerned with what the geologist, the meteorologist, even the biologist, has to say; she can afford to close her ears to no new truth of the physical order. her position of lofty isolation has been exchanged for one of community and mutual aid. the astronomer has become, in the highest sense of the term, a physicist; while the physicist is bound to be something of an astronomer. this, then, is what is designed to be conveyed by the "foundation of astronomical or cosmical physics." it means the establishment of a science of nature whose conclusions are not only presumed by analogy, but are ascertained by observation, to be valid wherever light can travel and gravity is obeyed--a science by which the nature of the stars can be studied upon the earth, and the nature of the earth can be made better known by study of the stars--a science, in a word, which is, or aims at being, one and universal, even as nature--the visible reflection of the invisible highest unity--is one and universal. it is not too much to say that a new birth of knowledge has ensued. the astronomy so signally promoted by bessel[ ]--the astronomy placed by comte[ ] at the head of the hierarchy of the physical sciences--was the science of the _movements_ of the heavenly bodies. and there were those who began to regard it as a science which, from its very perfection, had ceased to be interesting--whose tale of discoveries was told, and whose farther advance must be in the line of minute technical improvements, not of novel and stirring disclosures. but the science of the _nature_ of the heavenly bodies is one only in the beginning of its career. it is full of the audacities, the inconsistencies, the imperfections, the possibilities of youth. it promises everything; it has already performed much; it will doubtless perform much more. the means at its disposal are vast and are being daily augmented. what has so far been secured by them it must now be our task to extricate from more doubtful surroundings and place in due order before our readers. footnotes: [footnote : wolf, _gesch. der astr._, p. .] [footnote : manuel johnson, _mem. r.a.s._, vol. xxvi., p. .] [footnote : _astronomie théorique et pratique_, t. iii., p. .] [footnote : wolf, _gesch. der astr._, p. .] [footnote : _month. not._, vol. xvii., p. .] [footnote : _mem. r.a.s._, vol. xxvi., p. .] [footnote : _astr. nach._, no. .] [footnote : gehler's _physikalisches wörterbuch_, art. _sonnenflecken_, p. .] [footnote : _zweite abth._, p. .] [footnote : _annalen der physik_ (poggendorff's), bd. lxxxiv., p. .] [footnote : _phil. trans._, vol. cxlii., p. .] [footnote : _mittheilungen der naturforschenden gesellschaft_, , p. .] [footnote : _archives des sciences_, t. xxi., p. .] [footnote : _neue untersuchungen, mitth. naturf. ges._, , p. .] [footnote : _phil. trans._, vol. xci., p. .] [footnote : evidence of an eleven-yearly fluctuation in the price of food-grains in india was collected some years ago by mr. frederick chambers. _nature_, vol. xxxiv., p. .] [footnote : _bibl. un. de genève_, t. li., p. .] [footnote : _neue untersuchungen_, p. .] [footnote : _die sonne und ihre flecken_, p. . arago was the first who attempted to decide the question by keeping, through a series of years, a parallel register of sun-spots and weather; but the data regarding the solar condition amassed at the paris observatory from to were not sufficiently precise to support any inference.] [footnote : _phil. trans._, vol. xxix., p. .] [footnote : _ibid._, vols. cxliii., p. , cxlvi., p. .] [footnote : _observations on light and colours_, p. .] [footnote : _phil. trans._, vol. lxxv., p. .] [footnote : _denkschriften_ (munich. ac. of sc.), , , bd. v., p. .] [footnote : _edinburgh journal of science_, vol. v., p. . see also _phil. mag._, feb., , vol. iv., p. .] [footnote : _ed. phil. trans.,_ vol. xxi., p. .] [footnote : _on the absorption of light by coloured media, ed. phil. trans._, vol. ix., p. ( ).] [footnote : _phil. mag._, vol. xxvii, (ser. iii.), p. .] [footnote : _report brit. ass._, , p. (pt. ii.). _electrodes_ are the terminals from one to the other of which the electric spark passes, volatilising and rendering incandescent in its transit some particles of their substance, the characteristic light of which accordingly flashes out in the spectrum.] [footnote : _phil. mag._, vol. xx., p. .] [footnote : _annalen der physik_, bd. cxiii., p. .] [footnote : _phil. trans._, vol. xcii., p. .] [footnote : _denkschriften_, bd. v., p. .] [footnote : _ibid._, p. ; _edin. jour. of science_, vol. viii., p. .] [footnote : _denkschriften_, bd. v., p. .] [footnote : _arch. des sciences_, , p. .] [footnote : _phil. trans._, vol. cl., p. , _note_.] [footnote : _ed. phil. trans._, vol. xii., p. .] [footnote : _phil. trans._, vol. cxxvi., p. . "i conceive," he says, "that this result proves decisively that the sun's atmosphere has nothing to do with the production of this singular phenomenon" (p. ). and brewster's well-founded opinion that it had much to do with it was thereby, in fact, overthrown.] [footnote : _monatsberichte_, berlin, , p. .] [footnote : _abhandlungen_, berlin, , pp. , .] [footnote : _ibid._, , p. ; _annalen der physik_, bd. cxix., p. . a similar conclusion, reached by balfour stewart in , for heat-rays (_ed. phil. trans._, vol. xxii., p. ), was, in , without previous knowledge of kirchhoff's work, extended to light (_phil. mag._, vol. xx., p. ); but his experiments wanted the precision of those executed at heidelburg.] [footnote : _miscellaneous works_, vol. i., p. .] [footnote : _ed. phil. trans._, vol. ix., p. .] [footnote : _ibid._, vol. xii., p. .] [footnote : _quart. jour. chem. soc._, vol. x. p. .] [footnote : a facsimile accompanied sir h. roscoe's translation of kirchhoff's "researches on the solar spectrum" (london, - ).] [footnote : estimated by kirchhoff's at a _trillion to one_. _abhandl._, , p. .] [footnote : _phil. mag._, vol. xxvii. ( rd series), p. .] [footnote : _l'institut_, feb. , , p. ; _phil. mag._, vol. xix. ( th series), p. .] [footnote : _ann. d. phys._, vol. cxviii., p. .] [footnote : _phil. mag._, vol. ix. ( th series), p. .] [footnote : spectra may be produced by _diffraction_ as well as by _refraction_; but we are here only concerned with the subject in its simplest aspect.] [footnote : _astrologia gallica_ ( ), p. .] [footnote : _pos. phil._, vol. i., pp. , (martineau's trans.).] [footnote : _proem astronomiæ pars optica_ ( ), _op._, t. ii.] [footnote : _pop. vorl._, pp. , , .] [footnote : _pos. phil._, p. .] chapter ii _solar observations and theories_ the zeal with which solar studies have been pursued during the last half century has already gone far to redeem the neglect of the two preceding ones. since schwabe's discovery was published in , observers have multiplied, new facts have been rapidly accumulated, and the previous comparative quiescence of thought on the great subject of the constitution of the sun, has been replaced by a bewildering variety of speculations, conjectures, and more or less justifiable inferences. it is satisfactory to find this novel impulse not only shared, but to a large extent guided, by our countrymen. william rutter dawes, one of many clergymen eminent in astronomy, observed, in , with the help of a solar eye-piece of his own devising, some curious details of spot-structure.[ ] the umbra--heretofore taken for the darkest part of the spot--was seen to be suffused with a mottled, nebulous illumination, in marked contrast with the striated appearance of the penumbra; while through this "cloudy stratum" a "black opening" permitted the eye to divine farther unfathomable depths beyond. the _hole_ thus disclosed--evidently the true nucleus--was found to be present in all considerable, as well as in many small maculæ. again, the whirling motions of some of these objects were noticed by him. the remarkable form of one sketched at wateringbury, in kent, january , , gave him the means of detecting and measuring a rotatory movement of the whole spot round the black nucleus at the rate of degrees in six days. "it appeared," he said, "as if some prodigious ascending force of a whirlwind character, in bursting through the cloudy stratum and the two higher and luminous strata, had given to the whole a movement resembling its own."[ ] an interpretation founded, as is easily seen, on the herschelian theory, then still in full credit. an instance of the same kind was observed by mr. w. r. birt in ,[ ] and cyclonic movements are now a recognised feature of sun-spots. they are, however, as father secchi[ ] concluded from his long experience, but temporary and casual. scarcely three per cent. of all spots visible exhibit the spiral structure which should invariably result if a conflict of opposing, or the friction of unequal, currents were essential, and not merely incidental to their origin. a whirlpool phase not unfrequently accompanies their formation, and may be renewed at periods of recrudescence or dissolution; but it is both partial and inconstant, sometimes affecting only one side of a spot, sometimes slackening gradually its movement in one direction, to resume it, after a brief pause, in the opposite. persistent and uniform notions, such as the analogy of terrestrial storms would absolutely require, are not to be found. so that the "cyclonic theory" of sun-spots, suggested by herschel in ,[ ] and urged, from a different point of view, by faye in , may be said to have completely broken down. the drift of spots over the sun's surface was first systematically investigated by carrington, a self-constituted astronomer, gifted with the courage and the instinct of thoughtful labour. born at chelsea in may, , richard christopher carrington entered trinity college, cambridge, in . he was intended for the church, but professor challis's lectures diverted him to astronomy, and he resolved, as soon as he had taken his degree, to prepare, with all possible diligence, to follow his new vocation. his father, who was a brewer on a large scale at brentford, offered no opposition; ample means were at his disposal; nevertheless, he chose to serve an apprenticeship of three years as observer in the university of durham, as though his sole object had been to earn a livelihood. he quitted the post only when he found that its restricted opportunities offered no farther prospect of self-improvement. he now built an observatory of his own at redhill in surrey, with the design of completing bessel's and argelander's survey of the northern heavens by adding to it the circumpolar stars omitted from their view. this project, successfully carried out between and , had another and still larger one superposed upon it before it had even begun to be executed. in , while the redhill observatory was in course of erection, the discovery of the coincidence between the sun-spot and magnetic periods was announced. carrington was profoundly interested, and devoted his enforced leisure to the examination of records, both written and depicted, of past solar observations. struck with their fragmentary and inconsistent character, he resolved to "appropriate," as he said, by "close and methodical research," the eleven-year period next ensuing.[ ] he calculated rightly that he should have the field pretty nearly to himself; for many reasons conspire to make public observatories slow in taking up new subjects, and amateurs with freedom to choose, and means to treat them effectually, were scarcer then than they are now. the execution of this laborious task was commenced november , . it was intended to be merely a _parergon_--a "second subject," upon which daylight energies might be spent, while the hours of night were reserved for cataloguing those stars that "are bereft of the baths of ocean." its results, however, proved of the highest interest, although the vicissitudes of life barred the completion, in its full integrity, of the original design. by the death, in , of the elder carrington, the charge of the brewery devolved upon his son; and eventually absorbed so much of his care that it was found advisable to bring the solar observations to a premature close, on march , . his scientific life may be said to have closed with them. attacked four years later with severe, and, in its results, permanent illness, he disposed of the brentford business, and withdrew to churt, near farnham, in surrey. there, in a lonely spot, on the top of a detached conical hill known as the "devil's jump," he built a second observatory, and erected an instrument which he was no longer able to use with pristine effectiveness; and there, november , , he died of the rupture of a blood vessel on the brain, before he had completed his fiftieth year.[ ] his observations of sun-spots were of a geometrical character. they concerned positions and movements, leaving out of sight physical peculiarities. indeed, the prudence with which he limited his task to what came strictly within the range of his powers to accomplish, was one of carrington's most valuable qualities. the method of his observations, moreover, was chosen with the same practical sagacity as their objects. as early as , sir john herschel had recommended the daily self-registration of sun-spots,[ ] and he enforced the suggestion, with more immediate prospect of success, in .[ ] the art of celestial photography, however, was even then in a purely tentative stage, and carrington wisely resolved to waste no time on dubious experiments, but employ the means of registration and measurement actually at his command. these were very simple, yet very effective. to the "helioscope" employed by father scheiner[ ] two centuries and a quarter earlier, a species of micrometer was added. the image of the sun was projected upon a screen by means of a firmly-clamped telescope, in the focus of which were placed two cross-wires forming angles of ° with the meridian. the six instants were then carefully noted at which these were met by the edges of the disc as it traversed the screen, and by the nucleus of the spot to be measured.[ ] a short process of calculation then gave the exact position of the spot as referred to the sun's centre. from a series of , observations made in this way, together with a great number of accurate drawings, carrington derived conclusions of great importance on each of the three points which he had proposed to himself to investigate. these were: the law of the sun's rotation, the existence and direction of systematic currents, and the distribution of spots on the solar surface. grave discrepancies were early perceived to exist between determinations of the sun's rotation by different observers. galileo, with "comfortable generality," estimated the period at "about a lunar month";[ ] scheiner, at twenty-seven days.[ ] cassini, in , made it · ; delambre, in , no more than twenty-five days. later inquiries brought these divergences within no more tolerable limits. laugier's result of · days--obtained in --enjoyed the highest credit, yet it differed widely in one direction from that of böhm ( ), giving · days, and in the other from that of kysæus ( ), giving · days. now the cause of these variations was really obvious from the first, although for a long time strangely overlooked. scheiner pointed out in that different spots gave different periods, adding the significant remark that one at a distance from the solar equator revolved more slowly than those nearer to it.[ ] but the hint was wasted. for upwards of two centuries ideas on the subject were either retrograde or stationary. what were called the "proper motions" of spots were, however, recognised by schröter,[ ] and utterly baffled laugier,[ ] who despaired of obtaining any concordant result as to the sun's rotation except by taking the mean of a number of discordant ones. at last, in , a valuable course of observations made at capo di monte, naples, in - , enabled c. h. f. peters[ ] to set in the clearest light the insecurity of determinations based on the assumption of fixity in objects plainly affected by movements uncertain both in amount and direction. such was the state of affairs when carrington entered upon his task. everything was in confusion; the most that could be said was that the confusion had come to be distinctly admitted and referred to its true source. what he discovered was this: that the sun, or at least the outer shell of the sun visible to us, has _no single period of rotation_, but drifts round, carrying the spots with it, at a rate continually accelerated from the poles to the equator. in other words, the time of axial revolution is shortest at the equator and lengthens with increase of latitude. carrington devised a mathematical formula by which the rate or "law" of this lengthening was conveniently expressed; but it was a purely empirical one. it was a concise statement, but implied no physical interpretation. it summarised, but did not explain the facts. an assumed "mean period" for the solar rotation of · days (twenty-five days nine hours, very nearly), was thus found to be _actually_ conformed to only in two parallels of solar latitude ( ° north and south), while the equatorial period was slightly less than twenty-five, and that of latitude ° rose to twenty-seven days and a half.[ ] these curious results gave quite a new direction to ideas on solar physics. the other two "elements" of the sun's rotation were also ascertained by carrington with hitherto unattained precision. he fixed the inclination of its axis to the ecliptic at ° '; the longitude of the ascending node at ° ' (for the epoch a.d.). these data--which have scarcely yet been improved upon--suffice to determine the position in space of the sun's equator. its north pole is directed towards a star in the coils of the dragon, midway between vega and the pole-star; its plane intersects that of the earth's orbit in such a way that our planet finds itself in the same level on or about the rd of june and the th of december, when any spots visible on the disc cross it in apparently straight lines. at other times, the paths pursued by them seem curved--downward (to an observer in the northern hemisphere) between june and december, upward between december and june. a singular peculiarity in the distribution of sun-spots emerged from carrington's studies at the time of the minimum of . two broad belts of the solar surface, as we have seen, are frequented by them, of which the limits may be put at ° and ° of north and south latitude. individual equatorial spots are not uncommon, but nearer to the poles than ° they are a rare exception. carrington observed--as an extreme instance--in july, , one in south latitude °; and peters, in june, , watched, during several days, a spot in ° ' north latitude. but beyond this no true macula has ever been seen; for lahire's reported observation of one in latitude ° is now believed to have had its place on the solar globe erroneously assigned; and the "veiled spots" described by trouvelot in [ ] as occurring within ° of the pole can only be regarded as, at the most, the same kind of disturbance in an undeveloped form. but the novelty of carrington's observations consisted in the detection of certain changes in distribution concurrent with the progress of the eleven-year period. as the minimum approached, the spot-zones contracted towards the equator, and there finally vanished; then, as if by a fresh impulse, spots suddenly reappeared in high latitude, and spread downwards with the development of the new phase of activity. scarcely had this remark been made public,[ ] when wolf[ ] found a confirmation of its general truth in böhm's observations during the years - ; and a perfectly similar behaviour was noted both by spörer and secchi at the minimum epoch of . the ensuing period gave corresponding indications; and it may now be looked upon as established that the spot-zones close in towards the equator with the advance of each cycle, their activity culminating, as a rule, in a mean latitude of about °, and expiring when it is reduced to °. before this happens, however, a completely new disturbance will have manifested itself some ° north and south of the equator, and will have begun to travel over the same course as its predecessor. each series of sun-spots is thus, to some extent, overlapped by the succeeding one; so that while the average interval from one maximum to the next is eleven years, the period of each distinct wave of agitation is twelve or fourteen.[ ] curious evidence of the retarded character of the maximum of - was to be found in the unusually low latitude of the spot-zones when it occurred. their movement downward having gone on regularly while the crisis was postponed, its final symptoms were hence displaced locally as well as in time. the "law of zones" was duly obeyed at the minima of [ ] and , and spörer found evidence of conformity to it so far back as .[ ] his researches, however, also showed that it was in abeyance during some seventy years previously to , during which period sun-spots remained persistently scarce, and auroral displays were feeble and infrequent even in high northern latitudes. an unaccountable suspension of solar activity is, in fact, indicated.[ ] gustav spörer, born at berlin in , began to observe sun-spots with the view of assigning the law of solar rotation in december, . his assiduity and success with limited means attracted attention, and a government endowment was procured for his little solar observatory at anclam, in pomerania, the crown prince (afterwards emperor frederick) adding a five-inch refractor to its modest equipment. unaware of carrington's discovery (not made known until january, ), he arrived at and published, in june, ,[ ] a similar conclusion as to the equatorial quickening of the sun's movement on its axis. appointed observer in the new astrophysical establishment at potsdam in , he continued his sun-spot determinations there for twenty years, and died july , . the time had now evidently come for a fundamental revision of current notions respecting the nature of the sun. herschel's theory of a cool, dark, habitable globe, surrounded by, and protected against, the radiations of a luminous and heat-giving envelope, was shattered by the first _dicta_ of spectrum analysis. traces of it may be found for a few years subsequent to ,[ ] but they are obviously survivals from an earlier order of ideas, doomed to speedy extinction. it needs only a moment's consideration of the meaning at last found for the fraunhofer lines to see the incompatibility of the new facts with the old conceptions. they implied not only the presence near the sun, as glowing vapours, of bodies highly refractory to heat, but that these glowing vapours formed the relatively cool envelope of a still hotter internal mass. kirchhoff, accordingly, included in his great memoir "on the solar spectrum," read before the berlin academy of sciences, july , , an exposition of the views on the subject to which his memorable investigations had led him. they may be briefly summarised as follows: since the body of the sun gives a continuous spectrum, it must be either solid or liquid,[ ] while the interruptions in its light prove it to be surrounded by a complex atmosphere of metallic vapours, somewhat cooler than itself. spots are simply clouds due to local depressions of temperature, differing in no respect from terrestrial clouds except as regards the kinds of matter composing them. these _sun-clouds_ take their origin in the zones of encounter between polar and equatorial currents in the solar atmosphere. this explanation was liable to all the objections urged against the "cumulus theory" on the one hand, and the "trade-wind theory" on the other. setting aside its propounder, it was consistently upheld perhaps by no man eminent in science except spörer; and his advocacy of it proved ineffective to secure its general adoption. m. faye, of the paris academy of sciences, was the first to propose a coherent scheme of the solar constitution covering the whole range of new discovery. the fundamental ideas on the subject now in vogue here made their first connected appearance. much, indeed, remained to be modified and corrected; but the transition was finally made from the old to the new order of thought. the essence of the change may be conveyed in a single sentence. the sun was thenceforth regarded, not as a mere heated body, or--still more remotely from the truth--as a cool body unaccountably spun round with a cocoon of fire, but as a vast _heat-radiating machine_. the terrestrial analogy was abandoned in one more particular besides that of temperature. the solar system of circulation, instead of being adapted, like that of the earth, to the distribution of heat received from without, was seen to be directed towards the transportation towards the surface of the heat contained within. polar and equatorial currents, tending to a purely superficial equalisation of temperature, were replaced by vertical currents bringing up successive portions of the intensely heated interior mass, to contribute their share in turn to the radiation into space which might be called the proper function of a sun. faye's views, which were communicated to the academy of sciences, january , ,[ ] were avowedly based on the anomalous mode of solar rotation discovered by carrington. this may be regarded either as an acceleration increasing from the poles to the equator, or as a retardation increasing from the equator to the poles, according to the rate of revolution we choose to assume for the unseen nucleus. faye preferred to consider it a retardation produced by ascending currents continually left behind as the sphere widened in which the matter composing them was forced to travel. he further supposed that the depth from which these vertical currents rose, and consequently the amount of retardation effected by their ascent to the surface, became progressively greater as the poles were approached, owing to the considerable flattening of the spheroidal surface from which they started;[ ] but the adoption of this expedient has been shown to involve inadmissible consequences. the extreme internal mobility betrayed by carrington's and spörer's observations led to the inference that the matter composing the sun was mainly or wholly gaseous. this had already been suggested by father secchi[ ] a year earlier, and by sir john herschel in april, ;[ ] but it first obtained general currency through faye's more elaborate presentation. a physical basis was afforded for the view by cagniard de la tour's experiments in ,[ ] proving that, under conditions of great heat and pressure, the vaporous state was compatible with a very considerable density. the position was strengthened when andrews showed, in ,[ ] that above a fixed limit of temperature, varying for different bodies, true liquefaction is impossible, even though the pressure be so tremendous as to retain the gas within the same space that enclosed the liquid. the opinion that the mass of the sun is gaseous now commands a very general assent; although the gaseity admitted is of such a nature as to afford the consistence rather of honey or pitch than of the aeriform fluids with which we are familiar. on another important point the course of subsequent thought was powerfully influenced by faye's conclusions in . arago somewhat hastily inferred from experiments with the polariscope the wholly gaseous nature of the visible disc of the sun. kirchhoff, on the contrary, believed (erroneously, as we now know) that the brilliant continuous spectrum derived from it proved it to be a white-hot solid or liquid. herschel and secchi[ ] indicated a cloud-like structure as that which would best harmonise the whole of the evidence at command. the novelty introduced by faye consisted in regarding the photosphere no longer "as a defined surface, in the mathematical sense, but as a limit to which, in the general fluid mass, ascending currents carry the physical or chemical phenomena of incandescence."[ ] uprushing floods of mixed vapours with strong affinities--say of calcium or sodium and oxygen--at last attain a region cool enough to permit their combination; a fine dust of solid or liquid compound particles (of lime or soda, for example) there collects into the photospheric clouds, and descending by its own weight in torrents of incandescent rain, is dissociated by the fierce heat below, and replaced by ascending and combining currents of similar constitution. this first attempt to assign the part played in cosmical physics by chemical affinities was marked by the importation into the theory of the sun of the now familiar phrase _dissociation_. it is indeed tolerably certain that no such combinations as those contemplated by faye occur at the photospheric level, since the temperature there must be enormously higher than would be needed to reduce all metallic earths and oxides; but molecular changes of some kind, dependent perhaps in part upon electrical conditions, in part upon the effects of radiation into space, most likely replace them. the conjecture was emitted by dr. johnstone stoney in [ ] that the photospheric clouds are composed of carbon-particles precipitated from their mounting vapour just where the temperature is lowered by expansion and radiation to the boiling-point of that substance. but this view, though countenanced by Ångström,[ ] and advocated by hastings of baltimore,[ ] and other authorities,[ ] is open to grave objections.[ ] in faye's theory, sun-spots were regarded as simply breaks in the photospheric clouds, where the rising currents had strength to tear them asunder. it followed that they were regions of increased heat--regions, in fact, where the temperature was too high to permit the occurrence of the precipitations to which the photosphere is due. their obscurity was attributed, as in dr. brester's more recent _théorie du soleil_, to deficiency of emissive power. yet here the verdict of the spectroscope is adverse and irreversible. after every deduction, however, has been made, we still find that several ideas of permanent value were embodied in this comprehensive sketch of the solar constitution. the principal of these were; first, that the sun is a mainly gaseous body; secondly, that its stores of heat are rendered available at the surface by means of vertical convection-currents--by the bodily transport, that is to say, of intensely hot matter upward, and of comparatively cool matter downward; thirdly, that the photosphere is a surface of condensation, forming the limit set by the cold of space to this circulating process, and that a similar formation must attend, at a certain stage, the cooling of every cosmical body. to warren de la rue belongs the honour of having obtained the earliest results of substantial value in celestial photography. what had been done previously was interesting in the way of promise, but much could not be claimed for it as actual performance. some "pioneering experiments" were made by dr. j. w. draper of new york in , resulting in the production of a few "moon-pictures" one inch in diameter;[ ] but slight encouragement was derived from them, either to himself or others. bond of cambridge (u.s.), however, secured in with the harvard -inch refractor that daguerreotype of the moon with which the career of extra-terrestrial photography may be said to have formally opened. it was shown in london at the great exhibition of , and determined the direction of de la rue's efforts. yet it did little more than prove the art to be a possible one. warren de la rue was born in guernsey in , and died in london april , . educated at the École sainte-barbe in paris, he made a large fortune as a paper manufacturer in england, and thus amply and early provided the material supplies for his scientific campaign. towards the end of he took some successful lunar photographs. they were remarkable as the first examples of the application to astronomical light-painting of the collodion process, invented by archer in ; and also of the use of reflectors (de la rue's was one of thirteen inches, constructed by himself) for that kind of work. the absence of a driving apparatus was, however, very sensibly felt; the difficulty of moving the instrument by hand so as accurately to follow the moon's apparent motion being such as to cause the discontinuance of the experiments until , when the want was supplied. de la rue's new observatory, built in that year at cranford, was expressly dedicated to celestial photography; and there he applied to the heavenly bodies the stereoscopic method of obtaining relief, and turned his attention to the delicate business of photographing the sun. a solar daguerreotype was taken at paris, april , ,[ ] by foucault and fizeau, acting on a suggestion from arago. but the attempt, though far from being unsuccessful, does not, at that time, seem to have been repeated. its great difficulty consisted in the enormous light-power of the object to be represented, rendering an inconceivably short period of exposure indispensable, under pain of getting completely "burnt-up" plates. in de la rue was commissioned by the royal society to construct an instrument specially adapted to the purpose for the kew observatory. the resulting "photoheliograph" may be described as a small telescope (of - / inches aperture and focus), with a plate-holder at the eye-end, guarded in front by a spring-slide, the rapid movement of which across the field of view secured for the sensitive plate a virtually instantaneous exposure. by its means the first solar light-pictures of real value were taken, and the autographic record of the solar condition recommended by sir john herschel was commenced and continued at kew during fourteen years-- - . the work of photographing the sun is now carried on in every quarter of the globe, from mauritius to massachusetts, and the days are few indeed on which the self-betrayal of the camera can be evaded by our chief luminary. in the year the incorporation of indian with greenwich pictures afforded a record of the state of the solar surface on days; and were similarly provided for in and . the conclusions arrived at by photographic means at kew were communicated to the royal society in a series of papers drawn up jointly by de la rue, balfour stewart, and benjamin loewy, in and subsequent years. they influenced materially the progress of thought on the subject they were concerned with. by its rotation the sun itself offers opportunities for bringing the stereoscope to bear upon it. two pictures, taken at an interval of twenty-six minutes, show just the amount of difference needed to give, by their combination, the maximum effect of solidity.[ ] de la rue thus obtained, in , a stereoscopic view of a sun-spot and surrounding faculæ, representing the various parts in their true mutual relations. "i have ascertained in this way," he wrote,[ ] "that the faculæ occupy the highest portions of the sun's photosphere, the spots appearing like holes in the penumbræ, which appeared lower than the regions surrounding them; in one case, parts of the faculæ were discovered to be sailing over a spot apparently at some considerable height above it." thus wilson's inference as to the depressed nature of spots received, after the lapse of not far from a century, proof of the most simple, direct, and convincing kind. a careful application of wilson's own geometrical test gave results only a trifle less decisive. of spots observed, per cent. showed, as they traversed the disc, the expected effects of perspective;[ ] and their absence in the remaining per cent. might be explained by internal commotions producing irregularities of structure. the absolute depth of spot-cavities--at least of their sloping sides--was determined by father secchi through measurement of the "parallax of profundity"[ ]--that is, of apparent displacements attendant on the sun's rotation, due to depression below the sun's surface. he found that in every case it fell short of , miles, and averaged not more than , , corresponding, on the terrestrial scale, to an excavation in the earth's crust of - / miles. of late, however, the reality of even this moderate amount of depression has been denied. mr. howlett's persevering observations, extending over a third of a century, the results of which were presented to the royal astronomical society in december, ,[ ] availed to shatter the consensus of opinion which had so long been maintained on the subject of spot-structure.[ ] it has become impossible any longer to hold that it is uniformly cavernous; and what seem like actually protruding umbræ are occasionally vouched for on unimpeachable authority.[ ] we can only infer that the forms of sun-spots are really more various than had been supposed; that they are peculiarly subject to disturbance; and that the level of the nuclei may rise and fall during the phases of commotion, like lavas within volcanic craters. the opinion of the kew observers as to the nature of such disturbances was strongly swayed by another curious result of the "statistical method" of inquiry. they found that of , instances of spots accompanied by faculæ, had those faculæ chiefly or entirely on the left, showed a nearly equal distribution, while only had faculous appendages mainly on the right side.[ ] now the rotation of the sun, as we see it, is performed from left to right; so that the marked tendency of the faculæ was a lagging one. this was easily accounted for by supposing the matter composing them to have been flung upwards from a considerable depth, whence it would reach the surface with the lesser _absolute_ velocity belonging to a smaller circle of revolution, and would consequently fall behind the cavities or "spots" formed by its abstraction. an attempt, it is true, made by m. wilsing at potsdam in [ ] to determine the solar rotation from photographs of faculæ had an outcome inconsistent with this view of their origin. they unexpectedly gave a uniform period. no trace of the retardation poleward from the equator, shown by the spots, could be detected in their movements. but the experiment was obviously inconclusive;[ ] and m. stratonoff's[ ] repetition of it with ampler materials gave a full assurance that faculæ rotate like spots in periods lengthening as latitude augments. the ideas of m. faye were, on two fundamental points, contradicated by the kew investigators. he held spots to be regions of _uprush_ and of heightened temperature; they believed their obscurity to be due to a _downrush_ of comparatively cool vapours. now m. chacornac, observing, at ville-urbanne, march , , saw floods of photospheric matter visibly precipitating themselves into the abyss opened by a great spot, and carrying with them small neighbouring maculæ.[ ] similar instances were repeatedly noted by father secchi, who considered the existence of a kind of _suction_ in spots to be quite beyond question.[ ] the tendency in their vicinity, to put it otherwise, is _centripetal_, not _centrifugal_; and this alone seems to negative the supposition of a central uprush. a fresh witness was by this time at hand. the application of the spectroscope to the direct examination of the sun's surface dates from march , , when sir norman lockyer (to give him his present title) undertook an inquiry into the cause of the darkening in spots.[ ] it was made possible by the simple device of throwing upon the slit of the spectroscope an _image_ of the sun, any part of which could be subjected to special scrutiny, instead of, as had hitherto been done, admitting rays from every portion of his surface indiscriminately. the answer to the inquiry was prompt and unmistakable, and was again, in this case, adverse to the french theorist's view. the obscurations in question were found to be produced by no deficiency of emissive power, but by an increase of absorptive action. the background of variegated light remains unchanged, but more of it is stopped by the interposition of a dense mass of relatively cool vapours. the spectrum of a sun-spot is crossed by the same set of multitudinous dark lines, with some minor differences, visible in the ordinary solar spectrum. we must then conclude that the same vapours (speaking generally) which are dispersed over the unbroken solar surface are accumulated in the umbral cavity, the compression incident to such accumulation being betrayed by the thickening of certain lines of absorption. but there is also a general absorption, extending almost continuously from one end of the spot-spectrum to the other. using, however, a spectroscope of exceptionally high dispersive power, professor young of princeton, new jersey, succeeded in in "resolving" the supposed continuous obscurity of spot-spectra into a countless multitude of fine dark lines set very close together.[ ] their structure was seen still more perfectly, about five years later, by m. dunér,[ ] director of the upsala observatory, who traced besides some shadowy vestiges of the crowded doublets and triplets forming the array, from the spots on to the general solar surface. they cease to be separable in the blue part of the spectrum; and the ultra-violet radiations of spots show nothing distinctive.[ ] as to the movements of the constipated vapours forming spots, the spectroscope is also competent to supply information. the principle of the method by which it is procured will be explained farther on. suffice it here to say that the transport, at any considerable velocity, to or from the eye of the gaseous material giving bright or dark lines, can be measured by the displacement of such lines from their previously known normal positions. in this way movements have been detected in or above spots of enormous rapidity, ranging up to _miles per second_. but the result, so far, has been to negative the ascription to them of any systematic direction. uprushes and downrushes are doubtless, as father cortie remarks,[ ] "correlated phenomena in the production of a sun-spot"; but neither seem to predominate as part of its regular internal economy. the same kind of spectroscopic evidence tells heavily against a theory of sun-spots started by faye in . he had been foremost in pointing out that the observations of carrington and spörer absolutely forbade the supposition that any phenomenon at all resembling our trade-winds exists in the sun. they showed, indeed, that beyond the parallels of ° there is a general tendency in spots to a slow poleward displacement, while within that zone they incline to approach the equator; but their "proper movements" gave no evidence of uniformly flowing currents in latitude. the systematic drift of the photosphere is strictly a drift in longitude; its direction is everywhere parallel to the equator. this fact being once clearly recognised, the "solar tornado" hypothesis at once fell to pieces; but m. faye[ ] perceived another source of vorticose motion in the unequal rotating velocities of contiguous portions of the photosphere. the "pores" with which the whole surface of the sun is studded he took to be the smaller eddies resulting from these inequalities; the spots to be such eddies developed into whirlpools. it only needs to thrust a stick into a stream to produce the kind of effect designated. and it happens that the differences of angular movement adverted to attain a maximum just about the latitudes where spots are most frequent and conspicuous. there are, however, grave difficulties in identifying the two kinds of phenomena. one (already mentioned) is the total absence of the regular swirling motion--in a direction contrary to that of the hands of a watch north of the solar equator, in the opposite sense south of it--which should impress itself upon every lineament of a sun-spot if the cause assigned were a primary producing, and not merely (as it possibly may be) a secondary determining one. the other, pointed out by young,[ ] is that the cause is inadequate to the effect. the difference of movement, or _relative drift_, supposed to occasion such prodigious disturbances, amounts, at the utmost, for two portions of the photosphere miles apart, to about five yards a minute. thus the friction of contiguous sections must be quite insignificant. a view better justified by observation was urged by secchi in and after the year , and was presented in an improved form by professor young in his excellent little book on _the sun_, published in .[ ] spots are manifestly associated with violent eruptive action, giving rise to the faculæ and prominences which usually garnish their borders. it is accordingly contended that upon the withdrawal of matter from below by the flinging up of a prominence must ensue a sinking-in of the surface, into which the partially cooled erupted vapours rush and settle, producing just the kind of darkening by increased absorption told of by the spectroscope. round the edges of the cavity the rupture of the photospheric shell will form lines of weakness provocative of further eruptions, which will, in their turn, deepen and enlarge the cavity. the phenomenon thus tends to perpetuate itself, until equilibrium is at last restored by internal processes. a sun-spot might then be described as an inverted terrestrial volcano, in which the outbursts of heated matter take place on the borders instead of at the centre of the crater, while the cooled products gather in the centre instead of at the borders. but on the earth, the solid crust forcibly represses the steam gathering beneath until it has accumulated strength for an explosion, while there is no such restraining power that we know of in the sun. zöllner, indeed, adapted his theory of the solar constitution to the special purpose of procuring it; yet with very partial success, since almost every new fact has proved adverse to his assumptions. volcanic action is essentially spasmodic. it implies habitual constraint varied by temporary outbreaks, inconceivable in a gaseous globe, such as we believe the sun to be. if the "volcanic hypothesis" represented the truth, no spot could possibly appear without a precedent eruption. the real order of the phenomenon, however, is exceedingly difficult to ascertain; nor is it perhaps invariable. although, in most cases, the "opening" shows first, that may be simply because it is more easily seen. according to father sidgreaves,[ ] the disturbance has then already passed the incipient stage. he considers it indeed "highly probable that the preparatory sign of a new spot is always a small, bright patch of facula." this sequence, if established, would be fatal to lockyer's theory of sun-spots, communicated to the royal society, may , ,[ ] and further developed some months later in his work on _the chemistry of the sun_. spots are represented in it as incidental to a vast system of solar atmospheric circulation, starting with the polar out- and up-flows indicated by observations during some total eclipses, and eventuating in the plunge downward from great heights upon the photosphere of prodigious masses of condensed materials. from these falls result, primarily, spots; secondarily, through the answering uprushes in which chemical and mechanical forces co-operate, their girdles of flame-prominences. the evidence is, however, slight that such a circulatory flow as would be needed to maintain this supposed cycle of occurrences really prevails in the sun's atmosphere; and a similar objection applies to an "anticyclonic theory" (so to designate it) elaborated by egon von oppolzer in .[ ] august schmidt's optical rationale of solar phenomena[ ] was, on the other hand, a complete novelty, both in principle and development. attractive to speculators from its recondite nature and far-reaching scope, it by no means commended itself to practical observers, intolerant of finding the all but palpable realities of their daily experience dealt with as illusory products of "circular refraction." a singular circumstance has now to be recounted. on the st of september, , while carrington was engaged in his daily work of measuring the positions of sun-spots, he was startled by the sudden appearance of two patches of peculiarly intense light within the area of the largest group visible. his first idea was that a ray of unmitigated sunshine had penetrated the screen employed to reduce the brilliancy of the image; but, having quickly convinced himself to the contrary, he ran to summon an additional witness of an unmistakably remarkable occurrence. on his return he was disappointed to find the strange luminous outburst already on the wane; shortly afterwards the last trace vanished. its entire duration was five minutes--from . to . a.m., greenwich time; and during those five minutes it had traversed a space estimated at , miles! no perceptible change took place in the details of the group of spots visited by this transitory conflagration, which, it was accordingly inferred, took place at a considerable height above it.[ ] carrington's account was precisely confirmed by an observation made at highgate. mr. r. hodgson described the appearance seen by him as that "of a very brilliant star of light, much brighter than the sun's surface, most dazzling to the protected eye, illuminating the upper edges of the adjacent spots and streaks, not unlike in effect the edging of the clouds at sunset."[ ] this unique phenomenon seemed as if specially designed to accentuate the inference of a sympathetic relation between the earth and the sun. from the th of august to the th of september, , a magnetic storm of unparalleled intensity, extent, and duration, was in progress over the entire globe. telegraphic communication was everywhere interrupted--except, indeed, that it was, in some cases, found practicable to work the lines _without batteries_, by the agency of the earth-currents alone:[ ] sparks issued from the wires; gorgeous auroræ draped the skies in solemn crimson over both hemispheres, and even within the tropics; the magnetic needle lost all trace of continuity in its movements, and darted to and fro as if stricken with inexplicable panic. the coincidence was drawn even closer. _at the very instant_[ ] of the solar outburst witnessed by carrington and hodgson, the photographic apparatus at kew registered a marked disturbance of all the three magnetic elements; while, shortly after the ensuing midnight, the electric agitation culminated, thrilling the earth with subtle vibrations, and lighting up the atmosphere from pole to pole with the coruscating splendours which, perhaps, dimly recall the times when our ancient planet itself shone as a star. here then, at least, the sun was--in professor balfour stewart's phrase--"taken in the act"[ ] of stirring up terrestrial commotions. nor have instances since been wanting of an indubitable connection between outbreaks of individual spots and magnetic disturbances. four such were registered in ; and symptoms of the same kind, including the beautiful "rose aurora," marked the progress across the sun of the enormous spot-group of february, --the largest ever recorded at greenwich. this extraordinary formation, which covered about / of the sun's disc, survived through five complete rotations.[ ] it was remarkable for a persistent drift in latitude, its place altering progressively from ° to ° south of the solar equator. again, the central passage of an enormous spot on september , , synchronised with a sharp magnetic disturbance and brilliant aurora;[ ] and the coincidence was substantially repeated in march, ,[ ] when it was emphasised by the prevalent cosmic calm. the theory of the connection is indeed far from clear. lord kelvin, in ,[ ] pronounced against the possibility of any direct magnetic action by the sun upon the earth, on the ground of its involving an extravagant output of energy; but the fact is unquestionable that--in professor bigelow's words--"abnormal agitations affect the sun and the earth as a whole and at the same time."[ ] the nearer approach to the event of september , , was photographically observed by professor george e. hale at chicago, july , .[ ] an active spot in the southern hemisphere was the scene of this curiously sudden manifestation. during an interval of m. between two successive exposures, a bridge of dazzling light was found to have spanned the boundary-line dividing the twin-nuclei of the spot; and these, after another m., were themselves almost obliterated by an overflow of far-spreading brilliancy. yet two hours later, no trace of the outburst remained, the spot and its attendant faculæ remaining just as they had been previously to its occurrence. unlike that seen by carrington, it was accompanied by no exceptional magnetic phenomena, although a "storm" set in next day.[ ] possibly a terrestrial analogue to the former might be discovered in the "auroral beam" which traversed the heavens during a vivid display of polar lights, november , , and shared, there is every reason to believe, their electrical origin and character.[ ] meantime m. rudolf wolf, transferred to the direction of the zürich observatory, where he died, december , , had relaxed none of his zeal in the investigation of sun-spot periodicity. a laborious revision of the entire subject with the aid of fresh materials led him, in ,[ ] to the conclusion that while the _mean_ period differed little from that arrived at in of . years, very considerable fluctuations on either side of that mean were rather the rule than the exception. indeed, the phrase "sun-spot period" must be understood as fitting very loosely the great fact it is taken to represent; so loosely, that the interval between two maxima may rise to sixteen and a half or sink below seven and a half years.[ ] in [ ] wolf showed, and the remark was fully confirmed at kew, that the shortest periods brought the most acute crises, and _vice versâ_; as if for each wave of disturbance a strictly equal amount of energy were available, which might spend itself lavishly and rapidly, or slowly and parsimoniously, but could in no case be exceeded. the further inclusion of recurring solar commotions within a cycle of fifty-five and a half years was simultaneously pointed out; and hermann fritz showed soon afterwards that the aurora borealis is subject to an identical double periodicity.[ ] the same inquirer has more recently detected both for auroræ and sun-spots a "secular period" of years,[ ] and the kew observations indicate for the latter, oscillations accomplished within twenty-six and twenty-four days,[ ] depending, most likely, upon the rotation of the sun. this is certainly reflected in magnetic, and perhaps in auroral periodicity. the more closely, in fact, spot-fluctuations are looked into, the more complex they prove. maxima of one order are superposed upon, or in part neutralised by, maxima of another order;[ ] originating causes are masked by modifying causes; the larger waves of the commotion are indented with minor undulations, and these again crisped with tiny ripples, while the whole rises and falls with the swell of the great secular wave, scarcely perceptible in its progress because so vast in scale. the idea that solar maculation depends in some way upon the position of the planets occurred to galileo in .[ ] it has been industriously sifted by a whole bevy of modern solar physicists. wolf in [ ] found reason to believe that the eleven-year curve is determined by the action of jupiter, modified by that of saturn, and diversified by influences proceeding from the earth and venus. its tempting approach to agreement with jupiter's period of revolution round the sun, indeed, irresistibly suggested a causal connection; yet it does not seem that the most skilful "coaxing" of figures can bring about a fundamental harmony. carrington pointed out in , that while, during _eight successive periods_, from downwards, there were approximate coincidences between jupiter's aphelion passages and sun-spot maxima, the relation had been almost exactly reversed in the two periods preceding that date;[ ] and wolf himself finally concluded that the jovian origin must be abandoned.[ ] m. duponchel's[ ] prediction, nevertheless, of an abnormal retardation of the maximum due in through certain peculiarities in the positions of uranus and neptune about the time it fell due, was partially verified by the event, since, after an abortive phase of agitation in april, , the final outburst was postponed to january, . the interval was thus . instead of . years; and it is noticeable that the delay affected chiefly the southern hemisphere. alternations of activity in the solar hemispheres were indeed a marked feature of the maximum of , which, in m. faye's view,[ ] derived thence its indecisive character, while sharp, strong crises arise with the simultaneous advance of agitation north and south of the solar equator. the curve of magnetic disturbance followed with its usual strict fidelity the anomalous fluctuations of the sun-spot curve. the ensuing minimum occurred early in , and was succeeded in by a maximum slightly less feeble than its predecessor.[ ] it cannot be said that much progress has been made towards the disclosure of the cause, or causes, of the sun-spot cycle. no external influence adequate to the effect has, at any rate, yet been pointed out. most thinkers on this difficult subject provide a quasi-explanation of the periodicity in question through certain assumed vicissitudes affecting internal processes;[ ] sir norman lockyer and e. von oppolzer reach the same end by establishing self-compensatory fluctuations in the solar atmospheric circulation; dr. schuster resorts to changes in the electrical conductivity of space near the sun.[ ] in all these theories, however, the course of transition is arbitrarily arranged to suit a period, which imposes itself as a fact peremptorily claiming admittance, while obstinately defying explanation. the question so much discussed, as to the influence of sun-spots on weather, does not admit of a satisfactory answer. the facts of meteorology are too complex for easy or certain classification. effects owning dependence on one cause often wear the livery of another; the meaning of observed particulars may be inverted by situation; and yet it is only by the collection and collocation of particulars that we can hope to reach any general law. there is, however, a good deal of evidence to support the opinion--the grounds for which were primarily derived from the labours of dr. meldrum at mauritius--that increased rainfall and atmospheric agitation attend spot-maxima; while herschel's conjecture of a more copious emission of light and heat about the same epochs has recently obtained some countenance from savélieff's measures showing a gain in the strength of the sun's radiation _pari passu_ with increase in the number of spots visible on his surface.[ ] the examination of what we may call the _texture_ of the sun's surface derived new interest from a remarkable announcement made by mr. james nasmyth in .[ ] he had made (as he supposed) the discovery that the entire luminous stratum of the sun is composed of a multitude of elongated shining objects on a darker background, shaped much like willow-leaves, of vast size, crossing each other in all possible directions, and possessed of unceasing relative motions. a lively controversy ensued. in england and abroad the most powerful telescopes were directed to a scrutiny encompassed with varied difficulties. mr. dawes was especially emphatic in declaring that nasmyth's "willow-leaves" were nothing more than the "nodules" of sir william herschel seen under a misleading aspect of uniformity; and there is little doubt that he was right. it is, nevertheless, admitted that something of the kind may be seen in the penumbræ and "bridges" of spots, presenting an appearance compared by dawes himself in to that of a piece of coarse straw-thatching left untrimmed at the edges.[ ] the term "granulated," suggested by dawes in ,[ ] best describes the mottled aspect of the solar disc as shown by modern telescopes and cameras. the grains, or rather the "floccules," with which it is thickly strewn, have been resolved by langley, under exceptionally favourable conditions, into "granules" not above miles in diameter; and from these relatively minute elements, composing, jointly, about one-fifth of the visible photosphere,[ ] he estimates that three-quarters of the entire light of the sun are derived.[ ] janssen agrees, so far as to say that if the whole surface were as bright as its brightest parts, its luminous emission would be ten to twenty times greater than it actually is.[ ] the rapid changes in the forms of these solar cloud-summits are beautifully shown in the marvellous photographs taken by janssen at meudon, with exposures reduced at times to / of a second! by their means, also, the curious phenomenon known as the _réseau photosphérique_ has been made evident.[ ] this consists in the diffusion over the entire disc of fleeting blurred patches, separated by a reticulation of sharply-outlined and regularly-arranged granules. the imperfect definition in the smudged areas may be due to agitations in the solar or terrestrial atmosphere, unless it be--as dr. schemer thinks possible[ ]--merely a photographic effect. m. janssen considers that the photospheric cloudlets change their shape and character with the progress of the sun-spot period;[ ] but this is as yet uncertain. the "grains," or more brilliant parts of the photosphere, are now generally held to represent the upper termination of ascending and condensing currents, while the darker interstices (herschel's "pores") mark the positions of descending cooler ones. in the penumbræ of spots, the glowing streams rushing up from the tremendous sub-solar furnace are bent sideways by the powerful indraught, so as to change their vertical for a nearly horizontal motion, and are thus taken, as it were, in flank by the eye, instead of being seen end-on in mamelon-form. this gives a plausible explanation of the channelled structure of penumbræ which suggested the comparison to a rude thatch. accepting this theory as in the main correct, we perceive that the very same circulatory process which, in its spasms of activity, gives rise to spots, produces in its regular course the singular "marbled" appearance, for the recording of which we are no longer at the mercy of the fugitive or delusive impressions of the human retina. and precisely this circulatory process it is which gives to our great luminary its permance as a _sun_, or warming and illuminating body. footnotes: [footnote : _mem. r. a. s._, vol. xxi., p. .] [footnote : _ibid._, p. .] [footnote : _month. not._, vol. xxi., p. .] [footnote : _le soleil_, t. i., pp. - ( nd ed., ).] [footnote : see _ante_, p. .] [footnote : _observations at redhill ( )_, introduction.] [footnote : _month. not._, vol. xxxvi., p. .] [footnote : _cape observations_, p. , _note_.] [footnote : _month. not._, vol. x., p. .] [footnote : _rosa ursina_, lib. iii., p. .] [footnote : _observations at redhill_, p. .] [footnote : _op._, t. iii., p. .] [footnote : _rosa ursina_, lib. iv., p. . both galileo and scheiner spoke of the _apparent_ or "synodical" period, which is about one and a third days longer than the _true_ or "sidereal" one. the difference is caused by the revolution of the earth in its orbit in the same direction with the sun's rotation on its axis.] [footnote : _rosa ursina_, lib. iii., p. .] [footnote : faye, _comptes rendus_, t. lx., p. .] [footnote : _ibid._, t. xii., p. .] [footnote : _proc. am. ass. adv. of science_, , p. .] [footnote : _observations at redhill_, p. .] [footnote : _am. jour. of science_, vol. xi., p. .] [footnote : _month. not._, vol. xix., p. .] [footnote : _vierteljahrsschrift der naturfors. gesellschaft_ (zürich), , p. .] [footnote : lockyer, _chemistry of the sun_, p. .] [footnote : maunder, _knowledge_, vol. xv., p. .] [footnote : _month. mon._, vol. l., p. .] [footnote : maunder, _knowledge_, vol. xvii., p. .] [footnote : _astr. nach._, no. , .] [footnote : as late as an elaborate treatise in its support was written by f. coyteux, entitled _qu'est-ce que le soleil? peut-il être habité?_ and answering the question in the affirmative.] [footnote : the subsequent researches of plücker, frankland, wüllner, and others, showed that gases strongly compressed give an absolutely unbroken spectrum.] [footnote : _comptes rendus_, t. lx., pp. , .] [footnote : _ibid._, t. c., p. .] [footnote : _bull. meteor. dell osservatorio dell coll. rom._, jan. , , p. .] [footnote : _quart. jour. of science_, vol. i., p. .] [footnote : _ann. de chim. et de phys._, t. xxii., p. .] [footnote : _phil. trans._, vol. clix., p. .] [footnote : _les mondes_, dec. , , p. .] [footnote : _comptes rendus_, t. lx., p. .] [footnote : _proc. roy. society_, vol. xvi., p. .] [footnote : _recherches sur le spectre solaire_, p. .] [footnote : _am. jour. of science_, , vol. xxi., p. . hastings stipulated only for some member of the triad, carbon, silicon, and boron.] [footnote : ranyard, _knowledge_, vol. xvi., p. .] [footnote : young, _the sun_, p. , ed. .] [footnote : h. draper, _quart. journ. of sc._, vol. i., p. ; also _phil. mag._, vol. xvii., , p. .] [footnote : reproduced in arago's _popular astronomy_, plate xii., vol. .] [footnote : _report brit. ass._, , p. .] [footnote : _phil. trans._, vol. clii., p. .] [footnote : _researches in solar physics_, part i., p. .] [footnote : both the phrase and the method were suggested by faye, who estimated the average depth of the luminous sheath of spots at , miles. _comptes rendus_, t. lxi., p. ; t. xcvi., p. .] [footnote : _month. not._, vol. lv., p. .] [footnote : sidgreaves, _ibid._, p. ; cortie, _ibid._, vol. lviii., p. .] [footnote : explained by east as refraction-effects. _jour. brit. astr. ass._, vol. viii., p. .] [footnote : _proc. roy. soc._, vol. xiv., p. .] [footnote : _potsdam publicationen_, no. ; _astr. nach._, nos. , , , .] [footnote : faye, _comptes rendus_, t. cxi., p. ; bélopolsky, _astr. nach._, no. , .] [footnote : _ibid._, nos. , , , .] [footnote : lockyer, _contributions to solar physics_, p. .] [footnote : _le soleil_, p. .] [footnote : _proc. roy. soc._, vol. xv., p. .] [footnote : _phil. mag._, vol. xvi., p. .] [footnote : _recherches sur la rotation du soleil_, p. .] [footnote : hale, _astr. and astrophysics_, vol. xi., p. .] [footnote : _jour. brit. astr. ass._, vol. i., p. .] [footnote : _comptes rendus_, t. lxxv., p. ; _revue scientifique_, t. v., p. ( ). mr. herbert spencer had already (in _the reader_, feb. , ) put forward an opinion that spots were of the nature of "cyclonic clouds."] [footnote : _the sun_, p. . for faye's answer to the objection, see _comptes rendus_, t. xcv., p. .] [footnote : a revised edition appeared in .] [footnote : _astr. and astrophysics_, vol. xii., p. .] [footnote : _proc. roy. soc._, no. .] [footnote : _astr. nach._, no. , ; _astr. and astrophysics_, vol. xii., pp. , .] [footnote : _sirius_, sept., ; _ibid._, vol. xxiii., p. ; _astrophy. jour._, vol. i., p. (wilczynski), p. (keeler); vol. ii., p. (hale).] [footnote : _month. not._, vol. xx., p. .] [footnote : _ibid._, p. .] [footnote : _am. jour._, vol. xxix. ( nd series), pp. , .] [footnote : the magnetic disturbance took place at . a.m., three minutes before the solar blaze compelled the attention of carrington.] [footnote : _phil. trans._, vol. cli., p. .] [footnote : maunder, _journal brit. astr. ass._, vol. ii., p. ; miss e. brown, _ibid._, p. ; month. not., vol. lii., p. .] [footnote : _observatory_, vol. xxi., p. ; maunder, _knowledge_, vol. xxi., p. ; fényi, _astroph. jour._, vol. x., p. .] [footnote : _ibid._, p. ; w. anderson, observatory, vol. xxii., p. .] [footnote : _proc. roy. society_, vol. lii., p. ; rev. w. sidgreaves, _mem. r. a. s._, vol. liv., p. .] [footnote : _report on solar and terrestrial magnetism_, washington, , p. .] [footnote : _astr. and astrophysics_, vol. xi., p. .] [footnote : _ibid._, p. (sidgreaves).] [footnote : see j. rand capron, _phil. mag._, vol. xv., p. .] [footnote : _mittheilungen über die sonnenflecken_, no. ix., _vierteljahrsschrift der naturforschenden gesellschaft in zürich_, jahrgang .] [footnote : _mitth._, no. lii., p. ( ).] [footnote : _ibid._, no. xii., p. . baxendell, of manchester, reached independently a similar conclusion. see _month. not._, vol. xxi., p. .] [footnote : wolf, _mitth._, no. xv., p. , etc. olmsted, following hansteen, had already, in , sought to establish an auroral period of sixty-five years. _smithsonian contributions_, vol. viii., p. .] [footnote : hahn, _ueber die reziehungen der sonnenfleckenperiode zu meteorologischen erscheinungen_, p. ( ).] [footnote : _report brit. ass._, , p. ; , p. .] [footnote : the rev. a. cortie (_month. not._, vol. lx., p. ) detects the influence of a short subsidiary cycle, dr. w. j. s. lockyer that of a thirty-five year period (_nature_, june , ). professor newcomb (_astroph. jour._, vol. xiii., p. ) considers that solar activity oscillates uniformly in . years, with superposed periodic variations.] [footnote : _opere_, t. iii., p. .] [footnote : _mitth._, nos. vii. and xviii.] [footnote : _observations at redhill_, p. .] [footnote : _comptes rendus_, t. xcv., p. .] [footnote : _ibid._, t. xciii., p. ; t. xcvi., p. .] [footnote : _ibid._, t. c, p. .] [footnote : ellis, _proc. roy. society_, vol. lxiii., p. .] [footnote : schultz, _astr. nach._, nos. , - , , - ; wilsing, _ibid._, no. , ; bélopolsky, _ibid._, no. , .] [footnote : _report brit. ass._, , p. .] [footnote : a. w. augur, _astroph. jour._, vol. xiii., p. .] [footnote : _report brit. ass._, , p. (pt. ii.).] [footnote : _mem. r. a. s._, vol. xxi., p. .] [footnote : _month. not._, vol. xxiv., p. .] [footnote : _am. jour. of science_, vol. vii., , p. .] [footnote : young, _the sun_, p. .] [footnote : _ann. bur. long._, , p. .] [footnote : _ibid._, , p. .] [footnote : _himmelsphotographie_, p. .] [footnote : ranyard, _knowledge_, vols. xiv., p. , xvi., p. ; see also the accompanying photographs.] chapter iii _recent solar eclipses_ by observations made during a series of five remarkable eclipses, comprised within a period of eleven years, knowledge of the solar surroundings was advanced nearly to its present stage. each of these events brought with it a fresh disclosure of a definite and unmistakable character. we will now briefly review this orderly sequence of discovery. photography was first systematically applied to solve the problems presented by the eclipsed sun, july , . it is true that a daguerreotype,[ ] taken by berkowski with the königsberg heliometer during the eclipse of , is still valuable as a record of the corona of that year; and some subsequent attempts were made to register partial phases of solar occultation, notably by professor bartlett at west point in ;[ ] but the ground remained practically unbroken until . in that year the track of totality crossed spain, and thither, accordingly, warren de la rue transported his photo-heliograph, and father secchi his six-inch cauchoix refractor. the question then primarily at issue was that relating to the nature of the red protuberances. although, as already stated, the evidence collected in gave a reasonable certainty of their connection with the sun, objectors were not silenced; and when the side of incredulity was supported by so considerable an authority as m. faye, it was impossible to treat it with contempt. two crucial tests were available. if it could be shown that the fantastic shapes suspended above the edge of the dark moon were seen under an identical aspect from two distant stations, that fact alone would annihilate the theory of optical illusion or "mirage"; while the certainty that they were progressively concealed by the advancing moon on one side, and uncovered on the other, would effectually detach them from dependence on our satellite, and establish them as solar appendages. now both these tests were eminently capable of being applied by photography. but the difficulty arose that nothing was known as to the chemical power of the rosy prominence-light, while everything depended on its right estimation. a shot had to be fired, as it were, in the dark. it was a matter of some surprise, and of no small congratulation, that, in both cases, the shot took effect. de la rue occupied a station at rivabellosa, in the upper ebro valley; secchi set up his instrument at desierto de las palmas, about miles to the south-east, overlooking the mediterranean. from the totally eclipsed sun, with its strange garland of flames, each observer derived several perfectly successful impressions, which were found, on comparison, to agree in the most minute details. this at once settled the fundamental question as to the substantial reality of these objects; while their solar character was demonstrated by the passage of the moon _in front_ of them, indisputably attested by pictures taken at successive stages of the eclipse. that forms seeming to defy all laws of equilibrium were, nevertheless, not wholly evanescent, appeared from their identity at an interval of seven minutes, during which the lunar shadow was in transit from one station to the other; and the singular energy of their actinic rays was shown by the record on the sensitive plates of some prominences invisible in the telescope. moreover, photographic evidence strongly confirmed the inference--previously drawn by grant and others, and now with fuller assurance by secchi--that an uninterrupted stratum of prominence-matter encompasses the sun on all sides, forming a reservoir from which gigantic jets issue, and into which they subside. thus, first-fruits of accurate knowledge regarding the solar surroundings were gathered, while the value of the brief moments of eclipse gained indefinite increase, by supplementing transient visual impressions with the faithful and lasting records of the camera. in the year the history of eclipse spectroscopy virtually began, as that of eclipse photography in ; that is to say, the respective methods then first gave definite results. on the th of august, , the indian and malayan peninsulas were traversed by a lunar shadow producing total obscuration during five minutes and thirty-eight seconds. two english and two french expeditions were despatched to the distant regions favoured by an event so propitious to the advance of knowledge, chiefly to obtain the verdict of the prism as to the composition of prominences. nor were they despatched in vain. an identical discovery was made by nearly all the observers. at jamkandi, in the western ghauts, where lieutenant (now colonel) herschel was posted, unremitting bad weather threatened to baffle his eager expectations; but during the lapse of the critical five and a half minutes the clouds broke, and across the driving wrack a "long, finger-like projection" jutted out over the margin of the dark lunar globe. in another moment the spectroscope was pointed towards it; three bright lines--red, orange, and blue--flashed out, and the problem was solved.[ ] the problem was solved in this general sense, that the composition out of glowing vapours of the objects infelicitously termed "protuberances" or "prominences" was no longer doubtful; although further inquiry was needed for the determination of the particular species to which those vapours belonged. similar, but more complete observations were made, with less atmospheric hindrance, by tennant and janssen at guntoor, by pogson at masulipatam, and by rayet at wha-tonne, on the coast of the malay peninsula, the last observer counting as many as nine bright lines.[ ] among them it was not difficult to recognise the characteristic light of hydrogen; and it was generally, though over-hastily, assumed that the orange ray matched the luminous emissions of sodium. but fuller opportunities were at hand. the eclipse of is chiefly memorable for having taught astronomers to do without eclipses, so far, at least, as one particular branch of solar inquiry is concerned. inspired by the beauty and brilliancy of the variously tinted prominence-lines revealed to him by the spectroscope, janssen exclaimed to those about him, "je verrai ces lignes-là en dehors des éclipses!" on the following morning he carried into execution the plan which formed itself in his brain while the phenomenon which suggested it was still before his eyes. it rests upon an easily intelligible principle. the glare of our own atmosphere alone hides the appendages of the sun from our daily view. to a spectator on an airless planet, the central globe would appear attended by all its splendid retinue of crimson prominences, silvery corona, and far-spreading zodiacal light projected on the star-spangled black background of an absolutely unilluminated sky. now the spectroscope offers the means of indefinitely weakening atmospheric glare by diffusing a constant amount of it over an area widened _ad libitum_. but monochromatic or "bright-line" light is, by its nature, incapable of being so diffused. it can, of course, be _deviated_ by refraction to any extent desired; but it always remains equally concentrated, in whatever direction it may be thrown. hence, when it is mixed up with continuous light--as in the case of the solar flames shining through our atmosphere--it derives a _relative_ gain in intensity from every addition to the dispersive power of the spectroscope with which the heterogeneous mass of beams is analysed. employ prisms enough, and eventually the undiminished rays of persistent colour will stand out from the continually fading rainbow-tinted band, by which they were at first effectually veiled. this janssen saw by a flash of intuition while the eclipse was in progress; and this he realised at a.m. next morning, august , --the date of the beginning of spectroscopic work at the margin of the unobscured sun. during the whole of that day and many subsequent ones, he enjoyed, as he said, the advantage of a prolonged eclipse. the intense interest with which he surveyed the region suddenly laid bare to his scrutiny was heightened by evidences of rapid and violent change. on the th of august, during the eclipse, a vast spiral structure, _at least_ , miles high, was perceived, planted in surprising splendour on the rim of the interposed moon. if was formed as general tennant judged from its appearance in his photographs, by the encounter of two mounting torrents of flame, and was distinguished as the "great horn." next day it was in ruins; hardly a trace remained to show where it had been.[ ] janssen's spectroscope furnished him besides with the strongest confirmation of what had already been reported by the telescope and the camera as to the continuous nature of the scarlet "sierra" lying at the base of the prominences. everywhere at the sun's edge the same bright lines appeared. it was not until the th of september that janssen thought fit to send news of his discovery to europe. it seemed little likely to be anticipated; yet a few minutes before his despatch was handed to the secretary of the paris academy of sciences, a communication similar in purport had been received from sir norman lockyer. there is no need to discuss the narrow and wearisome question of priority; each of the competitors deserves, and has obtained, full credit for his invention. with noteworthy and confident prescience, lockyer, in , before anything was yet known regarding the constitution of the "red flames," ordered a strongly dispersive spectroscope for the express purpose of viewing, apart from eclipses, the bright-line spectrum which he expected them to give. various delays, however, supervened, and the instrument was not in his hands until october , . on the th he picked up the vivid rays, of which the presence and (approximately) the positions had in the interim become known. but there is little doubt that, even without that previous knowledge, they would have been found; and that the eclipse of august only accelerated a discovery already assured. sir william huggins, meanwhile, had been tending towards the same goal during two and a half years in his observatory at tulse hill. the principle of the spectroscopic visibility of prominence-lines at the edge of an uneclipsed sun was quite explicitly stated by him in february, ,[ ] and he devised various apparatus for bringing them into actual view; but not until he knew where to look did he succeed in seeing them. astronomers, thus liberated, by the acquisition of power to survey them at any time, from the necessity of studying prominences during eclipses, were able to concentrate the whole of their attention on the corona. the first thing to be done was to ascertain the character of its spectrum. this was seen in only as a faintly continuous one; for rayet, who seems to have perceived its distinctive bright line far above the summits of the flames, connected it, nevertheless, with those objects. on the other hand, lieutenant campbell ascertained on the same occasion the polarisation of the coronal light in planes passing through the sun's centre,[ ] thereby showing that light to be, in whole or in part, reflected sunshine. but if reflected sunshine, it was objected, the chief at least of the dark fraunhofer lines should be visible in it, as they are visible in moonbeams, sky illumination, and all other sun-derived light. the objection was well founded, but was prematurely urged, as we shall see. on the th of august, , a track of total eclipse crossed the continent of north america diagonally, entering at behring's straits, and issuing on the coast of north carolina. it was beset with observers; but the most effective work was done in iowa. at des moines, professor harkness of the naval observatory, washington, obtained from the corona an "absolutely continuous spectrum," slightly less bright than that of the full moon, but traversed by a single green ray.[ ] the same green ray was seen at burlington and its position measured by professor young of dartmouth college.[ ] it appeared to coincide with that of a dark line of iron in the solar spectrum, numbered , on kirchhoff's scale. but in young was able, by the use of greatly increased dispersion, to resolve the fraunhofer line " " into a pair, the more refrangible member of which he considered to be the reversal of the green coronal ray.[ ] scarcely called in question for over twenty years, the identification nevertheless broke down through the testimony of the eclipse-photographs of . sir norman lockyer derived from them a position for the line in question notably higher up in the spectrum than that previously assigned to it. instead of , , its true wave-length proved to be , ten millionths of a millimetre;[ ] nor does it make any show by absorption in dispersed sunlight. the originating substance, designated "coronium," of which nothing is known to terrestrial chemistry, continues luminous[ ] at least , miles above the sun's surface, and is hence presumably much lighter even than hydrogen. a further trophy was carried off by american skill[ ] sixteen months after the determination due to it of the distinctive spectrum of the corona. the eclipse of december , , though lasting only two minutes and ten seconds, drew observers from the new, as well as from the old world to the shores of the mediterranean. janssen issued from beleaguered paris in a balloon, carrying with him the _vital parts_ of a reflector specially constructed to collect evidence about the corona. but he reached oran only to find himself shut behind a cloud-curtain more impervious than the prussian lines. everywhere the sky was more or less overcast. lockyer's journey from england to sicily, and shipwreck in the _psyche_, were recompensed with a glimpse of the solar aureola during _one second and a half_! three parties stationed at various heights on mount etna saw absolutely nothing. nevertheless important information was snatched in despite of the elements. the prominent event was young's discovery of the "reversing layer." as the surviving solar crescent narrowed before the encroaching moon, "the dark lines of the spectrum," he tells us, "and the spectrum itself, gradually faded away, until all at once, as suddenly as a bursting rocket shoots out its stars, the whole field of view was filled with bright lines more numerous than one could count. the phenomenon was so sudden, so unexpected, and so wonderfully beautiful, as to force an involuntary exclamation."[ ] its duration was about two seconds, and the impression produced was that of a complete reversal of the fraunhofer spectrum--that is, the substitution of a bright for every dark line. now something of the kind was theoretically necessary to account for the dusky rays in sunlight which have taught us so much, and have yet much more to teach us; so that, although surprising from its transitory splendour, the appearance could not strictly be called "unexpected." moreover, its premonitory symptom in the fading out of these rays had been actually described by secchi in ,[ ] and looked for by young as the moon covered the sun in august . but with the slit of his spectroscope placed _normally_ to the sun's limb, the bright lines gave a flash too thin to catch the eye. in the position of the slit was _tangential_--it ran along the shallow bed of incandescent vapours, instead of cutting across it: hence his success. the same observation was made at xerez de la frontera by mr. pye, a member of young's party; and, although an exceedingly delicate one, has since frequently been repeated. the whole fraunhofer series appeared bright (omitting other instances) to maclear, herschel, and fyers in , at the beginning or end of totality; to pogson, at the break-up of an annual eclipse, june , ; to stone at klipfontein, april , , when he saw "the field full of bright lines."[ ] but between the picture presented by the "véritable pluie de lignes brilliantes,"[ ] which descended into m. trépied's spectroscope for three seconds after the disappearance of the sun, may , , and the familiar one of the dark-line solar spectrum, certain differences were perceiving, showing their relation to be not simply that of a positive to a negative impression. a "reversing layer," or stratum of mixed vapours, glowing, but at a lower temperature than that of the actual solar surface, was an integral part of kirchhoff's theory of the production of the fraunhofer lines. here it was assumed that the missing rays were stopped, and here also it was assumed that the missing rays would be seen bright, could they be isolated from the overpowering splendour of their background. this isolation is effected by eclipses, with the result--beautifully confirmatory of theory--of _reversing_, or turning from dark to bright, the fraunhofer spectrum. the completeness and precision of the reversal, however, could not be visually attested; and a quarter of a century elapsed before a successful "snap-shot" provided photographic evidence on the subject. it was taken at novaya zemlya by mr. shackleton, a member of the late sir george baden-powell's expedition to observe the eclipse of august , ;[ ] and similar records in abundance were secured during the indian eclipse of january , ,[ ] and the spanish-american eclipse of may , .[ ] the result of their leisurely examination has been to verify the existence of a "reversing-layer," in the literal sense of the term. it is true that no single "flash" photograph is an inverted transcript of the fraunhofer spectrum. the lines are, indeed, in each case--speaking broadly--the same; but their relative intensities are widely different. yet this need occasion no surprise when we remember that the fraunhofer spectrum integrates the absorption of multitudinous strata, various in density and composition, while only the upper section of the formation comes within view of the sensitive plates exposed at totalities, the low-lying vaporous beds being necessarily covered by the moon. the total depth of this glowing envelope may be estimated at to miles, and its normal state seems to be one of profound tranquillity, judging from the imperturbable aspect of the array of dark lines due to its sifting action upon light. the last of the five eclipses which we have grouped together for separate consideration was visible in southern india and australia, december , . some splendid photographs were secured by the english parties on the malabar coast, showing, for the first time, the remarkable branching forms of the coronal emanations; but the most conspicuous result was janssen's detection of some of the dark fraunhofer lines, long vainly sought in the continuous spectrum of the corona. chief among these was the d-line of sodium, the original index, it might be said, to solar chemistry. no proof could be afforded more decisive that this faint _echoing back_ of the distinctive notes of the fraunhofer spectrum, that the polariscope had spoken the truth in asserting a large part of the coronal radiance to be reflected sunlight. but it is usually so drenched in original luminosity, that its special features are almost obliterated. janssen's success in seizing them was due in part to the extreme purity of the air at sholoor, in the neilgherries, where he was stationed; in part to the use of an instrument adapted by its large aperture and short focus to give an image of the utmost brilliancy. his observation, repeated during the caroline island eclipse of , was photographically verified ten years later by m. de la baume pluvinel in senegal.[ ] an instrument of great value for particular purposes was introduced into eclipse-work in . the "slitless spectroscope" consists simply of a prism placed outside the object-glass of a telescope or the lens of a camera, whereby the radiance encompassing the eclipsed sun is separated into as many differently tinted rings as it contains different kinds of light. these tinted rings were simultaneously viewed by respighi at poodacottah, and by lockyer at baikul. their photographic registration by the latter in initiated the transformation of the slitless spectroscope into the prismatic camera.[ ] meanwhile, the use of an ordinary spectroscope by herschel and tennant at dodabetta showed the green ray of coronium to be just as bright in a rift as in the adjacent streamer. the visible structure of the corona was thus seen to be independent of the distribution of the gases which enter into its composition. by means, then, of the five great eclipses of - it was ascertained: first, that the prominences, and at least the lower part of the corona, are genuine solar appurtenances; secondly, that the prominences are composed of hydrogen and other gases in a state of incandescence, and rise, as irregular outliers, from a continuous envelope of the same materials, some thousands of miles in thickness; thirdly, that the corona is of a highly complex constitution, being made up in part of glowing vapours, in part of matter capable of reflecting sunlight. we may now proceed to consider the results of subsequent eclipses. these have raised, and have helped to solve, some very curious questions. indeed, every carefully watched total eclipse of the sun stimulates as well as appeases curiosity, and leaves a legacy of outstanding doubt, continually, as time and inquiry go on, removed, but continually replaced. it cannot be denied that the corona is a perplexing phenomenon, and that it does not become less perplexing as we know more about it. it presented itself under quite a new and strange aspect on the occasion of the eclipse which visited the western states of north america, july , . the conditions of observation were peculiarly favourable. the weather was superb; above the rocky mountains the sky was of such purity as to permit the detection of jupiter's satellites with the naked eye on several successive nights. the opportunity for advancing knowledge was made the most of. nearly a hundred astronomers, including several englishmen, occupied twelve separate posts, and prepared for an attack in force. the question had often suggested itself, and was a natural one to ask, whether the corona sympathises with the general condition of the sun? whether, either in shape or brilliancy, it varies with the progress of the sun-spot period? a more propitious moment for getting this question answered could hardly have been chosen than that at which the eclipse occurred. solar disturbance was just then at its lowest ebb. the development of spots for the month of july, , was represented on wolf's system of "relative numbers" by the fraction · , as against · for december, , an epoch of maximum activity. the "chromosphere"[ ] was, for the most part, shallow and quiescent; its depth, above the spot zones, had sunk from about , to , miles;[ ] prominences were few and faint. obviously, if a type of corona corresponding to a minimum of sun-spots existed, it should be seen then or never. it _was_ seen; but while, in some respects, it agreed with anticipation, in others it completely set it at naught. the corona of , as compared with those of , , and , was generally admitted to be shrunken in its main outlines and much reduced in brilliancy. lockyer pronounced it ten times fainter than in ; harkness estimated its light at less than one-seventh that derived from the mist-blotted aureola of .[ ] in shape, too, it was markedly different. when sun-spots are numerous, the corona appears to be most fully developed above the spot-zones, thus offering to our eyes a rudely quadrilateral contour. the four great luminous sheaves forming the corners of the square are made up of rays curving together from each side into "synclinal" or ogival groups, each of which may be compared to the petal of a flower. to janssen, in , the eclipsing moon seemed like the dark heart of a gigantic dahlia, painted in light on the sky; and the similitude to the ornament on a compass-card, used by airy in , well conveys the decorative effect of the beamy, radiated kind of aureola, never, it would appear, absent when solar activity is at a tolerably high pitch. in his splendid volume on eclipses,[ ] with which the systematic study of coronal structure may be said to have begun, mr. ranyard first generalised the synclinal peculiarity by a comparison of records; but the symmetry of the arrangement, though frequently striking, is liable to be confused by secondary formations. he further pointed out, with the help of careful drawings from the photographs of made by mr. wesley, the curved and branching shapes assumed by the component filaments of massive bundles of rays. nothing of all this, however, was visible in . instead, there was seen, as the groundwork of the corona, a ring of pearly light, nebulous to the eye, but shown by telescopes and in photographs to have a fibrous texture, as if made up of tufts of fine hairs. north and south, a series of short, vivid, electrical-looking flame-brushes diverged with conspicuous regularity from each of the solar poles. their direction was not towards the centre of the sun, but towards each summit of his axis, so that the farther rays on either side started almost tangentially to the surface. but the leading, and a truly amazing, characteristic of the phenomenon was formed by two vast, faintly-luminous _wings_ of light, expanded on either side of the sun in the direction of the ecliptic. these were missed by very few careful onlookers; but the extent assigned to them varied with skill in, and facilities for seeing. by far the most striking observations were made by newcomb at separation (wyoming), by cleveland abbe from the shoulder of pike's peak, and by langley at its summit, an elevation of , feet above the sea. never before had an eclipse been viewed from anything approaching that altitude, or under so translucent a sky. a proof of the great reduction in atmospheric glare was afforded by the perceptibility of the corona four minutes after totality was over. for the seconds of its duration, the remarkable streamers above alluded to continued "persistently visible," stretching away right and left of the sun to a distance of at least ten million miles! one branch was traced over an apparent extent of fully twelve lunar diameters, without sign of a definite termination having been reached; and there were no grounds for supposing the other more restricted. the resemblance to the zodiacal light was striking; and a community of origin between that enigmatical member of our system and the corona was irresistibly suggested. we should, indeed, expect to see, under such exceptionally favourable atmospheric conditions as professor langley enjoyed on pike's peak, the _roots_ of the zodiacal light presenting near the sun just such an appearance as he witnessed; but we can imagine no reason why their visibility should be associated with a low state of solar activity. nevertheless this seems to be the case with the streamers which astonished astronomers in . for in august, , when similar equatorial emanations, accompanied by similar symptoms of polar excitement, were described and depicted by grosch[ ] of the santiago observatory, sun-spots were at a minimum; while the corona of , which appears from the record of it by roger cotes[ ] to have been of the same type, preceded by three years the ensuing maximum. the eclipsed sun was seen by him at cambridge, may , , encompassed with a ring of light about one-sixth of the moon's diameter in breadth, upon which was superposed a luminous cross formed of long bright branches lying very nearly in the plane of the ecliptic, and shorter polar arms so faint as to be only intermittently visible. the resemblance between his sketch and cleveland abbe's drawing of the corona of is extremely striking. it should, nevertheless, be noted that some conspicuous spots were visible on the sun's disc at the time of cotes's eclipse, and that the preceding minimum (according to wolf) occurred in . thus, the coincidence of epochs is imperfect. professor cleveland abbe was fully persuaded that the long rays carefully observed by him from pike's peak were nothing else than streams of meteorites rushing towards or from perihelion; and it is quite certain that the solar neighbourhood must be crowded with such bodies. but the peculiar structure at the base of the streamers displayed in the photographs, the curved rays meeting in pointed arches like gothic windows, the visible upspringing tendency, the filamentous texture,[ ] speak unmistakably of the action of forces proceeding _from_ the sun, not of extraneous matter circling round him. a further proof of sympathetic change in the corona is afforded by the analysis of its light. in the bright line so conspicuous in the coronal spectrum in and had faded to the very limit of visibility. several skilled observers failed to see it at all; but young and eastman succeeded in tracing the green "coronium" ray all round the sun, to a height estimated at , miles. the substance emitting it was thus present, though in a low state of incandescence. the continuous spectrum was relatively strong; faint traces of the fraunhofer lines attested for it an origin, in part by reflection; and polarisation was undoubted, increasing towards the limb, whereas in it reached a maximum at a considerable distance from it. experiments with edison's tasimeter seemed to show that the corona radiates a sensible amount of heat. the next promising eclipse occurred may , . the concourse of astronomers which has become usual on such occasions assembled this time at sohag, in upper egypt. rarely have seventy-four seconds been turned to such account. to each observer a special task was assigned, and the advantages of a strict division of labour were visible in the variety and amount of the information gained. the year was one of numerous sun-spots. on the eve of the eclipse twenty-three separate maculæ were counted. if there were any truth in the theory which connected coronal forms with fluctuations in solar activity, it might be anticipated that the vast equatorial expansions and polar "brushes" of would be found replaced by the star-like structure of . this expectation was literally fulfilled. no lateral streamers were to be seen. the universal failure to perceive them, after express search in a sky of the most transparent purity, justifies the emphatic assertion that _they were not there_. instead, the type of corona observed in india eleven years earlier, was reproduced with its shining aigrettes, complex texture and brilliant radiated aspect. concordant testimony was given by the spectroscope. the reflected light derived from the corona was weaker than in , while its original emissions were proportionately intensified. nevertheless, most of the bright lines recorded as coronal[ ] were really due, there can be no doubt, to diffused chromospheric light. on this occasion, the first successful attempt was made to photograph the coronal spectrum procured in the ordinary way with a slit and prisms, while the prismatic camera was also profitably employed. it served to bring out at least one important fact--that of the uncommon strength in chromospheric regions of the twin violet beams of calcium, designated "h" and "k"; and prominence-photography signalised its improvement by the registration, in the spectrum of one such object, of twenty-nine rays, including many of the ultra-violet hydrogen series discovered by sir william huggins in the emission of white stars.[ ] dr. schuster's photographs of the corona itself were the most extensive, as well as the most detailed, of any yet secured. one rift imprinted itself on the plates to a distance of nearly a diameter and a half from the limb; and the transparency of the streamers was shown by the delineation through them of the delicate tracery beyond. the singular and picturesque feature was added of a bright comet, self-depicted in all the exquisite grace of swift movement betrayed by the fine curve of its tail, hurrying away from one of its rare visits to our sun, and rendered momentarily visible by the withdrawal of the splendour in which it had been, and was again quickly veiled. from a careful study of these valuable records sir william huggins derived the idea of a possible mode of photographing the corona _without an eclipse_.[ ] as already stated, its ordinary invisibility is entirely due to the "glare" or reflected light diffused through our atmosphere. but huggins found, on examining schuster's negatives, that a large proportion of the light in the coronal spectrum, both continuous and interrupted, is collected in the violet region between the fraunhofer lines g and h. there, then, he hoped that, all other rays being excluded, it might prove strong enough to vanquish inimical glare, and stamp on prepared plates, through _local_ superiority in illuminative power, the forms of the appendage by which it is emitted. his experiments were begun towards the end of may, , and by september he had obtained a fair earnest of success. the exclusion of all other qualities of light save that with which he desired to operate, was accomplished by using chloride of silver as his sensitive material, that substance being chemically inert to all other but those precise rays in which the corona has the advantage.[ ] plates thus sensitised received impressions which it was hardly possible to regard as spurious. "not only the general features," captain abney affirmed,[ ] "are the same, but details, such as rifts and streamers, have the same position and form." it was found, moreover, that the corona photographed during the total eclipse of may , , was intermediate in shape between the coronas photographed by sir william huggins before and after that event, each picture taking its proper place in a series of progressive modifications highly interesting in themselves, and full of promise for the value of the method employed to record them.[ ] but experiments on the subject were singularly interrupted. the volcanic explosion in the straits of sunda in august, , brought to astronomers a peculiarly unwelcome addition to their difficulties. the magnificent sunglows due to the diffractive effects on light of the vapours and fine dust flung in vast volumes into the air, and rapidly diffused all round the globe, betokened an atmospheric condition of all others the most prejudicial to delicate researches in the solar vicinity. the filmy coronal forms, accordingly, which had been hopefully traced on the tulse hill plates ceased to appear there; nor were any substantially better results obtained by mr. c. ray woods, in the purer air either of the riffel or the cape of good hope, during the three ensuing years. moreover, attempts to obtain coronal photographs during the partial phases of the eclipse of august , , completely failed. no part of the lunar globe became visible in relief against circumfluous solar radiance on any of the plates exposed at grenada; and what vestiges of "structure" there were, came out almost better _upon_ the moon than _beside_ her, thus stamping themselves at once as of atmospheric origin. that the effect sought is a perfectly possible one is proved by the distinct appearance of the moon projected on the corona, in photographs of the partially eclipsed sun in , , and , and very notably in and .[ ] in the spring of , professor hale[ ] attacked the problem of coronal daylight photography, employing the "double-slit" method so eminently serviceable for the delineation of prominences.[ ] but neither at kenwood nor at the summit of pike's peak, whither, in the course of the summer, he removed his apparatus, was any action of the desired kind secured. similar ill success attended his and professor riccò's employment, on mount etna in july, , of a specially designed coronagraph. yet discouragement did not induce despair. the end in view is indeed too important to be readily abandoned; but it can be reached only when a more particular acquaintance with the nature of coronal light than we now possess indicates the appropriate device for giving it a preferential advantage in self-portraiture. moreover, the effectiveness of this device may not improbably be enhanced, through changes in the coronal spectrum at epochs of sun-spot maximum. the prosperous result of the sohag observations stimulated the desire to repeat them on the first favourable opportunity. this offered itself one year later, may , , yet not without the drawbacks incident to terrestrial conditions. the eclipse promised was of rare length, giving no less than five minutes and twenty-three seconds of total obscurity, but its path was almost exclusively a "water-track." it touched land only on the outskirts of the marquesas group in the southern pacific, and presented, as the one available foothold for observers, a coral reef named caroline island, seven and a half miles long by one and a half wide, unknown previously to , and visited only for the sake of its stores of guano. seldom has a more striking proof been given of the vividness of human curiosity as to the condition of the worlds outside our own, than in the assemblage of a group of distinguished men from the chief centres of civilisation, on a barren ridge, isolated in a vast and tempestuous ocean, at a distance, in many cases, of , miles and upwards from the ordinary scene of their labours. and all these sacrifices--the cost and care of preparation, the transport and readjustment of delicate instruments, the contrivance of new and more subtle means of investigating phenomena--on the precarious chance of a clear sky during one particular five minutes! the event, though fortunate, emphasised the hazard of the venture. the observation of the eclipse was made possible only by the happy accident of a serene interval between two storms. the american expedition was led by professor edward s. holden, and to it were courteously permitted to be attached messrs. lawrance and woods, photographers, sent out by the royal society of london. m. janssen was chief of the french academy mission; he was accompanied from meudon by trouvelot, and joined from vienna by palisa, and from rome by tacchini. a large share of the work done was directed to assuring or negativing previous results. the circumstances of an eclipse favour illusion. a single observation by a single observer, made under unfamiliar conditions, and at a moment of peculiar excitement, can scarcely be regarded as offering more than a suggestion for future inquiry. but incredulity may be carried too far. janssen, for instance, felt compelled, by the survival of unwise doubts, to devote some of the precious minutes of obscurity at caroline island to confirming what, in his own persuasion, needed no confirmation--that is, the presence of reflected fraunhofer lines in the spectrum of the corona. trouvelot and palisa, on the other hand, instituted an exhaustive, but fruitless search for the spurious "intramercurian" planets announced by swift and watson in . new information, however, was not deficient. the corona proved identical in type with that of ,[ ] agreeably to what was expected at an epoch of protracted solar activity. the characteristic aigrettes were of even greater brilliancy than in the preceding year, and the chemical effects of the coronal light proved unusually intense. janssen's photographs, owing to the considerable apertures (six and eight inches) of his object-glasses, and the long exposures permitted by the duration of totality, were singularly perfect; they gave a greater extension to the coronal than could be traced with the telescope,[ ] and showed its forms as absolutely fixed and of remarkable complexity. the english pictures, taken with exposures up to sixty seconds, were likewise of great value. they exhibited details of structure from the limb to the tips of the streamers, which terminated definitely, and as it seemed actually, where the impressions on the plates ceased. the coronal spectrum was also successfully photographed, and although the reversing layer in its entirety evaded record, a print was caught of some of its more prominent rays just before and after totality. the use of the prismatic camera was baffled by the anomalous scarcity of prominences. using an ingenious apparatus for viewing simultaneously the spectrum from both sides of the sun, professor hastings noticed at caroline island alternations, with the advance of the moon, in the respective heights above the right and left solar limbs of the coronal green line, which were thought to imply that the corona, with its rifts and sheaves and "tangled hanks" of rays, is, after all, merely an illusive appearance produced by the diffraction of sunlight at the moon's edge.[ ] but the observation was assuredly misleading or misinterpreted. atmospheric _diffusion_ may indeed, under favouring circumstances, be effective in deceptively enlarging solar appendages; but always to a very limited extent. the controversy is an old one as to the part played by our air in producing the radiance visible round the eclipsed sun. in its original form, it is true, it came to an end when professor harkness, in ,[ ] pointed out that the shadow of the moon falls equally over the air and on the earth, and that if the sun had no luminous appendages, a circular space of almost absolute darkness would consequently surround the apparent places of the superposed sun and moon. mr. proctor,[ ] with his usual ability, impressed this mathematically certain truth upon public attention; and sir john herschel calculated that the diameter of the "negative halo" thus produced would be, in general, no less than °. but about the same time a noteworthy circumstance relating to the state of things in the solar vicinity was brought into view. on february , , messrs. frankland and lockyer communicated to the royal society a series of experiments on gaseous spectra under varying conditions of heat and density, leading them to the conclusion that the higher solar prominences exist in a medium of excessive tenuity, and that even at the base of the chromosphere the pressure is far below that at the earth's surface.[ ] this inference was fully borne out by the researches of wüllner; and janssen expressed the opinion that the chromospheric gases are rarefied almost to the degree of an air-pump vacuum.[ ] hence was derived a general and fully justified conviction that there could be outside, and incumbent upon the chromosphere, no such vast atmosphere as the corona appeared to represent. upon the strength of which conviction the "glare" theory entered, chiefly under the auspices of sir norman lockyer, upon the second stage of its existence. the genuineness of the "inner corona" to the height of ' or ' from the limb was admitted; but it was supposed that by the detailed reflection of its light in our air the far more extensive "outer corona" was optically created, the irregularities of the moon's edge being called in to account for the rays and rifts by which its structure was varied. this view received some countenance from admiral maclear's observation, during the eclipse of , of bright lines "everywhere"--even at the centre of the lunar disc. here, indeed, was an undoubted case of atmospheric diffusion; but here, also, was a safe index to the extent of its occurrence. light scatters equally in all directions; so that when the moon's face at the time of an eclipse shows (as is the common case) a blank in the spectroscope, it is quite certain that the corona is not noticeably enlarged by atmospheric causes. a sky drifted over with thin cirrus clouds and air changed with aqueous vapour amply accounted for the abnormal amount of scattering in . but even in positive evidence was obtained of the substantial reality of the radiated outer corona, in the appearance on the photographic plates exposed by willard in spain and by brothers in sicily of identical dark rifts. the truth is, that far from being developed by misty air, it is peculiarly liable to be effaced by it. the purer the sky, the more extensive, brilliant, and intricate in the details of its structure the corona appears. take as an example general myer's description of the eclipse of , as seen from the summit of white top mountain, virginia, at an elevation above the sea of , feet, in an atmosphere of peculiar clearness. "to the unaided eye," he wrote,[ ] "the eclipse presented, during the total obscuration, a vision magnificent beyond description. as a centre stood the full and intensely black disc of the moon, surrounded by the aureola of a soft bright light, through which shot out, as if from the circumference of the moon, straight, massive, silvery rays, seeming distinct and separate from each other, to a distance of two or three diameters of the solar disc; the whole spectacle showing as on a background of diffused rose-coloured light." on the same day, at des moines, newcomb could perceive, through somewhat hazy air, no long rays, and the four-pointed outline of the corona reached at its farthest only a _single semidiameter_ of the moon from the limb. the plain fact, that our atmosphere acts rather as a veil to hide the coronal radiance than as the medium through which it is visually formed, emerges from further innumerable records. no observations of importance were made during the eclipse of september , . the path of total obscurity touched land only on the shores of new zealand, and two minutes was the outside limit of available time. hence local observers had the phenomenon to themselves; nor were they even favoured by the weather in their efforts to make the most of it. one striking appearance was, however, disclosed. it was that of two "white" prominences of unusual brilliancy, shining like a pair of electric lamps hung one at each end of a solar diameter, right above the places of two large spots.[ ] this coincidence of diametrically opposite disturbances is of too frequent occurrence to be accidental. m. trouvelot observed at meudon, june , , two active and evanescent prominences thus situated, each rising to the enormous height of , miles; and on august , one scarcely less remarkable, balanced by an antipodal spot-group.[ ] it towered upward, as if by a process of _unrolling_, to a quarter of a million of miles; after which, in two minutes, the light died out of it; it had become completely extinct. the development, again from the ends of a diameter, of a pair of similar objects was watched, september and , , by father fényi, director of the kalocsa observatory; and the phenomenon has been too often repeated to be accidental. the eclipse of august , , was total during about four minutes over tropical atlantic regions; and an english expedition, led by sir norman lockyer, was accordingly despatched to grenada in the west indies, for the purpose of using the opportunity it offered. but the rainy season was just then at its height: clouds and squalls were the order of the day; and the elaborately planned programme of observation could only in part be carried through. some good work, none the less, was done. professor tacchini, who had been invited to accompany the party, ascertained besides some significant facts about prominences. from a comparison of their forms and sizes during and after the eclipse, it appeared that only the growing vaporous cores of these objects are shown by the spectroscope under ordinary circumstances; their upper sections, giving a faint continuous spectrum, and composed of presumably cooler materials, can only be seen when the veil of scattered light usually drawn over them is removed by an eclipse. thus all modestly tall prominences have silvery summits; but all do not appear to possess the "red heart of flame," by which alone they can be rendered perceptible to daylight observation. some prove to be ordinarily invisible, because silvery throughout--"sheeted ghosts," as it were, met only in the dark. specimens of the class had been noted as far back as , but tacchini first drew particular attention to them. the one observed by him in rose in a branching form to a height of , miles, and gave a brilliantly continuous spectrum, with bright lines at h and k, but no hydrogen-lines.[ ] hence the total invisibility of the object before and after the eclipse. during the eclipse, it was seen framed, as it were, in a pointed arch of coronal light, the symmetrical arrangement of which with regard to it was obviously significant. both its unspringing shape, and the violet rays of calcium strongly emitted by it, contradicted the supposition that "white prominences" represent a downrush of refrigerated materials. the corona of , as photographed by dr. schuster and mr. maunder, showed neither the petals and plumes of , nor the streamers of . it might be called of a transition type.[ ] wide polar rifts were filled in with tufted radiations, and bounded on either side by irregularly disposed, compound luminous masses. in the south-western quadrant, a triangular ray, conspicuous to the naked eye, represented, mr. w. h. pickering thought, the projection of a huge, hollow cone.[ ] branched and recurving jets were curiously associated with it. the intrinsic photographic brightness of the corona proved, from pickering's measures, to be about / that of the average surface of the full moon. the russian eclipse of august , , can only be remembered as a disastrous failure. much was expected of it. the shadow-path ran overland from leipsic to the japanese sea, so that the solar appurtenances would, it was hoped, be disclosed to observers echeloned along a line of , miles. but the incalculable element of weather rendered all forecasts nugatory. the clouds never parted, during the critical three minutes, over central russia, where many parties were stationed, and professor d. p. todd was equally unfortunate in japan. some good photographs were, nevertheless, secured by professor arai, director of the tokio observatory, as well as by mm. bélopolsky and glasenapp at petrovsk and jurjevitch respectively. they showed a corona of simpler form than that of the year before, but not yet of the pronounced type first associated by mr. ranyard with the lowest stage of solar activity. the genuineness of the association was ratified by the duplicate spectacle of the next-ensuing minimum year. two total eclipses of the sun distinguished . the first took place on new year's day, when a narrow shadow-path crossed california, allowing less than two minutes for the numerous experiments prompted by the varied nature of modern methods of research. american astronomers availed themselves of the occasion to the full. the heavens were propitious. photographic records were obtained in unprecedented abundance, and of unusual excellence. their comparison and study placed it beyond reasonable doubt that the radiated corona belonging to periods of maximum sun-spots gives place, at periods of minimum, to the "winged" type of . professor holden perceived further that the equatorial extensions characterising the latter tend to assume a "trumpet-shape."[ ] their extremities diverge, as if mutually repellent, instead of flowing together along a medial plane. the maximum actinic brilliancy of the corona of january , , was determined at lick to be twenty-one times less than that of the full moon.[ ] its colour was described as "of an intense luminous silver, with a bluish tinge, similar to the light of an electric arc."[ ] its spectrum was comparatively simple. very few bright lines besides those of hydrogen and coronium, and apparently no dark ones, stood out from the prismatic background. "the marked structural features of the corona, as presented by the negatives" taken by professors nipher and charroppin, were the filaments and the streamers. the filaments issued from polar calottes of ° radius. "the impression conveyed to the eye," professor pritchett wrote,[ ] "is that the equatorial stream of denser coronal matter extends across and through the filaments, simply obscuring them by its greater brightness. the effect is just as if the equatorial belt were superposed upon, or passed through, the filamentary structure. there is nothing in the photographs to prove that the filaments do not exist all round the sun.[ ] the testimony from negatives of different lengths of exposure goes to show that the equatorial streamers are made up of numerous interlacing parts inclined at varying angles to the sun's equator." the coronal extensions, perceptible with the naked eye to a distance of more than ° from the sun, appeared barely one-third of that length on the best negatives. little more could be seen of them either in barnard's exquisite miniature pictures, or in the photographs obtained by w. h. pickering with a thirteen-inch refractor--the largest instrument so far used in eclipse-photography. the total eclipse of december , , held out a prospect, unfortunately not realized, of removing some of the doubts and difficulties that impeded the progress of coronal photography.[ ] messrs. burnham and schaeberle secured at cayenne some excellent impressions, showing enough of the corona to prove its identical character with that depicted in the beginning of the year, but not enough to convey additional information about its terminal forms or innermost structure. any better result was indeed impossible, the moisture-laden air having cut down the actinic power of the coronal light to one-fourth its previous value. two english expeditions organized by the royal astronomical society fared still worse. mr. taylor was stationed on the west coast of africa, one hundred miles south of loanda; father perry chose as the scene of his operations the salut islands, off french guiana. each was supplied with a reflector constructed by dr. common, endowed, by its extremely short focal length of forty-five, combined with an aperture of twenty inches, with a light-concentrating force capable, it was hoped, of compelling the very filmiest coronal branches to self-registration. had things gone well two sets of coronal pictures, absolutely comparable in every respect, and taken at an interval of two hours and a half, would have been at the disposal of astronomers. but things went very far from well. clouds altogether obscured the sun in africa; they only separated to allow of his shining through a saturated atmosphere in south america. father perry's observations were the last heroic effort of a dying man. stricken with malaria, he crawled to the hospital as soon as the eclipse was over, and expired five days later, at sea, on board the _comus_. he was buried at barbados. and the sacrifice of his life had, after all, purchased no decisive success. most of the plates exposed by him suffered deterioration from the climate, or from an inevitably delayed development. a drawing from the best of them by miss violet common[ ] represented a corona differing from its predecessor of january , chiefly through the oppositely unsymmetrical relations of its parts. then the western wing had been broader at its base than the eastern; now the inequality was conspicuously the other way.[ ] the next opportunity for retrieving the mischances of the past was offered april , . the line of totality charted for that day ran from chili to senegambia. american parties appropriated the andes; both shores of the atlantic were in english occupation; french expeditions, led by deslandres and bigourdan, took up posts south of cape verde. a long totality of more than four minutes was favoured by serene skies; hence an ample store of photographic data was obtained. professor schaeberle, of the lick observatory, took, almost without assistance, at mina bronces, a mining station , feet above the pacific, fifty-two negatives, eight of them with a forty-foot telescope, on a scale of four and a half inches to the solar diameter. not only the inner corona, but the array of prominences then conspicuous, appeared in them to be composed of fibrous jets and arches, held to be sections of elliptic orbits described by luminous particles about the sun's centre.[ ] one plate received the impression of a curious object,[ ] entangled amidst coronal streamers, and the belief in its cometary nature was ratified by the bestowal of a comet-medal in recognition of the discovery. similiar paraboloidal forms had, nevertheless, occasionally been seen to make an integral part of earlier coronas; and it remains extremely doubtful whether schaeberle's "eclipse-comet" was justly entitled to the character claimed for it. the eclipse of disclosed a radiated corona such as a year of spot-maximum was sure to bring. an unexpected fact about it was, however, ascertained. the coronal has been believed to have much in common with the chromospheric spectrum; it proved, on investigation with a large prismatic camera, employed under sir norman lockyer's directions by mr. fowler at fundium, to be absolutely distinct from it. the fundamental green ray had, on the west african plates, seven more refrangible associates;[ ] but all alike are of unknown origin. they may be due to many substances, or to one; future research will perhaps decide; we can at present only say that the gaseous emission of the corona include none from hydrogen, helium, calcium, or any other recognisable terrestrial element. deslandres' attempt to determine the rotation of the corona through opposite displacements, east and west of the interposed moon, of the violet calcium-lines supposed to make part of the coronal spectrum, was thus rendered nugatory. yet it gave an earnest of success, by definitely introducing the subject into the constantly lengthened programme of eclipse-work. there is, however, little prospect of its being treated effectively until the green line is vivified by a fresh access of solar activity. the flight of the moon's shadow was, on august , , dogged by atrocious weather. it traversed, besides, some of the most inhospitable regions on the earth's surface, and afforded, at the best, but a brief interval of obscurity. at novaya zemlya, however, of all places, the conditions were tolerably favourable, and, as we have seen, the trophy of a "flash-spectrograph" was carried off. some coronal photographs, moreover, taken by the late sir george baden-powell[ ] and by m. hansky, a member of a russian party, were marked by features of considerable interest. they made apparent a close connection between coronal outflows and chromospheric jets, cone-shaped beams serving as the sheaths, or envelopes, of prominences. m. hansky,[ ] indeed, thought that every streamer had a chromospheric eruption at its base. further, dark veinings of singular shapes unmistakably interrupted the coronal light, and bordered brilliant prominences,[ ] reminding us of certain "black lines" traced by swift across the "anvil protuberance" august , .[ ] in type the corona of reproduced that of , as befitted its intermediate position in the solar cycle. the eclipse-track on january , , crossed the indian peninsula from viziadrug, on the malabar coast, to mount everest in the himalayas. not a cloud obstructed the view anywhere, and an unprecedented harvest of photographic records was garnered. the flash-spectrum, in its successive phases, appeared on plates taken by sir norman lockyer, mr. evershed, professor campbell,[ ] and others; professor turner[ ] set on foot a novel mode of research by picturing the corona in the polarised ingredient of its light; mrs. maunder[ ] practically solved the problem of photographing the faint coronal extensions, one ray on her plates running out to nearly six diameters from the moon's limb. yet she used a dallmeyer lens of only one and a half inches aperture. her success accorded perfectly with professor wadsworth's conclusion that effectiveness in delineation by slight contrasts of luminosity varies inversely with aperture. triple-coated plates, and a comparatively long exposure of twenty seconds, contributed to a result unlikely, for some time, to be surpassed. the corona of presented a mixed aspect. the polar plumes due at minimum were combined in it with the quadrilateral ogives belonging to spot-maxima. a slow course of transformation, in fact, seemed in progress; and it was found to be completed in , when the eclipse of may revealed the typical halo of a quiescent sun. the obscurity on this occasion was short--less than seconds--but was well observed east and west of the atlantic. no striking gain in knowledge, however, resulted. important experiments were indeed made on the heat of the corona with langley's bolometer, but their upshot can scarcely be admitted as decisive. they indicated a marked deficiency of thermal radiations, implying for coronal light, in professor langley's opinion,[ ] an origin analogous to that of the electric glow-discharge, which, at low pressures, was found by k. Ångström in to have no invisible heat-spectrum.[ ] the corona was photographed by professor barnard, at wadesborough, north carolina, with a - / -foot horizontal "coelostat." in this instrument, of a type now much employed in eclipse operations and first recommended by professor turner, a six-inch photographic objective preserved an invariable position, while a silvered plane mirror, revolving by clockwork once in forty-eight hours (since the angle of movement is doubled by reflection), supplied the light it brought to a focus. a temporary wooden tube connected the lens with the photographic house where the plates were exposed. pictures thus obtained with exposures of from one to fourteen seconds, were described as "remarkably sharp and perfectly defined, showing the prominences and inner corona very beautifully. the polar fans came out magnificently."[ ] the great sumatra eclipse left behind it manifold memories of foiled expectations. a totality of above six minutes drew observers to the far east from several continents, each cherishing a plan of inquiry which few were destined to execute. all along the line of shadow, which, on may , , crossed réunion and mauritius, and again met land at sumatra and borneo, the meteorological forecast was dubious, and the meteorological actuality in the main deplorable. nevertheless, the corona was seen, and fairly well photographed through drifting clouds, and proved to resemble in essentials the appendage viewed a year previously. negatives taken by members of the lick observatory expedition led by mr. perrine[ ] disclosed the unique phenomenon of a violent coronal disturbance, with a small compact prominence as its apparent focus. tumbling masses and irregular streamers radiating from a point subsequently shown by the greenwich photographs to be the seat of a conspicuous spot, suggested the recent occurrence of an explosion, the far-reaching effects of which might be traced in the confused floccular luminosity of a vast surrounding region. again, photographs in polarised light attested the radiance of the outer corona to be in large measure reflected, while that of the inner ring was original; and the inference was confirmed by spectrographs, recording many fraunhofer lines when the slit lay far from the sun's limb, but none in its immediate vicinity. on plates exposed by mr. dyson and dr. humphrys with special apparatus, the coronal spectrum, continuous and linear, impressed itself more extensively in the ultra-violet than on any previous occasion; and dr. mitchell succeeded in photographing the reversing layer by means of a grating spectroscope. finally, mrs. maunder, at mauritius, despite mischievous atmospheric tremors, obtained with the newbegin telescope an excellent series of coronal pictures.[ ] the principles of explanation applied to the corona may be briefly described as eruptive and electrical. the first was adopted by professor schaeberle in his "mechanical theory," advanced in .[ ] according to this view, the eclipse-halo consists of streams of matter shot out with great velocity from the spot-zones by forces acting perpendicularly to the sun's surface. the component particles return to the sun after describing sections of extremely elongated ellipses, unless their initial speed happen to equal or exceed the critical rate of miles a second, in which case they are finally driven off into space. the perspective overlapping and interlacing of these incandescent outflows was supposed to occasion the intricacies of texture visible in the corona; and it should be recorded that a virtually identical conclusion was reached by mr. perrine in ,[ ] by a different train of reasoning, based upon a distinct set of facts. a theory on very much the same lines was, moreover, worked out by m. bélopolsky in .[ ] schaeberle, however, had the merit of making the first adequate effort to deduce the real shape of the corona, as it exists in three dimensions, from its projection upon the surface of the sphere. he failed, indeed, to account for the variation in coronal types by the changes in our situation with regard to the sun's equator. it is only necessary to remark that, if this were so, they should be subject to an annual periodicity, of which no trace can be discerned. electro-magnetic theories have the charm, and the drawback, of dealing largely with the unknown. but they are gradually losing the vague and intangible character which long clung to them; and the improved definition of their outlines has not, so far, brought them into disaccord with truth. the most promising hypothesis of the kind is due to professor bigelow of washington. his able discussion of the eclipse photographs of january , ,[ ] showed a striking agreement between the observed coronal forms and the calculated effects of a repulsive influence obeying the laws of electric potential, also postulated by huggins in .[ ] finely subdivided matter, expelled from the sun along lines of force emanating from the neighbourhood of his poles, thus tends to accumulate at "equipotential surfaces." in deference, however, to a doubt more strongly felt then than now, whether the presence of free electricity is compatible with the solar temperature, he avoided any express assertion that the coronal structure is an electrical phenomenon, merely pointing out that, if it were, its details would be just what they are. later, in , pupin in america,[ ] and ebert in germany,[ ] imitated the coronal streamers by means of electrical discharges in low vacua between small conducting bodies and strips of tinfoil placed on the outside of the containing glass receptacles. finally, a critical experiment made by ebert in served, as bigelow justly said, "to clear up the entire subject, and put the theory on a working basis." having obtained coronoidal effects in the manner described, he proceeded to subject them to the action of a strong magnetic field, with the result of marshalling the scattered rays into a methodical and highly suggestive array. they followed the direction of the magnetic lines of force, and, forsaking the polar collar of the magnetised sphere, surrounded it like a ruffle. the obvious analogy with the aurora polaris and the solar corona was insisted upon by ebert himself, and has been further developed by bigelow.[ ] according to a recent modification of his hypothesis, the latter appendage is controlled by two opposing systems of forces; the magnetic causing the rays to diverge from the poles towards the equator, and the electrostatic urging their spread, through the mutual repulsion of the particles accumulated in the "wings," from the equator towards either pole. the cyclical change in the corona, he adds, is probably due to a variation in the balance of power thus established, the magnetic polar influence dominating at minima, the electrostatic at maxima. and he may well feel encouraged by the fortunate combination of many experimental details into one explanatory whole, no less than by the hopeful prospect of further developments, both practical and theoretical, along the same lines. what we really know about the corona can be summed up in a few words. it is certainly _not_ a solar atmosphere. it does not gravitate upon the sun's surface and share his rotation, as our air gravitates upon and shares the rotation of the earth; and this for the simple reason that there is no visible growth of pressure downwards (of which the spectroscope would infallibly give notice) in its gaseous constituents; whereas under the sole influence of the sun's attractive power, their density should be multiplied many million times in the descent through a mere fraction of their actual depth.[ ] they are apparently in a perpetual state of efflux from, and influx to our great luminary, under the stress of opposing forces. it is not unlikely that some part, at least, of the coronal materials are provided by eruptions from the body of the sun;[ ] it is almost certain that they are organized and arranged round it through electro-magnetic action. this, however, would seem to be influential only upon their white-hot or reflective ingredients, out of which the streamers and aigrettes are composed; since the coronal gases appear, from observations during eclipses, to form a shapeless envelope, with condensations above the spot-zones, or at the bases of equatorial extensions. the corona is undoubtedly affected both in shape and constitution by the periodic ebb and flow of solar activity, its low-tide form being winged, its high-tide form stellate; while the rays emitted by the gases contained in it fade, and the continuous spectrum brightens, at times of minimum sun-spots. the appendage, as a whole, must be of inconceivable tenuity, since comets cut their way through it without experiencing sensible retardation. not even sir william crookes's vacua can give an idea of the rarefaction which this fact implies. yet the observed luminous effects may not in reality bear witness contradictory of it. one solitary molecule in each cubic inch of space might, in professor young's opinion, produce them; while in the same volume of ordinary air at the sea-level, the molecules number (according to dr. johnstone stoney) , trillions! the most important lesson, however, derived from eclipses is that of partial independence of them. some of its fruits in the daily study of prominences the next chapter will collect; and the harvest has been rendered more abundant, as well as more valuable, since it has been found possible to enlist, in this department too, the versatile aid of the camera. footnotes: [footnote : _vierteljahrsschrift astr. ges._, jahrg. xxvi., p. .] [footnote : _astr. jour._, vol. iv., p. .] [footnote : _proc. roy. soc._, vol. xvii., p. .] [footnote : _comptes rendus_, t. lxvii., p. .] [footnote : _comptes rendus_, t. lxvii., p. .] [footnote : _month. not._, vol. xxvii., p. .] [footnote : _proc. roy. soc._, vol. xvii., p. .] [footnote : _washington observations_, , app. ii., harkness's report, p. .] [footnote : _am. jour._, vol. xlviii. ( nd series), p. .] [footnote : _am. jour._, vol. xi. ( rd series), p. .] [footnote : campbell, _astroph. jour._, vol. x., p. .] [footnote : keeler, _reports on eclipse of january , _, p. .] [footnote : everything in such observations depends upon the proper manipulation of the slit of the spectroscope.] [footnote : _mem. r. a. s._, vol. xli., p. .] [footnote : _comptes rendus_, t. lxvii., p. .] [footnote : _mem. r. a. s._, vol. xli., p. .] [footnote : _comptes rendus_, t. xciv., p. .] [footnote : young, _pop. astr._, oct., , p. .] [footnote : j. evershed, _indian eclipse_, , p. ; _month. not._, vol. lviii., p. ; _proc. roy. soc._, jan. , .] [footnote : frost, _astroph. jour._, vol. xii., p. ; lord, _ibid._, vol. xiii., p. .] [footnote : _comptes rendus_, t. cxvii., no. ; _jour. brit. astr. ass._, vol. iii., p. .] [footnote : lockyer, _phil. trans._, vol. clvii., p. .] [footnote : the rosy envelope of prominence-matter was so named by lockyer in (_phil. trans._, vol. clix., p. ).] [footnote : according to trouvelot (_wash. obs._, , app. iii., p. ), the subtracted matter was, at least to some extent, accumulated in the polar regions.] [footnote : _bull. phil. soc. washington_, vol. iii., p. .] [footnote : _mem. r. a. s._, vol. xli., .] [footnote : _astr. nach._, no. , .] [footnote : _correspondence with newton_, pp. - ; ranyard, _mem. astr. soc._, vol. xli., p. .] [footnote : s. p. langley, _wash. obs._, , app. iii., p. ; _nature_, vol. lxi., p. .] [footnote : schuster (_proc. roy. soc._, vol. xxxv., p. ) measured and photographed about thirty.] [footnote : abney, _phil. trans._, vol. clxxv., p. .] [footnote : _proc. roy. soc._, vol. xxxiv., p. . experiments directed to the same end had been made by dr. o. lohse at potsdam, - . _astr. nach._, no. , .] [footnote : the sensitiveness of chloride of silver extends from _h_ to h; that is, over the upper or more refrangible half of the space in which the main part of the coronal light is concentrated.] [footnote : _proc. roy. soc._, vol. xxxiv., p. .] [footnote : _report brit. assoc._, , p. .] [footnote : maunder, _indian eclipse_, p. ; _eclipse of _, p. .] [footnote : _astr. and astrophysics_, vol. xiii., p. .] [footnote : see _infra_, p. .] [footnote : abney, _phil. trans._, vol. clxxx., p. .] [footnote : _comptes rendus_, t. xcvii., p. .] [footnote : _memoirs national ac. of sciences_, vol. ii., p. .] [footnote : _wash. obs._, , app. ii., p. .] [footnote : _the sun_, p. .] [footnote : _proc. roy. soc._, vol. xvii., p. .] [footnote : _comptes rendus_, t. lxxiii., p. .] [footnote : _wash. obs._, , app. ii., p. .] [footnote : stokes, anniversary address, _nature_, vol. xxxv., p. .] [footnote : _comptes rendus_, t. ci., p. .] [footnote : _harvard annals_, vol. xviii., p. .] [footnote : wesley, _phil. trans._, vol. clxxx., p. .] [footnote : _harvard annals_, vol. xviii, p. .] [footnote : _lick report_, p. .] [footnote : _ibid._, p. .] [footnote : _ibid._, p. .] [footnote : _pub. astr. soc. of the pacific_, vol. iii., p. .] [footnote : professor holden concluded, with less qualification, "that so-called 'polar' rays exist at all latitudes on the sun's surface." _lick report_, p. .] [footnote : holden, _report on eclipse of december, _, p. ; charroppin, _pub. astr. soc. of the pacific_, vol. iii., p. .] [footnote : published as the frontispiece to the _observatory_, no. .] [footnote : wesley, _ibid._, p. .] [footnote : _lick observatory contributions_, no. , p. .] [footnote : _astr. and astrophysics_, vol. xiii. p. .] [footnote : lockyer, _phil. trans._, vol. clxxxvii., p. .] [footnote : he died in london, november , .] [footnote : _bull. acad. st. pétersbourg_, t. vi., p. .] [footnote : w. h. wesley, _phil. trans._, vol. cxc, p. .] [footnote : _lick reports on eclipse of january , _, p. .] [footnote : _astroph. jour._, vol. xi., p. .] [footnote : _observatory_, vol. xxi., p. .] [footnote : _the indian eclipse_, , p. .] [footnote : _science_, june , ; _astroph. jour._, vol. xii., p. .] [footnote : _ann. der physik_, bd. xlviii., p. . see also wood, _physical review_, vol. iv., p. , .] [footnote : _science_, august , .] [footnote : _lick observatory bulletin_, no. .] [footnote : _observatory_, vol. xxiv., pp. , .] [footnote : _lick report on eclipse of december , _, p. ; _month. not._, vol. l., p. .] [footnote : _lick obs. bull._, no. .] [footnote : _bull. de l'acad. st. pétersbourg_, t. iv., p. .] [footnote : _the solar corona discussed by spherical harmonics_, smithsonian institution, .] [footnote : bakerian lecture, _proc. roy. soc._, vol. xxxix.] [footnote : _astr. and astrophysics_, vol. xi., p. .] [footnote : _ibid._, vol. xii., p. .] [footnote : _am. journ. of science_, vol. xi., p. , .] [footnote : see huggins, _proc. roy. soc._, vol. xxxix., p. ; young, _north am. review_, february, , p. .] [footnote : professor w. a. norton, of yale college, appears to have been the earliest formal advocate of the expulsion theory of the solar surroundings, in the second ( ) and later editions of his _treatise on astronomy_.] chapter iv _solar spectroscopy_ the new way struck out by janssen and lockyer was at once and eagerly followed. in every part of europe, as well as in north america, observers devoted themselves to the daily study of the chromosphere and prominences. foremost among these were lockyer in england, zöllner at leipzig, spörer at anclam, young at hanover, new hampshire, secchi and respighi at rome. there were many others, but these names stood out conspicuously. the first point to be cleared up was that of chemical composition. leisurely measurements verified the presence above the sun's surface of hydrogen in prodigious volumes, but showed that sodium had nothing to do with the orange-yellow ray identified with it in the haste of the eclipse. from its vicinity to the d-pair (than which it is slightly more refrangible), the prominence-line was, however, designated d_ , and the unknown substance emitting it was named by lockyer "helium." its terrestrial discovery ensued after twenty-six years. in march, , professor ramsay obtained from the rare mineral clevite a volatile gas, the spectrum of which was found to include the yellow prominence-ray. helium was actually at hand, and available for examination. the identification cleared up many obscurities in chromospheric chemistry. several bright lines, persistently seen at the edge of the sun, and early suspected by young[ ] to emanate from the same source as d_ , were now derived from helium in the laboratory; and all the complex emissions of that exotic substance ranged themselves into six sets or series, the members of which are mutually connected by numerical relations of a definite and simple kind. helium is of rather more than twice the density of hydrogen, and has no chemical affinities. in almost evanescent quantities it lurks in the earth's crust, and is diffused through the earth's atmosphere. the importance of the part played in the prominence-spectrum by the violet line of calcium was noticed by professor young in , but since h and k lie near the limit of the visible spectrum, photography was needed for a thorough investigation of their appearances. aided by its resources, professor george e. hale, then at the beginning of his career, detected in their unfailing and conspicuous presence.[ ] the substance emitting them not only constitutes a fundamental ingredient of the chromosphere, but rises, in the fantastic jets thence issuing, to greater heights than hydrogen itself. the isolation of h and k in solar prominences from any other of the lines usually distinctive of calcium was experimentally proved by sir william and lady huggins in to be due to the extreme tenuity of the emitting vapour.[ ] hydrogen, helium, and calcium form, then, the chief and unvarying materials of the solar sierra and its peaks; but a number of metallic elements make their appearance spasmodically under the influence of disturbances in the layers beneath. in september, , young[ ] drew up at dartmouth college a list of lines significant of injections into the chromosphere of iron, titanium, chromium, magnesium, and many other substances. during two months' observation in the pure air of mount sherman ( , feet high) in the summer of , these tell-tale lines mounted up to ;[ ] and he believes their number might still be doubled by steady watching. indeed, both young and lockyer have more than once seen the whole field of the spectroscope momentarily inundated with bright rays, as if the "reversing layer" had been suddenly thrust upwards into the chromosphere, and as quickly allowed to drop back again. the opinion would thus appear to be well-grounded that the two form one continuous region, of which the lower parts are habitually occupied by the heaviest vapours, but where orderly arrangement is continually overturned by violent eruptive disturbances. the study of the _forms_ of prominences practically began with huggins's observation of one through an "open slit" february , .[ ] at first it had been thought possible to examine them only in sections--that is, by admitting mere narrow strips or "lines" of their various kinds of light; while the actual shape of the objects emitting those lines had been arrived at by such imperfect devices as that of giving to the slit of the spectroscope a vibratory moment rapid enough to enable the eye to retain the impression of one part while others were successively presented to it. it was an immense gain to find that their rays had strength to bear so much of dilution with ordinary light as was involved in opening the spectroscopic shutter wide enough to exhibit the tree-like, or horn-like, or flame-shaped bodies rising over the sun's rim in their undivided proportions. several diversely-coloured images of them are formed in the spectroscope; each may be seen under a crimson, a yellow, a green, and a deep blue aspect. the crimson, however (built up out of the c-line of hydrogen), is the most intense, and is commonly used for purposes of observation and illustration. friedrich zöllner was, by a few days, beforehand with huggins in describing the open-slit method, but was somewhat less prompt in applying it. his first survey of a complete prominence, pictured in, and not simply intersected by, the slit of his spectroscope, was obtained july , .[ ] shortly afterwards the plan was successfully adopted by the whole band of investigators. a difference in kind was very soon perceived to separate these objects into two well-marked classes. its natural and obvious character was shown by its having struck several observers independently. the distinction of "cloud-prominences" from "flame-prominences" was announced by lockyer, april ; by zöllner, june ; and by respighi, december , . the first description are tranquil and relatively permanent, sometimes enduring without striking change for many days. certain of the included species mimic terrestrial cloud-scenery--now appearing like fleecy cirrus transpenetrated with the red glow of sunset--now like prodigious masses of cumulo-stratus hanging heavily above the horizon. the solar clouds, however, have the peculiarity of possessing _stems_. slender columns can ordinarily be seen to connect the surface of the chromosphere with its outlying portions. hence the fantastic likeness to forest scenery presented by the long ranges of fiery trunks and foliage occasionally seeming to fringe the sun's limb. but while this mode of structure suggests an actual outpouring of incandescent material, certain facts require a different interpretation. at a distance, and quite apart from the chromosphere, prominences have been perceived, both by secchi and young, to _form_, just as clouds form in a clear sky, condensation being replaced by ignition. filaments were then thrown out downward towards the chromosphere, and finally the usual appearance of a "stemmed prominence" was assumed. still more remarkable was an observation made by trouvelot at harvard college observatory, june , .[ ] a gigantic comma-shaped prominence, , miles high, vanished from before his eyes by a withdrawal of light as sudden as the passage of a flash of lightning. the same observer has frequently witnessed a gradual illumination or gradual extinction of such objects, testifying to changes in the thermal or electrical condition of matter already _in situ_. the first photograph of a prominence, as shown by the spectroscope in daylight, was taken by professor young in .[ ] but neither his method, nor that described by dr. braun in ,[ ] had any practical success. this was reserved to reward the efforts towards the same end of professor hale. begun at harvard college in ,[ ] they were prosecuted soon afterwards at the kenwood observatory, chicago. the great difficulty was to extricate the coloured image of the gaseous structure, spectroscopically visible at the sun's limb, from the encompassing glare, a very little of which goes a long way in _fogging_ sensitive plates. to counteract its mischievous effects, a second slit,[ ] besides the usual narrow one in front of the collimator, was placed on guard, as it were, behind the dispersing apparatus, so as to shut out from the sensitised surface all light save that of the required quality. the sun's image being then allowed to drift across the outer slit, while the plate holder was kept moving at the same rate, the successive sectional impressions thus rapidly obtained finally "built up" a complete picture of the prominence. another expedient was soon afterwards contrived.[ ] the h and k rays of calcium are always, as we have seen, bright in the spectrum of prominences. they are besides fine and sharp, while the corresponding absorption-lines in the ordinary solar spectrum are wide and diffuse. hence, prominences formed by the spectroscope out of these particular qualities of violet light, can be photographed entire and at once, for the simple reason that they are projected upon a naturally darkened background. atmospheric glare is abolished by local absorption. this beautiful method was first realised by professor hale in june, . a "spectroheliograph," consisting of a spectroscopic and a photographic apparatus of special type, attached to the eye-end of an equatoreal twelve inches in aperture, was erected at kenwood in march, ; and with its aid, professor hale entered upon original researches of high promise for the advancement of solar physics. noteworthy above all is his achievement of photographing both prominences and faculæ on the very face of the sun. the latter had, until then, been very imperfectly observed. they were only visible, in fact, when relieved by their brilliancy against the dusky edge of the solar disc. their convenient emission of calcium light, however, makes it possible to photograph them in all positions, and emphasises their close relationship to prominences. the simultaneous picturing, moreover, of the entire chromospheric ring, with whatever trees or fountains of fire chance to be at the moment issuing from it, has been accomplished by a very simple device. the disc of the sun itself having been screened with a circular metallic diaphragm, it is only necessary to cause the slit to traverse the virtually eclipsed luminary, in order to get an impression of the whole round of its fringing appendages. and the record can be extended to the disc by removing the screen, and carrying the slit back at a quicker rate, when an "image of the sun's surface, with the faculæ and spots, is formed on the plate exactly within the image of the chromosphere formed during the first exposure. the whole operation," professor hale continues, "is completed in less than a minute, and the resulting photographs give the first true pictures of the sun, showing all of the various phenomena at its surface."[ ] most of these novel researches were, by a remarkable coincidence, pursued independently and contemporaneously by m. deslandres, of the paris observatory.[ ] the ultra-violet prominence spectrum was photographed for the first time from an uneclipsed sun, in june, , at chicago. besides h and k, four members of the huggins-series of hydrogen-lines imprinted themselves on the plate.[ ] meanwhile m. deslandres was enabled, by fitting quartz lenses to his spectroscope, and substituting a reflecting for a refracting telescope, to get rid of the obstructive action of glass upon the shorter light-waves, and thus to widen the scope of his inquiry into the peculiarities of those derived from prominences.[ ] as the result, not only all the nine white-star lines were photographed from a brilliant sun-flame, but five additional ones were found to continue the series upward. the wave-lengths of these last had, moreover, been calculated beforehand with singular exactness, from a simple formula known as "balmer's law."[ ] the new lines, accordingly, filled places in a manner already prepared for them, and were thus unmistakably associated with the hydrogen-spectrum. this is now known to be represented in prominences by twenty-seven lines,[ ] forming a kind of harmonic progression, only four of which are visibly darkened in the fraunhofer spectrum of the sun. plate i. [illustration: photographs of the solar chromosphere and prominences. taken with the spectroheliograph of the kenwood observatory, chicago, by professor george e. hale.] the chemistry of "cloud-prominences" is simple. hydrogen, helium, and calcium are their chief constituents. "flame-prominences," on the other hand, show, in addition, the characteristic rays of a number of metals, among which iron, titanium, barium, strontium, sodium, and magnesium are conspicuous. they are intensely brilliant; sharply defined in their varying forms of jets, spikes, fountains, waterspouts; of rapid formation and speedy dissolution, seldom attaining to the vast dimensions of the more tranquil kind. eruptive or explosive by origin, they occur in close connection with spots; whether causally, the materials ejected as "flames" cooling and settling down as dark, depressed patches of increased absorption;[ ] or consequentially, as a reactive effect of falls of solidified substances from great heights in the solar atmosphere.[ ] the two classes of phenomena, at any rate, stand in a most intimate relation; they obey the same law of periodicity, and are confined to the same portions of the sun's surface, while quiescent prominences may be found right up to the poles and close to the equator. the general distribution of prominences, including both genera, follows that of faculæ much more closely than that of spots. from father secchi's and professor respighi's observations, - , were derived the first clear ideas on the subject, which have been supplemented and modified by the later researches of professors tacchini and riccò at rome and palermo. the results are somewhat complicated, but may be stated broadly as follows. the district of greatest prominence-frequency covers and overlaps by several degrees that of the greatest spot-frequency. that is to say, it extends to about ° north and south of the equator.[ ] there is a visible tendency to a second pair of maxima nearer the poles. the poles themselves, as well as the equator, are regions of minimum occurrence. distribution in time is governed by the spot-cycle, but the maximum lasts longer for prominences than for spots. the structure of the chromosphere was investigated in and subsequent years by professor respighi, director of the capitoline observatory, as well as by spörer, and brédikhine of the moscow observatory. they found this supposed solar envelope to be of the same eruptive nature as the vast protrusions from it, and to be made up of a congeries of minute flames[ ] set close together like blades of grass. "the appearance," professor young writes,[ ] "which probably indicates a fact, is as if countless jets of heated gas were issuing through vents and spiracles over the whole surface, thus clothing it with flame which heaves and tosses like the blaze of a conflagration." the summits of these filaments of fire are commonly inclined, as if by a wind sweeping over them, when the sun's activity is near its height, but erect during his phase of tranquillity. spörer, in , inferred the influence of permanent polar currents,[ ] but tacchini showed in that the deflections upon which this inference was based ceased to be visible as the spot-minimum drew near.[ ] another peculiarity of the chromosphere, denoting the remoteness of its character from that of a true atmosphere,[ ] is the irregularity of its distribution over the sun's surface. there are no signs of its bulging out at the equator, as the laws of fluid equilibrium in a rotating mass would require; but there are some that the fluctuations in its depth are connected with the phases of solar agitation. at times of minimum it seems to accumulate and concentrate its activity at the poles; while maxima probably bring a more equable general distribution, with local depressions at the base of great prominences and above spots. a low-lying stratum of carbon-vapour was, in , detected in the chromosphere by professor hale with a grating-spectroscope attached to the -inch yerkes refractor.[ ] the eclipse-photographs of disclosed to hartley's examination the presence there of gallium;[ ] and those taken by evershed in were found by jewell[ ] to be crowded with ultra-violet lines of the equally rare metal scandium. the general rule had been laid down by sir norman lockyer that the metallic radiations from the chromosphere are those "enhanced" in the electric spark.[ ] hence, the comparative study of conditions prevalent in the arc and the spark has acquired great importance in solar physics. the reality of the appearance of violent disturbance presented by the "flaming" kind of prominence can be tested in a very remarkable manner. christian doppler,[ ] professor of mathematics at prague, enounced in the theorm that the colour of a luminous body, like the pitch of a sonorous body, must be changed by movements of approach or recession. the reason is this. both colour and pitch are physiological effects, depending, not upon absolute wave-length, but upon the number of waves entering the eye or ear in a given interval of time. and this number, it is easy to see, must be increased if the source of light or sound is diminishing its distance, and diminished if it is decreasing it. in the one case, the vibrating body _pursues_ and crowds together the waves emanating from it; in the other, it _retreats_ from them, and so lengthens out the space covered by an identical number. the principle may be thus illustrated. suppose shots to be fired at a target at fixed intervals of time. if the marksman advances, say twenty paces between each discharge of his rifle, it is evident that the shots will fall faster on the target than if he stood still; if, on the contrary, he retires by the same amount, they will strike at correspondingly longer intervals. the result will of course be the same whether the target or the marksman be in movement. so far doppler was altogether right. as regards sound, anyone can convince himself that the effect he predicted is a real one, by listening to the alternate shrilling and sinking of the steam-whistle when an express train rushes through a station. but in applying this principle to the colours of stars he went widely astray; for he omitted from consideration the double range of invisible vibrations which partake of, and to the eye exactly compensate, changes of refrangibility in the visible rays. there is, then, no possibility of finding a criterion of velocity in the hue of bodies shining, like the sun and stars, with continuous light. the entire spectrum is slightly shifted up or down in the scale of refrangibility; certain rays normally visible become exalted or degraded (as the case may be) into invisibility, and certain other rays at the opposite end undergo the converse process; but the sum total of impressions on the retina continues the same. we are not, however, without the means of measuring this sub-sensible transportation of the light-gamut. once more the wonderful fraunhofer lines came to the rescue. they were called by the earlier physicists "fixed lines;" but it is just because they are _not_ fixed that, in this instance, we find them useful. they share, and in sharing betray, the general shift of the spectrum. this aspect of doppler's principle was adverted to by fizeau in ,[ ] and the first tangible results in the estimation of movements of approach and recession between the earth and the stars, were communicated by sir william huggins to the royal society, april , . eighteen months later, zöllner devised his "reversion-spectroscope"[ ] for doubling the measurable effects of line-displacements; aided by which ingenious instrument, and following a suggestion of its inventor, professor h. c. vogel succeeded at bothkamp, june , ,[ ] in detecting effects of that nature due to the solar rotation. this application constitutes at once the test and the triumph of the method.[ ] the eastern edge of the sun is continually moving towards us with an equatorial speed of about a mile and a quarter per second, the western edge retreating at the same rate. the displacements--towards the violet on the east, towards the red on the west--corresponding to this velocity are very small; so small that it seems hardly credible that they should have been laid bare to perception. they amount to but / th part of the interval between the two constituents of the d-line of sodium; and the d-line of sodium itself can be separated into a pair only by a powerful spectroscope. nevertheless, professor young[ ] was able to show quite satisfactorily, in , not only deviations in the solar lines from their proper places indicating a velocity of rotation ( · miles per second) slightly in excess of that given by observations of spots, but the exemption of terrestrial lines (those produced by absorption in the earth's atmosphere) from the general push upwards or downwards. shortly afterwards, professor langley, then director of the allegheny observatory, having devised a means of comparing with great accuracy light from different portions of the sun's disc, found that while the obscure rays in two juxtaposed spectra derived from the solar poles were absolutely continuous, no sooner was the instrument rotated through °, so as to bring its luminous supplies from opposite extremities of the equator, than the same rays became perceptibly "notched." the telluric lines, meanwhile, remained unaffected, so as to be "virtually mapped" by the process.[ ] this rapid and unfailing mode of distinction was used by cornu with perfect ease during his investigation of atmospheric absorption near loiret in august and september, .[ ] a beautiful experiment of the same kind was performed by m. thollon, of m. bischoffsheim's observatory at nice, in the summer of .[ ] he confined his attention to one delicately defined group of four lines in the orange, of which the inner pair are solar (iron) and the outer terrestrial. at the centre of the sun the intervals separating them were sensibly equal; but when the light was taken alternately from the right and left limbs, a relative shift in alternate directions of the solar, towards and from the stationary telluric rays became apparent. a parallel observation was made at dunecht, december , , when it was noticed that a strong iron-line in the yellow part of the solar spectrum is permanently double on the sun's eastern, but single on his western limb;[ ] opposite motion-displacements bringing about this curious effect of coincidence with, and separation from, an adjacent stationary line of our own atmosphere's production, according as the spectrum is derived from the retreating or advancing margin of the solar globe. statements of fact so precise and authoritative amount to a demonstration that results of this kind are worthy of confidence; and they already occupy an important place among astronomical data. the subtle method of which they served to assure the validity was employed in - by m. dunér to test and extend carrington's and spörer's conclusions as to the anomalous nature of the sun's axial movement.[ ] his observations for the purpose, made with a fine diffraction-spectroscope, just then mounted at the observatory of upsala, were published in .[ ] their upshot was to confirm and widen the law of retardation with increasing latitude derived from the progressive motions of spots. determinations made within ° of the pole, consequently far beyond the region of spots, gave a rotation-period of - / , that of the equatorial belt being of - / days. spots near the equator indeed complete their rounds in a period shorter by at least half a day; and proportionate differences were found to exist elsewhere in corresponding latitudes; but dunér's observations, it must be remembered, apply to a distinct part of the complex solar machine from the disturbed photospheric surface. it is amply possible that the absorptive strata producing the fraunhofer lines, significant, by their varying displacements at either limb, of the inferred varying rates of rotation, may gyrate more slowly than the spot-generating level. moreover, faculæ appear to move at a quicker pace than either;[ ] so that we have, for three solar formations, three different periods of average rotation, the shortest of which belongs to the faculæ, one of intermediate length to the spots, and the most protracted to the reversing layer. all, however, agree in lengthening progressively from the equator towards the poles. professor holden aptly compared the sun to "a vast whirlpool where the velocities of rotation depend not only on the situation of the rotating masses as to latitude, but also as to depth beneath the exterior surface."[ ] sir norman lockyer[ ] promptly perceived the applicability of the surprising discovery of line-shiftings through end-on motion to the study of prominences, the discontinuous light of which affords precisely the same means of detecting movement without seeming change of place, as do lines of absorption in a continuous spectrum. indeed, his observations at the sun's edge almost compelled recourse to an explanation made available just when the need of it began to be felt. he saw bright lines, not merely pushed aside from their normal places by a barely perceptible amount, but bent, torn, broken, as if by the stress of some tremendous violence. these remarkable appearances were quite simply interpreted as the effects of movements varying in amount and direction in the different parts of the extensive mass of incandescent vapours falling within a single field of view. very commonly they are of a cyclonic character. the opposite distortions of the same coloured rays betray the fury of "counter-gales" rushing along at the rate of miles a second; while their undisturbed sections prove the persistence of a "heart of peace" in the midst of that unimaginable fiery whirlwind. velocities up to _miles a second_, or , times that of an express train at the top of its speed, were thus observed by young during his trip to mount sherman, august , ; and these were actually doubled in an extraordinary outburst observed by father jules fényi, on june , , at the haynald observatory in hungary, as well as by m. trouvelot at meudon.[ ] motions ascertainable in this way near the limb are, of course, horizontal as regards the sun's surface; the analogies they present might, accordingly, be styled _meteorological_ rather than _volcanic_. but vertical displacements on a scale no less stupendous can also be shown to exist. observations of the spectra of spots centrally situated (where motions in the line of sight are vertical) disclose the progress of violent uprushes and downrushes of ignited gases, for the most part in the penumbral or outlying districts. they appear to be occasioned by fitful and irregular disturbances, and have none of the systematic quality which would be required for the elucidation of sun-spot theories. indeed, they almost certainly take place at a great height above the actual openings in the photosphere. as to vertical motions above the limb, on the other hand, we have direct visual evidence of a truly amazing kind. the projected glowing matter has, by the aid of the spectroscope, been watched in its ascent. on september , , young examined at noon a vast hydrogen cloud , miles long, as it showed to the eye, and , high. it floated tranquilly above the chromosphere at an elevation of some , miles, and was connected with it by three or four upright columns, presenting the not uncommon aspect compared by lockyer to that of a grove of banyans. called away for a few minutes at . , on returning at . the observer found-- "that in the meantime the whole thing had been literally blown to shreds by some inconceivable uprush from beneath. in place of the quiet cloud i had left, the air, if i may use the expression, was filled with flying débris--a mass of detached, vertical, fusiform filaments, each from " to " long by " or " wide,[ ] brighter and closer together where the pillars had formerly stood, and rapidly ascending. they rose, with a velocity estimated at miles a second, to fully , miles above the sun's surface, then gradually faded away like a dissolving cloud, and at . only a few filmy wisps, with some brighter streamers low down near the photosphere, remained to mark the place."[ ] a velocity of projection of _at least_ miles per second was, by proctor's[ ] calculation, required to account for this extraordinary display, to which the earth immediately responded by a magnetic disturbance, and a fine aurora. it has proved by no means an isolated occurrence. young saw its main features repeated, october , ,[ ] on a still vaster scale; for the exploded prominence attained, this time, an altitude of , miles--the highest yet chronicled. lockyer, moreover, has seen a prominence , miles high shattered in ten minutes; while uprushes have been witnessed by respighi, of which the initial velocities were judged by him to be or miles a second. when it is remembered that a body starting from the sun's surface at the rate of miles a second would, if it encountered no resistance, escape for ever from his control, it is obvious that we have, in the enormous forces of eruption or repulsion manifested in the outbursts just described, the means of accounting for the vast diffusion of matter in the solar neighbourhood. nor is it possible to explain them away, as cornu,[ ] faye,[ ] and others have sought to do, by substituting for the rush of matter in motion, progressive illumination through electric discharges, chemical processes,[ ] or even through the mere reheating of gases cooled by expansion.[ ] all the appearances are against such evasions of the difficulty presented by velocities stigmatised as "fabulous" and "improbable," but which, there is the strongest reason to believe, really exist. on the th of december, , sir norman lockyer formally expounded before the royal society his hypothesis of the compound nature of the "chemical elements."[ ] an hypothesis, it is true, over and over again propounded from the simply terrestrial point of view. what was novel was the supra-terrestrial evidence adduced in its support; and even this had been, in a general and speculative way, anticipated by professor f. w. clarke of washington.[ ] lockyer had been led to his conclusion along several converging lines of research. in a letter to m. dumas, dated december , , he had sketched out the successive stages of "celestial dissociation" which he conceived to be represented in the sun and stars. the absence from the solar spectrum of metalloidal absorption he explained by the separation, in the fierce solar furnace, of such substances as oxygen, nitrogen, sulphur, and chlorine, into simpler constituents possessing unknown spectra; while metals were at that time still admitted to be capable of existing there in a state of integrity. three years later he shifted his position onward. he announced, as the result of a comparative study of the fraunhofer and electric-arc spectra of calcium, that the "molecular grouping" of that metal, which at low temperatures gives a spectrum with its chief line in the blue, is nearly broken up in the sun into another or others with lines in the violet.[ ] this came to be regarded by him as "a truly typical case."[ ] during four years ( - inclusive) this diligent observer was engaged in mapping a section of the more refrangible part of the solar spectrum (wave-lengths , - , ) on a scale of magnitude such that, if completed down to the infra-red, its length would have been about _half a furlong_. the attendant laborious investigation, by the aid of photography, of metallic spectra, seemed to indicate the existence of what he called "basic lines." these held their ground persistently in the spectra of two or more metals after all possible "impurities" had been eliminated, and were therefore held to attest the presence of a common substratum of matter in a simpler state of aggregation than any with which we are ordinarily acquainted. later inquiries have shown, however, that between the spectral lines of different substances there are probably no _absolute_ coincidences. "basic" lines are really formed of doublets or triplets merged together by insufficient dispersion. of thalèn's original list of seventy rays common to several spectra,[ ] very few resisted thollon's and young's powerful spectroscopes; and the process of resolution was completed by rowland. thus the argument from community of lines to community of substance has virtually collapsed. it was replaced by one founded on certain periodical changes on the spectra of sun-spots. they emerged from a series of observations begun at south kensington under sir norman lockyer's direction in , and continued for fifteen years.[ ] the principle of the method employed is this. the whole range of fraunhofer lines is visible when the light from a spot is examined with the spectroscope; but relatively few are widened. now these widened lines alone constitute (presumably) the true spot-spectrum; they, and they alone, tell what kinds of vapour are thrust down into the strange dusky pit of the nucleus, the unaffected lines taking their accustomed origin from the over-lying strata of the normal solar atmosphere. here then we have the criterion that was wanted--the means of distinguishing, spectroscopically and chemically, between the cavity and the absorbing layers piled up above it. by its persistent employment some marked peculiarities have been brought out, such as the unfamiliar character of numerous lines in spot-spectra, especially at epochs of disturbance; and the strange _individuality_ in the behaviour of every one of these darkened and distended rays. each seems to act on its own account; it comports itself as if it were the sole representative of the substance emitting it; its appearance is unconditioned by that of any of its terrestrial companions in the same spectrum. the most curious fact, however, elicited by these inquiries was that of the attendance of chemical vicissitudes upon the advance of the sun-spot period. as the maximum approached, unknown replaced known components of the spot-spectra in a most pronounced and unmistakable way.[ ] it seemed as if the vapours emitting lines of iron, titanium, nickel, etc., had ceased to exist as such, and their room been taken by others, total strangers in terrestrial laboratories. these were held by lockyer to be simply the finer constituents of their predecessors, dissociation having been effected by the higher temperature ensuing upon increased solar activity. but father cortie's supplementary investigations at stonyhurst[ ] modified, while they in the main substantiated, the south kensington results. they showed that the substitution of unknown for known lines characterizes disturbed spots, at all stages of the solar cycle, so that no systematic course of chemical change can be said to affect the sun as a whole. they showed further[ ]--from evidence independent of that obtained by young in [ ]--the remarkable conspicuousness in spot-spectra of vanadium lines excessively faint in the fraunhofer spectrum. lockyer's "unknown lines" may probably thus be accounted for. they represent absorption, not by new, but by scarce elements, especially, father cortie thinks, those with atomic weights of about . the circumstance of their development in solar commotions, largely to the exclusion of iron, is none the less curious; but it cannot be explained by any process of dissociation. the theory has, however, to be considered under still another aspect. it frequently happens that the contortions or displacements due to motion are seen to affect a single line belonging to a particular substance, while the other lines of _that same substance_ remain imperturbable. now, how is this most singular fact, which seems at first sight to imply that a body may be at rest and in motion at one and the same instant, to be accounted for? it is accounted for, on the present hypothesis, easily enough, by supposing that the rays thus discrepant in their testimony, do _not_ belong to one kind of matter, but to several, combined at ordinary temperatures to form a body in appearance "elementary." of these different vapours, one or more may of course be rushing rapidly towards or from the observer, while the others remain still; and since the line of sight across the average prominence-region penetrates, at the sun's edge, a depth of about , miles,[ ] all the incandescent materials separately occurring along which line are projected into a single "flame" or "cloud," it will be perceived that there is ample room for diversities of behaviour. the alternative mode of escape from the perplexity consists in assuming that the vapour in motion is rendered luminous under conditions which reduce its spectrum to a few rays, the unaffected lines being derived from a totally distinct mass of the same substance shining with its ordinary emissions.[ ] thus, calcium can be rendered virtually monochromatic by attenuation, and analogous cases are not rare. sir norman lockyer only asks us to believe that effects which follow certain causes on the earth are carried a stage further in the sun, where the same causes must be vastly intensified. we find that the bodies we call "compound" split asunder at fixed degrees of heat _within_ the range of our resources. why should we hesitate to admit that the bodies we call "simple" do likewise at degrees of heat _without_ the range of our resources? the term "element" simply expresses terrestrial incapability of reduction. that, in celestial laboratories, the means and their effect here absent should be present, would be an inference challenging, in itself, no expression of incredulity. there are indeed theoretical objections to it which, though probably not insuperable, are unquestionably grave. our seventy chemical "elements," for instance, are placed by the law of specific heats on a separate footing from their known compounds. we are not, it is true, compelled by it to believe their atoms to be really and absolutely such--to contain, that is, the "irreducible minimum" of material substance; but we do certainly gather from it that they are composed on a different principle from the salts and oxides made and unmade at pleasure by chemists. then the multiplication of the species of matter with which lockyer's results menace us, is at first sight startling. they may lead, we are told, to eventual unification, but the prospect appears remote. their only obvious outcome is the disruption into several constituents of each terrestrial "element." the components of iron alone should be counted by the dozen. and there are other metals, such as cerium, which, giving a still more complex spectrum, would doubtless be still more numerously resolved. sir norman lockyer interprets the observed phenomena as indicating the successive combinations, in varying proportions, of a very few original ingredients;[ ] but no definite sign of their existence is perceptible; "protyle" seems likely long to evade recognition; and the only intelligible underlying principle for the reasonings employed--that of "one line, one element"--implies a throng beyond counting of formative material units. thus, added complexity is substituted for that fundamental unity of matter which has long formed the dream of speculators. and it is extremely remarkable that sir william crookes, working along totally different lines, has been led to analogous conclusions. to take only one example. as the outcome of extremely delicate operations of sifting and testing carried on for years, he finds that the metal yttrium splits up into five, if not eight constituents.[ ] evidently, old notions are doomed, nor are any preconceived ones likely to take their place. it would seem, on the contrary, as if their complete reconstruction were at hand. subversive facts are steadily accumulating; the revolutionary ideas springing from them tend, if we interpret them aright, towards the substitution of electrical for chemical theories of matter. dissociation by the brute force of heat is already nearly superseded, in the thoughts of physicists, by the more delicate process of "ionisation." precisely what this implies and involves we do not know; but the symptoms of its occurrence are probably altogether different from those gathered by sir norman lockyer from the collation of celestial spectra. a. j. Ångström of upsala takes rank after kirchhoff as a subordinate founder, so to speak, of solar spectroscopy. his great map of the "normal" solar spectrum[ ] was published in , two years before he died. robert thalèn was his coadjutor in its execution, and the immense labour which it cost was amply repaid by its eminent and lasting usefulness. for more than a score of years it held its ground as the universal standard of reference in all spectroscopic inquiries within the range of the _visible_ emanations. those that are invisible by reason of the quickness of their vibrations were mapped by dr. henry draper, of new york, in , and with superior accuracy by m. cornu in . the infra-red part of the spectrum, investigated by langley, abney, and knut Ångström, reaches perhaps no definite end. the radiations oscillating too slowly to affect the eye as light may pass by insensible gradations into the long hertzian waves of electricity.[ ] professor rowland's photographic map of the solar spectrum, published in , and in a second enlarged edition in , opened fresh possibilities for its study, from far down in the red to high up in the ultra-violet, and the accompanying scale of absolute wave-lengths[ ] has been, with trifling modifications, universally adopted. his new table of standard solar lines was published in .[ ] through his work, indeed, knowledge of the solar spectrum so far outstripped knowledge of terrestrial spectra, that the recognition of their common constituents was hampered by intolerable uncertainties. thousands of the solar lines charted with minute precision remained unidentified for want of a corresponding precision in the registration of metallic lines. rowland himself, however, undertook to provide a remedy. aided by lewis e. jewell, he redetermined, at the johns hopkins university, the wave-lengths of about , solar lines,[ ] photographing for comparison with them the spectra of all the known chemical elements except gallium, of which he could procure no specimen. the labour of collation was well advanced when he died at the age of fifty-two, april , . investigations of metallic arc-spectra have also been carried out with signal success by hasselberg,[ ] kayser and runge, o. lohse,[ ] and others. another condition _sine quâ non_ of progress in this department is the separation of true solar lines from those produced by absorption in our own atmosphere. and here little remains to be done. thollon's great atlas[ ] was designed for this purpose of discrimination. each of its thirty-three maps exhibits in quadruplicate a subdivision of the solar spectrum under varied conditions of weather and zenith-distance. telluric effects are thus made easily legible, and they account wholly for , partly for , out of a total of , lines. but the death of the artist, april , , unfortunately interrupted the half-finished task of the last seven years of his life. a most satisfactory record, meanwhile, of selective atmospheric action has been supplied by the experiments and determinations of janssen, cornu and egoroff, by dr. becker's drawings,[ ] and mr. mcclean's photographs of the analysed light of the sun at high, low, and medium altitudes; and the autographic pictures obtained by mr. george higgs, of liverpool, of certain rhythmical groups in the red, emerging with surprising strength near sunset, excite general and well-deserved admiration.[ ] the main interest, however, of all these documents resides in the information afforded by them regarding the chemistry of the sun. the discovery that hydrogen exists in the atmosphere of the sun was made by Ångström in . his list of solar elements published in that year,[ ] the result of an investigation separate from, though conducted on the same principle as kirchhoff's, included the substance which we now know to be predominant among them. dr. plücker of bonn had identified in the fraunhofer line f with the green ray of hydrogen, but drew no inference from his observation. the agreement was verified by Ångström; two further coincidences were established; and in a fourth hydrogen line in the extreme violet (named _h_) was detected in the solar spectrum. with thalèn, he besides added manganese, titanium, and cobalt to the constituents of the sun enumerated by kirchhoff, and raised the number of identical rays in the solar and terrestrial spectra of iron to no less than .[ ] thus, when sir norman lockyer entered on that branch of inquiry in , fourteen substances were recognised as common to the earth and sun. early in he was able to increase the list provisionally to thirty-three,[ ] all except hydrogen metals. this rapid success was due to his adoption of the test of _length_ in lieu of that of _strength_ in the comparison of lines. he measured their relative significance, in other words, rather by their persistence through a wide range of temperature, than by their brilliancy at any one temperature. the distinction was easily drawn. photographs of the electric arc, in which any given metal had been volatilised, showed some of the rays emitted by it stretching across the axis of the light to a considerable distance on either side, while many others clung more or less closely to its central hottest core. the former "long lines," regarded as certainly representative, were those primarily sought in the solar spectrum; while the attendant "short lines," often, in point of fact, due to foreign admixtures, were set aside as likely to be misleading.[ ] the criterion is a valuable one, and its employment has greatly helped to quicken the progress of solar chemistry. carbon was the first non-metallic element discovered in the sun. messrs. trowbridge and hutchins of harvard college concluded in ,[ ] on the ground of certain spectral coincidences, that this protean substance is vaporised in the solar atmosphere at a temperature approximately that of the voltaic arc. partial evidence to the same effect had earlier been alleged by lockyer, as well as by liveing and dewar; and the case was rendered tolerably complete by photographs taken by kayser and runge in .[ ] it was by professor rowland shown to be irresistible. two hundred carbon-lines were, through his comparisons, sifted out from sunlight, and it contains others significant of the presence of silicon--a related substance, and one as important to rock-building on the earth, as carbon is to the maintenance of life. the general result of rowland's labours was the establishment among solar materials, not only of these two out of the fourteen metalloids, or non-metallic substances, but of thirty-three metals, including silver and tin. gold, mercury, bismuth, antimony, and arsenic were discarded from the catalogue; platinum and uranium, with six other metals, remained doubtful; while iron was recorded as crowding the spectrum with over two thousand obscure rays.[ ] gallium-absorption was detected in it by hartley and ramage in .[ ] dr. henry draper[ ] announced, in , his imagined discovery, in the solar spectrum, of eighteen especially brilliant spaces corresponding to oxygen-emissions. but the agreement proved, when put to the test of very high dispersion, to be wholly illusory.[ ] nor has it yet been found possible to identify, in analysed sunlight, any significant _bright_ beams.[ ] the book of solar chemistry must be read in characters exclusively of absorption. nevertheless, the whole truth is unlikely to be written there. that a substance displays none of its distinctive beams in the spectrum of the sun or of a star, affords scarcely a presumption against its presence. for it may be situated below the level where absorption occurs, or under a pressure such as to efface lines by widening and weakening them; it may be at a temperature so high that it gives out more light than it takes up, and yet its incandescence may be masked by the absorption of other bodies; finally, it may just balance absorption by emission, with the result of complete spectral neutrality. an instructive example is that of the chromospheric element helium. father secchi remarked in [ ] that there is no dark line in the solar spectrum matching its light; and his observation has been fully confirmed.[ ] helium-absorption is, however, occasionally noticed in the penumbræ of spots.[ ] our terrestrial vital element might then easily subsist unrecognisably in the sun. the inner organisation of the oxygen molecule is a considerably _plastic_ one. it is readily modified by heat, and these modifications are reflected in its varying modes of radiating light. dr. schuster enumerated in [ ] four distinct oxygen spectra, corresponding to various stages of temperature, or phases of electrical excitement; and a fifth has been added by m. egoroff's discovery in [ ] that certain well-known groups of dark lines in the red end of the solar spectrum (fraunhofer's a and b) are due to absorption by the cool oxygen of our air. these persist down to the lowest temperatures, and even survive a change of state. they are produced essentially the same by liquid, as by aërial oxygen.[ ] it seemed, however, possible to m. janssen that these bands owned a joint solar and terrestrial origin. oxygen in a fit condition to produce them might, he considered, exist in the outer atmosphere of the sun; and he resolved to decide the point. no one could bring more skill and experience to bear upon it than he.[ ] by observations on the summit of the faulhorn, as well as by direct experiment, he demonstrated, nearly thirty years ago, the leading part played by water-vapour in generating the atmospheric spectrum; and he had recourse to similar means for appraising the share in it assignable to oxygen. an electric beam, transmitted from the eiffel tower to meudon in the summer of , having passed through a weight of oxygen about equal to that piled above the surface of the earth, showed the groups a and b just as they appear in the high-sun spectrum.[ ] atmospheric action is then adequate to produce them. but m. janssen desired to prove, in addition, that they diminish proportionately to its amount. his ascent of mont blanc[ ] in was undertaken with this object. it was perfectly successful. in the solar spectrum, examined from that eminence, oxygen-absorption was so much enfeebled as to leave no possible doubt of its purely telluric origin. under another form, nevertheless, it has been detected as indubitably solar. a triplet of dark lines low down in the red, photographed from the sun by higgs and mcclean, was clearly identified by runge and paschen in [ ] with the fundamental group of an oxygen series, first seen by piazzi smyth in the spectrum of a vacuum-tube in .[ ] the _pabulum vitæ_ of our earth is then to some slight extent effective in arresting transmitted sunlight, and oxygen must be classed as a solar element. the rays of the sun, besides being stopped selectively in our atmosphere, suffer also a marked general absorption. this tells chiefly upon the shortest wave-lengths; the ultra-violet spectrum is in fact closed, as if by the interposition of an opaque screen. nor does the screen appear very sensibly less opaque from an elevation of , feet. dr. simony's spectral photographs, taken on the peak of teneriffe,[ ] extended but slightly further up than m. cornu's, taken in the valley of the loire. could the veil be withdrawn, some indications as to the originating temperature of the solar spectrum might be gathered from its range, since the proportion of quick vibrations given out by a glowing body grows with the intensity of its incandescence. and this brings us to the subject of our next chapter. footnotes: [footnote : _phil. mag._, vol. xlii., p. , .] [footnote : _astr. nach._, no. , , _amer. jour._, vol. xlii., p. ; deslandres, _comptes rendus_, t. cxiii., p. .] [footnote : _proc. roy. society_, vol. lxi., p. .] [footnote : _phil. mag._, vol. xlii., p. .] [footnote : frost-scheiner, _astr. spectroscopy_, pp. , .] [footnote : _proc. roy. soc._, vol. xvii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _am. jour. of science_, vol. xv., p. .] [footnote : _journ. franklin institute_, vol. xl., p. _a_.] [footnote : _pogg. annalen_, bd. cxlvi., p. ; _astr. nach._, no. , .] [footnote : _astr. nach._, nos. , , , .] [footnote : this device was suggested by janssen in .] [footnote : _astr. and astrophysics_, vol. xi., pp. , .] [footnote : _astr. and astrophysics_, vol. xi., p. .] [footnote : _comptes rendus_, t. cxiii., p. .] [footnote : _astr. and astrophysics_, vol. xi., p. .] [footnote : _ibid._, pp. , .] [footnote : wiedemann's _annalen der physik_, bd. xxv., p. .] [footnote : evershed, _knowledge_, vol. xxi., p. .] [footnote : secchi, _le soleil_, t. ii., p. .] [footnote : lockyer, _chemistry of the sun_, p. .] [footnote : _l'astronomie_, august, , p. (riccò); see also evershed, _jour. british astr. ass._, vol. ii., p. .] [footnote : averaging about miles across and high. _le soleil_, t. ii., p. .] [footnote : _the sun_, p. .] [footnote : _astr. nach._, no. , .] [footnote : _mem. degli spettroscopisti italiani_, t. v., p. ; secchi, _ibid._, t. vi., p. .] [footnote : its non-atmospheric character was early defined by proctor, _month. not._, vol. xxxi., p. .] [footnote : _astroph. jour._, vol. vi., p. .] [footnote : _ibid._, vol. xi., p. .] [footnote : _ibid._, p. .] [footnote : _sun's place in nature_, pp. , .] [footnote : _abh. d. kön. böhm ges. d. wiss._, bd. ii., - , p. .] [footnote : in a paper read before the société philomathique de paris, december , , and first published _in extenso_ in _ann. de chim. et de phys._, t. xix., p. ( ). hippolyte fizeau died in september, .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, no. , .] [footnote : a. cornu, _sur la méthode doppler-fizeau_, p. d. .] [footnote : _am. jour. of sc._, vol. xii., p. .] [footnote : _ibid._, vol. xiv., p. .] [footnote : _bull. astronom._, february, , p. .] [footnote : _comptes rendus_, t. xci., p. .] [footnote : _month. not._, vol. xliv., p. .] [footnote : see _ante_, p. .] [footnote : _recherches sur la rotation du soleil_, upsal, .] [footnote : harzer, _astr. nach._, no. , ; stratonoff, _ibid._, no. , .] [footnote : _publ. astr. pacific soc._, vol. ii., p. .] [footnote : _proc. roy. society_, vols. xvii., p. ; xviii., p. .] [footnote : _comptes rendus_, t. cxii., p. ; t. cxiii., p. .] [footnote : at the sun's distance, one second of arc represents about miles.] [footnote : _amer. jour. of sc._, vol. ii., p. , .] [footnote : _month. not._, vol. xxxii., p. .] [footnote : _nature_, vol. xxiii., p. .] [footnote : _comptes rendus_, t. lxxxvii., p. .] [footnote : _ibid._, t. xcvi., p. .] [footnote : a. brester, _théorie du soleil_, p. .] [footnote : such prominences as have been seen to grow by the spread of incandescence are of the quiescent kind, and present no deceptive appearance of violent motion.] [footnote : _proc. roy. soc._, vol. xxviii., p. .] [footnote : "evolution and the spectroscope," _pop. science monthly_, january, .] [footnote : _proc. roy. soc._, vol. xxiv., p. . these are the h and k of prominences. h. w. vogel discovered in a hydrogen-line nearly coincident with h (_monatsb. preuss. ak._, february, , p. ).] [footnote : _proc. roy. soc._, vol. xxviii., p. .] [footnote : many of these were referred by lockyer himself, who first sifted the matter, to traces of the metals concerned.] [footnote : _chemistry of the sun_, p. ; _proc. roy. society_, vol. lvii., p. .] [footnote : _lockyer's chemistry of the sun_, p. .] [footnote : _month. not._, vol. li., p. .] [footnote : _ibid._, vol. lviii., p. .] [footnote : _astr. and astrophysics_, vol. xi., p. .] [footnote : thollon's estimate (_comptes rendus_, t. xcvii., p. ) of , _kilometres_, seems considerably too low. limiting the "average prominence region" to a shell , miles deep ( ' of arc as seen from the earth), the visual line will, at mid-height ( , miles from the sun's surface), travel through (in round numbers) , miles of that region.] [footnote : liveing and dewar, _phil. mag._, vol. xvi. ( th ser.), p. .] [footnote : _chemistry of the sun_, p. .] [footnote : _nature_, october , .] [footnote : the normal spectrum is that depending exclusively upon wave-length--the fundamental constant given by nature as regards light. it is obtained by the interference of rays, in the manner first exemplified by fraunhofer, and affords the only unvarying standard for measurement. in the refraction spectrum (upon which kirchhoff's map was founded), the relative positions of the lines vary with the material of the prisms.] [footnote : scheiner, _die spectralanalyse der gestirne_, p. .] [footnote : _phil. mag._, vol. xxvii., p. .] [footnote : _astr. and astrophysics_, vol. xii., p. ; frost-scheiner, _astr. spectr._, p. .] [footnote : published in _astroph. jour._, vols. i. to vi.] [footnote : _astr. and astrophysics_, vol. xi., p. .] [footnote : _astroph. jour._, vol. vi., p. .] [footnote : _annales de l'observatoire de nice_, t. iii., .] [footnote : _trans. royal society of edinburgh_, vol. xxxvi., p. .] [footnote : rev. a. l. cortie, _astr. and astrophysics_, vol. xi., p. . specimens of his photographs were given by ranyard in _knowledge_, vol. xiii., p. .] [footnote : _ann. d. phys._, bd. cxvii., p. .] [footnote : _comptes rendus_, t. lxiii., p. .] [footnote : _ibid._, t. lxxxvi., p. . some half dozen of these identifications have proved fallacious.] [footnote : _chemistry of the sun_, p. .] [footnote : _amer. jour. of science_, vol. xxxiv., p. .] [footnote : _berlin abhandlungen_, .] [footnote : _amer. jour. of science_, vol. xli., p. . see appendix, table ii.] [footnote : _astrophy. jour._, vol. ix., p. ; fowler, _knowledge_, vol. xxiii., p. .] [footnote : _amer. jour, of science_, vol. xiv., p. ; _nature_, vol. xvi., p. ; _month. not._, vol. xxxix., p. .] [footnote : _month. not._, vol. xxxviii., p. ; trowbridge and hutchins, _amer. jour. of science_, vol. xxxiv., p. .] [footnote : scheiner, _die spectralanalyse_, p. .] [footnote : _comptes rendus_, t. lxvii., p. .] [footnote : rev. a. l. cortie, _month. not._, vol. li., p. .] [footnote : young, _the sun_, p. ; hale, _astr. and astrophysics_, vol. xi., p. buss, _jour. brit. astr. ass._, vol. ix., p. .] [footnote : _phil. trans._, vol. clxx., p. .] [footnote : _comptes rendus_, t. xcvii., p. ; t. ci., p. .] [footnote : liveing and dewar, _astr. and astrophysics_, vol. xi., p. .] [footnote : _comptes rendus_, t. lx., p. ; t. lxiii., p. .] [footnote : _ibid._, t. cviii., p. .] [footnote : _ibid._, t. cxi., p. .] [footnote : _astroph. jour._, vols. iv., p. ; vi., p. .] [footnote : _trans. roy. soc. edin._, vol. xxxii., p. .] [footnote : _comptes rendus_, t. cxi., p. ; huggins, _proc. roy. soc._, vol. xlvi., p. .] chapter v _temperature of the sun_ newton was the first who attempted to measure the quantity of heat received by the earth from the sun. his object in making the experiment was to ascertain the temperature encountered by the comet of at its passage through perihelion. he found it, by multiplying the observed heating effects of direct sunshine according to the familiar rule of the "inverse squares of the distances," to be about , times that of red-hot iron.[ ] determinations of the sun's thermal power, made with some scientific exactness, date, however, from . a few days previous to the beginning of that year, herschel began observing at the cape of good hope with an "actinometer," and obtained results agreeing quite satisfactorily with those derived by pouillet from experiments made in france some months later with a "pyrheliometer."[ ] pouillet found that the vertical rays of the sun falling on each square centimetre of the earth's surface are competent (apart from atmospheric absorption) to raise the temperature of · grammes of water one degree centigrade per minute. this number ( · ) he called the "solar constant"; and the unit of heat chosen is known as the "calorie." hence it was computed that the total amount of solar heat received during a year would suffice to melt a layer of ice covering the entire earth to a depth of · metres, or feet; while the heat emitted would melt, at the sun's surface, a stratum · metres thick each minute. a careful series of observations showed that nearly half the heat incident upon our atmosphere is stopped in its passage through it. herschel got somewhat larger figures, though he assigned only a third as the spoil of the air. taking a mean between his own and pouillet's, he calculated that the ordinary expenditure of the sun per minute would have power to melt a cylinder of ice feet in diameter, reaching from his surface to that of alpha centauri; or, putting it otherwise, that an ice-rod · miles across, continually darted into the sun with the velocity of light, would scarcely consume, in dissolving, the thermal supplies now poured abroad into space.[ ] it is nearly certain that this estimate should be increased by about two-thirds in order to bring it up to the truth. nothing would, at first sight, appear simpler than to pass from a knowledge of solar emission--a strictly measurable quantity--to a knowledge of the solar temperature; this being defined as the temperature to which a surface thickly coated with lamp-black (that is, of standard radiating power) should be raised to enable it to send us, from the sun's distance, the amount of heat actually received from the sun. sir john herschel showed that heat-rays at the sun's surface must be , times as dense as when they reach the earth; but it by no means follows that either the surface emitting, or a body absorbing those heat-rays must be , times hotter than a body exposed here to the full power of the sun. the reason is, that the rate of emission--consequently the rate of absorption, which is its correlative--increases very much faster than the temperature. in other words, a body radiates or cools at a continually accelerated pace as it becomes more and more intensely heated above its surroundings. newton, however, took it for granted that radiation and temperature advance _pari passu_--that you have only to ascertain the quantity of heat received from, and the distance of a remote body in order to know how hot it is.[ ] and the validity of this principle, known as "newton's law" of cooling, was never questioned until de la roche pointed out, in ,[ ] that it was approximately true only over a low range of temperature; while five years later, dulong and petit generalised experimental results into the rule, that while temperature grows by arithmetical, radiation increases by geometrical progression.[ ] adopting this formula, pouillet derived from his observations on solar heat a solar temperature of somewhere between , ° and , ° c. now, the higher of these points--which is nearly that of melting platinum--is undoubtedly surpassed at the focus of certain burning-glasses which have been constructed of such power as virtually to bring objects placed there within a quarter of a million of miles of the photosphere. in the rays thus concentrated, platinum and diamond become rapidly vaporised, notwithstanding the great loss of heat by absorption, first in passing through the air, and again in traversing the lens. pouillet's maximum is then manifestly too low, since it involves the absurdity of supposing a radiating mass capable of heating a distant body more than it is itself heated. less demonstrably, but scarcely less surely, mr. j. j. waterston, who attacked the problem in , erred in the opposite direction. working up, on newton's principle, data collected by himself in india and at edinburgh, he got for the "potential temperature" of the sun , , ° fahr.,[ ] equivalent to , , ° c. the phrase _potential temperature_ (for which violle substituted, in , _effective temperature_) was designed to express the accumulation in a single surface, postulated for the sake of simplicity, of the radiations not improbably received from a multitude of separate solar layers reinforcing each other; and might thus (it was explained) be considerably higher than the _actual_ temperature of any one stratum. at rome, in , father secchi repeated waterston's experiments, and reaffirmed his conclusion;[ ] while soret's observations, made on the summit of mont blanc in ,[ ] furnished him with materials for a fresh and even higher estimate of ten million degrees centigrade.[ ] yet from the very same data, substituting dulong and petit's for newton's law, vicaire deduced in a _provisional_ solar temperature of , °.[ ] this is below that at which iron melts, and we know that iron-vapour exists high up in the sun's atmosphere. the matter was taken into consideration on the other side of the atlantic by ericsson in . he attempted to re-establish the shaken credit of newton's principle, and arrived, by its means, at a temperature of , , ° fahrenheit.[ ] subsequently, an "underrated computation," based upon observation of the quantity of heat received by his "sun motor," gave him , , °. and the result, as he insisted, followed inevitably from the principle that the temperature produced by radiant heat is proportional to its density, or inversely as its diffusion.[ ] the principle, however, is demonstrably unsound. in the sun's temperature was proposed as the subject of a prize by the paris academy of sciences; but although the essay of m. jules violle was crowned, the problem was declared to remain unsolved. violle (who adhered to dulong and petit's formula) arrived at an _effective_ temperature of , ° c., but considered that it might _actually_ reach , ° c., if the emissive power of the photospheric clouds fell far short (as seemed probable) of the lamp-black standard.[ ] experiments made in april and may, , giving a somewhat higher result, he raised this figure to , ° c.[ ] appraisements so outrageously discordant as those of waterston, secchi, and ericsson on the one hand, and those of the french _savants_ on the other, served only to show that all were based upon a vicious principle. professor f. rosetti,[ ] accordingly, of the paduan university, at last perceived the necessity for getting out of the groove of "laws" plainly in contradiction with facts. the temperature, for instance, of the oxy-hydrogen flame was fixed by bunsen at , ° c.--an estimate certainly not very far from the truth. but if the two systems of measurement applied to the sun be used to determine the heat of a solid body rendered incandescent in this flame, it comes out, by newton's mode of calculation, , ° c.; by dulong and petit's, ° c.[ ] both, then, are justly discarded, the first as convicted of exaggeration, the second of undervaluation. the formula substituted by rosetti in was tested successfully up to , ° c.; but since, like its predecessors, it was a purely empirical rule, guaranteed by no principle, and hence not to be trusted out of sight, it was, like them, liable to break down at still higher elevations. radiation by this new prescription increases as the _square_ of the _absolute_ temperature--that is, of the number of degrees counted from the "absolute zero" of - ° c. its employment gave for the sun's radiating surface an effective temperature of , ° c. (including a supposed loss of one-half in the solar atmosphere); and setting a probable deficiency in emission (as compared with lamp-black) against a probable mutual reinforcement of superposed strata, professor rosetti considered "effective" as nearly equivalent to "actual" temperature. a "law of cooling," proposed by m. stefan at vienna in ,[ ] was shown by boltzmann, many years later, to have a certain theoretical validity.[ ] it is that emission grows as the fourth power of absolute temperature. hence the temperature of the photosphere would be proportional to the square root of the square root of its heating effects at a distance, and appeared, by stefan's calculations from violle's measures of solar radiative intensity, to be just , ° c.; while m. h. le chatelier[ ] derived , ° from a formula, conveying an intricate and unaccountable relation between the temperature of an incandescent body and the intensity of its red radiations. from a series of experiments carefully conducted at daramona, ireland, with a delicate thermal balance, of the kind invented by boys and designated a "radio-micrometer," messrs. wilson and gray arrived in , with the aid of stefan's law, at a photospheric temperature of , ° c.,[ ] reduced by the first-named investigator in to , °.[ ] dr. paschen, of hanover, on the other hand, ascribed to the sun a temperature of , ° from comparisons between solar radiative intensity and that of glowing platinum;[ ] while f. w. very showed in [ ] that a minimum value of , ° c. for the same datum resulted from paschen's formula connecting temperature with the position of maximum spectral energy. a new line of inquiry was struck out by zöllner in . instead of tracking the solar radiations backward with the dubious guide of empirical formulæ, he investigated their intensity at their source. he showed[ ] that, taking prominences to be simple effects of the escape of powerfully compressed gases, it was possible, from the known mechanical laws of heat and gaseous constitution, to deduce minimum values for the temperatures prevailing in the area of their development. these came out , ° c. for the strata lying immediately above, and , ° c. for the strata lying immediately below the photosphere, the former being regarded as the region _into_ which, and the latter as the region _from_ which the eruptions took place. in this calculation, no prominences exceeding , miles ( · ') in height were included. but in , g. a. hirn of colmar, having regard to the enormous velocities of projection observed in the interim, fixed two million degrees centigrade as the lowest _internal_ temperature by which they could be accounted for; although admitting the photospheric condensations to be incompatible with a higher _external_ temperature than , ° to , ° c.[ ] this method of going straight to the sun itself, observing what goes on there, and inferring conditions, has much to recommend it; but its profitable use demands knowledge we are still very far from possessing. we are quite ignorant, for instance, of the actual circumstances attending the birth of the solar flames. the assumption that they are nothing but phenomena of elasticity is a purely gratuitous one. spectroscopic indications, again, give hope of eventually affording a fixed point of comparison with terrestrial heat sources; but their interpretation is still beset with uncertainties; nor can, indeed, the expression of transcendental temperatures in degrees of impossible thermometers be, at the best, other than a futile attempt to convey notions respecting a state of things altogether outside the range of our experience. a more tangible, as well as a less disputable proof of solar radiative intensity than any mere estimates of temperature, was provided in some experiments made by professor langley in .[ ] using means of unquestioned validity, he found the sun's disc to radiate times as much heat, and , times as much light as an equal area of metal in a bessemer converter after the air-blast had continued about twenty minutes. the brilliancy of the incandescent steel, nevertheless, was so blinding, that melted iron, flowing in a dazzling white-hot stream into the crucible, showed "deep brown by comparison, presenting a contrast like that of dark coffee poured into a white cup." its temperature was estimated (not quite securely)[ ] at about , ° c.; and no allowances were made, in computing relative intensities, for atmospheric ravages on sunlight, for the extra impediments to its passage presented by the smoke-laden air of pittsburgh, or for the obliquity of its incidence. thus, a very large balance of advantage lay on the side of the metal. a further element of uncertainty in estimating the intrinsic strength of the sun's rays has still to be considered. from the time that his disc first began to be studied with the telescope, it was perceived to be less brilliant near the edges. lucas valerius, of the lyncean academy, seems to have been the first to note this fact, which, strangely enough, was denied by galileo in a letter to prince cesi of january , .[ ] father scheiner, however, fully admitted it, and devoted some columns of his bulky tome to the attempt to find its appropriate explanation.[ ] in bouguer measured, with much accuracy, the amount of this darkening; and from his data, laplace, adopting a principle of emission now known to be erroneous, concluded that the sun loses eleven-twelfths of his light through absorption in his own atmosphere.[ ] the real existence of this atmosphere, which is totally distinct from the beds of ignited vapours producing the fraunhofer lines, is not open to doubt, although its nature is still a matter of conjecture. the separate effects of its action on luminous, thermal, and chemical rays were carefully studied by father secchi, who in [ ] inferred the total absorption to be / of all radiations taken together, and added the important observation that the light from the limb is no longer white, but reddish-brown. absorptive effects were thus seen to be unequally distributed; and they could evidently be studied to advantage only by taking the various rays of the spectrum separately, and finding out how much each had suffered in transmission. this was done by h. c. vogel in .[ ] using a polarising photometer, he found that only per cent. of the violet rays escape at the edge of the solar disc, of the blue and green, of the yellow, and per cent. of the red. midway between centre and limb, · of violet light and · of red penetrate the absorbing envelope, the abolition of which would increase the intensity of the sun's visible spectrum above two and a half times in the most, and once and a half times in the least refrangible parts. the nucleus of a small spot was ascertained to be of the same luminous intensity as a portion of the unbroken surface about two and a half minutes from the limb. these experiments having been made during a spot-minimum when there is reason to think that absorption is below its average strength, vogel suggested their repetition at a time of greater activity. they were extended to the heat-rays by edwin b. frost. detailed inquiries made at potsdam in [ ] went to show that, were the sun's atmosphere removed, his thermal power, as regards ourselves, would be increased · times. they established, too, the practical uniformity in radiation of all parts of his disc. a confirmatory result was obtained about the same time by wilson and rambaut, who found that the unveiled sun would be once and a half times hotter than the actual sun.[ ] professor langley, now of washington, gave to measures of the kind a refinement previously undreamt of. reliable determinations of the "energy" of the individual spectral rays were, for the first time, rendered possible by his invention of the "bolometer" in .[ ] this exquisitely sensitive instrument affords the means of measuring heat, not directly, like the thermopile, but in its effects upon the conduction of electricity. it represents, in the phrase of the inventor, the finger laid upon the throttle-valve of a steam-engine. a minute force becomes the modulator of a much greater force, and thus from imperceptible becomes conspicuous. by locally raising the temperature of an inconceivably fine strip of platinum serving as the conducting-wire in a circuit, the flow of electricity is impeded at that point, and the included galvanometer records a disturbance of the electrical flow. amounts of heat were thus detected in less than ten seconds, which, expended during a thousand years on the melting of a kilogramme of ice, would leave a part of the work still undone; and further improvements rendered this marvellous instrument capable of thrilling to changes of temperature falling short of one ten-millionth of a degree centigrade.[ ] the heat contained in the diffraction spectrum is, with equal dispersions, barely one-tenth of that in the prismatic spectrum. it had, accordingly, never previously been found possible to measure it in detail--that is, ray by ray. but it is only from the diffraction, or normal spectrum that any true idea can be gained as to the real distribution of energy among the various constituents, visible and invisible, of a sunbeam. the effect of passage through a prism is to crowd together the red rays very much more than the blue. to this prismatic distortion was owing the establishment of a pseudo-maximum of heat in the infra-red, which disappeared when the natural arrangement by wave-length was allowed free play. langley's bolometer has shown that the hottest part of the normal spectrum virtually coincides with its most luminous part, both lying in the orange, close to the d-line.[ ] thus the last shred of evidence in favour of the threefold division of solar radiations vanished, and it became obvious that the varying effects--thermal, luminous, or chemical--produced by them are due, not to any distinction of quality in themselves, but to the different properties of the substances they impinge upon. they are simply bearers of energy, conveyed in shorter or longer vibrations; the result in each separate case depending upon the capacity of the material particles meeting them for taking up those shorter or longer vibrations, and turning them variously to account in their inner economy. a long series of experiments at allegheny was completed in the summer of on the crest of mount whitney in the sierra nevada. here, at an elevation of , feet, in the driest and purest air, perhaps, in the world, atmospheric absorptive inroads become less sensible, and the indications of the bolometer, consequently, surer and stronger. an enormous expansion was at once given to the invisible region in the solar spectrum below the red. captain abney had got chemical effects from undulations twelve ten-thousandths of a millimetre in length. these were the longest recognised as, or indeed believed, on theoretical grounds, to be capable of existing. professor langley now got heating effects from rays of above twice that wave-length, his delicate thread of platinum groping its way down nearly to thirty ten-thousandths of a millimetre, or three "microns." the known extent of the solar spectrum was thus at once more than doubled. its visible portion covers a range of about one octave; bolometric indications already in comprised between three and four. the great importance of the newly explored region appears from the fact that three-fourths of the entire energy of sunlight reside in the infra-red, while scarcely more than one-hundredth part of that amount is found in the better known ultra-violet space.[ ] these curious facts were reinforced, in ,[ ] by further particulars learned with the help of rock-salt lenses and prisms, glass being impervious to very slow, as to very rapid vibrations. traces were thus detected of solar heat distributed into bands of transmission alternating with bands of atmospheric absorption, far beyond the measurable limit of · microns. in , langley described at the oxford meeting of the british association[ ] his new "bolographic" researches, in which the sensitive plate was substituted for the eye in recording deflections of the galvanometer responding to variations of invisible heat. finally, in ,[ ] he embodied in a splendid map of the infra-red spectrum absorption-lines of determinate wave-lengths, ranging from · to · microns. their chemical origin, indeed, remains almost entirely unknown, no extensive investigations having yet been undertaken of the slower vibrations distinctive of particular substances; but there is evidence that seven of the nine great bands crossing the "new spectrum" (as langley calls it)[ ] are telluric, and subject to seasonal change. here, then, he thought, might eventually be found a sure standing-ground for vitally important previsions of famines, droughts, and bonanza-crops. atmospheric absorption had never before been studied with such precision as it was by langley on mount whitney. aided by simultaneous observations from lone pine, at the foot of the sierra, he was able to calculate the intensity belonging to each ray before entering the earth's gaseous envelope--in other words, to construct an extra-atmospheric curve of energy in the spectrum. the result showed that the blue end suffered far more than the red, absorption varying inversely as wave-length. this property of stopping predominantly the quicker vibrations is shared, as both vogel and langley[ ] have conclusively shown, by the solar atmosphere. the effect of this double absorption is as if two plates of reddish glass were interposed between us and the sun, the withdrawal of which would leave his orb, not only three or four times more brilliant, but in colour distinctly greenish-blue.[ ] the fact of the uncovered sun being _blue_ has an important bearing upon the question of his temperature, to afford a somewhat more secure answer to which was the ultimate object of professor langley's persevering researches; for it is well known that as bodies grow hotter, the proportionate representation in their spectra of the more refrangible rays becomes greater. the lowest stage of incandescence is the familiar one of _red_ heat. as it gains intensity, the quicker vibrations come in, and an optical balance of sensation is established at _white_ heat. the final term of _blue_ heat, as we now know, is attained by the photosphere. on this ground alone, then, of the large original preponderance of blue light, we must raise our estimate of solar heat; and actual measurements show the same upward tendency. until quite lately, pouillet's figure of . calories per minute per square centimetre of terrestrial surface, was the received value for the "solar constant." forbes had, it is true, got . from observations on the faulhorn in ;[ ] but they failed to obtain the confidence they merited. pouillet's result was not definitely superseded until violle, from actinometrical measures at the summit and base of mont blanc in , computed the intensity of solar radiation at . ,[ ] and crova, about the same time, at montpellier, showed it to be above two calories.[ ] langley went higher still. working out the results of the mount whitney expedition, he was led to conclude atmospheric absorption to be fully twice as effective as had hitherto been supposed. scarcely per cent., in fact, of those solar radiations which strike perpendicularly through a seemingly translucent sky, were estimated to attain the sea-level. the rest are reflected, dispersed, or absorbed. this discovery involved a large addition to the original supply so mercilessly cut down in transmission, and the solar constant rose at once to three calories. nor did the rise stop there. m. savélieff deduced for it a value of . from actinometrical observations made at kieff in ;[ ] and knut Ångström, taking account of the arrestive power of carbonic acid, inferred enormous atmospheric absorption, and a solar constant of four calories.[ ] in other words, the sun's heat reaching the outskirts of our atmosphere is capable of doing without cessation the work of an engine of four-horse power for each square yard of the earth's surface. thus, modern inquiries tend to render more and more evident the vastness of the thermal stores contained in the great central reservoir of our system, while bringing into fair agreement the estimates of its probable temperature. this is in great measure due to the acquisition of a workable formula by which to connect temperature with radiation. stefan's rule of a fourth-power relation, if not actually a law of nature, is a colourable imitation of one; and its employment has afforded a practical certainty that the sun's temperature, so far as it is definable, neither exceeds , ° c., nor falls short of , ° c. footnotes: [footnote : _principia_, p. ( st ed.).] [footnote : _comptes rendus_, t. vii., p. .] [footnote : _results of astr. observations_, p. .] [footnote : "est enim calor solis ut radiorum densitas, hoc est, reciproce ut quadratum distantiæ locorum a sole."--_principia_, p. ( d ed., ).] [footnote : _jour. de physique_, t. lxxv., p. .] [footnote : _ann. de chimie_, t. vii., , p. .] [footnote : _phil. mag._, vol. xxiii. ( th ser.), p. .] [footnote : _nuovo cimento_, t. xvi., p. .] [footnote : _comptes rendus_, t. lxv., p. .] [footnote : the direct result of - / million degrees was doubled in allowance for absorption in the sun's own atmosphere. _comptes rendus_, t. lxxiv., p. .] [footnote : _ibid._, p. .] [footnote : _nature_, vols. iv., p. ; v., p. .] [footnote : _ibid._, vol. xxx., p. .] [footnote : _ann. de chim._, t. x. ( th ser.), p. .] [footnote : _comptes rendus_, t. xcvi., p. .] [footnote : _phil. mag._, vol. viii., p. , .] [footnote : _ibid._, p. .] [footnote : _sitzungsberichte_, wien, bd. lxxix., ii., p. .] [footnote : _wiedemann's annalen_, bd. xxii., p. ; _scheiner, strahlung und temperatur der sonne_, p. .] [footnote : _comptes rendus_, march , ; _astr. and astrophysics_, vol. xi., p. .] [footnote : _phil. trans._, vol. clxxxv., p. .] [footnote : _proc. roy. society_, december , .] [footnote : scheiner, _temp. der sonne_, p. .] [footnote : _astroph. jour._, vol. ii., p. .] [footnote : _astr. nach._, nos. , - .] [footnote : _l'astronomie_, september, , p. .] [footnote : _amer. jour. of science_, vol. i. ( rd ser.), p. .] [footnote : young, _the sun_, p. .] [footnote : _op._, t. vi., p. .] [footnote : _rosa ursina_, lib. iv., p. .] [footnote : _méc. cél._, liv. x., p. .] [footnote : _le soleil_ ( st ed.), p. .] [footnote : _monatsber._, berlin, , p. .] [footnote : _astr. nach._, nos. , - ; _astr. and astrophysics_, vol. xi., p. .] [footnote : _proc. roy. irish acad._, vol. ii., no. , .] [footnote : _am. jour. of sc._, vol. xxi., p. .] [footnote : _amer. jour. of science_, vol. v., p. , .] [footnote : for j. w. draper's partial anticipation of this result, see _ibid_. vol. iv., , p. .] [footnote : _phil. mag._, vol. xiv., p. , .] [footnote : "the solar and the lunar spectrum," _memoirs national acad. of science_, vol. xxxii.; "on hitherto unrecognised wave-lengths," _amer. jour. of science_, vol. xxxii., august, .] [footnote : _astroph. jour._, vol. i., p. .] [footnote : _annals of the smithsonian astroph. observatory_, vol. i.; _comptes rendus_, t. cxxxi., p. ; _astroph. jour._, vol. iii., p. .] [footnote : _phil. mag._, july, .] [footnote : _comptes rendus_, t. xcii., p. .] [footnote : _nature_, vol. xxvi., p. .] [footnote : _phil. trans._, vol. cxxxii., p. .] [footnote : _ann. de chim._, t. x., p. .] [footnote : _ibid._, t. xi., p. .] [footnote : _comptes rendus_, t. cxii., p. .] [footnote : _wied. ann._, bd. xxxix., p. ; scheiner, _temperatur der sonne_, pp. , .] chapter vi _the sun's distance_ the question of the sun's distance arises naturally from the consideration of his temperature, since the intensity of the radiations emitted as compared with those received and measured, depends upon it. but the knowledge of that distance has a value quite apart from its connection with solar physics. the semi-diameter of the earth's orbit is our standard measure for the universe. it is the great fundamental datum of astronomy--the unit of space, any error in the estimation of which is multiplied and repeated in a thousand different ways, both in the planetary and sidereal systems. hence its determination was called by airy "the noblest problem in astronomy." it is also one of the most difficult. the quantities dealt with are so minute that their sure grasp tasks all the resources of modern science. an observational inaccuracy which would set the moon nearer to, or farther from us than she really is by one hundred miles, would vitiate an estimate of the sun's distance to the extent of sixteen million![ ] what is needed in order to attain knowledge of the desired exactness is no less than this: to measure an angle about equal to that subtended by a halfpenny , feet from the eye, within a little more than a thousandth part of its value. the angle thus represented is what is called the "horizontal parallax" of the sun. by this amount--the breadth of a halfpenny at , feet--he is, to a spectator on the rotating earth, removed at rising and setting from his meridian place in the heavens. such, in other terms, would be the magnitude of the terrestrial radius as viewed from the sun. if we knew this magnitude with certainty and precision, we should also know with certainty and precision--the dimensions of the earth being, as they are, well ascertained--the distance of the sun. in fact, the one quantity commonly stands for the other in works treating professedly of astronomy. but this angle of parallax or apparent displacement cannot be directly measured--cannot even be perceived with the finest instruments. not from its smallness. the parallactic shift of the nearest of the stars as seen from opposite sides of the earth's orbit, is many times smaller. but at the sun's limb, and close to the horizon, where the visual angle in question opens out to its full extent, atmospheric troubles become overwhelming, and altogether swamp the far more minute effects of parallax. there remain indirect methods. astronomers are well acquainted with the proportions which the various planetary orbits bear to each other. they are so connected, in the manner expressed by kepler's third law, that the periods being known, it only needs to find the interval between any two of them in order to infer at once the distances separating them all from one another and from the sun. the plan is given; what we want to discover is the scale upon which it is drawn; so that, if we can get a reliable measure of the distance of a single planet from the earth, our problem is solved. now some of our fellow-travellers in our unending journey round the sun, come at times well within the scope of celestial trigonometry. the orbit of mars lies at one point not more than thirty-five million miles outside that of the earth, and when the two bodies happen to arrive together in or near the favourable spot--a conjuncture which occurs every fifteen years--the desired opportunity is granted. mars is then "in opposition," or on the _opposite_ side of us from the sun, crossing the meridian consequently at midnight.[ ] it was from an opposition of mars, observed in by richer at cayenne in concert with cassini in paris, that the first scientific estimate of the sun's distance was derived. it appeared to be nearly eighty-seven millions of miles (parallax · "); while flamsteed deduced , , (parallax ") from his independent observations of the same occurrence--a difference quite insignificant at that stage of the inquiry. but picard's result was just half flamsteed's (parallax "; distance forty-one million miles); and lahire considered that we must be separated from the hearth of our system by an interval of _at least_ million miles.[ ] so that uncertainty continued to have an enormous range. venus, on the other hand, comes closest to the earth when she passes between it and the sun. at such times of "inferior conjunction" she is, however, still twenty-six million miles, or (in round numbers) times as distant as the moon. moreover, she is so immersed in the sun's rays that it is only when her path lies across his disc that the requisite facilities for measurement are afforded. these "partial eclipses of the sun by venus" (as encke termed them) are coupled together in pairs,[ ] of which the components are separated by eight years, recurring at intervals alternately of - / and - / years. thus, the first calculated transit took place in december, , and its companion (observed by horrocks) in the same month (n.s.), . then, after the lapse of - / years, came the june couple of and ; and again after - / , the two last observed, december , , and december , . throughout the twentieth century there will be no transit of venus; but the astronomers of the twenty-first will only have to wait four years for the first of a june pair. the rarity of these events is due to the fact that the orbits of the earth and venus do not lie in the same plane. if they did, there would be a transit each time that our twin-planet overtakes us in her more rapid circling--that is, on an average, every days. as things are actually arranged, she passes above or below the sun, except when she happens to be very near the line of intersection of the two tracks. such an occurrence as a transit of venus seems, at first sight, full of promise for solving the problem of the sun's distance. for nothing would appear easier than to determine exactly either the duration of the passage of a small, dark orb across a large brilliant disc, or the instant of its entry upon or exit from it. and the differences in these times (which, owing to the comparative nearness of venus, are quite considerable), as observed from remote parts of the earth, can be translated into differences of space--that is, into apparent or parallactic displacements, whereby the distance of venus becomes known, and thence, by a simple sum in proportion, the distance of the sun. but in that word "exactly" what snares and pitfalls lie hid! it is so easy to think and to say; so indefinitely hard to realise. the astronomers of the eighteenth century were full of hope and zeal. they confidently expected to attain, through the double opportunity offered them, to something like a permanent settlement of the statistics of our system. they were grievously disappointed. the uncertainty as to the sun's distance, which they had counted upon reducing to a few hundred thousand miles, remained at many millions. in , however, encke, then director of the seeberg observatory near gotha, undertook to bring order out of the confusion of discordant, and discordantly interpreted observations. his combined result for both transits ( and ) was published in ,[ ] and met universal acquiescence. the parallax of the sun thereby established was · ", corresponding to a mean distance[ ] of - / million miles. yet this abolition of doubt was far from being so satisfactory as it seemed. serenity on the point lasted exactly thirty years. it was disturbed in by hansen's announcement[ ] that the observed motions of the moon could be drawn into accord with theory only on the terms of bringing the sun considerably nearer to us than he was supposed to be. dr. matthew stewart, professor of mathematics in the university of edinburgh, had made a futile attempt in to deduce the sun's distance from his disturbing power over our satellite.[ ] tobias mayer of göttingen, however, whose short career was fruitful of suggestions, struck out the right way to the same end; and laplace, in the seventh book of the _mécanique céleste_,[ ] gave a solar parallax derived from the lunar "parallactic inequality" substantially identical with that issuing from encke's subsequent discussion of the eighteenth-century transits. thus, two wholly independent methods--the trigonometrical, or method by survey, and the gravitational, or method by perturbation--seemed to corroborate each the upshot of the use of the other until the nineteenth century was well past its meridian. it is singular how often errors conspire to lead conviction astray. hansen's note of alarm in was echoed by leverrier in .[ ] he found that an apparent monthly oscillation of the sun which reflects a real monthly movement of the earth round its common centre of gravity with the moon, and which depends for its amount solely on the mass of the moon and the distance of the sun, required a diminution in the admitted value of that distance by fully four million miles. three years later he pointed out that certain perplexing discrepancies between the observed and computed places both of venus and mars, would vanish on the adoption of a similar measure.[ ] moreover, a favourable opposition of mars gave the opportunity in for fresh observations, which, separately worked out by stone and winnecke, agreed with all the newer investigations in fixing the great unit at slightly over million miles. in newcomb's hands they gave - / million.[ ] the accumulating evidence in favour of a large reduction in the sun's distance was just then reinforced by an auxiliary result of a totally different and unexpected kind. the discovery that light does not travel instantaneously from point to point, but takes some short time in transmission, was made by olaus römer in , through observing that the eclipses of jupiter's satellites invariably occurred later, when the earth was on the far side, than when it was on the near side of its orbit. half the difference, or the time spent by a luminous vibration in crossing the "mean radius" of the earth's orbit, is called the "light-equation"; and the determination of its precise value has claimed the minute care distinctive of modern astronomy. delambre in made it seconds. glasenapp, a russian astronomer, raised the estimate in to , professor harkness adopts a safe medium value of seconds. hence, if we had any independent means of ascertaining how fast light travels, we could tell at once how far off the sun is. there is yet another way by which knowledge of the swiftness of light would lead us straight to the goal. the heavenly bodies are perceived, when carefully watched and measured, to be pushed forward out of their true places, in the direction of the earth's motion, by a very minute quantity. this effect (already adverted to) has been known since bradley's time as "aberration." it arises from a combination of the two movements of the earth round the sun and of the light-waves through the ether. if the earth stood still, or if light spent no time on the road from the stars, such an effect would not exist. its amount represents the proportion between the velocities with which the earth and the light-rays pursue their respective journeys. this proportion is, roughly, one to ten thousand. so that here again, if we knew the rate per second of luminous transmission, we should also know the rate per second of the earth's movement, consequently the size of its orbit and the distance of the sun. but, until lately, instead of finding the distance of the sun from the velocity of light, there has been no means of ascertaining the velocity of light except through the imperfect knowledge possessed as to the distance of the sun. the first successful terrestrial experiments on the point date from ; and it is certainly no slight triumph of human ingenuity to have taken rigorous account of the delay of a sunbeam in flashing from one mirror to another. fizeau led the way,[ ] and he was succeeded, after a few months, by léon foucault,[ ] who, in , had so far perfected wheatstone's method of revolving mirrors, as to be able to announce with authority that light travelled slower, and that the sun was in consequence nearer than had been supposed.[ ] thus a third line of separate research was found to converge to the same point with the two others. such a conspiracy of proof was not to be resisted, and at the anniversary meeting of the royal astronomical society in february, , the correction of the solar distance took the foremost place in the annals of the year. lest, however, a sudden bound of four million miles nearer to the centre of our system should shake public faith in astronomical accuracy, it was explained that the change in the solar parallax corresponding to that huge leap, amounted to no more than the breadth of a human hair feet from the eye![ ] the nautical almanac gave from the altered value of . ", for which newcomb's result of . ", adopted in in the berlin ephemeris, was substituted some ten years later. in astronomical literature the change was initiated by sir edmund beckett in the first edition ( ) of his _astronomy without mathematics_. if any doubt remained as to the misleading character of encke's deduction, so long implicitly trusted in, it was removed by powalky's and stone's rediscussions, in and respectively, of the transit observations of . using improved determinations of the longitude of the various stations, and a selective judgment in dealing with their materials, which, however indispensable, did not escape adverse criticism, they brought out results confirmatory of the no longer disputed necessity for largely increasing the solar parallax, and proportionately diminishing the solar distance. once more in , and this time with better success, the eighteenth-century transits were investigated by professor newcomb.[ ] turning to account the experience gained in the interim regarding the optical phenomena accompanying such events, he elicited from the mass of somewhat discordant observations at his command, a parallax ( · ") in close agreement with the value given by sundry modes of recent research. conclusions on the subject, however, were still regarded as purely provisional. a transit of venus was fast approaching, and to its arbitrament, as to that of a court of final appeal, the pending question was to be referred. it is true that the verdict in the same case by the same tribunal a century earlier had proved of so indecisive a character as to form only a starting-point for fresh litigation; but that century had not passed in vain, and it was confidently anticipated that observational difficulties, then equally unexpected and insuperable, would yield to the elaborate care and skill of forewarned modern preparation. the conditions of the transit of december , , were sketched out by sir george airy, then astronomer-royal, in ,[ ] and formed the subject of eager discussion in this and other countries down to the very eve of the occurrence. in these mr. proctor took a leading part; and it was due to his urgent representations that provision was made for the employment of the method identified with the name of halley,[ ] which had been too hastily assumed inapplicable to the first of each transit-pair. it depends upon the difference in the length of time taken by the planet to cross the sun's disc, as seen from various points of the terrestrial surface, and requires, accordingly, the visibility of both entrance and exit at the same station. since these were, in , separated by about three and a half hours, and the interval may be much longer, the choice of posts for the successful use of the "method of durations" is a matter of some difficulty. the system described by delisle in , on the other hand, involves merely noting the instant of ingress or egress (according to situation) from opposite extremities of a terrestrial diameter; the disparity in time giving a measure of the planet's apparent displacement, hence of its actual rate of travel in miles per minute, from which its distances severally from earth and sun are immediately deducible. its chief attendant difficulty is the necessity for accurately fixing the longitudes of the points of observation. but this was much more sensibly felt a century ago than it is now, the improved facility and certainty of modern determinations tending to give the delislean plan a decided superiority over its rival. these two traditional methods were supplemented in by the camera and the heliometer. from photography, above all, much was expected. observations made by its means would have the advantages of impartiality, multitude, and permanence. peculiarities of vision and bias of judgment would be eliminated; the slow progress of the phenomenon would permit an indefinite number of pictures to be taken, their epochs fixed to a fraction of a second; while subsequent leisurely comparison and measurement could hardly fail, it was thought, to educe approximate truth from the mass of accumulated evidence. the use of the heliometer (much relied on by german observers) was so far similar to that of the camera that the object aimed at by both was the determination of the relative positions of the _centres_ of the sun and venus viewed, at the same absolute instant, from opposite sides of the globe. so that the principle of the two older methods was to ascertain the exact times of meeting between the solar and planetary limbs; that of the two modern to determine the position of the dark body already thrown into complete relief by its shining background. the former are "methods by contact," the latter "methods by projection." every country which had a reputation to keep or to gain for scientific zeal was forward to co-operate in the great cosmopolitan enterprise of the transit. france and germany each sent out six expeditions; twenty-six stations were in russian, twelve in english, eight in american, three in italian, one in dutch occupation. in all, at a cost of nearly a quarter of a million, some fourscore distinct posts of observation were provided; among them such inhospitable, and all but inaccessible rocks in the bleak southern ocean, as st. paul's and campbell islands, swept by hurricanes, and fitted only for the habitation of seabirds, where the daring votaries of science, in the wise prevision of a long leaguer by the elements, were supplied with stores for many months, or even a whole year. siberia and the sandwich islands were thickly beset with observers; parties of three nationalities encamped within the mists of kerguelen island, expressively termed the "land of desolation," in the sanguine, though not wholly frustrated hope of a glimpse of the sun at the right moment. m. janssen narrowly escaped destruction from a typhoon in the china seas on his way to nagasaki; lord lindsay (now earl of crawford and balcarres) equipped, at his private expense, an expedition to mauritius, which was in itself an epitome of modern resource and ingenuity. during several years, the practical methods best suited to insure success for the impending enterprise formed a subject of european debate. official commissions were appointed to receive and decide upon evidence; and experiments were in progress for the purpose of defining the actual circumstances of contacts, the precise determination of which constituted the only tried, though by no means an assuredly safe road to the end in view. in england, america, france, and germany, artificial transits were mounted, and the members of the various expeditions were carefully trained to unanimity in estimating the phases of junction and separation between a moving dark circular body and a broad illuminated disc. in the previous century, a formidable and prevalent phenomenon, which acquired notoriety as the "black drop" or "black ligament," had swamped all pretensions to rigid accuracy. it may be described as substituting adhesion for contact, the limbs of the sun and planet, instead of meeting and parting with the desirable clean definiteness, _clinging_ together as if made of some glutinous material, and prolonging their connection by means of a dark band or dark threads stretched between them. some astronomers ascribed this baffling appearance entirely to instrumental imperfections; others to atmospheric agitation; others again to the optical encroachment of light upon darkness known as "irradiation." it is probable that all these causes conspired, in various measure, to produce it; and it is certain that its _conspicuous_ appearance may, by suitable precautions, be obviated. the organisation of the british forces reflected the utmost credit on the energy and ability of lieutenant-colonel tupman, who was responsible for the whole. no useful measure was neglected. each observer went out ticketed with his "personal equation," his senses drilled into a species of martial discipline, his powers absorbed, so far as possible, in the action of a cosmopolitan observing machine. instrumental uniformity and uniformity of method were obtainable, and were attained; but diversity of judgment unhappily survived the best-directed efforts for its extirpation. the eventful day had no sooner passed than telegrams began to pour in, announcing an outcome of considerable, though not unqualified success. the weather had proved generally favourable; the manifold arrangements had worked well; contacts had been plentifully observed; photographs in lavish abundance had been secured; a store of materials, in short, had been laid up, of which it would take years to work out the full results by calculation. gradually, nevertheless, it came to be known that the hope of a definitive issue must be abandoned. unanimity was found to be as remote as ever. the dreaded "black ligament" gave, indeed, less trouble than was expected; but another appearance supervened which took most observers by surprise. this was the illumination due to the atmosphere of venus. astronomers, it is true, were not ignorant that the planet had, on previous occasions, been seen girdled with a lucid ring; but its power to mar observations by the distorting effect of refraction had scarcely been reckoned with. it proved, however, to be very great. such was the difficulty of determining the critical instant of internal contact, that (in colonel tupman's words) "observers side by side, with adequate optical means, differed as much as twenty or thirty seconds in the times they recorded for phenomena which they have described in almost identical language."[ ] such uncertainties in the data admitted of a corresponding variety in the results. from the british observations of ingress and egress sir george airy[ ] derived, in , a solar parallax of · " (corrected to · "), indicating a mean distance of , , miles. mr. stone obtained a value of ninety-two millions (parallax · "), and held any parallax less than · " or more than · " to be "absolutely negatived" by the documents available.[ ] yet, from the same, colonel tupman deduced · ",[ ] implying a distance , miles greater than stone had obtained. the best french observations of contacts gave a parallax of about · "; french micrometric measures the obviously exaggerated one of · ".[ ] photography, as practised by most of the european parties, was a total failure. utterly discrepant values of the microscopic displacements designed to serve as sounding lines for the solar system, issued from attempts to measure even the most promising pictures. "you might as well try to measure the zodiacal light," it was remarked to sir george airy. those taken on the american plan of using telescopes of so great focal length as to afford, without further enlargement, an image of the requisite size, gave notably better results. from an elaborate comparison of those dating from vladivostock, nagasaki, and pekin, with others from kerguelen and chatham islands, professor d. p. todd, of amherst college, deduced a solar distance of about ninety-two million miles (parallax · " ± · "),[ ] and the value was much favoured by concurrent evidence. on the whole, estimates of the great spatial unit cannot be said to have gained any security from the combined effort of . a few months before the transit, mr. proctor considered that the uncertainty then amounted to , , miles;[ ] five years after the transit, professor harkness judged it to be still , , miles;[ ] yet it had been hoped that it would have been brought down to , . as regards the end for which it had been undertaken, the grand campaign had come to nothing. nevertheless, no sign of discouragement was apparent. there was a change of view, but no relaxation of purpose. the problem, it was seen, could be solved by no single heroic effort, but by the patient approximation of gradual improvements. astronomers, accordingly, looked round for fresh means or more refined expedients for applying those already known. a new phase of exertion was entered upon. on september , , mars came into opposition near the part of his orbit which lies nearest to that of the earth, and dr. gill (now sir david) took advantage of the circumstance to appeal once more to him for a decision on the _quæstio vexata_ of the sun's distance. he chose, as the scene of his labours, the island of ascension, and for their plan a method recommended by airy in ,[ ] but never before fairly tried. this is known as the "diurnal method of parallaxes." its principle consists in substituting successive morning and evening observations from the same spot, for simultaneous observations from remote spots, the rotation of the earth supplying the necessary difference in the points of view. its great advantage is that of unity in performance. a single mind, looking through the same pair of eyes, reinforced with the same optical appliances, is employed throughout, and the errors inseparable from the combination of data collected under different conditions are avoided. there are many cases in which one man can do the work of two better than two men can do the work of one. the result of gill's skilful determinations (made with lord lindsay's heliometer) was a solar parallax of · ", corresponding to a distance of , , miles.[ ] the bestowal of the royal astronomical society's gold medal stamped the merit of this distinguished service. but there are other subjects for this kind of inquiry besides mars and venus. professor galle of breslau suggested in [ ] that some of the minor planets might be got to repay astronomers for much disinterested toil spent in unravelling their motions, by lending aid to their efforts towards a correct celestial survey. ten or twelve come near enough, and are bright enough for the purpose; in fact, the absence of sensible magnitude is one of their chief recommendations, since a point of light offers far greater facilities for exact measurement than a disc. the first attempt to work this new vein was made at the opposition of phocæa in ; and from observations of flora in the following year at twelve observatories in the northern and southern hemispheres, galle deduced a solar parallax of · ".[ ] at mauritius in , lord lindsay and sir david gill applied the "diurnal method" to juno, then conveniently situated for the purpose; and the continued use of similar occasions affords an unexceptionable means for improving knowledge of the sun's distance. they frequently recur; they need no elaborate preparation; a single astronomer armed with a heliometer can do all the requisite work. dr. gill, however, organized a more complex plan of operations upon iris in , and upon victoria and sappho in . a novel method was adopted. its object was to secure simultaneous observations made from opposite sides of the globe just when the planet lay in the plane passing through the centre of the earth and the two observers, the same pair of reference-stars being used on each occasion. the displacements caused by parallax were thus in a sense doubled, since the star to which the planet seemed approximated in the northern hemisphere, showed as if slightly removed from it in the southern, and _vice versâ_. as the planet pursued its course, fresh star-couples came into play, during the weeks that the favourable period lasted. in these determinations, only heliometers were employed. dr. elkin, of yale college, co-operated throughout, and the heliometers of dresden, göttingen, bamberg, and leipzig, shared in the work, while dr. auwers of berlin was sir david gill's personal coadjutor at the cape. voluminous data were collected; meridian observations of the stars of reference for victoria occupied twenty-one establishments during four months; the direct work of triangulation kept four heliometers in almost exclusive use for the best part of a year; and the ensuing toilsome computations, carried out during three years at the cape observatory, filled two bulky tomes[ ] with their details. gill's final result, published in , was a parallax of · ", equivalent to a solar distance of , , ; and it was qualified by a probable error so small that the value might well have been accepted as definitive but for an unlooked-for discovery. the minor planet eros, detected august , , was found to pursue a course rendering it an almost ideal intermediary in solar parallax-determinations. once in thirty years, it comes within fifteen million miles of the earth; and although the next of these choice epochs must be awaited for some decades, an opposition too favourable to be neglected occurred in . at an international conference, accordingly, held at paris in july of that year, a plan of photographic operations was concerted between the representatives of no less than observatories.[ ] its primary object was to secure a large stock of negatives showing the planet with the comparison-stars along the route traversed by it from october, , to march, ,[ ] and this at least was successfully attained. their measurement will in due time educe the apparent displacements of the moving object as viewed simultaneously from remote parts of the earth; and the upshot should be a solar parallax adequate in accuracy to the exigent demands of the twentieth century. the second of the nineteenth-century pair of venus-transits was looked forward to with much abated enthusiasm. russia refused her active co-operation in observing it, on the ground that oppositions of the minor planets were trigonometrically more useful, and financially far less costly; and her example was followed by austria; while italian astronomers limited their sphere of action to their own peninsula. nevertheless, it was generally held that a phenomenon which the world could not again witness until it was four generations older should, at the price of any effort, not be allowed to pass in neglect. the persuasion of its importance justified the summoning of an international conference at paris in , from which, however, america, preferring independent action, held aloof. it was decided to give delisle's method another trial; and the ambiguities attending and marring its use were sought to be obviated by careful regulations for insuring agreement in the estimation of the critical moments of ingress and egress.[ ] but in fact (as m. puiseux had shown[ ]), contacts between the limbs of the sun and planet, so far from possessing the geometrical simplicity attributed to them, are really made up of a prolonged succession of various and varying phases, impossible either to predict or identify with anything like rigid exactitude. sir robert ball compared the task of determining the precise instant of their meeting or parting, to that of telling the hour with accuracy on a watch without a minute hand; and the comparison is admittedly inadequate. for not only is the apparent movement of venus across the sun extremely slow, being but the excess of her real motion over that of the earth; but three distinct atmospheres--the solar, terrestrial, and cytherean--combine to deform outlines and mask the geometrical relations which it is desired to connect with a strict count of time. the result was very much what had been expected. the arrangements were excellent, and were only in a few cases disconcerted by bad weather. the british parties, under the experienced guidance of mr. stone, the late radcliffe observer, took up positions scattered over the globe, from queensland to bermuda; the americans collected a whole library of photographs; the germans and belgians trusted to the heliometer; the french used the camera as an adjunct to the method of contacts. yet little or no approach was made to solving the problem. thus, from measures of venus on the sun, taken with a new kind of heliometer at santiago in chili, m. houzeau, of the brussels observatory, derived a solar parallax of . ", and a distance of , , miles.[ ] but the "probable errors" of this determination amounted to . " either way: it was subject to a "more or less" of , , or to a total uncertainty of , , miles. the "probable error" of the english result, published in , was less formidable,[ ] yet the details of the discussion showed that no great confidence could be placed in it. the sun's distance came out , , miles; while , , was given by professor harkness's investigation of , american photographs.[ ] finally, dr. auwers deduced from the german heliometric measures the unsatisfactorily small value of , , miles.[ ] the transit of had not, then, brought about the desired unanimity. the state and progress of knowledge on this important topic were summed up by faye and harkness in .[ ] the methods employed in its investigation fall (as we have seen) into three separate classes--the trigonometrical, the gravitational, and the "phototachymetrical"--an ungainly adjective used to describe the method by the velocity of light. each has its special difficulties and sources of error; each has counter-balancing advantages. the only trustworthy result from celestial surveys, was at that time furnished by gill's observations of mars in . but the method by lunar and planetary disturbances is unlike all the others in having time on its side. it is this which leverrier declared with emphasis must inevitably prevail, because its accuracy is continually growing.[ ] the scarcely perceptible errors which still impede its application are of such a nature as to accumulate year by year; eventually, then, they will challenge, and must receive, a more and more perfect correction. the light-velocity method, however, claimed, and for some years justified, m. faye's preference. by a beautiful series of experiments on foucault's principle, michelson fixed in the rate of luminous transmission at , (corrected later to , ) kilometres a second.[ ] this determination was held by professor todd to be entitled to four times as much confidence as any previous one; and if the solar parallax of · " deduced from it by professor harkness errs somewhat by defect, it is doubtless because glasenapp's "light-equation," with which it was combined, errs slightly by excess. but all earlier efforts of the kind were thrown into the shade by professor newcomb's arduous operations at washington in - .[ ] the scale upon which they were conducted was in itself impressive. foucault's entire apparatus in had been enclosed in a single room; newcomb's revolving and fixed mirrors, between which the rays of light were to run their timed course, were set up on opposite shores of the potomac, at a distance of nearly four kilometres. this advantage was turned to the utmost account by ingenuity and skill in contrivance and execution; and the deduced velocity of , kilometres = , miles a second, had an estimated error ( kilometres) only one-tenth that ascribed by cornu to his own result in . just as these experiments were concluded in , m. magnus nyrén, of st. petersburg, published an elaborate investigation of the small annular displacements of the stars due to the successive transmission of light, involving an increase of struve's "constant of aberration" from · " to · ". and from the new value, combined with newcomb's light-velocity, was derived a valuable approximation to the sun's distance, concluded at , , miles (parallax = · "). yet it is not quite certain that nyrén's correction was an improvement. a differential method of determining the amount of aberration, struck out by m. loewy of paris,[ ] avoids most of the objections to the absolute method previously in vogue; and the upshot of its application in was to show that struve's constant might better be retained than altered, loewy's of · " varying from it only to an insignificant extent. professor hall had, moreover, deduced nearly the same value ( · ") from the washington observations since , of alpha lyræ (vega); whence, in conjunction with newcomb's rate of light transmission, he arrived at a solar parallax of · ".[ ] inverting the process, sir david gill in derived the constant from the parallax. if the earth's orbit have a mean radius, as found by him, of , , miles, then, he calculated, the aberration of light--newcomb's measures of its velocity being supposed exact--amounts to . ". this figure can need very slight correction. professor harkness surveyed in ,[ ] from an eclectic point of view, the general situation as regarded the sun's parallax. convinced that no single method deserved an exclusive preference, he reached a plausible result through the combination, on the principle of least squares--that is, by the mathematical rules of probability--of all the various quantities upon which the great datum depends. it thus summed up and harmonised the whole of the multifarious evidence bearing upon the point, and, as modified in ,[ ] falls very satisfactorily into line with the cape determination. we may, then, at least provisionally, accept , , miles as the length of our measuring-rod for space. nor do we hazard much in fixing , miles as the outside limit of its future correction. footnotes: [footnote : airy, _month. not._, vol. xvii., p. .] [footnote : mars comes into opposition once in about days; but owing to the eccentricity of both orbits, his distance from the earth at those epochs varies from thirty-five to sixty-two million miles.] [footnote : j. d. cassini, _hist. abrégée de la parallaxe du soleil_, p. , .] [footnote : the present period of coupled eccentric transits will, in the course of ages, be succeeded by a period of single, nearly central transits. the alignments by which transits are produced, of the earth, venus, and the sun, close to the place of intersection of the two planetary orbits, now occur, the first a little in front of, the second, after eight years less two and a half days, a little behind the node. but when the first of these two meetings takes place very near the node, giving a nearly central transit, the second falls too far from it, and the planet escapes projection on the sun. the reason of the liability to an eight-yearly recurrence is that eight revolutions of the earth are accomplished in only a very little more time than thirteen revolutions of venus.] [footnote : _die entfernung der sonne: fortsetzung_, p. . encke slightly corrected his results of in _berlin abh._, , p. .] [footnote : owing to the ellipticity of its orbit, the earth is nearer to the sun in january than in june by , , miles. the quantity to be determined, or "mean distance," is that lying midway between these extremes--is, in other words, half the major axis of the ellipse in which the earth travels.] [footnote : _month. not._, vol. xv., p. .] [footnote : _the distance of the sun from the earth determined by the theory of gravity_, edinburgh, .] [footnote : _opera_, t. iii., p. .] [footnote : _comptes rendus_, t. xlvi., p. . the parallax · " derived by leverrier from the above-described inequality in the earth's motion, was corrected by stone to · ". _month. not._, vol. xxviii., p. .] [footnote : _month. not._, vol. xxxv., p. .] [footnote : _wash. obs._, , app. ii., p. .] [footnote : _comptes rendus_, t. xxix., p. .] [footnote : _ibid._, t. xxx., p. .] [footnote : _ibid._, t. lv., p. . the previously admitted velocity was million metres per second; foucault reduced it to million. combined with struve's "constant of aberration" this gave . " for the solar parallax, which exactly agreed with cornu's result from a repetition of fizeau's experiments in . _comptes rendus_, t. lxxvi., p. .] [footnote : _month. not._, vol. xxiv., p. .] [footnote : _astr. papers of the american ephemeris_, vol. ii., p. .] [footnote : _month. not._, vol. xvii., p. .] [footnote : because closely similar to that proposed by him in _phil. trans._ for .] [footnote : _month. not._, vol. xxxviii., p. .] [footnote : _ibid._, p. .] [footnote : _ibid._, p. .] [footnote : _ibid._, p. .] [footnote : _comptes rendus_, t. xcii., p. .] [footnote : _observatory_, vol. v., p. .] [footnote : _transits of venus_, p. ( st ed.).] [footnote : _am. jour. of sc._, vol. xx., p. .] [footnote : _month. not._, vol. xvii., p. .] [footnote : _mem. roy. astr. soc._, vol. xlvi., p. .] [footnote : _astr. nach._, no. , .] [footnote : hilfiker, _bern mittheilungen_, , p. .] [footnote : _annals of the cape observatory_, vols. vi., vii.] [footnote : _rapport sur l'État de l'observatoire de paris pour l'année _, p. .] [footnote : _observatory_, vol. xxiii., p. ; newcomb, _astr. jour._, no. .] [footnote : _comptes rendus_, t. xciii., p. .] [footnote : _ibid._, t. xcii., p. .] [footnote : _bull. de l'acad._, t. vi., p. .] [footnote : _month. not._, vol. xlviii., p. .] [footnote : _astr. jour._, no. .] [footnote : _astr. nach._, no. , .] [footnote : _comptes rendus_, t. xcii., p. ; _am. jour. of sc._, vol. xxii., p. .] [footnote : _month. not._, vol. xxxv., p. .] [footnote : _am. jour. of sc._, vol. xviii., p. .] [footnote : _nature_, vol. xxxiv., p. ; _astron. papers of the american ephemeris_, vol. ii., p. .] [footnote : _comptes rendus_, t. cxii., p. .] [footnote : _astr. journ._, nos. , ] [footnote : _the solar parallax and its related constants_, washington, .] [footnote : _astr. and astrophysics_, vol. xiii., p. .] chapter vii _planets and satellites_ johann hieronymus schröter was the herschel of germany. he did not, it is true, possess the more brilliant gifts of his rival. herschel's piercing discernment, comprehensive intelligence, and inventive splendour were wanting to him. he was, nevertheless, the founder of descriptive astronomy in germany, as herschel was in england. born at erfurt in , he prosecuted legal studies at göttingen, and there imbibed from kästner a life-long devotion to science. from the law, however, he got the means of living, and, what was to the full as precious to him, the means of observing. entering the sphere of hanoverian officialism in , he settled a few years later at lilienthal, near bremen, as "oberamtmann," or chief magistrate. here he built a small observatory, enriched in with a seven-foot reflector by herschel, then one of the most powerful instruments to be found anywhere out of england. it was soon surpassed, through his exertions, by the first-fruits of native industry in that branch. schrader of kiel transferred his workshops to lilienthal in , and constructed there, under the superintendence and at the cost of the astronomical oberamtmann, a thirteen-foot reflector, declared by lalande to be the finest telescope in existence, and one twenty-seven feet in focal length, probably as inferior to its predecessor in real efficiency as it was superior in size. thus, with instruments of gradually increasing power, schröter studied during thirty-four years the topography of the moon and planets. the field was then almost untrodden; he had but few and casual predecessors, and has since had no equal in the sustained and concentrated patience of his hourly watchings. both their prolixity and their enthusiasm are faithfully reflected in his various treatises. yet the one may be pardoned for the sake of the other, especially when it is remembered that he struck out a substantially new line, and that one of the main lines of future advance. moreover, his infectious zeal communicated itself; he set the example of observing when there was scarcely an observer in germany; and under his roof harding and bessel received their training as practical astronomers. but he was reserved to see evil days. early in the french under vandamme occupied bremen. on the night of april , the vale of lilies was, by their wanton destructiveness, laid waste with fire; the government offices were destroyed, and with them the chief part of schröter's property, including the whole stock of his books and writings. there was worse behind. a few days later, his observatory, which had escaped the conflagration, was broken into, pillaged, and ruined. his life was wrecked with it. he survived the catastrophe three years without the means to repair, or the power to forget it, and gradually sank from disappointment into decay, terminated by death, august , . he had, indeed, done all the work he was capable of; and though not of the first quality, it was far from contemptible. he laid the foundation of the _comparative_ study of the moon's surface, and the descriptive particulars of the planets laboriously collected by him constituted a store of more or less reliable information hardly added to during the ensuing half century. they rested, it is true, under some shadow of doubt; but the most recent observations have tended on several points to rehabilitate the discredited authority of the lilienthal astronomer. we may now briefly resume, and pursue in its further progress, the course of his studies, taking the planets in the order of their distances from the sun. in april, , schröter saw reason to conclude, from the gradual degradation of light on its partially illuminated disc, that mercury possesses a tolerably dense atmosphere.[ ] during the transit of may , , he was, moreover, struck with the appearance of a ring of softened luminosity encircling the planet to an apparent height of three seconds, or about a quarter of its own diameter.[ ] although a "mere thought" in texture, it remained persistently visible both with the seven-foot and the thirteen-foot reflectors, armed with powers up to . it had a well-marked grayish boundary, and reminded him, though indefinitely fainter, of the penumbra of a sun-spot. a similar appendage had been noticed by de plantade at montpellier, november , , and again in and by prosperin and flaugergues; but herschel, on november , , saw the preceding limb of the planet projected on the sun cut the luminous solar clouds with the most perfect sharpness.[ ] the presence, however, of a "halo" was unmistakable in , when professor moll, of utrecht, described it as a "nebulous ring of a darker tinge approaching to the violet colour."[ ] again, to huggins and stone, november , , it showed as lucid and most distinct. no change in the colour of the glasses used, or the powers applied, could get rid of it, and it lasted throughout the transit.[ ] it was next seen by christie and dunkin at greenwich, may , ,[ ] and with much precision of detail by trouvelot at cambridge (u.s.).[ ] professor holden, on the other hand, noted at hastings-on-hudson the total absence of all anomalous appearances.[ ] nor could any vestige of them be perceived by barnard at lick on november , .[ ] various effects of irradiation and diffraction were, however, observed by lowell and w. h. pickering at flagstaff;[ ] and davidson was favoured at san francisco with glimpses of the historic aureola,[ ] as well as of a central whitish spot, which often accompanies it. that both are somehow of optical production can scarcely be doubted. nothing can be learned from them regarding the planet's physical condition. airy showed that refraction in a mercurian atmosphere could not possibly originate the noted aureola, which must accordingly be set down as "strictly an ocular nervous phenomenon."[ ] it is the less easy to escape from this conclusion that we find the virtually airless moon capable of exhibiting a like appendage. professor stephen alexander, of the united states survey, with two other observers, perceived, during the eclipse of the sun of july , , the advancing lunar limb to be bordered with a bright band;[ ] and photographic effects of the same kind appear in pictures of transits of venus and partial solar eclipses. the spectroscope affords little information as to the constitution of mercury. its light is of course that of the sun reflected, and its spectrum is consequently a faint echo of the fraunhofer spectrum. dr. h. c. vogel, who first examined it in april, , _suspected_ traces of the action of an atmosphere like ours,[ ] but, it would seem, on slight grounds. it is, however, certainly very poor in blue rays. more definite conclusions were, in ,[ ] derived by zöllner from photometric observations of mercurian phases. a similar study of the waxing and waning moon had afforded him the curious discovery that light-changes dependent upon phase vary with the nature of the reflecting surface, following a totally different law on a smooth homogeneous globe and on a rugged and mountainous one. now the phases of mercury--so far as could be determined from only two sets of observations--correspond with the latter kind of structure. strictly analogous to those of the moon, they seem to indicate an analogous mode of surface-formation. this conclusion was fully borne out by müller's more extended observations at potsdam during the years - .[ ] practical assurance was gained from them that the innermost planet has a rough rind of dusky rock, absorbing all but per cent. of the light poured upon it by the fierce adjacent sun. its "albedo," in other words, is · ,[ ] which is precisely that ascribed to the moon. the absence of any appreciable mercurian atmosphere followed almost necessarily from these results. on march , , schröter, observing with his -foot reflector in a peculiarly clear sky, perceived the southern horn of mercury's crescent to be quite distinctly blunted.[ ] interception of sunlight by a mercurian mountain rather more than eleven english miles high explained the effect to his satisfaction. by carefully timing its recurrence, he concluded rotation on an axis in a period of hours minutes. the first determination of the kind rewarded twenty years of unceasing vigilance. it received ostensible confirmation from the successive appearances of a dusky streak and blotch in may and june, .[ ] these, however, were inferred to be no permanent markings on the body of the planet, but atmospheric formations, the streak at times drifting forwards (it was thought) under the fluctuating influence of mercurian breezes. from a rediscussion of these somewhat doubtful observations bessel inferred that mercury rotates on an axis inclined ° to the plane of its orbit in hours seconds. the rounded appearance of the southern horn seen by schröter was more or less doubtfully caught by noble ( ), burton, and franks ( );[ ] but was obvious to mr. w. f. denning at bristol on the morning of november , .[ ] that the southern polar regions are usually less bright than the northern is well ascertained; but the cause of the deficiency remains dubious. if inequalities of surface are in question, they must be on a considerable scale; and a similar explanation might be given of the deformations of the "terminator"--or dividing-line between darkness and light in the planet's phases--first remarked by schröter, and again clearly seen by trouvelot in and .[ ] the displacement, during four days, of certain brilliant and dusky spaces on the disc indicated to mr. denning in rotation in about twenty-five hours; while the general aspect of the planet reminded him of that of mars.[ ] but the difficulties in the way of its observation are enormously enhanced by its constant close attendance on the sun. in his sustained study of the features of mercury, schröter had no imitator until schiaparelli took up the task at milan in . his observations were made in daylight. it was found that much more could be seen, and higher magnifying powers used, high up in the sky near the sun, than at low altitudes, through the agitated air of morning or evening twilight. a notable discovery ensued.[ ] following the planet hour by hour, instead of making necessarily brief inspections at intervals of about a day, as previous observers had done, it was found that the markings faintly visible remained sensibly fixed, hence, that there was no rotation in a period at all comparable with that of the earth. and after long and patient watching, the conclusion was at last reached that mercury turns on his axis in the same time needed to complete a revolution in his orbit. one of his hemispheres, then, is always averted from the sun, as one of the moon's hemispheres from the earth, while the other never shifts from beneath his torrid rays. the "librations," however, of mercury are on a larger scale than those of the moon, because he travels in a more eccentric path. the temporary inequalities arising between his "even pacing" on an axis and his alternately accelerated and retarded elliptical movement occasion, in fact, an oscillation to and fro of the boundaries of light and darkness on his globe over an arc of ° ', in the course of his year of days. thus the regions of perpetual day and perpetual night are separated by two segments, amounting to one-fourth of the entire surface, where the sun rises and sets once in days. else there is no variation from the intense glare on one side of the globe, and the nocturnal blackness on the other. to schiaparelli's scrutiny, mercury appeared as a "spotty globe," enveloped in a tolerably dense atmosphere. the brownish stripes and streaks, discerned on his rose-tinged disc, and judged to be permanent, were made the basis of a chart. they were not indeed always equally well seen. they disappeared regularly near the limb, and were at times veiled even when centrally situated. some of them had been clearly perceived by de ball at bothkamp in .[ ] mr. lowell followed schiaparelli's example by observing mercury in the full glare of noon. "the best time to study him," he remarked, "is when planetary almanacs state 'mercury invisible.'" a remarkable series of drawings executed, some at flagstaff in , the remainder at mexico in , supplied grounds for the following, among other, conclusions.[ ] mercury rotates synchronously with its revolution--that is, once in days--on an axis sensibly perpendicular to its orbital plane. no certain signs of a mercurian atmosphere are visible. the globe is seamed and furrowed with long narrow markings, explicable as cracks in cooling. it is, and always was, a dead world. from micrometrical measures, moreover, the inferences were drawn that the planet's mass has a probable value about / that of the earth, while its mean density falls considerably short of the terrestrial standard. the theory of mercury's movements has always given trouble. in lalande's,[ ] as in mästlin's time, the planet seemed to exist for no other purpose than to throw discredit on astronomers; and even to leverrier's powerful analysis it long proved recalcitrant. on the th of september, , however, he was able to announce before the academy of sciences[ ] the terms of a compromise between observation and calculation. they involved the addition of a new member to the solar system. the hitherto unrecognised presence of a body about the size of mercury itself revolving at somewhat less than half its mean distance from the sun (or, if farther, then of less mass, and _vice versâ_), would, it was pointed out, produce exactly the effect required, of displacing the perihelion of the former planet " a century more than could otherwise be accounted for. the planes of the two orbits, however, should not lie far apart, as otherwise a nodal disturbance would arise not perceived to exist. it was added that a ring of asteroids similarly placed would answer the purpose equally well, and was more likely to have escaped notice. upon the heels of this forecast followed promptly a seeming verification. dr. lescarbault, a physician residing at orgères, whose slender opportunities had not blunted his hopes of achievement, had, ever since , when he witnessed a transit of mercury, cherished the idea that an unknown planet might be caught thus projected on the solar background. unable to observe continuously until , he, on march , , saw what he had expected--a small perfectly round object slowly traversing the sun's disc. the fruitless expectation of reobserving the phenomenon, however, kept him silent, and it was not until december , after the news of leverrier's prediction had reached him, that he wrote to acquaint him with his supposed discovery.[ ] the imperial astronomer thereupon hurried down to orgères, and by personal inspection of the simple apparatus used, by searching cross-examination and local inquiry, convinced himself of the genuine character and substantial accuracy of the reported observation. he named the new planet "vulcan," and computed elements giving it a period of revolution slightly under twenty days.[ ] but it has never since been seen. m. liais, director of the brazilian coast survey, thought himself justified in asserting that it never had been seen. observing the sun for twelve minutes after the supposed ingress recorded at orgères, he noted those particular regions of its surface as "très uniformes d'intensité."[ ] he subsequently, however, admitted lescarbault's good faith, at first rashly questioned. the planet-seeking doctor was, in truth, only one among many victims of similar illusions. waning interest in the subject was revived by a fresh announcement of a transit witnessed, it was asserted, by weber at peckeloh, april , .[ ] the pseudo-planet, indeed, was detected shortly afterwards on the greenwich photographs, and was found to have been seen by m. ventosa at madrid in its true character of a sun-spot without penumbra; but leverrier had meantime undertaken the investigation of a list of twenty similar dubious appearances, collected by haase, and republished by wolf in .[ ] from these, five were picked out as referring in all likelihood to the same body, the reality of whose existence was now confidently asserted, and of which more or less probable transits were fixed for march , , and october , .[ ] but, widespread watchfulness notwithstanding, no suspicious object came into view at either epoch. the next announcement of the discovery of "vulcan" was on the occasion of the total solar eclipse of july , .[ ] this time it was stated to have been seen at some distance south-west of the obscured sun, as a ruddy star with a minute planetary disc; and its simultaneous detection by two observers--the late professor james c. watson, stationed at rawlins (wyoming territory), and professor lewis swift at denver (colorado)--was at first readily admitted. but their separate observations could, on a closer examination, by no possibility be brought into harmony, and, if valid, certainly referred to two distinct objects, if not to four; each astronomer eventually claiming a pair of planets. nor could any one of the four be identified with lescarbault's and leverrier's vulcan, which, if a substantial body revolving round the sun, must then have been found on the _east_ side of that luminary.[ ] the most feasible explanation of the puzzle seems to be that watson and swift merely saw each the same two stars in cancer: haste and excitement doing the rest.[ ] nevertheless, they strenuously maintained their opposite conviction.[ ] intra-mercurian planets have since been diligently searched for when the opportunity of a total eclipse offered, especially during the long obscuration at caroline island. not only did professor holden "sweep" in the solar vicinity, but palisa and trouvelot agreed to divide the field of exploration, and thus make sure of whatever planetary prey there might be within reach; yet with only negative results. photographic explorations during recent eclipses have been equally fruitless. belief in the presence of any considerable body or bodies within the orbit of mercury is, accordingly, at a low ebb. yet the existence of the anomaly in the mercurian movements indicated by leverrier has been made only surer by further research.[ ] its elucidation constitutes one of the "pending problems" of astronomy. * * * * * from the observation at bologna in - of some very faint spots, domenico cassini concluded a rotation or libration of venus--he was not sure which--in about twenty-three hours.[ ] by bianchini in the period was augmented to twenty-four _days_ eight hours. j. j. cassini, however, in , showed that the data collected by both observers were consistent with rotation in twenty-three hours twenty minutes.[ ] so the matter rested until schröter's time. after watching nine years in vain, he at last, february , , perceived the ordinarily uniform brightness of the planet's disc to be marbled with a filmy streak, which returned periodically to the same position in about twenty-three hours twenty-eight minutes. this approximate estimate was corrected by the application of a more definite criterion. on december , , the southern horn of the crescent venus was seen truncated, an outlying lucid point interrupting the darkness beyond. precisely the same appearance recurred two years later, giving for the planet's rotation a period of h. m.[ ] to this only twenty-two seconds were added by de vico, as the result of over , observations made with the cauchoix refractor of the collegio romano, - .[ ] the axis of rotation was found to be much more bowed towards the orbital plane than that of the earth, the equator making with it an angle of ° '. these conclusions inspired, it is true, much distrust, consequently there were no received ideas on the subject to be subverted. nevertheless, a shock of surprise was felt at schiaparelli's announcement, early in ,[ ] that venus most probably rotates after the fashion just previously ascribed to mercury. a continuous series of observations, from november, , to february, , with their records in above a hundred drawings, supplied the chief part of the data upon which he rested his conclusions. they certainly appeared exceptionally well-grounded; and the doubts at first qualifying them were removed by a fresh set of determinations in july, .[ ] most observers had depended, in their attempts to ascertain the rotation-period of venus, upon evanescent shadings, most likely of atmospheric origin, and scarcely recognisable from day to day. schiaparelli fixed his attention upon round, defined, lustrously white spots, the presence of which near the cusps of the illuminated crescent has been attested for close upon two centuries. his steady watch over them showed the invariability of their position with regard to the terminator; and this is as much as to say that the regions of day and night do not shift on the surface of the planet. in other words, she keeps the same face always turned towards the sun. moreover, since her orbit is nearly circular, libratory effects are very small. they amount in fact to only just one-thirtieth of those serving to modify the severe contrasts of climate in mercury. confirmatory evidence of schiaparelli's result for venus is not wanting. thus, observations irreconcilable with a swift rate of rotation were made at bothkamp in by vogel and lohse;[ ] and a drawing executed by professor holden with the great washington reflector, december , , showed the same markings in the positions recorded at milan to have been occupied by them eight hours previously. further, a series of observations, carried out by m. perrotin at nice, may to october , , and from mount mounier in - , with the special aim of testing the inference of synchronous rotation and revolution, proved strongly corroborative of it.[ ] a remarkable collection of drawings made by mr. lowell in appeared decisive in its favour;[ ] tacchini at rome,[ ] mascari at catania and etna,[ ] cerulli at terano,[ ] obtained in - evidence similar in purport. on the other hand, niesten of brussels found reason to revert to vico's discarded elements for the planet's rotation;[ ] and trouvelot,[ ] stanley williams,[ ] villiger,[ ] and leo brenner,[ ] so far agreed with him as to adopt a period of approximately twenty-four hours. finally, e. von oppolzer suggested an appeal to the spectroscope;[ ] and bélopolsky secured in [ ] spectrograms apparently marked by the minute displacements corresponding to a rapid rate of axial movement. but they were avowedly taken only as an experiment, with unsuitable apparatus; and the desirable verification of their supposed import is not yet forthcoming. until it is, schiaparelli's period of days must be allowed to hold the field. effects attributed to great differences of level in the surface of venus have struck many observers. francesco fontana at naples in noticed irregularities along the inner edge of the crescent.[ ] lahire in considered them--regard being had to difference of distance--to be much more strongly marked than those visible in the moon.[ ] schröter's assertions to the same effect, though scouted with some unnecessary vehemence by herschel,[ ] have since been repeatedly confirmed; amongst others by mädler, de vico, langdon, who in saw the broken line of the terminator with peculiar distinctness through a veil of auroral cloud;[ ] by denning,[ ] march , , despite preliminary impressions to the contrary, as well as by c. v. zenger at prague, january , . the great mountain mass, presumed to occasion the periodical blunting of the southern horn, was precariously estimated by the lilienthal observer to rise to the prodigious height of nearly twenty-seven miles, or just five times the elevation of mount everest! yet the phenomenon persists, whatever may be thought of the explanation. moreover, the speck of light beyond, interpreted as the visible sign of a detached peak rising high enough above the encircling shadow to catch the first and last rays of the sun, was frequently discerned by baron van ertborn in ;[ ] while an object near the northern horn of the crescent, strongly resembling a lunar ring-mountain, was delineated both by de vico in and by denning forty years later. we are almost equally sure that venus, as that the earth is encompassed with an atmosphere. yet, notwithstanding luminous appearances plainly due to refraction during the transits both of and , schröter, in , took the initiative in coming to a definite conclusion on the subject.[ ] it was founded, first, on the rapid diminution of brilliancy towards the terminator, attributed to atmospheric absorption; next, on the extension beyond a semicircle of the horns of the crescent; lastly, on the presence of a bluish gleam illuminating the early hours of the cytherean night with what was taken to be genuine twilight. even herschel admitted that sunlight, by the same effect through which the heavenly bodies show _visibly above_ our horizons while still _geometrically below_ them, appeared to be bent round the shoulder of the globe of venus. ample confirmation of the fact has since been afforded. at dorpat in may, , the planet being within ° ' of inferior conjunction, mädler found the arms of waning light upon the disc to embrace no less than ° of its extent;[ ] and in december, , mr. guthrie, of bervie, n.b., actually observed, under similar conditions, the whole circumference to be lit up with a faint nebulous glow.[ ] the same curious phenomenon was intermittently seen by mr. leeson prince at uckfield in september, ;[ ] but with more satisfactory distinctness by mr. c. s. lyman of yale college,[ ] before and after the conjunction of december , , and during nearly five hours previous to the transit of , when the yellowish ring of refracted light showed at one point an approach to interruption, possibly through the intervention of a bank of clouds. again, on december , , venus being ° ' from the sun's centre, mr. h. n. russell, of the halsted observatory, descried the coalescence of the cusps, and founded on the observation a valuable discussion of such effects.[ ] taking account of certain features in the case left unnoticed by neison[ ] and proctor,[ ] he inferred from them the presence of a cytherean atmosphere considerably less refractive than our own, although possibly, in its lower strata, encumbered with dust or haze. similar appearances are conspicuous during transits. but while the mercurian halo is characteristically seen on the sun, the "silver thread" round the limb of venus commonly shows on the part _off_ the sun. there are, however, instances of each description in both cases. mr. grant, in collecting the records of physical phenomena accompanying the transits of and , remarks that no one person saw both kinds of annulus, and argues a dissimilarity in their respective modes of production.[ ] such a dissimilarity probably exists, in the sense that the inner section of the ring is illusory, the outer, a genuine result of the bending of light in a gaseous envelope; but the distinction of separate visibility has not been borne out by recent experience. several of the australian observers during the transit of witnessed the complete phenomenon. mr. j. macdonnell, at eden, saw a "shadowy nebulous ring" surround the whole disc when ingress was two-thirds accomplished; mr. tornaghi, at goulburn, perceived a halo, entire and unmistakable, at half egress.[ ] similar observations were made at sydney,[ ] and were renewed in by lescarbault at orgères, by metzger in java, and by barnard at vanderbilt university.[ ] spectroscopic indications of aqueous vapour as present in the atmosphere of venus, were obtained in and , by tacchini and riccò in italy, and by young in new jersey.[ ] janssen, however, who made a special study of the point subsequently to the transit of , found them much less certain than he had anticipated;[ ] and vogel, by repeated examinations, - , could detect only the very slightest variations from the pattern of the solar spectrum. some additions there indeed seem to be in the thickening of a few water and oxygen-lines; but so nearly evanescent as to induce the persuasion that most of the light we receive from venus has traversed only the tenuous upper portion of its atmosphere.[ ] it is reflected, at any rate, with comparatively slight diminution. on the th and th of september, , a close conjunction gave mr. james nasmyth the rare opportunity of watching venus and mercury for several hours side by side in the field of his reflector; when the former appeared to him like clean silver, the latter as dull as lead or zinc.[ ] yet the light _incident_ upon mercury is, on an average, three and a half times as strong as the light reaching venus. thus, the reflective power of venus must be singularly strong. and we find, accordingly, from a combination of zöllner's with müller's results, that its albedo is but little inferior to that of new-fallen snow; in other words, it gives back per cent. of the luminous rays impinging upon it. this extraordinary brilliancy would be intelligible were it permissible to suppose that we see nothing of the planet but a dense canopy of clouds. but the hypothesis is discountenanced by the flagstaff observations, and is irreconcilable with the visibility of mountainous elevations, and permanent surface-markings. to mr. lowell these were so distinct and unchanging as to furnish data for a chart of the cytherean globe, and the peculiar arrangement of divergent shading exhibited in it cannot off-hand be set down as unreal, in view of perrotin's earlier discernment of analogous linear traces. gruithuisen's "snow-caps,"[ ] however--it is safe to say--do not exist as such; although shining regions near the poles form a well-attested trait of the strange cytherean landscape. the "secondary," or "ashen light," of venus was first noticed by riccioli in ; it was seen by derham about , by kirch in , by schröter and harding in ;[ ] and the reality of the appearance has since been authenticated by numerous and trustworthy observations. it is precisely similar to that of the "old moon in the new moon's arms"; and zenger, who witnessed it with unusual distinctness, january , ,[ ] supposes it due to the same cause--namely, to the faint gleam of reflected earth-light from the night-side of the planet. when we remember, however, that "full earth-light" on venus, at its nearest, has little more than / its intensity on the moon, we see at once that the explanation is inadequate. nor can professor safarik's,[ ] by phosphorescence of the warm and teeming oceans with which zöllner[ ] regarded the globe of venus as mainly covered, be seriously entertained. vogel's suggestion is more plausible. he and o. lohse, at bothkamp, november to , , saw the dark hemisphere _partially_ illuminated by secondary light, extending ° from the terminator, and thought the effect might be produced by a very extensive twilight.[ ] others have had recourse to the analogy of our auroræ, and j. lamp suggested that the grayish gleam, visible to him at bothkamp, october and , ,[ ] might be an accompaniment of electrical processes connected with the planet's meteorology. whatever the origin of the phenomenon, it may serve, on a night-enwrapt hemisphere, to dissipate some of the thick darkness otherwise encroached upon only by "the pale light of stars." venus was once supposed to possess a satellite. but belief in its existence has died out. no one, indeed, has caught even a deceptive glimpse of such an object during the last years. yet it was repeatedly and, one might have thought, well observed in the seventeenth and eighteenth centuries. fontana "discovered" it in ; cassini--an adept in the art of seeing--recognised it in , and again in ; short watched it for a full hour in with varied instrumental means; tobias mayer in , montaigne in ; several astronomers at copenhagen in march, , noted what they considered its unmistakable presence; as did horrebow in . but m. paul stroobant,[ ] who in submitted all the available data on the subject to a searching examination, identified horrebow's satellite with theta libræ, a fifth-magnitude star; and a few other apparitions were, by his industry, similarly explained away. nevertheless, several withstood all efforts to account for them, and together form a most curious case of illusion. for it is quite certain that venus has no such conspicuous attendant. * * * * * the third planet encountered in travelling outward from the sun is the abode of man. he has in consequence opportunities for studying its physical habitudes altogether different from the baffling glimpse afforded to him of the other members of the solar family. regarding the earth, then, a mass of knowledge so varied and comprehensive has been accumulated as to form a science--or rather several sciences--apart. but underneath all lie astronomical relations, the recognition and investigation of which constitute one of the most significant intellectual events of the present century. it is indeed far from easy to draw a line of logical distinction between items of knowledge which have their proper place here, and those which should be left to the historian of geology. there are some, however, of which the cosmical connections are so close that it is impossible to overlook them. among these is the ascertainment of the solidity of the globe. at first sight it seems difficult to conceive what the apparent positions of the stars can have to do with subterranean conditions; yet it was from star measurements alone that hopkins, in , concluded the earth to be solid to a depth of at least or , miles.[ ] his argument was, that if it were a mere shell filled with liquid, precession and nutation would be much larger than they are observed to be. for the shell alone would follow the pull of the sun and moon on its equatorial girdle, leaving the liquid behind; and being thus so much the lighter, would move the more readily. there is, it is true, grave reason to doubt whether this reasoning corresponds with the actual facts of the case;[ ] but the conclusion to which it led has been otherwise affirmed and extended. indications of an identical purport have been derived from another kind of external disturbance, affecting our globe through the same agencies. lord kelvin (then sir william thomson) pointed out in [ ] that tidal influences are brought to bear on land as well as on water, although obedience to them is perceptible only in the mobile element. some bodily distortion of the earth's figure _must_, however, take place, unless we suppose it of absolute or "preternatural" rigidity, and the amount of such distortion can be determined from its effect in diminishing oceanic tides below their calculated value. for if the earth were perfectly plastic to the stresses of solar and lunar gravity, tides--in the ordinary sense--would not exist. continents and oceans would swell and subside together. it is to the _difference_ in the behaviour of solid and liquid terrestrial constituents that the ebb and flow of the waters are due. six years later, the distinguished glasgow professor suggested that this criterion might, by the aid of a prolonged series of exact tidal observations, be practically applied to test the interior condition of our planet.[ ] in , accordingly, suitable data extending over thirty-three years having at length become available, mr. g. h. darwin performed the laborious task of their analysis, with the general result that the "effective rigidity" of the earth's mass must be _at least_ as great as that of steel.[ ] ratification from an unexpected quarter has lately been brought to this conclusion. the question of a possible mobility in the earth's axis of rotation has often been mooted. now at last it has received an affirmative reply. dr. küstner detected, in his observations of - , effects apparently springing from a minute variation in the latitude of berlin. the matter having been brought before the international geodetic association in , special observations were set on foot at berlin, potsdam, prague, and strasbourg, the upshot of which was to bring plainly to view synchronous, and seemingly periodic fluctuations of latitude to the extent of half a second of arc. the reality of these was verified by an expedition to honolulu in - , the variations there corresponding inversely to those simultaneously determined in europe.[ ] their character was completely defined by mr. s. c. chandler's discussion in october, .[ ] he showed that they could be explained by supposing the pole of the earth to describe a circle with a radius of thirty feet in a period of fourteen months. confirmation of this hypothesis was found by dr. b. a. gould in the cordoba observations,[ ] and it was provided with a physical basis through the able co-operation of professor newcomb.[ ] the earth, owing to its ellipsoidal shape, should, apart from disturbance, rotate upon its "axis of figure," or shortest diameter; since thus alone can the centrifugal forces generated by its spinning balance each other. temporary causes, however, such as heavy falls of snow or rain limited to one continental area, the shifting of ice-masses, even the movements of winds, may render the globe slightly lop-sided, and thus oblige it to forsake its normal axis, and rotate on one somewhat divergent from it. this "instantaneous axis" (for it is incessantly changing) must, by mathematical theory, revolve round the axis of figure in a period of days. provided, that is to say, the earth were a perfectly rigid body. but it is far from being so; it yields sensibly to every strain put upon it; and this yielding tends to protract the time of circulation of the displaced pole. the length of its period, then, serves as a kind of measure of the plasticity of the globe; which, according to newcomb's and s. s. hough's independent calculations,[ ] seems to be a little less than that of steel. in an earth compacted of steel, the instantaneous axis would revolve in days; in the actual earth, the process is accomplished in days. by this new path, accordingly, astronomers have been led to an identical estimate of the consistence of our globe with that derived from tidal investigations. variations of latitude are intrinsically complex. to produce them, an incalculable interplay of causes must be at work, each with its proper period and law of action.[ ] all the elements of the phenomenon are then in a perpetual state of flux,[ ] and absorb for their continual redetermination, the arduous and combined labours of many astronomers. nor is this trouble superfluous. minute in extent though they be, the shiftings of the pole menace the very foundations of exact celestial science; their neglect would leave the entire fabric insecure. just at the beginning of the present century they reached a predicted minimum, but are expected again to augment their range after the year . the interesting suggestion has been made by mr. j. halm that such fluctuations are, in some obscure way, affected by changes in solar activity, and conform like them to an eleven-year cycle.[ ] in a paper read before the geological society, december , ,[ ] sir john herschel threw out the idea that the perplexing changes of climate revealed by the geological record might be explained through certain slow fluctuations in the eccentricity of the earth's orbit, produced by the disturbing action of the other planets. shortly afterwards, however, he abandoned the position as untenable;[ ] and it was left to the late dr. james croll, in [ ] and subsequent years, to reoccupy and fortify it. within restricted limits (as lagrange and, more certainly and definitely, leverrier proved), the path pursued by our planet round the sun alternately contracts, in the course of ages, into a moderate ellipse, and expands almost to a circle, the major axis, and consequently the mean distance, remaining invariable. even at present, when the eccentricity approaches a minimum, the sun is nearer to us in january than in july by above three million miles, and some , years ago this difference was more than four times as great. dr. croll brought together[ ] a mass of evidence to support the view, that, at epochs of considerable eccentricity, the hemisphere of which the winter, occurring at aphelion, was both intensified and prolonged, must have undergone extensive glaciation; while the opposite hemisphere, with a short, mild winter, and long, cool summer, enjoyed an approach to perennial spring. these conditions were exactly reversed at the end of , years, through the shifting of the perihelion combined with the precession of the equinoxes, the frozen hemisphere blooming into a luxuriant garden as its seasons came round to occur at the opposite sites of the terrestrial orbit, and the vernal hemisphere subsiding simultaneously into ice-bound rigour.[ ] thus a plausible explanation was offered of the anomalous alternations of glacial and semi-tropical periods, attested, on incontrovertible geological evidence, as having succeeded each other in times past over what are now temperate regions. they succeeded each other, it is true, with much less frequency and regularity than the theory demanded; but the discrepancy was overlooked or smoothed away. the most recent glacial epoch was placed by dr. croll about , years ago, when the eccentricity of the earth's orbit was · times as great as it is now. at present a faint representation of such a state of things is afforded by the southern hemisphere. one condition of glaciation in the coincidence of winter with the maximum of remoteness from the sun, is present; the other--a high eccentricity--is deficient. yet the ring of ice-bound territory hemming in the southern pole is well known to be far more extensive than the corresponding region in the north. the verification of this ingenious hypothesis depends upon a variety of intricate meteorological conditions, some of which have been adversely interpreted by competent authorities.[ ] what is still more serious, its acceptance seems precluded by time-relations of a simple kind. dr. wright[ ] has established with some approach to certainty that glacial conditions ceased in canada and the united states about ten or twelve thousand years ago. the erosive action of the falls of niagara qualifies them to serve as a clepsydra, or water-clock on a grand scale; and their chronological indications have been amply corroborated elsewhere and otherwise on the same continent. the astronomical ice age, however, should have been enormously more antique. no reconciliation of the facts with the theory appears possible. the first attempt at an experimental estimate of the "mean density" of the earth was maskelyne's observation in of the deflection of a plumb-line through the attraction of schehallien. the conclusion thence derived, that our globe weighs - / times as much as an equal bulk of water,[ ] was not very exact. it was considerably improved upon by cavendish, who, in , brought into use the "torsion-balance" constructed for the same purpose by john michell. the resulting estimate of · was raised to · by francis baily's elaborate repetition of the process in - . from experiments on the subject made in - by cornu and baille the slightly inferior value of · was derived; and it was further shown that the data collected by baily, when corrected for a systematic error, gave practically the same result ( · ).[ ] m. wilsing's of · , obtained at potsdam in ,[ ] nearly agreed with it; while professor poynting, by means of a common balance, arrived at a terrestrial mean density of · .[ ] professor boys next entered the field with an exquisite apparatus, in which a quartz fibre performed the functions of a torsion-rod; and the figure · determined by him, and exactly confirmed by dr. braun's research at mariaschein, bohemia, in ,[ ] may be called the standard value of the required datum. newton's guess at the average weight of the earth as five or six times that of water has thus been curiously verified. operations for determining the figure of the earth were carried out during the last century on an unprecedented scale. the russo-scandinavian arc, of which the measurement was completed under the direction of the elder struve in , reached from hammerfest to ismailia on the danube, a length of ° '. but little inferior to it was the indian arc, begun by lambton in the first years of the century, continued by everest, revised and extended by walker. both were surpassed in compass by the anglo-french arc, which embraced °; and considerable segments of meridians near the atlantic and pacific shores of north america were measured under the auspices of the united states coast survey. but these operations shrink into insignificance by comparison with sir david gill's grandiose scheme for uniting two hemispheres by a continuous network of triangulation. the history of geodesy in south africa began with lacaille's measurements in . they were repeated and enlarged in scope by sir thomas maclear in - ; and his determinations prepared the way for a complete survey of cape colony and natal, executed during the ten years - by colonel morris, r.e., under the direction of sir david gill.[ ] bechuanaland and rhodesia were subsequently included in the work; and the royal astronomer obtained, in , the support of the international geodetic association for its extension to the mouth of the nile. nor was this the limit of his design. by carrying the survey along the levantine coast, connection can be established with struve's system, and the magnificent amplitude of ° will be given to the conjoined african and european arcs. meantime, the french have undertaken the remeasurement of bouguer's peruvian arc, and a corresponding russo-swedish[ ] enterprise is progressing in spitzbergen; so that abundant materials will ere long be provided for fresh investigations of the shape and size of our planet. the smallness of the outstanding uncertainty can be judged of by comparing j. b. listing's[ ] with general clarke's[ ] results, published in the same year ( ). listing stated the dimensions of the terrestrial spheroid as follows: equatorial radius = , miles; polar radius = , miles; ellipticity = / · . clarke's corresponding figures were: , and , miles, giving an ellipticity of / · . the value of the latter fraction at present generally adopted is / ; that is to say, the thickness of the protuberant equatorial ring is held to be / of the equatorial radius. from astronomical considerations, it is true, newcomb estimated the ratio at / ;[ ] but for obtaining this particular datum, geodetical methods are unquestionably to be preferred. * * * * * the moon possesses for us a unique interest. she in all probability shared the origin of the earth; she perhaps prefigures its decay. she is at present its minister and companion. her existence, so far as we can see, serves no other purpose than to illuminate the darkness of terrestrial nights, and to measure, by swiftly-recurring and conspicuous changes of aspect, the long span of terrestrial time. inquiries stimulated by visible dependence, and aided by relatively close vicinity, have resulted in a wonderfully minute acquaintance with the features of the single lunar hemisphere open to our inspection. selenography, in the modern sense, is little more than a hundred years old. it originated with the publication in of schröter's _selenotopographische fragmente_.[ ] not but that the lunar surface had already been diligently studied, chiefly by hevelius, cassini, riccioli, and tobias mayer; the idea, however, of investigating the moon's physical condition, and detecting symptoms of the activity there of natural forces through minute topographical inquiry, first obtained effect at lilienthal. schröter's delineations, accordingly, imperfect though they were, afforded a starting-point for a _comparative_ study of the superficial features of our satellite. the first of the curious objects which he named "rills" was noted by him in . before he had found eleven; lohrmann added ; mädler ; schmidt published in a catalogue of , of which had been detected by himself;[ ] and he eventually brought the number up to nearly , . they are, then, a very persistent lunar feature, though wholly without terrestrial analogue. there is no difference of opinion as to their nature. they are quite obviously clefts in a rocky surface, to yards deep, usually a couple of miles across, and pursuing straight, curved, or branching tracks up to miles in length. as regards their origin, the most probable view is that they are fissures produced in cooling; but neison inclines to consider them rather as dried watercourses.[ ] on february , , schröter perceived what he took to be distinct traces of a lunar twilight, and continued to observe them during nine consecutive years.[ ] they indicated, he thought, the presence of a shallow atmosphere, about times more tenuous than our own. bessel, on the other hand, considered that the only way of "saving" a lunar atmosphere was to deny it any refractive power, the sharpness and suddenness of star-occultations negativing the possibility of gaseous surroundings of greater density (admitting an extreme supposition) than / that of terrestrial air.[ ] newcomb places the maximum at / . sir john herschel concluded "the non-existence of any atmosphere at the moon's edge having / part of the density of the earth's atmosphere."[ ] this decision was fully borne out by sir william huggins's spectroscopic observation of the disappearance behind the moon's limb of the small star eta piscium, january , .[ ] not the slightest sign of selective absorption or unequal refraction was discernible. the entire spectrum went out at once, as if a slide had suddenly dropped over it. the spectroscope has uniformly told the same tale; for m. thollon's observation during the total solar eclipse at sohag of a supposed thickening at the moon's rim, of certain dark lines in the solar spectrum, is now acknowledged to have been illusory. moonlight, analysed with the prism, is found to be pure reflected sunlight, diminished in _quantity_, owing to the low reflective capability of the lunar surface, to less than one-fifth its incident intensity, but wholly unmodified in _quality_. nevertheless, the diameter of the moon appeared from the greenwich observations discussed by airy in [ ] to be " smaller than when directly measured; and the effect would be explicable by refraction in a lunar atmosphere , times thinner than our own at the sea-level. but the difference was probably illusory. it resulted in part, if not wholly, from the visual enlargement by irradiation of the bright disc of the moon. professor comstock, employing the -inch clark equatoreal of the washburn observatory, found in the refractive displacements of occulted stars so trifling as to preclude the existence of a permanent lunar atmosphere of much more than / the density of the terrestrial envelope.[ ] the possibility, however, was admitted that, on the illuminated side of the moon, temporary exhalations of aqueous vapour might arise from ice-strata evaporated by sun-heat. meantime, some renewed evidence of actual crepuscular gleams on the moon had been gathered by mm. paul and prosper henry of the paris observatory, as well as by mr. w. h. pickering, in the pure air of arequipa, at an altitude of , feet above the sea.[ ] an occultation of jupiter, too, observed by him august , ,[ ] was attended with a slight flattening of the planet's disc through the effect, it was supposed, of lunar refraction--but of refraction in an atmosphere possessing, at the most, / the density at the sea-level of terrestrial air, and capable of holding in equilibrium no more than / of an inch of mercury. yet this small barometric value corresponds, mr. pickering remarks, "to a pressure of hundreds of tons per square mile of the lunar surface." the compression downward of gaseous strata on the moon should, in any case, proceed very gradually, owing to the slight power of lunar gravity,[ ] and they might hence play an important part in the economy of our satellite while evading spectroscopic and other tests. thus--as mr. ranyard remarked[ ]--the cliffs and pinnacles of the moon bear witness, by their unworn condition, to the efficiency of atmospheric protection against meteoric bombardment; and mr. pickering shows that it could be afforded by such a tenuous envelope as that postulated by him. the first to emulate schröter's selenographical zeal was wilhelm gotthelf lohrmann, a land-surveyor of dresden, who, in , published four out of twenty-five sections of the first scientifically executed lunar chart, on a scale of - / inches to a lunar diameter. his sight, however, began to fail three years later, and he died in , leaving materials from which the work was completed and published in by dr. julius schmidt, late director of the athens observatory. much had been done in the interim. beer and mädler began at berlin in their great trigonometrical survey of the lunar surface, as yet neither revised nor superseded. a map, issued in four parts, - , on nearly the same scale as lohrmann's, but more detailed and authoritative, embodied the results. it was succeeded, in , by a descriptive volume bearing the imposing title, _der mond; oder allgemeine vergleichende selenographie_. this summation of knowledge in that branch, though in truth leaving many questions open, had an air of finality which tended to discourage further inquiry.[ ] it gave form to a reaction against the sanguine views entertained by hevelius, schröter, herschel and gruithuisen as to the possibilities of agreeable residence on the moon, and relegated the "selenites," one of whose cities schröter thought he had discovered, and of whose festal processions gruithuisen had not despaired of becoming a spectator, to the shadowy land of the ivory gate. all examples of change in lunar formations were, moreover, dismissed as illusory. the light contained in the work was, in short, a "dry light," not stimulating to the imagination. "a mixture of a lie," bacon shrewdly remarks, "doth ever add pleasure." for many years, accordingly, schmidt had the field of selenography almost to himself. reviving interest in the subject was at once excited and displayed by the appointment, in , of a lunar committee of the british association. the indirect were of greater value than the direct fruits of its labours. an english school of selenography rose into importance. popularity was gained for the subject by the diffusion of works conspicuous for ingenuity and research. nasmyth's and carpenter's beautifully illustrated volume ( ) was succeeded, after two years, by a still more weighty contribution to lunar science in mr. neison's well-known book, accompanied by a map, based on the survey of beer and mädler, but adding some measures of positions, besides the representation of several thousand new objects. with schmidt's _charte der gebirge der mondes_, germany once more took the lead. this splendid delineation, built upon lohrmann's foundation, embraced the detail contained in upwards of , original drawings, representing the labour of thirty-four years. no less than , craters are represented in it, on a scale of seventy-five inches to a diameter. an additional help to lunar inquiries was provided at the same time in this country by the establishment, through the initiative of the late mr. w. r. birt, of the selenographical society. but the strongest incentive to diligence in studying the rugged features of our celestial helpmate has been the idea of probable or actual variation in them. a change always seems to the inquisitive intellect of man like a breach in the defences of nature's secrets, through which it may hope to make its way to the citadel. what is desirable easily becomes credible; and thus statements and rumours of lunar convulsions have successively, during the last hundred years, obtained credence, and successively, on closer investigation, been rejected. the subject is one as to which illusion is peculiarly easy. our view of the moon's surface is a bird's-eye view. its conformation reveals itself indirectly through irregularities in the distribution of light and darkness. the forms of its elevations and depressions can be inferred only from the shapes of the black, unmitigated shadows cast by them. but these shapes are in a state of perpetual and bewildering fluctuation, partly through changes in the angle of illumination, partly through changes in our point of view, caused by what are called the moon's "librations."[ ] the result is, that no single observation can be _exactly_ repeated by the same observer, since identical conditions recur only after the lapse of a great number of years. local peculiarities of surface, besides, are liable to produce perplexing effects. the reflection of earth-light at a particular angle from certain bright summits completely, though temporarily, deceived herschel into the belief that he had witnessed, in and , volcanic outbursts on the dark side of the moon. the persistent recurrence, indeed, of similar appearances under circumstances less amenable to explanation inclined webb to the view that effusions of native light actually occur.[ ] more cogent proofs must, however, be adduced before a fact so intrinsically improbable can be admitted as true. but from the publication of beer and mädler's work until , the received opinion was that no genuine sign of activity had ever been seen, or was likely to be seen, on our satellite; that her face was a stereotyped page, a fixed and irrevisable record of the past. a profound sensation, accordingly, was produced by schmidt's announcement, in october, , that the crater "linné," in the mare serenitatis, had disappeared,[ ] effaced, as it was supposed, by an igneous outflow. the case seemed undeniable, and is still dubious. linné had been known to lohrmann and mädler, - , as a deep crater, five or six miles in diameter, the third largest in the dusky plain known as the "mare serenitatis"; and schmidt had observed and drawn it, - , under a practically identical aspect. now it appears under high light as a whitish spot, in the centre of which, as the rays begin to fall obliquely, a pit, scarcely two miles across, emerges into view.[ ] the crateral character of this comparatively minute depression was detected by father secchi, february , . this is not all. schröter's description of linné, as seen by him november , , tallies quite closely with modern observation;[ ] while its inconspicuousness in is shown by its omission from russell's lunar globe and maps.[ ] we are thus driven to adopt one of two suppositions: either lohrmann, mädler, and schmidt were entirely mistaken in the size and importance of linné, or a real change in its outward semblance supervened during the first half of the century, and has since passed away, perhaps again to recur. the latter hypothesis seems the more probable: and its probability is strengthened by much evidence of actual obscuration or variation of tint in other parts of the lunar surface, more especially on the floor of the great "walled plain" named "plato."[ ] from a re-examination with a -inch refractor at arequipa in - , of this region, and of the mare serenitatis, mr. w. h. pickering inclines to the belief that lunar volcanic action, once apparently so potent, is not yet wholly extinct.[ ] an instance of an opposite kind of change was alleged by dr. hermann j. klein of cologne in march, .[ ] in linné the obliteration of an old crater had been assumed; in "hyginus n.," the formation of a new crater was asserted. yet, quite possibly, the same cause may have produced the effects thought to be apparent in both. it is, however, far from certain that any real change has affected the neighbourhood of hyginus. the novelty of klein's observation of may , , may have consisted simply in the detection of a hitherto unrecognised feature. the region is one of complex formation, consequently of more than ordinary liability to deceptive variations in aspect under rapid and entangled fluctuations of light and shade.[ ] moreover, it seems to be certain, from messrs. pratt and capron's attentive study, that "hyginus n." is no true crater, but a shallow, saucer-like depression, difficult of clear discernment.[ ] under suitable illumination, nevertheless, it contains, and is marked by, an ample shadow.[ ] in both these controverted instances of change, lunar photography was invoked as a witness; but, notwithstanding the great advances made in the art by de la rue in this country, by draper, and, above all, by rutherford in america, without decisive results. investigations of the kind began to assume a new aspect in , when professor holden organised them at the lick observatory.[ ] autographic moon-pictures were no longer taken casually, but on system; and dr. weinek's elaborate study, and skilful reproductions of them at prague,[ ] gave them universal value. they were designed to provide materials for an atlas on the scale of beer and mädler's, of which some beautiful specimen-plates have been issued. at paris, in , with the aid of a large "equatoreal coudé," a work of similar character was set on foot by mm. loewy and puiseux. its progress has been marked by the successive publication of five instalments of a splendid atlas, on a scale of about eight feet to the lunar diameter, accompanied by theoretical dissertations, designed to establish a science of "selenology." the moon's formations are thus not only delineated under every variety of light-incidence, but their meaning is sought to be elicited, and their history and mutual relations interpreted.[ ] henceforth, at any rate, the lunar volcanoes can scarcely, without notice taken, breathe hard in their age-long sleep. melloni was the first to get undeniable heating effects from moonlight. his experiments, made on mount vesuvius early in ,[ ] were repeated with like result by zantedeschi at venice four years later. a rough measure of the intensity of those effects was arrived at by piazzi smyth at guajara, on the peak of teneriffe, in . at a distance of fifteen feet from the thermomultiplier, a price's candle was found to radiate just twice as much heat as the full moon.[ ] then, after thirteen years, in - , an exact and extensive series of observations on the subject were made by the present earl of rosse. the lunar radiations, from the first to the last quarter, displayed, when concentrated with the parsonstown three-foot mirror, appreciable thermal energy, increasing with the phase, and largely due to "dark heat," distinguished from the quicker-vibrating sort by inability to traverse a plate of glass. this was supposed to indicate an actual heating of the surface, during the long lunar day of hours, to about ° f.[ ] (corrected later to °),[ ] the moon thus acting as a direct radiator no less than as a reflector of heat. but the conclusion was very imperfectly borne out by dr. boeddicker's observations with the same instrument and apparatus during the total lunar eclipse of october , .[ ] this initial opportunity of measuring the heat phases of an eclipsed moon was used with the remarkable result of showing that the heat disappeared almost completely, though not quite simultaneously, with the light. confirmatory evidence of the extraordinary promptitude with which our satellite parts with heat already to some extent appropriated, was afforded by professor langley's bolometric observations at allegheny of the partial eclipse of september , .[ ] yet it is certain that the moon sends us a perceptible quantity of heat _on its own account_, besides simply throwing back solar radiations. for in february, , professor langley succeeded, after many fruitless attempts, in getting measures of a "lunar heat-spectrum." the incredible delicacy of the operation may be judged of from the statement that the sum-total of the thermal energy dispersed by his rock-salt prisms was insufficient to raise a thermometer fully exposed to it one-thousandth of a degree centigrade! the singular fact was, however, elicited that this almost evanescent spectrum is made up of two superposed spectra, one due to reflection, the other, with a maximum far down in the infra-red, to radiation.[ ] the corresponding temperature of the moon's sunlit surface professor langley considers to be about that of freezing water.[ ] repeated experiments having failed to get any thermal effects from the dark part of the moon, it was inferred that our satellite "has no internal heat sensible at the surface"; so that the radiations from the lunar soil giving the low maximum in the heat-spectrum, "must be due purely to solar heat which has been absorbed and almost immediately re-radiated." professor langley's explorations of the terra incognita of immensely long wave-lengths where lie the unseen heat-emissions from the earth into space, led him to the discovery that these, contrary to the received opinion, are in good part transmissible by our atmosphere, although they are completely intercepted by glass. another important result of the allegheny work was the abolition of the anomalous notion of the "temperature of space," fixed by pouillet at - ° c. for space in itself can have no temperature, and stellar radiation is a negligible quantity. thus, it is safe to assume "that a perfect thermometer suspended in space at the distance of the earth or moon from the sun, but shielded from its rays, would sensibly indicate the absolute zero,"[ ] ordinarily placed at - ° c. a "prize essay on the distribution of the moon's heat" (the hague), , by mr. frank w. very, who had taken an active part in professor langley's long-sustained inquiry, embodies the fruits of its continuation. they show the lunar disc to be tolerably uniform in thermal power. the brighter parts are also indeed hotter, but not much. the traces perceived of a slight retention of heat by the substances forming the lunar surface, agreed well with the parsonstown observations of the total eclipse of the moon, january , .[ ] for they brought out an unmistakable divergence between the heat and light phases. a curious decrease of heat previous to the first touch of the earth's shadow upon the lunar globe remains unexplained, unless it be admissible to suppose the terrestrial atmosphere capable of absorbing heat at an elevation of miles. the probable range of temperature on the moon was discussed by professor very in .[ ] he concluded it to be very wide. hotter than boiling water under the sun's vertical rays, the arid surface of our dependent globe must, he found, cool in the -day lunar night to about the temperature of liquid air. although that fundamental part of astronomy known as "celestial mechanics" lies outside the scope of this work, and we therefore pass over in silence the immense labours of plana, damoiseau, hansen, delaunay, g. w. hill, and airy in reconciling the observed and calculated motions of the moon, there is one slight but significant discrepancy which is of such importance to the physical history of the solar system, that some brief mention must be made of it. halley discovered in , by examining the records of ancient eclipses, that the moon was going faster then than , years previously--so much faster, as to have got ahead of the place in the sky she would otherwise have occupied, by about two of her own diameters. it was one of laplace's highest triumphs to have found an explanation of this puzzling fact. he showed, in , that it was due to a very slow change in the ovalness of the earth's orbit, tending, during the present age of the world, to render it more nearly circular. the pull of the sun upon the moon is thereby lessened; the counter-pull of the earth gets the upper hand; and our satellite, drawn nearer to us by something less than an inch each year,[ ] proportionately quickens her pace. many thousands of years hence the process will be reversed; the terrestrial orbit will close in at the sides, the lunar orbit will open out under the growing stress of solar gravity, and our celestial chronometer will lose instead of gaining time. this is all quite true as laplace put it; but it is not enough. adams, the virtual discoverer of neptune, found with surprise in that the received account of the matter was "essentially incomplete," and explained, when the requisite correction was introduced, only half the observed acceleration.[ ] what was to be done with the remaining half? here delaunay, the eminent french mathematical astronomer, unhappily drowned at cherbourg in by the capsizing of a pleasure-boat, came to the rescue.[ ] it is obvious to anyone who considers the subject a little attentively, that the tides must act to some extent as a friction-brake upon the rotating earth. in other words, they must bring about an almost infinitely slow lengthening of the day. for the two masses of water piled up by lunar influence on the hither and farther sides of our globe, strive, as it were, to detach themselves from the unity of the terrestrial spheroid, and to follow the movements of the moon. the moon, accordingly, holds them _against_ the whirling earth, which revolves like a shaft in a fixed collar, slowly losing motion and gaining heat, eventually dissipated through space.[ ] this must go on (so far as we can see) until the periods of the earth's rotation and of the moon's revolution coincide. nay, the process will be continued--should our oceans survive so long--by the feebler tide-raising power of the sun, ceasing only when day and night cease to alternate, when one side of our planet is plunged in perpetual darkness and the other seared by unchanging light. here, then, we have the secret of the moon's turning always the same face towards the earth. it is that in primeval times, when the moon was liquid or plastic, an earth-raised tidal wave rapidly and forcibly reduced her rotation to its present exact agreement with her period of revolution. this was divined by kant[ ] nearly a century before the necessity for such a mode of action presented itself to any other thinker. in a weekly paper published at königsberg in , the modern doctrine of "tidal friction" was clearly outlined by him, both as regards its effects actually in progress on the rotation of the earth, and as regards its effects already consummated on the rotation of the moon--the whole forming a preliminary attempt at what he called a "natural history" of the heavens. his sagacious suggestion, however, remained entirely unnoticed until revived--it would seem independently--by julius robert mayer in ;[ ] while similar, and probably original, conclusions were reached by william ferrel of allensville, kentucky, in .[ ] delaunay was not then the inventor or discoverer of tidal friction; he merely displayed it as an effective cause of change. he showed reason for believing that its action in checking the earth's rotation, far from being, as ferrel had supposed, completely neutralised by the contraction of the globe through cooling, was a fact to be reckoned with in computing the movements, as well as in speculating on the history, of the heavenly bodies. the outstanding acceleration of the moon was thus at once explained. it was explained as apparent only--the reflection of a real lengthening, by one second in , years, of the day. but on this point the last word has not yet been spoken. professor newcomb undertook in the onerous task of investigating the errors of hansen's lunar tables as compared with observations prior to . the results, published in ,[ ] proved somewhat perplexing. they tend, in general, to reduce the amount of acceleration left unaccounted for by laplace's gravitational theory, and proportionately to diminish the importance of the part played by tidal friction. but, in order to bring about this diminution, and at the same time conciliate alexandrian and arabian observations, it is necessary to reject _as total_ the ancient solar eclipses known as those of thales and larissa. this may be a necessary, but it must be admitted to be a hazardous expedient. its upshot was to indicate a possibility that the observed and calculated values of the moon's acceleration might after all prove to be identical; and the small outstanding discrepancy was still further diminished by tisserand's investigation, differently conducted, of the same arabian eclipses discussed by newcomb.[ ] the necessity of having recourse to a lengthening day is then less pressing than it seemed some time ago; and the effect, if perceptible in the moon's motion, should, m. tisserand remarked, be proportionately so in the motions of all the other heavenly bodies. the presence of the apparent general acceleration that should ensue can be tested with most promise of success, according to the same authority, by delicate comparisons of past and future transits of mercury. newcomb further showed that small residual irregularities are still found in the movements of our satellite, inexplicable either by any known gravitational influence, or by any _uniform_ value that could be assigned to secular acceleration.[ ] if set down to the account of imperfections in the "time-keeping" of the earth, it could only be on the arbitrary supposition of fluctuations in its rate of going themselves needing explanation. this, it is true, might be found in very slight changes of figure,[ ] not altogether unlikely to occur. but into this cloudy and speculative region astronomers for the present decline to penetrate. they prefer, if possible, to deal only with calculable causes, and thus to preserve for their "most perfect of sciences" its special prerogative of assured prediction. footnotes: [footnote : _neueste beyträge zur erweiterung der sternkunde_, bd. iii., p. ( ).] [footnote : _ibid._, p. .] [footnote : _phil. trans._, vol. xciii., p. .] [footnote : _mem. roy. astr. soc._, vol. vi., p. .] [footnote : _month. not._, vol. xix., pp. , .] [footnote : _ibid._, vol. xxxviii., p. .] [footnote : _am. jour. of sc._, vol. xvi., p. .] [footnote : _wash. obs._ for , part ii., p. .] [footnote : _pop. astr._, vol. ii., p. ; _astr. jour._, no. .] [footnote : _astr. and astrophysics_, vol. xiii., p. .] [footnote : _ibid._, p. .] [footnote : _month. not._, vol. xxiv., p. .] [footnote : _ibid._, vol. xxiii., p. (challis).] [footnote : _untersuchungen über die spectra der planeten_, p. .] [footnote : _sirius_, vol. vii., p. .] [footnote : _potsdam publ._, no. ; _astr. nach._, no. , ; frost, _astr. and astrophysics_, vol. xii., p. .] [footnote : zöllner and winnecke made it=o· , _astr. nach._, no. , .] [footnote : _neueste beyträge_, bd. iii., p. .] [footnote : _astr. jahrbuch_, , pp. - .] [footnote : webb, _celestial objects_, p. ( th ed.).] [footnote : _l'astronomie_, t. ii., p. .] [footnote : _observations sur les planètes vénus et mercure_, p. .] [footnote : _observatory_, vol. vi., p. .] [footnote : _atti dell' accad. dei lincei_, t. v. ii., p. , ; _astr. nach._, no. , .] [footnote : _astr. nach._ no. , .] [footnote : _memoirs amer. acad._, vol. xii., no. , p. .] [footnote : _hist. de l'astr._, p. .] [footnote : _comptes rendus_, t. xlix., p. .] [footnote : _comptes rendus_, t. l., p. .] [footnote : _ibid._, p. .] [footnote : _astr. nach._, nos. , and , .] [footnote : _comptes rendus_, t. lxxxiii., pp. , .] [footnote : _handbuch der mathematik_, bd. ii., p. .] [footnote : _comptes rendus_, t. lxxxiii., p. .] [footnote : _nature_, vol. xviii., pp. , , .] [footnote : oppolzer, _astr. nach._, no. , .] [footnote : _ibid._, nos. , - (c. h. f. peters).] [footnote : _ibid._, nos. , and , . see also tisserand in _ann. bur. des long._, , p. .] [footnote : see j. bauschinger's _untersuchungen_ ( ), summarised in _bull. astr._, t. i., p. , and _astr. nach._, no. , . newcomb finds the anomalous motion of the perihelion to be even larger ( " instead of ") than leverrier made it. _month. not._, february, , p. . harzer's attempt to account for it in _astr. nach._, no. , , is more ingenious than successful.] [footnote : _jour. des sçavans_, december, , p. .] [footnote : _Élémens d'astr._, p. . cf. chandler, _pop. astr._, february, , p. .] [footnote : _beobachtungen über die sehr beträchtlichen gebirge und rotation der venus_, , p. . schröter's final result in was h. m. · s. _monat. corr._, bd. xxv., p. .] [footnote : _astr. nach._, no. .] [footnote : _rendiconti del r. istituto lombardo_, t. xxiii., serie ii.] [footnote : _astr. nach._, no. , .] [footnote : _bothkamp beobachtungen_, heft ii., p. .] [footnote : _comptes rendus_, t. cxi., p. ; t. cxxii., p. .] [footnote : _month. not._, vol. lvii., p. ; _astr. nach._, no. , .] [footnote : _mem. spettroscopisti italiani_, t. xxv., p. ; _nature_, vol. liii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._] [footnote : _bull. de l'acad. de belgique_, t. xxi., p. , .] [footnote : _observations sur les planètes vénus et mercure_, .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, no. , .] [footnote : _ibid._, no. , .] [footnote : _ibid._, no. , .] [footnote : _ibid._, no. , . the velocity of a point on the equator of venus, if brenner's period of h. m. were exact, would be · miles per second; but the displacements due to this rate would be doubled by reflection.] [footnote : _novæ observationes_, p. .] [footnote : _mém. de l'ac._, , p. .] [footnote : _phil. trans._, vol. lxxxiii., p. .] [footnote : webb, _cel. objects_, p. .] [footnote : _month. not._, vol. xlii., p. .] [footnote : _bull. ac. de bruxelles_, t. xliii., p. .] [footnote : _phil. trans._, vol. lxxxii., p. ; _aphroditographische fragmente_, p. ( ).] [footnote : _astr. nach._, no. .] [footnote : _month. not._, vol. xiv., p. .] [footnote : _ibid._, vol. xxiv., p. .] [footnote : _am. jour. of sc._, vol. xliii., p. ( d ser.); vol. ix., p. ( d ser.).] [footnote : _astroph. jour._, vol. ix., p. .] [footnote : _month. not._, vol. xxxvi., p. .] [footnote : _old and new astronomy_, p. .] [footnote : _hist. phys. astr._, p. .] [footnote : _mem. roy. astr. soc._, vol. xlvii., pp. , .] [footnote : _astr. reg._, vol. xiii., p. .] [footnote : _l'astronomie_, t. ii., p. ; _astr. nach._, no. , ; _am. jour. of sc._, vol. xxv., p. .] [footnote : _mem. spettr. ital._, dicembre, ; _am. jour. of sc._, vol. xxv., p. .] [footnote : _comptes rendus_, t. cxvi., p. .] [footnote : vogel, _spectra der planeten_, p. .] [footnote : _nature_, vol. xix., p. .] [footnote : _nova acta acad. naturæ curiosorum_, bd. x., .] [footnote : _astr. jahrbuch_, , p. .] [footnote : _month not._, vol. xliii., p. .] [footnote : _report brit. ass._, , p. . the paper contains a valuable record of observations of the phenomenon.] [footnote : _photom. untersuchungen_, p. .] [footnote : _bothkamp beobachtungen_, heft ii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _mémoires de l'acad. de bruxelles_, t. xlix., no. , to; _astr. nach._, no. , ; _f._ schorr, _der venusmond_, .] [footnote : _phil. trans._, , , .] [footnote : delaunay objected (_comptes rendus_, t. lxvii., p. ) that the viscosity of the contained liquid (of which hopkins took no account) would, where the movements were so excessively slow as those of the earth's axis, almost certainly cause it to behave like a solid. lord kelvin, however (_report brit. ass._, , ii., p. ), considered hopkins's argument valid as regards the comparatively quick solar semi-annual and lunar fortnightly nutations.] [footnote : _phil. trans._, cliii., p. .] [footnote : _report brit. ass._, , p. .] [footnote : _ibid._, , p. .] [footnote : albrecht, _astr. nach._, no. , .] [footnote : _astr. jour._, nos. , .] [footnote : _ibid._, no. .] [footnote : _month. not._, vol. lii., p. .] [footnote : _astr. nach._, no. , ; _phil. trans._, vol. clxxxvi., a., p. ; _proc. roy. soc._, vol. lix.] [footnote : see chandler's searching investigations, _astr. jour._, nos. , , , , , , , , , , , .] [footnote : rees, _pop. astr._, no. , .] [footnote : _nature_, vol. lxi., p. ; see also a. v. bäcklund, _astr. nach._, no. , .] [footnote : _trans. geol. soc._, vol. iii. ( d ser.), p. .] [footnote : see his _treatise on astronomy_, p. ( ).] [footnote : _phil. mag._, vol. xxviii. ( th ser.), p. .] [footnote : _climate and time_, ; _discussions on climate and cosmology_, .] [footnote : see for a popular account of the theory, sir r. ball's _the cause of an ice age_, .] [footnote : see a. woeikof, _phil. mag._, vol. xxi., p. .] [footnote : _the ice age in north america_, london, .] [footnote : _phil. trans._, vol. lxviii., p. .] [footnote : _comptes rendus_, t. lxxvi., p. .] [footnote : _potsdam publ._, nos. , .] [footnote : _phil. trans._, vol. clxxxii., p. ; _adams prize essay for ._] [footnote : _denkschriften akad. der wiss. wien_, bd. lxiv.; quoted by poynting. _nature_, vol. lxii., p. .] [footnote : _report on the geodetic survey of s. africa_, .] [footnote : _nature_, vol. lxii., p. ; hollis, _observatory_, vol. xxiii., p. ; poincaré, _comptes rendus_, july , .] [footnote : _astr. nach._, no. , .] [footnote : young's _gen. astr._, p. .] [footnote : _astr. constants_, p. .] [footnote : the second volume was published at göttingen in .] [footnote : _ueber rillen auf dem monde_, p. . _cf. the moon_, by t. gwyn elger, p. . w. h. pickering, _harvard annals_, vol. xxxii., p. .] [footnote : _the moon_, p. .] [footnote : _selen. fragm._, th. ii., p. .] [footnote : _astr. nach._, no. ( ); _pop. vorl._, pp. - ( ).] [footnote : _outlines of astr._, par. .] [footnote : _month. not._, vol. xxv., p. .] [footnote : _month. not._, vol. xxv., p. .] [footnote : _astroph. jour._, vol. vi., p. .] [footnote : _harvard annals_, vol. xxxii., p. .] [footnote : _astr. and astrophysics_, vol. xi., p. .] [footnote : neison, _the moon_, p. .] [footnote : _knowledge_, vol. xvii., p. .] [footnote : neison, _the moon_, p. .] [footnote : the combination of a uniform rotational with an unequal orbital movement causes a slight swaying of the moon's globe, now east, now west, by which we are able to see round the edges of the averted hemisphere. there is also a "parallactic" libration, depending on the earth's rotation; and a species of nodding movement--the "libration in latitude"--is produced by the inclination of the moon's axis to her orbit, and by her changes of position with regard to the terrestrial equator. altogether, about / of the _invisible_ side come into view.] [footnote : _cel. objects_, p. ( th ed.).] [footnote : _astr. nach._, no. , .] [footnote : cf. leo brenner, _naturwiss. wochenschrift_, january , ; _jour. brit. astr. ass._, vol. v., pp. , .] [footnote : respighi, _les mondes_, t. xiv., p. ; huggins, _month. not._, vol. xxvii., p. .] [footnote : birt, _ibid._, p. .] [footnote : _report brit. ass._, , p. .] [footnote : _observatory_, vol. xv., p. .] [footnote : _astr. reg._, vol. xvi., p. ; _astr. nach._, no. , .] [footnote : lindsay and copeland, _month. not._, vol. xxxix., p. .] [footnote : _observatory_, vols. ii., p. ; iv., p. . n. e. green (_astr. reg._, vol. xvii., p. ) concluded the object a mere "spot of colour," dark under oblique light.] [footnote : webb, _cel. objects_, p. .] [footnote : _publ. lick observatory_, vol. iii., p. .] [footnote : _ibid._, p. ; mee, _knowledge_, vol. xviii., p. .] [footnote : _comptes rendus_, t. cxxii., p. ; _bull. astr._, august, ; _ann. bureau des long._, ; _nature_, vols. lii., p. ; lvi., p. ; lix., p. ; lx., p. ; _astroph. jour._ vol. vi., p. .] [footnote : _comptes rendus_, t. xxii., p. .] [footnote : _phil. trans._, vol. cxlviii., p. .] [footnote : _proc. roy. soc._, vol. xvii., p. .] [footnote : _phil. trans._, vol. clxiii., p. .] [footnote : _trans. r. dublin soc._, vol. iii., p. .] [footnote : _science_, vol. vii., p. .] [footnote : _amer. jour. of science_, vol. xxxviii., p. .] [footnote : "the temperature of the moon," _memoirs national acad. of sciences_, vol. iv., p. , .] [footnote : _temperature of the moon_, p. iii.; see also app. ii., p. .] [footnote : _trans. r. dublin soc._, vol. iv., p. , ; rosse, _proc. roy. institution_, may , .] [footnote : _astroph. jour._, vol. viii., pp. , .] [footnote : airy, _observatory_, vol. iii., p. .] [footnote : _phil. trans._, vol. cxliii., p. ; _proc. roy. soc._, vol. vi., p. .] [footnote : _comptes rendus_, t. lxi., p. .] [footnote : professor darwin calculated that the heat generated by tidal friction in the course of lengthening the earth's period of rotation from to hours, equalled million times the amount of its present annual loss by cooling. _nature_, vol. xxxiv., p. .] [footnote : _sämmtl. werke_ (ed. ), th. vi., pp. - . see also c. j. monro's useful indications in _nature_, vol. vii., p. .] [footnote : _dynamik des himmels_, p. .] [footnote : gould's _astr. jour._, vol. iii., p. .] [footnote : _wash. obs._ for , vol. xxii., app. ii.] [footnote : _comptes rendus_, t. cxiii., p. ; _annuaire_, paris, .] [footnote : newcomb, _pop. astr._ ( th ed.), p. .] [footnote : sir w. thomson, _report brit. ass._, , p. .] chapter viii _planets and satellites_--(_continued_) "the analogy between mars and the earth is perhaps by far the greatest in the whole solar system." so herschel wrote in ,[ ] and so we may safely say to-day, after six score further years of scrutiny. the circumstance lends a particular interest to inquiries into the physical habitudes of our exterior planetary neighbour. fontana first caught glimpses, at naples in and ,[ ] of dusky stains on the ruddy disc of mars. they were next seen by hooke and cassini in , and this time with sufficient distinctness to serve as indexes to the planet's rotation, determined by the latter as taking place in a period of twenty-four hours forty minutes.[ ] increased confidence was given to this result through maraldi's precise verification of it in .[ ] among the spots observed by him, he distinguished two as stable in position, though variable in size. they were of a peculiar character, showing as bright patches round the poles, and had already been noticed during sixty years back. a current conjecture of their snowy nature obtained validity when herschel connected their fluctuations in extent with the progress of the martian seasons. the inference of frozen precipitations could scarcely be resisted when once it was clearly perceived that the shining polar zones did actually by turns diminish and grow with the alternations of summer and winter in the corresponding hemisphere. this, it may be said, was the opening of our acquaintance with the state of things prevailing on the surface of mars. it was accompanied by a steady assertion, on herschel's part, of permanence in the dark markings, notwithstanding partial obscurations by clouds and vapours floating in a "considerable but moderate atmosphere." hence the presumed inhabitants of the planet were inferred to "probably enjoy a situation in many respects similar to ours."[ ] schröter, on the other hand, went altogether wide of the truth as regards mars. he held that the surface visible to us is a mere shell of drifting cloud, deriving a certain amount of apparent stability from the influence on evaporation and condensation of subjacent but unseen areographical features;[ ] and his opinion prevailed with his contemporaries. it was, however, rejected by kunowsky in , and finally overthrown by beer and mädler's careful studies during five consecutive oppositions, - . they identified at each the same dark spots, frequently blurred with mists, especially when the local winter prevailed, but fundamentally unchanged.[ ] in lockyer established a "marvellous agreement" with beer and mädler's results of , leaving no doubt as to the complete fixity of the main features, amid "daily, nay, hourly," variations of detail through transits of clouds.[ ] on seventeen nights of the same opposition, f. kaiser of leyden obtained drawings in which nearly all the markings noted in at berlin reappeared, besides spots frequently seen respectively by arago in , by herschel in , and one sketched by huygens in with a writing-pen in his diary.[ ] from these data the leyden observer arrived at a period of rotation of h. m. · s., being just one second shorter than that deduced, exclusively from their own observations, by beer and mädler. the exactness of this result was practically confirmed by the inquiries of professor bakhuyzen of leyden.[ ] using for a middle term of comparison the disinterred observations of schröter, with those of huygens at one, and of schiaparelli at the other end of an interval of years, he was enabled to show, with something like certainty, that the time of rotation ( h. m. · s.) ascribed to mars by mr. proctor[ ] in reliance on a drawing executed by hooke in , was too long by _nearly one-tenth of a second_. the minuteness of the correction indicates the nicety of care employed. nor employed vainly; for, owing to the comparative antiquity of the records available in this case, an almost infinitesimal error becomes so multiplied by frequent repetition as to produce palpable discrepancies in the positions of the markings at distant dates. hence bakhuyzen's period of h. m. · s. is undoubtedly of a precision unapproached as regards any other heavenly body save the earth itself. two facts bearing on the state of things at the surface of mars were, then, fully acquired to science in or before the year . the first was that of the seasonal fluctuations of the polar spots; the second, that of the general permanence of certain dark gray or greenish patches, perceived with the telescope as standing out from the deep yellow ground of the disc. that these varieties of tint correspond to the real diversities of a terraqueous globe, the "ripe cornfield"[ ] sections representing land, the dusky spots and streaks, oceans and straits, has long been the prevalent opinion. sir j. herschel in led the way in ascribing the redness of the planet's light to an inherent peculiarity of soil.[ ] previously it had been assimilated to our sunset glows rather than to our red sandstone formations--set down, that is, to an atmospheric stoppage of blue rays. but the extensive martian atmosphere, implicitly believed in on the strength of some erroneous observations by cassini and römer in the seventeenth century, vanished before the sharp occultation of a small star in leo, witnessed by sir james south in ;[ ] and dawes's observation in ,[ ] that the ruddy tinge is deepest near the central parts of the disc, certified its non-atmospheric origin. the absolute whiteness of the polar snow-caps was alleged in support of the same inference by sir william huggins in .[ ] all recent operations tend to show that the atmosphere of mars is much thinner than our own. this was to have been expected _à priori_, since the same proportionate mass of air would on his smaller globe form a relatively sparse covering.[ ] besides, gravity there possesses less than four-tenths its force here, so that this sparser covering would weigh less, and be less condensed, than if it enveloped the earth. atmospheric pressure would accordingly be of about two and a quarter, instead of fifteen terrestrial pounds per square inch. this corresponds with what the telescope shows us. it is extremely doubtful whether any features of the earth's actual surface could be distinguished by a planetary spectator, however well provided with optical assistance. professor langley's inquiries[ ] led him to conclude that fully twice as much light is absorbed by our air as had previously been supposed--say per cent. of vertical rays in a clear sky. of the sixty reaching the earth, less than a quarter would be reflected even from white sandstone; and this quarter would again pay heavy toll in escaping back to space. thus not more than perhaps ten or twelve out of the original hundred sent by the sun would, under the most favourable circumstances, and from the very centre of the earth's disc, reach the eye of a martian or lunar observer. the light by which he views our world is, there is little doubt, light reflected from the various strata of our atmosphere, cloud or mist-laden or serene, as the case may be, with an occasional snow-mountain figuring as a permanent white spot. this consideration at once shows us how much more tenuous the martian air must be, since it admits of topographical delineations of the martian globe. the clouds, too, that form in it seem in general to be rather of the nature of ground-mists than of heavy cumulus.[ ] occasionally, indeed, durable and extensive strata become visible. during the latter half of october, , for instance, a region as large as europe remained apparently cloud-covered. yet most recent observers are unable to detect the traces of aqueous absorption in the martian spectrum noted by huggins in [ ] and by vogel in .[ ] campbell vainly looked for them,[ ] visually in , spectrographically in ; keeler was equally unsuccessful;[ ] jewell[ ] holds that they could, with present appliances, only be perceived if the atmosphere of mars were much richer in water-vapour than that of the earth. there can be little doubt, however, that its supply is about the minimum adequate to the needs of a _living_, and perhaps a life-nuturing planet. the climate of mars seems to be unexpectedly mild. its _theoretical_ mean temperature, taking into account both distance from the sun and albedo, is ° c. below freezing.[ ] yet its polar snows are both less extensive and less permanent than those on the earth. the southern white hood, noticed by schiaparelli in to have survived the summer only as a small lateral patch, melted completely in . moreover, mr. w. h. pickering observed with astonishment the disappearance, in the course of thirty-three days of june and july, , of , , square miles of southern snow.[ ] curiously enough, the initial stage of shrinkage in the white calotte was marked by its division into two unequal parts, as if in obedience to the mysterious principle of duplication governing so many martian phenomena.[ ] changes of the hues associated respectively with land and water accompanied in lower latitudes, and were thought to be occasioned by floods ensuing upon this rapid antarctic thaw. it is true that scarcity of moisture would account for the scantiness and transitoriness of snowy deposits easily liquefied because thinly spread. but we might expect to see the whole wintry hemisphere, at any rate, frost-bound, since the sun radiates less than half as much heat on mars as on the earth. water seems, nevertheless, to remain, as a rule, uncongealed everywhere outside the polar regions. we are at a loss to imagine by what beneficent arrangement the rigorous conditions naturally to be looked for can be modified into a climate which might be found tolerable by creatures constituted like ourselves. martian topography may be said to form nowadays a separate sub-department of descriptive astronomy. the amount of detail become legible by close scrutiny on a little disc which, once in fifteen years, attains a maximum of about / the area of the full moon, must excite surprise and might provoke incredulity. spurious discoveries, however, have little chance of holding their own where there are so many competitors quite as ready to dispute as to confirm. the first really good map of mars was constructed in by proctor from drawings by dawes. kaiser of leyden followed in with a representation founded upon data of his own providing in - ; and terby, in his valuable _aréographie_, presented to the brussels academy in [ ] a careful discussion of all important observations from the time of fontana downwards, thus virtually adding to knowledge by summarising and digesting it. the memorable opposition of september , , marked a fresh epoch in the study of mars. while executing a trigonometrical survey (the first attempted) of the disc, then of the unusual size of " across, g. v. schiaparelli, director of the milan observatory, detected a novel and curious feature. what had been taken for martian continents were found to be, in point of fact, agglomerations of islands, separated from each other by a network of so-called "canals" (more properly _channels_).[ ] these are obviously extensions of the "seas," originating and terminating in them, and sharing their gray-green hue, but running sometimes to a length of three or four thousand miles in a straight line, and preserving throughout a nearly uniform breadth of about sixty miles. further inquiries have fully substantiated the discovery made at the brera observatory. the "canals" of mars are an actually existent and permanent phenomenon. an examination of the drawings in his possession showed m. terby that they had been seen, though not distinctively recognised, by dawes, secchi, and holden; several were independently traced out by burton at the opposition of ; all were recovered by schiaparelli himself in and - ; and their indefinite multiplication resulted from lovell's observations in and . when the planet culminated at midnight, and was therefore in opposition, december , , its distance was greater, and its apparent diameter less than in , in the proportion of sixteen to twenty-five. its atmosphere was, however, more transparent, and ours of less impediment to northern observers, the object of scrutiny standing considerably higher in northern skies. never before, at any rate, had the true aspect of mars come out so clearly as at milan, with the - / -inch merz refractor of the observatory, between december, , and february, . the canals were all again there, but this time they were--in as many as twenty cases--_seen in duplicate_. that is to say, a twin-canal ran parallel to the original one at an interval of to miles.[ ] we are here brought face to face with an apparently insoluble enigma. schiaparelli regards the "germination" of his canals as a periodical phenomenon depending on the martian seasons. it is, assuredly, not an illusory one, since it was plainly apparent, during the opposition of , to mm. perrotin and thollon at nice,[ ] and to the former, using the new -inch refractor of that observatory, in ; mr. a. stanley williams, with the help of only a - / -inch reflector, distinctly perceived in seven of the duplicate objects noted at milan,[ ] and the lick observations, both of and of , together with the drawings made at flagstaff and mexico during the last favourable oppositions of the nineteenth century, brought unequivocal confirmation to the accuracy of schiaparelli's impressions.[ ] various conjectures have been hazarded in explanation of this bizarre appearance. the difficulty of conceiving a physical reality corresponding to it has suggested recourse to an optical rationale. proctor regarded it as an effect of diffraction;[ ] stanislas meunier, of oblique reflection from overlying mist-banks;[ ] flammarion considers it possible that companion-canals might, under special circumstances, be evoked by refraction as a kind of mirage.[ ] but none of these speculations are really admissible, when all the facts are taken into account. the view that the canals of mars are vast rifts due to the cooling of the globe, is recommended by the circumstance that they tend to follow great circles; nevertheless, it would break down if, as schiaparelli holds, the fluctuations in their visibility depend upon actual obliterations and re-emergencies. fantastic though the theory of their artificial origin appear, it is held by serious astronomers. its vogue is largely due to mr. lowell's ingenious advocacy. he considers the martian globe to be everywhere intersected by an elaborate system of irrigation-works, rendered necessary by a perennial water-famine, relieved periodically by the melting of the polar snows. nor does he admit the existence of oceans, or lakes. what have been taken for such are really tracts covered with vegetation, the bright areas intermixed with them representing sandy deserts. and it is noteworthy in this connection that professor barnard obtained in ,[ ] with the great lick refractor, "suggestive and impressive views" disclosing details of light and shade on the gray-green patches so intricate and minute as almost to preclude the supposition of their aqueous nature. the closeness of the terrestrial analogy has thus of late been much impaired. even if the surface of mars be composed of land and water, their distribution must be of a completely original type. the interlacing everywhere of continents with arms of the sea (if that be the correct interpretation of the visual effects) implies that their levels scarcely differ;[ ] and schiaparelli carries most observers with him in holding that their outlines are not absolutely constant, encroachments of dusky upon bright tints suggesting extensive inundations.[ ] the late n. e. green's observations at madeira in indicated, on the other hand, a rugged south polar region. the contour of the snow-cap not only appeared indented, as if by valleys and promontories, but brilliant points were discerned outside the white area, attributed to isolated snow-peaks.[ ] still more elevated, if similarly explained, must be the "ice island" first seen in a comparatively low latitude by dawes in january, . on august , , mars stood opposite to the sun at a distance of only , , miles from the earth. in point of vicinity, then, its situation was scarcely less favourable than in . the low altitude of the planet, however, practically neutralised this advantage for northern observers, and public expectation, which had been raised to the highest pitch by the announcements of sensation-mongers, was somewhat disappointed at the "meagreness" of the news authentically received from mars. valuable series of observations were, nevertheless, made at lick and arequipa; and they unite in testifying to the genuine prevalence of surface-variability, especially in certain regions of intermediate tint, and perhaps of the "crude consistence" of "boggy syrtes, neither sea, nor good dry land." professor holden insisted on the "enormous difficulties in the way of completely explaining the recorded phenomena by terrestrial analogies";[ ] mr. w. h. pickering spoke of "conspicuous and startling changes." they, however, merely overlaid, and partially disguised, a general stability. among the novelties detected by mr. pickering were a number of "lakes," or "oases" (in lowell's phraseology), under the aspect of black dots at the junctions of two or more canals;[ ] and he, no less than the lick astronomers and m. perrotin at nice,[ ] observed brilliant clouds projecting beyond the terminator, or above the limb, while carried round by the planet's rotation. they seemed to float at an altitude of at least twenty miles, or about four times the height of terrestrial cirrus; but this was not wonderful, considering the low power of gravity acting upon them. great capital was made in the journalistic interest out of these imaginary signals from intelligent martians, desirous of opening communications with (to them) problematical terrestrial beings. similar effects had, however, been seen before by mr. knobel in , by m. terby in , and at the lick observatory in ; and they were discerned again with particular distinctness by professor hussey at lick, august , .[ ] the first photograph of mars was taken by gould at cordoba in . little real service in planetary delineation has, it is true, been so far rendered by the art, yet one achievement must be recorded to its credit. a set of photographs obtained by mr. w. h. pickering on wilson's peak, california, april , , showed the southern polar cap of mars as of moderate dimensions, but with a large dim adjacent area. twenty-four hours later, on a corresponding set, the dim area was brilliantly white. the polar cap had become enlarged in the interim, apparently through a wide-spreading snow-fall, by the annexation of a territory equal to that of the united states. the season was towards the close of winter in mars. never until then had the process of glacial extension been actually (it might be said) superintended in that distant globe. mars was gratuitously supplied with a pair of satellites long before he was found actually to possess them. kepler interpreted galileo's anagram of the "triple" saturn in this sense; they were perceived by micromégas on his long voyage through space; and the laputan astronomers had even arrived at a knowledge, curiously accurate under the circumstances, of their distances and periods. but terrestrial observers could see nothing of them until the night of august , . the planet was then within one month of its second nearest approach to the earth during the last century; and in the washington -inch refractor was not in existence.[ ] professor asaph hall, accordingly, determined to turn the conjecture to account for an exhaustive inquiry into the surroundings of mars. keeping his glaring disc just outside the field of view, a minute attendant speck of light was "glimpsed" august . bad weather, however, intervened, and it was not until the th that it was ascertained to be what it appeared--a satellite. on the following evening a second, still nearer to the primary, was discovered, which, by the bewildering rapidity of its passages hither and thither, produced at first the effect of quite a crowd of little moons.[ ] both these delicate objects have since been repeatedly observed, both in europe and america, even with comparatively small instruments. at the opposition of , indeed, the distance of the planet was too great to permit of the detection of both elsewhere than at washington. but the lick equatoreal showed them, july , , when their brightness was only · its amount at the time of their discovery; so that they can now be followed for a considerable time before and after the least favourable oppositions. the names chosen for them were taken from the iliad, where "deimos" and "phobos" (fear and panic) are represented as the companions in battle of ares. in several respects, they are interesting and remarkable bodies. as to size, they may be said to stand midway between meteorites and satellites. from careful photometric measures executed at harvard in and , professor pickering concluded their diameters to be respectively six and seven miles.[ ] this is on the assumption that they reflect the same proportion of the light incident upon them that their primary does. but it may very well be that they are less reflective, in which case they would be more extensive. the albedo of mars is put by müller at · ; his surface, in other words, returns per cent. of the rays striking it. if we put the albedo of his satellites equal to that of our moon, · , their diameters will be increased from and to - / and miles, phobos, the inner one, being the larger. mr. lowell, however, formed a considerably larger estimate of their dimensions.[ ] it is interesting to note that deimos, according to professor pickering's very distinct perception, does not share the reddish tint of mars. deimos completes its nearly circular revolutions in thirty hours eighteen minutes, at a distance from the surface of its ruling body of , miles; phobos traverses an elliptical orbit[ ] in seven hours thirty-nine minutes twenty-two seconds, at a distance of only , miles. this is the only known instance of a satellite circulating faster than its primary rotates, and is a circumstance of some importance as regards theories of planetary development. to a martian spectator the curious effect would ensue of a celestial object, seemingly exempt from the general motion of the sphere, rising in the west, setting in the east, and culminating twice, or even thrice a day; which, moreover, in latitudes above ° north or south, would be permanently and altogether hidden by the intervening curvature of the globe. * * * * * the detection of new members of the solar system has come to be one of the most ordinary of astronomical events. since no single year has passed without bringing its tribute of asteroidal discovery. in the last of the seventies alone, a full score of miniature planets were distinguished from the thronging stars amid which they seem to move; brought seventeen such recognitions; their number touched a minimum of one in ; it rose in , and again in , to eleven; dropped to six in , and sprang up with the aid of photography to twenty-seven in . that high level has since, on an average, been maintained; and on january , , nearly asteroids were recognised as revolving between the orbits of mars and jupiter. of these, considerably more than one hundred are claimed by one investigator alone--dr. max wolf of heidelburg; m. charlois of nice comes second with ; while among the earlier observers palisa of vienna contributed , and c. h. f. peters of clinton (n. y.), whose varied and useful career terminated july , , to the grand total. the construction by chacornac and his successors at paris, and more recently by peters at clinton, of ecliptical charts showing all stars down to the thirteenth and fourteenth magnitudes respectively, rendered the picking out of moving objects above that brightness a mere question of time and diligence. both, however, are vastly economised by the photographic method. tedious comparisons of the sky with charts are no longer needed for the identification of unrecorded, because simulated stars. planetary bodies declare themselves by appearing upon the plate, not in circular, but in linear form. their motion converts their images into trails, long or short according to the time of exposure. the first asteroid (no. ) thus detected was by max wolf, december , .[ ] eighteen others were similarly discovered in , by the same skilful operator; and ten more through charlois's adoption at nice of the novel plan now in exclusive use for picking up errant light-specks. far more onerous than the task of their discovery is that of keeping them in view once discovered--of tracking out their paths, ixing their places, and calculating the disturbing effects upon them of the mighty jovian mass. these complex operations have come to be centralised at berlin under the superintendence of professor tietjen, and their results are given to the public through the medium of the _berliner astronomisches jahrbuch_. the _cui bono?_ however, began to be agitated. was it worth while to maintain a staff of astronomers for the sole purpose of keeping hold over the identity of the innumerable component particles of a cosmical ring? the prospect, indeed, of all but a select few of the asteroids being thrown back by their contemptuous captors into the sea of space seemed so imminent that professor watson provided by will against the dereliction of the twenty-two discovered by himself. but the fortunes of the whole family improved through the distinction obtained by one of them. on august , , the trail of a rapidly-moving, star-like object of the eleventh magnitude imprinted itself on a plate exposed by herr witt at the urania observatory, berlin. its originator proved to be unique among asteroids. "eros" is, in sober fact, 'one of those mysterious stars which hide themselves between the earth and mars,' divined or imagined by shelley.[ ] true, several of its congeners invade the martian sphere at intervals; but the proper habitat of eros is within that limit, although its excursions transcend it. in other words, its mean distance from the sun is about , as compared with the martian distance of million miles. further, its orbit being so fortunately circumstanced as to bring it once in sixty-seven years within some millions of miles of the earth, it is of extraordinary value to celestial surveyors. the calculation of its movements was much facilitated by detections, through a retrospective search,[ ] of many of its linear images among the star-dots on the harvard plates.[ ] the little body--which can scarcely be more than twenty miles in diameter--shows peculiarities of behaviour as well as of position. dr. von oppolzer, in february, ,[ ] announced it to be extensively and rapidly variable. once in hours minutes it lost about three-fourths of its light,[ ] but these fluctuations quickly diminished in range, and in the beginning of may ceased altogether.[ ] evidently, then, they depend upon the situation of the asteroid relatively to ourselves; and, so far, events lent countenance to m. andré's eclipse hypothesis, since mutual occultations of the supposed planetary twins could only take place when the plane of their revolutions passed through the earth, and this condition would be transitory. yet the recognition in eros of an "algol asteroid" seems on other grounds inadmissible;[ ] nor until the phenomenon is conspicuously renewed--as it probably will be at the opposition of --can there be much hope of finding its appropriate rationale. the crowd of orbits disclosed by asteroidal detections invites attentive study. d'arrest remarked in ,[ ] when only thirteen minor planets were known, that supposing their paths to be represented by solid hoops, not one of the thirteen could be lifted from its place without bringing the others with it. the complexity of interwoven tracks thus illustrated has grown almost in the numerical proportion of discovery. yet no two actually intersect, because no two lie exactly in the same plane, so that the chances of collision are at present _nil_. there is only one case, indeed, in which it seems to be eventually possible. m. lespiault has pointed out that the curves traversed by "fidés" and "maïa" approach so closely that a time may arrive when the bodies in question will either coalesce or unite to form a binary system.[ ] the maze threaded by the asteroids contrasts singularly with the harmoniously ordered and rhythmically separated orbits of the larger planets. yet the seeming confusion is not without a plan. the established rules of our system are far from being totally disregarded by its minor members. the orbit of pallas, with its inclination of ° ', touches the limit of departure from the ecliptic level; the average obliquity of the asteroidal paths is somewhat less than that of the sun's equator;[ ] their mean eccentricity is below that of the curve traced out by mercury, and all without exception are pursued in the planetary direction--from west to east. the zone in which these small bodies travel is about three times as wide as the interval separating the earth from the sun. it extends perilously near to jupiter, and dovetails into the sphere of mars. their distribution is very unequal. they are most densely congregated about the place where a single planet ought, by bode's law, to revolve; it may indeed be said that only stragglers from the main body are found more than fifty million miles within or without a mean distance from the sun · times that of the earth. significant gaps, too, occur where some force prohibitive of their presence would seem to be at work. the probable nature of that force was suggested by the late professor kirkwood, first in , when the number of known asteroids was only eighty-eight, and again with more confidence in , from the study of a list then run up to .[ ] it appears that these bare spaces are found just where a revolving body would have a period connected by a simple relation with that of jupiter. it would perform two or three circuits to his one, five to his two, nine to his five, and so on. kirkwood's inference was that the gaps in question were cleared of asteroids by the attractive influence of jupiter. for disturbances recurring time after time--owing to commensurability of periods--nearly at the same part of the orbit, would have accumulated until the shape of that orbit was notably changed. the body thus displaced would have come in contact with other cosmical particles of the same family with itself--then, it may be assumed, more evenly scattered than now--would have coalesced with them, and permanently left its original track. in this way the regions of maximum perturbation would gradually have become denuded of their occupants. we can scarcely doubt that this law of commensurability has largely influenced the present distribution of the asteroids. but its effects must have been produced while they were still in an unformed, perhaps a nebular condition. in a system giving room for considerable modification through disturbance, the recurrence of conjunctions with a dominating mass at the same orbital point need not involve instability.[ ] on the whole, the correspondence of facts with kirkwood's hypothesis has not been impaired by their more copious collection.[ ] some chasms of secondary importance have indeed been bridged; but the principal stand out more conspicuously through the denser scattering of orbits near their margins. nor is it doubtful that the influence of jupiter in some way produced them. m. de freycinet's study of the problem they present[ ] has, however, led him to the conclusion that they existed _ab origine_, thus testifying rather to the preventive than to the perturbing power of the giant planet. the existence, too, of numerous asteroidal pairs travelling in approximately coincident tracks, must date from a remote antiquity. they result, professor kirkwood[ ] believed, from the divellent action of jupiter upon embryo pigmy planets, just as comets moving in pursuit of one another are a consequence of the sundering influence of the sun. leverrier fixed, in ,[ ] one-fourth of the earth's mass as the outside limit for the combined masses of all the bodies circulating between mars and jupiter; but it is far from probable that this maximum is at all nearly approached. m. berberich[ ] held that the moon would more than outweigh the whole of them, a million of the lesser bodies shining like stars of the twelfth magnitude being needed, according to his judgment, to constitute her mass. and m. niesten estimated that the whole of the asteroids discovered up to august, , amounted in _volume_ to only / th of our globe,[ ] and we may safely add--since they are tolerably certain to be lighter, bulk for bulk, than the earth--that their proportionate _mass_ is smaller still. a fairly concordant result was published in by mr. b. m. roszel.[ ] he found that the lunar globe probably contains forty times, the terrestrial globe , times the quantity of matter parcelled out among the first minor planets. the actual size of a few of them may now be said to be known. professor pickering, from determinations of light-intensity, assigned to vesta a diameter of miles, to pallas , to juno , down to twelve and fourteen for the smaller members of the group.[ ] an albedo equal to that of mars was assumed as the basis of the calculation. moreover, professor g. müller[ ] of potsdam examined photometrically the phases of seven among them, of which four--namely, vesta, iris, massalia, and amphitrite--were found to conform precisely to the behaviour of mars as regards light-change from position, while ceres, pallas, and irene varied after the manner of the moon and mercury. the first group were hence inferred to resemble mars in physical constitution, nature of atmosphere, and reflective capacity; the second to be moon-like bodies. finally, professor barnard, directly measuring with the yerkes refractor the minute discs presented by the original quartette, obtained the following authentic data concerning them:[ ] diameter of ceres, miles, albedo = · ; diameter of pallas, miles, albedo = · ; diameter of vesta, miles, albedo = · ; diameter of juno, miles, albedo = · . thus, the rank of premier asteroid proves to belong to ceres, and to have been erroneously assigned to vesta in consequence of its deceptive brilliancy. what kind of surface this indicates, it is hard to say. the dazzling whiteness of snow can hardly be attributed to bare rock; yet the dynamical theory of gases--as dr. johnstone stoney pointed out in [ ]--prohibits the supposition that bodies of insignificant gravitative power can possess aerial envelopes. even our moon, it is calculated, could not permanently hold back the particles of oxygen, nitrogen, or water-gas from escaping into infinite space; still less, a globe one thousand times smaller. vogel's suspicion of an air-line in the spectrum of vesta[ ] has, accordingly, not been confirmed. * * * * * crossing the zone of asteroids on our journey outward from the sun, we meet with a group of bodies widely different from the "inferior" or terrestrial planets. their gigantic size, low specific gravity, and rapid rotation, obviously from the first threw the "superior" planets into a class apart; and modern research has added qualities still more significant of a dissimilar physical constitution. jupiter, a huge globe , miles in diameter, stands pre-eminent among them. he is, however, only _primus inter pares_; all the wider inferences regarding his condition may be extended, with little risk of error, to his fellows; and inferences in his case rest on surer grounds than in the case of the others, from the advantages offered for telescopic scrutiny by his comparative nearness. now the characteristic modern discovery concerning jupiter is that he is a body midway between the solar and terrestrial stages of cosmical existence--a decaying sun or a developing earth, as we choose to put it--whose vast unexpended stores of internal heat are mainly, if not solely, efficient in producing the interior agitations betrayed by the changing features of his visible disc. this view, impressed upon modern readers by mr. proctor's popular works, was anticipated in the last century. buffon wrote in his _Époques de la nature_ ( ):[ ]--"la surface de jupiter est, comme l'on sait, sujette à des changemens sensibles, qui semblent indiquer que cette grosse planète est encore dans un état d'inconstance et de bouillonnement." primitive incandescence, attendant, in his fantastic view, on planetary origin by cometary impacts with the sun, combined, he concluded, with vast bulk to bring about this result. jupiter has not yet had time to cool. kant thought similarly in ;[ ] but the idea did not commend itself to the astronomers of the time, and dropped out of sight until mr. nasmyth arrived at it afresh in .[ ] even still, however, terrestrial analogies held their ground. the dark belts running parallel to the equator, first seen at naples in , continued to be associated--as herschel had associated them in --with jovian trade-winds, in raising which the deficient power of the sun was supposed to be compensated by added swiftness of rotation. but opinion was not permitted to halt here. in g. p. bond of cambridge (u.s.) derived some remarkable indications from experiments on the light of jupiter.[ ] they showed that fourteen times more of the photographic rays striking it are reflected by the planet than by our moon, and that, unlike the moon, which sends its densest rays from the margin, jupiter is brightest near the centre. but the most perplexing part of his results was that jupiter actually seemed to give out more light than he received. bond, however, rightly considered his data too uncertain for the support of so bold an assumption as that of original luminosity, and, even if the presence of native light were proved, thought that it might emanate from auroral clouds of the terrestrial kind. the conception of a sun-like planet was still a remote, and seemed an extravagant one. only since it was adopted and enforced by zöllner in ,[ ] can it be regarded as permanently acquired to science. the rapid changes in the cloud-belts both of jupiter and saturn, he remarked, attest a high internal temperature. for we know that all atmospheric movements on the earth are sun-heat transformed into motion. but sun-heat at the distance of jupiter possesses but / , at that of saturn / of its force here. the large amount of energy, then, obviously exerted in those remote firmaments must have some other source, to be found nowhere else than in their own active and all-pervading fires, not yet banked in with a thick solid crust. the same acute investigator dwelt, in ,[ ] on the similarity between the modes of rotation of the great planets and of the sun, applying the same principles of explanation to each case. the fact of this similarity is undoubted. cassini[ ] and schröter both noticed that markings on jupiter travelled quicker the nearer they were to his equator; and cassini even hinted at their possible assimilation to sun-spots.[ ] it is now well ascertained that, as a rule (not without exceptions), equatorial spots give a period some - / minutes shorter than those in latitudes of about °. but, as mr. denning has pointed out,[ ] no single period will satisfy the observations either of different markings at the same epoch, or of the same markings at different epochs. accelerations and retardations, depending upon processes of growth or change, take place in very much the same kind of way as in solar maculæ, inevitably suggesting similarity of origin. the interesting query as to jupiter's surface incandescence has been studied since bond's time with the aid of all the appliances furnished to physical inquirers by modern inventiveness, yet without bringing to it a categorical reply. zöllner in , müller in , estimated his albedo at · and · respectively, that of fresh-fallen snow being · , and of white paper · .[ ] but the disc of jupiter is by no means purely white. the general ground is tinged with ochre; the polar zones are leaden or fawn coloured; large spaces are at times stained or suffused with chocolate-browns and rosy hues. it is occasionally seen ruled from pole to pole with dusky bars, and is never wholly free from obscure markings. the reflection, then, by it, as a whole, of about per cent. of the rays impinging upon it, might well suggest some original reinforcement. nevertheless, the spectroscope gives little countenance to the supposition of any considerable permanent light-emission. the spectrum of jupiter, as examined by huggins, - , and by vogel, - , shows the familiar fraunhofer rays belonging to reflected sunlight. but it also shows lines of native absorption. some of these are identical with those produced by the action of our own atmosphere, especially one or more groups due to aqueous vapours; others are of unknown origin; and it is remarkable that one among the latter--a strong band in the red--agrees in position with a dark line in the spectra of some ruddy stars.[ ] there is, besides, a general absorption of blue rays, intensified--as le sueur observed at melbourne in [ ]--in the dusky markings, evidently through an increase of depth in the atmospheric strata traversed by the light proceeding from them. all these observations, however (setting aside the stellar line as of doubtful significance), point to a cool planetary atmosphere. one spectrograph, it is true, taken by dr. henry draper, september , ,[ ] seemed to attest the action of intrinsic light; but the peculiarity was referred by dr. vogel, with convincing clearness, to a flaw in the film.[ ] so far, then, native emissions from any part of jupiter's diversified surface have not been detected; and, indeed, the blackness of the shadows cast by his satellites on his disc sufficiently proves that he sends out virtually none but reflected light.[ ] this conclusion, however, by no means invalidates that of his high internal temperature. the curious phenomena attending jovian satellite-transits may be explained, partly as effects of contrast, partly as due to temporary obscurations of the small discs projected on the large disc of jupiter. at their first entry upon its marginal parts, which are several times less luminous than those near the centre, they invariably show as bright spots, then usually vanish as the background gains lustre, to reappear, after crossing the disc, thrown into relief, as before, against the dusky limb. but instances are not rare, more especially of the third and fourth satellites standing out, during the entire middle part of their course, in such inky darkness as to be mistaken for their own shadows. the earliest witness of a "black transit" was cassini, september , ; römer in , and maraldi in and , made similar observations, which have been multiplied in recent years. in some cases the process of darkening has been visibly attended by the formation, or emergence into view, of spots on the transiting body, as noted by the two bonds at harvard, march , .[ ] the third satellite was seen by dawes, half dark, half bright, when crossing jupiter's disc, august , ;[ ] one-third dark by davidson of california, january , , under the same circumstances;[ ] and unmistakably spotted, both on and off the planet, by schröter, secchi, dawes, and lassell. the first satellite sometimes looks dusky, but never absolutely black, in travelling over the disc of jupiter. the second appears uniformly white--a circumstance attributed by dr. spitta[ ] to its high albedo. the singularly different aspects, even during successive transits, of the third and fourth satellites, are connected by professor holden[ ] with the varied luminosity of the segments of the planetary surface they are projected upon, and w. h. pickering inclines to the same opinion; but fluctuations in their own brightness[ ] may be a concurrent cause. herschel concluded in that, like our moon, they always turn the same face towards their primary, and as regards the outer satellite, engelmann's researches in , and c. e. burton's in , made this almost certain; while both for the third and fourth jovian moons it was completely assured by w. h. pickering's and a. e. douglass's observations at arequipa in ,[ ] and at flagstaff in - .[ ] strangely enough, however, the interior members of the system have preserved a relatively swift rotation, notwithstanding the enormous checking influence upon it of jove-raised tides. all the satellites are stated, on good authority, to be more or less egg-shaped. on september , , barnard saw the first elongated and bisected by a bright equatorial belt, during one of its dark transits;[ ] and his observation, repeated august , , was completely verified by schaeberle and campbell, who ascertained, moreover, that the longer axis of the prolate body was directed towards jupiter's centre.[ ] the ellipticity of its companions was determined by pickering and douglass; indeed, that of no. had long previously been noticed by secchi.[ ] no. also shows equatorial stripes, perceived in by schaeberle and campbell,[ ] and evident later to pickering and douglass;[ ] nor need we hesitate to admit as authentic their records of similar, though less conspicuous markings on the other satellites. a constitution analogous to that of jupiter himself was thus unexpectedly suggested; and vogel's detection of lines--or traces of lines--in their spectra, agreeing with absorption-rays derived from their primary, lends support to the conjecture that they possess gaseous envelopes similar to his. the system of jupiter, as it was discovered by galileo, and investigated by laplace, appeared in its outward aspect so symmetrical, and displayed in its inner mechanism such harmonious dynamical relations, that it might well have been deemed complete. nevertheless, a new member has been added to it. near midnight on september , , professor barnard discerned with the lick -inch "a tiny speck of light," closely following the planet.[ ] he instantly divined its nature, watched its hurried disappearance in the adjacent glare, and made sure of the reality of his discovery on the ensuing night. it was a delicate business throughout, the liliputian luminary subsiding into invisibility before the slightest glint of jovian light, and tarrying, only for brief intervals, far enough from the disc to admit of its exclusion by means of an occulting plate. the new satellite is estimated to be of the thirteenth stellar magnitude, and, if equally reflective of light with its next neighbour, io (satellite no. ), its diameter must be about one hundred miles. it revolves at a distance of , miles from jupiter's centre, and of , from his bulging equatorial surface. its period of h. m. s. is just two hours longer than jupiter's period of rotation, so that phobos still remains a unique example of a secondary body revolving faster than its primary rotates. jupiter's innermost moon conforms in its motions strictly, indeed inevitably, to the plane of his equatorial protuberance, following, however, a sensibly elliptical path the major axis of which is in rapid revolution.[ ] its very insignificance raises the suspicion that it may not prove solitary. possibly it belongs to a zone peopled by asteroidal satellites. more than fifteen thousand such small bodies could be furnished out of the materials of a single full-sized satellite spoiled in the making. but we must be content for the present to register the fact without seeking to penetrate the meaning of its existence. very high and very fine telescopic power is needed for its perception. outside the united states, it has been very little observed. the only instruments in this country successfully employed for its detection are, we believe, dr. common's -foot reflector and mr. newall's -inch refractor. in the course of his observations on jupiter at brussels in , m. niesten was struck with a rosy cloud attached to a whitish zone beneath the dark southern equatorial band.[ ] its size was enormous. at the distance of jupiter, its measured dimensions of " by " implied a real extension in longitude of , , in latitude of something short of , miles. the earliest record of its appearance seems to be by professor pritchett, director of the morrison observatory (u.s.), who figured and described it july , .[ ] it was again delineated august , by tempel at florence.[ ] in the following year it attracted the wonder and attention of almost every possessor of a telescope. its colour had by that time deepened into a full brick-red, and was set off by contrast with a white equatorial spot of unusual brilliancy. during three ensuing years these remarkable objects continued to offer a visible and striking illustration of the compound nature of the planet's rotation. the red spot completed a circuit in nine hours fifty-five minutes thirty-six seconds; the white spot in about five and a half minutes less. their _relative_ motion was thus no less than miles an hour, bringing them together in the same meridian at intervals of forty-four days ten hours forty-two minutes. neither, however, preserved continuously the same uniform rate of travel. the period of each had lengthened by some seconds in , while sudden displacements, associated with the recovery of lustre after recurrent fadings, were observed in the position of the white spot,[ ] recalling the leap forward of a reviving sun-spot. just the opposite effect attended the rekindling of the companion object. while semi-extinct, in - , it lost little motion; but a fresh access of retardation was observed by professor young[ ] in connection with its brightening in . this suggests very strongly that the red spot is _fed from below_. a shining aureola of "faculæ," described by bredichin at moscow, and by lohse at potsdam, as encircling it in september, ,[ ] was held to strengthen the solar analogy. the conspicuous visibility of this astonishing object lasted three years. when the planet returned to opposition in - , it had faded so considerably that riccò's uncertain glimpse of it at palermo, may , , was expected to be the last. it had, nevertheless, begun to recover in december, and presented to mr. denning in the beginning of much the same aspect as in october, .[ ] observed by him in an intermediate stage, february , , when "a mere skeleton of its former self," it bore a striking likeness to an "elliptical ring" descried in the same latitude by mr. gledhill in - . this, indeed, might be called the preliminary sketch for the famous object brought to perfection ten years later, but which mr. h. c. russell of sydney saw and drew still unfinished in june, ,[ ] before it had separated from its matrix, the dusky south tropical belt. in earlier times, too, a marking "at once fixed and transient" had been repeatedly perceived attached to the southernmost of the central belts. it gave cassini in a rotation-period of nine hours fifty-six minutes,[ ] reappeared and vanished eight times during the next forty-three years, and was last seen by maraldi in . it was, however, very much smaller than the recent object, and showed no unusual colour.[ ] the assiduous observations made on the "great red spot" by mr. denning at bristol and by professor hough at chicago afforded grounds only for negative conclusions as to its nature. it certainly did _not_ represent the outpourings of a jovian volcano; it was in no sense attached to the jovian soil--if the phrase have any application to that planet; it was _not_ a mere disclosure of a glowing mass elsewhere seethed over by rolling vapours. it was, indeed, certainly not self-luminous, a satellite projected upon it in transit having been seen to show as bright as upon the dusky equatorial bands. a fundamental objection to all three hypotheses is that the rotation of the spot was variable. it did not then ride at anchor, but floated free. some held that its surface was depressed below the average cloud-level, and that the cavity was filled with vapours. professor wilson, on the other hand, observing with the -inch equatorial of the goodsell observatory in minnesota, received a persistent impression of the object "being at a higher level than the other markings."[ ] a crucial experiment on this point was proposed by mr. stanley williams in .[ ] a dark spot moving faster along the same parallel was timed to overtake the red spot towards the end of july. a unique opportunity hence appeared to be at hand of determining the relative vertical depths of the two formations, one of which must inevitably, it was thought, pass above the other. no forecast included a third alternative, which was nevertheless adopted by the dark spot. it evaded the obstacle in its path by skirting round its southern edge.[ ] nothing, then, was gained by the conjunction, beyond an additional proof of the singular repellent influence exerted by the red spot over the markings in its vicinity. it has, for example, gradually carved out a deep bay for its accommodation in the gray belt just north of it. the effect was not at first steadily present. a premonitory excavation was drawn by schwabe at dessau, september , , and again by trouvelot, barnard, and elvins in ; yet there was no sign of it in the following year. its development can be traced in dr. boeddicker's beautiful delineations of jupiter, made with the parsonstown -foot reflector, from to .[ ] they record the belt as straight in , but as strongly indented from january, ; and the cavity now promises to outlast the spot. so long as it survives, however, the forces at work in the spot can have lost little of their activity. for it must be remembered that the belt has a shorter rotation-period than the red spot, which, accordingly (as mr. elvins of toronto has pointed out), breasts and diverts, by its interior energy, a current of flowing matter, ever ready to fill up its natural bed, and override the barrier of obstruction. the famous spot was described by keeler in , as "of a pale pink colour, slightly lighter in the middle. its outline was a fairly true ellipse, framed in by bright white clouds."[ ] the fading continuously in progress from was temporarily interrupted in . the revival, indeed, was brief. professor barnard wrote in august, : "the great red spot is still visible, but it has just passed through a crisis that seemingly threatened its very existence. for the past month it has been all but impossible to catch the feeblest trace of the spot, though the ever-persistent bay in the equatorial belt close north of it, and which has been so intimately connected with the history of the red spot, has been as conspicuous as ever. it is now, however, possible to detect traces of the entire spot. an obscuring medium seems to have been passing over it, and has now drifted somewhat preceding the spot."[ ] the object is now always inconspicuous, and often practically invisible, and may be said to float passively in the environing medium.[ ] yet there are sparks beneath the ashes. a rosy tinge faintly suffused it in april, ,[ ] and its absolute end may still be remote. the extreme complexity of the planet's surface-movements has been strikingly evinced by mr. stanley williams's detailed investigations. he enumerated in [ ] nine principal currents, all flowing parallel to the equator, but unsymmetrically placed north and south of it, and showing scant signs of conformity to the solar rule of retardation with increase of latitude. the linear rate of the planet's equatorial rotation was spectroscopically determined by bélopolsky and deslandres in . both found it to fall short of the calculated speed, whence an enlargement, by self-refraction, of the apparent disc was inferred.[ ] jupiter was systematically photographed with the lick -inch telescope during the oppositions of , , and , the image thrown on the plates (after eightfold direct enlargement) being one inch in diameter. mr. stanley williams's measurements and discussion of the set for showed the high value of the materials thus collected, although much more minute details can be seen than can at present be photographed. the red spot shows as "very distinctly annular" in several of these pictures.[ ] recently, the planet has been portrayed by deslandres with the -foot meudon refractor.[ ] the extreme actinic feebleness of the equatorial bands was strikingly apparent on his plates. in , mr. ranyard[ ]--whose death, december , , was a serious loss to astronomy--acting upon an earlier suggestion of sir william huggins, collected records of unusual appearances on the disc of jupiter, with a view to investigate the question of their recurrence at regular intervals. he concluded that the development of the deeper tinges of colour, and of the equatorial "port-hole" markings girdling the globe in regular alternations of bright and dusky, agreed, so far as could be ascertained, with epochs of sun-spot maximum. the further inquiries of dr. lohse at bothkamp in [ ] went to strengthen the coincidence, which had been anticipated _à priori_ by zöllner in .[ ] moreover, separate and distinct evidence was alleged by mr. denning in of decennial outbreaks of disturbance in north temperate regions.[ ] it may, indeed, be taken for granted that what hahn terms the universal pulse of the solar system[ ] affects the vicissitudes of jupiter; but the law of those vicissitudes is far from being so obviously subordinate to the rhythmical flow of central disturbance as are certain terrestrial phenomena. the great planet, being in fact himself a "semi-sun," may be regarded as an originator, no less than a recipient, of agitating influences, the combined effects of which may well appear insubordinate to any obvious law. it is likely that saturn is in a still earlier stage of planetary development than jupiter. he is the lightest for his size of all the planets. in fact, he would float in water. and since his density is shown, by the amount of his equatorial bulging, to increase centrally,[ ] it follows that his superficial materials must be of a specific gravity so low as to be inconsistent, on any probable supposition, with the solid or liquid states. moreover, the chief arguments in favour of the high temperature of jupiter, apply, with increased force, to saturn; so that it may be concluded, without much risk of error, that a large proportion of his bulky globe, , miles in diameter, is composed of heated vapours, kept in active and agitated circulation by the process of cooling. his unique set of appendages has, since the middle of the last century, formed the subject of searching and fruitful inquiries, both theoretical and telescopic. the mechanical problem of the stability of saturn's rings was left by laplace in a very unsatisfactory condition. considering them as rotating solid bodies, he pointed out that they could not maintain their position unless their weight were in some way unsymmetrically distributed; but made no attempt to determine the kind or amount of irregularity needed to secure this end. some observations by herschel gave astronomers an excuse for taking for granted the fulfilment of the condition thus vaguely postulated; and the question remained in abeyance until once more brought prominently forward by the discovery of the dusky ring in . the younger bond led the way, among modern observers, in denying the solidity of the structure. the fluctuations in its aspect were, he asserted in ,[ ] inconsistent with such a hypothesis. the fine dark lines of division, frequently detected in both bright rings, and as frequently relapsing into imperceptibility, were due, in his opinion, to the real nobility of their particles, and indicated a fluid formation. professor benjamin peirce of harvard university immediately followed with a demonstration, on abstract grounds, of their non-solidity.[ ] streams of some fluid denser than water were, he maintained, the physical reality giving rise to the anomalous appearance first disclosed by galileo's telescope. the mechanism of saturn's rings, proposed as the subject of the adams prize, was dealt with by james clerk maxwell in . his investigation forms the groundwork of all that is at present known in the matter. its upshot was to show that neither solid nor fluid rings could continue to exist, and that the only possible composition of the system was by an aggregated multitude of unconnected particles, each revolving independently in a period corresponding to its distance from the planet.[ ] this idea of a satellite-formation had been, remarkably enough, several times entertained and lost sight of. it was first put forward by roberval in the seventeenth century, again by jacques cassini in , and with perfect definiteness by wright of durham in .[ ] little heed, however, was taken of these casual anticipations of a truth which reappeared, a virtual novelty, as the legitimate outcome of the most refined modern methods. the details of telescopic observation accord, on the whole, admirably with this hypothesis. the displacements or disappearance of secondary dividing-lines--the singular striated appearance, first remarked by short in the eighteenth century, last by perrotin and lockyer at nice, march , [ ]--show the effects of waves of disturbance traversing a moving mass of gravitating particles;[ ] the broken and changing line of the planet's shadow on the ring gives evidence of variety in the planes of the orbits described by those particles. the whole ring-system, too, appears to be somewhat elliptical.[ ] the satellite-theory has derived unlooked-for support from photometric inquiries. professor seeliger pointed out in [ ] that the unvarying brilliancy of the outer rings under all angles of illumination, from ° to °, can be explained from no other point of view. nor does the constitution of the obscure inner ring offer any difficulty. for it is doubtless formed of similar small bodies to those aggregated in the lucid members of the system, only much more thinly strewn, and reflecting, consequently, much less light. it is not, indeed, at first easy to see why these sparser flights should show as a dense dark shading on the body of saturn. yet this is invariably the case. the objection has been urged by professor hastings of baltimore. the brightest parts of these appendages, he remarked,[ ] are more lustrous than the globe they encircle; but if the inner ring consists of identical materials, possessing presumably an equal reflective capacity, the mere fact of their scanty distribution would not cause them to show as dark against the same globe. professor seeliger, however, replied[ ] that the darkening is due to the never-ending swarms of their separate shadows transiting the planet's disc. sunlight is not, indeed, wholly excluded. many rays come and go between the open ranks of the meteorites. for the dusky ring is transparent. the planet it encloses shows through it, as if veiled with a strip of crape. a beautiful illustration of its quality in this respect was derived by professor barnard from an eclipse of japetus, november , .[ ] the eighth moon remained steadily visible during its passage through the shadow of the inner ring, but with a progressive loss of lustre in approaching its bright neighbour. there was no breach of continuity. the satellite met no gap, corresponding to that between the dusky ring and the body of saturn, through which it could shine with undiminished light, but was slowly lost sight of as it plunged into deeper and deeper gloom. the important facts were thus established, that the brilliant and obscure rings merge into each other, and that the latter thins out towards the saturnian globe. the meteoric constitution of these appendages was beautifully demonstrated in by professor keeler,[ ] then director of the alleghany observatory, pittsburgh. from spectrographs taken with the slit adjusted to coincidence with the equatorial plane of the system, he determined the comparative radial velocities of its different parts. and these supply a crucial test of clerk maxwell's theory. for if the rings were solid, the swiftest rates of rotation should be at their outer edges, corresponding to wider circles described in the same period; while, if they are pulverulent, the inverse relation must hold good. this proved to be actually the case. the motion slowed off outward, in agreement with the diminishing speed of particles travelling freely, each in its own orbit. keeler's result was promptly confirmed by campbell,[ ] as well as by deslandres and bélopolsky. a question of singular interest, and one which we cannot refrain from putting to ourselves, is--whether we see in the rings of saturn a finished structure, destined to play a permanent part in the economy of the system; or whether they represent merely a stage in the process of development out of the chaotic state in which it is impossible to doubt that the materials of all planets were originally merged. m. otto struve attempted to give a definite answer to this important query. a study of early and later records of observations disclosed to him, in , an apparent progressive approach of the inner edge of the bright ring to the planet. the rate of approach he estimated at about fifty-seven english miles a year, or , miles during the years elapsed since the time of huygens.[ ] were it to continue, a collapse of the system must be far advanced within three centuries. but was the change real or illusory--a plausible, but deceptive inference from insecure data? m. struve resolved to put it to the test. a set of elaborately careful micrometrical measures of the dimensions of saturn's rings, executed by himself at pulkowa in the autumn of , was provided as a standard of future comparison; and he was enabled to renew them, under closely similar circumstances, in .[ ] but the expected diminution of the space between saturn's globe and his rings had not taken place. a slight extension in the width of the system, both outward and inward, was indeed, hinted at; and it is worth notice that just such a separation of the rings was indicated by clerk maxwell's theory, so that there is an _à priori_ likelihood of its being in progress. yet hall's measures in - [ ] failed to supply evidence of alteration with time; and barnard's, executed at lick in - ,[ ] showed no sensible divergence from them. hence, much weight cannot be laid upon huygens's drawings and descriptions, which had been held to prove conclusively a partial filling up, since , of the interval between the ring and the planet.[ ] the rings of saturn replace, in professor g. h. darwin's view,[ ] an abortive satellite, scattered by tidal action into annular form. for they lie closer to the planet than is consistent with the integrity of a revolving body of reasonable bulk. the limit of possible existence for such a mass was fixed by roche of montpellier, in ,[ ] at · mean radii of its primary; while the outer edge of the ring-system is distant · radii of saturn from his centre. the virtual discovery of its pulverulent condition dates, then, according to professor darwin, from . he conjectures that the appendage will eventually disappear, partly through the dispersal of its constituent particles inward, and their subsidence upon the planet's surface, partly by their dispersal outward, to a region beyond "roche's limit," where coalescence might proceed unhindered by the strain of unequal attractions. one modest satellite, revolving inside mimas, would then be all that was left of the singular appurtenances we now contemplate with admiration. there seems reason to admit that kirkwood's law of commensurability has had some effect in bringing about the present distribution of the matter composing them. here the influential bodies are saturn's moons, while the divisions and boundaries of the rings represent the spaces where their disturbing action conspires to eliminate revolving particles. kirkwood, in fact, showed, in ,[ ] that a body circulating in the chasm between the bright rings known as "cassini's division," would have a period nearly commensurable with those of _four_ out of the eight moons; and meyer of geneva subsequently calculated all such combinations, with the result of bringing out coincidences between regions of maximum perturbation and the limiting and dividing lines of the system.[ ] this is in itself a strong confirmation of the view that the rings are made up of independently revolving small bodies. on december , , professor asaph hall discovered at washington a bright equatorial spot on saturn, which he followed and measured through above sixty rotations, each performed in ten hours fourteen minutes twenty-four seconds.[ ] this, he was careful to add, represented the period, not necessarily of the _planet_, but only of the individual spot. the only previous determination of saturn's axial movement (setting aside some insecure estimates by schröter) was herschel's in , giving a period of ten hours sixteen minutes. the substantial accuracy of hall's result was verified by mr. denning in .[ ] in may and june of that year, ten vague bright markings near the equator were watched by mr. stanley williams, who derived from them a rotation period only two seconds shorter than that determined at washington. nevertheless, similarly placed spots gave in and notably quicker rates;[ ] so that the task of timing the general drift of the saturnian surface by the displacements of such objects is hampered, to an indefinite extent, by their individual proper motions. saturn's outermost satellite, japetus, is markedly variable--so variable that it sends us, when brightest, just - / times as much light as when faintest. moreover, its fluctuations depend upon its orbital position in such a way as to make it a conspicuous telescopic object when west, a scarcely discernible one when east of the planet. herschel's inference[ ] of a partially obscured globe turning always the same face towards its primary seems the only admissible one, and is confirmed by pickering's measurements of the varying intensity of its light. he remarked further that the dusky and brilliant hemispheres must be so posited as to divide the disc, viewed from saturn, into nearly equal parts; so that this saturnian moon, even when "full," appears very imperfectly illuminated over one-half of its surface.[ ] zöllner estimated the albedo of saturn at · , müller at · , a value impossibly high, considering that the spectrum includes no vestige of original emissions. closely similar to that of jupiter, it shows the distinctive dark line in the red (wave-length ), which we may call the "red-star line"; and janssen, from the summit of etna in [ ] found traces in it of aqueous absorption. the light from the ring appears to be pure reflected sunshine unmodified by original atmospheric action.[ ] uranus, when favourably situated, can easily be seen with the naked eye as a star between the fifth and sixth magnitudes. there is indeed, some reason to suppose that he had been detected as a wandering orb by savage "watchers of the skies" in the pacific long before he swam into herschel's ken. nevertheless, inquiries into his physical habitudes are still in an early stage. they are exceedingly difficult of execution, even with the best and largest modern telescopes; and their results remain clouded with uncertainty. it will be remembered that uranus presents the unusual spectacle of a system of satellites travelling nearly at right angles to the plane of the ecliptic. the existence of this anomaly gives a special interest to investigations of his axial movement, which might be presumed, from the analogy of the other planets, to be executed in the same tilted plane. yet this is far from being certainly the case. mr. buffham in - caught traces of bright markings on the uranian disc, doubtfully suggesting a rotation in about twelve hours in a plane _not_ coincident with that in which his satellites circulate.[ ] dusky bands resembling those of jupiter, but very faint, were barely perceptible to professor young at princeton in . yet, though almost necessarily inferred to be equatorial, they made a considerable angle with the trend of the satellites' orbits.[ ] more distinctly by the brothers henry, with the aid of their fine refractor, two gray parallel rulings, separated by a brilliant zone, were discerned every clear night at paris from january to june, .[ ] what were taken to be the polar regions appeared comparatively dusky. the direction of the equatorial rulings (for so we may safely call them) made an angle of ° with the satellites' line of travel. similar observations were made at nice by mm. perrotin and thollon, march to june, , a lucid spot near the equator, in addition, indicating rotation in a period of about ten hours.[ ] the discrepancy was, however, considerably reduced by perrotin's study of the planet in with the new -inch equatoreal.[ ] the dark bands, thus viewed to better advantage than in , appeared to deviate no more than ° from the satellites' orbit-plane. no definitive results, on the other hand, were derived by professors holden, schaeberle, and keeler from their observations of uranus in - with the potent instrument on mount hamilton. shadings, it is true, were almost always, though faintly, seen; but they appeared under an anomalous, possibly an illusory aspect. they consisted, not of parallel, but of forked bands.[ ] measurements of the little sea-green disc which represents to us the massive bulk of uranus, by young, schiaparelli,[ ] safarik, h. c. wilson[ ] and perrotin, prove it to be quite distinctly _bulged_. the compression at once caught barnard's trained eye in ,[ ] when he undertook at lick a micrometrical investigation of the system; and he was surprised to perceive that the major axis of the elliptical surface made an angle of about ° with the line of travel pursued by the satellites. nothing more can be learned on this curious subject for some years, since the pole of the planet is just now turned nearly towards the earth; but barnard's conclusion is unlikely to be seriously modified. he fixed the mean diameter of uranus at , miles. but this estimate was materially reduced through dr. see's elimination of irradiative effects by means of daylight measures, executed at washington in .[ ] the visual spectrum of this planet was first examined by father secchi in , and later, with more advantages for accuracy, by huggins, vogel,[ ] and keeler.[ ] it is a very remarkable one. in lieu of the reflected fraunhofer lines, imperceptible perhaps through feebleness of light, six broad bands of original absorption appear, one corresponding to the blue-green ray of hydrogen (f), another to the "red-star line" of jupiter and saturn, the rest as yet unidentified. the hydrogen band seems much too strong and diffuse to be the mere echo of a solar line, and might accordingly be held to imply the presence of free hydrogen in the uranian atmosphere. this, however, would be difficult of reconcilement with keeler's identification of an absorption-group in the yellow with a telluric waterband. notwithstanding its high albedo-- · , according to zöllner--proof is wanting that any of the light of uranus is inherent. mr. albert taylor announced, indeed, in , his detection, with common's giant reflector, of bright flutings in its spectrum;[ ] but professor keeler's examination proved them to be merely contrast effects.[ ] sir william and lady huggins, moreover, obtained about the same time a photograph purely solar in character. the spectrum it represented was crossed by numerous fraunhofer lines, and by no others. it was, then, presumably composed entirely of reflected light. * * * * * judging from the indications of an almost evanescent spectrum, neptune, as regards physical condition, is the twin of uranus, as saturn of jupiter. of the circumstances of his rotation we are as good as completely ignorant. mr. maxwell hall, indeed, noticed at jamaica, in november and december, , certain rhythmical fluctuations of brightness, suggesting revolution on an axis in slightly less than eight hours;[ ] but professor pickering reduces the supposed variability to an amount altogether too small for certain perception, and dr. g. müller denies its existence _in toto_. it is true their observations were not precisely contemporaneous with those of mr. hall[ ] who believes the partial obscurations recorded by himself to have been of a passing kind, and to have suddenly ceased after a fortnight of prevalence. their less conspicuous renewal was visible to him in november, , confirming a rotation period of · hours. it was ascertained at first by indirect means that the orbit of neptune's satellite is inclined about ° to his equator. mr. marth[ ] having drawn attention to the rapid shifting of its plane of motion, m. tisserand and professor newcomb[ ] independently published the conclusion that such shifting necessarily results from neptune's ellipsoidal shape. the movement is of the kind exemplified--although with inverted relations--in the precession of the equinoxes. the pole of the satellite, owing to the pull of neptune's equatorial protuberance, describes a circle round the pole of his equator in a retrograde direction, and in a period of over five hundred years. the amount of compression indicated for the primary body is, at the outside, / ; whence it can be inferred that neptune possesses a lower rotatory velocity than the other giant planets. direct verification of the trend theoretically inferred for the satellite's movement was obtained by dr. see in . the washington -inch refractor disclosed to him, under exceptionally favourable conditions, a set of equatorial belts on the disc of neptune, and they took just the direction prescribed by theory. their objective reality cannot be doubted, although barnard was unable, either with the lick or the yerkes telescope,[ ] to detect any definite markings on this planet. its diameter was found by him to be , miles. the possibility that neptune may not be the most remote body circling round the sun has been contemplated ever since he has been known to exist. within the last few years the position at a given epoch of a planet far beyond his orbital verge has been approximately fixed by two separate investigators. professor george forbes of edinburgh adopted in a novel plan of search for unknown members of the solar system, the first idea of which was thrown out by m. flammarion in november, .[ ] it depends upon the movements of comets. it is well known that those of moderately short periods are, for a reason already explained, connected with the larger planets in such a way that the cometary aphelia fall near some planetary orbit. jupiter claims a large retinue of such partial dependents, neptune owns six, and there are two considerable groups, the farthest distances of which from the sun lie respectively near and times that of the earth. at each of these vast intervals, one involving a period of , , the other of , years, professor forbes maintains that an unseen planet circulates. he even computed elements for the nearer of the two, and fixed its place on the celestial sphere;[ ] but the photographic searches made for it by dr. roberts at crowborough and by mr. wilson at daramona proved unavailing. undeterred by deichmüller's discouraging opinion that cometary orbits extending beyond the recognised bounds of the solar system are too imperfectly known to serve as the basis of trustworthy conclusions,[ ] the edinburgh professor returned to the attack in .[ ] he now sought to prove that the lost comet of actually returned in , but with elements so transformed by ultra-neptunian perturbations as to have escaped immediate identification. if so, the "wanted" planet has just entered the sign libra, and, being larger than jupiter, should be possible to find. almost simultaneously with forbes, professor todd set about groping for the same object by the help of a totally different set of indications. adams's approved method commended itself to him; but the hypothetical divagations of neptune having scarcely yet had time to develop, he was thrown back upon the "residual errors" of uranus. they gave him a virtually identical situation for the new planet with that derived from the clustering of cometary aphelia.[ ] yet its assigned distance was little more than half that of the nearer of professor forbes's remote pair, and it completed a revolution in instead of , years. the agreement in them between the positions determined, on separate grounds, for the ultra-neptunian traveller was merely an odd coincidence; nor can we be certain, until it is seen, that we have really got into touch with it. footnotes: [footnote : _phil. trans._, vol. lxxiv., p. .] [footnote : _novæ observationes_, p. .] [footnote : _phil. trans._, vol. i., p. .] [footnote : _mém. de l'ac._, , p. .] [footnote : _phil. trans._, vol. lxxiv., p. .] [footnote : a large work, entitled _areographische fragmente_, in which schröter embodied the results of his labours on mars, - , narrowly escaped the conflagration of , and was published at leyden in .] [footnote : _beiträge_, p. .] [footnote : _mem. r. a. soc._, vol. xxxii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _observatory_, vol. viii., p. .] [footnote : _month. not._, vols. xxviii., p. ; xxix., p. ; xxxiii., p. .] [footnote : flammarion, _l'astronomie_, t. i., p. .] [footnote : smyth, _cel. cycle_, vol. i., p. ( st ed.).] [footnote : _phil. trans._, vol. cxxi., p. .] [footnote : _month. not._, vol. xxv., p. .] [footnote : _phil. mag._, vol. xxxiv., p. .] [footnote : proctor, _quart. jour. of science_, vol. x., p. ; maunder, _sunday mag._, january, february, march, ; campbell, _publ. astr. pac. soc._, vol. vi., p. .] [footnote : _am. jour. of sc._, vol. xxviii., p. .] [footnote : burton, _trans. roy. dublin soc._, vol. i., , p. .] [footnote : _month. not._, vol. xxvii., p. ; _astroph. journ._, vol. i., p. .] [footnote : _untersuchungen über die spectra der planeten_, p. ; _astroph. journ._, vol. i., p. .] [footnote : _publ. astr. pac. soc._, vols. vi., p. ; ix., p. ; _astr. and astroph._, vol. xiii., p. ; _astroph. jour._, vol. ii., p. .] [footnote : _ibid._, vol. v., p. .] [footnote : _ibid._, vols. i., p. ; iii., p. .] [footnote : c. christiansen, _beiblätter_, , p. .] [footnote : _astr. and astrophysics_, vol. xi., p. .] [footnote : flammarion, _la planète mars_, p. .] [footnote : _mémoires couronnés_, t. xxxix.] [footnote : lockyer, _nature_, vol. xlvi., p. .] [footnote : _mem. spettr. italiani_, t. xi., p. .] [footnote : _bull. astr._, t. iii., p. .] [footnote : _journ. brit. astr. ass._, vol. i., p. .] [footnote : _publ. pac. astr. soc._, vol. ii., p. ; percival lowell, _mars_, ; _annals of the lowell observatory_, vol. ii., .] [footnote : _old and new astr._, p. .] [footnote : _l'astronomie_, t. xi., p. .] [footnote : _la planète mars_, p. .] [footnote : _month. notices_, vol. lvi., p. .] [footnote : _l'astronomie_, t. viii.] [footnote : _astr. nach._, no. , ; _astr. and astrophysics_, vol. xiii., p. .] [footnote : _month. not._, vol. xxxviii., p. ; _mem. roy. astr. soc._, vol. xliv., p. .] [footnote : _astr. and astrophysics_, vol. xi., p. .] [footnote : _ibid._, p. .] [footnote : _comptes rendus_, t. cxv., p. .] [footnote : _astr. jour._, no. ; _publ. astr. pac. soc._, vol. vi., p. . _cf. observatory_ vol. xvii., pp. - .] [footnote : see mr. wentworth erck's remarks in _trans. roy. dublin soc._, vol. i., p. .] [footnote : _month. not._, vol. xxxviii., p. .] [footnote : _annals harvard coll. obs._, vol. xi., pt. ii., p. .] [footnote : young, _gen. astr._, p. .] [footnote : campbell, _publ. pac. astr. soc._, vol. vi., p. .] [footnote : _astr. nach._, no. , .] [footnote : _witch of atlas_, stanza iii. i am indebted to dr. garnett for the reference.] [footnote : recommended by chandler, _astr. jour._, no. .] [footnote : _harvard circulars_, nos. , , .] [footnote : _astr. nach._, no. , .] [footnote : montangerand, _comptes rendus_, march , .] [footnote : pickering, _astroph. jour._, vol. xiii., p. .] [footnote : _harvard circular_, no. .] [footnote : _astr. nach._, no. .] [footnote : l. niesten, _annuaire_, bruxelles, , p. .] [footnote : according to svedstrup (_astr. nach._, nos. , - ), the inclination to the ecliptic of the "mean asteroid's" orbit is = °.] [footnote : _smiths._ report, , p. ; _the asteroids_ (kirkwood), p. , .] [footnote : tisserand, _annuaire_, paris, , p. b. ; newcomb, _astr. jour._, no. ; backlund, _bull. astr._, t. xvii., p. ; parmentier, _bull. soc. astr. de france_, march, ; observatory, vol. xviii., p. .] [footnote : berberich, _astr. nach._, no. , .] [footnote : _bull. astr._, t. xviii., p. .] [footnote : _the asteroids_, p. ; _publ. astr. pac. soc._, vols. ii., p. ; iii., p. .] [footnote : _comptes rendus_, t. xxxvii., p. .] [footnote : _bull. astr._, t. v., p. .] [footnote : _annuaire_, bruxelles, , p. .] [footnote : _johns hopkins un. circular_, january, ; _observatory_, vol. xviii., p. .] [footnote : _harvard annals_, vol. xi., part ii., p. .] [footnote : _astr. nach._, nos. , - .] [footnote : _month. not._, vol. lxi., p. .] [footnote : _astroph. jour._, vol. vii., p. .] [footnote : _spectra der planeten_, p. .] [footnote : tome i., p. .] [footnote : _berlinische monatsschrift_, , p. .] [footnote : _month. not._, vol. xiii., p. .] [footnote : _mem. am. ac._, vol. viii., p. .] [footnote : _photom. unters._, p. .] [footnote : _astr. nach._, no. , .] [footnote : _mém. de l'ac._, t. x., p. .] [footnote : _ibid._, , p. .] [footnote : _month. not._, vol. xliv., p. .] [footnote : _photom. unters._, pp. , ; _potsdam publ._, no. .] [footnote : vogel, _sp. der planeten_, p. , _note_.] [footnote : _proc. roy. soc._, vol. xviii., p. .] [footnote : _month. not._, vol. xl., p. .] [footnote : _sitzungsberichte_, berlin, , ii., p. .] [footnote : the anomalous shadow-effects recorded by webb (_cel. objects_, p. , th ed.) are obviously of atmospheric and optical origin.] [footnote : engelmann, _ueber die helligkeitsverhältnisse der jupiterstrabanten_, p. .] [footnote : _month. not._, vol. xxviii., p. .] [footnote : _observatory_, vol. vii., p. .] [footnote : _month. not._, vol. xlviii., p. .] [footnote : _publ. astr. pac. soc._, vol. ii., p. .] [footnote : pickering failed to obtain any photometric evidence of their variability. _harvard annals_, vol. xi., p. .] [footnote : _astr. and astroph._, vol. xii., pp. , .] [footnote : _annals lowell obs._, vol. ii., pt. i.] [footnote : _astr. nach._, nos. , , , ; _month. not._, vols. li., p. ; liv., p. . barnard remains convinced that the oval forms attributed to jupiter's satellites are illusory effects of their markings. _astr. nach._, nos. , , , ; _astr. and astroph._, vol. xiii., p. .] [footnote : _publ. astr. pac. soc._, vol. iii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _publ. astr. pac. soc._, vol. iii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _astr. jour._, nos. , , , ; _observatory_, vol. xv., p. .] [footnote : tisserand, _comptes rendus_, october , ; cohn, _astr. nach._, no. , .] [footnote : _bull. ac. r. bruxelles_, t. xlviii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, no. , .] [footnote : denning, _month. not._, vol. xliv., pp. , ; _nature_, vol. xxv., p. .] [footnote : _sidereal mess._, december, , p. .] [footnote : _astr. nach._, nos. , , , .] [footnote : _month. not._, vol. xlvi., p. .] [footnote : _proc. roy. soc. n. s. wales_, vol. xiv., p. .] [footnote : _phil. trans._, vol. i., p. .] [footnote : for indications relative to the early history of the red spot, see holden, _publ. astr. pac. soc._, vol. ii., p. ; noble, _month. not._, vol. xlvii., p. ; a. s. williams, _observatory_, vol. xiii., p. .] [footnote : _astr. and astrophysics_, vol. xi., p. .] [footnote : _month. not._, vol. l., p. .] [footnote : _observatory_, vol. xiii., pp. , .] [footnote : _trans. r. dublin soc._, vol. iv., p. , .] [footnote : _publ. astr. pac. soc._, vol. ii., p. .] [footnote : _astr. and astrophysics_, vol. xi., p. .] [footnote : denning, _knowledge_, vol. xxiii., p. ; _observatory_, vol. xxiv., p. ; _pop. astr._, vol. ix., p. ; _nature_, vol. lv., p. .] [footnote : williams, _observatory_, vol. xxiii., p. .] [footnote : _month. not._, vol. lvi., p. .] [footnote : bélopolsky, _astr. nach._, no. , .] [footnote : _publ. astr. pac. soc._, vol. iv., p. .] [footnote : _bull. astr._, , p. .] [footnote : _month. not._, vol. xxxi., p. .] [footnote : _beobachtungen_, heft ii., p. .] [footnote : _ber. sächs. ges. der wiss._, , p. .] [footnote : _month. not._, vol. lix., p. .] [footnote : _beziehungen der sonnenfleckenperiode_, p. .] [footnote : a. hall, _astr. nach._, no. , .] [footnote : _astr. jour._ (gould's), vol. ii., p. .] [footnote : _ibid._, p. .] [footnote : _on the stability of the motion of saturn's rings_, p. .] [footnote : _mém. de l'ac._, , p. ; montucla, _hist. des math._, t. iv., p. ; _an original theory of the universe_, p. .] [footnote : _comptes rendus_, t. xcviii., p. .] [footnote : proctor, _saturn and its system_ ( ), p. .] [footnote : perrotin, _comptes rendus_, t. cvi., p. .] [footnote : _abhandl. akad. der wiss._, munich, bd. xvi., p. .] [footnote : _smiths. report_, (holden).] [footnote : quoted by dr. e. anding, _astr. nach._, no. , .] [footnote : _astr. and astrophysics_, vol. xi., p. ; _month. not._, vol. l., p. .] [footnote : _astroph. jour._, vol. i., p. .] [footnote : _ibid._, vol. ii., p. .] [footnote : _mém. de l'ac. imp._ (st. petersb.), t. vii., , p. .] [footnote : _astr. nach._, no. , .] [footnote : _washington observations_, app. ii., p. ] [footnote : _month. not._, vol. lvi., p. .] [footnote : t. lewis, _observatory_, vol. xviii., p. .] [footnote : _harper's magazine_, june, .] [footnote : _mém. de l'acad. de montpellier_, t. viii., p. , .] [footnote : _meteoric astronomy_, chap. xii. he carried the subject somewhat farther in . see _observatory_, vol. vi., p. .] [footnote : _astr. nach._, no. , .] [footnote : _amer. jour. of sc._, vol. xiv., p. .] [footnote : _observatory_, vol. xiv., p. .] [footnote : _month. not._, vol. liv., p. .] [footnote : _phil. trans._, vol. lxxxii., p. .] [footnote : _smiths. report_, .] [footnote : _comptes rendus_, t. lxiv., p. .] [footnote : huggins, _proc. r. soc._, vol. xlvi., p. ; keeler, _astr. nach._, no. , ; vogel, _astroph. jour._, vol. i., p. .] [footnote : _month. not._, vol. xxxiii., p. .] [footnote : _astr. nach._, no , .] [footnote : _comptes rendus_, t. xcviii., p. .] [footnote : _comptes rendus_, t. xcviii., pp. , .] [footnote : _v. j. s. astr. ges._, jahrg. xxiv., p. .] [footnote : _publ. astr. pac. soc._, vol. iii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, no. , .] [footnote : _astr. jour._, nos. , .] [footnote : _astr. nach._, no. , .] [footnote : _ann. der phys._, bd. clviii., p. ; _astroph. jour._, vol. i., p. .] [footnote : _astr. nach._, no. , .] [footnote : _month. not._, vol. xlix., p. .] [footnote : astr. nach., no. , ; scheiner's _spectralanalyse_, p. .] [footnote : _month. not._, vol. xliv., p. .] [footnote : _observatory_, vol. vii., pp. , , .] [footnote : _month. not._, vol. xlvi., p. .] [footnote : _comptes rendus_, t. cvii., p. ; _astr. and astroph._, vol. xiii., p. ; _astr. jour._, no. .] [footnote : _astr. jour._, nos. , , .] [footnote : _astr. pop._, p. ; _la nature_, january , .] [footnote : _proc. roy. soc. edinb._, vols. x., p. ; xi., p. .] [footnote : _vierteljahrsschrift. astr. ges._, jahrg. xxi., p. .] [footnote : _proc. roy. soc. edinb._, vol. xxiii., p. ; _nature_, vol. lxiv., p. .] [footnote : _amer. jour. of science_, vol. xx., p. .] chapter ix _theories of planetary evolution_ we cannot doubt that the solar system, as we see it, is the result of some process of growth--that, during innumerable ages, the forces of nature were at work upon its materials, blindly modelling them into the shape appointed for them from the beginning by omnipotent wisdom. to set ourselves to inquire what that process was may be an audacity, but it is a legitimate, nay, an inevitable one. for man's implanted instinct to "look before and after" does not apply to his own little life alone, but regards the whole history of creation, from the highest to the lowest--from the microscopic germ of an alga or a fungus to the visible frame and furniture of the heavens. kant considered that the inquiry into the mode of origin of the world was one of the easiest problems set by nature; but it cannot be said that his own solution of it was satisfactory. he, however, struck out in a track which thought still pursues. in his _allgemeine naturgeschichte_ the growth of sun and planets was traced from the cradle of a vast and formless mass of evenly diffused particles, and the uniformity of their movements was sought to be accounted for by the unvarying action of attractive and repulsive forces, under the dominion of which their development was carried forward. in its modern form, the "nebular hypothesis" made its appearance in .[ ] it was presented by laplace with diffidence, as a speculation unfortified by numerical buttresses of any kind, yet with visible exultation at having, as he thought, penetrated the birth-secret of our system. he demanded, indeed, more in the way of postulates than kant had done. he started with a sun ready made,[ ] and surrounded with a vast glowing atmosphere, extending into space out beyond the orbit of the farthest planet, and endowed with a slow rotatory motion. as this atmosphere or nebula cooled, it contracted; and as it contracted, its rotation, by a well-known mechanical law, became accelerated. at last a point arrived when tangential velocity at the equator increased beyond the power of gravity to control, and equilibrium was restored by the separation of a nebulous ring revolving in the same period as the generating mass. after a time, the ring broke up into fragments, all eventually reunited in a single revolving and rotating body. this was the first and farthest planet. meanwhile the parent nebula continued to shrink and whirl quicker and quicker, passing, as it did so, through successive crises of instability, each resulting in, and terminated by, the formation of a planet, at a smaller distance from the centre, and with a shorter period of revolution than its predecessor. in these secondary bodies the same process was repeated on a reduced scale, the birth of satellites ensuing upon their contraction, or not, according to circumstances. saturn's ring, it was added, afforded a striking confirmation of the theory of annular separation,[ ] and appeared to have survived in its original form in order to throw light on the genesis of the whole solar system; while the four first discovered asteroids offered an example in which the _débris_ of a shattered ring had failed to coalesce into a single globe. this scene of cosmical evolution was a characteristic bequest from the eighteenth century to the nineteenth. it possessed the self-sufficing symmetry and entireness appropriate to the ideas of a time of renovation, when the complexity of nature was little accounted of in comparison with the imperious orderliness of the thoughts of man. since its promulgation, however, knowledge has transgressed many boundaries, and set at naught much ingenious theorising. how has it fared with laplace's sketch of the origin of the world? it has at least not been discarded as effete. the groundwork of speculation on the subject is still furnished by it. it is, nevertheless, admittedly inadequate. of much that exists it gives no account, or an erroneous one. the march of events certainly did not everywhere--even if it did anywhere--follow the exact path prescribed for it. yet modern science attempts to supplement, but scarcely ventures to supersede it. thought has, in many directions, been profoundly modified by mayer's and joule's discovery, in , of the equivalence between heat and motion. its corollary was the grand idea of the "conservation of energy," now one of the cardinal principles of science. this means that, under the ordinary circumstances of observation, the old maxim _ex nihilo nihil fit_ applies to force as well as to matter. the supplies of heat, light, electricity, must be kept up, or the stream will cease to flow. the question of the maintenance of the sun's heat was thus inevitably raised; and with the question of maintenance that of origin is indissolubly connected. dr. julius robert mayer, a physician residing at heilbronn, was the first to apply the new light to the investigation of what sir john herschel had termed the "great secret." he showed that if the sun were a body either simply cooling or in a state of combustion, it must long since have "gone out." had an equal mass of coal been set alight four or five centuries after the building of the pyramid of cheops, and kept burning at such a rate as to supply solar light and heat during the interim, only a few cinders would now remain in lieu of our undiminished glorious orb. mayer looked round for an alternative. he found it in the "meteoric hypothesis" of solar conservation.[ ] the importance in the economy of our system of the bodies known as falling stars was then (in ) beginning to be recognised. it was known that they revolved in countless swarms round the sun; that the earth daily encountered millions of them; and it was surmised that the cone of the zodiacal light represented their visible condensation towards the attractive centre. from the zodiacal light, then, mayer derived the store needed for supporting the sun's radiations. he proved that, by the stoppage of their motion through falling into the sun, bodies would evolve from , to , times as much heat (according to their ultimate velocity) as would result from the burning of equal masses of coal, their precipitation upon the sun's surface being brought about by the resisting medium observed to affect the revolutions of encke's comet. there was, however, a difficulty. the quantity of matter needed to keep, by the sacrifice of its movement, the hearth of our system warm and bright would be very considerable. mayer's lowest estimate put it at , billion kilogrammes per second, or a mass equal to that of our moon bi-annually. but so large an addition to the gravitating power of the sun would quickly become sensible in the movement of the bodies dependent upon him. their revolutions would be notably accelerated. mayer admitted that each year would be shorter than the previous one by a not insignificant fraction of a second, and postulated an unceasing waste of substance, such as newton had supposed must accompany emission of the material corpuscles of light, to neutralise continual reinforcement. mayer's views obtained a very small share of publicity, and owned mr. waterston as their independent author in this country. the meteoric, or "dynamical," theory of solar sustentation was expounded by him before the british association in . it was developed with his usual ability by lord kelvin, in the following year. the inflow of meteorites, he remarked, "is the only one of all conceivable causes of solar heat which we know to exist from independent evidence."[ ] we know it to exist, but we now also know it to be entirely insufficient. the supplies presumed to be contained in the zodiacal light would be quickly exhausted; a constant inflow from space would be needed to meet the demand. but if moving bodies were drawn into the sun at anything like the required rate, the air, even out here at ninety-three millions of miles distance, would be thick with them; the earth would be red-hot from their impacts;[ ] geological deposits would be largely meteoric;[ ] to say nothing of the effects on the mechanism of the heavens. lord kelvin himself urged the inadmissibility of the "extra-planetary" theory of meteoric supply on the very tangible ground that, if it were true, the year would be shorter now, actually by six weeks, than at the opening of the christian era. the "intra-planetary" supply, however, is too scanty to be anything more than a temporary makeshift. the meteoric hypothesis was naturally extended from the maintenance of the sun's heat to the formation of the bodies circling round him. the earth--no less doubtless than the other planets--is still growing. cosmical matter in the shape of falling stars and aërolites, to the amount, adopting professor newton's estimate, of tons daily, is swept up by it as it pursues its orbital round. inevitably the idea suggested itself that this process of appropriation gives the key to the life-history of our globe, and that the momentary streak of fire in the summer sky represents a feeble survival of the glowing hailstorm by which in old times it was fashioned and warmed. mr. e. w. brayley supported this view of planetary production in ,[ ] and it has recommended itself to haidinger, helmholtz, proctor, and faye. but the negative evidence of geological deposits appears fatal to it. the theory of solar energy now generally regarded as the true one was enounced by helmholtz in a popular lecture in . it depends upon the same principle of the equivalence of heat and motion which had suggested the meteoric hypothesis. but here the movement surrendered and transformed belongs to the particles, not of any foreign bodies, but of the sun itself. drawn together from a wide ambit by the force of their own gravity, their fall towards the sun's centre must have engendered a vast thermal store, of which / are computed to be already spent. presumably, however, this stream of reinforcement is still flowing. in the very act of parting with heat, the sun develops a fresh stock. his radiations, in short, are the direct result of shrinkage through cooling. a diminution of the solar diameter by feet yearly would just suffice to cover the present rate of emission, and would for ages remain imperceptible with our means of observation, since, after the lapse of , years, the lessening of angular size would scarcely amount to one second.[ ] but the process, though not terminated, is strictly a terminable one. in less than five million years, the sun will have contracted to half its present bulk. in seven million more, it will be as dense as the earth. it is difficult to believe that it will then be a luminous body.[ ] nor can an unlimited past duration be admitted. helmholtz considered that radiation might have gone on with its actual intensity for twenty-two, langley allows only eighteen million years. the period can scarcely be stretched, by the most generous allowances, to double the latter figure. but this is far from meeting the demands of geologists and biologists. an attempt was made in to supply the sun with machinery analogous to that of a regenerative furnace, enabling it to consume the same fuel over and over again, and so to prolong indefinitely its beneficent existence. the inordinate "waste" of energy, which shocks our thrifty ideas, was simultaneously abolished. the earth stops and turns variously to account one , -millionth part of the solar radiations; each of the other planets and satellites takes a proportionate share; the rest, being all but an infinitesmal fraction of the whole, is dissipated through endless space, to serve what purpose we know not. now, on the late sir william siemens's plan, this reckless expenditure would cease; the solar incomings and outgoings would be regulated on approved economic principles, and the inevitable final bankruptcy would be staved off to remote ages. but there was a fatal flaw in its construction. he imagined a perpetual circulation of combustible materials, alternately surrendering and regaining chemical energy, the round being kept going by the motive force of the sun's rotation.[ ] this, however, was merely to perch the globe upon a tortoise, while leaving the tortoise in the air. the sun's rotation contains a certain definite amount of mechanical power--enough, according to lord kelvin, if directly converted into heat, to keep up the sun's emission during years and six days--a mere moment in cosmical time. more economically applied, it would no doubt go farther. its exhaustion would, nevertheless, under the most favourable circumstances, ensue in a comparatively short period.[ ] many other objections equally unanswerable have been urged to the "regenerative" hypothesis, but this one suffices. dr. croll's collision hypothesis[ ] is less demonstrably unsound, but scarcely less unsatisfactory. by the mutual impact of two dark masses rushing together with tremendous speed, he sought to provide the solar nebula with an immense _original_ stock of heat for the reinforcement of that subsequently evolved in the course of its progressive contraction. the sun, while still living on its capital, would thus have a larger capital to live on, and the time-demands of the less exacting geologists and biologists might be successfully met. but the primitive event, assumed for the purpose of dispensing them from the inconvenience of "hurrying up their phenomena," is not one that a sane judgment can readily admit to have ever, in point of actual fact, happened. there remains, then, as the only intelligible rationale of solar sustentation, helmholtz's shrinkage theory. and this has a very important bearing upon the nebular view of planetary formation; it may, in fact, be termed its complement. for it involves the idea that the sun's materials, once enormously diffused, gradually condensed to their present volume with development of heat and light, and, it may plausibly be added, with the separation of dependent globes. the data furnished by spectrum analysis, too, favour the supposition of a common origin for sun and planets by showing their community of substance; while gaseous nebulæ present examples of vast masses of tenuous vapour, such as our system may plausibly be conjectured to have primitively sprung from. but recent science raises many objections to the details, if it supplies some degree of confirmation to the fundamental idea of laplace's cosmogony. the detection of the retrograde movement of neptune's satellite made it plain that the anomalous conditions of the uranian world were due to no extraordinary disturbance, but to a systematic variety of arrangement at the outskirts of the solar domain. so that, were a trans-neptunian planet discovered, we should be fully prepared to find it rotating, and surrounded by satellites circulating from east to west. the uniformity of movement, upon the probabilities connected with which the french geometer mainly based his scheme, thus at once vanishes. the excessively rapid revolution of the inner martian moon is a further stumbling-block. on laplace's view, _no_ satellite can revolve in a shorter time than its primary rotates; for in its period of circulation survives the period of rotation of the parent mass which filled the sphere of its orbit at the time of giving it birth. and rotation quickens as contraction goes on; therefore, the older time of axial rotation should invariably be the longer. this obstacle can, however, as we shall presently see, be turned. more serious is one connected with the planetary periods, pointed out by babinet in .[ ] in order to make them fit in with the hypothesis of successive separation from a rotating and contracting body, certain arbitrary assumptions have to be made of fluctuations in the distribution of the matter forming that body at the various epochs of separation.[ ] such expedients usually merit the distrust which they inspire. primitive and permanent irregularities of density in the solar nebula, such as miss young's calculations suggest,[ ] do not, on the other hand, appear intrinsically improbable. again, it was objected by professor kirkwood in [ ] that there could be no sufficient cohesion in such an enormously diffused mass as the planets are supposed to have sprung from to account for the wide intervals between them. the matter separated through the growing excess of centrifugal speed would have been cast off, not by rarely recurring efforts, but continually, fragmentarily, _pari passu_ with condensation and acceleration. each wisp of nebula, as it found itself unduly hurried, would have declared its independence, and set about revolving and condensing on its own account. the result would have been a meteoric, not a planetary system. moreover, it is a question whether the relative ages of the planets do not follow an order just the reverse of that concluded by laplace. professor newcomb holds the opinion that the rings which eventually constituted the planets divided from the main body of the nebula almost simultaneously, priority, if there were any, being on the side of the inner and smaller ones;[ ] while in m. faye's cosmogony,[ ] the retrograde motion of the systems formed by the two outer planets is ascribed--on grounds, it is true, of dubious validity--to their comparatively late origin. this ingenious scheme was designed, not merely to complete, but to supersede that of laplace, which, undoubtedly, through the inclusion by our system of oppositely directed rotations, forfeits its claim simply and singly to account for the fundamental peculiarities of its structure. m. faye's leading contention is that, under the circumstances assumed by laplace, not the two outer planets alone, but the whole company must have been possessed of retrograde rotation. for they were formed--_ex hypothesi_--after the sun; central condensation had reached an advanced stage when the rings they were derived from separated; the principle of inverse squares consequently held good, and kepler's laws were in full operation. now, particles circulating in obedience to these laws can only--since their velocity decreases outward from the centre of attraction--coalesce into a globe with a _backward_ axial movement. nor was laplace blind to this flaw in his theory; but his effort to remove it, though it passed muster for the best part of a century,[ ] was scarcely successful. his planet-forming rings were made to rotate _all in one piece_, their outer parts thus necessarily travelling at a swifter linear rate than their inner parts, and eventually uniting, equally of necessity, into a _forward_-spinning body. the strength of cohesion involved may, however, safely be called impossible, especially when it is considered that nebulous materials were in question. the reform proposed by m. faye consists in admitting that all the planets inside uranus are of pre-solar origin--that they took globular form in the bosom of a nearly homogeneous nebula, revolving in a single period, with motion accelerated from centre to circumference, and hence agglomerating into masses with a direct rotation. uranus and neptune owe their exceptional characteristics to their later birth. when they came into existence, the development of the sun was already far advanced, central force had acquired virtually its present strength, unity of period had been abolished by its predominance, and motion was retarded outward. thus, what we may call the relative chronology of the solar system is thrown once more into confusion. the order of seniority of the planets is now no easier to determine than the "who first, who last?" among the victims of hector's spear. for m. faye's arrangements, notwithstanding the skill with which he has presented them, cannot be unreservedly accepted. the objections to them, thoughtfully urged by m. c. wolf[ ] and professor darwin,[ ] are grave. not the least so is his omission to take account of an agency of change presently to be noticed. a further valuable discussion of the matter was published by m. du ligondès in .[ ] his views are those of faye, modified to disarm the criticisms they had encountered; and special attention may be claimed for his weighty remark that each planet has a life-history of its own, essentially distinct from those of the others, and, despite original unity, not to be confounded with them. the drift of recent investigations seems, indeed, to be to find the embryonic solar system already potentially complete in the parent nebula, like the oak in an acorn, and to relegate detailed explanations of its peculiarities to the dim pre-nebular fore-time. we now come to a most remarkable investigation--one, indeed, unique in its profession to lead us back with mathematical certainty towards the origin of a heavenly body. we refer to professor darwin's inquiries into the former relations of the earth and moon.[ ] they deal exclusively with the effects of tidal friction, and primarily with those resulting, not from oceanic, but from "bodily" tides, such as the sun and moon must have raised in past ages on a liquid or viscous earth. the immediate effect of either is, as already explained, to destroy the rotation of the body on which the tide is raised, as regards the tide-raising body, bringing it to turn always the same face towards its disturber. this, we can see, has been completely brought about in the case of the moon. there is, however, a secondary or reactive effect. action is always mutual. precisely as much as the moon pulls the terrestrial tidal wave backward, the tidal wave pulls the moon forward. but pulling a body forward in its orbit implies the enlargement of that orbit; in other words, the moon is, as a consequence of tidal friction, very slowly receding from the earth. this will go on (other circumstances remaining unchanged) until the lengthening day overtakes the more tardily lengthening month, when each will be of about , hours.[ ] a position of what we may call tidal equilibrium between earth and moon will (apart from disturbance by other bodies) then be attained. if, however, it be true that, in the time to come, the moon will be much farther from us, it follows that in the time past she was much nearer to us than she now is. tracing back her history by the aid of professor darwin's clue, we at length find her revolving in a period of somewhere between two and four hours, almost in contact with an earth rotating just at the same rate. this was before tidal friction had begun its work of grinding down axial velocity and expanding orbital range. but the position was not one of stable equilibrium. the slightest inequality must have set on foot a series of uncompensated changes. if the moon had whirled the least iota faster than the earth spun she must have been precipitated upon it. her actual existence shows that the trembling balance inclined the other way. by a second or two to begin with, the month exceeded the day; the tidal wave crept ahead of the moon; tidal friction came into play, and our satellite started on its long spiral journey outward from the parent globe. this must have occurred, it is computed, _at least_ fifty-four million years ago. that this kind of tidal reactive effect played its part in bringing the moon into its present position, and is still, to some slight extent, at work in changing it, there can be no doubt whatever. an irresistible conjecture carried the explorer of its rigidly deducible consequences one step beyond them. the moon's time of revolution, when so near the earth as barely to escape contact with it, must have been, by kepler's law, more than two and less than two and a half hours. now it happens that the most rapid rate of rotation of a fluid mass of the earth's average density, consistent with spheroidal equilibrium, is two hours and twenty minutes. quicken the movement but by one second and the globe must fly asunder. hence the inference that the earth actually _did_ fly asunder through over-fast spinning, the ensuing disruption representing the birth-throes of the moon. it is likely that the event was hastened or helped by solar tidal disturbance. to recapitulate. analysis tracks backward the two bodies until it leaves them in very close contiguity, one rotating and the other revolving in approximately the same time, and that time certainly not far different from, and quite possibly identical with, the critical period of instability for the terrestrial spheroid. "is this," professor darwin asks, "a mere coincidence, or does it not rather point to the break-up of the primeval planet into two masses in consequence of a too rapid rotation?"[ ] we are tempted, but are not allowed to give an unqualified assent. mr. james nolan of victoria has made it clear that the moon could not have subsisted as a continuous mass under the powerful disruptive strain which would have acted upon it when revolving almost in contact with the present surface of the earth; and professor darwin, admitting the objection, concedes to our satellite, in its initial stage, the alternative form of a flock of meteorites.[ ] but such a congregation must have been quickly dispersed, by tidal action, into a meteoric ring. the same investigator subsequently fixed , miles from centre to centre as the minimum distance at which the moon could have revolved in its entirety; and he concluded it "necessary to suppose that, after the birth of a satellite, if it takes place at all in this way, a series of changes occur which are quite unknown."[ ] the evidence, however, for the efficiency of tidal friction in bringing about the actual configuration of the lunar-terrestrial system is not invalidated by this failure to penetrate its natal mystery. under its influence the principal elements of that system fall into interdependent mutual relations. it connects, casually and quantitatively, the periods of the moon's revolution and of the earth's rotation, the obliquity of the ecliptic, the inclination and eccentricity of the lunar orbit. all this can scarcely be accidental. professor darwin's first researches on this subject were communicated to the royal society, december , . they were followed, january , ,[ ] by an inquiry on the same principles into the earlier condition of the entire solar system. the results were a warning against hasty generalisation. they showed that the lunar-terrestrial system, far from being a pattern for their development, was a singular exception among the bodies swayed by the sun. its peculiarity resides in the fact that the moon is _proportionately_ by far the most massive attendant upon any known planet. its disturbing power over its primary is thus abnormally great, and tidal friction has, in consequence, played a predominant part in bringing their mutual relations into their present state. the comparatively late birth of the moon tends to ratify this inference. the dimensions of the earth did not differ (according to our present authority) very greatly from what they now are when her solitary offspring came, somehow, into existence. this is found not to have been the case with any other of the planets. it is unlikely that the satellites of jupiter, saturn, or mars (we may safely add, of uranus or neptune) ever revolved in much narrower orbits than those they now traverse; it is practically certain that they did not, like our moon, originate very near the _present_ surfaces of their primaries.[ ] what follows? the tide-raising power of a body grows with vicinity in a rapidly accelerated ratio. lunar tides must then have been on an enormous scale when the moon swung round at a fraction of its actual distance from the earth. but no other satellite with which we are acquainted occupied at any time a corresponding position. hence no other satellite ever possessed tide-raising capabilities in the least comparable to those of the moon. we conclude once more that tidal friction had an influence here very different from its influence elsewhere. quite possibly, however, that influence may be more nearly spent than in less advanced combinations of revolving globes. mr. nolan concluded in [ ] that it still retains appreciable efficacy in the several domains of the outer planets. the moons of jupiter and saturn are, by his calculations, in course of sensible retreat, under compulsion of the perennial ripples raised by them on the surfaces of their gigantic primaries. he thus connects the interior position of the fifth jovian satellite with its small mass. the feebleness of its tide-raising power obliged it to remain behind its companions; for there is no sign of its being more juvenile than the galilean quartette. the yielding of plastic bodies to the strain of unequal attractions is a phenomenon of far-reaching consequence. we know that the sun as well as the moon causes tides in our oceans. there must, then, be solar, no less than lunar, tidal friction. the question at once arises: what part has it played in the development of the solar system? has it ever been one of leading importance, or has its influence always been, as it now is, subordinate, almost negligible? to this, too, professor darwin supplies an answer. it can be stated without hesitation that the sun did _not_ give birth to the planets, as the earth has been supposed to have given birth to the moon, by the disruption of its already condensed, though viscous and glowing mass, pushing them then gradually backward from its surface into their present places. for the utmost possible increase in the length of the year through tidal friction is one hour; and five minutes is a more probable estimate.[ ] so far as the pull of tide-waves raised on the sun by the planets is concerned, then, the distances of the latter have never been notably different from what they now are; though that cause may have converted the paths traversed by them from circles into ellipses. over their _physical_ history, however, it was probably in a large measure influential. the first vital issue for each of them was--satellites or no satellites? were they to be governors as well as governed, or should they revolve in sterile isolation throughout the æons of their future existence? here there is strong reason to believe that solar tidal friction was the overruling power. it is remarkable that planetary fecundity increases--at least so far outward as saturn--with distance from the sun. can these two facts be in any way related? in other words, is there any conceivable way by which tidal influence could prevent or impede the throwingoff of secondary bodies? we have only to think for a moment in order to see that this is precisely one of its direct results.[ ] tidal friction, whether solar or lunar, tends to reduce the axial movement of the body it acts upon. but the separation of satellites depends--according to the received view--upon the attainment of a disruptive rate of rotation. hence, if solar tidal friction were strong enough to keep down the pace below this critical point, the contracting mass would remain intact--there would be no satellite-production. this, in all probability, actually occurred in the case both of mercury and venus. they cooled without dividing, because the solar friction-brake applied to them was too strong to permit acceleration to pass the limit of equilibrium. the complete destruction of their relative axial movement has been rendered probable by recent observations; and that the process went on rapidly is a reasonable further inference. the earth barely escaped the fate of loneliness incurred by her neighbours. her first and only epoch of instability was retarded until she had nearly reached maturity. the late appearance of the moon accounts for its large relative size--through the increased cohesion of an already strongly condensed parent mass--and for the distinctive peculiarities of its history and influence on the producing globe. solar tidal friction, although it did not hinder the formation of two minute dependents of mars, has been invoked to explain the anomalously rapid revolution of one of them. phobos, we have seen, completes more than three revolutions while mars rotates once. but this was probably not always so. the two periods were originally nearly equal. the difference, it is alleged, was brought about by tidal waves raised by the sun on the semi-fluid spheroid of mars. rotatory velocity was thereby destroyed, the martian day slowly lengthened, and, as a secondary consequence, the period of the inner satellite, become shorter than the augmented day, began progressively to diminish. so that phobos, unlike our moon, was in the beginning farther from its primary than now. but here again mr. nolan entered a _caveat_. applying the simple test of numerical evaluation, he showed that before solar tidal friction could lengthen the rotation-period of mars by so much as one minute, phobos should have been precipitated upon its surface.[ ] for the enormous disparity of mass between it and the sun is so far neutralised by the enormous disparity in their respective distances from mars that solar tidal force there is only fifty times that of the little satellite. but the tidal effects of a satellite circulating quicker than its primary rotates exactly reverse those of one moving, like our moon, comparatively slowly, so that the tides raised by phobos tend to _shorten_ both periods. its orbital momentum, however, is so extremely small in proportion to the rotational momentum of mars, that any perceptible inroad upon the latter is attended by a lavish and ruinous expenditure of the former. it is as if a man owning a single five-pound note were to play for equal stakes with a man possessing a million. the bankruptcy sure to ensue is typified by the coming fate of the martian inner satellite. the catastrophe of its fall needs to bring it about only a very feeble reactive pull compared with the friction which the sun should apply in order to protract the martian day by one minute. and from the proportionate strength of the forces at work, it is quite certain that one result cannot take place without the other. nor can things have been materially different in the past; hence the idea must be abandoned that the primitive time of rotation of mars survives in the period of its inner satellite. the anomalous shortness of the latter may, however, in m. wolf's opinion,[ ] be explained by the "traînées elliptiques" with which roche supplemented nebular annulation.[ ] these are traced back to the descent of separating strata from the _shoulders_ of the great nebulous spheroid towards its equatorial plane. their rotational velocity being thus relatively small, they formed "inner rings," very much nearer to the centre of condensation than would have been possible on the unmodified theory of laplace. phobos might, in this view, be called a polar offset of mars; and the rings of saturn are thought to own a similar origin. outside the orbit of mars, solar tidal friction can scarcely be said to possess at present any sensible power. but it is far from certain that this was always so. it seems not unlikely that its influence was the overruling one in determining the direction of planetary rotation. m. faye, as we have seen, objected to laplace's scheme that only retrograde secondary systems could be produced by it. in this he was anticipated by kirkwood, who, however, supplied an answer to his own objection.[ ] sun-raised tides must have acted with great power on the diffused masses of the embryo planets. by their means they doubtless very soon came to turn (in lunar fashion) the same hemisphere always towards their centre of motion. this amounts to saying that even if they started with retrograde rotation, it was, by solar tidal friction, quickly rendered direct.[ ] for it is scarcely necessary to point out that a planet turning an invariable face to the sun rotates in the same direction in which it revolves, and in the same period. as, with the progress of condensation, tides became feebler and rotation more rapid, the accelerated spinning necessarily proceeded in the sense thus prescribed for it. hence the backward axial movements of uranus and neptune may very well be a survival, due to the inefficiency of solar tides at their great distance, of a state of things originally prevailing universally throughout the system. the general outcome of mr. darwin's researches has been to leave laplace's cosmogony untouched. he concludes nothing against it, and, what perhaps tells with more weight in the long run, has nothing to substitute for it. in one form or the other, if we speculate at all on the development of the planetary system, our speculations are driven into conformity with the broad lines of the nebular hypothesis--to the extent, at least, of admitting an original material unity and motive uniformity. but we can see now, better than formerly, that these supply a bare and imperfect sketch of the truth. we should err gravely were we to suppose it possible to reconstruct, with the help of any knowledge our race is ever likely to possess, the real and complete history of our admirable system. "the subtlety of nature," bacon says, "transcends in many ways the subtlety of the intellect and senses of man." by no mere barren formula of evolution, indiscriminately applied all round, the results we marvel at, and by a fragment of which our life is conditioned, were brought forth; but by the manifold play of interacting forces, variously modified and variously prevailing, according to the local requirements of the design they were appointed to execute. footnotes: [footnote : _exposition du système du monde_, t. ii., p. .] [footnote : in later editions a retrospective clause was added admitting a prior condition of all but evanescent nebulosity.] [footnote : _méc. cél._, lib. xiv., ch. iii.] [footnote : _beiträge zur dynamik des himmels_, p. .] [footnote : _trans. roy. soc. of edinburgh_, vol. xxi., p. .] [footnote : newcomb, _pop. astr._, p. ( nd ed.).] [footnote : m. williams, _nature_, vol. iii., p. .] [footnote : _comp. brit. almanac_, p. .] [footnote : radau, _bull. astr._, t. ii., p. .] [footnote : newcomb, _pop. astr._, pp. - .] [footnote : _proc. roy. soc._, vol. xxxiii., p. .] [footnote : to this hostile argument, as urged by mr. e. douglas archibald, sir w. siemens opposed the increase of rotative velocity through contraction (_nature_, vol. xxv., p. ). but contraction cannot restore lost momentum.] [footnote : _stellar evolution, and its relations to geological time_, .] [footnote : _comptes rendus_, t. lii., p. . see also kirkwood, _observatory_, vol. iii., p. .] [footnote : fouché, _comptes rendus_, t. xcix., p. .] [footnote : _astroph. jour._, vol. xiii., p. .] [footnote : _month. not._, vol. xxix., p. .] [footnote : _pop. astr._, p. .] [footnote : _sur l'origine du monde_, .] [footnote : kirkwood adverted to it in , _am. jour._, vol. xxxviii., p. .] [footnote : _bull. astr._, t. ii.] [footnote : _nature_, vol. xxxi., p. .] [footnote : _formation mécanique du système du monde; bull. astr._, t. xiv., p. (o. callandreau). see also, _le problème solaire_, by l'abbé th. moreux, .] [footnote : _phil. trans._, vol. clxxi., p. .] [footnote : mr. j. nolan has pointed out (_nature_, vol. xxxiv., p. ) that the length of the equal day and month will be reduced to about , hours by the effects of _solar_ tidal friction.] [footnote : _phil. trans._, vol. clxxi., p. .] [footnote : _nature_, vol. xxxiii., p. ; see also nolan, _ibid._, vol. xxxiv., p. .] [footnote : _phil. trans._, vol. clxxviii., p. .] [footnote : _ibid._, vol. clxxii., p. .] [footnote : _ibid._, p. .] [footnote : _satellite evolution_, melbourne, ; _knowledge_, vol. xviii., p. .] [footnote : _phil. trans._, vol. clxxii., p. .] [footnote : this was perceived by m. ed. roche in . _mém. de l'acad. des sciences de montpellier_, t. viii., p. .] [footnote : _nature_, vol. xxxiv., p. .] [footnote : _bull. astr._, t. ii., p. .] [footnote : _montpellier méms._, t. viii., p. .] [footnote : _amer. jour._, vol. xxxviii. ( ), p. .] [footnote : wolf, _bull. astr._, t. ii., p. .] chapter x _recent comets_ on the nd of june, , giambattista donati discovered at florence a feeble round nebulosity in the constellation leo, about one-tenth the diameter of the full moon. it proved to be a comet approaching the sun. but it changed little in apparent place or brightness for some weeks. the gradual development of a central condensation of light was the first symptom of coming splendour. at harvard, in the middle of july, a strong stellar nucleus was seen; on august a tail began to be thrown out. as the comet wanted still over six weeks of the time of its perihelion-passage, it was obvious that great things might be expected of it. they did not fail of realisation. not before the early days of september was it generally recognised with the naked eye, though it had been detected without a glass at pulkowa, august . but its growth was thenceforward surprisingly rapid, as it swept with accelerated motion under the hindmost foot of the great bear, and past the starry locks of berenice. a sudden leap upward in lustre was noticed on september , when the nucleus shone with about the brightness of the pole-star, and the tail, notwithstanding large foreshortening, could be traced with the lowest telescopic power over six degrees of the sphere. the appendage, however, attained its full development only after perihelion, september , by which time, too, it lay nearly square to the line of sight from the earth. on october it stretched in a magnificent scimitar-like curve over a third and upwards of the visible hemisphere, representing a real extension in space of fifty-four million miles. but the most striking view was presented on october , when the brilliant star arcturus became involved in the brightest part of the tail, and during many hours contributed, its lustre undiminished by the interposed nebulous screen, to heighten the grandeur of the most majestic celestial object of which living memories retain the impress. donati's comet was, according to admiral smyth's testimony,[ ] outdone "as a mere _sight_-object" by the great comet of ; but what it lacked in splendour, it surely made up in grace, and variety of what we may call "scenic" effects. some of these were no less interesting to the student than impressive to the spectator. at pulkowa, on the th september, winnecke,[ ] the first director of the strasburg observatory, observed a faint outer envelope resembling a veil of almost evanescent texture flung somewhat widely over the head. next evening, the first of the "secondary" tails appeared, possibly as part of the same phenomenon. this was a narrow straight ray, forming a tangent to the strong curve of the primary tail, and reaching to a still greater distance from the nucleus. it continued faintly visible for about three weeks, during part of which time it was seen in duplicate. for from the chief train itself, at a point where its curvature abruptly changed, issued, as if through the rejection of some of its materials, a second beam nearly parallel to the first, the rigid line of which contrasted singularly with the softly diffused and waving aspect of the plume of light from which it sprang. olbers's theory of unequal repulsive forces was never more beautifully illustrated. the triple tail seemed a visible solar analysis of cometary matter. the processes of luminous emanation going on in this body forcibly recalled the observations made on the comets of and . from the middle of september, the nucleus, estimated by bond to be under five hundred miles in diameter, was the centre of action of the most energetic kind. seven distinct "envelopes" were detached in succession from the nebulosity surrounding the head, and after rising towards the sun during periods of from four to seven days, finally cast their material backward to form the right and left branches of the great train. the separation of these by an obscure axis--apparently as black, quite close up to the nucleus, as the sky--indicated for the tail a hollow, cone-like structure;[ ] while the repetition of certain spots and rays in the same corresponding situation on one envelope after another served to show that the nucleus--to some local peculiarity of which they were doubtless due--had no proper rotation, but merely shifted sufficiently on an axis to preserve the same aspect towards the sun as it moved round it.[ ] this observation of bond's was strongly confirmatory of bessel's hypothesis of opposite polarities in such bodies' opposite sides. the protrusion towards the sun, on september , of a brilliant luminous fan-shaped sector completed the resemblance to halley's comet. the appearance of the head was now somewhat that of a "bat's-wing" gaslight. there were, however, no oscillations to and fro, such as bessel had seen and speculated upon in . as the size of the nucleus contracted with approach to perihelion, its intensity augmented. on october , it outshone arcturus, and for a week or ten days was a conspicuous object half an hour after sunset. its lustre--setting aside the light derived from the tail--was, at that date, , times what it had been on june , though _theoretically_--taking into account, that is, only the differences of distance from sun and earth--it should have been only / of that amount. here, it might be thought, was convincing evidence of the comet itself becoming ignited under the growing intensity of the solar radiations. yet experiments with the polariscope were interpreted in an adverse sense, and bond's conclusion that the comet sent us virtually unmixed reflected sunshine was generally acquiesced in. it was, nevertheless, negatived by the first application of the spectroscope to these bodies. very few comets have been so well or so long observed as donati's. it was visible to the naked eye during days; it was telescopically discernible for , the last observation having been made by mr. william mann at the cape of good hope, march , . its course through the heavens combined singularly with the orbital place of the earth to favour curious inspection. the tail, when near its greatest development, lost next to nothing by the effects of perspective, and at the same time lay in a plane sufficiently inclined to the line of sight to enable it to display its exquisite curves to the greatest advantage. even the weather was, on both sides of the atlantic, propitious during the period of greatest interest, and the moon as little troublesome as possible. the volume compiled by the younger bond is a monument to the care and skill with which these advantages were turned to account. yet this stately apparition marked no turning-point in the history of cometary science. by its study knowledge was indeed materially advanced, but along the old lines. no quick and vivid illumination broke upon its path. quite insignificant objects--as we have already partly seen--have often proved more vitally instructive. donati's comet has been identified with no other. its path is an immensely elongated ellipse, lying in a plane far apart from that of the planetary movements, carrying it at perihelion considerably within the orbit of venus, and at aphelion out into space to - / times the distance from the sun of neptune. the entire circuit occupies over , years, and is performed in a retrograde direction, or against the order of the signs. before its next return, about the year a.d., the enigma of its presence and its purpose may have been to some extent--though we may be sure not completely--penetrated. on june , , the earth passed, for the second time in the century, through the tail of a great comet. some of our readers may remember the unexpected disclosure, on the withdrawal of the sun below the horizon on that evening, of an object so remarkable as to challenge universal attention. a golden-yellow planetary disc, wrapt in dense nebulosity, shone out while the june twilight of these latitudes was still in its first strength. the number and complexity of the envelopes surrounding the head produced, according to the late mr. webb,[ ] a magnificent effect. portions of six distinct emanations were traceable. "it was as though a number of light, hazy clouds were floating round a miniature full moon." as the sky darkened the tail emerged to view.[ ] although in brightness and sharpness of definition it could not compete with the display of , its dimensions proved to be extraordinary. it reached upwards beyond the zenith when the head had already set. by some authorities its extreme length was stated at °, and it showed no trace of curvature. most remarkable, however, was the appearance of two widely divergent rays, each pointing towards the head, though cut off from it by sky-illumination, of which one was seen by mr. webb, and both by mr. williams at liverpool, a quarter of an hour before midnight. there seems no doubt that webb's interpretation was the true one, and that these beams were, in fact, "the perspective representation of a conical or cylindrical tail, hanging closely above our heads, and probably just being lifted up out of our atmosphere."[ ] the cometary train was then rapidly receding from the earth, so that the sides of the "outspread fan" of light shown by it when we were right in the line of its axis must have appeared (as they did) to close up in departure. the swiftness with which the visually opened fan shut proved its vicinity; and, indeed, mr. hind's calculations showed that we were not so much near as actually within its folds at that very time. already m. liais, from his observations at rio de janeiro, june to , and mr. tebbutt, by whom the comet was discovered in new south wales on may , had anticipated such an encounter, while the former subsequently proved that it must have occurred in such a way as to cause an immersion of the earth in cometary matter to a depth of , miles.[ ] the comet then lay between the earth and the sun at a distance of about fourteen million miles from the former; its tail stretched outward just along the line of intersection of its own with the terrestrial orbit to an extent of fifteen million miles; so that our globe, happening to pass at the time, found itself during some hours involved in the flimsy appendage. no perceptible effects were produced by the meeting; it was known to have occurred by theory alone. a peculiar glare in the sky, thought by some to have distinguished the evening of june , was, at best, inconspicuous. nor were there any symptoms of unusual electric excitement. the greenwich instruments were, indeed, disturbed on the following night, but it would be rash to infer that the comet had art or part in their agitation. the perihelion-passage of this body occurred june , ; and its orbit has been shown by m. kreutz of bonn, from a very complete investigation founded on observations extending over nearly a year, to be an ellipse traversed in a period of years.[ ] towards the end of august, , a comet became visible to the naked eye high up in the northern hemisphere, with a nucleus equalling in brightness the lesser stars of the plough and a feeble tail ° in length. it thus occupied quite a secondary position among the members of its class. it was, nevertheless, a splendid object in comparison with a telescopic nebulosity discovered by tempel at marseilles, december , . this, the sole comet of , slipped past perihelion, january , without pomp of train or other appendages, and might have seemed hardly worth the trouble of pursuing. fortunately, this was not the view entertained by observers and computers; since upon the knowledge acquired of the movements of these two bodies has been founded one of the most significant discoveries of modern times. the first of them is now styled the comet ( iii.) of the august meteors, the second ( i.) that of the november meteors. the steps by which this curious connection came to be ascertained were many, and were taken in succession by a number of individuals. but the final result was reached by schiaparelli of milan, and remains deservedly associated with his name. the idea prevalent in the eighteenth century as to the nature of shooting stars was that they were mere aerial _ignes fatui_--inflammable vapours accidentally kindled in our atmosphere. but halley had already entertained the opinion of their cosmical origin; and chladni in formally broached the theory that space is filled with minute circulating atoms, which, drawn by the earth's attraction, and ignited by friction in its gaseous envelope, produce the luminous effects so frequently witnessed.[ ] acting on his suggestion, brandes and benzenberg, two students at the university of göttingen, began in to determine the heights of falling stars by simultaneous observations at a distance. they soon found that they move with planetary velocities in the most elevated regions of our atmosphere, and by the ascertainment of this fact laid a foundation of distinct knowledge regarding them. some of the data collected, however, served only to perplex opinion, and even caused chladni temporarily to renounce his. many high authorities, headed by laplace in , declared for the lunar-volcanic origin of meteorites; but thought on the subject was turbid, and inquiry seemed only to stir up the mud of ignorance. it needed one of those amazing spectacles, at which man assists, no longer in abject terror for his own frail fortunes, but with keen curiosity and the vivid expectation of new knowledge, to bring about a clarification. on the night of november - , , a tempest of falling stars broke over the earth. north america bore the brunt of its pelting. from the gulf of mexico to halifax, until daylight with some difficulty put an end to the display, the sky was scored in every direction with shining tracks and illuminated with majestic fireballs. at boston the frequency of meteors was estimated to be about half that of flakes of snow in an average snowstorm. their numbers, while the first fury of their coming lasted, were quite beyond counting; but as it waned, a reckoning was attempted, from which it was computed, on the basis of that much diminished rate, that , must have been visible during the nine hours they continued to fall.[ ] now there was one very remarkable feature common to the innumerable small bodies which traversed, or were consumed in our atmosphere that night. _they all seemed to come from the same part of the sky._ traced backward, their paths were invariably found to converge to a point in the constellation leo. moreover, that point travelled with the stars in their nightly round. in other words, it was entirely independent of the earth and its rotation. it was a point in inter-planetary space. the _effective_ perception of this fact[ ] amounted to a discovery, as olmsted and twining, who had "simultaneous ideas" on the subject, were the first to realize. denison olmsted was then professor of mathematics in yale college. he showed early in [ ] that the emanation of the showering meteors from a fixed "radiant" proved their approach to the earth along nearly parallel lines, appearing to diverge by an effect of perspective; and that those parallel lines must be sections of orbits described by them round the sun and intersecting that of the earth. for the november phenomenon was now seen to be a periodical one. on the same night of the year , although with less dazzling and universal splendour than in america in , it had been witnessed over great part of europe and in arabia. olmsted accordingly assigned to the cloud of cosmical particles (or "comet," as he chose to call it), by terrestrial encounters with which he supposed the appearances in question to be produced, a period of about days; its path a narrow ellipse, meeting, near its farthest end from the sun, the place occupied by the earth on november . once for all, then, as the result of the star-fall of , the study of luminous meteors became an integral part of astronomy. their membership of the solar system was no longer a theory or a conjecture--it was an established fact. the discovery might be compared to, if it did not transcend in importance, that of the asteroidal group. "c'est un nouveau monde planétaire," arago wrote,[ ] "qui commence à se révéler à nous." evidences of periodicity continued to accumulate. it was remembered that humboldt and bonpland had been the spectators at cumana, after midnight on november , , of a fiery shower little inferior to that of , and reported to have been visible from the equator to greenland. moreover, in and some subsequent years, there were waning repetitions of the display, as if through the gradual thinning-out of the meteoric supply. the extreme irregularity of its distribution was noted by olbers in , who conjectured that we might have to wait until to see the phenomenon renewed on its former scale of magnificence.[ ] this was the first hint of a thirty-three or thirty-four year period. the falling stars of november did not alone attract the attention of the learned. similar appearances were traditionally associated with august by the popular phrase in which they figured as "the tears of st. lawrence." but the association could not be taken on trust from mediæval authority. it had to be proved scientifically, and this quetelet of brussels succeeded in doing in december, .[ ] a second meteoric revolving system was thus shown to exist. but its establishment was at once perceived to be fatal to the "cosmical cloud" hypothesis of olmsted. for if it be a violation of probability to attribute to one such agglomeration a period of an exact year, or sub-multiple of a year, it would be plainly absurd to suppose the movements of _two_ or more regulated by such highly artificial conditions. an alternative was proposed by adolf erman of berlin in .[ ] no longer in _clouds_, but in closed _rings_, he supposed meteoric matter to revolve round the sun. thus the mere circumstance of intersection by a meteoric of the terrestrial orbit, without any coincidence of period, would account for the earth meeting some members of the system at each annual passage through the "node" or point of intersection. this was an important step in advance, yet it decided nothing as to the forms of the orbits of such annular assemblages; nor was it followed up in any direction for a quarter of a century. hubert a. newton took up, in ,[ ] the dropped thread of inquiry. the son of a mathematical mother, he attained, at the age of twenty-five, to the dignity of professor of mathematics in yale university, and occupied the post until his death in . the diversion of his powers, however, from purely abstract studies stimulated their effective exercise, and constituted him one of the founders of meteoric astronomy. a search through old records carried the november phenomenon back to the year a.d., long distinguished as "the year of the stars." for in the same night in which taormina was captured by the saracens, and the cruel aghlabite tyrant ibrahim ibn ahmed died "by the judgment of god" before cosenza, stars fell from heaven in such abundance as to amaze and terrify beholders far and near. this was on october , and recurrences were traced down through the subsequent centuries, always with a day's delay in about seventy years. it was easy, too, to derive from the dates a cycle of - / years, so that professor newton did not hesitate to predict the exhibition of an unusually striking meteoric spectacle on november - , .[ ] for the astronomical explanation of the phenomena, recourse was had to a method introduced by erman of computing meteoric orbits. it was found, however, that conspicuous recurrences every thirty-three or thirty-four years could be explained on the supposition of five widely different periods, combined with varying degrees of extension in the revolving group. professor newton himself gave the preference to the shortest--of - / days, but indicated the means of deciding with certainty upon the true one. it was furnished by the advancing motion of the node, or that day's delay of the november shower every seventy years, which the old chronicles had supplied data for detecting. for this is a strictly measurable effect of gravitational disturbance by the various planets, the amount of which naturally depends upon the course pursued by the disturbed bodies. here the great mathematical resources of professor adams were brought to bear. by laborious processes of calculation, he ascertained that four out of newton's five possible periods were entirely incompatible with the observed nodal displacement, while for the fifth--that of - / years--a perfectly harmonious result was obtained.[ ] this was the last link in the chain of evidence proving that the november meteors--or "leonids," as they had by that time come to be called--revolve round the sun in a period of · years, in an ellipse spanning the vast gulf between the orbits of the earth and uranus, the group being so extended as to occupy nearly three years in defiling past the scene of terrestrial encounters. but before it was completed in march, , the subject had assumed a new aspect and importance. professor newton's prediction of a remarkable star-shower in november, , was punctually fulfilled. this time, europe served as the main target of the celestial projectiles, and observers were numerous and forewarned. the display, although, according to mr. baxendell's memory,[ ] inferior to that of , was of extraordinary impressiveness. dense crowds of meteors, equal in lustre to the brightest stars, and some rivalling venus at her best,[ ] darted from east to west across the sky with enormous apparent velocities, and with a certain determinateness of aim, as if let fly with a purpose, and at some definite object.[ ] nearly all left behind them trains of emerald green or clear blue light, which occasionally lasted many minutes, before they shrivelled and curled up out of sight. the maximum rush occurred a little after one o'clock on the morning of november , when attempts to count were overpowered by frequency. but during a previous interval of seven minutes five seconds, four observers at mr. bishop's observatory at twickenham reckoned , and during an hour , .[ ] before daylight the earth had fairly cut her way through the star-bearing stratum; the "ethereal rockets" had ceased to fly. this event brought the subject of shooting stars once more vividly to the notice of astronomers. schiaparelli had, indeed, been already attracted by it. the results of his studies were made known in four remarkable letters, addressed, before the close of the year , to father secchi, and published in the _bulletino_ of the roman observatory.[ ] their upshot was to show, in the first place, that meteors possess a real velocity considerably greater than that of the earth, and travel, accordingly, to enormously greater distances from the sun along tracks resembling those of comets in being very eccentric, in lying at all levels indifferently, and in being pursued in either direction. it was next inferred that comets and meteors equally have an origin foreign to the solar system, but are drawn into it temporarily by the sun's attraction, and occasionally fixed in it by the backward pull of some planet. but the crowning fact was reserved for the last. it was the astonishing one that the august meteors move in the same orbit with the bright comet of --that the comet, in fact, is but a larger member of the family named "perseids" because their radiant point is situated in the constellation perseus. this discovery was quickly capped by others of the same kind. leverrier published, january , ,[ ] elements for the november swarm, founded on the most recent and authentic observations; at once identified by dr. c. f. w. peters of altona with oppolzer's elements for tempel's comet of .[ ] a few days later, schiaparelli, having recalculated the orbit of the meteors from improved data, arrived at the same conclusion; while professor weiss of vienna pointed to the agreement between the orbits of a comet which had appeared in and of a star-shower found to recur on april (lyraïds), as well as between those of biela's comet and certain conspicuous meteors of november .[ ] these instances do not seem to be exceptional. the number of known or suspected accordances of cometary tracks with meteor streams contained in a list drawn up in [ ] by professor alexander s. herschel (who has made the subject peculiarly his own) amounts to seventy-six; although the four first detected still remain the most conspicuous, and perhaps the only absolutely sure examples of a relation as significant as it was, to most astronomers, unexpected. there had, indeed, been anticipatory ideas. not that kepler's comparison of shooting stars to "minute comets," or maskelyne's "forse risulterà che essi sono comete," in a letter to the abate cesaris, december , ,[ ] need count for much. but chladni, in ,[ ] considered both to be fragments or particles of the same primitive matter, irregularly scattered through space as nebulæ; and morstadt of prague suggested about [ ] that the meteors of november might be dispersed atoms from the tail of biela's comet, the path of which is cut across by the earth near that epoch. professor kirkwood, however, by a luminous intuition, penetrated the whole secret, so far as it has yet been made known. in an article published, or rather buried, in the _danville quarterly review_ for december, , he argued, from the observed division of biela, and other less noted instances of the same kind, that the sun exercises a "divellent influence" on the nuclei of comets, which may be presumed to continue its action until their corporate existence (so to speak) ends in complete pulverisation. "may not," he continued, "our periodic meteors be the débris of ancient but now disintegrated comets, whose matter has become distributed round their orbits?"[ ] the gist of schiaparelli's discovery could not be more clearly conveyed. for it must be borne in mind that with the ultimate destiny of comets' tails this had nothing to do. the tenuous matter composing them is, no doubt, permanently lost to the body from which it emanated; but science does not pretend to track its further wanderings through space. it can, however, state categorically that these will no longer be conducted along the paths forsaken under solar compulsion. from the central, and probably solid parts of comets, on the other hand, are derived the granules by the swift passage of which our skies are seamed with periodic fires. it is certain that a loosely agglomerated mass (such as cometary nuclei most likely are) must gradually separate through the unequal action of gravity on its various parts--through, in short, solar tidal influence. thenceforward its fragments will revolve independently in parallel orbits, at first as a swarm, finally--when time has been given for the full effects of the lagging of the slower moving particles to develop--as a closed ring. the first condition is still, more or less, that of the november meteors; those of august have already arrived at the second. for this reason, leverrier pronounced, in , the perseid to be of older formation than the leonid system. he even assigned a date at which the introduction of the last-named bodies into their present orbit was probably effected through the influence of uranus. in a.d. a close approach must have taken place between the planet and the parent comet of the november stars, after which its regular returns to perihelion, and the consequent process of its disintegration, set in. though not complete, it is already far advanced. the view that meteorites are the dust of decaying comets was now to be put to a definite test of prediction. biela's comet had not been seen since its duplicate return in . yet it had been carefully watched for with the best telescopes; its path was accurately known; every perturbation it could suffer was scrupulously taken into account. under these circumstances, its repeated failure to come up to time might fairly be thought to imply a cessation from visible existence. might it not, however, be possible that it would appear under another form--that a star-shower might have sprung from and would commemorate its dissolution? an unusually large number of falling stars were seen by brandes, december , . similar displays were noticed in the years , , and , and the point from which they emanated was shown by heis at aix-la-chapelle to be situated near the bright star gamma andromedæ.[ ] now this is precisely the direction in which the orbit of biela's comet would seem to lie, as it runs down to cut the terrestrial track very near the place of the earth at the above dates. the inference was, then, an easy one, that the meteors were pursuing the same path with the comet; and it was separately arrived at, early in , by weiss, d'arrest, and galle.[ ] but biela travels in the opposite direction to tempel's comet and its attendant "leonids"; its motion is direct, or from west to east, while theirs is retrograde. consequently, the motion of its node is in the opposite direction too. in other words, the meeting-place of its orbit with that of the earth retreats (and very rapidly) along the ecliptic instead of advancing. so that if the "andromedes" stood in the supposed intimate relation to biela's comet, they might be expected to anticipate the times of their recurrence by as much as a week in half a century. all doubt as to the fact may be said to have been removed by signor zezioli's observation of the annual shower in more than usual abundance at bergamo, november , . the missing comet was next due at perihelion in the year , and the probability was contemplated by both weiss and galle of its being replaced by a copious discharge of falling stars. the precise date of the occurrence was not easily determinable, but galle thought the chances in favour of november . the event anticipated the prediction by twenty-four hours. scarcely had the sun set in western europe on november , when it became evident that biela's comet was shedding over us the pulverised products of its disintegration. the meteors came in volleys from the foot of the chained lady, their numbers at times baffling the attempt to keep a reckoning. at moncalieri, about p.m., they constituted (as father denza said[ ]) a "real rain of fire." four observers counted, on an average, four hundred each minute and a half; and not a few fireballs, equalling the moon in diameter, traversed the sky. on the whole, however, the stars of , though about equally numerous, were less brilliant than those of ; the phosphorescent tracks marking their passage were comparatively evanescent and their movements sluggish. this is easily understood when we remember that the andromedes _overtake_ the earth, while the leonids rush to meet it; the velocity of encounter for the first class of bodies being under twelve, for the second above forty-four miles a second. the spectacle was, nevertheless, magnificent. it presented itself successively to various parts of the earth, from bombay and the mauritius to new brunswick and venezuela, and was most diligently and extensively observed. here it had well-nigh terminated by midnight.[ ] it was attended by a slight aurora, and although tacchini had telegraphed that the state of the sun rendered some show of polar lights probable, it has too often figured as an accompaniment of star-showers to permit the coincidence to rank as fortuitous. admiral wrangel was accustomed to describe how, during the prevalence of an aurora on the siberian coast, the passage of a meteor never failed to extend the luminosity to parts of the sky previously dark;[ ] and an enhancement of electrical disturbance may well be associated with the flittings of such cosmical atoms. a singular incident connected with the meteors of has now to be recounted. the late professor klinkerfues, who had observed them very completely at göttingen, was led to believe that not merely the débris strewn along its path, but the comet itself must have been in immediate proximity to the earth during their appearance.[ ] if so, it might be possible, he thought, to descry it as it retreated in the diametrically opposite direction from that in which it had approached. on november , accordingly, he telegraphed to mr. pogson, the madras astronomer, "biela touched earth november ; search near theta centauri"--the "anti-radiant," as it is called, being situated close to that star. bad weather prohibited observation during thirty-six hours, but when the rain clouds broke on the morning of december , there a comet was, just in the indicated position. in appearance it might have passed well enough for one of the biela twins. it had no tail, but a decided nucleus, and was about seconds across, being thus altogether below the range of naked-eye discernment. it was again observed december , when a short tail was perceptible; but overcast skies supervened, and it has never since been seen. its identity accordingly remains in doubt. it seems tolerably certain, however, that it was _not_ the lost comet, which ought to have passed that spot twelve weeks earlier, and was subject to no conceivable disturbance capable of delaying to that extent its revolution. on the other hand, there is the strongest likelihood that it belonged to the same system[ ]--that it was a third fragment, torn from the parent-body of the andromedes at a period anterior to our first observations of it. in thirteen years biela's comet (or its relics) travels nearly twice round its orbit, so that a renewal of the meteoric shower of was looked for on the same day of the year , the probability being emphasised by an admonitory circular from dunecht. astronomers were accordingly on the alert, and were not disappointed. in england, observation was partially impeded by clouds; but at malta, palermo, beyrout, and other southern stations, the scene was most striking. the meteors were both larger and more numerous than in . their numbers in the densest part of the drift were estimated by professor newton at , per hour, visible from one spot to so large a group of spectators that practically none could be missed. yet each of these multitudinous little bodies was found by him to travel in a clear cubical space of which the edge measured twenty miles![ ] thus the dazzling effect of a luminous throng was produced without jostling or overcrowding, by particles, it might almost be said, isolated in the void. their aspect was strongly characteristic of the andromede family of meteors. "they invariably," mr. denning wrote,[ ] "traversed short paths with very slow motions, and became extinct in evolved streams of yellowish sparks." the conclusion seemed obvious "that these meteors are formed of very soft materials, which expand while incalescent, and are immediately crumbled and dissipated into exiguous dust." the biela meteors of did not merely gratify astronomers with a fulfilled prediction, but were the means of communicating to them some valuable information. although their main body was cut through by the moving earth in six hours, and was not more than , miles across, skirmishers were thrown out to nearly a million miles on either side of the compact central battalions. members of the system were, on the th of november, recorded by mr. denning at the hourly rate of about ; and they did not wholly cease to be visible until december . they afforded besides a particularly well-marked example of that diffuseness of radiation previously observed in some less conspicuous displays. their paths seemed to diverge from an area rather than from a point in the sky. they came so ill to focus that divergences of several degrees were found between the most authentically determined radiants. these incongruities are attributed by professor newton to the irregular shape of the meteoroids producing unsymmetrical resistance from the air, and hence causing them to glance from their original direction on entering it. thus, their luminous tracks did not always represent (even apart from the effects of the earth's attraction) the true prolongation of their course through space. the andromedes of were laggards behind the comet from which they sprang; those of were its avant-couriers. that wasted and disrupted body was not due at the node until january , , sixty days, that is, after the earth's encounter with its meteoric fragments. these are now probably scattered over more than five hundred million miles of its orbits;[ ] yet professor newton considers that all must have formed one compact group with biela at the time of its close approach to jupiter about the middle of . for otherwise both comet and meteorites could not have experienced, as they seem to have done, the same kind and amount of disturbance. the rapidity of cometary disintegration is thus curiously illustrated. a short-lived persuasion that the missing heavenly body itself had been recovered, was created by mr. edwin holms's discovery, at london, november , , of a tolerably bright, tailless comet, just in a spot which biela's comet must have traversed in approaching the intersection of its orbit with that of the earth. a hasty calculation by berberich assigned elements to the newcomer seeming not only to ratify the identity, but to promise a quasi-encounter with the earth on november . the only effect of the prediction, however, was to raise a panic among the negroes of the southern states of america. the comet quietly ignored it, and moved away from instead of towards the appointed meeting-place. its projection, then, on the night of its discovery, upon a point of the biela-orbit was by a mere caprice of chance. north america, nevertheless, was visited on november by a genuine andromede shower. although the meteors were less numerous than in , professor young estimated that , , at the least, of their orange fire-streaks came, during five hours, within the range of view at princeton.[ ] brédikhine estimated the width of the space containing them at about , , miles.[ ] the anticipation of their due time by four days implied--if they were a prolongation of the main biela group, the nucleus of which passed the spot of encounter five months previously--a recession of the node since by no less than three degrees. unless, indeed, mr. denning were right in supposing the display to have proceeded from "an associated branch of the main swarm through which we passed in and ."[ ] the existence of separated detachments of biela meteors, due to disturbing planetary action, was contemplated as highly probable by schiaparelli.[ ] such may have been the belated flights met with in , , , and , and such the advance flight plunged through in . a shower looked for november , , did not fall, and no further display from this quarter is probable until november , , although one is possible a year earlier.[ ] the leonids, through the adverse influence of jupiter and saturn, inflicted upon multitudes of eager watchers a still more poignant disappointment. a dense part of the swarm, having nearly completed a revolution since , should, travelling normally, have met the earth november , ; in point of fact, it swerved sunward, and the millions of meteorites which would otherwise have been sacrificed for the illumination of our skies escaped a fiery doom. the contingency had been forecast in the able calculations of dr. johnstone stoney and dr. a. m. w. downing,[ ] superintendent of the nautical almanac office; but the verification scarcely compensated the failure. nor was the situation retrieved in the following years. only ragged fringes of the great tempest-cloud here and there touched our globe. as the same investigators warned us to expect, the course of the meteorites had been not only rendered sinuous by perturbation, but also broken and irregular. we can no longer count upon the leonids. their glory, for scenic purposes, is departed. the comet associated with them also evaded observation. although it doubtless kept its tryst with the sun in the spring of , the attendant circumstances were too unfavourable to allow it to be seen from the earth.[ ] by an almost fantastic coincidence, nevertheless, a faint comet was photographed, november , ,[ ] by dr. chase, of the yale college observatory, close to the leonid radiant, whither a "meteorograph" was directed with a view to recording trails left by precursors of the main leonid body. a promising start, too, was made on the same occasion with meteoric researches from sensitive plates.[ ] indeed, schaeberle and colton[ ] had already, in , determined the height of a leonid by means of photographs taken at stations on different ridges of mount hamilton; and professor pickering has prosecuted similar work at harvard, with encouraging results. everything in this branch of science depends upon how far they can be carried. without the meteorograph, rigid accuracy in the observation of shooting stars is unattainable, and rigid accuracy is the _sine quâ non_ for obtaining exact knowledge. biela does not offer the only example of cometary disruption. setting aside the unauthentic reports of early chroniclers, we meet the "double comet" discovered by liais at olinda (brazil), february , , of which the division appeared recent, and about to be carried farther.[ ] but a division once established, separation must continually progress. the periodic times of the fragments will never be identical; one must drop a little behind the other at each revolution, until at length they come to travel in remote parts of nearly the same orbit. thus the comet predicted by klinkerfues and discovered by pogson had already lagged to the extent of twelve weeks, and we shall meet instances farther on where the retardation is counted, not by weeks, but by years. here original identity emerges only from calculation and comparison of orbits. comets, then, die, as kepler wrote long ago, _sicut bombyces filo fundendo_. this certainty, anticipated by kirkwood in , we have at least acquired from the discovery of their generative connection with meteors. nay, their actual materials become, in smaller or larger proportions, incorporated with our globe. it is not, indeed, universally admitted that the ponderous masses of which, according to daubrée's estimate,[ ] at least fall annually from space upon the earth, ever formed part of the bodies known to us as comets. some follow tschermak in attributing to aerolites a totally different origin from that of periodical shooting-stars. that no clear line of demarcation can be drawn is no valid reason for asserting that no real distinction exists; and it is certainly remarkable that a meteoric fusillade may be kept up for hours without a single solid projectile reaching its destination. it would seem as if the celestial army had been supplied with blank cartridges. yet, since a few detonating meteors have been found to proceed from ascertained radiants of shooting-stars, it is difficult to suppose that any generic difference separates them. their assimilation is further urged--though not with any demonstrative force--by two instances, the only two on record, of the tangible descent of an aerolite during the progress of a star-shower. on april , , the saxon chronicle informs us that stars fell "so thickly that no man could count them," and adds that one of them having struck the ground in france, a bystander "cast water upon it, which was raised in steam with a great noise of boiling."[ ] and again, on november , , while the skirts of the andromede-tempest were trailing over mexico, "a ball of fire" was precipitated from the sky at mazapil, within view of a ranchman.[ ] scientific examination proved it to be a "siderite," or mass of "nickel-iron"; its weight exceeded eight pounds, and it contained many nodules of graphite. we are not, however, authorised by the circumstances of its arrival to regard the mazapil fragment of cosmic metal as a specimen torn from biela's comet. in this, as in the preceding case, the coincidence of the fall with the shower may have been purely casual, since no hint is given of any sort of agreement between the tracks followed by the sample provided for curious study, and the swarming meteors consumed in the upper air. professor newton's inquiries into the tracks pursued by meteorites previous to their collisions with the earth tend to distinguish them, at least specifically, from shooting-stars. he found that nearly all had been travelling with a direct movement in orbits the perihelia of which lay in the outer half of the space separating the earth from the sun.[ ] shooting-stars, on the contrary, are entirely exempt from such limitations. the yale professor concluded "that the larger meteorites moving in our solar system are allied much more closely with the group of comets of short period than with the comets whose orbits are nearly parabolic." they would thus seem to be more at home than might have been expected amid the planetary family. father carbonelle has, moreover, shown[ ] that meteorites, if explosion-products of the earth or moon, should, with rare exceptions, follow just the kind of paths assigned to them, from data of observation, by professor newton. yet it is altogether improbable that projectiles from terrestrial volcanoes should, at any geological epoch, have received impulses powerful enough to enable them, not only to surmount the earth's gravity, but to penetrate its atmosphere. a striking--indeed, an almost startling--peculiarity, on the other hand, divides from their congeners a class of meteors identified by mr. denning during ten years' patient watching of such phenomena at bristol.[ ] these are described as "meteors with stationary radiants," since for months together they seem to come from the same fixed points in the sky. now this implies quite a portentous velocity. the direction of meteor-radiants is affected by a kind of _aberration_, analogous to the aberration of light. it results from a composition of terrestrial with meteoric motion. hence, unless that of the earth in its orbit be by comparison insignificant, the visual line of encounter must shift, if not perceptibly from day to day, at any rate conspicuously from month to month. the fixity, then, of many systems observed by mr. denning seems to demand the admission that their members travel so fast as to throw the earth's movement completely out of the account. the required velocity would be, by mr. ranyard's calculation, at least miles a second.[ ] but the aspect of the meteors justifies no such extravagant assumption. their seeming swiftness is very various, and--what is highly significant--it is notably less when they pursue than when they meet the earth. yet the "incredible and unaccountable"[ ] fact of the existence of these "long radiants," although doubted by tisserand[ ] because of its theoretical refractoriness, must apparently be admitted. the first plausible explanation of them was offered by professor turner in .[ ] they represent, in his view, the cumulative effects of the earth's attraction. the validity of his reasoning is, however, denied by m. brédikhine,[ ] who prefers to regard them as a congeries of separate streams. the enigma they present has evidently not yet received its definitive solution. the perseids afford, on the contrary, a remarkable instance of a "shifting radiant." mr. denning's observations of these yellowish, leisurely meteors extend over nearly six weeks, from july to august ; the point of radiation meantime progressing no less than ° in right ascension. doubts as to their common origin were hence freely expressed, especially by mr. monck of dublin.[ ] but the late dr. kleiber[ ] showed, by strict geometrical reasoning, that the forty-nine radiants successively determined for the shower were all, in fact, comprised within one narrowly limited region of space. in other words, the application of the proper correction for the terrestrial movement, and the effects of attraction by which each individual shooting-star is compelled to describe a hyperbola round the earth's centre, reduces the extended line of radiants to a compact group, with the cometary radiant for its central point; the cometary radiant being the spot in the sky met by a tangent to the orbit of the perseid comet of at its intersection with the orbit of the earth. the reality of the connection between the comet and the meteors could scarcely be more clearly proved; while the vast dimensions of the stream into which the latter are found to be diffused cannot but excite astonishment not unmixed with perplexity. the first successful application of the spectroscope to comets was by donati in .[ ] a comet discovered by tempel, july , brightened until it appeared like a star somewhat below the second magnitude, with a feeble tail ° in length. it was remarkable as having, on august , almost totally eclipsed a small star--a very rare occurrence.[ ] on august donati admitted its light through his train of prisms, and found it, thus analysed, to consist of three bright bands--yellow, green, and blue--separated by wider dark intervals. this implied a good deal. comets had previously been considered, as we have seen, to shine mainly, if not wholly, by reflected sunlight. they were now perceived to be self-luminous, and to be formed, to a large extent, of glowing gas. the next step was to determine what _kind_ of gas it was that was thus glowing in them; and this was taken by sir william huggins in .[ ] a comet of subordinate brilliancy, known as comet ii., or sometimes as winnecke's, was the subject of his experiment. on comparing its spectrum with that of an olefiant-gas "vacuum tube" rendered luminous by electricity, he found the agreement exact. it has since been abundantly confirmed. all the eighteen comets tested by light analysis, between and , showed the typical hydro-carbon spectrum[ ] common to the whole group of those compounds, but probably due immediately to the presence of acetylene. some minor deviations from the laboratory pattern, in the shifting of the maxima of light from the edge towards the middle of the yellow and blue bands, have been experimentally reproduced by vogel and hasselberg in tubes containing a mixture of carbonic oxide with olefiant gas.[ ] their illumination by disruptive electric discharges was, however, a condition _sine quâ non_ for the exhibition of the cometary type of spectrum. when a continuous current was employed, the carbonic oxide bands asserted themselves to the exclusion of the hydro-carbons. the distinction has great significance as regards the nature of comets. of particular interest in this connection is the circumstance that carbonic oxide is one of the gases evolved by meteoric stones and irons under stress of heat.[ ] for it must apparently have formed part of an aeriform mass in which they were immersed at an earlier stage of their history. plate ii. [illustration: great comet. photographed, may , , with the thirteen-inch astrographic refractor of the royal observatory, cape of good hope.] in a few exceptional comets the usual carbon-bands have been missed. two such were observed by sir william huggins in and respectively.[ ] in each a green ray, approximating in position to the fundamental nebular line, crossed an otherwise unbroken spectrum. and holmes's comet of displayed only a faint prismatic band devoid of any characteristic feature.[ ] now these three might well be set down as partially effete bodies; but a brilliant comet, visible in southern latitudes in april and may, , so far resembled them in the quality of its light as to give a spectrum mainly, if not purely, continuous. this, accordingly, is no symptom of decay. the earliest comet of first-class lustre to present itself for spectroscopic examination was that discovered by coggia at marseilles, april , . invisible to the naked eye till june, it blazed out in july a splendid ornament of our northern skies, with a just perceptibly curved tail, reaching more than half way from the horizon to the zenith, and a nucleus surpassing in brilliancy the brightest stars in the swan. brédikhine, vogel, and huggins[ ] were unanimous in pronouncing its spectrum to be that of marsh or olefiant gas. father secchi, in the clear sky of rome, was able to push the identification even closer than had heretofore been done. the _complete_ hydro-carbon spectrum consists of five zones of variously coloured light. three of these only--the three central ones--had till then been obtained from comets; owing, it was supposed, to their temperature not being high enough to develop the others. the light of coggia's comet, however, was found to contain all five, traces of the violet band emerging june , of the red, july .[ ] presumably, all five would show universally in cometary spectra, were the dispersed rays strong enough to enable them to be seen. the gaseous surroundings of comets are, then, largely made up of a compound of hydrogen with carbon. other materials are also present; but the hydro-carbon element is probably unfailing and predominant. its luminosity is, there is little doubt, an effect of electrical excitement. zöllner showed in [ ] that, owing to evaporation and other changes produced by rapid approach to the sun, electrical processes of considerable intensity must take place in comets; and that their original light is immediately connected with these, and depends upon solar radiation, rather through its direct or indirect electrifying effects, than through its more obvious thermal power, may be considered a truth permanently acquired to science.[ ] they are not, it thus seems, bodies incandescent through heat, but glowing by electricity; and this is compatible, under certain circumstances, with a relatively low temperature. the gaseous spectrum of comets is accompanied, in varying degrees, by a continuous spectrum. this is usually derived most strongly from the nucleus, but extends, more or less, to the nebulous appendages. in part, it is certainly due to reflected sunlight; in part, most likely, to the ignition of minute solid particles. footnotes: [footnote : _month. not._, vol. xix., p. .] [footnote : _mém. de l'ac. imp._, t. ii., , p. .] [footnote : _harvard annals_, vol. iii., p. .] [footnote : _ibid._, p. .] [footnote : _month. not._, vol. xxii., p. .] [footnote : stothard in _ibid._, vol. xxi., p. .] [footnote : _intell. observer_, vol. i., p. .] [footnote : _comptes rendus_, t. lxi., p. .] [footnote : _smiths. report_, (holden); _nature_, vol. xxv., p. ; _observatory_, vol. xxi., p. (w. t. lynn).] [footnote : _ueber den ursprung der von pallas gefundenen eisenmassen_, p. .] [footnote : arago, _annuaire_, , p. .] [footnote : humboldt had noticed the emanation of the shooting stars of from a single point, or "radiant," as greg long afterwards termed it; but no reasoning was founded on the observation.] [footnote : _am. journ. of sc._, vol. xxvi., p. .] [footnote : _annuaire_, , p. .] [footnote : _ann. de l'observ._, bruxelles, , p. .] [footnote : _ibid._, , p. .] [footnote : _astr. nach._, nos. , .] [footnote : _am. jour. of sc._, vol. xxxviii. ( nd ser.), p. .] [footnote : _ibid._, vol. xxxviii., p. .] [footnote : _month. not._, vol. xxvii., p. .] [footnote : _am. jour. of sc._, vol. xliii. ( nd ser.), p. .] [footnote : grant, _month. not._, vol. xxvii., p. .] [footnote : p. smyth, _ibid._, p. .] [footnote : hind, _ibid._, p. .] [footnote : reproduced in _les mondes_, t. xiii.] [footnote : _comptes rendus_, t. lxiv., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, no. , .] [footnote : _month. not._, vol. xxxviii., p. .] [footnote : schiaparelli, _le stelle cadenti_, p. .] [footnote : _ueber feuer-meteore_, p. .] [footnote : _astr. nach._, no. (mädler); see also boguslawski, _die kometen_, p. . .] [footnote : _nature_, vol. vi., p. .] [footnote : a. s. herschel, _month. not._, vol. xxxii., p. .] [footnote : _astr. nach._, nos. , , , , , .] [footnote : _nature_, vol. vii., p. .] [footnote : a. s. herschel, _report brit. ass._, , p. .] [footnote : humboldt, _cosmos_, vol. i., p. (otté's trans.).] [footnote : _month. not._, vol. xxxiii., p. .] [footnote : even this was denied by bruhns, _astr. nach._, no. , .] [footnote : _am. jour._, vol. xxxi., p. .] [footnote : _month. not._, vol. xlvi., p. .] [footnote : in schiaparelli's opinion, centuries must have elapsed while the observed amount of scattering was being produced. _le stelle cadenti_, , p. .] [footnote : _astr. and astroph._, vol. xi., p. .] [footnote : _bull. de l'acad. st. petersbourg_, t. xxxv., p. . .] [footnote : _observatory_, vol. xvi., p. .] [footnote : _le stelle cadenti_, p. ; _rendiconti dell' istituto lombardo_, t. iii., ser. ii., p. .] [footnote : denning, _memoirs roy. astr. soc._, vol. liii., p. ; abelmann, _astr. nach._, no. , .] [footnote : _proc. roy. soc._, march , ; _nature_, november , .] [footnote : berberich, _astr. nach._, no. , .] [footnote : elkin, _astroph. jour._, vol. ix., p. .] [footnote : elkin, _astroph. jour._, vol. x., p. .] [footnote : _pop. astr._, september, , p. .] [footnote : _month. not._, vol. xx., p. .] [footnote : _revue des deux mondes_, december , , p. .] [footnote : palgrave, _phil. trans._, vol. cxxv., p. .] [footnote : w. e. hidden, _century mag._, vol. xxxiv., p. .] [footnote : _amer. jour. of science_, vol. xxxvi., p. i., .] [footnote : _revue des questions scientifiques_, january, , p. ; tisserand, _bull. astr._, t. viii., p. .] [footnote : _month. not._, vol. xlv., p. .] [footnote : _observatory_, vol. viii., p. .] [footnote : denning, _month. not._, vol. xxxviii., p. .] [footnote : _comptes rendus_, t. cix., p. .] [footnote : _month. not._, vol. lix., p. .] [footnote : _bull. de l'acad. st. petersb._, t. xii., p. .] [footnote : _publ. astr. pac. soc._, vol. iii., p. .] [footnote : _month. not._, vol. lii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _annuaire_, paris, , p. .] [footnote : _phil. trans._, vol. clviii., p. .] [footnote : hasselberg, _mém. de l'ac. imp. de st. pétersbourg_, t. xxviii. ( th ser.), no. , p. .] [footnote : scheiner, _die spectralanalyse der gestirne_, p. . kayser (_astr. and astroph._, vol. xiii., p. ) refers the anomalies of the carbon-spectrum in comets wholly to instrumental sources.] [footnote : dewar, _proc. roy. inst._, vol. xi., p. .] [footnote : _proc. r. soc._, vol. xv., p. ; _month. not._, vol. xxvii., p. .] [footnote : keeler, _astr. and astrophysics_, vol. xi., p. ; vogel, _astr. nach._, no. , .] [footnote : _proc. roy. soc._, vol. xxiii., p. .] [footnote : hasselberg, _loc. cit._, p. .] [footnote : _ueber die natur der cometen_, p. .] [footnote : hasselberg, _loc. cit._, p. .] chapter xi _recent comets_ (_continued_) the mystery of comets' tails had been to some extent penetrated; so far, at least, that, by making certain assumptions strongly recommended by the facts of the case, their forms can be, with very approximate precision, calculated beforehand. we have, then, the assurance that these extraordinary appendages are composed of no ethereal or supersensual stuff, but of matter such as we know it, and subject to the ordinary laws of motion, though in a state of extreme tenuity. olbers, as already stated, originated in the view that the tails of comets are made up of particles subject to a force of electrical repulsion proceeding from the sun. it was developed and enforced by bessel's discussion of the appearances presented by halley's comet in . he, moreover, provided a formula for computing the movement of a particle under the influence of a repulsive force of any given intensity, and thus laid firmly the foundation of a mathematical theory of cometary emanations. professor w. a. norton, of yale college, considerably improved this by inquiries begun in , and resumed on the apparition of donati's comet; and dr. c. f. pape at altona[ ] gave numerical values for the impulses outward from the sun, which must have actuated the materials respectively of the curved and straight tails adorning the same beautiful and surprising object. the _physical_ theory of repulsion, however, was, it might be said, still in the air. nor did it even begin to assume consistency until zöllner took it in hand in .[ ] it is perfectly well ascertained that the energy of the push or pull produced by electricity depends (other things being the same) upon the _surface_ of the body acted on; that of gravity upon its _mass_. the efficacy of solar electrical repulsion relatively to solar gravitational attraction grows, consequently, as the size of the particle diminishes. make this small enough, and it will virtually cease to gravitate, and will unconditionally obey the impulse to recession. this principle zöllner was the first to realise in its application to comets. it gives the key to their constitution. admitting that the sun and they are similarly electrified, their more substantially aggregated parts will still follow the solicitations of his gravity, while the finely divided particles escaping from them will, simply by reason of their minuteness, fall under the sway of his repellent electric power. they will, in other words, form "tails." nor is any extravagant assumption called for as to the intensity of the electrical charge concerned in producing these effects. zöllner, in fact, showed[ ] that it need not be higher than that attributed by the best authorities to the terrestrial surface. forty years have elapsed since m. brédikhine, director successively of the moscow and of the pulkowa observatories, turned his attention to these curious phenomena. his persistent inquiries on the subject, however, date from the appearance of coggia's comet in . on computing the value of the repulsive force exerted in the formation of its tail, and comparing it with values of the same force arrived at by him in for some other conspicuous comets, it struck him that the numbers representing them fell into three well-defined classes. "i suspect," he wrote in , "that comets are divisible into groups, for each of which the repulsive force is perhaps the same."[ ] this idea was confirmed on fuller investigation. in the appendages of thirty-six well-observed comets had been reconstructed theoretically, without a single exception being met with to the rule of the three types. a further study of forty comets led, in , only to a modification of the numerical results previously arrived at. in the first of these, the repellent energy of the sun is fourteen times stronger than his attractive energy;[ ] the particles forming the enormously long straight rays projected outward from this kind of comet leave the nucleus with a mean velocity of just seven kilometres per second, which, becoming constantly accelerated, carries them in a few days to the limit of visibility. the great comets of , , and , that of (so far as its principal tail was concerned), and halley's comet at its various apparitions, belonged to this class. less narrow limits were assigned to the values of the repulsive force employed to produce the second type. for the axis of the tail, it exceeds by one-tenth (= · ) the power of solar gravity; for the anterior edge, it is more than twice ( · ), for the posterior only half as strong. the corresponding initial velocity (for the axis) is , metres a second, and the resulting appendage a scimitar-like or plumy tail, such as donati's and coggia's comets furnished splendid examples of. tails of the third type are constructed with forces of repulsion from the sun ranging from one-tenth to three-tenths that of his gravity, producing an accelerated movement of attenuated matter from the nucleus, beginning at the leisurely rate of to metres a second. they are short, strongly bent, brush-like emanations, and in bright comets seem to be only found in combination with tails of the higher classes. multiple tails, indeed--that is, tails of different types emitted simultaneously by one comet--are perceived, as experience advances and observation becomes closer, to be rather the rule than the exception.[ ] now what is the meaning of these three types? is any translation of them into physical fact possible? to this question brédikhine supplied, in , a plausible answer.[ ] it was already a current surmise that multiple tails are composed of different kinds of matter, differently acted on by the sun. both olbers and bessel had suggested this explanation of the straight and curved emanations from the comet of ; norton had applied it to the faint light tracks proceeding from that of donati;[ ] winnecke to the varying deviations of its more brilliant plumage. brédikhine defined and ratified the conjecture. he undertook to determine (provisionally as yet) the several kinds of matter appropriated severally to the three classes of tails. these he found to be hydrogen for the first, hydro-carbons for the second, and iron for the third. the ground of this apportionment is that the atomic weights of these substances bear to each other the same inverse proportion as the repulsive forces employed in producing the appendages they are supposed to form; and zöllner had pointed out in that the "heliofugal" power by which comets' tails are developed would, in fact, be effective just in that ratio.[ ] hydrogen, as the lightest known element--that is, the least under the influence of gravity--was naturally selected as that which yielded most readily to the counter-persuasions of electricity. hydro-carbons had been shown by the spectroscope to be present in comets, and were fitted by their specific weight, as compared with that of hydrogen, to form tails of the second type; while the atoms of iron were just heavy enough to compose those of the third, and, from the plentifulness of their presence in meteorites, might be presumed to enter, in no inconsiderable proportion, into the mass of comets. these three substances, however, were by no means supposed to be the sole constituents of the appendages in question. on the contrary, the great breadth of what, for the present, were taken to be characteristically "iron" tails was attributed to the presence of many kinds of matter of high and slightly different specific weights;[ ] while the expanded plume of donati was shown to be, in reality, a whole system of tails, made up of many substances, each spreading into a separate hollow cone, more or less deviating from, and partially superposed upon the others. yet these felicities of explanation must not make us forget that the chemical composition attributed to the first type of cometary trains has, so far, received no countenance from the spectroscope. the emission lines of free, incandescent hydrogen have never been derived from any part of these bodies. dissentient opinions, accordingly, were expressed as to the cause of their structural peculiarities. ranyard,[ ] zenker, and others advocated the agency of heat repulsion in producing them; kiaer somewhat obscurely explains them through the evolution of gases by colliding particles;[ ] herz of vienna concludes tails to be mere illusory appendages produced by electrical discharges through the rare medium assumed to fill space.[ ] but hirn[ ] conclusively showed that no such medium could possibly exist without promptly bringing ruin upon our "dædal earth" and its revolving companions. on the whole, modern researches tend to render superfluous the chemical diversities postulated by brédikhine. electricity alone seems competent to produce the varieties of cometary emanation they were designed to account for. the distinction of types rests on a solid basis of fact, but probably depends upon differences rather in the mode of action than in the kind of substance acted upon. suggestive sketches of electrical and "light-pressure" theories of comets have been published respectively by mr. fessenden of alleghany,[ ] and by m. arrhenius at stockholm.[ ] although evidently of a tentative character, they possess great interest. brédikhine's hypothesis was promptly and profusely illustrated. within three years of its promulgation, five bright comets made their appearance, each presenting some distinctive peculiarity by which knowledge of these curious objects was materially helped forward. the first of these is remembered as the "great southern comet." it was never visible in these latitudes, but made a short though stately progress through southern skies. its earliest detection was at cordoba on the last evening of january, ; and it was seen on february , as a luminous streak, extending just after sunset from the south-west horizon towards the pole, in new south wales, at monte video, and the cape of good hope. the head was lost in the solar rays until february , when dr. gould, then director of the national observatory of the argentine republic at cordoba, caught a glimpse of it very low in the west; and on the following evening, mr. eddie, at graham's town, discovered a faint nucleus, of a straw-coloured tinge, about the size of the annular nebula in lyra. its condensation, however, was very imperfect, and the whole apparition showed an exceedingly filmy texture. the tail was enormously long. on february it extended--large perspective retrenchment notwithstanding--over an arc of °; but its brightness nowhere exceeded that of the milky way in taurus. there was little curvature perceptible; the edges of the appendage ran parallel, forming a nebulous causeway from star to star; and the comparison to an auroral beam was appropriately used. the aspect of the famous comet of was forcibly recalled to the memory of mr. janisch, governor of st. helena; and the resemblance proved not merely superficial. but the comet of was less brilliant, and even more evanescent. after only eight days of visibility, it had faded so much as no longer to strike, though still discoverable by the unaided eye; and on february it was invisible with the great cordoba equatoreal pointed to its known place. but the most astonishing circumstance connected with this body is the identity of its path with that of its predecessor in . this is undeniable. dr. gould,[ ] mr. hind, and dr. copeland,[ ] each computed a separate set of elements from the first rough observations, and each was struck with an agreement between the two orbits so close as to render them virtually indistinguishable. "can it be possible," mr. hind wrote to sir george airy, "that there is such a comet in the system, almost grazing the sun's surface in perihelion, and revolving in less than thirty-seven years. i confess i feel a difficulty in admitting it, notwithstanding the above extraordinary resemblance of orbits."[ ] mr. hind's difficulty was shared by other astronomers. it would, indeed, be a violation of common-sense to suppose that a celestial visitant so striking in appearance had been for centuries back an unnoticed frequenter of our skies. various expedients, accordingly, were resorted to for getting rid of the anomaly. the most promising at first sight was that of the resisting medium. it was hard to believe that a body, largely vaporous, shooting past the sun at a distance of less than a hundred thousand miles from his surface, should have escaped powerful retardation. it must have passed through the very midst of the corona. it might easily have had an actual encounter with a prominence. escape from such proximity might, indeed, very well have been judged beforehand to be impossible. even admitting no other kind of opposition than that dubiously supposed to have affected encke's comet, the result in shortening the period ought to be of the most marked kind. it was proved by oppolzer[ ] that if the comet of had entered our system from stellar space with parabolic velocity it would, by the action of a medium such as encke postulated (varying in density inversely as the square of the distance from the sun), have been brought down, by its first perihelion passage, to elliptic movement in a period of twenty-four years, with such rapid diminution that its next return would be in about ten. but such restricted observations as were available on either occasion of its visibility gave no sign of such a rapid progress towards engulfment. another form of the theory was advocated by klinkerfues.[ ] he supposed that four returns of the same body had been witnessed within historical memory--the first in b.c., the next in , besides those of and ; an original period of , years being successively reduced by the withdrawal at each perihelion passage of / of the velocity acquired by falling from the far extremity of its orbit towards the sun, to and years. a continuance of the process would bring the comet of back in . unfortunately, the earliest of these apparitions cannot be identified with the recent ones unless by doing violence to the plain meaning of aristotle's words in describing it. he states that the comet was first seen "during the frosts and in the clear skies of winter," setting due west nearly at the same time as the sun.[ ] this implies some considerable north latitude. but the objects lately observed had practically _no_ north latitude. they accomplished their entire course _above_ the ecliptic in two hours and a quarter, during which space they were barely separated a hand's-breadth (one might say) from the sun's surface. for the purposes of the desired assimilation, aristotle's comet should have appeared in march. it is not credible, however, that even a native of thrace should have termed march "winter." with the comet of the case seemed more dubious. the circumstances of its appearance are barely reconcilable with the identity attributed to it, although too vaguely known to render certainty one way or the other attainable. it might however, be expected that recent observations would at least decide the questions whether the comet of could have returned in less than thirty-seven, and whether the comet of was to be looked for at the end of - / years. but the truth is that both these objects were observed over so small an arc-- ° and ° respectively--that their periods remained virtually undetermined. for while the shape and position of their orbits could be and were fixed with a very close approach to accuracy, the length of those orbits might vary enormously without any very sensible difference being produced in the small part of the curves traced out near the sun. dr. wilhelm meyer, however, arrived, by an elaborate discussion, at a period of thirty-seven years for the comet of ,[ ] while the observations of were admittedly best fitted by hubbard's ellipse of years; but these dr. meyer supposed to be affected by some constant source of error, such as would be produced by a mistaken estimate of the position of the comet's centre of gravity. he inferred finally that, in spite of previous non-appearances, the two comets represented a single regular denizen of our system, returning once in thirty-seven years along an orbit of such extreme eccentricity that its movement might be described as one of precipitation towards and rapid escape from the sun, rather than of sedate circulation round it. the _geometrical_ test of identity has hitherto been the only one which it was possible to apply to comets, and in the case before us it may fairly be said to have broken down. we may, then, tentatively, and with much hesitation, try a _physical_ test, though scarcely yet, properly speaking, available. we have seen that the comets of and were strikingly alike in general appearance, though the absence of a formed nucleus in the latter, and its inferior brilliancy, detracted from the convincing effect of the resemblance. nor was it maintained when tried by exact methods of inquiry. m. brédikhine found that the gigantic ray emitted in belonged to his type no. ; that of to type no. .[ ] the particles forming the one were actuated by a repulsive force ten times as powerful as those forming the other. it is true that a second noticeably curved tail was seen in chili, march , and at madras, march , ; and the conjecture was accordingly hazarded that the materials composing on that occasion the principal appendage having become exhausted, those of the secondary one remained predominant, and reappeared alone in the "hydro-carbon" train of . but the one known instance in point is against such a supposition. halley's comet, the only _great_ comet of which the returns have been securely authenticated and carefully observed, has preserved its "type" unchanged through many successive revolutions. the dilemma presented to astronomers by the great southern comet of was unexpectedly renewed in the following year. on the nd of may, , mr. john tebbutt of windsor, new south wales, scanning the western sky, discerned a hazy-looking object which he felt sure was a strange one. a marine telescope at once resolved it into two small stars and a comet, the latter of which quickly attracted the keen attention of astronomers; for dr. gould, computing its orbit from his first observations at cordoba, found it to agree so closely with that arrived at by bessel for the comet of that he telegraphed to europe, june , announcing the unexpected return of that body. so unexpected that theoretically it was not possible before the year ; and bessel's investigation was one which inspired and eminently deserved confidence. here, then, once more the perplexing choice had to be made between a premature and unaccountable reappearance and the admission of a plurality of comets moving nearly in the same path. but in this case facts proved decisive. tebbutt's comet passed the sun, june , at a distance of sixty-eight millions of miles, and became visible in europe six days later. it was, in the opinion of some, the finest object of the kind since . in traversing the constellation auriga on its _début_ in these latitudes, it outshone capella. on june and some subsequent nights, it was unmatched in brilliancy by any star in the heavens. in the telescope, the "two interlacing arcs of light" which had adorned the head of coggia's comet were reproduced; while a curious _dorsal spine_ of strong illumination formed the axis of the tail, which extended in clear skies over an arc of °. it belonged to the same "type" as donati's great plume; the particles composing it being driven _from_ the sun by a force twice as powerful as that urging them _towards_ it.[ ] but the appendage was, for a few nights, and by two observers perceived to be double. tempel, on june , and lewis boss, at albany (n.y.), june and , saw a long straight ray corresponding to a far higher rate of emission than the curved train, and shown by brédikhine to be a member of the (so-called) hydrogen class. it had vanished by july , but made a temporary reappearance july .[ ] the appendages of this comet were of remarkable transparency. small stars shone wholly undimmed across the tail, and a very nearly central transit of the head over one of the seventh magnitude on the night of june , produced--if any change--an increase of brilliancy in the object of this spontaneous experiment.[ ] dr. meyer, indeed, at the geneva observatory, detected apparent signs of refractive action upon rays thus transmitted;[ ] but his observations remain isolated, and were presumably illusory. the track pursued by this comet gave peculiar advantages for its observation. ascending from auriga through camelopardus, it stood, july , on a line between the pointers and the pole, within ° of the latter, and thus remained for a lengthened period constantly above the horizon of northern observers. its brightness, too, was no transient blaze, but had a lasting quality which enabled it to be kept steadily in view during nearly nine months. visible to the naked eye until the end of august, the last telescopic observation of it was made february , , when its distance from the earth considerably exceeded million miles. under these circumstances, the knowledge acquired of its orbit was of more than usual accuracy, and showed conclusively that the comet was not a simple return of bessel's; for this would involve a period of seventy-four years, whereas tebbutt's comet cannot revisit the sun until after the lapse of two and a half millenniums.[ ] nevertheless, the twin bodies move so nearly in the same path that an original connection of some kind is obvious; and the recent example of biela readily suggested a conjecture as to what the nature of that connection might have been. the comets of and are, then, regarded with much probability as fragments of a primitive disrupted body, one following in the wake of the other at an interval of seventy-four years. imperfect photographs were taken of donati's comet both in england and america;[ ] but tebbutt's comet was the first to which the process was satisfactorily applied. the difficulties to be overcome were very great. the chemical intensity of cometary light is, to begin with, extraordinarily small. janssen estimated it at / of moonlight.[ ] hence, if the ordinary process by which lunar photographs are taken had been applied to the comet of , an exposure of at least _three days_ would have been required in order to get an impression of the head with about a tenth part of the tail. but by that time a new method of vastly increased sensitiveness had been rendered available, by which dry gelatine-plates were substituted for the wet collodion-plates hitherto in use; and this improvement alone reduced the necessary time of exposure to two hours. it was brought down to half an hour by janssen's employment of a reflector specially adapted to give an image illuminated eight or ten times as strongly as that produced in the focus of an ordinary telescope.[ ] the photographic feebleness of cometary rays was not the only obstacle in the way of success. the proper motion of these bodies is so rapid as to render the usual devices for keeping a heavenly body steadily in view quite inapplicable. the machinery by which the diurnal movement of the sphere is followed, must be especially modified to suit each eccentric career. this, too, was done, and on june , , janssen secured a perfect photograph of the brilliant object then visible, showing the structure of the tail with beautiful distinctness to a distance of - / ° from the head. an impression to nearly ° was obtained about the same time by dr. henry draper at new york, with an exposure of minutes.[ ] tebbutt's (or comet iii.) was also the first comet of which the spectrum was so much as attempted to be chemically recorded. both huggins and draper were successful in this respect, but huggins was more completely so.[ ] the importance of the feat consisted in its throwing open to investigation a part of the spectrum invisible to the eye, and so affording an additional test of cometary constitution. the result was fully to confirm the origin from carbon-compounds assigned to the visible rays, by disclosing additional bands belonging to the same series in the ultra-violet; as well as to establish unmistakably the presence of a not inconsiderable proportion of reflected solar light by the clear impression of some of the principal fraunhofer lines. thus the polariscope was found to have told the truth, though not the whole truth. the photograph so satisfactorily communicative was taken by sir william huggins on the night of june ; and on the th, at greenwich, the tell-tale fraunhofer lines were perceived to interrupt the visible range of the spectrum. this was at first so vividly continuous, that the characteristic cometary bands could scarcely be detached from their bright background. but as the nucleus faded towards the end of june, they came out strongly, and were more and more clearly seen, both at greenwich and at princeton, to agree, not with the spectrum of hydro-carbons glowing in a vacuum tube, but with that of the same substances burning in a bunsen flame.[ ] it need not, however, be inferred that cometary materials are really in a state of combustion. this, from all that we know, may be called an impossibility. the additional clue furnished was rather to the manner of their electrical illumination.[ ] the spectrum of the tail was, in this comet, found to be not essentially different from that of the head. professor wright of yale college ascertained a large percentage of its light to be polarized in a plane passing through the sun, and hence to be reflected sunlight.[ ] a faint continuous spectrum corresponded to this portion of its radiance; but gaseous emissions were also present. at potsdam, on june , the hydro-carbon bands were indeed traced by vogel to the very end of the tail;[ ] and they were kept in sight by young at a greater distance from the nucleus than the more equably dispersed light. there seems little doubt that, as in the solar corona, the relative strength of the two orders of spectra is subject to fluctuations. the comet of iii. was thus of signal service to science. it afforded, when compared with the comet of , the first undeniable example of two such bodies travelling so nearly in the same orbit as to leave absolutely no doubt of the existence of a genetic tie between them. cometary photography came to its earliest fruition with it; and cometary spectroscopy made a notable advance by means of it. before it was yet out of sight, it was provided with a successor. at ann arbor observatory, michigan, on july , a comet was discovered by dr. schaeberle, which, as his claim to priority is undisputed, is often allowed to bear his name, although designated, in strict scientific parlance, comet iv. it was observed in europe after three days, became just discernible by the naked eye at the end of july, and brightened consistently up to its perihelion passage, august , when it was still about fifty million miles from the sun. during many days of that month, the uncommon spectacle was presented of two bright comets circling together, though at widely different distances, round the north pole of the heavens. the newcomer, however, never approached the pristine lustre of its predecessor. its nucleus, when brightest, was comparable to the star cor caroli, a narrow, perfectly straight ray proceeding from it to a distance of °. this was easily shown by brédikhine to belong to the hydrogen type of tails;[ ] while a "strange, faint second tail, or bifurcation of the first one," observed by captain noble, august ,[ ] fell into the hydro-carbon class of emanations. it was seen, august and , by dr. f. terby of louvain,[ ] as a short nebulous brush, like the abortive beginning of a congeries of curving trains; but appeared no more. its well-attested presence was significant of the complex constitution of such bodies, and the manifold kinds of action progressing in them. the only peculiarity in the spectrum of schaeberle's comet consisted in the almost total absence of continuous light. the carbon-bands were nearly isolated and very bright. barely from the nucleus proceeded a rainbow-tinted streak, indicative of solid or liquid matter, which, in this comet, must have been of very scanty amount. its visit to the sun in was, so far as is known, the first. the elements of its orbit showed no resemblance to those of any previous comet, nor any marked signs of periodicity. so that, although it may be considered probable, we do not _know_ that it is moving in a closed curve, or will ever again penetrate the precincts of the solar system. it was last seen from the southern hemisphere, october , . the third of a quartette of lucid comets visible within sixteen months, was discovered by mr. c. s. wells at the dudley observatory, albany, march , . two days later it was described by mr. lewis boss as "a great comet in miniature," so well defined and regularly developed were its various parts and appendages. discernible with optical aid early in may, it was on june observed on the meridian at albany just before noon--an astronomical event of extreme rarity. comet wells, however, never became an object so conspicuous as to attract general attention, owing to its immersion in the evening twilight of our northern june. but the study of its spectrum revealed new facts of the utmost interest. all the comets till then examined had been found (with the two transiently observed exceptions already mentioned) to conform to one invariable type of luminous emission. individual distinctions there had been, but no specific differences. now all these bodies had kept at a respectful distance from the sun; for of the great comet of no spectroscopic inquiries had been made. comet wells, on the other hand, approached its surface within little more than five million miles on june , ; and the vicinity had the effect of developing a novel feature in its incandescence. during the first half of april its spectrum was of the normal type, though the carbon bands were unusually weak; but with approach to the sun they died out, and the entire light seemed to become concentrated into a narrow, unbroken, brilliant streak, hardly to be distinguished from the spectrum of a star. this unusual behaviour excited attention, and a strict watch was kept. it was rewarded at the dunecht observatory, may , by the discernment of what had never before been seen in a comet--the yellow ray of sodium.[ ] by june , this had kindled into a blaze overpowering all other emissions. the light of the comet was practically monochromatic; and the image of the entire head, with the root of the tail, could be observed, like a solar prominence, depicted, in its new saffron vesture of vivid illumination, within the jaws of an open slit. at potsdam, the bright yellow line was perceived with astonishment by vogel on may , and was next evening identified with fraunhofer's "d." its character led him to infer a very considerable density in the glowing vapour emitting it.[ ] hasselberg founded an additional argument in favour of the electrical origin of cometary light on the changes in the spectrum of comet wells.[ ] for they were closely paralleled by some earlier experiments of wiedemann, in which the gaseous spectra of vacuum tubes were at once effaced on the introduction of metallic vapours. it seemed as if the metal had no sooner been rendered volatile by heat, than it usurped the entire office of carrying the discharge, the resulting light being thus exclusively of its production. had simple incandescence by heat been in question, the effect would have been different; the two spectra would have been superposed without prejudice to either. similarly, the replacement of the hydro-carbon bands in the spectrum of the comet by the sodium line proved electricity to be the exciting agent. for the increasing thermal power of the sun might, indeed, have ignited the sodium, but it could not have extinguished the hydro-carbons. sir william huggins succeeded in photographing the spectrum of comet wells by an exposure of one hour and a quarter.[ ] the result was to confirm the novelty of its character. none of the ultra-violet carbon groups were apparent; but certain bright rays, as yet unidentified, had imprinted themselves. otherwise the spectrum was strongly continuous, uninterrupted even by the fraunhofer lines detected in the spectrum of tebbutt's comet. hence it was concluded that a smaller proportion of reflected light was mingled with the native emissions of the later arrival. all that is certainly known about the _extent_ of the orbit traversed by the first comet of is that it came from, and is now retreating towards, vastly remote depths of space. an american computer[ ] found a period indicated for it of no less than , years; a. thraen of dingelstädt arrived at one of .[ ] both are perhaps equally insecure. we have now to give some brief account of one of the most remarkable cometary apparitions on record, and--with the single exception of that identified with the name of halley--the most instructive to astronomers. the lessons learned from it were as varied and significant as its aspect was splendid; although from the circumstance of its being visible in general only before sunrise, the spectators of its splendour were comparatively few. the discovery of a great comet at rio janeiro, september , , became known in europe through a telegram from m. cruls, director of the observatory at that place. it had, however (as appeared subsequently), been already seen on the th by mr. finlay of the cape observatory, and at auckland as early as september . a later, but very singularly conditioned detection, quite unconnected with any of the preceding, was effected by dr. common at ealing. since the eclipse of may , when a comet--named "tewfik" in honour of the khedive of egypt--was caught on dr. schuster's photographs, entangled, one might almost say, in the outer rays of the corona, he had scrutinized the neighbourhood of the sun on the infinitesimal chance of intercepting another such body on its rapid journey thence or thither. we record with wonder that, after an interval of exactly four months, that infinitesimal chance turned up in his favour. on the forenoon of sunday, september , he saw a great comet close to, and rapidly approaching the sun. it was, in fact, then within a few hours of perihelion. some measures of position were promptly taken; but a cloud-veil covered the interesting spectacle before mid-day was long past. mr. finlay at the cape was more completely fortunate. divided from his fellow-observer by half the world, he unconsciously finished, under a clearer sky, his interrupted observation. the comet, of which the silvery radiance contrasted strikingly with the reddish-yellow glare of the sun's margin it drew near to, was followed "continuously right into the boiling of the limb"--a circumstance without precedent in cometary history.[ ] dr. elkin, who watched the progress of the event with another instrument, thought the intrinsic brilliancy of the nucleus scarcely surpassed by that of the sun's surface. nevertheless it had no sooner touched it than it vanished as if annihilated. so sudden was the disappearance (at h. m. s., cape mean time), that the comet was at first believed to have passed _behind_ the sun. but this proved not to have been the case. the observers at the cape had witnessed a genuine transit. nor could non-visibility be explained by equality of lustre. for the gradations of light on the sun's disc are amply sufficient to bring out against the dusky background of the limb any object matching the brilliancy of the centre; while an object just equally luminous with the limb must inevitably show dark at the centre. the only admissible view, then, is that the bulk of the comet was of too filmy a texture, and its presumably solid nucleus too small, to intercept any noticeable part of the solar rays--a piece of information worth remembering. plate iii. [illustration: the great comet of september, . photographed at the royal observatory, cape of good hope] on the following morning, the object of this unique observation showed (in sir david gill's words) "an astonishing brilliancy as it rose behind the mountains on the east of table bay, and seemed in no way diminished in brightness when the sun rose a few minutes afterward. it was only necessary to shade the eye from direct sunlight with the hand at arm's length, to see the comet, with its brilliant white nucleus and dense white, sharply bordered tail of quite half a degree in length."[ ] all over the world, wherever the sky was clear during that day, september , it was obvious to ordinary vision. since nothing had been seen like it. from spain, italy, algeria, southern france, despatches came in announcing the extraordinary appearance. at cordoba, in south america, the "blazing star near the sun" was the one topic of discourse.[ ] moreover--and this is altogether extraordinary--the records of its daylight visibility to the naked eye extend over three days. at reus, near tarragona, it showed bright enough to be seen through a passing cloud when only three of the sun's diameters from his limb, just before its final rush past perihelion on september ; while at carthagena in spain, on september , it was kept in view during two hours before and two hours after noon, and was similarly visible in algeria on the same day.[ ] but still more surprising than the appearance of the body itself were the nature and relations of the path it moved in. the first rough elements computed for it by mr. tebbutt, dr. chandler, and mr. white, assistant at the melbourne observatory, showed at once a striking resemblance to those of the twin comets of and . this suggestive fact became known in this country, september , through the medium of a dunecht circular. it was fully confirmed by subsequent inquiries, for which ample opportunities were luckily provided. the likeness was not, indeed, so absolutely perfect as in the previous case; it included some slight, though real differences; but it bore a strong and unmistakable stamp, broadly challenging explanation. two hypotheses only were really available. either the comet of was an accelerated return of those of and , or it was a fragment of an original mass to which they also had belonged. for the purposes of the first view the "resisting medium" was brought into full play; the opinion of its efficacy was for some time both prevalent and popular, and formed the basis, moreover, of something of a sensational panic. for a comet which, at a single passage through the sun's atmosphere, encountered sufficient resistance to shorten its period from thirty-seven to two years and eight months, must, in the immediate future, be brought to rest on his surface; and the solar conflagration thence ensuing was represented in some quarters, with more licence of imagination than countenance from science, as likely to be of catastrophic import to the inhabitants of our little planet. but there was a test available in which it had not been possible to apply either in or in . the two bodies visible in those years had been observed only after they had already passed perihelion;[ ] the third member of the group, on the other hand, was accurately followed for a week before that event, as well as during many months after it. finlay's and elkin's observation of its disappearance at the sun's edge formed, besides, a peculiarly delicate test of its motion. the opportunity was thus afforded, by directly comparing the comet's velocity before and after its critical plunge through the solar surroundings, of ascertaining with approximate certainty whether any considerable retardation had been experienced in the course of that plunge. the answer distinctly given was that there had not. the computed and observed places on both sides of the sun fitted harmoniously together. the effect, if any were produced, was too small to be perceptible. this result is, in itself, a memorable one. it seems to give the _coup de grâce_ to encke's theory--discredited, in addition, by backlund's investigation--of a resisting medium growing rapidly denser inwards. for the perihelion distance of the comet of , though somewhat greater than that of its predecessors, was nevertheless extremely small. it passed at less than , miles of the sun's surface. but the ethereal substance long supposed to obstruct the movement of encke's comet would there be nearly , times denser than at the perihelion of the smaller body, and must have exerted a conspicuous retarding influence. that none such could be detected seems to argue that no such medium exists. further evidence of a decisive kind was not wanting on the question of identity. the "great september comet" of was in no hurry to withdraw itself from curious terrestrial scrutiny. it was discerned with the naked eye at cordoba as late as march , , and still showed in the field of the great equatoreal on june as an "excessively faint whiteness."[ ] it was then about millions of miles from the earth--a distance to which no other comet--not even excepting the peculiar one of --had been pursued.[ ] moreover, an arc of out of the entire degrees of its circuit had been described under the eyes of astronomers; so that its course came to be very well known. that its movement is in a very eccentric ellipse, traversed in several hundred years, was ascertained.[ ] the later inquiries of dr. kreutz,[ ] completed in a volume published in ,[ ] demonstrated the period to be of about years, while that of its predecessor in might possibly agree with it, but is much more probably estimated at years. the hypothesis that they, or any of the comets associated with them, were returns of an individual body is peremptorily excluded. they may all, however, have been separated from one original mass by the divellent action of the sun at close quarters. each has doubtless its own period, since each has most likely suffered retardations or accelerations special to itself, which, though trifling in amount, would avail materially to alter the length of the major axis, while leaving the remaining elements of the common orbit virtually unchanged.[ ] a fifth member was added to the family in . on the th of january in that year, m. thome discovered at cordoba a comet reproducing with curious fidelity the lineaments of that observed in the same latitudes seven years previously. the narrow ribbon of light, contracting towards the sun, and running outward from it to a distance of thirty-five degrees; the unsubstantial head--a veiled nothingness, as it appeared, since no distinct nucleus could be made out; the quick fading into invisibility, were all accordant peculiarities, and they were confirmed by some rough calculations of its orbit, showing geometrical affinity to be no less unmistakable than physical likeness. the observations secured were indeed, from the nature of the apparition, neither numerous nor over-reliable; and the earliest of them dated from a week after perihelion, passed, almost by a touch-and-go escape, january . on january , this mysterious object could barely be discerned telescopically at cordoba.[ ] that it belonged to the series of "southern comets" can scarcely be doubted; but the inference that it was an actual return of the comet of , improbable in itself, was negatived by its non-appearance in . meyer's incorporation with this extraordinary group of the "eclipse-comet" of [ ] has been approved by kreutz, after searching examination. the idea of cometary systems was first suggested by thomas clausen in .[ ] it was developed by the late m. hoek, director of the utrecht observatory, in and some following years.[ ] he found that in quite a considerable number of cases, the paths of two or three comets had a common point of intersection far out in space, indicating with much likelihood a community of origin. this consisted, according to his surmise, in the disruption of a parent mass during its sweep round the star latest visited. be this as it may, the fact is undoubted that numerous comets fall into groups, in which similar conditions of motion betray a pre-existent physical connection. never before, however, had geometrical relationship been so notorious as between the comets now under consideration; and never before, in a comet still, it might be said, in the prime of life, had physical peculiarities tending to account for that affinity been so obvious as in the chief member of the group. observation of a granular structure in cometary nuclei dates far back into the seventeenth century, when cysatus and hevelius described the central parts of the comets of and respectively as made up of a congeries of minute stars. analogous symptoms of a loose state of aggregation have of late been not unfrequently detected in telescopic comets, besides the instances of actual division offered by those connected with the names of biela and liais. the forces concerned in producing these effects seem to have been peculiarly energetic in the great comet of . the segmentation of the nucleus was first noticed in the united states and at the cape of good hope, september . it proceeded rapidly. at kiel, on october and , professor krüger perceived two centres of condensation. a definite and progressive separation into _three_ masses was observed by professor holden, october and .[ ] a few days later, m. tempel found the head to consist of _four_ lucid aggregations, ranged nearly along the prolongation of the caudal axis;[ ] and dr. common, january , , saw _five_ nuclei in a line "like pearls on a string."[ ] this remarkable character was preserved to the last moment of the comet's distinct visibility. it was a consequence, according to dr. kreutz, of violent interior action in the comet itself while close to the sun. there were, however, other curious proofs of a disaggregative tendency in this body. on october , schmidt discovered at athens a nebulous object ° south-west of the great comet, and travelling in the same direction. it remained visible for a few days, and, from oppenheim's and hind's calculations, there can be little doubt that it was really the offspring by fission of the body it accompanied.[ ] this is rendered more probable by the unexampled spectacle offered, october , to professor barnard, then of nashville, tennessee, of _six or eight_ distinct cometary masses within ° south by west of the comet's head, none of which reappeared on the next opportunity for a search.[ ] a week later, however, one similar object was discerned by mr. w. r. brooks, in the opposite direction from the comet. thus space appeared to be strewn with the filmy débris of this beautiful but fragile structure all along the track of its retreat from the sun. its tail was only equalled (if it were equalled) in length by that of the comet of . it extended in space to the vast distance of millions of miles from the head; but, so imperfectly were its proportions displayed to terrestrial observers, that it at no time covered an arc of the sky of more than °. this apparent extent was attained, during a few days previous to september , by a faint, thin, rigid streak, noticed only by a few observers--by elkin at the cape observatory, eddie at grahamstown, and cruls at rio janeiro. it diverged at a low angle from the denser curved train, and was produced, according to brédikhine,[ ] by the action of a repulsive force twelve times as strong as the counter-pull of gravity. it belonged, that is, to type ; while the great bifurcate appendage, obvious to all eyes, corresponded to the lower rate of emission characteristic of type . this was remarkable for the perfect definiteness of its termination, for its strongly-forked shape, and for its unusual permanence. down to the end of january, , its length, according to schmidt's observations, was still million miles; and a week later it remained visible to the naked eye, without notable abridgment. most singular of all was an anomalous extension of the appendage _towards_ the sun. during the greater part of october and november, a luminous "tube" or "sheath," of prodigious dimensions, seemed to surround the head, and project in a direction nearly opposite to that of the usual outpourings of attentuated matter. (see plate iii.) its diameter was computed by schmidt to be, october , no less than four million miles, and it was described by cruls as a "truncated cone of nebulosity," stretching ° or ° sunwards.[ ] this, and the entire anterior part of the comet, were again surrounded by a thin, but enormously voluminous paraboloidal envelope, observed by schiaparelli for a full month from october .[ ] there can be little doubt that these abnormal effluxes were a consequence of the tremendous physical disturbance suffered at perihelion; and it is worth remembering that something analogous was observed in the comet of (newton's), also noted for its excessively close approach to the sun, and possibly moving in a related orbit. the only plausible hypothesis as to the mode of their production is that of an opposite state of electrification in the particles composing the ordinary and extraordinary appendages. the spectrum of the great comet of was, in part, a repetition of that of its immediate predecessor, thus confirming the inference that the previously unexampled sodium-blaze was in both a direct result of the intense solar action to which they were exposed. but the d line was, this time, not seen alone. at dunecht, on the morning of september , drs. copeland and j. g. lohse succeeded in identifying six brilliant rays in the green and yellow with as many prominent iron-lines;[ ] a very significant addition to our knowledge of cometary constitution, and one which lent countenance to brédikhine's assumption of various kinds of matter issuing from the nucleus with velocities inversely as their atomic weights. all the lines equally showed a slight displacement, indicating a recession from the earth of the radiating body at the rate of to miles a second. a similar observation, made by m. thollon at nice on the same day, gave emphatic sanction to the spectroscopic method of estimating movement in the line of sight. before anything was as yet known of the comet's path or velocity, he announced, from the position of the double sodium-line alone, that at p.m. on september it was increasing its distance from our planet by from to kilometres per second.[ ] m. bigourdan's subsequent calculations showed that its actual swiftness of recession was at that moment kilometres. changes in the inverse order to those seen in the spectrum of comet wells soon became apparent. in the earlier body, carbon bands had died out with _approach_ to perihelion, and had been replaced by sodium emissions; in its successor, sodium emissions became weakened and disappeared with _retreat_ from perihelion, and found their substitute in carbon bands. professor riccò was, in fact, able to infer, from the sequence of prismatic phenomena, that the comet had already passed the sun; thus establishing a novel criterion for determining the position of a comet in its orbit by the varying quality of its radiations. recapitulating what was learnt from the five conspicuous comets of - , we find that the leading facts acquired to science were these three. first, that comets may be met with pursuing each other, after intervals of many years, in the same, or nearly the same, track; so that identity of orbit can no longer be regarded as a sure test of individual identity. secondly, that at least the outer corona may be traversed by such bodies with perfect apparent impunity. finally, that their chemical constitution is highly complex, and that they possess, in some cases at least, a metallic core resembling the meteoric masses which occasionally reach the earth from planetary space. a group of five comets, including halley's, own a sort of cliental dependence upon the planet neptune. they travel out from the sun just to about his distance from it, as if to pay homage to a powerful protector, who gets the credit of their establishment as periodical visitors to the solar system. the second of these bodies to affect a looked-for return was a comet--the sixteenth within ten years--discovered by pons, july , , and found by encke to revolve in an elliptic orbit, with a period of nearly years. it was not, however, until september , , that mr. brooks caught its reappearance; it passed perihelion january , and was last seen june , . at its brightest, it had the appearance of a second magnitude star, furnished with a poorly developed double tail, and was fairly conspicuous to the naked eye in southern europe, from december to march. one exceptional feature distinguished it. its fluctuations in form and luminosity were unprecedented in rapidity and extent. on september , dr. chandler[ ] observed it at harvard as a very faint, diffused nebulosity, with slight central condensation. on the next night, there was found in its place a bright star of the eighth magnitude, scarcely marked out, by a bare trace of environing haze, from the genuine stars it counterfeited. the change was attended by an eight-fold augmentation of light, and was proved by schiaparelli's confirmatory observations[ ] to have been accomplished within a few hours. the stellar disguise was quickly cast aside. the comet appeared on september as a wide nebulous disc, and soon after faded down to its original dimness. its distance from the sun was then no less than million miles, and its spectrum showed nothing unusual. these strange variations recurred slightly on october , and with marked emphasis on january , when they were witnessed with amazement, and photometrically studied by müller of potsdam.[ ] the entire cycle this time was run through in less than four hours--the comet having, in that brief space, condensed, with a vivid outburst of light, into a seeming star, and the seeming star having expanded back again into a comet. scarcely less transient, though not altogether similar, changes of aspect were noted by m. perrotin,[ ] january and , . on the latter date, the continuous spectrum given by a reddish-yellow disc surrounding the true nucleus seemed intensified by bright knots corresponding to the rays of sodium. a comet discovered by mr. sawerthal at the royal observatory, cape of good hope, february , , distinguished itself by blazing up, on may , to four or five times its normal brilliancy, at the same time throwing out from the head two lustrous lateral branches.[ ] these had, on june , spread backward so as to join the tail, with an effect like the playing of a fountain; ten or eleven days later, they had completely disappeared, leaving the comet in its former shape and insignificance. its abrupt display of vitality occurred two full months after perihelion. on the morning of july , , mr. w. r. brooks, of geneva, new york, eminent as a successful comet-hunter, secured one of his customary trophies. the faint object in question was moving through the constellation cetus, and turned out to be a member of jupiter's numerous family of comets, revolving round the sun in a period of seven years. its past history came then, to a certain extent, within the scope of investigation, and proved to have been singularly eventful; nor had the body escaped scatheless from the vicissitudes to which it had been exposed. observing from mount hamilton, august and , professor barnard noticed this comet ( , v.) to be attended in its progress through space by four _outriders_, "the two brighter companions" (the fainter pair survived a very short time) "were perfect miniatures," professor barnard tells us,[ ] "of the larger comet, each having a small, fairly defined head and nucleus, with a faint, hazy tail, the more distant one being the larger and less developed. the three comets were in a straight line, nearly east and west, their tails lying along this line. there was no connecting nebulosity between these objects, the tails of the two smaller not reaching each other, or the large comet. to all appearance they were absolutely independent comets." nevertheless, spitaler, at vienna, in the early days of august, perceived, as it were, a thin cocoon of nebulosity woven round the entire trio.[ ] one of them faded from view september ; the other actually outshone the original comet on august , but was plainly of inferior vitality. it was last seen by barnard on november , with the thirty-six inch refractor, while its primary afforded an observation for position with the twelve-inch, march , .[ ] a cause for the disruption it had presumably undergone had, before then, been plausibly assigned. the adventures of lexell's comet have long served to exemplify the effects of jupiter's despotic sway over such bodies. although bright enough in to be seen with the naked eye, and ascertained to be circulating in five and a half years, it had never previously been seen, and failed subsequently to present itself. the explanation of this anomaly, suggested by lexell, and fully confirmed by the analytical inquiries both of laplace and leverrier,[ ] was that a very close approach to jupiter in had completely changed the character of its orbit, and brought it within the range of terrestrial observation; while in , after having only twice traversed its new path (at its second return it was so circumstanced as to be invisible from the earth), it was, by a fresh encounter, diverted into one entirely different. yet the possibility was not lost sight of that the great planet, by inverting its mode of action, might undo its own work, and fling the comet once more into the inner part of the solar system. this possibility seemed to be realized by chandler's identification of brooks's and lexell's comet.[ ] an exceedingly close approach to jupiter in had, he found reason to believe, produced such extensive alterations in the elements of its motion as to bring the errant body back to our neighbourhood in . but his inference, though ratified by mr. charles lane poor's preliminary calculations, proved dubious on closer inquiry, and was rendered wholly inadmissible by the circumstances attending the return of brooks's comet in .[ ] the companion-objects watched by barnard in had by that time, perhaps, become dissipated in space, for they were not redetected. they represented, in all likelihood, wreckage from a collision with jupiter, dating, perhaps, so far back as , when mr. lane poor found that one of the fateful meetings to which short-period comets are especially subject had taken place. the lexell-brooks case was almost duplicated by the resemblance to de vico's lost comet of [ ] of one detected november , , by edward, son of lewis swift. schulhof[ ] announced the identity, and chandler,[ ] under reserve, vouched for it. had the comet continued to pursue the track laboriously laid down for it at boston, and shown itself at the due epoch in , its individuality might have been considered assured; but the formidable vicegerent of the sun once more interposed, and, in , swept it out of the terrestrial range of view. hence the recognition remains ambiguous. on the morning of march , , professor lewis swift discovered the brightest comet that had been seen by northern observers since . about the time of perihelion, which occurred on april , it was conspicuous, as it crossed the celestial equator from aquarius towards pegasus, with a nucleus equal to a third magnitude star, and a tail twenty degrees long. this tail was multiple, and multiple in a most curiously variable manner. it divided up into many thin nebulous streaks, the number and relative lustre of which underwent rapid and marked changes. their permanent record on barnard's and w. h. pickering's plates marked a noteworthy advance in cometary photography. plate iv. reproduces two of the lick pictures, taken with a six-inch camera, on april and respectively, with, in each case, an exposure of about one hour. the tail is in the first composed of three main branches, the middle one having sprung out since the previous morning, and the branches are, in their turn, split up into finer rays, to the number of perhaps a dozen in all. in the second a very different state of things is exhibited. "the southern component," professor barnard remarked, "which was the brightest on the th, had become diffused and fainter, while the middle tail was very bright and broad. its southern side, which was the best defined, was wavy in numerous places, the tail appearing as if disturbing currents were flowing at right angles to it. at ° from the head the tail made an abrupt bend towards the south, as if its current was deflected by some obstacle. in the densest portion of the tail, at the point of deflection, are a couple of dark holes, similar to those seen in some of the nebulæ. the middle portion of the tail is brighter, and looks like crumpled silk in places."[ ] next morning the southern was the prominent branch, and it was loaded, at ° ' from the head, with a strange excrescence, suggesting the budding-out of a fresh comet in that incongruous situation.[ ] some of these changes, professor barnard thought, might possibly be explained by a rotation of the tail on an axis passing through the nucleus, and pickering, who formed a similar opinion on independent grounds, assigned about hours as the period of the gyrating movement.[ ] he, moreover, determined accelerative velocities outward from the sun of definite condensations in the tail, indicating for its materials, on brédikhine's theory, a density less than one half that of hydrogen.[ ] this conclusion applied also to rordame's comet, which exhibited a year later phenomena analogous to those remarked in swift's. their photographic study led professor hussey[ ] to significant inferences as to the structure and rapid changes of cometary appendages. plate iv. [illustration: photographs of swift's comet. by professor e. e. barnard. no. . taken april , ; exposure h. no. . taken april , ; exposure h. m ] seven comets were detected in , and all, strange to say, were visible together towards the close of the year.[ ] among them was a faint object, which unexpectedly left a trail on a plate exposed by professor barnard to the stars in aquila[ ] on october . this was the first comet actually discovered by photography, the sohag comet having been simultaneously seen and pictured. it has a period of about six years. holmes's comet is likewise periodical, in rather less than seven years. its path, which is wholly comprised between the orbits of mars and jupiter, is less eccentric than that of any other known comet. subsequently to its discovery, on november , it underwent some curious vicissitudes. at first bright and condensed, it expanded rapidly with increasing distance from the sun (to which it had made its nearest approach on june ), until, by the middle of december, it was barely discernible with powerful telescopes as "a feebly luminous mist on the face of the sky."[ ] but on january , , observers in europe and america were bewildered to find, as if substituted for it, a yellow star of the seventh magnitude, enveloped in a thin nebulous husk, which enclosed a faint miniature tail.[ ] this condensation and recovery of light lasted in its full intensity only a couple of days. the almost evanescent faintness of holmes's comet at its next return accounted for its invisibility previous to , when it was evidently in a state of peculiar excitement. mr. perrine was barely able, with the lick -inch, to find the vague nebulous patch which occupied its predicted place on june , . the origin of comets has been long and eagerly inquired into, not altogether apart from the cheering guidance of ascertained facts. sir william herschel regarded them as fragments of nebulæ[ ]--scattered débris of embryo worlds; and laplace approved of and adopted the idea.[ ] but there was a difficulty. no comet has yet been observed to travel in a decided hyperbola. the typical cometary orbit, apart from disturbance, is parabolic--that is to say, it is indistinguishable from an enormously long ellipse. but this circumstance could only be reconciled with the view that the bodies thus moving were casual visitors from outer space, by making, as laplace did, the tacit assumption that the solar system was at rest. his reasoning was, indeed, thereby completely vitiated, as gauss pointed out in ;[ ] and the objections then urged were reiterated by schiaparelli,[ ] who demonstrated in that a large preponderance of well-marked hyperbolic orbits should result if comets were picked up _en route_ by a swiftly-advancing sun. the fact that their native movement is practically parabolic shows it to have been wholly imparted from without. they passively obeyed the pull exerted upon them. in other words, their condition previous to being attracted by the sun was one very nearly of relative repose.[ ] they shared, accordingly, the movement of translation through space of the solar system. this significant conclusion had been indicated, on other grounds, as the upshot of researches undertaken independently by carrington[ ] and mohn[ ] in , with a view to ascertaining the anticipated existence of a relationship between the general _lie_ of the paths of comets and the direction of the sun's journey. it is tolerably obvious that if they wander at haphazard through interstellar regions their apparitions should markedly aggregate towards the vicinity of the constellation lyra; that is to say, we should meet considerably more comets than would overtake us, for the very same reason that falling stars are more numerous after than before midnight. moreover, the comets met by us should be, apparently, swifter-moving objects than those coming up with us from behind; because, in the one case, our own real movement would be added to, in the other subtracted from, theirs. but nothing of all this can be detected. comets approach the sun indifferently from all quarters, and with velocities quite independent of direction. we conclude, then, that the "cosmical current" which bears the solar system towards its unknown goal carries also with it nebulous masses of undefined extent, and at an undefined remoteness, fragments detached from which, continually entering the sphere of the sun's attraction, flit across our skies under the form of comets. these are, however, almost certainly so far strangers to our system that they had no part in the long processes of development by which its present condition was attained. they are, perhaps, survivals of an earlier, and by us scarcely and dimly conceivable state of things, when the swirling chaos from which sun and planets were, by a supreme edict, to emerge, had not as yet separately begun to be. footnotes: [footnote : _astr. nach._, nos. , - .] [footnote : _berichte sächs. ges._, , p. .] [footnote : _natur der cometen_, p. ; _astr. nach._, no. , .] [footnote : _annales de l'obs. de moscou_, t. iii., pt. i., p. .] [footnote : _bull. astr._, t. iii., p. . the value of the repellent force for the comet of (which offered peculiar facilities for its determination) was found = · .] [footnote : faye, _comptes rendus_, t. xciii., p. .] [footnote : _annales_, t. v., pt. ii., p. .] [footnote : _am. jour. of sc._, vol. xxxii. ( nd ser.), p. .] [footnote : _astr. nach._, no. , .] [footnote : _annales de l'obs. de moscou_, t. vi., pt. i., p. .] [footnote : _astr. register_, march, .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, no. , .] [footnote : _constitution de l'espace céleste_, p. .] [footnote : _astroph. jour._, vol. iii., p. .] [footnote : _physikalische zeitschrift_, november and , ; _astroph. jour._, vol. xiii., p. . _cf._ schwarzschild, _sitzungsb._, münchen, , heft iii.; j. hahn, _nature_, vols. lxv., p. ; lxvi., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, no. , .] [footnote : _observatory_, vol. iii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ueber die kometen von v. chr._, , , i. und i. göttingen, .] [footnote : _meteor._, lib. i., cap. .] [footnote : _mém. soc. phys. de genève_, t. xxviii., p. .] [footnote : _annales de l'obs. de moscou_, t. vii., pt. i., p. .] [footnote : brédikhine, _annales_, t. viii., p. .] [footnote : _am. jour. of sc._, vol. xxii., p. .] [footnote : messrs. burton and green observed a dilatation of the stellar image into a nebulous patch by the transmission of its rays through a nuclear jet of the comet. _am. jour. of sc._, vol. xxii., p. .] [footnote : _archives des sciences_, t. viii., p. . _cf._ perrine's negative results for swift's comet in , _astr. nach._, no. , .] [footnote : riem concluded in for a definitive period of , years; _observatory_, vol. xix., p. .] [footnote : holden, _publ. astr. pac. soc._, vol. ix., p. .] [footnote : _annuaire_, paris, , p. .] [footnote : _annuaire_, , p. .] [footnote : _am. jour. of sc._, vol. xxii., p. .] [footnote : _report brit. assoc._, , p. .] [footnote : _month. not._, vol. xlii., p. ; _am. jour. of sc._, vol. xxii., p. .] [footnote : piazzi smyth, _nature_, vol. xxiv., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._] [footnote : _astr. nach._, no. , .] [footnote : _month. not._, vol. xlii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _copernicus_, vol. ii., p. .] [footnote : _astr. nach._, nos. , , , .] [footnote : _ibid._, no. , .] [footnote : _report brit. assoc._, , p. .] [footnote : j. j. parsons, _am. jour. of science_, vol. xxvii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _observatory_, vol. v., p. . the transit had been foreseen by mr. tebbutt, but it occurred after sunset in new south wales.] [footnote : _observatory_, vol. v., p. .] [footnote : gould, _astr. nach._, no. , .] [footnote : flammarion, _comptes rendus_, t. xcv., p. .] [footnote : captain ray's sextant observation of the comet of , a few hours before perihelion, was too rough to be of use.] [footnote : _astr. nach._, no. , .] [footnote : _nature_, vol. xxix., p. .] [footnote : _astr. nach._, no. , .] [footnote : _vierteljahrsschrift astr. ges._, jahrg. xxiv., p. ; _bull. astr._, t. vii., p. .] [footnote : _observatory_, vol. xxiv., p. .] [footnote : the attention of the author was kindly directed to this point by professor young of princeton (n. j.). _cf._ rebeur-paschwitz, _sirius_, bd. xvi., p. .] [footnote : oppenheim, _astr. nach._, no. , .] [footnote : _astr. nach._, no. , .] [footnote : gruithuisen's _analekten_, heft , p. .] [footnote : _month. not._, vols. xxv., xxvi., xxviii. _cf._ plummer, _observatory_, vol. xiii., p. .] [footnote : _nature_, vol. xxvii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _athenæum_, february , .] [footnote : _astr. nach._, nos. , , , .] [footnote : _ibid._, no. , .] [footnote : _annales_, moscow, t. ix., pt. ii., p. .] [footnote : _comptes rendus_, t. xcvii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _copernicus_, vol. ii., p. .] [footnote : _comptes rendus_, t. xcvi., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._] [footnote : _astr. nach._, no. , .] [footnote : _annales de l'observatoire de nice_, t. ii., c. .] [footnote : fényi, _astr. nach._, no. , ; kammermann, _ibid._, no. , .] [footnote : _publ. astr. pac. soc._, vol. i., p. .] [footnote : _annuaire_, paris, , p. .] [footnote : _astr. nach._, no. , .] [footnote : _comptes rendus_, t. xxv., p. .] [footnote : _astr. journ._, nos. , .] [footnote : _ibid._, nos. , , .] [footnote : _observatory_, vol. xviii., pp. , (denning and lynn).] [footnote : _astr. nach._, no. , ; plummer, _knowledge_, vol. xix., p. .] [footnote : _astr. jour._, nos. , .] [footnote : _astr. and astroph._, vol. xi., p. .] [footnote : _knowledge_, vol. xv., p. .] [footnote : _harvard annals_, vol. xxxii., pt. ii., p. .] [footnote : _ibid._, p. .] [footnote : _publ. astr. pac. soc._, vol. vii., p. .] [footnote : h. c. wilson, _astr. and astroph._, vol. xii., p. .] [footnote : _observatory_, vol. xvi., p. .] [footnote : barnard, _astr. and astroph._, vol. xii., p. ; _astroph. jour._, vol. iii., p. .] [footnote : palisa, _astr. nach._, no. , ; denning, _observatory_, vol. xvi., p. .] [footnote : _phil. trans._, vol. ci., p. .] [footnote : _conn. des temps_, , p. .] [footnote : _oeuvres_, t. vi., p. .] [footnote : _mem. dell' istit. lombardo_, t. xii., p. ; _rendiconti_, t. vii., p. , .] [footnote : w. förster, _pop. mitth._, , p. ; fabry, _Étude sur la probabilité des comètes hyperboliques_, marseille, , p. .] [footnote : _mem. r. a. soc._, vol. xxix., p. .] [footnote : _month. not._, vol. xxiii., p. .] chapter xii _stars and nebulÆ_ that a science of stellar chemistry should not only have become possible, but should already have made material advances, is assuredly one of the most amazing features in the swift progress of knowledge our age has witnessed. custom can never blunt the wonder with which we must regard the achievement of compelling rays emanating from a source devoid of sensible magnitude through immeasurable distance, to reveal, by its distinctive qualities, the composition of that source. the discovery of revolving double stars assured us that the great governing force of the planetary movements, and of our own material existence, sways equally the courses of the farthest suns in space; the application of prismatic analysis certified to the presence in the stars of the familiar materials, no less of the earth we tread, than of the human bodies built up out of its dust and circumambient vapours. we have seen that, as early as , fraunhofer ascertained the generic participation of stellar light in the peculiarity by which sunlight, spread out by transmission through a prism, shows numerous transverse rulings of interrupting darkness. no sooner had kirchhoff supplied the key to the hidden meaning of those ciphered characters than it was eagerly turned to the interpretation of the dim scrolls unfolded in the spectra of the stars. donati made at florence in the first efforts in this direction; but with little result, owing to the imperfections of the instrumental means at his command. his comparative failure, however, was a prelude to others' success. almost simultaneously, in , the novel line of investigation was entered upon by huggins near london, by father secchi at rome, and by lewis m. rutherfurd in new york. fraunhofer's device of using a cylindrical lens for the purpose of giving a second dimension to stellar spectra was adopted by all, and was, indeed, indispensable. for a luminous point, such as a star appears, becomes, when viewed through a prism, a variegated line, which, until broadened into a band by the intervention of a cylindrical lens, is all but useless for purposes of research. this process of _rolling out_ involves, it is true, much loss of light--a scanty and precious commodity, as coming from the stars; but the loss is an inevitable one. and so fully is it compensated by the great light-grasping power of modern telescopes that important information can now be gained from the spectroscopic examination of stars far below the range of the unarmed eye. the effective founders of stellar spectroscopy, then (since rutherfurd shortly turned his efforts elsewhither), were father secchi, the eminent jesuit astronomer of the collegio romano, where he died, february , , and sir william huggins, with whom the late professor w. a. miller was associated. the work of each was happily directed so as to supplement that of the other. with less perfect appliances, the roman astronomer sought to render his extensive rather than precise; at tulse hill searching accuracy over a narrow range was aimed at and attained. to father secchi is due the merit of having executed the first spectroscopic survey of the heavens. above , stars were passed in review by him, and classified according to the varying qualities of their light. his provisional establishment ( - ) of four types of stellar spectra[ ] has proved a genuine aid to knowledge through the facilities afforded by it for the arrangement and comparison of rapidly accumulating facts. moreover, it is scarcely doubtful that these spectral distinctions correspond to differences in physical condition of a marked kind. the first order comprises more than half the visible and probably an overwhelming proportion of the faintest stars. sirius, vega, regulus, altair, are amongst its leading members. their spectra are distinguished by the breadth and intensity of the four dark bars due to the absorption of hydrogen, and by the extreme faintness of the metallic lines, of which, nevertheless, hundreds are disclosed by careful examination. the light of these "sirian" orbs is white or bluish; and it is found to be rich in ultra-violet rays. capella and arcturus belong to the second, or solar type of stars, which is about one-sixth less numerously represented than the first. their spectra are quite closely similar to that of sunlight, in being ruled throughout by innumerable fine dark lines; and they share its yellowish tinge. the third class includes most red and variable stars (commonly synonymous), of which betelgeux in the shoulder of orion, and "mira" in the whale, are noted examples. their characteristic spectrum is of the "fluted" description. it shows like a strongly illuminated range of seven or eight variously tinted columns seen in perspective, the light falling from the red end towards the violet. this _kind_ of absorption is produced by the vapours of metalloids or of compound substances. to the fourth order of stars belongs also a colonnaded spectrum, but _reversed_; the light is thrown the other way. the three broad zones of absorption which interrupt it are sharp towards the red, insensibly gradated towards the violet end. the individuals composing class iv. are few and apparently insignificant, the brightest of them not exceeding the fifth magnitude. they are commonly distinguished by a deep red tint, and gleam like rubies in the field of the telescope. father secchi, who in detected the peculiarity of their analyzed light, ascribed it to the presence of carbon in some form in their atmospheres; and this was confirmed by the researches of h. c. vogel,[ ] director of the astro-physical observatory at potsdam. the hydro-carbon bands, in fact, seen bright in comets, are dark in these singular objects--the only ones in the heavens (save one bright-line star and a rare meteor)[ ] which display a cometary analogy of the fundamental sort revealed by the spectroscope. the members of all four orders are, however, emphatically suns. they possess, it would appear, photospheres radiating all kinds of light, and differ from each other mainly in the varying qualities of their absorptive atmospheres. the principle that the colours of stars depend, not on the intrinsic nature of their light, but on the kinds of vapours surrounding them, and stopping out certain portions of that light, was laid down by huggins in .[ ] the phenomena of double stars seem to indicate a connection between the state of the investing atmospheres, by the action of which their often brilliantly contrasted tints are produced, and their mutual physical relations. a tabular statement put forward by professor holden in june, ,[ ] made it, at any rate, clear that inequality of magnitude between the components of binary systems accompanies unlikeness in colour, and that stars more equally matched in one respect are pretty sure to be so in the other. besides, blue and green stars of a decided tinge are never solitary; they invariably form part of systems. so that association has undoubtedly a predominant influence upon colour. nevertheless, the crude notion thrown out by zöllner in ,[ ] that yellow and red stars are simply white stars in various stages of cooling, obtained for a time undeserved currency. d'arrest, indeed, protested against it, and Ångström, in ,[ ] substituted atmospheric quality for mere colour[ ] as a criterion of age and temperature. his lead was followed by lockyer in ,[ ] and by vogel in .[ ] the scheme of classification due to the potsdam astro-physicist differed from father secchi's only in presenting his third and fourth types as subdivisions of the same order, and in inserting three subordinate categories; but their variety was "rationalised" by the addition of the seductive idea of progressive development. thus, the white sirian stars were represented as the _youngest_ because the hottest of the sidereal family; those of the solar pattern as having already wasted much of their store by radiation, and being well advanced in middle life; while the red stars with banded spectra figured as effete suns, hastening rapidly down the road to final extinction. vogel's scheme is, however, incomplete. it traces the downward curve of decay, but gives no account of the slow ascent to maturity. the present splendour of vega, for instance, was prepared, according to all creative analogy, by almost endless processes of gradual change. what was its antecedent condition? the question has been variously answered. dr. johnstone stoney advocated, in , the comparative youth of red stars;[ ] a. ritter, of aix-la-chapelle, divided them, in ,[ ] into two squadrons, posted, the one on the ascending, the other on the descending branch of the temperature-curve, and corresponding, presumably, with secchi's third and fourth orders of stars with banded spectra. whether, in the interim, they should display spectra of the sirian or of the solar type was made to depend on their greater or less massiveness.[ ] but the relation actually existing perhaps inverts that contemplated by ritter. certainly, the evidence collected by mr. maunder in strongly supports the opinion that the average solar star is a weightier body than the average sirian star.[ ] on november , , sir norman lockyer communicated to the royal society the first of a series of papers embodying his "meteoritic hypothesis" of cosmical constitution, stated and supported more at large in a separate work bearing that name, published in . the fundamental proposition wrought out in it was that "all self-luminous bodies in the celestial space are composed either of swarms of meteorites or of masses of meteoric vapour produced by heat."[ ] on the basis of this supposed community of origin, sidereal objects were distributed in seven groups along a temperature-curve ascending from nebulæ and gaseous, or bright-line stars, through red stars of the third type, and a younger division of solar stars, to the high sirian level; then descending through the more strictly solar stars to red stars of the fourth type ("carbon-stars"), below which lay only the _caput mortuum_ entitled group vii. the ground-work of this classification was, however, insecure, and has given way. certain spectroscopic coincidences, avowedly only approximate, suggesting that stars and nebulæ of every species might be formed out of variously aggregated meteorites, failed of verification by exact inquiry. and spectroscopic coincidences admit of no compromise. those that are merely approximate are, as a rule, unmeaning. in his presidential address at the cardiff meeting of the british association in , dr. huggins adhered in the main to the line of advance traced by vogel. the inconspicuousness of metallic lines in the spectra of the white stars he attributed, not to the paucity, but to the high temperature of the vapours producing them, and the consequent deficiency of contrast between their absorption-rays and the continuous light of the photospheric background. "such a state of things would more probably," in his opinion, "be found in conditions anterior to the solar stage," while "a considerable cooling of the sun would probably give rise to banded spectra due to compounds." he adverted also to the influential effects upon stellar types of varying surface gravity, which being a function of both mass and bulk necessarily gains strength with wasting heat and consequent shrinkage. the same leading ideas were more fully worked out in "an atlas of representative stellar spectra," published by sir william and lady huggins in . they were, moreover, splendidly illustrated by a set of original spectrographic plates, while precision was added to the adopted classification by the separation of helium from hydrogen stars. the spectrum of the exotic substance terrestrially captured in is conspicuous by absorption, as vogel, lockyer, and deslandres promptly recognised in a considerable number of white stars, among them the pleiades and most of the brilliants in orion. mr. mcclean, whose valuable spectrographic survey of the heavens was completed at the cape in , found reason to conclude that they are in the first stage of development from gaseous nebulæ;[ ] and in this the tulse hill investigators unhesitatingly concur. the strongest evidence for the primitive state of white stars is found in their nebular relations. the components of groups, still involved and entangled with "silver braids" of cosmic mist, show, perhaps invariably, spectra of the helium type, occasionally crossed by bright rays. possibly all such stars have passed through a bright-line stage; but further evidence on the point is needed. relative density furnishes another important test of comparative age, and sirian stars are, on the whole, undoubtedly more bulky proportionately to their mass than solar stars. the rule, however, seems to admit of exceptions; hence the change from one kind of spectrum to the other is not inevitably connected with the attainment of a particular degree of condensation. there is reason to believe that it is anticipated in the more massive globes, despite their comparatively slow cooling, as a consequence of the greater power of gravity over their investing vaporous envelopes. this conclusion is enforced by the relations of double-star spectra. the fact that, in unequal pairs, the chief star most frequently shows a solar, its companion a sirian, spectrum can scarcely be otherwise explained than by admitting that, while the sequence of types is pursued in an invariable order, it is pursued much more rapidly in larger than in small orbs. it need not, indeed, be supposed that all stars are identical in constitution, and present identical life-histories.[ ] individualities in the one, and divergencies in the other, must be allowed for. yet the main track is plainly continuous, and leads by insensible gradations from nebulæ through helium stars to the sirian, and onward to the solar type, whence, by an inevitable transition, fluted, or "antarian,"[ ] spectra develop. the first-known examples of the class of gaseous stars--beta lyræ and gamma cassiopeiæ--were noticed by father secchi at the outset of his spectroscopic inquiries. both show _bright_ lines of hydrogen and helium, so that the peculiarity of their condition probably consists in the intense ignition of their chromospheric surroundings. their entire radiating surfaces might be described as _faculous_. that is to say, brilliant formations, such as have been photographed by professor hale on the sun's disc,[ ] cover, perhaps, the whole, instead of being limited to a small portion of the photospheric area. but this state of things is more or less inconstant. some at least of the bright rays indicative of it are subject to temporary extinctions. already in - , dr. vogel[ ] suspected the prevalence of such vicissitudes; and their reality was ascertained by m. eugen von gothard. after the completion of his new astrophysical observatory at herény in the autumn of , he repeatedly observed the spectra of both stars without perceiving a trace of bright lines; and was thus taken quite by surprise when he caught a twinkling of the crimson c in gamma cassiopeiæ, august , .[ ] a few days later, the whole range including d_ was lustrous. duly apprised of the recurrence of a phenomenon he had himself vainly looked for during some years, m. von konkoly took the opportunity of the great vienna refractor being placed at his disposal to examine with it the relighted spectrum on august .[ ] in its wealth of light c was dazzling; d_ and the green and blue hydrogen rays shone somewhat less vividly; d and the group _b_ showed faintly dark; while three broad absorption-bands, sharply terminated towards the red, diffuse towards the violet, shaded the spectrum near its opposite extremities. the previous absence of bright lines from the spectrum of this star was, however, by no means so protracted or complete as m. von gothard supposed. at dunecht, c was "superbly visible" december , [ ]; f was seen bright on october of the same year, and frequently at greenwich in - . the curious fact has, moreover, been adverted to by dr. copeland, that c _is much more variable than f_. to vogel, june , , the first was invisible, while the second was bright; at dunecht, january , , the conditions were so far inverted that c was resplendent, f comparatively dim. no spectral fluctuations were detected in gamma cassiopeiæ by keeler in ; but even with the giant telescope of mount hamilton, the helium-ray was completely invisible.[ ] it made, nevertheless, capricious appearances at south kensington during that autumn, and again october , ,[ ] while in september, , bélopolsky could obtain no trace of it on orthochromatic plates exposed with the -inch pulkowa refractor.[ ] still more noteworthy is the circumstance that the well-known green triplet of magnesium (_b_), recorded as dark by keeler in , came out bright on fifty-two spectrographs of the star taken by father sidgreaves during the years - .[ ] no fluctuations in the hydrogen-spectrum were betrayed by them; but subordinate lines of unknown origin showed alternate fading and vivification. the spectrum of beta lyræ undergoes transitions to some extent analogous, yet involving a different set of considerations. first noticed by von gothard in ,[ ] they were imperfectly made out, two years later, to be of a cyclical character.[ ] this, however, could only be effectively determined by photographic means. beta lyræ is a "short-period variable." its light changes with great regularity from · to · magnitude every twelve days and twenty-two hours, during which time it attains a twofold maximum, with an intervening secondary minimum. the question, then, is of singular interest, whether the changes of spectral quality visible in this object correspond to its changes in visual brightness. a distinct answer in the affirmative was supplied through mrs. fleming's examination of the harvard plates of the star's spectrum, upon which, in , she found recorded diverse complex changes of bright and dark lines obviously connected with the phases of luminous variation, and obeying, in the long-run, precisely the same period.[ ] something more will be said presently as to the import of this discovery. bright hydrogen lines have so far been detected--for the most part photographically at harvard college--in about sixty stars, including pleione, the surmised lost pleiad, p cygni, noted for instability of light in the seventeenth century, and the extraordinary southern variable, eta carinæ. in most of these objects other vivid rays are associated with those due to hydrogen. a blaze of hydrogen, moreover, accompanies the recurring outbursts of about one hundred and fifty "long-period variables," giving banded spectra of the third type. professor pickering discovered the first example of this class, towards the close of , in mira ceti; further detections were made visually by mr. espin; and the conjunction of bright hydrogen-lines with dusky bands has been proved by mrs. fleming's long experience in studying the harvard photographs, to indicate unerringly the subjection of the stars thus characterised to variations of lustre accomplished in some months. a third variety of gaseous star is named after mm. wolf and rayet, who discovered, at paris in ,[ ] its three typical representatives, close together in the constellation cygnus. six further specimens were discovered by dr. copeland, five of them in the course of a trip for the exploration of visual facilities in the andes in ;[ ] and a large number have been made known through spectral photographs taken in both hemispheres under professor pickering's direction. at the close of the nineteenth century, over a hundred such objects had been registered, none brighter than the sixth magnitude, with the single exception of gamma argûs, the resplendent continuous spectrum of which, first examined by respighi and lockyer in , is embellished with the yellow and blue rays distinctive of the type.[ ] here, then, we have a stellar globe apparently at the highest point of sunlike incandescence, sharing the peculiarities of bodies verging towards the nebulous state. examined with instruments of adequate power, their spectra are seen to be highly complex. they include a fairly strong continuous element, a numerous set of absorption-lines, and a range of emission-lines, more or less completely represented in different stars. especially conspicuous is a broad effluence of azure light, found by dr. vogel in ,[ ] and by sir william and lady huggins in ,[ ] to be of multiple structure, and hence to vary in its mode of display. its suggested identification with the blue carbon-fluting was disproved at tulse hill. metallic vapours give no certain sign of their presence in the atmospheres of these remarkable bodies; but nebulum is stated to shine in some.[ ] hydrogen and helium account for a large proportion of their spectral rays. thirty-two wolf-rayet stars were investigated, spectroscopically and spectrographically, by professor campbell with the great lick refractor in - ;[ ] and several disclosed the singularity, already noticed by him in gamma argûs, of giving out mixed series, the members of which change from vivid to obscure with increase of refrangibility. it is difficult to imagine by what chromospheric machinery this curious result can be produced. alcyone in the pleiades presents the same characteristic. alone among the hydrogen lines, crimson c glows in its spectrum, while all the others are dark. luminosity of the wolf-rayet kind is particularly constant, both in quantity and quality. it seems to be incapable of developing save under galactic conditions. all the stars marked by it lie near the central line of the milky way, or in the magellanic clouds. they tend also to gather into groups. circles of four degrees radius include respectively seven in argo, eight in cygnus. the first spectroscopic star catalogue was published by dr. vogel at potsdam in .[ ] it included , stars, distributed over a zone of the heavens extending from ° north to ° south of the celestial equator.[ ] more than half of these were white stars, while red stars with banded spectra occurred in the proportion of about one-thirteenth of the whole. to the latter genus, m. dunér, then of lund, now director of the upsala observatory, devoted a work of standard authority, issued at stockholm in . this was a catalogue with descriptive particulars of stars showing banded spectra, of which belong to secchi's third, to his fourth class (vogel's iii. _a_ and iii. _b_). since then discovery has progressed so rapidly, at first through the telescopic reviews of mr. espin, then in the course of the photographic survey carried on at harvard college, that considerably over one thousand stars are at present recognised as of the family of betelgeux and mira, while about have so far exhibited the spectral pattern of piscium. one fact well ascertained as regards both species is the invariability of the type. the prismatic flutings of the one, and the broader zones of the other, are as if stereotyped--they undergo, in their fundamental outlines, no modification, though varying in relative intensity from star to star. they are always accompanied by, or superposed upon, a spectrum of dark lines, in producing which sodium and iron have an obvious share; and certain bright rays, noticed by secchi with imperfect appliances as enhancing the chiaroscuro effects in carbon-stars, came out upon plates exposed by hale and ellerman in with the stellar spectrograph of the yerkes observatory.[ ] their genuineness was shortly afterwards visually attested by keeler, campbell, and dunér;[ ] but no chemical interpretation has been found for them. a fairly complete preliminary answer to the question, what are the stars made of? was given by sir william huggins in .[ ] by laborious processes of comparison between stellar dark lines and the bright rays emitted by terrestrial substances, he sought to assure his conclusions, regardless of cost in time and pains. he averred, indeed, that--taking into account restrictions by weather and position--the thorough investigation of a _single_ star-spectrum would be the work of some years. of two, however--those of betelgeux and aldebaran--he was able to furnish detailed and accurate drawings. the dusky flutings in the prismatic light of the first of these stars have not been identified with the absorption of any particular substance; but associated with them are metallic lines, of which were measured, and a good many identified by huggins, while the wave-lengths of were determined by vogel in .[ ] a photographic research, made by keeler at the alleghany observatory in , convinced him that the linear spectrum of third-type stars of the betelgeux pattern essentially repeats that of the sun, but with marked differences in the comparative strength of its components.[ ] hydrogen rays are inconspicuously present. that an exalted temperature reigns, at least in the lower strata of the atmosphere, is certified by the vaporisation there of matter so refractory to heat as iron.[ ] nine elements--among them iron, sodium, calcium, and magnesium--were recognised by huggins as having stamped their signature on the spectrum of aldebaran; while the existence in sirius, and nearly all the other stars inspected, of hydrogen, together with sundry metals, was rendered certain or highly probable. this was admitted to be a bare gleaning of results; nor is there reason to suppose any of his congeners inferior to our sun in complexity of constitution. definite knowledge on the subject, however, made little advance beyond the point to which it was brought by huggins's early experiments until spectroscopic photography became thoroughly effective as a means of research. in this, as in so many other directions, sir william huggins acted as pioneer. in march, , he obtained microscopic prints of the spectra of sirius and capella.[ ] but they told nothing. no lines were visible in them. they were mere characterless streaks of light. nine years later dr. henry draper of new york got an impression of four lines in the spectrum of vega. then huggins attacked the subject again in , when the -inch speculum of the royal society had come into his possession, using prisms of iceland spar and lenses of rock crystal; and this time with better success. a photograph of the spectrum of vega showed seven strong lines.[ ] still he was not satisfied. he waited and worked for three years longer. at length, on december , , he was able to communicate to the royal society[ ] results answering to his expectations. the delicacy of eye and hand needed to obtain them may be estimated from the single fact that the image of a star had to be kept, by continual minute adjustments, exactly projected upon a slit / of an inch in width during nearly an hour, in order to give it time to imprint the characters of its analyzed light upon a gelatine plate raised to the highest pitch of sensitiveness. but by this time he had secured in his wife a rarely qualified assistant. the ultra-violet spectrum of the white stars--of which vega was taken as the type--was thus shown to be a very remarkable one. a group of broad dark lines intersected it, arranged at intervals diminishing regularly upward, and falling into a rhythmical succession with the visible hydrogen lines. all belonged presumably to the same substance; and the presumption was rendered a certainty by direct photographs of the hydrogen spectrum taken by h. w. vogel at berlin a few months earlier.[ ] in them seven of the white-star series of grouped lines were visible; and the full complement of twelve appeared on cornu's plates in .[ ] in yellow stars, such as capella and arcturus, the same rhythmical series was _partially_ represented, but associated with a great number of other lines; their state, as regards ultra-violet absorption, approximating to that of the sun; while the redder stars betrayed so marked a deficiency in actinic rays that from betelgeux, with an exposure _forty times_ that required for sirius, only a faint spectral impression could be obtained, and from aldebaran, in the strictly invisible region, almost none at all. thus, by the means of stellar light-analysis, acquaintance was first made with the ultra-violet spectrum of hydrogen;[ ] and its harmonic character, as expressed by "balmer's law," supplies a sure test for discriminating, among newly discovered lines, those that appertain from those that are unrelated to it. deslandres' five additional prominence-rays, for instance, were at once seen to make part of the series, because conforming to its law;[ ] while a group of six dusky bands, photographed by sir william and lady huggins, april , ,[ ] near the extreme upper end of the spectrum of sirius, were pronounced without hesitation, for the opposite reason, to have nothing to do with hydrogen. their true affinities are still a matter for inquiry. as regards the hydrogen spectrum, however, the stars had further information in reserve. until recently, it was supposed to consist of a single harmonic series, although, by analogy, three should co-exist. in , accordingly, a second, bound to the first by unmistakable numerical relationships, was recognised by professor pickering in spectrographs of the · magnitude star zeta puppis,[ ] and the identification was shortly afterwards extended to prominent wolf-rayet emission lines. the discovery was capped by dr. rydberg's indication of the wolf-rayet blue band at lambda , as the fundamental member of the third, and principal, hydrogen series.[ ] none of the "pickering lines" (as they may be called to distinguish them from the "huggins series") can be induced to glimmer in vacuum-tubes. they seem to characterise bodies in a primitive state,[ ] and are in many cases associated with absorption rays of oxygen, the identification of which by mr. mcclean in [ ] was fully confirmed by sir david gill.[ ] the typical "oxygen star" is beta crucis, one of the brilliants of the southern cross; but the distinctive notes of its spectrum occur in not a few specimens of the helium class. thus, sir william and lady huggins photographed several ultra-violet oxygen lines in beta lyræ,[ ] and found in rigel signs of the presence of nitrogen,[ ] which, as well as silicium, proves to be a tolerably frequent constituent of such orbs.[ ] for some unknown reason, metalloids tend to become effaced, as metals, in the normal course of stellar development, exert a more and more conspicuous action. dr. scheiner's spectrographic researches at potsdam in and subsequently, exemplify the immense advantages of self-registration. in a restricted section of the spectrum of capella, he was enabled to determine nearly three hundred lines with more precision than had then been attained in the measurement of terrestrial spectra. this star appeared to be virtually identical with the sun in physical constitution, although it emits, according to the best available data, about times as much light, and is hence presumably , times more voluminous. an equally close examination of the spectrum of betelgeux showed the predominance in it of the linear absorption of iron;[ ] but its characteristic flutings do not extend to the photographic region. spectra of the second and third orders are for this reason not easily distinguished on the sensitive plate. a spectrographic investigation of all the brighter northern stars was set on foot in at the observatory of harvard college, under the form of a memorial to dr. h. draper, whose promising work in that line was brought to a close by his premature death in . no individual exertions could, however, have realized a tithe of what has been and is being accomplished under professor pickering's able direction, with the aid of the draper and other instruments, supplemented by mrs. draper's liberal provision of funds. a novel system was adopted, or, rather, an old one--originally used by fraunhofer--was revived.[ ] the use of a slit was discarded as unnecessary for objects like the stars, devoid of sensible dimensions, and giving hence a _naturally_ pure spectrum; and a large prism, placed in front of the object-glass, analysed at once, with slight loss of light, the rays of all the stars in the field. their spectra were taken, as it were, wholesale. as many as two hundred stars down to the eighth magnitude were occasionally printed on a single plate with a single exposure. no cylindrical lens was employed. the movement of the stars themselves was turned to account for giving the desirable width to their spectra. the star was allowed--by disconnecting or suitably regulating the clock--to travel slowly across the line of its own dispersed light, so broadening it gradually into a band. excellent results were secured in this way. about fifty lines appear in the photographed spectrum of aldebaran, and eight in that of vega. on january , , with an exposure of thirty-four minutes, a simultaneous impression was obtained of the spectra (among many others) of close upon forty pleiades. with few and doubtful exceptions, they all proved to belong to the same type. an additional argument for the common origin of the stars forming this beautiful group was thus provided.[ ] the "draper catalogue" of stellar spectra was published in .[ ] it gives the results of a rapid analytical survey of the heavens north of ° of southern declination, and includes , stars, down to about the eighth magnitude. the telescope used was of eight inches aperture and forty-five focus, its field of view--owing to the "portrait-lens" or "doublet" form given to it--embracing with fair definition no less than one hundred square degrees. an objective prism eight inches square was attached, and exposures of a few minutes were given to the most sensitive plates that could be procured. in this way the sky was twice covered in duplicate, each star appearing, as a rule, on four plates. the registration of their spectra was sought to be made more distinctive than had previously been attempted, secchi's first type being divided into four, his second into five subdivisions; but the differences regarded in them could be confidently established only for stars above the sixth magnitude. the work supplies none the less valuable materials for general inferences as to the distribution and relations of the spectral types. the labour of its actual preparation was borne by a staff of ladies under the direction of mrs. fleming. materials for its completion to the southern pole have been accumulated with the identical instrument used at cambridge, transferred for the purpose in to peru, and the forthcoming "second draper catalogue" will comprise , stars in both hemispheres. as supplements to this great enterprise, two important detailed discussions of stellar spectra were issued in and respectively.[ ] the first, by miss a. c. maury, dealt with bright stars visible in the northern hemisphere; the second, by miss a. j. cannon, with , southern stars. both authors traced, with care and ability, the minute gradations by which the long process of stellar evolution appears to be accomplished. the progress of the draper memorial researches was marked by discoveries of an unexampled kind. the principle upon which "motion in the line of sight" can be detected and measured with the spectroscope has already been explained.[ ] it depends, as our readers will remember, upon the removal of certain lines, dark or bright (it matters not which), from their normal places by almost infinitesimal amounts. the whole spectrum of the moving object, in fact, is very slightly _shoved_ hither or thither, according as it is travelling towards or from the eye; but, for convenience of measurement, one line is usually picked out from the rest, and attention concentrated upon it. the application of this method to the stars, however, is encompassed with difficulties. it needs a powerfully dispersive spectroscope to show line-displacements of the minute order in question; and powerful dispersion involves a strictly proportionate enfeeblement of light. this, where the supply is already to a deplorable extent niggardly, can ill be afforded; for which reason the operation of determining a star's approach or recession is, even apart from atmospheric obstacles, an excessively delicate one. it was first executed by sir william huggins early in .[ ] selecting the brightest star in the heavens as the most promising subject of experiment, he considered the f line in the spectrum of sirius to be just so much displaced towards the red as to indicate (the orbital motion of the earth being deducted) recession at the rate of twenty-nine miles a second; and the reality and direction of the movement were ratified by vogel and lohse's observation, march , , of a similar, but even more considerable displacement.[ ] the inquiry was resumed by huggins with improved apparatus in the following year, when the velocities of thirty stars were approximately determined.[ ] the retreat of sirius, which proved slower than had at first been supposed, was now announced to be shared, at rates varying from twelve to twenty-nine miles, by betelgeux, rigel, castor, regulus, and five of the principal stars in the plough. arcturus, on the contrary, gave signs of rapid approach, as well as pollux, vega, deneb in the swan, and the brightness of the pointers. numerically, indeed, these results were encompassed with uncertainty. thus, arcturus is now fully ascertained to be travelling towards the sun at the comparatively slow pace of less than five miles a second; and sirius moves twice as fast in the same direction. the great difficulty of measuring so distended a line as the sirian f might, indeed, well account for some apparent anomalies. the scope of sir william huggins's achievement was not, however, to provide definitive data, but to establish as practicable the method of procuring them. in this he was thoroughly successful, and his success was of incalculable value. spectroscopic investigations of stellar movements may confidently be expected to play a leading part in the unravelment of the vast and complex relations which we can dimly detect as prevailing among the innumerable orbs of the sidereal world; for it supplements the means which we possess of measuring by direct observation movements transverse to the line of sight, and thus completes our knowledge of the courses and velocities of stars at ascertained distances, while supplying for all a valuable index to the amount of perspective foreshortening of apparent movement. thus some, even if an imperfect, knowledge may at length be gained of the revolutions of the stars--of the systems they unite to form, of the paths they respectively pursue, and of the forces under the compulsion of which they travel. the applicability of the method to determining the orbital motions of double stars was pointed out by fox talbot in ;[ ] but its use for their discovery revealed itself spontaneously through the harvard college photographs. in "spectrograms" of zeta ursæ majoris (mizar), taken in , and again in , the k line was seen to be double; while on other plates it appeared single. a careful study of miss a. c. maury of a series of seventy impressions indicated for the doubling a period of fifty-two days, and showed it to affect all the lines in the spectrum.[ ] the only available, and no doubt the true, explanation of the phenomenon was that two similar and nearly equal stars are here merged into one telescopically indivisible; their combined light giving a single or double spectrum, according as their orbital velocities are directed across or along our line of sight. the movements of a revolving pair of stars must always be opposite in sense, and proportionately equal in amount. that is, they at all times travel with speeds in the inverse ratio of their masses. hence, unless the plane of their orbits be perpendicular to a plane passing through the eye, there must be two opposite points where their velocities in the line of sight reach a maximum, and two diametrically opposite points where they touch zero. the lines in their common spectrum would thus appear alternately double and single twice in the course of each revolution. to that of mizar, at first supposed to need days for its completion, a period of only twenty days fourteen hours was finally assigned by vogel.[ ] anomalous spectral effects, probably due to the very considerable eccentricity of the orbit, long impeded its satisfactory determination. the mean distance apart of the component stars, as now ascertained, is just twenty-two million miles, and their joint mass quadruples that of the sun. but these are minimum estimates. for if the orbital plane be inclined, much or little, to the line of sight, the dimensions and mass of the system should be proportionately increased. an analogous discovery was made by miss maury in . but in the spectrum of beta aurigæ, the lines open out and close up on alternate days, indicating a relative orbit[ ] with a radius of less than eight million miles, traversed in about four days. this implies a rate of travel for each star of sixty-five miles a second, and a combined mass · times that of the sun. the components are approximately equal, both in mass and light,[ ] and the system formed by them is transported towards us with a speed of some sixteen miles a second. the line-shiftings so singularly communicative proceed, in this star, with perfect regularity. this new class of "spectroscopic binaries" could never have been visually disclosed. the distance of beta aurigæ from the earth, as determined, somewhat doubtfully, by professor pritchard, is nearly three and a third million times that of the earth from the sun (parallax = · "); whence it has been calculated that the greatest angular separation of the revolving stars is only five-thousandths of a second of arc.[ ] to make this evanescent interval perceptible, a telescope eighty feet in aperture would be required. the zodiacal star, spica (alpha virginis), was announced by dr. vogel, april , ,[ ] to belong to the novel category, with the difference, however, of possessing a nearly dark, instead of a brilliantly lustrous companion. in this case, accordingly, the tell-tale spectroscopic variations consist merely in a slight swinging to and fro of single lines. no second spectrum leaves a legible trace on the plate. spica revolves in four days at the rate of fifty-seven miles a second,[ ] or quicker, in proportion as its orbit is more inclined to the line of sight, round a centre at a minimum distance of three millions of miles. but the position of the second star being unknown, the mass of the system remains indeterminate. the lesser component of the splendid, slowly revolving binary, castor, is also closely double. its spectral lines were found by bélopolsky in [ ] to oscillate once in nearly three days, the secondary globe being apparently quite obscure. further study of the movements thus betrayed elicited the fact that the major axis of the eclipse traversed revolves in a period of , days, as a consequence, most likely, of the flattened shape of the stars.[ ] still more unexpected was the simultaneous assignment, by campbell and newall, of a duplex character to capella.[ ] here both components shine, though with a different quality of light, one giving a pure solar spectrum, the other claiming prismatic affinity with procyon. their mutual circulation is performed in days, and the radius of their orbit cannot be less, and may be a great deal more, than , , miles. hence the possibility is not excluded that the star--which has an authentic parallax of · "--may be visually resolved. indeed, signs of "elongation" were thought to be perceptible with the greenwich -inch refractor,[ ] while only round images could be seen at lick.[ ] another noteworthy case is that of polaris, found by campbell to have certainly one, and probably two obscure attendants.[ ] through his systematic investigations of stellar radial velocities with the mills spectrograph, knowledge in this department has, since , progressed so rapidly that the spectroscopic binaries of our acquaintance already number half a hundred, and ten times as many more doubtless lie within easy range of detection. now it is evident that a spectroscopic binary, if the plane of its motion made a very small angle with the line of sight, would be a variable star. for, during a few hours of each revolution, some at least of its light should be cut off by a transit of its dusky companion. such "eclipse-stars" are actually found in the heavens. the best and longest-known member of the group is algol in the head of medusa, the "demon-star" of the arabs.[ ] this remarkable object, normally above the third magnitude, loses and regains three-fifths of its light once in · hours, the change being completed in about twelve hours. its definite and limited nature, and punctual recurrence, suggested to goodricke of york, by whom the periodicity of the star was discovered in ,[ ] the interposition of a large dark satellite. but the conditions involved by the explanation were first seriously investigated by pickering in .[ ] he found that the phenomena could be satisfactorily accounted for by supposing an obscure body · the bright star's diameter to revolve round it in a period identical with that of its observed variation. this theoretical forecast was verified with singular exactitude at potsdam in .[ ] a series of spectral photographs taken there showed each of algol's minima to be preceded by a rapid recession from the earth, and succeeded by a rapid movement of approach towards it. they take place, accordingly, when the star is at the furthest point from ourselves of an orbit described round an invisible companion, the transits of which across its disc betray themselves to notice by the luminous vicissitudes they occasion. the diameter of this orbit, traversed at the rate of twenty-six miles a second, is just , , miles; and it is an easy further inference from the duration and extent of the phases exhibited that algol itself must be (in round numbers) one million, its attendant , miles in diameter. assuming both to be of the same density, vogel found their respective masses to be four-ninths and two-ninths that of the sun, and their distance asunder to be , , miles. this singularly assorted pair of stars possibly form part of a larger system. their period of revolution is shorter now by six seconds than it was in goodricke's time; and dr. chandler has shown, by an exhaustive discussion, that its inequalities are comprised in a cycle of about years.[ ] they arise, in his view, from a common revolution, in that period, of the close couple about a third distant body, emitting little or no light, in an orbit inclined ° to our line of vision, and of approximately the size of that described by uranus round the sun. the time spent by light in crossing this orbit causes an apparent delay in the phases of the variable, when algol and its eclipsing satellite are on its further side from ourselves, balanced by acceleration while they traverse its hither side. dr. chandler derives confirmation for his plausible and ingenious theory from a supposed undulation in the line traced out by algol's small proper motion; but the reality of this disturbance has yet to be established.[ ] meanwhile, m. tisserand,[ ] late director of the paris observatory, preferred to account for algol's inequalities on the principle later applied by bélopolsky to those of castor. that is to say, he assumed a revolving line of apsides in an elliptical orbit traversed by a pretty strongly compressed pair of globes. the truth of this hypothesis can be tested by close observation of the phases of the star during the next few years. the variable in the head of medusa is the exemplar of a class including recognised members, all of which doubtless represent occulting combinations of stars. but their occultations result merely from the accident of their orbital planes passing through our line of sight; hence the heavens must contain numerous systems similarly constituted, though otherwise situated as regards ourselves, some of which, like spica virginis, will become known through their spectroscopic changes, while others, because revolving in planes nearly tangent to the sphere, or at right angles to the visual line, may never disclose to us their true nature. among eclipsing stars should probably be reckoned the peculiar variables, beta lyræ and v puppis, each believed to consist of a pair of bright stars revolving almost in contact.[ ] three stars, on the other hand, distinguished by rapid and regular fluctuations, have been proved by bélopolsky to be attended by non-occulting satellites, which circulate, nevertheless, in the identical periods of light-change. gore's "catalogue of known variables"[ ] included, in , entries, and the number was augmented to on its revision in .[ ] chandler's first list of such objects,[ ] published about the same time, received successive expansions in and ,[ ] and finally included entries. a new "catalogue of variable stars," still wider in scope, will shortly be issued by the german _astronomische gesellschaft_. mr. a. w. roberts's researches on southern variables[ ] have greatly helped to give precision, while adding to the extent of knowledge in this branch. dr. gould held the opinion that most stars fluctuate slightly in brightness through surface-alterations similar to, but on a larger scale than those of the sun; and the solar analogy might be pushed somewhat further. it perhaps affords a clue to much that is perplexing in stellar behaviour. wolf pointed out in the striking resemblance in character between curves representing sun-spot frequency and curves representing the changing luminous intensity of many variable stars. there were the same steep ascent to maximum and more gradual decline to minimum, the same irregularities in heights and hollows, and, it may be added, the same tendency to a double maximum, and complexity of superposed periods.[ ] it is impossible to compare the two sets of phenomena thus graphically portrayed without reaching the conclusion that they are of closely related origin. but the correspondence indicated is not, as has often been hastily assumed, between maxima of sun-spots and minima of stellar brightness, but just the reverse. the luminous outbursts, not the obscurations of variable stars, obey a law analogous to that governing the development of spots on the sun. objects of the kind do not, then, gain light through the closing-up of dusky chasms in their photospheres, but by an actual increase of surface-brilliancy, together with an immense growth of these brilliant formations--prominences and faculæ--which, in the sun, accompany, or are appended to spots. a comparison of light-curves with curves of spot-frequency leaves no doubt on this point, and the strongest corroborative evidence is derived from the emergence of bright lines in the spectra of long-period variables rising to their recurring maxima. every kind and degree of variability is exemplified in the heavens. at the bottom of the scales are stars like the sun, of which the lustre is--tried by our instrumental means--sensibly steady. at the other extreme are ranged the astounding apparitions of "new," or "temporary" stars. within the last thirty-six years eleven of these stellar guests (as the chinese call them) have presented themselves, and we meet with a twelfth no farther back than april , . but of the "new star" in ophiuchus found by mr. hind on that night, little more could be learnt than of the brilliant objects of the same kind observed by tycho and kepler. the spectroscope had not then been invented. let us hear what it had to tell of later arrivals. about thirty minutes before midnight of may , , mr. john birmingham of millbrook, near tuam, in ireland, saw with astonishment a bright star of the second magnitude unfamiliarly situated in the constellation of the northern crown. four hours earlier, schmidt of athens had been surveying the same part of the heavens, and was able to testify that it was not visible there. that is to say, a few hours, or possibly a few minutes, sufficed to bring about a conflagration, the news of which may have occupied hundreds of years in travelling to us across space. the rays which were its messengers, admitted within the slit of sir william huggins's spectroscope, may , proved to be of a composition highly significant as to the nature of the catastrophe. the star--which had already declined below the third magnitude--showed what was described as a double spectrum. to the dusky flutings of secchi's third type four brilliant rays were added.[ ] the chief of these agreed in position with lines of hydrogen; so that the immediate cause of the outburst was inferred to have been the eruption, or ignition, of vast masses of that subtle kind of matter, the universal importance of which throughout the cosmos is one of the most curious facts revealed by the spectroscope. t coronæ (as the new star was called) quickly lost its adventitious splendour. nine days after its discovery it was again invisible to the naked eye. it is now a pale yellow, slightly variable star near the tenth magnitude, and finds a place as such in argelander's charts.[ ] it was thus obscurely known before it made its sudden leap into notoriety. the next "temporary," discovered by dr. schmidt at athens, november , , could lay no claim to previous recognition even in that modest rank. it was strictly a parvenu. there was no record of its existence until it made its appearance as a star of nearly the third magnitude, in the constellation of the swan. its spectrum was examined, december , by cornu at paris,[ ] and a few days later by vogel and o. lohse at potsdam.[ ] it proved of a closely similar character to that of t coronæ. a range of bright lines, including those of hydrogen, and probably of helium, stood out from a continuous background impressed with strong absorption. it may be presumed that in reality the gaseous substances, which, by their sudden incandescence, had produced the apparent conflagration, lay comparatively near the surface of the star, while the screen of cooler materials intercepting large portions of its light was situated at a considerable elevation in its atmosphere. the object, meanwhile, steadily faded. by the end of the year it was of no more than seventh magnitude. after the second week of march, , strengthening twilight combined with the decline of its radiance to arrest further observation. it was resumed, september , at dunecht, with a strange result. practically the whole of its scanty light (it had then sunk below the tenth magnitude) was perceived to be gathered into a single bright line in the green, and that the most characteristic line of gaseous nebulæ.[ ] the star had, in fact, so far as outward appearance was concerned, become transformed into a planetary nebula, many of which are so minute as to be distinguishable from small stars only by the quality of their radiations. it is now, having sunk to about the fourteenth magnitude,[ ] entirely beyond the reach of spectroscopic scrutiny. perhaps none of the marvellous changes witnessed in the heavens has given a more significant hint as to their construction than the stellar blaze kindled in the heart of the great andromeda nebula some undetermined number of years or centuries before its rays reached the earth in the month of august, . the first published discovery was by dr. hartwig at dorpat on august ; but it was found to have been already seen, on the th, by mr. isaac w. ward of belfast, and on the th by m. ludovic gully of rouen. the _negative_ observations, on the th, of tempel[ ] and max wolf, limited very narrowly the epoch of the apparition. nevertheless, it did not, like most temporaries, attain its maximum brightness all at once. when first detected, it was of the ninth, by september it had risen to the seventh magnitude, from which it so rapidly fell off that in march it touched the limit of visibility (sixteenth magnitude) with the washington -inch. its light bleached very perceptibly as it faded.[ ] during the earlier stages of its decline, the contrast was striking between the sharply defined, ruddy disc of the star, and the hazy, greenish-white background upon which it was projected,[ ] and with which it was inevitably suggested to be in some sort of physical connection. let us consider what evidence was really available on this point. to begin with, the position of the star was not exactly central. it lay sixteen seconds of arc to the south-west of the true nebular nucleus. its appearance did not then signify a sudden advance of the nebula towards condensation, nor was it attended by any visible change in it save the transient effect of partial effacement through superior brightness. equally indecisive information was derived from the spectroscope. to vogel, hasselberg, and young, the light of the "nova" seemed perfectly continuous; but huggins caught traces of bright lines on september , confirmed on the th;[ ] and copeland succeeded, on september , in measuring three bright bands with an acute-angled prism specially constructed for the purpose.[ ] a shimmer of f was suspected, and had also been perceived by mr. o. t. sherman of yale college. still, the effect was widely different from that of the characteristic blazing spectrum of a temporary star, and prompted the surmise that here, too, a variable might be under scrutiny. the star, however, was certainly so far "new" that its rays, until their sudden accession of strength, were too feeble to affect even our reinforced senses. not one of the , small stars recorded in charts of the nebula could be identified with it; and a photograph taken by dr. common, august , , on which a multitude of stars down to the fifteenth magnitude had imprinted themselves, showed the uniform, soft gradation of nebulous light to be absolutely unbroken by a stellar indication in the spot reserved for the future occupation of the "nova."[ ] so far, then, the view that its relation to the nebula was a merely optical one might be justified; but it became altogether untenable when it was found that what was taken to be a chance coincidence had repeated itself within living memory. on the st of may, , m. auwers perceived at königsberg a seventh magnitude star shining close to the centre of a nebula in scorpio, numbered in messier's catalogue.[ ] three days earlier it certainly was not there, and three weeks later it had vanished. the effect to mr. pogson (who independently discovered the change, may )[ ] was as if the nebula had been _replaced_ by a star, so entirely were its dim rays overpowered by the concentrated blaze in their midst. now, it is simply incredible that two outbursts of so uncommon a character should have _accidentally_ occurred just on the line of sight between us and the central portions of two nebulæ; we must, then, conclude that they showed _on_ these objects because they took place _in_ them. the most favoured explanation is that they were what might be called effects of overcrowding--that some of the numerous small bodies, presumably composing the nebulæ, jostled together, in their intricate circlings, and obtained compensation in heat for their sacrifice of motion. but this is scarcely more than a plausible makeshift of perplexed thought. mr. w. h. s. monck, on the other hand, has suggested that new stars appear when dark bodies are rendered luminous by rushing through the gaseous fields of space,[ ] just as meteors kindle in our atmosphere. the idea, which has been revived and elaborated by dr. seeliger of munich,[ ] is ingenious, but was not designed to apply to our present case. neither of the objects distinguished by the striking variations just described is of gaseous constitution. that in scorpio appears under high magnifying powers as a "compressed cluster"; that in andromeda is perhaps, as sir j. herschel suggested, "optically nebulous through the smallness of its constituent stars"[ ]--if stars they deserve to be called. on the th of december, , dr. max wolf took a photograph of the region about chi aurigæ. no stranger so bright as the eighth magnitude was among the stars depicted upon it. on the th, nevertheless, a stellar object of the fifth magnitude, situated a couple of degrees to the north-east of beta tauri and previously unrecorded, where eleventh magnitude stars appeared, imprinted itself upon a harvard negative. subsequent photographs taken at the same place showed it to have gained about half a magnitude by the th; but the plates were not then examined, and the discovery was left to be modestly appropriated by an amateur, the rev. dr. anderson of edinburgh, by whom it was announced, february , , through the medium of an anonymous postcard, to dr. copeland, the astronomer royal for scotland.[ ] by him and others, the engines of modern research were promptly set to work. and to good purpose. nova aurigæ was the first star of its kind studied by the universal chemical method. it is the first, accordingly, of which authentic records can be handed down to posterity. they are of a most remarkable character. the spectrum of the new object was photographed at stonyhurst and south kensington on february ; a few days later, at harvard and lick in america, at potsdam and hérény on the continent of europe. but by far the most complete impression was secured, february , with an exposure of an hour and three-quarters, by sir william and lady huggins, through whose kindness it is reproduced in plate v., fig. . the range of bright lines displayed in it is of astonishing vividness and extent. it includes all the hydrogen rays dark in the spectrum of sirius (separately printed for comparison), besides many others still more refrangible, as yet unidentified. very significant, too, is the marked character of the great prominence lines h and k. the visual spectrum of the nova was splendidly effective. a quartette of brilliant green rays, two of them due to helium, caught the eye; and they had companions too numerous to be easily counted. the hydrogen lines were broad and bright; c blazed, as mr. espin said, "like a danger-signal on a dark night"; the sodium pair were identified at tulse hill, and the yellow helium ray was suspected to lurk close beside them. fig. in the same plate shows the spectrum as it was seen and mapped by lady huggins, february to , together with the spectra employed to test the nature of the emissions dispersed in it. one striking feature will be at once remarked. it is that of the pairing of bright with dark lines. both in the visible and the photographic regions this singular peculiarity was unmistakable; and since the two series plainly owned the same chemical origin, their separate visibility implied large displacement. otherwise they would have been superposed, not juxtaposed. measurements of the bright rays, accordingly, showed them to be considerably pushed down towards the red, while their dark companions were still more pushed up towards the blue end. thus the spectrum of nova aurigæ, like that of beta lyræ, with which it had many points in common, appeared to be really double. it was supposed to combine the light of two distinct bodies, one, of a gaseous nature, moving rapidly away from the earth, the other, giving a more sunlike spectrum, approaching it with even higher speed. the relative velocity determined at potsdam for these oppositely flying masses amounted to miles a second.[ ] and this prodigious rate of separation was fully maintained during six weeks! it did not then represent a mere periastral rush-past.[ ] to the bodies exhibiting its effects, and parting company for ever under its stress, it must have belonged, with slight diminution, in perpetuity. the luminous outburst by which they became visible was explained by sir william huggins, in a lecture delivered at the royal institution, may , , on the tidal theory of klinkerfues and wilsing. disturbances and deformations due to the mutual attraction of two bulky globes at a close approach would, he considered, "give rise to enormous eruptions of the hotter matters from within, immensely greater, but similar in kind, to solar eruptions; and accompanied, probably, by large electrical disturbances." the multiple aspect and somewhat variable character of both bright and dark lines were plausibly referred to processes of "reversal," such as are nearly always in progress above sun-spots; but the long duration of the star's suddenly acquired lustre did not easily fit in with the adopted rationale. a direct collision, on the other hand, was out of the question, since there had obviously been little, if any, sacrifice of motion; and the substitution of a nebula for one of the "stars"[ ] compelled recourse to scarcely conceivable modes of action for an explanation of the perplexing peculiarities of the compound spectrum. plate v. [illustration: photographic and visual spectrum of nova aurigæ. fig. .--from a photograph taken by sir william and lady huggins, feb. , . fig. .--from a drawing made by lady huggins, feb. to , .] an unexpected _dénouement_, however, threw all speculations off the track. the nova contained most of its brightness, fluctuations notwithstanding, until march ; after which date it ran swiftly and uniformly down towards what was apprehended to be total extinction. no marked change of spectrum attended its decline. when last examined at tulse hill, march , all the more essential features of its prismatic light were still faintly recognisable.[ ] the object was steadily sinking on april , when a (supposed) final glimpse of it was caught with the lick -inch.[ ] it was then of about the sixteenth magnitude. but on august it had sprung up to the tenth, as professors holden, schaeberle, and campbell perceived with amazement on turning the same instrument upon its place. and to professor barnard it appeared, two nights later, not only revived, but transformed into the nucleus of a planetary nebula, " across.[ ] the reality of this seeming distension, however, at once disputed, was eventually disproved. it unquestionably arose from the imperfect focussing power of the telescope for rays of unusual quality.[ ] the rekindled nova was detected in this country by mr. h. corder, on whose notification mr. espin, on august , examined its nearly monochromatic spectrum.[ ] the metamorphosis of nova cygni seemed repeated.[ ] the light of the new object, like that of its predecessor, was mainly concentrated in a vivid green band, identified with the chief nebular line by copeland,[ ] von gothard,[ ] and campbell.[ ] the second nebular line was also represented. indeed, the last-named observer recognised nearly all the eighteen lines measured by him in the nova as characteristic of planetary nebulæ.[ ] of particular interest is the emergence in the star-spectrum photographed by von gothard of an ultra-violet line originally discovered at tulse hill in the orion nebula, which is also very strong in the lyra annular nebula. obviously, then, the physical constitution of nova aurigæ became profoundly modified during the four months of its invisibility. the spectrum of february was or appeared compound; that of august was simple; it could be reasonably associated only with a single light-source. many of the former brilliant lines, too, had vanished, and been replaced by others, at first inconspicuous or absent. as a result, the solar-prominence type, to which the earlier spectrum had seemed to conform, was completely effaced in the later. the cause of these alterations remains mysterious, yet its effects continue. the chromatic behaviour of the semi-extinct nova, when scrutinised with great refractors, shows its waning light to be distinctly nebular.[ ] like nearly all its congeners, the star is situated in the full stream of the milky way, and we learn without surprise that micrometrical measures by burnham and barnard[ ] failed to elicit from it any sign of parallactic shifting. it is hence certain that the development of light, of which the news reached the earth in december, , must have been on a vast scale, and of ancient date. nova aurigæ at its maximum assuredly exceeded the sun many times in brightness; and its conflagration can scarcely have occurred less, and may have occurred much more, than a hundred years ago. by means of the photographic surveys of the skies, carried on in both hemispheres under professor pickering's superintendence, such amazing events have been proved to be of not infrequent occurrence. within six years five new stars were detected from draper memorial, or chart-plates by mrs fleming, besides the retrospective discovery of a sixth which had rapidly burnt itself out, eight years previously, in perseus.[ ] nova normæ was the immediate successor of nova aurigæ; nova carinæ and nova centauri lit up in , the latter in a pre-existent nebula; nova sagittarii and nova aquilæ attained brief maxima in and respectively. now, three out of these five stars reproduced with singular fidelity the spectrum of nova aurigæ; they displayed the same brilliant rays shadowed, invariably on their blue sides, by dark ones. palpably, then, the arrangement was systematic and significant; it could not result merely from the casually directed, opposite velocities of bodies meeting in space. the hypothesis of stellar encounters accordingly fell to the ground, and has been provided with no entire satisfactory substitute. most speculators now fully recognise that motion-displacements cannot be made to account for the doubled spectra of novæ, and seek recourse instead to some kind of physical agency for producing the observed effect.[ ] and since this is also visible in certain permanent, though peculiar objects--notably in p cygni, beta lyræ, and eta carinæ--the acting cause must also evidently be permanent and inherent. the "new star of the new century"[ ] was a visual discovery. dr. anderson duplicated, with added _éclat_, his performance of nine years back. in the early morning of february , , he perceived that algol had a neighbour of nearly its own brightness, which a photograph taken by mr. stanley williams, at brighton, proved to have risen from below the twelfth magnitude within the preceding hours. and it was still swiftly ascending. on the rd, it outshone capella; for a brief space it took rank as the premier star of the northern hemisphere. a decline set in promptly, but was pursued hesitatingly. the light fluctuated continually over a range of a couple of magnitudes, and with a close approach, during some weeks, to a three-day periodicity. a year after the original outburst, the star was still conspicuous with an opera-glass. the spectrum underwent amazing changes. at first continuous, save for fine dark lines of hydrogen and helium, it unfolded within forty-eight hours a composite range of brilliant and dusky bands disposed in the usual fashion of novæ. these lasted until far on in march, when hydrogen certainly, and probably other substances as well, ceased to exert any appreciable absorptive action. blue emissions of the wolf-rayet type then became occasionally prominent, in remarkable correspondence with the varying lustre of the star;[ ] finally, a band at lambda , found by wright at lick to characterise nebular spectra,[ ] assumed abnormal importance; and in july the nebular transformation might be said to be complete. striking alterations of colour attended these spectral vicissitudes. white to begin with, the star soon turned deep red, and its redness was visibly intensified at each of its recurring minima of light. blanching, however, ensued upon the development of its nebulous proclivities; and its surviving rays are of a steely hue. all the more important investigations of nova persei were conducted by photographic means. libraries of spectral plates were collected at the yerkes and lick observatories, at south kensington, stonyhurst, and potsdam, and await the more exhaustive interpretation of the future. meanwhile, extraordinary revelations have been supplied by immediate photographic delineation. on august and , , professor max wolf, by long exposures with the -inch bruce twin objectives of the königstuhl observatory (heidelberg), obtained indications of a large nebula finely ramified, extending south-east of the nova;[ ] and the entire formation came out in four hours with the yerkes -foot reflector, directed to it by mr. ritchey on september .[ ] it proved to be a great spiral encircling, and apparently emanating from, the star. but if so, tumultuously, and under stress of catastrophic impulsions. a picture obtained by mr. perrine with the crossley refractor, in h. m., on november and , disclosed the progress of a startling change.[ ] comparison with the yerkes photograph showed that during the intervening days four clearly identifiable condensations had become displaced, all to the same extent of about seconds of arc, and in fairly concordant directions, suggesting motion _round_ the nova as well as away from it. the velocity implied, however, is so prodigious as virtually to exclude the supposition of a bodily transport of matter. it should be at the rate of no less than twenty thousand miles a second, admitting the object to be at a distance from us corresponding to an annual parallax of one-tenth of a second, and actual measurements show it to be indefinitely more remote. the fact of rapid variations in the nebula was reaffirmed, though with less precision, from yerkes photographs of november and , mr. ritchey inferring a general expansion of its southern portions.[ ] much further evidence must be at hand before a sane judgment can be formed as to the nature of the strange events taking place in that secluded corner of the galaxy.[ ] and it is highly probable that the illumination of the nebulous wreaths round the star will prove no less evanescent than the blazing of the star itself. we have been compelled somewhat to anticipate our narrative as regards inquiries into the nature of nebulæ. the excursions of opinion on the point were abruptly restricted and defined by the application to them of the spectroscope. on august , , sir william huggins sifted through his prisms the rays of a bright planetary nebula in draco.[ ] to his infinite surprise, they proved to be mainly of one colour. in other words, they avowed their origin from a mass of glowing vapour. as to what _kind_ of vapour it might be by which herschel's conjecture of a "shining fluid" diffused at large throughout the cosmos was thus unexpectedly verified, an answer only partially satisfactory could be afforded. the conspicuous bright line of the draco nebula seemed to agree in position with one emitted by nitrogen, but has since proved to be distinct from it; of its two fainter companions, one was unmistakably the f line of hydrogen, while the other, in position intermediate between the two, still remains unidentified. by huggins had satisfactorily examined the spectra of about seventy nebulæ, of which one-third displayed a gaseous character.[ ] all of these gave the green ray fundamental to the nebular spectrum, and emanating from an unknown form of matter named by sir william huggins "nebulum." it is associated with seven or eight hydrogen lines, with three of "yellow" helium, and with a good many of undetermined origin. the absence of the crimson radiation of hydrogen--perceived with difficulty only in some highly condensed objects--is an anomaly very imperfectly explained as a physiological effect connected with the extreme faintness of nebular light.[ ] an approximate coincidence between the chief nebular line and a "fluting" of magnesium having been alleged by lockyer in support of his meteoritic hypothesis of nebular constitution, it became of interest to ascertain its reality. the task was accomplished by sir william and lady huggins in and ,[ ] and by professor keeler, with the advantages of the mount hamilton apparatus and atmosphere, in - .[ ] the upshot was to show a slight but sure discrepancy as to place, and a marked diversity as to character, between the two qualities of light. the nebular ray (wave-length , millionths of a millimetre) is slightly more refrangible than the magnesium fluting-edge, and it is sharp and fine, with no trace of the unilateral haze necessarily clinging even to the last "remnant" of a banded formation. planetary and annular nebulæ are, without exception, gaseous, as well as those termed "irregular," which frequent the region of the milky way. their constitution usually betrays itself to the eye by their blue or greenish colour; while those yielding a continuous spectrum are of a dull white. among the more remarkable of these are the well-known nebula in andromeda, and the great spiral in canes venatici; and, as a general rule, the emissions of all such nebulæ as present the appearance of star-clusters grown misty through excessive distance are of the same kind. it would, however, be eminently rash to conclude thence that they are really aggregations of sun-like bodies. the improbability of such an inference has been greatly enhanced by the occurrence, at an interval of a quarter of a century, of stellar outbursts in the midst of two of them. for it is practically certain that the temporary stars were equally remote with the hazy formations they illuminated; hence, if the constituent particles of the latter be suns, the incomparably vaster orbs by which their feeble light was well-nigh obliterated must, as was argued by mr. proctor, have been on a scale of magnitude such as the imagination recoils from contemplating. nevertheless, dr. scheiner, not without much difficulty, obtained, in january, , spectrographic prints of the andromeda nebula, indicative, he thought, of its being a cluster of solar stars.[ ] sir william and lady huggins, on the other hand, _saw_, in , bright intermixed with dark bands in the spectrum of the same object.[ ] and mr. maunder conjectures all "white" nebulæ to be made up of sunlets in which the coronal element predominates, while chromospheric materials assert their presence in nebulæ of the "green" variety.[ ] among the ascertained analogies between the stellar and nebular systems is that of variability of light. on october , , mr. hind discovered a small nebula in taurus. chacornac observed it at marseilles in , but was confounded four years later to find it vanished. d'arrest missed it october , and redetected it december , . it was easily seen in - , but invisible in the most powerful instruments from to .[ ] barnard, however, made out an almost evanescent trace of it, october , , with the great lick telescope,[ ] and saw it easily in the spring of , while six months later it evaded his most diligent search.[ ] then again, on september , , the yerkes -inch disclosed it to him as a mere shimmer at the last limit of visibility; and it came out in three diffuse patches on plates to which, on december and , , keeler gave prolonged exposures with the crossley reflector.[ ] moreover, a fairly bright adjacent nebula, perceived by o. struve in , and observed shortly afterwards by d'arrest, has totally vanished, and was most likely only a temporary apparition. these are the most authentic instances of nebular variability. many others have been more or less plausibly alleged;[ ] but professor holden's persuasion, acquired from an exhaustive study of the records since ,[ ] that the various parts of the orion nebula fluctuate continually in relative lustre, has not been ratified by photographic evidence. the case of the "trifid" nebula in sagittarius, investigated by holden in ,[ ] is less easily disposed of. what is certain is that a remarkable triple star, centrally situated, according to the observations of both the herschels, - , in a dark space between the three great _lobes_ of the nebula, is now, and has been since , densely involved in one of them; and since the hypothesis of relative motion is on many grounds inadmissible, the change that has apparently taken place must be in the distribution of light. one no less conspicuous was adduced by mr. h. c. russell, director of the sydney observatory.[ ] a particularly bright part of the great argo nebula, as drawn by sir john herschel, has, it would seem, almost totally disappeared. he noticed its absence in , using a -inch telescope, failed equally later on to find it with an - / -inch, and his long-exposure photographs show no vestige of it. the same structure is missing from, or scarcely traceable in, a splendid picture of the nebula taken by sir david gill in twelve hours distributed over four nights in march, .[ ] an immense gaseous expanse has, it would seem, sunk out of sight. materially it is no doubt there; but the radiance has left it. nebulæ have no ascertained proper motions. no genuine change of place in the heavens has yet been recorded for any one of them. all equally hold aloof, so far as telescopic observation shows, from the busy journeyings of the stars. this seeming immobility is partly an effect of vast distance. nebular parallax has, up to the present, proved evanescent, and nebular parallactic drift, in response to the sun's advance through space, remains likewise imperceptible.[ ] it may hence be presumed that no nebulæ occur within the sphere occupied by the nearer stars. but the difficulty of accurately measuring such objects must also be taken into account. displacements which would be conspicuous in stars might easily escape detection in ill-defined, hazy masses. thus the measures executed by d'arrest in [ ] have not yet proved effective for their designed purpose of contributing to the future detection of proper motions. some determinations made by mr. burnham with the lick refractor in ,[ ] will ultimately afford a more critical test. he found that nearly all planetary nebulæ include a sharp stellar nucleus, the position of which with reference to neighbouring stars could be fixed no less precisely than if it were devoid of nebulous surroundings. hence, the objects located by him cannot henceforward shift, were it only to the extent of a small fraction of a second, without the fact coming to the knowledge of astronomers. the spectroscope, however, here as elsewhere, can supplement the telescope; and what it has to tell, it tells at once, without the necessity of waiting on time to ripen results. sir william huggins made, in ,[ ] the earliest experiments on the radial movements of nebulæ. but with only a negative upshot. none of the six objects examined gave signs of spectral alteration, and it was estimated that they must have done so had they been in course of recession from or approach towards the earth by as much as twenty-five miles a second. with far more powerful appliances, professor keeler renewed the attempt at lick in - . his success was unequivocal. ten planetary nebulæ yielded perfectly satisfactory evidence of line-of-sight motion,[ ] the swiftest traveller being the well-known greenish globe in draco,[ ] found to be hurrying towards the earth at the rate of forty miles a second. for the orion nebula, a recession of about eleven miles was determined,[ ] the whole of which may, however, very well belong to the solar system itself, which, by its translation towards the constellation lyra, is certainly leaving the great nebula pretty rapidly behind. the anomaly of seeming nebular fixity has nevertheless been removed; and the problem of nebular motion has begun to be solved through the demonstrated possibility of its spectroscopic investigation. keeler's were the first trustworthy determinations of radial motion obtained visually. that the similar work on the stars begun at greenwich in , and carried on for thirteen years, remained comparatively unfruitful, was only what might have been expected, the instruments available there being altogether inadequate for the attainment of a high degree of accuracy. the various obstacles in the way of securing it were overcome by the substitution of the sensitive plate for the eye. air-tremors are thus rendered comparatively innocuous; and measurements of stellar lines displaced by motion with reference to fiducial lines from terrestrial sources, photographed on the same plates, can be depended upon within vastly reduced limits of error. studies for the realisation of the "spectrographic" method were begun by dr. vogel and his able assistant, dr. scheiner, at potsdam in . their preliminary results, communicated to the berlin academy of sciences, march , , already showed that the requirements for effective research in this important branch were at last about to be complied with. an improved instrument was erected in the autumn of the same year, and the fifty-one stars, bright enough for determination with a refractor of inches aperture, were promptly taken in hand. a list of their motions in the line of sight, published in ,[ ] was of high value, both in itself and for what it promised. one noteworthy inference from the data it collected was that the eye tends, under unfavourable circumstances, to exaggerate the line-displacements it attempts to estimate. the velocities photographically arrived at were of much smaller amounts than those visually assigned. the average speed of the potsdam stars came out only · miles a second, the quickest among them being aldebaran, with a recession of thirty miles a second. more lately, however, deslandres and campbell have determined for zeta herculis and eta cephei respectively approaching rates of forty-four and fifty-four miles a second. the installation, in , of a photographic refractor - / inches in aperture, coupled with a -inch guiding telescope, will enable dr. vogel to investigate spectrographically some hundreds of stars fainter than the second magnitude; and the materials thus accumulated should largely help to provide means for a definite and complete solution of the more than secular problem of the sun's advance through space. the solution should be complete, because including a genuine determination of the sun's velocity, apart from assumptions of any kind. m. homann's attempt, in ,[ ] to extract some provisional information on the subject from the radial movements of visually determined stars gave a fair earnest of what might be done with materials of a better quality. he arrived at a goal for the sun's way shifted eastward to the constellation cygnus--a result congruous with the marked tendency of recently determined apexes to collect in or near lyra; and the most probable corresponding velocity seemed to be about nineteen miles a second, or just that of the earth in its orbit. a more elaborate investigation of the same kind, based by professor campbell in [ ] upon the motions of stars, determined with extreme precision, suffered in completeness through lack of available data from the southern hemisphere. the outcome, accordingly, was an apex most likely correctly placed as regards right ascension, but displaced southward by some fifteen degrees. the speed of twelve miles a second, assigned to the solar translation, approximates doubtless very closely to the truth. a successful beginning was made in nebular spectrography by sir william huggins, march , .[ ] five lines in all stamped themselves upon the plate during forty-five minutes of exposure to the rays of the strange object in orion. of these, four were the known visible lines, and a fifth, high up in the ultra-violet, at wave-length , , has evidently peculiar relationships, as yet imperfectly apprehended. it is strong in the spectra of many planetaries; it helped to characterise the nebular metamorphosis of nova aurigæ, yet failed to appear in nova persei. two additional hydrogen lines, making six in all, were photographed at tulse hill, from the orion nebula, in ;[ ] and dr. copeland's detection in [ ] of the yellow ray d_ gave the first hint of the presence of helium in this prodigious formation. nor are there wanting spectroscopic indications of its physical connection with the stars visually involved in it. sir william and lady huggins found a plate exposed february , , impressed with four groups of fine bright lines, originating in the continuous light of two of the trapezium-stars, but extending some way into the surrounding nebula.[ ] and dr. scheiner[ ] argued a wider relationship from the common possession, by the nebula and the chief stars in the constellation orion, of a blue line, bright in the one case, dark in the others, since identified as a member of one of the helium series. the structural unity of the stellar and nebular orders in this extensive region of the sky has also, by direct photographic means, been unmistakably affirmed. the first promising autographic picture of the orion nebula was obtained by draper, september , .[ ] the marked approach towards a still more perfectly satisfactory result shown by his plates of march, and , was unhappily cut short by his death. meanwhile, m. janssen was at work in the same field from , with his accustomed success.[ ] but dr. a. ainslie common left all competitors far behind with a splendid picture, taken january , , by means of an exposure of thirty-seven minutes in the focus of his -foot silver-on-glass mirror.[ ] photography may thereby be said to have definitely assumed the office of historiographer to the nebulæ, since this one impression embodies a mass of facts hardly to be compassed by months of labour with the pencil, and affords a record of shape and relative brightness in the various parts of the stupendous object it delineates which must prove invaluable to the students of its future condition. its beauty and merit were officially recognised by the award of the astronomical society's gold medal in . a second picture of equal merit, obtained by the same means, february , , with an exposure of one hour, is reproduced in the frontispiece. the vignette includes two specimens of planetary photography. the jupiter, with the great red spot conspicuous in the southern hemisphere, is by dr. common. it dates from september , , and was accordingly one of the earliest results with his -inch, the direct image in which imprinted itself in a fraction of a second, and was subsequently enlarged on paper about twelve times. the exquisite little picture of saturn was taken at paris by mm. paul and prosper henry, december , , with their -inch photographic refractor. the telescopic image was in this case magnified eleven times previous to being photographed, an exposure of about five seconds being allowed; and the total enlargement, as it now appears, is nineteen times. a trace of the dusky ring perceptible on the original negative is lost in the print. a photograph of the orion nebula taken by dr. roberts in minutes, november , , made a striking disclosure of the extent of that prodigious object. more than six times the nebulous area depicted on dr. common's plates is covered by it, and it plainly shows an adjacent nebula, separately catalogued by messier, to belong to the same vast formation. this disposition to annex and appropriate has come out more strongly with every increase of photographic power. plates exposed at harvard college in march, , with an -inch portrait-lens (the same used in the preparation of the draper catalogue) showed the old-established "fish-mouth" nebula not only to involve the stars of the sword-handle, but to be in tolerably evident connection with the most easterly of the three belt-stars, from which a remarkable nebulous appendage was found to proceed.[ ] a still more curious discovery was made by w. h. pickering in .[ ] photographs taken in three hours from the summit of wilson's peak in california revealed the existence of an enormous, though faint spiral structure, enclosing in its span of nearly seventeen degrees the entire stellar and nebulous group of the belt and sword, from which it most likely, although not quite traceably, issues as if from a nucleus. a startling glimpse is thus afforded of the cosmical importance of that strange "hiatus" in the heavens which excited the wonder of huygens in . the inconceivable attenuation of the gaseous stuff composing it was virtually demonstrated by mr. ranyard.[ ] in march, , sir howard grubb mounted for dr. isaac roberts, at maghull, near liverpool (his observatory has since been transferred to crowborough in sussex), a silver-on-glass reflector of twenty inches aperture, constructed expressly for use in celestial photography. a series of nebula-pictures, obtained with this fine instrument, have proved highly instructive both as to the structure and extent of these wonderful objects; above all, one of the great andromeda nebula, to which an exposure of three hours was given on october , .[ ] in it a convoluted structure replaced and rendered intelligible the anomalously rifted mass seen by bond in .[ ] the effects of annular condensation appeared to have stamped themselves upon the plate, and two attendant nebulæ presented the aspect of satellites already separated from the parent body, and presumably revolving round it. the ring-nebula in lyra was photographed at paris in , and shortly afterwards by von gothard with a -inch reflector,[ ] and he similarly depicted in the two chief spiral and other nebulæ.[ ] photographs of the lyra nebula taken at algiers in ,[ ] and at the vatican observatory in ,[ ] were remarkable for the strong development of a central star, difficult of telescopic discernment, but evidently of primary importance to the annular structure around. the uses of photography in celestial investigations become every year more manifold and more apparent. the earliest chemical star-pictures were those of castor and vega, obtained with the cambridge refractor in by whipple of boston under the direction of w. c. bond. double-star photography was inaugurated under the auspices of g. p. bond, april , , with an impression, obtained in eight seconds, of mizar, the middle star in the handle of the plough. a series of measures from sixty-two similar images gave the distance and position-angle of its companion with about the same accuracy attainable by ordinary micrometrical operations; and the method and upshot of these novel experiments were described in three papers remarkably forecasting the purposes to be served by stellar photography.[ ] the matter next fell into the able hands of rutherfurd, who completed in a fine object glass (of - / inches) corrected for the ultra-violet rays, consequently useless for visual purposes. the sacrifice was recompensed by conspicuous success. a set of measurements from his photographs of nearly fifty stars in the pleiades, and their comparison with bessel's places, enabled dr. gould to announce, in , that during the intervening third of a century no changes of importance had occurred in their relative positions.[ ] and mr. harold jacoby[ ] similarly ascertained the fixity of seventy-five of rutherfurd's atlantids, between the epoch and that of dr. elkin's heliometric triangulation of the cluster in ,[ ] extending the interval to twenty-seven years by subsequent comparisons with plates taken at lick, september , .[ ] positive, however, as well as negative results have ensued from the application of modern methods to that antique group. on october , , wilhelm tempel, a saxon peasant by origin, later a skilled engraver, discovered with a small telescope, bought out of his scanty savings, an elliptical nebulosity, stretching far to the southward from the star merope. it attracted the attention of many observers, but was so often missed, owing to its extreme susceptibility to adverse atmospheric influences, as to rouse unfounded suspicions of its variability. the detection of this evasive object gave a hint, barely intelligible at the time, of further revelations of the same kind by more cogent means. a splendid photograph of , stars in the pleiades, taken by the mm. henry with three hours' exposure, november , , showed one of the brightest of them to have a small spiral nebula, somewhat resembling a strongly-curved comet's tail, attached to it. the reappearance of this strange appurtenance on three subsequent plates left no doubt of its real existence, visually attested at pulkowa, february , , by one of the first observations made with the -inch equatoreal.[ ] much smaller apertures, however, sufficed to disclose the "maia nebula," _once it was known to be there_. not only did it appear greatly extended in the vienna -inch,[ ] but mm. perrotin and thollon saw it with the nice -inch, and m. kammermann of geneva, employing special precautions, with a refractor of only ten inches aperture.[ ] the advantage derived by him for bringing it into view, from the insertion into the eye-piece of a uranium film, gives, with its photographic intensity, valid proof that a large proportion of the light of this remarkable object is of the ultra-violet kind. the beginning thus made was quickly followed up. a picture of the pleiades procured at maghull in eighty-nine minutes, october , , revealed nebulous surroundings to no less than four leading stars of the group, namely, alcyone, electra, merope, and maia; and a second impression, taken in three hours on the following night, showed further "that the nebulosity extends in streamers and fleecy masses till it seems almost to fill the spaces between the stars, and to extend far beyond them."[ ] the coherence of the entire mixed structure was, moreover, placed beyond doubt by the visibly close relationship of the stars to the nebulous formations surrounding them in dr. roberts's striking pictures. thus goldschmidt's notion that all the clustered pleiades constitute, as it were, a second orion trapezium in the midst of a huge formation of which tempel's nebula is but a fragment,[ ] has been to some extent verified. yet it seemed fantastic enough in . then in the mm. henry gave exposures of four hours each to several plates, which exhibited on development some new features of the entangled nebulæ. the most curious of these was the linking together of stars by nebulous chains. in one case seven aligned stars appeared strung on a silvery filament, "like beads on a rosary."[ ] the "rows of stars," so often noticed in the sky, may, then, be concluded to have more than an imaginary existence. of the , stars recorded in these pictures, a couple of hundred among the brightest can, at the outside, be reckoned as genuine pleiades. the great majority were relegated, by pickering's[ ] and stratonoff's[ ] counts of the stellar populace _in_ and _near_ the cluster, to the position of outsiders from it. they are undistinguished denizens of the abysmal background upon which it is projected. investigations of its condition were carried a stage further by barnard. on november , ,[ ] he discovered visually with the lick refractor a close nebulous satellite to merope, photographs of which were obtained by keeler in .[ ] it appears in them of a rudely pentagonal shape, a prominent angle being directed towards the adjacent star. finally, an exposure of ten hours made by barnard with the willard lens indicated the singular fact that the entire group is embedded in a nebulous matrix, streaky outliers of which blur a wide surface of the celestial vault.[ ] the artist's conviction of the reality of what his picture showed was confirmed by negatives obtained by bailey at arequipa in , and by h. c. wilson at northfield (minnesota) in .[ ] with the ealing -foot reflector, sold by dr. common to mr. crossley, and by him presented to the lick observatory, professor keeler took in a series of beautiful and instructive nebula[ ] photographs; one of the trifid may be singled out as of particular excellence. an astonishing multitude of new nebulæ were revealed by trial-exposures with this instrument. a "conservative estimate" gave , as the number coming within its scope. moreover, the majority of those actually recorded were of an unmistakable spiral character, and they included most of sir john herschel's "double nebulæ," previously supposed to exemplify the primitive history of binary stellar systems.[ ] dr. max wolf's explorations with a -inch voigtländer lens in emphatically reaffirmed the inexhaustible wealth of the nebular heavens. in one restricted region, midway between præsepe and the milky way, he located nebulæ, where only three had until then been catalogued; and he counted such objects clustering round the star comæ berenices,[ ] and so closely that all might be occulted together by the moon. the general photographic catalogue of nebulæ which dr. wolf has begun to prepare[ ] will thus be a most voluminous work. the history of celestial photography at the cape of good hope began with the appearance of the great comet of . no special apparatus was at hand; so sir david gill called in the services of a local artist, mr. allis of mowbray, with whose camera, strapped to the observatory equatoreal, pictures of conspicuous merit were obtained. but their particular distinction lay in the multitude of stars begemming the background. (see plate iii.) the sight of them at once opened to the royal astronomer a new prospect. he had already formed the project of extending argelander's "durchmusterung" from the point where it was left by schönfeld to the southern pole; and his ideas regarding the means of carrying it into execution crystallised at the needle touch of the cometary experiments. he resolved to employ photography for the purpose. the exposure of plates was accordingly begun, under the care of mr. ray woods, in ; and in less than six years, the sky, from ° of south latitude to the pole, had been covered in duplicate. their measurement, and the preparation of a catalogue of the stars imprinted upon them, were generously undertaken by professor kapteyn, and his laborious task has at length been successfully completed. the publication, in , of the third and concluding volume of the "cape photographic durchmusterung"[ ] placed at the disposal of astronomers a photographic census of the heavens fuller and surer than the corresponding visual enumeration executed at bonn. it includes , stars, nearly to the tenth magnitude, and their positions are reliable to about one second of arc. the production of this important work was thus a result of the cape comet-pictures; yet not the most momentous one. they turned the scale in favour of recourse to the camera when the mm. henry encountered, in their continuation of chacornac's half-finished enterprise of ecliptical charting, sections of the milky way defying the enumerating efforts of eye and hand. the perfect success of some preliminary experiments made with an instrument constructed by them expressly for the purpose was announced to the academy of sciences at paris, may , . by its means stars estimated as of the sixteenth magnitude clearly recorded their presence and their places; and the enormous increase of knowledge involved may be judged of from the fact that, in a space of the milky way in cygnus ° ' by °, where stars had been mapped by the old laborious method, about five thousand stamped their images on a single henry plate. these results suggested the grand undertaking of a general photographic survey of the heavens, and gill's proposal, june , , of an international congress for the purpose of setting it on foot was received with acclamation, and promptly acted upon. fifty-six delegates of seventeen different nationalities met in paris, april , , under the presidentship of admiral mouchez, to discuss measures and organise action. they resolved upon the construction of a photographic chart of the whole heavens, comprising stars of a fourteenth magnitude, to the surmised number of twenty millions; to be supplemented by a catalogue, framed from plates of comparatively short exposure, giving start to the eleventh magnitude. these will probably amount to about one million and a quarter. for procuring both sets of plates, instruments were constructed precisely similar to that of the mm. henry, which is a photographic refractor, thirteen inches in aperture, and eleven feet focus, attached to a guiding telescope of eleven inches aperture, corrected, of course, for the visual rays. each place covers an area of four square degrees, and since the series must be duplicated to prevent mistakes, about , plates will be needed for the chart alone. the task of preparing them was apportioned among eighteen observatories scattered over the globe, from mexico to melbourne; but three in south america having become disabled or inert, were replaced in by those at cordoba, montevideo, and perth, western australia. meanwhile, the publication of results has begun, and is likely to continue for at least a quarter of a century. the first volume of measures from the potsdam catalogue-plates was issued in , and its successors, if on the same scale, must number nearly . moreover, ninety-six heliogravure enlargements from the paris chart-plates, distributed in the same year, supplied a basis for the calculation that the entire atlas of the sky, composed of similar sheets, will form a pile thirty feet high and two tons in weight![ ] it will, however, possess an incalculable scientific value. for millions of stars can be determined by its means, from their imprinted images, with an accuracy comparable to that attainable by direct meridian observations. one of the most ardent promoters of the scheme it may be expected to realise was admiral mouchez, the successor of leverrier in the direction of the paris observatory. but it was not granted to him to see the fruition of his efforts. he died suddenly june , .[ ] although not an astronomer by profession, he had been singularly successful in pushing forward the cause of the science he loved, while his genial and open nature won for him wide personal regard. he was replaced by m. tisserand, whose mathematical eminence fitted him to continue the traditions of delaunay and leverrier. but his career, too, was unhappily cut short by an unforeseen death on october , ; and the more eminent among the many qualifications of his successor, m. maurice loewy, are of the practical kind. the sublime problem of the construction of the heavens has not been neglected amid the multiplicity of tasks imposed upon the cultivators of astronomy by its rapid development. but data of a far higher order of precision, and indefinitely greater in amount, than those at the disposal of herschel or struve must be accumulated before any definite conclusions on the subject are possible. the first organised effort towards realising this desideratum was made by the german astronomical society in , two years after its foundation at heidelberg. the original programme consisted in the _exact_ determination of the places of all argelander's stars to the ninth magnitude (exclusive of the polar zone), from the reobservation of which, say, in the year , astronomers of two generations hence may gather a vast store of knowledge--directly of the apparent motions, indirectly of the mutual relations binding together the suns and systems of space. thirteen observatories in europe and america joined in the work, now virtually terminated. its scope was, after its inception, widened to include southern zones as far as the tropic of capricorn; this having been rendered feasible by schönfeld's extension ( - ) of argelander's survey. thirty thousand additional stars thus taken in were allotted in zones to five observatories. another important undertaking of the same class is the reobservation of the , stars in lalande's _histoire céleste_. begun under arago in , its upshot has been the publication of the great paris catalogue, issued in eight volumes, between and . from a careful study of their secular changes in position, m. bossert has already derived the proper motions of a couple of thousand out of nearly fifty thousand stars enumerated in it. through dr. gould's unceasing labours during his fifteen years' residence at cordoba, a detailed acquaintance with southern stars was brought about. his _uranometria argentina_ ( ) enumerates the magnitudes of , out of , stars visible to the naked eye under those transparent skies; , down to - / magnitude are embraced in his "zones"; and the argentine general catalogue of , southern stars was published in . valuable work of the same kind has been done at the leander mccormick observatory, virginia, by professor o. stone; while the late redcliffe observer's "cape catalogue for " affords inestimable aid to the practical astronomer south of the line, which has been reinforced with several publications issued by the present astronomer royal at the cape. moreover, the gigantic task entered upon in by dr. c. h. f. peters, director of the litchfield observatory, clinton (n.y.), and of which a large instalment was finished in , deserves honourable mention. it was nothing less than to map all stars down to, and even below, the fourteenth magnitude, situated within ° on either side of the ecliptic, and so to afford "a sure basis for drawing conclusions with respect to the changes going on in the starry heavens."[ ] it is tolerably safe to predict that no work of its kind and for its purpose will ever again be undertaken. in a small part of one night stars can now be got to register themselves more numerously and more accurately than by the eye and hand of the most skilled observer in the course of a year. fundamental catalogues, constructed by the old, time-honoured method, will continue to furnish indispensable starting-points for measurement; and one of especial excellence was published by professor newcomb in ;[ ] but the relative places of the small crowded stars--the sidereal [greek: hoi pholloi]--will henceforth be derived from their autographic statements on the sensitive plate. even the secondary purpose--that of asteroidal discovery--served by detailed stellar enumeration, is more surely attained by photography than by laborious visual comparison. for planetary movement betrays itself in a comparatively short time by turning the imprinted image of the object affected by it from a dot into a trail. in the arduous matter of determining star distances progress has been steady, and bids fair to become rapidly accelerated. together, yet independently, gill and elkin carried out, at the cape observatory in - , an investigation of remarkable accuracy into the parallaxes of nine southern stars. one of these was the famous alpha centauri, the distance of which from the earth was ascertained to be just one-third greater than henderson had made it. the parallax of sirius, on the other hand, was doubled, or its distance halved; while canopus proved to be quite immeasurably remote--a circumstance which, considering that, among all the stellar multitude, it is outshone only by the radiant dog-star, gives a stupendous idea of its real splendour and dimensions. inquiries of this kind were, for some years, successfully pursued at the observatory of dunsink, near dublin. annual perspective displacements were by dr. brünnow detected in several stars, and in others remeasured with a care which inspired just confidence. his parallax for alpha lyræ ( · ") was authentic, though slightly too large (elkin's final results gave pi = · "); and the received value for the parallax of the swiftly travelling star "groombridge , " scarcely differs from that arrived at by him in (pi = · "). his successor as astronomer-royal for ireland, sir robert stawell ball (now lowndean professor of astronomy in the university of cambridge), has done good service in the same department. for besides verifying approximately struve's parallax of half a second of arc for cygni, he refuted, in , by a sweeping search for (so-called) "large" parallaxes, certain baseless conjectures of comparative nearness to the earth, in the case of red and temporary stars.[ ] of objects thus cursorily examined, only one star of the seventh magnitude, numbered , in groombridge's circumpolar catalogue, gave signs of measurable vicinity. similarly, a reconnaissance among rapidly moving stars lately made by dr. chase with the yale heliometer[ ] yielded no really large, and only eight appreciable parallaxes among the subjects of his experiments. a second campaign in stellar parallax was undertaken by gill and elkin in . but this time the two observers were in opposite hemispheres. both used heliometers. dr. elkin had charge of the fine instrument then recently erected in yale college observatory; sir david gill employed one of seven inches, just constructed under his directions, in first-rate style, by the repsolds of hamburg. dr. elkin completed in his share of the more immediate joint programme, which consisted in the determination, by direct measurement, of the average parallax of stars of the first magnitude. it came out, for the ten northern luminaries, after several revisions, · ", equivalent to a light-journey of thirty-three years. the deviations from this average were, indeed, exceedingly wide. two of the stars, betelgeux and alpha cygni, gave no certain sign of any perspective shifting; of the rest, procyon, with a parallax of · ", proved the nearest to our system. at the mean distance concluded for these ten brilliant stars, the sun would show as of only fifth magnitude; hence it claims a very subordinate rank among the suns of space. sir david gill's definitive results were published in .[ ] as the average parallax of the eleven brightest stars in the southern hemisphere, they gave · ", a value enhanced by the exceptional proximity of alpha centauri. yet four of these conspicuous objects--canopus, rigel, spica, and beta crucis--gave no sign of perspective response to the annual change in our point of view. the list included eleven fainter stars with notable proper motions, and most of these proved to have fairly large parallaxes. among other valuable contributions to this difficult branch may be instanced bruno peter's measurements of eleven stars with the leipzig heliometer, - ;[ ] kapteyn's application of the method by differences in right ascension to fifteen stars observed on the meridian - ;[ ] and flint's more recent similar determinations at madison, wisconsin.[ ] the great merit of having rendered photography available for the sounding of the celestial depths belongs to professor pritchard. the subject of his initial experiment was cygni. from measurements of negatives taken in , he derived for that classic star a parallax of · ", in satisfactory agreement with ball's of · ". a detailed examination convinced the astronomer-royal of its superior accuracy to bessel's result with the heliometer. the savilian professor carried out his project of determining all second magnitude stars to the number of about thirty,[ ] conveniently observable at oxford, obtaining as the general outcome of the research an average parallax of · ", for objects of that rank. but this value, though in itself probable, cannot be accepted as authoritative, in view of certain inaccuracies in the work adverted to by jacoby,[ ] hermann davis, and gill. the method has, nevertheless, very large capabilities. professor kapteyn showed, in ,[ ] the practicability of deriving parallaxes wholesale from plates exposed at due intervals, and applied his system, in , with encouraging success, to a group of stars.[ ] the apparent absence of spurious shiftings justified the proposal to follow up the completion of the astrographic chart with the initiation of a photographic "parallax durchmusterung." observers of double stars are among the most meritorious, and need to be among the most patient and painstaking workers in sidereal astronomy. they are scarcely as numerous as could be wished. dr. doberck, distinguished as a computer of stellar orbits, complained in [ ] that data sufficient for the purpose had not been collected for above or binaries out of between five and six hundred certainly or probably within reach. the progress since made is illustrated by mr. gore's useful catalogue of computed binaries, including fifty-nine entries, presented to the royal irish academy, june , .[ ] few have done more towards supplying the deficiency of materials than the late baron ercole dembowski of milan. he devoted the last thirty years of his life, which came to an end january , , to the revision of the dorpat catalogue, and left behind him a store of micrometrical measures as numerous as they are precise. of living observers in this branch, mr. s. w. burnham is beyond question the foremost. while pursuing legal avocations at chicago, he diverted his scanty leisure by exploring the skies with a -inch telescope mounted in his back-yard; and had discovered, in may, , one thousand close and mostly very difficult double stars.[ ] summoned as chief assistant to the new lick observatory in , he resumed the work of his predilection with the -inch and -inch refractors of that establishment. but although devoting most of his attention to much-needed remeasurements of known pairs, he incidentally divided no less than stars, the majority of which lay beyond the resolving power of less keen and effectually aided eyesight. one of his many interesting discoveries was that of a minute companion to alpha ursæ majoris (the first pointer), which already gives unmistakable signs of orbital movement round the shining orb it is attached to. another pair, kappa pegasi, detected in , was found in to have more than completed a circuit in the interim.[ ] its period of a little over eleven years is the shortest attributable to a _visible_ binary system, except that of delta equulei, provisionally determined by professor hussey in at · years,[ ] and indicated by spectroscopic evidence to be of uncommon brevity.[ ] burnham's catalogue of , double stars, discovered by him from to ,[ ] is a record of unprecedented interest. nearly all the pairs included in it, " or less than " apart, must be physically connected; and they offer a practically unlimited field for investigation; while the notes, diagrams, and orbits appended profusely to the various entries, are eminently helpful to students and computers. the author is continuing his researches at the yerkes observatory, having quitted the lick establishment in . the first complete enrolment of southern double stars was made by mr. r. t. a. innes in .[ ] the couples enumerated, twenty-one per cent. of which are separated by less than one second of arc, are , in number. they include discovered by himself. dr. see gathered a rich harvest of nearly new southern pairs with the lowell -inch refractor in .[ ] professor hough's discoveries in more northerly zones amount to ;[ ] hussey's at lick to ; and aitken's already to over . there is as yet no certainty that the stars of cygni form a true binary combination. mr. burnham, indeed, holds them to be in course of definitive separation; and professor hall's observations at washington, to , although favouring their physical connection, are far from decisive on the point.[ ] dr. wilsing, from certain anomalous displacements of their photographed images, concluded in [ ] the presence of an invisible third member of the system, revolving in a period of twenty-two months; but the effects noticed by him were probably illusory. important series of double-star observations were made by perrotin at nice in - ;[ ] by hall, with the -inch washington equatoreal, to ;[ ] by schiaparelli from onward; by glasenapp, o. stone, leavenworth, seabroke, and many besides. finally, professor hussey's revision of the pulkowa catalogue[ ] is a work of the _teres atque rotundus_ kind, which leaves little or nothing to be desired. the methods employed in double-star determinations remain, at the beginning of the twentieth century, essentially unchanged. the camera has scarcely encroached upon this part of the micrometer's domain.[ ] a research of striking merit into the origin of binary stars was published in by dr. t. j. j. see, in the form of an inaugural dissertation for his doctor's degree in the university of berlin. the main result was to show the powerful effects of tidal friction in prescribing the course of their development from double nebulæ, revolving almost in contact, to double suns, far apart, yet inseparable. the high eccentricities of their eventual orbits were shown to result necessarily from this mode of action, which must operate with enormous strength on closely conjoined, nearly equal masses, such as the rapidly revolving pairs disclosed by the spectroscope. that these are still in an early stage of their life-history is probable in itself, and is re-affirmed by the exceedingly small density indicated for eclipsing stars by the ratio of phase-duration to period. stellar photometry, initiated by the elder herschel, and provided with exact methods by his son at the cape, by steinheil and seidel at munich, has of late years assumed the importance of a separate department of astronomical research. two monumental works on the subject, compiled on opposite sides of the atlantic, were thus appropriately coupled in the bestowal of the royal astronomical society's gold medal in . harvard college observatory led the way under the able direction of professor e. c. pickering. his photometric catalogue of , stars,[ ] constructed from nearly , observations of light-intensity during the years - , constitutes a record of incalculable value for the detection and estimation of stellar variability. it was succeeded in by professor pritchard's "uranometria nova oxoniensis," including photometric determinations of the magnitude of all naked-eye stars, from the pole to ten degrees south of the equator to the number of , . the instrument employed was the "wedge photometer," which measures brightness by resistance to extinction. a wedge of neutral-tint glass, accurately divided to scale, is placed in the path of the stellar rays, when the thickness of it they have power to traverse furnishes a criterion of their intensity. professor pickering's "meridian photometer," on the other hand, is based upon zöllner's principle of equalization effected by a polarising apparatus. after all, however, as professor pritchard observed, "the eye is the real photometer," and its judgment can only be valid over a limited range.[ ] absolute uniformity, then, in estimates made by various means, under varying conditions, and by different observers, is not to be looked for; and it is satisfactory to find substantial agreement attainable and attained. only in an insignificant fraction of the stars common to the harvard and oxford catalogues discordances are found exceeding one-third of a magnitude; a large proportion ( per cent.) agree within one-fourth, a considerable minority ( per cent.) within one-tenth of a magnitude.[ ] the harvard photometry was extended, on the same scale, to the opposite pole in a catalogue of the magnitudes of , southern stars,[ ] founded on professor bailey's observations in peru, - . measurements still more comprehensive were subsequently executed at the primary establishment. with a meridian photometer of augmented power, the surprising number of , settings were made during the years - , nearly all by the indefatigable director himself, and they afforded materials for a "photometric durchmusterung," published in , including all stars to · magnitude north of declination - °.[ ] a photometric zone, ° wide, has for some time been in course of observation at potsdam by mm. müller and kempf. the instrument employed by them is constructed on the polarising principle as adapted by zöllner. photographic photometry has meanwhile risen to an importance if anything exceeding that of visual photometry. for the usefulness of the great international star-chart now being prepared would be gravely compromised by systematic mistakes regarding the magnitudes of the stars registered upon it. no entirely trustworthy means of determining them have, however, yet been found. there is no certainty as to the relative times of exposure needed to get images of stars representative of successive photometric ranks. all that can be done is to measure the proportionate diameters of such images, and to infer, by the application of a law learned from experience, the varied intensities of light to which they correspond. the law is, indeed, neither simple nor constant. different investigators have arrived at different formulæ, which, being purely empirical, vary their nature with the conditions of experiment. probably the best expedient for overcoming the difficulty is that devised by pickering, of simultaneously photographing a star and its secondary image, reduced in brightness by a known amount.[ ] the results of its use will be exhibited in a catalogue of , stars to the tenth magnitude, one for each square degree of the heavens. a photographic photometry of all the lucid stars, modelled on the visual photometry of , is promised from the same copious source of novelties. the magnitudes of the stars in the draper catalogue were determined, so to speak, spectrographically. the quantity measured in all cases was the intensity of the hydrogen line near g. by the employment of this definite and uniform test, results were obtained, of special value indeed, but in strong disaccord with those given by less exclusive determinations. thought, meantime, cannot be held aloof from the great subject upon the future illustration of which so much patient industry is being expended. nor are partial glimpses denied to us of relations fully discoverable, perhaps, only through centuries of toil. some important points in cosmical economy have, indeed, become quite clear within the last fifty years, and scarcely any longer admit of a difference of opinion. one of these is that of the true status of nebulæ. this was virtually settled by sir j. herschel's description in of the structure of the magellanic clouds; but it was not until whewell, in , and herbert spencer, in ,[ ] enforced the conclusions necessarily to be derived therefrom that the conception of the nebulæ as remote galaxies, which lord rosse's resolution of many into stellar points had appeared to support, began to withdraw into the region of discarded and half-forgotten speculations. in the nubeculæ, as whewell insisted,[ ] "there coexist, in a limited compass and in indiscriminate position, stars, clusters of stars, nebulæ, regular and irregular, and nebulous streaks and patches. these, then, are different kinds of things in themselves, not merely different to us. there are such things as nebulæ side by side with stars and with clusters of stars. nebulous matter resolvable occurs close to nebulous matter irresolvable." this argument from coexistence in nearly the same region of space, reiterated and reinforced with others by mr. spencer, was urged with his accustomed force and freshness by mr. proctor. it is unanswerable. there is no maintaining nebulæ to be simply remote worlds of stars in the face of an agglomeration like the nubecula major, containing in its (certainly capacious) bosom _both_ stars and nebulæ. add the facts that a considerable proportion of these perplexing objects are gaseous, and that an intimate relation obviously subsists between the mode of their scattering and the lie of the milky way, and it becomes impossible to resist the conclusion that both nebular and stellar systems are parts of a single scheme.[ ] as to the stars themselves, the presumption of their approximate uniformity in size and brightness has been effectually dissipated. differences of distance can no longer be invoked to account for dissimilarity in lustre. minute orbs, altogether invisible without optical aid, are found to be indefinitely nearer to us than such radiant objects as canopus, betelgeux, or rigel. moreover, intensity of light is perceived to be a very imperfect index to real magnitude. brilliant suns are swayed from their course by the attractive power of massive yet faintly luminous companions, and suffer eclipse from obscure interpositions. besides, effective lustre is now known to depend no less upon the qualities of the investing atmosphere than upon the extent and radiative power of the stellar surface. red stars must be far larger in proportion to the light diffused by them than white or yellow stars.[ ] there can be no doubt that our sun would at least double its brightness were the absorption suffered by its rays to be reduced to the sirian standard; and, on the other hand, that it would lose half its present efficiency as a light-source if the atmosphere partially veiling its splendours were rendered as dense as that of aldebaran. thus, variety of all kinds is seen to abound in the heavens; and it must be admitted that the consequent insecurity of all hypotheses as to the relative distances of individual stars singularly complicates the question of their allocation in space. nevertheless, something has been learnt even on that point; and the tendency of modern research is, on the whole, strongly confirmatory of the views expressed by herschel in . he then no longer regarded the milky way as the mere visual effect of an enormously extended stratum of stars, but as an actual aggregation, highly irregular in structure, made up of stellar clouds and groups and nodosities. all the facts since ascertained fit in with this conception, to which proctor added arguments favouring the view, since adopted by barnard[ ] and easton,[ ] that the stars forming the galactic stream are not only situated more closely together, but are also really, as well as apparently, of smaller dimensions than the lucid orbs studding our skies. by the laborious process of isographically charting the whole of argelander's , stars, he brought out in [ ] signs of relationship between the distribution of the brighter stars and the complex branchings of the milky way, which has been stamped as authentic by newcomb's recent statistical inquiries.[ ] there is, besides, a marked condensation of stars, especially in the southern hemisphere, towards a great circle inclined some twenty degrees to the galactic plane; and these were supposed by gould to form with the sun a subordinate cluster, of which the components are seen projected upon the sky as a zone of stellar brilliants.[ ] the zone has, however, galactic rather than solar affinities, and represents, perhaps, not a group, but a stream. the idea is gaining ground that the milky way is designed, in its main outlines, on a spiral pattern, and that its various branches and sections are consequently situated at very different distances from ourselves. proctor gave a preliminarily interpretation of their complexities on this principle, and easton of rotterdam[ ] has renewed the attempt with better success. a most suggestive delineation of the milky way, completed in , after five years of labour, by dr. otto boedicker, lord rosse's astronomer at parsonstown, was published by lithography in . it showed a curiously intricate structure, composed of dimly luminous streams, and shreds, and patches, intermixed with dark gaps and channels. ramifications from the main trunk ran out towards the andromeda nebula and the "bee-hive" cluster in cancer, involved the pleiades and hyades, and, winding round the constellation of orion, just attained the sword-handle nebula. the last delicate touches had scarcely been put to the picture, when the laborious eye-and-hand method was, in this quarter, as already in so many others, superseded by a more expeditious process. professor barnard took the first photographs ever secured of the true milky way, july , august and , , at the lick observatory. special conditions were required for success; above all, a wide field and a strong light-grasp, both complied with through the use of a -inch portrait-lens. even thus, the sensitive plate needed some hours to pick out the exceedingly faint stars collected in the galactic clouds. these cannot be photographed under the nebulous aspect they wear to the eye; the camera takes note of their real nature, and registers their constituent stars rank by rank. hence the difficulty of disclosing them. "in the photographs made with the -inch portrait-lens," professor barnard wrote, "besides myriads of stars, there are shown, for the first time, the vast and wonderful cloud-forms, with all their remarkable structure of lanes, holes, and black gaps, and sprays of stars. they present to us these forms in all their delicacy and beauty, as no eye or telescope can ever hope to see them."[ ] in plate vi. one of these strange galactic landscapes is reproduced. it occurs in the bow of sagittarius, not far from the trifid nebula, where the aggregations of the milky way are more than usually varied and characteristic. one of their distinctive features comes out with particular prominence. it will be noticed that the bright mass near the centre of the plate is tunnelled with dark holes and furrowed by dusky lanes. such interruptions recur perpetually in the milky way. they are exemplified on the largest scale in the great rift dividing it into two branches all the way from cygnus to crux; and they are reproduced in miniature in many clusters. plate vi. [illustration: region of the milky way in sagittarius--showing a double black aperture. photographed by professor e. e. barnard.] mr. h. c. russell, at sydney in , successfully imitated professor barnard's example.[ ] his photographs of the southern milky way have many points of interest. they show the great rift, black to the eye, yet densely star-strewn to the perception of the chemical retina; while the "coal-sack" appears absolutely dark only in its northern portion. his most remarkable discovery, however, was that of the spiral character of the two nubeculæ. with an effective exposure of four and a half hours, the greater cloud came out as "a complex spiral, with two centres"; while the similar conformation of its minor companion developed only after eight hours of persistent actinic action. the revelation is full of significance. scarcely less so, although after a different fashion, is the disclosure on plates exposed by dr. max wolf, with a -inch lens, in june, , of a vastly extended nebula, bringing some of the leading stars in cygnus into apparently organic connection with the piles of galactic star-dust likewise involved by it.[ ] barnard has similarly found great tracts of the milky way to be photographically nebulous, and the conclusion seems inevitable that we see in it a prodigious mixed system, resembling that of the pleiades in point of composition, though differing widely from it in plan of structure. of corroborative testimony, moreover, is the discovery independently resulting from gill's and pickering's photographic reviews, that stars of the first type of spectrum largely prevail in the galactic zone of the heavens.[ ] with approach to that zone, kapteyn noticed a steady growth of actinic intensity relative to visual brightness in the stars depicted on the cape durchmusterung plates.[ ] in other words, stellar light is, in the milky way, _bluer_ than elsewhere. and the reality of the primitive character hence to be inferred for the entire structure was, in a manner, certified by mr. mcclean's observation that helium stars--the supposed immediate products of nebulous matter--crowd towards its medial plane. the first step towards the unravelment of the tangled web of stellar movements was taken when herschel established the reality, and indicated the direction of the sun's journey. but the gradual shifting backward of the whole of the celestial scenery amid which we advance accounts for only a part of the observed displacements. the stars have motions of their own besides those reflected upon them from ours. all attempts, however, to grasp the general scheme of these motions have hitherto failed. yet they have not remained wholly fruitless. the community of slow movement in taurus, upon which mädler based his famous theory, has proved to be a fact, and one of very extended significance. in mr. proctor undertook to chart down the directions and proportionate amounts of about , proper motions, as determined by messrs. stone and main, with the result of bringing to light the remarkable phenomenon termed by him "star-drift."[ ] quite unmistakably, large groups of stars, otherwise apparently disconnected, were seen to be in progress together, in the same direction and at the same rate, across the sky. an example of this kind of unanimity was alleged by him in the five intermediate stars of the plough; and that the agreement in thwartwise motion is no casual one is practically demonstrated by the concordant radial velocities determined at potsdam for four out of the five objects in question. all of these approach the earth at the rate of about eighteen miles a second; and the fifth and faintest, delta ursæ, though not yet measured, may be held to share their advance. one of them, moreover, zeta ursæ, alias mizar, carries with it three other stars--alcor, the arab "rider" of the horse, visible to the naked eye, besides a telescopic and a spectroscopic attendant. so that the group may be regarded as octuple. it is of vast compass. dr. höffler assigned to it in [ ]--although on grounds more or less hypothetical--a mean parallax corresponding to a light-journey of years, which would give to the marching squadron a total extent of at least fourteen times the distance from the sun to alpha centauri, while implying for its brightest member--eta ursæ majoris--the lustre of six hundred suns. the organising principle of this grand scheme must long remain mysterious. it is no solitary example. particular association, indeed--as was surmised by michell far back in the eighteenth century--appears to be the rule rather than an exception in the sidereal system. stars are bound together by twos, by threes, by dozens, by hundreds. our own sun is, perhaps, not exempt from this gregarious tendency. yet the search for its companions has, up to the present, been unavailing. gould's cluster[ ] seems remote and intangible; kapteyn's collection of solar stars proved to have been a creation of erroneous data, and was abolished by his unrelenting industry. rather, we appear to have secured a compartment to ourselves for our long journey through space. a practical certainty has, at any rate, been gained that whatever aggregation holds the sun as a constituent is of a far looser build than the pleiades or præsepe. of all such majestic communities the laws and revolutions remain, as yet, inaccessible to inquiry; centuries may elapse before even a rudimentary acquaintance with them begins to develop; while the economy of the higher order of association, which we must reasonably believe that they unite to compose, will possibly continue to stimulate and baffle human curiosity to the end of time. footnotes: [footnote : _report brit. assoc._, , p. . rutherfurd gave a rudimentary sketch of a classification of the kind in december, , but based on imperfect observation. see _am. jour. of sc._, vol. xxxv., p. .] [footnote : _publicationem_, potsdam, no. , , p. .] [footnote : von konkoly _once_ derived from a slow-moving meteor a hydro-carbon spectrum. a. s. herschel, _nature_, vol. xxiv., p. .] [footnote : _phil. trans._, vol. cliv., p. .] [footnote : _am. jour. of sc._, vol. xix., p. .] [footnote : _photom. unters._, p. .] [footnote : _spectre solaire_, p. .] [footnote : mr. j. birmingham, in the introduction to his catalogue of red stars, adduced sundry instances of colour-change in a direction the opposite to that assumed by zöllner to be the inevitable result of time. _trans. r. irish acad._, vol. xxvi., p. . a learned discussion by dr. t. j. j. see, moreover, enforces the belief that sirius was absolutely _red_ eighteen hundred years ago. _astr. and astroph._, vol. xi., p. .] [footnote : _phil. trans.,_ vol. clxiv., p. .] [footnote : _astr. nach._, no. , .] [footnote : _proc. roy. soc._, vols. xvi., p. ; xvii., p. .] [footnote : _annalen der physik_, bd. xx., p. .] [footnote : _ibid._, p. .] [footnote : _knowledge_, vol. xiv., p. .] [footnote : _meteoritic hypothesis_, p. .] [footnote : _phil. trans._, vol. cxci. a., p. ; _spectra of southern stars_, p. .] [footnote : see the author's _system of the stars_, p. .] [footnote : a designation applied by sir norman lockyer to third-type stars.] [footnote : see _ante_, p. .] [footnote : _bothkamp beobachtungen_, heft ii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, no. , ; _observatory_, vol. vi., p. .] [footnote : _month. not._, vol. xlvii., p. .] [footnote : _publ. astr. pac. soc._, vol. i., p. ; _observatory_, vol. xiii., p. .] [footnote : _lockyer, proc. roy. soc._, vol. lvii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _month. not._, vol. lix., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, nos. , - .] [footnote : _ibid._, no. , ; _astr. and astrophysics_, vol. xi., p. ; bélopolsky, _astr. nach._, no. , .] [footnote : _comptes rendus_, t. lxv., p. .] [footnote : _copernicus_, vol. iii., p. .] [footnote : _system of the stars_, p. ; _harvard annals_, vol. xxviii., pt. ii., p. (miss cannon).] [footnote : _potsdam publ._, no. , p. .] [footnote : _proc. roy. soc._, vol. xlix., p. .] [footnote : miss a. j. cannon, _harvard annals_, vol. xxviii., pt. ii., p. .] [footnote : _astr. and astroph._, vol. xiii., p. .] [footnote : _potsdam publ._, no. .] [footnote : the results of von konkoly's extension of vogel's work to ° of south declination were published in _o gyalla beobachtungen_, bd. viii., th. ii., .] [footnote : _astroph. jour._, vols. viii., p. ; ix., p. .] [footnote : _ibid._, vol. ix., p. .] [footnote : _phil. trans._, vol. cliv., p. . some preliminary results were embodied in a "note" communicated to the royal society, february , (_proc. roy. soc._, vol. xii., p. ).] [footnote : _bothkamp beob._, heft i., p. .] [footnote : _astroph. jour._, vol. vi., p. .] [footnote : _phil. trans._, vol. cliv., p. , _note_.] [footnote : _month. not._, vol. xxiii., p. .] [footnote : _proc. roy. soc._, vol. xxv., p. .] [footnote : _phil. trans._, vol. clxxi., p. ; _atlas of stellar spectra_, p. .] [footnote : _astr. nach._, no. , ; _monatsb._, berlin, , p. ; , p. .] [footnote : _jour. de physique_, t. v., p. .] [footnote : _system of the stars_, p. .] [footnote : see _ante_, p. .] [footnote : _proc. roy. soc._, vol. xlviii., p. .] [footnote : _harvard circulars_, nos. , ; _astroph. jour._, vol. v., p. .] [footnote : _astroph. jour._, vol. vi., p. .] [footnote : mcclean, _phil. trans._, vol. cxci. a., p. .] [footnote : _proc. roy. soc._, vol. lxii., p. .] [footnote : _ibid._, april , ; _astroph. jour._, vol. x., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, no. , .] [footnote : lunt, _astroph. jour._, vol. xi., p. ; _proc. roy. soc._, vol. lxvi., p. ; lockyer, _ibid._, november , ; _nature_, vol. lxi., p. .] [footnote : _die spectralanalyse_, p. .] [footnote : _henry draper memorial, first ann. report_, .] [footnote : _mem. amer. acad._, vol. xi., p. .] [footnote : _harvard annals_, vol. xxvii.] [footnote : _harvard annals_, vol. xxviii., parts i. and ii.] [footnote : see _ante_, p. .] [footnote : _phil. trans._, vol. clviii., p. .] [footnote : schellen, _die spectralanalyse_, bd. ii., p. (ed. ).] [footnote : _proc. roy. soc._, vol. xx., p. .] [footnote : _system of the stars_, p. .] [footnote : pickering, _am. jour. of sc._, vol. xxxix., p. ; vogel, _astr. nach._ no. , .] [footnote : _sitzungsberichte_, berlin, may , ; _astroph. jour._, vol. xiii., p. .] [footnote : the "relative orbit" of a double star is that described by one round the other as a fixed point. micrometrical measures are always thus executed. but in reality both stars move in opposite directions, and at rates inversely as their masses round their common centre of gravity.] [footnote : vogel, _astr. nach._, nos. , , , .] [footnote : huggins, _pres. address_, ; cornu, _sur la méthode doppler-fizeau_ p. d. .] [footnote : _sitzungsb._, berlin, , p. ; _astr. nach._, no. , .] [footnote : _ibid._] [footnote : _astroph. jour._, vol. v., p. ; newall, _month. not._, vol. lvii., p. .] [footnote : _bull. de l'acad. de st. pétersb._, tt. vi., viii.] [footnote : _astroph. jour._, vol. x., p. ; _month. not._, vol. lx., p. ; vogel, _sitzungsb._, berlin, april , .] [footnote : _month. not._, vol. lx., p. .] [footnote : hussey, _astr. jour._, no. .] [footnote : _astroph. jour._, vols. x, p. ; xiv., p. ; _lick bulletin_, no. ; bélopolsky, _astr. nach._, no. , .] [footnote : the significance of the name "el ghoul" leaves little doubt that the arab astronomers took note of this star's variability. e. m. clerke, _observatory_, vol. xv., p. .] [footnote : _phil. trans._, vol. lxxiii., p. .] [footnote : _proc. amer. acad._, vol. xvi., p. ; _observatory_, vol. iv., p. . for a preliminary essay by t. s. aldis, see _phil. mag._, vol. xxxix., p. , .] [footnote : _astr. nach._, no. , .] [footnote : _astr. jour._, nos. - , - , . see also _knowledge_, vol. xv., p. .] [footnote : bauschinger, _v. j. s. astr. ges._, jahrg. xxix.; but _cf._ searle, _harvard annals_, vol. xxix., p. ; boss, _astr. jour._, no. .] [footnote : _comptes rendus_, t. cxx., p. .] [footnote : myers, _astroph. jour._, vol. vii., p. ; a. w. roberts, _ibid._, vol. xiii., p. .] [footnote : _proc. r. irish ac._, july, .] [footnote : _ibid._, vol. i., p. .] [footnote : _astr. jour._, nos. , .] [footnote : _ibid._, nos. , .] [footnote : _astr. jour._, nos. - .] [footnote : _system of the stars_, p. .] [footnote : _proc. roy. soc._, vol. xv., p. .] [footnote : weiss, _astr. nach._, no. , ; espin, _ibid._, no. , .] [footnote : _comptes rendus_, t. lxxxiii., p. .] [footnote : _monatsb._, berlin, , pp. , .] [footnote : _copernicus_, vol. ii., p. .] [footnote : burnham, _month. not._, vol. lii., p. .] [footnote : _astr. nach._, no. , .] [footnote : a. hall, _am. jour. of sc._, vol. xxxi., p. .] [footnote : young, _sid. messenger_, vol. iv., p. ; hasselberg, _astr. nach._, no. , .] [footnote : _report brit. assoc._, , p. .] [footnote : _month. not._, vol. xlvii., p. .] [footnote : _nature_, vol. xxxii., p. .] [footnote : _astr. nach._, nos. , , , .] [footnote : _month. not._, vol. xxi., p. .] [footnote : _observatory_, vol. viii., p. .] [footnote : _astr. nach._, no. , ; _astr. and astroph._, vol. xi., p. .] [footnote : _cape results_, p. .] [footnote : _trans. r. soc. of edinburgh_, vol. xxvii., p. ; _astr. and astroph._, august, , p. .] [footnote : vogel, _astr. nach._, no. , .] [footnote : _observatory_, vol. xv., p. ; seeliger, _astr. nach._, no. , ; _astr. and astroph._, vol. xi., p. .] [footnote : ranyard, _knowledge_, vol. xv., p. .] [footnote : _proc. roy. soc._, vol. li., p. .] [footnote : burnham, _month. not._, vol. liii., p. .] [footnote : _astr. nach._, nos. , , , .] [footnote : renz, _ibid._, nos. , , , ; huggins, _astr. and astroph._, vol. xiii., p. .] [footnote : _astr. nach._, no. , .] [footnote : bélopolsky, _astr. nach._, no. , .] [footnote : _nature_, september , .] [footnote : _astr. nach._, nos. , , , .] [footnote : _ibid._, no. , ; _astr. and astroph._, vol. xi., p. .] [footnote : _publ. astr. pac. soc._, vol. iv., p. .] [footnote : barnard, _astroph. jour._, vol. xiv., p. ; campbell, _observatory_, vol. xxiv., p. .] [footnote : _pop. astr._, march, , p. .] [footnote : _harvard circular_, no. , december , . the first nova persei was spectrographically recorded in .] [footnote : vogel, _sitzungsb._, berlin, april , , p. .] [footnote : sidgreaves, _observatory_, vol. xxiv., p. .] [footnote : _ibid._, _knowledge_, vol. xxv., p. .] [footnote : _lick bulletin_, no. .] [footnote : _astr. nach._, no. , .] [footnote : _astroph. jour._, vol. xiv., p. .] [footnote : _lick bulletin_, no. .] [footnote : _astroph. jour._, vols. xiv., p. ; xv., p. .] [footnote : _cf._ the theories on the subject of m. wolf, _astr. nach._, nos. , , , ; kapteyn, _ibid._, no. , ; f. w. very, _ibid._, no. , ; and w. e. wilson, _proc. roy. dublin soc._, no. , p. .] [footnote : _phil. trans._, vol. cliv., p. .] [footnote : _phil. trans._, vol. clviii., p. . the true proportion seems to be about one-tenth (_harvard annals_, vol. xxvi., pt. ii., p. ), the tulse hill working-list having been formed of specially selected objects.] [footnote : scheiner, _astr. nach._, no. , ; _astroph. jour._, vol. vii., p. ; campbell, _ibid._, vols. ix., p. ; x., p. .] [footnote : _proc. roy. soc._, vols. xlvi., p. ; xlviii., p. .] [footnote : _publ. astr. pac. soc._, vol. ii., p. ; _proc. roy. soc._, vol. xlix., p. .] [footnote : _astr. nach._, no , .] [footnote : _atlas of stellar spectra_, p. .] [footnote : _knowledge_, vol. xix., p. .] [footnote : _astr. nach._, nos. , , , , , ; chambers, _descriptive astr._ ( rd ed.), p. ; flammarion, _l'univers sidéral_, p. .] [footnote : _month. not._, vol. li., p. .] [footnote : _ibid._, vol. lix., p. .] [footnote : _ibid._, vol. lx., p. .] [footnote : dreyer, _ibid._, vol. lii., p. .] [footnote : _wash. obs._, vol. xxv., app. .] [footnote : _am. jour. of sc._, vol. xiv., p. ; c. dreyer, _month. not._, vol. xlvii., p. .] [footnote : _ibid._, vol. li., p. .] [footnote : reproduced in _knowledge_, april, .] [footnote : unless an exception be found in the pleiades nebulæ, which may be assumed to share the small apparent movement of the stars they adhere to.] [footnote : _abhandl. akad. der wiss._, leipzig, , bd. iii., p. .] [footnote : _month. not._, vol. lii., p. .] [footnote : _proc. roy. soc._, , p. .] [footnote : _publ. astr. pac. soc._, vol. ii., p. .] [footnote : _system of the stars_, p. .] [footnote : _proc. roy. soc._, vol. xlix., p. .] [footnote : _potsdam publ._, bd. vii., th. i.] [footnote : _astr. nach._, no. , ; schönfeld, _v. j. s. astr. ges._, jahrg. xxi., p. .] [footnote : _astroph. journ._, vol. xiii., p. .] [footnote : _proc. roy. soc._, vol. xxxiii., p. ; _report brit. assoc._, , p. . an impression of the four lower lines in the same spectrum was almost simultaneously obtained by dr. draper. _comptes rendus_, t. xciv., p. .] [footnote : _proc. roy. soc._, vol. xlviii., p. .] [footnote : _month. not._, vol. xlviii., p. .] [footnote : _proc. roy. soc._, vol. xlvi., p. ; _system of the stars_, p. .] [footnote : _sitzungsb._, berlin, february , .] [footnote : _wash. obs._, vol. xxv., app. i., p. .] [footnote : _comptes rendus_, t. xcii., p. .] [footnote : _month. not._, vol. xliii., p. .] [footnote : _harvard annals_, vol. xviii., p. .] [footnote : _sid. mess._, vol. ix., p. .] [footnote : _knowledge_, vol. xv., p. .] [footnote : _month. not._, vol. xlix., p. .] [footnote : _system of the stars_, p. .] [footnote : _astr. nach._, nos. , , , .] [footnote : vogel, _astr. nach._, , .] [footnote : _nature_, vol. xliii., p. .] [footnote : _l'astronomie_, t. xl., p. .] [footnote : _astr. nach._, bände xlvii., p. ; xlviii., p. ; xlix., p. . pickering, _mem. am. ac._, vol. xi., p. .] [footnote : gould on celestial photography, _observatory_, vol. ii., p. .] [footnote : _annals n. y. acad. of sciences_, vol. vi., p. , ; elkin, _publ. astr. pac. soc._, vol. iv., p. .] [footnote : _trans. yale observatory_, vol. i., pt. i.] [footnote : _astroph. jour._, vol. xiii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, no. , .] [footnote : _ibid._, no. , .] [footnote : _month. not._, vol. xlvii., p. .] [footnote : _les mondes_, t. iii., p. .] [footnote : mouchez, _comptes rendus_, t. cvi., p. .] [footnote : _astr. nach._, no. , .] [footnote : _ibid._, no. , .] [footnote : _ibid._, nos. , , , .] [footnote : _journ. brit. astr. assoc._, vol. ix., p. .] [footnote : _astr. nach._, no. , .] [footnote : _observatory_, vol. xxi., pp. , .] [footnote : reproduced in _astroph. journ._, vol. xi., p. .] [footnote : _ibid._, p. .] [footnote : _astr. nach._, no. , .] [footnote : _sitzungsb. bayer. akad._, march , .] [footnote : _annals of the cape observatory_, vols. iii., iv., v.] [footnote : _month. not._, vol. lx., p. .] [footnote : d. klumpke, _observatory_, vol. xv., p. .] [footnote : gilbert, _sid. mess._, vol. i., p. .] [footnote : _astr. papers for the amer. ephemeris_, vol. viii., pt. ii.] [footnote : _nature_, vol. xxiv., p. ; _dunsink observations_, pt. v., .] [footnote : elkin, _report for - _, p. ; newcomb, _the stars_, p. .] [footnote : _annals of the cape observatory_, vol. viii., pt. ii. some of the measures were made by messrs. finlay and de sitter.] [footnote : _astr. nach._, no. , ; _observatory_, vol. xxi., p. .] [footnote : _annalen der sternwarte in leiden_, bd. vii.] [footnote : _report of harvard conference in _ (snyder).] [footnote : _researches in stellar parallax_, pt. ii., .] [footnote : _v. j. s. astr. ges._, jahrg., xxviii., p. .] [footnote : _bulletin de la carte du ciel_, no. , p. .] [footnote : _publ. of the astr. laboratory at groningen_, no. .] [footnote : _nature_, vol. xxvi., p. .] [footnote : _proc. r. irish acad._, vol. i., p. , ser. iii.] [footnote : _mem. r. a. s._, vol. xlvii., p. .] [footnote : _astr. nach._, no. , .] [footnote : _publ. astr. pac. soc._, no. .] [footnote : campbell, _lick bulletin_, no. .] [footnote : _publ. yerkes observatory_, vol. i., .] [footnote : _annals cape observatory_, vol. ii., pt. ii.] [footnote : _astr. jour._, nos. - .] [footnote : w. j. hussey, _publ. astr. pac. soc._, no. .] [footnote : _astr. jour._, no. .] [footnote : _sitzungsberichte_, berlin, october , .] [footnote : _annales de l'obs. de nice_, t. ii.] [footnote : _washington observations_, , app. i.] [footnote : _publ. lick observatory_, vol. v., .] [footnote : t. lewis, _observatory_, vol. xvi., p. .] [footnote : _harvard annals_, vol. xiv., pt. i., .] [footnote : _observatory_, vol. viii., p. .] [footnote : _month. not._, vol. xlvi., p. .] [footnote : _harvard annals_, vol. xxxiv.] [footnote : _ibid._, vol. xlv.] [footnote : _carte phot. du ciel. réunion du comité permanent_, paris, , p. .] [footnote : _essays_ ( nd ser.), _the nebular hypothesis_.] [footnote : _on the plurality of worlds_, p. ( nd ed.).] [footnote : proctor, _month. not._, vol. xxix., p. .] [footnote : this remark was first made by j. michell, _phil. trans._, vol. lvii., p. ( ).] [footnote : _pop. astr._, no. .] [footnote : _astroph. jour._, vol. i., p. .] [footnote : _month. not._, vols. xxxi., p. ; xxxii., p. .] [footnote : _the stars_, p. .] [footnote : _system of the stars_, p. ; _old and new astronomy_, p. (ranyard).] [footnote : _astroph. jour._, vol. xii., p. .] [footnote : _publ. astr. pac. soc._, vol. ii., p. .] [footnote : _month. not._, vol. li., pp. , . for reproductions of some of the photographs in question, see _knowledge_, vol. xiv., p. .] [footnote : _astr. nach._, no. , ; _observatory_, vol. xiv., p. .] [footnote : _proc. roy. inst._, may , (gill).] [footnote : _annals cape obs._, iii., introduction, p. .] [footnote : _proc. roy. soc._, vol. xviii., p. .] [footnote : _astr. nach._, no. , ; _observatory_, vol. xxi., p. ; newcomb, _the stars_, p. .] [footnote : _month. not._, vol. xl., p. .] chapter xiii _methods of research_ comparing the methods now available for astronomical inquiries with those in use forty years ago, we are at once struck with the fact that they have multiplied. the telescope has been supplemented by the spectroscope and the photographic camera. now, this really involves a whole world of change. it means that astronomy has left the place where she dwelt apart in rapt union with mathematics, indifferent to all things on earth save only to those mechanical improvements which should aid her to penetrate further into the heavens, and has descended into the forum of human knowledge, at once a suppliant and a patron, alternately invoking help from and promising it to each of the sciences, and patiently waiting upon the advances of all. the science of the heavenly bodies has, in a word, become a branch of terrestrial physics, or rather a higher kind of integration of all their results. it has, however, this leading peculiarity, that the materials for the whole of its inquiries are telescopically furnished. they are such as come very imperfectly, or not at all, within the cognisance of the unarmed eye. spectroscopic and photographic apparatus are simply additions to the telescope. they do not supersede or render it of less importance. on the contrary, the efficacy of their action depends primarily upon the optical qualities of the instruments they are attached to. hence the development, to their fullest extent, of the powers of the telescope is of vital moment to the progress of modern physical astronomy, while the older mathematical astronomy could afford to remain comparatively indifferent to it. the colossal rosse reflector still marks, as to size, the _ne plus ultra_ of performance in that line. a mirror four feet in diameter was, however, sent out to melbourne by the late thomas grubb of dublin in . this is mounted in the cassegrainian manner, so that the observer looks straight through it towards the object viewed, of which he really sees a twice-reflected image. the dust-laden atmosphere of melbourne is said to impede very seriously the usefulness of this originally fine instrument. it may be doubted whether so large a spectrum will ever again be constructed. a new material for the mirrors of reflecting telescopes, proposed by steinheil in , and independently by foucault in ,[ ] has in a great measure superseded the use of a metallic alloy. this is glass upon which a thin film of silver has been deposited by a chemical process originally invented by liebig. it gives a peculiarly brilliant reflective surface, throwing back more light than a metallic mirror of the same area, in the proportion of about sixteen to nine. resilvering, too, involves much less risk and trouble than repolishing a speculum. the first use of this plan on a large scale was in an instrument of thirty-six inches aperture, finished by calver for dr. common in . to its excellent qualities turned to account with rare skill, his triumphs in celestial photography are mainly due. a more daring experiment was the construction and mounting, by dr. common himself, of a -foot reflector. but the first glass disc ordered from france for the purpose proved radically defective. when figured, polished, and silvered, towards the close of , it gave elliptical instead of circular star-images.[ ] a new one had to be procured, and was ready for astronomical use in . the satisfactory nature of its performance is vouched for by the observations made with it upon jupiter's new satellite in december, . this instrument, to which a newtonian form has been given, had no rival in respect of light-concentration at the time when it was built. it has now two--the paris -inch refractor and the yerkes -foot reflector. it is, however, in the construction of refracting telescopes that the most conspicuous advances have recently been made. the harvard college -inch achromatic was mounted and ready for work in june, . a similar instrument had already for some years been in its place at pulkowa. it was long before the possibility of surpassing these masterpieces of german skill presented itself to any optician. for fifteen years it seemed as if a line had been drawn just there. it was first transgressed in america. a portrait-painter of cambridgeport, massachusetts, named alvan clark, had for some time amused his leisure with grinding lenses, the singular excellence of which was discovered in england by mr. dawes in .[ ] seven years passed, and then an order came from the university of mississippi for an object-glass of the unexampled size of eighteen inches. an experimental glance through it to test its definition resulted, as we have seen, in the detection of the companion of sirius, january , . it never reached its destination in the south. war troubles supervened, and it was eventually sent to chicago, where it served professor hough in his investigations of jupiter, and mr. burnham in his scrutiny of double stars. the next step was an even longer one, and it was again taken by a self-taught optician, thomas cooke, the son of a shoemaker at allerthorpe, in the east riding of yorkshire. mr. newall of gateshead ordered from him in a -inch object-glass. it was finished early in , but at the cost of shortening the life of its maker, who died october , , before the giant refractor he had toiled at for five years was completely mounted. this instrument, the fine qualities of which had long been neutralized by an unfavourable situation, was presented by mr. newall to the university of cambridge, a few weeks before his death, april , . under the care of his son, mr. frank newall, it has proved highly efficient in the delicate work of measuring stellar radial motions. close upon its construction followed that of the washington -inch, for which twenty thousand dollars were paid to alvan clark. the most illustrious point in its career, entered upon in , has been the discovery of the satellites of mars. once known to be there, these were, indeed, found to be perceptible with very moderate optical means (mr. wentworth erck saw deimos with a -inch clark); but the first detection of such minute objects is a feat of a very different order from their subsequent observation. dr. see's perception with this instrument, in , of neptune's cloud-belts, and his refined series of micrometrical measures of the various planets, attest the unimpaired excellence of its optical qualities. it held the primacy for more than eight years. then, in december, , the place of honour had to be yielded to a -inch achromatic, built by howard grubb (son and successor of thomas grubb) for the vienna observatory. this, in its turn, was surpassed by two of respectively - / and inches, sent by gautier of paris to nice, and by alvan clark to pulkowa; and an object-glass, three feet in diameter, was in successfully turned out by the latter firm for the lick observatory in california. the difficulties, however, encountered in procuring discs of glass of the size and purity required for this last venture seemed to indicate that a term to progress in this direction was not far off. the flint was, indeed, cast with comparative ease in the workshops of m. feil at paris. the flawless mass weighed kilogrammes, was over inches across, and cost , dollars. but with the crown part of the designed achromatic combination things went less smoothly. the production of a perfect disc was only achieved after _nineteen_ failures, involving a delay of more than two years; and the glass for a third lens, designed to render the telescope available at pleasure for photographic purposes, proved to be strained, and consequently went to pieces in the process of grinding. it has been replaced by one of inches, with which a series of admirable lunar and other photographs have been taken. nor is the difficulty in obtaining suitable material the only obstacle to increasing the size of refractors. the "secondary spectrum," as it is called, also interposes a barrier troublesome to surmount. true achromatism cannot be obtained with ordinary flint and crown-glass; and although in lenses of "jena glass," outstanding colour is reduced to about one-sixth its usual amount, their term of service is fatally abridged by rapid deterioration. nevertheless, a -inch objective of the new variety was mounted at königsberg in ; and discs of jena crown and flint, inches across, were purchased by brashear at the chicago exhibition of . an achromatic combination of three kinds of glass, devised by mr. a. taylor[ ] for messrs. cooke of york, has less serious drawbacks, but has not yet come into extensive use. meanwhile, in giant telescopes affected to the full extent by chromatic aberration, such as the lick and yerkes refractors, the differences of focal length for the various colours are counted by inches,[ ] and this not through any lack of skill in the makers, but by the necessity of the case. embarrassing consequences follow. only a small part of the spectrum of a heavenly body, for instance, can be distinctly seen at one time; and a focal adjustment of half an inch is required in passing from the observation of a planetary nebula to that of its stellar nucleus. a refracting telescope loses, besides, one of its chief advantages over a reflector when its size is increased beyond a certain limit. that advantage is the greater luminosity of the images given by it. considerably more light is transmitted through a glass lens than is reflected from an equal metallic surface. but only so long as both are of moderate dimensions. for the glass necessarily grows in thickness as its area augments, and consequently stops a larger percentage of the rays it refracts. so that a point at length arrives--fixed by the late dr. robinson at a diameter a little short of feet[ ]--where the glass and the metal are, in this respect, on an equality; while above it the metal has the advantage. and since silvered glass gives back considerably more light than speculum metal, the stage of equalisation with lenses is reached proportionately sooner where this material is employed.[ ] the most distinctive faculty of reflectors, however, is that of bringing rays of all refrangibilities to a focus together. they are naturally achromatic. none of the beams they collect are thrown away in colour-fringes, obnoxious both in themselves and as a waste of the chief object of astrophysicists' greed--light. reflectors, then, are in this respect specially adapted to photographic and spectrographic use. but they have a countervailing drawback. the penalties imposed by bigness are for them peculiarly heavy. perfect definition becomes with increasing size, more and more difficult to attain; once attained, it becomes more and more difficult to keep. for the huge masses of material employed to form great object-glasses or mirrors tend with every movement to become deformed by their own weight. now, the slightest bending of a mirror is fatal to its performance, the effect being doubled by reflection; while in a lens alteration of figure is compensated by the equal and contrary flexures of the opposing surfaces, so that the emergent beams pursue much the same paths as if the curves of the refracting medium had remained theoretically perfect. for this reason work of precision must remain the province of refracting telescopes, although great reflectors retain the primacy in the portraiture of the heavenly bodies, as well as in certain branches of spectroscopy. professor hale, accordingly, summarised a valuable discussion on the subject by asserting[ ] "that the astrophysicist may properly consider the reflector to be an even more important part of his instrumental equipment than the refractor." a new era in its employment west of the atlantic opened with the transfer from halifax to mount hamilton of the crossley reflector. its prerogatives in nebular photography were splendidly indicated in by professor keeler's exquisite and searching portrayals of the cloud-worlds of space, and those obtained two years later, with a similar, though smaller, instrument, by professor ritchey of the yerkes observatory, were fully comparable with them. the performances of the yerkes -foot reflector still belong to the future. ambition as regards telescopic power is by no means yet satisfied. nor ought it to be. the advance of astrophysical researches of all kinds depends largely upon light-grasp. for the spectroscopic examination of stars, for the measurement of their motions in the line of sight, for the discovery and study of nebulæ, for stellar and nebular photography, the cry continually is "more light." there is no enterprising head of an observatory but must feel cramped in his designs if he can command no more than or inches of aperture, and he aspires to greater instrumental capacity, not merely with a view to the chances of discovery, but for the steady prosecution of some legitimate line of inquiry. thus projects of telescope-building on a large scale are rife, and some obtain realisation year by year. sir howard grubb finished in a -inch achromatic for the royal observatory, greenwich; the thompson equatoreal, mounted at the same establishment in , carries on a single axis a -inch photographic refractor and a -inch silvered-glass reflector; the victoria telescope, inaugurated at the cape in , comprises a powerful spectrographic apparatus, together with a chemically corrected -inch refractor, the whole being the munificent gift of mr. frank mcclean; at potsdam, at meudon, at paris, at alleghany, engines for light-concentration have been, or shortly will be, erected of dimensions which, two generations back, would have seemed extravagant and impossible. perhaps the finest, though not absolutely the greatest, among them, marked the summit and end of the performances of alvan g. clark, the last survivor of the cambridgeport firm. in october, , mr. yerkes of chicago offered an unlimited sum for the provision of the university of that city with a "superlative" telescope. and it happened, fortunately, that a pair of glass discs, nearly inches in diameter, and of perfect quality, were ready at hand. they had been cast by mantois for the university of southern california, when the erection of a great observatory on wilson's peak was under consideration. in the clark workshop they were combined into a superb objective, brought to perfection by trials and delicate touches extending over nearly five years. then the maker accompanied it to its destination, by the shore of a far western lake geneva, and died immediately after his return, june , . nor has the implement of celestial research he just lived to complete been allowed to "rust unburnished." manipulated by hale, burnham, and barnard, it has done work that would have been impracticable with less efficient optical aid. its construction thus marks a noticeable enlargement of astronomical possibilities, exemplified--to cite one among many performances--by barnard's success in keeping track of cluster-variables when below the common limit of visual perception. with the lick telescope results have also been achieved testifying to its unsurpassed excellence. holden's and schaeberle's views of planetary nebulæ, burnham's and hussey's hair's-breadth star-splitting operations, keeler's measurements of nebular radial motion, barnard's detections and prolonged pursuit of faint comets, his discovery of jupiter's tiny moon, campbell's spectroscopic determinations--all this could only have been accomplished, even by an exceptionally able and energetic staff, with the aid of an instrument of high power and quality. but there was another condition which should not be overlooked. the best telescope may be crippled by a bad situation. the larger it is, indeed, the more helpless is it to cope with atmospheric troubles. these are the worst plagues of all those that afflict the astronomer. no mechanical skill avails to neutralise or alleviate them. they augment with each increase of aperture; they grow with the magnifying powers applied. the rays from the heavenly bodies, when they can penetrate the cloud-veils that too often bar their path, reach us in an enfeebled, scattered, and disturbed condition. hence the twinkling of stars, the "boiling" effects at the edges of sun, moon, and planets; hence distortions of bright, effacements of feeble telescopic images; hence, too, the paucity of the results obtained with many powerful light-gathering machines. no sooner had the parsonstown telescope been built than it became obvious that the limit of profitable augmentation of size had, under climatic conditions at all nearly resembling those prevailing there, been reached, if not overpassed; and lord rosse himself was foremost to discern the need of pausing to look round the world for a clearer and stiller air than was to be found within the bounds of the united kingdom. with this express object mr. lassell transported his -foot newtonian to malta in , and mounted there, in , a similar instrument of fourfold capacity, with which, in the course of about two years, new nebulæ were discovered. professor piazzi smyth's experiences during a trip to the peak of teneriffe in in search of astronomical opportunities[ ] gave countenance to the most sanguine hopes of deliverance, at suitable elevated stations, from some of the oppressive conditions of low-level star-gazing; yet for a number of years nothing effectual was done for their realisation. now, at last, however, mountain observatories are not only an admitted necessity but an accomplished fact; and newton's long forecast of a time when astronomers would be compelled, by the developed powers of their telescopes, to mount high above the "grosser clouds" in order to use them,[ ] had been justified by the event. james lick, the millionaire of san francisco, had already chosen, when he died, october , , a site for the new observatory, to the building and endowment of which he had devoted a part of his large fortune. the situation of the establishment is exceptional and splendid. planted on one of the three peaks of mount hamilton, a crowning summit of the californian coast range, at an elevation of , feet above the sea, in a climate scarce rivalled throughout the world, it commands views both celestial and terrestrial which the lover of nature and astronomy may alike rejoice in. impediments to observation are there found to be most materially reduced. professor holden, who was appointed, in , president of the university of california and director of the new observatory affiliated to it, stated that during six or seven months of the year an unbroken serenity prevails, and that half the remaining nights are clear.[ ] the power of continuous work thus afforded is of itself an inestimable advantage; and the high visual excellences testified to by mr. burnham's discovery, during a two months' trip to mount hamilton in the autumn of , of forty-two new double stars with a -inch achromatic, gave hopes since fully realised of a brilliant future for the lick establishment. its advantages are shared, as professor holden desired them to be, by the whole astronomical world.[ ] a sort of appellate jurisdiction was at once accorded to the great equatoreal, and more than one disputed point has been satisfactorily settled by recourse to it. its performances, considered both as to quality and kind, are unlikely to be improved upon by merely outbidding it in size, unless the care expended upon the selection of its site be imitated. professor pickering thus showed his customary prudence in reserving his efforts to procure a great telescope until harvard college owned a dependent observatory where it could be employed to advantage. this was found by mr. w. h. pickering, after many experiments in colorado, california, and peru, at arequipa, on a slope of the andes, , feet above the sea-level. here the post provided for by the "boyden fund" was established in , under ideal meteorological conditions. temperature preserves a "golden mean"; the barometer is almost absolutely steady; the yearly rainfall amounts to no more than three or four inches. no wonder, then, that the "seeing" there is of the extraordinary excellence attested by mr. pickering's observations. in the absence of bright moonlight, he tells us,[ ] eleven pleiades can always be counted; the andromeda nebula appears to the naked eye conspicuously bright, and larger than the full moon; third magnitude stars have been followed to their disappearance at the true horizon; the zodiacal light spans the heavens as a complete arch, the "gegenschein" forming a regular part of the scenery of the heavens. corresponding telescopic facilities are enjoyed. the chief instrument at the station, a -inch equatoreal by clark, shows the fainter parts of the orion nebula, photographed at harvard college in , by which the dimensions given to it in bond's drawing are doubled; stars are at times seen encircled by half a dozen immovable diffraction rings, up to twelve of which have been counted round alpha centauri; while on many occasions no available increase of magnifying power availed to bring out any wavering in the limbs of the planets. moreover, the series of fine nights is nearly unbroken from march to november. the facilities thus offered for continuous photographic research rendered feasible professor bailey's amazing discovery of variable star-clusters. they belong exclusively to the "globular" class, and the peculiarity is most strikingly apparent in the groups known as omega centauri, and messier , , and . a large number of their minute components run through perfectly definite cycles of change in periods usually of a few hours.[ ] altogether, about "cluster-variables" have been recorded since . it should be mentioned that mr. david packer and dr. common discerned, about , some premonitory symptoms of light-fluctuation among the crowded stars of messier .[ ] with the bruce telescope, a photographic doublet inches in diameter, a store of , negatives was collected at arequipa between and . some were exposed directly, others with the intervention of a prism; and all are available for important purposes of detection or investigation. vapours and air-currents do not alone embarrass the use of giant telescopes. mechanical difficulties also oppose a formidable barrier to much further growth in size. but what seems a barrier often proves to be only a fresh starting-point; and signs are not wanting that it may be found so in this case. it is possible that the monumental domes and huge movable tubes of our present observatories will, in a few decades, be as much things of the past as huygens's "aerial" telescopes. it is certain that the thin edge of the wedge of innovation has been driven into the old plan of equatoreal mounting. m. loewy, the present director of the paris observatory, proposed to delaunay in the direction of a telescope on a novel system. the design seemed feasible, and was adopted; but the death of delaunay and the other untoward circumstances of the time interrupted its execution. its resumption, after some years, was rendered possible by m. bischoffsheim's gift of , francs for expenses, and the _coudé_ or "bent" equatoreal has been, since , one of the leading instruments at the paris establishment. its principle is briefly this: the telescope is, as it were, its own polar axis. the anterior part of the tube is supported at both ends, and is thus fixed in a direction pointing towards the pole, with only the power of twisting axially. the posterior section is joined on to it at right angles, and presents the object-glass, accordingly, to the celestial equator, in the plane of which it revolves. stars in any other part of the heavens have their beams reflected upon the object-glass by means of a plane rotating mirror placed in front of it. the observer, meanwhile, is looking steadfastly down the bent tube towards the invisible _southern_ pole. he would naturally see nothing whatever were it not that a second plane mirror is fixed at the "elbow" of the instrument, so as to send the rays which have traversed the object-glass to his eye. he never needs to move from his place. he watches the stars, seated in an arm-chair in a warm room, with as perfect convenience as if he were examining the seeds of a fungus with a microscope. nor is this a mere gain of personal ease. the abolition of hardship includes a vast accession of power.[ ] among other advantages of this method of construction are, first, that of added stability, the motion given to the ordinary equatoreal being transferred, in part, to an auxiliary mirror. next, that of increased focal length. the fixed part of the tube can be made almost indefinitely long without inconvenience, and with enormous advantage to the optical qualities of a large instrument. finally, the costly and unmanageable cupola is got rid of, a mere shed serving all purposes of protection required for the "coudé." the desirability of some such change as that which m. loewy has realised has been felt by others. professor pickering sketched, in , a plan for fixing large refractors in a permanently horizontal position, and reflecting into them, by means of a shifting mirror, the objects desired to be observed.[ ] the observations for his photometric catalogues are, in fact, made with a "broken transit," in which the line of sight remains permanently horizontal, whatever the altitude of the star examined. sir howard grubb, moreover, set up, in , a kind of siderostat at the crawford observatory, cork. in a paper read before the royal society, january , , he proposed to carry out the principle on a more extended scale;[ ] and shortly afterwards undertook its application to a telescope inches in aperture for the armagh observatory.[ ] the chief honours, however, remain to the paris inventor. none of the prognosticated causes of failure have proved effective. the loss of light from the double reflection is insignificant. the menaced deformation of images is, through the exquisite skill of the mm. henry in producing plane mirrors of all but absolute perfection, quite imperceptible. the definition was admitted to be singularly good. sir david gill stated in that he had never measured a double star so easily as he did gamma leonis by its means.[ ] sir norman lockyer pronounced it to be "one of the instruments of the future"; and the principle of its construction was immediately adopted by the directors of the besançon and algiers observatories, as well as for a -inch telescope destined for a new observatory at buenos ayres. at paris, it has since been carried out on a larger scale. a "coudé," of - / inches aperture and feet focal length was in installed at the national observatory, and has served m. loewy for his ingenious studies on refraction and aberration--above all, for taking the magnificent plates of his lunar atlas. the "bent" form is capable of being, but has not yet been, adapted to reflectors.[ ] the "coelostat," in the form given to it by professor turner, has proved an invaluable adjunct to eclipse-equipments. it consists essentially of a mirror rotating in forty-eight hours on an axis in its own plane, and parallel to the earth's axis. in the field of a telescope kept rigidly pointed towards such a mirror, stars appear immovably fixed. the employment of long-focus lenses for coronal photography is thus facilitated, and the size of the image is proportional to the length of the focus. professor barnard, accordingly, depicted the totality of with a horizontal telescope - / feet long, fed by a mirror inches across, the diameter of the moon on his plates being inches. the largest siderostat in the world is the paris -inch refractor, which formed the chief attraction of the palais d'optique at the exhibition of . it has a focal length of nearly feet, and can be used either for photographic or for visual purposes. celestial photography has not reached its grand climacteric; yet its earliest beginnings already seem centuries behind its present performances. the details of its gradual yet rapid improvement are of too technical a nature to find a place in these pages. suffice it to say that the "dry-plate" process, with which such wonderful results have been obtained, appears to have been first made available by sir william huggins in photographing the spectrum of vega in , and was then successively adopted by common, draper, and janssen. nor should captain abney's remarkable extension of the powers of the camera be left unnoticed. he began his experiments on the chemical action of red and infra-red rays in , and at length succeeded in obtaining a substance--the "blue" bromide of silver--highly sensitive to these slower vibrations of light. with its aid he explored a vast, unknown, and for ever invisible region of the solar spectrum, presenting to the royal society, december , ,[ ] a detailed map of its infra-red portion (wave-lengths , to , ), from which valuable inferences may yet be derived as to the condition of the various kinds of matter ignited in the solar atmosphere. upon plates rendered "orthochromatic" by staining with alizarine, or other dye-stuffs, the whole visible spectrum can now be photographed; but those with their maximum of sensitiveness near g are found preferable, except where the results of light-analysis are sought to be completely recorded. and since photographic refractors are corrected for the blue rays, exposures with them of orthochromatic surfaces would be entirely futile. the chemical plate has two advantages over the human retina:[ ] first, it is sensitive to rays which are utterly powerless to produce any visual effect; next, it can accumulate impression almost indefinitely, while from the retina they fade after one-tenth part of a second, leaving it a continually renewed _tabula rasa_. it is, accordingly, quite possible to photograph objects so faint as to be altogether beyond the power of any telescope to reveal--witness the chemical disclosure of the invisible nebula encircling nova persei--and we may thus eventually learn whether a blank space in the sky truly represents the end of the stellar universe in that direction, or whether farther and farther worlds roll and shine beyond, veiled in the obscurity of immeasurable distance. of many ingenious improvements in spectroscopic appliances the most fundamentally important relate to what are known as "gratings." these are very finely striated surfaces, by which light-waves are brought to interfere, and are thus sifted out, strictly according to their different lengths, into "normal" spectra. since no universally valid measures can be made in any others, their production is quite indispensable to spectroscopic science. fraunhofer, who initiated the study of the diffraction spectrum, used a real grating of very fine wires: but rulings on glass were adopted by his successors, and were by nobert executed with such consummate skill that a single square inch of surface was made to contain , hand-drawn lines. such rare and costly triumphs of art, however, found their way into very few hands, and practical availability was first given to this kind of instrument by the inventiveness and mechanical dexterity of two american investigators. both rutherfurd's and rowland's gratings are machine-ruled, and reflect instead of transmitting the rays they analyse; but rowland's present to them a very much larger diffractive surface, and consequently possess a higher resolving power. the first preliminary to his improvements was the production, in , of a faultless screw, those previously in use having been the inevitable source of periodical errors in striation, giving, in their turn, ghost-lines as subjects of spectroscopic study.[ ] their abolition was not one of rowland's least achievements. with his perfected machine a metallic area of - / by - / inches can be ruled with exquisite accuracy to almost any degree of fineness; he considered, however, , lines to the inch to be the limit of usefulness.[ ] the ruled surface is, moreover, concave, and hence brings the spectrum to a focus without a telescope. a slit and an eye-piece are alone needed to view it, and absorption of light by glass lenses is obviated--an advantage especially sensible in dealing with the ultra- or infra-visible rays. the high qualities of rowland's great photographic map of the solar spectrum were thus based upon his previous improvement of the instrumental means used in its execution. the amount of detail shown in it is illustrated by the appearance on the negatives of lines between h and k; and many lines depict themselves as double which, until examined with a concave grating, had passed for one and indivisible. a corresponding hand-drawing, for which m. thollon received in the lalande prize, exhibits, not the diffractive, but the prismatic spectrum as obtained with bisulphide of carbon prisms of large dispersive power. about one-third of the visible gamut of the solar radiations (a to _b_) is covered by it; it includes , lines, and is over ten metres long.[ ] the grating is an expensive tool in the way of light. where there is none to spare, its advantages must be foregone. they could not, accordingly, be turned to account in stellar spectroscopy until the lick telescope was at hand to supply more abundant material for research. by the use thus made possible of rowland's gratings, professor keeler was able to apply enormous dispersion to the rays of stars and nebulæ, and so to attain a previously unheard-of degree of accuracy in their measurement. his memorable detection of nebular movement in line of sight ensued as a consequence. professor campbell, his successor, has since obtained, by the same means, the first satisfactory photographs of stellar diffraction-spectra. the means at the disposal of astronomers have not multiplied faster than the tasks imposed upon them. looking back to the year , we cannot fail to be astonished at the change. the comparatively simple and serene science of the heavenly bodies known to our predecessors, almost perfect so far as it went, incurious of what lay beyond its grasp, has developed into a body of manifold powers and parts, each with its separate mode and means of growth, full of strong vitality, but animated by a restless and unsatisfied spirit, haunted by the sense of problems unsolved, and tormented by conscious impotence to sound the immensities it perpetually confronts. knowledge might be said, when the _mécanique céleste_ issued from the press, to be bounded by the solar system; but even the solar system presented itself under an aspect strangely different from what it now wears. it consisted of the sun, seven planets, and twice as many satellites, all circling harmoniously in obedience to a universal law, by the compensating action of which the indefinite stability of their mutual relations was secured. the occasional incursion of a comet, or the periodical presence of a single such wanderer chained down from escape to outer space by planetary attraction, availed nothing to impair the symmetry of the majestic spectacle. now, not alone the ascertained limits of the system have been widened by a thousand millions of miles, with the addition of one more giant planet and seven satellites to the ancient classes of its members, but a complexity has been given to its constitution baffling description or thought. five hundred circulating planetary bodies bridge the gap between jupiter and mars, the complete investigation of the movements of any one of which would overtask the energies of a lifetime. meteorites, strangers, apparently, to the fundamental ordering of the solar household, swarm, nevertheless, by millions in every cranny of its space, returning at regular intervals like the comets so singularly associated with them, or sweeping across it with hyperbolic velocities, brought, perhaps, from some distant star. and each of these cosmical grains of dust has a theory far more complex than that of jupiter; it bears within it the secret of its origin, and fulfils a function in the universe. the sun itself is no longer a semi-fabulous, fire-girt globe, but the vast scene of the play of forces as yet imperfectly known to us, offering a boundless field for the most arduous and inspiring researches. among the planets the widest variety in physical habitudes is seen to prevail, and each is recognised as a world apart, inviting inquiries which, to be effective, must necessarily be special and detailed. even our own moon threatens to break loose from the trammels of calculation, and commits "errors" which sap the very foundations of the lunar theory, and suggest the formidable necessity for its complete revision. nay, the steadfast earth has forfeited the implicit confidence placed in it as a time-keeper, and questions relating to the stability of the earth's axis and the constancy of the earth's rate of rotation are among those which it behoves the future to answer. everywhere there is multiformity and change, stimulating a curiosity which the rapid development of methods of research offers the possibility of at least partially gratifying. outside the solar system, the problems which demand a practical solution are virtually infinite in number and extent. and these have all arisen and crowded upon our thoughts within less than a hundred years. for sidereal science became a recognised branch of astronomy only through herschel's discovery of the revolutions of double stars in . yet already it may be, and has been called, "the astronomy of the future," so rapidly has the development of a keen and universal interest attended and stimulated the growth of power to investigate this sublime subject. what has been done is little--is scarcely a beginning; yet it is much in comparison with the total blank of a century past. and our knowledge will, we are easily persuaded, appear in turn the merest ignorance to those who come after us. yet it is not to be despised, since by it we reach up groping fingers to touch the hem of the garment of the most high. footnotes: [footnote : _comptes rendus_, t. xliv., p. .] [footnote : a. a. common, _memoirs r. astr. soc._, vol. i., p. .] [footnote : newcomb, _pop. astr._, p. .] [footnote : _month. not._, vol. liv., p. .] [footnote : keeler, _publ. astr. pac. soc._, vol. ii., p. .] [footnote : h. grubb, _trans. roy. dub. soc._, vol. i. (new ser.), p. .] [footnote : hale, nevertheless (_astroph. jour._, vol. v., p. ), considers that refractors preserve their superiority of visual light-grasp over newtonian reflectors up to an aperture of - / , while equalisation is reached for the photographic rays at inches.] [footnote : _astroph. jour._, vol. v., p. .] [footnote : _phil. trans._, vol. cxlviii., p. .] [footnote : _optics_, p. ( nd ed., ).] [footnote : _observatory_, vol. viii., p. .] [footnote : holden on celestial photography, _overland monthly_, nov., .] [footnote : _observatory_, vol. xv., p. .] [footnote : bailey, _astroph. jour._, vol. x., p. .] [footnote : _harvard circulars_, nos. , , , ;] [footnote : loewy, _bull. astr._, t. i., p. ; _nature_, vol. xxix., p. .] [footnote : _nature_, vol. xxiv., p. .] [footnote : _ibid._, vol. xxix., p. .] [footnote : _trans. r. dublin soc._, vol. iii., p. .] [footnote : _observatory_, vol. vii., p. .] [footnote : loewy, _bull. astr._, t. i., p. .] [footnote : _phil. trans._, vol. clxxi., p. .] [footnote : janssen, _l'astronomie_, t. ii., p. .] [footnote : rev. a. l. cortie, _astr. and astrophysics_, vol. xi., p. .] [footnote : _phil. mag._, vol. xiii., , p. .] [footnote : _bull. astr._, t. iii., p. .] appendix table i chronology, - , march herschel's first observation. subject, the orion nebula. sun-spots geometrically proved to be depressions by wilson. first experimental determination of the earth's mean density by maskelyne. , march discovery of uranus. herschel's first catalogue of double stars. herschel's first investigation of the sun's movement in space. goodricke's discovery of algol's law of variation. analogy between mars and the earth pointed out by herschel. construction of the heavens investigated by herschel's method of star-gauging. "cloven-disc" plan of the milky way. discovery of binary stars anticipated by michell. herschel's first catalogue of nebulæ. , jan. discovery by herschel of two uranian moons (oberon and titania). , nov. acceleration of the moon explained by laplace. herschel's second catalogue of nebulæ, and classification by age of these objects. completion of herschel's forty-foot reflector. , aug. his discovery with it of the two inner saturnian and sept. satellites. repeating-circle invented by borda. five-foot circle constructed by ramsden for piazzi. maskelyne's catalogue of thirty-six fundamental stars. herschel propounds the hypothesis of a fluid constitution for nebulæ. atmospheric refraction in venus announced by schröter. rotation-period of saturn fixed by herschel at h. m. herschel's theory of the solar constitution. herschel's first measures of comparative stellar brightness. laplace's nebular hypothesis published in _exposition du système du monde_. publication of olbers's method of computing cometary orbits. retrograde motions of uranian satellites detected by herschel. publication of first two volumes of _mécanique céleste_. , may transit of mercury observed by schröter. , nov. star-shower observed by humboldt at cumana. _monatliche correspondenz_ started by von zach. invisible heat-rays detected in the solar spectrum by herschel. , jan. discovery of ceres by piazzi. publication of lalande's _histoire céleste_. investigation by herschel of solar emissive variability in connection with spot-development. , march discovery of pallas by olbers. herschel's third catalogue of nebulæ. herschel's discovery of binary stars. marks of clustering in the milky way noted by herschel. wollaston records seven dark lines in the solar spectrum. , nov. transit of mercury observed by herschel. , sept. discovery of juno by harding. foundation of optical institute at munich. herschel's second determination of the solar apex. , march discovery of vesta by olbers. herschel's theory of the development of stars from nebulæ. , feb. death of maskelyne. pond appointed to succeed him as astronomer-royal. , sept. perihelion passage of great comet. theory of electrical repulsion in comets originated by olbers. , sept. perihelion passage of pons's comet. herschel demonstrates the irregular distribution of stars in space. fraunhofer maps dark lines in the solar spectrum. publication of bessel's _fundamenta astronomiæ_. recognition by encke of the first short-period comet. , june passage of the earth through the tail of a comet. foundation of the royal astronomical society. foundation of paramatta observatory. , september first number of _astronomische nachrichten_. , may first calculated return of encke's comet. , august death of herschel. bessel introduces the correction of observations for personal equation. fraunhofer examines the spectra of fixed stars. distance of the sun concluded by encke to be - / million miles. publication of lohrmann's lunar chart. dorpat refractor mounted equatoreally. commencement of schwabe's observations of sun-spots. , feb. biela's discovery of a comet. orbit of a binary star calculated by savary. completion of the royal observatory at the cape of good hope. the königsberg heliometer mounted. publication of bessel's _tabulæ regiomontanæ_. discovery by brewster of "atmospheric lines" in the solar spectrum. magnetic observatory established at göttingen. , nov. , star-shower visible in north america. completion of sir j. herschel's survey of the northern heavens. , jan. sir j. herschel's landing at the cape. , september airy appointed astronomer-royal in succession to pond. , nov. perihelion passage of halley's comet. solar movement determined by argelander. bessel's application of the heliometer to measurements of stellar parallax. publication of beer and mädler's _der mond_. publication of struve's _mensuræ micrometricæ_. , dec. outburst of eta carinæ observed by sir j. herschel. thermal power of the sun measured by herschel and pouillet. parallax of cygni determined by bessel. , jan. parallax of alpha centauri announced by henderson. completion of pulkowa observatory. solidity of the earth concluded by hopkins. , march death of olbers. first attempt to photograph the moon by j. w. draper. doppler enounces principle of colour-change by motion. conclusion of baily's experiments in weighing the earth. , july total solar eclipse. corona and prominences observed by airy, baily, arago, and struve. , feb. perihelion-passage of great comet. , february completion of parsonstown reflector. , april discovery with it of spiral nebulæ. , april daguerreotype of the sun taken by foucault and fizeau. , oct. place of neptune assigned by adams. , dec. discovery of astræa by hencke. , dec. duplication of biela's comet observed at yale college. melloni's detection of heating effects from moonlight. , march death of bessel. , sept. discovery of neptune by galle. , oct. neptune's satellite discovered by lassell. publication of sir j. herschel's _results of observations at the cape of good hope_. cyclonic theory of sun-spots stated by him. j. r. mayer's meteoric hypothesis of solar conservation. motion-displacements of fraunhofer lines adverted to by fizeau. , april new star in ophiuchus observed by hind. , sept. simultaneous discovery of hyperion by bond and lassell. first experimental determination of the velocity of light (fizeau). , july vega photographed at harvard college. , nov. discovery by bond of saturn's dusky ring. o. struve's first measurements of saturn's ring-system , july total solar eclipse observed in sweden. , oct. discovery by lassell of two inner uranian satellites. schwabe's discovery of sun-spot periodicity published by humboldt. , may coincidence of magnetic and sun-spot periods announced by sabine. , oct. variable nebula in taurus discovered by hind. lassell's two-foot reflector transported to malta. adams shows laplace's explanation of the moon's acceleration to be incomplete. hansen infers from lunar theory a reduced value for the distance of the sun. helmholtz's "gravitation theory" of solar energy. piazzi smyth's observations on the peak of teneriffe. saturn's rings shown by clerk maxwell to be of meteoric formation. , april double-star photography initiated at harvard college. solar photography begun at kew. , sept. perihelion of donati's comet. spectrum analysis established by kirchhoff and bunsen. carrington's discovery of the compound nature of the sun's rotation. , sept. luminous solar outburst and magnetic storm. , oct. merope nebula discovered by tempel. , dec. chemical constitution of the sun described by kirchhoff. , feb. discovery by liais of a "double comet." , may new star in scorpio detected by auwers. , july total solar eclipse observed in spain. prominences shown by photography to be solar appendages. , june the earth involved in the tail of a great comet. - kirchhoff's map of the solar spectrum. solar hydrogen-absorption recognised by Ångström. , jan. discovery by alvan g. clark of the companion of sirius. foucault determines the sun's distance by the velocity of light. opposition of mars. determination of solar parallax. completion of _bonner durchmusterung_. secchi's classification of stellar spectra. foundation of the german astronomical society. , march rotation period of mars determined by kaiser. huggins's first results in stellar spectrum analysis. , aug. spectroscopic examination of tempel's comet by donati shows it to be composed of glowing gas. , aug. discovery by huggins of gaseous nebulæ. value of , , miles adopted for the sun's distance. croll's explanation of glacial epochs. , nov. death of struve. , jan. spectroscopic observation by huggins of the occultation of eta piscium. , jan. faye's theory of the solar constitution. kew results published. zöllner argues for a high temperature in the great planets. identity of the orbits of the august meteors and of comet iii. demonstrated by schiaparelli. delaunay explains lunar acceleration by a lengthening of the day through tidal friction. , march spectroscopic study of the sun's surface by lockyer. , march new star in corona borealis detected by birmingham. , october schmidt announces the disappearance of the lunar crater linné. , nov. meteoric shower visible in europe. period of november meteors determined by adams. , aug. total solar eclipse. minimum sun-spot type of corona observed by grosch at santiago. discovery of gaseous stars in cygnus by wolf and rayet. , february principle of daylight spectroscopic visibility of prominences started by huggins. , aug. great indian eclipse. spectrum of prominences observed. , aug. janssen's first daylight view of a prominence. , oct. lockyer and janssen independently announce their discovery of the spectroscopic method. doppler's principle applied by huggins to measure stellar radial movements. publication of Ångström's map of the normal solar spectrum. spectrum of winnecke's comet found by huggins to agree with that of olefiant gas. , feb. tenuity of chromospheric gases inferred by lockyer and frankland. , feb. huggins observes a prominence with an "open slit." , aug. american eclipse. detection of bright-line coronal spectrum. mounting of newall's -inch achromatic at gateshead. proctor indicates the prevalence of drifting movements among the stars. a solar prominence photographed by young. , dec. sicilian eclipse. young discovers reversing layer. , may death of sir j. herschel. , june line-displacements due to solar rotation detected by vogel. , dec. total eclipse visible in india. janssen observes reflected fraunhofer lines in spectrum of corona. conclusion of a three years' series of observations on lunar heat by lord rosse. spectrum of vega photographed by h. draper. faye's cyclonic hypothesis of sun-spots. young's solar-spectroscopic observations at mount sherman. cornu's experiments on the velocity of light. , nov. meteoric shower connected with biela's comet. determination of mean density of the earth by cornu and baille. solar photographic work begun at greenwich. erection of -inch washington refractor. light-equation redetermined by glasenapp. vogel's classification of stellar spectra. , dec. transit of venus. publication of neison's _the moon_. , nov. new star in cygnus discovered by schmidt. spectrum of vega photographed by huggins. first use of dry gelatine plates in celestial photography. , may klein observes a supposed new lunar crater (hyginus n.). measurement by vogel of selective absorption in solar atmosphere. , aug. - discovery of two satellites of mars by hall at washington. , sept. death of leverrier. canals of mars discovered by schiaparelli. opposition of mars observed by gill at ascension. solar parallax deduced = . ". , january stationary meteor-radiants described by denning. publication of schmidt's _charte der gebirge des mondes_. first observations of great red spot on jupiter. conclusion of newcomb's researches on the lunar theory. , may transit of mercury. foundation of selenographical society. , july total eclipse visible in america. vast equatoreal extension of the corona. , october completion of potsdam astrophysical observatory. , dec. lockyer's theory of celestial dissociation communicated to the royal society. michelson's experiments on the velocity of light. publication of gould's _uranometria argentina_. , november observations of the spectra of sun-spots begun at south kensington. , dec. abney's map of the infra-red solar spectrum presented to the royal society. , dec. ultra-violet spectra of white stars described by huggins. , dec. communication of g. h. darwin's researches into the early history of the moon. , jan. discovery at cordoba of a great southern comet. conditions of algol's eclipses determined by pickering. pickering computes mass-brightness of binary stars. , sept. draper's photograph of the orion nebula. the bolometer invented by langley. , jan. communication of g. h. darwin's researches into the effects of tidal friction on the evolution of the solar system. langley's observations of atmospheric absorption on mount whitney. , june perihelion of tebbutt's comet. , june its spectrum photographed by huggins. , june photographs of tebbutt's comet by janssen and draper. , aug. retirement of sir george airy. succeeded by christie. , aug. perihelion of schaeberle's comet. publication of stone's cape catalogue for . struve's second measures of saturn's ring-system. newcomb's determination of the velocity of light. resulting solar parallax = · ". correction by nyrén of struve's constant of aberration. , march spectrum of orion nebula photographed by huggins. , may total solar eclipse observed at sohag in egypt. , may sodium-rays observed at dunecht in spectrum of comet wells. , june perihelion of comet wells. , sept. perihelion of great comet. daylight detection by common. transit observed at the cape. , sept. iron lines identified in spectrum by copeland and j. g. lohse. , september photographs of comet taken at the cape observatory, showing a background crowded with stars. , dec. transit of venus. duplication of martian canals observed by schiaparelli. completion by loewy at paris of first equatoreal coudé. rigidity of the earth concluded from tidal observations by g. h. darwin. experiments by huggins on photographing the corona without an eclipse. publication of holden's _monograph of the orion nebula_. , jan. orion nebula photographed by common. , may caroline island eclipse. , june great comet of observed from cordoba at a distance from the earth of million miles. parallaxes of nine southern stars measured by gill and elkin. catalogue of the spectra of , stars by vogel. , jan. return to perihelion of pons's comet. photometric catalogue by pickering of , stars. publication of gore's catalogue of variable stars. publication of faye's _origine du monde_. , oct. eclipse of the moon. heat-phases measured by boeddicker at parsonstown. dunér's catalogue of stars with banded spectra. backlund's researches into the movements of encke's comet. , february langley measures the lunar heat-spectrum. publication of _uranometria nova oxoniensis_. , aug. new star in andromeda nebula discerned by gully. , sept. thollon's drawing of the solar spectrum presented to the paris academy. , sept. solar eclipse visible in new zealand. , nov. photographic discovery by paul and prosper henry of a nebula in the pleiades. , nov. shower of biela meteors. thirty-inch achromatic mounted at pulkowa. publication of rowland's photographic map of the normal solar spectrum. bakhuyzen's determination of the rotation period of mars. stellar photographs by paul and prosper henry. , jan. spectra of forty pleiades simultaneously photographed at harvard college. , feb. first visual observation of the maia nebula with the pulkowa -inch refractor. , march photographs by the henrys of the pleiades, showing , stars with nebulæ intermixed. , may photographic investigations of stellar parallax undertaken by pritchard. , may periodical changes in spectra of sun-spots announced by lockyer. , june an international photographic congress proposed by gill. , aug. total eclipse of the sun observed at grenada. , oct. roberts's photograph showing annular structure of the andromeda nebula. , dec. roberts's photograph of the pleiades nebulosities. solar heat-spectrum extended by langley to below five microns. , dec. detection by copeland of helium-ray in spectrum of the orion nebula. thirty-inch refractor mounted at nice. publication of argentine general catalogue. completion of auwers's reduction of bradley's observations. draper memorial photographic work begun at harvard college. photographic detection at harvard college of bright hydrogen lines in spectra of variables (mira ceti and u orionis). , jan. discovery by thome at cordoba of a great comet belonging to the same group as the comet of . publication of lockyer's _chemistry of the sun_. , april meeting at paris of the international astrophotographic congress. heliometric triangulation of the pleiades by elkin. l. struve's investigation of the sun's motion, and redetermination of the constant of precession. von konkoly's extension to ° s. dec. of vogel's spectroscopic catalogue. auwers's investigation of the solar diameter. publication of schiaparelli's measures of double stars ( - ). , april death of thollon at nice. , aug. total eclipse of the sun. shadow-path crossed russia. observations marred by bad weather. , november langley's researches on the temperature of the moon. , nov. lockyer's _researches on meteorites_ communicated to the royal society. completion of -inch lick refractor. küstner's detection of variations in the latitude of berlin brought before the international geodetic association. chandler's first catalogue of variable stars. mean parallax of northern first magnitude stars determined by elkin. publication of dreyer's _new general catalogue_ of , nebulæ. vogel's first spectrographic determinations of stellar radial motion. carbon absorption recognised in solar spectrum by trowbridge and hutchins. , jan. total eclipse of the moon. heat-phases measured at parsonstown. , feb. remarkable photograph of the orion nebula spectrum taken at tulse hill. , june activity of the lick observatory begun. completion of dr. common's -foot reflector. heliometric measures of iris for solar parallax at the cape, newhaven (u.s.a.), and leipsic. loewy describes a comparative method of determining constant of aberration. presentation of the dunecht instrumental outfit to the nation by lord crawford. copeland succeeds piazzi smyth as astronomer-royal for scotland. , sept. death of r. a. proctor. photograph of the orion nebula taken by w. h. pickering, showing it to be the nucleus of a vast spiral. discovery at a harvard college of the first-known spectroscopic doubles, zeta ursæ majoris and beta aurigæ. eclipses of algol demonstrated spectrographically by vogel. completion of photographic work for the southern durchmusterung. boeddicker's drawing of the milky way. draper memorial photographs of southern star-spectra taken in peru. pernter's experiments on scintillation from the sonnblick. h. struve's researches on saturn's satellites. harkness's investigation of the masses of mercury, venus, and the earth. heliometric measures of victoria and sappho at the cape. , jan. total solar eclipse visible in california. , feb. foundation of the astronomical society of the pacific. , march investigation by sir william and lady huggins of the spectrum of the orion nebula. , july-aug. first photographs of the milky way taken by barnard. , august observation by barnard of four companions to brooks's comet. , nov. passage of japetus behind saturn's dusky ring observed by barnard. , december schiaparelli announces synchronous rotation and revolution of mercury. , dec. total eclipse of the sun visible in guiana. death of father perry, december . spectrum of uranus investigated visually by keeler, photographically by huggins. long-exposure photographs of ring-nebula in lyra. determinations of the solar translation by l. boss and o. stumpe. schiaparelli finds for venus an identical period of rotation and revolution. publication of thollon's map of the solar spectrum. bigelow's mathematical theory of coronal structures. foundation of the british astronomical association. measurements by keeler at lick of nebular radial movements. janssen's ascent of mont blanc, by which he ascertained the purely terrestrial origin of the oxygen-absorption in the solar spectrum. newcomb's discussion of the transits of venus of and . spiral structure of magellanic clouds displayed in photographs taken by h. c. russell of sydney. publication of the draper catalogue of stellar spectra. , april spica announced by vogel to be a spectroscopic binary. , june gore's catalogue of computed binaries. , november study by sir william and lady huggins of the spectra of wolf and rayet's stars in cygnus. , november discovery by barnard of a close nebulous companion to merope in the pleiades. , november mcclean spectrographs of the high and low sun. capture-theory of comets developed by callandreau, tisserand, and newton. dunér's spectroscopic researches on the sun's rotation. preponderance of sirian stars in the milky way concluded by pickering, gill, and kapteyn. detection by mrs. fleming of spectral variations corresponding to light-changes in beta lyræ. establishment of the harvard college station at arequipa in peru (height , feet). variations of latitude investigated by chandler. prominence-photography set on foot by hale at chicago and deslandres at paris. schmidt's theory of refraction in the sun. , april meeting at paris of the permanent committee for the photographic charting of the heavens. , may transit of mercury. , aug. presidential address by huggins at the cardiff meeting of the british association. , dec. nova aurigæ photographed at harvard college. , dec. photographic maximum of nova aurigæ. , dec. first photographic discovery of a minor planet by max wolf at heidelberg. commencement of international photographic charting work. photographic determination by scheiner of stars in the hercules cluster (m ). publication of vogel's spectrographic determinations for fifty-one stars. publication of pritchard's photographic parallaxes. , jan. death of sir george airy. , jan. death of professor adams. , feb. announcement by anderson of the outburst of a new star in auriga. , feb. appearance of the largest sun-spot ever photographed at greenwich. , march photograph of argo nebula taken by gill in twelve hours. , march discovery of a bright comet by swift. , june death of admiral mouchez. succeeded by tisserand as director of the national observatory, paris. , aug. favourable opposition of mars. , aug. rediscovery at lick of nova aurigæ. , sept. discovery by barnard of jupiter's inner satellite. , oct. first photographic discovery of a comet by barnard. , nov. discovery of holmes's comet. , nov. shower of andromede meteors visible in america. poynting's determination of the earth's mean density. dunér's investigation of the system of upsilon cygni. photographic investigation by deslandres of the spectra of prominences. photographs of the sun with faculæ and chromospheric surroundings taken by hale with a single exposure. investigation by t. j. j. see of the ancient colour of sirius. publication of t. j. j. see's thesis on the evolution of binary systems. chandler's theory of algol's inequalities. nebula in cygnus photographically discovered by max wolf. , jan. kapteyn's investigation of the structure of the stellar universe. , march gill announces his results from the opposition of victoria, among them a solar parallax = . ". , april total solar eclipse observed in south america and west africa. publication of kruger's _catalog der farbigen sterne_. conclusion of boys's series of experiments on the density of the earth. publication of _cordoba durchmusterung_, vol. i. fabry shows comets to be dependents of the solar system. publication of easton's _voie lactée_. campbell detects bright h alpha in gamma argûs and alcyone. nova normæ photographed july ; discovered on plates, october . , may death of professor pritchard. , july installation of -inch refractor at the royal observatory, greenwich. , december exterior nebulosities of pleiades photographed by barnard. , dec. death of rudolf wolf. , january sun-spot maximum. publication of potsdam _photometric durchmusterung_, part i. publication of roberts's _celestial photographs_, vol. i. wilson and gray's determination of the sun's temperature. barnard's micrometric measures of asteroids. mcclean's gift of an astrophysical outfit to the cape observatory. establishment of the lowell observatory at flagstaff, arizona. taylor's triple achromatic objective described. , april discovery of gale's comet. sampson's investigation of the sun's rotation. , oct. favourable opposition of mars. , nov. transit of mercury. , december howlett impugns the wilsonian theory of sun-spots. , dec. death of a. cowper ranyard. publication of newcomb's _astronomical constants_. bailey's photometric catalogue of , southern stars. bailey's photographic discovery of variable star clusters. publication of e. w. brown's _lunar theory_. tisserand's theory of the inequalities of algol. stratonoff's determination of the sun's rotation from photographs of faculæ. binary character of eta aquilæ spectroscopically recognised by bélopolsky. presentation of the crossley reflector to the lick observatory. , march great nebula in ophiuchus discovered photographically by barnard. , march ramsay's capture of helium. , april constitution of saturn's rings spectrographically demonstrated by keeler. binary character of delta cephei spectroscopically detected by bélopolsky. , june death of daniel kirkwood. , july death of f. w. g. spörer. , october nova carinæ spectrographically discovered by mrs. fleming. , dec. nova centauri spectrographically discovered by mrs. fleming. , dec. death of john russell hind. gill's report on the geodetic survey of south africa. appearance of loewy's photographic atlas of the moon, part i. , january fessenden's electrostatic theory of comets. chandler's third catalogue of variable stars. publication of lick observatory photographic atlas of the moon, part i. , february effects of pressure on wave-length described by humphreys and mohler. , april opening of new scottish royal observatory on blackford hill, edinburgh. , april pickering's photometric determinations of light curves of variable stars. one of the stars of castor spectroscopically resolved into two by bélopolsky. , may third astrographic chart conference at paris. , aug. total eclipse of the sun visible in novaya zemlya. reversing layer photographed by shackleton. , aug. death of hubert a. newton. , sept. death of hippolyte fizeau. , oct. death of f. tisserand. succeeded by maurice loewy. , nov. detection by schaeberle of procyon's missing satellite. , nov. death of benjamin apthorp gould. , november second series of hydrogen-lines discovered by pickering in stellar spectra. , december zeeman's discovery of spectral modifications through magnetic influence. , december oxygen-absorption identified in the sun by runge and paschen. study of lunar formations by loewy and puiseux. mounting of the mills spectrograph at the lick observatory. installation at greenwich of the thompson -inch photographic refractor. publication of miss maury's discussion of the photographed spectra of stars. callandreau's researches on cometary disaggregation. braun's determination of the earth's mean density. tenuity of calcium vapour in chromosphere demonstrated spectroscopically by sir william and lady huggins. completion at the cape observatory of mcclean's spectrographic survey of the heavens. twenty-one wolf-rayet stars found by mrs. fleming in magellanic cloud. percival lowell's _new observations on the planet mercury_ presented to the american academy. , april mcclean recognises oxygen-absorption in helium stars. , may death of e. j. stone, radcliffe observer. , june death of alvan g. clark. , june spectrum of a meteor photographed at arequipa. , oct. inauguration of the yerkes observatory. rabourdin's photographs of nebulæ with the meudon reflector. dr. see's discoveries of southern double stars with the lowell -inch refractor. , jan. total eclipse of the sun visible in india. , february binary character of zeta geminorum ascertained spectroscopically by bélopolsky. star with proper motion of nearly " discovered by innes and kapteyn from the cape durchmusterung plates. , march nova sagittarii photographed on draper memorial plates. , june opening of grand-ducal observatory at königsstuhl, heidelberg. keeler succeeds holden as director of the lick observatory. bruno peter's results in stellar parallax. lewis swift's discoveries of nebulæ at echo mountain, california. hale's photographic investigation of carbon stars. , aug. discovery of eros by witt. flint's investigations of stellar parallax by meridian differences. easton's spiral theory of the milky way. seeliger's research on star distribution. , october multiple hydrogen-bands observed by campbell in mira ceti. , november orbit of a leonid meteor photographically determined by elkin. publication of potsdam _photometric durchmusterung_, part ii. innes's _reference catalogue of southern double stars_. keeler's photographs of nebulæ with the crossley reflector and generalization of their spiral character. , january spectrum of andromeda nebula photographed by scheiner. , april photographic discovery of nova aquilæ by mrs. fleming. , aug. installation of -inch photographic refractor at potsdam. campbell's detection of polaris as spectroscopically triple. , october duplicate discovery by campbell and newall of capella as a spectroscopic binary. , nov. failure of the leonids. deflection of the stream predicted by johnstone stoney and downing. , december publication of sir william and lady huggins's _atlas of representative stellar spectra_. thirty-two-inch photographic refractor mounted at meudon. issue of first volume of potsdam measures of international catalogue plates. , jan. kapteyn's determination of the apex of solar motion. chase's measures for parallax of swiftly-moving stars. publication of gill's _researches on stellar parallax_. kapteyn proposes a method for a stellar parallax durchmusterung, and gives specimen results for stars. burnham's general catalogue of , double stars. publication of the concluding volume of the _cape photographic durchmusterung_. , may spanish-american total eclipse of the sun. , july international conference at paris. co-operation arranged of fifty-eight observatories in measures of eros for solar parallax. horizontal refractor, of inches aperture, feet focus, installed in paris exhibition. , aug. death of professor keeler. succeeded by campbell in direction of lick observatory. , november opposition of eros. publication of roberts's _celestial photographs_, vol. ii. complete publication of langley's researches on the infra-red spectrum. printing begun of paris section of international photographic catalogue. , feb. nova persei discovered by anderson. , february variability of eros announced by oppolzer. , april apparition of a great comet at the cape. publication of pickering's _photometric durchmusterung_. miss cannon's discussion of the spectra of , southern stars. kapteyn's investigation of mean stellar parallax. campbell's determination of the sun's velocity. porter's research on the solar motion in space. bigelow's magnetic theory of the solar corona. hussey's measurements of the pulkowa double stars. radial velocities of the components of delta equulei measured at lick. , april death of henry a. rowland. , june nebular spectrum derived from nova persei. , aug. nebula near nova persei photographed by max wolf. , sept. the same exhibited in spiral form on a plate taken by ritchey at the yerkes observatory. , nov. photograph taken by perrine with the crossley reflector showed nebula in course of rapid change. , sept. unveiling of the mcclean "victoria" telescope at the royal observatory, cape of good hope. sun-spot minimum. table ii. chemical elements in the sun (rowland, ). arranged according to the number of their representative lines in the solar spectrum. iron ( +). neodymium. cadmium. nickel. lanthanum. rhodium. titanium. yttrium. erbium. manganese. niobium. zinc. chromium. molybdenum. copper ( ). cobalt. palladium. silver ( ). carbon ( +). magnesium ( +). glucinum ( ). vanadium. sodium ( ). germanium. zirconium. silicon. tin. cerium. strontium. lead ( ). calcium ( +). barium. potassium ( ). scandium. aluminium ( ). _doubtful elements._--iridium, osmium, platinum, ruthenium, tantalum, thorium, tungsten, uranium. _not in solar spectrum._--antimony, arsenic, bismuth, boron, nitrogen (vacuum tube), cæsium, gold, iridium, mercury, phosphorus, rubidium, selenium, sulphur, thallium, praseodymium. oxygen was added to the solar ingredients by runge and paschen in , gallium by hartley and ramage in . lithium may be admitted provisionally, and the chromospheric constituent helium takes rank, since , as a chemical element. table iii. epochs of sun-spot maximum and minimum from to . +----------+----------++----------+----------+----------+----------+ | minima. | maxima. || minima. | maxima. | minima. | maxima. | +----------+----------++----------+----------+----------+----------+ | . | . || . | . | . | . | | . | . || . | . | . | . | | . | . || . | . | . | . | | . | . || . | . | . | . | | . | . || . | . | . | . | | . | . || . | . | . | . | | . | . || . | . | . | . | | . | . || . | . | . | . | | . | . || . | . | . | | +----------+----------++----------+----------+----------+----------+ table iv. movements of sun and stars. . translation of solar system. +--------------------+-------------------------+-------+ | apex of movement. | authority. | date. | +--------------------+-------------------------+-------+ | r. a. dec. | | | | | | | | ° ' + ° | newcomb | | | ° ' + ° ' | kapteyn | | | ° + ° | porter | | | ° + ° | boss | | | ° ' + ° | campbell (from stellar | | | | spectroscopic measures) | | +--------------------+-------------------------+-------+ | velocity = · miles per second (campbell). | +------------------------------------------------------+ . stellar velocities. +---------------------+------------+-------------+---------------------+ | name of star. | rate. | direction. | remarks. | | | miles per | | | | | sec. | | | +---------------------+------------+-------------+---------------------+ | delta leporis | | receding | campbell, | | eta cephei | | approaching | " | | theta canis majoris | | receding | " | | iota pegasi | | approaching | " " | | mu sagittarii | | approaching | " " | | eta andromedæ | | approaching | " " | | zeta herculis | | approaching | bélopolsky, | | cygni | | approaching | " " | | mu cassiopeiæ | | approaching | campbell, | | groombridge | | approaching | " " | | arcturus | . | approaching | keeler, | | arcturus | | tangential | accepting elkin's | | | | | parallax of · " | | groombridge | | tangential | parallax = · " | | mu cassiopeiæ | | tangential | parallax = · " | | | | | (peter) | | z. c. ^h | | tangential | parallax = · " | | | | | (gill) | | lacaille, , | | tangential | parallax = · " | | | | | (gill) | | lacaille, , | | tangential | parallax = · " | | | | | (gill) | | o_ , eridani | | tangential | parallax = · " | | | | | (gill) | | eta eridani | | tangential | parallax = · " | | | | | (gill) | +---------------------+------------+-------------+---------------------+ table v. list of great telescopes. . reflectors--a. metallic specula. +-------------+-----------+------------+-------------+------------------+ | locality. |aperture in|focal length| constructor.| remarks. | | | inches. | in feet. | | | +-------------+-----------+------------+-------------+------------------+ |birr castle, | | | third earl | | |parsonstown, | | | of rosse, |newtonian. | |ireland | | | | | +-------------+-----------+------------+-------------+------------------+ |melbourne | | | t. grubb, |cassegrain. | |observatory | | | | | +-------------+-----------+------------+-------------+------------------+ | | | | third earl |newtonian. | |birr castle | | -- | of rosse, |remounted | | | | | |equatoreally .| +-------------+-----------+------------+-------------+------------------+ | | | | william |newtonian. | |royal | | | lassell, |presented | |observatory | | | |by the missess | |greenwich | | | |lassell to the | | | | | |royal observatory | +-------------+-----------+------------+-------------+------------------+ | b. silvered glass mirrors. | +-------------+-----------+------------+-------------+------------------+ |ealing, near | | |a. a. common,|newtonian. | |london | | | | | +-------------+-----------+------------+-------------+------------------+ | | | |g. w. richey,|can be employed | |yerkes | | | |at choice as a | |observatory | | | |coudé or a | | | | | |cassegrain. | +-------------+-----------+------------+-------------+------------------+ |national | | -- |martin, |newtonian. | |observatory, | | | |remodelled for | |paris | | | |spectrographic | | | | | |work by | | | | | |deslandres in | | | | | | . | +-------------+-----------+------------+-------------+------------------+ |meudon | | · | | | |observatory | | | | | +-------------+-----------+------------+-------------+------------------+ |lick | | · |calver, |mounted by | |observatory | | | |common at | | | | | |ealing in . | | | | | |sold by him to | | | | | |crossley, . | | | | | |presented by | | | | | |crossley to the | | | | | |lick | | | | | |observatory, .| +-------------+-----------+------------+-------------+------------------+ |toulouse | · | · | brothers | | |observatory | | | henry | | +-------------+-----------+------------+-------------+------------------+ |marseilles | · | -- |foucault | | |observatory | | | | | +-------------+-----------+------------+-------------+------------------+ |royal | | -- |common, |cassegrain. | |observatory, | | | |mounted as a | |greenwich | | | |counterpoise | | | | | |to the | | | | | |thompson | | | | | |equatoreal. | +-------------+-----------+------------+-------------+------------------+ |westgate- | | | common, |the property | |on-sea | | -- | |of sir norman | | | | | |lockyer. | +-------------+-----------+------------+-------------+------------------+ |harvard | | | h. draper, |mounted for | |college | | -- | |spectrographic | |observatory | | | |work, . | +-------------+-----------+------------+-------------+------------------+ |royal | | | t. grubb, | | |observatory, | | -- | | | |edinburgh | | | | | +-------------+-----------+------------+-------------+------------------+ |daramona, | | | sir h. |remounted . | |ireland | | · | grubb, |owned by mr. w. e.| | | | | |wilson. | +-------------+-----------+------------+-------------+------------------+ | | | | |can be used as a | |yerkes | · | · | ritchey, |cassegrain, with | |observatory | | | |an equivalent | | | | | |focal length of | | | | | | feet. | +-------------+-----------+------------+-------------+------------------+ |harvard | | | | | |college | | -- | common, | | |observatory | | | | | +-------------+-----------+------------+-------------+------------------+ |crowborough, | | · | sir h. |mounted with a | |sussex | | | grubb, | -inch | | | | | |refractor. | +-------------+-----------+------------+-------------+------------------+ | . refractors. | +-------------+-----------+------------+-------------+------------------+ |palais de | | | gautier, |mounted as a | |l'optique, | · | | |siderostat in | |paris | | | |connection with | | | | | |a plane mirror | | | | | |inches across. | +-------------+-----------+------------+-------------+------------------+ |yerkes | | | alvan g. | | |observatory | | | clark, | | +-------------+-----------+------------+-------------+------------------+ | | | | |for photographic | |lick | | | a. clark and|purposes a | |observatory | | · | sons, |correcting lens is| | | | | |available, of | | | | | |inches aperture, | | | | | | · feet focus. | +-------------+-----------+------------+-------------+------------------+ | | | | |mounted with a | |meudon | · | · | henrys and |photographic | |observatory | | |gautier, |refractor of · | | | | | |inches aperture. | +-------------+-----------+------------+-------------+------------------+ | | | | |photographic. | |astrophysical| | |steinheil and|mounted with a | |observatory, | · | · |repsold, |visual refractor | |potsdam | | | | inches in | | | | | |aperture. | +-------------+-----------+------------+-------------+------------------+ |bischoffsheim| | | |visual. mounted | |observatory, | · | · | henrys and |on mont gros, | |nice | | |gautier, | , feet above | | | | | |sea level. | +-------------+-----------+------------+-------------+------------------+ |imperial | | | a. clark and|visual. mounted | |observatory, | | | sons, |by repshold. | |pulkowa | | | | | +-------------+-----------+------------+-------------+------------------+ |national | | | | | |observatory, | · | -- | martin | | |paris | | | | | +-------------+-----------+------------+-------------+------------------+ |royal | | | sir h. |visual and | |observatory, | | | grubb, |photographic. | |greenwich | | | |mounted by | | | | | |ransome and simms.| +-------------+-----------+------------+-------------+------------------+ | university | | |sir h. grubb,| visual. | | observatory,| | | | | | vienna | | | | | +-------------+-----------+------------+-------------+------------------+ | royal | | |sir h. grubb,| the thompson | | observatory,| | | | photographic | | greenwich | | | | equatoreal. | +-------------+-----------+------------+-------------+------------------+ | naval | | | a. clark and| | | observatory,| | | sons, | | | washington | | | | | +-------------+-----------+------------+-------------+------------------+ | leander | | | a. clark and| | | mccormick | | · | sons, | | | observatory,| | | | | | virginia | | | | | +-------------+-----------+------------+-------------+------------------+ | cambridge | | | t. cooke and| presented to the | | university | | -- | sons, | university in | | observatory | | | | by | | | | | | mr. r. s. newall.| +-------------+-----------+------------+-------------+------------------+ | meudon | | | henrys and | photographic. | | observatory | · | · | gautier, | mounted with a | | | | | | visual . | | | | | | refractor. | +-------------+-----------+------------+-------------+------------------+ | harvard | | | a. clark and| photographic | | college | | · | sons, | doublet. the gift| | observatory | | | | of miss bruce. | | | | | | transfered in | | | | | | to arequipa,| | | | | | peru. | +-------------+-----------+------------+-------------+------------------+ | royal | | |sir h. grubb,| photographic. | | observatory,| | | | the gift or mr. | | cape of | | · | | mcclean. mounted | | good hope | | | | with an -inch | | | | | | visual refractor.| +-------------+-----------+------------+-------------+------------------+ | lowell | | | alvan g. | visual. first | | observatory,| | | clark, | mounted near the | | flagstaff, | | | | city of mexico. | | arizona | | | | installed at | | | | | | flagstaff, . | +-------------+-----------+------------+-------------+------------------+ | national | | | henrys and | visual and | | observatory,| · | | gautier, | photographic. | | paris | | | | mounted as an | | | | | | equatoreal coudé.| +-------------+-----------+------------+-------------+------------------+ | halsted | | | a. clark and| | | observatory,| | | sons, | | | princeton, | | | | | | n.j. | | | | | +-------------+-----------+------------+-------------+------------------+ | city | | | | mounted as a | | observatory,| | | | visual | | edinburgh | | | | equatoreal on | | | | | | the calton hill, | | | | | | . | +-------------+-----------+------------+-------------+------------------+ | etna | | | merz, | | | observatory | · | -- | | | +-------------+-----------+------------+-------------+------------------+ | buckingham | | | buckingham | | | observatory | · | -- | and wragge | | +-------------+-----------+------------+-------------+------------------+ | porro | | | porro | | | observatory,| · | -- | | | | turin | | | | | +-------------+-----------+------------+-------------+------------------+ | chamberlin | | | alvan g. | visual. | | observatory,| | | clark and | with a reversible| | colorado | | | saegmüller, | crown lens for | | | | | | photography. | +-------------+-----------+------------+-------------+------------------+ | manila | | | merz and | visual. | | observatory | | -- | saegmüller, | provided with a | | | | | | photographic | | | | | | correcting lens. | +-------------+-----------+------------+-------------+------------------+ | strasburg | · | |merz and | | | observatory | | |repsold, | | +-------------+-----------+------------+-------------+------------------+ | brera | | | merz and | | | observatory,| · | | repsold | | | milan | | | | | +-------------+-----------+------------+-------------+------------------+ | dearborn | | | a. clark and| mounted . | | observatory,| · | | sons, | | | illinois | | | | | +-------------+-----------+------------+-------------+------------------+ | national | | | henrys and | coudé mount. | | observatory,| · | · | gautier, | visual. | | la plata | | | | | +-------------+-----------+------------+-------------+------------------+ | lowell | | | brashear, | mounted with a | | observatory,| | · | | -inch clark | | flagstaff, | | | | refractor as | | arizona | | | | counterpoise. | +-------------+-----------+------------+-------------+------------------+ | van der zee | | | fitz | dismounted. | | observatory,| | | | | | buffalo, | | -- | | | | n.y. | | | | | +-------------+-----------+------------+-------------+------------------+ |bischoffsheim| | | henrys and | coudé mount. | | observatory,| · | · | gautier, | visual. | | nice | | | | | +-------------+-----------+------------+-------------+------------------+ | university | | | henrys and | coudé mount. | | observatory,| · | · | gautier, | visual. | | vienna | | | | | +-------------+-----------+------------+-------------+------------------+ | jesuit | | | henrys and | photographic. | | observatory,| · | · | gautier, | mounted with a | | zi-ka-wei | | | | visual refractor | | | | | |of equal aperture.| +-------------+-----------+------------+-------------+------------------+ | goodsell | | | brashear, | | | observatory,| · | -- | | | | northfield, | | | | | | minnesota. | | | | | +-------------+-----------+------------+-------------+------------------+ | warner | | | a. clark and| | | observatory,| | | sons, | | | rochester, | | | | | | n.y. | | | | | +-------------+-----------+------------+-------------+------------------+ | grand-ducal | | | brashear and| a twin | observatory,| | · | grubb, | photographic | | königsstuhl,| | | |doublet. the gift | | heidelberg | | | | of miss bruce. | | | | | | mounted with a | | | | | | visual -inch | | | | | | refractor by | | | | | | pauly. | +-------------+-----------+------------+-------------+------------------+ | meudon | | | | | | observatory | · | · | | | +-------------+-----------+------------+-------------+------------------+ | washburn | | | a. clark and| | | observatory,| · | · | sons, | | | wisconsin | | | | | +-------------+-----------+------------+-------------+------------------+ | teramo | | | t. cooke and| formerly the | | observatory,| · | -- | sons, | property of | | italy | | | | mr. wigglesworth.| +-------------+-----------+------------+-------------+------------------+ | royal | | | t. grubb, | presented by | | observatory,| · | -- | | lord crawford. | | edinburgh | | | | | +-------------+-----------+------------+-------------+------------------+ | madrid | | | merz | | | observatory | | -- | | | +-------------+-----------+------------+-------------+------------------+ | tulse hill | | |sir h. grubb,| lent by the | | observatory | | | | royal society to | | | | | | sir william | | | | | | huggins. mounted | | | | | | with an -inch | | | | | | cassegrain | | | | | | reflector. | +-------------+-----------+------------+-------------+------------------+ | national | | | lerebours | | | observatory,| | | | | | paris | | | | | +-------------+-----------+------------+-------------+------------------+ | harvard | | | merz, | | | college | | | | | | observatory | | | | | +-------------+-----------+------------+-------------+------------------+ | national | | | | | | observatory,| | -- | | | | rio de | | | | | | janeiro | | | | | +-------------+-----------+------------+-------------+------------------+ | tacubaya | | |sir h. grubb,| | | observatory,| | | | | | mexico | | | | | +-------------+-----------+------------+-------------+------------------+ | stonyhurst | | |sir h. grubb,| | | college | | | | | | observatory | | | | | +-------------+-----------+------------+-------------+------------------+ | brera | | | | | | observatory,| | -- | | | | milan | | | | | +-------------+-----------+------------+-------------+------------------+ | university | | |sir h. grubb,| visual. | | of | | | | mounted with a | | mississippi | | | | photographic | | | | | | -inch refractor.| +-------------+-----------+------------+-------------+------------------+ | imperial | | | merz and | | | observatory,| | · | mahler, | | | pulkowa | | | | | +-------------+-----------+------------+-------------+------------------+ | maidenhead | | |sir h. grubb,| the property of | | observatory | | -- | | mr. dunn. | | | | | | mounted with a | | | | | | twin photographic| | | | | | reflector. | +-------------+-----------+------------+-------------+------------------+ | odessa | | | merz, | | | observatory | · | -- | | | +-------------+-----------+------------+-------------+------------------+ |bischoffsheim| | | henrys and | | | observatory,| · | | gautier | | | nice | | | | | +-------------+-----------+------------+-------------+------------------+ | brussels | | | merz and | | | observatory | · | | cooke, | | +-------------+-----------+------------+-------------+------------------+ | observatory | | | merz and | | | of bordeaux | · | · |gautier, | | +-------------+-----------+------------+-------------+------------------+ | observatory | | | merz and | | | of lisbon | · | -- | mahler | | +-------------+-----------+------------+-------------+------------------+ table vi. list of observatories employed in the construction of the photographic chart and catalogue of the heavens. all are provided with -inch photographic, coupled with -inch visual refractors: ---------------------------------------------------------- | name of observatory. | constructors of instruments.| | |-------------------------------| | | optical part.|mechanical part.| |------------------------|--------------|----------------| |paris | henrys | gautier | |algiers | " | " | |bordeaux | " | " | |toulouse | " | " | |san fernando (spain) | " | " | |vatican | " | " | |cordoba | " | " | |montevideo | " | " | |perth, western australia| " | " | |helsingfors | " | repsold | |potsdam | steinheil | " | |catania | " | salmoiraghi | |greenwich | sir h. grubb | sir h. grubb | |oxford | " | " | |the cape | " | " | |melbourne | " | " | |sydney | " | " | |tacubaya (mexico) | " | " | ---------------------------------------------------------- index abbe, cleveland, corona of , , aberdour, lord, solar chromosphere, aberration, discovered by bradley, , ; cause of, , ; investigations of, , abney, daylight coronal photographs, ; infra-red photography, , , absorption, terrestrial atmospheric, , , - , ; solar, - , , , , , , ; correlative with emission, , , adams, discovery of neptune, - ; lunar acceleration, ; orbit of november meteors, aerolites, falls of, , airy, solar translation, ; observations during eclipses, , , ; astronomer-royal, ; search for neptune, , ; corona of , ; solar parallax, , ; transit of venus, ; mercurian halo, ; lunar atmosphere, aitken, double star discoveries, albedo, of mercury, ; of venus, ; of mars, ; of minor planets, ; of jupiter, ; of saturn, ; of uranus, alexander, spiral nebulæ, ; observation during eclipse, algol, variability of light, , ; eclipses, ; nature of system, altitude and azimuth instrument, _note_, amici, comet of , anderson, discovery of nova aurigæ, ; of nova persei, andrews, conditions of liquefaction, Ångström, c. j., _optical researches_, ; spark spectrum, ; nature of photosphere, ; solar spectroscopy, , ; hydrogen in sun, ; temperature of stars, Ångström, k., infra-red solar spectrum, ; solar constant, arago, eclipse of , , , ; prominences, ; polarization in comets, ; magnetic relations of auroræ, ; nature of photosphere, ; meteor-systems, arai, photographs of corona of , arcturus, spectrum, , ; radial movement, argelander, bonn durchmusterung, , ; solar motion, ; centre of milky way, ; comet of , aristotle, description of a comet, arrhenius, light-pressure theory of comets, asten, movements of encke's comet, asteroids, so designated by herschel, astronomical circles, , astronomical physics, , , astronomical society founded, ; herschel its first president, astronomy, classification, ; popularity and progress, ; in united states, ; in germany, ; practical reform, ; of the invisible, ; physical, atmosphere, solar, , , , , ; of venus, , , , ; of mercury, - ; of the moon, , ; of mars, ; of minor planets, auroræ, periodicity, , ; excited by meteors, auwers, reduction of bradley's observations, ; system of procyon, ; opposition of victoria, ; solar parallax, ; new star in scorpio, babinet, nebular hypothesis, backlund, movements of encke's comet, , baden-powell, sir george, eclipse expedition, bailey, nebulosity round pleiades, ; stellar photometric observations, ; discovery of variable clusters, baily, early life and career, - ; observations of eclipses, - ; density of the earth, , baily's beads, , bakhuyzen, rotation of mars, ball, sir robert, parallaxes of stars, _note_, ; contacts in transits, balmer's law, , barnard, micrometrical measures of neptune, ; of minor planets, ; of saturn's rings, ; photographs of solar corona, , ; transit of mercury, ; halo round venus, ; surface of mars, ; ellipticity of jupiter's first satellite, ; of uranus, ; discovery of inner jovian satellite, , ; red spot on jupiter, ; eclipse of japetus, ; attendants on comet of , ; on brooks's comet, , ; swift's comet, ; photographic discovery of a comet, ; observations of nova aurigæ, , ; hind's variable nebula, ; exterior pleiades nebulosities, ; galactic stars, ; photographs of milky way, , ; cluster variables, ; horizontal telescope, bartlett, photograph of a partial eclipse, basic lines, , baxendell, meteors of , becker, drawings of solar spectrum, beckett, sir e. (lord grimthorpe), value of solar parallax, beer and mädler, surveys of lunar surface, , ; studies of mars, bélopolsky, coronal photographs, ; theory of corona, ; rotation of venus, ; of jupiter, ; spectroscopic determinations of saturn's rings, ; spectrum of gamma cassiopeiæ, ; system of castor, , ; detection of variable stars as spectroscopic binaries, berberich, mass of asteroids, ; orbit of holmes's comet, berkowski, daguerrotype of eclipsed sun, bessel, biographical sketch, - ; reduction of bradley's observations, ; parallax of cygni, ; disturbed motion of sirius and procyon, ; trans-uranian planet, ; halley's comet, ; theory of instrumental errors, ; personal equation, ; rotation of mercury, ; lunar atmosphere, ; cometary emanations, , ; multiple tails, ; comet of , betelgeux, remoteness, , ; spectrum, , , , ; radial movement, bianchini, rotation of venus, biela, discovery of a comet, bigelow, magnetic and solar disturbances, ; theory of corona, bigourdan, eclipse of , ; velocity of comet of , bird's quadrants, , , birmingham, colours of stars, _note_; discovery of t coronæ, birt, rotation of a sun-spot, ; selenographical society, bischoffsheim, coudé telescope, black ligament, bode, popular writings, ; solar constitution, ; missing planet, , bode's law, , , boeddicker, heat-phases during lunar eclipses, , ; drawings of jupiter, ; of the milky way, boehm, solar observations, , boguslawski, centre of sidereal revolutions, ; observation of halley's comet, bolometer, principle of construction, bond, g. p., his father's successor, ; light of jupiter, ; saturn's rings, ; donati's comet, , ; andromeda nebula, ; double-star photography, bond, w. c., observation of neptune's satellite, ; discovery of hyperion, ; of saturn's dusky ring, ; resolution of nebulæ, ; celestial photography, , ; satellite-transit on jupiter, borda, repeating circle, boss, solar translation, ; observations on comets, , bossert, proper motions of stars, bouguer, solar atmospheric absorption, boulliaud, period of mira, bouvard, tables of uranus, ; encke's comet, boys, radio-micrometer, ; density of the earth, bradley, discoveries of aberration and nutation, ; solar translation, ; star-distances, , ; observation on castor, ; instruments, , ; observations reduced by bessel and auwers, , brahe, tycho, star of , brandes, observations of meteors, , ; braun, prominence photography, ; density of the earth, brayley, meteoric origin of planets, brédikhine, theory of cometary appendages, , ; repulsive forces, , ; chemical differences, , ; formative types, , , , , ; structure of chromosphere, ; red spot on jupiter, ; andromede meteors, ; stationary radiants, ; spectrum of coggia's comet, bremiker, star maps, brenner, rotation of venus, brester, _théorie du soleil_, brewster, diffraction theory of corona, ; telluric lines in solar spectrum, ; absorption spectra, brinkley, ostensible stellar parallaxes, brisbane, establishment of paramatta observatory, , brooks, fragment of comet, ; cometary discoveries, , brünnow, stellar parallaxes, , bruno, giordano, motion of stars, buffham, rotation of uranus, buffon, internal heat of jupiter, bunsen, discovery of spectrum analysis, burchell, magnitude of eta carinæ, burnham, stellar orbits, ; coronal photographs, ; measures of nova aurigæ, ; of planetary nebulæ, ; discoveries of double stars, , , , ; catalogue, ; system of cygni, burton, canals of mars, ; rotation of jupiter's satellites, calandrelli, stellar parallaxes, callandreau, capture theory of comets, campani, saturn's dusky ring, campbell, lieutenant, polarisation of corona, campbell, professor, stellar radial velocities, , , ; flash spectrum, ; spectroscopic observations of saturn's rings, ; wolf-rayet stars, ; spectroscopic binaries, ; nova aurigæ, ; translation of solar system, ; stellar diffraction-spectra, canals of mars, - cannon, miss a. j., spectrographic researches, canopus, remoteness, , ; spectrum, capella, spectrum, , , ; a spectroscopic binary, carbon, material of photosphere, ; absorption by, in sun, ; in stars, carbonelle, origin of meteorites, carinæ, eta, light variation, , ; spectrum, carrington, astronomical career, , ; sun-spot observations, ; solar rotation, ; spot-distribution, ; luminous outburst on sun, , ; jovian and sun-spot periods, ; origin of comets, cassini, domenico, discoveries of saturnian satellites, ; of division in ring, ; solar rotation period, ; solar parallax, ; rotation of venus, ; of mars, ; of jupiter, , ; satellite of venus, ; satellite-transit on jupiter, cassini, j. j., stellar proper motions, ; sun-spots on limb, ; theory of corona, ; rotation of venus, ; structure of saturn's rings, castor, system of, , cavendish experiment, , ceres, discovery, , ; diameter, , chacornac, observation of sun-spot, ; star-maps, , ; variable nebula, challis, search for neptune, , ; duplication of biela's comet, charlois, discoveries of minor planets, charroppin, coronal photographs, chase, photographic discovery of a comet, ; stellar parallaxes, chladni, origin of meteors, , christie, mercurian halo, chromosphere, early indications, ; distinct recognition, , , ; depth, , , ; metallic injections, ; eruptive character, ; spectrum, clark, alvan, large refractors, , , , clark, alvan g., discovery of sirian companion, , ; -inch refractor, clarke, colonel, figure of the earth, clarke, f. w., celestial dissociation, clausen, period of comet, ; cometary systems, clerihew, secondary tail of comet, clusters, variable stars in, coggia, discovery of a comet, comet, halley's, return in , , ; orbit computed by bessel, ; capture by neptune, , ; return in , - , ; type of tail, , ; of , , - ; type of tail, , ; relationships, - ; newton's, , ; encke's, ; changes of volume, ; of brightness, ; acceleration, , ; capture by mercury, ; winnecke's, , ; biela's, - , ; brorsen's, ; vico's, , ; faye's, ; of , - , ; of , , , ; of , , ; lexell's, , ; tewfik, , , , ; donati's, - , , ; of , , , ; perseid, , ; leonid, , , , ; klinkerfues's, ; holmes's, , , ; coggia's, , , ; of , ; of , , ; aristotle's, ; tebbutt's, - ; schaeberle's, , ; wells's, , ; of september, , - , - ; thome's, ; pons-brooks, , ; sawerthal's, ; brooks's, of , , ; swift's, cometary tails, repulsive action upon, , , , - ; coruscations in, ; three types, - , , ; multiple, , , , , , , comets, subject to gravitation, ; of short period, , ; translucency, , , , ; small masses, , ; capture by planets, , , ; changes of volume, , , ; polarisation of light, , , ; refractive inertness, , ; relations to meteor-systems, , - ; disintegration, , , , ; spectra, - , , , - ; luminous by electricity, , , ; systems, , , , , ; origin, - common, reflectors for eclipse photography, ; jupiter's inner satellite, ; detection of great comet near the sun, ; its five nuclei, ; photographs of andromeda nebula, ; of orion nebula and jupiter, , ; great reflectors, , ; cluster variables, common, miss, drawing of eclipsed sun, comstock, lunar atmosphere, comte, celestial chemistry, ; astronomy, cooke, -inch refractor, copeland, comets of and , ; spectrum of comet of , ; of gamma cassiopeiæ, ; of nova andromedæ, ; of orion nebula, ; discoveries of gaseous stars, ; nova aurigæ, , copernicus, stellar parallax, cornu, telluric lines in solar spectrum, ; velocities in prominences, ; ultra-violet solar spectrum, , ; velocity of light, _note_, ; spectrum of hydrogen, ; of nova cygni, cornu and bailie, density of the earth, corona of , - , ; early records and theories, - ; photographs, , , , , - ; spectrum, , , , , , ; varying types, - , ; of , - ; of , ; of , ; of , ; of , - ; of , ; of , ; of , ; of , ; daylight photography of, - ; glare theory, ; mechanical theory, ; electro-magnetic theories, , coronium, , , cortie, movements in sun-spots, ; their spectral changes, cotes, corona of , croll, secular changes of climate, , ; derivation of solar energy, crookes, chemical elements, crova, solar constant, cruls, comet of , , cusa, solar constitution, cysatus, orion nebula, ; comet of , damoiseau, theory of halley's comet, d'arrest, orbits of minor planets, ; andromede meteors, ; ages of stars, ; variable nebulæ, ; measures of nebulæ, darwin, g. h., rigidity of the earth, ; saturn's ring system, ; origin of the moon, - ; development of solar system, , , ; solar tidal friction, daubrée, falls of aerolites, davidson, satellite-transit on jupiter, davis, stellar parallaxes, dawes, prominences in , ; saturn's dusky ring, ; a star behind a comet, ; solar observations, , ; observations and drawings of mars, , , ; satellite-transits on jupiter, , de ball, markings on mercury, delambre, greenwich observations, ; solar rotation, ; light-equation, de la roche, newton's law of cooling, de la rue, celestial photography, , , ; solar investigations, ; expedition to spain, , de la tour, experiments on liquefaction, delaunay, tidal friction, , ; coudé telescope, delisle, diffraction theory of corona, ; transits of venus, , dembowski, double star measurements, denning, observations of mercury, , ; mountain on venus, ; rotation of jupiter, ; red spot, ; periodicity of markings, ; rotation of saturn, ; meteors of , ; of , ; stationary radiants, denza, meteors of , derham, theory of sun-spots, ; ashen light on venus, deslandres, eclipse expedition, ; rotation of corona, ; prominence photography, ; hydrogen spectrum in prominences, , ; photographs of jupiter, ; radial movements of saturn's rings, ; helium absorption in stars, ; stellar radial velocities, diffraction, corona explained by, , , ; spectrum, , , , dissociation in the sun, , - ; in space, doberck, orbits of double stars, , dollond, discovery of achromatic telescope, , donati, discovery of comet, ; spectra of comets, ; of stars, doppler, effect of motion on light, douglass, observations of jupiter's satellites, downing, perturbations of the leonids, draper, h., ultra-violet spectrum, ; oxygen in sun, ; photographs of the moon, ; of jupiter's spectrum, ; of tebbutt's comet, ; of spectrum of vega, ; of orion nebula draper, j. w., lunar photographs, ; distribution of energy in spectrum, _note_ draper memorial, - dreyer, catalogue of nebulæ, dulong and petit, law of radiation, , dunér, spectra of sun-spots, ; spectroscopic measurement of solar rotation, ; spectroscopic star catalogue, dunkin, solar translation, duponchel, sun-spot period, durchmusterung, bonn, , ; cape photographic, ; parallax, ; photometric, dyson, coronal photographs, earth, mean density, , ; knowledge regarding, ; rigidity, , ; variation of latitude, , ; figure, , ; effects of tidal friction, - ; bodily tides, ; primitive disruption, easton, structure of milky way, , ebert, coronoidal discharges, eclipse, solar, of , ; of , - , , ; of , , , ; of , , ; of , - ; of , ; of , ; of , ; of , - ; of , , ; of , , ; of , ; of , ; of , ; of , - ; of , , ; of , ; of , ; of , , ; of , eclipses, lunar, heat-phases during, , eclipses, solar, importance, ; ancient, , ; classification, ; results, , eddie, comet of , ; of , edison, tasimeter, egoroff, telluric lines in solar spectrum, , elements, chemical, dissociation in sun, , , elkin, star parallaxes, , , ; photography of meteors, ; transit of great comet, , ; secondary tail, ; triangulation of the pleiades, elliot, opinions regarding the sun, elvins, red spot on jupiter, encke, star maps, ; calculation of short-period comet, ; resisting medium, ; distance of the sun, , ; period of pons's comet, engelmann, rotation of jupiter's satellites, ericsson, solar temperature, erman, meteoric rings, eros, measures of, for solar parallax, ; discovery, ; variability, ertborn, mountain in venus, espin, spectra of variable stars, ; stars with banded spectra, ; nova aurigæ, , euler, resisting medium, evershed, eclipse photographs, , evolution, of solar system, , , - , ; of earth-moon system, - ; of stellar systems, fabricius, david, discovery of mira ceti, fabricius, john, detection of sun-spots, faculæ, relation to spots, , , ; solar rotation from, ; photographed, , , faye, nature of prominences, , ; discovery of a comet, ; cyclonic theory of sun-spots, , ; solar constitution, - ; maximum of , ; velocities in prominences, ; distance of the sun, ; planetary evolution, , , feilitsch, solar appendages, fényi, solar observations, , ferrel, tidal friction, ferrer, nature of corona, ; prominences, fessenden, electrical theory of comets, finlay, transit of great comet, , fizeau, daguerrotype of the sun, ; doppler's principle, ; velocity of light, flammarion, canals of mars, ; trans-neptunian planet, flamsteed, solar constitution, ; distance, flaugergues, detection of comet, ; transit of mercury, fleming, mrs., spectrum of beta lyræ, ; preparation of draper catalogue, ; discoveries of new stars, flint, star-parallaxes, fontana, mountains of venus, ; satellite, ; spots on mars, forbes, george, trans-neptunian planets, , forbes, james d., spectrum of annularly eclipsed sun, ; solar constant, foucault, spectrum of voltaic arc, ; photograph of the sun, ; velocity of light, , ; silvered glass reflectors, fraunhofer, early accident, ; improvement of refractors, ; clockwork motion, ; spectra of flames, ; of sun and stars, , , ; objective prism, ; diffraction gratings, fraunhofer lines, mapped, , ; origin, - , , ; reflected in coronal spectrum, , , ; in cometary spectra, , ; shifted by radial motion, freycinet, distribution of minor planets, fritz, auroral periodicity, frost, solar heat radiation, galileo, descriptive astronomy, ; double-star method of parallaxes, ; discovery of sun-spots, ; solar rotation, ; planets and sun-spots, ; darkening at sun's edge, galle, discovery of neptune, , ; saturn's dusky ring, ; distance of the sun, ; path of andromede meteors, galloway, solar translation, gambart, discovery of comet, gauss, orbits of minor planets, ; _theoria motus_, ; magnetic observations, , ; cometary orbits, gautier, sun-spot and magnetic periods, , ; sun-spots and weather, german astronomical society, , gill, star-parallaxes, , , , ; expedition to ascension, ; distance of the sun, , , ; constant of aberration, ; arc measurements, , ; comet of , , ; oxygen-absorption in stars, ; photograph of argo nebula, ; cape durchmusterung, ; photographic celestial survey, ; actinic intensity of galactic stars, ; coudé telescope, gladstone, j. h., spectrum analysis, , glaisher, occultation by halley's comet, glasenapp, coronal photographs, ; light equation, , ; double star measures, glass, optical, excise duty on, , ; guinand's, , ; jena, gledhill, spot on jupiter, goldschmidt, nebulæ in the pleiades, goodricke, periodicity of algol, gore, catalogue of variable stars, ; of computed binaries, gothard, bright-line stellar spectra, , ; spectrum of nova aurigæ, ; photographs of nebulæ, gould, variation of latitude, ; photograph of mars, ; comets of and , , ; luminous instability of stars, ; photographic measures of the pleiades, ; _uranometria argentina_, ; solar cluster, , graham, discovery of metis, grant, solar envelope, , ; transit phenomena, green, observation of mars, greenwich observations, , , gregory, david, achromatic lenses, _note_ gregory, james, double star method of parallaxes, ; reflecting telescopes, groombridge, star catalogue, grosch, corona of , grubb, sir howard, photographic reflector, ; great refractors, , ; siderostat, grubb, thomas, melbourne reflector, _note_, gruithuisen, snow-caps of venus, ; lunar inhabitants, gully, detection of nova andromedæ, guthrie, nebulous glow round venus, hadley, saturn's dusky ring, ; reflecting telescope, haerdtl, winnecke's comet, hale, luminous outburst on sun, ; daylight coronal photography, ; spectrum of prominences, , ; prominence photography, , ; photographs of faculæ, , ; carbon in chromosphere, ; bright lines in fourth-type stars, ; reflectors and refractors, hall, asaph, parallax of the sun, ; discovery of martian satellites, ; rotation of saturn, ; double star measurements, hall, chester more, invention of achromatic telescope, hall, maxwell, rotation of neptune, halley, stellar proper motions, ; composition of nebulæ, ; observation of eta carinæ, ; eclipse of , , ; predicted return of comet, ; magnetic theory of auroræ, ; transits of venus, ; lunar acceleration, ; origin of meteors, halm, magnetic relations of latitude variation, hansen, solar parallax from lunar theory, hansky, coronal photographs, , harding, discovery of juno, ; celestial atlas, harkness, spectrum of corona, ; corona of , ; shadow of the moon in solar eclipses, ; light equation, ; distance of the sun, , , , harriot, observations on halley's comet, hartley, gallium in the sun, , hartwig, nova andromedæ, hasselberg, metallic spectra, ; spectra of comets, , ; of nova andromedæ, hastings, composition of photosphere, ; observations at caroline island, ; saturn's dusky ring, hegel, number of the planets, heis, radiant of andromedes, heliometer, , , , , helium, a constituent of prominences, , , ; no absorption by, in solar spectrum, ; absorptive action in first-type stars, ; bright in gaseous stars, , , ; in orion nebula, helmholtz, gravitational theory of sun-heat, - hencke, discoveries of minor planets, henderson, parallax of alpha centauri, , ; observation of chromosphere, henry, paul and prosper, lunar twilight, ; markings on uranus, ; photograph of saturn, ; photographs of nebulæ in the pleiades, , ; stellar photography, ; plane mirrors, herrick and bradley, duplication of biela's comet, herschel, alexander s., cometary and meteoric orbits, herschel, caroline, her brother's assistant, ; observation of encke's comet, herschel, colonel, spectrum of prominences, ; of reversing layer, ; of corona, herschel, sir john, life and work, - ; magellanic clouds, , ; sun-spots, , , ; solar flames, ; anticipation of neptune's discovery, ; status of hyperion, ; biela's comet, ; halley's, ; comet of , ; sixth star in "trapezium," ; grinding of specula, ; spectrum analysis, ; solar photography, , ; solar constitution, ; shadow round eclipsed sun, ; actinometrical experiments, ; solar heat, ; climate and eccentricity, ; lunar atmosphere, ; surface of mars, ; andromeda nebula, ; observations of nebulæ, ; double nebulæ, herschel, sir william, discovery of uranus, ; founder of sidereal astronomy, ; biographical sketch, - ; sun's motion in space, , , ; revolutions of double stars, , ; structure of milky way, - , ; nature of nebulæ, - , ; results of his observations, ; centre of sidereal system, ; theory of the sun, - , ; asteroids, ; discoveries of saturnian and uranian satellites, , , ; comet of , ; reflecting telescopes, - ; sun-spots and weather, ; transit of mercury, ; refraction in venus, ; lunar volcanoes, ; terrestrial affinity of mars, ; jovian trade-winds, ; rotation of jupiter's satellites, ; ring of saturn, ; rotation of saturn, ; origin of comets, ; stellar photometry, herz, comets' tails, hevelius, "mira" ceti, ; contraction of comets, ; granular structure of comet, higgs, photographs of solar spectrum, , hind, solar flames, ; iris and flora discovered by, ; distortion of biela's comet, ; transit of a comet, ; earth in a comet's tail, ; comets of and , ; schmidt's comet, ; new star, ; variable nebula, hirn, solar temperature, ; resistance in space, hodgson, outburst on the sun, hoeffler, star-drift in ursa major, hoek, cometary systems, holden, uranian satellites, ; eclipse expedition, ; coronal extensions, ; solar rotation, ; transit of mercury, ; intra-mercurian planets, ; drawing of venus, ; lunar photographs, ; canals on mars, ; surface of mars, ; transits of jupiter's satellites, ; markings on uranus, ; disintegration of comet, ; colours of double stars, ; nova aurigæ, ; orion and trifid nebulæ, , ; director of lick observatory, holden and schaeberle, observations of nebulæ, holmes, discovery of a comet, homann, solar translation, hooke, solar translation, ; stellar parallax, ; repulsive action on comets, _note_; automatic movement of telescopes, ; spots on mars, , hopkins, solidity of the earth, horrebow, sun-spot periodicity, ; satellite of venus, hough, g. w., red spot on jupiter, , ; observations of double stars, houzeau, solar parallax, howlett, sun-spot observations, hubbard, period of comet of , , huggins, sir william, spectroscopic observations of prominences, , ; hydrogen spectrum in stars, , ; daylight coronal photography, , ; repulsive action in corona, ; stellar motions in line of sight, , , ; transit of mercury, ; occultation of a star, ; snowcaps on mars, ; spectrum of mars, ; of jupiter, ; jovian markings and sun-spots, ; spectrum of uranus, ; of comets, , ; photographs, , ; stellar spectroscopy, ; colours of stars, ; classification of star spectra, ; photographs, , , ; stellar chemistry, , ; spectra of new stars, , ; theory of nova aurigæ, ; spectra of nebulæ, , , ; nebular radial movement, huggins, sir william and lady, photograph of uranian spectrum, ; spectra of wolf-rayet stars, ; ultra-violet spectrum of sirius, ; nitrogen in stars, ; spectrum of nova aurigæ, - ; of andromeda nebula, ; of orion nebula, humboldt, sun-spot period, ; magnetic observations, ; meteoric shower, hussey, t. j., search for neptune, hussey, w. j., cloud effects on mars, ; cometary appendages, ; period of delta equulei, ; discoveries of double stars, , huygens, stellar parallax, ; orion nebula, ; discovery of titan, ; saturn's ring, , ; spot on mars, hydrogen, a constituent of prominences, , , ; spectrum, , , , ; absorption in stars, , , - ; in sun, ; theoretical material of comets' tails, ; emissions in stars, - , , , ; in nebulæ, , innes, southern double stars, jacoby, measurement of rutherfurd's plates, ; pritchard's parallax work, janssen, photographs of the sun, ; spectroscopic observations of prominences, , ; escape from paris in a balloon, ; coronal spectrum, , ; coronal photographs, ; rarefaction of chromospheric gases, ; oxygen absorption in solar spectrum, ; transit of , ; spectrum of venus, ; of saturn, ; photographs of tebbutt's comet, , ; of orion nebula, japetus, eclipse of, ; variability in light, jewell, solar spectroscopy, , joule, heat and motion, jupiter, mass corrected, , ; conjectured influence on sun-spot development, ; physical condition, , ; spectrum, , ; satellite-transits, , ; discovery of inner satellite, ; red spot, - ; photographs, , ; periodicity of markings, kaiser, rotation of mars, ; map of mars, kammermann, observation of maia nebula, kant, status of nebulæ, ; sirius the central sun, ; planetary intervals, ; tidal friction, ; condition of jupiter, ; cosmogony, kapteyn, solar translation, ; cape durchmusterung, ; stellar parallaxes, , ; actinic intensity of galactic stars, ; solar cluster, kayser and runge, spectroscopic investigations, , keeler, red spot on jupiter, ; spectroscopic determination of movements in saturn's rings, ; spectrum of uranus, ; of third type stars, ; of nebulæ, ; photographs of nebulæ, , , , ; nebular radial movements, , , ; grating spectroscope, kepler, star of , ; solar corona, ; missing planets, ; cometary decay, , ; comet of , ; physical astronomy, kiaer, comets' tails, kirchhoff, foundation of spectrum analysis, , - , ; map of solar spectrum, ; solar constitution, , , kirkwood, distribution of minor planets, ; grouped orbits, ; divisions in saturn's rings, , ; origin of planets, ; their mode of rotation, ; comets and meteors, , kleiber, perseid radiants, klein, hyginus n., , klinkerfues, comet predicted by, , ; apparitions of southern comet, ; tidal theory of new stars, knobel, cloud effects on mars, konkoly, spectrum of gamma cassiopeiæ, ; spectroscopic survey, _note_ kreil, lunar magnetic action, kreutz, period of comet, ; orbit of comet, ; period of great september comet, ; cause of disintegration, ; eclipse-comet of , krüger, segmentation of great comet, küstner, variation of latitude, kunowsky, spots on mars, lacaille, southern nebulæ, ; eta carinæ, lagrange, theory of solar system, ; planetary disruption, lahire, diffraction theory of corona, ; distance of the sun, ; mountains of venus, lalande, popularisation of astronomy, ; revolving stars, ; _histoire céleste_, , ; nature of sun-spots, ; observations of neptune, lambert, solar motion, ; construction of the universe, , ; missing planets, lamont, magnetic period, , lamp, ashen light on venus, langdon, mountains of venus, langley, solar granules, ; corona of , ; spectroscopic effects of solar rotation, ; infra-red spectrum, , , ; experiments at pittsburg, ; bolometer, ; distribution of energy in spectrum, , ; atmospheric absorption, , , ; solar constant, ; lunar heat-spectrum, ; temperature of lunar surface, ; age of the sun, laplace, lunar acceleration, , ; _système du monde_, ; nebular hypothesis, , , , , , ; stability of saturn's rings, , ; solar atmosphere, , ; lexell's comet, , ; solar distance by lunar theory, ; origin of meteors, ; of comets, lassell, discovery of neptune's satellite, ; of hyperion, ; saturn's dusky ring, ; observations at malta, , ; reflectors, ; equatoreal mounting, latitude, variation of, , laugier, period of comet, ; solar rotation, le chatelier, temperature of the sun, lescarbault, pseudo-discovery of vulcan, ; halo round venus, lespiault, orbits of minor planets, le sueur, spectrum of jupiter, leverrier, discovery of neptune, - ; lexell's comet, , ; distance of the sun, , ; revolutions of mercury, ; supposed transits of vulcan, ; mass of asteroids, ; orbit of november meteors, ; perseids and leonids, lexell, comet of , , , liais, supposed transit of vulcan, ; comet of , ; division of a comet, librations, of mercury, ; of venus, ; of the moon, lick, foundation of observatory, light, velocity, , , ; extinction in space, ; refrangibility changed by movement, light-equation, , ligondès, development of solar system, lindsay, lord, expedition to mauritius, line of sight, movements in, , ; of solar limbs, , ; in prominences, , ; of stars, , , ; binaries detected by, - listing, dimensions of the globe, littrow, chromosphere, ; sun-spot periodicity, liveing and dewar, carbon in the sun, lockyer, solar spectroscopy, , ; theory of sun-spots, , ; daylight observations of prominences, , , , ; eclipse of , ; slitless spectroscope, ; corona of , ; glare theory of corona, ; eclipse of , ; chromospheric spectrum, ; classification of prominences, ; their radial movements, ; celestial dissociation, - ; chemistry of sun-spots, ; spots on mars, ; meteoritic hypothesis, , ; equatoreal coudé, loewy, constant of aberration, , ; lunar photographs, ; director of paris observatory, ; equatoreal coudé, , lohrmann, lunar chart, ; linné, lohse, j. g., spectrum of great comet, lohse, o., daylight coronal photography, _note_; spectral investigations, ; twilight on venus, ; red spot on jupiter, ; periodicity of jupiter's markings, ; motion of sirius, ; spectrum of nova cygni, louville, nature of corona, ; chromosphere, lowell, rotation of mercury, ; of venus, ; markings on venus, ; observations of mars, , ; satellites, lyman, atmosphere of venus, mcclean, photographs of solar spectrum, , ; helium stars, ; oxygen stars, ; equipment of cape observatory, macdonnell, luminous ring round venus, maclaurin, eclipse of , maclear, admiral, observations during eclipses, , maclear, sir thomas, maximum of eta carinæ, ; observation of halley's comet, mädler, central sun, ; observations of venus, ; lunar rills, ; aspect of linné, ; common proper motions, magellanic clouds, , ; spiral character, magnetism, terrestrial, international observations, ; periodicity, , ; solar relations, , , , , ; lunar influence, mann, last observation of donati's comet, maraldi, solar corona, ; rotation of mars, ; satellite-transits on jupiter, ; spot on jupiter, marius, andromeda nebula, ; sun-spots, mars, oppositions, ; solar parallax from, , , ; polar spots, , , , , ; permanent markings, - ; rotation, , ; atmosphere, , ; climate, , ; canals, - ; photographs, ; satellites, , , , , marth, revolutions of neptune's satellite, maskelyne, components of castor, ; astronomer-royal, ; experiment at schehallien, ; comets and meteors, maunder, photographs of corona of , ; comparative massiveness of stars, ; constitution of nebulæ, maunder, mrs., coronal photographs, , maury, director of naval observatory, ; duplication of biela's comet, maury, miss a. c., spectrographic investigations, ; discoveries of spectroscopic binaries, , maxwell, j. clerk, structure of saturn's rings, , mayer, c., star satellites, mayer, julius r., tidal friction, ; meteoric sustentation of sun's heat, mayer, tobias, stellar motions, ; solar translation, ; repeating circle, ; solar distance, ; satellite of venus, ; lunar surface, mazapil meteorite, meldrum, sun-spots and cyclones, melloni, lunar heat, melvill, spectra of flames, mercury, mass, ; luminous phenomena during transits, , ; spectrum, ; mountainous conformation, , ; rotation, , ; theory of movements, , mersenne, reflecting telescope, messier, catalogue of nebulæ, meteoric hypothesis of solar sustentation, ; of planetary formation, meteoritic hypothesis of cosmical constitution, , meteors, origin, , ; relations to comets, , - , ; leonids, - , ; perseids, , , , ; andromedes, - ; stationary radiants, meunier, canals of mars, meyer, divisions of saturn's rings, ; comet of , ; cometary refraction, ; comet tewfik, michell, double stars, ; torsion balance, ; star systems, michelson, velocity of light, milky way, grindstone theory, ; clustering power, , ; structure, , , , , - ; centre of gravity, , ; frequented by wolf-rayet, temporary, and helium stars, , , ; by gaseous nebulæ, ; drawings and photographs, , miller, w. a., spectrum analysis, , , ; stellar chemistry, mira, light changes, ; spectrum, , mitchel, lectures at cincinnati, mitchell, photograph of reversing layer, möller, theory of faye's comet, mohn, origin of comets, moll, transit of mercury, monck, perseid meteors, ; new stars, moon, acceleration, , , ; magnetic influence, ; photographs, , , ; solar parallax from disturbed motion, , ; study of surface, ; atmosphere, - ; charts, - ; librations, ; superficial changes, , ; thermal radiations, , ; rotation, ; tables, , ; origin, - morinus, celestial chemistry, morstadt, andromede meteors, mouchez, photographic survey of the heavens, ; death, müller, phases of mercury, ; of minor planets, ; albedo of mars, ; of jupiter, ; of saturn, ; variability of neptune, ; of pons's comet, ; stellar photometry, munich, optical institute, , myer, solar eclipse, nasmyth, lassell's reflector, ; solar willow-leaves, ; comparative lustre of mercury and venus, ; condition of jupiter, nasmyth and carpenter, _the moon_, nebula, andromeda, early observations, ; new star in, , ; photographs, , ; structure, ; spectrum, , ; visibility at arequipa, nebula, orion, observed by herschel, ; mentioned by cysatus, ; apparent resolvability, ; suspected variability, ; radial movement, ; spectrum, ; photographs, , , nebulæ, first discoveries, ; catalogues, , , , ; distribution, , , ; composition, , , , ; resolution, , , ; double, , ; spiral, , , ; new stars in, - , , ; spectra, - , ; variability, , ; radial movements, ; photographs, - , nebular hypothesis, herschel's, , ; laplace's, , , , ; objections, - neison, atmosphere of venus, ; rills on the moon, ; _the moon_, neptune, discovery, - ; satellite, , ; density, ; comets captured by, , , ; mode of rotation, , , , newall, f., duplicity of capella, ; stellar radial motions, newall, r. s., -inch refractor, newcomb, runaway stars, ; solar translation, ; origin of minor planets, ; telescopic powers, ; corona of , ; of , ; distance of the sun, - ; velocity of light, ; variation of latitude, ; lunar atmosphere, ; lunar theory, , ; disturbance of neptune's satellite, ; formation of planets, ; star catalogue, ; structure of milky way, newton, h. a., capture of comets by planets, ; falls of aerolites, ; november meteors, , ; meteors of , , ; orbits of aerolites, newton, sir isaac, founder of theoretical astronomy, , ; comets subject to gravitation, ; first speculum, ; solar radiations, ; law of cooling, - ; telescopes and atmosphere, niesten, volume of asteroids, ; red spot on jupiter, nobert, diffraction gratings, noble, observations of mercury, ; secondary tail of comet, nolan, origin of the moon, ; period of phobos, norton, expulsion theory of solar appendages, _note_; comets' tails, , nova andromedæ, , nova aurigæ, - nova cygni, , , nova persei, , nutation, discovered by bradley, , ; a uranographical correction, nyrén, constant of aberration, observatory, greenwich, , , ; cape of good hope, , , ; paramatta, , ; harvard college, , ; königsberg, ; dorpat, ; pulkowa, ; palermo, ; berlin, ; anclam, ; potsdam, ; kew, ; arequipa, , , ; yerkes, ; lick, occultations of stars by comets, , , ; by the moon, ; by mars, ; of jupiter by the moon, olbers, bessel's first patron, , ; discoveries of minor planets, , ; origin by explosion, , ; career, , ; biela's comet, ; comet of , ; electrical theory of comets, , , , ; multiple tails, ; comet of , ; cometary coruscations, ; november meteors, olmsted, radiant of leonids, ; orbit, oppenheim, calculation of schmidt's comet, oppolzer, e. von, theory of sun-spots, ; variability of eros, oppolzer, th. von, winnecke's comet, ; comet of , oxygen in sun, - ; telluric absorption, ; in stars, packer, variable stars in cluster, palisa, search for vulcan, , ; discoveries of minor planets, pallas, discovery, ; inclination of orbit, , ; diameter, , , pape, donati's comet, parallax, annual, of stars, , , , , - ; horizontal, of sun, ; encke's result, , ; improved values from oppositions of mars, , ; from light velocity, , , ; from recent transits, , ; from observations of minor planets, , ; general result, paris catalogue of stars, paschen, oxygen in sun, ; solar temperature, pastorff, drawings of the sun, peirce, structure of saturn's rings, perrine, eclipse photographs, ; nature of corona, ; observation of holmes's comet, ; nebula round nova persei, perrotin, rotation of venus, ; markings on, ; canals of mars, ; clouds on mars, ; striation of saturn's rings, ; rotation and compression of uranus, , ; changes of pons's comet, ; maia nebula, ; measures of double stars, perry, eclipse of december, , personal equation, , peter, star-parallaxes, peters, c. a. f., parallax of cygni, ; disturbed motion of sirius, peters, c. f. w., orbit of leonid meteors and comet, peters, c. h. f., sun-spot observations, , ; discoveries of minor planets, ; star maps, , peytal, description of chromosphere, phobos, rapid revolution, , , ; tidal relations, , photography, solar, , , , ; of corona, , , , , , - ; without an eclipse, - ; of prominences, , , ; of coronal spectrum, , , ; of prominence-spectrum, , ; of arc-spectrum, , ; of solar spectrum, , , , , ; of uranian spectrum, ; of cometary spectra, , ; of stellar and nebular spectra, - , , , , ; lunar, , , ; detection of comets by, , , ; of asteroids, ; of new stars, ; use of, in transits of venus, , , ; mars depicted by, , ; jupiter, , ; comets, , , , ; nebulæ, , , - , , ; milky way, , ; star-charting by, , ; star-parallaxes by, ; rapid improvement, photometry, stellar, , , ; of planetary phases, , ; of saturn's rings, ; photographic, photosphere, named by schröter, ; structure, , , , piazzi, star catalogues, ; parallaxes, ; motion of cygni, ; birth and training, ; -foot circle, , ; discovery of ceres, , picard, saturn's dark ring, ; sun's distance, pickering, e. c., photometric measures of martian satellites, ; of minor planets, ; variability of japetus, ; of neptune, ; meteoric photography, ; gaseous stars, ; hydrogen spectrum in stars ; spectrographic results, ; eclipses of algol, ; photographic celestial surveys, ; star density in pleiades, ; photometric catalogues, , ; photographic photometry, ; white stars in milky way, ; climate of arequipa, ; horizontal telescope, pickering, w. h., corona of , ; coronal photographs, january , , ; lunar twilight, ; lunar volcanic action, ; melting of snow on mars, ; martian snowfall, ; jupiter's satellites, ; photographs of comets, ; of orion nebula, ; observatory at arequipa, pingré, phenomena of comets, , planets, influence on sun-spots, ; periods and distances, ; intra-mercurian, - ; inferior and superior, ; trans-neptunian, , ; origin, , ; relative ages, , planets, minor, existence inferred, , ; discoveries, - , , , ; solar parallax from, - ; distribution of orbits, , ; collective volume, ; atmospheres, plantade, halo round mercury, pleiades, community of movement near, ; photographed spectra, ; measurements, ; photographs, , ; nebulæ, , plücker, hydrogen in sun, plummer, solar translation, ; encke's comet, plutarch, solar corona, pogson, prominence spectrum, ; reversing layer, ; discovery of a comet, , ; new star in cluster, pond, errors of greenwich quadrant, ; controversy with brinkley, pons, discoveries of comets, , , pontécoulant, return of halley's comet, poor, c. lane, calculation of lexell's comet, porter, solar translation, pouillet, solar constant, , ; temperature of the sun, ; of space, poynting, mean density of the earth, prince, glow round venus, pritchard, parallax of beta aurigæ, ; photographic determinations of stellar parallax, ; photometric catalogue, pritchett, corona of january, , ; red spot on jupiter, proctor, glare theory of corona, ; speed of ejections from sun, ; transit of venus, ; distance of sun, ; atmosphere of venus, ; rotation of mars, ; map and canals of mars, , ; condition of great planets, ; nova andromedæ, ; status of nebulæ, , ; structure of milky way, ; star drift, procyon, satellite, ; parallax, prominences, observed in , , , ; described by vassenius, ; observed in , ; photographed during eclipse, , , ; without eclipse, , ; spectrum, , , , , , ; spectroscopic method of observing, - , - ; white, , ; chemistry, , ; classification, ; distribution, ; movements in, - ; heat of development, quetelet, periodicity of august meteors, ranyard, drawing of sun-spot, ; coronal types, , ; lunar atmosphere, ; jupiter's markings, ; meteors from fixed radiants, ; cometary trains, ; tenuity of nebulæ, rayet, spectrum of prominences, , red spot on jupiter, , reduction of observations, ; bessel's improvements, , ; baily's, refraction, atmospheric, ; effects looked for in comets, , ; cytherean, , , ; lunar , reichenbach, foundation of optical institute, , , repsold, astronomical circles, , ; cape heliometer, resisting medium, , , respighi, slitless spectroscope, ; prominences and chromosphere, , , ; solar uprushes, ; spectrum of gamma argûs, reversing layer, detected, , ; photographed, , ; depth, riccioli, secondary light of venus, riccò, trials with coronagraph, ; distribution of prominences, ; spectrum of venus, ; spot on jupiter, ; spectrum of great comet, richer, distance of the sun, ristenpart, solar translation, ritchey, nebula round nova persei, ; photographs of nebulæ, ritter, development of stars, roberts, a. w., southern variables, roberts, isaac, search for ultra-neptunian planet, ; photographs of orion nebula, ; of andromeda nebula, ; of the pleiades, roberval, structure of saturn's rings, robinson, reflectors and refractors, roche, inner limit of satellite-formation, ; modification of nebular hypothesis, römer, star places, ; invention of equatoreal and transit instrument, ; of altazimuth, ; velocity of light, ; satellite transit on jupiter, rosenberger, return of halley's comet, rosetti, temperature of the sun, rosse, third earl of, biographical sketch, ; great specula, - ; discovery of spiral nebulæ, ; resolution of nebulæ, ; climate and telescopes, rosse, fourth earl of, experiments on lunar heat, rost, nature of sun-spots, roszel, mass of asteroids, rowland, photographic maps of solar spectrum, , ; elements in run, ; concave gratings, , rümker, observation of encke's comet, russell, h. c., red spot on jupiter, ; change in argo nebula, ; photographs of nubeculæ, russell, h. n., atmosphere of venus, rutherfurd, lunar photography, ; star spectra, ; photographs of the pleiades, ; diffraction gratings, sabine, magnetic and sun-spot periods, , , safarik, secondary light of venus, ; compression of uranus, satellites, discoveries, , , ; transits, , ; variability, , ; origin, , saturn, low specific gravity, ; rotation, ; spectrum, saturn's rings, first disclosure, ; dusky ring, ; stability, , ; meteoric constitution, ; eventual dispersal, savary, orbits of double stars, savélieff, solar radiation, , sawerthal, discovery of a comet, schaeberle, discovery of procyon's satellite, ; coronal photographs, , ; theory of corona, ; meteoric photography, ; discovery of a comet, schaeberle and campbell, observations of jupiter's satellites, scheiner, father, nature of sun-spots, , ; equatoreal instrument, _note_; solar rotation, ; darkening of sun's limb, schiener, dr. j., photospheric structure, ; spectrographic researches, , ; spectrum of andromeda nebula, ; stars and nebulæ in orion, schiaparelli, rotation of mercury, ; of venus, , ; spots on mars, ; snow-cap, ; canals, - ; compression of uranus, ; comets and meteors, , , , ; anomalous tail of great comet, ; pons's comet, ; origin of comets, ; measures of double stars, schmidt, a., circular refraction in sun, schmidt, j., sun-spot period, ; lunar rills, ; lunar maps, ; disappearance of linné, ; cometary appendages, ; new stars, schönfeld, extension of bonn durchmusterung, , schrader, construction of reflectors, schröter, a follower of herschel, ; motions of sun-spots, ; biographical sketch, , ; observations on mercury, , , ; on venus, - , ; on the moon, ; a lunar city, ; linné, ; spots on mars, ; jovian markings, schülen, perspective effects in sun-spots, schuster, photographs of corona, , ; spectra of oxygen, schwabe, sun-spot periodicity, , secchi, chromosphere, ; biela's comet, ; cyclonic movements in sun-spots, ; distribution, ; profundity, ; nature, , ; constitution of photosphere, ; eclipse observations, , ; reversing layer, ; observations of prominences, , , ; absence of helium absorption, ; temperature of the sun, ; solar atmospheric absorption, ; martian canals, ; spectrum of uranus, ; of coggia's comet, ; stellar spectral researches, , ; carbon stars, , ; gaseous stars, see, stellar orbits, , ; measures of neptune, ; measures of uranus, ; belts of neptune, ; colour of sirius, _note_; southern double stars, ; evolution of stellar systems, seeliger, photometry of saturn's rings, ; rationale of new stars, seidel, stellar photometry, sherman, spectrum of nova andromedæ, short, reflectors, , , , ; chromosphere, ; satellite of venus, ; striation of saturn's rings, sidereal science, foundation, , ; condition in , ; progress, sidgreaves, spots and faculæ, siemens, regenerative theory of the sun, simony, photographs of ultra-violet spectrum, sirius, a binary star, ; mass, ; parallax, , ; spectrum, , , ; former redness, _note_; radial movement, , smyth, admiral, donati's comet, smyth, piazzi, oxygen spectrum, ; lunar radiations, ; expedition to teneriffe, solar constant, , solar spectrum, fixed lines in, - ; maps, , , , , , , ; distribution of energy, , solar system, translation through space, , , , ; development, , , - , ; complexity, soret, solar temperature, south, observations of double stars, ; -inch lens, ; rosse reflector, ; occultation by mars, spectroscopic binaries, - spectrum analysis, defined, ; first experiments, , ; applied to the sun, - , ; to the stars, , , ; kirchhoff's theorem, ; elementary principles, , ; effects on science, , ; radial motion determined by, , ; investigations of comets by, , ; of new stars, , ; of nebulæ, - spencer, position of nebulæ, spitaler, attendants on brooks's comet, spitta, transits of jupiter's satellites, spörer, solar rotation, , ; chromosphere, , stannyan, early notice of chromosphere, star catalogues, , , , , , ; spectroscopic, , , ; photographic, - ; photometric, , star-drift, star-gauging, , , star-maps, , , , , , ; photographic, , stars, movements, , , , , , ; radial, , , ; comparative brightness, , , , , ; distances, - , - ; chemistry, , , ; spectroscopic orders, ; colours, ; development, - ; actual magnitudes, ; gregarious, stars, double, physical connection surmised, ; proved, , ; masses, , ; catalogues, , , , , , ; orbits, , ; discoveries, , , , , , ; photographs, ; evolution, stars, gaseous, - stars, temporary, , - stars, variable, early discoveries, ; eta carinæ, , , ; sun-spot analogy, , ; spectra, ; algol class, , ; catalogues, , stefan, law of cooling, steinheil, stellar photometry, ; silvered glass reflectors, stewart, balfour, kirchhoff's principle, _note_; solar investigations, , stewart, matthew, solar distance by lunar theory, stokes, prevision of spectrum analysis, stone, e. j., reversal of fraunhofer spectrum, ; distance of the sun, , , ; transit of venus, ; cape catalogue, ; proper motions, stone, o., star catalogues, ; measures of double stars, stoney, carbon in photosphere, ; dynamical theory of planetary atmospheres, ; perturbations of leonids, ; status of red stars, stratonoff, star counts in pleiades, stroobant, satellite of venus, struve, f. g. w., stellar parallax, ; career and investigations, - ; occultation by halley's comet, ; russo-scandinavian arc, , struve, ludwig, solar translation, struve, otto, parallax of eta cassiopeiæ, ; solar velocity, ; his father's successor at pulkowa, ; eclipse of , , ; neptune's satellite, ; research on saturn's rings, , ; variable nebula, stumpe, solar translation, sun, herschel's theory, - , , ; atmospheric circulation, , ; chemical composition, , - ; mode of rotation, , ; kirchhoff's theory, ; faye's, - ; convection currents in, , , ; dissociation, , - ; luminous outbursts, - ; explosions, ; heat emission, , , , , , ; temperature, - , ; problem of distance, ; results from transits, , , , ; from oppositions of mars, , ; from light-velocity, , ; from measurements of minor planets, ; concluded value, ; maintenance of heat supply, - ; past and future duration, sun-spots, speculations regarding, , ; wilson's demonstration, , ; distribution, , , ; cyclonic aspect, , , , ; periodicity, , , , ; magnetic relations, , , ; meteorological, , ; auroral, , , , ; photographs, , ; level, ; spectra, , , ; volcanic hypothesis, ; lockyer's rationale, ; planetary influence, ; relation to jovian markings, swan, chromosphere, ; sodium line, swift, e., discovery of a comet, swift, l., fallacious glimpse of vulcan, , ; discovery of a comet, tacchini, eclipse of , ; white prominences, ; prominences and chromosphere, , ; spectrum of venus, talbot, fox, spectrum analysis, ; spectroscopic method of determining stellar orbits, tarde, nature of sun-spots, taylor, eclipse expedition, ; spectrum of uranus, ; achromatic lenses, tebbutt, comets discovered by, , ; comet of , telescopes, achromatic, , , telescopes, equatoreal, , , telescopes, reflecting, short's, , , , ; herschel's, , - ; lassell's, , , ; varieties of construction, , ; rosse's, - , ; common's, , , telescopes, refracting, fraunhofer's, , , ; clark's, , , , , ; grubb's, , ; with bent and horizontal mountings, - tempel, red spot on jupiter, ; comet discoveries, ; cometary observations, , ; andromeda nebula, ; discovery of merope nebula, temperature, of the sun, - , ; of the moon, , ; of space, ; on mars, tennant, eclipse observations, , , terby, surface of mars, , , ; secondary tail of comet, thalén, basic lines, ; map of solar spectrum, ; solar elements, thollon, line-displacements by motion, , ; atlas of solar spectrum, , ; lunar atmospheric absorption, thome, comet discovered by, thomson, sir william (lord kelvin), solar chemistry, ; magnetic influence of the sun, ; tidal strains, ; rotation of the earth, ; dynamical theory of solar heat, , thraen, period of wells's comet, tidal friction, effects on moon's rotation, , , ; month lengthened by, , ; influence on planets, - ; on development of binary systems, tietjen, asteroidal orbits, tisserand, capture of comets, ; lunar acceleration, ; revolutions of neptune's satellite, ; stationary radiants, ; perturbations of algol, ; director of paris observatory, titius, law of planetary intervals, , , todd, eclipse of , ; solar distance, , ; trans-neptunian planet, tornaghi, halo round venus, transit instrument, trépied, reversal of fraunhofer spectrum, troughton, method of graduation, trouvelot, veiled spots, ; chromosphere in , ; intra-mercurian planets, , ; observations of prominences, , , ; of mercury, , ; rotation of venus, ; red spot on jupiter, trowbridge and hutchins, carbon in sun, tschermak, origin of meteorites, tupman, transit expedition, ; results, turner, polariscopic coronal photography, ; employment of coelostat, , ; stationary radiants, ulloa, eclipse of , united states, observatories founded in, , uranus, discovery, , , ; unexplained disturbances, , , ; satellites, , ; equatoreal markings, , ; spectrum, , ; retrograde rotation, , , valerius, darkening of sun's limb, vassenius, description of prominences, venus, transits, , , ; of , - ; of , , ; atmosphere, , , ; mountains, , ; spectrum, ; albedo, ; ashen light, , ; pseudo-satellite, ; effects upon, of solar tidal friction, very, temperature of sun, ; lunar heat, vesta, discovery, , ; diameter, ; spectrum, vicaire, solar temperature, vico, comet discovered by, ; rotation of venus, ; cytherean mountain, violle, solar temperature, , ; solar constant, vogel, h. c., solar rotation, ; solar atmospheric absorption, , ; spectrum of mercury, ; of venus, ; of vesta, ; of jupiter, ; of jupiter's satellites, ; of uranus, ; rotation of venus, ; ashen light, ; intrinsic light of jupiter, ; cometary spectra, , , , ; carbon in stars, ; stellar development, , ; spectrum of gamma cassiopeiæ, ; of nova cygni, ; of nova andromedæ, ; spectroscopic star catalogue, ; radial motion of sirius, ; period of mizar, ; eclipses of algol, ; components of nova aurigæ, ; spectrographic determinations of radial motion, , vogel, h. w., spectrum of hydrogen, _note_, vulcan, existence predicted, ; pseudo-discoveries, , wadsworth, coronal photography, ward, nova andromedæ, waterston, solar temperature, ; meteoric infalls, watson, fallacious observations of vulcan, , ; asteroidal discoveries, webb, comet of , weber, baily's beads, ; illusory transit of vulcan, weinek, study of lunar photographs, weiss, comets and meteors, , wells, comet discovered by, wesley, drawings of corona, wheatstone, spectrum of electric arc, ; method of ascertaining light-velocity, whewell, stars and nebulæ, williams, a. stanley, canals of mars, ; markings on jupiter, , ; rotation, ; nova persei, wilsing, solar rotation from faculæ, ; density of the earth, ; system of, cygni, wilson, alexander, perspective effects in sun-spots, , wilson, h. c., red spot on jupiter, ; compression of uranus, ; exterior nebulosities of pleiades, wilson, w. e., solar temperature, , ; ultra-neptunian planets, winnecke, comet discovered by, ; distance of the sun, ; donati's comet, , wisniewski, last glimpse of comet, witt, discovery of eros, wolf, c., objections to faye's cosmogony, ; origin of phobos, wolf, max, photographic discoveries of minor planets, , ; nova andromedæ, ; nova aurigæ, ; nebula near nova persei, ; photographic nebular survey, ; galactic nebulosity, wolf, r., sun-spot and magnetic periodicity, , , ; analogy of variable stars, , ; auroræ, ; suspicious transits, wollaston, ratio of moonlight to sunlight, ; flame spectra, ; lines in solar spectrum, woods, coronal photography, , ; cape durchmusterung, wrangel, auroræ and meteors, wright, g. f., ice age in north america, wright, thomas, theory of milky way, ; structure of saturn's rings, wright, w. h., polarisation of cometary light, ; spectrum of nebulæ, yerkes, donation of a telescope, young, miss anne, nebular hypothesis, young, c. a., spectrum of sun-spots, ; origin, ; spectrum of corona, , ; detection of reversing layer, , ; prominences and chromosphere, - , ; photograph of a prominence, ; spectroscopic measurement of sun's rotation, ; solar cyclones and explosions, , ; basic lines, ; spectrum of venus, ; red spot on jupiter, ; observations of uranus, , ; andromedes of , ; spectrum of tebbutt's comet, ; of nova andromedæ, young, thomas, absorption spectra, zach, baron von, promotion of astronomy, , , ; baily's beads, ; search for missing planet, ; rediscovery of ceres, ; use of a heliostat, zantedeschi, lines in solar spectrum, ; lunar radiation, zenger, observations on venus, , zenker, cometary tails, zezioli, observation of andromedes, zodiacal light, relation to medium of space, ; to solar corona, ; meteoric constitution, zöllner, electrical theory of comets, , , , ; solar constitution, ; observations of prominences, , ; reversion spectroscope, ; solar temperature, ; mercurian phases, ; albedo of venus, ; of jupiter, ; of saturn, ; of uranus, ; condition of venus, ; of great planets, ; jovian markings, ; ages of stars, ; polarising photometer, , the end billing and sons, ltd., printers, guildford the system of the stars by agnes m. clerke hon. member of the royal astronomical society; author of "history of astronomy during the nineteenth century" and "problems in astrophysics" _second edition_ _demy vo. cloth. with many illustrations. *price s. net.*_ (post free, price * s. d.*) from the preface fifteen years have elapsed since the original publication of the present work; and fifteen years count as a long spell of time where sidereal research is in question. in preparing the second edition, accordingly, i have introduced extensive modifications. considerable sections of the book have been recast, and all have been thoroughly revised. new chapters have been inserted, old ones have been in large part suppressed. drastic measures of reform have, in short, been adopted, with results that certainly import progress and (it is hoped) constitute improvements. most of the illustrations are entirely new; and i am under great obligations for the use of valuable photographs and drawings, among others, to sir. david gill, f.r.s., to professor hale and the university press of chicago, to the rev. w. sidgreaves, s.j., to professors e. c. pickering, campbell, barnard, and frost, and to dr. max wolf of heidelberg. "the work was admirable from the first, imparting the best knowledge of a decade and a half ago; now it retains its high quality by incorporating the newer knowledge."--_the guardian_. "it has the remarkable feature of combining extraordinary profusion of precise information with an elegance of literary style quite unusual in scientific authors."--_the academy_. published by adam & charles black . soho square . london by the same author problems in astrophysics demy vo. pages. cloth. containing illustrations in the text and plates price s. net (post free, price s. d.) note "this is emphatically a "new century" book. it aims at stimulating progress along lines carefully marked out as immediately practicable. the same author's "history of astronomy" is a survey of the past; "problems in astrophysics" looks to the future. what we already know is regarded in it as means to the end of augmenting knowledge. astrophysics is a science still at the outset of a magnificent career. its ways are beset with claimants for its attention. there is often much difficulty in choosing between them, yet rapidity of progress depends upon prudence in selection. many hints for its guidance are accordingly offered in the present work, which deals, so far as possible, with answerable questions. it should, then, find its way into the hands of every astronomer who desires to keep up with the drift of thought, and to be informed of the prospects of work and discovery in the various departments of research connected with the physics of the heavenly bodies." *some press opinions* "the book shows every sign of profound and careful study, and the sense of scientific imagination, which is one of the greatest means of independent discovery."--_st. james's gazette._ "the book is written with all the charm that has characterised the authoress's previous volumes, and contains a wealth of information and suggestion for work yet to be accomplished which will appeal to all who are interested in the problems of the universe."--_daily telegraph._ "we feel that miss clerke has earned, and will surely receive, the admiration and gratitude of astronomers for this new proof of her devotion to their science."--_the times._ modern cosmogonies crown vo. bound in cloth. price s. d. net (post free, price s. d.) note this volume contains a series of brief and attractive studies on current theories regarding the origin and history of the visible universe. the difficulties besetting cosmical doctrines of evolution are pointed out in them, and the expedients are described by which those difficulties have been met, though not wholly overcome. the widened possibilities connected with the new science of radiology, the unification of the physical forces that may ensue upon further discoveries concerning electrical action, the function in the world of the impalpable ether, the nature of gravity, are in turn discussed or adverted to; while the final chapter takes into consideration the crowning problem of life. "these sixteen chapters are short studies on modern theories about the origin and mystery of the universe, by an astronomer whose writings have done much to help and popularise that science."--_the times._ published by adam & charles black . soho square . london * * * * * transcriber's notes: original page line original text left as is (sic) --- ---------- -------------------------------------------------- the search for it, through confessedly scarcely the first description are tranquil page line original text replaced with --- ---------- ----------------------- --------------------------- footnote xviiie xviii^e (the e is superscript) byeways byways concentation concentration is appears from it appears from appearances seem by him appearances seen by him ecole militaire École militaire forgotton forgotten footnote / / (confirmed by looking up reference quoted) phenenoma phenomena bredikhin brédikhine identifiying identifying terrestial terrestrial appearence appearance bloodvessel blood vessel angström Ångström undimished undiminished sympton symptom familar familiar photograpic photographic by which i structure by which its structure bredikhine brédikhine stata strata - of its orbit hours of its orbit in hours seconds. seconds garden at its seasons garden as its seasons throngh through oparator operator recognised. in a recognised in a footnote applie applied the gaseous fields o the gaseous fields of relatiouship relationship footnote optice optics ofter some years after some years footnote (two references given, (split into two footnotes, within a single footnote. and corrected references in the text footnote in the text) used twice) conclusion of a conclusion of a spectographically spectrographically spectographic spectrographic lyrae lyræ index wolf, r., sun-spot and wolf, r., sun-spot and magnetic periodicity, magnetic periodicity, , , ; , , ; remarks concerning stones said to have fallen from the clouds, both in these days, and in antient times. by edward king, esq. f. r. s. and f. a. s. res ubi plurimum proficere, et valere possunt, collocari debent. cicero de orat. . london: printed for g. nicol, bookseller to his majesty, pall-mall. . [illustration: f. . f. . f. .] _an attempt to account for the production of a shower of stones, that fell in tuscany, on the th of june, ; and to shew that there are traces of similar events having taken place, in the highest ages of antiquity. in the course of which detail is also inserted, an account of an extraordinary hail-stone, that fell, with many others, in cornwall, on the th of october, ._ having received this last winter, from sir charles blagden, some very curious _manuscript_ accounts, concerning a surprising shower of stones; which is said, on the testimony of several persons, to have fallen in tuscany, on the th of june, ;--and having also perused, with much attention, a very interesting pamphlet, written in italian, by _abbate ambrose soldani_, professor of mathematics, in the university of siena, containing an extraordinary and full detail of such facts as could be collected relating to this shower; the whole has appeared to me to afford such an ample field for philosophical contemplation, and also for the illustration of antient historic facts; that (leaving the whole to rest upon such testimony as the learned professor has already collected together; and to be supported by such further corroboration, as i am informed is likely _soon_ to arrive in england,) i cannot but think it doing some service to the cause of literature, and science, to give to the world, in the earliest instance, a short abridgement of the substance of the whole of the information; expressed in the most concise and plainest language, in which it is possible for me to convey a full and exact idea of the phænomenon. it may be of some use, and afford satisfaction to several curious persons, to find the whole here compressed in so small a compass. and, as i shall add my own conclusions without reserve; because the whole of the phænomenon tends greatly to confirm some ideas which i had previously been led to form, many years ago, concerning the consolidation of certain species of stone; it may open a door for further curious investigation. and it may at least amuse, if not instruct; whilst i add a short detail of uncommon facts, recorded in antient history, and tending to shew clearly, that we are not without precedents of _similar events_ having happened, in the early ages of antiquity. on the th of june, , a tremendous cloud was seen in tuscany, near siena, and radacofani; coming from the north, about seven o'clock in the evening;--sending forth sparks, like rockets;--throwing out smoke like a furnace;--rendering violent explosions, and blasts, more like those of cannon, and of numerous muskets, than like thunder;--and casting down to the ground hot stones:--whilst the lightning that issued from the cloud was remarkably red; and moved with _less_ velocity than usual. the cloud appeared of different shapes; to persons in different situations; and remained suspended a long time: but every where was plainly seen to be burning, and smoking like a furnace. and its original height, from a variety of circumstances put together, seems to have been much above the common region of the clouds. the testimony, concerning the falling of the stones from it, appears to be almost unquestionable:--and is, evidently, from different persons, who had no communication with each other. for first; the fall of four stones is precisely ascertained: one of which was of an irregular figure, with a point like that of a diamond;--weighed five pounds and an half;--and had a vitriolic smell.--and another weighed three pounds and an half;--was black on the outside, as if from smoke;--and, internally, seemed composed of matter of the colour of ashes;--in which were perceived small spots of metals, of gold and silver. and, besides these, professor soldani of siena, was shewn about fifteen others: the surfaces of which were glazed black, like a sort of varnish;--resisted acids;--and were too hard to be scratched with the point of a penknife. signior _andrew montauli_, who saw the cloud, as he was travelling, described it as appearing much above the common region of the clouds; and as being clearly discerned to be on fire;--and becoming white, by degrees; not only where it had a communication, by a sort of stream of smoke and lightning, with a neighbouring similar cloud: but also, at last, in two-third parts of its whole mass, which was originally black. and yet he took notice, that it was not affected by the rays of the sun, though they shone full on its lower parts.--and he could discern as it were the bason of a fiery furnace, in the cloud, having a whirling motion. this curious observer gives an account also, of a stone, which he was assured fell from the cloud, at the feet of a farmer; and was dug out of the ground, into which it had penetrated.--and he says, that it was about five inches long, and four broad; nearly square; and polished: black on the surface, as if smoked; but within, like a sort of sand-stone, with various small particles of iron, and bright metallic stars. other stones are described by him; which were said to have fallen at the same time: were triangular; and terminated in a sort of (pyramidal) or conical figure.--and others were so small as to weigh not more than an ounce. professor soldani saw another stone, said to have fallen from the cloud, which had the figure of a parallelopiped, blunted at the angles; and was as it were varnished, on the outside, with a black crust; and quite unlike any stones whatever of the soil of the country where it had fallen. two ladies being at _cozone_, about miles from _siena_, saw a number of stones fall, with a great noise, in a neighbouring meadow: one of which, being soon after taken up by a young woman, burnt her hand: another burnt a countryman's hat: and a third was said to strike off the branch of a mulberry tree; and to cause the tree to wither. another stone, of about two ounces weight, fell near a girl watching sheep; a young person, whose veracity it is said could not be doubted.--this stone, the professor tells us, is also a parallelopiped, with the angles rounded; and its internal substance is like that of the others; only with more metallic spots; especially when viewed with a magnifying glass: and the black external crust appears to be minutely crystallized. many others, of a similar kind, were in the possession of different persons at siena. and besides the falling of these from the cloud, there is described to have been a fall of sand; seen by keepers of cattle near _cozone_, together with the falling of what appeared like squibs; and which proved afterwards to be stones, of the sort just described, weighing two or three ounces:--and some only a quarter of an ounce. amongst other stones that fell; was one weighing two pounds, and two ounces; which was also an oblong parallelopiped, with blunted angles, (as they are called, but which i think meant plainly prismatical terminations, and are said to have been about an inch in height;) and this was most remarkable for having, a small circle, or sort of belt round it, in one part; wherein the black crust appeared more smooth; and shining like glass; as if that part had suffered a greater degree of heat than the rest. another, also, was no less remarkable, for having many rounded cavities on its surface: as if the stone had been struck with small balls, whilst it was forming; and before it was hardened; which left their impressions.--and some appearances, of the same kind, were found on one of the four surfaces of another stone, in the possession of soldani. on minute examination, the professor found the stones were composed of blackish _crystals_, of different kinds; with metallic or pyritical spots, all united together by a kind of consolidated ashes.--and, on polishing them, they appeared to have a ground of a dark ash colour; intermixed with cubical blackish crystals, and shining pyritical specks, of a silver and gold colour. the conclusion which professor soldani evidently forms, is; _that the stones were generated in the air, by a combination of mineral substances, which had risen somewhere or other_, as exhalations, _from the earth_: but, as he seems to think, _not from_ vesuvius. the names of many persons, besides those already referred to, are mentioned; who were eye witnesses to the fall of the stones. and several _depositions_ were made, _in a regular juridical manner_, to ascertain the truth of the facts. the space of ground, within which the stones fell, was from three to four miles. the falling of them, was _the very day after_ the great eruption of vesuvius. and the distance of the place, from vesuvius, could not be less than two hundred miles, and seems to have been more. vesuvius is situated _to the south_ of the spot: and the cloud came _from the north_; about thirteen, or at most eighteen hours, after the eruption. now, putting all these circumstances together, i cannot but venture to form a conclusion, somewhat different from professor soldani's; though perfectly agreeing with his general principles. from a course of observations, and inquiries, which i have been led to pursue, for a great many years: tending to elucidate the history of extraneous fossils, and of the deluge; i have long been convinced, that stones in general, and strata of rocks, of all kinds, have been formed by _two_ very different operations of those elements, which the wisdom, and omnipotent hand of god, has ordained, and created. the one, by means of fire:--and the other, by means of water. and, of each sort, there are two subdivisions. of the stones, and rocks, formed by fire;--there are some, (besides lavas,) whose component parts, having been previously fused, and in a melted state, did merely cool, and harden _gradually_. and there are others; whose component parts, having been fused, and in a melted state, and having so become completely liquid; did instantly, by the operation of the powers of _attraction_, become crystallized. and, in like manner; of stones, and of strata of rocks, formed by means of water;--there are some, which having had their component parts brought together, in a fluid state; did then merely become gradually settled; and by the power of attraction, and the mixture of crystalline particles, were hardened by degrees. and there are others: which, having had their component parts, in like manner, brought together by water, did yet, on account of the peculiar nature, and more powerful _attraction_ of those parts, _instantly_ crystallize. and both of stones, and of strata of rocks, formed by fire; and of stones, and of strata of rocks formed by means of water; there are some such, as have been slowly consolidated by the first kind of operation; namely by the gradual cooling or settling of the substances; which yet do contain imbedded in them, crystals formed by the latter kind of operation. instances of which, we seem to have, in some granites, on the one hand;--and in some sorts of limestones on the other. to this i must add also; that there appear further, to have been some stones formed _by a sort of precipitation_: much in the same manner as _grew_ describes[a] the kernels, and stones of fruit to have been hardened. and i have met with many instances, wherein it appears unquestionably, that all these kind of processes in nature are going on continually: and that extraneous substances are actually inclosed, and _continually inclosing_, which could not be _antediluvian_; but must have been recent. to these short premises, i must beg leave to add; that in two papers formerly printed in the philosophical transactions,[b] i endeavoured, by some very remarkable instances, to prove, that iron, wherever it comes into combination with any substances that are tending to consolidation, _hastens the process exceedingly_;--and also renders the hardness of the body much greater. and i have also endeavoured, elsewhere,[c] to shew, in consequence of conclusions deduced from experiments of the most unquestionable authority, that _air_, in its various shapes and modifications, is indeed _itself_ the great consolidating fluid, out of which solid bodies are composed; and by means of which the various attractions take place, which form all the hard bodies, and visible substances upon earth. from all these premises then, it was impossible for me not to be led to conclude; that we have, in this august phænomenon of the fall of stones from the clouds, in tuscany, an obvious proof, as it were before our eyes, of the combined operation of those very powers, and processes, to which i have been alluding. it is well known; that pyrites, which are composed of iron, and sulphur, and other adventitious matter, when laid in heaps, and moistened, will take fire. it is also well known, that a mixture of pyrites of almost any kind, beaten small, and mixed with iron filings and water, when buried in the ground will take fire; and produce a sort of artificial volcano. and, surely then, wherever a vast quantity of such kind of matter should at any time become mixed together, as flying dust, or ashes; and be by any means condensed together, or compressed, the same effect might be produced, even in the atmosphere and air. instead, therefore, of having recourse to the supposition, of the cloud in tuscany having been produced by any other kind of exhalations from the earth; we may venture to believe, that an immense cloud of ashes, mixed with pyritical dust, and with numerous particles of iron, having been projected from vesuvius to a most prodigious height, became afterwards condensed in its descent;--took fire, both of itself, as well as by means of the electric fluid it contained;--produced many explosions;--melted the pyritical, and metallic, and argillaceous particles, of which the ashes were composed;--and, by this means, had a sudden crystallization, and consolidation of those particles taken place, which formed the stones of various sizes, that fell to the ground: _but did not harden the clayey ashes so rapidly as the metallic particles crystallized_; and, therefore, gave an opportunity for _impressions to be made_ on the surfaces of some of the stones, as they fell, by means of the impinging of the others. nor does it appear to me, to be any solid objection to this conclusion, either that vesuvius was so far distant; or that the cloud came from the north. for, if we examine sir william hamilton's account of the very eruption in question,[d] we shall find, that he had reason to conclude, that the _pine-like_ cloud of ashes projected from vesuvius, at one part of the time during this eruption, was twenty-five or thirty miles in height; and, if to this conclusion we add, not only that some ashes actually were carried to a greater distance than _two hundred miles_;[e] but that, when any substance is at a vast height in the atmosphere, a very small variation of the direction of its course, causes a most prodigious variation in the extent of the range of ground where it shall fall; (just as the least variation in the angle, at the vertex of an _isosceles_ triangle, causes a very great alteration in the extent of its base;) we may easily perceive, not only the possibility, but the probability, that the ashes in question, projected to so vast an height, were first carried even beyond _siena_ in tuscany, northward; and then brought back, by a contrary current of wind, in the direction in which they fell. sir william hamilton himself formed somewhat this sort of conclusion, on receiving the first intimation of this shower of stones from the earl of bristol.[f] i cannot therefore but allow my own conclusion to carry conviction with it to my own mind; and to send it forth into the world; as a ground, at least, for speculation, and reflection, to the minds of others. that ashes, and sand, and pyritical and sulphureous dust, mixed with metallic particles from volcanoes; fit for the instantaneous crystallization, and consolidation of such bodies as we have been describing, are often actually floating in the atmosphere, at incredible distances from volcanoes, and more frequently than the world are at all aware of, is manifest from several well attested facts. on the th of december, , captain _badily_, being in the gulph of volo, in the archipelago, riding at anchor, about ten o'clock at night, it began to rain _sand_ and _ashes_; and continued to do so till two o'clock the next morning. the ashes lay about two inches thick on the deck: so that they cast them overboard just as they had done snow the day before. there was no wind stirring, when the ashes fell: and yet this extraordinary shower was not confined merely to the place where _badily's_ ship was;[g] but, as it appeared afterwards, was extended so widely to other parts, that ships coming from _st. john d'acre_ to that port, being at the distance of _one hundred leagues_ from thence, were covered with the same sort of ashes. and no possible account could be given of them, except that they might come from vesuvius. on the d of october, , a ship belonging to a merchant of leith, bound for charles town, in carolina, being betwixt shetland and iceland, and about twenty-five leagues distant from the former, and therefore about three hundred miles from the latter, a shower of dust fell in the night upon the decks.[h] in october, , at _detroit_, in america, was a most surprising darkness, from day-break till four in the afternoon, during which time some rain falling, brought down, with the drops, sulphur and dirt; which rendered white paper black, and when burned fizzed like wet gunpowder:[i] and whence such matter could originally be brought, appeared to be past all conjecture, unless it came so far off as from the volcano in guadaloupe. condamine says, the ashes of the volcano of _sangay_, in south america, sometimes pass over the provinces of maca, and quito; and are even carried as far as guayaquil.[j] and hooke says,[k] that on occasion of a great explosion from a volcano, in the island of ternata, in the east indies, there followed so great a darkness, that the inhabitants could not see each other the next day: and he justly leads us to infer what an immense quantity of ashes must, by this means, have been showered down somewhere on the sea; because at _mindanao_, an hundred miles off, all the land was covered with ashes a foot thick. and now, i must add; that such kind of _falling of stones from the clouds_, as has been described to have happened in tuscany, seems to have happened also in very remote ages, of which we are not without sufficient testimony; and such as well deserves to be allowed and considered, on the present occasion; although the knowledge of the facts was, at first, in days of ignorance and gross darkness, soon perverted to the very worst purposes. in the acts of the holy apostles, we read, that the chief magistrate, at _ephesus_, begun his harangue to the people, by saying, "ye men of ephesus, _what man is there that knoweth not how that the city of the ephesians is a worshipper of the great goddess diana, and of the_ image _which fell down from jupiter_?" (or rather, as the original greek has it) "_of_ that _which fell down from jupiter_?" and the learned _greaves_ leads us to conclude this image of diana to have been nothing but _a conical, or pyramidal stone_, that fell from the clouds. for he tells us,[l] on unquestionable authorities, that many others of the images of heathen deities were merely such. herodian expressly declares,[m] that the phoenicians had no statue of the sun, polished by hand, to express an image; but only had a certain _great stone, circular below, and ending with a sharpness above, in the figure of a cone, of black colour. and they report it to have fallen from heaven, and to be the image of the sun_. so tacitus says,[n] that at cyprus, _the image of venus was not of human shape; but a figure rising continually round, from a larger bottom to a small top, in conical fashion_. and it is to be remarked, that _maximus tyrius_ (who perhaps was a more accurate mathematician,) says, the stone was _pyramidal_. and in corinth, we are told by _pausanias_,[o] that the images both of _jupiter melichius_, and of _diana_, were made (if made at all by hand) with little or no art. the former being represented by a pyramid, the latter by a column. _clemens alexandrinus_ was so well acquainted with these facts, that he even concludes[p] the worship of such stones to have been the first, and earliest idolatry, in the world. it is hard to conceive how mankind should ever have been led to so accursed an abomination, as the worship of stocks, and stones, at all: but, as far as any thing so horrid is to be accounted for, there is no way so likely of rendering a possible account; as that of concluding, that some of these pyramidal stones, at least, like the image of _diana_, actually did fall, in the earliest ages, from the clouds; in the same manner as these pyramidal stones fell, in , in tuscany. _plutarch_, it is well known, mentions[q] a stone which formerly fell from the clouds, in _thrace_, and which _anaxagoras_ fancied[r] to have fallen from the sun. and it is very remarkable, that the old writer, from whom plutarch had his account, described the cloud, from which this stone was said to fall, in a manner (if we only make some allowance for a little exaggeration in barbarous ages,) very similar to _soldani's_ account of the cloud in tuscany.--it hovered about for a long time; seemed to throw out splinters, which flew about, like wandering stars, before they fell; and at last it cast down to the earth a stone of extraordinary size. pliny,[s] who tells us that not only the remembrance of this event, but that the stone itself was preserved to his days, says, it was of a dark burnt colour. and though he does indeed speak of it as being of an extravagant weight and size, in which circumstance perhaps he was misled: yet he mentions _another_ of a moderate size, which fell in _abydos_, and was become an object of idolatrous worship in that place; as was still _another_, of the same sort, at _potidæa_. _livy_, who like _herodotus_, has been oftentimes censured as too credulous, and as a relater of falsehoods, for preserving traditions of _an extraordinary kind_; which, after all, in ages of more enlarged information, have proved to have been founded in truth; describes[t] a fall of stones to have happened on mount _alba_, during the reign of _tullus hostilius_, (that is about years before the christian æra), in words that exactly convey an idea of just such a phænomenon, as this which has so lately been observed in tuscany. he says, the senate were told, that _lapidibus pluisse_, it had rained stones. and, when they doubted of the fact; and sent to inquire; they were assured that stones had actually fallen; and had fallen just as hail does, which is concreted in a storm.[u] he mentions also shortly another shower of stones,[v] a. c. , and still a third,[w] which must have happened about the year before the christian æra. such are the records of antient history. and in holy writ also a remembrance of similar events is preserved. for when the royal psalmist says,[x] "_the lord also thundered out of heaven, and the highest gave his thunder: hail-stones_, and coals of fire,"--the latter expression, in consistency with common sense, and conformably to the right meaning of language, cannot but allude to some _such_ phænomenon as we have been describing. and especially, as in the cautious translation of the seventy, a greek word is used, which decidedly means _real hard substances made red hot_; and not mere appearances of fire or flame. whilst therefore, with the same sacred writer,[y] we should be led to consider all these powerful operations, as the works of god; _who casteth forth his ice like morsels_;[z] and should be led to consider "_fire and hail, snow and vapours, wind and storm as fulfilling his word_;"[aa] we should also be led to perceive, that the objections to holy writ, founded on a supposed _impossibility_ of the truth of what is written in the book of _joshua_,[bb] concerning the stones that fell from heaven, on the army of the canaanites; are only founded in ignorance, and error. and much more should we be led to do so; when, to these observations, and testimonies, concerning showers of hot burning stones, is added the consideration; that within the short period of our own lives, incredibly large _real hail-stones_, formed of consolidated ice;--_of ice consolidated in the atmosphere_, have fallen both in france, and in england. in france, on the th of july in the year ;--of which it is well known there has been a printed account: and concerning which it is said, and has been confirmed, on good authority, that some of the stones weighed three pounds: whilst others have been said to weigh even five pounds. and in england, on the th of october, , in cornwall. of one of the hail-stones of this latter, minor storm, i have had an opportunity of obtaining, by the favour of a friend, an exact model in glass; whereof i now add an engraving. this stone fell, with thousands of others of the same kind, near _menabilly_, the seat of _philip rashleigh_, esq.; well known for his science, and attention to whatever is curious; who having great copper works, and many ingenious miners, and workmen, on his estate, and directly under his eye; caused it to be instantly picked up: and having then, himself, first traced both its top, and bottom, upon paper; and having measured its thickness in every part, with a pair of compasses; caused a very exact mould to be formed: and afterwards, in that mould, had this model cast in glass: wherein, also, the appearances of the imbedded, common, small, roundish hail-stones, are seen transparently; just as they appeared in the great hail-stone itself originally. fig. , is a representation of the flat bottom of the stone. fig. , is a representation of the top of the stone. and fig. , shews the whole solid appearance sideways. whilst mr. rashleigh was taking the measures, it melted so fast, that he could not, in the end, take the _exact weight_, as he fully intended to have done. but as this model in glass weighs exactly ounce, pennyweights, grains, we may fairly conclude, that the hail-stone itself weighed much above half an ounce. for it is well known, that the specific gravity of common glass, of which sort this model is made, is to that of water, as . to . . and the specific gravity of common water, is to ice, as to .[cc]--and computing according to this standard, i make the exact weight of the hail-stone to have been grains. from the singular manner in which the small, prior, common hail-stones appear to have been imbedded in this larger one, whilst they were falling to the earth; there is reason to be convinced, that it was formed in the atmosphere, by a sudden extraordinary congelation _almost instantaneously_, out of rain suddenly condensed, which was mingled with the common hail. and it was very remarkable, that its dissolution, and melting, also, was much more rapid than that of the common small white hail-stones: as was the case, in like manner, with the other numerous large ones. perhaps it ought to be here added:--that on the th of may, in the year , some hail-stones are recorded to have fallen in london, near _gresham college_, which were seen and examined by the celebrated _dr. hooke_; and were some of them not less than two inches over, and others three inches. this which fell in cornwall was only about one inch and three quarters long; an inch, or in some parts an inch and a quarter broad; and between half an inch, and three quarters of an inch thick. and its weight was near an ounce.--how much more tremendous then were those others, that have been described as having fallen in france?--the accounts of some of them may very probably have been exaggerated: but the reality was nevertheless as wonderful, surely, as any thing related concerning the ages of antiquity. a proneness to credulity is ever blameable. and it is very possible, that sometimes, in a very wonderful narration, a jest may be intended to be palmed upon the world, instead of any elucidation of truth.--but facts, _positively affirmed_, should be hearkened to with patience: and, at least, so far recorded, as to give an opportunity of verifying whether similar events do afterwards happen; and of comparing such events one with another. to what has been said, therefore, concerning the fall of stones in tuscany, and concerning these strange showers of hail, in france, and in england, it might perhaps too justly be deemed an unwarrantable omission, on this occasion, not to mention the very strange fact that is affirmed to have happened the last year, near _the wold cottage_ in yorkshire. i leave the fact to rest on the support of the testimonies referred to in the printed paper, which is in so many persons' hands; and that is given to those who have the curiosity to examine the stone itself, now exhibiting in london;--and shall only relate the substance of the account shortly, as it is given to us. in the afternoon of the th of december, , near the wold cottage, noises were heard in the air, by various persons, like the report of a pistol; or of guns at a distance at sea; though there was neither any thunder or lightning at the time:--two distinct concussions of the earth were said to be perceived:--and an hissing noise, was also affirmed to be heard by other persons, as of something passing through the air;--and a labouring man plainly saw (as we are told) that something was so passing; and beheld a stone, as it seemed, at last, (about ten yards, or thirty feet, distant from the ground) descending, and striking into the ground, which flew up all about him: and in falling, sparks of fire, seemed to fly from it. afterwards he went to the place, in company with others; who had witnessed part of the phænomena, and dug the stone up from the place, where it was buried about twenty-one inches deep. it smelt, (as it is said,) very strongly of sulphur, when it was dug up: and was even warm, and smoked:--it was found to be thirty inches in length, and twenty-eight and a half inches in breadth. and it weighed fifty-six pounds. such is the account.--i affirm nothing.--neither do i pretend either absolutely to believe: or to disbelieve.--i have not an opportunity to examine the whole of the evidence.--but it may be examined: and so i leave it to be. this, however, i will say: that _first_ i saw a fragment of this stone; which had come into the hands of sir charles blagden, from the duke of leeds: and afterwards i saw the stone itself.--that it plainly had a dark, black crust; with several concave impressions on the outside, which must have been made before it was quite hardened; just like what is related concerning the crusts of those stones that fell in italy.--that its substance was not _properly_ of a _granite kind_, as described in the printed paper; but a sort of _grit stone_; composed (somewhat like the stones said to have fallen in italy) of sand and ashes.--that it contained very many particles, obviously of the appearance of gold, and silver, and iron; (or rather more truly of _pyrites_).--that there were also several small rusty specks; probably from decomposed pyrites;--and some striated marks;--that it does not effervesce with acids;--and that, as far as i have ever seen, or known, or have been able to obtain any information, no _such_ stone has ever been found, before this time, in yorkshire; or in any part of england. nor can i easily conceive that such a species of stone could be formed, by art, to impose upon the public. whether, therefore, it might, or might not, possibly be the effect of ashes flung out from _heckla_, and wafted to england; like those flung out from vesuvius, and (as i am disposed to believe) wafted to tuscany, i have nothing to affirm. i wish to be understood to preserve mere records, the full authority for which, deserves to be investigated more and more. having, nevertheless, gone so far as to say thus much; i ought to add, that the memorial of such sort of large stones having fallen from the clouds is still preserved also in germany. for one is recorded to have fallen in _alsace_, in the midst of a storm of hail, november th, a. d. ;[dd] which is said to be preserved in the great church of _anxissem_: and to be like a large dark sort of flint-stone; having its surface operated upon by fire: and to be of very many pounds weight. and another is said to be still preserved at vienna. this last is described by _abbé stutz_, assistant in the imperial cabinet of curiosities at vienna, in a book printed in german, at _leipsyc_, in : entitled _bergbaukimde_ (or _the science of mining_.) after describing two other stones, said to have fallen from the clouds: one in the _eichstedt_ country in germany; and another in the _bechin_ circle, in bohemia, in july, ; concerning the _real_ falling of which he had expressed some doubts; he proceeds to describe the falling of two, (whereof this was one,) not far from _agram_, the capital of _croatia_, in hungary; which caused him to change his opinion; and to believe, that the falling of such stones from heaven, was very possible. his words, fairly translated,[ee] in the beginning of his narrative, are, "these accounts put me in mind of a mass of iron, weighing seventy-one pounds, which was sent to the imperial collection of natural curiosities: about the origin of which _many mouths have been distorted with scoffing laughter_. if, in the _eichstedt_ specimen, the effects of fire appear _tolerably_ evident; they are, in this, not to be mistaken.--its surface is full of spherical impressions, like the mass of iron, which the celebrated _pallas_ found on the jenisei river; except that here the impressions are larger, and less deep; and it wants both the yellow glass, which fills up the hollows of the _siberian_ iron; and the _sand stone_, which is found in the _eichstedt_ specimen; the whole mass being solid, compact, and black, like hammered iron." and his words in the end of the narrative are, "there is a great step from the disbelief of tales, to the finding out the true cause of a phænomenon which appears wonderful to us. and probably i should have committed the fault into which we so naturally fall, respecting things we cannot explain; and have rather denied the whole history, than have determined to believe any thing _so incredible_; if various new writings, on electricity, and thunder, had not fortunately, at that time come into my hands; concerning remarkable experiments of reviving _metallic calces_ by the electric spark. lightning is an electrical stroke on a large scale.--if then the reduction of iron can be obtained, by the discharge of an electrical machine; why should not this be accomplished as well, and with much greater effect by the very powerful discharge of the lightning of the clouds?" the substance of the account of the fall of stones, in hungary, as given by him, after the most accurate inquiries, is what i shall now add in the following abridged detail; and it was verified by _wolfgang kukulyewich, spiritual vicar of francis baron clobuschiczky, bishop of agram_, who caused seven eye witnesses to be examined, concerning the actual falling of these stones on the th of may, ;--which witnesses were ready to testify all they affirmed, upon oath,--and one of them was mr. george marsich, curate, as we should call him, of the parish. according to their accounts; about six o'clock, in the afternoon of the day just mentioned, there was seen towards the east, a kind of fiery ball; which, after it had burst into two parts, with a great report, exceeding that of a cannon, fell from the sky, in the form, and appearance of _two chains_ entangled in one another:--and also with a loud noise, as of a great number of carriages rolled along. and after this a black smoke appeared; and a part of the ball seemed to fall in an arable field of one _michael koturnass_; on the fall of which to the ground a still greater noise was heard; and a shock perceived, something like an earthquake. this piece was afterwards soon dug out of the ground; which had been particularly noted to be plain and level, and ploughed just before; but where it was now found to have made a great fissure, or cleft, an ell wide, whilst it singed the earth on the sides. the other piece, which fell in a meadow, was also dug up; and weighed sixteen pounds. and it is fairly observed, that the unadorned manner in which the whole account from _agram_ is written; the agreement of the different witnesses, who had no reason to accord in a lie; and the similarity of this history to that of the _eichstedt_ stone; makes it at least very probable, that there was indeed something real, and worth notice, in the account. the _eichstedt_ stone (somewhat like that said to have fallen so lately in yorkshire) is described as having been composed of ash-grey sand stone, with fine grains intermixed all through it, partly of real native iron, and partly of yellowish brown ochre of iron: and as being about as hard as building stone.--it is said not to effervesce with acids, and evidently to consist of small particles of siliceous stone and iron.--it had also a solid malleable coat of native iron, as was supposed, quite free from sulphur, and about two lines thick; which quite covered its surface; resembling a blackish glazing. and the whole mass exhibited evident marks of having been exposed to fire. a plain testimony of the falling of this was affirmed to be, produced as follows; that a labourer, at a brick-kiln, in winter, when the earth was covered with snow, saw it fall down out of the air immediately after a violent clap of thunder;--and that he instantly ran up to take it out of the snow; but found he could not do so, on account of its heat; and was obliged therefore to wait, to let it cool. that it was about half a foot in diameter; and was entirely covered with a black coat like iron.[ff] and i must now add that there is a record;[gg] that stones, to the number of some hundreds, did once fall in the neighbourhood of a place called _abdua_; which were very large and heavy;--of the colour of rusty iron;--smooth, and hard;--and of a sulphureous smell:--and which were observed to fall from a vehement whirlwind; that appeared (like that in tuscany) as an atmosphere of fire. here i intended to have concluded all my observations. but a recent publication, which i knew not of, when these sheets were written, obliges me to add a few more pages. in a very singular tract, published in , at riga, by dr. _chladni_, concerning the supposed origin of the mass of iron found by dr. pallas in siberia; which the tartars still affirm to be _an holy thing_, and, _to have fallen from heaven_; and concerning what have been supposed, by him, to be similar phænomena; some circumstances are also mentioned, which it would be an unjust omission not to take notice of shortly, on the present occasion. with the author's hypothesis i do not presume to interfere; but surely his facts, which he affirms in support of his ideas, deserve much attention; and ought to be inserted, before i conclude these observations: and the rather, as they were adduced to maintain conclusions very different from these now offered to the consideration of the curious. on the st of may, , a fire ball was seen to come from dalmatia,[hh] proceeding over the adriatic sea; it passed obliquely over italy; where an hissing noise was heard; it burst ssw from leghorn, with a terrible report; _and the pieces are said to have fallen into the sea_, with the same sort of noise, as when red hot iron is quenched or extinguished in water. its height was computed to be not less than thirty-eight italian miles; and it is said to have moved with immense velocity. its form was oblong, at least as the luminous appearance seemed in its passage. _avicenna_ mentions, (averrhoes, lib. do meteor. cap. .) that he had seen at cordova, in spain, a sulphureous stone that had fallen from heaven. in _spangenberg_'s chron. saxon, an account is found, that at magdeburg, in a. d. , two great stones, fell down in a storm of thunder: one in the town itself; the other near the elbe, in the open country. the well known, and celebrated _cardan_, in his book, _de varietate rerum_, lib. . cap. . tells us, that he himself, in the year , had seen one hundred and twenty stones fall from heaven; among which one weighed one hundred and twenty; and another sixty pounds. that they were mostly of an _iron colour_, and very hard, and smelt of brimstone. he remarks, moreover, that about three o'clock, a great fire was to be seen in the heavens; and that about five o'clock the stones fell down with a rushing noise. and _julius scaliger_ (in his book _de subtilitate exerc._ p. .) affirms, that he had in his possession a piece of iron (as he calls it,) which had fallen from heaven in _savoy_. _wolf_ (in _lection. memorab._ tom. ii. p. .) mentions a great triangular stone, described by _sebastian brandt_, (which seems to have been the identical stone i have already mentioned as having been preserved in the church of anxissem,) and which was said to have fallen from heaven, in the year , at ensisheim or ensheim. _muschenbroek_,[ii] speaking of the same stone, says, that the stone was blackish, weighed about lb. and that marks of fire were to be seen upon it; but apprehended (in which he seems to have been mistaken) that the date of the fall was . _chladni_ also mentions another instance (from _nic. huknanfii_ hist. hungar. lib. . fol. .) of five stones, said to have fallen from heaven at _miscoz_, in transylvania, in a terrible thunder storm and commotion of the air, which were as big as a man's head, very heavy, of a pale yellow, and iron, or rusty colour; and of a strong sulphureous smell; and that four of them were kept in the treasury room at vienna. he adds, (from _john binbard_'s thuring. chron. p. .) that on the th of july, , between one and two o'clock in the afternoon, a stone fell down in _thuringia_, with a clap of thunder, which made the earth shake; at which time a small light cloud was to be seen, the sky being otherwise clear. it weighed lb.; was of a blue and brownish colour. it gave sparks, when struck with a flint, as steel does. it had sunk five quarters of an ell deep in the ground; so that the soil, at the time, was struck up to twice a man's height; and the stone itself was so hot, that no one could bear to touch it. it is said to have been afterwards carried to dresden. he adds, also, that in the st essay of the breslau collections, p. , is found an account by dr. _rost_; that on the d of june, , about two o'clock in the afternoon, in the country of pleskowicz, some miles from _reichstadt_, in bohemia, a small cloud was seen, the sky being otherwise clear; whereupon, at one place twenty-five, at another eight, great and small stones fell down, with a loud report, and without any lightning being perceived. the stones appeared externally black, internally like a metallic ore, and smelt strongly of brimstone. and i shall conclude all _chladni_'s remarkable facts, in addition to those which i had myself collected, before ever i heard of his curious book, with a short summary of what he calls one of the _newest_ accounts of this kind, extracted from the _histoire de l'académie des sciences_, , p. . it is an account of three masses, which fell down with thunder, in provinces very distant from one another; and which were sent to the academy in . they were sent from _maine_, _artois_, and _cotentin_: and it is affirmed, that when they fell an hissing was heard; and that they were found hot. all three were like one another; all three were of the same colour, and nearly of the same grain; and small metallic and pyritical particles could be distinguished in them; and, externally, all three were covered with an hard ferruginous coat: and, on chemical investigation, they were found to contain iron, and sulphur.[jj] considering, then, all these facts so positively affirmed, concerning these various, most curious phænomena:--the explosions;--the sparks;--the lights;--the hissing noises;--the stones seen to fall;--the stones dug up hot, and even smoking;--and some scorching, and even burning other bodies in their passage;--we cannot but also bring to remembrance, what sir john pringle affirmed to have been observed; concerning a fiery meteor, seen on sunday, the th of november, , in several parts of england and scotland.[kk] that the head, which appeared about half the diameter of the moon, was of a bright white, like iron when almost in a melting heat;[ll] the tail, which appeared about ° in length, was of a duskish red, burst in the atmosphere, when the head was about ° above the horizon, and disappeared; and in the room thereof were seen three bodies like stars, within the compass of a little more than three degrees from the head, which also kept descending with the head. that before this, in another place, near ancram in scotland, (where the same meteor was seen) one-third of the tail, towards the extremity, appeared _to break off_, and to separate into sparks, resembling stars.--that soon after this the body of the meteor had its light extinguished, with an explosion; but, as it seemed to the observer there, _the form of the entire figure of the body, quite black, was seen to go still forwards in the air_.[mm] by some persons, also, an hissing noise[nn] was apprehended to be heard. whether this might, or might not be an ignited body, of the kind we have been describing, falling to the earth, deserves consideration. sir john pringle seems to have been convinced that it was really _a solid substance_; but fairly adds,[oo] that if such meteors had really ever fallen to the earth, there must have been, long ago, so strong evidence of the fact, as to leave no room to doubt. perhaps, in the preceding accounts, we have such evidence, _now_ fairly collected together; at least in a certain degree. i take all the facts, just as i find them affirmed. i have preserved a faithful and an honest record. for the sake of possible philosophical use;--let the philosophical, and curious just preserve these facts in remembrance. for the sake of philological advantage;--let the discerning weigh, and judge. for (if such things be,) what has so often come to pass, according to what is commonly called _the usual course of nature_; may most undoubtedly, henceforth, without any hesitating doubts, be believed to have been brought to pass, on an extraordinary occasion, in a still more tremendous manner, by the immediate _fiat of the almighty_. let no man scoff; lest he drives away the means of real information.--and let all men _watch_, for the increase of science.-- the wisdom and power of god are far above not only the first apprehensions, but even the highest ideas of man. and our truest wisdom, and best improvement of knowledge, consist in searching out, and in attending diligently, to what he has actually done: ever bearing in mind those words of the holy psalmist.[pp] "_the works of the lord are great: sought out of all them that have pleasure therein._ "_the lord hath so done his marvellous works, that they ought to be had in remembrance._" postscript. since these sheets were printed, i have received from sir charles blagden, a present of one of the very small stones mentioned, p. , that are affirmed to have fallen in tuscany; and which has very lately been brought carefully from italy. its figure plainly indicates, that in the instant of its formation, there was a strong effort towards crystallization. for it is an irregular quadrilateral pyramid;--whose base, an imperfect kind of square, has two of its adjoining sides about six-tenths of an inch long, each; and the other two, each about five-tenths: whilst two of the triangular sides of the pyramid, are about six-tenths, on every side of each triangle, all of which are a little curved: and the other two triangular sides, are only five-tenths on the sides where these two last join. its black crust, or coating, is such as has been described in the preceding pages: and is also remarkable, for the appearance of a sort of minute chequer work, formed by very fine white lines on the black surface. footnotes: [a] in his anatomy of plants, p. - . [b] vol. lxiii. p. --and vol. lxix. p. . [c] in the morsels of criticism, p. . [d] in the philos. trans. for , p. , . [e] this is mentioned by sir william hamilton himself, p. . [f] see philos. trans. for , p. , . [g] see lowthorp's abridgement of the philos. trans. vol. ii. p. . [h] philos. trans. vol. xlix. p. . [i] philos. trans. vol. liii. p. . [j] condamine's journal, p. . [k] in his experiments, p. . [l] pyramidographia, vol. i. - . [m] lib. . [n] lib. . [o] in his corinthiaca. [p] clem. alex. lib. .--stromatum. [q] in vita lysandri. [r] diogenes in anaxag. [s] historia nat. lib. . cap. . [t] lib. . sec. . [u] haud aliter quam quum grandinem venti glomeratam in terras agunt, crebri cecidere coelo lapides. [v] lib. . sec. . [w] lib. . sec. . [x] psalm . v. . [y] psalm . v. . [z] psalm . v. . [aa] psalm . v. . [bb] joshua, ch. . v. . [cc] hooke's experiments, p. . [dd] vide gesner.--and ans de boot hist. lapidum. [ee] for which translation i am obliged to sir charles blagden. [ff] this account, from abbé _stutz_, and the following from dr. _chladni_, i received, translated from the german, by the favour of sir charles blagden. [gg] vide cardan _de variet_, lib. . c. . [hh] an account of this stone is given by dr. halley in the philosophical trans. no. . and also there is an account of it by montenari. [ii] essai de physique, tom. ii. sect. . [jj] all these facts are to be found mentioned in chladni's book; first at p. , and then from p. to . [kk] see the full account in the philosophical transactions, vol. li. for , p. , &c. [ll] this is according to the account sent by the rev. mr. michell, fellow of queen's college, cambridge, p. . [mm] ib. p. , , . [nn] ib. p. . [oo] ib. p. . [pp] psalm iii. v. and . transcriber's note the punctuation and spelling from the original text have been faithfully preserved. only obvious typographical errors have been corrected. there are several mathematical formulas within the text. they are represented as follows: superscripts: x^ subscripts: x_ square root: [square root] greek letters: [pi], [theta]. greek star names are represented as [alpha], [gamma], for example. pioneers of science [illustration] [illustration: newton _from the picture by kneller, , now at cambridge_] pioneers of science by oliver lodge, f.r.s. professor of physics in victoria university college, liverpool _with portraits and other illustrations_ london macmillan and co. and new york richard clay and sons, limited, london and bungay. preface this book takes its origin in a course of lectures on the history and progress of astronomy arranged for me in the year by three of my colleagues (a.c.b., j.m., g.h.r.), one of whom gave the course its name. the lectures having been found interesting, it was natural to write them out in full and publish. if i may claim for them any merit, i should say it consists in their simple statement and explanation of scientific facts and laws. the biographical details are compiled from all readily available sources, there is no novelty or originality about them; though it is hoped that there may be some vividness. i have simply tried to present a living figure of each pioneer in turn, and to trace his influence on the progress of thought. i am indebted to many biographers and writers, among others to mr. e.j.c. morton, whose excellent set of lives published by the s.p.c.k. saved me much trouble in the early part of the course. as we approach recent times the subject grows more complex, and the men more nearly contemporaries; hence the biographical aspect diminishes and the scientific treatment becomes fuller, but in no case has it been allowed to become technical and generally unreadable. to the friends (c.c.c., f.w.h.m., e.f.r.) who with great kindness have revised the proofs, and have indicated places where the facts could be made more readily intelligible by a clearer statement, i express my genuine gratitude. university college, liverpool, _november, _. contents _part i_ lecture i page copernicus and the motion of the earth lecture ii tycho brahÉ and the earliest observatory lecture iii kepler and the laws of planetary motion lecture iv galileo and the invention of the telescope lecture v galileo and the inquisition lecture vi descartes and his theory of vortices lecture vii sir isaac newton lecture viii newton and the law of gravitation lecture ix newton's "principia" _part ii_ lecture x roemer and bradley and the velocity of light lecture xi lagrange and laplace--the stability of the solar system, and the nebular hypothesis lecture xii herschel and the motion of the fixed stars lecture xiii the discovery of the asteroids lecture xiv bessel--the distances of the stars, and the discovery of stellar planets lecture xv the discovery of neptune lecture xvi comets and meteors lecture xvii the tides lecture xviii the tides, and planetary evolution illustrations fig. page . archimedes . leonardo da vinci . copernicus . homeric cosmogony . egyptian symbol of the universe . hindoo earth . order of ancient planets corresponding to the days of the week . ptolemaic system . specimens of apparent paths of venus and of mars among the stars . apparent epicyclic orbits of jupiter and saturn . egyptian system . true orbits of earth and jupiter . orbits of mercury and earth . copernican system as frequently represented . slow movement of the north pole in a circle among the stars . tychonic system, showing the sun with all the planets revolving round the earth . portrait of tycho . early out-door quadrant of tycho . map of denmark, showing the island of huen . uraniburg . astrolabe . tycho's large sextant . the quadrant in uraniburg . tycho's form of transit circle . a modern transit circle . orbits of some of the planets drawn to scale . many-sided polygon or approximate circle enveloped by straight lines . kepler's idea of the regular solids . diagram of equant . excentric circle supposed to be divided into equal areas . mode of drawing an ellipse . kepler's diagram proving equable description of areas for an ellipse . diagram of a planet's velocity in different parts of its orbit . portrait of kepler . curve described by a projectile . two forms of pulsilogy . tower of pisa . view of the half-moon in small telescope . portion of the lunar surface more highly magnified . another portion of the lunar surface . lunar landscape showing earth . galileo's method of estimating the height of lunar mountain . some clusters and nebulÆ . stages of the discovery of jupiter's satellites . eclipses of jupiter's satellites . old drawings of saturn by different observers, with the imperfect instruments of that day . phases of venus . sunspots as seen with low power . a portion of the sun's disk as seen in a powerful modern telescope . saturn and his rings . map of italy . portrait of galileo . portrait of descartes . descartes's eye diagram . descartes's diagram of vortices from his "principia" . manor-house of woolsthorpe . projectile diagram . } { . } diagrams illustrative of those near the beginning { . } of newton's "principia" { - . } { . prismatic dispersion . a single constituent of white light is capable of no more dispersion . parallel beam passing through a lens . newton's telescope . the sextant, as now made . newton when young . sir isaac newton . another "principia" diagram . well-known model exhibiting the oblate spheroidal form as a consequence of spinning about a central axis . jupiter . diagram of eye looking at a light reflected in a distant mirror through the teeth of a revolving wheel . fizeau's wheel, showing the appearance of distant image seen through its teeth . eclipses of one of jupiter's satellites . a transit instrument for the british astronomical expedition, . diagram of equatorially mounted telescope . aberration diagram . showing the three conjunction places in the orbits of jupiter and saturn . lord rosse's drawing of the spiral nebula in canes venatici . saturn . principle of newtonian reflector . herschel's -foot telescope . william herschel . caroline herschel . double stars . old drawing of the cluster in hercules . old drawing of the andromeda nebula . the great nebula in orion . planetary orbits to scale . diagram illustrating parallax . the kÖnigsberg heliometer . perturbations of uranus . uranus' and neptune's relative positions . meteorite . meteor stream crossing field of telescope . diagram of direction of earth's orbital motion . parabolic and elliptic orbits . orbit of halley's comet . various appearances of halley's comet when last seen . head of donati's comet of . comet . encke's comet . biela's comet as last seen in two portions . radiant point perspective . present orbit of november meteors . orbit of november meteors before and after encounter with uranus . the mersey . co-tidal lines, showing the way the tidal wave reaches the british isles from the atlantic . whirling earth model . earth and moon model . earth and moon (earth's rotation neglected) . maps showing how comparatively free from land obstruction the ocean in the southern hemisphere is . spring and neap tides . tidal clock . sir william thomson (lord kelvin) . tide-gauge for recording local tides . harmonic analyzer . tide-predicter . weekly sheet of curves pioneers of science part i _from dusk to daylight_ dates and summary of facts for lecture i _physical science of the ancients._ thales b.c., anaximander b.c., pythagoras b.c., anaxagoras b.c., eudoxus b.c., aristotle b.c., aristarchus b.c., archimedes b.c., eratosthenes b.c., hipparchus b.c., ptolemy a.d. _science of the middle ages._ cultivated only among the arabs; largely in the forms of astrology, alchemy, and algebra. _return of science to europe._ roger bacon , leonardo da vinci , (printing ), columbus , copernicus . _a sketch of copernik's life and work._ born at thorn in poland. studied mathematics at bologna. became an ecclesiastic. lived at frauenburg near mouth of vistula. substituted for the apparent motion of the heavens the real motion of the earth. published tables of planetary motions. motion still supposed to be in epicycles. worked out his ideas for years, and finally dedicated his work to the pope. died just as his book was printed, aged , a century before the birth of newton. a colossal statue by thorwaldsen erected at warsaw in . pioneers of science lecture i copernicus and the motion of the earth the ordinary run of men live among phenomena of which they know nothing and care less. they see bodies fall to the earth, they hear sounds, they kindle fires, they see the heavens roll above them, but of the causes and inner working of the whole they are ignorant, and with their ignorance they are content. "understand the structure of a soap-bubble?" said a cultivated literary man whom i know; "i wouldn't cross the street to know it!" and if this is a prevalent attitude now, what must have been the attitude in ancient times, when mankind was emerging from savagery, and when history seems composed of harassments by wars abroad and revolutions at home? in the most violently disturbed times indeed, those with which ordinary history is mainly occupied, science is quite impossible. it needs as its condition, in order to flourish, a fairly quiet, untroubled state, or else a cloister or university removed from the din and bustle of the political and commercial world. in such places it has taken its rise, and in such peaceful places and quiet times true science will continue to be cultivated. the great bulk of mankind must always remain, i suppose, more or less careless of scientific research and scientific result, except in so far as it affects their modes of locomotion, their health and pleasure, or their purse. but among a people hurried and busy and preoccupied, some in the pursuit of riches, some in the pursuit of pleasure, and some, the majority, in the struggle for existence, there arise in every generation, here and there, one or two great souls--men who seem of another age and country, who look upon the bustle and feverish activity and are not infected by it, who watch others achieving prizes of riches and pleasure and are not disturbed, who look on the world and the universe they are born in with quite other eyes. to them it appears not as a bazaar to buy and to sell in; not as a ladder to scramble up (or down) helter-skelter without knowing whither or why; but as a fact--a great and mysterious fact--to be pondered over, studied, and perchance in some small measure understood. by the multitude these men were sneered at as eccentric or feared as supernatural. their calm, clear, contemplative attitude seemed either insane or diabolic; and accordingly they have been pitied as enthusiasts or killed as blasphemers. one of these great souls may have been a prophet or preacher, and have called to his generation to bethink them of why and what they were, to struggle less and meditate more, to search for things of true value and not for dross. another has been a poet or musician, and has uttered in words or in song thoughts dimly possible to many men, but by them unutterable and left inarticulate. another has been influenced still more _directly_ by the universe around him, has felt at times overpowered by the mystery and solemnity of it all, and has been impelled by a force stronger than himself to study it, patiently, slowly, diligently; content if he could gather a few crumbs of the great harvest of knowledge, happy if he could grasp some great generalization or wide-embracing law, and so in some small measure enter into the mind and thought of the designer of all this wondrous frame of things. these last have been the men of science, the great and heaven-born men of science; and they are few. in our own day, amid the throng of inventions, there are a multitude of small men using the name of science but working for their own ends, jostling and scrambling just as they would jostle and scramble in any other trade or profession. these may be workers, they may and do advance knowledge, but they are never pioneers. not to them is it given to open out great tracts of unexplored territory, or to view the promised land as from a mountain-top. of them we shall not speak; we will concern ourselves only with the greatest, the epoch-making men, to whose life and work we and all who come after them owe so much. such a man was thales. such was archimedes, hipparchus, copernicus. such pre-eminently was newton. now i am not going to attempt a history of science. such a work in ten lectures would be absurd. i intend to pick out a few salient names here and there, and to study these in some detail, rather than by attempting to deal with too many to lose individuality and distinctness. we know so little of the great names of antiquity, that they are for this purpose scarcely suitable. in some departments the science of the greeks was remarkable, though it is completely overshadowed by their philosophy; yet it was largely based on what has proved to be a wrong method of procedure, viz the introspective and conjectural, rather than the inductive and experimental methods. they investigated nature by studying their own minds, by considering the meanings of words, rather than by studying things and recording phenomena. this wrong (though by no means, on the face of it, absurd) method was not pursued exclusively, else would their science have been valueless, but the influence it had was such as materially to detract from the value of their speculations and discoveries. for when truth and falsehood are inextricably woven into a statement, the truth is as hopelessly hidden as if it had never been stated, for we have no criterion to distinguish the false from the true. [illustration: fig. .--archimedes.] besides this, however, many of their discoveries were ultimately lost to the world, some, as at alexandria, by fire--the bigoted work of a mohammedan conqueror--some by irruption of barbarians; and all were buried so long and so completely by the night of the dark ages, that they had to be rediscovered almost as absolutely and completely as though they had never been. some of the names of antiquity we shall have occasion to refer to; so i have arranged some of them in chronological order on page , and as a representative one i may specially emphasize archimedes, one of the greatest men of science there has ever been, and the father of physics. the only effective link between the old and the new science is afforded by the arabs. the dark ages come as an utter gap in the scientific history of europe, and for more than a thousand years there was not a scientific man of note except in arabia; and with the arabs knowledge was so mixed up with magic and enchantment that one cannot contemplate it with any degree of satisfaction, and little real progress was made. in some of the _waverley novels_ you can realize the state of matters in these times; and you know how the only approach to science is through some arab sorcerer or astrologer, maintained usually by a monarch, and consulted upon all great occasions, as the oracles were of old. in the thirteenth century, however, a really great scientific man appeared, who may be said to herald the dawn of modern science in europe. this man was roger bacon. he cannot be said to do more than herald it, however, for we must wait two hundred years for the next name of great magnitude; moreover he was isolated, and so far in advance of his time that he left no followers. his own work suffered from the prevailing ignorance, for he was persecuted and imprisoned, not for the commonplace and natural reason that he frightened the church, but merely because he was eccentric in his habits and knew too much. the man i spoke of as coming two hundred years later is leonardo da vinci. true he is best known as an artist, but if you read his works you will come to the conclusion that he was the most scientific artist who ever lived. he teaches the laws of perspective (then new), of light and shade, of colour, of the equilibrium of bodies, and of a multitude of other matters where science touches on art--not always quite correctly according to modern ideas, but in beautiful and precise language. for clear and conscious power, for wide-embracing knowledge and skill, leonardo is one of the most remarkable men that ever lived. about this time the tremendous invention of printing was achieved, and columbus unwittingly discovered the new world. the middle of the next century must be taken as the real dawn of modern science; for the year marks the publication of the life-work of copernicus. [illustration: fig. .--leonardo da vinci.] nicolas copernik was his proper name. copernicus is merely the latinized form of it, according to the then prevailing fashion. he was born at thorn, in polish prussia, in . his father is believed to have been a german. he graduated at cracow as doctor in arts and medicine, and was destined for the ecclesiastical profession. the details of his life are few; it seems to have been quiet and uneventful, and we know very little about it. he was instructed in astronomy at cracow, and learnt mathematics at bologna. thence he went to rome, where he was made professor of mathematics; and soon afterwards he went into orders. on his return home, he took charge of the principal church in his native place, and became a canon. at frauenburg, near the mouth of the vistula, he lived the remainder of his life. we find him reporting on coinage for the government, but otherwise he does not appear as having entered into the life of the times. he was a quiet, scholarly monk of studious habits, and with a reputation which drew to him several earnest students, who received _vivâ voce_ instruction from him; so, in study and meditation, his life passed. he compiled tables of the planetary motions which were far more correct than any which had hitherto appeared, and which remained serviceable for long afterwards. the ptolemaic system of the heavens, which had been the orthodox system all through the christian era, he endeavoured to improve and simplify by the hypothesis that the sun was the centre of the system instead of the earth; and the first consequences of this change he worked out for many years, producing in the end a great book: his one life-work. this famous work, "de revolutionibus orbium coelestium," embodied all his painstaking calculations, applied his new system to each of the bodies in the solar system in succession, and treated besides of much other recondite matter. towards the close of his life it was put into type. he can scarcely be said to have lived to see it appear, for he was stricken with paralysis before its completion; but a printed copy was brought to his bedside and put into his hands, so that he might just feel it before he died. [illustration: fig. .--copernicus.] that copernicus was a giant in intellect or power--such as had lived in the past, and were destined to live in the near future--i see no reason whatever to believe. he was just a quiet, earnest, patient, and god-fearing man, a deep student, an unbiassed thinker, although with no specially brilliant or striking gifts; yet to him it was given to effect such a revolution in the whole course of man's thoughts as is difficult to parallel. you know what the outcome of his work was. it proved--he did not merely speculate, he proved--that the earth is a planet like the others, and that it revolves round the sun. yes, it can be summed up in a sentence, but what a revelation it contains. if you have never made an effort to grasp the full significance of this discovery you will not appreciate it. the doctrine is very familiar to us now, we have heard it, i suppose, since we were four years old, but can you realize it? i know it was a long time before i could. think of the solid earth, with trees and houses, cities and countries, mountains and seas--think of the vast tracts of land in asia, africa, and america--and then picture the whole mass spinning like a top, and rushing along its annual course round the sun at the rate of nineteen miles every second. were we not accustomed to it, the idea would be staggering. no wonder it was received with incredulity. but the difficulties of the conception are not only physical, they are still more felt from the speculative and theological points of view. with this last, indeed, the reconcilement cannot be considered complete even yet. theologians do not, indeed, now _deny_ the fact of the earth's subordination in the scheme of the universe, but many of them ignore it and pass it by. so soon as the church awoke to a perception of the tremendous and revolutionary import of the new doctrines, it was bound to resist them or be false to its traditions. for the whole tenor of men's thought must have been changed had they accepted it. if the earth were not the central and all-important body in the universe, if the sun and planets and stars were not attendant and subsidiary lights, but were other worlds larger and perhaps superior to ours, where was man's place in the universe? and where were the doctrines they had maintained as irrefragable? i by no means assert that the new doctrines were really utterly irreconcilable with the more essential parts of the old dogmas, if only theologians had had patience and genius enough to consider the matter calmly. i suppose that in that case they might have reached the amount of reconciliation at present attained, and not only have left scientific truth in peace to spread as it could, but might perhaps themselves have joined the band of earnest students and workers, as so many of the higher catholic clergy do at the present day. but this was too much to expect. such a revelation was not to be accepted in a day or in a century--the easiest plan was to treat it as a heresy, and try to crush it out. not in copernik's life, however, did they perceive the dangerous tendency of the doctrine--partly because it was buried in a ponderous and learned treatise not likely to be easily understood; partly, perhaps, because its propounder was himself an ecclesiastic; mainly because he was a patient and judicious man, not given to loud or intolerant assertion, but content to state his views in quiet conversation, and to let them gently spread for thirty years before he published them. and, when he did publish them, he used the happy device of dedicating his great book to the pope, and a cardinal bore the expense of printing it. thus did the roman church stand sponsor to a system of truth against which it was destined in the next century to hurl its anathemas, and to inflict on its conspicuous adherents torture, imprisonment, and death. to realize the change of thought, the utterly new view of the universe, which the copernican theory introduced, we must go back to preceding ages, and try to recall the views which had been held as probable concerning the form of the earth and the motion of the heavenly bodies. [illustration: fig. .--homeric cosmogony.] the earliest recorded notion of the earth is the very natural one that it is a flat area floating in an illimitable ocean. the sun was a god who drove his chariot across the heavens once a day; and anaxagoras was threatened with death and punished with banishment for teaching that the sun was only a ball of fire, and that it might perhaps be as big as the country of greece. the obvious difficulty as to how the sun got back to the east again every morning was got over--not by the conjecture that he went back in the dark, nor by the idea that there was a fresh sun every day; though, indeed, it was once believed that the moon was created once a month, and periodically cut up into stars--but by the doctrine that in the northern part of the earth was a high range of mountains, and that the sun travelled round on the surface of the sea behind these. sometimes, indeed, you find a representation of the sun being rowed round in a boat. later on it was perceived to be necessary that the sun should be able to travel beneath the earth, and so the earth was supposed to be supported on pillars or on roots, or to be a dome-shaped body floating in air--much like dean swift's island of laputa. the elephant and tortoise of the hindu earth are, no doubt, emblematic or typical, not literal. [illustration: fig. .--egyptian symbol of the universe. the earth a figure with leaves, the heaven a figure with stars, the principle of equilibrium and support, the boats of the rising and setting sun.] aristotle, however, taught that the earth must be a sphere, and used all the orthodox arguments of the present children's geography-books about the way you see ships at sea, and about lunar eclipses. to imagine a possible antipodes must, however, have been a tremendous difficulty in the way of this conception of a sphere, and i scarcely suppose that any one can at that time have contemplated the possibility of such upside-down regions being inhabited. i find that intelligent children invariably feel the greatest difficulty in realizing the existence of inhabitants on the opposite side of the earth. stupid children, like stupid persons in general, will of course believe anything they are told, and much good may the belief do them; but the kind of difficulties felt by intelligent and thoughtful children are most instructive, since it is quite certain that the early philosophers must have encountered and overcome those very same difficulties by their own genius. [illustration: fig. .--hindoo earth.] however, somehow or other the conception of a spherical earth was gradually grasped, and the heavenly bodies were perceived all to revolve round it: some moving regularly, as the stars, all fixed together into one spherical shell or firmament; some moving irregularly and apparently anomalously--these irregular bodies were therefore called planets [or wanderers]. seven of them were known, viz. moon, mercury, venus, sun, mars, jupiter, saturn, and there is little doubt that this number seven, so suggested, is the origin of the seven days of the week. the above order of the ancient planets is that of their supposed distance from the earth. not always, however, are they thus quoted by the ancients: sometimes the sun is supposed nearer than mercury or venus. it has always been known that the moon was the nearest of the heavenly bodies; and some rough notion of its distance was current. mars, jupiter, and saturn were placed in that order because that is the order of their apparent motions, and it was natural to suppose that the slowest moving bodies were the furthest off. the order of the days of the week shows what astrologers considered to be the order of the planets; on their system of each successive hour of the day being ruled over by the successive planets taken in order. the diagram (fig. ) shows that if the sun rule the first hour of a certain day (thereby giving its name to the day) venus will rule the second hour, mercury the third, and so on; the sun will thus be found to rule the eighth, fifteenth, and twenty-second hour of that day, venus the twenty-third, and mercury the twenty-fourth hour; so the moon will rule the first hour of the next day, which will therefore be monday. on the same principle (numbering round the hours successively, with the arrows) the first hour of the next day will be found to be ruled by mars, or by the saxon deity corresponding thereto; the first hour of the day after, by mercury (_mercredi_), and so on (following the straight lines of the pattern). the order of the planets round the circle counter-clockwise, _i.e._ the direction of their proper motions, is that quoted above in the text. to explain the motion of the planets and reduce them to any sort of law was a work of tremendous difficulty. the greatest astronomer of ancient times was hipparchus, and to him the system known as the ptolemaic system is no doubt largely due. but it was delivered to the world mainly by ptolemy, and goes by his name. this was a fine piece of work, and a great advance on anything that had gone before; for although it is of course saturated with error, still it is based on a large substratum of truth. its superiority to all the previously mentioned systems is obvious. and it really did in its more developed form describe the observed motions of the planets. each planet was, in the early stages of this system, as taught, say, by eudoxus, supposed to be set in a crystal sphere, which revolved so as to carry the planet with it. the sphere had to be of crystal to account for the visibility of other planets and the stars through it. outside the seven planetary spheres, arranged one inside the other, was a still larger one in which were set the stars. this was believed to turn all the others, and was called the _primum mobile_. the whole system was supposed to produce, in its revolution, for the few privileged to hear the music of the spheres, a sound as of some magnificent harmony. [illustration: fig. .--order of ancient planets corresponding to the days of the week.] the enthusiastic disciples of pythagoras believed that their master was privileged to hear this noble chant; and far be it from us to doubt that the rapt and absorbing pleasure of contemplating the harmony of nature, to a man so eminently great as pythagoras, must be truly and adequately represented by some such poetic conception. [illustration: fig. .--ptolemaic system.] the precise kind of motion supposed to be communicated from the _primum mobile_ to the other spheres so as to produce the observed motions of the planets was modified and improved by various philosophers until it developed into the epicyclic train of hipparchus and of ptolemy. it is very instructive to observe a planet (say mars or jupiter) night after night and plot down its place with reference to the fixed stars on a celestial globe or star-map. or, instead of direct observation by alignment with known stars, it is easier to look out its right ascension and declination in _whitaker's almanac_, and plot those down. if this be done for a year or two, it will be found that the motion of the planet is by no means regular, but that though on the whole it advances it sometimes is stationary and sometimes goes back.[ ] [illustration: fig. .--specimens of apparent paths of venus and of mars among the stars.] [illustration: fig. .--apparent epicyclic orbits of jupiter and saturn; the earth being supposed fixed at the centre, with the sun revolving in a small circle. a loop is made by each planet every year.] these "stations" and "retrogressions" of the planets were well known to the ancients. it was not to be supposed for a moment that the crystal spheres were subject to any irregularity, neither was uniform circular motion to be readily abandoned; so it was surmised that the main sphere carried, not the planet itself, but the centre or axis of a subordinate sphere, and that the planet was carried by this. the minor sphere could be allowed to revolve at a different uniform pace from the main sphere, and so a curve of some complexity could be obtained. a curve described in space by a point of a circle or sphere, which itself is carried along at the same time, is some kind of cycloid; if the centre of the tracing circle travels along a straight line, we get the ordinary cycloid, the curve traced in air by a nail on a coach-wheel; but if the centre of the tracing circle be carried round another circle the curve described is called an epicycloid. by such curves the planetary stations and retrogressions could be explained. a large sphere would have to revolve once for a "year" of the particular planet, carrying with it a subsidiary sphere in which the planet was fixed; this latter sphere revolving once for a "year" of the earth. the actual looped curve thus described is depicted for jupiter and saturn in the annexed diagram (fig. .) it was long ago perceived that real material spheres were unnecessary; such spheres indeed, though possibly transparent to light, would be impermeable to comets: any other epicyclic gearing would serve, and as a mere description of the motion it is simpler to think of a system of jointed bars, one long arm carrying a shorter arm, the two revolving at different rates, and the end of the short one carrying the planet. this does all that is needful for the first approximation to a planet's motion. in so far as the motion cannot be thus truly stated, the short arm may be supposed to carry another, and that another, and so on, so that the resultant motion of the planet is compounded of a large number of circular motions of different periods; by this device any required amount of complexity could be attained. we shall return to this at greater length in lecture iii. the main features of the motion, as shown in the diagram, required only two arms for their expression; one arm revolving with the average motion of the planet, and the other revolving with the apparent motion of the sun, and always pointing in the same direction as the single arm supposed to carry the sun. this last fact is of course because the motion to be represented does not really belong to the planet at all, but to the earth, and so all the main epicyclic motions for the superior planets were the same. as for the inferior planets (mercury and venus) they only appear to oscillate like the bob of a pendulum about the sun, and so it is very obvious that they must be really revolving round it. an ancient egyptian system perceived this truth; but the ptolemaic system imagined them to revolve round the earth like the rest, with an artificial system of epicycles to prevent their ever getting far away from the neighbourhood of the sun. it is easy now to see how the copernican system explains the main features of planetary motion, the stations and retrogressions, quite naturally and without any complexity. [illustration: fig. .--egyptian system.] let the outer circle represent the orbit of jupiter, and the inner circle the orbit of the earth, which is moving faster than jupiter (since jupiter takes days to make one revolution); then remember that the apparent position of jupiter is referred to the infinitely distant fixed stars and refer to fig. . let e_ , e_ , &c., be successive positions of the earth; j_ , j_ , &c., corresponding positions of jupiter. produce the lines e_ j_ , e_ j_ , &c., to an enormously greater circle outside, and it will be seen that the termination of these lines, representing apparent positions of jupiter among the stars, advances while the earth goes from e_ to e_ ; is almost stationary from somewhere about e_ to e_ ; and recedes from e_ to e_ ; so that evidently the recessions of jupiter are only apparent, and are due to the orbital motion of the earth. the apparent complications in the path of jupiter, shown in fig. , are seen to be caused simply by the motion of the earth, and to be thus completely and easily explained. [illustration: fig. .--true orbits of earth and jupiter.] the same thing for an inferior planet, say mercury, is even still more easily seen (_vide_ figure ). the motion of mercury is direct from m'' to m''', retrograde from m''' to m'', and stationary at m'' and m'''. it appears to oscillate, taking · days for its direct swing, and · for its return swing. [illustration: fig. .--orbit of mercury and earth.] on this system no artificiality is required to prevent mercury's ever getting far from the sun: the radius of its orbit limits its real and apparent excursions. even if the earth were stationary, the motions of mercury and venus would not be _essentially_ modified, but the stations and retrogressions of the superior planets, mars, jupiter, &c., would wholly cease. the complexity of the old mode of regarding apparent motion may be illustrated by the case of a traveller in a railway train unaware of his own motion. it is as though trees, hedges, distant objects, were all flying past him and contorting themselves as you may see the furrows of a ploughed field do when travelling, while you yourself seem stationary amidst it all. how great a simplicity would be introduced by the hypothesis that, after all, these things might be stationary and one's self moving. [illustration: fig. .--copernican system as frequently represented. but the cometary orbit is a much later addition, and no attempt is made to show the relative distances of the planets.] now you are not to suppose that the system of copernicus swept away the entire doctrine of epicycles; that doctrine can hardly be said to be swept away even now. as a description of a planet's motion it is not incorrect, though it is geometrically cumbrous. if you describe the motion of a railway train by stating that every point on the rim of each wheel describes a cycloid with reference to the earth, and a circle with reference to the train, and that the motion of the train is compounded of these cycloidal and circular motions, you will not be saying what is false, only what is cumbrous. the ptolemaic system demanded large epicycles, depending on the motion of the earth, these are what copernicus overthrew; but to express the minuter details of the motion smaller epicycles remained, and grew more and more complex as observations increased in accuracy, until a greater man than either copernicus or ptolemy, viz. kepler, replaced them all by a simple ellipse. one point i must not omit from this brief notice of the work of copernicus. hipparchus had, by most sagacious interpretation of certain observations of his, discovered a remarkable phenomenon called the precession of the equinoxes. it was a discovery of the first magnitude, and such as would raise to great fame the man who should have made it in any period of the world's history, even the present. it is scarcely expressible in popular language, and without some technical terms; but i can try. the plane of the earth's orbit produced into the sky gives the apparent path of the sun throughout a year. this path is known as the ecliptic, because eclipses only happen when the moon is in it. the sun keeps to it accurately, but the planets wander somewhat above and below it (fig. ), and the moon wanders a good deal. it is manifest, however, in order that there may be an eclipse of any kind, that a straight line must be able to be drawn through earth and moon and sun (not necessarily through their centres of course), and this is impossible unless some parts of the three bodies are in one plane, viz. the ecliptic, or something very near it. the ecliptic is a great circle of the sphere, and is usually drawn on both celestial and terrestrial globes. the earth's equator also produced into the sky, where it may still be called the equator (sometimes it is awkwardly called "the equinoctial"), gives another great circle inclined to the ecliptic and cutting it at two opposite points, labelled respectively [aries symbol] and [libra symbol], and together called "the equinoxes." the reason for the name is that when the sun is in that part of the ecliptic it is temporarily also on the equator, and hence is symmetrically situated with respect to the earth's axis of rotation, and consequently day and night are equal all over the earth. well, hipparchus found, by plotting the position of the sun for a long time,[ ] that these points of intersection, or equinoxes, were not stationary from century to century, but slowly moved among the stars, moving as it were to meet the sun, so that he gets back to one of these points again minutes - / seconds before it has really completed a revolution, _i.e._ before the true year is fairly over. this slow movement forward of the goal-post is called precession--the precession of the equinoxes. (one result of it is to shorten our years by about minutes each; for the shortened period has to be called a year, because it is on the position of the sun with respect to the earth's axis that our seasons depend.) copernicus perceived that, assuming the motion of the earth, a clearer account of this motion could be given. the ordinary approximate statement concerning the earth's axis is that it remains parallel to itself, _i.e._ has a fixed direction as the earth moves round the sun. but if, instead of being thus fixed, it be supposed to have a slow movement of revolution, so that it traces out a cone in the course of about , years, then, since the equator of course goes with it, the motion of its intersection with the fixed ecliptic is so far accounted for. that is to say, the precession of the equinoxes is seen to be dependent on, and caused by, a slow conical movement of the earth's axis. the prolongation of each end of the earth's axis into the sky, or the celestial north and south poles, will thus slowly trace out an approximate circle among the stars; and the course of the north pole during historic time is exhibited in the annexed diagram. it is now situated near one of the stars of the lesser bear, which we therefore call the pole star; but not always was it so, nor will it be so in the future. the position of the north pole years ago is shown in the figure; and a revolution will be completed in something like , years.[ ] [illustration: fig. .--slow movement of the north pole in a circle among the stars. (copied from sir r. ball.)] this perception of the conical motion of the earth's axis was a beautiful generalization of copernik's, whereby a multitude of facts were grouped into a single phenomenon. of course he did not explain the motion of the axis itself. he stated the fact that it so moved, and i do not suppose it ever struck him to seek for an explanation. an explanation was given later, and that a most complete one; but the idea even of seeking for it is a brilliant and striking one: the achievement of the explanation by a single individual in the way it actually was accomplished is one of the most astounding things in the history of science; and were it not that the same individual accomplished a dozen other things, equally and some still more extraordinary, we should rank that man as one of the greatest astronomers that ever lived. as it is, he is sir isaac newton. we are to remember, then, as the life-work of copernicus, that he placed the sun in its true place as the centre of the solar system, instead of the earth; that he greatly simplified the theory of planetary motion by this step, and also by the simpler epicyclic chain which now sufficed, and which he worked out mathematically; that he exhibited the precession of the equinoxes (discovered by hipparchus) as due to a conical motion of the earth's axis; and that, by means of his simpler theory and more exact planetary tables, he reduced to some sort of order the confused chaos of the ptolemaic system, whose accumulation of complexity and of outstanding errors threatened to render astronomy impossible by the mere burden of its detail. there are many imperfections in his system, it is true; but his great merit is that he dared to look at the facts of nature with his own eyes, unhampered by the prejudice of centuries. a system venerable with age, and supported by great names, was universally believed, and had been believed for centuries. to doubt this system, and to seek after another and better one, at a time when all men's minds were governed by tradition and authority, and when to doubt was sin--this required a great mind and a high character. such a mind and such a character had this monk of frauenburg. and it is interesting to notice that the so-called religious scruples of smaller and less truly religious men did not affect copernicus; it was no dread of consequences to one form of truth that led him to delay the publication of the other form of truth specially revealed to him. in his dedication he says:-- "if there be some babblers who, though ignorant of all mathematics, take upon them to judge of these things, and dare to blame and cavil at my work, because of some passage of scripture which they have wrested to their own purpose, i regard them not, and will not scruple to hold their judgment in contempt." i will conclude with the words of one of his biographers (mr. e.j.c. morton):-- "copernicus cannot be said to have flooded with light the dark places of nature--in the way that one stupendous mind subsequently did--but still, as we look back through the long vista of the history of science, the dim titanic figure of the old monk seems to rear itself out of the dull flats around it, pierces with its head the mists that overshadow them, and catches the first gleam of the rising sun, "'... like some iron peak, by the creator fired with the red glow of the rushing morn.'" dates and summary of facts for lecture ii copernicus lived from to , and was contemporary with paracelsus and raphael. tycho brahé from to . kepler from to . galileo from to . gilbert from to . francis bacon from to . descartes from to . _a sketch of tycho brahé's life and work._ tycho was a danish noble, born on his ancestral estate at knudstorp, near helsinborg, in . adopted by his uncle, and sent to the university of copenhagen to study law. attracted to astronomy by the occurrence of an eclipse on its predicted day, august st, . began to construct astronomical instruments, especially a quadrant and a sextant. observed at augsburg and wittenberg. studied alchemy, but was recalled to astronomy by the appearance of a new star. overcame his aristocratic prejudices, and delivered a course of lectures at copenhagen, at the request of the king. after this he married a peasant girl. again travelled and observed in germany. in was sent for to denmark by frederick ii., and established in the island of huen, with an endowment enabling him to devote his life to astronomy. built uraniburg, furnished it with splendid instruments, and became the founder of accurate instrumental astronomy. his theories were poor, but his observations were admirable. in frederick died, and five years later, tycho was impoverished and practically banished. after wandering till , he was invited to prague by the emperor rudolf, and there received john kepler among other pupils. but the sentence of exile was too severe, and he died in , aged years. a man of strong character, untiring energy, and devotion to accuracy, his influence on astronomy has been immense. lecture ii tycho brahÉ and the earliest observatory we have seen how copernicus placed the earth in its true position in the solar system, making it merely one of a number of other worlds revolving about a central luminary. and observe that there are two phenomena to be thus accounted for and explained: first, the diurnal revolution of the heavens; second, the annual motion of the sun among the stars. the effect of the diurnal motion is conspicuous to every one, and explains the rising, southing, and setting of the whole visible firmament. the effect of the annual motion, _i.e._ of the apparent annual motion, of the sun among the stars, is less obvious, but it may be followed easily enough by observing the stars visible at any given time of evening at different seasons of the year. at midnight, for instance, the position of the sun is definite, viz. due north always, but the constellation which at that time is due south or is rising or setting varies with the time of year; an interval of one month producing just the same effect on the appearance of the constellations as an interval of two hours does (because the day contains twice as many hours as the year contains months), _e.g._ the sky looks the same at midnight on the st of october as it does at p.m. on the st of november. all these simple consequences of the geocentric as opposed to the heliocentric point of view were pointed out by copernicus, in addition to his greater work of constructing improved planetary tables on the basis of his theory. but it must be admitted that he himself felt the hypothesis of the motion of the earth to be a difficulty. its acceptance is by no means such an easy and childish matter as we are apt now to regard it, and the hostility to it is not at all surprising. the human race, after having ridiculed and resisted the truth for a long time, is apt to end in accepting it so blindly and unimaginatively as to fail to recognize the real achievement of its first propounders, or the difficulties which they had to overcome. the majority of men at the present day have grown accustomed to hear the motion of the earth spoken of: their acceptance of it means nothing: the attitude of the paradoxer who denies it is more intelligent. it is not to be supposed that the idea of thus explaining some of the phenomena of the heavens, especially the daily motion of the entire firmament, by a diurnal rotation of the earth had not struck any one. it was often at this time referred to as the pythagorean theory, and it had been taught, i believe, by aristarchus. but it was new to the modern world, and it had the great weight of aristotle against it. consequently, for long after copernicus, only a few leading spirits could be found to support it, and the long-established venerable ptolemaic system continued to be taught in all universities. the main objections to the motion of the earth were such as the following:-- . the motion is unfelt and difficult to imagine. that it is unfelt is due to its uniformity, and can be explained mechanically. that it is difficult to imagine is and remains true, but a most important lesson we have to learn is that difficulty of conception is no valid argument against reality. . that the stars do not alter their relative positions according to the season of the year, but the constellations preserve always the same aspect precisely, even to careful measurement. this is indeed a difficulty, and a great one. in june the earth is million miles away from where it was in december: how can we see precisely the same fixed stars? it is not possible, unless they are at a practically infinite distance. that is the only answer that can be given. it was the tentative answer given by copernicus. it is the correct answer. not only from every position of the earth, but from every planet of the solar system, the same constellations are visible, and the stars have the same aspect. the whole immensity of the solar system shrinks to practically a point when confronted with the distance of the stars. not, however, so entirely a speck as to resist the terrific accuracy of the present century, and their microscopic relative displacement with the season of the year has now at length been detected, and the distance of many thereby measured. . that, if the earth revolved round the sun, mercury and venus ought to show phases like the moon. so they ought. any globe must show phases if it live nearer the sun than we do and if we go round it, for we shall see varying amounts of its illuminated half. the only answer that copernicus could give to this was that they might be difficult to see without extra powers of sight, but he ventured to predict that the phases would be seen if ever our powers of vision should be enhanced. . that if the earth moved, or even revolved on its own axis, a stone or other dropped body ought to be left far behind. this difficulty is not a real one, like the two last, and it is based on an ignorance of the laws of mechanics, which had not at that time been formulated. we know now that a ball dropped from a high tower, so far from lagging, drops a minute trifle _in front_ of the foot of a perpendicular, because the top of the tower is moving a trace faster than the bottom, by reason of the diurnal rotation. but, ignoring this, a stone dropped from the lamp of a railway carriage drops in the centre of the floor, whether the carriage be moving steadily or standing still; a slant direction of fall could only be detected if the carriage were being accelerated or if the brake were applied. a body dropped from a moving carriage shares the motion of the carriage, and starts with that as its initial velocity. a ball dropped from a moving balloon does not simply drop, but starts off in whatever direction the car was moving, its motion being immediately modified by gravity, precisely in the same way as that of a thrown ball is modified. this is, indeed, the whole philosophy of throwing--to drop a ball from a moving carriage. the carriage is the hand, and, to throw far, a run is taken and the body is jerked forward; the arm is also moved as rapidly as possible on the shoulder as pivot. the fore-arm can be moved still faster, and the wrist-joint gives yet another motion: the art of throwing is to bring all these to bear at the same instant, and then just as they have all attained their maximum velocity to let the ball go. it starts off with the initial velocity thus imparted, and is abandoned to gravity. if the vehicle were able to continue its motion steadily, as a balloon does, the ball when let go from it would appear to the occupant simply to drop; and it would strike the ground at a spot vertically under the moving vehicle, though by no means vertically below the place where it started. the resistance of the air makes observations of this kind inaccurate, except when performed inside a carriage so that the air shares in the motion. otherwise a person could toss and catch a ball out of a train window just as well as if he were stationary; though to a spectator outside he would seem to be using great skill to throw the ball in the parabola adapted to bring it back to his hand. the same circumstance enhances the apparent difficulty of the circus rider's jumping feats. all he has to do is to jump up and down on the horse; the forward motion which carries him through hoops belongs to him by virtue of the motion of the horse, without effort on his part. thus, then, it happens that a stone dropped sixteen feet on the earth appears to fall straight down, although its real path in space is a very flat trajectory of nineteen miles base and sixteen feet height; nineteen miles being the distance traversed by the earth every second in the course of its annual journey round the sun. no wonder that it was thought that bodies must be left behind if the earth was subject to such terrific speed as this. all that copernicus could suggest on this head was that perhaps the atmosphere might help to carry things forward, and enable them to keep pace with the earth. there were thus several outstanding physical difficulties in the way of the acceptance of the copernican theory, besides the biblical difficulty. it was quite natural that the idea of the earth's motion should be repugnant, and take a long time to sink into the minds of men; and as scientific progress was vastly slower then than it is now, we find not only all priests but even some astronomers one hundred years afterwards still imagining the earth to be at rest. and among them was a very eminent one, tycho brahé. it is interesting to note, moreover, that the argument about the motion of the earth being contrary to scripture appealed not only to ecclesiastics in those days, but to scientific men also; and tycho brahé, being a man of great piety, and highly superstitious also, was so much influenced by it, that he endeavoured to devise some scheme by which the chief practical advantages of the copernican system could be retained, and yet the earth be kept still at the centre of the whole. this was done by making all the celestial sphere, with stars and everything, rotate round the earth once a day, as in the ptolemaic scheme; and then besides this making all the planets revolve round the sun, and this to revolve round the earth. such is the tychonic system. so far as _relative_ motion is concerned it comes to the same thing; just as when you drop a book you may say either that the earth rises to meet the book, or that the book falls to meet the earth. or when a fly buzzes round your head, you may say that you are revolving round the fly. but the absurdity of making the whole gigantic system of sun and planets and stars revolve round our insignificant earth was too great to be swallowed by other astronomers after they had once had a taste of the copernican theory; and accordingly the tychonic system died a speedy and an easy death at the same time as its inventor. wherein then lay the magnitude of the man?--not in his theories, which were puerile, but in his observations, which were magnificent. he was the first observational astronomer, the founder of the splendid system of practical astronomy which has culminated in the present greenwich observatory. [illustration: fig. .--tychonic system showing the sun with all the planets revolving round the earth.] up to tycho the only astronomical measurements had been of the rudest kind. copernicus even improved upon what had gone before, with measuring rules made with his own hands. ptolemy's observations could never be trusted to half a degree. tycho introduced accuracy before undreamed of, and though his measurements, reckoned by modern ideas, are of course almost ludicrously rough (remember no such thing as a telescope or microscope was then dreamed of), yet, estimated by the era in which they were made, they are marvels of accuracy, and not a single mistake due to carelessness has ever been detected in them. in fact they may be depended on almost to minutes of arc, _i.e._ to sixtieths of a degree. for certain purposes connected with the proper motion of stars they are still appealed to, and they served as the certain and trustworthy data for succeeding generations of theorists to work upon. it was long, indeed, after tycho's death before observations approaching in accuracy to his were again made. in every sense, therefore, he was a pioneer: let us proceed to trace his history. born the eldest son of a noble family--"as noble and ignorant as sixteen undisputed quarterings could make them," as one of his biographers says--in a period when, even more than at present, killing and hunting were the only natural aristocratic pursuits, when all study was regarded as something only fit for monks, and when science was looked at askance as something unsavoury, useless, and semi-diabolic, there was little in his introduction to the world urging him in the direction where his genius lay. of course he was destined for a soldier; but fortunately his uncle, george brahé, a more educated man than his father, having no son of his own, was anxious to adopt him, and though not permitted to do so for a time, succeeded in getting his way on the birth of a second son, steno--who, by the way, ultimately became privy councillor to the king of denmark. tycho's uncle gave him what he would never have got at home--a good education; and ultimately put him to study law. at the age of thirteen he entered the university of copenhagen, and while there occurred the determining influence of his life. an eclipse of the sun in those days was not regarded with the cold-blooded inquisitiveness or matter-of-fact apathy, according as there is or is not anything to be learnt from it, with which such an event is now regarded. every occurrence in the heavens was then believed to carry with it the destiny of nations and the fate of individuals, and accordingly was of surpassing interest. ever since the time of hipparchus it had been possible for some capable man here and there to predict the occurrence of eclipses pretty closely. the thing is not difficult. the prediction was not, indeed, to the minute and second, as it is now; but the day could usually be hit upon pretty accurately some time ahead, much as we now manage to hit upon the return of a comet--barring accidents; and the hour could be predicted as the event approached. well, the boy tycho, among others, watched for this eclipse on august st, ; and when it appeared at its appointed time, every instinct for the marvellous, dormant in his strong nature, awoke to strenuous life, and he determined to understand for himself a science permitting such wonderful possibilities of prediction. he was sent to leipzig with a tutor to go on with his study of law, but he seems to have done as little law as possible: he spent all his money on books and instruments, and sat up half the night studying and watching the stars. in he observed a conjunction of jupiter and saturn, the precursor, and _cause_ as he thought it, of the great plague. he found that the old planetary tables were as much as a month in error in fixing this event, and even the copernican tables were several days out; so he formed the resolve to devote his life to improving astronomical tables. this resolve he executed with a vengeance. his first instrument was a jointed ruler with sights for fixing the position of planets with respect to the stars, and observing their stations and retrogressions. by thus measuring the angles between a planet and two fixed stars, its position can be plotted down on a celestial map or globe. [illustration: fig. .--portrait of tycho.] in his uncle george died, and made tycho his heir. he returned to denmark, but met with nothing but ridicule and contempt for his absurd drivelling away of time over useless pursuits. so he went back to germany--first to wittenberg, thence, driven by the plague, to rostock. here his fiery nature led him into an absurd though somewhat dangerous adventure. a quarrel at some feast, on a mathematical point, with a countryman, manderupius, led to the fixing of a duel, and it was fought with swords at p.m. at the end of december, when, if there was any light at all, it must have been of a flickering and unsatisfactory nature. the result of this insane performance was that tycho got his nose cut clean off. he managed however to construct an artificial one, some say of gold and silver, some say of putty and brass; but whatever it was made of there is no doubt that he wore it for the rest of his life, and it is a most famous feature. it excited generally far more interest than his astronomical researches. it is said, moreover, to have very fairly resembled the original, but whether this remark was made by a friend or by an enemy i cannot say. one account says that he used to carry about with him a box of cement to apply whenever his nose came off, which it periodically did. about this time he visited augsburg, met with some kindred and enlightened spirits in that town, and with much enthusiasm and spirit constructed a great quadrant. these early instruments were tremendous affairs. a great number of workmen were employed upon this quadrant, and it took twenty men to carry it to its place and erect it. it stood in the open air for five years, and then was destroyed by a storm. with it he made many observations. [illustration: fig. .--early out-door quadrant of tycho; for observing altitudes by help of the sights _d_, _l_ and the plumb line.] on his return to denmark in , his fame preceded him, and he was much better received; and in order to increase his power of constructing instruments he took up the study of alchemy, and like the rest of the persuasion tried to make gold. the precious metals were by many old philosophers considered to be related in some way to the heavenly bodies: silver to the moon, for instance--as we still see by the name lunar caustic applied to nitrate of silver; gold to the sun, copper to mars, lead to saturn. hence astronomy and alchemy often went together. tycho all his life combined a little alchemy with his astronomical labours, and he constructed a wonderful patent medicine to cure all disorders, which had as wide a circulation in europe in its time as holloway's pills; he gives a tremendous receipt for it, with liquid gold and all manner of ingredients in it; among them, however, occurs a little antimony--a well-known sudorific--and to this, no doubt, whatever efficacy the medicine possessed was due. so he might have gone on wasting his time, were it not that in november, , a new star made its appearance, as they have done occasionally before and since. on the average one may say that about every fifty years a new star of fair magnitude makes its temporary appearance. they are now known to be the result of some catastrophe or collision, whereby immense masses of incandescent gas are produced. this one seen by tycho became as bright as jupiter, and then died away in about a year and a half. tycho observed all its changes, and endeavoured to measure its distance from the earth, with the result that it was proved to belong to the region of the fixed stars, at an immeasurable distance, and was not some nearer and more trivial phenomenon. he was asked by the university of copenhagen to give a course of lectures on astronomy; but this was a step he felt some aristocratic aversion to, until a little friendly pressure was brought to bear upon him by a request from the king, and delivered they were. he now seems to have finally thrown off his aristocratic prejudices, and to have indulged himself in treading on the corns of nearly all the high and mighty people he came into contact with. in short, he became what we might now call a violent radical; but he was a good-hearted man, nevertheless, and many are the tales told of his visits to sick peasants, of his consulting the stars as to their fate--all in perfect good faith--and of the medicines which he concocted and prescribed for them. the daughter of one of these peasants he married, and very happy the marriage seems to have been. [illustration: fig. .--map of denmark, showing the island of huen. _walker & boutallse._] now comes the crowning episode in tycho's life. frederick ii., realizing how eminent a man they had among them, and how much he could do if only he had the means--for we must understand that tycho, though of good family and well off, was by no means what we would call a wealthy man--frederick ii. made him a splendid and enlightened offer. the offer was this: that if tycho would agree to settle down and make his astronomical observations in denmark, he should have an estate in norway settled upon him, a pension of £ a year for life, a site for a large observatory, and £ , to build it with. [illustration: fig. .--uraniburg.] [illustration: fig. .--astrolabe. an old instrument with sights for marking the positions of the celestial bodies roughly. a sort of skeleton celestial globe.] [illustration: sextans astronomicvs trigonicvs pro distantiis rimandis. fig. .--tycho's large sextant; for measuring the angular distance between two bodies by direct sighting.] well, if ever money was well spent, this was. by its means denmark before long headed the nations of europe in the matter of science--a thing it has not done before or since. the site granted was the island of huen, between copenhagen and elsinore; and here the most magnificent observatory ever built was raised, and called uraniburg--the castle of the heavens. it was built on a hill in the centre of the island, and included gardens, printing shops, laboratory, dwelling-houses, and four observatories--all furnished with the most splendid instruments that tycho could devise, and that could then be constructed. it was decorated with pictures and sculptures of eminent men, and altogether was a most gorgeous place. £ , no doubt went far in those days, but the original grant was supplemented by tycho himself, who is said to have spent another equal sum out of his own pocket on the place. [illustration: qvadrans maximvs chalibeus quadrato inclusus, et horizonti azimuthali chalybeo insistens. fig. .--the quadrant in uraniburg; or altitude and azimuth instrument.] for twenty years this great temple of science was continually worked in by him, and he soon became the foremost scientific man in europe. philosophers, statesmen, and occasionally kings, came to visit the great astronomer, and to inspect his curiosities. [illustration: qvadrans mvralis sive tichonicus. fig. .--tycho's form of transit circle. the method of utilising the extremely uniform rotation of the earth by watching the planets and stars as they cross the meridian, and recording their times of transit; observing also at the same time their meridian altitudes (see observer _f_), was the invention of tycho, and constitutes his greatest achievement. his method is followed to this day in all observatories.] [illustration: fig. .--a modern transit circle, showing essentially the same parts as in tycho's instrument, viz. the observer watching the transit, the clock, the recorder of the observation, and the graduated circle; the latter to be read by a second observer.] and very wholesome for some of these great personages must have been the treatment they met with. for tycho was no respecter of persons. his humbly-born wife sat at the head of the table, whoever was there; and he would snub and contradict a chancellor just as soon as he would a serf. whatever form his pride may have taken when a youth, in his maturity it impelled him to ignore differences of rank not substantially justified, and he seemed to take a delight in exposing the ignorance of shallow titled persons, to whom contradiction and exposure were most unusual experiences. for sick peasants he would take no end of trouble, and went about doctoring them for nothing, till he set all the professional doctors against him; so that when his day of misfortune came, as come it did, their influence was not wanting to help to ruin one who spoilt their practice, and whom they derided as a quack. but some of the great ignorant folk who came to visit his temple of science, and to inspect its curiosities, felt themselves insulted--not always without reason. he kept a tame maniac in the house, named lep, and he used to regard the sayings of this personage as oracular, presaging future events, and far better worth listening to than ordinary conversation. consequently he used to have him at his banquets and feed him himself; and whenever lep opened his mouth to speak, every one else was peremptorily ordered to hold his tongue, so that lep's words might be written down. in fact it was something like an exaggerated edition of betsy trotwood and mr. dick. "it must have been an odd dinner party" (says prof. stuart), "with this strange, wild, terribly clever man, with his red hair and brazen nose, sometimes flashing with wit and knowledge, sometimes making the whole company, princes and servants alike, hold their peace and listen humbly to the ravings of a poor imbecile." to people he despised he did not show his serious instruments. he had other attractions, in the shape of a lot of toy machinery, little windmills, and queer doors, and golden globes, and all manner of ingenious tricks and automata, many of which he had made himself, and these he used to show them instead; and no doubt they were well enough pleased with them. those of the visitors, however, who really cared to see and understand his instruments, went away enchanted with his genius and hospitality. i may, perhaps, be producing an unfair impression of imperiousness and insolence. tycho was fiery, no doubt, but i think we should wrong him if we considered him insolent. most of the nobles of his day were haughty persons, accustomed to deal with serfs, and very likely to sneer at and trample on any meek man of science whom they could easily despise. so tycho was not meek; he stood up for the honour of his science, and paid them back in their own coin, with perhaps a little interest. that this behaviour was not worldly-wise is true enough, but i know of no commandment enjoining us to be worldly-wise. if we knew more about his so-called imbecile _protégé_ we should probably find some reason for the interest which tycho took in him. whether he was what is now called a "clairvoyant" or not, tycho evidently regarded his utterances as oracular, and of course when one is receiving what may be a revelation from heaven it is natural to suppress ordinary conversation. among the noble visitors whom he received and entertained, it is interesting to notice james i. of england, who spent eight days at uraniburg on the occasion of his marriage with anne of denmark in , and seems to have been deeply impressed by his visit. among other gifts, james presented tycho with a dog (depicted in fig. ), and this same animal was subsequently the cause of trouble. for it seems that one day the chancellor of denmark, walchendorf, brutally kicked the poor beast; and tycho, who was very fond of animals, gave him a piece of his mind in no measured language. walchendorf went home determined to ruin him. king frederick, however, remained his true friend, doubtless partly influenced thereto by his queen sophia, an enlightened woman who paid many visits to uraniburg, and knew tycho well. but unfortunately frederick died; and his son, a mere boy, came to the throne. now was the time for the people whom tycho had offended, for those who were jealous of his great fame and importance, as well as for those who cast longing eyes on his estate and endowments. the boy-king, too, unfortunately paid a visit to tycho, and, venturing upon a decided opinion on some recondite subject, received a quiet setting down which he ill relished. letters written by tycho about this time are full of foreboding. he greatly dreads having to leave uraniburg, with which his whole life has for twenty years been bound up. he tries to comfort himself with the thought that, wherever he is sent, he will have the same heavens and the same stars over his head. gradually his norwegian estate and his pension were taken away, and in five years poverty compelled him to abandon his magnificent temple, and to take a small house in copenhagen. not content with this, walchendorf got a royal commission appointed to inquire into the value of his astronomical labours. this sapient body reported that his work was not only useless, but noxious; and soon after he was attacked by the populace in the public street. nothing was left for him now but to leave the country, and he went into germany, leaving his wife and instruments to follow him whenever he could find a home for them. his wanderings in this dark time--some two years--are not quite clear; but at last the enlightened emperor of bohemia, rudolph ii., invited him to settle in prague. thither he repaired, a castle was given him as an observatory, a house in the city, and crowns a year for life. so his instruments were set up once more, students flocked to hear him and to receive work at his hands--among them a poor youth, john kepler, to whom he was very kind, and who became, as you know, a still greater man than his master. but the spirit of tycho was broken, and though some good work was done at prague--more observations made, and the rudolphine tables begun--yet the hand of death was upon him. a painful disease seized him, attended with sleeplessness and temporary delirium, during the paroxysms of which he frequently exclaimed, _ne frustra vixisse videar_. ("oh that it may not appear that i have lived in vain!") quietly, however, at last, and surrounded by his friends and relatives, this fierce, passionate soul passed away, on the th of october, . his beloved instruments, which were almost a part of himself, were stored by rudolph in a museum with scrupulous care, until the taking of prague by the elector palatine's troops. in this disturbed time they got smashed, dispersed, and converted to other purposes. one thing only was saved--the great brass globe, which some thirty years after was recognized by a later king of denmark as having belonged to tycho, and deposited in the library of the academy of sciences at copenhagen, where i believe it is to this day. the island of huen was overrun by the danish nobility, and nothing now remains of uraniburg but a mound of earth and two pits. as to the real work of tycho, that has become immortal enough,--chiefly through the labours of his friend and scholar whose life we shall consider in the next lecture. summary of facts for lecture iii _life and work of kepler._ kepler was born in december, , at weil in würtemberg. father an officer in the duke's army, mother something of a virago, both very poor. kepler was utilized as a tavern pot-boy, but ultimately sent to a charity school, and thence to the university of tübingen. health extremely delicate; he was liable to violent attacks all his life. studied mathematics, and accepted an astronomical lectureship at graz as the first post which offered. endeavoured to discover some connection between the number of the planets, their times of revolution, and their distances from the sun. ultimately hit upon his fanciful regular-solid hypothesis, and published his first book in . in was invited by tycho to prague, and there appointed imperial mathematician, at a handsome but seldom paid salary. observed the new star of . endeavoured to find the law of refraction of light from vitellio's measurements, but failed. analyzed tycho's observations to find the true law of motion of mars. after incredible labour, through innumerable wrong guesses, and six years of almost incessant calculation, he at length emerged in his two "laws"--discoveries which swept away all epicycles, deferents, equants, and other remnants of the greek system, and ushered in the dawn of modern astronomy. law i. _planets move in ellipses, with the sun in one focus._ law ii. _the radius vector (or line joining sun and planet) sweeps out equal areas in equal times._ published his second book containing these laws in . death of rudolph in , and subsequent increased misery and misfortune of kepler. ultimately discovered the connection between the times and distances of the planets for which he had been groping all his mature life, and announced it in :-- law iii. _the square of the time of revolution (or year) of each planet is proportional to the cube of its mean distance from the sun._ the book in which this law was published ("on celestial harmonies") was dedicated to james of england. in had to intervene to protect his mother from being tortured for witchcraft. accepted a professorship at linz. published the rudolphine tables in , embodying tycho's observations and his own theory. made a last effort to overcome his poverty by getting the arrears of his salary paid at prague, but was unsuccessful, and, contracting brain fever on the journey, died in november, , aged . a man of keen imagination, indomitable perseverance, and uncompromising love of truth, kepler overcame ill-health, poverty, and misfortune, and placed himself in the very highest rank of scientific men. his laws, so extraordinarily discovered, introduced order and simplicity into what else would have been a chaos of detailed observations; and they served as a secure basis for the splendid erection made on them by newton. _seven planets of the ptolemaic system--_ moon, mercury, venus, sun, mars, jupiter, saturn. _six planets of the copernican system--_ mercury, venus, earth, mars, jupiter, saturn. _the five regular solids, in appropriate order--_ octahedron, icosahedron, dodecahedron, tetrahedron, cube. _table illustrating kepler's third law._ +---------+---------------+-----------+---------------+----------------+ | | mean distance | length | cube of the | square of the | | planet. | from sun. | of year. | distance. | time. | | | d | t | d^ | t^ | +---------+---------------+-----------+---------------+----------------+ | mercury | · | · | · | · | | venus | · | · | · | · | | earth | · | · | · | · | | mars | · | · | · | · | | jupiter | · | · | · | · | | saturn | · | · | · | · | +---------+---------------+-----------+---------------+----------------+ the length of the earth's year is · days; its mean distance from the sun, taken above as unity, is , , miles. lecture iii kepler and the laws of planetary motion it is difficult to imagine a stronger contrast between two men engaged in the same branch of science than exists between tycho brahé, the subject of last lecture, and kepler, our subject on the present occasion. the one, rich, noble, vigorous, passionate, strong in mechanical ingenuity and experimental skill, but not above the average in theoretical and mathematical power. the other, poor, sickly, devoid of experimental gifts, and unfitted by nature for accurate observation, but strong almost beyond competition in speculative subtlety and innate mathematical perception. the one is the complement of the other; and from the fact of their following each other so closely arose the most surprising benefits to science. the outward life of kepler is to a large extent a mere record of poverty and misfortune. i shall only sketch in its broad features, so that we may have more time to attend to his work. he was born (so his biographer assures us) in longitude ° ', latitude ° ', on the st of december, . his parents seem to have been of fair condition, but by reason, it is said, of his becoming surety for a friend, the father lost all his slender income, and was reduced to keeping a tavern. young john kepler was thereupon taken from school, and employed as pot-boy between the ages of nine and twelve. he was a sickly lad, subject to violent illnesses from the cradle, so that his life was frequently despaired of. ultimately he was sent to a monastic school and thence to the university of tübingen, where he graduated second on the list. meanwhile home affairs had gone to rack and ruin. his father abandoned the home, and later died abroad. the mother quarrelled with all her relations, including her son john; who was therefore glad to get away as soon as possible. all his connection with astronomy up to this time had been the hearing the copernican theory expounded in university lectures, and defending it in a college debating society. an astronomical lectureship at graz happening to offer itself, he was urged to take it, and agreed to do so, though stipulating that it should not debar him from some more brilliant profession when there was a chance. for astronomy in those days seems to have ranked as a minor science, like mineralogy or meteorology now. it had little of the special dignity with which the labours of kepler himself were destined so greatly to aid in endowing it. well, he speedily became a thorough copernican, and as he had a most singularly restless and inquisitive mind, full of appreciation of everything relating to number and magnitude--was a born speculator and thinker just as mozart was a born musician, or bidder a born calculator--he was agitated by questions such as these: why are there exactly six planets? is there any connection between their orbital distances, or between their orbits and the times of describing them? these things tormented him, and he thought about them day and night. it is characteristic of the spirit of the times--this questioning why there should be six planets. nowadays, we should simply record the fact and look out for a seventh. then, some occult property of the number six was groped for, such as that it was equal to + + and likewise equal to × × , and so on. many fine reasons had been given for the seven planets of the ptolemaic system (see, for instance, p. ), but for the six planets of the copernican system the reasons were not so cogent. again, with respect to their successive distances from the sun, some law would seem to regulate their distance, but it was not known. (parenthetically i may remark that it is not known even now: a crude empirical statement known as bode's law--see page --is all that has been discovered.) once more, the further the planet the slower it moved; there seemed to be some law connecting speed and distance. this also kepler made continual attempts to discover. [illustration: fig. .--orbits of some of the planets drawn to scale: showing the gap between mars and jupiter.] one of his ideas concerning the law of the successive distances was based on the inscription of a triangle in a circle. if you inscribe in a circle a large number of equilateral triangles, they envelop another circle bearing a definite ratio to the first: these might do for the orbits of two planets (see fig. ). then try inscribing and circumscribing squares, hexagons, and other figures, and see if the circles thus defined would correspond to the several planetary orbits. but they would not give any satisfactory result. brooding over this disappointment, the idea of trying solid figures suddenly strikes him. "what have plane figures to do with the celestial orbits?" he cries out; "inscribe the regular solids." and then--brilliant idea--he remembers that there are but five. euclid had shown that there could be only five regular solids.[ ] the number evidently corresponds to the gaps between the six planets. the reason of there being only six seems to be attained. this coincidence assures him he is on the right track, and with great enthusiasm and hope he "represents the earth's orbit by a sphere as the norm and measure of all"; round it he circumscribes a dodecahedron, and puts another sphere round that, which is approximately the orbit of mars; round that, again, a tetrahedron, the corners of which mark the sphere of the orbit of jupiter; round that sphere, again, he places a cube, which roughly gives the orbit of saturn. [illustration: fig. .--many-sided polygon or approximate circle enveloped by straight lines, as for instance by a number of equilateral triangles.] on the other hand, he inscribes in the sphere of the earth's orbit an icosahedron; and inside the sphere determined by that, an octahedron; which figures he takes to inclose the spheres of venus and of mercury respectively. the imagined discovery is purely fictitious and accidental. first of all, eight planets are now known; and secondly, their real distances agree only very approximately with kepler's hypothesis. [illustration: fig. .--frameworks with inscribed and circumscribed spheres, representing the five regular solids distributed as kepler supposed them to be among the planetary orbits. (see "summary" at beginning of this lecture, p. .)] nevertheless, the idea gave him great delight. he says:--"the intense pleasure i have received from this discovery can never be told in words. i regretted no more the time wasted; i tired of no labour; i shunned no toil of reckoning, days and nights spent in calculation, until i could see whether my hypothesis would agree with the orbits of copernicus, or whether my joy was to vanish into air." he then went on to speculate as to the cause of the planets' motion. the old idea was that they were carried round by angels or celestial intelligences. kepler tried to establish some propelling force emanating from the sun, like the spokes of a windmill. this first book of his brought him into notice, and served as an introduction to tycho and to galileo. tycho brahé was at this time at prague under the patronage of the emperor rudolph; and as he was known to have by far the best planetary observations of any man living, kepler wrote to him to know if he might come and examine them so as to perfect his theory. tycho immediately replied, "come, not as a stranger, but as a very welcome friend; come and share in my observations with such instruments as i have with me, and as a dearly beloved associate." after this visit, tycho wrote again, offering him the post of mathematical assistant, which after hesitation was accepted. part of the hesitation kepler expresses by saying that "for observations his sight was dull, and for mechanical operations his hand was awkward. he suffered much from weak eyes, and dare not expose himself to night air." in all this he was, of course, the antipodes of tycho, but in mathematical skill he was greatly his superior. on his way to prague he was seized with one of his periodical illnesses, and all his means were exhausted by the time he could set forward again, so that he had to apply for help to tycho. it is clear, indeed, that for some time now he subsisted entirely on the bounty of tycho, and he expresses himself most deeply grateful for all the kindness he received from that noble and distinguished man, the head of the scientific world at that date. to illustrate tycho's kindness and generosity, i must read you a letter written to him by kepler. it seems that kepler, on one of his absences from prague, driven half mad with poverty and trouble, fell foul of tycho, whom he thought to be behaving badly in money matters to him and his family, and wrote him a violent letter full of reproaches and insults. tycho's secretary replied quietly enough, pointing out the groundlessness and ingratitude of the accusation. kepler repents instantly, and replies:-- "most noble tycho," (these are the words of his letter), "how shall i enumerate or rightly estimate your benefits conferred on me? for two months you have liberally and gratuitously maintained me, and my whole family; you have provided for all my wishes; you have done me every possible kindness; you have communicated to me everything you hold most dear; no one, by word or deed, has intentionally injured me in anything; in short, not to your children, your wife, or yourself have you shown more indulgence than to me. this being so, as i am anxious to put on record, i cannot reflect without consternation that i should have been so given up by god to my own intemperance as to shut my eyes on all these benefits; that, instead of modest and respectful gratitude, i should indulge for three weeks in continual moroseness towards all your family, in headlong passion and the utmost insolence towards yourself, who possess so many claims on my veneration, from your noble family, your extraordinary learning, and distinguished reputation. whatever i have said or written against the person, the fame, the honour, and the learning of your excellency; or whatever, in any other way, i have injuriously spoken or written (if they admit no other more favourable interpretation), as, to my grief, i have spoken and written many things, and more than i can remember; all and everything i recant, and freely and honestly declare and profess to be groundless, false, and incapable of proof." tycho accepted the apology thus heartily rendered, and the temporary breach was permanently healed. in , kepler was appointed "imperial mathematician," to assist tycho in his calculations. the emperor rudolph did a good piece of work in thus maintaining these two eminent men, but it is quite clear that it was as astrologers that he valued them; and all he cared for in the planetary motions was limited to their supposed effect on his own and his kingdom's destiny. he seems to have been politically a weak and superstitious prince, who was letting his kingdom get into hopeless confusion, and entangling himself in all manner of political complications. while bohemia suffered, however, the world has benefited at his hands; and the tables upon which tycho was now engaged are well called the rudolphine tables. these tables of planetary motion tycho had always regarded as the main work of his life; but he died before they were finished, and on his death-bed he intrusted the completion of them to kepler, who loyally undertook their charge. the imperial funds were by this time, however, so taxed by wars and other difficulties that the tables could only be proceeded with very slowly, a staff of calculators being out of the question. in fact, kepler could not get even his own salary paid: he got orders, and promises, and drafts on estates for it; but when the time came for them to be honoured they were worthless, and he had no power to enforce his claims. so everything but brooding had to be abandoned as too expensive, and he proceeded to study optics. he gave a very accurate explanation of the action of the human eye, and made many hypotheses, some of them shrewd and close to the mark, concerning the law of refraction of light in dense media: but though several minor points of interest turned up, nothing of the first magnitude came out of this long research. the true law of refraction was discovered some years after by a dutch professor, willebrod snell. we must now devote a little time to the main work of kepler's life. all the time he had been at prague he had been making a severe study of the motion of the planet mars, analyzing minutely tycho's books of observations, in order to find out, if possible, the true theory of his motion. aristotle had taught that circular motion was the only perfect and natural motion, and that the heavenly bodies therefore necessarily moved in circles. so firmly had this idea become rooted in men's minds, that no one ever seems to have contemplated the possibility of its being false or meaningless. when hipparchus and others found that, as a matter of fact, the planets did _not_ revolve in simple circles, they did not try other curves, as we should at once do now, but they tried combinations of circles, as we saw in lecture i. the small circle carried by a bigger one was called an epicycle. the carrying circle was called the deferent. if for any reason the earth had to be placed out of the centre, the main planetary orbit was called an excentric, and so on. but although the planetary paths might be roughly represented by a combination of circles, their speeds could not, on the hypothesis of uniform motion in each circle round the earth as a fixed body. hence was introduced the idea of an equant, _i.e._ an arbitrary point, not the earth, about which the speed might be uniform. copernicus, by making the sun the centre, had been able to simplify a good deal of this, and to abolish the equant. but now that kepler had the accurate observations of tycho to refer to, he found immense difficulty in obtaining the true positions of the planets for long together on any such theory. he specially attacked the motion of the planet mars, because that was sufficiently rapid in its changes for a considerable collection of data to have accumulated with respect to it. he tried all manner of circular orbits for the earth and for mars, placing them in all sorts of aspects with respect to the sun. the problem to be solved was to choose such an orbit and such a law of speed, for both the earth and mars, that a line joining them, produced out to the stars, should always mark correctly the apparent position of mars as seen from the earth. he had to arrange the size of the orbits that suited best, then the positions of their centres, both being supposed excentric with respect to the sun; but he could not get any such arrangement to work with uniform motion about the sun. so he reintroduced the equant, and thus had another variable at his disposal--in fact, two, for he had an equant for the earth and another for mars, getting a pattern of the kind suggested in fig. . the equants might divide the line in any arbitrary ratio. all sorts of combinations had to be tried, the relative positions of the earth and mars to be worked out for each, and compared with tycho's recorded observations. it was easy to get them to agree for a short time, but sooner or later a discrepancy showed itself. [illustration: fig. .--_s_ represents the sun; _ec_, the centre of the earth's orbit, to be placed as best suited; _mc_, the same for mars; _ee_, the earth's equant, or point about which the earth uniformly revolved (_i.e._ the point determining the law of speed about the sun), likewise to be placed anywhere, but supposed to be in the line joining _s_ to _ec_; _me_, the same thing for mars; with _?me_ for an alternative hypothesis that perhaps mars' equant was on line joining _ec_ with _mc_.] i need not say that all these attempts and gropings, thus briefly summarized, entailed enormous labour, and required not only great pertinacity, but a most singularly constituted mind, that could thus continue groping in the dark without a possible ray of theory to illuminate its search. grope he did, however, with unexampled diligence. at length he hit upon a point that seemed nearly right. he thought he had found the truth; but no, before long the position of the planet, as calculated, and as recorded by tycho, differed by eight minutes of arc, or about one-eighth of a degree. could the observation be wrong by this small amount? no, he had known tycho, and knew that he was never wrong eight minutes in an observation. so he set out the whole weary way again, and said that with those eight minutes he would yet find out the law of the universe. he proceeded to see if by making the planet librate, or the plane of its orbit tilt up and down, anything could be done. he was rewarded by finding that at any rate the plane of the orbit did not tilt up and down: it was fixed, and this was a simplification on copernicus's theory. it is not an absolute fixture, but the changes are very small (see laplace, page ). [illustration: fig. .--excentric circle supposed to be divided into equal areas. the sun, _s_, being placed at a selected point, it was possible to represent the varying speed of a planet by saying that it moved from _a_ to _b_, from _b_ to _c_, and so on, in equal times.] at last he thought of giving up the idea of _uniform_ circular motion, and of trying _varying_ circular motion, say inversely as its distance from the sun. to simplify calculation, he divided the orbit into triangles, and tried if making the triangles equal would do. a great piece of luck, they did beautifully: the rate of description of areas (not arcs) is uniform. over this discovery he greatly rejoices. he feels as though he had been carrying on a war against the planet and had triumphed; but his gratulation was premature. before long fresh little errors appeared, and grew in importance. thus he announces it himself:-- "while thus triumphing over mars, and preparing for him, as for one already vanquished, tabular prisons and equated excentric fetters, it is buzzed here and there that the victory is vain, and that the war is raging anew as violently as before. for the enemy left at home a despised captive has burst all the chains of the equations, and broken forth from the prisons of the tables." still, a part of the truth had been gained, and was not to be abandoned any more. the law of speed was fixed: that which is now known as his second law. but what about the shape of the orbit--was it after all possible that aristotle, and every philosopher since aristotle, had been wrong? that circular motion was not the perfect and natural motion, but that planets might move in some other closed curve? suppose he tried an oval. well, there are a great variety of ovals, and several were tried: with the result that they could be made to answer better than a circle, but still were not right. now, however, the geometrical and mathematical difficulties of calculation, which before had been tedious and oppressive, threatened to become overwhelming; and it is with a rising sense of despondency that kepler sees his six years' unremitting labour leading deeper and deeper into complication. one most disheartening circumstance appeared, viz. that when he made the circuit oval his law of equable description of areas broke down. that seemed to require the circular orbit, and yet no circular orbit was quite accurate. while thinking and pondering for weeks and months over this new dilemma and complication of difficulties, till his brain reeled, an accidental ray of light broke upon him in a way not now intelligible, or barely intelligible. half the extreme breadth intercepted between the circle and oval was / , of the radius, and he remembered that the "optical inequality" of mars was also about / , . this coincidence, in his own words, woke him out of sleep; and for some reason or other impelled him instantly to try making the planet oscillate in the diameter of its epicycle instead of revolve round it--a singular idea, but copernicus had had a similar one to explain the motions of mercury. [illustration: fig. .--mode of drawing an ellipse. the two pins _f_ are the foci.] away he started through his calculations again. a long course of work night and day was rewarded by finding that he was now able to hit off the motions better than before; but what a singularly complicated motion it was. could it be expressed no more simply? yes, the curve so described by the planet is a comparatively simple one: it is a special kind of oval--the ellipse. strange that he had not thought of it before. it was a famous curve, for the greek geometers had studied it as one of the sections of a cone, but it was not so well known in kepler's time. the fact that the planets move in it has raised it to the first importance, and it is familiar enough to us now. but did it satisfy the law of speed? could the rate of description of areas be uniform with it? well, he tried the ellipse, and to his inexpressible delight he found that it did satisfy the condition of equable description of areas, if the sun was in one focus. so, moving the planet in a selected ellipse, with the sun in one focus, at a speed given by the equable area description, its position agreed with tycho's observations within the limits of the error of experiment. mars was finally conquered, and remains in his prison-house to this day. the orbit was found. [illustration: fig. .] in a paroxysm of delight kepler celebrates his victory by a triumphant figure, sketched actually on his geometrical diagram--the diagram which proves that the law of equable description of areas can hold good with an ellipse. the above is a tracing of it. such is a crude and bald sketch of the steps by which kepler rose to his great generalizations--the two laws which have immortalized his name. all the complications of epicycle, equant, deferent, excentric, and the like, were swept at once away, and an orbit of striking and beautiful properties substituted. well might he be called, as he was, "the legislator," or law interpreter, "of the heavens." [illustration: fig. .--if _s_ is the sun, a planet or comet moves from _p_ to _p_ _, from _p_ _ to _p_ _, and from _p_ _ to _p_ _ in the same time; if the shaded areas are equal.] he concludes his book on the motions of mars with a half comic appeal to the emperor to provide him with the sinews of war for an attack on mars's relations--father jupiter, brother mercury, and the rest--but the death of his unhappy patron in put an end to all these schemes, and reduced kepler to the utmost misery. while at prague his salary was in continual arrear, and it was with difficulty that he could provide sustenance for his family. he had been there eleven years, but they had been hard years of poverty, and he could leave without regret were it not that he should have to leave tycho's instruments and observations behind him. while he was hesitating what best to do, and reduced to the verge of despair, his wife, who had long been suffering from low spirits and despondency, and his three children, were taken ill; one of the sons died of small-pox, and the wife eleven days after of low fever and epilepsy. no money could be got at prague, so after a short time he accepted a professorship at linz, and withdrew with his two quite young remaining children. he provided for himself now partly by publishing a prophesying almanack, a sort of zadkiel arrangement--a thing which he despised, but the support of which he could not afford to do without. he is continually attacking and throwing sarcasm at astrology, but it was the only thing for which people would pay him, and on it after a fashion he lived. we do not find that his circumstances were ever prosperous, and though , crowns were due to him from bohemia he could not manage to get them paid. about this time occurred a singular interruption to his work. his old mother, of whose fierce temper something has already been indicated, had been engaged in a law-suit for some years near their old home in würtemberg. a change of judge having in process of time occurred, the defendant saw his way to turn the tables on the old lady by accusing her of sorcery. she was sent to prison, and condemned to the torture, with the usual intelligent idea of extracting a "voluntary" confession. kepler had to hurry from linz to interpose. he succeeded in saving her from the torture, but she remained in prison for a year or so. her spirit, however, was unbroken, for no sooner was she released than she commenced a fresh action against her accuser. but fresh trouble was averted by the death of the poor old dame at the age of nearly eighty. this narration renders the unflagging energy shown by her son in his mathematical wrestlings less surprising. interspersed with these domestic troubles, and with harassing and unsuccessful attempts to get his rights, he still brooded over his old problem of some possible connection between the distances of the planets from the sun and their times of revolution, _i.e._ the length of their years. it might well have been that there was no connection, that it was purely imaginary, like his old idea of the law of the successive distances of the planets, and like so many others of the guesses and fancies which he entertained and spent his energies in probing. but fortunately this time there was a connection, and he lived to have the joy of discovering it. the connection is this, that if one compares the distance of the different planets from the sun with the length of time they take to go round him, the cube of the respective distances is proportional to the square of the corresponding times. in other words, the ratio of r^ to t^ for every planet is the same. or, again, the length of a planet's year depends on the / th power of its distance from the sun. or, once more, the speed of each planet in its orbit is as the inverse square-root of its distance from the sun. the product of the distance into the square of the speed is the same for each planet. this (however stated) is called kepler's third law. it welds the planets together, and shows them to be one system. his rapture on detecting the law was unbounded, and he breaks out into an exulting rhapsody:-- "what i prophesied two-and-twenty years ago, as soon as i discovered the five solids among the heavenly orbits--what i firmly believed long before i had seen ptolemy's _harmonies_--what i had promised my friends in the title of this book, which i named before i was sure of my discovery--what sixteen years ago, i urged as a thing to be sought--that for which i joined tycho brahé, for which i settled in prague, for which i have devoted the best part of my life to astronomical contemplations, at length i have brought to light, and recognized its truth beyond my most sanguine expectations. it is not eighteen months since i got the first glimpse of light, three months since the dawn, very few days since the unveiled sun, most admirable to gaze upon, burst upon me. nothing holds me; i will indulge my sacred fury; i will triumph over mankind by the honest confession that i have stolen the golden vases of the egyptians to build up a tabernacle for my god far away from the confines of egypt. if you forgive me, i rejoice; if you are angry, i can bear it; the die is cast, the book is written, to be read either now or by posterity, i care not which; it may well wait a century for a reader, as god has waited six thousand years for an observer." soon after this great work his third book appeared: it was an epitome of the copernican theory, a clear and fairly popular exposition of it, which had the honour of being at once suppressed and placed on the list of books prohibited by the church, side by side with the work of copernicus himself, _de revolutionibus orbium coelestium_. this honour, however, gave kepler no satisfaction--it rather occasioned him dismay, especially as it deprived him of all pecuniary benefit, and made it almost impossible for him to get a publisher to undertake another book. still he worked on at the rudolphine tables of tycho, and ultimately, with some small help from vienna, completed them; but he could not get the means to print them. he applied to the court till he was sick of applying: they lay idle four years. at last he determined to pay for the type himself. what he paid it with, god knows, but he did pay it, and he did bring out the tables, and so was faithful to the behest of his friend. this great publication marks an era in astronomy. they were the first really accurate tables which navigators ever possessed; they were the precursors of our present _nautical almanack_. after this, the grand duke of tuscany sent kepler a golden chain, which is interesting inasmuch as it must really have come from galileo, who was in high favour at the italian court at this time. once more kepler made a determined attempt to get his arrears of salary paid, and rescue himself and family from their bitter poverty. he travelled to prague on purpose, attended the imperial meeting, and pleaded his own cause, but it was all fruitless; and exhausted by the journey, weakened by over-study, and disheartened by the failure, he caught a fever, and died in his fifty-ninth year. his body was buried at ratisbon, and a century ago a proposal was made to erect a marble monument to his memory, but nothing was done. it matters little one way or the other whether germany, having almost refused him bread during his life, should, a century and a half after his death, offer him a stone. [illustration: fig. .--portrait of kepler, older.] the contiguity of the lives of kepler and tycho furnishes a moral too obvious to need pointing out. what kepler might have achieved had he been relieved of those ghastly struggles for subsistence one cannot tell, but this much is clear, that had tycho been subjected to the same misfortune, instead of being born rich and being assisted by generous and enlightened patrons, he could have accomplished very little. his instruments, his observatory--the tools by which he did his work--would have been impossible for him. frederick and sophia of denmark, and rudolph of bohemia, are therefore to be remembered as co-workers with him. kepler, with his ill-health and inferior physical energy, was unable to command the like advantages. much, nevertheless, he did; more one cannot but feel he might have done had he been properly helped. besides, the world would have been free from the reproach of accepting the fruits of his bright genius while condemning the worker to a life of misery, relieved only by the beauty of his own thoughts and the ecstasy awakened in him by the harmony and precision of nature. concerning the method of kepler, the mode by which he made his discoveries, we must remember that he gives us an account of all the steps, unsuccessful as well as successful, by which he travelled. he maps out his route like a traveller. in fact he compares himself to columbus or magellan, voyaging into unknown lands, and recording his wandering route. this being remembered, it will be found that his methods do not differ so utterly from those used by other philosophers in like case. his imagination was perhaps more luxuriant and was allowed freer play than most men's, but it was nevertheless always controlled by rigid examination and comparison of hypotheses with fact. brewster says of him:--"ardent, restless, burning to distinguish himself by discovery, he attempted everything; and once having obtained a glimpse of a clue, no labour was too hard in following or verifying it. a few of his attempts succeeded--a multitude failed. those which failed seem to us now fanciful, those which succeeded appear to us sublime. but his methods were the same. when in search of what really existed he sometimes found it; when in pursuit of a chimæra he could not but fail; but in either case he displayed the same great qualities, and that obstinate perseverance which must conquer all difficulties except those really insurmountable." to realize what he did for astronomy, it is necessary for us now to consider some science still in its infancy. astronomy is so clear and so thoroughly explored now, that it is difficult to put oneself into a contemporary attitude. but take some other science still barely developed: meteorology, for instance. the science of the weather, the succession of winds and rain, sunshine and frost, clouds and fog, is now very much in the condition of astronomy before kepler. we have passed through the stage of ascribing atmospheric disturbances--thunderstorms, cyclones, earthquakes, and the like--to supernatural agency; we have had our copernican era: not perhaps brought about by a single individual, but still achieved. something of the laws of cyclone and anticyclone are known, and rude weather predictions across the atlantic are roughly possible. barometers and thermometers and anemometers, and all their tribe, represent the astronomical instruments in the island of huen; and our numerous meteorological observatories, with their continual record of events, represent the work of tycho brahé. observation is heaped on observation; tables are compiled; volumes are filled with data; the hours of sunshine are recorded, the fall of rain, the moisture in the air, the kind of clouds, the temperature--millions of facts; but where is the kepler to study and brood over them? where is the man to spend his life in evolving the beginnings of law and order from the midst of all this chaos? perhaps as a man he may not come, but his era will come. through this stage the science must pass, ere it is ready for the commanding intellect of a newton. but what a work it will be for the man, whoever he be that undertakes it--a fearful monotonous grind of calculation, hypothesis, hypothesis, calculation, a desperate and groping endeavour to reconcile theories with facts. a life of such labour, crowned by three brilliant discoveries, the world owes (and too late recognizes its obligation) to the harshly treated german genius, kepler. summary of facts for lectures iv and v in , michael angelo died and galileo was born; in , galileo died and newton was born. milton lived from to . for teaching the plurality of worlds, with other heterodox doctrines, and refusing to recant, bruno, after six years' imprisonment in rome, was burnt at the stake on the th of february, a.d. a "natural" death in the dungeons of the inquisition saved antonio de dominis, the explainer of the rainbow, from the same fate, but his body and books were publicly burned at rome in . the persecution of galileo began in , became intense in , and so lasted till his death and after. * * * * * galileo galilei, eldest son of vincenzo de bonajuti de galilei, a noble florentine, was born at pisa, th of february, . at the age of was sent to the university of pisa to study medicine. observed the swing of a pendulum and applied it to count pulse-beats. read euclid and archimedes, and could be kept at medicine no more. at was appointed lecturer in mathematics at pisa. read bruno and became smitten with the copernican theory. controverted the aristotelians concerning falling bodies, at pisa. hence became unpopular and accepted a chair at padua, . invented a thermometer. wrote on astronomy, adopting the ptolemaic system provisionally, and so opened up a correspondence with kepler, with whom he formed a friendship. lectured on the new star of , and publicly renounced the old systems of astronomy. invented a calculating compass or "gunter's scale." in invented a telescope, after hearing of a dutch optician's discovery. invented the microscope soon after. rapidly completed a better telescope and began a survey of the heavens. on the th of january, , discovered jupiter's satellites. observed the mountains in the moon, and roughly measured their height. explained the visibility of the new moon by _earth-shine_. was invited to the grand ducal court of tuscany by cosmo de medici, and appointed philosopher to that personage. discovered innumerable new stars, and the nebulæ. observed a triple appearance of saturn. discovered the phases of venus predicted by copernicus, and spots on the sun. wrote on floating bodies. tried to get his satellites utilized for determining longitude at sea. went to rome to defend the copernican system, then under official discussion, and as a result was formally forbidden ever to teach it. on the accession of pope urban viii. in , galileo again visited rome to pay his respects, and was well received. in appeared his "dialogues" on the ptolemaic and copernican systems. summoned to rome, practically imprisoned, and "rigorously questioned." was made to recant nd of june, . forbidden evermore to publish anything, or to teach, or receive friends. retired to arcetri in broken down health. death of his favourite daughter, sister maria celeste. wrote and meditated on the laws of motion. discovered the moon's libration. in he became blind. the rigour was then slightly relaxed and many visited him: among them john milton. died th of january, , aged . as a prisoner of the inquisition his right to make a will or to be buried in consecrated ground was disputed. many of his manuscripts were destroyed. galileo, besides being a singularly clear-headed thinker and experimental genius, was also something of a musician, a poet, and an artist. he was full of humour as well as of solid common-sense, and his literary style is brilliant. of his scientific achievements those now reckoned most weighty, are the discovery of the laws of motion, and the laying of the foundations of mechanics. _particulars of jupiter's satellites, illustrating their obedience to kepler's third law._ -------------------------------------------------------------------------- | | | distance| | | t^ | | time of | from | | | ---- satellite.|diameter revolution | jupiter, | t^ | d^ | d^ | miles.| in hours. |in jovian | | | which is | miles | (t) | radii. | | |practically | | | (d) | | | constant. ----------|-------|------------|----------|---------|---------|----------- no. . | | · | · | · | · | · no. . | | · | · | · | · | · no. . | | · | · | · | · | · no. . | | · | · | · | · | · -------------------------------------------------------------------------- the diameter of jupiter is , miles. _falling bodies._ since all bodies fall at the same rate, except for the disturbing effect of the resistance of the air, a statement of their rates of fall is of interest. in one second a freely falling body near the earth is found to drop feet. in two seconds it drops feet altogether, viz. feet in the first, and feet in the next second; because at the beginning of every second after the first it has the accumulated velocity of preceding seconds. the height fallen by a dropped body is not proportional to the time simply, but to what is rather absurdly called the square of the time, _i.e._ the time multiplied by itself. for instance, in seconds it drops × = feet; in seconds × , or feet, and so on. the distances travelled in , , , , &c., seconds by a body dropped from rest and not appreciably resisted by the air, are , , , , , &c., respectively, each multiplied by the constant feet. another way of stating the law is to say that the heights travelled in successive seconds proceed in the proportion , , , , , &c.; again multiplied by feet in each case. [illustration: fig. .--curve described by a projectile, showing how it drops from the line of fire, _o d_, in successive seconds, the same distances _ap_, _bq_, _cr_, &c., as are stated above for a dropped body.] all this was experimentally established by galileo. a body takes half a second to drop feet; and a quarter of a second to drop foot. the easiest way of estimating a quarter of a second with some accuracy is to drop a bullet one foot. a bullet thrown or shot in any direction falls just as much as if merely dropped; but instead of falling from the starting-point it drops vertically from the line of fire. (see fig. ). the rate of fall depends on the intensity of gravity; if it could be doubled, a body would fall twice as far in the same time; but to make it fall a given distance in half the time the intensity of gravity would have to be quadrupled. at a place where the intensity of gravity is / of what it is here, a body would fall as far in a minute as it now falls in a second. such a place occurs at about the distance of the moon (_cf._ page ). the fact that the height fallen through is proportional to the square of the time proves that the attraction of the earth or the intensity of gravity is sensibly constant throughout ordinary small ranges. over great distances of fall, gravity cannot be considered constant; so for things falling through great spaces the galilean law of the square of the time does not hold. the fact that things near the earth fall feet in the first second proves that the intensity of ordinary terrestrial gravity is british units of force per pound of matter. the fact that all bodies fall at the same rate (when the resistance of the air is eliminated), proves that weight is proportional to mass; or more explicitly, that the gravitative attraction of the earth on matter near its surface depends on the amount of that matter, as estimated by its inertia, and on nothing else. lecture iv galileo and the invention of the telescope contemporary with the life of kepler, but overlapping it at both ends, comes the great and eventful life of galileo galilei,[ ] a man whose influence on the development of human thought has been greater than that of any man whom we have yet considered, and upon whom, therefore, it is necessary for us, in order to carry out the plan of these lectures, to bestow much time. a man of great and wide culture, a so-called universal genius, it is as an experimental philosopher that he takes the first rank. in this capacity he must be placed alongside of archimedes, and it is pretty certain that between the two there was no man of magnitude equal to either in experimental philosophy. it is perhaps too bold a speculation, but i venture to doubt whether in succeeding generations we find his equal in the domain of purely experimental science until we come to faraday. faraday was no doubt his superior, but i know of no other of whom the like can unhesitatingly be said. in mathematical and deductive science, of course, it is quite otherwise. kepler, for instance, and many men before and since, have far excelled galileo in mathematical skill and power, though at the same time his achievements in this department are by no means to be despised. born at pisa three centuries ago, on the very day that michael angelo lay dying in rome, he inherited from his father a noble name, cultivated tastes, a keen love of truth, and an impoverished patrimony. vincenzo de galilei, a descendant of the important bonajuti family, was himself a mathematician and a musician, and in a book of his still extant he declares himself in favour of free and open inquiry into scientific matters, unrestrained by the weight of authority and tradition. in all probability the son imbibed these precepts: certainly he acted on them. vincenzo, having himself experienced the unremunerative character of scientific work, had a horror of his son's taking to it, especially as in his boyhood he was always constructing ingenious mechanical toys, and exhibiting other marks of precocity. so the son was destined for business--to be, in fact, a cloth-dealer. but he was to receive a good education first, and was sent to an excellent convent school. here he made rapid progress, and soon excelled in all branches of classics and literature. he delighted in poetry, and in later years wrote several essays on dante, tasso, and ariosto, besides composing some tolerable poems himself. he played skilfully on several musical instruments, especially on the lute, of which indeed he became a master, and on which he solaced himself when quite an old man. besides this he seems to have had some skill as an artist, which was useful afterwards in illustrating his discoveries, and to have had a fine sensibility as an art critic, for we find several eminent painters of that day acknowledging the value of the opinion of the young galileo. perceiving all this display of ability, the father wisely came to the conclusion that the selling of woollen stuffs would hardly satisfy his aspirations for long, and that it was worth a sacrifice to send him to the university. so to the university of his native town he went, with the avowed object of studying medicine, that career seeming the most likely to be profitable. old vincenzo's horror of mathematics or science as a means of obtaining a livelihood is justified by the fact that while the university professor of medicine received , scudi a year, the professor of mathematics had only , that is £ a year, or - / _d._ a day. so the son had been kept properly ignorant of such poverty-stricken subjects, and to study medicine he went. but his natural bent showed itself even here. for praying one day in the cathedral, like a good catholic as he was all his life, his attention was arrested by the great lamp which, after lighting it, the verger had left swinging to and fro. galileo proceeded to time its swings by the only watch he possessed--viz., his own pulse. he noticed that the time of swing remained as near as he could tell the same, notwithstanding the fact that the swings were getting smaller and smaller. by subsequent experiment he verified the law, and the isochronism of the pendulum was discovered. an immensely important practical discovery this, for upon it all modern clocks are based; and huyghens soon applied it to the astronomical clock, which up to that time had been a crude and quite untrustworthy instrument. the best clock which tycho brahé could get for his observatory was inferior to one that may now be purchased for a few shillings; and this change is owing to the discovery of the pendulum by galileo. not that he applied it to clocks; he was not thinking of astronomy, he was thinking of medicine, and wanted to count people's pulses. the pendulum served; and "pulsilogies," as they were called, were thus introduced to and used by medical practitioners. the tuscan court came to pisa for the summer months, for it was then a seaside place, and among the suite was ostillio ricci, a distinguished mathematician and old friend of the galileo family. the youth visited him, and one day, it is said, heard a lesson in euclid being given by ricci to the pages while he stood outside the door entranced. anyhow he implored ricci to help him into some knowledge of mathematics, and the old man willingly consented. so he mastered euclid and passed on to archimedes, for whom he acquired a great veneration. his father soon heard of this obnoxious proclivity, and did what he could to divert him back to medicine again. but it was no use. underneath his galen and hippocrates were secreted copies of euclid and archimedes, to be studied at every available opportunity. old vincenzo perceived the bent of genius to be too strong for him, and at last gave way. [illustration: fig. .--two forms of pulsilogy. the string is wound up till the swinging weight keeps time with the pulse, and the position of a bead or of an index connected with the string is then read on a scale or dial.] with prodigious rapidity the released philosopher now assimilated the elements of mathematics and physics, and at twenty-six we find him appointed for three years to the university chair of mathematics, and enjoying the paternally dreaded stipend of - / _d._ a day. now it was that he pondered over the laws of falling bodies. he verified, by experiment, the fact that the velocity acquired by falling down any slope of given height was independent of the angle of slope. also, that the height fallen through was proportional to the square of the time. another thing he found experimentally was that all bodies, heavy and light, fell at the same rate, striking the ground at the same time.[ ] now this was clean contrary to what he had been taught. the physics of those days were a simple reproduction of statements in old books. aristotle had asserted certain things to be true, and these were universally believed. no one thought of trying the thing to see if it really were so. the idea of making an experiment would have savoured of impiety, because it seemed to tend towards scepticism, and cast a doubt on a reverend authority. young galileo, with all the energy and imprudence of youth (what a blessing that youth has a little imprudence and disregard of consequences in pursuing a high ideal!), as soon as he perceived that his instructors were wrong on the subject of falling bodies, instantly informed them of the fact. whether he expected them to be pleased or not is a question. anyhow, they were not pleased, but were much annoyed by his impertinent arrogance. it is, perhaps, difficult for us now to appreciate precisely their position. these doctrines of antiquity, which had come down hoary with age, and the discovery of which had reawakened learning and quickened intellectual life, were accepted less as a science or a philosophy, than as a religion. had they regarded aristotle as a verbally inspired writer, they could not have received his statements with more unhesitating conviction. in any dispute as to a question of fact, such as the one before us concerning the laws of falling bodies, their method was not to make an experiment, but to turn over the pages of aristotle; and he who could quote chapter and verse of this great writer was held to settle the question and raise it above the reach of controversy. it is very necessary for us to realize this state of things clearly, because otherwise the attitude of the learned of those days towards every new discovery seems stupid and almost insane. they had a crystallized system of truth, perfect, symmetrical--it wanted no novelty, no additions; every addition or growth was an imperfection, an excrescence, a deformity. progress was unnecessary and undesired. the church had a rigid system of dogma, which must be accepted in its entirety on pain of being treated as a heretic. philosophers had a cast-iron system of truth to match--a system founded upon aristotle--and so interwoven with the great theological dogmas that to question one was almost equivalent to casting doubt upon the other. in such an atmosphere true science was impossible. the life-blood of science is growth, expansion, freedom, development. before it could appear it must throw off these old shackles of centuries. it must burst its old skin, and emerge, worn with the struggle, weakly and unprotected, but free and able to grow and to expand. the conflict was inevitable, and it was severe. is it over yet? i fear not quite, though so nearly as to disturb science hardly at all. then it was different; it was terrible. honour to the men who bore the first shock of the battle! now aristotle had said that bodies fell at rates depending on their weight. a lb. weight would fall five times as quick as a lb. weight; a lb. weight fifty times as quick, and so on. why he said so nobody knows. he cannot have tried. he was not above trying experiments, like his smaller disciples; but probably it never occurred to him to doubt the fact. it seems so natural that a heavy body should fall quicker than a light one; and perhaps he thought of a stone and a feather, and was satisfied. galileo, however, asserted that the weight did not matter a bit, that everything fell at the same rate (even a stone and a feather, but for the resistance of the air), and would reach the ground in the same time. and he was not content to be pooh-poohed and snubbed. he knew he was right, and he was determined to make every one see the facts as he saw them. so one morning, before the assembled university, he ascended the famous leaning tower, taking with him a lb. shot and a lb. shot. he balanced them on the edge of the tower, and let them drop together. together they fell, and together they struck the ground. the simultaneous clang of those two weights sounded the death-knell of the old system of philosophy, and heralded the birth of the new. but was the change sudden? were his opponents convinced? not a jot. though they had seen with their eyes, and heard with their ears, the full light of heaven shining upon them, they went back muttering and discontented to their musty old volumes and their garrets, there to invent occult reasons for denying the validity of the observation, and for referring it to some unknown disturbing cause. they saw that if they gave way on this one point they would be letting go their anchorage, and henceforward would be liable to drift along with the tide, not knowing whither. they dared not do this. no; they _must_ cling to the old traditions; they could not cast away their rotting ropes and sail out on to the free ocean of god's truth in a spirit of fearless faith. [illustration: fig. .--tower of pisa.] yet they had received a shock: as by a breath of fresh salt breeze and a dash of spray in their faces, they had been awakened out of their comfortable lethargy. they felt the approach of a new era. yes, it was a shock; and they hated the young galileo for giving it them--hated him with the sullen hatred of men who fight for a lost and dying cause. we need scarcely blame these men; at least we need not blame them overmuch. to say that they acted as they did is to say that they were human, were narrow-minded, and were the apostles of a lost cause. but _they_ could not know this; _they_ had no experience of the past to guide them; the conditions under which they found themselves were novel, and had to be met for the first time. conduct which was excusable then would be unpardonable now, in the light of all this experience to guide us. are there any now who practically repeat their error, and resist new truth? who cling to any old anchorage of dogma, and refuse to rise with the tide of advancing knowledge? there may be some even now. well, the unpopularity of galileo smouldered for a time, until, by another noble imprudence, he managed to offend a semi-royal personage, giovanni de medici, by giving his real opinion, when consulted, about a machine which de medici had invented for cleaning out the harbour of leghorn. he said it was as useless as it in fact turned out to be. through the influence of the mortified inventor he lost favour at court; and his enemies took advantage of the fact to render his chair untenable. he resigned before his three years were up, and retired to florence. his father at this time died, and the family were left in narrow circumstances. he had a brother and three sisters to provide for. he was offered a professorship at padua for six years by the senate of venice, and willingly accepted it. now began a very successful career. his introductory address was marked by brilliant eloquence, and his lectures soon acquired fame. he wrote for his pupils on the laws of motion, on fortifications, on sundials, on mechanics, and on the celestial globe: some of these papers are now lost, others have been printed during the present century. kepler sent him a copy of his new book, _mysterium cosmographicum_, and galileo in thanking him for it writes him the following letter:--[ ] "i count myself happy, in the search after truth, to have so great an ally as yourself, and one who is so great a friend of the truth itself. it is really pitiful that there are so few who seek truth, and who do not pursue a perverse method of philosophising. but this is not the place to mourn over the miseries of our times, but to congratulate you on your splendid discoveries in confirmation of truth. i shall read your book to the end, sure of finding much that is excellent in it. i shall do so with the more pleasure, because _i have been for many years an adherent of the copernican system_, and it explains to me the causes of many of the appearances of nature which are quite unintelligible on the commonly accepted hypothesis. _i have collected many arguments for the purpose of refuting the latter_; but i do not venture to bring them to the light of publicity, for fear of sharing the fate of our master, copernicus, who, although he has earned immortal fame with some, yet with very many (so great is the number of fools) has become an object of ridicule and scorn. i should certainly venture to publish my speculations if there were more people like you. but this not being the case, i refrain from such an undertaking." kepler urged him to publish his arguments in favour of the copernican theory, but he hesitated for the present, knowing that his declaration would be received with ridicule and opposition, and thinking it wiser to get rather more firmly seated in his chair before encountering the storm of controversy. the six years passed away, and the venetian senate, anxious not to lose so bright an ornament, renewed his appointment for another six years at a largely increased salary. soon after this appeared a new star, the stella nova of , not the one tycho had seen--that was in --but the same that kepler was so much interested in. galileo gave a course of three lectures upon it to a great audience. at the first the theatre was over-crowded, so he had to adjourn to a hall holding persons. at the next he had to lecture in the open air. he took occasion to rebuke his hearers for thronging to hear about an ephemeral novelty, while for the much more wonderful and important truths about the permanent stars and facts of nature they had but deaf ears. but the main point he brought out concerning the new star was that it upset the received aristotelian doctrine of the immutability of the heavens. according to that doctrine the heavens were unchangeable, perfect, subject neither to growth nor to decay. here was a body, not a meteor but a real distant star, which had not been visible and which would shortly fade away again, but which meanwhile was brighter than jupiter. the staff of petrified professorial wisdom were annoyed at the appearance of the star, still more at galileo's calling public attention to it; and controversy began at padua. however, he accepted it; and now boldly threw down the gauntlet in favour of the copernican theory, utterly repudiating the old ptolemaic system which up to that time he had taught in the schools according to established custom. the earth no longer the only world to which all else in the firmament were obsequious attendants, but a mere insignificant speck among the host of heaven! man no longer the centre and cynosure of creation, but, as it were, an insect crawling on the surface of this little speck! all this not set down in crabbed latin in dry folios for a few learned monks, as in copernicus's time, but promulgated and argued in rich italian, illustrated by analogy, by experiment, and with cultured wit; taught not to a few scholars here and there in musty libraries, but proclaimed in the vernacular to the whole populace with all the energy and enthusiasm of a recent convert and a master of language! had a bombshell been exploded among the fossilized professors it had been less disturbing. but there was worse in store for them. a dutch optician, hans lippershey by name, of middleburg, had in his shop a curious toy, rigged up, it is said, by an apprentice, and made out of a couple of spectacle lenses, whereby, if one looked through it, the weather-cock of a neighbouring church spire was seen nearer and upside down. the tale goes that the marquis spinola, happening to call at the shop, was struck with the toy and bought it. he showed it to prince maurice of nassau, who thought of using it for military reconnoitring. all this is trivial. what is important is that some faint and inaccurate echo of this news found its way to padua, and into the ears of galileo. the seed fell on good soil. all that night he sat up and pondered. he knew about lenses and magnifying glasses. he had read kepler's theory of the eye, and had himself lectured on optics. could he not hit on the device and make an instrument capable of bringing the heavenly bodies nearer? who knew what marvels he might not so perceive! by morning he had some schemes ready to try, and one of them was successful. singularly enough it was not the same plan as the dutch optician's, it was another mode of achieving the same end. he took an old small organ pipe, jammed a suitably chosen spectacle glass into either end, one convex the other concave, and behold, he had the half of a wretchedly bad opera glass capable of magnifying three times. it was better than the dutchman's, however; it did not invert. it is easy to understand the general principle of a telescope. a general knowledge of the common magnifying glass may be assumed. roger bacon knew about lenses; and the ancients often refer to them, though usually as burning glasses. the magnifying power of globes of water must have been noticed soon after the discovery of glass and the art of working it. a magnifying glass is most simply thought of as an additional lens to the eye. the eye has a lens by which ordinary vision is accomplished, an extra glass lens strengthens it and enables objects to be seen nearer and therefore apparently bigger. but to apply a magnifying glass to distant objects is impossible. in order to magnify distant objects, another function of lenses has also to be employed, viz., their power of forming real images, the power on which their use as burning-glasses depends: for the best focus is an image of the sun. although the object itself is inaccessible, the image of it is by no means so, and to the image a magnifier can be applied. this is exactly what is done in the telescope; the object glass or large lens forms an image, which is then looked at through a magnifying glass or eye-piece. of course the image is nothing like so big as the object. for astronomical objects it is almost infinitely less; still it is an exact representation at an accessible place, and no one expects a telescope to show distant bodies as big as they really are. all it does is to show them bigger than they could be seen without it. but if the objects are not distant, the same principle may still be applied, and two lenses may be used, one to form an image, the other to magnify it; only if the object can be put where we please, we can easily place it so that its image is already much bigger than the object even before magnification by the eye lens. this is the compound microscope, the invention of which soon followed the telescope. in fact the two instruments shade off into one another, so that the reading telescope or reading microscope of a laboratory (for reading thermometers, and small divisions generally) goes by either name at random. the arrangement so far described depicts things on the retina the unaccustomed way up. by using a concave glass instead of a convex, and placing it so as to prevent any image being formed, except on the retina direct, this inconvenience is avoided. [illustration: fig. .--view of the half-moon in small telescope. the darker regions, or plains, used to be called "seas."] such a thing as galileo made may now be bought at a toy-shop for i suppose half a crown, and yet what a potentiality lay in that "glazed optic tube," as milton called it. away he went with it to venice and showed it to the signoria, to their great astonishment. "many noblemen and senators," says galileo, "though of advanced age, mounted to the top of one of the highest towers to watch the ships, which were visible through my glass two hours before they were seen entering the harbour, for it makes a thing fifty miles off as near and clear as if it were only five." among the people too the instrument excited the greatest astonishment and interest, so that he was nearly mobbed. the senate hinted to him that a present of the instrument would not be unacceptable, so galileo took the hint and made another for them. [illustration: fig. .--portion of the lunar surface more highly magnified, showing the shadows of a mountain range, deep pits, and other details.] they immediately doubled his salary at padua, making it florins, and confirmed him in the enjoyment of it for life. he now eagerly began the construction of a larger and better instrument. grinding the lenses with his own hands with consummate skill, he succeeded in making a telescope magnifying thirty times. thus equipped he was ready to begin a survey of the heavens. [illustration: fig. .--another portion of the lunar surface, showing a so-called crater or vast lava pool and other evidences of ancient heat unmodified by water.] the first object he carefully examined was naturally the moon. he found there everything at first sight very like the earth, mountains and valleys, craters and plains, rocks, and apparently seas. you may imagine the hostility excited among the aristotelian philosophers, especially no doubt those he had left behind at pisa, on the ground of his spoiling the pure, smooth, crystalline, celestial face of the moon as they had thought it, and making it harsh and rugged and like so vile and ignoble a body as the earth. [illustration: fig. .--lunar landscape showing earth. the earth would be a stationary object in the moon's sky: its only apparent motion being a slow oscillation as of a pendulum (the result of the moon's libration).] he went further, however, into heterodoxy than this--he not only made the moon like the earth, but he made the earth shine like the moon. the visibility of "the old moon in the new moon's arms" he explained by earth-shine. leonardo had given the same explanation a century before. now one of the many stock arguments against copernican theory of the earth being a planet like the rest was that the earth was dull and dark and did not shine. galileo argued that it shone just as much as the moon does, and in fact rather more--especially if it be covered with clouds. one reason of the peculiar brilliancy of venus is that she is a very cloudy planet.[ ] seen from the moon the earth would look exactly as the moon does to us, only a little brighter and sixteen times as big (four times the diameter). [illustration: fig. .--galileo's method of estimating the height of lunar mountain. _ab'bc_ is the illuminated half of the moon. _sa_ is a solar ray just catching the peak of the mountain _m_. then by geometry, as _mn_ is to _ma_, so is _ma_ to _mb'_; whence the height of the mountain, _mn_, can be determined. the earth and spectator are supposed to be somewhere in the direction _ba_ produced, _i.e._ towards the top of the page.] galileo made a very good estimate of the height of lunar mountains, of which many are five miles high and some as much as seven. he did this simply by measuring from the half-moon's straight edge the distance at which their peaks caught the rising or setting sun. the above simple diagram shows that as this distance is to the diameter of the moon, so is the height of the sun-tipped mountain to the aforesaid distance. wherever galileo turned his telescope new stars appeared. the milky way, which had so puzzled the ancients, was found to be composed of stars. stars that appeared single to the eye were some of them found to be double; and at intervals were found hazy nebulous wisps, some of which seemed to be star clusters, while others seemed only a fleecy cloud. [illustration: fig. .--some clusters and nebulæ.] [illustration: fig. .--jupiter's satellites, showing the stages of their discovery.] now we come to his most brilliant, at least his most sensational, discovery. examining jupiter minutely on january , , he noticed three little stars near it, which he noted down as fixing its then position. on the following night jupiter had moved to the other side of the three stars. this was natural enough, but was it moving the right way? on examination it appeared not. was it possible the tables were wrong? the next evening was cloudy, and he had to curb his feverish impatience. on the th there were only two, and those on the other side. on the th two again, but one bigger than the other. on the th the three re-appeared, and on the th there were four. no more appeared. jupiter then had moons like the earth, four of them in fact, and they revolved round him in periods which were soon determined. the reason why they were not all visible at first, and why their visibility so rapidly changes, is because they revolve round him almost in the plane of our vision, so that sometimes they are in front and sometimes behind him, while again at other times they plunge into his shadow and are thus eclipsed from the light of the sun which enables us to see them. a large modern telescope will show the moons when in front of jupiter, but small telescopes will only show them when clear of the disk and shadow. often all four can be thus seen, but three or two is a very common amount of visibility. quite a small telescope, such as a ship's telescope, if held steadily, suffices to show the satellites of jupiter, and very interesting objects they are. they are of habitable size, and may be important worlds for all we know to the contrary. the news of the discovery soon spread and excited the greatest interest and astonishment. many of course refused to believe it. some there were who having been shown them refused to believe their eyes, and asserted that although the telescope acted well enough for terrestrial objects, it was altogether false and illusory when applied to the heavens. others took the safer ground of refusing to look through the glass. one of these who would not look at the satellites happened to die soon afterwards. "i hope," says galileo, "that he saw them on his way to heaven." the way in which kepler received the news is characteristic, though by adding four to the supposed number of planets it might have seemed to upset his notions about the five regular solids. he says,[ ] "i was sitting idle at home thinking of you, most excellent galileo, and your letters, when the news was brought me of the discovery of four planets by the help of the double eye-glass. wachenfels stopped his carriage at my door to tell me, when such a fit of wonder seized me at a report which seemed so very absurd, and i was thrown into such agitation at seeing an old dispute between us decided in this way, that between his joy, my colouring, and the laughter of us both, confounded as we were by such a novelty, we were hardly capable, he of speaking, or i of listening.... "on our separating, i immediately fell to thinking how there could be any addition to the number of planets without overturning my _mysterium cosmographicon_, published thirteen years ago, according to which euclid's five regular solids do not allow more than six planets round the sun. "but i am so far from disbelieving the existence of the four circumjovial planets that i long for a telescope to anticipate you if possible in discovering two round mars (as the proportion seems to me to require) six or eight round saturn, and one each round mercury and venus." [illustration: fig. .--eclipses of jupiter's satellites. the diagram shows the first (_i.e._ the nearest) moon in jupiter's shadow, the second as passing between earth and jupiter, and appearing to transit his disk, the third as on the verge of entering his shadow, and the fourth quite plainly and separately visible.] as an illustration of the opposite school, i will take the following extract from francesco sizzi, a florentine astronomer, who argues against the discovery thus:-- "there are seven windows in the head, two nostrils, two eyes, two ears, and a mouth; so in the heavens there are two favourable stars, two unpropitious, two luminaries, and mercury alone undecided and indifferent. from which and many other similar phenomena of nature, such as the seven metals, &c., which it were tedious to enumerate, we gather that the number of planets is necessarily seven. "moreover, the satellites are invisible to the naked eye, and therefore can have no influence on the earth, and therefore would be useless, and therefore do not exist. "besides, the jews and other ancient nations as well as modern europeans have adopted the division of the week into seven days, and have named them from the seven planets: now if we increase the number of the planets this whole system falls to the ground." to these arguments galileo replied that whatever their force might be as a reason for believing beforehand that no more than seven planets would be discovered, they hardly seemed of sufficient weight to destroy the new ones when actually seen. writing to kepler at this time, galileo ejaculates: "oh, my dear kepler, how i wish that we could have one hearty laugh together! here, at padua, is the principal professor of philosophy whom i have repeatedly and urgently requested to look at the moon and planets through my glass, which he pertinaciously refuses to do. why are you not here? what shouts of laughter we should have at this glorious folly! and to hear the professor of philosophy at pisa labouring before the grand duke with logical arguments, as if with magical incantations, to charm the new planets out of the sky." a young german _protégé_ of kepler, martin horkey, was travelling in italy, and meeting galileo at bologna was favoured with a view through his telescope. but supposing that kepler must necessarily be jealous of such great discoveries, and thinking to please him, he writes, "i cannot tell what to think about these observations. they are stupendous, they are wonderful, but whether they are true or false i cannot tell." he concludes, "i will never concede his four new planets to that italian from padua though i die for it." so he published a pamphlet asserting that reflected rays and optical illusions were the sole cause of the appearance, and that the only use of the imaginary planets was to gratify galileo's thirst for gold and notoriety. when after this performance he paid a visit to his old instructor kepler, he got a reception which astonished him. however, he pleaded so hard to be forgiven that kepler restored him to partial favour, on this condition, that he was to look again at the satellites, and this time to see them and own that they were there. by degrees the enemies of galileo were compelled to confess to the truth of the discovery, and the next step was to outdo him. scheiner counted five, rheiter nine, and others went as high as twelve. some of these were imaginary, some were fixed stars, and four satellites only are known to this day.[ ] here, close to the summit of his greatness, we must leave him for a time. a few steps more and he will be on the brow of the hill; a short piece of table-land, and then the descent begins. lecture v galileo and the inquisition one sinister event occurred while galileo was at padua, some time before the era we have now arrived at, before the invention of the telescope--two years indeed after he had first gone to padua; an event not directly concerning galileo, but which i must mention because it must have shadowed his life both at the time and long afterwards. it was the execution of giordano bruno for heresy. this eminent philosopher had travelled largely, had lived some time in england, had acquired new and heterodox views on a variety of subjects, and did not hesitate to propound them even after he had returned to italy. the copernican doctrine of the motion of the earth was one of his obnoxious heresies. being persecuted to some extent by the church, bruno took refuge in venice--a free republic almost independent of the papacy--where he felt himself safe. galileo was at padua hard by: the university of padua was under the government of the senate of venice: the two men must in all probability have met. well, the inquisition at rome sent messengers to venice with a demand for the extradition of bruno--they wanted him at rome to try him for heresy. in a moment of miserable weakness the venetian republic gave him up, and bruno was taken to rome. there he was tried, and cast into the dungeons for six years, and because he entirely refused to recant, was at length delivered over to the secular arm and burned at the stake on th february, anno domini . this event could not but have cast a gloom over the mind of lovers and expounders of truth, and the lesson probably sank deep into galileo's soul. in dealing with these historic events will you allow me to repudiate once for all the slightest sectarian bias or meaning. i have nothing to do with catholic or protestant as such. i have nothing to do with the church of rome as such. i am dealing with the history of science. but historically at one period science and the church came into conflict. it was not specially one church rather than another--it was the church in general, the only one that then existed in those countries. historically, i say, they came into conflict, and historically the church was the conqueror. it got its way; and science, in the persons of bruno, galileo, and several others, was vanquished. such being the facts, there is no help but to mention them in dealing with the history of science. doubtless _now_ the church regards it as an unhappy victory, and gladly would ignore this painful struggle. this, however, is impossible. with their creed the churchmen of that day could act in no other way. they were bound to prosecute heresy, and they were bound to conquer in the struggle or be themselves shattered. but let me insist on the fact that no one accuses the ecclesiastical courts of crime or evil motives. they attacked heresy after their manner, as the civil courts attacked witchcraft after _their_ manner. both erred grievously, but both acted with the best intentions. we must remember, moreover, that his doctrines were scientifically heterodox, and the university professors of that day were probably quite as ready to condemn them as the church was. to realise the position we must think of some subjects which _to-day_ are scientifically heterodox, and of the customary attitude adopted towards them by persons of widely differing creeds. if it be contended now, as it is, that the ecclesiastics treated galileo well, i admit it freely: they treated him as well as they possibly could. they overcame him, and he recanted; but if he had not recanted, if he had persisted in his heresy, they would--well, they would still have treated his soul well, but they would have set fire to his body. their mistake consisted not in cruelty, but in supposing themselves the arbiters of eternal truth; and by no amount of slurring and glossing over facts can they evade the responsibility assumed by them on account of this mistaken attitude. i am not here attacking the dogma of papal infallibility: it is historically, i believe, quite unaffected by the controversy respecting the motion of the earth, no papal edict _ex cathedrâ_ having been promulgated on the subject. we left galileo standing at his telescope and beginning his survey of the heavens. we followed him indeed through a few of his first great discoveries--the discovery of the mountains and other variety of surface in the moon, of the nebulæ and a multitude of faint stars, and lastly of the four satellites of jupiter. this latter discovery made an immense sensation, and contributed its share to his removal from padua, which quickly followed it, as i shall shortly narrate; but first i think it will be best to continue our survey of his astronomical discoveries without regard to the place whence they were made. before the end of the year galileo had made another discovery--this time on saturn. but to guard against the host of plagiarists and impostors, he published it in the form of an anagram, which, at the request of the emperor rudolph (a request probably inspired by kepler), he interpreted; it ran thus: the furthest planet is triple. very soon after he found that venus was changing from a full moon to a half moon appearance. he announced this also by an anagram, and waited till it should become a crescent, which it did. this was a dreadful blow to the anti-copernicans, for it removed the last lingering difficulty to the reception of the copernican doctrine. [illustration: fig. .--old drawings of saturn by different observers, with the imperfect instruments of that day. the first is galileo's idea of what he saw.] copernicus had predicted, indeed, a hundred years before, that, if ever our powers of sight were sufficiently enhanced, venus and mercury would be seen to have phases like the moon. and now galileo with his telescope verifies the prediction to the letter. here was a triumph for the grand old monk, and a bitter morsel for his opponents. castelli writes: "this must now convince the most obstinate." but galileo, with more experience, replies:--"you almost make me laugh by saying that these clear observations are sufficient to convince the most obstinate; it seems you have yet to learn that long ago the observations were enough to convince those who are capable of reasoning, and those who wish to learn the truth; but that to convince the obstinate, and those who care for nothing beyond the vain applause of the senseless vulgar, not even the testimony of the stars would suffice, were they to descend on earth to speak for themselves. let us, then, endeavour to procure some knowledge for ourselves, and rest contented with this sole satisfaction; but of advancing in popular opinion, or of gaining the assent of the book-philosophers, let us abandon both the hope and the desire." [illustration: fig. .--phases of venus. showing also its apparent variations in size by reason of its varying distance from the earth. when fully illuminated it is necessarily most distant. it looks brightest to us when a broad crescent.] what a year's work it had been! in twelve months observational astronomy had made such a bound as it has never made before or since. why did not others make any of these observations? because no one could make telescopes like galileo. he gathered pupils round him however, and taught them how to work the lenses, so that gradually these instruments penetrated europe, and astronomers everywhere verified his splendid discoveries. but still he worked on, and by march in the very next year, he saw something still more hateful to the aristotelian philosophers, viz. spots on the sun. [illustration: fig. .] if anything was pure and perfect it was the sun, they said. was this impostor going to blacken its face too? well, there they were. they slowly formed and changed, and by moving all together showed him that the sun rotated about once a month. before taking leave of galileo's astronomical researches, i must mention an observation made at the end of , that the apparent triplicity of saturn (fig. ) had vanished. [illustration: fig. .--a portion of the sun's disk as seen in a powerful modern telescope.] "looking on saturn within these few days, i found it solitary, without the assistance of its accustomed stars, and in short perfectly round and defined, like jupiter, and such it still remains. now what can be said of so strange a metamorphosis? are perhaps the two smaller stars consumed like spots on the sun? have they suddenly vanished and fled? or has saturn devoured his own children? or was the appearance indeed fraud and illusion, with which the glasses have so long time mocked me and so many others who have so often observed with me? now perhaps the time is come to revive the withering hopes of those, who, guided by more profound contemplations, have fathomed all the fallacies of the new observations and recognized their impossibility! i cannot resolve what to say in a chance so strange, so new, so unexpected. the shortness of time, the unexampled occurrence, the weakness of my intellect, the terror of being mistaken, have greatly confounded me." however, he plucked up courage, and conjectured that the two attendants would reappear, by revolving round the planet. [illustration: fig. .--saturn and his rings, as seen under the most favourable circumstances.] the real reason of their disappearance is well known to us now. the plane of saturn's rings oscillates slowly about our line of sight, and so we sometimes see them edgeways and sometimes with a moderate amount of obliquity. the rings are so thin that, when turned precisely edgeways, they become invisible. the two imaginary attendants were the most conspicuous portions of the ring, subsequently called _ansæ_. i have thought it better not to interrupt this catalogue of brilliant discoveries by any biographical details; but we must now retrace our steps to the years and , the era of the invention of the telescope. by this time galileo had been eighteen years at padua, and like many another man in like case, was getting rather tired of continual lecturing. moreover, he felt so full of ideas that he longed to have a better opportunity of following them up, and more time for thinking them out. now in the holidays he had been accustomed to return to his family home at pisa, and there to come a good deal into contact with the grand-ducal house of tuscany. young cosmo di medici became in fact his pupil, and arrived at man's estate with the highest opinion of the philosopher. this young man had now come to the throne as cosmo ii., and to him galileo wrote saying how much he should like more time and leisure, how full he was of discoveries if he only had the chance of a reasonable income without the necessity of consuming so large a portion of his time in elementary teaching, and practically asking to be removed to some position in the court. nothing was done for a time, but negotiations proceeded, and soon after the discovery of jupiter's satellites cosmo wrote making a generous offer, which galileo gladly and enthusiastically accepted, and at once left padua for florence. all his subsequent discoveries date from florence. thus closed his brilliant and happy career as a professor at the university of padua. he had been treated well: his pay had become larger than that of any professor of mathematics up to that time; and, as you know, immediately after his invention of the telescope the venetian senate, in a fit of enthusiasm, had doubled it and secured it to him for life wherever he was. to throw up his chair and leave the place the very next year scarcely seems a strictly honourable procedure. it was legal enough no doubt, and it is easy for small men to criticize a great one, but nevertheless i think we must admit that it is a step such as a man with a keen sense of honour would hardly have taken. one quite feels and sympathizes with the temptation. not emolument, but leisure; freedom from harassing engagements and constant teaching, and liberty to prosecute his studies day and night without interference: this was the golden prospect before him. he yielded, but one cannot help wishing he had not. as it turned out it was a false step--the first false step of his public career. when made it was irretrievable, and it led to great misery. at first it seemed brilliant enough. the great philosopher of the tuscan court was courted and flattered by princes and nobles, he enjoyed a world-wide reputation, lived as luxuriously as he cared for, had his time all to himself, and lectured but very seldom, on great occasions or to a few crowned heads. his position was in fact analogous to that of tycho brahé in his island of huen. misfortune overtook both. in tycho's case it arose mainly from the death of his patron. in galileo's it was due to a more insidious cause, to understand which cause aright we must remember the political divisions of italy at that date. tuscany was a papal state, and thought there was by no means free. venice was a free republic, and was even hostile to the papacy. in the pope had placed it under an interdict. in reply it had ejected every jesuit. out of this atmosphere of comparative enlightenment and freedom into that hotbed of mediævalism and superstition went galileo with his eyes open. keen was the regret of his paduan and venetian friends; bitter were their remonstrances and exhortations. but he was determined to go, and, not without turning some of his old friends into enemies, he went. seldom has such a man made so great a mistake: never, i suppose, has one been so cruelly punished for it. [illustration: fig. .--map of italy.] we must remember, however, that galileo, though by no means a saint, was yet a really religious man, a devout catholic and thorough adherent of the church, so that he would have no dislike to place himself under her sway. moreover, he had been born a tuscan, his family had lived at florence or pisa, and it felt like going home. his theological attitude is worthy of notice, for he was not in the least a sceptic. he quite acquiesces in the authority of the bible, especially in all matters concerning faith and conduct; as to its statements in scientific matters, he argues that we are so liable to misinterpret their meaning that it is really easier to examine nature for truth in scientific matters, and that when direct observation and scripture seem to clash, it is because of our fallacious interpretation of one or both of them. he is, in fact, what one now calls a "reconciler." it is curious to find such a man prosecuted for heresy, when to-day his opinions are those of the orthodox among the orthodox. but so it ever is, and the heresy of one generation becomes the commonplace of the next. he accepts joshua's miracle, for instance, not as a striking poem, but as a literal fact; and he points out how much more simply it could be done on the copernican system by stopping the earth's rotation for a short time, than by stopping the sun and moon and all the host of heaven as on the old ptolemaic system, or again by stopping only the sun and not any of the other bodies, and so throwing astronomy all wrong. this reads to us like satire, but no doubt it was his genuine opinion. these scriptural reconciliations of his, however, angered the religious authorities still more. they said it was bad enough for this heretic to try and upset old _scientific_ beliefs, and to spoil the face of _nature_ with his infidel discoveries, but at least he might leave the bible alone; and they addressed an indignant remonstrance to rome, to protect it from the hands of ignorant laymen. thus, wherever he turned he encountered hostility. of course he had many friends--some of them powerful like cosmo, all of them faithful and sincere. but against the power of rome what could they do? cosmo dared no more than remonstrate, and ultimately his successor had to refrain from even this, so enchained and bound was the spirit of the rulers of those days; and so when his day of tribulation came he stood alone and helpless in the midst of his enemies. you may wonder, perhaps, why this man should excite so much more hostility than many another man who was suffered to believe and teach much the same doctrines unmolested. but no other man had made such brilliant and exciting discoveries. no man stood so prominently forward in the eyes of all christendom as the champion of the new doctrines. no other man stated them so clearly and forcibly, nor drove them home with such brilliant and telling illustrations. and again, there was the memory of his early conflict with the aristotelians at pisa, of his scornful and successful refutation of their absurdities. all this made him specially obnoxious to the aristotelian jesuits in their double capacity both of priests and of philosophers, and they singled him out for relentless official persecution. not yet, however, is he much troubled by them. the chief men at rome have not yet moved. messages, however, keep going up from tuscany to rome respecting the teachings of this man, and of the harm he is doing by his pertinacious preaching of the copernican doctrine that the earth moves. at length, in , pope paul v. wrote requesting him to come to rome to explain his views. he went, was well received, made a special friend of cardinal barberino--an accomplished man in high position, who became in fact the next pope. galileo showed cardinals and others his telescope, and to as many as would look through it he showed jupiter's satellites and his other discoveries. he had a most successful visit. he talked, he harangued, he held forth in the midst of fifteen or twenty disputants at once, confounding his opponents and putting them to shame. his method was to let the opposite arguments be stated as fully and completely as possible, himself aiding, and often adducing the most forcible and plausible arguments against his own views; and then, all having been well stated, he would proceed to utterly undermine and demolish the whole fabric, and bring out the truth in such a way as to convince all honest minds. it was this habit that made him such a formidable antagonist. he never shrank from meeting an opposing argument, never sought to ignore it, or cloak it in a cloud of words. every hostile argument he seemed to delight in, as a foe to be crushed, and the better and stronger they sounded the more he liked them. he knew many of them well, he invented a number more, and had he chosen could have out-argued the stoutest aristotelian on his own grounds. thus did he lead his adversaries on, almost like socrates, only to ultimately overwhelm them in a more hopeless rout. all this in rome too, in the heart of the catholic world. had he been worldly-wise, he would certainly have kept silent and unobtrusive till he had leave to go away again. but he felt like an apostle of the new doctrines, whose mission it was to proclaim them even in this centre of the world and of the church. well, he had an audience with the pope--a chat an hour long--and the two parted good friends, mutually pleased with each other. he writes that he is all right now, and might return home when he liked. but the question began to be agitated whether the whole system of copernicus ought not to be condemned as impious and heretical. this view was persistently urged upon the pope and college of cardinals, and it was soon to be decided upon. had galileo been unfaithful to the church he could have left them to stultify themselves in any way they thought proper, and himself have gone; but he felt supremely interested in the result, and he stayed. he writes:-- "so far as concerns the clearing of my own character, i might return home immediately; but although this new question regards me no more than all those who for the last eighty years have supported those opinions both in public and private, yet, as perhaps i may be of some assistance in that part of the discussion which depends on the knowledge of truths ascertained by means of the sciences which i profess, i, as a zealous and catholic christian, neither can nor ought to withhold that assistance which my knowledge affords, and this business keeps me sufficiently employed." it is possible that his stay was the worst thing for the cause he had at heart. anyhow, the result was that the system was condemned, and both the book of copernicus and the epitome of it by kepler were placed on the forbidden list,[ ] and galileo himself was formally ordered never to teach or to believe the motion of the earth. he quitted rome in disgust, which before long broke out in satire. the only way in which he could safely speak of these views now was as if they were hypothetical and uncertain, and so we find him writing to the archduke leopold, with a presentation copy of his book on the tides, the following:-- "this theory occurred to me when in rome whilst the theologians were debating on the prohibition of copernicus's book, and of the opinion maintained in it of the motion of the earth, which i at that time believed: until it pleased those gentlemen to suspend the book, and declare the opinion false and repugnant to the holy scriptures. now, as i know how well it becomes me to obey and believe the decisions of my superiors, which proceed out of more knowledge than the weakness of my intellect can attain to, this theory which i send you, which is founded on the motion of the earth, i now look upon as a fiction and a dream, and beg your highness to receive it as such. but as poets often learn to prize the creations of their fancy, so in like manner do i set some value on this absurdity of mine. it is true that when i sketched this little work i did hope that copernicus would not, after eighty years, be convicted of error; and i had intended to develop and amplify it further, but a voice from heaven suddenly awakened me, and at once annihilated all my confused and entangled fancies." this sarcasm, if it had been in print, would probably have been dangerous. it was safe in a private letter, but it shows us his real feelings. however, he was left comparatively quiet for a time. he was getting an old man now, and passed the time studiously enough, partly at his house in florence, partly at his villa in arcetri, a mile or so out of the town. here was a convent, and in it his two daughters were nuns. one of them, who passed under the name of sister maria celeste, seems to have been a woman of considerable capacity--certainly she was of a most affectionate disposition--and loved and honoured her father in the most dutiful way. this was a quiet period of his life, spoiled only by occasional fits of illness and severe rheumatic pains, to which the old man was always liable. many little circumstances are known of this peaceful time. for instance, the convent clock won't go, and galileo mends it for them. he is always doing little things for them, and sending presents to the lady superior and his two daughters. he was occupied now with problems in hydrostatics, and on other matters unconnected with astronomy: a large piece of work which i must pass over. most interesting and acute it is, however. in , when the old pope died, there was elected to the papal throne, as urban viii., cardinal barberino, a man of very considerable enlightenment, and a personal friend of galileo's, so that both he and his daughters rejoice greatly, and hope that things will come all right, and the forbidding edict be withdrawn. the year after this election he manages to make another journey to rome to compliment his friend on his elevation to the pontifical chair. he had many talks with urban, and made himself very agreeable. urban wrote to the grand duke ferdinand, son of cosmo:-- "for we find in him not only literary distinction but also love of piety, and he is strong in those qualities by which pontifical good will is easily obtainable. and now, when he has been brought to this city to congratulate us on our elevation, we have very lovingly embraced him; nor can we suffer him to return to the country whither your liberality recalls him without an ample provision of pontifical love. and that you may know how dear he is to us, we have willed to give him this honourable testimonial of virtue and piety. and we further signify that every benefit which you shall confer upon him, imitating or even surpassing your father's liberality, will conduce to our gratification." encouraged, doubtless, by these marks of approbation, and reposing too much confidence in the individual good will of the pope, without heeding the crowd of half-declared enemies who were seeking to undermine his reputation, he set about, after his return to florence, his greatest literary and most popular work, _dialogues on the ptolemaic and copernican systems_. this purports to be a series of four conversations between three characters: salviati, a copernican philosopher; sagredo, a wit and scholar, not specially learned, but keen and critical, and who lightens the talk with chaff; simplicio, an aristotelian philosopher, who propounds the stock absurdities which served instead of arguments to the majority of men. the conversations are something between plato's _dialogues_ and sir arthur helps's _friends in council_. the whole is conducted with great good temper and fairness; and, discreetly enough, no definite conclusion is arrived at, the whole being left in abeyance as if for a fifth and decisive dialogue, which, however, was never written, and perhaps was only intended in case the reception was favourable. the preface also sets forth that the object of the writer is to show that the roman edict forbidding the copernican doctrine was not issued in ignorance of the facts of the case, as had been maliciously reported, and that he wishes to show how well and clearly it was all known beforehand. so he says the dialogue on the copernican side takes up the question purely as a mathematical hypothesis or speculative figment, and gives it every artificial advantage of which the theory is capable. this piece of caution was insufficient to blind the eyes of the cardinals; for in it the arguments in favour of the earth's motion are so cogent and unanswerable, and are so popularly stated, as to do more in a few years to undermine the old system than all that he had written and spoken before. he could not get it printed for two years after he had written it, and then only got consent through a piece of carelessness or laziness on the part of the ecclesiastical censor through whose hands the manuscript passed--for which he was afterwards dismissed. however, it did appear, and was eagerly read; the more, perhaps, as the church at once sought to suppress it. the aristotelians were furious, and represented to the pope that he himself was the character intended by simplicio, the philosopher whose opinions get alternately refuted and ridiculed by the other two, till he is reduced to an abject state of impotence. the idea that galileo had thus cast ridicule upon his friend and patron is no doubt a gratuitous and insulting libel: there is no telling whether or not urban believed it, but certainly his countenance changed to galileo henceforward, and whether overruled by his cardinals, or actuated by some other motive, his favour was completely withdrawn. the infirm old man was instantly summoned to rome. his friends pleaded his age--he was now seventy--his ill-health, the time of year, the state of the roads, the quarantine existing on account of the plague. it was all of no avail, to rome he must go, and on the th of february he arrived. [illustration: fig. .--portrait of galileo.] his daughter at arcetri was in despair; and anxiety and fastings and penances self-inflicted on his account, dangerously reduced her health. at rome he was not imprisoned, but he was told to keep indoors, and show himself as little as possible. he was allowed, however, to stay at the house of the tuscan ambassador instead of in gaol. by april he was removed to the chambers of the inquisition, and examined several times. here, however, the anxiety was too much, and his health began to give way seriously; so, before long, he was allowed to return to the ambassador's house; and, after application had been made, was allowed to drive in the public garden in a half-closed carriage. thus in every way the inquisition dealt with him as leniently as they could. he was now their prisoner, and they might have cast him into their dungeons, as many another had been cast. by whatever they were influenced--perhaps the pope's old friendship, perhaps his advanced age and infirmities--he was not so cruelly used. still, they had their rules; he _must_ be made to recant and abjure his heresy; and, if necessary, torture must be applied. this he knew well enough, and his daughter knew it, and her distress may be imagined. moreover, it is not as if they had really been heretics, as if they hated or despised the church of rome. on the contrary, they loved and honoured the church. they were sincere and devout worshippers, and only on a few scientific matters did galileo presume to differ from his ecclesiastical superiors: his disagreement with them occasioned him real sorrow; and his dearest hope was that they could be brought to his way of thinking and embrace the truth. every time he was sent for by the inquisition he was in danger of torture unless he recanted. all his friends urged him repeatedly to submit. they said resistance was hopeless and fatal. within the memory of men still young, giordano bruno had been burnt alive for a similar heresy. this had happened while galileo was at padua. venice was full of it. and since that, only eight years ago indeed, antonio de dominis, archbishop of salpetria, had been sentenced to the same fate: "to be handed over to the secular arm to be dealt with as mercifully as possible without the shedding of blood." so ran the hideous formula condemning a man to the stake. after his sentence, this unfortunate man died in the dungeons in which he had been incarcerated six years--died what is called a "natural" death; but the sentence was carried out, notwithstanding, on his lifeless body and his writings. his writings for which he had been willing to die! these were the tender mercies of the inquisition; and this was the kind of meaning lurking behind many of their well-sounding and merciful phrases. for instance, what they call "rigorous examination," we call "torture." let us, however, remember in our horror at this mode of compelling a prisoner to say anything they wished, that they were a legally constituted tribunal; that they acted with well established rules, and not in passion; and that torture was a recognized mode of extracting evidence, not only in ecclesiastical but in civil courts, at that date. all this, however, was but poor solace to the pitiable old philosopher, thus ruthlessly haled up and down, questioned and threatened, threatened and questioned, receiving agonizing letters from his daughter week by week, and trying to keep up a little spirit to reply as happily and hopefully as he could. this condition of things could not go on. from february to june the suspense lasted. on the th of june he was summoned again, and told he would be wanted all next day for a rigorous examination. early in the morning of the st he repaired thither, and the doors were shut. out of those chambers of horror he did not reappear till the th. what went on all those three days no one knows. he himself was bound to secrecy. no outsider was present. the records of the inquisition are jealously guarded. that he was technically tortured is certain; that he actually underwent the torment of the rack is doubtful. much learning has been expended upon the question, especially in germany. several eminent scholars have held the fact of actual torture to be indisputable (geometrically certain, one says), and they confirm it by the hernia from which he afterwards suffered, this being a well-known and frequent consequence. other equally learned commentators, however, deny that the last stage was reached. for there are five stages all laid down in the rules of the inquisition, and steadily adhered to in a rigorous examination, at each stage an opportunity being given for recantation, every utterance, groan, or sigh being strictly recorded. the recantation so given has to be confirmed a day or two later, under pain of a precisely similar ordeal. the five stages are:-- st. the official threat in the court. nd. the taking to the door of the torture chamber and renewing the official threat. rd. the taking inside and showing the instruments. th. undressing and binding upon the rack. th. _territio realis._ through how many of these ghastly acts galileo passed i do not know. i hope and believe not the last. there are those who lament that he did not hold out, and accept the crown of martyrdom thus offered to him. had he done so we know his fate--a few years' languishing in the dungeons, and then the flames. whatever he ought to have done, he did not hold out--he gave way. at one stage or another of the dread ordeal he said: "i am in your hands. i will say whatever you wish." then was he removed to a cell while his special form of perjury was drawn up. the next day, clothed as a penitent, the venerable old man was taken to the convent of minerva, where the cardinals and prelates were assembled for the purpose of passing judgment upon him. the text of the judgment i have here, but it is too long to read. it sentences him-- st. to the abjuration. nd. to formal imprisonment for life. rd. to recite the seven penitential psalms every week. ten cardinals were present; but, to their honour be it said, three refused to sign; and this blasphemous record of intolerance and bigoted folly goes down the ages with the names of seven cardinals immortalized upon it. this having been read, he next had to read word for word the abjuration which had been drawn up for him, and then sign it. the abjuration of galileo. "i, galileo galilei, son of the late vincenzo galilei, of florence, aged seventy years, being brought personally to judgment, and kneeling before you most eminent and most reverend lords cardinals, general inquisitors of the universal christian republic against heretical depravity, having before my eyes the holy gospels, which i touch with my own hands, swear that i have always believed, and now believe, and with the help of god will in future believe, every article which the holy catholic and apostolic church of rome holds, teaches, and preaches. but because i have been enjoined by this holy office altogether to abandon the false opinion which maintains that the sun is the centre and immovable, and forbidden to hold, defend, or teach the said false doctrine in any manner, and after it hath been signified to me that the said doctrine is repugnant with the holy scripture, i have written and printed a book, in which i treat of the same doctrine now condemned, and adduce reasons with great force in support of the same, without giving any solution, and therefore have been judged grievously suspected of heresy; that is to say, that i held and believed that the sun is the centre of the universe and is immovable, and that the earth is not the centre and is movable; willing, therefore, to remove from the minds of your eminences, and of every catholic christian, this vehement suspicion rightfully entertained towards me, with a sincere heart and unfeigned faith, i abjure, curse, and detest the said errors and heresies, and generally every other error and sect contrary to holy church; and i swear that i will never more in future say or assert anything verbally, or in writing, which may give rise to a similar suspicion of me; but if i shall know any heretic, or any one suspected of heresy, that i will denounce him to this holy office, or to the inquisitor or ordinary of the place where i may be; i swear, moreover, and promise, that i will fulfil and observe fully, all the penances which have been or shall be laid on me by this holy office. but if it shall happen that i violate any of my said promises, oaths, and protestations (which god avert!), i subject myself to all the pains and punishments which have been decreed and promulgated by the sacred canons, and other general and particular constitutions, against delinquents of this description. so may god help me, and his holy gospels which i touch with my own hands. i, the above-named galileo galilei, have abjured, sworn, promised, and bound myself as above, and in witness thereof with my own hand have subscribed this present writing of my abjuration, which i have recited word for word. at rome, in the convent of minerva, nd june, . i, galileo galilei, have abjured as above with my own hand." those who believe the story about his muttering to a friend, as he rose from his knees, "e pur si muove," do not realize the scene. st. there was no friend in the place. nd. it would have been fatally dangerous to mutter anything before such an assemblage. rd. he was by this time an utterly broken and disgraced old man; wishful, of all things, to get away and hide himself and his miseries from the public gaze; probably with his senses deadened and stupefied by the mental sufferings he had undergone, and no longer able to think or care about anything--except perhaps his daughter,--certainly not about any motion of this wretched earth. far and wide the news of the recantation spread. copies of the abjuration were immediately sent to all universities, with instructions to the professors to read it publicly. at florence, his home, it was read out in the cathedral church, all his friends and adherents being specially summoned to hear it. for a short time more he was imprisoned in rome; but at length was permitted to depart, never more of his own will to return. he was allowed to go to siena. here his daughter wrote consolingly, rejoicing at his escape, and saying how joyfully she already recited the penitential psalms for him, and so relieved him of that part of his sentence. but the poor girl was herself, by this time, ill--thoroughly worn out with anxiety and terror; she lay, in fact, on what proved to be her death-bed. her one wish was to see her dearest lord and father, so she calls him, once more. the wish was granted. his prison was changed, by orders from rome, from siena to arcetri, and once more father and daughter embraced. six days after this she died. the broken-hearted old man now asks for permission to go to live in florence, but is met with the stern answer that he is to stay at arcetri, is not to go out of the house, is not to receive visitors, and that if he asks for more favours, or transgresses the commands laid upon him, he is liable to be haled back to rome and cast into a dungeon. these harsh measures were dictated, not by cruelty, but by the fear of his still spreading heresy by conversation, and so he was to be kept isolated. idle, however, he was not and could not be. he often complains that his head is too busy for his body. in the enforced solitude of arcetri he was composing those dialogues on motion which are now reckoned his greatest and most solid achievement. in these the true laws of motion are set forth for the first time (see page ). one more astronomical discovery also he was to make--that of the moon's libration. and then there came one more crushing blow. his eyes became inflamed and painful--the sight of one of them failed, the other soon went; he became totally blind. but this, being a heaven-sent infliction, he could bear with resignation, though it must have been keenly painful to a solitary man of his activity. "alas!" says he, in one of his letters, "your dear friend and servant is totally blind. henceforth this heaven, this universe, which by wonderful observations i had enlarged a hundred and a thousand times beyond the conception of former ages, is shrunk for me into the narrow space which i myself fill in it. so it pleases god; it shall therefore please me also." he was now allowed an amanuensis, and the help of his pupils torricelli, castelli, and viviani, all devotedly attached to him, and torricelli very famous after him. visitors also were permitted, after approval by a jesuit supervisor; and under these circumstances many visited him, among them a man as immortal as himself--john milton, then only twenty-nine, travelling in italy. surely a pathetic incident, this meeting of these two great men--the one already blind, the other destined to become so. no wonder that, as in his old age he dictated his masterpiece, the thoughts of the english poet should run on the blind sage of tuscany, and the reminiscence of their conversation should lend colour to the poem. well, it were tedious to follow the petty annoyances and troubles to which galileo was still subject--how his own son was set to see that no unauthorized procedure took place, and that no heretic visitors were admitted; how it was impossible to get his new book printed till long afterwards; and how one form of illness after another took possession of him. the merciful end came at last, and at the age of seventy-eight he was released from the inquisition. they wanted to deny him burial--they did deny him a monument; they threatened to cart his bones away from florence if his friends attempted one. and so they hoped that he and his work might be forgotten. poor schemers! before the year was out an infant was born in lincolnshire, whose destiny it was to round and complete and carry forward the work of their victim, so that, until man shall cease from the planet, neither the work nor its author shall have need of a monument. * * * * * here might i end, were it not that the same kind of struggle as went on fiercely in the seventeenth century is still smouldering even now. not in astronomy indeed, as then; nor yet in geology, as some fifty years ago; but in biology mainly--perhaps in other subjects. i myself have heard charles darwin spoken of as an atheist and an infidel, the theory of evolution assailed as unscriptural, and the doctrine of the ascent of man from a lower state of being, as opposed to the fall of man from some higher condition, denied as impious and un-christian. men will not learn by the past; still they brandish their feeble weapons against the truths of nature, as if assertions one way or another could alter fact, or make the thing other than it really is. as galileo said before his spirit was broken, "in these and other positions certainly no man doubts but his holiness the pope hath always an absolute power of admitting or condemning them; but it is not in the power of any creature to make them to be true or false, or otherwise than of their own nature and in fact they are." i know nothing of the views of any here present; but i have met educated persons who, while they might laugh at the men who refused to look through a telescope lest they should learn something they did not like, yet also themselves commit the very same folly. i have met persons who utterly refuse to listen to any view concerning the origin of man other than that of a perfect primæval pair in a garden, and i am constrained to say this much: take heed lest some prophet, after having excited your indignation at the follies and bigotry of a bygone generation, does not turn upon you with the sentence, "thou art the man." summary of facts for lecture vi _science before newton_ _dr. gilbert_, of colchester, physician to queen elizabeth, was an excellent experimenter, and made many discoveries in magnetism and electricity. he was contemporary with tycho brahé, and lived from to . _francis bacon_, lord verulam, - , though a brilliant writer, is not specially important as regards science. he was not a scientific man, and his rules for making discoveries, or methods of induction, have never been consciously, nor often indeed unconsciously, followed by discoverers. they are not in fact practical rules at all, though they were so intended. his really strong doctrines are that phenomena must be studied direct, and that variations in the ordinary course of nature must be induced by aid of experiment; but he lacked the scientific instinct for pursuing these great truths into detail and special cases. he sneered at the work and methods of both gilbert and galileo, and rejected the copernican theory as absurd. his literary gifts have conferred on him an artificially high scientific reputation, especially in england; at the same time his writings undoubtedly helped to make popular the idea of there being new methods for investigating nature, and, by insisting on the necessity for freedom from preconceived ideas and opinions, they did much to release men from the bondage of aristotelian authority and scholastic tradition. the greatest name between galileo and newton is that of descartes. _rené descartes_ was born at la haye in touraine, , and died at stockholm in . he did important work in mathematics, physics, anatomy, and philosophy. was greatest as a philosopher and mathematician. at the age of twenty-one he served as a volunteer under prince maurice of nassau, but spent most of his later life in holland. his famous _discourse on method_ appeared at leyden in , and his _principia_ at amsterdam in ; great pains being taken to avoid the condemnation of the church. descartes's main scientific achievement was the application of algebra to geometry; his most famous speculation was the "theory of vortices," invented to account for the motion of planets. he also made many discoveries in optics and physiology. his best known immediate pupils were the princess elizabeth of bohemia, and christina, queen of sweden. he founded a distinct school of thought (the cartesian), and was the precursor of the modern mathematical method of investigating science, just as galileo and gilbert were the originators of the modern experimental method. lecture vi descartes and his theory of vortices after the dramatic life we have been considering in the last two lectures, it is well to have a breathing space, to look round on what has been accomplished, and to review the state of scientific thought, before proceeding to the next great era. for we are still in the early morning of scientific discovery: the dawn of the modern period, faintly heralded by copernicus, brought nearer by the work of tycho and kepler, and introduced by the discoveries of galileo--the dawn has occurred, but the sun is not yet visible. it is hidden by the clouds and mists of the long night of ignorance and prejudice. the light is sufficient, indeed, to render these earth-born vapours more visible: it is not sufficient to dispel them. a generation of slow and doubtful progress must pass, before the first ray of sunlight can break through the eastern clouds and the full orb of day itself appear. it is this period of hesitating progress and slow leavening of men's ideas that we have to pass through in this week's lecture. it always happens thus: the assimilation of great and new ideas is always a slow and gradual process: there is no haste either here or in any other department of nature. _die zeit ist unendlich lang._ steadily the forces work, sometimes seeming to accomplish nothing; sometimes even the motion appears retrograde; but in the long run the destined end is reached, and the course, whether of a planet or of men's thoughts about the universe, is permanently altered. then, the controversy was about the _earth's_ place in the universe; now, if there be any controversy of the same kind, it is about _man's_ place in the universe; but the process is the same: a startling statement by a great genius or prophet, general disbelief, and, it may be, an attitude of hostility, gradual acceptance by a few, slow spreading among the many, ending in universal acceptance and faith often as unquestioning and unreasoning as the old state of unfaith had been. now the process is comparatively speedy: twenty years accomplishes a great deal: then it was tediously slow, and a century seemed to accomplish very little. periodical literature may be responsible for some waste of time, but it certainly assists the rapid spread of ideas. the rate with which ideas are assimilated by the general public cannot even now be considered excessive, but how much faster it is than it was a few centuries ago may be illustrated by the attitude of the public to darwinism now, twenty-five years after _the origin of species_, as compared with their attitude to the copernican system a century after _de revolutionibus_. by the way, it is, i know, presumptuous for me to have an opinion, but i cannot hear darwin compared to or mentioned along with newton without a shudder. the stage in which he found biology seems to me far more comparable with the ptolemaic era in astronomy, and he himself to be quite fairly comparable to copernicus. let us proceed to summarize the stage at which the human race had arrived at the epoch with which we are now dealing. the copernican view of the solar system had been stated, restated, fought, and insisted on; a chain of brilliant telescopic discoveries had made it popular and accessible to all men of any intelligence: henceforth it must be left to slowly percolate and sink into the minds of the people. for the nations were waking up now, and were accessible to new ideas. england especially was, in some sort, at the zenith of its glory; or, if not at the zenith, was in that full flush of youth and expectation and hope which is stronger and more prolific of great deeds and thoughts than a maturer period. a common cause against a common and detested enemy had roused in the hearts of englishmen a passion of enthusiasm and patriotism; so that the mean elements of trade, their cheating yard-wands, were forgotten for a time; the armada was defeated, and the nation's true and conscious adult life began. commerce was now no mere struggle for profit and hard bargains; it was full of the spirit of adventure and discovery; a new world had been opened up; who could tell what more remained unexplored? men awoke to the splendour of their inheritance, and away sailed drake and frobisher and raleigh into the lands of the west. for literature, you know what a time it was. the author of _hamlet_ and _othello_ was alive: it is needless to say more. and what about science? the atmosphere of science is a more quiet and less stirring one; it thrives best when the fever of excitement is allayed; it is necessarily a later growth than literature. already, however, our second great man of science was at work in a quiet country town--second in point of time, i mean, roger bacon being the first. dr. gilbert, of colchester, was the second in point of time, and the age was ripening for the time when england was to be honoured with such a galaxy of scientific luminaries--hooke and boyle and newton--as the world had not yet known. yes, the nations were awake. "in all directions," as draper says, "nature was investigated: in all directions new methods of examination were yielding unexpected and beautiful results. on the ruins of its ivy-grown cathedrals ecclesiasticism [or scholasticism], surprised and blinded by the breaking day, sat solemnly blinking at the light and life about it, absorbed in the recollection of the night that had passed, dreaming of new phantoms and delusions in its wished-for return, and vindictively striking its talons at any derisive assailant who incautiously approached too near." of the work of gilbert there is much to say; so there is also of roger bacon, whose life i am by no means sure i did right in omitting. but neither of them had much to do with astronomy, and since it is in astronomy that the most startling progress was during these centuries being made, i have judged it wiser to adhere mainly to the pioneers in this particular department. only for this reason do i pass gilbert with but slight mention. he knew of the copernican theory and thoroughly accepted it (it is convenient to speak of it as the copernican theory, though you know that it had been considerably improved in detail since the first crude statement by copernicus), but he made in it no changes. he was a cultivated scientific man, and an acute experimental philosopher; his main work lay in the domain of magnetism and electricity. the phenomena connected with the mariner's compass had been studied somewhat by roger bacon; and they were now examined still more thoroughly by gilbert, whose treatise _de magnete_, marks the beginning of the science of magnetism. as an appendix to that work he studied the phenomenon of amber, which had been mentioned by thales. he resuscitated this little fact after its burial of , years, and greatly extended it. he it was who invented the name electricity--i wish it had been a shorter one. mankind invents names much better than do philosophers. what can be better than "heat," "light," "sound"? how favourably they compare with electricity, magnetism, galvanism, electro-magnetism, and magneto-electricity! the only long-established monosyllabic name i know invented by a philosopher is "gas"--an excellent attempt, which ought to be imitated.[ ] of lord bacon, who flourished about the same time (a little later), it is necessary to say something, because many persons are under the impression that to him and his _novum organon_ the reawakening of the world, and the overthrow of aristotelian tradition, are mainly due. his influence, however, has been exaggerated. i am not going to enter into a discussion of the _novum organon_, and the mechanical methods which he propounded as certain to evolve truth if patiently pursued; for this is what he thought he was doing--giving to the world an infallible recipe for discovering truth, with which any ordinarily industrious man could make discoveries by means of collection and discrimination of instances. you will take my statement for what it is worth, but i assert this: that many of the methods which bacon lays down are not those which the experience of mankind has found to be serviceable; nor are they such as a scientific man would have thought of devising. true it is that a real love and faculty for science are born in a man, and that to the man of scientific capacity rules of procedure are unnecessary; his own intuition is sufficient, or he has mistaken his vocation,--but that is not my point. it is not that bacon's methods are useless because the best men do not need them; if they had been founded on a careful study of the methods actually employed, though it might be unconsciously employed, by scientific men--as the methods of induction, stated long after by john stuart mill, were founded--then, no doubt, their statement would have been a valuable service and a great thing to accomplish. but they were not this. they are the ideas of a brilliant man of letters, writing in an age when scientific research was almost unknown, about a subject in which he was an amateur. i confess i do not see how he, or john stuart mill, or any one else, writing in that age, could have formulated the true rules of philosophizing; because the materials and information were scarcely to hand. science and its methods were only beginning to grow. no doubt it was a brilliant attempt. no doubt also there are many good and true points in the statement, especially in his insistence on the attitude of free and open candour with which the investigation of nature should be approached. no doubt there was much beauty in his allegories of the errors into which men were apt to fall--the _idola_ of the market-place, of the tribe, of the theatre, and of the den; but all this is literature, and on the solid progress of science may be said to have had little or no effect. descartes's _discourse on method_ was a much more solid production. you will understand that i speak of bacon purely as a scientific man. as a man of letters, as a lawyer, a man of the world, and a statesman, he is beyond any criticism of mine. i speak only of the purely scientific aspect of the _novum organon_. _the essays_ and _the advancement of learning_ are masterly productions; and as a literary man he takes high rank. the over-praise which, in the british isles, has been lavished upon his scientific importance is being followed abroad by what may be an unnecessary amount of detraction. this is always the worst of setting up a man on too high a pinnacle; some one has to undertake the ungrateful task of pulling him down again. justus von liebig addressed himself to this task with some vigour in his _reden und abhandlung_ (leipzig, ), where he quotes from bacon a number of suggestions for absurd experimentation.[ ] the next paragraph i read, not because i endorse it, but because it is always well to hear both sides of a question. you have probably been long accustomed to read over-estimates of bacon's importance, and extravagant laudation of his writings as making an epoch in science; hear what draper says on the opposite side:--[ ] "the more closely we examine the writings of lord bacon, the more unworthy does he seem to have been of the great reputation which has been awarded to him. the popular delusion to which he owes so much originated at a time when the history of science was unknown. they who first brought him into notice knew nothing of the old school of alexandria. this boasted founder of a new philosophy could not comprehend, and would not accept, the greatest of all scientific doctrines when it was plainly set before his eyes. "it has been represented that the invention of the true method of physical science was an amusement of bacon's hours of relaxation from the more laborious studies of law, and duties of a court. "his chief admirers have been persons of a literary turn, who have an idea that scientific discoveries are accomplished by a mechanico-mental operation. bacon never produced any great practical result himself, no great physicist has ever made any use of his method. he has had the same to do with the development of modern science that the inventor of the orrery has had to do with the discovery of the mechanism of the world. of all the important physical discoveries, there is not one which shows that its author made it by the baconian instrument. "newton never seems to have been aware that he was under any obligation to bacon. archimedes, and the alexandrians, and the arabians, and leonardo da vinci did very well before he was born; the discovery of america by columbus and the circumnavigation by magellan can hardly be attributed to him, yet they were the consequences of a truly philosophical reasoning. but the investigation of nature is an affair of genius, not of rules. no man can invent an _organon_ for writing tragedies and epic poems. bacon's system is, in its own terms, an idol of the theatre. it would scarcely guide a man to a solution of the riddle of Ælia lælia crispis, or to that of the charade of sir hilary. "few scientific pretenders have made more mistakes than lord bacon. he rejected the copernican system, and spoke insolently of its great author; he undertook to criticize adversely gilbert's treatise _de magnete_; he was occupied in the condemnation of any investigation of final causes, while harvey was deducing the circulation of the blood from aquapendente's discovery of the valves in the veins; he was doubtful whether instruments were of any advantage, while galileo was investigating the heavens with the telescope. ignorant himself of every branch of mathematics, he presumed that they were useless in science but a few years before newton achieved by their aid his immortal discoveries. "it is time that the sacred name of philosophy should be severed from its long connection with that of one who was a pretender in science, a time-serving politician, an insidious lawyer, a corrupt judge, a treacherous friend, a bad man." this seems to me a depreciation as excessive as are the eulogies commonly current. the truth probably lies somewhere between the two extremes. it is unfair to judge bacon's methods by thinking of physical science in its present stage. to realise his position we must think of a subject still in its very early infancy, one in which the advisability of applying experimental methods is still doubted; one which has been studied by means of books and words and discussion of normal instances, instead of by collection and observation of the unusual and irregular, and by experimental production of variety. if we think of a subject still in this infantile and almost pre-scientific stage, bacon's words and formulæ are far from inapplicable; they are, within their limitations, quite necessary and wholesome. a subject in this stage, strange to say, exists,--psychology; now hesitatingly beginning to assume its experimental weapons amid a stifling atmosphere of distrust and suspicion. bacon's lack of the modern scientific instinct must be admitted, but he rendered humanity a powerful service in directing it from books to nature herself, and his genius is indubitable. a judicious account of his life and work is given by prof. adamson, in the _encyclopædia britannica_, and to this article i now refer you. * * * * * who, then, was the man of first magnitude filling up the gap in scientific history between the death of galileo and the maturity of newton? unknown and mysterious are the laws regulating the appearance of genius. we have passed in review a pole, a dane, a german, and an italian,--the great man is now a frenchman, rené descartes, born in touraine, on the st of march, . his mother died at his birth; the father was of no importance, save as the owner of some landed property. the boy was reared luxuriously, and inherited a fair fortune. nearly all the men of first rank, you notice, were born well off. genius born to poverty might, indeed, even then achieve name and fame--as we see in the case of kepler--but it was terribly handicapped. handicapped it is still, but far less than of old; and we may hope it will become gradually still less so as enlightenment proceeds, and the tremendous moment of great men to a nation is more clearly and actively perceived. it is possible for genius, when combined with strong character, to overcome all obstacles, and reach the highest eminence, but the struggle must be severe; and the absence of early training and refinement during the receptive years of youth must be a lifelong drawback. descartes had none of these drawbacks; life came easily to him, and, as a consequence perhaps, he never seems to have taken it quite seriously. great movements and stirring events were to him opportunities for the study of men and manners; he was not the man to court persecution, nor to show enthusiasm for a losing or struggling cause. in this, as in many other things, he was imbued with a very modern spirit, a cynical and sceptical spirit, which, to an outside and superficial observer like myself, seems rather rife just now. he was also imbued with a phase of scientific spirit which you sometimes still meet with, though i believe it is passing away, viz. an uncultured absorption in his own pursuits, and some feeling of contempt for classical and literary and æsthetic studies. in politics, art, and history he seems to have had no interest. he was a spectator rather than an actor on the stage of the world; and though he joined the army of that great military commander prince maurice of nassau, he did it not as a man with a cause at heart worth fighting for, but precisely in the spirit in which one of our own gilded youths would volunteer in a similar case, as a good opportunity for frolic and for seeing life. he soon tired of it and withdrew--at first to gay society in paris. here he might naturally have sunk into the gutter with his companions, but for a great mental shock which became the main epoch and turning-point of his life, the crisis which diverted him from frivolity to seriousness. it was a purely intellectual emotion, not excited by anything in the visible or tangible world; nor could it be called conversion in the common acceptation of that term. he tells us that on the th of november, , at the age of twenty-four, a brilliant idea flashed upon him--the first idea, namely, of his great and powerful mathematical method, of which i will speak directly; and in the flush of it he foresaw that just as geometers, starting with a few simple and evident propositions or axioms, ascend by a long and intricate ladder of reasoning to propositions more and more abstruse, so it might be possible to ascend from a few data, to all the secrets and facts of the universe, by a process of mathematical reasoning. "comparing the mysteries of nature with the laws of mathematics, he dared to hope that the secrets of both could be unlocked with the same key." that night he lapsed gradually into a state of enthusiasm, in which he saw three dreams or visions, which he interpreted at the time, even before waking, to be revelations from the spirit of truth to direct his future course, as well as to warn him from the sins he had already committed. his account of the dreams is on record, but is not very easy to follow; nor is it likely that a man should be able to convey to others any adequate idea of the deepest spiritual or mental agitation which has shaken him to his foundations. his associates in paris were now abandoned, and he withdrew, after some wanderings, to holland, where he abode the best part of his life and did his real work. even now, however, he took life easily. he recommends idleness as necessary to the production of good mental work. he worked and meditated but a few hours a day: and most of those in bed. he used to think best in bed, he said. the afternoon he devoted to society and recreation. after supper he wrote letters to various persons, all plainly intended for publication, and scrupulously preserved. he kept himself free from care, and was most cautious about his health, regarding himself, no doubt, as a subject of experiment, and wishful to see how long he could prolong his life. at one time he writes to a friend that he shall be seriously disappointed if he does not manage to see years. [illustration: fig. .--descartes.] this plan of not over-working himself, and limiting the hours devoted to serious thought, is one that might perhaps advantageously be followed by some over-laborious students of the present day. at any rate it conveys a lesson; for the amount of ground covered by descartes, in a life not very long, is extraordinary. he must, however, have had a singular aptitude for scientific work; and the judicious leaven of selfishness whereby he was able to keep himself free from care and embarrassments must have been a great help to him. and what did his versatile genius accomplish during his fifty-four years of life? in philosophy, using the term as meaning mental or moral philosophy and metaphysics, as opposed to natural philosophy or physics, he takes a very high rank, and it is on this that perhaps his greatest fame rests. (he is the author, you may remember, of the famous aphorism, "_cogito, ergo sum_.") in biology i believe he may be considered almost equally great: certainly he spent a great deal of time in dissecting, and he made out a good deal of what is now known of the structure of the body, and of the theory of vision. he eagerly accepted the doctrine of the circulation of the blood, then being taught by harvey, and was an excellent anatomist. you doubtless know professor huxley's article on descartes in the _lay sermons_, and you perceive in what high estimation he is there held. he originated the hypothesis that animals are automata, for which indeed there is much to be said from some points of view; but he unfortunately believed that they were unconscious and non-sentient automata, and this belief led his disciples into acts of abominable cruelty. professor huxley lectured on this hypothesis and partially upheld it not many years since. the article is included in his volume called _science and culture_. concerning his work in mathematics and physics i can speak with more confidence. he is the author of the cartesian system of algebraic or analytic geometry, which has been so powerful an engine of research, far easier to wield than the old synthetic geometry. without it newton could never have written the _principia_, or made his greatest discoveries. he might indeed have invented it for himself, but it would have consumed some of his life to have brought it to the necessary perfection. the principle of it is the specification of the position of a point in a plane by two numbers, indicating say its distance from two lines of reference in the plane; like the latitude and longitude of a place on the globe. for instance, the two lines of reference might be the bottom edge and the left-hand vertical edge of a wall; then a point on the wall, stated as being for instance feet along and feet up, is precisely determined. these two distances are called co-ordinates; horizontal ones are usually denoted by _x_, and vertical ones by _y_. if, instead of specifying two things, only one statement is made, such as _y_ = , it is satisfied by a whole row of points, all the points in a horizontal line feet above the ground. hence _y_ = may be said to represent that straight line, and is called the equation to that straight line. similarly _x_ = represents a vertical straight line feet (or inches or some other unit) from the left-hand edge. if it is asserted that _x_ = and _y_ = , only one point can be found to satisfy both conditions, viz. the crossing point of the above two straight lines. suppose an equation such as _x_ = _y_ to be given. this also is satisfied by a row of points, viz. by all those that are equidistant from bottom and left-hand edges. in other words, _x_ = _y_ represents a straight line slanting upwards at °. the equation _x_ = _y_ represents another straight line with a different angle of slope, and so on. the equation x^ + y^ = represents a circle of radius . the equation x^ + y^ = represents an ellipse; and in general every algebraic equation that can be written down, provided it involve only two variables, _x_ and _y_, represents some curve in a plane; a curve moreover that can be drawn, or its properties completely investigated without drawing, from the equation. thus algebra is wedded to geometry, and the investigation of geometric relations by means of algebraic equations is called analytical geometry, as opposed to the old euclidian or synthetic mode of treating the subject by reasoning consciously directed to the subject by help of figures. if there be three variables--_x_, _y_, and _z_,--instead of only two, an equation among them represents not a curve in a plane but a surface in space; the three variables corresponding to the three dimensions of space: length, breadth, and thickness. an equation with four variables usually requires space of four dimensions for its geometrical interpretation, and so on. thus geometry can not only be reasoned about in a more mechanical and therefore much easier, manner, but it can be extended into regions of which we have and can have no direct conception, because we are deficient in sense organs for accumulating any kind of experience in connexion with such ideas. [illustration: fig. .--the eye diagram. [from descartes' _principia_.] three external points are shown depicted on the retina: the image being appreciated by a representation of the brain.] in physics proper descartes' tract on optics is of considerable historical interest. he treats all the subjects he takes up in an able and original manner. in astronomy he is the author of that famous and long upheld theory, the doctrine of vortices. he regarded space as a plenum full of an all-pervading fluid. certain portions of this fluid were in a state of whirling motion, as in a whirlpool or eddy of water; and each planet had its own eddy, in which it was whirled round and round, as a straw is caught and whirled in a common whirlpool. this idea he works out and elaborates very fully, applying it to the system of the world, and to the explanation of all the motions of the planets. [illustration: fig. .--descartes's diagram of vortices, from his _principia_.] this system evidently supplied a void in men's minds, left vacant by the overthrow of the ptolemaic system, and it was rapidly accepted. in the english universities it held for a long time almost undisputed sway; it was in this faith that newton was brought up. something was felt to be necessary to keep the planets moving on their endless round; the _primum mobile_ of ptolemy had been stopped; an angel was sometimes assigned to each planet to carry it round, but though a widely diffused belief, this was a fantastic and not a serious scientific one. descartes's vortices seemed to do exactly what was wanted. it is true they had no connexion with the laws of kepler. i doubt whether he knew about the laws of kepler; he had not much opinion of other people's work; he read very little--found it easier to think. (he travelled through florence once when galileo was at the height of his renown without calling upon or seeing him.) in so far as the motion of a planet was not circular, it had to be accounted for by the jostling and crowding and distortion of the vortices. gravitation he explained by a settling down of bodies toward the centre of each vortex; and cohesion by an absence of relative motion tending to separate particles of matter. he "can imagine no stronger cement." the vortices, as descartes imagined them, are not now believed in. are we then to regard the system as absurd and wholly false? i do not see how we can do this, when to this day philosophers are agreed in believing space to be completely full of fluid, which fluid is certainly capable of vortex motion, and perhaps everywhere does possess that motion. true, the now imagined vortices are not the large whirls of planetary size, they are rather infinitesimal whirls of less than atomic dimensions; still a whirling fluid is believed in to this day, and many are seeking to deduce all the properties of matter (rigidity, elasticity, cohesion gravitation, and the rest) from it. further, although we talk glibly about gravitation and magnetism, and so on, we do not really know what they are. progress is being made, but we do not yet properly know. much, overwhelmingly much, remains to be discovered, and it ill-behoves us to reject any well-founded and long-held theory as utterly and intrinsically false and absurd. the more one gets to know, the more one perceives a kernel of truth even in the most singular statements; and scientific men have learned by experience to be very careful how they lop off any branch of the tree of knowledge, lest as they cut away the dead wood they lose also some green shoot, some healthy bud of unperceived truth. however, it may be admitted that the idea of a cartesian vortex in connexion with the solar system applies, if at all, rather to an earlier--its nebulous--stage, when the whole thing was one great whirl, ready to split or shrink off planetary rings at their appropriate distances. soon after he had written his great work, the _principia mathematica_, and before he printed it, news reached him of the persecution and recantation of galileo. "he seems to have been quite thunderstruck at the tidings," says mr. mahaffy, in his _life of descartes_.[ ] "he had started on his scientific journeys with the firm determination to enter into no conflict with the church, and to carry out his system of pure mathematics and physics without ever meddling with matters of faith. he was rudely disillusioned as to the possibility of this severance. he wrote at once--apparently, november th, --to mersenne to say he would on no account publish his work--nay, that he had at first resolved to burn all his papers, for that he would never prosecute philosophy at the risk of being censured by his church. 'i could hardly have believed,' he says, 'that an italian, and in favour with the pope as i hear, could be considered criminal for nothing else than for seeking to establish the earth's motion; though i know it has formerly been censured by some cardinals. but i thought i had heard that since then it was constantly being taught, even at rome; and i confess that if the opinion of the earth's movement is false, all the foundations of my philosophy are so also, because it is demonstrated clearly by them. it is so bound up with every part of my treatise that i could not sever it without making the remainder faulty; and although i consider all my conclusions based on very certain and clear demonstrations, i would not for all the world sustain them against the authority of the church.'" ten years later, however, he did publish the book, for he had by this time hit on an ingenious compromise. he formally denied that the earth moved, and only asserted that it was carried along with its water and air in one of those larger motions of the celestial ether which produce the diurnal and annual revolutions of the solar system. so, just as a passenger on the deck of a ship might be called stationary, so was the earth. he gives himself out therefore as a follower of tycho rather than of copernicus, and says if the church won't accept this compromise he must return to the ptolemaic system; but he hopes they won't compel him to do that, seeing that it is manifestly untrue. this elaborate deference to the powers that be did not indeed save the work from being ultimately placed upon the forbidden list by the church, but it saved himself, at any rate, from annoying persecution. he was not, indeed, at all willing to be persecuted, and would no doubt have at once withdrawn anything they wished. i should be sorry to call him a time-server, but he certainly had plenty of that worldly wisdom in which some of his predecessors had been so lamentably deficient. moreover, he was really a sceptic, and cared nothing at all about the church or its dogmas. he knew the church's power, however, and the advisability of standing well with it: he therefore professed himself a catholic, and studiously kept his science and his christianity distinct. in saying that he was a sceptic you must not understand that he was in the least an atheist. very few men are; certainly descartes never thought of being one. the term is indeed ludicrously inapplicable to him, for a great part of his philosophy is occupied with what he considers a rigorous proof of the existence of the deity. at the age of fifty-three he was sent for to stockholm by christina, queen of sweden, a young lady enthusiastically devoted to study of all kinds and determined to surround her court with all that was most famous in literature and science. thither, after hesitation, descartes went. he greatly liked royalty, but he dreaded the cold climate. born in touraine, a swedish winter was peculiarly trying to him, especially as the energetic queen would have lessons given her at five o'clock in the morning. she intended to treat him well, and was immensely taken with him; but this getting up at five o'clock on a november morning, to a man accustomed all his life to lie in bed till eleven, was a cruel hardship. he was too much of a courtier, however, to murmur, and the early morning audience continued. his health began to break down: he thought of retreating, but suddenly he gave way and became delirious. the queen's physician attended him, and of course wanted to bleed him. this, knowing all he knew of physiology, sent him furious, and they could do nothing with him. after some days he became quiet, was bled twice, and gradually sank, discoursing with great calmness on his approaching death, and duly fortified with all the rites of the catholic church. his general method of research was as nearly as possible a purely deductive one:--_i.e._, after the manner of euclid he starts with a few simple principles, and then, by a chain of reasoning, endeavours to deduce from them their consequences, and so to build up bit by bit an edifice of connected knowledge. in this he was the precursor of newton. this method, when rigorously pursued, is the most powerful and satisfactory of all, and results in an ordered province of science far superior to the fragmentary conquests of experiment. but few indeed are the men who can handle it safely and satisfactorily: and none without continual appeals to experiment for verification. it was through not perceiving the necessity for verification that he erred. his importance to science lies not so much in what he actually discovered as in his anticipation of the right conditions for the solution of problems in physical science. he in fact made the discovery that nature could after all be interrogated mathematically--a fact that was in great danger of remaining unknown. for, observe, that the mathematical study of nature, the discovery of truth with a piece of paper and a pen, has a perilous similarity at first sight to the straw-thrashing subtleties of the greeks, whose methods of investigating nature by discussing the meaning of words and the usage of language and the necessities of thought, had proved to be so futile and unproductive. a reaction had set in, led by galileo, gilbert, and the whole modern school of experimental philosophers, lasting down to the present day:--men who teach that the only right way of investigating nature is by experiment and observation. it is indeed a very right and an absolutely necessary way; but it is not the only way. a foundation of experimental fact there must be; but upon this a great structure of theoretical deduction can be based, all rigidly connected together by pure reasoning, and all necessarily as true as the premises, provided no mistake is made. to guard against the possibility of mistake and oversight, especially oversight, all conclusions must sooner or later be brought to the test of experiment; and if disagreeing therewith, the theory itself must be re-examined, and the flaw discovered, or else the theory must be abandoned. of this grand method, quite different from the gropings in the dark of kepler--this method, which, in combination with experiment, has made science what it now is--this which in the hands of newton was to lead to such stupendous results, we owe the beginning and early stages to rené descartes. summary of facts for lectures vii and viii otto guericke - hon. robert boyle - huyghens - christopher wren - robert hooke - newton - edmund halley - james bradley - _chronology of newton's life._ isaac newton was born at woolsthorpe, near grantham, lincolnshire, on christmas day, . his father, a small freehold farmer, also named isaac, died before his birth. his mother, _née_ hannah ayscough, in two years married a mr. smith, rector of north witham, but was again left a widow in . his uncle, w. ayscough, was rector of a near parish and a graduate of trinity college, cambridge. at the age of fifteen isaac was removed from school at grantham to be made a farmer of, but as it seemed he would not make a good one his uncle arranged for him to return to school and thence to cambridge, where he entered trinity college as a sub-sizar in . studied descartes's geometry. found out a method of infinite series in , and began the invention of fluxions. in the same year and the next he was driven from cambridge by the plague. in , at woolsthorpe, the apple fell. in he was elected a fellow of his college, and in was specially noted as possessing an unparalleled genius by dr. barrow, first lucasian professor of mathematics. the same year dr. barrow retired from his chair in favour of newton, who was thus elected at the age of twenty-six. he lectured first on optics with great success. early in he was elected a fellow of the royal society, and communicated his researches in optics, his reflecting telescope, and his discovery of the compound nature of white light. annoying controversies arose; but he nevertheless contributed a good many other most important papers in optics, including observations in diffraction, and colours of thin plates. he also invented the modern sextant. in a letter from paris was read at the royal society concerning a new and accurate determination of the size of the earth by picard. when newton heard of it he began the _principia_, working in silence. in arose a discussion between wren, hooke, and halley concerning the law of inverse square as applied to gravity and the path it would cause the planets to describe. hooke asserted that he had a solution, but he would not produce it. after waiting some time for it halley went to cambridge to consult newton on the subject, and thus discovered the existence of the first part of the _principia_, wherein all this and much more was thoroughly worked out. on his representations to the royal society the manuscript was asked for, and when complete was printed and published in at halley's expense. while it was being completed newton and seven others were sent to uphold the dignity of the university, before the court of high commission and judge jeffreys, against a high-handed action of james ii. in he was sent to parliament, and was present at the coronation of william and mary. made friends with locke. in montague, lord halifax, made him warden, and in master, of the mint. whiston succeeded him as lucasian professor. in the method of fluxions was published. in newton was made president of the royal society, and held the office to the end of his life. in he was knighted by anne. in cotes helped him to bring out a new edition of the _principia_, completed as we now have it. on the th of march , he died: having lived from charles i. to george ii. the laws of motion, discovered by galileo, stated by newton. _law ._--if no force acts on a body in motion, it continues to move uniformly in a straight line. _law ._--if force acts on a body, it produces a change of motion proportional to the force and in the same direction. _law ._--when one body exerts force on another, that other reacts with equal force upon the one. lecture vii sir isaac newton the little hamlet of woolsthorpe lies close to the village of colsterworth, about six miles south of grantham, in the county of lincoln. in the manor house of woolsthorpe, on christmas day, , was born to a widowed mother a sickly infant who seemed not long for this world. two women who were sent to north witham to get some medicine for him scarcely expected to find him alive on their return. however, the child lived, became fairly robust, and was named isaac, after his father. what sort of a man this father was we do not know. he was what we may call a yeoman, that most wholesome and natural of all classes. he owned the soil he tilled, and his little estate had already been in the family for some hundred years. he was thirty-six when he died, and had only been married a few months. of the mother, unfortunately, we know almost as little. we hear that she was recommended by a parishioner to the rev. barnabas smith, an old bachelor in search of a wife, as "the widow newton--an extraordinary good woman:" and so i expect she was, a thoroughly sensible, practical, homely, industrious, middle-class, mill-on-the-floss sort of woman. however, on her second marriage she went to live at north witham, and her mother, old mrs. ayscough, came to superintend the farm at woolsthorpe, and take care of young isaac. by her second marriage his mother acquired another piece of land, which she settled on her first son; so isaac found himself heir to two little properties, bringing in a rental of about £ a year. [illustration: fig. .--manor-house of woolsthorpe.] he had been sent to a couple of village schools to acquire the ordinary accomplishments taught at those places, and for three years to the grammar school at grantham, then conducted by an old gentleman named mr. stokes. he had not been very industrious at school, nor did he feel keenly the fascinations of the latin grammar, for he tells us that he was the last boy in the lowest class but one. he used to pay much more attention to the construction of kites and windmills and waterwheels, all of which he made to work very well. he also used to tie paper lanterns to the tail of his kite, so as to make the country folk fancy they saw a comet, and in general to disport himself as a boy should. it so happened, however, that he succeeded in thrashing, in fair fight, a bigger boy who was higher in the school, and who had given him a kick. his success awakened a spirit of emulation in other things than boxing, and young newton speedily rose to be top of the school. under these circumstances, at the age of fifteen, his mother, who had now returned to woolsthorpe, which had been rebuilt, thought it was time to train him for the management of his land, and to make a farmer and grazier of him. the boy was doubtless glad to get away from school, but he did not take kindly to the farm--especially not to the marketing at grantham. he and an old servant were sent to grantham every week to buy and sell produce, but young isaac used to leave his old mentor to do all the business, and himself retire to an attic in the house he had lodged in when at school, and there bury himself in books. after a time he didn't even go through the farce of visiting grantham at all; but stopped on the road and sat under a hedge, reading or making some model, until his companion returned. we hear of him now in the great storm of , the storm on the day cromwell died, measuring the force of the wind by seeing how far he could jump with it and against it. he also made a water-clock and set it up in the house at grantham, where it kept fairly good time so long as he was in the neighbourhood to look after it occasionally. at his own home he made a couple of sundials on the side of the wall (he began by marking the position of the sun by the shadow of a peg driven into the wall, but this gradually developed into a regular dial) one of which remained of use for some time; and was still to be seen in the same place during the first half of the present century, only with the gnomon gone. in the stone on which it was carved was carefully extracted and presented to the royal society, who preserve it in their library. the letters wton roughly carved on it are barely visible. all these pursuits must have been rather trying to his poor mother, and she probably complained to her brother, the rector of burton coggles: at any rate this gentleman found master newton one morning under a hedge when he ought to have been farming. but as he found him working away at mathematics, like a wise man he persuaded his sister to send the boy back to school for a short time, and then to cambridge. on the day of his finally leaving school old mr. stokes assembled the boys, made them a speech in praise of newton's character and ability, and then dismissed him to cambridge. at trinity college a new world opened out before the country-bred lad. he knew his classics passably, but of mathematics and science he was ignorant, except through the smatterings he had picked up for himself. he devoured a book on logic, and another on kepler's optics, so fast that his attendance at lectures on these subjects became unnecessary. he also got hold of a euclid and of descartes's geometry. the euclid seemed childishly easy, and was thrown aside, but the descartes baffled him for a time. however, he set to it again and again and before long mastered it. he threw himself heart and soul into mathematics, and very soon made some remarkable discoveries. first he discovered the binomial theorem: familiar now to all who have done any algebra, unintelligible to others, and therefore i say nothing about it. by the age of twenty-one or two he had begun his great mathematical discovery of infinite series and fluxions--now known by the name of the differential calculus. he wrote these things out and must have been quite absorbed in them, but it never seems to have occurred to him to publish them or tell any one about them. in he noticed some halos round the moon, and, as his manner was, he measured their angles--the small ones and degrees each, the larger one °· . later he gave their theory. small coloured halos round the moon are often seen, and are said to be a sign of rain. they are produced by the action of minute globules of water or cloud particles upon light, and are brightest when the particles are nearly equal in size. they are not like the rainbow, every part of which is due to light that has entered a raindrop, and been refracted and reflected with prismatic separation of colours; a halo is caused by particles so small as to be almost comparable with the size of waves of light, in a way which is explained in optics under the head "diffraction." it may be easily imitated by dusting an ordinary piece of window-glass over with lycopodium, placing a candle near it, and then looking at the candle-flame through the dusty glass from a fair distance. or you may look at the image of a candle in a dusted looking-glass. lycopodium dust is specially suitable, for its granules are remarkably equal in size. the large halo, more rarely seen, of angular radius °· , is due to another cause again, and is a prismatic effect, although it exhibits hardly any colour. the angle - / ° is characteristic of refraction in crystals with angles of ° and refractive index about the same as water; in other words this halo is caused by ice crystals in the higher regions of the atmosphere. he also the same year observed a comet, and sat up so late watching it that he made himself ill. by the end of the year he was elected to a scholarship and took his b.a. degree. the order of merit for that year never existed or has not been kept. it would have been interesting, not as a testimony to newton, but to the sense or non-sense of the examiners. the oldest professorship of mathematics at the university of cambridge, the lucasian, had not then been long founded, and its first occupant was dr. isaac barrow, an eminent mathematician, and a kind old man. with him newton made good friends, and was helpful in preparing a treatise on optics for the press. his help is acknowledged by dr. barrow in the preface, which states that he had corrected several errors and made some capital additions of his own. thus we see that, although the chief part of his time was devoted to mathematics, his attention was already directed to both optics and astronomy. (kepler, descartes, galileo, all combined some optics with astronomy. tycho and the old ones combined alchemy; newton dabbled in this also.) newton reached the age of twenty-three in , the year of the great plague. the plague broke out in cambridge as well as in london, and the whole college was sent down. newton went back to woolsthorpe, his mind teeming with ideas, and spent the rest of this year and part of the next in quiet pondering. somehow or other he had got hold of the notion of centrifugal force. it was six years before huyghens discovered and published the laws of centrifugal force, but in some quiet way of his own newton knew about it and applied the idea to the motion of the planets. we can almost follow the course of his thoughts as he brooded and meditated on the great problem which had taxed so many previous thinkers,--what makes the planets move round the sun? kepler had discovered how they moved, but why did they so move, what urged them? even the "how" took a long time--all the time of the greeks, through ptolemy, the arabs, copernicus, tycho: circular motion, epicycles, and excentrics had been the prevailing theory. kepler, with his marvellous industry, had wrested from tycho's observations the secret of their orbits. they moved in ellipses with the sun in one focus. their rate of description of area, not their speed, was uniform and proportional to time. yes, and a third law, a mysterious law of unintelligible import, had also yielded itself to his penetrating industry--a law the discovery of which had given him the keenest delight, and excited an outburst of rapture--viz. that there was a relation between the distances and the periodic times of the several planets. the cubes of the distances were proportional to the squares of the times for the whole system. this law, first found true for the six primary planets, he had also extended, after galileo's discovery, to the four secondary planets, or satellites of jupiter (p. ). but all this was working in the dark--it was only the first step--this empirical discovery of facts; the facts were so, but how came they so? what made the planets move in this particular way? descartes's vortices was an attempt, a poor and imperfect attempt, at an explanation. it had been hailed and adopted throughout europe for want of a better, but it did not satisfy newton. no, it proceeded on a wrong tack, and kepler had proceeded on a wrong tack in imagining spokes or rays sticking out from the sun and driving the planets round like a piece of mechanism or mill work. for, note that all these theories are based on a wrong idea--the idea, viz., that some force is necessary to maintain a body in motion. but this was contrary to the laws of motion as discovered by galileo. you know that during his last years of blind helplessness at arcetri, galileo had pondered and written much on the laws of motion, the foundation of mechanics. in his early youth, at pisa, he had been similarly occupied; he had discovered the pendulum, he had refuted the aristotelians by dropping weights from the leaning tower (which we must rejoice that no earthquake has yet injured), and he had returned to mechanics at intervals all his life; and now, when his eyes were useless for astronomy, when the outer world has become to him only a prison to be broken by death, he returns once more to the laws of motion, and produces the most solid and substantial work of his life. for this is galileo's main glory--not his brilliant exposition of the copernican system, not his flashes of wit at the expense of a moribund philosophy, not his experiments on floating bodies, not even his telescope and astronomical discoveries--though these are the most taking and dazzling at first sight. no; his main glory and title to immortality consists in this, that he first laid the foundation of mechanics on a firm and secure basis of experiment, reasoning, and observation. he first discovered the true laws of motion. i said little of this achievement in my lecture on him; for the work was written towards the end of his life, and i had no time then. but i knew i should have to return to it before we came to newton, and here we are. you may wonder how the work got published when so many of his manuscripts were destroyed. horrible to say, galileo's own son destroyed a great bundle of his father's manuscripts, thinking, no doubt, thereby to save his own soul. this book on mechanics was not burnt, however. the fact is it was rescued by one or other of his pupils, toricelli or viviani, who were allowed to visit him in his last two or three years; it was kept by them for some time, and then published surreptitiously in holland. not that there is anything in it bearing in any visible way on any theological controversy; but it is unlikely that the inquisition would have suffered it to pass notwithstanding. i have appended to the summary preceding this lecture (p. ) the three axioms or laws of motion discovered by galileo. they are stated by newton with unexampled clearness and accuracy, and are hence known as newton's laws, but they are based on galileo's work. the first is the simplest; though ignorance of it gave the ancients a deal of trouble. it is simply a statement that force is needed to change the motion of a body; _i.e._ that if no force act on a body it will continue to move uniformly both in speed and direction--in other words, steadily, in a straight line. the old idea had been that some force was needed to maintain motion. on the contrary, the first law asserts, some force is needed to destroy it. leave a body alone, free from all friction or other retarding forces, and it will go on for ever. the planetary motion through empty space therefore wants no keeping up; it is not the motion that demands a force to maintain it, it is the curvature of the path that needs a force to produce it continually. the motion of a planet is approximately uniform so far as speed is concerned, but it is not constant in direction; it is nearly a circle. the real force needed is not a propelling but a deflecting force. the second law asserts that when a force acts, the motion changes, either in speed or in direction, or both, at a pace proportional to the magnitude of the force, and in the same direction as that in which the force acts. now since it is almost solely in direction that planetary motion alters, a deflecting force only is needed; a force at right angles to the direction of motion, a force normal to the path. considering the motion as circular, a force along the radius, a radial or centripetal force, must be acting continually. whirl a weight round and round by a bit of elastic, the elastic is stretched; whirl it faster, it is stretched more. the moving mass pulls at the elastic--that is its centrifugal force; the hand at the centre pulls also--that is centripetal force. the third law asserts that these two forces are equal, and together constitute the tension in the elastic. it is impossible to have one force alone, there must be a pair. you can't push hard against a body that offers no resistance. whatever force you exert upon a body, with that same force the body must react upon you. action and reaction are always equal and opposite. sometimes an absurd difficulty is felt with respect to this, even by engineers. they say, "if the cart pulls against the horse with precisely the same force as the horse pulls the cart, why should the cart move?" why on earth not? the cart moves because the horse pulls it, and because nothing else is pulling it back. "yes," they say, "the cart is pulling back." but what is it pulling back? not itself, surely? "no, the horse." yes, certainly the cart is pulling at the horse; if the cart offered no resistance what would be the good of the horse? that is what he is for, to overcome the pull-back of the cart; but nothing is pulling the cart back (except, of course, a little friction), and the horse is pulling it forward, hence it goes forward. there is no puzzle at all when once you realise that there are two bodies and two forces acting, and that one force acts on each body.[ ] if, indeed, two balanced forces acted on one body that would be in equilibrium, but the two equal forces contemplated in the third law act on two different bodies, and neither is in equilibrium. so much for the third law, which is extremely simple, though it has extraordinarily far-reaching consequences, and when combined with a denial of "action at a distance," is precisely the principle of the conservation of energy. attempts at perpetual motion may all be regarded as attempts to get round this "third law." [illustration: fig. .] on the subject of the _second_ law a great deal more has to be said before it can be in any proper sense even partially appreciated, but a complete discussion of it would involve a treatise on mechanics. it is _the_ law of mechanics. one aspect of it we must attend to now in order to deal with the motion of the planets, and that is the fact that the change of motion of a body depends solely and simply on the force acting, and not at all upon what the body happens to be doing at the time it acts. it may be stationary, or it may be moving in any direction; that makes no difference. thus, referring back to the summary preceding lecture iv, it is there stated that a dropped body falls feet in the first second, that in two seconds it falls feet, and so on, in proportion to the square of the time. so also will it be the case with a thrown body, but the drop must be reckoned from its line of motion--the straight line which, but for gravity, it would describe. thus a stone thrown from _o_ with the velocity _oa_ would in one second find itself at _a_, in two seconds at _b_, in three seconds at _c_, and so on, in accordance with the first law of motion, if no force acted. but if gravity acts it will have fallen feet by the time it would have got to _a_, and so will find itself at _p_. in two seconds it will be at _q_, having fallen a vertical height of feet; in three seconds it will be at _r_, feet below _c_; and so on. its actual path will be a curve, which in this case is a parabola. (fig. .) if a cannon is pointed horizontally over a level plain, the cannon ball will be just as much affected by gravity as if it were dropped, and so will strike the plain at the same instant as another which was simply dropped where it started. one ball may have gone a mile and the other only dropped a hundred feet or so, but the time needed by both for the vertical drop will be the same. the horizontal motion of one is an extra, and is due to the powder. as a matter of fact the path of a projectile in vacuo is only approximately a parabola. it is instructive to remember that it is really an ellipse with one focus very distant, but not at infinity. one of its foci is the centre of the earth. a projectile is really a minute satellite of the earth's, and in vacuo it accurately obeys all kepler's laws. it happens not to be able to complete its orbit, because it was started inconveniently close to the earth, whose bulk gets in its way; but in that respect the earth is to be reckoned as a gratuitous obstruction, like a target, but a target that differs from most targets in being hard to miss. [illustration: fig. .] now consider circular motion in the same way, say a ball whirled round by a string. (fig. .) attending to the body at _o_, it is for an instant moving towards _a_, and if no force acted it would get to _a_ in a time which for brevity we may call a second. but a force, the pull of the string, is continually drawing it towards _s_, and so it really finds itself at _p_, having described the circular arc _op_, which may be considered to be compounded of, and analyzable into the rectilinear motion _oa_ and the drop _ap_. at _p_ it is for an instant moving towards _b_, and the same process therefore carries it to _q_; in the third second it gets to _r_; and so on: always falling, so to speak, from its natural rectilinear path, towards the centre, but never getting any nearer to the centre. the force with which it has thus to be constantly pulled in towards the centre, or, which is the same thing, the force with which it is tugging at whatever constraint it is that holds it in, is _mv^ /r_; where _m_ is the mass of the particle, _v_ its velocity, and _r_ the radius of its circle of movement. this is the formula first given by huyghens for centrifugal force. we shall find it convenient to express it in terms of the time of one revolution, say _t_. it is easily done, since plainly t = circumference/speed = _ [pi]r/v_; so the above expression for centrifugal force becomes _ [pi]^ mr/t^ _. as to the fall of the body towards the centre every microscopic unit of time, it is easily reckoned. for by euclid iii. , and fig. , _ap.aa' = ao^ _. take _a_ very near _o_, then _oa = vt_, and _aa' = r_; so _ap = v^ t^ / r = [pi]^ r t^ /t^ _; or the fall per second is _ [pi]^ r/t^ _, _r_ being its distance from the centre, and _t_ its time of going once round. in the case of the moon for instance, _r_ is earth radii; more exactly · ; and _t_ is a lunar month, or more precisely days, hours, minutes, and - / seconds. hence the moon's deflection from the tangential or rectilinear path every minute comes out as very closely feet (the true size of the earth being used). returning now to the case of a small body revolving round a big one, and assuming a force directly proportional to the mass of both bodies, and inversely proportional to the square of the distance between them: _i.e._ assuming the known force of gravity, it is _v mm/r^ _ where _v_ is a constant, called the gravitation constant, to be determined by experiment. if this is the centripetal force pulling a planet or satellite in, it must be equal to the centrifugal force of this latter, viz. (see above). _ [pi]^ mr/t^ equate the two together, and at once we get _r^ /t^ = v/ [pi]^ m;_ or, in words, the cube of the distance divided by the square of the periodic time for every planet or satellite of the system under consideration, will be constant and proportional to the mass of the central body. this is kepler's third law, with a notable addition. it is stated above for circular motion only, so as to avoid geometrical difficulties, but even so it is very instructive. the reason of the proportion between _r^ _ and _t^ _ is at once manifest; and as soon as the constant _v_ became known, _the mass of the central body_, the sun in the case of a planet, the earth in the case of the moon, jupiter in the case of his satellites, was at once determined. newton's reasoning at this time might, however, be better displayed perhaps by altering the order of the steps a little, as thus:-- the centrifugal force of a body is proportional to _r^ /t^ _, but by kepler's third law _r^ /t^ _ is constant for all the planets, reckoning _r_ from the sun. hence the centripetal force needed to hold in all the planets will be a single force emanating from the sun and varying inversely with the square of the distance from that body. such a force is at once necessary and sufficient. such a force would explain the motion of the planets. but then all this proceeds on a wrong assumption--that the planetary motion is circular. will it hold for elliptic orbits? will an inverse square law of force keep a body moving in an elliptic orbit about the sun in one focus? this is a far more difficult question. newton solved it, but i do not believe that even he could have solved it, except that he had at his disposal two mathematical engines of great power--the cartesian method of treating geometry, and his own method of fluxions. one can explain the elliptic motion now mathematically, but hardly otherwise; and i must be content to state that the double fact is true--viz., that an inverse square law will move the body in an ellipse or other conic section with the sun in one focus, and that if a body so moves it _must_ be acted on by an inverse square law. [illustration: fig. .] this then is the meaning of the first and third laws of kepler. what about the second? what is the meaning of the equable description of areas? well, that rigorously proves that a planet is acted on by a force directed to the centre about which the rate of description of areas is equable. it proves, in fact, that the sun is the attracting body, and that no other force acts. for first of all if the first law of motion is obeyed, _i.e._ if no force acts, and if the path be equally subdivided to represent equal times, and straight lines be drawn from the divisions to any point whatever, all these areas thus enclosed will be equal, because they are triangles on equal base and of the same height (euclid, i). see fig. ; _s_ being any point whatever, and _a_, _b_, _c_, successive positions of a body. now at each of the successive instants let the body receive a sudden blow in the direction of that same point _s_, sufficient to carry it from _a_ to _d_ in the same time as it would have got to _b_ if left alone. the result will be that there will be a compromise, and it will really arrive at _p_, travelling along the diagonal of the parallelogram _ap_. the area its radius vector sweeps out is therefore _sap_, instead of what it would have been, _sab_. but then these two areas are equal, because they are triangles on the same base _as_, and between the same parallels _bp_, _as_; for by the parallelogram law _bp_ is parallel to _ad_. hence the area that would have been described is described, and as all the areas were equal in the case of no force, they remain equal when the body receives a blow at the end of every equal interval of time, _provided_ that every blow is actually directed to _s_, the point to which radii vectores are drawn. [illustration: fig. .] [illustration: fig. .] it is instructive to see that it does not hold if the blow is any otherwise directed; for instance, as in fig. , when the blow is along _ae_, the body finds itself at _p_ at the end of the second interval, but the area _sap_ is by no means equal to _sab_, and therefore not equal to _soa_, the area swept out in the first interval. in order to modify fig. so as to represent continuous motion and steady forces, we have to take the sides of the polygon _oapq_, &c., very numerous and very small; in the limit, infinitely numerous and infinitely small. the path then becomes a curve, and the series of blows becomes a steady force directed towards _s_. about whatever point therefore the rate of description of areas is uniform, that point and no other must be the centre of all the force there is. if there be no force, as in fig. , well and good, but if there be any force however small not directed towards _s_, then the rate of description of areas about _s_ cannot be uniform. kepler, however, says that the rate of description of areas of each planet about the sun is, by tycho's observations, uniform; hence the sun is the centre of all the force that acts on them, and there is no other force, not even friction. that is the moral of kepler's second law. we may also see from it that gravity does not travel like light, so as to take time on its journey from sun to planet; for, if it did, there would be a sort of aberration, and the force on its arrival could no longer be accurately directed to the centre of the sun. (see _nature_, vol. xlvi., p. .) it is a matter for accuracy of observation, therefore, to decide whether the minutest trace of such deviation can be detected, _i.e._ within what limits of accuracy kepler's second law is now known to be obeyed. i will content myself by saying that the limits are extremely narrow. [reference may be made also to p. .] thus then it became clear to newton that the whole solar system depended on a central force emanating from the sun, and varying inversely with the square of the distance from him: for by that hypothesis all the laws of kepler concerning these motions were completely accounted for; and, in fact, the laws necessitated the hypothesis and established it as a theory. similarly the satellites of jupiter were controlled by a force emanating from jupiter and varying according to the same law. and again our moon must be controlled by a force from the earth, decreasing with the distance according to the same law. grant this hypothetical attracting force pulling the planets towards the sun, pulling the moon towards the earth, and the whole mechanism of the solar system is beautifully explained. if only one could be sure there was such a force! it was one thing to calculate out what the effects of such a force would be: it was another to be able to put one's finger upon it and say, this is the force that actually exists and is known to exist. we must picture him meditating in his garden on this want--an attractive force towards the earth. if only such an attractive force pulling down bodies to the earth existed. an apple falls from a tree. why, it does exist! there is gravitation, common gravity that makes bodies fall and gives them their weight. wanted, a force tending towards the centre of the earth. it is to hand! it is common old gravity that had been known so long, that was perfectly familiar to galileo, and probably to archimedes. gravity that regulates the motion of projectiles. why should it only pull stones and apples? why should it not reach as high as the moon? why should it not be the gravitation of the sun that is the central force acting on all the planets? surely the secret of the universe is discovered! but, wait a bit; is it discovered? is this force of gravity sufficient for the purpose? it must vary inversely with the square of the distance from the centre of the earth. how far is the moon away? sixty earth's radii. hence the force of gravity at the moon's distance can only be / of what it is on the earth's surface. so, instead of pulling it ft. per second, it should pull it / ft. per second, or ft. a minute.[ ] how can one decide whether such a force is able to pull the moon the actual amount required? to newton this would seem only like a sum in arithmetic. out with a pencil and paper and reckon how much the moon falls toward the earth in every second of its motion. is it / ? that is what it ought to be: but is it? the size of the earth comes into the calculation. sixty miles make a degree, degrees a circumference. this gives as the earth's diameter , miles; work it out. the answer is not feet a minute, it is · feet. surely a mistake of calculation? no, it is no mistake: there is something wrong in the theory, gravity is too strong. instead of falling toward the earth - / hundredths of an inch every second, as it would under gravity, the moon only falls - / hundredths of an inch per second. with such a discovery in his grasp at the age of twenty-three he is disappointed--the figures do not agree, and he cannot make them agree. either gravity is not the force in action, or else something interferes with it. possibly, gravity does part of the work, and the vortices of descartes interfere with it. he must abandon the fascinating idea for the time. in his own words, "he laid aside at that time any further thought of the matter." so far as is known, he never mentioned his disappointment to a soul. he might, perhaps, if he had been at cambridge, but he was a shy and solitary youth, and just as likely he might not. up in lincolnshire, in the seventeenth century, who was there for him to consult? true, he might have rushed into premature publication, after our nineteenth century fashion, but that was not his method. publication never seemed to have occurred to him. his reticence now is noteworthy, but later on it is perfectly astonishing. he is so absorbed in making discoveries that he actually has to be reminded to tell any one about them, and some one else always has to see to the printing and publishing for him. i have entered thus fully into what i conjecture to be the stages of this early discovery of the law of gravitation, as applicable to the heavenly bodies, because it is frequently and commonly misunderstood. it is sometimes thought that he discovered the force of gravity; i hope i have made it clear that he did no such thing. every educated man long before his time, if asked why bodies fell, would reply just as glibly as they do now, "because the earth attracts them," or "because of the force of gravity." his discovery was that the motions of the solar system were due to the action of a central force, directed to the body at the centre of the system, and varying inversely with the square of the distance from it. this discovery was based upon kepler's laws, and was clear and certain. it might have been published had he so chosen. but he did not like hypothetical and unknown forces; he tried to see whether the known force of gravity would serve. this discovery at that time he failed to make, owing to a wrong numerical datum. the size of the earth he only knew from the common doctrine of sailors that miles make a degree; and that threw him out. instead of falling feet a minute, as it ought under gravity, it only fell · feet, so he abandoned the idea. we do not find that he returned to it for sixteen years. lecture viii newton and the law of gravitation we left newton at the age of twenty-three on the verge of discovering the mechanism of the solar system, deterred therefrom only by an error in the then imagined size of the earth. he had proved from kepler's laws that a centripetal force directed to the sun, and varying as the inverse square of the distance from that body, would account for the observed planetary motions, and that a similar force directed to the earth would account for the lunar motion; and it had struck him that this force might be the very same as the familiar force of gravitation which gave to bodies their weight: but in attempting a numerical verification of this idea in the case of the moon he was led by the then received notion that sixty miles made a degree on the earth's surface into an erroneous estimate of the size of the moon's orbit. being thus baffled in obtaining such verification, he laid the matter aside for a time. the anecdote of the apple we learn from voltaire, who had it from newton's favourite niece, who with her husband lived and kept house for him all his later life. it is very like one of those anecdotes which are easily invented and believed in, and very often turn out on scrutiny to have no foundation. fortunately this anecdote is well authenticated, and moreover is intrinsically probable; i say fortunately, because it is always painful to have to give up these child-learnt anecdotes, like alfred and the cakes and so on. this anecdote of the apple we need not resign. the tree was blown down in and part of its wood is preserved. i have mentioned voltaire in connection with newton's philosophy. this acute critic at a later stage did a good deal to popularise it throughout europe and to overturn that of his own countryman descartes. cambridge rapidly became newtonian, but oxford remained cartesian for fifty years or more. it is curious what little hold science and mathematics have ever secured in the older and more ecclesiastical university. the pride of possessing newton has however no doubt been the main stimulus to the special pursuits of cambridge. he now began to turn his attention to optics, and, as was usual with him, his whole mind became absorbed in this subject as if nothing else had ever occupied him. his cash-book for this time has been discovered, and the entries show that he is buying prisms and lenses and polishing powder at the beginning of . he was anxious to improve telescopes by making more perfect lenses than had ever been used before. accordingly he calculated out their proper curves, just as descartes had also done, and then proceeded to grind them as near as he could to those figures. but the images did not please him; they were always blurred and rather indistinct. at length, it struck him that perhaps it was not the lenses but the light which was at fault. perhaps light was so composed that it _could_ not be focused accurately to a sharp and definite point. perhaps the law of refraction was not quite accurate, but only an approximation. so he bought a prism to try the law. he let in sunlight through a small round hole in a window shutter, inserted the prism in the light, and received the deflected beam on a white screen; turning the prism about till it was deviated as little as possible. the patch on the screen was not a round disk, as it would have been without the prism, but was an elongated oval and was coloured at its extremities. evidently refraction was not a simple geometrical deflection of a ray, there was a spreading out as well. [illustration: fig. .--a prism not only _deviates_ a beam of sunlight, but also spreads it out or _disperses_ it.] why did the image thus spread out? if it were due to irregularities in the glass a second prism should rather increase them, but a second prism when held in appropriate position was able to neutralise the dispersion and to reproduce the simple round white spot without deviation. evidently the spreading out of the beam was connected in some definite way with its refraction. could it be that the light particles after passing through the prism travelled in variously curved lines, as spinning racquet balls do? to examine this he measured the length of the oval patch when the screen was at different distances from the prism, and found that the two things were directly proportional to each other. doubling the distance of the screen doubled the length of the patch. hence the rays travelled in straight lines from the prism, and the spreading out was due to something that occurred within its substance. could it be that white light was compound, was a mixture of several constituents, and that its different constituents were differently bent? no sooner thought than tried. pierce the screen to let one of the constituents through and interpose a second prism in its path. if the spreading out depended on the prism only it should spread out just as much as before, but if it depended on the complex character of white light, this isolated simple constituent should be able to spread out no more. it did not spread out any more: a prism had no more dispersive power over it; it was deflected by the appropriate amount, but it was not analysed into constituents. it differed from sunlight in being simple. with many ingenious and beautifully simple experiments, which are quoted in full in several books on optics, he clinched the argument and established his discovery. white light was not simple but compound. it could be sorted out by a prism into an infinite number of constituent parts which were differently refracted, and the most striking of which newton named violet, indigo, blue, green, yellow, orange, and red. [illustration: fig. .--a single constituent of white light, obtained by the use of perforated screens is capable of no more dispersion.] at once the true nature of colour became manifest. colour resided not in the coloured object as had till now been thought, but in the light which illuminated it. red glass for instance adds nothing to sunlight. the light does not get dyed red by passing through the glass; all that the red glass does is to stop and absorb a large part of the sunlight; it is opaque to the larger portion, but it is transparent to that particular portion which affects our eyes with the sensation of red. the prism acts like a sieve sorting out the different kinds of light. coloured media act like filters, stopping certain kinds but allowing the rest to go through. leonardo's and all the ancient doctrines of colour had been singularly wrong; colour is not in the object but in the light. goethe, in his _farbenlehre_, endeavoured to controvert newton, and to reinstate something more like the old views; but his failure was complete. refraction analysed out the various constituents of white light and displayed them in the form of a series of overlapping images of the aperture, each of a different colour; this series of images we call a spectrum, and the operation we now call spectrum analysis. the reason of the defect of lenses was now plain: it was not so much a defect of the lens as a defect of light. a lens acts by refraction and brings rays to a focus. if light be simple it acts well, but if ordinary white light fall upon a lens, its different constituents have different foci; every bright object is fringed with colour, and nothing like a clear image can be obtained. [illustration: fig. .--showing the boundary rays of a parallel beam passing through a lens.] a parallel beam passing through a lens becomes conical; but instead of a single cone it is a sheaf or nest of cones, all having the edge of the lens as base, but each having a different vertex. the violet cone is innermost, near the lens, the red cone outermost, while the others lie between. beyond the crossing point or focus the order of cones is reversed, as the above figure shows. only the two marginal rays of the beam are depicted. if a screen be held anywhere nearer the lens than the place marked there will be a whitish centre to the patch of light and a red and orange fringe or border. held anywhere beyond the region , the border of the patch will be blue and violet. held about the colour will be less marked than elsewhere, but nowhere can it be got rid of. each point of an object will be represented in the image not by a point but by a coloured patch: a fact which amply explains the observed blurring and indistinctness. newton measured and calculated the distance between the violet and red foci--vr in the diagram--and showed that it was / th the diameter of the lens. to overcome this difficulty (called chromatic aberration) telescope glasses were made small and of very long focus: some of them so long that they had no tube, all of them egregiously cumbrous. yet it was with such instruments that all the early discoveries were made. with such an instrument, for instance, huyghens discovered the real shape of saturn's ring. the defects of refractors seemed irremediable, being founded in the nature of light itself. so he gave up his "glass works"; and proceeded to think of reflexion from metal specula. a concave mirror forms an image just as a lens does, but since it does so without refraction or transmission through any substance, there is no accompanying dispersion or chromatic aberration. the first reflecting telescope he made was in. diameter and in. long, and magnified forty times. it acted as well as a three or four feet refractor of that day, and showed jupiter's moons. so he made a larger one, now in the library of the royal society, london, with an inscription: "the first reflecting telescope, invented by sir isaac newton, and made with his own hands." this has been the parent of most of the gigantic telescopes of the present day. fifty years elapsed before it was much improved on, and then, first by hadley and afterwards by herschel and others, large and good reflectors were constructed. the largest telescope ever made, that of lord rosse, is a newtonian reflector, fifty feet long, six feet diameter, with a mirror weighing four tons. the sextant, as used by navigators, was also invented by newton. the year after the plague, in , newton returned to trinity college, and there continued his experiments on optics. it is specially to be noted that at this time, at the age of twenty-four, newton had laid the foundations of all his greatest discoveries:-- [illustration: fig. .--newton's telescope.] the theory of fluxions; or, the differential calculus. the law of gravitation; or, the complete theory of astronomy. the compound nature of white light; or, the beginning of spectrum analysis. [illustration: fig. .--the sextant, as now made.] his later life was to be occupied in working these incipient discoveries out. but the most remarkable thing is that no one knew about any one of them. however, he was known as an accomplished young mathematician, and was made a fellow of his college. you remember that he had a friend there in the person of dr. isaac barrow, first lucasian professor of mathematics in the university. it happened, about , that a mathematical discovery of some interest was being much discussed, and dr. barrow happened to mention it to newton, who said yes, he had worked out that and a few other similar things some time ago. he accordingly went and fetched some papers to dr. barrow, who forwarded them to other distinguished mathematicians, and it thus appeared that newton had discovered theorems much more general than this special case that was exciting so much interest. dr. barrow, being anxious to devote his time more particularly to theology, resigned his chair the same year in favour of newton, who was accordingly elected to the lucasian professorship, which he held for thirty years. this chair is now the most famous in the university, and it is commonly referred to as the chair of newton. still, however, his method of fluxions was unknown, and still he did not publish it. he lectured first on optics, giving an account of his experiments. his lectures were afterwards published both in latin and english, and are highly valued to this day. the fame of his mathematical genius came to the ears of the royal society, and a motion was made to get him elected a fellow of that body. the royal society, the oldest and most famous of all scientific societies with a continuous existence, took its origin in some private meetings, got up in london by the hon. robert boyle and a few scientific friends, during all the trouble of the commonwealth. after the restoration, charles ii. in incorporated it under royal charter; among the original members being boyle, hooke, christopher wren, and other less famous names. boyle was a great experimenter, a worthy follower of dr. gilbert. hooke began as his assistant, but being of a most extraordinary ingenuity he rapidly rose so as to exceed his master in importance. fate has been a little unkind to hooke in placing him so near to newton; had he lived in an ordinary age he would undoubtedly have shone as a star of the first magnitude. with great ingenuity, remarkable scientific insight, and consummate experimental skill, he stands in many respects almost on a level with galileo. but it is difficult to see stars even of the first magnitude when the sun is up, and thus it happens that the name and fame of this brilliant man are almost lost in the blaze of newton. of christopher wren i need not say much. he is well known as an architect, but he was a most accomplished all-round man, and had a considerable taste and faculty for science. these then were the luminaries of the royal society at the time we are speaking of, and to them newton's first scientific publication was submitted. he communicated to them an account of his reflecting telescope, and presented them with the instrument. their reception of it surprised him; they were greatly delighted with it, and wrote specially thanking him for the communication, and assuring him that all right should be done him in the matter of the invention. the bishop of salisbury (bishop burnet) proposed him for election as a fellow, and elected he was. in reply, he expressed his surprise at the value they set on the telescope, and offered, if they cared for it, to send them an account of a discovery which he doubts not will prove much more grateful than the communication of that instrument, "being in my judgment the oddest, if not the most considerable detection that has recently been made into the operations of nature." so he tells them about his optical researches and his discovery of the nature of white light, writing them a series of papers which were long afterwards incorporated and published as his _optics_. a magnificent work, which of itself suffices to place its author in the first rank of the world's men of science. the nature of white light, the true doctrine of colour, and the differential calculus! besides a good number of minor results--binomial theorem, reflecting telescope, sextant, and the like; one would think it enough for one man's life-work, but the masterpiece remains still to be mentioned. it is as when one is considering shakspeare: _king lear_, _macbeth_, _othello_,--surely a sufficient achievement,--but the masterpiece remains. comparisons in different departments are but little help perhaps, nevertheless it seems to me that in his own department, and considered simply as a man of science, newton towers head and shoulders over, not only his contemporaries--that is a small matter--but over every other scientific man who has ever lived, in a way that we can find no parallel for in other departments. other nations admit his scientific pre-eminence with as much alacrity as we do. well, we have arrived at the year and his election to the royal society. during the first year of his membership there was read at one of the meetings a paper giving an account of a very careful determination of the length of a degree (_i.e._ of the size of the earth), which had been made by picard near paris. the length of the degree turned out to be not sixty miles, but nearly seventy miles. how soon newton heard of this we do not learn--probably not for some years,--cambridge was not so near london then as it is now, but ultimately it was brought to his notice. armed with this new datum, his old speculation concerning gravity occurred to him. he had worked out the mechanics of the solar system on a certain hypothesis, but it had remained a hypothesis somewhat out of harmony with apparent fact. what if it should turn out to be true after all! he took out his old papers and began again the calculation. if gravity were the force keeping the moon in its orbit, it would fall toward the earth sixteen feet every minute. how far did it fall? the newly known size of the earth would modify the figures: with intense excitement he runs through the working, his mind leaps before his hand, and as he perceives the answer to be coming out right, all the infinite meaning and scope of his mighty discovery flashes upon him, and he can no longer see the paper. he throws down the pen; and the secret of the universe is, to one man, known. but of course it had to be worked out. the meaning might flash upon him, but its full detail required years of elaboration; and deeper and deeper consequences revealed themselves to him as he proceeded. for two years he devoted himself solely to this one object. during those years he lived but to calculate and think, and the most ludicrous stories are told concerning his entire absorption and inattention to ordinary affairs of life. thus, for instance, when getting up in a morning he would sit on the side of the bed half-dressed, and remain like that till dinner time. often he would stay at home for days together, eating what was taken to him, but without apparently noticing what he was doing. one day an intimate friend, dr. stukely, called on him and found on the table a cover laid for his solitary dinner. after waiting a long time, dr. stukely removed the cover and ate the chicken underneath it, replacing and covering up the bones again. at length newton appeared, and after greeting his friend, sat down to dinner, but on lifting the cover he said in surprise, "dear me, i thought i had not dined, but i see i have." it was by this continuous application that the _principia_ was accomplished. probably nothing of the first magnitude can be accomplished without something of the same absorbed unconsciousness and freedom from interruption. but though desirable and essential for the _work_, it was a severe tax upon the powers of the _man_. there is, in fact, no doubt that newton's brain suffered temporary aberration after this effort for a short time. the attack was slight, and it has been denied; but there are letters extant which are inexplicable otherwise, and moreover after a year or two he writes to his friends apologizing for strange and disjointed epistles, which he believed he had written without understanding clearly what he wrote. the derangement was, however, both slight and temporary: and it is only instructive to us as showing at what cost such a work as the _principia_ must be produced, even by so mighty a mind as that of newton. the first part of the work having been done, any ordinary mortal would have proceeded to publish it; but the fact is that after he had sent to the royal society his papers on optics, there had arisen controversies and objections; most of them rather paltry, to which he felt compelled to find answers. many men would have enjoyed this part of the work, and taken it as evidence of interest and success. but to newton's shy and retiring disposition these discussions were merely painful. he writes, indeed, his answers with great patience and ability, and ultimately converts the more reasonable of his opponents, but he relieves his mind in the following letter to the secretary of the royal society: "i see i have made myself a slave to philosophy, but if i get free of this present business i will resolutely bid adieu to it eternally, except what i do for my private satisfaction or leave to come out after me; for i see a man must either resolve to put out nothing new, or to become a slave to defend it." and again in a letter to leibnitz: "i have been so persecuted with discussions arising out of my theory of light that i blamed my own imprudence for parting with so substantial a blessing as my quiet to run after a shadow." this shows how much he cared for contemporary fame. so he locked up the first part of the _principia_ in his desk, doubtless intending it to be published after his death. but fortunately this was not so to be. in , among the leading lights of the royal society, the same sort of notions about gravity and the solar system began independently to be bruited. the theory of gravitation seemed to be in the air, and wren, hooke, and halley had many a talk about it. hooke showed an experiment with a pendulum, which he likened to a planet going round the sun. the analogy is more superficial than real. it does not obey kepler's laws; still it was a striking experiment. they had guessed at a law of inverse squares, and their difficulty was to prove what curve a body subject to it would describe. they knew it ought to be an ellipse if it was to serve to explain the planetary motion, and hooke said he could prove that an ellipse it was; but he was nothing of a mathematician, and the others scarcely believed him. undoubtedly he had shrewd inklings of the truth, though his guesses were based on little else than a most sagacious intuition. he surmised also that gravity was the force concerned, and asserted that the path of an ordinary projectile was an ellipse, like the path of a planet--which is quite right. in fact the beginnings of the discovery were beginning to dawn upon him in the well-known way in which things do dawn upon ordinary men of genius: and had newton not lived we should doubtless, by the labours of a long chain of distinguished men, beginning with hooke, wren, and halley, have been now in possession of all the truths revealed by the _principia_. we should never have had them stated in the same form, nor proved with the same marvellous lucidity and simplicity, but the facts themselves we should by this time have arrived at. their developments and completions, due to such men as clairaut, euler, d'alembert, lagrange, laplace, airy, leverrier, adams, we should of course not have had to the same extent; because the lives and energies of these great men would have been partially consumed in obtaining the main facts themselves. the youngest of the three questioners at the time we are speaking of was edmund halley, an able and remarkable man. he had been at cambridge, doubtless had heard newton lecture, and had acquired a great veneration for him. in january, , we find wren offering hooke and halley a prize, in the shape of a book worth forty shillings, if they would either of them bring him within two months a demonstration that the path of a planet subject to an inverse square law would be an ellipse. not in two months, nor yet in seven, was there any proof forthcoming. so at last, in august, halley went over to cambridge to speak to newton about the difficult problem and secure his aid. arriving at his rooms he went straight to the point. he said, "what path will a body describe if it be attracted by a centre with a force varying as the inverse square of the distance." to which newton at once replied, "an ellipse." "how on earth do you know?" said halley in amazement. "why, i have calculated it," and began hunting about for the paper. he actually couldn't find it just then, but sent it him shortly by post, and with it much more--in fact, what appeared to be a complete treatise on motion in general. with his valuable burden halley hastened to the royal society and told them what he had discovered. the society at his representation wrote to mr. newton asking leave that it might be printed. to this he consented; but the royal society wisely appointed mr. halley to see after him and jog his memory, in case he forgot about it. however, he set to work to polish it up and finish it, and added to it a great number of later developments and embellishments, especially the part concerning the lunar theory, which gave him a deal of trouble--and no wonder; for in the way he has put it there never was a man yet living who could have done the same thing. mathematicians regard the achievement now as men might stare at the work of some demigod of a bygone age, wondering what manner of man this was, able to wield such ponderous implements with such apparent ease. to halley the world owes a great debt of gratitude--first, for discovering the _principia_; second, for seeing it through the press; and third, for defraying the cost of its publication out of his own scanty purse. for though he ultimately suffered no pecuniary loss, rather the contrary, yet there was considerable risk in bringing out a book which not a dozen men living could at the time comprehend. it is no small part of the merit of halley that he recognized the transcendent value of the yet unfinished work, that he brought it to light, and assisted in its becoming understood to the best of his ability. though halley afterwards became astronomer-royal, lived to the ripe old age of eighty-six, and made many striking observations, yet he would be the first to admit that nothing he ever did was at all comparable in importance with his discovery of the _principia_; and he always used to regard his part in it with peculiar pride and pleasure. and how was the _principia_ received? considering the abstruse nature of its subject, it was received with great interest and enthusiasm. in less than twenty years the edition was sold out, and copies fetched large sums. we hear of poor students copying out the whole in manuscript in order to possess a copy--not by any means a bad thing to do, however many copies one may possess. the only useful way really to read a book like that is to pore over every sentence: it is no book to be skimmed. while the _principia_ was preparing for the press a curious incident of contact between english history and the university occurred. it seems that james ii., in his policy of catholicising the country, ordered both universities to elect certain priests to degrees without the ordinary oaths. oxford had given way, and the dean of christ church was a creature of james's choosing. cambridge rebelled, and sent eight of its members, among them mr. newton, to plead their cause before the court of high commission. judge jeffreys presided over the court, and threatened and bullied with his usual insolence. the vice-chancellor of cambridge was deprived of office, the other deputies were silenced and ordered away. from the precincts of this court of justice newton returned to trinity college to complete the _principia_. by this time newton was only forty-five years old, but his main work was done. his method of fluxions was still unpublished; his optics was published only imperfectly; a second edition of the _principia_, with additions and improvements, had yet to appear; but fame had now come upon him, and with fame worries of all kinds. by some fatality, principally no doubt because of the interest they excited, every discovery he published was the signal for an outburst of criticism and sometimes of attack. i shall not go into these matters: they are now trivial enough, but it is necessary to mention them, because to newton they evidently loomed large and terrible, and occasioned him acute torment. [illustration: fig. .--newton when young. (_from an engraving by b. reading after sir peter lely._)] no sooner was the _principia_ put than hooke put in his claims for priority. and indeed his claims were not altogether negligible; for vague ideas of the same sort had been floating in his comprehensive mind, and he doubtless felt indistinctly conscious of a great deal more than he could really state or prove. by indiscreet friends these two great men were set somewhat at loggerheads, and worse might have happened had they not managed to come to close quarters, and correspond privately in a quite friendly manner, instead of acting through the mischievous medium of third parties. in the next edition newton liberally recognizes the claims of both hooke and wren. however, he takes warning betimes of what he has to expect, and writes to halley that he will only publish the first two books, those containing general theorems on motion. the third book--concerning the system of the world, _i.e._ the application to the solar system--he says "i now design to suppress. philosophy is such an impertinently litigious lady that a man had as good be engaged in law-suits as have to do with her. i found it so formerly, and now i am no sooner come near her again but she gives me warning. the two books without the third will not so well bear the title 'mathematical principles of natural philosophy,' and therefore i had altered it to this, 'on the free motion of two bodies'; but on second thoughts i retain the former title: 'twill help the sale of the book--which i ought not to diminish now 'tis yours." however, fortunately, halley was able to prevail upon him to publish the third book also. it is, indeed, the most interesting and popular of the three, as it contains all the direct applications to astronomy of the truths established in the other two. some years later, when his method of fluxions was published, another and a worse controversy arose--this time with leibnitz, who had also independently invented the differential calculus. it was not so well recognized then how frequently it happens that two men independently and unknowingly work at the very same thing at the same time. the history of science is now full of such instances; but then the friends of each accused the other of plagiarism. i will not go into the controversy: it is painful and useless. it only served to embitter the later years of two great men, and it continued long after newton's death--long after both their deaths. it can hardly be called ancient history even now. but fame brought other and less unpleasant distractions than controversies. we are a curious, practical, and rather stupid people, and our one idea of honouring a man is to _vote_ for him in some way or other; so they sent newton to parliament. he went, i believe, as a whig, but it is not recorded that he spoke. it is, in fact, recorded that he was once expected to speak when on a royal commission about some question of chronometers, but that he would not. however, i dare say he made a good average member. then a little later it was realized that newton was poor, that he still had to teach for his livelihood, and that though the crown had continued his fellowship to him as lucasian professor without the necessity of taking orders, yet it was rather disgraceful that he should not be better off. so an appeal was made to the government on his behalf, and lord halifax, who exerted himself strongly in the matter, succeeding to office on the accession of william iii., was able to make him ultimately master of the mint, with a salary of some £ , a year. i believe he made rather a good master, and turned out excellent coins: certainly he devoted his attention to his work there in a most exemplary manner. but what a pitiful business it all is! here is a man sent by heaven to do certain things which no man else could do, and so long as he is comparatively unknown he does them; but so soon as he is found out, he is clapped into a routine office with a big salary: and there is, comparatively speaking, an end of him. it is not to be supposed that he had lost his power, for he frequently solved problems very quickly which had been given out by great continental mathematicians as a challenge to the world. we may ask why newton allowed himself to be thus bandied about instead of settling himself down to the work in which he was so pre-eminently great. well, i expect your truly great man never realizes how great he is, and seldom knows where his real strength lies. certainly newton did not know it. he several times talks of giving up philosophy altogether; and though he never really does it, and perhaps the feeling is one only born of some temporary overwork, yet he does not sacrifice everything else to it as he surely must had he been conscious of his own greatness. no; self-consciousness was the last thing that affected him. it is for a great man's contemporaries to discover him, to make much of him, and to put him in surroundings where he may flourish luxuriantly in his own heaven-intended way. however, it is difficult for us to judge of these things. perhaps if he had been maintained at the national expense to do that for which he was preternaturally fitted, he might have worn himself out prematurely; whereas by giving him routine work the scientific world got the benefit of his matured wisdom and experience. it was no small matter to the young royal society to be able to have him as their president for twenty-four years. his portrait has hung over the president's chair ever since, and there i suppose it will continue to hang until the royal society becomes extinct. the events of his later life i shall pass over lightly. he lived a calm, benevolent life, universally respected and beloved. his silver-white hair when he removed his peruke was a venerable spectacle. a lock of it is still preserved, with many other relics, in the library of trinity college. he died quietly, after a painful illness, at the ripe age of eighty-five. his body lay in state in the jerusalem chamber, and he was buried in westminster abbey, six peers bearing the pall. these things are to be mentioned to the credit of the time and the country; for after we have seen the calamitous spectacle of the way tycho and kepler and galileo were treated by their ungrateful and unworthy countries, it is pleasant to reflect that england, with all its mistakes, yet recognized _her_ great man when she received him, and honoured him with the best she knew how to give. [illustration: fig. .--sir isaac newton.] concerning his character, one need only say that it was what one would expect and wish. it was characterized by a modest, calm, dignified simplicity. he lived frugally with his niece and her husband, mr. conduit, who succeeded him as master of the mint. he never married, nor apparently did he ever think of so doing. the idea, perhaps, did not naturally occur to him, any more than the idea of publishing his work did. he was always a deeply religious man and a sincere christian, though somewhat of the arian or unitarian persuasion--so, at least, it is asserted by orthodox divines who understand these matters. he studied theology more or less all his life, and towards the end was greatly interested in questions of biblical criticism and chronology. by some ancient eclipse or other he altered the recognized system of dates a few hundred years; and his book on the prophecies of daniel and the revelation of st. john, wherein he identifies the beast with the church of rome in quite the orthodox way, is still by some admired. but in all these matters it is probable that he was a merely ordinary man, with natural acumen and ability doubtless, but nothing in the least superhuman. in science, the impression he makes upon me is only expressible by the words inspired, superhuman. and yet if one realizes his method of work, and the calm, uninterrupted flow of all his earlier life, perhaps his achievements become more intelligible. when asked how he made his discoveries, he replied: "by always thinking unto them. i keep the subject constantly before me, and wait till the first dawnings open slowly by little and little into a full and clear light." that is the way--quiet, steady, continuous thinking, uninterrupted and unharassed brooding. much may be done under those conditions. much ought to be sacrificed to obtain those conditions. all the best thinking work of the world has been thus done.[ ] buffon said: "genius is patience." so says newton: "if i have done the public any service this way, it is due to nothing but industry and patient thought." genius patience? no, it is not quite that, or, rather, it is much more than that; but genius without patience is like fire without fuel--it will soon burn itself out. notes for lecture ix the _principia_ published . newton died . the law of gravitation.--every particle of matter attracts every other particle of matter with a force proportional to the mass of each and to the inverse square of the distance between them. some of newton's deductions. . kepler's second law (equable description of areas) proves that each planet is acted on by a force directed towards the sun as a centre of force. . kepler's first law proves that this central force diminishes in the same proportion as the square of the distance increases. . kepler's third law proves that all the planets are acted on by the same kind of force; of an intensity depending on the mass of the sun.[ ] . so by knowing the length of year and distance of any planet from the sun, the sun's mass can be calculated, in terms of that of the earth. . for the satellites, the force acting depends on the mass of _their_ central body, a planet. hence the mass of any planet possessing a satellite becomes known. . the force constraining the moon in her orbit is the same gravity as gives terrestrial bodies their weight and regulates the motion of projectiles. [because, while a stone drops feet in a second, the moon, which is times as far from the centre of the earth, drops feet in a minute.] * * * * * . the moon is attracted not only by the earth, but by the sun also; hence its orbit is perturbed, and newton calculated out the chief of these perturbations, viz.:-- (the equation of the centre, discovered by hipparchus.) (_a_) the evection, discovered by hipparchus and ptolemy. (_b_) the variation, discovered by tycho brahé. (_c_) the annual equation, discovered by tycho brahé. (_d_) the retrogression of the nodes, then being observed at greenwich by flamsteed. (_e_) the variation of inclination, then being observed at greenwich by flamsteed. (_f_) the progression of the apses (with an error of one-half). (_g_) the inequality of apogee, previously unknown. (_h_) the inequality of nodes, previously unknown. . each planet is attracted not only by the sun but by the other planets, hence their orbits are slightly affected by each other. newton began the theory of planetary perturbations. . he recognized the comets as members of the solar system, obedient to the same law of gravity and moving in very elongated ellipses; so their return could be predicted (_e.g._ halley's comet). . applying the idea of centrifugal force to the earth considered as a rotating body, he perceived that it could not be a true sphere, and calculated its oblateness, obtaining miles greater equatorial than polar diameter. . conversely, from the observed shape of jupiter, or any planet, the length of its day could be estimated. . the so-calculated shape of the earth, in combination with centrifugal force, causes the weight of bodies to vary with latitude; and newton calculated the amount of this variation. lbs. at pole balance lbs. at equator. . a homogeneous sphere attracts as if its mass were concentrated at its centre. for any other figure, such as an oblate spheroid, this is not exactly true. a hollow concentric spherical shell exerts no force on small bodies inside it. . the earth's equatorial protuberance, being acted on by the attraction of the sun and moon, must disturb its axis of rotation in a calculated manner; and thus is produced the precession of the equinoxes. [the attraction of the planets on the same protuberance causes a smaller and rather different kind of precession.] . the waters of the ocean are attracted towards the sun and moon on one side, and whirled a little further away than the solid earth on the other side: hence newton explained all the main phenomena of the tides. . the sun's mass being known, he calculated the height of the solar tide. . from the observed heights of spring and neap tides he determined the lunar tide, and thence made an estimate of the mass of the moon. reference table of numerical data. +---------+---------------+----------------------+-----------------+ | |masses in solar| height dropped by a | length of day or| | | system. |stone in first second.|time of rotation.| +---------+---------------+----------------------+-----------------+ |mercury | · | · feet | hours | |venus | · | · " | - / " | |earth | · | · " | " | |mars | · | · " | - / " | |jupiter | · | · " | " | |saturn | · | · " | - / " | |the sun | · | · " | " | |the moon | about · | · " | " | +---------+---------------+----------------------+-----------------+ the mass of the earth, taken above as unity, is , trillion tons. _observatories._--uraniburg flourished from to ; the observatory of paris was founded in ; greenwich observatory in . _astronomers-royal._--flamsteed, halley, bradley, bliss, maskelyne, pond, airy, christie. lecture ix newton's "principia" the law of gravitation, above enunciated, in conjunction with the laws of motion rehearsed at the end of the preliminary notes of lecture vii., now supersedes the laws of kepler and includes them as special cases. the more comprehensive law enables us to criticize kepler's laws from a higher standpoint, to see how far they are exact and how far they are only approximations. they are, in fact, not precisely accurate, but the reason for every discrepancy now becomes abundantly clear, and can be worked out by the theory of gravitation. we may treat kepler's laws either as immediate consequences of the law of gravitation, or as the known facts upon which that law was founded. historically, the latter is the more natural plan, and it is thus that they are treated in the first three statements of the above notes; but each proposition may be worked inversely, and we might state them thus:-- . the fact that the force acting on each planet is directed to the sun, necessitates the equable description of areas. . the fact that the force varies as the inverse square of the distance, necessitates motion in an ellipse, or some other conic section, with the sun in one focus. . the fact that one attracting body acts on all the planets with an inverse square law, causes the cubes of their mean distances to be proportional to the squares of their periodic times. not only these but a multitude of other deductions follow rigorously from the simple datum that every particle of matter attracts every other particle with a force directly proportional to the mass of each and to the inverse square of their mutual distance. those dealt with in the _principia_ are summarized above, and it will be convenient to run over them in order, with the object of giving some idea of the general meaning of each, without attempting anything too intricate to be readily intelligible. [illustration: fig. .] no. . kepler's second law (equable description of areas) proves that each planet is acted on by a force directed towards the sun as a centre of force. the equable description of areas about a centre of force has already been fully, though briefly, established. (p. .) it is undoubtedly of fundamental importance, and is the earliest instance of the serious discussion of central forces, _i.e._ of forces directed always to a fixed centre. we may put it afresh thus:--oa has been the motion of a particle in a unit of time; at a it receives a knock towards c, whereby in the next unit it travels along ad instead of ab. now the area of the triangle cad, swept out by the radius vector in unit time, is / _bh_; _h_ being the perpendicular height of the triangle from the base ac. (fig. .) now the blow at a, being along the base, has no effect upon _h_; and consequently the area remains just what it would have been without the blow. a blow directed to any point other than c would at once alter the area of the triangle. one interesting deduction may at once be drawn. if gravity were a radiant force emitted from the sun with a velocity like that of light, the moving planet would encounter it at a certain apparent angle (aberration), and the force experienced would come from a point a little in advance of the sun. the rate of description of areas would thus tend to increase; whereas in reality it is constant. hence the force of gravity, if it travel at all, does so with a speed far greater than that of light. it appears to be practically instantaneous. (cf. "modern views of electricity," § , end of chap. xii.) again, anything like a retarding effect of the medium through which the planets move would constitute a tangential force, entirely un-directed towards the sun. hence no such frictional or retarding force can appreciably exist. it is, however, conceivable that both these effects might occur and just neutralize each other. the neutralization is unlikely to be exact for all the planets; and the fact is, that no trace of either effect has as yet been discovered. (see also p. .) the planets are, however, subject to forces not directed towards the sun, viz. their attractions for each other; and these perturbing forces do produce a slight discrepancy from kepler's second law, but a discrepancy which is completely subject to calculation. no. . kepler's first law proves that this central force diminishes in the same proportion as the square of the distance increases. to prove the connection between the inverse-square law of distance, and the travelling in a conic section with the centre of force in one focus (the other focus being empty), is not so simple. it obviously involves some geometry, and must therefore be left to properly armed students. but it may be useful to state that the inverse-square law of distance, although the simplest possible law for force emanating from a point or sphere, is not to be regarded as self-evident or as needing no demonstration. the force of a magnetic pole on a magnetized steel scrap, for instance, varies as the inverse cube of the distance; and the curve described by such a particle would be quite different from a conic section--it would be a definite class of spiral (called cotes's spiral). again, on an iron filing the force of a single pole might vary more nearly as the inverse fifth power; and so on. even when the thing concerned is radiant in straight lines, like light, the law of inverse squares is not universally true. its truth assumes, first, that the source is a point or sphere; next, that there is no reflection or refraction of any kind; and lastly, that the medium is perfectly transparent. the law of inverse squares by no means holds from a prairie fire for instance, or from a lighthouse, or from a street lamp in a fog. mutual perturbations, especially the pull of jupiter, prevent the path of a planet from being really and truly an ellipse, or indeed from being any simple re-entrant curve. moreover, when a planet possesses a satellite, it is not the centre of the planet which ever attempts to describe the keplerian ellipse, but it is the common centre of gravity of the two bodies. thus, in the case of the earth and moon, the point which really does describe a close attempt at an ellipse is a point displaced about miles from the centre of the earth towards the moon, and is therefore only miles beneath the surface. no. . kepler's third law proves that all the planets are acted on by the same kind of force; of an intensity depending on the mass of the sun. the third law of kepler, although it requires geometry to state and establish it for elliptic motion (for which it holds just as well as it does for circular motion), is very easy to establish for circular motion, by any one who knows about centrifugal force. if _m_ is the mass of a planet, _v_ its velocity, _r_ the radius of its orbit, and _t_ the time of describing it; [pi]_r_ = _vt_, and the centripetal force needed to hold it in its orbit is mv^ [pi]^ _mr_ -------- or ----------- _r_ t^ now the force of gravitative attraction between the planet and the sun is _vms_ -----, r^ where _v_ is a fixed quantity called the gravitation-constant, to be determined if possible by experiment once for all. now, expressing the fact that the force of gravitation _is_ the force holding the planet in, we write, [pi]^ _mr_ _vms_ ----------- = ---------, t^ r^ whence, by the simplest algebra, r^ _vs_ ------ = ---------. t^ [pi]^ the mass of the planet has been cancelled out; the mass of the sun remains, multiplied by the gravitation-constant, and is seen to be proportional to the cube of the distance divided by the square of the periodic time: a ratio, which is therefore the same for all planets controlled by the sun. hence, knowing _r_ and _t_ for any single planet, the value of _vs_ is known. no. . so by knowing the length of year and distance of any planet from the sun, the sun's mass can be calculated, in terms of that of the earth. no. . for the satellites, the force acting depends on the mass of _their_ central body, a planet. hence the mass of any planet possessing a satellite becomes known. the same argument holds for any other system controlled by a central body--for instance, for the satellites of jupiter; only instead of _s_ it will be natural to write _j_, as meaning the mass of jupiter. hence, knowing _r_ and _t_ for any one satellite of jupiter, the value of _vj_ is known. apply the argument also to the case of moon and earth. knowing the distance and time of revolution of our moon, the value of _ve_ is at once determined; _e_ being the mass of the earth. hence, _s_ and _j_, and in fact the mass of any central body possessing a visible satellite, are now known in terms of _e_, the mass of the earth (or, what is practically the same thing, in terms of _v_, the gravitation-constant). observe that so far none of these quantities are known absolutely. their relative values are known, and are tabulated at the end of the notes above, but the finding of their absolute values is another matter, which we must defer. but, it may be asked, if kepler's third law only gives us the mass of a _central_ body, how is the mass of a _satellite_ to be known? well, it is not easy; the mass of no satellite is known with much accuracy. their mutual perturbations give us some data in the case of the satellites of jupiter; but to our own moon this method is of course inapplicable. our moon perturbs at first sight nothing, and accordingly its mass is not even yet known with exactness. the mass of comets, again, is quite unknown. all that we can be sure of is that they are smaller than a certain limit, else they would perturb the planets they pass near. nothing of this sort has ever been detected. they are themselves perturbed plentifully, but they perturb nothing; hence we learn that their mass is small. the mass of a comet may, indeed, be a few million or even billion tons; but that is quite small in astronomy. but now it may be asked, surely the moon perturbs the earth, swinging it round their common centre of gravity, and really describing its own orbit about this point instead of about the earth's centre? yes, that is so; and a more precise consideration of kepler's third law enables us to make a fair approximation to the position of this common centre of gravity, and thus practically to "weigh the moon," i.e. to compare its mass with that of the earth; for their masses will be inversely as their respective distances from the common centre of gravity or balancing point--on the simple steel-yard principle. hitherto we have not troubled ourselves about the precise point about which the revolution occurs, but kepler's third law is not precisely accurate unless it is attended to. the bigger the revolving body the greater is the discrepancy: and we see in the table preceding lecture iii., on page , that jupiter exhibits an error which, though very slight, is greater than that of any of the other planets, when the sun is considered the fixed centre. let the common centre of gravity of earth and moon be displaced a distance _x_ from the centre of the earth, then the moon's distance from the real centre of revolution is not _r_, but _r-x_; and the equation of centrifugal force to gravitative-attraction is strictly [pi]^ _ve_ --------- (_r-x_) = ------, t^ r^ instead of what is in the text above; and this gives a slightly modified "third law." from this equation, if we have any distinct method of determining _ve_ (and the next section gives such a method), we can calculate _x_ and thus roughly weigh the moon, since _r-x_ e ----- = -----, _r_ e+m but to get anything like a reasonable result the data must be very precise. no. . the force constraining the moon in her orbit is the same gravity as gives terrestrial bodies their weight and regulates the motion of projectiles. here we come to the newtonian verification already several times mentioned; but because of its importance i will repeat it in other words. the hypothesis to be verified is that the force acting on the moon is the same kind of force as acts on bodies we can handle and weigh, and which gives them their weight. now the weight of a mass _m_ is commonly written _mg_, where _g_ is the intensity of terrestrial gravity, a thing easily measured; being, indeed, numerically equal to twice the distance a stone drops in the first second of free fall. [see table p. .] hence, expressing that the weight of a body is due to gravity, and remembering that the centre of the earth's attraction is distant from us by one earth's radius (r), we can write _vm_e _mg_ = ------, r^ or _v_e = gr^ = , cubic miles-per-second per second. but we already know _v_e, in terms of the moon's motion, as [pi]^ r^ ----------- t^ approximately, [more accurately, see preceding note, this quantity is _v_(e + m)]; hence we can easily see if the two determinations of this quantity agree.[ ] all these deductions are fundamental, and may be considered as the foundation of the _principia_. it was these that flashed upon newton during that moment of excitement when he learned the real size of the earth, and discovered his speculations to be true. the next are elaborations and amplifications of the theory, such as in ordinary times are left for subsequent generations of theorists to discover and work out. newton did not work out these remoter consequences of his theory completely by any means: the astronomical and mathematical world has been working them out ever since; but he carried the theory a great way, and here it is that his marvellous power is most conspicuous. it is his treatment of no. , the perturbations of the moon, that perhaps most especially has struck all future mathematicians with amazement. no. , no. , no. , these are the most inspired of the whole. no. . the moon is attracted not only by the earth, but by the sun also; hence its orbit is perturbed, and newton calculated out the chief of these perturbations. now running through the perturbations (p. ) in order:--the first is in parenthesis, because it is mere excentricity. it is not a true perturbation at all, and more properly belongs to kepler. (_a_) the first true perturbation is what ptolemy called "the evection," the principal part of which is a periodic change in the ellipticity or excentricity of the moon's orbit, owing to the pull of the sun. it is a complicated matter, and newton only partially solved it. i shall not attempt to give an account of it. (_b_) the next, "the variation," is a much simpler affair. it is caused by the fact that as the moon revolves round the earth it is half the time nearer to the sun than the earth is, and so gets pulled more than the average, while for the other fortnight it is further from the sun than the earth is, and so gets pulled less. for the week during which it is changing from a decreasing half to a new moon it is moving in the direction of the extra pull, and hence becomes new sooner than would have been expected. all next week it is moving against the same extra pull, and so arrives at quadrature (half moon) somewhat late. for the next fortnight it is in the region of too little pull, the earth gets pulled more than it does; the effect of this is to hurry it up for the third week, so that the full moon occurs a little early, and to retard it for the fourth week, so that the decreasing half moon like the increasing half occurs behind time again. thus each syzygy (as new and full are technically called) is too early; each quadrature is too late; the maximum hurrying and slackening force being felt at the octants, or intermediate ° points. (_c_) the "annual equation" is a fluctuation introduced into the other perturbations by reason of the varying distance of the disturbing body, the sun, at different seasons of the year. its magnitude plainly depends simply on the excentricity of the earth's orbit. both these perturbations, (_b_) and (_c_), newton worked out completely. (_d_) and (_e_) next come the retrogression of the nodes and the variation of the inclination, which at the time were being observed at greenwich by flamsteed, from whom newton frequently, but vainly, begged for data that he might complete their theory while he had his mind upon it. fortunately, halley succeeded flamsteed as astronomer-royal [see list at end of notes above], and then newton would have no difficulty in gaining such information as the national observatory could give. the "inclination" meant is the angle between the plane of the moon's orbit and that of the earth. the plane of the earth's orbit round the sun is called the ecliptic; the plane of the moon's orbit round the earth is inclined to it at a certain angle, which is slowly changing, though in a periodic manner. imagine a curtain ring bisected by a sheet of paper, and tilted to a certain angle; it may be likened to the moon's orbit, cutting the plane of the ecliptic. the two points at which the plane is cut by the ring are called "nodes"; and these nodes are not stationary, but are slowly regressing, _i.e._ travelling in a direction opposite to that of the moon itself. also the angle of tilt is varying slowly, oscillating up and down in the course of centuries. (_f_) the two points in the moon's elliptic orbit where it comes nearest to or farthest from the earth, _i.e._ the points at the extremity of the long axis of the ellipse, are called separately perigee and apogee, or together "the apses." now the pull of the sun causes the whole orbit to slowly revolve in its own plane, and consequently these apses "progress," so that the true path is not quite a closed curve, but a sort of spiral with elliptic loops. but here comes in a striking circumstance. newton states with reference to this perturbation that theory only accounts for - / ° per annum, whereas observation gives °, or just twice as much. this is published in the _principia_ as a fact, without comment. it was for long regarded as a very curious thing, and many great mathematicians afterwards tried to find an error in the working. d'alembert, clairaut, and others attacked the problem, but were led to just the same result. it constituted the great outstanding difficulty in the way of accepting the theory of gravitation. it was suggested that perhaps the inverse square law was only a first approximation; that perhaps a more complete expression, such as a b ---- + -----, r^ r^ must be given for it; and so on. ultimately, clairaut took into account a whole series of neglected terms, and it came out correct; thus verifying the theory. but the strangest part of this tale is to come. for only a few years ago, prof. adams, of cambridge (neptune adams, as he is called), was editing various old papers of newton's, now in the possession of the duke of portland, and he found manuscripts bearing on this very point, and discovered that newton had reworked out the calculations himself, had found the cause of the error, had taken into account the terms hitherto neglected, and so, fifty years before clairaut, had completely, though not publicly, solved this long outstanding problem of the progression of the apses. (_g_) and (_h_) two other inequalities he calculated out and predicted, viz. variation in the motions of the apses and the nodes. neither of these had then been observed, but they were afterwards detected and verified. a good many other minor irregularities are now known--some thirty, i believe; and altogether the lunar theory, or problem of the moon's exact motion, is one of the most complicated and difficult in astronomy; the perturbations being so numerous and large, because of the enormous mass of the perturbing body. the disturbances experienced by the planets are much smaller, because they are controlled by the sun and perturbed by each other. the moon is controlled only by the earth, and perturbed by the sun. planetary perturbations can be treated as a series of disturbances with some satisfaction: not so those of the moon. and yet it is the only way at present known of dealing with the lunar theory. to deal with it satisfactorily would demand the solution of such a problem as this:--given three rigid spherical masses thrown into empty space with any initial motions whatever, and abandoned to gravity: to determine their subsequent motions. with two masses the problem is simple enough, being pretty well summed up in kepler's laws; but with three masses, strange to say, it is so complicated as to be beyond the reach of even modern mathematics. it is a famous problem, known as that of "the three bodies," but it has not yet been solved. even when it is solved it will be only a close approximation to the case of earth, moon, and sun, for these bodies are not spherical, and are not rigid. one may imagine how absurdly and hopelessly complicated a complete treatment of the motions of the entire solar system would be. no. . each planet is attracted not only by the sun but by the other planets, hence their orbits are slightly affected by each other. the subject of planetary perturbation was only just begun by newton. gradually (by laplace and others) the theory became highly developed; and, as everybody knows, in neptune was discovered by means of it. no. . he recognized the comets as members of the solar system, obedient to the same law of gravity and moving in very elongated ellipses; so their return could be predicted. it was a long time before newton recognized the comets as real members of the solar system, and subject to gravity like the rest. he at first thought they moved in straight lines. it was only in the second edition of the _principia_ that the theory of comets was introduced. halley observed a fine comet in , and calculated its orbit on newtonian principles. he also calculated when it ought to have been seen in past times; and he found the year , when one was seen by kepler; also the year , when one was seen by appian; again, he reckoned , , . all these appearances were the same comet, in all probability, returning every seventy-five or seventy-six years. the period was easily allowed to be not exact, because of perturbing planets. he then predicted its return for , or perhaps , a date he could not himself hope to see. he lived to a great age, but he died sixteen years before this date. as the time drew nigh, three-quarters of a century afterwards, astronomers were greatly interested in this first cometary prediction, and kept an eager look-out for "halley's comet." clairaut, a most eminent mathematician and student of newton, proceeded to calculate out more exactly the perturbing influence of jupiter, near which it had passed. after immense labour (for the difficulty of the calculation was extreme, and the mass of mere figures something portentous), he predicted its return on the th of april, , but he considered that he might have made a possible error of a month. it returned on the th of march, , and established beyond all doubt the rule of the newtonian theory over comets. [illustration: fig. .--well-known model exhibiting the oblate spheroidal form as a consequence of spinning about a central axis. the brass strip _a_ looks like a transparent globe when whirled, and bulges out equatorially.] no. . applying the idea of centrifugal force to the earth considered as a rotating body, he perceived that it could not be a true sphere, and calculated its oblateness, obtaining miles greater equatorial than polar diameter. here we return to one of the more simple deductions. a spinning body of any kind tends to swell at its circumference (or equator), and shrink along its axis (or poles). if the body is of yielding material, its shape must alter under the influence of centrifugal force; and if a globe of yielding substance subject to known forces rotates at a definite pace, its shape can be calculated. thus a plastic sphere the size of the earth, held together by its own gravity, and rotating once a day, can be shown to have its equatorial diameter twenty-eight miles greater than its polar diameter: the two diameters being , and , respectively. now we have no guarantee that the earth is of yielding material: for all newton could tell it might be extremely rigid. as a matter of fact it is now very nearly rigid. but he argued thus. the water on it is certainly yielding, and although the solid earth might decline to bulge at the equator in deference to the diurnal rotation, that would not prevent the ocean from flowing from the poles to the equator and piling itself up as an equatorial ocean fourteen miles deep, leaving dry land everywhere near either pole. nothing of this sort is observed: the distribution of land and water is not thus regulated. hence, whatever the earth may be now, it must once have been plastic enough to accommodate itself perfectly to the centrifugal forces, and to take the shape appropriate to a perfectly plastic body. in all probability it was once molten, and for long afterwards pasty. thus, then, the shape of the earth can be calculated from the length of its day and the intensity of its gravity. the calculation is not difficult: it consists in imagining a couple of holes bored to the centre of the earth, one from a pole and one from the equator; filling these both with water, and calculating how much higher the water will stand in one leg of the gigantic v tube so formed than in the other. the answer comes out about fourteen miles. the shape of the earth can now be observed geodetically, and it accords with calculation, but the observations are extremely delicate; in newton's time the _size_ was only barely known, the _shape_ was not observed till long after; but on the principles of mechanics, combined with a little common-sense reasoning, it could be calculated with certainty and accuracy. no. . from the observed shape of jupiter or any planet the length of its day could be estimated. jupiter is much more oblate than the earth. its two diameters are to one another as is to ; the ellipticity of its disk is manifest to simple inspection. hence we perceive that its whirling action must be more violent--it must rotate quicker. as a matter of fact its day is ten [illustration: fig. .--jupiter.] hours long--five hours daylight and five hours night. the times of rotation of other bodies in the solar system are recorded in a table above. no. . the so-calculated shape of the earth, in combination with centrifugal force, causes the weight of bodies to vary with latitude; and newton calculated the amount of this variation. lbs. at pole balance lbs. at equator. but following from the calculated shape of the earth follow several interesting consequences. first of all, the intensity of gravity will not be the same everywhere; for at the equator a stone is further from the average bulk of the earth (say the centre) than it is at the poles, and owing to this fact a mass of pounds at the pole; would suffice to balance pounds at the equator, if the two could be placed in the pans of a gigantic balance whose beam straddled along an earth's quadrant. this is a _true_ variation of gravity due to the shape of the earth. but besides this there is a still larger _apparent_ variation due to centrifugal force, which affects all bodies at the equator but not those at the poles. from this cause, even if the earth were a true sphere, yet if it were spinning at its actual pace, pounds at the pole could balance pounds at the equator; because at the equator the true weight of the mass would not be fully appreciated, centrifugal force would virtually diminish it by / th of its amount. in actual fact both causes co-exist, and accordingly the total variation of gravity observed is compounded of the real and the apparent effects; the result is that pounds at a pole weighs as much as pounds at the equator. no. . a homogeneous sphere attracts as if its mass were concentrated at its centre. for any other figure, such as an oblate spheroid, this is not exactly true. a hollow concentric spherical shell exerts no force on small bodies inside it. a sphere composed of uniform material, or of materials arranged in concentric strata, can be shown to attract external bodies as if its mass were concentrated at its centre. a hollow sphere, similarly composed, does the same, but on internal bodies it exerts no force at all. hence, at all distances above the surface of the earth, gravity decreases in inverse proportion as the square of the distance from the centre of the earth increases; but, if you descend a mine, gravity decreases in this case also as you leave the surface, though not at the same rate as when you went up. for as you penetrate the crust you get inside a concentric shell, which is thus powerless to act upon you, and the earth you are now outside is a smaller one. at what rate the force decreases depends on the distribution of density; if the density were uniform all through, the law of variation would be the direct distance, otherwise it would be more complicated. anyhow, the intensity of gravity is a maximum at the surface of the earth, and decreases as you travel from the surface either up or down. no. . the earth's equatorial protuberance, being acted on by the attraction of the sun and moon, must disturb its axis of rotation in a calculated manner; and thus is produced the precession of the equinoxes. here we come to a truly awful piece of reasoning. a sphere attracts as if its mass were concentrated at its centre (no. ), but a spheroid does not. the earth is a spheroid, and hence it pulls and is pulled by the moon with a slightly uncentric attraction. in other words, the line of pull does not pass through its precise centre. now when we have a spinning body, say a top, overloaded on one side so that gravity acts on it unsymmetrically, what happens? the axis of rotation begins to rotate cone-wise, at a pace which depends on the rate of spin, and on the shape and mass of the top, as well as on the amount and leverage of the overloading. newton calculated out the rapidity of this conical motion of the axis of the earth, produced by the slightly unsymmetrical pull of the moon, and found that it would complete a revolution in , years--precisely what was wanted to explain the precession of the equinoxes. in fact he had discovered the physical cause of that precession. observe that there were three stages in this discovery of precession:-- first, the observation by hipparchus, that the nodes, or intersections of the earth's orbit (the sun's apparent orbit) with the plane of the equator, were not stationary, but slowly moved. second, the description of this motion by copernicus, by the statement that it was due to a conical motion of the earth's axis of rotation about its centre as a fixed point. third, the explanation of this motion by newton as due to the pull of the moon on the equatorial protuberance of the earth. the explanation _could_ not have been previously suspected, for the shape of the earth, on which the whole theory depends, was entirely unknown till newton calculated it. another and smaller motion of a somewhat similar kind has been worked out since: it is due to the unsymmetrical attraction of the other planets for this same equatorial protuberance. it shows itself as a periodic change in the obliquity of the ecliptic, or so-called recession of the apses, rather than as a motion of the nodes.[ ] no. . the waters of the ocean are attracted towards the sun and moon on one side, and whirled a little farther away than the solid earth on the other side: hence newton explained all the main phenomena of the tides. and now comes another tremendous generalization. the tides had long been an utter mystery. kepler likens the earth to an animal, and the tides to his breathings and inbreathings, and says they follow the moon. galileo chaffs him for this, and says that it is mere superstition to connect the moon with the tides. descartes said the moon pressed down upon the waters by the centrifugal force of its vortex, and so produced a low tide under it. everything was fog and darkness on the subject. the legend goes that an astronomer threw himself into the sea in despair of ever being able to explain the flux and reflux of its waters. newton now with consummate skill applied his theory to the effect of the moon upon the ocean, and all the main details of tidal action gradually revealed themselves to him. he treated the water, rotating with the earth once a day, somewhat as if it were a satellite acted on by perturbing forces. the moon as it revolves round the earth is perturbed by the sun. the ocean as it revolves round the earth (being held on by gravitation just as the moon is) is perturbed by both sun and moon. the perturbing effect of a body varies directly as its mass, and inversely as the cube of its distance. (the simple law of inverse square does not apply, because a perturbation is a differential effect: the satellite or ocean when nearer to the perturbing body than the rest of the earth, is attracted more, and when further off it is attracted less than is the main body of the earth; and it is these differences alone which constitute the perturbation.) the moon is the more powerful of the two perturbing bodies, hence the main tides are due to the moon; and its chief action is to cause a pair of low waves or oceanic humps, of gigantic area, to travel round the earth once in a lunar day, _i.e._ in about hours and minutes. the sun makes a similar but still lower pair of low elevations to travel round once in a solar day of hours. and the combination of the two pairs of humps, thus periodically overtaking each other, accounts for the well-known spring and neap tides,--spring tides when their maxima agree, neap tides when the maximum of one coincides with the minimum of the other: each of which events happens regularly once a fortnight. these are the main effects, but besides these there are the effects of varying distances and obliquity to be taken into account; and so we have a whole series of minor disturbances, very like those discussed in no. , under the lunar theory, but more complex still, because there are two perturbing bodies instead of only one. the subject of the tides is, therefore, very recondite; and though one may give some elementary account of its main features, it will be best to defer this to a separate lecture (lecture xvii). i had better, however, here say that newton did not limit himself to the consideration of the primary oceanic humps: he pursued the subject into geographical detail. he pointed out that, although the rise and fall of the tide at mid-ocean islands would be but small, yet on stretches of coast the wave would fling itself, and by its momentum would propel the waters, to a much greater height--for instance, or feet; especially in some funnel-shaped openings like the bristol channel and the bay of fundy, where the concentrated impetus of the water is enormous. he also showed how the tidal waves reached different stations in successive regular order each day; and how some places might be fed with tide by two distinct channels; and that if the time of these channels happened to differ by six hours, a high tide might be arriving by one channel and a low tide by the other, so that the place would only feel the difference, and so have a very small observed rise and fall; instancing a port in china (in the gulf of tonquin) where that approximately occurs. in fact, although his theory was not, as we now know, complete or final, yet it satisfactorily explained a mass of intricate detail as well as the main features of the tides. no. . the sun's mass being known, he calculated the height of the solar tide. no. . from the observed heights of spring and neap tides he determined the lunar tide, and thence made an estimate of the mass of the moon. knowing the sun's mass and distance, it was not difficult for newton to calculate the height of the protuberance caused by it in a pasty ocean covering the whole earth. i say pasty, because, if there was any tendency for impulses to accumulate, as timely pushes given to a pendulum accumulate, the amount of disturbance might become excessive, and its calculation would involve a multitude of data. the newtonian tide ignored this, thus practically treating the motion as either dead-beat, or else the impulses as very inadequately timed. with this reservation the mid-ocean tide due to the action of the sun alone comes out about one foot, or let us say one foot for simplicity. now the actual tide observed in mid-atlantic is at the springs about four feet, at the neaps about two. the spring tide is lunar plus solar; the neap tide is lunar minus solar. hence it appears that the tide caused by the moon alone must be about three feet, when unaffected by momentum. from this datum newton made the first attempt to approximately estimate the mass of the moon. i said that the masses of satellites must be estimated, if at all, by the perturbation they are able to cause. the lunar tide is a perturbation in the diurnal motion of the sea, and its amount is therefore a legitimate mode of calculating the moon's mass. the available data were not at all good, however; nor are they even now very perfect; and so the estimate was a good way out. it is now considered that the mass of the moon is about one-eightieth that of the earth. * * * * * such are some of the gems extracted from their setting in the _principia_, and presented as clearly as i am able before you. do you realize the tremendous stride in knowledge--not a stride, as whewell says, nor yet a leap, but a flight--which has occurred between the dim gropings of kepler, the elementary truths of galileo, the fascinating but wild speculations of descartes, and this magnificent and comprehensive system of ordered knowledge. to some his genius seemed almost divine. "does mr. newton eat, drink, sleep, like other men?" said the marquis de l'hôpital, a french mathematician of no mean eminence; "i picture him to myself as a celestial genius, entirely removed from the restrictions of ordinary matter." to many it seemed as if there was nothing more to be discovered, as if the universe were now explored, and only a few fragments of truth remained for the gleaner. this is the attitude of mind expressed in pope's famous epigram:-- "nature and nature's laws lay hid in night, god said, let newton be, and all was light." this feeling of hopelessness and impotence was very natural after the advent of so overpowering a genius, and it prevailed in england for fully a century. it was very natural, but it was very mischievous; for, as a consequence, nothing of great moment was done by england in science, and no englishman of the first magnitude appeared, till some who are either living now or who have lived within the present century. it appeared to his contemporaries as if he had almost exhausted the possibility of discovery; but did it so appear to newton? did it seem to him as if he had seen far and deep into the truths of this great and infinite universe? it did not. when quite an old man, full of honour and renown, venerated, almost worshipped, by his contemporaries, these were his words:-- "i know not what the world will think of my labours, but to myself it seems that i have been but as a child playing on the sea-shore; now finding some pebble rather more polished, and now some shell rather more agreeably variegated than another, while the immense ocean of truth extended itself unexplored before me." and so it must ever seem to the wisest and greatest of men when brought into contact with the great things of god--that which they know is as nothing, and less than nothing, to the infinitude of which they are ignorant. newton's words sound like a simple and pleasing echo of the words of that great unknown poet, the writer of the book of job:-- "lo, these are parts of his ways, but how little a portion is heard of him; the thunder of his power, who can understand?" end of part i. part ii _a couple of centuries' progress._ notes to lecture x _science during the century after newton_ the _principia_ published, roemer - james bradley - clairaut - euler - d'alembert - lagrange - laplace - william herschel - _olaus roemer_ was born in jutland, and studied at copenhagen. assisted picard in to determine the exact position of tycho's observatory on huen. accompanied picard to paris, and in read before the academy his paper "on successive propagation of light as revealed by a certain inequality in the motion of jupiter's first satellite." in he returned to copenhagen as professor of mathematics and astronomy, and died in . he invented the transit instrument, mural circle, equatorial mounting for telescopes, and most of the other principal instruments now in use in observatories. he made as many observations as tycho brahé, but the records of all but the work of three days were destroyed by a great fire in . _bradley_, professor of astronomy at oxford, discovered the aberration of light in , while examining stars for parallax, and the nutation of the earth's axis in . was appointed astronomer-royal in . lecture x roemer and bradley and the velocity of light at newton's death england stood pre-eminent among the nations of europe in the sphere of science. but the pre-eminence did not last long. two great discoveries were made very soon after his decease, both by professor bradley, of oxford, and then there came a gap. a moderately great man often leaves behind him a school of disciples able to work according to their master's methods, and with a healthy spirit of rivalry which stimulates and encourages them. newton left, indeed, a school of disciples, but his methods of work were largely unknown to them, and such as were known were too ponderous to be used by ordinary men. only one fresh result, and that a small one, has ever been attained by other men working according to the methods of the _principia_. the methods were studied and commented on in england to the exclusion of all others for nigh a century, and as a consequence no really important work was done. on the continent, however, no such system of slavish imitation prevailed. those methods of newton's which had been simultaneously discovered by leibnitz were more thoroughly grasped, modified, extended, and improved. there arose a great school of french and german mathematicians, and the laurels of scientific discovery passed to france and germany--more especially, perhaps, at this time to france. england has never wholly recovered them. during the present century this country has been favoured with some giants who, as they become distant enough for their true magnitude to be perceived, may possibly stand out as great as any who have ever lived; but for the mass and bulk of scientific work at the present day we have to look to germany, with its enlightened government and extensive intellectual development. england, however, is waking up, and what its government does not do, private enterprise is beginning to accomplish. the establishment of centres of scientific and literary activity in the great towns of england, though at present they are partially encumbered with the supply of education of an exceedingly rudimentary type, is a movement that in the course of another century or so will be seen to be one of the most important and fruitful steps ever taken by this country. on the continent such centres have long existed; almost every large town is the seat of a university, and they are now liberally endowed. the university of bologna (where, you may remember, copernicus learnt mathematics) has recently celebrated its th anniversary. the scientific history of the century after newton, summarized in the above table of dates, embraces the labours of the great mathematicians clairaut, euler, d'alembert, and especially of lagrange and laplace. but the main work of all these men was hardly pioneering work. it was rather the surveying, and mapping out, and bringing into cultivation, of lands already discovered. probably herschel may be justly regarded as the next true pioneer. we shall not, however, properly appreciate the stages through which astronomy has passed, nor shall we be prepared adequately to welcome the discoveries of modern times unless we pay some attention to the intervening age. moreover, during this era several facts of great moment gradually came into recognition; and the importance of the discovery we have now to speak of can hardly be over-estimated. our whole direct knowledge of the planetary and stellar universe, from the early observations of the ancients down to the magnificent discoveries of a herschel, depends entirely upon our happening to possess a sense of sight. to no other of our senses do any other worlds than our own in the slightest degree appeal. we touch them or hear them never. consequently, if the human race had happened to be blind, no other world but the one it groped its way upon could ever have been known or imagined by it. the outside universe would have existed, but man would have been entirely and hopelessly ignorant of it. the bare idea of an outside universe beyond the world would have been inconceivable, and might have been scouted as absurd. we do possess the sense of sight; but is it to be supposed that we possess every sense that can be possessed by finite beings? there is not the least ground for such an assumption. it is easy to imagine a deaf race or a blind race: it is not so easy to imagine a race more highly endowed with senses than our own; and yet the sense of smell in animals may give us some aid in thinking of powers of perception which transcend our own in particular directions. if there were a race with higher or other senses than our own, or if the human race should ever in the process of development acquire such extra sense-organs, a whole universe of existent fact might become for the first time perceived by us, and we should look back upon our past state as upon a blind chrysalid form of existence in which we had been unconscious of all this new wealth of perception. it cannot be too clearly and strongly insisted on and brought home to every mind, that the mode in which the universe strikes us, our view of the universe, our whole idea of matter, and force, and other worlds, and even of consciousness, depends upon the particular set of sense-organs with which we, as men, happen to be endowed. the senses of force, of motion, of sound, of light, of touch, of heat, of taste, and of smell--these we have, and these are the things we primarily know. all else is inference founded upon these sensations. so the world appears to us. but given other sense-organs, and it might appear quite otherwise. what it is actually and truly like, therefore, is quite and for ever beyond us--so long as we are finite beings. without eyes, astronomy would be non-existent. light it is which conveys all the information we possess, or, as it would seem, ever can possess, concerning the outer and greater universe in which this small world forms a speck. light is the channel, the messenger of information; our eyes, aided by telescopes, spectroscopes, and many other "scopes" that may yet be invented, are the means by which we read the information that light brings. light travels from the stars to our eyes: does it come instantaneously? or does it loiter by the way? for if it lingers it is not bringing us information properly up to date--it is only telling us what the state of affairs was when it started on its long journey. now, it is evidently a matter of interest to us whether we see the sun as he is now, or only as he was some three hundred years ago. if the information came by express train it would be three hundred years behind date, and the sun might have gone out in the reign of queen anne without our being as yet any the wiser. the question, therefore, "at what rate does our messenger travel?" is evidently one of great interest for astronomers, and many have been the attempts made to solve it. very likely the ancient greeks pondered over this question, but the earliest writer known to me who seriously discussed the question is galileo. he suggests a rough experimental means of attacking it. first of all, it plainly comes quicker than sound. this can be perceived by merely watching distant hammering, or by noticing that the flash of a pistol is seen before its report is heard, or by listening to the noise of a flash of lightning. sound takes five seconds to travel a mile--it has about the same speed as a rifle bullet; but light is much quicker than that. the rude experiment suggested by galileo was to send two men with lanterns and screens to two distant watch-towers or neighbouring mountain tops, and to arrange that each was to watch alternate displays and obscurations of the light made by the other, and to imitate them as promptly as possible. either man, therefore, on obscuring or showing his own light would see the distant glimmer do the same, and would be able to judge if there was any appreciable interval between his own action and the response of the distant light. the experiment was actually tried by the florentine academicians,[ ] with the result that, as practice improved, the interval became shorter and shorter, so that there was no reason to suppose that there was any real interval at all. light, in fact, seemed to travel instantaneously. well might they have arrived at this result. even if they had made far more perfect arrangements--for instance, by arranging a looking-glass at one of the stations in which a distant observer might see the reflection of his own lantern, and watch the obscurations and flashings made by himself, without having to depend on the response of human mechanism--even then no interval whatever could have been detected. if, by some impossibly perfect optical arrangement, a lighthouse here were made visible to us after reflection in a mirror erected at new york, so that the light would have to travel across the atlantic and back before it could be seen, even then the appearance of the light on removing a shutter, or the eclipse on interposing it, would seem to happen quite instantaneously. there would certainly be an interval: the interval would be the fiftieth part of a second (the time a stone takes to drop / th of an inch), but that is too short to be securely detected without mechanism. with mechanism the thing might be managed, for a series of shutters might be arranged like the teeth of a large wheel; so that, when the wheel rotates, eclipses follow one another very rapidly; if then an eye looked through the same opening as that by which the light goes on its way to the distant mirror, a tooth might have moved sufficiently to cover up this space by the time the light returned; in which case the whole would appear dark, for the light would be stopped by a tooth, either at starting or at returning, continually. at higher speeds of rotation some light would reappear, and at lower speeds it would also reappear; by noticing, therefore, the precise speed at which there was constant eclipse the velocity of light could be determined. [illustration: fig. .--diagram of eye looking at a light reflected in a distant mirror through the teeth of a revolving wheel.] this experiment has now been made in a highly refined form by fizeau, and repeated by m. cornu with prodigious care and accuracy. but with these recent matters we have no concern at present. it may be instructive to say, however, that if the light had to travel two miles altogether, the wheel would have to possess teeth and to spin times a second (at the risk of flying to pieces) in order that the ray starting through any one of the gaps might be stopped on returning by the adjacent tooth. well might the velocity of light be called instantaneous by the early observers. an ordinary experiment seemed (and was) hopeless, and light was supposed to travel at an infinite speed. but a phenomenon was noticed in the heavens by a quick-witted and ingenious danish astronomer, which was not susceptible of any ordinary explanation, and which he perceived could at once be explained if light had a certain rate of travel--great, indeed, but something short of infinite. this phenomenon was connected with the satellites of jupiter, and the astronomer's name was roemer. i will speak first of the observation and then of the man. [illustration: fig. .--fizeau's wheel, shewing the appearance of distant image seen through its teeth. st, when stationary, next when revolving at a moderate speed, last when revolving at the high speed just sufficient to cause eclipse.] jupiter's satellites are visible, precisely as our own moon is, by reason of the shimmer of sunlight which they reflect. but as they revolve round their great planet they plunge into his shadow at one part of their course, and so become eclipsed from sunshine and invisible to us. the moment of disappearance can be sharply observed. take the first satellite as an example. the interval between successive eclipses ought to be its period of revolution round jupiter. observe this period. it was not uniform. on the average it was hours minutes, but it seemed to depend on the time of year. when roemer observed in spring it was less, and in autumn it was more than usual. this was evidently a puzzling fact: what on earth can our year have to do with the motion of a moon of jupiter's? it was probably, therefore, only an apparent change, caused either by our greater or less distance from jupiter, or else by our greater or less speed of travelling to or from him. considering it thus, he was led to see that, when the time of revolution seemed longest, we were receding fastest from jupiter, and when shortest, approaching fastest. _if_, then, light took time on its journey, _if_ it travelled progressively, the whole anomaly would be explained. in a second the earth goes nineteen miles; therefore in - / hours (the time of revolution of jupiter's first satellite) it goes · million (say three million) miles. the eclipse happens punctually, but we do not see it till the light conveying the information has travelled the extra three million miles and caught up the earth. evidently, therefore, by observing how much the apparent time of revolution is lengthened in one part of the earth's orbit and shortened in another, getting all the data accurately, and assuming the truth of our hypothetical explanation, we can calculate the velocity of light. this is what roemer did. now the maximum amount of retardation is just about fifteen seconds. hence light takes this time to travel three million miles; therefore its velocity is three million divided by fifteen, say , , or, as we now know more exactly, , miles every second. note that the delay does not depend on our _distance_, but on our _speed_. one can tell this by common-sense as soon as we grasp the general idea of the explanation. a velocity cannot possibly depend on a distance only. [illustration: fig. .--eclipses of one of jupiter's satellites. a diagram intended to illustrate the dependence of its apparent time of revolution (from eclipse to eclipse) on the motion of the earth; but not illustrating the matter at all well. tt' t'' are successive positions of the earth, while jj' j'' are corresponding positions of jupiter.] roemer's explanation of the anomaly was not accepted by astronomers. it excited some attention, and was discussed, but it was found not obviously applicable to any of the satellites except the first, and not very simply and satisfactorily even to that. i have, of course, given you the theory in its most elementary and simple form. in actual fact a host of disturbing and complicated considerations come in--not so violently disturbing for the first satellite as for the others, because it moves so quickly, but still complicated enough. the fact is, the real motion of jupiter's satellites is a most difficult problem. the motion even of our own moon (the lunar theory) is difficult enough: perturbed as its motion is by the sun. you know that newton said it cost him more labour than all the rest of the _principia_. but the motion of jupiter's satellites is far worse. no one, in fact, has yet worked their theory completely out. they are perturbed by the sun, of course, but they also perturb each other, and jupiter is far from spherical. the shape of jupiter, and their mutual attractions, combine to make their motions most peculiar and distracting. hence an error in the time of revolution of a satellite was not _certainly_ due to the cause roemer suggested, unless one could be sure that the inequality was not a real one, unless it could be shown that the theory of gravitation was insufficient to account for it. this had not then been done; so the half-made discovery was shelved, and properly shelved, as a brilliant but unverified speculation. it remained on the shelf for half a century, and was no doubt almost forgotten. [illustration: fig. .--a transit-instrument for the british astronomical expedition, . shewing in its essential features the simplest form of such an instrument.] now a word or two about the man. he was a dane, educated at copenhagen, and learned in the mathematics. we first hear of him as appointed to assist picard, the eminent french geodetic surveyor (whose admirable work in determining the length of a degree you remember in connection with newton), who had come over to denmark with the object of fixing the exact site of the old and extinct tychonic observatory in the island of huen. for of course the knowledge of the exact latitude and longitude of every place whence numerous observations have been taken must be an essential to the full interpretation of those observations. the measurements being finished, young roemer accompanied picard to paris, and here it was, a few years after, that he read his famous paper concerning "an inequality in the motion of jupiter's first satellite," and its explanation by means of an hypothesis of "the successive propagation of light." the later years of his life he spent in copenhagen as a professor in the university and an enthusiastic observer of the heavens,--not a descriptive observer like herschel, but a measuring observer like sir george airy or tycho brahé. he was, in fact, a worthy follower of tycho, and the main work of his life is the development and devising of new and more accurate astronomical instruments. many of the large and accurate instruments with which a modern observatory is furnished are the invention of this dane. one of the finest observatories in the world is the russian one at pulkowa, and a list of the instruments there reads like an extended catalogue of roemer's inventions. he not only _invented_ the instruments, he had them made, being allowed money for the purpose; and he used them vigorously, so that at his death he left great piles of manuscript stored in the national observatory. unfortunately this observatory was in the heart of the city, and was thus exposed to a danger from which such places ought to be as far as possible exempt. some eighteen years after roemer's death a great conflagration broke out in copenhagen, and ruined large portions of the city. the successor to roemer, horrebow by name, fled from his house, with such valuables as he possessed, to the observatory, and there went on with his work. but before long the wind shifted, and to his horror he saw the flames coming his way. he packed up his own and his predecessor's manuscript observations in two cases, and prepared to escape with them, but the neighbours had resorted to the observatory as a place of safety, and so choked up the staircase with their property that he was barely able to escape himself, let alone the luggage, and everything was lost. [illustration: fig. .--diagram of equatorially mounted telescope; ce is the polar axis parallel to the axis of the earth; ab the declination axis. the diurnal motion is compensated by motion about the polar axis only, the other being clamped.] of all the observations, only three days' work remains, and these were carefully discussed by dr. galle, of berlin, in , and their nutriment extracted. these ancient observations are of great use for purposes of comparison with the present state of the heavens, and throw light upon possible changes that are going on. of course nowadays such a series of observations would be printed and distributed in many libraries, and so made practically indestructible. sad as the disaster was to the posthumous fame of the great observer, a considerable compensation was preparing. the very year that the fire occurred in denmark a quiet philosopher in england was speculating and brooding on a remarkable observation that he had made concerning the apparent motion of certain stars, and he was led thereby to a discovery of the first magnitude concerning the speed of light--a discovery which resuscitated the old theory of roemer about jupiter's satellites, and made both it and him immortal. james bradley lived a quiet, uneventful, studious life, mainly at oxford but afterwards at the national observatory at greenwich, of which he was third astronomer-royal, flamsteed and halley having preceded him in that office. he had taken orders, and lectured at oxford as savilian professor. it is said that he pondered his great discovery while pacing the long walk at magdalen college--and a beautiful place it is to meditate in. bradley was engaged in making observations to determine if possible the parallax of some of the fixed stars. parallax means the apparent relative shift of bodies due to a change in the observer's position. it is parallax which we observe when travelling by rail and looking out of window at the distant landscape. things at different distances are left behind at different apparent rates, and accordingly they seem to move relatively to each other. the most distant objects are least affected; and anything enormously distant, like the moon, is not subject to this effect, but would retain its position however far we travelled, unless we had some extraordinarily precise means of observation. so with the fixed stars: they were being observed from a moving carriage--viz. the earth--and one moving at the rate of nineteen miles a second. unless they were infinitely distant, or unless they were all at the same distance, they must show relative apparent motions among themselves. seen from one point of the earth's orbit, and then in six months from an opposite point, nearly million miles away, surely they must show some difference of aspect. remember that the old copernican difficulty had never been removed. if the earth revolved round the sun, how came it that the fixed stars showed no parallax? the fact still remained a surprise, and the question a challenge. picard, like other astronomers, supposed that it was only because the methods of observation had not been delicate enough; but now that, since the invention of the telescope and the founding of national observatories, accuracy hitherto undreamt of was possible, why not attack the problem anew? this, then, he did, watching the stars with great care to see if in six months they showed any change in absolute position with reference to the pole of the heavens; any known secular motion of the pole, such as precession, being allowed for. already he thought he detected a slight parallax for several stars near the pole, and the subject was exciting much interest. bradley determined to attempt the same investigation. he was not destined to succeed in it. not till the present century was success in that most difficult observation achieved; and even now it cannot be done by the absolute methods then attempted; but, as so often happens, bradley, in attempting one thing, hit upon another, and, as it happened, one of still greater brilliance and importance. let us trace the stages of his discovery. atmospheric refraction made horizon observations useless for the delicacy of his purpose, so he chose stars near the zenith, particularly one--[gamma] draconis. this he observed very carefully at different seasons of the year by means of an instrument specially adapted for zenith observations, viz. a zenith sector. the observations were made in conjunction with a friend of his, an amateur astronomer named molyneux, and they were made at kew. molyneux was shortly made first lord of the admiralty, or something important of that sort, and gave up frivolous pursuits. so bradley observed alone. they observed the star accurately early in the month of december, and then intended to wait six months. but from curiosity bradley observed it again only about a week later. to his surprise, he found that it had already changed its position. he recorded his observation on the back of an old envelope: it was his wont thus to use up odd scraps of paper--he was not, i regret to say, a tidy or methodical person--and this odd piece of paper turned up long afterwards among his manuscripts. it has been photographed and preserved as an historical relic. again and again he repeated the observation of the star, and continually found it moving still a little further and further south, an excessively small motion, but still an appreciable one--not to be set down to errors of observation. so it went on till march. it then waited, and after a bit longer began to return, until june. by september it was displaced as much to the north as it had been to the south, and by december it had got back to its original position. it had described, in fact, a small oscillation in the course of the year. the motion affected neighbouring stars in a similar way, and was called an "aberration," or wandering from their true place. for a long time bradley pondered over this observation, and over others like them which he also made. he found one group of stars describing small circles, while others at a distance from them were oscillating in straight lines, and all the others were describing ellipses. unless this state of things were cleared up, accurate astronomy was impossible. the fixed stars!--they were not fixed a bit. to refined and accurate observation, such as was now possible, they were all careering about in little orbits having a reference to the earth's year, besides any proper motion which they might really have of their own, though no such motion was at present known. not till herschel was that discovered; not till this extraordinary aberration was allowed for could it be discovered. the effect observed by bradley and molyneux must manifestly be only an apparent motion: it was absurd to suppose a real stellar motion regulating itself according to the position of the earth. parallax could not do it, for that would displace stars relatively among each other--it would not move similarly a set of neighbouring stars. at length, four years after the observation, the explanation struck him, while in a boat upon the thames. he noticed the apparent direction of the wind changed whenever the boat started. the wind veered when the boat's motion changed. of course the cause of this was obvious enough--the speed of the wind and the speed of the boat were compounded, and gave an apparent direction of the wind other than the true direction. but this immediately suggested a cause for what he had observed in the heavens. he had been observing an apparent direction of the stars other than the true direction, because he was observing from a moving vehicle. the real direction was doubtless fixed: the apparent direction veered about with the motion of the earth. it must be that light did not travel instantaneously, but gradually, as roemer had surmised fifty years ago; and that the motion of the light was compounded with the motion of the earth. think of a stream of light or anything else falling on a moving carriage. the carriage will run athwart the stream, the occupants of the carriage will mistake its true direction. a rifle fired through the windows of a railway carriage by a man at rest outside would make its perforations not in the true line of fire unless the train is stationary. if the train is moving, the line joining the holes will point to a place in advance of where the rifle is really located. so it is with the two glasses of a telescope, the object-glass and eye-piece, which are pierced by the light; an astronomer, applying his eye to the tube and looking for the origin of the disturbance, sees it apparently, but not in its real position--its apparent direction is displaced in the direction of the telescope's motion; by an amount depending on the ratio of the velocity of the earth to the velocity of light, and on the angle between those two directions. [illustration: fig. .--aberration diagram. the light-ray l penetrates the object-glass of the moving telescope at o, but does not reach the eye-piece until the telescope has travelled to the second position. consequently a moving telescope does not point out the true direction of the light, but aims at a point a little in advance.] but how minute is the displacement! the greatest effect is obtained when the two motions are at right angles to each other, _i.e._ when the star seen is at right angles to the direction of the earth's motion, but even then it is only ", or / th part of a degree; one-ninetieth of the moon's apparent diameter. it could not be detected without a cross-wire in the telescope, and would only appear as a slight displacement from the centre of the field, supposing the telescope accurately pointed to the true direction. but if this explanation be true, it at once gives a method of determining the velocity of light. the maximum angle of deviation, represented as a ratio of arc ÷ radius, amounts to ------------ - · = ------ × - / , (a gradient of foot in two miles). in other words, the velocity of light must be , times as great as the velocity of the earth in its orbit. this amounts to a speed of , miles a second--not so very different from what roemer had reckoned it in order to explain the anomalies of jupiter's first satellite. stars in the direction in which the earth was moving would not be thus affected; there would be nothing in mere approach or recession to alter direction or to make itself in any way visible. stars at right angles to the earth's line of motion would be most affected, and these would be all displaced by the full amount of seconds of arc. stars in intermediate directions would be displaced by intermediate amounts. but the line of the earth's motion is approximately a circle round the sun, hence the direction of its advance is constantly though slowly changing, and in one year it goes through all the points of the compass. the stars, being displaced always in the line of advance, must similarly appear to describe little closed curves, always a quadrant in advance of the earth, completing their orbits once a year. those near the pole of the ecliptic will describe circles, being always at right angles to the motion. those in the plane of the ecliptic (near the zodiac) will be sometimes at right angles to the motion, but at other times will be approached or receded from; hence these will oscillate like pendulums once a year; and intermediate stars will have intermediate motions--that is to say, will describe ellipses of varying excentricity, but all completed in a year, and all with the major axis ". this agreed very closely with what was observed. the main details were thus clearly and simply explained by the hypothesis of a finite velocity for light, "the successive propagation of light in time." this time there was no room for hesitation, and astronomers hailed the discovery with enthusiasm. not yet, however, did bradley rest. the finite velocity of light explained the major part of the irregularities he had observed, but not the whole. the more carefully he measured the amount of the deviation, the less completely accurate became its explanation. there clearly was a small outstanding error or discrepancy; the stars were still subject to an unexplained displacement--not, indeed, a displacement that repeated itself every year, but one that went through a cycle of changes in a longer period. the displacement was only about half that of aberration, and having a longer period was rather more difficult to detect securely. but the major difficulty was the fact that the two sorts of disturbances were co-existent, and the skill of disentangling them, and exhibiting the true and complete cause of each inequality, was very brilliant. for nineteen years did bradley observe this minor displacement, and in that time he saw it go through a complete cycle. its cause was now clear to him; the nineteen-year period suggested the explanation. it is the period in which the moon goes through all her changes--a period known to the ancients as the lunar cycle, or metonic cycle, and used by them to predict eclipses. it is still used for the first rough approximation to the prediction of eclipses, and to calculate easter. the "golden number" of the prayer-book is the number of the year in this cycle. the cause of the second inequality, or apparent periodic motion of the stars, bradley made out to be a nodding motion of the earth's axis. the axis of the earth describes its precessional orbit or conical motion every , years, as had long been known; but superposed upon this great movement have now been detected minute nods, each with a period of nineteen years. the cause of the nodding is completely accounted for by the theory of gravitation, just as the precession of the equinoxes was. both disturbances result from the attraction of the moon on the non-spherical earth--on its protuberant equator. "nutation" is, in fact, a small perturbation of precession. the motion may be observed in a non-sleeping top. the slow conical motion of the top's slanting axis represents the course of precession. sometimes this path is loopy, and its little nods correspond to nutation. the probable existence of some such perturbation had not escaped the sagacity of newton, and he mentions something about it in the _principia_, but thinks it too small to be detected by observation. he was thinking, however, of a solar disturbance rather than a lunar one, and this is certainly very small, though it, too, has now been observed. newton was dead before bradley made these great discoveries, else he would have been greatly pleased to hear of them. these discoveries of aberration and nutation, says delambre, the great french historian of science, secure to their author a distinguished place after hipparchus and kepler among the astronomers of all ages and all countries. notes to lecture xi _lagrange_ and _laplace_, both tremendous mathematicians, worked very much in alliance, and completed newton's work. the _mécanique céleste_ contains the higher intricacies of astronomy mathematically worked out according to the theory of gravitation. they proved the solar system to be stable; all its inequalities being periodic, not cumulative. and laplace suggested the "nebular hypothesis" concerning the origin of sun and planets: a hypothesis previously suggested, and to some extent, elaborated, by kant. a list of some of the principal astronomical researches of lagrange and laplace:--libration of the moon. long inequality of jupiter and saturn. perturbations of jupiter's satellites. perturbations of comets. acceleration of the moon's mean motion. improved lunar theory. improvements in the theory of the tides. periodic changes in the form and obliquity of the earth's orbit. stability of the solar system considered as an assemblage of rigid bodies subject to gravity. the two equations which establish the stability of the solar system are:-- _sum (me^ [square root]d) = constant,_ and _sum (m tan^ [theta][square root]d) = constant;_ where _m_ is the mass of each planet, _d_ its mean distance from the sun, _e_ the excentricity of its orbit, and [theta] the inclination of its plane. however the expressions above formulated may change for individual planets, the sum of them for all the planets remains invariable. the period of the variations in excentricity of the earth's orbit is , years; the period of conical revolution of the earth's axis is , years. about , years ago the excentricity was at a maximum. lecture xi lagrange and laplace--the stability of the solar system, and the nebular hypothesis laplace was the son of a small farmer or peasant of normandy. his extraordinary ability was noticed by some wealthy neighbours, and by them he was sent to a good school. from that time his career was one brilliant success, until in the later years of his life his prominence brought him tangibly into contact with the deteriorating influence of politics. perhaps one ought rather to say trying than deteriorating; for they seem trying to a strong character, deteriorating to a weak one--and unfortunately, laplace must be classed in this latter category. it has always been the custom in france for its high scientific men to be conspicuous also in politics. it seems to be now becoming the fashion in this country also, i regret to say. the _life_ of laplace is not specially interesting, and i shall not go into it. his brilliant mathematical genius is unquestionable, and almost unrivalled. he is, in fact, generally considered to come in this respect next after newton. his talents were of a more popular order than those of lagrange, and accordingly he acquired fame and rank, and rose to the highest dignities. nevertheless, as a man and a politician he hardly commands our respect, and in time-serving adjustability he is comparable to the redoubtable vicar of bray. his scientific insight and genius were however unquestionably of the very highest order, and his work has been invaluable to astronomy. i will give a short sketch of some of his investigations, so far as they can be made intelligible without overmuch labour. he worked very much in conjunction with lagrange, a more solid though a less brilliant man, and it is both impossible and unnecessary for us to attempt to apportion respective shares of credit between these two scientific giants, the greatest scientific men that france ever produced. first comes a research into the libration of the moon. this was discovered by galileo in his old age at arcetri, just before his blindness. the moon, as every one knows, keeps the same face to the earth as it revolves round it. in other words, it does not rotate with reference to the earth, though it does rotate with respect to outside bodies. its libration consists in a sort of oscillation, whereby it shows us now a little more on one side, now a little more on the other, so that altogether we are cognizant of more than one-half of its surface--in fact, altogether of about three-fifths. it is a simple and unimportant matter, easily explained. the motion of the moon may be analyzed into a rotation about its own axis combined with a revolution about the earth. the speed of the rotation is quite uniform, the speed of the revolution is not quite uniform, because the orbit is not circular but elliptical, and the moon has to travel faster in perigee than in apogee (in accordance with kepler's second law). the consequence of this is that we see a little too far round the body of the moon, first on one side, then on the other. hence it _appears_ to oscillate slightly, like a lop-sided fly-wheel whose revolutions have been allowed to die away so that they end in oscillations of small amplitude.[ ] its axis of rotation, too, is not precisely perpendicular to its plane of revolution, and therefore we sometimes see a few hundred miles beyond its north pole, sometimes a similar amount beyond its south. lastly, there is a sort of parallax effect, owing to the fact that we see the rising moon from one point of view, and the setting moon from a point , miles distant; and this base-line of the earth's diameter gives us again some extra glimpses. this diurnal or parallactic libration is really more effective than the other two in extending our vision into the space-facing hemisphere of the moon. these simple matters may as well be understood, but there is nothing in them to dwell upon. the far side of the moon is probably but little worth seeing. its features are likely to be more blurred with accumulations of meteoric dust than are those of our side, but otherwise they are likely to be of the same general character. the thing of real interest is the fact that the moon does turn the same face towards us; _i.e._ has ceased to rotate with respect to the earth (if ever it did so). the stability of this state of things was shown by lagrange to depend on the shape of the moon. it must be slightly egg-shape, or prolate--extended in the direction of the earth; its earth-pointing diameter being a few hundred feet longer than its visible diameter; a cause slight enough, but nevertheless sufficient to maintain stability, except under the action of a distinct disturbing cause. the prolate or lemon-like shape is caused by the gravitative pull of the earth, balanced by the centrifugal whirl. the two forces balance each other as regards motion, but between them they have strained the moon a trifle out of shape. the moon has yielded as if it were perfectly plastic; in all probability it once was so. it may be interesting to note for a moment the correlative effect of this aspect of the moon, if we transfer ourselves to its surface in imagination, and look at the earth (cf. fig. ). the earth would be like a gigantic moon of four times our moon's diameter, and would go through its phases in regular order. but it would not rise or set: it would be fixed in the sky, and subject only to a minute oscillation to and fro once a month, by reason of the "libration" we have been speaking of. its aspect, as seen by markings on its surface, would rapidly change, going through a cycle in twenty-four hours; but its permanent features would be usually masked by lawless accumulations of cloud, mainly aggregated in rude belts parallel to the equator. and these cloudy patches would be the most luminous, the whitest portions; for of course it would be their silver lining that we would then be looking on.[ ] next among the investigations of lagrange and laplace we will mention the long inequality of jupiter and saturn. halley had found that jupiter was continually lagging behind its true place as given by the theory of gravitation; and, on the other hand, that saturn was being accelerated. the lag on the part of jupiter amounted to about - / minutes in a century. overhauling ancient observations, however, halley found signs of the opposite state of things, for when he got far enough back jupiter was accelerated and saturn was being retarded. here was evidently a case of planetary perturbation, and laplace and lagrange undertook the working of it out. they attacked it as a case of the problem of three bodies, viz. the sun, jupiter, and saturn; which are so enormously the biggest of the known bodies in the system that insignificant masses like the earth, mars, and the rest, may be wholly neglected. they succeeded brilliantly, after a long and complex investigation: succeeded, not in solving the problem of the three bodies, but, by considering their mutual action as perturbations superposed on each other, in explaining the most conspicuous of the observed anomalies of their motion, and in laying the foundation of a general planetary theory. [illustration: fig. .--shewing the three conjunction places in the orbits of jupiter and saturn. the two planets are represented as leaving one of the conjunctions where jupiter was being pulled back and saturn being pulled forward by their mutual attraction.] one of the facts that plays a large part in the result was known to the old astrologers, viz. that jupiter and saturn come into conjunction with a certain triangular symmetry; the whole scheme being called a trigon, and being mentioned several times by kepler. it happens that five of jupiter's years very nearly equal two of saturn's,[ ] so that they get very nearly into conjunction three times in every five jupiter years, but not exactly. the result of this close approach is that periodically one pulls the other on and is itself pulled back; but since the three points progress, it is not always the same planet which gets pulled back. the complete theory shows that in the year there was no marked perturbation: before that it was in one direction, while afterwards it was in the other direction, and the period of the whole cycle of disturbances is of our years. the solution of this long outstanding puzzle by the theory of gravitation was hailed with the greatest enthusiasm by astronomers, and it established the fame of the two french mathematicians. next they attacked the complicated problem of the motions of jupiter's satellites. they succeeded in obtaining a theory of their motions which represented fact very nearly indeed, and they detected the following curious relationship between the satellites:--the speed of the first satellite + twice the speed of the second is equal to the speed of the third. they found this, not empirically, after the manner of kepler, but as a deduction from the law of gravitation; for they go on to show that even if the satellites had not started with this relation they would sooner or later, by mutual perturbation, get themselves into it. one singular consequence of this, and of another quite similar connection between their positions, is that all three satellites can never be eclipsed at once. the motion of the fourth satellite is less tractable; it does not so readily form an easy system with the others. after these great successes the two astronomers naturally proceeded to study the mutual perturbations of all other bodies in the solar system. and one very remarkable discovery they made concerning the earth and moon, an account of which will be interesting, though the details and processes of calculation are quite beyond us in a course like this. astronomical theory had become so nearly perfect by this time, and observations so accurate, that it was possible to calculate many astronomical events forwards or backwards, over even a thousand years or more, with admirable precision. now, halley had studied some records of ancient eclipses, and had calculated back by means of the lunar theory to see whether the calculation of the time they ought to occur would agree with the record of the time they did occur. to his surprise he found a discrepancy, not a large one, but still one quite noticeable. to state it as we know it now:--an eclipse a century ago happened twelve seconds later than it ought to have happened by theory; two centuries back the error amounted to forty-eight seconds, in three centuries it would be seconds, and so on; the lag depending on the square of the time. by research, and help from scholars, he succeeded in obtaining the records of some very ancient eclipses indeed. one in egypt towards the end of the tenth century a.d.; another in a.d.; another a little before christ; and one, the oldest of all of which any authentic record has been preserved, observed by the chaldæan astronomers in babylon in the reign of hezekiah. calculating back to this splendid old record of a solar eclipse, over the intervening , years, the calculated and the observed times were found to disagree by nearly two hours. pondering over an explanation of the discrepancy, halley guessed that it must be because the moon's motion was not uniform, it must be going quicker and quicker, gaining twelve seconds each century on its previous gain--a discovery announced by him as "the acceleration of the moon's mean motion." the month was constantly getting shorter. what was the physical cause of this acceleration according to the theory of gravitation? many attacked the question, but all failed. this was the problem laplace set himself to work out. a singular and beautiful result rewarded his efforts. you know that the earth describes an elliptic orbit round the sun: and that an ellipse is a circle with a certain amount of flattening or "excentricity."[ ] well, laplace found that the excentricity of the earth's orbit must be changing, getting slightly less; and that this change of excentricity would have an effect upon the length of the month. it would make the moon go quicker. one can almost see how it comes about. a decrease in excentricity means an increase in mean distance of the earth from the sun. this means to the moon a less solar perturbation. now one effect of the solar perturbation is to keep the moon's orbit extra large: if the size of its orbit diminishes, its velocity must increase, according to kepler's third law. laplace calculated the amount of acceleration so resulting, and found it ten seconds a century; very nearly what observation required; for, though i have quoted observation as demanding twelve seconds per century, the facts were not then so distinctly and definitely ascertained. this calculation for a long time seemed thoroughly satisfactory, but it is not the last word on the subject. quite lately an error has been found in the working, which diminishes the theoretical gravitation-acceleration to six seconds a century instead of ten, thus making it insufficient to agree exactly with fact. the theory of gravitation leaves an outstanding error. (the point is now almost thoroughly understood, and we shall return to it in lecture xviii). but another question arises out of this discussion. i have spoken of the excentricity of the earth's orbit as decreasing. was it always decreasing? and if so, how far back was it so excentric that at perihelion the earth passed quite near the sun? if it ever did thus pass near the sun, the inference is manifest--the earth must at one time have been thrown off, or been separated off, from the sun. if a projectile could be fired so fast that it described an orbit round the earth--and the speed of fire to attain this lies between five and seven miles a second (not less than the one, nor more than the other)--it would ever afterwards pass through its point of projection as one point of its elliptic orbit; and its periodic return through that point would be the sign of its origin. similarly, if a satellite does _not_ come near its central orb, and can be shown never to have been near it, the natural inference is that it has _not_ been born from it, but has originated in some other way. the question which presented itself in connexion with the variable ellipticity of the earth's orbit was the following:--had it always been decreasing, so that once it was excentric enough just to graze the sun at perihelion as a projected body would do? into the problem thus presented lagrange threw himself, and he succeeded in showing that no such explanation of the origin of the earth is possible. the excentricity of the orbit, though now decreasing, was not always decreasing; ages ago it was increasing: it passes through periodic changes. eighteen thousand years ago its excentricity was a maximum; since then it has been diminishing, and will continue to diminish for , years more, when it will be an almost perfect circle; it will then begin to increase again, and so on. the obliquity of the ecliptic is also changing periodically, but not greatly: the change is less than three degrees. this research has, or ought to have, the most transcendent interest for geologists and geographers. you know that geologists find traces of extraordinary variations of temperature on the surface of the earth. england was at one time tropical, at another time glacial. far away north, in spitzbergen, evidence of the luxuriant vegetation of past ages has been found; and the explanation of these great climatic changes has long been a puzzle. does not the secular variation in excentricity of the earth's orbit, combined with the precession of the equinoxes, afford a key? and if a key at all, it will be an accurate key, and enable us to calculate back with some precision to the date of the glacial epoch; and again to the time when a tropical flora flourished in what is now northern europe, _i.e._ to the date of the carboniferous era. this aspect of the subject has recently been taught with vigour and success by dr. croll in his book "climate and time." a brief and partial explanation of the matter may be given, because it is a point of some interest and is also one of fair simplicity. every one knows that the climatic conditions of winter and summer are inverted in the two hemispheres, and that at present the sun is nearest to us in our (northern) winter. in other words, the earth's axis is inclined so as to tilt its north pole away from the sun at perihelion, or when the earth is at the part of its elliptic orbit nearest the sun's focus; and to tilt it towards the sun at aphelion. the result of this present state of things is to diminish the intensity of the average northern winter and of the average northern summer, and on the other hand to aggravate the extremes of temperature in the southern hemisphere; all other things being equal. of course other things are not equal, and the distribution of land and sea is a still more powerful climatic agent than is the three million miles or so extra nearness of the sun. but it is supposed that the antarctic ice-cap is larger than the northern, and increased summer radiation with increased winter cold would account for this. but the present state of things did not always obtain. the conical movement of the earth's axis (now known by a curious perversion of phrase as "precession") will in the course of , years or so cause the tilt to be precisely opposite, and then we shall have the more extreme winters and summers instead of the southern hemisphere. if the change were to occur now, it might not be overpowering, because now the excentricity is moderate. but if it happened some time back, when the excentricity was much greater, a decidedly different arrangement of climate may have resulted. there is no need to say _if_ it happened some time back: it did happen, and accordingly an agent for affecting the distribution of mean temperature on the earth is to hand; though whether it is sufficient to achieve all that has been observed by geologists is a matter of opinion. once more, the whole diversity of the seasons depends on the tilt of the earth's axis, the ° by which it is inclined to a perpendicular to the orbital plane; and this obliquity or tilt is subject to slow fluctuations. hence there will come eras when all causes combine to produce a maximum extremity of seasons in the northern hemisphere, and other eras when it is the southern hemisphere which is subject to extremes. but a grander problem still awaited solution--nothing less than the fate of the whole solar system. here are a number of bodies of various sizes circulating at various rates round one central body, all attracted by it, and all attracting each other, the whole abandoned to the free play of the force of gravitation: what will be the end of it all? will they ultimately approach and fall into the sun, or will they recede further and further from him, into the cold of space? there is a third possible alternative: may they not alternately approach and recede from him, so as on the whole to maintain a fair approximation to their present distances, without great and violent extremes of temperature either way? if any one planet of the system were to fall into the sun, more especially if it were a big one like jupiter or saturn, the heat produced would be so terrific that life on this earth would be destroyed, even at its present distance; so that we are personally interested in the behaviour of the other planets as well as in the behaviour of our own. the result of the portentously difficult and profoundly interesting investigation, here sketched in barest outline, is that the solar system is stable: that is to say, that if disturbed a little it will oscillate and return to its old state; whereas if it were unstable the slightest disturbance would tend to accumulate, and would sooner or later bring about a catastrophe. a hanging pendulum is stable, and oscillates about a mean position; its motion is periodic. a top-heavy load balanced on a point is unstable. all the changes of the solar system are periodic, _i.e._ they repeat themselves at regular intervals, and they never exceed a certain moderate amount. the period is something enormous. they will not have gone through all their changes until a period of , , years has elapsed. this is the period of the planetary oscillation: "a great pendulum of eternity which beats ages as our pendulums beat seconds." enormous it seems; and yet we have reason to believe that the earth has existed through many such periods. the two laws of stability discovered and stated by lagrange and laplace i can state, though they may be difficult to understand:-- represent the masses of the several planets by m_ , m_ , &c.; their mean distances from the sun (or radii vectores) by r_ , r_ , &c.; the excentricities of their orbits by e_ , e_ , &c.; and the obliquity of the planes of these orbits, reckoned from a single plane of reference or "invariable plane," by [theta]_ , [theta]_ , &c.; then all these quantities (except m) are liable to fluctuate; but, however much they change, an increase for one planet will be accompanied by a decrease for some others; so that, taking all the planets into account, the sum of a set of terms like these, m_ e_ ^ [square root]r_ + m_ e_ ^ [square root]r_ + &c., will remain always the same. this is summed up briefly in the following statement: [sigma](me^ [square root]r) = constant. that is one law, and the other is like it, but with inclination of orbit instead of excentricity, viz.: [sigma](m[theta]^ [square root]r) = constant. the value of each of these two constants can at any time be calculated. at present their values are small. hence they always were and always will be small; being, in fact, invariable. hence neither _e_ nor _r_ nor [theta] can ever become infinite, nor can their average value for the system ever become zero. the planets may share the given amount of total excentricity and obliquity in various proportions between themselves; but even if it were all piled on to one planet it would not be very excessive, unless the planet were so small a one as mercury; and it would be most improbable that one planet should ever have all the excentricity of the solar system heaped upon itself. the earth, therefore, never has been, nor ever will be, enormously nearer the sun than it is at present: nor can it ever get very much further off. its changes are small and are periodic--an increase is followed by a decrease, like the swing of a pendulum. the above two laws have been called the magna charta of the solar system, and were long supposed to guarantee its absolute permanence. so far as the theory of gravitation carries us, they do guarantee its permanence; but something more remains to be said on the subject in a future lecture (xviii). and now, finally, we come to a sublime speculation, thrown out by laplace, not as the result of profound calculation, like the results hitherto mentioned, not following certainly from the theory of gravitation, or from any other known theory, and therefore not to be accepted as more than a brilliant hypothesis, to be confirmed or rejected as our knowledge extends. this speculation is the "nebular hypothesis." since the time of laplace the nebular hypothesis has had ups and downs of credence, sometimes being largely believed in, sometimes being almost ignored. at the present time it holds the field with perhaps greater probability of ultimate triumph than has ever before seemed to belong to it--far greater than belonged to it when first propounded. it had been previously stated clearly and well by the philosopher kant, who was intensely interested in "the starry heavens" as well as in the "mind of man," and who shewed in connexion with astronomy also a most surprising genius. the hypothesis ought by rights perhaps to be known rather by his name than by that of laplace. the data on which it was founded are these:--every motion in the solar system known at that time took place in one direction, and in one direction only. thus the planets revolve round the sun, all going the same way round; moons revolve round the planets, still maintaining the same direction of rotation, and all the bodies that were known to rotate on their own axis did so with still the same kind of spin. moreover, all these motions take place in or near a single plane. the ancients knew that sun moon and planets all keep near to the ecliptic, within a belt known as the zodiac: none strays away into other parts of the sky. satellites also, and rings, are arranged in or near the same plane; and the plane of diurnal spin, or equator of the different bodies, is but slightly tilted. now all this could not be the result of chance. what could have caused it? is there any connection or common ancestry possible, to account for this strange family likeness? there is no connection now, but there may have been once. must have been, we may almost say. it is as though they had once been parts of one great mass rotating as a whole; for if such a rotating mass broke up, its parts would retain its direction of rotation. but such a mass, filling all space as far as or beyond saturn, although containing the materials of the whole solar system in itself, must have been of very rare consistency. occupying so much bulk it could not have been solid, nor yet liquid, but it might have been gaseous. are there any such gigantic rotating masses of gas in the heaven now? certainly there are; there are the nebulæ. some of the nebulæ are now known to be gaseous, and some of them at least are in a state of rotation. laplace could not have known this for certain, but he suspected it. the first distinctly spiral nebula was discovered by the telescope of lord rosse; and quite recently a splendid photograph of the great andromeda nebula, by our townsman, mr. isaac roberts, reveals what was quite unsuspected--and makes it clear that this prodigious mass also is in a state of extensive and majestic whirl. very well, then, put this problem:--a vast mass of rotating gas is left to itself to cool for ages and to condense as it cools: how will it behave? a difficult mathematical problem, worthy of being attacked to-day; not yet at all adequately treated. there are those who believe that by the complete treatment of such a problem all the history of the solar system could be evolved. [illustration: fig. .--lord rosse's drawing of the spiral nebula in canes venatici, with the stub marks of the draughtsman unduly emphasised into features by the engraver.] laplace pictured to himself this mass shrinking and thereby whirling more and more rapidly. a spinning body shrinking in size and retaining its original amount of rotation, as it will unless a brake is applied, must spin more and more rapidly as it shrinks. it has what mathematicians call a constant moment of momentum; and what it loses in leverage, as it shrinks, it gains in speed. the mass is held together by gravitation, every particle attracting every other particle; but since all the particles are describing curved paths, they will tend to fly off tangentially, and only a small excess of the gravitation force over the centrifugal is left to pull the particles in, and slowly to concentrate the nebula. the mutual gravitation of the parts is opposed by the centrifugal force of the whirl. at length a point is reached where the two forces balance. a portion outside a certain line will be in equilibrium; it will be left behind, and the rest must contract without it. a ring is formed, and away goes the inner nucleus contracting further and further towards a centre. after a time another ring will be left behind in the same way, and so on. what happens to these rings? they rotate with the motion they possess when thrown or shrunk off; but will they remain rings? if perfectly regular they may; if there be any irregularity they are liable to break up. they will break into one or two or more large masses, which are ultimately very likely to collide and become one. the revolving body so formed is still a rotating gaseous mass; and it will go on shrinking and cooling and throwing off rings, like the larger nucleus by which it has been abandoned. as any nucleus gets smaller, its rate of rotation increases, and so the rings last thrown off will be spinning faster than those thrown off earliest. the final nucleus or residual central body will be rotating fastest of all. the nucleus of the whole original mass we now see shrunk up into what we call the sun, which is spinning on its axis once every twenty-five days. the rings successively thrown off by it are now the planets--some large, some small--those last thrown off rotating round him comparatively quickly, those outside much more slowly. the rings thrown off by the planetary gaseous masses as they contracted have now become satellites; except one ring which has remained without breaking up, and is to be seen rotating round saturn still. one other similar ring, an abortive attempt at a planet, is also left round the sun (the zone of asteroids). such, crudely and baldly, is the famous nebular hypothesis of laplace. it was first stated, as has been said above, by the philosopher kant, but it was elaborated into much fuller detail by the greatest of french mathematicians and astronomers. the contracting masses will condense and generate great quantities of heat by their own shrinkage; they will at a certain stage condense to liquid, and after a time will begin to cool and congeal with a superficial crust, which will get thicker and thicker; but for ages they will remain hot, even after they have become thoroughly solid. the small ones will cool fastest; the big ones will retain their heat for an immense time. bullets cool quickly, cannon-balls take hours or days to cool, planets take millions of years. our moon may be nearly cold, but the earth is still warm--indeed, very hot inside. jupiter is believed by some observers still to glow with a dull red heat; and the high temperature of the much larger and still liquid mass of the sun is apparent to everybody. not till it begins to scum over will it be perceptibly cooler. [illustration: fig. .--saturn.] many things are now known concerning heat which were not known to laplace (in the above paragraph they are only hinted at), and these confirm and strengthen the general features of his hypothesis in a striking way; so do the most recent telescopic discoveries. but fresh possibilities have now occurred to us, tidal phenomena are seen to have an influence then wholly unsuspected, and it will be in a modified and amplified form that the philosopher of next century will still hold to the main features of this famous old nebular hypothesis respecting the origin of the sun and planets--the evolution of the solar system. notes to lecture xii the subject of stellar astronomy was first opened up by sir william herschel, the greatest observing astronomer. _frederick william herschel_ was born in hanover in , and brought up as a musician. came to england in . first saw a telescope in . made a great many himself, and began a survey of the heavens. his sister caroline, born in , came to england in , and became his devoted assistant to the end of his life. uranus discovered in . music finally abandoned next year, and the -foot telescope begun. discovered two moons of saturn and two of uranus. reviewed, described, and gauged all the visible heavens. discovered and catalogued , nebulæ and double stars. speculated concerning the milky way, the nebulosity of stars, the origin and growth of solar systems. discovered that the stars were in motion, not fixed, and that the sun as one of them was journeying towards a point in the constellation hercules. died in , eighty-four years old. caroline herschel discovered eight comets, and lived on to the age of ninety-eight. lecture xii herschel and the motion of the fixed stars we may admit, i think, that, with a few notable exceptions, the work of the great men we have been recently considering was rather to complete and round off the work of newton, than to strike out new and original lines. this was the whole tendency of eighteenth century astronomy. it appeared to be getting into an adult and uninteresting stage, wherein everything could be calculated and predicted. labour and ingenuity, and a severe mathematical training, were necessary to work out the remote consequences of known laws, but nothing fresh seemed likely to turn up. consequently men's minds began turning in other directions, and we find chemistry and optics largely studied by some of the greatest minds, instead of astronomy. but before the century closed there was destined to arise one remarkable exception--a man who was comparatively ignorant of that which had been done before--a man unversed in mathematics and the intricacies of science, but who possessed such a real and genuine enthusiasm and love of nature that he overcame the force of adverse circumstances, and entering the territory of astronomy by a by-path, struck out a new line for himself, and infused into the science a healthy spirit of fresh life and activity. this man was william herschel. "the rise of herschel," says miss clerke, "is the one conspicuous anomaly in the otherwise somewhat quiet and prosy eighteenth century. it proved decisive of the course of events in the nineteenth. it was unexplained by anything that had gone before, yet all that came after hinged upon it. it gave a new direction to effort; it lent a fresh impulse to thought. it opened a channel for the widespread public interest which was gathering towards astronomical subjects to flow in." herschel was born at hanover in , the son of an oboe player in a military regiment. the father was a good musician, and a cultivated man. the mother was a german _frau_ of the period, a strong, active, business-like woman, of strong character and profound ignorance. herself unable to write, she set her face against learning and all new-fangled notions. the education of the sons she could not altogether control, though she lamented over it, but the education of her two daughters she strictly limited to cooking, sewing, and household management. these, however, she taught them well. it was a large family, and william was the fourth child. we need only remember the names of his younger brother alexander, and of his much younger sister caroline. they were all very musical--the youngest boy was once raised upon a table to play the violin at a public performance. the girls were forbidden to learn music by their mother, but their father sometimes taught them a little on the sly. alexander was besides an ingenious mechanician. at the age of seventeen, william became oboist to the hanoverian guards, shortly before the regiment was ordered to england. two years later he removed himself from the regiment, with the approval of his parents, though probably without the approbation or consent of the commanding officer, by whom such removal would be regarded as simple desertion, which indeed it was; and george iii. long afterwards handed him an official pardon for it. at the age of nineteen, he was thus launched in england with an outfit of some french, latin, and english, picked up by himself; some skill in playing the hautboy, the violin, and the organ, as taught by his father; and some good linen and clothing, and an immense stock of energy, provided by his mother. he lived as musical instructor to one or two militia bands in yorkshire, and for three years we hear no more than this of him. but, at the end of that time, a noted organist, dr. miller, of durham, who had heard his playing, proposed that he should come and live with him and play at concerts, which he was very glad to do. he next obtained the post of organist at halifax; and some four or five years later he was invited to become organist at the octagon chapel in bath, and soon led the musical life of that then very fashionable place. about this time he went on a short visit to his family at hanover, by all of whom he was very much beloved, especially by his young sister caroline, who always regarded him as specially her own brother. it is rather pitiful, however, to find that her domestic occupations still unfairly repressed and blighted her life. she says:-- "of the joys and pleasures which all felt at this long-wished-for meeting with my--let me say my dearest--brother, but a small portion could fall to my share; for with my constant attendance at church and school, besides the time i was employed in doing the drudgery of the scullery, it was but seldom i could make one in the group when the family were assembled together." while at bath he wrote many musical pieces--glees, anthems, chants, pieces for the harp, and an orchestral symphony. he taught a large number of pupils, and lived a hard and successful life. after fourteen hours or so spent in teaching and playing, he would retire at night to instruct his mind with a study of mathematics, optics, italian, or greek, in all of which he managed to make some progress. he also about this time fell in with some book on astronomy. in his father was struck with paralysis, and two years later he died. william then proposed that alexander should come over from hanover and join him at bath, which was done. next they wanted to rescue their sister caroline from her humdrum existence, but this was a more difficult matter. caroline's journal gives an account of her life at this time that is instructive. here are a few extracts from it:-- "my father wished to give me something like a polished education, but my mother was particularly determined that it should be a rough, but at the same time a useful one; and nothing further she thought was necessary but to send me two or three months to a sempstress to be taught to make household linen.... "my mother would not consent to my being taught french, ... so all my father could do for me was to indulge me (and please himself) sometimes with a short lesson on the violin, when my mother was either in good humour or out of the way.... she had cause for wishing me not to know more than was necessary for being useful in the family; for it was her certain belief that my brother william would have returned to his country, and my eldest brother not have looked so high, if they had had a little less learning." however, seven years after the death of their father, william went over to germany and returned to england in triumph, bringing caroline with him: she being then twenty-two. so now began a busy life in bath. for caroline the work must have been tremendous. for, besides having to learn singing, she had to learn english. she had, moreover, to keep accounts and do the marketing. when the season at bath was over, she hoped to get rather more of her brother william's society; but he was deep in optics and astronomy, used to sleep with the books under his pillow, read them during meals, and scarcely ever thought of anything else. he was determined to see for himself all the astronomical wonders; and there being a small gregorian reflector in one of the shops, he hired it. but he was not satisfied with this, and contemplated making a telescope feet long. he wrote to opticians inquiring the price of a mirror suitable, but found there were none so large, and that even the smaller ones were beyond his means. nothing daunted, he determined to make some for himself. alexander entered into his plans: tools, hones, polishers, and all sorts of rubbish were imported into the house, to the sister's dismay, who says:-- [illustration: fig. .--principle of newtonian reflector.] "and then, to my sorrow, i saw almost every room turned into a workshop. a cabinet-maker making a tube and stands of all descriptions in a handsomely furnished drawing-room; alex. putting up a huge turning-machine (which he had brought in the autumn from bristol, where he used to spend the summer) in a bed-room, for turning patterns, grinding glasses, and turning eye-pieces, &c. at the same time music durst not lie entirely dormant during the summer, and my brother had frequent rehearsals at home." finally, in , at the age of thirty-six, he had made himself a - / -foot telescope, and began to view the heavens. so attached was he to the instrument that he would run from the concert-room between the parts, and take a look at the stars. he soon began another telescope, and then another. he must have made some dozen different telescopes, always trying to get them bigger and bigger; at last he got a -foot and then a -foot instrument, and began a systematic survey of the heavens; he also began to communicate his results to the royal society. he now took a larger house, with more room for workshops, and a grass plot for a -foot telescope, and still he went on grinding mirrors--literally hundreds of them. i read another extract from the diary of his sister, who waited on him and obeyed him like a spaniel:-- "my time was taken up with copying music and practising, besides attendance on my brother when polishing, since by way of keeping him alive i was constantly obliged to feed him by putting the victuals by bits into his mouth. this was once the case when, in order to finish a -foot mirror, he had not taken his hands from it for sixteen hours together. in general he was never unemployed at meals, but was always at those times contriving or making drawings of whatever came in his mind. generally i was obliged to read to him whilst he was at the turning-lathe, or polishing mirrors--_don quixote_, _arabian nights' entertainments_, the novels of sterne, fielding, &c.; serving tea and supper without interrupting the work with which he was engaged, ... and sometimes lending a hand. i became, in time, as useful a member of the workshop as a boy might be to his master in the first year of his apprenticeship.... but as i was to take a part the next year in the oratorios, i had, for a whole twelvemonth, two lessons per week from miss fleming, the celebrated dancing-mistress, to drill me for a gentlewoman (god knows how she succeeded). so we lived on without interruption. my brother alex. was absent from bath for some months every summer, but when at home he took much pleasure in executing some turning or clockmaker's work for his brother." the music, and the astronomy, and the making of telescopes, all went on together, each at high pressure, and enough done in each to satisfy any ordinary activity. but the herschels knew no rest. grinding mirrors by day, concerts and oratorios in the evening, star-gazing at night. it is strange his health could stand it. the star-gazing, moreover, was no _dilettante_ work; it was based on a serious system--a well thought out plan of observation. it was nothing less than this--to pass the whole heavens steadily and in order through the telescope, noting and describing and recording every object that should be visible, whether previously known or unknown. the operation is called sweeping; but it is not a rapid passage from one object to another, as the term might suggest; it is a most tedious business, and consists in following with the telescope a certain field of view for some minutes, so as to be sure that nothing is missed, then shifting it to the next overlapping field, and watching again. and whatever object appears must be scrutinized anxiously to see what there is peculiar about it. if a star, it may be double, or it may be coloured, or it may be nebulous; or again it may be variable, and so its brightness must be estimated in order to compare with a subsequent observation. four distinct times in his life did herschel thus pass the whole visible heavens under review; and each survey occupied him several years. he discovered double stars, variable stars, nebulæ, and comets; and mr. william herschel, of bath, the amateur astronomer, was gradually emerging from his obscurity, and becoming a known man. tuesday, the th of march, , is a date memorable in the annals of astronomy. "on this night," he writes to the royal society, "in examining the small stars near _[eta]_ geminorum, i perceived one visibly larger than the rest. struck with its uncommon appearance, i compared it to _[eta]_ geminorum and another star, and finding it so much larger than either, i suspected it to be a comet." the "comet" was immediately observed by professional astronomers, and its orbit was computed by some of them. it was thus found to move in nearly a circle instead of an elongated ellipse, and to be nearly twice as far from the sun as saturn. it was no comet, it was a new planet; more than times as big as the earth, and nearly twice as far away as saturn. it was presently christened "uranus." this was a most striking discovery, and the news sped over europe. to understand the interest it excited we must remember that such a discovery was unique. since the most ancient times of which men had any knowledge, the planets mercury, venus, mars, jupiter, saturn, had been known, and there had been no addition to their number. galileo and others had discovered satellites indeed, but a new primary planet was an entire and utterly unsuspected novelty. one of the most immediate consequences of the event was the discovery of herschel himself. the royal society made him a fellow the same year. the university of oxford dubbed him a doctor; and the king sent for him to bring his telescope and show it at court. so to london and windsor he went, taking with him his best telescope. maskelyne, the then astronomer-royal, compared it with the national one at greenwich, and found herschel's home-made instrument far the better of the two. he had a stand made after herschel's pattern, but was so disgusted with his own instrument now that he scarcely thought it worthy of the stand when it was made. at windsor, george iii. was very civil, and mr. herschel was in great request to show the ladies of the court saturn and other objects of interest. mr. herschel exhibited a piece of worldly wisdom under these circumstances, that recalls faintly the behaviour of tycho brahé under similar circumstances. the evening when the exhibition was to take place threatened to become cloudy and wet, so herschel rigged up an artificial saturn, constructed of card and tissue paper, with a lamp behind it, in the distant wall of a garden; and, when the time came, his new titled friends were regaled with a view of this imitation saturn through the telescope--the real one not being visible. they went away much pleased. he stayed hovering between windsor and greenwich, and uncertain what was to be the outcome of all this regal patronizing. he writes to his sister that he would much rather be back grinding mirrors at bath. and she writes begging him to come, for his musical pupils were getting impatient. they had to get the better of their impatience, however, for the king ultimately appointed him astronomer or rather telescope-maker to himself, and so caroline and the whole household were sent for, and established in a small house at datchet. from being a star-gazing musician, herschel thus became a practical astronomer. henceforth he lived in his observatory; only on wet and moonlight nights could he be torn away from it. the day-time he devoted to making his long-contemplated -foot telescope. not yet, however, were all their difficulties removed. the house at datchet was a tumble-down barn of a place, chosen rather as a workshop and observatory than as a dwelling-house. and the salary allowed him by george iii. was scarcely a princely one. it was, as a matter of fact, £ a year. the idea was that he would earn his living by making telescopes, and so indeed he did. he made altogether some hundreds. among others, four for the king. but this eternal making of telescopes for other people to use or play with was a weariness to the flesh. what he wanted was to observe, observe, observe. sir william watson, an old friend of his, and of some influence at court, expressed his mind pretty plainly concerning herschel's position; and as soon as the king got to understand that there was anything the matter, he immediately offered £ , for a gigantic telescope to be made for herschel's own use. nothing better did he want in life. the whole army of carpenters and craftsmen resident in datchet were pressed into the service. furnaces for the speculum metal were built, stands erected, and the -foot telescope fairly begun. it cost £ , before it was finished, but the king paid the whole. [illustration: fig. .--herschel's -foot telescope.] with it he discovered two more satellites to saturn (five hitherto had been known), and two moons to his own planet uranus. these two are now known as oberon and titania. they were not seen again till some forty years after, when his son, sir john herschel, reobserved them. and in , mr. lassell, at his house, "starfield," near liverpool, discovered two more, called ariel and umbriel, making the number four, as now known. mr. lassell also discovered, with a telescope of his own making, an eighth satellite of saturn--hyperion--and a satellite to neptune. a letter from a foreign astronomer about this period describes herschel and his sister's method of work:-- "i spent the night of the th of january at herschel's, in datchet, near windsor, and had the good luck to hit on a fine evening. he has his -foot newtonian telescope in the open air, and mounted in his garden very simply and conveniently. it is moved by an assistant, who stands below it.... near the instrument is a clock regulated to sidereal time.... in the room near it sits herschel's sister, and she has flamsteed's atlas open before her. as he gives her the word, she writes down the declination and right ascension, and the other circumstances of the observation. in this way herschel examines the whole sky without omitting the least part. he commonly observes with a magnifying power of one hundred and fifty, and is sure that after four or five years he will have passed in review every object above our horizon. he showed me the book in which his observations up to this time are written, and i am astonished at the great number of them. each sweep covers ° ' in declination, and he lets each star pass at least three times through the field of his telescope, so that it is impossible that anything can escape him. he has already found about double stars, and almost as many nebulæ. i went to bed about one o'clock, and up to that time he had found that night four or five new nebulæ. the thermometer in the garden stood at ° fahrenheit; but, in spite of this, herschel observes the whole night through, except that he stops every three or four hours and goes into the room for a few moments. for some years herschel has observed the heavens every hour when the weather is clear, and this always in the open air, because he says that the telescope only performs well when it is at the same temperature as the air. he protects himself against the weather by putting on more clothing. he has an excellent constitution, and thinks about nothing else in the world but the celestial bodies. he has promised me in the most cordial way, entirely in the service of astronomy, and without thinking of his own interest, to see to the telescopes i have ordered for european observatories, and he will himself attend to the preparation of the mirrors." [illustration: _painted by abbott._ _engraved by ryder._ fig. .--william herschel. _from an original picture in the possession of_ wm. watson, m.d., f.r.s.] in , herschel married an estimable lady who sympathized with his pursuits. she was the only daughter of a city magnate, so his pecuniary difficulties, such as they were (they were never very troublesome to him), came to an end. they moved now into a more commodious house at slough. their one son, afterwards the famous sir john herschel, was born some nine years later. but the marriage was rather a blow to his devoted sister: henceforth she lived in lodgings, and went over at night-time to help him observe. for it must be remarked that this family literally turned night into day. whatever sleep they got was in the day-time. every fine night without exception was spent in observing: and the quite incredible fierceness of the pursuit is illustrated, as strongly as it can be, by the following sentence out of caroline's diary, at the time of the move from datchet to slough: "the last night at datchet was spent in sweeping till daylight, and by the next evening the telescope stood ready for observation at slough." caroline was now often allowed to sweep with a small telescope on her own account. in this way she picked up a good many nebulæ in the course of her life, and eight comets, four of which were quite new, and one of which, known since as encke's comet, has become very famous. the work they got through between them is something astonishing. he made with his own hands parabolic mirrors for reflecting telescopes, besides a great number of complete instruments. he was forty-two when he began contributing to the royal society; yet before he died he had sent them sixty-nine long and elaborate treatises. one of these memoirs is a catalogue of nebulæ. fifteen years after he sends in another ; and some years later another . he also discovered double stars, which he proved were really corrected from the fact that they revolved round each other (p. ). he lived to see some of them perform half a revolution. for him the stars were not fixed: they moved slowly among themselves. he detected their proper motions. he passed the whole northern firmament in review four distinct times; counted the stars in , gauge-fields, and estimated the brightness of hundreds of stars. he also measured as accurately as he could their proper motions, devising for this purpose the method which still to this day remains in use. and what is the outcome of it all? it is not uranus, nor the satellites, nor even the double stars and the nebulæ considered as mere objects: it is the beginning of a science of the stars. [illustration: fig. .--caroline herschel. _from a drawing from life, by_ george mÜller, .] hitherto the stars had only been observed for nautical and practical purposes. their times of rising and southing and setting had been noted; they had been treated as a clock or piece of dead mechanism, and as fixed points of reference. all the energies of astronomers had gone out towards the solar system. it was the planets that had been observed. tycho had observed and tabulated their positions. kepler had found out some laws of their motion. galileo had discovered their peculiarities and attendants. newton and laplace had perceived every detail of their laws. but for the stars--the old ptolemaic system might still have been true. they might still be mere dots in a vast crystalline sphere, all set at about one distance, and subservient to the uses of the earth. herschel changed all this. instead of sameness, he found variety; instead of uniformity of distance, limitless and utterly limitless fields and boundless distances; instead of rest and quiescence, motion and activity; instead of stagnation, life. [illustration: fig. .--the double-double star [epsilon] lyræ as seen under three different powers.] yes, that is what herschel discovered--the life and activity of the whole visible universe. no longer was our little solar system to be the one object of regard, no longer were its phenomena to be alone interesting to man. with herschel every star was a solar system. and more than that: he found suns revolving round suns, at distances such as the mind reels at, still obeying the same law of gravitation as pulls an apple from a tree. he tried hard to estimate the distance of the stars from the earth, but there he failed: it was too hopeless a problem. it was solved some time after his death by bessel, and the distances of many stars are now known but these distances are awful and unspeakable. our distance from the sun shrinks up into a mere speck--the whole solar system into a mere unit of measurement, to be repeated hundreds of thousands of times before we reach the stars. yet their motion is visible--yes, to very accurate measurement quite plain. one star, known as cygni, was then and is now rushing along at the rate of miles every second. not that you must imagine that this makes any obvious and apparent change in its position. no, for all ordinary and practical purposes they are still fixed stars; thousands of years will show us no obvious change; "adam" saw precisely the same constellations as we do: it is only by refined micrometric measurement with high magnifying power that their flight can be detected. but the sun is one of the stars--not by any means a specially large or bright one; sirius we now know to be twenty times as big as the sun. the sun is one of the stars: then is it at rest? herschel asked this question and endeavoured to answer it. he succeeded in the most astonishing manner. it is, perhaps, his most remarkable discovery, and savours of intuition. this is how it happened. with imperfect optical means and his own eyesight to guide him, he considered and pondered over the proper motion of the stars as he had observed it, till he discovered a kind of uniformity running through it all. mixed up with irregularities and individualities, he found that in a certain part of the heavens the stars were on the whole opening out--separating slowly from each other; on the opposite side of the heavens they were on the average closing up--getting slightly nearer to each other; while in directions at right angles to this they were fairly preserving their customary distances asunder. now, what is the moral to be drawn from such uniformity of behaviour among unconnected bodies? surely that this part of their motion is only apparent--that it is we who are moving. travelling over a prairie bounded by a belt of trees, we should see the trees in our line of advance opening out, and those behind closing up; we should see in fact the same kind of apparent motion as herschel was able to detect among the stars: the opening out being most marked near the constellation hercules. the conclusion is obvious: the sun, with all its planets, must be steadily moving towards a point in the constellation hercules. the most accurate modern research has been hardly able to improve upon this statement of herschel's. possibly the solar system may ultimately be found to revolve round some other body, but what that is no one knows. all one can tell is the present direction of the majestic motion: since it was discovered it has continued unchanged, and will probably so continue for thousands of years. [illustration: fig. .--old drawing of the cluster in hercules.] and, finally, concerning the nebulæ. these mysterious objects exercised a strong fascination for herschel, and many are the speculations he indulges in concerning them. at one time he regards them all as clusters of stars, and the milky way as our cluster; the others he regards as other universes almost infinitely distant; and he proceeds to gauge and estimate the shape of our own universe or galaxy of suns, the milky way. later on, however, he pictures to himself the nebulæ as nascent suns: solar systems before they are formed. some he thinks have begun to aggregate, while some are still glowing gas. [illustration: fig. .--old drawing of the andromeda nebula.] he likens the heavens to a garden in which there are plants growing in all manner of different stages: some shooting, some in leaf, some in flower, some bearing seed, some decaying; and thus at one inspection we have before us the whole life-history of the plant. just so he thinks the heavens contain worlds, some old, some dead, some young and vigorous, and some in the act of being formed. the nebulæ are these latter, and the nebulous stars are a further stage in the condensation towards a sun. and thus, by simple observation, he is led towards something very like the nebular hypothesis of laplace; and his position, whether it be true or false, is substantially the same as is held to-day. [illustration: fig. .--the great nebula in orion.] we _know_ now that many of the nebulæ consist of innumerable isolated particles and may be spoken of as gas. we know that some are in a state of whirling motion. we know also that such gas left to itself will slowly as it cools condense and shrink, so as to form a central solid nucleus; and also, if it were in whirling motion, that it would send off rings from itself, and that these rings could break up into planets. in two familiar cases the ring has not yet thus aggregated into planet or satellite--the zone of asteroids, and saturn's ring. the whole of this could not have been asserted in herschel's time: for further information the world had to wait. these are the problems of modern astronomy--these and many others, which are the growth of this century, aye, and the growth of the last thirty or forty, and indeed of the last ten years. even as i write, new and very confirmatory discoveries are being announced. the milky way _does_ seem to have some affinity with our sun. and the chief stars of the constellation of orion constitute another family, and are enveloped in the great nebula, now by photography perceived to be far greater than had ever been imagined. what is to be the outcome of it all i know not; but sure i am of this, that the largest views of the universe that we are able to frame, and the grandest manner of its construction that we can conceive, are certain to pale and shrink and become inadequate when confronted with the truth. notes to lecture xiii bode's law.--write down the series , , , , , , &c.; add to each, and divide by ; you get the series: · · · · · · · · · mercury venus earth mars ---- jupiter saturn uranus ---- numbers which very fairly represent the distances of the then known planets from the sun in the order specified. ceres was discovered on the st of january, , by piazzi; pallas in march, , by olbers; juno in , by harding; and vesta in , by olbers. no more asteroids were discovered till , but there are now several hundreds known. their diameters range from to miles. neptune was discovered from the perturbations of uranus by sheer calculation, carried on simultaneously and independently by leverrier in paris, and adams in cambridge. it was first knowingly seen by galle, of berlin, on the rd of september, . lecture xiii the discovery of the asteroids up to the time of herschel, astronomical interest centred on the solar system. since that time it has been divided, and a great part of our attention has been given to the more distant celestial bodies. the solar system has by no means lost its interest--it has indeed gained in interest continually, as we gain in knowledge concerning it; but in order to follow the course of science it will be necessary for us to oscillate to and fro, sometimes attending to the solar system--the planets and their satellites--sometimes extending our vision to the enormously more distant stellar spaces. those who have read the third lecture in part i. will remember the speculation in which kepler indulged respecting the arrangements of the planets, the order in which they succeeded one another in space, and the law of their respective distances from the sun; and his fanciful guess about the five regular solids inscribed and circumscribed about their orbits. the rude coincidences were, however, accidental, and he failed to discover any true law. no thoroughly satisfactory law is known at the present day. and yet, if the nebular hypothesis or anything like it be true, there must be some law to be discovered hereafter, though it may be a very complicated one. an empirical relation is, however, known: it was suggested by tatius, and published by bode, of berlin, in . it is always known as bode's law. bode's law asserts that the distance of each planet is approximately double the distance of the inner adjacent planet from the sun, but that the rate of increase is distinctly slower than this for the inner ones; consequently a better approximation will be obtained by adding a constant to each term of an appropriate geometrical progression. thus, form a doubling series like this, - / , , , , , &c. doubling each time; then add to each, and you get a series which expresses very fairly the relative distances of the successive planets from the sun, except that the number for mercury is rather erroneous, and we now know that at the other extreme the number for neptune is erroneous too. i have stated it in the notes above in a form calculated to give the law every chance, and a form that was probably fashionable after the discovery of uranus; but to call the first term of the doubling series is evidently not quite fair, though it puts mercury's distance right. neptune's distance, however, turns out to be more nearly times the earth's distance than · . the others are very nearly right: compare column d of the table preceding lecture iii. on p. , with the numbers in the notes on p. . the discovery of uranus a few years afterwards, in , at · times the earth's distance from the sun, lent great _éclât_ to the law, and seemed to establish its right to be regarded as at least a close approximation to the truth. the gap between mars and jupiter, which had often been noticed, and which kepler filled with a hypothetical planet too small to see, comes into great prominence by this law of bode. so much so, that towards the end of last century an enthusiastic german, von zach, after some search himself for the expected planet, arranged a committee of observing astronomers, or, as he termed it, a body of astronomical detective police, to begin a systematic search for this missing subject of the sun. [illustration: fig. .--planetary orbits to scale; showing the asteroidal region between jupiter and mars. (the orbits of satellites are exaggerated.)] in the preliminaries were settled: the heavens near the zodiac were divided into twenty-four regions, each of which was intrusted to one observer to be swept. meanwhile, however, quite independently of these arrangements in germany, and entirely unknown to this committee, a quiet astronomer in sicily, piazzi, was engaged in making a catalogue of the stars. his attention was directed to a certain region in taurus by an error in a previous catalogue, which contained a star really non-existent. in the course of his scrutiny, on the st of january, , he noticed a small star which next evening appeared to have shifted. he watched it anxiously for successive evenings, and by the th of january he was quite sure he had got hold of some moving body, not a star: probably, he thought, a comet. it was very small, only of the eighth magnitude; and he wrote to two astronomers (one of them bode himself) saying what he had observed. he continued to observe till the th of february, when he was attacked by illness and compelled to cease. his letters did not reach their destination till the end of march. directly bode opened his letter he jumped to the conclusion that this must be the missing planet. but unfortunately he was unable to verify the guess, for the object, whatever it was, had now got too near the sun to be seen. it would not be likely to be out again before september, and by that time it would be hopelessly lost again, and have just as much to be rediscovered as if it had never been seen. mathematical astronomers tried to calculate a possible orbit for the body from the observations of piazzi, but the observed places were so desperately few and close together. it was like having to determine a curve from three points close together. three observations ought to serve,[ ] but if they are taken with insufficient interval between them it is extremely difficult to construct the whole circumstances of the orbit from them. all the calculations gave different results, and none were of the slightest use. the difficulty as it turned out was most fortunate. it resulted in the discovery of one of the greatest mathematicians, perhaps the greatest, that germany has ever produced--gauss. he was then a young man of twenty-five, eking out a living by tuition. he had invented but not published several powerful mathematical methods (one of them now known as "the method of least squares"), and he applied them to piazzi's observations. he was thus able to calculate an orbit, and to predict a place where, by the end of the year, the planet should be visible. on the st of december of that same year, very near the place predicted by gauss, von zach rediscovered it, and olbers discovered it also the next evening. piazzi called it ceres, after the tutelary goddess of sicily. its distance from the sun as determined by gauss was · times the earth's distance. bode's law made it · . it was undoubtedly the missing planet. but it was only one hundred and fifty or two hundred miles in diameter--the smallest heavenly body known at the time of its discovery. it revolves the same way as other planets, but the plane of its orbit is tilted ° to the plane of the ecliptic, which was an exceptionally large amount. very soon, a more surprising discovery followed. olbers, while searching for ceres, had carefully mapped the part of the heavens where it was expected; and in march, , he saw in this place a star he had not previously noticed. in two hours he detected its motion, and in a month he sent his observations to gauss, who returned as answer the calculated orbit. it was distant · , like ceres, and was a little smaller, but it had a very excentric orbit: its plane being tilted - / °, an extraordinary inclination. this was called pallas. olbers at once surmised that these two planets were fragments of a larger one, and kept an eager look out for other fragments. in two years another was seen, in the course of charting the region of the heavens traversed by ceres and pallas. it was smaller than either, and was called juno. in the persevering search of olbers resulted in the discovery of another, with a very oblique orbit, which gauss named vesta. vesta is bigger than any of the others, being five hundred miles in diameter, and shines like a star of the sixth magnitude. gauss by this time had become so practised in the difficult computations that he worked out the complete orbit of vesta within ten hours of receiving the observational data from olbers. for many weary years olbers kept up a patient and unremitting search for more of these small bodies, or fragments of the large planet as he thought them; but his patience went unrewarded, and he died in without seeing or knowing of any more. in another was found, however, in germany, and a few weeks later two others by mr. hind in england. since then there seems no end to them; numbers have been discovered in america, where professors peters and watson have made a specialty of them, and have themselves found something like a hundred. vesta is the largest--its area being about the same as that of central europe, without russia or spain--and the smallest known is about twenty miles in diameter, or with a surface about the size of kent. the whole of them together do not nearly equal the earth in bulk. the main interest of these bodies to us lies in the question, what is their history? can they have been once a single planet broken up? or are they rather an abortive attempt at a planet never yet formed into one? the question is not _entirely_ settled, but i can tell you which way opinion strongly tends at the present time. imagine a shell travelling in an elliptic orbit round the earth to suddenly explode: the centre of gravity of all its fragments would continue moving along precisely the same path as had been traversed by the centre of the shell before explosion, and would complete its orbit quite undisturbed. each fragment would describe an orbit of its own, because it would be affected by a different initial velocity; but every orbit would be a simple ellipse, and consequently every piece would in time return through its starting-point--viz. the place at which the explosion occurred. if the zone of asteroids had a common point through which they all successively passed, they could be unhesitatingly asserted to be the remains of an exploded planet. but they have nothing of the kind; their orbits are scattered within a certain broad zone--a zone everywhere as broad as the earth's distance from the sun, , , miles--with no sort of law indicating an origin of this kind. it must be admitted, however, that the fragments of our supposed shell might in the course of ages, if left to themselves, mutually perturb each other into a different arrangement of orbits from that with which they began. but their perturbations would be very minute, and moreover, on laplace's theory, would only result in periodic changes, provided each mass were rigid. it is probable that the asteroids were at one time not rigid, and hence it is difficult to say what may have happened to them; but there is not the least reason to believe that their present arrangement is derivable in any way from an explosion, and it is certain that an enormous time must have elapsed since such an event if it ever occurred. it is far more probable that they never constituted one body at all, but are the remains of a cloudy ring thrown off by the solar system in shrinking past that point: a small ring after the immense effort which produced jupiter and his satellites: a ring which has aggregated into a multitude of little lumps instead of a few big ones. such an event is not unique in the solar system; there is a similar ring round saturn. at first sight, and to ordinary careful inspection, this differs from the zone of asteroids in being a solid lump of matter, like a quoit. but it is easy to show from the theory of gravitation, that a solid ring could not possibly be stable, but would before long get precipitated excentrically upon the body of the planet. devices have been invented, such as artfully distributed irregularities calculated to act as satellites and maintain stability; but none of these things really work. nor will it do to imagine the rings fluid; they too would destroy each other. the mechanical behaviour of a system of rings, on different hypotheses as to their constitution, has been worked out with consummate skill by clerk maxwell; who finds that the only possible constitution for saturn's assemblage of rings is a multitude of discrete particles each pursuing its independent orbit. saturn's ring is, in fact, a very concentrated zone of minor asteroids, and there is every reason to conclude that the origin of the solar asteroids cannot be very unlike the origin of the saturnian ones. the nebular hypothesis lends itself readily to both. the interlockings and motions of the particles in saturn's rings are most beautiful, and have been worked out and stated by maxwell with marvellous completeness. his paper constituted what is called "the adams prize essay" for . sir george airy, one of the adjudicators (recently astronomer-royal), characterized it as "one of the most remarkable applications of mathematics to physics that i have ever seen." there are several distinct constituent rings in the entire saturnian zone, and each perturbs the other, with the result that they ripple and pulse in concord. the waves thus formed absorb the effect of the mutual perturbations, and prevent an accumulation which would be dangerous to the persistence of the whole. the only effect of gravitational perturbation and of collisions is gradually to broaden out the whole ring, enlarging its outer and diminishing its inner diameter. but if there were any frictional resistance in the medium through which the rings spin, then other effects would slowly occur, which ought to be looked for with interest. so complete and intimate is the way maxwell works out and describes the whole circumstances of the motion of such an assemblage of particles, and so cogent his argument as to the necessity that they must move precisely so, and no otherwise, else the rings would not be stable, that it was a cambridge joke concerning him that he paid a visit to saturn one evening, and made his observations on the spot. notes to lecture xiv the total number of stars in the heavens visible to a good eye is about , . the total number at present seen by telescope is about , , . the number able to impress a photographic plate has not yet been estimated; but it is enormously greater still. of those which we can see in these latitudes, about are of the first magnitude, of the second, of the third, of the fourth, of the fifth, and , of the sixth; total, , . the quickest-moving stars known are a double star of the sixth magnitude, called cygni, and one of the seventh magnitude, called groombridge . the velocity of the latter is miles a second. the nearest known stars are cygni and [alpha] centauri. the distance of these from us is about , times the distance of the sun. their parallax is accordingly half a second of arc. sirius is more than a million times further from us than our sun is, and twenty times as big; many of the brightest stars are at more than double this distance. the distance of arcturus is too great to measure even now. stellar parallax was first securely detected in , by bessel, for cygni. bessel was born in , and died in , shortly before the discovery of neptune. the stars are suns, and are most likely surrounded by planets. one planet belonging to sirius has been discovered. it was predicted by bessel, its position calculated by peters, and seen by alvan clark in . another predicted one, belonging to procyon, has not yet been seen. a velocity of miles a second could carry a projectile right round the earth. a velocity of miles a second would carry it away from the earth, and round the sun. a velocity of miles a second would carry a projectile right out of the solar system never to return. lecture xiv bessel--the distances of the stars, and the discovery of stellar planets we will now leave the solar system for a time, and hastily sketch the history of stellar astronomy from the time of sir william herschel. you remember how greatly herschel had changed the aspect of the heavens for man,--how he had found that none of the stars were really fixed, but were moving in all manner of ways: some of this motion only apparent, much of it real. nevertheless, so enormously distant are they, that if we could be transported back to the days of the old chaldæan astronomers, or to the days of noah, we should still see the heavens with precisely the same aspect as they wear now. only by refined apparatus could any change be discoverable in all those centuries. for all practical purposes, therefore, the stars may still be well called fixed. another thing one may notice, as showing their enormous distances, is that from every planet of the solar system the aspect of the heavens will be precisely the same. inhabitants of mars, or jupiter, or saturn, or uranus, will see exactly the same constellations as we do. the whole dimensions of the solar system shrink up into a speck when so contemplated. and from the stars none of the planetary orbs of our system are visible at all; nothing but the sun is visible, and that merely as a twinkling star, brighter than some, but fainter than many others. the sun and the stars are one. try to realize this distinctly, and keep it in mind. i find it often difficult to drive this idea home. after some talk on the subject a friendly auditor will report, "the lecturer then described the stars, including that greatest and most magnificent of all stars, the sun." it would be difficult more completely to misapprehend the entire statement. when i say the sun is one of the stars, i mean one among the others; we are a long way from them, they are a long way from each other. they need be no more closely packed among each other than we are closely packed among them; except that some of them are double or multiple, and we are not double. it is highly desirable to acquire an intimate knowledge of the constellations and a nodding acquaintance with their principal stars. a description of their peculiarities is dull and uninteresting unless they are at least familiar by name. a little _vivâ voce_ help to begin with, supplemented by patient night scrutiny with a celestial globe or star maps under a tent or shed, is perhaps the easiest way: a very convenient instrument for the purpose of learning the constellations is the form of map called a "planisphere," because it can be made to show all the constellations visible at a given time at a given date, and no others. the greek alphabet also is a thing that should be learnt by everybody. the increased difficulty in teaching science owing to the modern ignorance of even a smattering of greek is becoming grotesque. the stars are named from their ancient grouping into constellations, and by the prefix of a greek letter to the larger ones, and of numerals to the smaller ones. the biggest of all have special arabic names as well. the brightest stars are called of "the first magnitude," the next are of "the second magnitude," and so on. but this arrangement into magnitudes has become technical and precise, and intermediate or fractional magnitudes are inserted. those brighter than the ordinary first magnitude are therefore now spoken of as of magnitude / , for instance, or · , which is rather confusing. small telescopic stars are often only named by their numbers in some specified catalogue--a dull but sufficient method. here is a list of the stars visible from these latitudes, which are popularly considered as of the first magnitude. all of them should be familiarly recognized in the heavens, whenever seen. star. constellation. sirius canis major procyon canis minor rigel orion betelgeux orion castor gemini pollux gemini aldebaran taurus arcturus boötes vega lyra capella auriga regulus leo altair aquila fomalhaut southern fish spica virgo [alpha] cygni is a little below the first magnitude. so, perhaps, is castor. in the southern heavens, canopus and [alpha] centauri rank next after sirius in brightness. [illustration: fig. .--diagram illustrating parallax.] the distances of the fixed stars had, we know, been a perennial problem, and many had been the attempts to solve it. all the methods of any precision have depended on the copernican fact that the earth in june was million miles away from its position in december, and that accordingly the grouping and aspect of the heavens should be somewhat different when seen from so different a point of view. an apparent change of this sort is called generally parallax; _the_ parallax of a star being technically defined as the angle subtended at the star by the radius of the earth's orbit: that is to say, the angle e[sigma]s; where e is the earth, s the sun, and [sigma] a star (fig. ). plainly, the further off [sigma] is, the more nearly parallel will the two lines to it become. and the difficulty of determining the parallax was just this, that the more accurately the observations were made, the more nearly parallel did those lines become. the angle was, in fact, just as likely to turn out negative as positive--an absurd result, of course, to be attributed to unavoidable very minute inaccuracies. for a long time absolute methods of determining parallax were attempted; for instance, by observing the position of the star with respect to the zenith at different seasons of the year. and many of these determinations appeared to result in success. hooke fancied he had measured a parallax for vega in this way, amounting to " of arc. flamsteed obtained " for [gamma] draconis. roemer made a serious attempt by comparing observations of vega and sirius, stars almost the antipodes of each other in the celestial vault; hoping to detect some effect due to the size of the earth's orbit, which should apparently displace them with the season of the year. all these fancied results however, were shown to be spurious, and their real cause assigned, by the great discovery of the aberration of light by bradley. after this discovery it was possible to watch for still outstanding very minute discrepancies; and so the problem of stellar parallax was attacked with fresh vigour by piazzi, by brinkley, and by struve. but when results were obtained, they were traced after long discussion to age and gradual wear of the instrument, or to some other minute inaccuracy. the more carefully the observation was made, the more nearly zero became the parallax--the more nearly infinite the distance of the stars. the brightest stars were the ones commonly chosen for the investigation, and vega was a favourite, because, going near the zenith, it was far removed from the fluctuating and tiresome disturbances of atmospheric refraction. the reason bright stars were chosen was because they were presumably nearer than the others; and indeed a rough guess at their probable distance was made by supposing them to be of the same size as the sun, and estimating their light in comparison with sunlight. by this confessedly unsatisfactory method it had been estimated that sirius must be , times further away than the sun is, if he be equally big. we now know that sirius is much further off than this; and accordingly that he is much brighter, perhaps sixty times as bright, though not necessarily sixty times as big, as our sun. but even supposing him of the same light-giving power as the sun, his parallax was estimated as "· , a quantity very difficult to be sure of in any absolute determination. relative methods were, however, also employed, and the advantages of one of these (which seems to have been suggested by galileo) so impressed themselves upon william herschel that he made a serious attempt to compass the problem by its means. the method was to take two stars in the same telescopic field and carefully to estimate their apparent angular distance from each other at different seasons of the year. all such disturbances as precession, aberration, nutation, refraction, and the like, would affect them both equally, and could thus be eliminated. if they were at the same distance from the solar system, relative parallax would, indeed, also be eliminated; but if, as was probable, they were at different distances, then they would apparently shift relatively to one another, and the amount of shift, if it could be observed, would measure, not indeed the distance of either from the earth, but their distance from each other. and this at any rate would be a step. it might be completed by similarly treating other stars in the same field, taking them in pairs together. a bright and a faint star would naturally be suitable, because their distances were likely to be unequal; and so herschel fixed upon a number of doublets which he knew of, containing one bright and one faint component. for up to that time it had been supposed that such grouping in occasional pairs or triplets was chance coincidence, the two being optically foreshortened together, but having no real connection or proximity. herschel failed in what he was looking for, but instead of that he discovered the real connection of a number of these doublets, for he found that they were slowly revolving round each other. there are a certain number of merely optical or accidental doublets, but the majority of them are real pairs of suns revolving round each other. this relative method of mapping micrometrically a field of neighbouring stars, and comparing their configuration now and six months hence, was, however, the method ultimately destined to succeed; and it is, i believe, the only method which has succeeded down to the present day. certainly it is the method regularly employed, at dunsink, at the cape of good hope, and everywhere else where stellar parallax is part of the work. between and the question was ripe for settlement, and, as frequently happens with a long-matured difficulty, it gave way in three places at once. bessel, henderson, and struve almost simultaneously announced a stellar parallax which could reasonably be accepted. bessel was a little the earliest, and by far the most accurate. his, indeed, was the result which commanded confidence, and to him the palm must be awarded. he was largely a self-taught student, having begun life in a counting-house, and having abandoned business for astronomy. but notwithstanding these disadvantages, he became a highly competent mathematician as well as a skilful practical astronomer. he was appointed to superintend the construction of germany's first great astronomical observatory, that of königsberg, which, by his system, zeal, and genius, he rapidly made a place of the first importance. struve at dorpat, bessel at königsberg, and henderson at the cape of good hope--all of them at newly-equipped observatories--were severally engaged at the same problem. but the russian and german observers had the advantage of the work of one of the most brilliant opticians--i suppose the most brilliant--that has yet appeared: fraunhofer, of munich. an orphan lad, apprenticed to a maker of looking-glasses, and subject to hard struggles and privations in early life, he struggled upwards, and ultimately became head of the optical department of a munich firm of telescope-makers. here he constructed the famous "dorpat refractor" for struve, which is still at work; and designed the "königsberg heliometer" for bessel. he also made a long and most skilful research into the solar spectrum, which has immortalized his name. but his health was broken by early trials, and he died at the age of thirty-nine, while planning new and still more important optical achievements. a heliometer is the most accurate astronomical instrument for relative measurements of position, as a transit circle is the most accurate for absolute determinations. it consists of an equatorial telescope with object-glass cut right across, and each half movable by a sliding movement one past the other, the amount by which the two halves are dislocated being read off by a refined method, and the whole instrument having a multitude of appendages conducive to convenience and accuracy. its use is to act as a micrometer or measurer of small distances.[ ] each half of the object-glass gives a distinct image, which may be allowed to coincide or may be separated as occasion requires. if it be the components of a double star that are being examined, each component will in general be seen double, so that four images will be seen altogether; but by careful adjustment it will be possible to arrange that one image of each pair shall be superposed on or coincide with each other, in which case only three images are visible; the amount of dislocation of the halves of the object-glass necessary to accomplish this is what is read off. the adjustment is one that can be performed with extreme accuracy, and by performing it again and again with all possible modifications, an extremely accurate determination of the angular distance between the two components is obtained. [illustration: fig. .--heliometer.] bessel determined to apply this beautiful instrument to the problem of stellar parallax; and he began by considering carefully the kind of star for which success was most likely. hitherto the brightest had been most attended to, but bessel thought that quickness of proper motion would be a still better test of nearness. not that either criterion is conclusive as to distance, but there was a presumption in favour of either a very bright or an obviously moving star being nearer than a faint or a stationary one; and as the "bright" criterion had already been often applied without result, he decided to try the other. he had already called attention to a record by piazzi in of a double star in cygnus whose proper motion was five seconds of arc every year--a motion which caused this telescopic object, cygni, to be known as "the flying star." its motion is not really very perceptible, for it will only have traversed one-third of a lunar diameter in the course of a century; still it was the quickest moving star then known. the position of this interesting double he compared with two other stars which were seen simultaneously in the field of the heliometer, by the method i have described, throughout the whole year ; and in the last month of that year he was able to announce with confidence a distinct though very small parallax; substantiating it with a mass of detailed evidence which commanded the assent of astronomers. the amount of it he gave as one-third of a second. we know now that he was very nearly right, though modern research makes it more like half a second.[ ] soon afterwards, struve announced a quarter of a second as the parallax of vega, but that is distinctly too great; and henderson announced for [alpha] centauri (then thought to be a double) a parallax of one second, which, if correct, would make it quite the nearest of all the stars, but the result is now believed to be about twice too big. knowing the distance of cygni, we can at once tell its real rate of travel--at least, its rate across our line of sight: it is rather over three million miles a day. now just consider the smallness of the half second of arc, thus triumphantly though only approximately measured. it is the angle subtended by twenty-six feet at a distance of , miles. if a telescope planted at new york could be directed to a house in england, and be then turned so as to set its cross-wire first on one end of an ordinary room and then on the other end of the same room, it would have turned through half a second, the angle of greatest stellar parallax. or, putting it another way. if the star were as near us as new york is, the sun, on the same scale, would be nine paces off. as twenty-six feet is to the distance of new york, so is ninety-two million miles to the distance of the nearest fixed star. suppose you could arrange some sort of telegraphic vehicle able to carry you from here to new york in the tenth part of a second--_i.e._ in the time required to drop two inches--such a vehicle would carry you to the moon in twelve seconds, to the sun in an hour and a quarter. travelling thus continually, in twenty-four hours you would leave the last member of the solar system behind you, and begin your plunge into the depths of space. how long would it be before you encountered another object? a month, should you guess? twenty years you must journey with that prodigious speed before you reach the nearest star, and then another twenty years before you reach another. at these awful distances from one another the stars are scattered in space, and were they not brilliantly self-luminous and glowing like our sun, they would be hopelessly invisible. i have spoken of cygni as a flying star, but there is another which goes still quicker, a faint star, in groombridge's catalogue. its distance is far greater than that of cygni, and yet it is seen to move almost as quickly. its actual speed is about miles a second--greater than the whole visible firmament of fifty million stars can control; and unless the universe is immensely larger than anything we can see with the most powerful telescopes, or unless there are crowds of invisible non-luminous stars mixed up with the others, it can only be a temporary visitor to this frame of things; it is rushing from an infinite distance to an infinite distance; it is passing through our visible universe for the first and only time--it will never return. but so gigantic is the extent of visible space, that even with its amazing speed of miles every second, this star will take two or three million years to get out of sight of our present telescopes, and several thousand years before it gets perceptibly fainter than it is now. have we any reason for supposing that the stars we see are all there are? in other words, have we any reason for supposing all celestial objects to be sufficiently luminous to be visible? we have every ground for believing the contrary. every body in the solar system is dull and dark except the sun, though probably jupiter is still red-hot. why may not some of the stars be dark too? the genius of bessel surmised this, and consistently upheld the doctrine that the astronomy of the future would have to concern itself with dark and invisible bodies; he preached "an astronomy of the invisible." moreover he predicted the presence of two such dark bodies--one a companion of sirius, the other of procyon. he noticed certain irregularities in the motions of these stars which he asserted must be caused by their revolving round other bodies in a period of half a century. he announced in that both sirius and procyon were double stars, but that their companions, though large, were dark, and therefore invisible. no one accepted this view, till peters, in america, found in that the hypothesis accurately explained the anomalous motion of sirius, and, in fact, indicated an exact place where the companion ought to be. the obscure companion of sirius became now a recognized celestial object, although it had never been seen, and it was held to revolve round sirius in fifty years, and to be about half as big. in , the firm of alvan clark and sons, of new york, were completing a magnificent -inch refractor, and the younger clark was trying it on sirius, when he said: "why, father, the star has a companion!" the elder clark also looked, and sure enough there was a faint companion due east of the bright star, and in just the position required by theory. not that the clarks knew anything about the theory. they were keen-sighted and most skilful instrument-makers, and they made the discovery by accident. after it had once been seen, it was found that several of the large telescopes of the world were able to show it. it is half as big, but it only gives / th part of the light that sirius gives. no doubt it shines partly with a borrowed light and partly with a dull heat of its own. it is a real planet, but as yet too hot to live on. it will cool down in time, as our earth has cooled and as jupiter is cooling, and no doubt become habitable enough. it does revolve round sirius in a period of · years--almost exactly what bessel assigned to it. but bessel also assigned a dark companion to procyon. it and its luminous neighbour are considered to revolve round each other in a period of forty years, and astronomers feel perfectly assured of its existence, though at present it has not been seen by man. lecture xv the discovery of neptune we approach to-night perhaps the greatest, certainly the most conspicuous, triumphs of the theory of gravitation. the explanation by newton of the observed facts of the motion of the moon, the way he accounted for precession and nutation and for the tides, the way in which laplace explained every detail of the planetary motions--these achievements may seem to the professional astronomer equally, if not more, striking and wonderful; but of the facts to be explained in these cases the general public are necessarily more or less ignorant, and so no beauty or thoroughness of treatment appeals to them, nor can excite their imaginations. but to predict in the solitude of the study, with no weapons other than pen, ink, and paper, an unknown and enormously distant world, to calculate its orbit when as yet it had never been seen, and to be able to say to a practical astronomer, "point your telescope in such a direction at such a time, and you will see a new planet hitherto unknown to man"--this must always appeal to the imagination with dramatic intensity, and must awaken some interest in almost the dullest. prediction is no novelty in science; and in astronomy least of all is it a novelty. thousands of years ago, thales, and others whose very names we have forgotten, could predict eclipses with some certainty, though with only rough accuracy. and many other phenomena were capable of prediction by accumulated experience. we have seen, for instance (coming to later times), how a gap between mars and jupiter caused a missing planet to be suspected and looked for, and to be found in a hundred pieces. we have seen, also, how the abnormal proper-motion of sirius suggested to bessel the existence of an unseen companion. and these last instances seem to approach very near the same class of prediction as that of the discovery of neptune. wherein, then, lies the difference? how comes it that some classes of prediction--such as that if you put your finger in fire it will get burnt--are childishly easy and commonplace, while others excite in the keenest intellects the highest feelings of admiration? mainly, the difference lies, first, in the grounds on which the prediction is based; second, on the difficulty of the investigation whereby it is accomplished; third, in the completeness and the accuracy with which it can be verified. in all these points, the discovery of neptune stands out pre-eminently among the verified predictions of science, and the circumstances surrounding it are of singular interest. * * * * * in , sir william herschel discovered the planet uranus. now you know that three distinct observations suffice to determine the orbit of a planet completely, and that it is well to have the three observations as far apart as possible so as to minimize the effects of minute but necessary errors of observation. (see p. .) directly uranus was found, therefore, old records of stellar observations were ransacked, with the object of discovering whether it had ever been unwittingly seen before. if seen, it had been thought of course to be a star (for it shines like a star of the sixth magnitude, and can therefore be just seen without a telescope if one knows precisely where to look for it, and if one has good sight), but if it had been seen and catalogued as a star it would have moved from its place, and the catalogue would by that entry be wrong. the thing to detect, therefore, was errors in the catalogues: to examine all entries, and see if the stars entered actually existed, or were any of them missing. if a wrong entry were discovered, it might of course have been due to some clerical error, though that is hardly probable considering the care taken over these things, or it might have been some tailless comet or other, or it might have been the newly found planet. so the next thing was to calculate backwards, and see if by any possibility the planet could have been in that place at that time. examined in this way the tabulated observations of flamsteed showed that he had unwittingly observed uranus five distinct times, the first time in , nearly a century before herschel discovered its true nature. but more remarkable still, le monnier, of paris, had observed it eight times in one month, cataloguing it each time as a different star. if only he had reduced and compared his observations, he would have anticipated herschel by twelve years. as it was, he missed it altogether. it was seen once by bradley also. altogether it had been seen twenty times. these old observations of flamsteed and those of le monnier, combined with those made after herschel's discovery, were very useful in determining an exact orbit for the new planet, and its motion was considered thoroughly known. it was not an _exact_ ellipse, of course: none of the planets describe _exact_ ellipses--each perturbs all the rest, and these small perturbations must be taken into account, those of jupiter and saturn being by far the most important. for a time uranus seemed to travel regularly and as expected, in the orbit which had been calculated for it; but early in the present century it began to be slightly refractory, and by its actual place showed quite a distinct discrepancy from its position as calculated with the aid of the old observations. it was at first thought that this discrepancy must be due to inaccuracies in the older observations, and they were accordingly rejected, and tables prepared for the planet based on the newer and more accurate observations only. but by it became apparent that it would not accurately obey even these. the error amounted to some ". by it was as much as ', or a minute and a half. this discrepancy is quite distinct, but still it is very small, and had two objects been in the heavens at once, the actual uranus and the theoretical uranus, no unaided eye could possibly have distinguished them or detected that they were other than a single star. [illustration: fig. .--perturbations of uranus. the chance observations by flamsteed, by le monnier, and others, are plotted in this diagram, as well as the modern determinations made after herschel had discovered the nature of the planet. the decades are laid off horizontally. vertical distance represents the difference between observed and subsequently calculated longitudes--in other words, the principal perturbations caused by neptune. to show the scale, a number of standard things are represented too by lengths measured upwards from the line of time, viz: the smallest quantity perceptible to the naked eye,--the maximum angle of aberration, of nutation, and of stellar parallax; though this last is too small to be properly indicated. the perturbations are much bigger than these; but compared with what can be seen without a telescope they are small--the distance between the component pairs of [epsilon] lyræ ( ") (see fig. , page ), which a few keen-eyed persons can see as a simple double star, being about twice the greatest perturbation.] the diagram shows all the irregularities plotted in the light of our present knowledge; and, to compare with their amounts, a few standard things are placed on the same scale, such as the smallest interval capable of being detected with the unaided eye, the distance of the component stars in [epsilon] lyræ, the constants of aberration, of nutation, and of stellar parallax. the errors of uranus therefore, though small, were enormously greater than things which had certainly been observed; there was an unmistakable discrepancy between theory and observation. some cause was evidently at work on this distant planet, causing it to disagree with its motion as calculated according to the law of gravitation. some thought that the exact law of gravitation did not apply to so distant a body. others surmised the presence of some foreign and unknown body, some comet, or some still more distant planet perhaps, whose gravitative attraction for uranus was the cause of the whole difficulty--some perturbations, in fact, which had not been taken into account because of our ignorance of the existence of the body which caused them. but though such an idea was mentioned among astronomers, it was not regarded with any special favour, and was considered merely as one among a number of hypotheses which could be suggested as fairly probable. it is perfectly right not to attach much importance to unelaborated guesses. not until the consequences of an hypothesis have been laboriously worked out--not until it can be shown capable of producing the effect quantitatively as well as qualitatively--does its statement rise above the level of a guess, and attain the dignity of a theory. a later stage still occurs when the theory has been actually and completely verified by agreement with observation. now the errors in the motion of uranus, _i.e._ the discrepancy between its observed and calculated longitudes--all known disturbing causes, such as jupiter and saturn, being allowed for--are as follows (as quoted by dr. haughton) in seconds of arc:-- ancient observations (casually made, as of a star). flamsteed + · " + · " + · le monnier - · bradley - · mayer - · le monnier - · " - · " - · modern observations. + · + · + · + · + · + · + · + · + · - · - · - · - · these are the numbers plotted in the above diagram (fig. ), where h marks the discovery of the planet and the beginning of its regular observation. something was evidently the matter with the planet. if the law of gravitation held exactly at so great a distance from the sun, there must be some perturbing force acting on it besides all those known ones which had been fully taken into account. could it be an outer planet? the question occurred to several, and one or two tried if they could solve the problem, but were soon stopped by the tremendous difficulties of calculation. the ordinary problem of perturbation is difficult enough: given a disturbing planet in such and such a position, to find the perturbations it produces. this problem it was that laplace worked out in the _mécanique céleste_. but the inverse problem: given the perturbations, to find the planet which causes them--such a problem had never yet been attacked, and by only a few had its possibility been conceived. bessel made preparations for trying what he could do at it in , but he was prevented by fatal illness. in the difficulties of the problem presented by these residual perturbations of uranus excited the imagination of a young student, an undergraduate of st. john's college, cambridge--john couch adams by name--and he determined to have a try at it as soon as he was through his tripos. in january, , he graduated as senior wrangler, and shortly afterwards he set to work. in less than two years he reached a definite conclusion; and in october, , he wrote to the astronomer-royal, at greenwich, professor airy, saying that the perturbations of uranus would be explained by assuming the existence of an outer planet, which he reckoned was now situated in a specified latitude and longitude. we know now that had the astronomer-royal put sufficient faith in this result to point his big telescope to the spot indicated and commence sweeping for a planet, he would have detected it within - / ° of the place assigned to it by mr. adams. but any one in the position of the astronomer-royal knows that almost every post brings an absurd letter from some ambitious correspondent or other, some of them having just discovered perpetual motion, or squared the circle, or proved the earth flat, or discovered the constitution of the moon, or of ether, or of electricity; and out of this mass of rubbish it requires great skill and patience to detect such gems of value as there may be. now this letter of mr. adams's was indeed a jewel of the first water, and no doubt bore on its face a very different appearance from the chaff of which i have spoken; but still mr. adams was an unknown man: he had graduated as senior wrangler it is true, but somebody must graduate as senior wrangler every year, and every year by no means produces a first-rate mathematician. those behind the scenes, as professor airy of course was, having been a senior wrangler himself, knew perfectly well that the labelling of a young man on taking his degree is much more worthless as a testimony to his genius and ability than the general public are apt to suppose. was it likely that a young and unknown man should have successfully solved so extremely difficult a problem? it was altogether unlikely. still, he would test him: he would ask for further explanations concerning some of the perturbations which he himself had specially noticed, and see if mr. adams could explain these also by his hypothesis. if he could, there might be something in his theory. if he failed--well, there was an end of it. the questions were not difficult. they concerned the error of the radius vector. mr. adams could have answered them with perfect ease; but sad to say, though a brilliant mathematician, he was not a man of business. he did not answer professor airy's letter. it may to many seem a pity that the greenwich equatoreal was not pointed to the place, just to see whether any foreign object did happen to be in that neighbourhood; but it is no light matter to derange the work of an observatory, and alter the work mapped out for the staff into a sudden sweep for a new planet, on the strength of a mathematical investigation just received by post. if observatories were conducted on these unsystematic and spasmodic principles, they would not be the calm, accurate, satisfactory places they are. of course, if any one could have known that a new planet was to be had for the looking, _any_ course would have been justified; but no one could know this. i do not suppose that mr. adams himself could feel all that confidence in his attempted prediction. so there the matter dropped. mr. adams's communication was pigeon-holed, and remained in seclusion for eight or nine months. meanwhile, and quite independently, something of the same sort was going on in france. a brilliant young mathematician, born in normandy in , had accepted the post of astronomical professor at the École polytechnique, then recently founded by napoleon. his first published papers directed attention to his wonderful powers; and the official head of astronomy in france, the famous arago, suggested to him the unexplained perturbations of uranus as a worthy object for his fresh and well-armed vigour. at once he set to work in a thorough and systematic way. he first considered whether the discrepancies could be due to errors in the tables or errors in the old observations. he discussed them with minute care, and came to the conclusion that they were not thus to be explained away. this part of the work he published in november, . he then set to work to consider the perturbations produced by jupiter and saturn, to see if they had been with perfect accuracy allowed for, or whether some minute improvements could be made sufficient to destroy the irregularities. he introduced several fresh terms into these perturbations, but none of them of sufficient magnitude to do more than slightly lessen the unexplained perturbations. he next examined the various hypotheses that had been suggested to account for them:--was it a failure in the law of gravitation? was it due to the presence of a resisting medium? was it due to some unseen but large satellite? or was it due to a collision with some comet? all these he examined and dismissed for various reasons one after the other. it was due to some steady continuous cause--for instance, some unknown planet. could this planet be inside the orbit of uranus? no, for then it would perturb saturn and jupiter also, and they were not perturbed by it. it must, therefore, be some planet outside the orbit of uranus, and in all probability, according to bode's empirical law, at nearly double the distance from the sun that uranus is. lastly he proceeded to examine where this planet was, and what its orbit must be to produce the observed disturbances. [illustration: fig. .--uranus's and neptune's relative positions. the above diagram, drawn to scale by dr. haughton, shows the paths of uranus and neptune, and their positions from to , and illustrates the _direction_ of their mutual perturbing force. in the planets were in conjunction, and the force would then perturb the radius vector (or distance from the sun), but not the longitude (or place in orbit). before that date uranus had been hurried along, and after that date it had been retarded, by the pull of neptune, and thus the observed discrepancies from its computed place were produced. the problem was first to disentangle the outstanding perturbations from those which would be caused by jupiter and saturn and all other known causes, and then to assign the place of an outer planet able to produce precisely those perturbations in uranus.] not without failures and disheartening complications was this part of the process completed. this was, after all, the real tug of war. so many unknown quantities: its mass, its distance, its excentricity, the obliquity of its orbit, its position at any time--nothing known, in fact, about the planet except the microscopic disturbance it caused in uranus, some thousand million miles away from it. without going into further detail, suffice it to say that in june, , he published his last paper, and in it announced to the world his theoretical position for the planet. professor airy received a copy of this paper before the end of the month, and was astonished to find that leverrier's theoretical place for the planet was within ° of the place mr. adams had assigned to it eight months before. so striking a coincidence seemed sufficient to justify a herschelian "sweep" for a week or two. but a sweep for so distant a planet would be no easy matter. when seen in a large telescope it would still only look like a star, and it would require considerable labour and watching to sift it out from the other stars surrounding it. we know that uranus had been seen twenty times, and thought to be a star, before its true nature was by herschel discovered; and uranus is only about half as far away as neptune is. neither in paris nor yet at greenwich was any optical search undertaken; but professor airy wrote to ask m. leverrier the same old question as he had fruitlessly put to mr. adams: did the new theory explain the errors of the radius vector or not? the reply of leverrier was both prompt and satisfactory--these errors were explained, as well as all the others. the existence of the object was then for the first time officially believed in. the british association met that year at southampton, and sir john herschel was one of its sectional presidents. in his inaugural address, on september th, , he called attention to the researches of leverrier and adams in these memorable words:-- "the past year has given to us the new [minor] planet astræa; it has done more--it has given us the probable prospect of another. we see it as columbus saw america from the shores of spain. its movements have been felt trembling along the far-reaching line of our analysis with a certainty hardly inferior to ocular demonstration." it was about time to begin to look for it. so the astronomer-royal thought on reading leverrier's paper. but as the national telescope at greenwich was otherwise occupied, he wrote to professor challis, at cambridge, to know if he would permit a search to be made for it with the northumberland equatoreal, the large telescope of cambridge university, presented to it by one of the dukes of northumberland. professor challis said he would conduct the search himself; and shortly commenced a leisurely and dignified series of sweeps round about the place assigned by theory, cataloguing all the stars which he observed, intending afterwards to sort out his observations, compare one with another, and find out whether any one star had changed its position; because if it had it must be the planet. he thus, without giving an excessive time to the business, accumulated a host of observations, which he intended afterwards to reduce and sift at his leisure. the wretched man thus actually saw the planet twice--on august th and august th, --without knowing it. if only he had had a map of the heavens containing telescopic stars down to the tenth magnitude, and if he had compared his observations with this map as they were made, the process would have been easy, and the discovery quick. but he had no such map. nevertheless one was in existence: it had just been completed in that country of enlightened method and industry--germany. dr. bremiker had not, indeed, completed his great work--a chart of the whole zodiac down to stars of the tenth magnitude--but portions of it were completed, and the special region where the new planet was expected happened to be among the portions already just done. but in england this was not known. meanwhile, mr. adams wrote to the astronomer-royal several additional communications, making improvements in his theory, and giving what he considered nearer and nearer approximations for the place of the planet. he also now answered quite satisfactorily, but too late, the question about the radius vector sent to him months before. let us return to leverrier. this great man was likewise engaged in improving his theory and in considering how best the optical search could be conducted. actuated, probably, by the knowledge that in such matters as cataloguing and mapping germany was then, as now, far ahead of all the other nations of the world, he wrote in september (the same september as sir john herschel delivered his eloquent address at southampton) to berlin. leverrier wrote, i say, to dr. galle, head of the observatory at berlin, saying to him, clearly and decidedly, that the new planet was now in or close to such and such a position, and that if he would point his telescope to that part of the heavens he would see it; and, moreover, that he would be able to tell it from a star by its having a sensible magnitude, or disk, instead of being a mere point. galle got the letter on the rd of september, . that same evening he did point his telescope to the place leverrier told him, and he saw the planet that very night. he recognized it first by its appearance. to his practised eye it did seem to have a small disk, and not quite the same aspect as an ordinary star. he then consulted bremiker's great star chart, the part just engraved and finished, and sure enough on that chart there was no such star there. undoubtedly it was the planet. the news flashed over europe at the maximum speed with which news could travel at that date (which was not very fast); and by the st of october professor challis and mr. adams heard it at cambridge, and had the pleasure of knowing that they were forestalled, and that england was out of the race. it was an unconscious race to all concerned, however. those in france knew nothing of the search going on in england. mr. adams's papers had never been published; and very annoyed the french were when a claim was set up on his behalf to a share in this magnificent discovery. controversies and recriminations, excuses and justifications, followed; but the discussion has now settled down. all the world honours the bright genius and mathematical skill of mr. adams, and recognizes that he first solved the problem by calculation. all the world, too, perceives clearly the no less eminent mathematical talents of m. leverrier, but it recognizes in him something more than the mere mathematician--the man of energy, decision, and character. lecture xvi comets and meteors we have now considered the solar system in several aspects, and we have passed in review something of what is known about the stars. we have seen how each star is itself, in all probability, the centre of another and distinct solar system, the constituents of which are too dark and far off to be visible to us; nothing visible here but the central sun alone, and that only as a twinkling speck. but between our solar system and these other suns--between each of these suns and all the rest--there exist vast empty spaces, apparently devoid of matter. we have now to ask, are these spaces really empty? is there really nothing in space but the nebulæ, the suns, their planets, and their satellites? are all the bodies in space of this gigantic size? may there not be an infinitude of small bodies as well? the answer to this question is in the affirmative. there appears to be no special size suited to the vastness of space; we find, as a matter of fact, bodies of all manner of sizes, ranging by gradations from the most tremendous suns, like sirius, down through ordinary suns to smaller ones, then to planets of all sizes, satellites still smaller, then the asteroids, till we come to the smallest satellite of mars, only about ten miles in diameter, and weighing only some billion tons--the smallest of the regular bodies belonging to the solar system known. but, besides all these, there are found to occur other masses, not much bigger and some probably smaller, and these we call comets when we see them. below these, again, we find masses varying from a few tons in weight down to only a few pounds or ounces, and these when we see them, which is not often, we call meteors or shooting-stars; and to the size of these meteorites there would appear to be no limit: some may be literal grains of dust. there seems to be a regular gradation of size, therefore, ranging from sirius to dust; and apparently we must regard all space as full of these cosmic particles--stray fragments, as it were, perhaps of some older world, perhaps going to help to form a new one some day. as kepler said, there are more "comets" in the sky than fish in the sea. not that they are at all crowded together, else they would make a cosmic haze. the transparency of space shows that there must be an enormous proportion of clear space between each, and they are probably much more concentrated near one of the big bodies than they are in interstellar space.[ ] even during the furious hail of meteors in november it was estimated that their average distance apart in the thickest of the shower was miles. consider the nature of a meteor or shooting-star. we ordinarily see them as a mere streak of light; sometimes they leave a luminous tail behind them; occasionally they appear as an actual fire-ball, accompanied by an explosion; sometimes, but very seldom, they are seen to drop, and may subsequently be dug up as a lump of iron or rock, showing signs of rough treatment by excoriation and heat. these last are the meteorites, or siderites, or aërolites, or bolides, of our museums. they are popularly spoken of as thunderbolts, though they have nothing whatever to do with atmospheric electricity. [illustration: fig. .--meteorite.] they appear to be travelling rocky or metallic fragments which in their journey through space are caught in the earth's atmosphere and instantaneously ignited by the friction. far away in the depths of space one of these bodies felt the attracting power of the sun, and began moving towards him. as it approached, its speed grew gradually quicker and quicker continually, until by the time it has approached to within the distance of the earth, it whizzes past with the velocity of twenty-six miles a second. the earth is moving on its own account nineteen miles every second. if the two bodies happened to be moving in opposite directions, the combined speed would be terrific; and the faintest trace of atmosphere, miles above the earth's surface, would exert a furious grinding action on the stone. a stream of particles would be torn off; if of iron, they would burn like a shower of filings from a firework, thus forming a trail; and the mass itself would be dissipated, shattered to fragments in an instant. [illustration: fig. .--meteor stream crossing field of telescope.] [illustration: fig. .--diagram of direction of earth's orbital motion, showing that after midnight, _i.e._ between midnight and noon, more asteroids are likely to be swept up by any locality than between noon and midnight. [from sir r.s. ball.]] even if the earth were moving laterally, the same thing would occur. but if earth and stone happened to be moving in the same direction, there would be only the differential velocity of seven miles a second; and though this is in all conscience great enough, yet there might be a chance for a residue of the nucleus to escape entire destruction, though it would be scraped, heated, and superficially molten by the friction; but so much of its speed would be rubbed out of it, that on striking the earth it might bury itself only a few feet or yards in the soil, so that it could be dug out. the number of those which thus reach the earth is comparatively infinitesimal. nearly all get ground up and dissipated by the atmosphere; and fortunate it is for us that they are so. this bombardment of the exposed face of the moon must be something terrible.[ ] thus, then, every shooting-star we see, and all the myriads that we do not and cannot see because they occur in the day-time, all these bright flashes or streaks, represent the death and burial of one of these flying stones. it had been careering on its own account through space for untold ages, till it meets a planet. it cannot strike the actual body of the planet--the atmosphere is a sufficient screen; the tremendous friction reduces it to dust in an instant, and this dust then quietly and leisurely settles down on to the surface. evidence of the settlement of meteoric dust is not easy to obtain in such a place as england, where the dust which accumulates is seldom of a celestial character; but on the snow-fields of greenland or the himalayas dust can be found; and by a committee of the british association distinct evidence of molten globules of iron and other materials appropriate to aërolites has been obtained, by the simple process of collecting, melting, and filtering long exposed snow. volcanic ash may be mingled with it, but under the microscope the volcanic and the meteoric constituents have each a distinctive character. the quantity of meteoric material which reaches the earth as dust must be immensely in excess of the minute quantity which arrives in the form of lumps. hundreds or thousands of tons per annum must be received; and the accretion must, one would think, in the course of ages be able to exert some influence on the period of the earth's rotation--the length of the day. it is too small, however, to have been yet certainly detected. possibly, it is altogether negligible. it has been suggested that those stones which actually fall are not the true cosmic wanderers, but are merely fragments of our own earth, cast up by powerful volcanoes long ago when the igneous power of the earth was more vigorous than now--cast up with a speed of close upon seven miles a second; and now in these quiet times gradually being swept up by the earth, and so returning whence they came. i confess i am unable to draw a clear distinction between one set and the other. some falling stars may have had an origin of this sort, but certainly others have not; and it would seem very unlikely that one set only should fall bodily upon the earth, while the others should always be rubbed to powder. still, it is a possibility to be borne in mind. we have spoken of these cosmic visitors as wandering masses of stone or iron; but we should be wrong if we associated with the term "wandering" any ideas of lawlessness and irregularity of path. these small lumps of matter are as obedient to the law of gravity as any large ones can be. they must all, therefore, have definite orbits, and these orbits will have reference to the main attracting power of our system--they will, in fact, be nearly all careering round the sun. each planet may, in truth, have a certain following of its own. within the limited sphere of the earth's predominant attraction, for instance, extending some way beyond the moon, we may have a number of satellites that we never see, all revolving regularly in elliptic orbits round the earth. but, comparatively speaking, these satellite meteorites are few. the great bulk of them will be of a planetary character--they will be attendant upon the sun. it may seem strange that such minute bodies should have regular orbits and obey kepler's laws, but they must. all three laws must be as rigorously obeyed by them as by the planets themselves. there is nothing in the smallness of a particle to excuse it from implicit obedience to law. the only consequence of their smallness is their inability to perturb others. they cannot appreciably perturb either the planets they approach or each other. the attracting power of a lump one million tons in weight is very minute. a pound, on the surface of such a body of the same density as the earth, would be only pulled to it with a force equal to that with which the earth pulls a grain. so the perturbing power of such a mass on distant bodies is imperceptible. it is a good thing it is so: accurate astronomy would be impossible if we had to take into account the perturbations caused by a crowd of invisible bodies. astronomy would then approach in complexity some of the problems of physics. but though we may be convinced from the facts of gravitation that these meteoric stones, and all other bodies flying through space near our solar system, must be constrained by the sun to obey kepler's laws, and fly round it in some regular elliptic or hyperbolic orbit, what chance have we of determining that orbit? at first sight, a very poor chance, for we never see them except for the instant when they splash into our atmosphere; and for them that instant is instant death. it is unlikely that any escape that ordeal, and even if they do, their career and orbit are effectually changed. henceforward they must become attendants on the earth. they may drop on to its surface, or they may duck out of our atmosphere again, and revolve round us unseen in the clear space between earth and moon. nevertheless, although the problem of determining the original orbit of any given set of shooting-stars before it struck us would seem nearly insoluble, it has been solved, and solved with some approach to accuracy; being done by the help of observations of certain other bodies. the bodies by whose help this difficult problem has been attacked and resolved are comets. what are comets? i must tell you that the scientific world is not entirely and completely decided on the structure of comets. there are many floating ideas on the subject, and some certain knowledge. but the subject is still, in many respects, an open one, and the ideas i propose to advocate you will accept for no more than they are worth, viz. as worthy to be compared with other and different views. up to the time of newton, the nature of comets was entirely unknown. they were regarded with superstitious awe as fiery portents, and were supposed to be connected with the death of some king, or with some national catastrophe. even so late as the first edition of the _principia_ the problem of comets was unsolved, and their theory is not given; but between the first and the second editions a large comet appeared, in , and newton speculated on its appearance and behaviour. it rushed down very close to the sun, spun half round him very quickly, and then receded from him again. if it were a material substance, to which the law of gravitation applied, it must be moving in a conic section with the sun in one focus, and its radius vector must sweep out equal areas in equal times. examining the record of its positions made at observatories, he found its observed path quite accordant with theory; and the motion of comets was from that time understood. up to that time no one had attempted to calculate an orbit for a comet. they had been thought irregular and lawless bodies. now they were recognized as perfectly obedient to the law of gravitation, and revolving round the sun like everything else--as members, in fact, of our solar system, though not necessarily permanent members. but the orbit of a comet is very different from a planetary one. the excentricity of its orbit is enormous--in other words, it is either a very elongated ellipse or a parabola. the comet of , newton found to move in an orbit so nearly a parabola that the time of describing it must be reckoned in hundreds of years at the least. it is now thought possible that it may not be quite a parabola, but an ellipse so elongated that it will not return till . until that date arrives, however, uncertainty will prevail as to whether it is a periodic comet, or one of those that only visit our system once. if it be periodic, as suspected, it is the same as appeared when julius cæsar was killed, and which likewise appeared in the years and a.d. should it appear in , our posterity will probably regard it as a memorial of newton. [illustration: fig. .--parabolic and elliptic orbits. the _a b_ (visible) portions are indistinguishable.] the next comet discussed in the light of the theory of gravitation was the famous one of halley. you know something of the history of this. its period is - / years. halley saw it in , and predicted its return in or --the first cometary prediction. clairaut calculated its return right within a month (p. ). it has been back once more, in ; and this time its date was correctly predicted within three days, because uranus was now known. it was away at its furthest point in . it will be back again in . [illustration: fig. .--orbit of halley's comet.] coming to recent times, we have the great comets of and of , the history of neither being known. quite possibly they arrived then for the first time. possibly the second will appear again in . but besides these great comets, there are a multitude of telescopic ones, which do not show these striking features, and have no gigantic tail. some have no tail at all, others have at best a few insignificant streamers, and others show a faint haze looking like a microscopic nebula. all these comets are of considerable extent--some millions of miles thick usually, and yet stars are clearly visible through them. hence they must be matter of very small density; their tails can be nothing more dense than a filmy mist, but their nucleus must be something more solid and substantial. [illustration: fig. .--various appearances of halley's comet when last seen.] i have said that comets arrive from the depths of space, rush towards and round the sun, whizzing past the earth with a speed of twenty-six miles a second, on round the sun with a far greater velocity than that, and then rush off again. now, all the time they are away from the sun they are invisible. it is only as they get near him that they begin to expand and throw off tails and other appendages. the sun's heat is evidently evaporating them, and driving away a cloud of mist and volatile matter. this is when they can be seen. the comet is most gorgeous when it is near the sun, and as soon as it gets a reasonable distance away from him it is perfectly invisible. the matter evaporated from the comet by the sun's heat does not return--it is lost to the comet; and hence, after a few such journeys, its volatile matter gets appreciably diminished, and so old-established periodic comets have no tails to speak of. but the new visitants, coming from the depths of space for the first time--these have great supplies of volatile matter, and these are they which show the most magnificent tails. [illustration: fig. .--head of donati's comet of .] the tail of a comet is always directed away from the sun as if it were repelled. to this rule there is no exception. it is suggested, and held as most probable, that the tail and sun are similarly electrified, and that the repulsion of the tail is electrical repulsion. some great force is obviously at work to account for the enormous distance to which the tail is shot in a few hours. the pressure of the sun's light can do something, and is a force that must not be ignored when small particles are being dealt with. (cf. _modern views of electricity_, nd edition, p. .) now just think what analogies there are between comets and meteors. both are bodies travelling in orbits round the sun, and both are mostly invisible, but both become visible to us under certain circumstances. meteors become visible when they plunge into the extreme limits of our atmosphere. comets become visible when they approach the sun. is it possible that comets are large meteors which dip into the solar atmosphere, and are thus rendered conspicuously luminous? certainly they do not dip into the actual main atmosphere of the sun, else they would be utterly destroyed; but it is possible that the sun has a faint trace of atmosphere extending far beyond this, and into this perhaps these meteors dip, and glow with the friction. the particles thrown off might be, also by friction, electrified; and the vaporous tail might be thus accounted for. [illustration: fig. .--halley's comet.] let us make this hypothesis provisionally--that comets are large meteors, or a compact swarm of meteors, which, coming near the sun, find a highly rarefied sort of atmosphere, in which they get heated and partly vaporized, just as ordinary meteorites do when they dip into the atmosphere of the earth. and let us see whether any facts bear out the analogy and justify the hypothesis. i must tell you now the history of three bodies, and you will see that some intimate connection between comets and meteors is proved. the three bodies are known as, first, encke's comet; second, biela's comet; third, the november swarm of meteors. encke's comet (one of those discovered by miss herschel) is an insignificant-looking telescopic comet of small period, the orbit of which was well known, and which was carefully observed at each reappearance after encke had calculated its orbit. it was the quickest of the comets, returning every - / years. [illustration: fig. .--encke's comet.] it was found, however, that its period was not quite constant; it kept on getting slightly shorter. the comet, in fact, returned to the sun slightly before its time. now this effect is exactly what friction against a solar atmosphere would bring about. every time it passed near the sun a little velocity would be rubbed out of it. but the velocity is that which carries it away, hence it would not go quite so far, and therefore would return a little sooner. any revolving body subject to friction must revolve quicker and quicker, and get nearer and nearer its central body, until, if the process goes on long enough, it must drop upon its surface. this seems the kind of thing happening to encke's comet. the effect is very small, and not thoroughly proved; but, so far as it goes, the evidence points to a greatly extended rare solar atmosphere, which rubs some energy out of it at every perihelion passage. [illustration: fig. .--biela's comet as last seen, in two portions.] next, biela's comet. this also was a well known and carefully observed telescopic comet, with a period of six years. in one of its distant excursions, it was calculated that it must pass very near jupiter, and much curiosity was excited as to what would happen to it in consequence of the perturbation it must experience. as i have said, comets are only visible as they approach the sun, and a watch was kept for it about its appointed time. it was late, but it did ultimately arrive. the singular thing about it, however, was that it was now double. it had apparently separated into two. this was in . it was looked for again in , and this time the components were further separated. sometimes one was brighter, sometimes the other. next time it ought to have come round no one could find either portion. the comet seemed to have wholly disappeared. it has never been seen since. it was then recorded and advertised as the missing comet. but now comes the interesting part of the story. the orbit of this biela comet was well known, and it was found that on a certain night in the earth would cross the orbit, and had some chance of encountering the comet. not a very likely chance, because it need not be in that part of its orbit at the time; but it was suspected not to be far off--if still existent. well, the night arrived, the earth did cross the orbit, and there was seen, not the comet, but a number of shooting-stars. not one body, nor yet two, but a multitude of bodies--in fact, a swarm of meteors. not a very great swarm, such as sometimes occurs, but still a quite noticeable one; and this shower of meteors is definitely recognized as flying along the track of biela's comet. they are known as the andromedes. this observation has been generalized. every cometary orbit is marked by a ring of meteoric stones travelling round it, and whenever a number of shooting-stars are seen quickly one after the other, it is an evidence that we are crossing the track of some comet. but suppose instead of only crossing the track of a comet we were to pass close to the comet itself, we should then expect to see an extraordinary swarm--a multitude of shooting-stars. such phenomena have occurred. the most famous are those known as the november meteors, or leonids. this is the third of those bodies whose history i had to tell you. professor h.a. newton, of america, by examining ancient records arrived at the conclusion that the earth passed through a certain definite meteor shoal every thirty-three years. he found, in fact, that every thirty-three years an unusual flight of shooting-stars was witnessed in november, the earliest record being a.d. their last appearance had been in , and he therefore predicted their return in or . sure enough, in november, , they appeared; and many must remember seeing that glorious display. although their hail was almost continuous, it is estimated that their average distance apart was thirty-five miles! their radiant point was and always is in the constellation leo, and hence their name leonids. [illustration: fig. .--radiant point perspective. the arrows represent a number of approximately parallel meteor-streaks foreshortened from a common vanishing-point.] a parallel stream fixed in space necessarily exhibits a definite aspect with reference to the fixed stars. its aspect with respect to the earth will be very changeable, because of the rotation and revolution of that body, but its position with respect to constellations will be steady. hence each meteor swarm, being a steady parallel stream of rushing masses, always strikes us from the same point in stellar space, and by this point (or radiant) it is identified and named. the paths do not appear to us to be parallel, because of perspective: they seem to radiate and spread in all directions from a fixed centre like spokes, but all these diverging streaks are really parallel lines optically foreshortened by different amounts so as to produce the radiant impression. the annexed diagram (fig. ) clearly illustrates the fact that the "radiant" is the vanishing point of a number of parallel lines. [illustration: fig. .--orbit of november meteors.] this swarm is specially interesting to us from the fact that we cross its orbit every year. its orbit and the earth's intersect. every november we go through it, and hence every november we see a few stragglers of this immense swarm. the swarm itself takes thirty-three years on its revolution round the sun, and hence we only encounter it every thirty-three years. the swarm is of immense size. in breadth it is such that the earth, flying nineteen miles a second, takes four or five hours to cross it, and this is therefore the time the display lasts. but in length it is far more enormous. the speed with which it travels is twenty-five miles a second, (for its orbit extends as far as uranus, although by no means parabolic), and yet it takes more than a year to pass. imagine a procession , miles broad, every individual rushing along at the rate of twenty-five miles every second, and the whole procession so long that it takes more than a year to pass. it is like a gigantic shoal of herrings swimming round and round the sun every thirty-three years, and travelling past the earth with that tremendous velocity of twenty-five miles a second. the earth dashes through the swarm and sweeps up myriads. think of the countless numbers swept up by the whole earth in crossing such a shoal as that! but heaps more remain, and probably the millions which are destroyed every thirty-three years have not yet made any very important difference to the numbers still remaining. the earth never misses this swarm. every thirty-three years it is bound to pass through some part of them, for the shoal is so long that if the head is just missed one november the tail will be encountered next november. this is a plain and obvious result of its enormous length. it may be likened to a two-foot length of sewing silk swimming round and round an oval sixty feet in circumference. but, you will say, although the numbers are so great that destroying a few millions or so every thirty-three years makes but little difference to them, yet, if this process has been going on from all eternity, they ought to be all swept up. granted; and no doubt the most ancient swarms have already all or nearly all been swept up. [illustration: fig. .--orbit of november meteors; showing their probable parabolic orbit previous to a.d., and its sudden conversion into an elliptic orbit by the violent perturbation caused by uranus, which at that date occupied the position shown.] the august meteors, or perseids, are an example. every august we cross their path, and we have a small meteoric display radiating from the sword-hand of perseus, but never specially more in one august than another. it would seem as if the main shoal has disappeared, and nothing is now left but the stragglers; or perhaps it is that the shoal has gradually become uniformly distributed all along the path. anyhow, these august meteors are reckoned much more ancient members of the solar system than are the november meteors. the november meteors are believed to have entered the solar system in the year a.d. this may seem an extraordinary statement. it is not final, but it is based on the calculations of leverrier--confirmed recently by mr. adams. a few moments will suffice to make the grounds of it clear. leverrier calculated the orbit of the november meteors, and found them to be an oval extending beyond uranus. it was perturbed by the outer planets near which it went, so that in past times it must have moved in a slightly different orbit. calculating back to their past positions, it was found that in a certain year it must have gone very near to uranus, and that by the perturbation of this planet its path had been completely changed. originally it had in all probability been a comet, flying in a parabolic orbit towards the sun like many others. this one, encountering uranus, was pulled to pieces as it were, and its orbit made elliptical as shown in fig. . it was no longer free to escape and go away into the depths of space: it was enchained and made a member of the solar system. it also ceased to be a comet; it was degraded into a shoal of meteors. this is believed to be the past history of this splendid swarm. since its introduction to the solar system it has made revolutions: its next return is due in november, , and i hope that it may occur in the english dusk, and (see fig. ) in a cloudless after-midnight sky, as it did in . notes for lecture xvii the tide-generating force of one body on another is directly as the mass of the one body and inversely as the cube of the distance between them. hence the moon is more effective in producing terrestrial tides than the sun. the tidal wave directly produced by the moon in the open ocean is about feet high, that produced by the sun is about feet. hence the average spring tide is to the average neap as about to . the lunar tide varies between apogee and perigee from · to · . the solar tide varies between aphelion and perihelion from · to · . hence the highest spring tide is to the lowest neap as · + · is to · - · , or as to · . the semi-synchronous oscillation of the southern ocean raises the magnitude of oceanic tides somewhat above these directly generated values. oceanic tides are true waves, not currents. coast tides are currents. the momentum of the water, when the tidal wave breaks upon a continent and rushes up channels, raises coast tides to a much greater height--in some places up to or feet, or even more. early observed connections between moon and tides would be these:-- st. spring tides at new and full moon. nd. average interval between tide and tide is half a lunar, not a solar, day--a lunar day being the interval between two successive returns of the moon to the meridian: hours and minutes. rd. the tides of a given place at new and full moon occur always at the same time of day whatever the season of the year. lecture xvii the tides persons accustomed to make use of the mersey landing-stages can hardly fail to have been struck with two obvious phenomena. one is that the gangways thereto are sometimes almost level, and at other times very steep; another is that the water often rushes past the stage rather violently, sometimes south towards garston, sometimes north towards the sea. they observe, in fact, that the water has two periodic motions--one up and down, the other to and fro--a vertical and a horizontal motion. they may further observe, if they take the trouble, that a complete swing of the water, up and down, or to and fro, takes place about every twelve and a half hours; moreover, that soon after high and low water there is no current--the water is stationary, whereas about half-way between high and low it is rushing with maximum speed either up or down the river. to both these motions of the water the name _tide_ is given, and both are extremely important. sailors usually pay most attention to the horizontal motion, and on charts you find the tide-races marked; and the places where there is but a small horizontal rush of the water are labelled "very little tide here." landsmen, or, at any rate, such of the more philosophic sort as pay any attention to the matter at all, think most of the vertical motion of the water--its amount of rise and fall. dwellers in some low-lying districts in london are compelled to pay attention to the extra high tides of the thames, because it is, or was, very liable to overflow its banks and inundate their basements. sailors, however, on nearing a port are also greatly affected by the time and amount of high water there, especially when they are in a big ship; and we know well enough how frequently atlantic liners, after having accomplished their voyage with good speed, have to hang around for hours waiting till there is enough water to lift them over the bar--that standing obstruction, one feels inclined to say disgrace, to the liverpool harbour. [illustration: fig. .--the mersey] to us in liverpool the tides are of supreme importance--upon them the very existence of the city depends--for without them liverpool would not be a port. it may be familiar to many of you how this is, and yet it is a matter that cannot be passed over in silence. i will therefore call your attention to the ordnance survey of the estuaries of the mersey and the dee. you see first that there is a great tendency for sand-banks to accumulate all about this coast, from north wales right away round to southport. you see next that the port of chester has been practically silted up by the deposits of sand in the wide-mouthed dee, while the port of liverpool remains open owing to the scouring action of the tide in its peculiarly shaped channel. without the tides the mersey would be a wretched dribble not much bigger than it is at warrington. with them, this splendid basin is kept open, and a channel is cut of such depth that the _great eastern_ easily rode in it in all states of the water. the basin is filled with water every twelve hours through its narrow neck. the amount of water stored up in this basin at high tide i estimate as million tons. all this quantity flows through the neck in six hours, and flows out again in the next six, scouring and cleansing and carrying mud and sand far out to sea. just at present the currents set strongest on the birkenhead side of the river, and accordingly a "pluckington bank" unfortunately grows under the liverpool stage. should this tendency to silt up the gates of our docks increase, land can be reclaimed on the other side of the river between tranmere and rock ferry, and an embankment made so as to deflect the water over liverpool way, and give us a fairer proportion of the current. after passing new brighton the water spreads out again to the left; its velocity forward diminishes; and after a few miles it has no power to cut away that sandbank known as the bar. should it be thought desirable to make it accomplish this, and sweep the bar further out to sea into deeper water, it is probable that a rude training wall (say of old hulks, or other removable partial obstruction) on the west of queen's channel, arranged so as to check the spreading out over all this useless area, may be quite sufficient to retain the needed extra impetus in the water, perhaps even without choking up the useful old rock channel, through which smaller ships still find convenient exit. now, although the horizontal rush of the tide is necessary to our existence as a port, it does not follow that the accompanying rise and fall of the water is an unmixed blessing. to it is due the need for all the expensive arrangements of docks and gates wherewith to store up the high-level water. quebec and new york are cities on such magnificent rivers that the current required to keep open channel is supplied without any tidal action, although quebec is nearly , miles from the open ocean; and accordingly, atlantic liners do not hover in mid-river and discharge passengers by tender, but they proceed straight to the side of the quays lining the river, or, as at new york, they dive into one of the pockets belonging to the company running the ship, and there discharge passengers and cargo without further trouble, and with no need for docks or gates. however, rivers like the st. lawrence and the hudson are the natural property of a gigantic continent; and we in england may be well contented with the possession of such tidal estuaries as the mersey, the thames, and the humber. that by pertinacious dredging the citizens of glasgow manage to get large ships right up their small river, the clyde, to the quays of the town, is a remarkable fact, and redounds very highly to their credit. we will now proceed to consider the connection existing between the horizontal rush of water and its vertical elevation, and ask, which is cause and which is effect? does the elevation of the ocean cause the tidal flow, or does the tidal flow cause the elevation? the answer is twofold: both statements are in some sense true. the prime cause of the tide is undoubtedly a vertical elevation of the ocean, a tidal wave or hump produced by the attraction of the moon. this hump as it passes the various channels opening into the ocean raises their level, and causes water to flow up them. but this simple oceanic tide, although the cause of all tide, is itself but a small affair. it seldom rises above six or seven feet, and tides on islands in mid-ocean have about this value or less. but the tides on our coasts are far greater than this--they rise twenty or thirty feet, or even fifty feet occasionally, at some places, as at bristol. why is this? the horizontal motion of the water gives it such an impetus or momentum that its motion far transcends that of the original impulse given to it, just as a push given to a pendulum may cause it to swing over a much greater arc than that through which the force acts. the inrushing water flowing up the english channel or the bristol channel or st. george's channel has such an impetus that it propels itself some twenty or thirty feet high before it has exhausted its momentum and begins to descend. in the bristol channel the gradual narrowing of the opening so much assists this action that the tides often rise forty feet, occasionally fifty feet, and rush still further up the severn in a precipitous and extraordinary hill of water called "the bore." some places are subject to considerable rise and fall of water with very little horizontal flow; others possess strong tidal races, but very little elevation and depression. the effect observed at any given place entirely depends on whether the place has the general character of a terminus, or whether it lies _en route_ to some great basin. you must understand, then, that all tide takes its rise in the free and open ocean under the action of the moon. no ordinary-sized sea like the north sea, or even the mediterranean, is big enough for more than a just appreciable tide to be generated in it. the pacific, the atlantic, and the southern oceans are the great tidal reservoirs, and in them the tides of the earth are generated as low flat humps of gigantic area, though only a few feet high, oscillating up and down in the period of approximately twelve hours. the tides we, and other coast-possessing nations, experience are the overflow or back-wash of these oceanic humps, and i will now show you in what manner the great atlantic tide-wave reaches the british isles twice a day. [illustration: fig. .--co-tidal lines.] fig. shows the contour lines of the great wave as it rolls in east from the atlantic, getting split by the land's end and by ireland into three portions; one of which rushes up the english channel and through the straits of dover. another rolls up the irish sea, with a minor offshoot up the bristol channel, and, curling round anglesey, flows along the north wales coast and fills liverpool bay and the mersey. the third branch streams round the north coast of ireland, past the mull of cantyre and rathlin island; part fills up the firth of clyde, while the rest flows south, and, swirling round the west side of the isle of man, helps the southern current to fill the bay of liverpool. the rest of the great wave impinges on the coast of scotland, and, curling round it, fills up the north sea right away to the norway coast, and then flows down below denmark, joining the southern and earlier arriving stream. the diagram i show you is a rough chart of cotidal lines, which i made out of the information contained in _whitaker's almanac_. a place may thus be fed with tide by two distinct channels, and many curious phenomena occur in certain places from this cause. thus it may happen that one channel is six hours longer than the other, in which case a flow will arrive by one at the same time as an ebb arrives by the other; and the result will be that the place will have hardly any tide at all, one tide interfering with and neutralizing the other. this is more markedly observed at other parts of the world than in the british isles. whenever a place is reached by two channels of different length, its tides are sure to be peculiar, and probably small. another cause of small tide is the way the wave surges to and fro in a channel. the tidal wave surging up the english channel, for instance, gets largely reflected by the constriction at dover, and so a crest surges back again, as we may see waves reflected in a long trough or tilted bath. the result is that southampton has two high tides rapidly succeeding one another, and for three hours the high-water level varies but slightly--a fact of evident convenience to the port. places on a nodal line, so to speak, about the middle of the length of the channel, have a minimum of rise and fall, though the water rushes past them first violently up towards dover, where the rise is considerable, and then back again towards the ocean. at portland, for instance, the total rise and fall is very small: it is practically on a node. yarmouth, again, is near a less marked node in the north sea, where stationary waves likewise surge to and fro, and accordingly the tidal rise and fall at yarmouth is only about five feet (varying from four and a half to six), whereas at london it is twenty or thirty feet, and at flamborough head or leith it is from twelve to sixteen feet. it is generally supposed that water never flows up-hill, but in these cases of oscillation it flows up-hill for three hours together. the water is rushing up the english channel towards dover long after it is highest at the dover end; it goes on piling itself up, until its momentum is checked by the pressure, and then it surges back. it behaves, in fact, very like the bob of a pendulum, which rises against gravity at every quarter swing. to get a very large tide, the place ought to be directly accessible by a long sweep of a channel to the open ocean, and if it is situate on a gradually converging opening, the ebb and flow may be enormous. the severn is the best example of this on the british isles; but the largest tides in the world are found, i believe, in the bay of fundy, on the coast of north america, where they sometimes rise one hundred and twenty feet. excessive or extra tides may be produced occasionally in any place by the propelling force of a high wind driving the water towards the shore; also by a low barometer, _i.e._ by a local decrease in the pressure of the air. well, now, leaving these topographical details concerning tides, which we see to be due to great oceanic humps (great in area that is, though small in height), let us proceed to ask what causes these humps; and if it be the moon that does it, how does it do it? the statement that the moon causes the tides sounds at first rather an absurdity, and a mere popular superstition. galileo chaffed kepler for believing it. who it was that discovered the connection between moon and tides we know not--probably it is a thing which has been several times rediscovered by observant sailors or coast-dwellers--and it is certainly a very ancient piece of information. probably the first connection observed was that about full moon and about new moon the tides are extra high, being called spring tides, whereas about half-moon the tides are much less, and are called neap tides. the word spring in this connection has no reference to the season of the year; except that both words probably represent the same idea of energetic uprising or upspringing, while the word neap comes from nip, and means pinched, scanty, nipped tide. the next connection likely to be observed would be that the interval between two day tides was not exactly a solar day of twenty-four hours, but a lunar day of fifty minutes longer. for by reason of the moon's monthly motion it lags behind the sun about fifty minutes a day, and the tides do the same, and so perpetually occur later and later, about fifty minutes a day later, or hours and minutes on the average between tide and tide. a third and still more striking connection was also discovered by some of the ancient great navigators and philosophers--viz. that the time of high water at a given place at full moon is always the same, or very nearly so. in other words, the highest or spring tides always occur nearly at the same time of day at a given place. for instance, at liverpool this time is noon and midnight. london is about two hours and a half later. each port has its own time for receiving a given tide, and the time is called the "establishment" of the port. look out a day when the moon is full, and you will find the liverpool high tide occurs at half-past eleven, or close upon it. the same happens when the moon is new. a day after full or new moon the spring tides rise to their highest, and these extra high tides always occur in liverpool at noon and at midnight, whatever the season of the year. about the equinoxes they are liable to be extraordinarily high. the extra low tides here are therefore at a.m. and p.m., and the p.m. low tide is a nuisance to the river steamers. the spring tides at london are highest about half-past two. * * * * * it is, therefore, quite clear that the moon has to do with the tides. it and the sun together are, in fact, the whole cause of them; and the mode in which these bodies act by gravitative attraction was first made out and explained in remarkably full detail by sir isaac newton. you will find his account of the tides in the second and third books of the _principia_; and though the theory does not occupy more than a few pages of that immortal work, he succeeds not only in explaining the local tidal peculiarities, much as i have done to-night, but also in calculating the approximate height of mid-ocean solar tide; and from the observed lunar tide he shows how to determine the then quite unknown mass of the moon. this was a quite extraordinary achievement, the difficulty of which it is not easy for a person unused to similar discussions fully to appreciate. it is, indeed, but a small part of what newton accomplished, but by itself it is sufficient to confer immortality upon any ordinary philosopher, and to place him in a front rank. [illustration: fig. .--whirling earth model.] to make intelligible newton's theory of the tides, i must not attempt to go into too great detail. i will consider only the salient points. first, you know that every mass of matter attracts every other piece of matter; second, that the moon revolves round the earth, or rather that the earth and moon revolve round their common centre of gravity once a month; third, that the earth spins on its own axis once a day; fourth, that when a thing is whirled round, it tends to fly out from the centre and requires a force to hold it in. these are the principles involved. you can whirl a bucket full of water vertically round without spilling it. make an elastic globe rotate, and it bulges out into an oblate or orange shape; as illustrated by the model shown in fig. . this is exactly what the earth does, and newton calculated the bulging of it as fourteen miles all round the equator. make an elastic globe revolve round a fixed centre outside itself, and it gets pulled into a prolate or lemon shape; the simplest illustrative experiment is to attach a string to an elastic bag or football full of water, and whirl it round and round. its prolateness is readily visible. now consider the earth and moon revolving round each other like a man whirling a child round. the child travels furthest, but the man cannot merely rotate, he leans back and thus also describes a small circle: so does the earth; it revolves round the common centre of gravity of earth and moon (_cf._ p. ). this is a vital point in the comprehension of the tides: the earth's centre is not at rest, but is being whirled round by the moon, in a circle about / as big as the circle which the moon describes, because the earth weighs eighty times as much as the moon. the effect of the revolution is to make both bodies slightly protrude in the direction of the line joining them; they become slightly "prolate" as it is called--that is, lemon-shaped. illustrating still by the man and child, the child's legs fly outwards so that he is elongated in the direction of a radius; the man's coat-tails fly out too, so that he too is similarly though less elongated. these elongations or protuberances constitute the tides. [illustration: fig. .--earth and moon model, illustrating the production of statical or "equilibrium" tides when the whole is whirled about the point g.] fig. shows a model to illustrate the mechanism. a couple of cardboard disks (to represent globes of course), one four times the diameter of the other, and each loaded so as to have about the correct earth-moon ratio of weights, are fixed at either end of a long stick, and they balance about a certain point, which is their common centre of gravity. for convenience this point is taken a trifle too far out from the centre of the earth--that is, just beyond its surface. through the balancing point g a bradawl is stuck, and on that as pivot the whole readily revolves. now, behind the circular disks, you see, are four pieces of card of appropriate shape, which are able to slide out under proper forces. they are shown dotted in the figure, and are lettered a, b, c, d. the inner pair, b and c, are attached to each other by a bit of string, which has to typify the attraction of gravitation; the outer pair, a and d, are not attached to anything, but have a certain amount of play against friction in slots parallel to the length of the stick. the moon-disk is also slotted, so a small amount of motion is possible to it along the stick or bar. these things being so arranged, and the protuberant pieces of card being all pushed home, so that they are hidden behind their respective disks, the whole is spun rapidly round the centre of gravity, g. the result of a brief spin is to make a and d fly out by centrifugal force and show, as in the figure; while the moon, flying out too in its slot, tightens up the string, which causes b and c to be pulled out too. thus all four high tides are produced, two on the earth and two on the moon, a and d being caused by centrifugal force, b and c by the attraction of gravitation. each disk has become prolate in the same sort of fashion as yielding globes do. of course the fluid ocean takes this shape more easily and more completely than the solid earth can, and so here are the very oceanic humps we have been talking about, and about three feet high (fig. ). if there were a sea on the _moon_, its humps would be a good deal bigger; but there probably is no sea there, and if there were, the earth's tides are more interesting to us, at any rate to begin with. [illustration: fig. .--earth and moon (earth's rotation neglected).] the humps as so far treated are always protruding in the earth-moon line, and are stationary. but now we have to remember that the earth is spinning inside them. it is not easy to see what precise effect this spin will have upon the humps, even if the world were covered with a uniform ocean; but we can see at any rate that however much they may get displaced, and they do get displaced a good deal, they cannot possibly be carried round and round. the whole explanation we have given of their causes shows that they must maintain some steady aspect with respect to the moon--in other words, they must remain stationary as the earth spins round. not that the same identical water remains stationary, for in that case it would have to be dragged over the earth's equator at the rate of , miles an hour, but the hump or wave-crest remains stationary. it is a true wave, or form only, and consists of continuously changing individual particles. the same is true of all waves, except breaking ones. given, then, these stationary humps and the earth spinning on its axis, we see that a given place on the earth will be carried round and round, now past a hump, and six hours later past a depression: another six hours and it will be at the antipodal hump, and so on. thus every six hours we shall travel from the region in space where the water is high to the region where it is low; and ignoring our own motion we shall say that the sea first rises and then falls; and so, with respect to the place, it does. thus the succession of high and low water, and the two high tides every twenty-four hours, are easily understood in their easiest and most elementary aspect. a more complete account of the matter it will be wisest not to attempt: suffice it to say that the difficulties soon become formidable when the inertia of the water, its natural time of oscillation, the varying obliquity of the moon to the ecliptic, its varying distance, and the disturbing action of the sun are taken into consideration. when all these things are included, the problem becomes to ordinary minds overwhelming. a great many of these difficulties were successfully attacked by laplace. others remained for modern philosophers, among whom are sir george airy, sir william thomson, and professor george darwin. i may just mention that the main and simplest effect of including the inertia or momentum of the water is to dislocate the obvious and simple connexion between high water and high moon; inertia always tends to make an effect differ in phase by a quarter period from the cause producing it, as may be illustrated by a swinging pendulum. hence high water is not to be expected when the tide-raising force is a maximum, but six hours later; so that, considering inertia and neglecting friction, there would be low water under the moon. including friction, something nearer the equilibrium state of things occurs. with _sufficient_ friction the motion becomes dead-beat again, _i.e._ follows closely the force that causes it. returning to the elementary discussion, we see that the rotation of the earth with respect to the humps will not be performed in exactly twenty-four hours, because the humps are travelling slowly after the moon, and will complete a revolution in a month in the same direction as the earth is rotating. hence a place on the earth has to catch them up, and so each high tide arrives later and later each day--roughly speaking, an hour later for each day tide; not by any means a constant interval, because of superposed disturbances not here mentioned, but on the average about fifty minutes. we see, then, that as a result of all this we get a pair of humps travelling all over the surface of the earth, about once a day. if the earth were all ocean (and in the southern hemisphere it is nearly all ocean), then they would go travelling across the earth, tidal waves three feet high, and constituting the mid-ocean tides. but in the northern hemisphere they can only thus journey a little way without striking land. as the moon rises at a place on the east shores of the atlantic, for instance, the waters begin to flow in towards this place, or the tide begins to rise. this goes on till the moon is overhead and for some time afterwards, when the tide is at its highest. the hump then follows the moon in its apparent journey across to america, and there precipitates itself upon the coast, rushing up all the channels, and constituting the land tide. at the same time, the water is dragged away from the east shores, and so _our_ tide is at its lowest. the same thing repeats itself in a little more than twelve hours again, when the other hump passes over the atlantic, as the moon journeys beneath the earth, and so on every day. in the free southern ocean, where land obstruction is comparatively absent, the water gets up a considerable swing by reason of its accumulated momentum, and this modifies and increases the open ocean tides there. also for some reason, i suppose because of the natural time of swing of the water, one of the humps is there usually much larger than the other; and so places in the indian and other offshoots of the southern ocean get their really high tide only once every twenty-four hours. these southern tides are in fact much more complicated than those the british isles receive. ours are singularly simple. no doubt some trace of the influence of the southern ocean is felt in the north atlantic, but any ocean extending over ° of longitude is big enough to have its own tides generated; and i imagine that the main tides we feel are thus produced on the spot, and that they are simple because the damping-out being vigorous, and accumulated effects small, we feel the tide-producing forces more directly. but for authoritative statements on tides, other books must be read. i have thought, and still think, it best in an elementary exposition to begin by a consideration of the tide-generating forces as if they acted on a non-rotating earth. it is the tide generating forces, and not the tides themselves, that are really represented in figs. and . the rotation of the earth then comes in as a disturbing cause. a more complete exposition would begin with the rotating earth, and would superpose the attraction of the moon as a disturbing cause, treating it as a problem in planetary perturbation, the ocean being a sort of satellite of the earth. this treatment, introducing inertia but ignoring friction and land obstruction, gives low water in the line of pull, and high water at right angles, or where the pull is zero; in the same sort of way as a pendulum bob is highest where most force is pulling it down, and lowest where no force is acting on it. for a clear treatment of the tides as due to the perturbing forces of sun and moon, see a little book by mr. t.k. abbott of trinity college, dublin. (longman.) [illustration: fig. .--maps showing how comparatively free from land obstruction the ocean in the southern hemisphere is.] if the moon were the only body that swung the earth round, this is all that need be said in an elementary treatment; but it is not the only one. the moon swings the earth round once a month, the sun swings it round once a year. the circle of swing is bigger, but the speed is so much slower that the protuberance produced is only one-third of that caused by the monthly whirl; _i.e._ the simple solar tide in the open sea, without taking momentum into account, is but a little more than a foot high, while the simple lunar tide is about three feet. when the two agree, we get a spring tide of four feet; when they oppose each other, we get a neap tide of only two feet. they assist each other at full moon and at new moon. at half-moon they oppose each other. so we have spring tides regularly once a fortnight, with neap tides in between. [illustration: fig. .--spring and neap tides.] fig. gives the customary diagrams to illustrate these simple things. you see that when the moon and sun act at right angles (_i.e._ at every half-moon), the high tides of one coincide with the low tides of the other; and so, as a place is carried round by the earth's rotation, it always finds either solar or else lunar high water, and only experiences the difference of their two effects. whereas, when the sun and moon act in the same line (as they do at new and full moon), their high and low tides coincide, and a place feels their effects added together. the tide then rises extra high and falls extra low. [illustration: fig. .--tidal clock. the position of the disk b shows the height of the tide. the tide represented is a nearly high tide eight feet above mean level.] utilizing these principles, a very elementary form of tidal-clock, or tide-predicter, can be made, and for an open coast station it really would not give the tides so very badly. it consists of a sort of clock face with two hands, one nearly three times as long as the other. the short hand, ca, should revolve round c once in twelve hours, and the vertical height of its end a represents the height of the solar tide on the scale of horizontal lines ruled across the face of the clock. the long hand, ab, should revolve round a once in twelve hours and twenty-five minutes, and the height of its end b (if a were fixed on the zero line) would represent the lunar tide. the two revolutions are made to occur together, either by means of a link-work parallelogram, or, what is better in practice, by a string and pulleys, as shown; and the height of the end point, b, of the third side or resultant, cb, read off on a scale of horizontal parallel lines behind, represents the combination or actual tide at the place. every fortnight the two will agree, and you will get spring tides of maximum height ca + ab; every other fortnight the two will oppose, and you will get neap tides of maximum height ca-ab. such a clock, if set properly and driven in the ordinary way, would then roughly indicate the state of the tide whenever you chose to look at it and read the height of its indicating point. it would not indeed be very accurate, especially for such an inclosed station as liverpool is, and that is probably why they are not made. a great number of disturbances, some astronomical, some terrestrial, have to be taken into account in the complete theory. it is not an easy matter to do this, but it can be, and has been, done; and a tide-predicter has not only been constructed, but two of them are in regular work, predicting the tides for years hence--one, the property of the indian government, for coast stations of india; the other for various british and foreign stations, wherever the necessary preliminary observations have been made. these machines are the invention of sir william thomson. the tide-tables for indian ports are now always made by means of them. [illustration: fig. .--sir william thomson (lord kelvin).] [illustration: fig. .--tide-gauge for recording local tides, a pencil moved up and down by a float writes on a drum driven by clockwork.] the first thing to be done by any port which wishes its tides to be predicted is to set up a tide-gauge, or automatic recorder, and keep it working for a year or two. the tide-gauge is easy enough to understand: it marks the height of the tide at every instant by an irregular curved line like a barometer chart (fig. ). these observational curves so obtained have next to be fed into a fearfully complex machine, which it would take a whole lecture to make even partially intelligible, but fig. shows its aspect. it consists of ten integrating machines in a row, coupled up and working together. this is the "harmonic analyzer," and the result of passing the curve through this machine is to give you all the constituents of which it is built up, viz. the lunar tide, the solar tide, and eight of the sub-tides or disturbances. these ten values are then set off into a third machine, the tide-predicter proper. the general mode of action of this machine is not difficult to understand. it consists of a string wound over and under a set of pulleys, which are each set on an excentric, so as to have an up-and-down motion. these up-and-down motions are all different, and there are ten of these movable pulleys, which by their respective excursions represent the lunar tide, the solar tide, and the eight disturbances already analyzed out of the tide-gauge curve by the harmonic analyzer. one end of the string is fixed, the other carries a pencil which writes a trace on a revolving drum of paper--a trace which represents the combined motion of all the pulleys, and so predicts the exact height of the tide at the place, at any future time you like. the machine can be turned quite quickly, so that a year's tides can be run off with every detail in about half-an-hour. this is the easiest part of the operation. nothing has to be done but to keep it supplied with paper and pencil, and turn a handle as if it were a coffee-mill instead of a tide-mill. (figs. and .) [illustration: fig. .--harmonic analyzer; for analyzing out the constituents from a set of observational curves.] my subject is not half exhausted. i might go on to discuss the question of tidal energy--whether it can be ever utilized for industrial purposes; and also the very interesting question whence it comes. tidal energy is almost the only terrestrial form of energy that does not directly or indirectly come from the sun. the energy of tides is now known to be obtained at the expense of the earth's rotation; and accordingly our day must be slowly, very slowly, lengthening. the tides of past ages have destroyed the moon's rotation, and so it always turns the same face to us. there is every reason to believe that in geologic ages the moon was nearer to us than it is now, and that accordingly our tides were then far more violent, rising some hundreds of feet instead of twenty or thirty, and sweeping every six hours right over the face of a country, ploughing down hills, denuding rocks, and producing a copious sedimentary deposit. [illustration: fig. .--tide-predicter, for combining the ascertained constituents into a tidal curve for the future.] in thus discovering the probable violent tides of past ages, astronomy has, within the last few years, presented geology with the most powerful denuding agent known; and the study of the earth's past history cannot fail to be greatly affected by the modern study of the intricate and refined conditions attending prolonged tidal action on incompletely rigid bodies. [read on this point the last chapter of sir r. ball's _story of the heavens_.] [illustration: fig. .--weekly sheet of curves. tides for successive days are predicted on the same sheet of paper, to economise space.] i might also point out that the magnitude of our terrestrial tides enables us to answer the question as to the internal fluidity of the earth. it used to be thought that the earth's crust was comparatively thin, and that it contained a molten interior. we now know that this is not the case. the interior of the earth is hot indeed, but it is not fluid. or at least, if it be fluid, the amount of fluid is but very small compared with the thickness of the unyielding crust. all these, and a number of other most interesting questions, fringe the subject of the tides; the theoretical study of which, started by newton, has developed, and is destined in the future to further develop, into one of the most gigantic and absorbing investigations--having to do with the stability or instability of solar systems, and with the construction and decay of universes. these theories are the work of pioneers now living, whose biographies it is therefore unsuitable for us to discuss, nor shall i constantly mention their names. but helmholtz, and thomson, are household words, and you well know that in them and their disciples the race of pioneers maintains its ancient glory. notes for lecture xviii tides are due to incomplete rigidity of bodies revolving round each other under the action of gravitation, and at the same time spinning on their axes. two spheres revolving round each other can only remain spherical if rigid; if at all plastic they become prolate. if either rotate on its axis, in the same or nearly the same plane as it revolves, that one is necessarily subject to tides. the axial rotation tends to carry the humps with it, but the pull of the other body keeps them from moving much. hence the rotation takes place against a pull, and is therefore more or less checked and retarded. this is the theory of von helmholtz. the attracting force between two such bodies is no longer _exactly_ towards the centre of revolution, and therefore kepler's second law is no longer precisely obeyed: the rate of description of areas is subject to slight acceleration. the effect of this tangential force acting on the tide-compelling body is gradually to increase its distance from the other body. applying these statements to the earth and moon, we see that tidal energy is produced at the expense of the earth's rotation, and that the length of the day is thereby slowly increasing. also that the moon's rotation relative to the earth has been destroyed by past tidal action in it (the only residue of ancient lunar rotation now being a scarcely perceptible libration), so that it turns always the same face towards us. moreover, that its distance from the earth is steadily increasing. this last is the theory of professor g.h. darwin. long ago the moon must therefore have been much nearer the earth, and the day was much shorter. the tides were then far more violent. halving the distance would make them eight times as high; quartering it would increase them sixty-four-fold. a most powerful geological denuding agent. trade winds and storms were also more violent. if ever the moon were close to the earth, it would have to revolve round it in about three hours. if the earth rotated on its axis in three hours, when fluid or pasty, it would be unstable, and begin to separate a portion of itself as a kind of bud, which might then get detached and gradually pushed away by the violent tidal action. hence it is possible that this is the history of the moon. if so, it is probably an exceptional history. the planets were not formed from the sun in this way. mars' moons revolve round him more quickly than the planet rotates: hence with them the process is inverted, and they must be approaching him and may some day crash along his surface. the inner moon is now about , miles away, and revolves in - / hours. it appears to be about miles in diameter, and weighs therefore, if composed of rock, billion tons. mars rotates in - / hours. a similar fate may _possibly_ await our moon ages hence--by reason of the action of terrestrial tides produced by the sun. lecture xviii the tides, and planetary evolution in the last lecture we considered the local peculiarities of the tides, the way in which they were formed in open ocean under the action of the moon and the sun, and also the means by which their heights and times could be calculated and predicted years beforehand. towards the end i stated that the subject was very far from being exhausted, and enumerated some of the large and interesting questions which had been left untouched. it is with some of these questions that i propose now to deal. i must begin by reminding you of certain well-known facts, a knowledge of which i may safely assume. and first we must remind ourselves of the fact that almost all the rocks which form the accessible crust of the earth were deposited by the agency of water. nearly all are arranged in regular strata, and are composed of pulverized materials--materials ground down from pre-existing rocks by some denuding and grinding action. they nearly all contain vestiges of ancient life embedded in them, and these vestiges are mainly of marine origin. the strata which were once horizontal are now so no longer--they have been tilted and upheaved, bent and distorted, in many places. some of them again have been metamorphosed by fire, so that their organic remains have been destroyed, and the traces of their aqueous origin almost obliterated. but still, to the eye of the geologist, all are of aqueous or sedimentary origin: roughly speaking, one may say they were all deposited at the bottom of some ancient sea. the date of their formation no man yet can tell, but that it was vastly distant is certain. for the geological era is not over. aqueous action still goes on: still does frost chip the rocks into fragments; still do mountain torrents sweep stone and mud and _débris_ down the gulleys and watercourses; still do rivers erode their channels, and carry mud and silt far out to sea. and, more powerful than any of these agents of denudation, the waves and the tides are still at work along every coast-line, eating away into the cliffs, undermining gradually and submerging acre after acre, and making with the refuse a shingly, or a sandy, or a muddy beach--the nucleus of a new geological formation. of all denuding agents, there can be no doubt that, to the land exposed to them, the waves of the sea are by far the most powerful. think how they beat and tear, and drive and drag, until even the hardest rock, like basalt, becomes honeycombed into strange galleries and passages--fingal's cave, for instance--and the softer parts are crumbled away. but the area now exposed to the teeth of the waves is not great. the fury of a winter storm may dash them a little higher than usual, but they cannot reach cliffs feet high. they can undermine such cliffs indeed, and then grind the fragments to powder, but their direct action is limited. not so limited, however, as they would be without the tides. consider for a moment the denudation import of the tides: how does the existence of tidal rise and fall affect the geological problem? the scouring action of the tidal currents themselves is not to be despised. it is the tidal ebb and flow which keeps open channel in the mersey, for instance. but few places are so favourably situated as liverpool in this respect, and the direct scouring action of the tides in general is not very great. their geological import mainly consists in this--that they raise and lower the surface waves at regular intervals, so as to apply them to a considerable stretch of coast. the waves are a great planing machine attacking the land, and the tides raise and lower this planing machine, so that its denuding tooth is applied, now twenty feet vertically above mean level, now twenty feet below. making all allowance for the power of winds and waves, currents, tides, and watercourses, assisted by glacial ice and frost, it must be apparent how slowly the work of forming the rocks is being carried on. it goes on steadily, but so slowly that it is estimated to take years to wear away one foot of the american continent by all the denuding causes combined. to erode a stratum feet thick will require at this rate thirty million years. the age of the earth is not at all accurately known, but there are many grounds for believing it not to be much older than some thirty million years. that is to say, not greatly more than this period of time has elapsed since it was in a molten condition. it may be as old as a hundred million years, but its age is believed by those most competent to judge to be more likely within this limit than beyond it. but if we ask what is the thickness of the rocks which in past times have been formed, and denuded, and re-formed, over and over again, we get an answer, not in feet, but in miles. the laurentian and huronian rocks of canada constitute a stratum ten miles thick; and everywhere the rocks at the base of our stratified system are of the most stupendous volume and thickness. it has always been a puzzle how known agents could have formed these mighty masses, and the only solution offered by geologists was, unlimited time. given unlimited time, they could, of course, be formed, no matter how slowly the process went on. but inasmuch as the time allowable since the earth was cool enough for water to exist on it except as steam is not by any means unlimited, it becomes necessary to look for a far more powerful engine than any now existing; there must have been some denuding agent in those remote ages--ages far more distant from us than the carboniferous period, far older than any forms of life, fossil or otherwise, ages among the oldest known to geology--a denuding agent must have then existed, far more powerful than any we now know. such an agent it has been the privilege of astronomy and physics, within the last ten years, to discover. to this discovery i now proceed to lead up. our fundamental standard of time is the period of the earth's rotation--the length of the day. the earth is our one standard clock: all time is expressed in terms of it, and if it began to go wrong, or if it did not go with perfect uniformity, it would seem a most difficult thing to discover its error, and a most puzzling piece of knowledge to utilize when found. that it does not go much wrong is proved by the fact that we can calculate back to past astronomical events--ancient eclipses and the like--and we find that the record of their occurrence, as made by the old magi of chaldæa, is in very close accordance with the result of calculation. one of these famous old eclipses was observed in babylon about thirty-six centuries ago, and the chaldæan astronomers have put on record the time of its occurrence. modern astronomers have calculated back when it should have occurred, and the observed time agrees very closely with the actual, but not exactly. why not exactly? partly because of the acceleration of the moon's mean motion, as explained in the lecture on laplace (p. ). the orbit of the earth was at that time getting rounder, and so, as a secondary result, the speed of the moon was slightly increasing. it is of the nature of a perturbation, and is therefore a periodic not a progressive or continuous change, and in a sufficiently long time it will be reversed. still, for the last few thousand years the moon's motion has been, on the whole, accelerated (though there seems to be a very slight retarding force in action too). laplace thought that this fact accounted for the whole of the discrepancy; but recently, in , professor adams re-examined the matter, and made a correction in the details of the theory which diminishes its effect by about one-half, leaving the other half to be accounted for in some other way. his calculations have been confirmed by professor cayley. this residual discrepancy, when every known cause has been allowed for, amounts to about one hour. the eclipse occurred later than calculation warrants. now this would have happened from either of two causes, either an acceleration of the moon in her orbit, or a retardation of the earth in her diurnal rotation--a shortening of the month or a lengthening of the day, or both. the total discrepancy being, say, two hours, an acceleration of six seconds-per-century per century will in thirty-six centuries amount to one hour; and this, according to the corrected laplacian theory, is what has occurred. but to account for the other hour some other cause must be sought, and at present it is considered most probably due to a steady retardation of the earth's rotation--a slow, very slow, lengthening of the day. the statement that a solar eclipse thirty-six centuries ago was an hour late, means that a place on the earth's surface came into the shadow one hour behind time--that is, had lagged one twenty-fourth part of a revolution. the earth, therefore, had lost this amount in the course of × - / revolutions. the loss per revolution is exceedingly small, but it accumulates, and at any era the total loss is the sum of all the losses preceding it. it may be worth while just to explain this point further. suppose the earth loses a small piece of time, which i will call an instant, per day; a locality on the earth will come up to a given position one instant late on the first day after an event. on the next day it would come up two instants late by reason of the previous loss; but it also loses another instant during the course of the second day, and so the total lateness by the end of that day amounts to three instants. the day after, it will be going slower from the beginning at the rate of two instants a day, it will lose another instant on the fresh day's own account, and it started three instants late; hence the aggregate loss by the end of the third day is + + = . by the end of the fourth day the whole loss will be + + + , and so on. wherefore by merely losing one instant every day the total loss in _n_ days is ( + + + ... + _n_) instants, which amounts to / _n_ (_n_ + ) instants; or practically, when _n_ is big, to / n^ . now in thirty-six centuries there have been × - / days, and the total loss has amounted to an hour; hence the length of "an instant," the loss per diem, can be found from the equation / ( × )^ instants = hour; whence one "instant" equals the millionth part of a second. this minute quantity represents the retardation of the earth per day. in a year the aggregate loss mounts up to / th part of a second, in a century to about three seconds, and in thirty-six centuries to an hour. but even at the end of the thirty-six centuries the day is barely any longer; it is only × instants, that is / th of a second, longer than it was at the beginning. and even a million years ago, unless the rate of loss was different (as it probably was), the day would only be thirty-five minutes shorter, though by that time the aggregate loss, as measured by the apparent lateness of any perfectly punctual event reckoned now, would have amounted to nine years. (these numbers are to be taken as illustrative, not as precisely representing terrestrial fact.) what can have caused the slowing down? swelling of the earth by reason of accumulation of meteoric dust might do something, but probably very little. contraction of the earth as it goes on cooling would act in the opposite direction, and probably more than counterbalance the dust effect. the problem is thus not a simple one, for there are several disturbing causes, and for none of them are the data enough to base a quantitative estimate upon; but one certain agent in lengthening the day, and almost certainly the main agent, is to be found in the tides. remember that the tidal humps were produced as the prolateness of a sphere whirled round and round a fixed centre, like a football whirled by a string. these humps are pulled at by the moon, and the earth rotates on its axis against this pull. hence it tends to be constantly, though very slightly, dragged back. in so far as the tidal wave is allowed to oscillate freely, it will swing with barely any maintaining force, giving back at one quarter-swing what it has received at the previous quarter; but in so far as it encounters friction, which it does in all channels where there is an actual ebb and flow of the water, it has to receive more than it gives back, and the balance of energy has to be made up to it, or the tides would cease. the energy of the tides is, in fact, continually being dissipated by friction, and all the energy so dissipated is taken from the rotation of the earth. if tidal energy were utilized by engineers, the machines driven would be really driven at the expense of the earth's rotation: it would be a mode of harnessing the earth and using the moon as fixed point or fulcrum; the moon pulling at the tidal protuberance, and holding it still as the earth rotates, is the mechanism whereby the energy is extracted, the handle whereby the friction brake is applied. winds and ocean currents have no such effect (as mr. fronde in _oceania_ supposes they have), because they are all accompanied by a precisely equal counter-current somewhere else, and no internal rearrangement of fluid can affect the motion of a mass as a whole; but the tides are in different case, being produced, not by internal inequalities of temperature, but by a straightforward pull from an external body. the ultimate effect of tidal friction and dissipation of energy will, therefore, be to gradually retard the earth till it does not rotate with reference to the moon, _i.e._ till it rotates once while the moon revolves once; in other words, to make the day and the month equal. the same cause must have been in operation, but with eighty-fold greater intensity, on the moon. it has ceased now, because the rotation has stopped, but if ever the moon rotated on its axis with respect to the earth, and if it were either fluid itself or possessed any liquid ocean, then the tides caused by the pull of the earth must have been prodigious, and would tend to stop its rotation. have they not succeeded? is it not probable that this is _why_ the moon always now turns the same face towards us? it is believed to be almost certainly the cause. if so, there was a time when the moon behaved differently--when it rotated more quickly than it revolved, and exhibited to us its whole surface. and at this era, too, the earth itself must have rotated a little faster, for it has been losing speed ever since. we have thus arrived at this fact, that a thousand years ago the day was a trifle shorter than it is now. a million years ago it was, perhaps, an hour shorter. twenty million years ago it must have been much shorter. fifty million years ago it may have been only a few hours long. the earth may have spun round then quite quickly. but there is a limit. if it spun too fast it would fly to pieces. attach shot by means of wax to the whirling earth model, fig. , and at a certain speed the cohesion of the wax cannot hold them, so they fly off. the earth is held together not by cohesion but by gravitation; it is not difficult to reckon how fast the earth must spin for gravity at its surface to be annulled, and for portions to fly off. we find it about one revolution in three hours. this is a critical speed. if ever the day was three hours long, something must have happened. the day can never have been shorter than that; for if it were, the earth would have a tendency to fly in pieces, or, at least, to separate into two pieces. remember this, as a natural result of a three-hour day, which corresponds to an unstable state of things; remember also that in some past epoch a three-hour day is a probability. if we think of the state of things going on in the earth's atmosphere, if it had an atmosphere at that remote date, we shall recognize the existence of the most fearful tornadoes. the trade winds, which are now peaceful agents of commerce, would then be perpetual hurricanes, and all the denudation agents of the geologist would be in a state of feverish activity. so, too, would the tides: instead of waiting six hours between low and high tide, we should have to wait only three-quarters of an hour. every hour-and-a-half the water would execute a complete swing from high tide to high again. very well, now leave the earth, and think what has been happening to the moon all this while. we have seen that the moon pulls the tidal hump nearest to it back; but action and reaction are always equal and opposite--it cannot do that without itself getting pulled forward. the pull of the earth on the moon will therefore not be quite central, but will be a little in advance of its centre; hence, by kepler's second law, the rate of description of areas by its radius vector cannot be constant, but must increase (p. ). and the way it increases will be for the radius vector to lengthen, so as to sweep out a bigger area. or, to put it another way, the extra speed tending to be gained by the moon will fling it further away by extra centrifugal force. this last is not so good a way of regarding the matter; though it serves well enough for the case of a ball whirled at the end of an elastic string. after having got up the whirl, the hand holding the string may remain almost fixed at the centre of the circle, and the motion will continue steadily; but if the hand be moved so as always to pull the string a little in advance of the centre, the speed of whirl will increase, the elastic will be more and more stretched, until the whirling ball is describing a much larger circle. but in this case it will likewise be going faster--distance and speed increase together. this is because it obeys a different law from gravitation--the force is not inversely as the square, or any other single power, of the distance. it does not obey any of kepler's laws, and so it does not obey the one which now concerns us, viz. the third; which practically states that the further a planet is from the centre the slower it goes; its velocity varies inversely with the square root of its distance (p. ). if, instead of a ball held by elastic, it were a satellite held by gravity, an increase in distance must be accompanied by a diminution in speed. the time of revolution varies as the square of the cube root of the distance (kepler's third law). hence, the tidal reaction on the moon, having as its primary effect, as we have seen, the pulling the moon a little forward, has also the secondary or indirect effect of making it move slower and go further off. it may seem strange that an accelerating pull, directed in front of the centre, and therefore always pulling the moon the way it is going, should retard it; and that a retarding force like friction, if such a force acted, should hasten it, and make it complete its orbit sooner; but so it precisely is. gradually, but very slowly, the moon is receding from us, and the month is becoming longer. the tides of the earth are pushing it away. this is not a periodic disturbance, like the temporary acceleration of its motion discovered by laplace, which in a few centuries, more or less, will be reversed; it is a disturbance which always acts one way, and which is therefore cumulative. it is superposed upon all periodic changes, and, though it seems smaller than they, it is more inexorable. in a thousand years it makes scarcely an appreciable change, but in a million years its persistence tells very distinctly; and so, in the long run, the month is getting longer and the moon further off. working backwards also, we see that in past ages the moon must have been nearer to us than it is now, and the month shorter. now just note what the effect of the increased nearness of the moon was upon our tides. remember that the tide-generating force varies inversely as the cube of distance, wherefore a small change of distance will produce a great difference in the tide-force. the moon's present distance is thousand miles. at a time when it was only thousand miles, the earth's tides would have been twice as high as they are now. the pushing away action was then a good deal more violent, and so the process went on quicker. the moon must at some time have been just half its present distance, and the tides would then have risen, not or feet, but or feet. a little further back still, we have the moon at one-third of its present distance from the earth, and the tides feet high. now just contemplate the effect of a -foot tide. we are here only about feet above the level of the sea; hence, the tide would sweep right over us and rush far away inland. at high tide we should have some feet of blue water over our heads. there would be nothing to stop such a tide as that in this neighbourhood till it reached the high lands of derbyshire. manchester would be a seaport then with a vengeance! the day was shorter then, and so the interval between tide and tide was more like ten than twelve hours. accordingly, in about five hours, all that mass of water would have swept back again, and great tracts of sand between here and ireland would be left dry. another five hours, and the water would come tearing and driving over the country, applying its furious waves and currents to the work of denudation, which would proceed apace. these high tides of enormously distant past ages constitute the denuding agent which the geologist required. they are very ancient--more ancient than the carboniferous period, for instance, for no trees could stand the furious storms that must have been prevalent at this time. it is doubtful whether any but the very lowest forms of life then existed. it is the strata at the bottom of the geological scale that are of the most portentous thickness, and the only organism suspected in them is the doubtful _eozoon canadense_. sir robert ball believes, and several geologists agree with him, that the mighty tides we are contemplating may have been coæval with this ancient laurentian formation, and others of like nature with it. but let us leave geology now, and trace the inverted progress of events as we recede in imagination back through the geological era, beyond, into the dim vista of the past, when the moon was still closer and closer to the earth, and was revolving round it quicker and quicker, before life or water existed on it, and when the rocks were still molten. suppose the moon once touched the earth's surface, it is easy to calculate, according to the principles of gravitation, and with a reasonable estimate of its size as then expanded by heat, how fast it must then have revolved round the earth, so as just to save itself from falling in. it must have gone round once every three hours. the month was only three hours long at this initial epoch. remember, however, the initial length of the day. we found that it was just possible for the earth to rotate on its axis in three hours, and that when it did so, something was liable to separate from it. here we find the moon in contact with it, and going round it in this same three-hour period. surely the two are connected. surely the moon was a part of the earth, and was separating from it. that is the great discovery--the origin of the moon. once, long ages back, at date unknown, but believed to be certainly as much as fifty million years ago, and quite possibly one hundred million, there was no moon, only the earth as a molten globe, rapidly spinning on its axis--spinning in about three hours. gradually, by reason of some disturbing causes, a protuberance, a sort of bud, forms at one side, and the great inchoate mass separates into two--one about eighty times as big as the other. the bigger one we now call earth, the smaller we now call moon. round and round the two bodies went, pulling each other into tremendously elongated or prolate shapes, and so they might have gone on for a long time. but they are unstable, and cannot go on thus: they must either separate or collapse. some disturbing cause acts again, and the smaller mass begins to revolve less rapidly. tides at once begin--gigantic tides of molten lava hundreds of miles high; tides not in free ocean, for there was none then, but in the pasty mass of the entire earth. immediately the series of changes i have described begins, the speed of rotation gets slackened, the moon's mass gets pushed further and further away, and its time of revolution grows rapidly longer. the changes went on rapidly at first, because the tides were so gigantic; but gradually, and by slow degrees, the bodies get more distant, and the rate of change more moderate. until, after the lapse of ages, we find the day twenty-four hours long, the moon , miles distant, revolving in - / days, and the tides only existing in the water of the ocean, and only a few feet high. this is the era we call "to-day." the process does not stop here: still the stately march of events goes on; and the eye of science strives to penetrate into the events of the future with the same clearness as it has been able to descry the events of the past. and what does it see? it will take too long to go into full detail: but i will shortly summarize the results. it sees this first--the day and the month both again equal, but both now about , hours long. neither of these bodies rotating with respect to each other--the two as if joined by a bar--and total cessation of tide-generating action between them. the date of this period is one hundred and fifty millions of years hence, but unless some unforeseen catastrophe intervenes, it must assuredly come. yet neither will even this be the final stage; for the system is disturbed by the tide-generating force of the sun. it is a small effect, but it is cumulative; and gradually, by much slower degrees than anything we have yet contemplated, we are presented with a picture of the month getting gradually shorter than the day, the moon gradually approaching instead of receding, and so, incalculable myriads of ages hence, precipitating itself upon the surface of the earth whence it arose. such a catastrophe is already imminent in a neighbouring planet--mars. mars' principal moon circulates round him at an absurd pace, completing a revolution in - / hours, and it is now only , miles from his surface. the planet rotates in twenty-four hours as we do; but its tides are following its moon more quickly than it rotates after them; they are therefore tending to increase its rate of spin, and to retard the revolution of the moon. mars is therefore slowly but surely pulling its moon down on to itself, by a reverse action to that which separated our moon. the day shorter than the month forces a moon further away; the month shorter than the day tends to draw a satellite nearer. this moon of mars is not a large body: it is only twenty or thirty miles in diameter, but it weighs some forty billion tons, and will ultimately crash along the surface with a velocity of , miles an hour. such a blow must produce the most astounding effects when it occurs, but i am unable to tell you its probable date. so far we have dealt mainly with the earth and its moon; but is the existence of tides limited to these bodies? by no means. no body in the solar system is rigid, no body in the stellar universe is rigid. all must be susceptible of some tidal deformation, and hence, in all of them, agents like those we have traced in the history of the earth and moon must be at work: the motion of all must be complicated by the phenomena of tides. it is prof. george darwin who has worked out the astronomical influence of the tides, on the principles of sir william thomson: it is sir robert ball who has extended mr. darwin's results to the past history of our own and other worlds.[ ] tides are of course produced in the sun by the action of the planets, for the sun rotates in twenty-five days or thereabouts, while the planets revolve in much longer periods than that. the principal tide-generating bodies will be venus and jupiter; the greater nearness of one rather more than compensating for the greater mass of the other. it may be interesting to tabulate the relative tide-producing powers of the planets on the sun. they are as follows, calling that of the earth , :-- relative tide-producing powers of the planets on the sun. mercury , venus , earth , mars jupiter , saturn , uranus neptune the power of all of them is very feeble, and by acting on different sides they usually partly neutralize each other's action; but occasionally they get all on one side, and in that case some perceptible effect may be produced; the probable effect seems likely to be a gentle heaving tide in the solar surface, with breaking up of any incipient crust; and such an effect may be considered as evidenced periodically by the great increase in the number of solar spots which then break out. the solar tides are, however, much too small to appreciably push any planet away, hence we are not to suppose that the planets originated by budding from the sun, in contradiction of the nebular hypothesis. nor is it necessary to assume that the satellites, as a class, originated in the way ours did; though they may have done so. they were more probably secondary rings. our moon differs from other satellites in being exceptionally large compared with the size of its primary; it is as big as some of the moons of jupiter and saturn. the earth is the only one of the small planets that has an appreciable moon, and hence there is nothing forced or unnatural in supposing that it may have had an exceptional history. evidently, however, tidal phenomena must be taken into consideration in any treatment of the solar system through enormous length of time, and it will probably play a large part in determining its future. when laplace and lagrange investigated the question of the stability or instability of the solar system, they did so on the hypothesis that the bodies composing it were rigid. they reached a grand conclusion--that all the mutual perturbations of the solar system were periodic--that whatever changes were going on would reach a maximum and then begin to diminish; then increase again, then diminish, and so on. the system was stable, and its changes were merely like those of a swinging pendulum. but this conclusion is not final. the hypothesis that the bodies are rigid is not strictly true: and directly tidal deformation is taken into consideration it is perceived to be a potent factor, able in the long run to upset all their calculations. but it is so utterly and inconceivably minute--it only produces an appreciable effect after millions of years--whereas the ordinary perturbations go through their swings in some hundred thousand years or so at the most. granted it is small, but it is terribly persistent; and it always acts in one direction. never does it cease: never does it begin to act oppositely and undo what it has done. it is like the perpetual dropping of water. there may be only one drop in a twelvemonth, but leave it long enough, and the hardest stone must be worn away at last. * * * * * we have been speaking of millions of years somewhat familiarly; but what, after all, is a million years that we should not speak familiarly of it? it is longer than our lifetime, it is true. to the ephemeral insects whose lifetime is an hour, a year might seem an awful period, the mid-day sun might seem an almost stationary body, the changes of the seasons would be unknown, everything but the most fleeting and rapid changes would appear permanent and at rest. conversely, if our life-period embraced myriads of æons, things which now seem permanent would then appear as in a perpetual state of flux. a continent would be sometimes dry, sometimes covered with ocean; the stars we now call fixed would be moving visibly before our eyes; the earth would be humming on its axis like a top, and the whole of human history might seem as fleeting as a cloud of breath on a mirror. evolution is always a slow process. to evolve such an animal as a greyhound from its remote ancestors, according to mr. darwin, needs immense tracts of time; and if the evolution of some feeble animal crawling on the surface of this planet is slow, shall the stately evolution of the planetary orbs themselves be hurried? it may be that we are able to trace the history of the solar system for some thousand million years or so; but for how much longer time must it not have a history--a history, and also a future--entirely beyond our ken? those who study the stars have impressed upon them the existence of the most immeasurable distances, which yet are swallowed up as nothing in the infinitude of space. no less are we compelled to recognize the existence of incalculable æons of time, and yet to perceive that these are but as drops in the ocean of eternity. footnotes: [ ] the following account of mars's motion is from the excellent small manual of astronomy by dr. haughton of trinity college, dublin:--(p. ) "mars's motion is very unequal; when he first appears in the morning emerging from the rays of the sun, his motion is direct and rapid; it afterwards becomes slower, and he becomes stationary when at an elongation of ° from the sun; then his motion becomes retrograde, and its velocity increases until he is in opposition to the sun at °; at this time the retrograde motion is most rapid, and afterwards diminishes until he is ° distant from the sun on the other side, when mars again becomes stationary; his motion then becomes direct, and increases in velocity until it reaches a maximum, when the planet is again in conjunction with the sun. the retrograde motion of this planet lasts for days: and its arc of retrogradation is °." [ ] it is not so easy to plot the path of the sun among the stars by direct observation, as it is to plot the path of a planet; because sun and stars are not visible together. hipparchus used the moon as an intermediary; since sun and moon are visible together, and also moon and stars. [ ] this is, however, by no means the whole of the matter. the motion is not a simple circle nor has it a readily specifiable period. there are several disturbing causes. all that is given here is a first rough approximation. [ ] the proof is easy, and ought to occur in books on solid geometry. by a "regular" solid is meant one with all its faces, edges, angles, &c., absolutely alike: it is of these perfectly symmetrical bodies that there are only five. crystalline forms are practically infinite in number. [ ] best known to us by his christian name, as so many others of that time are known, _e.g._ raphael sanzio, dante alighieri, michael angelo buonarotti. the rule is not universal. tasso and ariosto are surnames. [ ] it would seem that the fact that all bodies of every material tend to fall at the same rate is still not clearly known. confusion is introduced by the resistance of the air. but a little thought should make it clear that the effect of the air is a mere disturbance, to be eliminated as far as possible, since the atmosphere has nothing to do with gravitation. the old fashioned "guinea and feather experiment" illustrates that in a vacuum things entirely different in specific gravity or surface drop at the same pace. [ ] karl von gebler (galileo), p. . [ ] it is of course the "silver lining" of clouds that outside observers see. [ ] l.u.k., _life of galileo_, p. . [ ] _note added september, ._ news from the lick observatory makes a very small fifth satellite not improbable. [ ] they remained there till this century. in they were quietly dropped. [ ] it was invented by van helmont, a belgian chemist, who died in . he suggested two names _gas_ and _blas_, and the first has survived. blas was, i suppose, from _blasen_, to blow, and gas seems to be an attempt to get at the sanskrit root underlying all such words as _geist_. [ ] such as this, among many others:--the duration of a flame under different conditions is well worth determining. a spoonful of warm spirits of wine burnt pulsations. the same spoonful of spirits of wine with addition of one-sixth saltpetre burnt pulsations. with one-sixth common salt, ; with one-sixth gunpowder, ; a piece of wax in the middle of the spirit, ; a piece of _kieselstein_, ; one-sixth water, ; and with equal parts water, only pulse-beats. this, says liebig, is given as an example of a "_licht-bringende versuch_." [ ] draper, _history of civilization in europe_, vol. ii. p. . [ ] professor knight's series of philosophical classics. [ ] to explain why the entire system, horse and cart together, move forward, the forces acting on the ground must be attended to. [ ] the distance being proportional to the _square_ of the time, see p. . [ ] the following letter, recently unearthed and published in _nature_, may , , seems to me well worth preserving. the feeling of a respiratory interval which it describes is familiar to students during the too few periods of really satisfactory occupation. the early guess concerning atmospheric electricity is typical of his extraordinary instinct for guessing right. "london, _dec. , _. "dear doctor,--he that in ye mine of knowledge deepest diggeth, hath, like every other miner, ye least breathing time, and must sometimes at least come to terr. alt. for air. "in one of these respiratory intervals i now sit down to write to you, my friend. "you ask me how, with so much study, i manage to retene my health. ah, my dear doctor, you have a better opinion of your lazy friend than he hath of himself. morpheous is my last companion; without or hours of him yr correspondent is not worth one scavenger's peruke. my practices did at ye first hurt my stomach, but now i eat heartily enou' as y' will see when i come down beside you. "i have been much amused at ye singular [greek: _phenomena_] resulting from bringing of a needle into contact with a piece of amber or resin fricated on silke clothe. ye flame putteth me in mind of sheet lightning on a small--how very small--scale. but i shall in my epistles abjure philosophy whereof when i come down to sakly i'll give you enou'. i began to scrawl at mins. from of ye clk. and have in writing consmd. mins. my ld. somerset is announced. "farewell, gd. bless you and help yr sincere friend. "isaac newton. "_to_ dr. law, suffolk." [ ] kepler's laws may be called respectively, the law of path, the law of speed, and the relationship law. by the "mass" of a body is meant the number of pounds or tons in it: the amount of matter it contains. the idea is involved in the popular word "massive." [ ] the equation we have to verify is [pi]^ r^ gr^ = -----------, t^ with the data that _r_, the moon's distance, is times r, the earth's radius, which is , miles; while t, the time taken to complete the moon's orbit, is days, hours, minutes, seconds. hence, suppose we calculate out _g_, the intensity of terrestrial gravity, from the above equation, we get [pi]^ · × × miles _g_ = ---------- × ( )^ r = ----------------------------- t ( days, hours, &c.)^ = · feet-per-second per second, which is not far wrong. [ ] the two motions may be roughly compounded into a single motion, which for a few centuries may without much error be regarded as a conical revolution about a different axis with a different period; and lieutenant-colonel drayson writes books emphasizing this simple fact, under the impression that it is a discovery. [ ] members of the accademia dei lyncei, the famous old scientific society established in the time of cosmo de medici--older than our own royal society. [ ] newton suspected that the moon really did so oscillate, and so it may have done once; but any real or physical libration, if existing at all, is now extremely minute. [ ] an interesting picture in the new gallery this year ( ), attempting to depict "earth-rise in moon-land," unfortunately errs in several particulars. first of all, the earth does not "rise," but is fixed relatively to each place on the moon; and two-fifths of the moon never sees it. next, the earth would not look like a map of the world with a haze on its edge. lastly, whatever animal remains the moon may contain would probably be rather in the form of fossils than of skeletons. the skeleton is of course intended as an image of death and desolation. it is a matter of taste: but a skeleton, it seems to me, speaks too recently of life to be as appallingly weird and desolate as a blank stone or ice landscape, unshaded by atmosphere or by any trace of animal or plant life, could be made. [ ] five of jupiter's revolutions occupy , days; two of saturn's revolutions occupy , days. [ ] _excircularity_ is what is meant by this term. it is called "excentricity" because the foci (not the centre) of an ellipse are regarded as the representatives of the centre of a circle. their distance from the centre, compared with the radius of the unflattened circle, is called the excentricity. [ ] a curve of the _n_th degree has / _n_(_n_+ ) arbitrary constants in its equation, hence this number of points specifically determine it. but special points, like focus or vertex, count as two ordinary ones. hence three points plus the focus act as five points, and determine a conic or curve of the second degree. three observations therefore fix an orbit round the sun. [ ] its name suggests a measure of the diameter of the sun's disk, and this is one of its functions; but it can likewise measure planetary and other disks; and in general behaves as the most elaborate and expensive form of micrometer. the königsberg instrument is shewn in fig. . [ ] it may be supposed that the terms "minute" and "second" have some necessary connection with time, but they are mere abbreviations for _partes minutæ_ and _partes minutæ secundæ_, and consequently may be applied to the subdivision of degrees just as properly as to the subdivision of hours. a "second" of arc means the th part of a degree, just as a second of time means the th part of an hour. [ ] a group of flying particles, each one invisible, obstructs light singularly little, even when they are close together, as one can tell by the transparency of showers and snowstorms. the opacity of haze may be due not merely to dust particles, but to little eddies set up by radiation above each particle, so that the air becomes turbulent and of varying density. (see a similar suggestion by mr. poynting in _nature_, vol. , p. .) [ ] the moon ought to be watched during the next great shower, if the line of fire happens to take effect on a visible part of the dark portion. [ ] address to birmingham midland institute, "a glimpse through the corridors of time." index index a abbott, t.k., on tides, adams, john couch, , , , , , , , , , , airy, sir george, , , , , , , anaxagoras, appian, arabs, the, form a link between the old and new science, archimedes, , , , , , aristarchus, aristotle, , , , , , . he taught that the earth was a sphere, ; his theories did not allow of the earth's motion, ; he was regarded as inspired, b bacon, francis, , , , . his _novum organum_, bacon, roger, , , . the herald of the dawn of science, brahé, george, uncle of tycho brahé, brahé, steno, brother of tycho brahé, brahé, tycho, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , . he tried to adopt the main features of the copernican theory without admitting the motion of the earth, ; he was a poor theorist but a great observer, ; his medicine, ; his personal history, , _seq._; his observatory, uraniburg, ; his greatest invention, , note; his maniac lep, ; his kindness to kepler, ball, sir r., , ; his _story of the heavens_, barrow, dr., , bessel, , , , , , , , biela, , , bode's law, , , , , boyle, , bradley, prof. james, , , , , , , , bremiker, , brewster, on kepler, brinkley, bruno, giordano, , c castelli, , cayley, prof., challis, prof., , clairut, , , , , , clark, alvan and sons, columbus, , copernicus, , , _seq._, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ; his _de revolutionibus orbium coelestium_, , , ; he _proved_ that the earth went round the sun, ; the influence of his theory on the church, , _seq._; his life-work summarised, ; his life by mr. e.j.c. morton, copernican tables, ; copernican theory, , , , , copernik, nicolas; see copernicus cornu, croll, dr., his _climate and time_, d d'alembert, , darwin, charles, , , darwin, prof. george, , delambre, descartes, , , , , , , , , , , , , , ; his _discourse on method_, ; his dream, ; his system of algebraic geometry, , _seq._; his doctrine of vortices, , _seq._; his _principia mathematica_, ; his life by mr. mahaffy, e earth, the difficulties in the way of believing that it moved, , _seq._ "earth-rise in moon-land," , note encke, , epicyclic orbits explained, , _seq._ equinoxes, their precession discovered by hipparchus, eudoxus, euler, , f faraday, fizeau, , flamsteed, , , , , fraunhofer, froude, prof.; his _oceania_, g galen, galileo, galilei, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ; his youth, ; his discovery of the pendulum, ; his first observations about falling bodies, , _seq._; he invents a telescope, ; he adopts the copernican theory, ; he conceives "earth-shine," ; he discovers jupiter's moons, ; he studies saturn, , _seq._; his _dialogues on the ptolemaic and copernican systems_, ; his abjuration, ; he becomes blind, ; he discovered the laws of motion, , _seq._; he guessed that sight was not instantaneous, , galle, dr., , gauss, , gilbert, dr., , , , ; his _de magnete_, , greeks, their scientific methods, groombridge's catalogue, h hadley, halley, , , , , , , , , , , , , , ; he discovered the _principia_, harvey, , haughton, dr., ; his manual on astronomy, , note heliometer, described, helmholtz, helmont, van, invented the word "gas," henderson, , herschel, alexander, , , , herschel, caroline, , , , , ; her journal quoted, , _seq._; her work with william h. described, herschel, sir john, , , , herschel, william, , , , , , , , , , , , , , , , , , , , , , ; he "sweeps" the heavens, ; his discovery of uranus, , ; his artificial saturn, , ; his methods of work with his sister, described, ; he founded the science of astronomy, hind, hipparchus, , , , , , and note, , , , , ; an explanation of his discovery of the precession of the equinoxes, , seq. hippocrates, homeric cosmogony, , _seq._ hooke, , , , , , , hôpital, marquis de l', horkey, martin, horrebow, huxley, prof., huyghens, , , k kant, , kelvin, lord, see thomson, sir w. kepler, john, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ; he replaced epicycles by an ellipse, ; he was a pupil of tycho brahé, ; he was a speculator more than an observer, ; his personal life, , _seq._; his theories about the numbers and distances of the planets, , ; he was helped by tycho, ; his main work, , _seq._; he gave up circular motion, ; his _mysterium cosmographicon_, ; his laws, , , , , , , , , _seq._ l lagrange, , , , , , , lagrange and laplace, , , ; they laid the foundations of the planetary theory, laplace, , , , , , , , , , , , , , , , , , ; his nebular hypothesis, , ; his _mécanique céleste_, lassell, mr., , leibnitz, , , le monnier, leonardo, see vinci, leonardo da leverrier, , , , , , lippershey, hans, m maskelyne, maxwell, clerk, , molyneux, , morton, mr. e.j. c, his life of copernicus, n newton, prof. h.a., newton, sir isaac, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ; his _principia_, , , , , , , , , , , , , , , ; his early life, , _seq._; his first experiments, ; his work at cambridge, ; his laws, ; his application of the laws of gravity to astronomy, , , , , ; his reticence, ; his discoveries in optics, , _seq._; his work summarised, ; his _optics_, ; anecdotes of him, ; his appearance in a court of justice, ; some of his manuscripts very recently discovered, ; his theories of the equinoxes and tides, , _seq._, , , _seq._ o olbers, , p peters, prof., , piazzi, , , , picard, , , , pioneers, genuine, planets and days of the week, poynting, printing, ptolemy, , , , , , , , ; his system of the heavens simplified by copernicus, , ; his system described, , _seq._; his system taught, ; his harmonies, pythagoras, , , q quadrant, an early, , r rheiter, ricci, ostillio, , roberts, isaac, roemer, , , , , , , rosse, lord, his telescope, , rudolphine tables, s scheiner, sizzi, francesca, an orthodox astronomer, snell, willebrod, and the law of refraction, solar system, its fate, stars, a list of, struve, , , , stuart, prof., quoted, t tatius, telescopes, early, thales, , , thomson, sir william, , , , , tide-gauge, described, , _seq._ tides, , _seq._ time, is not exactly uniform, torricelli, , tycho, see brahé, tycho v vinci, leonardo da, , , , viviani, , voltaire, w watson, prof., whewell, wren, sir christopher, , , , z zach, von, , zone of asteroids, , _seq._ the end. richard clay and sons, limited, london and bungay. catalog a. . astronomical instruments ... and ... accessories. [illustration] wm. gaertner & co. - lake ave. chicago. catalog a. . astronomical instruments ... and ... accessories. [illustration] wm. gaertner & co. - lake ave. chicago. notice. this catalogue supersedes former editions. the prices given in this catalogue are net and do not include packing which will be charged at cost. to avoid mistakes and delays when ordering please give catalogue number and shipping instructions. most of the instruments listed in this catalogue are constructed to order only but the smaller sizes of telescopes with accessories, chronographs, simpler measuring machines, etc., are usually kept in stock. the apparatus listed in this catalogue is of our own manufacture, excepting the astronomical regulator clocks, which we have listed for the convenience of our customers. all orders will be filled as promptly as possible with due regard to thorough workmanship and efficient inspection. everything that leaves our establishment is carefully tested and inspected and we can guarantee our apparatus to be, in every respect, fully as represented. any piece, which does not come up to the most exacting requirements will always be promptly replaced within the shortest possible time. we shall be glad to satisfy any special requirements of our customers and will make any desired alterations and additions on the standard designs. introduction. in the following pages we have endeavored to give to our customers an idea of the line of astronomical instruments of the latest and most improved types which we have constructed for some of the leading observatories as mentioned in the text. we feel that this series of illustrations may not only be of general interest, but also of service in helping to determine further developments of modern astronomical and astrophysical equipment. we have not thought it advisable to describe in detail the various instruments of precision, which we have been called upon to construct from time to time for the scientists in the astronomical as well as other branches of science during the last ten years. as a rule every astronomer is interested in some special line of research, which for a successful investigation requires a special adaptation of the instrument he proposes to use, and in such cases we are confident we can fill the requirements and shall be happy to correspond with interested parties. we are in position to undertake the design of special apparatus and can furnish sketches and estimates in accordance with the suggestions of the customer on short notice in such cases, where the importance of the prospective business will warrant it. among the various instruments which we have built, but which are not included in this catalogue, we wish to mention, spectroheliographs, planetary cameras, photographic plate holders, domes for observatories, etc. the optical parts which we furnish are of the highest degree of excellence and are made for us by several of the most reliable firms in this line. mr. o. l. petitdidier is closely located to our works, and during the last ten years has supplied most of our objectives, etc. it is and has been the aim of our firm to produce apparatus of the highest grade and the constant growth of our business is a mute but eloquent testimonial that our efforts have been appreciated by our customers. we are glad to acknowledge our indebtedness to many of the foremost astronomers, who have greatly assisted us by suggestions as to their particular requirements, and by supplying certain of the photographs here reproduced. we take this opportunity to thank our customers for the favors rendered us in the past, and trust that in the future these mutually pleasant relations may be renewed. [illustration: a ] #a . alt-azimuth mounting# on strong hardwood tripod. the vertical and horizontal axis have large bearing surfaces, assuring stability and steadiness of motion. all parts excepting the tripod head are made of brass and are nicely finished. the telescope is fitted with long rack and pinion motion. three celestial eye pieces are included. price with ½" telescope #$ . # #a .# the same as above with " telescope. price #$ . # #a .# the same as above with ½" telescope. price #$ . # #a .# the same as above with " telescope. price #$ . # #note.#--all our astronomical telescope have objectives of the standard focal length (focus = to about times diameter of aperture). the objective is mounted in the most approved manner and is provided with adjustment for collimation. #note.#--for accessories to these telescopes see page . [illustration: a ] #a . universal equatorial mounting.# this form of mounting can be used equally well for celestial and terrestrial observations. the mounting is made to swivel on the tripod head, in order to set the instrument in the meridian. the polar axis can be set at any latitude and a graduated arc gives the exact position. the instrument is set level by means of two small levels attached to the tripod top. the polar axis is fitted with worm wheel and worm for slow motion. the handle with the universal joint can be clamped on either side of the worm shaft. telescope of " aperture and three eye pieces. price #$ . # #a .# same as above with ½" telescope. price #$ . # #a .# same as above with " telescope. price #$ . # for accessories see page . [illustration: a ] #a . portable equatorial mounting with driving clock.# this instrument was designed to meet the demand for portable low priced telescope suitable for the study of astronomy in the college, high school or for the amateur astronomer. every observer and teacher in astronomy will appreciate the great usefulness of a driving mechanism, which will keep the star in the field during observation. after several years of experimenting we have succeeded in constructing a reliable clock which can easily be attached to our portable telescope mounting. the instrument is mounted on strong hardwood tripod fitted with iron shoes. it is attached to the tripod top by a single screw which holds it firmly and allows adjustment in azimuth. the clock case carrying the bearing for the polar axis is made to swivel in the base casting, so that the polar axis may be easily set and firmly clamped at an angle from horizontal to vertical. a level is fitted to the tripod top and a graduated arc is fastened to the clock case. if the polar axis is set vertical and the worm wheel unclamped, the instrument is transformed into an alt-azimuth mounting. this feature will be found to be a great convenience especially for terrestrial observations. both the polar and declination axes are carefully fitted to their bearings and carry finding circles. the right ascension circle reads to min., the declination circle to single degrees. the declination axis is fitted with clamp screw which is within convenient reach. the clock has ample power and is enclosed in a heavy case which protects it from dust and injury. it is driven by two strong springs and will run about hours without rewinding. a lever for starting or stopping is provided. motion from clock to polar axis is transmitted by means of a set of bevel gears and worm and worm wheel. the worm wheel is held by friction to the polar axis so that the telescope can be moved without loosening any screw and without affecting the clock. the clock will give steady and accurate motion to the telescope and with ordinary care it will keep in good repair for years. a slow motion adjustment independent of the clock is fitted to the polar axis. with the instrument are furnished three celestial eye pieces giving a magnifying power of about , and diameters respectively. the instrument is easily portable, the total weight of a " telescope being about lbs. yet it is made heavy enough, and the material well distributed to insure strength and steadiness. workmanship and finish of the instrument are the best. the brass parts are either lacquered yellow or bronzed. the iron parts are durably enameled and all exposed steel parts are nickel-plated. #a .# telescope of " aperture. price #$ . # #a .# telescope of ½" aperture, the same as above but the bearing parts made proportionally heavier. price #$ . # #a .# telescope of " aperture. price #$ . # for accessories see page . #note.#--an iron column can be provided for above telescopes in place of of the tripod, at an extra cost of $ . . [illustration: a ] equatorial mountings on iron pillar with driving clock, etc. on page is shown an illustration of our standard high grade type of in. equatorial. a description of the same will apply to practically all larger and smaller sizes. the pillar is of rectangular cross section and well proportioned. the clock case is securely fastened to the top of the pillar but with provision for adjustment in azimuth. the clock has ample driving power, is very carefully constructed and regulated by friction governor (design prof. young.) maintaining gears are provided on the main shaft which allows the winding of the clock without retarding motion. worm wheel and worm are carefully cut, and protected by brass shields. right ascension and declination circles have fine graduation on solid silver and coarse finding graduation on the edge. electric illumination and magnifying glasses are fitted to the verniers. the handles for all clamps and slow motions are fastened conveniently near the eye end of the telescope and are of different shape so as to distinguish in the dark right ascension and declination. the axes are of tool steel carefully fitted to their bearings, and on the larger instruments friction rollers are fitted to the polar axis. the telescope tube is made of steel, light but strongly constructed. the eye end has long and heavy rack and pinion motion and the whole can be easily removed. #note.#--the fine divided circles are often omitted on smaller equatorials, as they are not essential, the electrical illumination for the circles is also left off, and this will amount to a material saving. #a . equatorial mounting with in. telescope.# according to above description with first class objective, and five eye pieces. price #$ . # the same instrument without fine circles and electrical illumination. price #$ . # #a . equatorial mounting with in. telescope.# complete as a . price #$ . # #a . equatorial mounting with in. telescope.# same as above. price #$ . # #a . equatorial mounting with in. telescope.# same as above. price #$ . # #a . equatorial mounting with in. telescope.# same as above. price #$ . # #a . equatorial mounting with in. telescope.# same as above. price #$ . # #note.#--we are equipped to construct larger instruments and are glad to give prices on application. [illustration: a ] [illustration] the above cut shows a driving clock for our standard in. telescope. we have constructed clocks for a number of larger telescopes among others the clock for the in. lowell refractor. accessories to telescopes a to a . #a . finder# fitted to " telescope. price #$ . # #a . finder# fitted to ½" telescope. price #$ . # #a . positive eye pieces.# (ramsden), focus mm. to mm. price #$ . # #a . negative eye pieces.# (huygenian), focus mm. to mm. price #$ . # #a . diagonal eye piece.# the prism of the eye piece has guaranteed optically plane surfaces and will not affect the definition of the telescope. price #$ . # #a . terrestrial eye piece#, focus mm. price #$ . # #a . sun caps#, to fit above eye pieces. price #$ . # the outside diameter of all our eye pieces is ¼" excepting those used in our micrometers. [illustration: a ] #a . position micrometers for " to " telescopes.# circle cm. diameter, divisions on solid silver, verniers reading to min., slow gear motion for rotating, electrical illumination, provided with different color screens. screw guaranteed of highest accuracy. price #$ . # #a . position micrometers for " to " telescopes.# similar to above but rotation by hand and construction somewhat simpler. price #$ . # [illustration: a ] #a . small position micrometer for " to " telescopes.# circle reads to single degrees. electric illumination. price #$ . # [illustration: a ] #transit instrument#, with reversing arrangement, illumination with oil or electric lamp, filar micrometer with two eye pieces. weight of axis balanced by springs and rollers. the circle has a diameter of mm., verniers read to seconds. the instrument is mounted on an iron base plate, which is fitted with azimuth adjustment (not shown in cut). #a . transit# with " telescope. price #$ . # #a . transit# with " telescope. price #$ . # [illustration: a ] #a . universal instrument.# telescope with objective prism. vertical circle has a diameter of cm. is divided to min. and reads by means of two micrometer microscopes to sec. the telescope is fitted with hard phosphor bronze bearing rings and is reversible. aperture of objective is mm. two eye pieces are furnished giving magnification of and diameters. the prism has absolutely plane surfaces and will not affect the definition. striding level reads to sec. the horizontal circle is protected and reads by means of two verniers to min. the instrument is packed in a light but strong case. price complete with tripod #$ . # #note.#--this instrument was first constructed for mr. e. de k. leffingwell, who has found it very satisfactory for his work in the polar regions. [illustration: a ] #a . zoellner astrophotometer.# the instrument is made to attach to the eye end of the telescope and may be used with any size of refractor or reflector. an axis parallel to the telescope tube allows the instrument to rotate as a whole. a clamp is provided to hold it in position. the artificial star is formed by a small incandescent lamp which is adjustable in any direction, and in front of which is mounted a small diaphragm. the color changing device, consisting of nicol prism and quartz plate, is fitted with a divided circle reading to single degrees. the circle revolving with the nicol prism for changing the intensity of the star, has a diameter of cm. and reads by means of two verniers to min. a concave lens is mounted in the path of the artificial star to make the light diverging at the proper angle and a plane parallel plate is adjustably fixed in the center of the box in order to throw the light in the eye piece. an achromatic objective in front of the eye piece brings the images of the real and artificial star to a focus in the same plane. price #$ . # [illustration: a ] [illustration: a ] #a . small spectroscope.# suitable for telescope from to in. aperture. the collimator and observing telescope have an aperture of mm., focus of mm. both are fitted with rack and pinion. the spectroscope may be used with a grating or a degree prism, and for this purpose has openings for the telescopes at the proper angles. a position circle of mm. diameter, reading to degrees, is fitted to the instrument. the slit has micrometer head. price, without grating, but including degree prism #$ . # #note.#--this spectroscope can also be used on a support for laboratory work. #support# for above spectroscope. price #$ . # #a . spectroscope# for telescopes from in. to in. aperture and detailed description on application. [illustration: a ] #a . chronograph# for physical and astronomical work. this chronograph is designed to meet the demand for a medium sized, well-made and accurate instrument of this kind and has given very good satisfaction. the cylinder is cm. in diameter cm. long, driven by strong, carefully made clock work, fitted with friction governor. two different speeds for the cylinder can be obtained by means of change gears. the slow speed of the drum is one revolution per minute, and at this speed the drum will run for a full hour. the fast speed is seconds per revolution. the carriage is driven by means of a screw, the nut of which is made to disengage easily. price of the instrument complete with two pens and glass cover #$ . # #a a. chronograph#, the same as a but with only one pen. the carriage is so constructed that a second pen can easily be added later. price with glass cover #$ . # #note.#--every instrument is carefully tested before being sent out and we can guarantee the speed not to vary over - second during the full run of one hour. large temperature changes will not affect the speed of the clock. a large number of the chronographs are in use and have given excellent satisfaction. [illustration: a ] #a . prof. g. w. hough's printing chronograph.# the instrument consists of two carefully and accurately constructed clock movements, which are driven by gravity and controlled electrically by the sidereal clock. the movements revolve three type wheels. one of these turns once per second, its edge is divided in parts and it is driven by a separate movement. the second wheel turns once per minute and the third once per hour and they will print the seconds and minutes, while the first will give the hundredths of seconds. a strip of paper is carried over these wheels and moved forward by the same electro-magnet, which operates the printing hammers. the paper is sufficiently long for observations including spacing between records. the operation of the printing hammers is such that the uniform motion of the type wheel is not disturbed in the act of printing. the whole instrument is mounted on a heavy slate plate cm. by cm., and protected by a glass cover. the manipulation of the apparatus is extremely simple and convenient and the records obtained are perfectly reliable and accurate within about . of a second. the saving of time and labor by the printing chronograph is very considerable and the filing of the records very convenient. this printing chronograph has been in use at the dearborn observatory for about twenty years and during the last five years the following observatories have been equipped with the instrument: amherst college observatory; case school of applied science, cleveland; philadelphia observatory; durham observatory, durham, england; observatory of laplatta, argentine; and dominion observatory, ottawa, canada. prof. hough has kindly consented to inspect every chronograph before it leaves our shop. price, complete on stand, glass cover, etc. #$ . # [illustration: a ] [illustration: a ] #a . laboratory clock.# this is an eight-day clock; has a movement of the best workmanship, is driven by two strong springs, and keeps accurate time. the dial is inches in diameter and has hour, minute and second hands. pendulum beats seconds and makes electric contact by means of an adjustable mercury cup. it is mounted in a hardwood case with glass door. price #$ . # #a . laboratory clock.# the same as preceding one, but with better clock movement, gravity driven. price #$ . # #a . regulator clock.# this clock has a first-class eight day movement with cut steel pinions. it is fitted with mercury compensation pendulum and electrical seconds contact. price #$ . # #note.#--we can furnish free of duty to educational institutions astronomical precision clocks made by c. riefler, germany, and will be pleased to quote prices to interested parties. [illustration: a ] #a . simple comparator.# (fig. a .) built up of #micrometer slide# m and #microscope# m , #with support# m #fitted with stage#. the stage is mm. long and provided with clips for holding objects, such as spectrum photographs, scales, etc., and is fitted with illuminating mirror. price #$ . # #a . small comparator.# the instrument is intended for measuring spectra photographs, gratings, divided scales, or such objects which can be focused by the microscope and will allow rapid measurements of the highest possible accuracy. the measurement depends on the accuracy of the micrometer screw, which is cut and corrected with great care. the screw has a pitch of . mm. and diameter of mm. the index head attached to the screw is of considerable diameter so as to allow the direct reading of . mm. the head is divided on solid silver in parts, and carries two rows of figures indicating the first and second half of the mm. the full mm. are read by means of a scale in front of the instrument. the bed plate is heavy, of cast iron, and the guides are carefully scraped true within . mm. the carriage has a movement of mm., is made of gun metal and fitted exactly to the guides; it is also provided with a second or top carriage with mm. motion. the top carriage can be moved by hand and accurately set by means of a micrometer screw. the microscope is of variable magnifying power, focused by rack and pinion. illumination for transparent objects is given from below by means of a plane mirror. the instrument is mounted on heavy supports, under an angle to make it convenient for the observer. the instrument is finished in first-class manner, and the iron bed plate heavily copper and nickel plated. the design of this machine was suggested by prof. edwin frost of the yerkes observatory, where a number of these machines have been in constant use during the last five years. careful tests have shown the screw accurate within . of a mm. throughout the full length. price #$ . # #note.#--the micrometer on microscope as shown in cut is not furnished with instrument. a tangent screw for revolving the eye piece with the spider thread can be attached at a cost of $ . . [illustration: a ] [illustration: a ] #a . comparator for measuring spectra photographs, scales, etc.# range cm. the microscope travels on carefully straightened ways, and is moved by a screw of mm. pitch. the screw head is faced with a silver band and is divided in parts. the handle for turning the screw is placed on the left side of the instrument so as to have the right hand free for recording readings. a set of change gears can easily be engaged to give a fast motion to the carriage. the stage will hold plates from to cm. in width and of cm. in length. it can be shifted lengthwise for a distance of cm. and is provided with adjustment for aligning the plates. the whole stage can be easily removed from the instrument. the microscope is fitted with variable magnifying power and is of standard size. price #$ . # #note.#--the design of this comparator was suggested by prof. humphries, director laboratory of the u. s. weather bureau, for whom the first one was built. [illustration: a ] #a . comparator for star photographs#, measures in one direction mm. and under right angle mm. the lower part is constructed similar to our comparator a . the top slide carries a divided circle for measuring position angles. the circle is arranged similar to a position micrometer. it is fitted with quick gear motion and tangent screw and the two verniers read to - degree. the microscope has variable magnifying power and is provided with reversing eye piece. this machine has been furnished for and is in use at yerkes, lick and kirkwood observatories. price #$ . # [illustration: a ] #a . comparator.# for plates × inches. this machine was constructed in accordance with suggestions given by dr. frank schlesinger, director of allegheny observatory. the ways are carefully straightened to within . millimeter. the carriage is moved by two racks and pinions and has a large handle on each side. two concentric circles are fitted to the carriage, the inner circle carries the plate with the film in a fixed plane, no matter what the thickness of the glass may be. both circles are provided with clamps and tangent screws, so that each one may be clamped and adjusted independently. the outside circle carries four index points degrees apart. one of these marks is made adjustable. these four marks serve to turn the plate exactly degrees so as to measure rectangular coordinates. the guide carrying the measuring microscope is adjusted exactly at right angles to the ways of the bed plate. the carriage supporting the microscope is moved by rack and pinion. the microscope is arranged to tilt, so as to view either the plate or the scale above. the eye piece is provided with a reversing prism. measurements are made on the scale divided in millimeters. the smaller measurements, to - of a mm., are made by the micrometer. the micrometer is fitted with sliding eye piece and counter for full revolutions. the scale is not marked in the usual way with single lines, but each millimeter is marked with a double line. this double line allows the use of one single spider thread, which is preferable to use for bisecting the star, and the double line on the scale allows a clear setting with the single spider thread. the stage is fitted with an adaptor for holding plates × inches. price #$ . # this machine has been in use during the last years at the yerkes observatory and has been found very convenient and satisfactory in every way. a graduated circle can be provided if desirable. such division on solid silver, with verniers reading to - of a degree will increase the cost $ . . #a . comparator#, for plates × in. of the same design as a . price #$ . # [illustration: a ] #a . comparator.# a larger machine of the same design as a with scale of cm. length, divided in double lines to . mm., and with two carriages, one for spectrum plates of in. length and the other with divided circles for star photographs "× ". this instrument was constructed for the solar observatory of the carnegie institution at mount wilson, california. [illustration: coelostat. as built for the united states naval observatory after design furnished by mr. w. w. dinwiddie.] [illustration: armillary sphere. built after design of prof. j. f. lanneau, wake forest college, n. c.] _november, _ wm. gaertner & co. astronomical, physical and physiological apparatus - lake avenue chicago [illustration] spectrometers and accessories #cat. no. l laboratory spectrometer.# this instrument is very rigid and accurate in construction and every part of its design has been carefully considered. the circle has a diameter of cm. and the two verniers read to sec. it is fitted with protecting plate which adds greatly to the value of the instrument if put in the hands of students. the graduation is on solid silver. the verniers are fitted with adjustable magnifying glasses, the telescopes have an aperture of mm., focal length of mm. a horizontal adjustment for the telescope is provided. no provision is made for radial adjustment, which correction is made before the instrument is sent out. the slit is accurately constructed; the jaws are of german silver and it is provided with comparison prism. the eye end of the observing telescope is of standard size so as to receive our micrometers m or m , or the auto collimating eye piece (lamont & abbe) m . gauze eye piece is fitted to the instrument. the prism table and observing telescope have independent movements and each is provided with clamp and tangent screw. the prism table can be clamped to any part of the vernier plate. the prism holder has convenient leveling arrangement and will hold prism up to mm. in height. the instrument is arranged to receive fuess centering apparatus and crystal holder. a degree heavy flint glass prism of best optical quality is included. price $ . #l b camera attachment# will fit in place of the observing telescope. it is fitted with long focus objective and standard plate holder ½"× ½". the plate holder is provided with swivel for proper focusing of the spectrum. the slit is so arranged that four exposures can be made on one plate. price $ . #cat. no. l c scale tube.# the third arm of the instrument can be easily attached and rotated and clamped in any position. it may also be used for holding a second collimator. price $ . #cat. no. l student's spectrometer.# this instrument is in every respect similar to our l , but some of the higher finish has been omitted. all the essentials are carefully constructed and the optical parts are of the best quality. the circle is protected but only one vernier reading to sec. is fitted to the instrument. the magnifying glass is omitted. price, including prism $ . [illustration] [illustration] transcribers' notes general: corrections to punctuation have not been individually noted. general: inconsistent spelling of catalog/catalogue preserved as in original. page : magnifiing corrected to magnifying and celestrial corrected to celestial. page : intensisy corrected to intensity. page : hundreths corrected to hundredths. laplatta as in original. nb the final two pages of the book, on astronomical, physical and physiological apparatus, are in a slightly different format in the original. particularly, prices are not in bold. that has been preserved in this version. generously made available by the internet archive/american libraries.) transcriber's note the punctuation and spelling from the original text have been faithfully preserved. only obvious typographical errors have been corrected. the story of the heavens [illustration: plate i. the planet saturn, in .] the story of the heavens sir robert stawell ball, ll.d. d.sc. _author of_ "_star-land_" fellow of the royal society of london, honorary fellow of the royal society of edinburgh, fellow of the royal astronomical society, scientific adviser to the commissioners of irish lights, lowndean professor of astronomy and geometry in the university of cambridge, and formerly royal astronomer of ireland _with twenty-four coloured plates and numerous illustrations_ new and revised edition cassell and company, limited _london, paris, new york & melbourne_ all rights reserved [illustration: la·belle sauvage] preface to original edition. i have to acknowledge the kind aid which i have received in the preparation of this book. mr. nasmyth has permitted me to use some of the beautiful drawings of the moon, which have appeared in the well-known work published by him in conjunction with mr. carpenter. to this source i am indebted for plates vii., viii., ix., x., and figs. , , . professor pickering has allowed me to copy some of the drawings made at harvard college observatory by mr. trouvelot, and i have availed myself of his kindness for plates i., iv., xii., xv. i am indebted to professor langley for plate ii., to mr. de la rue for plates iii. and xiv., to mr. t.e. key for plate xvii., to professor schiaparelli for plate xviii., to the late professor c. piazzi smyth for fig. , to mr. chambers for fig. , which has been borrowed from his "handbook of descriptive astronomy," to dr. stoney for fig. , and to dr. copeland and dr. dreyer for fig. . i have to acknowledge the valuable assistance derived from professor newcomb's "popular astronomy," and professor young's "sun." in revising the volume i have had the kind aid of the rev. maxwell close. i have also to thank dr. copeland and mr. steele for their kindness in reading through the entire proofs; while i have also occasionally availed myself of the help of mr. cathcart. robert s. ball. observatory, dunsink, co. dublin. _ th may, ._ note to this edition. i have taken the opportunity in the present edition to revise the work in accordance with the recent progress of astronomy. i am indebted to the royal astronomical society for the permission to reproduce some photographs from their published series, and to mr. henry f. griffiths, for beautiful drawings of jupiter, from which plate xi. was prepared. robert s. ball. cambridge, _ st may, _. contents. page introduction chapter i. the astronomical observatory ii. the sun iii. the moon iv. the solar system v. the law of gravitation vi. the planet of romance vii. mercury viii. venus ix. the earth x. mars xi. the minor planets xii. jupiter xiii. saturn xiv. uranus xv. neptune xvi. comets xvii. shooting stars xviii. the starry heavens xix. the distant suns xx. double stars xxi. the distances of the stars xxii. star clusters and nebulÆ xxiii. the physical nature of the stars xxiv. the precession and nutation of the earth's axis xxv. the aberration of light xxvi. the astronomical significance of heat xxvii. the tides appendix list of plates. plate i. the planet saturn _frontispiece_ ii. a typical sun-spot _to face page_ a. the sun " " iii. spots and faculæ on the sun " " iv. solar prominences or flames " " v. the solar corona " " vi. chart of the moon's surface " " b. portion of the moon " " vii. the lunar crater triesnecker " " viii. a normal lunar crater " " ix. the lunar crater plato " " x. the lunar crater tycho " " xi. the planet jupiter " " xii. coggia's comet " " c. comet a., , swift " " xiii. spectra of the sun and of three stars " " d. the milky way, near messier ii. " " xiv. the great nebula in orion " " xv. the great nebula in andromeda " " e. nebulæ in the pleiades " " f. ô centauri " " xvi. nebulæ observed with lord rosse's telescope " " xvii. the comet of " " xviii. schiaparelli's map of mars " " list of illustrations. fig. page . principle of the refracting telescope . dome of the south equatorial at dunsink observatory, co. dublin . section of the dome of dunsink observatory . the telescope at yerkes observatory, chicago . principle of herschel's reflecting telescope . south front of the yerkes observatory, chicago . lord rosse's telescope . meridian circle . the great bear . comparative sizes of the earth and the sun . the sun, photographed september , . photograph of the solar surface . an ordinary sun-spot . scheiner's observations on sun-spots . zones on the sun's surface in which spots appear . texture of the sun and a small spot . the prism . dispersion of light by the prism . prominences seen in total eclipses . view of the corona in a total eclipse . view of corona during eclipse of january , . the zodiacal light in . comparative sizes of the earth and the moon . the moon's path around the sun . the phases of the moon . the earth's shadow and penumbra . key to chart of the moon (plate vi.) . lunar volcano in activity: nasmyth's theory . lunar volcano: subsequent feeble activity . " " formation of the level floor by lava . orbits of the four interior planets . the earth's movement . orbits of the four giant planets . apparent size of the sun from various planets . comparative sizes of the planets . illustration of the moon's motion . drawing an ellipse . varying velocity of elliptic motion . equal areas in equal times . transit of the planet of romance . variations in phase and apparent size of mercury . mercury as a crescent . venus, may , . different aspects of venus in the telescope . venus on the sun at the transit of . paths of venus across the sun in the transits of and . a transit of venus, as seen from two localities . orbits of the earth and of mars . apparent movements of mars in . relative sizes of mars and the earth , . drawings of mars . elevations and depressions on the terminator of mars . the southern polar cap on mars . the zone of minor planets between mars and jupiter . relative dimensions of jupiter and the earth - . the occultation of jupiter . jupiter and his four satellites . disappearances of jupiter's satellites . mode of measuring the velocity of light . saturn . relative sizes of saturn and the earth . method of measuring the rotation of saturn's rings . method of measuring the rotation of saturn's rings . transit of titan and its shadow . parabolic path of a comet . orbit of encke's comet . tail of a comet directed from the sun . bredichin's theory of comets' tails . tails of the comet of . the comet of . the path of the fireball of november , . the orbit of a shoal of meteors . radiant point of shooting stars . the history of the leonids . section of the chaco meteorite . the great bear and pole star . the great bear and cassiopeia . the great square of pegasus . perseus and its neighbouring stars . the pleiades . orion, sirius, and neighbouring stars . castor and pollux . the great bear and the lion . boötes and the crown . virgo and neighbouring constellations . the constellation of lyra . vega, the swan, and the eagle . the orbit of sirius . the parallactic ellipse . cygni and the comparison stars . parallax in declination of cygni . globular cluster in hercules . position of the great nebula in orion . the multiple star th orionis . the nebula n.g.c. . star-map, showing precessional movement . illustration of the motion of precession the story of the heavens. "the story of the heavens" is the title of our book. we have indeed a wondrous story to narrate; and could we tell it adequately it would prove of boundless interest and of exquisite beauty. it leads to the contemplation of grand phenomena in nature and great achievements of human genius. let us enumerate a few of the questions which will be naturally asked by one who seeks to learn something of those glorious bodies which adorn our skies: what is the sun--how hot, how big, and how distant? whence comes its heat? what is the moon? what are its landscapes like? how does our satellite move? how is it related to the earth? are the planets globes like that on which we live? how large are they, and how far off? what do we know of the satellites of jupiter and of the rings of saturn? how was uranus discovered? what was the intellectual triumph which brought the planet neptune to light? then, as to the other bodies of our system, what are we to say of those mysterious objects, the comets? can we discover the laws of their seemingly capricious movements? do we know anything of their nature and of the marvellous tails with which they are often decorated? what can be told about the shooting-stars which so often dash into our atmosphere and perish in a streak of splendour? what is the nature of those constellations of bright stars which have been recognised from all antiquity, and of the host of smaller stars which our telescopes disclose? can it be true that these countless orbs are really majestic suns, sunk to an appalling depth in the abyss of unfathomable space? what have we to tell of the different varieties of stars--of coloured stars, of variable stars, of double stars, of multiple stars, of stars that seem to move, and of stars that seem at rest? what of those glorious objects, the great star clusters? what of the milky way? and, lastly, what can we learn of the marvellous nebulæ which our telescopes disclose, poised at an immeasurable distance? such are a few of the questions which occur when we ponder on the mysteries of the heavens. the history of astronomy is, in one respect, only too like many other histories. the earliest part of it is completely and hopelessly lost. the stars had been studied, and some great astronomical discoveries had been made, untold ages before those to which our earliest historical records extend. for example, the observation of the apparent movement of the sun, and the discrimination between the planets and the fixed stars, are both to be classed among the discoveries of prehistoric ages. nor is it to be said that these achievements related to matters of an obvious character. ancient astronomy may seem very elementary to those of the present day who have been familiar from childhood with the great truths of nature, but, in the infancy of science, the men who made such discoveries as we have mentioned must have been sagacious philosophers. of all the phenomena of astronomy the first and the most obvious is that of the rising and the setting of the sun. we may assume that in the dawn of human intelligence these daily occurrences would form one of the first problems to engage the attention of those whose thoughts rose above the animal anxieties of everyday existence. a sun sets and disappears in the west. the following morning a sun rises in the east, moves across the heavens, and it too disappears in the west; the same appearances recur every day. to us it is obvious that the sun, which appears each day, is the same sun; but this would not seem reasonable to one who thought his senses showed him that the earth was a flat plain of indefinite extent, and that around the inhabited regions on all sides extended, to vast distances, either desert wastes or trackless oceans. how could that same sun, which plunged into the ocean at a fabulous distance in the west, reappear the next morning at an equally great distance in the east? the old mythology asserted that after the sun had dipped in the western ocean at sunset (the iberians, and other ancient nations, actually imagined that they could hear the hissing of the waters when the glowing globe was plunged therein), it was seized by vulcan and placed in a golden goblet. this strange craft with its astonishing cargo navigated the ocean by a northerly course, so as to reach the east again in time for sunrise the following morning. among the earlier physicists of old it was believed that in some manner the sun was conveyed by night across the northern regions, and that darkness was due to lofty mountains, which screened off the sunbeams during the voyage. in the course of time it was thought more rational to suppose that the sun actually pursued his course below the solid earth during the course of the night. the early astronomers had, moreover, learned to recognise the fixed stars. it was noticed that, like the sun, many of these stars rose and set in consequence of the diurnal movement, while the moon obviously followed a similar law. philosophers thus taught that the various heavenly bodies were in the habit of actually passing beneath the solid earth. by the acknowledgment that the whole contents of the heavens performed these movements, an important step in comprehending the constitution of the universe had been decidedly taken. it was clear that the earth could not be a plane extending to an indefinitely great distance. it was also obvious that there must be a finite depth to the earth below our feet. nay, more, it became certain that whatever the shape of the earth might be, it was at all events something detached from all other bodies, and poised without visible support in space. when this discovery was first announced it must have appeared a very startling truth. it was so difficult to realise that the solid earth on which we stand reposed on nothing! what was to keep it from falling? how could it be sustained without tangible support, like the legendary coffin of mahomet? but difficult as it may have been to receive this doctrine, yet its necessary truth in due time commanded assent, and the science of astronomy began to exist. the changes of the seasons and the recurrence of seed-time and harvest must, from the earliest times, have been associated with certain changes in the position of the sun. in the summer at mid-day the sun rises high in the heavens, in the winter it is always low. our luminary, therefore, performs an annual movement up and down in the heavens, as well as a diurnal movement of rising and setting. but there is a third species of change in the sun's position, which is not quite so obvious, though it is still capable of being detected by a few careful observations, if combined with a philosophical habit of reflection. the very earliest observers of the stars can hardly have failed to notice that the constellations visible at night varied with the season of the year. for instance, the brilliant figure of orion, though so well seen on winter nights, is absent from the summer skies, and the place it occupied is then taken by quite different groups of stars. the same may be said of other constellations. each season of the year can thus be characterised by the sidereal objects that are conspicuous by night. indeed, in ancient days, the time for commencing the cycle of agricultural occupations was sometimes indicated by the position of the constellations in the evening. by reflecting on these facts the early astronomers were enabled to demonstrate the apparent annual movement of the sun. there could be no rational explanation of the changes in the constellations with the seasons, except by supposing that the place of the sun was altering, so as to make a complete circuit of the heavens in the course of the year. this movement of the sun is otherwise confirmed by looking at the west after sunset, and watching the stars. as the season progresses, it may be noticed each evening that the constellations seem to sink lower and lower towards the west, until at length they become invisible from the brightness of the sky. the disappearance is explained by the supposition that the sun appears to be continually ascending from the west to meet the stars. this motion is, of course, not to be confounded with the ordinary diurnal rising and setting, in which all the heavenly bodies participate. it is to be understood that besides being affected by the common motion our luminary has a slow independent movement in the opposite direction; so that though the sun and a star may set at the same time to-day, yet since by to-morrow the sun will have moved a little towards the east, it follows that the star must then set a few minutes before the sun.[ ] the patient observations of the early astronomers enabled the sun's track through the heavens to be ascertained, and it was found that in its circuit amid the stars and constellations our luminary invariably followed the same path. this is called the _ecliptic_, and the constellations through which it passes form a belt around the heavens known as the _zodiac_. it was anciently divided into twelve equal portions or "signs," so that the stages on the sun's great journey could be conveniently indicated. the duration of the year, or the period required by the sun to run its course around the heavens, seems to have been first ascertained by astronomers whose names are unknown. the skill of the early oriental geometers was further evidenced by their determination of the position of the ecliptic with regard to the celestial equator, and by their success in the measurement of the angle between these two important circles on the heavens. the principal features of the motion of the moon have also been noticed with intelligence at an antiquity more remote than history. the attentive observer perceives the important truth that the moon does not occupy a fixed position in the heavens. during the course of a single night the fact that the moon has moved from west to east across the heavens can be perceived by noting its position relatively to adjacent stars. it is indeed probable that the motion of the moon was a discovery prior to that of the annual motion of the sun, inasmuch as it is the immediate consequence of a simple observation, and involves but little exercise of any intellectual power. in prehistoric times also, the time of revolution of the moon had been ascertained, and the phases of our satellite had been correctly attributed to the varying aspect under which the sun-illuminated side is turned towards the earth. but we are far from having exhausted the list of great discoveries which have come down from unknown antiquity. correct explanations had been given of the striking phenomenon of a lunar eclipse, in which the brilliant surface is plunged temporarily into darkness, and also of the still more imposing spectacle of a solar eclipse, in which the sun itself undergoes a partial or even a total obscuration. then, too, the acuteness of the early astronomers had detected the five wandering stars or planets: they had traced the movements of mercury and venus, mars, jupiter, and saturn. they had observed with awe the various configurations of these planets: and just as the sun, and in a lesser degree the moon, were intimately associated with the affairs of daily life, so in the imagination of these early investigators the movements of the planets were thought to be pregnant with human weal or human woe. at length a certain order was perceived to govern the apparently capricious movements of the planets. it was found that they obeyed certain laws. the cultivation of the science of geometry went hand in hand with the study of astronomy: and as we emerge from the dim prehistoric ages into the historical period, we find that the theory of the phenomena of the heavens possessed already some degree of coherence. ptolemy, following pythagoras, plato, and aristotle, acknowledged that the earth's figure was globular, and he demonstrated it by the same arguments that we employ at the present day. he also discerned how this mighty globe was isolated in space. he admitted that the diurnal movement of the heavens could be accounted for by the revolution of the earth upon its axis, but unfortunately he assigned reasons for the deliberate rejection of this view. the earth, according to him, was a fixed body; it possessed neither rotation round an axis nor translation through space, but remained constantly at rest in what he supposed to be the centre of the universe. according to ptolemy's theory the sun and the moon moved in circular orbits around the earth in the centre. the explanation of the movements of the planets he found to be more complicated, because it was necessary to account for the fact that a planet sometimes advanced and that it sometimes retrograded. the ancient geometers refused to believe that any movement, except revolution in a circle, was possible for a celestial body: accordingly a contrivance was devised by which each planet was supposed to revolve in a circle, of which the centre described another circle around the earth. although the ptolemaic doctrine is now known to be framed on quite an extravagant estimate of the importance of the earth in the scheme of the heavens, yet it must be admitted that the apparent movements of the celestial bodies can be thus accounted for with considerable accuracy. this theory is described in the great work known as the "almagest," which was written in the second century of our era, and was regarded for fourteen centuries as the final authority on all questions of astronomy. such was the system of astronomy which prevailed during the middle ages, and was only discredited at an epoch nearly simultaneous with that of the discovery of the new world by columbus. the true arrangement of the solar system was then expounded by copernicus in the great work to which he devoted his life. the first principle established by these labours showed the diurnal movement of the heavens to be due to the rotation of the earth on its axis. copernicus pointed out the fundamental difference between real motions and apparent motions; he proved that the appearances presented in the daily rising and setting of the sun and the stars could be accounted for by the supposition that the earth rotated, just as satisfactorily as by the more cumbrous supposition of ptolemy. he showed, moreover, that the latter supposition must attribute an almost infinite velocity to the stars, so that the rotation of the entire universe around the earth was clearly a preposterous supposition. the second great principle, which has conferred immortal glory on copernicus, assigned to the earth its true position in the universe. copernicus transferred the centre, about which all the planets revolve, from the earth to the sun; and he established the somewhat humiliating truth, that our earth is merely a planet pursuing a track between the paths of venus and of mars, and subordinated like all the other planets to the supreme sway of the sun. this great revolution swept from astronomy those distorted views of the earth's importance which arose, perhaps not unnaturally, from the fact that we happen to be domiciled on that particular planet. the achievements of copernicus were soon to be followed by the invention of the telescope, that wonderful instrument by which the modern science of astronomy has been created. to the consideration of this important subject we shall devote the first chapter of our book. [illustration: plate ii. a typical sun-spot. (after langley.)] chapter i. the astronomical observatory. early astronomical observations--the observatory of tycho brahe--the pupil of the eye--vision of faint objects--the telescope--the object-glass--advantages of large telescopes--the equatorial--the observatory--the power of a telescope--reflecting telescopes--lord rosse's great reflector at parsonstown--how the mighty telescope is used--instruments of precision--the meridian circle--the spider lines--delicacy of pointing a telescope--precautions necessary in making observations--the ideal instrument and the practical one--the elimination of error--greenwich observatory--the ordinary opera-glass as an astronomical instrument--the great bear--counting the stars in the constellation--how to become an observer. the earliest rudiments of the astronomical observatory are as little known as the earliest discoveries in astronomy itself. probably the first application of instrumental observation to the heavenly bodies consisted in the simple operation of measuring the shadow of a post cast by the sun at noonday. the variations in the length of this shadow enabled the primitive astronomers to investigate the apparent movements of the sun. but even in very early times special astronomical instruments were employed which possessed sufficient accuracy to add to the amount of astronomical knowledge, and displayed considerable ingenuity on the part of the designers. professor newcomb[ ] thus writes: "the leader was tycho brahe, who was born in , three years after the death of copernicus. his attention was first directed to the study of astronomy by an eclipse of the sun on august st, , which was total in some parts of europe. astonished that such a phenomenon could be predicted, he devoted himself to a study of the methods of observation and calculation by which the prediction was made. in the king of denmark founded the celebrated observatory of uraniborg, at which tycho spent twenty years assiduously engaged in observations of the positions of the heavenly bodies with the best instruments that could then be made. this was just before the invention of the telescope, so that the astronomer could not avail himself of that powerful instrument. consequently, his observations were superseded by the improved ones of the centuries following, and their celebrity and importance are principally due to their having afforded kepler the means of discovering his celebrated laws of planetary motion." the direction of the telescope to the skies by galileo gave a wonderful impulse to the study of the heavenly bodies. this extraordinary man is prominent in the history of astronomy, not alone for his connection with this supreme invention, but also for his achievements in the more abstract parts of astronomy. he was born at pisa in , and in the first telescope used for astronomical observation was constructed. galileo died in , the year in which newton was born. it was galileo who laid with solidity the foundations of that science of dynamics, of which astronomy is the most splendid illustration; and it was he who, by promulgating the doctrines taught by copernicus, incurred the wrath of the inquisition. the structure of the human eye in so far as the exquisite adaptation of the pupil is concerned presents us with an apt illustration of the principle of the telescope. to see an object, it is necessary that the light from it should enter the eye. the portal through which the light is admitted is the pupil. in daytime, when the light is brilliant, the iris decreases the size of the pupil, and thus prevents too much light from entering. at night, or whenever the light is scarce, the eye often requires to grasp all it can. the pupil then expands; more and more light is admitted according as the pupil grows larger. the illumination of the image on the retina is thus effectively controlled in accordance with the requirements of vision. a star transmits to us its feeble rays of light, and from those rays the image is formed. even with the most widely-opened pupil, it may, however, happen that the image is not bright enough to excite the sensation of vision. here the telescope comes to our aid: it catches all the rays in a beam whose original dimensions were far too great to allow of its admission through the pupil. the action of the lenses concentrates those rays into a stream slender enough to pass through the small opening. we thus have the brightness of the image on the retina intensified. it is illuminated with nearly as much light as would be collected from the same object through a pupil as large as the great lenses of the telescope. [illustration: fig. .--principle of the refracting telescope.] in astronomical observatories we employ telescopes of two entirely different classes. the more familiar forms are those known as _refractors_, in which the operation of condensing the rays of light is conducted by refraction. the character of the refractor is shown in fig. . the rays from the star fall upon the object-glass at the end of the telescope, and on passing through they become refracted into a converging beam, so that all intersect at the focus. diverging from thence, the rays encounter the eye-piece, which has the effect of restoring them to parallelism. the large cylindrical beam which poured down on the object-glass has been thus condensed into a small one, which can enter the pupil. it should, however, be added that the composite nature of light requires a more complex form of object-glass than the simple lens here shown. in a refracting telescope we have to employ what is known as the achromatic combination, consisting of one lens of flint glass and one of crown glass, adjusted to suit each other with extreme care. [illustration: fig. .--the dome of the south equatorial at dunsink observatory co dublin.] [illustration: fig. .--section of the dome of dunsink observatory.] the appearance of an astronomical observatory, designed to accommodate an instrument of moderate dimensions, is shown in the adjoining figures. the first (fig. ) represents the dome erected at dunsink observatory for the equatorial telescope, the object-glass of which was presented to the board of trinity college, dublin, by the late sir james south. the main part of the building is a cylindrical wall, on the top of which reposes a hemispherical roof. in this roof is a shutter, which can be opened so as to allow the telescope in the interior to obtain a view of the heavens. the dome is capable of revolving so that the opening may be turned towards that part of the sky where the object happens to be situated. the next view (fig. ) exhibits a section through the dome, showing the machinery by which the attendant causes it to revolve, as well as the telescope itself. the eye of the observer is placed at the eye-piece, and he is represented in the act of turning a handle, which has the power of slowly moving the telescope, in order to adjust the instrument accurately on the celestial body which it is desired to observe. the two lenses which together form the object-glass of this instrument are twelve inches in diameter, and the quality of the telescope mainly depends on the accuracy with which these lenses have been wrought. the eye-piece is a comparatively simple matter. it consists merely of one or two small lenses; and various eye-pieces can be employed, according to the magnifying power which may be desired. it is to be observed that for many purposes of astronomy high magnifying powers are not desirable. there is a limit, too, beyond which the magnification cannot be carried with advantage. the object-glass can only collect a certain quantity of light from the star; and if the magnifying power be too great, this limited amount of light will be thinly dispersed over too large a surface, and the result will be found unsatisfactory. the unsteadiness of the atmosphere still further limits the extent to which the image may be advantageously magnified, for every increase of power increases in the same degree the atmospheric disturbance. a telescope mounted in the manner here shown is called an _equatorial_. the convenience of this peculiar style of supporting the instrument consists in the ease with which the telescope can be moved so as to follow a star in its apparent journey across the sky. the necessary movements of the tube are given by clockwork driven by a weight, so that, once the instrument has been correctly pointed, the star will remain in the observer's field of view, and the effect of the apparent diurnal movement will be neutralised. the last refinement in this direction is the application of an electrical arrangement by which the driving of the instrument is controlled from the standard clock of the observatory. [illustration: fig. .--the telescope at yerkes observatory, chicago. (_from the astrophysical journal, vol. vi., no. ._)] the power of a refracting telescope--so far as the expression has any definite meaning--is to be measured by the diameter of its object-glass. there has, indeed, been some honourable rivalry between the various civilised nations as to which should possess the greatest refracting telescope. among the notable instruments that have been successfully completed is that erected in by sir howard grubb, of dublin, at the splendid observatory at vienna. its dimensions may be estimated from the fact that the object-glass is two feet and three inches in diameter. many ingenious contrivances help to lessen the inconvenience incident to the use of an instrument possessing such vast proportions. among them we may here notice the method by which the graduated circles attached to the telescope are brought within view of the observer. these circles are necessarily situated at parts of the instrument which lie remote from the eye-piece where the observer is stationed. the delicate marks and figures are, however, easily read from a distance by a small auxiliary telescope, which, by suitable reflectors, conducts the rays of light from the circles to the eye of the observer. [illustration: fig. .--principle of herschel's refracting telescope.] numerous refracting telescopes of exquisite perfection have been produced by messrs. alvan clark, of cambridgeport, boston, mass. one of their most famous telescopes is the great lick refractor now in use on mount hamilton in california. the diameter of this object-glass is thirty-six inches, and its focal length is fifty-six feet two inches. a still greater effort has recently been made by the same firm in the refractor of forty inches aperture for the yerkes observatory of the university of chicago. the telescope, which is seventy-five feet in length, is mounted under a revolving dome ninety feet in diameter, and in order to enable the observer to reach the eye-piece without using very large step-ladders, the floor of the room can be raised and lowered through a range of twenty-two feet by electric motors. this is shown in fig. , while the south front of the yerkes observatory is represented in fig. . [illustration: fig. .--south front of the yerkes observatory, chicago. (_from the astrophysical journal, vol. vi., no. ._)] [illustration: fig. .--lord rosse's telescope.] within the last few years two fine telescopes have been added to the instrumental equipment of the royal observatory, greenwich, both by sir h. grubb. one of these, containing a -inch object-glass, has been erected on a mounting originally constructed for a smaller instrument by sir g. airy. the other, presented by sir henry thompson, is of inches aperture, and is adapted for photographic work. there is a limit to the size of the refractor depending upon the material of the object-glass. glass manufacturers seem to experience unusual difficulties in their attempts to form large discs of optical glass pure enough and uniform enough to be suitable for telescopes. these difficulties are enhanced with every increase in the size of the discs, so that the cost has a tendency to increase at a very much greater rate. it may be mentioned in illustration that the price paid for the object-glass of the lick telescope exceeded ten thousand pounds. there is, however, an alternative method of constructing a telescope, in which the difficulty we have just mentioned does not arise. the principle of the simplest form of _reflector_ is shown in fig. , which represents what is called the herschelian instrument. the rays of light from the star under observation fall on a mirror which is both carefully shaped and highly polished. after reflection, the rays proceed to a focus, and diverging from thence, fall on the eye-piece, by which they are restored to parallelism, and thus become adapted for reception in the eye. it was essentially on this principle (though with a secondary flat mirror at the upper end of the tube reflecting the rays at a right angle to the side of the tube, where the eye-piece is placed) that sir isaac newton constructed the little reflecting telescope which is now treasured by the royal society. a famous instrument of the newtonian type was built, half a century ago, by the late earl of rosse, at parsonstown. it is represented in fig. . the colossal aperture of this instrument has never been surpassed; it has, indeed, never been rivalled. the mirror or speculum, as it is often called, is a thick metallic disc, composed of a mixture of two parts of copper with one of tin. this alloy is so hard and brittle as to make the necessary mechanical operations difficult to manage. the material admits, however, of a brilliant polish, and of receiving and retaining an accurate figure. the rosse speculum--six feet in diameter and three tons in weight--reposes at the lower end of a telescope fifty-five feet long. the tube is suspended between two massive castellated walls, which form an imposing feature on the lawn at birr castle. this instrument cannot be turned about towards every part of the sky, like the equatorials we have recently been considering. the great tube is only capable of elevation in altitude along the meridian, and of a small lateral movement east and west of the meridian. every star or nebula visible in the latitude of parsonstown (except those very near the pole) can, however, be observed in the great telescope, if looked for at the right time. [illustration: fig. .--meridian circle.] before the object reaches the meridian, the telescope must be adjusted at the right elevation. the necessary power is transmitted by a chain from a winch at the northern end of the walls to a point near the upper end of the tube. by this contrivance the telescope can be raised or lowered, and an ingenious system of counterpoises renders the movement equally easy at all altitudes. the observer then takes his station in one of the galleries which give access to the eye-piece; and when the right moment has arrived, the star enters the field of view. powerful mechanism drives the great instrument, so as to counteract the diurnal movement, and thus the observer can retain the object in view until he has made his measurements or finished his drawing. of late years reflecting telescopes have been generally made with mirrors of glass covered with a thin film of silver, which is capable of reflecting much more light than the surface of a metallic mirror. among great reflectors of this kind we may mention two, of three and five feet aperture respectively, with which dr. common has done valuable work. we must not, however, assume that for the general work in an observatory a colossal instrument is the most suitable. the mighty reflector, or refractor, is chiefly of use where unusually faint objects are being examined. for work in which accurate measurements are made of objects not particularly difficult to see, telescopes of smaller dimensions are more suitable. the fundamental facts about the heavenly bodies have been chiefly learned from observations obtained with instruments of moderate optical power, specially furnished so as to enable precise measures of position to be secured. indeed, in the early stages of astronomy, important determinations of position were effected by contrivances which showed the direction of the object without any telescopic aid. perhaps the most valuable measurements obtained in our modern observatories are yielded by that instrument of precision known as the _meridian circle_. it is impossible, in any adequate account of the story of the heavens, to avoid some reference to this indispensable aid to astronomical research, and therefore we shall give a brief account of one of its simpler forms, choosing for this purpose a great instrument in the paris observatory, which is represented in fig. . the telescope is attached at its centre to an axis at right angles to its length. pivots at each extremity of this axis rotate upon fixed bearings, so that the movements of the telescope are completely restricted to the plane of the meridian. inside the eye-piece of the telescope extremely fine vertical fibres are stretched. the observer watches the moon, or star, or planet enter the field of view; and he notes by the clock the exact time, to the fraction of a second, at which the object passes over each of the lines. a silver band on the circle attached to the axis is divided into degrees and subdivisions of a degree, and as this circle moves with the telescope, the elevation at which the instrument is pointed will be indicated. for reading the delicately engraved marks and figures on the silver, microscopes are necessary. these are shown in the sketch, each one being fixed into an aperture in the wall which supports one end of the instrument. at the opposite side is a lamp, the light from which passes through the perforated axis of the pivot, and is thence ingeniously deflected by mirrors so as to provide the requisite illumination for the lines at the focus. the fibres which the observer sees stretched over the field of view of the telescope demand a few words of explanation. we require for this purpose a material which shall be very fine and fairly durable, as well as somewhat elastic, and of no appreciable weight. these conditions cannot be completely fulfilled by any metallic wire, but they are exquisitely realised in the beautiful thread which is spun by the spider. the delicate fibres are stretched with nice skill across the field of view of the telescope, and cemented in their proper places. with instruments so beautifully appointed we can understand the precision attained in modern observations. the telescope is directed towards a star, and the image of the star is a minute point of light. when that point coincides with the intersection of the two central spider lines the telescope is properly sighted. we use the word sighted designedly, because we wish to suggest a comparison between the sighting of a rifle at the target and the sighting of a telescope at a star. instead of the ordinary large bull's-eye, suppose that the target only consisted of a watch-dial, which, of course, the rifleman could not see at the distance of any ordinary range. but with the telescope of the meridian circle the watch-dial would be visible even at the distance of a mile. the meridian circle is indeed capable of such precision as a sighting instrument that it could be pointed separately to each of two stars which subtend at the eye an angle no greater than that subtended by an adjoining pair of the sixty minute dots around the circumference of a watch-dial a mile distant from the observer. this power of directing the instrument so accurately would be of but little avail unless it were combined with arrangements by which, when once the telescope has been pointed correctly, the position of the star can be ascertained and recorded. one element in the determination of the position is secured by the astronomical clock, which gives the moment when the object crosses the central vertical wire; the other element is given by the graduated circle which reads the angular distance of the star from the zenith or point directly overhead. superb meridian instruments adorn our great observatories, and are nightly devoted to those measurements upon which the great truths of astronomy are mainly based. these instruments have been constructed with refined skill; but it is the duty of the painstaking astronomer to distrust the accuracy of his instrument in every conceivable way. the great tube may be as rigid a structure as mechanical engineers can produce; the graduations on the circle may have been engraved by the most perfect of dividing machines; but the conscientious astronomer will not be content with mere mechanical precision. that meridian circle which, to the uninitiated, seems a marvellous piece of workmanship, possessing almost illimitable accuracy, is viewed in a very different light by the astronomer who makes use of it. no one can appreciate more fully than he the skill of the artist who has made that meridian circle, and the beautiful contrivances for illumination and reading off which give to the instrument its perfection; but while the astronomer recognises the beauty of the actual machine he is using, he has always before his mind's eye an ideal instrument of absolute perfection, to which the actual meridian circle only makes an approximation. contrasted with the ideal instrument, the finest meridian circle is little more than a mass of imperfections. the ideal tube is perfectly rigid, the actual tube is flexible; the ideal divisions of the circle are perfectly uniform, the actual divisions are not uniform. the ideal instrument is a geometrical embodiment of perfect circles, perfect straight lines, and perfect right angles; the actual instrument can only show approximate circles, approximate straight lines, and approximate right angles. perhaps the spider's part of the work is on the whole the best; the stretched web gives us the nearest mechanical approach to a perfectly straight line; but we mar the spider's work by not being able to insert those beautiful threads with perfect uniformity, while our attempts to adjust two of them across the field of view at right angles do not succeed in producing an angle of exactly ninety degrees. nor are the difficulties encountered by the meridian observer due solely to his instrument. he has to contend against his own imperfections; he has often to allow for personal peculiarities of an unexpected nature; the troubles that the atmosphere can give are notorious; while the levelling of his instrument warns him that he cannot even rely on the solid earth itself. we learn that the earthquakes, by which the solid ground is sometimes disturbed, are merely the more conspicuous instances of incessant small movements in the earth which every night in the year derange the delicate adjustment of the instrument. when the existence of these errors has been recognised, the first great step has been taken. by an alliance between the astronomer and the mathematician it is possible to measure the discrepancies between the actual meridian circle and the instrument that is ideally perfect. once this has been done, we can estimate the effect which the irregularities produce on the observations, and finally, we succeed in purging the observations from the grosser errors by which they are contaminated. we thus obtain results which are not indeed mathematically accurate, but are nevertheless close approximations to those which would be obtained by a perfect observer using an ideal instrument of geometrical accuracy, standing on an earth of absolute rigidity, and viewing the heavens without the intervention of the atmosphere. in addition to instruments like those already indicated, astronomers have other means of following the motions of the heavenly bodies. within the last fifteen years photography has commenced to play an important part in practical astronomy. this beautiful art can be utilised for representing many objects in the heavens by more faithful pictures than the pencil of even the most skilful draughtsman can produce. photography is also applicable for making charts of any region in the sky which it is desired to examine. when repeated pictures of the same region are made from time to time, their comparison gives the means of ascertaining whether any star has moved during the interval. the amount and direction of this motion may be ascertained by a delicate measuring apparatus under which the photographic plate is placed. if a refracting telescope is to be used for taking celestial photographs, the lenses of the object-glass must be specially designed for this purpose. the rays of light which imprint an image on the prepared plate are not exactly the same as those which are chiefly concerned in the production of the image on the retina of the human eye. a reflecting mirror, however, brings all the rays, both those which are chemically active and those which are solely visual, to one and the same focus. the same reflecting instrument may therefore be used either for looking at the heavens or for taking pictures on a photographic plate which has been substituted for the observer's eye. a simple portrait camera has been advantageously employed for obtaining striking photographs of larger areas of the sky than can be grasped in a long telescope; but for purposes of accurate measurement those taken with the latter are incomparably better. it is needless to say that the photographic apparatus, whatever it may be, must be driven by delicately-adjusted clockwork to counteract the apparent daily motion of the stars caused by the rotation of the earth. the picture would otherwise be spoiled, just as a portrait is ruined if the sitter does not remain quiet during the exposure. among the observatories in the united kingdom the royal observatory at greenwich is of course the most famous. it is specially remarkable among all the similar institutions in the world for the continuity of its labours for several generations. greenwich observatory was founded in for the promotion of astronomy and navigation, and the observations have from the first been specially arranged with the object of determining with the greatest accuracy the positions of the principal fixed stars, the sun, the moon, and the planets. in recent years, however, great developments of the work of the observatory have been witnessed, and the most modern branches of the science are now assiduously pursued there. the largest equatorial at greenwich is a refractor of twenty-eight inches aperture and twenty-eight feet long, constructed by sir howard grubb. a remarkable composite instrument from the same celebrated workshop has also been recently added to our national institution. it consists of a great refractor specially constructed for photography, of twenty-six inches aperture (presented by sir henry thompson) and a reflector of thirty inches diameter, which is the product of dr. common's skill. the huge volume published annually bears witness to the assiduity with which the astronomer royal and his numerous staff of assistant astronomers make use of the splendid means at their disposal. the southern part of the heavens, most of which cannot be seen in this country, is watched from various observatories in the southern hemisphere. foremost among them is the royal observatory at the cape of good hope, which is furnished with first-class instruments. we may mention a great photographic telescope, the gift of mr. m'clean. astronomy has been greatly enriched by the many researches made by dr. gill, the director of the cape observatory. [illustration: fig. .--the great bear.] it is not, however, necessary to use such great instruments to obtain some idea of the aid the telescope will afford. the most suitable instrument for commencing astronomical studies is within ordinary reach. it is the well-known binocular that a captain uses on board ship; or if that cannot be had, then the common opera-glass will answer nearly as well. this is, no doubt, not so powerful as a telescope, but it has some compensating advantages. the opera-glass will enable us to survey a large region of the sky at one glance, while a telescope, generally speaking, presents a much smaller field of view. let us suppose that the observer is provided with an opera-glass and is about to commence his astronomical studies. the first step is to become acquainted with the conspicuous group of seven stars represented in fig. . this group is often called the plough, or charles's wain, but astronomers prefer to regard it as a portion of the constellation of the great bear (ursa major). there are many features of interest in this constellation, and the beginner should learn as soon as possible to identify the seven stars which compose it. of these the two marked a and b, at the head of the bear, are generally called the "pointers." they are of special use, because they serve to guide the eye to that most important star in the whole sky, known as the "pole star." fix the attention on that region in the great bear, which forms a sort of rectangle, of which the stars a b g d are the corners. the next fine night try to count how many stars are visible within that rectangle. on a very fine night, without a moon, perhaps a dozen might be perceived, or even more, according to the keenness of the eyesight. but when the opera-glass is directed to the same part of the constellation an astonishing sight is witnessed. a hundred stars can now be seen with the greatest ease. but the opera-glass will not show nearly all the stars in this region. any good telescope will reveal many hundreds too faint for the feebler instrument. the greater the telescope the more numerous the stars: so that seen through one of the colossal instruments the number would have to be reckoned in thousands. we have chosen the great bear because it is more generally known than any other constellation. but the great bear is not exceptionally rich in stars. to tell the number of the stars is a task which no man has accomplished; but various estimates have been made. our great telescopes can probably show at least , , stars. the student who uses a good refracting telescope, having an object-glass not less than three inches in diameter, will find occupation for many a fine evening. it will greatly increase the interest of his work if he have the charming handbook of the heavens known as webb's "celestial objects for common telescopes." chapter ii. the sun. the vast size of the sun--hotter than melting platinum--is the sun the source of heat for the earth?--the sun is , , miles distant--how to realise the magnitude of this distance--day and night--luminous and non-luminous bodies--contrast between the sun and the stars--the sun a star--granulated appearance of the sun--the spots on the sun--changes in the form of a spot--the faculæ--the rotation of the sun on its axis--view of a typical sun-spot--periodicity of the sun-spots--connection between the sun-spots and terrestrial magnetism--principles of spectrum analysis--substances present in the sun--spectrum of a spot--the prominences surrounding the sun--total eclipse of the sun--size and movement of the prominences--their connection with the spots--spectroscopic measurement of motion on the sun--the corona surrounding the sun--constitution of the sun. in commencing our examination of the orbs which surround us, we naturally begin with our peerless sun. his splendid brilliance gives him the pre-eminence over all other celestial bodies. the dimensions of our luminary are commensurate with his importance. astronomers have succeeded in the difficult task of ascertaining the exact figures, but they are so gigantic that the results are hard to realise. the diameter of the orb of day, or the length of the axis, passing through the centre from one side to the other, is , miles. yet this bare statement of the dimensions of the great globe fails to convey an adequate idea of its vastness. if a railway were laid round the sun, and if we were to start in an express train moving sixty miles an hour, we should have to travel for five years without intermission night or day before we had accomplished the journey. when the sun is compared with the earth the bulk of our luminary becomes still more striking. suppose his globe were cut up into one million parts, each of these parts would appreciably exceed the bulk of our earth. fig. exhibits a large circle and a very small one, marked s and e respectively. these circles show the comparative sizes of the two bodies. the mass of the sun does not, however, exceed that of the earth in the same proportion. were the sun placed in one pan of a mighty weighing balance, and were , bodies as heavy as our earth placed in the other, the luminary would turn the scale. [illustration: fig. .--comparative size of the earth and the sun.] the sun has a temperature far surpassing any that we artificially produce, either in our chemical laboratories or our metallurgical establishments. we can send a galvanic current through a piece of platinum wire. the wire first becomes red hot, then white hot; then it glows with a brilliance almost dazzling until it fuses and breaks. the temperature of the melting platinum wire could hardly be surpassed in the most elaborate furnaces, but it does not attain the temperature of the sun. it must, however, be admitted that there is an apparent discrepancy between a fact of common experience and the statement that the sun possesses the extremely high temperature that we have just tried to illustrate. "if the sun were hot," it has been said, "then the nearer we approach to him the hotter we should feel; yet this does not seem to be the case. on the top of a high mountain we are nearer to the sun, and yet everybody knows that it is much colder up there than in the valley beneath. if the mountain be as high as mont blanc, then we are certainly two or three miles nearer the glowing globe than we were at the sea-level; yet, instead of additional warmth, we find eternal snow." a simple illustration may help to lessen this difficulty. in a greenhouse on a sunshiny day the temperature is much hotter than it is outside. the glass will permit the hot sunbeams to enter, but it refuses to allow them out again with equal freedom, and consequently the temperature rises. the earth may, from this point of view, be likened to a greenhouse, only, instead of the panes of glass, our globe is enveloped by an enormous coating of air. on the earth's surface, we stand, as it were, inside the greenhouse, and we benefit by the interposition of the atmosphere; but when we climb very high mountains, we gradually pass through some of the protecting medium, and then we suffer from the cold. if the earth were deprived of its coat of air, it seems certain that eternal frost would reign over whole continents as well as on the tops of the mountains. the actual distance of the sun from the earth is about , , miles; but by merely reciting the figures we do not receive a vivid impression of the real magnitude. it would be necessary to count as quickly as possible for three days and three nights before one million was completed; yet this would have to be repeated nearly ninety-three times before we had counted all the miles between the earth and the sun. every clear night we see a vast host of stars scattered over the sky. some are bright, some are faint, some are grouped into remarkable forms. with regard to this multitude of brilliant points we have now to ask an important question. are they bodies which shine by their own light like the sun, or do they only shine with borrowed light like the moon? the answer is easily stated. most of those bodies shine by their own light, and they are properly called _stars_. suppose that the sun and the multitude of stars, properly so called, are each and all self-luminous brilliant bodies, what is the great distinction between the sun and the stars? there is, of course, a vast and obvious difference between the unrivalled splendour of the sun and the feeble twinkle of the stars. yet this distinction does not necessarily indicate that our luminary has an intrinsic splendour superior to that of the stars. the fact is that we are nestled up comparatively close to the sun for the benefit of his warmth and light, while we are separated from even the nearest of the stars by a mighty abyss. if the sun were gradually to retreat from the earth, his light would decrease, so that when he had penetrated the depths of space to a distance comparable with that by which we are separated from the stars, his glory would have utterly departed. no longer would the sun seem to be the majestic orb with which we are familiar. no longer would he be a source of genial heat, or a luminary to dispel the darkness of night. our great sun would have shrunk to the insignificance of a star, not so bright as many of those which we see every night. momentous indeed is the conclusion to which we are now led. that myriad host of stars which studs our sky every night has been elevated into vast importance. each one of those stars is itself a mighty sun, actually rivalling, and in many cases surpassing, the splendour of our own luminary. we thus open up a majestic conception of the vast dimensions of space, and of the dignity and splendour of the myriad globes by which that space is tenanted. there is another aspect of the picture not without its utility. we must from henceforth remember that our sun is only a star, and not a particularly important star. if the sun and the earth, and all which it contains, were to vanish, the effect in the universe would merely be that a tiny star had ceased its twinkling. viewed simply as a star, the sun must retire to a position of insignificance in the mighty fabric of the universe. but it is not as a star that we have to deal with the sun. to us his comparative proximity gives him an importance incalculably transcending that of all the other stars. we imagined ourselves to be withdrawn from the sun to obtain his true perspective in the universe; let us now draw near, and give him that attention which his supreme importance to us merits. [illustration: fig. .--the sun, photographed on september , .] to the unaided eye the sun appears to be a flat circle. if, however, it be examined with the telescope, taking care of course to interpose a piece of dark-coloured glass, or to employ some similar precaution to screen the eye from injury, it will then be perceived that the sun is not a flat surface, but a veritable glowing globe. the first question which we must attempt to answer enquires whether the glowing matter which forms the globe is a solid mass, or, if not solid, which is it, liquid or gaseous? at the first glance we might think that the sun cannot be fluid, and we might naturally imagine that it was a solid ball of some white-hot substance. but this view is not correct; for we can show that the sun is certainly not a solid body in so far at least as its superficial parts are concerned. a general view of the sun as shown by a telescope of moderate dimensions may be seen in fig. , which is taken from a photograph obtained by mr. rutherford at new york on the nd of september, . it is at once seen that the surface of the luminary is by no means of uniform texture or brightness. it may rather be described as granulated or mottled. this appearance is due to the luminous clouds which float suspended in a somewhat less luminous layer of gas. it is needless to say that these solar clouds are very different from the clouds which we know so well in our own atmosphere. terrestrial clouds are, of course, formed from minute drops of water, while the clouds at the surface of the sun are composed of drops of one or more chemical elements at an exceedingly high temperature. the granulated appearance of the solar surface is beautifully shown in the remarkable photographs on a large scale which m. janssen, of meudon, has succeeded in obtaining during the last twenty years. we are enabled to reproduce one of them in fig. . it will be observed that the interstices between the luminous dots are of a greyish tint, the general effect (as remarked by professor young) being much like that of rough drawing paper seen from a little distance. we often notice places over the surface of such a plate where the definition seems to be unsatisfactory. these are not, however, the blemishes that might at first be supposed. they arise neither from casual imperfections of the photographic plate nor from accidents during the development; they plainly owe their origin to some veritable cause in the sun itself, nor shall we find it hard to explain what that cause must be. as we shall have occasion to mention further on, the velocities with which the glowing gases on the sun are animated must be exceedingly great. even in the hundredth part of a second (which is about the duration of the exposure of this plate) the movements of the solar clouds are sufficiently great to produce the observed indistinctness. [illustration: fig. .--photograph of the solar surface. (_by janssen._)] irregularly dispersed over the solar surface small dark objects called sun-spots are generally visible. these spots vary greatly both as to size and as to number. sun-spots were first noticed in the beginning of the seventeenth century, shortly after the invention of the telescope. their general appearance is shown in fig. , in which the dark central nucleus appears in sharp contrast with the lighter margin or penumbra. fig. shows a small spot developing out of one of the pores or interstices between the granules. [illustration: fig. .--an ordinary sun-spot.] the earliest observers of these spots had remarked that they seem to have a common motion across the sun. in fig. we give a copy of a remarkable drawing by father scheiner, showing the motion of two spots observed by him in march, . the figure indicates the successive positions assumed by the spots on the several days from the nd to the th march. those marks which are merely given in outline represent the assumed positions on the th and the th, on which days it happened that the weather was cloudy, so that no observations could be made. it is invariably found that these objects move in the same direction--namely, from the eastern to the western limb[ ] of the sun. they complete the journey across the face of the sun in twelve or thirteen days, after which they remain invisible for about the same length of time until they reappear at the eastern limb. these early observers were quick to discern the true import of their discovery. they deduced from these simple observations the remarkable fact that the sun, like the earth, performs a rotation on its axis, and in the same direction. but there is the important difference between these rotations that whereas the earth takes only twenty-four hours to turn once round, the solar globe takes about twenty-six days to complete one of its much more deliberate rotations. [illustration: plate iii. spots and faculÆ on the sun. (from a photograph by mr. warren de la rue, th sept., .)] if we examine sun-spots under favourable atmospheric conditions and with a telescope of fairly large aperture, we perceive a great amount of interesting detail which is full of information with regard to the structure of the sun. the penumbra of a spot is often found to be made up of filaments directed towards the middle of the spot, and generally brighter at their inner ends, where they adjoin the nucleus. in a regularly formed spot the outline of the penumbra is of the same general form as that of the nucleus, but astronomers are frequently deeply interested by witnessing vast spots of very irregular figure. in such cases the bright surface-covering of the sun (the photosphere, as it is called) often encroaches on the nucleus and forms a peninsula stretching out into, or even bridging across, the gloomy interior. this is well shown in professor langley's fine drawing (plate ii.) of a very irregular spot which he observed on december - , . the details of a spot vary continually; changes may often be noticed even from day to day, sometimes from hour to hour. a similar remark may be made with respect to the bright streaks or patches which are frequently to be observed especially in the neighbourhood of spots. these bright marks are known by the name of _faculæ_ (little torches). they are most distinctly seen near the margin of the sun, where the light from its surface is not so bright as it is nearer to the centre of the disc. the reduction of light at the margin is due to the greater thickness of absorbing atmosphere round the sun, through which the light emitted from the regions near the margin has to pass in starting on its way towards us. none of the markings on the solar disc constitute permanent features on the sun. some of these objects may no doubt last for weeks. it has, indeed, occasionally happened that the same spot has marked the solar globe for many months; but after an existence of greater or less duration those on one part of the sun may disappear, while as frequently fresh marks of the same kind become visible in other places. the inference from these various facts is irresistible. they tell us that the visible surface of the sun is not a solid mass, is not even a liquid mass, but that the globe, so far as we can see it, consists of matter in the gaseous, or vaporous, condition. [illustration: fig. .--scheiner's observations on sun-spots.] it often happens that a large spot divides into two or more separate portions, and these have been sometimes seen to fly apart with a velocity in some cases not less than a thousand miles an hour. "at times, though very rarely" (i quote here professor young,[ ] to whom i am frequently indebted), "a different phenomenon of the most surprising and startling character appears in connection with these objects: patches of intense brightness suddenly break out, remaining visible for a few minutes, moving, while they last, with velocities as great as one hundred miles _a second_." [illustration: fig. .--zones on the sun's surface in which spots appear.] "one of these events has become classical. it occurred on the forenoon (greenwich time) of september st, , and was independently witnessed by two well-known and reliable observers--mr. carrington and mr. hodgson--whose accounts of the matter may be found in the monthly notices of the royal astronomical society for november, . mr. carrington at the time was making his usual daily observations upon the position, configuration, and size of the spots by means of an image of the solar disc upon a screen--being then engaged upon that eight years' series of observations which lie at the foundation of so much of our present solar science. mr. hodgson, at a distance of many miles, was at the same time sketching details of sun-spot structure by means of a solar eye-piece and shade-glass. they simultaneously saw two luminous objects, shaped something like two new moons, each about eight thousand miles in length and two thousand wide, at a distance of some twelve thousand miles from each other. these burst suddenly into sight at the edge of a great sun-spot with a dazzling brightness at least five or six times that of the neighbouring portions of the photosphere, and moved eastward over the spot in parallel lines, growing smaller and fainter, until in about five minutes they disappeared, after traversing a course of nearly thirty-six thousand miles." the sun-spots do not occur at all parts of the sun's surface indifferently. they are mainly found in two zones (fig. ) on each side of the solar equator between the latitudes of ° and °. on the equator the spots are rare except, curiously enough, near the time when there are few spots elsewhere. in high latitudes they are never seen. closely connected with these peculiar principles of their distribution is the remarkable fact that spots in different latitudes do not indicate the same values for the period of rotation of the sun. by watching a spot near the sun's equator carrington found that it completed a revolution in twenty-five days and two hours. at a latitude of ° the period is about twenty-five days and eighteen hours, at ° it is no less than twenty-six days and twelve hours, while the comparatively few spots observed in the latitude of ° require twenty-seven and a half days to complete their circuit. as the sun, so far at least as its outer regions are concerned, is a mass of gas and not a solid body, there would be nothing incredible in the supposition that spots are occasionally endowed with movements of their own like ships on the ocean. it seems, however, from the facts before us that the different zones on the sun, corresponding to what we call the torrid and temperate zones on the earth, persist in rotating with velocities which gradually decrease from the equator towards the poles. it seems probable that the interior parts of the sun do not rotate as if the whole were a rigidly connected mass. the mass of the sun, or at all events its greater part, is quite unlike a rigid body, and the several portions are thus to some extent free for independent motion. though we cannot actually see how the interior parts of the sun rotate, yet here the laws of dynamics enable us to infer that the interior layers of the sun rotate more rapidly than the outer layers, and thus some of the features of the spot movements can be accounted for. but at present it must be confessed that there are great difficulties in the way of accounting for the distribution of spots and the law of rotation of the sun. in the year schwabe, a german astronomer, commenced to keep a regular register of the number of spots visible on the sun. after watching them for seventeen years he was able to announce that the number of spots seemed to fluctuate from year to year, and that there was a period of about ten years in their changes. subsequent observations have confirmed this discovery, and old books and manuscripts have been thoroughly searched for information of early date. thus a more or less complete record of the state of the sun as regards spots since the beginning of the seventeenth century has been put together. this has enabled astronomers to fix the period of the recurring maximum with greater accuracy. the course of one of the sun-spot cycles may be described as follows: for two or three years the spots are both larger and more numerous than on the average; then they begin to diminish, until in about six or seven years from the maximum they decline to a minimum; the number of the spots then begins to increase, and in about four and a half years the maximum is once more attained. the length of the cycle is, on an average, about eleven years and five weeks, but both its length and the intensity of the maxima vary somewhat. for instance, a great maximum occurred in the summer of , after which a very low minimum occurred in , followed by a feeble maximum at the end of ; next came an average minimum about august, , followed by the last observed maximum in january, . it is not unlikely that a second period of about sixty or eighty years affects the regularity of the eleven-year period. systematic observations carried on through a great many years to come will be required to settle this question, as the observations of sun-spots previous to are far too incomplete to decide the issues which arise. a curious connection seems to exist between the periodicity of the spots and their distribution over the surface of the sun. when a minimum is about to pass away the spots generally begin to show themselves in latitudes about ° north and south of the sun's equator; they then gradually break out somewhat nearer to the equator, so that at the time of maximum frequency most of them appear at latitudes not greater than °. this distance from the sun's equator goes on decreasing till the time of minimum. indeed, the spots linger on very close to the equator for a couple of years more, until the outbreak signalising the commencement of another period has commenced in higher latitudes. we have still to note an extraordinary feature which points to an intimate connection between the phenomena of sun-spots and the purely terrestrial phenomena of magnetism. it is of course well known that the needle of a compass does not point exactly to the north, but diverges from the meridian by an angle which is different in different places and is not even constant at the same place. for instance, at greenwich the needle at present points in a direction ° west of north, but this amount is subject to very slow and gradual changes, as well as to very small daily oscillations. it was found about fifty years ago by lamont (a bavarian astronomer, but a native of scotland) that the extent of this daily oscillation increases and decreases regularly in a period which he gave as - / years, but which was subsequently found to be - / years, exactly the same as the period of the spots on the sun. from a diligent study of the records of magnetic observations it has been found that the time of sun-spot maximum always coincides almost exactly with that of maximum daily oscillation of the compass needle, while the minima agree similarly. this close relationship between the periodicity of sun-spots and the daily movements of the magnetic needle is not the sole proof we possess that there is a connection of some sort between solar phenomena and terrestrial magnetism. a time of maximum sun-spots is a time of great magnetic activity, and there have even been special cases in which a peculiar outbreak on the sun has been associated with remarkable magnetic phenomena on the earth. a very interesting instance of this kind is recorded by professor young, who, when observing at sherman on the rd august, , perceived a very violent disturbance of the sun's surface. he was told the same day by a member of his party, who was engaged in magnetic observations and who was quite in ignorance of what professor young had seen, that he had been obliged to desist from his magnetic work in consequence of the violent motion of his magnet. it was afterwards found from the photographic records at greenwich and stonyhurst that the magnetic "storm" observed in america had simultaneously been felt in england. a similar connection between sun-spots and the aurora borealis has also been noticed, this fact being a natural consequence of the well-known connection between the aurora and magnetic disturbances. on the other hand, it must be confessed that many striking magnetic storms have occurred without any corresponding solar disturbance,[ ] but even those who are inclined to be sceptical as to the connection between these two classes of phenomena in particular cases can hardly doubt the remarkable parallelism between the general rise and fall in the number of sun-spots and the extent of the daily movements of the compass needle. [illustration: fig. .--the texture of the sun and a small spot.] we have now described the principal solar phenomena with which the telescope has made us acquainted. but there are many questions connected with the nature of the sun which not even the most powerful telescope would enable us to solve, but which the spectroscope has given us the means of investigating. what we receive from the sun is warmth and light. the intensely heated mass of the sun radiates forth its beams in all directions with boundless prodigality. each beam we feel to be warm, and we see to be brilliantly white, but a more subtle analysis than mere feeling or mere vision is required. each sunbeam bears marks of its origin. these marks are not visible until a special process has been applied, but then the sunbeam can be made to tell its story, and it will disclose to us much of the nature of the constitution of the great luminary. we regard the sun's light as colourless, just as we speak of water as tasteless, but both of those expressions relate rather to our own feelings than to anything really characteristic of water or of sunlight. we regard the sunlight as colourless because it forms, as it were, the background on which all other colours are depicted. the fact is, that white is so far from being colourless that it contains every known hue blended together in certain proportions. the sun's light is really extremely composite; nature herself tells us this if we will but give her the slightest attention. whence come the beautiful hues with which we are all familiar? look at the lovely tints of a garden; the red of the rose is not in the rose itself. all the rose does is to grasp the sunbeams which fall upon it, extract from these beams the red which they contain, and radiate that red light to our eyes. were there not red rays conveyed with the other rays in the sunbeam, there could be no red rose to be seen by sunlight. the principle here involved has many other applications; a lady will often say that a dress which looks very well in the daylight does not answer in the evening. the reason is that the dress is intended to show certain colours which exist in the sunlight; but these colours are not contained to the same degree in gaslight, and consequently the dress has a different hue. the fault is not in the dress, the fault lies in the gas; and when the electric light is used it sends forth beams more nearly resembling those from the sun, and the colours of the dress appear with all their intended beauty. the most glorious natural indication of the nature of the sunlight is seen in the rainbow. here the sunbeams are refracted and reflected from tiny globes of water in the clouds; these convey to us the sunlight, and in doing so decompose the white beams into the seven primary hues--red, orange, yellow, green, blue, indigo, and violet. [illustration: plate a. the sun. _royal observatory, greenwich, july , ._] [illustration: fig. .--the prism.] the bow set in the cloud is typical of that great department of modern science of which we shall now set forth the principles. the globes of water decompose the solar beams; and we follow the course suggested by the rainbow, and analyse the sunlight into its constituents. we are enabled to do this with scientific accuracy when we employ that remarkable key to nature's secrets known as the spectroscope. the beams of white sunlight consist of innumerable beams of every hue in intimate association. every shade of red, of yellow, of blue, and of green, can be found in a sunbeam. the magician's wand, with which we strike the sunbeam and sort the tangled skein into perfect order, is the simple instrument known as the glass prism. we have represented this instrument in its simplest form in the adjoining figure (fig. ). it is a piece of pure and homogeneous glass in the shape of a wedge. when a ray of light from the sun or from any source falls upon the prism, it passes through the transparent glass and emerges on the other side; a remarkable change is, however, impressed on the ray by the influence of the glass. it is bent by refraction from the path it originally pursued, and is compelled to follow a different path. if, however, the prism bent all rays of light equally, then it would be of no service in the analysis of light; but it fortunately happens that the prism acts with varying efficiency on the rays of different hues. a red ray is not refracted so much as a yellow ray; a yellow ray is not refracted so much as a blue one. it consequently happens that when the composite beam of sunlight, in which all the different rays are blended, passes through the prism, they emerge in the manner shown in the annexed figure (fig. ). here then we have the source of the analysing power of the prism; it bends the different hues unequally and consequently the beam of composite sunlight, after passing through the prism, no longer shows mere white light, but is expanded into a coloured band of light, with hues like the rainbow, passing from deep red at one end through every intermediate grade to the violet. [illustration: fig. .--dispersion of light by the prism.] we have in the prism the means of decomposing the light from the sun, or the light from any other source, into its component parts. the examination of the quality of the light when analysed enables us to learn something of the constitution of the body from which this light has emanated. indeed, in some simple cases the mere colour of a light will be sufficient to indicate the source from which it has come. there is, for instance, a splendid red light sometimes seen in displays of fireworks, due to the metal strontium. the eye can identify the element by the mere colour of the flame. there is also a characteristic yellow light produced by the flame of common salt burned with spirits of wine. sodium is the important constituent of salt, so here we recognise another substance merely by the colour it emits when burning. we may also mention a third substance, magnesium, which burns with a brilliant white light, eminently characteristic of the metal. [illustration: plate xiii. spectra of the sun and stars. i. sun. ii. sirius. iii. aldebaran. iv. betelgeuze.] the three metals, strontium, sodium, and magnesium, may thus be identified by the colours they produce when incandescent. in this simple observation lies the germ of the modern method of research known as spectrum analysis. we may now examine with the prism the colours of the sun and the colours of the stars, and from this examination we can learn something of the materials which enter into their composition. we are not restricted to the use of merely a single prism, but we may arrange that the light which it is desired to analyse shall pass through several prisms in succession in order to increase the _dispersion_ or the spreading out of the different colours. to enter the spectroscope the light first passes through a narrow slit, and the rays are then rendered parallel by passing through a lens; these parallel rays next pass through one or more prisms, and are finally viewed through a small telescope, or they may be intercepted by a photographic plate on which a picture will then be made. if the beam of light passing through the slit has radiated from an incandescent solid or liquid body, or from a gas under high pressure, the coloured band or _spectrum_ is found to contain all the colours indicated on plate xiii., without any interruption between the colours. this is known as a continuous spectrum. but if we examine light from a gas under low pressure, as can be done by placing a small quantity of the gas in a glass tube and making it glow by an electric current, we find that it does not emit rays of all colours, but only rays of certain distinct colours which are different for different gases. the spectrum of a gas, therefore, consists of a number of detached luminous lines. when we study the sunlight through the prism, it is found that the spectrum does not extend quite continuously from one end to the other, but is shaded over by a multitude of dark lines, only a few of which are shown in the adjoining plate. (plate xiii.) these lines are a permanent feature in the solar spectrum. they are as characteristic of the sunlight as the prismatic colours themselves, and are full of interest and information with regard to the sun. these lines are the characters in which the history and the nature of the sun are written. viewed through an instrument of adequate power, dark lines are to be found crossing the solar spectrum in hundreds and in thousands. they are of every variety of strength and faintness; their distribution seems guided by no simple law. at some parts of the spectrum there are but few lines; in other regions they are crowded so closely together that it is difficult to separate them. they are in some places exquisitely fine and delicate, and they never fail to excite the admiration of every one who looks at this interesting spectacle in a good instrument. there can be no better method of expounding the rather difficult subject of spectrum analysis than by actually following the steps of the original discovery which first gave a clear demonstration of the significance of the dark "fraunhofer" lines. let us concentrate our attention specially upon that line of the solar spectrum marked d. this, when seen in the spectroscope, is found to consist of two lines, very delicately separated by a minute interval, one of these lines being slightly thicker than the other. suppose that while the attention is concentrated on these lines the flame of an ordinary spirit-lamp coloured by common salt be held in front of the instrument, so that the ray of direct solar light passes through the flame before entering the spectroscope. the observer sees at once the two lines known as d flash out with a greatly increased blackness and vividness, while there is no other perceptible effect on the spectrum. a few trials show that this intensification of the d lines is due to the vapour of sodium arising from the salt burning in the lamp through which the sunlight has passed. it is quite impossible that this marvellous connection between sodium and the d lines of the spectrum can be merely casual. even if there were only a single line concerned, it would be in the highest degree unlikely that the coincidence should arise by accident; but when we find the sodium affecting both of the two close lines which form d, our conviction that there must be some profound connection between these lines and sodium rises to absolute certainty. suppose that the sunlight be cut off, and that all other light is excluded save that emanating from the glowing vapour of sodium in the spirit flame. we shall then find, on looking through the spectroscope, that we no longer obtain all the colours of the rainbow; the light from the sodium is concentrated into two bright yellow lines, filling precisely the position which the dark d lines occupied in the solar spectrum, and the darkness of which the sodium flame seemed to intensify. we must here endeavour to remove what may at first sight appear to be a paradox. how is it, that though the sodium flame produces two _bright_ lines when viewed in the absence of other light, yet it actually appears to intensify the two _dark_ lines in the sun's spectrum? the explanation of this leads us at once to the cardinal doctrine of spectrum analysis. the so-called dark lines in the solar spectrum are only dark _by contrast_ with the brilliant illumination of the rest of the spectrum. a good deal of solar light really lies in the dark lines, though not enough to be seen when the eye is dazzled by the brilliancy around. when the flame of the spirit-lamp charged with sodium intervenes, it sends out a certain amount of light, which is entirely localised in these two lines. so far it would seem that the influence of the sodium flame ought to be manifested in diminishing the darkness of the lines and rendering them less conspicuous. as a matter of fact, they are far more conspicuous with the sodium flame than without it. this arises from the fact that the sodium flame possesses the remarkable property of cutting off the sunlight which was on its way to those particular lines; so that, though the sodium contributes some light to the lines, yet it intercepts a far greater quantity of the light that would otherwise have illuminated those lines, and hence they became darker with the sodium flame than without it. we are thus conducted to a remarkable principle, which has led to the interpretation of the dark lines in the spectrum of the sun. we find that when the sodium vapour is heated, it gives out light of a very particular type, which, viewed through the prism, is concentrated in two lines. but the sodium vapour possesses also this property, that light from the sun can pass through it without any perceptible absorption, except of those particular rays which are of the same characters as the two lines in question. in other words, we say that if the heated vapour of a substance gives a spectrum of bright lines, corresponding to lights of various kinds, this same vapour will act as an opaque screen to lights of those special kinds, while remaining transparent to light of every other description. this principle is of such importance in the theory of spectrum analysis that we add a further example. let us take the element iron, which in a very striking degree illustrates the law in question. in the solar spectrum some hundreds of the dark lines are known to correspond with the spectrum of iron. this correspondence is exhibited in a vivid manner when, by a suitable contrivance, the light of an electric spark from poles of iron is examined in the spectroscope side by side with the solar spectrum. the iron lines in the sun are identical in position with the lines in the spectrum of glowing iron vapour. but the spectrum of iron, as here described, consists of bright lines; while those with which it is compared in the sun are dark on a bright background. they can be completely understood if we suppose the vapour arising from intensely heated iron to be present in the atmosphere which surrounds the luminous strata on the sun. this vapour would absorb or stop precisely the same rays as it emits when incandescent, and hence we learn the important fact that iron, no less than sodium, must, in one form or another, be a constituent of the sun. such is, in brief outline, the celebrated discovery of modern times which has given an interpretation to the dark lines of the solar spectrum. the spectra of a large number of terrestrial substances have been examined in comparison with the solar spectrum, and thus it has been established that many of the elements known on the earth are present in the sun. we may mention calcium, iron, hydrogen, sodium, carbon, nickel, magnesium, cobalt, aluminium, chromium, strontium, manganese, copper, zinc, cadmium, silver, tin, lead, potassium. some of the elements which are of the greatest importance on the earth would appear to be missing from the sun. sulphur, phosphorus, mercury, gold, nitrogen may be mentioned among the elements which have hitherto given no indication of their being solar constituents. it is also possible that the lines of a substance in the sun's atmosphere may be so very bright that the light of the continuous spectrum, on which they are superposed, is not able to "reverse" them--_i.e._ turn them into dark lines. we know, for instance, that the bright lines of sodium vapour may be made so intensely bright that the spectrum of an incandescent lime-cylinder placed behind the sodium vapour does not reverse these lines. if, then, we make the sodium lines fainter, they may be reduced to exactly the intensity prevailing in that part of the spectrum of the lime-light, in which case the lines, of course, could not be distinguished. the question as to what elements are really missing from the sun must therefore, like many other questions concerning our great luminary, at present be considered an open one. we shall shortly see that an element previously unknown has actually been discovered by means of a line representing it in the solar spectrum. let us now return to the sun-spots and see what the spectroscope can teach us as to their nature. we attach a powerful spectroscope to the eye-end of a telescope in order to get as much light as possible concentrated on the slit; the latter has therefore to be placed exactly at the focus of the object-glass. the instrument is then pointed to a spot, so that its image falls on the slit, and the presence of the dark central part called the _umbra_ reveals itself by a darkish stripe which traverses the ordinary sun-spectrum from end to end. it is bordered on both sides by the spectrum of the _penumbra_, which is much brighter than that of the umbra, but fainter than that of the adjoining regions of the sun. from the fact that the spectrum is darkened we learn that there is considerable general absorption of light in the umbra. this absorption is not, however, such as would be caused by the presence of volumes of minute solid or liquid particles like those which constitute smoke or cloud. this is indicated by the fact, first discovered by young in , that the spectrum is not uniformly darkened as it would be if the absorption were caused by floating particles. in the course of examination of many large and quiescent spots, he perceived that the middle green part of the spectrum was crossed by countless fine, dark lines, generally touching each other, but here and there separated by bright intervals. each line is thicker in the middle (corresponding to the centre of the spot) and tapers to a fine thread at each end; indeed, most of these lines can be traced across the spectrum of the penumbra and out on to that of the solar surface. the absorption would therefore seem to be caused by gases at a much lower temperature than that of the gases present outside the spot. in the red and yellow parts of the spot-spectrum, which have been specially studied for many years by sir norman lockyer at the south kensington observatory, interesting details are found which confirm this conclusion. many of the dark lines are not thicker and darker in the spot than they are in the ordinary sun-spectrum, while others are very much thickened in the spot-spectrum, such as the lines of iron, calcium, and sodium. the sodium lines are sometimes both widened and doubly reversed--that is, on the thick dark line a bright line is superposed. the same peculiarity is not seldom seen in the notable calcium lines h and k at the violet end of the spectrum. these facts indicate the presence of great masses of the vapours of sodium and calcium over the nucleus. the observations at south kensington have also brought to light another interesting peculiarity of the spot-spectra. at the time of minimum frequency of spots the lines of iron and other terrestrial elements are prominent among the most widened lines; at the maxima these almost vanish, and the widening is found only amongst lines of unknown origin. the spectroscope has given us the means of studying other interesting features on the sun, which are so faint that in the full blaze of sunlight they cannot be readily observed with a mere telescope. we can, however, see them easily enough when the brilliant body of the sun is obscured during the rare occurrence of a total eclipse. the conditions necessary for the occurrence of an eclipse will be more fully considered in the next chapter. for the present it will be sufficient to observe that by the movement of the moon it may so happen that the moon completely hides the sun, and thus for certain parts of the earth produces what we call a total eclipse. the few minutes during which a total eclipse lasts are of much interest to the astronomer. darkness reigns over the landscape, and in that darkness rare and beautiful sights are witnessed. [illustration: fig. .--prominences seen in total eclipse.] we have in fig. a diagram of a total eclipse, showing some of the remarkable objects known as prominences (_a_, _b_, _c_, _d_, _e_) which project from behind the dark body of the moon. that they do not belong to the moon, but are solar appendages of some sort, is easily demonstrated. they first appear on the eastern limb at the commencement of totality. those first seen are gradually more or less covered by the advancing moon, while others peep out behind the western limb of the moon, until totality is over and the sunlight bursts out again, when they all instantly vanish. the first total eclipse which occurred after the spectroscope had been placed in the hands of astronomers was in . on the th august in that year a total eclipse was visible in india. several observers, armed with spectroscopes, were on the look-out for the prominences, and were able to announce that their spectrum consisted of detached bright lines, thus demonstrating that these objects were masses of glowing gas. on the following day the illustrious astronomer, janssen, one of the observers of the eclipse, succeeded in seeing the lines in full sunlight, as he now knew exactly where to look for them. many months before the eclipse sir norman lockyer had been preparing to search for the prominences, as he expected them to yield a line spectrum which would be readily visible, if only the sun's ordinary light could be sufficiently winnowed away. he proposed to effect this by using a spectroscope of great dispersion, which would spread out the continuous spectrum considerably and make it fainter. the effect of the great dispersion on the isolated bright lines he expected to see would be only to widen the intervals between them without interfering with their brightness. the new spectroscope, which he ordered to be constructed for this purpose, was not completed until some weeks after the eclipse was over, though before the news of janssen's achievement reached europe from india. when that news did arrive sir n. lockyer had already found the spectrum of unseen prominences at the sun's limb. the honour of the practical application of a method of observing solar prominences without the help of an eclipse must therefore be shared between the two astronomers. when a spectroscope is pointed to the margin of the sun so that the slit is radial, certain short luminous lines become visible which lie exactly in the prolongation of the corresponding dark lines in the solar spectrum. from due consideration of the circumstances it can be shown that the gases which form the prominences are also present as a comparatively shallow atmospheric layer all round the great luminary. this layer is about five or six thousand miles deep, and is situated immediately above the dense layer of luminous clouds which forms the visible surface of the sun and which we call the photosphere. the gaseous envelope from which the prominences spring has been called the chromosphere on account of the coloured lines displayed in its spectrum. such lines are very numerous, but those pertaining to the single substance, hydrogen, predominate so greatly that we may say the chromosphere consists chiefly of this element. it is, however, to be noted that calcium and one other element are also invariably present, while iron, manganese and magnesium are often apparent. the remarkable element, of which we have not yet mentioned the name, has had an astonishing history. during the eclipse of a fine yellow line was noticed among the lines of the prominence spectrum, and it was not unnaturally at first assumed that it must be the yellow sodium line. but when careful observations were afterwards made without hurry in full sunshine, and accurate measures were obtained, it was at once remarked that this line was not identical with either of the components of the double sodium line. the new line was, no doubt, quite close to the sodium lines, but slightly towards the green part of the spectrum. it was also noticed there was not generally any corresponding line to be seen among the dark lines in the ordinary solar spectrum, though a fine dark one has now and then been detected, especially near a sun-spot. sir norman lockyer and sir edward frankland showed that this was not produced by any known terrestrial element. it was, therefore, supposed to be caused by some hitherto unknown body to which the name of _helium_, or the sun element, was given. about a dozen less conspicuous lines were gradually identified in the spectrum of the prominences and the chromosphere, which appeared also to be caused by this same mysterious helium. these same remarkable lines have in more recent years also been detected in the spectra of various stars. this gas so long known in the heavens was at last detected on earth. in april, , professor ramsay, who with lord rayleigh had discovered the new element argon, detected the presence of the famous helium line in the spectrum of the gas liberated by heating the rare mineral known as cleveite, found in norway. thus this element, the existence of which had first been detected on the sun, ninety-three million miles away, has at last been proved to be a terrestrial element also. when it was announced by runge that the principal line in the spectrum of the terrestrial helium had a faint and very close companion line on the red-ward side, some doubt seemed at first to be cast on the identity of the new terrestrial gas discovered by ramsay with the helium of the chromosphere. the helium line of the latter had never been noticed to be double. subsequently, however, several observers provided with very powerful instruments found that the famous line in the chromosphere really had a very faint companion line. thus the identity between the celestial helium and the gas found on our globe was established in the most remarkable manner. certain circumstances have seemed to indicate that the new gas might possibly be a mixture of two gases of different densities, but up to the present this has not been proved to be the case. after it had been found possible to see the spectra of prominences without waiting for an eclipse, sir w. huggins, in an observation on the th of february, , successfully applied a method for viewing the remarkable solar objects themselves instead of their mere spectra in full sunshine. it is only necessary to adjust the spectroscope so that one of the brightest lines--_e.g._ the red hydrogen line--is in the middle of the field of the viewing telescope, and then to open wide the slit of the spectroscope. a red image of the prominence will then be displayed instead of the mere line. in fact, when the slit is opened wide, the prisms produce a series of detached images of the prominence under observation, one for each kind of light which the object emits. we have spoken of the spectroscope as depending upon the action of glass prisms. it remains to be added that in the highest class of spectroscopes the prisms are replaced by ruled gratings from which the light is reflected. the effect of the ruling is to produce by what is known as diffraction the required breaking up of the beam of light into its constituent parts. [illustration: plate iv. solar prominences. (drawn by trouvelot at harvard college, cambridge, u.s., in .)] majestic indeed are the proportions of some of those mighty prominences which leap from the luminous surface; yet they flicker, as do our terrestrial flames, when we allow them time comparable to their gigantic dimensions. drawings of the same prominence made at intervals of a few hours, or even less, often show great changes. the magnitude of the displacements that have been noticed sometimes attains many thousands of miles, and the actual velocity with which such masses move frequently exceeds miles a second. still more violent are the convulsions when, from the surface of the chromosphere, as from a mighty furnace, vast incandescent masses of gas are projected upwards. plate iv. gives a view of a number of prominences as seen by trouvelot at harvard college observatory, cambridge, u.s.a. trouvelot has succeeded in exhibiting in the different pictures the wondrous variety of aspect which these objects assume. the dimensions of the prominences may be inferred from the scale appended to the plate. the largest of those here shown is fully , miles high; and trustworthy observers have recorded prominences of an altitude even much greater. the rapid changes which these objects sometimes undergo are well illustrated in the two sketches on the left of the lowest line, which were drawn on april th, . these are both drawings of the same prominence taken at an interval no greater than twenty minutes. this mighty flame is so vast that its length is ten times as great as the diameter of the earth, yet in this brief period it has completely changed its aspect; the upper part of the flame has, indeed, broken away, and is now shown in that part of the drawing between the two figures on the line above. the same plate also shows various instances of the remarkable spike-like objects, taken, however, at different times and at various parts of the sun. these spikes attain altitudes not generally greater than , miles, though sometimes they soar aloft to stupendous distances. we may refer to one special object of this kind, the remarkable history of which has been chronicled by professor young. on october th, , a prominence was seen, at about . a.m., on the south-east limb of the sun. it was then about , miles high, and attracted no special attention. half an hour later a marvellous transformation had taken place. during that brief interval the prominence became very brilliant and doubled its length. for another hour the mighty flame still soared upwards, until it attained the unprecedented elevation of , miles--a distance more than one-third the diameter of the great luminary itself. at this climax the energy of the mighty outbreak seems to have at last become exhausted: the flame broke up into fragments, and by . --an interval of only two hours from the time when it was first noticed--the phenomenon had completely faded away. no doubt this particular eruption was exceptional in its vehemence, and in the vastness of the changes of which it was an indication. the velocity of upheaval must have been at least , miles an hour, or, to put it in another form, more than fifty miles a second. this mighty flame leaped from the sun with a velocity more than times as great as that of the swiftest bullet ever fired from a rifle. the prominences may be generally divided into two classes. we have first those which are comparatively quiescent, and in form somewhat resemble the clouds which float in our earth's atmosphere. the second class of prominences are best described as eruptive. they are, in fact, thrown up from the chromosphere like gigantic jets of incandescent material. these two classes of objects differ not only in appearance but also in the gases of which they are composed. the cloud-like prominences consist mainly of hydrogen, with helium and calcium, while many metals are present in the eruptive discharges. the latter are never seen in the neighbourhood of the sun's poles, but generally appear close to a sun-spot, thus confirming the conclusion that the spots are associated with violent disturbances on the surface of the sun. when a spot has reached the limb of the sun it is frequently found to be surrounded by prominences. it has even been possible in a few instances to detect powerful gaseous eruptions in the neighbourhood of a spot, the spectroscope rendering them visible against the background of the solar surface just as the prominences are observed at the limb against the background of the sky. in order to photograph a prominence we have, of course, to substitute a photographic plate for the observer's eye. owing, however, to the difficulty of preventing the feeble light from the prominence from being overpowered by extraneous light, the photography of these bodies was not very successful until professor hale, of chicago, designed his spectro-heliograph. in this instrument there is (in addition to the usual slit through which the light falls on the prisms, or grating,) a second slit immediately in front of the photographic plate through which the light of a given wave-length can be permitted to pass to the exclusion of all the rest. the light chosen for producing an image of the prominences is that radiated in the remarkable "k line," due to calcium. this lies at the extreme end of the violet. the light from that part of the spectrum, though it is invisible to the eye, is much more active photographically than the light from the red, yellow, or green parts of the spectrum. the front slit is adjusted so that the k line falls upon the second slit, and as the front slit is slowly swept by clockwork over the whole of a prominence, the second slit keeps pace with it by a mechanical contrivance. if the image of the solar disc is hidden by a screen of exactly the proper size, the slits may be made to sweep over the whole sun, thus giving us at one exposure a picture of the chromospheric ring round the sun's limb with its prominences. the screen may now be withdrawn, and the slits may be made to sweep rapidly over the disc itself. they reveal the existence of glowing calcium vapours in many parts of the surface of the sun. thus we get a striking picture of the sun as drawn by this particular light. in this manner professor hale confirmed the observation made long before by professor young, that the spectra of faculæ always show the two great calcium bands. the velocity with which a prominence shoots upward from the sun's limb can, of course, be measured directly by observations of the ordinary kind with a micrometer. the spectroscope, however, enables us to estimate the speed with which disturbances at the surface of the sun travel in the direction towards the earth or from the earth. we can measure this speed by watching the peculiar behaviour of the spectral lines representing the rapidly moving masses. this opens up a remarkable line of investigation with important applications in many branches of astronomy. it is, of course, now generally understood that the sensation of light is caused by waves or undulations which impinge on the retina of the eye after having been transmitted through that medium which we call the ether. to the different colours correspond different wave-lengths--that is to say, different distances between two successive waves. a beam of white light is formed by the union of innumerable different waves whose lengths have almost every possible value lying between certain limits. the wave-length of red light is such that there are , waves in an inch, while that of violet light is but little more than half that of red light. the position of a line in the spectrum depends solely on the wave-length of the light to which it is due. suppose that the source of light is approaching directly towards the observer; obviously the waves follow each other more closely than if the source were at rest, and the number of undulations which his eye receives in a second must be proportionately increased. thus the distance between two successive ether waves will be very slightly diminished. a well-known phenomenon of a similar character is the change of pitch of the whistle of a locomotive engine as it rushes past. this is particularly noticeable if the observer happens to be in a train which is moving rapidly in the opposite direction. in the case of sound, of course, the vibrations or waves take place in the air and not in the ether. but the effect of motion to or from the observer is strictly analogous in the two cases. as, however, light travels , miles a second, the source of light will also have to travel with a very high velocity in order to produce even the smallest perceptible change in the position of a spectral line. we have already seen that enormously high velocities are by no means uncommon in some of these mighty disturbances on the sun; accordingly, when we examine the spectrum of a sun-spot, we often see that some of the lines are shifted a little towards one end of the spectrum and sometimes towards the other, while in other cases the lines are seen to be distorted or twisted in the most fantastic manner, indicating very violent local commotions. if the spot happens to be near the centre of the sun's disc, the gases must be shooting upwards or downwards to produce these changes in the lines. the velocities indicated in observations of this class sometimes amount to as much as two or even three hundred miles per second. we find it difficult to conceive the enormous internal pressures which are required to impel such mighty masses of gases aloft from the photosphere with speeds so terrific, or the conditions which bring about the downrush of such gigantic masses of vapour from above. in the spectra of the prominences on the sun's limb also we often see the bright lines bent or shifted to one side. in such cases what we witness is evidently caused by movements along the surface of the chromosphere, conveying materials towards us or away from us. an interesting application of this beautiful method of measuring the speed of moving bodies has been made in various attempts to determine the period of rotation of the sun spectroscopically. as the sun turns round on its axis, a point on the eastern limb is moving towards the observer and a point on the western limb is moving away from him. in each case the velocity is a little over a mile per second. at the eastern limb the lines in the solar spectrum are very slightly shifted towards the violet end of the spectrum, while the lines in the spectrum of the western limb are equally shifted towards the red end. by an ingenious optical contrivance it is possible to place the spectra from the two limbs side by side, which doubles the apparent displacement, and thus makes it much more easy to measure. even with this contrivance the visual quantities to be measured remain exceedingly minute. all the parts of the instrument have to be most accurately adjusted, and the observations are correspondingly delicate. they have been attempted by various observers. among the most successful investigations of this kind we may mention that of the swedish astronomer, dunér, who, by pointing his instrument to a number of places on the limb, found values in good agreement with the peculiar law of rotation which has been deduced from the motion of sun-spots. this result is specially interesting, as it shows that the atmospheric layers, in which that absorption takes place which produces the dark lines in the spectrum, shares in the motion of the photosphere at the same latitude. [illustration: fig. .--view of the corona (and a comet) in a total eclipse.] [illustration: plate v. total solar eclipse, july th, . the corona from the photographs. (harkness.)] we have yet to mention one other striking phenomenon which is among the chief attractions to observers of total eclipses, and which it has hitherto not been found possible to see in full daylight. this is the corona or aureole of light which is suddenly seen to surround the sun in an eclipse when the moon has completely covered the last remaining crescent of the sun. a general idea of the appearance of the corona is given in fig. , and we further present in plate v. the drawing of the corona made by professor harkness from a comparison of a large number of photographs obtained at different places in the united states during the total eclipse of july th, . in fig. we are permitted by the kindness of mr. and mrs. maunder to reproduce the remarkable photograph of the corona which they obtained in india during the eclipse of january nd, . [illustration: fig. .--view of corona during the eclipse of jan. nd, (_reproduced by kind permission of mr. and mrs. maunder and of the proprietors of "knowledge._")] the part of the corona nearest the sun is very bright, though not so brilliant as the prominences, which (as professor young says) blaze through it like carbuncles. this inner portion is generally of fairly regular outline, forming a white ring about a tenth part of the solar diameter in width. the outer parts of the corona are usually very irregular and very extensive. they are often interrupted by narrow "rifts," or narrow dark bands, which reach from the limb of the sun through the entire corona. on the other hand, there are also sometimes narrow bright streamers, inclined at various angles to the limb of the sun and not seldom curved. in the eclipses of , , and , all of which occurred at periods of sun-spot minimum, the corona showed long and faint streamers nearly in the direction of the sun's equator, and short but distinct brushes of light near the poles. in the eclipses of , , and , near sun-spot maxima, the corona was more regularly circular, and chiefly developed over the spot zones. we have here another proof (if one were necessary) of the intimate connection between the periodicity of the spots and the development of all other solar phenomena. in the spectrum of the corona there is a mysterious line in the green, as to the origin of which nothing is at present certainly known. it is best seen during eclipses occurring near the time of sun-spot maximum. it is presented in the ordinary solar spectrum as a very thin, dark line, which generally remains undisturbed even when lines of hydrogen and other substances are twisted and distorted by the violent rush of disturbed elements. the line is always present among the bright lines of the chromosphere spectrum. in addition to it the corona shows a few other bright lines, belonging, no doubt, to the same unknown element ("coronium"), and also a faint continuous spectrum, in which even a few of the more prominent dark lines of the solar spectrum have been sometimes detected. this shows that in addition to glowing gas (represented by the bright lines) the corona also contains a great deal of matter like dust, or fog, the minute particles of which are capable of reflecting the sunlight and thereby producing a feeble continuous spectrum. this matter seems to form the principal constituent of the long coronal rays and streamers, as the latter are not visible in the detached images of the corona which appear instead of the bright lines when the corona is viewed, or photographed, during an eclipse, in a spectroscope without a slit. if the long rays were composed of the gas or gases which constitute the inner corona, it is evident that they ought to appear in these detached images. as to the nature of the forces which are continually engaged in shooting out these enormously long streamers, we have at present but little information. it is, however, certain that the extensive atmospheric envelope round the sun, which shows itself as the inner corona, must be extremely attenuated. comets have on several occasions been known to rush through this coronal atmosphere without evincing the slightest appreciable diminution in their speed from the resistance to which they were exposed. we have accumulated by observation a great number of facts concerning the sun, but when we try to draw from these facts conclusions as to the physical constitution of that great body, it cannot be denied that the difficulties seem to be very great indeed. we find that the best authorities differ considerably in the opinions they entertain as to its nature. we shall here set forth the principal conclusions as to which there is little or no controversy. we shall see in a following chapter that astronomers have been able to determine the relative densities of the bodies in the solar system; in other words, they have found the relation between the quantities of matter contained in an equally large volume of each. it has thus been ascertained that the average density of the sun is about a quarter that of the earth. if we compare the weight of the sun with that of an equally great globe of water, we find that the luminary would be barely one and a half times as heavy as the water. of course, the actual mass of the sun is very enormous; it is no less than , times as great as that of the earth. the solar material itself is, however, relatively light, so that the sun is four times as big as it would have to be if, while its weight remained the same, its density equalled that of the earth. bearing in mind this lightness of the sun, and also the exceedingly high temperature which we know to prevail there, no other conclusion seems possible than that the body of the sun must be in a gaseous state. the conditions under which such gases exist in the sun are, no doubt, altogether different from those with which we are acquainted on the earth. at the surface of the sun the force of gravity is more than twenty-seven times as great as it is on the earth. a person who on the earth could just lift twenty-seven equal pieces of metal would, if he were transferred to the sun, only be able to lift one of the pieces at a time. the pressure of the gases below the surface must therefore be very great, and it might be supposed that they would become liquefied in consequence. it was, however, discovered by andrews that so long as a gas is kept at a temperature higher than a certain point, known as the "critical temperature" (which is different for different gases), the gas will not be turned into a liquid however great be the pressure to which it is submitted. the temperature on the sun cannot be lower than the critical temperatures of the gases there existing; so it would seem that even the enormous pressure can hardly reduce the gases in the great luminary to the liquid form. of the interior of the sun we can, of course, expect to learn little or nothing. what we observe is the surface-layer, the so-called photosphere, in which the cold of space produces the condensation of the gases into those luminous clouds which we see in our drawings and photographs as "rice grains" or "willow leaves." it has been suggested by dr. johnstone stoney (and afterwards by professor hastings, of baltimore) that these luminous clouds are mainly composed of carbon with those of the related elements silicon and boron, the boiling points of which are much higher than those of other elements which might be considered likely to form the photospheric clouds. the low atomic weight of carbon must also have the effect of giving the molecules of this element a very high velocity, and thereby enabling them to work their way into the upper regions, where the temperature has so fallen that the vapour becomes chilled into cloud. a necessary consequence of the rapid cooling of these clouds, and the consequent radiation of heat on a large scale, would be the formation of what we may perhaps describe as smoke, which settles by degrees through the intervals between the clouds (making these intervals appear darker) until it is again volatilised on reaching a level of greater heat below the clouds. this same smoke is probably the cause of the well-known fact that the solar limb is considerably fainter than the middle of the disc. this seems to arise from the greater absorption caused by the longer distance which a ray of light from a point near the limb has to travel through this layer of smoke before reaching the earth. it is shown that this absorption cannot be attributed to a gaseous atmosphere, since this would have the effect of producing more dark absorption lines in the spectrum. there would thus be a marked difference between the solar spectrum from a part near the middle of the disc and the spectrum from a part near the limb. this, however, we do not find to be the case. with regard to the nature of sun-spots, the idea first suggested by secchi and lockyer, that they represent down rushes of cooler vapours into the photosphere (or to its surface), seems on the whole to accord best with the observed phenomena. we have already mentioned that the spots are generally accompanied by faculæ and eruptive prominences in their immediate neighbourhood, but whether these eruptions are caused by the downfall of the vapour which makes the photospheric matter "splash up" in the vicinity, or whether the eruptions come first, and by diminishing the upward pressure from below form a "sink," into which overlying cooler vapour descends, are problems as to which opinions are still much divided. a remarkable appendage to the sun, which extends to a distance very much greater than that of the corona, produces the phenomenon of the zodiacal light. a pearly glow is sometimes seen in the spring to spread over a part of the sky in the vicinity of the point where the sun has disappeared after sunset. the same spectacle may also be witnessed before sunrise in the autumn, and it would seem as if the material producing the zodiacal light, whatever it may be, had a lens-shaped form with the sun in the centre. the nature of this object is still a matter of uncertainty, but it is probably composed of a kind of dust, as the faint spectrum it affords is of a continuous type. a view of the zodiacal light is shown in fig. . in all directions the sun pours forth, with the most prodigal liberality, its torrents of light and of heat. the earth can only grasp the merest fraction, less than the , , , th part of the whole. our fellow planets and the moon also intercept a trifle; but how small is the portion of the mighty flood which they can utilise! the sip that a flying swallow takes from a river is as far from exhausting the water in the river as are the planets from using all the heat which streams from the sun. the sun's gracious beams supply the magic power that enables the corn to grow and ripen. it is the heat of the sun which raises water from the ocean in the form of vapour, and then sends down that vapour as rain to refresh the earth and to fill the rivers which bear our ships down to the ocean. it is the heat of the sun beating on the large continents which gives rise to the breezes and winds that waft our vessels across the deep; and when on a winter's evening we draw around the fire and feel its invigorating rays, we are only enjoying sunbeams which shone on the earth countless ages ago. the heat in those ancient sunbeams developed the mighty vegetation of the coal period, and in the form of coal that heat has slumbered for millions of years, till we now call it again into activity. it is the power of the sun stored up in coal that urges on our steam-engines. it is the light of the sun stored up in coal that beams from every gaslight in our cities. for the power to live and move, for the plenty with which we are surrounded, for the beauty with which nature is adorned, we are immediately indebted to one body in the countless hosts of space, and that body is the sun. [illustration: fig. .--the zodiacal light in .] chapter iii. the moon. the moon and the tides--the use of the moon in navigation--the changes of the moon--the moon and the poets--whence the light of the moon?--sizes of the earth and the moon--weight of the moon--changes in apparent size--variations in its distance--influence of the earth on the moon--the path of the moon--explanation of the moon's phases--lunar eclipses--eclipses of the sun, how produced--visibility of the moon in a total eclipse--how eclipses are predicted--uses of the moon in finding longitude--the moon not connected with the weather--topography of the moon--nasmyth's drawing of triesnecker--volcanoes on the moon--normal lunar crater--plato--the shadows of lunar mountains--the micrometer--lunar heights--former activity on the moon--nasmyth's view of the formation of craters--gravitation on the moon--varied sizes of the lunar craters--other features of the moon--is there life on the moon?--absence of water and of air--dr. stoney's theory--explanation of the rugged character of lunar scenery--possibility of life on distant bodies in space. if the moon were suddenly struck out of existence, we should be immediately apprised of the fact by a wail from every seaport in the kingdom. from london and from liverpool we should hear the same story--the rise and fall of the tide had almost ceased. the ships in dock could not get out; the ships outside could not get in; and the maritime commerce of the world would be thrown into dire confusion. the moon is the principal agent in causing the daily ebb and flow of the tide, and this is the most important work which our satellite has to do. the fleets of fishing boats around the coasts time their daily movements by the tide, and are largely indebted to the moon for bringing them in and out of harbour. experienced sailors assure us that the tides are of the utmost service to navigation. the question as to how the moon causes the tides is postponed to a future chapter, in which we shall also sketch the marvellous part which the tides seem to have played in the early history of our earth. who is there that has not watched, with admiration, the beautiful series of changes through which the moon passes every month? we first see her as an exquisite crescent of pale light in the western sky after sunset. if the night is fine, the rest of the moon is visible inside the crescent, being faintly illumined by light reflected from our own earth. night after night she moves further and further to the east, until she becomes full, and rises about the same time that the sun sets. from the time of the full the disc of light begins to diminish until the last quarter is reached. then it is that the moon is seen high in the heavens in the morning. as the days pass by, the crescent shape is again assumed. the crescent wanes thinner and thinner as the satellite draws closer to the sun. finally she becomes lost in the overpowering light of the sun, again to emerge as the new moon, and again to go through the same cycle of changes. the brilliance of the moon arises solely from the light of the sun, which falls on the not self-luminous substance of the moon. out of the vast flood of light which the sun pours forth with such prodigality into space the dark body of the moon intercepts a little, and of that little it reflects a small fraction to illuminate the earth. the moon sheds so much light, and seems so bright, that it is often difficult at night to remember that the moon has no light except what falls on it from the sun. nevertheless, the actual surface of the brightest full moon is perhaps not much brighter than the streets of london on a clear sunshiny day. a very simple observation will suffice to show that the moon's light is only sunlight. look some morning at the moon in daylight, and compare the moon with the clouds. the brightness of the moon and of the clouds are directly comparable, and then it can be readily comprehended how the sun which illuminates the clouds has also illumined the moon. an attempt has been made to form a comparative estimate of the brightness of the sun and the full moon. if , full moons were shining at once, their collective brilliancy would equal that of the sun. the beautiful crescent moon has furnished a theme for many a poet. indeed, if we may venture to say so, it would seem that some poets have forgotten that the moon is not to be seen every night. a poetical description of evening is almost certain to be associated with the appearance of the moon in some phase or other. we may cite one notable instance in which a poet, describing an historical event, has enshrined in exquisite verse a statement which cannot be correct. every child who speaks our language has been taught that the burial of sir john moore took place "by the struggling moonbeams' misty light." there is an appearance of detail in this statement which wears the garb of truth. we are not inclined to doubt that the night was misty, nor as to whether the moonbeams had to struggle into visibility; the question at issue is a much more fundamental one. we do not know who was the first to raise the point as to whether any moon shone on that memorable event at all or not; but the question having been raised, the nautical almanac immediately supplies an answer. from it we learn in language, whose truthfulness constitutes its only claim to be poetry, that the moon was new at one o'clock in the morning of the day of the battle of corunna ( th january, ). the ballad evidently implies that the funeral took place on the night following the battle. we are therefore assured that the moon can hardly have been a day old when the hero was consigned to his grave. but the moon in such a case is practically invisible, and yields no appreciable moonbeams at all, misty or otherwise. indeed, if the funeral took place at the "dead of night," as the poet asserts, then the moon must have been far below the horizon at the time.[ ] in alluding to this and similar instances, mr. nasmyth gives a word of advice to authors or to artists who desire to bring the moon on a scene without knowing as a matter of fact that our satellite was actually present. he recommends them to follow the example of bottom in _a midsummer's night's dream_, and consult "a calendar, a calendar! look in the almanac; find out moonshine, find out moonshine!" [illustration: fig. .--comparative sizes of the earth and the moon.] among the countless host of celestial bodies--the sun, the moon, the planets, and the stars--our satellite enjoys one special claim on our attention. the moon is our nearest permanent neighbour. it is just possible that a comet may occasionally approach the earth more closely than the moon but with this exception the other celestial bodies are all many hundreds or thousands, or even many millions, of times further from us than the moon. it is also to be observed that the moon is one of the smallest visible objects which the heavens contain. every one of the thousands of stars that can be seen with the unaided eye is enormously larger than our satellite. the brilliance and apparent vast proportions of the moon arise from the fact that it is only , miles away, which is a distance almost immeasurably small when compared with the distances between the earth and the stars. fig. exhibits the relative sizes of the earth and its attendant. the small globe shows the moon, while the larger globe represents the earth. when we measure the actual diameters of the two globes, we find that of the earth to be , miles and of the moon , miles, so that the diameter of the earth is nearly four times greater than the diameter of the moon. if the earth were cut into fifty pieces, all equally large, then one of these pieces rolled into a globe would equal the size of the moon. the superficial extent of the moon is equal to about one thirteenth part of the surface of the earth. the hemisphere our neighbour turns towards us exhibits an area equal to about one twenty-seventh part of the area of the earth. this, to speak approximately, is about double the actual extent of the continent of europe. the average materials of the earth are, however, much heavier than those contained in the moon. it would take more than eighty globes, each as ponderous as the moon, to weigh down the earth. amid the changes which the moon presents to us, one obvious fact stands prominently forth. whether our satellite be new or full, at first quarter or at last, whether it be high in the heavens or low near the horizon, whether it be in process of eclipse by the sun, or whether the sun himself is being eclipsed by the moon, the apparent size of the latter is nearly constant. we can express the matter numerically. a globe one foot in diameter, at a distance of feet from the observer, would under ordinary circumstances be just sufficient to hide the disc of the moon; occasionally, however, the globe would have to be brought in to a distance of only feet, or occasionally it might have to be moved out to so much as feet, if the moon is to be exactly hidden. it is unusual for the moon to approach either of its extreme limits of position, so that the distance from the eye at which the globe must be situated so as to exactly cover the moon is usually more than feet, and less than feet. these fluctuations in the apparent size of our satellite are contained within such narrow limits that in the first glance at the subject they may be overlooked. it will be easily seen that the apparent size of the moon must be connected with its real distance from the earth. suppose, for the sake of illustration, that the moon were to recede into space, its size would seem to dwindle, and long ere it had reached the distance of even the very nearest of the other celestial bodies it would have shrunk into insignificance. on the other hand, if the moon were to come nearer to the earth, its apparent size would gradually increase until, when close to our globe, it would seem like a mighty continent stretching over the sky. we find that the apparent size of the moon is nearly constant, and hence we infer that the average distance of the same body is also nearly constant. the average value of that distance is , miles. in rare circumstances it may approach to a distance but little more than , miles, or recede to a distance hardly less than , miles, but the ordinary fluctuations do not exceed more than about , miles on either side of its mean value. from the moon's incessant changes we perceive that she is in constant motion, and we now further see that whatever these movements may be, the earth and the moon must at present remain at _nearly_ the same distance apart. if we further add that the path pursued by the moon around the heavens lies nearly in a plane, then we are forced to the conclusion that our satellite must be revolving in a nearly circular path around the earth at the centre. it can, indeed, be shown that the constant distance of the two bodies involves as a necessary condition the revolution of the moon around the earth. the attraction between the moon and the earth tends to bring the two bodies together. the only way by which such a catastrophe can be permanently avoided is by making the satellite move as we actually find it to do. the attraction between the earth and the moon still exists, but its effect is not then shown in bringing the moon in towards the earth. the attraction has now to exert its whole power in restraining the moon in its circular path; were the attraction to cease, the moon would start off in a straight line, and recede never to return. [illustration: fig. .--the moon's path around the sun.] the fact of the moon's revolution around the earth is easily demonstrated by observations of the stars. the rising and setting of our satellite is, of course, due to the rotation of the earth, and this apparent diurnal movement the moon possesses in common with the sun and with the stars. it will, however, be noticed that the moon is continually changing its place among the stars. even in the course of a single night the displacement will be conspicuous to a careful observer without the aid of a telescope. the moon completes each revolution around the earth in a period of · days. [illustration: fig. .--the phases of the moon.] in fig. we have a view of the relative positions of the earth, the sun, and the moon, but it is to be observed that, for the convenience of illustration, we have been obliged to represent the orbit of the moon on a much larger scale than it ought to be in comparison with the distance of the sun. that half of the moon which is turned towards the sun is brilliantly illuminated, and, according as we see more or less of that brilliant half, we say that the moon is more or less full, the several "phases" being visible in the succession shown by the numbers in fig. . a beginner sometimes finds considerable difficulty in understanding how the light on the full moon at night can have been derived from the sun. "is not," he will say, "the earth in the way? and must it not intercept the sunlight from every object on the other side of the earth to the sun?" a study of fig. will explain the difficulty. the plane in which the moon revolves does not coincide with the plane in which the earth revolves around the sun. the line in which the plane of the earth's motion is intersected by that of the moon divides the moon's path into two semicircles. we must imagine the moon's path to be tilted a little, so that the upper semicircle is somewhat above the plane of the paper, and the other semicircle below. it thus follows that when the moon is in the position marked full, under the circumstances shown in the figure, the moon will be just above the line joining the earth and the sun; the sunlight will thus pass over the earth to the moon, and the moon will be illuminated. at new moon, the moon will be under the line joining the earth and the sun. as the relative positions of the earth and the sun are changing, it happens twice in each revolution that the sun comes into the position of the line of intersection of the two planes. if this occurs at the time of full moon, the earth lies directly between the moon and the sun; the moon is thus plunged into the shadow of the earth, the light from the sun is intercepted, and we say that the moon is eclipsed. the moon sometimes only partially enters the earth's shadow, in which case the eclipse is a partial one. when, on the other hand, the sun is situated on the line of intersection at the time of new moon, the moon lies directly between the earth and the sun, and the dark body of the moon will then cut off the sunlight from the earth, producing a solar eclipse. usually only a part of the sun is thus obscured, forming the well-known partial eclipse; if, however, the moon pass centrally over the sun, then we must have one or other of two very remarkable kinds of eclipse. sometimes the moon entirely blots out the sun, and thus is produced the sublime spectacle of a total eclipse, which tells us so much as to the nature of the sun, and to which we have already referred in the last chapter. even when the moon is placed centrally over the sun, a thin rim of sunlight is occasionally seen round the margin of the moon. we then have what is known as an annular eclipse. it is remarkable that the moon is sometimes able to hide the sun completely, while on other occasions it fails to do so. it happens that the average apparent size of the moon is nearly equal to the average apparent size of the sun, but, owing to the fluctuations in their distances, the actual apparent sizes of both bodies undergo certain changes. on certain occasions the apparent size of the moon is greater than that of the sun. in this case a central passage produces a total eclipse; but it may also happen that the apparent size of the sun exceeds that of the moon, in which case a central passage can only produce an annular eclipse. [illustration: fig. .--form of the earth's shadow, showing the penumbra, or partially shaded region. within the penumbra, the moon is visible; in the shadow it is nearly invisible.] there are hardly any more interesting celestial phenomena than the different descriptions of eclipses. the almanac will always give timely notice of the occurrence, and the more striking features can be observed without a telescope. in an eclipse of the moon (fig. ) it is interesting to note the moment when the black shadow is first detected, to watch its gradual encroachment over the bright surface of the moon, to follow it, in case the eclipse is total, until there is only a thin crescent of moonlight left, and to watch the final extinction of that crescent when the whole moon is plunged into the shadow. but now a spectacle of great interest and beauty is often manifested; for though the moon is so hidden behind the earth that not a single direct ray of the sunlight could reach its surface, yet we often find that the moon remains visible, and, indeed, actually glows with a copper-coloured hue bright enough to permit several of the markings on the surface to be discerned. this illumination of the moon even in the depth of a total eclipse is due to the sunbeams which have just grazed the edge of the earth. in doing so they have become bent by the refraction of the atmosphere, and have thus been turned inwards into the shadow. such beams have passed through a prodigious thickness of the earth's atmosphere, and in this long journey through hundreds of miles of air they have become tinged with a ruddy or copper-like hue. nor is this property of our atmosphere an unfamiliar one. the sun both at sunrise and at sunset glows with a light which is much more ruddy than the beams it dispenses at noonday. but at sunset or at sunrise the rays which reach our eyes have much more of our atmosphere to penetrate than they have at noon, and accordingly the atmosphere imparts to them that ruddy colour so characteristic and often so lovely. if the spectrum of the sun when close to the horizon is examined it is seen to be filled with numerous dark lines and bands situated chiefly towards the blue and violet end. these are caused by the increased absorption which the light suffers in the atmosphere, and give rise to the preponderating red light on the sun under such conditions. in the case of the eclipsed moon, the sunbeams have to take an atmospheric journey more than double as long as that at sunrise or sunset, and hence the ruddy glow of the eclipsed moon may be accounted for. the almanacs give the full particulars of each eclipse that happens in the corresponding year. these predictions are reliable, because astronomers have been carefully observing the moon for ages, and have learned from these observations not only how the moon moves at present, but also how it will move for ages to come. the actual calculations are so complicated that we cannot here discuss them. there is, however, one leading principle about eclipses which is so simple that we must refer to it. the eclipses occurring this year have no very obvious relation to the eclipses that occurred last year, or to those that will occur next year. yet, when we take a more extended view of the sequence of these phenomena, a very definite principle becomes manifest. if we observe all the eclipses in a period of eighteen years, or nineteen years, then we can predict, with at least an approximation to the truth, all the future eclipses for many years. it is only necessary to recollect that in , - / days after one eclipse a nearly similar eclipse follows. for instance, a beautiful eclipse of the moon occurred on the th of december, . if we count back , days from that date, or, that is, eighteen years and eleven days, we come to november th, , and a similar eclipse of the moon took place then. again, there were four eclipses in the year . if we add , - / days to the date of each eclipse, it will give the dates of all the four eclipses in the year . it was this rule which enabled the ancient astronomers to predict the recurrence of eclipses, at a time when the motions of the moon were not understood nearly so well as they now are. during a long voyage, and perhaps in critical circumstances, the moon will often render invaluable information to the sailor. to navigate a ship, suppose from liverpool to china, the captain must frequently determine the precise position which his ship then occupies. if he could not do this, he would never find his way across the trackless ocean. observations of the sun give him his latitude and tell him his local time, but the captain further requires to know the greenwich time before he can place his finger at a point of the chart and say, "my ship is here." to ascertain the greenwich time the ship carries a chronometer which has been carefully rated before starting, and, as a precaution, two or three chronometers are usually provided to guard against the risk of error. an unknown error of a minute in the chronometer might perhaps lead the vessel fifteen miles from its proper course. [illustration: plate vi. chart of the moon's surface.] [illustration: fig. .--key to chart of the moon (plate vi.).] it is important to have the means of testing the chronometers during the progress of the voyage; and it would be a great convenience if every captain, when he wished, could actually consult some infallible standard of greenwich time. we want, in fact, a greenwich clock which may be visible over the whole globe. there is such a clock; and, like any other clock, it has a face on which certain marks are made, and a hand which travels round that face. the great clock at westminster shrinks into insignificance when compared with the mighty clock which the captain uses for setting his chronometer. the face of this stupendous dial is the face of the heavens. the numbers engraved on the face of a clock are replaced by the twinkling stars; while the hand which moves over the dial is the beautiful moon herself. when the captain desires to test his chronometer, he measures the distance of the moon from a neighbouring star. for example, he may see that the moon is three degrees from the star regulus. in the nautical almanac he finds the greenwich time at which the moon was three degrees from regulus. comparing this with the indications of the chronometer, he finds the required correction. there is one widely-credited myth about the moon which must be regarded as devoid of foundation. the idea that our satellite and the weather bear some relation has no doubt been entertained by high authority, and appears to be an article in the belief of many an excellent mariner. careful comparison between the state of the weather and the phases of the moon has, however, quite discredited the notion that any connection of the kind does really exist. we often notice large blank spaces on maps of africa and of australia which indicate our ignorance of parts of the interior of those great continents. we can find no such blank spaces in the map of the moon. astronomers know the surface of the moon better than geographers know the interior of africa. every spot on the face of the moon which is as large as an english parish has been mapped, and all the more important objects have been named. a general map of the moon is shown in plate vi. it has been based upon drawings made with small telescopes, and it gives an entire view of that side of our satellite which is presented towards us. the moon is shown as it appears in an astronomical telescope, which inverts everything, so that the south is at the top and the north at the bottom (to show objects upright a telescope requires an additional pair of lenses in the eye-piece, and as this diminishes the amount of light reaching the eye they are dispensed with in astronomical telescopes). we can see on the map some of the characteristic features of lunar scenery. those dark regions so conspicuous in the ordinary full moon are easily recognised on the map. they were thought to be seas by astronomers before the days of telescopes, and indeed the name "mare" is still retained, though it is obvious that they contain no water at present. the map also shows certain ridges or elevated portions, and when we apply measurement to these objects we learn that they must be mighty mountain ranges. but the most striking features on the moon are those ring-like objects which are scattered over the surface in profusion. these are known as the lunar craters. to facilitate reference to the chief points of interest we have arranged an index map (fig. ) which will give a clue to the names of the several objects depicted upon the plate. the so-called seas are represented by capital letters; so that a is the mare crisium, and h the oceanus procellarum. the ranges of mountains are indicated by small letters; thus a on the index is the site of the so-called caucasus mountains, and similarly the apennines are denoted by _c_. the numerous craters are distinguished by numbers; for example, the feature on the map corresponding to on the index is the crater designated ptolemy. a. mare crisium. b. mare foecunditatis. c. mare tranquillitatis. d. mare serenitatis. e. mare imbrium. f. sinus iridum. g. mare vaporum. h. oceanus procellarum. i. mare humorum. j. mare nubium. k. mare nectaris. _a._ caucasus. _b._ alps. _c._ apennines. _d._ carpathians. _f._ cordilleras & d'alembert mountains. _g._ rook mountains. _h._ doerfel mountains. _i._ leibnitz mountains. . posidonius. . linné. . aristotle. . great valley of the alps. . aristillus. . autolycus. . archimedes. . plato. . eratosthenes. . copernicus. . kepler. . aristarchus. . grimaldi. . gassendi. . schickard. . wargentin. . clavius. . tycho. . alphonsus. . ptolemy. . catharina. . cyrillus. . theophilus. . petavius. . hyginus. . triesnecker. in every geographical atlas there is a map showing the two hemispheres of the earth, the eastern and the western. in the case of the moon we can only give a map of one hemisphere, for the simple reason that the moon always turns the same side towards us, and accordingly we never get a view of the other side. this is caused by the interesting circumstance that the moon takes exactly the same time to turn once round its own axis as it takes to go once round the earth. the rotation is, however, performed with uniform speed, while the moon does not move in its orbit with a perfectly uniform velocity (_see_ chapter iv.). the consequence is that we now get a slight glimpse round the east limb, and now a similar glimpse round the west limb, as if the moon were shaking its head very gently at us. but it is only an insignificant margin of the far side of the moon which this _libration_ permits us to examine. lunar objects are well suited for observation when the sunlight falls upon them in such a manner as to exhibit strongly contrasted lights and shadows. it is impossible to observe the moon satisfactorily when it is full, for then no conspicuous shadows are cast. the most opportune moment for seeing any particular lunar object is when it lies just at the illuminated side of the boundary between light and shade, for then the features are brought out with exquisite distinctness. plate vii.[ ] gives an illustration of lunar scenery, the object represented being known to astronomers by the name of triesnecker. the district included is only a very small fraction of the entire surface of the moon, yet the actual area is very considerable, embracing as it does many hundreds of square miles. we see in it various ranges of lunar mountains, while the central object in the picture is one of those remarkable lunar craters which we meet with so frequently in every lunar landscape. this crater is about twenty miles in diameter, and it has a lofty mountain in the centre, the peak of which is just illuminated by the rising sun in that phase of our satellite which is represented in the picture. a typical view of a lunar crater is shown in plate viii. this is, no doubt, a somewhat imaginary sketch. the point of view from which the artist is supposed to have taken the picture is one quite unattainable by terrestrial astronomers, yet there can be little doubt that it is a fair representation of objects on the moon. we should, however, recollect the scale on which it is drawn. the vast crater must be many miles across, and the mountain at its centre must be thousands of feet high. the telescope will, even at its best, only show the moon as well as we could see it with the unaided eye if it were miles away instead of being , . we must not, therefore, expect to see any details on the moon even with the finest telescopes, unless they were coarse enough to be visible at a distance of miles. england from such a point of view would only show london as a coloured spot, in contrast with the general surface of the country. we return, however, from a somewhat fancy sketch to a more prosaic examination of what the telescope does actually reveal. plate ix. represents the large crater plato, so well known to everyone who uses a telescope. the floor of this remarkable object is nearly flat, and the central mountain, so often seen in other craters, is entirely wanting. we describe it more fully in the general list of lunar objects. the mountain peaks on the moon throw long, well-defined shadows, characterised by a sharpness which we do not find in the shadows of terrestrial objects. the difference between the two cases arises from the absence of air from the moon. our atmosphere diffuses a certain amount of light, which mitigates the blackness of terrestrial shadows and tends to soften their outline. no such influences are at work on the moon, and the sharpness of the shadows is taken advantage of in our attempts to measure the heights of the lunar mountains. it is often easy to compute the altitude of a church steeple, a lofty chimney, or any similar object, from the length of its shadow. the simplest and the most accurate process is to measure at noon the number of feet from the base of the object to the end of the shadow. the elevation of the sun at noon on the day in question can be obtained from the almanac, and then the height of the object follows by a simple calculation. indeed, if the observations can be made either on the th of april or the th of september, at or near the latitude of london, then calculations would be unnecessary. the noonday length of the shadow on either of the dates named is equal to the altitude of the object. in summer the length of the noontide shadow is less than the altitude; in winter the length of the shadow exceeds the altitude. at sunrise or sunset the shadows are, of course, much longer than at noon, and it is shadows of this kind that we observe on the moon. the necessary measurements are made by that indispensable adjunct to the equatorial telescope known as the _micrometer_. this word denotes an instrument for measuring _small_ distances. in one sense the term is not a happy one. the objects to which the astronomer applies the micrometer are usually anything but small. they are generally of the most transcendent dimensions, far exceeding the moon or the sun, or even our whole system. still, the name is not altogether inappropriate, for, vast though the objects may be, they generally seem minute, even in the telescope, on account of their great distance. we require for such measurements an instrument capable of the greatest nicety. here, again, we invoke the aid of the spider, to whose assistance in another department we have already referred. in the filar micrometer two spider lines are parallel, and one intersects them at right angles. one or both of the parallel lines can be moved by means of screws, the threads of which have been shaped by consummate workmanship. the distance through which the line has been moved is accurately indicated by noting the number of revolutions and parts of a revolution of the screw. suppose the two lines be first brought into coincidence, and then separated until the apparent length of the shadow of the mountain on the moon is equal to the distance between the lines: we then know the number of revolutions of the micrometer screw which is equivalent to the length of the shadow. the number of miles on the moon which correspond to one revolution of the screw has been previously ascertained by other observations, and hence the length of the shadow can be determined. the elevation of the sun, as it would have appeared to an observer at this point of the moon, at the moment when the measures were being made, is also obtainable, and hence the actual elevation of the mountain can be calculated. by measurements of this kind the altitudes of other lunar objects, such, for example, as the height of the rampart surrounding a circular-walled plane, can be determined. the beauty and interest of the moon as a telescopic object induces us to give to the student a somewhat detailed account of the more remarkable features which it presents. most of the objects we are to describe can be effectively exhibited with very moderate telescopic power. it is, however, to be remembered that all of them cannot be well seen at one time. the region most distinctly shown is the boundary between light and darkness. the student will, therefore, select for observation such objects as may happen to lie near that boundary at the time when he is observing. . _posidonius._--the diameter of this large crater is nearly miles. although its surrounding wall is comparatively slender, it is so distinctly marked as to make the object very conspicuous. as so frequently happens in lunar volcanoes, the bottom of the crater is below the level of the surrounding plain, in the present instance to the extent of nearly , feet. . _linné._--this small crater lies in the mare serenitatis. about sixty years ago it was described as being about - / miles in diameter, and seems to have been sufficiently conspicuous. in schmidt, of athens, announced that the crater had disappeared. since then an exceedingly small shallow depression has been visible, but the whole object is now very inconsiderable. this seems to be the most clearly attested case of change in a lunar object. apparently the walls of the crater have tumbled into the interior and partly filled it up, but many astronomers doubt that a change has really taken place, as schröter, a hanoverian observer at the end of the eighteenth century, appears not to have seen any conspicuous crater in the place, though it must be admitted that his observations are rather incomplete. to give some idea of schmidt's amazing industry in lunar researches, it may be mentioned that in six years he made nearly , individual settings of his micrometer in the measurement of lunar altitudes. his great chart of the mountains in the moon is based on no less than , drawings and sketches, if those are counted twice that may have been used for two divisions of the map. . _aristotle._--this great philosopher's name has been attached to a grand crater miles in diameter, the interior of which, although very hilly, shows no decidedly marked central cone. but the lofty wall of the crater, exceeding , feet in height, overshadows the floor so continuously that its features are never seen to advantage. . _the great valley of the alps._--a wonderfully straight valley, with a width ranging from - / to miles, runs right through the lunar alps. it is, according to mädler, at least , feet deep, and over miles in length. a few low ridges which are parallel to the sides of the valley may possibly be the result of landslips. . _aristillus._--under favourable conditions lord rosse's great telescope has shown the exterior of this magnificent crater to be scored with deep gullies radiating from its centre. aristillus is about miles wide and , feet in depth. . _autolycus_ is somewhat smaller than the foregoing, to which it forms a companion in accordance with what mädler thought a well-defined relation amongst lunar craters, by which they frequently occurred in pairs, with the smaller one more usually to the south. towards the edge this arrangement is generally rather apparent than real, and is merely a result of foreshortening. . _archimedes._--this large plain, about miles in diameter, has its vast smooth interior divided by unequally bright streaks into seven distinct zones, running east and west. there is no central mountain or other obvious internal sign of former activity, but its irregular wall rises into abrupt towers, and is marked outside by decided terraces. [illustration: plate b. portion of the moon. (alps, archimedes, apennines.) _messrs. loewy & puiseux_.] . _plato._--we have already referred to this extensive circular plain, which is noticeable with the smallest telescope. the average height of the rampart is about , feet on the eastern side; the western side is somewhat lower, but there is one peak rising to the height of nearly , feet. the plain girdled by this vast rampart is of ample proportions. it is a somewhat irregular circle, about miles in diameter, and containing an area of , square miles. on its floor the shadows of the western wall are shown in plate ix., as are also three of the small craters, of which a large number have been detected by persevering observers. the narrow sharp line leading from the crater to the left is one of those remarkable "clefts" which traverse the moon in so many directions. another may be seen further to the left. above plato are several detached mountains, the loftiest of which is pico, about , feet in height. its long and pointed shadow would at first sight lead one to suppose that it must be very steep; but schmidt, who specially studied the inclinations of the lunar slopes, is of opinion that it cannot be nearly so steep as many of the swiss mountains that are frequently ascended. as many as thirty minute craters have been carefully observed on the floor of plato, and variations have been thought by mr. w.h. pickering to be perceptible. . _eratosthenes._--this profound crater, upwards of miles in diameter, lies at the end of the gigantic range of the apennines. not improbably, eratosthenes once formed the volcanic vent for the stupendous forces that elevated the comparatively craterless peaks of these great mountains. . _copernicus._--of all the lunar craters this is one of the grandest and best known. the region to the west is dotted over with innumerable minute craterlets. it has a central many-peaked mountain about , feet in height. there is good reason to believe that the terracing shown in its interior is mainly due to the repeated alternate rise, partial congelation, and subsequent retreat of a vast sea of lava. at full moon the crater of copernicus is seen to be surrounded by radiating streaks. . _kepler._--although the internal depth of this crater is scarcely less than , feet, it has but a very low surrounding wall, which is remarkable for being covered with the same glistening substance that also forms a system of bright rays not unlike those surrounding the last object. . _aristarchus_ is the most brilliant of the lunar craters, being specially vivid with a low power in a large telescope. so bright is it, indeed, that it has often been seen on the dark side just after new moon, and has thus given rise to marvellous stories of active lunar volcanoes. to the south-east lies another smaller crater, herodotus, north of which is a narrow, deep valley, nowhere more than - / miles broad, which makes a remarkable zigzag. it is one of the largest of the lunar "clefts." . _grimaldi_ calls for notice as the darkest object of its size in the moon. under very exceptional circumstances it has been seen with the naked eye, and as its area has been estimated at nearly , square miles, it gives an idea of how little unaided vision can discern in the moon; it must, however, be added that we always see grimaldi considerably foreshortened. . the great crater _gassendi_ has been very frequently mapped on account of its elaborate system of "clefts." at its northern end it communicates with a smaller but much deeper crater, that is often filled with black shadow after the whole floor of gassendi has been illuminated. . _schickard_ is one of the largest walled plains on the moon, about miles in breadth. within its vast expanse mädler detected minor craters. with regard to this object chacornac pointed out that, owing to the curvature of the surface of the moon, a spectator at the centre of the floor "would think himself in a boundless desert," because the surrounding wall, although in one place nearly , feet high, would lie entirely beneath his horizon. . close to the foregoing is _wargentin_. there can be little doubt that this is really a huge crater almost filled with congealed lava, as there is scarcely any fall towards the interior. . _clavius._--near the th parallel of lunar south latitude lies this enormous enclosure, the area of which is not less than , square miles. both in its interior and on its walls are many peaks and secondary craters. the telescopic view of a sunrise upon the surface of clavius is truly said by mädler to be indescribably magnificent. one of the peaks rises to a height of , feet above the bottom of one of the included craters. mädler even expressed the opinion that in this wild neighbourhood there are craters so profound that no ray of sunlight ever penetrated their lowest depths, while, as if in compensation, there are peaks whose summits enjoy a mean day almost twice as long as their night. . if the full moon be viewed through an opera-glass or any small hand-telescope, one crater is immediately seen to be conspicuous beyond all others, by reason of the brilliant rays or streaks that radiate from it. this is the majestic _tycho_, , feet in depth and miles in diameter (plate x.). a peak , feet in height rises in the centre of its floor, while a series of terraces diversity its interior slopes; but it is the mysterious bright rays that chiefly surprise us. when the sun rises on tycho, these streaks are utterly invisible; indeed, the whole object is then so obscure that it requires a practised eye to recognise tycho amidst its mountainous surroundings. but as soon as the sun has attained a height of about ° above its horizon, the rays emerge from their obscurity and gradually increase in brightness until the moon becomes full, when they are the most conspicuous objects on her surface. they vary in length, from a few hundred miles to two or, in one instance, nearly three thousand miles. they extend indifferently across vast plains, into the deepest craters, or over the loftiest elevations. we know of nothing on our earth to which they can be compared. as these rays are only seen about the time of full moon, their visibility obviously depends on the light falling more or less closely in the line of sight, quite regardless of the inclination of the surfaces, mountains or valleys, on which they appear. each small portion of the surface of the streak must therefore be of a form which is symmetrical to the spectator from whatever point it is seen. the sphere alone appears to fulfil this condition, and professor copeland therefore suggests that the material constituting the surface of the streak must be made up of a large number of more or less completely spherical globules. the streaks must represent parts of the lunar surface either pitted with minute cavities of spherical figure, or strewn over with minute transparent spheres.[ ] near the centre of the moon's disc is a fine range of ring plains fully open to our view under all illuminations. of these, two may be mentioned--_alphonsus_ ( ), the floor of which is strangely characterised by two bright and several dark markings which cannot be explained by irregularities in the surface.--_ptolemy_ ( ). besides several small enclosed craters, its floor is crossed by numerous low ridges, visible when the sun is rising or setting. , , .--when the moon is five or six days old this beautiful group of three craters will be favourably placed for observation. they are named _catharina_, _cyrillus_, and _theophilus_. catharina, the most southerly of the group, is more than , feet deep, and connected with cyrillus by a wide valley; but between cyrillus and theophilus there is no such connection. indeed, cyrillus looks as if its huge surrounding ramparts, as high as mont blanc, had been completely finished before the volcanic forces commenced the formation of theophilus, the rampart of which encroaches considerably on its older neighbour. theophilus stands as a well-defined circular crater about miles in diameter, with an internal depth of , to , feet, and a beautiful central group of mountains, one-third of that height, on its floor. although theophilus is the deepest crater we can see in the moon, it has suffered little or no deformation from secondary eruptions, while the floor and wall of catharina show complete sequences of lesser craters of various sizes that have broken in upon and partly destroyed each other. in the spring of the year, when the moon is somewhat before the first quarter, this instructive group of extinct volcanoes can be seen to great advantage at a convenient hour in the evening. [illustration: plate vii. triesnecker. (after nasmyth.)] . _petavius_ is remarkable not only for its great size, but also for the rare feature of having a double rampart. it is a beautiful object soon after new moon, or just after full moon, but disappears absolutely when the sun is more than ° above its horizon. the crater floor is remarkably convex, culminating in a central group of hills intersected by a deep cleft. . _hyginus_ is a small crater near the centre of the moon's disc. one of the largest of the lunar chasms passes right through it, making an abrupt turn as it does so. . _triesnecker._--this fine crater has been already described, but is again alluded to in order to draw attention to the elaborate system of chasms so conspicuously shown in plate vii. that these chasms are depressions is abundantly evident by the shadows inside. very often their margins are appreciably raised. they seem to be fractures in the moon's surface. of the various mountains that are occasionally seen as projections on the actual edge of the moon, those called after leibnitz (_i_) seem to be the highest. schmidt found the highest peak to be upwards of , feet above a neighbouring valley. in comparing these altitudes with those of mountains on our earth, we must for the latter add the depth of the sea to the height of the land. reckoned in this way, our highest mountains are still higher than any we know of in the moon. we must now discuss the important question as to the origin of these remarkable features on the surface of the moon. we shall admit at the outset that our evidence on this subject is only indirect. to establish by unimpeachable evidence the volcanic origin of the remarkable lunar craters, it would seem almost necessary that volcanic outbursts should have been witnessed on the moon, and that such outbursts should have been seen to result in the formation of the well-known ring, with or without the mountain rising from the centre. to say that nothing of the kind has ever been witnessed would be rather too emphatic a statement. on certain occasions careful observers have reported the occurrence of minute local changes on the moon. as we have already remarked, a crater named linné, of dimensions respectable, no doubt, to a lunar inhabitant, but forming a very inconsiderable telescopic object, was thought to have undergone some change. on another occasion a minute crater was thought to have arisen near the well-known object named hyginus. the mere enumeration of such instances gives real emphasis to the statement that there is at the present time no appreciable source of disturbance of the moon's surface. even were these trifling cases of suspected change really established--and this is perhaps rather farther than many astronomers would be willing to go--they are still insignificant when compared with the mighty phenomena that gave rise to the host of great craters which cover so large a portion of the moon's surface. we are led inevitably to the conclusion that our satellite must have once possessed much greater activity than it now displays. we can also give a reasonable, or, at all events, a plausible, explanation of the cessation of that activity in recent times. let us glance at two other bodies of our system, the earth and the sun, and compare them with the moon. of the three bodies, the sun is enormously the largest, while the moon is much less than the earth. we have also seen that though the sun must have a very high temperature, there can be no doubt that it is gradually parting with its heat. the surface of the earth, formed as it is of solid rocks and clay, or covered in great part by the vast expanse of ocean, bears but few obvious traces of a high temperature. nevertheless, it is highly probable from ordinary volcanic phenomena that the interior of the earth still possesses a temperature of incandescence. a large body when heated takes a longer time to cool than does a small body raised to the same temperature. a large iron casting will take days to cool; a small casting will become cold in a few hours. whatever may have been the original source of heat in our system--a question which we are not now discussing--it seems demonstrable that the different bodies were all originally heated, and have now for ages been gradually cooling. the sun is so vast that he has not yet had time to cool; the earth, of intermediate bulk, has become cold on the outside, while still retaining vast stores of internal heat; while the moon, the smallest body of all, has lost its heat to such an extent that changes of importance on its surface can no longer be originated by internal fires. we are thus led to refer the origin of the lunar craters to some ancient epoch in the moon's history. we have no moans of knowing the remoteness of that epoch, but it is reasonable to surmise that the antiquity of the lunar volcanoes must be extremely great. at the time when the moon was sufficiently heated to originate those convulsions, of which the mighty craters are the survivals, the earth must also have been much hotter than it is at present. when the moon possessed sufficient heat for its volcanoes to be active, the earth was probably so hot that life was impossible on its surface. this supposition would point to an antiquity for the lunar craters far too great to be estimated by the centuries and the thousands of years which are adequate for such periods as those with which the history of human events is concerned. it seems not unlikely that millions of years may have elapsed since the mighty craters of plato or of copernicus consolidated into their present form. we shall now attempt to account for the formation of the lunar craters. the most probable views on the subject seem to be those which have been set forth by mr. nasmyth, though it must be admitted that his doctrines are by no means free from difficulty. according to his theory we can explain how the rampart around the lunar crater has been formed, and how the great mountain arose which so often adorns the centre of the plain. the view in fig. contains an imaginary sketch of a volcanic vent on the moon in the days when the craters were active. the eruption is here shown in the fulness of its energy, when the internal forces are hurling forth ashes or stones which fall at a considerable distance from the vent. the materials thus accumulated constitute the rampart surrounding the crater. the second picture (fig. ) depicts the crater in a later stage of its history. the prodigious explosive power has now been exhausted, and has perhaps been intermitted for some time. again, the volcano bursts into activity, but this time with only a small part of its original energy. a comparatively feeble eruption now issues from the same vent, deposits materials close around the orifice, and raises a mountain in the centre. finally, when the activity has subsided, and the volcano is silent and still, we find the evidence of the early energy testified to by the rampart which surrounds the ancient crater, and by the mountain which adorns the interior. the flat floor which is found in some of the craters may not improbably have arisen from an outflow of lava which has afterwards consolidated. subsequent outbreaks have also occurred in many cases. one of the principal difficulties attending this method of accounting for the structure of a crater arises from the great size which some of these objects attain. there are ancient volcanoes on the moon forty or fifty miles in diameter; indeed, there is one well-formed ring, with a mountain rising in the centre, the diameter of which is no less than seventy-eight miles (petavius). it seems difficult to conceive how a blowing cone at the centre could convey the materials to such a distance as the thirty-nine miles between the centre of petavius and the rampart. the explanation is, however, facilitated when it is borne in mind that the force of gravitation is much less on the moon than on the earth. [illustration: plate viii. a normal lunar crater.] [illustration: fig. .--volcano in activity.] [illustration: fig. .--subsequent feeble activity.] have we not already seen that our satellite is so much smaller than the earth that eighty moons rolled into one would not weigh as much as the earth? on the earth an ounce weighs an ounce and a pound weighs a pound; but a weight of six ounces here would only weigh one ounce on the moon, and a weight of six pounds here would only weigh one pound on the moon. a labourer who can carry one sack of corn on the earth could, with the same exertion, carry six sacks of corn on the moon. a cricketer who can throw a ball yards on the earth could with precisely the same exertion throw the same ball yards on the moon. hiawatha could shoot ten arrows into the air one after the other before the first reached the ground; on the moon he might have emptied his whole quiver. the volcano, which on the moon drove projectiles to the distance of thirty-nine miles, need only possess the same explosive power as would have been sufficient to drive the missiles six or seven miles on the earth. a modern cannon properly elevated would easily achieve this feat. [illustration: fig. .--formation of the level floor by lava.] it must also be borne in mind that there are innumerable craters on the moon of the same general type but of the most varied dimensions; from a tiny telescopic object two or three miles in diameter, we can point out gradually ascending stages until we reach the mighty petavius just considered. with regard to the smaller craters, there is obviously little or no difficulty in attributing to them a volcanic origin, and as the continuity from the smallest to the largest craters is unbroken, it seems quite reasonable to suppose that even the greatest has arisen in the same way. it should, however, be remarked that some lunar features might be explained by actions from without rather than from within. mr. g.k. gilbert has marshalled the evidence in support of the belief that lunar sculptures arise from the impact of bodies falling on the moon. the mare imbrium, according to this view, has been the seat of a collision to which the surrounding lunar scenery is due. mr. gilbert explains the furrows as hewn out by mighty projectiles moving with such velocities as meteors possess. the lunar landscapes are excessively weird and rugged. they always remind us of sterile deserts, and we cannot fail to notice the absence of grassy plains or green forests such as we are familiar with on our globe. in some respects the moon is not very differently circumstanced from the earth. like it, the moon has the pleasing alternations of day and night, though the day in the moon is as long as twenty-nine of our days, and the night of the moon is as long as twenty-nine of our nights. we are warmed by the rays of the sun; so, too, is the moon; but, whatever may be the temperature during the long day on the moon, it seems certain that the cold of the lunar night would transcend that known in the bleakest regions of our earth. the amount of heat radiated to us by the moon has been investigated by lord rosse, and more recently by professor langley. though every point on the moon's surface is exposed to the sunlight for a fortnight without any interruption, the actual temperature to which the soil is raised cannot be a high one. the moon does not, like the earth, possess a warm blanket, in the shape of an atmosphere, which can keep in and accumulate the heat received. even our largest telescopes can tell nothing directly as to whether life can exist on the moon. the mammoth trees of california might be growing on the lunar mountains, and elephants might be walking about on the plains, but our telescopes could not show them. the smallest object that we can see on the moon must be about as large as a good-sized cathedral, so that organised beings resembling in size any that we are familiar with, if they existed, could not make themselves visible as telescopic objects. we are therefore compelled to resort to indirect evidence as to whether life would be possible on the moon. we may say at once that astronomers believe that life, as we know it, could not exist. among the necessary conditions of life, water is one of the first. take every form of vegetable life, from the lichen which grows on the rock to the giant tree of the forest, and we find the substance of every plant contains water, and could not exist without it. nor is water less necessary to the existence of animal life. deprived of this element, all organic life, the life of man himself, would be inconceivable. unless, therefore, water be present in the moon, we shall be bound to conclude that life, as we know it, is impossible. if anyone stationed on the moon were to look at the earth through a telescope, would he be able to see any water here? most undoubtedly he would. he would see the clouds and he would notice their incessant changes, and the clouds alone would be almost conclusive evidence of the existence of water. an astronomer on the moon would also see our oceans as coloured surfaces, remarkably contrasted with the land, and he would perhaps frequently see an image of the sun, like a brilliant star, reflected from some smooth portion of the sea. in fact, considering that much more than half of our globe is covered with oceans, and that most of the remainder is liable to be obscured by clouds, the lunar astronomer in looking at our earth would often see hardly anything but water in one form or other. very likely he would come to the conclusion that our globe was only fitted to be a residence for amphibious animals. but when we look at the moon with our telescopes we see no direct evidence of water. close inspection shows that the so-called lunar seas are deserts, often marked with small craters and rocks. the telescope reveals no seas and no oceans, no lakes and no rivers. nor is the grandeur of the moon's scenery ever impaired by clouds over her surface. whenever the moon is above our horizon, and terrestrial clouds are out of the way, we can see the features of our satellite's surface with distinctness. there are no clouds in the moon; there are not even the mists or the vapours which invariably arise wherever water is present, and therefore astronomers have been led to the conclusion that the surface of the globe which attends the earth is a sterile and a waterless desert. another essential element of organic life is also absent from the moon. our globe is surrounded with a deep clothing of air resting on the surface, and extending above our heads to the height of about or miles. we need hardly say how necessary air is to life, and therefore we turn with interest to the question as to whether the moon can be surrounded with an atmosphere. let us clearly understand the problem we are about to consider. imagine that a traveller started from the earth on a journey to the moon; as he proceeded, the air would gradually become more and more rarefied, until at length, when he was a few hundred miles above the earth's surface, he would have left the last perceptible traces of the earth's envelope behind him. by the time he had passed completely through the atmosphere he would have advanced only a very small fraction of the whole journey of , miles, and there would still remain a vast void to be traversed before the moon would be reached. if the moon were enveloped in the same way as the earth, then, as the traveller approached the end of his journey, and came within a few hundred miles of the moon's surface, he would meet again with traces of an atmosphere, which would gradually increase in density until he arrived at the moon's surface. the traveller would thus have passed through one stratum of air at the beginning of his journey, and through another at the end, while the main portion of the voyage would have been through space more void than that to be found in the exhausted receiver of an air-pump. such would be the case if the moon were coated with an atmosphere like that surrounding our earth. but what are the facts? the traveller as he drew near the moon would seek in vain for air to breathe at all resembling ours. it is possible that close to the surface there are faint traces of some gaseous material surrounding the moon, but it can only be equal to a very small fractional part of the ample clothing which the earth now enjoys. for all purposes of respiration, as we understand the term, we may say that there is no air on the moon, and an inhabitant of our earth transferred thereto would be as certainly suffocated as he would be in the middle of space. it may, however, be asked how we learn this. is not air transparent, and how, therefore, could our telescopes be expected to show whether the moon really possessed such an envelope? it is by indirect, but thoroughly reliable, methods of observation that we learn the destitute condition of our satellite. there are various arguments to be adduced; but the most conclusive is that obtained on the occurrence of what is called an "occultation." it sometimes happens that the moon comes directly between the earth and a star, and the temporary extinction of the latter is an "occultation." we can observe the moment when the phenomenon takes place, and the suddenness of the disappearance of the star is generally remarked. if the moon were enveloped in a copious atmosphere, the interposition of this gaseous mass by the movement of the moon would produce a gradual evanescence of the star wholly wanting the abruptness which marks the obscuration.[ ] let us consider how we can account for the absence of an atmosphere from the moon. what we call a gas has been found by modern research to be a collection of an immense number of molecules, each of which is in exceedingly rapid motion. this motion is only pursued for a short distance in one direction before a molecule comes into collision with some other molecule, whereby the directions and velocities of the individual molecules are continually changed. there is a certain average speed for each gas which is peculiar to the molecules of that gas at a certain temperature. when several gases are mixed, as oxygen and nitrogen are in our atmosphere, the molecules of each gas continue to move with their own characteristic velocities. so far as we can estimate the temperature at the boundary of the earth's atmosphere, we may assume that the average of the velocities of the oxygen molecules there found is about a quarter of a mile per second. the velocities for nitrogen are much the same, while the average speed of a molecule of hydrogen is about one mile per second, being, in fact, by far the greatest molecular velocity possessed by any gas. [illustration: plate ix. plato. (after nasmyth.)] a stone thrown into the air soon regains the earth. a rifle bullet fired vertically upwards will ascend higher and higher, until at length its motion ceases, it begins to return, and falls to the ground. let us for the moment suppose that we had a rifle of infinite strength and gunpowder of unlimited power. as we increase the charge we find that the bullet will ascend higher and higher, and each time it will take a longer period before it returns to the ground. the descent of the bullet is due to the attraction of the earth. gravitation must necessarily act on the projectile throughout its career, and it gradually lessens the velocity, overcomes the upward motion, and brings the bullet back. it must be remembered that the efficiency of the attraction decreases when the height is increased. consequently when the body has a prodigiously great initial velocity, in consequence of which it ascends to an enormous height, its return is retarded by a twofold cause. in the first place, the distance through which it has to be recalled is greatly increased, and in the second place the efficiency of gravitation in effecting its recall has decreased. the greater the velocity, the feebler must be the capacity of gravitation for bringing back the body. we can conceive the speed to be increased to that point at which the gravitation, constantly declining as the body ascends, is never quite able to neutralise the velocity, and hence we have the remarkable case of a body projected away never to return. it is possible to exhibit this reasoning in a numerical form, and to show that a velocity of six or seven miles a second directed upwards would suffice to convey a body entirely away from the gravitation of the earth. this speed is far beyond the utmost limits of our artillery. it is, indeed, at least a dozen times as swift as a cannon shot; and even if we could produce it, the resistance of the air would present an insuperable difficulty. such reflections, however, do not affect the conclusion that there is for each planet a certain specific velocity appropriate to that body, and depending solely upon its size and mass, with which we should have to discharge a projectile, in order to prevent the attraction of that body from pulling the projectile back again. it is a simple matter of calculation to determine this "critical velocity" for any celestial body. the greater the body the greater in general must be the initial speed which will enable the projectile to forsake for ever the globe from which it has been discharged. as we have already indicated, this speed is about seven miles per second on the earth. it would be three on the planet mercury, three and a half on mars, twenty-two on saturn, and thirty-seven on jupiter; while for a missile to depart from the sun without prospect of return, it must leave the brilliant surface at a speed not less than miles per second. supposing that a quantity of free hydrogen was present in our atmosphere, its molecules would move with an average velocity of about one mile per second. it would occasionally happen by a combination of circumstances that a molecule would attain a speed which exceeded seven miles a second. if this happened on the confines of the atmosphere where it escaped collision with other molecules, the latter object would fly off into space, and would not be recaptured by the earth. by incessant repetitions of this process, in the course of countless ages, all the molecules of hydrogen gas would escape from the earth, and in this manner we may explain the fact that there is no free hydrogen present in the earth's atmosphere.[ ] the velocities which can be attained by the molecules of gases other than hydrogen are far too small to permit of their escape from the attraction of the earth. we therefore find oxygen, nitrogen, water vapour, and carbon dioxide remaining as permanent components of our air. on the other hand, the enormous mass of the sun makes the "critical velocity" at the surface of that body to be so great ( miles per second) that not even the molecules of hydrogen can possibly emulate it. consequently, as we have seen, hydrogen is a most important component of the sun's atmospheric envelope. if we now apply this reasoning to the moon, the critical velocity is found by calculation to be only a mile and a half per second. this seems to be well within the maximum velocities attainable by the molecules of oxygen, nitrogen, and other gases. it therefore follows that none of these gases could remain permanently to form an atmosphere at the surface of so small a body as the moon. this seems to be the reason why there are no present traces of any distinct gaseous surroundings to our satellite. the absence of air and of water from the moon explains the sublime ruggedness of the lunar scenery. we know that on the earth the action of wind and of rain, of frost and of snow, is constantly tending to wear down our mountains and reduce their asperities. no such agents are at work on the moon. volcanoes sculptured the surface into its present condition, and, though they have ceased to operate for ages, the traces of their handiwork seem nearly as fresh to-day as they were when the mighty fires were extinguished. "the cloud-capped towers, the gorgeous palaces, the solemn temples" have but a brief career on earth. it is chiefly the incessant action of water and of air that makes them vanish like the "baseless fabric of a vision." on the moon these causes of disintegration and of decay are all absent, though perhaps the changes of temperature in the transition from lunar day to lunar night would be attended with expansions and contractions that might compensate in some slight degree for the absence of more potent agents of dissolution. it seems probable that a building on the moon would remain for century after century just as it was left by the builders. there need be no glass in the windows, for there is no wind and no rain to keep out. there need not be fireplaces in the rooms, for fuel cannot burn without air. dwellers in a lunar city would find that no dust could rise, no odours be perceived, no sounds be heard. man is a creature adapted for life under circumstances which are very narrowly limited. a few degrees of temperature more or less, a slight variation in the composition of air, the precise suitability of food, make all the difference between health and sickness, between life and death. looking beyond the moon, into the length and breadth of the universe, we find countless celestial globes with every conceivable variety of temperature and of constitution. amid this vast number of worlds with which space is tenanted, are there any inhabited by living beings? to this great question science can make no response: we cannot tell. yet it is impossible to resist a conjecture. we find our earth teeming with life in every part. we find life under the most varied conditions that can be conceived. it is met with under the burning heat of the tropics and in the everlasting frost at the poles. we find life in caves where not a ray of light ever penetrates. nor is it wanting in the depths of the ocean, at the pressure of tons on the square inch. whatever may be the external circumstances, nature generally provides some form of life to which those circumstances are congenial. it is not at all probable that among the million spheres of the universe there is a single one exactly like our earth--like it in the possession of air and of water, like it in size and in composition. it does not seem probable that a man could live for one hour on any body in the universe except the earth, or that an oak-tree could live in any other sphere for a single season. men can dwell on the earth, and oak-trees can thrive therein, because the constitutions of the man and of the oak are specially adapted to the particular circumstances of the earth. could we obtain a closer view of some of the celestial bodies, we should probably find that they, too, teem with life, but with life specially adapted to the environment--life in forms strange and weird; life far stranger to us than columbus found it to be in the new world when he first landed there. life, it may be, stranger than ever dante described or doré sketched. intelligence may also have a home among those spheres no less than on the earth. there are globes greater and globes less--atmospheres greater and atmospheres less. the truest philosophy on this subject is crystallised in the language of tennyson:-- "this truth within thy mind rehearse, that in a boundless universe is boundless better, boundless worse. "think you this mould of hopes and fears could find no statelier than his peers in yonder hundred million spheres?" [illustration: plate x. tycho and its surroundings. (after nasmyth.)] chapter iv. the solar system. exceptional importance of the sun and moon--the course to be pursued--the order of distance--the neighbouring orbs--how are they to be discriminated?--the planets venus and jupiter attract notice by their brilliancy--sirius not a neighbour--the planets saturn and mercury--telescopic planets--the criterion as to whether a body is to be ranked as a neighbour--meaning of the word _planet_--uranus and neptune--comets--the planets are illuminated by the sun--the stars are not--the earth is really a planet--the four inner planets, mercury, venus, the earth, and mars--velocity of the earth--the outer planets, jupiter, saturn, uranus, neptune--light and heat received by the planets from the sun--comparative sizes of the planets--the minor planets--the planets all revolve in the same direction--the solar system--an island group in space. in the two preceding chapters of this work we have endeavoured to describe the heavenly bodies in the order of their relative importance to mankind. could we doubt for a moment as to which of the many orbs in the universe should be the first to receive our attention? we do not now allude to the intrinsic significance of the sun when compared with other bodies or groups of bodies scattered through space. it may be that numerous globes rival the sun in real splendour, in bulk, and in mass. we shall, in fact, show later on in this volume that this is the case; and we shall then be in a position to indicate the true rank of the sun amid the countless hosts of heaven. but whatever may be the importance of the sun, viewed merely as one of the bodies which teem through space, there can be no hesitation in asserting how immeasurably his influence on the earth surpasses that of all other bodies in the universe together. it was therefore natural--indeed inevitable--that our first examination of the orbs of heaven should be directed to that mighty body which is the source of our life itself. nor could there be much hesitation as to the second step which ought to be taken. the intrinsic importance of the moon, when compared with other celestial bodies, may be small; it is, indeed, as we shall afterwards see, almost infinitesimal. but in the economy of our earth the moon has played, and still plays, a part second only in importance to that of the sun himself. the moon is so close to us that her brilliant rays pale to invisibility countless orbs of a size and an intrinsic splendour incomparably greater than her own. the moon also occupies an exceptional position in the history of astronomy; for the law of gravitation, the greatest discovery that science has yet witnessed, was chiefly accomplished by observations of the moon. it was therefore natural that an early chapter in our story of the heavens should be devoted to a body the interest of which approximated so closely to that of the sun himself. but the sun and the moon having been partly described (we shall afterwards have to refer to them again), some hesitation is natural in the choice of the next step. the two great luminaries being abstracted from our view, there remains no other celestial body of such exceptional interest and significance as to make it quite clear what course to pursue; we desire to unfold the story of the heavens in the most natural manner. if we made the attempt to describe the celestial bodies in the order of their actual magnitude, our ignorance must at once pronounce the task to be impossible. we cannot even make a conjecture as to which body in the heavens is to stand first on the list. even if that mightiest body be within reach of our telescopes (in itself a highly improbable supposition), we have not the least idea in what part of the heavens it is to be sought. and even if this were possible--if we were able to arrange all the visible bodies rank by rank in the order of their magnitude and their splendour--still the scheme would be impracticable, for of most of them we know little or nothing. we are therefore compelled to adopt a different method of procedure, and the simplest, as well as the most natural, will be to follow as far as possible the order of distance of the different bodies. we have already spoken of the moon as the nearest neighbour to the earth; we shall next consider some of the other celestial bodies which are comparatively near to us; then, as the subject unfolds, we shall discuss the objects further and further away, until towards the close of the volume we shall be engaged in considering the most distant bodies in the universe which the telescope has yet revealed to us. even when we have decided on this principle, our course is still not free from ambiguity. many of the bodies in the heavens are in motion, so that their relative distances from the earth are in continual change; this is, however, a difficulty which need not detain us. we shall make no attempt to adhere closely to the principle in all details. it will be sufficient if we first describe those great bodies--not a very numerous class--which are, comparatively speaking, in our vicinity, though still at varied distances; and then we shall pass on to the uncounted bodies which are separated from us by distances so vast that the imagination is baffled in the attempt to realise them. let us, then, scan the heavens to discover those orbs which lie in our neighbourhood. the sun has set, the moon has not risen; a cloudless sky discloses a heaven glittering with countless gems of light. some are grouped together into well-marked constellations; others seem scattered promiscuously, with every degree of lustre, from the very brightest down to the faintest point that the eye can just glimpse. amid all this host of objects, how are we to identify those which lie nearest to the earth? look to the west: and there, over the spot where the departing sunbeams still linger, we often see the lovely evening star shining forth. this is the planet venus--a beauteous orb, twin-sister to the earth. the brilliancy of this planet, its rapid changes both in position and in lustre, would suggest at once that it was much nearer to the earth than other star-like objects. this presumption has been amply confirmed by careful measurements, and therefore venus is to be included in the list of the orbs which constitute our neighbours. another conspicuous planet--almost rivalling venus in lustre, and vastly surpassing venus in the magnificence of its proportions and its retinue--has borne from antiquity the majestic name of jupiter. no doubt jupiter is much more distant from us than venus. indeed, he is always at least twice as far, and sometimes as much as ten times. but still we must include jupiter among our neighbours. compared with the host of stars which glitter on the heavens, jupiter must be regarded as quite contiguous. the distance of the great planet requires, it is true, hundreds of millions of miles for its expression; yet, vast as is that distance, it would have to be multiplied by tens of thousands, or hundreds of thousands, before it would be long enough to span the abyss which intervenes between the earth and the nearest of the stars. venus and jupiter have invited our attention by their exceptional brilliancy. we should, however, fall into error if we assumed generally that the brightest objects were those nearest to the earth. an observer unacquainted with astronomy might not improbably point to the dog star--or sirius, as astronomers more generally know it--as an object whose exceptional lustre showed it to be one of our neighbours. this, however, would be a mistake. we shall afterwards have occasion to refer more particularly to this gem of our southern skies, and then it will appear that sirius is a mighty globe far transcending our own sun in size as well as in splendour, but plunged into the depths of space to such an appalling distance that his enfeebled rays, when they reach the earth, give us the impression, not of a mighty sun, but only of a brilliant star. the principle of selection, by which the earth's neighbours can be discriminated, will be explained presently; in the meantime, it will be sufficient to observe that our list is to be augmented first by the addition of the unique object known as saturn, though its brightness is far surpassed by that of sirius, as well as by a few other stars. then we add mars, an object which occasionally approaches so close to the earth that it shines with a fiery radiance which would hardly prepare us for the truth that this planet is intrinsically one of the smallest of the celestial bodies. besides the objects we have mentioned, the ancient astronomers had detected a fifth, known as mercury--a planet which is usually invisible amid the light surrounding the sun. mercury, however, occasionally wanders far enough from our luminary to be seen before sunrise or after sunset. these five--mercury, venus, mars, jupiter, and saturn--comprised the planets known from remote antiquity. we can, however, now extend the list somewhat further by adding to it the telescopic objects which have in modern times been found to be among our neighbours. here we must no longer postpone the introduction of the criterion by which we can detect whether a body is near the earth or not. the brighter planets can be recognised by the steady radiance of their light as contrasted with the incessant twinkling of the stars. a little attention devoted to any of the bodies we have named will, however, point out a more definite contrast between the planets and the stars. observe, for instance, jupiter, on any clear night when the heavens can be well seen, and note his position with regard to the constellations in his neighbourhood--how he is to the right of this star, or to the left of that; directly between this pair, or directly pointed to by that. we then mark down the place of jupiter on a celestial map, or we make a sketch of the stars in the neighbourhood showing the position of the planet. after a month or two, when the observations are repeated, the place of jupiter is to be compared again with those stars by which it was defined. it will be found that, while the stars have preserved their relative positions, the place of jupiter has changed. hence this body is with propriety called a _planet_, or a wanderer, because it is incessantly moving from one part of the starry heavens to another. by similar comparisons it can be shown that the other bodies we have mentioned--venus and mercury, saturn and mars--are also wanderers, and belong to that group of heavenly bodies known as planets. here, then, we have the simple criterion by which the earth's neighbours are readily to be discriminated from the stars. each of the bodies near the earth is a planet, or a wanderer, and the mere fact that a body is a wanderer is alone sufficient to prove it to be one of the class which we are now studying. provided with this test, we can at once make an addition to our list of neighbours. amid the myriad orbs which the telescope reveals, we occasionally find one which is a wanderer. two other mighty planets, known as uranus and neptune, must thus be added to the five already mentioned, making in all a group of seven great planets. a vastly greater number may also be reckoned when we admit to our view bodies which not only seem to be minute telescopic objects, but really are small globes when compared with the mighty bulk of our earth. these lesser planets, to the number of more than four hundred, are also among the earth's neighbours. we should remark that another class of heavenly bodies widely differing from the planets must also be included in our system. these are the comets, and, indeed, it may happen that one of these erratic bodies will sometimes draw nearer to the earth than even the closest approach ever made by a planet. these mysterious visitors will necessarily engage a good deal of our attention later on. for the present we confine our attention to those more substantial globes, whether large or small, which are always termed planets. imagine for a moment that some opaque covering could be clasped around our sun so that all his beams were extinguished. that our earth would be plunged into the darkness of midnight is of course an obvious consequence. a moment's consideration will show that the moon, shining as it does by the reflected rays of the sun, would become totally invisible. but would this extinction of the sunlight have any other effect? would it influence the countless brilliant points that stud the heavens at midnight? such an obscuration of the sun would indeed produce a remarkable effect on the sky at night, which a little attention would disclose. the stars, no doubt, would not exhibit the slightest change in brilliancy. each star shines by its own light and is not indebted to the sun. the constellations would thus twinkle on as before, but a wonderful change would come over the planets. were the sun to be obscured, the planets would also disappear from view. the midnight sky would thus experience the effacement of the planets one by one, while the stars would remain unaltered. it may seem difficult to realise how the brilliancy of venus or the lustre of jupiter have their origin solely in the beams which fall upon these bodies from the distant sun. the evidence is, however, conclusive on the question; and it will be placed before the reader more fully when we come to discuss the several planets in detail. suppose that we are looking at jupiter high in mid-heavens on a winter's night, it might be contended that, as the earth lies between jupiter and the sun, it must be impossible for the rays of the sun to fall upon the planet. this is, perhaps, not an unnatural view for an inhabitant of this earth to adopt until he has become acquainted with the relative sizes of the various bodies concerned, and with the distances by which those bodies are separated. but the question would appear in a widely different form to an inhabitant of the planet jupiter. if such a being were asked whether he suffered much inconvenience by the intrusion of the earth between himself and the sun, his answer would be something of this kind:--"no doubt such an event as the passage of the earth between me and the sun is possible, and has occurred on rare occasions separated by long intervals; but so far from the transit being the cause of any inconvenience, the whole earth, of which you think so much, is really so minute, that when it did come in front of the sun it was merely seen as a small telescopic point, and the amount of sunlight which it intercepted was quite inappreciable." the fact that the planets shine by the sun's light points at once to the similarity between them and our earth. we are thus led to regard our sun as a central fervid globe associated with a number of much smaller bodies, each of which, being dark itself, is indebted to the sun both for light and for heat. that was, indeed, a grand step in astronomy which demonstrated the nature of the solar system. the discovery that our earth must be a globe isolated in space was in itself a mighty exertion of human intellect; but when it came to be recognised that this globe was but one of a whole group of similar objects, some smaller, no doubt, but others very much larger, and when it was further ascertained that these bodies were subordinated to the supreme control of the sun, we have a chain of discoveries that wrought a fundamental transformation in human knowledge. we thus see that the sun presides over a numerous family. the members of that family are dependent upon the sun, and their dimensions are suitably proportioned to their subordinate position. even jupiter, the largest member of that family, does not contain one-thousandth part of the material which forms the vast bulk of the sun. yet the bulk of jupiter alone would exceed that of the rest of the planets were they all rolled together. around the central luminary in fig. we have drawn four circles in dotted lines which sufficiently illustrate the orbits in which the different bodies move. the innermost of these four paths represents the orbit of the planet mercury. the planet moves around the sun in this path, and regains the place from which it started in eighty-eight days. the next orbit, proceeding outwards from the sun, is that of the planet venus, which we have already referred to as the well-known evening star. venus completes the circuit of its path in days. one step further from the sun and we come to the orbit of another planet. this body is almost the same size as venus, and is therefore much larger than mercury. the planet now under consideration accomplishes each revolution in days. this period sounds familiar to our ears. it is the length of the year; and the planet is the earth on which we stand. there is an impressive way in which to realise the length of the road along which the earth has to travel in each annual journey. the circumference of a circle is about three and one-seventh times the diameter of the same figure; so that taking the distance from the earth to the centre of the sun as , , miles, the diameter of the circle which the earth describes around the sun will be , , miles, and consequently the circumference of the mighty circle in which the earth moves round the sun is fully , , miles. the earth has to travel this distance every year. it is merely a sum in division to find how far we have to move each second in order to accomplish this long journey in a twelvemonth. it will appear that the earth must actually complete eighteen miles every second, as otherwise it would not finish its journey within the allotted time. [illustration: fig. .--the orbits of the four interior planets.] pause for a moment to think what a velocity of eighteen miles a second really implies. can we realise a speed so tremendous? let us compare it with our ordinary types of rapid movement. look at that express train how it crashes under the bridge, how, in another moment, it is lost to view! can any velocity be greater than that? let us try it by figures. the train moves a mile a minute; multiply that velocity by eighteen and it becomes eighteen miles a _minute_, but we must further multiply it by sixty to make it eighteen miles a _second_. the velocity of the express train is not even the thousandth part of the velocity of the earth. let us take another illustration. we stand at the rifle ranges to see a rifle fired at a target , feet away, and we find that a second or two is sufficient to carry the bullet over that distance. the earth moves nearly one hundred times as fast as the rifle bullet. [illustration: fig. .--the earth's movement.] viewed in another way, the stupendous speed of the earth does not seem immoderate. the earth is a mighty globe, so great indeed that even when moving at this speed it takes almost eight minutes to pass over its own diameter. if a steamer required eight minutes to traverse a distance equal to its own length, its pace would be less than a mile an hour. to illustrate this method of considering the subject, we show here a view of the progress made by the earth (fig. ). the distance between the centres of these circles is about six times the diameter; and, accordingly, if they be taken to represent the earth, the time required to pass from one position to the other is about forty-eight minutes. outside the path of the earth, we come to the orbit of the fourth planet, mars, which requires days, or nearly two years, to complete its circuit round the sun. with our arrival at mars we have gained the limit to the inner portion of the solar system. the four planets we have mentioned form a group in themselves, distinguished by their comparative nearness to the sun. they are all bodies of moderate dimensions. venus and the earth are globes of about the same size. mercury and mars are both smaller objects which lie, so far as bulk is concerned, between the earth and the moon. the four planets which come nearest to the sun are vastly surpassed in bulk and weight by the giant bodies of our system--the stately group of jupiter and saturn, uranus and neptune. [illustration: fig. .--the orbits of the four giant planets.] these giant planets enjoy the sun's guidance equally with their weaker brethren. in the diagram on this page (fig. ) parts of the orbits of the great outer planets are represented. the sun, as before, presides at the centre, but the inner planets would on this scale be so close to the sun that it is only possible to represent the orbit of mars. after the orbit of mars comes a considerable interval, not, however, devoid of planetary activity, and then follow the orbits of jupiter and saturn; further still, we have uranus, a great globe on the verge of unassisted vision; and, lastly, the whole system is bounded by the grand orbit of neptune--a planet of which we shall have a marvellous story to narrate. the various circles in fig. show the apparent sizes of the sun as seen from the different planets. taking the circle corresponding to the earth to represent the amount of heat and light which the earth derives from the sun then the other circles indicate the heat and the light enjoyed by the corresponding planets. the next outer planet to the earth is mars, whose share of solar blessings is not so very inferior to that of the earth; but we fail to see how bodies so remote as jupiter or saturn can enjoy climates at all comparable with those of the planets which are more favourably situated. [illustration: fig. .--comparative apparent size of the sun as seen from the various planets.] fig. shows a picture of the whole family of planets surrounding the sun--represented on the same scale, so as to exhibit their comparative sizes. measured by bulk, jupiter is more than , times as great as the earth, so that it would take at least , earths rolled into one to form a globe equal to the globe of jupiter. measured by weight, the disparity between the earth and jupiter, though still enormous, is not quite so great; but this is a matter to be discussed more fully in a later chapter. [illustration: fig. .--comparative sizes of the planets.] even in this preliminary survey of the solar system we must not omit to refer to the planets which attract our attention, not by their bulk, but by their multitude. in the ample zone bounded on the inside by the orbit of mars and on the outside by the orbit of jupiter it was thought at one time that no planet revolved. modern research has shown that this region is tenanted, not by one planet, but by hundreds. the discovery of these planets is a charge which has been undertaken by various diligent astronomers of the present day, while the discussion of their movements affords labour to other men of science. we shall find something to learn from the study of these tiny bodies, and especially from another small planet called eros, which lies nearer to the earth than the limit above indicated. a chapter will be devoted to these objects. but we do not propose to enter deeply into the mere statistics of the planetary system at present. were such our intention, the tables at the end of the volume would show that ample materials are available. astronomers have taken an inventory of each of the planets. they have measured their distances, the shapes of their orbits and the positions of those orbits, their times of revolution, and, in the case of all the larger planets, their sizes and their weights. such results are of interest for many purposes. it is, however, the more general features of the science which at present claim our attention. let us, in conclusion, note one or two important truths with reference to our planetary system. we have seen that all the planets revolve in nearly circular paths around the sun. we have now to add another fact possessing much significance. each of the planets pursues its path in the same direction. it thus happens that one such body may overtake another, but it can never happen that two planets pass by each other as do the trains on adjacent lines of railway. we shall subsequently find that the whole welfare of our system, nay, its continuous existence, is dependent upon this remarkable uniformity taken in conjunction with other features of the system. such is our solar system; a mighty organised group of planets circulating under the control of the sun, and completely isolated from all external interference. no star, no constellation, has any appreciable influence on our solar system. we constitute a little island group, separated from the nearest stars by the most amazing distances. it may be that as the other stars are suns, so they too may have systems of planets circulating around them; but of this we know nothing. of the stars we can only say that they appear to us as points of light, and any planets they may possess must for ever remain invisible to us, even if they were many times larger than jupiter. we need not repine at this limitation to our possible knowledge, for just as we find in the solar system all that is necessary for our daily bodily wants, so shall we find ample occupation for whatever faculties we may possess in endeavouring to understand those mysteries of the heavens which lie within our reach. chapter v. the law of gravitation. gravitation--the falling of a stone to the ground--all bodies fall equally, sixteen feet in a second--is this true at great heights?--fall of a body at a height of a quarter of a million miles--how newton obtained an answer from the moon--his great discovery--statement of the law of gravitation--illustrations of the law--how is it that all the bodies in the universe do not rush together?--the effect of motion--how a circular path can be produced by attraction--general account of the moon's motion--is gravitation a force of great intensity?--two weights of lbs.--two iron globes, yards in diameter, and a mile apart, attract with a force of lb.--characteristics of gravitation--orbits of the planets not strictly circles--the discoveries of kepler--construction of an ellipse--kepler's first law--does a planet move uniformly?--law of the changes of velocity--kepler's second law--the relation between the distances and the periodic times--kepler's third law--kepler's laws and the law of gravitation--movement in a straight line--a body unacted on by disturbing forces would move in a straight line with constant velocity--application to the earth and the planets--the law of gravitation deduced from kepler's laws--universal gravitation. our description of the heavenly bodies must undergo a slight interruption, while we illustrate with appropriate detail an important principle, known as the law of gravitation, which underlies the whole of astronomy. by this law we can explain the movements of the moon around the earth, and of the planets around the sun. it is accordingly incumbent upon us to discuss this subject before we proceed to the more particular account of the separate planets. we shall find, too, that the law of gravitation sheds some much-needed light on the nature of the stars situated at the remotest distances in space. it also enables us to cast a glance through the vistas of time past, and to trace with plausibility, if not with certainty, certain early phases in the history of our system. the sun and the moon, the planets and the comets, the stars and the nebulæ, all alike are subject to this universal law, which is now to engage our attention. what is more familiar than the fact that when a stone is dropped it will fall to the ground? no one at first thinks the matter even worthy of remark. people are often surprised at seeing a piece of iron drawn to a magnet. yet the fall of a stone to the ground is the manifestation of a force quite as interesting as the force of magnetism. it is the earth which draws the stone, just as the magnet draws the iron. in each case the force is one of attraction; but while the magnetic attraction is confined to a few substances, and is of comparatively limited importance, the attraction of gravitation is significant throughout the universe. let us commence with a few very simple experiments upon the force of gravitation. hold in the hand a small piece of lead, and then allow it to drop upon a cushion. the lead requires a certain time to move from the fingers to the cushion, but that time is always the same when the height is the same. take now a larger piece of lead, and hold one piece in each hand at the same height. if both are released at the same moment, they will both reach the cushion simultaneously. it might have been thought that the heavy body would fall more quickly than the light body; but when the experiment is tried, it is seen that this is not the case. repeat the experiment with various other substances. an ordinary marble will be found to fall in the same time as the piece of lead. with a piece of cork we again try the experiment, and again obtain the same result. at first it seems to fail when we compare a feather with the piece of lead; but that is solely on account of the air, which resists the feather more than it resists the lead. if, however, the feather be placed upon the top of a penny, and the penny be horizontal when dropped, it will clear the air out of the way of the feather in its descent, and then the feather will fall as quickly as the penny, as quickly as the marble, or as quickly as the lead. if the observer were in a gallery when trying these experiments, and if the cushion were sixteen feet below his hands, then the time the marble would take to fall through the sixteen feet would be one second. the time occupied by the cork or by the lead would be the same; and even the feather itself would fall through sixteen feet in one second, if it could be screened from the interference of the air. try this experiment where we like, in london, or in any other city, in any island or continent, on board a ship at sea, at the north pole, or the south pole, or the equator, it will always be found that any body, of any size or any material, will fall about sixteen feet in one second of time. lest any erroneous impression should arise, we may just mention that the distance traversed in one second does vary slightly at different parts of the earth, but from causes which need not at this moment detain us. we shall for the present regard sixteen feet as the distance through which any body, free from interference, would fall in one second at any part of the earth's surface. but now let us extend our view above the earth's surface, and enquire how far this law of sixteen feet in a second may find obedience elsewhere. let us, for instance, ascend to the top of a mountain and try the experiment there. it would be found that at the top of the mountain a marble would take a little longer to fall through sixteen feet than the same marble would if let fall at its base. the difference would be very small; but yet it would be measurable, and would suffice to show that the power of the earth to pull the marble to the ground becomes somewhat weakened at a point high above the earth's surface. whatever be the elevation to which we ascend, be it either the top of a high mountain, or the still greater altitudes that have been reached in balloon ascents, we shall never find that the tendency of bodies to fall to the ground ceases, though no doubt the higher we go the more is that tendency weakened. it would be of great interest to find how far this power of the earth to draw bodies towards it can really extend. we cannot attain more than about five or six miles above the earth's surface in a balloon; yet we want to know what would happen if we could ascend miles, or , miles, or still further, into the regions of space. conceive that a traveller were endowed with some means of soaring aloft for miles and thousands of miles, still up and up, until at length he had attained the awful height of nearly a quarter of a million of miles above the ground. glancing down at the surface of that earth, which is at such a stupendous depth beneath, he would be able to see a wonderful bird's-eye view. he would lose, no doubt, the details of towns and villages; the features in such a landscape would be whole continents and whole oceans, in so far as the openings between the clouds would permit the earth's surface to be exposed. at this stupendous elevation he could try one of the most interesting experiments that was ever in the power of a philosopher. he could test whether the earth's attraction was felt at such a height, and he could measure the amount of that attraction. take for the experiment a cork, a marble, or any other object, large or small; hold it between the fingers, and let it go. everyone knows what would happen in such a case down here; but it required sir isaac newton to tell what would happen in such a case up there. newton asserts that the power of the earth to attract bodies extends even to this great height, and that the marble would fall. this is the doctrine that we can now test. we are ready for the experiment. the marble is released, and, lo! our first exclamation is one of wonder. instead of dropping instantly, the little object appears to remain suspended. we are on the point of exclaiming that we must have gone beyond the earth's attraction, and that newton is wrong, when our attention is arrested; the marble is beginning to move, so slowly that at first we have to watch it carefully. but the pace gradually improves, so that the attraction is beyond all doubt, until, gradually acquiring more and more velocity, the marble speeds on its long journey of a quarter of a million of miles to the earth. but surely, it will be said, such an experiment must be entirely impossible; and no doubt it cannot be performed in the way described. the bold idea occurred to newton of making use of the moon itself, which is almost a quarter of a million of miles above the earth, for the purpose of answering the question. never was our satellite put to such noble use before. it is actually at each moment falling in towards the earth. we can calculate how much it is deflected towards the earth in each second, and thus obtain a measure of the earth's attractive power. from such enquiries newton was able to learn that a body released at the distance of , miles above the surface of the earth would still be attracted by the earth, that in virtue of the attraction the body would commence to move off towards the earth--not, indeed, with the velocity with which a body falls in experiments on the surface, but with a very much lesser speed. a body dropped down from the distance of the moon would commence its long journey so slowly that a _minute_, instead of a _second_, would have elapsed before the distance of sixteen feet had been accomplished.[ ] it was by pondering on information thus won from the moon that newton made his immortal discovery. the gravitation of the earth is a force which extends far and wide through space. the more distant the body, the weaker the gravitation becomes; here newton found the means of determining the great problem as to the law according to which the intensity of the gravitation decreased. the information derived from the moon, that a body , miles away requires a minute to fall through a space equal to that through which it would fall in a second down here, was of paramount importance. in the first place, it shows that the attractive power of the earth, by which it draws all bodies earthwards, becomes weaker at a distance. this might, indeed, have been anticipated. it is as reasonable to suppose that as we retreated further and further into the depths of space the power of attraction should diminish, as that the lustre of light should diminish as we recede from it; and it is remarkable that the law according to which the attraction of gravitation decreases with the increase of distance is precisely the same as the law according to which the brilliancy of a light decreases as its distance increases. the law of nature, stated in its simplest form, asserts that the intensity of gravitation varies inversely as the square of the distance. let me endeavour to elucidate this somewhat abstract statement by one or two simple illustrations. suppose a body were raised above the surface of the earth to a height of nearly , miles, so as to be at an altitude equal to the radius of the earth. in other words, a body so situated would be twice as far from the centre of the earth as a body which lay on the surface. the law of gravitation says that the intensity of the attraction is then to be decreased to one-fourth part, so that the pull of the earth on a body , miles high is only one quarter of the pull of the earth on that body so long as it lies on the ground. we may imagine the effect of this pull to be shown in different ways. allow the body to fall, and in the interval of one second it will only drop through four feet, a mere quarter of the distance that gravity would cause near the earth's surface. we may consider the matter in another way by supposing that the attraction of the earth is measured by one of those little weighing machines known as a spring balance. if a weight of four pounds be hung on such a contrivance, at the earth's surface, the index of course shows a weight of four pounds; but conceive this balance, still bearing the weight appended thereto, were to be carried up and up, the _indicated_ strain would become less and less, until by the time the balance reached , miles high, where it was _twice_ as far away from the earth's centre as at first, the indicated strain would be reduced to the _fourth_ part, and the balance would only show one pound. if we could imagine the instrument to be carried still further into the depths of space, the indication of the scale would steadily continue to decline. by the time the apparatus had reached a distance of , miles high, being then _three_ times as far from the earth's centre as at first, the law of gravitation tells us that the attraction must have decreased to one-ninth part. the strain thus shown on the balance would be only the ninth part of four pounds, or less than half a pound. but let the voyage be once again resumed, and let not a halt be made this time until the balance and its four-pound weight have retreated to that orbit which the moon traverses in its monthly course around the earth. the distance thus attained is about sixty times the radius of the earth, and consequently the attraction of gravitation is diminished in the proportion of one to the square of sixty; the spring will then only be strained by the inappreciable fraction of - , part of four pounds. it therefore appears that a weight which on the earth weighed a ton and a half would, if raised , miles, weigh less than a pound. but even at this vast distance we are not to halt; imagine that we retreat still further and further; the strain shown by the balance will ever decrease, but it will still exist, no matter how far we go. astronomy appears to teach us that the attraction of gravitation can extend, with suitably enfeebled intensity, across the most profound gulfs of space. the principle of gravitation is of far wider scope than we have yet indicated. we have spoken merely of the attraction of the earth, and we have stated that this force extends throughout space. but the law of gravitation is not so limited. not only does the earth attract every other body, and every other body attract the earth, but each of these bodies attracts the other; so that in its more complete shape the law of gravitation announces that "every body in the universe attracts every other body with a force which varies inversely as the square of the distance." it is impossible for us to over-estimate the importance of this law. it supplies the clue by which we can unravel the complicated movements of the planets. it has led to marvellous discoveries, in which the law of gravitation has enabled us to anticipate the telescope, and to feel the existence of bodies before those bodies have even been seen. an objection which may be raised at this point must first be dealt with. it seems to be, indeed, a plausible one. if the earth attracts the moon, why does not the moon tumble down on the earth? if the earth is attracted by the sun, why does it not tumble into the sun? if the sun is attracted by other stars, why do they not rush together with a frightful collision? it may not unreasonably be urged that if all these bodies in the heavens are attracting each other, it would seem that they must all rush together in consequence of that attraction, and thus weld the whole material universe into a single mighty mass. we know, as a matter of fact, that these collisions do not often happen, and that there is extremely little likelihood of their taking place. we see that although our earth is said to have been attracted by the sun for countless ages, yet the earth is just as far from the sun as ever it was. is not this in conflict with the doctrine of universal gravitation? in the early days of astronomy such objections would be regarded, and doubtless were regarded, as well-nigh insuperable; even still we occasionally hear them raised, and it is therefore the more incumbent on us to explain how it happens that the solar system has been able to escape from the catastrophe by which it seems to be threatened. there can be no doubt that if the moon and the earth had been initially placed _at rest_, they would have been drawn together by their mutual attraction. so, too, if the system of planets surrounding the sun had been left initially _at rest_ they would have dashed into the sun, and the system would have been annihilated. it is the fact that the planets are _moving_, and that the moon is _moving_, which has enabled these bodies successfully to resist the attraction in so far, at least, as that they are not drawn thereby to total destruction. it is so desirable that the student should understand clearly how a central attraction is compatible with revolution in a nearly circular path, that we give an illustration to show how the moon pursues its monthly orbit under the guidance and the control of the attracting earth. [illustration: fig. .--illustration of the moon's motion.] the imaginary sketch in fig. denotes a section of the earth with a high mountain thereon.[ ] if a cannon were stationed on the top of the mountain at c, and if the cannonball were fired off in the direction c e with a moderate charge of powder, the ball would move down along the first curved path. if it be fired a second time with a heavier charge, the path will be along the second curved line, and the ball would again fall to the ground. but let us try next time with a charge still further increased, and, indeed, with a far stronger cannon than any piece of ordnance ever yet made. the velocity of the projectile must now be assumed to be some miles per second, but we can conceive that the speed shall be so adjusted that the ball shall move along the path c d, always at the same height above the earth, though still curving, as every projectile must curve, from the horizontal line in which it moved at the first moment. arrived at d, the ball will still be at the same height above the surface, and its velocity must be unabated. it will therefore continue in its path and move round another quadrant of the circle without getting nearer to the surface. in this manner the projectile will travel completely round the whole globe, coming back again to c and then taking another start in the same path. if we could abolish the mountain and the cannon at the top, we should have a body revolving for ever around the earth in consequence of the attraction of gravitation. make now a bold stretch of the imagination. conceive a terrific cannon capable of receiving a round bullet not less than , miles in diameter. discharge this enormous bullet with a velocity of about , feet per second, which is two or three times as great as the velocity actually attainable in modern artillery. let this notable bullet be fired horizontally from some station nearly a quarter of a million miles above the surface of the earth. that fearful missile would sweep right round the earth in a nearly circular orbit, and return to where it started in about four weeks. it would then commence another revolution, four weeks more would find it again at the starting point, and this motion would go on for ages. do not suppose that we are entirely romancing. we cannot indeed show the cannon, but we can point to a great projectile. we see it every month; it is the beautiful moon herself. no one asserts that the moon was ever shot from such a cannon; but it must be admitted that she moves as if she had been. in a later chapter we shall enquire into the history of the moon, and show how she came to revolve in this wonderful manner. as with the moon around the earth, so with the earth around the sun. the illustration shows that a circular or nearly circular motion harmonises with the conception of the law of universal gravitation. we are accustomed to regard gravitation as a force of stupendous magnitude. does not gravitation control the moon in its revolution around the earth? is not even the mighty earth itself retained in its path around the sun by the surpassing power of the sun's attraction? no doubt the actual force which keeps the earth in its path, as well as that which retains the moon in our neighbourhood, is of vast intensity, but that is because gravitation is in such cases associated with bodies of enormous mass. no one can deny that all bodies accessible to our observation appear to attract each other in accordance with the law of gravitation; but it must be confessed that, unless one or both of the attracting bodies is of gigantic dimensions, the intensity is almost immeasurably small. let us attempt to illustrate how feeble is the gravitation between masses of easily manageable dimensions. take, for instance, two iron weights, each weighing about lb., and separated by a distance of one foot from centre to centre. there is a certain attraction of gravitation between these weights. the two weights are drawn together, yet they do not move. the attraction between them, though it certainly exists, is an extremely minute force, not at all comparable as to intensity with magnetic attraction. everyone knows that a magnet will draw a piece of iron with considerable vigour, but the intensity of gravitation is very much less on masses of equal amount. the attraction between these two lb. weights is less than the ten-millionth part of a single pound. such a force is utterly infinitesimal in comparison with the friction between the weights and the table on which they stand, and hence there is no response to the attraction by even the slightest movement. yet, if we can conceive each of these weights mounted on wheels absolutely devoid of friction, and running on absolutely perfect horizontal rails, then there is no doubt that the bodies would slowly commence to draw together, and in the course of time would arrive in actual contact. if we desire to conceive gravitation as a force of measurable intensity, we must employ masses immensely more ponderous than those lb. weights. imagine a pair of globes, each composed of , tons of cast iron, and each, if solid, being about yards in diameter. imagine these globes placed at a distance of one mile apart. each globe attracts the other by the force of gravitation. it does not matter that buildings and obstacles of every description intervene; gravitation will pass through such impediments as easily as light passes through glass. no screen can be devised dense enough to intercept the passage of this force. each of these iron globes will therefore under all circumstances attract the other; but, notwithstanding their ample proportions, the intensity of that attraction is still very small, though appreciable. the attraction between these two globes is a force no greater than the pressure exerted by a single pound weight. a child could hold back one of these massive globes from its attraction by the other. suppose that all was clear, and that friction could be so neutralised as to permit the globes to follow the impulse of their mutual attractions. the two globes will then commence to approach, but the masses are so large, while the attraction is so small, that the speed will be accelerated very slowly. a microscope would be necessary to show when the motion has actually commenced. an hour and a half must elapse before the distance is diminished by a single foot; and although the pace improves subsequently, yet three or four days must elapse before the two globes will come together. the most remarkable characteristic of the force of gravitation must be here specially alluded to. the intensity appears to depend only on the quantity of matter in the bodies, and not at all on the nature of the substances of which these bodies are composed. we have described the two globes as made of cast iron, but if either or both were composed of lead or copper, of wood or stone, of air or water, the attractive power would still be the same, provided only that the masses remain unaltered. in this we observe a profound difference between the attraction of gravitation and magnetic attraction. in the latter case the attraction is not perceptible at all in the great majority of substances, and is only considerable in the case of iron. in our account of the solar system we have represented the moon as revolving around the earth in a _nearly_ circular path, and the planets as revolving around the sun in orbits which are also approximately circular. it is now our duty to give a more minute description of these remarkable paths; and, instead of dismissing them as being _nearly_ circles, we must ascertain precisely in what respects they differ therefrom. if a planet revolved around the sun in a truly circular path, of which the sun was always at the centre, it is then obvious that the distance from the sun to the planet, being always equal to the radius of the circle, must be of constant magnitude. now, there can be no doubt that the distance from the sun to each planet is approximately constant; but when accurate observations are made, it becomes clear that the distance is not absolutely so. the variations in distance may amount to many millions of miles, but, even in extreme cases, the variation in the distance of the planet is only a small fraction--usually a very small fraction--of the total amount of that distance. the circumstances vary in the case of each of the planets. the orbit of the earth itself is such that the distance from the earth to the sun departs but little from its mean value. venus makes even a closer approach to perfectly circular movement; while, on the other hand, the path of mars, and much more the path of mercury, show considerable relative fluctuations in the distance from the planet to the sun. it has often been noticed that many of the great discoveries in science have their origin in the nice observation and explanation of minute departures from some law approximately true. we have in this department of astronomy an excellent illustration of this principle. the orbits of the planets are nearly circles, but they are not exactly circles. now, why is this? there must be some natural reason. that reason has been ascertained, and it has led to several of the grandest discoveries that the mind of man has ever achieved in the realms of nature. in the first place, let us see the inferences to be drawn from the fact that the distance of a planet from the sun is not constant. the motion in a circle is one of such beauty and simplicity that we are reluctant to abandon it, unless the necessity for doing so be made clearly apparent. can we not devise any way by which the circular motion might be preserved, and yet be compatible with the fluctuations in the distance from the planet to the sun? this is clearly impossible with the sun at the centre of the circle. but suppose the sun did not occupy the centre, while the planet, as before, revolved around the sun. the distance between the two bodies would then necessarily fluctuate. the more eccentric the position of the sun, the larger would be the proportionate variation in the distance of the planet when at the different parts of its orbit. it might further be supposed that by placing a series of circles around the sun the various planetary orbits could be accounted for. the centre of the circle belonging to venus is to coincide very nearly with the centre of the sun, and the centres of the orbits of all the other planets are to be placed at such suitable distances from the sun as will render a satisfactory explanation of the gradual increase and decrease of the distance between the two bodies. there can be no doubt that the movements of the moon and of the planets would be, to a large extent, explained by such a system of circular orbits; but the spirit of astronomical enquiry is not satisfied with approximate results. again and again the planets are observed, and again and again the observations are compared with the places which the planets would occupy if they moved in accordance with the system here indicated. the centres of the circles are moved hither and thither, their radii are adjusted with greater care; but it is all of no avail. the observations of the planets are minutely examined to see if they can be in error; but of errors there are none at all sufficient to account for the discrepancies. the conclusion is thus inevitable--astronomers are forced to abandon the circular motion, which was thought to possess such unrivalled symmetry and beauty, and are compelled to admit that the orbits of the planets are not circular. then if these orbits be not circles, what are they? such was the great problem which kepler proposed to solve, and which, to his immortal glory, he succeeded in solving and in proving to demonstration. the great discovery of the true shape of the planetary orbits stands out as one of the most conspicuous events in the history of astronomy. it may, in fact, be doubted whether any other discovery in the whole range of science has led to results of such far-reaching interest. we must here adventure for a while into the field of science known as geometry, and study therein the nature of that curve which the discovery of kepler has raised to such unparalleled importance. the subject, no doubt, is a difficult one, and to pursue it with any detail would involve us in many abstruse calculations which would be out of place in this volume; but a general sketch of the subject is indispensable, and we must attempt to render it such justice as may be compatible with our limits. the curve which represents with perfect fidelity the movements of a planet in its revolution around the sun belongs to that well-known group of curves which mathematicians describe as the conic sections. the particular form of conic section which denotes the orbit of a planet is known by the name of the _ellipse_: it is spoken of somewhat less accurately as an oval. the ellipse is a curve which can be readily constructed. there is no simpler method of doing so than that which is familiar to draughtsmen, and which we shall here briefly describe. we represent on the next page (fig. ) two pins passing through a sheet of paper. a loop of twine passes over the two pins in the manner here indicated, and is stretched by the point of a pencil. with a little care the pencil can be guided so as to keep the string stretched, and its point will then describe a curve completely round the pins, returning to the point from which it started. we thus produce that celebrated geometrical figure which is called an ellipse. it will be instructive to draw a number of ellipses, varying in each case the circumstances under which they are formed. if, for instance, the pins remain placed as before, while the length of the loop is increased, so that the pencil is farther away from the pins, then it will be observed that the ellipse has lost some of its elongation, and approaches more closely to a circle. on the other hand, if the length of the cord in the loop be lessened, while the pins remain as before, the ellipse will be found more oval, or, as a mathematician would say, its _eccentricity_ is increased. it is also useful to study the changes which the form of the ellipse undergoes when one of the pins is altered, while the length of the loop remains unchanged. if the two pins be brought nearer together the eccentricity will decrease, and the ellipse will approximate more closely to the shape of a circle. if the pins be separated more widely the eccentricity of the ellipse will be increased. that the circle is an extreme form of ellipse will be evident, if we suppose the two pins to draw in so close together that they become coincident; the point will then simply trace out a circle as the pencil moves round the figure. [illustration: fig. .--drawing an ellipse.] the points marked by the pins obviously possess very remarkable relations with respect to the curve. each one is called a _focus_, and an ellipse can only have one pair of foci. in other words, there is but a single pair of positions possible for the two pins, when an ellipse of specified size, shape, and position is to be constructed. the ellipse differs principally from a circle in the circumstance that it possesses variety of form. we can have large and small ellipses just as we can have large and small circles, but we can also have ellipses of greater or less eccentricity. if the ellipse has not the perfect simplicity of the circle it has, at least, the charm of variety which the circle has not. the oval curve has also the beauty derived from an outline of perfect grace and an association with ennobling conceptions. the ancient geometricians had studied the ellipse: they had noticed its foci; they were acquainted with its geometrical relations; and thus kepler was familiar with the ellipse at the time when he undertook his celebrated researches on the movements of the planets. he had found, as we have already indicated, that the movements of the planets could not be reconciled with circular orbits. what shape of orbit should next be tried? the ellipse was ready to hand, its properties were known, and the comparison could be made; memorable, indeed, was the consequence of this comparison. kepler found that the movement of the planets could be explained, by supposing that the path in which each one revolved was an ellipse. this in itself was a discovery of the most commanding importance. on the one hand it reduced to order the movements of the great globes which circulate round the sun; while on the other, it took that beautiful class of curves which had exercised the geometrical talents of the ancients, and assigned to them the dignity of defining the highways of the universe. but we have as yet only partly enunciated the first discovery of kepler. we have seen that a planet revolves in an ellipse around the sun, and that the sun is, therefore, at some point in the interior of the ellipse--but at what point? interesting, indeed, is the answer to this question. we have pointed out how the foci possess a geometrical significance which no other points enjoy. kepler showed that the sun must be situated in one of the foci of the ellipse in which each planet revolves. we thus enunciate the first law of planetary motion in the following words:-- _each planet revolves around the sun in an elliptic path, having the sun at one of the foci._ we are now enabled to form a clear picture of the orbits of the planets, be they ever so numerous, as they revolve around the sun. in the first place, we observe that the ellipse is a plane curve; that is to say, each planet must, in the course of its long journey, confine its movements to one plane. each planet has thus a certain plane appropriated to it. it is true that all these planes are very nearly coincident, at least in so far as the great planets are concerned; but still they are distinct, and the only feature in which they all agree is that each one of them passes through the sun. all the elliptic orbits of the planets have one focus in common, and that focus lies at the centre of the sun. it is well to illustrate this remarkable law by considering the circumstances of two or three different planets. take first the case of the earth, the path of which, though really an ellipse, is very nearly circular. in fact, if it were drawn accurately to scale on a sheet of paper, the difference between the elliptic orbit and the circle would hardly be detected without careful measurement. in the case of venus the ellipse is still more nearly a circle, and the two foci of the ellipse are very nearly coincident with the centre of the circle. on the other hand, in the case of mercury, we have an ellipse which departs from the circle to a very marked extent, while in the orbits of some of the minor planets the eccentricity is still greater. it is extremely remarkable that every planet, no matter how far from the sun, should be found to move in an ellipse of some shape or other. we shall presently show that necessity compels each planet to pursue an elliptic path, and that no other form of path is possible. started on its elliptic path, the planet pursues its stately course, and after a certain duration, known as the _periodic time_, regains the position from which its departure was taken. again the planet traces out anew the same elliptic path, and thus, revolution after revolution, an identical track is traversed around the sun. let us now attempt to follow the body in its course, and observe the history of its motion during the time requisite for the completion of one of its circuits. the dimensions of a planetary orbit are so stupendous that the planet must run its course very rapidly in order to finish the journey within the allotted time. the earth, as we have already seen, has to move eighteen miles a second to accomplish one of its voyages round the sun in the lapse of - / days. the question then arises as to whether the rate at which a planet moves is uniform or not. does the earth, for instance, actually move at all times with the velocity of eighteen miles a second, or does our planet sometimes move more rapidly and sometimes more slowly, so that the average of eighteen miles a second is still maintained? this is a question of very great importance, and we are able to answer it in the clearest and most emphatic manner. the velocity of a planet is _not_ uniform, and the variations of that velocity can be explained by the adjoining figure (fig. ). [illustration: fig. .--varying velocity of elliptic motion.] let us first of all imagine the planet to be situated at that part of its path most distant from the sun towards the right of the figure. in this position the body's velocity is at its lowest; as the planet begins to approach the sun the speed gradually improves until it attains its mean value. after this point has been passed, and the planet is now rapidly hurrying on towards the sun, the velocity with which it moves becomes gradually greater and greater, until at length, as it dashes round the sun, its speed attains a maximum. after passing the sun, the distance of the planet from the luminary increases, and the velocity of the motion begins to abate; gradually it declines until the mean value is again reached, and then it falls still lower, until the body recedes to its greatest distance from the sun, by which time the velocity has abated to the value from which we supposed it to commence. we thus observe that the nearer the planet is to the sun the quicker it moves. we can, however, give numerical definiteness to the principle according to which the velocity of the planet varies. the adjoining figure (fig. ) shows a planetary orbit, with, of course, the sun at the focus s. we have taken two portions, a b and c d, round the ellipse, and joined their extremities to the focus. kepler's second law may be stated in these words:-- "_every planet moves round the sun with such a velocity at every point, that a straight line drawn from it to the sun passes over equal areas in equal times._" [illustration: fig. .--equal areas in equal times.] for example, if the two shaded portions, a b s and d c s, are equal in area, then the times occupied by the planet in travelling over the portions of the ellipse, a b and c d, are equal. if the one area be greater than the other, then the times required are in the proportion of the areas. this law being admitted, the reason of the increase in the planet's velocity when it approaches the sun is at once apparent. to accomplish a definite area when near the sun, a larger arc is obviously necessary than at other parts of the path. at the opposite extremity, a small arc suffices for a large area, and the velocity is accordingly less. these two laws completely prescribe the motion of a planet round the sun. the first defines the path which the planet pursues; the second describes how the velocity of the body varies at different points along its path. but kepler added to these a third law, which enables us to compare the movements of two different planets revolving round the same sun. before stating this law, it is necessary to explain exactly what is meant by the _mean_ distance of a planet. in its elliptic path the distance from the sun to the planet is constantly changing; but it is nevertheless easy to attach a distinct meaning to that distance which is an average of all the distances. this average is called the mean distance. the simplest way of finding the mean distance is to add the greatest of these quantities to the least, and take half the sum. we have already defined the periodic time of the planet; it is the number of days which the planet requires for the completion of a journey round its path. kepler's third law establishes a relation between the mean distances and the periodic times of the various planets. that relation is stated in the following words:-- "_the squares of the periodic times are proportional to the cubes of the mean distances._" kepler knew that the different planets had different periodic times; he also saw that the greater the mean distance of the planet the greater was its periodic time, and he was determined to find out the connection between the two. it was easily found that it would not be true to say that the periodic time is merely proportional to the mean distance. were this the case, then if one planet had a distance twice as great as another, the periodic time of the former would have been double that of the latter; observation showed, however, that the periodic time of the more distant planet exceeded twice, and was indeed nearly three times, that of the other. by repeated trials, which would have exhausted the patience of one less confident in his own sagacity, and less assured of the accuracy of the observations which he sought to interpret, kepler at length discovered the true law, and expressed it in the form we have stated. to illustrate the nature of this law, we shall take for comparison the earth and the planet venus. if we denote the mean distance of the earth from the sun by unity then the mean distance of venus from the sun is · . omitting decimals beyond the first place, we can represent the periodic time of the earth as · days, and the periodic time of venus as · days. now the law which kepler asserts is that the square of · is to the square of · in the same proportion as unity is to the cube of · . the reader can easily verify the truth of this identity by actual multiplication. it is, however, to be remembered that, as only four figures have been retained in the expressions of the periodic times, so only four figures are to be considered significant in making the calculations. the most striking manner of making the verification will be to regard the time of the revolution of venus as an unknown quantity, and deduce it from the known revolution of the earth and the mean distance of venus. in this way, by assuming kepler's law, we deduce the cube of the periodic time by a simple proportion, and the resulting value of · days can then be obtained. as a matter of fact, in the calculations of astronomy, the distances of the planets are usually ascertained from kepler's law. the periodic time of the planet is an element which can be measured with great accuracy; and once it is known, then the square of the mean distance, and consequently the mean distance itself, is determined. such are the three celebrated laws of planetary motion, which have always been associated with the name of their discoverer. the profound skill by which these laws were elicited from the mass of observations, the intrinsic beauty of the laws themselves, their widespread generality, and the bond of union which they have established between the various members of the solar system, have given them quite an exceptional position in astronomy. as established by kepler, these planetary laws were merely the results of observation. it was found, as a matter of fact, that the planets did move in ellipses, but kepler assigned no reason why they should adopt this curve rather than any other. still less was he able to offer a reason why these bodies should sweep over equal areas in equal times, or why that third law was invariably obeyed. the laws as they came from kepler's hands stood out as three independent truths; thoroughly established, no doubt, but unsupported by any arguments as to why these movements rather than any others should be appropriate for the revolutions of the planets. it was the crowning triumph of the great law of universal gravitation to remove this empirical character from kepler's laws. newton's grand discovery bound together the three isolated laws of kepler into one beautiful doctrine. he showed not only that those laws are true, but he showed why they must be true, and why no other laws could have been true. he proved to demonstration in his immortal work, the "principia," that the explanation of the famous planetary laws was to be sought in the attraction of gravitation. newton set forth that a power of attraction resided in the sun, and as a necessary consequence of that attraction every planet must revolve in an elliptic orbit round the sun, having the sun as one focus; the radius of the planet's orbit must sweep over equal areas in equal times; and in comparing the movements of two planets, it was necessary to have the squares of the periodic times proportional to the cubes of the mean distances. as this is not a mathematical treatise, it will be impossible for us to discuss the proofs which newton has given, and which have commanded the immediate and universal acquiescence of all who have taken the trouble to understand them. we must here confine ourselves only to a very brief and general survey of the subject, which will indicate the character of the reasoning employed, without introducing details of a technical character. let us, in the first place, endeavour to think of a globe freely poised in space, and completely isolated from the influence of every other body in the universe. let us imagine that this globe is set in motion by some impulse which starts it forward on a rapid voyage through the realms of space. when the impulse ceases the globe is in motion, and continues to move onwards. but what will be the path which it pursues? we are so accustomed to see a stone thrown into the air moving in a curved path, that we might naturally think a body projected into free space will also move in a curve. a little consideration will, however, show that the cases are very different. in the realms of free space we find no conception of upwards or downwards; all paths are alike; there is no reason why the body should swerve to the right or to the left; and hence we are led to surmise that in these circumstances a body, once started and freed from all interference, would move in a straight line. it is true that this statement is one which can never be submitted to the test of direct experiment. circumstanced as we are on the surface of the earth, we have no means of isolating a body from external forces. the resistance of the air, as well as friction in various other forms, no less than the gravitation towards the earth itself, interfere with our experiments. a stone thrown along a sheet of ice will be exposed to but little resistance, and in this case we see that the stone will take a straight course along the frozen surface. a stone similarly cast into empty space would pursue a course absolutely rectilinear. this we demonstrate, not by any attempts at an experiment which would necessarily be futile, but by indirect reasoning. the truth of this principle can never for a moment be doubted by one who has duly weighed the arguments which have been produced in its behalf. admitting, then, the rectilinear path of the body, the next question which arises relates to the velocity with which that movement is performed. the stone gliding over the smooth ice on a frozen lake will, as everyone has observed, travel a long distance before it comes to rest. there is but little friction between the ice and the stone, but still even on ice friction is not altogether absent; and as that friction always tends to stop the motion, the stone will at length be brought to rest. in a voyage through the solitudes of space, a body experiences no friction; there is no tendency for the velocity to be reduced, and consequently we believe that the body could journey on for ever with unabated speed. no doubt such a statement seems at variance with our ordinary experience. a sailing ship makes no progress on the sea when the wind dies away. a train will gradually lose its velocity when the steam has been turned off. a humming-top will slowly expend its rotation and come to rest. from such instances it might be plausibly argued that when the force has ceased to act, the motion that the force generated gradually wanes, and ultimately vanishes. but in all these cases it will be found, on reflection, that the decline of the motion is to be attributed to the action of resisting forces. the sailing ship is retarded by the rubbing of the water on its sides; the train is checked by the friction of the wheels, and by the fact that it has to force its way through the air; and the atmospheric resistance is mainly the cause of the stopping of the humming-top, for if the air be withdrawn, by making the experiment in a vacuum, the top will continue to spin for a greatly lengthened period. we are thus led to admit that a body, once projected freely in space and acted upon by no external resistance, will continue to move on for ever in a straight line, and will preserve unabated to the end of time the velocity with which it originally started. this principle is known as the _first law of motion_. let us apply this principle to the important question of the movement of the planets. take, for instance, the case of our earth, and let us discuss the consequences of the first law of motion. we know that the earth is moving each moment with a velocity of about eighteen miles a second, and the first law of motion assures us that if this globe were submitted to no external force, it would for ever pursue a straight track through the universe, nor would it depart from the precise velocity which it possesses at the present moment. but is the earth moving in this manner? obviously not. we have already found that our globe is moving round the sun, and the comprehensive laws of kepler have given to that motion the most perfect distinctness and precision. the consequence is irresistible. the earth cannot be free from external force. some potent influence on our globe must be in ceaseless action. that influence, whatever it may be, constantly deflects the earth from the rectilinear path which it tends to pursue, and constrains it to trace out an ellipse instead of a straight line. the great problem to be solved is now easily stated. there must be some external agent constantly influencing the earth. what is that agent, whence does it proceed, and to what laws is it submitted? nor is the question confined to the earth. mercury and venus, mars, jupiter, and saturn, unmistakably show that, as they are not moving in rectilinear paths, they must be exposed to some force. what is this force which guides the planets in their paths? before the time of newton this question might have been asked in vain. it was the splendid genius of newton which supplied the answer, and thus revolutionised the whole of modern science. the data from which the question is to be answered must be obtained from observation. we have here no problem which can be solved by mere mathematical meditation. mathematics is no doubt a useful, indeed, an indispensable, instrument in the enquiry; but we must not attribute to mathematics a potency which it does not possess. in a case of this kind, all that mathematics can do is to interpret the results obtained by observation. the data from which newton proceeded were the observed phenomena in the movement of the earth and the other planets. those facts had found a succinct expression by the aid of kepler's laws. it was, accordingly, the laws of kepler which newton took as the basis of his labours, and it was for the interpretation of kepler's laws that newton invoked the aid of that celebrated mathematical reasoning which he created. the question is then to be approached in this way: a planet being subject to _some_ external influence, we have to determine what that influence is, from our knowledge that the path of each planet is an ellipse, and that each planet sweeps round the sun over equal areas in equal times. the influence on each planet is what a mathematician would call a force, and a force must have a line of direction. the most simple conception of a force is that of a pull communicated along a rope, and the direction of the rope is in this case the direction of the force. let us imagine that the force exerted on each planet is imparted by an invisible rope. kepler's laws will inform us with regard to the direction of this rope and the intensity of the strain transmitted through it. the mathematical analysis of kepler's laws would be beyond the scope of this volume. we must, therefore, confine ourselves to the results to which they lead, and omit the details of the reasoning. newton first took the law which asserted that the planet moved over equal areas in equal times, and he showed by unimpeachable logic that this at once gave the direction in which the force acted on the planet. he showed that the imaginary rope by which the planet is controlled must be invariably directed towards the sun. in other words, the force exerted on each planet was at all times pointed from the planet towards the sun. it still remained to explain the intensity of the force, and to show how the intensity of that force varied when the planet was at different points of its path. kepler's first law enables this question to be answered. if the planet's path be elliptic, and if the force be always directed towards the sun at one focus of that ellipse, then mathematical analysis obliges us to say that the intensity of the force must vary inversely as the square of the distance from the planet to the sun. the movements of the planets, in conformity with kepler's laws, would thus be accounted for even in their minutest details, if we admit that an attractive power draws the planet towards the sun, and that the intensity of this attraction varies inversely as the square of the distance. can we hesitate to say that such an attraction does exist? we have seen how the earth attracts a falling body; we have seen how the earth's attraction extends to the moon, and explains the revolution of the moon around the earth. we have now learned that the movement of the planets round the sun can also be explained as a consequence of this law of attraction. but the evidence in support of the law of universal gravitation is, in truth, much stronger than any we have yet presented. we shall have occasion to dwell on this matter further on. we shall show not only how the sun attracts the planets, but how the planets attract each other; and we shall find how this mutual attraction of the planets has led to remarkable discoveries, which have elevated the law of gravitation beyond the possibility of doubt. admitting the existence of this law, we can show that the planets must revolve around the sun in elliptic paths with the sun in the common focus. we can show that they must sweep over equal areas in equal times. we can prove that the squares of the periodic times must be proportional to the cubes of their mean distances. still further, we can show how the mysterious movements of comets can be accounted for. by the same great law we can explain the revolutions of the satellites. we can account for the tides, and for other phenomena throughout the solar system. finally, we shall show that when we extend our view beyond the limits of our solar system to the beautiful starry systems scattered through space, we find even there evidence of the great law of universal gravitation. chapter vi. the planet of romance. outline of the subject--is mercury the planet nearest the sun?--transit of an interior planet across the sun--has a transit of vulcan ever been seen?--visibility of planets during a total eclipse of the sun--professor watson's researches in . provided with a general survey of the solar system, and with such an outline of the law of universal gravitation as the last chapter has afforded us, we commence the more detailed examination of the planets and their satellites. we shall begin with the planets nearest to the sun, and then we shall gradually proceed outwards to one planet after another, until we reach the confines of the system. we shall find much to occupy our attention. each planet is itself a globe, and it will be for us to describe as much as is known of it. the satellites by which so many of the planets are accompanied possess many points of interest. the circumstances of their discovery, their sizes, their movements, and their distances must be duly considered. it will also be found that the movements of the planets present much matter for reflection and examination. we shall have occasion to show how the planets mutually disturb each other, and what remarkable consequences have arisen from these influences. we must also occasionally refer to the important problems of celestial measuring and celestial weighing. we must show how the sizes, the weights, and the distances of the various members of our system are to be discovered. the greater part of our task will fortunately lead us over ground which is thoroughly certain, and where the results have been confirmed by frequent observation. it happens, however, that at the very outset of our course we are obliged to deal with observations which are far from certain. the existence of a planet much closer to the sun than those hitherto known has been asserted by competent authority. the question is still unsettled, but the planet cannot at present be found. hence it is that we have called the subject of this chapter, the planet of romance. it had often been thought that mercury, long supposed to be the nearest planet to the sun, was perhaps not really the body entitled to that distinction. mercury revolves round the sun at an average distance of about , , miles. in the interval between it and the sun there might have been one or many other planets. there might have been one revolving at ten million miles, another at fifteen, and so on. but did such planets exist? did even one planet revolve inside the orbit of mercury? there were certain reasons for believing in such a planet. in the movements of mercury indications were perceptible of an influence that it was at one time thought might have been accounted for by the supposition of an interior planet.[ ] but there was necessarily a great difficulty about seeing this object. it must always be close to the sun, and even in the best telescope it is generally impossible to see a star-like point in that position. nor could such a planet be seen after sunset, for under the most favourable conditions it would set almost immediately after the sun, and a like difficulty would make it invisible at sunrise. our ordinary means of observing a planet have therefore completely failed. we are compelled to resort to extraordinary methods if we would seek to settle the great question as to the existence of the intra-mercurial planets. there are at least two lines of observation which might be expected to answer our purpose. an opportunity for the first would arise when it happened that the unknown planet came directly between the earth and the sun. in the diagram (fig. ) we show the sun at the centre; the internal dotted circle denotes the orbit of the unknown planet, which has received the name of vulcan before even its very existence has been at all satisfactorily established. the outer circle denotes the orbit of the earth. as vulcan moves more rapidly than the earth, it will frequently happen that the planet will overtake the earth, so that the three bodies will have the positions represented in the diagram. it would not, however, necessarily follow that vulcan was exactly between the earth and the luminary. the path of the planet may be tilted, so that, as seen from the earth, vulcan would be over or under the sun, according to circumstances. if, however, vulcan really does exist, we might expect that sometimes the three bodies will be directly in line, and this would then give the desired opportunity of making the telescopic discovery of the planet. we should expect on such an occasion to observe the planet as a dark spot, moving slowly across the face of the sun. the two other planets interior to the earth, namely, mercury and venus, are occasionally seen in the act of transit; and there cannot be a doubt that if vulcan exists, its transits across the sun must be more numerous than those of mercury, and far more numerous than those of venus. on the other hand, it may reasonably be anticipated that vulcan is a small globe, and as it will be much more distant from us than mercury at the time of its transit, we could not expect that the transit of the planet of romance would be at all comparable as a spectacle with those of either of the two other bodies we have named. the question arises as to whether telescopic research has ever disclosed anything which can be regarded as a transit of vulcan. on this point it is not possible to speak with any certainty. it has, on more than one occasion, been asserted by observers that a spot has been seen traversing the sun, and from its shape and general appearance they have presumed it to have been an intra-mercurial planet. but a close examination of the circumstances in which such observations have been made has not tended to increase confidence in this presumption. such discoveries have usually been made by persons little familiar with telescopic observations. it is certainly a significant fact that, notwithstanding the diligent scrutiny to which the sun has been subjected during the past century by astronomers who have specially devoted themselves to this branch of research, no telescopic discovery of vulcan on the sun has been announced by any really experienced astronomer. the last announcement of a planet having crossed the sun dates from , and was made by a german amateur, but what he thought to have been a planet was promptly shown to have been a small sun-spot, which had been photographed at greenwich in the course of the daily routine work, and had also been observed at madrid. from an examination of the whole subject, we are inclined to believe that there is not at this moment any reliable telescopic evidence of the transit of an intra-mercurial planet over the face of the central luminary. [illustration: fig. .--the transit of the planet of romance.] but there is still another method by which we might reasonably hope to detect new planets in the vicinity of the sun. this method is, however, but seldom available. it is only possible when the sun is totally eclipsed. when the moon is interposed directly between the earth and the sun, the brightness of day is temporarily exchanged for the gloom of night. if the sky be free from clouds the stars spring forth, and can be seen around the obscured sun. even if a planet were quite close to the luminary it would be visible on such an occasion if its magnitude were comparable with that of mercury. careful preparation is necessary when it is proposed to make a trial of this kind. the danger to be specially avoided is that of confounding the planet with the ordinary stars, which it will probably resemble. the late distinguished american astronomer, professor watson, specially prepared to devote himself to this research during the notable total eclipse in . when the eclipse occurred the light of the sun vanished and the stars burst forth. among them professor watson saw an object which to him seemed to be the long-sought intra-mercurial planet. we should add that this zealous observer saw another object which he at first took to be the star known as zeta in the constellation cancer. when he afterwards found that the recorded place of this object did not agree so well as he expected with the known position of this star, he came to the conclusion that it could not be zeta but must be some other unknown planet. the relative positions of the two objects which he took to be planets agree, however, sufficiently well, considering the difficulties of the observation, with the relative positions of the stars theta and zeta cancri, and it can now hardly be doubted that watson merely saw these two stars. he maintained, however, that he had noticed theta cancri as well as the two planets, but without recording its position. there is, however, a third star, known as cancri, near the same place, and this watson probably mistook for theta. it is necessary to record that vulcan has not been observed, though specially looked for, during the eclipses which have occurred since , and it is accordingly the general belief among astronomers that no planet has yet been detected within the orbit of mercury. chapter vii. mercury. the ancient astronomical discoveries--how mercury was first found--not easily seen--mercury was known from the earliest ages--skill necessary in the discovery--the distinction of mercury from a star--mercury in the east and in the west--the prediction--how to observe mercury--its telescopic appearance--difficulty of observing its appearance--orbit of mercury--velocity of the planet--can there be life on the planet?--changes in its temperature--transit of mercury over the sun--gassendi's observations--rotation of mercury--the weight of mercury. long and glorious is the record of astronomical discovery. the discoveries of modern days have succeeded each other with such rapidity, they have so often dazzled our imaginations with their brilliancy, that we are sometimes apt to think that astronomical discovery is a purely modern product. but no idea could be more fundamentally wrong. while we appreciate to the utmost the achievements of modern times, let us endeavour to do justice to the labours of the astronomers of antiquity. and when we speak of the astronomers of antiquity, let us understand clearly what is meant. the science is now growing so rapidly that each century witnesses a surprising advance; each generation, each decade, each year, has its own rewards for those diligent astronomers by whom the heavens are so carefully scanned. we must, however, project our glance to a remote epoch in time past, if we would view the memorable discovery of mercury. compared with it, the discoveries of newton are to be regarded as very modern achievements; even the announcement of the copernican system of the heavens is itself a recent event in comparison with the detection of this planet now to be discussed. by whom was this great discovery made? let us see if the question can be answered by the examination of astronomical records. at the close of his memorable life copernicus was heard to express his sincere regret that he never enjoyed an opportunity of beholding the planet mercury. he had specially longed to see this body, the movements of which were to such a marked extent illustrative of the theory of the celestial motions which it was his immortal glory to have established, but he had never been successful. mercury is not generally to be seen so easily as are some of the other planets, and it may well have been that the vapours from the immense lagoon at the mouth of the vistula obscured the horizon at frauenburg, where copernicus dwelt, and thus his opportunities of viewing mercury were probably even rarer than they are at other places. the existence of mercury was certainly quite a familiar fact in the time of copernicus, and therefore we must look to some earlier epoch for its discovery. in the scanty astronomical literature of the middle ages we find occasional references to the existence of this object. we can trace observations of mercury through remote centuries to the commencement of our era. records from dates still earlier are not wanting, until at length we come on an observation which has descended to us for more than , years, having been made in the year before the christian era. it is not pretended, however, that this observation records the _discovery_ of the planet. earlier still we find the chief of the astronomers at nineveh alluding to mercury in a report which he made to assurbanipal, the king of assyria. it does not appear in the least degree likely that the discovery was even then a recent one. it may have been that the planet was independently discovered in two or more localities, but all records of such discoveries are totally wanting; and we are ignorant alike of the names of the discoverers, of the nations to which they belonged, and of the epochs at which they lived. although this discovery is of such vast antiquity, although it was made before correct notions were entertained as to the true system of the universe, and, it is needless to add, long before the invention of the telescope, yet it must not be assumed that the detection of mercury was by any means a simple or obvious matter. this will be manifest when we try to conceive the manner in which the discovery must probably have been made. some primæval astronomer, long familiar with the heavens, had learned to recognise the various stars and constellations. experience had impressed upon him the permanence of these objects; he had seen that sirius invariably appeared at the same seasons of the year, and he had noticed how it was placed with regard to orion and the other neighbouring constellations. in the same manner each of the other bright stars was to him a familiar object always to be found in a particular region of the heavens. he saw how the stars rose and set in such a way, that though each star appeared to move, yet the relative positions of the stars were incapable of alteration. no doubt this ancient astronomer was acquainted with venus; he knew the evening star; he knew the morning star; and he may have concluded that venus was a body which oscillated from one side of the sun to the other. we can easily imagine how the discovery of mercury was made in the clear skies over an eastern desert. the sun has set, the brief twilight has almost ceased, when lo, near that part of the horizon where the glow of the setting sun still illuminates the sky, a bright star is seen. the primæval astronomer knows that there is no bright star at this place in the heavens. if the object of his attention be not a star, what, then, can it be? eager to examine this question, the heavens are watched next night, and there again, higher above the horizon, and more brilliant still, is the object seen the night before. each successive night the body gains more and more lustre, until at length it becomes a conspicuous gem. perhaps it will rise still higher and higher; perhaps it will increase till it attains the brilliancy of venus itself. such were the surmises not improbably made by those who first watched this object; but they were not realised. after a few nights of exceptional splendour the lustre of this mysterious orb declines. the planet again draws near the horizon at sunset, until at length it sets so soon after the sun that it has become invisible. is it lost for ever? years may elapse before another opportunity of observing the object after sunset may be available; but then again it will be seen to run through the same series of changes, though, perhaps, under very different circumstances. the greatest height above the horizon and the greatest brightness both vary considerably. long and careful observations must have been made before the primæval astronomer could assure himself that the various appearances might all be attributed to a single body. in the eastern deserts the phenomena of sunrise must have been nearly as familiar as those of sunset, and in the clear skies, at the point where the sunbeams were commencing to dawn above the horizon, a bright star-like point might sometimes be perceived. each successive day this object rose higher and higher above the horizon before the moment of sunrise, and its lustre increased with the distance; then again it would draw in towards the sun, and return for many months to invisibility. such were the data which were presented to the mind of the primitive astronomer. one body was seen after sunset, another body was seen before sunrise. to us it may seem an obvious inference from the observed facts that the two bodies were identical. the inference is a correct one, but it is in no sense an obvious one. long and patient observation established the remarkable law that one of these bodies was never seen until the other had disappeared. hence it was inferred that the phenomena, both at sunrise and at sunset, were due to the same body, which oscillated to and fro about the sun. we can easily imagine that the announcement of the identity of these two objects was one which would have to be carefully tested before it could be accepted. how are the tests to be applied in a case of this kind? there can hardly be a doubt that the most complete and convincing demonstration of scientific truth is found in the fulfilment of prediction. when mercury had been observed for years, a certain regularity in the recurrence of its visibility was noticed. once a periodicity had been fully established, prediction became possible. the time when mercury would be seen after sunset, the time when it would be seen before sunrise, could be foretold with accuracy! when it was found that these predictions were obeyed to the letter--that the planet was always seen when looked for in accordance with the predictions--it was impossible to refuse assent to the hypothesis on which these predictions were based. underlying that hypothesis was the assumption that all the various appearances arose from the oscillations of a single body, and hence the discovery of mercury was established on a basis as firm as the discovery of jupiter or of venus. in the latitudes of the british islands it is generally possible to see mercury some time during the course of the year. it is not practicable to lay down, within reasonable limits, any general rule for finding the dates at which the search should be made; but the student who is determined to see the planet will generally succeed with a little patience. he must first consult an almanac which gives the positions of the body, and select an occasion when mercury is stated to be an evening or a morning star. such an occasion during the spring months is especially suitable, as the elevation of mercury above the horizon is usually greater then than at other seasons; and in the evening twilight, about three-quarters of an hour after sunset, a view of this shy but beautiful object will reward the observer's attention. to those astronomers who are provided with equatorial telescopes such instructions are unnecessary. to enjoy a telescopic view of mercury, we first turn to the nautical almanac, and find the position in which the planet lies. if it happen to be above the horizon, we can at once direct the telescope to the place, and even in broad daylight the planet will very often be seen. the telescopic appearance of mercury is, however, disappointing. though it is much larger than the moon, yet it is sufficiently far off to seem insignificant. there is, however, one feature in a view of this planet which would immediately attract attention. mercury is not usually observed to be a circular object, but more or less crescent-shaped, like a miniature moon. the phases of the planet are also to be accounted for on exactly the same principles as the phases of the moon. mercury is a globe composed, like our earth, of materials possessing in themselves no source of illumination. one hemisphere of the planet must necessarily be turned towards the sun, and this side is accordingly lighted up brilliantly by the solar rays. when we look at mercury we see nothing of the non-illuminated side, and the crescent is due to the foreshortened view which we obtain of the illuminated part. the planet is such a small object that, in the glitter of the naked-eye view, the _shape_ of the luminous body cannot be defined. indeed, even in the much larger crescent of venus, the aid of the telescope has to be invoked before the characteristic form can be observed. beyond, however, the fact that mercury is a crescent, and that it undergoes varying phases in correspondence with the changes in its relative position to the earth and the sun, we cannot see much of the planet. it is too small and too bright to admit of easy delineation of details on its surface. no doubt attempts have been made, and observations have been recorded, as to certain very faint and indistinct markings on the planet, but such statements must be received with great hesitation. [illustration: fig. .--the movement of mercury, showing the variations in phase and in apparent size.] [illustration: fig. .--mercury as a crescent.] the facts which have been thoroughly established with regard to mercury are mainly numerical statements as to the path it describes around the sun. the time taken by the planet to complete one of its revolutions is eighty-eight days nearly. the average distance from the sun is about , , miles, and the mean velocity with which the body moves is over twenty-nine miles a second. we have already alluded to the most characteristic and remarkable feature of the orbit of mercury. that orbit differs from the paths of all the other large planets by its much greater departure from the circular form. in the majority of cases the planetary orbits are so little elliptic that a diagram of the orbit drawn accurately to scale would not be perceived to differ from a circle unless careful measurements were made. in the case of mercury the circumstances are different. the elliptic form of the path would be quite unmistakable by the most casual observer. the distance from the sun to the planet fluctuates between very considerable limits. the lowest value it can attain is about , , miles; the highest value is about , , miles. in accordance with kepler's second law, the velocity of the planet must exhibit corresponding changes. it must sweep rapidly around that part of his path near the sun, and more slowly round the remote parts of his path. the greatest velocity is about thirty-five miles a second, and the least is twenty-three miles a second. for an adequate conception of the movements of mercury we ought not to dissociate the velocity from the true dimensions of the body by which it is performed. no doubt a speed of twenty-nine miles a second is enormous when compared with the velocities with which daily life makes us familiar. the speed of the planet is not less than a hundred times as great as the velocity of the rifle bullet. but when we compare the sizes of the bodies with their velocities, the velocity of mercury seems relatively much less than that of the bullet. a rifle bullet traverses a distance equal to its own diameter many thousands of times in a second. but even though mercury is moving so much faster, yet the dimensions of the planet are so considerable that a period of two minutes will be required for it to move through a distance equal to its diameter. viewing the globe of the planet as a whole, the velocity of its movement is but a stately and dignified progress appropriate to its dimensions. as we can learn little or nothing of the true surface of mercury, it is utterly impossible for us to say whether life can exist on the surface of that planet. we may, however, reasonably conclude that there cannot be life on mercury in any respect analogous to the life which we know on the earth. the heat of the sun and the light of the sun beat down on mercury with an intensity many times greater than that which we experience. when this planet is at its utmost distance from the sun the intensity of solar radiation is even then more than four times greater than the greatest heat which ever reaches the earth. but when mercury, in the course of its remarkable changes of distance, draws in to the warmest part of its orbit, it is exposed to a terrific scorching. the intensity of the sun's heat must then be not less than nine times as great as the greatest radiation to which we are exposed. these tremendous climatic changes succeed each other much more rapidly than do the variations of our seasons. on mercury the interval between midsummer and midwinter is only forty-four days, while the whole year is only eighty-eight days. such rapid variations in solar heat must in themselves exercise a profound effect on the habitability of mercury. mr. ledger well remarks, in his interesting work,[ ] that if there be inhabitants on mercury the notions of "perihelion" and "aphelion," which are here often regarded as expressing ideas of an intricate or recondite character, must on the surface of that planet be familiar to everybody. the words imply "near the sun," and "away from the sun;" but we do not associate these expressions with any obvious phenomena, because the changes in the distance of the earth from the sun are inconsiderable. but on mercury, where in six weeks the sun rises to more than double his apparent size, and gives more than double the quantity of light and of heat, such changes as are signified by perihelion and aphelion embody ideas obviously and intimately connected with the whole economy of the planet. it is nevertheless rash to found any inferences as to climate merely upon the proximity or the remoteness of the sun. climate depends upon other matters besides the sun's distance. the atmosphere surrounding the earth has a profound influence on heat and cold, and if mercury have an atmosphere--as has often been supposed--its climate may be thereby modified to any necessary extent. it seems, however, hardly possible to suppose that any atmosphere could form an adequate protection for the inhabitants from the violent and rapid fluctuations of solar radiation. all we can say is, that the problem of life in mercury belongs to the class of unsolved, and perhaps unsolvable, mysteries. it was in the year that kepler made an important announcement as to impending astronomical events. he had been studying profoundly the movements of the planets; and from his study of the past he had ventured to predict the future. kepler announced that in the year the planets venus and mercury would both make a transit across the sun, and he assigned the dates to be november th for mercury, and december th for venus. this was at the time a very remarkable prediction. we are so accustomed to turn to our almanacs and learn from them all the astronomical phenomena which are anticipated during the year, that we are apt to forget how in early times this was impossible. it has only been by slow degrees that astronomy has been rendered so perfect as to enable us to foretell, with accuracy, the occurrence of the more delicate phenomena. the prediction of those transits by kepler, some years before they occurred, was justly regarded at the time as a most remarkable achievement. the illustrious gassendi prepared to apply the test of actual observation to the announcements of kepler. we can now assign the time of the transit accurately to within a few minutes, but in those early attempts equal precision was not practicable. gassendi considered it necessary to commence watching for the transit of mercury two whole days before the time indicated by kepler, and he had arranged an ingenious plan for making his observations. the light of the sun was admitted into a darkened room through a hole in the shutter, and an image of the sun was formed on a white screen by a lens. this is, indeed, an admirable and a very pleasing way of studying the surface of the sun, and even at the present day, with our best telescopes, one of the methods of viewing our luminary is founded on the same principle. gassendi commenced his watch on the th of november, and carefully studied the sun's image at every available opportunity. it was not, however, until five hours after the time assigned by kepler that the transit of mercury actually commenced. gassendi's preparations had been made with all the resources which he could command, but these resources seem very imperfect when compared with the appliances of our modern observatories. he was anxious to note the time when the planet appeared, and for this purpose he had stationed an assistant in the room beneath, who was to observe the altitude of the sun at the moment indicated by gassendi. the signal to the assistant was to be conveyed by a very primitive apparatus. gassendi was to stamp on the floor when the critical moment had arrived. in spite of the long delay, which exhausted the patience of the assistant, some valuable observations were obtained, and thus the first passage of a planet across the sun was observed. the transits of mercury are not rare phenomena (there have been thirteen of them during the nineteenth century), and they are chiefly of importance on account of the accuracy which their observation infuses into our calculations of the movements of the planet. it has often been hoped that the opportunities afforded by a transit would be available for procuring information as to the physical character of the globe of mercury, but these hopes have not been realised. spectroscopic observations of mercury are but scanty. they seem to indicate that water vapour is a probable constituent in the atmosphere of mercury, as it is in our own. a distinguished italian astronomer, professor schiaparelli, some years ago announced a remarkable discovery with respect to the rotation of the planet mercury. he found that the planet rotates on its axis in the same period as it revolves around the sun. the practical consequence of the identity between these two periods is that mercury always turns the same face to the sun. if our earth were to rotate in a similar fashion, then the hemisphere directed to the sun would enjoy eternal day, while the opposite hemisphere would be relegated to perpetual night. according to this discovery, mercury revolves around the sun in the same way as the moon revolves around the earth. as the velocity with which mercury travels round the sun is very variable, owing to the highly elliptic shape of its orbit, while the rotation about its axis is performed with uniform speed, it follows that rather more than a hemisphere (about five-eighths of the surface) enjoys more or less the light of the sun in the course of a mercurial year. this important discovery of schiaparelli has lately been confirmed by an american astronomer, mr. lowell, of arizona, u.s.a., who observed the planet under very favourable conditions with a refractor of twenty-four inches aperture. he has detected on the globe of mercury certain narrow, dark lines, the very slow shifting of which points to a period of rotation about its axis exactly coincident with the period of revolution round the sun. the same observer shows that the axis of rotation of mercury is perpendicular to the plane of the orbit. mr. lowell has perceived no sign of clouds or obscurations, and indeed no indication of any atmospheric envelope; the surface of mercury is colourless, "a geography in black and white." we may assert that, there is a strong _à priori_ probability in favour of the reality of schiaparelli's discovery. mercury, being one of the planets devoid of a moon, will be solely influenced by the sun in so far as tidal phenomena are concerned. owing, moreover, to the proximity of mercury to the sun, the solar tides on that planet possess an especial vehemence. as the tendency of tides is to make mercury present a constant face to the sun, there need be little hesitation in accepting testimony that tides have wrought exactly the result that we know they were competent to perform. here we take leave of the planet mercury--an interesting and beautiful object, which stimulates our intellectual curiosity, while at the same time it eludes our attempts to make a closer acquaintance. there is, however, one point of attainable knowledge which we must mention in conclusion. it is a difficult, but not by any means an impossible, task to weigh mercury in the celestial balance, and determine his mass in comparison with the other globes of our system. this is a delicate operation, but it leads us through some of the most interesting paths of astronomical discovery. the weight of the planet, as recently determined by von asten, is about one twenty-fourth part of the weight of the earth, but the result is more uncertain than the determinations of the mass of any of the other larger planets. chapter viii. venus. interest attaching to this planet--the unexpectedness of its appearance--the evening star--visibility in daylight--lighted only by the sun--the phases of venus--why the crescent is not visible to the unaided eye--variations in the apparent size of the planet--the rotation of venus--resemblance of venus to the earth--the transit of venus--why of such especial interest--the scale of the solar system--orbits of the earth and venus not in the same plane--recurrence of the transits in pairs--appearance of venus in transit--transits of and --the early transits of and --the observations of horrocks and crabtree--the announcement of halley--how the track of the planet differs from different places--illustrations of parallax--voyage to otaheite--the result of encke--probable value of the sun's distance--observations at dunsink of the last transit of venus--the question of an atmosphere to venus--other determinations of the sun's distance--statistics about venus. it might, for one reason, have been not inappropriate to have commenced our review of the planetary system by the description of the planet venus. this body is not especially remarkable for its size, for there are other planets hundreds of times larger. the orbit of venus is no doubt larger than that of mercury, but it is much smaller than that of the outer planets. venus has not even the splendid retinue of minor attendants which gives such dignity and such interest to the mighty planets of our system. yet the fact still remains that venus is peerless among the planetary host. we speak not now of celestial bodies only seen in the telescope; we refer to the ordinary observation which detected venus ages before telescopes were invented. who has not been delighted with the view of this glorious object? it is not to be seen at all times. for months together the star of evening is hidden from mortal gaze. its beauties are even enhanced by the caprice and the uncertainty which attend its appearance. we do not say that there is any caprice in the movements of venus, as known to those who diligently consult their almanacs. the movements of the lovely planet are there prescribed with a prosaic detail hardly in harmony with the character usually ascribed to the goddess of love. but to those who do not devote particular attention to the stars, the very unexpectedness of its appearance is one of its greatest charms. venus has not been noticed, not been thought of, for many months. it is a beautifully clear evening; the sun has just set. the lover of nature turns to admire the sunset, as every lover of nature will. in the golden glory of the west a beauteous gem is seen to glitter; it is the evening star--the planet venus. a few weeks later another beautiful sunset is seen, and now the planet is no longer a point low down in the western glow; it has risen high above the horizon, and continues a brilliant object long after the shades of night have descended. again, a little later, and venus has gained its full brilliancy and splendour. all the heavenly host--even sirius and even jupiter--must pale before the splendid lustre of venus, the unrivalled queen of the firmament. after weeks of splendour, the height of venus at sunset diminishes, and its lustre begins gradually to decline. it sinks to invisibility, and is forgotten by the great majority of mankind; but the capricious goddess has only moved from one side of the sky to the other. ere the sun rises, the morning star will be seen in the east. its splendour gradually augments until it rivals the beauty of the evening star. then again the planet draws near to the sun, and remains lost to view for many months, until the same cycle of changes recommences, after an interval of a year and seven months. when venus is at its brightest it can be easily seen in broad daylight with the unaided eye. this striking spectacle proclaims in an unmistakable manner the unrivalled supremacy of this planet as compared with its fellow-planets and with the fixed stars. indeed, at this time venus is from forty to sixty times more brilliant than any stellar object in the northern heavens. the beautiful evening star is often such a very conspicuous object that it may seem difficult at first to realise that the body is not self-luminous. yet it is impossible to doubt that the planet is really only a dark globe, and to that extent resembles our own earth. the brilliance of the planet is not so very much greater than that of the earth on a sunshiny day. the splendour of venus entirely arises from the reflected light of the sun, in the manner already explained with respect to the moon. we cannot distinguish the characteristic crescent shape of the planet with the unaided eye, which merely shows a brilliant point too small to possess sensible form. this is to be explained on physiological grounds. the optical contrivances in the eye form an image of the planet on the retina which is necessarily very small. even when venus is nearest to the earth the diameter of the planet subtends an angle not much more than one minute of arc. on the delicate membrane a picture of venus is thus drawn about one six-thousandth part of an inch in diameter. great as may be the delicacy of the retina, it is not adequate to the perception of form in a picture so minute. the nervous structure, which has been described as the source of vision, forms too coarse a canvas for the reception of the details of this tiny picture. hence it is that to the unaided eye the brilliant venus appears merely as a bright spot. ordinary vision cannot tell what shape it has; still less can it reveal the true beauty of the crescent. if the diameter of venus were several times as great as it actually is; were this body, for instance, as large as jupiter or some of the other great planets, then its crescent could be readily discerned by the unaided eye. it is curious to speculate on what might have been the history of astronomy had venus only been as large as jupiter. were everyone able to see the crescent form without a telescope, it would then have been an elementary and almost obvious truth that venus must be a dark body revolving round the sun. the analogy between venus and our earth would have been at once perceived; and the doctrine which was left to be discovered by copernicus in comparatively modern times might not improbably have been handed down to us with the other discoveries which have come from the ancient nations of the east. [illustration: fig. . venus, may th, .] perhaps the most perfect drawing of venus that has been hitherto obtained is that made (fig. ) by professor e.e. barnard, on th may, , with a -inch equatorial, at the lick observatory, which for this purpose and on this occasion professor barnard found to be superior to the -inch. the markings shown seem undoubtedly to exist on the planet, and in professor barnard writes: "the circumstances under which this drawing was made are memorable with me, for i never afterwards had such perfect conditions to observe venus." in fig. we show three views of venus under different aspects. the planet is so much closer to the earth when the crescent is seen, that it appears to be part of a much larger circle than that made by venus when more nearly full. this drawing shows the different aspects of the globe in their true relative proportions. it is very difficult to perceive distinctly any markings on the brilliantly lighted surface. sometimes observers have seen spots or other features, and occasionally the pointed extremities of the horns have been irregular, as if to show that the surface of venus is not smooth. some observers report having seen white spots at the poles of venus, in some degree resembling the more conspicuous features of the same character to be seen on mars. [illustration: fig. .--different aspects of venus in the telescope.] as it is so very difficult to see any markings on venus, we are hardly yet able to give a definite answer to the important question as to the period of rotation of this planet round its axis. various observers during the last two hundred years have from very insufficient data concluded that venus rotated in about twenty-three hours. schiaparelli, of milan, turned his attention to this planet in and noticed a dark shade and two bright spots, all situated not far from the southern end of the crescent. this most painstaking astronomer watched these markings for three months, and found that there was no change perceptible in the position which they occupied. this was particularly the case when he continued his watch for some consecutive hours. this fact seemed to show conclusively that venus could not rotate in twenty-three hours nor in any other short period. week after week the spots remained unaltered, until schiaparelli felt convinced that his observations could only be reconciled with a period of rotation between six and nine months. he naturally concluded that the period was days--that is to say, the period which venus takes to complete one revolution round the sun; in other words, venus always turns the same face to the sun. this remarkable result was confirmed by observations made at nice; but it has been vigorously assailed by several observers, who maintain that their own drawings can only agree with a period about equal to that of the rotation of our own earth. schiaparelli's result is, however, well supported by the letters of mr. lowell. he has published a number of drawings of venus made with his -inch refractor, and he finds that the rotation is performed in the same time as the planet's orbital revolution, the axis of rotation being perpendicular to the plane of the orbit. the markings seen by mr. lowell were long and streaky, and they were always visible whenever his own atmospheric conditions were fairly good. we have seen that the moon revolves so as to keep the same face always turned towards the earth. we have now seen that the planets venus and mercury each appear to revolve in such a way that they keep the same face towards the sun. all these phenomena are of profound interest in the higher departments of astronomical research. they are not mere coincidences. they arise from the operation of the tides, in a manner that will be explained in a later chapter. it happens that our earth and venus are very nearly equal in bulk. the difference is hardly perceptible, but the earth has a diameter a few miles greater than that of venus. there are indications of the existence of an atmosphere around venus, and the evidence of the spectroscope shows that water vapour is there present. if there be oxygen in the atmosphere of venus, then it would seem possible that there might be life on that globe not essentially different in character from some forms of life on the earth. no doubt the sun's heat on venus is greatly in excess of the sun's heat with which we are acquainted, but this is not an insuperable difficulty. we see at present on the earth, life in very hot regions and life in very cold regions. indeed, with each approach to the equator we find life more and more exuberant; so that, if water be present on the surface of venus and if oxygen be a constituent of its atmosphere, we might expect to find in that planet a luxuriant tropical life, of a kind perhaps analogous in some respects to life on the earth. in our account of the planet mercury, as well as in the brief description of the hypothetical planet vulcan, it has been necessary to allude to the phenomena presented by the transit of a planet over the face of the sun. such an event is always of interest to astronomers, and especially so in the case of venus. we have in recent years had the opportunity of witnessing two of these rare occurrences. it is perhaps not too much to assert that the transits of and have received a degree of attention never before accorded to any astronomical phenomenon. the transit of venus cannot be described as a very striking or beautiful spectacle. it is not nearly so fine a sight as a great comet or a shower of shooting stars. why is it, then, that it is regarded as of so much scientific importance? it is because the phenomenon helps us to solve one of the greatest problems which has ever engaged the mind of man. by the transit of venus we may determine the scale on which our solar system is constructed. truly this is a noble problem. let us dwell upon it for a moment. in the centre of our system we have the sun--a majestic globe more than a million times as large as the earth. circling round the sun we have the planets, of which our earth is but one. there are hundreds of small planets. there are a few comparable with our earth; there are others vastly surpassing the earth. besides the planets there are other bodies in our system. many of the planets are accompanied by systems of revolving moons. there are hundreds, perhaps thousands, of comets. each member of this stupendous host moves in a prescribed orbit around the sun, and collectively they form the solar system. it is comparatively easy to learn the proportions of this system, to measure the relative distances of the planets from the sun, and even the relative sizes of the planets themselves. peculiar difficulties are, however, experienced when we seek to ascertain the actual _size_ of the system as well as its shape. it is this latter question which the transit of venus offers us a method of solving. look, for instance, at an ordinary map of europe. we see the various countries laid down with precision; we can tell the courses of the rivers; we can say that france is larger than england, and russia larger than france; but no matter how perfectly the map be constructed, something else is necessary before we can have a complete conception of the dimensions of the country. we must know _the scale on which the map is drawn_. the map contains a reference line with certain marks upon it. this line is to give the scale of the map. its duty is to tell us that an inch on the map corresponds with so many miles on the actual surface. unless it be supplemented by the scale, the map would be quite useless for many purposes. suppose that we consulted it in order to choose a route from london to vienna, we can see at once the direction to be taken and the various towns and countries to be traversed; but unless we refer to the little scale in the corner, the map will not tell how many miles long the journey is to be. a map of the solar system can be readily constructed. we can draw on it the orbits of some of the planets and of their satellites, and we can include many of the comets. we can assign to the planets and to the orbits their proper proportions. but to render the map quite efficient something more is necessary. we must have the scale which is to tell us how many millions of miles on the heavens correspond to one inch of the map. it is at this point we encounter a difficulty. there are, however, several ways of solving the problem, though they are all difficult and laborious. the most celebrated method (though far from the best) is that presented on an occasion of the transit of venus. herein, then, lies the importance of this rare event. it is one of the best-known means of finding the actual scale on which our system is constructed. observe the full importance of the problem. once the scale has been determined, then all is known. we know the size of the sun; we know his distance; we know the bulk of jupiter, and the distances at which his satellites revolve; we know the dimensions of the comets, and the number of miles to which they recede in their wanderings; we know the velocity of the shooting stars; and we learn the important lesson that our earth is but one of the minor members of the sun's family. as the path of venus lies inside that of the earth, and as venus moves more quickly than the earth, it follows that the earth is frequently passed by the planet, and just at the critical moment it will sometimes happen that the earth, the planet, and the sun lie in the same straight line. we can then see venus on the face of the sun, and this is the phenomenon which we call the _transit of venus_. it is, indeed, quite plain that if the three bodies were exactly in a line, an observer on the earth, looking at the planet, would see it brought out vividly against the brilliant background of the sun. considering that the earth is overtaken by venus once every nineteen months, it might seem that the transits of the planet should occur with corresponding frequency. this is not the case; the transit of venus is an exceedingly rare occurrence, and a hundred years or more will often elapse without a single one taking place. the rarity of these phenomena arises from the fact that the path of the planet is inclined to the plane of the earth's orbit; so that for half of its path venus is above the plane of the earth's orbit, and in the other half it is below. when venus overtakes the earth, the line from the earth to venus will therefore usually pass over or under the sun. if, however, it should happen that venus overtakes the earth at or near either of the points in which the plane of the orbit of venus passes through that of the earth, then the three bodies will be in line, and a transit of venus will be the consequence. the rarity of the occurrence of a transit need no longer be a mystery. the earth passes through one of the critical parts every december, and through the other every june. if it happens that the conjunction of venus occurs on, or close to, june th or december th, then a transit of venus will occur at that conjunction, but in no other circumstances. the most remarkable law with reference to the repetition of the phenomenon is the well-known eight-year interval. the transits may be all grouped together into pairs, the two transits of any single pair being separated by an interval of eight years. for instance, a transit of venus took place in , and again in . no further transits occurred until those witnessed in and in . then, again, comes a long interval, for another transit will not occur until , but it will be followed by another in . this arrangement of the transits in pairs admits of a very simple explanation. it happens that the periodic time of venus bears a remarkable relation to the periodic time of the earth. the planet accomplishes thirteen revolutions around the sun in very nearly the same time that the earth requires for eight revolutions. if, therefore, venus and the earth were in line with the sun in , then in eight years more the earth will again be found in the same place; and so will venus, for it has just been able to accomplish thirteen revolutions. a transit of venus having occurred on the first occasion, a transit must also occur on the second. it is not, however, to be supposed that every eight years the planets will again resume the same position with sufficient precision for a regular eight-year transit interval. it is only approximately true that thirteen revolutions of venus are coincident with eight revolutions of the earth. each recurrence of conjunction takes place at a slightly different position of the planets, so that when the two planets came together again in the year the point of conjunction was so far removed from the critical point that the line from the earth to venus did not intersect the sun, and thus, although venus passed very near the sun, yet no transit took place. [illustration: fig. .--venus on the sun at the transit of .] fig. represents the transit of venus in . it is taken from a photograph obtained, during the occurrence, by m. janssen. his telescope was directed towards the sun during the eventful minutes while it lasted, and thus an image of the sun was depicted on the photographic plate placed in the telescope. the lighter circle represents the disc of the sun. on that disc we see the round, sharp image of venus, showing the characteristic appearance of the planet during the progress of the transit. the only other features to be noticed are a few of the solar spots, rather dimly shown, and a network of lines which were marked on a glass plate across the field of view of the telescope to facilitate measurements. the adjoining sketch (fig. ) exhibits the course which the planet pursued in its passage across the sun on the two occasions in and . our generation has had the good fortune to witness the two occurrences indicated on this picture. the white circle denotes the disc of the sun; the planet encroaches on the white surface, and at first is like a bite out of the sun's margin. gradually the black spot steals in front of the sun, until, after nearly half an hour, the black disc is entirely visible. slowly the planet wends its way across, followed by hundreds of telescopes from every accessible part of the globe whence the phenomenon is visible, until at length, in the course of a few hours, it emerges at the other side. it will be useful to take a brief retrospect of the different transits of venus of which there is any historical record. they are not numerous. hundreds of such phenomena have occurred since man first came on the earth. it was not until the approach of the year that attention began to be directed to the matter, though the transit which undoubtedly occurred in that year was not noticed by anyone. the success of gassendi in observing the transit of mercury, to which we have referred in the last chapter, led him to hope that he would be equally fortunate in observing the transit of venus, which kepler had also foretold. gassendi looked at the sun on the th, th, and th december. he looked at it again on the th, but he saw no sign of the planet. we now know the reason. the transit of venus took place during the night, between the th and the th, and must therefore have been invisible to european observers. kepler had not noticed that another transit would occur in . this discovery was made by another astronomer, and it is the one with which the history of the subject may be said to commence. it was the first occasion on which the phenomenon was ever actually witnessed; nor was it then seen by many. so far as is known, it was witnessed by only two persons. [illustration: fig. .--the path of venus across the sun in the transits of and .] a young and ardent english astronomer, named horrocks, had undertaken some computations about the motions of venus. he made the discovery that the transit of venus would be repeated in , and he prepared to verify the fact. the sun rose bright on the morning of the day--which happened to be a sunday. the clerical profession, which horrocks followed, here came into collision with his desires as an astronomer. he tells us that at nine he was called away by business of the highest importance--referring, no doubt, to his official duties; but the service was quickly performed, and a little before ten he was again on the watch, only to find the brilliant face of the sun without any unusual feature. it was marked with a spot, but nothing that could be mistaken for a planet. again, at noon, came an interruption; he went to church, but he was back by one. nor were these the only impediments to his observations. the sun was also more or less clouded over during part of the day. however, at a quarter past three in the afternoon his clerical work was over; the clouds had dispersed, and he once more resumed his observations. to his intense delight he then saw on the sun the round, dark spot, which was at once identified as the planet venus. the observations could not last long; it was the depth of winter, and the sun was rapidly setting. only half an hour was available, but he had made such careful preparations beforehand that it sufficed to enable him to secure some valuable measurements. horrocks had previously acquainted his friend, william crabtree, with the impending occurrence. crabtree was therefore on the watch, and succeeded in seeing the transit; a striking picture of crabtree's famous observation is shown in one of the beautiful frescoes in the town hall at manchester. but to no one else had horrocks communicated the intelligence; as he says, "i hope to be excused for not informing other of my friends of the expected phenomenon, but most of them care little for trifles of this kind, rather preferring their hawks and hounds, to say no worse; and although england is not without votaries of astronomy, with some of whom i am acquainted, i was unable to convey to them the agreeable tidings, having myself had so little notice." it was not till long afterwards that the full importance of the transit of venus was appreciated. nearly a century had rolled away when the great astronomer, halley ( - ), drew attention to the subject. the next transit was to occur in , and forty-five years before that event halley explained his celebrated method of finding the distance of the sun by means of the transit of venus.[ ] he was then a man sixty years of age; he could have no expectation that he would live to witness the event; but in noble language he commends the problem to the notice of the learned, and thus addresses the royal society of london:--"and this is what i am now desirous to lay before this illustrious society, which i foretell will continue for ages, that i may explain beforehand to young astronomers, who may, perhaps, live to observe these things, a method by which the immense distance of the sun may be truly obtained.... i recommend it, therefore, again and again to those curious astronomers who, when i am dead, will have an opportunity of observing these things, that they would remember this my admonition, and diligently apply themselves with all their might in making the observations, and i earnestly wish them all imaginable success--in the first place, that they may not by the unseasonable obscurity of a cloudy sky be deprived of this most desirable sight, and then that, having ascertained with more exactness the magnitudes of the planetary orbits, it may redound to their immortal fame and glory." halley lived to a good old age, but he died nineteen years before the transit occurred. the student of astronomy who desires to learn how the transit of venus will tell the distance from the sun must prepare to encounter a geometrical problem of no little complexity. we cannot give to the subject the detail that would be requisite for a full explanation. all we can attempt is to render a general account of the method, sufficient to enable the reader to see that the transit of venus really does contain all the elements necessary for the solution of the problem. we must first explain clearly the conception which is known to astronomers by the name of _parallax_; for it is by parallax that the distance of the sun, or, indeed, the distance of any other celestial body, must be determined. let us take a simple illustration. stand near a window whence you can look at buildings, or the trees, the clouds, or any distant objects. place on the glass a thin strip of paper vertically in the middle of one of the panes. close the right eye, and note with the left eye the position of the strip of paper relatively to the objects in the background. then, while still remaining in the same position, close the left eye and again observe the position of the strip of paper with the right eye. you will find that the position of the paper on the background has changed. as i sit in my study and look out of the window i see a strip of paper, with my right eye, in front of a certain bough on a tree a couple of hundred yards away; with my left eye the paper is no longer in front of that bough, it has moved to a position near the outline of the tree. this apparent displacement of the strip of paper, relatively to the distant background, is what is called parallax. move closer to the window, and repeat the observation, and you find that _the apparent displacement of the strip increases_. move away from the window, and the displacement decreases. move to the other side of the room, the displacement is much less, though probably still visible. we thus see that the change in the apparent place of the strip of paper, as viewed with the right eye or the left eye, varies in amount as the distance changes; but it varies in the opposite way to the distance, for as either becomes greater the other becomes less. we can thus associate with each particular distance a corresponding particular displacement. from this it will be easy to infer that if we have the means of measuring the amount of displacement, then we have the means of calculating the distance from the observer to the window. it is this principle, applied on a gigantic scale, which enables us to measure the distances of the heavenly bodies. look, for instance, at the planet venus; let this correspond to the strip of paper, and let the sun, on which venus is seen in the act of transit, be the background. instead of the two eyes of the observer, we now place two observatories in distant regions of the earth; we look at venus from one observatory, we look at it from the other; we measure the amount of the displacement, and from that we calculate the distance of the planet. all depends, then, on the means which we have of measuring the displacement of venus as viewed from the two different stations. there are various ways of accomplishing this, but the most simple is that originally proposed by halley. from the observatory at a venus seems to pursue the upper of the two tracks shown in the adjoining figure (fig. ). from the observatory at b it follows the lower track, and it is for us to measure the distance between the two tracks. this can be accomplished in several ways. suppose the observer at a notes the time that venus has occupied in crossing the disc, and that similar observations be made at b. as the track seen from b is the larger, it must follow that the time observed at b will be greater than that at a. when the observations from the different hemispheres are compared, the _times_ observed will enable the lengths of the tracks to be calculated. the lengths being known, their places on the circular disc of the sun are determined, and hence the amount of displacement of venus in transit is ascertained. thus it is that the distance of venus is measured, and the scale of the solar system is known. [illustration: fig. .--to illustrate the observation of the transit of venus from two localities, a and b, on the earth.] the two transits to which halley's memorable researches referred occurred in the years and . the results of the first were not very successful, in spite of the arduous labours of those who undertook the observations. the transit of is of particular interest, not only for the determination of the sun's distance, but also because it gave rise to the first of the celebrated voyages of captain cook. it was to see the transit of venus that captain cook was commissioned to sail to otaheite, and there, on the rd of june, on a splendid day in that exquisite climate, the phenomenon was carefully observed and measured by different observers. simultaneously with these observations others were obtained in europe and elsewhere, and from the combination of all the observations an approximate knowledge of the sun's distance was gained. the most complete discussion of these observations did not, however, take place for some time. it was not until the year that the illustrious encke computed the distance of the sun, and gave as the definite result , , miles. for many years this number was invariably adopted, and many of the present generation will remember how they were taught in their school-days that the sun was , , miles away. at length doubts began to be whispered as to the accuracy of this result. the doubts arose in different quarters, and were presented with different degrees of importance; but they all pointed in one direction, they all indicated that the distance of the sun was not really so great as the result which encke had obtained. it must be remembered that there are several ways of finding the distance of the sun, and it will be our duty to allude to some other methods later on. it has been ascertained that the result obtained by encke from the observations made in and , with instruments inferior to our modern ones, was too great, and that the distance of the sun may probably be now stated at , , miles. i venture to record our personal experience of the last transit of venus, which we had the good fortune to view from dunsink observatory on the afternoon of the th of december, . the morning of the eventful day appeared to be about as unfavourable for a grand astronomical spectacle as could well be imagined. snow, a couple of inches thick, covered the ground, and more was falling, with but little intermission, all the forenoon. it seemed almost hopeless that a view of the phenomenon could be obtained from that observatory; but it is well in such cases to bear in mind the injunction given to the observers on a celebrated eclipse expedition. they were instructed, no matter what the day should be like, that they were to make all their preparations precisely as they would have done were the sun shining with undimmed splendour. by this advice no doubt many observers have profited; and we acted upon it with very considerable success. there were at that time at the observatory two equatorials, one of them an old, but tolerably good, instrument, of about six inches aperture; the other the great south equatorial, of twelve inches aperture, already referred to. at eleven o'clock the day looked worse than ever; but we at once proceeded to make all ready. i stationed mr. rambaut at the small equatorial, while i myself took charge of the south instrument. the snow was still falling when the domes were opened; but, according to our prearranged scheme, the telescopes were directed, not indeed upon the sun, but to the place where we knew the sun was, and the clockwork was set in motion which carried round the telescopes, still constantly pointing towards the invisible sun. the predicted time of the transit had not yet arrived. the eye-piece employed on the south equatorial must also receive a brief notice. it will, of course, be obvious that the full glare of the sun has to be greatly mitigated before the eye can view it with impunity. the light from the sun falls upon a piece of transparent glass inclined at a certain angle, and the chief portion of the sun's heat, as well as a certain amount of its light, pass through the glass and are lost. a certain fraction of the light is, however, reflected from the glass, and enters the eye-piece. this light is already much reduced in intensity, but it undergoes as much further reduction as we please by an ingenious contrivance. the glass which reflects the light does so at what is called the polarising angle, and between the eye-piece and the eye is a plate of tourmaline. this plate of tourmaline can be turned round by the observer. in one position it hardly interferes with the polarised light at all, while in the position at right angles thereto it cuts off nearly the whole of it. by simply adjusting the position of the tourmaline, the observer has it in his power to render the image of any brightness that may be convenient, and thus the observations of the sun can be conducted with the appropriate degree of illumination. but such appliances seemed on this occasion to be a mere mockery. the tourmaline was all ready, but up to one o'clock not a trace of the sun could be seen. shortly after one o'clock, however, we noticed that the day was getting lighter; and, on looking to the north, whence the wind and the snow were coming, we saw, to our inexpressible delight, that the clouds were clearing. at length, the sky towards the south began to improve, and at last, as the critical moment approached, we could detect the spot where the sun was becoming visible. but the predicted moment arrived and passed, and still the sun had not broken through the clouds, though every moment the certainty that it would do so became more apparent. the external contact was therefore missed. we tried to console ourselves by the reflection that this was not, after all, a very important phase, and hoped that the internal contact would be more successful. at length the struggling beams pierced the obstruction, and i saw the round, sharp disc of the sun in the finder, and eagerly glanced at the point on which attention was concentrated. some minutes had now elapsed since the predicted moment of first contact, and, to my delight, i saw the small notch in the margin of the sun showing that the transit had commenced, and that the planet was then one-third on the sun. but the critical moment had not yet arrived. by the expression "first internal contact" we are to understand the moment when the planet has completely entered _on_ the sun. this first contact was timed to occur twenty-one minutes later than the external contact already referred to. but the clouds again disappointed our hope of seeing the internal contact. while steadily looking at the exquisitely beautiful sight of the gradual advance of the planet, i became aware that there were other objects besides venus between me and the sun. they were the snowflakes, which again began to fall rapidly. i must admit the phenomenon was singularly beautiful. the telescopic effect of a snowstorm with the sun as a background i had never before seen. it reminded me of the golden rain which is sometimes seen falling from a flight of sky-rockets during pyrotechnic displays; i would gladly have dispensed with the spectacle, for it necessarily followed that the sun and venus again disappeared from view. the clouds gathered, the snowstorm descended as heavily as ever, and we hardly dared to hope that we should see anything more; hr. min. came and passed, the first internal contact was over, and venus had fully entered on the sun. we had only obtained a brief view, and we had not yet been able to make any measurements or other observations that could be of service. still, to have seen even a part of a transit of venus is an event to remember for a lifetime, and we felt more delight than can be easily expressed at even this slight gleam of success. but better things were in store. my assistant came over with the report that he had also been successful in seeing venus in the same phase as i had. we both resumed our posts, and at half-past two the clouds began to disperse, and the prospect of seeing the sun began to improve. it was now no question of the observations of contact. venus by this time was well on the sun, and we therefore prepared to make observations with the micrometer attached to the eye-piece. the clouds at length dispersed, and at this time venus had so completely entered on the sun that the distance from the edge of the planet to the edge of the sun was about twice the diameter of the planet. we measured the distance of the inner edge of venus from the nearest limb of the sun. these observations were repeated as frequently as possible, but it should be added that they were only made with very considerable difficulty. the sun was now very low, and the edges of the sun and of venus were by no means of that steady character which is suitable for micrometrical measurement. the margin of the luminary was quivering, and venus, though no doubt it was sometimes circular, was very often distorted to such a degree as to make the measures very uncertain. we succeeded in obtaining sixteen measures altogether; but the sun was now getting low, the clouds began again to interfere, and we saw that the pursuit of the transit must be left to the thousands of astronomers in happier climes who had been eagerly awaiting it. but before the phenomena had ceased i spared a few minutes from the somewhat mechanical work at the micrometer to take a view of the transit in the more picturesque form which the large field of the finder presented. the sun was already beginning to put on the ruddy hues of sunset, and there, far in on its face, was the sharp, round, black disc of venus. it was then easy to sympathise with the supreme joy of horrocks, when, in , he for the first time witnessed this spectacle. the intrinsic interest of the phenomenon, its rarity, the fulfilment of the prediction, the noble problem which the transit of venus helps us to solve, are all present to our thoughts when we look at this pleasing picture, a repetition of which will not occur again until the flowers are blooming in the june of a.d. . the occasion of a transit of venus also affords an opportunity of studying the physical nature of the planet, and we may here briefly indicate the results that have been obtained. in the first place, a transit will throw some light on the question as to whether venus is accompanied by a satellite. if venus were attended by a small body in close proximity, it would be conceivable that in ordinary circumstances the brilliancy of the planet would obliterate the feeble beam of rays from the minute companion, and thus the satellite would remain undiscovered. it was therefore a matter of great interest to scrutinise the vicinity of the planet while in the act of transit. if a satellite existed--and the existence of one or more of such bodies has often been suspected--then it would be capable of detection against the brilliant background of the sun. special attention was directed to this point during the recent transits, but no satellite of venus was to be found. it seems, therefore, to be very unlikely that venus can be attended by any companion globe of appreciable dimensions. the observations directed to the investigation of the atmosphere surrounding venus have been more successful. if the planet were devoid of an atmosphere, then it would be totally invisible just before commencing to enter on the sun, and would relapse into total invisibility as soon as it had left the sun. the observations made during the transits are not in conformity with such suppositions. special attention has been directed to this point during the recent transits. the result has been very remarkable, and has proved in the most conclusive manner the existence of an atmosphere around venus. as the planet gradually moved off the sun, the circular edge of the planet extending out into the darkness was seen to be bounded by a circular arc of light, and dr. copeland, who observed this transit in very favourable circumstances, was actually able to follow the planet until it had passed entirely away from the sun, at which time the globe, though itself invisible, was distinctly marked by the girdle of light by which it was surrounded. this luminous circle is inexplicable save by the supposition that the globe of venus is surrounded by an atmospheric shell in the same way as the earth. it may be asked, what is the advantage of devoting so much time and labour to a celestial phenomenon like the transit of venus which has so little bearing on practical affairs? what does it matter whether the sun be , , miles off, or whether it be only , , , or any other distance? we must admit at once that the enquiry has but a slender bearing on matters of practical utility. no doubt a fanciful person might contend that to compute our nautical almanacs with perfect accuracy we require a precise knowledge of the distance of the sun. our vast commerce depends on skilful navigation, and one factor necessary for success is the reliability of the "nautical almanac." the increased perfection of the almanac must therefore bear some relation to increased perfection in navigation. now, as good authorities tell us that in running for a harbour on a tempestuous night, or in other critical emergencies, even a yard of sea-room is often of great consequence, so it may conceivably happen that to the infinitesimal influence of the transit of venus on the "nautical almanac" is due the safety of a gallant vessel. but the time, the labour, and the money expended in observing the transit of venus are really to be defended on quite different grounds. we see in it a fruitful source of information. it tells us the distance of the sun, which is the foundation of all the great measurements of the universe. it gratifies the intellectual curiosity of man by a view of the true dimensions of the majestic solar system, in which the earth is seen to play a dignified, though still subordinate, part; and it leads us to a conception of the stupendous scale on which the universe is constructed. it is not possible for us, with a due regard to the limits of this volume, to protract any longer our discussion of the transit of venus. when we begin to study the details of the observations, we are immediately confronted with a multitude of technical and intricate matters. unfortunately, there are very great difficulties in making the observations with the necessary precision. the moments when venus enters on and leaves the solar disc cannot be very accurately observed, partly owing to a peculiar optical illusion known as "the black drop," whereby venus seems to cling to the sun's limb for many seconds, partly owing to the influence of the planet's atmosphere, which helps to make the observed time of contact uncertain. these circumstances make it difficult to determine the distance of the sun from observations of transits of venus with the accuracy which modern science requires. it seems therefore likely that the final determination of the sun's distance will be obtained in quite a different manner. this will be explained in chapter xi., and hence we feel the less reluctance in passing any from the consideration of the transit of venus as a method of celestial surveying. we must now close our description of this lovely planet; but before doing so, let us add--or in some cases repeat--a few statistical facts as to the size and the dimensions of the planet and its orbit. the diameter of venus is about , miles, and the planet shows no measurable departure from the globular form, though we can hardly doubt that its polar diameter must really be somewhat shorter than the equatorial diameter. this diameter is only about miles less than that of the earth. the mass of venus is about three-quarters of the mass of the earth; or if, as is more usual, we compare the mass of venus with the sun, it is to be represented by the fraction divided by , . it is to be observed that the mass of venus is not quite so great in comparison with its bulk as might have been expected. the density of this planet is about · of that of the earth. venus would weigh · times as much as a globe of water of equal size. the gravitation at its surface will, to a slight extent, be less than the gravitation at the surface of the earth. a body here falls sixteen feet in a second; a body let fall at the surface of venus would fall about three feet less. it seems not unlikely that the time of rotation of venus may be equal to the period of its revolution around the sun. the orbit of venus is remarkable for the close approach which it makes to a circle. the greatest distance of this planet from the sun does not exceed the least distance by one per cent. its mean distance from the sun is about , , miles, and the movement in the orbit amounts to a mean velocity of nearly miles per second, the entire journey being accomplished in · days. chapter ix. the earth. the earth is a great globe--how the size of the earth is measured--the base line--the latitude found by the elevation of the pole--a degree of the meridian--the earth not a sphere--the pendulum experiment--is the motion of the earth slow or fast?--coincidence of the axis of rotation and the axis of figure--the existence of heat in the earth--the earth once in a soft condition--effects of centrifugal force--comparison with the sun and jupiter--the protuberance of the equator--the weighing of the earth--comparison between the weight of the earth and an equal globe of water--comparison of the earth with a leaden globe--the pendulum--use of the pendulum in measuring the intensity of gravitation--the principle of isochronism--shape of the earth measured by the pendulum. that the earth must be a round body is a truth immediately suggested by simple astronomical considerations. the sun is round, the moon is round, and telescopes show that the planets are round. no doubt comets are not round, but then a comet seems to be in no sense a solid body. we can see right through one of these frail objects, and its weight is too small for our methods of measurement to appreciate. if, then, all the solid bodies we can see are round globes, is it not likely that the earth is a globe also? but we have far more direct information than mere surmise. there is no better way of actually seeing that the surface of the ocean is curved than by watching a distant ship on the open sea. when the ship is a long way off and is still receding, its hull will gradually disappear, while the masts will remain visible. on a fine summer's day we can often see the top of the funnel of a steamer appearing above the sea, while the body of the steamer is below. to see this best the eye should be brought as close as possible to the surface of the sea. if the sea were perfectly flat, there would be nothing to obscure the body of the vessel, and it would therefore be visible so long as the funnel remains visible. if the sea be really curved, the protuberant part intercepts the view of the hull, while the funnel is still to be seen. we thus learn how the sea is curved at every part, and therefore it is natural to suppose that the earth is a sphere. when we make more careful measurements we find that the globe is not perfectly round. it is flattened to some extent at each of the poles. this may be easily illustrated by an indiarubber ball, which can be compressed on two opposite sides so as to bulge out at the centre. the earth is similarly flattened at the poles, and bulged out at the equator. the divergence of the earth from the truly globular form is, however, not very great, and would not be noticed without very careful measurements. the determination of the size of the earth involves operations of no little delicacy. very much skill and very much labour have been devoted to the work, and the dimensions of the earth are known with a high degree of accuracy, though perhaps not with all the precision that we may ultimately hope to attain. the scientific importance of an accurate measurement of the earth can hardly be over-estimated. the radius of the earth is itself the unit in which many other astronomical magnitudes are expressed. for example, when observations are made with the view of finding the distance of the moon, the observations, when discussed and reduced, tell us that the distance of the moon is equal to fifty-nine times the equatorial radius of the earth. if we want to find the distance of the moon in miles, we require to know the number of miles in the earth's radius. a level part of the earth's surface having been chosen, a line a few miles long is measured. this is called the base, and as all the subsequent measures depend ultimately on the base, it is necessary that this measurement shall be made with scrupulous accuracy. to measure a line four or five miles long with such precision as to exclude any errors greater than a few inches demands the most minute precautions. we do not now enter upon a description of the operations that are necessary. it is a most laborious piece of work, and many ponderous volumes have been devoted to the discussion of the results. but when a few base lines have been obtained in different places on the earth's surface, the measuring rods are to be laid aside, and the subsequent task of the survey of the earth is to be conducted by the measurement of angles from one station to another and trigonometrical calculations based thereon. starting from a base line a few miles long, distances of greater length are calculated, until at length stretches miles long, or even more, can be accomplished. it is thus possible to find the length of a long line running due north and south. so far the work has been merely that of the terrestrial surveyor. the distance thus ascertained is handed over to the astronomer to deduce from it the dimensions of the earth. the astronomer fixes his observatory at the northern end of the long line, and proceeds to determine his latitude by observation. there are various ways by which this can be accomplished. they will be found fully described in works on practical astronomy. we shall here only indicate in a very brief manner the principle on which such observations are to be made. everyone ought to be familiar with the pole star, which, though by no means the most brilliant, is probably the most important star in the whole heavens. in these latitudes we are accustomed to find the pole star at a considerable elevation, and there we can invariably find it, always in the same place in the northern sky. but suppose we start on a voyage to the southern hemisphere: as we approach the equator we find, night after night, the pole star coming closer to the horizon. at the equator it is on the horizon; while if we cross the line, we find on entering the southern hemisphere that this useful celestial body has become invisible. this is in itself sufficient to show us that the earth cannot be the flat surface that untutored experience seems to indicate. on the other hand, a traveller leaving england for norway observes that the pole star is every night higher in the heavens than he has been accustomed to see it. if he extend his journey farther north, the same object will gradually rise higher and higher, until at length, when approaching the pole of the earth, the pole star is high up over his head. we are thus led to perceive that the higher our latitude, the higher, in general, is the elevation of the pole star. but we cannot use precise language until we replace the twinkling point by the pole of the heavens itself. the pole of the heavens is near the pole star, which itself revolves around the pole of the heavens, as all the other stars do, once every day. the circle described by the pole star is, however, so small that, unless we give it special attention, the motion will not be perceived. the true pole is not a visible point, but it is capable of being accurately defined, and it enables us to state with the utmost precision the relation between the pole and the latitude. the statement is, that the elevation of the pole above the horizon is equal to the latitude of the place. the astronomer stationed at one end of the long line measures the elevation of the pole above the horizon. this is an operation of some delicacy. in the first place, as the pole is invisible, he has to obtain its position indirectly. he measures the altitude of the pole star when that altitude is greatest, and repeats the operation twelve hours later, when the altitude of the pole star is least; the mean between the two, when corrected in various ways which it is not necessary for us now to discuss, gives the true altitude of the pole. suffice it to say that by such operations the latitude of one end of the line is determined. the astronomer then, with all his equipment of instruments, moves to the other end of the line. he there repeats the process, and he finds that the pole has now a different elevation, corresponding to the different latitude. the difference of the two elevations thus gives him an accurate measure of the number of degrees and fractional parts of a degree between the latitudes of the two stations. this can be compared with the actual distance in miles between the two stations, which has been ascertained by the trigonometrical survey. a simple calculation will then show the number of miles and fractional parts of a mile corresponding to one degree of latitude--or, as it is more usually expressed, the length of a degree of the meridian. this operation has to be repeated in different parts of the earth--in the northern hemisphere and in the southern, in high latitudes and in low. if the sea-level over the entire earth were a perfect sphere, an important consequence would follow--the length of a degree of the meridian would be everywhere the same. it would be the same in peru as in sweden, the same in india as in england. but the lengths of the degrees are not all the same, and hence we learn that our earth is not really a sphere. the measured lengths of the degrees enable us to see to what extent the shape of the earth departs from a perfect sphere. near the pole the length of a degree is longer than near the equator. this shows that the earth is flattened at the poles and protuberant at the equator, and it provides the means by which we may calculate the actual lengths of the polar and the equatorial axes. in this way the equatorial diameter has been found equal to , miles, while the polar diameter is miles shorter. the polar axis of the earth may be defined as the diameter about which the earth rotates. this axis intersects the surface at the north and south poles. the time which the earth occupies in making a complete rotation around this axis is called a sidereal day. the sidereal day is a little shorter than the ordinary day, being only hours, minutes, and seconds. the rotation is performed just as if a rigid axis passed through the centre of the earth; or, to use the old and homely illustration, the earth rotates just as a ball of worsted may be made to rotate around a knitting-needle thrust through its centre. it is a noteworthy circumstance that the axis about which the earth rotates occupies a position identical with that of the shortest diameter of the earth as found by actual surveying. this is a coincidence which would be utterly inconceivable if the shape of the earth was not in some way physically connected with the fact that the earth is rotating. what connection can then be traced? let us enquire into the subject, and we shall find that the shape of the earth is a consequence of its rotation. the earth at the present time is subject, at various localities, to occasional volcanic outbreaks. the phenomena of such eruptions, the allied occurrence of earthquakes, the well-known fact that the heat increases the deeper we descend into the earth, the existence of hot springs, the geysers found in iceland and elsewhere, all testify to the fact that heat exists in the interior of the earth. whether that heat be, as some suppose, universal in the interior of the earth, or whether it be merely local at the several places where its manifestations are felt, is a matter which need not now concern us. all that is necessary for our present purpose is the admission that heat is present to some extent. this internal heat, be it much or little, has obviously a different origin from the heat which we know on the surface. the heat we enjoy is derived from the sun. the internal heat cannot have been derived from the sun; its intensity is far too great, and there are other insuperable difficulties attending the supposition that it has come from the sun. where, then, has this heat come from? this is a question which at present we can hardly answer--nor, indeed, does it much concern our argument that we should answer it. the fact being admitted that the heat is there, all that we require is to apply one or two of the well-known thermal laws to the interpretation of the facts. we have first to consider the general principle by which heat tends to diffuse itself and spread away from its original source. the heat, deep-seated in the interior of the earth, is transmitted through the superincumbent rocks, and slowly reaches the surface. it is true that the rocks and materials with which our earth is covered are not good conductors of heat; most of them are, indeed, extremely inefficient in this way; but, good or bad, they are in some shape conductors, and through them the heat must creep to the surface. it cannot be urged against this conclusion that we do not feel this heat. a few feet of brickwork will so confine the heat of a mighty blast furnace that but little will escape through the bricks; but _some_ heat does escape, and the bricks have never been made, and never could be made, which would absolutely intercept all the heat. if a few feet of brickwork can thus nearly mask the heat of a furnace, cannot some scores of miles of rock nearly mask the heat in the depths of the earth, even though that heat were seven times hotter than the mightiest furnace that ever existed? the heat would escape slowly, and perhaps imperceptibly, but, unless all our knowledge of nature is a delusion, no rocks, however thick, can prevent, in the course of time, the leakage of the heat to the surface. when this heat arrives at the surface of the earth it must, in virtue of another thermal law, gradually radiate away and be lost to the earth. it would lead us too far to discuss fully the objections which may perhaps be raised against what we have here stated. it is often said that the heat in the interior of the earth is being produced by chemical combination or by mechanical process, and thus that the heat may be constantly renewed as fast or even faster than it escapes. this, however, is more a difference in form than in substance. if heat be produced in the way just supposed (and there can be no doubt that there may be such an origin for some of the heat in the interior of the globe) there must be a certain expenditure of chemical or mechanical energies that produces a certain exhaustion. for every unit of heat which escapes there will either be a loss of an unit of heat from the globe, or, what comes nearly to the same thing, a loss of an unit of heat-making power from the chemical or the mechanical energies. the substantial result is the same; the heat, actual or potential, of the earth must be decreasing. it should, of course, be observed that a great part of the thermal losses experienced by the earth is of an obvious character, and not dependent upon the slow processes of conduction. each outburst of a volcano discharges a stupendous quantity of heat, which disappears very speedily from the earth; while in the hot springs found in so many places there is a perennial discharge of the same kind, which in the course of years attains enormous proportions. the earth is thus losing heat, while it never acquires any fresh supplies of the same kind to replace the losses. the consequence is obvious; the interior of the earth must be growing colder. no doubt this is an extremely slow process; the life of an individual, the life of a nation, perhaps the life of the human race itself, has not been long enough to witness any pronounced change in the store of terrestrial heat. but the law is inevitable, and though the decline in heat may be slow, yet it is continuous, and in the lapse of ages must necessarily produce great and important results. it is not our present purpose to offer any forecast as to the changes which must necessarily arise from this process. we wish at present rather to look back into past time and see what consequences we may legitimately infer. such intervals of time as we are familiar with in ordinary life, or even in ordinary history, are for our present purpose quite inappreciable. as our earth is daily losing internal heat, or the equivalent of heat, it must have contained more heat yesterday than it does to-day, more last year than this year, more twenty years ago than ten years ago. the effect has not been appreciable in historic time; but when we rise from hundreds of years to thousands of years, from thousands of years to hundreds of thousands of years, and from hundreds of thousands of years to millions of years, the effect is not only appreciable, but even of startling magnitude. there must have been a time when the earth contained much more heat than at present. there must have been a time when the surface of the earth was sensibly hot from this source. we cannot pretend to say how many thousands or millions of years ago this epoch must have been; but we may be sure that earlier still the earth was even hotter, until at length we seem to see the temperature increase to a red heat, from a red heat we look back to a still earlier age when the earth was white hot, back further till we find the surface of our now solid globe was actually molten. we need not push the retrospect any further at present, still less is it necessary for us to attempt to assign the probable origin of that heat. this, it will be observed, is not required in our argument. we find heat now, and we know that heat is being lost every day. from this the conclusion that we have already drawn seems inevitable, and thus we are conducted back to some remote epoch in the abyss of time past when our solid earth was a globe molten and soft throughout. a dewdrop on the petal of a flower is nearly globular; but it is not quite a globe, because the gravitation presses it against the flower and somewhat distorts the shape. a falling drop of rain is a globe; a drop of oil suspended in a liquid with which it does not mix forms a globe. passing from small things to great things, let us endeavour to conceive a stupendous globe of molten matter. let that globe be as large as the earth, and let its materials be so soft as to obey the forces of attraction exerted by each part of the globe on all the other parts. there can be no doubt as to the effect of these attractions; they would tend to smooth down any irregularities on the surface just in the same way as the surface of the ocean is smooth when freed from the disturbing influences of the wind. we might, therefore, expect that our molten globe, isolated from all external interference, would assume the form of a sphere. but now suppose that this great sphere, which we have hitherto assumed to be at rest, is made to rotate round an axis passing through its centre. we need not suppose that this axis is a material object, nor are we concerned with any supposition as to how the velocity of rotation was caused. we can, however, easily see what the consequence of the rotation would be. the sphere would become deformed, the centrifugal force would make the molten body bulge out at the equator and flatten down at the poles. the greater the velocity of rotation the greater would be the bulging. to each velocity of rotation a certain degree of bulging would be appropriate. the molten earth thus bulged out to an extent which was dependent upon the fact that it turned round once a day. now suppose that the earth, while still rotating, commences to pass from the liquid to the solid state. the form which the earth would assume on consolidation would, no doubt, be very irregular on the surface; it would be irregular in consequence of the upheavals and the outbursts incident to the transformation of so mighty a mass of matter; but irregular though it be, we can be sure that, on the whole, the form of the earth's surface would coincide with the shape which it had assumed by the movement of rotation. hence we can explain the protuberant form of the equator of the earth, and we can appeal to that form in corroboration of the view that this globe was once in a soft or molten condition. the argument may be supported and illustrated by comparing the shape of our earth with the shapes of some of the other celestial bodies. the sun, for instance, seems to be almost a perfect globe. no measures that we can make show that the polar diameter of the sun is shorter than the equatorial diameter. but this is what we might have expected. no doubt the sun is rotating on its axis, and, as it is the rotation that causes the protuberance, why should not the rotation have deformed the sun like the earth? the probability is that a difference really does exist between the two diameters of the sun, but that the difference is too small for us to measure. it is impossible not to connect this with the _slowness_ of the sun's rotation. the sun takes twenty-five days to complete a rotation, and the protuberance appropriate to so low a velocity is not appreciable. on the other hand, when we look at one of the quickly-rotating planets, we obtain a very different result. let us take the very striking instance which is presented in the great planet jupiter. viewed in the telescope, jupiter is at once seen not to be a globe. the difference is so conspicuous that accurate measures are not necessary to show that the polar diameter of jupiter is shorter than the equatorial diameter. the departure of jupiter from the truly spherical shape is indeed much greater than the departure of the earth. it is impossible not to connect this with the much more rapid rotation of jupiter. we shall presently have to devote a chapter to the consideration of this splendid orb. we may, however, so far anticipate what we shall then say as to state that the time of jupiter's rotation is under ten hours, and this notwithstanding the fact that jupiter is more than one thousand times greater than the earth. his enormously rapid rotation has caused him to bulge out at the equator to a remarkable extent. the survey of our earth and the measurement of its dimensions having been accomplished, the next operation for the astronomer is the determination of its weight. here, indeed, is a problem which taxes the resources of science to the very uttermost. of the interior of the earth we know little--i might almost say we know nothing. no doubt we sink deep mines into the earth. these mines enable us to penetrate half a mile, or even a whole mile, into the depths of the interior. but this is, after all, only a most insignificant attempt to explore the interior of the earth. what is an advance of one mile in comparison with the distance to the centre of the earth? it is only about one four-thousandth part of the whole. our knowledge of the earth merely reaches to an utterly insignificant depth below the surface, and we have not a conception of what may be the nature of our globe only a few miles below where we are standing. seeing, then, our almost complete ignorance of the solid contents of the earth, does it not seem a hopeless task to attempt to weigh the entire globe? yet that problem has been solved, and the result is known--not, indeed, with the accuracy attained in other astronomical researches, but still with tolerable approximation. it is needless to enunciate the weight of the earth in our ordinary units. the enumeration of billions of tons does not convey any distinct impression. it is a far more natural course to compare the mass of the earth with that of an equal globe of water. we should be prepared to find that our earth was heavier than a like volume of water. the rocks which form its surface are heavier, bulk for bulk, than the oceans which repose on those rocks. the abundance of metals in the earth, the gradual increase in the density of the earth, which must arise from the enormous pressure at great depths--all these considerations will prepare us to learn that the earth is very much heavier than a globe of water of equal size. newton supposed that the earth was between five and six times as heavy as an equal bulk of water. nor is it hard to see that such a suggestion is plausible. the rocks and materials on the surface are usually about two or three times as heavy as water, but the density of the interior must be much greater. there is good reason to believe that down in the remote depths of the earth there is a very large proportion of iron. an iron earth would weigh about seven times as much as an equal globe of water. we are thus led to see that the earth's weight must be probably more than three, and probably less than seven, times an equal globe of water; and hence, in fixing the density between five and six, newton adopted a result plausible at the moment, and since shown to be probably correct. several methods have been proposed by which this important question can be solved with accuracy. of all these methods we shall here only describe one, because it illustrates, in a very remarkable manner, the law of universal gravitation. in the chapter on gravitation it was pointed out that the intensity of this force between two masses of moderate dimensions was extremely minute, and the difficulty in weighing the earth arises from this cause. the practical application of the process is encumbered by multitudinous details, which it will be unnecessary for us to consider at present. the principle of the process is simple enough. to give definiteness to our description, let us conceive a large globe about two feet in diameter; and as it is desirable for this globe to be as heavy as possible, let us suppose it to be made of lead. a small globe brought near the large one is attracted by the force of gravitation. the amount of this attraction is extremely small, but, nevertheless, it can be measured by a refined process which renders extremely small forces sensible. the intensity of the attraction depends both on the masses of the globes and on their distance apart, as well as on the force of gravitation. we can also readily measure the attraction of the earth upon the small globe. this is, in fact, nothing more nor less than the weight of the small globe in the ordinary acceptation of the word. we can thus compare the attraction exerted by the leaden globe with the attraction exerted by the earth. if the centre of the earth and the centre of the leaden globe were at the same distance from the attracted body, then the intensity of their attractions would give at once the ratio of their masses by simple proportion. in this case, however, matters are not so simple: the leaden ball is only distant by a few inches from the attracted ball, while the centre of the earth's attraction is nearly , miles away at the centre of the earth. allowance has to be made for this difference, and the attraction of the leaden sphere has to be reduced to what it would be were it removed to a distance of , miles. this can fortunately be effected by a simple calculation depending upon the general law that the intensity of gravitation varies inversely as the square of the distance. we can thus, partly by calculation and partly by experiment, compare the intensity of the attraction of the leaden sphere with the attraction of the earth. it is known that the attractions are proportional to the masses, so that the comparative masses of the earth and of the leaden sphere have been measured; and it has been ascertained that the earth is about half as heavy as a globe of lead of equal size would be. we may thus state finally that the mass of the earth is about five and a half times as great as the mass of a globe of water equal to it in bulk. in the chapter on gravitation we have mentioned the fact that a body let fall near the surface of the earth drops through sixteen feet in the first second. this distance varies slightly at different parts of the earth. if the earth were a perfect sphere, then the attraction would be the same at every part, and the body would fall through the same distance everywhere. the earth is not round, so the distance which the body falls in one second differs slightly at different places. at the pole the radius of the earth is shorter than at the equator, and accordingly the attraction of the earth at the pole is greater than at the equator. had we accurate measurements showing the distance a body would fall in one second both at the pole and at the equator, we should have the means of ascertaining the shape of the earth. it is, however, difficult to measure correctly the distance a body will fall in one second. we have, therefore, been obliged to resort to other means for determining the force of attraction of the earth at the equator and other accessible parts of its surface. the methods adopted are founded on the pendulum, which is, perhaps, the simplest and certainly one of the most useful of philosophical instruments. the ideal pendulum is a small and heavy weight suspended from a fixed point by a fine and flexible wire. if we draw the pendulum aside from its vertical position and then release it, the weight will swing to and fro. for its journey to and fro the pendulum requires a small period of time. it is very remarkable that this period does not depend appreciably on the length of the circular arc through which the pendulum swings. to verify this law we suspend another pendulum beside the first, both being of the same length. if we draw both pendulums aside and then release them, they swing together and return together. this might have been expected. but if we draw one pendulum a great deal to one side, and the other only a little, the two pendulums still swing sympathetically. this, perhaps, would not have been expected. try it again, with even a still greater difference in the arc of vibration, and still we see the two weights occupy the same time for the swing. we can vary the experiment in another way. let us change the weights on the pendulums, so that they are of unequal size, though both of iron. shall we find any difference in the periods of vibration? we try again: the period is the same as before; swing them through different arcs, large or small, the period is still the same. but it may be said that this is due to the fact that both weights are of the same material. try it again, using a leaden weight instead of one of the iron weights; the result is identical. even with a ball of wood the period of oscillation is the same as that of the ball of iron, and this is true no matter what be the arc through which the vibration takes place. if, however, we change the _length_ of the wire by which the weight is supported, then the period will not remain unchanged. this can be very easily illustrated. take a short pendulum with a wire only one-fourth of the length of that of the long one; suspend the two close together, and compare the periods of vibration of the short pendulum with that of the long one, and we find that the former has a period only half that of the latter. we may state the result generally, and say that the time of vibration of a pendulum is proportional to the square root of its length. if we quadruple the length of the suspending cord we double the time of its vibration; if we increase the length of the pendulum ninefold, we increase its period of vibration threefold. it is the gravitation of the earth which makes the pendulum swing. the greater the attraction, the more rapidly will the pendulum oscillate. this may be easily accounted for. if the earth pulls the weight down very vigorously, the time will be short; if the power of the earth's attraction be lessened, then it cannot pull the weight down so quickly, and the period will be lengthened. the time of vibration of the pendulum can be determined with great accuracy. let it swing for , oscillations, and measure the time that these oscillations have consumed. the arc through which the pendulum swings may not have remained quite constant, but this does not appreciably affect the _time_ of its oscillation. suppose that an error of a second is made in the determination of the time of , oscillations; this will only entail an error of the ten-thousandth part of the second in the time of a single oscillation, and will afford a correspondingly accurate determination of the force of gravity at the place where the experiment was made. take a pendulum to the equator. let it perform , oscillations, and determine carefully the _time_ that these oscillations have required. bring the same pendulum to another part of the earth, and repeat the experiment. we have thus a means of comparing the gravitation at the two places. there are, no doubt, a multitude of precautions to be observed which need not here concern us. it is not necessary to enter into details as to the manner in which the motion of the pendulum is to be sustained, nor as to the effect of changes of temperature in the alteration of its length. it will suffice for us to see how the time of the pendulum's swing can be measured accurately, and how from that measurement the intensity of gravitation can be calculated. the pendulum thus enables us to make a gravitational survey of the surface of the earth with the highest degree of accuracy. we cannot, however, infer that gravity alone affects the oscillations of the pendulum. we have seen how the earth rotates on its axis, and we have attributed the bulging of the earth at the equator to this influence. but the centrifugal force arising from the rotation has the effect of decreasing the apparent weight of bodies, and the change is greatest at the equator, and lessens gradually as we approach the poles. from this cause alone the attraction of the pendulum at the equator is less than elsewhere, and therefore the oscillations of the pendulum will take a longer time there than at other localities. a part of the apparent change in gravitation is accordingly due to the centrifugal force; but there is, in addition, a real alteration. in a work on astronomy it does not come within our scope to enter into further detail on the subject of our planet. the surface of the earth, its contour and its oceans, its mountain chains and its rivers, are for the physical geographer; while its rocks and their contents, its volcanoes and its earthquakes, are to be studied by the geologists and the physicists. chapter x. mars. our nearer neighbours in the heavens--surface of mars can be examined in the telescope--remarkable orbit of mars--resemblance of mars to a star--meaning of opposition--the eccentricity of the orbit of mars--different oppositions of mars--apparent movements of the planet--effect of the earth's movement--measurement of the distance of mars--theoretical investigation of the sun's distance--drawings of the planet--is there snow on mars?--the rotation of the planet--gravitation on mars--has mars any satellites?--prof. asaph hall's great discovery--the revolutions of the satellites--deimos and phobos--"gulliver's travels." the special relation in which we stand to one planet of our system has necessitated a somewhat different treatment of that globe from the treatment appropriate to the others. we discussed mercury and venus as distant objects known chiefly by telescopic research, and by calculations of which astronomical observations were the foundation. our knowledge of the earth is of a different character, and attained in a different way. yet it was necessary for symmetry that we should discuss the earth after the planet venus, in order to give to the earth its true position in the solar system. but now that the earth has been passed in our outward progress from the sun, we come to the planet mars; and here again we resume, though in a somewhat modified form, the methods that were appropriate to venus and to mercury. venus and mars have, from one point of view, quite peculiar claims on our attention. they are our nearest planetary neighbours, on either side. we may naturally expect to learn more of them than of the other planets farther off. in the case of venus, however, this anticipation can hardly be realised, for, as we have already pointed out, its dense atmosphere prevents us from making a satisfactory telescopic examination. when we turn to our other planetary neighbour, mars, we are enabled to learn a good deal with regard to his appearance. indeed, with the exception of the moon, we are better acquainted with the details of the surface of mars than with those of any other celestial body. this beautiful planet offers many features for consideration besides those presented by its physical structure. the orbit of mars is one of remarkable proportions, and it was by the observations of this orbit that the celebrated laws of kepler were discovered. during the occasional approaches of mars to the earth it has been possible to measure its distance with accuracy, and thus another method of finding the sun's distance has arisen which, to say the least, may compete in precision with that afforded by the transit of venus. it must also be observed that the greatest achievement in pure telescopic research which this century has witnessed was that of the discovery of the satellites of mars. to the unaided eye this planet generally appears like a star of the first magnitude. it is usually to be distinguished by its ruddy colour, but the beginner in astronomy cannot rely on its colour only for the identification of mars. there are several stars nearly, if not quite, as ruddy as this globe. the bright star aldebaran, the brightest star in the constellation of the bull, has often been mistaken for the planet. it often resembles betelgeuze, a brilliant point in the constellation of orion. mistakes of this kind will be impossible if the learner has first studied the principal constellations and the more brilliant stars. he will then find great interest in tracing out the positions of the planets, and in watching their ceaseless movements. [illustration: fig. .--the orbits of the earth and of mars, showing the favourable opposition of .] the position of each orb can always be ascertained from the almanac. sometimes the planet will be too near the sun to be visible. it will rise with the sun and set with the sun, and consequently will not be above the horizon during the night. the best time for seeing one of the planets situated like mars will be during what is called its opposition. this state of things occurs when the earth intervenes directly between the planet and the sun. in this case, the distance from mars to the earth is less than at any other time. there is also another advantage in viewing mars during opposition. the planet is then at one side of the earth and the sun at the opposite side, so that when mars is high in the heavens the sun is directly beneath the earth; in other words, the planet is then at its greatest elevation above the horizon at midnight. some oppositions of mars are, however, much more favourable than others. this is distinctly shown in fig. , which represents the orbit of mars and the orbit of the earth accurately drawn to scale. it will be seen that while the orbit of the earth is very nearly circular, the orbit of mars has a very decided degree of eccentricity; indeed, with the exception of the orbit of mercury, that of mars has the greatest eccentricity of any orbit of the larger planets in our system. the value of an opposition of mars for telescopic purposes will vary greatly according to circumstances. the favourable oppositions will be those which occur as near as possible to the th of august. the other extreme will be found in an opposition which occurs near the nd of february. in the latter case the distance between the planet and the earth is nearly twice as great as the former. the last opposition which was suitable for the highest class of work took place in the year . mars was then a magnificent object, and received much, and deserved, attention. the favourable oppositions follow each other at somewhat irregular intervals; the last occurred in the year , and another will take place in the year . the apparent movements of mars are by no means simple. we can imagine the embarrassment of the early astronomer who first undertook the task of attempting to decipher these movements. the planet is seen to be a brilliant and conspicuous object. it attracts the astronomer's attention; he looks carefully, and he sees how it lies among the constellations with which he is familiar. a few nights later he observes the same body again; but is it exactly in the same place? he thinks not. he notes more carefully than before the place of the planet. he sees how it is situated with regard to the stars. again, in a few days, his observations are repeated. there is no longer a trace of doubt about the matter--mars has decidedly changed his position. it is veritably a wanderer. night after night the primitive astronomer is at his post. he notes the changes of mars. he sees that it is now moving even more rapidly than it was at first. is it going to complete the circuit of the heavens? the astronomer determines to watch the orb and see whether this surmise is justified. he pursues his task night after night, and at length he begins to think that the body is not moving quite so rapidly as at first. a few nights more, and he is sure of the fact: the planet is moving more slowly. again a few nights more, and he begins to surmise that the motion may cease; after a short time the motion does cease, and the object seems to rest; but is it going to remain at rest for ever? has its long journey been finished? for many nights this seems to be the case, but at length the astronomer suspects that the planet must be commencing to move backwards. a few nights more, and the fact is confirmed beyond possibility of doubt, and the extraordinary discovery of the direct and the retrograde movement of mars has been accomplished. [illustration: fig. .--the apparent movements of mars in .] in the greater part of its journey around the heavens mars seems to move steadily from the west to the east. it moves backwards, in fact, as the moon moves and as the sun moves. it is only during a comparatively small part of its path that those elaborate movements are accomplished which presented such an enigma to the primitive observer. we show in the adjoining picture (fig. ) the track of the actual journey which mars accomplished in the opposition of . the figure only shows that part of its path which presents the anomalous features; the rest of the orbit is pursued, not indeed with uniform velocity, but with unaltered direction. this complexity of the apparent movements of mars seems at first sight fatal to the acceptance of any simple and elementary explanation of the planetary motion. if the motion of mars were purely elliptic, how, it may well be said, could it perform this extraordinary evolution? the elucidation is to be found in the fact that the earth on which we stand is itself in motion. even if mars were at rest, the fact that the earth moves would make the planet appear to move. the apparent movements of mars are thus combined with the real movements. this circumstance will not embarrass the geometer. he is able to disentangle the true movement of the planet from its association with the apparent movement, and to account completely for the complicated evolutions exhibited by mars. could we transfer our point of view from the ever-shifting earth to an immovable standpoint, we should then see that the shape of the orbit of mars was an ellipse, described around the sun in conformity with the laws which kepler discovered by observations of this planet. mars takes days to travel round the sun, its average distance from that body being , , miles. under the most favourable circumstances the planet, at the time of opposition, may approach the earth to a distance not greater than about , , miles. no doubt this seems an enormous distance, when estimated by any standard adapted for terrestrial measurements; it is, however, hardly greater than the distance of venus when nearest, and it is much less than the distance from the earth to the sun. we have explained how the _form_ of the solar system is known from kepler's laws, and how the absolute size of the system and of its various parts can be known when the direct measurement of any one part has been accomplished. a close approach of mars affords a favourable opportunity for measuring his distance, and thus, in a different way, solving the same problem as that investigated by the transit of venus. we are thus led a second time to a knowledge of the distance of the sun and the distances of the planets generally, and to many other numerical facts about the solar system. on the occasion of the opposition of mars in a successful attempt was made to apply this refined process to the solution of the problem of celestial measurement. it cannot be said to have been the first occasion on which this method was suggested, or even practically attempted. the observations of were, however, conducted with such skill and with such minute attention to the necessary precautions as to render them an important contribution to astronomy. dr. david gill, now her majesty's astronomer at the cape of good hope, undertook a journey to the island of ascension for the purpose of observing the parallax of mars in . on this occasion mars approached to the earth so closely as to afford an admirable opportunity for the application of the method. dr. gill succeeded in obtaining a valuable series of measurements, and from them he concluded the distance of the sun with an accuracy somewhat superior to that attainable by the transit of venus. there is yet another method by which mars can be made to give us information as to the distance of the sun. this method is one of some delicacy, and is interesting from its connection with the loftiest enquiries in mathematical astronomy. it was foreshadowed in the dynamical theory of newton, and was wrought to perfection by le verrier. it is based upon the great law of gravitation, and is intimately associated with the splendid discoveries in planetary perturbation which form so striking a chapter in modern astronomical discovery. there is a certain relation between two quantities which at first sight seems quite independent. these quantities are the mass of the earth and the distance of the sun. the distance of the sun bears to a certain distance (which can be calculated when we know the intensity of gravitation at the earth's surface, the size of the earth and the length of the year) the same proportion that the cube root of the sun's mass bears to the cube root of that of the earth. there is no uncertainty about this result, and the consequence is obvious. if we have the means of weighing the earth in comparison with the sun, then the distance of the sun can be immediately deduced. how are we to place our great earth in the weighing scales? this is the problem which le verrier has shown us how to solve, and he does so by invoking the aid of the planet mars. if mars in his revolution around the sun were solely swayed by the attraction of the sun, he would, in accordance with the well-known laws of planetary motion, follow for ever the same elliptic path. at the end of one century, or even of many centuries, the shape, the size, and the position of that ellipse would remain unaltered. fortunately for our present purpose, a disturbance in the orbit of mars is produced by the earth. although the mass of our globe is so much less than that of the sun, yet the earth is still large enough to exercise an appreciable attraction on mars. the ellipse described by the planet is consequently not invariable. the shape of that ellipse and its position gradually change, so that the position of the planet depends to some extent upon the mass of the earth. the place in which the planet is found can be determined by observation; the place which the planet would have had if the earth were absent can be found by calculation. the difference between the two is due to the attraction of the earth, and, when it has been measured, the mass of the earth can be ascertained. the amount of displacement increases from one century to another, but as the rate of growth is small, ancient observations are necessary to enable the measures to be made with accuracy. a remarkable occurrence which took place more than two centuries ago fortunately enables the place of mars to be determined with great precision at that date. on the st of october, , three independent observers witnessed the occultation of a star in aquarius by the ruddy planet. the place of the star is known with accuracy, and hence we are provided with the means of indicating the exact point in the heavens occupied by mars on the day in question. from this result, combined with the modern meridian observations, we learn that the displacement of mars by the attraction of the earth has, in the lapse of two centuries, grown to about five minutes of arc ( seconds). it has been maintained that this cannot be erroneous to the extent of more than a second, and hence it would follow that the earth's mass is determined to about one three-hundredth part of its amount. if no other error were present, this would give the sun's distance to about one nine-hundredth part. [illustration: fig. .--relative sizes of mars and the earth.] notwithstanding the intrinsic beauty of this method, and the very high auspices under which it has been introduced, it is, we think, at present hardly worthy of reliance in comparison with some of the other methods. as the displacement of mars, due to the perturbing influence of the earth, goes on increasing continually, it will ultimately attain sufficient magnitude to give a very exact value of the earth's mass, and then this method will give us the distance of the sun with great precision. but interesting and beautiful though this method may be, we must as yet rather regard it as a striking confirmation of the law of gravitation than as affording an accurate means of measuring the sun's distance. [illustration: fig. .--drawing of mars (july th, ).] [illustration: fig. .--drawing of mars (august th, ).] [illustration: fig. .--elevations and depressions on the "terminator" of mars (august th, ).] [illustration: fig. .--the southern polar cap on mars (july , ).] the close approaches of mars to the earth afford us opportunities for making a careful telescopic scrutiny of his surface. it must not be expected that the details on mars could be inspected with the same minuteness as those on the moon. even under the most favourable circumstances, mars is still more than a hundred times as far as the moon, and, therefore, the features of the planet have to be at least one hundred times as large if they are to be seen as distinctly as the features on the moon. mars is much smaller than the earth. the diameter of the planet is , miles, but little more than half that of the earth. fig. shows the comparative sizes of the two bodies. we here reproduce two of the remarkable drawings[ ] of mars made by professor william h. pickering at the lowell observatory, flagstaff a.t. fig. was taken on the th of july, , and fig. on the th of august, . the southern polar cap on mars, as seen by professor william h. pickering at lowell observatory on the st of july, , is represented in fig. .[ ] the remarkable black mark intruding into the polar area will be noticed. in fig. are shown a series of unusually marked elevations and depressions upon the "terminator" of the planet, drawn as accurately as possible to scale by the same skilful hand on the th of august, . in making an examination of the planet it is to be observed that it does not, like the moon, always present the same face towards the observer. mars rotates upon an axis in exactly the same manner as the earth. it is not a little remarkable that the period required by mars for the completion of one rotation should be only about half an hour greater than the period of rotation of the earth. the exact period is hours, minutes, - / seconds. it therefore follows that the aspect of the planet changes from hour to hour. the western side gradually sinks from view, the eastern side gradually assumes prominence. in twelve hours the aspect of the planet is completely changed. these changes, together with the inevitable effects of foreshortening, render it often difficult to correlate the objects on the planet with those on the maps. the latter, it must be confessed, fall short of the maps of the moon in definiteness and in certainty; yet there is no doubt that the main features of the planet are to be regarded as thoroughly established, and some astronomers have given names to all the prominent objects. the markings on the surface of mars are of two classes. some of them are of an iron-grey hue verging on green, while the others are generally dark yellow or orange, occasionally verging on white. the former have usually been supposed to represent the tracts of ocean, the latter the continental masses on the ruddy planet. we possess a great number of drawings of mars, the earliest being taken in the middle of the seventeenth century. though these early sketches are very rough, and are not of much value for the solution of questions of topography, they have been found very useful in aiding us to fix the period of rotation of the planet on its axis by comparison with our modern drawings. early observers had already noticed that each of the poles of mars is distinguished by a white spot. it is, however, to william herschel that we owe the first systematic study of these remarkable polar caps. this illustrious astronomer was rewarded by a very interesting discovery. he found that these arctic tracts on mars vary both in extent and distinctness with the seasons of the hemisphere on which they are situated. they attain a maximum development from three to six months after the winter solstice on that planet, and then diminish until they are smallest about three to six months after the summer solstice. the analogy with the behaviour of the masses of snow and ice which surround our own poles is complete, and there has until lately been hardly any doubt that the white polar spots of mars are somewhat similarly constituted. as the period of revolution of mars around the sun is so much longer than our year, days instead of , the seasons of the planet are, of course, also much longer than the terrestrial seasons. in the northern hemisphere of mars the summer lasts for no fewer than days, and the winter must be days. in both hemispheres the white polar cap in the course of the long winter season increases until it reaches a diameter of ° to °, while the long summer reduces it to a small area only ° or ° in diameter. it is remarkable that one of these white regions--that at the south pole--seems not to be concentric with the pole, but is placed so much to one side that the south pole of mars appears to be quite free from ice or snow once a year. although many valuable observations of mars were made in the course of the nineteenth century, it is only since the very favourable opposition of that the study of the surface of mars has made that immense progress which is one of the most remarkable features of modern astronomy. among the observers who produced valuable drawings of the planet in we may mention mr. green, whose exquisite pictures were published by the royal astronomical society, and professor schiaparelli, of milan, who almost revolutionised our knowledge of this planet. schiaparelli had a refractor of only eight inches aperture at his disposal, but he was doubtless much favoured by the purity of the italian sky, which enabled him to detect in the bright portions of the surface of mars a considerable number of long, narrow lines. to these he gave the name of canals, inasmuch as they issued from the so-called oceans, and could be traced across the reputed continents for considerable distances, which sometimes reached thousands of miles. the canals seemed to form a kind of network, which connected the various seas with each other. a few of the more conspicuous of these so-called canals appeared indeed on some of the drawings made by dawes and others before schiaparelli's time. it was, however, the illustrious italian astronomer who detected that these narrow lines are present in such great numbers as to form a notable feature of the planet. some of these remarkable features are shown in figs. and , which are copied from drawings made by professor william h. pickering at the lowell observatory in . great as had been the surprise of astronomers when schiaparelli first proclaimed the discovery of these numerous canals, it was, perhaps, surpassed by the astonishment with which his announcement was received in that most of the canals had become double. between december, , and february, , thirty of these duplications appear to have taken place. nineteen of these were cases of a well-traced parallel line being formed near a previously existing canal. the remaining canals were less certainly established, or were cases where the two lines did not seem to be quite parallel. a copy of the map of mars which schiaparelli formed from his observations of - is given in plate xviii. it brings out clearly these strange double canals, so unlike any features that we know on any other globe. [illustration: plate xviii. schiaparelli's map of mars in - .] subsequent observations by schiaparelli and several other observers seem to indicate that this phenomenon of the duplication of the canals is of a periodic character. it is produced about the times when mars passes through its equinoxes. one of the two parallel lines is often superposed as exactly as possible upon the track of the old canal. it does, however, sometimes happen that both the lines occupy opposite sides of the former canal and are situated on entirely new ground. the distance between the two lines varies from about miles as a maximum down to the smallest limit distinguishable in our large telescopes, which is something less than thirty miles. the breadth of each of these remarkable channels may range from the limits of visibility, say, up to more than sixty miles. the duplication of the canals is perhaps the most difficult problem which mars offers to us for solution. even if we admit that the canals themselves represent inlets or channels through which the melted polar snow makes its way across the equatorial continents, it is not easy to see how the duplicate canals can arise. this is especially true in those cases where the original channel seems to vanish and to be replaced by two quite new canals, each about the breadth of the english channel, and lying one on each side of the course of the old one. the very obvious explanation that the whole duplication is an optical illusion has been brought forward more than once, but never in a conclusive manner. we must, perhaps, be content to let the solution of this matter rest for the present, in the hope that the extraordinary attention which this planet is now receiving will in due time explain the present enigma. the markings on the surface of this planet are, generally speaking, of a permanent character, so that when we compare drawings made one or two hundred years ago with drawings made more recently we can recognise in each the same features. this permanence is, however, not nearly so absolute as it is in the case of the moon. in addition to the canals which we have already considered, many other parts of the surface of mars alter their outlines from time to time. this is particularly the case with those dark spots which we call oceans, the contours of which sometimes undergo modifications in matters of detail which are quite unmistakable. changes of colour are often observed on parts of the planet, and though some of these observations may perhaps be attributed to the influence of our own atmosphere on the planet's appearance, they cannot be all thus accounted for. some of the phenomena must certainly be due to actual changes which have taken place on the surface of mars. as an example of such changes, we may refer to the north-western part of the notable feature, to which schiaparelli has given the name of _syrtis major_.[ ] this has at various times been recorded as grey, green, blue, brown, and even violet. when this region (about the time of the autumnal equinox of the northern hemisphere) is situated in the middle of the visible disc, the eastern part is distinctly greener than the western. as the season progresses this characteristic colour gets feebler, until the green tint is to be perceived only on the shores of the syrtis. the atmosphere of mars is usually very transparent, and fortunately allows us to scrutinise the surface of the planet without putting obstacles in the way m the shape of martian clouds. such clouds, however, are not invariably absent. our view of the surface is occasionally obstructed in such a manner as to make it certain that clouds or mist in the atmosphere of mars must be the cause of the trouble. would we form an idea of the physical constitution of the surface of mars, then the question as to the character of the atmosphere of the planet is among the first to be considered. spectroscopic observations do not in this case render us much assistance. of course, we know that the planet has no intrinsic light. it merely shines by reflected sunlight. the hemisphere which is turned towards the sun is bright, and the hemisphere which is turned away from the sun is dark. the spectrum ought, therefore, like that of the moon, to be an exact though faint copy of the solar spectrum, unless the sun's rays, by passing twice through the atmosphere of mars, suffered some absorption which could give rise to additional dark lines. some of the earlier observers thought that they could distinctly make out some such lines due, as was supposed, to water vapour. the presence of such lines is, however, denied by mr. campbell, of the lick observatory, and professor keeler, at the allegheny observatory,[ ] who, with their unrivalled opportunities, both instrumental and climatic, could find no difference between the spectra of mars and the moon. if mars had an atmosphere of appreciable extent, its absorptive effect should be noticeable, especially at the limb of the planet; but mr. campbell's observations do not show any increased absorption at the limb. it would therefore seem that mars cannot have an extensive atmosphere, and this conclusion is confirmed in several other ways. the distinctness with which we see the surface of this planet tends to show that the atmosphere must be very thin as compared with our own. there can hardly be any doubt that an observer on mars with a good telescope would be unable to distinguish much of the features of the earth's surface. this would be the case not only by reason of the strong absorption of the light during the double passage through our atmosphere, but also on account of the great diffusion of the light caused by this same atmosphere. also, it is needless to say, the great amount of cloud generally floating over the earth would totally obscure many parts of our planet from a martian observer. but though, as already mentioned, we occasionally find parts of mars rendered indistinct, it must be acknowledged that the clouds on mars are very slight. we should expect that the polar caps, if composed of snow, would, when melting, produce clouds which would more or less hide the polar regions from our inspection; yet nothing of the kind has ever been seen. we have seen that there are very grave doubts as to the existence of water on mars. no doubt we have frequently spoken of the dark markings as "oceans" and of the bright parts as "continents." that this language was just has been the opinion of astronomers for a very long time. a few years ago mr. schaeberle, of the lick observatory, came to the very opposite conclusion. he contended that the dark parts were the continents and the bright ones were the oceans of water, or some other fluid. he pointed to the irregular shading of the dark parts, which does not suggest the idea of light reflected from a spherical surface of water, especially as the contrasts between light and shade are strongest about the middle of the disc. it is also to be noticed that the dark regions are not infrequently traversed by still darker streaks, which can be traced for hundreds of miles almost in straight lines, while the so-called canals in the bright parts often seem to be continuations of these same lines. mr. schaeberle therefore suggests that the canals may be chains of mountains stretching over sea and land! the late professor phillips and mr. h.d. taylor have pointed out that if there were lakes or seas in the tropical regions of mars we should frequently see the sun directly reflected from them, thus producing a bright, star-like point which could not escape observation. even moderately disturbed water would make its presence known in this manner, and yet nothing of the kind has ever been recorded. on the question as to the possibility of life on mars a few words may be added. if we could be certain of the existence of water on mars, then one of the fundamental conditions would be fulfilled; and even though the atmosphere on mars had but few points of resemblance either in composition or in density to the atmosphere of the earth, life might still be possible. even if we could suppose that a man would find suitable nutriment for his body and suitable air for his respiration, it seems very doubtful whether he would be able to live. owing to the small size of mars and the smallness of its mass in comparison with the earth, the intensity of the gravitation on the neighbouring planet would be different from the attraction on the surface of the earth. we have already alluded to the small gravitation on the moon, and in a lesser degree the same remarks will apply to mars. a body which weighs on the earth two pounds would on the surface of mars weigh rather less than one pound. nearly the same exertion which will raise a -lb. weight on the earth would lift two similar weights on mars. the earth is attended by one moon. jupiter is attended by four conspicuous moons. mars is a planet revolving between the orbits of the earth and of jupiter. it is a body of the same general type as the earth and jupiter. it is ruled by the same sun, and all three planets form part of the same system; but as the earth has one moon and jupiter four moons, why should not mars also have a moon? no doubt mars is a small body, less even than the earth, and much less than jupiter. we could not expect mars to have large moons, but why should it be unlike its two neighbours, and not have any moon at all? so reasoned astronomers, but until modern times no satellite of mars could be found. for centuries the planet has been diligently examined with this special object, and as failure after failure came to be recorded, the conclusion seemed almost to be justified that the chain of analogical reasoning had broken down. the moonless mars was thought to be an exception to the rule that all the great planets outside venus were dignified by an attendant retinue of satellites. it seemed almost hopeless to begin again a research which had often been tried, and had invariably led to disappointment; yet, fortunately, the present generation has witnessed still one more attack, conducted with perfect equipment and with consummate skill this attempt has obtained the success it so well merited, and the result has been the memorable detection of two satellites of mars. this discovery was made by professor asaph hall, the distinguished astronomer at the observatory of washington. mr. hall was provided with an instrument of colossal proportions and of exquisite workmanship, known as the great washington refractor. it is the product of the celebrated workshop of messrs. alvan clark and sons, from which so many large telescopes have proceeded, and in its noble proportions far surpassed any other telescope ever devoted to the same research. the object-glass measures twenty-six inches in diameter, and is hardly less remarkable for the perfection of its definition than for its size. but even the skill of mr. hall, and the space-penetrating power of his telescope, would not have been able on ordinary occasions to discover the satellites of mars. advantage was accordingly taken of that memorable opposition of mars in , when, as we have already described, the planet came unusually near the earth. had mars been attended by a moon one-hundredth part of the bulk of our moon it must long ago have been discovered. mr. hall, therefore, knew that if there were any satellites they must be extremely small bodies, and he braced himself for a severe and diligent search. the circumstances were all favourable. not only was mars as near as it well could be to the earth; not only was the great telescope at washington the most powerful refractor then in existence; but the situation of washington is such that mars was seen from the observatory at a high elevation. it was while the british association were meeting at plymouth, in , that a telegram flashed across the atlantic. brilliant success had rewarded mr. hall's efforts. he had hoped to discover one satellite. the discovery of even one would have made the whole scientific world ring; but fortune smiled on mr. hall. he discovered first one satellite, and then he discovered a second; and, in connection with these satellites, he further discovered a unique fact in the solar system. deimos, the outer of the satellites, revolves around the planet in the period of hours, mins., secs.; it is the inner satellite, phobos, which has commanded the more special attention of every astronomer in the world. mars turns round on his axis in a martial day, which is very nearly the same length as our day of twenty-four hours. the inner satellite of mars moves round in hours, mins., secs. phobos, in fact, revolves three times round mars in the same time that mars can turn round once. this circumstance is unparalleled in the solar system; indeed, as far as we know, it is unparalleled in the universe. in the case of our own planet, the earth rotates twenty-seven times for one revolution of the moon. to some extent the same may be said of jupiter and of saturn; while in the great system of the sun himself and the planets, the sun rotates on his axis several times for each revolution of even the most rapidly moving of the planets. there is no other known case where the satellite revolves around the primary more quickly than the primary rotates on its axis. the anomalous movement of the satellite of mars has, however, been accounted for. in a subsequent chapter we shall again allude to this, as it is connected with an important department of modern astronomy. the satellites are so small that we are unable to measure their diameters directly, but from observations of their brightness it is evident that their diameters cannot exceed twenty or thirty miles, and may be even smaller. owing to their rapid motion the two satellites must present some remarkable peculiarities to an observer on mars. phobos rises in the west, passes across the heavens, and sets in the east after about five and a half hours, while deimos rises in the east and remains more than two days above the horizon. as the satellites revolve in paths vertically above the equator of their primary, the one less than , miles and the other only some , miles above the surface, it follows that they can never be visible from the poles of mars; indeed, to see phobos, the observer's planetary latitude must not be above - / °. if it were so, the satellite would be hidden by the body of mars, just as we, in the british islands, would be unable to see an object revolving round the earth a few hundred miles above the equator. before passing from the attractive subject of the satellites, we may just mention two points of a literary character. mr. hall consulted his classical friends as to the designation to be conferred on the two satellites. homer was referred to, and a passage in the "iliad" suggested the names of deimos and phobos. these personages were the attendants of mars, and the lines in which they occur have been thus construed by my friend professor tyrrell:-- "mars spake, and called dismay and rout to yoke his steeds, and he did on his harness sheen." a curious circumstance with respect to the satellites of mars will be familiar to those who are acquainted with "gulliver's travels." the astronomers on board the flying island of laputa had, according to gulliver, keen vision and good telescopes. the traveller says that they had found two satellites to mars, one of which revolved around him in ten hours, and the other in twenty-one and a half. the author has thus not only made a correct guess about the number of the satellites, but he actually stated the periodic time with considerable accuracy! we do not know what can have suggested the latter guess. a few years ago any astronomer reading the voyage to laputa would have said this was absurd. there might be two satellites to mars, no doubt; but to say that one of them revolves in ten hours would be to assert what no one could believe. yet the truth has been even stranger than the fiction. and now we must bring to a close our account of this beautiful and interesting planet. there are many additional features over which we are tempted to linger, but so many other bodies claim our attention in the solar system, so many other bodies which exceed mars in size and intrinsic importance, that we are obliged to desist. our next step will not, however, at once conduct us to the giant planets. we find outside mars a host of objects, small indeed, but of much interest; and with these we shall find abundant occupation for the following chapter. chapter xi. the minor planets. the lesser members of our system--bode's law--the vacant region in the planetary system--the research--the discovery of piazzi--was the small body a planet?--the planet becomes invisible--gauss undertakes the search by mathematics--the planet recovered--further discoveries--number of minor planets now known--the region to be searched--the construction of the chart for the search for small planets--how a minor planet is discovered--physical nature of the minor planets--small gravitation on the minor planets--the berlin computations--how the minor planets tell us the distance of the sun--accuracy of the observations--how they may be multiplied--victoria and sappho--the most perfect method. in our chapters on the sun and moon, on the earth and venus, and on mercury and mars, we have been discussing the features and the movements of globes of vast dimensions. the least of all these bodies is the moon, but even that globe is , miles from one side to the other. in approaching the subject of the minor planets we must be prepared to find objects of dimensions quite inconsiderable in comparison with the great spheres of our system. no doubt these minor planets are all of them some few miles, and some of them a great many miles, in diameter. were they close to the earth they would be conspicuous, and even splendid, objects; but as they are so distant they do not, even in our greatest telescopes, become very remarkable, while to the unaided eye they are almost all invisible. in the diagram (p. ) of the orbits of the various planets, it is shown that a wide space exists between the orbit of mars and that of jupiter. it was often surmised that this ample region must be tenanted by some other planet. the presumption became much stronger when a remarkable law was discovered which exhibited, with considerable accuracy, the relative distances of the great planets of our system. take the series of numbers, , , , , , , , whereof each number (except the second) is double of the number which precedes it. if we now add four to each, we have the series , , , , , , . with the exception of the fifth of these numbers ( ), they are all sensibly proportional to the distances of the various planets from the sun. in fact, the distances are as follows:--mercury, · ; venus, · ; earth, ; mars, · ; jupiter, · ; saturn, · . although we have no physical reason to offer why this law--generally known as bode's--should be true, yet the fact that it is so nearly true in the case of all the known planets tempts us to ask whether there may not also be a planet revolving around the sun at the distance represented by . so strongly was this felt at the end of the eighteenth century that some energetic astronomers decided to make a united effort to search for the unknown planet. it seemed certain that the planet could not be a large one, as otherwise it must have been found long ago. if it should exist, then means were required for discriminating between the planet and the hosts of stars strewn along its path. the search for the small planet was soon rewarded by a success which has rendered the evening of the first day in the nineteenth century memorable in astronomy. it was in the pure skies of palermo that the observatory was situated where the memorable discovery of the first known minor planet was made by piazzi. this laborious and accomplished astronomer had organised an ingenious system of exploring the heavens which was eminently calculated to discriminate a planet among the starry host. on a certain night he would select a series of stars to the number of fifty, more or less, according to circumstances. with his meridian circle he determined the places of the chosen objects. the following night, or, at all events, as soon as convenient, he re-observed the whole fifty stars with the same instrument and in the same manner, and the whole operation was afterwards repeated on two, or perhaps more, nights. when the observations were compared together he was in possession of some four or more places of each one of the stars on different nights, and the whole series was complete. he was persevering enough to carry on these observations for very many groups, and at length he was rewarded by a success which amply compensated him for all his toil. it was on the st of january, , that piazzi commenced for the one hundred and fifty-ninth time to observe a new series. fifty stars this night were viewed in his telescope, and their places were carefully recorded. of these objects the first twelve were undoubtedly stellar, and so to all appearance was the thirteenth, a star of the eighth magnitude in the constellation of taurus. there was nothing to distinguish the telescopic appearance of this object from all the others which preceded or followed it. the following night piazzi, according to his custom, re-observed the whole fifty stars, and he did the same again on the rd of january, and once again on the th. he then, as usual, brought together the four places he had found for each of the several bodies. when this was done it was at once seen that the thirteenth object on the list was quite a different body from the remainder and from all the other stars which he had ever observed before. the four places of this mysterious object were all different; in other words, it was in movement, and was therefore a planet. a few days' observation sufficed to show how this little body, afterwards called ceres, revolved around the sun, and how it circulated in that vacant path intermediate between the path of mars and the path of jupiter. great, indeed, was the interest aroused by this discovery and the influence which it has exercised on the progress of astronomy. the majestic planets of our system had now to admit a much more humble object to a share of the benefits dispensed by the sun. after piazzi had obtained a few further observations, the season for observing this part of the heavens passed away, and the new planet of course ceased to be visible. in a few months, no doubt, the same part of the sky would again be above the horizon after dark, and the stars would of course be seen as before. the planet, however, was moving, and would continue to move, and by the time the next season had arrived it would have passed off into some distant region, and would be again confounded with the stars which it so closely resembled. how, then, was the planet to be pursued through its period of invisibility and identified when it again came within reach of observation? this difficulty attracted the attention of astronomers, and they sought for some method by which the place of the planet could be recovered so as to prevent piazzi's discovery from falling into oblivion. a young german mathematician, whose name was gauss, opened his distinguished career by a successful attempt to solve this problem. a planet, as we have shown, describes an ellipse around the sun, and the sun lies at a focus of that curve. it can be demonstrated that when three positions of a planet are known, then the ellipse in which the planet moves is completely determined. piazzi had on each occasion measured the place which it then occupied. this information was available to gauss, and the problem which he had to solve may be thus stated. knowing the place of the planet on three nights, it is required, without any further observations, to tell what the place of the planet will be on a special occasion some months in the future. mathematical calculations, based on the laws of kepler, will enable this problem to be solved, and gauss succeeded in solving it. gauss demonstrated that though the telescope of the astronomer was unable to detect the wanderer during its season of invisibility, yet the pen of the mathematician could follow it with unfailing certainty. when, therefore, the progress of the seasons permitted the observations to be renewed, the search was recommenced. the telescope was directed to the point which gauss's calculations indicated, and there was the little ceres. ever since its re-discovery, the planet has been so completely bound in the toils of mathematical reasoning that its place every night of the year can be indicated with a fidelity approaching to that attainable in observing the moon or the great planets of our system. the discovery of one minor planet was quickly followed by similar successes, so that within seven years pallas, juno, and vesta were added to the solar system. the orbits of all these bodies lie in the region between the orbit of mars and of jupiter, and for many years it seems to have been thought that our planetary system was now complete. forty years later systematic research was again commenced. planet after planet was added to the list; gradually the discoveries became a stream of increasing volume, until in the total number reached about . their distribution in the solar system is somewhat as represented in fig. . by the improvement of astronomical telescopes, and by the devotion with which certain astronomers have applied themselves to this interesting research, a special method of observing has been created for the distinct purpose of searching out these little objects. it is known that the paths in which all the great planets move through the heavens coincide very nearly with the path which the sun appears to follow among the stars, and which is known as the ecliptic. it is natural to assume that the small planets also move in the same great highway, which leads them through all the signs of the zodiac in succession. some of the small planets, no doubt, deviate rather widely from the track of the sun, but the great majority are approximately near it. this consideration at once simplifies the search for new planets. a certain zone extending around the heavens is to be examined, but there is in general little advantage in pushing the research into other parts of the sky. the next step is to construct a map containing all the stars in this region. this is a task of very great labour; the stars visible in the large telescopes are so numerous that many tens of thousands, perhaps we should say hundreds of thousands, are included in the region so narrowly limited. the fact is that many of the minor planets now known are objects of extreme minuteness; they can only be seen with very powerful telescopes, and for their detection it is necessary to use charts on which even the faintest stars have been depicted. many astronomers have concurred in the labour of producing these charts; among them may be mentioned palisa, of vienna, who by means of his charts has found eighty-three minor planets, and the late professor peters, of clinton, new york, who in a similar way found forty-nine of these bodies. [illustration: fig. .--the zone of minor planets between mars and jupiter.] the astronomer about to seek for a new planet directs his telescope towards that part of the sun's path which is on the meridian at midnight; there, if anywhere, lies the chance of success, because that is the region in which such a body is nearer to the earth than at any other part of its course. he steadfastly compares his chart with the heavens, and usually finds the stars in the heavens and the stars in the chart to correspond; but sometimes it will happen that a point in the heavens is missing from the chart. his attention is at once arrested; he follows the object with care, and if it moves it is a planet. still he cannot be sure that he has really made a discovery; he has found a planet, no doubt, but it may be one of the large number already known. to clear up this point he must undertake a further, and sometimes a very laborious, enquiry; he must search the berlin year-book and other ephemerides of such planets and see whether it is possible for one of them to have been in the position on the night in question. if he can ascertain that no previously discovered body could have been there, he is then entitled to announce to his brother astronomers the discovery of a new member of the solar system. it seems certain that all the more important of the minor planets have been long since discovered. the recent additions to the list are generally extremely minute objects, beyond the powers of small telescopes. since the method of searching for minor planets which we have just described has been almost abandoned in favour of a process greatly superior. it has been found feasible to employ photography for making charts of the heavens. a photographic plate is exposed in the telescope to a certain region of the sky sufficiently long to enable very faint telescopic stars to imprint their images. care has to be taken that the clock which moves the camera shall keep pace most accurately with the rotation of the earth, so that fixed stars appear on the plate as sharp points. if, on developing the plate, a star is found to have left a trail, it is evident that this star must during the time of exposure (generally some hours) have had an independent motion of its own; in other words, it must be a planet. for greater security a second picture is generally taken of the same region after a short interval. if the place occupied by the trail on the first plate is now vacant, while on the second plate a new trail appears in a line with the first one, there remains no possible doubt that we have genuine indications of a planet, and that we have not been led astray by some impurity on the plate or by a few minute stars which happened to lie very closely together. wolf, of heidelberg, and following in his footsteps charlois, of nice, have in this manner discovered a great number of new minor planets, while they have also recovered a good many of those which had been lost sight of owing to an insufficiency of observations. on the th of august, , herr g. witt, of the observatory of urania in berlin, discovered a new asteroid by the photographic method. this object was at first regarded merely as forming an addition of no special importance to the asteroids whose discovery had preceded it. it received, as usual, a provisional designation in accordance with a simple alphabetical device. this temporary label affixed to witt's asteroid was "d q." but the formal naming of the asteroid has now superseded this label. herr witt has given to his asteroid the name of "eros." this has been duly accepted by astronomers, and thus for all time the planet is to be known. the feature which makes the discovery of eros one of the most remarkable incidents in recent astronomy is that on those rare occasions when this asteroid comes nearest to the earth it is closer to the earth than the planet mars can ever be. closer than the planet venus can ever be. closer than any other known asteroid can ever be. thus we assign to eros the exceptional position of being our nearest planetary neighbour in the whole host of heaven. under certain circumstances it will have a distance from the earth not exceeding one-seventh of the mean distance of the sun. of the physical composition of the asteroids and of the character of their surfaces we are entirely ignorant. it may be, for anything we can tell, that these planets are globes like our earth in miniature, diversified by continents and by oceans. if there be life on such bodies, which are often only a few miles in diameter, that life must be something totally different from anything with which we are familiar. setting aside every other difficulty arising from the possible absence of water and from the great improbability of finding there an atmosphere of a density and a composition suitable for respiration, gravitation itself would prohibit organic beings adapted for this earth from residing on a minor planet. let us attempt to illustrate this point, and suppose that we take the case of a minor planet eight miles in diameter, or, in round numbers, one-thousandth part of the diameter of the earth. if we further suppose that the materials of the planet are of the same nature as the substances in the earth, it is easy to prove that the gravity on the surface of the planet will be only one-thousandth part of the gravity of the earth. it follows that the weight of an object on the earth would be reduced to the thousandth part if that object were transferred to the planet. this would not be disclosed by an ordinary weighing scales, where the weights are to be placed in one pan and the body to be weighed in the other. tested in this way, a body would, of course, weigh precisely the same anywhere; for if the gravitation of the body is altered, so is also in equal proportion the gravitation of the counterpoising weights. but, weighed with a spring balance, the change would be at once evident, and the effort with which a weight could be raised would be reduced to one-thousandth part. a load of one thousand pounds could be lifted from the surface of the planet by the same effort which would lift one pound on the earth; the effects which this would produce are very remarkable. in our description of the moon it was mentioned (p. ) that we can calculate the velocity with which it would be necessary to discharge a projectile so that it would never again fall back on the globe from which it was expelled. we applied this reasoning to explain why the moon has apparently altogether lost any atmosphere it might have once possessed. if we assume for the sake of illustration that the densities of all planets are identical, then the law which expresses the critical velocity for each planet can be readily stated. it is, in fact, simply proportional to the diameter of the globe in question. thus, for a minor planet whose diameter was one-thousandth part of that of the earth, or about eight miles, the critical velocity would be the thousandth part of six miles a second--that is, about thirty feet per second. this is a low velocity compared with ordinary standards. a child easily tosses a ball up fifteen or sixteen feet high, yet to carry it up this height it must be projected with a velocity of thirty feet per second. a child, standing upon a planet eight miles in diameter, throws his ball vertically upwards; up and up the ball will soar to an amazing elevation. if the original velocity were less than thirty feet per second, the ball would at length cease to move, would begin to turn, and fall with a gradually accelerating pace, until at length it regained the surface with a speed equal to that with which it had been projected. if the original velocity had been as much as, or more than, thirty feet per second, then the ball would soar up and up never to return. in a future chapter it will be necessary to refer again to this subject. a few of the minor planets appear in powerful telescopes as discs with appreciable dimensions, and they have even been measured with the micrometer. in this way professor barnard, late of the lick observatory, determined the following values for the diameters of the four first discovered minor planets:-- ceres miles. pallas miles. juno miles. vesta miles. the value for juno is, however, very uncertain, and by far the greater number of the minor planets are very much smaller than the figures here given would indicate. it is possible by a certain calculation to form an estimate of the aggregate mass of all the minor planets, inasmuch as observations disclose to us the extent of their united disturbing influences on the motion of mars. in this manner le verrier concluded that the collected mass of the small planets must be about equal to one-fourth of the mass of the earth. harzer, repeating the enquiry in an improved manner, deduced a collected mass one-sixth of that of the earth. there can be no doubt that the total mass of all the minor planets at present known is not more than a very small fraction of the amount to which these calculations point. we therefore conclude that there must be a vast number of minor planets which have not yet been recognised in the observatory. these unknown planets must be extremely minute. the orbits of this group of bodies differ in remarkable characteristics from those of the larger planets. some of them are inclined at angles of ° to the plane of the earth's orbit, the inclinations of the great planets being not more than a few degrees. some of the orbits of the minor planets are also greatly elongated ellipses, while, of course, the orbits of the large planets do not much depart from the circular form. the periods of revolution of these small objects round the sun range from three years to nearly nine years. a great increase in the number of minor planets has rewarded the zeal of those astronomers who have devoted their labours to this subject. their success has entailed a vast amount of labour on the computers of the "berlin year-book." that useful work occupies in this respect a position which has not been taken by our own "nautical almanac," nor by the similar publications of other countries. a skilful band of computers make it their duty to provide for the "berlin year-book" detailed information as to the movements of the minor planets. as soon as a few complete observations have been obtained, the little object passes into the secure grasp of the mathematician; he is able to predict its career for years to come, and the announcements with respect to all the known minor planets are to be found in the annual volumes of the work referred to. the growth of discovery has been so rapid that the necessary labour for the preparation of such predictions is now enormous. it must be confessed that many of the minor planets are very faint and otherwise devoid of interest, so that astronomers are sometimes tempted to concur with the suggestion that a portion of the astronomical labour now devoted to the computation of the paths of these bodies might be more profitably applied. for this it would be only necessary to cast adrift all the less interesting members of the host, and allow them to pursue their paths unwatched by the telescope, or by the still more ceaseless tables of the mathematical computer. the sun, which controls the mighty orbs of our system, does not disdain to guide, with equal care, the tiny globes which form the minor planets. at certain times some of them approach near enough to the earth to merit the attention of those astronomers who are specially interested in determining the dimensions of the solar system. the observations are of such a nature that they can be made with considerable precision; they can also be multiplied to any extent that may be desired. some of these little bodies have consequently a great astronomical future, inasmuch as they seem destined to indicate the true distance from the earth to the sun more accurately than venus or than mars. the smallest of these planets will not answer for this purpose; they can only be seen in powerful telescopes, and they do not admit of being measured with the necessary accuracy. it is also obvious that the planets to be chosen for observation must come as near the earth as possible. in favourable circumstances, some of the minor planets will approach the earth to a distance which is about three-quarters of the distance of the sun. these various conditions limit the number of bodies available for this purpose to about a dozen, of which one or two will usually be suitably placed each year. for the determination of the sun's distance this method by the minor planets offers unquestionable advantages. the orb itself is a minute star-like point in the telescope, and the measures are made from it to the stars which are seen near it. a few words will, perhaps, be necessary at this place as to the nature of the observations referred to. when we speak of the measures from the planet to the star, we do not refer to what would be perhaps the most ordinary acceptation of the expression. we do _not_ mean the actual measurement of the number of miles in a straight line between the planet and the star. this element, even if attainable, could only be the result of a protracted series of observations of a nature which will be explained later on when we come to speak of the distances of the stars. the measures now referred to are of a more simple character; they are merely to ascertain the apparent distance of the objects expressed in angular measure. this angular measurement is of a wholly different character from the linear measurement, and the two methods may, indeed, lead to results that would at first seem paradoxical. we may take, as an illustration, the case of the group of stars forming the pleiades, and those which form the great bear. the latter is a large group, the former is a small one. but why do we think the words large and small rightly applied here? each pair of stars of the great bear makes a large angle with the eye. each pair of stars in the pleiades makes a small angle, and it is these angles which are the direct object of astronomical measurement. we speak of the distance of two stars, meaning thereby the angle which is bounded by the two lines from the eye to the two stars. this is what our instruments are able to measure, and it is to be observed that no reference to linear magnitude is implied. indeed, if we are to mention actual dimensions, it is quite possible, for anything we can tell, that the pleiades may form a much larger group than the great bear, and that the apparent superiority of the latter is merely due to its being closer to us. the most accurate of these angular measures are obtained when two stars, or two star-like points, are so close together as to enable them to be included in one field of view of the telescope. there are special forms of apparatus which enable the astronomer in this case to give to his observations a precision unattainable in the measurement of objects less definitely marked, or at a greater apparent distance. the determination of the distance of the small star-like planet from a star is therefore characterised by great accuracy. but there is another and, perhaps, a weightier argument in favour of the determination of the scale of the solar system by this process. the real strength of the minor planet method rests hardly so much on the individual accuracy of the observations, as on the fact that from the nature of the method a considerable number of repetitions can be concentrated on the result. it will, of course, be understood that when we speak of the accuracy of an observation, it is not to be presumed that it can ever be entirely free from error. errors always exist, and though they may be small, yet if the quantity to be measured is minute, an error of intrinsic insignificance may amount to an appreciable fraction of the whole. the one way by which their effect can be subdued is by taking the mean of a large number of observations. this is the real source of the value of the minor planet method. we have not to wait for the occurrence of rare events like the transit of venus. each year will witness the approach of some one or more minor planets sufficiently close to the earth to render the method applicable. the varied circumstances attending each planet, and the great variety of the observations which may be made upon it, will further conduce to eliminate error. as the planet pursues its course through the sky, which is everywhere studded over with countless myriads of minute stars, it is evident that this body, itself so like a star, will always have some stars in its immediate neighbourhood. as the movements of the planet are well known, we can foretell where it will be on each night that it is to be observed. it is thus possible to prearrange with observers in widely-different parts of the earth as to the observations to be made on each particular night. an attempt has been made, on the suggestion of dr. gill, to carry out this method on a scale commensurate with its importance. the planets iris, victoria, and sappho happened, in the years and , to approach so close to the earth that arrangements were made for simultaneous measurements in both the northern and the southern hemispheres. a scheme was completely drawn up many months before the observations were to commence. each observer who participated in the work was thus advised beforehand of the stars which were to be employed each night. viewed from any part of the earth, from the cape of good hope or from great britain, the positions of the stars remain absolutely unchanged. their distance is so stupendous that a change of place on the earth displaces them to no appreciable extent. but the case is different with a minor planet. it is hardly one-millionth part of the distance of the stars, and the displacement of the planet when viewed from the cape and when viewed from europe is a measurable quantity. the magnitude we are seeking is to be elicited by comparison between the measurements made in the northern hemisphere with those made in the southern. the observations in the two localities must be as nearly simultaneous as possible, due allowance being made for the motion of the planet in whatever interval may have elapsed. although every precaution is taken to eliminate the errors of each observation, yet the fact remains that we compare the measures made by observers in the northern hemisphere with those made by different observers, using of course different instruments, thousands of miles away. but in this respect we are at no greater disadvantage than in observing the transit of venus. it is, however, possible to obviate even this objection, and thus to give the minor planet method a supremacy over its rival which cannot be disputed. the difficulty would be overcome if we could arrange that an astronomer, after making a set of observations on a fine night in the northern hemisphere, should be instantly transferred, instruments and all, to the southern station, and there repeat the observations. an equivalent transformation can be effected without any miraculous agency, and in it we have undoubtedly the most perfect mode of measuring the sun's distance with which we are acquainted. this method has already been applied with success by dr. gill in the case of juno, and there are other members of the host of minor planets still more favourably circumstanced. consider, for instance, a minor planet, which sometimes approaches to within , , miles of the earth. when the opposition is drawing near, a skilled observer is to be placed at some suitable station near the equator. the instrument he is to use should be that marvellous piece of mechanical and optical skill known as the heliometer.[ ] it can be used to measure the angular distance between objects too far apart for the filar micrometer. the measurements are to be made in the evening as soon as the planet has risen high enough to enable it to be seen distinctly. the observer and the observatory are then to be transferred to the other side of the earth. how is this to be done? say, rather, how we could prevent it from being done. is not the earth rotating on its axis, so that in the course of a few hours the observatory on the equator is carried bodily round for thousands of miles? as the morning approaches the observations are to be repeated. the planet is found to have changed its place very considerably with regard to the stars. this is partly due to its own motion, but it is also largely due to the parallactic displacement arising from the rotation of the earth, which may amount to so much as twenty seconds. the measures on a single night with the heliometer should not have a mean error greater than one-fifth of a second, and we might reasonably expect that observations could be secured on about twenty-five nights during the opposition. four such groups might be expected to give the sun's distance without any uncertainty greater than the thousandth part of the total amount. the chief difficulty of the process arises from the movement of the planet during the interval which divides the evening from the morning observations. this drawback can be avoided by diligent and repeated measurements of the place of the planet with respect to the stars among which it passes. in the monumental piece of work which issued in from the cape observatory, under the direction of dr. gill, the final results from the observations of iris, victoria, and sappho have been obtained. from this it appears that the angle which the earth's equatorial radius subtends at the centre of the sun when at its mean distance has the value "· . if we employ the best value of the earth's equatorial radius we obtain , , miles as the mean distance of the centre of the sun from the centre of the earth. this is probably the most accurate determination of the scale of the solar system which has yet been made. chapter xii. jupiter. the great size of jupiter--comparison of his diameter with that of the earth--dimensions of the planet and his orbit--his rotation--comparison of his weight and bulk with that of the earth--relative lightness of jupiter--how explained--jupiter still probably in a heated condition--the belts on jupiter--spots on his surface--time of rotation of different spots various--storms on jupiter--jupiter not incandescent--the satellites--their discovery--telescopic appearance--their orbits--the eclipses and occultations--a satellite in transit--the velocity of light discovered--how is this velocity to be measured experimentally?--determination of the sun's distance by the eclipses of jupiter's satellites--jupiter's satellites demonstrating the copernican system. in our exploration of the beautiful series of bodies which form the solar system, we have proceeded step by step outwards from the sun. in the pursuit of this method we have now come to the splendid planet jupiter, which wends its majestic way in a path immediately outside those orbits of the minor planets which we have just been considering. great, indeed, is the contrast between these tiny globes and the stupendous globe of jupiter. had we adopted a somewhat different method of treatment--had we, for instance, discussed the various bodies of our planetary system in the order of their magnitude--then the minor planets would have been the last to be considered, while the leader of the host would be jupiter. to this position jupiter is entitled without an approach to rivalry. the next greatest on the list, the beautiful and interesting saturn, comes a long distance behind. another great descent in the scale of magnitude has to be made before we reach uranus and neptune, while still another step downwards must be made before we reach that lesser group of planets which includes our earth. so conspicuously does jupiter tower over the rest, that even if saturn were to be augmented by all the other globes of our system rolled into one, the united mass would still not equal the great globe of jupiter. [illustration: fig. .--the relative dimensions of jupiter and the earth.] the adjoining picture (fig. ) shows the relative dimensions of jupiter and the earth, and it conveys to the eye a more vivid impression of the enormous bulk of jupiter than we can readily obtain by merely considering the numerical statements by which his bulk is to be accurately estimated. as, however, it will be necessary to place the numerical facts before our readers, we do so at the outset of this chapter. jupiter revolves in an elliptic orbit around the sun in the focus, at a mean distance of , , miles. the path of jupiter is thus about · times as great in diameter as the path pursued by the earth. the shape of jupiter's orbit departs very appreciably from a circle, the greatest distance from the sun being · , while the least distance is about · , the earth's distance from the sun being taken as unity. in the most favourable circumstances for seeing jupiter at opposition, it must still be about four times as far from the earth as the earth is from the sun. this great globe will also illustrate the law that the more distant a planet is, the slower is the velocity with which its orbital motion is accomplished. while the earth passes over eighteen miles each second, jupiter only accomplishes eight miles. thus for a twofold reason the time occupied by an exterior planet in completing a revolution is greater than the period of the earth. not only has the outer planet to complete a longer course than the earth, but the speed is less; it thus happens that jupiter requires , · days, or about fifty days less than twelve years, to make a circuit of the heavens. the mean diameter of the great planet is about , miles. we say the _mean_ diameter, because there is a conspicuous difference in the case of jupiter between his equatorial and his polar diameters. we have already seen that there is a similar difference in the case of the earth, where we find the polar diameter to be shorter than the equatorial; but the inequality of these two dimensions is very much larger in jupiter than in the earth. the equatorial diameter of jupiter is , miles, while the polar is not more than , miles. the ellipticity of jupiter indicated by these figures is sufficiently marked to be obvious without any refined measures. around the shortest diameter the planet spins with what must be considered an enormous velocity when we reflect on the size of the globe. each rotation is completed in about hrs. mins. we may naturally contrast the period of rotation of jupiter with the much slower rotation of our earth in twenty-four hours. the difference becomes much more striking if we consider the relative speeds at which an object on the equator of the earth and on that of jupiter actually moves. as the diameter of jupiter is nearly eleven times that of the earth, it will follow that the speed of the equator on jupiter must be about twenty-seven times as great as that on the earth. it is no doubt to this high velocity of rotation that we must ascribe the extraordinary ellipticity of jupiter; the rapid rotation causes a great centrifugal force, and this bulges out the pliant materials of which he seems to be formed. jupiter is not, so far as we can see, a solid body. this is an important circumstance; and therefore it will be necessary to discuss the matter at some little length, as we here perceive a wide contrast between this great planet and the other planets which have previously occupied our attention. from the measurements already given it is easy to calculate the bulk or the volume of jupiter. it will be found that this planet is about , times as large as the earth; in other words, it would take , globes, each as large as our earth, all rolled into one, to form a single globe as large as jupiter. if the materials of which jupiter is composed were of a nature analogous to the materials of the earth, we might expect that the weight of the planet would exceed the weight of the earth in something like the proportion of their volumes. this is the matter now proposed to be brought to trial. here we may at once be met with the query, as to how we are to find the weight of jupiter. it is not even an easy matter to weigh the earth on which we stand. how, then, can we weigh a mighty planet vastly larger than the earth, and distant from us by some hundreds of millions of miles? truly, this is a bold problem. yet the intellectual resources of man have proved sufficient to achieve this feat of celestial engineering. they are not, it is true, actually able to make the ponderous weighing scales in which the great planet is to be cast, but they are able to divert to this purpose certain natural phenomena which yield the information that is required. such investigations are based on the principle of universal gravitation. the mass of jupiter attracts other masses in the solar system. the efficiency of that attraction is more particularly shown on the bodies which are near the planet. in virtue of this attraction certain movements are performed by those bodies. we can observe their character with our telescopes, we can ascertain their amount, and from our measurements we can calculate the mass of the body by which the movements have been produced. this is the sole method which we possess for the investigation of the masses of the planets; and though it may be difficult in its application--not only from the observations which are required, but also from the intricacy and the profundity of the calculations to which those observations must be submitted--yet, in the case of jupiter at least, there is no uncertainty about the result. the task is peculiarly simplified in the case of the greatest planet of our system by the beautiful system of moons with which he is attended. these little moons revolve under the guidance of jupiter, and their movements are not otherwise interfered with so as to prevent their use for our present purpose. it is from the observations of the satellites of jupiter that we are enabled to measure his attractive power, and thence to calculate the mass of the mighty planet. to those not specially conversant with the principles of mechanics, it may seem difficult to realise the degree of accuracy of which such a method is capable. yet there can be no doubt that his moons inform us of the mass of jupiter, and do not leave a margin of inaccuracy so great as one hundredth part of the total amount. if other confirmation be needed, then it is forthcoming in abundance. a minor planet occasionally draws near the orbit of jupiter and experiences his attraction; the planet is forced to swerve from its path, and the amount of the deviation can be measured. from that measurement the mass of jupiter can be computed by a calculation, of which it would be impossible to give an account in this place. the mass of jupiter, as determined by this method, agrees with the mass obtained in a totally different manner from the satellites. nor have we yet exhausted the resources of astronomy in its bearing on this question. we can discard the planetary system, and invite the assistance of a comet which, flashing through the orbits of the planets, occasionally experiences large and sometimes enormous disturbances. for the present it suffices to remark, that on one or two occasions it has happened that venturous comets have been near enough to jupiter to be much disturbed by his attraction, and then to proclaim in their altered movements the magnitude of the mass which has affected them. the satellites of jupiter, the minor planets, and the comets, all tell the weight of the giant orb; and, as they all concur in the result (at least within extremely narrow limits), we cannot hesitate to conclude that the mass of the greatest planet of our system has been determined with accuracy. the results of these measures must now be stated. they show, of course, that jupiter is vastly inferior to the sun--that, in fact, it would take about , jupiters, all rolled into one, to form a globe equal in _weight_ to the sun. they also show us that it would take globes as heavy as our earth to counterbalance the weight of jupiter. no doubt this proves jupiter to be a body of magnificent proportions; but the remarkable circumstance is not that jupiter should be times as heavy as the earth, but that he is not a great deal more. have we not stated that jupiter is , times as _large_ as the earth? how then comes it that he is only times as _heavy_? this points at once to some fundamental contrast between the constitution of jupiter and of the earth. how are we to account for this difference? we can conceive of two explanations. in the first place, it might be supposed that jupiter is constituted of materials partly or wholly unknown on the earth. there is, however, an alternative supposition at once more philosophical and more consistent with the evidence. it is true that we know little or nothing of what the elementary substances on jupiter may be, but one of the great discoveries of modern astronomy has taught us something of the elementary bodies present in other bodies of the universe, and has demonstrated that to a large extent they are identical with the elementary bodies on the earth. if jupiter be composed of bodies resembling those on the earth, there is one way, and only one, in which we can account for the disparity between his size and his mass. perhaps the best way of stating the argument will be found in a glance at the remote history of the earth itself, for it seems not impossible that the present condition of jupiter was itself foreshadowed by the condition of our earth countless ages ago. in a previous chapter we had occasion to point out how the earth seemed to be cooling from an earlier and highly heated condition. the further we look back, the hotter our globe seems to have been; and if we project our glance back to an epoch sufficiently remote, we see that it must once have been so hot that life on its surface would have been impossible. back still earlier, we find the heat to have been such that water could not rest on the earth; and hence it seems likely that at some incredibly remote epoch all the oceans now reposing in the deeps on the surface, and perhaps a considerable portion of its now solid crust, must have been in a state of vapour. such a transformation of the globe would not alter its _mass_, for the materials weigh the same whatever be their condition as to temperature, but it would alter the _size_ of our globe to a very considerable extent. if these oceans were transformed into vapour, then the atmosphere, charged with mighty clouds, would have a bulk some hundreds of times greater than that which it has at present. viewed from a distant planet, the cloud-laden atmosphere would indicate the visible size of our globe, and its average density would accordingly appear to be very much less than it is at present. from these considerations it will be manifest that the discrepancy between the size and the weight of jupiter, as contrasted with our earth, would be completely removed if we supposed that jupiter was at the present day a highly heated body in the condition of our earth countless ages ago. every circumstance of the case tends to justify this argument. we have assigned the smallness of the moon as a reason why the moon has cooled sufficiently to make its volcanoes silent and still. in the same way the smallness of the earth, as compared with jupiter, accounts for the fact that jupiter still retains a large part of its original heat, while the smaller earth has dissipated most of its store. this argument is illustrated and strengthened when we introduce other planets into the comparison. as a general rule we find that the smaller bodies, like the earth and mars, have a high density, indicative of a low temperature, while the giant planets, like jupiter and saturn, have a low density, suggesting that they still retain a large part of their original heat. we say "original heat" for the want, perhaps, of a more correct expression; it will, however, indicate that we do not in the least refer to the solar heat, of which, indeed, the great outer planets receive much less than those nearer the sun. where the original heat may have come from is a matter still confined to the province of speculation. a complete justification of these views with regard to jupiter is to be found when we make a minute telescopic scrutiny of its surface; and it fortunately happens that the size of the planet is so great that, even at a distance of more millions of miles than there are days in the year, we can still trace on its surface some significant features. plate xi. gives a series of four different views of jupiter. they have been taken from a series of admirable drawings of the great planet made by mr. griffiths in . the first picture shows the appearance of the globe at h. m. greenwich time on february th, , through a powerful refracting telescope. we at once notice in this drawing that the outline of jupiter is distinctly elliptical. the surface of the planet usually shows the remarkable series of belts here represented. they are nearly parallel to each other and to the planet's equator. when jupiter is observed for some hours, the appearance of the belts undergoes certain changes. these are partly due to the regular rotation of the planet on its axis, which, in a period of less than five hours, will completely carry away the hemisphere we first saw, and replace it by the hemisphere originally at the other side. but besides the changes thus arising, the belts and other features on the planet are also very variable. sometimes new stripes or marks appear, and old ones disappear; in fact, a thorough examination of jupiter will demonstrate the remarkable fact that there are no permanent features whatever to be discerned. we are here immediately struck by the contrast between jupiter and mars; on the smaller planet the main topographical outlines are almost invariable, and it has been feasible to construct maps of the surface with tolerably accurate detail; a map of jupiter is, however, an impossibility--the drawing of the planet which we make to-night will be different from the drawing of the same hemisphere made a few weeks hence. it should, however, be noticed that objects occasionally appear on the planet which seem of a rather more persistent character than the belts. we may especially mention the object known as the great oblong red spot, which has been a very remarkable feature upon the southern hemisphere of jupiter since . this object, which has attracted a great deal of attention from observers, is about , miles long by about , in breadth. professor barnard remarks that the older the spots on jupiter are, the more ruddy do they tend to become. the conclusion is irresistibly forced upon us that when we view the surface of jupiter we are not looking at any solid body. the want of permanence in the features of the planet would be intelligible if what we see be merely an atmosphere laden with clouds of impenetrable density. the belts especially support this view; we are at once reminded of the equatorial zones on our own earth, and it is not at all unlikely that an observer sufficiently remote from the earth to obtain a just view of its appearance would detect upon its surface more or less perfect cloud-belts suggestive of those on jupiter. a view of our earth would be, as it were, intermediate between a view of jupiter and of mars. in the latter case the appearance of the permanent features of the planet is only to a trifling extent obscured by clouds floating over the surface. our earth would always be partly, and often perhaps very largely, covered with cloud, while jupiter seems at all times completely enveloped. from another class of observations we are also taught the important truth that jupiter is not, superficially at least, a solid body. the period of rotation of the planet around its axis is derived from the observation of certain marks, which present sufficient definiteness and sufficient permanence to be suitable for the purpose. suppose one of these objects to lie at the centre of the planet's disc; its position is carefully measured, and the time is noted. as the hours pass on, the mark moves to the edge of the disc, then round the other side of the planet, and back again to the visible disc. when it has returned to the position originally occupied the time is again taken, and the interval which has elapsed is called the period of rotation of the spot. if jupiter were a solid, and if these features were engraved upon its surface, then it is perfectly clear that the time of rotation as found by any one spot must coincide precisely with the time yielded by any other spot; but this is not observed to be the case. in fact, it would be nearer the truth to say that each spot gives a special period of its own. nor are the differences very minute. it has been found that the time in which the red spot (the latitude of which is about ° south) is carried round is five minutes longer than that required by some peculiar white marks near the equator. the red spot has now been watched for about twenty years, and during most of that time has had a tendency to rotate more and more slowly, as may be seen from the following values of its rotation period:-- in , h. m. · s. in , h. m. · s. in , h. m. · s. since this tendency seems to have ceased, while the spot has been gradually fading away. generally speaking, we may say that the equatorial regions rotate in about h. m. s., and the temperate zones in about h. m. s. remarkable exceptions are occasionally met with. some small black spots in north latitude °, which broke out in and again in , rotated in h. m. to h. - / m. it may, therefore, be regarded as certain that the globe of jupiter, so far as we can see it, is not a solid body. it consists, on the exterior at all events, of clouds and vaporous masses, which seem to be agitated by storms of the utmost intensity, if we are to judge from the ceaseless changes of the planet's surface. [illustration: plate xi. feb. nd. feb. th. feb. th. feb. th. the planet jupiter. .] [illustration: fig. .--the occultation of jupiter ( ).] [illustration: fig. .--the occultation of jupiter ( ).] [illustration: fig. .--the occultation of jupiter ( ).] [illustration: fig. .--the occultation of jupiter ( ).] various photographs of jupiter have been obtained; those which have been taken at the lick observatory being specially interesting and instructive. pictures of the planet obtained with the camera in remarkable circumstances are represented in figs. - , which were taken by professor wm. h. pickering at arequipa, peru, on the th of august, .[ ] the small object with the belts is the planet jupiter. the large advancing disc (of which only a small part can be shown) is the moon. the phenomenon illustrated is called the "occultation" of the planet. the planet is half-way behind the moon in fig. , while in fig. half of the planet is still hidden by the dark limb of the moon. it is well known that the tempests by which the atmosphere surrounding the earth is convulsed are to be ultimately attributed to the heat of the sun. it is the rays from the great luminary which, striking on the vast continents, warm the air in contact therewith. this heated air becomes lighter, and rises, while air to supply its place must flow in along the surface. the currents so produced form a breeze or a wind; while, under exceptional circumstances, we have the phenomena of cyclones and hurricanes, all originated by the sun's heat. need we add that the rains, which so often accompany the storms, have also arisen from the solar beams, which have distilled from the wide expanse of ocean the moisture by which the earth is refreshed? the storms on jupiter seem to be vastly greater than those on the earth. yet the intensity of the sun's heat on jupiter is only a mere fraction--less, indeed, than the twenty-fifth part--of that received by the earth. it is incredible that the motive power of the appalling tempests on the great planet can be entirely, or even largely, due to the feeble influence of solar heat. we are, therefore, led to seek for some other source of such disturbances. what that source is to be will appear obvious when we admit that jupiter still retains a large proportion of primitive internal heat. just as the sun itself is distracted by violent tempests as an accompaniment of its intense internal fervour, so, in a lesser degree, do we observe the same phenomena in jupiter. it may also be noticed that the spots on the sun usually lie in more or less regular zones, parallel to its equator, the arrangement being in this respect not dissimilar to that of the belts on jupiter. it being admitted that the mighty planet still retains some of its internal heat, the question remains as to how much. it is, of course, obvious that the heat of the planet is inconsiderable when compared with the heat of the sun. the brilliance of jupiter, which makes it an object of such splendour in our midnight sky, is derived from the same great source which illuminates the earth, the moon, or the other planets. jupiter, in fact, shines by reflected sunlight, and not in virtue of any intrinsic light in his globe. a beautiful proof of this truth is familiar to every user of a telescope. the little satellites of the planet sometimes intrude between him and the sun, and cast a shadow on jupiter. the shadow is black, or, at all events, it seems black, relatively to the brilliant surrounding surface of the planet. it must, therefore, be obvious that jupiter is indebted to the sun for its brilliancy. the satellites supply another interesting proof of this truth. one of these bodies sometimes enters the shadow of jupiter and lo! the little body vanishes. it does so because jupiter has cut off the supply of sunlight which previously rendered the satellite visible. but the planet is not himself able to offer the satellite any light in compensation for the sunlight which he has intercepted.[ ] enough, however, has been demonstrated to enable us to pronounce on the question as to whether the globe of jupiter can be inhabited by living creatures resembling those on this earth. obviously this cannot be so. the internal heat and the fearful tempests seem to preclude the possibility of organic life on the great planet, even were there not other arguments tending to the same conclusion. it may, however, be contended, with perhaps some plausibility, that jupiter has in the distant future the prospect of a glorious career as the residence of organic life. the time will assuredly come when the internal heat must decline, when the clouds will gradually condense into oceans. on the surface dry land may then appear, and jupiter be rendered habitable. from this sketch of the planet itself we now turn to the interesting and beautiful system of five satellites by which jupiter is attended. we have, indeed, already found it necessary to allude more than once to these little bodies, but not to such an extent as to interfere with the more formal treatment which they are now to receive. the discovery of the four chief satellites may be regarded as an important epoch in the history of astronomy. they are objects situated in a remarkable manner on the border-line which divides the objects visible to the unaided eye from those which require telescopic aid. it has been frequently asserted that these objects have been seen with the unaided eye; but without entering into any controversy on the matter, it is sufficient to recite the well-known fact that, although jupiter had been a familiar object for countless centuries, yet the sharpest eyes under the clearest skies never discovered the satellites until galileo turned the newly invented telescope upon them. this tube was no doubt a very feeble instrument, but very little power suffices to show objects so dose to the limit of visibility. [illustration: fig. .--jupiter and his four satellites as seen in a telescope of low power.] the view of the planet and its elaborate system of satellites as shown in a telescope of moderate power, is represented in fig. . we here see the great globe, and nearly in a line parsing through its centre lie four small objects, three on one side and one on the other. these little bodies resemble stars, but they can be distinguished therefrom by their ceaseless movements around the planet, which they never fail to accompany during his entire circuit of the heavens. there is no more pleasing spectacle for the student than to follow with his telescope the movements of this beautiful system. [illustration: fig. .--disappearances of jupiter's satellites.] in fig. we have represented some of the various phenomena which the satellites present. the long black shadow is that produced by the interposition of jupiter in the path of the sun's rays. in consequence of the great distance of the sun this shadow will extend, in the form of a very elongated cone, to a distance far beyond the orbit of the outer satellite. the second satellite is immersed in this shadow, and consequently eclipsed. the eclipse of a satellite must not be attributed to the intervention of the body of jupiter between the satellite and the earth. such an occurrence is called an occultation, and the third satellite is shown in this condition. the second and the third satellites are thus alike invisible, but the cause of the invisibility is quite different in the two cases. the eclipse is much the more striking phenomenon of the two, because the satellite, at the moment it plunges into the darkness, may be still at some apparent distance from the edge of the planet, and is thus seen up to the moment of the eclipse. in an occultation the satellite in passing behind the planet is, at the time of disappearance, close to the planet's bright edge, and the extinction of the light from the small body cannot be observed with the same impressiveness as the occurrence of an eclipse. a satellite also assumes another remarkable situation when in the course of transit over the face of the planet. the satellite itself is not always very easy to see in such circumstances, but the beautiful shadow which it casts forms a sharp black spot on the brilliant orb: the satellite will, indeed, frequently cast a visible shadow when it passes between the planet and the sun, even though it be not actually at the moment in front of the planet, as it is seen from the earth. the periods in which the four principal moons of jupiter revolve around their primary are respectively, day hrs. min. secs. for the first; days hrs. min. secs., for the second; days hrs. min. secs, for the third; and days hrs. min. secs. for the fourth. we thus observe that the periods of jupiter's satellites are decidedly briefer than that of our moon. even the satellite most distant from the great planet requires for each revolution less than two-thirds of an ordinary lunar month. the innermost of these bodies, revolving as it does in less than two days, presents a striking series of ceaseless and rapid changes, and it becomes eclipsed during every revolution. the distance from the centre of jupiter to the orbit of the innermost of these four attendants is a quarter of a million miles, while the radius of the outermost is a little more than a million miles. the second of the satellites proceeding outwards from the planet is almost the same size as our moon; the other three bodies are larger; the third being the greatest of all (about , miles in diameter). owing to the minuteness of the satellites as seen from the earth, it is extremely difficult to perceive any markings on their surfaces, but the few observations made seem to indicate that the satellites (like our moon) always turn the same face towards their primary. professor barnard has, with the great lick refractor, seen a white equatorial belt on the first satellite, while its poles were very dark. mr. douglass, observing with mr. lowell's great refractor, has also reported certain streaky markings on the third satellite. a very interesting astronomical discovery was that made by professor e.e. barnard in . he detected with the -inch lick refractor an extremely minute fifth satellite to jupiter at a distance of , miles, and revolving in a period of hrs. min. · secs. it can only be seen with the most powerful telescopes. the eclipses of jupiter's satellites had been observed for many years, and the times of their occurrence had been recorded. at length it was perceived that a certain order reigned among the eclipses of these bodies, as among all other astronomical phenomena. when once the laws according to which the eclipses recurred had been perceived, the usual consequence followed. it became possible to foretell the time at which the eclipses would occur in future. predictions were accordingly made, and it was found that they were approximately verified. further improvements in the calculations were then perfected, and it was sought to predict the times with still greater accuracy. but when it came to naming the actual minute at which the eclipse should occur, expectations were not always realised. sometimes the eclipse was five or ten minutes too soon. sometimes it was five or ten minutes too late. discrepancies of this kind always demand attention. it is, indeed, by the right use of them that discoveries are often made, and one of the most interesting examples is that now before us. the irregularity in the occurrence of the eclipses was at length perceived to observe certain rules. it was noticed that when the earth was near to jupiter the eclipse generally occurred before the predicted time; while when the earth happened to be at the side of its orbit away from jupiter, the eclipse occurred after the predicted time. once this was proved, the great discovery was quickly made by roemer, a danish astronomer, in . when the satellite enters the shadow, its light gradually decreases until it disappears. it is the last ray of light from the eclipsed satellite that gives the time of the eclipse; but that ray of light has to travel from the satellite to the earth, and enter our telescope, before we can note the occurrence. it used to be thought that light travelled instantaneously, so that the moment the eclipse occurred was assumed to be the moment when the eclipse was seen in the telescope. this was now perceived to be incorrect. it was found that light took time to travel. when the earth was comparatively near jupiter the light had only a short journey, the intelligence of the eclipse arrived quickly, and the eclipse was seen sooner than the calculations indicated. when the earth occupied a position far from jupiter, the light had a longer journey, and took more than the average time, so that the eclipse was later than the prediction. this simple explanation removed the difficulty attending the predictions of the eclipses of the satellites. but the discovery had a significance far more momentous. we learned from it that light had a measurable velocity, which, according to recent researches, amounts to , miles per second. one of the most celebrated attempts to ascertain the distance of the sun is derived from a combination of experiments on the velocity of light with astronomical measurements. this is a method of considerable refinement and interest, and although it does not so fulfil all the necessary conditions as to make it perfectly satisfactory, yet it is impossible to avoid some reference to it here. notwithstanding that the velocity of light is so stupendous, it has been found possible to measure that velocity by actual trial. this is one of the most delicate experimental researches that have ever been undertaken. if it be difficult to measure the speed of a rifle bullet, what shall we say of the speed of a ray of light, which is nearly a million times as great? how shall we devise an apparatus subtle enough to determine the velocity which would girdle the earth at the equator no less than seven times in a single second of time? ordinary contrivances for measurement are here futile; we have to devise an instrument of a wholly different character. in the attempt to discover the speed of a moving body we first mark out a certain distance, and then measure the time which the body requires to traverse that distance. we determine the velocity of a railway train by the time it takes to pass from one mile-post to the next. we learn the speed of a rifle bullet by an ingenious contrivance really founded on the same principle. the greater the velocity, the more desirable is it that the distance traversed during the experiment shall be as large as possible. in dealing with the measurement of the velocity of light, we therefore choose for our measured distance the greatest length that may be convenient. it is, however, necessary that the two ends of the line shall be visible from each other. a hill a mile or two away will form a suitable site for the distant station, and the distance of the selected point on the hill from the observer must be carefully measured. the problem is now easily stated. a ray of light is to be sent from the observer to the distant station, and the time occupied by that ray in the journey is to be measured. we may suppose that the observer, by a suitable contrivance, has arranged a lantern from which a thin ray of light issues. let us assume that this travels all the way to the distant station, and there falls upon the surface of a reflecting mirror. instantly it will be diverted by reflection into a new direction depending upon the inclination of the mirror. by suitable adjustment the latter can be so placed that the light shall fall perpendicularly upon it, in which case the ray will of course return along the direction in which it came. let the mirror be fixed in this position throughout the course of the experiments. it follows that a ray of light starting from the lantern will be returned to the lantern after it has made the journey to the distant station and back again. imagine, then, a little shutter placed in front of the lantern. we open the shutter, the ray streams forth to the remote reflector, and back again through the opening. but now, after having allowed the ray to pass through the shutter, suppose we try and close it before the ray has had time to get back again. what fingers could be nimble enough to do this? even if the distant station were ten miles away, so that the light had a journey of ten miles in going to the mirror and ten miles in coming back, yet the whole course would be accomplished in about the nine-thousandth part of a second--a period so short that even were it a thousand times as long it would hardly enable manual dexterity to close the aperture. yet a shutter can be constructed which shall be sufficiently delicate for the purpose. [illustration: fig. .--mode of measuring the velocity of light.] the principle of this beautiful method will be sufficiently obvious from the diagram on this page (fig. ), which has been taken from newcomb's "popular astronomy." the figure exhibits the lantern and the observer, and a large wheel with projecting teeth. each tooth as it passes round eclipses the beam of light emerging from the lantern, and also the eye, which is of course directed to the mirror at the distant station. in the position of the wheel here shown the ray from the lantern will pass to the mirror and back so as to be visible to the eye; but if the wheel be rotating, it may so happen that the beam after leaving the lantern will not have time to return before the next tooth of the wheel comes in front of the eye and screens it. if the wheel be urged still faster, the next tooth may have passed the eye, so that the ray again becomes visible. the speed at which the wheel is rotating can be measured. we can thus determine the time taken by one of the teeth to pass in front of the eye; we have accordingly a measure of the time occupied by the ray of light in the double journey, and hence we have a measurement of the velocity of light. it thus appears that we can tell the velocity of light either by the observations of jupiter's satellites or by experimental enquiry. if we take the latter method, then we are entitled to deduce remarkable astronomical consequences. we can, in fact, employ this method for solving that great problem so often referred to--the distance from the earth to the sun--though it cannot compete in accuracy with some of the other methods. the dimensions of the solar system are so considerable that a sunbeam requires an appreciable interval of time to span the abyss which separates the earth from the sun. eight minutes is approximately the duration of the journey, so that at any moment we see the sun as it appeared eight minutes earlier to an observer in its immediate neighbourhood. in fact, if the sun were to be suddenly blotted out it would still be seen shining brilliantly for eight minutes after it had really disappeared. we can determine this period from the eclipses of jupiter's satellites. so long as the satellite is shining it radiates a stream of light across the vast space between jupiter and the earth. when the eclipse has commenced, the little orb is no longer luminous, but there is, nevertheless, a long stream of light on its way, and until all this has poured into our telescopes we still see the satellite shining as before. if we could calculate the moment when the eclipse really took place, and if we could observe the moment at which the eclipse is seen, the difference between the two gives the time which the light occupies on the journey. this can be found with some accuracy; and, as we already know the velocity of light, we can ascertain the distance of jupiter from the earth; and hence deduce the scale of the solar system. it must, however, be remarked that at both extremities of the process there are characteristic sources of uncertainty. the occurrence of the eclipse is not an instantaneous phenomenon. the satellite is large enough to require an appreciable time in crossing the boundary which defines the shadow, so that the observation of an eclipse cannot be sufficiently precise to form the basis of an important and accurate measurement.[ ] still greater difficulties accompany the attempt to define the true moment of the occurrence of the eclipse as it would be seen by an observer in the vicinity of the satellite. for this we should require a far more perfect theory of the movements of jupiter's satellites than is at present attainable. this method of finding the sun's distance holds out no prospect of a result accurate to the one-thousandth part of its amount, and we may discard it, inasmuch as the other methods available seem to admit of much higher accuracy. the four chief satellites of jupiter have special interest for the mathematician, who finds in them a most striking instance of the universality of the law of gravitation. these bodies are, of course, mainly controlled in their movements by the attraction of the great planet; but they also attract each other, and certain curious consequences are the result. the mean motion of the first satellite in each day about the centre of jupiter is °· . that of the second is °· , and that of the third is °· . these quantities are so related that the following law will be found to be observed: the mean motion of the first satellite added to twice the mean motion of the third is exactly equal to three times the mean motion of the second. there is another law of an analogous character, which is thus expressed (the mean longitude being the angle between a fixed line and the radius to the mean place of the satellite): if to the mean longitude of the first satellite we add twice the mean longitude of the third, and subtract three times the mean longitude of the second, the difference is always °. it was from observation that these principles were first discovered. laplace, however, showed that if the satellites revolved nearly in this way, then their mutual perturbations, in accordance with the law of gravitation, would preserve them in this relative position for ever. we shall conclude with the remark, that the discovery of jupiter's satellites afforded the great confirmation of the copernican theory. copernicus had asked the world to believe that our sun was a great globe, and that the earth and all the other planets were small bodies revolving round the great one. this doctrine, so repugnant to the theories previously held, and to the immediate evidence of our senses, could only be established by a refined course of reasoning. the discovery of jupiter's satellites was very opportune. here we had an exquisite ocular demonstration of a system, though, of course, on a much smaller scale, precisely identical with that which copernicus had proposed. the astronomer who had watched jupiter's moons circling around their primary, who had noticed their eclipses and all the interesting phenomena attendant on them, saw before his eyes, in a manner wholly unmistakable, that the great planet controlled these small bodies, and forced them to revolve around him, and thus exhibited a miniature of the great solar system itself. "as in the case of the spots on the sun, galileo's announcement of this discovery was received with incredulity by those philosophers of the day who believed that everything in nature was described in the writings of aristotle. one eminent astronomer, clavius, said that to see the satellites one must have a telescope which would produce them; but he changed his mind as soon as he saw them himself. another philosopher, more prudent, refused to put his eye to the telescope lest he should see them and be convinced. he died shortly afterwards. 'i hope,' said the caustic galileo, 'that he saw them while on his way to heaven'"[ ] chapter xiii. saturn. the position of saturn in the system--saturn one of the three most interesting objects in the heavens--compared with jupiter--saturn to the unaided eye--statistics relating to the planet--density of saturn--lighter than water--the researches of galileo--what he found in saturn--a mysterious object--the discoveries made by huyghens half a century later--how the existence of the ring was demonstrated--invisibility of the rings every fifteen years--the rotation of the planet--the celebrated cypher--the explanation--drawing of saturn--the dark line--w. herschel's researches--is the division in the ring really a separation?--possibility of deciding the question--the ring in a critical position--are there other divisions in the ring?--the dusky ring--physical nature of saturn's rings--can they be solid?--can they even be slender rings?--a fluid?--true nature of the rings--a multitude of small satellites--analogy of the rings of saturn to the group of minor planets--problems suggested by saturn--the group of satellites to saturn--the discoveries of additional satellites--the orbit of saturn not the frontier of our system. at a profound distance in space, which, on an average, is , , miles, the planet saturn performs its mighty revolution around the sun in a period of twenty-nine and a half years. this gigantic orbit formed the boundary to the planetary system, so far as it was known to the ancients. although saturn is not so great a body as jupiter, yet it vastly exceeds the earth in bulk and in mass, and is, indeed, much greater than any one of the planets, jupiter alone excepted. but while saturn must yield the palm to jupiter so far as mere dimensions are concerned, yet it will be generally admitted that even jupiter, with all the retinue by which he is attended, cannot compete in beauty with the marvellous system of saturn. to the present writer it has always seemed that saturn is one of the three most interesting celestial objects visible to observers in northern latitudes. the other two will occupy our attention in future chapters. they are the great nebula in orion, and the star cluster in hercules. so far as the globe of saturn is concerned, we do not meet with any features which give to the planet any exceptional interest. the globe is less than that of jupiter, and as the latter is also much nearer to us, the apparent size of saturn is in a twofold way much smaller than that of jupiter. it should also be noticed that, owing to the greater distance of saturn from the sun, its intrinsic brilliancy is less than that of jupiter. there are, no doubt, certain marks and bands often to be seen on saturn, but they are not nearly so striking nor so characteristic as the ever-variable belts upon jupiter. the telescopic appearance of the globe of saturn must also be ranked as greatly inferior in interest to that of mars. the delicacy of detail which we can see on mars when favourably placed has no parallel whatever in the dim and distant saturn. nor has saturn, regarded again merely as a globe, anything like the interest of venus. the great splendour of venus is altogether out of comparison with that of saturn, while the brilliant crescent of the evening star is infinitely more pleasing than any telescopic view of the globe of saturn. yet even while we admit all this to the fullest extent, it does not invalidate the claim of saturn to be one of the most supremely beautiful and interesting objects in the heavens. this interest is not due to his globe; it is due to that marvellous system of rings by which saturn is surrounded--a system wonderful from every point of view, and, so far as our knowledge goes, without a parallel in the wide extent of the universe. [illustration: fig. . saturn. (july nd, . -in. equatorial.) (prof. e.e. barnard.)] to the unaided eye saturn usually appears like a star of the first magnitude. its light alone would hardly be sufficient to discriminate it from many of the brighter fixed stars. yet the ancients were acquainted with saturn, and they knew it as a planet. it was included with the other four great planets--mercury, venus, mars, and jupiter--in the group of wanderers, which were bound to no fixed points of the sky like the stars. on account of the great distance of saturn, its movements are much slower than those of the other planets known to the ancients. twenty-nine years and a half are required for this distant object to complete its circuit of the heavens; and, though this movement is slow compared with the incessant changes of venus, yet it is rapid enough to attract the attention of any careful observer. in a single year saturn moves through a distance of about twelve degrees, a quantity sufficiently large to be conspicuous to casual observation. even in a month, or sometimes in a week, the planet traverses an arc of the sky which can be detected by anyone who will take the trouble to mark the place of the planet with regard to the stars in its vicinity. those who are privileged to use accurate astronomical instruments can readily detect the motion of saturn in a few hours. the average distance from the sun to saturn is about millions of miles. the path of saturn, as of every other planet, is really an ellipse with the sun in one focus. in the case of saturn the shape of this ellipse is very appreciably different from a purely circular path. around this path saturn moves with an average velocity of · miles per second. the mean diameter of the globe of saturn is about , miles. its equatorial diameter is about , miles, and its polar diameter , miles--the ratio of these numbers being approximately that of to . it is thus obvious that saturn departs from the truly spherical shape to a very marked extent. the protuberance at its equator must, no doubt, be attributed to the high velocity with which the planet is rotating. the velocity of rotation of saturn is more than double as fast as that of the earth, though it is not quite so fast as that of jupiter. saturn makes one complete rotation in about hrs. min. mr. stanley williams has, however, observed with great care a number of spots which he has discovered, and he finds that some of these spots in about ° north latitude indicate rotation in a period of hrs. mins. to min., while equatorial spots require no more than hrs. min. to min. there is, however, the peculiarity that spots in the same latitude, but at different parts of the planet, rotate at rates which differ by a minute or more, while the period found by various groups of spots seems to change from year to year. these facts prove that saturn and the spots do not form a rigid system. the lightness of this planet is such as to be wholly incompatible with the supposition that its globe is constituted of solid materials at all comparable with those of which the crust of our earth is composed. the satellites, which surround saturn and form a system only less interesting than the renowned rings themselves, enable us to weigh the planet in comparison with the sun, and hence to deduce its actual mass relatively to the earth. the result is not a little remarkable. it appears that the density of the earth is eight times as great as that of saturn. in fact, the density of the latter is less than that of water itself, so that a mighty globe of water, equal in bulk to saturn, would actually weigh more. if we could conceive a vast ocean into which a globe equal to saturn in size and weight were cast, the great globe would not sink like our earth or like any of the other planets; it would float buoyantly at the surface with one-fourth of its bulk out of the water. we thus learn with high probability that what our telescopes show upon saturn is not a solid surface, but merely a vast envelope of clouds surrounding a heated interior. it is impossible to resist the suggestion that this planet, like jupiter, has still retained its heat because its mass is so large. we must, however, allude to a circumstance which perhaps may seem somewhat inconsistent with the view here taken. we have found that jupiter and saturn are, both of them, much less dense than the earth. when we compare the two planets together, it appears that saturn is much less dense than jupiter. in fact, every cubic mile of jupiter weighs nearly twice as much as each cubic mile of saturn. this would seem to point to the conclusion that saturn is the more heated of the two bodies. yet, as jupiter is the larger, it might more reasonably have been expected to be hotter than the other planet. we do not attempt to reconcile this discrepancy; in fact, in our ignorance as to the material constitution of these bodies, it would be idle to discuss the question. even if we allow for the lightness of saturn, as compared bulk for bulk with the earth, yet the volume of saturn is so enormous that the planet weighs more than ninety-five times as much as the earth. the adjoining view represents the relative sizes of saturn and the earth (fig. ). [illustration: fig. .--relative sizes of saturn and the earth.] as the unaided eye discloses none of those marvels by which saturn is surrounded, the interest which attaches to this planet may be said to commence from the time when it began to be observed with the telescope. the history must be briefly alluded to, for it was only by degrees that the real nature of this complicated object was understood. when galileo completed his little refracting telescope, which, though it only magnified thirty times, was yet an enormous addition to the powers of unaided vision, he made with it his memorable review of the heavens. he saw the spots on the sun and the mountains on the moon; he noticed the crescent of venus and the satellites of jupiter. stimulated and encouraged by such brilliant discoveries, he naturally sought to examine the other planets, and accordingly directed his telescope to saturn. here, again, galileo at once made a discovery. he saw that saturn presented a visible form like the other planets, but that it differed from any other telescopic object, inasmuch as it appeared to him to be composed of three bodies which always touched each other and always maintained the same relative positions. these three bodies were in a line--the central one was the largest, and the two others were east and west of it. there was nothing he had hitherto seen in the heavens which filled his mind with such astonishment, and which seemed so wholly inexplicable. in his endeavours to understand this mysterious object, galileo continued his observations during the year , and, to his amazement, he saw the two lesser bodies gradually become smaller and smaller, until, in the course of the two following years, they had entirely vanished, and the planet simply appeared with a round disc like jupiter. here, again, was a new source of anxiety to galileo. he had at that day to contend against the advocates of the ancient system of astronomy, who derided his discoveries and refused to accept his theories. he had announced his observation of the composite nature of saturn; he had now to tell of the gradual decline and the ultimate extinction of these two auxiliary globes, and he naturally feared that his opponents would seize the opportunity of pronouncing that the whole of his observations were illusory.[ ] "what," he remarks, "is to be said concerning so strange a metamorphosis? are the two lesser stars consumed after the manner of the solar spots? have they vanished and suddenly fled? has saturn perhaps, devoured his own children? or were the appearances indeed illusion or fraud, with which the glasses have so long deceived me, as well as many others to whom i have shown them? now, perhaps, is the time come to revive the well-nigh withered hopes of those who, guided by more profound contemplations, have discovered the fallacy of the new observations, and demonstrated the utter impossibility of their existence. i do not know what to say in a case so surprising, so unlooked for, and so novel. the shortness of the time, the unexpected nature of the event, the weakness of my understanding, and the fear of being mistaken, have greatly confounded me." but galileo was not mistaken. the objects were really there when he first began to observe, they really did decline, and they really disappeared; but this disappearance was only for a time--they again came into view. they were then subjected to ceaseless examination, until gradually their nature became unfolded. with increased telescopic power it was found that the two bodies which galileo had described as globes on either side of saturn were not really spherical--they were rather two luminous crescents with the concavity of each turned towards the central globe. it was also perceived that these objects underwent a remarkable series of periodic changes. at the beginning of such a series the planet was found with a truly circular disc. the appendages first appeared as two arms extending directly outwards on each side of the planet; then these arms gradually opened into two crescents, resembling handles to the globe, and attained their maximum width after about seven or eight years; then they began to contract, until after the lapse of about the same time they vanished again. the true nature of these objects was at length discovered by huyghens in , nearly half a century after galileo had first detected their appearance. he perceived the shadow thrown by the ring upon the globe, and his explanation of the phenomena was obtained in a very philosophical manner. he noticed that the earth, the sun, and the moon rotated upon their axes, and he therefore regarded it as a general law that each one of the bodies in the system rotates about an axis. it is true, observations had not yet been made which actually showed that saturn was also rotating; but it would be highly, nay, indeed, infinitely, improbable that any planet should be devoid of such movement. all the analogies of the system pointed to the conclusion that the velocity of rotation would be considerable. one satellite of saturn was already known to revolve in a period of sixteen days, being little more than half our month. huyghens assumed--and it was a most reasonable assumption--that saturn in all probability rotated rapidly on its axis. it was also to be observed that if these remarkable appendages were attached by an actual bodily connection to the planet they must rotate with saturn. if, however, the appendages were not actually attached it would still be necessary that they should rotate if the analogy of saturn to other objects in the system were to be in any degree preserved. we see satellites near jupiter which revolve around him. we see, nearer home, how the moon revolves around the earth. we see how all the planetary system revolves around the sun. all these considerations were present to huyghens when he came to the conclusion that, whether the curious appendages were actually attached to the planet or were physically free from it, they must still be in rotation. provided with such reasonings, it soon became easy to conjecture the true nature of the saturnian system. we have seen how the appendages declined to invisibility once every fifteen years, and then gradually reappeared in the form, at first, of rectilinear arms projecting outwards from the planet. the progressive development is a slow one, and for weeks and months, night after night, the same appearance is presented with but little change. but all this time both saturn and the mysterious objects around him are rotating. whatever these may be, they present the same appearance to the eye, notwithstanding their ceaseless motion of rotation. what must be the shape of an object which satisfies the conditions here implied? it will obviously not suffice to regard the projections as two spokes diverging from the planet. they would change from visibility to invisibility in every rotation, and thus there would be ceaseless alterations of the appearance instead of that slow and gradual change which requires fifteen years for a complete period. there are, indeed, other considerations which preclude the possibility of the objects being anything of this character, for they are always of the same length as compared with the diameter of the planet. a little reflection will show that one supposition--and indeed only one--will meet all the facts of the case. if there were a thin symmetrical ring rotating in its own plane around the equator of saturn, then the persistence of the object from night to night would be accounted for. this at once removes the greater part of the difficulty. for the rest, it was only necessary to suppose that the ring was so thin that when turned actually edgewise to the earth it became invisible, and then as the illuminated side of the plane became turned more and more towards the earth the appendages to the planet gradually increased. the handle-shaped appearance which the object periodically assumed demonstrated that the ring could not be attached to the globe. at length huyghens found that he had the clue to the great enigma which had perplexed astronomers for the last fifty years. he saw that the ring was an object of astonishing interest, unique at that time, as it is, indeed, unique still. he felt, however, that he had hardly demonstrated the matter with all the certainty which it merited, and which he thought that by further attention he could secure. yet he was loath to hazard the loss of his discovery by an undue postponement of its announcement, lest some other astronomer might intervene. how, then, was he to secure his priority if the discovery should turn out correct, and at the same time be enabled to perfect it at his leisure? he adopted the course, usual at the time, of making his first announcement in cipher, and accordingly, on march th, , he published a tract, which contained the following proposition:-- aaaaaaa ccccc d eeeee g h iiiiiii llll mm nnnnnnnnn oooo pp q rr s ttttt uuuuu perhaps some of those curious persons whose successors now devote so much labour to double acrostics may have pondered on this renowned cryptograph, and even attempted to decipher it. but even if such attempts were made, we do not learn that they were successful. a few years of further study were thus secured to huyghens. he tested his theory in every way that he could devise, and he found it verified in every detail. he therefore thought that it was needless for him any longer to conceal from the world his great discovery, and accordingly in the year --about three years after the appearance of his cryptograph--he announced the interpretation of it. by restoring the letters to their original arrangement the discovery was enunciated in the following words:--"_annulo cingitur_, _tenui_, _plano_, _nusquam cohærente_, _ad eclipticam inclinato_," which may be translated into the statement:--"the planet is surrounded by a slender flat ring everywhere distinct from its surface, and inclined to the elliptic." huyghens was not content with merely demonstrating how fully this assumption explained all the observed phenomena. he submitted it to the further and most delicate test which can be applied to any astronomical theory. he attempted by its aid to make a prediction the fulfilment of which would necessarily give his theory the seal of certainty. from his calculations he saw that the planet would appear circular about july or august in . this anticipation was practically verified, for the ring was seen to vanish in may of that year. no doubt, with our modern calculations founded on long-continued and accurate observation, we are now enabled to make forecasts as to the appearance or the disappearance of saturn's ring with far greater accuracy; but, remembering the early stage in the history of the planet at which the prediction of huyghens was made, we must regard its fulfilment as quite sufficient, and as confirming in a satisfactory manner the theory of saturn and his ring. the ring of saturn having thus been thoroughly established as a fact in celestial architecture, each generation of astronomers has laboured to find out more and more of its marvellous features. in the frontispiece (plate i.) we have a view of the planet as seen at the harvard college observatory, u.s.a., between july th and october th, . it has been drawn by the skilful astronomer and artist--mr. l. trouvelot--and gives a faithful and beautiful representation of this unique object. fig. is a drawing of the same object taken on july nd, , by prof. e.e. barnard, at the lick observatory. the next great discovery in the saturnian system after those of huyghens showed that the ring surrounding the planet was marked by a dark concentric line, which divided it into two parts--the outer being narrower than the inner. this line was first seen by j.d. cassini, when saturn emerged from the rays of the sun in . that this black line is not merely a black mark on the ring, but that it is actually a separation, was rendered very probable by the researches of maraldi in , followed many years later by those of sir william herschel, who, with that thoroughness which was a marked characteristic of the man, made a minute and scrupulous examination of saturn. night after night he followed it for hours with his exquisite instruments, and considerably added to our knowledge of the planet and his system. herschel devoted very particular attention to the examination of the line dividing the ring. he saw that the colour of this line was not to be distinguished from the colour of the space intermediate between the globe and the ring. he observed it for ten years on the northern face of the ring, and during that time it continued to present the same breadth and colour and sharpness of outline. he was then fortunate enough to observe the southern side of the ring. there again could the black line be seen, corresponding both in appearance and in position with the dark line as seen on the northern side. no doubt could remain as to the fact that saturn was girdled by two concentric rings equally thin, the outer edge of one closely approaching to the inner edge of the other. at the same time it is right to add that the only absolutely indisputable proof of the division between the rings has not yet been yielded by the telescope. the appearances noted by herschel would be consistent with the view that the black line was merely a part of the ring extending through its thickness, and composed of materials very much less capable of reflecting light than the rest of the ring. it is still a matter of doubt how far it is ever possible actually to see through the dark line. there is apparently only one satisfactory method of accomplishing this. it would only occur in rare circumstances, and it does not seem that the opportunity has as yet arisen. suppose that in the course of its motion through the heavens the path of saturn happened to cross directly between the earth and a fixed star. the telescopic appearance of a star is merely a point of light much smaller than the globes and rings of saturn. if the ring passed in front of the star and the black line on the ring came over the star, we should, if the black line were really an opening, see the star shining through the narrow aperture. up to the present, we believe, there has been no opportunity of submitting the question of the duplex character of the ring to this crucial test. let us hope that as there are now so many telescopes in use adequate to deal with the subject, there may, ere long, be observations made which will decide the question. it can hardly be expected that a very small star would be suitable. no doubt the smallness of the star would render the observations more delicate and precise if the star were visible; but we must remember that it will be thrown into contrast with the bright rings of saturn on each margin so that unless the star were of considerable magnitude it would hardly answer. it has, however, been recently observed that the globe of the planet can be, in some degree, discerned through the dark line; this is practically a demonstration of the fact that the line is at all events partly transparent. the outer ring is also divided into two by a line much fainter than that just described. it requires a good telescope and a fine night, combined with a favourable position of the planet, to render this line a well-marked object. it is most easily seen at the extremities of the ring most remote from the planet. to the present writer, who has examined the planet with the twelve-inch refractor of the south equatorial at dunsink observatory, this outer line appears as broad as the well-known line; but it is unquestionably fainter, and has a more shaded appearance. it certainly does not suggest the appearance of being actually an opening in the ring, and it is often invisible for a long time. it seems rather as if the ring were at this place thinner and less substantial without being actually void of substance. on these points it may be expected that much additional information will be acquired when next the ring places itself in such a position that its plane, if produced, would pass between the earth and the sun. such occasions are but rare, and even when they do occur it may happen that the planet will not be well placed for observation. the next really good opportunity will not be till . in this case the sunlight illuminates one side of the ring, while it is the other side of the ring that is presented towards the earth. powerful telescopes are necessary to deal with the planet under such circumstances; but it may be reasonably hoped that the questions relating to the division of the ring, as well as to many other matters, will then receive some further elucidation. occasionally, other divisions of the ring, both inner and outer, have been recorded. it may, at all events, be stated that no such divisions can be regarded as permanent features. yet their existence has been so frequently enunciated by skilful observers that it is impossible to doubt that they have been sometimes seen. it was about years after huyghens had first explained the true theory of saturn that another very important discovery was effected. it had, up to the year , been always supposed that the two rings, divided by the well-known black line, comprised the entire ring system surrounding the planet. in the year just mentioned, professor bond, the distinguished astronomer of cambridge, mass., startled the astronomical world by the announcement of his discovery of a third ring surrounding saturn. as so often happens in such cases, the same object was discovered independently by another--an english astronomer named dawes. this third ring lies just inside the inner of the two well-known rings, and extends to about half the distance towards the body of the planet. it seems to be of a totally different character from the two other rings in so far as they present a comparatively substantial appearance. we shall, indeed, presently show that they are not solid--not even liquid bodies--but still, when compared with the third ring, the others were of a substantial character. they can receive and exhibit the deeply-marked shadow of saturn, and they can throw a deep and black shadow upon saturn themselves; but the third ring is of a much less compact texture. it has not the brilliancy of the others, it is rather of a dusky, semi-transparent appearance, and the expression "crape ring," by which it is often designated, is by no means inappropriate. it is the faintness of this crape ring which led to its having been so frequently overlooked by the earlier observers of saturn. it has often been noticed that when an astronomical discovery has been made with a good telescope, it afterwards becomes possible for the same object to be observed with instruments of much inferior power. no doubt, when the observer knows what to look for, he will often be able to see what would not otherwise have attracted his attention. it may be regarded as an illustration of this principle, that the crape ring of saturn has become an object familiar to those who are accustomed to work with good telescopes; but it may, nevertheless, be doubted whether the ease and distinctness with which the crape ring is now seen can be entirely accounted for by this supposition. indeed, it seems possible that the crape ring has, from some cause or other, gradually become more and more visible. the supposed increased brightness of the crape ring is one of those arguments now made use of to prove that in all probability the rings of saturn are at this moment undergoing gradual transformation; but observations of hadley show that the crape ring was seen by him in , and it was previously seen by campani and picard, as a faint belt crossing the planet. the partial transparency of the crape ring was beautifully illustrated in an observation by professor barnard of the eclipse of iapetus on november st, . the satellite was faintly visible in the shadow of the crape ring, while wholly invisible in the shadow of the better known rings. the various features of the rings are well shown in the drawing of trouvelot already referred to. we here see the inner and the outer ring, and the line of division between them. we see in the outer ring the faint traces of the line by which it is divided, and inside the inner ring we have a view of the curious and semi-transparent crape ring. the black shadow of the planet is cast upon the ring, thus proving that the ring, no less than the body of the planet, shines only in virtue of the sunlight which falls upon it. this shadow presents some anomalous features, but its curious irregularity may be, to some extent, an optical illusion. there can be no doubt that any attempt to depict the rings of saturn only represents the salient features of that marvellous system. we are situated at such a great distance that all objects not of colossal dimensions are invisible. we have, indeed, only an outline, which makes us wish to be able to fill in the details. we long, for instance, to see the actual texture of the rings, and to learn of what materials they are made; we wish to comprehend the strange and filmy crape ring, so unlike any other object known to us in the heavens. there is no doubt that much may even yet be learned under all the disadvantageous conditions of our position; there is still room for the labour of whole generations of astronomers provided with splendid instruments. we want accurate drawings of saturn under every conceivable aspect in which it may be presented. we want incessantly repeated measurements, of the most fastidious accuracy. these measures are to tell us the sizes and the shapes of the rings; they are to measure with fidelity the position of the dark lines and the boundaries of the rings. these measures are to be protracted for generations and for centuries; then and then only can terrestrial astronomers learn whether this elaborate system has really the attributes of permanence, or whether it may be undergoing changes. we have been accustomed to find that the law of universal gravitation pervades every part of our system, and to look to gravitation for the explanation of many phenomena otherwise inexplicable. we have good reasons for knowing that in this marvellous saturnian system the law of gravitation is paramount. there are satellites revolving around saturn as well as a ring; these satellites move, as other satellites do, in conformity with the laws of kepler; and, therefore, any theory as to the nature of saturn's ring must be formed subject to the condition that it shall be attracted by the gigantic planet situated in its interior. to a hasty glance nothing might seem easier than to reconcile the phenomena of the ring with the attraction of the planet. we might suppose that the ring stands at rest symmetrically around the planet. at its centre the planet pulls in the ring equally on all sides, so that there is no tendency in it to move in one way rather than another; and, therefore, it will stay at rest. this will not do. a ring composed of materials almost infinitely rigid might possibly, under such circumstances, be for a moment at rest; but it could not remain permanently at rest any more than can a needle balanced vertically on its point. in each case the equilibrium is unstable. if the slightest cause of disturbance arise, the equilibrium is destroyed, and the ring would inevitably fall in upon the planet. such causes of derangement are incessantly present, so that unstable equilibrium cannot be an appropriate explanation of the phenomena. even if this difficulty could be removed, there is still another, which would be quite insuperable if the ring be composed of any materials with which we are acquainted. let us ponder for a moment on the matter, as it will lead up naturally to that explanation of the rings of saturn which is now most generally accepted. imagine that you stood on the planet saturn, near his equator; over your head stretches the ring, which sinks down to the horizon in the east and in the west. the half-ring above your horizon would then resemble a mighty arch, with a span of about a hundred thousand miles. every particle of this arch is drawn towards saturn by gravitation, and if the arch continue to exist, it must do so in obedience to the ordinary mechanical laws which regulate the railway arches with which we are familiar. the continuance of these arches depends upon the resistance of the stones forming them to a crushing force. each stone of an arch is subjected to a vast pressure, but stone is a material capable of resisting such pressure, and the arch remains. the wider the span of the arch the greater is the pressure to which each stone is exposed. at length a span is reached which corresponds to a pressure as great as the stones can safely bear, and accordingly we thus find the limiting span over which a single arch of masonry can be constructed. apply these principles to the stupendous arch formed by the ring of saturn. it can be shown that the pressure on the materials of the arch capable of spanning an abyss of such awful magnitude would be something so enormous that no materials we know of would be capable of bearing it. were the ring formed of the toughest steel that was ever made, the pressure would be so great that the metal would be squeezed like a liquid, and the mighty structure would collapse and fall down on the surface of the planet. it is not credible that any materials could exist capable of sustaining a stress so stupendous. the law of gravitation accordingly bids us search for a method by which the intensity of this stress can be mitigated. one method is at hand, and is obviously suggested by analogous phenomena everywhere in our system. we have spoken of the ring as if it were at rest; let us now suppose it to be animated by a motion of rotation in its plane around saturn as a centre. instantly we have a force developed antagonistic to the gravitation of saturn. this force is the so-called centrifugal force. if we imagine the ring to rotate, the centrifugal force at all points acts in an opposite direction to the attractive force, and hence the enormous stress on the ring can be abated and one difficulty can be overcome. we can thus attribute to each ring a rotation which will partly relieve it from the stress the arch would otherwise have to sustain. but we cannot admit that the difficulty has been fully removed. suppose that the outer ring revolve at such a rate as shall be appropriate to neutralise the gravitation on its outer edge, the centrifugal force will be less at the interior of the ring, while the gravitation will be greater; and hence vast stresses will be set up in the interior parts of the outer ring. suppose the ring to rotate at such a rate as would be adequate to neutralise the gravitation at its inner margin; then the centrifugal force at the outer parts will largely exceed the gravitation, and there will be a tendency to disruption of the ring outwards. to obviate this tendency we may assume the outer parts of each ring to rotate more slowly than the inner parts. this naturally requires that the parts of the ring shall be mobile relatively to one another, and thus we are conducted to the suggestion that perhaps the rings are really composed of matter in a fluid state. the suggestion is, at first sight, a plausible one; each part of each ring would then move with an appropriate velocity, and the rings would thus exhibit a number of concentric circular currents with different velocities. the mathematician can push this inquiry a little farther, and he can study how this fluid would behave under such circumstances. his symbols can pursue the subject into the intricacies which cannot be described in general language. the mathematician finds that waves would originate in the supposed fluid, and that as these waves would lead to disruption of the rings, the fluid theory must be abandoned. but we can still make one or two more suppositions. what if it be really true that the ring consist of an incredibly large number of concentric rings, each animated precisely with the velocity which would be suitable to the production of a centrifugal force just adequate to neutralise the attraction? no doubt this meets many of the difficulties: it is also suggested by those observations which have shown the presence of several dark lines on the ring. here again dynamical considerations must be invoked for the reply. such a system of solid rings is not compatible with the laws of dynamics. we are, therefore, compelled to make one last attempt, and still further to subdivide the ring. it may seem rather startling to abandon entirely the supposition that the ring is in any sense a continuous body, but there remains no alternative. look at it how we will, we seem to be conducted to the conclusion that the ring is really an enormous shoal of extremely minute bodies; each of these little bodies pursues an orbit of its own around the planet, and is, in fact, merely a satellite. these bodies are so numerous and so close together that they seem to us to be continuous, and they may be very minute--perhaps not larger than the globules of water found in an ordinary cloud over the surface of the earth, which, even at a short distance, seems like a continuous body. until a few years ago this theory of the constitution of saturn's rings, though unassailable from a mathematical point of view, had never been confirmed by observation. the only astronomer who maintained that he had actually seen the rings rotate was w. herschel, who watched the motion of some luminous points on the ring in , at which time the plane of the ring happened to pass through the earth. from these observations herschel concluded that the ring rotated in ten hours and thirty-two minutes. but none of the subsequent observers, even though they may have watched saturn with instruments very superior to that used by herschel, were ever able to succeed in verifying his rotation of these appendages of saturn. if the ring were composed of a vast number of small bodies, then the third law of kepler will enable us to calculate the time which these tiny satellites would require to travel completely round the planet. it appears that any satellite situated at the outer edge of the ring would require as long a period as hrs. min., those about the middle would not need more than hrs. min., while those at the inner edge of the ring would accomplish their rotation in hrs. min. even our mightiest telescopes, erected in the purest skies and employed by the most skilful astronomers, refuse to display this extremely delicate phenomenon. it would, indeed, have been a repetition on a grand scale of the curious behaviour of the inner satellite of mars, which revolves round its primary in a shorter time than the planet itself takes to turn round on its own axis. [illustration: fig. .--prof. keeler's method of measuring the rotation of saturn's ring.] but what the telescope could not show, the spectroscope has lately demonstrated in a most effective and interesting manner. we have explained in the chapter on the sun how the motion of a source of light along the line of vision, towards or away from the observer, produces a slight shift in the position of the lines of the spectrum. by the measurement of the displacement of the lines the direction and amount of the motion of the source of light may be determined. we illustrated the method by showing how it had actually been used to measure the speed of rotation of the solar surface. in professor keeler,[ ] director of the allegheny observatory, succeeded in measuring the rotation of saturn's ring in this manner. he placed the slit of his spectroscope across the ball, in the direction of the major axis of the elliptic figure which the effect of perspective gives the ring as shown by the parallel lines in fig. stretching from e to w. his photographic plate should then show three spectra close together, that of the ball of saturn in the middle, separated by dark intervals from the narrower spectra above and below it of the two handles (or ansæ, as they are generally called) of the ring. in fig. we have represented the behaviour of any one line of the spectrum under various suppositions as to rotation or non-rotation of saturn and the ring. at the top ( ) we see how each line would look if there was no rotatory motion; the three lines produced by ring, planet, and ring are in a straight line. of course the spectrum, which is practically a very faint copy of the solar spectrum, shows the principal dark fraunhofer lines, so that the reader must imagine these for himself, parallel to the one we show in the figure. but saturn and the ring are not standing still, they are rotating, the eastern part (at e) moving towards us, and the western part (w) moving away from us.[ ] at e the line will therefore be shifted towards the violet end of the spectrum and at w towards the red, and as the actual linear velocity is greater the further we get away from the centre of saturn (assuming ring and planet to rotate together), the lines would be turned as in fig. ( ), but the three would remain in a straight line. if the ring consisted of two independent rings separated by cassini's division and rotating with different velocities, the lines would be situated as in fig. ( ), the lines due to the inner ring being more deflected than those due to the outer ring, owing to the greater velocity of the inner ring. [illustration: fig. .--prof. keeler's method of measuring the rotation of saturn's ring.] finally, let us consider the case of the rings, consisting of innumerable particles moving round the planet in accordance with kepler's third law. the actual velocities of these particles would be per second:-- at outer edge of ring · miles. at middle of ring · miles. at inner edge of ring · miles. rotation speed at surface of planet · miles. the shifting of the lines of the spectrum should be in accordance with these velocities, and it is easy to see that the lines ought to lie as in the fourth figure. when professor keeler came to examine the photographed spectra, he found the lines of the three spectra tilted precisely in this manner, showing that the outer edge of the ring was travelling round the planet with a smaller linear velocity than the inner one, as it ought to do if the sources of light (or, rather, the reflectors of sunlight) were independent particles free to move according to kepler's third law, and as it ought not to do if the ring, or rings, were rigid, in which case the outer edge would have the greatest linear speed, as it had to travel through the greatest distance. here, at last, was the proof of the meteoritic composition of saturn's ring. professor keeler's beautiful discovery has since been verified by repeated observations at the allegheny, lick, paris, and pulkova observatories; the actual velocities resulting from the observed displacements of the lines have been measured and found to agree well (within the limits of the errors of observation) with the calculated velocities, so that this brilliant confirmation of the mathematical deductions of clerk maxwell is raised beyond the possibility of doubt. the spectrum of saturn is so faint that only the strongest lines of the solar spectrum can be seen in it, but the atmosphere of the planet seems to exert a considerable amount of general absorption in the blue and violet parts of the spectrum, which is especially strong near the equatorial belt, while a strong band in the red testifies to the density of the atmosphere. this band is not seen in the spectrum of the rings, around which there can therefore be no atmosphere. as saturn's ring is itself unique, we cannot find elsewhere any very pertinent illustration of a structure so remarkable as that now claimed for the ring. yet the solar system does show some analogous phenomena. there is, for instance, one on a very grand scale surrounding the sun himself. we allude to the multitude of minor planets, all confined within a certain region of the system. imagine these planets to be vastly increased in number, and those orbits which are much inclined to the rest flattened down and otherwise adjusted, and we should have a ring surrounding the sun, thus producing an arrangement not dissimilar from that now attributed to saturn. it is tempting to linger still longer over this beautiful system, to speculate on the appearance which the ring would present to an inhabitant of saturn, to conjecture whether it is to be regarded as a permanent feature of our system in the same way as we attribute permanence to our moon or to the satellites of jupiter. looked at from every point of view, the question is full of interest, and it provides occupation abundant for the labours of every type of astronomer. if he be furnished with a good telescope, then has he ample duties to fulfil in the task of surveying, of sketching, and of measuring. if he be one of those useful astronomers who devote their energies not to actual telescopic work, but to forming calculations based on the observations of others, then the beautiful system of saturn provides copious material. he has to foretell the different phases of the ring, to announce to astronomers when each feature can be best seen, and at what hour each element can be best determined. he has also to predict the times of the movements of saturn's satellites, and the other phenomena of a system more elaborate than that of jupiter. lastly, if the astronomer be one of that class--perhaps, from some points of view, the highest class of all--who employ the most profound researches of the human intellect to unravel the dynamical problems of astronomy, he, too, finds in saturn problems which test to the utmost, even if they do not utterly transcend, the loftiest flights of analysis. he discovers in saturn's ring an object so utterly unlike anything else, that new mathematical weapons have to be forged for the encounter. he finds in the system so many extraordinary features, and such delicacy of adjustment, that he is constrained to admit that if he did not actually see saturn's rings before him, he would not have thought that such a system was possible. the mathematician's labours on this wondrous system are at present only in their infancy. not alone are the researches of so abstruse a character as to demand the highest genius for this branch of science, but even yet the materials for the inquiry have not been accumulated. in a discussion of this character, observation must precede calculation. the scanty observations hitherto obtained, however they may illustrate the beauty of the system, are still utterly insufficient to form the basis of that great mathematical theory of saturn which must eventually be written. but saturn possesses an interest for a far more numerous class of persons than those who are specially devoted to astronomy. it is of interest, it must be of interest, to every cultivated person who has the slightest love for nature. a lover of the picturesque cannot behold saturn in a telescope without feelings of the liveliest emotion; while, if his reading and reflection have previously rendered him aware of the colossal magnitude of the object at which he is looking, he will be constrained to admit that no more remarkable spectacle is presented in the whole of nature. we have pondered so long over the fascinations of saturn's ring that we can only give a very brief account of that system of satellites by which the planet is attended. we have already had occasion to allude more than once to these bodies; it only remains now to enumerate a few further particulars. it was on the th of march, , that the first satellite of saturn was detected by huyghens, to whose penetration we owe the discovery of the true form of the ring. on the evening of the day referred to, huyghens was examining saturn with a telescope constructed with his own hands, when he observed a small star-like object near the planet. the next night he repeated his observations, and it was found that the star was accompanying the planet in its progress through the heavens. this showed that the little object was really a satellite to saturn, and further observations revealed the fact that it was revolving around him in a period of days, hours, minutes. such was the commencement of that numerous series of discoveries of satellites which accompany saturn. one by one they were detected, so that at the present time no fewer than nine are known to attend the great planet through his wanderings. the subsequent discoveries were, however, in no case made by huyghens, for he abandoned the search for any further satellites on grounds which sound strange to modern ears, but which were quite in keeping with the ideas of his time. it appears that from some principle of symmetry, huyghens thought that it would accord with the fitness of things that the number of satellites, or secondary planets, should be equal in number to the primary planets themselves. the primary planets, including the earth, numbered six; and huyghens' discovery now brought the total number of satellites to be also six. the earth had one, jupiter had four, saturn had one, and the system was complete. nature, however, knows no such arithmetical doctrines as those which huyghens attributed to her. had he been less influenced by such prejudices, he might, perhaps, have anticipated the labours of cassini, who, by discovering other satellites of saturn, demonstrated the absurdity of the doctrine of numerical equality between planets and satellites. as further discoveries were made, the number of satellites was at first raised above the number of planets; but in recent times, when the swarm of minor planets came to be discovered, the number of planets speedily reached and speedily passed the number of their attendant satellites. it was in , about sixteen years after the discovery of the first satellite of saturn, that a second was discovered by cassini. this is the outermost of the older satellites; it takes days to travel round saturn. in the following year he discovered another; and twelve years later, in , still two more; thus making a total of five satellites to this planet. [illustration: fig. .--transit of titan and its shadow, by f. terby louvain, th april, .] the complexity of the saturnian system had now no rival in the heavens. saturn had five satellites, and jupiter had but four, while at least one of the satellites of saturn, named titan, was larger than any satellite of jupiter.[ ] some of the discoveries of cassini had been made with telescopes of quite monstrous dimensions. the length of the instrument, or rather the distance at which the object-glass was placed, was one hundred feet or more from the eye of the observer. it seemed hardly possible to push telescopic research farther with instruments of this cumbrous type. at length, however, the great reformation in the construction of astronomical instruments began to dawn. in the hands of herschel, it was found possible to construct reflecting telescopes of manageable dimensions, which were both more powerful and more accurate than the long-focussed lenses of cassini. a great instrument of this kind, forty feet long, just completed by herschel, was directed to saturn on the th of august, . never before had the wondrous planet been submitted to a scrutiny so minute. herschel was familiar with the labours of his predecessors. he had often looked at saturn and his five moons in inferior telescopes; now again he saw the five moons and a star-like object so near the plane of the ring that he conjectured this to be a sixth satellite. a speedy method of testing this conjecture was at hand. saturn was then moving rapidly over the heavens. if this new object were in truth a satellite, then it must be carried on by saturn. herschel watched with anxiety to see whether this would be the case. a short time sufficed to answer the question; in two hours and a half the planet had moved to a distance quite appreciable, and had carried with him not only the five satellites already known, but also this sixth object. had this been a star it would have been left behind; it was not left behind, and hence it, too, was a satellite. thus, after the long lapse of a century, the telescopic discovery of satellites to saturn recommenced. herschel, as was his wont, observed this object with unremitting ardour, and discovered that it was much nearer to saturn than any of the previously known satellites. in accordance with the general law, that the nearer the satellite the shorter the period of revolution, herschel found that this little moon completed a revolution in about day, hours, minutes. the same great telescope, used with the same unrivalled skill, soon led herschel to a still more interesting discovery. an object so small as only to appear like a very minute point in the great forty-foot reflector was also detected by herschel, and was by him proved to be a satellite, so close to the planet that it completed a revolution in the very brief period of hours and minutes. this is an extremely delicate object, only to be seen by the best telescopes in the brief intervals when it is not entirely screened from view by the ring. again another long interval elapsed, and for almost fifty years the saturnian system was regarded as consisting of the series of rings and of the seven satellites. the next discovery has a singular historical interest. it was made simultaneously by two observers--professor bond, of cambridge, mass., and mr. lassell, of liverpool--for on the th september, , both of these astronomers verified that a small point which they had each seen on previous nights was really a satellite. this object is, however, at a considerable distance from the planet, and requires days, hours, minutes for each revolution; it is the seventh in order from the planet. yet one more extremely faint outer satellite was discerned by photography on the th, th, and th august, , by professor w.h. pickering. this object is much more distant from the planet than the larger and older satellites. its motion has not yet been fully determined, but probably it requires not less than days to perform a single revolution. from observations of the satellites it has been found that , globes as heavy as saturn would weigh as much as the sun. a law has been observed by professor kirkwood, which connects together the movements of the four interior satellites of saturn. this law is fulfilled in such a manner as leads to the supposition that it arises from the mutual attraction of the satellites. we have already described a similar law relative to three of the satellites of jupiter. the problem relating to saturn, involving as it does no fewer than four satellites, is one of no ordinary complexity. it involves the theory of perturbations to a greater degree than that to which mathematicians are accustomed in their investigation of the more ordinary features of our system. to express this law it is necessary to have recourse to the daily movements of the satellites; these are respectively-- satellite. daily movement. i. °· . ii. °· . iii. °· . iv. °· . the law states that if to five times the movement of the first satellite we add that of the third and four times that of the fourth, the whole will equal ten times the movement of the second satellite. the calculation stands thus:-- times i. equals °· iii. equals °· ii. °· times iv. equals °· -------- -------- °· equal °· nearly. nothing can be simpler than the verification of this law; but the task of showing the physical reason why it should be fulfilled has not yet been accomplished. saturn was the most distant planet known to the ancients. it revolves in an orbit far outside the other ancient planets, and, until the discovery of uranus in the year , the orbit of saturn might well be regarded as the frontier of the solar system. the ringed planet was indeed a worthy object to occupy a position so distinguished. but we now know that the mighty orbit of saturn does not extend to the frontiers of the solar system; a splendid discovery, leading to one still more splendid, has vastly extended the boundary, by revealing two mighty planets, revolving in dim telescopic distance, far outside the path of saturn. these objects have not the beauty of saturn; they are, indeed, in no sense effective telescopic pictures. yet these outer planets awaken an interest of a most special kind. the discovery of each is a classical event in the history of astronomy, and the opinion has been maintained, and perhaps with reason, that the discovery of neptune, the more remote of the two, is the greatest achievement in astronomy made since the time of newton. chapter xiv uranus. contrast between uranus and the other great planets--william herschel--his birth and parentage--herschel's arrival in england--his love of learning--commencement of his astronomical studies--the construction of telescopes--construction of mirrors--the professor of music becomes an astronomer--the methodical research--the th march, --the discovery of uranus--delicacy of observation--was the object a comet?--the significance of this discovery--the fame of herschel--george iii. and the bath musician--the king's astronomer at windsor--the planet uranus--numerical data with reference thereto--the four satellites of uranus--their circular orbits--early observations of uranus--flamsteed's observations--lemonnier saw uranus--utility of their measurements--the elliptic path--the great problem thus suggested. to the present writer it has always seemed that the history of uranus, and of the circumstances attending its discovery, forms one of the most pleasing and interesting episodes in the whole history of science. we here occupy an entirely new position in the study of the solar system. all the other great planets were familiarly known from antiquity, however erroneous might be the ideas entertained in connection with them. they were conspicuous objects, and by their movements could hardly fail to attract the attention of those whose pursuits led them to observe the stars. but now we come to a great planet, the very existence of which was utterly unknown to the ancients; and hence, in approaching the subject, we have first to describe the actual discovery of this object, and then to consider what we can learn as to its physical nature. we have, in preceding pages, had occasion to mention the revered name of william herschel in connection with various branches of astronomy; but we have hitherto designedly postponed any more explicit reference to this extraordinary man until we had arrived at the present stage of our work. the story of uranus, in its earlier stages at all events, is the story of the early career of william herschel. it would be alike impossible and undesirable to attempt to separate them. william herschel, the illustrious astronomer, was born at hanover in . his father was an accomplished man, pursuing, in a somewhat humble manner, the calling of a professor of music. he had a family of ten children, of whom william was the fourth; and it may be noted that all the members of the family of whom any record has been preserved inherited their father's musical talents, and became accomplished performers. pleasing sketches have been given of this interesting family, of the unusual aptitude of william, of the long discussions on music and on philosophy, and of the little sister caroline, destined in later years for an illustrious career. william soon learned all that his master could teach him in the ordinary branches of knowledge, and by the age of fourteen he was already a competent performer on the oboe and the viol. he was engaged in the court orchestra at hanover, and was also a member of the band of the hanoverian guards. troublous times were soon to break up herschel's family. the french invaded hanover, the hanoverian guards were overthrown in the battle of hastenbeck, and young william herschel had some unpleasant experience of actual warfare. his health was not very strong, and he decided that he would make a change in his profession. his method of doing so is one which his biographers can scarcely be expected to defend; for, to speak plainly, he deserted, and succeeded in making his escape to england. it is stated on unquestionable authority that on herschel's first visit to king george iii., more than twenty years afterwards, his pardon was handed to him by the king himself, written out in due form. at the age of nineteen the young musician began to seek his fortunes in england. he met at first with very considerable hardship, but industry and skill conquered all difficulties, and by the time he was twenty-six years of age he was thoroughly settled in england, and doing well in his profession. in the year we find herschel occupying a position of some distinction in the musical world; he had become the organist of the octagon chapel at bath, and his time was fully employed in giving lessons to his numerous pupils, and with his preparation for concerts and oratorios. notwithstanding his busy professional life, herschel still retained that insatiable thirst for knowledge which he had when a boy. every moment he could snatch from his musical engagements was eagerly devoted to study. in his desire to perfect his knowledge of the more abstruse parts of the theory of music he had occasion to learn mathematics; from mathematics the transition to optics was a natural one; and once he had commenced to study optics, he was of course brought to a knowledge of the telescope, and thence to astronomy itself. his beginnings were made on a very modest scale. it was through a small and imperfect telescope that the great astronomer obtained his first view of the celestial glories. no doubt he had often before looked at the heavens on a clear night, and admired the thousands of stars with which they were adorned; but now, when he was able to increase his powers of vision even to a slight extent, he obtained a view which fascinated him. the stars he had seen before he now saw far more distinctly; but, more than this, he found that myriads of others previously invisible were now revealed to him. glorious, indeed, is this spectacle to anyone who possesses a spark of enthusiasm for natural beauty. to herschel this view immediately changed the whole current of his life. his success as a professor of music, his oratorios, and his pupils were speedily to be forgotten, and the rest of his life was to be devoted to the absorbing pursuit of one of the noblest of the sciences. herschel could not remain contented with the small and imperfect instrument which first interested him. throughout his career he determined to see everything for himself in the best manner which his utmost powers could command. he at once decided to have a better instrument, and he wrote to a celebrated optician in london with the view of making a purchase. but the price which the optician demanded seemed more than herschel thought he could or ought to give. instantly his resolution was taken. a good telescope he must have, and as he could not buy one he resolved to make one. it was alike fortunate, both for herschel and for science, that circumstances impelled him to this determination. yet, at first sight, how unpromising was the enterprise! that a music teacher, busily employed day and night, should, without previous training, expect to succeed in a task where the highest mechanical and optical skill was required, seemed indeed unlikely. but enthusiasm and genius know no insuperable difficulties. from conducting a brilliant concert in bath, when that city was at the height of its fame, herschel would rush home, and without even delaying to take off his lace ruffles, he would plunge into his manual labours of grinding specula and polishing lenses. no alchemist of old was ever more deeply absorbed in a project for turning lead into gold than was herschel in his determination to have a telescope. he transformed his home into a laboratory; of his drawing-room he made a carpenter's shop. turning lathes were the furniture of his best bedroom. a telescope he must have, and as he progressed he determined, not only that he should have a good telescope, but a very good one; and as success cheered his efforts he ultimately succeeded in constructing the greatest telescope that the world had up to that time ever seen. though it is as an astronomer that we are concerned with herschel, yet we must observe even as a telescope maker also great fame and no small degree of commercial success flowed in upon him. when the world began to ring with his glorious discoveries, and when it was known that he used no other telescopes than those which were the work of his own hands, a demand sprang up for instruments of his construction. it is stated that he made upwards of eighty large telescopes, as well as many others of smaller size. several of these instruments were purchased by foreign princes and potentates.[ ] we have never heard that any of these illustrious personages became celebrated astronomers, but, at all events, they seem to have paid herschel handsomely for his skill, so that by the sale of large telescopes he was enabled to realise what may be regarded as a fortune in the moderate horizon of the man of science. up to the middle of his life herschel was unknown to the public except as a laborious musician, with considerable renown in his profession, not only in bath, but throughout the west of england. his telescope-making was merely the occupation of his spare moments, and was unheard of by most of those who knew and respected his musical attainments. it was in that herschel first enjoyed a view of the heavens through an instrument built with his own hands. it was but a small one in comparison with those which he afterwards fashioned, but at once he experienced the advantage of being his own instrument maker. night after night he was able to add the improvements which experience suggested; at one time he was enlarging the mirrors; at another he was reconstructing the mounting, or trying to remedy defects in the eye-pieces. with unwearying perseverance he aimed at the highest excellence, and with each successive advance he found that he was able to pierce further into the sky. his enthusiasm attracted a few friends who were, like himself, ardently attached to science. the mode in which he first made the acquaintance of sir william watson, who afterwards became his warmest friend, was characteristic of both. herschel was observing the mountains in the moon, and as the hours passed on, he had occasion to bring his telescope into the street in front of his house to enable him to continue his work. sir william watson happened to pass by, and was arrested by the unusual spectacle of an astronomer in the public street, at the dead of night, using a large and quaint-looking instrument. having a taste for astronomy, sir william stopped, and when herschel took his eye from the telescope, asked if he might be allowed to have a look at the moon. the request was readily granted. probably herschel found but few in the gay city who cared for such matters; he was quickly drawn to sir w. watson, who at once reciprocated the feeling, and thus began a friendship which bore important fruit in herschel's subsequent career. at length the year approached, which was to witness his great achievement. herschel had made good use of seven years' practical experience in astronomy, and he had completed a telescope of exquisite optical perfection, though greatly inferior in size to some of those which he afterwards erected. with this reflector herschel commenced a methodical piece of observation. he formed the scheme of systematically examining all the stars which were above a certain degree of brightness. it does not quite appear what object herschel proposed to himself when he undertook this labour, but, in any case, he could hardly have anticipated the extraordinary success with which the work was to be crowned. in the course of this review the telescope was directed to a star; that star was examined; then another was brought into the field of view, and it too was examined. every star under such circumstances merely shows itself as a point of light; the point may be brilliant or not, according as the star is bright or not; the point will also, of course, show the colour of the star, but it cannot exhibit recognisable size or shape. the greater, in fact, the perfection of the telescope, the smaller is the telescopic image of a star. how many stars herschel inspected in this review we are not told; but at all events, on the ever-memorable night of the th of march, , he was pursuing his self-allotted task among the hosts in the constellation gemini. doubtless, one star after another was admitted to view, and was allowed to pass away. at length, however, an object was placed in the field which differed from every other star. it was not a mere point of light; it had a minute, but still a perfectly recognisable, disc. we say the disc was perfectly recognisable, but we should be careful to add that it was so in the excellent telescope of herschel alone. other astronomers had seen this object before. its position had actually been measured no fewer than nineteen times before the bath musician, with his home-made telescope, looked at it, but the previous observers had only seen it in small meridian instruments with low magnifying powers. even after the discovery was made, and when well-trained observers with good instruments looked again under the direction of herschel, one after another bore testimony to the extraordinary delicacy of the great astronomer's perception, which enabled him almost at the first glance to discriminate between it and a star. if not a star, what, then, could it be? the first step to enable this question to be answered was to observe the body for some time. this herschel did. he looked at it one night after another, and soon he discovered another fundamental difference between this object and an ordinary star. the stars are, of course, characterised by their fixity, but this object was not fixed; night after night the place it occupied changed with respect to the stars. no longer could there be any doubt that this body was a member of the solar system, and that an interesting discovery had been made; many months, however, elapsed before herschel knew the real merit of his achievement. he did not realise that he had made the superb discovery of another mighty planet revolving outside saturn; he thought that it could only be a comet. no doubt this object looked very different from a great comet, decorated with a tail. it was not, however, so entirely different from some forms of telescopic comets as to make the suggestion of its being a body of this kind unlikely; and the discovery was at first announced in accordance with this view. time was necessary before the true character of the object could be ascertained. it must be followed for a considerable distance along its path, and measures of its position at different epochs must be effected, before it is practicable for the mathematician to calculate the path which the body pursues; once, however, attention was devoted to the subject, many astronomers aided in making the necessary observations. these were placed in the hands of mathematicians, and the result was proclaimed that this body was not a comet, but that, like all the planets, it revolved in nearly a circular path around the sun, and that the path lay millions of miles outside the path of saturn, which had so long been regarded as the boundary of the solar system. it is hardly possible to over-estimate the significance of this splendid discovery. the five planets had been known from all antiquity; they were all, at suitable seasons, brilliantly conspicuous to the unaided eye. but it was now found that, far outside the outermost of these planets revolved another splendid planet, larger than mercury or mars, larger--far larger--than venus and the earth, and only surpassed in bulk by jupiter and by saturn. this superb new planet was plunged into space to such a depth that, notwithstanding its noble proportions, it seemed merely a tiny star, being only on rare occasions within reach of the unaided eye. this great globe required a period of eighty-four years to complete its majestic path, and the diameter of that path was , , , miles. although the history of astronomy is the record of brilliant discoveries--of the labours of copernicus, and of kepler--of the telescopic achievements of galileo, and the splendid theory of newton--of the refined discovery of the aberration of light--of many other imperishable triumphs of intellect--yet this achievement of the organist at the octagon chapel occupies a totally different position from any other. there never before had been any historic record of the discovery of one of the bodies of the particular system to which the earth belongs. the older planets were no doubt discovered by someone, but we can say little more about these discoveries than we can about the discovery of the sun or of the moon; all are alike prehistoric. here was the first recorded instance of the discovery of a planet which, like the earth, revolves around the sun, and, like our earth, may conceivably be an inhabited globe. so unique an achievement instantly arrested the attention of the whole scientific world. the music-master at bath, hitherto unheard of as an astronomer, was speedily placed in the very foremost rank of those entitled to the name. on all sides the greatest interest was manifested about the unknown philosopher. the name of herschel, then unfamiliar to english ears, appeared in every journal, and a curious list has been preserved of the number of blunders which were made in spelling the name. the different scientific societies hastened to convey their congratulations on an occasion so memorable. tidings of the discovery made by the hanoverian musician reached the ears of george iii., and he sent for herschel to come to the court, that the king might learn what his achievement actually was from the discoverer's own lips. herschel brought with him one of his telescopes, and he provided himself with a chart of the solar system, with which to explain precisely wherein the significance of the discovery lay. the king was greatly interested in herschel's narrative, and not less in herschel himself. the telescope was erected at windsor, and, under the astronomer's guidance, the king was shown saturn and other celebrated objects. it is also told how the ladies of the court the next day asked herschel to show them the wonders which had so pleased the king. the telescope was duly erected in a window of one of the queen's apartments, but when evening arrived the sky was found to be overcast with clouds, and no stars could be seen. this was an experience with which herschel, like every other astronomer, was unhappily only too familiar. but it is not every astronomer who would have shown the readiness of herschel in escaping gracefully from the position. he showed to his lady pupils the construction of the telescope; he explained the mirror, and how he had fashioned it and given the polish; and then, seeing the clouds were inexorable, he proposed that, as he could not show them the real saturn, he should exhibit an artificial one as the best substitute. the permission granted, herschel turned the telescope away from the sky, and pointed it towards the wall of a distant garden. on looking into the telescope there was saturn, his globe and his system of rings, so faithfully shown that, says herschel, even a skilful astronomer might have been deceived. the fact was that during the course of the day herschel saw that the sky would probably be overcast in the evening, and he had provided for the emergency by cutting a hole in a piece of cardboard, the shape of saturn, which was then placed against the distant garden wall, and illuminated by a lamp at the back. this visit to windsor was productive of consequences momentous to herschel, momentous to science. he had made so favourable an impression, that the king proposed to create for him the special appointment of king's astronomer at windsor. the king was to provide the means for erecting the great telescopes, and he allocated to herschel a salary of £ a year, the figures being based, it must be admitted, on a somewhat moderate estimate of the requirements of an astronomer's household. herschel mentioned these particulars to no one save to his constant and generous friend, sir w. watson, who exclaimed, "never bought monarch honour so cheap." to other enquirers, herschel merely said that the king had provided for him. in accepting this post, the great astronomer took no doubt a serious step. he at once sacrificed entirely his musical career, now, from many sources, a lucrative one; but his determination was speedily taken. the splendid earnest that he had already given of his devotion to astronomy was, he knew, only the commencement of a series of memorable labours. he had indeed long been feeling that it was his bounden duty to follow that path in life which his genius indicated. he was no longer a young man. he had attained middle age, and the years had become especially precious to one who knew that he had still a life-work to accomplish. he at one stroke freed himself from all distractions; his pupils and concerts, his whole connection at bath, were immediately renounced; he accepted the king's offer with alacrity, and after one or two changes settled permanently at slough, near windsor. it has, indeed, been well remarked that the most important event in connection with the discovery of uranus was the discovery of herschel's unrivalled powers of observation. uranus must, sooner or later, have been found. had herschel not lived, we would still, no doubt, have known uranus long ere this. the really important point for science was that herschel's genius should be given full scope, by setting him free from the engrossing details of an ordinary professional calling. the discovery of uranus secured all this, and accordingly obtained for astronomy all herschel's future labours.[ ] uranus is so remote that even the best of our modern telescopes cannot make of it a striking picture. we can see, as herschel did, that it has a measurable disc, and from measurements of that disc we conclude that the diameter of the planet is about , miles. this is about four times as great as the diameter of the earth, and we accordingly see that the volume of uranus must be about sixty-four times as great as that of the earth. we also find that, like the other giant planets, uranus seems to be composed of materials much lighter, on the whole, than those we find here; so that, though sixty-four times as large as the earth, uranus is only fifteen times as heavy. if we may trust to the analogies of what we see everywhere else in our system, we can feel but little doubt that uranus must rotate about an axis. the ordinary means of demonstrating this rotation can be hardly available in a body whose surface appears so small and so faint. the period of rotation is accordingly unknown. the spectroscope tells us that a remarkable atmosphere, containing apparently some gases foreign to our own, deeply envelops uranus. there is, however, one feature about uranus which presents many points of interest to those astronomers who are possessed of telescopes of unusual size and perfection. uranus is accompanied by a system of satellites, some of which are so faint as to require the closest scrutiny for their detection. the discovery of these satellites was one of the subsequent achievements of herschel. it is, however, remarkable that even his penetration and care did not preserve him from errors with regard to these very delicate objects. some of the points which he thought to be satellites must, it would now seem, have been merely stars enormously more distant, which happened to lie in the field of view. it has been since ascertained that the known satellites of uranus are four in number, and their movements have been made the subject of prolonged and interesting telescopic research. the four satellites bear the names of ariel, umbriel, titania, and oberon. arranged in order of their distance from the central body, ariel, the nearest, accomplishes its journey in days and hours. oberon, the most distant, completes its journey in days and hours. the law of kepler declares that the path of a satellite around its primary, no less than of the primary around the sun, must be an ellipse. it leaves, however, boundless latitude in the actual eccentricity of the curve. the ellipse may be nearly a circle, it may be absolutely a circle, or it may be something quite different from a circle. the paths pursued by the planets are, generally speaking, nearly circles; but we meet with no exact circle among planetary orbits. so far as we at present know, the closest approach made to a perfectly circular movement is that by which the satellites of uranus revolve around their primary. we are not prepared to say that these paths are absolutely circular. all that can be said is that our telescopes fail to show any measurable departure therefrom. it is also to be noted as an interesting circumstance that the orbits of the satellites of uranus all lie in the same plane. this is not true of the orbits of the planets around the sun, nor is it true of the orbits of any other system of satellites around their primary. the most singular circumstance attending the uranian system is, however, found in the position which this plane occupies. this is indeed almost as great an anomaly in our system as are the rings of saturn themselves. we have already had occasion to notice that the plane in which the earth revolves around the sun is very nearly coincident with the planes in which all the other great planets revolve. the same is true, to a large extent, of the orbits of the minor planets; though here, no doubt, we meet with a few cases in which the plane of the orbit is inclined at no inconsiderable angle to the plane in which the earth moves. the plane in which the moon revolves also approximates to this system of planetary planes. so, too, do the orbits of the satellites of saturn and of jupiter, while even the more recently discovered satellites of mars form no exception to the rule. the whole solar system--at least so far as the great planets are concerned--would require comparatively little alteration if the orbits were to be entirely flattened down into one plane. there are, however, some notable exceptions to this rule. the satellites of uranus revolve in a plane which is far from coinciding with the plane to which all other orbits approximate. in fact, the paths of the satellites of uranus lie in a plane nearly at right angles to the orbit of uranus. we are not in a position to give any satisfactory explanation of this circumstance. it is, however, evident that in the genesis of the uranian system there must have been some influence of a quite exceptional and local character. soon after the discovery of the planet uranus, in , sufficient observations were accumulated to enable the orbit it follows to be determined. when the path was known, it was then a mere matter of mathematical calculation to ascertain where the planet was situated at any past time, and where it would be situated at any future time. an interesting enquiry was thus originated as to how far it might be possible to find any observations of the planet made previously to its discovery by herschel. uranus looks like a star of the sixth magnitude. not many astronomers were provided with telescopes of the perfection attained by herschel, and the personal delicacy of perception characteristic of herschel was a still more rare possession. it was, therefore, to be expected that, if such previous observations existed, they would merely record uranus as a star visible, and indeed bright, in a moderate telescope, but still not claiming any exceptional attention over thousands of apparently similar stars. many of the early astronomers had devoted themselves to the useful and laborious work of forming catalogues of stars. in the preparation of a star catalogue, the telescope was directed to the heavens, the stars were observed, their places were carefully measured, the brightness of the star was also estimated, and thus the catalogue was gradually compiled in which each star had its place faithfully recorded, so that at any future time it could be identified. the stars were thus registered, by hundreds and by thousands, at various dates from the birth of accurate astronomy till the present time. the suggestion was then made that, as uranus looked so like a star, and as it was quite bright enough to have engaged the attention of astronomers possessed of even very moderate instrumental powers, there was a possibility that it had already been observed, and thus actually lay recorded as a star in some of the older catalogues. this was indeed an idea worthy of every attention, and pregnant with the most important consequences in connection with the immortal discovery to be discussed in our next chapter. but how was such an examination of the catalogues to be conducted? uranus is constantly moving about; does it not seem that there is every element of uncertainty in such an investigation? let us consider a notable example. the great national observatory at greenwich was founded in , and the first astronomer-royal was the illustrious flamsteed, who in commenced that series of observations of the heavenly bodies which has been continued to the present day with such incalculable benefits to science. at first the instruments were of a rather primitive description, but in the course of some years flamsteed succeeded in procuring instruments adequate to the production of a catalogue of stars, and he devoted himself with extraordinary zeal to the undertaking. it is in this memorable work, the "historia coelestis" of flamsteed, that the earliest observation of uranus is recorded. in the first place it was known that the orbit of this body, like the orbit of every other great planet, was inclined at a very small angle to the ecliptic. it hence follows that uranus is at all times only to be met with along the ecliptic, and it is possible to calculate where the planet has been in each year. it was thus seen that in the planet was situated in that part of the ecliptic where flamsteed was at the same date making his observations. it was natural to search the observations of flamsteed, and see whether any of the so-called stars could have been uranus. an object was found in the "historia coelestis" which occupied a position identical with that which uranus must have filled on the same date. could this be uranus? a decisive test was at once available. the telescope was directed to the spot in the heavens where flamsteed saw a sixth-magnitude star. if that were really a star, then would it still be visible. the trial was made: no such star could be found, and hence the presumption that this was really uranus could hardly be for a moment doubted. speedily other confirmation flowed in. it was shown that uranus had been observed by bradley and by tobias mayer, and it also became apparent that flamsteed had observed uranus not only once, but that he had actually measured its place four times in the years and . yet flamsteed was never conscious of the discovery that lay so nearly in his grasp. he was, of course, under the impression that all these observations related to different stars. a still more remarkable case is that of lemonnier, who had actually observed uranus twelve times, and even recorded it on four consecutive days in january, . if lemonnier had only carefully looked over his own work; if he had perceived, as he might have done, how the star he observed yesterday was gone to-day, while the star visible to-day had moved away by to-morrow, there is no doubt that uranus would have been discovered, and william herschel would have been anticipated. would lemonnier have made as good use of his fame as herschel did? this seems a question which can never be decided, but those who estimate herschel as the present writer thinks he ought to be estimated, will probably agree in thinking that it was most fortunate for science that lemonnier did _not_ compare his observations.[ ] these early accidental observations of uranus are not merely to be regarded as matters of historical interest or curiosity. that they are of the deepest importance with regard to the science itself a few words will enable us to show. it is to be remembered that uranus requires no less than eighty-four years to accomplish his mighty revolution around the sun. the planet has completed one entire revolution since its discovery, and up to the present time ( ) has accomplished more than one-third of another. for the careful study of the nature of the orbit, it was desirable to have as many measurements as possible, and extending over the widest possible interval. this was in a great measure secured by the identification of the early observations of uranus. an approximate knowledge of the orbit was quite capable of giving the places of the planet with sufficient accuracy to identify it when met with in the catalogues. but when by their aid the actual observations have been discovered, they tell us precisely the place of uranus; and hence, instead of our knowledge of the planet being limited to but little more than one revolution, we have at the present time information with regard to it extending over considerably more than two revolutions. from the observations of the planet the ellipse in which it moves can be ascertained. we can compute this ellipse from the observations made during the time since the discovery. we can also compute the ellipse from the early observations made before the discovery. if kepler's laws were rigorously verified, then, of course, the ellipse performed in the present revolution must differ in no respect from the ellipse performed in the preceding, or indeed in any other revolution. we can test this point in an interesting manner by comparing the ellipse derived from the ancient observations with that deduced from the modern ones. these ellipses closely resemble each other; they are nearly the same; but it is most important to observe that they are not _exactly_ the same, even when allowance has been made for every known source of disturbance in accordance with the principles explained in the next chapter. the law of kepler seems thus not absolutely true in the case of uranus. here is, indeed, a matter demanding our most earnest and careful attention. have we not repeatedly laid down the universality of the laws of kepler in controlling the planetary motions? how then can we reconcile this law with the irregularities proved beyond a doubt to exist in the motions of uranus? let us look a little more closely into the matter. we know that the laws of kepler are a consequence of the laws of gravitation. we know that the planet moves in an elliptic path around the sun, in virtue of the sun's attraction, and we know that the ellipse will be preserved without the minutest alteration if the sun and the planet be left to their mutual attractions, and if no other force intervene. we can also calculate the influence of each of the known planets on the form and position of the orbit. but when allowance is made for all such perturbing influences it is found that the observed and computed orbits do not agree. the conclusion is irresistible. uranus does not move solely in consequence of the sun's attraction and that of the planets of our system interior to uranus; there must therefore be some further influence acting upon uranus besides those already known. to the development of this subject the next chapter will be devoted. chapter xv. neptune. discovery of neptune--a mathematical achievement--the sun's attraction--all bodies attract--jupiter and saturn--the planetary perturbations--three bodies--nature has simplified the problem--approximate solution--the sources of success--the problem stated for the earth--the discoveries of lagrange--the eccentricity--necessity that all the planets revolve in the same direction--lagrange's discoveries have not the dramatic interest of the more recent achievements--the irregularities of uranus--the unknown planet must revolve outside the path of uranus--the data for the problem--le verrier and adams both investigate the question--adams indicates the place of the planet--how the search was to be conducted--le verrier also solves the problem--the telescopic discovery of the planet--the rival claims--early observation of neptune--difficulty of the telescopic study of neptune--numerical details of the orbit--is there any outer planet?--contrast between mercury and neptune. we describe in this chapter a discovery so extraordinary that the whole annals of science may be searched in vain for a parallel. we are not here concerned with technicalities of practical astronomy. neptune was first revealed by profound mathematical research rather than by minute telescopic investigation. we must develop the account of this striking epoch in the history of science with the fulness of detail which is commensurate with its importance; and it will accordingly be necessary, at the outset of our narrative, to make an excursion into a difficult but attractive department of astronomy, to which we have as yet made little reference. the supreme controlling power in the solar system is the attraction of the sun. each planet of the system experiences that attraction, and, in virtue thereof, is constrained to revolve around the sun in an elliptic path. the efficiency of a body as an attractive agent is directly proportional to its mass, and as the mass of the sun is more than a thousand times as great as that of jupiter, which, itself, exceeds that of all the other planets collectively, the attraction of the sun is necessarily the chief determining force of the movements in our system. the law of gravitation, however, does not merely say that the sun attracts each planet. gravitation is a doctrine much more general, for it asserts that every body in the universe attracts every other body. in obedience to this law, each planet must be attracted, not only by the sun, but by innumerable bodies, and the movement of the planet must be the joint effect of all such attractions. as for the influence of the stars on our solar system, it may be at once set aside as inappreciable. the stars are no doubt enormous bodies, in many cases possibly transcending the sun in magnitude, but the law of gravitation tells us that the intensity of the attraction decreases as the square of the distance increases. most of the stars are a million times as remote as the sun, and consequently their attraction is so slight as to be absolutely inappreciable in the discussion of this question. the only attractions we need consider are those which arise from the action of one body of the system upon another. let us take, for instance, the two largest planets of our system, jupiter and saturn. each of these globes revolves mainly in consequence of the sun's attraction, but every planet also attracts every other, and the consequence is that each one is slightly drawn away from the position it would have otherwise occupied. in the language of astronomy, we would say that the path of jupiter is perturbed by the attraction of saturn; and, conversely, that the path of saturn is perturbed by the attraction of jupiter. for many years these irregularities of the planetary motions presented problems with which astronomers were not able to cope. gradually, however, one difficulty after another has been vanquished, and though there are no doubt some small irregularities still outstanding which have not been completely explained, yet all the larger and more important phenomena of the kind are well understood. the subject is one of the most difficult which the astronomer has to encounter in the whole range of his science. he has here to calculate what effect one planet is capable of producing on another planet. such calculations bristle with formidable difficulties, and can only be overcome by consummate skill in the loftiest branches of mathematics. let us state what the problem really is. when two bodies move in virtue of their mutual attraction, both of them will revolve in a curve which admits of being exactly ascertained. each path is, in fact, an ellipse, and they must have a common focus at the centre of gravity of the two bodies, considered as a single system. in the case of a sun and a planet, in which the mass of the sun preponderates enormously over the mass of the planet, the centre of gravity of the two lies very near the centre of the sun; the path of the great body is in such a case very small in comparison with the path of the planet. all these matters admit of perfectly accurate calculation of a somewhat elementary character. but now let us add a third body to the system which attracts each of the others and is attracted by them. in consequence of this attraction, the third body is displaced, and accordingly its influence on the others is modified; they in turn act upon it, and these actions and reactions introduce endless complexity into the system. such is the famous "problem of three bodies," which has engaged the attention of almost every great mathematician since the time of newton. stated in its mathematical aspect, and without having its intricacy abated by any modifying circumstances, the problem is one that defies solution. mathematicians have not yet been able to deal with the mutual attractions of three bodies moving freely in space. if the number of bodies be greater than three, as is actually the case in the solar system, the problem becomes still more hopeless. nature, however, has in this matter dealt kindly with us. she has, it is true, proposed a problem which cannot be accurately solved; but she has introduced into the problem, as proposed in the solar system, certain special features which materially reduce the difficulty. we are still unable to make what a mathematician would describe as a rigorous solution of the question; we cannot solve it with the completeness of a sum in arithmetic; but we can do what is nearly if not quite as useful. we can solve the problem approximately; we can find out what the effect of one planet on the other is _very nearly_, and by additional labour we can reduce the limits of uncertainty to as low a point as may be desired. we thus obtain a practical solution of the problem adequate for all the purposes of science. it avails us little to know the place of a planet with absolute mathematical accuracy. if we can determine what we want with so close an approximation to the true position that no telescope could possibly disclose the difference, then every practical end will have been attained. the reason why in this case we are enabled to get round the difficulties which we cannot surmount lies in the exceptional character of the problem of three bodies as exhibited in the solar system. in the first place, the sun is of such pre-eminent mass that many matters may be overlooked which would be of moment were he rivalled in mass by any of the planets. another source of our success arises from the small inclinations of the planetary orbits to each other; while the fact that the orbits are nearly circular also greatly facilitates the work. the mathematicians who may reside in some of the other parts of the universe are not equally favoured. among the sidereal systems we find not a few cases where the problem of three bodies, or even of more than three, would have to be faced without any of the alleviating circumstances which our system presents. in such groups as the marvellous star th orionis, we have three or four bodies comparable in size, which must produce movements of the utmost complexity. even if terrestrial mathematicians shall ever have the hardihood to face such problems, there is no likelihood of their being able to do so for ages to come; such researches must repose on accurate observations as their foundation; and the observations of these distant systems are at present utterly inadequate for the purpose. the undisturbed revolution of a planet around the sun, in conformity with kepler's law, would assure for that planet permanent conditions of climate. the earth, for instance, if guided solely by kepler's laws, would return each day of the year exactly to the same position which it had on the same day of last year. from age to age the quantity of heat received by the earth would remain constant if the sun continued unaltered, and the present climate might thus be preserved indefinitely. but since the existence of planetary perturbation has become recognised, questions arise of the gravest importance with reference to the possible effects which such perturbations may have. we now see that the path of the earth is not absolutely fixed. that path is deranged by venus and by mars; it is deranged, it must be deranged, by every planet in our system. it is true that in a year, or even in a century, the amount of alteration produced is not very great; the ellipse which represents the path of our earth this year does not differ considerably from the ellipse which represented the movement of the earth one hundred years ago. but the important question arises as to whether the slight difference which does exist may not be constantly increasing, and may not ultimately assume such proportions as to modify our climates, or even to render life utterly impossible. indeed, if we look at the subject without attentive calculation, nothing would seem more probable than that such should be the fate of our system. this globe revolves in a path inside that of the mighty jupiter. it is, therefore, constantly attracted by jupiter, and when it overtakes the vast planet, and comes between him and the sun, then the two bodies are comparatively close together, and the earth is pulled outwards by jupiter. it might be supposed that the tendency of such disturbances would be to draw the earth gradually away from the sun, and thus to cause our globe to describe a path ever growing wider and wider. it is not, however, possible to decide a dynamical question by merely superficial reasoning of this character. the question has to be brought before the tribunal of mathematical analysis, where every element in the case is duly taken into account. such an enquiry is by no means a simple one. it worthily occupied the splendid talents of lagrange and laplace, whose discoveries in the theory of planetary perturbation are some of the most remarkable achievements in astronomy. we cannot here attempt to describe the reasoning which these great mathematicians employed. it can only be expressed by the formulæ of the mathematician, and would then be hardly intelligible without previous years of mathematical study. it fortunately happens, however, that the results to which lagrange and laplace were conducted, and which have been abundantly confirmed by the labours of other mathematicians, admit of being described in simple language. let us suppose the case of the sun, and of two planets circulating around him. these two planets are mutually disturbing each other, but the amount of the disturbance is small in comparison with the effect of the sun on each of them. lagrange demonstrated that, though the ellipse in which each planet moved was gradually altered in some respects by the attraction of the other planet, yet there is one feature of the curve which the perturbation is powerless to alter permanently: the longest axis of the ellipse, and, therefore, the mean distance of the planet from the sun, which is equal to one-half of it, must remain unchanged. this is really a discovery as important as it was unexpected. it at once removes all fear as to the effect which perturbations can produce on the stability of the system. it shows that, notwithstanding the attractions of mars and of venus, of jupiter and of saturn, our earth will for ever continue to revolve at the same mean distance from the sun, and thus the succession of the seasons and the length of the year, so far as this element at least is concerned, will remain for ever unchanged. but lagrange went further into the enquiry. he saw that the mean distance did not alter, but it remained to be seen whether the eccentricity of the ellipse described by the earth might not be affected by the perturbations. this is a matter of hardly less consequence than that just referred to. even though the earth preserved the same average distance from the sun, yet the greatest and least distance might be widely unequal: the earth might pass very close to the sun at one part of its orbit, and then recede to a very great distance at the opposite part. so far as the welfare of our globe and its inhabitants is concerned, this is quite as important as the question of the mean distance; too much heat in one half of the year would afford but indifferent compensation for too little during the other half. lagrange submitted this question also to his analysis. again he vanquished the mathematical difficulties, and again he was able to give assurance of the permanence of our system. it is true that he was not this time able to say that the eccentricity of each path will remain constant; this is not the case. what he does assert, and what he has abundantly proved, is that the eccentricity of each orbit will always remain small. we learn that the shape of the earth's orbit gradually swells and gradually contracts; the greatest length of the ellipse is invariable, but sometimes it approaches more to a circle, and sometimes becomes more elliptical. these changes are comprised within narrow limits; so that, though they may probably correspond with measurable climatic changes, yet the safety of the system is not imperilled, as it would be if the eccentricity could increase indefinitely. once again lagrange applied the resources of his calculus to study the effect which perturbations can have on the inclination of the path in which the planet moves. the result in this case was similar to that obtained with respect to the eccentricities. if we commence with the assumption that the mutual inclinations of the planets are small, then mathematics assure us that they must always remain small. we are thus led to the conclusion that the planetary perturbations are unable to affect the stability of the solar system. we shall perhaps more fully appreciate the importance of these memorable researches if we consider how easily matters might have been otherwise. let us suppose a system resembling ours in every respect save one. let that system have a sun, as ours has; a system of planets and of satellites like ours. let the masses of all the bodies in this hypothetical system be identical with the masses in our system, and let the distances and the periodic times be the same in the two cases. let all the planes of the orbits be similarly placed; and yet this hypothetical system might contain seeds of decay from which ours is free. there is one point in the imaginary scheme which we have not yet specified. in our system all the planets revolve in the _same direction_ around the sun. let us suppose this law violated in the hypothetical system by reversing one planet on its path. that slight change alone would expose the system to the risk of destruction by the planetary perturbations. here, then, we find the necessity of that remarkable uniformity of the directions in which the planets revolve around the sun. had these directions not been uniform, our system must, in all probability, have perished ages ago, and we should not be here to discuss perturbations or any other subject. great as was the success of the eminent french mathematician who made these beautiful discoveries, it was left for this century to witness the crowning triumph of mathematical analysis applied to the law of gravitation. the work of lagrange lacks the dramatic interest of the discovery made by le verrier and adams, which gave still wider extent to the solar system by the discovery of the planet neptune revolving far outside uranus. we have already alluded to the difficulties which were experienced when it was sought to reconcile the early observations of uranus with those made since its discovery. we have shown that the path in which this planet revolved experienced change, and that consequently uranus must be exposed to the action of some other force besides the sun's attraction. the question arises as to the nature of these disturbing forces. from what we have already learned of the mutual deranging influence between any two planets, it seems natural to inquire whether the irregularities of uranus could not be accounted for by the attraction of the other planets. uranus revolves just outside saturn. the mass of saturn is much larger than the mass of uranus. could it not be that saturn draws uranus aside, and thus causes the changes? this is a question to be decided by the mathematician. he can compute what saturn is able to do, and he finds, no doubt, that saturn is capable of producing some displacement of uranus. in a similar manner jupiter, with his mighty mass, acts on uranus, and produces a disturbance which the mathematician calculates. when the figures had been worked out for all the known planets they were applied to uranus, and we might expect to find that they would fully account for the observed irregularities of his path. this was, however, not the case. after every known source of disturbance had been carefully allowed for, uranus was still shown to be influenced by some further agent; and hence the conclusion was established that uranus must be affected by some unknown body. what could this unknown body be, and where must it be situated? analogy was here the guide of those who speculated on this matter. we know no cause of disturbance of a planet's motion except it be the attraction of another planet. could it be that uranus was really attracted by some other planet at that time utterly unknown? this suggestion was made by many astronomers, and it was possible to determine some conditions which the unknown body should fulfil. in the first place its orbit must lie outside the orbit of uranus. this was necessary, because the unknown planet must be a large and massive one to produce the observed irregularities. if, therefore, it were nearer than uranus, it would be a conspicuous object, and must have been discovered long ago. other reasonings were also available to show that if the disturbances of uranus were caused by the attraction of a planet, that body must revolve outside the globe discovered by herschel. the general analogies of the planetary system might also be invoked in support of the hypothesis that the path of the unknown planet, though necessarily elliptic, did not differ widely from a circle, and that the plane in which it moved must also be nearly coincident with the plane of the earth's orbit. the measured deviations of uranus at the different points of its orbit were the sole data available for the discovery of the new planet. we have to fit the orbit of the unknown globe, as well as the mass of the planet itself, in such a way as to account for the various perturbations. let us, for instance, assume a certain distance for the hypothetical body, and try if we can assign both an orbit and a mass for the planet, at that distance, which shall account for the perturbations. our first assumption is perhaps too great. we try again with a lesser distance. we can now represent the observations with greater accuracy. a third attempt will give the result still more closely, until at length the distance of the unknown planet is determined. in a similar way the mass of the body can be also determined. we assume a certain value, and calculate the perturbations. if the results seem greater than those obtained by observations, then the assumed mass is too great. we amend the assumption, and recompute with a lesser amount, and so on until at length we determine a mass for the planet which harmonises with the results of actual measurement. the other elements of the unknown orbit--its eccentricity and the position of its axis--are all to be ascertained in a similar manner. at length it appeared that the perturbations of uranus could be completely explained if the unknown planet had a certain mass, and moved in an orbit which had a certain position, while it was also manifest that no very different orbit or greatly altered mass would explain the observed facts. these remarkable computations were undertaken quite independently by two astronomers--one in england and one in france. each of them attacked, and each of them succeeded in solving, the great problem. the scientific men of england and the scientific men of france joined issue on the question as to the claims of their respective champions to the great discovery; but in the forty years which have elapsed since these memorable researches the question has gradually become settled. it is the impartial verdict of the scientific world outside england and france, that the merits of this splendid triumph of science must be divided equally between the late distinguished professor j.c. adams, of cambridge, and the late u.j.j. le verrier, the director of the paris observatory. shortly after mr. adams had taken his degree at cambridge, in , when he obtained the distinction of senior wrangler, he turned his attention to the perturbations of uranus, and, guided by these perturbations alone, commenced his search for the unknown planet. long and arduous was the enquiry--demanding an enormous amount of numerical calculation, as well as consummate mathematical resource; but gradually mr. adams overcame the difficulties. as the subject unfolded itself, he saw how the perturbations of uranus could be fully explained by the existence of an exterior planet, and at length he had ascertained, not alone the orbit of this outer body, but he was even able to indicate the part of the heavens in which the unknown globe must be sought. with his researches in this advanced condition, mr. adams called on the astronomer-royal, sir george airy, at greenwich, in october, , and placed in his hands the computations which indicated with marvellous accuracy the place of the yet unobserved planet. it thus appears that seven months before anyone else had solved this problem mr. adams had conquered its difficulties, and had actually located the planet in a position but little more than a degree distant from the spot which it is now known to have occupied. all that was wanted to complete the discovery, and to gain for professor adams and for english science the undivided glory of this achievement, was a strict telescopic search through the heavens in the neighbourhood indicated. why, it may be said, was not such an enquiry instituted at once? no doubt this would have been done, if the observatories had been generally furnished forty years ago with those elaborate star-charts which they now possess. in the absence of a chart (and none had yet been published of the part of the sky where the unknown planet was) the search for the planet was a most tedious undertaking. it had been suggested that the new globe could be detected by its visible disc; but it must be remembered that even uranus, so much closer to us, had a disc so small that it was observed nearly a score of times without particular notice, though it did not escape the eagle glance of herschel. there remained then only one available method of finding neptune. it was to construct a chart of the heavens in the neighbourhood indicated, and then to compare this chart night after night with the stars in the heavens. before recommending the commencement of a labour so onerous, the astronomer-royal thought it right to submit mr. adams's researches to a crucial preliminary test. mr. adams had shown how his theory rendered an exact account of the perturbations of uranus in longitude. the astronomer-royal asked mr. adams whether he was able to give an equally clear explanation of the notable variations in the distance of uranus. there can be no doubt that his theory would have rendered a satisfactory account of these variations also; but, unfortunately, mr. adams seems not to have thought the matter of sufficient importance to give the astronomer-royal any speedy reply, and hence it happened that no less than nine months elapsed between the time when mr. adams first communicated his results to the astronomer-royal and the time when the telescopic search for the planet was systematically commenced. up to this time no account of mr. adams's researches had been published. his labours were known to but few besides the astronomer-royal and professor challis of cambridge, to whom the duty of making the search was afterwards entrusted. in the meantime the attention of le verrier, the great french mathematician and astronomer, had been specially directed by arago to the problem of the perturbations of uranus. with exhaustive analysis le verrier investigated every possible known source of disturbance. the influences of the older planets were estimated once more with every precision, but only to confirm the conclusion already arrived at as to their inadequacy to account for the perturbations. le verrier then commenced the search for the unknown planet by the aid of mathematical investigation, in complete ignorance of the labours of adams. in november, , and again on the st of june, , portions of the french astronomer's results were announced. the astronomer-royal then perceived that his calculations coincided practically with those of adams, insomuch that the places assigned to the unknown planet by the two astronomers were not more than a degree apart! this was, indeed, a remarkable result. here was a planet unknown to human sight, yet felt, as it were, by mathematical analysis with a certainty so great that two astronomers, each in total ignorance of the other's labours, concurred in locating the planet in almost the same spot of the heavens. the existence of the new globe was thus raised nearly to a certainty, and it became incumbent on practical astronomers to commence the search forthwith. in june, , the astronomer-royal announced to the visitors of the greenwich observatory the close coincidence between the calculations of le verrier and of adams, and urged that a strict scrutiny of the region indicated should be at once instituted. professor challis, having the command of the great northumberland equatorial telescope at cambridge, was induced to undertake the work, and on the th july, , he began his labours. the plan of search adopted by professor challis was an onerous one. he first took the theoretical place of the planet, as given by mr. adams, and after allowing a very large margin for the uncertainties of a calculation so recondite, he marked out a certain region of the heavens, near the ecliptic, in which it might be anticipated that the unknown planet must be found. he then determined to observe all the stars in this region and measure their relative positions. when this work was once done it was to be repeated a second time. his scheme even contemplated a third complete set of observations of the stars contained within this selected region. there could be no doubt that this process would determine the planet if it were bright enough to come within the limits of stellar magnitude which professor challis adopted. the globe would be detected by its motion relatively to the stars, when the three series of measures came to be compared. the scheme was organised so thoroughly that it must have led to the expected discovery--in fact, it afterwards appeared that professor challis did actually observe the planet more than once, and a subsequent comparison of its positions must infallibly have led to the detection of the new globe. le verrier was steadily maturing his no less elaborate investigations in the same direction. he felt confident of the existence of the planet, and he went so far as to predict not only the situation of the globe but even its actual appearance. he thought the planet would be large enough (though still of course only a telescopic object) to be distinguished from the stars by the possession of a disc. these definite predictions strengthened the belief that we were on the verge of another great discovery in the solar system, so much so that when sir john herschel addressed the british association on the th of september, , he uttered the following words:--"the past year has given to us the new planet astræa--it has done more, it has given us the probable prospect of another. we see it as columbus saw america from the shores of spain. its movements have been felt trembling along the far-reaching line of our analysis, with a certainty hardly inferior to ocular demonstration." the time of the discovery was now rapidly approaching. on the th of september, , le verrier wrote to dr. galle of the berlin observatory, describing the place of the planet indicated by his calculations, and asking him to make its telescopic discovery. the request thus preferred was similar to that made on behalf of adams to professor challis. both at berlin and at cambridge the telescopic research was to be made in the same region of the heavens. the berlin astronomers were, however, fortunate in possessing an invaluable aid to the research which was not at the time in the hands of professor challis. we have mentioned how the search for a telescopic planet can be facilitated by the use of a carefully-executed chart of the stars. in fact, a mere comparison of the chart with the sky is all that is necessary. it happened that the preparation of a series of star charts had been undertaken by the berlin academy of sciences some years previously. on these charts the place of every star, down even to the tenth magnitude, had been faithfully engraved. this work was one of much utility, but its originators could hardly have anticipated the brilliant discovery which would arise from their years of tedious labour. it was found convenient to publish such an extensive piece of surveying work by instalments, and accordingly, as the chart was completed, it issued from the press sheet by sheet. it happened that just before the news of le verrier's labours reached berlin the chart of that part of the heavens had been engraved and printed. it was on the rd of september that le verrier's letter reached dr. galle at berlin. the sky that night was clear, and we can imagine with what anxiety dr. galle directed his telescope to the heavens. the instrument was pointed in accordance with le verrier's instructions. the field of view showed a multitude of stars, as does every part of the heavens. one of these was really the planet. the new chart was unrolled, and, star by star, the heavens were compared with it. as the identification of the stars went on, one object after another was found to lie in the heavens as it was engraved on the chart, and was of course rejected. at length a star of the eighth magnitude--a brilliant object--was brought into review. the chart was examined, but there was no star there. this object could not have been in its present place when the chart was formed. the object was therefore a wanderer--a planet. yet it was necessary to be cautious in such a matter. many possibilities had to be guarded against. it was, for instance, at least conceivable that the object was really a star which, by some mischance, eluded the careful eye of the astronomer who had constructed the map. it was even possible that the star might be one of the large class of variables which alternate in brightness, and it might have been too faint to have been visible when the chart was made. or it might be one of the minor planets moving between mars and jupiter. even if none of these explanations would answer, it was still necessary to show that the object was moving with that particular velocity and in that particular direction which the theory of le verrier indicated. the lapse of a single day was sufficient to dissipate all doubts. the next night the object was again observed. it had moved, and when its motion was measured it was found to accord precisely with what le verrier had foretold. indeed, as if no circumstance in the confirmation should be wanting, the diameter of the planet, as measured by the micrometers at berlin, proved to be practically coincident with that anticipated by le verrier. the world speedily rang with the news of this splendid achievement. instantly the name of le verrier rose to a pinnacle hardly surpassed by that of any astronomer of any age or country. the circumstances of the discovery were highly dramatic. we picture the great astronomer buried in profound meditation for many months; his eyes are bent, not on the stars, but on his calculations. no telescope is in his hand; the human intellect is the instrument he alone uses. with patient labour, guided by consummate mathematical artifice, he manipulates his columns of figures. he attempts one solution after another. in each he learns something to avoid; by each he obtains some light to guide him in his future labours. at length he begins to see harmony in those results where before there was but discord. gradually the clouds disperse, and he discerns with a certainty little short of actual vision the planet glittering in the far depths of space. he rises from his desk and invokes the aid of a practical astronomer; and lo! there is the planet in the indicated spot. the annals of science present no such spectacle as this. it was the most triumphant proof of the law of universal gravitation. the newtonian theory had indeed long ere this attained an impregnable position; but, as if to place its truth in the most conspicuous light, this discovery of neptune was accomplished. for a moment it seemed as if the french were to enjoy the undivided honour of this splendid triumph; nor would it, indeed, have been unfitting that the nation which gave birth to lagrange and to laplace, and which developed the great newtonian theory by their immortal labours, should have obtained this distinction. up to the time of the telescopic discovery of the planet by dr. galle at berlin, no public announcement had been made of the labours of challis in searching for the planet, nor even of the theoretical researches of adams on which those observations were based. but in the midst of the pæans of triumph with which the enthusiastic french nation hailed the discovery of le verrier, there appeared a letter from sir john herschel in the _athenæum_ for rd october, , in which he announced the researches made by adams, and claimed for him a participation in the glory of the discovery. subsequent enquiry has shown that this claim was a just one, and it is now universally admitted by all independent authorities. yet it will easily be imagined that the french _savants_, jealous of the fame of their countryman, could not at first be brought to recognise a claim so put forward. they were asked to divide the unparalleled honour between their own illustrious countryman and a young foreigner of whom but few had ever heard, and who had not even published a line of his work, nor had any claim been made on his part until after the work had been completely finished by le verrier. the demand made on behalf of adams was accordingly refused any acknowledgment in france; and an embittered controversy was the consequence. point by point the english astronomers succeeded in establishing the claim of their countryman. it was true that adams had not published his researches to the world, but he had communicated them to the astronomer-royal, the official head of the science in this country. they were also well known to professor challis, the professor of astronomy at cambridge. then, too, the work of adams was published, and it was found to be quite as thorough and quite as successful as that of le verrier. it was also found that the method of search adopted by professor challis not only must have been eventually successful, but that it actually was in a sense already successful. when the telescopic discovery of the planet had been achieved, challis turned naturally to see whether he had observed the new globe also. it was on the st october that he heard of the success of dr. galle, and by that time challis had accumulated observations in connection with this research of no fewer than , stars. among them he speedily found that an object observed on the th of august was not in the same place on the th of july. this was really the planet; and its discovery would thus have been assured had challis had time to compare his measurements. in fact, if he had only discussed his observations at once, there cannot be much doubt that the entire glory of the discovery would have been awarded to adams. he would then have been first, no less in the theoretical calculations than in the optical verification of the planet's existence. it may also be remarked that challis narrowly missed making the discovery of neptune in another way. le verrier had pointed out in his paper the possibility of detecting the sought-for globe by its disc. challis made the attempt, and before the intelligence of the actual discovery at berlin had reached him he had made an examination of the region indicated by le verrier. about stars passed through the field of view, and among them he selected one on account of its disc; it afterwards appeared that this was indeed the planet. if the researches of le verrier and of adams had never been undertaken it is certain that the distant neptune must have been some time discovered; yet that might have been made in a manner which every true lover of science would now deplore. we hear constantly that new minor planets are observed, yet no one attaches to such achievements a fraction of the consequence belonging to the discovery of neptune. the danger was, that neptune should have been merely dropped upon by simple survey work, just as uranus was discovered, or just as the hosts of minor planets are now found. in this case theoretical astronomy, the great science founded by newton, would have been deprived of its most brilliant illustration. neptune had, in fact, a very narrow escape on at least one previous occasion of being discovered in a very simple way. this was shown when sufficient observations had been collected to enable the path of the planet to be calculated. it was then possible to trace back the movements of the planet among the stars and thus to institute a search in the catalogues of earlier astronomers to see whether they contained any record of neptune, erroneously noted as a star. several such instances have been discovered. i shall, however, only refer to one, which possesses a singular interest. it was found that the place of the planet on may th, , must have coincided with that of a so-called star recorded on that day in the "histoire céleste" of lalande. by actual examination of the heavens it further appeared that there was no star in the place indicated by lalande, so the fact that here was really an observation of neptune was placed quite beyond doubt. when reference was made to the original manuscripts of lalande, a matter of great interest was brought to light. it was there found that he had observed the same star (for so he regarded it) both on may th and on may th; on each day he had determined its position, and both observations are duly recorded. but when he came to prepare his catalogue and found that the places on the two occasions were different, he discarded the earlier result, and merely printed the latter. had lalande possessed a proper confidence in his own observations, an immortal discovery lay in his grasp; had he manfully said, "i was right on the th of may and i was right on the th of may; i made no mistake on either occasion, and the object i saw on the th must have moved between that and the th," then he must without fail have found neptune. but had he done so, how lamentable would have been the loss to science! the discovery of neptune would then merely have been an accidental reward to a laborious worker, instead of being one of the most glorious achievements in the loftiest department of human reason. besides this brief sketch of the discovery of neptune, we have but little to tell with regard to this distant planet. if we fail to see in uranus any of those features which make mars or venus, jupiter or saturn, such attractive telescopic objects, what can we expect to find in neptune, which is half as far again as uranus? with a good telescope and a suitable magnifying power we can indeed see that neptune has a disc, but no features on that disc can be identified. we are consequently not in a position to ascertain the period in which neptune rotates around its axis, though from the general analogy of the system we must feel assured that it really does rotate. more successful have been the attempts to measure the diameter of neptune, which is found to be about , miles, or more than four times the diameter of the earth. it would also seem that, like jupiter and like saturn, the planet must be enveloped with a vast cloud-laden atmosphere, for the mean density of the globe is only about one-fifth that of the earth. this great globe revolves around the sun at a mean distance of no less than , millions of miles, which is about thirty times as great as the mean distance from the earth to the sun. the journey, though accomplished at the rate of more than three miles a second, is yet so long that neptune requires almost years to complete one revolution. since its discovery, some fifty years ago, neptune has moved through about one-third of its path, and even since the date when it was first casually seen by lalande, in , it has only had time to traverse three-fifths of its mighty circuit. neptune, like our earth, is attended by a single satellite; this delicate object was discovered by mr. lassell with his two-foot reflecting telescope shortly after the planet itself became known. the motion of the satellite of neptune is nearly circular. its orbit is inclined at an angle of about ° to the ecliptic, and it is specially noteworthy that, like the satellites of uranus, the direction of the motion runs counter to the planetary movements generally. the satellite performs its journey around neptune in a period of a little less than six days. by observing the motions of this moon we are enabled to determine the mass of the planet, and thus it appears that the weight of neptune is about one nineteen-thousandth part of that of the sun. no planets beyond neptune have been seen, nor is there at present any good ground for believing in their existence as visual objects. in the chapter on the minor planets i have entered into a discussion of the way in which these objects are discovered. it is by minute and diligent comparison of the heavens with elaborate star charts that these bodies are brought to light. such enquiries would be equally efficacious in searching for an ultra-neptunian planet; in fact, we could design no better method to seek for such a body, if it existed, than that which is at this moment in constant practice at many observatories. the labours of those who search for small planets have been abundantly rewarded with discoveries now counted by hundreds. yet it is a noteworthy fact that all these planets are limited to one region of the solar system. it has sometimes been conjectured that time may disclose perturbations in the orbit of neptune, and that these perturbations may lead to the discovery of a planet still more remote, even though that planet be so distant and so faint that it eludes all telescopic research. at present, however, such an enquiry can hardly come within the range of practical astronomy. its movements have no doubt been studied minutely, but it must describe a larger part of its orbit before it would be feasible to conclude, from the perturbations of its path, the existence of an unknown and still more remote planet. we have thus seen that the planetary system is bounded on one side by mercury and on the other by neptune. the discovery of mercury was an achievement of prehistoric times. the early astronomer who accomplished that feat, when devoid of instrumental assistance and unsupported by accurate theoretical knowledge, merits our hearty admiration for his untutored acuteness and penetration. on the other hand, the discovery of the exterior boundary of the planetary system is worthy of special attention from the fact that it was founded solely on profound theoretical learning. though we here close our account of the planets and their satellites, we have still two chapters to add before we shall have completed what is to be said with regard to the solar system. a further and notable class of bodies, neither planets nor satellites, own allegiance to the sun, and revolve round him in conformity with the laws of universal gravitation. these bodies are the comets, and their somewhat more humble associates, the shooting stars. we find in the study of these objects many matters of interest, which we shall discuss in the ensuing chapters. chapter xvi. comets. comets contrasted with planets in nature as well as in their movements--coggia's comet--periodic returns--the law of gravitation--parabolic and elliptic orbits--theory in advance of observations--most cometary orbits are sensibly parabolic--the labours of halley--the comet of --halley's memorable prediction--the retardation produced by disturbance--successive returns of halley's comet--encke's comet--effect of perturbations--orbit of encke's comet--attraction of mercury and of jupiter--how the identity of the comet is secured--how to weigh mercury--distance from the earth to the sun found by encke's comet--the disturbing medium--remarkable comets--spectrum of a comet--passage of a comet between the earth and the stars--can the comet be weighed?--evidence of the small mass of the comet derived from the theory of perturbation--the tail of the comet--its changes--views as to its nature--carbon present in comets--origin of periodic comets. in our previous chapters, which treated of the sun and the moon, the planets and their satellites, we found in all cases that the celestial bodies with which we were concerned were nearly globular in form, and many are undoubtedly of solid substance. all these objects possess a density which, even if in some cases it be much less than that of the earth, is still hundreds of times greater than the density of merely gaseous materials. we now, however, approach the consideration of a class of objects of a widely different character. we have no longer to deal with globular objects possessing considerable mass. comets are of altogether irregular shape; they are in large part, at all events, formed of materials in the utmost state of tenuity, and their masses are so small that no means we possess have enabled them to be measured. not only are comets different in constitution from planets or from the other more solid bodies of our system, but the movements of such bodies are quite distinct from the orderly return of the planets at their appointed seasons. the comets appear sometimes with almost startling unexpectedness; they rapidly swell in size to an extent that in superstitious ages called forth the utmost terror; presently they disappear, in many cases never again to return. modern science has, no doubt, removed a great deal of the mystery which once invested the whole subject of comets. their movements are now to a large extent explained, and some additions have been made to our knowledge of their nature, though we must still confess that what we do know bears but a very small proportion to what remains unknown. let me first describe in general terms the nature of a comet, in so far as its structure is disclosed by the aid of a powerful refracting telescope. we represent in plate xii. two interesting sketches made at harvard college observatory of the great comet of , distinguished by the name of its discoverer coggia. we see here the head of the comet, containing as its brightest spot what is called the nucleus, and in which the material of the comet seems to be much denser than elsewhere. surrounding the nucleus we find certain definite layers of luminous material, the coma, or head, from , to , , miles in diameter, from which the tail seems to stream away. this view may be regarded as that of a typical object of this class, but the varieties of structure presented by different comets are almost innumerable. in some cases we find the nucleus absent; in other cases we find the tail to be wanting. the tail is, no doubt, a conspicuous feature in those great comets which receive universal attention; but in the small telescopic objects, of which a few are generally found every year, this feature is usually absent. not only do comets present great varieties in appearance, but even the aspect of a single object undergoes great change. the comet will sometimes increase enormously in bulk; sometimes it will diminish; sometimes it will have a large tail, or sometimes no tail at all. measurements of a comet's size are almost futile; they may cease to be true even during the few hours in which a comet is observed in the course of a night. it is, in fact, impossible to identify a comet by any description of its personal appearance. yet the question as to identity of a comet is often of very great consequence. we must provide means by which it can be established, entirely apart from what the comet may look like. it is now well known that several of these bodies make periodic returns. after having been invisible for a certain number of years, a comet comes into view, and again retreats into space to perform another revolution. the question then arises as to how we are to recognise the body when it does come back? the personal features of its size or brightness, the presence or absence of a tail, large or small, are fleeting characters of no value for such a purpose. fortunately, however, the law of elliptic motion established by kepler has suggested the means of defining the identity of a comet with absolute precision. after newton had made his discovery of the law of gravitation, and succeeded in demonstrating that the elliptic paths of the planets around the sun were necessary consequences of that law, he was naturally tempted to apply the same reasoning to explain the movements of comets. here, again, he met with marvellous success, and illustrated his theory by completely explaining the movements of the remarkable body which was visible from december, , to march, . [illustration: fig. .--the parabolic path of a comet.] there is a certain beautiful curve known to geometricians by the name of the parabola. its form is shown in the adjoining figure; it is a curved line which bends in towards and around a certain point known as the focus. this would not be the occasion for any allusion to the geometrical properties of this curve; they should be sought in works on mathematics. it will here be only necessary to point to the connection which exists between the parabola and the ellipse. in a former chapter we have explained the construction of the latter curve, and we have shown how it possesses two foci. let us suppose that a series of ellipses are drawn, each of which has a greater distance between its foci than the preceding one. imagine the process carried on until at length the distance between the foci became enormously great in comparison with the distance from each focus to the curve, then each end of this long ellipse will practically have the same form as a parabola. we may thus look on the latter curve represented in fig. as being one end of an ellipse of which the other end is at an indefinitely great distance. in doerfel, a clergyman of saxony, proved that the great comet then recently observed moved in a parabola, in the focus of which the sun was situated. newton showed that the law of gravitation would permit a body to move in an ellipse of this very extreme type no less than in one of the more ordinary proportions. an object revolving in a parabolic orbit about the sun at the focus moves in gradually towards the sun, sweeps around the great luminary, and then begins to retreat. there is a necessary distinction between parabolic and elliptic motion. in the latter case the body, after its retreat to a certain distance, will turn round and again draw in towards the sun; in fact, it must make periodic circuits of its orbit, as the planets are found to do. but in the case of the true parabola the body can never return; to do so it would have to double the distant focus, and as that is infinitely remote, it could not be reached except in the lapse of infinite time. the characteristic feature of the movement in a parabola may be thus described. the body draws in gradually towards the focus from an indefinitely remote distance on one side, and after passing round the focus gradually recedes to an indefinitely remote distance on the other side, never again to return. when newton had perceived that parabolic motion of this type could arise from the law of gravitation, it at once occurred to him (independently of doerfel's discovery, of which he was not aware) that by its means the movements of a comet might be explained. he knew that comets must be attracted by the sun; he saw that the usual course of a comet was to appear suddenly, to sweep around the sun and then retreat, never again to return. was this really a case of parabolic motion? fortunately, the materials for the trial of this important suggestion were ready to his hand. he was able to avail himself of the known movements of the comet of , and of observations of several other bodies of the same nature which had been collected by the diligence of astronomers. with his usual sagacity, newton devised a method by which, from the known facts, the path which the comet pursues could be determined. he found that it was a parabola, and that the velocity of the comet was governed by the law that the straight line from the sun to the comet swept over equal areas in equal times. here was another confirmation of the law of universal gravitation. in this case, indeed, the theory may be said to have been actually in advance of calculation. kepler had determined from observation that the paths of the planets were ellipses, and newton had shown how this fact was a consequence of the law of gravitation. but in the case of the comets their highly erratic orbits had never been reduced to geometrical form until the theory of newton showed him that they were parabolic, and then he invoked observation to verify the anticipations of his theory. [illustration: plate xii. coggia's comet. (as seen on june th and july th, .)] the great majority of comets move in orbits which cannot be sensibly discriminated from parabolæ, and any body whose orbit is of this character can only be seen at a single apparition. the theory of gravitation, though it admits the parabola as a possible orbit for a comet, does not assert that the path must necessarily be of this type. we have pointed out that this curve is only a very extreme type of ellipse, and it would still be in perfect accordance with the law of gravitation for a comet to pursue a path of any elliptical form, provided that the sun was placed at the focus, and that the comet obeyed the rule of describing equal areas in equal times. if a body move in an elliptic path, then it will return to the sun again, and consequently we shall have periodical visits from the same object. an interesting field of enquiry was here presented to the astronomer. nor was it long before the discovery of a periodic comet was made which illustrated, in a striking manner, the soundness of the anticipation just expressed. the name of the celebrated astronomer halley is, perhaps, best known from its association with the great comet whose periodicity was discovered by his calculations. when halley learned from the newtonian theory the possibility that a comet might move in an elliptic orbit, he undertook a most laborious investigation; he collected from various records of observed comets all the reliable particulars that could be obtained, and thus he was enabled to ascertain, with tolerable accuracy, the nature of the paths pursued by about twenty-four large comets. one of these was the great body of , which halley himself observed, and whose path he computed in accordance with the principles of newton. halley then proceeded to investigate whether this comet of could have visited our system at any previous epoch. to answer this question he turned to the list of recorded comets which he had so carefully compiled, and he found that his comet very closely resembled, both in appearance and in orbit, a comet observed in , and also another observed in . could these three bodies be identical? it was only necessary to suppose that a comet, instead of revolving in a parabolic orbit, really revolved in an extremely elongated ellipse, and that it completed each revolution in a period of about seventy-five or seventy-six years. he submitted this hypothesis to every test that he could devise; he found that the orbits, determined on each of the three occasions, were so nearly identical that it would be contrary to all probability that the coincidence should be accidental. accordingly, he decided to submit his theory to the most supreme test known to astronomy. he ventured to make a prediction which posterity would have the opportunity of verifying. if the period of the comet were seventy-five or seventy-six years, as the former observations seemed to show, then halley estimated that, if unmolested, it ought to return in or . there were, however, certain sources of disturbance which he pointed out, and which would be quite powerful enough to affect materially the time of return. the comet in its journey passes near the path of jupiter, and experiences great perturbations from that mighty planet. halley concluded that the expected return might be accordingly delayed till the end of or the beginning of . this prediction was a memorable event in the history of astronomy, inasmuch as it was the first attempt to foretell the apparition of one of those mysterious bodies whose visits seemed guided by no fixed law, and which were usually regarded as omens of awful import. halley felt the importance of his announcement. he knew that his earthly course would have run long before the comet had completed its revolution; and, in language almost touching, the great astronomer writes: "wherefore if it should return according to our prediction about the year , impartial posterity will not refuse to acknowledge that this was first discovered by an englishman." as the time drew near when this great event was expected, it awakened the liveliest interest among astronomers. the distinguished mathematician clairaut undertook to compute anew, by the aid of improved methods, the effect which would be wrought on the comet by the attraction of the planets. his analysis of the perturbations was sufficient to show that the object would be kept back for days by saturn, and for days by jupiter. he therefore gave some additional exactness to the prediction of halley, and finally concluded that this comet would reach the perihelion, or the point of its path nearest to the sun, about the middle of april, . the sagacious astronomer (who, we must remember, lived long before the discovery of uranus and of neptune) further adds that as this body retreats so far, it may possibly be subject to influences of which we do not know, or to the disturbance even of some planet too remote to be ever perceived. he, accordingly, qualified his prediction with the statement that, owing to these unknown possibilities, his calculations might be a month wrong one way or the other. clairaut made this memorable communication to the academy of sciences on the th of november, . the attention of astronomers was immediately quickened to see whether the visitor, who last appeared seventy-six years previously, was about to return. night after night the heavens were scanned. on christmas day in the comet was first detected, and it passed closest to the sun about midnight on the th of march, just a month earlier than the time announced by clairaut, but still within the limits of error which he had assigned as being possible. the verification of this prediction was a further confirmation of the theory of gravitation. since then, halley's comet has returned once again, in , in circumstances somewhat similar to those just narrated. further historical research has also succeeded in identifying halley's comet with numerous memorable apparitions of comets in former times. it has even been shown that a splendid object, which appeared eleven years before the commencement of the christian era, was merely halley's comet in one of its former returns. among the most celebrated visits of this body was that of , when the apparition attracted universal attention. a picture of the comet on this occasion forms a quaint feature in the bayeux tapestry. the next return of halley's comet is expected about the year . there are now several comets known which revolve in elliptic paths, and are, accordingly, entitled to be termed periodic. these objects are chiefly telescopic, and are thus in strong contrast to the splendid comet of halley. most of the other periodic comets have periods much shorter than that of halley. of these objects, by far the most celebrated is that known as encke's comet, which merits our careful attention. the object to which we refer has had a striking career during which it has provided many illustrations of the law of gravitation. we are not here concerned with the prosaic routine of a mere planetary orbit. a planet is mainly subordinated to the compelling sway of the sun's gravitation. it is also to some slight extent affected by the attractions which it experiences from the other planets. mathematicians have long been accustomed to anticipate the movements of these globes by actual calculation. they know how the place of the planet is approximately decided by the sun's attraction, and they can discriminate the different adjustments which that place is to receive in consequence of the disturbances produced by the other planets. the capabilities of the planets for producing disturbance are greatly increased when the disturbed body follows the eccentric path of a comet. it is frequently found that the path of such a body comes very near the track of a planet, so that the comet may actually sweep by the planet itself, even if the two bodies do not actually run into collision. on such an occasion the disturbing effect is enormously augmented, and we therefore turn to the comets when we desire to illustrate the theory of planetary perturbations by some striking example. having decided to choose a comet, the next question is, _what_ comet? there cannot here be much room for hesitation. those splendid comets which appear so capriciously may be at once excluded. they are visitors apparently coming for the first time, and retreating without any distinct promise that mankind shall ever see them again. a comet of this kind moves in a parabolic path, sweeps once around the sun, and thence retreats into the space whence it came. we cannot study the effect of perturbations on a comet completely until it has been watched during successive returns to the sun. our choice is thus limited to the comparatively small class of objects known as periodic comets; and, from a survey of the entire group, we select the most suitable to our purpose. it is the object generally known as encke's comet, for, though encke was not the discoverer, yet it is to his calculations that the comet owes its fame. this body is rendered more suitable for our purpose by the researches to which it has recently given rise. in the year a comet was discovered by the painstaking astronomer pons at marseilles. we are not to imagine that this body produced a splendid spectacle. it was a small telescopic object, not unlike one of those dim nebulæ which are scattered in thousands over the heavens. the comet is, however, readily distinguished from a nebula by its movement relatively to the stars, while the nebula remains at rest for centuries. the position of this comet was ascertained by its discoverer, as well as by other astronomers. encke found from the observations that the comet returned to the sun once in every three years and a few months. this was a startling announcement. at that time no other comet of short period had been detected, so that this new addition to the solar system awakened the liveliest interest. the question was immediately raised as to whether this comet, which revolved so frequently, might not have been observed during previous returns. the historical records of the apparitions of comets are counted by hundreds, and how among this host are we to select those objects which were identical with the comet discovered by pons? [illustration: fig. .--the orbit of encke's comet.] we may at once relinquish any hope of identification from drawings of the object, but, fortunately, there is one feature of a comet on which we can seize, and which no fluctuations of the actual structure can modify or disguise. the path in which the body travels through space is independent of the bodily changes in its structure. the shape of that path and its position depend entirely upon those other bodies of the solar system which are specially involved in the theory of encke's comet. in fig. we show the orbits of three of the planets. they have been chosen with such proportions as shall make the innermost represent the orbit of mercury; the next is the orbit of the earth, while the outermost is the orbit of jupiter. besides these three we perceive in the figure a much more elliptical path, representing the orbit of encke's comet, projected down on the plane of the earth's motion. the sun is situated at the focus of the ellipse. the comet is constrained to revolve in this curve by the attraction of the sun, and it requires a little more than three years to accomplish a complete revolution. it passes close to the sun at perihelion, at a point inside the path of mercury, while at its greatest distance it approaches the path of jupiter. this elliptic orbit is mainly determined by the attraction of the sun. whether the comet weighed an ounce, a ton, a thousand tons, or a million tons, whether it was a few miles, or many thousands of miles in diameter, the orbit would still be the same. it is by the shape of this ellipse, by its actual size, and by the position in which it lies, that we identify the comet. it had been observed in , , and , but on these occasions it had not been noticed that the comet's path deviated from the parabola. encke's comet is usually so faint that even the most powerful telescope in the world would not show a trace of it. after one of its periodical visits, the body withdraws until it recedes to the outermost part of its path, then it will turn, and again approach the sun. it would seem that it becomes invigorated by the sun's rays, and commences to dilate under their genial influence. while moving in this part of its path the comet lessens its distance from the earth. it daily increases in splendour, until at length, partly by the intrinsic increase in brightness and partly by the decrease in distance from the earth, it comes within the range of our telescopes. we can generally anticipate when this will occur, and we can tell to what point of the heavens the telescope is to be pointed so as to discern the comet at its next return to perihelion. the comet cannot elude the grasp of the mathematician. he can tell when and where the comet is to be found, but no one can say what it will be like. were all the other bodies of the system removed, then the path of encke's comet must be for ever performed in the same ellipse and with absolute regularity. the chief interest for our present purpose lies not in the regularity of its path, but in the _irregularities_ introduced into that path by the presence of the other bodies of the solar system. let us, for instance, follow the progress of the comet through its perihelion passage, in which the track lies near that of the planet mercury. it will usually happen that mercury is situated in a distant part of its path at the moment the comet is passing, and the influence of the planet will then be comparatively small. it may, however, sometimes happen that the planet and the comet come close together. one of the most interesting instances of a close approach to mercury took place on the nd november, . on that day the comet and the planet were only separated by an interval of about one-thirtieth of the earth's distance from the sun, _i.e._ about , , miles. on several other occasions the distance between encke's comet and mercury has been less than , , miles--an amount of trifling import in comparison with the dimensions of our system. approaches so close as this are fraught with serious consequences to the movements of the comet. mercury, though a small body, is still sufficiently massive. it always attracts the comet, but the efficacy of that attraction is enormously enhanced when the comet in its wanderings comes near the planet. the effect of this attraction is to force the comet to swerve from its path, and to impress certain changes upon its velocity. as the comet recedes, the disturbing influence of mercury rapidly abates, and ere long becomes insensible. but time cannot efface from the orbit of the comet the effect which the disturbance of mercury has actually accomplished. the disturbed orbit is different from the undisturbed ellipse which the comet would have occupied had the influence of the sun alone determined its shape. we are able to calculate the movements of the comet as determined by the sun. we can also calculate the effects arising from the disturbance produced by mercury, provided we know the mass of the latter. though mercury is one of the smallest of the planets, it is perhaps the most troublesome to the astronomer. it lies so close to the sun that it is seen but seldom in comparison with the other great planets. its orbit is very eccentric, and it experiences disturbances by the attraction of other bodies in a way not yet fully understood. a special difficulty has also been found in the attempt to place mercury in the weighing scales. we can weigh the whole earth, we can weigh the sun, the moon, and even jupiter and other planets, but mercury presents difficulties of a peculiar character. le verrier, however, succeeded in devising a method of weighing it. he demonstrated that our earth is attracted by this planet, and he showed how the amount of attraction may be disclosed by observations of the sun, so that, from an examination of the observations, he made an approximate determination of the mass of mercury. le verrier's result indicated that the weight of the planet was about the fourteenth part of the weight of the earth. in other words, if our earth was placed in a balance, and fourteen globes, each equal to mercury, were laid in the other, the scales would hang evenly. it was necessary that this result should be received with great caution. it depended upon a delicate interpretation of somewhat precarious measurements. it could only be regarded as of provisional value, to be discarded when a better one should be obtained. the approach of encke's comet to mercury, and the elaborate investigations of von asten and backlund, in which the observations of the body were discussed, have thrown much light on the subject; but, owing to a peculiarity in the motion of this comet, which we shall presently mention, the difficulties of this investigation are enormous. backlund's latest result is, that the sun is , , times as heavy as mercury, and he considers that this is worthy of great confidence. there is a considerable difference between this result (which makes the earth about thirty times as heavy as mercury) and that of le verrier; and, on the other hand, haerdtl has, from the motion of winnecke's periodic comet, found a value of the mass of mercury which is not very different from le verrier's. mercury is, however, the only planet about the mass of which there is any serious uncertainty, and this must not make us doubt the accuracy of this delicate weighing-machine. look at the orbit of jupiter, to which encke's comet approaches so nearly when it retreats from the sun. it will sometimes happen that jupiter and the comet are in close proximity, and then the mighty planet seriously disturbs the pliable orbit of the comet. the path of the latter bears unmistakable traces of the jupiter perturbations, as well as of the mercury perturbations. it might seem a hopeless task to discriminate between the influences of the two planets, overshadowed as they both are by the supreme control of the sun, but contrivances of mathematical analysis are adequate to deal with the problem. they point out how much is due to mercury, how much is due to jupiter; and the wanderings of encke's comet can thus be made to disclose the mass of jupiter as well as that of mercury. here we have a means of testing the precision of our weighing appliances. the mass of jupiter can be measured by his moons, in the way mentioned in a previous chapter. as the satellites revolve round and round the planet, they furnish a method of measuring his weight by the rapidity of their motion. they tell us that if the sun were placed in one scale of the celestial balance, it would take , bodies equal to jupiter in the other to weigh him down. hardly a trace of uncertainty clings to this determination, and it is therefore of great interest to test the theory of encke's comet by seeing whether it gives an accordant result. the comparison has been made by von asten. encke's comet tells us that the sun is , times as heavy as jupiter; so the results are practically identical, and the accuracy of the indications of the comet are confirmed. but the calculation of the perturbations of encke's comet is so extremely intricate that asten's result is not of great value. from the motion of winnecke's periodic comet, haerdtl has found that the sun is , · times as heavy as jupiter, in perfect accordance with the best results derived from the attraction of jupiter on his satellites and the other planets. we have hitherto discussed the adventures of encke's comet in cases where they throw light on questions otherwise more or less known to us. we now approach a celebrated problem, on which encke's comet is our only authority. every , days that comet revolves completely around its orbit, and returns again to the neighbourhood of the sun. the movements of the comet are, however, somewhat irregular. we have already explained how perturbations arise from mercury and from jupiter. further disturbances arise from the attraction of the earth and of the other remaining planets; but all these can be allowed for, and then we are entitled to expect, if the law of gravitation be universally true, that the comet shall obey the calculations of mathematics. encke's comet has not justified this anticipation; at each revolution the period is getting steadily shorter! each time the comet comes back to perihelion in two and a half hours less than on the former occasion. two and a half hours is, no doubt, a small period in comparison with that of an entire revolution; but in the region of its path visible to us the comet is moving so quickly that its motion in two and a half hours is considerable. this irregularity cannot be overlooked, inasmuch as it has been confirmed by the returns during about twenty revolutions. it has sometimes been thought that the discrepancies might be attributed to some planetary perturbations omitted or not fully accounted for. the masterly analysis of von asten and backlund has, however, disposed of this explanation. they have minutely studied the observations down to , but only to confirm the reality of this diminution in the periodic time of encke's comet. an explanation of these irregularities was suggested by encke long ago. let us briefly attempt to describe this memorable hypothesis. when we say that a body will move in an elliptic path around the sun in virtue of gravitation, it is always assumed that the body has a free course through space. it is assumed that there is no friction, no air, or other source of disturbance. but suppose that this assumption should be incorrect; suppose that there really is some medium pervading space which offers resistance to the comet in the same way as the air impedes the flight of a rifle bullet, what effect ought such a medium to produce? this is the idea which encke put forward. even if the greater part of space be utterly void, so that the path of the filmy and almost spiritual comet is incapable of feeling resistance, yet in the neighbourhood of the sun it was supposed that there might be some medium of excessive tenuity capable of affecting so light a body. it can be demonstrated that a resisting medium such as we have supposed would lessen the size of the comet's path, and diminish the periodic time. this hypothesis has, however, now been abandoned. it has always appeared strange that no other comet showed the least sign of being retarded by the assumed resisting medium. but the labours of backlund have now proved beyond a doubt that the acceleration of the motion of encke's comet is not a constant one, and cannot be accounted for by assuming a resisting medium distributed round the sun, no matter how we imagine this medium to be constituted with regard to density at different distances from the sun. backlund found that the acceleration was fairly constant from to ; it commenced to decrease between and , and continued to diminish till some time between and , since which time it has remained fairly constant. he considers that the acceleration can only be produced by the comet encountering periodically a swarm of meteors, and if we could only observe the comet during its motion through the greater part of its orbit we should be able to point out the locality where this encounter takes place. we have selected the comets of halley and of encke as illustrations of the class of periodic comets, of which, indeed, they are the most remarkable members. another very remarkable periodic comet is that of biela, of which we shall have more to say in the next chapter. of the much more numerous class of non-periodic comets, examples in abundance may be cited. we shall mention a few which have appeared during the present century. there is first the splendid comet of , which appeared suddenly in february of that year, and was so brilliant that it could be seen during full daylight. this comet followed a path which could not be certainly distinguished from a parabola, though there is no doubt that it might have been a very elongated ellipse. it is frequently impossible to decide a question of this kind, during the brief opportunities available for finding the place of the comet. we can only see the object during a very small arc of its orbit, and even then it is not a very well-defined point which admits of being measured with the precision attainable in observations of a star or a planet. this comet of is, however, especially remarkable for the rapidity with which it moved, and for the close approach which it made to the sun. the heat to which it was exposed during its passage around the sun must have been enormously greater than the heat which can be raised in our mightiest furnaces. if the materials had been agate or cornelian, or the most infusible substances known on the earth, they would have been fused and driven into vapour by the intensity of the sun's rays. the great comet of was one of the celestial spectacles of modern times. it was first observed on june nd of that year by donati, whose name the comet has subsequently borne; it was then merely a faint nebulous spot, and for about three months it pursued its way across the heavens without giving any indications of the splendour which it was so soon to attain. the comet had hardly become visible to the unaided eye at the end of august, and was then furnished with only a very small tail, but as it gradually drew nearer and nearer to the sun in september, it soon became invested with splendour. a tail of majestic proportions was quickly developed, and by the middle of october, when the maximum brightness was attained, its length extended over an arc of forty degrees. the beauty and interest of this comet were greatly enhanced by its favourable position in the sky at a season when the nights were sufficiently dark. on the nd may, , mr. tebbutt, of windsor, in new south wales, discovered a comet which speedily developed into one of the most interesting celestial objects seen by this generation. about the nd of june it became visible from these latitudes in the northern sky at midnight. gradually it ascended higher and higher until it passed around the pole. the nucleus of the comet was as bright as a star of the first magnitude, and its tail was about ° long. on the nd of september it ceased to be visible to the unaided eye, but remained visible in telescopes until the following february. this was the first comet which was successfully photographed, and it may be remarked that comets possess very little actinic power. it has been estimated that moonlight possesses an intensity , times greater than that of a comet where the purposes of photography are concerned. another of the bodies of this class which have received great and deserved attention was that discovered in the southern hemisphere early in september, . it increased so much in brilliancy that it was seen in daylight by mr. common on the th of that month, while on the same day the astronomers at the cape of good hope were fortunate enough to have observed the body actually approach the sun's limb, where it ceased to be visible. we know that the comet must have passed between the earth and the sun, and it is very interesting to learn from the cape observers that it was totally invisible when it was actually projected on the sun's disc. the following day it was again visible to the naked eye in full daylight, not far from the sun, and valuable spectroscopic observations were secured at dunecht and palermo. at that time the comet was rushing through the part of its orbit closest to the sun, and about a week later it began to be visible in the morning before sunrise, near the eastern horizon, exhibiting a fine long tail. (_see_ plate xvii.) the nucleus gradually lengthened until it broke into four separate pieces, lying in a straight line, while the comet's head became enveloped in a sort of faint, nebulous tube, pointing towards the sun. several small detached nebulous masses became also visible, which travelled along with the comet, though not with the same velocity. the comet became invisible to the naked eye in february, and was last observed telescopically in south america on the st june, . there is a remarkable resemblance between the orbit of this comet and the orbits in which the comet of , the great comet of , and a great comet seen in in the southern hemisphere, travelled round the sun. in fact, these four comets moved along very nearly the same track and rushed round the sun within a couple of hundred thousand miles of the surface of the photosphere. it is also possible that the comet which, according to aristotle, appeared in the year b.c. followed the same orbit. and yet we cannot suppose that all these were apparitions of one and the same comet, as the observations of the comet of give the period of revolution of that body equal to about years. it is not impossible that the comets of and are one and the same, but in both years the observations extend over too short a time to enable us to decide whether the orbit was a parabola or an ellipse. but as the comet of was in any case a distinct body, it seems more likely that we have here a family of comets approaching the sun from the same region of space and pursuing almost the same course. we know a few other instances of such resemblances between the orbits of distinct comets. of other interesting comets seen within the last few years we may mention one discovered by mr. holmes in london on the th november, . it was then situated not far from the bright nebula in the constellation andromeda, and like it was just visible to the naked eye. the comet became gradually fainter and more diffused, but on the th january following it appeared suddenly with a central condensation, like a star of the eighth magnitude, surrounded by a small coma. gradually it expanded again, and grew fainter, until it was last observed on the th april.[ ] the orbit was found to be an ellipse more nearly circular than the orbit of any other known comet, the period being nearly seven years. another comet of is remarkable as having been discovered by professor barnard, of the lick observatory, on a photograph of a region in aquila; he was at once able to distinguish the comet from a nebula by its motion. since the light of every comet which has made its appearance has been analysed by the spectroscope. the slight surface-brightness of these bodies renders it necessary to open the slit of the spectroscope rather wide, and the dispersion employed cannot be very great, which again makes accurate measurements difficult. the spectrum of a comet is chiefly characterised by three bright bands shading gradually off towards the violet, and sharply defined on the side towards the red. this appearance is caused by a large number of fine and close lines, whose intensity and distance apart decrease towards the violet. these three bands reveal the existence of hydrocarbon in comets. the important _rôle_ which we thus find carbon playing in the constitution of comets is especially striking when we reflect on the significance of the same element on the earth. we see it as the chief constituent of all vegetable life, we find it to be invariably present in animal life. it is an interesting fact that this element, of such transcendent importance on the earth, should now have been proved to be present in these wandering bodies. the hydrocarbon bands are, however, not always the only features visible in cometary spectra. in a comet seen in the spring months of , professor copeland discovered that a new bright yellow line, coinciding in position with the d-line of sodium, had suddenly appeared, and it was subsequently, both by him and by other observers, seen beautifully double. in fact, sodium was so strongly represented in this comet, that both the head and the tail could be perfectly well seen in sodium light by merely opening the slit of the spectroscope very wide, just as a solar prominence may be seen in hydrogen light. the sodium line attained its greatest brilliance at the time when the comet was nearest to the sun, while the hydrocarbon bands were either invisible or very faint. the same connection between the intensity of the sodium line and the distance from the sun was noticed in the great september comet of . the spectrum of the great comet of was observed by copeland and lohse on the th september in daylight, and, in addition to the sodium line, they saw a number of other bright lines, which seemed to be due to iron vapour, while the only line of manganese visible at the temperature of a bunsen burner was also seen. this very remarkable observation was made less than a day after the perihelion passage, and illustrates the wonderful activity in the interior of a comet when very close to the sun. [illustration: plate xvii. the comet of , as seen from streatham, nov. th, a.m. from a drawing by t.e. key.] in addition to the bright lines comets generally show a faint continuous spectrum, in which dark fraunhofer lines can occasionally be distinguished. of course, this shows that the continuous spectrum is to a great extent due to reflected sunlight, but there is no doubt that part of it is often due to light actually developed in the comets. this was certainly the case in the first comet of , as a sudden outburst of light in this body was accompanied by a considerable increase of brightness of the continuous spectrum. a change in the relative brightness of the three hydrocarbon bands indicated a considerable rise of temperature, during the continuance of which the comet emitted white light. as comets are much nearer to the earth than the stars, it will occasionally happen that the comet must arrive at a position directly between the earth and a star. there is quite a similar phenomenon in the movement of the moon. a star is frequently occulted in this way, and the observations of such phenomena are familiar to astronomers; but when a comet passes in front of a star the circumstances are widely different. the star is indeed seen nearly as well through the comet as it would be if the comet were entirely out of the way. this has often been noticed. one of the most celebrated observations of this kind was made by the late sir john herschel on biela's comet, which is one of the periodic class, and will be alluded to in the next chapter. the illustrious astronomer saw on one occasion this object pass over a star cluster. it consisted of excessively minute stars, which could only be seen by a powerful telescope, such as the one sir john was using. the faintest haze or the merest trace of a cloud would have sufficed to hide all the stars. it was therefore with no little interest that the astronomer watched the progress of biela's comet. gradually the wanderer encroached on the group of stars, so that if it had any appreciable solidity the numerous twinkling points would have been completely screened. but what were the facts? down to the most minute star in that cluster, down to the smallest point of light which the great telescope could show, every object in the group was distinctly seen to twinkle right through the mass of biela's comet. this was an important observation. we must recollect that the veil drawn between the cluster and the telescope was not a thin curtain; it was a volume of cometary substance many thousands of miles in thickness. contrast, then, the almost inconceivable tenuity of a comet with the clouds to which we are accustomed. a cloud a few hundred feet thick will hide not only the stars, but even the great sun himself. the lightest haze that ever floated in a summer sky would do more to screen the stars from our view than would one hundred thousand miles of such cometary material as was here interposed. the great comet of donati passed over many stars which were visible distinctly through its tail. among these stars was a very bright one--the well-known arcturus. the comet, fortunately, happened to pass over arcturus, and though nearly the densest part of the comet was interposed between the earth and the star, yet arcturus twinkled on with undiminished lustre through the thickness of this stupendous curtain. recent observations have, however, shown that stars in some cases experience change in lustre when the denser part of the comet passes over them. it is, indeed, difficult to imagine that a star would remain visible if the nucleus of a really large comet passed over it; but it does not seem that an opportunity of testing this supposition has yet arisen. as a comet contains transparent gaseous material we might expect that the place of a star would be deranged when the comet approached it. the refractive power of air is very considerable. when we look at the sunset, we see the sun appearing to pass below the horizon; yet the sun has actually sunk beneath the horizon before any part of its disk appears to have commenced its descent. the refractive power of the air bends the luminous rays round and shows the sun, though it is directly screened by the intervening obstacles. the refractive power of the material of comets has been carefully tested. a comet has been observed to approach two stars; one of which was seen through the comet, while the other could be observed directly. if the body had any appreciable quantity of gas in its composition the relative places of the two stars would be altered. this question has been more than once submitted to the test of actual measurement. it has sometimes been found that no appreciable change of position could be detected, and that accordingly in such cases the comet has no perceptible density. careful measurements of the great comet in showed, however, that in the neighbourhood of the nucleus there was some refractive power, though quite insignificant in comparison with the refraction of our atmosphere. [illustration: plate c. comet a , . swift. _photographed by e.e. barnard, th april, ._] from these considerations it will probably be at once admitted that the _mass_ of a comet must be indeed a very small quantity in comparison with its bulk. when we attempt actually to weigh the comet, our efforts have proved abortive. we have been able to weigh the mighty planets jupiter and saturn; we have been even able to weigh the vast sun himself; the law of gravitation has provided us with a stupendous weighing apparatus, which has been applied in all these cases with success, but the same methods applied to comets are speedily seen to be illusory. no weighing machinery known to the astronomer is delicate enough to determine the weight of a comet. all that we can accomplish in any circumstances is to weigh one heavenly body in comparison with another. comets seem to be almost imponderable when estimated by such robust masses as those of the earth, or any of the other great planets. of course, it will be understood that when we say the weight of a comet is inappreciable, we mean with regard to the other bodies of our system. perhaps no one now doubts that a great comet must really weigh tons; though whether those tons are to be reckoned in tens, in hundreds, in thousands, or in millions, the total seems quite insignificant when compared with the weight of a body like the earth. the small mass of comets is also brought before us in a very striking way when we recall what has been said in the last chapter on the important subject of the planetary perturbations. we have there treated of the permanence of our system, and we have shown that this permanence depends upon certain laws which the planetary motions must invariably fulfil. the planets move nearly in circles, their orbits are all nearly in the same plane, and they all move in the same direction. the permanence of the system would be imperilled if any one of these conditions was not fulfilled. in that discussion we made no allusion to the comets. yet they are members of our system, and they far outnumber the planets. the comets repudiate these rules of the road which the planets so rigorously obey. their orbits are never like circles; they are, indeed, more usually parabolic, and thus differ as widely as possible from the circular path. nor do the planes of the orbits of comets affect any particular aspect; they are inclined at all sorts of angles, and the directions in which they move seem to be mere matters of caprice. all these articles of the planetary convention are violated by comets, but yet our system lasts; it has lasted for countless ages, and seems destined to last for ages to come. the comets are attracted by the planets, and conversely, the comets must attract the planets, and must perturb their orbits to some extent; but to what extent? if comets moved in orbits subject to the same general laws which characterise planetary motion, then our argument would break down. the planets might experience considerable derangements from cometary attraction, and yet in the lapse of time those disturbances would neutralise each other, and the permanence of the system would be unaffected. but the case is very different when we deal with the actual cometary orbits. if comets could appreciably disturb planets, those disturbances would not neutralise each other, and in the lapse of time the system would be wrecked by a continuous accumulation of irregularities. the facts, however, show that the system has lived, and is living, notwithstanding comets; and hence we are forced to the conclusion that their masses must be insignificant in comparison with those of the great planetary bodies. these considerations exhibit the laws of universal gravitation and their relations to the permanence of our system in a very striking light. if we include the comets, we may say that the solar system includes many thousands of bodies, in orbits of all sizes, shapes, and positions, only agreeing in the fact that the sun occupies a focus common to all. the majority of these bodies are imponderable in comparison with planets, and their orbits are placed anyhow, so that, although they may suffer much from the perturbations of the other bodies, they can in no case inflict any appreciable disturbance. there are, however, a few great planets capable of producing vast disturbances; and if their orbits were not properly adjusted, chaos would sooner or later be the result. by the mutual adaptations of their orbits to a nearly circular form, to a nearly coincident plane, and to a uniformity of direction, a permanent truce has been effected among the great planets. they cannot now permanently disorganise each other, while the slight mass of the comets renders them incompetent to do so. the stability of the great planets is thus assured; but it is to be observed that there is no guarantee of stability for comets. their eccentric and irregular paths may undergo the most enormous derangements; indeed, the history of astronomy contains many instances of the vicissitudes to which a cometary career is exposed. great comets appear in the heavens in the most diverse circumstances. there is no part of the sky, no constellation or region, which is not liable to occasional visits from these mysterious bodies. there is no season of the year, no hour of the day or of the night when comets may not be seen above the horizon. in like manner, the size and aspect of the comets are of every character, from the dim spot just visible to an eye fortified by a mighty telescope, up to a gigantic and brilliant object, with a tail stretching across the heavens for a distance which is as far as from the horizon to the zenith. so also the direction of the tail of the comet seems at first to admit of every possible position: it may stand straight up in the heavens, as if the comet were about to plunge below the horizon; it may stream down from the head of the comet, as if the body had been shot up from below; it may slope to the right or to the left. amid all this variety and seeming caprice, can we discover any feature common to the different phenomena? we shall find that there is a very remarkable law which the tails of comets obey--a law so true and satisfactory, that if we are given the place of a comet in the heavens, it is possible at once to point out in what direction the tail will lie. a beautiful comet appears in summer in the northern sky. it is near midnight; we are gazing on the faintly luminous tail, which stands up straight and points towards the zenith; perhaps it may be curved a little or possibly curved a good deal, but still, on the whole, it is directed from the horizon to the zenith. we are not here referring to any particular comet. every comet, large or small, that appears in the north must at midnight have its tail pointed up in a nearly vertical direction. this fact, which has been verified on numerous occasions, is a striking illustration of the law of direction of comets' tails. think for one moment of the facts of the case. it is summer; the twilight at the north shows the position of the sun, and the tail of the comet points directly away from the twilight and away from the sun. take another case. it is evening; the sun has set, the stars have begun to shine, and a long-tailed comet is seen. let that comet be high or low, north or south, east or west, its tail invariably points _away_ from that point in the west where the departing sunlight still lingers. again, a comet is watched in the early morning, and if the eye be moved from the place where the first streak of dawn is appearing to the head of the comet, then along that direction, streaming away from the sun, is found the tail of the comet. this law is of still more general application. at any season, at any hour of the night, the tail of a comet is directed away from the sun. more than three hundred years ago this fact in the movement of comets arrested the attention of those who pondered on the movements of the heavenly bodies. it is a fact patent to ordinary observation, it gives some degree of consistency to the multitudinous phenomena of comets, and it must be made the basis of our enquiries into the structure of the tails. in the adjoining figure, fig. , we show a portion of the parabolic orbit of a comet, and we also represent the position of the tail of the comet at various points of its path. it would be, perhaps, going too far to assert that throughout the whole vast journey of the comet, its tail must always be directed from the sun. in the first place, it must be recollected that we can only see the comet during that small part of its journey when it is approaching to or receding from the sun. it is also to be remembered that, while actually passing round the sun, the brilliancy of the comet is so overpowered by the sun that the comet often becomes invisible, just as the stars are invisible in daylight. indeed, in certain cases, jets of cometary material are actually projected towards the sun. [illustration: fig. .--the tail of a comet directed from the sun.] in a hasty consideration of the subject, it might be thought that as the comet was dashing along with enormous velocity the tail was merely streaming out behind, just as the shower of sparks from a rocket are strewn along the path which it follows. this would be an entirely erroneous analogy; the comet is moving not through an atmosphere, but through open space, where there is no medium sufficient to sweep the tail into the line of motion. another very remarkable feature is the gradual growth of the tail as the comet approaches the sun. while the body is still at a great distance it has usually no perceptible tail, but as it draws in the tail gradually develops, and in some cases reaches stupendous dimensions. it is not to be supposed that this increase is a mere optical consequence of the diminution of distance. it can be shown that the growth of the tail takes place much more rapidly than it would be possible to explain in this way. we are thus led to connect the formation of the tail with the approach to the sun, and we are accordingly in the presence of an enigma without any analogy among the other bodies of our system. that the comet as a whole is attracted by the sun there can be no doubt whatever. the fact that the comet moves in an ellipse or in a parabola proves that the two bodies act and react on each other in obedience to the law of universal gravitation. but while this is true of the comet as a whole, it is no less certain that the tail of the comet is _repelled_ by the sun. it is impossible to speak with certainty as to how this comes about, but the facts of the case seem to point to an explanation of the following kind. we have seen that the spectroscope has proved with certainty the presence of hydrocarbon and other gases in comets. but we are not to conclude from this that comets are merely masses of gas moving through space. though the total quantity of matter in a comet, as we have seen, is exceedingly small, it is quite possible that the comet may consist of a number of widely scattered particles of appreciable density; indeed, we shall see in the next chapter, when describing the remarkable relationship between comets and meteors, that we have reason to believe this to be the case. we may therefore look on a comet as a swarm of tiny solid particles, each surrounded by gas. when we watch a great comet approaching the sun the nucleus is first seen to become brighter and more clearly defined; at a later stage luminous matter appears to be projected from it towards the sun, often in the shape of a fan or a jet, which sometimes oscillates to and fro like a pendulum. in the head of halley's comet, for instance, bessel observed in october, , that the jet in the course of eight hours swung through an angle of °. on other occasions concentric arcs of light are formed round the nucleus, one after another, getting fainter as they travel further from the nucleus. evidently the material of the fan or the arcs is repelled by the nucleus of the comet; but it is also repelled by the sun, and this latter repulsive force compels the luminous matter to overcome the attraction of gravitation, and to turn back all round the nucleus in the direction away from the sun. in this manner the tail is formed. (_see_ plate xii.) the mathematical theory of the formation of comets' tails has been developed on the assumption that the matter which forms the tail is repelled both by the nucleus and by the sun. this investigation was first undertaken by the great astronomer bessel, in his memoir on the appearance of halley's comet in , and it has since been considerably developed by roche and the russian astronomer bredichin. though we are, perhaps, hardly in a position to accept this theory as absolutely true, we can assert that it accounts well for the principal phenomena observed in the formation of comets' tails. professor bredichin has conducted his labours in the philosophical manner which has led to many other great discoveries in science. he has carefully collated the measurements and drawings of the tails of various comets. one result has been obtained from this preliminary part of his enquiry, which possesses a value that cannot be affected even if the ulterior portion of his labours should be found to require qualification. in the examination of the various tails, he observed that the curvilinear shapes of the outlines fall into one or other of three special types. in the first we have the straightest tails, which point almost directly away from the sun. in the second are classed tails which, after starting away from the sun, are curved backwards from the direction towards which the comet is moving. in the third we find the appendage still more curved in towards the comet's path. it can be shown that the tails of comets can almost invariably be identified with one or other of these three types; and in cases where the comet exhibits two tails, as has sometimes happened, then they will be found to belong to two of the types. the adjoining diagram (fig. ) gives a sketch of an imaginary comet furnished with tails of the three different types. the direction in which the comet is moving is shown by the arrow-head on the line passing through the nucleus. bredichin concludes that the straightest of the three tails, marked as type i., is most probably due to the element hydrogen; the tails of the second form are due to the presence of some of the hydrocarbons in the body of the comet; while the small tails of the third type may be due to iron or to some other element with a high atomic weight. it will, of course, be understood that this diagram does not represent any actual comet. [illustration: fig. .--bredichin's theory of comets' tails.] [illustration: fig. .--tails of the comet of .] an interesting illustration of this theory is afforded in the case of the celebrated comet of already referred to, of which a drawing is shown in fig. . we find here, besides the great tail, which is the characteristic feature of the body, two other faint streaks of light. these are the edges of the hollow cone which forms a tail of type i. when we look through the central regions it will be easily understood that the light is not sufficiently intense to be visible; at the edges, however, a sufficient thickness of the cometary matter is presented, and thus we have the appearance shown in this figure. it would seem that donati's comet possessed one tail due to hydrogen, and another due to some of the compounds of carbon. the carbon compounds involved appear to be of considerable variety, and there is, in consequence, a disposition in the tails of the second type to a more indefinite outline than in the hydrogen tails. cases have been recorded in which several tails have been seen simultaneously on the same comet. the most celebrated of these is that which appeared in the year . professor bredichin has devoted special attention to the theory of this marvellous object, and he has shown with a high degree of probability how the multiform tail could be accounted for. the adjoining figure (fig. ) is from a sketch of this object made on the morning of the th march by mademoiselle kirch at the berlin observatory. the figure shows eleven streaks, of which the first ten (counting from the left) represent the bright edges of five of the tails, while the sixth and shortest tail is at the extreme right. sketches of this rare phenomenon were also made by chéseaux at lausanne and de l'isle at st. petersburg. before the perihelion passage the comet had only had one tail, but a very splendid one. [illustration: fig. .--the comet of .] it is possible to submit some of the questions involved to the test of calculation, and it can be shown that the repulsive force adequate to produce the straight tail of type i. need only be about twelve times as large as the attraction of gravitation. tails of the second type could be produced by a repulsive force which was about equal to gravitation, while tails of the third type would only require a repulsive force about one-quarter the power of gravitation.[ ] the chief repulsive force known in nature is derived from electricity, and it has naturally been surmised that the phenomena of comets' tails are due to the electric condition of the sun and of the comet. it would be premature to assert that the electric character of the comet's tail has been absolutely demonstrated; all that can be said is that, as it seems to account for the observed facts, it would be undesirable to introduce some mere hypothetical repulsive force. it must be remembered that on quite other grounds it is known that the sun is the seat of electric phenomena. as the comet gradually recedes from the sun the repulsive force becomes weaker, and accordingly we find that the tail of the comet declines. if the comet be a periodic one, the same series of changes may take place at its next return to perihelion. a new tail is formed, which also gradually disappears as the comet regains the depths of space. if we may employ the analogy of terrestrial vapours to guide us in our reasoning, then it would seem that, as the comet retreats, its tail would condense into myriads of small particles. over these small particles the law of gravitation would resume its undivided sway, no longer obscured by the superior efficiency of the repulsion. the mass of the comet is, however, so extremely small that it would not be able to recall these particles by the mere force of attraction. it follows that, as the comet at each perihelion passage makes a tail, it must on each occasion expend a corresponding quantity of tail-making material. let us suppose that the comet was endowed in the beginning with a certain capital of those particular materials which are adapted for the production of tails. each perihelion passage witnesses the formation of a tail, and the expenditure of a corresponding amount of the capital. it is obvious that this operation cannot go on indefinitely. in the case of the great majority of comets the visits to perihelion are so extremely rare that the consequences of the extravagance are not very apparent; but to those periodic comets which have short periods and make frequent returns, the consequences are precisely what might have been anticipated: the tail-making capital has been gradually squandered, and thus at length we have the spectacle of a comet without any tail at all. we can even conceive that a comet may in this manner be completely dissipated, and we shall see in the next chapter how this fate seems to have overtaken biela's periodic comet. but as it sweeps through the solar system the comet may chance to pass very near one of the larger planets, and, in passing, its motion may be seriously disturbed by the attraction of the planet. if the velocity of the comet is accelerated by this disturbing influence, the orbit will be changed from a parabola into another curve known as a hyperbola, and the comet will swing round the sun and pass away never to return. but if the planet is so situated as to retard the velocity of the comet, the parabolic orbit will be changed into an ellipse, and the comet will become a periodic one. we can hardly doubt that some periodic comets have been "captured" in this manner and thereby made permanent members of our solar system, if we remark that the comets of short periods (from three to eight years) come very near the orbit of jupiter at some point or other of their paths. each of them must, therefore, have been near the giant planet at some moment during their past history. similarly the other periodic comets of longer period approach near to the orbits of either saturn, uranus, or neptune, the last-mentioned planet being probably responsible for the periodicity of halley's comet. we have, indeed, on more than one occasion, actually witnessed the violent disturbance of a cometary orbit. the most interesting case is that of lexell's comet. in the french astronomer messier (who devoted himself with great success to the discovery of comets) detected a comet for which lexell computed the orbit, and found an ellipse with a period of five years and some months. yet the comet had never been seen before, nor did it ever come back again. long afterwards it was found, from most laborious investigations by burckhardt and le verrier, that the comet had moved in a totally different orbit previous to . but at the beginning of the year it happened to come so close to jupiter that the powerful attraction of this planet forced it into a new orbit, with a period of five and a half years. it passed the perihelion on the th august, , and again in , but in the latter year it was not conveniently situated for being seen from the earth. in the summer of the comet was again in the neighbourhood of jupiter, and was thrown out of its elliptic orbit, so that we have never seen it since, or, perhaps, it would be safer to say that we have not with certainty identified lexell's comet with any comet observed since then. we are also, in the case of several other periodic comets, able to fix in a similar way the date when they started on their journeys in their present elliptic orbits. such is a brief outline of the principal facts known with regard to these interesting but perplexing bodies. we must be content with the recital of what we know, rather than hazard guesses about matters beyond our reach. we see that they are obedient to the great laws of gravitation, and afford a striking illustration of their truth. we have seen how modern science has dissipated the superstition with which, in earlier ages, the advent of a comet was regarded. we no longer regard such a body as a sign of impending calamity; we may rather look upon it as an interesting and a beautiful visitor, which comes to please us and to instruct us, but never to threaten or to destroy. chapter xvii. shooting stars. small bodies of our system--their numbers--how they are observed--the shooting star--the theory of heat--a great shooting star--the november meteors--their ancient history--the route followed by the shoal--diagram of the shoal of meteors--how the shoal becomes spread out along its path--absorption of meteors by the earth--the discovery of the relation between meteors and comets--the remarkable investigations concerning the november meteors--two showers in successive years--no particles have ever been identified from the great shooting star showers--meteoric stones--chladni's researches--early cases of stone-falls--the meteorite at ensisheim--collections of meteorites--the rowton siderite--relative frequency of iron and stony meteorites--fragmentary character of meteorites--tschermak's hypothesis--effects of gravitation on a missile ejected from a volcano--can they have come from the moon?--the claims of the minor planets to the parentage of meteorites--possible terrestrial origin--the ovifak iron. in the preceding chapters we have dealt with the gigantic bodies which form the chief objects in what we know as the solar system. we have studied mighty planets measuring thousands of miles in diameter, and we have followed the movements of comets whose dimensions are often to be told by millions of miles. once, indeed, in a previous chapter we have made a descent to objects much lower in the scale of magnitude, and we have examined that numerous class of small bodies which we call the minor planets. it is now, however, our duty to make a still further, and this time a very long step, downwards in the scale of magnitude. even the minor planets must be regarded as colossal objects when compared with those little bodies whose presence is revealed to us in an interesting and sometimes in a striking manner. these small bodies compensate in some degree for their minute size by the profusion in which they exist. no attempt, indeed, could be made to tell in figures the myriads in which they swarm throughout space. they are probably of very varied dimensions, some of them being many pounds or perhaps tons in weight, while others seem to be not larger than pebbles, or even than grains of sand. yet, insignificant as these bodies may seem, the sun does not disdain to undertake their control. each particle, whether it be as small as the mote in a sunbeam or as mighty as the planet jupiter, must perforce trace out its path around the sun in conformity with the laws of kepler. who does not know that beautiful occurrence which we call a shooting star, or which, in its more splendid forms, is sometimes called a meteor or fireball? it is to objects of this class that we are now to direct our attention. a small body is moving round the sun. just as a mighty planet revolves in an ellipse, so even a small object will be guided round and round in an ellipse with the sun in the focus. there are, at the present moment, inconceivable myriads of such meteors moving in this manner. they are too small and too distant for our telescopes, and we never see them except under extraordinary circumstances. when the meteor flashes into view it is moving with such enormous velocity that it often traverses more than twenty miles in a second of time. such a velocity is almost impossible near the earth's surface: the resistance of the air would prevent it. aloft, in the emptiness of space, there is no air to impede its flight. it may have been moving round and round the sun for thousands, perhaps for millions of years, without suffering any interference; but the supreme moment arrives, and the meteor perishes in a streak of splendour. in the course of its wanderings the body comes near the earth, and within a few hundred miles of its surface begins to encounter the upper surface of the atmosphere with which the earth is enclosed. to a body moving with the appalling velocity of a meteor, a plunge into the atmosphere is usually fatal. even though the upper layers of air are excessively attenuated, yet they suddenly check the velocity almost as a rifle bullet would be checked when fired into water. as the meteor rushes through the atmosphere the friction of the air warms its surface; gradually it becomes red-hot, then white-hot, and is finally driven off into vapour with a brilliant light, while we on the earth, one or two hundred miles below, exclaim: "oh, look, there is a shooting star!" we have here an experiment illustrating the mechanical theory of heat. it may seem incredible that mere friction should be sufficient to generate heat enough to produce so brilliant a display, but we must recollect two facts: first, that the velocity of the meteor is, perhaps, one hundred times that of a rifle bullet; and, second, that the efficiency of friction in developing heat is proportional to the square of the velocity. the meteor in passing through the air may therefore develop by the friction of the air about ten thousand times as much heat as the rifle bullet. we do not make an exaggerated estimate in supposing that the latter missile becomes heated ten degrees by friction; yet if this be admitted, we must grant that there is such an enormous development of heat attending the flight of the meteor that even a fraction of it would be sufficient to drive the object into vapour. let us first consider the circumstances in which these external bodies are manifested to us, and, for the sake of illustration, we may take a remarkable fireball which occurred on november th, . this body was seen from many different places in england; and by combining and comparing these observations, we obtain accurate information as to the height of the object and the velocity with which it travelled. it appears that this meteor commenced to be visible at a point ninety miles above frome, in somersetshire, and that it vanished twenty-seven miles over the sea, near st. ives, in cornwall. the path of the body, and the principal localities from which it was observed, are shown in the map (fig. ). the whole length of its visible course was about miles, which was performed in a period of five seconds, thus giving an average velocity of thirty-four miles per second. a remarkable feature in the appearance which this fireball presented was the long persistent streak of luminous cloud, about fifty miles long and four miles wide, which remained in sight for fully fifty minutes. we have in this example an illustration of the chief features of the phenomena of a shooting star presented on a very grand scale. it is, however, to be observed that the persistent luminous streak is not a universal, nor, indeed, a very common characteristic of a shooting star. [illustration: fig. .--the path of the fireball of november th, .] the small objects which occasionally flash across the field of the telescope show us that there are innumerable telescopic shooting stars, too small and too faint to be visible to the unaided eye. these objects are all dissipated in the way we have described; it is, in fact, only at the moment, and during the process of their dissolution, that we become aware of their existence. small as these missiles probably are, their velocity is so prodigious that they would render the earth uninhabitable were they permitted to rain down unimpeded on its surface. we must, therefore, among the other good qualities of our atmosphere, not forget that it constitutes a kindly screen, which shields us from a tempest of missiles, the velocity of which no artillery could equal. it is, in fact, the very fury of these missiles which is the cause of their utter destruction. their anxiety to strike us is so great, that friction dissolves them into harmless vapour. next to a grand meteor such as that we have just described, the most striking display in connection with shooting stars is what is known as a shower. these phenomena have attracted a great deal of attention within the last century, and they have abundantly rewarded the labour devoted to them by affording some of the most interesting astronomical discoveries of modern times. the showers of shooting stars do not occur very frequently. no doubt the quickened perception of those who especially attend to meteors will detect a shower when others see only a few straggling shooting stars; but, speaking generally, we may say that the present generation can hardly have witnessed more than two or three such occurrences. i have myself seen two great showers, one of which, in november, , has impressed itself on my memory as a glorious spectacle. to commence the history of the november meteors it is necessary to look back for nearly a thousand years. on the th of october, in the year , occurred the death of a moorish king, and in connection with this event an old chronicler relates how "that night there were seen, as it were lances, an infinite number of stars, which scattered themselves like rain to right and left, and that year was called the year of the stars." no one now believes that the heavens intended to commemorate the death of the king by that display. the record is, however, of considerable importance, for it indicates the year as one in which a great shower of shooting stars occurred. it was with the greatest interest astronomers perceived that this was the first recorded instance of that periodical shower, the last of whose regular returns were seen in , , and . further diligent literary research has revealed here and there records of startling appearances in the heavens, which fit in with successive returns of the november meteors. from the first instance, in , to the present day there have been twenty-nine visits of the shower; and it is not unlikely that these may have all been seen in some parts of the earth. sometimes they may have been witnessed by savages, who had neither the inclination nor the means to place on record an apparition which to them was a source of terror. sometimes, however, these showers were observed by civilised communities. their nature was not understood, but the records were made; and in some cases, at all events, these records have withstood the corrosion of time, and have now been brought together to illustrate this curious subject. we have altogether historical notices of twelve of these showers, collected mainly by the industry of professor h.a. newton whose labours have contributed so much to the advancement of our knowledge of shooting stars. let us imagine a swarm of small objects roaming through space. think of a shoal of herrings in the ocean, extending over many square miles, and containing countless myriads of individuals; or think of those enormous flocks of wild pigeons in the united states of which audubon has told us. the shoal of shooting stars is perhaps much more numerous than the herrings or the pigeons. the shooting stars are, however, not very close together; they are, on an average, probably some few miles apart. the actual bulk of the shoal is therefore prodigious; and its dimensions are to be measured by hundreds of thousands of miles. [illustration: fig. .--the orbit of a shoal of meteors.] the meteors cannot choose their own track, like the shoal of herrings, for they are compelled to follow the route which is prescribed to them by the sun. each one pursues its own ellipse in complete independence of its neighbours, and accomplishes its journey, thousands of millions of miles in length, every thirty-three years. we cannot observe the meteors during the greater part of their flight. there are countless myriads of these bodies at this very moment coursing round their path. we never see them till the earth catches them. every thirty-three years the earth makes a haul of these meteors just as successfully as the fisherman among the herrings, and in much the same way, for while the fisherman spreads his net in which the fishes meet their doom, so the earth has an atmosphere wherein the meteors perish. we are told that there is no fear of the herrings becoming exhausted, for those the fishermen catch are as nothing compared to the profusion in which they abound in ocean. we may say the same with regard to the meteors. they exist in such myriads, that though the earth swallows up millions every thirty-three years, plenty are left for future showers. the diagram (fig. ) will explain the way in which the earth makes her captures. we there see the orbit in which our globe moves around the sun, as well as the elliptic path of the meteors, though it should be remarked that it is not convenient to draw the figure exactly to scale, so that the path of the meteors is relatively much larger than here represented. once each year the earth completes its revolution, and between the th and the th of november crosses the track in which the meteors move. it will usually happen that the great shoal is not at this point when the earth is passing. there are, however, some stragglers all along the path, and the earth generally catches a few of these at this date. they dart into our atmosphere as shooting stars, and form what we usually speak of as the november meteors. it will occasionally happen that when the earth is in the act of crossing the track it encounters the bulk of the meteors. through the shoal our globe then plunges, enveloped, of course, with the surrounding coat of air. into this net the meteors dash in countless myriads, never again to emerge. in a few hours' time, the earth, moving at the rate of eighteen miles a second, has crossed the track and emerges on the other side, bearing with it the spoils of the encounter. some few meteors, which have only narrowly escaped capture, will henceforth bear evidence of the fray by moving in slightly different orbits, but the remaining meteors of the shoal continue their journey without interruption; perhaps millions have been taken, but probably hundreds of millions have been left. such was the occurrence which astonished the world on the night between november th and th, . we then plunged into the middle of the shoal. the night was fine; the moon was absent. the meteors were distinguished not only by their enormous multitude, but by their intrinsic magnificence. i shall never forget that night. on the memorable evening i was engaged in my usual duty at that time of observing nebulæ with lord rosse's great reflecting telescope. i was of course aware that a shower of meteors had been predicted, but nothing that i had heard prepared me for the splendid spectacle so soon to be unfolded. it was about ten o'clock at night when an exclamation from an attendant by my side made me look up from the telescope, just in time to see a fine meteor dash across the sky. it was presently followed by another, and then again by more in twos and in threes, which showed that the prediction of a great shower was likely to be verified. at this time the earl of rosse (then lord oxmantown) joined me at the telescope, and, after a brief interval, we decided to cease our observations of the nebulæ and ascend to the top of the wall of the great telescope (fig. , p. ), whence a clear view of the whole hemisphere of the heavens could be obtained. there, for the next two or three hours, we witnessed a spectacle which can never fade from my memory. the shooting stars gradually increased in number until sometimes several were seen at once. sometimes they swept over our heads, sometimes to the right, sometimes to the left, but they all diverged from the east. as the night wore on, the constellation leo ascended above the horizon, and then the remarkable character of the shower was disclosed. all the tracks of the meteors radiated from leo. (_see_ fig. , p. .) sometimes a meteor appeared to come almost directly towards us, and then its path was so foreshortened that it had hardly any appreciable length, and looked like an ordinary fixed star swelling into brilliancy and then as rapidly vanishing. occasionally luminous trains would linger on for many minutes after the meteor had flashed across, but the great majority of the trains in this shower were evanescent. it would be impossible to say how many thousands of meteors were seen, each one of which was bright enough to have elicited a note of admiration on any ordinary night. the adjoining figure (fig. ) shows the remarkable manner in which the shooting stars of this shower diverged from a point. it is not to be supposed that all these objects were in view at the same moment. the observer of a shower is provided with a map of that part of the heavens in which the shooting stars appear. he then fixes his attention on one particular shooting star, and observes carefully its track with respect to the fixed stars in its vicinity. he then draws a line upon his map in the direction in which the shooting star moved. repeating the same observation for several other shooting stars belonging to the shower, his map will hardly fail to show that their different tracks almost all tend from one point or region of the figure. there are, it is true, a few erratic ones, but the majority observe this law. it certainly looks, at first sight, as if all the shooting stars did actually dart from this point; but a little reflection will show that this is a case in which the real motion is different from the apparent. if there actually were a point from which these meteors diverged, then from different parts of the earth the point would be seen in different positions with respect to the fixed stars; but this is not the case. the radiant, as this point is called, is seen in the same part of the heavens from whatever station the shower is visible. [illustration: fig. .--the radiant point of shooting stars.] we are, therefore, led to accept the simple explanation afforded by the theory of perspective. those who are acquainted with the principles of this science know that when a number of parallel lines in an object have to be represented in a drawing, they must all be made to pass through the same point in the plane of the picture. when we are looking at the shooting stars, we see the projections of their paths upon the surface of the heavens. from the fact that those paths pass through the same point, we are to infer that the shooting stars belonging to the same shower are moving in parallel lines. we are now able to ascertain the actual direction in which the shooting stars are moving, because a line drawn from the eye of the observer to the radiant point must be parallel to that direction. of course, it is not intended to convey the idea that throughout all space the shooting stars of one shower are moving in parallel lines; all we mean is that during the short time in which we see them the motion of each of the shooting stars is sensibly a straight line, and that all these straight lines are parallel. in the year the great meteor shoal of the leonids (for so this shower is called) attained its greatest distance from the sun, and then commenced to return. each year the earth crossed the orbit of the meteors; but the shoal was not met with, and no noteworthy shower of stars was perceived. every succeeding year found the meteors approaching the critical point, and the year brought the shoal to the earth's track. in that year a brilliant meteoric shower was expected, but the result fell far short of expectation. the shoal of meteors is of such enormous length that it takes more than a year for the mighty procession to pass through the critical portion of its orbit which lies across the track of the earth. we thus see that the meteors cannot escape the earth. it may be that when the shoal begins to reach this neighbourhood the earth will have just left this part of its path, and a year will have elapsed before the earth gets round again. those meteors that have the good fortune to be in the front of the shoal will thus escape the net, but some of those behind will not be so fortunate, and the earth will again devour an incredible host. it has sometimes happened that casts into the shoal have been obtained in two consecutive years. if the earth happened to pass through the front part in one year, then the shoal is so long that the earth will have moved right round its orbit of , , miles, and will again dash through the critical spot before the entire number have passed. history contains records of cases when, in two consecutive novembers, brilliant showers of leonids have been seen. as the earth consumes such myriads of leonids each thirty-three years, it follows that the total number must be decreasing. the splendour of the showers in future ages will, no doubt, be affected by this circumstance. they cannot be always so bright as they have been. it is also of interest to notice that the shape of the shoal is gradually changing. each meteor of the shoal moves in its own ellipse round the sun, and is quite independent of the rest of these bodies. each one has thus a special period of revolution which depends upon the length of the ellipse in which it happens to revolve. two meteors will move around the sun in the same time if the lengths of their ellipses are exactly equal, but not otherwise. the lengths of these ellipses are many hundreds of millions of miles, and it is impossible that they can be all absolutely equal. in this may be detected the origin of a gradual change in the character of the shower. suppose two meteors a and b be such that a travels completely round in thirty-three years, while b takes thirty-four years. if the two start together, then when a has finished the first round b will be a year behind; the next time b will be two years behind, and so on. the case is exactly parallel to that of a number of boys who start for a long race, in which they have to run several times round the course before the distance has been accomplished. at first they all start in a cluster, and perhaps for the first round or two they may remain in comparative proximity; gradually, however, the faster runners get ahead and the slower ones lag behind, so the cluster becomes elongated. as the race continues, the cluster becomes dispersed around the entire course, and perhaps the first boy will even overtake the last. such seems the destiny of the november meteors in future ages. the cluster will in time come to be spread out around the whole of this mighty track, and no longer will a superb display have to be recorded every thirty-three years. it was in connection with the shower of november meteors in that a very interesting and beautiful discovery in mathematical astronomy was made by professor adams. we have seen that the leonids must move in an elliptic path, and that they return every thirty-three years, but the telescope cannot follow them during their wanderings. all that we know by observation is the date of their occurrence, the point of the heavens from which they radiate, and the great return every thirty-three years. putting these various facts together, it is possible to determine the ellipse in which the meteors move--not exactly: the facts do not go so far--they only tell us that the ellipse must be one of five possible orbits. these five possible orbits are--firstly, the immense ellipse in which we now know the meteorites do revolve, and for which they require the whole thirty-three years to complete a revolution; secondly, a nearly circular orbit, very little larger than the earth's path, which the meteors would traverse in a few days more than a year; another similar orbit, in which the time would be a few days short of the year; and two other small elliptical orbits lying inside the earth's orbit. it was clearly demonstrated by professor newton, of new haven, u.s.a., that the observed facts would be explained if the meteors moved in any one of these paths, but that they could not be explained by any other hypothesis. it remained to see which of these orbits was the true one. professor newton himself made the suggestion of a possible method of solving the problem. the test he proposed was one of some difficulty, for it involved certain intricate calculations in the theory of perturbations. fortunately, however, professor adams undertook the inquiry, and by his successful labours the path of the leonids has been completely ascertained. [illustration: fig. .--the history of the leonids.] when the ancient records of the appearance of great leonid showers were examined, it was found that the date of their occurrence undergoes a gradual and continuous change, which professor newton fixed at one day in seventy years. it follows as a necessary consequence that the point where the path of the meteors crosses the earth's track is not fixed, but that at each successive return they cross at a point about half a degree further on in the direction in which the earth is travelling. it follows that the orbit in which the meteors are revolving is undergoing change; the path they follow in one revolution varies slightly from that pursued in the next. as, however, these changes proceed in the same direction, they may gradually attain considerable dimensions; and the amount of change which is produced in the path of the meteors in the lapse of centuries may be estimated by the two ellipses shown in fig. . the continuous line represents the orbit in a.d. ; the dotted line represents it at present. this unmistakable change in the orbit is one that astronomers attribute to what we have already spoken of as perturbation. it is certain that the elliptic motion of these bodies is due to the sun, and that if they were only acted on by the sun the ellipse would remain absolutely unaltered. we see, then, in this gradual change of the ellipse the influence of the attractions of the planets. it was shown that if the meteors moved in the large orbit, this shifting of the path must be due to the attraction of the planets jupiter, saturn, uranus, and the earth; while if the meteors followed one of the smaller orbits, the planets that would be near enough and massive enough to act sensibly on them would be the earth, venus, and jupiter. here, then, we see how the question may be answered by calculation. it is difficult, but it is possible, to calculate what the attraction of the planets would be capable of producing for each of the five different suppositions as to the orbit. this is what adams did. he found that if the meteors moved in the great orbit, then the attraction of jupiter would account for two-thirds of the observed change, while the remaining third was due to the influence of saturn, supplemented by a small addition on account of uranus. in this way the calculation showed that the large orbit was a possible one. professor adams also computed the amount of displacement in the path that could be produced if the meteors revolved in any of the four smaller ellipses. this investigation was one of an arduous character, but the results amply repaid the labour. it was shown that with the smaller ellipses it would be impossible to obtain a displacement even one-half of that which was observed. these four orbits must, therefore, be rejected. thus the demonstration was complete that it is in the large path that the meteors revolve. the movements in each revolution are guided by kepler's laws. when at the part of its path most distant from the sun the velocity of a meteor is at its lowest, being then but little more than a mile a second; as it draws in, the speed gradually increases, until, when the meteor crosses the earth's track, its velocity is no less than twenty-six miles a second. the earth is moving very nearly in the opposite direction at the rate of eighteen miles a second, so that, if the meteor happen to strike the earth's atmosphere, it does so with the enormous velocity of nearly forty-four miles a second. if a collision is escaped, then the meteor resumes its onward journey with gradually declining velocity, and by the time it has completed its circuit a period of thirty-three years and a quarter will have elapsed. the innumerable meteors which form the leonids are arranged in an enormous stream, of a breadth very small in comparison with its length. if we represent the orbit by an ellipse whose length is seven feet, then the meteor stream will be represented by a thread of the finest sewing-silk, about a foot and a half or two feet long, creeping along the orbit.[ ] the size of this stream may be estimated from the consideration that even its width cannot be less than , miles. its length may be estimated from the circumstance that, although its velocity is about twenty-six miles a second, yet the stream takes about two years to pass the point where its orbit crosses the earth's track. on the memorable night between the th and th of november, , the earth plunged into this stream near its head, and did not emerge on the other side until five hours later. during that time it happened that the hemisphere of the earth which was in front contained the continents of europe, asia, and africa, and consequently it was in the old world that the great shower was seen. on that day twelvemonth, when the earth had regained the same spot, the shoal had not entirely passed, and the earth made another plunge. this time the american continent was in the van, and consequently it was there that the shower of was seen. even in the following year the great shoal had not entirely passed, and since then a few stragglers along the route have been encountered at each annual transit of the earth across this meteoric highway. the diagram is also designed to indicate a remarkable speculation which was put forward on the high authority of le verrier, with the view of explaining how the shoal came to be introduced into the solar system. the orbit in which the meteors revolve does not intersect the paths of jupiter, saturn, or mars, but it does intersect the orbit of uranus. it must sometimes happen that uranus is passing through this point of its path just as the shoal arrives there. le verrier has demonstrated that such an event took place in the year a.d. , but that it has not happened since. we thus seem to have a clue to a very wonderful history by which the meteors are shown to have come into our system in the year named. the expectations or a repetition of the great shower in which had been widely entertained, and on good grounds, were not realised. hardly more than a few meteors of the ordinary type were observed. assuming that the orbit of the august meteors was a parabola, schiaparelli computed the dimensions and position in space of this orbit, and when he had worked this out, he noticed that the orbit corresponded in every particular with the orbit of a fine comet which had appeared in the summer of . this could not be a mere matter of accident. the plane in which the comet moved coincided exactly with that in which the meteors moved; so did the directions of the axes of their orbits, while the direction of the motion is the same, and the shortest distance from the sun to the orbit is also in the two cases identical. this proved to demonstration that there must be some profound physical connection between comets and swarms of meteors. and a further proof of this was shortly afterwards furnished, when le verrier had computed the orbit of the november meteors, for this was at once noticed to be precisely the same as the orbit of a comet which had passed its perihelion in january, , and for which the period of revolution had been found to be thirty-three years and two months. among the leonids we see occasionally fireballs brighter than venus, and even half the apparent size of the moon, bursting out with lightning-like flashes, and leaving streaks which last from a minute to an hour or more. but the great majority are only as bright as stars of the second, third, or fourth magnitude. as the amount of light given by a meteor depends on its mass and velocity, we can form some idea as to the actual weight of one of these meteors, and it appears that most of them do not weigh nearly as much as a quarter of an ounce; indeed, it is probable that many do not weigh a single grain. but we have seen that a comet in all probability is nothing but a very loose swarm of small particles surrounded by gas of very slight density, and we have also seen that the material of a comet must by degrees be more or less dissipated through space. we have still to tell a wonderful story of the breaking up of a comet and what appears to have become of the particles thereof. a copious meteoric shower took place on the night of the th november, . on this occasion the shooting stars diverged from a radiant point in the constellation of andromeda. as a spectacle, it was unquestionably inferior to the magnificent display of , but it is difficult to say which of the two showers has been of greater scientific importance. it surely is a remarkable coincidence that the earth should encounter the andromedes (for so this shower is called) at the very moment when it is crossing the track of biela's comet. we have observed the direction from which the andromedes come when they plunge into the atmosphere; we can ascertain also the direction in which biela's comet is moving when it passes the earth's track, and we find that the direction in which the comet moves and the direction in which the meteors move are identical. this is, in itself, a strong and almost overwhelming presumption that the comet and the shooting stars are connected; but it is not all. we have observations of this swarm dating back to the eighteenth century, and we find that the date of its appearance has changed from the th or th of december to the end of november in perfect accordance with the retrograde motion of the crossing-point of the earth's orbit and the orbit of biela's comet. this comet was observed in , and again in - , before its periodic return every seven years was discovered. it was discovered by biela in , and was observed again in . in the astronomical world was startled to find that there were now two comets in place of one, and the two fragments were again perceived at the return in . in biela's comet could not be seen, owing to its unfavourable situation with regard to the earth. no trace of biela's comet was seen in - , when its return was also due, nor has it ever been seen since. it therefore appears that in the autumn of the time had arrived for the return of biela's comet, and thus the occurrence of the great shower of the andromedes took place about the time when biela's comet was actually due. the inference is irresistible that the shooting stars, if not actually a part of the comet itself, are at all events most intimately connected therewith. this shower is also memorable for the telegram sent from professor klinkerfues to mr. pogson at madras. the telegram ran as follows:--"biela touched earth on th. search near theta centauri." pogson did search and did find a comet, but, unfortunately, owing to bad weather he only secured observations of it on two nights. as we require three observations to determine the orbit of a planet or comet, it is not possible to compute the orbit of pogson's, but it seems almost certain that the latter cannot be identical with either of the two components of biela's comet. it is, however, likely that it really was a comet moving along the same track as biela and the meteors. another display of the biela meteors took place in , just giving time for two complete revolutions of the swarm since . the display on the th november, , was magnificent; professor newton estimated that at the time of maximum the meteors came on at the rate of , per hour. in the comet ought again to have returned to perihelion, but in that year no meteors were seen on the th november, while many were seen on the rd from the same radiant. the change in the point of intersection between the orbit of the meteors and the orbit of the earth indicated by this difference of four days was found by bredichin to be due to the perturbing action of jupiter on the motion of the swarm. it is a noticeable circumstance that the great meteoric showers seem never yet to have projected a missile which has reached the earth's surface. out of the myriads of leonids, of perseids, or of andromedes, not one particle has ever been seized and identified.[ ] those bodies which fall from the sky to the earth, and which we call meteorites, do not seem to come from the great showers, so far as we know. they may, indeed, have quite a different origin from that of the periodic meteors. it is somewhat curious that the belief in the celestial origin of meteorites is of modern growth. in ancient times there were, no doubt, rumours of wonderful stones which had fallen down from the heavens to the earth, but these reports seem to have obtained but little credit. they were a century ago regarded as perfectly fabulous, though there was abundant testimony on the subject. eye-witnesses averred that they had seen the stones fall. the bodies themselves were unlike other objects in the neighbourhood, and cases were even authenticated where men had been killed by these celestial visitors. no doubt the observations were generally made by ignorant and illiterate persons. the true parts of the record were so mixed up with imaginary additions, that cautious men refused to credit the statements that such objects really fell from the sky. even at the present day it is often extremely difficult to obtain accurate testimony on such matters. for instance, the fall of a meteorite was observed by a hindoo in the jungle. the stone was there, its meteoric character was undoubted, and the witness was duly examined as to the details of the occurrence; but he was so frightened by the noise and by the danger he believed himself to have narrowly escaped, that he could tell little or nothing. he felt certain, however, that the meteorite had hunted him for two hours through the jungle before it fell to the earth! in the year chladni published an account of the remarkable mass of iron which the traveller pallas had discovered in siberia. it was then for the first time recognised that this object and others similar to it must have had a celestial origin. but even chladni's reputation and the arguments he brought forward failed to procure universal assent. shortly afterwards a stone of fifty-six pounds was exhibited in london, which several witnesses declared they had seen fall at wold cottage, in yorkshire, in . this body was subsequently deposited in our national collection, and is now to be seen in the natural history museum at south kensington. the evidence then began to pour in from other quarters; portions of stone from italy and from benares were found to be of identical composition with the yorkshire stone. the incredulity of those who had doubted the celestial origin of these objects began to give way. a careful memoir on the benares meteorite, by howard, was published in the "philosophical transactions" for , while, as if to complete the demonstration, a great shower of stones took place in the following year at l'aigle, in normandy. the french academy deputed the physicist biot to visit the locality and make a detailed examination of the circumstances attending this memorable shower. his enquiry removed every trace of doubt, and the meteoric stones have accordingly been transferred from the dominions of geology to those of astronomy. it may be noted that the recognition of the celestial origin of meteorites happens to be simultaneous with the discovery of the first of the minor planets. in each case our knowledge of the solar system has been extended by the addition of numerous minute bodies, which, notwithstanding their insignificant dimensions, are pregnant with information. when the possibility of stone-falls has been admitted, we can turn to the ancient records, and assign to them the credit they merit, which was withheld for so many centuries. perhaps the earliest of all these stone-falls which can be said to have much pretension to historical accuracy is that of the shower which livy describes as having fallen, about the year b.c., on the alban mount, near rome. among the more modern instances, we may mention one which was authenticated in a very emphatic manner. it occurred in the year at ensisheim, in alsace. the emperor maximilian ordered a minute narrative of the circumstances to be drawn up and deposited with the stone in the church. the stone was suspended in the church for three centuries, until in the french revolution it was carried off to colmar, and pieces were broken from it, one of which is now in our national collection. fortunately, this interesting object has been restored to its ancient position in the church at ensisheim, where it remains an attraction to sight-seers at this day. the account is as follows:--"in the year of the lord , on the wednesday before st. martin's day, november th, a singular miracle occurred, for between eleven o'clock and noon there was a loud clap of thunder and a prolonged confused noise, which was heard at a great distance, and a stone fell from the air in the jurisdiction of ensisheim which weighed pounds, and the confused noise was at other places much louder than here. then a boy saw it strike on ploughed ground in the upper field towards the rhine and the ill, near the district of gisgang, which was sown with wheat, and it did no harm, except that it made a hole there; and then they conveyed it from the spot, and many pieces were broken from it, which the land vogt forbade. they therefore caused it to be placed in the church, with the intention of suspending it as a miracle, and there came here many people to see this stone, so there were many remarkable conversations about this stone; the learned said they knew not what it was, for it was beyond the ordinary course of nature that such a large stone should smite from the height of the air, but that it was really a miracle from god, for before that time never was anything heard like it, nor seen, nor written. when they found that stone, it had entered into the earth to half the depth of a man's stature, which everybody explained to be the will of god that it should be found, and the noise of it was heard at lucerne, at villingen, and at many other places, so loud that the people thought that the houses had been overturned; and as the king maximilian was here, the monday after st. catherine's day of the same year, his royal excellency ordered the stone which had fallen to be brought to the castle, and after having conversed a long time about it with the noblemen, he said that the people of ensisheim should take it and order it to be hung up in the church, and not to allow anybody to take anything from it. his excellency, however, took two pieces of it, of which he kept one, and sent the other to duke sigismund of austria, and there was a great deal of talk about the stone, which was suspended in the choir, where it still is, and a great many people came to see it." admitting the celestial origin of the meteorites, they surely claim our closest attention. they afford the only direct method we possess of obtaining a knowledge of the materials of bodies exterior to our planet. we can take a meteorite in our hands, we can analyse it, and find the elements of which it is composed. we shall not attempt to enter into any very detailed account of the structure of meteorites; it is rather a matter for the consideration of chemists and mineralogists than for astronomers. a few of the more obvious features will be all that we require. they will serve as a preliminary to the discussion of the probable origin of these bodies. in the natural history museum at south kensington we may examine a superb collection of meteorites. they have been brought together from all parts of the earth, and vary in size from bodies not much larger than a pin's head up to vast masses weighing many hundredweights. there are also models of celebrated meteorites, of which the originals are dispersed through various other museums. many meteorites have nothing very remarkable in their external appearance. if they were met with on the sea beach, they would be passed by without more notice than would be given to any other stone. yet, what a history a meteorite might tell us if we could only manage to obtain it! it fell; it was seen to fall from the sky; but what was its course anterior to that movement? where was it years ago, , years ago? through what regions of space has it wandered? why did it never fall before? why has it actually now fallen? such are some of the questions which crowd upon us as we ponder over these most interesting bodies. some of these objects are composed of very characteristic materials; take, for example, one of the more recent arrivals, known as the rowton siderite. this body differs very much from the more ordinary kind of stony meteorite. it is an object which even a casual passer-by would hardly pass without notice. its great weight would also attract attention, while if it be scratched or rubbed with a file, it would appear to be a mass of nearly pure iron. we know the circumstances in which that piece of iron fell to the earth. it was on the th of april, , about . p.m., that a strange rumbling noise, followed by a startling explosion, was heard over an area of several miles in extent among the villages in shropshire, eight or ten miles north of the wrekin. about an hour after this occurrence a farmer noticed that the ground in one of his grass-fields had been disturbed, and he probed the hole which the meteorite had made, and found it, still warm, about eighteen inches below the surface. some men working at no great distance had heard the noise made in its descent. this remarkable object, weighs - / lbs. it is an irregular angular mass of iron, though all its edges seem to have been rounded by fusion in its transit through the air. it is covered with a thick black pellicle of the magnetic oxide of iron, except at the point where it first struck the ground. the duke of cleveland, on whose property it fell, afterwards presented it to our national institution already referred to, where, as the rowton siderite, it attracts the attention of everyone who is interested in these wonderful bodies. this siderite is specially interesting on account of its distinctly metallic character. falls of objects of this particular type are not so frequent as are those of the stony meteorites; in fact, there are only a few known instances of meteoric irons having been actually seen to fall, while the observed falls of stony meteorites are to be counted in scores or in hundreds. the inference is that the iron meteorites are much less frequent than the stony ones. this is, however, not the impression that the visitor to the museum would be likely to receive. in that extensive collection the meteoric irons are by far the most striking objects. the explanation is not difficult. those gigantic masses of iron are unquestionably meteoric: no one doubts that this is the case. yet the vast majority of them have never been seen to fall; they have simply been found, in circumstances which point unmistakably to their meteoric nature. suppose, for instance, that a traveller on one of the plains of siberia or of central america finds a mass of metallic iron lying on the surface of the ground, what explanation can be rendered of such an occurrence? no one has brought the iron there, and there is no iron within hundreds of miles. man never fashioned that object, and the iron is found to be alloyed with nickel in a manner that is always observed in known meteorites, and is generally regarded as a sure indication of a meteoric origin. observe also, that as iron perishes by corrosion in our atmosphere, that great mass of iron cannot have lain where it is for indefinite ages; it must have been placed there at some finite time. only one source for such an object is conceivable; it must have fallen from the sky. on the same plains the stony meteorites have also fallen in hundreds and in thousands, but they crumble away in the course of time, and in any case would not arrest the attention of the traveller as the irons are likely to do. hence it follows, that although the stony meteorites seem to fall much more frequently, yet, unless they are actually observed at the moment of descent, they are much more liable to be overlooked than the meteoric irons. hence it is that the more prominent objects of the british collection are the meteoric irons. we have said that a noise accompanied the descent of the rowton siderite, and it is on record that a loud explosion took place when the meteorite fell at ensisheim. in this we have a characteristic feature of the phenomenon. nearly all the descents of meteorites that have been observed seem to have been ushered in by a detonation. we do not, however, assert that this is quite an invariable feature; and it is also the case that meteors often detonate without throwing down any solid fragments that have been collected. the violence associated with the phenomenon is forcibly illustrated by the butsura meteorite. this object fell in india in . a loud explosion was heard, several fragments of stone were collected from distances three or four miles apart; and when brought together, they were found to fit, so as to enable the primitive form of the meteorite to be reconstructed. a few of the pieces are wanting (they were, no doubt, lost by falling unobserved into localities from which they could not be recovered), but we have obtained pieces quite numerous enough to permit us to form a good idea of the irregular shape of the object before the explosion occurred which shattered it into fragments. this is one of the ordinary stony meteorites, and is thus contrasted with the rowton siderite which we have just been considering. there are also other types of meteorites. the breitenbach iron, as it is called, is a good representative of a class of these bodies which lie intermediate between the meteoric irons and the stones. it consists of a coarsely cellular mass of iron, the cavities being filled with mineral substances. in the museum, sections of intermediate forms are shown in which this structure is exhibited. look first at the most obvious characteristic of these meteorites. we do not now allude to their chemical composition, but to their external appearance. what is the most remarkable feature in the shape of these objects?--surely it is that they are fragments. they are evidently pieces that are _broken_ from some larger object. this is apparent by merely looking at their form; it is still more manifest when we examine their mechanical structure. it is often found that meteorites are themselves composed of smaller fragments. such a structure may be illustrated by a section of an aërolite found on the sierra of chaco, weighing about lbs. (fig. ). the section here represented shows the composite structure of this object, which belongs to the class of stony meteorites. its shape shows that it was really a fragment with angular edges and corners. no doubt it may have been much more considerable when it first dashed into the atmosphere. the angular edges now seen on the exterior may be due to an explosion which then occurred; but this will not account for the structure of the interior. we there see irregular pieces of varied form and material agglomerated into a single mass. if we would seek for analogous objects on the earth, we must look to some of the volcanic rocks, where we have multitudes of irregular angular fragments cemented together by a matrix in which they are imbedded. the evidence presented by this meteorite is conclusive as to one circumstance with regard to the origin of these objects. they must have come as fragments, from some body of considerable, if not of vast, dimensions. in this meteorite there are numerous small grains of iron mingled with mineral substances. the iron in many meteorites has, indeed, characters resembling those produced by the actual blasting of iron by dynamite. thus, a large meteoric iron from brazil has been found to have been actually shivered into fragments at some time anterior to its fall on the earth. these fragments have been cemented together again by irregular veins of mineral substances. [illustration: fig. .--section of the chaco meteorite.] for an aërolite of a very different type we may refer to the carbonaceous meteorite of orgueil, which fell in france on the th may, . on the occasion of its descent a splendid meteor was seen, rivalling the full moon in size. the actual diameter of this globe of fire must have been some hundreds of yards. nearly a hundred fragments of the body were found scattered over a tract of country fifteen miles long. this object is of particular interest, inasmuch as it belongs to a rare group of aërolites, from which metallic iron is absent. it contains many of the same minerals which are met with in other meteorites, but in these fragments they are _associated with carbon_, and with substances of a white or yellowish crystallisable material, soluble in ether, and resembling some of the hydrocarbons. such a substance, if it had not been seen falling to the earth, would probably be deemed a product resulting from animal or vegetable life! we have pointed out how a body moving with great velocity and impinging upon the air may become red-hot and white-hot, or even be driven off into vapour. how, then, does it happen that meteorites escape this fiery ordeal, and fall down to the earth, with a great velocity, no doubt, but still, with very much less than that which would have sufficed to drive them off into vapour? had the rowton siderite, for instance, struck our atmosphere with a velocity of twenty miles a second, it seems unquestionable that it would have been dissipated by heat, though, no doubt, the particles would ultimately coalesce so as to descend slowly to the earth in microscopic beads of iron. how has the meteorite escaped this fate? it must be remembered that our earth is also moving with a velocity of about eighteen miles per second, and that the _relative_ velocity with which the meteorite plunges into the air is that which will determine the degree to which friction is operating. if the meteorite come into direct collision with the earth, the velocity of the collision will be extremely great; but it may happen that though the actual velocities of the two bodies are both enormous, yet the relative velocity may be comparatively small. this is, at all events, one conceivable explanation of the arrival of a meteorite on the surface of the earth. we have shown in the earlier parts of the chapter that the well-known star showers are intimately connected with comets. in fact, each star shower revolves in the path pursued by a comet, and the shooting star particles have, in all probability, been themselves derived from the comet. showers of shooting stars have, therefore, an intimate connection with comets, but it is doubtful whether meteorites have any connection with comets. it has already been remarked that meteorites have never been known to fall in the great star showers. no particle of a meteorite is known to have dropped from the countless host of the leonids or of the perseids; as far as we know, the lyrids never dropped a meteorite, nor did the quadrantids, the geminids, or the many other showers with which every astronomer is familiar. there is no reason to connect meteorites with these showers, and it is, therefore, doubtful whether we should connect meteorites with comets. with reference to the origin of meteorites it is difficult to speak with any great degree of confidence. every theory of meteorites presents difficulties, so it seems that the only course open to us is to choose that view of their origin which seems least improbable. it appears to me that this condition is fulfilled in the theory entertained by the austrian mineralogist, tschermak. he has made a study of the meteorites in the rich collection at vienna, and he has come to the conclusion that the "meteorites have had a volcanic source on some celestial body." let us attempt to pursue this reasoning and discuss the problem, which may be thus stated:--assuming that at least some of the meteorites have been ejected from volcanoes, on what body or bodies in the universe must these volcanoes be situated? this is really a question for astronomers and mathematicians. once the mineralogists assure us that these bodies are volcanic, the question becomes one of calculation and of the balance of probabilities. the first step in the enquiry is to realise distinctly the dynamical conditions of the problem. conceive a volcano to be located on a planet. the volcano is supposed to be in a state of eruption, and in one of its mighty throes projects a missile aloft: this missile will ascend, it will stop, and fall down again. such is the case at present in the eruptions of terrestrial volcanoes. cotopaxi has been known to hurl prodigious stones to a vast height, but these stones assuredly return to earth. the gravitation of the earth has gradually overcome the velocity produced by the explosion, and down the body falls. but let us suppose that the eruption is still more violent, and that the stones are projected from the planet to a still greater height above its surface. suppose, for instance, that the stone should be shot up to a height equal to the planet's radius, the attraction of gravitation will then be reduced to one-fourth of what it was at the surface, and hence the planet will find greater difficulty in pulling back the stone. not only is the distance through which the stone has to be pulled back increased as the height increases, but the efficiency of gravitation is weakened, so that in a twofold way the difficulty of recalling the stone is increased. we have already more than once alluded to this subject, and we have shown that there is a certain critical velocity appropriate to each planet, and depending on its mass and its radius. if the missile be projected upwards with a velocity equal to or greater than this, then it will ascend never to return. we all recollect jules verne's voyage to the moon, in which he described the columbiad, an imaginary cannon, capable of shooting out a projectile with a velocity of six or seven miles a second. this is the critical velocity for the earth. if we could imagine the air removed, then a cannon of seven-mile power would project a body upwards which would never fall down. the great difficulty about tschermak's view of the volcanic origin of the meteorites lies in the tremendous initial velocity which is required. the columbiad is a myth, and we know no agent, natural or artificial, at the present time on the earth, adequate to the production of a velocity so appalling. the thunders of krakatoa were heard thousands of miles away, but in its mightiest throes it discharged no missiles with a velocity of six miles a second. we are therefore led to enquire whether any of the other celestial bodies are entitled to the parentage of the meteorites. we cannot see volcanoes on any other body except the moon; all the other bodies are too remote for an inspection so minute. does it seem likely that volcanoes on the moon can ever launch forth missiles which fall upon the earth? this belief was once sustained by eminent authority. the mass of the moon is about one-eightieth of the mass of the earth. it would not be true to assert that the critical velocity of projection varies directly as the mass of the planet. the correct law is, that it varies directly as the square root of the mass, and inversely as the square root of the radius. it is hence shown that the velocity required to project a missile away from the moon is only about one-sixth of that which would be required to project a missile away from the earth. if the moon had on its surface volcanoes of one-mile power, it is quite conceivable that these might be the source of meteorites. we have seen how the whole surface of the moon shows traces of intense volcanic activity. a missile thus projected from the moon could undoubtedly fall on the earth, and it is not impossible that some of the meteorites may really have come from this source. there is, however, one great difficulty about the volcanoes on the moon. suppose an object were so projected, it would, under the attraction of the earth, in accordance with kepler's laws, move around the earth as a focus. if we set aside the disturbances produced by all other bodies, as well as the disturbance produced by the moon itself, we see that the meteorite if it once misses the earth can never fall thereon. it would be necessary that the shortest distance of the earth's centre from the orbit of the projectile should be less than the radius of the earth, so that if a lunar meteorite is to fall on the earth, it must do so the first time it goes round. the journey of a meteorite from the moon to the earth is only a matter of days, and therefore, as meteorites are still falling, it would follow that they must still be constantly ejected from the moon. the volcanoes on the moon are, however, not now active; observers have long studied its surface, and they find no reliable traces of volcanic activity at the present day. it is utterly out of the question, whatever the moon may once have been able to do, that at the present date she could still continue to launch forth meteorites. it is just possible that a meteorite expelled from the moon in remote antiquity, when its volcanoes were active, may, under the influence of the disturbances of the other bodies of the system, have its orbit so altered, that at length it comes within reach of the atmosphere and falls to the earth, but in no circumstances could the moon send us a meteorite at present. it is therefore reasonable to look elsewhere in our search for volcanoes fulfilling the conditions of the problem. let us now direct our attention to the planets, and examine the circumstances in which volcanoes located thereon could eject a meteorite which should ultimately tumble on the earth. we cannot see the planets well enough to tell whether they have or ever had any volcanoes; but the almost universal presence of heat in the large celestial masses seems to leave us in little doubt that some form of volcanic action might be found in the planets. we may at once dismiss the giant planets, such as jupiter or saturn: their appearance is very unlike a volcanic surface; while their great mass would render it necessary to suppose that the meteorites were expelled with terrific velocity if they should succeed in escaping from the gravitation of the planet. applying the rule already given, a volcano on jupiter would have to be five or six times as powerful as the volcano on the earth. to avoid this difficulty, we naturally turn to the smaller planets of the system; take, for instance, one of that innumerable host of minor planets, and let us enquire how far this body is likely to have ejected a missile which should fall upon the earth. some of these globes are only a few miles in diameter. there are bodies in the solar system so small that a very moderate velocity would be sufficient to project a missile away from them altogether. we have, indeed, already illustrated this point in discussing the minor planets. it has been suggested that a volcano placed on one of the minor planets might be quite powerful enough to start the meteorites on a long ramble through space until the chapter of accidents brought them into collision with the earth. there is but little difficulty in granting that there might be such volcanoes, and that they might be sufficiently powerful to drive bodies from the surface of the planet; but we must remember that the missiles are to fall on the earth, and dynamical considerations are involved which merit our close attention. to concentrate our ideas, we shall consider one of the minor planets, and for this purpose let us take ceres. if a meteorite is to fall upon the earth, it must pass through the narrow ring, some , miles wide, which marks the earth's path; it will not suffice for the missile to pass through the ecliptic on the inside or on the outside of the ring, it must be actually through this narrow strip, and then if the earth happens to be there at the same moment the meteorite will fall. the first condition to be secured is, therefore, that the path of the meteorite shall traverse this narrow ring. this is to be effected by projection from some point in the orbit of ceres. but it can be shown on purely dynamical grounds that although the volcanic energy sufficient to remove the projectile from ceres may be of no great account, yet if that projectile is to cross the earth's track, the dynamical requirements of the case demand a volcano on ceres at the very least of three-mile power. we have thus gained but little by the suggestion of a minor planet, for we have not found that a moderate volcanic power would be adequate. but there is another difficulty in the case of ceres, inasmuch as the ring on the ecliptic is very narrow in comparison with the other dimensions of the problem. ceres is a long way off, and it would require very great accuracy in volcanic practice on ceres to project a missile so that it should just traverse this ring and fall neither inside nor outside, neither above nor below. there must be a great many misses for every hit. we have attempted to make the calculation by the aid of the theory of probabilities, and we find that the chances against this occurrence are about , to , so that out of every , projectiles hurled from a point in the orbit of ceres only a single one can be expected to satisfy even the first of the conditions necessary if it is ever to tumble on our globe. it is thus evident that there are two objections to ceres (and the same may be said of the other minor planets) as a possible source of the meteorites. firstly, that notwithstanding the small mass of the planet a very powerful volcano would still be required; and secondly, that we are obliged to assume that for every one which ever reached the earth at least , must have been ejected. it is thus plain that if the meteorites have really been driven from some planet of the solar system, large or small, the volcano must, from one cause or another, have been a very powerful one. we are thus led to enquire which planet possesses on other grounds the greatest probability in its favour. we admit of course that at the present time the volcanoes on the earth are utterly devoid of the necessary power; but were the terrestrial volcanoes always so feeble as they are in these later days? grounds are not wanting for the belief that in the very early days of geological time the volcanic energy on the earth was much greater than at present. we admit fully the difficulties of the view that the meteorites have really come from the earth; but they must have some origin, and it is reasonable to indicate the source which seems to have most probability in its favour. grant for a moment that in the primæval days of volcanic activity there were some mighty throes which hurled forth missiles with the adequate velocity: these missiles would ascend, they would pass from the gravitation of the earth, they would be seized by the gravitation of the sun, and they would be compelled to revolve around the sun for ever after. no doubt the resistance of the air would be a very great difficulty, but this resistance would be greatly lessened were the crater at a very high elevation above the sea level, while, if a vast volume of ejected gases or vapours accompanied the more solid material, the effect of the resistance of the air would be still further reduced. some of these objects might perhaps revolve in hyperbolic orbits, and retreat never to return; while others would be driven into elliptic paths. round the sun these objects would revolve for ages, but at each revolution--and here is the important point--they would traverse the point from which they were originally launched. in other words, every object so projected from the earth would at each revolution cross the track of the earth. we have in this fact an enormous probability in favour of the earth as contrasted with ceres. only one ceres-ejected meteorite out of every , would probably cross the earth's track, while every earth-projected meteorite would necessarily do so. if this view be true, then there must be hosts of meteorites traversing space in elliptic orbits around the sun. these orbits have one feature in common: they all intersect the track of the earth. it will sometimes happen that the earth is found at this point at the moment the meteorite is crossing; when this is the case the long travels of the little body are at an end, and it tumbles back on the earth from which it parted so many ages ago. it is well to emphasise the contrast between the lunar theory of meteorites (which we think improbable) and the terrestrial theory (which appears to be probable). for the lunar theory it would, as we have seen, be necessary that some of the lunar volcanoes should be still active. in the terrestrial theory it is only necessary to suppose that the volcanoes on the earth once possessed sufficient explosive power. no one supposes that the volcanoes at present on the earth eject now the fragments which are to form future meteorites; but it seems possible that the earth may be now slowly gathering back, in these quiet times, the fragments she ejected in an early stage of her history. assuming, therefore, with tschermak, that many meteorites have had a volcanic origin on some considerable celestial body, we are led to agree with those who think that most probably that body is the earth. it is interesting to notice a few circumstances which seem to corroborate the view that many meteorites are of ancient terrestrial origin. the most characteristic constituent of these bodies is the alloy of iron and nickel, which is almost universally present. sometimes, as in the rowton siderite, the whole object consists of little else; sometimes this alloy is in grains distributed through the mass. when nordenskjöld discovered in greenland a mass of native iron containing nickel, this was at once regarded as a celestial visitor. it was called the ovifak meteorite, and large pieces of the iron were conveyed to our museums. there is, for instance, in the national collection a most interesting exhibit of the ovifak substance. close examination shows that this so-called meteorite lies in a bed of basalt which has been vomited from the interior of the earth. those who believe in the meteoric origin of the ovifak iron are constrained to admit that shortly after the eruption of the basalt, and while it was still soft, this stupendous iron meteorite of gigantic mass and bulk happened to fall into this particular soft bed. the view is, however, steadily gaining ground that this great iron mass was no celestial visitor at all, but that it simply came forth from the interior of the earth with the basalt itself. the beautiful specimens in the british museum show how the iron graduates into the basalt in such a way as to make it highly probable that the source of the iron is really to be sought in the earth and not external thereto. should further research establish this, as now seems probable, a most important step will have been taken in proving the terrestrial origin of meteorites. if the ovifak iron be really associated with the basalt, we have a proof that the iron-nickel alloy is indeed a terrestrial substance, found deep in the interior of the earth, and associated with volcanic phenomena. this being so, it will be no longer difficult to account for the iron in undoubted meteorites. when the vast volcanoes were in activity they ejected masses of this iron-alloy, which, having circulated round the sun for ages, have at last come back again. as if to confirm this view, professor andrews discovered particles of native iron in the basalt of the giant's causeway, while the probability that large masses of iron are there associated with the basaltic formation was proved by the researches on magnetism of the late provost lloyd. besides the more solid meteorites there can be no doubt that the _débris_ of the ordinary shooting stars must rain down upon the earth in gentle showers of celestial dust. the snow in the arctic regions has often been found stained with traces of dust which contains particles of iron. similar particles have been found on the towers of cathedrals and in many other situations where it could only have been deposited from the air. there can be hardly a doubt that some of the motes in the sunbeam, and many of the particles which good housekeepers abhor as dust, have indeed a cosmical origin. in the famous cruise of the _challenger_ the dredges brought up from the depths of the atlantic no "wedges of gold, great anchors, heaps of pearl," but among the mud which they raised are to be found numerous magnetic particles which there is every reason to believe fell from the sky, and thence subsided to the depths of the ocean. sand from the deserts of africa, when examined under the microscope, yield traces of minute iron particles which bear the marks of having experienced a high temperature. the earth draws in this cosmic dust continuously, but the earth now never parts with a particle of its mass. the consequence is inevitable; the mass of the earth must be growing, and though the change may be a small one, yet to those who have studied darwin's treatise on "earth-worms," or to those who are acquainted with the modern theory of evolution, it will be manifest that stupendous results can be achieved by slight causes which tend in one direction. it is quite probable that an appreciable part of the solid substance of our globe may have been derived from meteoric matter which descends in perennial showers upon its surface. chapter xviii. the starry heavens. the constellations--the great bear and the pointers--the pole star--cassiopeia--andromeda, pegasus, and perseus--the pleiades: auriga, capella, aldebaran--taurus, orion, sirius; castor and pollux--the lion--boötes, corona, and hercules--virgo and spica--vega and lyra--the swan. the student of astronomy should make himself acquainted with the principal constellations in the heavens. this is a pleasing acquirement, and might well form a part of the education of every child in the kingdom. we shall commence our discussion of the sidereal system with a brief account of the principal constellations visible in the northern hemisphere, and we accompany our description with such outline maps of the stars as will enable the beginner to identify the chief features of the starry heavens. in an earlier chapter we directed the attention of the student to the remarkable constellation of stars which is known to astronomers as ursa major, or the great bear. it forms the most conspicuous group in the northern skies, and in northern latitudes it never sets. at eleven p.m. in the month of april the great bear is directly overhead (for an observer in the united kingdom); at the same hour in september it is low down in the north; at the same hour july it is in the west; by christmas it is at the east. from the remotest antiquity this group of stars has attracted attention. the stars in the great bear were comprised in a great catalogue of stars, made two thousand years ago, which has been handed down to us. from the positions of the stars given in this catalogue it is possible to reconstruct the great bear as it appeared in those early days. this has been done, and it appears that the seven principal stars have not changed in this lapse of time to any large extent, so that the configuration of the great bear remains practically the same now as it was then. the beginner must first obtain an acquaintance with this group of seven stars, and then his further progress in this branch of astronomy will be greatly facilitated. the great bear is, indeed, a splendid constellation, and its only rival is to be found in orion, which contains more brilliant stars, though it does not occupy so large a region in the heavens. [illustration: fig. .--the great bear and pole star.] [illustration: fig. .--the great bear and cassiopeia.] in the first place, we observe how the great bear enables the pole star, which is the most important object in the northern heavens, to be readily found. the pole star is very conveniently indicated by the direction of the two stars, b and a, of the great bear, which are, accordingly, generally known as the "pointers." this use of the great bear is shown on the diagram in fig. , in which the line b a, produced onwards and slightly curved, will conduct to the pole star. there is no likelihood of making any mistake in this star, as it is the only bright one in the neighbourhood. once it has been seen it will be readily identified on future occasions, and the observer will not fail to notice how constant is the position which it preserves in the heavens. the other stars either rise or set, or, like the great bear, they dip down low in the north without actually setting, but the pole star exhibits no considerable changes. in summer or winter, by night or by day, the pole star is ever found in the same place--at least, so far as ordinary observation is concerned. no doubt, when we use the accurate instruments of the observatory the notion of the fixity of the pole star is abandoned; we then see that it has a slow motion, and that it describes a small circle every twenty-four hours around the true pole of the heavens, which is not coincident with the pole star, though closely adjacent thereto. the distance is at present a little more than a degree, and it is gradually lessening, until, in the year a.d. , the distance will be under half a degree. the pole star itself belongs to another inconsiderable group of stars known as the little bear. the two principal members of this group, next in brightness to the pole star, are sometimes called the "guards." the great bear and the little bear, with the pole star, form a group in the northern sky not paralleled by any similarly situated constellation in the southern heavens. at the south pole there is no conspicuous star to indicate its position approximately--a circumstance disadvantageous to astronomers and navigators in the southern hemisphere. it will now be easy to add a third constellation to the two already acquired. on the opposite side of the pole star to the great bear, and at about the same distance, lies a very pleasing group of five bright stars, forming a w. these are the more conspicuous members of the constellation cassiopeia, which contains altogether about sixty stars visible to the naked eye. when the great bear is low down in the north, then cassiopeia is high overhead. when the great bear is high overhead, then cassiopeia is to be looked for low down in the north. the configuration of the leading stars is so striking that once the eye has recognised them future identification will be very easy--the more so when it is borne in mind that the pole star lies midway between cassiopeia and the great bear (fig. ). these important constellations will serve as guides to the rest. we shall accordingly show how the learner may distinguish the various other groups visible from the british islands or similar northern latitudes. the next constellation to be recognised is the imposing group which contains the great square of pegasus. this is not, like ursa major, or like cassiopeia, said to be "circumpolar." the great square of pegasus sets and rises daily. it cannot be seen conveniently during the spring and the summer, but in autumn and in winter the four stars which mark the corners of the square can be easily recognised. there are certain small stars within the region so limited; perhaps about thirty can be counted by an unaided eye of ordinary power in these latitudes. in the south of europe, with its pure and bright skies, the number of visible stars appears to be greatly increased. an acute observer at athens has counted in the same region. [illustration: fig. .--the great square of pegasus.] the great square of pegasus can be reached by a line from the pole star over the end of cassiopeia. if it be produced about as far again it will conduct the eye to the centre of the great square of pegasus (fig. ). the line through b and a in pegasus continued ° to the south points out the important star fomalhaut in the mouth of the southern fish. to the right of this line, nearly half-way down, is the rather vague constellation of aquarius, where a small equilateral triangle with a star in the centre may be noticed. the square of pegasus is not a felicitous illustration of the way in which the boundaries of the constellations should be defined. there can be no more naturally associated group than the four stars of this square, and they ought surely to be included in the same constellation. three of the stars--marked a, b, g--do belong to pegasus; but that at the fourth corner--also marked a--is placed in a different figure, known as andromeda, whereof it is, indeed, the brightest member. the remaining bright stars of andromeda are marked b and g, and they are readily identified by producing one side of the square of pegasus in a curved direction. we have thus a remarkable array of seven stars, which it is both easy to identify and easy to remember, notwithstanding that they are contributed to by three different constellations. they are respectively a, b, and g of pegasus; a, b, and g of andromeda; and a of perseus. the three form a sort of handle, as it were, extending from one side of the square, and are a group both striking in appearance, and useful in the further identification of celestial objects. b andromedæ, with two smaller stars, form the girdle of the unfortunate heroine. a persei lies between two other stars (g and d) of the same constellation. if we draw a curve through these three and prolong it in a bold sweep, we are conducted to one of the gems of the northern heavens--the beautiful star capella, in auriga (fig. ). close to capella are three small stars forming an isosceles triangle--these are the hoedi or kids. capella and vega are, with the exception of arcturus, the two most brilliant stars in the northern heavens; and though vega is probably the more lustrous of the two, yet the opposite opinion has been entertained. different eyes will frequently form various estimates of the relative brilliancy of stars which approach each other in brightness. the difficulty of making a satisfactory comparison between vega and capella is greatly increased by the wide distance in the heavens at which they are separated, as well as by a slight difference in colour, for vega is distinctly whiter than capella. this contrast between the colour of stars is often a source of uncertainty in the attempt to compare their relative brilliancy; so that when actual measurements have to be effected by instrumental means, it is necessary to compare the two stars alternately with some object of intermediate hue. [illustration: fig. .--perseus and its neighbouring stars.] on the opposite side of the pole to capella, but not quite so far away, will be found four small stars in a quadrilateral. they form the head of the dragon, the rest of whose form coils right round the pole. if we continue the curve formed by the three stars g, a, and d in perseus, and if we bend round this curve gracefully into one of an opposite flexion, in the manner shown in fig. , we are first conducted to two other principal stars in perseus, marked e and z. the region of perseus is one of the richest in the heavens. we have here a most splendid portion of the milky way, and the field of the telescope is crowded with stars beyond number. even a small telescope or an opera-glass directed to this teeming constellation cannot fail to delight the observer, and convey to him a profound impression of the extent of the sidereal heavens. we shall give in a subsequent paragraph a brief enumeration of some of the remarkable telescopic objects in perseus. pursuing in the same figure the line e and z, we are conducted to the remarkable little group known as the pleiades. [illustration: fig. .--the pleiades.] the pleiades form a group so universally known and so easily identified that it hardly seems necessary to give any further specific instructions for their discovery. it may, however, be observed that in these latitudes they cannot be seen before midnight during the summer. let us suppose that the search is made at about p.m. at night: on the st of january the pleiades will be found high up in the sky in the south-west; on the st of march, at the same hour, they will be seen to be setting in the west. on the st of may they are not visible; on the st of july they are not visible; on the st of september they will be seen low down in the east. on the st of november they will be high in the heavens in the south-east. on the ensuing st of january the pleiades will be in the same position as they were on the same date in the previous year, and so on from year to year. it need, perhaps, hardly be explained here that these changes are not really due to movements of the constellations; they are due, of course, to the apparent annual motion of the sun among the stars. [illustration: fig. .--orion, sirius, and the neighbouring stars.] the pleiades are shown in the figure (fig. ), where a group of ten stars is represented, this being about the number visible with the unaided eye to those who are gifted with very acute vision. the lowest telescopic power will increase the number of stars to thirty or forty (galileo saw more than forty with his first telescope), while with telescopes of greater power the number is largely increased; indeed, no fewer than have been counted with the aid of a powerful telescope. the group is, however, rather too widely scattered to make an effective telescopic object, except with a large field and low power. viewed through an opera-glass it forms a very pleasing spectacle. [illustration: fig. .--castor and pollux.] if we draw a ray from the pole star to capella, and produce it sufficiently far, as shown in fig. , we come to the great constellation of our winter sky, the splendid group of orion. the brilliancy of the stars in orion, the conspicuous belt, and the telescopic objects which it contains, alike render this group remarkable, and place it perhaps at the head of the constellations. the leading star in orion is known either as a orionis, or as betelgeuze, by which name it is here designated. it lies above the three stars, d, e, z, which form the belt. betelgeuze is a star of the first magnitude, and so also is rigel, on the opposite side of the belt. orion thus enjoys the distinction of containing two stars of the first magnitude in its group, while the five other stars shown in fig. are of the second magnitude. the neighbourhood of orion contains some important stars. if we carry on the line of the belt upwards to the right, we are conducted to another star of the first magnitude, aldebaran, which strongly resembles betelgeuze in its ruddy colour. aldebaran is the brightest star in the constellation of taurus. it is this constellation which contains the pleiades already referred to, and another more scattered group known as the hyades, which can be discovered near aldebaran. [illustration: fig. .--the great bear and the lion.] the line of the belt of orion continued downwards to the left conducts the eye to the gem of the sky, the splendid sirius, which is the most brilliant star in the heavens. it has, indeed, been necessary to create a special order of magnitude for the reception of sirius alone; all the other first magnitude stars, such as vega and capella, betelgeuze and aldebaran, coming a long way behind. sirius, with a few other stars of much less lustre, form the constellation of canis major. it is useful for the learner to note the large configuration, of an irregular lozenge shape, of which the four corners are the first magnitude stars, aldebaran, betelgeuze, sirius, and rigel (fig. ). the belt of orion is placed symmetrically in the centre of the group, and the whole figure is so striking that once perceived it is not likely to be forgotten. about half way from the square of pegasus to aldebaran is the chief star in the ram--a bright orb of the second magnitude; with two others it forms a curve, at the other end of which will be found g of the same constellation, which was the first double star ever noticed. we can again invoke the aid of the great bear to point out the stars in the constellation of gemini (fig. ). if the diagonal joining the stars d and b of the body of the bear be produced in the direction opposite to the tail, it will lead to castor and pollux, two remarkable stars of the second magnitude. this same line carried a little further on passes near the star procyon, of the first magnitude, which is the only conspicuous object in the constellation of the little dog. [illustration: fig. .--boötes and the crown.] [illustration: fig. .--virgo and the neighbouring constellations.] the pointers in the great bear marked a b will also serve to indicate the constellation of the lion. if we produce the line joining them in the direction opposite from that used in finding the pole, we are brought into the body of the lion. this group will be recognised by the star of the first magnitude called regulus. it is one of a series of stars forming an object somewhat resembling a sickle: three of the group are of the second magnitude. the sickle has a special claim on our notice because it contains the radiant point from which the periodic shooting star shower known as the leonids diverges. regulus lies alongside the sun's highway through the stars, at a point which he passes on the st of august every year. between gemini and leo the inconspicuous constellation of the crab may be found; the most striking object it contains is the misty patch called præsepe or the bee-hive, which the smallest opera-glass will resolve into its component stars. [illustration: fig. .--the constellation of lyra.] the tail of the great bear, when prolonged with a continuation of the curve which it possesses, leads to a brilliant star of the first magnitude known as arcturus, the principal star in the constellation of boötes (fig. ). a few other stars, marked b, g, d, and e in the same constellation, are also shown in the figure. among the stars visible in these latitudes arcturus is to be placed next to sirius in point of brightness. two stars in the southern hemisphere, invisible in these latitudes, termed a centauri and canopus, are nearly as bright as vega and capella, but not quite as bright as arcturus. in the immediate neighbourhood of boötes is a striking semicircular group known as the crown or corona borealis. it will be readily found from its position as indicated in the figure, or it may be identified by following the curved line indicated by b, d, e, and z in the great bear. [illustration: fig. .--vega, the swan, and the eagle.] the constellation of virgo is principally characterised by the first magnitude star called spica, or a virginis. this may be found from the great bear; for if the line joining the two stars a and g in that constellation be prolonged with a slight curve, it will conduct the eye to spica. we may here notice another of those large configurations which are of great assistance in the study of the stars. there is a fine equilateral triangle, whereof arcturus and spica form two of the corners, while the third is indicated by denebola, the bright star near the tail of the lion (fig. ). in the summer evenings when the crown is overhead, a line from the pole star through its fainter edge, continued nearly to the southern horizon, encounters the brilliant red star cor scorpionis, or the scorpion's heart (antares), which was the first star mentioned as having been seen with the telescope in the daytime. the first magnitude star, vega, in the constellation of the lyre, can be readily found at the corner of a bold triangle, of which the pole star and arcturus form the base (fig. ). the brilliant whiteness of vega will arrest the attention, while the small group of neighbouring stars which form the lyre produces one of the best defined constellations. near vega is another important constellation, known as the swan or cygnus. the brightest star will be identified as the vertex of a right-angled triangle, of which the line from vega to the pole star is the base, as shown in fig. . there are in cygnus five principal stars, which form a constellation of rather remarkable form. the last constellation which we shall here describe is that of aquila or the eagle, which contains a star of the first magnitude, known as altair; this group can be readily found by a line from vega over b cygni, which passes near the line of three stars, forming the characteristic part of the eagle. we have taken the opportunity to indicate in these sketches of the constellations the positions of some other remarkable telescopic objects, the description of which we must postpone to the following chapters. chapter xix. the distant suns. sirius contrasted with the sun--stars can be weighed, but not in general measured--the companion of sirius--determination of the weights of sirius and his companion--dark stars--variable and temporary stars--enormous number of stars. the splendid pre-eminence of sirius has caused it to be observed with minute care from the earliest times in the history of astronomy. each generation of astronomers devoted time and labour to determine the exact places of the brightest stars in the heavens. a vast mass of observations as to the place of sirius among the stars had thus been accumulated, and it was found that, like many other stars, sirius had what astronomers call _proper motion_. comparing the place of sirius with regard to the other stars now with the place which it occupied one hundred years ago, there is a difference of two minutes ( ") in its situation. this is a small quantity: it is so small that the unaided eye could not see it. could we now see the sky as it appeared one century ago, we should still see this star in its well-known place to the left of orion. careful alignment by the eye would hardly detect that sirius was moving in two, or even in three or in four centuries. but the accuracy of the meridian circle renders these minute quantities evident, and gives to them their true significance. to the eye of the astronomer, sirius, instead of creeping along with a movement which centuries will not show, is pursuing its majestic course with a velocity appropriate to its dimensions. though the velocity of sirius is _about_ , miles a minute, yet it is sometimes a little more and sometimes a little less than its mean value. to the astronomer this fact is pregnant with information. were sirius an isolated star, attended only by planets of comparative insignificance, there could be no irregularity in its motion. if it were once started with a velocity of , miles a minute, then it must preserve that velocity. neither the lapse of centuries nor the mighty length of the journey could alter it. the path of sirius would be inflexible in its direction; and it would be traversed with unalterable velocity. [illustration: fig. .--the orbit of sirius (professor burnham).] the fact that sirius had not been moving uniformly was of such interest that it arrested the attention of bessel when he discovered the irregularities in . believing, as bessel did, that there must be some adequate cause for these disturbances, it was hardly possible to doubt what the cause must be. when motion is disturbed there must be force in action, and the only force that we recognise in such cases is that known as gravitation. but gravity can only act from one body to another body; so that when we seek for the derangement of sirius by gravitation, we are obliged to suppose that there must be some mighty and massive body near sirius. the question was taken up again by peters and by auwers, who were able to discover, from the irregularities of sirius, the nature of the path of the disturbing body. they were able to show that it must revolve around sirius in a period of about fifty years, and although they could not tell its distance from sirius, yet they were able to point out the direction in which it must lie. fig. shows the orbit of sirius as given by mr. burnham, of yerkes observatory. the detection of the attendant of sirius, and the measures which have been made thereon, enable us to determine the weight of this famous star. let us attempt to illustrate this subject. it must, no doubt, be admitted that the numerical estimates we employ have to be received with a certain degree of caution. the companion of sirius is a difficult object to observe, and previous to it had only been followed through an arc of °. we are, therefore, hardly as yet in a position to speak with absolute accuracy as to the periodic time in which the companion completes its revolution. we may, however, take this time to be fifty-two years. we also know the distance from sirius to his companion, and we may take it to be about twenty-one times the distance from the earth to the sun. it is useful, in the first place, to compare the revolution of the companion around sirius with the revolution of the planet uranus around the sun. taking the earth's distance as unity, the radius of the orbit of uranus is about nineteen, and uranus takes eighty-four years to accomplish a complete revolution. we have no planet in the solar system at a distance of twenty-one; but from kepler's third law it may be shown that, if there were such a planet, its periodic time would be about ninety-nine years. we have now the necessary materials for making the comparison between the mass of sirius and the mass of the sun. a body revolving around sirius at a certain distance completes its journey in fifty-two years. to revolve around the sun at the same distance a body should complete its journey in ninety-nine years. the quicker the body is moving the greater must be the centrifugal force, and the greater must be the attractive power of the central body. it can be shown from the principles of dynamics that the attractive power is inversely proportional to the square of the periodic time. hence, then, the attractive power of sirius must bear to the attractive power of the sun the proportion which the square of ninety-nine has to the square of fifty-two. as the distances are in each case supposed to be equal, the attractive powers will be proportional to the masses, and hence we conclude that the mass of sirius, together with that of his companion, is to the mass of the sun, together with that of his planet, in the ratio of three and a half to one. we had already learned that sirius was much brighter than the sun; now we have learned that it is also much more massive. before we leave the consideration of sirius, there is one additional point of very great interest which it is necessary to consider. there is a remarkable contrast between the brilliancy of sirius and his companion. sirius is a star far transcending all other stars of the first magnitude, while his companion is extremely faint. even if it were completely withdrawn from the dazzling proximity of sirius, the companion would be only a small star of the eighth or ninth magnitude, far below the limits of visibility to the unaided eye. to put the matter in numerical language, sirius is , times as bright as its companion, but only about twice as heavy! here is a very great contrast; and this point will appear even more forcible if we contrast the companion of sirius with our sun. the companion is slightly heavier than our sun; but in spite of its slightly inferior bulk, our sun is much more powerful as a light-giver. one hundred of the companions of sirius would not give as much light as our sun! this is a result of very considerable significance. it teaches us that besides the great bodies in the universe which attract attention by their brilliancy, there are also other bodies of stupendous mass which have but little brilliancy--probably some of them possess none at all. this suggests a greatly enhanced conception of the majestic scale of the universe. it also invites us to the belief that the universe which we behold bears but a small ratio to the far larger part which is invisible in the sombre shades of night. in the wide extent of the material universe we have here or there a star or a mass of gaseous matter sufficiently heated to be luminous, and thus to become visible from the earth; but our observation of these luminous points can tell us little of the remaining contents of the universe. the most celebrated of all the variable stars is that known as algol, whose position in the constellation of perseus is shown in fig. . this star is conveniently placed for observation, being visible every night in our latitude, and its interesting changes can be observed without any telescopic aid. everyone who desires to become acquainted with the great truths of astronomy should be able to recognise this star, and should have also followed it during one of its periods of change. algol is usually a star of the second magnitude; but in a period between two and three days, or, more accurately, in an interval of days hours minutes and seconds, its brilliancy goes through a most remarkable cycle of variations. the series commences with a gradual decline of the star's brightness, which in the course of four and a half hours falls from the second magnitude down to the fourth. at this lowest stage of brightness algol remains for about twenty minutes, and then begins to increase, until in three and a half hours it regains the second magnitude, at which it continues for about days hours, when the same series commences anew. it seems that the period required by algol to go through its changes is itself subject to a slow but certain variation. we shall see in a following chapter how it has been proved that the variability of algol is due to the occasional interposition of a dark companion which cuts off a part of the lustre of the star. all the circumstances can thus be accounted for, and even the weight and the size of algol and its dark companion be determined. there are, however, other classes of variable stars, the fluctuation of whose light can hardly be due to occasional obscuration by dark bodies. this is particularly the case with those variables which are generally faint, but now and then flare up for a short time, after which temporary exaltation they again sink down to their original condition. the periods of such changes are usually from six months to two years. the best known example of a star of this class was discovered more than three hundred years ago. it is situated in the constellation cetus, a little south of the equator. this object was the earliest known case of a variable star, except the so-called temporary stars, to which we shall presently refer. the variable in cetus received the name of mira, or the wonderful. the period of the fluctuations of mira ceti is about eleven months, during the greater part of which time the star is of the ninth magnitude, and consequently invisible to the naked eye. when the proper time has arrived, its brightness begins to increase rather suddenly. it soon becomes a conspicuous object of the second or third magnitude. in this condition it remains for eight or ten days, and then declines more slowly than it rose until it is reduced to its original faintness, about three hundred days after the rise commenced. more striking to the general observer than the ordinary variable stars are the _temporary stars_ which on rare occasions suddenly make their appearance in the heavens. the most famous object of this kind was that which blazed out in the beginning of november, , and which when first seen was as bright as venus at its maximum brightness. it could, indeed, be seen in full daylight by sharp-sighted people. as far as history can tell us, no other temporary star has ever been as bright as this one. it is specially associated with the name of tycho brahe, for although he was not the discoverer, he made the best observations of the object, and he proved that it was at a distance comparable with that of the ordinary fixed stars. tycho described carefully the gradual decline of the wonderful star until it disappeared from his view about the end of march, , for the telescope, by which it could doubtless have been followed further, had not yet been invented. during the decline the colour of the object gradually changed; at first it was white, and by degrees became yellow, and in the spring of reddish, like aldebaran. about may, , we are told somewhat enigmatically that it "became like lead, or somewhat like saturn," and so it remained as long as it was visible. what a fund of information our modern spectroscopes and other instruments would supply us with if so magnificent a star were to burst out in these modern days! but though we have not in our own times been favoured with a view of a temporary star as splendid as the one seen by tycho brahe and his contemporaries, it has been our privilege to witness several minor outbursts of this kind. it seems likely that we should possess more records of temporary stars from former times if a better watch had been kept for them. that is at any rate the impression we get when we see how several of the modern stars of this kind have nearly escaped us altogether, notwithstanding the great number of telescopes which are now pointed to the sky on every clear night. in a star of the second magnitude suddenly appeared in the constellation of the crown (corona borealis). it was first seen on the th may, and a few days afterwards it began to fade away. argelander's maps of the northern heavens had been published some years previously, and when the position of the new star had been accurately determined, it was found that it was identical with an insignificant looking star marked on one of the maps as of the - / magnitude. the star exists in the same spot to this day, and it is of the same magnitude as it was prior to its spasmodic outburst in . this was the first new star which was spectroscopically examined. we shall give in chapter xxiii. a short account of the features of its spectrum. the next of these temporary bright stars, nova cygni, was first seen by julius schmidt at athens on the th november, , when it was between the third and fourth magnitudes, and he maintains that it cannot have been conspicuous four days earlier, when he was looking at the same constellation. by some inadvertence the news of the discovery was not properly circulated, and the star was not observed elsewhere for about ten days, when it had already become considerably fainter. the decrease of brightness went on very slowly; in october, , the star was only of the tenth magnitude, and it continued getting fainter until it reached the fifteenth magnitude; in other words, it became a minute telescopic star, and it is so still in the very same spot. as this star did not reach the first or second magnitude it would probably have escaped notice altogether if schmidt had not happened to look at the swan on that particular evening. we are not so likely to miss seeing a new star since astronomers have pressed the photographic camera into their service. this became evident in , when the last conspicuous temporary star appeared in auriga. on the th january, dr. anderson, an astronomer in edinburgh, noticed a yellowish star of the fifth magnitude in the constellation auriga, and a week later, when he had compared a star-map with the heavens and made sure that the object was really a new star, he made his discovery public. in the case of this star we are able to fix fairly closely the moment when it first blazed out. in the course of the regular photographic survey of the heavens undertaken at the harvard college observatory (cambridge, massachusetts) the region of the sky where the new star appeared had been photographed on thirteen nights from october st to december st, , and on twelve nights from december th to january th, . on the first series of plates there was no trace of the nova, while it was visible on the very first plate of the second series as a star of the fifth magnitude. fortunately it turned out that professor max wolf of heidelberg, a most successful celestial photographer, had photographed the same region on the th december, and this photograph does not show the star, so that it cannot on that night have been as bright as the ninth magnitude. nova auriga must therefore have flared up suddenly between the th and the th of december. according to the harvard photographs, the first maximum of brightness occurred about the th of december, when the magnitude was - / . the decrease of the brightness was very irregular; the star fluctuated for the five weeks following the first of february between the fourth and the sixth magnitude, but after the beginning of march, , the brightness declined very rapidly, and at the end of april the star was seen as an exceedingly faint one (sixteenth magnitude) with the great lick refractor. when this mighty instrument was again pointed to the nova in the following august, it had risen nearly to the tenth magnitude, after which it gradually became extremely faint again, and is so still. the temporary and the variable stars form but a very small section of the vast number of stars with which the vault of the heavens is studded. that the sun is no more than a star, and the stars are no less than suns, is a cardinal doctrine of astronomy. the imposing magnificence of this truth is only realised when we attempt to estimate the countless myriads of stars. this is a problem on which our calculations are necessarily vain. let us, therefore, invoke the aid of the poet to attempt to express the innumerable, and conclude this chapter with the following lines of mr. allingham:-- "but number every grain of sand, wherever salt wave touches land; number in single drops the sea; number the leaves on every tree, number earth's living creatures, all that run, that fly, that swim, that crawl; of sands, drops, leaves, and lives, the count add up into one vast amount, and then for every separate one of all those, let a flaming sun whirl in the boundless skies, with each its massy planets, to outreach all sight, all thought: for all we see encircled with infinity, is but an island." chapter xx. double stars. interesting stellar objects--stars optically double--the great discovery of the binary stars made by herschel--the binary stars describe elliptic paths--why is this so important?--the law of gravitation--special double stars--castor--mizar--the coloured double stars--b cygni. the sidereal heavens contain few more interesting objects for the telescope than can be found in the numerous class of double stars. they are to be counted in thousands; indeed, _many_ thousands can be found in the catalogues devoted to this special branch of astronomy. many of these objects are, no doubt, small and comparatively uninteresting, but some of them are among the most conspicuous stars in the heavens, such as sirius, whose system we have already described. we shall in this brief account select for special discussion and illustration a few of the more remarkable double stars. we shall particularly notice some of those that can be readily observed with a small telescope, and we have indicated on the sketches of the constellations in a previous chapter how the positions of these objects in the heavens can be ascertained. it had been shown by cassini in that certain stars, which appeared to the unaided eye as single points of light, really consisted of two or more stars, so close together that the telescope was required for their separation.[ ] the number of these objects was gradually increased by fresh discoveries, until in (the same year in which herschel discovered uranus) a list containing eighty double stars was published by the astronomer bode. these interesting objects claimed the attention of herschel during his memorable researches. the list of known doubles rapidly swelled. herschel's discoveries are to be enumerated by hundreds, while he also commenced systematic measurements of the distance by which the stars were separated, and the direction in which the line joining them pointed. it was these measurements which ultimately led to one of the most important and instructive of all herschel's discoveries. when, in the course of years, his observations were repeated, herschel found that in some cases the relative position of the stars had changed. he was thus led to the discovery that in many of the double stars the components are so related that they revolve around each other. mark the importance of this result. we must remember that the stars are suns, comparable, it may be, with our sun in magnitude; so that here we have the astonishing spectacle of pairs of suns in mutual revolution. there is nothing very surprising in the fact that movements should be observed, for in all probability every body in the universe is in motion. it is the particular character of the movement which is specially interesting and instructive. it had been imagined that the proximity of the two stars forming a double must be only accidental. it was thought that amid the vast host of stars in the heavens it not unfrequently happened that one star was so nearly behind another (as seen from the earth) that when the two were viewed in the telescope they produced the effect of a double star. no doubt many of the so-called double stars are produced in this way. herschel's discovery shows that this explanation will not always answer, but that in many cases we really have two stars close together, and in motion round their common centre of gravity. when the measurements of the distances and the positions of double stars had been accumulated during many years, they were taken over by the mathematicians to be treated by their methods. there is one peculiarity about double star observations: they have not--they cannot have--the accuracy which the computer of an orbit demands. if the distance between the pair of stars forming a binary be four seconds, the orbit we have to scrutinise is only as large as the apparent size of a penny-piece at the distance of one mile. it would require very careful measurement to make out the form of a penny a mile off, even with good telescopes. if the penny were tilted a little, it would appear, not circular, but oval; and it would be possible, by measuring this oval, to determine how much the penny was tilted. all this requires skilful work: the errors, viewed intrinsically, may not be great, but viewed with reference to the whole size of the quantities under consideration, they are very appreciable. we therefore find the errors of observation far more prominent in observations of this class than is generally the case when the mathematician assumes the task of discussing the labours of the observer. the interpretation of herschel's discovery was not accomplished by himself; the light of mathematics was turned on his observations of the binary stars by savary, and afterwards by other mathematicians. under their searching enquiries the errors of the measurements were disclosed, and the observations were purified from the grosser part of their inaccuracy. mathematicians could then apply to their corrected materials the methods of enquiry with which they were familiar; they could deduce with fair precision the actual shape of the orbit of the binary stars, and the position of the plane in which that orbit is contained. the result is not a little remarkable. it has been proved that the motion of each of the stars is performed in an ellipse which contains the centre of gravity of the two stars in its focus. this has been actually shown to be true in many binary stars; it is believed to be true in all. but why is this so important? is not motion in an ellipse common enough? does not the earth revolve in an ellipse round the sun? and do not the planets also revolve in ellipses? it is this very fact that elliptic motion is so common in the planets of the solar system which renders its discovery in binary stars of such importance. from what does the elliptic motion in the solar system arise? is it not due to the law of attraction, discovered by newton, which states that every mass attracts every other mass with a force which varies inversely as the square of the distance? that law of attraction had been found to pervade the whole solar system, and it explained the movements of the bodies of our system with marvellous fidelity. but the solar system, consisting of the sun, and the planets, with their satellites, the comets, and a host of smaller bodies, formed merely a little island group in the universe. in the economy of this tiny cosmical island the law of gravitation reigns supreme; before herschel's discovery we never could have known whether that law was not merely a piece of local legislation, specially contrived for the exigencies of our particular system. this discovery gave us the knowledge which we could have gained from no other source. from the binary stars came a whisper across the vast abyss of space. that whisper told us that the law of gravitation was not peculiar to the solar system. it told us the law extended to the distant shores of the abyss in which our island is situated. it gives us grounds for believing that the law of gravitation is obeyed throughout the length, breadth, and depth of the entire visible universe. one of the finest binary stars is that known as castor, the brighter of the two principal stars in the constellation of gemini. the position of castor on the heavens is indicated in fig. , page . viewed by the unaided eye, castor resembles a single star; but with a moderately good telescope it is found that what seems to be one star is really two separate stars, one of which is of the third magnitude, while the other is somewhat less. the angular distance of these two stars in the heavens is not so great as the angle subtended by a line an inch long viewed at a distance of half a mile. castor is one of the double stars in which the components have been observed to possess a motion of revolution. the movement is, however, extremely slow, and the lapse of centuries will be required before a revolution is completely effected. a beautiful double star can be readily identified in the constellation of ursa major (_see_ fig. , page ). it is known as mizar, and is the middle star (z) of the three which form the tail. in the close neighbourhood of mizar is the small star alcor, which can be readily seen with the unaided eye; but when we speak of mizar as a double star, it is not to be understood that alcor is one of the components of the double. under the magnifying power of the telescope alcor is seen to be transferred a long way from mizar, while mizar itself is split up into two suns close together. these components are of the second and the fourth magnitudes respectively, and as the apparent distance is nearly three times as great as in castor, they are observed with facility even in a small telescope. this is, indeed, the best double star in the heavens for the beginner to commence his observations upon. we cannot, however, assert that mizar is a binary, inasmuch as observations have not yet established the existence of a motion of revolution. still less are we able to say whether alcor is also a member of the same group, or whether it may not merely be a star which happens to fall nearly in the line of vision. recent spectroscopic observations have shown that the larger component of mizar is itself a double, consisting of a pair of suns so close together that there is not the slightest possibility of their ever being seen separately by the most powerful telescope in the world. a pleasing class of double stars is that in which we have the remarkable phenomenon of colours, differing in a striking degree from the colours of ordinary stars. among the latter we find, in the great majority of cases, no very characteristic hue; some are, however, more or less tinged with red, some are decidedly ruddy, and some are intensely red. stars of a bluish or greenish colour are much more rare,[ ] and when a star of this character does occur, it is almost invariably as one of a pair which form a double. the other star of the double is sometimes of the same hue, but more usually it is yellow or ruddy. one of the loveliest of these objects, which lies within reach of telescopes of very moderate pretensions, is that found in the constellation of the swan, and known as b cygni (fig. ). this exquisite object is composed of two stars. the larger, about the third magnitude, is of a golden-yellow, or topaz, colour; the smaller, of the sixth magnitude, is of a light blue. these colours are nearly complementary, but still there can be no doubt that the effect is not merely one of contrast. that these two stars are both tinged with the hues we have stated can be shown by hiding each in succession behind a bar placed in the field of view. it has also been confirmed in a very striking manner by spectroscopic investigation; for we see that the blue star has experienced a special absorption of the red rays, while the more ruddy light of the other star has arisen from the absorption of the blue rays. the contrast of the colours in this object can often be very effectively seen by putting the eye-piece out of focus. the discs thus produced show the contrast of colours better than when the telescope exhibits merely two stellar points. such are a few of these double and multiple stars. their numbers are being annually augmented; indeed, one observer--mr. burnham, formerly on the staff of the lick observatory, and now an observer in the yerkes observatory--has added by his own researches more than , new doubles to the list of those previously known. the interest in this class of objects must necessarily be increased when we reflect that, small as the stars appear to be in our telescopes, they are in reality suns of great size and splendour, in many cases rivalling our own sun, or, perhaps, even surpassing him. whether these suns have planets attending upon them we cannot tell; the light reflected from the planet would be utterly inadequate to the penetration of the vast extent of space which separates us from the stars. if there be planets surrounding these objects, then, instead of a single sun, such planets will be illuminated by two, or, perhaps, even more suns. what wondrous effects of light and shade must be the result! sometimes both suns will be above the horizon together, sometimes only one sun, and sometimes both will be absent. especially remarkable would be the condition of a planet whose suns were of the coloured type. to-day we have a red sun illuminating the heavens, to-morrow it would be a blue sun, and, perhaps, the day after both the red sun and the blue sun will be in the firmament together. what endless variety of scenery such a thought suggests! there are, however, grave dynamical reasons for doubting whether the conditions under which such a planet would exist could be made compatible with life in any degree resembling the life with which we are familiar. the problem of the movement of a planet under the influence of two suns is one of the most difficult that has ever been proposed to mathematicians, and it is, indeed, impossible in the present state of analysis to solve with accuracy all the questions which it implies. it seems not at all unlikely that the disturbances of the planet's orbit would be so great that it would be exposed to vicissitudes of light and of temperature far transcending those experienced by a planet moving, like the earth, under the supreme control of a single sun. chapter xxi. the distances of the stars. sounding-line for space--the labours of bessel--meaning of annual parallax--minuteness of the parallactic ellipse illustrated--the case of cygni--different comparison stars used--the proper motion of the star--struve's investigations--can they be reconciled?--researches at dunsink--conclusion obtained--accuracy which such observations admit examined--the proper motion of cygni--the permanence of the sidereal heavens--the new star in cygnus--its history--no appreciable parallax--a mighty outburst of light--the movement of the solar system through space--herschel's discovery--journey towards lyra--probabilities. we have long known the dimensions of the solar system with more or less accuracy. our knowledge includes the distances of the planets and the comets from the sun, as well as their movements. we have also considerable knowledge of the diameters and the masses of many of the different bodies which belong to the solar system. we have long known, in fact, many details of the isolated group nestled together under the protection of the sun. the problem for consideration in the present chapter involves a still grander survey than is required for measures of our solar system. we propose to carry the sounding-line across the vast abyss which separates the group of bodies closely associated about our sun from the other stars which are scattered through the realms of space. for centuries the great problem of star distance has engaged the attention of those who have studied the heavens. it would be impossible to attempt here even an outline of the various researches which have been made on the subject. in the limited survey which we can make, we must glance first at the remarkable speculative efforts which have been directed to the problem, and then we shall refer to those labours which have introduced the problem into the region of accurate astronomy. no attempt to solve the problem of the absolute distances of the stars was successful until many years after herschel's labours were closed. fresh generations of astronomers, armed with fresh appliances, have for many years pursued the subject with unremitting diligence, but for a long time the effort seemed hopeless. the distances of the stars were so great that they could not be ascertained until the utmost refinements of mechanical skill and the most elaborate methods of mathematical calculation were brought to converge on the difficulty. at last it was found that the problem was beginning to yield. a few stars have been induced to disclose the secret of their distance. we are able to give some answer to the question--how far are the stars? though it must be confessed that our reply up to the present moment is both hesitating and imperfect. even the little knowledge which has been gained possesses interest and importance. as often happens in similar cases, the discovery of the distance of a star was made independently about the same time by two or three astronomers. the name of bessel stands out conspicuously in this memorable chapter of astronomy. bessel proved ( ) that the distance of the star known as cygni was a measurable quantity. his demonstration possessed such unanswerable logic that universal assent could not be withheld. almost simultaneously with the classical labours of bessel we have struve's measurement of the distance of vega, and henderson's determination of the distance of the southern star a centauri. great interest was excited in the astronomical world by these discoveries, and the royal astronomical society awarded its gold medal to bessel. it appropriately devolved on sir john herschel to deliver the address on the occasion of the presentation of the medal: that address is a most eloquent tribute to the labours of the three astronomers. we cannot resist quoting the few lines in which sir john said:-- "gentlemen of the royal astronomical society,--i congratulate you and myself that we have lived to see the great and hitherto impassable barrier to our excursion into the sidereal universe, that barrier against which we have chafed so long and so vainly--_æstuantes angusto limite mundi_--almost simultaneously overleaped at three different points. it is the greatest and most glorious triumph which practical astronomy has ever witnessed. perhaps i ought not to speak so strongly; perhaps i should hold some reserve in favour of the bare possibility that it may be all an illusion, and that future researches, as they have repeatedly before, so may now fail to substantiate this noble result. but i confess myself unequal to such prudence under such excitement. let us rather accept the joyful omens of the time, and trust that, as the barrier has begun to yield, it will speedily be effectually prostrated." before proceeding further, it will be convenient to explain briefly how the distance of a star can be measured. the problem is one of a wholly different character from that of the sun's distance, which we have already discussed in these pages. the observations for the determination of stellar parallax are founded on the familiar truth that the earth revolves around the sun. we may for our present purpose assume that the earth revolves in a circular path. the centre of that path is at the centre of the sun, and the radius of the path is , , miles. owing to our position on the earth, we observe the stars from a point of view which is constantly changing. in summer the earth is , , miles distant from the position which it occupied in winter. it follows that the apparent positions of the stars, as projected on the background of the sky, must present corresponding changes. we do not now mean that the actual positions of the stars are really displaced. the changes are only apparent, and while oblivious of our own motion, which produces the displacements, we attribute the changes to the stars. on the diagram in fig. is an ellipse with certain months--viz., january, april, july, october--marked upon its circumference. this ellipse may be regarded as a miniature picture of the earth's orbit around the sun. in january the earth is at the spot so marked; in april it has moved a quarter of the whole journey; and so on round the whole circle, returning to its original position in the course of one year. when we look from the position of the earth in january, we see the star a projected against the point of the sky marked . three months later the observer with his telescope is carried round to april; but he now sees the star projected to the position marked . thus, as the observer moves around the whole orbit in the annual revolution of the earth, so the star appears to move round in an ellipse on the background of the sky. in the technical language of astronomers, we speak of this as the parallactic ellipse, and it is by measuring the major axis of this ellipse that we determine the distance of the star from the sun. half of this major axis, or, what comes to the same thing, the angle which the radius of the earth's orbit subtends as seen from the star, is called the star's "annual parallax." [illustration: fig. .--the parallactic ellipse.] the figure shows another star, b, more distant from the earth and the solar system generally than the star previously considered. this star also describes an elliptic path. we cannot, however, fail to notice that the parallactic ellipse belonging to b is much smaller than that of a. the difference in the sizes of the ellipses arises from the different distances of the stars from the earth. the nearer the star is to the earth the greater is the ellipse, so that the nearest star in the heavens will describe the largest ellipse, while the most distant star will describe the smallest ellipse. we thus see that the distance of the star is inversely proportional to the size of the ellipse, and if we measure the angular value of the major axis of the ellipse, then, by an exceedingly simple mathematical manipulation, the distance of the star can be expressed as a multiple of a radius of the earth's orbit. assuming that radius to be , , miles, the distance of the star is obtained by simple arithmetic. the difficulty in the process arises from the fact that these ellipses are so small that our micrometers often fail to detect them. how shall we adequately describe the extreme minuteness of the parallactic ellipses in the case of even the nearest stars? in the technical language of astronomers, we may state that the longest diameter of the ellipse never subtends an angle of more than one and a half seconds. in a somewhat more popular manner, we would say that one thousand times the major axis of the very largest parallactic ellipse would not be as great as the diameter of the full moon. for a still more simple illustration, let us endeavour to think of a penny-piece placed at a distance of two miles. if looked at edgeways it will be linear, if tilted a little it would be elliptic; but the ellipse would, even at that distance, be greater than the greatest parallactic ellipse of any star in the sky. suppose a sphere described around an observer, with a radius of two miles. if a penny-piece were placed on this sphere, in front of each of the stars, every parallactic ellipse would be totally concealed. the star in the swan known as cygni is not remarkable either for its size or for its brightness. it is barely visible to the unaided eye, and there are some thousands of stars which are apparently larger and brighter. it is, however, a very interesting example of that remarkable class of objects known as double stars. it consists of two nearly equal stars close together, and evidently connected by a bond of mutual attraction. the attention of astronomers is also specially directed towards the star by its large proper motion. in virtue of that proper motion, the two components are carried together over the sky at the rate of five seconds annually. a proper motion of this magnitude is extremely rare, yet we do not say it is unparalleled, for there are some few stars which have a proper motion even more rapid; but the remarkable duplex character of cygni, combined with the large proper motion, render it an unique object, at all events, in the northern hemisphere. when bessel proposed to undertake the great research with which his name will be for ever connected, he determined to devote one, or two, or three years to the continuous observations of one star, with the view of measuring carefully its parallactic ellipse. how was he to select the object on which so much labour was to be expended? it was all-important to choose a star which should prove sufficiently near to reward his efforts by exhibiting a measurable parallax. yet he could have but little more than surmise and analogy as a guide. it occurred to him that the exceptional features of cygni afforded the necessary presumption, and he determined to apply the process of observation to this star. he devoted the greater part of three years to the work, and succeeded in discovering its distance from the earth. since the date of sir john herschel's address, cygni has received the devoted and scarcely remitted attention of astronomers. in fact, we might say that each succeeding generation undertakes a new discussion of the distance of this star, with the view of confirming or of criticising the original discovery of bessel. the diagram here given (fig. ) is intended to illustrate the recent history of cygni. when bessel engaged in his labours, the pair of stars forming the double were at the point indicated on the diagram by the date . the next epoch occurred fifteen years later, when otto struve undertook his researches, and the pair of stars had by that time moved to the position marked . finally, when the same object was more recently observed at dunsink observatory, the pair had made still another advance, to the position indicated by the date . thus, in forty years this double star had moved over an arc of the heavens upwards of three minutes in length. the actual path is, indeed, more complicated than a simple rectilinear movement. the two stars which form the double have a certain relative velocity, in consequence of their mutual attraction. it will not, however, be necessary to take this into account, as the displacement thus arising in the lapse of a single year is far too minute to produce any inconvenient effect on the parallactic ellipse. [illustration: fig. .-- cygni and the comparison stars.] the case of cygni is, however, exceptional. it is one of our nearest neighbours in the heavens. we can never find its distance accurately to one or two billions of miles; but still we have a consciousness that an uncertainty amounting to twenty billions is too large a percentage of the whole. we shall presently show that we believe struve was right, yet it does not necessarily follow that bessel was wrong. the apparent paradox can be easily explained. it would not be easily explained if struve had used the _same comparison star_ as bessel had done; but struve's comparison star was different from either of bessel's, and this is probably the cause of the discrepancy. it will be recollected that the essence of the process consists of the comparison of the small ellipse made by the distant star with the larger ellipse made by the nearer star. if the two stars were at the same distance, the process would be wholly inapplicable. in such a case, no matter how near the stars were to the earth, no parallax could be detected. for the method to be completely successful, the comparison star should be at least eight times as far as the principal star. bearing this in mind, it is quite possible to reconcile the measures of bessel with those of struve. we need only assume that bessel's comparison stars are about three times as far as cygni, while struve's comparison star is at least eight or ten times as far. we may add that, as the comparison stars used by bessel are brighter than that of struve, there really is a presumption that the latter is the most distant of the three. we have here a characteristic feature of this method of determining parallax. even if all the observations and the reductions of a parallax series were mathematically correct, we could not with strict propriety describe the final result as the parallax of one star. it is only the _difference_ between the parallax of the star and that of the comparison star. we can therefore only assert that the parallax sought cannot be less than the quantity determined. viewed in this manner, the discrepancy between struve and bessel vanishes. bessel asserted that the distance of cygni could not be _more_ than sixty billions of miles. struve did not contradict this--nay, he certainly confirmed it--when he showed that the distance could not be more than forty billions. nearly half a century has elapsed since struve made his observations. those observations have certainly been challenged; but they are, on the whole, confirmed by other investigations. in a critical review of the subject auwers showed that struve's determination is worthy of considerable confidence. yet, notwithstanding this authoritative announcement, the study of cygni has been repeatedly resumed. dr. brünnow, when astronomer royal of ireland, commenced a series of observations on the parallax of cygni, which were continued and completed by the present writer, his successor. brünnow chose a fourth comparison star (marked on the diagram), different from any of those which had been used by the earlier observers. the method of observing which brünnow employed was quite different from that of struve, though the filar micrometer was used in both cases. brünnow sought to determine the parallactic ellipse by measuring the difference in declination between cygni and the comparison star.[ ] in the course of a year it is found that the difference in declination undergoes a periodic change, and from that change the parallactic ellipse can be computed. in the first series of observations i measured the difference of declination between the preceding star of cygni and the comparison star; in the second series i took the other component of cygni and the same comparison star. we had thus two completely independent determinations of the parallax resulting from two years' work. the first of these makes the distance forty billions of miles, and the second makes it almost exactly the same. there can be no doubt that this work supports struve's determination in correction of bessel's, and therefore we may perhaps sum up the present state of our knowledge of this question by saying that the distance of cygni is much nearer to the forty billions of miles which struve found than to the sixty billions which bessel found.[ ] it is desirable to give the reader the means of forming his own opinion as to the quality of the evidence which is available in such researches. the diagram in fig. here shown has been constructed with this object. it is intended to illustrate the second series of observations of difference of declination which i made at dunsink. each of the dots represents one night's observations. the height of the dot is the observed difference of declination between (b) cygni and the comparison star. the distance along the horizontal line--or the abscissa, as a mathematician would call it--represents the date. these observations are grouped more or less regularly in the vicinity of a certain curve. that curve expresses where the observations should have been, had they been absolutely perfect. the distances between the dots and the curve may be regarded as the errors which have been committed in making the observations. [illustration: fig. .--parallax in declination of cygni.] perhaps it will be thought that in many cases these errors appear to have attained very undesirable dimensions. let us, therefore, hasten to say that it was precisely for the purpose of setting forth these errors that this diagram has been shown; we have to exhibit the weakness of the case no less than its strength. the errors of the observations are not, however, intrinsically so great as might at first sight be imagined. to perceive this, it is only necessary to interpret the scale on which this diagram has been drawn by comparison with familiar standards. the distance from the very top of the curve to the horizontal line denotes an angle of only four-tenths of a second. this is about the apparent diameter of a penny-piece at a distance of _ten miles_! we can now appraise the true magnitude of the errors which have been made. it will be noticed that no one of the dots is distant from the curve by much more than half of the height of the curve. it thus appears that the greatest error in the whole series of observations amounts to but two or three tenths of a second. this is equivalent to our having pointed the telescope to the upper edge of a penny-piece fifteen or twenty miles off, instead of to the lower edge. this is not a great blunder. a rifle team whose errors in pointing were more than a hundred times as great might still easily win every prize at bisley. we have entered into the history of cygni with some detail, because it is the star whose distance has been most studied. we do not say that cygni is the nearest of all the stars; it would, indeed, be very rash to assert that any particular star was the nearest of all the countless millions in the heavenly host. we certainly know one star which seems nearer than cygni; it lies in one of the southern constellations, and its name is a centauri. this star is, indeed, of memorable interest in the history of the subject. its parallax was first determined at the cape of good hope by henderson; subsequent researches have confirmed his observations, and the elaborate investigations of dr. gill have proved that the parallax of this star is about three-quarters of a second, so that it is only two-thirds of the distance of cygni. cygni arrested our attention, in the first instance, by the circumstance that it had the large proper motion of five seconds annually. we have also ascertained that the annual parallax is about half a second. the combination of these two statements leads to a result of considerable interest. it teaches us that cygni must each year traverse a distance of not less than ten times the radius of the earth's orbit. translating this into ordinary figures, we learn that this star must travel nine hundred and twenty million miles per annum. it must move between two and three million miles each day, but this can only be accomplished by maintaining the prodigious velocity of thirty miles per second. there seems to be no escape from this conclusion. the facts which we have described, and which are now sufficiently well established, are inconsistent with the supposition that the velocity of cygni is less than thirty miles per second; the velocity may be greater, but less it cannot be. for the last hundred and fifty years we know that cygni has been moving in the same direction and with the same velocity. prior to the existence of the telescope we have no observation to guide us; we cannot, therefore, be absolutely certain as to the earlier history of this star, yet it is only reasonable to suppose that cygni has been moving from remote antiquity with a velocity comparable with that it has at present. if disturbing influences were entirely absent, there could be no trace of doubt about the matter. _some_ disturbing influence, however, there must be; the only question is whether that disturbing influence is sufficient to modify seriously the assumption we have made. a powerful disturbing influence might greatly alter the velocity of the star; it might deflect the star from its rectilinear course; it might even force the star to move around a closed orbit. we do not, however, believe that any disturbing influence of this magnitude need be contemplated, and there can be no reasonable doubt that cygni moves at present in a path very nearly straight, and with a velocity very nearly uniform. as the distance of cygni from the sun is forty billions of miles, and its velocity is thirty miles a second, it is easy to find how long the star would take to accomplish a journey equal to its distance from the sun. the time required will be about , years. in the last , years cygni will have moved over a distance ten times as great as its present distance from the sun, whatever be the direction of motion. this star must therefore have been about ten times as far from the earth , years ago as it is at present. though this epoch is incredibly more remote than any historical record, it is perhaps not incomparable with the duration of the human race; while compared with the vast lapse of geological time, such periods seem trivial and insignificant. geologists have long ago repudiated mere thousands of years; they now claim millions, and many millions of years, for the performance of geological phenomena. if the earth has existed for the millions of years which geologists assert, it becomes reasonable for astronomers to speculate on the phenomena which have transpired in the heavens in the lapse of similar ages. by the aid of our knowledge of star distances, combined with an assumed velocity of thirty miles per second, we can make the attempt to peer back into the remote past, and show how great are the changes which our universe seems to have undergone. in a million years cygni will apparently have moved through a distance which is twenty-five times as great as its present distance from the sun. whatever be the direction in which cygni is moving--whether it be towards the earth or from the earth, to the right or to the left, it must have been about twenty-five times as far off a million years ago as it is at present; but even at its present distance cygni is a small star; were it ten times as far it could only be seen with a good telescope; were it twenty-five times as far it would barely be a visible point in our greatest telescopes. the conclusions arrived at with regard to cygni may be applied with varying degrees of emphasis to other stars. we are thus led to the conclusion that many of the stars with which the heavens are strewn are apparently in slow motion. but this motion though apparently slow may really be very rapid. when standing on the sea-shore, and looking at a steamer on the distant horizon, we can hardly notice that the steamer is moving. it is true that by looking again in a few minutes we can detect a change in its place; but the motion of the steamer seems slow. yet if we were near the steamer we would find that it was rushing along at the rate of many miles an hour. it is the distance which causes the illusion. so it is with the stars: they seem to move slowly because they are very distant, but were we near them, we could see that in the majority of cases their motions are a thousand times as fast as the quickest steamer that ever ploughed the ocean. it thus appears that the permanence of the sidereal heavens, and the fixity of the constellations in their relative positions, are only ephemeral. when we rise to the contemplation of such vast periods of time as the researches of geology disclose, the durability of the constellations vanishes! in the lapse of those stupendous ages stars and constellations gradually dissolve from view, to be replaced by others of no greater permanence. it not unfrequently happens that a parallax research proves abortive. the labour has been finished, the observations are reduced and discussed, and yet no value of the parallax can be obtained. the distance of the star is so vast that our base-line, although it is nearly two hundred millions of miles long, is too short to bear any appreciable ratio to the distance of the star. even from such failures, however, information may often be drawn. let me illustrate this by an account derived from my own experience at dunsink. we have already mentioned that on the th november, , a well-known astronomer--dr. schmidt, of athens--noticed a new bright star of the third magnitude in the constellation cygnus. on the th of november nova cygni was invisible. whether it first burst forth on the st, nd, or rd no one can tell; but on the th it was discovered. its brilliancy even then seemed to be waning; so, presumably, it was brightest at some moment between the th and th of november. the outbreak must thus have been comparatively sudden, and we know of no cause which would account for such a phenomenon more simply than a gigantic collision. the decline in the brilliancy was much more tardy than its growth, and more than a fortnight passed before the star relapsed into insignificance--two or three days (or less) for the rise, two or three weeks for the fall. yet even two or three weeks was a short time in which to extinguish so mighty a conflagration. it is comparatively easy to suggest an explanation of the sudden outbreak; it is not equally easy to understand how it can have been subdued in a few weeks. a good-sized iron casting in one of our foundries takes nearly as much time to cool as sufficed to abate the celestial fires in nova cygni! on this ground it seemed not unreasonable to suppose that perhaps nova cygni was not really a very extensive conflagration. but, if such were the case, the star must have been comparatively _near_ to the earth, since it presented so brilliant a spectacle and attracted so much attention. it therefore appeared a plausible object for a parallax research; and consequently a series of observations were made some years ago at dunsink. i was at the time too much engaged with other work to devote very much labour to a research which might, after all, only prove illusory. i simply made a sufficient number of micrometric measurements to test whether a _large_ parallax existed. it has been already pointed out how each star appears to describe a minute parallactic ellipse, in consequence of the annual motion of the earth, and by measurement of this ellipse the parallax--and therefore the distance--of the star can be determined. in ordinary circumstances, when the parallax of a star is being investigated, it is necessary to measure the position of the star in its ellipse on many different occasions, distributed over a period of at least an entire year. the method we adopted was much less laborious. it was sufficiently accurate to test whether or not nova cygni had a _large_ parallax, though it might not have been delicate enough to disclose a small parallax. at a certain date, which can be readily computed, the star is at one end of the parallactic ellipse, and six months later the star is at the other end. by choosing suitable times in the year for our observations, we can measure the star in those two positions when it is most deranged by parallax. it was by observations of this kind that i sought to detect the parallax of nova cygni. its distance from a neighbouring star was carefully measured by the micrometer at the two seasons when, if parallax existed, those distances should show their greatest discrepancy; but no certain difference between these distances could be detected. the observations, therefore, failed to reveal the existence of a parallactic ellipse--or, in other words, the distance of nova cygni was too great to be measured by observations of this kind. it is certain that if nova cygni had been one of the nearest stars these observations would not have been abortive. we are therefore entitled to believe that nova cygni must be at least , , , , miles from the solar system; and the suggestion that the brilliant outburst was of small dimensions must, it seems, be abandoned. the intrinsic brightness of nova cygni, when at its best, cannot have been greatly if at all inferior to the brilliancy of our sun himself. if the sun were withdrawn from us to the distance of nova cygni, it would seemingly have dwindled down to an object not more brilliant than the variable star. how the lustre of such a stupendous object declined so rapidly remains, therefore, a mystery not easy to explain. have we not said that the outbreak of brilliancy in this star occurred between the th and the th of november, ? it would be more correct to say that the tidings of that outbreak reached our system at the time referred to. the real outbreak must have taken place at least three years previously. indeed, at the time that the star excited such commotion in the astronomical world here, it had already relapsed again into insignificance. in connection with the subject of the present chapter we have to consider a great problem which was proposed by sir william herschel. he saw that the stars were animated by proper motion; he saw also that the sun is a star, one of the countless host of heaven, and he was therefore led to propound the stupendous question as to whether the sun, like the other stars which are its peers, was also in motion. consider all that this great question involves. the sun has around it a retinue of planets and their attendant satellites, the comets, and a host of smaller bodies. the question is, whether all this superb system is revolving around the sun _at rest_ in the middle, or whether the whole system--sun, planets, and all--is not moving on bodily through space. herschel was the first to solve this noble problem; he discovered that our sun and the splendid retinue by which it is attended are moving in space. he not only discovered this, but he ascertained the direction in which the system was moving, as well as the approximate velocity with which that movement was probably performed. it has been shown that the sun and his system is now hastening towards a point of the heavens near the constellation lyra. the velocity with which the motion is performed corresponds to the magnitude of the system; quicker than the swiftest rifle-bullet that was ever fired, the sun, bearing with it the earth and all the other planets, is now sweeping onwards. we on the earth participate in that motion. every half hour we are something like ten thousand miles nearer to the constellation of lyra than we should have been if the solar system were not animated by this motion. as we are proceeding at this stupendous rate towards lyra, it might at first be supposed that we ought soon to get there; but the distances of the stars in that neighbourhood seem not less than those of the stars elsewhere, and we may be certain that the sun and his system must travel at the present rate for far more than a million years before we have crossed the abyss between our present position and the frontiers of lyra. it must, however, be acknowledged that our estimate of the actual _speed_ with which our solar system is travelling is exceedingly uncertain, but this does not in the least affect the fact that we are moving in the direction first approximately indicated by herschel (_see_ chapter xxiii.). it remains to explain the method of reasoning which herschel adopted, by which he was able to make this great discovery. it may sound strange to hear that the detection of the motion of the sun was not made by looking at the sun; all the observations of the luminary itself with all the telescopes in the world would never tell us of that motion, for the simple reason that the earth, whence our observations must be made, participates in it. a passenger in the cabin of a ship usually becomes aware that the ship is moving by the roughness of the sea; but if the sea be perfectly calm, then, though the tables and chairs in the cabin are moving as rapidly as the ship, yet we do not see them moving, because we are also travelling with the ship. if we could not go out of the cabin, nor look through the windows, we would never know whether the ship was moving or at rest; nor could we have any idea as to the direction in which the ship was going, or as to the velocity with which that motion was performed. the sun, with his attendant host of planets and satellites, may be likened to the ship. the planets may revolve around the sun just as the passengers may move about in the cabin, but as the passengers, by looking at objects on board, can never tell whither the ship is going, so we, by merely looking at the sun, or at the other planets or members of the solar system, can never tell if our system as a whole is in motion. the conditions of a perfectly uniform movement along a perfectly calm sea are not often fulfilled on the waters with which we are acquainted, but the course of the sun and his system is untroubled by any disturbance, so that the majestic progress is conducted with absolute uniformity. we do not feel the motion; and as all the planets are travelling with us, we can get no information from them as to the common motion by which the whole system is animated. the passengers are, however, at once apprised of the ship's motion when they go on deck, and when they look at the sea surrounding them. let us suppose that their voyage is nearly accomplished, that the distant land appears in sight, and, as evening approaches, the harbour is discerned into which the ship is to enter. let us suppose that the harbour has, as is often the case, a narrow entrance, and that its mouth is indicated by a lighthouse on each side. when the harbour is still a long way off, near the horizon, the two lights are seen close together, and now that the evening has closed in, and the night has become quite dark, these two lights are all that remain visible. while the ship is still some miles from its destination the two lights seem close together, but as the distance decreases the two lights seem to open out; gradually the ship gets nearer, while the lights are still opening, till finally, when the ship enters the harbour, instead of the two lights being directly in front, as at the commencement, one of the lights is passed by on the right hand, while the other is similarly found on the left. if, then, we are to discover the motion of the solar system, we must, like the passenger, look at objects unconnected with our system, and learn our own motion by their apparent movements. but are there any objects in the heavens unconnected with our system? if all the stars were like the earth, merely the appendages of our sun, then we never could discover whether we were at rest or whether we were in motion: our system might be in a condition of absolute rest, or it might be hurrying on with an inconceivably great velocity, for anything we could tell to the contrary. but the stars do not belong to the system of our sun; they are, rather, suns themselves, and do not recognise the sway of our sun, as this earth is obliged to do. the stars will, therefore, act as the external objects by which we can test whether our system is voyaging through space. with the stars as our beacons, what ought we to expect if our system be really in motion? remember that when the ship was approaching the harbour the lights gradually opened out to the right and left. but the astronomer has also lights by which he can observe the navigation of that vast craft, our solar system, and these lights will indicate the path along which he is borne. if our solar system be in motion, we should expect to find that the stars were gradually spreading away from that point in the heavens towards which our motion tends. this is precisely what we do find. the stars in the constellations are gradually spreading away from a central point near the constellation of lyra, and hence we infer that it is towards lyra that the motion of the solar system is directed. there is one great difficulty in the discussion of this question. have we not had occasion to observe that the stars themselves are in actual motion? it seems certain that every star, including the sun himself as a star, has each an individual motion of its own. the motions of the stars as we see them are partly apparent as well as partly real; they partly arise from the actual motion of each star and partly from the motion of the sun, in which we partake, and which produces an apparent motion of the star. how are these to be discriminated? our telescopes and our observations can never effect this decomposition directly. to accomplish the analysis, herschel resorted to certain geometrical methods. his materials at that time were but scanty, but in his hands they proved adequate, and he boldly announced his discovery of the movement of the solar system. so astounding an announcement demanded the severest test which the most refined astronomical resources could suggest. there is a certain powerful and subtle method which astronomers use in the effort to interpret nature. bishop butler has said that probability is the guide of life. the proper motion of a star has to be decomposed into two parts, one real and the other apparent. when several stars are taken, we may conceive an infinite number of ways into which the movements of each star can be so decomposed. each one of these conceivable divisions will have a certain element of probability in its favour. it is the business of the mathematician to determine the amount of that probability. the case, then, is as follows:--among all the various systems one must be true. we cannot lay our finger for certain on the true one, but we can take that which has the highest degree of probability in its favour, and thus follow the precept of butler to which we have already referred. a mathematician would describe his process by calling it the method of least squares. since herschel's discovery, one hundred years ago, many an astronomer using observations of hundreds of stars has attacked the same problem. mathematicians have exhausted every refinement which the theory of probabilities can afford, but only to confirm the truth of that splendid theory which seems to have been one of the flashes of herschel's genius. chapter xxii. star clusters and nebulÆ. interesting sidereal objects--stars not scattered uniformly--star clusters--their varieties--the cluster in perseus--the globular cluster in hercules--the milky way--a cluster of minute stars--the magellanic clouds--nebulæ distinct from clouds--number of known nebulæ--the constellation of orion--the position of the great nebula--the wonderful star th orionis--the drawing of the great nebula in lord rosse's telescope--photographs of this wonderful object--the great nebula in andromeda--the annular nebula in lyra--resemblance to vortex rings--planetary nebulæ--drawings of several remarkable nebulæ--nature of nebulæ--spectra of nebulæ--their distribution; the milky way. we have already mentioned saturn as one of the most glorious telescopic spectacles in the heavens. setting aside the obvious claims of the sun and of the moon, there are, perhaps, two other objects visible from these latitudes which rival saturn in the splendour and the interest of their telescopic picture. one of these objects is the star cluster in hercules; the other is the great nebula in orion. we take these objects as typical of the two great classes of bodies to be discussed in this chapter, under the head of star clusters and nebulæ. the stars, which to the number of several millions bespangle the sky, are not scattered uniformly. we can see that while some regions are comparatively barren, others contain stars in profusion. sometimes we have a small group, like the pleiades; sometimes we have a stupendous region of the heavens strewn over with stars, as in the milky way. such objects are called star clusters. we find every variety in the clusters; sometimes the stars are remarkable for their brilliancy, sometimes for their enormous numbers, and sometimes for the remarkable form in which they are grouped. sometimes a star cluster is adorned with brilliantly-coloured stars; sometimes the luminous points are so close together that their separate rays cannot he disentangled; sometimes the stars are so minute or so distant that the cluster is barely distinguishable from a nebula. of the clusters remarkable at once both for richness and brilliancy of the individual stars, we may mention the cluster in the sword-handle of perseus. the position of this object is marked on fig. , page . to the unaided eye a hazy spot is visible, which in the telescope expands into two clusters separated by a short distance. in each of them we have innumerable stars, crowded together so as to fill the field of view of the telescope. the splendour of this object may be appreciated when we reflect that each one of these stars is itself a brilliant sun, perhaps rivalling our own sun in lustre. there are, however, regions in the heavens near the southern cross, of course invisible from northern latitudes, in which parts of the milky way present a richer appearance even than the cluster in perseus. the most striking type of star cluster is well exhibited in the constellation of hercules. in this case we have a group of minute stars apparently in a roughly globular form. fig. represents this object as seen in lord rosse's great telescope, and it shows three radiating streaks, in which the stars seem less numerous than elsewhere. it is estimated that this cluster must contain from , to , stars, all concentrated into an extremely small part of the heavens. viewed in a very small telescope, this object resembles a nebula. the position of the cluster in hercules is shown in a diagram previously given (fig. , page ). we have already referred to this glorious aggregation of stars as one of the three especially interesting objects in the heavens. [illustration: plate d. milky way near messier ii. _photographed by e.e. barnard, th june, ._] the milky way forms a girdle which, with more or less regularity, sweeps completely around the heavens; and when viewed with the telescope, is seen to consist of myriads of minute stars. in some places the stars are much more numerous than elsewhere. all these stars are incomparably more distant than the sun, which they surround, so it is evident that our sun and, of course, the system which attends him lie actually inside the milky way. it seems tempting to pursue the thought here suggested, and to reflect that the whole milky way may, after all, be merely a star cluster, comparable in size with some of the other star clusters which we see, and that, viewed from a remote point in space, the milky way would seem to be but one of the many clusters of stars containing our sun as an indistinguishable unit. [illustration: fig. .--the globular cluster in hercules.] in the southern hemisphere there are two immense masses which are conspicuously visible to the naked eye, and resemble detached portions of the milky way. they cannot be seen by observers in our latitude, and are known as the magellanic clouds or the two nubeculæ. their structure, as revealed to an observer using a powerful telescope, is of great complexity. sir john herschel, who made a special study of these remarkable objects, gives the following description of them: "the general ground of both consists of large tracts and patches of nebulosity in every stage of resolution, from light irresolvable, in a reflector of eighteen inches aperture, up to perfectly separated stars like the milky way, and clustering groups sufficiently insulated and condensed to come under the designation of irregular and in some cases pretty rich clusters. but besides these there are also nebulæ in abundance and globular clusters in every state of condensation." it can hardly be doubted that the two nubeculæ, which are, roughly speaking, round, or, rather, oval, are not formed accidentally by a vast number of very different objects being ranged at various distances along the same line of sight, but that they really represent two great systems of objects, widely different in constitution, which here are congregated in each other's neighbourhood, whereas they generally do not co-exist close to each other in the milky way, with which the mere naked-eye view would otherwise lead us to associate the magellanic clouds. when we direct a good telescope to the heavens, we shall occasionally meet with one of the remarkable celestial objects which are known as nebulæ. they are faint cloudy spots, or stains of light on the black background of the sky. they are nearly all invisible to the naked eye. these celestial objects must not for a moment be confounded with clouds, in the ordinary meaning of the word. the latter exist only suspended in the atmosphere, while nebulæ are immersed in the depths of space. clouds shine by the light of the sun, which they reflect to us; nebulæ shine with no borrowed light; they are self-luminous. clouds change from hour to hour; nebulæ do not change even from year to year. clouds are far smaller than the earth; while the smallest nebula known to us is incomparably greater than the sun. clouds are within a few miles of the earth; the nebulæ are almost inconceivably remote. immediately after herschel and his sister had settled at slough he commenced his review of the northern heavens in a systematic manner. for observations of this kind it is essential that the sky be free from cloud, while even the light of the moon is sufficient to obliterate the fainter and more interesting objects. it was in the long and fine winter nights, when the stars were shining brilliantly and the pale path of the milky way extended across the heavens, that the labour was to be done. the telescope being directed to the heavens, the ordinary diurnal motion by which the sun and stars appear to rise and set carries the stars across the field of view in a majestic panorama. the stars enter slowly into the field of view, slowly move across it, and slowly leave it, to be again replaced by others. thus the observer, by merely remaining passive at the eye-piece, sees one field after another pass before him, and is enabled to examine their contents. it follows, that even without moving the telescope a long narrow strip of the heavens is brought under review, and by moving the telescope slightly up and down the width of this strip can be suitably increased. on another night the telescope is brought into a different position, and another strip of the sky is examined; so that in the course of time the whole heavens can be carefully scrutinised. herschel stands at the eye-piece to watch the glorious procession of celestial objects. close by, his sister caroline sits at her desk, pen in hand, to take down the observations as they fall from her brother's lips. in front of her is a chronometer from which she can note the time, and a contrivance which indicates the altitude of the telescope, so that she can record the exact position of the object in connection with the description which her brother dictated. such was the splendid scheme which this brother and sister had arranged to carry out as the object of their life-long devotion. the discoveries which herschel was destined to make were to be reckoned not by tens or by hundreds, but by thousands. the records of these discoveries are to be found in the "philosophical transactions of the royal society," and they are among the richest treasures of those volumes. it was left to sir john herschel, the only son of sir william, to complete his father's labour by repeating the survey of the northern heavens and extending it to the southern hemisphere. he undertook with this object a journey to the cape of good hope, and sojourned there for the years necessary to complete the great work. [illustration: fig. .--the constellation of orion, showing the position of the great nebula.] as the result of the gigantic labours thus inaugurated and continued by other observers, there are now about eight thousand nebulæ known to us, and with every improvement of the telescope fresh additions are being made to the list. they differ from one another as eight thousand pebbles selected at random on a sea-beach might differ--namely, in form, size, colour, and material--but yet, like the pebbles, bear a certain generic resemblance to each other. to describe this class of bodies in any detail would altogether exceed the limits of this chapter; we shall merely select a few of the nebulæ, choosing naturally those of the most remarkable character, and also those which are representatives of the different groups into which nebulæ may be divided. [illustration: plate xiv. the great nebula in orion.] we have already stated that the great nebula in the constellation of orion is one of the most interesting objects in the heavens. it is alike remarkable whether we consider its size or its brilliancy, the care with which it has been studied, or the success which has attended the efforts to learn something of its character. to find this object, we refer to fig. for the sketch of the chief stars in this constellation, where the letter a indicates the middle one of the three stars which form the sword-handle of orion. above the handle will be seen the three stars which form the well-known belt so conspicuous in the wintry sky. the star a, when viewed attentively with the unaided eye, presents a somewhat misty appearance. in the year cysat directed a telescope to this star, and saw surrounding it a curious luminous haze, which proved to be the great nebula. ever since his time this object has been diligently studied by many astronomers, so that very many observations have been made of the great nebula, and even whole volumes have been written which treat of nothing else. any ordinary telescope will show the object to some extent, but the more powerful the telescope the more are the curious details revealed. [illustration: fig. .--the multiple star (th orionis) in the great nebula of orion.] in the first place, the object which we have denoted by a (th orionis, also called the trapezium of orion) is in itself the most striking multiple star in the whole heavens. it consists really of six stars, represented in the next diagram (fig. ). these points are so close together that their commingled rays cannot be distinguished without a telescope. four of them are, however, easily seen in quite small instruments, but the two smaller stars require telescopes of considerable power. and yet these stars are suns, comparable, it may be, with our sun in magnitude. it is not a little remarkable that this unrivalled group of six suns should be surrounded by the renowned nebula; the nebula or the multiple star would, either of them alone, be of exceptional interest, and here we have a combination of the two. it seems impossible to resist drawing the conclusion that the multiple star really lies in the nebula, and not merely along the same line of vision. it would, indeed, seem to be at variance with all probability to suppose that the presentation of these two exceptional objects in the same field of view was merely accidental. if the multiple star be really in the nebula, then this object affords evidence that in one case at all events the distance of a nebula is a quantity of the same magnitude as the distance of a star. this is unhappily almost the entire extent of our knowledge of the distances of the nebulæ from the earth. the great nebula of orion surrounds the multiple star, and extends out to a vast distance into the neighbouring space. the dotted circle drawn around the star marked a in fig. represents approximately the extent of the nebula, as seen in a moderately good telescope. the nebula is of a faint bluish colour, impossible to represent in a drawing. its brightness is much greater in some places than in others; the central parts are, generally speaking, the most brilliant, and the luminosity gradually fades away as the edge of the nebula is approached. in fact, we can hardly say that the nebula has any definite boundary, for with each increase of telescopic power faint new branches can be seen. there seems to be an empty space in the nebula immediately surrounding the multiple star, but this is merely an illusion, produced by the contrast of the brilliant light of the stars, as the spectroscopic examination of the nebula shows that the nebulous matter is continuous between the stars. the plate of the great nebula in orion which is here shown (plate xiv.) represents, in a reduced form, the elaborate drawing of this object, which has been made with the earl of rosse's great reflecting telescope at parsonstown.[ ] a telescopic view of the nebula shows two hundred stars or more, scattered over its surface. it is not necessary to suppose that these stars are immersed in the substance of the nebula as the multiple star appears to be; they may be either in front of it, or, less probably, behind it, so as to be projected on the same part of the sky. [illustration: plate xv. photograph of the nebula m andromedÆ exposure hours, enlarged times. taken by mr. isaac roberts, december, .] a considerable number of drawings of this unique object have been made by other astronomers. among these we must mention that executed by professor bond, in cambridge, mass., which possesses a faithfulness in detail that every student of this object is bound to acknowledge. of late years also successful attempts have been made to photograph the great nebula. the late professor draper was fortunate enough to obtain some admirable photographs. in england mr. common was the first to take most excellent photographs of the nebula, and superb photographs of the same object have also been obtained by dr. roberts and mr. w.e. wilson, which show a vast extension of the nebula into regions which it was not previously known to occupy. the great nebula in andromeda, which is faintly visible to the unaided eye, is shown in plate xv., which has been copied with permission from one of the astonishing photographs that dr. isaac roberts has obtained. two dark channels in the nebula cannot fail to be noticed, and the number of faint stars scattered over its surface is also a point to which attention may be drawn. to find this object we must look out for cassiopeia and the great square of pegasus, and then the nebula will be easily perceived in the position shown on p. . in the year a new star of the seventh magnitude suddenly appeared close to the brightest part of the nebula, and declined again to invisibility after the lapse of a few months. the nebula in lyra is the most conspicuous ring nebula in the heavens, but it is not to be supposed that it is the only member of this class. altogether, there are about a dozen of these objects. it seems difficult to form any adequate conception of the nature of such a body. it is, however, impossible to view the annular nebulæ without being, at all events, reminded of those elegant objects known as vortex rings. who has not noticed a graceful ring of steam which occasionally escapes from the funnel of a locomotive, and ascends high into the air, only dissolving some time after the steam not so specialised has disappeared? such vortex rings can be produced artificially by a cubical box, one open side of which is covered with canvas, while on the opposite side of the box is a circular hole. a tap on the canvas will cause a vortex ring to start from the hole; and if the box be filled with smoke, this ring will be visible for many feet of its path. it would certainly be far too much to assert that the annular nebulæ have any real analogy to vortex rings; but there is, at all events, no other object known to us with which they can be compared. the heavens contain a number of minute but brilliant objects known as the planetary nebulæ. they can only be described as globes of glowing bluish-coloured gas, often small enough to be mistaken for a star when viewed through a telescope. one of the most remarkable of these objects lies in the constellation draco, and can be found half-way between the pole star and the star g draconis. some of the more recently discovered planetary nebulæ are extremely small, and they have indeed only been distinguished from small stars by the spectroscope. it is also to be noticed that such objects are a little out of the stellar focus in the refracting telescope in consequence of their blue colour. this remark does not apply to a reflecting telescope, as this instrument conducts all the rays to a common focus. there are many other forms of nebulæ: there are long nebulous rays; there are the wondrous spirals which have been disclosed in lord rosse's great reflector; there are the double nebulæ. but all these various objects we must merely dismiss with this passing reference. there is a great difficulty in making pictorial representations of such nebulæ. most of them are very faint--so faint, indeed, that they can only be seen with close attention even in powerful instruments. in making drawings of these objects, therefore, it is impossible to avoid intensifying the fainter features if an intelligible picture is to be made. with this caution, however, we present plate xvi., which exhibits several of the more remarkable nebulæ as seen through lord rosse's great telescope. [illustration: fig. .--the nebula n.g.c., , . (_by e.e. barnard, lick observatory, september , ._)] the actual nature of the nebulæ offers a problem of the greatest interest, which naturally occupied the mind of the first assiduous observer of nebulæ, william herschel, for many years. at first he assumed all nebulæ to be nothing but dense aggregations of stars--a very natural conclusion for one who had so greatly advanced the optical power of telescopes, and was accustomed to see many objects which in a small telescope looked nebulous become "resolved" into stars when scrutinised with a telescope of large aperture. but in , when sir william huggins first directed a telescope armed with a spectroscope to one of the planetary nebulæ, it became evident that at least some nebulæ were really clouds of fiery mist and not star clusters. we shall in our next chapter deal with the spectra of the fixed stars, but we may here in anticipation remark that these spectra are continuous, generally showing the whole length of spectrum, from red to violet, as in the sun's spectrum, though with many and important differences as to the presence of dark and bright lines. a star cluster must, of course, give a similar spectrum, resulting from the superposition of the spectra of the single stars in the cluster. many nebulæ give a spectrum of this kind; for instance, the great nebula in andromeda. but it does not by any means follow from this that these objects are only clusters of ordinary stars, as a continuous spectrum may be produced not only by matter in the liquid or solid state, or by gases at high pressure, but also by gases at lower pressure but high temperature under certain conditions. a continuous spectrum in the case of a nebula, therefore, need not indicate that the nebula is a cluster of bodies comparable in size and general constitution with our sun. but if a spectrum of bright lines is given by a nebula, we can be certain that gases at low pressure are present in the object under examination. and this was precisely what sir william huggins discovered to be the case in many nebulæ. when he first decided to study the spectra of nebulæ, he selected for observation those objects known as planetary nebulæ--small, round, or slightly oval discs, generally without central condensation, and looking like ill-defined planets. the colour of their light, which often is blue tinted with green, is remarkable, since this is a colour very rare among single stars. the spectrum was found to be totally different to that of any star, consisting merely of three or four bright lines. the brightest one is situated in the bluish-green part of the spectrum, and was at first thought to be identical with a line of the spectrum of nitrogen, but subsequent more accurate measures have shown that neither this nor the second nebular line correspond to any dark line in the solar spectrum, nor can they be produced experimentally in the laboratory, and we are therefore unable to ascribe them to any known element. the third and fourth lines were at once seen to be identical with the two hydrogen lines which in the solar spectrum are named f and g. [illustration: plate e. nebulÆ in the pleiades. _from a photograph by dr. isaac roberts._] spectrum analysis has here, as on so many other occasions, rendered services which no telescope could ever have done. the spectra of nebulæ have, after huggins, been studied, both visually and photographically, by vogel, copeland, campbell, keeler, and others, and a great many very faint lines have been detected in addition to those four which an instrument of moderate dimensions shows. it is remarkable that the red c-line of hydrogen, ordinarily so bright, is either absent or excessively faint in the spectra of nebulæ, but experiments by frankland and lockyer have shown that under certain conditions of temperature and pressure the complicated spectrum of hydrogen is reduced to one green line, the f-line. it is, therefore, not surprising that the spectra of gaseous nebulæ are comparatively simple, as the probably low density of the gases in them and the faintness of these bodies would tend to reduce the spectra to a small number of lines. some gaseous nebulæ also show faint continuous spectra, the place of maximum brightness of which is not in the yellow (as in the solar spectrum), but about the green. it is probable that these continuous spectra are really an aggregate of very faint luminous lines. a list of all the nebulæ known to have a gaseous spectrum would now contain about eighty members. in addition to the planetary nebulæ, many large and more diffused nebulæ belong to this class, and this is also the case with the annular nebula in lyra and the great nebula of orion. it is needless to say that it is of special interest to find this grand object enrolled among the nebulæ of a gaseous nature. in this nebula copeland detected the wonderful d line of helium at a time when "helium" was a mere name, a hypothetical something, but which we now know to be an element very widely distributed through the universe. it has since been found in several other nebulæ. the ease with which the characteristic gaseous spectrum is recognised has suggested the idea of sweeping the sky with a spectroscope in order to pick up new planetary nebulæ, and a number of objects have actually been discovered by pickering and copeland in this manner, as also more recently by pickering by examining spectrum photographs of various regions of the sky. most of these new objects when seen through a telescope look like ordinary stars, and their real nature could never have been detected without the spectroscope. when we look up at the starry sky on a clear night, the stars seem at first sight to be very irregularly distributed over the heavens. here and there a few bright stars form characteristic groups, like orion or the great bear, while other equally large tracts are almost devoid of bright stars and only contain a few insignificant ones. if we take a binocular, or other small telescope, and sweep the sky with it, the result seems to be the same--now we come across spaces rich in stars; now we meet with comparatively empty places. but when we approach the zone of the milky way, we are struck with the rapid increase of the number of stars which fill the field of the telescope; and when we reach the milky way itself, the eye is almost unable to separate the single points of light, which are packed so closely together that they produce the appearance to the naked eye of a broad, but very irregular, band of dim light, which even a powerful telescope in some places can hardly resolve into stars. how are we to account for this remarkable arrangement of the stars? what is the reason of our seeing so few at the parts of the heavens farthest from the milky way, and so very many in or near that wonderful belt? the first attempt to give an answer to these questions was made by thomas wright, an instrument maker in london, in a book published in . he supposed the stars of our sidereal system to be distributed in a vast stratum of inconsiderable thickness compared with its length and breadth. if we had a big grindstone made of glass, in which had become uniformly imbedded a vast quantity of grains of sand or similar minute particles, and if we were able to place our eye somewhere near the centre of this grindstone, it is easy to see that we should see very few particles near the direction of the axle of the grindstone, but a great many if we looked towards any point of the circumference. this was wright's idea of the structure of the milky way, and he supposed the sun to be situated not very far from the centre of this stellar stratum. [illustration: plate f. ô centauri. _from a drawing in the publications of harvard college observatory._] if the milky way itself did not exist--and we had simply the fact to build on that the stars appeared to increase rapidly in number towards a certain circle (almost a great circle) spanning the heavens--then the disc theory might have a good deal in its favour. but the telescopic study of the milky way, and even more the marvellous photographs of its complicated structure produced by professor barnard, have given the death blow to the old theory, and have made it most reasonable to conclude that the milky way is really, and not only apparently, a mighty stream of stars encircling the heavens. we shall shortly mention a few facts which point in this direction. a mere glance is sufficient to show that the milky way is not a single belt of light; near the constellation aquila it separates into two branches with a fairly broad interval between them, and these branches do not meet again until they have proceeded far into the southern hemisphere. the disc theory had, in order to explain this, to assume that the stellar stratum was cleft in two nearly to the centre. but even if we grant this, how can we account for the numerous more or less dark holes in the milky way, the largest and most remarkable of which is the so-called "coal sack" in the southern hemisphere? obviously we should have to assume the existence of a number of tunnels, drilled through the disc-like stratum, and by some strange sympathy all directed towards the spot where our solar system is situated. and the many small arms which stretch out from the milky way would have to be either planes seen edgeways or the convexities of curved surfaces viewed tangentially. the improbability of these various assumptions is very great. but evidence is not wanting that the relatively bright stars are crowded together along the same zone where the excessively faint ones are so closely packed. the late mr. proctor plotted all the stars which occur in argelander's great atlas of the northern hemisphere, , in number, on a single chart, and though these stars are all above the tenth magnitude, and thus superior in brightness to that innumerable host of stars of which the individual members are more or less lost in the galactic zone, and on the hypothesis of uniform distribution ought to be relatively near to us, the chart shows distinctly the whole course of the milky way by the clustering of these stars. this disposes sufficiently of the idea that the milky way is nothing but a disc-like stratum seen projected on the heavenly sphere; after this it is hardly necessary to examine professor barnard's photographs and see how fairly bright and very faint regions alternate without any attempt at regularity, in order to become convinced that the milky way is more probably a stream of stars clustered together, a stream or ring of incredibly enormous dimensions, inside which our solar system happens to be situated. but it must be admitted that it is premature to attempt to find the actual figure of this stream or to determine the relative distance of the various portions of it. [illustration: plate xvi. nebulÆ observed with lord rosse's great telescope.] chapter xxiii. the physical nature of the stars. star spectroscopes--classification of stellar spectra--type i., with very few absorption lines--type ii., like the sun--type iii., with strongly marked dark bands--distribution of these classes over the heavens--motion in the line of sight--orbital motion discovered with the spectroscope: new class of binaries--spectra of temporary stars--nature of these bodies. we have frequently in the previous chapters had occasion to refer to the revelations of the spectroscope, which form an important chapter in the history of modern science. by its aid a mighty stride has been taken in our attempt to comprehend the physical constitution of the sun. in the present chapter we propose to give an account of what the spectroscope tells us about the physical constitution of the fixed stars. quite a new phase of astronomy is here opened up. every improvement in telescopes revealed fainter and fainter objects, but all the telescopes in the world could not answer the question as to whether iron and other elements are to be found in the stars. the ordinary star is a mighty glowing globe, hotter than a bessemer converter or a siemens furnace; if iron is in the star, it must be not only white-hot and molten, but actually converted into vapour. but the vapour of iron is not visible in the telescope. how would you recognise it? how would you know if it commingled with the vapour of many other metals or other substances? it is, in truth, a delicate piece of analysis to discriminate iron in the glowing atmosphere of a star. but the spectroscope is adequate to the task, and it renders its analysis with an amount of evidence that is absolutely convincing. that the spectra of the moon and planets are practically nothing but faint reproductions of the spectrum of the sun was discovered by the great german optician fraunhofer about the year . by placing a prism in front of the object glass of a small theodolite (an instrument used for geodetic measurements) he was able to ascertain that venus and mars showed the same spectrum as the sun, while sirius gave a very different one. this important observation encouraged him to procure better instrumental means with which to continue the work, and he succeeded in distinguishing the chief characteristics of the various types of stellar spectra. the form of instrument which fraunhofer adopted for this work, in which the prism was placed outside the object glass of the telescope, has not been much used until within the last few years, owing to the difficulty of obtaining prisms of large dimensions (for it is obvious that the prism ought to be as large as the object glass if the full power of the latter is to be made use of), but this is the simplest form of spectroscope for observing spectra of objects of no sensible angular diameter, like the fixed stars. the parallel rays from the stars are dispersed by the prism into a spectrum, and this is viewed by means of the telescope. but as the image of the star in the telescope is nothing but a luminous point, its spectrum will be merely a line in which it would not be possible to distinguish any lines crossing it laterally such as those we see in the spectrum of the sun. a cylindrical lens is, therefore, placed before the eye-piece of the telescope, and as this has the effect of turning a point into a line and a line into a band, the narrow spectrum of the star is thereby broadened out into a luminous band in which we can distinguish any details that exist. in other forms of stellar spectroscope we require a slit which must be placed in the focus of the object glass, and the general arrangement is similar to that which we have described in the chapter on the sun, except that a cylindrical lens is required. the study of the spectra of the fixed stars made hardly any progress until the principles of spectrum analysis had been established by kirchhoff in . when the dark lines in the solar spectrum had been properly interpreted, it was at once evident that science had opened wide the gates of a new territory for human exploration, of the very existence of which hardly anyone had been aware up to that time. we have seen to what splendid triumphs the study of the sun has led the investigators in this field, and we have seen how very valuable results have been obtained by the new method when applied to observations of comets and nebulæ. we shall now give some account of what has been learned with regard to the constitution of the fixed stars by the researches which were inaugurated by sir william huggins and continued and developed by him, as well as by secchi, vogel, pickering, lockyer, dunér, scheiner and others. here, as in the other modern branches of astronomy, photography has played a most important part, not only because photographed spectra of stars extend much farther at the violet end than the observer can follow them with his eye, but also because the positions of the lines can be very accurately measured on the photographs. the first observer who reduced the apparently chaotic diversity of stellar spectra to order was secchi, who showed that they might all be grouped according to four types. within the last thirty years, however, so many modifications of the various types have been found that it has become necessary to subdivide secchi's types, and most observers now make use of vogel's classification, which we shall also for convenience adopt in this chapter. _type i._--in the spectra of stars of this class the metallic lines, which are so very numerous and conspicuous in the sun's violet spectrum, are very faint and thin, or quite invisible, and the blue and white parts are very intensely bright. vogel subdivides the class into three groups. in the first (i.a) the hydrogen lines are present, and are remarkably broad and intense; sirius, vega, and regulus are examples of this group. the great breadth of the lines probably indicates that these stars are surrounded by hydrogen atmospheres of great dimensions. it is generally acknowledged that stars of this group must be the hottest of all, and support is lent to this view by the appearance in their spectra of a certain magnesium line, which, as sir norman lockyer showed many years ago, by laboratory experiments, does not appear in the ordinary spectrum of magnesium, but is indicative of the presence of the substance at a very high temperature. in the spectra of stars of group i.b the hydrogen lines and the few metallic lines are of equal breadth, and the magnesium line just mentioned is the strongest of all. rigel and several other bright stars in orion belong to this group, and it is remarkable that helium is present at least in some of these stars, so that (as professor keeler remarks) the spectrum of rigel may almost be regarded as the nebular spectrum reversed (lines dark instead of bright), except that the two chief nebular lines are not reversed in the star. this fact will doubtless eventually be of great importance to our understanding the successive development of a star from a nebula; and a star like rigel is no doubt also of very high temperature. this is probably not the case with stars of the third subdivision of type i. (i.c), the spectra of which are distinguished by the presence of bright hydrogen lines and the bright helium line d . among the stars having this very remarkable kind of spectrum is a very interesting variable star in the constellation lyra (b) and the star known as g cassiopeiæ, both of which have been assiduously observed, their spectra possessing numerous peculiarities which render an explanation of the physical constitution of the stars of this subdivision a very difficult matter. passing to _type ii._, we find spectra in which the metallic lines are strong. the more refrangible end of the spectrum is fainter than in the previous class, and absorption bands are sometimes found towards the red end. in its first subdivision (ii.a) are contained spectra with a large number of strong and well-defined lines due to metals, the hydrogen lines being also well seen, though they are not specially conspicuous. among the very numerous stars of this group are capella, aldebaran, arcturus, pollux, etc. the spectra of these stars are in fact practically identical with the spectrum of our own sun, as shown, for instance, by dr. scheiner, of the potsdam astrophysical observatory, who has measured several hundred lines on photographs of the spectrum of capella, and found a very close agreement between these lines and corresponding ones in the solar spectrum. we can hardly doubt that the physical constitution of these stars is very similar to that of our sun. this cannot be the case with the stars of the second subdivision (ii.b), the spectra of which are very complex, each consisting of a continuous spectrum crossed by numerous dark lines, on which is superposed a second spectrum of bright lines. upwards of seventy stars are known to possess this extraordinary spectrum, the only bright one among them being a star of the third magnitude in the southern constellation argus. here again we have hydrogen and helium represented by bright lines, while the origin of the remaining bright lines is doubtful. with regard to the physical constitution of the stars of this group it is very difficult to come to a definite conclusion, but it would seem not unlikely that we have here to do with stars which are not only surrounded by an atmosphere of lower temperature, causing the dark lines, but which, outside of that, have an enormous envelope of hydrogen and other gases. in one star at least of this group professor campbell, of the lick observatory, has seen the f line as a long line extending a very appreciable distance on each side of the continuous spectrum, and with an open slit it was seen as a large circular disc about six seconds in diameter; two other principal hydrogen lines showed the same appearance. as far as this observation goes, the existence of an extensive gaseous envelope surrounding the star seems to be indicated. _type iii._ contains comparatively few stars, and the spectra are characterised by numerous dark bands in addition to dark lines, while the more refrangible parts are very faint, for which reason the stars are more or less red in colour. this class has two strongly marked subdivisions. in the first (iii.a) the principal absorption lines coincide with similar ones in the solar spectrum, but with great differences as to intensity, many lines being much stronger in these stars than in the sun, while many new lines also appear. these dissimilarities are, however, of less importance than the peculiar absorption bands in the red, yellow, and green parts of the spectrum, overlying the metallic lines, and being sharply defined on the side towards the violet and shading off gradually towards the red end of the spectrum. bands of this kind belong to chemical combinations, and this appears to show that somewhere in the atmospheres of these distant suns the temperature is low enough to allow stable chemical combinations to be formed. the most important star of this kind is betelgeuze or a orionis, the red star of the first magnitude in the shoulder of orion; but it is of special importance to note that many variable stars of long period have spectra of type iii.a. sir norman lockyer predicted in that bright lines, probably of hydrogen, would eventually be found to appear at the maximum of brightness, when the smaller swarm is supposed to pass through the larger one, and this was soon afterwards confirmed by the announcement that professor pickering had found a number of hydrogen lines bright on photographs, obtained at harvard college observatory, of the spectrum of the remarkable variable, mira ceti, at the time of maximum. professor pickering has since then reported that bright lines have been found on the plates of forty-one previously known variables of this class, and that more than twenty other stars have been detected as variables by this peculiarity of their spectrum; that is, bright lines being seen in them suggested that the stars were variable, and further photometric investigations corroborated the fact. the second subdivision (iii.b) contains only comparatively faint stars, of which none exceed the fifth magnitude, and is limited to a small number of red stars. the strongly marked bands in their spectra are sharply defined and dark on the red side, while they fade away gradually towards the violet, exactly the reverse of what we see in the spectra of iii.a. these bands appear to arise from the absorption due to hydrocarbon vapours present in the atmospheres of these stars; but there are also some lines visible which indicate the presence of metallic vapours, sodium being certainly among these. there can be little doubt that these stars represent the last stage in the life of a sun, when it has cooled down considerably and is not very far from actual extinction, owing to the increasing absorption of its remaining light in the atmosphere surrounding it. the method employed for the spectroscopic determination of the motion of a star in the line of sight is the same as the method we have described in the chapter on the sun. the position of a certain line in the spectrum of a star is compared with the position of the corresponding bright line of an element in an artificially produced spectrum, and in this manner a displacement of the stellar line either towards the violet (indicating that the star is approaching us) or towards the red (indicating that it is receding) may be detected. the earliest attempt of this sort was made in by sir william huggins, who compared the f line in the spectrum of sirius with the same line of the spectrum of hydrogen contained in a vacuum tube reflected into the field of his astronomical spectroscope, so that the two spectra appeared side by side. the work thus commenced and continued by him was afterwards taken up at the greenwich observatory; but the results obtained by these direct observations were never satisfactory, as remarkable discrepancies appeared between the values obtained by different observers, and even by the same observer on different nights. this is not to be wondered at when we bear in mind that the velocity of light is so enormous compared with any velocity with which a heavenly body may travel, that the change of wave length resulting from the latter motion can only be a very minute one, difficult to perceive, and still more difficult to measure. but since photography was first made use of for these investigations by dr. vogel, of potsdam, much more accordant and reliable results have been obtained, though even now extreme care is required to avoid systematic errors. to give some idea of the results obtainable, we present in the following table the values of the velocity per second of a number of stars observed in and by mr. h.f. newall with the bruce spectrograph attached to the great -inch newall refractor of the cambridge observatory, and we have added the values found at potsdam by vogel and scheiner. the results are expressed in kilometres ( km. = · english mile). the sign + means that the star is receding from us,-that it is approaching. newall. vogel. scheiner. aldebaran + · + · + · betelgeuze + · + · + · procyon - · - · - · pollux - · + · + · g leonis - · - · - · arcturus - · - · - · these results have been corrected for the earth's orbital motion round the sun, but not for the sun's motion through space, as the amount of the latter is practically unknown, or at least very uncertain; so that the above figures really represent the velocity per second of the various stars relative to the sun. we may add that the direction and velocity of the sun's motion may eventually be ascertained from spectroscopic measures of a great number of stars, and it seems likely that the sun's velocity will be much more accurately found in this way than by the older method of combining proper motions of stars with speculations as to the average distances of the various classes of stars. this has already been attempted by dr. kempf, who from the potsdam spectrographic observations found the sun's velocity to be · kilometres, or · miles per second, a result which is probably not far from the truth. but the spectra of the fixed stars can also tell us something about orbital motion in these extremely distant systems. if one star revolved round another in a plane passing through the sun, it must on one side of the orbit move straight towards us and on the other side move straight away from us, while it will not alter its distance from us while it is passing in front of, or behind, the central body. if we therefore find from the spectroscopic observations that a star is alternately moving towards and away from the earth in a certain period, there can be no doubt that this star is travelling round some unseen body (or, rather, round the centre of gravity of both) in the period indicated by the shifting of the spectral lines. in chapter xix. we mentioned the variable star algol in the constellation perseus, which is one of a class of variable stars distinguished by the fact that for the greater part of the period they remain of unaltered brightness, while for a very short time they become considerably fainter. that this was caused by some sort of an eclipse--or, in other words, by the periodic passage of a dark body in front of the star, hiding more or less of the latter from us--was the simplest possible hypothesis, and it had already years ago been generally accepted. but it was not possible to prove that this was the true explanation of the periodicity of stars like algol until professor vogel, from the spectroscopic observations made at potsdam, found that before every minimum algol is receding from the sun, while it is approaching us after the minimum. assuming the orbit to be circular, the velocity of algol was found to be twenty-six miles per second. from this and the length of the period ( d. h. m. s.) and the time of obscuration it was easy to compute the size of the orbit and the actual dimensions of the two bodies. it was even possible to go a step further and to calculate from the orbital velocities the masses of the two bodies,[ ] assuming them to be of equal density--an assumption which is no doubt very uncertain. the following are the approximate elements of the algol system found by vogel:-- diameter of algol , , miles. diameter of companion , miles. distance between their centres , , miles. orbital velocity of algol miles per sec. orbital velocity of companion miles per sec. mass of algol / of sun's mass. mass of companion / of sun's mass. the period of algol has been gradually decreasing during the last century (by six or seven seconds), but whether this is caused by the motion of the pair round a third and very much more distant body, as suggested by mr. chandler, has still to be found out. we have already mentioned that in order to produce eclipses, and thereby variations of light, it is necessary that the line of sight should lie nearly in the plane of the orbit. it is also essential that there should be a considerable difference of brightness between the two bodies. these conditions must be fulfilled in the fifteen variable stars of the algol class now known; but according to the theory of probability, there must be many more binary systems like that of algol where these conditions are not fulfilled, and in those cases no variations will occur in the stars' brightness. of course, we know many cases of a luminous star travelling round another, but there must also be cases of a large companion travelling round another at so small a distance that our telescopes are unable to "divide" the double star. this has actually been discovered by means of the spectroscope. if we suppose an extremely close double star to be examined with the spectroscope, the spectra of the two components will be superposed, and we shall not be aware that we really see two different spectra. but during the revolution of the two bodies round their common centre of gravity there must periodically come a time when one body is moving towards us and the other moving from us, and consequently the lines in the spectrum of the former will be subject to a minute, relative shift towards the violet end of the spectrum, and those of the other to a minute shift towards the red. those lines which are common to the two spectra will therefore periodically become double. a discovery of this sort was first made in by professor pickering from photographs of the spectrum of mizar, or z ursa majoris, the larger component of the well-known double star in the tail of the great bear. certain of the lines were found to be double at intervals of fifty-two days. the maximum separation of the two components of each line corresponds to a relative velocity of one star as compared with the other of about a hundred miles per second, but subsequent observations have shown the case to be very complicated, either with a very eccentric elliptic orbit or possibly owing to the presence of a third body. the harvard college photographs also showed periodic duplicity of lines in the star b aurigæ, the period being remarkably short, only three days and twenty-three hours and thirty-seven minutes. in vogel found, from photographs of the spectrum of spica, the first magnitude star in virgo, that this star alternately recedes from and approaches to the solar system, the period being four days. certain other "spectroscopic binaries" have since then been found, notably one component of castor, with a period of three days, found by m. belopolsky, and a star in the constellation scorpio, with a period of only thirty-four hours, detected on the harvard spectrograms. quite recently mr. h.f. newall, at cambridge, and mr. campbell, of the lick observatory, have shown that a aurigæ, or capella, consists of a sun-like star and a procyon-like star, revolving in days. at first sight there is something very startling in the idea of two suns circling round each other, separated by an interval which, in comparison with their diameters, is only a very small one. in the algol system, for instance, we have two bodies, one the size of our own sun and the other slightly larger, moving round their common centre of gravity in less than three days, and at a distance between their surfaces equal to only twice the diameter of the larger one. again, in the system of spica we have two great suns swinging round each other in only four days, at a distance equal to that between saturn and his sixth satellite. but although we have at present nothing analogous to this in our solar system, it can be proved mathematically that it is perfectly possible for a system of this kind to preserve its stability, if not for ever, at any rate for ages, and we shall see in our last chapter that there was in all probability a time when the earth and the moon formed a peculiar system of two bodies revolving rapidly at a very small distance compared to the diameters of the bodies. it is possible that we have a more complicated system in the star known as b lyræ. this is a variable star of great interest, having a period of twelve days and twenty-two hours, in which time it rises from magnitude - / to a little above - / , sinks nearly to the fourth magnitude, rises again to fully - / , and finally falls to magnitude - / . in professor pickering discovered that the bright lines in the spectrum of this star changed their position from time to time, appearing now on one side, now on the other side of corresponding dark lines. obviously these bright lines change their wave length, the light-giving source alternately receding from and approaching to the earth, and the former appeared to be the case during one-half of the period of variation of the star's light, the latter during the other half. the spectrum of this star has been further examined by belopolsky and others, who have found that the lines are apparently double, but that one of the components either disappears or becomes very narrow from time to time. on the assumption that these lines were really single (the apparent duplicity resulting from the superposition of a dark line), belopolsky determined the amount of their displacement by measuring the distances from the two edges of a line of hydrogen (f) to the artificial hydrogen line produced by gas glowing in a tube and photographed along with the star-spectrum. assuming the alternate approach and recession to be caused by orbital revolution, belopolsky found that the body emitting the light of the bright lines moved with an orbital velocity of forty-one miles. he succeeded in in observing the displacement of a dark line due to magnesium, and found that the body emitting it was also moving in an orbit, but while the velocities given by the bright f line are positive after the principal minimum of the star's light, those given by the dark line are negative. therefore, during the principal minimum it is a star giving the dark line which is eclipsed, and during the secondary minimum another star giving the bright line is eclipsed. this wonderful variable will, however, require more observatioêns before the problem of its constitution is finally solved, and the same may be said of several variable stars, _e.g._ ê aquilæ and d cephei, in which a want of harmony has been found between the changes of velocity and the fluctuations of the light. there are some striking analogies between the complicated spectrum of b lyræ and the spectra of temporary stars. the first "new star" which could be spectroscopically examined was that which appeared in corona borealis in , and which was studied by sir w. huggins. it showed a continuous spectrum with dark absorption lines, and also the bright lines of hydrogen; practically the same spectrum as the stars of type ii.b. this was also the case with schmidt's star of , which showed the helium line (d ) and the principal nebula line in addition to the lines of hydrogen; but in the autumn of , when the star had fallen to the tenth magnitude, dr. copeland was surprised to find that only one line was visible, the principal nebula line, in which almost the whole light of the star was concentrated, the continuous spectrum being hardly traceable. it seemed, in fact, that the star had been transformed into a planetary nebula, but later the spectrum seems to have lost this peculiar monochromatic character, the nebula line having disappeared and a faint continuous spectrum alone being visible, which is also the case with the star of since it sank down to the tenth magnitude. a continuous spectrum was all that could be seen of the new star which broke out in the nebula of andromeda in , much the same as the spectrum of the nebula itself. when the new star in auriga was announced, in february, , astronomers were better prepared to observe it spectroscopically, as it was now possible by means of photography to study the ultra-violet part of the spectrum which to the eye is invisible. the visible spectrum was very like that of nova cygni of , but when the wave-lengths of all the bright lines seen and photographed at the lick observatory and at potsdam were measured, a strong resemblance to the bright line spectrum of the chromosphere of the sun became very evident. the hydrogen lines were very conspicuous, while the iron lines were very numerous, and calcium and magnesium were also represented. the most remarkable revelation made by the photographs was, however, that the bright lines were in many cases accompanied, on the side next the violet, by broad dark bands, while both bright and dark lines were of a composite character. many of the dark lines had a thin bright line superposed in the middle, while on the other hand many of the bright lines had two or three points maxima of brightness. the results of the measures of motion in the line of sight were of special importance. they showed that the source of light, whence came the thin bright lines within the dark ones, was travelling towards the sun at the enormous rate of miles per second, and if the bright lines were actual "reversals" of the dark ones, then the source of the absorption spectrum must have been endowed with much the same velocity. on the other hand, if the two or three maxima of brightness in the bright lines really represent two or three separate bodies giving bright lines, the measures indicate that the principal one was almost at rest as regards the sun, while the others were receding from us at the extraordinary rates of and miles per second. and as if this were not sufficiently puzzling, the star on its revival in august, , as a tenth magnitude star had a totally different spectrum, showing nothing but a number of the bright lines belonging to planetary nebulæ! it is possible that the principal ones of these were really present in the spectrum from the first, but that their wave lengths had been different owing to change of the motion in the line of sight, so that the nebula lines seen in the autumn were identical with others seen in the spring at slightly different places. subsequent observations of these nebula lines seemed to point to a motion of the nova towards the solar system (of about miles per second) which gradually diminished. but although we are obliged to confess our inability to say for certain why a temporary star blazes up so suddenly, we have every cause to think that these strange bodies will by degrees tell us a great deal about the constitution of the fixed stars. the great variety of spectra which we see in the starry universe, nebula spectra with bright lines, stellar spectra of the same general character, others with broad absorption bands, or numerous dark lines like our sun, or a few absorption lines only--all this shows us the universe as teeming with bodies in various stages of evolution. we shall have a few more words to say on this matter when we come to consider the astronomical significance of heat; but we have reached a point where man's intellect can hardly keep pace with the development of our instrumental resources, and where our imagination stands bewildered when we endeavour to systematise the knowledge we have gained. that great caution will have to be exercised in the interpretation of the observed phenomena is evident from the recent experience of professor rowland, of baltimore, from which we learn that spectral lines are not only widened by increased pressure of the light-giving vapour, but that they may be bodily shifted thereby. dr. zeeman's discovery, that a line from a source placed in a strong magnetic field may be both widened, broadened, and doubled, will also increase our difficulties in the interpretation of these obscure phenomena. chapter xxiv. the precession and nutation of the earth's axis. the pole is not a fixed point--its effect on the apparent places of the stars--the illustration of the peg-top--the disturbing force which acts on the earth--attraction of the sun on a globe--the protuberance at the equator--the attraction of the protuberance by the sun and by the moon produces precession--the efficiency of the precessional agent varies inversely as the cube of the distance--the relative efficiency of the sun and the moon--how the pole of the earth's axis revolves round the pole of the ecliptic--variation of latitude. the position of the pole of the heavens is most conveniently indicated by the bright star known as the pole star, which lies in its immediate vicinity. around this pole the whole heavens appear to rotate once in a sidereal day; and we have hitherto always referred to the pole as though it were a fixed point in the heavens. this language is sufficiently correct when we embrace only a moderate period of time in our review. it is no doubt true that the pole lies near the pole star at the present time. it did so during the lives of the last generation, and it will do so during the lives of the next generation. all this time, however, the pole is steadily moving in the heavens, so that the time will at length come when the pole will have departed a long way from the present pole star. this movement is incessant. it can be easily detected and measured by the instruments in our observatories, and astronomers are familiar with the fact that in all their calculations it is necessary to hold special account of this movement of the pole. it produces an apparent change in the position of a star, which is known by the term "precession." [illustration: fig. .] the movement of the pole is very clearly shown in the accompanying figure (fig. ), for which i am indebted to the kindness of the late professor c. piazzi smyth. the circle shows the track along which the pole moves among the stars. the centre of the circle in the constellation of draco is the pole of the ecliptic. a complete journey of the pole occupies the considerable period of about , years. the drawing shows the position of the pole at the several dates from b.c. to a.d. a glance at this map brings prominently before us how casual is the proximity of the pole to the pole star. at present, indeed, the distance of the two is actually lessening, but afterwards the distance will increase until, when half of the revolution has been accomplished, the pole will be at a distance of twice the radius of the circle from the pole star. it will then happen that the pole will be near the bright star vega or a lyræ, so that our successors some , years hence may make use of vega for many of the purposes for which the pole star is at present employed! looking back into past ages, we see that some , or , years b.c. the star a draconis was suitably placed to serve as the pole star, when b and d of the great bear served as pointers. it need hardly be added, that since the birth of accurate astronomy the course of the pole has only been observed over a very small part of the mighty circle. we are not, however, entitled to doubt that the motion of the pole will continue to pursue the same path. this will be made abundantly clear when we proceed to render an explanation of this very interesting phenomenon. the north pole of the heavens is the point of the celestial sphere towards which the northern end of the axis about which the earth rotates is directed. it therefore follows that this axis must be constantly changing its position. the character of the movement of the earth, so far as its rotation is concerned, may be illustrated by a very common toy with which every boy is familiar. when a peg-top is set spinning, it has, of course, a very rapid rotation around its axis; but besides this rotation there is usually another motion, whereby the axis of the peg-top does not remain in a constant direction, but moves in a conical path around the vertical line. the adjoining figure (fig. ) gives a view of the peg-top. it is, of course, rotating with great rapidity around its axis, while the axis itself revolves around the vertical line with a very deliberate motion. if we could imagine a vast peg-top which rotated on its axis once a day, and if that axis were inclined at an angle of twenty-three and a half degrees to the vertical, and if the slow conical motion of the axis were such that the revolution of the axis were completed in about , years, then the movements would resemble those actually made by the earth. the illustration of the peg-top comes, indeed, very close to the actual phenomenon of precession. in each case the rotation about the axis is far more rapid than that of the revolution of the axis itself; in each case also the slow movement is due to an external interference. looking at the figure of the peg-top (fig. ) we may ask the question, why does it not fall down? the obvious effect of gravity would seem to say that it is impossible for the peg-top to be in the position shown in the figure. yet everybody knows that this is possible so long as the top is spinning. if the top were not spinning, it would, of course, fall. it therefore follows that the effect of the rapid rotation of the top so modifies the effect of gravitation that the latter, instead of producing its apparently obvious consequence, causes the slow conical motion of the axis of rotation. this is, no doubt, a dynamical question of some difficulty, but it is easy to verify experimentally that it is the case. if a top be constructed so that the point about which it is spinning shall coincide with the centre of gravity, then there is no effect of gravitation on the top, and there is no conical motion perceived. [illustration: fig. .--illustration of the motion of precession.] if the earth were subject to no external interference, then the direction of the axis about which it rotates must remain for ever constant; but as the direction of the axis does not remain constant, it is necessary to seek for a disturbing force adequate to the production of the phenomena which are observed. we have invariably found that the dynamical phenomena of astronomy can be accounted for by the law of universal gravitation. it is therefore natural to enquire how far gravitation will render an account of the phenomenon of precession; and to put the matter in its simplest form, let us consider the effect which a distant attracting body can have upon the rotation of the earth. to answer this question, it becomes necessary to define precisely what we mean by the earth; and as for most purposes of astronomy we regard the earth as a spherical globe, we shall commence with this assumption. it seems also certain that the interior of the earth is, on the whole, heavier than the outer portions. it is therefore reasonable to assume that the density increases as we descend; nor is there any sufficient ground for thinking that the earth is much heavier in one part than at any other part equally remote from the centre. it is therefore usual in such calculations to assume that the earth is formed of concentric spherical shells, each one of which is of uniform density; while the density decreases from each shell to the one exterior thereto. a globe of this constitution being submitted to the attraction of some external body, let us examine the effects which that external body can produce. suppose, for instance, the sun attracts a globe of this character, what movements will be the result? the first and most obvious result is that which we have already so frequently discussed, and which is expressed by kepler's laws: the attraction will compel the earth to revolve around the sun in an elliptic path, of which the sun is in the focus. with this movement we are, however, not at this moment concerned. we must enquire how far the sun's attraction can modify the earth's rotation around its axis. it can be demonstrated that the attraction of the sun would be powerless to derange the rotation of the earth so constituted. this is a result which can be formally proved by mathematical calculation. it is, however, sufficiently obvious that the force of attraction of any distant point on a symmetrical globe must pass through the centre of that globe: and as the sun is only an enormous aggregate of attracting points, it can only produce a corresponding multitude of attractive forces; each of these forces passes through the centre of the earth, and consequently the resultant force which expresses the joint result of all the individual forces must also be directed through the centre of the earth. a force of this character, whatever other potent influence it may have, will be powerless to affect the rotation of the earth. if the earth be rotating on an axis, the direction of that axis would be invariably preserved; so that as the earth revolves around the sun, it would still continue to rotate around an axis which always remained parallel to itself. nor would the attraction of the earth by any other body prove more efficacious than that of the sun. if the earth really were the symmetrical globe we have supposed, then the attraction of the sun and moon, and even the influence of all the planets as well, would never be competent to make the earth's axis of rotation swerve for a single second from its original direction. we have thus narrowed very closely the search for the cause of the "precession." if the earth were a perfect sphere, precession would be inexplicable. we are therefore forced to seek for an explanation of precession in the fact that the earth is not a perfect sphere. this we have already demonstrated to be the case. we have shown that the equatorial axis of the earth is longer than the polar axis, so that there is a protuberant zone girdling the equator. the attraction of external bodies is able to grasp this protuberance, and thereby force the earth's axis of rotation to change its direction. there are only two bodies in the universe which sensibly contribute to the precessional movement of the earth's axis: these bodies are the sun and the moon. the shares in which the labour is borne by the sun and the moon are not what might have been expected from a hasty view of the subject. this is a point on which it will be desirable to dwell, as it illustrates a point in the theory of gravitation which is of very considerable importance. the law of gravitation asserts that the intensity of the attraction which a body can exercise is directly proportional to the mass of that body, and inversely proportional to the square of its distance from the attracted point. we can thus compare the attraction exerted upon the earth by the sun and by the moon. the mass of the sun exceeds the mass of the moon in the proportion of about , , to . on the other hand, the moon is at a distance which, on an average, is about one- th part of that of the sun. it is thus an easy calculation to show that the efficiency of the sun's attraction on the earth is about times as great as the attraction of the moon. hence it is, of course, that the earth obeys the supremely important attraction of the sun, and pursues an elliptic path around the sun, bearing the moon as an appendage. but when we come to that particular effect of attraction which is competent to produce precession, we find that the law by which the efficiency of the attracting body is computed assumes a different form. the measure of efficiency is, in this case, to be found by taking the mass of the body and dividing it by the _cube_ of the distance. the complete demonstration of this statement must be sought in the formulæ of mathematics, and cannot be introduced into these pages; we may, however, adduce one consideration which will enable the reader in some degree to understand the principle, though without pretending to be a demonstration of its accuracy. it will be obvious that the nearer the disturbing body approaches to the earth the greater is the _leverage_ (if we may use the expression) which is afforded by the protuberance at the equator. the efficiency of a given force will, therefore, on this account alone, increase in the inverse proportion of the distance. the actual intensity of the force itself augments in the inverse square of the distance, and hence the capacity of the attracting body for producing precession will, for a double reason, increase when the distance decreases. suppose, for example, that the disturbing body is brought to half its original distance from the disturbed body, the leverage is by this means doubled, while the actual intensity of the force is at the same time quadrupled according to the law of gravitation. it will follow that the effect produced in the latter case must be eight times as great as in the former case. and this is merely equivalent to the statement that the precession-producing capacity of a body varies inversely as the cube of the distance. it is this consideration which gives to the moon an importance as a precession-producing agent to which its mere attractive capacity would not have entitled it. even though the mass of the sun be , , times as great as the mass of the moon, yet when this number is divided by the cube of the relative value of the distances of the bodies ( ), it is seen that the efficiency of the moon is more than twice as great as that of the sun. in other words, we may say that one-third of the movement of precession is due to the sun, and two-thirds to the moon. for the study of the joint precessional effect due to the sun and the moon acting simultaneously, it will be advantageous to consider the effect produced by the two bodies separately; and as the case of the sun is the simpler of the two, we shall take it first. as the earth travels in its annual path around the sun, the axis of the earth is directed to a point in the heavens which is - / ° from the pole of the ecliptic. the precessional effect of the sun is to cause this point--the pole of the earth--to revolve, always preserving the same angular distance from the pole of the ecliptic; and thus we have a motion of the type represented in the diagram. as the ecliptic occupies a position which for our present purpose we may regard as fixed in space, it follows that the pole of the ecliptic is a fixed point on the surface of the heavens; so that the path of the pole of the earth must be a small circle in the heavens, fixed in its position relatively to the surrounding stars. in this we find a motion strictly analogous to that of the peg-top. it is the gravitation of the earth acting upon the peg-top which forces it into the conical motion. the immediate effect of the gravitation is so modified by the rapid rotation of the top, that, in obedience to a profound dynamical principle, the axis of the top revolves in a cone rather than fall down, as it would do were the top not spinning. in a similar manner the immediate effect of the sun's attraction on the protuberance at the equator would be to bring the pole of the earth's axis towards the pole of the ecliptic, but the rapid rotation of the earth modifies this into the conical movement of precession. the circumstances with regard to the moon are much more complicated. the moon describes a certain orbit around the earth; that orbit lies in a certain plane, and that plane has, of course, a certain pole on the celestial sphere. the precessional effect of the moon would accordingly tend to make the pole of the earth's axis describe a circle around that point in the heavens which is the pole of the moon's orbit. this point is about ° from the pole of the ecliptic. the pole of the earth is therefore solicited by two different movements--one a revolution around the pole of the ecliptic, the other a revolution about another point ° distant, which is the pole of the moon's orbit. it would thus seem that the earth's pole should make a certain composite movement due to the two separate movements. this is really the case, but there is a point to be very carefully attended to, which at first seems almost paradoxical. we have shown how the potency of the moon as a precessional agent exceeds that of the sun, and therefore it might be thought that the composite movement of the earth's pole would conform more nearly to a rotation around the pole of the plane of the moon's orbit than to a rotation around the pole of the ecliptic; but this is not the case. the precessional movement is represented by a revolution around the pole of the ecliptic, as is shown in the figure. here lies the germ of one of those exquisite astronomical discoveries which delight us by illustrating some of the most subtle phenomena of nature. the plane in which the moon revolves does not occupy a constant position. we are not here specially concerned with the causes of this change in the plane of the moon's orbit, but the character of the movement must be enunciated. the inclination of this plane to the ecliptic is about °, and this inclination does not vary (except within very narrow limits); but the line of intersection of the two planes does vary, and, in fact, varies so quickly that it completes a revolution in about - / years. this movement of the plane of the moon's orbit necessitates a corresponding change in the position of its pole. we thus see that the pole of the moon's orbit must be actually revolving around the pole of the ecliptic, always remaining at the same distance of °, and completing its revolution in - / years. it will, therefore, be obvious that there is a profound difference between the precessional effect of the sun and of the moon in their action on the earth. the sun invites the earth's pole to describe a circle around a fixed centre; the moon invites the earth's pole to describe a circle around a centre which is itself in constant motion. it fortunately happens that the circumstances of the case are such as to reduce considerably the complexity of the problem. the movement of the moon's plane, only occupying about - / years, is a very rapid motion compared with the whole precessional movement, which occupies about , years. it follows that by the time the earth's axis has completed one circuit of its majestic cone, the pole of the moon's plane will have gone round about , times. now, as this pole really only describes a comparatively small cone of ° in radius, we may for a first approximation take the average position which it occupies; but this average position is, of course, the centre of the circle which it describes--that is, the pole of the ecliptic. we thus see that the average precessional effect of the moon simply conspires with that of the sun to produce a revolution around the pole of the ecliptic. the grosser phenomena of the movements of the earth's axis are to be explained by the uniform revolution of the pole in a circular path; but if we make a minute examination of the track of the earth's axis, we shall find that though it, on the whole, conforms with the circle, yet that it really traces out a sinuous line, sometimes on the inside and sometimes on the outside of the circle. this delicate movement arises from the continuous change in the place of the pole of the moon's orbit. the period of these undulations is - / years, agreeing exactly with the period of the revolution of the moon's nodes. the amount by which the pole departs from the circle on either side is only about · seconds--a quantity rather less than the twenty-thousandth part of the radius of the sphere. this phenomenon, known as "nutation," was discovered by the beautiful telescopic researches of bradley, in . whether we look at the theoretical interest of the subject or at the refinement of the observations involved, this achievement of the "vir incomparabilis," as bradley has been called by bessel, is one of the masterpieces of astronomical genius. the phenomena of precession and nutation depend on movements of the earth itself, and not on movements of the axis of rotation within the earth. therefore the distance of any particular spot on the earth from the north or south pole is not disturbed by either of these phenomena. the latitude of a place is the distance of the place from the earth's equator, and this quantity remains unaltered in the course of the long precession cycle of , years. but it has been discovered within the last few years that latitudes are subject to a small periodic change of a few tenths of a second of arc. this was first pointed out about by dr. küstner, of berlin, and by a masterly analysis of all available observations, made in the course of many years past at various observatories, dr. chandler, of boston, has shown that the latitude of every point on the earth is subject to a double oscillation, the period of one being days and the other about a year, the mean amplitude of each being o"· . in other words, the spot in the arctic regions, directly in the prolongation of the earth's axis of rotation, is not absolutely fixed; the end of the imaginary axis moves about in a complicated manner, but always keeping within a few yards of its average position. this remarkable discovery is not only of value as introducing a new refinement in many astronomical researches depending on an accurate knowledge of the latitude, but theoretical investigations show that the periods of this variation are incompatible with the assumption that the earth is an absolutely rigid body. though this assumption has in other ways been found to be untenable, the confirmation of this view by the discovery of dr. chandler is of great importance. chapter xxv. the aberration of light. the real and apparent movements of the stars--how they can be discriminated--aberration produces effects dependent on the position of the stars--the pole of the ecliptic--aberration makes stars seem to move in a circle, an ellipse, or a straight line according to position--all the ellipses have equal major axes--how is this movement to be explained?--how to be distinguished from annual parallax--the apex of the earth's way--how this is to be explained by the velocity of light--how the scale of the solar system can be measured by the aberration of light. we have in this chapter to narrate a discovery of a recondite character, which illustrates in a forcible manner some of the fundamental truths of astronomy. our discussion of it will naturally be divided into two parts. in the first part we must describe the nature of the phenomenon, and then we must give the extremely elegant explanation afforded by the properties of light. the telescopic discovery of aberration, as well as its explanation, are both due to the illustrious bradley. the expression _fixed_ star, so often used in astronomy, is to be received in a very qualified sense. the stars are, no doubt, well fixed in their places, so far as coarse observation is concerned. the lineaments of the constellations remain unchanged for centuries, and, in contrast with the ceaseless movements of the planets, the stars are not inappropriately called fixed. we have, however, had more than one occasion to show throughout the course of this work that the expression "fixed star" is not an accurate one when minute quantities are held in estimation. with the exact measures of modern instruments, many of these quantities are so perceptible that they have to be always reckoned with in astronomical enquiry. we can divide the movements of the stars into two great classes: the real movements and the apparent movements. the proper motion of the stars and the movements of revolution of the binary stars constitute the real movements of these bodies. these movements are special to each star, so that two stars, although close together in the heavens, may differ in the widest degree as to the real movements which they possess. it may, indeed, sometimes happen that stars in a certain region are animated with a common movement. in this phenomenon we have traces of a real movement shared by a number of stars in a certain group. with this exception, however, the real movements of the stars seem to be governed by no systematic law, and the rapidly moving stars are scattered here and there indiscriminately over the heavens. the apparent movements of the stars have a different character, inasmuch as we find the movement of each star determined by the place which it occupies in the heavens. it is by this means that we discriminate the real movements of the star from its apparent movements, and examine the character of both. in the present chapter we are concerned with the apparent movements only, and of these there are three, due respectively to precession, to nutation, and to aberration. each of these apparent movements obeys laws peculiar to itself, and thus it becomes possible to analyse the total apparent motion, and to discriminate the proportions in which the precession, the nutation, and the aberration have severally contributed. we are thus enabled to isolate the effect of aberration as completely as if it were the sole agent of apparent displacement, so that, by an alliance between mathematical calculation and astronomical observation, we can study the effects of aberration as clearly as if the stars were affected by no other motions. concentrating our attention solely on the phenomena of aberration we shall describe its particular effect upon stars in different regions of the sky, and thus ascertain the laws according to which the effects of aberration are exhibited. when this step has been taken, we shall be in a position to give the beautiful explanation of those laws dependent upon the velocity of light. at one particular region of the heavens the effect of aberration has a degree of simplicity which is not manifested anywhere else. this region lies in the constellation draco, at the pole of the ecliptic. at this pole, or in its immediate neighbourhood, each star, in virtue of aberration, describes a circle in the heavens. this circle is very minute; it would take something like , of these circles together to form an area equal to the area of the moon. expressed in the usual astronomical language, we should say that the diameter of this small circle is about · seconds of arc. this is a quantity which, though small to the unaided eye, is really of great relative magnitude in the present state of telescopic research. it is not only large enough to be perceived, but it can be measured, with an accuracy which actually does not admit of a doubt, to the hundredth part of the whole. it is also observed that each star describes its little circle in precisely the same period of time; and that period is one year, or, in other words, the time of the revolution of the earth around the sun. it is found that for all stars in this region, be they large stars or small, single or double, white or coloured, the circles appropriate to each have all the same size, and are all described in the same time. even from this alone it would be manifest that the cause of the phenomenon cannot lie in the star itself. this unanimity in stars of every magnitude and distance requires some simpler explanation. further examination of stars in different regions sheds new light on the subject. as we proceed from the pole of the ecliptic, we still find that each star exhibits an annual movement of the same character as the stars just considered. in one respect, however, there is a difference. the apparent path of the star is no longer a circle; it has become an ellipse. it is, however, soon perceived that the shape and the position of this ellipse are governed by the simple law that the further the star is from the pole of the ecliptic the greater is the eccentricity of the ellipse. the apparent path of the stars at the same distance from the pole have equal eccentricity, and of the axes of the ellipse the shorter is always directed to the pole, the longer being, of course, perpendicular to it. it is, however, found that no matter how great the eccentricity may become, the major axis always retains its original length. it is always equal to about · seconds--that is, to the diameter of the circle of aberration at the pole itself. as we proceed further and further from the pole of the ecliptic, we find that each star describes a path more and more eccentric, until at length, when we examine a star on the ecliptic, the ellipse has become so attenuated that it has flattened into a line. each star which happens to lie on the ecliptic oscillates to and fro along the ecliptic through an amplitude of · seconds. half a year accomplishes the journey one way, and the other half of the year restores the star to its original position. when we pass to stars on the southern side of the ecliptic, we see the same series of changes proceed in an inverse order. the ellipse, from being actually linear, gradually grows in width, though still preserving the same length of major axis, until at length the stars near the southern pole of the ecliptic are each found to describe a circle equal to the paths pursued by the stars at the north pole of the ecliptic. the circumstance that the major axes of all those ellipses are of equal length suggests a still further simplification. let us suppose that every star, either at the pole of the ecliptic or elsewhere, pursues an absolutely circular path, and that all these circles agree not only in magnitude, but also in being all parallel to the plane of the ecliptic: it is easy to see that this simple supposition will account for the observed facts. the stars at the pole of the ecliptic will, of course, show their circles turned fairly towards us, and we shall see that they pursue circular paths. the circular paths of the stars remote from the pole of the ecliptic will, however, be only seen somewhat edgewise, and thus the apparent paths will be elliptical, as we actually find them. we can even calculate the degree of ellipticity which this surmise would require, and we find that it coincides with the observed ellipticity. finally, when we observe stars actually moving in the ecliptic, the circles they follow would be seen edgewise, and thus the stars would have merely the linear movement which they are seen to possess. all the observed phenomena are thus found to be completely consistent with the supposition that every star of all the millions in the heavens describes once each year a circular path; and that, whether the star be far or near, this circle has always the same apparent diameter, and lies in a plane always parallel to the plane of the ecliptic. we have now wrought the facts of observation into a form which enables us to examine into the cause of a movement so systematic. why is it that each star should seem to describe a small circular path? why should that path be parallel to the ecliptic? why should it be completed exactly in a twelvemonth? we are at once referred to the motion of the earth around the sun. that movement takes place in the ecliptic. it is completed in a year. the coincidences are so obvious that we feel almost necessarily compelled to connect in some way this apparent movement of the stars with the annual movement of the earth around the sun. if there were no such connection, it would be in the highest degree improbable that the planes of the circles should be all parallel to the ecliptic, or that the time of revolution of each star in its circle should equal that of the revolution of the earth around the sun. as both these conditions are fulfilled, the probability of the connection rises to a value almost infinite. the important question has then arisen as to why the movement of the earth around the sun should be associated in so remarkable a manner with this universal star movement. there is here one obvious point to be noticed and to be dismissed. we have in a previous chapter discussed the important question of the annual parallax of stars, and we have shown how, in virtue of annual parallax, each star describes an ellipse. it can further be demonstrated that these ellipses are really circles parallel to the ecliptic; so that we might hastily assume that annual parallax was the cause of the phenomenon discovered by bradley. a single circumstance will, however, dispose of this suggestion. the circle described by a star in virtue of annual parallax has a magnitude dependent on the distance of the star, so that the circles described by various stars are of various dimensions, corresponding to the varied distances of different stars. the phenomena of aberration, however, distinctly assert that the circular path of each star is of the same size, quite independently of what its distance may be, and hence annual parallax will not afford an adequate explanation. it should also be noticed that the movements of a star produced by annual parallax are much smaller than those due to aberration. there is not any known star whose circular path due to annual parallax has a diameter one-twentieth part of that of the circle due to aberration; indeed, in the great majority of cases the parallax of the star is an absolutely insensible quantity. there is, however, a still graver and quite insuperable distinction between the parallactic path and the aberrational path. let us, for simplicity, think of a star situated near the pole of the ecliptic, and thus appearing to revolve annually in a circle, whether we regard either the phenomenon of parallax or of aberration. as the earth revolves, so does the star appear to revolve; and thus to each place of the earth in its orbit corresponds a certain place of the star in its circle. if the movement arise from annual parallax, it is easy to see where the place of the star will be for any position of the earth. it is, however, found that in the movement discovered by bradley the star never has the position which parallax assigns to it, but is, in fact, a quarter of the circumference of its little circle distant therefrom. a simple rule will find the position of the star due to aberration. draw from the centre of the ellipse a radius parallel to the direction in which the earth is moving at the moment in question, then the extremity of this radius gives the point on its ellipse where the star is to be found. tested at all seasons, and with all stars, this law is found to be always verified, and by its means we are conducted to the true explanation of the phenomenon. we can enunciate the effects of aberration in a somewhat different manner, which will show even more forcibly how the phenomenon is connected with the motion of the earth in its orbit. as the earth pursues its annual course around the sun, its movement at any moment may be regarded as directed towards a certain point of the ecliptic. from day to day, and even from hour to hour, the point gradually moves along the ecliptic, so as to complete the circuit in a year. at each moment, however, there is always a certain point in the heavens towards which the earth's motion is directed. it is, in fact, the point on the celestial sphere towards which the earth would travel continuously if, at the moment, the attraction of the sun could be annihilated. it is found that this point is intimately connected with the phenomenon of aberration. in fact, the aberration is really equivalent to drawing each star from its mean place towards the apex of the earth's way, as the point is sometimes termed. it can also be shown by observation that the amount of aberration depends upon the distance from the apex. a star which happened to lie on the ecliptic will not be at all deranged by aberration from its mean place when it happens that the apex coincides with the star. all the stars ° from the apex will be displaced each by the same amount, and all directly in towards the apex. a star ° from the apex will undergo a larger degree of displacement, though still in the same direction, exactly towards the apex; and all stars at the same distance will be displaced by the same amount. proceeding thus from the apex, we come to stars at a distance of ° therefrom. here the amount of displacement will be a maximum. each one will be about twenty seconds from its average place; but in every case the imperative law will be obeyed, that the displacement of the star from its mean place lies towards the apex of the earth's way. we have thus given two distinct descriptions of the phenomenon of aberration. in the first we find it convenient to speak of a star as describing a minute circular path; in the other we have regarded aberration as merely amounting to a derangement of the star from its mean place in accordance with specified laws. these descriptions are not inconsistent: they are, in fact, geometrically equivalent; but the latter is rather the more perfect, inasmuch as it assigns completely the direction and extent of the derangement caused by aberration in any particular star at any particular moment. the question has now been narrowed to a very definite form. what is it which makes each star seem to close in towards the point towards which the earth is travelling? the answer will be found when we make a minute enquiry into the circumstances in which we view a star in the telescope. the beam of rays from a star falls on the object-glass of a telescope; those rays are parallel, and after they pass through the object-glass they converge to a focus near the eye end of the instrument. let us first suppose that the telescope is at rest; then if the telescope be pointed directly towards the star, the rays will converge to a point at the centre of the field of view where a pair of cross wires are placed, whose intersection defines the axis of the telescope. the case will, however, be altered if the telescope be moved after the light has passed through the objective; the rays of light in the interior of the tube will pursue a direct path, as before, and will proceed to a focus at the same precise point as before. as, however, the telescope has moved, it will, of course, have carried with it the pair of cross wires; they will no longer be at the same point as at first, and consequently the image of the star will not now coincide with their intersection. the movement of the telescope arises from its connection with the earth: for as the earth hurries along at a speed of eighteen miles a second, the telescope is necessarily displaced with this velocity. it might at first be thought, that in the incredibly small fraction of time necessary for light to pass from the object-glass to the eye-piece, the change in the position of the telescope must be too minute to be appreciable. let us suppose, for instance, that the star is situated near the pole of the ecliptic, then the telescope will be conveyed by the earth's motion in a direction perpendicular to its length. if the tube of the instrument be about twenty feet long, it can be readily demonstrated that during the time the light travels down the tube the movement of the earth will convey the telescope through a distance of about one-fortieth of an inch.[ ] this is a quantity very distinctly measurable with the magnifying power of the eye-piece, and hence this derangement of the star's place is very appreciable. it therefore follows that if we wish the star to be shown at the centre of the instrument, the telescope is not to be pointed directly at the star, as it would have to be were the earth at rest, but the telescope must be pointed a little in advance of the star's true position; and as we determine the apparent place of the star by the direction in which the telescope is pointed, it follows that the apparent place of the star is altered by the motion of the earth. every circumstance of the change in the star's place admits of complete explanation in this manner. take, for instance, the small circular path which each star appears to describe. we shall, for simplicity, refer only to a star at the pole of the ecliptic. suppose that the telescope is pointed truly to the place of the star, then, as we have shown, the image of the star will be at a distance of one-fortieth of an inch from the cross wires. this distance will remain constant, but each night the direction of the star from the cross wires will change, so that in the course of the year it completes a circle, and returns to its original position. we shall not pursue the calculations relative to other stars; suffice it here to say that the movement of the earth has been found adequate to account for the phenomena, and thus the doctrine of the aberration of light is demonstrated. it remains to allude to one point of the utmost interest and importance. we have seen that the magnitude of the aberration can be measured by astronomical observation. the amount of this aberration depends upon the velocity of light, and on the velocity with which the earth's motion is performed. we can measure the velocity of light by independent measurements, in the manner already explained in chapter xii. we are thus enabled to calculate what the velocity of the earth must be, for there is only one particular velocity for the earth which, when combined with the measured velocity of light, will give the measured value of aberration. the velocity of the earth being thus ascertained, and the length of the year being known, it is easy to find the circumference of the earth's path, and therefore its radius; that is, the distance from the earth to the sun. here is indeed a singular result, and one which shows how profoundly the various phenomena of science are interwoven. we make experiments in our laboratory, and find the velocity of light. we observe the fixed stars, and measure the aberration. we combine these results, and deduce therefrom the distance from the earth to the sun! although this method of finding the sun's distance is one of very great elegance, and admits of a certain amount of precision, yet it cannot be relied upon as a perfectly unimpeachable method of deducing the great constant. a perfect method must be based on the operations of mere surveying, and ought not to involve recondite physical considerations. we cannot, however, fail to regard the discovery of aberration by bradley as a most pleasing and beautiful achievement, for it not only greatly improves the calculations of practical astronomy, but links together several physical phenomena of the greatest interest. chapter xxvi. the astronomical significance of heat. heat and astronomy--distribution of heat--the presence of heat in the earth--heat in other celestial bodies--varieties of temperature--the law of cooling--the heat of the sun--can its temperature be measured?--radiation connected with the sun's bulk--can the sun be exhausting his resources?--no marked change has occurred--geological evidence as to the changes of the sun's heat doubtful--the cooling of the sun--the sun cannot be merely an incandescent solid cooling--combustion will not explain the matter--some heat is obtained from meteoric matter, but this is not adequate to the maintenance of the sun's heat--the contraction of a heated globe of gas--an apparent paradox--the doctrine of energy--the nebular theory--evidence in support of this theory--sidereal evidence of the nebular theory--herschel's view of sidereal aggregation--the nebulæ do not exhibit changes within the limits of our observation. that a portion of a work on astronomy should bear the title placed at the head of this chapter will perhaps strike some of our readers as unusual, if not actually inappropriate. is not heat, it may be said, a question merely of experimental physics? and how can it be legitimately introduced into a treatise upon the heavenly bodies and their movements? whatever weight such objections might have once had need not now be considered. the recent researches on heat have shown not only that heat has important bearings on astronomy, but that it has really been one of the chief agents by which the universe has been moulded into its actual form. at the present time no work on astronomy could be complete without some account of the remarkable connection between the laws of heat and the astronomical consequences which follow from those laws. in discussing the planetary motions and the laws of kepler, or in discussing the movements of the moon, the proper motions of the stars, or the revolutions of the binary stars, we proceed on the supposition that the bodies we are dealing with are rigid particles, and the question as to whether these particles are hot or cold does not seem to have any especial bearing. no doubt the ordinary periodic phenomena of our system, such as the revolution of the planets in conformity with kepler's laws, will be observed for countless ages, whether the planets be hot or cold, or whatever may be the heat of the sun. it must, however, be admitted that the laws of heat introduce certain modifications into the statement of these laws. the effects of heat may not be immediately perceptible, but they exist--they are constantly acting; and in the progress of time they are adequate to effecting the mightiest changes throughout the universe. let us briefly recapitulate the circumstances of our system which give to heat its potency. look first at our earth, which at present seems--on its surface, at all events--to be a body devoid of internal heat; a closer examination will dispel this idea. have we not the phenomena of volcanoes, of geysers, and of hot springs, which show that in the interior of the earth heat must exist in far greater intensity than we find on the surface? these phenomena are found in widely different regions of the earth. their origin is, no doubt, involved in a good deal of obscurity, but yet no one can deny that they indicate vast reservoirs of heat. it would indeed seem that heat is to be found everywhere in the deep inner regions of the earth. if we take a thermometer down a deep mine, we find it records a temperature higher than at the surface. the deeper we descend the higher is the temperature; and if the same rate of progress should be maintained through those depths of the earth which we are not able to penetrate, it can be demonstrated that at twenty or thirty miles below the surface the temperature must be as great as that of red-hot iron. we find in the other celestial bodies abundant evidence of the present or the past existence of heat. our moon, as we have already mentioned, affords a very striking instance of a body which must once have been very highly heated. the extraordinary volcanoes on its surface place this beyond any doubt. it is equally true that those volcanoes have been silent for ages, so that, whatever may be the interior condition of the moon, the surface has now cooled down. extending our view further, we see in the great planets jupiter and saturn evidence that they are still endowed with a temperature far in excess of that which the earth has retained; while, when we look at our sun, we see a body in a state of brilliant incandescence, and glowing with a fervour to which we cannot approximate in our mightiest furnaces. the various fixed stars are bodies which glow with heat, like our sun; while we have in the nebulæ objects the existence of which is hardly intelligible to us, unless we admit that they are possessed of heat. from this rapid survey of the different bodies in our universe one conclusion is obvious. we may have great doubts as to the actual temperature of any individual body of the system; but it cannot be doubted that there is a wide range of temperature among the different bodies. some are hotter than others. the stars and suns are perhaps the hottest of all; but it is not improbable that they may be immeasurably outnumbered by the cold and dark bodies of the universe, which are to us invisible, and only manifest their existence in an indirect and casual manner. the law of cooling tells us that every body radiates heat, and that the quantity of heat which it radiates increases when the temperature of the body increases relatively to the surrounding medium. this law appears to be universal. it is obeyed on the earth, and it would seem that it must be equally obeyed by every other body in space. we thus see that each of the planets and each of the stars is continuously pouring forth in all directions a never-ceasing stream of heat. this radiation of heat is productive of very momentous consequences. let us study them, for instance, in the case of the sun. our great luminary emits an incessant flood of radiant heat in all directions. a minute fraction of that heat is intercepted by our earth, and is, directly or indirectly, the source of all life, and of nearly all movement, on our earth. to pour forth heat as the sun does, it is necessary that his temperature be enormously high. and there are some facts which permit us to form an estimate of what that temperature must actually be. it is difficult to form any numerical statement of the actual temperature of the sun. the intensity of that temperature vastly transcends the greatest artificial heat, and any attempt to clothe such estimates in figures is necessarily very precarious. but assuming the greatest artificial temperature to be about , ° fahr., we shall probably be well within the truth if we state the effective temperature of the sun to be about , ° fahr. this is the result of a recent investigation by messrs. wilson and gray, which seems to be entitled to considerable weight. the copious outflow of heat from the sun corresponds with its enormous temperature. we can express the amount of heat in various ways, but it must be remembered that considerable uncertainty still attaches to such measurements. the old method of measuring heat by the quantity of ice melted may be used as an illustration. it is computed that a shell of ice - / feet thick surrounding the whole sun would in one minute be melted by the sun's heat underneath. a somewhat more elegant illustration was also given by sir john herschel, who showed that if a cylindrical glacier miles in diameter were to be continually flowing into the sun with the velocity of light, the end of that glacier would be melted as quickly as it advanced. from each square foot in the surface of the sun emerges a quantity of heat as great as could be produced by the daily combustion of sixteen tons of coal. this is, indeed, an amount of heat which, properly transformed into work, would keep an engine of many hundreds of horse-power running from one year's end to the other. the heat radiated from a few acres on the sun would be adequate to drive all the steam engines in the world. when we reflect on the vast intensity of the radiation from each square foot of the sun's surface, and when we combine with this the stupendous dimensions of the sun, imagination fails to realise how vast must be the actual expenditure of heat. in presence of the prodigal expenditure of the sun's heat, we are tempted to ask a question which has the most vital interest for the earth and its inhabitants. we live from hour to hour by the sun's splendid generosity; and, therefore, it is important for us to know what security we possess for the continuance of his favours. when we witness the terrific disbursement of the sun's heat each hour, we are compelled to ask whether our great luminary may not be exhausting its resources; and if so, what are the prospects of the future? this question we can partly answer. the whole subject is indeed of surpassing interest, and redolent with the spirit of modern scientific thought. our first attempt to examine this question must lie in an appeal to the facts which are attainable. we want to know whether the sun is showing any symptoms of decay. are the days as warm and as bright now as they were last year, ten years ago, one hundred years ago? we can find no evidence of any change since the beginning of authentic records. if the sun's heat had perceptibly changed within the last two thousand years, we should expect to find corresponding changes in the distribution of plants and of animals; but no such changes have been detected. there is no reason to think that the climate of ancient greece or of ancient rome was appreciably different from the climates of the greece and the rome that we know at this day. the vine and the olive grow now where they grew two thousand years ago. we must not, however, lay too much stress on this argument; for the effects of slight changes in the sun's heat may have been neutralised by corresponding adaptations in the pliable organisms of cultivated plants. all we can certainly conclude is that no marked change has taken place in the heat of the sun during historical time. but when we come to look back into much earlier ages, we find copious evidence that the earth has undergone great changes in climate. geological records can on this question hardly be misinterpreted. yet it is curious to note that these changes are hardly such as could arise from the gradual exhaustion of the sun's radiation. no doubt, in very early times we have evidence that the earth's climate must have been much warmer than at present. we had the great carboniferous period, when the temperature must almost have been tropical in arctic latitudes. yet it is hardly possible to cite this as evidence that the sun was then much more powerful; for we are immediately reminded of the glacial period, when our temperate zones were overlaid by sheets of solid ice, as northern greenland is at present. if we suppose the sun to have been hotter than it is at present to account for the vegetation which produced coal, then we ought to assume the sun to be colder than it is now to account for the glacial period. it is not reasonable to attribute such phenomena to fluctuations in the radiation from the sun. the glacial periods prove that we cannot appeal to geology in aid of the doctrine that a secular cooling of the sun is now in progress. the geological variations of climate may have been caused by changes in the earth itself, or by changes in its actual orbit; but however they have been caused, they hardly tell us much with regard to the past history of our sun. the heat of the sun has lasted countless ages; yet we cannot credit the sun with the power of actually creating heat. we must apply to the tremendous mass of the sun the same laws which we have found by our experiments on the earth. we must ask, whence comes the heat sufficient to supply this lavish outgoing? let us briefly recount the various suppositions that have been made. place two red-hot spheres of iron side by side, a large one and a small one. they have been taken from the same fire; they were both equally hot; they are both cooling, but the small sphere cools more rapidly. it speedily becomes dark, while the large sphere is still glowing, and would continue to do so for some minutes. the larger the sphere, the longer it will take to cool; and hence it has been supposed that a mighty sphere of the prodigious dimensions of our sun would, if once heated, cool gradually, but the duration of the cooling would be so long that for thousands and for millions of years it could continue to be a source of light and heat to the revolving system of planets. this suggestion will not bear the test of arithmetic. if the sun had no source of heat beyond that indicated by its high temperature, we can show that radiation would cool the sun a few degrees every year. two thousand years would then witness a very great decrease in the sun's heat. we are certain that no such decrease can have taken place. the source of the sun's radiation cannot be found in the mere cooling of an incandescent mass. can the fires in the sun be maintained by combustion, analogous to that which goes on in our furnaces? here we would seem to have a source of gigantic heat; but arithmetic also disposes of this supposition. we know that if the sun were made of even solid coal itself, and if that coal were burning in pure oxygen, the heat that could be produced would only suffice for , years. if the sun which shone upon the builders of the great pyramid had been solid coal from surface to centre, it must by this time have been in great part burned away in the attempt to maintain its present rate of expenditure. we are thus forced to look to other sources for the supply of the sun's heat, since neither the heat of incandescence nor the heat of combustion will suffice. there is probably--indeed, we may say certainly--one external source from which the heat of the sun is recruited. it will be necessary for us to consider this source with some care, though i think we shall find it to be merely an auxiliary of comparatively trifling moment. according to this view, the solar heat receives occasional accessions from the fall upon the sun's surface of masses of meteoric matter. there can be hardly a doubt that such masses do fall upon the sun; there is certainly no doubt that if they do, the sun must gain some heat thereby. we have experience on the earth of a very interesting kind, which illustrates the development of heat by meteoric matter. there lies a world of philosophy in a shooting star. some of these myriad objects rush into our atmosphere and are lost; others, no doubt, rush into the sun with the same result. we also admit that the descent of a shooting star into the atmosphere of the sun must be attended with a flash of light and of heat. the heat acquired by the earth from the flashing of the shooting stars through our air is quite insensible. it has been supposed, however, that the heat accruing to the sun from the same cause may be quite sensible--nay, it has been even supposed that the sun may be re-invigorated from this source. here, again, we must apply the cold principles of weights and measures to estimate the plausibility of this suggestion. we first calculate the actual weight of meteoric indraught to the sun which would be adequate to sustain the fires of the sun at their present vigour. the mass of matter that would be required is so enormous that we cannot usefully express it by imperial weights; we must deal with masses of imposing magnitude. it fortunately happens that the weight of our moon is a convenient unit. conceive that our moon--a huge globe, , miles in diameter--were crushed into a myriad of fragments, and that these fragments were allowed to rain in on the sun; there can be no doubt that this tremendous meteoric shower would contribute to the sun rather more heat than would be required to supply his radiation for a whole year. if we take our earth itself, conceive it comminuted into dust, and allow that dust to fall on the sun as a mighty shower, each fragment would instantly give out a quantity of heat, and the whole would add to the sun a supply of heat adequate to sustain the present rate of radiation for nearly one hundred years. the mighty mass of jupiter treated in the same way would generate a meteoric display greater in the ratio in which the mass of jupiter exceeds the mass of earth. were jupiter to fall into the sun, enough heat would be thereby produced to scorch the whole solar system; while all the planets together would be capable of producing heat which, if properly economised, would supply the radiation of the sun for , years. it must be remembered that though the moon could supply one year's heat, and jupiter , years' heat, yet the practical question is not whether the solar system could supply the sun's heat, but whether it does. is it likely that meteors equal in mass to the moon fall into the sun every year? this is the real question, and i think we are bound to reply to it in the negative. it can be shown that the quantity of meteors which could be caught by the sun in any one year can be only an excessively minute fraction of the total amount. if, therefore, a moon-weight of meteors were caught every year, there must be an incredible mass of meteoric matter roaming at large through the system. there must be so many meteors that the earth would be incessantly pelted with them, and heated to such a degree as to be rendered uninhabitable. there are also other reasons which preclude the supposition that a stupendous quantity of meteoric matter exists in the vicinity of the sun. such matter would produce an appreciable effect on the movement of the planet mercury. there are, no doubt, some irregularities in the movements of mercury not yet fully explained, but these irregularities are very much less than would be the case if meteoric matter existed in quantity adequate to the sustentation of the sun. astronomers, then, believe that though meteors may provide a rate in aid of the sun's current expenditure, yet that the greater portion of that expenditure must be defrayed from other resources. it is one of the achievements of modern science to have effected the solution of the problem--to have shown how it is that, notwithstanding the stupendous radiation, the sun still maintains its temperature. the question is not free from difficulty in its exposition, but the matter is one of such very great importance that we are compelled to make the attempt. let us imagine a vast globe of heated gas in space. this is not an entirely gratuitous supposition, inasmuch as there are globes apparently of this character; they have been already alluded to as planetary nebulæ. this globe will radiate heat, and we shall suppose that it emits more heat than it receives from the radiation of other bodies. the globe will accordingly lose heat, or what is equivalent thereto, but it will be incorrect to assume that the globe will necessarily fall in temperature. that the contrary is, indeed, the case is a result almost paradoxical at the first glance; but yet it can be shown to be a necessary consequence of the laws of heat and of gases. let us fix our attention on a portion of the gas lying on the surface of the globe. this is, of course, attracted by all the rest of the globe, and thus tends in towards the centre of the globe. if equilibrium subsists, this tendency must be neutralised by the pressure of the gas beneath; so that the greater the gravitation, the greater is the pressure. when the globe of gas loses heat by radiation, let us suppose that it grows colder--that its temperature accordingly falls; then, since the pressure of a gas decreases when the temperature falls, the pressure beneath the superficial layer of the gas will decrease, while the gravitation is unaltered. the consequence will inevitably be that the gravitation will now conquer the pressure, and the globe of gas will accordingly contract. there is, however, another way in which we can look at the matter. we know that heat is equivalent to energy, so that when the globe radiates forth heat, it must expend energy. a part of the energy of the globe will be due to its temperature; but another, and in some respects a more important, part is that due to the separation of its particles. if we allow the particles to come closer together we shall diminish the energy due to separation, and the energy thus set free can take the form of heat. but this drawing in of the particles necessarily involves a shrinking of the globe. and now for the remarkable consequence, which seems to have a very important application in astronomy. as the globe contracts, a part of its energy of separation is changed into heat; that heat is partly radiated away, but not so rapidly as it is produced by the contraction. the consequence is, that although the globe is really losing heat and really contracting, yet that its temperature is actually rising.[ ] a simple case will suffice to demonstrate this result, paradoxical as it may at first seem. let us suppose that by contraction of the sphere it had diminished to one-half its diameter; and let us fix our attention on a cubic inch of the gaseous matter in any point of the mass. after the contraction has taken place each edge of the cube would be reduced to half an inch, and the volume would therefore be reduced to one-eighth part of its original amount. the law of gases tells us that if the temperature be unaltered the pressure varies inversely as the volume, and consequently the internal pressure in the cube would in that case be increased eightfold. as, however, in the case before us, the distance between every two particles is reduced to one-half, it will follow that the gravitation between every two particles is increased fourfold, and as the area is also reduced to one-fourth, it will follow that the pressure inside the reduced cube is increased sixteenfold; but we have already seen that with a constant temperature it only increases eightfold, and hence the temperature cannot be constant, but must rise with the contraction. we thus have the somewhat astonishing result that a gaseous globe in space radiating heat, and thereby growing smaller, is all the time actually increasing in temperature. but, it may be said, surely this cannot go on for ever. are we to suppose that the gaseous mass will go on contracting and contracting with a temperature ever fiercer and fiercer, and actually radiating out more and more heat the more it loses? where lies the limit to such a prospect? as the body contracts, its density must increase, until it either becomes a liquid, or a solid, or, at any rate, until it ceases to obey the laws of a purely gaseous body which we have supposed. once these laws cease to be observed the argument disappears; the loss of heat may then really be attended with a loss of temperature, until in the course of time the body has sunk to the temperature of space itself. it is not assumed that this reasoning can be applied in all its completeness to the present state of the sun. the sun's density is now so great that the laws of gases cannot be there strictly followed. there is, however, good reason to believe that the sun was once more gaseous than at present; possibly at one time he may have been quite gaseous enough to admit of this reasoning in all its fulness. at present the sun appears to be in some intermediate stage of its progress from the gaseous condition to the solid condition. we cannot, therefore, say that the temperature of the sun is now increasing in correspondence with the process of contraction. this may be true or it may not be true; we have no means of deciding the point. we may, however, feel certain that the sun is still sufficiently gaseous to experience in some degree the rise of temperature associated with the contraction. that rise in temperature may be partly or wholly obscured by the fall in temperature which would be the more obvious consequence of the radiation of heat from the partially solid body. it will, however, be manifest that the cooling of the sun may be enormously protracted if the fall of temperature from the one cause be nearly compensated by the rise of temperature from the other. it can hardly be doubted that in this we find the real explanation of the fact that we have no historical evidence of any appreciable alteration in the radiation of heat from the sun. this question is one of such interest that it may be worth while to look at it from a slightly different point of view. the sun contains a certain store of energy, part of which is continually disappearing in the form of radiant heat. the energy remaining in the sun is partly transformed in character; some of it is transformed into heat, which goes wholly or partly to supply the loss by radiation. the total energy of the sun must, however, be decreasing; and hence it would seem the sun must at some time or other have its energy exhausted, and cease to be a source of light and of heat. it is true that the rate at which the sun contracts is very slow. we are, indeed, not able to measure with certainty the decrease in the sun's bulk. it is a quantity so minute, that the contraction since the birth of accurate astronomy is not large enough to be perceptible in our telescopes. it is, however, possible to compute what the contraction of the sun's bulk must be, on the supposition that the energy lost by that contraction just suffices to supply the daily radiation of heat. the change is very small when we consider the present size of the sun. at the present time the sun's diameter is about , miles. if each year this diameter decreases by about feet, sufficient energy will be yielded to account for the entire radiation. this gradual decrease is always in progress. these considerations are of considerable interest when we apply them retrospectively. if it be true that the sun is at this moment shrinking, then in past times his globe must have been greater than it is at present. assuming the figures already given, it follows that one hundred years ago the diameter of the sun must have been nearly six miles greater than it is now; one thousand years ago the diameter was fifty-seven miles greater; ten thousand years ago the diameter of the sun was five hundred and seventy miles greater than it is to-day. when man first trod this earth it would seem that the sun must have been many hundreds, perhaps many thousands, of miles greater than it is at this time. we must not, however, over-estimate the significance of this statement. the diameter of the sun is so great, that a diminution of , miles would be but little more than the hundredth part of its diameter. if it were suddenly to shrink to the extent of , miles, the change would not be appreciable to ordinary observation, though a much smaller change would not elude delicate astronomical measurement. it does not necessarily follow that the climates on our earth in these early times must have been very different from those which we find at this day, for the question of climate depends upon other matters besides sunbeams. yet we need not abruptly stop our retrospect at any epoch, however remote. we may go back earlier and earlier, through the long ages which geologists claim for the deposition of the stratified rocks; and back again still further, to those very earliest epochs when life began to dawn on the earth. still we can find no reason to suppose that the law of the sun's decreasing heat is not maintained; and thus we would seem bound by our present knowledge to suppose that the sun grows larger and larger the further our retrospect extends. we cannot assume that the rate of that growth is always the same. no such assumption is required; it is sufficient for our purpose that we find the sun growing larger and larger the further we peer back into the remote abyss of time past. if the present order of things in our universe has lasted long enough, then it would seem that there was a time when the sun must have been twice as large as it is at present; it must once have been ten times as large. how long ago that was no one can venture to say. but we cannot stop at the stage when the sun was even ten times as large as it is at present; the arguments will still apply in earlier ages. we see the sun swelling and swelling, with a corresponding decrease in its density, until at length we find, instead of our sun as we know it, a mighty nebula filling a gigantic region of space. such is, in fact, the doctrine of the origin of our system which has been advanced in that celebrated speculation known as the nebular theory of laplace. nor can it be ever more than a speculation; it cannot be established by observation, nor can it be proved by calculation. it is merely a conjecture, more or less plausible, but perhaps in some degree necessarily true, if our present laws of heat, as we understand them, admit of the extreme application here required, and if also the present order of things has reigned for sufficient time without the intervention of any influence at present unknown to us. this nebular theory is not confined to the history of our sun. precisely similar reasoning may be extended to the individual planets: the farther we look back, the hotter and the hotter does the whole system become. it has been thought that if we could look far enough back, we should see the earth too hot for life; back further still, we should find the earth and all the planets red-hot; and back further still, to an exceedingly remote epoch, when the planets would be heated just as much as our sun is now. in a still earlier stage the whole solar system is thought to have been one vast mass of glowing gas, from which the present forms of the sun, with the planets and their satellites, have been gradually evolved. we cannot be sure that the course of events has been what is here indicated; but there are sufficient grounds for thinking that this doctrine substantially represents what has actually occurred. many of the features in the solar system harmonise with the supposition that the origin of the system has been that suggested by the nebular theory. we have already had occasion in an earlier chapter to allude to the fact that all the planets perform their revolutions around the sun in the same direction. it is also to be observed that the rotation of the planets on their axes, as well as the movements of the satellites around their primaries, all follow the same law, with two slight exceptions in the case of the uranian and neptunian systems. a coincidence so remarkable naturally suggests the necessity for some physical explanation. such an explanation is offered by the nebular theory. suppose that countless ages ago a mighty nebula was slowly rotating and slowly contracting. in the process of contraction, portions of the condensed matter of the nebula would be left behind. these portions would still revolve around the central mass, and each portion would rotate on its axis in the same direction. as the process of contraction proceeded, it would follow from dynamical principles that the velocity of rotation would increase; and thus at length these portions would consolidate into planets, while the central mass would gradually contract to form the sun. by a similar process on a smaller scale the systems of satellites were evolved from the contracting primary. these satellites would also revolve in the same direction, and thus the characteristic features of the solar system could be accounted for. the nebular origin of the solar system receives considerable countenance from the study of the sidereal heavens. we have already dwelt upon the resemblance between the sun and the stars. if, then, our sun has passed through such changes as the nebular theory requires, may we not anticipate that similar phenomena should be met with in other stars? if this be so, it is reasonable to suppose that the evolution of some of the stars may not have progressed so far as has that of the sun, and thus we may be able actually to witness stars in the earlier phases of their development. let us see how far the telescope responds to these anticipations. the field of view of a large telescope usually discloses a number of stars scattered over a black background of sky; but the blackness of the background is not uniform: the practised eye of the skilled observer will detect in some parts of the heavens a faint luminosity. this will sometimes be visible over the whole extent of the field, or it may even occupy several fields. years may pass on, and still there is no perceptible change. there can be no illusion, and the conclusion is irresistible that the object is a stupendous mass of faintly luminous glowing gas or vapour. this is the simplest type of nebula; it is characterised by extreme faintness, and seems composed of matter of the utmost tenuity. on the other hand we are occasionally presented with the beautiful and striking phenomenon of a definite and brilliant star surrounded by a luminous atmosphere. between these two extreme types of a faint diffused mass on the one hand, and a bright star with a nebula surrounding it on the other, a graduated series of various other nebulæ can be arranged. we thus have a series of links passing by imperceptible gradations from the most faintly diffused nebulæ on the one side, into stars on the other. the nebulæ seemed to herschel to be vast masses of phosphorescent vapour. this vapour gradually cools down, and ultimately condenses into a star, or a cluster of stars. when the varied forms of nebulæ were classified, it almost seemed as if the different links in the process could be actually witnessed. in the vast faint nebulæ the process of condensation had just begun; in the smaller and brighter nebulæ the condensation had advanced farther; while in others, the star, or stars, arising from the condensation had already become visible. but, it may be asked, how did herschel know this? what is his evidence? let us answer this question by an illustration. go into a forest, and look at a noble old oak which has weathered the storm for centuries; have we any doubt that the oak-tree was once a young small plant, and that it grew stage by stage until it reached maturity? yet no one has ever followed an oak-tree through its various stages; the brief span of human life has not been long enough to do so. the reason why we believe the oak-tree to have passed through all these stages is, because we are familiar with oak-trees of every gradation in size, from the seedling up to the noble veteran. having seen this gradation in a vast multitude of trees, we are convinced that each individual passes through all these stages. it was by a similar train of reasoning that herschel was led to adopt the view of the origin of the stars which we have endeavoured to describe. the astronomer's life is not long enough, the life of the human race might not be long enough, to watch the process by which a nebula condenses down so as to form a solid body. but by looking at one nebula after another, the astronomer thinks he is able to detect the various stages which connect the nebula in its original form with the final form. he is thus led to believe that each of the nebulæ passes, in the course of ages, through these stages. and thus herschel adopted the opinion that stars--some, many, or all--have each originated from what was once a glowing nebula. such a speculation may captivate the imagination, but it must be carefully distinguished from the truths of astronomy, properly so called. remote posterity may perhaps obtain evidence on the subject which to us is inaccessible: our knowledge of nebulæ is too recent. there has not yet been time enough to detect any appreciable changes: for the study of nebulæ can only be said to date from messier's catalogue in . since herschel's time, no doubt, many careful drawings and observations of the nebulæ have been obtained; but still the interval has been much too short, and the earlier observations are too imperfect, to enable any changes in the nebulæ to be investigated with sufficient accuracy. if the human race lasts for very many centuries, and if our present observations are preserved during that time for comparison, then herschel's theory may perhaps be satisfactorily tested. a hundred years have passed since laplace, with some diffidence, set forth his hypothesis as to the mode of formation of the solar system. on the whole it must be said that this "nebular hypothesis" has stood the test of advancing science well, though some slight modifications have become necessary in the light of more recent discoveries. laplace (and herschel also) seems to have considered a primitive nebula to consist of a "fiery mist" or glowing gas at a very high temperature. but this is by no means necessary, as we have seen that the gradual contraction of the vast mass supplies energy which may be converted into heat, and the spectroscopic evidence seems also to point to the existence of a moderate temperature in the gaseous nebulæ, which must be considered to be representatives of the hypothetical primitive chaos out of which our sun and planets have been evolved. another point which has been reconsidered is the formation of the various planets. it was formerly thought that the rotation of the original mass had by degrees caused a number of rings of different dimensions to be separated from the central part, the material of which rings in time collected into single planets. the ring of saturn was held to be a proof of this process, since we here have a ring, the condensation of which into one or more satellites has somehow been arrested. but while it is not impossible that matter in the shape of rings may have been left behind during the contraction of the nebulous mass (indeed, the minor planets between mars and jupiter have perhaps originated in this way), it seems likely that the larger planets were formed from the agglomeration of matter at a point on the equator of the rotating nebula. the actual steps of the process by which the primeval nebula became transformed into the solar system seem to lie beyond reach of discovery. chapter xxvii. the tides.[ ] mathematical astronomy--lagrange's theories: how far they are really true--the solar system not made of rigid bodies--kepler's laws true to observation, but not absolutely true when the bodies are not rigid--the errors of observation--the tides--how the tides were observed--discovery of the connection between the tides and the moon--solar and lunar tides--work done by the tides--whence do the tides obtain the power to do the work?--tides are increasing the length of the day--limit to the shortness of the day--early history of the earth-moon system--unstable equilibrium--ratio of the month to the day--the future course of the system--equality of the month and the day--the future critical epoch--the constant face of the moon accounted for--the other side of the moon--the satellites of mars--their remarkable motions--have the tides possessed influence in moulding the solar system generally?--moment of momentum--tides have had little or no appreciable effect on the orbit of jupiter--conclusion. that the great discoveries of lagrange on the stability of the planetary system are correct is in one sense strictly true. no one has ever ventured to impugn the mathematics of lagrange. given the planetary system in the form which lagrange assumed and the stability of that system is assured for all time. there is, however, one assumption which lagrange makes, and on which his whole theory was founded: his assumption is that the planets are _rigid_ bodies. no doubt our earth seems a rigid body. what can be more solid and unyielding than the mass of rocks and metals which form the earth, so far as it is accessible to us? in the wide realms of space the earth is but as a particle; it surely was a natural and a legitimate assumption to suppose that that particle was a rigid body. if the earth were absolutely rigid--if every particle of the earth were absolutely at a fixed distance from every other particle--if under no stress of forces, and in no conceivable circumstance, the earth experienced even the minutest change of form--if the same could be said of the sun and of all the other planets--then lagrange's prediction of the eternal duration of our system must be fulfilled. but what are the facts of the case? is the earth really rigid? we know from experiment that a rigid body in the mathematical sense of the word does not exist. rocks are not rigid; steel is not rigid; even a diamond is not perfectly rigid. the whole earth is far from being rigid even on the surface, while part of the interior is still, perhaps, more or less fluid. the earth cannot be called a perfectly rigid body; still less can the larger bodies of our system be called rigid. jupiter and saturn are perhaps hardly even what could be called solid bodies. the solar system of lagrange consisted of a rigid sun and a number of minute rigid planets; the actual solar system consists of a sun which is in no sense rigid, and planets which are only partially so. the question then arises as to whether the discoveries of the great mathematicians of the last century will apply, not only to the ideal solar system which they conceived, but to the actual solar system in which our lot has been cast. there can be no doubt that these discoveries are approximately true: they are, indeed, so near the absolute truth, that observation has not yet satisfactorily shown any departure from them. but in the present state of science we can no longer overlook the important questions which arise when we deal with bodies not rigid in the mathematical sense of the word. let us, for instance, take the simplest of the laws to which we have referred, the great law of kepler, which asserts that a planet will revolve for ever in an elliptic path of which the sun is one focus. this is seen to be verified by actual observation; indeed, it was established by observation before any theoretical explanation of that movement was propounded. if, however, we state the matter with a little more precision, we shall find that what newton really demonstrated was, that if two _rigid_ particles attract each other by a law of force which varies with the inverse square of the distance between the particles, then each of the particles will describe an ellipse with the common centre of gravity in the focus. the earth is, to some extent, rigid, and hence it was natural to suppose that the relative behaviour of the earth and the sun would, to a corresponding extent, observe the simple elliptic law of kepler; as a matter of fact, they do observe it with such fidelity that, if we make allowance for other causes of disturbance, we cannot, even by most careful observation, detect the slightest variation in the motion of the earth arising from its want of rigidity. there is, however, a subtlety in the investigations of mathematics which, in this instance at all events, transcends the most delicate observations which our instruments enable us to make. the principles of mathematics tell us that though kepler's laws may be true for bodies which are absolutely and mathematically rigid, yet that if the sun or the planets be either wholly, or even in their minutest part, devoid of perfect rigidity, then kepler's laws can be no longer true. do we not seem here to be in the presence of a contradiction? observation tells us that kepler's laws are true in the planetary system; theory tells us that these laws cannot be true in the planetary system, because the bodies in that system are not perfectly rigid. how is this discrepancy to be removed? or is there really a discrepancy at all? there is not. when we say that kepler's laws have been proved to be true by observation, we must reflect on the nature of the proofs which are attainable. we observe the places of the planets with the instruments in our observatories; these places are measured by the help of our clocks and of the graduated circles on the instruments. these observations are no doubt wonderfully accurate; but they do not, they cannot, possess absolute accuracy in the mathematical sense of the word. we can, for instance, determine the place of a planet with such precision that it is certainly not one second of arc wrong; and one second is an extremely small quantity. a foot-rule placed at a distance of about forty miles subtends an angle of a second, and it is surely a delicate achievement to measure the place of a planet, and feel confident that no error greater than this can have intruded into our result. when we compare the results of observation with the calculations conducted on the assumption of the truth of kepler's laws, and when we pronounce on the agreement of the observations with the calculations, there is always a reference, more or less explicit, to the inevitable errors of the observations. if the calculations and observations agree so closely that the differences between the two are minute enough to have arisen in the errors inseparable from the observations, then we are satisfied with the accordance; for, in fact, no closer agreement is attainable, or even conceivable. the influence which the want of rigidity exercises on the fulfilment of the laws of kepler can be estimated by calculation; it is found, as might be expected, to be extremely small--so small, in fact, as to be contained within that slender margin of error by which observations are liable to be affected. we are thus not able to discriminate by actual measurement the effects due to the absence of rigidity; they are inextricably hid among the small errors of observation. the argument on which we are to base our researches is really founded on a very familiar phenomenon. there is no one who has ever visited the sea-side who is not familiar with that rise and fall of the sea which we call the tide. twice every twenty-four hours the sea advances on the beach to produce high tide; twice every day the sea again retreats to produce low tide. these tides are not merely confined to the coasts; they penetrate for miles up the courses of rivers; they periodically inundate great estuaries. in a maritime country the tides are of the most profound practical importance; they also possess a significance of a far less obvious character, which it is our object now to investigate. these daily pulses of the ocean have long ceased to be a mystery. it was in the earliest times perceived that there was a connection between the tides and the moon. ancient writers, such as pliny and aristotle, have referred to the alliance between the times of high water and the age of the moon. i think we sometimes do not give the ancient astronomers as much credit as their shrewdness really entitles them to. we have all read--we have all been taught--that the moon and the tides are connected together; but how many of us are in a position to say that we have actually noticed that connection by direct personal observation? the first man who studied this matter with sufficient attention to convince himself and to convince others of its reality must have been a great philosopher. we know not his name, we know not his nation, we know not the age in which he lived; but our admiration of his discovery must be increased by the reflection that he had not the theory of gravitation to guide him. a philosopher of the present day who had never seen the sea could still predict the necessity of tides as a consequence of the law of universal gravitation; but the primitive astronomer, who knew not of the invisible bond by which all bodies in the universe are drawn together, made a splendid--indeed, a typical--inductive discovery, when he ascertained the relation between the moon and the tides. we can surmise that this discovery, in all probability, first arose from the observations of experienced navigators. in all matters of entering port or of leaving port, the state of the tide is of the utmost concern to the sailor. even in the open sea he has sometimes to shape his course in accordance with the currents produced by the tides; or, in guiding his course by taking soundings, he has always to bear in mind that the depth varies with the tide. all matters relating to the tide would thus come under his daily observation. his daily work, the success of his occupation, the security of his life, depend often on the tides; and hence he would be solicitous to learn from his observation all that would be useful to him in the future. to the coasting sailor the question of the day is the time of high water. that time varies from day to day; it is an hour or more later to-morrow than to-day, and there is no very simple rule which can be enunciated. the sailor would therefore welcome gladly any rule which would guide him in a matter of such importance. we can make a conjecture as to the manner in which such a rule was first discovered. let us suppose that a sailor at calais, for example, is making for harbour. he has a beautiful night--the moon is full; it guides him on his way; he gets safely into harbour; and the next morning he finds the tide high between and .[ ] he often repeats the same voyage, but he finds sometimes a low and inconvenient tide in the morning. at length, however, it occurs to him that _when he has a moonlight night_ he has a high tide at . this occurs once or twice: he thinks it but a chance coincidence. it occurs again and again. at length he finds it always occurs. he tells the rule to other sailors; they try it too. it is invariably found that when the moon is full, the high tide always recurs at the same hour at the same place. the connection between the moon and the tide is thus established, and the intelligent sailor will naturally compare other phases of the moon with the times of high water. he finds, for example, that the moon at the first quarter always gives high water at the same hour of the day; and finally, he obtains a practical rule, by which, from the state of the moon, he can at once tell the time when the tide will be high at the port where his occupation lies. a diligent observer will trace a still further connection between the moon and the tides; he will observe that some high tides rise higher than others, that some low tides fall lower than others. this is a matter of much practical importance. when a dangerous bar has to be crossed, the sailor will feel much additional security in knowing that he is carried over it on the top of a spring tide; or if he has to contend against tidal currents, which in some places have enormous force, he will naturally prefer for his voyage the neap tides, in which the strength of these currents is less than usual. the spring tides and the neap tides will become familiar to him, and he will perceive that the spring tides occur when the moon is full or new--or, at all events, that the spring tides are within a certain constant number of days of the full or new moon. it was, no doubt, by reasoning such as this, that in primitive times the connection between the moon and the tides came to be perceived. it was not, however, until the great discovery of newton had disclosed the law of universal gravitation that it became possible to give a physical explanation of the tides. it was then seen how the moon attracts the whole earth and every particle of the earth. it was seen how the fluid particles which form the oceans on the earth were enabled to obey the attraction in a way that the solid parts could not. when the moon is overhead it tends to draw the water up, as it were, into a heap underneath, and thus to give rise to the high tide. the water on the opposite side of the earth is also affected in a way that might not be at first anticipated. the moon attracts the solid body of the earth with greater intensity than it attracts the water at the other side which lies more distant from it. the earth is thus drawn away from the water, and there is therefore a tendency to a high tide as well on the side of the earth away from the moon as on that towards the moon. the low tides occupy the intermediate positions. the sun also excites tides on the earth; but owing to the great distance of the sun, the difference between its attraction on the sea and on the solid interior of the earth is not so appreciable. the solar tides are thus smaller than the lunar tides. when the two conspire, they cause a spring tide; when the solar and lunar tides are opposed, we have the neap tide. there are, however, a multitude of circumstances to be taken into account when we attempt to apply this general reasoning to the conditions of a particular case. owing to local peculiarities the tides vary enormously at the different parts of the coast. in a confined area like the mediterranean sea, the tides have only a comparatively small range, varying at different places from one foot to a few feet. in mid-ocean also the tidal rise and fall is not large, amounting, for instance, to a range of three feet at st. helena. near the great continental masses the tides become very much modified by the coasts. we find at london a tide of eighteen or nineteen feet; but the most remarkable tides in the british islands are those in the bristol channel, where, at chepstow or cardiff, there is a rise and fall during spring tides to the height of thirty-seven or thirty-eight feet, and at neap tides to a height of twenty-eight or twenty-nine. these tides are surpassed in magnitude at other parts of the world. the greatest of all tides are those in the bay of fundy, at some parts of which the rise and fall at spring tides is not less than fifty feet. the rising and falling of the tide is necessarily attended with the formation of currents. such currents are, indeed, well known, and in some of our great rivers they are of the utmost consequence. these currents of water can, like water-streams of any other kind, be made to do useful work. we can, for instance, impound the rising water in a reservoir, and as the tide falls we can compel the enclosed water to work a water-wheel before it returns to the sea. we have, indeed, here a source of actual power; but it is only in very unusual circumstances that it is found to be economical to use the tides for this purpose. the question can be submitted to calculation, and the area of the reservoir can be computed which would retain sufficient water to work a water-wheel of given horse-power. it can be shown that the area of the reservoir necessary to impound water enough to produce horse-power would be acres. the whole question is then reduced to the simple one of expense: would the construction and the maintenance of this reservoir be more or less costly than the erection and the maintenance of a steam-engine of equivalent power? in most cases it would seem that the latter would be by far the cheaper; at all events, we do not practically find tidal engines in use, so that the power of the tides is now running to waste. the economical aspects of the case may, however, be very profoundly altered at some remote epoch, when our stores of fuel, now so lavishly expended, give appreciable signs of approaching exhaustion. the tides are, however, _doing work_ of one kind or another. a tide in a river estuary will sometimes scour away a bank and carry its materials elsewhere. we have here work done and energy consumed, just as much as if the same task had been accomplished by engineers directing the powerful arms of navvies. we know that work cannot be done without the consumption of energy in some of its forms; whence, then, comes the energy which supplies the power of the tides? at a first glance the answer to this question seems a very obvious one. have we not said that the tides are caused by the moon? and must not the energy, therefore, be derived from the moon? this seems plain enough, but, unfortunately, it is not true. it is one of those cases by no means infrequent in dynamics, where the truth is widely different from that which seems to be the case. an illustration will perhaps make the matter clearer. when a rifle is fired, it is the finger of the rifleman that pulls the trigger; but are we, then, to say that the energy by which the bullet has been driven off has been supplied by the rifleman? certainly not; the energy is, of course, due to the gunpowder, and all the rifleman did was to provide the means by which the energy stored up in the powder could be liberated. to a certain extent we may compare this with the tidal problem; the tides raised by the moon are the originating cause whereby a certain store of energy is drawn upon and applied to do such work as the tides are competent to perform. this store of energy, strange to say, does not lie in the moon; it is in the earth itself. indeed, it is extremely remarkable that the moon actually gains energy from the tides by itself absorbing some of the store which exists in the earth. this is not put forward as an obvious result; it depends upon a refined dynamical theorem. we must clearly understand the nature of this mighty store of energy from which the tides draw their power, and on which the moon is permitted to make large and incessant drafts. let us see in what sense the earth is said to possess a store of energy. we know that the earth rotates on its axis once every day. it is this rotation which is the source of the energy. let us compare the rotation of the earth with the rotation of the fly-wheel belonging to a steam-engine. the rotation of the fly-wheel is really a reservoir, into which the engine pours energy at each stroke of the piston. the various machines in the mill worked by the engine merely draw upon the store of energy accumulated in the fly-wheel. the earth may be likened to a gigantic fly-wheel detached from the engine, though still connected with the machines in the mill. from its stupendous dimensions and from its rapid velocity, that great fly-wheel possesses an enormous store of energy, which must be expended before the fly-wheel comes to rest. hence it is that, though the tides are caused by the moon, yet the energy they require is obtained by simply appropriating some of the vast supply available from the rotation of the earth. there is, however, a distinction of a very fundamental character between the earth and the fly-wheel of an engine. as the energy is withdrawn from the fly-wheel and consumed by the various machines in the mill, it is continually replaced by fresh energy, which flows in from the exertions of the steam-engine, and thus the velocity of the fly-wheel is maintained. but the earth is a fly-wheel without the engine. when the tides draw upon the store of energy and expend it in doing work, that energy is not replaced. the consequence is irresistible: the energy in the rotation of the earth must be decreasing. this leads to a consequence of the utmost significance. if the engine be cut off from the fly-wheel, then, as everyone knows, the massive fly-wheel may still give a few rotations, but it will speedily come to rest. a similar inference must be made with regard to the earth; but its store of energy is so enormous, in comparison with the demands which are made upon it, that the earth is able to hold out. ages of countless duration must elapse before the energy of the earth's rotation can be completely exhausted by such drafts as the tides are capable of making. nevertheless, it is necessarily true that the energy is decreasing; and if it be decreasing, then the speed of the earth's rotation must be surely, if slowly, abating. now we have arrived at a consequence of the tides which admits of being stated in the simplest language. if the speed of rotation be abating, then the length of the day must be increasing; and hence we are conducted to the following most important statement: that the _tides are increasing the length of the day_. to-day is longer than yesterday--to-morrow will be longer than to-day. the difference is so small that even in the course of ages it can hardly be said to have been distinctly established by observation. we do not pretend to say how many centuries have elapsed since the day was even one second shorter than it is at present; but centuries are not the units which we employ in tidal evolution. a million years ago it is quite probable that the divergence of the length of the day from its present value may have been very considerable. let us take a glance back into the profound depths of times past, and see what the tides have to tell us. if the present order of things has lasted, the day must have been shorter and shorter the farther we look back into the dim past. the day is now twenty-four hours; it was once twenty hours, once ten hours; it was once six hours. how much farther can we go? once the six hours is past, we begin to approach a limit which must at some point bound our retrospect. the shorter the day the more is the earth bulged at the equator; the more the earth is bulged at the equator the greater is the strain put upon the materials of the earth by the centrifugal force of its rotation. if the earth were to go too fast it would be unable to cohere together; it would separate into pieces, just as a grindstone driven too rapidly is rent asunder with violence. here, therefore, we discern in the remote past a barrier which stops the present argument. there is a certain critical velocity which is the greatest that the earth could bear without risk of rupture, but the exact amount of that velocity is a question not very easy to answer. it depends upon the nature of the materials of the earth; it depends upon the temperature; it depends upon the effect of pressure, and on other details not accurately known to us. an estimate of the critical velocity has, however, been made, and it has been shown mathematically that the shortest period of rotation which the earth could have, without flying into pieces, is about three or four hours. the doctrine of tidal evolution has thus conducted us to the conclusion that, at some inconceivably remote epoch, the earth was spinning round its axis in a period approximating to three or four hours. we thus learn that we are indebted to the moon for the gradual elongation of the day from its primitive value up to twenty-four hours. in obedience to one of the most profound laws of nature, the earth has reacted on the moon, and the reaction of the earth has taken a tangible form. it has simply consisted in gradually driving the moon away from the earth. you may observe that this driving away of the moon resembles a piece of retaliation on the part of the earth. the consequence of the retreat of the moon is sufficiently remarkable. the path in which the moon is revolving has at the present time a radius of , miles. this radius must be constantly growing larger, in consequence of the tides. provided with this fact, let us now glance back into the past history of the moon. as the moon's distance is increasing when we look forwards, so we find it decreasing when we look backwards. the moon must have been nearer the earth yesterday than it is to-day; the difference is no doubt inappreciable in years, in centuries, or in thousands of years; but when we come to millions of years, the moon must have been significantly closer than it is at present, until at length we find that its distance, instead of , miles, has dwindled down to , , to , , to , miles. nor need we stop--nor can we stop--until we find the moon actually close to the earth's surface. if the present laws of nature have operated long enough, and if there has been no external interference, then it cannot be doubted that the moon and the earth were once in immediate proximity. we can, indeed, calculate the period in which the moon must have been revolving round the earth. the nearer the moon is to the earth the quicker it must revolve; and at the critical epoch when the satellite was in immediate proximity to our earth it must have completed each revolution in about three or four hours. this has led to one of the most daring speculations which has ever been made in astronomy. we cannot refrain from enunciating it; but it must be remembered that it is only a speculation, and to be received with corresponding reserve. the speculation is intended to answer the question, what brought the moon into that position, close to the surface of the earth? we will only say that there is the gravest reason to believe that the moon was, at some very early period, fractured off from the earth when the earth was in a soft or plastic condition. at the beginning of the history we found the earth and the moon close together. we found that the rate of rotation of the earth was only a few hours, instead of twenty-four hours. we found that the moon completed its journey round the primitive earth in exactly the same time as the primitive earth rotated on its axis, so that the two bodies were then constantly face to face. such a state of things formed what a mathematician would describe as a case of unstable dynamical equilibrium. it could not last. it may be compared to the case of a needle balanced on its point; the needle must fall to one side or the other. in the same way, the moon could not continue to preserve this position. there were two courses open: the moon must either have fallen back on the earth, and been reabsorbed into the mass of the earth, or it must have commenced its outward journey. which of these courses was the moon to adopt? we have no means, perhaps, of knowing exactly what it was which determined the moon to one course rather than to another, but as to the course which was actually taken there can be no doubt. the fact that the moon exists shows that it did not return to the earth, but commenced its outward journey. as the moon recedes from the earth it must, in conformity with kepler's laws, require a longer time to complete its revolution. it has thus happened that, from the original period of only a few hours, the duration has increased until it has reached the present number of hours. the rotation of the earth has, of course, also been modified, in accordance with the retreat of the moon. once the moon had commenced to recede, the earth was released from the obligation which required it constantly to direct the same face to the moon. when the moon had receded to a certain distance, the earth would complete the rotation in less time than that required by the moon for one revolution. still the moon gets further and further away, and the duration of the revolution increases to a corresponding extent, until three, four, or more days (or rotations of the earth) are identical with the month (or revolution of the moon). although the number of days in the month increases, yet we are not to suppose that the rate of the earth's rotation is increasing; indeed, the contrary is the fact. the earth's rotation is getting slower, and so is the revolution of the moon, but the retardation of the moon is greater than that of the earth. even though the period of rotation of the earth has greatly increased from its primitive value, yet the period of the moon has increased still more, so that it is several times as large as that of the rotation of the earth. as ages roll on the moon recedes further and further, its orbit increases, the duration of the revolution augments, until at length a very noticeable epoch is attained, which is, in one sense, a culminating point in the career of the moon. at this epoch the revolution periods of the moon, when measured in rotation periods of the earth, attain their greatest value. it would seem that the month was then twenty-nine days. it is not, of course, meant that the month and the day at that epoch were the month and the day as our clocks now measure time. both were shorter then than now. but what we mean is, that at this epoch the earth rotated twenty-nine times on its axis while the moon completed one circuit. this epoch has now been passed. no attempt can be made at present to evaluate the date of that epoch in our ordinary units of measurement. at the same time, however, no doubt can be entertained as to the immeasurable antiquity of the event, in comparison with all historic records; but whether it is to be reckoned in hundreds of thousands of years, in millions of years, or in tens of millions of years, must be left in great degree to conjecture. this remarkable epoch once passed, we find that the course of events in the earth-moon system begins to shape itself towards that remarkable final stage which has points of resemblance to the initial stage. the moon still continues to revolve in an orbit with a diameter steadily, though very slowly, growing. the length of the month is accordingly increasing, and the rotation of the earth being still constantly retarded, the length of the day is also continually growing. but the ratio of the length of the month to the length of the day now exhibits a change. that ratio had gradually increased, from unity at the commencement, up to the maximum value of somewhere about twenty-nine at the epoch just referred to. the ratio now begins again to decline, until we find the earth makes only twenty-eight rotations, instead of twenty-nine, in one revolution of the moon. the decrease in the ratio continues until the number twenty-seven expresses the days in the month. here, again, we have an epoch which it is impossible for us to pass without special comment. in all that has hitherto been said we have been dealing with events in the distant past; and we have at length arrived at the present state of the earth-moon system. the days at this epoch are our well-known days, the month is the well-known period of the revolution of our moon. at the present time the month is about twenty-seven of our days, and this relation has remained sensibly true for thousands of years past. it will continue to remain sensibly true for thousands of years to come, but it will not remain true indefinitely. it is merely a stage in this grand transformation; it may possess the attributes of permanence to our ephemeral view, just as the wings of a gnat seem at rest when illuminated by the electric spark; but when we contemplate the history with time conceptions sufficiently ample for astronomy we realise how the present condition of the earth-moon system can have no greater permanence than any other stage in the history. our narrative must, however, now assume a different form. we have been speaking of the past; we have been conducted to the present; can we say anything of the future? here, again, the tides come to our assistance. if we have rightly comprehended the truth of dynamics (and who is there now that can doubt them?), we shall be enabled to make a forecast of the further changes of the earth-moon system. if there be no interruption from any external source at present unknown to us, we can predict--in outline, at all events--the subsequent career of the moon. we can see how the moon will still follow its outward course. the path in which it revolves will grow with extreme slowness, but yet it will always grow; the progress will not be reversed, at all events, before the final stage of our history has been attained. we shall not now delay to dwell on the intervening stages; we will rather attempt to sketch the ultimate type to which our system tends. in the dim future--countless millions of years to come--this final stage will be approached. the ratio of the month to the day, whose decline we have already referred to, will continue to decline. the period of revolution of the moon will grow longer and longer, but the length of the day will increase much more rapidly than the increase in the duration of the moon's period. from the month of twenty-seven days we shall pass to a month of twenty-six days, and so on, until we shall reach a month of ten days, and, finally, a month of one day. let us clearly understand what we mean by a month of one day. we mean that the time in which the moon revolves around the earth will be equal to the time in which the earth rotates around its axis. the length of this day will, of course, be vastly greater than our day. the only element of uncertainty in these enquiries arises when we attempt to give numerical accuracy to the statements. it seems to be as true as the laws of dynamics that a state of the earth-moon system in which the day and the month are equal must be ultimately attained; but when we attempt to state the length of that day we introduce a hazardous element into the enquiry. in giving any estimate of its length, it must be understood that the magnitude is stated with great reserve. it may be erroneous to some extent, though, perhaps, not to any considerable amount. the length of this great day would seem to be about equal to fifty-seven of our days. in other words, at some critical time in the excessively distant future, the earth will take something like , hours to perform a rotation, while the moon will complete its journey precisely in the same time. we thus see how, in some respects, the first stage of the earth-moon system and the last stage resemble each other. in each case we have the day equal to the month. in the first case the day and the month were only a small fraction of our day; in the last stage the day and the month are each a large multiple of our day. there is, however, a profound contrast between the first critical epoch and the last. we have already mentioned that the first epoch was one of unstability--it could not last; but this second state is one of dynamical stability. once that state has been acquired, it would be permanent, and would endure for ever if the earth and the moon could be isolated from all external interference. there is one special feature which characterises the movement when the month is equal to the day. a little reflection will show that when this is the case the earth must constantly direct the same face towards the moon. if the day be equal to the month, then the earth and moon must revolve together, as if bound by invisible bands; and whatever hemisphere of the earth be directed to the moon when this state of things commences will remain there so long as the day remains equal to the month. at this point it is hardly possible to escape being reminded of that characteristic feature of the moon's motion which has been observed from all antiquity. we refer, of course, to the fact that the moon at the present time constantly turns the same face to the earth. it is incumbent upon astronomers to provide a physical explanation of this remarkable fact. the moon revolves around our earth once in a definite number of seconds. if the moon always turns the same face to the earth, then it is demonstrated that the moon rotates on its axis once in the same number of seconds also. now, this would be a coincidence wildly improbable unless there were some physical cause to account for it. we have not far to seek for a cause: the tides on the moon have produced the phenomenon. we now find the moon has a rugged surface, which testifies to the existence of intense volcanic activity in former times. those volcanoes are now silent--the internal fires in the moon seem to have become exhausted; but there was a time when the moon must have been a heated and semi-molten mass. there was a time when the materials of the moon were so hot as to be soft and yielding, and in that soft and yielding mass the attraction of our earth excited great tides. we have no historical record of these tides (they were long anterior to the existence of telescopes, they were probably long anterior to the existence of the human race), but we know that these tides once existed by the work they have accomplished, and that work is seen to-day in the constant face which the moon turns towards the earth. the gentle rise and fall of the oceans which form our tides present a picture widely different from the tides by which the moon was once agitated. the tides on the moon were vastly greater than those of the earth. they were greater because the weight of the earth is greater than that of the moon, so that the earth was able to produce much more powerful tides in the moon than the moon has ever been able to raise on the earth. that the moon should bend the same face to the earth depends immediately upon the condition that the moon shall rotate on its axis in precisely the same period as that which it requires to revolve around the earth. the tides are a regulating power of unremitting efficiency to ensure that this condition shall be observed. if the moon rotated more slowly than it ought, then the great lava tides would drag the moon round faster and faster until it attained the desired velocity; and then, but not till then, they would give the moon peace. or if the moon were to rotate faster on its axis than in its orbit, again the tides would come furiously into play; but this time they would be engaged in retarding the moon's rotation, until they had reduced the speed of the moon to one rotation for each revolution. can the moon ever escape from the thraldom of the tides? this is not very easy to answer, but it seems perhaps not impossible that the moon may, at some future time, be freed from tidal control. it is, indeed, obvious that the tides, even at present, have not the extremely stringent control over the moon which they once exercised. we now see no ocean on the moon, nor do the volcanoes show any trace of molten lava. there can hardly be tides _on_ the moon, but there may be tides _in_ the moon. it may be that the interior of the moon is still hot enough to retain an appreciable degree of fluidity, and if so, the tidal control would still retain the moon in its grip; but the time will probably come, if it have not come already, when the moon will be cold to the centre--cold as the temperature of space. if the materials of the moon were what a mathematician would call absolutely rigid, there can be no doubt that the tides could no longer exist, and the moon would be emancipated from tidal control. it seems impossible to predicate how far the moon can ever conform to the circumstances of an actual rigid body, but it may be conceivable that at some future time the tidal control shall have practically ceased. there would then be no longer any necessary identity between the period of rotation and that of revolution. a gleam of hope is thus projected over the astronomy of the distant future. we know that the time of revolution of the moon is increasing, and so long as the tidal governor could act, the time of rotation must increase sympathetically. we have now surmised a state of things in which the control is absent. there will then be nothing to prevent the rotation remaining as at present, while the period of revolution is increasing. the privilege of seeing the other side of the moon, which has been withheld from all previous astronomers, may thus in the distant future be granted to their successors. the tides which the moon raises in the earth act as a brake on the rotation of the earth. they now constantly tend to bring the period of rotation of the earth to coincide with the period of revolution of the moon. as the moon revolves once in twenty-seven days, the earth is at present going too fast, and consequently the tidal control at the present moment endeavours to retard the rotation of the earth. the rotation of the moon long since succumbed to tidal control, but that was because the moon was comparatively small and the tidal power of the earth was enormous. but this is the opposite case. the earth is large and more massive than the moon, the tides raised by the moon are but small and weak, and the earth has not yet completely succumbed to the tidal action. but the tides are constant, they never for an instant relax the effort to control, and they are gradually tending to render the day and the month coincident, though the progress is a very slow one. the theory of the tides leads us to look forward to a remote state of things, in which the moon revolves around the earth in a period equal to the day, so that the two bodies shall constantly bend the same face to each other, provided the tidal control be still able to guide the moon's rotation. so far as the mutual action of the earth and the moon is concerned, such an arrangement possesses all the attributes of permanence. if, however, we venture to project our view to a still more remote future, we can discern an external cause which must prevent this mutual accommodation between the earth and the moon from being eternal. the tides raised by the moon on the earth are so much greater than those raised by the sun, that we have, in the course of our previous reasoning, held little account of the sun-raised tides. this is obviously only an approximate method of dealing with the question. the influence of the solar tide is appreciable, and its importance relatively to the lunar tide will gradually increase as the earth and moon approach the final critical stage. the solar tides will have the effect of constantly applying a further brake to the rotation of the earth. it will therefore follow that, after the day and the month have become equal, a still further retardation awaits the length of the day. we thus see that in the remote future we shall find the moon revolving around the earth in a shorter time than that in which the earth rotates on its axis. a most instructive corroboration of these views is afforded by the discovery of the satellites of mars. the planet mars is one of the smaller members of our system. it has a mass which is only the eighth part of the mass of the earth. a small planet like mars has much less energy of rotation to be destroyed than a larger one like the earth. it may therefore be expected that the small planet will proceed much more rapidly in its evolution than the large one; we might, therefore, anticipate that mars and his satellites have attained a more advanced stage of their history than is the case with the earth and her satellite. when the discovery of the satellites of mars startled the world, in , there was no feature which created so much amazement as the periodic time of the interior satellite. we have already pointed out in chapter x. how phobos revolves around mars in a period of hours minutes. the period of rotation of mars himself is hours minutes, and hence we have the fact, unparalleled in the solar system, that the satellite is actually revolving three times as rapidly as the planet is rotating. there can hardly be a doubt that the solar tides on mars have abated its velocity of rotation in the manner just suggested. it has always seemed to me that the matter just referred to is one of the most interesting and instructive in the whole history of astronomy. we have, first, a very beautiful telescopic discovery of the minute satellites of mars, and we have a determination of the anomalous movement of one of them. we have then found a satisfactory physical explanation of the cause of this phenomenon, and we have shown it to be a striking instance of tidal evolution. finally, we have seen that the system of mars and his satellite is really a forecast of the destiny which, after the lapse of ages, awaits the earth-moon system. it seems natural to enquire how far the influence of tides can have contributed towards moulding the planetary orbits. the circumstances are here very different from those we have encountered in the earth-moon system. let us first enunciate the problem in a definite shape. the solar system consists of the sun in the centre, and of the planets revolving around the sun. these planets rotate on their axes; and circulating round some of the planets we have their systems of satellites. for simplicity, we may suppose all the planets and their satellites to revolve in the same plane, and the planets to rotate about axes which are perpendicular to that plane. in the study of the theory of tidal evolution we must be mainly guided by a profound dynamical principle known as the conservation of the "moment of momentum." the proof of this great principle is not here attempted; suffice it to say that it can be strictly deduced from the laws of motion, and is thus only second in certainty to the fundamental truths of ordinary geometry or of algebra. take, for instance, the giant planet, jupiter. in one second he moves around the sun through a certain angle. if we multiply the mass of jupiter by that angle, and if we then multiply the product by the square of the distance from jupiter to the sun, we obtain a certain definite amount. a mathematician calls this quantity the "orbital" moment of momentum of jupiter.[ ] in the same way, if we multiply the mass of saturn by the angle through which the planet moves in one second, and this product by the square of the distance between the planet and the sun, then we have the orbital moment of momentum of saturn. in a similar manner we ascertain the moment of momentum for each of the other planets due to revolution around the sun. we have also to define the moment of momentum of the planets around their axes. in one second jupiter rotates through a certain angle; we multiply that angle by the mass of jupiter, and by the square of a certain line which depends on his internal constitution: the product forms the "rotational" moment of momentum. in a similar manner we find the rotational moment of momentum for each of the other planets. each satellite revolves through a certain angle around its primary in one second; we obtain the moment of momentum of each satellite by multiplying its mass into the angle described in one second, and then multiplying the product into the square of the distance of the satellite from its primary. finally, we compute the moment of momentum of the sun due to its rotation. this we obtain by multiplying the angle through which the sun turns in one second by the whole mass of the sun, and then multiplying the product by the square of a certain line of prodigious length, which depends upon the details of the sun's internal structure. if we have succeeded in explaining what is meant by the moment of momentum, then the statement of the great law is comparatively simple. we are, in the first place, to observe that the moment of momentum of any planet may alter. it would alter if the distance of the planet from the sun changed, or if the velocity with which the planet rotates upon its axis changed; so, too, the moment of momentum of the sun may change, and so may those of the satellites. in the beginning a certain total quantity of moment of momentum was communicated to our system, and not one particle of that total can the solar system, as a whole, squander or alienate. no matter what be the mutual actions of the various bodies of the system, no matter what perturbations they may undergo--what tides may be produced, or even what mutual collisions may occur--the great law of the conservation of moment of momentum must be obeyed. if some bodies in the solar system be losing moment of momentum, then other bodies in the system must be gaining, so that the total quantity shall remain unaltered. this consideration is one of supreme importance in connection with the tides. the distribution of moment of momentum in the system is being continually altered by the tides; but, however the tides may ebb or flow, the total moment of momentum can never alter so long as influences external to the system are absent. we must here point out the contrast between the endowment of our system with energy and with moment of momentum. the mutual actions of our system, in so far as they produce heat, tend to squander the energy, a considerable part of which can be thus dissipated and lost; but the mutual actions have no power of dissipating the moment of momentum. the total moment of momentum of the solar system being taken to be , this is at present distributed as follows:-- orbital moment of momentum of jupiter orbital moment of momentum of saturn orbital moment of momentum of uranus orbital moment of momentum of neptune rotational moment of momentum of sun -- the contributions of the other items are excessively minute. the orbital moments of momentum of the few interior planets contain but little more than one thousandth part of the total amount. the rotational contributions of all the planets and of their satellites is very much less, being not more than one sixty-thousandth part of the whole. when, therefore, we are studying the general effects of tides on the planetary orbits these trifling matters may be overlooked. we shall, however, find it desirable to narrow the question still more, and concentrate our attention on one splendid illustration. let us take the sun and the planet jupiter, and, supposing all other bodies of our system to be absent, let us discuss the influence of tides produced in jupiter by the sun, and of tides in the sun by jupiter. it might be hastily thought that, just as the moon was born of the earth, so the planets were born of the sun, and have gradually receded by tides into their present condition. we have the means of enquiry into this question by the figures just given, and we shall show that it is impossible that jupiter, or any of the other planets, can ever have been very much closer to the sun than they are at present. in the case of jupiter and the sun we have the moment of momentum made up of three items. by far the largest of these items is due to the orbital revolution of jupiter, the next is due to the sun, the third is due to the rotation of jupiter on its axis. we may put them in round numbers as follows:-- orbital moment of momentum of jupiter , rotational moment of momentum of sun , rotational moment of momentum of jupiter the sun produces tides in jupiter, those tides retard the rotation of jupiter. they make jupiter rotate more and more slowly, therefore the moment of momentum of jupiter is decreasing, therefore its present value of must be decreasing. even the mighty sun himself may be distracted by tides. jupiter raises tides in the sun, those tides retard the motion of the sun, and therefore the moment of momentum of the sun is decreasing, and it follows from both causes that the item of , must be increasing; in other words, the orbital motion of jupiter must be increasing, or jupiter must be receding from the sun. to this extent, therefore, the sun-jupiter system is analogous to the earth-moon system. as the tides on the earth are driving away the moon, so the tides in jupiter and the sun are gradually driving the two bodies apart. but there is a profound difference between the two cases. it can be proved that the tides produced in jupiter by the sun are more effective than those produced in the sun by jupiter. the contribution of the sun may, therefore, be at present omitted; so that, practically, the augmentations of the orbital moment of momentum of jupiter are now achieved at the expense of that stored up by jupiter's rotation. but what is compared with , . even when the whole of jupiter's rotational moment of momentum and that of his satellites has become absorbed into the orbital motion, there will hardly be an appreciable difference in the latter. in ancient days we may indeed suppose that jupiter being hotter was larger than at present, and that he had considerably more rotational moment of momentum. but it is hardly credible that jupiter can ever have had one hundred times the moment of momentum that he has at present. yet even if , units of rotational momentum had been transferred to the orbital motion it would only correspond with the most trivial difference in the distance of jupiter from the sun. we are hence assured that the tides have not appreciably altered the dimensions of the orbit of jupiter, or of the other great planets. the time will, however, come when the rotation of jupiter on his axis will be gradually abated by the influence of the tides. it will then be found that the moment of momentum of the sun's rotation will be gradually expended in increasing the orbits of the planets, but as this reserve only holds about two per cent. of the whole amount in our system it cannot produce any considerable effect. the theory of tidal evolution, which in the hands of professor darwin has taught us so much with regard to the past history of the systems of satellites in the solar system, will doubtless also, as pointed out by dr. see, be found to account for the highly eccentric orbits of double star systems. in the earth-moon system we have two bodies exceedingly different in bulk, the mass of the earth being about eighty times as great as that of the moon. but in the case of most double stars we have to do with two bodies not very different as regards mass. it can be demonstrated that the orbit must have been originally of slight eccentricity, but that tidal friction is capable not only of extending, but also of elongating it. the accelerating force is vastly greater at periastron (when the two bodies are nearest each other) than at apastron (when their distance is greatest). at periastron the disturbing force will, therefore, increase the apastron distance by an enormous amount, while at apastron it increases the periastron distance by a very small amount. thus, while the ellipse is being gradually expanded, the orbit grows more and more eccentric, until the axial rotations have been sufficiently reduced by the transfer of axial to orbital moment of momentum. and now we must draw this chapter to a close, though there are many other subjects that might be included. the theory of tidal evolution is, indeed, one of quite exceptional interest. the earlier mathematicians expended their labour on the determination of the dynamics of a system which consisted of rigid bodies. we are indebted to contemporary mathematicians for opening up celestial mechanics upon the more real supposition that the bodies are not rigid; in other words, that they are subject to tides. the mathematical difficulties are enormously enhanced, but the problem is more true to nature, and has already led to some of the most remarkable astronomical discoveries made in modern times. * * * * * our story of the heavens has now been told. we commenced this work with some account of the mechanical and optical aids to astronomy; we have ended it with a brief description of an intellectual method of research which reveals some of the celestial phenomena that occurred ages before the human race existed. we have spoken of those objects which are comparatively near to us, and then, step by step, we have advanced to the distant nebulæ and clusters which seem to lie on the confines of the visible universe. yet how little can we see with even our greatest telescopes, when compared with the whole extent of infinite space! no matter how vast may be the depth which our instruments have sounded, there is yet a beyond of infinite extent. imagine a mighty globe described in space, a globe of such stupendous dimensions that it shall include the sun and his system, all the stars and nebulæ, and even all the objects which our finite capacities can imagine. yet, what ratio must the volume of this great globe bear to the whole extent of infinite space? the ratio is infinitely less than that which the water in a single drop of dew bears to the water in the whole atlantic ocean. appendix. astronomical quantities. the sun. the sun's mean distance from the earth is , , miles; his diameter is , miles; his mean density, as compared with water, is · ; his ellipticity is insensible; he rotates on his axis in a period between and days. the moon. the moon's mean distance from the earth is , miles. the diameter of the moon is , miles; and her mean density, as compared with water, is · . the time of a revolution around the earth is · days. the planets. ___________________________________________________________________________ | |distance from the sun in | | mean | |density | | | millions of miles. | periodic |diameter| axial |compared| | |-------------------------| time | in | rotation.| with | | | mean. | least.|greatest.| in days. | miles. | | water. | |-------+-------+-------+---------+----------+--------+----------+--------| |mercury| · | · | · | · | , | (?) | · (?)| |venus | · | · | · | · | , | (?) | · | |earth | · | · | · | · | , | · | · | |mars | | | | · | , | · | · | |jupiter| | | | , · | , | -- | · | |saturn | | | | , | , | -- | · | |uranus | , | , | , | , | , | unknown | · | |neptune| , | , | , | , | , | unknown | · | --------------------------------------------------------------------------- the satellites of mars. mean distance from periodic time. name. centre of mars. hrs. mins. secs. phobos , miles deimos , miles the satellites of jupiter. mean distance from periodic time. name. centre of jupiter. days. hrs. mins. secs. new inner satellite barnard , miles i. , miles ii. , miles iii. , miles iv. , , miles the satellites of saturn. mean distance from periodic time. name. centre of saturn. days. hrs. mins. secs. mimas , miles enceladus , miles tethys , miles dione , miles rhea , miles titan , miles hyperion , miles iapetus , , miles the satellites of uranus. mean distance from periodic time. name. centre of uranus. days. hrs. mins. secs. ariel , miles umbriel , miles titania , miles oberon , miles the satellite of neptune. mean distance from periodic time. name. centre of neptune. days. hrs. mins. secs. satellite , miles index. a aberration of light, - ; and the apparent movements of stars, , ; bradley's discoveries, ; causes, - ; circles of stars, - ; dependent upon the velocity of light, ; effect on draco, ; telescopic investigation, achromatic combination of glasses, adams, professor j.c., and the discovery of neptune, - , - ; and the ellipse of the leonids, aërolite, the chaco, ; the orgueil, airy, sir george, alban mount meteorites, the, alcor, aldebaran, , , ; spectrum of, ; value of velocity of, algol, , almagest, the, alphonsus, alps, the great valley of the (lunar), altair, aluminium in the sun, ancients, astronomy of the, - andrews, professor, and basaltic formation at giant's causeway, andromeda, ; nebula in, , andromedes, the, shooting star shower, and biela's comet, antares, apennines (lunar), aphelion, aquarius, , aquila, or the eagle, arago, archimedes, arcturus, , ; value of velocity of, argelander's catalogue of stars, , argus, ariel, , aristarchus, aristillus, aristotle, lunar crater named after him, ; credulity respecting his writings, ; the moon and the tides and, asteroids, - astrea, astronomers of nineveh, astronomical quantities, astronomy, ancient, - ; galileo's achievements in, ; the first phenomenon of, _athenæum_, the, and sir john herschel's letter on adams's share in the discovery of neptune, atmosphere, height of the earth's, attraction, between the moon and the earth, ; between the planets, ; between the sun and the planets, , ; of jupiter, , ; producing precession, auriga, , aurora borealis, autolycus, auwers and star distances, ; and the irregularity in movement of sirius, axis, polar, , ; precession and nutation of the earth's, - b backlund, and encke's comet, , barnard, professor e.e., and saturn, , , ; and titan, ; and the comet of , ; and the milky way, beehive, the, belopolsky, m., and binaries, , benares meteorite, the, bessel, and bradley, ; and the distance of cygni, , , ; and the distances of stars, ; and the irregular movements of sirius, ; receives gold medal of royal astronomical society, betelgeuze, , , , ; value of velocity of, biela's comet, and sir john herschel, ; and the andromedes, binaries, spectroscopic, binocular glass, biot and the l'aigle meteorites, bode's law, ; list of double stars, bond, professor, and saturn's satellites, ; and the nebula in orion, ; and the third ring of saturn, boötes, bradley, and nutation, ; and the aberration of light, ; his observations of uranus, bredichin, professor, and the tails of comets, , , breitenbach iron, the, bristol channel, tides in the, brünnow, dr., observations on the parallax of cygni, _burial of sir john moore_, burnham, mr., and the orbit of sirius, ; his additions to the known number of double stars, butler, bishop, and probability, butsura meteorite, c cadmium in the sun, calais, tides at, calcium in the sun, campbell, mr., and argus, ; and mars, canals on mars, cancri , cancri, z, cancri, th, canis major, canopus, cape observatory, capella, , , carboniferous period, cardiff, tides at, cassini, j.d., and double stars, ; and saturn's satellites, ; and the rings of saturn, cassiopeia, castor, , ; a binary star, ; revolution of, catalogues of stars, , ; messier's, catharina, centauri, a, ; dr. gill's observations of, ; henderson's measurement of distance of, , ceres, , , ; and meteorites, , chaco meteorite, the, chacornac, and the lunar crater schickard, _challenger_, the cruise of the, and magnetic particles in the atlantic, challis, professor, ; his search for neptune, , , , chandler, mr., and algol, charles's wain, chepstow, tides at, chéseaux, discoverer of comet of , chicago, telescope at yerkes observatory, chladni and the meteorite of siberia, chromium in the sun, chromosphere, the, chronometers tested by the moon, clairaut and the attraction of planets on comets, , clavius, ; and jupiter's satellites, clock, astronomical, clusters, star, - cobalt in the sun, coggia's comet, , colour of light and indication of its source, colours, the seven primary, columbiad, the, columbus, comets, , , , ; and the spectroscope, ; attraction from planets, , ; biela's, ; biela's and the andromedes, ; clairaut's investigations, , ; coggia's, ; common's ( ), ; connection of, with shooting star showers, ; constitution of, ; containing sodium and iron, ; donati's ( ), , , ; eccentricity of, ; encke's, - ; existence of carbon in, , ; gravitation and, , ; halley's investigations about, - ; head or nucleus of, ; lexell's, ; mass of, ; movements of, ; newton's explanations of, ; non-periodic, - ; of , ; of , ; of , , ; of , ; of (chéseaux's), ; of , ; of , ; of , ; of , ; of , ; origin of, ; parabolic orbits of, - , ; periodic return of, - ; shape of, ; size of, ; tailless, ; tails of, , ; bredichin's researches, ; chéseaux's, ; composition of, , ; condensation of, ; electricity and, ; gradual growth of, ; law of direction of, ; repelled by the sun, ; repulsive force of, , ; various types of, ; tebbutt's ( ), ; tenuity of, common, dr., constructor of reflectors, ; and the comet of , ; and the nebula in orion, cook, captain, and the transit of venus, copeland, dr., and schmidt's star, ; and the lunar crater, tycho, ; and the spectra of nebula, ; and the transit of venus, copernicus and mercury, ; confirmation of his theory by the discovery of jupiter's satellites, ; his theory of astronomy, ; lunar crater called after him, copper in the sun, cor scorpionis, corona borealis, , corona of sun, during an eclipse, - , coronium, cotopaxi and meteorites, crab, the, crabtree, and the transit of venus, crape ring of saturn, craters in the moon, - , - critical velocity, , , crown, the, cryptograph of huyghens, the, cygni, b, cygni , annual parallax of, ; bessel's measurement of distance of, , , ; brünnow's observations of, ; distance from the sun of, ; disturbing influence of, ; double, ; professor a. hall's measurement of, ; professor pritchard's photographic researches concerning, ; proper motion of, ; struve's observations of, , ; velocity of, cygnus, cyrillus, cysat, and the belt of orion, d d line in solar spectrum, darwin, professor g.h., and tidal evolution, dawes, professor, and saturn's third ring, day, length of, and the moon, ; and the tides, deimos, , denebola, diffraction, dione, dispersion of colours, distances, astronomical, , doerfel, and comets, dog star (_see_ sirius) dog, the little, donati's comet, , ; tails, double stars, - d q, draco, nebula in, dragon, the, draper, professor, and the nebula in orion, dunsink observatory, , , , dynamical stability, ; theory of newton, dynamics and the earth-moon system, dynamics, galileo the founder of, e eagle, the, earth, the, ancient ideas respecting, ; annual movement of, and the apparent movement of the stars, , ; attraction of jupiter, ; attraction of on encke's comet, ; attraction of, on the leonids, ; attraction of saturn, ; attraction of the moon, , ; attraction of the sun, ; axial rotation of, ; carboniferous period on, ; change of climate on, ; composition of, ; contact of atmosphere of, with meteors, - ; density of, ; diameter of, ; distance of, from mars, ; distance of, from the moon, , ; distance of, from the sun, , , , , , , , ; energy from rotation of, ; formerly a molten globe, , ; geological records and, ; glacial period on, ; gravitation and, , , , ; heat in the interior of, , , , , ; how it is measured, - ; its mass increasing owing to the fall of meteoric matter, ; its oceans once vapour, ; once in immediate proximity to the moon, ; orbit of, ; orbit of, its elliptic form, ; path of deranged by venus and mars, ; periodic time of, ; plane of orbit of, ; polar axis of, , - ; position of, relatively to the sun and the moon, , ; precession and nutation of axis of, - ; radius of, , ; rotation of, , , , , ; shape of, , , , , ; size of, compared with jupiter, , and with other planets, ; size and weight of, compared with those of the sun, , and moon, , ; velocity of, , , , , and periodic time, ; volcanic outbreaks on, , and the origin of meteorites, ; weight of, , , as compared with saturn, , earthquakes, astronomical instruments disturbed by, eccentricity of planetary ellipses, , eclipse of jupiter's satellites, , , - ellipse of the moon, - ; of the sun, eclipses, ancient explanations of, ; calculations of the recurrence of, , ecliptic, the, , ; pole of the, , , electric light, the, ellipse, the, ; eccentricity of, ; focus of, ; kepler's discoveries respecting, , , - , ; the form which the orbit of a planet takes, ; the parallactic, ; variety of form of, enceladus, encke, and the distance of the sun from the earth, , ; his comet, - encke's comet, - ; approach to jupiter of, ; and mercury, ; and the sun, ; diminution in periodic time of, ; distance from mercury of, ; disturbed by the earth, , and by mercury, ; irregularities of, , ; orbit of, ; periodical return of, ; von asten's calculations concerning, - energy supplying the tides, ensisheim meteorite, the, equatorial diameter, , ; telescope, eratosthenes, eros, eruptions, evening star, , eye, structure of the, f faculæ of the sun, fire ball of , fire balls, "fixed" stars, flamsteed, first astronomer-royal, ; his _historia coelestis_, focus of planetary ellipse, - fomalhaut, fraunhofer, fraunhofer lines, fundy, bay of, tides in, g galileo, achievements of, ; and jupiter's satellites, ; and saturn's rings, , ; and the pleiades, galle, dr., and neptune, - gassendi, and the transit of mercury, ; and the transit of venus, ; lunar crater named after him, gauss, and the minor planet ceres, gemini, constellation of, , geminids, the, geologists and the lapse of time, geometers, oriental, geometry, cultivation by the ancients of, george iii. and sir w. herschel, , giant's causeway, gill, dr. d., ; and juno, ; and the minor planets, ; and the parallax of a centauri, ; and the parallax of mars, glacial period, gravitation, law of, - ; and binary stars, ; and precession, ; and the earth's axis, , , ; and the parabolic path of comets, ; and the periodical return of comets, ; and the weight of the earth, , ; illustrated by experiments, , , , - ; its discovery aided by lunar observations, , ; its influence on the satellites, ; its influence on stars, ; its influence on tides, ; le verrier's triumphant proof of, ; newton's discoveries, , , ; on the moon, ; universality amongst the heavenly bodies, , great bear, , , ; configuration, ; double star in the, ; positions of, , green, mr., and mars, greenwich observatory, , griffiths, mr., and jupiter, grimaldi, grubb, sir howard, "guards," the, _gulliver's travels_ and the satellites of mars, h hadley's observations of saturn, hall, professor asaph, and the satellites of mars, halley, and the periodicity of comets, - ; and the transit of venus, heat, bearings on astronomy, ; in the interior of the earth, - , ; of the sun, - heliometer, the, helium, henderson, and the distance of a centauri, , hercules, star cluster in, , herodotus (lunar crater), herschel, caroline, , herschel, sir john, address to british association, ; address on the presentation of gold medal to bessel, ; and biela's comet, ; and nebulæ, ; letter to _athenoeum_ on adams's share in the discovery of neptune, herschel, sir w., and double stars, , ; and saturn, ; and saturn's satellites, ; and the empress catherine, ; and the movement of solar system towards lyra, ; discovery of satellite of uranus by, , ; discovery of uranus by, , ; early life of, ; friendship with sir w. watson of ; he makes his own telescopes, ; "king's astronomer," ; method of making his telescopes, ; musical talent of, ; organist of octagon chapel, bath, ; pardon for desertion from george iii., ; passion for astronomy of, , ; relinquishes musical profession, ; sidereal aggregation theory of, ; study of the nebulæ by, - , herschelian telescope, _historia coelestis_, hoedi, the, holmes's, mr., comet ( ), horrocks, and the transit of venus, howard, mr., and the benares meteorite, huggins, sir w., , ; and nebulæ, huyghens, and saturn's rings, - ; discovers first satellite of saturn, hyades, the, hydrogen in sirius and vega, ; in the sun, hyginus, hyperion, i iapetus, iberians, the, inquisition, the, and galileo, iris, iron, dust in the arctic regions, ; in the sun, ; of meteorites, the, ; spectrum of, j janssen, m., , ; and the transit of venus, juno, , jupiter, ancient study of, ; and the leonids, ; attraction of, ; axial rotation of, ; belts of, ; brilliancy of, ; composition of, ; covered with an atmosphere of clouds, , ; density of, ; diameter of, , ; distance from the earth of, , ; distance from the sun of, , ; habitability of, ; heat received from the sun by, ; internal heat of, , , ; lack of permanent features of, ; lack of solidity of, , , ; moment of momentum of, , ; occultation of, ; orbit of, , , ; path of, perturbed by the attraction of saturn, ; periodic time of, ; a planet, or "wanderer," ; red spot in , ; revolution of, ; rotation of, , ; satellites of, , , - , , ; satellites of, and gravitation, ; satellites of, and the copernican theory, ; shadow from satellites of, ; shape of, , , , ; size of, compared with the earth, , , , and other planets, ; and the sun, ; storms on, ; tides on, ; weight of, , , and encke's comet, k keeler, professor, and saturn's ring, kempf, dr., and the sun's velocity, kepler, and comets, ; and laws of planetary motion, ; and meteors, ; and the orbit of mars, ; explanation of his laws, , , ; his discovery of the shape of the planetary orbits, , ; his first planetary law, ; lunar crater called after him, ; prediction of the transit of venus and mercury, , ; second law, ; third law, kids, the, kirchhoff, and spectrum analysis, kirkwood, professor, and the movements of saturn's satellites, klinkerfues, professor, l lagrange, and the theory of planetary perturbation, - ; his assumption of planetary rigidity, l'aigle meteorites, the, lalande, and neptune, , landscapes, lunar, lane, mr. j. homer, laplace, and the nebular theory, ; and the satellites of jupiter, ; and the theory of planetary perturbation, lassell, mr., and saturn's eighth satellite, ; discovers neptune's satellite, law of gravitation (_see_ gravitation) laws of planetary motion (_see_ planetary motion) lead in the sun, ledger, mr., and mercury, leibnitz, lunar mountains named after him, lemonnier, and uranus, leo, and shooting stars, , leonids, attractions of planets on, ; breadth of stream of, ; change of shape of, ; decrease of, ; enormous number of, ; historical records, ; length of stream of, ; le verrier, and the cause of their introduction into the solar system, ; meteor shoal of, ; periodic return of, ; their connection with comets and professor, schiaparelli, leonis g, value of velocity of, leverage by equatorial protuberance, le verrier, and mars, ; and the discovery of neptune, - ; and the introduction of the leonids into the solar system, ; and the weight of mercury, lexell's comet, libration, lick observatory, light, aberration of, - ; velocity of, , , , , linné, , lion, the, , little bear, the, little dog, the, livy, and meteorites, lloyd, provost, lockyer, sir norman, and betelgeuze, ; and solar light, london, tides at, louvain, f. terby, and titan, lowell, mr., and mercury, lunar tides, , lyra, motion of solar system towards, lyre, the, ; nebula in, lyrids, the, m mädler, and the lunar craters, , , magellanic clouds, magnesium, colour of flame from, ; in the sun, magnetism, connection with sun spots, manganese in the sun, maraldi, and the rings of saturn, mare crisium, ; foecunditatis, ; humorum, ; imbrium, , ; nectaris ; nubium, ; serenitatis, ; tranquillitatis, ; vaporum, mars, ancient study of, ; appearance of, through the telescope, ; atmosphere of, ; axial rotation of, ; canals on, ; density of, ; diameter of, ; distance, from the earth of, ; distance from the sun of, , ; gravitation on, ; le verrier's discovery of, ; life improbable on, ; marking on, ; movements of, - ; opposition of, - ; orbit of, , , , ; orbit of, and the laws of kepler, ; parallax ( ), and dr. d. gill, ; periodic time of, ; a planet or "wanderer," ; "polar caps" on, , ; proximity to the earth of, ; rising and setting of, ; rotation of, ; satellites of, - , ; size of compared with other planets, , ; tides on, ; water and ice on, , maximilian, emperor, mayer, tobias, and uranus, measurement of the earth, - mediterranean, tides in the, mercury, ancient study of, ; antiquity of its discovery, - ; atmosphere of, ; attraction on comets of, ; climate of, ; comparative proximity to the earth of, ; composition of, ; crescent-shaped, ; density of, ; diameter of, ; distance from the sun of, , ; habitability of, ; movement of, , ; its elliptic form, , ; orbit of, ; period of revolution of, ; periodic appearances of, ; periodic time of, ; perturbations of, ; a planet or "wanderer," ; revolution of, ; rotation of, and professor schiaparelli, ; size of, compared with other planets, ; surface of, ; transit of, ; transit of, and gassendi's observations, ; transit of, predicted by kepler, ; velocity of, ; weight of, , meridian circle, , messier's catalogue of stars, meteors (_see_ stars, shooting) meteorites, ; alban mount, ; ancient accounts, , ; benares, ; butsura, ; chaco, ; characteristics of, ; chladni's account of discovery in siberia, ; composition of, - ; ensisheim ( ), ; hindoo account of, ; l'aigle, ; not connected with comets, ; not connected with star showers, ; orgueil, ; origin, - ; ovifak, ; rowton, - ; wold cottage, micrometer, milky way, - , - mimas, minor planets, - mira ceti, , mizar, , moment of momentum, the, - month of one day, moon, the, absence of air on, , ; absence of heat on, ; agent in causing the tides, , - ; ancient discoveries respecting, ; apparent size of, ; attraction to the earth of, ; brightness of, as compared with that of the sun, ; changes during the month of, , ; chart of surface of, ; craters on, , , - , ; density of, ; diameter of, ; distance from the earth of, , , ; eclipses of, , - ; illustration of the law of gravitation, , , ; landscapes on, ; life impossible on, ; measuring heights of mountains, etc., of, , ; micrometer, ; motion of, ; mountains on, , , , , , ; phases of, , ; plane of orbit of, , , ; poets and artists and, ; pole, ; possibility of ejecting meteorites, ; possibly fractured off from the earth, ; prehistoric tides on, , ; produces precession, - ; proximity to the earth of, , ; receding from the earth, ; relative position of with regard to the earth and the sun, , ; revolution of, round the earth, , , ; "seas" on, , ; shadows of, ; size of, compared with that of the earth, ; test for chronometers, a, ; thraldom of terrestrial tides, ; waterless, ; weather not a affected by the phases of, ; weight of, motion, laws of planetary, , , , , mountains of the moon, , , n nasmyth, mr., and the formation of lunar craters, natural history museum, meteorites, _nautical almanack_, neap tides, nebula, in andromeda, ; annular, in lyra, ; in orion, , , - ; colour of, ; magnitude of, ; nature of, ; planetary, in draco, ; simplest type of a, ; various grades of, nebulæ, - ; condensation, ; distances of, ; double, ; herschel's labours respecting, - , , ; number of, ; planetary, ; self-luminous, ; smallest greater than the sun, ; spiral, nebular theory, the, neptune, ; adams's researches, - , ; challis's observations of, - ; density of, ; diameter of, , ; disc of, ; discovery ( ) of, ; distance from the sun of, , ; lalande's observations of, , ; le verrier's calculations, - ; moment of momentum of, ; orbit of, ; periodic time of, ; revolution of, ; rotation of uncertain, ; satellite of, discovered by mr. lassell, ; size of, compared with other planets, ; vaporous atmosphere of, ; weight of, newall, mr. h.f., and capella, ; and the values of velocity of stars, newcomb, professor, , , , newton, professor, and meteoric showers, , newton, sir isaac, discovery of gravitation verified kepler's laws, ; dynamical theory, ; illustrations of his teaching, - ; law of gravitation and, , , ; parabolic path of comets and, - ; reflecting telescope, ; weight of the earth and, nickel in the sun, nineveh, astronomers of, nordenskjöld, and the ovifak meteorite, nova cygni, ; brilliancy of, ; decline of, ; distance of, ; parallax of, november meteors, , , nutation, and bradley, o oberon, , object-glasses, , , , , observatories, - observatory, cape of good hope, ; dunsink, , ; greenwich, , ; lick, ; paris, ; uraniborg, ; vienna, ; washington, ; yerkes, occultation, , oceanus procellarum, opera-glass, , opposition of mars, orbital moment of momentum, orbits of planets, , , ; dimensions, - ; elliptical form, - ; minor planets, , , ; not exactly circles, ; of satellites of uranus, ; sun the common focus, orgueil meteorite, the, orion, , orion, belt of, , ; brilliancy of, ; nebula in, , , - orionis, a, , orionis, th, a multiple star, , ovifak meteorite, the, p palisa and the minor planets, pallas, , parabolic path of comets, - parallactic ellipse, parallax, , , , ; of stars, paris telescope, , pegasus, great square of, , peg-top, the, and the rotation of the earth, pendulum for determining the force of the earth's attraction, penumbra of sun-spot, perihelion, periodic times of planets, - , periodicity of sun-spots, perseids, perseus, , , ; sword-handle, perturbation, planetary, - , perturbations, theory of, petavius, peters, professor, and charts of minor planets, ; and the derangement of sirius, phases of the moon, , phobos, , , photography, and practical astronomy, ; and the distance of cygni, ; dr. roberts and the nebula in andromeda, ; mr. common and the nebula in orion, ; sir w. huggins and the spectra of nebulæ, photosphere, the, , physical nature of the stars, piazzi, discoverer of the first known minor planet, pickering, professor, , , , ; and betelgeuze, ; and planetary nebulæ, ; and saturn's satellites, ; and spectroscopic binaries, , pico, planetary motion, kepler's laws of, , , , , planetary nebulæ, planetary perturbation, - planets, ancient ideas respecting, , ; approximate number of, ; attract each other, , ; attracted by comets, ; bode's law, ; comparative sizes of, , ; distance of, from the earth, - ; distance of, from the sun, ; how distinguished from stars, ; irregularity of motions of, - ; lagrange's theory of rigidity of, ; light of, derived from the sun, ; minor, - ; orbits of the four giant, ; orbits of the four interior, ; orbits have their focus in the centre of the sun, ; orbits not exactly circles, ; orbits take the form of an ellipse, - ; origin of, as suggested by the nebular theory, ; periodic times of, - , ; relative distances of, ; uniformity of direction in their revolution, , ; velocity of, - , , , plato (lunar crater), pleiades, , ; invisible in the summer, pliny, the tides and the moon, plough, the, pogson, mr., pointers in the great bear, , polar axis, polar caps on mars, , pole, the, distance of from pole star lessening, ; elevation of, ; movement of, ; near a draconis, ; near vega or a lyra, pole star, ; belongs to the little bear, ; distance of, from the pole of the heavens, , , ; position of, ; slow motion of, pollux, , ; value of velocity of, pons, and the comet of , posidonius, potassium in the sun, præsepe, precession and nutation of the earth's axis, - proctor, and the stars in argelander's atlas, prism, the, ; its analysing power, pritchard, professor, stellar photographic researches of, procyon, ; value of velocity of, prominences on the sun, - ptolemy, his theory of astronomy, ; lunar crater named after him, q quarantids, the, r radius of the earth, , rainbow, the, ram, the, reflectors, , , refraction by the prism, refractors, , , regulus, , reservoir formed from tidal water, retina, the, and the telescope, , rhea, rigel, , , rigidity of the planets, , roberts, dr. isaac, and the nebula in andromeda, ; and the nebula in orion, roemer, and the velocity of light, romance, planet of, - rosse telescope, the, , , , rotational moment of momentum, rowland, professor, and spectral lines, rowton siderite, royal astronomical society and bessel, s sappho, satellites of jupiter, , , - , , ; confirmation of the copernican theory, satellites of mars, , - , , satellites of neptune, , satellites of saturn, ; bond's discoveries, ; cassini's discoveries, ; distances, ; herschel's discoveries, ; huyghens' discovery, ; kirkwood's deduction, ; lassell's deduction, ; movements, ; origin as suggested by the nebular theory, satellites of uranus, , , , saturn, ancient study of, ; attraction on uranus, ; axial rotation of, ; beauty of, ; comparative proximity to the earth of, ; density of, ; diameter of, , ; distance of, from the sun, , , ; elliptic path of, ; gravitation paramount, ; internal heat of, , ; leonids and, ; low density of, ; moment of momentum of, ; motion of, ; orbit of, , ; path of, perturbed by the attraction of jupiter, ; periodic time of, ; period of revolution of, ; picturesqueness of, ; position of, in the solar system, ; rings of, ; rings, bonds discovery, ; rings, cassini's discovery, ; rings, consistency, ; rings, dawes's discovery, ; rings, galileo's discovery, , ; rings, hadley's observations, ; rings, herschel's researches, ; rings, huyghens' discovery, - ; rings, keeler's measurement of the rotation, ; rings, maraldi's researches, ; rings, rotation of, , ; rings, spectrum of, ; rings, trouvelot's drawing, ; satellites of, , , , , ; size of, compared with other planets, , , ; spectrum of, ; unequal in appearance to mars and venus, ; velocity of, ; weight of, compared with the earth, savary and binary stars, schaeberle, mr., and mars, scheiner, and the values of velocity of stars, ; observations on sun-spots, schiaparelli, professor, and mars, ; and the connection between shooting-star showers and comets, ; and the rotation of mercury, schickard, schmidt, and nova cygni, , ; and the crater linné, ; and the leibnitz mountains, schröter, and the crater posidonius, schwabe, and sun-spots, seas in the moon, secchi, and stellar spectra, shoal of shooting stars, ; dimensions, shooting stars (_see_ stars, shooting) sickle, the, sidereal aggregation theory of sir w. herschel, siderite, rowton, sinus iridum, sirius, change in position of, ; companion of, , ; exceptional lustre of, ; irregularities of movement of, ; larger than the sun, ; most brilliant star, ; periodical appearances of, ; proper motion of, ; spectrum of, ; velocity of, ; weight of, smyth, professor c.p., sodium, colour of flame from, ; in the sun, solar corona, prominences etc. (_see under_ sun) solar system, - ; copernican exposition of the, ; influence of gravitation on, ; information respecting, obtained by observing the transit of venus, ; island in the universe, ; minor planets, - ; moment of momentum, ; movement of, towards lyra, ; origin of, as suggested by the nebular theory, ; position of saturn and uranus in, , south, sir james, spectra of stars, spectro-heliograph, spectroscope, - ; detection of iron in the sun by the, spectroscopic binaries, spectrum analysis, ; dark lines, , ; gaseous nebulæ, ; line d, , speculum, the rosse, spica, , spider-threads for adjusting the micrometer, ; for sighting telescopes, spots on the sun, - ; connection with magnetism, ; cycles, ; duration, ; epochs of maximum, ; motion, ; period of revolution, ; scheiner's observations, ; zones in which they occur, star clusters, - ; in hercules, ; in perseus, stars, apparent movements due to precession, nutation, and aberration, ; approximate number of, ; attraction inappreciable, ; catalogues of, , , , ; charts of, , ; circular movement of, - stars, distances of, ; bessel's labours, - ; henderson's labours, ; method of measuring, - ; struve's work, , , ; parallactic ellipse, - stars, double, ; bode's list, ; burnham's additions, ; cassini, ; herschel, , ; measurement, , ; revolution, ; savary, ; shape of orbit, ; variation in colour, stars, elliptic movement of, ; gravitation and, ; how distinguished from planets, ; physical nature of, ; probability of their possessing a planetary system, ; real and apparent movements of, ; really suns, , stars, shooting, attractions of the planets, ; connection with comets, - ; countless in number, ; dimensions of shoal, ; features of, ; length of orbit, ; orbit, ; orbit, gradual change, ; period of revolution, ; periodic return, , ; shower of november, , , - ; shower of november, , and professor adams, , ; shower of november, , radiation of tracks from leo, ; shower of november, , ; showers, ; showers and professor newton, ; track, ; transformed into vapour by friction with the earth's atmosphere, , ; velocity, , stars, spectra of, ; teaching of ancients respecting, ; temperature of, ; temporary, , ; values of velocity of, ; variable, stoney, dr. g.j., strontium, flame from, ; in the sun, struve, otto, and the distance of vega, , ; and the distance of cygni, , sun, the, and the velocity of light, ; apparent size of, as seen from the planets, , ; as a star, ; axial rotation of, ; compared with the earth, ; connection of, with the seasons, ; corona of, during eclipse, - ; density of, , ; diameter of, ; distance of, from mars, ; distance of, from saturn, ; distance of, from the earth, , , , , ; eclipse of, , ; ellipticity of, ; faculæ on surface of, ; focus of planets' orbits, ; gradually parting with its heat, ; granules on surface of, ; heat of, and its sources, - ; heat of, thrown on jupiter, ; minor planets and, ; movement of, towards lyra, ; nebular theory of its heat, ; photographed, ; precession of the earth's axis, ; prominences of, - ; relation of, to the moon, ; rising and setting of, ; rotation of, , ; size of, ; spectrum of, ; spots on, - ; spots, connection with magnetism, ; storms and convulsions on, , ; surface of, gaseous matter, ; surface of, mottled, ; teaching of early astronomers concerning, - ; temperature of, , , ; texture of, ; tides on, ; velocity of, ; weight of, compared with jupiter, , ; zodiacal light and, ; zones on the surface of, sunbeam, revelations of a, swan, the, , , sword-handle of perseus, syrtis major, t taurus, constellation of, , tebbutt's comet, telescope, construction of the first, ; equatorial (dunsink), - , ; greenwich, ; herschelian, ; lick, , ; paris, , ; reflecting, , ; refracting, , ; rosse, , , , ; sighting of a, ; structure of the eye illustrates the principle of the, ; vienna, - ; washington, ; yerkes, temporary stars, , tethys, theophilus, tides, the, actual energy derived from the earth, ; affected by the law of gravitation, , ; affected by the moon, , - ; at bay of fundy, ; at cardiff, ; at chepstow, ; at london, ; at st. helena, ; excited by the sun, ; formation of currents, ; in bristol channel, ; in mediterranean, ; in mid-ocean, ; jupiter and, ; length of the day and, ; lunar, , ; moment of momentum and, ; neap, ; rotation of the earth, and revolution of the moon, ; satellites of mars, ; solar, ; spring, ; variations in, ; waste of water power, ; work effected, tin in the sun, titan, , , titania, , transit of mercury, , , transit of venus, ; captain cook, ; copeland's observations of, ; crabtree's observations of, ; gassendi's observations of, ; halley's method, , ; horrocks' observations of, , ; importance of, ; kepler's prediction of, ; observations of, at dunsink, - transit of vulcan, - triesnecker, , trouvelot, mr. l., and saturn's rings, tschermak, and the origin of meteorites, , tycho (lunar crater), tycho brahe, and the observatory of uraniborg, , , u umbra of sun-spot, umbriel, , unstable dynamical equilibrium, uraniborg, observatory of, uranus, ; attraction of saturn, ; bradley's observations of, ; composition of, ; density of, ; diameter of, , ; diameter of orbit of, ; disc of, ; discovery of, by herschel, , ; distance from sun of, ; ellipse of, ; first taken for a comet, ; flamsteed's observations of, , ; formerly regarded as a star, , ; investigations to discover a planet outside the orbit, - ; irregular motion of, , ; lemonnier's observations of, ; leonids and, ; mayer's observations of, ; moment of momentum of, ; orbit of, , ; periodic time of, ; period of revolution of, ; rotation of, ; satellites of, ; satellites, discovery by herschel, ; satellites, movement nearly circular, ; satellites, periodical movements, ; satellites, plane of orbits, , ; size of compared with the earth, ; and with other planets, ; subject to another attraction besides the sun, ursa major (_see_ great bear) v variable stars, vega, , , , ; struve's measurement of, velocity, of light, , , ; of light, laws dependent upon, ; of planets, - , , ; of stars, values of, - venus, ancient study of, ; aspects of, ; atmosphere of, ; brilliancy of, ; density of, ; diameter of, , ; distance of, from the sun, , ; habitability of, ; movement of, ; neighbour to the earth, ; orbit of, , ; orbit form of, , ; periodic time of, ; a planet or "wanderer," ; rotation of, ; shape of, ; size of, compared with other planets, , ; surface of, ; transit of, , - ; transit, importance of, ; transit predicted by kepler, ; velocity and periodic time of, , , ; view of the ancients about, vesta, , victoria, vienna telescope, - virgo, vogel and algol, ; and spica, , ; and the spectra of the stars, , volcanic origin of meteorites, ; outbreaks on the earth, von asten and encke's comet, , ; and the distance of the sun, ; and the weight of mercury, vortex rings, vulcan, , ; and the sun, w wargentin, watson, professor, and mercury, watson, sir william, friendship with herschel, wave-lengths, weather, not affected by the moon, wilson, mr. w.e., and the nebula in orion, witt, herr g., and eros, wold cottage meteorite, the, wright, thomas, and the milky way, y "year of stars," the, yerkes observatory, chicago, young, professor, account of a marvellous sun-prominence, ; and sun-spots, ; observations on magnetic storms, z zeeman, dr., and spectral lines, zinc in the sun, zodiac, the, zodiacal light, zone of minor planets, printed by cassell & company, limited, la belle sauvage, london, e.c. footnotes: [ ] it may, however, be remarked that a star is never _seen_ to set, as, owing to our atmosphere, it ceases to be visible before it reaches the horizon. [ ] "popular astronomy," p. . [ ] _limb_ is the word used by astronomers to denote the _edge_ or circumference of the apparent disc of a heavenly body. [ ] "the sun," p. . [ ] it has been frequently stated that the outburst in , witnessed by carrington and hodgson, was immediately followed by an unusually intense magnetic storm, but the records at kew and greenwich show that the magnetic disturbances on that day were of a very trivial character. [ ] some ungainly critic has observed that the poet himself seems to have felt a doubt on the matter, because he has supplemented the dubious moonbeams by the "lantern dimly burning." the more generous, if somewhat a sanguine remark has been also made, that "the time will come when the evidence of this poem will prevail over any astronomical calculations." [ ] this sketch has been copied by permission from the very beautiful view in messrs. nasmyth and carpenter's book, of which it forms plate xi. so have also the other illustrations of lunar scenery in plates viii., ix. the photographs were obtained by mr. nasmyth from models carefully constructed from his drawings to illustrate the features on the moon. during the last twenty years photography has completely superseded drawing by eye in the delineation of lunar objects. long series of magnificent photographs of lunar scenery have been published by the paris and lick observatories. [ ] at the british association's meeting at cardiff in , prof. copeland exhibited a model of the moon, on which the appearance of the streaks near full moon was perfectly shown by means of small spheres of transparent glass attached to the surface. [ ] the duration of an occultation, or, in other words, the length of time during which the moon hides the star, would be slightly shorter than the computed time, if the moon had an atmosphere capable of sensibly refracting the light from the star. but, so far, our observations do not indicate this with certainty. [ ] i owe my knowledge of this subject to dr. g. johnstone stoney, f.r.s. there has been some controversy as to who originated the ingenious and instructive doctrine here sketched. [ ] the space described by a falling body is proportional to the product of the force and the square of the time. the force varies inversely as the square of the distance from the earth, so that the space will vary as the square of the time, and inversely as the square of the distance. if, therefore, the distance be increased sixty-fold, the time must also be increased sixty-fold, if the space fallen through is to remain the same. [ ] see newcomb's "popular astronomy," p. . [ ] recent investigation by newcomb on the motion of mercury have led to the result that the hypothesis of a planet or a ring of very small planets between the orbit of mercury and the sun cannot account for the difference between theory and observation in the movements of mercury. harzer has come to the same result, and has shown that the disturbing element may possibly be the material of the solar corona. [ ] "the sun: its planets, and their satellites." london: (page ). [ ] james gregory, in a book on optics written in , had already suggested the use of the transit of venus for this purpose. [ ] _see_ "astronomy and astrophysics," no. . [ ] _see_ "astronomy and astrophysics," no. . [ ] this is the curved marking which on plate xviii. appears in longitude ° and north of (that is, below) the equator. here, as in all astronomical drawings, north is at the foot and south at the top. _see_ above, p. (chapter iii.). [ ] now director of the lick observatory. [ ] the heliometer is a telescope with its object-glass cut in half along a diameter. one or both of these halves is movable transversely by a screw. each half gives a complete image of the object. the measures are effected by observing how many turns of the screw convey the image of the star formed by one half of the object-glass to coincide with the image of the planet formed by the other. [ ] see "astronomy and astrophysics," no. . [ ] it is only right to add that some observers believe that, in exceptional circumstances, points of jupiter have shown some slight degree of intrinsic light. [ ] professor pickering, of cambridge, mass., has, however, effected the important improvement of measuring the decline of light of the satellite undergoing eclipse by the photometer. much additional precision may be anticipated in the results of such observations. [ ] "newcomb's popular astronomy," p. . [ ] _see_ grant, "history of physical astronomy," page . [ ] now director of the lick observatory. [ ] we are here neglecting the orbital motion of saturn, by which the whole system is moved towards or from the earth, but as this motion is common to the ball and the ring, it will not disturb the relative positions of the three spectra. [ ] according to prof. barnard's recent measures, the diameter of titan is , miles. this is the satellite discovered by huyghens; it is the sixth in order from the planet. [ ] extract from "three cities of russia," by c. piazzi smyth, vol. ii., p. : "in the year . it then chanced that george iii., of great britain, was pleased to send as a present to the empress catharine of russia a ten-foot reflecting telescope constructed by sir william herschel. her majesty immediately desired to try its powers, and roumovsky was sent for from the academy to repair to tsarskoe-selo, where the court was at the time residing. the telescope was accordingly unpacked, and for eight long consecutive evenings the empress employed herself ardently in observing the moon, planets, and stars; and more than this, in inquiring into the state of astronomy in her dominions. then it was that roumovsky set before the imperial view the academy's idea of removing their observatory, detailing the necessity for, and the advantages of, such a proceeding. graciously did the 'semiramis of the north,' the 'polar star,' enter into all these particulars, and warmly approve of the project; but death closed her career within a few weeks after, and prevented her execution of the design." [ ] _see_ professor holden's "sir william herschel, his life and works." [ ] arago says that "lemonnier's records were the image of chaos." bouvard showed to arago one of the observations of uranus which was written on a paper bag that in its time had contained hair-powder. [ ] the first comet of also suddenly increased in brightness, while a distinct disc, which hitherto had formed the nucleus, became transformed into a fine point of light. [ ] the three numbers , , and / are nearly inversely proportional to the atomic weights of hydrogen, hydrocarbon gas, and iron vapour, and it is for this reason that bredichin suggested the above-mentioned composition of the various types of tail. spectroscopic evidence of the presence of hydrogen is yet wanting. [ ] this illustration, as well as the figure of the path of the meteors, has been derived from dr. g.j. stoney's interesting lecture on "the story of the november meteors," at the royal institution, in . [ ] on the th november, , a piece of meteoric iron fell at mazapil, in mexico, during the shower of andromedes, but whether it formed part of the swarm is not known. it is, however, to be noticed that meteorites are said to have fallen on several other occasions at the end of november. [ ] hooke had noticed, in , that the star gamma arietis was double. [ ] perhaps if we could view the stars without the intervention of the atmosphere, blue stars would be more common. the absorption of the atmosphere specially affects the greenish and bluish colours. professor langley gives us good reason for believing that the sun itself would be blue if it were not for the effect of the air. [ ] the declination of a star is the arc drawn from the star to the equator at right angles to the latter. [ ] the distance of cygni has, however, again been investigated by professor asaph hall, of washington, who has obtained a result considerably less than had been previously supposed; on the other hand, professor pritchard's photographic researches are in confirmation of struve's and those obtained at dunsink. [ ] i am indebted for this drawing to the kindness of messrs. de la rue. [ ] _see_ chapter xix., on the mass of sirius and his satellite. [ ] as the earth carries on the telescope at the rate of miles a second, and as light moves with the velocity of , miles a second very nearly, it follows that the velocity of the telescope is about one ten-thousandth part of that of light. while the light moves down the tube feet long, the telescope will therefore have moved the ten-thousandth part of feet--_i.e._, the fortieth of an inch. [ ] _see_ newcomb's "popular astronomy," p. , where the discovery of this law is attributed to mr. j. homer lane, of washington. the contraction theory is due to helmholtz. [ ] the theory of tidal evolution sketched in this chapter is mainly due to the researches of professor g.h. darwin, f.r.s. [ ] the hour varies with the locality: it would be . at calais; at liverpool, . ; at swansea bay, . , etc. [ ] having decided upon the units of mass, of angle, and of distance which we intend to use for measuring these quantities, then any mass, or angle, or distance is expressed by a certain definite number. thus if we take the mass of the earth as the unit of mass, the angle through which it moves in a second as the unit of angle, and its distance from the sun as the unit of distance, we shall find that the similar quantities for jupiter are expressed by the numbers , · , and · respectively. hence its orbital moment of momentum is × · × ( · )². sir william herschel his life and works [illustration: sir william herschel] sir william herschel his life and works by edward s. holden united states naval observatory, washington [illustration: coelis exploratis] new york charles scribner's sons and broadway copyright, , by charles scribner's sons. press of j. j. little & co., nos. to astor place, new york. please see the end of the text for transcriber's notes preface. in the following account of the life and works of sir william herschel, i have been obliged to depend strictly upon data already in print--the _memoir_ of his sister, his own scientific writings and the memoirs and diaries of his cotemporaries. the review of his published works will, i trust, be of use. it is based upon a careful study of all his papers in the _philosophical transactions_ and elsewhere. a life of herschel which shall be satisfactory in every particular can only be written after a full examination of the materials which are preserved at the family seat in england; but as two generations have passed since his death, and as no biography yet exists which approaches to completeness, no apology seems to me to be needed for a conscientious attempt to make the best use of the scanty material which we do possess. this study will, i trust, serve to exhibit so much of his life as belongs to the whole public. his private life belongs to his family, until the time is come to let the world know more of the greatest of practical astronomers and of the inner life of one of its most profound philosophers,--of a great and ardent mind, whose achievements are and will remain the glory of england. contents. page chapter i. early years; - , chapter ii. life in bath; - , chapter iii. life at datchet, clay hall, and slough; - , chapter iv. review of the scientific labors of herschel, bibliography, index of names, life and works of william herschel. chapter i. early years; - . of the great modern philosophers, that one of whom least is known, is william herschel. we may appropriate the words which escaped him when the barren region of the sky near the body of _scorpio_ was passing slowly through the field of his great reflector, during one of his sweeps, to express our own sense of absence of light and knowledge: _hier ist wahrhaftig ein loch im himmel._ herschel prepared, about the year , a biographical memorandum, which his sister carolina placed among his papers. this has never been made public. the only thoroughly authentic sources of information in possession of the world, are a letter written by herschel himself, in answer to a pressing request for a sketch of his life, and the _memoir and correspondence of caroline herschel_ (london, ), a precious memorial not only of his life, but of one which otherwise would have remained almost unknown, and one, too, which the world could ill afford to lose. the latter, which has been ably edited by mrs. mary cornwallis herschel,[ ] is the only source of knowledge in regard to the early years of the great astronomer, and together with the all too scanty materials to be gained from a diligent search through the biography of the time, affords the data for those personal details of his life, habits, and character, which seem to complete the distinct, though partial conception of him which the student of his philosophical writings acquires. the letter referred to was published in the göttingen magazine of science and literature, iii., , shortly after the name of herschel had become familiar to every ear through his discovery of _uranus_, but while the circumstances of the discovery, and the condition of the amateur who made it, were still entirely unknown. the editor (lichtenberg) says: "herr herschel was good enough to send me, some time since, through herr magellan, copies of his dissertations on double stars, on the parallax of the fixed stars, and on a new micrometer. in the letter which conveyed to him my thanks for his gift, i requested him to note down a few facts in regard to his life, for publication in this magazine, since various accounts, more or less incorrect, had appeared in several journals. in answer, i received a very obliging letter from him and what follows is that portion of it relating to my request, which was sent me with full permission to make it public." "datchet, near windsor, _nov. , ._ "i was born in hanover, november, . my father, who was a musician, destined me to the same profession, hence i was instructed betimes in his art. that i might acquire a perfect knowledge of the theory as well as of the practice of music, i was set at an early age to study mathematics in all its branches--algebra, conic sections, infinitesimal analysis, and the rest. "the insatiable desire for knowledge thus awakened resulted next in a course of languages; i learned french, english, and latin, and steadfastly resolved henceforth to devote myself wholly to those sciences from the pursuit of which i alone looked for all my future happiness and enjoyment. i have never been either necessitated or disposed to alter this resolve. my father, whose means were limited, and who consequently could not be as liberal to his children as he would have desired, was compelled to dispose of them in one way or another at an early age; consequently in my fifteenth year i enlisted in military service, only remaining in the army, however, until i reached my nineteenth year, when i resigned and went over to england. "my familiarity with the organ, which i had carefully mastered previously, soon procured for me the position of organist in yorkshire, which i finally exchanged for a similar situation at bath in , and while here the peculiar circumstances of my post, as agreeable as it was lucrative, made it possible for me to occupy myself once more with my studies, especially with mathematics. when, in the course of time, i took up astronomy, i determined to accept nothing on faith, but to see with my own eyes everything which others had seen before me. having already some knowledge of the science of optics, i resolved to manufacture my own telescopes, and after many continuous, determined trials, i finally succeeded in completing a so-called newtonian instrument, seven feet in length. from this i advanced to one of ten feet, and at last to one of twenty, for i had fully made up my mind to carry on the improvement of my telescopes as far as it could possibly be done. when i had carefully and thoroughly perfected the great instrument in all its parts, i made systematic use of it in my observations of the heavens, first forming a determination never to pass by any, the smallest, portion of them without due investigation. this habit, persisted in, led to the discovery of the new planet (_georgium sidus_). this was by no means the result of chance, but a simple consequence of the position of the planet on that particular evening, since it occupied precisely that spot in the heavens which came in the order of the minute observations that i had previously mapped out for myself. had i not seen it just when i did, i must inevitably have come upon it soon after, since my telescope was so perfect that i was able to distinguish it from a fixed star in the first minute of observation. "now to bring this sketch to a close. as the king had expressed a desire to see my telescope, i took it by his command to greenwich, where it was compared with the instruments of my excellent friend, dr. maskelyne, not only by himself, but by other experts, who pronounced it as their opinion that my instrument was superior to all the rest. thereupon the king ordered that the instrument be brought to windsor, and since it there met with marked approval, his majesty graciously awarded me a yearly pension, that i might be enabled to relinquish my profession of music, and devote my whole time to astronomy and the improvement of the telescope. gratitude, as well as other considerations specified by me in a paper presented to the royal society, of which i am a member, has induced me to call the new planet _georgium sidus_. "'georgium sidus.--jam nunc assuesce vocari.'--(_virgil._) and i hope it will retain the name." we know but little of the family of herschel. the name is undoubtedly jewish, and is found in poland, germany, and england. we learn that the ancestors of the present branch left moravia about the beginning of the xviith century, on account of their change of religion to protestantism. they became possessors of land in saxony. hans herschel, the great-grandfather of william, was a brewer in pirna (a small town near dresden). of the two sons of hans, one, abraham (born in , died ), was employed in the royal gardens at dresden, and seems to have been a man of taste and skill in his calling. of his eldest son, eusebius, there appears to be little trace in the records of the family. the second son, benjamin, died in infancy; the third, isaac, was born in (jan. ), and was thus an orphan at eleven years of age. isaac was the father of the great astronomer. he appears to have early had a passionate fondness for music, and this, added to a distaste for his father's calling, determined his career. he was taught music by an oboe-player in the royal band, and he also learned the violin. at the age of twenty-one he studied music for a year under the cappelmeister pabrich, at potsdam, and in august, , he became oboist in the band of the guards, at hanover. in august, , he married anna ilse moritzen. she appears to have been a careful and busy wife and mother, possessed of no special faculties which would lead us to attribute to her care any great part of the abilities of her son. she could not herself write the letters which she sent to her husband during his absences with his regiment. it was her firm belief that the separations and some of the sorrows of the family came from too much learning; and while she could not hinder the education of the sons of the family, she prevented their sisters from learning french and dancing. it is but just to say that the useful accomplishments of cooking, sewing, and the care of a household, were thoroughly taught by her to her two daughters. the father, isaac, appears to have been of a different mould, and to him, no doubt, the chief intellectual characteristics of the family are due. his position obliged him to be often absent from hanover, with his regiment, but his hand appears to have been always present, smoothing over difficulties, and encouraging his sons to such learning and improvement as was to be had. his health was seriously injured by the exposures of the campaigns, and he was left, after the seven years' war, with a broken constitution. after his final return home, in , his daughter gives this record of him-- "copying music employed every vacant moment, even sometimes throughout half the night. . . . with my brother [dietrich]--now a little engaging creature of between four and five years old--he was very much pleased, and [on the first evening of his arrival at home] before he went to rest, the adempken (a little violin) was taken from the shelf and newly strung, and the daily lessons immediately commenced. . . . i do not recollect that he ever desired any other society than what he had opportunities of enjoying in many of the parties where he was introduced by his profession, though far from being of a morose disposition; he would frequently encourage my mother in keeping up a social intercourse among a few acquaintances, whilst his afternoon hours generally were taken up in giving lessons to some scholars at home, who gladly saved him the troublesome exertion of walking. . . . he also found great pleasure in seeing dietrich's improvement, who, young as he was, and of the most lively temper imaginable, was always ready to receive his lessons, leaving his little companions with the greatest cheerfulness to go to his father, who was so pleased with his performances that he made him play a solo on the adempken in rake's concert, being placed on a table before a crowded company, for which he was very much applauded and caressed, particularly by an english lady, who put a gold coin in his little pocket. "it was not long before my father had as many scholars as he could find time to attend. and when they assembled at my father's to make little concerts, i was frequently called to join the second violin in an overture, for my father found pleasure in giving me sometimes a lesson before the instruments were laid by, after practising with dietrich, for i never was missing at those hours, sitting in a corner with my knitting and listening all the while." here, as in all her writing, carolina is simple, true, direct to awkwardness, and unconsciously pathetic even in joy. the family of isaac and anna herschel consisted of ten children. six of these lived to adult age. they were: . sophia elizabeth; born , married griesbach, a musician in the guard, by whom she had children. five of her sons were afterwards musicians at the court, in england, where they obtained places through the influence of william. . henry anton jacob; born , november . . frederic william (the astronomer) born , november . . john alexander; born , november . . carolina lucretia; born , march . . dietrich; born , september . of this family group, the important figures to us are william, alexander, and carolina. jacob was organist at the garrison church of hanover in , a member of the guards' band in , and first violin in the hanover court orchestra in . afterwards he joined his brother william in bath, but again returned to hanover. in he published in amsterdam his opus i., a set of six quartettes, and later, in london, he published two symphonies and six trios. he appears to have been a clever musician, and his letters to his younger brother william are full of discussion on points of musical composition, etc. he died in . dietrich, the youngest brother, shared in the musical abilities of his family, and when only fifteen years old was so far advanced as to be able to supply his brother jacob's place in the court orchestra, and to give his lessons to private pupils. there is no one of the family, except the eldest daughter, whom we do not know to have possessed marked ability in music, and this taste descended truly for four generations. in the letters of chevalier bunsen,[ ] he describes meeting, in , the eldest granddaughter of william herschel, who, he says, "is a musical genius." three members of the family, william, alexander, and carolina, formed a group which was inseparable for many years, and while the progress of the lives of alexander and carolina was determined by the energy and efforts of william, these two lent him an aid without which his career would have been strangely different. it is necessary to understand a little better the early life of all three. the sons of the herschel family all attended the garrison school in hanover until they were about fourteen years old. they were taught the ordinary rudiments of knowledge--to read, to write, to cipher--and a knowledge of french and english was added. william especially distinguished himself in his studies, learning french very rapidly, and studying latin and arithmetic with his master out of hours. the household life seems to have been active, harmonious, and intelligent, especially during the presence of the father, who took a great delight in the rapid progress of all his sons in music, and who encouraged them with his companionship in their studies and in their reading on all intellectual subjects. from the _memoir_ of carolina, on which we must depend for our knowledge of this early life, we take the following paragraph: "my brothers were often introduced as solo performers and assistants in the orchestra of the court, and i remember that i was frequently prevented from going to sleep by the lively criticism on music on coming from a concert, or by conversations on philosophical subjects, which lasted frequently till morning, in which my father was a lively partaker and assistant of my brother william, by contriving self-made instruments. . . . often i would keep myself awake that i might listen to their animating remarks, for it made _me so happy_ to see _them so happy_. but generally their conversation would branch out on philosophical subjects, when my brother william and my father often argued with such warmth that my mother's interference became necessary, when the names leibnitz, newton, and euler sounded rather too loud for the repose of her little ones, who ought to be in school by seven in the morning. but it seems that on the brothers retiring to their own room, where they shared the same bed, my brother william had still a great deal to say; and frequently it happened that when he stopped for an assent or reply, he found his hearer was gone to sleep, and i suppose it was not till then that he bethought himself to do the same. "the recollection of these happy scenes confirms me in the belief, that had my brother william not then been interrupted in his philosophical pursuits, we should have had much earlier proofs of his inventive genius. my father was a great admirer of astronomy, and had some knowledge of that science; for i remember his taking me, on a clear frosty night, into the street, to make me acquainted with several of the most beautiful constellations, after we had been gazing at a comet which was then visible. and i well remember with what delight he used to assist my brother william in his various contrivances in the pursuit of his philosophical studies, among which was a neatly turned -inch globe, upon which the equator and ecliptic were engraved by my brother." the mechanical genius was not confined to william, for we read that alexander used often to "sit by us and amuse us and himself by making all sorts of things out of pasteboard, or contriving how to make a twelve-hour cuckoo clock go a week." this ability of alexander's was turned later to the best account when he became his brother william's right hand in the manufacture of reflectors, eye-pieces, and stands in england. his abilities were great, and a purpose which might otherwise have been lacking was supplied through the younger brother's ardor in all that he undertook. his musical talent was remarkable; he played "divinely" on the violoncello. he returned to hanover in , where he lived in comfortable independence, through the never-failing generosity of his brother, until his death in . a notice of him in a bristol paper says: "died, march , , at hanover, alexander herschel, esqr., well known to the public of bath and bristol as a performer and elegant musician; and who for forty-seven years was the admiration of the frequenters of concerts and theatres of both those cities as principal violoncello. to the extraordinary merits of mr. herschel was united considerable acquirement in the superior branches of mechanics and philosophy, and his affinity to his brother, sir william herschel, was not less in science than in blood." we shall learn more of the sister, carolina, as time goes on. now in these early years she was a silent and persistent child, growing up with a feeling that she was uncared for and neglected, and lavishing all her childish affection, as she did all that of her womanly life, on her brother william. throughout her long life, "my brother" was william, "my nephew" _his_ son. the brothers jacob and william were, with their father, members of the band of the guards in , when the regiment was ordered to england, and they were absent from hanover a year. william (then seventeen years old) went as oboist, and out of his scanty pay brought back to hanover, in , only one memento of his stay--a copy of locke _on the human understanding_. he appears to have served with the guard during part of the campaign of . his health was then delicate, and his parents "determined to remove him from the service--a step attended by no small difficulties."[ ] this "removal" was hurriedly and safely effected, so hurriedly that the copy of locke was not put in the parcels sent after him to hamburg by his mother; "she, dear woman, knew no other wants than good linen and clothing." thus, at last, the young william herschel, the son of an oboe-player in the king's guard, is launched in life for himself, in the year , at the age of nineteen. all his equipment is the "good linen and clothing," a knowledge of french, latin, and english, some skill in playing the violin, the organ, and the oboe, and an "uncommon precipitancy" in doing what there is to be done. a slender outfit truly; but we are not to overlook what he said of himself on another occasion. "i have, nevertheless, several resources in view, and do not despair of succeeding pretty well in the end." from to --three years--we know nothing of his life. we can imagine what it was. his previous visit to england had given him a good knowledge of the language, and perhaps a few uninfluential acquaintances. on his return he would naturally seek these out, and, by means of his music, he could gain a livelihood. we first hear of him as charged with the organization of the music of a corps of the militia of durham, under the auspices of the earl of darlington. "la manière dont il remplit cette mission, le fit connaître avantageusement."[ ] the nature of the service of these militia corps, which were then forming all over england, is well described in the autobiography of gibbon. every county-gentleman felt constrained to serve his country, and the regimental mess-rooms were filled with men of rank and fashion. in we hear of him again. he has attracted the notice of those about him. "about the year , as miller[ ] was dining at pontefract with the officers of the durham militia, one of them, knowing his love of music, told him they had a young german in their band as a performer on the hautboy, who had only been a few months in england, and yet spoke english almost as well as a native, and who was also an excellent performer on the violin; the officer added that if miller would come into another room, this german should entertain him with a solo. the invitation was gladly accepted, and miller heard a solo of giardini's executed in a manner that surprised him. he afterwards took an opportunity of having some private conversation with the young musician, and asked him whether he had engaged himself for any long period to the durham militia. the answer was, 'only from month to month.' 'leave them, then,' said the organist, 'and come and live with me. i am a single man, and think we shall be happy together; and, doubtless, your merit will soon entitle you to a more eligible situation.' the offer was accepted as frankly as it was made, and the reader may imagine with what satisfaction dr. miller must have remembered this act of generous feeling when he hears that this young german was herschel, the astronomer. 'my humble mansion,' says miller, 'consisted, at that time, but of two rooms. however, poor as i was, my cottage contained a library of well-chosen books; and it must appear singular that a foreigner who had been so short a time in england should understand even the peculiarities of the language so well as to fix upon swift for his favorite author.' "he took an early opportunity of introducing his new friend at mr. cropley's concerts; the first violin was resigned to him; 'and never,' says the organist, 'had i heard the concertos of corelli, geminiani, and avison, or the overtures of handel performed more chastely, or more according to the original intention of the composers, than by mr. herschel. i soon lost my companion; his fame was presently spread abroad; he had the offer of pupils, and was solicited to lead the public concerts both at wakefield and halifax. a new organ for the parish church of halifax was built about this time, and herschel was one of the seven candidates for the organist's place. they drew lots how they were to perform in succession. herschel drew the third, the second fell to dr. wainwright of manchester, whose finger was so rapid that old snetzler, the organ-builder, ran about the church exclaiming: '_te tevel! te tevel! he run over te keys like one cat; he will not give my piphes room for to shpeak._' 'during mr. wainwright's performance,' says miller, 'i was standing in the middle aisle with herschel. 'what chance have you,' said i, 'to follow this man?' he replied, 'i don't know; i am sure fingers will not do.' on which he ascended the organ loft, and produced from the organ so uncommon a fulness, such a volume of slow, solemn harmony, that i could by no means account for the effect. after this short _ex tempore_ effusion, he finished with the old hundredth psalm-tune, which he played better than his opponent. "'_ay, ay_,' cried old snetzler, '_tish is very goot, very goot indeet; i vil luf tish man, for he gives my piphes room for to shpeak._' having afterwards asked mr. herschel by what means, in the beginning of his performance, he produced so uncommon an effect, he replied, 'i told you fingers would not do!' and producing two pieces of lead from his waistcoat pocket, 'one of these,' said he, 'i placed on the lowest key of the organ, and the other upon the octave above; thus by accommodating the harmony, i produced the effect of four hands, instead of two.'"[ ] the dates in this extract are not so well defined as might be wished. herschel had certainly been more than a few months in england at the time of his meeting with dr. miller, which was probably about . the appointment as organist at halifax was in , and the pupils and public concerts must have filled up the intervening five years. during a part of this time he lived in leeds, with the family of mr. bulman, whom he afterwards provided with a place as clerk to the octagon chapel, in his usual generous manner. all during his life he was placing some of the less fortunate and energetic members of his family. we cannot be too grateful to dr. miller, who, seeing his opportunity, used it. their frank friendship does honor to both. herschel's organ-playing, which no doubt had been begun when his brother was the organist of the garrison chapel at hanover, must have been perfected at this time, and it was through his organ-playing that he was able to leave the needy life in yorkshire. he was sure to have emerged sooner or later, but every year spared to him as a struggling musician was a year saved to astronomy. during all this period, a constant correspondence was maintained between the family at hanover and the absent son. many of william's letters were written in english, and addressed to his brother jacob, and treated of such subjects as the theory of music, in which he was already far advanced. his little sister was still faithful to the memory of her _dearest_ brother, and his father, whose health was steadily declining, became painfully eager for his return. in (april ), he returned to hanover on a very brief visit. he was attached to england, he was prospering there, and he had no inclination towards returning to a life in hanover. his sister says: "of the joys and pleasures which all felt at this long-wished-for meeting with my--let me say my _dearest_--brother, but a small portion could fall to my share; for with my constant attendance at church and school, besides the time i was employed in doing the drudgery of the scullery, it was but seldom i could make one in the group when the family were assembled together. "in the first week, some of the orchestra were invited to a concert, at which some of my brother william's compositions, overtures, etc., and some of my eldest brother jacob's were performed, to the great delight of my dear father, who hoped and expected that they would be turned to some profit by publishing them, but there was no printer who bid high enough. "sunday, the th, was the--to me--eventful day of my confirmation, and i left home not a little proud and encouraged by my dear brother william's approbation of my appearance in my new gown." the engagement of herschel at halifax did not long continue. in he obtained an advantageous engagement as oboist at bath, and soon after the position of organist at the octagon chapel was offered to him and accepted. this was a great and important change. bath was then, as now, one of the most beautiful cities in england, and the resort of the fashion and rank of the kingdom, who came to take the waters. it is beautifully situated on both sides of the avon, and has many fine walks and public buildings. the aspect of the city is markedly cheerful and brilliant, owing to the nature of the white stone of which the principal houses are built, and to the exquisite amphitheatre of hills in which they lie. the society was then gay and polite, and herschel was at once thrown into a far more intelligent atmosphere than that he had just left in yorkshire. it was easy to get new books, to see new faces, to hear new things. the assembly rooms (built in ) were noted for their size and elegance; the theatre was the best out of london. his position as organist of the fashionable chapel placed him in the current. his charming and engaging manners made him friends. his talents brought him admirers and pupils, and pupils brought him money.[ ] he began in a life of unceasing activity, which continued. in he published in london a symphony (in c) for two violins, viola, bass, two oboes, and two horns, and in the same year two military concertos for two oboes, two horns, two trumpets, and two bassoons.[ ] he wrote pieces for the harp, glees, "catches," and other songs for the voice. one of these, the _echo catch_, was published and had even considerable vogue. a competent musical critic writes to me of this work: "the counterpoint is clear and flowing, and is managed with considerable taste and effect. it would be difficult to explain the great cleverness shown in the construction of the _catch_ without diagrams to illustrate the movements of the parts. it is certainly an ingenious bit of musical writing." when he left bath (in ), many of these musical writings were lost, in his great haste to take up his new profession. one, specially, his sister remembers to have written out for the printer, "but he could not find a moment to send it off, nor answer the printer's letters." this was a four-part song, "in thee i bear so dear a part." he wrote very many anthems, chants, and psalm-tunes for the excellent cathedral choir of the octagon chapel. unfortunately, most of this music is now not to be found. a notice of herschel's life which appeared in the _european magazine_ for , january, gives a very lively picture of his life at this time, and it is especially valuable as showing how he appeared to his cotemporaries. "although mr. herschel loved music to an excess, and made a considerable progress in it, he yet determined with a sort of enthusiasm to devote every moment he could spare from business to the pursuit of knowledge, which he regarded as the sovereign good, and in which he resolved to place all his views of future happiness in life.". . . "his situation at the octagon chapel proved a very profitable one, as he soon fell into all the public business of the concerts, the rooms, the theatre, and the oratorios, besides many scholars and private concerts. this great run of business, instead of lessening his propensity to study, increased it, so that many times, after a fatiguing day of fourteen or sixteen hours spent in his vocation, he would retire at night with the greatest avidity to _unbend the mind_, if it may be so called, with a few propositions in maclaurin's _fluxions_, or other books of that sort." it was in these years that he mastered italian and made some progress in greek. "we may hazard a natural conjecture respecting the course of herschel's early studies. music conducted him to mathematics, or, in other words, impelled him to study smith's _harmonics_. now this robert smith was the author of _a complete system of optics_, a masterly work, which, notwithstanding the rapid growth of that branch of the science, is not yet wholly superseded. it seems to us not unlikely that herschel, studying the _harmonics_, conceived a reverence for the author, who was at that time still living, so that from the _philosophy of music_ he passed to the _optics_, a work on which smith's great reputation chiefly rested; and thus undesignedly prepared himself for the career on which he was shortly about to enter with so much glory."[ ] there is no doubt that this conjecture is a true one. the _optics_ of dr. smith is one of the very few books quoted by herschel throughout his writings, and there is every evidence of his complete familiarity with its conclusions and methods; and this familiarity is of the kind which a student acquires with his early text-books. one other work he quotes in the same way, lalande's _astronomy_, and this too must have been deeply studied. during the years - , while herschel was following his profession and his studies at bath, the family life at hanover went on in much the same way. in his father isaac had a stroke of paralysis, which ended his violin-playing forever, and forced him to depend entirely upon pupils and copying of music for a livelihood. he died on march , , leaving behind him a good name, and living in the affectionate remembrance of his children and of all who knew him. carolina had now lost her best friend, and transferred to her brother william the affection she had before divided between him and her father. "my father wished to give me something like a polished education, but my mother was particularly determined that it should be a rough, but at the same time a useful one; and nothing farther she thought was necessary but to send me two or three months to a sempstress to be taught to make household linen. . . . my mother would not consent to my being taught french, and my brother dietrich was even denied a dancing-master, because she would not permit my learning along with him, though the entrance had been paid for us both; so all my father could do for me was to indulge me (and please himself) sometimes with a short lesson on the violin, when my mother was either in good humor or out of the way. though i have often felt myself exceedingly at a loss for the want of those few accomplishments of which i was thus, by an erroneous though well-meant opinion of my mother, deprived, i could not help thinking but that she had cause for wishing me not to know more than was necessary for being useful in the family; for it was her certain belief that my brother william would have returned to his country, and my eldest brother not have looked so high, if they had had a little less learning. * * * * * but sometimes i found it scarcely possible to get through with the work required, and felt very unhappy that no time at all was left for improving myself in music or fancy work, in which i had an opportunity of receiving some instruction from an ingenious young woman whose parents lived in the same house with us. but the time wanted for spending a few hours together could only be obtained by our meeting at daybreak, because by the time of the family's rising at seven, i was obliged to be at my daily business. though i had neither time nor means for producing anything immediately either for show or use, i was content with keeping samples of all possible patterns in needlework, beads, bugles, horse-hair, etc., for i could not help feeling troubled sometimes about my future destiny; yet i could not bear the idea of being turned into an abigail or housemaid, and thought that with the above and such like acquirements, with a little notion of music, i might obtain a place as governess in some family where the want of a knowledge of french would be no objection." a change was soon to come in her life too; her brother william wrote to propose that she should join him at bath-- . . . "to make the trial, if, by his instruction, i might not become a useful singer for his winter concerts and oratorios; he advised my brother jacob to give me some lessons by way of beginning; but that if, after a trial of two years, we should not find it answer our expectation, he would bring me back again. this at first seemed to be agreeable to all parties, but by the time i had set my heart upon this change in my situation, jacob began to turn the whole scheme into ridicule, and, of course, he never heard the sound of my voice except in speaking, and yet i was left in the harassing uncertainty whether i was to go or not. i resolved at last to prepare, as far as lay in my power, for both cases, by taking, in the first place, every opportunity, when all were from home, to imitate, with a gag between my teeth, the solo parts of concertos, _shake and all_, such as i had heard them play on the violin; in consequence i had gained a tolerable execution before i knew how to sing. i next began to knit ruffles, which were intended for my brother william, in case i remained at home--else they were to be jacob's. for my mother and brother d. i knitted as many cotton stockings as would last two years at least." in august, , her brother arrived at hanover, to take her back to england with him. the journey to london was made between august th and th, and soon after they went together to herschel's house, no. new king's street, bath. footnotes: [ ] wife of major john herschel, of the royal engineers, grandson of sir william. [ ] page . [ ] _memoir_ of carolina herschel, p. . sir george airy, astronomer royal, relates in the _academy_ that this "removal" was a desertion, as he was told by the duke of sussex that on the first visit of herschel to the king, after the discovery of the _georgium sidus_, the pardon of herschel was handed to him by the king himself, written out in due form. [ ] fÉtis; _biographie universelle des musiciens_, tome v. ( ) p. . [ ] dr. miller, a noted organist, and afterwards historian of doncaster. [ ] _the doctor_; by robert southey, edition of , p. . [ ] he frequently gave thirty-five and thirty-eight lessons a week to pupils at this time. [ ] according to fÉtis. a search for these in london has led me to the belief that fÉtis, who is usually very accurate, is here mistaken, and that these writings are by jacob herschel. [ ] _foreign quarterly review_, volume . chapter ii. life in bath; - . it was to a busy life in bath that herschel took his sister carolina, then twenty-two years old. she was a perfectly untried girl, of very small accomplishments and outwardly with but little to attract. the basis of her character was the possibility of an unchanging devotion to one object; for the best years of her life this object was the happiness and success of her brother william, whom she profoundly loved. her love was headstrong and full of a kind of obstinate pride, which refused to see anything but the view she had adopted. as long as her life continued to be with her dearest brother, all was well with her. she had a noble aim, and her heart was more than full. later on, this very singleness of character brought her other years of wretchedness. it is necessary to understand the almost spaniel-like allegiance she gave, in order to comprehend the value which her services were to herschel. she supplied him with an aid which was utterly loyal, entire, and devoted. her obedience was unquestioning, her reverence amounted almost to adoration. in their relation, he gave everything in the way of incentive and initiative, and she returned her entire effort loyally. at first her business was to gain a knowledge of the language, and to perfect herself in singing, so that she might become a soloist in the concerts and oratorios which he was constantly giving. in the beginning it was not easy. . . . "as the season for the arrival of visitors to the baths does not begin till october, my brother had leisure to try my capacity for becoming a useful singer for his concerts and oratorios, and being very well satisfied with my voice, i had two or three lessons every day, and the hours which were not spent at the harpsichord, were employed in putting me in the way of managing the family. . . . on the second morning, on meeting my brother at breakfast, he began immediately to give me a lesson in english and arithmetic, and showed me the way of booking and keeping accounts of cash received and laid out. . . . by way of relaxation we talked of astronomy and the bright constellations with which i had made acquaintance during the fine nights we spent on the postwagen travelling through holland. "my brother alexander, who had been some time in england, boarded and lodged with his elder brother, and, with myself, occupied the attic. the first floor, which was furnished in the newest and most handsome style, my brother kept for himself. the front room, containing the harpsichord, was always in order to receive his musical friends and scholars at little private concerts or rehearsals. . . . sundays i received a sum for the weekly expenses, of which my housekeeping book (written in english) showed the amount laid out, and my purse the remaining cash. one of the principal things required was to market, and about six weeks after coming to england i was sent alone among fishwomen, butchers, basket-women, etc., and i brought home whatever in my fright i could pick up. . . . my brother alex., who was now returned from his summer engagement, used to watch me at a distance, unknown to me, till he saw me safe on my way home. but all attempts to introduce any order in our little household proved vain, owing to the servant my brother then had. and what still further increased my difficulty was, that my brother's time was entirely taken up with business, so that i only saw him at meals. breakfast was at seven o'clock or before--much too early for me, who would rather have remained up all night than be obliged to rise at so early an hour. . . . "the three winter months passed on very heavily. i had to struggle against _heimwehe_ (home sickness) and low spirits, and to answer my sister's melancholy letters on the death of her husband, by which she became a widow with six children. i knew too little english to derive any consolation from the society of those who were about me, so that, dinner-time excepted, i was entirely left to myself." so the winter passed. "the time when i could hope to receive a little more of my brother's instruction and attention was now drawing near; for after easter, bath becomes very empty, only a few of his scholars, whose families were resident in the neighborhood, remaining. but i was greatly disappointed; for, in consequence of the harassing and fatiguing life he had led during the winter months, he used to retire to bed with a basin of milk or glass of water, and smith's _harmonics_ and _optics_, ferguson's _astronomy_, etc., and so went to sleep buried under his favorite authors; and his first thoughts on rising were how to obtain instruments for viewing those objects himself of which he had been reading. there being in one of the shops a two-and-a-half-foot gregorian telescope to be let, it was for some time taken in requisition, and served not only for viewing the heavens, but for making experiments on its construction. . . . it soon appeared that my brother was not contented with knowing what former observers had seen, for he began to contrive a telescope eighteen or twenty feet long (i believe after huyghens' description). . . . i was much hindered in my musical practice by my help being continually wanted in the execution of the various contrivances, and i had to amuse myself with making the tube of pasteboard for the glasses, which were to arrive from london, for at that time no optician had settled at bath. but when all was finished, no one besides my brother could get a glimpse of jupiter or saturn, for the great length of the tube would not allow it to be kept in a straight line. this difficulty, however, was soon removed by substituting tin tubes. . . . my brother wrote to inquire the price of a reflecting mirror for (i believe) a five or six foot telescope. the answer was, there were none of so large a size, but a person offered to make one at a price much above what my brother thought proper to give. . . . about this time he bought of a quaker, resident at bath, who had formerly made attempts at polishing mirrors, all his rubbish of patterns, tools, hones, polishers, unfinished mirrors, etc., but all for small gregorians, and none above two or three inches diameter. "but nothing serious could be attempted, for want of time, till the beginning of june, when some of my brother's scholars were leaving bath; and then, to my sorrow, i saw almost every room turned into a workshop. a cabinet-maker making a tube and stands of all descriptions in a handsomely furnished drawing-room; alex. putting up a huge turning machine (which he had brought in the autumn from bristol, where he used to spend the summer) in a bedroom, for turning patterns, grinding glasses, and turning eye-pieces, etc. at the same time music durst not lie entirely dormant during the summer, and my brother had frequent rehearsals at home, where miss farinelli, an italian singer, was met by several of the principal performers he had engaged for the winter concerts." finally, in , he had made himself a gregorian telescope,[ ] and had begun to view the heavens. he was then thirty-six years old. the writer in the _european magazine_ describes this period: "all this time he continued his astronomical observations, and nothing now seemed wanting to complete his felicity, but sufficient leisure to enjoy his telescopes, to which he was so much attached, that at the theatre he used frequently to run from the harpsichord to look at the stars, during the time between the acts." in an extract from his _journal no. _, now at the rooms of the royal society, may be seen a copy of his first observation of the nebula of _orion_, on march , . this was made with his five-and-a-half-foot gregorian reflector. it was at this time ( ), between the acts of the theatre, that he made his first review of the heavens, with a newtonian telescope, of an aperture of four and a half inches and a magnifying power of times. this telescope was one of the first made by himself. the review consisted of the examination of every star in the sky of the first, second, third, and fourth magnitudes, and of all planets visible. there are no records of these observations now extant, and they are noteworthy only as a preparation for more serious work. he was carrying out his resolve to see everything for himself. his assiduity may be judged of by the fact that between and herschel had observed a single object--the nebula of _orion_--no less than fourteen times. the success of his first telescopes incited him to new efforts. his house became a complete _atelier_, where everything that could tend to excellence in this manufacture was tried and re-tried a hundred different ways. when a difficulty arose, experiments were begun which continued till it was conquered. when a success was gained, it was prosecuted to the utmost. in the first seven-foot reflector was made, in a ten-foot was finished, in a "very good" ten-foot took its place. it must not be thought that the telescopes mentioned were the only ones completed. on the contrary, they were but the best ones selected out of many. in a new house had been engaged, which had "more room for workshops," and whose roof gave space for observing. the grass-plat near it was soon utilized to hold the stand of a twenty-foot telescope, which he had even then projected. his projects were unending, no success was final; his mind was at the height of activity; his whole effort was thrown into every undertaking. the mirrors for all these telescopes were made by hand. every portion of the grinding down to rough dimensions, the shaping to something near the correct form, the polishing till the accurately exact curves were obtained, all this must be done by hand. the machines for the purpose were not invented until .[ ] alexander and william worked together at this, but most of the work was done by the latter. the sister's part was to attend in the workshop and lend a hand wherever and whenever it was needed. . . . "my time was taken up with copying music and practising, besides attendance on my brother when polishing, since by way of keeping him alive i was constantly obliged to feed him by putting the victuals by bits into his mouth. this was once the case when, in order to finish a seven-foot mirror, he had not taken his hands from it for sixteen hours together. in general he was never unemployed at meals, but was always at those times contriving or making drawings of whatever came in his mind. generally i was obliged to read to him whilst he was at the turning-lathe, or polishing mirrors, _don quixote_, _arabian nights' entertainment_, the novels of sterne, fielding, etc.; serving tea and supper without interrupting the work with which he was engaged, . . . and sometimes lending a hand. i became, in time, as useful a member of the workshop as a boy might be to his master in the first year of his apprenticeship. . . . but as i was to take a part the next year in the oratorios, i had, for a whole twelvemonth, two lessons per week from miss fleming, the celebrated dancing-mistress, to drill me for a gentlewoman (god knows how she succeeded). so we lived on without interruption. my brother alex. was absent from bath for some months every summer, but when at home he took much pleasure in executing some turning or clockmaker's work for his brother." news from hanover put a sudden stop, for a time, to all these labors. the mother wrote, in the utmost distress, to say that dietrich had disappeared from his home, it was supposed with the intention of going to india "with a young idler not older than himself." his brother immediately left the lathe at which he was turning an eye-piece in cocoa-nut, and started for holland, whence he proceeded to hanover, failing to meet his brother, as he expected. meanwhile the sister received a letter to say that dietrich was "laid up very ill" at an inn in wapping. alexander posted to town, removed him to a lodging, and, after a fortnight's nursing, brought him to bath, where, on his brother william's return, he found him being well cared for by his sister. about this time another change was made to the house new king street, which was the last move in bath. it was here that the _georgium sidus_ was discovered. the music still went on. the oratorios of the _messiah_, _judas maccabeus_, and _samson_ were to be performed under herschel's direction, with an orchestra of nearly one hundred pieces. the scores and vocal parts of these carolina copied with her own hands, and the _soprani_ were instructed by her, she being the leading soloist. along with the music went the astronomy. not only were new telescopes made, but they were made for immediate use. the variable star _mira ceti_ was observed, and a long series of lunar observations begun. "in , , and i measured the heights of about one hundred mountains of the moon, by three different methods. "some of these observations are given in _philosophical transactions_, vol. lxx., but most remain uncalculated in my journal _till some proper opportunity."[ ]_ while herschel was measuring these lunar mountains, in december, , he made by chance an acquaintance of much value to him. dr. william watson, a fellow of the royal society, distinguished for his researches in electricity, happened to see him at his telescope, and this led to a visit and an invitation to herschel to join the philosophical society of bath, then forming. this he gladly did, and it was of use to him in many ways. he there formed acquaintance with men of his own way of thinking, and he himself became known. better than all, he learned to measure himself with other men, and by his early papers read to the society, he gained skill in putting his thoughts before his hearers. this skill he never lost, and the merely literary art of his memoirs would make his papers remarkable without their other merits. he is always clear, and in his early papers especially, he appeals to his particular audience--the royal society--in a way which shows that he is conscious of all its weaknesses as well as of its dignity. later, his tone slightly changed. he became less anxious to win his audience, for he had become an authority. this knowledge lent a quiet strength to his style, but never induced the slightest arrogance of spirit or manner. the bath philosophical society has left no printed proceedings. herschel was one of its earliest members, and many papers were communicated to it by his hand. these appear to have been of a very miscellaneous nature. some of them at least would be of the highest interest to us now. in the _philosophical transactions_ for , p. , herschel tells us that he communicated to that society "certain mathematical papers" relating to central forces other than the force of gravity, which are or may be concerned in the construction of the sidereal heavens. this early idea was still entertained by herschel in , and the mathematical papers referred to must be contained in the _minutes_ of the society, which on its dissolution were torn from the minute-book and returned to the writers. the earliest published writing of herschel is the answer to the prize question in the "ladies' diary" for , proposed by the celebrated landen, namely: "the length, tension, and weight of a musical string being given, it is required to find how many vibrations it will make in a given time, when a small given weight is fastened to its middle and vibrates with it." in the _philosophical transactions_ of the royal society for , are two papers of his. the title of the first is, _astronomical observations on the periodical star in collo ceti_, by mr. william herschel, of bath. this was communicated to the society by dr. william watson, jr., and was read may , , at the same time as the other paper on the mountains of the moon. it is to be noted that herschel was at this time plain "mr. william herschel, of bath." it was only in that he became "dr. herschel," through the oxford degree of ll.d. neither of these two papers is specially remarkable on its purely astronomical side. the problems examined were such as lay open before all, and the treatment of them was such as would naturally be suggested. the second of these two contained, however, a short description of his newtonian telescope, and he speaks of it with a just pride: "i believe that for distinctness of vision this instrument is perhaps equal to any that was ever made." he was, at least, certain of having obtained excellence in the making of his instruments. in his next paper, however, read january , , a subject is approached which shows a different kind of thought. it is the first obvious proof of the truth of the statement which he made long afterwards ( ), when he said: "a knowledge of the construction of the heavens has always been the ultimate object of my observations." the title of this paper was _astronomical observations on the rotation of the planets round their axes, made with a view to determine whether the earth's diurnal motion is perfectly equable_. here the question is a difficult and a remote one, and the method adopted for its solution is perfectly suitable in principle. it marks a step onward from mere observations to philosophizing upon their results. in practical astronomy, too, we note an advance. not only are his results given, but also careful estimates of the errors to be feared in them, and a discussion of the sources of such errors. the same volume of the _philosophical transactions_ which contains this paper, also contains another, _account of a comet_, read april , . this comet was the major planet _uranus_, or, as herschel named it, _georgium sidus_. he had found it on the night of tuesday, march , . "in examining the small stars in the neighborhood of h _geminorum_, i perceived one that appeared visibly larger than the rest; being struck with its uncommon appearance, i compared it to h _geminorum_ and the small star in the quartile between _auriga_ and _gemini_, and finding it so much larger than either of them, i suspected it to be a comet." the "comet" was observed over all europe. its orbit was computed by various astronomers, and its distance from the sun was found to be nineteen times that of our earth. this was no comet, but a new major planet. the discovery of the amateur astronomer of bath was the most striking since the invention of the telescope. it had absolutely no parallel, for every other major planet had been known from time immemorial.[ ] the effect of the discoveries of galileo was felt almost more in the moral than in the scientific world. the mystic number of the planets was broken up by the introduction of four satellites to _jupiter_. that _venus_ emulated the phases of our moon, overthrew superstition and seated the copernican theory firmly. the discovery of "an innumerable multitude of fixed stars" in the milky way confounded the received ideas. this was the great mission of the telescope in galileo's hands. the epoch of mere astronomical discovery began with the detection of the large satellite of _saturn_ by huyghens, in . even then superstition was not dead. huyghens did not search for more moons, because by that discovery he had raised the number of known satellites to six,[ ] and because these, with the six planets, made "the perfect number twelve." from to cassini discovered four more moons revolving about _saturn_. since no new body had been added to the solar system. it was thought complete for nearly a century. in england, the remarkable discoveries of bradley ( - ) had been in the field of practical astronomy, and his example had set the key-note for further researches. france was just about beginning the brilliant period of her discoveries in mathematical astronomy, and had no observatory devoted to investigations like herschel's, with the possible exception of darquier's and flaugergues'. the observatories of schroeter and von hahn, in germany, were not yet active. the field which herschel was created to fill was vacant, the whole world over. it was especially so in england. the royal observatory at greenwich, under maskelyne, a skilful observer, whose work was mostly confined to meridian observations, was no rival to a private observatory like herschel's. the private observatories themselves were but small affairs; those of the king, at kew, of dr. wilson, at glasgow, of mr. aubert, at loampit hill, of the count von bruhl, in london, being perhaps the most important. the whole field was open. what was perhaps more remarkable, there was in england, during herschel's lifetime, no astronomer, public or private, whose talents, even as an observer, lay in the same direction. it hardly need be said that as a philosopher in his science, he had then no rival, as he has had none since. his only associates even, were michell and wilson.[ ] without depreciating the abilities of the astronomers of england, his cotemporaries, we may fairly say that herschel stood a great man among a group of small ones. let us endeavor to appreciate the change effected in the state of astronomy not only in england but in the whole world, simply by the discovery of _uranus_. suppose, for example, that the last planet in our system had been _saturn_. no doubt herschel would have gone on. in spite of one and another difficulty, he would have made his ten-foot, his twenty-foot telescopes. his forty-foot would never have been built, and the two satellites which he found with it might not have been discovered. certainly _mimas_ would not have been. his researches on the construction of the heavens would have been made; those were in his brain, and must have been ultimated. the mass of observations of _saturn_, of _jupiter_, of _mars_, of _venus_, would have been made and published. the researches on the sun, on the "invisible rays" of heat, on comets and nebulæ--all these might have been made, printed, and read. but these would have gone into the _philosophical transactions_ as the work of an amateur astronomer, "mr. herschel, of bath." they would have been praised, and they would have been doubted. it would have taken a whole generation to have appreciated them. they would have been severely tried, entirely on their merits, and finally they would have stood where they stand to-day--unrivalled. but through what increased labors these successes would have been gained! it is not merely that the patronage of the king, the subsidies for the forty-foot telescope (£ , ), the comparative ease of herschel's life would have been lacking. it is more than this. it would have been necessary for him to have created the audience to which he appealed, and to have conquered the most persistent of enemies--indifference. certainly, if herschel's mind had been other than it was, the discovery of _uranus_, which brought him honors from every scientific society in the world, and which gave him authority, might have had a hurtful effect. but, as he was, there was nothing which could have aided his career more than this startling discovery. it was needed for him. it completed the solar system far more by affording a free play to a profoundly philosophical mind, than by occupying the vacant spaces beyond _saturn_. his opportunities would have been profoundly modified, though his personal worth would have been the same. "the star that from the zenith darts its beams, visible though it be to half the earth, though half a sphere be conscious of its brightness, is yet of no diviner origin, no purer essence, than the one that burns like an untended watchfire, on the ridge of some dark mountain; or than those that seem humbly to hang, like twinkling winter lamps, among the branches of the leafless trees." to show how completely unknown the private astronomer of bath was at this time, i transcribe a sentence from bode's account of the discovery of _uranus_. "in the _gazette littéraire_ of june, , this worthy man is called mersthel; in julius' _journal encyclopédique_, hertschel; in a letter from mr. maskelyne to m. messier, herthel; in another letter of maskelyne's to herr mayer, at mannheim, herrschell; m. darquier calls him hermstel. what may his name be? he must have been born a _german_."[ ] this obscurity did not long continue. the news spread quickly from fashionable bath to london. on the th of december, , herschel was elected a fellow of the royal society, to which he was formally "admitted" may , . he was forty-three years old. he also received the copley medal in for his "discovery of a new and singular star."[ ] . . . "he was now frequently interrupted by visitors who were introduced by some of his resident scholars, among whom i remember sir harry engelfield, dr. blagden, and dr. maskelyne. with the latter he was engaged in a long conversation, which to me sounded like quarrelling, and the first words my brother said after he was gone were: 'that is a devil of a fellow.'. . . "i suppose their names were often not known, or were forgotten; for it was not till the year or that a memorandum of the names of visitors was thought of.". . . "my brother now applied himself to perfect his mirrors, erecting in his garden a stand for his twenty-foot telescope; many trials were necessary before the required motions for such an unwieldy machine could be contrived. many attempts were made by way of experiment before an intended thirty-foot telescope could be completed, for which, between whiles (not interrupting the observations with seven, ten, and twenty-foot, and writing papers for both the royal and bath philosophical societies), gauges, shapes, weight, etc., of the mirror were calculated, and trials of the composition of the metal were made. in short, i saw nothing else and heard nothing else talked of but these things when my brothers were together. alex. was always very alert, assisting when anything new was going forward, but he wanted perseverance, and never liked to confine himself at home for many hours together. and so it happened that my brother william was obliged to make trial of my abilities in copying for him catalogues, tables, etc., and sometimes whole papers which were lent him for his perusal. among them was one by mr. michell and a catalogue of christian mayer, in latin, which kept me employed when my brother was at the telescope at night. when i found that a hand was sometimes wanted when any particular measures were to be made with the lamp micrometer, etc., or a fire to be kept up, or a dish of coffee necessary during a long night's watching, i undertook with pleasure what others might have thought a hardship. . . . since the discovery of the _georgium sidus_ [march , ], i believe few men of learning or consequence left bath before they had seen and conversed with its discoverer, and thought themselves fortunate in finding him at home on their repeated visits. sir william watson was almost an intimate, for hardly a day passed but he had something to communicate from the letters which he received from sir joseph banks, and other members of the royal society, from which it appeared that my brother was expected in town to receive the gold medal. the end of november was the most precarious season for absenting himself. but sir william watson went with him, and it was arranged so that they set out with the diligence at night, and by that means his absence did not last above three or four days, when my brother returned alone, sir william remaining with his father. "now a very busy winter was commencing; for my brother had engaged himself to conduct the oratorios conjointly with ronzini, and had made himself answerable for the payment of the engaged performers, for his credit ever stood high in the opinion of every one he had to deal with. (he lost considerably by this arrangement.) but, though at times much harassed with business, the mirror for the thirty-foot reflector was never out of his mind, and if a minute could but be spared in going from one scholar to another, or giving one the slip, he called at home to see how the men went on with the furnace, which was built in a room below, even with the garden. "the mirror was to be cast in a mould of loam, of which an immense quantity was to be pounded in a mortar and sifted through a fine sieve. it was an endless piece of work, and served me for many an hour's exercise; and alex. frequently took his turn at it, for we were all eager to do something towards the great undertaking. even sir william watson would sometimes take the pestle from me when he found me in the work-room, where he expected to find his friend, in whose concerns he took so much interest that he felt much disappointed at not being allowed to pay for the metal. but i do not think my brother ever accepted pecuniary assistance from any one of his friends, and on this occasion he declined the offer by saying it was paid for already. "among the bath visitors were many philosophical gentlemen who used to frequent the levées at st. james's, when in town. colonel walsh, in particular, informed my brother that from a conversation he had had with his majesty, it appeared that in the spring he was to come with his seven-foot telescope to the king. similar reports he received from many others, but they made no great impression nor caused any interruption in his occupation or study, and as soon as the season for the concerts was over, and the mould, etc., in readiness, a day was set apart for casting, and the metal was in the furnace. unfortunately it began to leak at the moment when ready for pouring, and both my brothers and the caster, with his men, were obliged to run out at opposite doors, for the stone flooring (which ought to have been taken up) flew about in all directions as high as the ceiling. before the second casting was attempted, everything which could insure success had been attended to, and a very perfect metal was found in the mould. "but a total stop and derangement now took place, and nearly six or seven months elapsed before my brother could return to the undisturbed enjoyment of his instruments and observations. for one morning in passion week, as sir william watson was with my brother, talking about the pending journey to town, my eldest nephew arrived to pay us a visit, and brought the confirmation that his uncle was expected with his instrument in town. . . . we had not one night in the week, except friday, but what was set apart for an oratorio either at bath or bristol. soon after easter, a new organ being erected in st. james's church, it was opened with two performances of the 'messiah;' this again took up some of my brother's time.". . . in may of herschel went to london. "but when almost double the time had elapsed which my brother could safely be absent from his scholars, alex., as well as myself, were much at a loss how to answer their inquiries, for, from the letters we received, we could learn nothing but that he had been introduced to the king and queen, and had permission to come to the concerts at buckingham house, where the king conversed with him about astronomy." it was during his absence at this time that the three following letters were written and received: "dear lina:-- "i have had an audience of his majesty this morning, and met with a very gracious reception. i presented him with the drawing of the solar system, and had the honor of explaining it to him and the queen. my telescope is in three weeks' time to go to richmond, and meanwhile to be put up at greenwich, where i shall accordingly carry it to-day. so you see, lina, that you must not think of seeing me in less than a month. i shall write to miss lee myself; and other scholars who inquire for me, you may tell that i cannot wait on them till his majesty shall be pleased to give me leave to return, or rather to dismiss me, for till then i must attend. i will also write to mr. palmer to acquaint him with it. "i am in a great hurry, therefore can write no more at present. tell alexander that everything looks very likely as if i were to stay here. the king inquired after him, and after my great speculum. he also gave me leave to come to hear the griesbachs play at the private concert which he has every evening. my having seen the king need not be kept a secret, but about my staying here it will be best not to say anything, but only that i must remain here till his majesty has observed the planets with my telescope. "yesterday i dined with colonel walsh, who inquired after you. there were mr. aubert and dr. maskelyne. dr. maskelyne in public declared his obligations to me for having introduced to them the high powers, for mr. aubert has so much succeeded with them that he says he looks down upon , , or with contempt, and immediately begins with . he has used , very completely, and seen my fine double stars with them. all my papers are printing, with the postscript and all, and are allowed to be very valuable. you see, lina, i tell you all these things. you know vanity is not my foible, therefore i need not fear your censure. farewell. "i am, your affectionate brother, "wm. herschel. "saturday morning, "probably _may , _." to miss herschel. "monday evening, _june , ._ "dear lina:-- "i pass my time between greenwich and london agreeably enough, but am rather at a loss for work that i like. company is not always pleasing, and i would much rather be polishing a speculum. last friday i was at the king's concert to hear george play. the king spoke to me as soon as he saw me, and kept me in conversation for half an hour. he asked george to play a solo-concerto on purpose that i might hear him; and george plays extremely well, is very much improved, and the king likes him very much. these two last nights i have been star-gazing at greenwich with dr. maskelyne and mr. aubert. we have compared our telescopes together, and mine was found very superior to any of the royal observatory. double stars which they could not see with their instruments i had the pleasure to show them very plainly, and my mechanism is so much approved of that dr. maskelyne has already ordered a model to be taken from mine, and a stand to be made by it to his reflector. he is, however, now so much out of love with his instrument that he begins to doubt whether it _deserves_ a new stand. "i am introduced to the best company. to-morrow i dine at lord palmerston's, next day with sir joseph banks, etc., etc. among opticians and astronomers nothing now is talked of but _what they call_ my great discoveries. alas! this shows how far they are behind, when such trifles as i have seen and done are called _great_. let me but get at it again! i will make such telescopes, and see such things--that is, i will endeavor to do so." to miss herschel. "_july , ._" "dear carolina:-- "i have been so much employed that you will not wonder at my not writing sooner. the letter you sent me last monday came very safe to me. as dr. watson has been so good as to acquaint you and alexander with my situation, i was still more easy in my silence to you. last night the king, the queen, the prince of wales, the princess royal, princess sophia, princess augusta, etc., duke of montague, dr. heberden, m. de luc, etc., etc., saw my telescope, and it was a very fine evening. my instrument gave general satisfaction. the king has very good eyes, and enjoys observations with telescopes exceedingly. "this evening, as the king and queen are gone to kew, the princesses were desirous of seeing my telescope, but wanted to know if it was possible to see without going out on the grass, and were much pleased when they heard that my telescope could be carried into any place they liked best to have it. about eight o'clock it was moved into the queen's apartments, and we waited some time in hopes of seeing _jupiter_ or _saturn_. meanwhile i showed the princesses, and several other ladies who were present, the speculum, the micrometers, the movements of the telescopes, and other things that seemed to excite their curiosity. when the evening appeared to be totally unpromising, i proposed an artificial _saturn_ as an object, since we could not have the real one. i had beforehand prepared this little piece, as i guessed by the appearance of the weather in the afternoon we should have no stars to look at. this being accepted with great pleasure, i had the lamps lighted up which illuminated the picture of a _saturn_ (cut out in pasteboard) at the bottom of the garden wall. the effect was fine, and so natural that the best astronomer might have been deceived. their royal highnesses and other ladies seemed to be much pleased with the artifice. "i remained in the queen's apartment with the ladies till about half after ten; when in conversation with them i found them extremely well instructed in every subject that was introduced, and they seemed to be most amiable characters. to-morrow evening they hope to have better luck, and nothing will give me greater happiness than to be able to show them some of those beautiful objects with which the heavens are so gloriously ornamented." carolina's diary goes on: "sir william watson returned to bath after a fort-night or three weeks' stay. from him we heard that my brother was invited to greenwich with the telescope, where he was met by a numerous party of astronomical and learned gentlemen, and trials of his instrument were made. in these letters he complained of being obliged to lead an idle life, having nothing to do but to pass between london and greenwich. sir william received many letters, which he was so kind as to communicate to us. by these, and from those to alexander or to me, we learned that the king wished to see the telescope at windsor. at last a letter, dated july , arrived from therese, and from this and several succeeding ones we gathered that the king would not suffer my brother to return to his profession again, and by his writing several times for a supply of money we could only suppose that he himself was in uncertainty about the time of his return. "in the last week of july my brother came home, and immediately prepared for removing to datchet, where he had taken a house with a garden and grass-plat annexed, quite suitable for the purpose of an observing-place. sir william watson spent nearly the whole time at our house, and he was not the only friend who truly grieved at my brother's going from bath; or feared his having perhaps agreed to no very advantageous offers; their fears were, in fact, not without reason. . . . the prospect of entering again on the toils of teaching, etc., which awaited my brother at home (the months of leisure being now almost gone by), appeared to him an intolerable waste of time, and by way of alternative he chose to be royal astronomer, with a salary of £ a year. sir william watson was the only one to whom the sum was mentioned, and he exclaimed, 'never bought monarch honor so cheap!' to every other inquirer, my brother's answer was that the king had provided for him." on the st of august, , the family removed to datchet. the last musical duty was performed on whit-sunday, , in st. margaret's chapel, bath, when the anthem for the day was of herschel's own composition. the end of the introductory epoch of his life is reached. henceforth he lived in his observatory, and from his forty-fourth year onwards he only left it for short periods to go to london to submit his classic memoirs to the royal society. even for these occasions he chose periods of moonlight, when no observations could be made. he was a private man no longer. henceforth he belongs to the whole world. footnotes: [ ] probably on the model of one of short's gregorian telescopes, which were then the best instruments of the kind. [ ] for a description of the main points of herschel's processes of making reflectors, which will illustrate his strong mechanical talents, see _encyclopædia britannica_, eighth edition, article _telescope_. [ ] these have never been published, nor is it likely at this day, when our measuring instruments are so greatly improved, that they would be of any material value to science, although of interest as giving the proofs of herschel's assiduity and skill. he was always more than the maker of telescopes, for he was never content until they were applied to the problems of astronomy. [ ] arago has implied that if herschel had directed his telescope to _uranus_ only eleven days earlier than he did, this discovery would have escaped him, since at that time (march , ) the planet was at its _station_, and had no motion relative to the star. this is an entire misconception, since the new planet was detected by its physical appearance, and not by its motion. does any one suppose that "a new and singular star" like this would have been once viewed and then forgotten? [ ] four of _jupiter_, one of the earth, and one of _saturn_. [ ] john michell had been a member of the royal society since : he died in . he was a philosophical thinker, as is shown by his memoirs on the distances of the stars, and by his invention of the method for determining the earth's density. it is not certain that he was personally known to herschel, although his writings were familiar to the latter. alexander wilson was professor of astronomy at glasgow, and is chiefly known to us by his theory of the nature of the solar spots, which was adopted and enlarged by herschel. he died in ; but the families of wilson and herschel remained close friends. [ ] _berliner jahrbuch_, , p. . in the _connaissance des tems_ for he is called "horochelle." [ ] at the presentation sir joseph banks, the president of the royal society, said: "in the name of the royal society i present to you this gold medal, the reward which they have assigned to your successful labors, and i exhort you to continue diligently to cultivate those fields of science which have produced to you a harvest of so much honor. your attention to the improvement of telescopes has already amply repaid the labor which you have bestowed upon them; but the treasures of the heavens are well known to be inexhaustible. who can say but your new star, which exceeds _saturn_ in its distance from the sun, may exceed him as much in magnificence of attendance? who knows what new rings, new satellites, or what other nameless and numberless phenomena remain behind, waiting to reward future industry and improvement?" chapter iii. life at datchet, clay hall, and slough; - . the new house at datchet, which was occupied from till , was a source of despair to carolina herschel, who looked upon its desolate and isolated condition with a housekeeper's eyes. this was nothing to her brother, who gayly consented to live upon "eggs and bacon," now that he was free at last to mind the heavens. the ruinous state of the place had no terrors in his eyes, for was there not a laundry which would serve as a library, a large stable which was just the place for the grinding of mirrors, and a grass-plat for the small twenty-foot reflector? here they set to work at astronomy; the brother with the twenty-foot, the sister aiding him, and at odd times sweeping for comets. in the course of her life she discovered no less than eight, and five of these were first seen by her. * * * * * in herschel wrote his paper "on three volcanoes in the moon," which he had observed in april of that year. in this he mentions previous observations of the same sort. i do not remember that the following account of these has ever been put on record in english. baron von zach writes from london to bode:[ ] "probably you have heard also of the volcanoes in the moon, which herschel has observed. . . . i will give you an account of it as i heard it from his own lips. dr. lind, a worthy physician in windsor, who has made himself known through his two journeys in china, and who is a friend of our herschel's, was with his wife one evening on a visit to herschel in datchet [ , may ]. on this evening there was to be an occultation of a star at the moon's dark limb. this was observed by herschel and doctor lind. mrs. lind wished also to see what was occurring, and placed herself at a telescope and watched attentively. "scarcely had the star disappeared before mrs. lind thought she saw it again, and exclaimed that the star had gone in front of, and not behind the moon. this provoked a short astronomical lecture on the question, but still she would not credit it, because she _saw_ differently. finally herschel stepped to the telescope, and in fact he saw a bright point on the dark disc of the moon, which he followed attentively. it gradually became fainter and finally vanished.". . . the life at datchet was not free from its annoyances. "much of my brother's time was taken up in going, when the evenings were clear, to the queen's lodge, to show the king, etc., objects through the seven-foot. but when the days began to shorten, this was found impossible, for the telescope was often (at no small expense and risk of damage) obliged to be transported in the dark back to datchet, for the purpose of spending the rest of the night with observations on double stars for a second catalogue. my brother was, besides, obliged to be absent for a week or ten days, for the purpose of bringing home the metal of the cracked thirty-foot mirror, and the remaining materials from his work-room. before the furnace was taken down at bath, a second twenty-foot mirror, twelve inches diameter, was cast, which happened to be very fortunate, for on the st of january, , a very fine one cracked by frost in the tube. . . . "in my brother's absence from home i was, of course, left alone to amuse myself with my own thoughts, which were anything but cheerful. i found i was to be trained for an assistant astronomer, and, by way of encouragement, a telescope adapted for 'sweeping,' consisting of a tube with two glasses, such as are commonly used in a 'finder,' was given me. i was 'to sweep for comets,' and i see, by my journal, that i began august d, , to write down and describe all remarkable appearances i saw in my 'sweeps,' which were horizontal. but it was not till the last two months of the same year that i felt the least encouragement to spend the star-light nights on a grass-plot covered with dew or hoar-frost, without a human being near enough to be within call. i knew too little of the real heavens to be able to point out every object so as to find it again, without losing much time by consulting the atlas. but all these troubles were removed when i knew my brother to be at no great distance making observations, with his various instruments, on double stars, planets, etc., and when i could have his assistance immediately if i found a nebula or cluster of stars, of which i intended to give a catalogue; but, at the end of , i had only marked fourteen, when my sweeping was interrupted by being employed to write down my brother's observations with the large twenty-foot. i had, however, the comfort to see that my brother was satisfied with my endeavors to assist him when he wanted another person either to run to the clocks, write down a memorandum, fetch and carry instruments, or measure the ground with poles, etc., etc., of which something of the kind every moment would occur. for the assiduity with which the measurements on the diameter of the _georgium sidus_, and observations of other planets, double stars, etc., etc., were made, was incredible, as may be seen by the various papers that were given to the royal society in , which papers were written in the daytime, or when cloudy nights interfered. besides this, the twelve-inch speculum was perfected before the spring, and many hours were spent at the turning-bench, as not a night clear enough for observing ever passed but that some improvements were planned for perfecting the mounting and motions of the various instruments then in use, or some trials were made of new constructed eye-pieces, which were mostly executed by my brother's own hands. wishing to save his time, he began to have some work of that kind done by a watchmaker who had retired from business and lived on datchet common; but the work was so bad, and the charges so unreasonable, that he could not be employed. it was not till some time afterwards, in his frequent visits to the meetings of the royal society (made in moonlight nights), that he had an opportunity of looking about for mathematical workmen, opticians, and founders. but the work seldom answered expectation, and it was kept, to be executed with improvements by alexander during the few months he spent with us. "the summer months passed in the most active preparation for getting the large twenty-foot ready against the next winter. the carpenters and smiths of datchet were in daily requisition, and, as soon as patterns for tools and mirrors were ready, my brother went to town to have them cast, and, during the three or four months alexander could be absent from bath, the mirrors and optical parts were nearly completed. "but that the nights after a day of toil were not given to rest, may be seen by the observations on _mars_, of which a paper, dated december , , was given to the royal society. some trouble, also, was often thrown away, during those nights, in the attempt to teach me to remeasure double stars with the same micrometers with which former measures had been taken, and the small twenty-foot was given me for that purpose. . . . i had also to ascertain their places by a transit instrument lent for that purpose by mr. dalrymple; but, after many fruitless attempts, it was seen that the instrument was, perhaps, as much in fault as my observations." in herschel says: "i have now finished my third review of the heavens. the first was made with a newtonian telescope something less than seven feet focal length, a power of , and an aperture of four and a half inches. it extended only to stars of the first, second, third, and fourth magnitudes. my second review was made with an instrument much superior to the other, of . inches focus, . inches aperture, and power . it extended to all the stars of harris's maps and the telescopic ones near them, as far as the eighth magnitude. the catalogue of double stars and the discovery of the _georgium sidus_, were the results of that review. the third was with the same instrument and aperture, but with a power of . this review extended to all the stars of flamsteed's catalogue, together with every small star about them, to the amount of a great many thousands of stars. i have, many a night, in the course of eleven or twelve hours of observation, carefully and singly examined not less than celestial objects, besides taking measures, and sometimes viewing a particular star for half an hour together." the fourth review began with the twenty-foot, in . "my brother began his series of sweeps when the instrument was yet in a very unfinished state, and my feelings were not very comfortable when every moment i was alarmed by a crack or fall, knowing him to be elevated fifteen feet or more on a temporary cross-beam, instead of a safe gallery. the ladders had not even their braces at the bottom; and one night, in a very high wind, he had hardly touched the ground before the whole apparatus came down. some laboring men were called up to help in extricating the mirror, which was, fortunately, uninjured, but much work was cut out for carpenters next day. i could give a pretty long list of accidents which were near proving fatal to my brother as well as myself. to make observations with such large machinery, where all around is in darkness, is not unattended with danger, especially when personal safety is the last thing with which the mind is occupied; even poor piazzi did not go home without getting broken shins by falling over the rack-bar. "in the long days of the summer months many ten and seven foot mirrors were finished; there was nothing but grinding and polishing to be seen. for ten-foot, several had been cast with ribbed backs, by way of experiment, to reduce the weight in large mirrors. in my leisure hours i ground seven-foot and plain mirrors from rough to fining down, and was _indulged_ with polishing and the last finishing of a very beautiful mirror for sir william watson. "an account of the discoveries made with the twenty-foot and the improvements of the mechanical parts of the instrument during the winter of is given with the catalogue of the first , new nebulæ. by which account it must plainly appear that the expenses of these improvements, and those which were yet to be made in the apparatus of the twenty-foot (which, in fact, proved to be a model of a larger instrument), could not be supplied out of a salary of £ a year, especially as my brother's finances had been too much reduced during the six months before he received his _first_ quarterly payment of _fifty pounds_ (which was michaelmas, ). travelling from bath to london, greenwich, windsor, backwards and forwards, transporting the telescope, etc., breaking up his establishment at bath and forming a new one near the court, all this, even leaving such personal conveniences as he had for many years been used to, out of the question, could not be obtained for a trifle; a good large piece of ground was required for the use of the instruments, and a habitation in which he could receive and offer a bed to an astronomical friend, was necessary after a night's observation. "it seemed to be supposed that enough had been done when my brother was enabled to leave his profession that he might have time to make and sell telescopes. the king ordered four ten-foot himself, and many seven-foot besides had been bespoke, and much time had already been expended on polishing the mirrors for the same. but all this was only retarding the work of a thirty or forty foot instrument, which it was my brother's chief object to obtain as soon as possible; for he was then on the wrong side of forty-five, and felt how great an injustice he would be doing to himself and to the cause of astronomy by giving up his time to making telescopes for other observers. "sir william watson, who often in the lifetime of his father came to make some stay with us at datchet, saw my brother's difficulties, and expressed great dissatisfaction. on his return to bath he met, among the visitors there, several belonging to the court, to whom he gave his opinion concerning his friend and his situation very freely. in consequence of this, my brother had soon after, through sir j. banks, the promise that £ , would be granted for enabling him to make himself an instrument. "immediately every preparation for beginning the great work commenced. a very ingenious smith (campion), who was seeking employment, was secured by my brother, and a temporary forge erected in an upstairs room." the sale of these telescopes of herschel's must have produced a large sum, for he had made before more than two hundred seven-feet, one hundred and fifty ten-feet, and eighty twenty-feet mirrors. for many of the telescopes sent abroad no stands were constructed. the mirrors and eye-pieces alone were furnished, and a drawing of the stand sent with them by which the mirrors could be mounted. in the cost of a seven-foot telescope, six and four-tenths inches aperture, stand, eye-pieces, etc., complete, was two hundred guineas, a ten-foot was six hundred guineas, and a twenty-foot about , to , guineas. he had made four ten-foot telescopes like this for the king. in schroeter got the mirrors and eye-pieces only for a four-and-three-quarter-inch reflector for five guineas; those for his seven-foot telescope were twenty-three guineas. later a seven-foot telescope, complete, was sold for one hundred guineas, and the twenty-five-foot reflector, made for the madrid observatory, cost them , francs = $ , .[ ] it was ordered in , but not delivered for several years, the spanish government being short of money. for a ten and a seven foot telescope, the prince of canino paid £ , . von magellan writes to bode concerning a visit to herschel:[ ] "i spent the night of the th of january at herschel's, in datchet, near windsor, and had the good luck to hit on a fine evening. he has his twenty-foot newtonian telescope in the open air and mounted in his garden very simply and conveniently. it is moved by an assistant, who stands below it. . . . near the instrument is a clock regulated to sidereal time. . . . in the room near it sits herschel's sister, and she has flamsteed's atlas open before her. as he gives her the word, she writes down the declination and right ascension and the other circumstances of the observation. in this way herschel examines the whole sky without omitting the least part. he commonly observes with a magnifying power of one hundred and fifty, and is sure that after four or five years he will have passed in review every object above our horizon. he showed me the book in which his observations up to this time are written, and i am astonished at the great number of them. each sweep covers ° ' in declination, and he lets each star pass at least three times through the field of his telescope, so that it is impossible that anything can escape him. he has already found about double stars and almost as many nebulæ. i went to bed about one o'clock, and up to that time, he had found that night four or five new nebulæ. the thermometer in the garden stood at ° fahrenheit; but, in spite of this, herschel observes the whole night through, except that he stops every three or four hours and goes in the room for a few moments. for some years herschel has observed the heavens every hour when the weather is clear, and this always in the open air, because he says that the telescope only performs well when it is at the same temperature as the air. he protects himself against the weather by putting on more clothing. he has an excellent constitution, and thinks about nothing else in the world but the celestial bodies. he has promised me in the most cordial way, entirely in the service of astronomy, and without thinking of his own interest, to see to the telescopes i have ordered for european observatories, and he will himself attend to the preparation of the mirrors." it was at this time, , may , that herschel married. his wife was the daughter of mr. james baldwin, a merchant of the city of london, and the widow of john pitt, esq. she is described as a lady of singular amiability and gentleness of character. she was entirely interested in his scientific pursuits, and the jointure which she brought removed all further anxiety about money affairs. they had but one child, john frederick william, born march , .[ ] * * * * * the house at datchet became more and more unfit for the needs of the family, and in june, , a move was made to clay hall, in old windsor. the residence here was but short, and finally a last change was made to slough on april, d, . the ardor of the work during these years can be judged of by a single sentence from carolina herschel's diary: "the last night at clay hall was spent in sweeping till daylight, and by the next evening the telescope stood ready for observation at slough." from until his death, herschel remained at slough; his life, truly speaking, was in his observatory. it is indeed true, as arago has said in his eloquent tribute to him: "on peut dire hardiment du jardin et de la petite maison de slough, que c'est le lieu du monde où il a été fait le plus de découvertes. le nom de ce village ne périra pas; les sciences le transmettront religieusement à nos derniers neveux." herschel's first contribution to the _philosophical transactions_ was printed in the volume for , his last in that for . of these thirty-nine volumes, there are only two ( and ) which contain no paper from his hand, and many volumes contain more than one, as he published no less than sixty-eight memoirs in this place. and yet it must not be thought that his was an austere and grave existence. music, which he loved to enthusiasm, was still a delight to him. all the more that his devotion was free. the glimpses which we get of his life with his friends show him always cheerful, ardent, and devoted. even in his later years, he had not lost a "boyish earnestness to explain;" his simplicity and the charm of his manner struck every one. "herschel, you know, and everybody knows, is one of the most pleasing and well-bred natural characters of the present age," says dr. burney, who had opportunity to know. the portrait which is given in the frontispiece must have been painted about this time ( ), and the eager, ardent face shows his inner life far better than any words can do. even in his scientific writings, which everything conspired to render grave and sober, the almost poetic nature of his mind shows forth. in one of his (unpublished) note-books, now in the royal society's library, i found this entry: " th sweep--november , .--the nebula of _orion_, which i saw by the front view, was so glaring and beautiful that i could not think of taking any place of its extent." he was quite alone under the perfectly silent sky when this was written, and he was at his post simply to make this and other such observations. but the sky was beautiful to him, and his faithful sister, carolina, sitting below, has preserved for us the words as they dropped from his lips. on the th of january, , herschel discovered two satellites to _uranus_. after he had well assured himself of their existence, but before he communicated his discovery to the world, he made this crucial test. he prepared a sketch of _uranus_ attended by his two satellites, as it would appear on the night of february , , and when the night came, "the heavens displayed the original of my drawings, by showing in the situation i had delineated them _the georgian planet attended by two satellites_. i confess that this scene appeared to me with additional beauty, as the little secondary planets seemed to give a dignity to the primary one which raises it into a more conspicuous situation among the great bodies of the solar system.". . . in a memoir of , he has a few sentences which show the living way in which the heavens appeared to him: "this method of viewing the heavens seems to throw them into a new kind of light. "they are now seen to resemble a luxuriant garden, which contains the greatest variety of productions in different flourishing beds; and one advantage we may at least reap from it is, that we can, as it were, extend the range of our experience to an immense duration. for is it not almost the same thing whether we live successively to witness the germination, blooming, foliage, fecundity, fading, withering, and corruption of a plant, or whether a vast number of specimens selected from every stage through which the plant passes in the course of its existence be brought at once to our view?" the thought here is no less finely expressed than it is profound. the simile is perfect, if we have the power to separate among the vast variety each state of being from every other, and if the very luxuriance of illustration in the heavens does not bewilder and overpower the mind. it was precisely this discriminating power that herschel possessed in perfection. there is a kind of humor in the way he records a change of opinion: "i formerly supposed the surface of _saturn's_ ring to be rough, owing to luminous points like mountains seen on it, till one of these was kind enough to venture off the edge of the ring and appear as a satellite." in he replies with a certain concealed sharpness to the idea that he used magnifying powers which were too high. there is a tone almost of impatience, as if he were conscious he was replying to a criticism based on ignorance: "we are told that we gain nothing by magnifying too much. i grant it; but shall never believe i magnify too much till by experience i find that i can see better with a lower power." ( .) by , when he returns to this subject, in answer to a formal request to explain his use of high magnifiers, he is quite over any irritation, and treats the subject almost with playfulness: "soon after my first essay of using high powers with the newtonian telescope, i began to doubt whether an opinion which has been entertained by several eminent authors, 'that vision will grow indistinct when the optic pencils are less than the fiftieth part of an inch,' would hold good in all cases. i perceived that according to this criterion i was not entitled to see distinctly with a power of much more than about in a seven-foot telescope of an aperture of six and four-tenths inches, whereas in many experiments i found myself very well pleased with magnifiers which far exceeded such narrow limits. this induced me, as it were, by way of apology to myself for seeing well where i ought to have seen less distinctly, to make a few experiments." it is needless to say that these experiments proved that from the point of view taken by herschel, he was quite right, and that his high powers had numerous valuable applications. he goes on to say: "had it not been for a late conversation with some of my highly esteemed and learned friends, i might probably have left the papers on which these experiments were recorded, among the rest of those that are laid aside, when they have afforded me the information i want." the last sentence seems to be a kind of notice to his learned friends that there is yet more unsaid. as a warning to those to whose criticisms he had replied, he gives them this picture of the kind of assiduity which will be required, if some of his observations on double stars are to be repeated: "it is in vain to look for these stars if every circumstance is not favorable. the observer as well as the instrument must have been long enough out in the open air to acquire the same temperature. in very cold weather an hour at least will be required." ( .) we may gain some further insight into his character from the following chance extracts from his writings: "i have all along had truth and reality in view as the sole object of my endeavors." ( .) "not being satisfied when i thought it possible to obtain more accurate measures, i employed [a more delicate apparatus]." ( .) "to this end i have already begun a series of observations upon several zones of double stars, and should the result of them be against these conjectures, i shall be the first to point out their fallacy." ( .) "there is a great probability of succeeding still farther in this laborious but delightful research, so as to be able at last to say not only how much the annual parallax _is not_, but how much it really _is_." ( .) the nature of his philosophizing, and the limits which he set to himself, may be more clearly seen in further extracts: "by taking more time [before printing these observations] i should undoubtedly be enabled to speak more confidently of the _interior_ _construction of the heavens_, and of its various _nebulous_ and sidereal strata. as an apology for this prematurity it may be said that, the end of all discoveries being communication, we can never be too ready in giving facts and observations, whatever we may be in reasoning upon them." ( .) "in an investigation of this delicate nature we ought to avoid two opposite extremes. if we indulge a fanciful imagination, and build worlds of our own, we must not wonder at our going wide from the path of truth and nature. on the other hand, if we add observation to observation without attempting to draw not only certain conclusions but also conjectural views from them, we offend against the very end for which only observations ought to be made. i will endeavor to keep a proper medium, but if i should deviate from that, i could wish not to fall into the latter error." ( .) "as observations carefully made should always take the lead of theories, i shall not be concerned if what i have to say contradicts what has been said in my last paper on this subject." ( .) no course of reasoning could be more simple, more exact, more profound, and more beautiful than this which follows: "as it has been shown that the spherical figure of a cluster is owing to the action of central powers, it follows that those clusters which, _cæteris paribus_, are the most complete in this figure, must have been the longest exposed to the action of these causes. thus the maturity of a sidereal system may be judged from the disposition of the component parts. "hence planetary nebulæ may be looked on as very aged. though we cannot see any individual nebula pass through all its stages of life, we can select particular ones in each peculiar stage." ( .) there is something almost grandiose and majestic in his statement of the ultimate destiny of the galaxy: "to him the fates were known of orbs dim hovering on the skirts of space." "--since the stars of the milky way are permanently exposed to the action of a power whereby they are irresistibly drawn into groups, we may be certain that from mere clustering stars they will be gradually compressed, through successive stages of accumulation, till they come up to what may be called the ripening period of the globular form, and total insulation; from which it is evident that the milky way must be finally broken up and cease to be a stratum of scattered stars. "the state into which the incessant action of the clustering power has brought it at present, is a kind of chronometer that may be used to measure the time of its past and future existence; and although we do not know the rate of going of this mysterious chronometer, it is nevertheless certain that since the breaking up of the milky way affords a proof that it cannot last forever, it equally bears witness that its past duration cannot be admitted to be infinite." ( .) herschel's relations with his cotemporaries were usually of the most pleasant character, though seldom intimate. this peace was broken but by one unpleasant occurrence. in the _philosophical transactions_ for , schroeter had communicated a series of observations made with one of herschel's own telescopes on the atmospheres of _venus_, the moon, etc. it was not only an account of phenomena which had been seen; it was accompanied by measures, and the computations based on these led to heights and dimensions for mountains on _venus_ which were, to say the least, extravagant. the adjective will not seem too strong when we say that the very existence of the mountains themselves is to-day more than doubtful. the appearances seen by schroeter were described by him in perfectly good faith, and similar ones have been since recorded. his reasoning upon them was defective, and the measures which he made were practically valueless. this paper, printed in the _transactions_ of the royal society, to which schroeter had not before contributed, appears to have irritated herschel. no doubt there were not wanting members of his own society who hinted that on the continent, too, there were to be found great observers, and that here, at least, herschel had been anticipated even in his own field. i have always thought that the memoir of herschel which appeared in the next volume of the _transactions_ ( ), _observations on the planet venus_, was a rejoinder intended far more for the detractors at home than for the astronomer abroad. the review is conceived in a severe spirit. the first idea seems to be to crush an opposition which he feels. the truth is established, but its establishment is hardly the _first_ object. it seems as if herschel had almost allowed himself to be forced into a position of arrogance, which his whole life shows was entirely foreign to his nature. all through the review he does not once mention schroeter's name. he says: "a series of observations on _venus_, begun by me in april, , has been continued down to the present time. . . . the result of my observations would have been communicated long ago if i had not flattered myself with the hope of some better success concerning the diurnal motion of _venus_, which has still eluded my constant attention as far as concerns its period and direction. . . . even at this present time i should hesitate to give the following extracts if it did not seem incumbent on me to examine by what accident i came to overlook mountains in this planet of such enormous height as to exceed four, five, or even six times the perpendicular height of chimboraço, the highest of our mountains. . . . the same paper contains other particulars concerning _venus_ and _saturn_. all of which being things of which i have never taken any notice, it will not be amiss to show, by what follows, that neither want of attention, nor a deficiency of instruments, would occasion my not perceiving these mountains of more than twenty-three miles in height, this jagged border of _venus_, and these flat, spherical forms on _saturn_." the reply of schroeter ( ) is temperate and just. it does him honor, and he generously gives full justice to his critic. it would hardly be worth while to mention this slight incident if it were not that during these years there certainly existed a feeling that herschel undervalued the labors of his cotemporaries. this impression was fostered no doubt by his general habit of not quoting previous authorities in the fields which he was working. a careful reading of his papers will, i think, show that his definite indebtedness to his _cotemporaries_ was vanishingly small. the work of michell and wilson he alludes to again and again, and always with appreciation. certainly he seems to show a vein of annoyance that the papers of christian mayer, _de novis in coelo sidereo phænomenis_ ( ), and _beobachtungen von fixsterntrabanten_ ( ), should have been quoted to prove that the method proposed by herschel in for ascertaining the parallax of the fixed stars by means of observations of those which were double, was not entirely original with himself. there is direct proof that it was so,[ ] and if this was not forthcoming it would be unnecessary, as he has amply shown in his catalogue of double stars. one is reminded of his remarks on the use of the high magnifying powers by the impatience of his comments. his proposal to call the newly discovered minor planets _asteroids_ ( ) was received as a sign that he wished to discriminate between the discoveries of piazzi and olbers and his own discovery of uranus.[ ] he takes pains to quietly put this on one side in one of his papers, showing that he was cognizant of the existence of such a feeling. i am tempted to resurrect from a deserved obscurity a notice of herschel's _observations on the two lately discovered celestial bodies_ (_philosophical transactions_, ), printed in the first volume of the _edinburgh review_, simply to show the kind of envy to which even he, the glory of england, was subject. the reviewer sets forth the principal results of herschel's observations, and, after quoting his definition of the new term asteroid, goes on to say: "if a new name must be found, why not call them by some appellation which shall, in some degree, be descriptive of, or at least consistent with, their properties? why not, for instance, call them _concentric comets_, or _planetary comets_, or _cometary planets_? or, if a single term must be found, why may we not coin such a phrase as _planetoid_ or _cometoid_?" then follows a general arraignment of herschel's methods of expression and thought, as distinguished from his powers of mere observation. this distinction, it may be said, exists only in the reviewer's mind; there was no such distinction in fact. if ever a series of observations was directed by profound and reasonable thought, it was herschel's own. "dr. herschel's passion for coining words and idioms has often struck us as a weakness wholly unworthy of him. the invention of a name is but a poor achievement for him who has discovered whole worlds. why, for instance, do we hear him talking of the _space-penetrating power_ of his instrument--a compound epithet and metaphor which he ought to have left to the poets, who, in some future age, shall acquire glory by celebrating his name. the other papers of dr. herschel, in the late volumes of the _transactions_, do not deserve such particular attention. his catalogue of new nebulæ, though extremely valuable to the practical astronomer, leads to no general conclusions of importance, and abounds with the defects which are peculiar to the doctor's writings--a great prolixity and tediousness of narration--loose and often unphilosophical reflections, which give no very favorable idea of his scientific powers, however great his merit may be as an observer--above all, that idle fondness for inventing names without any manner of occasion, to which we have already alluded, and a use of novel and affected idioms. * * * * * "to the speculations of the doctor on the nature of the sun, we have many similar objections; but they are all eclipsed by the grand absurdity which he has there committed, in his hasty and erroneous theory concerning the influence of the solar spots on the price of grain. since the publication of gulliver's voyage to laputa, nothing so ridiculous has ever been offered to the world. we heartily wish the doctor had suppressed it; or, if determined to publish it, that he had detailed it in language less confident and flippant." one is almost ashamed to give space and currency to a forgotten attack, but it yields a kind of perspective; and it is instructive and perhaps useful to view herschel's labors from all sides, even from wrong and envious ones. the study of the original papers, together with a knowledge of the circumstances in which they were written, will abundantly show that herschel's ideas sprung from a profound meditation of the nature of things in themselves. what the origin of trains of thought prosecuted for years may have been we cannot say, nor could he himself have expressed it. a new path in science was to be found out, and he found it. it was not in his closet, surrounded by authorities, but under the open sky, that he meditated the construction of the heavens. as he says, "my situation permitted me not to consult large libraries; nor, indeed, was it very material; for as i intended to view the heavens myself, nature, that great volume, appeared to me to contain the best catalogue." his remarkable memoirs on the invisible and other rays of the solar spectrum were received with doubt, and with open denial by many of the scientific bodies of europe. the reviews and notices of his work in this direction were often quite beyond the bounds of a proper scientific criticism; but herschel maintained a dignified silence. the discoveries were true, the proofs were open to all, and no response was needed from him. he may have been sorely tempted to reply, but i am apt to believe that the rumors that reached him from abroad and at home did not then affect him as they might have done earlier. he was at his grand climacteric, he had passed his sixty-third year, his temper was less hasty than it had been in his youth, and his nerves had not yet received the severe strain from whose effects he suffered during the last years of his life. * * * * * we have some glimpses of his personal life in the reminiscences of him in the _diary and letters_ of madame d'arblay, who knew him well: " .--in the evening mr. herschel came to tea. i had once seen that very extraordinary man at mrs. de luc's, but was happy to see him again, for he has not more fame to awaken curiosity than sense and modesty to gratify it. he is perfectly unassuming, yet openly happy, and happy in the success of those studies which would render a mind less excellently formed presumptuous and arrogant. "the king has not a happier subject than this man, who owes it wholly to his majesty that he is not wretched; for such was his eagerness to quit all other pursuits to follow astronomy solely, that he was in danger of ruin, when his talents and great and uncommon genius attracted the king's patronage. he has now not only his pension, which gives him the felicity of devoting all his time to his darling study, but he is indulged in license from the king to make a telescope according to his new ideas and discoveries, that is to have no cost spared in its construction, and is wholly to be paid for by his majesty. "this seems to have made him happier even than the pension, as it enables him to put in execution all his wonderful projects, from which his expectations of future discoveries are so sanguine as to make his present existence a state of almost perfect enjoyment. mr. locke himself would be quite charmed with him. "he seems a man without a wish that has its object in the terrestrial globe. at night mr. herschel, by the king's command, came to exhibit to his majesty and the royal family the new comet lately discovered by his sister, miss herschel; and while i was playing at piquet with mrs. schwellenburg, the princess augusta came into the room and asked her if she chose to go into the garden and look at it. she declined the offer, and the princess then made it to me. i was glad to accept it for all sorts of reasons. we found him at his telescope. the comet was very small, and had nothing grand or striking in its appearance; but it is the first lady's comet, and i was very desirous to see it. mr. herschel then showed me some of his new discovered universes, with all the good humor with which he would have taken the same trouble for a brother or a sister astronomer; there is no possibility of admiring his genius more than his gentleness." "_ , december th_.--this morning my dear father carried me to dr. herschel. that great and very extraordinary man received us almost with open arms. he is very fond of my father, who is one of the council of the royal society this year, as well as himself. . . . at this time of day there was nothing to see but his instruments; those, however, are curiosities sufficient. . . . i wished very much to have seen his sister, . . . but she had been up all night, and was then in bed." "_ , september_.--dr. herschel is a delightful man; so unassuming with his great knowledge, so willing to dispense it to the ignorant, and so cheerful and easy in his general manners, that, were he no genius, it would be impossible not to remark him as a pleasing and sensible man." "_ , october d_.--we returned to windsor at noon, and mrs. de luc sent me a most pressing invitation to tea and to hear a little music. two young ladies were to perform at her house in a little concert. dr. herschel was there, and accompanied them very sweetly on the violin; his new-married wife was with him, and his sister. his wife seems good-natured; she was rich, too! and astronomers are as able as other men to discern that gold can glitter as well as stars." dr. burney to madame d'arblay. "chelsea college, _september , _. "* * * * * "i drove through slough in order to ask at dr. herschel's door when my visit would be least inconvenient to him--that night or next morning. the good soul was at dinner, but came to the door himself, to press me to alight immediately and partake of his family repast; and this he did so heartily that i could not resist. * * * * * * * "i expected (not knowing that herschel was married) only to have found miss herschel; but there was a very old lady, the mother, i believe, of mrs. herschel, who was at the head of the table herself, and a scots lady (a miss wilson, daughter of dr. wilson, of glasgow, an eminent astronomer), miss herschel, and a little boy. they rejoiced at the accident which had brought me there, and hoped i would send my carriage away and take a bed with them. they were sorry they had no stables for my horses. "we soon grew acquainted--i mean the ladies and i--and before dinner was over we seemed old friends just met after a long absence. mrs. herschel is sensible, good-humored, unpretending, and well bred; miss herschel all shyness and virgin modesty; the scots lady sensible and harmless; and the little boy entertaining, promising, and comical. herschel, you know, and everybody knows, is one of the most pleasing and well-bred natural characters of the present age, as well as the greatest astronomer. "your health was drunk after dinner (put that into your pocket), and after much social conversation and a few hearty laughs, the ladies proposed to take a walk, in order, i believe, to leave herschel and me together. we walked and talked round his great telescopes till it grew damp and dusk, then retreated into his study to philosophize. * * * * * "he made a discovery to me, which, had i known it sooner, would have overset me, and prevented my reading any part of my work.[ ] he said that he had almost always had an aversion to poetry, which he regarded as the arrangement of fine words, without any useful meaning or adherence to truth; but that when truth and science were united to these fine words, he liked poetry very well." , december . dr. burney to madame d'arblay. "herschel has been in town for short spurts, and back again two or three times, leaving mrs. herschel behind (in town) to transact law business. i had him here two whole days." the reading of the manuscript of the _poetical history of astronomy_ was continued, "and herschel was so humble as to confess that i knew more of the history of astronomy than he did, and had surprised him with the mass of information i had got together. "he thanked me for the entertainment and instruction i had given him. 'can anything be grander?' and all this before he knows a word of what i have said of himself--all his discoveries, as you may remember, being kept back for the twelfth and last book." dr. burney to madame d'arblay. "slough, _monday morning._ _july , _, in bed at dr. herschel's, half-past five, where i can neither sleep nor lie idle. "my dear fanny:--i believe i told you on friday that i was going to finish the perusal of my astronomical verses to the great astronomer on saturday. * * * * * "after tea dr. herschel proposed that we two should retire into a quiet room in order to resume the perusal of my work, in which no progress has been made since last december. the evening was finished very cheerfully; and we went to our bowers not much out of humor with each other or the world. . . . after dinner we all agreed to go to the terrace [at windsor]--mr., mrs., and miss h., with their nice little boy, and three young ladies. here i met with almost everybody i wished and expected to see previous to the king's arrival. * * * * * "but now here comes will, and i must get up, and make myself up to go down to the perusal of my last book, entitled _herschel_. so good-morrow." "chelsea, _tuesday._ "not a moment could i get to write till now. . . . i must tell you that herschel proposed to me to go with him to the king's concert at night, he having permission to go when he chooses, his five nephews (griesbachs) making a principal part of the band. 'and,' says he, 'i know you will be welcome.'" an intimacy was gradually established between herschel and dr. burney. they saw each other often at the meetings of the royal society, and herschel frequently stayed at the doctor's house. "on the first evening herschel spent at chelsea, when i called for my argand lamp, herschel, who had not seen one of those lamps, was surprised at the great effusion of light, and immediately calculated the difference between that and a single candle, and found it sixteen to one."[ ] in we find herschel as a witness for his friend james watt, in the celebrated case of watt _vs._ bull, which was tried in the court of common pleas. and from muirhead's life of watt, it appears that herschel visited watt at heathfield in . a delightful picture of the old age of herschel is given by the poet campbell,[ ] whose nature was fitted to perceive the beauties of a grand and simple character like herschel's: "[brighton], _september , _. . . . "i wish you had been with me the day before yesterday, when you would have joined me, i am sure, deeply in admiring a great, simple, good old man--dr. herschel. do not think me vain, or at least put up with my vanity, in saying that i almost flatter myself i have made him my friend. i have got an invitation, and a pressing one, to go to his house; and the lady who introduced me to him, says he spoke of me as if he would really be happy to see me. . . . i spent all sunday with him and his family. his son is a prodigy in sciences, and fond of poetry, but very unassuming. . . . now, for the old astronomer himself. his simplicity, his kindness, his anecdotes, his readiness to explain--and make perfectly conspicuous too--his own sublime conceptions of the universe are indescribably charming. he is seventy-six, but fresh and stout; and there he sat, nearest the door, at his friend's house, alternately smiling at a joke, or contentedly sitting without share or notice in the conversation. any train of conversation he follows implicitly; anything you ask he labors with a sort of boyish earnestness to explain. "i was anxious to get from him as many particulars as i could about his interview with buonaparte.[ ] the latter, it was reported, had astonished him by his astronomical knowledge. "'no,' he said, 'the first consul did surprise me by his quickness and versatility on all subjects; but in science he seemed to know little more than any well-educated gentleman, and of astronomy much less for instance than our own king. his general air,' he said, 'was something like affecting to know more than he did know.' he was high, and tried to be great with herschel, i suppose, without success; and 'i remarked,' said the astronomer, 'his hypocrisy in concluding the conversation on astronomy by observing how all these glorious views gave proofs of an almighty wisdom.' i asked him if he thought the system of laplace to be quite certain, with regard to the total security of the planetary system from the effects of gravitation losing its present balance? he said, no; he thought by no means that the universe was secured from the chance of sudden losses of parts. "he was convinced that there had existed a planet between _mars_ and _jupiter_, in our own system, of which the little asteroids, or planetkins, lately discovered, are indubitably fragments; and 'remember,' said he, 'that though they have discovered only four of those parts, there will be thousands--perhaps thirty thousand more--yet discovered.' this planet he believed to have been lost by explosion. "with great kindness and patience he referred me, in the course of my attempts to talk with him, to a theorem in newton's 'principles of natural philosophy' in which the time that the light takes to travel from the sun is proved with a simplicity which requires but a few steps in reasoning. in talking of some inconceivably distant bodies, he introduced the mention of this plain theorem, to remind me that the progress of light could be measured in the one case as well as the other. then, speaking of himself, he said, with a modesty of manner which quite overcame me, when taken together with the greatness of the assertion: 'i have looked _further into space than ever human being did before me_. i have observed stars, of which the light, it can be proved, must take two millions of years to reach this earth.' "i really and unfeignedly felt at this moment as if i had been conversing with a supernatural intelligence. 'nay, more,' said he, 'if those distant bodies had ceased to exist two millions of years ago, we should still see them, as the light would travel after the body was gone. . . .' these were herschel's words; and if you had heard him speak them, you would not think he was apt to tell more than the truth. "after leaving herschel i felt elevated and overcome; and have in writing to you made only this memorandum of some of the most interesting moments of my life." campbell's conscientious biographer appears to have felt that the value of this charming account of his interview with herschel was in its report of astronomical facts and opinions, and he adds a foot-note to explain that "herschel's opinion never amounted to more than _hypothesis_ having some degree of probability. sir john herschel remembers his father saying, 'if that hypothesis were true, and _if_ the planet destroyed were as large as the earth, there must have been at least thirty-thousand such fragments,' but always as an hypothesis--he was never heard to declare any degree of conviction that it was so." for us, the value of this sympathetic account of a day in herschel's life is in its conception of the simplicity, the modesty, the "boyish earnestness," the elevation of thought and speech of the old philosopher; and in the impression made on the feelings, not the mind, of the poet, then thirty-five years old. in a letter to alison, campbell reverts with great pleasure to the day spent with herschel: "sydenham, _december , _. "my dearest alison:-- * * * * * "i spent three weeks with my family at brighton, in charming weather, and was much pleased with, as well as benefited by, the place. there i met a man with whom you will stare at the idea of my being congenial, or having the vanity to think myself so--the great herschel. he is a simple, great being. . . . i once in my life looked at newton's _principia_, and attended an astronomical class at glasgow; wonderful it seemed to myself, that the great man condescended to understand my questions; to become apparently earnest in communicating to me as much information as my limited capacity and preparation for such knowledge would admit. he invited me to see him at his own abode, and so kindly that i could not believe that it was mere good breeding; but a sincere wish to see me again. i had a full day with him; he described to me his whole interview with buonaparte; said it was not true, as reported, that buonaparte understood astronomical subjects deeply, but affected more than he knew. "in speaking of his great and chief telescope, he said with an air, not of the least pride, but with a greatness and simplicity of expression that struck me with wonder, 'i have looked further into space than ever human being did before me. i have observed stars, of which the light takes _two millions_ of years to travel to this globe.' i mean to pay him a reverential visit at slough, as soon as my book is out, this winter." * * * * * in carolina herschel has this entry in her diary: "_october_ .--my brother came from brighton. the same night two parties from the castle came to see the comet, and during the whole month my brother had not an evening to himself. as he was then in the midst of polishing the forty-foot mirror, rest became absolutely necessary after a day spent in that most laborious work; and it has ever been my opinion that on the th of october his nerves received a shock of which he never got the better afterwards." in the spring of he was quite seriously ill; but in may the observing went on again. in and his principal investigations were upon physical subjects (newton's rings), and in the only long series of observations was upon the comet of that year. after the state of herschel's health required that his observations should be much less frequent. much of the time after he was absent, and his work at home consisted largely in arranging the results of his previous labors, and in computations connected with them. all through the years to , herschel's health was very feeble. the severe winter of - had told materially upon him. in , however, he undertook to repolish the forty-foot mirror, but was obliged to give it over. he now found it necessary to make frequent little excursions for change of air and scene. his faithful sister remained at home, bringing order into the masses of manuscript, and copying the papers for the royal society. she was sick at heart, fearing that each time she saw her brother it would be the last. in she says: "feb. , i went to my brother and remained with him till the d. we spent our time, though not in idleness, in sorrow and sadness. he is not only unwell, but low in spirits." in (december ), herschel went to london to have his portrait painted by artaud. while he was in london his will was made.[ ] in there is a glimmer of the old-time light. in a note herschel says: "lina:--there is a great comet. i want you to assist me. come to dine and spend the day here. if you can come soon after one o'clock, we shall have time to prepare maps and telescopes. i saw its situation last night. it has a long tail. "_july , ._" this note has been carefully kept by his sister, and on it she has written: "i keep this as a relic. every line _now_ traced by the hand of my dear brother becomes a treasure to me." so the next three years passed away. sir william[ ] was daily more and more feeble. he spent his time in putting his works in order, but could devote only a few moments each day to this. his sister says: "_aug. th_, _ th_, _ th_, and _ th_ [ ], i went as usual to spend some hours of the forenoon with my brother. "_aug. th._--i hastened to the spot where i was wont to find him, with the newspaper which i was to read to him. but instead i found mrs. monson, miss baldwin, and mr. bulman, from leeds, the grandson of my brother's earliest acquaintance in this country. i was informed my brother had been obliged to return to his room, whither i flew immediately. lady h. and the housekeeper were with him, administering everything which could be thought of for supporting him. i found him much irritated at not being able to grant mr. bulman's request for some token of remembrance for his father. as soon as he saw me, i was sent to the library to fetch one of his last papers and a plate of the forty-foot telescope. but for the universe i could not have looked twice at what i had snatched from the shelf, and when he faintly asked if the breaking up of the milky way was in it, i said 'yes,' and he looked content. i cannot help remembering this circumstance; it was the last time i was sent to the library on such an occasion. that the anxious care for his papers and workrooms never ended but with his life, was proved by his frequent whispered inquiries if they were locked and the key safe, of which i took care to assure him that they were, and the key in lady herschel's hands. "after half an hour's vain attempt to support himself, my brother was obliged to consent to be put to bed, leaving no hope ever to see him rise again." on the th of august, , herschel died peacefully at the age of eighty-four years. his remains lie in the little church at upton, near windsor, where a memorial tablet has been erected by his son. the epitaph is as follows:[ ] h. s. e. gulielmus herschel eques guelphicus hanoviæ natus angliam elegit patriam astronomis ætatis suæ præstantissimis merito annumeratus ut leviora sileantur inventa planetam ille extra saturni orbitam primus detexit novis artis adjumentis innixus quæ ipse excogitavit et perfecit coelorum perrupit claustra et remotiora penetrans et explorans spatia incognitos astrorum ignes astronomorum oculis et intellectui subjecit qua sedulitate qua solertia corporum et phantasmatum extra systematis nostri fines lucentium naturam indagaverit quidquid paulo audacius conjecit ingenita temperans verecundia ultro testantur hodie æquales vera esse quæ docuit pleraque siquidem certiora futuris ingeniis subsidia debitura est astronomia agnoscent forte posteri vitam utilem innocuam amabilem non minus felici laborum exitu quam virtutibus ornatam et vere eximiam morte suis et bonis omnibus deflenda nec tamen immatura clausit die xxv augusti a. d. ci[c]i[c]cccxxii Ætatis vero suæ lxxxiv. footnotes: [ ] bode's _jahrbuch_, , p. . [ ] zach's _monatlich correspondenz_, , p. . [ ] bode's _jahrbuch_, , p. . [ ] through sir john herschel there is preserved to us an incident of his early boyhood, which shows the nature of the training his young mind received in the household at slough. walking with his father, he asked him "what was the oldest of all things?" the father replied, after the socratic manner, "and what do you suppose is the oldest of all things?" the boy was not successful in his answers, whereon the old astronomer took up a small stone from the garden walk: "there, my child, there is the oldest of all the things that i certainly know." on another occasion the father asked his son, "what sort of things do you think are most alike?" the boy replied, "the leaves of the same tree are most like each other." "gather, then, a handful of leaves from that tree," rejoined the philosopher, "and choose two which are alike."--_monthly notices royal astronomical society_, vol. xxxii., page . [ ] _memoir of caroline herschel_, p. . [ ] "of late years these expectations have been more than accomplished by the discovery of no fewer than four planetary bodies, almost all in the same place; but so small that dr. herschel refuses to honor them with the name of planets, and chooses to call them asteroids, though for what reason it is not easy to determine, unless it be to deprive the discoverers of these bodies of any pretence for rating themselves as high in the list of astronomical discoverers as himself."--_history of the royal society_, by thomas thomson, p. . this work was published in , and therefore during the lifetime of herschel. [ ] _poetical history of astronomy_: this work was nearly completed, but was never published. the whole of it was read to herschel, in order that burney might have the benefit of his criticism on its technical terms. [ ] _memoirs of dr. burney_, vol. iii., p. . [ ] life and letters of thomas campbell, edited by william beattie, vol. ii., p. . [ ] this interview must have taken place in , during herschel's journey to paris. we have no other record of it. [ ] the will of herschel was dated december th, . "the personal effects were sworn under £ , . the copyhold and other lands and tenements at upton-cum-chalvey, in the county of bucks, and at slough, he decrees to his son, with £ , in the per cent. reduced annuities. £ , are given to his brother johann dietrich, and annuities of £ each to his brother johann alexander and to his sister carolina; £ each to his nephews and nieces, and the residue (with the exception of astronomical instruments, telescopes, observations, etc., which he declares to have given, on account of his advanced age, to his son for the purpose of continuing his studies) is left solely to lady herschel."--_gentleman's magazine_, vol. xcii., , p. . it is not necessary to say here how nobly sir john herschel redeemed the trust confided in him. all the world knows of his survey of the southern heavens, in which he completed the review of the sky which had been begun and completed for the northern heavens by the same instruments in his father's hands. a glance at the bibliography at the end of this book will show the titles of several papers by sir john, written with the sole object of rendering his father's labors more complete. [ ] he was created a knight of the royal hanoverian guelphic order in , and was the first president of the royal astronomical society in , his son being its first foreign secretary. [ ] bode's _jahrbuch_, , p. . chapter iv. review of the scientific labors of william herschel. in this chapter i shall endeavor to give such explanations as will enable the general reader to follow the course of discovery in each branch of astronomy and physics, regularly through the period of herschel's life, and up to the state in which he left it. a more detailed and precise account, which should appeal directly to the professional astronomer, will not be needed, since arago has already fulfilled this want in his "_analyse de la vie et des travaux de sir william herschel_," published in . the few misconceptions there contained will be easily corrected by those to whom alone they are of consequence. the latter class of readers may also consult the abstracts of herschel's memoirs, which have been given in "_a subject-index and a synopsis of the scientific writings of sir william herschel_," prepared by dr. hastings and myself, and published by the smithsonian institution. an accurate sketch of the state of astronomy in england and on the continent, in the years - , need not be given. it will be enough if we remember that of the chief observatories of europe, public and private, no one was actively devoted to such labors as were undertaken by herschel at the very beginning of his career. his observations on variable stars, indeed, were in the same line as those of pigott; flaugergues and darquier, in france, had perhaps preceded him in minute scrutiny of the sun's surface, etc.; but, even in that department of observation, he at once put an immense distance between himself and others by the rapid and extraordinary advances in the size and in the excellence of his telescopes. before his time the principal aids to observation were the gregorian and newtonian telescopes of short, and the small achromatics of dollond.[ ] we have seen, in what goes before, how his patient zeal had succeeded in improving upon these. there was no delay, and no rest. steadily the art of making reflectors was urged forward, until he had finally in his hands the forty-foot telescope. it must be admitted that this was the limit to which the manufacture of powerful telescopes could be pushed in his generation. the optical and mechanical difficulties which prevented a farther advance required time for their solution; and, indeed, some of these difficulties are scarcely solved at this day. it may fairly be said that no reflector larger than three feet in aperture has yet realized our expectations. _the improvement of telescopes and optical apparatus._ it will be of interest to give in this place some connected account of the large forty-foot reflector, of four feet aperture, made by herschel. its history extends from to . its manufacture was considered by his cotemporaries as his greatest triumph. as a machine, it was extremely ingenious in all its parts, as may be seen from the elaborate description and plates of it published in the _philosophical transactions_ for . one of its mirrors certainly had good definition, for, by means of it, the two small satellites of _saturn_ (_mimas_ and _enceladus_) were discovered, and these discoveries alone would make it famous. perhaps more was expected of it by the public in general than it absolutely performed. its merits were after a while decried, and herschel even felt obliged to state why he did not always employ it in his observations. his reasons were perfectly valid, and such as any one may understand. the time required to get so large a machine into working order was a serious tax; it required more assistants than his twenty-foot telescope, and he says, "i have made it a rule never to employ a larger telescope when a smaller will answer the purpose." it still remains as a remarkable feat of engineering and an example of great optical and mechanical skill. it led the way to the large reflectors of lord rosse, some sixty years later, and several of the forty-foot telescopes of the present day even have done less useful work. its great feat, however, was to have added two satellites to the solar system. from the published accounts of it the following is taken: "when i resided at bath i had long been acquainted with the theory of optics and mechanics, and wanted only that experience so necessary in the practical part of these sciences. this i acquired by degrees at that place, where in my leisure hours, by way of amusement, i made several two-foot, five-foot, seven-foot, ten-foot, and twenty-foot newtonian telescopes, beside others, of the gregorian form, of eight, twelve, and eighteen inches, and two, three, five, and ten feet focal length. in this way i made not less than two hundred seven-foot, one hundred and fifty ten-foot, and about eighty twenty-foot mirrors, not to mention the gregorian telescopes.[ ] "the number of stands i invented for these telescopes it would not be easy to assign. . . . in i began to construct a thirty-foot aërial reflector, and having made a stand for it, i cast the mirror thirty-six inches in diameter. this was cracked in cooling. i cast it a second time, and the furnace i had built in my house broke." soon after, the georgian planet was discovered, and this interrupted the work for a time. "in the year i finished a very good twenty-foot reflector with a large aperture, and mounted it upon the plan of my present telescope. after two years' observation with it, the great advantage of such apertures appeared so clearly to me that i recurred to my former intention of increasing them still further; and being now sufficiently provided with experience in the work which i wished to undertake, the president of the royal society, who is always ready to promote useful undertakings, had the goodness to lay my design before the king. his majesty was graciously pleased to approve of it, and with his usual liberality to support it with his royal bounty. "in consequence of this arrangement i began to construct the forty-foot telescope about the latter end of .[ ] the woodwork of the stand and machines for giving the required motions to the instrument were immediately put in hand. in the whole of the apparatus none but common workmen were employed, for i made drawings of every part of it, by which it was easy to execute the work, as i constantly inspected and directed every person's labor; though sometimes there were not less than forty different workmen employed at the same time. while the stand of the telescope was preparing, i also began the construction of the great mirror, of which i inspected the casting, grinding, and polishing, and the work was in this manner carried on with no other interruption than that occasioned by the removal of all the apparatus and materials from where i then lived, to my present situation at slough. "here, soon after my arrival, i began to lay the foundation upon which by degrees the whole structure was raised as it now stands, and the speculum being highly polished and put into the tube, i had the first view through it on february , . i do not, however, date the completing of the instrument till much later. for the first speculum, by a mismanagement of the person who cast it, came out thinner on the centre of the back than was intended, and on account of its weakness would not permit a good figure to be given to it. "a second mirror was cast january , , but it cracked in cooling. february we recast it, and it proved to be of a proper degree of strength. october it was brought to a pretty good figure and polish, and i observed the planet _saturn_ with it. but not being satisfied, i continued to work upon it till august , , when it was tried upon the fixed stars, and i found it to give a pretty sharp image. large stars were a little affected with scattered light, owing to many remaining scratches on the mirror. august the th, , having brought the telescope to the parallel of _saturn_, i discovered a _sixth_ satellite of that planet, and also saw the spots upon _saturn_ better than i had ever seen them before, so that i may date the finishing of the forty-foot telescope from that time." another satellite of _saturn_ was discovered with the forty-foot on the th of september ( ). it was used for various observations so late as . on january , of that year, herschel observed the nebula of _orion_ with it. this was one of his last observations. the final disposition of the telescope is told in the following extract from a letter of sir john herschel's to mr. weld, secretary of the royal society: "collingwood, _march , _. . . . "in reply to your queries, respecting the forty-foot reflecting telescope constructed by my father, i have to state that king george iii. munificently defrayed the _entire_ cost of that instrument (including, of course, all preparatory cost in the nature of construction of tools, and of the apparatus for casting, grinding, and figuring the reflectors, of which two were constructed), at a total cost of £ , . the woodwork of the telescope being so far decayed as to be dangerous, in the year i pulled it down, and piers were erected on which the tube was placed, _that_ being of iron and so well preserved, that, although not more than one-twentieth of an inch thick, when in the horizontal position it sustained within it all my family, and continues to sustain inclosed within it, to this day, not only the heavier of the two reflectors, but also all the more important portions of the machinery. . . . the mirror and the rest of the polishing apparatus are on the premises. the iron grinding tools and polishers are placed underneath the tube, let into the ground, and level with the surface of the gravelled area in which it stands.". . . the closing of the tube was done with appropriate ceremony on new-year's-day, , when, after a procession through it by the family at slough, a poem, written by sir john, was read, the machinery put into its present position, and the tube sealed. the memoir on the forty-foot telescope shows throughout that herschel's prime object was not the making of the telescope itself, but that his mind was constantly directed towards the uses to which it was to be put--towards the questions which he wished it to answer. again and again, in his various papers, he returns to the question of the _limit of vision_. as bessel has said: "the naked eye has its limit of vision in the stars of the sixth magnitude. the light of fainter stars than these does not affect the retina enough for them to be seen. a very small telescope penetrates to smaller, and, in general, without doubt, to more distant stars. a more powerful one penetrates deeper into space, and as its power is increased, so the boundaries of the visible universe are widened, and the number of stars increased to millions and millions. whoever has followed the history of the series of herschel's telescopes will have observed this. but herschel was not content with the bare fact, but strove ever to know _how far_ a telescope of a certain construction and size could penetrate, compared with the naked and unassisted eye. these investigations were never for the discovery of new facts concerning the working of his instruments; it was for the knowledge of the distribution of the fixed stars in space itself that he strove. . . . herschel's instruments were designed to aid vision to the last extent. they were only secondarily for the taking of measures. his efforts were not for a knowledge of the _motions_, but of the _constitution_ and _construction_ of the heavenly bodies." besides the stands for his telescopes, which were both ingenious and convenient, herschel devised many forms of apparatus for facilitating the art of observation. his micrometers for measuring position angles, his lamp micrometer, the method of limiting apertures, and the methods he used for viewing the sun may be mentioned among these. points in practical astronomy are considered all through the years of observation. a reference to his original papers will show how numerous, how varied, and how valuable these are. i cannot forbear quoting here the account of a precaution observed during his examination of the belts on _saturn_ ( ). it is the most striking example of how fully herschel realized that the eye of the observer is a material part of the optical apparatus of astronomy. simple as this principle may appear, it was an absolute novelty in his day. in making these observations, he says: "i took care to bend my head so as to receive the picture of the belt in the same direction as i did formerly. this was a precaution that occurred to me, as there was a possibility that the vertical diameter of the retina might be more or less sensitive than the horizontal one." astronomers will recognize in this the first suggestion of the processes which have led to important results in the hands of dr. otto struve and others in the comparison of the measures of double stars by different observers, each of whom has a personal habit of observation, which, if not corrected, may affect his results in the way which herschel was striving to avoid. _researches on the relative brightness of the stars: variable stars._ no research of herschel's was more laborious than the elaborate classification of the stars according to their comparative brightness, which he executed during the years to . it was directly in the line of his main work--to find out the construction of the heavens. his first paper had been upon the variable star _mira ceti_. here was a sun, shining by its native brightness, which waxed and waned like the moon itself. this star is periodic. it is for a long period invisible to the unassisted eye. then it can just be seen, and increases in brightness for a little over a month, and attains a maximum brilliancy. from this it decreases for nearly three months, and after becoming invisible, remains so for five or six months. its whole period is about days. are all other stars constant in brightness? the example of _mira ceti_ and of other known variables makes this at least doubtful. but the sun itself may vary for all that we know. it is a simple star like the rest. this question of variability in general is an important one, then. it can only be tested by making accurate catalogues of the relative brilliance of stars at various times, and by comparing these. no such general catalogue existed before herschel's time, and led by the discrepancies in isolated cases, which he found between his own estimates and those of his predecessors, he made from observation a series of four catalogues, in which were set down the order of sequence of the stars of each constellation. the method adopted by herschel was perfectly simple in principle, though most laborious in practice. suppose any number of stars, a, b, c, d, e, . . . etc., near enough to each other to be well compared. the process consists simply in writing down the names of the stars, a, b, c, etc., in the order of their relative brightness. thus if for a group of eight stars we have found at one epoch a, b, c, d, e, f, g, h, and if at another time the order was a, b, c, d, f, e, g, h, symptoms of variability are pointed out. repeated observations, where the same star is found in different sequences, will decide the question. thus, for the stars visible to the naked eye, we know exactly the state of the sky in herschel's day, now nearly a century ago. any material change cannot escape us. these catalogues have been singularly overlooked by the observers of our generation who have followed this branch of observation, and it was not till that they received proper attention and a suitable reduction (at the hands of mr. c. s. pierce). we owe to herschel the first trustworthy account of the stars visible to the naked eye, and since the date of his labors (about ) we have similar views published by argelander ( ), heis ( ), argelander and schÖnfeld ( ), gould ( and ), and houzeau ( ). thus his labors have been well followed up. in the prosecution of this work herschel found stars whose light was progressively diminishing, others which regularly increased, one star whose light periodically varies (_[alpha] herculis_), and at least one star ( _herculis_) which has utterly disappeared. on october , , and april , , he observed this latter star, but in may, , it had totally vanished. there was no trace remaining. the discovery of the variability of _[alpha] herculis_ was a more important one than would at first sight appear. up to that time the only variable stars known were seven in number. their periods were four hundred and ninety-four, four hundred and four, three hundred and thirty-four, seven, six, five, and three days. these periods seemed to fall into two groups, one of from three hundred to five hundred days, the other comparatively much shorter, of three to seven days. _[alpha] herculis_ came to occupy the middle place between these groups, its period being about sixty days. the cause of these strange and regular variations of brightness was supposed by herschel to be the rotation of the star bodily on an axis, by which revolution different parts of its surface, of different brilliancy, were successively and periodically presented to us. this explanation it might have been difficult to receive, when the periods of the known variables were so markedly various in length. his own discovery came to bridge over the interval, and quite confirmed him in his belief. he returned to the subject of the revolution of stars about their axes again and again, and connected it with the revolution of satellites. he found that the satellites of _jupiter_ and one of _saturn's_ periodically changed in brightness, and by quite simple means showed that their periods of rotation were at least approximately the same as their periods of revolution about their primaries. in this case, as in every other, he considered a discovery in each and every one of its possible bearings. there are no instances where he has singularly overlooked the consequences of his observations. _researches on double stars._ the double stars were the subject of herschel's earliest and of his latest papers. in he published his "_catalogue of double stars_," and his last published memoir ( ) was on the same subject. the question of determining the parallax of stars first brought herschel to the discovery of double stars. if two stars, a and b, appear very close together, and if, in reality, the star b is very many times more distant from the earth than a, although seen along the same line of sight, then the revolution of the earth in its orbit will produce changes in the relative situation of a and b, and, in fact, b will describe a small orbit about a, due to this revolution. this idea had been proposed by galileo, and measures on this plan had been made by long, with negative results. but herschel, in reviewing their work, declares that the stars chosen by long were not suitable to the purpose. it is necessary, among other things, to the success of this method, that it should be certain that the star b is really very much more distant than the star a. the only general test of the distance of stars is their brilliancy, and herschel decided to use only stars for this research which had two components very greatly different in brightness. a must be very bright (and presumably near to us), and b must be very close to a, and very faint (and thus, presumably, very distant). it was in the search for such pairs of stars that the _catalogue of double stars_ ( ) was formed. herschel's first idea of a double star made such pairs as he found, to consist of two stars _accidentally_ near to each other. a was near to us, and appeared projected in a certain place on the celestial sphere. b was many times more distant, but, by chance, was seen along the same line, and made with a an _optical_ double. if the two stars were at the same distance from the earth, if they made part of the same physical system, if one revolved around the other, then this method of gaining a knowledge of their distance failed. even in his first memoir on the subject, a surmise that this latter state might occur in some cases, was expressed by herschel. the notes on some of the pairs declare that a motion of one of them was suspected. but this motion might be truly orbital--of one star about the other as a centre--or it might simply be that one star was moving by its own _proper_ motion, and leaving the other behind. it was best to wait and see. the first catalogue of double stars contained two hundred and three instances of such associations. these were observed from time to time, and new pairs discovered. the paper of michell, "an inquiry into the probable parallax and magnitude of the fixed stars, from the quantity of light which they afford, and the particular circumstances of their situation" ( ), was read and pondered. by herschel had become certain that there existed in the heavens real pairs of stars, both at the same distance from the earth, which were physically connected with each other. the arguments of michell have been applied by bessel to the case of one of herschel's double stars, in much the same order in which the argument ran in herschel's own mind, as follows: the star _castor_ (_[alpha] geminorum_) is a double star, where a is of the second, and b of the fourth, magnitude. to the naked eye these two appear as one star. with a telescope this is seen to be two stars, some " apart. in the whole sky there are not above fifty such stars as the brighter of the two, and about four hundred of the brilliancy of b. these fifty and four hundred stars are scattered over the vault of heaven, almost at random. no law has yet been traced by which we can say that here or here there shall be a bright star like a, or a fainter one like b. in general the distribution appears to be fortuitous. how then can we account for one of the four hundred stars like b placed so close to one of the fifty like a? the chances are over four hundred thousand to one that the association in position is not accidental. this argument becomes overwhelming when the same association is found in many other cases. there were two hundred and three doubles in the catalogue of alone, and many thousands are now known. by a process like this, herschel reached his grand discovery of true binary systems, where one sun revolves about another. for he saw that if the two stars are near together in space, they could not stand still in face of each other, but that they must revolve in true orbits. here was the discovery which came to take the place of the detection of the parallaxes of the fixed stars. he had failed in one research, but he was led to grand conclusions. was the force that these distant pairs of suns obeyed, the force of gravitation? this he could not settle, but his successors have done so. it was not till about that savary, of the paris observatory, showed that one of herschel's doubles was subjected to the law of gravitation, and thus extended the power of this law from our system to the universe at large. herschel himself lived to see some of his double stars perform half a revolution. of herschel's discoveries, arago thinks this has "le plus d'avenir." it may well be so. the laws which govern our solar system have been extended, through his researches, to regions of unknown distance. the binary stars will afford the largest field for research into the laws which govern them, and together with the clusters and groups, they will give a firm basis by which to study the distribution of stars in general, since here we have the great advantage of knowing, if not the real distance of the two stars from the earth, at least that this distance is alike for both. _researches on planets and satellites._ after herschel's first publication on the mountains of the moon ( ), our satellite appears to have occupied him but little. the observation of volcanoes ( ) and of a lunar eclipse are his only published ones. the planets _mercury_, _venus_, _mars_, and _jupiter_, although they were often studied, were not the subjects of his more important memoirs. the planet _saturn_, on the contrary, seems never to have been lost sight of from the time of his first view of it in . the field of discovery always appears to be completely occupied until the advent of a great man, who, even by his way of putting old and familiar facts, shows the paths along which discoveries must come, if at all. this faculty comes from profound reflection on the nature of the subject itself, from a sort of transmuting power which changes the words of the books into the things of reality. herschel's paper on _saturn_, in , is an admirable example of this. herschel's observations on _saturn_ began in . from to he published six memoirs on the figure, the ring, and the satellites of this planet. the spheroidal shape of the ball was first discovered by him, and we owe much of our certain knowledge of the constitution of the rings to his work. the sixth and seventh satellites, _mimas_ and _enceladus_, were discovered by him in . the periods of rotation of the ball and of the ring were also fixed. in his conclusions as to the real figure of the rings, there is a degree of scientific caution which is truly remarkable, and which to-day seems almost excessive. in his paper of , herschel shows that the most distant satellite of _saturn_--_japetus_--turns once on its axis in each revolution about its primary, just as our moon does. he says of this: "i cannot help reflecting with some pleasure on the discovery of an analogy which shows that a certain uniform plan is carried on among the secondary planets of our solar system; and we may conjecture that probably most of the satellites are governed by the same law; especially if it be founded on such a construction of their figure as makes them more ponderous towards their primary planets." i believe the last suggestion to have been the first statement of the possible arrangement of matter in satellites, which was afterwards so forcibly maintained by hansen in his theory of the moon. hansen's researches show the consequences of such an arrangement, although they do not prove its existence. it should be recorded that the explanation which is to-day received of the belts and bands upon _jupiter_, is, i believe, first found in herschel's memoir on _venus_ ( ). his memoir of , on the changeable brightness of the satellites of _jupiter_, has already been referred to. the times of the rotation of the satellites on their axes was first determined by herschel from these observations, which also contain accounts of the curious, and as yet unexplained, phenomena attending their appearances on the disc of the planet. herschel discovered in january, , the two brighter satellites of _uranus_, now called _oberon_ and _titania_. they are among the faintest objects in the solar system. a later discussion of all his observations led him to the belief that there were four more, and he gives his observations and computations in full. he says that of the existence of additional satellites he has no doubt. of these four, three were exterior to the most distant satellite _oberon_, the other was "interior" to _titania_. it was not until that even _oberon_ and _titania_ were again observed (by sir john herschel) with a telescope of twenty feet, similar to that which had discovered them, and not until was the true state of this system known, when mr. lassell discovered _ariel_ and _umbriel_, two satellites interior to _titania_, neither of which was herschel's "interior" satellite. in and later years mr. lassell, by the aid of telescopes constructed by himself, fully settled the fact that only four satellites of this planet existed. in i examined the observations of herschel on his supposed "interior" satellite, thinking that it might be possible that among the very few glimpses of it which he recorded, some might have belonged to _ariel_ and some to _umbriel_, and that by combining rare and almost accidental observations of two satellites which really existed, he had come to announce the existence of an "interior" satellite which had no existence in fact. such i believe to be the case. in , april , herschel describes an interior satellite in the position angle °, distant " from the planet. at that instant _umbriel_, one of mr. lassell's satellites, was in the position °, and distant " from _uranus_, in the most favorable position for seeing it. the observation of , march , _may_ belong to _ariel_. at the best the investigation is of passing interest only, and has nothing to do with the question of the discovery of the satellites. herschel discovered the two brighter ones, and it was only sixty years later that they were properly re-observed by mr. lassell, who has the great honor of having added as many more, and who first settled the vexed question of satellites _exterior_ to _oberon_, and this with a reflecting telescope made by himself, which is unequalled by any other of its dimensions. _researches on the nature of the sun._ in the introduction to his paper on the _nature and construction of the sun and fixed stars_ ( ), herschel recounts what was known of the nature of the sun at that time. newton had shown that it was the centre of the system; galileo and his successors had determined its rotation, the place of its equator, its real diameter, magnitude, density, distance, and the force of gravity on its surface. he says: "i should not wonder if, considering all this, we were induced to think that nothing remained to be added; and yet we are still very ignorant in regard to the internal construction of the sun." "the spots have been supposed to be solid bodies, the smoke of volcanoes, the scum floating on an ocean of fluid matter, clouds, opaque masses, and to be many other things." "the sun itself has been called a globe of fire, though, perhaps, metaphorically." "it is time now to profit by the observations we are in possession of. i have availed myself of the labors of preceding astronomers, but have been induced thereto by my own actual observation of the solar phenomena." herschel then refers to the theories advanced by his friend, prof. wilson, of glasgow, in . wilson maintained that the spots were depressions below the sun's atmosphere, vast hollows as it were, at the bases of which the true surface of the sun could be seen. the essence of his theory was the existence of two different kinds of matter in the sun: one solid and non-luminous--the nucleus--the other gaseous and incandescent--the atmosphere. vacant places in the atmosphere, however caused, would show the black surface of the solid mass below. these were the spots. no explanation could be given of the _faculæ_, bright streaks, which appear on the sun's surface from time to time; but his theory accounted for the existence of the black _nuclei_ of the spots, and for the existence of the _penumbræ_ about these. the penumbra of a spot was formed by the thinner parts of the atmosphere about the vacancy which surrounded the nucleus. this theory of wilson's was adopted by herschel as a basis for his own, and he brought numerous observations to confirm it, in the modified shape which he gave to it. according to herschel, the sun consisted of three essentially different parts. first, there was a solid nucleus, non-luminous, cool, and even capable of being inhabited. second, above this was an atmosphere proper; and, lastly, outside of this was a layer in which floated the clouds, or bodies which gave to the solar surface its intense brilliancy: "according to my theory, a dark spot in the sun is a place in its atmosphere which happens to be free from luminous decompositions" above it. the two atmospheric layers, which will be of varying thickness about a spot, will account for all the shades of darkness seen in the penumbra. ascending currents from the solar surface will elevate certain regions, and may increase the solar activity near by, and will thus give rise to faculæ, which herschel shows to be elevated above the general surface. it will not be necessary to give a further account of this theory. the data in the possession of the modern theorist is a thousand-fold that to be derived from herschel's observations, and, while the subject of the internal construction of the sun is to-day unsettled, we know that many important, even fundamental, portions of his theory are untenable. a remark of his should be recorded, however, as it has played a great part in such theories: "that the emission of light must waste the sun, is not a difficulty that can be opposed to our hypothesis. many of the operations of nature are carried on in her great laboratory which we cannot comprehend. perhaps the many telescopic comets may restore to the sun what is lost by the emission of light." arguments in favor of the habitability of both sun and moon are contained in this paper; but they rest more on a metaphysical than a scientific basis, and are to-day justly forgotten. _researches on the motion of the sun and of the solar system in space._ in herschel writes, in regard to some of his discoveries of double stars: "these may serve another very important end. i will just mention it, though it is foreign to my present purpose. several stars of the first magnitude have been observed or suspected to have a proper motion; hence we may surmise that our sun, with all its planets and comets, may also have a motion towards some particular point of the heavens. . . . if this surmise should have any foundation, it will show itself in a series of some years in a kind of systematical parallax, or change, due to the motion of the whole solar system." in he published his paper _on the proper motion of the solar system_, which contained the proofs of his surmises of a year before. that certain of the stars had in fact a _proper_ motion had been well established by the astronomers of the eighteenth century. after all allowances had been made for the effects of precession and other displacements of a star's position which were produced by motions of the earth, it was found that there were still small outstanding differences which must be due to the motion of the star itself--its proper motion. the quantity of this motion was not well known for any star when herschel's researches began. before they were concluded, however, maskelyne had deduced the proper motions of thirty-six stars--the fundamental stars, so called--which included in their number _sirius_, _procyon_, _arcturus_, and generally the brightest stars. it is _à priori_ evident that stars, in general, must have proper motions, when once we admit the universality of gravitation. that any fixed star should be entirely at rest would require that the attractions on all sides of it should be exactly balanced. any change in the position of this star would break up this balance, and thus, in general, it follows that stars must be in motion, since all of them cannot occupy such a critical position as has to be assumed. if but one fixed star is in motion, this affects all the rest, and we cannot doubt but that every star, our sun included, is in motion by an amount which varies from small to great. if the sun alone had a motion, and the other stars were at rest, the consequence of this would be that all the fixed stars would appear to be retreating _en masse_ from that point in the sky towards which we were moving. those nearest us would move more rapidly, those more distant less so. and in the same way, the stars from which the solar system was receding would seem to be approaching each other. if the stars, instead of being quite at rest, as just supposed, had motions proper to themselves, then we should have a double complexity. they would still appear to an observer in the solar system to have motions, and part of these motions would be truly proper to the stars, and part would be due to the advance of the sun itself in space. observations can show us only the _resultant_ of these two motions. it is for reasoning to separate this resultant into its two components. at first the question is to determine whether the results of observation indicate any solar motion at all. if there is none, the proper motions of stars will be directed along all possible lines. if the sun does truly move, then there will be a general agreement in the resultant motions of the stars near the ends of the line along which it moves, while those at the sides, so to speak, will show comparatively less systematic effect. it is as if one were riding in the rear of a railway train and watching the rails over which it has just passed. as we recede from any point, the rails at that point seem to come nearer and nearer together. if we were passing through a forest, we should see the trunks of the trees from which we were going apparently come nearer and nearer together, while those on the sides of us would remain at their constant distance, and those in front would grow further and further apart. these phenomena, which occur in a case where we are sensible of our own motion, serve to show how we may deduce a motion, otherwise unknown, from the appearances which are presented by the stars in space. in this way, acting upon suggestions which had been thrown out previously to his own time by lambert, mayer, and bradley, herschel demonstrated that the sun, together with all its system, was moving through space in an unknown and majestic orbit of its own. the centre round which this motion is directed cannot yet be assigned. we can only know the point in the heavens towards which our course is directed--"the apex of solar motion." by a study of the proper motions assigned by maskelyne to the brighter stars, herschel was able to define the position of the solar apex with an astonishing degree of accuracy. his calculations have been several times repeated with the advantage of modern analytical methods, and of the hundred-fold material now at our disposition, but nothing essential has been added to his results of , which were based upon such scanty data; and his paper of contains the announcement of the discovery itself. his second paper on the _direction_ and _velocity_ of the solar system ( ) is the best example that can possibly be given of his marvellous skill in reaching the heart of a matter, and it may be the one in which his philosophical powers appear in their highest exercise. for sustained reflection and high philosophic thought it is to be ranked with the researches of newton in the _principia_. _researches on the construction of the heavens._ herschel's papers on the construction of the heavens, as he named it, extended over his whole scientific life. by this he specially means the method according to which the stars, the clusters, the nebulæ, are spread through the regions of space, the causes that have led to this distribution, and the laws to which it is subjected. no single astronomical fact is unimportant in the light which it may throw on the scheme of the whole, and each fact is to be considered in this light. as an instance: his discovery of the variable star _[alpha] herculis_, which has a period of sixty days, was valuable in itself as adding one more to the number of those strange suns whose light is now brighter, now fainter, in a regular and periodic order. but the chief value of the discovery was that now we had an instance of a periodic star which went through all its phases in sixty days, and connected, as it were, the stars of short periods (three to seven days) with those of very long ones (three hundred to five hundred days), which two groups had, until then, been the only ones known. in the same way all his researches on the parallaxes of stars were not alone for the discovery of the distance of any one or two single stars, but to gain a unit of celestial measure, by means of which the depths of space might be sounded. astronomy in herschel's day considered the bodies of the solar system as separated from each other by distances, and as filling a cubical space. the ideas of near and far, of up and down, were preserved, in regard to them, by common astronomical terms. but the vast number of stars seemed to be thought of, as they appear in fact to exist, lying on the surface of a hollow sphere. the immediate followers of bradley used these fixed stars as points of reference by which the motions within the solar system could be determined, or, like lacaille and lalande, gathered those immense catalogues of their positions which are so indispensable to the science. michell and herschel alone, in england, occupied their thoughts with the nature and construction of the heavens--the one in his study, the other through observation.[ ] they were concerned with all three of the dimensions of space. in his memoir of , herschel says: "hitherto the sidereal heavens have, not inadequately for the purpose designed, been represented by the concave surface of a sphere, in the centre of which the eye of an observer might be supposed to be placed. "it is true the various magnitudes of the fixed stars even then plainly suggested to us, and would have better suited, the idea of an expanded firmament of three dimensions; but the observations upon which i am now going to enter still farther illustrate and enforce the necessity of considering the heavens in this point of view. in future, therefore, we shall look upon those regions into which we may now penetrate by means of such large telescopes, as a naturalist regards a rich extent of ground or chain of mountains containing strata variously inclined and directed, as well as consisting of very different materials. the surface of a globe or map, therefore, will but ill delineate the interior parts of the heavens." herschel's method of study was founded on a mode of observation which he called _star-gauging_. it consisted in pointing a powerful telescope toward various parts of the heavens, and ascertaining by actual count how thick the stars were in each region. his twenty-foot reflector was provided with such an eye-piece that, in looking into it, he saw a portion of the heavens about ' in diameter. a circle of this size on the celestial sphere has about one quarter the apparent surface of the sun, or of the full moon. on pointing the telescope in any direction, a greater or less number of stars were visible. these were counted, and the direction in which the telescope pointed was noted. gauges of this kind were made in all parts of the sky, and the results were tabulated in the order of right ascension. the following is an extract from the gauges, and gives the average number of stars in each field at the points noted in right ascension and north polar distance: ---------------------------------------------------------- | n. p. d. || | n. p. d. r. a. | ° to °. || r. a. | ° to °. | no. of stars. || | no. of stars. ------------|-----------------||-----------|-------------- h. m. | || h. m. | | . || | . | . || | . | . || | . | . || | . | . || | . | . || | . ---------------------------------------------------------- in this small table, it is plain that a different law of clustering or of distribution obtains in the two regions. such differences are still more marked, if we compare the extreme cases found by herschel, as r. a. = h m, n. p. d. = ° ', number of stars per field = ; and r. a. = h m, n. p. d. = ° ', number of stars = . . the number of stars in certain portions is very great. for example, in the milky way, near _orion_, six fields of view promiscuously taken gave , , , , , and stars each, or a mean of stars per field. the most vacant space in this neighborhood gave stars. so that as herschel's sweeps were two degrees wide in declination, in one hour ( °) there would pass through the field of his telescope , or more stars. in some of the sweeps this number was as great as , stars in a quarter of an hour. when herschel first applied his telescope to the milky way, he believed that it completely resolved the whole whitish appearance into small stars. this conclusion he subsequently modified. he says: "it is very probable that the great stratum called the milky way is that in which the sun is placed, though perhaps not in the very centre of its thickness. "we gather this from the appearance of the galaxy, which seems to encompass the whole heavens, as it certainly must do if the sun is within it. for, suppose a number of stars arranged between two parallel planes, indefinitely extended every way, but at a given considerable distance from each other; and calling this a sidereal stratum, an eye placed somewhere within it will see all the stars in the direction of the planes of the stratum projected into a great circle, which will appear lucid on account of the accumulation of the stars, while the rest of the heavens, at the sides, will only seem to be scattered over with constellations, more or less crowded according to the distance of the planes, or number of stars contained in the thickness or sides of the stratum. "if the eye were placed somewhere without the stratum, at no very great distance, the appearance of the stars within it would assume the form of one of the smaller circles of the sphere, which would be more or less contracted according to the distance of the eye; and, if this distance were exceedingly increased, the whole stratum might at last be drawn together into a lucid spot of any shape, according to the length, breadth, and height of the stratum. "suppose that a smaller stratum should branch out from the former in a certain direction, and that it also is contained between two parallel planes, so that the eye is contained within the great stratum somewhere before the separation, and not far from the place where the strata are still united. then this second stratum will not be projected into a bright circle like the former, but it will be seen as a lucid branch proceeding from the first, and returning into it again at a distance less than a semicircle. if the bounding surfaces are not parallel planes, but irregularly curved surfaces, analogous appearances must result." the milky way, as we see it, presents the aspect which has been just accounted for, in its general appearance of a girdle around the heavens and in its bifurcation at a certain point, and herschel's explanation of this appearance, as just given, has never been seriously questioned. one doubtful point remains: are the stars scattered all through space? or are they near its bounding planes, or clustered in any way within this space so as to produce the same result to the eye as if uniformly distributed? herschel assumed that they were nearly equably arranged all through the space in question. he only examined one other arrangement, _viz._, that of a ring of stars surrounding the sun, and he pronounced against such an arrangement, for the reason that there is absolutely nothing in the size or brilliancy of the sun to cause us to suppose it to be the centre of such a gigantic system. no reason, except its importance to us personally, can be alleged for such a supposition. every star will have its own appearance of a galaxy or milky way, which will vary according to the situation of the star. such an explanation will account for the general appearances of the milky way and of the rest of the sky, supposing the stars equally or nearly equally distributed in space. on this supposition, the system must be deeper where the stars appear most numerous. herschel endeavored, in his early memoirs, to explain this inequality of distribution on the fundamental assumption that the stars were nearly equably distributed in space. if they were so distributed, then the number of stars visible in any gauge would show the thickness of the stellar system in the direction in which the telescope was pointed. at each pointing, the field of view of the instrument includes all the visible stars situated within a cone, having its vortex at the observer's eye, and its base at the very limits of the system, the angle of the cone (at the eye) being '. then the cubes of the perpendiculars let fall from the eye, on the plane of the bases of the various visual cones, are proportional to the solid contents of the cones themselves, or, as the stars are supposed equally scattered within all the cones, the cube roots of the numbers of stars in each of the fields express the relative lengths of the perpendiculars. a _section_ of the sidereal system along any great circle can be constructed from the data furnished by the gauges in the following way: the solar system is within the mass of stars. from this point lines are drawn along the different directions in which the gauging telescope was pointed. on these lines are laid off lengths proportional to the cube roots of the number of stars in each gauge. the irregular line joining the terminal points will be approximately the bounding curve of the stellar system in the great circle chosen. within this line the space is nearly uniformly filled with stars. without it is empty space. a similar section can be constructed in any other great circle, and a combination of all such would give a representation of the shape of our stellar system. the more numerous and careful the observations, the more elaborate the representation, and the gauges of herschel are sufficient to mark out with great precision the main features of the milky way, and even to indicate some of its chief irregularities. on the fundamental assumption of herschel (equable distribution), no other conclusion can be drawn from his statistics but the one laid down by him. this assumption he subsequently modified in some degree, and was led to regard his gauges as indicating not so much the _depth of the system_ in any direction, as the _clustering power or tendency_ of the stars in those special regions. it is clear that if in any given part of the sky, where, on the average, there are ten stars (say) to a field, we should find a certain small portion having or more to a field, then, on herschel's first hypothesis, rigorously interpreted, it would be necessary to suppose a spike-shaped protuberance directed from the earth, in order to explain the increased number of stars. if many such places could be found, then the probability is great that this explanation is wrong. we should more rationally suppose some real inequality of star distribution here. it is, in fact, in just such details that the method of herschel breaks down, and a careful examination of his system leads to the belief that it must be greatly modified to cover all the known facts, while it undoubtedly has, in the main, a strong basis. the stars are certainly not uniformly distributed, and any general theory of the sidereal system must take into account the varied tendency to aggregation in various parts of the sky. in , herschel published an important memoir on the same subject, in which his first method was largely modified, though not abandoned. its fundamental principle was stated by him as follows: "it is evident that we cannot mean to affirm that the stars of the fifth, sixth, and seventh magnitudes are really smaller than those of the first, second, or third, and that we must ascribe the cause of the difference in the apparent magnitudes of the stars to a difference in their relative distances from us. on account of the great number of stars in each class, we must also allow that the stars of each succeeding magnitude, beginning with the first, are, one with another, further from us than those of the magnitude immediately preceding. the relative magnitudes give only relative distances, and can afford no information as to the real distances at which the stars are placed. "a standard of reference for the arrangement of the stars may be had by comparing their distribution to a certain properly modified equality of scattering. the equality which i propose does not require that the stars should be at equal distances from each other, nor is it necessary that all those of the same nominal magnitude should be equally distant from us." it consisted in allotting a certain equal portion of space to every star, so that, on the whole, each equal portion of space within the stellar system contains an equal number of stars. the space about each star can be considered spherical. suppose such a sphere to surround our own sun. its radius will not differ greatly from the distance of the nearest fixed star, and this is taken as the unit of distance. suppose a series of larger spheres, all drawn around our sun as a centre, and having the radii , , , , etc. the contents of the spheres being as the cubes of their diameters, the first sphere will have × × = times the volume of the unit sphere, and will therefore be large enough to contain stars; the second will have times the volume, and will therefore contain stars, and so on with the successive spheres. for instance, the sphere of radius has room for stars, but of this space parts belong to the spheres inside of it; there is, therefore, room for stars between the spheres of radii and . herschel designates the several distances of these layers of stars as orders; the stars between spheres and are of the first order of distance, those between and of the second order, and so on. comparing the room for stars between the several spheres with the number of stars of the several magnitudes which actually exists in the sky, he found the result to be as follows: -------------------------------------------------------- order of | number of | | number of distance. | stars there | magnitude. | stars of that | is room for. | | magnitude. -------------------------------------------------------- ........ | | | ........ | | | ........ | | | ........ | | | ........ | | | , ........ | | | , ........ | , | | , ........ | , | | --------------------------------------------------------- the result of this comparison is, that if the order of magnitudes could indicate the distance of the stars, it would denote at first a gradual and afterward a very abrupt condensation of them, at and beyond the region of the sixth-magnitude stars. if we assume the brightness of any star to be inversely proportional to the square of its distance, it leads to a scale of distance different from that adopted by herschel, so that a sixth-magnitude star on the common scale would be about of the eighth order of distance according to this scheme--that is, we must remove a star of the first magnitude to eight times its actual distance to make it shine like a star of the sixth magnitude. on the scheme here laid down, herschel subsequently assigned the _order_ of distance of various objects, mostly star-clusters, and his estimates of these distances are still quoted. they rest on the fundamental hypothesis which has been explained, and the error in the assumption of equal intrinsic brilliancy for all stars affects these estimates. it is perhaps probable that the hypothesis of equal brilliancy for all stars is still more erroneous than the hypothesis of equal distribution, and it may well be that there is a very large range indeed in the actual dimensions and in the intrinsic brilliancy of stars at the same order of distance from us, so that the tenth-magnitude stars, for example, may be scattered throughout the spheres which herschel would assign to the seventh, eighth, ninth, tenth, eleventh, twelfth, and thirteenth magnitudes. however this may be, the fact remains that it is from herschel's groundwork that future investigators must build. he found the whole subject in utter confusion. by his observations, data for the solution of some of the most general questions were accumulated, and in his memoirs, which struve well calls "immortal," he brought the scattered facts into order and gave the first bold outlines of a reasonable theory. he is the founder of a new branch of astronomy. _researches for a scale of celestial measures. distances of the stars._ if the stars are _supposed_ all of the same absolute brightness, their brightness to the eye will depend only upon their distance from us. if we call the brightness of one of the fixed stars at the distance of _sirius_, which may be used as the unity of distance, , then if it is moved to the distance , its apparent brightness will be one-fourth; if to the distance , one-ninth; if to the distance , one-sixteenth, and so on, the apparent brightness diminishing as the square of the distance increases. the distance may be taken as an order of magnitude. stars at the _distances_ two, three, four, etc., herschel called of the second, third, and fourth magnitudes. by a series of experiments, the details of which cannot be given here, herschel determined the space-penetrating power of each of his telescopes. the twenty-foot would penetrate into space seventy-five times farther than the naked eye; the twenty-five foot, ninety-six times; and the forty-foot, one hundred and ninety-two times. if the seventh-magnitude stars are those just visible to the naked eye, and if we still suppose all stars to be of equal intrinsic brightness, such seventh-magnitude stars would remain visible in the forty-foot, even if removed to , times the distance of _sirius_ ( , = × ). if, further, we suppose that the visibility of a star is strictly proportional to the total intensity of the light from it which strikes the eye, then a condensed cluster of , stars of the , th magnitude could still be seen in the forty-foot at a distance where each star would have become , times fainter, that is, at about times the distance of _sirius_ ( × = , ). the light from the nearest star requires some three years to reach the earth. from a star , times farther it would require about , years, and for such a cluster as we have imagined no less than , years are needed. that is, the light by which we see such a group has not just now left it. on the contrary, it has been travelling through space for centuries and centuries since it first darted forth. it is the ancient history of such groups that we are studying now, and it was thus that herschel declared that telescopes penetrated into time as well as into space. other more exact researches on the relative light of stars were made by herschel. these were only one more attempt to obtain a scale of celestial distances, according to which some notion of the limits and of the interior dimensions of the universe could be gained. two telescopes, _exactly equal_ in every respect, were chosen and placed side by side. pairs of stars which were _exactly equal_, were selected by means of them. by diminishing the aperture of one telescope directed to a bright star, and keeping the other telescope unchanged and directed to a fainter star, the two stars could be equalized in light, and, from the relative size of the apertures, the relative light of this pair of stars could be accurately computed, and so on for other pairs. this was the first use of the method of _limiting apertures_. his general results were that the stars of the first magnitude would still remain visible to the naked eye, even if they were at a distance from us _twelve_ times their actual distance. this method received a still further development at his hands. he did not leave it until he had gained all the information it was capable of giving. he prepared a set of telescopes collecting , , , etc. ( × , × , × , etc.), times as much light as the naked eye. these were to extend the determinations of distance to the telescopic stars. for example, a certain portion of the heavens which he examined contained no star visible to the naked eye, but many telescopic stars. we cannot say that no one of these is as bright in itself as some of our first-magnitude stars. the smallest telescope of the set showed a large number of stars; these must, then, be _twice_ as far from us, on the average, as the stars just visible to the naked eye. but first-magnitude stars, like _sirius_, _procyon_, _arcturus_, etc., become just visible to the eye if removed to twelve times their present distance. hence the stars seen in this first telescope of the set were between twelve and twenty-four times as far from us as _arcturus_, for example. "at least," as herschel says, "we are certain that if stars of the size and lustre of _sirius_, _arcturus_, etc., were removed into the profundity of space i have mentioned, they would then appear like the stars which i saw." with the next telescope, which collected nine times more light than the eye, and brought into view objects three times more distant, other and new stars appeared, which were then ( × ) thirty-six times farther from us than _arcturus_. in the same way, the seven-foot reflector showed stars times, the ten-foot times, the twenty-foot times farther from us than the average first-magnitude star. as the light from such a star requires three years to reach us, the light from the faintest stars seen by the twenty-foot would require , years ( × ). but herschel was now ( ) convinced that the twenty-foot telescope could not penetrate to the boundaries of the milky way; the faintest stars of the galaxy must then be farther from us even than nine hundred times the distance of _arcturus_, and their light must be at least , years old when it reaches us. there is no escaping a certain part of the consequences established by herschel. it is indeed true that unless a particular star is of the same intrinsic brightness as our largest stars, this reasoning does not apply to it; in just so far as the average star is less bright than the average brightness of our largest stars, will the numbers which herschel obtained be diminished. but for every star of which his hypothesis is true, we may assert that his conclusions are true, and no one can deny, with any show of reason, that, on the whole, his suppositions must be valid. on the whole, the stars which we call faint are farther from us than the brighter ones; and, on the whole, the brilliancy of our brightest and nearest stars is not very far from the brilliancy of the average star in space. we cannot yet define the word _very_ by a numerical ratio. the _method_ struck out by herschel was correct; it is for his successors to look for the special cases and limitations, to answer the question, at a certain distance from us, what are the variations which actually take place in the brilliancy and the sizes of stars? the answer to this question is to be found in the study of the clusters of regular forms, where we _know_ the stars to be all at the same distance from us. _researches on light and heat, etc._ frequently in the course of his astronomical work, herschel found himself confronted by questions of physics which could not be immediately answered in the state of the science at that time. in his efforts to find a method for determining the dimensions of the stellar universe, he was finally led, as has been shown, to regard the brightness of a star as, in general, the best attainable measure of its distance from us. his work, however, was done with telescopes of various dimensions and powers, and it was therefore necessary to find some law for comparing the different results among themselves as well as with those given by observations with an unassisted eye. this necessity prompted an investigation, published in , in which, after drawing the distinction between absolute and intrinsic brightness, herschel gave an expression for the _space-penetrating power_ of a telescope. the reasoning at the base of this conception was as follows. the ratio of the light entering the eye when directed toward a star, to the whole light given out by the star, would be as the area of the pupil of the eye to the area of the whole sphere having the star as a centre and our distance from the star as a radius. if the eye is assisted by a telescope, the ratio is quite different. in that case the ratio of the light which enters the eye to the whole light, would be as the area of the mirror or object-glass to the area of the whole sphere having the star as a centre and its distance as a radius. thus the light received by the _eye_ in the two cases would be as the area of the pupil is to the area of the object-glass. for instance, if the pupil has a diameter of two-fifths of an inch, and the mirror a diameter of four inches, then a hundred times as much light would enter the eye when assisted by the telescope as when unarmed, since the _area_ of the pupil is one-hundredth the _area_ of the objective. if a particular star is just visible to the naked eye, it will be quite bright if viewed with this special telescope, which makes it one hundred times more brilliant in appearance. if we could move the star bodily away from us to a distance ten times its present distance, we could thus reduce its brightness, as seen with the telescope, to what it was at first, as seen with the eye alone, _i. e._, to bare visibility. moving the star to ten times its present distance would increase the surface of the sphere which it illuminates a hundred-fold. we cannot move any special star, but we can examine stars of all brightnesses, and thus (presumably) of all distances. herschel's argument was, then, as follows: since with such a telescope one can see a star ten times as far off as is possible to the naked eye, this telescope has the power of penetrating into space ten times farther than the eye alone. but this number ten, also, expresses the ratio of the diameter of the objective to that of the pupil of the eye, consequently the general law is that the _space-penetrating power_ of a telescope is found by dividing the diameter of the mirror in inches by two-fifths. the diameter of the pupil of the eye (two-fifths of an inch) herschel determined by many measures. this simple ratio would only hold good, however, provided no more light were lost by the repeated reflections and refractions in the telescope than in the eye. that light must be so lost was evident, but no data existed for determining the loss. herschel was thus led to a long series of photometric experiments on the reflecting powers of the metals used in his mirrors, and on the amount of light transmitted by lenses. applying the corrections thus deduced experimentally, he found that the space-penetrating power of his twenty-foot telescope, with which he made his star-gauges, was sixty-one times that of the unassisted eye, while the space-penetrating power of his great forty-foot telescope was one hundred and ninety-two times that of the eye. in support of his important conclusions herschel had an almost unlimited amount of experimental data in the records of his observations, of which he made effective use. by far the most important of herschel's work in the domain of pure physics was published in the same year ( ), and related to radiant heat. the investigation of the space-penetrating powers of telescopes was undertaken for the sole purpose of aiding him in measuring the dimensions of the stellar universe, and there was no temptation for him to pursue it beyond the limits of its immediate usefulness. but here, though the first hint leading to remarkable discoveries was a direct consequence of his astronomical work, the novelty and interest of the phenomena observed induced him to follow the investigation very far beyond the mere solution of the practical question in which it originated. having tried many varieties of shade-glasses between the eye-piece of his telescope and the eye, in order to reduce the inordinate degree of heat and light transmitted by the instrument when directed towards the sun, he observed that certain combinations of colored glasses permitted very little light to pass, but transmitted so much heat that they could not be used; while, on the other hand, different combinations and differently colored glasses would stop nearly all the heat, but allow an inconveniently great amount of light to pass. at the same time he noticed, in the various experiments, that the images of the sun were of different colors. this suggested the question as to whether there was not a different heating power proper to each color of the spectrum. on comparing the readings of sensitive thermometers exposed in different portions of an intense solar spectrum, he found that, beginning with the violet end, he came to the maximum of light long before that of heat, which lay at the other extremity, that is, near the red. by several experiments it appeared that the maximum of illumination, _i. e._, the yellow, had little more than half the heat of the full red rays; and from other experiments he concluded that even the full red fell short of the maximum of heat, which, perhaps, lay even a little beyond the limits of the visible spectrum. "in this case," he says, "radiant heat will at least partly, if not chiefly, consist, if i may be permitted the expression, of invisible light; that is to say, of rays coming from the sun, that have such a momentum[ ] as to be unfit for vision. and admitting, as is highly probable, that the organs of sight are only adapted to receive impressions from particles of a certain momentum, it explains why the maximum of illumination should be in the middle of the refrangible rays; as those which have greater or less momenta are likely to become equally unfit for the impression of sight." in his second paper on this subject, published in the same year, herschel describes the experiments which led to the conclusion given above. this paper contains a remarkably interesting passage which admirably illustrates herschel's philosophic method. "to conclude, if we call light, those rays which illuminate objects, and radiant heat, those which heat bodies, it may be inquired whether light be essentially different from radiant heat? in answer to which i would suggest that we are not allowed, by the rules of philosophizing, to admit two different causes to explain certain effects, if they may be accounted for by one. . . . if this be a true account of the solar heat, for the support of which i appeal to my experiments, it remains only for us to admit that such of the rays of the sun as have the refrangibility of those which are contained in the prismatic spectrum, by the construction of the organs of sight, are admitted under the appearance of light and colors, and that the rest, being stopped in the coats and humors of the eye, act on them, as they are known to do on all the other parts of our body, by occasioning a sensation of heat." we now know that the reasoning and conclusion here given are entirely correct, but they have for their basis only a philosophical conception, and not a series of experiments designed especially to test their correctness. such an experimental test of this important question was the motive for a third and last paper in this department of physics. this paper was published in volume ninety of the _philosophical transactions_, and gave the results of two hundred and nineteen quantitative experiments. here we are at a loss to know which to admire most--the marvellous skill evinced in acquiring such accurate data with such inadequate means, and in varying and testing such a number of questions as were suggested in the course of the investigation--or the intellectual power shown in marshalling and reducing to a system such intricate and apparently self-contradictory phenomena. it is true that this discussion led him to a different conclusion from that announced in the previous paper, and, consequently, to a false conclusion; but almost the only escape from his course of reasoning lay in a principle which belongs to a later period of intellectual development than that of herschel's own time. herschel made a careful determination of the quantitative distribution of light and of heat in the prismatic spectrum, and discovered the surprising fact that not only where the light was at a maximum the heat was very inconsiderable, but that where there was a maximum exhibition of heat, there was not a trace of light. "this consideration," he writes, "must alter the form of our proposed inquiry; for the question being thus at least partly decided, since it is ascertained that we have rays of heat which give no light, it can only become a subject of inquiry whether some of these heat-making rays may not have a power of rendering objects visible, superadded to their now already established power of heating bodies. this being the case, it is evident that the _onus probandi_ ought to lie with those who are willing to establish such an hypothesis, for it does not appear that nature is in the habit of using one and the same mechanism with any two of our senses. witness the vibration of air that makes sound, the effluvia that occasion smells, the particles that produce taste, the resistance or repulsive powers that affect the touch--all these are evidently suited to their respective organs of sense." it is difficult to see how the fallacy of this argument could have been detected by any one not familiar with the fundamental physiological law that the nature of a sensation is in no wise determined by the character of the agent producing it, but only by the character of the nerves acted upon; but, as already intimated, this law belongs to a later epoch than the one we are considering. herschel thus finally concluded that light and radiant heat were of essentially different natures, and upon this supposition he explained all of the phenomena which his numerous experiments had shown him. so complete and satisfactory did this work appear to the scientific world, that for a long time the question was looked upon as closed, and not until thirty-five years later was there any dissent. then the italian physicist, melloni, with instrumental means a thousand times more delicate than that of herschel, and with a far larger store of cognate phenomena, collected during the generation which had elapsed, to serve as a guide, discovered the true law. this, as we have seen, was at first adopted by herschel on philosophical grounds, and then rejected, since he did not at that time possess the key which alone could have enabled him to properly interpret his experiments. it is well to summarize the capital discoveries in this field made by herschel, more particularly because his claims as a discoverer seem to have been strangely overlooked by historians of the development of physical science. he, before any other investigator, showed that radiant heat is refracted according to the laws governing the refraction of light by transparent media; that a portion of the radiation from the sun is incapable of exciting the sensation of vision, and that this portion is the less refrangible; that the different colors of the spectrum possess very unequal heating powers, which are not proportional to their luminosity; that substances differ very greatly in their power of transmitting radiant heat, and that this power does not depend solely upon their color; and that the property of diffusing heat is possessed to a varying degree by different bodies, independently of their color. nor should we neglect to emphasize, in this connection, the importance of his measurements of the intensity of the heat and light in the different portions of the solar spectrum. it is the more necessary to state herschel's claims clearly, as his work has been neglected by those who should first have done him justice. in his "history of physics," poggendorff has no reference to herschel. in the collected works of verdet, long bibliographical notes are appended to each chapter, with the intention of exhibiting the progress and order of discovery. but all of herschel's work is overlooked, or indexed under the name of his son. one little reference in the text alone shows that his very name was not unknown. even in the great work of helmholtz on physiological optics, herschel's labors are not taken account of. it is easy to account for this seemingly strange neglect. herschel is known to this generation only as an astronomer. a study of his memoirs will show that his physical work alone should give him a very high rank indeed, and i trust that the brief summaries, which alone can be given here, will have made this plain. * * * * * we may conclude from the time expended, the elaborate nature of the experiments involved, and the character of the papers devoted to their consideration, that the portion of herschel's researches in physics which interested him to the greatest degree, was the investigation of the optical phenomena known as newton's rings. in he obtained the two object-glasses of huyghens, which were in the possession of the royal society, for the purpose of repeating newton's experiments, and in he read the last of his three papers on the subject. sir isaac newton had given some of his most vigorous efforts to the study of the phenomena of interference of light, which are exemplified in the colors of thin and of thick plates. the colors of thin plates are most conveniently studied in the regular form which they present when produced by a thin plate of air, limited on one side by a plane polished surface, and on the other by a spherical surface of long radius, such as the exterior surface of a convex lens, for example. the colors are then arranged in concentric circles, and, though others had so produced them before newton, these rings have, ever since the publication of his remarkable work, been known by his name. to explain the phenomena, newton was obliged to supplement his theory of the corpuscular nature of light, by supposing that the inconceivably minute particles constituting light are not always equally susceptible of reflection, but that they have periodically recurring "fits of easy reflection" and of "easy transmission." this conception, though by no means unphilosophical, seemed to herschel too artificial and improbable for ready acceptance, and his effort was to supply a more probable explanation. the developments of optical science have justified herschel in his objections, but we cannot accord to him must any considerable part in making clear the true nature of the phenomenon. indeed, it must be recognized that his position was distinctly less advanced than that of newton. that great philosopher announced the true law governing the relation between the color and the thickness of the film. herschel did not recognize such a relation. newton showed exactly how the phenomenon depended upon the obliquity at which it was viewed. herschel found no place in his theory for this evident variation. in the series of experiments described in the first paper on this subject, herschel mistook the locus of a certain set of rings which he was observing. this mistake, though so slight as hardly to be detected without the guidance of the definite knowledge acquired in later times, not only vitiated the conclusion from the experiments, but gave an erroneous direction to the whole investigation. to him these experiments proved that newton's conception of a periodic phenomenon was untenable. thus cut loose from all hypothesis, his fertility in ideas and ingenuity in experimentation are as striking as ever. he tried the effect of having a polished metal as one of the surfaces limiting the thin plate of air. observing the so-called "blue bow" of newton at the limit of total reflection in a prism, he was led to the discovery of its complement, the "red bow" by refraction. here he thought he had found the solution of his problem, and attributed the rings to the reflection of the light which passed through in the red bow. though mistaken, he had presented to the world of science two experiments which have since played very prominent parts in the undulatory theory of light, namely, the rings formed upon polished metal, and the bands produced by a thin plate near the critical angle. as in his later researches upon the nature of radiant heat, he was wrong in his conclusions, and perhaps with less excuse. his experiments were skilfully devised and most ingenious. his philosophizing was distinctly faulty. we can see not only that he was wrong, but exactly where he began to go wrong. yet these papers are full of interest to the physicist, and by no means deserve the neglect into which they have fallen. _researches on the dimensions of the stars._ herschel examined a number of bright stars, using extremely high magnifying powers, in order to determine whether the stars have sensible dimensions. in a good telescope stars present round and pretty uniformly illuminated disks. if these disks really represent the angular diameter of the stars, they should admit of magnifying, like other objects; but, instead of this, herschel found that they appeared smaller as the telescopic power was increased. he accordingly called the disk of light seen in the telescope a spurious disk. this singular phenomenon gave its discoverer a ready criterion for determining whether a small bright body has an appreciable size, or only impresses the sense of sight by virtue of its intrinsic brightness. if the first were the case, the apparent size would increase with increased magnifying power, while, if the angular dimensions were inappreciable, the apparent size would, on the contrary, diminish with additional magnifying. an occasion for using this criterion came in the first years of this century, with the discovery of three small planets having orbits lying between those of _mars_ and _jupiter_. herschel gave the name _asteroids_ to these bodies. as the appropriateness of this term had been violently assailed, the discovery of _juno_, in , the third one of the group, led to a careful experimental study of the defining power of the telescope used, and of the laws governing the phenomena of spurious disks. with a telescope of about nine inches in aperture, herschel found that if _juno_ subtended an angle greater than a quarter of a second of arc, a certain indication of the fact would have shown itself in the course of the experiments. this conclusion was a justification of the name asteroid, since the appearance of the new planet was strictly stellar. on other grounds, a better name might have been selected. in the paper giving the results of the experiments, the phenomena of the spurious disks are very completely described; but they did not attract the attention which they deserved, and they only became an object of especial interest to students of physics when they were again studied by the famous german optician fraunhofer, a generation later. incidentally the experiments are of interest, as yielding us a measure of the excellence of herschel's telescopes, and a measure which is quite independent of the keenness of his vision. from them we may be sure that the efficiency of the nine-inch mirror used was not sensibly less than that of the highest theoretically attainable excellence. in this connection, too, we may refer to the _philosophical transactions_ for , pp. and , where herschel gives observations of both _enceladus_ and _mimas_ seen in contact with the ball of _saturn_. i have never seen so good definition, telescopic and atmospheric, as he must have had on these occasions. _researches on the spectra of the fixed stars._ the spectroscope was applied by secchi to the study of the spectra of the fixed stars visible to the naked eye in the years to . he examined the nature of the spectrum of each of the larger stars, and found that these stars could be arranged in three general classes or _types_. his results have been verified and extended by other astronomers, and his classification has been generally accepted. according to secchi, the lucid stars may be separated into three groups, distinguished by marked differences in their spectra. secchi's type i. contains stars whose spectra are like those of _sirius_, _procyon_, and _[alpha] lyræ_; his type ii. stars like _arcturus_ and _aldebaran_; his type iii. stars like _[alpha] orionis_. herschel also made some trials in this direction. in the _philosophical transactions_ for (p. ), he says: "by some experiments on the light of a few of the stars of the first magnitude, made in , by a prism applied to the eye-glasses of my reflectors, adjustable to any angle and to any direction, i had the following analyses: "the light of _sirius_ consists of red, orange, yellow, green, blue, purple, and violet. _[alpha] orionis_ contains the same colors, but the red is more intense, and the orange and yellow are less copious in proportion than they are in _sirius_. _procyon_ contains all the colors, but proportionately more blue and purple than _sirius_. _arcturus_ contains more red and orange, and less yellow in proportion than _sirius_. _aldebaran_ contains much orange and very little yellow. _[alpha] lyræ_ contains much yellow, green, blue, and purple." here the essential peculiarities of the spectrum of each of the stars investigated by herschel is pointed out, and if we were to use his observations alone to classify these stars into types, we should put _sirius_ and _procyon_ into one type of stars which have "all the colors" in their spectra; _arcturus_ and _aldebaran_ would represent another group of stars, with a deficiency of yellow and an excess of orange and red in the spectrum; and _[alpha] orionis_ would stand as a type of those stars with an excess of red and a deficiency of orange. _[alpha] lyræ_ would represent a sub-group of the first class. herschel's immediate object was not classification, and his observations are only recorded in a passing way. but the fact remains that he clearly distinguished the essential differences of the spectra of these stars, and that he made these observations in support of his statement that the fixed stars, "like the planets, also shine with differently colored light. that of _arcturus_ and _aldebaran_, for instance, is as different from the light of _sirius_ and _capella_ as that of _mars_ and _saturn_ is from the light of _venus_ and _jupiter_." of course, no special discovery can be claimed for him on these few instances. we can see, however, a good example of the manner in which he examined a subject from every side, and used the most remote evidence exactly in its proper place and time. _researches on the variable emission of light and heat from the sun._ it is certainly a remarkable fact that herschel was the first observer to recognize the real importance of the aperture or diameter of a telescope. before his time it was generally assumed that this element only conditioned the amount of light transmitted to the eye, or, in other words, merely determined the brightness of the image. hence the conclusion that if an object is sufficiently bright, the telescope may be made as small as desired without loss of power. thus, in observing the sun, astronomers before herschel had been accustomed to reduce the aperture of their telescopes, in order to moderate the heat and light transmitted. scheiner, it is true, nearly two centuries before the time we are considering, had invented a method for observing the sun without danger, still employing the full aperture. this was by projecting the image of the sun on a white screen beyond the eye-piece, the telescope being slightly lengthened. for special purposes this ingenious method has even been found useful in modern times, though for sharpness of definition it bears much the same relation to the more usual manner of observing, that a photographic picture does to direct vision. although herschel saw the advantages of using the whole aperture of a telescope in such observations, the practical difficulties in the way were very great. we have noted his attempts to find screens which would effectively cut off a large portion of the heat and light without impairing vision, and have considered, somewhat in detail, the remarkable discoveries in radiant heat to which these attempts led him. his efforts were not unsuccessful. a green glass smoked, and a glass cell containing a solution of black writing ink in water--were found to work admirably. thus provided with more powerful instrumental means than had ever been applied to the purpose, herschel turned his attention to the sun. in a very short time he exhausted nearly all there was to be discovered, so that since the publication of his last paper on this subject, in , until the present time, there has been but a single telescopic phenomenon, connected with the physical appearance of the sun, which was unknown to herschel. that phenomenon is the frequent occurrence of a darker central shade or kernel in large spots, discovered by dawes about . herschel, though observing a hundred and ninety years after the earliest discovery of sun spots, seems to have been the first to suspect their periodic character. to establish this as a fact, and to measure the period, was left for our own times and for the indefatigable observer schwabe. the probable importance of such a period in its relation to terrestrial meteorology was not only clearly pointed out by herschel, but he even attempted to demonstrate, from such data as were obtainable, the character of this influence. perhaps no one thing which this great philosopher has done better exhibits the catholic character of his mind than this research. when the possible connection of solar and terrestrial phenomena occurred to him as a question to be tested, there were no available meteorological records, and he could find but four or five short series of observations, widely separated in time. to an ordinary thinker the task would have seemed hopeless until more data had been collected. but herschel's fertile mind, though it could not recall lost opportunities for solar observations, did find a substitute for meteorological records in the statistics of the prices of grain during the various epochs. it is clear that the price of wheat must have depended upon the supply, and the supply, in turn, largely upon the character of the season. the method, as ingenious as it is, failed in herschel's hands on account of the paucity of solar statistics; but it has since proved of value, and has taken its place as a recognized method of research. _researches on nebulæ and clusters._ when herschel first began to observe the nebulæ in , there were very few of these objects known. the nebulæ of _orion_ and _andromeda_ had been known in europe only a little over a hundred years. in messier published a list of sixty-eight such objects which he had found in his searches for comets, and twenty-eight nebulæ had been found by lacaille in his observations at the cape of good hope. in the mere discovery of these objects herschel quickly surpassed all others. in he published a catalogue of one thousand new nebulæ, in a catalogue of a second thousand, and in one of five hundred. in all he discovered and described two thousand five hundred and eight new nebulæ and clusters. this branch of astronomy may almost be said to be proper to the herschels, father and son. sir john herschel re-observed all his father's nebulæ in the northern hemisphere, and added many new ones, and in his astronomical expedition to the cape of good hope he recorded almost an equal number in the southern sky. of the six thousand two hundred nebulæ now known the herschels discovered at least eight-tenths. the mere discovery of twenty-five hundred nebulæ would have been a brilliant addition to our knowledge of celestial statistics. herschel did more than merely point out the existence and position of these new bodies. each observation was accompanied by a careful and minute description of the object viewed, and with sketches and diagrams which gave the position of the small stars in it and near it.[ ] as the nebulæ and clusters were discovered they were placed in classes, each class covering those nebulæ which resembled each other in their general features. even at the telescope herschel's object was not discovery merely, but to know the inner constitution of the heavens. his classes were arranged with this end, and they are to-day adopted. they were: class i. "bright nebulæ ( in all). ii. "faint nebulæ ( in all). iii. "very faint nebulæ ( in all). iv. "planetary nebulæ, stars with burs, with milky chevelure, with short rays, remarkable shapes, etc. ( in all). v. "very large nebulæ ( in all). vi. "very compressed and rich clusters of stars ( in all). vii. "pretty much compressed clusters ( in all). viii. "coarsely scattered clusters of stars" ( in all). the lists of these classes were the storehouses of rich material from which herschel drew the examples by which his later opinions on the physical conditions of nebulous matter were enforced. as the nebulæ were discovered and classified they were placed upon a star-map in their proper positions ( ), and, as the discoveries went on, the real laws of the distribution of the nebulæ and of the clusters over the surface of the sky showed themselves more and more plainly. it was by this means that herschel was led to the announcement of the law that the spaces richest in nebulæ are distant from the milky way, etc. by no other means could he have detected this, and i believe this to have been the first example of the use of the graphical method, now become common in treating large masses of statistics. it is still in his capacity of an observer--an acute and wise one--that herschel is considered. but this was the least of his gifts. this vast mass of material was not left in this state: it served him for a stepping-stone to larger views of the nature and extent of the nebulous matter itself. his views on the nature of nebulæ underwent successive changes. at first he supposed all nebulæ to be but aggregations of stars. the logic was simple. to the naked eye there are many groups of stars which appear nebulous. _praesepe_ is, perhaps, the best example. the slightest telescopic power applied to such groups alters the nebulous appearance, and shows that it comes from the combined and confused light of discrete stars. other groups which remain nebulous in a seven-foot telescope, become stellar in a ten-foot. the nebulosity of the ten-foot can be resolved into stars by the twenty-foot, and so on. the nebulæ which remained still unresolved, it was reasonable to conclude, would yield to higher power, and generally a nebula was but a group of stars removed to a great distance. an increase of telescopic power was alone necessary to demonstrate this.[ ] "nebulæ can be selected so that an insensible gradation shall take place from a coarse cluster like the _pleiades_ down to a milky nebulosity like that in _orion_, every intermediate step being represented. this tends to confirm the hypothesis that all are composed of stars more or less remote." so, at first, herschel believed that his twenty-foot telescope was of power sufficient to fathom the milky way, that is, to see through it and beyond it, and to reduce all its nebulosities to true groups of stars. in he published a memoir on _nebulous stars_, in which his views were completely changed. he had found a nebulous star, the sixty-ninth of his class iv., to which his reasons would not apply. in the centre of it was a bright star; around the star was a halo gradually diminishing in brightness from the star outward, and perfectly circular. it was clear the two parts, star and nebula, were connected, and thus at the same distance from us. there were two possible solutions only. either the whole mass was, _first_, composed of stars, in which case the nucleus would be enormously larger than the other stars of its stellar magnitude elsewhere in the sky, or the stars which made up the halo indefinitely small; or, _second_, the central nucleus was indeed a star, but a star surrounded with "a shining fluid, of a nature totally unknown to us." the long strata of nebulæ, which he had before described under the name of "telescopic milky ways," might well be accounted for by masses of this fluid lying beyond the regions of the seventh-magnitude stars. this fluid might exist independently of stars. if it is self-luminous, it seems more fit to produce a star by its condensation, than to depend upon the star for its own existence. such were a few of the theorems to which his discovery of this nebula led him. the hypothesis of an elastic _shining fluid_ existing in space, sometimes in connection with stars, sometimes distinct from them, was adopted and never abandoned. how well the spectroscope has confirmed this idea it is not necessary to say. we know the shining fluid does exist, and in late years we have seen the reverse of the process imagined by herschel. a star has actually, under our eyes, become a planetary nebula, and the cycle of which he gave the first terms is complete. in five separate memoirs ( , , , , and ) herschel elaborated his views of the sidereal system. the whole extent of his views must be gained from the extended memoirs themselves. here only the merest outline can be given. in there is a marshaling of the various objects beyond our solar system. the stars themselves may be _insulated_, or may belong to _binary_ or _multiple_ systems, to _clusters_ and groups, or to grand groups like the milky way. nebulæ may have any of the forms which have been described; and, in , he gives examples of immense spaces in the sky covered with diffused and very faint nebulosity. "its abundance exceeds all imagination."[ ] these masses of nebular matter are the seats of attracting forces, and these forces must produce condensation. when a nebula has more than one preponderating seat of attracting matter, it may in time be divided, and the double nebulæ have had such an origin. when nebulæ appear to us as round masses, they are in reality globular in form, and this form is at once the effect and the proof of a gravitating cause. the central brightness of nebulæ points out the seat of the attraction; and the completeness of the approximation to a spherical form points out the length of time that the gravitating forces have been at work. those nebulæ (and clusters) which are most perfect in the globular form, have been longest exposed to central forces. the planetary nebulæ are the oldest in our system. they must have a rotatory motion on their axes. by progressive condensation planetary nebulæ may be successively converted into bright stellar nebulæ, or into nebulous stars, and these again, by the effects of the same cause, into insulated or double stars. this chain of theorems, laid down in the memoir of , is enforced in with examples which show how the nebulous appearance may grow into the sidereal. herschel selects from the hundreds of instances in his note-books, nebulæ in every stage of progress, and traces the effect of condensation and of clustering power through all its course, even to the final breaking up of the milky way itself. the memoirs of and add little to the general view of the physical constitution of the heavens. they are attempts to gain a scale of celestial measures by which we may judge of the distances of the stars and clusters in which these changes are going on. there is little to change in herschel's statement of the general construction of the heavens. it is the groundwork upon which we have still to build. every astronomical discovery and every physical fact well observed is material for the elaboration of its details or for the correction of some of its minor points. as a scientific conception it is perhaps the grandest that has ever entered into the human mind. as a study of the height to which the efforts of one man may go, it is almost without a parallel. the philosopher who will add to it to-day, will have his facts and his methods ready to his hands. herschel presents the almost unique example of an eager observer marshaling the multitude of single instances, which he himself has laboriously gathered, into a compact and philosophic whole. in spite of minor errors and defects, his ideas of the nature of the sidereal universe have prevailed, and are to-day the unacknowledged basis of our every thought upon it. some of its most secret processes have been worked out by him, and the paths which he pointed out are those along which our advances must be made. in concluding this condensed account of herschel's scientific labors, it behoves us to remember that there was nothing due to accident in his long life. he was born with the faculties which fitted him for the gigantic labors which he undertook, and he had the firm basis of energy and principle which kept him steadily to his work. as a practical astronomer he remains without an equal. in profound philosophy he has few superiors. by a kindly chance he can be claimed as the citizen of no one country. in very truth his is one of the few names which belong to the whole world. footnotes: [ ] james short, f.r.s. ( - ), and john dollond, f.r.s. ( - ), were the most celebrated makers of telescopes of their day. the six-foot newtonian reflectors of short (aperture . inches), and the forty-six-inch achromatics of dollond (aperture . inches), were highly esteemed. the royal observatory of greenwich possessed, in , one of each class. in a comparative trial of short's telescope, at greenwich, and one of herschel's first telescopes, the latter was adjudged greatly superior. [ ] at least _one_ of these telescopes had the principal mirror made of glass instead of metal.--_philosophical transactions_, . [ ] the following extract from fourier's _Éloge_ of herschel is of interest in this connection. the sum first appropriated by the king was £ , . this was afterwards raised to £ , , and a sum of £ yearly was given for maintenance. "l'histoire doit conserver à jamais la réponse de ce prince à un étranger célèbre [lalande?] qui le remerciait des sommes considérables accordées pour les progrès de l'astronomie. 'je fais les dépenses de la guerre,' dit le roi, 'parcequ'elles sont nécessaires; quant à celles des sciences, il m'est agréable des les ordonner; leur objet ne coûte point des larmes, et honore l'humanité.'" lalande's own account is a little different. he says the king exclaimed: "ne vaut-il pas mieux employer son argent à cela qu'à faire tuer des hommes?" [ ] the memoirs on the parallaxes of stars, written by various astronomers from to , were mainly directed to the improvement of the methods, or to the discovery of the parallax of some particular star. for example, lacaille's observations of _sirius_, at the cape of good hope, had resulted in a parallax of " for that star--a quantity over forty times too large. [ ] herschel accepted, as did all his cotemporaries, the newtonian or corpuscular theory of light. [ ] thus the position of small stars critically situated in the centre, or on the edges of the nebulæ was always noted. many of the descriptions are given in the published papers, but the publication of the diagrams would be an immense help to this branch of astronomy. d'arrest in his reduction of herschel's nebula observations ( ) writes: "gewiss wäre es vom höchsten interesse für die entwickelung, welche hoffentlich auch dieser zweig der beobachtenden astronomie zukünftig erhalten wird, wenn die herschel'schen beobachtungen in der ausführlichkeit in welcher sie, verschiedenen andeutungen zufolge, _handschriftlich_ vorhanden sind, veröffentlicht würden. es schliesst sich dieser wunsch in betreff der nebelflecken lebhaft an den an, welcher, schon vor einem jahrzehnt nach veröffentlichung der noch unedirten _star-gauges_ von gewichtigerer seite her geäussert wurde." in this all must agree who have a knowledge of the direction in which we must look for advances in the difficult and important questions of the distance, the motions, and the changes of the nebulæ. almost the only aid to be looked for from the older observations must come from such diagrams, and we may safely say that the publication of this priceless material, just as it stands, would carry our exact data back from to , or no less than forty-seven years. [ ] long after herschel had abandoned this idea, it continued current among astronomers. the successes of lord rosse's telescope perpetuated to the middle of the nineteenth century an erroneous view which herschel had given up in . [ ] these have never been re-observed. they should be sought for with a powerful refractor, taking special precautions against the illumination of the field of view from neighboring bright stars. herschel's reflectors were specially open to illusions produced in this way. his observations probably will remain untested until some large telescope is used in the way he adopted, _i. e._, in sweeping. bibliography. i.--list of the published writings of william herschel on astronomical subjects. [in chronological order.] _n.b.--in general, translations and abstracts of those which appeared in periodicals are not noticed here. i have made exceptions in the more important cases._ [solution of a prize question. _see_ this book, page .] _ladies' diary_, . astronomical observations on the periodical star in _collo ceti_. _phil. trans._, , p. . astronomical observations relating to the mountains of the moon. _phil. trans._, , p. . astronomical observations on the rotation of the planets round their axes, made with a view to determine whether the earth's diurnal motion is perfectly equable. _phil. trans._, , p. . account of a comet. [dated th march, . this was _uranus_.] _phil. trans._, , p. . on the parallax of the fixed stars. _phil. trans._, , p. . catalogue of double stars. _phil. trans._, , p. : translation in _bode's jahrbuch_, , p. . description of a lamp micrometer and the method of using it. _phil. trans._, , p. . a paper to obviate some doubts concerning the great magnifying powers used. _phil. trans._, , p. . a letter from william herschel, esq., f.r.s., to sir joseph banks, _bart._, p.r.s. _phil. trans._, , p. . aus einem schreiben des hrn. herschel an mich [bode], datirt london, den ten august, . [this is a letter forwarding herschel's memoir on the parallax of the fixed stars, etc.] _bode's jahrbuch_, , p. . on the diameter and magnitude of the _georgium sidus_, with a description of the dark and lucid disk and periphery micrometers. _phil. trans._, , p. . on the proper motion of the sun and solar system, with an account of several changes that have happened among the fixed stars since the time of mr. flamsteed. _phil. trans._, , p. . _bode's jahrbuch_, , p. , p. . astronomische nachrichten und entdeckungen, aus einem französischen schreiben desselben an mich [bode], datirt datchet, nahe bey windsor, den . mai, . [this letter is on the subject of the use of high magnifying powers, and gives a _résumé_ of his recent papers.] _bode's jahrbuch_, , p. . on the remarkable appearances at the polar regions of the planet _mars_, the inclination of its axis, the position of its poles and its spheroidical figure; with a few hints relating to its real diameter and atmosphere. _phil. trans._, , p. . account of some observations tending to investigate the construction of the heavens. _phil. trans._, , p. . [_bode's jahrbuch_, , p. , has a summary of this paper by baron von zach. see, also, _bode's jahrbuch_, , p. .] catalogue of double stars. _phil. trans._, , p. . on the construction of the heavens. _phil. trans._, , p. . _bode's jahrbuch_, , p. . see, also, _same_, , p. , and , p. . aus einem schreiben des hrn. herschel an mich [bode], datirt clay hall, nahe bey windsor, den . jul., . [this is a letter forwarding two memoirs, and giving the prices of telescopes] _bode's jahrbuch_, , p. . catalogue of one thousand new nebulæ and clusters of stars. _phil. trans._, , p. . _bode's jahrbuch_, , p. , and _same_, , p. . investigation of the cause of that indistinctness of vision which has been ascribed to the smallness of the optic pencil. _phil. trans._, , p. . remarks on the new comet [ , ii.]. _phil. trans._, , p. . [letter from herschel to bode on the discovery of two satellites to _uranus_, dated slough, , feb. .] _bode's jahrbuch_, , p. . an account of the discovery of two satellites revolving round the _georgian planet_. _phil. trans._, , p. . _bode's jahrbuch_, , p. . an account of three volcanoes in the moon. _phil. trans._, , p. . _bode's jahrbuch_, , p. . note on m. mÉchain's comet. [ , i.] [added to preceding paper.] _phil. trans._, , p. . on the _georgian planet_ and its satellites. _phil. trans._, , p. . _bode's jahrbuch_, , p. . observations on a comet [ , ii.]. _phil. trans._, , p. . catalogue of a second thousand of new nebulæ and clusters of stars, with a few introductory remarks on the construction of the heavens. _phil. trans._, , p. . _bode's jahrbuch_, , p. . also, _same_, , p. . account of the discovery of a sixth and seventh satellite of the planet _saturn_, with remarks on the construction of its ring, its atmosphere, its rotation on an axis, and its spheroidical figure. _phil. trans._, , p. . _bode's jahrbuch_, , p. ; _same_, , p. ; , p. . on the satellites of the planet _saturn_, and the rotation of its ring on an axis. _phil. trans._, , p. . on nebulous stars properly so called. _phil. trans._, , p. . _bode's jahrbuch_, , p. . on the ring of _saturn_ and the rotation of the fifth satellite upon its axis. _phil. trans._, , p. . _bode's jahrbuch_, , p. . miscellaneous observations. [account of a comet], p. [ , i.]. [on the periodical appearance of omicron ceti], p. . [on the disappearance of the th _herculis_], p. . [remarkable phenomenon in an eclipse of the moon], p. . _phil. trans._, , p. . observations on the planet _venus_. _phil. trans._, , p. . observations of a quintuple belt on the planet _saturn_. _phil. trans._, , p. . _bode's jahrbuch_, , p. . account of some particulars observed during the late eclipse of the sun. [ , september th.] _phil. trans._, , p. . on the rotation of the planet _saturn_ upon its axis. _phil. trans._, , p. . _bode's jahrbuch_, , p. . on the nature and construction of the sun and fixed stars. _phil. trans._, , p. . _bode's jahrbuch_, ii. suppl. band, p. . description of a forty-foot reflecting telescope. _phil. trans._, , p. . _bode's jahrbuch_, iii. suppl. band, p. . additional observations on the comet. [ , i.] _phil. trans._, , p. . on the method of observing the changes that happen to the fixed stars; with some remarks on the stability of the light of our sun. to which is added a catalogue of comparative brightness for ascertaining the permanency of the lustre of stars. _phil. trans._, , p. . _bode's jahrbuch_, , p. . on the periodical star _[alpha] herculis_; with remarks tending to establish the rotatory motion of the stars on their axes; to which is added a second catalogue of the comparative brightness of the stars. _phil. trans._, , p. . _bode's jahrbuch_, , p. . a third catalogue of the comparative brightness of the stars, with an introductory account of an index to mr. flamsteed's observations of the fixed stars, contained in the second volume of the historia coelestis. to which are added several useful results derived from that index. _phil. trans._, , p. . _bode's jahrbuch_, , p. . observations of the changeable brightness of the satellites of _jupiter_, and of the variation in their apparent magnitudes, with a determination of the time of their rotatory motions on their axes. to which is added a measure of the diameter of the second satellite, and an estimate of the comparative size of all the four. _phil. trans._, , p. . _bode's jahrbuch_, , p. . on the discovery of four additional satellites of the _georgium sidus_. the retrograde motion of its old satellites announced, and the cause of their disappearance at certain distances from the planet explained. _phil. trans._, , p. . _bode's jahrbuch_, , p. . a fourth catalogue of the comparative brightness of the stars. _phil. trans._, , p. . _bode's jahrbuch_, , p. . on the power of penetrating into space by telescopes, with a comparative determination of the extent of that power in natural vision, and in telescopes of various sizes and constructions, illustrated by select observations. _phil. trans._, , pp. - . _bode's jahrbuch_, , p. . investigation of the powers of the prismatic colors to heat and illuminate objects, with remarks that prove the different refrangibility of radiant heat. to which is added an inquiry into the method of viewing the sun advantageously with telescopes of large apertures and high magnifying powers. _phil. trans._, , pp. - . _bode's jahrbuch_, , p. . experiments on the refrangibility of the invisible rays of the sun. _phil. trans._, , pp. - . _bode's jahrbuch_, , p. . experiments on the solar and on the terrestrial rays that occasion heat, with a comparative view of the laws by which light and heat, or rather the rays that occasion them, are subject, in order to determine whether they are the same or different. _phil. trans._, , pp. - , - . _gilbert annal._, x. ( ), pp. - ; _same_, xii. ( ), pp. - . observations tending to investigate the nature of the sun, in order to find the causes or symptoms of its variable emission of light and heat, with remarks on the use that may possibly be drawn from solar observations. _phil. trans._, , pp. - . _bode's jahrbuch_, , p. , and , p. . ueber den nebelfleck der sten classe des herschel'schen verzeichniss, und ueber _ceres_ und _pallas_, vom herrn doctor herschel, aus zwey briefen desselben. _bode's jahrbuch_, , p. . additional observations tending to investigate the symptoms of the variable emission of the light and heat of the sun, with trials to set aside darkening glasses by transmitting the solar rays through liquids, and a few remarks to remove objections that might be made against some of the arguments contained in the former paper. _phil. trans._, , pp. - . observations on the two lately discovered celestial bodies [_ceres and pallas_]. _phil. trans._, , pp. - . _nicholson journal_, iv. ( ), pp. - , - . catalogue of five hundred new nebulæ, nebulous stars, planetary nebulæ, and clusters of stars, with remarks on the construction of the heavens. _phil. trans._, , pp. - . _bode's jahrbuch_, , p. . observations of the transit of _mercury_ over the sun's disk, to which is added an investigation of the causes which often prevent the proper action of mirrors. _phil. trans._, , pp. - . account of the changes which have happened during the last twenty-five years in the relative situation of double stars, with an investigation of the cause to which they are owing. _phil. trans._, , pp. - . _bode's jahrbuch_, , pp. - . continuation of the account of the changes that have happened in the relative situation of double stars. _phil. trans._, , pp. - . _bode's jahrbuch_, , p. . aus einem schreiben des herrn doctor herschel, datirt slough, bey windsor, den . may, . [relates to his theory of the relation between the solar radiation and the price of wheat.] _bode's jahrbuch_, , p. . experiments for ascertaining how far telescopes will enable us to determine very small angles, and to distinguish the real from the spurious diameters of celestial and terrestrial objects, with an application of the results of those experiments to a series of observations on the nature and magnitude of mr. harding's lately discovered star [_juno_ ( ),]. _phil. trans._, , pp. - . on the direction and velocity of the motion of the sun and solar system. _phil. trans._, , pp. - . _bode's jahrbuch_, iv. suppl. band, p. . observations on the singular figure of the planet _saturn_. _phil. trans._, , pp. - . _bode's jahrbuch_, , p. . on the quantity and velocity of solar motion. _phil. trans._, , pp. - . _bode's jahrbuch_, , p. . observations and remarks on the figure, climate, and atmosphere of _saturn_ and its ring. _phil. trans._, , pp. - . _gilbert annal._, xxxiv. ( ), pp. - . _bode's jahrbuch_, , p. . experiments for investigating the cause of the colored concentric rings discovered by sir i. newton between two object-glasses laid one upon another. _phil. trans._, , pp. - . _annal. de chimie_, lxx., , pp. - , - ; _same_, lxxi., , pp. - . observations on the nature of the new celestial body [_vesta_] discovered by dr. olbers, and of the comet which was expected to appear last january in its return from the sun. [ , ii.] _phil. trans._, , pp. - . observations of a comet [ , i.] made with a view to investigate its magnitude and the nature of its illumination, to which is added an account of a new irregularity lately perceived in the apparent figure of the planet _saturn_. _phil. trans._, , pp. - . _gilbert annal._, xxxvi. ( ), pp. - . _zach, monat. corresp._, xx. ( ), pp. - . continuation of experiments for investigating the cause of colored concentric rings and other appearances of a similar nature. _phil. trans._, , pp. - . supplement to the first and second part of the paper of experiments for investigating the cause of colored concentric rings between object-glasses, and other appearances of a similar nature. _phil. trans._, , pp. - . _gilbert annal._, xlvi., , pp. - . astronomical observations relating to the construction of the heavens, arranged for the purpose of a critical examination, the result of which appears to throw some new light upon the organization of the celestial bodies. _phil. trans._, , pp. - . _journ. de phys._, lxxv., , pp. - . observations of a comet, with remarks on the construction of its different parts [ , i.]. _phil. trans._, , pp. - . _journ. de phys._, lxxvii., , pp. - . _zach, monat. corresp._, xxviii., , pp. - , - . _bode's jahrbuch_, , p. . observations of a second comet, with remarks on its construction [ , ii.]. _phil. trans._, , pp. - . _nicholson journ._, xxxv., , pp. - . _bode's jahrbuch_, , p. . astronomical observations relating to the sidereal part of the heavens, and its connection with the nebulous part, arranged for the purpose of a critical examination. _phil. trans._, . pp. - . _bode's jahrbuch_, , pp. - . a series of observations of the satellites of the _georgian planet_, including a passage through the node of their orbits, with an introductory account of the telescopic apparatus that has been used on this occasion, and a final exposition of some calculated particulars deduced from the observations. _phil. trans._, , pp. - . _bode's jahrbuch_, , p. - . astronomical observations and experiments tending to investigate the local arrangement of the celestial bodies in space, and to determine the extent and condition of the milky way. _phil. trans._, , pp. - . _bode's jahrbuch_, , p. . astronomical observations and experiments selected for the purpose of ascertaining the relative distances of clusters of stars, and of investigating how far the power of our telescopes may be expected to reach into space, when directed to ambiguous celestial objects. _phil. trans._, , pp. - . on the places of one hundred and forty-five new double stars ( ). _mem. roy. ast. soc._, , , pp. - . ii.--list of works relating to the life and writings of william herschel. [arranged alphabetically by authors.] _n.b.--in general, the notices of his life to be found in encyclopædias of biography, etc., are not included here._ arago (f.) analyse de la vie et des travaux de sir william herschel [from _annuaire du bureau des longitudes_, ]. paris, . mo. [see also the _annuaire_ for , for an account of herschel's work on double stars.] arago (f.) biographies of distinguished scientific men. translated by admiral w. h. smyth, rev. b. powell, and robert grant, esq. herschel. first series, p. . boston, . vo. arago (f.) herschel. [translated from the french.] smithsonian report. . p. . vo. auwers (a.) william herschel's verzeichnisse von nebelflecken und sternhaufen bearbeitet von a. auwers. from the _königsberg observations_. . folio. bessel (f. w.) sir william herschel. [from the _königsberger allgemeine zeitung_, , , no. , _et seq._, reprinted in his] _abhandlungen_, vol iii., p. . leipzig, . to. d'arrest (h. l.) verzeichniss von sir william herschel's nebelflecken _erster_ und _vierter_ classe, aus den beobachtungen berechnet und auf reducirt. _abhandlungen der math. phys. classe der k. sächs gesells. d. wissenschaften_, band iii. [ ], p. . dunkin (e.) obituary notices of astronomers, p. . sir william herschel, k.c.h., f.r.s., - . london, . mo. fÉtis (f. j.) biographie universelle des musiciens [article herschel]. paris, - . vo. forbes (j. d.) sir william herschel [being § of dissertation vi.]. encyclopædia britannica, eighth edition. vol. i.,_dissertations_, p. . fourier (j.) Éloge historique de sir william herschel, prononcé dans la séance publique de l'académie royale des sciences le juin, . _historie de l'académie royale des sciences de l'institut de france_, _tome_ vi., _année_ , p. lxi. harding (c. l.) des herrn dr. herschel's untersuchungen über die natur der sonnenstrahlen, aus dem englischen übersetzt. erstes heft. [translations from _phil. trans._, .] celle, . mo. hastings (c. s.) see holden _and_ hastings. herschel (carolina.) an account of a new comet. [ , ii.] _phil. trans._, , vol. lxxvii., p. . herschel (carolina.) an account of the discovery of a comet. [ , i.] _phil. trans._, , vol. lxxxiv., p. . herschel (carolina.) account of the discovery of a comet. [ , ii.] _phil. trans._, , vol. lxxxvi., p. . herschel (carolina.) catalogue of stars taken from flamsteed's observations contained in the second volume of his _historia coelestis_, and not inserted in the british catalogue; to which is added a collection of errata which should be noticed in the same volume; with remarks by w. herschel. london, . folio. herschel (carolina.) verzeichniss von sternen flamsteeds von denen keine beobachtungen in der _hist. coel. brit._ vorkommen. _bode's jahrbuch_, , p. [herschel (carolina.)] [notice of her life.] _monthly notices roy. ast. soc._, vol. , p. ; _also_, _memoirs roy. ast. soc._, vol. , p. . [herschel (carolina.)] memoir and correspondence of caroline herschel. by mrs. john herschel. with portraits. london, . mo. herschel (j. f. w.) article _telescope_, in encyclopædia britannica, eighth edition. [this article (illustrated) gives most of the important features of sir william herschel's manner of grinding and polishing specula.] herschel (j. f. w.) catalogue of nebulæ and clusters of stars. [general and systematic reduction of all sir w. herschel's observations brought into connection with all other similar ones.] _phil. trans._, . page . to. herschel (j. f. w.) a synopsis of all sir william herschel's micrometrical measurements, etc., of double stars, together with a catalogue of those stars . . . for . _mem. roy. ast. soc._, vol. , p. . lond., . to. herschel (j. f. w.) additional identifications of double stars in the synoptic catalogues of sir william herschel's micrometrical measurements, etc. _monthly notices roy. ast. soc._, vol. , p. . london, . vo. herschel (mrs. john.) memoir and correspondence of caroline herschel. with portraits. london, . mo. herschel (w.) [solution of a prize question. _see_ this book, page .] _ladies' diary_, . herschel (w.) the favorite eccho catch . . . and the preceding glee [by s. leach]. to which is added the . . . catch sung by three old women . . . in the pantomime called "the genius of nonsense" [by h. harington]. london, (?). obl. folio. [a ms. copy of this was kindly furnished me by dr. r. garnett, of the british museum.] herschel (w.) _göttingen magazin der wissenschaften und literatur_ ( ), vol. iii., p. . lichtenberg and forster, editors. [letter from herschel, giving a brief account of his life. _see_ this book, page .] herschel (w.) i. _manuscripts in possession of the royal society._ . a series of register sheets in which are entered up _all_ the observations of _each_ nebula, copied _verbatim_ from the sweeps. . a similar set of register sheets for messier's nebulæ. . a general index of the , nebulæ of w. herschel; given the class and number, to find the general number. . an index list; given the general number, to find the class and number. . a more complete list like . . a manuscript catalogue of all the nebulæ and clusters, reduced to , , and arranged in zones of ° in polar distance; by miss carolina herschel. . the original sweeps with the -foot reflector at slough, in three small to and four folio vols. of ms. ii. _manuscripts in possession of the royal astronomical society._ this library contains "the whole series of autograph observations of each double star [observed by herschel], brought together on separate sheets by sir william herschel and miss carolina herschel." [herschel (w.)] some account of the life and writings of william herschel, esq. [with a portrait.] the _european magazine and london review_ for january, . vo. [herschel (w.)] _edinburgh review_, vol. i., p, . [a review of herschel's memoir, "observations on the two lately discovered bodies," from _phil. trans._, . _see_ this book, page .] [herschel (w.)] "sir william herschel, from a london paper." [this is a short obituary notice "furnished by a gentleman well acquainted with sir william and his family, and its accuracy may be relied on."] _niles' register_, vol. , p. . nov. , . vo. [herschel (w.)] obituary: sir william herschel, knt., ll.d., f.r.s. _the gentleman's magazine and historical chronicle_, vol. xcii., , p. . vo. [herschel (w.)] _annual register_, , p. . vo. [herschel (w.)] w. herschel's sämmtliche schriften, erster band. ueber den bau des himmels. mit kupfertafeln. [edited by j. w. pfaff. a second edition was published in .] dresden and leipzig, . vo. [herschel (w.)] _new york mirror_, vol. vi., - , p. . [herschel (w.)] _living age_, vol. ii., p. ( ). vo. [reprinted from _chambers' journal_.] [herschel (w.)] _foreign quarterly review_, vol. , p. . vo. [review of arago's "analyse de la vie et des travaux de sir william herschel."] [herschel (w.)] arago's life of herschel. _eclectic museum_, vol. ii., p. . [reprinted from the _foreign quarterly review_, vol. .] holden (e. s.) on the inner satellites of _uranus_. [reduction of sir william herschel's observations.] _proceedings amer. assn. adv. science_, _august_, , p. . vo. holden (e. s.) index catalogue of books and memoirs relative to nebulæ, clusters, etc. _smithsonian miscellaneous collections_, no. , pp. - . [abstracts of sir william herschel's memoirs (on nebulæ) in the _philosophical transactions_.] washington, . vo. holden (e. s.) and c. s. hastings. a subject-index and a synopsis of the scientific writings of sir william herschel. [reprinted from the _report_ of the smithsonian institution ( ).] washington, . vo. krafft (j. g. f.) kurze nachricht von dem berühmten astronomen herschel und einigen seiner entdeckungen. bayreuth, . vo. peirce (c. s.) photometric researches. [a reduction of herschel's observations on the comparative brightness of the stars.] _annals harvard college observatory_, vol. ix. leipzig, . to. sommer (g. m.) william herschel . . . ueber den bau des himmels; drei abhandlungen aus dem englischen uebersetzt, nebst einem authentischen auszug aus kants allgemeiner naturgeschichte und theorie des himmels. koenigsberg, . vo. struve (w.) Études d'astronomie stellaire. sur la voie lactée et sur la distance des étoiles fixes. [p. _et seq._ contains an elaborate review of the construction of the heavens according to herschel.] st. petersburg, . vo. wolf (r.) william herschel. zurich, . vo. zach (f. von.) dr. william herschel [translated from _public characters_ and printed in zach's _monatlich correspondenz_, , part i., p. _et seq._] iii.--list of the published portraits of william herschel. _artist_, mme. dupiery. _engraver_, thÖnert. vo. early portrait. some copies in red. profile. _artist_, f. rehburg. _engraver_, f. w. bollinger. vo. late portrait. _artist_, ----? _engraver_, c. westermayr. vo. medallion. _artist_, c. brand. _engraver_, ----? vo. lithograph. _artist_, ----? _engraver_, j. sewell. vo. profile, . _artist_, ----? _engraver_, ----? vo. profile. _artist_, f. bonneville. _engraver_, f. bonneville. vo. profile. _artist_, j. russell, r.a. _engraver_, e. scriven. vo. engraved from a crayon in the possession of his son, and published by the s. d. u. k. in the _gallery of portraits_, vol. . _artist_, ----? _engraver_, ----? vo. _european magazine_, jan., . this is a bust in profile, showing the left side of the face. _artist_, ----? _engraver_, thomson. vo. published by caxton, . this must have been engraved before since the legend is william herschel, ll.d., f.r.s. _artist_, lady gordon. from the painting by abbott in the national portrait gallery. _engraver_, joseph brown. vo. published in memoir of caroline herschel. this is of the date , or thereabouts. _see_ frontispiece. _artist_, ----? _engraver_, c. mÜller. to. medallion, (?). _artist_, ----? _engraver_, h. pinhas. to. legend in russian. _artist_, baisch. _engraver_, ----? to. lithograph. _artist_, h. grÉvedon. _engraver_, ----? fol. lithograph. _artist_, ----? _engraver_, f. mÜller. fol. _artist_, abbott. _engraver_, ryder. fol. . _artist_, j. boilly. _engraver_, ----? fol. . lithograph. _artist_, ----? _engraver_, j. godby. fol. r. w. s. lutwidge, esq., f.r.a.s., has an original seal with a head of sir william herschel, which is shown on the title-page of this work. a cut of it has been courteously furnished me by john browning, esq., f.r.a.s., etc. in a bust of herschel was made by lockie for sir william watson. a picture of herschel was painted by mr. artaud about the beginning of . a portrait of herschel by abbott is in the national portrait gallery, london. there are no doubt many other paintings in england, though i can find notices of these only. the royal society of london has nearly a hundred portraits of its most distinguished members, but owns none of sir william herschel. index of names. _n.b.--this index is intended to refer to the proper names occurring in the body of the work only, and not to the bibliography._ airy (sir george), . alison (sir archibald), . arago (françois), , , . artaud (m.), . aubert (alexander), , , . baldwin (miss), . banks (sir joseph), , , . bessel (f. w.), , . blagden (dr.), . bonaparte (napoleon), , . bradley (james), , . bruhl (count von), . bulman (mr.), , , . bunsen (chevalier), . burney (dr.), , , , , , . campbell (thomas), . cassini (j. d.), . cropley (mr.), . dalrymple (mr.), . d'arblay (madame), _et seq._ darlington (earl of), . darquier, , . d'arrest (h. l.), . dawes (w. r.), . de luc (m.), . de luc (mrs.), , . dollond (j.), . engelfield (sir harry), . farinelli (miss), . flaugergues (h.), , . fleming (miss), . fourier (j.), . frauenhofer (j.), . galileo, , . george iii., , , , . griesbach (george), . griesbachs (the), , . hansen (p. a.), . hastings (c. s.), . heberden (dr.), . helmholtz (h.), . herschel (abraham) [ - ], . herschel (alexander) [ - ], , , , , , , , , , , , , , . herschel (benjamin), . herschel (carolina) [ - ], , , , , , , , , , , ; discovers five comets, , , , , , , , , , . herschel (carolina), her _memoir_ quoted, , , , , , , , , , , , , , , , . herschel (dietrich), , , , , , . herschel (eusebius), . herschel (hans) [_circa_ ], . herschel (isaac) [ - ], , , . herschel (jacob) [ - ], , , , , . herschel (sir john frederick william) [ - ], , , , , , , , . herschel (lady), , , , , . herschel (major john), . herschel (mrs. mary cornwallis), . herschel (sophia elizabeth), b. , married griesbach, . herschel (william), born , november ; . oboist in the band of the guards ( ), and goes to england for the first time, returning in , . deserts from the guards and goes to england ( ), . organizes the band of the durham militia ( ), . leaves the band and lives with dr. miller, . leads the public concerts at wakefield and halifax, . organist at halifax ( ), . organist of the octagon chapel at bath ( ), . his musical writings, . studies smith's harmonies and optics, . visits hanover, august, , . hires a small telescope, . makes his first telescope ( ), . visits hanover ( ?), . st review of the heavens, , . d review of the heavens, . d review of the heavens ( ), . th review of the heavens ( ), . manufacture of telescopes, , , , , , , , , , , , . moves to new king st., bath, . conducts oratorios of handel, . begins astronomical _measures_ ( ), . joins philosophical society of bath, . first published scientific writing ( ), . first communication to the royal society ( ), . discovery of _uranus_ ( , march ), . its effect on his career, . elected a member of the royal society ( ), and receives the copley medal, . attempts a thirty-foot reflector, . goes to london, (may, june, july), . appointed royal astronomer (£ ), , . removes to datchet, , august , . his assiduity, , , , . his mechanical genius, , , . cost of his telescopes, . marries mrs. john pitt, _née_ baldwin ( ), . only child born ( ), . removes to slough ( ), . ll.d. (_oxon._), , . his account of the discovery of _uranus_ ( , march ), . discovers two satellites to _uranus_, , jan. , . discovers two satellites to _saturn_, , august-september, . invents machines for making reflectors ( ), . began forty-foot telescope, , finished it, , . biographical letter ( ), . list of published portraits of him, . value of his sister's assistance to him, . letters to carolina herschel, , , , . his personal character ( - ), _et seq._ his relations to his cotemporaries, , , , , , , , , , . list of writings relating to him and to his works, . his literary skill, . examples of his style, _et seq._ failure of health, _et seq._ created a knight of the royal hanoverian guelphic order ( ), . herschel (sir william), first president of the royal astronomical society ( ), . his will, . his death, august th, , . his epitaph, . list of his scientific writings, . review of his scientific labors, . the improvement of telescopes and apparatus, . the relative brightness of the stars; variable stars, . researches on double stars, . researches on planets and satellites, . researches on the nature of the sun, , , . the motion of the solar system in space, . researches on the construction of the heavens, . scale of celestial measures; distances of the stars, . researches on light, heat, etc., . researches on the dimensions of the stars, . on the spectra of the fixed stars, . on the variable emission of light and heat from the sun, . researches on nebulæ and clusters, . huyghens (c.), , . king george iii., , , , . lacaille (n. l.), , . lalande (jerome), , , . lambert (j. h.), . lassell (w.), . lee (miss), . lichtenberg (herr), . lind (dr. and mrs.), . long (dr.), . magellan (herr), , . maskelyne (nevil), , , , , , , . mayer (christian), , . melloni (m.), . messier (c.), . michell (john), , , , . miller (dr.), , , , . monson (mrs.), . moritzen (anna ilse), m. isaac herschel, , . napoleon i., , . newton (sir isaac), , , , . olbers (william), . pabrich (cappelmeister), . palmerston (lord), . piazzi (joseph), , . pierce (charles s.), . pigott (j.), . poggendorff (j. g.), . ronzoni (signor), . rosse (lord), , . savary (m.), . secchi (angelo), , . scheiner (c.), . schroeter (j. h.), , , , , . schwabe (h.), . short (james), . smith (dr. robert), . snetzler (herr), , . struve (otto von), . thomson (thomas), . verdet (e.), . wainwright (dr.), . walsh (colonel), . watson (sir william), , , , , , , , , . watt (james), . weld (r.), . wilson (alexander), , , . zach (baron von), . the end. start of transcriber's notes: i have used [alpha] to represent the greek letter used in the text. page line in the original text, left as is. übersetzt uebersetzt biographie universelle des musiciens biographie universelle des musiciens koenigsberg, . vo. königsberger allgemeine zeitung fraunhofer index frauenhofer (j.), . vol. (all other occurances are) vol. index ronzoni (signor), . ronzini original page line original text replaced with ornamented. ornamented." [c] for letter c reversed ci[c]i[c]cccxxii den . may den . mai suppl. band., p. . suppl. band, p. . end of transcriber's notes: none half-hours with the stars a plain and easy guide to the knowledge of the constellations showing, in twelve maps, the position for the united states of the principal star groups night after night throughout the year, with introduction and a separate explanation of each map. true for every year maps and text specially prepared for american students by richard a. proctor, f.r.a.s. author of "half hours with the telescope," "easy star lessons," "a larger star atlas," and the article on astronomy in the "american cyclopÆdia" and the "cyclopÆdia brittanica." etc., etc. "here i may sit and rightly spell of every star that heav'n doth show."--milton. the heavens declare the glory of god; and the firmament showeth his handiwork.--psalms xix: . g. p. putnam's sons new york and london the knickerbocker press introduction on the use of the maps. it is very easy to gain a knowledge of the stars, if the learner sets to work in the proper manner. but he commonly meets with a difficulty at the outset of his task. he provides himself with a set of the ordinary star-maps, and then finds himself at a loss how to make use of them. such maps tell him nothing of the position of the constellations _on the sky_. if he happen to recognize a constellation, then indeed his maps, if properly constructed, will tell him the names of the stars forming the constellation, and also he may be able to recognize a few of the neighboring constellations. but when he has done this he may meet with a new difficulty, even as respects this very constellation. for if he look for it again some months later, he will neither find it in its former place nor will it present the same aspect,--if indeed it happen to be above the horizon at all. it is clear, then, that what the learner wants is a set of maps specially constructed to show him in what part of the sky the constellations are to be looked for. he ought on any night of the year to be able to turn at once to the proper map, and in that map he ought to see at once what to look for, toward what point of the compass each visible constellation lies, and how high it is above the horizon. and, if possible (as the present work shows is the case), _one_ map ought to suffice to exhibit the aspect of the whole heavens, in order that the beginner may not be confused by turning from map to map, and trying to find out how each fits in with the others. it is to fulfil these requirements that the present maps have been constructed. each exhibits the aspect of the whole sky at a given day and hour. the circumference of the map represents the natural horizon, the middle of the map representing the part of the sky which lies immediately overhead. if the learner hold one of these maps over his head, so as to look vertically upwards at it, the different parts of the horizon marked in round the circumference being turned towards the proper compass points, he will see the same view of the heavens as he would if he were to lie on his back and look upwards at the sky, only that the map is a planisphere and the sky a hemisphere. but although this illustration serves to indicate the nature of the maps, the actual mode of using them is more convenient. let it first be noted that properly speaking the maps have neither top, bottom, nor sides. each map may be held with any part of the circumference downward: then the centre of the map is to be looked upon as the top for that part of the circumference. the portion of the map lying beneath the centre represents the portion of the sky lying between the point overhead and a certain part of the horizon--the part in fact corresponding to the particular part of the circumference which is turned downwards. thus if on any night we wish to learn what are the stars towards the north, we look for the map corresponding to that night. at the hour named the stars toward the north will be those shown between the centre of the map and the top; and, of course, we hold the map upside down so as to bring the centre above the northern part of the circumference. but this matter will be more clearly understood by comparing the account of any of the accompanying maps with the map itself. again, it must be noted that, although the maps are necessarily arranged in a certain order, there is in reality no first or last in the series. the map numbered i. follows the map numbered xii. in exactly the same manner that the latter follows the map numbered xi. the maps form a circular series, in fact. the only reason for numbering the maps as at present, is that the map numbered i. happens to exhibit the aspect of the sky at a convenient hour on the night of january st. it will be found that the dates follow on with intervals of seven or eight days right round the year, the end of the year falling in the left-hand column of the table under map i., while the beginning of the year is in the right-hand column of the same map.[*] [footnote *: it may be mentioned in passing, that the dates have not been thrown in so as to fall regularly round the year, but correspond with the variations due to the earth's variable motion round the sun.] it will be seen at once that a map can always be found corresponding to a convenient hour on any night of the year. (in midsummer, on a few of the dates mentioned under the maps, night has not begun at the hour named.) on any date named under a map, the aspect of the sky two hours later than that named is that represented in the following map. thus at eight o'clock in the evening of january d, the aspect of the stars is as shown in map i. at ten o'clock on the same night the aspect of the sky is that shown in map ii., as a date under that map shows. applying this rule to the few occasions on which the hour named is not available for observation (five or six in all out of ninety-six dates), the observer can manage as well for those occasions as for any others. next, as to finding the north point, or any point of the compass which will enable the observer to determine the rest. if he is only familiar with the aspect of those seven bright stars of the great bear which have been called the dipper, charles' wain, (really "the churl's wain,") the butcher's cleaver, and by other names, he can always determine the north point by means of the two stars called the pointers, since these seven stars never set. in the explanation of each map i have shown where the great bear is to be looked for on each night, the observer being assumed to have such a general knowledge of the direction of the compass-points, as will suffice for the purpose of finding so marked a collection of stars. thus the pole-star is found, and for the purpose of such observations as are here considered, this star may be looked upon as marking the exact direction of the north. perhaps nothing further is required; but if the observer prefer it he can determine the north point conveniently _at noon_ by setting up a vertical stick in the sunlight and noting the direction in which the shadow lies. once the observation has been made, he can note what objects (these should be distant) lie towards the different points of the compass, and from that time he can use the accompanying maps without any reference to the great bear and the pointers. it is worth noticing that the stars called the guardians of the pole form no bad time-piece when used with the aid of such maps as the present. they revolve round the pole once in twenty-four hours (less about four minutes), in a direction contrary to that of a clock's hands. but stars near the equator, whose motions are much more rapid, afford a yet better measure of time, if the direction of the south point is well determined. of course, the observer who really wishes to become an astronomer will not rest satisfied by learning only the principal stars shown in these maps. by means of the regular star-maps, such as those of my school star atlas, he will be able to explore the depths of all the constellations, having once learned their position and general appearance from the accompanying maps. it will be well for the student to remember that the planets venus, mars, jupiter, and saturn will at times appear among the constellations here shown. venus and jupiter can always be recognized by their superior light, mars and saturn by the steadiness with which they shine. the almanac will always show when these planets and mercury (often very bright in the clear skies of america) are above the horizon, and where they are situate. they never appear except among the zodiacal constellations. for particulars and pictures of the different constellations, and other details associated with the study of the star-groupings, the reader is referred to my "easy star lessons," published like the present maps by messrs. putnam's sons. i have to thank the proprietors of the _scientific american_ for permission to publish these maps, which originally appeared (though in a slightly different form) in the pages of that excellent magazine. the latin names of the constellations included in the maps of this series are as follows: the little bear, ursa minor (a, the _pole star_; b, g, _the guardians_). the dragon, draco (a, _thuban_). king cepheus, cepheus. the lady in the chair, cassiopeia. the champion, perseus (b, _algol_, remarkable variable). the charioteer, auriga (a, _capella_). the greater bear, ursa major (a, b, the _pointers_). the hunting dogs, canes venatici (a, _cor caroli_). queen berenice's hair, coma berenices. the herdsman, boÖtes (a, _arcturus_). the northern crown, corona borealis. the serpent, serpens. the kneeler, hercules. the lyre, lyra (a, _vega_). the swan, cygnus (a, _arided_; b, _albireo_). the winged horse, pegasus. the chained lady, andromeda. the triangles, triangula. the ram, aries. the bull, taurus (a, _aldebaran_; ae, _alcyone_, the chief _pleiad_). the twins, gemini (a, _castor_; b, _pollux_). the crab, cancer (the cluster between g and d is the _beehive_). the lion, leo (a, _regulus_). the virgin, virgo (a, _spica_). the scales, libra. the serpent-holder, ophiuchus. the eagle, aquila (a, _altair_). the dolphin, delphinus. the water carrier, aquarius. the fishes, pisces. the sea monster, cetus (o, _mira_, remarkable variable) the river, eridanus. the giant hunter, orion (a, _betelgeux_; b, _rigel_). the lesser dog, canis minor (a, _procyon_). the sea serpent, hydra (a, _alphard_). the cup, crater (a, _alkes_). the crow, corvus. the scorpion, scorpio (a, _antares_). the archer, sagittarius. the sea-goat, capricornus. the southern fish, piscis australis (a, _fomalhaut_). the hare, lepus. the dove, columba. the greater dog, canis major, (a, _sirius_). the ship, argo. the centaur, centaurus. the following table exhibits the names of all the stars of the first three magnitudes to which astronomers have given names; at least, all those whose names are in common use: a andromedæ, _alpheratz_ b ----, _mirach, mizar_. g ----, _almach_. a aquarii, _sadalmelik_ b ----, _sadalsund_ g ----, _skat_ a aquilæ, _altair_ b ----, _alshain_ g ----, _tarazed_ a arietis, _hamal_ b ----, _sheratan_ g ----, _mesartim_ a aurigæ, _capella_ b ----, _menkalinan_ a boötis, _arcturus_ b ----, _nekkar_ e ----, _izar, mizar, mirach_ ae ----, _muphrid_ a canum ven., _cor caroli_ a canis majoris, _sirius_ b ----, _mirzam_ e ----, _adara_ a canis minoris, _procyon_ b ----, _gomeisa_ a capricorni, _secunda giedi_ d ----, _deneb algiedi_ a cassiopeiæ, _schedar_ b ----, _chaph_ a cephei, _alderamin_ b ----, _alphirk_ g ----, _errai_ a ceti, _menkar_ b ----, _diphda_ z ceti, _baten kaitos_ o ----, _mira_ a columbæ, _phact_ a coronæ bor, _alphecca_ a corvi, _alchiba_ d ----, _algores_ a crateris, _alkes_ a cygni, _arided, deneb adige_ b ----, _albireo_ a draconis, _thuban_ b ----, _alwaid_ g ----, _etanin_ b eridani, _cursa_ g ----, _zaurac_ a geminorum, _castor_. b ----, _pollux_ g ----, _alhena_ d ----, _wesat_ e ----, _mebsuta_ a herculis, _ras algethi_ b ----, _korneforos_ a hydræ, _al fard, cor hydroe_ a leonis, _regulus, cor leonis_ b ----, _deneb aleet, denebola, deneb_ g ----, _algeiba_ d ----, _zosma_ a leporis, _arneb_ a libræ, _zuben el genubi_ b ----, _zuben el chamali_ g ----, _zuben hakrabi_ a lyræ, _vega_ b ----, _sheliak_ g ----, _salaphat_ a ophiuchi, _ras alhague_ b ----, _cebalrai_ a orionis, _betelgeux_ b ----, _rigel_ g ----, _bellatrix_ d ----, _mintaka_ e ----, _alnilam_ a pegasi, _markab_ b ----, _scheat_ g ----, _algenib_ e ----, _enif_ z ----, _homan_ a persei, _mirfak_ b ----, _algol_ a piscis aust., _fomalhaut_ e sagittarii, _kaus australis_ a scorpionis, _antares, cor scorpionis_ a serpentis, _unukalhai_ a tauri, _aldebaran_ b ----, _nath_ ae ----, _alcyone_ (pleiad) a ursæ majoris, _dubhe_ b ----, _merak_ g ----, _phecda_ e ----, _alioth_ z ----, _mizar_ ae ----, _alkaid, benetnasch_ i ----, _talitha_ a ursa minoris, _polaris_ b ----, _kochab_ a virginis, _spica azimech, spica_ b ----, _zavijava_ e ----, _vindemiatrix_ [illustration: map i. night sky.--december and january. at o'clock: dec. . | |at o'clock: jan. . at - / o'clock: dec. .|at - / o'clock:|at - / o'clock: jan. . at o'clock: dec. . | dec. . |at o'clock: jan. . stars of the first magnitude are eight-pointed; second magnitude, six-pointed; third magnitude, five-pointed; fourth magnitude (a few), four-pointed; fifth magnitude (very few), three-pointed. for star names refer to page .] night sky.--december and january. the great bear (_ursa major_) is now rising well above the horizon, in the northeast, the pointers about midway between north and northeast. a line from the pole star to the guardians of the pole is now in the position of the minute hand of a clock about minutes past an hour. the dragon (_draco_) lies due north, curving round under the little bear, its head close to the horizon. low down in the northwest is a part of the swan (_cygnus_). higher up we see king cepheus, his wife _cassiopeia_, and their daughter _andromeda_ (the seated lady and chained lady, respectively), with the rescuer (_perseus_) nearly overhead. the winged horse is setting, his head close by the western horizon, and near the jar of the water bearer (_aquarius_). in the southwest is the whale; and close by, the constellation _pisces_, or the fishes; above them the ram (_aries_), between which and _andromeda_ the triangles can be seen. in the south the river (_eridanus_) makes now its best show. its leading brilliant, _achernar_, is, however, never seen in the united states. in the southeast the great dog, with the splendid sirius ("which brightliest shines when laved of ocean's wave"), shows resplendently. above is orion now standing upright, treading on the hare (_lepus_) and facing the bull (_taurus_), now at its highest. the dove (_columba_) below the hare is a modern and not very interesting constellation. the little dog (_canis minor_) is on the east of orion. in the east the sea serpent (_hydra_) is rising, and due east a little higher we find _cancer_, the crab, (note the pretty cluster called the beehive (_proesepe_)); above are the twins (_gemini_), and above them the charioteer (_auriga_), with the bright _capella_, nearly overhead. the lion is rising in the northeast, his heart star _regulus_ (a) being low down a little north of east. lastly, due north, high up, the absurd giraffe (_camelopardus_) stands proudly on his ridiculous head. night sky.--january and february. the great bear (_ursa major_) with its dipper and pointers, occupies the northeasterly mid-heaven. a line from the pole star (a of the little bear, _ursa minor_) to the guardians, b and g, lies in the position of the minute hand of a clock minutes after an hour. the camelopard (_camelopardus_) is above. the dragon (_draco_), whose head is below the horizon, curves round the little bear to between the guardians and the pointers. in the northwest, fairly high up, we find _cassiopeia_, the seated lady, and on her right, lower down, the inconspicuous constellation _cephius_. _andromeda_, the chained lady, is on _cassiopeia's_ left. the great nebula will be noticed in the map--it is faintly visible to the naked eye. above _andromeda_ is _perseus_, the rescuing knight, and above him the charioteer (_auriga_), nearly overhead. on the left of _andromeda_ is _aries_, the ram, the small constellation the triangles lying between them. toward the southwest, the whale (_cetus_) is beginning to set. the river (_eridanus_) occupies the lower part of the southwesterly sky, and extends also to the mid-heavens in that direction. the dove (_columba_) is nearly due south, and at its best--which is not saying much. above is the hare (_lepus_), on which _orion_ treads. the giant now presents his noblest aspect--prince of all the constellations as he is. he faces the bull (_taurus_), known by the _pleiades_ and the bright _aldebaran_. close by the poor hare, on the left, leaps _canis major_, the greater dog, with the bright sirius, which "bickers into green and emerald." the stern of the star ship (_argo_) is nearing the south. very high in the southeast we find the twins (_gemini_), with the twin stars, _castor_ and _pollux_ (a and b); and below them the little dog (_canis minor_). the sea serpent (_hydra_) is rearing its tall neck above the eastern horizon (by south), as if aiming either for the little dog or for the crab (_cancer_), now high up in the east, with its pretty beehive cluster showing well in clear weather. the lion (_leo_) is due east, the sickle (marked by the stars a, ae, g, m, and e) being easily recognized. queen berenice's hair (_coma berenices_, not _berenicis_, as often ignorantly given) is in the northeast. it used to mark the tip of the real lion's tail, just as the stars of the crab marked his head. the hunting dogs occupy the space between berenice's hair and the great bear. [illustration: map ii. night sky.--january and february. at o'clock: jan. . | |at o'clock: feb. . at - / o'clock: jan. .|at - / o'clock:|at - / o'clock: feb. . at o'clock: jan. . | jan. . |at o'clock: jan. . stars of the first magnitude are eight-pointed; second magnitude, six-pointed; third magnitude, five-pointed; fourth magnitude (a few), four-pointed; fifth magnitude (very few), three-pointed. for star names refer to page .] [illustration: map iii. night sky.--february and march. at o'clock: feb. . | |at o'clock: mar. . at - / o'clock: feb. .|at - / o'clock:|at - / o'clock: mar. . at o'clock: feb. . | mar. . |at o'clock: mar. . stars of the first magnitude are eight-pointed; second magnitude, six-pointed; third magnitude, five-pointed; fourth magnitude (a few), four-pointed; fifth magnitude (very few), three-pointed. for star names refer to page .] night sky.--february and march. the great bear (_ursa major_), with its dipper and pointers, is now high up in the northeastern sky. the pointers direct us to the pole star, (a of the little bear _ursa minor_). a line from the pole star to the guardians of the pole (b and g) lies in the position of the minute hand of a clock minutes after an hour. the dragon (_draco_) extends from between the bears to the horizon--east of north--where its head with its two bright eyes can be seen. _cepheus_ is low down, somewhat to the west of north; his queen (_cassiopeia_) the seated lady, beside him (a and b mark the top rail of her chair's back); while above her lies the poor constellation _camelopardus_, the giraffe. _andromeda_, the chained lady, is in the northwest, low down--in fact, partly set; the triangles and the ram (_aries_) beside her, toward the west. above them is _perseus_, the rescuing knight; and above him, somewhat to the west, the charioteer (_auriga_). the bull (_taurus_), with the _pleiades_ and the bright _aldebaran_, is in the mid-heaven, due west; _gemini_, the twins, higher, and toward the southwest. _orion_, below them, is already slanting toward "his grave, low down in the west"; beneath him the hare, and in the southwest a part of the river (_eridanus_). due south is a part of the star ship (_argo_), beside which, low down, is the foolish dove (_columba_), while above leaps the great dog (_canis major_), with the splendid _sirius_, chief of all the stars in the sky, marking his mouth. high up, a little west of north, is the little dog (_canis minor_); and higher, a little east of north, the crab (_cancer_), the "dark constellation," as it was called of old, with the pretty cluster _proesepe_, or the beehive. the sea serpent (_hydra_) is rearing his long neck high above the horizon, bearing on his back, absurdly enough, noah's cup (_crater_) and noah's raven or crow (_corvus_). nearly due east, the virgin (_virgo_) has risen, spica shining brightly just above the horizon. the lion (_leo_) occupies the mid-space above; the "sickle in the lion"--its handle marked by ae and a, its curved blade by g, m, and e--will at once be recognized. the hair of queen berenice (_coma berenices_) is nearly due east, and fairly high. between this small but remarkable group and the great bear, lies hevelius's foolish constellation, the hunting dogs (_canes venatici_). lastly, in the northeast, the herdsman (_boötes_), with the orange-yellow brilliant, arcturus, is rising, though at present, paradoxical as it may seem, he lies on his back. night sky.--march and april. the great bear (_ursa major_) is now nearing the point overhead, the pointers (a and b) aiming almost directly downward toward the pole star. the line from this star (a of the little bear, _ursa minor_) to the guardians (b and g) is now in the position of the minute hand of a clock about minutes after an hour. _cepheus_ lies north, low down, _cassiopeia_ on his left, the camelopard above her, _andromeda_ just setting, almost due northwest, on the left. _perseus_ is due northwest, rather low, the charioteer (_auriga_) on his left, but higher. setting between west and northwest we see the bull (_taurus_), with the _pleiades_ and the ruddy _aldebaran_. _orion_ is almost prone in his descent toward his western grave. the twins (_gemini_) are due west, in the mid-heavens; the little dog (_canis minor_) beside them on their left, the crab (_cancer_) above, the greater dog (_canis major_) below, chasing the hare (_lepus_) below the horizon. just behind the dog the poop of the great ship (_argo_) is also setting. the sea serpent (_hydra_) now shows his full length, rearing his head high in the south. observe the darkness of the region around his heart, marked by the star a, _alfard_, the solitary one. the cup (_crater_) and crow (_corvus_) stand on his back. the sickle in the lion (_leo_) now stands with handle upright, due south. below the tail stars of the lion we see the virgin (_virgo_), with the bright _spica azimech_. the set of five third magnitude stars, above, was called by the arabs, for reasons not explained, the "retreat of the howling she dog." behind the lion, due east and high up, we see _coma berenices_, the hair of queen berenice, between which and the tail of the great bear we see in the chart one star only of the hunting dogs (_canes venatici_). the herdsman (_boötes_), still on his back, pursues in that striking and effective position the great bear. below the shoulder stars of the herdsman we see the crown (_corona borealis_), near which, on the right, low down and due east, the head of the serpent (_serpens_) is rising. _hercules_ is also rising, but in the northeast. lastly, the stars of the dragon (_draco_) can be seen curving from between the pointers and the pole, round the little bear, then back toward _hercules_, the head of the dragon, with the bright eyes, b and g, being rather low down, and somewhat north of northeast. [illustration: map iv. night sky.--march and april. at o'clock: mar. . | |at o'clock: apr. . at - / o'clock: mar. .|at - / o'clock:|at - / o'clock: apr. . at o'clock: mar. . | mar. . |at o'clock: apr. . stars of the first magnitude are eight-pointed; second magnitude, six-pointed; third magnitude, five-pointed; fourth magnitude (a few), four-pointed; fifth magnitude (very few), three-pointed. for star names refer to page .] [illustration: map v. night sky.--april and may. at o'clock: apr. . | |at o'clock: may . at - / o'clock: apr. .|at - / o'clock:|at - / o'clock: may . at o'clock: apr. . | apr. . |at o'clock: may. . stars of the first magnitude are eight-pointed; second magnitude, six-pointed; third magnitude, five-pointed; fourth magnitude (a few), four-pointed; fifth magnitude (very few), three-pointed. for star names refer to page .] night sky.--april and may. the great bear (_ursa major_) is now at its highest and nearly overhead, the pointers aiming downward from high up, slightly west of due north. a line from the pole star, (a of the little bear, _ursa minor_) to the guardians of the pole, (b and g) is now in the position of the minute hand of a clock minutes after an hour. below the little bear we find _cepheus_ low down to the east of north, and _cassiopeia_ low down to the west of north. _perseus_, the rescuer, is setting in the northwest; the camelopard is above, trying to get on his feet. the charioteer (_auriga_), with the bright _capella_, is nearing the northwestern horizon, followed by the twins (_gemini_), in the west. further west and higher we find the crab (_cancer_), below which is the little dog (_canis minor_). the southwestern sky is very barren of bright stars. _alfard_, the heart of the sea serpent, _hydra_, shines here alone in a great blank space. above the sea serpent's head we see the sickle in the lion, _leo_ himself stretching his tail to due south, very high up. _coma berenices_ is close by, and the hunting dogs (_canes venatici_) between _coma_ and the great bear. in the south, lower down, we find the crow (_corvus_), and the cup (_crater_), on the serpent's back; the virgin (_virgo_), extending in the mid-heavens from southeast to south, between the lion's tail and the crow. in the same direction, but low down, we find the head and body of the centaur (_centaurus_), supposed to have typified the patriarchal noah. in the southeast the scorpion's heart has just risen, and between the head of _scorpio_ and the virgin's robes we see the stars of the scales (_libra_). due east, low down, is the serpent-holder (_ophiuchus_), on his back--it is the customary attitude of heavenly bodies when rising. the serpent (_serpens_) held by him is seen curving upward toward the crown (_corona borealis_). the serpent's head is due west, and above it we see the bright arcturus, chief brilliant of the herdsman (_boötes_). in the northeast is _hercules_, his head close to the head of the serpent-holder. beneath his feet is the lyre (_lyra_) with the brilliant _vega_; and the swan (_cygnus_) has already half risen above the northeastern horizon. lastly, the dragon (_draco_) curves from between the pointers and the pole, round the guardians toward _cepheus_, and then retorts its head, with gleaming eyes (b and g), toward the heel of _hercules_. night sky.--may and june. the great bear (_ursa major_) occupies all the upper sky from the west to north, except a small space occupied by the hunting dogs (_canes venatici_). the pointers are in the northwest, almost horizontal. a line from the pole star (a of the little bear--_ursa minor_) to the guardians of the pole (b and g) now occupies the position of the minute hand of a clock minutes past an hour. due north, low down, lies _cassiopeia_, while above, somewhat toward the east, we find the inconspicuous constellation _cepheus_. the camelopard is in the west of north, and getting upright. low down in the northwest lie the charioteer (_auriga_), and the head stars of the twins (_gemini_) further west. the crab (_cancer_) is nearly due west, the sea serpent (_hydra_) holding his head almost exactly to the west point. above is the sickle in the lion, its blade curved downward, and the tail of the lion (_leo_) lies above, toward the south of west. on the serpent's back we find the cup (_crater_) and the crow (_corvus_), in the southwest and to the south of southwest respectively. above these constellations, and extending beyond the south toward the east, the virgin (_virgo_) occupies the mid-heavens. above the virgin we see the herdsman (_boötes_), his head and shoulders nearly overhead. low down in the south is the centaur (_centaurus_), bearing on his spear the wolf (_lupus_) as an offering for the altar (_ara_), which, however, is invisible in these latitudes. above the wolf we see the scales (_libra_), while the scorpion (_scorpio_), one of the few constellations which can at once be recognized by its shape, is rising balefully in the southeast. the serpent bearer (_ophiuchus_) bears the serpent (_serpens_) in the mid-heavens toward the southeast, the crown (_corona borealis_) being high up in the east, close by the serpent's head. low down in the east is the eagle (_aquila_), with the fine steel-blue star _altair_, the swan on the left about northeast, and above it the lyre (_lyra_), with the still more brilliant steel-blue star _vega_. hercules occupies the space between the lyre on the one side and the crown and the serpent's head on the other. he is high up, due east. lastly, the dragon winds from between the pointers and the pole round the little bear, toward cepheus, and then eastward toward the feet of hercules, close by which we see his head and gleaming eyes (b and g). [illustration: map vi. night sky.--may and june. at o'clock: may . | |at o'clock: june . at - / o'clock: may .|at - / o'clock: |at - / o'clock: june . at o'clock: may . | may . |at o'clock: june . stars of the first magnitude are eight-pointed; second magnitude, six-pointed; third magnitude, five-pointed; fourth magnitude (a few), four-pointed; fifth magnitude (very few), three-pointed. for star names refer to page .] [illustration: map vii. night sky.--june and july. at o'clock: june . | |at o'clock: july . at - / o'clock: june .|at - / o'clock:|at - / o'clock: july . at o'clock: june . | june . |at o'clock: july . stars of the first magnitude are eight-pointed; second magnitude, six-pointed; third magnitude, five-pointed; fourth magnitude (a few), four-pointed; fifth magnitude (very few), three-pointed. for star names refer to page .] night sky.--june and july. the great bear (_ursa major_) is in the mid-heavens toward the northwest, the pointers not far from the horizontal position. they direct us to the pole star (a of the little bear, _ursa minor_). the line from this star to the guardians of the pole, b and g, is in about the position of the minute hand of a clock minutes before an hour. the dragon (_draco_) curls over the little bear, curving upward on the east, to where its head, high up in the northeast, is marked by the gleaming eyes, b and g. under the little bear, the camelopard has at last come upright. low down in the west the lion (_leo_) is setting. the point of the "sickle in the lion" is turned toward the horizon; the handle (marked by a and ae) is nearly horizontal. above the lion's tail is berenice's hair (_coma berenices_); and between that and the great bear's tail our chart shows a solitary star of the hunting dogs (_canes venatici_). the crow (_corvus_) is low down in the southwest, the cup (_crater_) beside it, partly set, on the right. above is _virgo_, the virgin. still higher in the southwest--in fact, with head close to the point overhead--is the herdsman (_boötes_), the crown (_corona borealis_) near his southern shoulder marking what was once the herdsman's uplifted arm. low down between the south and southwest we find the head and shoulders of the centaur (_centaurus_), who holds the wolf (_lupus_) due south. above the wolf are the scales (_libra_), and above these the serpent (_serpens_), his head in the south, stretching toward the crown. in the mid-sky, toward the southeast, we find the serpent bearer (_ophiuchus_--one star of the serpent lies east of him). below the serpent bearer we find the scorpion (_scorpio_), now fully risen, and showing truly scorpionic form. beside the scorpion is the archer (_sagittarius_), low down in the southeast. to his left we see, low down, two stars marking the head of the sea goat (_capricornus_), and one belonging to the water bearer (_aquarius_). above the sea goat flies the eagle (_aquila_), with the bright star _altair_; and above, near the point overhead, is the kneeling _hercules_. due east, we see part of the winged horse (_pegasus_); above that, the little dolphin (_delphinus_), and higher, the swan (_cygnus_) and the lyre (_lyra_), with the beautiful bluish-white star _vega_. lastly, low down, between north and northeast, we find the seated lady (_cassiopeia_); and above, somewhat eastwardly, the inconspicuous constellation _cepheus, cassiopeia's_ royal husband. night sky.--july and august. the great bear (_ursa major_) is now in the northwest, his paws near the horizon. the pointers (a and b) direct us to the pole star, (a of the little bear, _ursa minor_). a line from the pole star to the guardians of the pole is in the position of the minute hand of a clock about minutes before an hour. below the little bear we see the camelopard, a little to the east of due north. the dragon (_draco_) curves round from between the pointers and the pole, above the little bear toward the east, then upward to near the point overhead, its head, with the bright stars b and g, being highest. low down in the west we see berenice's hair (_coma berenices_), and one star of the hunting dogs (_canes venatici_) is seen in the chart between _coma_ and the great bear. the herdsman {_boötes_) occupies the mid-heaven in the west, the crown (_corona borealis_) higher up, and due west, hercules, between the crown and the point overhead. low down, extending from the west to near the southwest, we find the virgin (_virgo_), the bright _spica_ near its setting place. in the southwest are the scales (_libra_), and farther to the left, extending from the scales to low down near the south, we find the scorpion (_scorpio_), one of the finest of the constellations, _antares_, the rival of mars (as the name means), marking its heart. above the scorpion and the scales are the serpent bearer (_serpentarius_ or _ophiuchus_) and the serpent (_serpens_), extending right across him to near the crown, after which the serpent seems reaching. a little east due south, low down, we find the archer (_sagittarius_); in the southeast, low down, the sea goat (_capricornus_); and farther east, and lower down, the water bearer (_aquarius_). above the sea goat is the eagle (_aquila_), with the bright bluish-white star _altair_; on its left the pretty little dolphin (_delphinus_), and above the dolphin, nearly overhead, the lyre (_lyra_), with the bluish-white star _vega_ (even brighter than _altair_) nearly overhead. below the lyre we see the swan (_cygnus_), due east; and below the swan the winged horse (_pegasus_), upside down, as usual. in the northeast, _andromeda_, the chained lady, is rising, her head marked by the star a (which was also called d of _pegasus_). (the "square of pegasus" is formed by a of _andromeda_ and a, b, and g of _pegasus_.) between the north and northeast is _cassiopeia_, the seated lady, and above her, her husband, king _cepheus_. and lastly _perseus_ is just rising, between the north and northeast. [illustration: map viii. night sky.--july and august. at o'clock: july . | |at o'clock: aug. . at - / o'clock: july .|at - / o'clock:|at - / o'clock: aug. . at o'clock: july . | july . |at o'clock: aug. . stars of the first magnitude are eight-pointed; second magnitude, six-pointed; third magnitude, five-pointed; fourth magnitude (a few), four-pointed; fifth magnitude (very few), three-pointed. for star names refer to page .] [illustration: map ix. night sky.--august and september. at o'clock: aug. . | |at o'clock: sept. . at - / o'clock: aug. .|at - / o'clock:|at - / o'clock: sept. . at o'clock: aug. . | aug. . |at o'clock: sept. . stars of the first magnitude are eight-pointed; second magnitude, six-pointed; third magnitude, five-pointed; fourth magnitude (a few), four-pointed; fifth magnitude (very few), three-pointed. for star names refer to page .] night sky.--august and september. the great bear (_ursa major_) is low down, between northwest and north, the pointers (a and b) directed slantingly upward toward the pole. a line from the pole star (a of the little bear, _ursa minor_) to the guardians of the pole (b and g), is in the position of the minute hand of a clock minutes before an hour. between the great bear and the little bear run the stars of the dragon (_draco_), round the little bear toward the north, thence toward the northwest, where we see the head of the dragon high up, its two bright eyes, b and g, directed toward _hercules_, which occupies the western mid-heaven. above hercules is _lyra_, the lyre, with the bright steel-blue star vega high up toward the point overhead. right overhead is the swan (_cygnus_). low down in the northwest we see in the chart one star of the hunting dogs (_canes venatici_). nearer the west stands the herdsman, rather slanting forward, however, with the crown (_corona borealis_) on his left, almost due west. the long winding serpent (_serpens_) runs from near the crown (where we see its head due west) to farther south than southwest, high up on the western side of the serpent holder (_serpentarius_ or _ophiuchus_), now standing upright in the southwest. low down creeps the scorpion (_scorpio_), its heart antares, rival of mars, in the southwest, the end of its tail between south and southwest. above and south of the scorpion's tail we see the archer (_sagittarius_). due south, and high up, is the eagle (_aquila_), its tail at z and e, its head at th, the bright steel-blue altair marking its body. on the left, or east, of the eagle lies the neat little dolphin (_delphinus_). midway between the dolphin and the horizon is the tip of the tail of the sea goat (_capricornus_), whose head lies nearly due south. on the southern horizon is the head of the indian (_indus_); on its left a part of the crane (_grus_), and low down in the southeast lies fomalhaut, the chief brilliant of the southern fish (_piscis australis_). above lies the water bearer (_aquarius_), in the southwestern mid-heaven. due east, fairly high, is "the square of pegasus," the head of the winged horse, pegasus lying close by the water pitcher of aquarius (marked by the stars z, g, and a). the fishes (_pisces_) are low down in the east. a few stars of the whale (_cetus_) are seen on their right, very low down. on the left of pisces we see the ram (_aries_), low down; above it the triangle; and above that the chained lady (_andromeda_). low down in the northeast is the rescuing knight (_perseus_); above whom is _cassiopeia_, and on her left, higher up, the inconspicuous constellation _cepheus_. lastly, immediately below _cepheus_, we find the camelopard, below which, very low down, between north and northeast, is the charioteer (_auriga_), the brilliant capella being just above the horizon. night sky.--september and october. low down, between north and northwest, we find the seven stars of the dipper, the pointers on the right nearly due north. they direct us to the pole star. the guardians of the pole (b and g of the little bear, _ursa minor_) lie in a direction from the pole star corresponding to that of the minute hand of a clock about minutes before an hour. between the pointers and the pole star we find the tip of the dragon's tail: then passing round the little bear with the dragon's long train of third magnitude stars, we come, after a bend, to the dragon's head, with the two bright eyes, a and b--(part of the dragon's nose has been borrowed by hercules). these two stars are almost exactly midway between the horizon and the point overhead, and nearly northwest. king cepheus--not a very conspicuous constellation--lies between the point overhead and the little bear. low down in the northwest we find the head of the herdsman (_boötes_). the crown (_corona borealis_), which no one can mistake, lies on his left; and close by is the setting head of the serpent. above these three groups we see hercules--the kneeler--his head at a, his upraised club by g. above the head of hercules we find the lyre, with the bright star vega; and above that the swan. passing southward, we see the serpent-holder (_serpentarius_ or _ophiuchus_), beyond whom lies the serpent's tail; a most inconvenient arrangement, as the serpent is divided into two parts. almost exactly southeast, and low down, are the stars of the archer (_sagittarius_); while above, in the mid-sky, we see the eagle (_aquila_), with the bright altair. note the neat little constellation the dolphin (_delphinus_), close by. due south is the crane (_grus_); above it the southern fish, with the bright star fomalhaut. above that the sea goat (_capricornus_), and on the left of this the water bearer (_aquarius_); one can recognize his water pitcher, marked by the stars b, g, and a. toward the west, high up, is the winged horse (_pegasus_); he is upside down just now. below lies the whale (_cetus_), or rather the sea monster. i have my own notion about cetus, regarding him as an icthyosaurus: but that is neither here nor there. the star o of this constellation is called mira; it is a wonderful variable star. the fishes (_pisces_) may be seen between the whale and pegasus. few constellations have suffered more than pisces by the breaking up of star groups. the fishes themselves are now lost in andromeda and pegasus. note how on the left of pisces the ram (_aries_) "bears aloft" andromeda, the chained lady (whose head lies at a), as milton set aries doing long since. the triangles serve only as a saddle. between andromeda and her father, cepheus, we find her mother, cassiopeia, or rather cassiopeia's chair. (of course b, a, and g mark the chair's back.) perseus, the rescuer, lies below; b is the famous variable _algol_. below him lies the bull (_taurus_), with the pleiades and the bright aldebaran. low down to the left of the bull, we find the charioteer (_auriga_), with the bright capella. and lastly, anyone who likes may admire the camelopard (_camelopardalis_), between the great bear, cepheus, and the charioteer. [illustration: map x. night sky.--september and october. at o'clock: sept. . | |at o'clock: oct. . at - / o'clock: sept. .|at - / o'clock:|at - / o'clock: oct. . at o'clock: sept. . | sept. . |at o'clock: oct. . stars of the first magnitude are eight-pointed; second magnitude, six-pointed; third magnitude, five-pointed; fourth magnitude (a few), four-pointed; fifth magnitude (very few), three-pointed. for star names refer to page .] [illustration: map xi. night sky.--october and november. at o'clock: oct. . | |at o'clock: nov. . at - / o'clock: oct. .|at - / o'clock:|at - / o'clock: nov. . at o'clock: oct. . | oct. . |at o'clock: nov. . stars of the first magnitude are eight-pointed; second magnitude, six-pointed; third magnitude, five-pointed; fourth magnitude (a few), four-pointed; fifth magnitude (very few), three-pointed. for star names refer to page .] night sky.--october and november. the dipper lies low, the pointers a little east of north. they direct to the pole star. the guardians of the pole (b and g of the little bear, _ursa minor_) lie in a direction from the pole star corresponding to that of the minute hand of a clock about minutes before an hour. between the pointers and pole star lies the tip of the dragon's tail. sweeping around the little bear (_ursa minor_) we find the stars of the dragon (_draco_) curving back by the star d to the dragon's head, with the two bright eyes, g and b. above is the inconspicuous constellation cepheus; and somewhat higher, the stars of cassiopeia, a and b, marking the top rail of the seated lady's chair. low down in the northwest hercules is setting. above is the lyre, with the bright steel-blue vega; and above that the stars of the swan (_cygnus_), which has sometimes been called the northern cross. nearly due west we find the eagle (_aquila_), z and e marking its tail, th the head. above the eagle is the pretty little constellation _delphinus_, the dolphin. in the southwest, rather low, is the sea goat (_capricornus_); above and to the south of him the water bearer (_aquarius_), with his pitcher, marked by the stars, a, g, and z. the head of the winged horse, _pegasus_, now upside down (in fact, he is seldom otherwise), is just above this group. the "square of pegasus" will be noticed high up, due south. the star a of andromeda, one of the corners of this square, used to be also called d of pegasus. much attention need not be directed to the phoenix, low in the southern horizon. the river _eridanus_ is coming well into view; and the great sea monster (_cetus_) now shows finely, his head at a and g, his paddles at z and t. the fishes (_pisces_) are above; the ram (_aries_) above them and eastward, lying toward the southeast; then the triangle (_triangula_, or the triangles, according to modern maps), and the chained lady (_andromeda_) too nearly overhead to be very pleasantly observed. the great nebula in which the new star recently appeared is near the point overhead. the grand giant orion is rising in the east; above him the bull (_taurus_) with the pleiades. low down in the northeast the twins (_gemini_) are rising; above is the charioteer (_auriga_), and above him the rescuing knight (_perseus_), "of fair-haired danae born." the camelopard is hardly worth noticing, except as marking a barren region of the heavens. night sky.--november and december. the great bear (_ursa major_) is beginning to rise above the northeast (by north) horizon. the end of the dipper's handle is hidden. a line from the pole star (toward which the pointers direct the observer) to the guardians of the pole (b and g of the little bear, _ursa minor_), is now in the position of the minute hand of a clock minutes before an hour. the stars of the dragon wind round below the little bear toward the west, the head of the dragon with the gleaming eyes ("oblique retorted that askant cast gleaming fire") being low down, a little north of northwest. above is king cepheus, and above him his queen, the seated lady (_cassiopeia_); their daughter, the chained lady (_andromeda_) being nearly overhead. low down in the northwest we see the lyre (_lyra_), with the bright vega; and close by toward the west the swan (_cygnus_), or northern cross. the eagle is setting in the west, and the little dolphin nears the western horizon. toward the southwest (by west) we see the water bearer (_aquarius_), with his pitcher (b, g, a), close by which is the head of the winged horse (_pegasus_). in the south, low down, is the absurd phoenix; above, the sea monster, or whale (_cetus_); above him, the fishes (_pisces_); above them, the ram (_aries_); while nearly overhead lies the triangle, in reality the triangles (_triangula_). the river (_eridanus_) occupies the southeasterly sky. the dove and great dog (_columba_ and _canis major_) are rising in the southeast. the glorious _orion_ has now come well into view, though not yet so upright as we could wish a knightly hunter to be. he treads on the hare (_lepus_), and faces the bull (_taurus_) above. due east we find the crab (_cancer_), and little dog (_canis minor_) low down; the twins (_gemini_) higher; above them the charioteer (_auriga_), with the bright _capella_, and _perseus_ the rescuer nearing the point overhead. in the mid-space between _perseus, auriga_, and the two bears, we find the ridiculous constellation _camelopardus_, or the giraffe. [illustration: map xii. night sky.--november and december. at o'clock: nov. . | |at o'clock: dec. . at - / o'clock: nov. .|at - / o'clock:|at - / o'clock: dec. . at o'clock: nov. . | nov. . |at o'clock: dec. . stars of the first magnitude are eight-pointed; second magnitude, six-pointed; third magnitude, five-pointed; fourth magnitude (a few), four-pointed; fifth magnitude (very few), three-pointed. for star names refer to page .] side-lights on astronomy and kindred fields of popular science essays and addresses by simon newcomb contents preface i. the unsolved problems of astronomy ii. the new problems of the universe iii. the structure of the universe iv. the extent of the universe v. making and using a telescope vi. what the astronomers are doing vii. life in the universe viii. how the planets are weighed ix. the mariner's compass x. the fairyland of geometry xi. the organization of scientific research xii. can we make it rain? xiii. the astronomical ephemeris and nautical almanac xiv. the world's debt to astronomy xv. an astronomical friendship xvi. the evolution of the scientific investigator xvii. the evolution of astronomical knowledge xviii. aspects of american astronomy xix. the universe as an organism xx. the relation of scientific method to social progress xxi. the outlook for the flying-machine illustrations simon newcomb photograph of the corona of the sun, taken in tripoli during total eclipse of august , . a typical star cluster-centauri the glass disk the optician's tool the optician's tool grinding a large lens image of candle-flame in object-glass testing adjustment of object-glass a very primitive mounting for a telescope the huyghenian eye-piece section of the primitive mounting spectral images of stars, the upper line showing how they appear with the eye-piece pushed in, the lower with the eye-piece drawn out the great refractor of the national observatory at washington the "broken-backed comet-seeker" nebula in orion dip of the magnetic needle in various latitudes star spectra professor langley's air-ship preface in preparing and issuing this collection of essays and addresses, the author has yielded to what he could not but regard as the too flattering judgment of the publishers. having done this, it became incumbent to do what he could to justify their good opinion by revising the material and bringing it up to date. interest rather than unity of thought has determined the selection. a prominent theme in the collection is that of the structure, extent, and duration of the universe. here some repetition of ideas was found unavoidable, in a case where what is substantially a single theme has been treated in the various forms which it assumed in the light of constantly growing knowledge. if the critical reader finds this a defect, the author can plead in extenuation only the difficulty of avoiding it under the circumstances. although mainly astronomical, a number of discussions relating to general scientific subjects have been included. acknowledgment is due to the proprietors of the various periodicals from the pages of which most of the essays have been taken. besides harper's magazine and the north american review, these include mcclure's magazine, from which were taken the articles "the unsolved problems of astronomy" and "how the planets are weighed." "the structure of the universe" appeared in the international monthly, now the international quarterly; "the outlook for the flying-machine" is mainly from the new york independent, but in part from mcclure's magazine; "the world's debt to astronomy" is from the chautauquan; and "an astronomical friendship" from the atlantic monthly. simon newcomb. washington, june, . i the unsolved problems of astronomy the reader already knows what the solar system is: an immense central body, the sun, with a number of planets revolving round it at various distances. on one of these planets we dwell. vast, indeed, are the distances of the planets when measured by our terrestrial standards. a cannon-ball fired from the earth to celebrate the signing of the declaration of independence, and continuing its course ever since with a velocity of eighteen hundred feet per second, would not yet be half-way to the orbit of neptune, the outer planet. and yet the thousands of stars which stud the heavens are at distances so much greater than that of neptune that our solar system is like a little colony, separated from the rest of the universe by an ocean of void space almost immeasurable in extent. the orbit of the earth round the sun is of such size that a railway train running sixty miles an hour, with never a stop, would take about three hundred and fifty years to cross it. represent this orbit by a lady's finger-ring. then the nearest fixed star will be about a mile and a half away; the next more than two miles; a few more from three to twenty miles; the great body at scores or hundreds of miles. imagine the stars thus scattered from the atlantic to the mississippi, and keep this little finger-ring in mind as the orbit of the earth, and one may have some idea of the extent of the universe. one of the most beautiful stars in the heavens, and one that can be seen most of the year, is a lyrae, or alpha of the lyre, known also as vega. in a spring evening it may be seen in the northeast, in the later summer near the zenith, in the autumn in the northwest. on the scale we have laid down with the earth's orbit as a finger-ring, its distance would be some eight or ten miles. the small stars around it in the same constellation are probably ten, twenty, or fifty times as far. now, the greatest fact which modern science has brought to light is that our whole solar system, including the sun, with all its planets, is on a journey towards the constellation lyra. during our whole lives, in all probability during the whole of human history, we have been flying unceasingly towards this beautiful constellation with a speed to which no motion on earth can compare. the speed has recently been determined with a fair degree of certainty, though not with entire exactness; it is about ten miles a second, and therefore not far from three hundred millions of miles a year. but whatever it may be, it is unceasing and unchanging; for us mortals eternal. we are nearer the constellation by five or six hundred miles every minute we live; we are nearer to it now than we were ten years ago by thousands of millions of miles, and every future generation of our race will be nearer than its predecessor by thousands of millions of miles. when, where, and how, if ever, did this journey begin--when, where, and how, if ever, will it end? this is the greatest of the unsolved problems of astronomy. an astronomer who should watch the heavens for ten thousand years might gather some faint suggestion of an answer, or he might not. all we can do is to seek for some hints by study and comparison with other stars. the stars are suns. to put it in another way, the sun is one of the stars, and rather a small one at that. if the sun is moving in the way i have described, may not the stars also be in motion, each on a journey of its own through the wilderness of space? to this question astronomy gives an affirmative answer. most of the stars nearest to us are found to be in motion, some faster than the sun, some more slowly, and the same is doubtless true of all; only the century of accurate observations at our disposal does not show the motion of the distant ones. a given motion seems slower the more distant the moving body; we have to watch a steamship on the horizon some little time to see that she moves at all. thus it is that the unsolved problem of the motion of our sun is only one branch of a yet more stupendous one: what mean the motions of the stars--how did they begin, and how, if ever, will they end? so far as we can yet see, each star is going straight ahead on its own journey, without regard to its neighbors, if other stars can be so called. is each describing some vast orbit which, though looking like a straight line during the short period of our observation, will really be seen to curve after ten thousand or a hundred thousand years, or will it go straight on forever? if the laws of motion are true for all space and all time, as we are forced to believe, then each moving star will go on in an unbending line forever unless hindered by the attraction of other stars. if they go on thus, they must, after countless years, scatter in all directions, so that the inhabitants of each shall see only a black, starless sky. mathematical science can throw only a few glimmers of light on the questions thus suggested. from what little we know of the masses, distances, and numbers of the stars we see a possibility that the more slow-moving ones may, in long ages, be stopped in their onward courses or brought into orbits of some sort by the attraction of their millions of fellows. but it is hard to admit even this possibility in the case of the swift-moving ones. attraction, varying as the inverse square of the distance, diminishes so rapidly as the distance increases that, at the distances which separate the stars, it is small indeed. we could not, with the most delicate balance that science has yet invented, even show the attraction of the greatest known star. so far as we know, the two swiftest-moving stars are, first, arcturus, and, second, one known in astronomy as groombridge, the latter so called because it was first observed by the astronomer groombridge, and is numbered in his catalogue of stars. if our determinations of the distances of these bodies are to be relied on, the velocity of their motion cannot be much less than two hundred miles a second. they would make the circuit of the earth every two or three minutes. a body massive enough to control this motion would throw a large part of the universe into disorder. thus the problem where these stars came from and where they are going is for us insoluble, and is all the more so from the fact that the swiftly moving stars are moving in different directions and seem to have no connection with each other or with any known star. it must not be supposed that these enormous velocities seem so to us. not one of them, even the greatest, would be visible to the naked eye until after years of watching. on our finger-ring scale, groombridge would be some ten miles and arcturus thirty or forty miles away. either of them would be moving only two or three feet in a year. to the oldest assyrian priests lyra looked much as it does to us to-day. among the bright and well-known stars arcturus has the most rapid apparent motion, yet job himself would not to-day see that its position had changed, unless he had noted it with more exactness than any astronomer of his time. another unsolved problem among the greatest which present themselves to the astronomer is that of the size of the universe of stars. we know that several thousand of these bodies are visible to the naked eye; moderate telescopes show us millions; our giant telescopes of the present time, when used as cameras to photograph the heavens, show a number past count, perhaps one hundred millions. are all these stars only those few which happen to be near us in a universe extending out without end, or do they form a collection of stars outside of which is empty infinite space? in other words, has the universe a boundary? taken in its widest scope this question must always remain unanswered by us mortals because, even if we should discover a boundary within which all the stars and clusters we ever can know are contained, and outside of which is empty space, still we could never prove that this space is empty out to an infinite distance. far outside of what we call the universe might still exist other universes which we can never see. it is a great encouragement to the astronomer that, although he cannot yet set any exact boundary to this universe of ours, he is gathering faint indications that it has a boundary, which his successors not many generations hence may locate so that the astronomer shall include creation itself within his mental grasp. it can be shown mathematically that an infinitely extended system of stars would fill the heavens with a blaze of light like that of the noonday sun. as no such effect is produced, it may be concluded that the universe has a boundary. but this does not enable us to locate the boundary, nor to say how many stars may lie outside the farthest stretches of telescopic vision. yet by patient research we are slowly throwing light on these points and reaching inferences which, not many years ago, would have seemed forever beyond our powers. every one now knows that the milky way, that girdle of light which spans the evening sky, is formed of clouds of stars too minute to be seen by the unaided vision. it seems to form the base on which the universe is built and to bind all the stars into a system. it comprises by far the larger number of stars that the telescope has shown to exist. those we see with the naked eye are almost equally scattered over the sky. but the number which the telescope shows us become more and more condensed in the milky way as telescope power is increased. the number of new stars brought out with our greatest power is vastly greater in the milky way than in the rest of the sky, so that the former contains a great majority of the stars. what is yet more curious, spectroscopic research has shown that a particular kind of stars, those formed of heated gas, are yet more condensed in the central circle of this band; if they were visible to the naked eye, we should see them encircling the heavens as a narrow girdle forming perhaps the base of our whole system of stars. this arrangement of the gaseous or vaporous stars is one of the most singular facts that modern research has brought to light. it seems to show that these particular stars form a system of their own; but how such a thing can be we are still unable to see. the question of the form and extent of the milky way thus becomes the central one of stellar astronomy. sir william herschel began by trying to sound its depths; at one time he thought he had succeeded; but before he died he saw that they were unfathomable with his most powerful telescopes. even today he would be a bold astronomer who would profess to say with certainty whether the smallest stars we can photograph are at the boundary of the system. before we decide this point we must have some idea of the form and distance of the cloudlike masses of stars which form our great celestial girdle. a most curious fact is that our solar system seems to be in the centre of this galactic universe, because the milky way divides the heavens into two equal parts, and seems equally broad at all points. were we looking at such a girdle as this from one side or the other, this appearance would not be presented. but let us not be too bold. perhaps we are the victims of some fallacy, as ptolemy was when he proved, by what looked like sound reasoning, based on undeniable facts, that this earth of ours stood at rest in the centre of the heavens! a related problem, and one which may be of supreme importance to the future of our race, is, what is the source of the heat radiated by the sun and stars? we know that life on the earth is dependent on the heat which the sun sends it. if we were deprived of this heat we should in a few days be enveloped in a frost which would destroy nearly all vegetation, and in a few months neither man nor animal would be alive, unless crouching over fires soon to expire for want of fuel. we also know that, at a time which is geologically recent, the whole of new england was covered with a sheet of ice, hundreds or even thousands of feet thick, above which no mountain but washington raised its head. it is quite possible that a small diminution in the supply of heat sent us by the sun would gradually reproduce the great glacier, and once more make the eastern states like the pole. but the fact is that observations of temperature in various countries for the last two or three hundred years do not show any change in climate which can be attributed to a variation in the amount of heat received from the sun. the acceptance of this theory of the heat of those heavenly bodies which shine by their own light--sun, stars, and nebulae--still leaves open a problem that looks insoluble with our present knowledge. what becomes of the great flood of heat and light which the sun and stars radiate into empty space with a velocity of one hundred and eighty thousand miles a second? only a very small fraction of it can be received by the planets or by other stars, because these are mere points compared with their distance from us. taking the teaching of our science just as it stands, we should say that all this heat continues to move on through infinite space forever. in a few thousand years it reaches the probable confines of our great universe. but we know of no reason why it should stop here. during the hundreds of millions of years since all our stars began to shine, has the first ray of light and heat kept on through space at the rate of one hundred and eighty thousand miles a second, and will it continue to go on for ages to come? if so, think of its distance now, and think of its still going on, to be forever wasted! rather say that the problem, what becomes of it? is as yet unsolved. thus far i have described the greatest of problems; those which we may suppose to concern the inhabitants of millions of worlds revolving round the stars as much as they concern us. let us now come down from the starry heights to this little colony where we live, the solar system. here we have the great advantage of being better able to see what is going on, owing to the comparative nearness of the planets. when we learn that these bodies are like our earth in form, size, and motions, the first question we ask is, could we fly from planet to planet and light on the surface of each, what sort of scenery would meet our eyes? mountain, forest, and field, a dreary waste, or a seething caldron larger than our earth? if solid land there is, would we find on it the homes of intelligent beings, the lairs of wild beasts, or no living thing at all? could we breathe the air, would we choke for breath or be poisoned by the fumes of some noxious gas? to most of these questions science cannot as yet give a positive answer, except in the case of the moon. our satellite is so near us that we can see it has no atmosphere and no water, and therefore cannot be the abode of life like ours. the contrast of its eternal deadness with the active life around us is great indeed. here we have weather of so many kinds that we never tire of talking about it. but on the moon there is no weather at all. on our globe so many things are constantly happening that our thousands of daily journals cannot begin to record them. but on the dreary, rocky wastes of the moon nothing ever happens. so far as we can determine, every stone that lies loose on its surface has lain there through untold ages, unchanged and unmoved. we cannot speak so confidently of the planets. the most powerful telescopes yet made, the most powerful we can ever hope to make, would scarcely shows us mountains, or lakes, rivers, or fields at a distance of fifty millions of miles. much less would they show us any works of man. pointed at the two nearest planets, venus and mars, they whet our curiosity more than they gratify it. especially is this the case with venus. ever since the telescope was invented observers have tried to find the time of rotation of this planet on its axis. some have reached one conclusion, some another, while the wisest have only doubted. the great herschel claimed that the planet was so enveloped in vapor or clouds that no permanent features could be seen on its surface. the best equipped recent observers think they see faint, shadowy patches, which remain the same from day to day, and which show that the planet always presents the same face to the sun, as the moon does to the earth. others do not accept this conclusion as proved, believing that these patches may be nothing more than variations of light, shade, and color caused by the reflection of the sun's light at various angles from different parts of the planet. there is also some mystery about the atmosphere of this planet. when venus passes nearly between us and the sun, her dark hemisphere is turned towards us, her bright one being always towards the sun. but she is not exactly on a line with the sun except on the very rare occasions of a transit across the sun's disk. hence, on ordinary occasions, when she seems very near on a line with the sun, we see a very small part of the illuminated hemisphere, which now presents the form of a very thin crescent like the new moon. and this crescent is supposed to be a little broader than it would be if only half the planet were illuminated, and to encircle rather more than half the planet. now, this is just the effect that would be produced by an atmosphere refracting the sun's light around the edge of the illuminated hemisphere. the difficulty of observations of this kind is such that the conclusion may be open to doubt. what is seen during transits of venus over the sun's disk leads to more certain, but yet very puzzling, conclusions. the writer will describe what he saw at the cape of good hope during the transit of december , . as the dark planet impinged on the bright sun, it of course cut out a round notch from the edge of the sun. at first, when this notch was small, nothing could be seen of the outline of that part of the planet which was outside the sun. but when half the planet was on the sun, the outline of the part still off the sun was marked by a slender arc of light. a curious fact was that this arc did not at first span the whole outline of the planet, but only showed at one or two points. in a few moments another part of the outline appeared, and then another, until, at last, the arc of light extended around the complete outline. all this seems to show that while the planet has an atmosphere, it is not transparent like ours, but is so filled with mist and clouds that the sun is seen through it only as if shining in a fog. not many years ago the planet mars, which is the next one outside of us, was supposed to have a surface like that of our earth. some parts were of a dark greenish gray hue; these were supposed to be seas and oceans. other parts had a bright, warm tint; these were supposed to be the continents. during the last twenty years much has been learned as to how this planet looks, and the details of its surface have been mapped by several observers, using the best telescopes under the most favorable conditions of air and climate. and yet it must be confessed that the result of this labor is not altogether satisfactory. it seems certain that the so-called seas are really land and not water. when it comes to comparing mars with the earth, we cannot be certain of more than a single point of resemblance. this is that during the martian winter a white cap, as of snow, is formed over the pole, which partially melts away during the summer. the conclusion that there are oceans whose evaporation forms clouds which give rise to this snow seems plausible. but the telescope shows no clouds, and nothing to make it certain that there is an atmosphere to sustain them. there is no certainty that the white deposit is what we call snow; perhaps it is not formed of water at all. the most careful studies of the surface of this planet, under the best conditions, are those made at the lowell observatory at flagstaff, arizona. especially wonderful is the system of so-called canals, first seen by schiaparelli, but mapped in great detail at flagstaff. but the nature and meaning of these mysterious lines are still to be discovered. the result is that the question of the real nature of the surface of mars and of what we should see around us could we land upon it and travel over it are still among the unsolved problems of astronomy. if this is the case with the nearest planets that we can study, how is it with more distant ones? jupiter is the only one of these of the condition of whose surface we can claim to have definite knowledge. but even this knowledge is meagre. the substance of what we know is that its surface is surrounded by layers of what look like dense clouds, through which nothing can certainly be seen. i have already spoken of the heat of the sun and its probable origin. but the question of its heat, though the most important, is not the only one that the sun offers us. what is the sun? when we say that it is a very hot globe, more than a million times as large as the earth, and hotter than any furnace that man can make, so that literally "the elements melt with fervent heat" even at its surface, while inside they are all vaporized, we have told the most that we know as to what the sun really is. of course we know a great deal about the spots, the rotation of the sun on its axis, the materials of which it is composed, and how its surroundings look during a total eclipse. but all this does not answer our question. there are several mysteries which ingenious men have tried to explain, but they cannot prove their explanations to be correct. one is the cause and nature of the spots. another is that the shining surface of the sun, the "photosphere," as it is technically called, seems so calm and quiet while forces are acting within it of a magnitude quite beyond our conception. flames in which our earth and everything on it would be engulfed like a boy's marble in a blacksmith's forge are continually shooting up to a height of tens of thousands of miles. one would suppose that internal forces capable of doing this would break the surface up into billows of fire a thousand miles high; but we see nothing of the kind. the surface of the sun seems almost as placid as a lake. yet another mystery is the corona of the sun. this is something we should never have known to exist if the sun were not sometimes totally eclipsed by the dark body of the moon. on these rare occasions the sun is seen to be surrounded by a halo of soft, white light, sending out rays in various directions to great distances. this halo is called the corona, and has been most industriously studied and photographed during nearly every total eclipse for thirty years. thus we have learned much about how it looks and what its shape is. it has a fibrous, woolly structure, a little like the loose end of a much-worn hempen rope. a certain resemblance has been seen between the form of these seeming fibres and that of the lines in which iron filings arrange themselves when sprinkled on paper over a magnet. it has hence been inferred that the sun has magnetic properties, a conclusion which, in a general way, is supported by many other facts. yet the corona itself remains no less an unexplained phenomenon. [illustration with caption: photograph of the corona of the sun, taken in tripoli during total eclipse of august , ] a phenomenon almost as mysterious as the solar corona is the "zodiacal light," which any one can see rising from the western horizon just after the end of twilight on a clear winter or spring evening. the most plausible explanation is that it is due to a cloud of small meteoric bodies revolving round the sun. we should hardly doubt this explanation were it not that this light has a yet more mysterious appendage, commonly called the gegenschein, or counter-glow. this is a patch of light in the sky in a direction exactly opposite that of the sun. it is so faint that it can be seen only by a practised eye under the most favorable conditions. but it is always there. the latest suggestion is that it is a tail of the earth, of the same kind as the tail of a comet! we know that the motions of the heavenly bodies are predicted with extraordinary exactness by the theory of gravitation. when one finds that the exact path of the moon's shadow on the earth during a total eclipse of the sun can be mapped out many years in advance, and that the planets follow the predictions of the astronomer so closely that, if you could see the predicted planet as a separate object, it would look, even in a good telescope, as if it exactly fitted over the real planet, one thinks that here at least is a branch of astronomy which is simply perfect. and yet the worlds themselves show slight deviations in their movements which the astronomer cannot always explain, and which may be due to some hidden cause that, when brought to light, shall lead to conclusions of the greatest importance to our race. one of these deviations is in the rotation of the earth. sometimes, for several years at a time, it seems to revolve a little faster, and then again a little slower. the changes are very slight; they can be detected only by the most laborious and refined methods; yet they must have a cause, and we should like to know what that cause is. the moon shows a similar irregularity of motion. for half a century, perhaps through a whole century, she will go around the earth a little ahead of her regular rate, and then for another half-century or more she will fall behind. the changes are very small; they would never have been seen with the unaided eye, yet they exist. what is their cause? mathematicians have vainly spent years of study in trying to answer this question. the orbit of mercury is found by observations to have a slight motion which mathematicians have vainly tried to explain. for some time it was supposed to be caused by the attraction of an unknown planet between mercury and the sun, and some were so sure of the existence of this planet that they gave it a name, calling it vulcan. but of late years it has become reasonably certain that no planet large enough to produce the effect observed can be there. so thoroughly has every possible explanation been sifted out and found wanting, that some astronomers are now inquiring whether the law of gravitation itself may not be a little different from what has always been supposed. a very slight deviation, indeed, would account for the facts, but cautious astronomers want other proofs before regarding the deviation of gravitation as an established fact. intelligent men have sometimes inquired how, after devoting so much work to the study of the heavens, anything can remain for astronomers to find out. it is a curious fact that, although they were never learning so fast as at the present day, yet there seems to be more to learn now than there ever was before. great and numerous as are the unsolved problems of our science, knowledge is now advancing into regions which, a few years ago, seemed inaccessible. where it will stop none can say. ii the new problems of the universe the achievements of the nineteenth century are still a theme of congratulation on the part of all who compare the present state of the world with that of one hundred years ago. and yet, if we should fancy the most sagacious prophet, endowed with a brilliant imagination, to have set forth in the year the problems that the century might solve and the things which it might do, we should be surprised to see how few of his predictions had come to pass. he might have fancied aerial navigation and a number of other triumphs of the same class, but he would hardly have had either steam navigation or the telegraph in his picture. in an article appeared in harper's magazine depicting some anticipated features of life in a.d. . we have since made great advances, but they bear little resemblance to what the writer imagined. he did not dream of the telephone, but did describe much that has not yet come to pass and probably never will. the fact is that, much as the nineteenth century has done, its last work was to amuse itself by setting forth more problems for this century to solve than it has ever itself succeeded in mastering. we should not be far wrong in saying that to-day there are more riddles in the universe than there were before men knew that it contained anything more than the objects they could see. so far as mere material progress is concerned, it may be doubtful whether anything so epoch-making as the steam-engine or the telegraph is held in store for us by the future. but in the field of purely scientific discovery we are finding a crowd of things of which our philosophy did not dream even ten years ago. the greatest riddles which the nineteenth century has bequeathed to us relate to subjects so widely separated as the structure of the universe and the structure of atoms of matter. we see more and more of these structures, and we see more and more of unity everywhere, and yet new facts difficult of explanation are being added more rapidly than old facts are being explained. we all know that the nineteenth century was marked by a separation of the sciences into a vast number of specialties, to the subdivisions of which one could see no end. but the great work of the twentieth century will be to combine many of these specialties. the physical philosopher of the present time is directing his thought to the demonstration of the unity of creation. astronomical and physical researches are now being united in a way which is bringing the infinitely great and the infinitely small into one field of knowledge. ten years ago the atoms of matter, of which it takes millions of millions to make a drop of water, were the minutest objects with which science could imagine itself to be concerned, now a body of experimentalists, prominent among whom stand professors j. j. thompson, becquerel, and roentgen, have demonstrated the existence of objects so minute that they find their way among and between the atoms of matter as rain-drops do among the buildings of a city. more wonderful yet, it seems likely, although it has not been demonstrated, that these little things, called "corpuscles," play an important part in what is going on among the stars. whether this be true or not, it is certain that there do exist in the universe emanations of some sort, producing visible effects, the investigation of which the nineteenth century has had to bequeath to the twentieth. for the purpose of the navigator, the direction of the magnetic needle is invariable in any one place, for months and even years; but when exact scientific observations on it are made, it is found subject to numerous slight changes. the most regular of these consists in a daily change of its direction. it moves one way from morning until noon, and then, late in the afternoon and during the night, turns back again to its original pointing. the laws of this change have been carefully studied from observations, which show that it is least at the equator and larger as we go north into middle latitudes; but no explanation of it resting on an indisputable basis has ever been offered. besides these regular changes, there are others of a very irregular character. every now and then the changes in the direction of the magnet are wider and more rapid than those which occur regularly every day. the needle may move back and forth in a way so fitful as to show the action of some unusual exciting cause. such movements of the needle are commonly seen when there is a brilliant aurora. this connection shows that a magnetic storm and an aurora must be due to the same or some connected causes. those of us who are acquainted with astronomical matters know that the number of spots on the sun goes through a regular cycle of change, having a period of eleven years and one or two months. now, the curious fact is, when the number and violence of magnetic storms are recorded and compared, it is found that they correspond to the spots on the sun, and go through the same period of eleven years. the conclusion seems almost inevitable: magnetic storms are due to some emanation sent out by the sun, which arises from the same cause that produces the spots. this emanation does not go on incessantly, but only in an occasional way, as storms follow each other on the earth. what is it? every attempt to detect it has been in vain. professor hale, at the yerkes observatory, has had in operation from time to time, for several years, his ingenious spectroheliograph, which photographs the sun by a single ray of the spectrum. this instrument shows that violent actions are going on in the sun, which ordinary observation would never lead us to suspect. but it has failed to show with certainty any peculiar emanation at the time of a magnetic storm or anything connected with such a storm. a mystery which seems yet more impenetrable is associated with the so-called new stars which blaze forth from time to time. these offer to our sight the most astounding phenomena ever presented to the physical philosopher. one hundred years ago such objects offered no mystery. there was no reason to suppose that the creator of the universe had ceased his functions; and, continuing them, it was perfectly natural that he should be making continual additions to the universe of stars. but the idea that these objects are really new creations, made out of nothing, is contrary to all our modern ideas and not in accord with the observed facts. granting the possibility of a really new star--if such an object were created, it would be destined to take its place among the other stars as a permanent member of the universe. instead of this, such objects invariably fade away after a few months, and are changed into something very like an ordinary nebula. a question of transcendent interest is that of the cause of these outbursts. it cannot be said that science has, up to the present time, been able to offer any suggestion not open to question. the most definite one is the collision theory, according to which the outburst is due to the clashing together of two stars, one or both of which might previously have been dark, like a planet. the stars which may be actually photographed probably exceed one hundred millions in number, and those which give too little light to affect the photographic plate may be vastly more numerous than those which do. dark stars revolve around bright ones in an infinite variety of ways, and complex systems of bodies, the members of which powerfully attract each other, are the rule throughout the universe. moreover, we can set no limit to the possible number of dark or invisible stars that may be flying through the celestial spaces. while, therefore, we cannot regard the theory of collision as established, it seems to be the only one yet put forth which can lay any claim to a scientific basis. what gives most color to it is the extreme suddenness with which the new stars, so far as has yet been observed, invariably blaze forth. in almost every case it has been only two or three days from the time that the existence of such an object became known until it had attained nearly its full brightness. in fact, it would seem that in the case of the star in perseus, as in most other cases, the greater part of the outburst took place within the space of twenty-four hours. this suddenness and rapidity is exactly what would be the result of a collision. the most inexplicable feature of all is the rapid formation of a nebula around this star. in the first photographs of the latter, the appearance presented is simply that of an ordinary star. but, in the course of three or four months, the delicate photographs taken at the lick observatory showed that a nebulous light surrounded the star, and was continually growing larger and larger. at first sight, there would seem to be nothing extraordinary in this fact. great masses of intensely hot vapor, shining by their own light, would naturally be thrown out from the star. or, if the star had originally been surrounded by a very rare nebulous fog or vapor, the latter would be seen by the brilliant light emitted by the star. on this was based an explanation offered by kapteyn, which at first seemed very plausible. it was that the sudden wave of light thrown out by the star when it burst forth caused the illumination of the surrounding vapor, which, though really at rest, would seem to expand with the velocity of light, as the illumination reached more and more distant regions of the nebula. this result may be made the subject of exact calculation. the velocity of light is such as would make a circuit of the earth more than seven times in a second. it would, therefore, go out from the star at the rate of a million of miles in between five and six seconds. in the lapse of one of our days, the light would have filled a sphere around the star having a diameter more than one hundred and fifty times the distance of the sun from the earth, and more than five times the dimensions of the whole solar system. continuing its course and enlarging its sphere day after day, the sight presented to us would have been that of a gradually expanding nebulous mass--a globe of faint light continually increasing in size with the velocity of light. the first sentiment the reader will feel on this subject is doubtless one of surprise that the distance of the star should be so great as this explanation would imply. six months after the explosion, the globe of light, as actually photographed, was of a size which would have been visible to the naked eye only as a very minute object in the sky. is it possible that this minute object could have been thousands of times the dimensions of our solar system? to see how the question stands from this point of view, we must have some idea of the possible distance of the new star. to gain this idea, we must find some way of estimating distances in the universe. for a reason which will soon be apparent, we begin with the greatest structure which nature offers to the view of man. we all know that the milky way is formed of countless stars, too minute to be individually visible to the naked eye. the more powerful the telescope through which we sweep the heavens, the greater the number of the stars that can be seen in it. with the powerful instruments which are now in use for photographing the sky, the number of stars brought to light must rise into the hundreds of millions, and the greater part of these belong to the milky way. the smaller the stars we count, the greater their comparative number in the region of the milky way. of the stars visible through the telescope, more than one-half are found in the milky way, which may be regarded as a girdle spanning the entire visible universe. of the diameter of this girdle we can say, almost with certainty, that it must be more than a thousand times as great as the distance of the nearest fixed star from us, and is probably two or three times greater. according to the best judgment we can form, our solar system is situate near the central region of the girdle, so that the latter must be distant from us by half its diameter. it follows that if we can imagine a gigantic pair of compasses, of which the points extend from us to alpha centauri, the nearest star, we should have to measure out at least five hundred spaces with the compass, and perhaps even one thousand or more, to reach the region of the milky way. with this we have to connect another curious fact. of eighteen new stars which have been observed to blaze forth during the last four hundred years, all are in the region of the milky way. this seems to show that, as a rule, they belong to the milky way. accepting this very plausible conclusion, the new star in perseus must have been more than five hundred times as far as the nearest fixed star. we know that it takes light four years to reach us from alpha centauri. it follows that the new star was at a distance through which light would require more than two thousand years to travel, and quite likely a time two or three times this. it requires only the most elementary ideas of geometry to see that if we suppose a ray of light to shoot from a star at such a distance in a direction perpendicular to the line of sight from us to the star, we can compute how fast the ray would seem to us to travel. granting the distance to be only two thousand light years, the apparent size of the sphere around the star which the light would fill at the end of one year after the explosion would be that of a coin seen at a distance of two thousand times its radius, or one thousand times its diameter--say, a five-cent piece at the distance of sixty feet. but, as a matter of fact, the nebulous illumination expanded with a velocity from ten to twenty times as great as this. the idea that the nebulosity around the new star was formed by the illumination caused by the light of the explosion spreading out on all sides therefore fails to satisfy us, not because the expansion of the nebula seemed to be so slow, but because it was many times as swift as the speed of light. another reason for believing that it was not a mere wave of light is offered by the fact that it did not take place regularly in every direction from the star, but seemed to shoot off at various angles. up to the present time, the speed of light has been to science, as well as to the intelligence of our race, almost a symbol of the greatest of possible speeds. the more carefully we reflect on the case, the more clearly we shall see the difficulty in supposing any agency to travel at the rate of the seeming emanations from the new star in perseus. as the emanation is seen spreading day after day, the reader may inquire whether this is not an appearance due to some other cause than the mere motion of light. may not an explosion taking place in the centre of a star produce an effect which shall travel yet faster than light? we can only reply that no such agency is known to science. but is there really anything intrinsically improbable in an agency travelling with a speed many times that of light? in considering that there is, we may fall into an error very much like that into which our predecessors fell in thinking it entirely out of the range of reasonable probability that the stars should be placed at such distances as we now know them to be. accepting it as a fact that agencies do exist which travel from sun to planet and from star to star with a speed which beggars all our previous ideas, the first question that arises is that of their nature and mode of action. this question is, up to the present time, one which we do not see any way of completely answering. the first difficulty is that we have no evidence of these agents except that afforded by their action. we see that the sun goes through a regular course of pulsations, each requiring eleven years for completion; and we see that, simultaneously with these, the earth's magnetism goes through a similar course of pulsations. the connection of the two, therefore, seems absolutely proven. but when we ask by what agency it is possible for the sun to affect the magnetism of the earth, and when we trace the passage of some agent between the two bodies, we find nothing to explain the action. to all appearance, the space between the earth and the sun is a perfect void. that electricity cannot of itself pass through a vacuum seems to be a well-established law of physics. it is true that electromagnetic waves, which are supposed to be of the same nature with those of light, and which are used in wireless telegraphy, do pass through a vacuum and may pass from the sun to the earth. but there is no way of explaining how such waves would either produce or affect the magnetism of the earth. the mysterious emanations from various substances, under certain conditions, may have an intimate relation with yet another of the mysteries of the universe. it is a fundamental law of the universe that when a body emits light or heat, or anything capable of being transformed into light or heat, it can do so only by the expenditure of force, limited in supply. the sun and stars are continually sending out a flood of heat. they are exhausting the internal supply of something which must be limited in extent. whence comes the supply? how is the heat of the sun kept up? if it were a hot body cooling off, a very few years would suffice for it to cool off so far that its surface would become solid and very soon cold. in recent years, the theory universally accepted has been that the supply of heat is kept up by the continual contraction of the sun, by mutual gravitation of its parts as it cools off. this theory has the advantage of enabling us to calculate, with some approximation to exactness, at what rate the sun must be contracting in order to keep up the supply of heat which it radiates. on this theory, it must, ten millions of years ago, have had twice its present diameter, while less than twenty millions of years ago it could not have existed except as an immense nebula filling the whole solar system. we must bear in mind that this theory is the only one which accounts for the supply of heat, even through human history. if it be true, then the sun, earth, and solar system must be less than twenty million years old. here the geologists step in and tell us that this conclusion is wholly inadmissible. the study of the strata of the earth and of many other geological phenomena, they assure us, makes it certain that the earth must have existed much in its present condition for hundreds of millions of years. during all that time there can have been no great diminution in the supply of heat radiated by the sun. the astronomer, in considering this argument, has to admit that he finds a similar difficulty in connection with the stars and nebulas. it is an impossibility to regard these objects as new; they must be as old as the universe itself. they radiate heat and light year after year. in all probability, they must have been doing so for millions of years. whence comes the supply? the geologist may well claim that until the astronomer explains this mystery in his own domain, he cannot declare the conclusions of geology as to the age of the earth to be wholly inadmissible. now, the scientific experiments of the last two years have brought this mystery of the celestial spaces right down into our earthly laboratories. m. and madame curie have discovered the singular metal radium, which seems to send out light, heat, and other rays incessantly, without, so far as has yet been determined, drawing the required energy from any outward source. as we have already pointed out, such an emanation must come from some storehouse of energy. is the storehouse, then, in the medium itself, or does the latter draw it from surrounding objects? if it does, it must abstract heat from these objects. this question has been settled by professor dewar, at the royal institution, london, by placing the radium in a medium next to the coldest that art has yet produced--liquid air. the latter is surrounded by the only yet colder medium, liquid hydrogen, so that no heat can reach it. under these circumstances, the radium still gives out heat, boiling away the liquid air until the latter has entirely disappeared. instead of the radiation diminishing with time, it rather seems to increase. called on to explain all this, science can only say that a molecular change must be going on in the radium, to correspond to the heat it gives out. what that change may be is still a complete mystery. it is a mystery which we find alike in those minute specimens of the rarest of substances under our microscopes, in the sun, and in the vast nebulous masses in the midst of which our whole solar system would be but a speck. the unravelling of this mystery must be the great work of science of the twentieth century. what results shall follow for mankind one cannot say, any more than he could have said two hundred years ago what modern science would bring forth. perhaps, before future developments, all the boasted achievements of the nineteenth century may take the modest place which we now assign to the science of the eighteenth century--that of the infant which is to grow into a man. iii the structure of the universe the questions of the extent of the universe in space and of its duration in time, especially of its possible infinity in either space or time, are of the highest interest both in philosophy and science. the traditional philosophy had no means of attacking these questions except considerations suggested by pure reason, analogy, and that general fitness of things which was supposed to mark the order of nature. with modern science the questions belong to the realm of fact, and can be decided only by the results of observation and a study of the laws to which these results may lead. from the philosophic stand-point, a discussion of this subject which is of such weight that in the history of thought it must be assigned a place above all others, is that of kant in his "kritik." here we find two opposing propositions--the thesis that the universe occupies only a finite space and is of finite duration; the antithesis that it is infinite both as regards extent in space and duration in time. both of these opposing propositions are shown to admit of demonstration with equal force, not directly, but by the methods of reductio ad absurdum. the difficulty, discussed by kant, was more tersely expressed by hamilton in pointing out that we could neither conceive of infinite space nor of space as bounded. the methods and conclusions of modern astronomy are, however, in no way at variance with kant's reasoning, so far as it extends. the fact is that the problem with which the philosopher of konigsberg vainly grappled is one which our science cannot solve any more than could his logic. we may hope to gain complete information as to everything which lies within the range of the telescope, and to trace to its beginning every process which we can now see going on in space. but before questions of the absolute beginning of things, or of the boundary beyond which nothing exists, our means of inquiry are quite powerless. another example of the ancient method is found in the great work of copernicus. it is remarkable how completely the first expounder of the system of the world was dominated by the philosophy of his time, which he had inherited from his predecessors. this is seen not only in the general course of thought through the opening chapters of his work, but among his introductory propositions. the first of these is that the universe--mundus--as well as the earth, is spherical in form. his arguments for the sphericity of the earth, as derived from observation, are little more than a repetition of those of ptolemy, and therefore not of special interest. his proposition that the universe is spherical is, however, not based on observation, but on considerations of the perfection of the spherical form, the general tendency of bodies--a drop of water, for example--to assume this form, and the sphericity of the sun and moon. the idea retained its place in his mind, although the fundamental conception of his system did away with the idea of the universe having any well-defined form. the question as attacked by modern astronomy is this: we see scattered through space in every direction many millions of stars of various orders of brightness and at distances so great as to defy exact measurement, except in the case of a few of the nearest. has this collection of stars any well-defined boundary, or is what we see merely that part of an infinite mass which chances to lie within the range of our telescopes? if we were transported to the most distant star of which we have knowledge, should we there find ourselves still surrounded by stars on all sides, or would the space beyond be void? granting that, in any or every direction, there is a limit to the universe, and that the space beyond is therefore void, what is the form of the whole system and the distance of its boundaries? preliminary in some sort to these questions are the more approachable ones: of what sort of matter is the universe formed? and into what sort of bodies is this matter collected? to the ancients the celestial sphere was a reality, instead of a mere effect of perspective, as we regard it. the stars were set on its surface, or at least at no great distance within its crystalline mass. outside of it imagination placed the empyrean. when and how these conceptions vanished from the mind of man, it would be as hard to say as when and how santa claus gets transformed in the mind of the child. they are not treated as realities by any astronomical writer from ptolemy down; yet, the impressions and forms of thought to which they gave rise are well marked in copernicus and faintly evident in kepler. the latter was perhaps the first to suggest that the sun might be one of the stars; yet, from defective knowledge of the relative brightness of the latter, he was led to the conclusion that their distances from each other were less than the distance which separated them from the sun. the latter he supposed to stand in the centre of a vast vacant region within the system of stars. for us the great collection of millions of stars which are made known to us by the telescope, together with all the invisible bodies which may be contained within the limits of the system, form the universe. here the term "universe" is perhaps objectionable because there may be other systems than the one with which we are acquainted. the term stellar system is, therefore, a better one by which to designate the collection of stars in question. it is remarkable that the first known propounder of that theory of the form and arrangement of the system which has been most generally accepted seems to have been a writer otherwise unknown in science--thomas wright, of durham, england. he is said to have published a book on the theory of the universe, about . it does not appear that this work was of a very scientific character, and it was, perhaps, too much in the nature of a speculation to excite notice in scientific circles. one of the curious features of the history is that it was kant who first cited wright's theory, pointed out its accordance with the appearance of the milky way, and showed its general reasonableness. but, at the time in question, the work of the philosopher of konigsberg seems to have excited no more notice among his scientific contemporaries than that of wright. kant's fame as a speculative philosopher has so eclipsed his scientific work that the latter has but recently been appraised at its true value. he was the originator of views which, though defective in detail, embodied a remarkable number of the results of recent research on the structure and form of the universe, and the changes taking place in it. the most curious illustration of the way in which he arrived at a correct conclusion by defective reasoning is found in his anticipation of the modern theory of a constant retardation of the velocity with which the earth revolves on its axis. he conceived that this effect must result from the force exerted by the tidal wave, as moving towards the west it strikes the eastern coasts of asia and america. an opposite conclusion was reached by laplace, who showed that the effect of this force was neutralized by forces producing the wave and acting in the opposite direction. and yet, nearly a century later, it was shown that while laplace was quite correct as regards the general principles involved, the friction of the moving water must prevent the complete neutralization of the two opposing forces, and leave a small residual force acting towards the west and retarding the rotation. kant's conclusion was established, but by an action different from that which he supposed. the theory of wright and kant, which was still further developed by herschel, was that our stellar system has somewhat the form of a flattened cylinder, or perhaps that which the earth would assume if, in consequence of more rapid rotation, the bulging out at its equator and the flattening at its poles were carried to an extreme limit. this form has been correctly though satirically compared to that of a grindstone. it rests to a certain extent, but not entirely, on the idea that the stars are scattered through space with equal thickness in every direction, and that the appearance of the milky way is due to the fact that we, situated in the centre of this flattened system, see more stars in the direction of the circumference of the system than in that of its poles. the argument on which the view in question rests may be made clear in the following way. let us chose for our observations that hour of the night at which the milky way skirts our horizon. this is nearly the case in the evenings of may and june, though the coincidence with the horizon can never be exact except to observers stationed near the tropics. using the figure of the grindstone, we at its centre will then have its circumference around our horizon, while the axis will be nearly vertical. the points in which the latter intersects the celestial sphere are called the galactic poles. there will be two of these poles, the one at the hour in question near the zenith, the other in our nadir, and therefore invisible to us, though seen by our antipodes. our horizon corresponds, as it were, to the central circle of the milky way, which now surrounds us on all sides in a horizontal direction, while the galactic poles are degrees distant from every part of it, as every point of the horizon is degrees from the zenith. let us next count the number of stars visible in a powerful telescope in the region of the heavens around the galactic pole, now our zenith, and find the average number per square degree. this will be the richness of the region in stars. then we take regions nearer the horizontal milky way--say that contained between degrees and degrees from the zenith--and, by a similar count, find its richness in stars. we do the same for other regions, nearer and nearer to the horizon, till we reach the galaxy itself. the result of all the counts will be that the richness of the sky in stars is least around the galactic pole, and increases in every direction towards the milky way. without such counts of the stars we might imagine our stellar system to be a globular collection of stars around which the object in question passed as a girdle; and we might take a globe with a chain passing around it as representative of the possible figure of the stellar system. but the actual increase in star-thickness which we have pointed out shows us that this view is incorrect. the nature and validity of the conclusions to be drawn can be best appreciated by a statement of some features of this tendency of the stars to crowd towards the galactic circle. most remarkable is the fact that the tendency is seen even among the brighter stars. without either telescope or technical knowledge, the careful observer of the stars will notice that the most brilliant constellations show this tendency. the glorious orion, canis major containing the brightest star in the heavens, cassiopeia, perseus, cygnus, and lyra with its bright-blue vega, not to mention such constellations as the southern cross, all lie in or near the milky way. schiaparelli has extended the investigation to all the stars visible to the naked eye. he laid down on planispheres the number of such stars in each region of the heavens of degrees square. each region was then shaded with a tint that was darker as the region was richer in stars. the very existence of the milky way was ignored in this work, though his most darkly shaded regions lie along the course of this belt. by drawing a band around the sky so as to follow or cover his darkest regions, we shall rediscover the course of the milky way without any reference to the actual object. it is hardly necessary to add that this result would be reached with yet greater precision if we included the telescopic stars to any degree of magnitude--plotting them on a chart and shading the chart in the same way. what we learn from this is that the stellar system is not an irregular chaos; and that notwithstanding all its minor irregularities, it may be considered as built up with special reference to the milky way as a foundation. another feature of the tendency in question is that it is more and more marked as we include fainter stars in our count. the galactic region is perhaps twice as rich in stars visible to the naked eye as the rest of the heavens. in telescopic stars to the ninth magnitude it is three or four times as rich. in the stars found on the photographs of the sky made at the harvard and other observatories, and in the stargauges of the herschels, it is from five to ten times as rich. another feature showing the unity of the system is the symmetry of the heavens on the two sides of the galactic belt let us return to our supposition of such a position of the celestial sphere, with respect to the horizon, that the latter coincides with the central line of this belt, one galactic pole being near our zenith. the celestial hemisphere which, being above our horizon, is visible to us, is the one to which we have hitherto directed our attention in describing the distribution of the stars. but below our horizon is another hemisphere, that of our antipodes, which is the counterpart of ours. the stars which it contains are in a different part of the universe from those which we see, and, without unity of plan, would not be subject to the same law. but the most accurate counts of stars that have been made fail to show any difference in their general arrangement in the two hemispheres. they are just as thick around the south galactic poles as around the north one. they show the same tendency to crowd towards the milky way in the hemisphere invisible to us as in the hemisphere which we see. slight differences and irregularities, are, indeed, found in the enumeration, but they are no greater than must necessarily arise from the difficulty of stopping our count at a perfectly fixed magnitude. the aim of star-counts is not to estimate the total number of stars, for this is beyond our power, but the number visible with a given telescope. in such work different observers have explored different parts of the sky, and in a count of the same region by two observers we shall find that, although they attempt to stop at the same magnitude, each will include a great number of stars which the other omits. there is, therefore, room for considerable difference in the numbers of stars recorded, without there being any actual inequality between the two hemispheres. a corresponding similarity is found in the physical constitution of the stars as brought out by the spectroscope. the milky way is extremely rich in bluish stars, which make up a considerable majority of the cloudlike masses there seen. but when we recede from the galaxy on one side, we find the blue stars becoming thinner, while those having a yellow tinge become relatively more numerous. this difference of color also is the same on the two sides of the galactic plane. nor can any systematic difference be detected between the proper motions of the stars in these two hemispheres. if the largest known proper motion is found in the one, the second largest is in the other. counting all the known stars that have proper motions exceeding a given limit, we find about as many in one hemisphere as in the other. in this respect, also, the universe appears to be alike through its whole extent. it is the uniformity thus prevailing through the visible universe, as far as we can see, in two opposite directions, which inspires us with confidence in the possibility of ultimately reaching some well-founded conclusion as to the extent and structure of the system. all these facts concur in supporting the view of wright, kant, and herschel as to the form of the universe. the farther out the stars extend in any direction, the more stars we may see in that direction. in the direction of the axis of the cylinder, the distances of the boundary are least, so that we see fewer stars. the farther we direct our attention towards the equatorial regions of the system, the greater the distance from us to the boundary, and hence the more stars we see. the fact that the increase in the number of stars seen towards the equatorial region of the system is greater, the smaller the stars, is the natural consequence of the fact that distant stars come within our view in greater numbers towards the equatorial than towards the polar regions. objections have been raised to the herschelian view on the ground that it assumes an approximately uniform distribution of the stars in space. it has been claimed that the fact of our seeing more stars in one direction than in another may not arise merely from our looking through a deeper stratum, as herschel supposed, but may as well be due to the stars being more thinly scattered in the direction of the axis of the system than in that of its equatorial region. the great inequalities in the richness of neighboring regions in the milky way show that the hypothesis of uniform distribution does not apply to the equatorial region. the claim has therefore been made that there is no proof of the system extending out any farther in the equatorial than in the polar direction. the consideration of this objection requires a closer inquiry as to what we are to understand by the form of our system. we have already pointed out the impossibility of assigning any boundary beyond which we can say that nothing exists. and even as regards a boundary of our stellar system, it is impossible for us to assign any exact limit beyond which no star is visible to us. the analogy of collections of stars seen in various parts of the heavens leads us to suppose that there may be no well-defined form to our system, but that, as we go out farther and farther, we shall see occasional scattered stars to, possibly, an indefinite distance. the truth probably is that, as in ascending a mountain, we find the trees, which may be very dense at its base, thin out gradually as we approach the summit, where there may be few or none, so we might find the stars to thin out could we fly to the distant regions of space. the practical question is whether, in such a flight, we should find this sooner by going in the direction of the axis of our system than by directing our course towards the milky way. if a point is at length reached beyond which there are but few scattered stars, such a point would, for us, mark the boundary of our system. from this point of view the answer does not seem to admit of doubt. if, going in every direction, we mark the point, if any, at which the great mass of the stars are seen behind us, the totality of all these points will lie on a surface of the general form that herschel supposed. there is still another direct indication of the finitude of our stellar system upon which we have not touched. if this system extended out without limit in any direction whatever, it is shown by a geometric process which it is not necessary to explain in the present connection, but which is of the character of mathematical demonstration, that the heavens would, in every direction where this was true, blaze with the light of the noonday sun. this would be very different from the blue-black sky which we actually see on a clear night, and which, with a reservation that we shall consider hereafter, shows that, how far so-ever our stellar system may extend, it is not infinite. beyond this negative conclusion the fact does not teach us much. vast, indeed, is the distance to which the system might extend without the sky appearing much brighter than it is, and we must have recourse to other considerations in seeking for indications of a boundary, or even of a well-marked thinning out, of stars. if, as was formerly supposed, the stars did not greatly differ in the amount of light emitted by each, and if their diversity of apparent magnitude were due principally to the greater distance of the fainter stars, then the brightness of a star would enable us to form a more or less approximate idea of its distance. but the accumulated researches of the past seventy years show that the stars differ so enormously in their actual luminosity that the apparent brightness of a star affords us only a very imperfect indication of its distance. while, in the general average, the brighter stars must be nearer to us than the fainter ones, it by no means follows that a very bright star, even of the first magnitude, is among the nearer to our system. two stars are worthy of especial mention in this connection, canopus and rigel. the first is, with the single exception of sirius, the brightest star in the heavens. the other is a star of the first magnitude in the southwest corner of orion. the most long-continued and complete measures of parallax yet made are those carried on by gill, at the cape of good hope, on these two and some other bright stars. the results, published in , show that neither of these bodies has any parallax that can be measured by the most refined instrumental means known to astronomy. in other words, the distance of these stars is immeasurably great. the actual amount of light emitted by each is certainly thousands and probably tens of thousands of times that of the sun. notwithstanding the difficulties that surround the subject, we can at least say something of the distance of a considerable number of the stars. two methods are available for our estimate--measures of parallax and determination of proper motions. the problem of stellar parallax, simple though it is in its conception, is the most delicate and difficult of all which the practical astronomer has to encounter. an idea of it may be gained by supposing a minute object on a mountain-top, we know not how many miles away, to be visible through a telescope. the observer is allowed to change the position of his instrument by two inches, but no more. he is required to determine the change in the direction of the object produced by this minute displacement with accuracy enough to determine the distance of the mountain. this is quite analogous to the determination of the change in the direction in which we see a star as the earth, moving through its vast circuit, passes from one extremity of its orbit to the other. representing this motion on such a scale that the distance of our planet from the sun shall be one inch, we find that the nearest star, on the same scale, will be more than four miles away, and scarcely one out of a million will be at a less distance than ten miles. it is only by the most wonderful perfection both in the heliometer, the instrument principally used for these measures, and in methods of observation, that any displacement at all can be seen even among the nearest stars. the parallaxes of perhaps a hundred stars have been determined, with greater or less precision, and a few hundred more may be near enough for measurement. all the others are immeasurably distant; and it is only by statistical methods based on their proper motions and their probable near approach to equality in distribution that any idea can be gained of their distances. to form a conception of the stellar system, we must have a unit of measure not only exceeding any terrestrial standard, but even any distance in the solar system. for purely astronomical purposes the most convenient unit is the distance corresponding to a parallax of ", which is a little more than , times the sun's distance. but for the purposes of all but the professional astronomer the most convenient unit will be the light-year--that is, the distance through which light would travel in one year. this is equal to the product of , miles, the distance travelled in one second, by , , , the number of seconds in a year. the reader who chooses to do so may perform the multiplication for himself. the product will amount to about , times the distance of the sun. [illustration with caption: a typical star cluster--centauri] the nearest star whose distance we know, alpha centauri, is distant from us more than four light-years. in all likelihood this is really the nearest star, and it is not at all probable that any other star lies within six light-years. moreover, if we were transported to this star the probability seems to be that the sun would now be the nearest star to us. flying to any other of the stars whose parallax has been measured, we should probably find that the average of the six or eight nearest stars around us ranges somewhere between five and seven light-years. we may, in a certain sense, call eight light-years a star-distance, meaning by this term the average of the nearest distances from one star to the surrounding ones. to put the result of measures of parallax into another form, let us suppose, described around our sun as a centre, a system of concentric spheres each of whose surfaces is at the distance of six light-years outside the sphere next within it. the inner is at the distance of six light-years around the sun. the surface of the second sphere will be twelve light-years away, that of the third eighteen, etc. the volumes of space within each of these spheres will be as the cubes of the diameters. the most likely conclusion we can draw from measures of parallax is that the first sphere will contain, beside the sun at its centre, only alpha centauri. the second, twelve light-years away, will probably contain, besides these two, six other stars, making eight in all. the third may contain twenty-one more, making twenty-seven stars within the third sphere, which is the cube of three. within the fourth would probably be found sixty-four stars, this being the cube of four, and so on. beyond this no measures of parallax yet made will give us much assistance. we can only infer that probably the same law holds for a large number of spheres, though it is quite certain that it does not hold indefinitely. for more light on the subject we must have recourse to the proper motions. the latest words of astronomy on this subject may be briefly summarized. as a rule, no star is at rest. each is moving through space with a speed which differs greatly with different stars, but is nearly always swift, indeed, when measured by any standard to which we are accustomed. slow and halting, indeed, is that star which does not make more than a mile a second. with two or three exceptions, where the attraction of a companion comes in, the motion of every star, so far as yet determined, takes place in a straight line. in its outward motion the flying body deviates neither to the right nor left. it is safe to say that, if any deviation is to take place, thousands of years will be required for our terrestrial observers to recognize it. rapid as the course of these objects is, the distances which we have described are such that, in the great majority of cases, all the observations yet made on the positions of the stars fail to show any well-established motion. it is only in the case of the nearer of these objects that we can expect any motion to be perceptible during the period, in no case exceeding one hundred and fifty years, through which accurate observations extend. the efforts of all the observatories which engage in such work are, up to the present time, unequal to the task of grappling with the motions of all the stars that can be seen with the instruments, and reaching a decision as to the proper motion in each particular case. as the question now stands, the aim of the astronomer is to determine what stars have proper motions large enough to be well established. to make our statement on this subject clear, it must be understood that by this term the astronomer does not mean the speed of a star in space, but its angular motion as he observes it on the celestial sphere. a star moving forward with a given speed will have a greater proper motion according as it is nearer to us. to avoid all ambiguity, we shall use the term "speed" to express the velocity in miles per second with which such a body moves through space, and the term "proper motion" to express the apparent angular motion which the astronomer measures upon the celestial sphere. up to the present time, two stars have been found whose proper motions are so large that, if continued, the bodies would make a complete circuit of the heavens in less than , years. one of these would require about , ; the other about , years for the circuit. of other stars having a rapid motion only about one hundred would complete their course in less than a million of years. quite recently a system of observations upon stars to the ninth magnitude has been nearly carried through by an international combination of observatories. the most important conclusion from these observations relates to the distribution of the stars with reference to the milky way, which we have already described. we have shown that stars of every magnitude, bright and faint, show a tendency to crowd towards this belt. it is, therefore, remarkable that no such tendency is seen in the case of those stars which have proper motions large enough to be accurately determined. so far as yet appears, such stars are equally scattered over the heavens, without reference to the course of the milky way. the conclusion is obvious. these stars are all inside the girdle of the milky way, and within the sphere which contains them the distribution in space is approximately uniform. at least there is no well-marked condensation in the direction of the galaxy nor any marked thinning out towards its poles. what can we say as to the extent of this sphere? to answer this question, we have to consider whether there is any average or ordinary speed that a star has in space. a great number of motions in the line of sight--that is to say, in the direction of the line from us to the star--have been measured with great precision by campbell at the lick observatory, and by other astronomers. the statistical investigations of kaptoyn also throw much light on the subject. the results of these investigators agree well in showing an average speed in space--a straight-ahead motion we may call it--of twenty-one miles per second. some stars may move more slowly than this to any extent; others more rapidly. in two or three cases the speed exceeds one hundred miles per second, but these are quite exceptional. by taking several thousand stars having a given proper motion, we may form a general idea of their average distance, though a great number of them will exceed this average to a considerable extent. the conclusion drawn in this way would be that the stars having an apparent proper motion of " per century or more are mostly contained within, or lie not far outside of a sphere whose surface is at a distance from us of light-years. granting the volume of space which we have shown that nature seems to allow to each star, this sphere should contain , stars in all. there are about , stars known to have so large a proper motion as ". but there is no actual discordance between these results, because not only are there, in all probability, great numbers of stars of which the proper motion is not yet recognized, but there are within the sphere a great number of stars whose motion is less than the average. on the other hand, it is probable that a considerable number of the , stars lie at a distance at least one-half greater than that of the radius of the sphere. on the whole, it seems likely that, out to a distance of or even light-years, there is no marked inequality in star distribution. if we should explore the heavens to this distance, we should neither find the beginning of the milky way in one direction nor a very marked thinning out in the other. this conclusion is quite accordant with the probabilities of the case. if all the stars which form the groundwork of the milky way should be blotted out, we should probably find , , , perhaps even more, remaining. assigning to each star the space already shown to be its quota, we should require a sphere of about light-years radius to contain such a number of stars. at some such distance as this, we might find a thinning out of the stars in the direction of the galactic poles, or the commencement of the milky way in the direction of this stream. even if this were not found at the distance which we have supposed, it is quite certain that, at some greater distance, we should at least find that the region of the milky way is richer in stars than the region near the galactic poles. there is strong reason, based on the appearance of the stars of the milky way, their physical constitution, and their magnitudes as seen in the telescope, to believe that, were we placed on one of these stars, we should find the stars around us to be more thickly strewn than they are around our system. in other words, the quota of space filled by each star is probably less in the region of the milky way than it is near the centre where we seem to be situated. we are, therefore, presented with what seems to be the most extraordinary spectacle that the universe can offer, a ring of stars spanning it, and including within its limits by far the great majority of the stars within our system. we have in this spectacle another example of the unity which seems to pervade the system. we might imagine the latter so arranged as to show diversity to any extent. we might have agglomerations of stars like those of the milky way situated in some corner of the system, or at its centre, or scattered through it here and there in every direction. but such is not the case. there are, indeed, a few star-clusters scattered here and there through the system; but they are essentially different from the clusters of the milky way, and cannot be regarded as forming an important part of the general plan. in the case of the galaxy we have no such scattering, but find the stars built, as it were, into this enormous ring, having similar characteristics throughout nearly its whole extent, and having within it a nearly uniform scattering of stars, with here and there some collected into clusters. such, to our limited vision, now appears the universe as a whole. we have already alluded to the conclusion that an absolutely infinite system of stars would cause the entire heavens to be filled with a blaze of light as bright as the sun. it is also true that the attractive force within such a universe would be infinitely great in some direction or another. but neither of these considerations enables us to set a limit to the extent of our system. in two remarkable papers by lord kelvin which have recently appeared, the one being an address before the british association at its glasgow meeting, in , are given the results of some numerical computations pertaining to this subject. granting that the stars are scattered promiscuously through space with some approach to uniformity in thickness, and are of a known degree of brilliancy, it is easy to compute how far out the system must extend in order that, looking up at the sky, we shall see a certain amount of light coming from the invisible stars. granting that, in the general average, each star is as bright as the sun, and that their thickness is such that within a sphere of light-years there are , , , stars, if we inquire how far out such a system must be continued in order that the sky shall shine with even four per cent of the light of the sun, we shall find the distance of its boundary so great that millions of millions of years would be required for the light of the outer stars to reach the centre of the system. in view of the fact that this duration in time far exceeds what seems to be the possible life duration of a star, so far as our knowledge of it can extend, the mere fact that the sky does not glow with any such brightness proves little or nothing as to the extent of the system. we may, however, replace these purely negative considerations by inquiring how much light we actually get from the invisible stars of our system. here we can make a definite statement. mark out a small circle in the sky degree in diameter. the quantity of light which we receive on a cloudless and moonless night from the sky within this circle admits of actual determination. from the measures so far available it would seem that, in the general average, this quantity of light is not very different from that of a star of the fifth magnitude. this is something very different from a blaze of light. a star of the fifth magnitude is scarcely more than plainly visible to ordinary vision. the area of the whole sky is, in round numbers, about , times that of the circle we have described. it follows that the total quantity of light which we receive from all the stars is about equal to that of , stars of the fifth magnitude--somewhat more than of the first magnitude. this whole amount of light would have to be multiplied by , , to make a light equal to that of the sun. it is, therefore, not at all necessary to consider how far the system must extend in order that the heavens should blaze like the sun. adopting lord kelvin's hypothesis, we shall find that, in order that we may receive from the stars the amount of light we have designated, this system need not extend beyond some light-years. but this hypothesis probably overestimates the thickness of the stars in space. it does not seem probable that there are as many as , , , stars within the sphere of light-years. nor is it at all certain that the light of the average star is equal to that of the sun. it is impossible, in the present state of our knowledge, to assign any definite value to this average. to do so is a problem similar to that of assigning an average weight to each component of the animal creation, from the microscopic insects which destroy our plants up to the elephant. what we can say with a fair approximation to confidence is that, if we could fly out in any direction to a distance of , , perhaps even of , , light-years, we should find that we had left a large fraction of our system behind us. we should see its boundary in the direction in which we had travelled much more certainly than we see it from our stand-point. we should not dismiss this branch of the subject without saying that considerations are frequently adduced by eminent authorities which tend to impair our confidence in almost any conclusion as to the limits of the stellar system. the main argument is based on the possibility that light is extinguished in its passage through space; that beyond a certain distance we cannot see a star, however bright, because its light is entirely lost before reaching us. that there could be any loss of light in passing through an absolute vacuum of any extent cannot be admitted by the physicist of to-day without impairing what he considers the fundamental principles of the vibration of light. but the possibility that the celestial spaces are pervaded by matter which might obstruct the passage of light is to be considered. we know that minute meteoric particles are flying through our system in such numbers that the earth encounters several millions of them every day, which appear to us in the familiar phenomena of shooting-stars. if such particles are scattered through all space, they must ultimately obstruct the passage of light. we know little of the size of these bodies, but, from the amount of energy contained in their light as they are consumed in the passage through our atmosphere, it does not seem at all likely that they are larger than grains of sand or, perhaps, minute pebbles. they are probably vastly more numerous in the vicinity of the sun than in the interstellar spaces, since they would naturally tend to be collected by the sun's attraction. in fact there are some reasons for believing that most of these bodies are the debris of comets; and the latter are now known to belong to the solar system, and not to the universe at large. but whatever view we take of these possibilities, they cannot invalidate our conclusion as to the general structure of the stellar system as we know it. were meteors so numerous as to cut off a large fraction of the light from the more distant stars, we should see no milky way, but the apparent thickness of the stars in every direction would be nearly the same. the fact that so many more of these objects are seen around the galactic belt than in the direction of its poles shows that, whatever extinction light may suffer in going through the greatest distances, we see nearly all that comes from stars not more distant than the milky way itself. intimately connected with the subject we have discussed is the question of the age of our system, if age it can be said to have. in considering this question, the simplest hypothesis to suggest itself is that the universe has existed forever in some such form as we now see it; that it is a self-sustaining system, able to go on forever with only such cycles of transformation as may repeat themselves indefinitely, and may, therefore, have repeated themselves indefinitely in the past. ordinary observation does not make anything known to us which would seem to invalidate this hypothesis. in looking upon the operations of the universe, we may liken ourselves to a visitor to the earth from another sphere who has to draw conclusions about the life of an individual man from observations extending through a few days. during that time, he would see no reason why the life of the man should have either a beginning or an end. he sees a daily round of change, activity and rest, nutrition and waste; but, at the end of the round, the individual is seemingly restored to his state of the day before. why may not this round have been going on forever, and continue in the future without end? it would take a profounder course of observation and a longer time to show that, notwithstanding this seeming restoration, an imperceptible residual of vital energy, necessary to the continuance of life, has not been restored, and that the loss of this residuum day by day must finally result in death. the case is much the same with the great bodies of the universe. although, to superficial observation, it might seem that they could radiate their light forever, the modern generalizations of physics show that such cannot be the case. the radiation of light necessarily involves a corresponding loss of heat and with it the expenditure of some form of energy. the amount of energy within any body is necessarily limited. the supply must be exhausted unless the energy of the light sent out into infinite space is, in some way, restored to the body which expended it. the possibility of such a restoration completely transcends our science. how can the little vibration which strikes our eye from some distant star, and which has been perhaps thousands of years in reaching us, find its way back to its origin? the light emitted by the sun , years ago is to-day pursuing its way in a sphere whose surface is , light-years distant on all sides. science has nothing even to suggest the possibility of its restoration, and the most delicate observations fail to show any return from the unfathomable abyss. up to the time when radium was discovered, the most careful investigations of all conceivable sources of supply had shown only one which could possibly be of long duration. this is the contraction which is produced in the great incandescent bodies of the universe by the loss of the heat which they radiate. as remarked in the preceding essay, the energy generated by the sun's contraction could not have kept up its present supply of heat for much more than twenty or thirty millions of years, while the study of earth and ocean shows evidence of the action of a series of causes which must have been going on for hundreds of millions of years. the antagonism between the two conclusions is even more marked than would appear from this statement. the period of the sun's heat set by the astronomical physicist is that during which our luminary could possibly have existed in its present form. the period set by the geologist is not merely that of the sun's existence, but that during which the causes effecting geological changes have not undergone any complete revolution. if, at any time, the sun radiated much less than its present amount of heat, no water could have existed on the earth's surface except in the form of ice; there would have been scarcely any evaporation, and the geological changes due to erosion could not have taken place. moreover, the commencement of the geological operations of which we speak is by no means the commencement of the earth's existence. the theories of both parties agree that, for untold aeons before the geological changes now visible commenced, our planet was a molten mass, perhaps even an incandescent globe like the sun. during all those aeons the sun must have been in existence as a vast nebulous mass, first reaching as far as the earth's orbit, and slowly contracting its dimensions. and these aeons are to be included in any estimate of the age of the sun. the doctrine of cosmic evolution--the theory which in former times was generally known as the nebular hypothesis--that the heavenly bodies were formed by the slow contraction of heated nebulous masses, is indicated by so many facts that it seems scarcely possible to doubt it except on the theory that the laws of nature were, at some former time, different from those which we now see in operation. granting the evolutionary hypothesis, every star has its lifetime. we can even lay down the law by which it passes from infancy to old age. all stars do not have the same length of life; the rule is that the larger the star, or the greater the mass of matter which composes it, the longer will it endure. up to the present time, science can do nothing more than point out these indications of a beginning, and their inevitable consequence, that there is to be an end to the light and heat of every heavenly body. but no cautious thinker can treat such a subject with the ease of ordinary demonstration. the investigator may even be excused if he stands dumb with awe before the creation of his own intellect. our accurate records of the operations of nature extend through only two or three centuries, and do not reach a satisfactory standard until within a single century. the experience of the individual is limited to a few years, and beyond this period he must depend upon the records of his ancestors. all his knowledge of the laws of nature is derived from this very limited experience. how can he essay to describe what may have been going on hundreds of millions of years in the past? can he dare to say that nature was the same then as now? it is a fundamental principle of the theory of evolution, as developed by its greatest recent expounder, that matter itself is eternal, and that all the changes which have taken place in the universe, so far as made up of matter, are in the nature of transformations of this eternal substance. but we doubt whether any physical philosopher of the present day would be satisfied to accept any demonstration of the eternity of matter. all he would admit is that, so far as his observation goes, no change in the quantity of matter can be produced by the action of any known cause. it seems to be equally uncreatable and indestructible. but he would, at the same time, admit that his experience no more sufficed to settle the question than the observation of an animal for a single day would settle the question of the duration of its life, or prove that it had neither beginning nor end. he would probably admit that even matter itself may be a product of evolution. the astronomer finds it difficult to conceive that the great nebulous masses which he sees in the celestial spaces--millions of times larger than the whole solar system, yet so tenuous that they offer not the slightest obstruction to the passage of a ray of light through their whole length--situated in what seems to be a region of eternal cold, below anything that we can produce on the earth's surface, yet radiating light, and with it heat, like an incandescent body--can be made up of the same kind of substance that we have around us on the earth's surface. who knows but that the radiant property that becquerel has found in certain forms of matter may be a residuum of some original form of energy which is inherent in great cosmical masses, and has fed our sun during all the ages required by the geologist for the structure of the earth's crusts? it may be that in this phenomenon we have the key to the great riddle of the universe, with which profounder secrets of matter than any we have penetrated will be opened to the eyes of our successors. iv the extent of the universe we cannot expect that the wisest men of our remotest posterity, who can base their conclusions upon thousands of years of accurate observation, will reach a decision on this subject without some measure of reserve. such being the case, it might appear the dictate of wisdom to leave its consideration to some future age, when it may be taken up with better means of information than we now possess. but the question is one which will refuse to be postponed so long as the propensity to think of the possibilities of creation is characteristic of our race. the issue is not whether we shall ignore the question altogether, like eve in the presence of raphael; but whether in studying it we shall confine our speculations within the limits set by sound scientific reasoning. essaying to do this, i invite the reader's attention to what science may suggest, admitting in advance that the sphere of exact knowledge is small compared with the possibilities of creation, and that outside this sphere we can state only more or less probable conclusions. the reader who desires to approach this subject in the most receptive spirit should begin his study by betaking himself on a clear, moonless evening, when he has no earthly concern to disturb the serenity of his thoughts, to some point where he can lie on his back on bench or roof, and scan the whole vault of heaven at one view. he can do this with the greatest pleasure and profit in late summer or autumn--winter would do equally well were it possible for the mind to rise so far above bodily conditions that the question of temperature should not enter. the thinking man who does this under circumstances most favorable for calm thought will form a new conception of the wonder of the universe. if summer or autumn be chosen, the stupendous arch of the milky way will pass near the zenith, and the constellation lyra, led by its beautiful blue vega of the first magnitude, may be not very far from that point. south of it will be seen the constellation aquila, marked by the bright altair, between two smaller but conspicuous stars. the bright arcturus will be somewhere in the west, and, if the observation is not made too early in the season, aldebaran will be seen somewhere in the east. when attention is concentrated on the scene the thousands of stars on each side of the milky way will fill the mind with the consciousness of a stupendous and all-embracing frame, beside which all human affairs sink into insignificance. a new idea will be formed of such a well-known fact of astronomy as the motion of the solar system in space, by reflecting that, during all human history, the sun, carrying the earth with it, has been flying towards a region in or just south of the constellation lyra, with a speed beyond all that art can produce on earth, without producing any change apparent to ordinary vision in the aspect of the constellation. not only lyra and aquila, but every one of the thousand stars which form the framework of the sky, were seen by our earliest ancestors just as we see them now. bodily rest may be obtained at any time by ceasing from our labors, and weary systems may find nerve rest at any summer resort; but i know of no way in which complete rest can be obtained for the weary soul--in which the mind can be so entirely relieved of the burden of all human anxiety--as by the contemplation of the spectacle presented by the starry heavens under the conditions just described. as we make a feeble attempt to learn what science can tell us about the structure of this starry frame, i hope the reader will allow me to at least fancy him contemplating it in this way. the first question which may suggest itself to the inquiring reader is: how is it possible by any methods of observation yet known to the astronomer to learn anything about the universe as a whole? we may commence by answering this question in a somewhat comprehensive way. it is possible only because the universe, vast though it is, shows certain characteristics of a unified and bounded whole. it is not a chaos, it is not even a collection of things, each of which came into existence in its own separate way. if it were, there would be nothing in common between two widely separate regions of the universe. but, as a matter of fact, science shows unity in the whole structure, and diversity only in details. the milky way itself will be seen by the most ordinary observer to form a single structure. this structure is, in some sort, the foundation on which the universe is built. it is a girdle which seems to span the whole of creation, so far as our telescopes have yet enabled us to determine what creation is; and yet it has elements of similarity in all its parts. what has yet more significance, it is in some respects unlike those parts of the universe which lie without it, and even unlike those which lie in that central region within it where our system is now situated. the minute stars, individually far beyond the limit of visibility to the naked eye, which form its cloudlike agglomerations, are found to be mostly bluer in color, from one extreme to the other, than the general average of the stars which make up the rest of the universe. in the preceding essay on the structure of the universe, we have pointed out several features of the universe showing the unity of the whole. we shall now bring together these and other features with a view of showing their relation to the question of the extent of the universe. the milky way being in a certain sense the foundation on which the whole system is constructed, we have first to notice the symmetry of the whole. this is seen in the fact that a certain resemblance is found in any two opposite regions of the sky, no matter where we choose them. if we take them in the milky way, the stars are more numerous than elsewhere; if we take opposite regions in or near the milky way, we shall find more stars in both of them than elsewhere; if we take them in the region anywhere around the poles of the milky way, we shall find fewer stars, but they will be equally numerous in each of the two regions. we infer from this that whatever cause determined the number of the stars in space was of the same nature in every two antipodal regions of the heavens. another unity marked with yet more precision is seen in the chemical elements of which stars are composed. we know that the sun is composed of the same elements which we find on the earth and into which we resolve compounds in our laboratories. these same elements are found in the most distant stars. it is true that some of these bodies seem to contain elements which we do not find on earth. but as these unknown elements are scattered from one extreme of the universe to the other, they only serve still further to enforce the unity which runs through the whole. the nebulae are composed, in part at least, of forms of matter dissimilar to any with which we are acquainted. but, different though they may be, they are alike in their general character throughout the whole field we are considering. even in such a feature as the proper motions of the stars, the same unity is seen. the reader doubtless knows that each of these objects is flying through space on its own course with a speed comparable with that of the earth around the sun. these speeds range from the smallest limit up to more than one hundred miles a second. such diversity might seem to detract from the unity of the whole; but when we seek to learn something definite by taking their average, we find this average to be, so far as can yet be determined, much the same in opposite regions of the universe. quite recently it has become probable that a certain class of very bright stars known as orion stars--because there are many of them in the most brilliant of our constellations--which are scattered along the whole course of the milky way, have one and all, in the general average, slower motions than other stars. here again we have a definable characteristic extending through the universe. in drawing attention to these points of similarity throughout the whole universe, it must not be supposed that we base our conclusions directly upon them. the point they bring out is that the universe is in the nature of an organized system; and it is upon the fact of its being such a system that we are able, by other facts, to reach conclusions as to its structure, extent, and other characteristics. one of the great problems connected with the universe is that of its possible extent. how far away are the stars? one of the unities which we have described leads at once to the conclusion that the stars must be at very different distances from us; probably the more distant ones are a thousand times as far as the nearest; possibly even farther than this. this conclusion may, in the first place, be based on the fact that the stars seem to be scattered equally throughout those regions of the universe which are not connected with the milky way. to illustrate the principle, suppose a farmer to sow a wheat-field of entirely unknown extent with ten bushels of wheat. we visit the field and wish to have some idea of its acreage. we may do this if we know how many grains of wheat there are in the ten bushels. then we examine a space two or three feet square in any part of the field and count the number of grains in that space. if the wheat is equally scattered over the whole field, we find its extent by the simple rule that the size of the field bears the same proportion to the size of the space in which the count was made that the whole number of grains in the ten bushels sown bears to the number of grains counted. if we find ten grains in a square foot, we know that the number of square feet in the whole field is one-tenth that of the number of grains sown. so it is with the universe of stars. if the latter are sown equally through space, the extent of the space occupied must be proportional to the number of stars which it contains. but this consideration does not tell us anything about the actual distance of the stars or how thickly they may be scattered. to do this we must be able to determine the distance of a certain number of stars, just as we suppose the farmer to count the grains in a certain small extent of his wheat-field. there is only one way in which we can make a definite measure of the distance of any one star. as the earth swings through its vast annual circuit round the sun, the direction of the stars must appear to be a little different when seen from one extremity of the circuit than when seen from the other. this difference is called the parallax of the stars; and the problem of measuring it is one of the most delicate and difficult in the whole field of practical astronomy. the nineteenth century was well on its way before the instruments of the astronomer were brought to such perfection as to admit of the measurement. from the time of copernicus to that of bessel many attempts had been made to measure the parallax of the stars, and more than once had some eager astronomer thought himself successful. but subsequent investigation always showed that he had been mistaken, and that what he thought was the effect of parallax was due to some other cause, perhaps the imperfections of his instrument, perhaps the effect of heat and cold upon it or upon the atmosphere through which he was obliged to observe the star, or upon the going of his clock. thus things went on until , when bessel announced that measures with a heliometer--the most refined instrument that has ever been used in measurement--showed that a certain star in the constellation cygnus had a parallax of one-third of a second. it may be interesting to give an idea of this quantity. suppose one's self in a house on top of a mountain looking out of a window one foot square, at a house on another mountain one hundred miles away. one is allowed to look at that distant house through one edge of the pane of glass and then through the opposite edge; and he has to determine the change in the direction of the distant house produced by this change of one foot in his own position. from this he is to estimate how far off the other mountain is. to do this, one would have to measure just about the amount of parallax that bessel found in his star. and yet this star is among the few nearest to our system. the nearest star of all, alpha centauri, visible only in latitudes south of our middle ones, is perhaps half as far as bessel's star, while sirius and one or two others are nearly at the same distance. about stars, all told, have had their parallax measured with a greater or less degree of probability. the work is going on from year to year, each successive astronomer who takes it up being able, as a general rule, to avail himself of better instruments or to use a better method. but, after all, the distances of even some of the stars carefully measured must still remain quite doubtful. let us now return to the idea of dividing the space in which the universe is situated into concentric spheres drawn at various distances around our system as a centre. here we shall take as our standard a distance , times that of the sun from the earth. regarding this as a unit, we imagine ourselves to measure out in any direction a distance twice as great as this--then another equal distance, making one three times as great, and so indefinitely. we then have successive spheres of which we take the nearer one as the unit. the total space filled by the second sphere will be times the unit; that of the third space times, and so on, as the cube of each distance. since each sphere includes all those within it, the volume of space between each two spheres will be proportional to the difference of these numbers--that is, to , , , etc. comparing these volumes with the number of stars probably within them, the general result up to the present time is that the number of stars in any of these spheres will be about equal to the units of volume which they comprise, when we take for this unit the smallest and innermost of the spheres, having a radius , times the sun's distance. we are thus enabled to form some general idea of how thickly the stars are sown through space. we cannot claim any numerical exactness for this idea, but in the absence of better methods it does afford us some basis for reasoning. now we can carry on our computation as we supposed the farmer to measure the extent of his wheat-field. let us suppose that there are , , stars in the heavens. this is an exceedingly rough estimate, but let us make the supposition for the time being. accepting the view that they are nearly equally scattered throughout space, it will follow that they must be contained within a volume equal to , , times the sphere we have taken as our unit. we find the distance of the surface of this sphere by extracting the cube root of this number, which gives us . we may, therefore, say, as the result of a very rough estimate, that the number of stars we have supposed would be contained within a distance found by multiplying , times the distance of the sun by ; that is, that they are contained within a region whose boundary is , , times the distance of the sun. this is a distance through which light would travel in about years. it is not impossible that the number of stars is much greater than that we have supposed. let us grant that there are eight times as many, or , , , . then we should have to extend the boundary of our universe twice as far, carrying it to a distance which light would require years to travel. there is another method of estimating the thickness with which stars are sown through space, and hence the extent of the universe, the result of which will be of interest. it is based on the proper motion of the stars. one of the greatest triumphs of astronomy of our time has been the measurement of the actual speed at which many of the stars are moving to or from us in space. these measures are made with the spectroscope. unfortunately, they can be best made only on the brighter stars--becoming very difficult in the case of stars not plainly visible to the naked eye. still the motions of several hundreds have been measured and the number is constantly increasing. a general result of all these measures and of other estimates may be summed up by saying that there is a certain average speed with which the individual stars move in space; and that this average is about twenty miles per second. we are also able to form an estimate as to what proportion of the stars move with each rate of speed from the lowest up to a limit which is probably as high as miles per second. knowing these proportions we have, by observation of the proper motions of the stars, another method of estimating how thickly they are scattered in space; in other words, what is the volume of space which, on the average, contains a single star. this method gives a thickness of the stars greater by about twenty-five per cent, than that derived from the measures of parallax. that is to say, a sphere like the second we have proposed, having a radius , times the distance of the sun, and therefore a diameter , , times this distance, would, judging by the proper motions, have ten or twelve stars contained within it, while the measures of parallax only show eight stars within the sphere of this diameter having the sun as its centre. the probabilities are in favor of the result giving the greater thickness of the stars. but, after all, the discrepancy does not change the general conclusion as to the limits of the visible universe. if we cannot estimate its extent with the same certainty that we can determine the size of the earth, we can still form a general idea of it. the estimates we have made are based on the supposition that the stars are equally scattered in space. we have good reason to believe that this is true of all the stars except those of the milky way. but, after all, the latter probably includes half the whole number of stars visible with a telescope, and the question may arise whether our results are seriously wrong from this cause. this question can best be solved by yet another method of estimating the average distance of certain classes of stars. the parallaxes of which we have heretofore spoken consist in the change in the direction of a star produced by the swing of the earth from one side of its orbit to the other. but we have already remarked that our solar system, with the earth as one of its bodies, has been journeying straightforward through space during all historic times. it follows, therefore, that we are continually changing the position from which we view the stars, and that, if the latter were at rest, we could, by measuring the apparent speed with which they are moving in the opposite direction from that of the earth, determine their distance. but since every star has its own motion, it is impossible, in any one case, to determine how much of the apparent motion is due to the star itself, and how much to the motion of the solar system through space. yet, by taking general averages among groups of stars, most of which are probably near each other, it is possible to estimate the average distance by this method. when an attempt is made to apply it, so as to obtain a definite result, the astronomer finds that the data now available for the purpose are very deficient. the proper motion of a star can be determined only by comparing its observed position in the heavens at two widely separate epochs. observations of sufficient precision for this purpose were commenced about at the greenwich observatory, by bradley, then astronomer royal of england. but out of stars which he determined, only a few are available for the purpose. even since his time, the determinations made by each generation of astronomers have not been sufficiently complete and systematic to furnish the material for anything like a precise determination of the proper motions of stars. to determine a single position of any one star involves a good deal of computation, and if we reflect that, in order to attack the problem in question in a satisfactory way, we should have observations of , , of these bodies made at intervals of at least a considerable fraction of a century, we see what an enormous task the astronomers dealing with this problem have before them, and how imperfect must be any determination of the distance of the stars based on our motion through space. so far as an estimate can be made, it seems to agree fairly well with the results obtained by the other methods. speaking roughly, we have reason, from the data so far available, to believe that the stars of the milky way are situated at a distance between , , and , , times the distance of the sun. at distances less than this it seems likely that the stars are distributed through space with some approach to uniformity. we may state as a general conclusion, indicated by several methods of making the estimate, that nearly all the stars which we can see with our telescopes are contained within a sphere not likely to be much more than , , times the distance of the sun. the inquiring reader may here ask another question. granting that all the stars we can see are contained within this limit, may there not be any number of stars outside the limit which are invisible only because they are too far away to be seen? this question may be answered quite definitely if we grant that light from the most distant stars meets with no obstruction in reaching us. the most conclusive answer is afforded by the measure of starlight. if the stars extended out indefinitely, then the number of those of each order of magnitude would be nearly four times that of the magnitude next brighter. for example, we should have nearly four times as many stars of the sixth magnitude as of the fifth; nearly four times as many of the seventh as of the sixth, and so on indefinitely. now, it is actually found that while this ratio of increase is true for the brighter stars, it is not so for the fainter ones, and that the increase in the number of the latter rapidly falls off when we make counts of the fainter telescopic stars. in fact, it has long been known that, were the universe infinite in extent, and the stars equally scattered through all space, the whole heavens would blaze with the light of countless millions of distant stars separately invisible even with the telescope. the only way in which this conclusion can be invalidated is by the possibility that the light of the stars is in some way extinguished or obstructed in its passage through space. a theory to this effect was propounded by struve nearly a century ago, but it has since been found that the facts as he set them forth do not justify the conclusion, which was, in fact, rather hypothetical. the theories of modern science converge towards the view that, in the pure ether of space, no single ray of light can ever be lost, no matter how far it may travel. but there is another possible cause for the extinction of light. during the last few years discoveries of dark and therefore invisible stars have been made by means of the spectroscope with a success which would have been quite incredible a very few years ago, and which, even to-day, must excite wonder and admiration. the general conclusion is that, besides the shining stars which exist in space, there may be any number of dark ones, forever invisible in our telescopes. may it not be that these bodies are so numerous as to cut off the light which we would otherwise receive from the more distant bodies of the universe? it is, of course, impossible to answer this question in a positive way, but the probable conclusion is a negative one. we may say with certainty that dark stars are not so numerous as to cut off any important part of the light from the stars of the milky way, because, if they did, the latter would not be so clearly seen as it is. since we have reason to believe that the milky way comprises the more distant stars of our system, we may feel fairly confident that not much light can be cut off by dark bodies from the most distant region to which our telescopes can penetrate. up to this distance we see the stars just as they are. even within the limit of the universe as we understand it, it is likely that more than one-half the stars which actually exist are too faint to be seen by human vision, even when armed with the most powerful telescopes. but their invisibility is due only to their distance and the faintness of their intrinsic light, and not to any obstructing agency. the possibility of dark stars, therefore, does not invalidate the general conclusions at which our survey of the subject points. the universe, so far as we can see it, is a bounded whole. it is surrounded by an immense girdle of stars, which, to our vision, appears as the milky way. while we cannot set exact limits to its distance, we may yet confidently say that it is bounded. it has uniformities running through its vast extent. could we fly out to distances equal to that of the milky way, we should find comparatively few stars beyond the limits of that girdle. it is true that we cannot set any definite limit and say that beyond this nothing exists. what we can say is that the region containing the visible stars has some approximation to a boundary. we may fairly anticipate that each successive generation of astronomers, through coming centuries, will obtain a little more light on the subject--will be enabled to make more definite the boundaries of our system of stars, and to draw more and more probable conclusions as to the existence or non-existence of any object outside of it. the wise investigator of to-day will leave to them the task of putting the problem into a more positive shape. v making and using a telescope the impression is quite common that satisfactory views of the heavenly bodies can be obtained only with very large telescopes, and that the owner of a small one must stand at a great disadvantage alongside of the fortunate possessor of a great one. this is not true to the extent commonly supposed. sir william herschel would have been delighted to view the moon through what we should now consider a very modest instrument; and there are some objects, especially the moon, which commonly present a more pleasing aspect through a small telescope than through a large one. the numerous owners of small telescopes throughout the country might find their instruments much more interesting than they do if they only knew what objects were best suited to examination with the means at their command. there are many others, not possessors of telescopes, who would like to know how one can be acquired, and to whom hints in this direction will be valuable. we shall therefore give such information as we are able respecting the construction of a telescope, and the more interesting celestial objects to which it may be applied. whether the reader does or does not feel competent to undertake the making of a telescope, it may be of interest to him to know how it is done. first, as to the general principles involved, it is generally known that the really vital parts of the telescope, which by their combined action perform the office of magnifying the object looked at, are two in number, the objective and the eye-piece. the former brings the rays of light which emanate from the object to the focus where the image of the object is formed. the eye-piece enables the observer to see this image to the best advantage. the functions of the objective as well as those of the eye-piece may, to a certain extent, each be performed by a single lens. galileo and his contemporaries made their telescopes in this way, because they knew of no way in which two lenses could be made to do better than one. but every one who has studied optics knows that white light passing through a single lens is not all brought to the same focus, but that the blue light will come to a focus nearer the objective than the red light. there will, in fact, be a succession of images, blue, green, yellow, and red, corresponding to the colors of the spectrum. it is impossible to see these different images clearly at the same time, because each of them will render all the others indistinct. the achromatic object-glass, invented by dollond, about , obviates this difficulty, and brings all the rays to nearly the same focus. nearly every one interested in the subject is aware that this object-glass is composed of two lenses--a concave one of flint-glass and a convex one of crown-glass, the latter being on the side towards the object. this is the one vital part of the telescope, the construction of which involves the greatest difficulty. once in possession of a perfect object-glass, the rest of the telescope is a matter of little more than constructive skill which there is no difficulty in commanding. the construction of the object-glass requires two completely distinct processes: the making of the rough glass, which is the work of the glass-maker; and the grinding and polishing into shape, which is the work of the optician. the ordinary glass of commerce will not answer the purpose of the telescope at all, because it is not sufficiently clear and homogeneous. optical glass, as it is called, must be made of materials selected and purified with the greatest care, and worked in a more elaborate manner than is necessary in any other kind of glass. in the time of dollond it was found scarcely possible to make good disks of flint-glass more than three or four inches in diameter. early in the present century, guinand, of switzerland, invented a process by which disks of much larger size could be produced. in conjunction with the celebrated fraunhofer he made disks of nine or ten inches in diameter, which were employed by his colaborer in constructing the telescopes which were so famous in their time. he was long supposed to be in possession of some secret method of avoiding the difficulties which his predecessors had met. it is now believed that this secret, if one it was, consisted principally in the constant stirring of the molten glass during the process of manufacture. however this may be, it is a curious historical fact that the most successful makers of these great disks of glass have either been of the family of guinand, or successors, in the management of the family firm. it was feil, a son-in-law or near relative, who made the glass from which clark fabricated the lenses of the great telescope of the lick observatory. his successor, mantois, of paris, carried the art to a point of perfection never before approached. the transparency and uniformity of his disks as well as the great size to which he was able to carry them would suggest that he and his successors have out-distanced all competitors in the process. he it was who made the great -inch lens for the yerkes observatory. as optical glass is now made, the material is constantly stirred with an iron rod during all the time it is melting in the furnace, and after it has begun to cool, until it becomes so stiff that the stirring has to cease. it is then placed, pot and all, in the annealing furnace, where it is kept nearly at a melting heat for three weeks or more, according to the size of the pot. when the furnace has cooled off, the glass is taken out, and the pot is broken from around it, leaving only the central mass of glass. having such a mass, there is no trouble in breaking it up into pieces of all desirable purity, and sufficiently large for moderate-sized telescopes. but when a great telescope of two feet aperture or upward is to be constructed, very delicate and laborious operations have to be undertaken. the outside of the glass has first to be chipped off, because it is filled with impurities from the material of the pot itself. but this is not all. veins of unequal density are always found extending through the interior of the mass, no way of avoiding them having yet been discovered. they are supposed to arise from the materials of the pot and stirring rod, which become mixed in with the glass in consequence of the intense heat to which all are subjected. these veins must, so far as possible, be ground or chipped out with the greatest care. the glass is then melted again, pressed into a flat disk, and once more put into the annealing oven. in fact, the operation of annealing must be repeated every time the glass is melted. when cooled, it is again examined for veins, of which great numbers are sure to be found. the problem now is to remove these by cutting and grinding without either breaking the glass in two or cutting a hole through it. if the parts of the glass are once separated, they can never be joined without producing a bad scar at the point of junction. so long, however, as the surface is unbroken, the interior parts of the glass can be changed in form to any extent. having ground out the veins as far as possible, the glass is to be again melted, and moulded into proper shape. in this mould great care must be taken to have no folding of the surface. imagining the latter to be a sort of skin enclosing the melted glass inside, it must be raised up wherever the glass is thinnest, and the latter allowed to slowly run together beneath it. [illustration with caption: the glass disk.] if the disk is of flint, all the veins must be ground out on the first or second trial, because after two or three mouldings the glass will lose its transparency. a crown disk may, however, be melted a number of times without serious injury. in many cases--perhaps the majority--the artisan finds that after all his months of labor he cannot perfectly clear his glass of the noxious veins, and he has to break it up into smaller pieces. when he finally succeeds, the disk has the form of a thin grindstone two feet or upward in diameter, according to the size of the telescope to be made, and from two to three inches in thickness. the glass is then ready for the optician. [illustration with caption: the optician's tool.] the first process to be performed by the optician is to grind the glass into the shape of a lens with perfectly spherical surfaces. the convex surface must be ground in a saucer-shaped tool of corresponding form. it is impossible to make a tool perfectly spherical in the first place, but success may be secured on the geometrical principle that two surfaces cannot fit each other in all positions unless both are perfectly spherical. the tool of the optician is a very simple affair, being nothing more than a plate of iron somewhat larger, perhaps a fourth, than the lens to be ground to the corresponding curvature. in order to insure its changing to fit the glass, it is covered on the interior with a coating of pitch from an eighth to a quarter of an inch thick. this material is admirably adapted to the purpose because it gives way certainly, though very slowly, to the pressure of the glass. in order that it may have room to change its form, grooves are cut through it in both directions, so as to leave it in the form of squares, like those on a chess-board. [illustration with caption: the optician's tool.] it is then sprinkled over with rouge, moistened with water, and gently warmed. the roughly ground lens is then placed upon it, and moved from side to side. the direction of the motion is slightly changed with every stroke, so that after a dozen or so of strokes the lines of motion will lie in every direction on the tool. this change of direction is most readily and easily effected by the operator slowly walking around as he polishes, at the same time the lens is to be slowly turned around either in the opposite direction or more rapidly yet in the same direction, so that the strokes of the polisher shall cross the lens in all directions. this double motion insures every part of the lens coming into contact with every part of the polisher, and moving over it in every direction. then whatever parts either of the lens or of the polisher may be too high to form a spherical surface will be gradually worn down, thus securing the perfect sphericity of both. [illustration with caption: grinding a large lens.] when the polishing is done by machinery, which is the custom in europe, with large lenses, the polisher is slid back and forth over the lens by means of a crank attached to a revolving wheel. the polisher is at the same time slowly revolving around a pivot at its centre, which pivot the crank works into, and the glass below it is slowly turned in an opposite direction. thus the same effect is produced as in the other system. those who practice this method claim that by thus using machinery the conditions of a uniform polish for every part of the surface can be more perfectly fulfilled than by a hand motion. the results, however, do not support this view. no european optician will claim to do better than the american firm of alvan clark & sons in producing uniformly good object-glasses, and this firm always does the work by hand, moving the glass over the polisher, and not the polisher over the glass. having brought both flint and crown glasses into proper figure by this process, they are joined together, and tested by observations either upon a star in the heavens, or some illuminated point at a little distance on the ground. the reflection of the sun from a drop of quicksilver, a thermometer bulb, or even a piece of broken bottle, makes an excellent artificial star. the very best optician will always find that on a first trial his glass is not perfect. he will find that he has not given exactly the proper curves to secure achromatism. he must then change the figure of one or both the glasses by polishing it upon a tool of slightly different curvature. he may also find that there is some spherical aberration outstanding. he must then alter his curve so as to correct this. the correction of these little imperfections in the figures of the lenses so as to secure perfect vision through them is the most difficult branch of the art of the optician, and upon his skill in practising it will depend more than upon anything else his ultimate success and reputation. the shaping of a pair of lenses in the way we have described is not beyond the power of any person of ordinary mechanical ingenuity, possessing the necessary delicacy of touch and appreciation of the problem he is attacking. but to make a perfect objective of considerable size, which shall satisfy all the wants of the astronomer, is an undertaking requiring such accuracy of eyesight, and judgment in determining where the error lies, and such skill in manipulating so as to remove the defects, that the successful men in any one generation can be counted on one's fingers. in order that the telescope may finally perform satisfactorily it is not sufficient that the lenses should both be of proper figure; they must also both be properly centred in their cells. if either lens is tipped aside, or slid out from its proper central line, the definition will be injured. as this is liable to happen with almost any telescope, we shall explain how the proper adjustment is to be made. the easiest way to test this adjustment is to set the cell with the two glasses of the objective in it against a wall at night, and going to a short distance, observe the reflection in the glass of the flame of a candle held in the hand. three or four reflections will be seen from the different surfaces. the observer, holding the candle before his eye, and having his line of sight as close as possible to the flame, must then move until the different images of the flame coincide with each other. if he cannot bring them into coincidence, owing to different pairs coinciding on different sides of the flame, the glasses are not perfectly centred upon each other. when the centring is perfect, the observer having the light in the line of the axes of the lenses, and (if it were possible to do so) looking through the centre of the flame, would see the three or four images all in coincidence. as he cannot see through the flame itself, he must look first on one side and then on the other, and see if the arrangement of the images seen in the lenses is symmetrical. if, going to different distances, he finds no deviation from symmetry, in this respect the adjustment is near enough for all practical purposes. a more artistic instrument than a simple candle is a small concave reflector pierced through its centre, such as is used by physicians in examining the throat. [illustration with caption: image of candle-flame in object-glass.] [illustration with caption: testing adjustment of object-glass.] place this reflector in the prolongation of the optical axis, set the candle so that the light from the reflector shall be shown through the glass, and look through the opening. images of the reflector itself will then be seen in the object-glass, and if the adjustment is perfect, the reflector can be moved so that they will all come into coincidence together. when the objective is in the tube of the telescope, it is always well to examine this adjustment from time to time, holding the candle so that its light shall shine through the opening perpendicularly upon the object-glass. the observer looks upon one side of the flame, and then upon the other, to see if the images are symmetrical in the different positions. if in order to see them in this way the candle has to be moved to one side of the central line of the tube, the whole objective must be adjusted. if two images coincide in one position of the candle-flame, and two in another position, so that they cannot all be brought together in any position, it shows that the glasses are not properly adjusted in their cell. it may be remarked that this last adjustment is the proper work of the optician, since it is so difficult that the user of the telescope cannot ordinarily effect it. but the perpendicularity of the whole objective to the tube of the telescope is liable to be deranged in use, and every one who uses such an instrument should be able to rectify an error of this kind. the question may be asked, how much of a telescope can an amateur observer, under any circumstances, make for himself? as a general rule, his work in this direction must be confined to the tube and the mounting. we should not, it is true, dare to assert that any ingenious young man, with a clear appreciation of optical principles, could not soon learn to grind and polish an object-glass for himself by the method we have described, and thus obtain a much better instrument than galileo ever had at his command. but it would be a wonderful success if his home-made telescope was equal to the most indifferent one which can be bought at an optician's. the objective, complete in itself, can be purchased at prices depending upon the size. [footnote: the following is a rough rule for getting an idea of the price of an achromatic objective, made to order, of the finest quality. take the cube of the diameter in inches, or, which is the same thing, calculate the contents of a cubical box which would hold a sphere of the same diameter as the clear aperture of the glass. the price of the glass will then range from $ to $ . for each cubic inch in this box. for example, the price of a four-inch objective will probably range from $ to $ . very small object-glasses of one or two inches may be a little higher than would be given by this rule. instruments which are not first-class, but will answer most of the purposes of the amateur, are much cheaper.] [illustration with caption: a very primitive mounting for a telescope.] the tube for the telescope may be made of paper, by pasting a great number of thicknesses around a long wooden cylinder. a yet better tube is made of a simple wooden box. the best material, however, is metal, because wood and pasteboard are liable both to get out of shape, and to swell under the influence of moisture. tin, if it be of sufficient thickness, would be a very good material. the brighter it is kept, the better. the work of fitting the objective into one end of a tin tube of double thickness, and properly adjusting it, will probably be quite within the powers of the ordinary amateur. the fitting of the eye-piece into the other end of the tube will require some skill and care both on his own part and that of his tinsmith. although the construction of the eye-piece is much easier than that of the objective, since the same accuracy in adjusting the curves is not necessary, yet the price is lower in a yet greater degree, so that the amateur will find it better to buy than to make his eye-piece, unless he is anxious to test his mechanical powers. for a telescope which has no micrometer, the huyghenian or negative eye-piece, as it is commonly called, is the best. as made by huyghens, it consists of two plano-convex lenses, with their plane sides next the eye, as shown in the figure. [illustration with caption: the huyghenian eye-piece.] so far as we have yet described our telescope it is optically complete. if it could be used as a spy-glass by simply holding it in the hand, and pointing at the object we wish to observe, there would be little need of any very elaborate support. but if a telescope, even of the smallest size, is to be used with regularity, a proper "mounting" is as essential as a good instrument. persons unpractised in the use of such instruments are very apt to underrate the importance of those accessories which merely enable us to point the telescope. an idea of what is wanted in the mounting may readily be formed if the reader will try to look at a star with an ordinary good-sized spy-glass held in the hand, and then imagine the difficulties he meets with multiplied by fifty. the smaller and cheaper telescopes, as commonly sold, are mounted on a simple little stand, on which the instrument admits of a horizontal and vertical motion. if one only wants to get a few glimpses of a celestial object, this mounting will answer his purpose. but to make anything like a study of a celestial body, the mounting must be an equatorial one; that is, one of the axes around which the telescope moves must be inclined so as to point towards the pole of the heavens, which is near the polar star. this axis will then make an angle with the horizon equal to the latitude of the place. the telescope cannot, however, be mounted directly on this axis, but must be attached to a second one, itself fastened to this one. [illustration with caption: section of the primitive mounting. p p. polar axis, bearing a fork at the upper end a. declination axis passing through the fork e. section of telescope tube c. weight to balance the tube.] when mounted in this way, an object can be followed in its diurnal motion from east to west by turning on the polar axis alone. but if the greatest facility in use is required, this motion must be performed by clock-work. a telescope with this appendage will commonly cost one thousand dollars and upward, so that it is not usually applied to very small ones. we will now suppose that the reader wishes to purchase a telescope or an object-glass for himself, and to be able to judge of its performance. he must have the object-glass properly adjusted in its tube, and must use the highest power; that is, the smallest eye-piece, which he intends to use in the instrument. of course he understands that in looking directly at a star or a celestial object it must appear sharp in outline and well defined. but without long practice with good instruments, this will not give him a very definite idea. if the person who selects the telescope is quite unpractised, it is possible that he can make the best test by ascertaining at what distance he can read ordinary print. to do this he should have an eye-piece magnifying about fifty times for each inch of aperture of the telescope. for instance, if his telescope is three inches clear aperture, then his eye-piece should magnify one hundred and fifty times; if the aperture is four inches, one magnifying two hundred times may be used. this magnifying power is, as a general rule, about the highest that can be advantageously used with any telescope. supposing this magnifying power to be used, this page should be legible at a distance of four feet for every unit of magnifying power of the telescope. for example, with a power of , it should be legible at a distance of feet; with a power of , at feet, and so on. to put the condition into another shape: if the telescope will read the print at a distance of feet for each inch of aperture with the best magnifying power, its performance is at least not very bad. if the magnifying power is less than would be given by this rule, the telescope should perform a little better; for instance, a three-inch telescope with a power of should make this page legible at a distance of feet, or four feet for each unit of power. the test applied by the optician is much more exact, and also more easy. he points the instrument at a star, or at the reflection of the sun's rays from a small round piece of glass or a globule of quicksilver several hundred yards away, and ascertains whether the rays are all brought to a focus. this is not done by simply looking at the star, but by alternately pushing the eye-piece in beyond the point of distinct vision and drawing it out past the point. in this way the image of the star will appear, not as a point, but as a round disk of light. if the telescope is perfect, this disk will appear round and of uniform brightness in either position of the eye-piece. but if there is any spherical aberration or differences of density in different parts of the glass, the image will appear distorted in various ways. if the spherical aberration is not correct, the outer rim of the disk will be brighter than the centre when the eye-piece is pushed in, and the centre will be the brighter when it is drawn out. if the curves of the glass are not even all around, the image will appear oval in one or the other position. if there are large veins of unequal density, wings or notches will be seen on the image. if the atmosphere is steady, the image, when the eye-piece is pushed in, will be formed of a great number of minute rings of light. if the glass is good, these rings will be round, unbroken, and equally bright. we present several figures showing how these spectral images, as they are sometimes called, will appear; first, when the eye-piece is pushed in, and secondly, when it is drawn out, with telescopes of different qualities. we have thus far spoken only of the refracting telescope, because it is the kind with which an observer would naturally seek to supply himself. at the same time there is little doubt that the construction of a reflector of moderate size is easier than that of a corresponding refractor. the essential part of the reflector is a slightly concave mirror of any metal which will bear a high polish. this mirror may be ground and polished in the same way as a lens, only the tool must be convex. [illustration with caption: spectral images of stars; the upper line showing how they appear with the eye-piece pushed in, the lower with the eye-piece drawn out. a the telescope is all right b spherical aberration shown by the light and dark centre c the objective is not spherical but elliptical d the glass not uniform--a very bad and incurable case e one side of the objective nearer than the other. adjust it] of late years it has become very common to make the mirror of glass and to cover the reflecting face with an exceedingly thin film of silver, which can be polished by hand in a few minutes. such a mirror differs from our ordinary looking-glass in that the coating of silver is put on the front surface, so that the light does not pass through the glass. moreover, the coating of silver is so thin as to be almost transparent: in fact, the sun may be seen through it by direct vision as a faint blue object. silvered glass reflectors made in this way are extensively manufactured in london, and are far cheaper than refracting telescopes of corresponding size. their great drawback is the want of permanence in the silver film. in the city the film will ordinarily tarnish in a few months from the sulphurous vapors arising from gaslights and other sources, and even in the country it is very difficult to preserve the mirror from the contact of everything that will injure it. in consequence, the possessor of such a telescope, if he wishes to keep it in order, must always be prepared to resilver and repolish it. to do this requires such careful manipulation and management of the chemicals that it is hardly to be expected that an amateur will take the trouble to keep his telescope in order, unless he has a taste for chemistry as well as for astronomy. the curiosity to see the heavenly bodies through great telescopes is so wide-spread that we are apt to forget how much can be seen and done with small ones. the fact is that a large proportion of the astronomical observations of past times have been made with what we should now regard as very small instruments, and a good deal of the solid astronomical work of the present time is done with meridian circles the apertures of which ordinarily range from four to eight inches. one of the most conspicuous examples in recent times of how a moderate-sized instrument may be utilized is afforded by the discoveries of double stars made by mr. s. w. burnham, of chicago. provided with a little six-inch telescope, procured at his own expense from the messrs. clark, he has discovered many hundred double stars so difficult that they had escaped the scrutiny of maedler and the struves, and gained for himself one of the highest positions among the astronomers of the day engaged in the observation of these objects. it was with this little instrument that on mount hamilton, california--afterward the site of the great lick observatory--he discovered forty-eight new double stars, which had remained unnoticed by all previous observers. first among the objects which show beautifully through moderate instruments stands the moon. people who want to see the moon at an observatory generally make the mistake of looking when the moon is full, and asking to see it through the largest telescope. nothing can then be made out but a brilliant blaze of light, mottled with dark spots, and crossed by irregular bright lines. the best time to view the moon is near or before the first quarter, or when she is from three to eight days old. the last quarter is of course equally favorable, so far as seeing is concerned, only one must be up after midnight to see her in that position. seen through a three or four inch telescope, a day or two before the first quarter, about half an hour after sunset, and with a magnifying power between fifty and one hundred, the moon is one of the most beautiful objects in the heavens. twilight softens her radiance so that the eye is not dazzled as it will be when the sky is entirely dark. the general aspect she then presents is that of a hemisphere of beautiful chased silver carved out in curious round patterns with a more than human skill. if, however, one wishes to see the minute details of the lunar surface, in which many of our astronomers are now so deeply interested, he must use a higher magnifying power. the general beautiful effect is then lessened, but more details are seen. still, it is hardly necessary to seek for a very large telescope for any investigation of the lunar surface. i very much doubt whether any one has ever seen anything on the moon which could not be made out in a clear, steady atmosphere with a six-inch telescope of the first class. next to the moon, saturn is among the most beautiful of celestial objects. its aspect, however, varies with its position in its orbit. twice in the course of a revolution, which occupies nearly thirty years, the rings are seen edgewise, and for a few days are invisible even in a powerful telescope. for an entire year their form may be difficult to make out with a small telescope. these unfavorable conditions occur in and . between these dates, especially for some years after , the position of the planet in the sky will be the most favorable, being in northern declination, near its perihelion, and having its rings widely open. we all know that saturn is plainly visible to the naked eye, shining almost like a star of the first magnitude, so that there is no difficulty in finding it if one knows when and where to look. in - its oppositions occur in the month of september. in subsequent years, it will occur a month later every two and a half years. the ring can be seen with a common, good spy-glass fastened to a post so as to be steady. a four or five-inch telescope will show most of the satellites, the division in the ring, and, when the ring is well opened, the curious dusky ring discovered by bond. this "crape ring," as it is commonly called, is one of the most singular phenomena presented by that planet. it might be interesting to the amateur astronomer with a keen eye and a telescope of four inches aperture or upward to frequently scrutinize saturn, with a view of detecting any extraordinary eruptions upon his surface, like that seen by professor hall in . on december th of that year a bright spot was seen upon saturn's equator. it elongated itself from day to day, and remained visible for several weeks. such a thing had never before been known upon this planet, and had it not been that professor hall was engaged in observations upon the satellites, it would not have been seen then. a similar spot on the planet was recorded in , and much more extensively noticed. on this occasion the spot appeared in a higher latitude from the planet's equator than did professor hall's. at this appearance the time of the planet's revolution on its axis was found to be somewhat greater than in , in accordance with the general law exhibited in the rotations of the sun and of jupiter. notwithstanding their transient character, these two spots have afforded the only determination of the time of revolution of saturn which has been made since herschel the elder. [illustration with caption: the great refractor of the national observatory at washington] of the satellites of saturn the brightest is titan, which can be seen with the smallest telescope, and revolves around the planet in fifteen days. iapetus, the outer satellite, is remarkable for varying greatly in brilliancy during its revolution around the planet. any one having the means and ability to make accurate photometrical estimates of the light of this satellite in all points of its orbit, can thereby render a valuable service to astronomy. the observations of venus, by which the astronomers of the last century supposed themselves to have discovered its time of rotation on its axis, were made with telescopes much inferior to ours. although their observations have not been confirmed, some astronomers are still inclined to think that their results have not been refuted by the failure of recent observers to detect those changes which the older ones describe on the surface of the planet. with a six-inch telescope of the best quality, and with time to choose the most favorable moment, one will be as well equipped to settle the question of the rotation of venus as the best observer. the few days near each inferior conjunction are especially to be taken advantage of. the questions to be settled are two: first, are there any dark spots or other markings on the disk? second, are there any irregularities in the form of the sharp cusps? the central portions of the disk are much darker than the outline, and it is probably this fact which has given rise to the impression of dark spots. unless this apparent darkness changes from time to time, or shows some irregularity in its outline, it cannot indicate any rotation of the planet. the best time to scrutinize the sharp cusps will be when the planet is nearly on the line from the earth to the sun. the best hour of the day is near sunset, the half-hour following sunset being the best of all. but if venus is near the sun, she will after sunset be too low down to be well seen, and must be looked at late in the afternoon. the planet mars must always be an object of great interest, because of all the heavenly bodies it is that which appears to bear the greatest resemblance to the earth. it comes into opposition at intervals of a little more than two years, and can be well seen only for a month or two before and after each opposition. it is hopeless to look for the satellites of mars with any but the greatest telescopes of the world. but the markings on the surface, from which the time of rotation has been determined, and which indicate a resemblance to the surface of our own planet, can be well seen with telescopes of six inches aperture and upward. one or both of the bright polar spots, which are supposed to be due to deposits of snow, can be seen with smaller telescopes when the situation of the planet is favorable. the case is different with the so-called canals discovered by schiaparelli in , which have ever since excited so much interest, and given rise to so much discussion as to their nature. the astronomer who has had the best opportunities for studying them is mr. percival lowell, whose observatory at flaggstaff, arizona, is finely situated for the purpose, while he also has one of the best if not the largest of telescopes. there the canals are seen as fine dark lines; but, even then, they must be fifty miles in breadth, so that the word "canal" may be regarded as a misnomer. although the planet jupiter does not present such striking features as saturn, it is of even more interest to the amateur astronomer, because he can study it with less optical power, and see more of the changes upon its surface. every work on astronomy tells in a general way of the belts of jupiter, and many speculate upon their causes. the reader of recent works knows that jupiter is supposed to be not a solid mass like the earth, but a great globe of molten and vaporous matter, intermediate in constitution between the earth and the sun. the outer surface which we see is probably a hot mass of vapor hundreds of miles deep, thrown up from the heated interior. the belts are probably cloudlike forms in this vaporous mass. certain it is that they are continually changing, so that the planet seldom looks exactly the same on two successive evenings. the rotation of the planet can be very well seen by an hour's watching. in two hours an object at the centre of the disk will move off to near the margin. the satellites of this planet, in their ever-varying phases, are objects of perennial interest. their eclipses may be observed with a very small telescope, if one knows when to look for them. to do this successfully, and without waste of time, it is necessary to have an astronomical ephemeris for the year. all the observable phenomena are there predicted for the convenience of observers. perhaps the most curious observation to be made is that of the shadow of the satellite crossing the disk of jupiter. the writer has seen this perfectly with a six-inch telescope, and a much smaller one would probably show it well. with a telescope of this size, or a little larger, the satellites can be seen between us and jupiter. sometimes they appear a little brighter than the planet, and sometimes a little fainter. of the remaining large planets, mercury, the inner one, and uranus and neptune, the two outer ones, are of less interest than the others to an amateur with a small telescope, because they are more difficult to see. mercury can, indeed, be observed with the smallest instrument, but no physical configurations or changes have ever been made out upon his surface. the question whether any such can be observed is still an open one, which can be settled only by long and careful scrutiny. a small telescope is almost as good for this purpose as a large one, because the atmospheric difficulties in the way of getting a good view of the planet cannot be lessened by an increase of telescopic power. uranus and neptune are so distant that telescopes of considerable size and high magnifying power are necessary to show their disks. in small telescopes they have the appearance of stars, and the observer has no way of distinguishing them from the surrounding stars unless he can command the best astronomical appliances, such as star maps, circles on his instrument, etc. it is, however, to be remarked, as a fact not generally known, that uranus can be well seen with the naked eye if one knows where to look for it. to recognize it, it is necessary to have an astronomical ephemeris showing its right ascension and declination, and star maps showing where the parallels of right ascension and declination lie among the stars. when once found by the naked eye, there will, of course, be no difficulty in pointing the telescope upon it. of celestial objects which it is well to keep a watch upon, and which can be seen to good advantage with inexpensive instruments, the sun may be considered as holding the first place. astronomers who make a specialty of solar physics have, especially in this country, so many other duties, and their view is so often interrupted by clouds, that a continuous record of the spots on the sun and the changes they undergo is hardly possible. perhaps one of the most interesting and useful pieces of astronomical work which an amateur can perform will consist of a record of the origin and changes of form of the solar spots and faculae. what does a spot look like when it first comes into sight? does it immediately burst forth with considerable magnitude, or does it begin as the smallest visible speck, and gradually grow? when several spots coalesce into one, how do they do it? when a spot breaks up into several pieces, what is the seeming nature of the process? how do the groups of brilliant points called faculae come, change, and grow? all these questions must no doubt be answered in various ways, according to the behavior of the particular spot, but the record is rather meagre, and the conscientious and industrious amateur will be able to amuse himself by adding to it, and possibly may make valuable contributions to science in the same way. still another branch of astronomical observation, in which industry and skill count for more than expensive instruments, is the search for new comets. this requires a very practised eye, in order that the comet may be caught among the crowd of stars which flit across the field of view as the telescope is moved. it is also necessary to be well acquainted with a number of nebulae which look very much like comets. the search can be made with almost any small telescope, if one is careful to use a very low power. with a four-inch telescope a power not exceeding twenty should be employed. to search with ease, and in the best manner, the observer should have what among astronomers is familiarly known as a "broken-backed telescope." this instrument has the eye-piece on the end of the axis, where one would never think of looking for it. by turning the instrument on this axis, it sweeps from one horizon through the zenith and over to the other horizon without the observer having to move his head. this is effected by having a reflector in the central part of the instrument, which throws the rays of light at right angles through the axis. [illustration: the "broken-backed comet-seeker"] how well this search can be conducted by observers with limited means at their disposal is shown by the success of several american observers, among whom messrs. w. r. brooks, e. e. barnard, and lewis swift are well known. the cometary discoveries of these men afford an excellent illustration of how much can be done with the smallest means when one sets to work in the right spirit. the larger number of wonderful telescopic objects are to be sought for far beyond the confines of the solar system, in regions from which light requires years to reach us. on account of their great distance, these objects generally require the most powerful telescopes to be seen in the best manner; but there are quite a number within the range of the amateur. looking at the milky way, especially its southern part, on a clear winter or summer evening, tufts of light will be seen here and there. on examining these tufts with a telescope, they will be found to consist of congeries of stars. many of these groups are of the greatest beauty, with only a moderate optical power. of all the groups in the milky way the best known is that in the sword-handle of perseus, which may be seen during the greater part of the year, and is distinctly visible to the naked eye as a patch of diffused light. with the telescope there are seen in this patch two closely connected clusters of stars, or perhaps we ought rather to say two centres of condensation. another object of the same class is proesepe in the constellation cancer. this can be very distinctly seen by the naked eye on a clear moonless night in winter or spring as a faint nebulous object, surrounded by three small stars. the smallest telescope shows it as a group of stars. of all stellar objects, the great nebula of orion is that which has most fascinated the astronomers of two centuries. it is distinctly visible to the naked eye, and may be found without difficulty on any winter night. the three bright stars forming the sword-belt of orion are known to every one who has noticed that constellation. below this belt is seen another triplet of stars, not so bright, and lying in a north and south direction. the middle star of this triplet is the great nebula. at first the naked eye sees nothing to distinguish it from other stars, but if closely scanned it will be seen to have a hazy aspect. a four-inch telescope will show its curious form. not the least interesting of its features are the four stars known as the "trapezium," which are located in a dark region near its centre. in fact, the whole nebula is dotted with stars, which add greatly to the effect produced by its mysterious aspect. the great nebula of andromeda is second only to that of orion in interest. like the former, it is distinctly visible to the naked eye, having the aspect of a faint comet. the most curious feature of this object is that although the most powerful telescopes do not resolve it into stars, it appears in the spectroscope as if it were solid matter shining by its own light. the above are merely selections from the countless number of objects which the heavens offer to telescopic study. many such are described in astronomical works, but the amateur can gratify his curiosity to almost any extent by searching them out for himself. [illustration with caption: nebula in orion] ever since a red spot, unlike any before noticed, has generally been visible on jupiter. at first it was for several years a very conspicuous object, but gradually faded away, so that since it has been made out only with difficulty. but it is now regarded as a permanent feature of the planet. there is some reason to believe it was occasionally seen long before attention was first attracted to it. doubtless, when it can be seen at all, practice in observing such objects is more important than size of telescope. vi what the astronomers are doing in no field of science has human knowledge been more extended in our time than in that of astronomy. forty years ago astronomical research seemed quite barren of results of great interest or value to our race. the observers of the world were working on a traditional system, grinding out results in an endless course, without seeing any prospect of the great generalizations to which they might ultimately lead. now this is all changed. a new instrument, the spectroscope, has been developed, the extent of whose revelations we are just beginning to learn, although it has been more than thirty years in use. the application of photography has been so extended that, in some important branches of astronomical work, the observer simply photographs the phenomenon which he is to study, and then makes his observation on the developed negative. the world of astronomy is one of the busiest that can be found to-day, and the writer proposes, with the reader's courteous consent, to take him on a stroll through it and see what is going on. we may begin our inspection with a body which is, for us, next to the earth, the most important in the universe. i mean the sun. at the greenwich observatory the sun has for more than twenty years been regularly photographed on every clear day, with the view of determining the changes going on in its spots. in recent years these observations have been supplemented by others, made at stations in india and mauritius, so that by the combination of all it is quite exceptional to have an entire day pass without at least one photograph being taken. on these observations must mainly rest our knowledge of the curious cycle of change in the solar spots, which goes through a period of about eleven years, but of which no one has as yet been able to establish the cause. this greenwich system has been extended and improved by an american. professor george e. hale, formerly director of the yerkes observatory, has devised an instrument for taking photographs of the sun by a single ray of the spectrum. the light emitted by calcium, the base of lime, and one of the substances most abundant in the sun, is often selected to impress the plate. the carnegie institution has recently organized an enterprise for carrying on the study of the sun under a combination of better conditions than were ever before enjoyed. the first requirement in such a case is the ablest and most enthusiastic worker in the field, ready to devote all his energies to its cultivation. this requirement is found in the person of professor hale himself. the next requirement is an atmosphere of the greatest transparency, and a situation at a high elevation above sea-level, so that the passage of light from the sun to the observer shall be obstructed as little as possible by the mists and vapors near the earth's surface. this requirement is reached by placing the observatory on mount wilson, near pasadena, california, where the climate is found to be the best of any in the united states, and probably not exceeded by that of any other attainable point in the world. the third requirement is the best of instruments, specially devised to meet the requirements. in this respect we may be sure that nothing attainable by human ingenuity will be found wanting. thus provided, professor hale has entered upon the task of studying the sun, and recording from day to day all the changes going on in it, using specially devised instruments for each purpose in view. photography is made use of through almost the entire investigation. a full description of the work would require an enumeration of technical details, into which we need not enter at present. let it, therefore, suffice to say in a general way that the study of the sun is being carried on on a scale, and with an energy worthy of the most important subject that presents itself to the astronomer. closely associated with this work is that of professor langley and dr. abbot, at the astro-physical observatory of the smithsonian institution, who have recently completed one of the most important works ever carried out on the light of the sun. they have for years been analyzing those of its rays which, although entirely invisible to our eyes, are of the same nature as those of light, and are felt by us as heat. to do this, langley invented a sort of artificial eye, which he called a bolometer, in which the optic nerve is made of an extremely thin strip of metal, so slight that one can hardly see it, which is traversed by an electric current. this eye would be so dazzled by the heat radiated from one's body that, when in use, it must be protected from all such heat by being enclosed in a case kept at a constant temperature by being immersed in water. with this eye the two observers have mapped the heat rays of the sun down to an extent and with a precision which were before entirely unknown. the question of possible changes in the sun's radiation, and of the relation of those changes to human welfare, still eludes our scrutiny. with all the efforts that have been made, the physicist of to-day has not yet been able to make anything like an exact determination of the total amount of heat received from the sun. the largest measurements are almost double the smallest. this is partly due to the atmosphere absorbing an unknown and variable fraction of the sun's rays which pass through it, and partly to the difficulty of distinguishing the heat radiated by the sun from that radiated by terrestrial objects. in one recent instance, a change in the sun's radiation has been noticed in various parts of the world, and is of especial interest because there seems to be little doubt as to its origin. in the latter part of an extraordinary diminution was found in the intensity of the sun's heat, as measured by the bolometer and other instruments. this continued through the first part of , with wide variations at different places, and it was more than a year after the first diminution before the sun's rays again assumed their ordinary intensity. this result is now attributed to the eruption of mount pelee, during which an enormous mass of volcanic dust and vapor was projected into the higher regions of the air, and gradually carried over the entire earth by winds and currents. many of our readers may remember that something yet more striking occurred after the great cataclasm at krakatoa in , when, for more than a year, red sunsets and red twilights of a depth of shade never before observed were seen in every part of the world. what we call universology--the knowledge of the structure and extent of the universe--must begin with a study of the starry heavens as we see them. there are perhaps one hundred million stars in the sky within the reach of telescopic vision. this number is too great to allow of all the stars being studied individually; yet, to form the basis for any conclusion, we must know the positions and arrangement of as many of them as we can determine. to do this the first want is a catalogue giving very precise positions of as many of the brighter stars as possible. the principal national observatories, as well as some others, are engaged in supplying this want. up to the present time about , stars visible in our latitudes have been catalogued on this precise plan, and the work is still going on. in that part of the sky which we never see, because it is only visible from the southern hemisphere, the corresponding work is far from being as extensive. sir david gill, astronomer at the cape of good hope, and also the directors of other southern observatories, are engaged in pushing it forward as rapidly as the limited facilities at their disposal will allow. next in order comes the work of simply listing as many stars as possible. here the most exact positions are not required. it is only necessary to lay down the position of each star with sufficient exactness to distinguish it from all its neighbors. about , stars were during the last half-century listed in this way at the observatory of bonn by argelander, schonfeld, and their assistants. this work is now being carried through the southern hemisphere on a large scale by thome, director of the cordoba observatory, in the argentine republic. this was founded thirty years ago by our dr. b. a. gould, who turned it over to dr. thome in . the latter has, up to the present time, fixed and published the positions of nearly half a million stars. this work of thome extends to fainter stars than any other yet attempted, so that, as it goes on, we have more stars listed in a region invisible in middle northern latitudes than we have for that part of the sky we can see. up to the present time three quarto volumes giving the positions and magnitudes of the stars have appeared. two or three volumes more, and, perhaps, ten or fifteen years, will be required to complete the work. about twenty years ago it was discovered that, by means of a telescope especially adapted to this purpose, it was possible to photograph many more stars than an instrument of the same size would show to the eye. this discovery was soon applied in various quarters. sir david gill, with characteristic energy, photographed the stars of the southern sky to the number of nearly half a million. as it was beyond his power to measure off and compute the positions of the stars from his plates, the latter were sent to professor j. c. kapteyn, of holland, who undertook the enormous labor of collecting them into a catalogue, the last volume of which was published in . one curious result of this enterprise is that the work of listing the stars is more complete for the southern hemisphere than for the northern. another great photographic work now in progress has to do with the millions of stars which it is impossible to handle individually. fifteen years ago an association of observatories in both hemispheres undertook to make a photographic chart of the sky on the largest scale. some portions of this work are now approaching completion, but in others it is still in a backward state, owing to the failure of several south american observatories to carry out their part of the programme. when it is all done we shall have a picture of the sky, the study of which may require the labor of a whole generation of astronomers. quite independently of this work, the harvard university, under the direction of professor pickering, keeps up the work of photographing the sky on a surprising scale. on this plan we do not have to leave it to posterity to learn whether there is any change in the heavens, for one result of the enterprise has been the discovery of thirteen of the new stars which now and then blaze out in the heavens at points where none were before known. professor pickering's work has been continually enlarged and improved until about , photographic plates, showing from time to time the places of countless millions of stars among their fellows are now stored at the harvard observatory. not less remarkable than this wealth of material has been the development of skill in working it up. some idea of the work will be obtained by reflecting that, thirty years ago, careful study of the heavens by astronomers devoting their lives to the task had resulted in the discovery of some two or three hundred stars, varying in their light. now, at harvard, through keen eyes studying and comparing successive photographs not only of isolated stars, but of clusters and agglomerations of stars in the milky way and elsewhere, discoveries of such objects numbering hundreds have been made, and the work is going on with ever-increasing speed. indeed, the number of variable stars now known is such that their study as individual objects no longer suffices, and they must hereafter be treated statistically with reference to their distribution in space, and their relations to one another, as a census classifies the entire population without taking any account of individuals. the works just mentioned are concerned with the stars. but the heavenly spaces contain nebulae as well as stars; and photography can now be even more successful in picturing them than the stars. a few years ago the late lamented keeler, at the lick observatory, undertook to see what could be done by pointing the crossley reflecting telescope at the sky and putting a sensitive photographic plate in the focus. he was surprised to find that a great number of nebulae, the existence of which had never before been suspected, were impressed on the plate. up to the present time the positions of about of these objects have been listed. keeler found that there were probably , nebulae in the heavens capable of being photographed with the crossley reflector. but the work of taking these photographs is so great, and the number of reflecting telescopes which can be applied to it so small, that no one has ventured to seriously commence it. it is worthy of remark that only a very small fraction of these objects which can be photographed are visible to the eye, even with the most powerful telescope. this demonstration of what the reflecting telescope can do may be regarded as one of the most important discoveries of our time as to the capabilities of astronomical instruments. it has long been known that the image formed in the focus of the best refracting telescope is affected by an imperfection arising from the different action of the glasses on rays of light of different colors. hence, the image of a star can never be seen or photographed with such an instrument, as an actual point, but only as a small, diffused mass. this difficulty is avoided in the reflecting telescope; but a new difficulty is found in the bending of the mirror under the influence of its own weight. devices for overcoming this had been so far from successful that, when mr. crossley presented his instrument to the lick observatory, it was feared that little of importance could be done with it. but as often happens in human affairs outside the field of astronomy, when ingenious and able men devote their attention to the careful study of a problem, it was found that new results could be reached. thus it was that, before a great while, what was supposed to be an inferior instrument proved not only to have qualities not before suspected, but to be the means of making an important addition to the methods of astronomical investigation. in order that our knowledge of the position of a star may be complete, we must know its distance. this can be measured only through the star's parallax--that is to say, the slight change in its direction produced by the swing of our earth around its orbit. but so vast is the distance in question that this change is immeasurably small, except for, perhaps, a few hundred stars, and even for these few its measurement almost baffles the skill of the most expert astronomer. progress in this direction is therefore very slow, and there are probably not yet a hundred stars of which the parallax has been ascertained with any approach to certainty. dr. chase is now completing an important work of this kind at the yale observatory. to the most refined telescopic observations, as well as to the naked eye, the stars seem all alike, except that they differ greatly in brightness, and somewhat in color. but when their light is analyzed by the spectroscope, it is found that scarcely any two are exactly alike. an important part of the work of the astro-physical observatories, especially that of harvard, consists in photographing the spectra of thousands of stars, and studying the peculiarities thus brought out. at harvard a large portion of this work is done as part of the work of the henry draper memorial, established by his widow in memory of the eminent investigator of new york, who died twenty years ago. by a comparison of the spectra of stars sir william huggins has developed the idea that these bodies, like human beings, have a life history. they are nebulae in infancy, while the progress to old age is marked by a constant increase in the density of their substance. their temperature also changes in a way analogous to the vigor of the human being. during a certain time the star continually grows hotter and hotter. but an end to this must come, and it cools off in old age. what the age of a star may be is hard even to guess. it is many millions of years, perhaps hundreds, possibly even thousands, of millions. some attempt at giving the magnitude is included in every considerable list of stars. the work of determining the magnitudes with the greatest precision is so laborious that it must go on rather slowly. it is being pursued on a large scale at the harvard observatory, as well as in that of potsdam, germany. we come now to the question of changes in the appearance of bright stars. it seems pretty certain that more than one per cent of these bodies fluctuate to a greater or less extent in their light. observations of these fluctuations, in the case of at least the brighter stars, may be carried on without any instrument more expensive than a good opera-glass--in fact, in the case of stars visible to the naked eye, with no instrument at all. as a general rule, the light of these stars goes through its changes in a regular period, which is sometimes as short as a few hours, but generally several days, frequently a large fraction of a year or even eighteen months. observations of these stars are made to determine the length of the period and the law of variation of the brightness. any person with a good eye and skill in making estimates can make the observations if he will devote sufficient pains to training himself; but they require a degree of care and assiduity which is not to be expected of any one but an enthusiast on the subject. one of the most successful observers of the present time is mr. w. a. roberts, a resident of south africa, whom the boer war did not prevent from keeping up a watch of the southern sky, which has resulted in greatly increasing our knowledge of variable stars. there are also quite a number of astronomers in europe and america who make this particular study their specialty. during the past fifteen years the art of measuring the speed with which a star is approaching us or receding from us has been brought to a wonderful degree of perfection. the instrument with which this was first done was the spectroscope; it is now replaced with another of the same general kind, called the spectrograph. the latter differs from the other only in that the spectrum of the star is photographed, and the observer makes his measures on the negative. this method was first extensively applied at the potsdam observatory in germany, and has lately become one of the specialties of the lick observatory, where professor campbell has brought it to its present degree of perfection. the yerkes observatory is also beginning work in the same line, where professor frost is already rivalling the lick observatory in the precision of his measures. let us now go back to our own little colony and see what is being done to advance our knowledge of the solar system. this consists of planets, on one of which we dwell, moons revolving around them, comets, and meteoric bodies. the principal national observatories keep up a more or less orderly system of observations of the positions of the planets and their satellites in order to determine the laws of their motion. as in the case of the stars, it is necessary to continue these observations through long periods of time in order that everything possible to learn may be discovered. our own moon is one of the enigmas of the mathematical astronomer. observations show that she is deviating from her predicted place, and that this deviation continues to increase. true, it is not very great when measured by an ordinary standard. the time at which the moon's shadow passed a given point near norfolk during the total eclipse of may , , was only about seven seconds different from the time given in the astronomical ephemeris. the path of the shadow along the earth was not out of place by more than one or two miles but, small though these deviations are, they show that something is wrong, and no one has as yet found out what it is. worse yet, the deviation is increasing rapidly. the observers of the total eclipse in august, , were surprised to find that it began twenty seconds before the predicted time. the mathematical problems involved in correcting this error are of such complexity that it is only now and then that a mathematician turns up anywhere in the world who is both able and bold enough to attack them. there now seems little doubt that jupiter is a miniature sun, only not hot enough at its surface to shine by its own light the point in which it most resembles the sun is that its equatorial regions rotate in less time than do the regions near the poles. this shows that what we see is not a solid body. but none of the careful observers have yet succeeded in determining the law of this difference of rotation. twelve years ago a suspicion which had long been entertained that the earth's axis of rotation varied a little from time to time was verified by chandler. the result of this is a slight change in the latitude of all places on the earth's surface, which admits of being determined by precise observations. the national geodetic association has established four observatories on the same parallel of latitude--one at gaithersburg, maryland, another on the pacific coast, a third in japan, and a fourth in italy--to study these variations by continuous observations from night to night. this work is now going forward on a well-devised plan. a fact which will appeal to our readers on this side of the atlantic is the success of american astronomers. sixty years ago it could not be said that there was a well-known observatory on the american continent. the cultivation of astronomy was confined to a professor here and there, who seldom had anything better than a little telescope with which he showed the heavenly bodies to his students. but during the past thirty years all this has been changed. the total quantity of published research is still less among us than on the continent of europe, but the number of men who have reached the highest success among us may be judged by one fact. the royal astronomical society of england awards an annual medal to the english or foreign astronomer deemed most worthy of it. the number of these medals awarded to americans within twenty-five years is about equal to the number awarded to the astronomers of all other nations foreign to the english. that this preponderance is not growing less is shown by the award of medals to americans in three consecutive years-- , , and . the recipients were hale, boss, and campbell. of the fifty foreign associates chosen by this society for their eminence in astronomical research, no less than eighteen--more than one-third--are americans. vii life in the universe so far as we can judge from what we see on our globe, the production of life is one of the greatest and most incessant purposes of nature. life is absent only in regions of perpetual frost, where it never has an opportunity to begin; in places where the temperature is near the boiling-point, which is found to be destructive to it; and beneath the earth's surface, where none of the changes essential to it can come about. within the limits imposed by these prohibitory conditions--that is to say, within the range of temperature at which water retains its liquid state, and in regions where the sun's rays can penetrate and where wind can blow and water exist in a liquid form--life is the universal rule. how prodigal nature seems to be in its production is too trite a fact to be dwelt upon. we have all read of the millions of germs which are destroyed for every one that comes to maturity. even the higher forms of life are found almost everywhere. only small islands have ever been discovered which were uninhabited, and animals of a higher grade are as widely diffused as man. if it would be going too far to claim that all conditions may have forms of life appropriate to them, it would be going as much too far in the other direction to claim that life can exist only with the precise surroundings which nurture it on this planet. it is very remarkable in this connection that while in one direction we see life coming to an end, in the other direction we see it flourishing more and more up to the limit. these two directions are those of heat and cold. we cannot suppose that life would develop in any important degree in a region of perpetual frost, such as the polar regions of our globe. but we do not find any end to it as the climate becomes warmer. on the contrary, every one knows that the tropics are the most fertile regions of the globe in its production. the luxuriance of the vegetation and the number of the animals continually increase the more tropical the climate becomes. where the limit may be set no one can say. but it would doubtless be far above the present temperature of the equatorial regions. it has often been said that this does not apply to the human race, that men lack vigor in the tropics. but human vigor depends on so many conditions, hereditary and otherwise, that we cannot regard the inferior development of humanity in the tropics as due solely to temperature. physically considered, no men attain a better development than many tribes who inhabit the warmer regions of the globe. the inferiority of the inhabitants of these regions in intellectual power is more likely the result of race heredity than of temperature. we all know that this earth on which we dwell is only one of countless millions of globes scattered through the wilds of infinite space. so far as we know, most of these globes are wholly unlike the earth, being at a temperature so high that, like our sun, they shine by their own light. in such worlds we may regard it as quite certain that no organized life could exist. but evidence is continually increasing that dark and opaque worlds like ours exist and revolve around their suns, as the earth on which we dwell revolves around its central luminary. although the number of such globes yet discovered is not great, the circumstances under which they are found lead us to believe that the actual number may be as great as that of the visible stars which stud the sky. if so, the probabilities are that millions of them are essentially similar to our own globe. have we any reason to believe that life exists on these other worlds? the reader will not expect me to answer this question positively. it must be admitted that, scientifically, we have no light upon the question, and therefore no positive grounds for reaching a conclusion. we can only reason by analogy and by what we know of the origin and conditions of life around us, and assume that the same agencies which are at play here would be found at play under similar conditions in other parts of the universe. if we ask what the opinion of men has been, we know historically that our race has, in all periods of its history, peopled other regions with beings even higher in the scale of development than we are ourselves. the gods and demons of an earlier age all wielded powers greater than those granted to man--powers which they could use to determine human destiny. but, up to the time that copernicus showed that the planets were other worlds, the location of these imaginary beings was rather indefinite. it was therefore quite natural that when the moon and planets were found to be dark globes of a size comparable with that of the earth itself, they were made the habitations of beings like unto ourselves. the trend of modern discovery has been against carrying this view to its extreme, as will be presently shown. before considering the difficulties in the way of accepting it to the widest extent, let us enter upon some preliminary considerations as to the origin and prevalence of life, so far as we have any sound basis to go upon. a generation ago the origin of life upon our planet was one of the great mysteries of science. all the facts brought out by investigation into the past history of our earth seemed to show, with hardly the possibility of a doubt, that there was a time when it was a fiery mass, no more capable of serving as the abode of a living being than the interior of a bessemer steel furnace. there must therefore have been, within a certain period, a beginning of life upon its surface. but, so far as investigation had gone--indeed, so far as it has gone to the present time--no life has been found to originate of itself. the living germ seems to be necessary to the beginning of any living form. whence, then, came the first germ? many of our readers may remember a suggestion by sir william thomson, now lord kelvin, made twenty or thirty years ago, that life may have been brought to our planet by the falling of a meteor from space. this does not, however, solve the difficulty--indeed, it would only make it greater. it still leaves open the question how life began on the meteor; and granting this, why it was not destroyed by the heat generated as the meteor passed through the air. the popular view that life began through a special act of creative power seemed to be almost forced upon man by the failure of science to discover any other beginning for it. it cannot be said that even to-day anything definite has been actually discovered to refute this view. all we can say about it is that it does not run in with the general views of modern science as to the beginning of things, and that those who refuse to accept it must hold that, under certain conditions which prevail, life begins by a very gradual process, similar to that by which forms suggesting growth seem to originate even under conditions so unfavorable as those existing in a bottle of acid. but it is not at all necessary for our purpose to decide this question. if life existed through a creative act, it is absurd to suppose that that act was confined to one of the countless millions of worlds scattered through space. if it began at a certain stage of evolution by a natural process, the question will arise, what conditions are favorable to the commencement of this process? here we are quite justified in reasoning from what, granting this process, has taken place upon our globe during its past history. one of the most elementary principles accepted by the human mind is that like causes produce like effects. the special conditions under which we find life to develop around us may be comprehensively summed up as the existence of water in the liquid form, and the presence of nitrogen, free perhaps in the first place, but accompanied by substances with which it may form combinations. oxygen, hydrogen, and nitrogen are, then, the fundamental requirements. the addition of calcium or other forms of matter necessary to the existence of a solid world goes without saying. the question now is whether these necessary conditions exist in other parts of the universe. the spectroscope shows that, so far as the chemical elements go, other worlds are composed of the same elements as ours. hydrogen especially exists everywhere, and we have reason to believe that the same is true of oxygen and nitrogen. calcium, the base of lime, is almost universal. so far as chemical elements go, we may therefore take it for granted that the conditions under which life begins are very widely diffused in the universe. it is, therefore, contrary to all the analogies of nature to suppose that life began only on a single world. it is a scientific inference, based on facts so numerous as not to admit of serious question, that during the history of our globe there has been a continually improving development of life. as ages upon ages pass, new forms are generated, higher in the scale than those which preceded them, until at length reason appears and asserts its sway. in a recent well-known work alfred russel wallace has argued that this development of life required the presence of such a rare combination of conditions that there is no reason to suppose that it prevailed anywhere except on our earth. it is quite impossible in the present discussion to follow his reasoning in detail; but it seems to me altogether inconclusive. not only does life, but intelligence, flourish on this globe under a great variety of conditions as regards temperature and surroundings, and no sound reason can be shown why under certain conditions, which are frequent in the universe, intelligent beings should not acquire the highest development. now let us look at the subject from the view of the mathematical theory of probabilities. a fundamental tenet of this theory is that no matter how improbable a result may be on a single trial, supposing it at all possible, it is sure to occur after a sufficient number of trials--and over and over again if the trials are repeated often enough. for example, if a million grains of corn, of which a single one was red, were all placed in a pile, and a blindfolded person were required to grope in the pile, select a grain, and then put it back again, the chances would be a million to one against his drawing out the red grain. if drawing it meant he should die, a sensible person would give himself no concern at having to draw the grain. the probability of his death would not be so great as the actual probability that he will really die within the next twenty-four hours. and yet if the whole human race were required to run this chance, it is certain that about fifteen hundred, or one out of a million, of the whole human family would draw the red grain and meet his death. now apply this principle to the universe. let us suppose, to fix the ideas, that there are a hundred million worlds, but that the chances are one thousand to one against any one of these taken at random being fitted for the highest development of life or for the evolution of reason. the chances would still be that one hundred thousand of them would be inhabited by rational beings whom we call human. but where are we to look for these worlds? this no man can tell. we only infer from the statistics of the stars--and this inference is fairly well grounded--that the number of worlds which, so far as we know, may be inhabited, are to be counted by thousands, and perhaps by millions. in a number of bodies so vast we should expect every variety of conditions as regards temperature and surroundings. if we suppose that the special conditions which prevail on our planet are necessary to the highest forms of life, we still have reason to believe that these same conditions prevail on thousands of other worlds. the fact that we might find the conditions in millions of other worlds unfavorable to life would not disprove the existence of the latter on countless worlds differently situated. coming down now from the general question to the specific one, we all know that the only worlds the conditions of which can be made the subject of observation are the planets which revolve around the sun, and their satellites. the question whether these bodies are inhabited is one which, of course, completely transcends not only our powers of observation at present, but every appliance of research that we can conceive of men devising. if mars is inhabited, and if the people of that planet have equal powers with ourselves, the problem of merely producing an illumination which could be seen in our most powerful telescope would be beyond all the ordinary efforts of an entire nation. an unbroken square mile of flame would be invisible in our telescopes, but a hundred square miles might be seen. we cannot, therefore, expect to see any signs of the works of inhabitants even on mars. all that we can do is to ascertain with greater or less probability whether the conditions necessary to life exist on the other planets of the system. the moon being much the nearest to us of all the heavenly bodies, we can pronounce more definitely in its case than in any other. we know that neither air nor water exists on the moon in quantities sufficient to be perceived by the most delicate tests at our command. it is certain that the moon's atmosphere, if any exists, is less than the thousandth part of the density of that around us. the vacuum is greater than any ordinary air-pump is capable of producing. we can hardly suppose that so small a quantity of air could be of any benefit whatever in sustaining life; an animal that could get along on so little could get along on none at all. but the proof of the absence of life is yet stronger when we consider the results of actual telescopic observation. an object such as an ordinary city block could be detected on the moon. if anything like vegetation were present on its surface, we should see the changes which it would undergo in the course of a month, during one portion of which it would be exposed to the rays of the unclouded sun, and during another to the intense cold of space. if men built cities, or even separate buildings the size of the larger ones on our earth, we might see some signs of them. in recent times we not only observe the moon with the telescope, but get still more definite information by photography. the whole visible surface has been repeatedly photographed under the best conditions. but no change has been established beyond question, nor does the photograph show the slightest difference of structure or shade which could be attributed to cities or other works of man. to all appearances the whole surface of our satellite is as completely devoid of life as the lava newly thrown from vesuvius. we next pass to the planets. mercury, the nearest to the sun, is in a position very unfavorable for observation from the earth, because when nearest to us it is between us and the sun, so that its dark hemisphere is presented to us. nothing satisfactory has yet been made out as to its condition. we cannot say with certainty whether it has an atmosphere or not. what seems very probable is that the temperature on its surface is higher than any of our earthly animals could sustain. but this proves nothing. we know that venus has an atmosphere. this was very conclusively shown during the transits of venus in and . but this atmosphere is so filled with clouds or vapor that it does not seem likely that we ever get a view of the solid body of the planet through it. some observers have thought they could see spots on venus day after day, while others have disputed this view. on the whole, if intelligent inhabitants live there, it is not likely that they ever see sun or stars. instead of the sun they see only an effulgence in the vapory sky which disappears and reappears at regular intervals. when we come to mars, we have more definite knowledge, and there seems to be greater possibilities for life there than in the case of any other planet besides the earth. the main reason for denying that life such as ours could exist there is that the atmosphere of mars is so rare that, in the light of the most recent researches, we cannot be fully assured that it exists at all. the very careful comparisons of the spectra of mars and of the moon made by campbell at the lick observatory failed to show the slightest difference in the two. if mars had an atmosphere as dense as ours, the result could be seen in the darkening of the lines of the spectrum produced by the double passage of the light through it. there were no lines in the spectrum of mars that were not seen with equal distinctness in that of the moon. but this does not prove the entire absence of an atmosphere. it only shows a limit to its density. it may be one-fifth or one-fourth the density of that on the earth, but probably no more. that there must be something in the nature of vapor at least seems to be shown by the formation and disappearance of the white polar caps of this planet. every reader of astronomy at the present time knows that, during the martian winter, white caps form around the pole of the planet which is turned away from the sun, and grow larger and larger until the sun begins to shine upon them, when they gradually grow smaller, and perhaps nearly disappear. it seems, therefore, fairly well proved that, under the influence of cold, some white substance forms around the polar regions of mars which evaporates under the influence of the sun's rays. it has been supposed that this substance is snow, produced in the same way that snow is produced on the earth, by the evaporation of water. but there are difficulties in the way of this explanation. the sun sends less than half as much heat to mars as to the earth, and it does not seem likely that the polar regions can ever receive enough of heat to melt any considerable quantity of snow. nor does it seem likely that any clouds from which snow could fall ever obscure the surface of mars. but a very slight change in the explanation will make it tenable. quite possibly the white deposits may be due to something like hoar-frost condensed from slightly moist air, without the actual production of snow. this would produce the effect that we see. even this explanation implies that mars has air and water, rare though the former may be. it is quite possible that air as thin as that of mars would sustain life in some form. life not totally unlike that on the earth may therefore exist upon this planet for anything that we know to the contrary. more than this we cannot say. in the case of the outer planets the answer to our question must be in the negative. it now seems likely that jupiter is a body very much like our sun, only that the dark portion is too cool to emit much, if any, light. it is doubtful whether jupiter has anything in the nature of a solid surface. its interior is in all likelihood a mass of molten matter far above a red heat, which is surrounded by a comparatively cool, yet, to our measure, extremely hot, vapor. the belt-like clouds which surround the planet are due to this vapor combined with the rapid rotation. if there is any solid surface below the atmosphere that we can see, it is swept by winds such that nothing we have on earth could withstand them. but, as we have said, the probabilities are very much against there being anything like such a surface. at some great depth in the fiery vapor there is a solid nucleus; that is all we can say. the planet saturn seems to be very much like that of jupiter in its composition. it receives so little heat from the sun that, unless it is a mass of fiery vapor like jupiter, the surface must be far below the freezing-point. we cannot speak with such certainty of uranus and neptune; yet the probability seems to be that they are in much the same condition as saturn. they are known to have very dense atmospheres, which are made known to us only by their absorbing some of the light of the sun. but nothing is known of the composition of these atmospheres. to sum up our argument: the fact that, so far as we have yet been able to learn, only a very small proportion of the visible worlds scattered through space are fitted to be the abode of life does not preclude the probability that among hundreds of millions of such worlds a vast number are so fitted. such being the case, all the analogies of nature lead us to believe that, whatever the process which led to life upon this earth--whether a special act of creative power or a gradual course of development--through that same process does life begin in every part of the universe fitted to sustain it. the course of development involves a gradual improvement in living forms, which by irregular steps rise higher and higher in the scale of being. we have every reason to believe that this is the case wherever life exists. it is, therefore, perfectly reasonable to suppose that beings, not only animated, but endowed with reason, inhabit countless worlds in space. it would, indeed, be very inspiring could we learn by actual observation what forms of society exist throughout space, and see the members of such societies enjoying themselves by their warm firesides. but this, so far as we can now see, is entirely beyond the possible reach of our race, so long as it is confined to a single world. viii how the planets are weighed you ask me how the planets are weighed? i reply, on the same principle by which a butcher weighs a ham in a spring-balance. when he picks the ham up, he feels a pull of the ham towards the earth. when he hangs it on the hook, this pull is transferred from his hand to the spring of the balance. the stronger the pull, the farther the spring is pulled down. what he reads on the scale is the strength of the pull. you know that this pull is simply the attraction of the earth on the ham. but, by a universal law of force, the ham attracts the earth exactly as much as the earth does the ham. so what the butcher really does is to find how much or how strongly the ham attracts the earth, and he calls that pull the weight of the ham. on the same principle, the astronomer finds the weight of a body by finding how strong is its attractive pull on some other body. if the butcher, with his spring-balance and a ham, could fly to all the planets, one after the other, weigh the ham on each, and come back to report the results to an astronomer, the latter could immediately compute the weight of each planet of known diameter, as compared with that of the earth. in applying this principle to the heavenly bodies, we at once meet a difficulty that looks insurmountable. you cannot get up to the heavenly bodies to do your weighing; how then will you measure their pull? i must begin the answer to this question by explaining a nice point in exact science. astronomers distinguish between the weight of a body and its mass. the weight of objects is not the same all over the world; a thing which weighs thirty pounds in new york would weigh an ounce more than thirty pounds in a spring-balance in greenland, and nearly an ounce less at the equator. this is because the earth is not a perfect sphere, but a little flattened. thus weight varies with the place. if a ham weighing thirty pounds were taken up to the moon and weighed there, the pull would only be five pounds, because the moon is so much smaller and lighter than the earth. there would be another weight of the ham for the planet mars, and yet another on the sun, where it would weigh some eight hundred pounds. hence the astronomer does not speak of the weight of a planet, because that would depend on the place where it was weighed; but he speaks of the mass of the planet, which means how much planet there is, no matter where you might weigh it. at the same time, we might, without any inexactness, agree that the mass of a heavenly body should be fixed by the weight it would have in new york. as we could not even imagine a planet at new york, because it may be larger than the earth itself, what we are to imagine is this: suppose the planet could be divided into a million million million equal parts, and one of these parts brought to new york and weighed. we could easily find its weight in pounds or tons. then multiply this weight by a million million million, and we shall have a weight of the planet. this would be what the astronomers might take as the mass of the planet. with these explanations, let us see how the weight of the earth is found. the principle we apply is that round bodies of the same specific gravity attract small objects on their surface with a force proportional to the diameter of the attracting body. for example, a body two feet in diameter attracts twice as strongly as one of a foot, one of three feet three times as strongly, and so on. now, our earth is about , , feet in diameter; that is , , times four feet. it follows that if we made a little model of the earth four feet in diameter, having the average specific gravity of the earth, it would attract a particle with one ten-millionth part of the attraction of the earth. the attraction of such a model has actually been measured. since we do not know the average specific gravity of the earth--that being in fact what we want to find out--we take a globe of lead, four feet in diameter, let us suppose. by means of a balance of the most exquisite construction it is found that such a globe does exert a minute attraction on small bodies around it, and that this attraction is a little more than the ten-millionth part of that of the earth. this shows that the specific gravity of the lead is a little greater than that of the average of the whole earth. all the minute calculations made, it is found that the earth, in order to attract with the force it does, must be about five and one-half times as heavy as its bulk of water, or perhaps a little more. different experimenters find different results; the best between . and . , so that . is, perhaps, as near the number as we can now get. this is much more than the average specific gravity of the materials which compose that part of the earth which we can reach by digging mines. the difference arises from the fact that, at the depth of many miles, the matter composing the earth is compressed into a smaller space by the enormous weight of the portions lying above it. thus, at the depth of miles, the pressure on every cubic inch is more than tons, a weight which would greatly condense the hardest metal. we come now to the planets. i have said that the mass or weight of a heavenly body is determined by its attraction on some other body. there are two ways in which the attraction of a planet may be measured. one is by its attraction on the planets next to it. if these bodies did not attract one another at all, but only moved under the influence of the sun, they would move in orbits having the form of ellipses. they are found to move very nearly in such orbits, only the actual path deviates from an ellipse, now in one direction and then in another, and it slowly changes its position from year to year. these deviations are due to the pull of the other planets, and by measuring the deviations we can determine the amount of the pull, and hence the mass of the planet. the reader will readily understand that the mathematical processes necessary to get a result in this way must be very delicate and complicated. a much simpler method can be used in the case of those planets which have satellites revolving round them, because the attraction of the planet can be determined by the motions of the satellite. the first law of motion teaches us that a body in motion, if acted on by no force, will move in a straight line. hence, if we see a body moving in a curve, we know that it is acted on by a force in the direction towards which the motion curves. a familiar example is that of a stone thrown from the hand. if the stone were not attracted by the earth, it would go on forever in the line of throw, and leave the earth entirely. but under the attraction of the earth, it is drawn down and down, as it travels onward, until finally it reaches the ground. the faster the stone is thrown, of course, the farther it will go, and the greater will be the sweep of the curve of its path. if it were a cannon-ball, the first part of the curve would be nearly a right line. if we could fire a cannon-ball horizontally from the top of a high mountain with a velocity of five miles a second, and if it were not resisted by the air, the curvature of the path would be equal to that of the surface of our earth, and so the ball would never reach the earth, but would revolve round it like a little satellite in an orbit of its own. could this be done, the astronomer would be able, knowing the velocity of the ball, to calculate the attraction of the earth as well as we determine it by actually observing the motion of falling bodies around us. thus it is that when a planet, like mars or jupiter, has satellites revolving round it, astronomers on the earth can observe the attraction of the planet on its satellites and thus determine its mass. the rule for doing this is very simple. the cube of the distance between the planet and satellite is divided by the square of the time of revolution of the satellite. the quotient is a number which is proportional to the mass of the planet. the rule applies to the motion of the moon round the earth and of the planets round the sun. if we divide the cube of the earth's distance from the sun, say , , miles, by the square of / , the days in a year, we shall get a certain quotient. let us call this number the sun-quotient. then, if we divide the cube of the moon's distance from the earth by the square of its time of revolution, we shall get another quotient, which we may call the earth-quotient. the sun-quotient will come out about , times as large as the earth-quotient. hence it is concluded that the mass of the sun is , times that of the earth; that it would take this number of earths to make a body as heavy as the sun. i give this calculation to illustrate the principle; it must not be supposed that the astronomer proceeds exactly in this way and has only this simple calculation to make. in the case of the moon and earth, the motion and distance of the former vary in consequence of the attraction of the sun, so that their actual distance apart is a changing quantity. so what the astronomer actually does is to find the attraction of the earth by observing the length of a pendulum which beats seconds in various latitudes. then, by very delicate mathematical processes, he can find with great exactness what would be the time of revolution of a small satellite at any given distance from the earth, and thus can get the earth-quotient. but, as i have already pointed out, we must, in the case of the planets, find the quotient in question by means of the satellites; and it happens, fortunately, that the motions of these bodies are much less changed by the attraction of the sun than is the motion of the moon. thus, when we make the computation for the outer satellite of mars, we find the quotient to be / that of the sun-quotient. hence we conclude that the mass of mars is / that of the sun. by the corresponding quotient, the mass of jupiter is found to be about / that of the sun, saturn / , uranus / , neptune / . we have set forth only the great principle on which the astronomer has proceeded for the purpose in question. the law of gravitation is at the bottom of all his work. the effects of this law require mathematical processes which it has taken two hundred years to bring to their present state, and which are still far from perfect. the measurement of the distance of a satellite is not a job to be done in an evening; it requires patient labor extending through months and years, and then is not as exact as the astronomer would wish. he does the best he can, and must be satisfied with that. ix the mariner's compass among those provisions of nature which seem to us as especially designed for the use of man, none is more striking than the seeming magnetism of the earth. what would our civilization have been if the mariner's compass had never been known? that columbus could never have crossed the atlantic is certain; in what generation since his time our continent would have been discovered is doubtful. did the reader ever reflect what a problem the captain of the finest ocean liner of our day would face if he had to cross the ocean without this little instrument? with the aid of a pilot he gets his ship outside of sandy hook without much difficulty. even later, so long as the sun is visible and the air is clear, he will have some apparatus for sailing by the direction of the sun. but after a few hours clouds cover the sky. from that moment he has not the slightest idea of east, west, north, or south, except so far as he may infer it from the direction in which he notices the wind to blow. for a few hours he may be guided by the wind, provided he is sure he is not going ashore on long island. thus, in time, he feels his way out into the open sea. by day he has some idea of direction with the aid of the sun; by night, when the sky is clear he can steer by the great bear, or "cynosure," the compass of his ancient predecessors on the mediterranean. but when it is cloudy, if he persists in steaming ahead, he may be running towards the azores or towards greenland, or he may be making his way back to new york without knowing it. so, keeping up steam only when sun or star is visible, he at length finds that he is approaching the coast of ireland. then he has to grope along much like a blind man with his staff, feeling his way along the edge of a precipice. he can determine the latitude at noon if the sky is clear, and his longitude in the morning or evening in the same conditions. in this way he will get a general idea of his whereabouts. but if he ventures to make headway in a fog, he may find himself on the rocks at any moment. he reaches his haven only after many spells of patient waiting for favoring skies. the fact that the earth acts like a magnet, that the needle points to the north, has been generally known to navigators for nearly a thousand years, and is said to have been known to the chinese at a yet earlier period. and yet, to-day, if any professor of physical science is asked to explain the magnetic property of the earth, he will acknowledge his inability to do so to his own satisfaction. happily this does not hinder us from finding out by what law these forces act, and how they enable us to navigate the ocean. i therefore hope the reader will be interested in a short exposition of the very curious and interesting laws on which the science of magnetism is based, and which are applied in the use of the compass. the force known as magnetic, on which the compass depends, is different from all other natural forces with which we are familiar. it is very remarkable that iron is the only substance which can become magnetic in any considerable degree. nickel and one or two other metals have the same property, but in a very slight degree. it is also remarkable that, however powerfully a bar of steel may be magnetized, not the slightest effect of the magnetism can be seen by its action on other than magnetic substances. it is no heavier than before. its magnetism does not produce the slightest influence upon the human body. no one would know that it was magnetic until something containing iron was brought into its immediate neighborhood; then the attraction is set up. the most important principle of magnetic science is that there are two opposite kinds of magnetism, which are, in a certain sense, contrary in their manifestations. the difference is seen in the behavior of the magnet itself. one particular end points north, and the other end south. what is it that distinguishes these two ends? the answer is that one end has what we call north magnetism, while the other has south magnetism. every magnetic bar has two poles, one near one end, one near the other. the north pole is drawn towards the north pole of the earth, the south pole towards the south pole, and thus it is that the direction of the magnet is determined. now, when we bring two magnets near each other we find another curious phenomenon. if the two like poles are brought together, they do not attract but repel each other. but the two opposite poles attract each other. the attraction and repulsion are exactly equal under the same conditions. there is no more attraction than repulsion. if we seal one magnet up in a paper or a box, and then suspend another over the box, the north pole of the one outside will tend to the south pole of the one in the box, and vice versa. our next discovery is, that whenever a magnet attracts a piece of iron it makes that iron into a magnet, at least for the time being. in the case of ordinary soft or untempered iron the magnetism disappears instantly when the magnet is removed. but if the magnet be made to attract a piece of hardened steel, the latter will retain the magnetism produced in it and become itself a permanent magnet. this fact must have been known from the time that the compass came into use. to make this instrument it was necessary to magnetize a small bar or needle by passing a natural magnet over it. in our times the magnetization is effected by an electric current. the latter has curious magnetic properties; a magnetic needle brought alongside of it will be found placing itself at right angles to the wire bearing the current. on this principle is made the galvanometer for measuring the intensity of a current. moreover, if a piece of wire is coiled round a bar of steel, and a powerful electric current pass through the coil, the bar will become a magnet. another curious property of magnetism is that we cannot develop north magnetism in a bar without developing south magnetism at the same time. if it were otherwise, important consequences would result. a separate north pole of a magnet would, if attached to a floating object and thrown into the ocean, start on a journey towards the north all by itself. a possible method of bringing this result about may suggest itself. let us take an ordinary bar magnet, with a pole at each end, and break it in the middle; then would not the north end be all ready to start on its voyage north, and the south end to make its way south? but, alas! when this experiment is tried it is found that a south pole instantly develops itself on one side of the break, and a north pole on the other side, so that the two pieces will simply form two magnets, each with its north and south pole. there is no possibility of making a magnet with only one pole. it was formerly supposed that the central portions of the earth consisted of an immense magnet directed north and south. although this view is found, for reasons which need not be set forth in detail, to be untenable, it gives us a good general idea of the nature of terrestrial magnetism. one result that follows from the law of poles already mentioned is that the magnetism which seems to belong to the north pole of the earth is what we call south on the magnet, and vice versa. careful experiment shows us that the region around every magnet is filled with magnetic force, strongest near the poles of the magnet, but diminishing as the inverse square of the distance from the pole. this force, at each point, acts along a certain line, called a line of force. these lines are very prettily shown by the familiar experiment of placing a sheet of paper over a magnet, and then scattering iron filings on the surface of the paper. it will be noticed that the filings arrange themselves along a series of curved lines, diverging in every direction from each pole, but always passing from one pole to the other. it is a universal law that whenever a magnet is brought into a region where this force acts, it is attracted into such a position that it shall have the same direction as the lines of force. its north pole will take the direction of the curve leading to the south pole of the other magnet, and its south pole the opposite one. the fact of terrestrial magnetism may be expressed by saying that the space within and around the whole earth is filled by lines of magnetic force, which we know nothing about until we suspend a magnet so perfectly balanced that it may point in any direction whatever. then it turns and points in the direction of the lines of force, which may thus be mapped out for all points of the earth. we commonly say that the pole of the needle points towards the north. the poets tell us how the needle is true to the pole. every reader, however, is now familiar with the general fact of a variation of the compass. on our eastern seaboard, and all the way across the atlantic, the north pointing of the compass varies so far to the west that a ship going to europe and making no allowance for this deviation would find herself making more nearly for the north cape than for her destination. the "declination," as it is termed in scientific language, varies from one region of the earth to another. in some places it is towards the west, in others towards the east. the pointing of the needle in various regions of the world is shown by means of magnetic maps. such maps are published by the united states coast survey, whose experts make a careful study of the magnetic force all over the country. it is found that there is a line running nearly north and south through the middle states along which there is no variation of the compass. to the east of it the variation of the north pole of the magnet is west; to the west of it, east. the most rapid changes in the pointing of the needle are towards the northeast and northwest regions. when we travel to the northeastern boundary of maine the westerly variation has risen to degrees. towards the northwest the easterly variation continually increases, until, in the northern part of the state of washington, it amounts to degrees. when we cross the atlantic into europe we find the west variation diminishing until we reach a certain line passing through central russia and western asia. this is again a line of no variation. crossing it, the variation is once more towards the east. this direction continues over most of the continent of asia, but varies in a somewhat irregular manner from one part of the continent to another. as a general rule, the lines of the earth's magnetic force are not horizontal, and therefore one end or the other of a perfectly suspended magnet will dip below the horizontal position. this is called the "dip of the needle." it is observed by means of a brass circle, of which the circumference is marked off in degrees. a magnet is attached to this circle so as to form a diameter, and suspended on a horizontal axis passing through the centre of gravity, so that the magnet shall be free to point in the direction indicated by the earth's lines of magnetic force. armed with this apparatus, scientific travellers and navigators have visited various points of the earth in order to determine the dip. it is thus found that there is a belt passing around the earth near the equator, but sometimes deviating several degrees from it, in which there is no dip; that is to say, the lines of magnetic force are horizontal. taking any point on this belt and going north, it will be found that the north pole of the magnet gradually tends downward, the dip constantly increasing as we go farther north. in the southern part of the united states the dip is about degrees, and the direction of the needle is nearly perpendicular to the earth's axis. in the northern part of the country, including the region of the great lakes, the dip increases to degrees. noticing that a dip of degrees would mean that the north end of the magnet points straight downward, it follows that it would be more nearly correct to say that, throughout the united states, the magnetic needle points up and down than that it points north and south. going yet farther north, we find the dip still increasing, until at a certain point in the arctic regions the north pole of the needle points downward. in this region the compass is of no use to the traveller or the navigator. the point is called the magnetic pole. its position has been located several times by scientific observers. the best determinations made during the last eighty years agree fairly well in placing it near degrees north latitude and degrees longitude west from greenwich. this point is situated on the west shore of the boothian peninsula, which is bounded on the south end by mcclintock channel. it is about five hundred miles north of the northwest part of hudson bay. there is a corresponding magnetic pole in the antarctic ocean, or rather on victoria land, nearly south of australia. its position has not been so exactly located as in the north, but it is supposed to be at about degrees of south latitude and degrees of east longitude from greenwich. the magnetic poles used to be looked upon as the points towards which the respective ends of the needle were attracted. and, as a matter of fact, the magnetic force is stronger near the poles than elsewhere. when located in this way by strength of force, it is found that there is a second north pole in northern siberia. its location has not, however, been so well determined as in the case of the american pole, and it is not yet satisfactorily shown that there is any one point in siberia where the direction of the force is exactly downward. [illustration with caption: dip of the magnetic needle in various latitudes. the arrow points show the direction of the north end of the magnetic needle, which dips downward in north latitudes, while the south end dips in south latitudes.] the declination and dip, taken together, show the exact direction of the magnetic force at any place. but in order to complete the statement of the force, one more element must be given--its amount. the intensity of the magnetic force is determined by suspending a magnet in a horizontal position, and then allowing it to oscillate back and forth around the suspension. the stronger the force, the less the time it will take to oscillate. thus, by carrying a magnet to various parts of the world, the magnetic force can be determined at every point where a proper support for the magnet is obtainable. the intensity thus found is called the horizontal force. this is not really the total force, because the latter depends upon the dip; the greater the dip, the less will be the horizontal force which corresponds to a certain total force. but a very simple computation enables the one to be determined when the value of the other is known. in this way it is found that, as a general rule, the magnetic force is least in the earth's equatorial regions and increases as we approach either of the magnetic poles. when the most exact observations on the direction of the needle are made, it is found that it never remains at rest. beginning with the changes of shortest duration, we have a change which takes place every day, and is therefore called diurnal. in our northern latitudes it is found that during the six hours from nine o'clock at night until three in the morning the direction of the magnet remains nearly the same. but between three and four a.m. it begins to deviate towards the east, going farther and farther east until about a.m. then, rather suddenly, it begins to swing towards the west with a much more rapid movement, which comes to an end between one and two o'clock in the afternoon. then, more slowly, it returns in an easterly direction until about nine at night, when it becomes once more nearly quiescent. happily, the amount of this change is so small that the navigator need not trouble himself with it. the entire range of movement rarely amounts to one-quarter of a degree. it is a curious fact that the amount of the change is twice as great in june as it is in december. this indicates that it is caused by the sun's radiation. but how or why this cause should produce such an effect no one has yet discovered. another curious feature is that in the southern hemisphere the direction of the motion is reversed, although its general character remains the same. the pointing deviates towards the west in the morning, then rapidly moves towards the east until about two o'clock, after which it slowly returns to its original direction. the dip of the needle goes through a similar cycle of daily changes. in northern latitudes it is found that at about six in the morning the dip begins to increase, and continues to do so until noon, after which it diminishes until seven or eight o'clock in the evening, when it becomes nearly constant for the rest of the night. in the southern hemisphere the direction of the movement is reversed. when the pointing of the needle is compared with the direction of the moon, it is found that there is a similar change. but, instead of following the moon in its course, it goes through two periods in a day, like the tides. when the moon is on the meridian, whether above or below us, the effect is in one direction, while when it is rising or setting it is in the opposite direction. in other words, there is a complete swinging backward and forward twice in a lunar day. it might be supposed that such an effect would be due to the moon, like the earth, being a magnet. but were this the case there would be only one swing back and forth during the passage of the moon from the meridian until it came back to the meridian again. the effect would be opposite at the rising and setting of the moon, which we have seen is not the case. to make the explanation yet more difficult, it is found that, as in the case of the sun, the change is opposite in the northern and southern hemispheres and very small at the equator, where, by virtue of any action that we can conceive of, it ought to be greatest. the pointing is also found to change with the age of the moon and with the season of the year. but these motions are too small to be set forth in the present article. there is yet another class of changes much wider than these. the observations recorded since the time of columbus show that, in the course of centuries, the variation of the compass, at any one point, changes very widely. it is well known that in the needle pointed east of north in the mediterranean, as well as in those portions of the atlantic which were then navigated. columbus was therefore much astonished when, on his first voyage, in mid-ocean, he found that the deviation was reversed, and was now towards the west. it follows that a line of no variation then passed through the atlantic ocean. but this line has since been moving towards the east. about it passed the meridian of paris. during the two hundred and forty years which have since elapsed, it has passed over central europe, and now, as we have already said, passes through european russia. the existence of natural magnets composed of iron ore, and their property of attracting iron and making it magnetic, have been known from the remotest antiquity. but the question as to who first discovered the fact that a magnetized needle points north and south, and applied this discovery to navigation, has given rise to much discussion. that the property was known to the chinese about the beginning of our era seems to be fairly well established, the statements to that effect being of a kind that could not well have been invented. historical evidence of the use of the magnetic needle in navigation dates from the twelfth century. the earliest compass consisted simply of a splinter of wood or a piece of straw to which the magnetized needle was attached, and which was floated in water. a curious obstacle is said to have interfered with the first uses of this instrument. jack is a superstitious fellow, and we may be sure that he was not less so in former times than he is today. from his point of view there was something uncanny in so very simple a contrivance as a floating straw persistently showing him the direction in which he must sail. it made him very uncomfortable to go to sea under the guidance of an invisible power. but with him, as with the rest of us, familiarity breeds contempt, and it did not take more than a generation to show that much good and no harm came to those who used the magic pointer. the modern compass, as made in the most approved form for naval and other large ships, is the liquid one. this does not mean that the card bearing the needle floats on the liquid, but only that a part of the force is taken off from the pivot on which it turns, so as to make the friction as small as possible, and to prevent the oscillation back and forth which would continually go on if the card were perfectly free to turn. the compass-card is marked not only with the thirty-two familiar points of the compass, but is also divided into degrees. in the most accurate navigation it is probable that very little use of the points is made, the ship being directed according to the degrees. a single needle is not relied upon to secure the direction of the card, the latter being attached to a system of four or even more magnets, all pointing in the same direction. the compass must have no iron in its construction or support, because the attraction of that substance on the needle would be fatal to its performance. from this cause the use of iron as ship-building material introduced a difficulty which it was feared would prove very serious. the thousands of tons of iron in a ship must exert a strong attraction on the magnetic needle. another complication is introduced by the fact that the iron of the ship will always become more or less magnetic, and when the ship is built of steel, as modern ones are, this magnetism will be more or less permanent. we have already said that a magnet has the property of making steel or iron in its neighborhood into another magnet, with its poles pointing in the opposite direction. the consequence is that the magnetism of the earth itself will make iron or steel more or less magnetic. as a ship is built she thus becomes a great repository of magnetism, the direction of the force of which will depend upon the position in which she lay while building. if erected on the bank of an east and west stream, the north end of the ship will become the north pole of a magnet and the south end the south pole. accordingly, when she is launched and proceeds to sea, the compass points not exactly according to the magnetism of the earth, but partly according to that of the ship also. the methods of obviating this difficulty have exercised the ingenuity of the ablest physicists from the beginning of iron ship building. one method is to place in the neighborhood of the compass, but not too near it, a steel bar magnetized in the opposite direction from that of the ship, so that the action of the latter shall be neutralized. but a perfect neutralization cannot be thus effected. it is all the more difficult to effect it because the magnetism of a ship is liable to change. the practical method therefore adopted is called "swinging the ship," an operation which passengers on ocean liners may have frequently noticed when approaching land. the ship is swung around so that her bow shall point in various directions. at each pointing the direction of the ship is noticed by sighting on the sun, and also the direction of the compass itself. in this way the error of the pointing of the compass as the ship swings around is found for every direction in which she may be sailing. a table can then be made showing what the pointing, according to the compass, should be in order that the ship may sail in any given direction. this, however, does not wholly avoid the danger. the tables thus made are good when the ship is on a level keel. if, from any cause whatever, she heels over to one side, the action will be different. thus there is a "heeling error" which must be allowed for. it is supposed to have been from this source of error not having been sufficiently determined or appreciated that the lamentable wreck of the united states ship huron off the coast of hatteras occurred some twenty years ago. x the fairyland of geometry if the reader were asked in what branch of science the imagination is confined within the strictest limits, he would, i fancy, reply that it must be that of mathematics. the pursuer of this science deals only with problems requiring the most exact statements and the most rigorous reasoning. in all other fields of thought more or less room for play may be allowed to the imagination, but here it is fettered by iron rules, expressed in the most rigid logical form, from which no deviation can be allowed. we are told by philosophers that absolute certainty is unattainable in all ordinary human affairs, the only field in which it is reached being that of geometric demonstration. and yet geometry itself has its fairyland--a land in which the imagination, while adhering to the forms of the strictest demonstration, roams farther than it ever did in the dreams of grimm or andersen. one thing which gives this field its strictly mathematical character is that it was discovered and explored in the search after something to supply an actual want of mathematical science, and was incited by this want rather than by any desire to give play to fancy. geometricians have always sought to found their science on the most logical basis possible, and thus have carefully and critically inquired into its foundations. the new geometry which has thus arisen is of two closely related yet distinct forms. one of these is called non-euclidian, because euclid's axiom of parallels, which we shall presently explain, is ignored. in the other form space is assumed to have one or more dimensions in addition to the three to which the space we actually inhabit is confined. as we go beyond the limits set by euclid in adding a fourth dimension to space, this last branch as well as the other is often designated non-euclidian. but the more common term is hypergeometry, which, though belonging more especially to space of more than three dimensions, is also sometimes applied to any geometric system which transcends our ordinary ideas. in all geometric reasoning some propositions are necessarily taken for granted. these are called axioms, and are commonly regarded as self-evident. yet their vital principle is not so much that of being self-evident as being, from the nature of the case, incapable of demonstration. our edifice must have some support to rest upon, and we take these axioms as its foundation. one example of such a geometric axiom is that only one straight line can be drawn between two fixed points; in other words, two straight lines can never intersect in more than a single point. the axiom with which we are at present concerned is commonly known as the th of euclid, and may be set forth in the following way: we have given a straight line, a b, and a point, p, with another line, c d, passing through it and capable of being turned around on p. euclid assumes that this line c d will have one position in which it will be parallel to a b, that is, a position such that if the two lines are produced without end, they will never meet. his axiom is that only one such line can be drawn through p. that is to say, if we make the slightest possible change in the direction of the line c d, it will intersect the other line, either in one direction or the other. the new geometry grew out of the feeling that this proposition ought to be proved rather than taken as an axiom; in fact, that it could in some way be derived from the other axioms. many demonstrations of it were attempted, but it was always found, on critical examination, that the proposition itself, or its equivalent, had slyly worked itself in as part of the base of the reasoning, so that the very thing to be proved was really taken for granted. [illustration with caption: fig. ] this suggested another course of inquiry. if this axiom of parallels does not follow from the other axioms, then from these latter we may construct a system of geometry in which the axiom of parallels shall not be true. this was done by lobatchewsky and bolyai, the one a russian the other a hungarian geometer, about . to show how a result which looks absurd, and is really inconceivable by us, can be treated as possible in geometry, we must have recourse to analogy. suppose a world consisting of a boundless flat plane to be inhabited by reasoning beings who can move about at pleasure on the plane, but are not able to turn their heads up or down, or even to see or think of such terms as above them and below them, and things around them can be pushed or pulled about in any direction, but cannot be lifted up. people and things can pass around each other, but cannot step over anything. these dwellers in "flatland" could construct a plane geometry which would be exactly like ours in being based on the axioms of euclid. two parallel straight lines would never meet, though continued indefinitely. but suppose that the surface on which these beings live, instead of being an infinitely extended plane, is really the surface of an immense globe, like the earth on which we live. it needs no knowledge of geometry, but only an examination of any globular object--an apple, for example--to show that if we draw a line as straight as possible on a sphere, and parallel to it draw a small piece of a second line, and continue this in as straight a line as we can, the two lines will meet when we proceed in either direction one-quarter of the way around the sphere. for our "flat-land" people these lines would both be perfectly straight, because the only curvature would be in the direction downward, which they could never either perceive or discover. the lines would also correspond to the definition of straight lines, because any portion of either contained between two of its points would be the shortest distance between those points. and yet, if these people should extend their measures far enough, they would find any two parallel lines to meet in two points in opposite directions. for all small spaces the axioms of their geometry would apparently hold good, but when they came to spaces as immense as the semi-diameter of the earth, they would find the seemingly absurd result that two parallel lines would, in the course of thousands of miles, come together. another result yet more astonishing would be that, going ahead far enough in a straight line, they would find that although they had been going forward all the time in what seemed to them the same direction, they would at the end of , miles find themselves once more at their starting-point. one form of the modern non-euclidian geometry assumes that a similar theorem is true for the space in which our universe is contained. although two straight lines, when continued indefinitely, do not appear to converge even at the immense distances which separate us from the fixed stars, it is possible that there may be a point at which they would eventually meet without either line having deviated from its primitive direction as we understand the case. it would follow that, if we could start out from the earth and fly through space in a perfectly straight line with a velocity perhaps millions of times that of light, we might at length find ourselves approaching the earth from a direction the opposite of that in which we started. our straight-line circle would be complete. another result of the theory is that, if it be true, space, though still unbounded, is not infinite, just as the surface of a sphere, though without any edge or boundary, has only a limited extent of surface. space would then have only a certain volume--a volume which, though perhaps greater than that of all the atoms in the material universe, would still be capable of being expressed in cubic miles. if we imagine our earth to grow larger and larger in every direction without limit, and with a speed similar to that we have described, so that to-morrow it was large enough to extend to the nearest fixed stars, the day after to yet farther stars, and so on, and we, living upon it, looked out for the result, we should, in time, see the other side of the earth above us, coming down upon us? as it were. the space intervening would grow smaller, at last being filled up. the earth would then be so expanded as to fill all existing space. this, although to us the most interesting form of the non-euclidian geometry, is not the only one. the idea which lobatchewsky worked out was that through a point more than one parallel to a given line could be drawn; that is to say, if through the point p we have already supposed another line were drawn making ever so small an angle with cd, this line also would never meet the line ab. it might approach the latter at first, but would eventually diverge. the two lines ab and cd, starting parallel, would eventually, perhaps at distances greater than that of the fixed stars, gradually diverge from each other. this system does not admit of being shown by analogy so easily as the other, but an idea of it may be had by supposing that the surface of "flat-land," instead of being spherical, is saddle-shaped. apparently straight parallel lines drawn upon it would then diverge, as supposed by bolyai. we cannot, however, imagine such a surface extended indefinitely without losing its properties. the analogy is not so clearly marked as in the other case. to explain hypergeometry proper we must first set forth what a fourth dimension of space means, and show how natural the way is by which it may be approached. we continue our analogy from "flat-land" in this supposed land let us make a cross--two straight lines intersecting at right angles. the inhabitants of this land understand the cross perfectly, and conceive of it just as we do. but let us ask them to draw a third line, intersecting in the same point, and perpendicular to both the other lines. they would at once pronounce this absurd and impossible. it is equally absurd and impossible to us if we require the third line to be drawn on the paper. but we should reply, "if you allow us to leave the paper or flat surface, then we can solve the problem by simply drawing the third line through the paper perpendicular to its surface." [illustration with caption: fig. ] now, to pursue the analogy, suppose that, after we have drawn three mutually perpendicular lines, some being from another sphere proposes to us the drawing of a fourth line through the same point, perpendicular to all three of the lines already there. we should answer him in the same way that the inhabitants of "flat-land" answered us: "the problem is impossible. you cannot draw any such line in space as we understand it." if our visitor conceived of the fourth dimension, he would reply to us as we replied to the "flat-land" people: "the problem is absurd and impossible if you confine your line to space as you understand it. but for me there is a fourth dimension in space. draw your line through that dimension, and the problem will be solved. this is perfectly simple to me; it is impossible to you solely because your conceptions do not admit of more than three dimensions." supposing the inhabitants of "flat-land" to be intellectual beings as we are, it would be interesting to them to be told what dwellers of space in three dimensions could do. let us pursue the analogy by showing what dwellers in four dimensions might do. place a dweller of "flat-land" inside a circle drawn on his plane, and ask him to step outside of it without breaking through it. he would go all around, and, finding every inch of it closed, he would say it was impossible from the very nature of the conditions. "but," we would reply, "that is because of your limited conceptions. we can step over it." "step over it!" he would exclaim. "i do not know what that means. i can pass around anything if there is a way open, but i cannot imagine what you mean by stepping over it." but we should simply step over the line and reappear on the other side. so, if we confine a being able to move in a fourth dimension in the walls of a dungeon of which the sides, the floor, and the ceiling were all impenetrable, he would step outside of it without touching any part of the building, just as easily as we could step over a circle drawn on the plane without touching it. he would simply disappear from our view like a spirit, and perhaps reappear the next moment outside the prison. to do this he would only have to make a little excursion in the fourth dimension. [illustration with caption: fig. ] another curious application of the principle is more purely geometrical. we have here two triangles, of which the sides and angles of the one are all equal to corresponding sides and angles of the other. euclid takes it for granted that the one triangle can be laid upon the other so that the two shall fit together. but this cannot be done unless we lift one up and turn it over. in the geometry of "flat-land" such a thing as lifting up is inconceivable; the two triangles could never be fitted together. [illustration with caption: fig ] now let us suppose two pyramids similarly related. all the faces and angles of the one correspond to the faces and angles of the other. yet, lift them about as we please, we could never fit them together. if we fit the bases together the two will lie on opposite sides, one being below the other. but the dweller in four dimensions of space will fit them together without any trouble. by the mere turning over of one he will convert it into the other without any change whatever in the relative position of its parts. what he could do with the pyramids he could also do with one of us if we allowed him to take hold of us and turn a somersault with us in the fourth dimension. we should then come back into our natural space, but changed as if we were seen in a mirror. everything on us would be changed from right to left, even the seams in our clothes, and every hair on our head. all this would be done without, during any of the motion, any change having occurred in the positions of the parts of the body. it is very curious that, in these transcendental speculations, the most rigorous mathematical methods correspond to the most mystical ideas of the swedenborgian and other forms of religion. right around us, but in a direction which we cannot conceive any more than the inhabitants of "flat-land" can conceive up and down, there may exist not merely another universe, but any number of universes. all that physical science can say against the supposition is that, even if a fourth dimension exists, there is some law of all the matter with which we are acquainted which prevents any of it from entering that dimension, so that, in our natural condition, it must forever remain unknown to us. another possibility in space of four dimensions would be that of turning a hollow sphere, an india-rubber ball, for example, inside out by simple bending without tearing it. to show the motion in our space to which this is analogous, let us take a thin, round sheet of india-rubber, and cut out all the central part, leaving only a narrow ring round the border. suppose the outer edge of this ring fastened down on a table, while we take hold of the inner edge and stretch it upward and outward over the outer edge until we flatten the whole ring on the table, upside down, with the inner edge now the outer one. this motion would be as inconceivable in "flat-land" as turning the ball inside out is to us. xi the organization of scientific research the claims of scientific research on the public were never more forcibly urged than in professor ray lankester's recent romanes lecture before the university of oxford. man is here eloquently pictured as nature's rebel, who, under conditions where his great superior commands "thou shalt die," replies "i will live." in pursuance of this determination, civilized man has proceeded so far in his interference with the regular course of nature that he must either go on and acquire firmer control of the conditions, or perish miserably by the vengeance certain to be inflicted on the half-hearted meddler in great affairs. this rebel by every step forward renders himself liable to greater and greater penalties, and so cannot afford to pause or fail in one single step. one of nature's most powerful agencies in thwarting his determination to live is found in disease-producing parasites. "where there is one man of first-rate intelligence now employed in gaining knowledge of this agency, there should be a thousand. it should be as much the purpose of civilized nations to protect their citizens in this respect as it is to provide defence against human aggression." it was no part of the function of the lecturer to devise a plan for carrying on the great war he proposes to wage. the object of the present article is to contribute some suggestions in this direction; with especial reference to conditions in our own country; and no better text can be found for a discourse on the subject than the preceding quotation. in saying that there should be a thousand investigators of disease where there is now one, i believe that professor lankester would be the first to admit that this statement was that of an ideal to be aimed at, rather than of an end to be practically reached. every careful thinker will agree that to gather a body of men, young or old, supply them with laboratories and microscopes, and tell them to investigate disease, would be much like sending out an army without trained leaders to invade an enemy's country. there is at least one condition of success in this line which is better fulfilled in our own country than in any other; and that is liberality of support on the part of munificent citizens desirous of so employing their wealth as to promote the public good. combining this instrumentality with the general public spirit of our people, it must be admitted that, with all the disadvantages under which scientific research among us has hitherto labored, there is still no country to which we can look more hopefully than to our own as the field in which the ideal set forth by professor lankester is to be pursued. some thoughts on the question how scientific research may be most effectively promoted in our own country through organized effort may therefore be of interest. our first step will be to inquire what general lessons are to be learned from the experience of the past. the first and most important of these lessons is that research has never reached its highest development except at centres where bodies of men engaged in it have been brought together, and stimulated to action by mutual sympathy and support. we must call to mind that, although the beginnings of modern science were laid by such men as copernicus, galileo, leonardo da vinci, and torricelli, before the middle of the seventeenth century, unbroken activity and progress date from the foundations of the academy of sciences of paris and the royal society of london at that time. the historic fact that the bringing of men together, and their support by an intelligent and interested community, is the first requirement to be kept in view can easily be explained. effective research involves so intricate a network of problems and considerations that no one engaged in it can fail to profit by the suggestions of kindred spirits, even if less acquainted with the subject than he is himself. intelligent discussion suggests new ideas and continually carries the mind to a higher level of thought. we must not regard the typical scientific worker, even of the highest class, as one who, having chosen his special field and met with success in cultivating it, has only to be supplied with the facilities he may be supposed to need in order to continue his work in the most efficient way. what we have to deal with is not a fixed and permanent body of learned men, each knowing all about the field of work in which he is engaged, but a changing and growing class, constantly recruited by beginners at the bottom of the scale, and constantly depleted by the old dropping away at the top. no view of the subject is complete which does not embrace the entire activity of the investigator, from the tyro to the leader. the leader himself, unless engaged in the prosecution of some narrow specialty, can rarely be so completely acquainted with his field as not to need information from others. without this, he is constantly liable to be repeating what has already been better done than he can do it himself, of following lines which are known to lead to no result, and of adopting methods shown by the experience of others not to be the best. even the books and published researches to which he must have access may be so voluminous that he cannot find time to completely examine them for himself; or they may be inaccessible. all this will make it clear that, with an occasional exception, the best results of research are not to be expected except at centres where large bodies of men are brought into close personal contact. in addition to the power and facility acquired by frequent discussion with his fellows, the appreciation and support of an intelligent community, to whom the investigator may, from time to time, make known his thoughts and the results of his work, add a most effective stimulus. the greater the number of men of like minds that can be brought together and the larger the community which interests itself in what they are doing, the more rapid will be the advance and the more effective the work carried on. it is thus that london, with its munificently supported institutions, and paris and berlin, with their bodies of investigators supported either by the government or by various foundations, have been for more than three centuries the great centres where we find scientific activity most active and most effective. looking at this undoubted fact, which has asserted itself through so long a period, and which asserts itself today more strongly than ever, the writer conceives that there can be no question as to one proposition. if we aim at the single object of promoting the advance of knowledge in the most effective way, and making our own country the leading one in research, our efforts should be directed towards bringing together as many scientific workers as possible at a single centre, where they can profit in the highest degree by mutual help, support, and sympathy. in thus strongly setting forth what must seem an indisputable conclusion, the writer does not deny that there are drawbacks to such a policy, as there are to every policy that can be devised aiming at a good result. nature offers to society no good that she does not accompany by a greater or less measure of evil the only question is whether the good outweighs the evil. in the present case, the seeming evil, whether real or not, is that of centralization. a policy tending in this direction is held to be contrary to the best interests of science in quarters entitled to so much respect that we must inquire into the soundness of the objection. it would be idle to discuss so extreme a question as whether we shall take all the best scientific investigators of our country from their several seats of learning and attract them to some one point. we know that this cannot be done, even were it granted that success would be productive of great results. the most that can be done is to choose some existing centre of learning, population, wealth, and influence, and do what we can to foster the growth of science at that centre by attracting thither the greatest possible number of scientific investigators, especially of the younger class, and making it possible for them to pursue their researches in the most effective way. this policy would not result in the slightest harm to any institution or community situated elsewhere. it would not be even like building up a university to outrank all the others of our country; because the functions of the new institution, if such should be founded, would in its relations to the country be radically different from those of a university. its primary object would not be the education of youth, but the increase of knowledge. so far as the interests of any community or of the world at large are concerned, it is quite indifferent where knowledge may be acquired, because, when once acquired and made public, it is free to the world. the drawbacks suffered by other centres would be no greater than those suffered by our western cities, because all the great departments of the government are situated at a single distant point. strong arguments could doubtless be made for locating some of these departments in the far west, in the mississippi valley, or in various cities of the atlantic coast; but every one knows that any local advantages thus gained would be of no importance compared with the loss of that administrative efficiency which is essential to the whole country. there is, therefore, no real danger from centralization. the actual danger is rather in the opposite direction; that the sentiment against concentrating research will prove to operate too strongly. there is a feeling that it is rather better to leave every investigator where he chances to be at the moment, a feeling which sometimes finds expression in the apothegm that we cannot transplant a genius. that such a proposition should find acceptance affords a striking example of the readiness of men to accept a euphonious phrase without inquiring whether the facts support the doctrine which it enunciates. the fact is that many, perhaps the majority, of the great scientific investigators of this and of former times have done their best work through being transplanted. as soon as the enlightened monarchs of europe felt the importance of making their capitals great centres of learning, they began to invite eminent men of other countries to their own. lagrange was an italian transplanted to paris, as a member of the academy of sciences, after he had shown his powers in his native country. his great contemporary, euler, was a swiss, transplanted first to st. petersburg, then invited by frederick the great to become a member of the berlin academy, then again attracted to st. petersburg. huyghens was transplanted from his native country to paris. agassiz was an exotic, brought among us from switzerland, whose activity during the generation he passed among us was as great and effective as at any time of his life. on the continent, outside of france, the most eminent professors in the universities have been and still are brought from distant points. so numerous are the cases of which these are examples that it would be more in accord with the facts to claim that it is only by transplanting a genius that we stimulate him to his best work. having shown that the best results can be expected only by bringing into contact as many scientific investigators as possible, the next question which arises is that of their relations to one another. it may be asked whether we shall aim at individualism or collectivism. shall our ideal be an organized system of directors, professors, associates, assistants, fellows; or shall it be a collection of individual workers, each pursuing his own task in the way he deems best, untrammelled by authority? the reply to this question is that there is in this special case no antagonism between the two ideas. the most effective organization will aim both at the promotion of individual effort, and at subordination and co-operation. it would be a serious error to formulate any general rule by which all cases should be governed. the experience of the past should be our guide, so far as it applies to present and future conditions; but in availing ourselves of it we must remember that conditions are constantly changing, and must adapt our policy to the problems of the future. in doing this, we shall find that different fields of research require very different policies as regards co-operation and subordination. it will be profitable to point out those special differences, because we shall thereby gain a more luminous insight into the problems which now confront the scientific investigator, and better appreciate their variety, and the necessity of different methods of dealing with them. at one extreme, we have the field of normative science, work in which is of necessity that of the individual mind alone. this embraces pure mathematics and the methods of science in their widest range. the common interests of science require that these methods shall be worked out and formulated for the guidance of investigators generally, and this work is necessarily that of the individual brain. at the other extreme, we have the great and growing body of sciences of observation. through the whole nineteenth century, to say nothing of previous centuries, organizations, and even individuals, have been engaged in recording the innumerable phases of the course of nature, hoping to accumulate material that posterity shall be able to utilize for its benefit. we have observations astronomical, meteorological, magnetic, and social, accumulating in constantly increasing volume, the mass of which is so unmanageable with our present organizations that the question might well arise whether almost the whole of it will not have to be consigned to oblivion. such a conclusion should not be entertained until we have made a vigorous effort to find what pure metal of value can be extracted from the mass of ore. to do this requires the co-operation of minds of various orders, quite akin in their relations to those necessary in a mine or great manufacturing establishment. laborers whose duties are in a large measure matters of routine must be guided by the skill of a class higher in quality and smaller in number than their own, and these again by the technical knowledge of leaders in research. between these extremes we have a great variety of systems of co-operation. there is another feature of modern research the apprehension of which is necessary to the completeness of our view. a cursory survey of the field of science conveys the impression that it embraces only a constantly increasing number of disconnected specialties, in which each cultivator knows little or nothing of what is being done by others. measured by its bulk, the published mass of scientific research is increasing in a more than geometrical ratio. not only do the publications of nearly every scientific society increase in number and volume, but new and vigorous societies are constantly organized to add to the sum total. the stately quartos issued from the presses of the leading academies of europe are, in most cases, to be counted by hundreds. the philosophical transactions of the royal society already number about two hundred volumes, and the time when the memoirs of the french academy of sciences shall reach the thousand mark does not belong to the very remote future. besides such large volumes, these and other societies publish smaller ones in a constantly growing number. in addition to the publications of learned societies, there are journals devoted to each scientific specialty, which seem to propagate their species by subdivision in much the same way as some of the lower orders of animal life. every new publication of the kind is suggested by the wants of a body of specialists, who require a new medium for their researches and communications. the time has already come when we cannot assume that any specialist is acquainted with all that is being done even in his own line. to keep the run of this may well be beyond his own powers; more he can rarely attempt. what is the science of the future to do when this huge mass outgrows the space that can be found for it in the libraries, and what are we to say of the value of it all? are all these scientific researches to be classed as really valuable contributions to knowledge, or have we only a pile in which nuggets of gold are here and there to be sought for? one encouraging answer to such a question is that, taking the interests of the world as a whole, scientific investigation has paid for itself in benefits to humanity a thousand times over, and that all that is known to-day is but an insignificant fraction of what nature has to show us. apart from this, another feature of the science of our time demands attention. while we cannot hope that the multiplication of specialties will cease, we find that upon the process of differentiation and subdivision is now being superposed a form of evolution, tending towards the general unity of all the sciences, of which some examples may be pointed out. biological science, which a generation ago was supposed to be at the antipodes of exact science, is becoming more and more exact, and is cultivated by methods which are developed and taught by mathematicians. psychophysics--the study of the operations of the mind by physical apparatus of the same general nature as that used by the chemist and physicist--is now an established branch of research. a natural science which, if any comparisons are possible, may outweigh all others in importance to the race, is the rising one of "eugenics,"--the improvement of the human race by controlling the production of its offspring. no better example of the drawbacks which our country suffers as a seat of science can be given than the fact that the beginning of such a science has been possible only at the seat of a larger body of cultivated men than our land has yet been able to bring together. generations may elapse before the seed sown by mr. francis galton, from which grew the eugenic society, shall bear full fruit in the adoption of those individual efforts and social regulations necessary to the propagation of sound and healthy offspring on the part of the human family. but when this comes about, then indeed will professor lankester's "rebel against nature" find his independence acknowledged by the hitherto merciless despot that has decreed punishment for his treason. this new branch of science from which so much may be expected is the offshoot of another, the rapid growth of which illustrates the rapid invasion of the most important fields of thought by the methods of exact science. it is only a few years since it was remarked of professor karl pearson's mathematical investigations into the laws of heredity, and the biological questions associated with these laws, that he was working almost alone, because the biologists did not understand his mathematics, while the mathematicians were not interested in his biology. had he not lived at a great centre of active thought, within the sphere of influence of the two great universities of england, it is quite likely that this condition of isolation would have been his to the end. but, one by one, men were found possessing the skill and interest in the subject necessary to unite in his work, which now has not only a journal of its own, but is growing in a way which, though slow, has all the marks of healthy progress towards an end the importance of which has scarcely dawned upon the public mind. admitting that an organized association of investigators is of the first necessity to secure the best results in the scientific work of the future, we meet the question of the conditions and auspices under which they are to be brought together. the first thought to strike us at this point may well be that we have, in our great universities, organizations which include most of the leading men now engaged in scientific research, whose personnel and facilities we should utilize. admitting, as we all do, that there are already too many universities, and that better work would be done by a consolidation of the smaller ones, a natural conclusion is that the end in view will be best reached through existing organizations. but it would be a great mistake to jump at this conclusion without a careful study of the conditions. the brief argument--there are already too many institutions--instead of having more we should strengthen those we have--should not be accepted without examination. had it been accepted thirty years ago, there are at least two great american universities of to-day which would not have come into being, the means devoted to their support having been divided among others. these are the johns hopkins and the university of chicago. what would have been gained by applying the argument in these cases? the advantage would have been that, instead of so-called universities which appear to-day in the annual report of the bureau of education, we should have had only . the work of these would have been strengthened by an addition, to their resources, represented by the endowments of baltimore and chicago, and sufficient to add perhaps one professor to the staff of each. would the result have been better than it actually has been? have we not gained anything by allowing the argument to be forgotten in the cases of these two institutions? i do not believe that any who carefully look at the subject will hesitate in answering this question in the affirmative. the essential point is that the johns hopkins university did not merely add one to an already overcrowded list, but that it undertook a mission which none of the others was then adequately carrying out. if it did not plant the university idea in american soil, it at least gave it an impetus which has now made it the dominant one in the higher education of almost every state. the question whether the country at large would have reaped a greater benefit, had the professors of the university of chicago, with the appliances they now command, been distributed among fifty or a hundred institutions in every quarter of the land, than it has actually reaped from that university, is one which answers itself. our two youngest universities have attained success, not because two have thus been added to the number of american institutions of learning, but because they had a special mission, required by the advance of the age, for which existing institutions were inadequate. the conclusion to which these considerations lead is simple. no new institution is needed to pursue work on traditional lines, guided by traditional ideas. but, if a new idea is to be vigorously prosecuted, then a young and vigorous institution, specially organized to put the idea into effect, is necessary. the project of building up in our midst, at the most appropriate point, an organization of leading scientific investigators, for the single purpose of giving a new impetus to american science and, if possible, elevating the thought of the country and of the world to a higher plane, involves a new idea, which can best be realized by an institution organized for the special purpose. while this purpose is quite in line with that of the leading universities, it goes too far beyond them to admit of its complete attainment through their instrumentality. the first object of a university is the training of the growing individual for the highest duties of life. additions to the mass of knowledge have not been its principal function, nor even an important function in our own country, until a recent time. the primary object of the proposed institution is the advance of knowledge and the opening up of new lines of thought, which, it may be hoped, are to prove of great import to humanity. it does not follow that the function of teaching shall be wholly foreign to its activities. it must take up the best young men at the point where universities leave them, and train them in the arts of thinking and investigating. but this training will be beyond that which any regular university is carrying out. in pursuing our theme the question next arises as to the special features of the proposed association. the leading requirement is one that cannot be too highly emphasized. how clearly soever the organizers may have in their minds' eye the end in view, they must recognize the fact that it cannot be attained in a day. in every branch of work which is undertaken, there must be a single leader, and he must be the best that the country, perhaps even the world, can produce. the required man is not to be found without careful inquiry; in many branches he may be unattainable for years. when such is the case, wait patiently till he appears. prudence requires that the fewest possible risks would be taken, and that no leader should be chosen except one of tried experience and world-wide reputation. yet we should not leave wholly out of sight the success of the johns hopkins university in selecting, at its very foundation, young men who were to prove themselves the leaders of the future. this experience may admit of being repeated, if it be carefully borne in mind that young men of promise are to be avoided and young men of performance only to be considered. the performance need not be striking: ex pede herculem may be possible; but we must be sure of the soundness of our judgment before accepting our hercules. this requires a master. clerk-maxwell, who never left his native island to visit our shores, is entitled to honor as a promoter of american science for seeing the lion's paw in the early efforts of rowland, for which the latter was unable to find a medium of publication in his own country. it must also be admitted that the task is more serious now than it was then, because, from the constantly increasing specialization of science, it has become difficult for a specialist in one line to ascertain the soundness of work in another. with all the risks that may be involved in the proceeding, it will be quite possible to select an effective body of leaders, young and old, with whom an institution can begin. the wants of these men will be of the most varied kind. one needs scarcely more than a study and library; another must have small pieces of apparatus which he can perhaps design and make for himself. another may need apparatus and appliances so expensive that only an institution at least as wealthy as an ordinary university would be able to supply them. the apparatus required by others will be very largely human--assistants of every grade, from university graduates of the highest standing down to routine drudges and day-laborers. workrooms there must be; but it is hardly probable that buildings and laboratories of a highly specialized character will be required at the outset. the best counsel will be necessary at every step, and in this respect the institution must start from simple beginnings and grow slowly. leaders must be added one by one, each being judged by those who have preceded him before becoming in his turn a member of the body. as the body grows its members must be kept in personal touch, talk together, pull together, and act together. the writer submits these views to the great body of his fellow-citizens interested in the promotion of american science with the feeling that, though his conclusions may need amendment in details, they rest upon facts of the past and present which have not received the consideration which they merit. what he most strongly urges is that the whole subject of the most efficient method of promoting research upon a higher plane shall be considered with special reference to conditions in our own country; and that the lessons taught by the history and progress of scientific research in all countries shall be fully weighed and discussed by those most interested in making this form of effort a more important feature of our national life. when this is done, he will feel that his purpose in inviting special consideration to his individual views has been in great measure reached. xii can we make it rain? to the uncritical observer the possible achievements of invention and discovery seem boundless. half a century ago no idea could have appeared more visionary than that of holding communication in a few seconds of time with our fellows in australia, or having a talk going on viva voce between a man in washington and another in boston. the actual attainment of these results has naturally given rise to the belief that the word "impossible" has disappeared from our vocabulary. to every demonstration that a result cannot be reached the answer is, did not one lardner, some sixty years ago, demonstrate that a steamship could not cross the atlantic? if we say that for every actual discovery there are a thousand visionary projects, we are told that, after all, any given project may be the one out of the thousand. in a certain way these hopeful anticipations are justified. we cannot set any limit either to the discovery of new laws of nature or to the ingenious combination of devices to attain results which now look impossible. the science of to-day suggests a boundless field of possibilities. it demonstrates that the heat which the sun radiates upon the earth in a single day would suffice to drive all the steamships now on the ocean and run all the machinery on the land for a thousand years. the only difficulty is how to concentrate and utilize this wasted energy. from the stand-point of exact science aerial navigation is a very simple matter. we have only to find the proper combination of such elements as weight, power, and mechanical force. whenever mr. maxim can make an engine strong and light enough, and sails large, strong, and light enough, and devise the machinery required to connect the sails and engine, he will fly. science has nothing but encouraging words for his project, so far as general principles are concerned. such being the case, i am not going to maintain that we can never make it rain. but i do maintain two propositions. if we are ever going to make it rain, or produce any other result hitherto unattainable, we must employ adequate means. and if any proposed means or agency is already familiar to science, we may be able to decide beforehand whether it is adequate. let us grant that out of a thousand seemingly visionary projects one is really sound. must we try the entire thousand to find the one? by no means. the chances are that nine hundred of them will involve no agency that is not already fully understood, and may, therefore, be set aside without even being tried. to this class belongs the project of producing rain by sound. as i write, the daily journals are announcing the brilliant success of experiments in this direction; yet i unhesitatingly maintain that sound cannot make rain, and propose to adduce all necessary proof of my thesis. the nature of sound is fully understood, and so are the conditions under which the aqueous vapor in the atmosphere may be condensed. let us see how the case stands. a room of average size, at ordinary temperature and under usual conditions, contains about a quart of water in the form of invisible vapor. the whole atmosphere is impregnated with vapor in about the same proportion. we must, however, distinguish between this invisible vapor and the clouds or other visible masses to which the same term is often applied. the distinction may be very clearly seen by watching the steam coming from the spout of a boiling kettle. immediately at the spout the escaping steam is transparent and invisible; an inch or two away a white cloud is formed, which we commonly call steam, and which is seen belching out to a distance of one or more feet, and perhaps filling a considerable space around the kettle; at a still greater distance this cloud gradually disappears. properly speaking, the visible cloud is not vapor or steam at all, but minute particles or drops of water in a liquid state. the transparent vapor at the mouth of the kettle is the true vapor of water, which is condensed into liquid drops by cooling; but after being diffused through the air these drops evaporate and again become true vapor. clouds, then, are not formed of true vapor, but consist of impalpable particles of liquid water floating or suspended in the air. but we all know that clouds do not always fall as rain. in order that rain may fall the impalpable particles of water which form the cloud must collect into sensible drops large enough to fall to the earth. two steps are therefore necessary to the formation of rain: the transparent aqueous vapor in the air must be condensed into clouds, and the material of the clouds must agglomerate into raindrops. no physical fact is better established than that, under the conditions which prevail in the atmosphere, the aqueous vapor of the air cannot be condensed into clouds except by cooling. it is true that in our laboratories it can be condensed by compression. but, for reasons which i need not explain, condensation by compression cannot take place in the air. the cooling which results in the formation of clouds and rain may come in two ways. rains which last for several hours or days are generally produced by the intermixture of currents of air of different temperatures. a current of cold air meeting a current of warm, moist air in its course may condense a considerable portion of the moisture into clouds and rain, and this condensation will go on as long as the currents continue to meet. in a hot spring day a mass of air which has been warmed by the sun, and moistened by evaporation near the surface of the earth, may rise up and cool by expansion to near the freezing-point. the resulting condensation of the moisture may then produce a shower or thunder-squall. but the formation of clouds in a clear sky without motion of the air or change in the temperature of the vapor is simply impossible. we know by abundant experiments that a mass of true aqueous vapor will never condense into clouds or drops so long as its temperature and the pressure of the air upon it remain unchanged. now let us consider sound as an agent for changing the state of things in the air. it is one of the commonest and simplest agencies in the world, which we can experiment upon without difficulty. it is purely mechanical in its action. when a bomb explodes, a certain quantity of gas, say five or six cubic yards, is suddenly produced. it pushes aside and compresses the surrounding air in all directions, and this motion and compression are transmitted from one portion of the air to another. the amount of motion diminishes as the square of the distance; a simple calculation shows that at a quarter of a mile from the point of explosion it would not be one ten-thousandth of an inch. the condensation is only momentary; it may last the hundredth or the thousandth of a second, according to the suddenness and violence of the explosion; then elasticity restores the air to its original condition and everything is just as it was before the explosion. a thousand detonations can produce no more effect upon the air, or upon the watery vapor in it, than a thousand rebounds of a small boy's rubber ball would produce upon a stonewall. so far as the compression of the air could produce even a momentary effect, it would be to prevent rather than to cause condensation of its vapor, because it is productive of heat, which produces evaporation, not condensation. the popular notion that sound may produce rain is founded principally upon the supposed fact that great battles have been followed by heavy rains. this notion, i believe, is not confirmed by statistics; but, whether it is or not, we can say with confidence that it was not the sound of the cannon that produced the rain. that sound as a physical factor is quite insignificant would be evident were it not for our fallacious way of measuring it. the human ear is an instrument of wonderful delicacy, and when its tympanum is agitated by a sound we call it a "concussion" when, in fact, all that takes place is a sudden motion back and forth of a tenth, a hundredth, or a thousandth of an inch, accompanied by a slight momentary condensation. after these motions are completed the air is exactly in the same condition as it was before; it is neither hotter nor colder; no current has been produced, no moisture added. if the reader is not satisfied with this explanation, he can try a very simple experiment which ought to be conclusive. if he will explode a grain of dynamite, the concussion within a foot of the point of explosion will be greater than that which can be produced by the most powerful bomb at a distance of a quarter of a mile. in fact, if the latter can condense vapor a quarter of a mile away, then anybody can condense vapor in a room by slapping his hands. let us, therefore, go to work slapping our hands, and see how long we must continue before a cloud begins to form. what we have just said applies principally to the condensation of invisible vapor. it may be asked whether, if clouds are already formed, something may not be done to accelerate their condensation into raindrops large enough to fall to the ground. this also may be the subject of experiment. let us stand in the steam escaping from a kettle and slap our hands. we shall see whether the steam condenses into drops. i am sure the experiment will be a failure; and no other conclusion is possible than that the production of rain by sound or explosions is out of the question. it must, however, be added that the laws under which the impalpable particles of water in clouds agglomerate into drops of rain are not yet understood, and that opinions differ on this subject. experiments to decide the question are needed, and it is to be hoped that the weather bureau will undertake them. for anything we know to the contrary, the agglomeration may be facilitated by smoke in the air. if it be really true that rains have been produced by great battles, we may say with confidence that they were produced by the smoke from the burning powder rising into the clouds and forming nuclei for the agglomeration into drops, and not by the mere explosion. if this be the case, if it was the smoke and not the sound that brought the rain, then by burning gunpowder and dynamite we are acting much like charles lamb's chinamen who practised the burning of their houses for several centuries before finding out that there was any cheaper way of securing the coveted delicacy of roast pig. but how, it may be asked, shall we deal with the fact that mr. dyrenforth's recent explosions of bombs under a clear sky in texas were followed in a few hours, or a day or two, by rains in a region where rain was almost unknown? i know too little about the fact, if such it be, to do more than ask questions about it suggested by well-known scientific truths. if there is any scientific result which we can accept with confidence, it is that ten seconds after the sound of the last bomb died away, silence resumed her sway. from that moment everything in the air--humidity, temperature, pressure, and motion--was exactly the same as if no bomb had been fired. now, what went on during the hours that elapsed between the sound of the last bomb and the falling of the first drop of rain? did the aqueous vapor already in the surrounding air slowly condense into clouds and raindrops in defiance of physical laws? if not, the hours must have been occupied by the passage of a mass of thousands of cubic miles of warm, moist air coming from some other region to which the sound could not have extended. or was jupiter pluvius awakened by the sound after two thousand years of slumber, and did the laws of nature become silent at his command? when we transcend what is scientifically possible, all suppositions are admissible; and we leave the reader to take his choice between these and any others he may choose to invent. one word in justification of the confidence with which i have cited established physical laws. it is very generally supposed that most great advances in applied science are made by rejecting or disproving the results reached by one's predecessors. nothing could be farther from the truth. as huxley has truly said, the army of science has never retreated from a position once gained. men like ohm and maxwell have reduced electricity to a mathematical science, and it is by accepting, mastering, and applying the laws of electric currents which they discovered and expounded that the electric light, electric railway, and all other applications of electricity have been developed. it is by applying and utilizing the laws of heat, force, and vapor laid down by such men as carnot and regnault that we now cross the atlantic in six days. these same laws govern the condensation of vapor in the atmosphere; and i say with confidence that if we ever do learn to make it rain, it will be by accepting and applying them, and not by ignoring or trying to repeal them. how much the indisposition of our government to secure expert scientific evidence may cost it is strikingly shown by a recent example. it expended several million dollars on a tunnel and water-works for the city of washington, and then abandoned the whole work. had the project been submitted to a commission of geologists, the fact that the rock-bed under the district of columbia would not stand the continued action of water would have been immediately reported, and all the money expended would have been saved. the fact is that there is very little to excite popular interest in the advance of exact science. investigators are generally quiet, unimpressive men, rather diffident, and wholly wanting in the art of interesting the public in their work. it is safe to say that neither lavoisier, galvani, ohm, regnault, nor maxwell could have gotten the smallest appropriation through congress to help make discoveries which are now the pride of our century. they all dealt in facts and conclusions quite devoid of that grandeur which renders so captivating the project of attacking the rains in their aerial stronghold with dynamite bombs. xiii the astronomical ephemeris and the nautical almanac [footnote: read before the u s naval institute, january , .] although the nautical almanacs of the world, at the present time, are of comparatively recent origin, they have grown from small beginnings, the tracing of which is not unlike that of the origin of species by the naturalist of the present day. notwithstanding its familiar name, it has always been designed rather for astronomical than for nautical purposes. such a publication would have been of no use to the navigator before he had instruments with which to measure the altitudes of the heavenly bodies. the earlier navigators seldom ventured out of sight of land, and during the night they are said to have steered by the "cynosure" or constellation of the great bear, a practice which has brought the name of the constellation into our language of the present day to designate an object on which all eyes are intently fixed. this constellation was a little nearer the pole in former ages than at the present time; still its distance was always so great that its use as a mark of the northern point of the horizon does not inspire us with great respect for the accuracy with which the ancient navigators sought to shape their course. the nautical almanac of the present day had its origin in the astronomical ephemerides called forth by the needs of predictions of celestial motions both on the part of the astronomer and the citizen. so long as astrology had a firm hold on the minds of men, the positions of the planets were looked to with great interest. the theories of ptolemy, although founded on a radically false system, nevertheless sufficed to predict the position of the sun, moon, and planets, with all the accuracy necessary for the purposes of the daily life of the ancients or the sentences of their astrologers. indeed, if his tables were carried down to the present time, the positions of the heavenly bodies would be so few degrees in error that their recognition would be very easy. the times of most of the eclipses would be predicted within a few hours, and the conjunctions of the planets within a few days. thus it was possible for the astronomers of the middle ages to prepare for their own use, and that of the people, certain rude predictions respecting the courses of the sun and moon and the aspect of the heavens, which served the purpose of daily life and perhaps lessened the confusion arising from their complicated calendars. in the signs of the zodiac and the different effects which follow from the sun and moon passing from sign to sign, still found in our farmers' almanacs, we have the dying traces of these ancient ephemerides. the great kepler was obliged to print an astrological almanac in virtue of his position as astronomer of the court of the king of austria. but, notwithstanding the popular belief that astronomy had its origin in astrology, the astronomical writings of all ages seem to show that the astronomers proper never had any belief in astrology. to kepler himself the necessity for preparing this almanac was a humiliation to which he submitted only through the pressure of poverty. subsequent ephemerides were prepared with more practical objects. they gave the longitudes of the planets, the position of the sun, the time of rising and setting, the prediction of eclipses, etc. they have, of course, gradually increased in accuracy as the tables of the celestial motions were improved from time to time. at first they were not regular, annual publications, issued by governments, as at the present time, but the works of individual astronomers who issued their ephemerides for several years in advance, at irregular intervals. one man might issue one, two, or half a dozen such volumes, as a private work, for the benefit of his fellows, and each might cover as many years as he thought proper. the first publication of this sort, which i have in my possession, is the ephemerides of manfredi, of bonn, computed for the years to , in two volumes. of the regular annual ephemerides the earliest, so far as i am aware, is the connaissance des temps or french nautical almanac. the first issue was in the year , by picard, and it has been continued without interruption to the present time. its early numbers were, of course, very small, and meagre in their details. they were issued by the astronomers of the french academy of sciences, under the combined auspices of the academy and the government. they included not merely predictions from the tables, but also astronomical observations made at the paris observatory or elsewhere. when the bureau of longitudes was created in , the preparation of the work was intrusted to it, and has remained in its charge until the present time. as it is the oldest, so, in respect at least to number of pages, it is the largest ephemeris of the present time. the astronomical portion of the volume for fills more than seven hundred pages, while the table of geographical positions, which has always been a feature of the work, contains nearly one hundred pages more. the first issue of the british nautical almanac was that for the year and appeared in . it differs from the french almanac in owing its origin entirely to the needs of navigation. the british nation, as the leading maritime power of the world, was naturally interested in the discovery of a method by which the longitude could be found at sea. as most of my hearers are probably aware, there was, for many years, a standing offer by the british government, of ten thousand pounds for the discovery of a practical and sufficiently accurate method of attaining this object. if i am rightly informed, the requirement was that a ship should be able to determine the greenwich time within two minutes, after being six months at sea. when the office of astronomer royal was established in , the duty of the incumbent was declared to be "to apply himself with the most exact care and diligence to the rectifying the tables of the motions of the heavens, and the places of the fixed stars in order to find out the so much desired longitude at sea for the perfecting the art of navigation." about the middle of the last century the lunar tables were so far improved that dr. maskelyne considered them available for attaining this long-wished-for object. the method which i think was then, for the first time, proposed was the now familiar one of lunar distances. several trials of the method were made by accomplished gentlemen who considered that nothing was wanting to make it practical at sea but a nautical ephemeris. the tables of the moon, necessary for the purpose, were prepared by tobias mayer, of gottingen, and the regular annual issue of the work was commenced in , as already stated. of the reward which had been offered, three thousand pounds were paid to the widow of mayer, and three thousand pounds to the celebrated mathematician euler for having invented the methods used by mayer in the construction of his tables. the issue of the nautical ephemeris was intrusted to dr. maskelyne. like other publications of this sort this ephemeris has gradually increased in volume. during the first sixty or seventy years the data were extremely meagre, including only such as were considered necessary for the determination of positions. in the subject of improving the nautical almanac was referred by the lord commissioners of the admiralty to a committee of the astronomical society of london. a subcommittee, including eleven of the most distinguished astronomers and one scientific navigator, made an exhaustive report, recommending a radical rearrangement and improvement of the work. the recommendations of this committee were first carried into effect in the nautical almanac for the year . the arrangement of the navigator's ephemeris then devised has been continued in the british almanac to the present time. a good deal of matter has been added to the british almanac during the forty years and upwards which have elapsed, but it has been worked in rather by using smaller type and closer printing than by increasing the number of pages. the almanac for contains five hundred and seventeen pages and that for five hundred and nineteen pages. the general aspect of the page is now somewhat crowded, yet, considering the quantity of figures on each page the arrangement is marvellously clear and legible. the spanish "almanaque nautico" has been issued since the beginning of the century. like its fellows it has been gradually enlarged and improved, in recent times, and is now of about the same number of pages with the british and american almanacs. as a rule there is less matter on a page, so that the data actually given are not so complete as in some other publications. in germany two distinct publications of this class are issued, the one purely astronomical, the other purely nautical. the astronomical publication has been issued for more than a century under the title of "berliner astronomisches jahrbuch." it is intended principally for the theoretical astronomer, and in respect to matter necessary to the determinations of positions on the earth it is rather meagre. it is issued by the berlin observatory, at the expense of the government. the companion of this work, intended for the use of the german marine, is the "nautisches jahrbuch," prepared and issued under the direction of the minister of commerce and public works. it is copied largely from the british nautical almanac, and in respect to arrangement and data is similar to our american nautical almanac, prepared for the use of navigators, giving, however, more matter, but in a less convenient form. the right ascension and declination of the moon are given for every three hours instead of for every hour; one page of each month is devoted to eclipses of jupiter's satellites, phenomena which we never consider necessary in the nautical portion of our own almanac. at the end of the work the apparent positions of seventy or eighty of the brightest stars are given for every ten days, while it is considered that our own navigators will be satisfied with the mean places for the beginning of the year. at the end is a collection of tables which i doubt whether any other than a german navigator would ever use. whether they use them or not i am not prepared to say. the preceding are the principal astronomical and nautical ephemerides of the world, but there are a number of minor publications, of the same class, of which i cannot pretend to give a complete list. among them is the portuguese astronomical ephemeris for the meridian of the university of coimbra, prepared for portuguese navigators. i do not know whether the portuguese navigators really reckon their longitudes from this point: if they do the practice must be attended with more or less confusion. all the matter is given by months, as in the solar and lunar ephemeris of our own and the british almanac. for the sun we have its longitude, right ascension, and declination, all expressed in arc and not in time. the equation of time and the sidereal time of mean noon complete the ephemeris proper. the positions of the principal planets are given in no case oftener than for every third day. the longitude and latitude of the moon are given for noon and midnight. one feature not found in any other almanac is the time at which the moon enters each of the signs of the zodiac. it may be supposed that this information is designed rather for the benefit of the portuguese landsman than of the navigator. the right ascensions and declinations of the moon and the lunar distances are also given for intervals of twelve hours. only the last page gives the eclipses of the satellites of jupiter. the fixed stars are wholly omitted. an old ephemeris, and one well known in astronomy is that published by the observatory of milan, italy, which has lately entered upon the second century of its existence. its data are extremely meagre and of no interest whatever to the navigator. the greater part of the volume is taken up with observations at the milan observatory. since taking charge of the american ephemeris i have endeavored to ascertain what nautical almanacs are actually used by the principal maritime nations of europe. i have been able to obtain none except those above mentioned. as a general rule i think the british nautical almanac is used by all the northern nations, as already indicated. the german nautical jahrbuch is principally a reprint from the british. the swedish navigators, being all well acquainted with the english language, use the british almanac without change. the russian government, however, prints an explanation of the various terms in the language of their own people and binds it in at the end of the british almanac. this explanation includes translations of the principal terms used in the heading of pages, such as the names of the months and days, the different planets, constellations, and fixed stars, and the phenomena of angle and time. they have even an index of their own in which the titles of the different articles are given in russian. this explanation occupies, in all, seventy-five pages--more than double that taken up by the original explanation. one of the first considerations which strikes us in comparing these multitudinous publications is the confusion which must arise from the use of so many meridians. if each of these southern nations, the spanish and portuguese for instance, actually use a meridian of their own, the practice must lead to great confusion. if their navigators do not do so but refer their longitudes to the meridian of greenwich, then their almanacs must be as good as useless. they would find it far better to buy an ephemeris referred to the meridian of greenwich than to attempt to use their own the northern nations, i think, have all begun to refer to the meridian of greenwich, and the same thing is happily true of our own marine. we may, therefore, hope that all commercial nations will, before long, refer their longitudes to one and the same meridian, and the resulting confusion be thus avoided. the preparation of the american ephemeris and nautical almanac was commenced in , under the superintendence of the late rear-admiral, then lieutenant, charles henry davis. the first volume to be issued was that for the year . both in the preparation of that work and in the connected work of mapping the country, the question of the meridian to be adopted was one of the first importance, and received great attention from admiral davis, who made an able report on the subject. our situation was in some respects peculiar, owing to the great distance which separated us from europe and the uncertainty of the exact difference of longitude between the two continents. it was hardly practicable to refer longitudes in our own country to any european meridian. the attempt to do so would involve continual changes as the transatlantic longitude was from time to time corrected. on the other hand, in order to avoid confusion in navigation, it was essential that our navigators should continue to reckon from the meridian of greenwich. the trouble arising from uncertainty of the exact longitude does not affect the navigator, because, for his purpose, astronomical precision is not necessary. the wisest solution was probably that embodied in the act of congress, approved september , , on the recommendation of lieutenant davis, if i mistake not. "the meridian of the observatory at washington shall be adopted and used as the american meridian for all astronomical purposes, and the meridian of greenwich shall be adopted for all nautical purposes." the execution of this law necessarily involves the question, "what shall be considered astronomical and what nautical purposes?" whether it was from the difficulty of deciding this question, or from nobody's remembering the law, the latter has been practically a dead letter. surely, if there is any region of the globe which the law intended should be referred to the meridian of washington, it is the interior of our own country. yet, notwithstanding the law, all acts of congress relating to the territories have, so far as i know, referred everything to the meridian of greenwich and not to that of washington. even the maps issued by our various surveys are referred to the same transatlantic meridian. the absurdity culminated in a local map of the city of washington and the district of columbia, issued by private parties, in , in which we find even the meridians passing through the city of washington referred to a supposed greenwich. this practice has led to a confusion which may not be evident at first sight, but which is so great and permanent that it may be worth explaining. if, indeed, we could actually refer all our longitudes to an accurate meridian of greenwich in the first place; if, for instance, any western region could be at once connected by telegraph with the greenwich observatory, and thus exchange longitude signals night after night, no trouble or confusion would arise from referring to the meridian of greenwich. but this, practically, cannot be done. all our interior longitudes have been and are determined differentially by comparison with some point in this country. one of the most frequent points of reference used this way has been the cambridge observatory. suppose, then, a surveyor at omaha makes a telegraphic longitude determination between that point and the cambridge observatory. since he wants his longitude reduced to greenwich, he finds some supposed longitude of the cambridge observatory from greenwich and adds that to his own longitude. thus, what he gives is a longitude actually determined, plus an assumed longitude of cambridge, and, unless the assumed longitude of cambridge is distinctly marked on his maps, we may not know what it is. after a while a second party determines the longitude of ogden from cambridge. in the mean time, the longitude of cambridge from greenwich has been corrected, and we have a longitude of ogden which will be discordant with that of omaha, owing to the change in the longitude of cambridge. a third party determines the longitudes of, let us suppose, st. louis from washington, he adds the assumed longitudes of washington from greenwich which may not agree with either of the longitudes of cambridge and gets his longitude. thus we have a series of results for our western longitude all nominally referred to the meridian of greenwich, but actually referred to a confused collection of meridians, nobody knows what. if the law had only provided that the longitude of washington from greenwich should be invariably fixed at a certain quantity, say degrees ', this confusion would not have arisen. it is true that the longitude thus established by law might not have been perfectly correct, but this would not cause any trouble nor confusion. our longitude would have been simply referred to a certain assumed greenwich, the small error of which would have been of no importance to the navigator or astronomer. it would have differed from the present system only in that the assumed greenwich would have been invariable instead of dancing about from time to time as it has done under the present system. you understand that when the astronomer, in computing an interior longitude, supposes that of cambridge from greenwich to be a certain definite amount, say h m s, what he actually does is to count from a meridian just that far east of cambridge. when he changes the assumed longitude of cambridge he counts from a meridian farther east or farther west of his former one: in other words, he always counts from an assumed greenwich, which changes its position from time to time, relative to our own country. having two meridians to look after, the form of the american ephemeris, to be best adapted to the wants both of navigators and astronomers was necessarily peculiar. had our navigators referred their longitudes to any meridian of our own country the arrangement of the work need not have differed materially from that of foreign ones. but being referred to a meridian far outside our limits and at the same time designed for use within those limits, it was necessary to make a division of the matter. accordingly, the american ephemeris has always been divided into two parts: the first for the use of navigators, referred to the meridian of greenwich, the second for that of astronomers, referred to the meridian of washington. the division of the matter without serious duplication is more easy than might at first be imagined. in explaining it, i will take the ephemeris as it now is, with the small changes which have been made from time to time. one of the purposes of any ephemeris, and especially of that of the navigators, is to give the position of the heavenly bodies at equidistant intervals of time, usually one day. since it is noon at some point of the earth all the time, it follows that such an ephemeris will always be referred to noon at some meridian. what meridian this shall be is purely a practical question, to be determined by convenience and custom. greenwich noon, being that necessarily used by the navigator, is adopted as the standard, but we must not conclude that the ephemeris for greenwich noon is referred to the meridian of greenwich in the sense that we refer a longitude to that meridian. greenwich noon is h m s, washington mean time; so the ephemeris which gives data for every greenwich noon may be considered as referred to the meridian of washington giving the data for h m s, washington time, every day. the rule adopted, therefore, is to have all the ephemerides which refer to absolute time, without any reference to a meridian, given for greenwich noon, unless there may be some special reason to the contrary. for the needs of the navigator and the theoretical astronomer these are the most convenient epochs. another part of the ephemeris gives the position of the heavenly bodies, not at equidistant intervals, but at transit over some meridian. for this purpose the meridian of washington is chosen for obvious reasons. the astronomical part of our ephemeris, therefore, gives the positions of the principal fixed stars, the sun, moon, and all the larger planets at the moment of transit over our own meridian. the third class of data in the ephemeris comprises phenomena to be predicted and observed. such are eclipses of the sun and moon, occultations of fixed stars by the moon, and eclipses of jupiter's satellites. these phenomena are all given in washington mean time as being most convenient for observers in our own country. there is a partial exception, however, in the case of eclipses of the sun and moon. the former are rather for the world in general than for our own country, and it was found difficult to arrange them to be referred to the meridian of washington without having the maps referred to the same meridian. since, however, the meridian of greenwich is most convenient outside of our own territory, and since but a small portion of the eclipses are visible within it, it is much the best to have the eclipses referred entirely to the meridian of greenwich. i am the more ready to adopt this change because when the eclipses are to be computed for our own country the change of meridians will be very readily understood by those who make the computation. it may be interesting to say something of the tables and theories from which the astronomical ephemerides are computed. to understand them completely it is necessary to trace them to their origin. the problem of calculating the motions of the heavenly bodies and the changes in the aspect of the celestial sphere was one of the first with which the students of astronomy were occupied. indeed, in ancient times, the only astronomical problems which could be attacked were of this class, for the simple reason that without the telescope and other instruments of research it was impossible to form any idea of the physical constitution of the heavenly bodies. to the ancients the stars and planets were simply points or surfaces in motion. they might have guessed that they were globes like that on which we live, but they were unable to form any theory of the nature of these globes. thus, in the almagest of ptolemy, the most complete treatise on the ancient astronomy which we possess, we find the motions of all the heavenly bodies carefully investigated and tables given for the convenient computation of their positions. crude and imperfect though these tables may be, they were the beginnings from which those now in use have arisen. no radical change was made in the general principles on which these theories and tables were constructed until the true system of the world was propounded by copernicus. on this system the apparent motion of each planet in the epicycle was represented by a motion of the earth around the sun, and the problem of correcting the position of the planet on account of the epicycle was reduced to finding its geocentric from its heliocentric position. this was the greatest step ever taken in theoretical astronomy, yet it was but a single step. so far as the materials were concerned and the mode of representing the planetary motions, no other radical advance was made by copernicus. indeed, it is remarkable that he introduced an epicycle which was not considered necessary by ptolemy in order to represent the inequalities in the motions of the planets around the sun. the next great advance made in the theory of the planetary motion was the discovery by kepler of the celebrated laws which bear his name. when it was established that each planet moved in an ellipse having the sun in one focus it became possible to form tables of the motions of the heavenly bodies much more accurate than had before been known. such tables were published by kepler in , under the name of rudolphine tables, in memory of his patron, the emperor rudolph. but the laws of kepler took no account of the action of the planets on one another. it is well known that if each planet moved only under the influence of the gravitating force of the sun its motion would accord rigorously with the laws of kepler, and the problems of theoretical astronomy would be greatly simplified. when, therefore, the results of kepler's laws were compared with ancient and modern observations it was found that they were not exactly represented by the theory. it was evident that the elliptic orbits of the planets were subject to change, but it was entirely beyond the power of investigation, at that time, to assign any cause for such changes. notwithstanding the simplicity of the causes which we now know to produce them, they are in form extremely complex. without the knowledge of the theory of gravitation it would be entirely out of the question to form any tables of the planetary motions which would at all satisfy our modern astronomers. when the theory of universal gravitation was propounded by newton he showed that a planet subjected only to the gravitation of a central body, like the sun, would move in exact accordance with kepler's laws. but by his theory the planets must attract one another and these attractions must cause the motions of each to deviate slightly from the laws in question. since such deviations were actually observed it was very natural to conclude that they were due to this cause, but how shall we prove it? to do this with all the rigor required in a mathematical investigation it is necessary to calculate the effect of the mutual action of the planets in changing their orbits. this calculation must be made with such precision that there shall be no doubt respecting the results of the theory. then its results must be compared with the best observations. if the slightest outstanding difference is established there is something wrong and the requirements of astronomical science are not satisfied. the complete solution of this problem was entirely beyond the power of newton. when his methods of research were used he was indeed able to show that the mutual action of the planets would produce deviations in their motions of the same general nature with those observed, but he was not able to calculate these deviations with numerical exactness. his most successful attempt in this direction was perhaps made in the case of the moon. he showed that the sun's disturbing force on this body would produce several inequalities the existence of which had been established by observation, and he was also able to give a rough estimate of their amount, but this was as far as his method could go. a great improvement had to be made, and this was effected not by english, but by continental mathematicians. the latter saw, clearly, that it was impossible to effect the required solution by the geometrical mode of reasoning employed by newton. the problem, as it presented itself to their minds, was to find algebraic expressions for the positions of the planets at any time. the latitude, longitude, and radius-vector of each planet are constantly varying, but they each have a determined value at each moment of time. they may therefore be regarded as functions of the time, and the problem was to express these functions by algebraic formulae. these algebraic expressions would contain, besides the time, the elements of the planetary orbits to be derived from observation. the time which we may suppose to be represented algebraically by the symbol t, would remain as an unknown quantity to the end. what the mathematician sought to do was to present the astronomer with a series of algebraic expressions containing t as an indeterminate quantity, and so, by simply substituting for t any year and fraction of a year whatever-- , , , for example, the result would give the latitude, longitude, or radius-vector of a planet. the problem as thus presented was one of the most difficult we can perceive of, but the difficulty was only an incentive to attacking it with all the greater energy. so long as the motion was supposed purely elliptical, so long as the action of the planets was neglected, the problem was a simple one, requiring for its solution only the analytic geometry of the ellipse. the real difficulties commenced when the mutual action of the planets was taken into account. it is, of course, out of the question to give any technical description or analysis of the processes which have been invented for solving the problem; but a brief historical sketch may not be out of place. a complete and rigorous solution of the problem is out of the question--that is, it is impossible by any known method to form an algebraic expression for the co-ordinates of a planet which shall be absolutely exact in a mathematical sense. in whatever way we go to work the expression comes out in the form of an infinite series of terms, each term being, on the whole, a little smaller as we increase the number. so, by increasing the number of these various terms, we can approach nearer and nearer to a mathematical exactness, but can never reach it. the mathematician and astronomer have to be satisfied when they have carried the solution so far that the neglected quantities are entirely beyond the powers of observation. mathematicians have worked upon the problem in its various phases for nearly two centuries, and many improvements in detail have, from time to time, been made, but no general method, applicable to all cases, has been devised. one plan is to be used in treating the motion of the moon, another for the interior planets, another for jupiter and saturn, another for the minor planets, and so on. under these circumstances it will not surprise you to learn that our tables of the celestial motions do not, in general, correspond in accuracy to the present state of practical astronomy. there is no authority and no office in the world whose duty it is to look after the preparations of the formulae i have described. the work of computing them has been almost entirely left to individual mathematicians whose taste lay in that direction, and who have sometimes devoted the greater part of their lives to calculations on a single part of the work. as a striking instance of this, the last great work on the motion of the moon, that of delaunay, of paris, involved some fifteen years of continuous hard labor. hansen, of germany, who died five years ago, devoted almost his whole life to investigations of this class and to the development of new methods of computation. his tables of the moon are those now used for predicting the places of the moon in all the ephemerides of the world. the only successful attempt to prepare systematic tables for all the large planets is that completed by le verrier just before his death; but he used only a small fraction of the material at his disposal, and did not employ the modern methods, confining himself wholly to those invented by his countrymen about the beginning of the present century. for him jacobi and hansen had lived in vain. the great difficulty which besets the subject arises from the fact that mathematical processes alone will not give us the position of a planet, there being seven unknown quantities for each planet which must be determined by observations. a planet, for instance, may move in any ellipse whatever, having the sun in one focus, and it is impossible to tell what ellipse it is, except from observation. the mean motion of a planet, or its period of revolution, can only be determined by a long series of observations, greater accuracy being obtained the longer the observations are continued. before the time of bradley, who commenced work at the greenwich observatory about , the observations were so far from accurate that they are now of no use whatever, unless in exceptional cases. even bradley's observations are in many cases far less accurate than those made now. in consequence, we have not heretofore had a sufficiently extended series of observations to form an entirely satisfactory theory of the celestial motions. as a consequence of the several difficulties and drawbacks, when the computation of our ephemeris was started, in the year , there were no tables which could be regarded as really satisfactory in use. in the british nautical almanac the places of the moon were derived from the tables of burckhardt published in the year . you will understand, in a case like this, no observations subsequent to the issue of the tables are made use of; the place of the moon of any day, hour, and minute of greenwich time, mean time, was precisely what burckhardt would have computed nearly a half a century before. of the tables of the larger planets the latest were those of bouvard, published in , while the places of venus were from tables published by lindenau in . of course such tables did not possess astronomical accuracy. at that time, in the case of the moon, completely new tables were constructed from the results reached by professor airy in his reduction of the greenwich observations of the moon from to . these were constructed under the direction of professor pierce and represented the places of the moon with far greater accuracy than the older tables of burckhardt. for the larger planets corrections were applied to the older tables to make them more nearly represent observations before new ones were constructed. these corrections, however, have not proved satisfactory, not being founded on sufficiently thorough investigations. indeed, the operation of correcting tables by observation, as we would correct the dead-reckoning of a ship, is a makeshift, the result of which must always be somewhat uncertain, and it tends to destroy that unity which is an essential element of the astronomical ephemeris designed for permanent future use. the result of introducing them, while no doubt an improvement on the old tables, has not been all that should be desired. the general lack of unity in the tables hitherto employed is such that i can only state what has been done by mentioning each planet in detail. for mercury, new tables were constructed by professor winlock, from formulae published by le verrier in . these tables have, however, been deviating from the true motion of the planet, owing to the motion of the perihelion of mercury, subsequently discovered by le verrier himself. they are now much less accurate than the newer tables published by le verrier ten years later. of venus new tables were constructed by mr. hill in . they are more accurate than any others, being founded on later data than those of le verrier, and are therefore satisfactory so far as accuracy of prediction is concerned. the place of mars, jupiter, and saturn are still computed from the old tables, with certain necessary corrections to make them better represent observations. the places of uranus and neptune are derived from new tables which will probably be sufficiently accurate for some time to come. for the moon, pierce's tables have been employed up to the year inclusive. commencing with the ephemeris for the year , hansen's tables are introduced with corrections to the mean longitude founded on two centuries of observation. with so great a lack of uniformity, and in the absence of any existing tables which have any other element of unity than that of being the work of the same authors, it is extremely desirable that we should be able to compute astronomical ephemerides from a single uniform and consistent set of astronomical data. i hope, in the course of years, to render this possible. when our ephemeris was first commenced, the corrections applied to existing tables rendered it more accurate than any other. since that time, the introduction into foreign ephemerides of the improved tables of le verrier have rendered them, on the whole, rather more accurate than our own. in one direction, however, our ephemeris will hereafter be far ahead of all others. i mean in its positions of the fixed stars. this portion of it is of particular importance to us, owing to the extent to which our government is engaged in the determination of positions on this continent, and especially in our western territories. although the places of the stars are determined far more easily than those of the planets, the discussion of star positions has been in almost as backward a state as planetary positions. the errors of old observers have crept in and been continued through two generations of astronomers. a systematic attempt has been made to correct the places of the stars for all systematic errors of this kind, and the work of preparing a catalogue of stars which shall be completely adapted to the determination of time and longitude, both in the fixed observatory and in the field, is now approaching completion. the catalogue cannot be sufficiently complete to give places of the stars for determining the latitude by the zenith telescope, because for such a purpose a much greater number of stars is necessary than can be incorporated in the ephemeris. from what i have said, it will be seen that the astronomical tables, in general, do not satisfy the scientific condition of completely representing observations to the last degree of accuracy. few, i think, have an idea how unsystematically work of this kind has hitherto been performed. until very lately the tables we have possessed have been the work of one man here, another there, and another one somewhere else, each using different methods and different data. the result of this is that there is nothing uniform and systematic among them, and that they have every range of precision. this is no doubt due in part to the fact that the construction of such tables, founded on the mass of observation hitherto made, is entirely beyond the power of any one man. what is wanted is a number of men of different degrees of capacity, all co-operating on a uniform system, so as to obtain a uniform result, like the astronomers in a large observatory. the greenwich observatory presents an example of co-operative work of this class extending over more than a century. but it has never extended its operations far outside the field of observation, reduction, and comparison with existing tables. it shows clearly, from time to time, the errors of the tables used in the british nautical almanac, but does nothing further, occasional investigations excepted, in the way of supplying new tables. an exception to this is a great work on the theory of the moon's motion, in which professor airy is now engaged. it will be understood that several distinct conditions not yet fulfilled are desirable in astronomical tables; one is that each set of tables shall be founded on absolutely consistent data, for instance, that the masses of the planets shall be the same throughout. another requirement is that this data shall be as near the truth as astronomical data will suffice to determine them. the third is that the results shall be correct in theory. that is, whether they agree or disagree with observations, they shall be such as result mathematically from the adopted data. tables completely fulfilling these conditions are still a work of the future. it is yet to be seen whether such co-operation as is necessary to their production can be secured under any arrangement whatever. xiv the world's debt to astronomy astronomy is more intimately connected than any other science with the history of mankind. while chemistry, physics, and we might say all sciences which pertain to things on the earth, are comparatively modern, we find that contemplative men engaged in the study of the celestial motions even before the commencement of authentic history. the earliest navigators of whom we know must have been aware that the earth was round. this fact was certainly understood by the ancient greeks and egyptians, as well as it is at the present day. true, they did not know that the earth revolved on its axis, but thought that the heavens and all that in them is performed a daily revolution around our globe, which was, therefore, the centre of the universe. it was the cynosure, or constellation of the little bear, by which the sailors used to guide their ships before the discovery of the mariner's compass. thus we see both a practical and contemplative side to astronomy through all history. the world owes two debts to that science: one for its practical uses, and the other for the ideas it has afforded us of the immensity of creation. the practical uses of astronomy are of two kinds: one relates to geography; the other to times, seasons, and chronology. every navigator who sails long out of sight of land must be something of an astronomer. his compass tells him where are east, west, north, and south, but it gives him no information as to where on the wide ocean he may be, or whither the currents may be carrying him. even with the swiftest modern steamers it is not safe to trust to the compass in crossing the atlantic. a number of years ago the steamer city of washington set out on her usual voyage from liverpool to new york. by rare bad luck the weather was stormy or cloudy during her whole passage, so that the captain could not get a sight on the sun, and therefore had to trust to his compass and his log-line, the former telling him in what direction he had steamed, and the latter how fast he was going each hour. the result was that the ship ran ashore on the coast of nova scotia, when the captain thought he was approaching nantucket. not only the navigator but the surveyor in the western wilds must depend on astronomical observations to learn his exact position on the earth's surface, or the latitude and longitude of the camp which he occupies. he is able to do this because the earth is round, and the direction of the plumb-line not exactly the same at any two places. let us suppose that the earth stood still, so as not to revolve on its axis at all. then we should always see the stars at rest and the star which was in the zenith of any place, say a farm-house in new york, at any time, would be there every night and every hour of the year. now the zenith is simply the point from which the plumb-line seems to drop. lie on the ground; hang a plummet above your head, sight on the line with one eye, and the direction of the sight will be the zenith of your place. suppose the earth was still, and a certain star was at your zenith. then if you went to another place a mile away, the direction of the plumb-line would be slightly different. the change would, indeed, be very small, so small that you could not detect it by sighting with the plumb-line. but astronomers and surveyors have vastly more accurate instruments than the plumb-line and the eye, instruments by which a deviation that the unaided eye could not detect can be seen and measured. instead of the plumb-line they use a spirit-level or a basin of quicksilver. the surface of quicksilver is exactly level and so at right angles to the true direction of the plumb-line or the force of gravity. its direction is therefore a little different at two different places on the surface, and the change can be measured by its effect on the apparent direction of a star seen by reflection from the surface. it is true that a considerable distance on the earth's surface will seem very small in its effect on the position of a star. suppose there were two stars in the heavens, the one in the zenith of the place where you now stand, and the other in the zenith of a place a mile away. to the best eye unaided by a telescope those two stars would look like a single one. but let the two places be five miles apart, and the eye could see that there were two of them. a good telescope could distinguish between two stars corresponding to places not more than a hundred feet apart. the most exact measurements can determine distances ranging from thirty to sixty feet. if a skilful astronomical observer should mount a telescope on your premises, and determine his latitude by observations on two or three evenings, and then you should try to trick him by taking up the instrument and putting it at another point one hundred feet north or south, he would find out that something was wrong by a single night's work. within the past three years a wobbling of the earth's axis has been discovered, which takes place within a circle thirty feet in radius and sixty feet in diameter. its effect was noticed in astronomical observations many years ago, but the change it produced was so small that men could not find out what the matter was. the exact nature and amount of the wobbling is a work of the exact astronomy of the present time. we cannot measure across oceans from island to island. until a recent time we have not even measured across the continent, from new york to san francisco, in the most precise way. without astronomy we should know nothing of the distance between new york and liverpool, except by the time which it took steamers to run it, a measure which would be very uncertain indeed. but by the aid of astronomical observations and the atlantic cables the distance is found within a few hundred yards. without astronomy we could scarcely make an accurate map of the united states, except at enormous labor and expense, and even then we could not be sure of its correctness. but the practical astronomer being able to determine his latitude and longitude within fifty yards, the positions of the principal points in all great cities of the country are known, and can be laid down on maps. the world has always had to depend on astronomy for all its knowledge concerning times and seasons. the changes of the moon gave us the first month, and the year completes its round as the earth travels in its orbit. the results of astronomical observation are for us condensed into almanacs, which are now in such universal use that we never think of their astronomical origin. but in ancient times people had no almanacs, and they learned the time of year, or the number of days in the year, by observing the time when sirius or some other bright star rose or set with the sun, or disappeared from view in the sun's rays. at alexandria, in egypt, the length of the year was determined yet more exactly by observing when the sun rose exactly in the east and set exactly in the west, a date which fixed the equinox for them as for us. more than seventeen hundred years ago, ptolemy, the great author of the almagest, had fixed the length of the year to within a very few minutes. he knew it was a little less than / days. the dates of events in ancient history depend very largely on the chronological cycles of astronomy. eclipses of the sun and moon sometimes fixed the date of great events, and we learn the relation of ancient calendars to our own through the motions of the earth and moon, and can thus measure out the years for the events in ancient history on the same scale that we measure out our own. at the present day, the work of the practical astronomer is made use of in our daily life throughout the whole country in yet another way. our fore-fathers had to regulate their clocks by a sundial, or perhaps by a mark at the corner of the house, which showed where the shadow of the house fell at noon. very rude indeed was this method; and it was uncertain for another reason. it is not always exactly twenty-four hours between two noons by the sun, sometimes for two or three months the sun will make it noon earlier and earlier every day; and during several other months later and later every day. the result is that, if a clock is perfectly regulated, the sun will be sometimes a quarter of an hour behind it, and sometimes nearly the same amount before it. any effort to keep the clock in accord with this changing sun was in vain, and so the time of day was always uncertain. now, however, at some of the principal observatories of the country astronomical observations are made on every clear night for the express purpose of regulating an astronomical clock with the greatest exactness. every day at noon a signal is sent to various parts of the country by telegraph, so that all operators and railway men who hear that signal can set their clock at noon within two or three seconds. people who live near railway stations can thus get their time from it, and so exact time is diffused into every household of the land which is at all near a railway station, without the trouble of watching the sun. thus increased exactness is given to the time on all our railroads, increased safety is obtained, and great loss of time saved to every one. if we estimated the money value of this saving alone we should no doubt find it to be greater than all that our study of astronomy costs. it must therefore be conceded that, on the whole, astronomy is a science of more practical use than one would at first suppose. to the thoughtless man, the stars seem to have very little relation to his daily life; they might be forever hid from view without his being the worse for it. he wonders what object men can have in devoting themselves to the study of the motions or phenomena of the heavens. but the more he looks into the subject, and the wider the range which his studies include, the more he will be impressed with the great practical usefulness of the science of the heavens. and yet i think it would be a serious error to say that the world's greatest debt to astronomy was owing to its usefulness in surveying, navigation, and chronology. the more enlightened a man is, the more he will feel that what makes his mind what it is, and gives him the ideas of himself and creation which he possesses, is more important than that which gains him wealth. i therefore hold that the world's greatest debt to astronomy is that it has taught us what a great thing creation is, and what an insignificant part of the creator's work is this earth on which we dwell, and everything that is upon it. that space is infinite, that wherever we go there is a farther still beyond it, must have been accepted as a fact by all men who have thought of the subject since men began to think at all. but it is very curious how hard even the astronomers found it to believe that creation is as large as we now know it to be. the greeks had their gods on or not very far above olympus, which was a sort of footstool to the heavens. sometimes they tried to guess how far it probably was from the vault of heaven to the earth, and they had a myth as to the time it took vulcan to fall. ptolemy knew that the moon was about thirty diameters of the earth distant from us, and he knew that the sun was many times farther than the moon; he thought it about twenty times as far, but could not be sure. we know that it is nearly four hundred times as far. when copernicus propounded the theory that the earth moved around the sun, and not the sun around the earth, he was able to fix the relative distances of the several planets, and thus make a map of the solar system. but he knew nothing about the scale of this map. he knew, for example, that venus was a little more than two-thirds the distance of the earth from the sun, and that mars was about half as far again as the earth, jupiter about five times, and saturn about ten times; but he knew nothing about the distance of any one of them from the sun. he had his map all right, but he could not give any scale of miles or any other measurements upon it. the astronomers who first succeeded him found that the distance was very much greater than had formerly been supposed; that it was, in fact, for them immeasurably great, and that was all they could say about it. the proofs which copernicus gave that the earth revolved around the sun were so strong that none could well doubt them. and yet there was a difficulty in accepting the theory which seemed insuperable. if the earth really moved in so immense an orbit as it must, then the stars would seem to move in the opposite direction, just as, if you were in a train that is shunting off cars one after another, as the train moves back and forth you see its motion in the opposite motion of every object around you. if then the earth at one side of its orbit was exactly between two stars, when it moved to the other side of its orbit it would not be in a line between them, but each star would have seemed to move in the opposite direction. for centuries astronomers made the most exact observations that they were able without having succeeded in detecting any such apparent motion among the stars. here was a mystery which they could not solve. either the copernican system was not true, after all, and the earth did not move in an orbit, or the stars were at such immense distances that the whole immeasurable orbit of the earth is a mere point in comparison. philosophers could not believe that the creator would waste room by allowing the inconceivable spaces which appeared to lie between our system and the fixed stars to remain unused, and so thought there must be something wrong in the theory of the earth's motion. not until the nineteenth century was well in progress did the most skilful observers of their time, bessel and struve, having at command the most refined instruments which science was then able to devise, discover the reality of the parallax of the stars, and show that the nearest of these bodies which they could find was more than , times as far as the , , of miles which separate the earth from the sun. during the half-century and more which has elapsed since this discovery, astronomers have been busily engaged in fathoming the heavenly depths. the nearest star they have been able to find is about , times the sun's distance. a dozen or a score more are within , , times that distance. beyond this all is unfathomable by any sounding-line yet known to man. the results of these astronomical measures are stupendous beyond conception. no mere statement in numbers conveys any idea of it. nearly all the brighter stars are known to be flying through space at speeds which generally range between ten and forty or fifty miles per second, some slower and some swifter, even up to one or two hundred miles a second. such a speed would carry us across the atlantic while we were reading two or three of these sentences. these motions take place some in one direction and some in another. some of the stars are coming almost straight towards us. should they reach us, and pass through our solar system, the result would be destructive to our earth, and perhaps to our sun. are we in any danger? no, because, however madly they may come, whether ten, twenty, or one hundred miles per second, so many millions of years must elapse before they reach us that we need give ourselves no concern in the matter. probably none of them are coming straight to us; their course deviates just a hair's-breadth from our system, but that hair's-breadth is so large a quantity that when the millions of years elapse their course will lie on one side or the other of our system and they will do no harm to our planet; just as a bullet fired at an insect a mile away would be nearly sure to miss it in one direction or the other. our instrument makers have constructed telescopes more and more powerful, and with these the whole number of stars visible is carried up into the millions, say perhaps to fifty or one hundred millions. for aught we know every one of those stars may have planets like our own circling round it, and these planets may be inhabited by beings equal to ourselves. to suppose that our globe is the only one thus inhabited is something so unlikely that no one could expect it. it would be very nice to know something about the people who may inhabit these bodies, but we must await our translation to another sphere before we can know anything on the subject. meanwhile, we have gained what is of more value than gold or silver; we have learned that creation transcends all our conceptions, and our ideas of its author are enlarged accordingly. xv an astronomical friendship there are few men with whom i would like so well to have a quiet talk as with father hell. i have known more important and more interesting men, but none whose acquaintance has afforded me a serener satisfaction, or imbued me with an ampler measure of a feeling that i am candid enough to call self-complacency. the ties that bind us are peculiar. when i call him my friend, i do not mean that we ever hobnobbed together. but if we are in sympathy, what matters it that he was dead long before i was born, that he lived in one century and i in another? such differences of generation count for little in the brotherhood of astronomy, the work of whose members so extends through all time that one might well forget that he belongs to one century or to another. father hell was an astronomer. ask not whether he was a very great one, for in our science we have no infallible gauge by which we try men and measure their stature. he was a lover of science and an indefatigable worker, and he did what in him lay to advance our knowledge of the stars. let that suffice. i love to fancy that in some other sphere, either within this universe of ours or outside of it, all who have successfully done this may some time gather and exchange greetings. should this come about there will be a few--hipparchus and ptolemy, copernicus and newton, galileo and herschel--to be surrounded by admiring crowds. but these men will have as warm a grasp and as kind a word for the humblest of their followers, who has merely discovered a comet or catalogued a nebula, as for the more brilliant of their brethren. my friend wrote the letters s. j. after his name. this would indicate that he had views and tastes which, in some points, were very different from my own. but such differences mark no dividing line in the brotherhood of astronomy. my testimony would count for nothing were i called as witness for the prosecution in a case against the order to which my friend belonged. the record would be very short: deponent saith that he has at various times known sundry members of the said order; and that they were lovers of sound learning, devoted to the discovery and propagation of knowledge; and further deponent saith not. if it be true that an undevout astronomer is mad, then was father hell the sanest of men. in his diary we find entries like these: "benedicente deo, i observed the sun on the meridian to-day.... deo quoque benedicente, i to-day got corresponding altitudes of the sun's upper limb." how he maintained the simplicity of his faith in the true spirit of the modern investigator is shown by his proceedings during a momentous voyage along the coast of norway, of which i shall presently speak. he and his party were passengers on a norwegian vessel. for twelve consecutive days they had been driven about by adverse storms, threatened with shipwreck on stony cliffs, and finally compelled to take refuge in a little bay, with another ship bound in the same direction, there to wait for better weather. father hell was philosopher enough to know that unusual events do not happen without cause. perhaps he would have undergone a week of storm without its occurring to him to investigate the cause of such a bad spell of weather. but when he found the second week approaching its end and yet no sign of the sun appearing or the wind abating, he was satisfied that something must be wrong. so he went to work in the spirit of the modern physician who, when there is a sudden outbreak of typhoid fever, looks at the wells and examines their water with the microscope to find the microbes that must be lurking somewhere. he looked about, and made careful inquiries to find what wickedness captain and crew had been guilty of to bring such a punishment. success soon rewarded his efforts. the king of denmark had issued a regulation that no fish or oil should be sold along the coast except by the regular dealers in those articles. and the vessel had on board contraband fish and blubber, to be disposed of in violation of this law. the astronomer took immediate and energetic measures to insure the public safety. he called the crew together, admonished them of their sin, the suffering they were bringing on themselves, and the necessity of getting back to their families. he exhorted them to throw the fish overboard, as the only measure to secure their safety. in the goodness of his heart, he even offered to pay the value of the jettison as soon as the vessel reached drontheim. but the descendants of the vikings were stupid and unenlightened men--"educatione sua et professione homines crassissimi"--and would not swallow the medicine so generously offered. they claimed that, as they had bought the fish from the russians, their proceedings were quite lawful. as for being paid to throw the fish overboard, they must have spot cash in advance or they would not do it. after further fruitless conferences, father hell determined to escape the danger by transferring his party to the other vessel. they had not more than got away from the wicked crew than heaven began to smile on their act--"factum comprobare deus ipse videtur"--the clouds cleared away, the storm ceased to rage, and they made their voyage to copenhagen under sunny skies. i regret to say that the narrative is silent as to the measure of storm subsequently awarded to the homines crassissimi of the forsaken vessel. for more than a century father hell had been a well-known figure in astronomical history. his celebrity was not, however, of such a kind as the royal astronomer of austria that he was ought to enjoy. a not unimportant element in his fame was a suspicion of his being a black sheep in the astronomical flock. he got under this cloud through engaging in a trying and worthy enterprise. on june , , an event occurred which had for generations been anticipated with the greatest interest by the whole astronomical world. this was a transit of venus over the disk of the sun. our readers doubtless know that at that time such a transit afforded the most accurate method known of determining the distance of the earth from the sun. to attain this object, parties were sent to the most widely separated parts of the globe, not only over wide stretches of longitude, but as near as possible to the two poles of the earth. one of the most favorable and important regions of observation was lapland, and the king of denmark, to whom that country then belonged, interested himself in getting a party sent thither. after a careful survey of the field he selected father hell, chief of the observatory at vienna, and well known as editor and publisher of an annual ephemeris, in which the movements and aspects of the heavenly bodies were predicted. the astronomer accepted the mission and undertook what was at that time a rather hazardous voyage. his station was at vardo in the region of the north cape. what made it most advantageous for the purpose was its being situated several degrees within the arctic circle, so that on the date of the transit the sun did not set. the transit began when the sun was still two or three hours from his midnight goal, and it ended nearly an equal time afterwards. the party consisted of hell himself, his friend and associate, father sajnovics, one dominus borgrewing, of whom history, so far as i know, says nothing more, and an humble individual who in the record receives no other designation than "familias." this implies, we may suppose, that he pitched the tent and made the coffee. if he did nothing but this we might pass him over in silence. but we learn that on the day of the transit he stood at the clock and counted the all-important seconds while the observations were going on. the party was favored by cloudless weather, and made the required observations with entire success. they returned to copenhagen, and there father hell remained to edit and publish his work. astronomers were naturally anxious to get the results, and showed some impatience when it became known that hell refused to announce them until they were all reduced and printed in proper form under the auspices of his royal patron. while waiting, the story got abroad that he was delaying the work until he got the results of observations made elsewhere, in order to "doctor" his own and make them fit in with the others. one went so far as to express a suspicion that hell had not seen the transit at all, owing to clouds, and that what he pretended to publish were pure fabrications. but his book came out in a few months in such good form that this suspicion was evidently groundless. still, the fears that the observations were not genuine were not wholly allayed, and the results derived from them were, in consequence, subject to some doubt. hell himself considered the reflections upon his integrity too contemptible to merit a serious reply. it is said that he wrote to some one offering to exhibit his journal free from interlineations or erasures, but it does not appear that there is any sound authority for this statement. what is of some interest is that he published a determination of the parallax of the sun based on the comparison of his own observations with those made at other stations. the result was ". . it was then, and long after, supposed that the actual value of the parallax was about ". , and the deviation of hell's result from this was considered to strengthen the doubt as to the correctness of his work. it is of interest to learn that, by the most recent researches, the number in question must be between ". and ". , so that in reality hell's computations came nearer the truth than those generally current during the century following his work. thus the matter stood for sixty years after the transit, and for a generation after father hell had gone to his rest. about it was found that the original journal of his voyage, containing the record of his work as first written down at the station, was still preserved at the vienna observatory. littrow, then an astronomer at vienna, made a critical examination of this record in order to determine whether it had been tampered with. his conclusions were published in a little book giving a transcript of the journal, a facsimile of the most important entries, and a very critical description of the supposed alterations made in them. he reported in substance that the original record had been so tampered with that it was impossible to decide whether the observations as published were genuine or not. the vital figures, those which told the times when venus entered upon the sun, had been erased, and rewritten with blacker ink. this might well have been done after the party returned to copenhagen. the case against the observer seemed so well made out that professors of astronomy gave their hearers a lesson in the value of truthfulness, by telling them how father hell had destroyed what might have been very good observations by trying to make them appear better than they really were. in i paid a visit to vienna for the purpose of examining the great telescope which had just been mounted in the observatory there by grubb, of dublin. the weather was so unfavorable that it was necessary to remain two weeks, waiting for an opportunity to see the stars. one evening i visited the theatre to see edwin booth, in his celebrated tour over the continent, play king lear to the applauding viennese. but evening amusements cannot be utilized to kill time during the day. among the works i had projected was that of rediscussing all the observations made on the transits of venus which had occurred in and , by the light of modern discovery. as i have already remarked, hell's observations were among the most important made, if they were only genuine. so, during my almost daily visits to the observatory, i asked permission of the director to study hell's manuscript, which was deposited in the library of the institution. permission was freely given, and for some days i pored over the manuscript. it is a very common experience in scientific research that a subject which seems very unpromising when first examined may be found more and more interesting as one looks further into it. such was the case here. for some time there did not seem any possibility of deciding the question whether the record was genuine. but every time i looked at it some new point came to light. i compared the pages with littrow's published description and was struck by a seeming want of precision, especially when he spoke of the ink with which the record had been made. erasers were doubtless unknown in those days--at least our astronomer had none on his expedition--so when he found he had written the wrong word he simply wiped the place off with, perhaps, his finger and wrote what he wanted to say. in such a case littrow described the matter as erased and new matter written. when the ink flowed freely from the quill pen it was a little dark. then littrow said a different kind of ink had been used, probably after he had got back from his journey. on the other hand, there was a very singular case in which there had been a subsequent interlineation in ink of quite a different tint, which littrow said nothing about. this seemed so curious that i wrote in my notes as follows: "that littrow, in arraying his proofs of hell's forgery, should have failed to dwell upon the obvious difference between this ink and that with which the alterations were made leads me to suspect a defect in his sense of color." the more i studied the description and the manuscript the stronger this impression became. then it occurred to me to inquire whether perhaps such could have been the case. so i asked director weiss whether anything was known as to the normal character of littrow's power of distinguishing colors. his answer was prompt and decisive. "oh yes, littrow was color-blind to red. he could not distinguish between the color of aldebaran and the whitest star." no further research was necessary. for half a century the astronomical world had based an impression on the innocent but mistaken evidence of a color-blind man--respecting the tints of ink in a manuscript. it has doubtless happened more than once that when an intimate friend has suddenly and unexpectedly passed away, the reader has ardently wished that it were possible to whisper just one word of appreciation across the dark abyss. and so it is that i have ever since felt that i would like greatly to tell father hell the story of my work at vienna in . xvi the evolution of the scientific investigator [footnote: presidential address at the opening of the international congress of arts and science, st. louis exposition, september : .] as we look at the assemblage gathered in this hall, comprising so many names of widest renown in every branch of learning--we might almost say in every field of human endeavor--the first inquiry suggested must be after the object of our meeting. the answer is that our purpose corresponds to the eminence of the assemblage. we aim at nothing less than a survey of the realm of knowledge, as comprehensive as is permitted by the limitations of time and space. the organizers of our congress have honored me with the charge of presenting such preliminary view of its field as may make clear the spirit of our undertaking. certain tendencies characteristic of the science of our day clearly suggest the direction of our thoughts most appropriate to the occasion. among the strongest of these is one towards laying greater stress on questions of the beginnings of things, and regarding a knowledge of the laws of development of any object of study as necessary to the understanding of its present form. it may be conceded that the principle here involved is as applicable in the broad field before us as in a special research into the properties of the minutest organism. it therefore seems meet that we should begin by inquiring what agency has brought about the remarkable development of science to which the world of to-day bears witness. this view is recognized in the plan of our proceedings by providing for each great department of knowledge a review of its progress during the century that has elapsed since the great event commemorated by the scenes outside this hall. but such reviews do not make up that general survey of science at large which is necessary to the development of our theme, and which must include the action of causes that had their origin long before our time. the movement which culminated in making the nineteenth century ever memorable in history is the outcome of a long series of causes, acting through many centuries, which are worthy of especial attention on such an occasion as this. in setting them forth we should avoid laying stress on those visible manifestations which, striking the eye of every beholder, are in no danger of being overlooked, and search rather for those agencies whose activities underlie the whole visible scene, but which are liable to be blotted out of sight by the very brilliancy of the results to which they have given rise. it is easy to draw attention to the wonderful qualities of the oak; but, from that very fact, it may be needful to point out that the real wonder lies concealed in the acorn from which it grew. our inquiry into the logical order of the causes which have made our civilization what it is to-day will be facilitated by bringing to mind certain elementary considerations--ideas so familiar that setting them forth may seem like citing a body of truisms--and yet so frequently overlooked, not only individually, but in their relation to each other, that the conclusion to which they lead may be lost to sight. one of these propositions is that psychical rather than material causes are those which we should regard as fundamental in directing the development of the social organism. the human intellect is the really active agent in every branch of endeavor--the primum mobile of civilization--and all those material manifestations to which our attention is so often directed are to be regarded as secondary to this first agency. if it be true that "in the world is nothing great but man; in man is nothing great but mind," then should the key-note of our discourse be the recognition of this first and greatest of powers. another well-known fact is that those applications of the forces of nature to the promotion of human welfare which have made our age what it is are of such comparatively recent origin that we need go back only a single century to antedate their most important features, and scarcely more than four centuries to find their beginning. it follows that the subject of our inquiry should be the commencement, not many centuries ago, of a certain new form of intellectual activity. having gained this point of view, our next inquiry will be into the nature of that activity and its relation to the stages of progress which preceded and followed its beginning. the superficial observer, who sees the oak but forgets the acorn, might tell us that the special qualities which have brought out such great results are expert scientific knowledge and rare ingenuity, directed to the application of the powers of steam and electricity. from this point of view the great inventors and the great captains of industry were the first agents in bringing about the modern era. but the more careful inquirer will see that the work of these men was possible only through a knowledge of the laws of nature, which had been gained by men whose work took precedence of theirs in logical order, and that success in invention has been measured by completeness in such knowledge. while giving all due honor to the great inventors, let us remember that the first place is that of the great investigators, whose forceful intellects opened the way to secrets previously hidden from men. let it be an honor and not a reproach to these men that they were not actuated by the love of gain, and did not keep utilitarian ends in view in the pursuit of their researches. if it seems that in neglecting such ends they were leaving undone the most important part of their work, let us remember that nature turns a forbidding face to those who pay her court with the hope of gain, and is responsive only to those suitors whose love for her is pure and undefiled. not only is the special genius required in the investigator not that generally best adapted to applying the discoveries which he makes, but the result of his having sordid ends in view would be to narrow the field of his efforts, and exercise a depressing effect upon his activities. the true man of science has no such expression in his vocabulary as "useful knowledge." his domain is as wide as nature itself, and he best fulfils his mission when he leaves to others the task of applying the knowledge he gives to the world. we have here the explanation of the well-known fact that the functions of the investigator of the laws of nature, and of the inventor who applies these laws to utilitarian purposes, are rarely united in the same person. if the one conspicuous exception which the past century presents to this rule is not unique, we should probably have to go back to watt to find another. from this view-point it is clear that the primary agent in the movement which has elevated man to the masterful position he now occupies is the scientific investigator. he it is whose work has deprived plague and pestilence of their terrors, alleviated human suffering, girdled the earth with the electric wire, bound the continent with the iron way, and made neighbors of the most distant nations. as the first agent which has made possible this meeting of his representatives, let his evolution be this day our worthy theme. as we follow the evolution of an organism by studying the stages of its growth, so we have to show how the work of the scientific investigator is related to the ineffectual efforts of his predecessors. in our time we think of the process of development in nature as one going continuously forward through the combination of the opposite processes of evolution and dissolution. the tendency of our thought has been in the direction of banishing cataclysms to the theological limbo, and viewing nature as a sleepless plodder, endowed with infinite patience, waiting through long ages for results. i do not contest the truth of the principle of continuity on which this view is based. but it fails to make known to us the whole truth. the building of a ship from the time that her keel is laid until she is making her way across the ocean is a slow and gradual process; yet there is a cataclysmic epoch opening up a new era in her history. it is the moment when, after lying for months or years a dead, inert, immovable mass, she is suddenly endowed with the power of motion, and, as if imbued with life, glides into the stream, eager to begin the career for which she was designed. i think it is thus in the development of humanity. long ages may pass during which a race, to all external observation, appears to be making no real progress. additions may be made to learning, and the records of history may constantly grow, but there is nothing in its sphere of thought, or in the features of its life, that can be called essentially new. yet, nature may have been all along slowly working in a way which evades our scrutiny, until the result of her operations suddenly appears in a new and revolutionary movement, carrying the race to a higher plane of civilization. it is not difficult to point out such epochs in human progress. the greatest of all, because it was the first, is one of which we find no record either in written or geological history. it was the epoch when our progenitors first took conscious thought of the morrow, first used the crude weapons which nature had placed within their reach to kill their prey, first built a fire to warm their bodies and cook their food. i love to fancy that there was some one first man, the adam of evolution, who did all this, and who used the power thus acquired to show his fellows how they might profit by his example. when the members of the tribe or community which he gathered around him began to conceive of life as a whole--to include yesterday, to-day, and to-morrow in the same mental grasp--to think how they might apply the gifts of nature to their own uses--a movement was begun which should ultimately lead to civilization. long indeed must have been the ages required for the development of this rudest primitive community into the civilization revealed to us by the most ancient tablets of egypt and assyria. after spoken language was developed, and after the rude representation of ideas by visible marks drawn to resemble them had long been practised, some cadmus must have invented an alphabet. when the use of written language was thus introduced, the word of command ceased to be confined to the range of the human voice, and it became possible for master minds to extend their influence as far as a written message could be carried. then were communities gathered into provinces; provinces into kingdoms, kingdoms into great empires of antiquity. then arose a stage of civilization which we find pictured in the most ancient records--a stage in which men were governed by laws that were perhaps as wisely adapted to their conditions as our laws are to ours--in which the phenomena of nature were rudely observed, and striking occurrences in the earth or in the heavens recorded in the annals of the nation. vast was the progress of knowledge during the interval between these empires and the century in which modern science began. yet, if i am right in making a distinction between the slow and regular steps of progress, each growing naturally out of that which preceded it, and the entrance of the mind at some fairly definite epoch into an entirely new sphere of activity, it would appear that there was only one such epoch during the entire interval. this was when abstract geometrical reasoning commenced, and astronomical observations aiming at precision were recorded, compared, and discussed. closely associated with it must have been the construction of the forms of logic. the radical difference between the demonstration of a theorem of geometry and the reasoning of every-day life which the masses of men must have practised from the beginning, and which few even to-day ever get beyond, is so evident at a glance that i need not dwell upon it. the principal feature of this advance is that, by one of those antinomies of human intellect of which examples are not wanting even in our own time, the development of abstract ideas preceded the concrete knowledge of natural phenomena. when we reflect that in the geometry of euclid the science of space was brought to such logical perfection that even to-day its teachers are not agreed as to the practicability of any great improvement upon it, we cannot avoid the feeling that a very slight change in the direction of the intellectual activity of the greeks would have led to the beginning of natural science. but it would seem that the very purity and perfection which was aimed at in their system of geometry stood in the way of any extension or application of its methods and spirit to the field of nature. one example of this is worthy of attention. in modern teaching the idea of magnitude as generated by motion is freely introduced. a line is described by a moving point; a plane by a moving line; a solid by a moving plane. it may, at first sight, seem singular that this conception finds no place in the euclidian system. but we may regard the omission as a mark of logical purity and rigor. had the real or supposed advantages of introducing motion into geometrical conceptions been suggested to euclid, we may suppose him to have replied that the theorems of space are independent of time; that the idea of motion necessarily implies time, and that, in consequence, to avail ourselves of it would be to introduce an extraneous element into geometry. it is quite possible that the contempt of the ancient philosophers for the practical application of their science, which has continued in some form to our own time, and which is not altogether unwholesome, was a powerful factor in the same direction. the result was that, in keeping geometry pure from ideas which did not belong to it, it failed to form what might otherwise have been the basis of physical science. its founders missed the discovery that methods similar to those of geometric demonstration could be extended into other and wider fields than that of space. thus not only the development of applied geometry but the reduction of other conceptions to a rigorous mathematical form was indefinitely postponed. there is, however, one science which admitted of the immediate application of the theorems of geometry, and which did not require the application of the experimental method. astronomy is necessarily a science of observation pure and simple, in which experiment can have no place except as an auxiliary. the vague accounts of striking celestial phenomena handed down by the priests and astrologers of antiquity were followed in the time of the greeks by observations having, in form at least, a rude approach to precision, though nothing like the degree of precision that the astronomer of to-day would reach with the naked eye, aided by such instruments as he could fashion from the tools at the command of the ancients. the rude observations commenced by the babylonians were continued with gradually improving instruments--first by the greeks and afterwards by the arabs--but the results failed to afford any insight into the true relation of the earth to the heavens. what was most remarkable in this failure is that, to take a first step forward which would have led on to success, no more was necessary than a course of abstract thinking vastly easier than that required for working out the problems of geometry. that space is infinite is an unexpressed axiom, tacitly assumed by euclid and his successors. combining this with the most elementary consideration of the properties of the triangle, it would be seen that a body of any given size could be placed at such a distance in space as to appear to us like a point. hence a body as large as our earth, which was known to be a globe from the time that the ancient phoenicians navigated the mediterranean, if placed in the heavens at a sufficient distance, would look like a star. the obvious conclusion that the stars might be bodies like our globe, shining either by their own light or by that of the sun, would have been a first step to the understanding of the true system of the world. there is historic evidence that this deduction did not wholly escape the greek thinkers. it is true that the critical student will assign little weight to the current belief that the vague theory of pythagoras--that fire was at the centre of all things--implies a conception of the heliocentric theory of the solar system. but the testimony of archimedes, confused though it is in form, leaves no serious doubt that aristarchus of samos not only propounded the view that the earth revolves both on its own axis and around the sun, but that he correctly removed the great stumbling-block in the way of this theory by adding that the distance of the fixed stars was infinitely greater than the dimensions of the earth's orbit. even the world of philosophy was not yet ready for this conception, and, so far from seeing the reasonableness of the explanation, we find ptolemy arguing against the rotation of the earth on grounds which careful observations of the phenomena around him would have shown to be ill-founded. physical science, if we can apply that term to an uncoordinated body of facts, was successfully cultivated from the earliest times. something must have been known of the properties of metals, and the art of extracting them from their ores must have been practised, from the time that coins and medals were first stamped. the properties of the most common compounds were discovered by alchemists in their vain search for the philosopher's stone, but no actual progress worthy of the name rewarded the practitioners of the black art. perhaps the first approach to a correct method was that of archimedes, who by much thinking worked out the law of the lever, reached the conception of the centre of gravity, and demonstrated the first principles of hydrostatics. it is remarkable that he did not extend his researches into the phenomena of motion, whether spontaneous or produced by force. the stationary condition of the human intellect is most strikingly illustrated by the fact that not until the time of leonardo was any substantial advance made on his discovery. to sum up in one sentence the most characteristic feature of ancient and medieval science, we see a notable contrast between the precision of thought implied in the construction and demonstration of geometrical theorems and the vague indefinite character of the ideas of natural phenomena generally, a contrast which did not disappear until the foundations of modern science began to be laid. we should miss the most essential point of the difference between medieval and modern learning if we looked upon it as mainly a difference either in the precision or the amount of knowledge. the development of both of these qualities would, under any circumstances, have been slow and gradual, but sure. we can hardly suppose that any one generation, or even any one century, would have seen the complete substitution of exact for inexact ideas. slowness of growth is as inevitable in the case of knowledge as in that of a growing organism. the most essential point of difference is one of those seemingly slight ones, the importance of which we are too apt to overlook. it was like the drop of blood in the wrong place, which some one has told us makes all the difference between a philosopher and a maniac. it was all the difference between a living tree and a dead one, between an inert mass and a growing organism. the transition of knowledge from the dead to the living form must, in any complete review of the subject, be looked upon as the really great event of modern times. before this event the intellect was bound down by a scholasticism which regarded knowledge as a rounded whole, the parts of which were written in books and carried in the minds of learned men. the student was taught from the beginning of his work to look upon authority as the foundation of his beliefs. the older the authority the greater the weight it carried. so effective was this teaching that it seems never to have occurred to individual men that they had all the opportunities ever enjoyed by aristotle of discovering truth, with the added advantage of all his knowledge to begin with. advanced as was the development of formal logic, that practical logic was wanting which could see that the last of a series of authorities, every one of which rested on those which preceded it, could never form a surer foundation for any doctrine than that supplied by its original propounder. the result of this view of knowledge was that, although during the fifteen centuries following the death of the geometer of syracuse great universities were founded at which generations of professors expounded all the learning of their time, neither professor nor student ever suspected what latent possibilities of good were concealed in the most familiar operations of nature. every one felt the wind blow, saw water boil, and heard the thunder crash, but never thought of investigating the forces here at play. up to the middle of the fifteenth century the most acute observer could scarcely have seen the dawn of a new era. in view of this state of things it must be regarded as one of the most remarkable facts in evolutionary history that four or five men, whose mental constitution was either typical of the new order of things, or who were powerful agents in bringing it about, were all born during the fifteenth century, four of them at least, at so nearly the same time as to be contemporaries. leonardo da vinci, whose artistic genius has charmed succeeding generations, was also the first practical engineer of his time, and the first man after archimedes to make a substantial advance in developing the laws of motion. that the world was not prepared to make use of his scientific discoveries does not detract from the significance which must attach to the period of his birth. shortly after him was born the great navigator whose bold spirit was to make known a new world, thus giving to commercial enterprise that impetus which was so powerful an agent in bringing about a revolution in the thoughts of men. the birth of columbus was soon followed by that of copernicus, the first after aristarchus to demonstrate the true system of the world. in him more than in any of his contemporaries do we see the struggle between the old forms of thought and the new. it seems almost pathetic and is certainly most suggestive of the general view of knowledge taken at that time that, instead of claiming credit for bringing to light great truths before unknown, he made a labored attempt to show that, after all, there was nothing really new in his system, which he claimed to date from pythagoras and philolaus. in this connection it is curious that he makes no mention of aristarchus, who i think will be regarded by conservative historians as his only demonstrated predecessor. to the hold of the older ideas upon his mind we must attribute the fact that in constructing his system he took great pains to make as little change as possible in ancient conceptions. luther, the greatest thought-stirrer of them all, practically of the same generation with copernicus, leonardo and columbus, does not come in as a scientific investigator, but as the great loosener of chains which had so fettered the intellect of men that they dared not think otherwise than as the authorities thought. almost coeval with the advent of these intellects was the invention of printing with movable type. gutenberg was born during the first decade of the century, and his associates and others credited with the invention not many years afterwards. if we accept the principle on which i am basing my argument, that in bringing out the springs of our progress we should assign the first place to the birth of those psychic agencies which started men on new lines of thought, then surely was the fifteenth the wonderful century. let us not forget that, in assigning the actors then born to their places, we are not narrating history, but studying a special phase of evolution. it matters not for us that no university invited leonardo to its halls, and that his science was valued by his contemporaries only as an adjunct to the art of engineering. the great fact still is that he was the first of mankind to propound laws of motion. it is not for anything in luther's doctrines that he finds a place in our scheme. no matter for us whether they were sound or not. what he did towards the evolution of the scientific investigator was to show by his example that a man might question the best-established and most venerable authority and still live--still preserve his intellectual integrity--still command a hearing from nations and their rulers. it matters not for us whether columbus ever knew that he had discovered a new continent. his work was to teach that neither hydra, chimera nor abyss--neither divine injunction nor infernal machination--was in the way of men visiting every part of the globe, and that the problem of conquering the world reduced itself to one of sails and rigging, hull and compass. the better part of copernicus was to direct man to a view-point whence he should see that the heavens were of like matter with the earth. all this done, the acorn was planted from which the oak of our civilization should spring. the mad quest for gold which followed the discovery of columbus, the questionings which absorbed the attention of the learned, the indignation excited by the seeming vagaries of a paracelsus, the fear and trembling lest the strange doctrine of copernicus should undermine the faith of centuries, were all helps to the germination of the seed--stimuli to thought which urged it on to explore the new fields opened up to its occupation. this given, all that has since followed came out in regular order of development, and need be here considered only in those phases having a special relation to the purpose of our present meeting. so slow was the growth at first that the sixteenth century may scarcely have recognized the inauguration of a new era. torricelli and benedetti were of the third generation after leonardo, and galileo, the first to make a substantial advance upon his theory, was born more than a century after him. only two or three men appeared in a generation who, working alone, could make real progress in discovery, and even these could do little in leavening the minds of their fellowmen with the new ideas. up to the middle of the seventeenth century an agent which all experience since that time shows to be necessary to the most productive intellectual activity was wanting. this was the attrition of like minds, making suggestions to one another, criticising, comparing, and reasoning. this element was introduced by the organization of the royal society of london and the academy of sciences of paris. the members of these two bodies seem like ingenious youth suddenly thrown into a new world of interesting objects, the purposes and relations of which they had to discover. the novelty of the situation is strikingly shown in the questions which occupied the minds of the incipient investigators. one natural result of british maritime enterprise was that the aspirations of the fellows of the royal society were not confined to any continent or hemisphere. inquiries were sent all the way to batavia to know "whether there be a hill in sumatra which burneth continually, and a fountain which runneth pure balsam." the astronomical precision with which it seemed possible that physiological operations might go on was evinced by the inquiry whether the indians can so prepare that stupefying herb datura that "they make it lie several days, months, years, according as they will, in a man's body without doing him any harm, and at the end kill him without missing an hour's time." of this continent one of the inquiries was whether there be a tree in mexico that yields water, wine, vinegar, milk, honey, wax, thread and needles. among the problems before the paris academy of sciences those of physiology and biology took a prominent place. the distillation of compounds had long been practised, and the fact that the more spirituous elements of certain substances were thus separated naturally led to the question whether the essential essences of life might not be discoverable in the same way. in order that all might participate in the experiments, they were conducted in open session of the academy, thus guarding against the danger of any one member obtaining for his exclusive personal use a possible elixir of life. a wide range of the animal and vegetable kingdom, including cats, dogs and birds of various species, were thus analyzed. the practice of dissection was introduced on a large scale. that of the cadaver of an elephant occupied several sessions, and was of such interest that the monarch himself was a spectator. to the same epoch with the formation and first work of these two bodies belongs the invention of a mathematical method which in its importance to the advance of exact science may be classed with the invention of the alphabet in its relation to the progress of society at large. the use of algebraic symbols to represent quantities had its origin before the commencement of the new era, and gradually grew into a highly developed form during the first two centuries of that era. but this method could represent quantities only as fixed. it is true that the elasticity inherent in the use of such symbols permitted of their being applied to any and every quantity; yet, in any one application, the quantity was considered as fixed and definite. but most of the magnitudes of nature are in a state of continual variation; indeed, since all motion is variation, the latter is a universal characteristic of all phenomena. no serious advance could be made in the application of algebraic language to the expression of physical phenomena until it could be so extended as to express variation in quantities, as well as the quantities themselves. this extension, worked out independently by newton and leibnitz, may be classed as the most fruitful of conceptions in exact science. with it the way was opened for the unimpeded and continually accelerated progress of the last two centuries. the feature of this period which has the closest relation to the purpose of our coming together is the seemingly unending subdivision of knowledge into specialties, many of which are becoming so minute and so isolated that they seem to have no interest for any but their few pursuers. happily science itself has afforded a corrective for its own tendency in this direction. the careful thinker will see that in these seemingly diverging branches common elements and common principles are coming more and more to light. there is an increasing recognition of methods of research, and of deduction, which are common to large branches, or to the whole of science. we are more and more recognizing the principle that progress in knowledge implies its reduction to more exact forms, and the expression of its ideas in language more or less mathematical. the problem before the organizers of this congress was, therefore, to bring the sciences together, and seek for the unity which we believe underlies their infinite diversity. the assembling of such a body as now fills this hall was scarcely possible in any preceding generation, and is made possible now only through the agency of science itself. it differs from all preceding international meetings by the universality of its scope, which aims to include the whole of knowledge. it is also unique in that none but leaders have been sought out as members. it is unique in that so many lands have delegated their choicest intellects to carry on its work. they come from the country to which our republic is indebted for a third of its territory, including the ground on which we stand; from the land which has taught us that the most scholarly devotion to the languages and learning of the cloistered past is compatible with leadership in the practical application of modern science to the arts of life; from the island whose language and literature have found a new field and a vigorous growth in this region; from the last seat of the holy roman empire; from the country which, remembering a monarch who made an astronomical observation at the greenwich observatory, has enthroned science in one of the highest places in its government; from the peninsula so learned that we have invited one of its scholars to come and tells us of our own language; from the land which gave birth to leonardo, galileo, torricelli, columbus, volta--what an array of immortal names!--from the little republic of glorious history which, breeding men rugged as its eternal snow-peaks, has yet been the seat of scientific investigation since the day of the bernoullis; from the land whose heroic dwellers did not hesitate to use the ocean itself to protect it against invaders, and which now makes us marvel at the amount of erudition compressed within its little area; from the nation across the pacific, which, by half a century of unequalled progress in the arts of life, has made an important contribution to evolutionary science through demonstrating the falsity of the theory that the most ancient races are doomed to be left in the rear of the advancing age--in a word, from every great centre of intellectual activity on the globe i see before me eminent representatives of that world--advance in knowledge which we have met to celebrate. may we not confidently hope that the discussions of such an assemblage will prove pregnant of a future for science which shall outshine even its brilliant past. gentlemen and scholars all! you do not visit our shores to find great collections in which centuries of humanity have given expression on canvas and in marble to their hopes, fears, and aspirations. nor do you expect institutions and buildings hoary with age. but as you feel the vigor latent in the fresh air of these expansive prairies, which has collected the products of human genius by which we are here surrounded, and, i may add, brought us together; as you study the institutions which we have founded for the benefit, not only of our own people, but of humanity at large; as you meet the men who, in the short space of one century, have transformed this valley from a savage wilderness into what it is today--then may you find compensation for the want of a past like yours by seeing with prophetic eye a future world-power of which this region shall be the seat. if such is to be the outcome of the institutions which we are now building up, then may your present visit be a blessing both to your posterity and ours by making that power one for good to all man-kind. your deliberations will help to demonstrate to us and to the world at large that the reign of law must supplant that of brute force in the relations of the nations, just as it has supplanted it in the relations of individuals. you will help to show that the war which science is now waging against the sources of diseases, pain, and misery offers an even nobler field for the exercise of heroic qualities than can that of battle. we hope that when, after your all too-fleeting sojourn in our midst, you return to your own shores, you will long feel the influence of the new air you have breathed in an infusion of increased vigor in pursuing your varied labors. and if a new impetus is thus given to the great intellectual movement of the past century, resulting not only in promoting the unification of knowledge, but in widening its field through new combinations of effort on the part of its votaries, the projectors, organizers and supporters of this congress of arts and science will be justified of their labors. xvii the evolution of astronomical knowledge [footnote: address at the dedication of the flower observatory, university of pennsylvania, may , --science, may , ] assembled, as we are, to dedicate a new institution to the promotion of our knowledge of the heavens, it appeared to me that an appropriate and interesting subject might be the present and future problems of astronomy. yet it seemed, on further reflection, that, apart from the difficulty of making an adequate statement of these problems on such an occasion as the present, such a wording of the theme would not fully express the idea which i wish to convey. the so-called problems of astronomy are not separate and independent, but are rather the parts of one great problem, that of increasing our knowledge of the universe in its widest extent. nor is it easy to contemplate the edifice of astronomical science as it now stands, without thinking of the past as well as of the present and future. the fact is that our knowledge of the universe has been in the nature of a slow and gradual evolution, commencing at a very early period in human history, and destined to go forward without stop, as we hope, so long as civilization shall endure. the astronomer of every age has built on the foundations laid by his predecessors, and his work has always formed, and must ever form, the base on which his successors shall build. the astronomer of to-day may look back upon hipparchus and ptolemy as the earliest ancestors of whom he has positive knowledge. he can trace his scientific descent from generation to generation, through the periods of arabian and medieval science, through copernicus, kepler, newton, laplace, and herschel, down to the present time. the evolution of astronomical knowledge, generally slow and gradual, offering little to excite the attention of the public, has yet been marked by two cataclysms. one of these is seen in the grand conception of copernicus that this earth on which we dwell is not a globe fixed in the centre of the universe, but is simply one of a number of bodies, turning on their own axes and at the same time moving around the sun as a centre. it has always seemed to me that the real significance of the heliocentric system lies in the greatness of this conception rather than in the fact of the discovery itself. there is no figure in astronomical history which may more appropriately claim the admiration of mankind through all time than that of copernicus. scarcely any great work was ever so exclusively the work of one man as was the heliocentric system the work of the retiring sage of frauenburg. no more striking contrast between the views of scientific research entertained in his time and in ours can be found than that afforded by the fact that, instead of claiming credit for his great work, he deemed it rather necessary to apologize for it and, so far as possible, to attribute his ideas to the ancients. a century and a half after copernicus followed the second great step, that taken by newton. this was nothing less than showing that the seemingly complicated and inexplicable motions of the heavenly bodies were only special cases of the same kind of motion, governed by the same forces, that we see around us whenever a stone is thrown by the hand or an apple falls to the ground. the actual motions of the heavens and the laws which govern them being known, man had the key with which he might commence to unlock the mysteries of the universe. when huyghens, in , published his systema saturnium, where he first set forth the mystery of the rings of saturn, which, for nearly half a century, had perplexed telescopic observers, he prefaced it with a remark that many, even among the learned, might condemn his course in devoting so much time and attention to matters far outside the earth, when he might better be studying subjects of more concern to humanity. notwithstanding that the inventor of the pendulum clock was, perhaps, the last astronomer against whom a neglect of things terrestrial could be charged, he thought it necessary to enter into an elaborate defence of his course in studying the heavens. now, however, the more distant objects are in space--i might almost add the more distant events are in time--the more they excite the attention of the astronomer, if only he can hope to acquire positive knowledge about them. not, however, because he is more interested in things distant than in things near, but because thus he may more completely embrace in the scope of his work the beginning and the end, the boundaries of all things, and thus, indirectly, more fully comprehend all that they include. from his stand-point, "all are but parts of one stupendous whole, whose body nature is and god the soul." others study nature and her plans as we see them developed on the surface of this little planet which we inhabit, the astronomer would fain learn the plan on which the whole universe is constructed. the magnificent conception of copernicus is, for him, only an introduction to the yet more magnificent conception of infinite space containing a collection of bodies which we call the visible universe. how far does this universe extend? what are the distances and arrangements of the stars? does the universe constitute a system? if so, can we comprehend the plan on which this system is formed, of its beginning and of its end? has it bounds outside of which nothing exists but the black and starless depths of infinity itself? or are the stars we see simply such members of an infinite collection as happen to be the nearest our system? a few such questions as these we are perhaps beginning to answer; but hundreds, thousands, perhaps even millions, of years may elapse without our reaching a complete solution. yet the astronomer does not view them as kantian antinomies, in the nature of things insoluble, but as questions to which he may hopefully look for at least a partial answer. the problem of the distances of the stars is of peculiar interest in connection with the copernican system. the greatest objection to this system, which must have been more clearly seen by astronomers themselves than by any others, was found in the absence of any apparent parallax of the stars. if the earth performed such an immeasurable circle around the sun as copernicus maintained, then, as it passed from side to side of its orbit, the stars outside the solar system must appear to have a corresponding motion in the other direction, and thus to swing back and forth as the earth moved in one and the other direction. the fact that not the slightest swing of that sort could be seen was, from the time of ptolemy, the basis on which the doctrine of the earth's immobility rested. the difficulty was not grappled with by copernicus or his immediate successors. the idea that nature would not squander space by allowing immeasurable stretches of it to go unused seems to have been one from which medieval thinkers could not entirely break away. the consideration that there could be no need of any such economy, because the supply was infinite, might have been theoretically acknowledged, but was not practically felt. the fact is that magnificent as was the conception of copernicus, it was dwarfed by the conception of stretches from star to star so vast that the whole orbit of the earth was only a point in comparison. an indication of the extent to which the difficulty thus arising was felt is seen in the title of a book published by horrebow, the danish astronomer, some two centuries ago. this industrious observer, one of the first who used an instrument resembling our meridian transit of the present day, determined to see if he could find the parallax of the stars by observing the intervals at which a pair of stars in opposite quarters of the heavens crossed his meridian at opposite seasons of the year. when, as he thought, he had won success, he published his observations and conclusions under the title of copernicus triumphans. but alas! the keen criticism of his successors showed that what he supposed to be a swing of the stars from season to season arose from a minute variation in the rate of his clock, due to the different temperatures to which it was exposed during the day and the night. the measurement of the distance even of the nearest stars evaded astronomical research until bessel and struve arose in the early part of the present century. on some aspects of the problem of the extent of the universe light is being thrown even now. evidence is gradually accumulating which points to the probability that the successive orders of smaller and smaller stars, which our continually increasing telescopic power brings into view, are not situated at greater and greater distances, but that we actually see the boundary of our universe. this indication lends a peculiar interest to various questions growing out of the motions of the stars. quite possibly the problem of these motions will be the great one of the future astronomer. even now it suggests thoughts and questions of the most far-reaching character. i have seldom felt a more delicious sense of repose than when crossing the ocean during the summer months i sought a place where i could lie alone on the deck, look up at the constellations, with lyra near the zenith, and, while listening to the clank of the engine, try to calculate the hundreds of millions of years which would be required by our ship to reach the star a lyrae, if she could continue her course in that direction without ever stopping. it is a striking example of how easily we may fail to realize our knowledge when i say that i have thought many a time how deliciously one might pass those hundred millions of years in a journey to the star a lyrae, without its occurring to me that we are actually making that very journey at a speed compared with which the motion of a steamship is slow indeed. through every year, every hour, every minute, of human history from the first appearance of man on the earth, from the era of the builders of the pyramids, through the times of caesar and hannibal, through the period of every event that history records, not merely our earth, but the sun and the whole solar system with it, have been speeding their way towards the star of which i speak on a journey of which we know neither the beginning nor the end. we are at this moment thousands of miles nearer to a lyrae than we were a few minutes ago when i began this discourse, and through every future moment, for untold thousands of years to come, the earth and all there is on it will be nearer to a lyrae, or nearer to the place where that star now is, by hundreds of miles for every minute of time come and gone. when shall we get there? probably in less than a million years, perhaps in half a million. we cannot tell exactly, but get there we must if the laws of nature and the laws of motion continue as they are. to attain to the stars was the seemingly vain wish of an ancient philosopher, but the whole human race is, in a certain sense, realizing this wish as rapidly as a speed of ten miles a second can bring it about. i have called attention to this motion because it may, in the not distant future, afford the means of approximating to a solution of the problem already mentioned--that of the extent of the universe. notwithstanding the success of astronomers during the present century in measuring the parallax of a number of stars, the most recent investigations show that there are very few, perhaps hardly more than a score, of stars of which the parallax, and therefore the distance, has been determined with any approach to certainty. many parallaxes determined about the middle of the nineteenth century have had to disappear before the powerful tests applied by measures with the heliometer; others have been greatly reduced and the distances of the stars increased in proportion. so far as measurement goes, we can only say of the distances of all the stars, except the few whose parallaxes have been determined, that they are immeasurable. the radius of the earth's orbit, a line more than ninety millions of miles in length, not only vanishes from sight before we reach the distance of the great mass of stars, but becomes such a mere point that when magnified by the powerful instruments of modern times the most delicate appliances fail to make it measurable. here the solar motion comes to our help. this motion, by which, as i have said, we are carried unceasingly through space, is made evident by a motion of most of the stars in the opposite direction, just as passing through a country on a railway we see the houses on the right and on the left being left behind us. it is clear enough that the apparent motion will be more rapid the nearer the object. we may therefore form some idea of the distance of the stars when we know the amount of the motion. it is found that in the great mass of stars of the sixth magnitude, the smallest visible to the naked eye, the motion is about three seconds per century. as a measure thus stated does not convey an accurate conception of magnitude to one not practised in the subject, i would say that in the heavens, to the ordinary eye, a pair of stars will appear single unless they are separated by a distance of or seconds. let us, then, imagine ourselves looking at a star of the sixth magnitude, which is at rest while we are carried past it with the motion of six to eight miles per second which i have described. mark its position in the heavens as we see it to-day; then let its position again be marked five thousand years hence. a good eye will just be able to perceive that there are two stars marked instead of one. the two would be so close together that no distinct space between them could be perceived by unaided vision. it is due to the magnifying power of the telescope, enlarging such small apparent distances, that the motion has been determined in so small a period as the one hundred and fifty years during which accurate observations of the stars have been made. the motion just described has been fairly well determined for what, astronomically speaking, are the brighter stars; that is to say, those visible to the naked eye. but how is it with the millions of faint telescopic stars, especially those which form the cloud masses of the milky way? the distance of these stars is undoubtedly greater, and the apparent motion is therefore smaller. accurate observations upon such stars have been commenced only recently, so that we have not yet had time to determine the amount of the motion. but the indication seems to be that it will prove quite a measurable quantity and that before the twentieth century has elapsed, it will be determined for very much smaller stars than those which have heretofore been studied. a photographic chart of the whole heavens is now being constructed by an association of observatories in some of the leading countries of the world. i cannot say all the leading countries, because then we should have to exclude our own, which, unhappily, has taken no part in this work. at the end of the twentieth century we may expect that the work will be repeated. then, by comparing the charts, we shall see the effect of the solar motion and perhaps get new light upon the problem in question. closely connected with the problem of the extent of the universe is another which appears, for us, to be insoluble because it brings us face to face with infinity itself. we are familiar enough with eternity, or, let us say, the millions or hundreds of millions of years which geologists tell us must have passed while the crust of the earth was assuming its present form, our mountains being built, our rocks consolidated, and successive orders of animals coming and going. hundreds of millions of years is indeed a long time, and yet, when we contemplate the changes supposed to have taken place during that time, we do not look out on eternity itself, which is veiled from our sight, as it were, by the unending succession of changes that mark the progress of time. but in the motions of the stars we are brought face to face with eternity and infinity, covered by no veil whatever. it would be bold to speak dogmatically on a subject where the springs of being are so far hidden from mortal eyes as in the depths of the universe. but, without declaring its positive certainty, it must be said that the conclusion seems unavoidable that a number of stars are moving with a speed such that the attraction of all the bodies of the universe could never stop them. one such case is that of arcturus, the bright reddish star familiar to mankind since the days of job, and visible near the zenith on the clear evenings of may and june. yet another case is that of a star known in astronomical nomenclature as groombridge, which exceeds all others in its angular proper motion as seen from the earth. we should naturally suppose that it seems to move so fast because it is near us. but the best measurements of its parallax seem to show that it can scarcely be less than two million times the distance of the earth from the sun, while it may be much greater. accepting this result, its velocity cannot be much less than two hundred miles per second, and may be much more. with this speed it would make the circuit of our globe in two minutes, and had it gone round and round in our latitudes we should have seen it fly past us a number of times since i commenced this discourse. it would make the journey from the earth to the sun in five days. if it is now near the centre of our universe it would probably reach its confines in a million of years. so far as our knowledge goes, there is no force in nature which would ever have set it in motion and no force which can ever stop it. what, then, was the history of this star, and, if there are planets circulating around, what the experience of beings who may have lived on those planets during the ages which geologists and naturalists assure us our earth has existed? was there a period when they saw at night only a black and starless heaven? was there a time when in that heaven a small faint patch of light began gradually to appear? did that patch of light grow larger and larger as million after million of years elapsed? did it at last fill the heavens and break up into constellations as we now see them? as millions more of years elapse will the constellations gather together in the opposite quarter and gradually diminish to a patch of light as the star pursues its irresistible course of two hundred miles per second through the wilderness of space, leaving our universe farther and farther behind it, until it is lost in the distance? if the conceptions of modern science are to be considered as good for all time--a point on which i confess to a large measure of scepticism--then these questions must be answered in the affirmative. the problems of which i have so far spoken are those of what may be called the older astronomy. if i apply this title it is because that branch of the science to which the spectroscope has given birth is often called the new astronomy. it is commonly to be expected that a new and vigorous form of scientific research will supersede that which is hoary with antiquity. but i am not willing to admit that such is the case with the old astronomy, if old we may call it. it is more pregnant with future discoveries today than it ever has been, and it is more disposed to welcome the spectroscope as a useful handmaid, which may help it on to new fields, than it is to give way to it. how useful it may thus become has been recently shown by a dutch astronomer, who finds that the stars having one type of spectrum belong mostly to the milky way, and are farther from us than the others. in the field of the newer astronomy perhaps the most interesting work is that associated with comets. it must be confessed, however, that the spectroscope has rather increased than diminished the mystery which, in some respects, surrounds the constitution of these bodies. the older astronomy has satisfactorily accounted for their appearance, and we might also say for their origin and their end, so far as questions of origin can come into the domain of science. it is now known that comets are not wanderers through the celestial spaces from star to star, but must always have belonged to our system. but their orbits are so very elongated that thousands, or even hundreds of thousands, of years are required for a revolution. sometimes, however, a comet passing near to jupiter is so fascinated by that planet that, in its vain attempts to follow it, it loses so much of its primitive velocity as to circulate around the sun in a period of a few years, and thus to become, apparently, a new member of our system. if the orbit of such a comet, or in fact of any comet, chances to intersect that of the earth, the latter in passing the point of intersection encounters minute particles which cause a meteoric shower. but all this does not tell us much about the nature and make-up of a comet. does it consist of nothing but isolated particles, or is there a solid nucleus, the attraction of which tends to keep the mass together? no one yet knows. the spectroscope, if we interpret its indications in the usual way, tells us that a comet is simply a mass of hydrocarbon vapor, shining by its own light. but there must be something wrong in this interpretation. that the light is reflected sunlight seems to follow necessarily from the increased brilliancy of the comet as it approaches the sun and its disappearance as it passes away. great attention has recently been bestowed upon the physical constitution of the planets and the changes which the surfaces of those bodies may undergo. in this department of research we must feel gratified by the energy of our countrymen who have entered upon it. should i seek to even mention all the results thus made known i might be stepping on dangerous ground, as many questions are still unsettled. while every astronomer has entertained the highest admiration for the energy and enthusiasm shown by mr. percival lowell in founding an observatory in regions where the planets can be studied under the most favorable conditions, they cannot lose sight of the fact that the ablest and most experienced observers are liable to error when they attempt to delineate the features of a body , , or , , miles away through such a disturbing medium as our atmosphere. even on such a subject as the canals of mars doubts may still be felt. that certain markings to which schiaparelli gave the name of canals exist, few will question. but it may be questioned whether these markings are the fine, sharp, uniform lines found on schiaparelli's map and delineated in lowell's beautiful book. it is certainly curious that barnard at mount hamilton, with the most powerful instrument and under the most favorable circumstances, does not see these markings as canals. i can only mention among the problems of the spectroscope the elegant and remarkable solution of the mystery surrounding the rings of saturn, which has been effected by keeler at allegheny. that these rings could not be solid has long been a conclusion of the laws of mechanics, but keeler was the first to show that they really consist of separate particles, because the inner portions revolve more rapidly than the outer. the question of the atmosphere of mars has also received an important advance by the work of campbell at mount hamilton. although it is not proved that mars has no atmosphere, for the existence of some atmosphere can scarcely be doubted, yet the mount hamilton astronomer seems to have shown, with great conclusiveness, that it is so rare as not to produce any sensible absorption of the solar rays. i have left an important subject for the close. it belongs entirely to the older astronomy, and it is one with which i am glad to say this observatory is expected to especially concern itself. i refer to the question of the variation of latitudes, that singular phenomenon scarcely suspected ten years ago, but brought out by observations in germany during the past eight years, and reduced to law with such brilliant success by our own chandler. the north pole is not a fixed point on the earth's surface, but moves around in rather an irregular way. true, the motion is small; a circle of sixty feet in diameter will include the pole in its widest range. this is a very small matter so far as the interests of daily life are concerned; but it is very important to the astronomer. it is not simply a motion of the pole of the earth, but a wobbling of the solid earth itself. no one knows what conclusions of importance to our race may yet follow from a study of the stupendous forces necessary to produce even this slight motion. the director of this new observatory has already distinguished himself in the delicate and difficult work of investigating this motion, and i am glad to know that he is continuing the work here with one of the finest instruments ever used for the purpose, a splendid product of american mechanical genius. i can assure you that astronomers the world over will look with the greatest interest for professor doolittle's success in the arduous task he has undertaken. there is one question connected with these studies of the universe on which i have not touched, and which is, nevertheless, of transcendent interest. what sort of life, spiritual and intellectual, exists in distant worlds? we cannot for a moment suppose that our little planet is the only one throughout the whole universe on which may be found the fruits of civilization, family affection, friendship, the desire to penetrate the mysteries of creation. and yet this question is not to-day a problem of astronomy, nor can we see any prospect that it ever will be, for the simple reason that science affords us no hope of an answer to any question that we may send through the fathomless abyss. when the spectroscope was in its infancy it was suggested that possibly some difference might be found in the rays reflected from living matter, especially from vegetation, that might enable us to distinguish them from rays reflected by matter not endowed with life. but this hope has not been realized, nor does it seem possible to realize it. the astronomer cannot afford to waste his energies on hopeless speculation about matters of which he cannot learn anything, and he therefore leaves this question of the plurality of worlds to others who are as competent to discuss it as he is. all he can tell the world is: he who through vast immensity can pierce, see worlds on worlds compose one universe; observe how system into system runs, what other planets circle other suns, what varied being peoples every star, may tell why heaven has made us as we are. xviii aspects of american astronomy [footnote: address delivered at the university of chicago, october , , in connection with the dedication of the yerkes observatory. printed in the astro physical journal. november, .] the university of chicago yesterday accepted one of the most munificent gifts ever made for the promotion of any single science, and with appropriate ceremonies dedicated it to the increase of our knowledge of the heavenly bodies. the president of your university has done me the honor of inviting me to supplement what was said on that occasion by some remarks of a more general nature suggested by the celebration. one is naturally disposed to say first what is uppermost in his mind. at the present moment this will naturally be the general impression made by what has been seen and heard. the ceremonies were attended, not only by a remarkable delegation of citizens, but by a number of visiting astronomers which seems large when we consider that the profession itself is not at all numerous in any country. as one of these, your guests, i am sure that i give expression only to their unanimous sentiment in saying that we have been extremely gratified in many ways by all that we have seen and heard. the mere fact of so munificent a gift to science cannot but excite universal admiration. we knew well enough that it was nothing more than might have been expected from the public spirit of this great west; but the first view of a towering snowpeak is none the less impressive because you have learned in your geography how many feet high it is, and great acts are none the less admirable because they correspond to what you have heard and read, and might therefore be led to expect. the next gratifying feature is the great public interest excited by the occasion. that the opening of a purely scientific institution should have led so large an assemblage of citizens to devote an entire day, including a long journey by rail, to the celebration of yesterday is something most suggestive from its unfamiliarity. a great many scientific establishments have been inaugurated during the last half-century, but if on any such occasion so large a body of citizens has gone so great a distance to take part in the inauguration, the fact has at the moment escaped my mind. that the interest thus shown is not confined to the hundreds of attendants, but must be shared by your great public, is shown by the unfailing barometer of journalism. here we have a field in which the non-survival of the unfit is the rule in its most ruthless form. the journals that we see and read are merely the fortunate few of a countless number, dead and forgotten, that did not know what the public wanted to read about. the eagerness shown by the representatives of your press in recording everything your guests would say was accomplished by an enterprise in making known everything that occurred, and, in case of an emergency requiring a heroic measure, what did not occur, showing that smart journalists of the east must have learned their trade, or at least breathed their inspiration, in these regions. i think it was some twenty years since i told a european friend that the eighth wonder of the world was a chicago daily newspaper. since that time the course of journalistic enterprise has been in the reverse direction to that of the course of empire, eastward instead of westward. it has been sometimes said--wrongfully, i think--that scientific men form a mutual admiration society. one feature of the occasion made me feel that we, your guests, ought then and there to have organized such a society and forthwith proceeded to business. this feature consisted in the conferences on almost every branch of astronomy by which the celebration of yesterday was preceded. the fact that beyond the acceptance of a graceful compliment i contributed nothing to these conferences relieves me from the charge of bias or self-assertion in saying that they gave me a new and most inspiring view of the energy now being expended in research by the younger generation of astronomers. all the experience of the past leads us to believe that this energy will reap the reward which nature always bestows upon those who seek her acquaintance from unselfish motives. in one way it might appear that little was to be learned from a meeting like that of the present week. each astronomer may know by publications pertaining to the science what all the others are doing. but knowledge obtained in this way has a sort of abstractness about it a little like our knowledge of the progress of civilization in japan, or of the great extent of the australian continent. it was, therefore, a most happy thought on the part of your authorities to bring together the largest possible number of visiting astronomers from europe, as well as america, in order that each might see, through the attrition of personal contact, what progress the others were making in their researches. to the visitors at least i am sure that the result of this meeting has been extremely gratifying. they earnestly hope, one and all, that the callers of the conference will not themselves be more disappointed in its results; that, however little they may have actually to learn of methods and results, they will feel stimulated to well-directed efforts and find themselves inspired by thoughts which, however familiar, will now be more easily worked out. we may pass from the aspects of the case as seen by the strictly professional class to those general aspects fitted to excite the attention of the great public. from the point of view of the latter it may well appear that the most striking feature of the celebration is the great amount of effort which is shown to be devoted to the cultivation of a field quite outside the ordinary range of human interests. the workers whom we see around us are only a detachment from an army of investigators who, in many parts of the world, are seeking to explore the mysteries of creation. why so great an expenditure of energy? certainly not to gain wealth, for astronomy is perhaps the one field of scientific work which, in our expressive modern phrase, "has no money in it." it is true that the great practical use of astronomical science to the country and the world in affording us the means of determining positions on land and at sea is frequently pointed out. it is said that an astronomer royal of england once calculated that every meridian observation of the moon made at greenwich was worth a pound sterling, on account of the help it would afford to the navigation of the ocean. an accurate map of the united states cannot be constructed without astronomical observations at numerous points scattered over the whole country, aided by data which great observatories have been accumulating for more than a century, and must continue to accumulate in the future. but neither the measurement of the earth, the making of maps, nor the aid of the navigator is the main object which the astronomers of to-day have in view. if they do not quite share the sentiment of that eminent mathematician, who is said to have thanked god that his science was one which could not be prostituted to any useful purpose, they still know well that to keep utilitarian objects in view would only prove & handicap on their efforts. consequently they never ask in what way their science is going to benefit mankind. as the great captain of industry is moved by the love of wealth, and the political leader by the love of power over men, so the astronomer is moved by the love of knowledge for its own sake, and not for the sake of its useful applications. yet he is proud to know that his science has been worth more to mankind than it has cost. he does not value its results merely as a means of crossing the ocean or mapping the country, for he feels that man does not live by bread alone. if it is not more than bread to know the place we occupy in the universe, it is certainly something which we should place not far behind the means of subsistence. that we now look upon a comet as something very interesting, of which the sight affords us a pleasure unmixed with fear of war, pestilence, or other calamity, and of which we therefore wish the return, is a gain we cannot measure by money. in all ages astronomy has been an index to the civilization of the people who cultivated it. it has been crude or exact, enlightened or mingled with superstition, according to the current mode of thought. when once men understand the relation of the planet on which they dwell to the universe at large, superstition is doomed to speedy extinction. this alone is an object worth more than money. astronomy may fairly claim to be that science which transcends all others in its demands upon the practical application of our reasoning powers. look at the stars that stud the heavens on a clear evening. what more hopeless problem to one confined to earth than that of determining their varying distances, their motions, and their physical constitution? everything on earth we can handle and investigate. but how investigate that which is ever beyond our reach, on which we can never make an experiment? on certain occasions we see the moon pass in front of the sun and hide it from our eyes. to an observer a few miles away the sun was not entirely hidden, for the shadow of the moon in a total eclipse is rarely one hundred miles wide. on another continent no eclipse at all may have been visible. who shall take a map of the world and mark upon it the line on which the moon's shadow will travel during some eclipse a hundred years hence? who shall map out the orbits of the heavenly bodies as they are going to appear in a hundred thousand years? how shall we ever know of what chemical elements the sun and the stars are made? all this has been done, but not by the intellect of any one man. the road to the stars has been opened only by the efforts of many generations of mathematicians and observers, each of whom began where his predecessor had left off. we have reached a stage where we know much of the heavenly bodies. we have mapped out our solar system with great precision. but how with that great universe of millions of stars in which our solar system is only a speck of star-dust, a speck which a traveller through the wilds of space might pass a hundred times without notice? we have learned much about this universe, though our knowledge of it is still dim. we see it as a traveller on a mountain-top sees a distant city in a cloud of mist, by a few specks of glimmering light from steeples or roofs. we want to know more about it, its origin and its destiny; its limits in time and space, if it has any; what function it serves in the universal economy. the journey is long, yet we want, in knowledge at least, to make it. hence we build observatories and train observers and investigators. slow, indeed, is progress in the solution of the greatest of problems, when measured by what we want to know. some questions may require centuries, others thousands of years for their answer. and yet never was progress more rapid than during our time. in some directions our astronomers of to-day are out of sight of those of fifty years ago; we are even gaining heights which twenty years ago looked hopeless. never before had the astronomer so much work--good, hard, yet hopeful work--before him as to-day. he who is leaving the stage feels that he has only begun and must leave his successors with more to do than his predecessors left him. to us an interesting feature of this progress is the part taken in it by our own country. the science of our day, it is true, is of no country. yet we very appropriately speak of american science from the fact that our traditional reputation has not been that of a people deeply interested in the higher branches of intellectual work. men yet living can remember when in the eyes of the universal church of learning, all cisatlantic countries, our own included, were partes infidelium. yet american astronomy is not entirely of our generation. in the middle of the last century professor winthrop, of harvard, was an industrious observer of eclipses and kindred phenomena, whose work was recorded in the transactions of learned societies. but the greatest astronomical activity during our colonial period was that called out by the transit of venus in , which was visible in this country. a committee of the american philosophical society, at philadelphia, organized an excellent system of observations, which we now know to have been fully as successful, perhaps more so, than the majority of those made on other continents, owing mainly to the advantages of air and climate. among the observers was the celebrated rittenhouse, to whom is due the distinction of having been the first american astronomer whose work has an important place in the history of the science. in addition to the observations which he has left us, he was the first inventor or proposer of the collimating telescope, an instrument which has become almost a necessity wherever accurate observations are made. the fact that the subsequent invention by bessel may have been independent does not detract from the merits of either. shortly after the transit of venus, which i have mentioned, the war of the revolution commenced. the generation which carried on that war and the following one, which framed our constitution and laid the bases of our political institutions, were naturally too much occupied with these great problems to pay much attention to pure science. while the great mathematical astronomers of europe were laying the foundation of celestial mechanics their writings were a sealed book to every one on this side of the atlantic, and so remained until bowditch appeared, early in the present century. his translation of the mecanique celeste made an epoch in american science by bringing the great work of laplace down to the reach of the best american students of his time. american astronomers must always honor the names of rittenhouse and bowditch. and yet in one respect their work was disappointing of results. neither of them was the founder of a school. rittenhouse left no successor to carry on his work. the help which bowditch afforded his generation was invaluable to isolated students who, here and there, dived alone and unaided into the mysteries of the celestial motions. his work was not mainly in the field of observational astronomy, and therefore did not materially influence that branch of science. in professor airy, afterwards astronomer royal of england, made a report to the british association on the condition of practical astronomy in various countries. in this report he remarked that he was unable to say anything about american astronomy because, so far as he knew, no public observatory existed in the united states. william c. bond, afterwards famous as the first director of the harvard observatory, was at that time making observations with a small telescope, first near boston and afterwards at cambridge. but with so meagre an outfit his establishment could scarcely lay claim to being an astronomical observatory, and it was not surprising if airy did not know anything of his modest efforts. if at this time professor airy had extended his investigations into yet another field, with a view of determining the prospects for a great city at the site of fort dearborn, on the southern shore of lake michigan, he would have seen as little prospect of civic growth in that region as of a great development of astronomy in the united states at large. a plat of the proposed town of chicago had been prepared two years before, when the place contained perhaps half a dozen families. in the same month in which professor airy made his report, august, , the people of the place, then numbering twenty-eight voters, decided to become incorporated, and selected five trustees to carry on their government. in a city charter was obtained from the legislature of illinois. the growth of this infant city, then small even for an infant, into the great commercial metropolis of the west has been the just pride of its people and the wonder of the world. i mention it now because of a remarkable coincidence. with this civic growth has quietly gone on another, little noted by the great world, and yet in its way equally wonderful and equally gratifying to the pride of those who measure greatness by intellectual progress. taking knowledge of the universe as a measure of progress, i wish to invite attention to the fact that american astronomy began with your city, and has slowly but surely kept pace with it, until to-day our country stands second only to germany in the number of researches being prosecuted, and second to none in the number of men who have gained the highest recognition by their labors. in professor albert hopkins, of williams college, and professor elias loomis, of western reserve college, ohio, both commenced little observatories. professor loomis went to europe for all his instruments, but hopkins was able even then to get some of his in this country. shortly afterwards a little wooden structure was erected by captain gilliss on capitol hill, at washington, and supplied with a transit instrument for observing moon culminations, in conjunction with captain wilkes, who was then setting out on his exploring expedition to the southern hemisphere. the date of these observatories was practically the same as that on which a charter for the city of chicago was obtained from the legislature. with their establishment the population of your city had increased to . the next decade, to , was that in which our practical astronomy seriously commenced. the little observatory of captain gilliss was replaced by the naval, then called the national observatory, erected at washington during the years - , and fitted out with what were then the most approved instruments. about the same time the appearance of the great comet of led the citizens of boston to erect the observatory of harvard college. thus it is little more than a half-century since the two principal observatories in the united states were established. but we must not for a moment suppose that the mere erection of an observatory can mark an epoch in scientific history. what must make the decade of which i speak ever memorable in american astronomy was not merely the erection of buildings, but the character of the work done by astronomers away from them as well as in them. the national observatory soon became famous by two remarkable steps which raised our country to an important position among those applying modern science to practical uses. one of these consisted of the researches of sears cook walker on the motion of the newly discovered planet neptune. he was the first astronomer to determine fairly good elements of the orbit of that planet, and, what is yet more remarkable, he was able to trace back the movement of the planet in the heavens for half a century and to show that it had been observed as a fixed star by lalande in , without the observer having any suspicion of the true character of the object. the other work to which i refer was the application to astronomy and to the determination of longitudes of the chronographic method of registering transits of stars or other phenomena requiring an exact record of the instant of their occurrence. it is to be regretted that the history of this application has not been fully written. in some points there seems to be as much obscurity as with the discovery of ether as an anaesthetic, which took place about the same time. happily, no such contest has been fought over the astronomical as over the surgical discovery, the fact being that all who were engaged in the application of the new method were more anxious to perfect it than they were to get credit for themselves. we know that saxton, of the coast survey; mitchell and locke, of cincinnati; bond, at cambridge, as well as walker, and other astronomers at the naval observatory, all worked at the apparatus; that maury seconded their efforts with untiring zeal; that it was used to determine the longitude of baltimore as early as by captain wilkes, and that it was put into practical use in recording observations at the naval observatory as early as . at the cambridge observatory the two bonds, father and son, speedily began to show the stuff of which the astronomer is made. a well-devised system of observations was put in operation. the discovery of the dark ring of saturn and of a new satellite to that planet gave additional fame to the establishment. nor was activity confined to the observational side of the science. the same decade of which i speak was marked by the beginning of professor pierce's mathematical work, especially his determination of the perturbations of uranus and neptune. at this time commenced the work of dr. b. a. gould, who soon became the leading figure in american astronomy. immediately on graduating at harvard in , he determined to devote all the energies of his life to the prosecution of his favorite science. he studied in europe for three years, took the doctor's degree at gottingen, came home, founded the astronomical journal, and took an active part in that branch of the work of the coast survey which included the determination of longitudes by astronomical methods. an episode which may not belong to the history of astronomy must be acknowledged to have had a powerful influence in exciting public interest in that science. professor o. m. mitchell, the founder and first director of the cincinnati observatory, made the masses of our intelligent people acquainted with the leading facts of astronomy by courses of lectures which, in lucidity and eloquence, have never been excelled. the immediate object of the lectures was to raise funds for establishing his observatory and fitting it out with a fine telescope. the popular interest thus excited in the science had an important effect in leading the public to support astronomical research. if public support, based on public interest, is what has made the present fabric of american astronomy possible, then should we honor the name of a man whose enthusiasm leavened the masses of his countrymen with interest in our science. the civil war naturally exerted a depressing influence upon our scientific activity. the cultivator of knowledge is no less patriotic than his fellow-citizens, and vies with them in devotion to the public welfare. the active interest which such cultivators took, first in the prosecution of the war and then in the restoration of the union, naturally distracted their attention from their favorite pursuits. but no sooner was political stability reached than a wave of intellectual activity set in, which has gone on increasing up to the present time. if it be true that never before in our history has so much attention been given to education as now; that never before did so many men devote themselves to the diffusion of knowledge, it is no less true that never was astronomical work so energetically pursued among us as at the present time. one deplorable result of the civil war was that gould's astronomical journal had to be suspended. shortly after the restoration of peace, instead of re-establishing the journal, its founder conceived the project of exploring the southern heavens. the northern hemisphere being the seat of civilization, that portion of the sky which could not be seen from our latitudes was comparatively neglected. what had been done in the southern hemisphere was mostly the occasional work of individuals and of one or two permanent observatories. the latter were so few in number and so meagre in their outfit that a splendid field was open to the inquirer. gould found the patron which he desired in the government of the argentine republic, on whose territory he erected what must rank in the future as one of the memorable astronomical establishments of the world. his work affords a most striking example of the principle that the astronomer is more important than his instruments. not only were the means at the command of the argentine observatory slender in the extreme when compared with those of the favored institutions of the north, but, from the very nature of the case, the argentine republic could not supply trained astronomers. the difficulties thus growing out of the administration cannot be overestimated. and yet the sixteen great volumes in which the work of the institution has been published will rank in the future among the classics of astronomy. another wonderful focus of activity, in which one hardly knows whether he ought most to admire the exhaustless energy or the admirable ingenuity which he finds displayed, is the harvard observatory. its work has been aided by gifts which have no parallel in the liberality that prompted them. yet without energy and skill such gifts would have been useless. the activity of the establishment includes both hemispheres. time would fail to tell how it has not only mapped out important regions of the heavens from the north to the south pole, but analyzed the rays of light which come from hundreds of thousands of stars by recording their spectra in permanence on photographic plates. the work of the establishment is so organized that a new star cannot appear in any part of the heavens nor a known star undergo any noteworthy change without immediate detection by the photographic eye of one or more little telescopes, all-seeing and never-sleeping policemen that scan the heavens unceasingly while the astronomer may sleep, and report in the morning every case of irregularity in the proceedings of the heavenly bodies. yet another example, showing what great results may be obtained with limited means, is afforded by the lick observatory, on mount hamilton, california. during the ten years of its activity its astronomers have made it known the world over by works and discoveries too varied and numerous to be even mentioned at the present time. the astronomical work of which i have thus far spoken has been almost entirely that done at observatories. i fear that i may in this way have strengthened an erroneous impression that the seat of important astronomical work is necessarily connected with an observatory. it must be admitted that an institution which has a local habitation and a magnificent building commands public attention so strongly that valuable work done elsewhere may be overlooked. a very important part of astronomical work is done away from telescopes and meridian circles and requires nothing but a good library for its prosecution. one who is devoted to this side of the subject may often feel that the public does not appreciate his work at its true relative value from the very fact that he has no great buildings or fine instruments to show. i may therefore be allowed to claim as an important factor in the american astronomy of the last half-century an institution of which few have heard and which has been overlooked because there was nothing about it to excite attention. in the american nautical almanac office was established by a congressional appropriation. the title of this publication is somewhat misleading in suggesting a simple enlargement of the family almanac which the sailor is to hang up in his cabin for daily use. the fact is that what started more than a century ago as a nautical almanac has since grown into an astronomical ephemeris for the publication of everything pertaining to times, seasons, eclipses, and the motions of the heavenly bodies. it is the work in which astronomical observations made in all the great observatories of the world are ultimately utilized for scientific and public purposes. each of the leading nations of western europe issues such a publication. when the preparation and publication of the american ephemeris was decided upon the office was first established in cambridge, the seat of harvard university, because there could most readily be secured the technical knowledge of mathematics and theoretical astronomy necessary for the work. a field of activity was thus opened, of which a number of able young men who have since earned distinction in various walks of life availed themselves. the head of the office, commander davis, adopted a policy well fitted to promote their development. he translated the classic work of gauss, theoria motus corporum celestium, and made the office a sort of informal school, not, indeed, of the modern type, but rather more like the classic grove of hellas, where philosophers conducted their discussions and profited by mutual attrition. when, after a few years of experience, methods were well established and a routine adopted, the office was removed to washington, where it has since remained. the work of preparing the ephemeris has, with experience, been reduced to a matter of routine which may be continued indefinitely, with occasional changes in methods and data, and improvements to meet the increasing wants of investigators. the mere preparation of the ephemeris includes but a small part of the work of mathematical calculation and investigation required in astronomy. one of the great wants of the science to-day is the reduction of the observations made during the first half of the present century, and even during the last half of the preceding one. the labor which could profitably be devoted to this work would be more than that required in any one astronomical observatory. it is unfortunate for this work that a great building is not required for its prosecution because its needfulness is thus very generally overlooked by that portion of the public interested in the progress of science. an organization especially devoted to it is one of the scientific needs of our time. in such an epoch-making age as the present it is dangerous to cite any one step as making a new epoch. yet it may be that when the historian of the future reviews the science of our day he will find the most remarkable feature of the astronomy of the last twenty years of our century to be the discovery that this steadfast earth of which the poets have told us is not, after all, quite steadfast; that the north and south poles move about a very little, describing curves so complicated that they have not yet been fully marked out. the periodic variations of latitude thus brought about were first suspected about , and announced with some modest assurance by kustner, of berlin, a few years later. the progress of the views of astronomical opinion from incredulity to confidence was extremely slow until, about , chandler, of the united states, by an exhaustive discussion of innumerable results of observations, showed that the latitude of every point on the earth was subject to a double oscillation, one having a period of a year, the other of four hundred and twenty-seven days. notwithstanding the remarkable parallel between the growth of american astronomy and that of your city, one cannot but fear that if a foreign observer had been asked only half a dozen years ago at what point in the united states a great school of theoretical and practical astronomy, aided by an establishment for the exploration of the heavens, was likely to be established by the munificence of private citizens, he would have been wiser than most foreigners had he guessed chicago. had this place been suggested to him, i fear he would have replied that were it possible to utilize celestial knowledge in acquiring earthly wealth, here would be the most promising seat for such a school. but he would need to have been a little wiser than his generation to reflect that wealth is at the base of all progress in knowledge and the liberal arts; that it is only when men are relieved from the necessity of devoting all their energies to the immediate wants of life that they can lead the intellectual life, and that we should therefore look to the most enterprising commercial centre as the likeliest seat for a great scientific institution. now we have the school, and we have the observatory, which we hope will in the near future do work that will cast lustre on the name of its founder as well as on the astronomers who may be associated with it. you will, i am sure, pardon me if i make some suggestions on the subject of the future needs of the establishment. we want this newly founded institution to be a great success, to do work which shall show that the intellectual productiveness of your community will not be allowed to lag behind its material growth the public is very apt to feel that when some munificent patron of science has mounted a great telescope under a suitable dome, and supplied all the apparatus which the astronomer wants to use, success is assured. but such is not the case. the most important requisite, one more difficult to command than telescopes or observatories, may still be wanting. a great telescope is of no use without a man at the end of it, and what the telescope may do depends more upon this appendage than upon the instrument itself. the place which telescopes and observatories have taken in astronomical history are by no means proportional to their dimensions. many a great instrument has been a mere toy in the hands of its owner. many a small one has become famous. twenty years ago there was here in your own city a modest little instrument which, judged by its size, could not hold up its head with the great ones even of that day. it was the private property of a young man holding no scientific position and scarcely known to the public. and yet that little telescope is to-day among the famous ones of the world, having made memorable advances in the astronomy of double stars, and shown its owner to be a worthy successor of the herschels and struves in that line of work. a hundred observers might have used the appliances of the lick observatory for a whole generation without finding the fifth satellite of jupiter; without successfully photographing the cloud forms of the milky way; without discovering the extraordinary patches of nebulous light, nearly or quite invisible to the human eye, which fill some regions of the heavens. when i was in zurich last year i paid a visit to the little, but not unknown, observatory of its famous polytechnic school. the professor of astronomy was especially interested in the observations of the sun with the aid of the spectroscope, and among the ingenious devices which he described, not the least interesting was the method of photographing the sun by special rays of the spectrum, which had been worked out at the kenwood observatory in chicago. the kenwood observatory is not, i believe, in the eye of the public, one of the noteworthy institutions of your city which every visitor is taken to see, and yet this invention has given it an important place in the science of our day. should you ask me what are the most hopeful features in the great establishment which you are now dedicating, i would say that they are not alone to be found in the size of your unequalled telescope, nor in the cost of the outfit, but in the fact that your authorities have shown their appreciation of the requirements of success by adding to the material outfit of the establishment the three men whose works i have described. gentlemen of the trustees, allow me to commend to your fostering care the men at the end of the telescope. the constitution of the astronomer shows curious and interesting features. if he is destined to advance the science by works of real genius, he must, like the poet, be born, not made. the born astronomer, when placed in command of a telescope, goes about using it as naturally and effectively as the babe avails itself of its mother's breast. he sees intuitively what less gifted men have to learn by long study and tedious experiment. he is moved to celestial knowledge by a passion which dominates his nature. he can no more avoid doing astronomical work, whether in the line of observations or research, than a poet can chain his pegasus to earth. i do not mean by this that education and training will be of no use to him. they will certainly accelerate his early progress. if he is to become great on the mathematical side, not only must his genius have a bend in that direction, but he must have the means of pursuing his studies. and yet i have seen so many failures of men who had the best instruction, and so many successes of men who scarcely learned anything of their teachers, that i sometimes ask whether the great american celestial mechanician of the twentieth century will be a graduate of a university or of the backwoods. is the man thus moved to the exploration of nature by an unconquerable passion more to be envied or pitied? in no other pursuit does success come with such certainty to him who deserves it. no life is so enjoyable as that whose energies are devoted to following out the inborn impulses of one's nature. the investigator of truth is little subject to the disappointments which await the ambitious man in other fields of activity. it is pleasant to be one of a brotherhood extending over the world, in which no rivalry exists except that which comes out of trying to do better work than any one else, while mutual admiration stifles jealousy. and yet, with all these advantages, the experience of the astronomer may have its dark side. as he sees his field widening faster than he can advance he is impressed with the littleness of all that can be done in one short life. he feels the same want of successors to pursue his work that the founder of a dynasty may feel for heirs to occupy his throne. he has no desire to figure in history as a napoleon of science whose conquests must terminate with his life. even during his active career his work may be such a kind as to require the co-operation of others and the active support of the public. if he is disappointed in commanding these requirements, if he finds neither co-operation nor support, if some great scheme to which he may have devoted much of his life thus proves to be only a castle in the air, he may feel that nature has dealt hardly with him in not endowing him with passions like to those of other men. in treating a theme of perennial interest one naturally tries to fancy what the future may have in store if the traveller, contemplating the ruins of some ancient city which in the long ago teemed with the life and activities of generations of men, sees every stone instinct with emotion and the dust alive with memories of the past, may he not be similarly impressed when he feels that he is looking around upon a seat of future empire--a region where generations yet unborn may take a leading part in moulding the history of the world? what may we not expect of that energy which in sixty years has transformed a straggling village into one of the world's great centres of commerce? may it not exercise a powerful influence on the destiny not only of the country but of the world? if so, shall the power thus to be exercised prove an agent of beneficence, diffusing light and life among nations, or shall it be the opposite? the time must come ere long when wealth shall outgrow the field in which it can be profitably employed. in what direction shall its possessors then look? shall they train a posterity which will so use its power as to make the world better that it has lived in it? will the future heir to great wealth prefer the intellectual life to the life of pleasure? we can have no more hopeful answer to these questions than the establishment of this great university in the very focus of the commercial activity of the west. its connection with the institution we have been dedicating suggests some thoughts on science as a factor in that scheme of education best adapted to make the power of a wealthy community a benefit to the race at large. when we see what a factor science has been in our present civilization, how it has transformed the world and increased the means of human enjoyment by enabling men to apply the powers of nature to their own uses, it is not wonderful that it should claim the place in education hitherto held by classical studies. in the contest which has thus arisen i take no part but that of a peace-maker, holding that it is as important to us to keep in touch with the traditions of our race, and to cherish the thoughts which have come down to us through the centuries, as it is to enjoy and utilize what the present has to offer us. speaking from this point of view, i would point out the error of making the utilitarian applications of knowledge the main object in its pursuit. it is an historic fact that abstract science--science pursued without any utilitarian end--has been at the base of our progress in the utilization of knowledge. if in the last century such men as galvani and volta had been moved by any other motive than love of penetrating the secrets of nature they would never have pursued the seemingly useless experiments they did, and the foundation of electrical science would not have been laid. our present applications of electricity did not become possible until ohm's mathematical laws of the electric current, which when first made known seemed little more than mathematical curiosities, had become the common property of inventors. professional pride on the part of our own henry led him, after making the discoveries which rendered the telegraph possible, to go no further in their application, and to live and die without receiving a dollar of the millions which the country has won through his agency. in the spirit of scientific progress thus shown we have patriotism in its highest form--a sentiment which does not seek to benefit the country at the expense of the world, but to benefit the world by means of one's country. science has its competition, as keen as that which is the life of commerce. but its rivalries are over the question who shall contribute the most and the best to the sum total of knowledge; who shall give the most, not who shall take the most. its animating spirit is love of truth. its pride is to do the greatest good to the greatest number. it embraces not only the whole human race but all nature in its scope. the public spirit of which this city is the focus has made the desert blossom as the rose, and benefited humanity by the diffusion of the material products of the earth. should you ask me how it is in the future to use its influence for the benefit of humanity at large, i would say, look at the work now going on in these precincts, and study its spirit. here are the agencies which will make "the voice of law the harmony of the world." here is the love of country blended with love of the race. here the love of knowledge is as unconfined as your commercial enterprise. let not your youth come hither merely to learn the forms of vertebrates and the properties of oxides, but rather to imbibe that catholic spirit which, animating their growing energies, shall make the power they are to wield an agent of beneficence to all mankind. xix the universe as an organism [footnote: address before the astronomical and astrophysical society of america, december , ] if i were called upon to convey, within the compass of a single sentence, an idea of the trend of recent astronomical and physical science, i should say that it was in the direction of showing the universe to be a connected whole. the farther we advance in knowledge, the clearer it becomes that the bodies which are scattered through the celestial spaces are not completely independent existences, but have, with all their infinite diversity, many attributes in common. in this we are going in the direction of certain ideas of the ancients which modern discovery long seemed to have contradicted. in the infancy of the race, the idea that the heavens were simply an enlarged and diversified earth, peopled by beings who could roam at pleasure from one extreme to the other, was a quite natural one. the crystalline sphere or spheres which contained all formed a combination of machinery revolving on a single plan. but all bonds of unity between the stars began to be weakened when copernicus showed that there were no spheres, that the planets were isolated bodies, and that the stars were vastly more distant than the planets. as discovery went on and our conceptions of the universe were enlarged, it was found that the system of the fixed stars was made up of bodies so vastly distant and so completely isolated that it was difficult to conceive of them as standing in any definable relation to one another. it is true that they all emitted light, else we could not see them, and the theory of gravitation, if extended to such distances, a fact not then proved, showed that they acted on one another by their mutual gravitation. but this was all. leaving out light and gravitation, the universe was still, in the time of herschel, composed of bodies which, for the most part, could not stand in any known relation one to the other. when, forty years ago, the spectroscope was applied to analyze the light coming from the stars, a field was opened not less fruitful than that which the telescope made known to galileo. the first conclusion reached was that the sun was composed almost entirely of the same elements that existed upon the earth. yet, as the bodies of our solar system were evidently closely related, this was not remarkable. but very soon the same conclusion was, to a limited extent, extended to the fixed stars in general. such elements as iron, hydrogen, and calcium were found not to belong merely to our earth, but to form important constituents of the whole universe. we can conceive of no reason why, out of the infinite number of combinations which might make up a spectrum, there should not be a separate kind of matter for each combination. so far as we know, the elements might merge into one another by insensible gradations. it is, therefore, a remarkable and suggestive fact when we find that the elements which make up bodies so widely separate that we can hardly imagine them having anything in common, should be so much the same. in recent times what we may regard as a new branch of astronomical science is being developed, showing a tendency towards unity of structure throughout the whole domain of the stars. this is what we now call the science of stellar statistics. the very conception of such a science might almost appall us by its immensity. the widest statistical field in other branches of research is that occupied by sociology. every country has its census, in which the individual inhabitants are classified on the largest scale and the combination of these statistics for different countries may be said to include all the interest of the human race within its scope. yet this field is necessarily confined to the surface of our planet. in the field of stellar statistics millions of stars are classified as if each taken individually were of no more weight in the scale than a single inhabitant of china in the scale of the sociologist. and yet the most insignificant of these suns may, for aught we know, have planets revolving around it, the interests of whose inhabitants cover as wide a range as ours do upon our own globe. the statistics of the stars may be said to have commenced with herschel's gauges of the heavens, which were continued from time to time by various observers, never, however, on the largest scale. the subject was first opened out into an illimitable field of research through a paper presented by kapteyn to the amsterdam academy of sciences in . the capital results of this paper were that different regions of space contain different kinds of stars and, more especially, that the stars of the milky way belong, in part at least, to a different class from those existing elsewhere. stars not belonging to the milky way are, in large part, of a distinctly different class. the outcome of kapteyn's conclusions is that we are able to describe the universe as a single object, with some characters of an organized whole. a large part of the stars which compose it may be considered as divisible into two groups. one of these comprises the stars composing the great girdle of the milky way. these are distinguished from the others by being bluer in color, generally greater in absolute brilliancy, and affected, there is some reason to believe, with rather slower proper motions the other classes are stars with a greater or less shade of yellow in their color, scattered through a spherical space of unknown dimensions, but concentric with the milky way. thus a sphere with a girdle passing around it forms the nearest approach to a conception of the universe which we can reach to-day. the number of stars in the girdle is much greater than that in the sphere. the feature of the universe which should therefore command our attention is the arrangement of a large part of the stars which compose it in a ring, seemingly alike in all its parts, so far as general features are concerned. so far as research has yet gone, we are not able to say decisively that one region of this ring differs essentially from another. it may, therefore, be regarded as forming a structure built on a uniform plan throughout. all scientific conclusions drawn from statistical data require a critical investigation of the basis on which they rest. if we are going, from merely counting the stars, observing their magnitudes and determining their proper motions, to draw conclusions as to the structure of the universe in space, the question may arise how we can form any estimate whatever of the possible distance of the stars, a conclusion as to which must be the very first step we take. we can hardly say that the parallaxes of more than one hundred stars have been measured with any approach to certainty. the individuals of this one hundred are situated at very different distances from us. we hope, by long and repeated observations, to make a fairly approximate determination of the parallaxes of all the stars whose distance is less than twenty times that of a centauri. but how can we know anything about the distance of stars outside this sphere? what can we say against the view of kepler that the space around our sun is very much thinner in stars than it is at a greater distance; in fact, that, the great mass of the stars may be situated between the surfaces of two concentrated spheres not very different in radius. may not this universe of stars be somewhat in the nature of a hollow sphere? this objection requires very careful consideration on the part of all who draw conclusions as to the distribution of stars in space and as to the extent of the visible universe. the steps to a conclusion on the subject are briefly these: first, we have a general conclusion, the basis of which i have already set forth, that, to use a loose expression, there are likenesses throughout the whole diameter of the universe. there is, therefore, no reason to suppose that the region in which our system is situated differs in any essential degree from any other region near the central portion. again, spectroscopic examinations seem to show that all the stars are in motion, and that we cannot say that those in one part of the universe move more rapidly than those in another. this result is of the greatest value for our purpose, because, when we consider only the apparent motions, as ordinarily observed, these are necessarily dependent upon the distance of the star. we cannot, therefore, infer the actual speed of a star from ordinary observations until we know its distance. but the results of spectroscopic measurements of radial velocity are independent of the distance of the star. but let us not claim too much. we cannot yet say with certainty that the stars which form the agglomerations of the milky way have, beyond doubt, the same average motion as the stars in other regions of the universe. the difficulty is that these stars appear to us so faint individually, that the investigation of their spectra is still beyond the powers of our instruments. but the extraordinary feat performed at the lick observatory of measuring the radial motion of groombridge, a star quite invisible to the naked eye, and showing that it is approaching our system with a speed of between fifty and sixty miles a second, may lead us to hope for a speedy solution of this question. but we need not await this result in order to reach very probable conclusions. the general outcome of researches on proper motions tends to strengthen the conclusions that the keplerian sphere, if i may use this expression, has no very well marked existence. the laws of stellar velocity and the statistics of proper motions, while giving some color to the view that the space in which we are situated is thinner in stars than elsewhere, yet show that, as a general rule, there are no great agglomerations of stars elsewhere than in the region of the milky way. with unity there is always diversity; in fact, the unity of the universe on which i have been insisting consists in part of diversity. it is very curious that, among the many thousands of stars which have been spectroscopically examined, no two are known to have absolutely the same physical constitution. it is true that there are a great many resemblances. alpha centauri, our nearest neighbor, if we can use such a word as "near" in speaking of its distance, has a spectrum very like that of our sun, and so has capella. but even in these cases careful examination shows differences. these differences arise from variety in the combinations and temperature of the substances of which the star is made up. quite likely also, elements not known on the earth may exist on the stars, but this is a point on which we cannot yet speak with certainty. perhaps the attribute in which the stars show the greatest variety is that of absolute luminosity. one hundred years ago it was naturally supposed that the brighter stars were the nearest to us, and this is doubtless true when we take the general average. but it was soon found that we cannot conclude that because a star is bright, therefore it is near. the most striking example of this is afforded by the absence of measurable parallaxes in the two bright stars, canopus and rigel, showing that these stars, though of the first magnitude, are immeasurably distant. a remarkable fact is that these conclusions coincide with that which we draw from the minuteness of the proper motions. rigel has no motion that has certainly been shown by more than a century of observation, and it is not certain that canopus has either. from this alone we may conclude, with a high degree of probability, that the distance of each is immeasurably great. we may say with certainty that the brightness of each is thousands of times that of the sun, and with a high degree of probability that it is hundreds of thousands of times. on the other hand, there are stars comparatively near us of which the light is not the hundredth part of the sun. [illustration with caption: star spectra] the universe may be a unit in two ways. one is that unity of structure to which our attention has just been directed. this might subsist forever without one body influencing another. the other form of unity leads us to view the universe as an organism. it is such by mutual action going on between its bodies. a few years ago we could hardly suppose or imagine that any other agents than gravitation and light could possibly pass through spaces so immense as those which separate the stars. the most remarkable and hopeful characteristic of the unity of the universe is the evidence which is being gathered that there are other agencies whose exact nature is yet unknown to us, but which do pass from one heavenly body to another. the best established example of this yet obtained is afforded in the case of the sun and the earth. the fact that the frequency of magnetic storms goes through a period of about eleven years, and is proportional to the frequency of sun-spots, has been well established. the recent work of professor bigelow shows the coincidence to be of remarkable exactness, the curves of the two phenomena being practically coincident so far as their general features are concerned. the conclusion is that spots on the sun and magnetic storms are due to the same cause. this cause cannot be any change in the ordinary radiation of the sun, because the best records of temperature show that, to whatever variations the sun's radiation may be subjected, they do not change in the period of the sun-spots. to appreciate the relation, we must recall that the researches of hale with the spectro-heliograph show that spots are not the primary phenomenon of solar activity, but are simply the outcome of processes going on constantly in the sun which result in spots only in special regions and on special occasions. it does not, therefore, necessarily follow that a spot does cause a magnetic storm. what we should conclude is that the solar activity which produces a spot also produces the magnetic storm. when we inquire into the possible nature of these relations between solar activity and terrestrial magnetism, we find ourselves so completely in the dark that the question of what is really proved by the coincidence may arise. perhaps the most obvious explanation of fluctuations in the earth's magnetic field to be inquired into would be based on the hypothesis that the space through which the earth is moving is in itself a varying magnetic field of vast extent. this explanation is tested by inquiring whether the fluctuations in question can be explained by supposing a disturbing force which acts substantially in the same direction all over the globe. but a very obvious test shows that this explanation is untenable. were it the correct one, the intensity of the force in some regions of the earth would be diminished and in regions where the needle pointed in the opposite direction would be increased in exactly the same degree. but there is no relation traceable either in any of the regular fluctuations of the magnetic force, or in those irregular ones which occur during a magnetic storm. if the horizontal force is increased in one part of the earth, it is very apt to show a simultaneous increase the world over, regardless of the direction in which the needle may point in various localities. it is hardly necessary to add that none of the fluctuations in terrestrial magnetism can be explained on the hypothesis that either the moon or the sun acts as a magnet. in such a case the action would be substantially in the same direction at the same moment the world over. such being the case, the question may arise whether the action producing a magnetic storm comes from the sun at all, and whether the fluctuations in the sun's activity, and in the earth's magnetic field may not be due to some cause external to both. all we can say in reply to this is that every effort to find such a cause has failed and that it is hardly possible to imagine any cause producing such an effect. it is true that the solar spots were, not many years ago, supposed to be due in some way to the action of the planets. but, for reasons which it would be tedious to go into at present, we may fairly regard this hypothesis as being completely disproved. there can, i conclude, be little doubt that the eleven-year cycle of change in the solar spots is due to a cycle going on in the sun itself. such being the case, the corresponding change in the earth's magnetism must be due to the same cause. we may, therefore, regard it as a fact sufficiently established to merit further investigation that there does emanate from the sun, in an irregular way, some agency adequate to produce a measurable effect on the magnetic needle. we must regard it as a singular fact that no observations yet made give us the slightest indication as to what this emanation is. the possibility of defining it is suggested by the discovery within the past few years, that under certain conditions, heated matter sends forth entities known as rontgen rays, becquerel corpuscles and electrons. i cannot speak authoritatively on this subject, but, so far as i am aware, no direct evidence has yet been gathered showing that any of these entities reach us from the sun. we must regard the search for the unknown agency so fully proved as among the most important tasks of the astronomical physicist of the present time. from what we know of the history of scientific discovery, it seems highly probable that, in the course of his search, he will, before he finds the object he is aiming at, discover many other things of equal or greater importance of which he had, at the outset, no conception. the main point i desire to bring out in this review is the tendency which it shows towards unification in physical research. heretofore differentiation--the subdivision of workers into a continually increasing number of groups of specialists--has been the rule. now we see a coming together of what, at first sight, seem the most widely separated spheres of activity. what two branches could be more widely separated than that of stellar statistics, embracing the whole universe within its scope, and the study of these newly discovered emanations, the product of our laboratories, which seem to show the existence of corpuscles smaller than the atoms of matter? and yet, the phenomena which we have reviewed, especially the relation of terrestrial magnetism to the solar activity, and the formation of nebulous masses around the new stars, can be accounted for only by emanations or forms of force, having probably some similarity with the corpuscles, electrons, and rays which we are now producing in our laboratories. the nineteenth century, in passing away, points with pride to what it has done. it has become a word to symbolize what is most important in human progress yet, perhaps its greatest glory may prove to be that the last thing it did was to lay a foundation for the physical science of the twentieth century. what shall be discovered in the new fields is, at present, as far without our ken as were the modern developments of electricity without the ken of the investigators of one hundred years ago. we cannot guarantee any special discovery. what lies before us is an illimitable field, the existence of which was scarcely suspected ten years ago, the exploration of which may well absorb the activities of our physical laboratories, and of the great mass of our astronomical observers and investigators for as many generations as were required to bring electrical science to its present state. we of the older generation cannot hope to see more than the beginning of this development, and can only tender our best wishes and most hearty congratulations to the younger school whose function it will be to explore the limitless field now before it. xx the relation of scientific method to social progress [footnote: an address before the washington philosophical society] among those subjects which are not always correctly apprehended, even by educated men, we may place that of the true significance of scientific method and the relations of such method to practical affairs. this is especially apt to be the case in a country like our own, where the points of contact between the scientific world on the one hand, and the industrial and political world on the other, are fewer than in other civilized countries. the form which this misapprehension usually takes is that of a failure to appreciate the character of scientific method, and especially its analogy to the methods of practical life. in the judgment of the ordinary intelligent man there is a wide distinction between theoretical and practical science. the latter he considers as that science directly applicable to the building of railroads, the construction of engines, the invention of new machinery, the construction of maps, and other useful objects. the former he considers analogous to those philosophic speculations in which men have indulged in all ages without leading to any result which he considers practical. that our knowledge of nature is increased by its prosecution is a fact of which he is quite conscious, but he considers it as terminating with a mere increase of knowledge, and not as having in its method anything which a person devoted to material interests can be expected to appreciate. this view is strengthened by the spirit with which he sees scientific investigation prosecuted. it is well understood on all sides that when such investigations are pursued in a spirit really recognized as scientific, no merely utilitarian object is had in view. indeed, it is easy to see how the very fact of pursuing such an object would detract from that thoroughness of examination which is the first condition of a real advance. true science demands in its every research a completeness far beyond what is apparently necessary for its practical applications. the precision with which the astronomer seeks to measure the heavens and the chemist to determine the relations of the ultimate molecules of matter has no limit, except that set by the imperfections of the instruments of research. there is no such division recognized as that of useful and useless knowledge. the ultimate aim is nothing less than that of bringing all the phenomena of nature under laws as exact as those which govern the planetary motions. now the pursuit of any high object in this spirit commands from men of wide views that respect which is felt towards all exertion having in view more elevated objects than the pursuit of gain. accordingly, it is very natural to classify scientists and philosophers with the men who in all ages have sought after learning instead of utility. but there is another aspect of the question which will show the relations of scientific advance to the practical affairs of life in a different light. i make bold to say that the greatest want of the day, from a purely practical point of view, is the more general introduction of the scientific method and the scientific spirit into the discussion of those political and social problems which we encounter on our road to a higher plane of public well being. far from using methods too refined for practical purposes, what most distinguishes scientific from other thought is the introduction of the methods of practical life into the discussion of abstract general problems. a single instance will illustrate the lesson i wish to enforce. the question of the tariff is, from a practical point of view, one of the most important with which our legislators will have to deal during the next few years. the widest diversity of opinion exists as to the best policy to be pursued in collecting a revenue from imports. opposing interests contend against one another without any common basis of fact or principle on which a conclusion can be reached. the opinions of intelligent men differ almost as widely as those of the men who are immediately interested. but all will admit that public action in this direction should be dictated by one guiding principle--that the greatest good of the community is to be sought after. that policy is the best which will most promote this good. nor is there any serious difference of opinion as to the nature of the good to be had in view; it is in a word the increase of the national wealth and prosperity. the question on which opinions fundamentally differ is that of the effects of a higher or lower rate of duty upon the interests of the public. if it were possible to foresee, with an approach to certainty, what effect a given tariff would have upon the producers and consumers of an article taxed, and, indirectly, upon each member of the community in any way interested in the article, we should then have an exact datum which we do not now possess for reaching a conclusion. if some superhuman authority, speaking with the voice of infallibility, could give us this information, it is evident that a great national want would be supplied. no question in practical life is more important than this: how can this desirable knowledge of the economic effects of a tariff be obtained? the answer to this question is clear and simple. the subject must be studied in the same spirit, and, to a certain extent, by the same methods which have been so successful in advancing our knowledge of nature. every one knows that, within the last two centuries, a method of studying the course of nature has been introduced which has been so successful in enabling us to trace the sequence of cause and effect as almost to revolutionize society. the very fact that scientific method has been so successful here leads to the belief that it might be equally successful in other departments of inquiry. the same remarks will apply to the questions connected with banking and currency; the standard of value; and, indeed, all subjects which have a financial bearing. on every such question we see wide differences of opinion without any common basis to rest upon. it may be said, in reply, that in these cases there are really no grounds for forming an opinion, and that the contests which arise over them are merely those between conflicting interests. but this claim is not at all consonant with the form which we see the discussion assume. nearly every one has a decided opinion on these several subjects; whereas, if there were no data for forming an opinion, it would be unreasonable to maintain any whatever. indeed, it is evident that there must be truth somewhere, and the only question that can be open is that of the mode of discovering it. no man imbued with a scientific spirit can claim that such truth is beyond the power of the human intellect. he may doubt his own ability to grasp it, but cannot doubt that by pursuing the proper method and adopting the best means the problem can be solved. it is, in fact, difficult to show why some exact results could not be as certainly reached in economic questions as in those of physical science. it is true that if we pursue the inquiry far enough we shall find more complex conditions to encounter, because the future course of demand and supply enters as an uncertain element. but a remarkable fact to be considered is that the difference of opinion to which we allude does not depend upon different estimates of the future, but upon different views of the most elementary and general principles of the subject. it is as if men were not agreed whether air were elastic or whether the earth turns on its axis. why is it that while in all subjects of physical science we find a general agreement through a wide range of subjects, and doubt commences only where certainty is not attained, yet when we turn to economic subjects we do not find the beginning of an agreement? no two answers can be given. it is because the two classes of subjects are investigated by different instruments and in a different spirit. the physicist has an exact nomenclature; uses methods of research well adapted to the objects he has in view; pursues his investigations without being attacked by those who wish for different results; and, above all, pursues them only for the purpose of discovering the truth. in economic questions the case is entirely different. only in rare cases are they studied without at least the suspicion that the student has a preconceived theory to support. if results are attained which oppose any powerful interest, this interest can hire a competing investigator to bring out a different result. so far as the public can see, one man's result is as good as another's, and thus the object is as far off as ever. we may be sure that until there is an intelligent and rational public, able to distinguish between the speculations of the charlatan and the researches of the investigator, the present state of things will continue. what we want is so wide a diffusion of scientific ideas that there shall be a class of men engaged in studying economic problems for their own sake, and an intelligent public able to judge what they are doing. there must be an improvement in the objects at which they aim in education, and it is now worth while to inquire what that improvement is. it is not mere instruction in any branch of technical science that is wanted. no knowledge of chemistry, physics, or biology, however extensive, can give the learner much aid in forming a correct opinion of such a question as that of the currency. if we should claim that political economy ought to be more extensively studied, we would be met by the question, which of several conflicting systems shall we teach? what is wanted is not to teach this system or that, but to give such a training that the student shall be able to decide for himself which system is right. it seems to me that the true educational want is ignored both by those who advocate a classical and those who advocate a scientific education. what is really wanted is to train the intellectual powers, and the question ought to be, what is the best method of doing this? perhaps it might be found that both of the conflicting methods could be improved upon. the really distinctive features, which we should desire to see introduced, are two in number: the one the scientific spirit; the other the scientific discipline. although many details may be classified under each of these heads, yet there is one of pre-eminent importance on which we should insist. the one feature of the scientific spirit which outweighs all others in importance is the love of knowledge for its own sake. if by our system of education we can inculcate this sentiment we shall do what is, from a public point of view, worth more than any amount of technical knowledge, because we shall lay the foundation of all knowledge. so long as men study only what they think is going to be useful their knowledge will be partial and insufficient. i think it is to the constant inculcation of this fact by experience, rather than to any reasoning, that is due the continued appreciation of a liberal education. every business-man knows that a business-college training is of very little account in enabling one to fight the battle of life, and that college-bred men have a great advantage even in fields where mere education is a secondary matter. we are accustomed to seeing ridicule thrown upon the questions sometimes asked of candidates for the civil service because the questions refer to subjects of which a knowledge is not essential. the reply to all criticisms of this kind is that there is no one quality which more certainly assures a man's usefulness to society than the propensity to acquire useless knowledge. most of our citizens take a wide interest in public affairs, else our form of government would be a failure. but it is desirable that their study of public measures should be more critical and take a wider range. it is especially desirable that the conclusions to which they are led should be unaffected by partisan sympathies. the more strongly the love of mere truth is inculcated in their nature the better this end will be attained. the scientific discipline to which i ask mainly to call your attention consists in training the scholar to the scientific use of language. although whole volumes may be written on the logic of science there is one general feature of its method which is of fundamental significance. it is that every term which it uses and every proposition which it enunciates has a precise meaning which can be made evident by proper definitions. this general principle of scientific language is much more easily inculcated by example than subject to exact description; but i shall ask leave to add one to several attempts i have made to define it. if i should say that when a statement is made in the language of science the speaker knows what he means, and the hearer either knows it or can be made to know it by proper definitions, and that this community of understanding is frequently not reached in other departments of thought, i might be understood as casting a slur on whole departments of inquiry. without intending any such slur, i may still say that language and statements are worthy of the name scientific as they approach this standard; and, moreover, that a great deal is said and written which does not fulfil the requirement. the fact that words lose their meaning when removed from the connections in which that meaning has been acquired and put to higher uses, is one which, i think, is rarely recognized. there is nothing in the history of philosophical inquiry more curious than the frequency of interminable disputes on subjects where no agreement can be reached because the opposing parties do not use words in the same sense. that the history of science is not free from this reproach is shown by the fact of the long dispute whether the force of a moving body was proportional to the simple velocity or to its square. neither of the parties to the dispute thought it worth while to define what they meant by the word "force," and it was at length found that if a definition was agreed upon the seeming difference of opinion would vanish. perhaps the most striking feature of the case, and one peculiar to a scientific dispute, was that the opposing parties did not differ in their solution of a single mechanical problem. i say this is curious, because the very fact of their agreeing upon every concrete question which could have been presented ought to have made it clear that some fallacy was lacking in the discussion as to the measure of force. the good effect of a scientific spirit is shown by the fact that this discussion is almost unique in the history of science during the past two centuries, and that scientific men themselves were able to see the fallacy involved, and thus to bring the matter to a conclusion. if we now turn to the discussion of philosophers, we shall find at least one yet more striking example of the same kind. the question of the freedom of the human will has, i believe, raged for centuries. it cannot yet be said that any conclusion has been reached. indeed, i have heard it admitted by men of high intellectual attainments that the question was insoluble. now a curious feature of this dispute is that none of the combatants, at least on the affirmative side, have made any serious attempt to define what should be meant by the phrase freedom of the will, except by using such terms as require definition equally with the word freedom itself. it can, i conceive, be made quite clear that the assertion, "the will is free," is one without meaning, until we analyze more fully the different meanings to be attached to the word free. now this word has a perfectly well-defined signification in every-day life. we say that anything is free when it is not subject to external constraint. we also know exactly what we mean when we say that a man is free to do a certain act. we mean that if he chooses to do it there is no external constraint acting to prevent him. in all cases a relation of two things is implied in the word, some active agent or power, and the presence or absence of another constraining agent. now, when we inquire whether the will itself is free, irrespective of external constraints, the word free no longer has a meaning, because one of the elements implied in it is ignored. to inquire whether the will itself is free is like inquiring whether fire itself is consumed by the burning, or whether clothing is itself clad. it is not, therefore, at all surprising that both parties have been able to dispute without end, but it is a most astonishing phenomenon of the human intellect that the dispute should go on generation after generation without the parties finding out whether there was really any difference of opinion between them on the subject. i venture to say that if there is any such difference, neither party has ever analyzed the meaning of the words used sufficiently far to show it. the daily experience of every man, from his cradle to his grave, shows that human acts are as much the subject of external causal influences as are the phenomena of nature. to dispute this would be little short of the ludicrous. all that the opponents of freedom, as a class, have ever claimed is the assertion of a causal connection between the acts of the will and influences independent of the will. true, propositions of this sort can be expressed in a variety of ways connoting an endless number of more or less objectionable ideas, but this is the substance of the matter. to suppose that the advocates on the other side meant to take issue on this proposition would be to assume that they did not know what they were saying. the conclusion forced upon us is that though men spend their whole lives in the study of the most elevated department of human thought it does not guard them against the danger of using words without meaning. it would be a mark of ignorance, rather than of penetration, to hastily denounce propositions on subjects we are not well acquainted with because we do not understand their meaning. i do not mean to intimate that philosophy itself is subject to this reproach. when we see a philosophical proposition couched in terms we do not understand, the most modest and charitable view is to assume that this arises from our lack of knowledge. nothing is easier than for the ignorant to ridicule the propositions of the learned. and yet, with every reserve, i cannot but feel that the disputes to which i have alluded prove the necessity of bringing scientific precision of language into the whole domain of thought. if the discussion had been confined to a few, and other philosophers had analyzed the subject, and showed the fictitious character of the discussion, or had pointed out where opinions really might differ, there would be nothing derogatory to philosophers. but the most suggestive circumstance is that although a large proportion of the philosophic writers in recent times have devoted more or less attention to the subject, few, or none, have made even this modest contribution. i speak with some little confidence on this subject, because several years ago i wrote to one of the most acute thinkers of the country, asking if he could find in philosophic literature any terms or definitions expressive of the three different senses in which not only the word freedom, but nearly all words implying freedom were used. his search was in vain. nothing of this sort occurs in the practical affairs of life. all terms used in business, however general or abstract, have that well-defined meaning which is the first requisite of the scientific language. now one important lesson which i wish to inculcate is that the language of science in this respect corresponds to that of business; in that each and every term that is employed has a meaning as well defined as the subject of discussion can admit of. it will be an instructive exercise to inquire what this peculiarity of scientific and business language is. it can be shown that a certain requirement should be fulfilled by all language intended for the discovery of truth, which is fulfilled only by the two classes of language which i have described. it is one of the most common errors of discourse to assume that any common expression which we may use always conveys an idea, no matter what the subject of discourse. the true state of the case can, perhaps, best be seen by beginning at the foundation of things and examining under what conditions language can really convey ideas. suppose thrown among us a person of well-developed intellect, but unacquainted with a single language or word that we use. it is absolutely useless to talk to him, because nothing that we say conveys any meaning to his mind. we can supply him no dictionary, because by hypothesis he knows no language to which we have access. how shall we proceed to communicate our ideas to him? clearly there is but one possible way--namely, through his senses. outside of this means of bringing him in contact with us we can have no communication with him. we, therefore, begin by showing him sensible objects, and letting him understand that certain words which we use correspond to those objects. after he has thus acquired a small vocabulary, we make him understand that other terms refer to relations between objects which he can perceive by his senses. next he learns, by induction, that there are terms which apply not to special objects, but to whole classes of objects. continuing the same process, he learns that there are certain attributes of objects made known by the manner in which they affect his senses, to which abstract terms are applied. having learned all this, we can teach him new words by combining words without exhibiting objects already known. using these words we can proceed yet further, building up, as it were, a complete language. but there is one limit at every step. every term which we make known to him must depend ultimately upon terms the meaning of which he has learned from their connection with special objects of sense. to communicate to him a knowledge of words expressive of mental states it is necessary to assume that his own mind is subject to these states as well as our own, and that we can in some way indicate them by our acts. that the former hypothesis is sufficiently well established can be made evident so long as a consistency of different words and ideas is maintained. if no such consistency of meaning on his part were evident, it might indicate that the operations of his mind were so different from ours that no such communication of ideas was possible. uncertainty in this respect must arise as soon as we go beyond those mental states which communicate themselves to the senses of others. we now see that in order to communicate to our foreigner a knowledge of language, we must follow rules similar to those necessary for the stability of a building. the foundation of the building must be well laid upon objects knowable by his five senses. of course the mind, as well as the external object, may be a factor in determining the ideas which the words are intended to express; but this does not in any manner invalidate the conditions which we impose. whatever theory we may adopt of the relative part played by the knowing subject, and the external object in the acquirement of knowledge, it remains none the less true that no knowledge of the meaning of a word can be acquired except through the senses, and that the meaning is, therefore, limited by the senses. if we transgress the rule of founding each meaning upon meanings below it, and having the whole ultimately resting upon a sensuous foundation, we at once branch off into sound without sense. we may teach him the use of an extended vocabulary, to the terms of which he may apply ideas of his own, more or less vague, but there will be no way of deciding that he attaches the same meaning to these terms that we do. what we have shown true of an intelligent foreigner is necessarily true of the growing child. we come into the world without a knowledge of the meaning of words, and can acquire such knowledge only by a process which we have found applicable to the intelligent foreigner. but to confine ourselves within these limits in the use of language requires a course of severe mental discipline. the transgression of the rule will naturally seem to the undisciplined mind a mark of intellectual vigor rather than the reverse. in our system of education every temptation is held out to the learner to transgress the rule by the fluent use of language to which it is doubtful if he himself attaches clear notions, and which he can never be certain suggests to his hearer the ideas which he desires to convey. indeed, we not infrequently see, even among practical educators, expressions of positive antipathy to scientific precision of language so obviously opposed to good sense that they can be attributed only to a failure to comprehend the meaning of the language which they criticise. perhaps the most injurious effect in this direction arises from the natural tendency of the mind, when not subject to a scientific discipline, to think of words expressing sensible objects and their relations as connoting certain supersensuous attributes. this is frequently seen in the repugnance of the metaphysical mind to receive a scientific statement about a matter of fact simply as a matter of fact. this repugnance does not generally arise in respect to the every-day matters of life. when we say that the earth is round we state a truth which every one is willing to receive as final. if without denying that the earth was round, one should criticise the statement on the ground that it was not necessarily round but might be of some other form, we should simply smile at this use of language. but when we take a more general statement and assert that the laws of nature are inexorable, and that all phenomena, so far as we can show, occur in obedience to their requirements, we are met with a sort of criticism with which all of us are familiar, but which i am unable adequately to describe. no one denies that as a matter of fact, and as far as his experience extends, these laws do appear to be inexorable. i have never heard of any one professing, during the present generation, to describe a natural phenomenon, with the avowed belief that it was not a product of natural law; yet we constantly hear the scientific view criticised on the ground that events may occur without being subject to natural law. the word "may," in this connection, is one to which we can attach no meaning expressive of a sensuous relation. the analogous conflict between the scientific use of language and the use made by some philosophers is found in connection with the idea of causation. fundamentally the word cause is used in scientific language in the same sense as in the language of common life. when we discuss with our neighbors the cause of a fit of illness, of a fire, or of cold weather, not the slightest ambiguity attaches to the use of the word, because whatever meaning may be given to it is founded only on an accurate analysis of the ideas involved in it from daily use. no philosopher objects to the common meaning of the word, yet we frequently find men of eminence in the intellectual world who will not tolerate the scientific man in using the word in this way. in every explanation which he can give to its use they detect ambiguity. they insist that in any proper use of the term the idea of power must be connoted. but what meaning is here attached to the word power, and how shall we first reduce it to a sensible form, and then apply its meaning to the operations of nature? whether this can be done, i do not inquire. all i maintain is that if we wish to do it, we must pass without the domain of scientific statement. perhaps the greatest advantage in the use of symbolic and other mathematical language in scientific investigation is that it cannot possibly be made to connote anything except what the speaker means. it adheres to the subject matter of discourse with a tenacity which no criticism can overcome. in consequence, whenever a science is reduced to a mathematical form its conclusions are no longer the subject of philosophical attack. to secure the same desirable quality in all other scientific language it is necessary to give it, so far as possible, the same simplicity of signification which attaches to mathematical symbols. this is not easy, because we are obliged to use words of ordinary language, and it is impossible to divest them of whatever they may connote to ordinary hearers. i have thus sought to make it clear that the language of science corresponds to that of ordinary life, and especially of business life, in confining its meaning to phenomena. an analogous statement may be made of the method and objects of scientific investigation. i think professor clifford was very happy in defining science as organized common-sense. the foundation of its widest general creations is laid, not in any artificial theories, but in the natural beliefs and tendencies of the human mind. its position against those who deny these generalizations is quite analogous to that taken by the scottish school of philosophy against the scepticism of hume. it may be asked, if the methods and language of science correspond to those of practical life, why is not the every-day discipline of that life as good as the discipline of science? the answer is, that the power of transferring the modes of thought of common life to subjects of a higher order of generality is a rare faculty which can be acquired only by scientific discipline. what we want is that in public affairs men shall reason about questions of finance, trade, national wealth, legislation, and administration, with the same consciousness of the practical side that they reason about their own interests. when this habit is once acquired and appreciated, the scientific method will naturally be applied to the study of questions of social policy. when a scientific interest is taken in such questions, their boundaries will be extended beyond the utilities immediately involved, and one important condition of unceasing progress will be complied with. xxi the outlook for the flying-machine mr. secretary langley's trial of his flying-machine, which seems to have come to an abortive issue for the time, strikes a sympathetic chord in the constitution of our race. are we not the lords of creation? have we not girdled the earth with wires through which we speak to our antipodes? do we not journey from continent to continent over oceans that no animal can cross, and with a speed of which our ancestors would never have dreamed? is not all the rest of the animal creation so far inferior to us in every point that the best thing it can do is to become completely subservient to our needs, dying, if need be, that its flesh may become a toothsome dish on our tables? and yet here is an insignificant little bird, from whose mind, if mind it has, all conceptions of natural law are excluded, applying the rules of aerodynamics in an application of mechanical force to an end we have never been able to reach, and this with entire ease and absence of consciousness that it is doing an extraordinary thing. surely our knowledge of natural laws, and that inventive genius which has enabled us to subordinate all nature to our needs, ought also to enable us to do anything that the bird can do. therefore we must fly. if we cannot yet do it, it is only because we have not got to the bottom of the subject. our successors of the not distant future will surely succeed. this is at first sight a very natural and plausible view of the case. and yet there are a number of circumstances of which we should take account before attempting a confident forecast. our hope for the future is based on what we have done in the past. but when we draw conclusions from past successes we should not lose sight of the conditions on which success has depended. there is no advantage which has not its attendant drawbacks; no strength which has not its concomitant weakness. wealth has its trials and health its dangers. we must expect our great superiority to the bird to be associated with conditions which would give it an advantage at some point. a little study will make these conditions clear. we may look on the bird as a sort of flying-machine complete in itself, of which a brain and nervous system are fundamentally necessary parts. no such machine can navigate the air unless guided by something having life. apart from this, it could be of little use to us unless it carried human beings on its wings. we thus meet with a difficulty at the first step--we cannot give a brain and nervous system to our machine. these necessary adjuncts must be supplied by a man, who is no part of the machine, but something carried by it. the bird is a complete machine in itself. our aerial ship must be machine plus man. now, a man is, i believe, heavier than any bird that flies. the limit which the rarity of the air places upon its power of supporting wings, taken in connection with the combined weight of a man and a machine, make a drawback which we should not too hastily assume our ability to overcome. the example of the bird does not prove that man can fly. the hundred and fifty pounds of dead weight which the manager of the machine must add to it over and above that necessary in the bird may well prove an insurmountable obstacle to success. i need hardly remark that the advantage possessed by the bird has its attendant drawbacks when we consider other movements than flying. its wings are simply one pair of its legs, and the human race could not afford to abandon its arms for the most effective wings that nature or art could supply. another point to be considered is that the bird operates by the application of a kind of force which is peculiar to the animal creation, and no approach to which has ever been made in any mechanism. this force is that which gives rise to muscular action, of which the necessary condition is the direct action of a nervous system. we cannot have muscles or nerves for our flying-machine. we have to replace them by such crude and clumsy adjuncts as steam-engines and electric batteries. it may certainly seem singular if man is never to discover any combination of substances which, under the influence of some such agency as an electric current, shall expand and contract like a muscle. but, if he is ever to do so, the time is still in the future. we do not see the dawn of the age in which such a result will be brought forth. another consideration of a general character may be introduced. as a rule it is the unexpected that happens in invention as well as discovery. there are many problems which have fascinated mankind ever since civilization began which we have made little or no advance in solving. the only satisfaction we can feel in our treatment of the great geometrical problems of antiquity is that we have shown their solution to be impossible. the mathematician of to-day admits that he can neither square the circle, duplicate the cube or trisect the angle. may not our mechanicians, in like manner, be ultimately forced to admit that aerial flight is one of that great class of problems with which man can never cope, and give up all attempts to grapple with it? [illustration with caption: professor langley's air-ship] the fact is that invention and discovery have, notwithstanding their seemingly wide extent, gone on in rather narrower lines than is commonly supposed. if, a hundred years ago, the most sagacious of mortals had been told that before the nineteenth century closed the face of the earth would be changed, time and space almost annihilated, and communication between continents made more rapid and easy than it was between cities in his time; and if he had been asked to exercise his wildest imagination in depicting what might come--the airship and the flying-machine would probably have had a prominent place in his scheme, but neither the steamship, the railway, the telegraph, nor the telephone would have been there. probably not a single new agency which he could have imagined would have been one that has come to pass. it is quite clear to me that success must await progress of a different kind from that which the inventors of flying-machines are aiming at. we want a great discovery, not a great invention. it is an unfortunate fact that we do not always appreciate the distinction between progress in scientific discovery and ingenious application of discovery to the wants of civilization. the name of marconi is familiar to every ear; the names of maxwell and herz, who made the discoveries which rendered wireless telegraphy possible, are rarely recalled. modern progress is the result of two factors: discoveries of the laws of nature and of actions or possibilities in nature, and the application of such discoveries to practical purposes. the first is the work of the scientific investigator, the second that of the inventor. in view of the scientific discoveries of the past ten years, which, after bringing about results that would have seemed chimerical if predicted, leading on to the extraction of a substance which seems to set the laws and limits of nature at defiance by radiating a flood of heat, even when cooled to the lowest point that science can reach--a substance, a few specks of which contain power enough to start a railway train, and embody perpetual motion itself, almost--he would be a bold prophet who would set any limit to possible discoveries in the realm of nature. we are binding the universe together by agencies which pass from sun to planet and from star to star. we are determined to find out all we can about the mysterious ethereal medium supposed to fill all space, and which conveys light and heat from one heavenly body to another, but which yet evades all direct investigation. we are peering into the law of gravitation itself with the full hope of discovering something in its origin which may enable us to evade its action. from time to time philosophers fancy the road open to success, yet nothing that can be practically called success has yet been reached or even approached. when it is reached, when we are able to state exactly why matter gravitates, then will arise the question how this hitherto unchangeable force may be controlled and regulated. with this question answered the problem of the interaction between ether and matter may be solved. that interaction goes on between ethers and molecules is shown by the radiation of heat by all bodies. when the molecules are combined into a mass, this interaction ceases, so that the lightest objects fly through the ether without resistance. why is this? why does ether act on the molecule and not the mass? when we can produce the latter, and when the mutual action can be controlled, then may gravitation be overcome and then may men build, not merely airships, but ships which shall fly above the air, and transport their passengers from continent to continent with the speed of the celestial motions. the first question suggested to the reader by these considerations is whether any such result is possible; whether it is within the power of man to discover the nature of luminiferous ether and the cause of gravitation. to this the profoundest philosopher can only answer, "i do not know." quite possibly the gates at which he is beating are, in the very nature of things, incapable of being opened. it may be that the mind of man is incapable of grasping the secrets within them. the question has even occurred to me whether, if a being of such supernatural power as to understand the operations going on in a molecule of matter or in a current of electricity as we understand the operations of a steam-engine should essay to explain them to us, he would meet with any more success than we should in explaining to a fish the engines of a ship which so rudely invades its domain. as was remarked by william k. clifford, perhaps the clearest spirit that has ever studied such problems, it is possible that the laws of geometry for spaces infinitely small may be so different from those of larger spaces that we must necessarily be unable to conceive them. still, considering mere possibilities, it is not impossible that the twentieth century may be destined to make known natural forces which will enable us to fly from continent to continent with a speed far exceeding that of the bird. but when we inquire whether aerial flight is possible in the present state of our knowledge, whether, with such materials as we possess, a combination of steel, cloth, and wire can be made which, moved by the power of electricity or steam, shall form a successful flying-machine, the outlook may be altogether different. to judge it sanely, let us bear in mind the difficulties which are encountered in any flying-machine. the basic principle on which any such machine must be constructed is that of the aeroplane. this, by itself, would be the simplest of all flyers, and therefore the best if it could be put into operation. the principle involved may be readily comprehended by the accompanying figure. a m is the section of a flat plane surface, say a thin sheet of metal or a cloth supported by wires. it moves through the air, the latter being represented by the horizontal rows of dots. the direction of the motion is that of the horizontal line a p. the aeroplane has a slight inclination measured by the proportion between the perpendicular m p and the length a p. we may raise the edge m up or lower it at pleasure. now the interesting point, and that on which the hopes of inventors are based, is that if we give the plane any given inclination, even one so small that the perpendicular m p is only two or three per cent of the length a m, we can also calculate a certain speed of motion through the air which, if given to the plane, will enable it to bear any required weight. a plane ten feet square, for example, would not need any great inclination, nor would it require a speed higher than a few hundred feet a second to bear a man. what is of yet more importance, the higher the speed the less the inclination required, and, if we leave out of consideration the friction of the air and the resistance arising from any object which the machine may carry, the less the horse-power expended in driving the plane. [illustration] maxim exemplified this by experiment several years ago. he found that, with a small inclination, he could readily give his aeroplane, when it slid forward upon ways, such a speed that it would rise from the ways of itself. the whole problem of the successful flying-machine is, therefore, that of arranging an aeroplane that shall move through the air with the requisite speed. the practical difficulties in the way of realizing the movement of such an object are obvious. the aeroplane must have its propellers. these must be driven by an engine with a source of power. weight is an essential quality of every engine. the propellers must be made of metal, which has its weakness, and which is liable to give way when its speed attains a certain limit. and, granting complete success, imagine the proud possessor of the aeroplane darting through the air at a speed of several hundred feet per second! it is the speed alone that sustains him. how is he ever going to stop? once he slackens his speed, down he begins to fall. he may, indeed, increase the inclination of his aeroplane. then he increases the resistance to the sustaining force. once he stops he falls a dead mass. how shall he reach the ground without destroying his delicate machinery? i do not think the most imaginative inventor has yet even put upon paper a demonstratively successful way of meeting this difficulty. the only ray of hope is afforded by the bird. the latter does succeed in stopping and reaching the ground safely after its flight. but we have already mentioned the great advantages which the bird possesses in the power of applying force to its wings, which, in its case, form the aeroplanes. but we have already seen that there is no mechanical combination, and no way of applying force, which will give to the aeroplanes the flexibility and rapidity of movement belonging to the wings of a bird. with all the improvements that the genius of man has made in the steamship, the greatest and best ever constructed is liable now and then to meet with accident. when this happens she simply floats on the water until the damage is repaired, or help reaches her. unless we are to suppose for the flying-machine, in addition to everything else, an immunity from accident which no human experience leads us to believe possible, it would be liable to derangements of machinery, any one of which would be necessarily fatal. if an engine were necessary not only to propel a ship, but also to make her float--if, on the occasion of any accident she immediately went to the bottom with all on board--there would not, at the present day, be any such thing as steam navigation. that this difficulty is insurmountable would seem to be a very fair deduction, not only from the failure of all attempts to surmount it, but from the fact that maxim has never, so far as we are aware, followed up his seemingly successful experiment. there is, indeed, a way of attacking it which may, at first sight, seem plausible. in order that the aeroplane may have its full sustaining power, there is no need that its motion be continuously forward. a nearly horizontal surface, swinging around in a circle, on a vertical axis, like the wings of a windmill moving horizontally, will fulfil all the conditions. in fact, we have a machine on this simple principle in the familiar toy which, set rapidly whirling, rises in the air. why more attempts have not been made to apply this system, with two sets of sails whirling in opposite directions, i do not know. were there any possibility of making a flying-machine, it would seem that we should look in this direction. the difficulties which i have pointed out are only preliminary ones, patent on the surface. a more fundamental one still, which the writer feels may prove insurmountable, is based on a law of nature which we are bound to accept. it is that when we increase the size of any flying-machine without changing its model we increase the weight in proportion to the cube of the linear dimensions, while the effective supporting power of the air increases only as the square of those dimensions. to illustrate the principle let us make two flying-machines exactly alike, only make one on double the scale of the other in all its dimensions. we all know that the volume and therefore the weight of two similar bodies are proportional to the cubes of their dimensions. the cube of two is eight. hence the large machine will have eight times the weight of the other. but surfaces are as the squares of the dimensions. the square of two is four. the heavier machine will therefore expose only four times the wing surface to the air, and so will have a distinct disadvantage in the ratio of efficiency to weight. mechanical principles show that the steam pressures which the engines would bear would be the same, and that the larger engine, though it would have more than four times the horse-power of the other, would have less than eight times. the larger of the two machines would therefore be at a disadvantage, which could be overcome only by reducing the thickness of its parts, especially of its wings, to that of the other machine. then we should lose in strength. it follows that the smaller the machine the greater its advantage, and the smallest possible flying-machine will be the first one to be successful. we see the principle of the cube exemplified in the animal kingdom. the agile flea, the nimble ant, the swift-footed greyhound, and the unwieldy elephant form a series of which the next term would be an animal tottering under its own weight, if able to stand or move at all. the kingdom of flying animals shows a similar gradation. the most numerous fliers are little insects, and the rising series stops with the condor, which, though having much less weight than a man, is said to fly with difficulty when gorged with food. now, suppose that an inventor succeeds, as well he may, in making a machine which would go into a watch-case, yet complete in all its parts, able to fly around the room. it may carry a button, but nothing heavier. elated by his success, he makes one on the same model twice as large in every dimension. the parts of the first, which are one inch in length, he increases to two inches. every part is twice as long, twice as broad, and twice as thick. the result is that his machine is eight times as heavy as before. but the sustaining surface is only four times as great. as compared with the smaller machine, its ratio of effectiveness is reduced to one-half. it may carry two or three buttons, but will not carry over four, because the total weight, machine plus buttons, can only be quadrupled, and if he more than quadruples the weight of the machine, he must less than quadruple that of the load. how many such enlargements must he make before his machine will cease to sustain itself, before it will fall as an inert mass when we seek to make it fly through the air? is there any size at which it will be able to support a human being? we may well hesitate before we answer this question in the affirmative. dr. graham bell, with a cheery optimism very pleasant to contemplate, has pointed out that the law i have just cited may be evaded by not making a larger machine on the same model, but changing the latter in a way tantamount to increasing the number of small machines. this is quite true, and i wish it understood that, in laying down the law i have cited, i limit it to two machines of different sizes on the same model throughout. quite likely the most effective flying-machine would be one carried by a vast number of little birds. the veracious chronicler who escaped from a cloud of mosquitoes by crawling into an immense metal pot and then amused himself by clinching the antennae of the insects which bored through the pot until, to his horror, they became so numerous as to fly off with the covering, was more scientific than he supposed. yes, a sufficient number of humming-birds, if we could combine their forces, would carry an aerial excursion party of human beings through the air. if the watch-maker can make a machine which will fly through the room with a button, then, by combining ten thousand such machines he may be able to carry a man. but how shall the combined forces be applied? the difficulties i have pointed out apply only to the flying-machine properly so-called, and not to the dirigible balloon or airship. it is of interest to notice that the law is reversed in the case of a body which is not supported by the resistance of a fluid in which it is immersed, but floats in it, the ship or balloon, for example. when we double the linear dimensions of a steamship in all its parts, we increase not only her weight but her floating power, her carrying capacity, and her engine capacity eightfold. but the resistance which she meets with when passing through the water at a given speed is only multiplied four times. hence, the larger we build the steamship the more economical the application of the power necessary to drive it at a given speed. it is this law which has brought the great increase in the size of ocean steamers in recent times. the proportionately diminishing resistance which, in the flying-machine, represents the floating power is, in the ship, something to be overcome. thus there is a complete reversal of the law in its practical application to the two cases. the balloon is in the same class with the ship. practical difficulties aside, the larger it is built the more effective it will be, and the more advantageous will be the ratio of the power which is necessary to drive it to the resistance to be overcome. if, therefore, we are ever to have aerial navigation with our present knowledge of natural capabilities, it is to the airship floating in the air, rather than the flying-machine resting on the air, to which we are to look. in the light of the law which i have laid down, the subject, while not at all promising, seems worthy of more attention than it has received. it is not at all unlikely that if a skilful and experienced naval constructor, aided by an able corps of assistants, should design an airship of a diameter of not less than two hundred feet, and a length at least four or five times as great, constructed, possibly, of a textile substance impervious to gas and borne by a light framework, but, more likely, of exceedingly thin plates of steel carried by a frame fitted to secure the greatest combination of strength and lightness, he might find the result to be, ideally at least, a ship which would be driven through the air by a steam-engine with a velocity far exceeding that of the fleetest atlantic liner. then would come the practical problem of realizing the ship by overcoming the mechanical difficulties involved in the construction of such a huge and light framework. i would not be at all surprised if the result of the exact calculation necessary to determine the question should lead to an affirmative conclusion, but i am quite unable to judge whether steel could be rolled into parts of the size and form required in the mechanism. in judging of the possibility of commercial success the cheapness of modern transportation is an element in the case that should not be overlooked. i believe the principal part of the resistance which a limited express train meets is the resistance of the air. this would be as great for an airship as for a train. an important fraction of the cost of transporting goods from chicago to london is that of getting them into vehicles, whether cars or ships, and getting them out again. the cost of sending a pair of shoes from a shop in new york to the residence of the wearer is, if i mistake not, much greater than the mere cost of transporting them across the atlantic. even if a dirigible balloon should cross the atlantic, it does not follow that it could compete with the steamship in carrying passengers and freight. i may, in conclusion, caution the reader on one point. i should be very sorry if my suggestion of the advantage of the huge airship leads to the subject being taken up by any other than skilful engineers or constructors, able to grapple with all problems relating to the strength and resistance of materials. as a single example of what is to be avoided i may mention the project, which sometimes has been mooted, of making a balloon by pumping the air from a very thin, hollow receptacle. such a project is as futile as can well be imagined; no known substance would begin to resist the necessary pressure. our aerial ship must be filled with some substance lighter than air. whether heated air would answer the purpose, or whether we should have to use a gas, is a question for the designer. to return to our main theme, all should admit that if any hope for the flying-machine can be entertained, it must be based more on general faith in what mankind is going to do than upon either reasoning or experience. we have solved the problem of talking between two widely separated cities, and of telegraphing from continent to continent and island to island under all the oceans--therefore we shall solve the problem of flying. but, as i have already intimated, there is another great fact of progress which should limit this hope. as an almost universal rule we have never solved a problem at which our predecessors have worked in vain, unless through the discovery of some agency of which they have had no conception. the demonstration that no possible combination of known substances, known forms of machinery, and known forms of force can be united in a practicable machine by which men shall fly long distances through the air, seems to the writer as complete as it is possible for the demonstration of any physical fact to be. but let us discover a substance a hundred times as strong as steel, and with that some form of force hitherto unsuspected which will enable us to utilize this strength, or let us discover some way of reversing the law of gravitation so that matter may be repelled by the earth instead of attracted--then we may have a flying-machine. but we have every reason to believe that mere ingenious contrivances with our present means and forms of force will be as vain in the future as they have been in the past. the martyrs of science, or the lives of galileo, tycho brahe, and kepler. by sir david brewster, k.h. d.c.l., principal of the united college of st salvator and st leonard, st andrews; fellow of the royal society of london; vice-president of the royal society of edinburgh; corresponding member of the institute of france; and member of the academies of st petersburg, stockholm, berlin, copenhagen, gottingen, philadelphia, &c. &c. london: john murray, albemarle street. . g. s. tullis, printer, cupar. to the right hon. francis lord gray, f.r.s., f.r.s.e. my lord, in submitting this volume to the public under your lordship's auspices, i avail myself of the opportunity thus afforded me of expressing the deep sense which i entertain of the friendship and kindness with which your lordship has so long honoured me. although in these days, when science constitutes the power and wealth of nations, and encircles the domestic hearth with its most substantial comforts, there is no risk of its votaries being either persecuted or neglected, yet the countenance of those to whom providence has given rank and station will ever be one of the most powerful incitements to scientific enterprise, as well as one of its most legitimate rewards. next to the satisfaction of cultivating science, and thus laying up the only earthly treasure which we can carry along with us into a better state, is that of having encouraged and assisted others in the same beneficent labours. that your lordship may long continue to enjoy these sources of happiness is the earnest prayer of, my lord, your lordship's most faithful and obedient servant, david brewster. st leonards, st andrews, october , . contents. life of galileo. page. chapter i. peculiar interest attached to his life--his birth--his early studies--his passion for mathematics--his work on the hydrostatic balance--appointed lecturer on mathematics at pisa--his antipathy to the philosophy of aristotle--his contentions with the aristotelians--chosen professor of mathematics in padua--adopts the copernican system, but still teaches the ptolemaic doctrine--his alarming illness--he observes the new star in --his magnetical experiments, chapter ii. cosmo, grand duke of tuscany, invites galileo to pisa--galileo visits venice in , where he first hears of the telescope--he invents and constructs one, which excites a great sensation--discovers mountains in the moon, and forty stars in the pleiades--discovers jupiter's satellites in --effect of this discovery on kepler--manner in which these discoveries were received--galileo appointed mathematician to cosmo--mayer claims the discovery of the satellites of jupiter--harriot observes them in england in october , chapter iii. galileo announces his discoveries in enigmas--discovers the crescent of venus--the ring of saturn--the spots on the sun--similar observations made in england by harriot--claims of fabricius and scheiner to the discovery of the solar spots--galileo's letters to velser on the claims of scheiner--his residence at the villa of salviati--composes his work on floating bodies, which involves him in new controversies, chapter iv. galileo treats his opponents with severity and sarcasm--he is aided by the sceptics of the day--the church party the most powerful--galileo commences the attack, and is answered by caccini, a dominican--galileo's letter to the grand duchess of tuscany, in support of the motion of the earth and the stability of the sun--galileo visits rome--is summoned before the inquisition--and renounces his opinions as heretical--the inquisition denounces the copernican system--galileo has an audience of the pope, but still maintains his opinions in private society--proposes to find out the longitude at sea by means of jupiter's satellites--his negotiation on this subject with the court of spain--its failure--he is unable to observe the three comets of , but is involved in the controversy to which they gave rise, chapter v. urban viii., galileo's friend, raised to the pontificate--galileo goes to rome to offer his congratulations--the pope loads galileo with presents, and promises a pension to his son--galileo in pecuniary difficulties, owing to the death of his patron, cosmo--galileo again rashly attacks the church, notwithstanding the pope's kindness--he composes his system of the world, to demonstrate the copernican system--artfully obtains a license to print it--nature of the work--its influence on the public mind--the pope resolves on suppressing it--galileo summoned before the inquisition--his trial--his defence--his formal abjuration of his opinions--observations on his conduct--the pope shews great indulgence to galileo, who is allowed to return to his own house at arcetri as the place of his confinement, chapter vi. galileo loses his favourite daughter--he falls into a state of melancholy and ill health--is allowed to go to florence for its recovery in --but is prevented from leaving his house or receiving his friends--his friend castelli permitted to visit him in the presence of an officer of the inquisition--he composes his celebrated dialogues on local motion--discovers the moon's libration--loses the sight of one eye--the other eye attacked by the same disease--is struck blind--negociates with the dutch government respecting his method of finding the longitude--he is allowed free intercourse with his friends--his illness and death in --his epitaph--his social, moral, and scientific character, * * * * * life of tycho brahe. chapter i. tycho's birth, family, and education--an eclipse of the sun turns his attention to astronomy--studies law at leipsic--but pursues astronomy by stealth--his uncle's death--he returns to copenhagen, and resumes his observations--revisits germany--fights a duel, and loses his nose--visits augsburg, and meets hainzel--who assists him in making a large quadrant--revisits denmark--and is warmly received by the king--he settles at his uncle's castle of herritzvold--his observatory and laboratory--discovers the new star in cassiopeia--account of this remarkable body--tycho's marriage with a peasant girl--which irritates his friends--his lectures on astronomy--he visits the prince of hesse--attends the coronation of the emperor rudolph at ratisbon--he returns to denmark, chapter ii. frederick ii. patronizes tycho--and resolves to establish him in denmark--grants him the island of huen for life--and builds the splendid observatory of uraniburg--description of the island, and of the observatory--account of its astronomical instruments--tycho begins his observations--his pupils--tycho is made canon of rothschild, and receives a large pension--his hospitality to his visitors--ingratitude of wittichius--tycho sends an assistant to take the latitude of frauenburg and konigsberg--is visited by ulric, duke of mecklenburg--change in tycho's fortunes, chapter iii. tycho's labours do honour to his country--death of frederick ii.--james vi. of scotland visits tycho at uraniburg--christian iv. visits tycho--the duke of brunswick's visit to tycho--the danish nobility, jealous of his fame, conspire against him--he is compelled to quit uraniburg--and to abandon his studies--cruelty of the minister walchendorp--tycho quits denmark with his family and instruments--is hospitably received by count rantzau--who introduces him to the emperor rudolph--the emperor invites him to prague--he gives him a pension of crowns--and the castle of benach as a residence and an observatory--kepler visits tycho--who obtains for him the appointment of mathematician to rudolph, chapter iv. tycho resumes his astronomical observations--is attacked with a painful disease--his sufferings and death in --his funeral--his temper--his turn for satire and raillery--his piety--account of his astronomical discoveries--his love of astrology and alchymy--observations on the character of the alchymists--tycho's elixir--his fondness for the marvellous--his automata and invisible bells--account of the idiot, called lep, whom he kept as a prophet--history of tycho's instruments--his great brass globe preserved at copenhagen--present state of the island of huen, * * * * * life of john kepler. chapter i. kepler's birth in --his family--and early education--the distresses and poverty of his family--he enters the monastic school of maulbronn--and is admitted into the university of tubingen, where he distinguishes himself, and takes his degree--he is appointed professor of astronomy and greek in --his first speculations on the orbits of the planets--account of their progress and failure--his "cosmographical mystery" published--he marries a widow in --religious troubles at gratz--he retires from thence to hungary--visits tycho at prague in --returns to gratz, which he again quits for prague--he is taken ill on the road--is appointed tycho's assistant in --succeeds tycho as imperial mathematician--his work on the new star of --singular specimen of it, chapter ii. kepler's pecuniary embarrassments--his inquiries respecting the law of refraction--his supplement to vitellio--his researches on vision--his treatise on dioptrics--his commentaries on mars--he discovers that the orbit of mars is an ellipse, with the sun in one focus--and extends this discovery to all the other planets--he establishes the two first laws of physical astronomy--his family distresses--death of his wife--he is appointed professor of mathematics at linz--his method of choosing a second wife--her character, as given by himself--origin of his treatise on gauging--he goes to ratisbon to give his opinion to the diet on the change of style--he refuses the mathematical chair at bologna, chapter iii. kepler's continued embarrassments--death of mathias--liberality of ferdinand--kepler's "harmonies of the world"--the epitome of the copernican astronomy--it is prohibited by the inquisition--sir henry wotton, the british ambassador, invites kepler to england--he declines the invitation--neglect of genius by the english government--trial of kepler's mother--her final acquittal--and death at the age of seventy-five--the states of styria burn publicly kepler's calendar--he receives his arrears of salary from ferdinand--the rudolphine tables published in --he receives a gold chain from the grand duke of tuscany--he is patronised by the duke of friedland--he removes to sagan, in silesia--is appointed professor of mathematics at rostoch--goes to ratisbon to receive his arrears--his death, funeral, and epitaph--monument erected to his memory in --his family--his posthumous volume, entitled "the dream, or lunar astronomy," chapter iv. number of kepler's published works--his numerous manuscripts in folio volumes--purchased by hevelius, and afterwards by hansch--who publishes kepler's life and correspondence at the expense of charles vi.--the history of the rest of his manuscripts, which are deposited in the library of the academy of sciences at st petersburg--general character of kepler--his candour in acknowledging his errors--his moral and religious character--his astrological writings and opinions considered--his character as an astronomer and a philosopher--the splendour of his discoveries--account of his method of investigating truth, life of galileo. chapter i. _peculiar interest attached to his life--his birth--his early studies--his passion for mathematics--his work on the hydrostatic balance--appointed lecturer on mathematics at pisa--his antipathy to the philosophy of aristotle--his contentions with the aristotelians--chosen professor of mathematics in padua--adopts the copernican system, but still teaches the ptolemaic doctrine--his alarming illness--he observes the new star in --his magnetical experiments._ the history of the life and labours of galileo is pregnant with a peculiar interest to the general reader, as well as to the philosopher. his brilliant discoveries, the man of science regards as his peculiar property; the means by which they were made, and the development of his intellectual character, belong to the logician and to the philosopher; but the triumphs and the reverses of his eventful life must be claimed for our common nature, as a source of more than ordinary instruction. the lengthened career which providence assigned to galileo was filled up throughout its rugged outline with events even of dramatic interest. but though it was emblazoned with achievements of transcendent magnitude, yet his noblest discoveries were the derision of his contemporaries, and were even denounced as crimes which merited the vengeance of heaven. though he was the idol of his friends, and the favoured companion of princes, yet he afterwards became the victim of persecution, and spent some of his last hours within the walls of a prison; and though the almighty granted him, as it were, a new sight to descry unknown worlds in the obscurity of space, yet the eyes which were allowed to witness such wonders, were themselves doomed to be closed in darkness. such were the lights and shadows in which history delineates "the starry galileo with his woes."[ ] [ ] childe harold, canto iv. stanza liv. but, however powerful be their contrasts, they are not unusual in their proportions. the balance which has been struck between his days of good and evil, is that which regulates the lot of man, whether we study it in the despotic sway of the autocrat, in the peaceful inquiries of the philosopher, or in the humbler toils of ordinary life. galileo galilei was born at pisa, on the th of february, , and was the eldest of a family of three sons and three daughters. under the name of bonajuti, his noble ancestors had filled high offices at florence; but about the middle of the th century they seem to have abandoned this surname for that of galileo. vincenzo galilei, our author's father, was himself a philosopher of no mean powers; and though his talents seem to have been exercised only in the composition of treatises on the theory and practice of music, yet he appears to have anticipated even his son in a just estimate of the philosophy of the age, and in a distinct perception of the true method of investigating truth.[ ] [ ] life of galileo, library of useful knowledge, p. . the early years of galileo were, like those of almost all great experimental philosophers, spent in the construction of instruments and pieces of machinery, which were calculated chiefly to amuse himself and his schoolfellows. this employment of his hands, however, did not interfere with his regular studies; and though, from the straitened circumstances of his father, he was educated under considerable disadvantages, yet he acquired the elements of classical literature, and was initiated into all the learning of the times. music, drawing, and painting were the occupations of his leisure hours; and such was his proficiency in these arts, that he was reckoned a skilful performer on several musical instruments, especially the lute; and his knowledge of pictures was held in great esteem by some of the best artists of his day. galileo seems to have been desirous of following the profession of a painter: but his father had observed decided indications of early genius; and, though by no means able to afford it, he resolved to send him to the university to pursue the study of medicine. he accordingly enrolled himself as a scholar in arts at the university of pisa, on the th of november, , and pursued his medical studies under the celebrated botanist andrew cæsalpinus, who filled the chair of medicine from to . in order to study the principles of music and drawing, galileo found it necessary to acquire some knowledge of geometry. his father seems to have foreseen the consequences of following this new pursuit, and though he did not prohibit him from reading euclid under ostilio ricci, one of the professors at pisa, yet he watched his progress with the utmost jealousy, and had resolved that it should not interfere with his medical studies. the demonstrations, however, of the greek mathematician had too many charms for the ardent mind of galileo. his whole attention was engrossed with the new truths which burst upon his understanding; and after many fruitless attempts to check his ardour and direct his thoughts to professional objects, his father was obliged to surrender his parental control, and allow the fullest scope to the genius of his son. from the elementary works of geometry, galileo passed to the writings of archimedes; and while he was studying the hydrostatical treatise[ ] of the syracusan philosopher, he wrote his essay on the hydrostatical balance,[ ] in which he describes the construction of the instrument, and the method by which archimedes detected the fraud committed by the jeweller in the composition of hiero's crown. this work gained for its author the esteem of guido ubaldi, who had distinguished himself by his mechanical and mathematical acquirements, and who engaged his young friend to investigate the subject of the centre of gravity in solid bodies. the treatise on this subject, which galileo presented to his patron, proved the source of his future success in life. [ ] de insidentibus in fluido. [ ] opere di galileo. milano, , vol. iv. p. - . through the cardinal del monte, the brother-in-law of ubaldi, the reigning duke of tuscany, ferdinand de medici was made acquainted with the merits of our young philosopher; and, in , he was appointed lecturer on mathematics at pisa. as the salary, however, attached to this office was only sixty crowns, he was compelled to enlarge this inadequate income by the additional occupation of private teaching, and thus to encroach upon the leisure which he was anxious to devote to science. with this moderate competency, galileo commenced his philosophical career. at the early age of eighteen, when he had entered the university, his innate antipathy to the aristotelian philosophy began to display itself. this feeling was strengthened by his earliest inquiries; and upon his establishment at pisa he seems to have regarded the doctrines of aristotle as the intellectual prey which, in his chace of glory, he was destined to pursue. nizzoli, who flourished near the beginning of the sixteenth century, and giordano bruno, who was burned at rome in , led the way in this daring pursuit; but it was reserved for galileo to track the thracian boar through its native thickets, and, at the risk of his own life, to strangle it in its den. with the resolution of submitting every opinion to the test of experiment, galileo's first inquiries at pisa were directed to the mechanical doctrines of aristotle. their incorrectness and absurdity soon became apparent; and with a zeal, perhaps, bordering on indiscretion, he denounced them to his pupils with an ardour of manner and of expression proportioned to his own conviction of the truth. the detection of long-established errors is apt to inspire the young philosopher with an exultation which reason condemns. the feeling of triumph is apt to clothe itself in the language of asperity; and the abettor of erroneous opinions is treated as a species of enemy to science. like the soldier who fleshes his first spear in battle, the philosopher is apt to leave the stain of cruelty on his early achievements. it is only from age and experience, indeed, that we can expect the discretion of valour, whether it is called forth in controversy or in battle. galileo seems to have waged this stern warfare against the followers of aristotle; and such was the exasperation which was excited by his reiterated and successful attacks, that he was assailed, during the rest of his life, with a degree of rancour which seldom originates in a mere difference of opinion. forgetting that all knowledge is progressive, and that the errors of one generation call forth the comments, and are replaced by the discoveries, of the next, galileo did not anticipate that his own speculations and incompleted labours might one day provoke unmitigated censure; and he therefore failed in making allowance for the prejudices and ignorance of his opponents. he who enjoys the proud lot of taking a position in advance of his age, need not wonder that his less gifted contemporaries are left behind. men are not necessarily obstinate because they cleave to deeply rooted and venerable errors, nor are they absolutely dull when they are long in understanding and slow in embracing newly discovered truths. it was one of the axioms of the aristotelian mechanics, that the heavier of two falling bodies would reach the ground sooner than the other, and that their velocities would be proportional to their weights. galileo attacked the arguments by which this opinion was supported; and when he found his reasoning ineffectual, he appealed to direct experiment. he maintained, that all bodies would fall through the same height in the same time, if they were not unequally retarded by the resistance of the air: and though he performed the experiment with the most satisfactory results, by letting heavy bodies fall from the leaning tower of pisa, yet the aristotelians, who with their own eyes saw the unequal weights strike the ground at the same instant, ascribed the effect to some unknown cause, and preferred the decision of their master to that of nature herself. galileo could not brook this opposition to his discoveries; nor could the aristotelians tolerate the rebukes of their young instructor. the two parties were, consequently, marshalled in hostile array; when, fortunately for both, an event occurred, which placed them beyond the reach of danger. don giovanni de medici, a natural son of cosmo, had proposed a method of clearing out the harbour of leghorn. galileo, whose opinion was requested, gave such an unfavourable report upon it, that the disappointed inventor directed against him all the force of his malice. it was an easy task to concentrate the malignity of his enemies at pisa; and so effectually was this accomplished, that galileo resolved to accept another professorship, to which he had been previously invited. the chair of mathematics in the university of padua having been vacant for five years, the republic of venice had resolved to fill it up; and, on the recommendation of guido ubaldi, galileo was appointed to it, in , for a period of six years. previous to this event, galileo had lost his father, who died, in , at an advanced age. as he was the eldest son, the support of the family naturally devolved upon him; and this sacred obligation must have increased his anxiety to better his circumstances, and therefore added to his other inducements to quit pisa. in september , he removed to padua, where he had a salary of only florins, and where he was again obliged to add to his income by the labours of tuition. notwithstanding this fruitless occupation of his time, he appears to have found leisure for composing several of his works, and completing various inventions, which will be afterwards described. his manuscripts were circulated privately among his friends and pupils; but some of them strayed beyond this sacred limit, and found their way into the hands of persons, who did not scruple to claim and publish, as their own, the discoveries and inventions which they contained. it is not easy to ascertain the exact time when galileo became a convert to the doctrines of copernicus, or the particular circumstances under which he was led to adopt them. it is stated by gerard voss, that a public lecture of moestlin, the instructor of kepler, was the means of making galileo acquainted with the true system of the universe. this assertion, however, is by no means probable; and it has been ably shown, by the latest biographer of galileo,[ ] that, in his dialogues on the copernican system, our author gives the true account of his own conversion. this passage is so interesting, that we shall give it entire. [ ] life of galileo, in library of useful knowledge, p. . "i cannot omit this opportunity of relating to you what happened to myself at the time when this opinion (the copernican system) began to be discussed. i was then a very young man, and had scarcely finished my course of philosophy, which other occupations obliged me to leave off, when there arrived in this country, from rostoch, a foreigner, whose name, i believe, was christian vurstisius (wurteisen), a follower of copernicus. this person delivered, on this subject, two or three lectures in a certain academy, and to a crowded audience. believing that several were attracted more by the novelty of the subject than by any other cause, and being firmly persuaded that this opinion was a piece of solemn folly, i was unwilling to be present. upon interrogating, however, some of those who were there, i found that they all made it a subject of merriment, with the exception of one, who assured me that it was not a thing wholly ridiculous. as i considered this individual to be both prudent and circumspect, i repented that i had not attended the lectures; and, whenever i met any of the followers of copernicus, i began to inquire if they had always been of the same opinion. i found that there was not one of them who did not declare that he had long maintained the very opposite opinions, and had not gone over to the new doctrines till he was driven by the force of argument. i next examined them one by one, to see if they were masters of the arguments on the opposite side; and such was the readiness of their answers, that i was satisfied they had not taken up this opinion from ignorance or vanity. on the other hand, whenever i interrogated the peripatetics and the ptolemeans--and, out of curiosity, i have interrogated not a few--respecting their perusal of copernicus's work, i perceived that there were few who had seen the book, and not one who understood it. nor have i omitted to inquire among the followers of the peripatetic doctrines, if any of them had ever stood on the opposite side; and the result was, that there was not one. considering, then, that nobody followed the copernican doctrine, who had not previously held the contrary opinion, and who was not well acquainted with the arguments of aristotle and ptolemy; while, on the other hand, nobody followed ptolemy and aristotle, who had before adhered to copernicus, and had gone over from him into the camp of aristotle;--weighing, i say, these things, i began to believe that, if any one who rejects an opinion which he has imbibed with his milk, and which has been embraced by an infinite number, shall take up an opinion held only by a few, condemned by all the schools, and really regarded as a great paradox, it cannot be doubted that he must have been induced, not to say driven, to embrace it by the most cogent arguments. on this account i have become very curious to penetrate to the very bottom of the subject."[ ] [ ] systema cosmicum, dial. ii. p. . it appears, on the testimony of galileo himself, that he taught the ptolemaic system, in compliance with the popular feeling, after he had convinced himself of the truth of the copernican doctrines. in the treatise on the sphere, indeed, which bears his name,[ ] and which must have been written soon after he went to padua, and subsequently to , the stability of the earth, and the motion of the sun, are supported by the very arguments which galileo afterwards ridiculed; but we have no means of determining whether or not he had then adopted the true system of the universe. although he might have taught the ptolemaic system in his lectures after he had convinced himself of its falsehood, yet it is not likely that he would go so far as to publish to the world, as true, the very doctrines which he despised. in a letter to kepler, dated in , he distinctly states that he _had, many years ago, adopted the opinions of copernicus_; but that _he had not yet dared to publish his arguments in favour of them, and his refutation of the opposite opinions_. these facts would leave us to place galileo's conversion somewhere between and , although _many_ years cannot be said to have elapsed between these two dates. [ ] the authenticity of this work has been doubted. it was printed at rome, in , from a ms. in the library of somaschi, at venice. see opere di galileo, tom. vii. p. . at this early period of galileo's life, in the year , he met with an accident which had nearly proved fatal. a party at padua, of which he was one, were enjoying, at an open window, a current of air, which was artificially cooled by a fall of water. galileo unfortunately fell asleep under its influence; and so powerful was its effect upon his robust constitution, that he contracted a severe chronic disorder, accompanied with acute pains in his body, and loss of sleep and appetite, which attacked him at intervals during the rest of his life. others of the party suffered still more severely, and perished by their own rashness. galileo's reputation was now widely extended over europe. the archduke ferdinand (afterwards emperor of germany), the landgrave of hesse, and the princes of alsace and mantua, honoured his lectures with their presence; and prince gustavus adolphus of sweden also received instructions from him in mathematics, during his sojourn in italy. when galileo had completed the first period of his engagement at padua, he was re-elected for other six years, with an increased salary of florins. this liberal addition to his income is ascribed by fabbroni to the malice of one of his enemies, who informed the senate that galileo was living in illicit intercourse with marina gamba. without inquiring into the truth of the accusation, the senate is said to have replied, that if "he had a family to support, he had the more need of an increased salary." it is more likely that the liberality of the republic had been called forth by the high reputation of their professor, and that the terms of their reply were intended only to rebuke the malignity of the informer. the mode of expression would seem to indicate that one or more of galileo's children had been born previous to his re-election in ; but as this is scarcely consistent with other facts, we are disposed to doubt the authenticity of fabbroni's anecdote. the new star which attracted the notice of astronomers in , excited the particular attention of galileo. the observations which he made upon it, and the speculations which they suggested, formed the subject of three lectures, the beginning of the first of which only has reached our times. from the absence of parallax, he proved that the common hypothesis of its being a meteor was erroneous, and that, like the fixed stars, it was situated far beyond the bounds of our own system. the popularity of the subject attracted crowds to his lecture-room; and galileo had the boldness to reproach his hearers for taking so deep an interest in a temporary phenomenon, while they overlooked the wonders of creation which were daily presented to their view. in the year , galileo was again appointed to the professorship at padua, with an augmented stipend of florins. his popularity had now risen so high, that his audience could not be accommodated in his lecture-room; and even when he had assembled them in the school of medicine, which contained persons, he was frequently obliged to adjourn to the open air. among the variety of pursuits which occupied his attention, was the examination of the properties of the loadstone. in , he commenced his experiments; but, with the exception of a method of arming loadstones, which, according to the report of sir kenelm digby, enabled them to carry twice as much weight as before, he does not seem to have made any additions to our knowledge of magnetism. he appears to have studied with care the admirable work of our countryman, dr gilbert, "de magnete," which was published in ; and he recognised in the experiments and reasonings of the english philosopher the principles of that method of investigating truth which he had himself adopted. gilbert died in , in the d year of his age, and probably never read the fine compliment which was paid to him by the italian philosopher--"i extremely praise, admire, and envy this author." chapter ii. _cosmo, grand duke of tuscany, invites galileo to pisa--galileo visits venice in , where he first hears of the telescope--he invents and constructs one, which excites a great sensation--discovers mountains in the moon, and forty stars in the pleiades--discovers jupiter's satellites in --effect of this discovery on kepler--manner in which these discoveries were received--galileo appointed mathematician to cosmo--mayer claims the discovery of the satellites of jupiter--harriot observes them in england in october ._ in the preceding chapter we have brought down the history of galileo's labours to that auspicious year in which he first directed the telescope to the heavens. no sooner was that noble instrument placed in his hands, than providence released him from his professional toils, and supplied him with the fullest leisure and the amplest means for pursuing and completing the grandest discoveries. although he had quitted the service and the domains of his munificent patron, the grand duke of tuscany, yet he maintained his connection with the family, by visiting florence during his academic vacations, and giving mathematical instruction to the younger branches of that distinguished house. cosmo, who had been one of his pupils, now succeeded his father ferdinand; and having his mind early imbued with a love of knowledge, which had become hereditary in his family, he felt that the residence of galileo within his dominions, and still more his introduction into his household, would do honour to their common country, and reflect a lustre upon his own name. in the year , accordingly, cosmo made proposals to galileo to return to his original situation at pisa. these overtures were gratefully received; and in the arrangements which galileo on this occasion suggested, as well as in the manner in which they were urged, we obtain some insight into his temper and character. he informs the correspondent through whom cosmo's offer was conveyed, that his salary of florins at padua would be increased to as many crowns at his re-election, and that he could enlarge his income to any extent he pleased, by giving private lectures and receiving pupils. his public duties, he stated, occupied him only sixty half-hours in the year; but his studies suffered such interruptions from his domestic pupils and private lectures, that his most ardent wish was to be relieved from them, in order that he might have sufficient rest and leisure, before the close of his life, to finish and publish those great works which he had projected. in the event, therefore, of his returning to pisa, he hoped that it would be the first object of his serene highness to give him leisure to complete his works without the drudgery of lecturing. he expresses his anxiety to gain his bread by his writings, and he promises to dedicate them to his serene master. he enumerates, among these books, two on the system of the universe, three on local motion, three books of mechanics, two on the demonstration of principles, and one of problems; besides treatises on sound and speech, on light and colours, on the tides, on the composition of continuous quantity, on the motions of animals, and on the military art. on the subject of his salary, he makes the following curious observations:-- "i say nothing," says he, "on the amount of my salary; being convinced that, as i am to live upon it, the graciousness of his highness would not deprive me of any of those comforts, of which, however, i feel the want of less than many others; and, therefore, i say nothing more on the subject. finally, on the title and profession of my service, i should wish that, to the title of mathematician, his highness would add that of philosopher, as i profess to have studied a greater number of years in philosophy, than months in pure mathematics; and how i have profited by it, and if i can or ought to deserve this title, i may let their highnesses see, as often as it shall please them to give me an opportunity of discussing such subjects in their presence with those who are most esteemed in this knowledge." during the progress of this negotiation, galileo went to venice, on a visit to a friend, in the month of april or may . here he learned, from common rumour, that a dutchman had presented to prince maurice of nassau an optical instrument, which possessed the singular property of causing distant objects to appear nearer the observer. this dutchman was hans or john lippershey, who, as has been clearly proved by the late professor moll of utrecht,[ ] was in the possession of a telescope made by himself so early as d october . a few days afterwards, the truth of this report was confirmed by a letter which galileo received from james badorere at paris, and he immediately applied himself to the consideration of the subject. on the first night after his return to padua, he found, in the doctrines of refraction, the principle which he sought. he placed at the ends of a leaden tube two spectacle glasses, both of which were plain on one side, while one of them had its other side convex, and the other its second side concave, and having applied his eye to the concave glass, he saw objects pretty large and pretty near him. this little instrument, which magnified only three times, he carried in triumph to venice, where it excited the most intense interest. crowds of the principal citizens flocked to his house to see the magical toy; and after nearly a month had been spent in gratifying this epidemical curiosity, galileo was led to understand from leonardo deodati, the doge of venice, that the senate would be highly gratified by obtaining possession of so extraordinary an instrument. galileo instantly complied with the wishes of his patrons, who acknowledged the present by a mandate conferring upon him for life his professorship at padua, and generously raising his salary from to florins.[ ] [ ] on the first invention of telescopes.--_journ. r. instit._, ., vol i., p. . [ ] viviani _vita del' galileo_, p. . although we cannot doubt the veracity of galileo, when he affirms that he had never seen any of the dutch telescopes, yet it is expressly stated by fuccarius, that one of these instruments had at this time been brought to florence; and sirturus assures us that a frenchman, calling himself a partner of the dutch inventor, came to milan in may , and offered a telescope to the count de fuentes. in a letter from lorenzo pignoria to paolo gualdo, dated from padua, on the st of august , it is expressly said, that, at the re-election of the professors, galileo had contrived to obtain florins for life, which was alleged to be on account of an eye-glass like the one which was sent from flanders to the cardinal borghese. in a memoir so brief and general as the present, it would be out of place to discuss the history of this extraordinary invention. we have no hesitation in asserting that a method of magnifying distant objects was known to baptista porta and others; but it seems to be equally certain that an _instrument_ for producing these effects was first constructed in holland, and that it was from that kingdom that galileo derived the knowledge of its existence. in considering the contending claims, which have been urged with all the ardour and partiality of national feeling, it has been generally overlooked, _that a single convex lens_, whose focal length exceeds the distance at which we examine minute objects, performs the part of a telescope, when an eye, placed behind it, sees distinctly the inverted image which it forms. a lens, twenty feet in focal length, will in this manner magnify twenty times; and it was by the same principle that sir william herschel discovered a new satellite of saturn, by using only the mirror of his forty-feet telescope. the instrument presented to prince maurice, and which the marquis spinola found in the shop of john lippershey, the spectacle maker of middleburg, must have been an astronomical telescope consisting of two convex lenses. upon this supposition, it differed from that which galileo constructed; and the italian philosopher will be justly entitled to the honour of having invented that form of the telescope which still bears his name, while we must accord to the dutch optician the honour of having previously invented the astronomical telescope. the interest which the exhibition of the telescope excited at venice did not soon subside: sirturi[ ] describes it as amounting almost to phrensy. when he himself had succeeded in making one of these instruments, he ascended the tower of st mark, where he might use it without molestation. he was recognised, however, by a crowd in the street; and such was the eagerness of their curiosity, that they took possession of the wondrous tube, and detained the impatient philosopher for several hours, till they had successively witnessed its effects. desirous of obtaining the same gratification for their friends, they endeavoured to learn the name of the inn at which he lodged; but sirturi fortunately overheard their inquiries, and quitted venice early next morning, in order to avoid a second visitation of this new school of philosophers. the opticians speedily availed themselves of the new instrument. galileo's tube,--or the double eye-glass, or the cylinder, or the trunk, as it was then called, for demisiano had not yet given it the appellation of _telescope_,--was manufactured in great quantities, and in a very superior manner. the instruments were purchased merely as philosophical toys, and were carried by travellers into every corner of europe. [ ] de telescopio. the art of grinding and polishing lenses was at this time very imperfect. galileo, and those whom he instructed, were alone capable of making tolerable instruments. it appears, from the testimony of gassendi and gærtner, that, in , a good telescope could not be procured in paris, venice, or amsterdam; and that, even in , there was not one in holland which could shew jupiter's disc well defined. after galileo had completed his first instrument, which magnified only _three_ times, he executed a larger and a better one, with a power of about _eight_. "at length," as he himself remarks, "sparing neither labour nor expense," he constructed an instrument so excellent, that it bore a magnifying power of more than _thirty_ times. the first celestial object to which galileo applied his telescope was the moon, which, to use his own words, appeared as near as if it had been distant only two semidiameters of the earth. he then directed it to the planets and the fixed stars, which he frequently observed with "incredible delight."[ ] [ ] incredibili animi jucunditate. the observations which he made upon the moon possessed a high degree of interest. the general resemblance of its surface to that of our own globe naturally fixed his attention; and he was soon able to trace, in almost every part of the lunar disc, ranges of mountains, deep hollows, and other inequalities, which reverberated from their summits and margins the rays of the rising sun, while the intervening hollows were still buried in darkness. the dark and luminous spaces he regarded as indicating seas and continents, which reflected, in different degrees, the incidental light of the sun; and he ascribed the phosphorescence, as it has been improperly called, or the secondary light, which is seen on the dark limb of the moon in her first and last quarters, to the reflection of the sun's light from the earth. these discoveries were ill received by the followers of aristotle. according to their preconceived opinions, the moon was perfectly spherical, and absolutely smooth; and to cover it with mountains, and scoop it out into valleys, was an act of impiety which defaced the regular forms which nature herself had imprinted. it was in vain that galileo appealed to the evidence of observation, and to the actual surface of our own globe. the very irregularities on the moon were, in his opinion, the proof of divine wisdom; and had its surface been absolutely smooth, it would have been "but a vast unblessed desert, void of animals, of plants, of cities, and of men--the abode of silence and inaction--senseless, lifeless, soulless, and stripped of all those ornaments which now render it so varied and so beautiful." in examining the fixed stars, and comparing them with the planets, galileo observed a remarkable difference in the appearance of their discs. all the planets appeared with round globular discs like the moon; whereas the fixed stars never exhibited any disc at all, but resembled lucid points sending forth twinkling rays. stars of all magnitudes he found to have the same appearance; those of the fifth and sixth magnitude having the same character, when seen through a telescope, as sirius, the largest of the stars, when seen by the naked eye. upon directing his telescope to nebulæ and clusters of stars, he was delighted to find that they consisted of great numbers of stars which could not be recognised by unassisted vision. he counted no fewer than _forty_ in the cluster called the _pleiades_, or _seven stars_; and he has given us drawings of this constellation, as well as of the belt and sword of orion, and of the nebula of præsepe. in the great nebula of the milky way, he descried crowds of minute stars; and he concluded that this singular portion of the heavens derived its whiteness from still smaller stars, which his telescope was unable to separate. important and interesting as these discoveries were, they were thrown into the shade by those to which he was led during an accurate examination of the planets with a more powerful telescope. on the th of january , at one o'clock in the morning, when he directed his telescope to jupiter, he observed three stars near the body of the planet, two being to the east and one to the west of him. they were all in a straight line, and parallel to the ecliptic, and appeared brighter than other stars of the same magnitude. believing them to be fixed stars, he paid no great attention to their distances from jupiter and from one another. on the th of january, however, when, from some cause or other,[ ] he had been led to observe the stars again, he found a very different arrangement of them: all the three were on the west side of jupiter, _nearer one another than before_, and almost at equal distances. though he had not turned his attention to the extraordinary fact of the mutual approach of the stars, yet he began to consider how jupiter could be found to the east of the three stars, when but the day before he had been to the west of two of them. the only explanation which he could give of this fact was, that the motion of jupiter was _direct_, contrary to astronomical calculations, and that he had got before these two stars by his own motion. [ ] nescio quo fato ductus. in this dilemma between the testimony of his senses and the results of calculation, he waited for the following night with the utmost anxiety; but his hopes were disappointed, for the heavens were wholly veiled in clouds. on the th, two only of the stars appeared, and both on the east of the planet. as it was obviously impossible that jupiter could have advanced from west to east on the th of january, and from east to west on the th, galileo was forced to conclude that the phenomenon which he had observed arose from the motion of the stars, and he set himself to observe diligently their change of place. on the th, there were still only two stars, and both to the east of jupiter; but the more eastern star was now _twice as large as the other one_, though on the preceding night they had been perfectly equal. this fact threw a new light upon galileo's difficulties, and he immediately drew the conclusion, which he considered to be indubitable, "_that there were in the heavens three stars which revolved round jupiter, in the same manner as venus and mercury revolve round the sun_." on the th of january, he again observed them in new positions, and of different magnitudes; and, on the th, he discovered a fourth star, which completed the _four_ secondary planets with which jupiter is surrounded. galileo continued his observations on these bodies every clear night till the d of march, and studied their motions in reference to fixed stars that were at the same time within the field of his telescope. having thus clearly established that the four new stars were satellites or moons, which revolved round jupiter in the same manner as the moon revolves round our own globe, he drew up an account of his discovery, in which he gave to the four new bodies the names of the _medicean stars_, in honour of his patron, cosmo de medici, grand duke of tuscany. this work, under the title of "nuncius sidereus," or the "sidereal messenger," was dedicated to the same prince; and the dedication bears the date of the th of march, only two days after he concluded his observations. the importance of this great discovery was instantly felt by the enemies as well as by the friends of the copernican system. the planets had hitherto been distinguished from the fixed stars only by their relative change of place, but the telescope proved them to be bodies so near to our own globe as to exhibit well-defined discs, while the fixed stars retained, even when magnified, the minuteness of remote and lucid points. the system of jupiter, illuminated by four moons performing their revolutions in different and regular periods, exhibited to the proud reason of man the comparative insignificance of the globe he inhabits, and proclaimed in impressive language that that globe was not the centre of the universe. the reception which these discoveries met with from kepler is highly interesting, and characteristic of the genius of that great man. he was one day sitting idle, and thinking of galileo, when his friend wachenfels stopped his carriage at his door, to communicate to him the intelligence. "such a fit of wonder," says he, "seized me at a report which seemed to be so very absurd, and i was thrown into such agitation at seeing an old dispute between us decided in this way, that between his joy, my colouring, and the laughter of both, confounded as we were by such a novelty, we were hardly capable, he of speaking, or i of listening. on our parting, i immediately began to think how there could be any addition to the number of the planets without overturning my 'cosmographic mystery,' according to which euclid's five regular solids do not allow more than six planets round the sun.... i am so far from disbelieving the existence of the four circumjovial planets, that i long for a telescope, to anticipate you, if possible, in discovering _two_ round mars, as the proportion seems to require, _six_ or _eight_ round saturn, and perhaps _one_ each round mercury and venus." in a very different spirit did the aristotelians receive the "sidereal messenger" of galileo. the principal professor of philosophy at padua resisted galileo's repeated and urgent entreaties to look at the moon and planets through his telescope; and he even laboured to convince the grand duke that the satellites of jupiter could not possibly exist. sizzi, an astronomer of florence, maintained that as there were only _seven_ apertures in the head--_two_ eyes, _two_ ears, _two_ nostrils, and _one_ mouth--and as there were only _seven_ metals, and _seven_ days in the week, so there could be only _seven_ planets. he seems, however, to have admitted the visibility of the four satellites through the telescope; but he argues, that as they are invisible to the naked eye, they can exercise no influence on the earth; and being useless, they do not therefore exist. a _protegé_ of kepler's, of the name of horky, wrote a volume against galileo's discovery, after having declared, "that he would never concede his four new planets to that italian from padua, even if he should die for it." this resolute aristotelian was at no loss for arguments. he asserted that he had examined the heavens _through galileo's own glass_, and that no such thing as a satellite existed round jupiter. he affirmed, that he did not more surely know that he had a soul in his body, than that reflected rays are the sole cause of galileo's erroneous observations; and that the only use of the new planets was to gratify galileo's thirst for gold, and afford to himself a subject of discussion. when horky first presented himself to kepler, after the publication of this work, the opinion of his patron was announced to him by a burst of indignation which overwhelmed the astonished author. horky supplicated mercy for his offence; and, as kepler himself informed galileo, he took him again into favour, on the condition that kepler was to show him jupiter's satellites, and that horky was not only to see them, but to admit their existence. when the spirit of philosophy had thus left the individuals who bore so unworthily her sacred name, it was fortunate for science that it found a refuge among princes. notwithstanding the reiterated logic of his philosophical professor at padua, cosmo de medici preferred the testimony of his senses to the syllogisms of his instructor. he observed the new planets several times, along with galileo, at pisa; and when he parted with him, he gave him a present worth more than florins, and concluded that liberal arrangement to which we have already referred. as philosopher and principal mathematician to the grand duke of tuscany, galileo now took up his residence at florence, with a salary of florins. no official duties, excepting that of lecturing occasionally to sovereign princes, were attached to this appointment; and it was expressly stipulated that he should enjoy the most perfect leisure to complete his treatises on the constitution of the universe, on mechanics, and on local motion. the resignation of his professorship in the university of padua, which was the necessary consequence of his new appointment, created much dissatisfaction: but though many of his former friends refused at first to hold any communication with him, this excitement gradually subsided; and the venetian senate at last appreciated the feelings, as well as the motives, which induced a stranger to accept of promotion in his native land. while galileo was enjoying the reward and the fame of his great discovery, a new species of enmity was roused against him. simon mayer, an astronomer of no character, pretended that he had discovered the satellites of jupiter before galileo, and that his first observation was made on the th of december, . other astronomers announced the discovery of new satellites: scheiner reckoned five, rheita nine, and others found even so many as twelve: these satellites, however, were found to be only fixed stars. the names of _vladislavian_, _agrippine_, _uranodavian_, and _ferdinandotertian_, which were hastily given to these common telescopic stars, soon disappeared from the page of science, and even the splendid telescopes of modern times have not been able to add another gem to the diadem of jupiter. a modern astronomer of no mean celebrity has, even in the present day, endeavoured to rob galileo of this staple article of his reputation. from a careless examination of the papers of our celebrated countryman, thomas harriot, which baron zach had made in , at petworth, the seat of lord egremont, this astronomer has asserted[ ] that harriot first observed the satellites of jupiter on the th of january, ; and continued his observations till the th of february, . baron zach adds the following extraordinary conclusion:--"galileo pretends to have discovered them on the th of january, ; so that it is not improbable that harriot was likewise the first discoverer of these attendants of jupiter." in a communication which i received from dr robertson, of oxford, in ,[ ] he informed me that he had examined a portion of harriot's papers, entitled, "de jovialibus planetis;" and that it appears, from two pages of these papers, _that harriot first observed jupiter's satellites on the th of october, _. these observations are accompanied with rough drawings of the positions of the satellites, and rough calculations of their periodical revolutions. my friend, professor rigaud,[ ] who has very recently examined the harriot mss., has confirmed the accuracy of dr robertson's observations, and has thus restored to galileo the honour of being the first and the sole discoverer of these secondary planets. [ ] berlin ephemeris, . [ ] edin. phil. journ. vol. vi. p. . [ ] life and correspondence of dr bradley, oxford, , p. , see also his supplement. oxford, , p. . chapter iii. _galileo announces his discoveries in enigmas--discovers the crescent of venus--the ring of saturn--the spots on the sun--similar observations made in england by harriot--claims of fabricius and scheiner to the discovery of the solar spots--galileo's letters to velser on the claims of scheiner--his residence at the villa of salviati--composes his work on floating bodies, which involves him in new controversies._ the great success which attended the first telescopic observations of galileo, induced him to apply his best instruments to the other planets of our system. the attempts which had been made to deprive him of the honour of some of his discoveries, combined, probably, with a desire to repeat his observations with better telescopes, led him to announce his discoveries under the veil of an enigma, and to invite astronomers to declare, within a given time, if they had observed any new phenomena in the heavens. before the close of , galileo excited the curiosity of astronomers by the publication of his first enigma. kepler and others tried in vain to decipher it; but in consequence of the emperor rodolph requesting a solution of the puzzle, galileo sent him the following clue:-- "altissimam planetam tergeminam observavi." i have observed that the most remote planet is triple. in explaining more fully the nature of his observation, galileo remarked that saturn was not a single star, but three together, nearly touching one another. he described them as having no relative motion, and as having the form of three o's, namely, ooo, the central one being larger than those on each side of it. although galileo had announced that nothing new appeared in the other planets, yet he soon communicated to the world another discovery of no slight interest. the enigmatical letters in which it was concealed formed the following sentence:-- "cynthiæ figuras æmulatur mater amorum." venus rivals the phases of the moon. hitherto, galileo had observed venus when her disc was largely illuminated; but having directed his telescope to her when she was not far removed from the sun, he saw her in the form of a crescent, resembling exactly the moon at the same elongation. he continued to observe her night after night, during the whole time that she could be seen in the course of her revolution round the sun, and he found that she exhibited the very same phases which resulted from her motion round that luminary. galileo had long contemplated a visit to the metropolis of italy, and he accordingly carried his intentions into effect in the early part of the year . here he was received with that distinction which was due to his great talents and his extended reputation. princes, cardinals, and prelates hastened to do him honour; and even those who discredited his discoveries, and dreaded their results, vied with the true friends of science in their anxiety to see the intellectual wonder of the age. in order to show the new celestial phenomena to his friends at rome, galileo took with him his best telescope; and as he had discovered the spots on the sun's surface in october or november , or even earlier,[ ] he had the gratification of exhibiting them to his admiring disciples. he accordingly erected his telescope in the quirinal garden, belonging to cardinal bandini; and in april he shewed them to his friends in many of their most interesting variations. from their change of position on the sun's disc, galileo at first inferred, either that the sun revolved about an axis, or that other planets, like venus and mercury, revolved so near the sun as to appear like black spots when they were opposite to his disc. upon continuing his observations, however, he saw reason to abandon this hasty opinion. he found that the spots must be in contact with the surface of the sun,--that their figures were irregular,--that they had different degrees of darkness,--that one spot would often divide itself into three or four,--that three or four spots would often unite themselves into one,--and that all the spots revolved regularly with the sun, which appeared to complete its revolution in about twenty-eight days. [ ] professor rigaud is of opinion that galileo had discovered the solar spots at an earlier period than eighteen months before may . previous to the invention of the telescope, spots had been more than once seen on the sun's disc with the unassisted eye. but even if these were of the same character as those which galileo and others observed, we cannot consider them as anticipations of their discovery by the telescope. as the telescope was now in the possession of several astronomers, galileo began to have many rivals in discovery; but notwithstanding the claims of harriot, fabricius, and scheiner, it is now placed beyond the reach of doubt that he was the first discoverer of the solar spots. from the communication which i received in from the late dr robertson, of oxford,[ ] it appeared that thomas harriot had observed the solar spots on the th of december ; but his manuscripts, in lord egremont's possession,[ ] incontestably prove that his regular observations on the spots did not commence till december , , although he had seen the spots at the date above mentioned, and that they were continued till the th of january . the observations which he has recorded are in number, and the accounts of them are accompanied with rough drawings representing the number, position, and magnitude of the spots.[ ] in the observation of harriot, made on the th december , before he knew of galileo's discovery, he saw three spots on the sun, which he has represented in a diagram. the sun was then ° or ° high, and there was a frost and a mist, which no doubt acted as a darkening glass. harriot does not apply the name of spots to what he noticed in this observation, and he does not enumerate it among the observations above mentioned. professor rigaud[ ] considers it "a misapplication of terms to call such an observation a discovery;" but, with all the respect which we feel for the candour of this remark, we are disposed to confer on harriot the merit of an original discoverer of the spots on the sun. [ ] see page . [ ] these interesting mss. i have had the good fortune of seeing in the possession of my much valued friend, the late professor rigaud of oxford. [ ] edin. phil. journ. , vol. vi. p. . see rigaud's life of bradley, supplement, p. . [ ] id. it., p. , . another candidate for the honour of discovering the spots of the sun, was john fabricius, who undoubtedly saw them previous to june . the dedication of the work[ ] in which he has recorded his observation, bears the date of the th of june ; and it is obvious, from the work itself, that he had seen the spots about the end of the year ; but as there is no proof that he saw them before october, we are compelled to assign the priority of the discovery to the italian astronomer. [ ] joh. fabricii phrysii de maculis in sole observatis, et apparente earum cum sole conversione, narratio. wittemb. . the claim of scheiner, professor of mathematics at ingolstadt, is more intimately connected with the history of galileo. this learned astronomer having, early in , turned his telescope to the sun, necessarily discovered the spots which at that time covered his disc. light flying clouds happened, at the time, to weaken the intensity of his light, so that he was able to show the spots to his pupils. these observations were not published till january ; and they appeared in the form of three letters, addressed to mark velser, one of the magistrates of augsburg, under the signature of _appelles post tabulam_. scheiner, who, many years afterwards, published an elaborate work on the subject, adopted the same idea which had at first occurred to galileo--that the spots were the dark sides of planets revolving round and near the sun.[ ] [ ] it does not appear from the history of solar observations at what time, and by whom, coloured glasses were first introduced for permitting the eye to look at the sun with impunity. fabricius was obviously quite ignorant of the use of coloured glasses. he observed the sun when he was in the horizon, and when his brilliancy was impaired by the interposition of thin clouds and floating vapours; and he advises those who may repeat his observations to admit at first to the eye a small portion of the sun's light, till it is gradually accustomed to its full splendour. when the sun's altitude became considerable, fabricius gave up his observations, which he often continued so long that he was scarcely able, for two days together, to see objects with their usual distinctness. fabricius speaks of observing the sun by admitting his rays through a small _hole_ into a dark room, and receiving his image on paper; but he says nothing about a lens or a telescope being applied to the hole; and he does not say that he saw the spots of the sun in this way. harriot also viewed the solar spots when the sun was near the horizon, or was visible through "thick layer and thin cloudes," or through thin mist. on december , , at a quarter past p.m., he observed the spots when the sky was perfectly clear, but his "sight was after dim for an houre." scheiner, in his "appelles post tabulam," describes four different ways of viewing the spots; one of which is by the _interposition of blue or green glasses_. his first method was to observe the sun near the horizon; the second was to view him through a transparent cloud; the third was to look at him through his telescope with a blue or a green glass of a proper thickness, and plane on both sides, or to use a thin blue glass when the sun was covered with a thin vapour or cloud; and the fourth method was to begin and observe the sun at his margin, till the eye gradually reached the middle of his disc. on the publication of scheiner's letters, velser transmitted a copy of them to his friend galileo, with the request that he would favour him with his opinion of the new phenomena. after some delay, galileo addressed three letters to velser, in which he combated the opinions of scheiner on the cause of the spots. the first of these letters was dated the th of may ;[ ] but though the controversy was carried on in the language of mutual respect and esteem, it put an end to the friendship which had existed between the two astronomers. in these letters galileo showed that the spots often dispersed like vapours or clouds; that they sometimes had a duration of only one or two days, and at other times of thirty or forty days; that they contracted in their breadth when they approached the sun's limb, without any diminution of their length; that they describe circles parallel to each other; that the monthly rotation of the sun again brings the same spots into view; and that they are seldom seen at a greater distance than ° from the sun's equator. galileo likewise discovered on the sun's disc _faculæ_, or _luculi_, as they were called, which differ in no respect from the common ones but in their being brighter than the rest of the sun's surface.[ ] [ ] the original of this letter is in the british museum. [ ] see istoria e dimonstrazioni, intorno alle macchie solare. _roma_, . see opere di galileo, vol, v., p. - . in the last of the letters which our author addressed to velser, and which was written in december , he recurs to his former discovery of the elongated shape, or rather the triple structure, of saturn. the singular figure which he had observed in this planet had entirely disappeared; and he evidently announces the fact to velser, lest it should be used by his enemies to discredit the accuracy of his observations. "looking on saturn," says he, "within these few days, i found it solitary, without the assistance of its accustomed stars, and, in short, perfectly round and defined like jupiter; and such it still remains. now, what can be said of so strange a metamorphosis? are the two smaller stars consumed like the spots on the sun? have they suddenly vanished and fled? or has saturn devoured his own children? or was the appearance indeed fraud and illusion, with which the glasses have for so long a time mocked me, and so many others who have often observed with me? now, perhaps, the time is come to revive the withering hopes of those who, guided by more profound contemplations, have followed all the fallacies of the new observations, and recognised their impossibilities. i cannot resolve what to say in a chance so strange, so new, and so unexpected; the shortness of the time, the unexampled occurrence, the weakness of my intellect, and the terror of being mistaken, have greatly confounded me." although galileo struggled to obtain a solution of this mystery, yet he had not the good fortune to succeed. he imagined that the two smaller stars would reappear, in consequence of the supposed revolution of the planet round its axis; but the discovery of the ring of saturn, and of the obliquity of its plane to the ecliptic, was necessary to explain the phenomena which were so perplexing to our author. the ill health to which galileo was occasionally subject, and the belief that the air of florence was prejudicial to his complaints, induced him to spend much of his time at selve, the villa of his friend salviati. this eminent individual had ever been the warmest friend of galileo, and seems to have delighted in drawing round him the scientific genius of the age. he was a member of the celebrated lyncæan society, founded by prince frederigo cesi; and though he is not known as the author of any important discovery, yet he has earned, by his liberality to science, a glorious name, which will be indissolubly united with the immortal destiny of galileo. the subject of floating bridges having been discussed at one of the scientific parties which had assembled at the house of salviati, a difference of opinion arose respecting the influence of the shape of bodies on their disposition to float or to sink in a fluid. contrary to the general opinion, galileo undertook to prove that it depended on other causes; and he was thus led to compose his discourse on floating bodies,[ ] which was published in , and dedicated to cosmo de medici. this work contains many ingenious experiments, and much acute reasoning in support of the true principles of hydrostatics; and it is now chiefly remarkable as a specimen of the sagacity and intellectual power of its author. like all his other works, it encountered the most violent opposition; and galileo was more than once summoned into the field to repel the aggressions of his ignorant and presumptuous opponents. the first attack upon it was made by ptolemy nozzolini, in a letter to marzemedici, archbishop of florence;[ ] and to this galileo replied in a letter addressed to his antagonist.[ ] a more elaborate examination of it was published by lodovico delle colombe, and another by m. vincenzo di grazia. to these attacks, a minute and overwhelming answer was printed in the name of benedetti castelli, the friend and pupil of galileo; but it was discovered, some years after galileo's death, that he was himself the author of this work.[ ] [ ] discorso intorno alle cose che stanno in su l'acqua, o che in quella si muovono. opere di galileo, vol. ii. pp. - . [ ] opere di galileo, vol. ii. pp. - . [ ] ibid. - . [ ] these three treatises occupy the whole of the third volume of the opere di galileo. chapter iv. _galileo treats his opponents with severity and sarcasm--he is aided by the sceptics of the day--the church party the most powerful--galileo commences the attack, and is answered by caccini, a dominican--galileo's letter to the grand duchess of tuscany, in support of the motion of the earth and the stability of the sun--- galileo visits rome--is summoned before the inquisition, and renounces his opinions as heretical--the inquisition denounces the copernican system--galileo has an audience of the pope, but still maintains his opinions in private society--proposes to find out the longitude at sea by means of jupiter's satellites--his negociation on this subject with the court of spain--its failure--he is unable to observe the three comets of , but is involved in the controversy to which they gave rise._ the current of galileo's life had hitherto flowed in a smooth and unobstructed channel. he had now attained the highest objects of earthly ambition. his discoveries had placed him at the head of the great men of the age; he possessed a professional income far beyond his wants, and even beyond his anticipations; and, what is still dearer to a philosopher, he enjoyed the most perfect leisure for carrying on and completing his discoveries. the opposition which these discoveries encountered, was to him more a subject for triumph than for sorrow. prejudice and ignorance were his only enemies; and if they succeeded for a while in harassing his march, it was only to lay a foundation for fresh achievements. he who contends for truths which he has himself been permitted to discover, may well sustain the conflict in which presumption and error are destined to fall. the public tribunal may neither be sufficiently pure nor enlightened to decide upon the issue; but he can appeal to posterity, and reckon with confidence on "its sure decree." the ardour of galileo's mind, the keenness of his temper, his clear perception of truth, and his inextinguishable love of it, combined to exasperate and prolong the hostility of his enemies. when argument failed to enlighten their judgment, and reason to dispel their prejudices, he wielded against them his powerful weapons of ridicule and sarcasm; and in this unrelenting warfare, he seems to have forgotten that providence had withheld from his enemies those very gifts which he had so liberally received. he who is allowed to take the start of his species, and to penetrate the veil which conceals from common minds the mysteries of nature, must not expect that the world will be patiently dragged at the chariot wheels of his philosophy. mind has its inertia as well as matter; and its progress to truth can only be insured by the gradual and patient removal of the obstructions which surround it. the boldness--may we not say the recklessness--with which galileo insisted upon making proselytes of his enemies, served but to alienate them from the truth. errors thus assailed speedily entrench themselves in general feelings, and become embalmed in the virulence of the passions. the various classes of his opponents marshalled themselves for their mutual defence. the aristotelian professors, the temporising jesuits, the political churchmen, and that timid but respectable body who at all times dread innovation, whether it be in religion or in science, entered into an alliance against the philosophical tyrant who threatened them with the penalties of knowledge. the party of galileo, though weak in numbers, was not without power and influence. he had trained around him a devoted band, who idolised his genius and cherished his doctrines. his pupils had been appointed to several of the principal professorships in italy. the enemies of religion were on this occasion united with the christian philosopher; and there were, even in these days, many princes and nobles who had felt the inconvenience of ecclesiastical jurisdiction, and who secretly abetted galileo in his crusade against established errors. although these two parties had been long dreading each others power, and reconnoitring each others position, yet we cannot exactly determine which of them hoisted the first signal for war. the church party, particularly its highest dignitaries, were certainly disposed to rest on the defensive. flanked on one side by the logic of the schools, and on the other by the popular interpretation of scripture, and backed by the strong arm of the civil power, they were not disposed to interfere with the prosecution of science, however much they may have dreaded its influence. the philosophers, on the contrary, united the zeal of innovators with that firmness of purpose which truth alone can inspire. victorious in every contest, they were flushed with success, and they panted for a struggle in which they knew they must triumph. in this state of warlike preparation galileo addressed a letter, in , to his friend and pupil, the abbé castelli, the object of which was to prove that the scriptures were not intended to teach us science and philosophy. hence he inferred, that the language employed in the sacred volume in reference to such subjects should be interpreted only in its common acceptation; and that it was in reality as difficult to reconcile the ptolemaic as the copernican system to the expressions which occur in the bible. a demonstration was about this time made by the opposite party, in the person of caccini, a dominican friar, who made a personal attack upon galileo from the pulpit. this violent ecclesiastic ridiculed the astronomer and his followers, by addressing them sarcastically in the sacred language of scripture--"ye men of _galilee_, why stand ye here looking up into heaven?" but this species of warfare was disapproved of even by the church; and luigi maraffi, the general of the dominicans, not only apologised to galileo, who had transmitted to him a formal complaint against caccini, but expressed the acuteness of his own feelings on being implicated in the "brutal conduct of thirty or forty thousand monks." from the character of caccini, and the part which he afterwards played in the persecution of galileo, we can scarcely avoid the opinion that his attack from the pulpit was intended as a snare for the unwary philosopher. it roused galileo from his wonted caution; and stimulated, no doubt, by the nature of the answer which he received from maraffi, he published a long letter of seventy pages, defending and illustrating his former views respecting the influence of scriptural language on the two contending systems. as if to give the impress of royal authority to this new appeal, he addressed it to christian, grand duchess of tuscany, the mother of cosmo; and in this form it seems to have excited a new interest, as if it had expressed the opinion of the grand ducal family. these external circumstances gave additional weight to the powerful and unanswerable reasoning which this letter contains; and it was scarcely possible that any man, possessed of a sound mind, and willing to learn the truth, should refuse his assent to the judicious views of our author. he expresses his belief that the scriptures were designed to instruct mankind respecting their salvation, and that the faculties of our minds were given us for the purpose of investigating the phenomena of nature. he considers scripture and nature as proceeding from the same divine author, and, therefore, incapable of speaking a different language; and he points out the absurdity of supposing that professors of astronomy will shut their eyes to the phenomena which they discover in the heavens, or will refuse to believe those deductions of reason which appeal to their judgment with all the power of demonstration. he supports these views by quotations from the ancient fathers; and he refers to the dedication of copernicus's own work to the roman pontiff, paul iii., as a proof that the pope himself did not regard the new system of the world as hostile to the sacred writings. copernicus, on the contrary, tells his holiness, that the reason of inscribing to him his new system was, that the authority of the pontiff might put to silence the calumnies of some individuals, who attacked it by arguments drawn from passages of scripture twisted for their own purpose. it was in vain to meet such reasoning by any other weapons than those of the civil power. the enemies of galileo saw that they must either crush the dangerous innovation, or allow it the fullest scope; and they determined upon an appeal to the inquisition. lorini, a monk of the dominican order, had already denounced to this body galileo's letter to castelli; and caccini, bribed by the mastership of the convent of st mary of minerva, was invited to settle at rome for the purpose of embodying the evidence against galileo. though these plans had been carried on in secret, yet galileo's suspicions were excited; and he obtained leave from cosmo to go to rome about the end of .[ ] here he was lodged in the palace of the grand duke's ambassador, and kept up a constant correspondence with the family of his patron at florence; but, in the midst of this external splendour, he was summoned before the inquisition to answer for the heretical doctrines which he had published. he was charged with maintaining the motion of the earth, and the stability of the sun--with teaching this doctrine to his pupils--with corresponding on the subject with several german mathematicians--and with having published it, and attempted to reconcile it to scripture, in his letters to mark velser in . the inquisition assembled to consider these charges on the th of february ; and it was decreed that galileo should be enjoined by cardinal bellarmine to renounce the obnoxious doctrines, and to pledge himself that he would neither teach, defend, nor publish them in future. in the event of his refusing to acquiesce in this sentence, it was decreed that he should be thrown into prison. galileo did not hesitate to yield to this injunction. on the day following, the th of february, he appeared before cardinal bellarmine, to renounce his heretical opinions; and, having declared that he abandoned the doctrine of the earth's motion, and would neither defend nor teach it, in his conversation or in his writings, he was dismissed from the bar of the inquisition. [ ] it is said that galileo was cited to appear at rome on this occasion; and the opinion is not without foundation. having thus disposed of galileo, the inquisition conceived the design of condemning the whole system of copernicus as heretical. galileo, with more hardihood than prudence, remained at rome for the purpose of giving his assistance in frustrating this plan; but there is reason to think that he injured by his presence the very cause which he meant to support. the inquisitors had determined to put down the new opinions; and they now inserted among the prohibited books galileo's letters to castelli and the grand duchess, kepler's epitome of the copernican theory, and copernicus's own work on the revolutions of the heavenly bodies. notwithstanding these proceedings, galileo had an audience of the pope, paul v., in march . he was received very graciously, and spent nearly an hour with his holiness. when they were about to part, the pope assured galileo, that the congregation were not disposed to receive upon light grounds any calumnies which might be propagated by his enemies, and that, as long as he occupied the papal chair, he might consider himself as safe. these assurances were no doubt founded on the belief that galileo would adhere to his pledges; but so bold and inconsiderate was he in the expression of his opinions, that even in rome he was continually engaged in controversial discussions. the following very interesting account of these disputes is given by querenghi, in a letter to the cardinal d'este:-- "your eminence would be delighted with galileo if you heard him holding forth, as he often does, in the midst of fifteen or twenty, all violently attacking him, sometimes in one house, sometimes in another. but he is armed after such fashion that he laughs all of them to scorn; and even if the novelty of his opinions prevents entire persuasion, he at least convicts of emptiness most of the arguments with which his adversaries endeavour to overwhelm him. he was particularly admirable on monday last in the house of signor frederico ghisilieri; and what especially pleased me was, that before replying to the contrary arguments, he amplified and enforced them with new grounds of great plausibility, so as to leave his adversaries in a more ridiculous plight, when he afterwards overturned them all." the discovery of jupiter's satellites suggested to galileo a new method of finding the longitude at sea. philip iii. had encouraged astronomers to direct their attention to this problem, by offering a reward for its solution; and in those days, when new discoveries in science were sometimes rejected as injurious to mankind, it was no common event to see a powerful sovereign courting the assistance of astronomers in promoting the commercial interests of his empire. galileo seems to have regarded the solution of this problem as an object worthy of his ambition; and he no doubt anticipated the triumph which he would obtain over his enemies, if the medicean stars, which they had treated with such contempt, could be made subservient to the great interests of mankind. during his residence at rome in and , galileo had communicated his views on this subject to the comte di lemos, the viceroy of naples, who had presided over the council of the spanish indies. this nobleman advised him to apply to the spanish minister the duke of lerma; and, through the influence of the grand duke cosmo, his ambassador at the court of madrid was engaged to manage the affair. the anxiety of galileo on this subject was singularly great. he assured the tuscan ambassador that, in order to accomplish this object, "he was ready to leave all his comforts, his country, his friends, and his family, to cross over into spain, and to stay as long as he might be wanted at seville or at lisbon, or wherever it might be convenient to communicate a knowledge of his method." the lethargy of the spanish court seems to have increased with the enthusiasm of galileo; and though the negotiations were occasionally revived for ten or twelve years, yet no steps were taken to bring them to a close. this strange procrastination has been generally ascribed to jealousy or indifference on the part of spain; but nelli, one of galileo's biographers, declares, on the authority of florentine records, that cosmo had privately requested from the government the privilege of sending annually to the spanish indies two leghorn merchantmen free of duty, as a compensation for the loss of galileo! the failure of this negotiation must have been a source of extreme mortification to the high spirit and sanguine temperament of galileo. he had calculated, however, too securely on his means of putting the new method to a successful trial. the great imperfection of the time-keepers of that day, and the want of proper telescopes, would have baffled him in all his efforts, and he would have been subject to a more serious mortification from the failure and rejection of his plan, than that which he actually experienced from the avarice of his patron, or the indifference of spain. even in the present day, no telescope has been invented which is capable of observing at sea the eclipses of jupiter's satellites; and though this method of finding the longitude has great advantages on shore, yet it has been completely abandoned at sea, and superseded by easier and more correct methods. in the year , when no fewer than _three_ comets visited our system, and attracted the attention of all the astronomers of europe, galileo was unfortunately confined to his bed by a severe illness; but, though he was unable to make a single observation upon these remarkable bodies, he contrived to involve himself in the controversies which they occasioned. marco guiducci, an astronomer of florence, and a friend of galileo, had delivered a discourse on comets before the florentine academy. the heads of this discourse, which was published in ,[ ] were supposed to have been communicated to him by galileo, and this seems to have been universally admitted during the controversy to which it gave rise. the opinion maintained in this treatise, that comets are nothing but meteors which occasionally appear in our atmosphere, like halos and rainbows, savours so little of the sagacity of galileo that we should be disposed to question its paternity. his inability to partake in the general interest which these three comets excited, and to employ his powerful telescope in observing their phenomena, and their movements, might have had some slight share in the formation of an opinion which deprived them of their importance as celestial bodies. but, however this may have been, the treatise of guiducci afforded a favourable point of attack to galileo's enemies, and the dangerous task was entrusted to horatio grassi, a learned jesuit, who, in a work entitled _the astronomical and philosophical balance_, criticised the discourse on comets, under the feigned name of lotario sarsi. [ ] discorso delle comete. printed in the opere di galileo, vol. vi., pp. - . galileo replied to this attack in a volume entitled _il saggiatore_, or _the assayer_, which, owing to the state of his health, was not published till the autumn of .[ ] this work was written in the form of a letter to virginio cesarini, a member of the lyncæan academy, and master of the chamber to urban viii., who had just ascended the papal throne. it was dedicated to the pontiff himself, and has been long celebrated among literary men for the beauty of its language, though it is doubtless one of the least important of galileo's writings. [ ] printed in the opere di galileo, vol. vi., pp. - . chapter v. _urban viii., galileo's friend, raised to the pontificate--galileo goes to rome to offer his congratulations--the pope loads galileo with presents, and promises a pension to his son--galileo in pecuniary difficulties, owing to the death of his patron, cosmo--galileo again rashly attacks the church, notwithstanding the pope's kindness--he composes his system of the world, to demonstrate the copernican system--artfully obtains a license to print it--nature of the work--its influence on the public mind--the pope resolves on suppressing it--galileo summoned before the inquisition--his trial--his defence--his formal abjuration of his opinions--observations on his conduct--the pope shews great indulgence to galileo, who is allowed to return to his own house at arcetri, as the place of his confinement._ the succession of the cardinal maffeo barberini to the papal throne, under the name of urban viii., was hailed by galileo and his friends as an event favourable to the promotion of science. urban had not only been the personal friend of galileo and of prince cesi, the founder of the lyncæan academy, but had been intimately connected with that able and liberal association; and it was therefore deemed prudent to secure his favour and attachment. if paul iii. had, nearly a century before, patronised copernicus, and accepted of the dedication of his great work, it was not unreasonable to expect that, in more enlightened times, another pontiff might exhibit the same liberality to science. the plan of securing to galileo the patronage of urban viii. seems to have been devised by prince cesi. although galileo had not been able for some years to travel, excepting in a litter, yet he was urged by the prince to perform a journey to rome, for the express purpose of congratulating his friend upon his elevation to the papal chair. this request was made in october ; and though galileo's health was not such as to authorise him to undergo so much fatigue, yet he felt the importance of the advice, and, after visiting cesi at acqua sparta, he arrived at rome in the spring of . the reception which he here experienced far exceeded his most sanguine expectations. during the two months which he spent in the capital he was permitted to have no fewer than six long and gratifying audiences of the pope. the kindness of his holiness was of the most marked description. he not only loaded galileo with presents,[ ] and promised him a pension for his son vincenzo, but he wrote a letter to ferdinand, who had just succeeded cosmo as grand duke of tuscany, recommending galileo to his particular patronage. "for we find in him," says he, "not only literary distinction, but the love of piety; and he is strong in those qualities by which pontifical good-will is easily obtained. and now, when he has been brought to this city to congratulate us on our elevation, we have very lovingly embraced him; nor can we suffer him to return to the country whither your liberality recalls him, without an ample provision of pontifical love. and that you may know how dear he is to us, we have willed to give him this honourable testimonial of virtue and piety. and we further signify, that every benefit which you shall confer upon him, imitating or even surpassing your father's liberality, will conduce to our gratification." [ ] a fine painting in gold, and a silver medal, and "a good quantity of agnus dei." not content with thus securing the friendship of the pope, galileo endeavoured to bespeak the good-will of the cardinals towards the copernican system. he had, accordingly, many interviews with several of these dignitaries; and he was assured, by cardinal hohenzoller, that in a representation which he had made to the pope on the subject of copernicus, he stated to his holiness, "that as all the heretics considered that system as undoubted, it would be necessary to be very circumspect in coming to any resolution on the subject." to this remark his holiness replied--"that the church had not condemned this system; and that it should not be condemned as heretical, but only as rash;" and he added, "that there was no fear of any person undertaking to prove that it must necessarily be true." the recent appointment of the abbé castelli, the friend and pupil of galileo, to be mathematician to the pope, was an event of a most gratifying nature; and when we recollect that it was to castelli that he addressed the famous letter which was pronounced heretical by the inquisition, we must regard it also as an event indicative of a new and favourable feeling towards the friends of science. the opinions of urban, indeed, had suffered no change. he was one of the few cardinals who had opposed the inquisitorial decree of , and his subsequent demeanour was in every respect conformable to the liberality of his early views. the sincerity of his conduct was still further evinced by the grant of a pension of one hundred crowns to galileo, a few years after his visit to rome; though there is reason to think that this allowance was not regularly paid. the death of cosmo, whose liberality had given him both affluence and leisure, threatened galileo with pecuniary difficulties. he had been involved in a "great load of debt," owing to the circumstances of his brother's family; and, in order to relieve himself, he had requested castelli to dispose of the pension of his son vincenzo. in addition to this calamity he was now alarmed at the prospect of losing his salary as an extraordinary professor at pisa. the great youth of ferdinand, who was scarcely of age, induced galileo's enemies, in , to raise doubts respecting the payment of a salary to a professor who neither resided nor lectured in the university; but the question was decided in his favour, and we have no doubt that the decision was facilitated by the friendly recommendation of the pope, to which we have already referred. although galileo had made a narrow escape from the grasp of the inquisition, yet he was never sufficiently sensible of the lenity which he experienced. when he left rome in , under the solemn pledge of never again teaching the obnoxious doctrine, it was with a hostility against the church, suppressed but deeply cherished; and his resolution to propagate the heresy seems to have been coeval with the vow by which he renounced it. in the year , when he communicated his theory of the tides to the archduke leopold, he alludes in the most sarcastic manner to the conduct of the church. the same hostile tone, more or less, pervaded all his writings, and, while he laboured to sharpen the edge of his satire, he endeavoured to guard himself against its effects, by an affectation of the humblest deference to the decisions of theology. had galileo stood alone, his devotion to science might have withdrawn him from so hopeless a contest; but he was spurred on by the violence of a party. the lyncæan academy never scrupled to summon him from his researches. they placed him in the forlorn hope of their combat, and he at last fell a victim to the rashness of his friends. but whatever allowance we may make for the ardour of galileo's temper, and the peculiarity of his position; and however we may justify and even approve of his past conduct, his visit to urban viii., in , placed him in a new relation to the church, which demanded on his part a new and corresponding demeanour. the noble and generous reception which he met with from urban, and the liberal declaration of cardinal hohenzoller on the subject of the copernican system, should have been regarded as expressions of regret for the past, and offers of conciliation for the future. thus honoured by the head of the church, and befriended by its dignitaries, galileo must have felt himself secure against the indignities of its lesser functionaries, and in the possession of the fullest license to prosecute his researches and publish his discoveries, provided he avoided that dogma of the church which, even in the present day, it has not ventured to renounce. but galileo was bound to the romish hierarchy by even stronger ties. his son and himself were pensioners of the church, and, having accepted of its alms, they owed to it, at least, a decent and respectful allegiance. the pension thus given by urban was not a remuneration which sovereigns sometimes award to the services of their subjects. galileo was a foreigner at rome. the sovereign of the papal state owed him no obligation; and hence we must regard the pension of galileo as a donation from the roman pontiff to science itself, and as a declaration to the christian world that religion was not jealous of philosophy, and that the church of rome was willing to respect and foster even the genius of its enemies. galileo viewed all these circumstances in a different light. he resolved to compose a work in which the copernican system should be demonstrated; but he had not the courage to do this in a direct and open manner. he adopted the plan of discussing the subject in a dialogue between three speakers, in the hope of eluding by this artifice the censure of the church. this work was completed in , but, owing to some difficulties in obtaining a license to print it, it was not published till . in obtaining this license, galileo exhibited considerable address, and his memory has not escaped from the imputation of having acted unfairly, and of having involved his personal friends in the consequences of his imprudence. the situation of master of the palace was, fortunately for galileo's designs, filled by nicolo riccardi, a friend and pupil of his own. this officer was a sort of censor of new publications, and when he was applied to on the subject of printing his work, galileo soon found that attempts had previously been made to thwart his views. he instantly set off for rome, and had an interview with his friend, who was in every respect anxious to oblige him. riccardi examined the manuscript, pointed out some incautious expressions which he considered it necessary to erase, and returned it with his written approbation, on the understanding that the alterations he suggested would be made. dreading to remain in rome during the unhealthy season, which was fast approaching, galileo returned to florence, with the intention of completing the index and dedication, and of sending the ms. to rome, to be printed under the care of prince cesi. the death of that distinguished individual, in august , frustrated galileo's plan, and he applied for leave to have the book printed in florence. riccardi was at first desirous to examine the ms. again, but, after inspecting only the beginning and the end of it, he gave galileo leave to print it wherever he chose, providing it bore the license of the inquisitor-general of florence, and one or two other persons whom he named. having overcome all these difficulties, galileo's work was published in , under the title of "_the system of the world of galileo galilei_, &c., in which, in four dialogues concerning the two principal systems of the world--the ptolemaic and the copernican--he discusses, indeterminately and firmly, the arguments proposed on both sides." it is dedicated to ferdinand, grand duke of tuscany, and is prefaced by an "address to the prudent reader," which is itself characterised by the utmost imprudence. he refers to the decree of the inquisition in the most insulting and ironical language. he attributes it to passion and to ignorance, not by direct assertion, but by insinuations ascribed to others; and he announces his intention to defend the copernican system, as a pure mathematical hypothesis, and not as an opinion having an advantage over that of the stability of the earth absolutely. the dialogue is conducted by three persons, salviati, sagredo, and simplicio. salviati, who is the true philosopher in the dialogue, was the real name of a nobleman whom we have already had occasion to mention. sagredo, the name of another noble friend of galileo's, performs a secondary part under salviati. he proposes doubts, suggests difficulties, and enlivens the gravity of the dialogue with his wit and pleasantry. simplicio is a resolute follower of ptolemy and aristotle, and, with a proper degree of candour and modesty, he brings forward all the common arguments in favour of the ptolemaic system. between the wit of sagredo, and the powerful philosophy of salviati, the peripatetic sage is baffled in every discussion; and there can be no doubt that galileo aimed a more fatal blow at the ptolemaic system by this mode of discussing it, than if he had endeavoured to overturn it by direct arguments. the influence of this work on the public mind was such as might have been anticipated. the obnoxious doctrines which it upheld were eagerly received, and widely disseminated; and the church of rome became sensible of the shock which was thus given to its intellectual supremacy. pope urban viii., attached though he had been to galileo, never once hesitated respecting the line of conduct which he felt himself bound to pursue. his mind was, nevertheless, agitated with conflicting sentiments. he entertained a sincere affection for science and literature, and yet he was placed in the position of their enemy. he had been the personal friend of galileo, and yet his duty compelled him to become his accuser. embarrassing as these feelings were, other considerations contributed to soothe him. he had, in his capacity of a cardinal, opposed the first persecution of galileo. he had, since his elevation to the pontificate, traced an open path for the march of galileo's discoveries; and he had finally endeavoured to bind the recusant philosopher by the chains of kindness and gratitude. all these means, however, had proved abortive, and he was now called upon to support the doctrine which he had subscribed, and administer the law of which he was the guardian. it has been supposed, without any satisfactory evidence, that urban may have been influenced by less creditable motives. salviati and sagredo being well-known personages, it was inferred that simplicio must also have a representative. the enemies of galileo are said to have convinced his holiness that simplicio was intended as a portraiture of himself; and this opinion received some probability from the fact, that the peripatetic disputant had employed many of the arguments which urban had himself used in his discussions with galileo. the latest biographer of galileo[ ] regards this motive as necessary to account for "the otherwise inexplicable change which took place in the conduct of urban to his old friend;"--but we cannot admit the truth of this supposition. the church had been placed in hostility to a powerful and liberal party, which was adverse to its interests. the dogmas of the catholic faith had been brought into direct collision with the deductions of science. the leader of the philosophic band had broken the most solemn armistice with the inquisition: he had renounced the ties of gratitude which bound him to the pontiff; and urban was thus compelled to entrench himself in a position to which he had been driven by his opponents. [ ] library of useful knowledge, life of galileo, chap. viii. the design of summoning galileo before the inquisition, seems to have been formed almost immediately after the publication of his book; for even in august , the preliminary proceedings had reached the ears of the grand duke ferdinand. the tuscan ambassador at rome was speedily acquainted with the dissatisfaction which his sovereign felt at these proceedings; and he was instructed to forward to florence a written statement of the charges against galileo, in order to enable him to prepare for his defence. although this request was denied, ferdinand again interposed, and transmitted a letter to his ambassador, recommending the admission of campanella and castelli into the congregation of ecclesiastics by whom galileo was to be judged. circumstances, however, rendered it prudent to withhold this letter. castelli was sent away from rome, and scipio chiaramonte, a bigotted ecclesiastic, was summoned from pisa to complete the number of the judges. it appears from a despatch of the tuscan minister, that ferdinand was enraged at the transaction; and he instructed his ambassador, niccolini, to make the strongest representations to the pope. niccolini had several interviews with his holiness; but all his expostulations were fruitless. he found urban highly incensed against galileo; and his holiness begged niccolini to advise the archduke not to interfere any farther, as he would not "get through it with honour." on the th of september the pope caused it to be intimated to niccolini, as a mark of his especial esteem for the grand duke, that he was obliged to refer the work to the inquisition; but both the prince and his ambassador were declared liable to the usual censures if they divulged the secret. from the measures which this tribunal had formerly pursued, it was not difficult to foresee the result of their present deliberations. they summoned galileo to appear before them at rome, to answer in person the charges under which he lay. the tuscan ambassador expostulated warmly with the court of rome on the inhumanity of this proceeding. he urged his advanced age, his infirm health, the discomforts of the journey, and the miseries of the quarantine,[ ] as motives for reconsidering their decision: but the pope was inexorable, and though it was agreed to relax the quarantine as much as possible in his favour, yet it was declared indispensable that he should appear in person before the inquisition. [ ] the communication between florence and rome was at this time interrupted by a contagious disease which had broken out in tuscany. worn out with age and infirmities, and exhausted with the fatigues of his journey, galileo arrived at rome on the th of february, . the tuscan ambassador announced his arrival in an official form to the commissary of the holy office, and galileo awaited in calm dignity the approach of his trial. among those who proffered their advice in this distressing emergency, we must enumerate the cardinal barberino, the pope's nephew, who, though he may have felt the necessity of an interference on the part of the church, was yet desirous that it should be effected with the least injury to galileo and to science. he accordingly visited galileo, and advised him to remain as much at home as possible, to keep aloof from general society, and to see only his most intimate friends. the same advice was given from different quarters; and galileo, feeling its propriety, remained in strict seclusion in the palace of the tuscan ambassador. during the whole of the trial which had now commenced, galileo was treated with the most marked indulgence. abhorring, as we must do, the principles and practice of this odious tribunal, and reprobating its interference with the cautious deductions of science, we must yet admit that, on this occasion, its deliberations were not dictated by passion, nor its power directed by vengeance. though placed at their judgment-seat as a heretic, galileo stood there with the recognised attributes of a sage; and though an offender against the laws of which they were the guardian, yet the highest respect was yielded to his genius, and the kindest commiseration to his infirmities. in the beginning of april, when his examination in person was to commence, it became necessary that he should be removed to the holy office; but instead of committing him, as was the practice, to solitary confinement, he was provided with apartments in the house of the fiscal of the inquisition. his table was provided by the tuscan ambassador, and his servant was allowed to attend him at his pleasure, and to sleep in an adjoining apartment. even this nominal confinement, however, galileo's high spirit was unable to brook. an attack of the disease to which he was constitutionally subject contributed to fret and irritate him, and he became impatient for a release from his anxiety as well as from his bondage. cardinal barberino seems to have received notice of the state of galileo's feelings, and, with a magnanimity which posterity will ever honour, he liberated the philosopher on his own responsibility; and in ten days after his first examination, and on the last day of april, he was restored to the hospitable roof of the tuscan ambassador. though this favour was granted on the condition of his remaining in strict seclusion, galileo recovered his health, and to a certain degree his usual hilarity, amid the kind attentions of niccolini and his family; and when the want of exercise had begun to produce symptoms of indisposition, the tuscan minister obtained for him leave to go into the public gardens in a half-closed carriage. after the inquisition had examined galileo personally, they allowed him a reasonable time for preparing his defence. he felt the difficulty of adducing any thing like a plausible justification of his conduct; and he resorted to an ingenious, though a shallow artifice, which was regarded by the court as an aggravation of the crime. after his first appearance before the inquisition in , he was publicly and falsely charged by his enemies with having then abjured his opinions; and he was taunted as a criminal who had been actually punished for his offences. as a refutation of these calumnies, cardinal bellarmine had given him a certificate in his own handwriting, declaring that he neither abjured his opinions, nor suffered punishment for them; and that the doctrine of the earth's motion, and the sun's stability, was only denounced to him as contrary to scripture, and as one which could not be defended. to this certificate the cardinal did not add, because he was not called upon to do it, that galileo was enjoined not _to teach in any manner_ the doctrine thus denounced; and galileo ingeniously avails himself of this supposed omission, to account for his having, in the lapse of fourteen or sixteen years, forgotten the injunction. he assigned the same excuse for his having omitted to mention this injunction to riccardi, and to the inquisitor-general at florence, when he obtained the licence to print his dialogues. the court held the production of this certificate to be at once a proof and an aggravation of his offence, because the certificate itself declared that the obnoxious doctrines had been pronounced contrary to the holy scriptures. having duly weighed the confessions and excuses of their prisoner, and considered the general merits of the case, the inquisition came to an agreement upon the sentence which they were to pronounce, and appointed the d of june as the day on which it was to be delivered. two days previous to this, galileo was summoned to appear at the holy office; and on the morning of the st, he obeyed the summons. on the d of june he was clothed in a penitential dress, and conducted to the convent of minerva, where the inquisition was assembled to give judgment. a long and elaborate sentence was pronounced, detailing the former proceedings of the inquisition, and specifying the offences which he had committed in teaching heretical doctrines, in violating his former pledges, and in obtaining by improper means a license for the printing of his dialogues. after an invocation of the name of our saviour, and of the holy virgin, galileo is declared to have brought himself under strong suspicions of heresy, and to have incurred all the censures and penalties which are enjoined against delinquents of this kind; but from all these consequences he is to be held absolved, provided that with a sincere heart, and a faith unfeigned, he abjures and curses the heresies he has cherished, as well as every other heresy against the catholic church. in order that his offence might not go altogether unpunished, that he might be more cautious in future, and be a warning to others to abstain from similar delinquencies, it was also decreed that his dialogues should be prohibited by public edict; that he himself should be condemned to the prison of the inquisition during their pleasure, and that, in the course of the next three years, he should recite once a week the seven penitential psalms. the ceremony of galileo's abjuration was one of exciting interest, and of awful formality. clothed in the sackcloth of a repentant criminal, the venerable sage fell upon his knees before the assembled cardinals; and laying his hands upon the holy evangelists, he invoked the divine aid in abjuring and detesting, and vowing never again to teach, the doctrine of the earth's motion, and of the sun's stability. he pledged himself that he would never again, either in words or in writing, propagate such heresies; and he swore that he would fulfil and observe the penances which had been inflicted upon him.[ ] at the conclusion of this ceremony, in which he recited his abjuration word for word, and then signed it, he was conveyed, in conformity with his sentence, to the prison of the inquisition. [ ] it has been said, but upon what authority we cannot state, that when galileo rose from his knees, he stamped on the ground, and said in a whisper to one of his friends, "_e pur si muove._" "it does move, though."--life of galileo, lib. useful knowledge, part ii. p. . the account which we have now given of the trial and the sentence of galileo, is pregnant with the deepest interest and instruction. human nature is here drawn in its darkest colouring; and in surveying the melancholy picture, it is difficult to decide whether religion or philosophy has been most degraded. while we witness the presumptuous priest pronouncing infallible the decrees of his own erring judgment, we see the high-minded philosopher abjuring the eternal and immutable truths which he had himself the glory of establishing. in the ignorance and prejudices of the age--in a too literal interpretation of the language of scripture--in a mistaken respect for the errors that had become venerable from their antiquity--and in the peculiar position which galileo had taken among the avowed enemies of the church, we may find the elements of an apology, poor though it be, for the conduct of the inquisition. but what excuse can we devise for the humiliating confession and abjuration of galileo? why did this master-spirit of the age--this high-priest of the stars--this representative of science--this hoary sage, whose career of glory was near its consummation--why did he reject the crown of martyrdom which he had himself coveted, and which, plaited with immortal laurels, was about to descend upon his head? if, in place of disavowing the laws of nature, and surrendering in his own person the intellectual dignity of his species, he had boldly asserted the truth of his opinions, and confided his character to posterity, and his cause to an all-ruling providence, he would have strung up the hair-suspended sabre, and disarmed for ever the hostility which threatened to overwhelm him. the philosopher, however, was supported only by philosophy; and in the love of truth he found a miserable substitute for the hopes of the martyr. galileo cowered under the fear of man, and his submission was the salvation of the church. the sword of the inquisition descended on his prostrate neck; and though its stroke was not physical, yet it fell with a moral influence fatal to the character of its victim, and to the dignity of science. in studying with attention this portion of scientific history, the reader will not fail to perceive that the church of rome was driven into a dilemma, from which the submission and abjuration of galileo could alone extricate it. he who confesses a crime and denounces its atrocity, not only sanctions but inflicts the punishment which is annexed to it. had galileo declared his innocence, and avowed his sentiments, and had he appealed to the past conduct of the church itself, to the acknowledged opinions of its dignitaries, and even to the acts of its pontiffs, he would have at once confounded his accusers, and escaped from their toils. after copernicus, himself a catholic priest, had _openly_ maintained the motion of the earth, and the stability of the sun:--after he had dedicated the work which advocated these opinions to pope paul iii., on the express ground that the _authority of the pontiff_ might silence the calumnies of those who attacked these opinions by arguments drawn from scripture:--after the cardinal schonberg and the bishop of culm had urged copernicus to publish the new doctrines;--and after the bishop of ermeland had erected a monument to commemorate his great discoveries;--how could the church of rome have appealed to its pontifical decrees as the ground of persecuting and punishing galileo? even in later times, the same doctrines had been propagated with entire toleration: nay, in the very year of galileo's first persecution, paul anthony foscarinus, a learned carmelite monk, wrote a pamphlet, in which he illustrates and defends the mobility of the earth, and endeavours to reconcile to this new doctrine the passages of scripture which had been employed to subvert it. this very singular production was dated from the carmelite convent at naples; was dedicated to the very reverend sebastian fantoni, general of the carmelite order; and, sanctioned by the ecclesiastical authorities, it was published at naples in , the very year of the first persecution of galileo. nor was this the only defence of the copernican system which issued from the bosom of the church. thomas campanella, a calabrian monk, published, in , "_an apology for galileo_," and he even dedicates it to d. boniface, cardinal of cajeta. nay, it appears from the dedication, that he undertook the work at the command of the cardinal, and that the examination of the question had been entrusted to the cardinal by the holy senate. after an able defence of his friend, campanella refers, at the conclusion of his apology, to the suppression of galileo's writings, and justly observes, that the effect of such a measure would be to make them more generally read, and more highly esteemed. the boldness of the apologist, however, is wisely tempered with the humility of the ecclesiastic, and he concludes his work with the declaration, that in all his opinions, whether written or to be written, he submits himself to the opinions of the holy mother church of rome and to the judgment of his superiors. by these proceedings of the dignitaries, as well as the clergy of the church of rome, which had been tolerated for more than a century, the decrees of the pontiffs against the doctrine of the earth's motion were virtually repealed; and galileo might have pleaded them with success in arrest of judgment. unfortunately, however, for himself and for science, he acted otherwise. by admitting their authority, he revived in fresh force these obsolete and obnoxious enactments; and, by yielding to their power, he riveted for another century the almost broken chains of spiritual despotism. it is a curious fact in the annals of heresy and sedition, that opinions maintained with impunity by one individual, have, in the same age, brought others to the stake or to the scaffold. the results of deep research or extravagant speculation seldom provoke hostility, when meekly announced as the deductions of reason or the convictions of conscience. as the dreams of a recluse or of an enthusiast, they may excite pity or call forth contempt; but, like seed quietly cast into the earth, they will rot and germinate according to the vitality with which they are endowed. but, if new and startling opinions are thrown in the face of the community--if they are uttered in triumph or in insult--in contempt of public opinion, or in derision of cherished errors, they lose the comeliness of truth in the rancour of their propagation; and they are like seed scattered in a hurricane, which only irritates and blinds the husbandman. had galileo concluded his _system of the world_ with the quiet peroration of his apologist campanella, and dedicated it to the pope, it might have stood in the library of the vatican, beside the cherished though equally heretical volume of copernicus. in the abjuration of his opinions by galileo, pope urban vii. did not fail to observe the full extent of his triumph; and he exhibited the utmost sagacity in the means which he employed to secure it. while he endeavoured to overawe the enemies of the church by the formal promulgation of galileo's sentence and abjuration, and by punishing the officials who had assisted in obtaining the license to print his work, he treated galileo with the utmost lenity, and yielded to every request that was made to diminish, and almost suspend, the constraint under which he lay. the sentence of abjuration was ordered to be publicly read at several universities. at florence the ceremonial was performed in the church of santa croce, and the friends and disciples of galileo were especially summoned to witness the public degradation of their master. the inquisitor at florence was ordered to be reprimanded for his conduct; and riccardi, the master of the sacred palace, and ciampoli, the secretary of pope urban himself, were dismissed from their situations. galileo had remained only four days in the prison of the inquisition, when, on the application of niccolini, the tuscan ambassador, he was allowed to reside with him in his palace. as florence still suffered under the contagious disease which we have already mentioned, it was proposed that sienna should be the place of galileo's confinement, and that his residence should be in one of the convents of that city. niccolini, however, recommended the palace of the archbishop piccolomoni as a more suitable residence; and though the archbishop was one of galileo's best friends, the pope agreed to the arrangement, and in the beginning of july galileo quitted rome for sienna. after having spent nearly six months under the hospitable roof of his friend, with no other restraint than that of being confined to the limits of the palace, galileo was permitted to return to his villa near florence under the same restrictions; and as the contagious disease had disappeared in tuscany, he was able in the month of december to re-enter his own house at arcetri, where he spent the remainder of his days. chapter vi. _galileo loses his favourite daughter--he falls into a state of melancholy and ill health--is allowed to go to florence for its recovery in --but is prevented from leaving his house or receiving his friends--his friend castelli permitted to visit him in the presence of an officer of the inquisition--he composes his celebrated dialogues on local motion--discovers the moon's libration--loses the sight of one eye--the other eye attacked by the same disease--is struck blind--negociates with the dutch government respecting his method of finding the longitude--he is allowed free intercourse with his friends--his illness and death in --his epitaph--his social, moral, and scientific character._ although galileo had now the happiness of rejoining his family under their paternal roof, yet, like all sublunary blessings, it was but of short duration. his favourite daughter maria, who along with her sister had joined the convent of st matthew in the neighbourhood of arcetri, had looked forward to the arrival of her father with the most affectionate anticipations. she hoped that her filial devotion might form some compensation for the malignity of his enemies, and she eagerly assumed the labour of reciting weekly the seven penitentiary psalms which formed part of her father's sentence. these sacred duties, however, were destined to terminate almost at the moment they were begun. she was seized with a fatal illness in the same month in which she rejoined her parent, and before the month of april she was no more. this heavy blow, so suddenly struck, overwhelmed galileo in the deepest agony. owing to the decline of his health, and the recurrence of his old complaints, he was unable to oppose to this mental suffering the constitutional energy of his mind. the bulwarks of his heart broke down, and a flood of grief desolated his manly and powerful mind. he felt, as he expressed it, that he was incessantly called by his daughter--his pulse intermitted--his heart was agitated with unceasing palpitations--his appetite entirely left him, and he considered his dissolution so near at hand, that he would not permit his son vicenzo to set out upon a journey which he had contemplated. from this state of melancholy and indisposition, galileo slowly, though partially, recovered, and, with the view of obtaining medical assistance, he requested leave to go to florence. his enemies, however, refused this application, and he was given to understand that any additional importunities would be visited with a more vigilant surveillance. he remained, therefore, five years at arcetri, from to , without any remission of his confinement, and pursuing his studies under the influence of a continued and general indisposition. there is no reason to think that galileo or his friends renewed their application to the church of rome; but, in , the pope transmitted, through the inquisitor fariano, his permission that he might remove to florence for the recovery of his health, on the condition that he should present himself at the office of the inquisitor to learn the terms upon which this indulgence was granted. galileo accepted of the kindness thus unexpectedly proffered. but the conditions upon which it was given were more severe than he expected. he was prohibited from leaving his house or admitting his friends; and so sternly was this system pursued, that he required a special order for attending mass during passion week. the severity of this order was keenly felt by galileo. while he remained at arcetri, his seclusion from the world would have been an object of choice, if it had not been the decree of a tribunal; but to be debarred from the conversation of his friends in florence--in that city where his genius had been idolised, and where his fame had become immortal, was an aggravation of punishment which he was unable to bear. with his accustomed kindness, the grand duke made a strong representation on the subject to his ambassador at the court of rome. he stated that, from his great age and infirmities, galileo's career was near its close; that he possessed many valuable ideas, which the world might lose if they were not matured and conveyed to his friends; and that galileo was anxious to make these communications to father castelli, who was then a stipendiary of the court of rome. the grand duke commanded his ambassador to see castelli on the subject--to urge him to obtain leave from the pope to spend a few months in florence--and to supply him with money and every thing that was necessary for his journey. influenced by this kind and liberal message, castelli obtained an audience of the pope, and requested leave to pay a visit to florence. urban instantly suspected the object of his journey; and, upon castelli's acknowledging that he could not possibly refrain from seeing galileo, he received permission to visit him in the company of an officer of the inquisition. castelli accordingly went to florence, and, a few months afterwards, galileo was ordered to return to arcetri. during galileo's confinement at sienna and arcetri, between and , his time was principally occupied in the composition of his "dialogues on local motion," in which he treats of the strength and cohesion of solid bodies, of the laws of uniform and accelerated motions, of the motion of projectiles, and of the centre of gravity of solids. this remarkable work, which was considered by its author as the best of his productions, was printed by louis elzevir, at amsterdam, and dedicated to the count de noailles, the french ambassador at rome. various attempts to have it printed in germany had failed; and, in order to save himself from the malignity of his enemies, he was obliged to pretend that the edition published in holland had been printed from a ms. entrusted to the french ambassador. although galileo had for a long time abandoned his astronomical studies, yet his attention was directed, about the year , to a curious appearance in the lunar disc, which is known by the name of the moon's libration. when we examine with a telescope the outline of the moon, we observe that certain parts of her disc, which are seen at one time, are invisible at another. this change or libration is of four different kinds, viz. the diurnal libration, the libration in longitude, the libration in latitude, and the spheroidal libration. galileo discovered the first of these kinds of libration, and appears to have had some knowledge of the second; but the third was discovered by hevelius, and the fourth by lagrange. this curious discovery was the result of the last telescopic observations of galileo. although his right eye had for some years lost its power, yet his general vision was sufficiently perfect to enable him to carry on his usual researches. in , however, this affection of his eye became more serious; and, in , his left eye was attacked with the same disease. his medical friends at first supposed that cataracts were formed in the crystalline lens, and anticipated a cure from the operation of couching. these hopes were fallacious. the disease turned out to be in the cornea, and every attempt to restore its transparency was fruitless. in a few months the white cloud covered the whole aperture of the pupil, and galileo became totally blind. this sudden and unexpected calamity had almost overwhelmed galileo and his friends. in writing to a correspondent he exclaims, "alas! your dear friend and servant has become totally and irreparably blind. these heavens, this earth, this universe, which by wonderful observation i had enlarged a thousand times beyond the belief of past ages, are henceforth shrunk into the narrow space which i myself occupy. so it pleases god; it shall, therefore, please me also." his friend, father castelli, deplores the calamity in the same tone of pathetic sublimity:--"the noblest eye," says he, "which nature ever made, is darkened; an eye so privileged, and gifted with such rare powers, that it may truly be said to have seen more than the eyes of all that are gone, and to have opened the eyes of all that are to come." although galileo had been thwarted in his attempt to introduce into the spanish marine his new method of finding the longitude at sea, yet he never lost sight of an object to which he attached the highest importance. as the formation of correct tables of the motion of jupiter's satellites was a necessary preliminary to its introduction, he had occupied himself for twenty-four years in observations for this purpose, and he had made considerable progress in this laborious task. after the publication of his "dialogues on motion," in , he renewed his attempts to bring his method into actual use. for this purpose he addressed himself to lorenzo real, who had been the dutch governor-general in india, and offered the free use of his method to the states-general of holland.[ ] the dutch government received this proposal with an anxious desire to have it carried into effect. at the instigation of constantine huygens, the father of the illustrious huygens, and the secretary to the prince of orange, they appointed commissioners to communicate with galileo; and while they transmitted him a gold chain as a mark of their esteem, they at the same time assured him, that if his plan should prove successful it should not pass unrewarded. the commissioners entered into an active correspondence with galileo, and had even appointed one of their number to communicate personally with him in italy. lest this, however, should excite the jealousy of the court of rome, galileo objected to the arrangement, so that the negociation was carried on solely by correspondence. [ ] it is a curious fact that morin had about this time proposed to determine the longitude by the moon's distance from a fixed star, and that the commissioners assembled in paris to examine it requested galileo's opinion of its value and practicability. galileo's opinion was highly unfavourable. he saw clearly, and explained distinctly, the objection to morin's method, arising from the imperfection of the lunar tables, and the inadequacy of astronomical instruments; but he seemed not to be conscious that the very same objections applied with even greater force to his own method, which has since been supplanted by that of the french savant. see life of galileo, library of useful knowledge, p. . it was at this time that galileo was struck with blindness. his friend and pupil, renieri, undertook in this emergency to arrange and complete his observations and calculations; but before he had made much progress in the arduous task, each of the four commissioners died in succession, and it was with great difficulty that constantine huygens succeeded in renewing the scheme. it was again obstructed, however, by the death of galileo; and when renieri was about to publish, by the order of the grand duke, the "ephemeris," and "tables of the jovian planets," he was attacked with a mortal disease, and the manuscripts of galileo, which he was on the eve of publishing, were never more heard of. by such a series of misfortunes were the plans of galileo and of the states-general completely overthrown. it is some consolation, however, to know that neither science nor navigation suffered any severe loss. notwithstanding the perfection of our present tables of jupiter's satellites, and of the astronomical instruments by which their eclipses may be observed, the method of galileo is still impracticable at sea. in consequence of the strict seclusion to which galileo had been subjected, he was in the practice of dating his letters from his prison at arcetri; but after he had lost the use of his eyes, the inquisition seems to have relaxed its severity, and to have allowed him the freest intercourse with his friends. the grand duke of tuscany paid him frequent visits; and among the celebrated strangers who came from distant lands to see the ornament of italy, were gassendi, deodati, and our illustrious countryman milton. during the last three years of his life, his eminent pupil viviani formed one of his family; and in october , the celebrated torricelli, another of his pupils, was admitted to the same distinction. though the powerful mind of galileo still retained its vigour, yet his debilitated frame was exhausted with mental labour. he often complained that his head was too busy for his body; and the continuity of his studies was frequently broken with attacks of hypochondria, want of sleep, and acute rheumatic pains. along with these calamities, he was afflicted with another still more severe--with deafness almost total; but though he was now excluded from all communication with the external world, yet his mind still grappled with the material universe, and while he was studying the force of percussion, and preparing for a continuation of his "dialogues on motion," he was attacked with fever and palpitation of the heart, which, after continuing two months, terminated fatally on the th of january , in the th year of his age. having died in the character of a prisoner of the inquisition, this odious tribunal disputed his right of making a will, and of being buried in consecrated ground. these objections, however, were withdrawn; but though a large sum was subscribed for erecting a monument to him in the church of santa croce, in florence, the pope would not permit the design to be carried into execution. his sacred remains were, therefore, deposited in an obscure corner of the church, and remained for more than thirty years unmarked with any monumental tablet. the following epitaph, given without any remark in the leyden edition of his dialogues, is, we presume, the one which was inscribed on a tablet in the church of santa croce:-- galilÃ�o galilÃ�i florentino, philosopho et geometræ vere lynceo, naturæ oedipo, mirabilium semper inventorum machinatori, qui inconcessa adhuc mortalibus gloria cælorum provincias auxit et universo dedit incrementum: non enim vitreos spherarum orbes fragilesque stellas conflavit: sed æterna mundi corpore mediceæ beneficentiæ dedicavit, cujus inextincta gloriæ cupiditas ut oculos nationum sæculorumque omnium videre doceret, proprios impendit oculos. cum jam nil amplius haberet natura quod ipse videret. cujus inventa vix intra rerum limites comprehensa firmamentum ipsum non solum continet, sed etiam recipit. qui relictis tot scientiarum monumentis plura secum tulit, quam reliquit. gravi enim sed nondum affecta senectute, novis contemplationibus majorem gloriam affectans inexplebilem sapientiæ animam immaturo nobis obitu exhalavit anno domini mcxlii. Ã�tatis suæ lxxviii. at his death, in , viviani purchased his property, with the charge of erecting a monument over galileo's remains and his own. this design was not carried into effect till , at the expense of the family of nelli, when both their bodies were disinterred, and removed to the site of the splendid monument which now covers them. this monument contains the bust of galileo, with figures of geometry and astronomy. it was designed by giulio foggini. galileo's bust was executed by giovanni battista foggini; the figure of astronomy by vincenzio foggini, his son; and that of geometry by girolamo ticciati. galileo's house at arcetri still remains. in it belonged to one signor alimari, having been preserved in the state in which it was left by galileo; it stands very near the convent of st matthew, and about a mile to the s. e. of florence. an inscription by nelli, over the door of the house, still remains. the character of galileo, whether we view him as a member of the social circle, or as a man of science, presents many interesting and instructive points of contemplation. unfortunate, and to a certain extent immoral, in his domestic relations, he did not derive from that hallowed source all the enjoyments which it generally yields; and it was owing to this cause, perhaps, that he was more fond of society than might have been expected from his studious habits. his habitual cheerfulness and gaiety, and his affability and frankness of manner, rendered him an universal favourite among his friends. without any of the pedantry of exclusive talent, and without any of that ostentation which often marks the man of limited though profound acquirements, galileo never conversed upon scientific or philosophical subjects except among those who were capable of understanding them. the extent of his general information, indeed, his great literary knowledge, but, above all, his retentive memory, stored with the legends and the poetry of ancient times, saved him from the necessity of drawing upon his own peculiar studies for the topics of his conversation. galileo was not less distinguished for his hospitality and benevolence; he was liberal to the poor, and generous in the aid which he administered to men of genius and talent, who often found a comfortable asylum under his roof. in his domestic economy he was frugal without being parsimonious. his hospitable board was ever ready for the reception of his friends; and, though he was himself abstemious in his diet, he seems to have been a lover of good wines, of which he received always the choicest varieties out of the grand duke's cellar. this peculiar taste, together with his attachment to a country life, rendered him fond of agricultural pursuits, and induced him to devote his leisure hours to the cultivation of his vineyards. in his personal appearance galileo was about the middle size, and of a square-built, but well-proportioned, frame. his complexion was fair, his eyes penetrating, and his hair of a reddish hue. his expression was cheerful and animated, and though his temper was easily ruffled, yet the excitement was transient, and the cause of it speedily forgotten. one of the most prominent traits in the character of galileo was his invincible love of truth, and his abhorrence of that spiritual despotism which had so long brooded over europe. his views, however, were too liberal, and too far in advance of the age which he adorned; and however much we may admire the noble spirit which he evinced, and the personal sacrifices which he made, in his struggle for truth, we must yet lament the hotness of his zeal and the temerity of his onset. in his contest with the church of rome, he fell under her victorious banner; and though his cause was that of truth, and hers that of superstition, yet the sympathy of europe was not roused by his misfortunes. under the sagacious and peaceful sway of copernicus, astronomy had effected a glorious triumph over the dogmas of the church; but under the bold and uncompromising sceptre of galileo all her conquests were irrecoverably lost. the scientific character of galileo, and his method of investigating truth, demand our warmest admiration. the number and ingenuity of his inventions, the brilliant discoveries which he made in the heavens, and the depth and beauty of his researches respecting the laws of motion, have gained him the admiration of every succeeding age, and have placed him next to newton in the lists of original and inventive genius. to this high rank he was doubtless elevated by the inductive processes which he followed in all his inquiries. under the sure guidance of observation and experiment, he advanced to general laws; and if bacon had never lived, the student of nature would have found, in the writings and labours of galileo, not only the boasted principles of the inductive philosophy, but also their practical application to the highest efforts of invention and discovery. life of tycho brahe. chapter i. _tycho's birth, family, and education--an eclipse of the sun turns his attention to astronomy--studies law at leipsic--but pursues astronomy by stealth--his uncle's death--he returns to copenhagen, and resumes his observations--revisits germany--fights a duel, and loses his nose--visits augsburg, and meets hainzel--who assists him in making a large quadrant--revisits denmark--and is warmly received by the king--he settles at his uncle's castle of herritzvold--his observatory and laboratory--discovers the new star in cassiopeia--account of this remarkable body--tycho's marriage with a peasant girl--which irritates his friends--his lectures on astronomy--he visits the prince of hesse--attends the coronation of the emperor rudolph at ratisbon--he returns to denmark._ among the distinguished men who were destined to revive the sciences, and to establish the true system of the universe, tycho brahe holds a conspicuous place. he was born on the th december , at knudstorp, the estate of his ancestors, which is situated near helsingborg, in scania, and was the eldest son and the second child of a family of five sons and five daughters. his father, otto brahe, who was descended from a noble swedish family, was in such straitened circumstances, that he resolved to educate his sons for the military profession; but tycho seems to have disliked the choice that was made for him; and his next brother, steno, who appears to have had a similar feeling, exchanged the sword for the more peaceful occupation of privy councillor to the king. the rest of his brothers, though of senatorial rank, do not seem to have extended the renown of their family; but their youngest sister, sophia, is represented as an accomplished mathematician, and is said to have devoted her mind to astronomy as well as to the astrological reveries of the age. george brahe, the brother of otto, having no children of his own, resolved to adopt and to educate one of his nephews. on the birth of tycho, accordingly, he was desirous of having him placed under his wife's care; but his parents could not be prevailed upon to part with their child till after the birth of steno, their second son. having been instructed in reading and writing under proper masters, tycho began the study of latin in his seventh year; and, in opposition to his father's views, he prosecuted it for five years under private teachers, from whom he received also occasional instruction in poetry and the belles lettres. in april , about three years after his father's death, tycho was sent to the university of copenhagen, to study rhetoric and philosophy, with the view of preparing for the study of the law, and qualifying himself for some of those political offices which his rank entitled him to expect. in this situation he contracted no fondness for any particular study; but after he had been sixteen months at college, an event occurred which directed all the powers of his mind to the science of astronomy. the attention of the public had been long fixed on a great eclipse of the sun, which was to happen on the st august ; and as in those days a phenomenon of this kind was linked with the destinies of nations as well as of individuals, the interest which it excited was as intense as it was general. tycho watched its arrival with peculiar anxiety. he read the astrological diaries of the day, in which its phases and its consequences were described; and when he saw the sun darkened at the very moment that had been predicted, and to the very extent that had been delineated, he resolved to make himself master of a science which was capable of predicting future events, and especially that branch of it which connected these events with the fortunes and destinies of man. with this view he purchased the _tabulæ bergenses_, calculated by john stadius, and began with ardour the study of the planetary motions. when tycho had completed his course at copenhagen, he was sent, in february , under the charge of a tutor to study jurisprudence at leipsic. astronomy, however, engrossed all his thoughts; and he had no sooner escaped from the daily surveillance of his master, than he rushed with headlong impetuosity into his favourite pursuits. with his pocket money he purchased astronomical books, which he read in secret; and by means of a celestial globe, the size of his fist, he made himself acquainted with the stars, and followed them night after night through the heavens, when sleep had lulled the vigilance of his preceptor. by means of the ephemerides of stadius, he learned to distinguish the planets, and to trace them through their direct and retrograde movements; and having obtained the alphonsine and prutenic tables, and compared his own calculations and observations with those of stadius, he observed great differences in the results, and from that moment he seems to have conceived the design of devoting his life to the accurate construction of tables, which he justly regarded as the basis of astronomy. with this view, he applied himself secretly to the study of arithmetic and geometry; and, without the assistance of a master, he acquired that mathematical knowledge which enabled him to realise these early aspirations. his ardour for astronomy was still farther inflamed, and the resolution which it inspired still farther strengthened, by the great conjunction of jupiter and saturn, which took place in august . the calculated time of this phenomenon differed considerably from the true time which was observed; and in determining the instant of conjunction tycho felt in the strongest manner the imperfection of the instruments which he used. for this purpose he employed a sort of compass, one leg of which was directed to one planet and the second to the other planet or fixed star; and, by measuring the angular opening between them, he determined the distance of the two celestial bodies. by this rude contrivance he found that the alphonsine tables erred a whole month in the time of conjunction, while the copernican ones were at least several days in error. to this celebrated conjunction tycho ascribed the great plague which in subsequent years desolated europe, because it took place in the beginning of _leo_, and not far from the nebulous stars of _cancer_, two of the zodiacal signs which are reckoned by ptolemy "suffocating and pestilent!" there dwelt at this time at leipsic an ingenious artisan named scultetus, who was employed by homelius, the professor of mathematics in that city, to assist him in the construction of his instruments. having become acquainted with this young man, tycho put into his hand a wooden radius, such as was recommended by gemma frisius, for the purpose of having it divided in the manner adopted by homelius; and with this improved instrument he made a great number of astronomical observations out of his window, without ever exciting the suspicions of his tutor. having spent three years at leipsic, he was about to make the tour of germany, when, in consequence of his uncle's death, he was summoned to his native country to inherit the fortune which had been left him. he accordingly quitted leipsic about the middle of may , and after having arranged his domestic concerns in denmark, he continued his astronomical observations with the radius constructed for him by scultetus. the ardour with which he pursued his studies gave great umbrage to his friends as well as to his relations. he was reproached for having abandoned the profession of the law; his astronomical observations were ridiculed as not only useless but degrading, and, among his numerous connexions, his maternal uncle, steno bille, was the only one who applauded him for following the bent of his genius. under these uncomfortable circumstances he resolved to quit his country, and pay a visit to the most interesting cities of germany. at wittemberg, where he arrived in april , he resumed his astronomical observations; but, in consequence of the plague having broken out in that city, he removed to rostoch in the following autumn. here an accident occurred which had nearly deprived him of his life. on the th december he was invited to a wedding feast; and, among other guests, there was present a noble countryman of his own, manderupius pasbergius. some difference having arisen between them on this occasion, they parted with feelings of mutual displeasure. on the th of the same month they met again at some festive games, and having revived their former quarrel, they agreed to settle their differences by the sword. they accordingly met at o'clock in the evening of the th, and fought in total darkness. in this blind combat, manderupius cut off the whole of the front of tycho's nose, and it was fortunate for astronomy that his more valuable organs were defended by so faithful an outpost. the quarrel, which is said to have originated in a difference of opinion respecting their mathematical acquirements, terminated here; and tycho repaired his loss by cementing upon his face a nose of gold and silver, which is said to have formed a good imitation of the original. during the years and , tycho continued to reside at rostoch, with the exception of a few months, during which he made a rapid journey into denmark. he lived in a house in the college of the jesuits, which he had rented on account of its fitness for celestial observations; but, though he intended to spend the winter under its roof, he had made no arrangement respecting his future life, leaving it, as he said, in the hands of providence. a desire, however, to visit the south of germany induced him to quit rostoch, and having crossed the danube, he paid a visit to augsburg. upon entering this ancient city, tycho was particularly struck with the grandeur of its fortifications, the splendour of its private houses, and the beauty of its fountains; and, after a short residence within its walls, he was still more delighted with the industry of the people, the refinement of the higher classes, and the love of literature and science which was cherished by its wealthy citizens. among the interesting acquaintances which he formed at augsburg, were two brothers, john and paul hainzel, the one a septemvir, and the other the consul or burgomaster. they were both distinguished by their learning, and both of them, particularly paul, were ardent lovers of astronomy. tycho had hitherto no other astronomical instrument than the coarse radius which was made for him by scultetus, and he waited only for a proper occasion to have a larger and better instrument constructed for his use. having now the command of workmen who could execute his plans, he conceived the bold design of making a divided instrument which should distinctly exhibit single minutes of a degree. while he was transferring the first rude conception of his instrument to paper, paul hainzel entered his study, and was so struck with the grandeur of the plan, that he instantly undertook to have it executed at his own expense. the projected instrument was a quadrant of fourteen cubits radius! and tycho and his friend entered upon its construction with that intense ardour which is ever crowned with success. in the village of gegginga, about half a mile to the south of the city, paul hainzel had a country house, the garden of which was chosen as the spot where the quadrant was to be fixed. the best artists in augsburg, clockmakers, jewellers, smiths, and carpenters, were engaged to execute the work, and from the zeal which so novel an instrument inspired, the quadrant was completed in less than a month. its size was so great that twenty men could with difficulty transport it to its place of fixture. the two principal rectangular radii were beams of oak; the arch which lay between their extremities was made of solid wood of a particular kind, and the whole was bound together by twelve beams. it received additional strength from several iron bands, and the arch was covered with plates of brass, for the purpose of receiving the divisions into which it was to be subdivided. a large and strong pillar of oak, shod with iron, was driven into the ground, and kept in its place by solid mason work. to this pillar the quadrant was fixed in a vertical plane, and steps were prepared to elevate the observer, when stars of a low altitude required his attention. as the instrument could not be conveniently covered with a roof, it was protected from the weather by a covering made of skins, but notwithstanding this and other precautions, it was broken to pieces by a violent storm, after having remained uninjured for the space of five years. as this quadrant was fitted only to determine the altitudes of the celestial bodies, tycho constructed a large sextant for the purpose of measuring their distances. it consisted of two radii, which opened and shut round a centre, and which were nearly four cubits long, and also of two arches, one of which was graduated, while the other served to keep the radii in the same plane. after the radii had been opened or shut till they nearly comprehended the angle between the stars to be observed, the adjustment was completed by means of a very fine tangent screw. with this instrument tycho made many excellent observations during his stay at augsburg. he began also the construction of a wooden globe about six feet in diameter. its outer surface was turned with great accuracy into a sphere, and kept from warping by interior bars of wood supported at its centre. after receiving a visit from the celebrated peter ramus, who subsequently fell a victim at the massacre of st bartholomew, tycho left augsburg, having received a promise from his friend hainzel that he would communicate to him the observations made with his large quadrant, and with the sextant which he had given him in a present. he paid a visit to philip appian in passing through ingolstadt, and returned to his native country about the end of . the fame which he had acquired as an astronomer procured for him a warmer reception than that which he had formerly experienced. the king invited him to court, and his friends and admirers loaded him with kindness. his uncle, steno bille, who now lived at the ancient convent of herritzvold, and who had always taken a deep interest in the scientific character of his nephew, not only invited him to his house, but assigned to him for an observatory the part of it which was best adapted for that purpose. tycho cheerfully accepted of this liberal offer. the immediate proximity of herritzvold to knudstorp, rendered this arrangement peculiarly convenient, and in the house of his uncle he experienced all that kindness and consideration which natural affection and a love of science combined to cherish. when steno learned that the study of chemistry was one of the pursuits of his nephew, he granted him a spacious house, a few yards distant from the convent, for his laboratory. tycho lost no time in fitting up his observatory, and in providing his furnaces; and regarding gold and silver and the other metals as the stars of the earth, he used to represent his two opposite pursuits as forming only one science, namely, celestial and terrestrial astronomy. in the hopes of enriching himself by the pursuits of alchemy, tycho devoted most of his attention to those satellites of gold and silver which now constituted his own system, and which disturbed by their powerful action the hitherto uniform movements of their primary. his affections were ever turning to germany, where astronomers of kindred views, and artists of surpassing talent were to be found in almost every city. the want of money alone prevented him from realizing his wishes; and it was in the hope of attaining the means of travelling, that he in a great measure forsook his sextants for his crucibles. in order, however, that he might have one good instrument in his observatory, he constructed a sextant similar to, but somewhat larger than, that which he had presented to hainzel. its limb was made of solid brass, and was exquisitely divided into single minutes of a degree. its radii were strengthened with plates of brass, and the apparatus for opening and shutting them was made with great accuracy. the possession of this instrument was peculiarly fortunate for tycho, for an event now occurred which roused him from his golden visions, and directed all his faculties into their earlier and purer current. on the th november , when he was returning to supper from his laboratory, the clearness of the sky inspired him with the desire of completing some particular observations. on looking up to the starry firmament he was surprised to see an extraordinary light in the constellation of cassiopeia, which was then above his head. he felt confident that he had never before observed such a star in that constellation, and distrusting the evidence of his own senses, he called out the servants and the peasants, and having received their testimony that it was a huge star such as they had never seen before, he was satisfied of the correctness of his own vision. regarding it as a new and unusual phenomenon, he hastened to his observatory, adjusted his sextant, and measured its distances from the nearest stars in cassiopeia. he noted also its form, its magnitude, its light, and its colour, and he waited with great anxiety for the next night that he might determine the important point whether it was a fixed star, or a body within, or near to, our own system. for several years tycho had been in the practice of calculating, at the beginning of each year, a sort of almanac for his own use, and in this he inserted all the observations which he had made on the new star, and the conclusions which he had drawn from them. having gone to copenhagen in the course of the ensuing spring, he shewed this manuscript to john pratensis, a professor, in whose house he was always hospitably received. charles danzeus, the french ambassador, and a person of great learning, having heard of tycho's arrival, invited himself to dine with him at the house of pratensis. the conversation soon turned upon the new star, and tycho found his companion very sceptical about its existence. danzeus was particularly jocular on the subject, and attacked the danes for their inattention to so important a science as astronomy. tycho received this lecture in good temper, and with the anxious expectation that a clear sky would enable him to give a practical refutation of the attack which was made upon his country. the night turned out serene, and the whole party saw with astonishment the new star under the most favourable circumstances. pratensis conceived that it was similar to the one observed by hipparchus, and urged tycho to publish the observations which he had made upon it. tycho refused to accede to this request, on the pretext that his work was not sufficiently perfect; but the true reason, as he afterwards acknowledged, was, that he considered it would be a disgrace for a nobleman, either to study such subjects, or to communicate them to the public. this absurd notion was with some difficulty overcome, and through the earnest entreaties and assistance of pratensis, his work on the new star was published in . this remarkable body presents to us one of the most interesting phenomena in astronomy. the date of its first appearance has not been exactly ascertained. tycho saw it on the th november, but cornelius gemma had seen it on the th, paul hainzel saw it on the th of august at augsburg, and wolfgangus schulerus observed it at wittenberg on the th. tycho conjectures that it was first seen on the th, and hieronymus munosius asserts that at valentia, in spain, it was not seen on the d, when he was shewing that part of the heavens to his pupils. this singular body continued to be seen during months, and did not disappear till march . in its appearance it was exactly like a star, having none of the distinctive marks of a comet. it twinkled strongly, and grew larger than _lyra_ or _sirius_, or any other fixed star. it seemed to be somewhat larger than _jupiter_, when he is nearest the earth, and rivalled _venus_ in her greatest brightness. in the _first_ month of its appearance it was less than jupiter; in the _second_ it equalled him; in the _third_ it surpassed him in splendour; in the _fourth_ it was equal to _sirius_; in the _fifth_ to _lyra_; in the _sixth_ and _seventh_ to stars of the _second_ magnitude; in the _eighth_, _ninth_, and _tenth_, to stars of the _third_ magnitude; in the _eleventh_, _twelfth_, and _thirteenth_, to stars of the _fourth_ magnitude; in the _fourteenth_ and _fifteenth_ to stars of the _fifth_ magnitude; and in the _sixteenth_ month to stars of the _sixth_ magnitude. after this it became so small that it at last disappeared. its colour changed also with its size. at first it was white and bright; in the third month it began to become yellowish; in the fifth it became reddish like aldebaran; and in the seventh and eighth it became bluish like saturn; growing afterwards duller and duller. its place in the heavens was invariable. its longitude was in the th degree and th minute of taurus; and its latitude ° ´ north. its right ascension was ° - / ´ and its declination ° - / ´. it had no parallax, and was unquestionably situated in the region of the fixed stars. after tycho had published his book, he proposed to travel into germany and italy, but he was seized with a fever, and he had no sooner recovered from it, than he became involved in a love affair, which frustrated all his schemes. although tycho was afraid of casting a stain upon his nobility by publishing his observations on the new star, yet he did not scruple to debase his lineage by marrying a peasant girl of the village of knudstorp. this event took place in , and in his wife gave birth to his daughter magdalene. tycho's noble relations were deeply offended at this imprudent step; and so far did the mutual animosity of the parties extend, that the king himself was obliged to effect a reconciliation. the fame of our author as an astronomer and mathematician was now so high, that several young danish nobles requested him to deliver a course of lectures upon these interesting subjects. this application was seconded by pratensis, danzeus, and all his best friends; but their solicitations were vain. the king at last made the request in a way which ensured its being granted, and tycho delivered a course of lectures, in which he not only gave a full view of the science of astronomy, but defended and explained all the reveries of astrology. having finished his lectures, and arranged his domestic affairs, he set out on his projected journey about the beginning of the spring of , leaving behind him his wife and daughter, till he should fix upon a place of permanent residence. the first town which he visited was hesse-cassel, the residence of william, landgrave of hesse, whose patronage of astronomy, and whose skill in making celestial observations, have immortalized his name. here tycho spent eight or ten delightful days, during which the two astronomers were occupied one half of the day in scientific conversation, and the other half in astronomical observations; and he would have prolonged a visit which gave him so much pleasure, had not the death of one of the landgrave's daughters interrupted their labours. passing through frankfort, tycho went into switzerland; and, after visiting many cities on his way, he fixed upon basle as a place of residence, not only from its centrical position, but from the salubrity of the air, and the cheapness of living. from switzerland he went to venice, and, in returning through germany, he came to ratisbon, at the time of the congress, which had been called together on the st of november, for the coronation of the emperor rudolph. on this occasion he met with several distinguished individuals, who were not only skilled in astronomy, but who were among its warmest patrons. from ratisbon he passed to saalfeld, and thence to wittemburg, where he saw the parallactic instruments and the wooden quadrant which had been used by john pratensis in determining the latitude of the city, and in measuring the altitudes of the new star. tycho was now impatient for home, and he lost no time in returning to denmark, where events were awaiting him which frustrated all his schemes, by placing him in the most favourable situation for promoting his own happiness, and advancing the interests of astronomy. chapter ii. _frederick ii. patronises tycho--and resolves to establish him in denmark--grants him the island of huen for life--and builds the splendid observatory of uraniburg--description of the island, and of the observatory--account of its astronomical instruments--tycho begins his observations--his pupils--tycho is made canon of rothschild, and receives a large pension--his hospitality to his visitors--ingratitude of witichius--tycho sends an assistant to take the latitude of frauenburg and konigsberg--is visited by ulric, duke of mecklenburg--change in tycho's fortunes._ the patronage which had been extended to astronomers by several of the reigning princes of germany, especially by the landgrave of hesse, and augustus, elector of saxony, had begun to excite a love of science in the minds of other sovereigns. the king of denmark seems to have felt it as a stain upon his character, that the only astronomer in his dominions should carry on his observations in distant kingdoms and adorn by his discoveries other courts than his own. with this feeling he sent ambassadors to hesse-cassel to inquire after tycho, and to intimate to him his wish that he should return to denmark, and his anxiety to promote the advancement of astronomy in his own dominions. tycho had left cassel when these messengers arrived, and had heard nothing of the king's intentions till he was about to quit knudstorp with his family for basle. at this time he was surprised at the arrival of a noble messenger, who brought a letter requesting him to meet the king as soon as possible at copenhagen. tycho lost no time in obeying the royal summons. the king received him with the most flattering kindness. he offered to give him a grant for life of the island of huen, between denmark and sweden, and to construct and furnish with instruments, at his own expense, an observatory, as well as a house for the accommodation of his family, together with a laboratory for carrying on his chemical inquiries. tycho, who truly loved his country, was deeply affected with the munificence of the royal offer. he accepted of it with that warmth of gratitude which it was calculated to inspire; and he particularly rejoiced in the thought that if any success should attend his future labours, the glory of it would belong to his native land. the island of huen is about sxix miles from the coast of zealand, three from that of sweden, and fourteen from copenhagen. it is six miles in circumference, and rises into the form of a mountain, which, though very high, terminates in a plain. it is nowhere rocky, and even in the time of tycho it produced the best kinds of grain, afforded excellent pasturage for horses, cattle, and sheep, and possessed deer, hares, rabbits, and partridges in abundance. it contained at that time only one village, with about forty inhabitants. having surveyed his new territory, tycho resolved to build a magnificent tower in the centre of the elevated plain, which he resolved to call uraniburg, or _the city of the heavens_. having made the necessary arrangements, he repaired to the island on the th of august, and his friend charles danzeus laid the foundation stone of the new observatory, which consisted of a slab of porphyry, with the following inscription:-- regnante in dania frederico ii., carolus danzÃ�us aquitanus r. g. i. d. l.,[ ] domui huic philosophiÃ�, imprimisque astrorum contemplationi, regis decreto a nobili viro tychone brahe de knudstrup extructÃ� votivum hunc lapidem memoriÃ� et felicis auspicii ergo p. anno cic.ic.lxxvi.[ ] vi id. augusti. [ ] regis gallorum in dania legatus. [ ] transcriber's footnote: the second cs in cic and ic are printed reversed in the original. this ceremony was performed early in the morning of a splendid day, in which the rising sun threw its blessing upon frederick, and upon the party of noblemen and philosophers who had assembled to testify their love of science. an entertainment was provided for the occasion, and copious libations of a variety of wines were offered for the success of the undertaking. the observatory was surrounded by a rampart, each face of which was three hundred feet long. about the middle of each face the rampart became a semicircle, the inner diameter of which was ninety feet. the height of the rampart was twenty-two feet, and its thickness at the base twenty. its four angles corresponded exactly with the four cardinal points, and at the north and south angles were erected turrets, of which one was a printing-house, and the other the residence of the servants. gates were erected at the east and west angles, and above them were apartments for the reception of strangers. within the rampart was a shrubbery with about three hundred varieties of trees; and at the centre of each semicircular part of the rampart was a bower or summer-house. this shrubbery surrounded the flower-garden, which was terminated within by a circular wall about forty-five feet high, which enclosed a more elevated area, in the centre of which stood the principal building in the observatory, and from which four paths led to the above-mentioned angles, with as many doors for entering the garden. the principal building was about sixty feet square. the doors were placed on the east and west sides; and to the north and south fronts were attached two round towers, whose inner diameter was about thirty-two feet, and which formed the observatories which had windows in their roof, that could be opened towards any part of the heavens. the accommodations for the family were numerous and splendid. under the observatory, in the south tower, was the museum and library, and below this again was the laboratory in a subterraneous crypt, containing sixteen furnaces of various kinds. beneath this was a well forty feet deep, from which water was distributed by syphons to every part of the building. besides the principal building there were other two situated without the rampart, one to the north, containing a workshop for the construction of astronomical and other instruments, and the other to the south, which was occupied as a sort of farm-house. these buildings cost the king of denmark , rix-dollars (£ , ), and tycho is said to have expended upon them a similar sum. as the two towers could not accommodate the instruments which tycho required for his observations, he found it necessary to erect, on the hill about sixty paces to the south of uraniburg, a subterranean observatory, in which he might place his larger instruments, which required to be firmly fixed, and to be protected from the wind and the weather. this observatory, which he called stiern-berg, or the mountain, of the stars, consisted of several crypts, separated by solid walls, and to these there was a subterranean passage from the laboratory in uraniburg. the various buildings which tycho erected were built in a regular style of architecture, and were highly ornamented, not only with external decorations, but with the statues and pictures of the most distinguished astronomers, from hipparchus and ptolemy down to copernicus, and with inscriptions and poems in honour of astronomers. while these buildings were erecting, and after their completion, tycho was busily occupied in preparing instruments for observation. these were of the most splendid description, and the reader will form some notion of their grandeur and their expense from the following list:-- _in the south and greater observatory._ . a semicircle of solid iron, covered with brass, four cubits radius. . a sextant of the same materials and size. . a quadrant of one and a half cubits radius, and an azimuth circle of three cubits. . ptolemy's parallactic rules, covered with brass, four cubits in the side. . the sextant already described in page . . another quadrant, like no. . . zodiacal armillaries of melted brass, and turned out of the solid, of three cubits in diameter. near this observatory was a large clock, with one wheel two cubits in diameter, and two smaller ones, which, like it, indicated hours, minutes, and seconds. _in the south and lesser observatory._ . an armillary sphere of brass, with a steel meridian, whose diameter was about cubits. _in the north observatory._ . brass parallactic rules, which revolved in azimuth above a brass horizon, twelve feet in diameter. . a half sextant, of four cubits radius. . a steel sextant. . another half sextant, with steel limb, four cubits radius. . the parallactic rules of copernicus. . equatorial armillaries. . a quadrant of a solid plate of brass, five cubits in radius, shewing every ten seconds. . in the museum was the large globe made at augsburg, see p. . _in the stiern-berg observatory._ . in the central part, a large semicircle, with a brass limb, and three clocks, shewing hours, minutes, and seconds. . equatorial armillaries of seven cubits, with semi-armillaries of nine cubits. . a sextant of four cubits radius. . a geometrical square of iron, with an intercepted quadrant of five cubits, and divided into fifteen seconds. . a quadrant of four cubits radius, shewing ten seconds, with an azimuth circle. . zodiacal armillaries of brass, with steel meridians, three cubits in diameter. . a sextant of brass, kept together by screws, and capable of being taken to pieces for travelling with. its radius was four cubits. . a moveable armillary sphere, three cubits in diameter. . a quadrant of solid brass, one cubit radius, and divided into minutes by nonian circles. . an astronomical radius of solid brass, three cubits long. . an astronomical ring of brass, a cubit in diameter. . a small brass astrolabe. in almost all the instruments now enumerated, the limb was subdivided by diagonal lines, a method which tycho first brought into use, but which, in modern times, has been superseded by the inventions of nonius and vernier. when tycho had thus furnished his observatory, he devoted himself to the examination of the stars; and during the twenty-one years which he spent in this delightful occupation, he made vast additions to astronomical science. in order to instruct the young in the art of observation, and educate assistants for his observatory, he had sometimes under his roof from six to twelve pupils, whom he boarded and educated. some of these were named by the king, and educated at his expense. others were sent by different academies and cities; and several, who had presented themselves of their own accord, were liberally admitted by the generous astronomer. as tycho had spent nearly a ton of gold (about , dollars) in his outlay at uraniburg, his own income was reduced to very narrow limits. to supply this defect, frederick gave him an annual pension of dollars, beside an estate in norway, and made him canon of the episcopal church of rothschild, or prebend of st laurence,[ ] which had an annual income of dollars, and which was burdened only with the expense of keeping up the chapel containing the mausolea of the kings of the family of oldenburg. [ ] this office had been usually conferred on the king's chancellor. it would be an unprofitable task, and one by no means interesting to the general reader, to give a detailed history of the various astronomical observations and discoveries which were made by tycho during the twenty years that he spent at uraniburg. every phenomenon that appeared in the heavens, he observed with the greatest care; while he at the same time carried on regular series of observations for determining the places of the fixed stars, and for improving the tables of the sun, moon, and planets. though almost wholly devoted to these noble pursuits, yet he kept an open house, and received, with unbounded hospitality, the crowds of philosophers, nobles, and princes who came to be introduced to the first astronomer of the age, and to admire the splendid temple which the danish sovereign had consecrated to science. among the strangers whom he received under his roof, there were some who returned his kindness with ingratitude. among these was paul witichius, a mathematician; who, under the pretence of devoting his whole life to astronomy, insinuated himself into the utmost familiarity with tycho. the unsuspecting astronomer explained to his guest all his inventions, described all his methods, and even made him acquainted with those views which he had not realised, and with instruments which he had not yet executed. when witichius had thus obtained possession of the methods, and inventions, and views of tycho, and had enjoyed his hospitality for three months, he pretended that he was obliged to return to germany to receive an inheritance to which he had succeeded. after quitting uraniburg, this ungrateful mathematician neither returned to see tycho, nor kept up any correspondence with him; and it was not till five years after his departure that tycho learned, from the letters of the prince of hesse to ranzau, that witichius had passed through hesse, and had described, as his own, the various inventions and methods which had been shewn to him in huen. being unable to reconcile his own observations with those of copernicus, and with the prutenic tables, tycho resolved to obtain new determinations of the latitude of frauenburg, in prussia, where copernicus made his observations, and of konigsberg, to the meridian of which rheinhold had adapted his prutenic tables. for these purposes he sent one of his assistants, elias morsianus, with a proper instrument, under the protection of bylovius, ambassador of the margrave of anspach, to the king of denmark, who was returning by sea to germany; and after receiving the greatest attention and assistance from the noble canons of ermeland, he determined, from nearly a month's observations on the sun and stars, that the latitude of frauenburg was ° ½´, in place of ° ½´, as given by copernicus. in like manner he determined that the latitude of konigsberg was ° ´, in place of ° ´, as adopted by rheinhold. when morsianus returned to huen in july, he brought with him, as a present to tycho, from john hannovius, one of the canons of ermeland, the ptolemaic rules, or the parallactic instrument which copernicus had used and made with his own hands. it consisted of two equal wooden rules, five cubits long, and divided into parts. tycho preserved this gift as one peculiarly dear to him, and, on the day of his receiving it, he composed a set of verses in honour of the great astronomer to whom it belonged. among the distinguished visits which were paid to tycho, we must enumerate that of ulric, duke of mecklenburg, in . although his daughter, sophia, queen of denmark, had already paid two visits to uraniburg in the same year, yet such was her love of astronomy, that she accompanied her father and his wife elizabeth on this occasion. ulric was not only fond of science in general, but had for many years devoted himself to chemical pursuits, and he was therefore peculiarly gratified in examining the splendid laboratory and extensive apparatus which tycho possessed. it has been said by some of the biographers of tycho, that the landgrave of hesse visited uraniburg about this period; but this opinion is not correct, as it was only his astronomer and optician, rothman, who made a journey to huen in for the recovery of his health. tycho had long carried on a correspondence with this able astronomer respecting the observations made at the observatory of hesse-cassel, and, during the few months which they now spent together, they discussed in the amplest manner all the questions which had previously been agitated. rothman was astonished at the wonderful apparatus which he saw at uraniburg, and returned to his native country charmed with the hospitality of the danish astronomer. hitherto we have followed tycho through a career of almost unexampled prosperity. when he had scarcely reached his thirtieth year he was established, by the kindness and liberality of his sovereign, in the most splendid observatory that had ever been erected in europe; and a thriving family, an ample income, and a widely extended reputation were added to his blessings. of the value of these gifts he was deeply sensible, and he enjoyed them the more that he received them with a grateful heart. tycho was a christian as well as a philosopher. the powers of his gifted mind have been amply displayed in his astronomical labours; but we shall now have occasion to witness his piety and resignation in submitting to an unexpected and an adverse destiny. chapter iii. _tycho's labours do honour to his country--death of frederick ii.--james vi. of scotland visits tycho at uraniburg--christian iv. visits tycho--the duke of brunswick's visit to tycho--the danish nobility, jealous of his fame, conspire against him--he is compelled to quit uraniburg--and to abandon his studies--cruelty of the minister walchendorp--tycho quits denmark with his family and instruments--is hospitably received by count rantzau--who introduces him to the emperor rudolph--the emperor invites him to prague--he gives him a pension of crowns--and the castle of benach as a residence and an observatory--kepler visits tycho--who obtains for him the appointment of mathematician to rudolph._ the love of astronomy which had been so unequivocally exhibited by frederick ii. and his royal consort, inspired their courtiers with at least an outward respect for science; and among the ministers and advisers of the king, tycho reckoned many ardent friends. it was every where felt that denmark had elevated herself among the nations of europe by her liberality to tycho; and the peaceful glory which he had in return conferred upon his country was not of a kind to dissatisfy even rival nations. in the conquests of science no widow's or orphan's tears are shed, no captives are dragged from their homes, and no devoted victims are yoked to the chariot wheels of the triumphant philosopher. the newly acquired domains of knowledge belong, in right of conquest, to all nations, and denmark had now earned the gratitude of europe by the magnitude as well as the success of her contingent. an event, however, now occurred which threatened with destruction the interests of danish science. in the beginning of april , frederick ii. died in the th year of his age, and the th of his reign. his remains were conveyed to rothschild, and deposited in the chapel under tycho's care, where a finely executed bust of him was afterwards placed. his son and successor, christian iv., was only in the th year of his age, and though his temper and disposition were good, yet tycho had reason to be alarmed at the possibility of his discontinuing the patronage of astronomy. the taste for science, however, which had sprung up in the danish court had extended itself no wider than the influence of the reigning sovereign. the parasites of royalty saw themselves eclipsed in the bright renown which tycho had acquired, and every new visit to uraniburg by a foreign prince supplied fresh fuel to the rancour which had long been smothering in their breasts. the accession of a youthful king held out to his enemies an opportunity of destroying the influence of tycho; and though no adverse step was taken, yet he had the sagacity to foresee, in "trifles light as air," the approaching confirmation of his fears. hope, however, still cheered him amid his labours, but that hope was founded chiefly on the learning and character of nicolas caasius, the chancellor of the kingdom, from whom he had experienced the warmest attentions. among the princes who visited uraniburg, there were none who conducted themselves with more condescension and generosity than our own sovereign, james vi. in the year , when the scottish king repaired to denmark to celebrate his marriage with the princess anne, the king's sister, he paid a visit to tycho, attended by his councillors and a large suite of nobility. during the eight days which he spent at uraniburg, james carried on long discussions with tycho on various subjects, but chiefly on the motion which copernicus had ascribed to the earth. he examined narrowly all the astronomical instruments, and made himself acquainted with the principles of their construction and the method of using them. he inspected the busts and pictures in the museum, and when he perceived the portrait of george buchanan, his own preceptor, he could not refrain from the strongest expressions of delight. upon quitting the hospitable roof of tycho, james not only presented him with a magnificent donation, but afterwards gave him his royal license to publish his works in england during seventy years. this license was accompanied with the following high eulogium on his abilities and learning:--"nor have i become acquainted with these things only from the relation of others, or from a bare inspection of your works, but i have seen then before my own eyes, and have heard them with my own ears, in your residence at uraniburg, and have drawn them from the various learned and agreeable conversations which i there held with you, and which even now affect my mind to such a degree, that it is difficult to determine whether i recollect them with greater pleasure or admiration; as i now willingly testify, by this license, to present and to future generations," &c. at the request of tycho, the king also composed and wrote in his own hand some latin verses, which were more complimentary than classical. his chancellor had also composed some verses of a similar character during his visit to tycho. a short specimen of these will be deemed sufficient by the classical reader:-- "vidit et obstupuit rex huennum scoticus almam; miratus clari tot monumenta viri." in the year , when christian iv. had reached his th year, he expressed a desire to pay a visit to uraniburg. he accordingly set out with a large party, consisting of his three principal senators, and other councillors and noblemen; and having examined the various instruments in the observatories and laboratory, he proposed to tycho various questions on mechanics and mathematics, but particularly on the principles of fortification and ship building. having observed that he particularly admired a brass globe, which, by means of internal wheelwork, imitated the diurnal motion of the heavens, the rising and setting of the sun, and the phases of the moon, tycho made him a present of it, and received in return an elegant gold chain, with his majesty's picture, with an assurance of his unalterable attachment and protection. notwithstanding this assurance, tycho had already, as we have stated, begun to suspect the designs of his enemies; and in a letter addressed to the landgrave of hesse, early in , he throws out some hints which indicated the anxieties that agitated his mind. the landgrave of hesse, as if he had heard some rumours unfavourable to the prospects of tycho, requested him to write him respecting the state of the kingdom, and concerning his own private affairs. to this letter, which was dated early in february, tycho replied about the beginning of april. he informed the landgrave that he led a private life in his own island, exempt from all official functions, and never willingly taking a part in public affairs. he was desirous of leaving the ambition of public honours to others, and of devoting himself wholly to the study of philosophy and astronomy; and he expressed a hope that if he should be involved in the tumults and troubles of life, either by his own destiny or by evil counsels, he might be able, by the blessing of god, to extricate himself by the force of his mind and the integrity of his life. he comforted himself with the idea that every soil was the country of a great man, and that wherever he went the blue sky would still be over his head;[ ] and he distinctly states at the close of his letter, that he had thought of transferring his residence to some other place, as there were some of the king's councillors who had already begun to calumniate his studies, and to grudge him his pension from the treasury. [ ] omne solum forti patria, et coelum undique supra est. the causes which led to this change of feeling on the part of christian iv.'s advisers have not been explained by the biographers of tycho. it has been stated, in general terms, that he had made many enemies, by the keenness of his temper and the severity of his satire; but i have not been able to discover any distinct examples of these peculiarities of his mind. in an event, indeed, which occurred about this time, he slightly resented a piece of marked incivility on the part of henry julius, duke of brunswick, who had married the princess eliza of denmark; but it is not likely that so trivial an affair, if it were known at court, could have called down upon him the hostility of the king's advisers. the duke of brunswick had, in , paid a visit to uraniburg, and had particularly admired an antique brass statue of mercury, about a cubit long, which tycho had placed in the roof of the hypocaust or central crypt of the stiern-berg observatory. by means of a concealed mechanism, it moved round in a circular orbit. the duke requested the statue and its machinery, which tycho gave him, on the condition that he should obtain a model of it, for the purpose of having another executed by a skilful workman. the duke not only forgot his promise, but paid no attention to the letters which were addressed to him. tycho was justly irritated at this unprincely conduct, and ordered this anecdote to be inserted in the description of uraniburg which he was now preparing for publication. in the year , tycho lost his distinguished friend and correspondent the prince of hesse, and astronomy one of its most active and intelligent cultivators. his grief on this occasion was deep and sincere, and he gave utterance to his feelings in an impassioned elegy, in which he recorded the virtues and talents of his friend. prince maurice, the son and successor of the landgrave, continued, with the assistance of able observers, to keep up the reputation of the observatory of hesse-cassel; and the observations which were there made were afterwards published by snellius. the extensive and valuable correspondence between tycho and the landgrave was prepared for publication about the beginning of , and contains also the letters of rothman and rantzau. for several years the studies of tycho had been treated with an unwilling toleration by the danish court. many of the nobles envied the munificent establishment which he had received from frederick, and the liberal pension which he drew from his treasury. but among his most active enemies were some physicians, who envied his reputation as a successful and a gratuitous practitioner of the healing art. numbers of invalids flocked to huen, and diseases, which resisted all other methods of cure, are said to have yielded to the panaceal prescription of the astrologer. under the influence of such motives, these individuals succeeded in exciting against tycho the hostility of the court. they drew the public attention to the exhausted state of the treasury. they maintained that he had possessed too long the estate in norway, which might be given to men who laboured more usefully for the commonwealth; and they accused him of allowing the chapel at rothschild to fall into decay. the president of the council, christopher walchendorp, and the king's chancellor, were the most active of the enemies of tycho; and, having poisoned the mind of their sovereign against the most meritorious of his subjects, tycho was deprived of his canonry, his estate in norway, and his pension. being no longer able to bear the expenses of his establishment in huen, and dreading that the feelings which had been excited against him might be still further roused, so as to deprive him of the island of huen itself, he resolved to transfer his instruments to some other situation. notwithstanding this resolution, he remained with his family in the island, and continued his observations till the spring of , when he took a house in copenhagen, and removed to it all his smaller and more portable instruments, leaving those which were large or fixed in the crypts of stiern-berg. his first plan was to remove every thing from huen as a measure of security; but the public feeling began to turn in his favour, and there were many good men in copenhagen who did not scruple to reprobate the conduct of the government. the president of the council, walchendorp--a name which, while the heavens revolve, will be pronounced with horror by astronomers--saw the change of sentiment which his injustice had produced, and adopted an artful method of sheltering himself from public odium. in consequence of a quarrel with tycho, the recollection of which had rankled in his breast, he dreaded to be the prime mover in his persecution. he therefore appointed a committee of two persons, one of whom was thomas feuchius, to report to the government on the nature and utility of the studies of tycho. these two individuals were entirely ignorant of astronomy and the use of instruments; and even if they had not, they would have been equally subservient to the views of the minister. they reported that the studies of tycho were of no value, and that they were not only useless, but noxious. armed with this report, walchendorp prohibited tycho, in the king's name, from continuing his chemical experiments; and instigated, no doubt, by this wicked minister, an attack was made upon himself, and his shepherd or his steward was injured in the affray. tycho was provoked to revenge himself upon his enemies, and the judge was commanded not to interfere in the matter. thus persecuted by his enemies, tycho resolved to remain no longer in an ungrateful country. he carried from huen every thing that was moveable, and having packed up his instruments, his crucibles, and his books, he hired a ship to convey them to some foreign land. his wife, his five sons and four daughters, his male and his female servants, and many of his pupils and assistants, among whom were tengnagel, his future son-in-law, and the celebrated longomontanus, embarked at copenhagen, to seek the hospitality of some better country than their own. freighted with the glory of denmark, this interesting bark made the best of its way across the baltic, and arrived safely at rostoch. here the exiled patriarch found many of his early friends, particularly henry bruce, an able astronomer, to whom he had formerly presented one of his brass quadrants. the approach of the plague, however, prevented tycho from making any arrangements for a permanent residence; and, having received a warm invitation from count henry rantzau, who lived in holstein at the castle of wandesberg, near hamburg, he went with all his family, about the end of , to enjoy the hospitality of his friend. though tycho derived the highest pleasure from the kindness and conversation of count rantzau, yet a cloud overshadowed the future, and he had yet to seek for a patron and a home. his hopes were fixed on the emperor rudolph, who was not only fond of science, but who was especially addicted to alchemy and astrology, and his friend rantzau promised to have him introduced to the emperor by proper letters. when tycho learned that rudolph was particularly fond of mechanical instruments and of chemistry, he resolved to complete and to dedicate to him his work on the mechanics of astronomy, and to add to it an account of his chemical labours. this task he soon performed, and his work appeared in under the title of _tychonis brahe, astronomiæ instauratæ mechanica_. along with this work he transmitted to the emperor a copy of his ms. catalogue of fixed stars. with these proofs of his services to science, and instigated by various letters in his favour, the emperor rudolph desired his vice-chancellor to send for tycho, and to assure him that he would be received according to his great merits, and that nothing should be wanting to promote his scientific studies. leaving his wife and daughters at wandesberg, and taking with him his sons and his pupils, tycho went to wittemberg; but having learned that the plague had broken out at prague, and that the emperor had gone to pilsen, he deferred for a while his journey into bohemia. early in the spring of , when the pestilence had ceased at prague, and the emperor had returned to his capital, tycho set out for bohemia. on his arrival at prague, he found a splendid house ready for his reception, and a kind message from the emperor, prohibiting him from paying his respects to him till he had recovered from the fatigues of his journey. on his presentation to rudolph, the generous emperor received him with the most distinguished kindness. he announced to him that he was to receive an annual pension of crowns; that an estate would as soon as possible be settled upon him and his family and their successors; that a town house would be provided for him; and that he might have his choice of various castles and houses in the country as the site of his observatory and laboratory. the emperor had also taken care to provide every thing that was necessary for tycho's immediate wants; and so overwhelmed was he with such unexpected kindness, that he remarked that, as he could not find words to express his gratitude, the whole heavens would speak for him, and posterity should know what a refuge his great and good sovereign had been to the queen of the arts. among the numerous friends whom tycho found at prague, were his correspondents coroducius and hagecius, and his benefactor barrovitius, the emperor's secretary. he was congratulated by them all on his distinguished reception at court, and was regarded as the Ã�neas of science, who had been driven from his peaceful home, and who had carried with him to the latium of germany his wife, his children, and his household gods. if external circumstances could remove the sorrows of the past, tycho must now have been supremely happy. in his spacious mansion, which had belonged to his friend curtius, he found a position for one of his best instruments, and having covered with poetical inscriptions the four sides of the pedestal on which it stood, in honour of his benefactors, as well as of former astronomers, he resumed with diligence his examination of the stars. when rudolph saw the magnificent instruments which tycho had brought along with him, and had acquired some knowledge of their use, he pressed him to send to denmark for the still larger ones which he had left at stiern-berg. in the meantime, he gave him the choice of the castles of brandisium, lyssa, and benach as his country residence; and after visiting them about the end of may, tycho gave the preference to benach, which was situated upon a rising ground, and commanded an extensive horizon. it contained splendid and commodious buildings, and was almost, as he calls it, a small city, situated on the stream lisor, near its confluence with the albis. it stood a little to the east and north of prague, and was distant from that city only five german miles, or about six hours' journey. on the th of august, the prefect of brandisium gave tycho possession of his new residence. his gratitude to his royal patron was copiously displayed, not only in a latin poem written on the occasion, but in latin inscriptions which he placed above the doors of his observatory and his laboratory. in order that he might establish an astronomical school at prague, he wrote to longomontanus, kepler, muller, david fabricius, and two students at wittemberg, who were good calculators, requesting them to reside with him at benach, as his assistants and pupils: he at the same time dispatched his destined son-in-law, tengnagel, accompanied by pascal muleus, to bring home his wife and daughters from wandesberg, and his instruments from huen; and he begged that longomontanus would accompany them to denmark, and return in the same carriage with them to bohemia. kepler arrived at prague in january , and, after spending three or four months at benach, in carrying on his inquiries and in making astronomical observations, he returned to gratz. tycho had undertaken to obtain for him the appointment of his assistant. it was arranged that the emperor should allow him a hundred florins, on the condition that the states of styria would permit him to retain his salary for two years. this scheme, however, failed, and kepler was about to study medicine, and offer himself for a professorship of medicine at tubingen, when tycho undertook to obtain him a permanent appointment from the emperor. kepler, accordingly, returned in september , and, on the recommendation of his friend, he was named imperial mathematician, on the condition of assisting tycho in his observations. tycho had experienced much inconvenience in his residence at benach, from his ignorance of the language and customs of the country, as well as from other causes. he was therefore anxious to transfer his instruments to prague; and no sooner were his wishes conveyed to the emperor than he gave him leave to send them to the royal gardens and the adjacent buildings. his family and his larger instruments having now arrived from huen, the astronomer with his family and his property were safely lodged in the royal edifice. having found that there was no house in prague more suited for his purposes than that of his late friend curtius, the emperor purchased it from his widow, and tycho removed into it on the th february . chapter iv. _tycho resumes his astronomical observations--is attacked with a painful disease--his sufferings and death in --his funeral--his temper--his turn for satire and raillery--his piety--account of his astronomical discoveries--his love of astrology and alchymy--observations on the character of the alchymists--tycho's elixir--his fondness for the marvellous--his automata and invisible bells--account of the idiot, called lep, whom he kept as a prophet--history of tycho's instruments--his great brass globe preserved at copenhagen--present state of the island of huen._ although tycho continued in this new position to observe the planets with his usual assiduity, yet the recollection of his sufferings, and the inconveniences and disappointments which he had experienced, began to prey upon his mind, and to affect his health. notwithstanding the continued liberality of the emperor, and the kindness of his friends and pupils, he was yet a stranger in a distant land. misfortune was unable to subdue that love of country which was one of the most powerful of his affections; and, though its ingratitude might have broken the chain which bound him to the land of his nativity, it seems only to have rivetted it more firmly. his imagination, thus influenced, acquired an undue predominance over his judgment. he viewed the most trifling occurrences as supernatural indications; and in those azure moments when the clouds broke from his mind, and when he displayed his usual wit and pleasantry, he frequently turned the conversation to the subject of his latter end. this state of mind was the forerunner, though probably the effect, of a painful disease, which had, doubtless, its origin in the severity and continuity of his studies. on the th october, when he was supping at the house of a nobleman called rosenberg, he was seized with a retention of urine, which forced him to leave the party. this attack continued with little intermission for more than a week, and, during this period, he suffered great pain, attended with want of sleep and temporary delirium, during which, he frequently exclaimed, _ne frustra vixisse videor_. on the th he recovered from this painful situation, and became perfectly tranquil. his strength, however, was gone, and he saw that he had not many hours to live. he expressed an anxious wish that his labours would redound to the glory of his maker, to whom he offered up the most ardent prayers. he enjoined his sons and his son-in-law not to allow them to be lost. he encouraged his pupils not to abandon their pursuits, he requested kepler to complete the rudolphine tables, and to his family he recommended piety and resignation to the divine will. among those who never quitted tycho in his illness, was erick brahe, count wittehorn, a swede, and a relation of his own, and counsellor to the king of poland. this amiable individual never left the bedside of his friend, and administered to him all those attentions which his situation required. tycho, turning to him, thanked him for his affectionate kindness, and requested him to maintain the relationship with his family. he then expired without pain, amid the consolations, the prayers, and the tears of his friends. this event took place on the th of october , when he was only fifty-four years and ten months old. the emperor rudolph evinced the greatest sorrow when he was informed of the death of his friend, and he gave orders that he should be buried in the most honourable manner, in the principal church of the ancient city.[ ] the funeral took place on the th november, and he was interred in the dress of a nobleman, and with the ceremonies of his order. the funeral oration was pronounced by jessenius, before a distinguished assemblage, and many elegies were written on his death. [ ] the church of tiers, where a monument has been erected to his memory. tycho was a little above the middle size, and in the last years of his life he was slightly corpulent. he had reddish yellow hair and a ruddy complexion. he was of a sanguine temperament, and is said to have been sometimes irritable, and even obstinate. this failing, however, if he did possess it, was not exhibited towards his pupils or his scientific friends, who ever entertained for him the warmest affection and esteem. some of his pupils had remained in his house more than twenty years; and in the quarrel which arose between him and kepler,[ ] and which is allowed to have originated entirely in the temper of the latter, he conducted himself with the greatest patience and forbearance. there is reason to think that the irritability with which he has been charged was less an affection of his mind than the effect of that noble independence of character which belonged to him, and that it has been inferred chiefly from his conduct to some of those high personages with whom he was brought in contact. when walchendorp, the president of the council, kicked his favourite hound, it was no proof of irritability of character that tycho expressed in strong terms his disapprobation of the deed. [ ] see the life of kepler. it was, doubtless, a greater weakness in his character that he indulged his turn for satire, without being able to bear retaliation. his jocular habits, too, sometimes led him into disagreeable positions. when the duke of brunswick was dining with him at uraniburg, the duke said, towards the end of the dinner, that, as it was late, he must be going. tycho jocularly remarked that this could not be done without his permission; upon which the duke rose and left the party, without taking leave of his host. tycho became indignant in his turn, and continued to sit at table; but, as if repenting of what he had done, he followed the duke, who was on his way to the ship, and, calling upon him, displayed the cup in his hand, as if he had washed out his offence by a draught of wine. tycho was a man of true piety, and cherished the deepest veneration for the sacred scriptures, and for the great truths which they reveal. their principles regulated his conduct, and their promises animated his hopes. his familiarity with the wonders of the heavens increased, instead of diminishing, his admiration of divine wisdom, and his daily conversation was elevated by a constant reference to a superintending providence. as a practical astronomer, tycho has not been surpassed by any observer of ancient or of modern times. the splendour and number of his instruments, the ingenuity which he exhibited in inventing new ones and in improving and adding to those which were formerly known, and his skill and assiduity as an observer, have given a character to his labours, and a value to his observations, which will be appreciated to the latest posterity. the appearance of the new star in led him to form a catalogue of stars, vastly superior in accuracy to those of hipparchus and ulugh beig. his improvements on the lunar theory were still more valuable. he discovered the important inequality called the _variation_, and also the annual inequality which depends on the position of the earth in its orbit. he discovered, also, the inequality in the inclination of the moon's orbit, and in the motion of her nodes. he determined with new accuracy the astronomical refractions from an altitude of ° down to the horizon, where he found it to be ´; and he made a vast collection of observations on the planets, which formed the groundwork of kepler's discoveries and the basis of the rudolphine tables. tycho's powers of observation were not equalled by his capacity for general views. it was, perhaps, owing more to his veneration for the scriptures than to the vanity of giving his name to a new system that he rejected the copernican hypothesis. hence he was led to propose a new system, called the tychonic, in which the earth is stationary in the centre of the universe, while the sun, with all the other planets and comets revolving round him, performs his daily revolution about the earth. this arrangement of the planets afforded a sufficient explanation of the various phenomena of the heavens; and as it was consistent with the language of scripture, and conformable to the indications of the senses, it found many supporters, notwithstanding the physical absurdity of making the whole system revolve round one of the smallest of the planets. it is a painful transition to pass from the astronomical labours of tycho to his astrological and chemical pursuits. that tycho studied and practised astrology has been universally admitted. he calculated the nativity of the emperor rudolph, and foretold that his relations would make some attempts upon his life. the credulous emperor confided in the prediction, and when the conduct of his brother seemed to justify his belief, he confined himself to his palace, and fell a prey to the fear which it inspired. tycho, however, seems to have entirely renounced his astrological faith in his latter days; and kepler states,[ ] in the most pointed manner, that tycho carried on his astronomical labours with his mind entirely free from the superstitions of astrology; that he derided and detested the vanity and knavery of astrologers, and was convinced that the stars exercised no influence on the destinies of men. [ ] in his preface to the rudolphine tables. although tycho informed rothman that he devoted as much labour and expense to the study of terrestrial (chemistry) as he did to that of celestial astronomy, yet it is a singular fact that he never published any account of his experiments, nor has he left among his writings any trace of his chemical inquiries. he pretended, however, to have made discoveries in the science, and we should have been disposed to reprobate the apology which he makes for not publishing them, did we not know that it had been frequently given by the other alchemists of the age--"on consideration," says he, "and by the advice of the most learned men, i thought it improper to unfold the secrets of the art (of alchemy) to the vulgar, as few persons were capable of using its mysteries to advantage and without detriment." admitting then, as we must do, that tycho was not only a professed alchemist, but that he was practically occupied with its pursuits, and continually misled by its delusions, it may not be uninteresting to the reader to consider how far a belief in alchemy, and a practice of its arts, have a foundation in the weakness of human nature; and to what extent they are compatible with the piety and elevated moral feeling by which our author was distinguished. in the history of human errors two classes of impostors, of very different characters, present themselves to our notice--those who wilfully deluded their species, and those who permitted their species to delude themselves. the first of those classes consisted of the selfish tyrants who upheld an unjust supremacy by systematic delusions, and of grovelling mountebanks who quenched their avaricious thirst at the fountains of credulity and ignorance. the second class comprehended spirits of a nobler mould: it embraced the speculative enthusiasts, whom the love of fame and of truth urged onward, in a fruitless research, and those great lights of knowledge and of virtue, who, while they stood forward as the landmarks of the age which they adorned, had neither the intellectual nor the moral courage to divest themselves of the supernatural radiance with which the ignorance of the vulgar had encircled them. the thrones and shrines, which delusion once sustained even in the civilized quarter of the globe, are for ever fallen, and that civil and religious liberty, which in past ages was kept down by the marvellous exhibitions of science to the senses, is now maintained by its application to the reason of man. the charlatans, whether they deal in moral or in physical wonders, form a race which is never extinct. they migrate to the different zones of the social system, and though they change their place, and their purposes, and their victims, yet their character and motives remain the same. the philosophical mind, therefore, is not disposed to study either of these varieties of impostors; but the other two families which compose the second class are objects of paramount interest. the eccentricities and even the obliquities of great minds merit the scrutiny of the metaphysician and the moralist, and they derive a peculiar interest from the state of society in which they are exhibited. had cardan and cornelius agrippa lived in modern times, their vanity and self-importance would have been checked by the forms of society, and even if their harmless pretensions had been displayed, they would have disappeared in the blaze of their genius and knowledge. but nursed in superstition, and educated in dark and turbulent times, when every thing intellectual was in a state of restless transition, the genius and character of great men necessarily reflected the peculiarities of the age in which they lived. had history transmitted to us correct details of the leading alchemists and scientific magicians of the dark ages, we should have been able to analyse their actions and their opinions, and trace them, probably, to the ordinary principles by which the human mind is in every age influenced and directed. but when a great man has once become an object either of interest or of wonder, and still more when he is considered as the possessor of knowledge and skill which transcend the capacity of the age, he is soon transformed into the hero of romance. his powers are overrated, his deeds exaggerated, and he becomes the subject of idle legends, which acquire a firmer hold on credulity from the slight sprinkling of truth with which they are seasoned. to disclaim the possession of lofty attributes thus ascribed to great men is a degree of humility which is not often exercised. but even when this species of modesty is displayed, it never fails to defeat its object. it but calls forth a deeper homage, and fixes the demigod more firmly in his shrine. the history of learning furnishes us with many examples of that species of delusion in which a great mind submits itself to vulgar adulation, and renounces unwillingly, if it renounces at all, the unenviable reputation of supernatural agency. in cases where self-interest and ambition are the basis of this peculiarity of temperament, and in an age when the conjuror and the alchemist were the companions and even the idols of princes, it is easy to trace the steps by which a gifted sage retains his ascendancy among the ignorant. the hecatomb which is sacrificed to the magician, he receives as an oblation to his science, and conscious of possessing real endowments, the idol devours the meats that are offered to him without analysing the motives and expectations under which he is fed. but even when the idolater and his god are not placed in this transverse relation, the love of power or of notoriety is sufficient to induce good men to lend a too willing ear to vulgar testimony in favour of themselves; and in our own times it is not common to repudiate the unmerited cheers of a popular assembly, or to offer a contradiction to fictitious tales which record our talents or our courage, our charity or our piety. the conduct of the scientific alchemists of the thirteenth, fourteenth, and fifteenth centuries presents a problem of very difficult solution. when we consider that a gas, a fluid, and a solid may consist of the very same ingredients in different proportions; that a virulent poison may differ from the most wholesome food only in the difference of quantity of the very same elements; that gold and silver, and lead and mercury, and indeed all the metals, may be extracted from transparent crystals, which scarcely differ in their appearance from a piece of common salt or a bit of sugarcandy; and that diamond is nothing more than charcoal,--we need not greatly wonder at the extravagant expectation that the precious metals and the noblest gems might be procured from the basest materials. these expectations, too, must have been often excited by the startling results of their daily experiments. the most ignorant compounder of simples could not fail to witness the magical transformations of chemical action; and every new product must have added to the probability that the tempting doublets of gold and silver might be thrown from the dice-box with which he was gambling. but when the precious metals were found in lead and copper by the action of powerful re-agents, it was natural to suppose that they had been actually formed during the process; and men of well-regulated minds even might have thus been led to embark in new adventures to procure a more copious supply, without any insult being offered to sober reason, or any injury inflicted on sound morality. when an ardent and ambitious mind is once dazzled with the fascination of some lofty pursuit, where gold is the object, or fame the impulse, it is difficult to pause in a doubtful career, and to make a voluntary shipwreck of the reputation which has been staked. hope still cheers the aspirant from failure to failure, till the loss of fortune and the decay of credit disturb the serenity of his mind, and hurry him on to the last resource of baffled ingenuity and disappointed ambition. the philosopher thus becomes an impostor; and by the pretended transmutation of the baser metals into gold, or the discovery of the philosopher's stone, he attempts to sustain his sinking reputation, and recover the fortune he has lost. the communication of the great secret is now the staple commodity with which he is to barter, and the grand talisman with which he is to conjure. it can be imparted only to a chosen few--to those among the opulent who merit it by their virtues, and can acquire it by their diligence, and the divine vengeance is threatened against its disclosure. a process commencing in fraud and terminating in mysticism is conveyed to the wealthy aspirant, or instilled into the young enthusiast, and the grand mystery passes current for a season, till some cautious professor of the art, like tycho, denounces its publication as detrimental to society. among the extravagant pretensions of the alchemists, that of forming a universal medicine was perhaps not the most irrational. it was only when they pretended to cure every disease, and to confer longevity, that they did violence to reason. the success of the arabian physicians in the use of mercurial preparations naturally led to the belief that other medicines, still more general in their application, and efficacious in their healing powers, might yet be brought to light; and we have no doubt that many substantial discoveries were the result of such overstrained expectations. tycho was not merely a believer in the medical dogmas of the alchemists, he was actually the discoverer of a new _elixir_, which went by his name, and which was sold in every apothecary's shop as a specific against the epidemic diseases which were then ravaging germany. the emperor rudolph having heard of this celebrated medicine, obtained a small portion of it from tycho by the hands of the governor of brandisium; but, not satisfied with the gift, he seems to have applied to tycho for an account of the method of preparing it. tycho accordingly addressed to the emperor a long letter, dated september , , containing a minute account of the process. the base of this remarkable medicine is venetian treacle, which undergoes an infinity of chemical operations and admixtures before it is ready for the patient. when properly prepared he assures the emperor that it is better than gold, and that it may be made still more valuable by mixing with it a single scruple either of the tincture of corals, or sapphire, or hyacinth, or a solution of pearls, or of potable gold, if it can be obtained free of all corrosive matter! in order to render the medicine _universal_ for all diseases which can be cured by perspiration, and which, he says, form a third of those which attack the human frame, he combines it with antimony, a well known sudorific in the present practice of physic. tycho concludes his letter by humbly beseeching the emperor to keep the process secret, and reserve the medicine for himself alone! the same disposition of mind which made tycho an astrologer and an alchemist, inspired him with a singular love of the marvellous. he had various automata with which he delighted to astonish the peasants; and by means of invisible bells, which communicated with every part of his establishment, and which rung with the gentlest touch, he had great pleasure in bringing any of his pupils suddenly before strangers, muttering at a particular time the words "come hither, peter," as if he had commanded their presence by some supernatural agency. if, on leaving home, he met with an old woman or a hare, he returned immediately to his house: but the most extraordinary of all his peculiarities remains to be noticed. when he lived at uraniburg he maintained an idiot of the name of lep, who lay at his feet whenever he sat down to dinner, and whom he fed with his own hand. persuaded that his mind, when moved, was capable of foretelling future events, tycho carefully marked every thing he said. lest it should be supposed that this was done to no purpose, longomontanus relates that when any person in the island was sick, lep never, when interrogated, failed to predict whether the patient would live or die. it is stated also in the letters of wormius, both to gassendi and peyter, that when tycho was absent, and his pupils became very noisy and merry in consequence of not expecting him soon home, the idiot, who was present, exclaimed, _juncher xaa laudit_, "your master has arrived." on another occasion, when tycho had sent two of his pupils to copenhagen on business, and had fixed the day of their return, lep surprised him on that day while he was at dinner, by exclaiming, "behold your pupils are bathing in the sea." tycho, suspecting that they were shipwrecked, sent some person to the observatory to look for their boat. the messenger brought back word that he saw some persons wet on the shore, and in distress, with a boat upset at a great distance. these stories have been given by gassendi, and may be viewed as specimens of the superstition of the age. tycho left behind him a wife and six children, but even in the time of gassendi nothing was known of their history, excepting that tengnagel, who married one of the daughters, gave up his scientific pursuits, and, having been admitted among the emperor's counsellors, was employed in several of his embassies. the instruments of tycho were purchased from his heirs, by the emperor, for , crowns. they were shut up in the house of curtius, and were treated with such veneration, that no astronomer, not even kepler himself, was permitted to see or to use them. here they remained till the death of the emperor matthias, in , when the troubles in bohemia took place. when prague was taken by the forces of the elector palatine, the instruments were carried off, and some were destroyed, and others converted to different purposes. the great brass globe, however, was saved. it was first carried to niessa, the episcopal city of silesia; and having been presented to the college of jesuits, it was preserved in their museum, till udalric, the son of christian, king of denmark, took niessa in . the globe was recognized as having belonged to tycho, and it was carried in triumph to denmark. an inscription was written upon it by longomontanus, and it was deposited with some pomp in the library of the academy of sciences. after tycho left huen, the island was transferred to some of the danish nobility, and the following brief but melancholy description of it was given by wormius. "there is, in the island, a field where uraniburg was." the scientific antiquities of huen, have been more recently described by mr cox, in his travels through denmark. "we landed," says he, "on the south west part in a small bay, just below the place where a stream, supplied by numerous pools and fish ponds, falls into the sea. we ascended the shore, which is clothed with short herbage, crossed the stream, and passed over a gently waving surface, gradually sloping towards the sea, and walked a mile to a farm house, standing in the middle of the island, inhabited by mr schaw, a swedish gentleman, to whom the greater part of the island belongs. he lives here in summer, but in winter resides at landscrona. this dwelling is the same as existed in tycho brahe's time, and was the farm house belonging to his estate. a guide, whom we obtained from mr schaw, conducted us to the remains of tycho's mansion, which are near the house, and consist of little more than a mound of earth which enclosed the garden, and two pits, the sites of his mansion and observatory."[ ] [ ] cox's travels in poland, &c., vol. v., p. , . life of john kepler. chapter i. _kepler's birth in --his family--and early education--the distresses and poverty of his family--he enters the monastic school of maulbronn--and is admitted into the university of tubingen, where he distinguishes himself, and takes his degrees--he is appointed professor of astronomy and greek in --his first speculations on the orbits of the planets--account of their progress and failure--his "cosmographical mystery" published--he marries a widow in --religious troubles at gratz--he retires from thence to hungary--visits tycho at prague in --returns to gratz, which he again quits for prague--he is taken ill on the road--is appointed tycho's assistant in --succeeds tycho as imperial mathematician--his work on the new star of --singular specimen of it._ it is a remarkable circumstance in the history of science, that astronomy should have been cultivated at the same time by three such distinguished men as tycho, kepler, and galileo. while tycho, in the th year of his age, was observing the heavens at prague, kepler, only years old, was applying his wild genius to the determination of the orbit of mars, and galileo, at the age of , was about to direct the telescope to the unexplored regions of space. the diversity of gifts which providence assigned to these three philosophers was no less remarkable. tycho was destined to lay the foundation of modern astronomy, by a vast series of accurate observations made with the largest and the finest instruments; it was the proud lot of kepler to deduce the laws of the planetary orbits from the observations of his predecessors; while galileo enjoyed the more dazzling honour of discovering by the telescope new celestial bodies, and new systems of worlds. john kepler, the youngest of this illustrious band, was born at the imperial city of weil, in the duchy of wirtemberg, on the st december . his parents, henry kepler and catherine guldenmann, were both of noble family, but had been reduced to indigence by their own bad conduct. henry kepler had been long in the service of the duke of wirtemberg as a petty officer, and in that capacity had wasted his fortune. upon setting out for the army, he left his wife in a state of pregnancy; and, at the end of seven months, she gave premature birth to john kepler, who was, from this cause, a sickly child during the first years of his life. being obliged to join the army in the netherlands, his wife followed him into the field, and left her son, then five years old, under the charge of his grandfather at limberg. sometime afterwards he was attacked with the smallpox, and having with difficulty recovered from this severe malady, he was sent to school in . having become security for one of his friends, who absconded from his creditors, henry kepler was obliged to sell his house and all his property, and was driven to the necessity of keeping a tavern at elmendingen. owing to these misfortunes, young kepler was taken from school about two years afterwards, and was obliged to perform the functions of a servant in his father's house. in , he was again placed in the school of elmendingen; but his father and mother having been both attacked with the smallpox, and he himself having been seized with a violent illness in , his education had been much neglected, and he was prohibited from all mental application. in the year , on the th of november, kepler was admitted into the school at the monastery of maulbronn, which had been established at the reformation, and which was maintained at the expense of the duke of wirtemberg, as a preparatory seminary for the university of tubingen. after remaining a year at the upper classes, the scholars presented themselves for examination at the college for the degree of bachelor; and having received this, they returned to the school with the title of veterans. here they completed the usual course of study; and being admitted as resident students at tubingen, they took their degree of master. in prosecuting this course of study, kepler was sadly interrupted, not only by periodical returns of his former complaints, but by family quarrels of the most serious import. these dissensions, arising greatly from the perverseness of his mother, drove his father to a foreign land, where he soon died; and his mother having quarrelled with all her relations, the affairs of the family were involved in inextricable disorder. notwithstanding these calamities, kepler took his degree of bachelor on the th september , and his degree of master in august , on which occasion he held the second place at the annual examination. in his early studies, kepler devoted himself with intense pleasure to philosophy in general, but he entertained no peculiar affection for astronomy. being well grounded in arithmetic and geometry, he had no difficulty in making himself master of the geometrical and astronomical theorems which occurred in the course of his studies. while attending the lectures of moestlin, professor of mathematics, who had distinguished himself by an oration in favour of the copernican system, kepler not only became a convert to the opinions of his master, but defended them in the physical disputations of the students, and even wrote an essay on the primary motion, in order to prove that it was produced by the daily rotation of the earth. in , the astronomical chair at gratz, in styria, fell vacant by the death of george stadt, and, according to kepler's own statement, he was forced to accept this situation by the authority of his professional tutors, who recommended him to the nobles of styria. though kepler had little knowledge of the science, and no passion for it whatever, yet the nature of his office forced him to attend to astronomy; and, in the year , when he enjoyed some leisure from his lectures, he directed the whole energy of his mind to the three important topics of the number, the size, and the motion of the orbits of the planets. he first tried if the size of the planets' orbits, or the difference of their sizes, had any regular proportion to each other. finding no proof of this, he inserted a new planet between mars and jupiter, and another between venus and mercury, which he supposed might be invisible from their smallness; but even with these assumptions the distances of the planets exhibited no regular progression. kepler next tried if these distances varied as the cosines of the quadrant, and if their motion varied as the sun's, the sine of representing the motion at the sun, and the sine of ° that at the fixed stars; but in this trial he was also disappointed. having spent the whole summer in these fruitless speculations, and praying constantly to his maker for success, he was accidentally drawing a diagram in his lecture-room, in july , when he observed the relation between the circle inscribed in a triangle, and that described round it; and the ratio of these circles, which was that of to , appeared to his eye to be identical with that of jupiter's and saturn's orbits. hence he was led to compare the orbits of the other planets' circles described in pentagons and hexagons. as this hypothesis was as inapplicable to the heavens as its predecessors, kepler asked himself in despair, "what have _plane_ figures to do with _solid_ orbits? solid bodies ought to be used for solid orbits." on the strength of this conceit, he supposed that the distances of the planets were regulated by the sizes of the five regular solids described within one another. "the earth is the circle, the measurer of all. round it describe a dodecahedron; the circle including this will be mars. round mars describe a tetrahedron; the circle including this will be jupiter. describe a cube round jupiter; the circle including this will be saturn. then inscribe in the earth an icosahedron; the circle described in it will be venus. inscribe an octohedron in venus; the circle inscribed in it will be mercury." this discovery, as he considered it, harmonized in a very rude way with the measures of the planetary orbits given by copernicus; but kepler was so enamoured with it, that he ascribed the differences to errors of observation, and declared that he would not renounce the glory of having made it for the whole electorate of saxony. in his attempt to discover the relation between the periodic times of the planets and their distances from the sun, he was not more successful; but as this relation had a real existence, he made some slight approach to its determination. these extraordinary researches, which indicate the wildness and irregularity of kepler's genius, were published in , in a work entitled, "prodromus of cosmographical dissertations; containing the cosmographical mystery respecting the admirable proportion of the celestial orbits, and the genuine and real causes of the number, magnitude, and periods of the planets demonstrated by the five regular geometrical solids." notwithstanding the speculative character of this volume, it obtained for its author a high name among astronomers. galileo and tycho, whose opinions of it he requested, spoke of it with some commendation. the former praised the ingenuity and good faith which it displayed; and tycho, though he requested him to try to adapt something of the same nature to the tychonic system, saw the speculative character of his mind, and advised him "to lay a solid foundation for his views by actual observation, and then, by ascending from these, to strive to reach the causes of things." in , before kepler had quitted tubingen, he was on the eve of entering into the married state. though the foolish scheme was fortunately broken off, yet he resumed it again in , when he paid his addresses to barbara millar of muleckh, who was a widow for the second time, though only twenty-three years of age. her parents, however, would not consent to the match till kepler proved his nobility; and, owing to the delay which arose from this circumstance, the marriage did not take place till . the income which kepler derived from his professorship was very small, and as his wife's fortune turned out much less than he had been led to expect, he not only was annoyed with pecuniary difficulties, but was involved in disputes with his wife's relations. these evils were greatly increased by the religious troubles which took place in styria. the catholics at gratz rose against the protestants, and threatened to expell them from the city. kepler, who openly professed the protestant religion, saw the risks to which he was exposed, and retired with his wife into hungary. here he continued nearly a year, during which he composed and transmitted to his friend zehentmaier, at tubingen, several small treatises, "on the magnet," "on the cause of the obliquity of the ecliptic," and "on the divine wisdom, as shewn in the creation"--all of which seem to have been lost. in , kepler was recalled to gratz by the states of styria, and resumed his professorship; but the city was still divided into two factions, and kepler, who was a lover of peace, found his situation very uncomfortable. having learned from tycho that he had been able to determine more accurately than had been done the eccentricities of the orbits of the planets, kepler was anxious to avail himself of these observations, and set out on a visit to tycho at prague, where he arrived in january . tycho received him with great kindness, notwithstanding the part which he had taken against him along with raimar, and he spent three or four months with him at benach. it was then arranged that kepler should be appointed tycho's assistant in the observatory, with a salary of florins, provided the states of styria should, on the emperor's application, allow him to be absent for two years and retain his salary. kepler had returned to gratz before this arrangement was completed, and new troubles having broke out in that city, he resigned his professorship. dreading lest this step would frustrate his scheme of joining tycho, he resolved to ask the patronage of the duke of wirtemberg for the professorship of medicine at tubingen; and with this view he corresponded with moestlin and his other friends in that university. when tycho heard of this plan, he pressed him to abandon it, and promised his best exertions to procure a permanent situation for him from the emperor. encouraged by these promises, kepler and his wife set off for prague, but he was unfortunately attacked on the road with a quartan ague, which lasted seven months; and having exhausted the little money which he had along with him, he was obliged to apply to tycho for a supply. after his arrival at prague he was supported entirely by the bounty of his friend, and he endeavoured to make some return for this kindness by attacking in a controversial pamphlet two of the scientific opponents of tycho. kepler's total dependence on the generosity of his friend had made him suspicious of his sincerity. he imagined that tycho had not freely communicated to him all his observations, and that he had not been sufficiently liberal in supplying his wife with money in his absence. while absent a second time from prague, and influenced by these feelings, he addressed a violent letter to tycho, filled with reproaches. on the plea of being occupied with his daughter's marriage, tycho requested ericksen, one of his assistants, to reply to kepler's letter; and he did this with so much effect, that kepler saw his mistake, and in the noblest and most generous manner supplicated the forgiveness of his friend. tycho exhibited the same good feeling; and the kindness of hoffman, president of the states of styria, completed the reconciliation of the two astronomers. on his return to prague in , he was presented by tycho to the emperor, who conferred upon him the title of imperial mathematician, on the condition that he would assist tycho in his calculations. this connexion was peculiarly valuable to kepler, as the observations of his colleague were the only ones made in the world which could enable him to carry on his own theoretical inquiries. these two astronomers now undertook to compute, from tycho's observations, a new set of astronomical tables, to be called the rudolphine tables, in honour of the emperor. this scheme flattered the vanity of their master, and he pledged himself to pay all the expenses of the work. longomontanus, tycho's principal assistant, took upon himself the labour of arranging and discussing the observations on the stars, while kepler devoted himself to the more congenial task of examining those on the planet mars, with which tycho was at that time particularly occupied. the appointment of longomontanus to a professorship in denmark, and the death of tycho in october , put a stop to these important schemes. kepler succeeded tycho as principal mathematician to the emperor, and was provided with a handsome salary, which was partly charged on the imperial treasury, and partly on the states of silesia, and the first instalment of which was to be paid in march . the generosity of the emperor did not fail to excite the jealousy of ignorant individuals, who were not aware of the value of science to the state; but the increasing fame of kepler, and the valuable works which he published, soon silenced their opposition. in september , astronomers were surprised with the appearance of a new star in the foot of serpentarius. it was not seen before the th of september, and moestlin informs us that, on account of clouds, he did not obtain a good view of it till the th of october. like that of ,[ ] it at first surpassed jupiter in brightness, and rivalled even venus, but it afterwards became as small as regulus, and as dull as saturn, and disappeared at the end of a few months. it constantly changed its colour, and was at first tawny, then yellow, then purple and red, and often white at great altitudes. it had no parallax, and therefore was a fixed star. kepler wrote a short account of this remarkable body, and maintained its superiority to that of , as this last came in an ordinary year, while the other appeared in the year of the _fiery trigon_, or that in which saturn, jupiter, and mars, are in the three fiery signs, aries, leo, and sagittarius, an event which occurs only every years. after discussing a great variety of topics, but little connected with his subject, and in a style of absurd jocularity, he attacks the opinions of the epicureans, that the star was a fortuitous concourse of atoms, in the following remarkable paragraph, which is a good specimen of the work:--"when i was a youth with plenty of idle time on my hands, i was much taken with the vanity, of which some grown men are not ashamed, of making anagrams by transposing the letters of my name, written in latin. out of _joannes keplerus_ came _serpens in akuleo_ (a serpent in his sting); but not being satisfied with the meaning of these words, and being unable to make another, i trusted the thing to chance, and taking out of a pack of playing cards as many as there were letters in the name, i wrote one upon each, and then began to shuffle them, and at each shuffle to read them in the order they came, to see if any meaning came of it. now, may all the epicurean gods and goddesses confound this same chance, which, although i have spent a good deal of time over it, never shewed me anything like sense even from a distance. so i gave up my cards to the epicurean eternity, to be carried away into infinity; and, it is said, they are still flying about there in the utmost confusion among the atoms, and have never yet come to any meaning. i will tell those disputants, my opponents, not my own opinion, but my wife's. yesterday, when weary with writing, and my mind quite dusty with considering these atoms, i was called to supper, and a salad i had asked for was set before me. 'it seems then,' said i, aloud, 'that if pewter dishes, leaves of lettuce, grains of salt, drops of water, vinegar, and oil, and slices of egg, had been flying about in the air from all eternity, it might at last happen by chance that there would come a salad.' 'yes,' says my wife, 'but not so nice and well dressed as this of mine is.'" [ ] see the life of tycho, page . chapter ii. _kepler's pecuniary embarrassments--his inquiries respecting the law of refraction--his supplement to vitellio--his researches on vision--his treatise on dioptrics--his commentaries on mars--he discovers that the orbit of mars is an ellipse, with the sun in one focus--and extends this discovery to all the other planets--he establishes the two first laws of physical astronomy--his family distresses--death of his wife--he is appointed professor of mathematics at linz--his method of choosing a second wife--her character, as given by himself--origin of his treatise on gauging--he goes to ratisbon to give his opinion to the diet on the change of style--he refuses the mathematical chair at bologna._ although kepler now filled one of the most honourable situations to which a philosopher could aspire, and possessed a large salary fitted to supply his most reasonable wants, yet, as the imperial treasury was drained by the demands of an expensive war, his salary was always in arrear. owing to this cause he was constantly involved in pecuniary difficulties, and, as he himself described his situation, he was perpetually begging his bread from the emperor at prague. his increasing family rendered the want of money still more distressing, and he was driven to the painful alternative of drawing his income from casting nativities. from the same cause he was obliged to abandon his plan of publishing the rudolphine tables, and to devote himself to works of a less expensive kind, and which were more likely to yield some pecuniary advantages. in spite of these embarrassments, and the occupation of his time in the practice of astrology, kepler found leisure for his favourite pursuits. no adverse circumstances were capable of extinguishing his scientific ardour, and whenever he directed his vigorous mind to the investigation of phenomena, he never failed to obtain interesting and original results. since the death of tycho, his attention had been much occupied with the subject of refraction and vision; and, in , he published the result of his researches in a work, entitled "a supplement to vitellio, in which the optical part of astronomy is treated, but chiefly on the artificial observation and estimation of diameters, and of the eclipses of the sun and moon." astronomers had long been perplexed with the refraction of the atmosphere, and so little was known of the general subject, as well as of this branch of it, that tycho believed the refraction of the atmosphere to cease at ° of altitude. even at the beginning of the second century, claudius ptolemy of alexandria had unravelled its principal mysteries, and had given in his optics a theory of astronomical refraction more complete than that of any astronomer before the time of cassini;[ ] but the mss. had unfortunately been mislaid, and alhazen and vitellio and kepler were obliged to take up the subject from its commencement. ptolemy had not only determined that the refraction of the atmosphere had gradually increased from the zenith to the horizon, but he had measured with singular accuracy the angles of refraction for water and glass, from a perpendicular incidence to a horizontal one. [ ] cassini was born in , and died in . kepler treated this branch of science in his own peculiar way, "hunting down," as he expressed it, every hypothesis which his fertile imagination had successively presented to him. in his various attempts to discover the law of refraction, or a measure of it, as varying with the density of the body and the angle of incidence of the light, he was nearer the goal, in his first speculation, than in any of the rest; and he seems to have failed in consequence of his not separating the question as it related to density from the question as it related to incidence. "i did not leave untried," says he, "whether, by assuming a horizontal refraction according to the density of the medium, the rest would correspond to the sines of the distances from a vertical direction, but calculation proved that it was not so: and, indeed, there was no occasion to have tried it, for thus the _refraction would increase according to the same law in all mediums, which is contradicted by experiment_." although completely foiled in his search after the law of refraction, which was subsequently discovered by willebrord snell, and sometime afterwards by james gregory, he was, singularly successful in his inquiries respecting vision. regarding the eye as analogous in its structure with the camera obscura of baptista porta, he discovered that the images of external objects were painted in an inverted position on the retina, by the union of the pencils of rays which issued from every point of the object. he ascribed an erect vision to an operation of the mind, by which it traces the rays back to the pupil, where they cross one another, and thus refers the lower parts of the image to the higher parts of the object. he also explained the cause of long-sighted and short-sighted vision, and shewed how convex and concave lenses enabled those who possessed these peculiarities of vision to see distinctly, by accurately converging the pencils of rays to a focus on the retina. kepler likewise observed the power of accommodating the eye to different distances, and he ascribed it to the contraction of the ciliary processes, which drew the sides of the eyeball towards the crystalline lens, and thus elongated the eye so as to produce an adjustment of it for near objects. kepler wisely declined to inquire into the way in which the mind perceives the images painted on the retina, and he blames vitellio for attempting to determine a question which he considered as not belonging to optics. the work of kepler, now under consideration, contains the method of calculating eclipses which is now in use at the present day. the only other optical treatise written by kepler, was his _dioptrics_, with an appendix on the use of optics in philosophy. this admirable work, which laid the foundation of the science, was published at augsburg in , and reprinted at london in . although maurolycus had made some slight progress in studying the passage of light through different media, yet it is to kepler that we owe the methods of tracing the progress of rays through transparent bodies with convex and concave surfaces, and of determining the foci of lenses, and of the relative positions of the images which they form, and the objects from which the rays proceed. he was thus led to explain the _rationale_ of the telescope, and to invent the astronomical telescope, which consists of two convex lenses, by which objects are seen inverted. kepler also discovered the important fact, that spherical surfaces were not capable of converging rays to a single focus, and he conjectured, what descartes afterwards proved, that this property might be possessed by lenses having the figure of some of the sections of the cone. the total reflection of light at the second surface of bodies was likewise studied by kepler, and he determined that the total reflection commenced when the angle of incidence was equal to the angle of refraction, which corresponded to an incidence of . two years before the publication of his dioptrics, viz. in , kepler had given to the world his great work, entitled "the new astronomy, or commentaries on the motions of mars." the discoveries which this volume records form the basis of physical astronomy. the inquiries by which he was led to them began in that memorable year , when he became the colleague or assistant of tycho. the powers of original genius were then for the first time associated with inventive skill and patient observation; and though the astronomical data provided by tycho were sure of finding their application in some future age, yet without them kepler's speculations would have been vain, and the laws which they enabled him to determine would have adorned the history of another century. having tried in vain to represent the motion of mars by an uniform motion in a circular orbit, and by the cycles and epicycles with which copernicus had endeavoured to explain the planetary inequalities, kepler was led, after many fruitless speculations,[ ] to suppose the orbit of the planet to be oval; and, from his knowledge of the conic sections, he afterwards determined it to be an ellipse, with the sun placed in one of its foci. he then ascertained the dimensions of the orbit; and, by a comparison of the times employed by the planet to complete a whole revolution or any part of one, he discovered that the time in which mars describes any arches of his elliptic orbit, were always to one another as the areas contained by lines drawn from the focus or the centre of the sun to the extremities of the respective arches; or, in other words, that the radius vector, or the line joining the sun and mars described equal areas in equal times. by examining the inequalities of the other planets he found that they all moved in elliptic orbits, and that the radius vector of each described areas proportional to the times. these two great results are known by the name of the first and second laws of kepler. the third law, or that which relates to the connexion between the periodic times and the distances of the planets, was not discovered till a later period of his life. [ ] an interesting account of the steps by which kepler proceeded will be found in mr drinkwater bethune's admirable life of kepler, in the library of useful knowledge. when kepler presented to rudolph the volume which contained these fine discoveries, he reminded him jocularly of his requiring the sinews of war to make similar attacks upon the other planets. the emperor, however, had more formidable enemies than jupiter and saturn, and from the treasury, which war had exhausted, he found it difficult to supply the wants of science. while kepler was thus involved in the miseries of poverty, misfortunes of every kind filled up the cup of his adversity. his wife, who had long been the victim of low spirits, was seized, towards the end of , with fever, epilepsy, and phrenitis, and before she had completely recovered, all his three children were simultaneously attacked with the smallpox. his favourite son fell a victim to this malady, and at the same time prague was partially occupied by the troops of leopold. the part of the city where kepler resided was harassed by the bohemian levies, and, to crown this list of evils, the austrian troops introduced the plague into the city. sometime afterwards kepler set out for austria with the view of obtaining the professorship of mathematics at linz, which was now vacant; but, upon his return in june, he found his wife in a decline, brought on by grief for the loss of her son, and she was sometime afterwards seized with an infectious fever, of which she died. the emperor rudolph was unwilling to allow kepler to quit prague. he encouraged him with hopes that the arrears of his salary would be paid from saxony; but these hopes were fallacious, and it was not till the death of rudolph, in , that kepler was freed from these distressing embarrassments. on the accession of mathias, rudolph's brother, kepler was re-appointed imperial mathematician, and was allowed to accept the professorship at linz. his family now consisted of two children--a daughter, susannah, born in , and a son, louis, born in . his own time was so completely occupied by his new professorial duties, as well as by his private studies, that he found it necessary to seek another parent for his children. for this purpose, he gave a commission to his friends to look out for him a suitable wife, and, in a long and jocular letter to baron strahlendorf, he has given an amusing account of the different negotiations which preceded his marriage. the substance of this letter is so well given by mr drinkwater bethune, that we shall follow his account of it. the first of the eleven ladies among whom his inclinations wavered, "was a widow, an intimate friend of his first wife; and who, on many accounts, appeared a most eligible match. at first," says kepler, "she seemed favourably inclined to the proposal; it is certain that she took time to consider it, but at last she very quietly excused herself." it must have been from a recollection of this lady's good qualities, that kepler was induced to make his offer; for we learn rather unexpectedly, after being informed of her decision, that when he soon afterwards paid his respects to her, it was the first time that he had seen her during the last six years; and he found, to his great relief, that "there was no single pleasing part about her." the truth seems to be, that he was nettled by her answer, and he is at greater pains than appears necessary, considering this last discovery, to determine why she would not accept his offered hand. among other reasons, he suggested her children, among whom were two marriageable daughters; and it is diverting afterwards to find them also in the catalogue, which kepler appeared to be making, of all his female acquaintance.... of the other ladies, one was too old, another in bad health, another too proud of her birth and quarterings, a fourth had learned nothing but shewy accomplishments, "not at all suitable to the sort of life she would have to lead with me," another grew impatient, and married a more decided admirer, whilst he was hesitating. "the mischief," says he, "in all these attachments was, that whilst i was delaying, comparing and balancing conflicting reasons, every day saw me inflamed with a new passion." by the time he reached the th, he found his match in this respect. "fortune at length has avenged herself on my doubtful inclinations. at first she was quite complying, and her friends also; presently, whether she did or did not consent, not only i, but she herself did not know. after the lapse of a few days came a renewed promise, which, however, had to be confirmed a third time; and four days after that, she again repeated her confirmation, and begged to be excused from it. upon this i gave her up, and this time all my counsellors were of one opinion." this was the longest courtship in the list, having lasted three whole months; and, quite disheartened by its bad success, kepler's next attempt was of a more timid complexion. his advances to no. were made by confiding to her the whole story of his recent disappointment, prudently determining to be guided in his behaviour, by observing whether the treatment he had experienced met with a proper degree of sympathy. apparently the experiment did not succeed; and, almost reduced to despair, kepler betook himself to the advice of a friend, who had for some time past complained that she was not consulted in this difficult negotiation. when she produced no. , and the first visit was paid, the report upon her was as follows:--"she has, undoubtedly, a good fortune, is of good family, and of economical habits: but her physiognomy is most horribly ugly; she would be stared at in the streets, not to mention the striking disproportion in our figures. i am lank, lean, and spare; she short and thick: in a family notorious for fulness, she is considered superfluously fat." the only objection to no. seems to have been her excessive youth; and when this treaty was broken off on that account, kepler turned his back upon all his advisers, and chose for himself one who had figured as no. in the list, to whom he professes to have felt attached throughout, but from whom the representations of his friends had hitherto detained him, probably on account of her humble station. the following is kepler's summary of her character:--"her name is susannah, the daughter of john reuthinger and barbara, citizens of the town of eferdingen. the father was by trade a cabinetmaker, but both her parents are dead. she has received an education well worth the largest dowry, by favour of the lady of stahrenberg, the strictness of whose household is famous throughout the province. her person and manners are suitable to mine--no pride, no extravagance. she can bear to work; she has a tolerable knowledge how to manage a family; middle-aged, and of a disposition and capability to acquire what she still wants. her i shall marry, by favour of the noble baron of stahrenberg, at o'clock on the th of next october, with all eferdingen assembled to meet us, and we shall eat the marriage dinner at maurice's at the golden lion."[ ] [ ] life of kepler, chap. vi. kepler's marriage seems to have taken place at the time here mentioned; for, in his book on gauging, published at linz in , he informs us that he took home his new wife in november, on which occasion he found it necessary to stock his cellar with a few casks of wine. when the wine-merchant came to measure the casks, kepler objected to his method, as he made no allowance for the different sizes of the bulging parts of the cask. from this accident, kepler was led to study the subject of gauging, and to write the book which we have mentioned, and which contains the earliest specimens of the modern analysis. about this period, kepler was summoned to the diet at ratisbon, to give his opinion on the reformation of the kalendar, and he published a short essay on the subject; but though the government did not scruple to avail themselves of his services, yet his pension was allowed to fall in arrear, and, in order to support his family, he was obliged to publish an almanac, suited to the taste of the age. "in order," says he, "to defray the expense of the ephemeris for two years,[ ] i have been obliged to compose _a vile prophesying almanac, which is scarcely more respectable than begging_, unless from its saving the emperor's credit, who abandons me entirely, and would suffer me to perish with hunger." [ ] these ephemerides, from to , were published at linz in . the one for was dedicated to baron napier of merchiston. although kepler's residence at linz was rendered uncomfortable by the roman catholics, who had excommunicated him on account of his refusing to subscribe to some opinions respecting the ubiquity of our saviour, or, as others maintain, on account of some opinions which he had expressed respecting transubstantiation, yet he refused, in , to accept of an invitation to fill the mathematical chair at bologna. the prospect of his fortune being bettered by such a change could not reconcile him to live in a country where his freedom of speech and manners might expose him to suspicion; and he accordingly declined, in the most respectful manner, the offer which was made him. chapter iii. _kepler's continued embarrassments--death of mathias--liberality of ferdinand--kepler's "harmonies of the world"--the epitome of the copernican astronomy--it is prohibited by the inquisition--sir henry wotton, the british ambassador, invites kepler to england--he declines the invitation--neglect of genius by the english government--trial of kepler's mother--her final acquittal--and death at the age of seventy-five--the states of styria burn publicly kepler's calendar--he receives his arrears of salary from ferdinand--the rudolphine tables published in --he receives a gold chain from the grand dulce of tuscany--he is patronised by the duke of friedland--he removes to sagan, in silesia--is appointed professor of mathematics at rostoch--goes to ratisbon to receive his arrears--his death, funeral, and epitaph--monument erected to his memory in --his family--his posthumous volume, entitled "the dream, or lunar astronomy."_ kepler was kept in a state of constant anxiety from the delay in the government to pay up the arrears of his pension, while their repeated promises prevented him from accepting of other employments. he had hoped that the affair of the bolognese chair would rouse the imperial treasury to a sense of its duty, and enable him to publish the rudolphine tables,--that great work which he owed to the memory both of tycho and of rudolph. but though he was disappointed in this expectation, an event now occurred which at least held out the prospect of a favourable change in his circumstances. the emperor mathias died in , and was succeeded by ferdinand iii., who not only continued him in the situation of his principal mathematician, with his former pension, but promised to pay up the arrears of it, and to furnish the means for publishing the rudolphine tables. the year , so favourable to kepler's prospects in life, was distinguished also by the publication, at linz, of one of his most remarkable productions, entitled "the harmonies of the world." it is dedicated to james i. of england, and will be for ever memorable in the history of science, as containing the celebrated law that the squares of the periodic times of the planets are to one another as the cubes of their distances. this singular volume, which is marked with all the peculiarities which distinguish his cosmographical mystery, is divided into five books. the two first books are principally geometrical, and relate to regular polygons inscribed in a circle; the third book is a treatise on music, in which musical proportions are derived from figures; the fourth book is astrological, and treats of the harmony of rays emanating on the earth from the heavenly bodies, and on their influence over the sublunary or human soul; the fifth book is astronomical and metaphysical, and treats of the exquisite harmonies of the celestial motions, and of the celebrated third law of the universe, which we have already referred to. this law, as he himself informs us, first entered his mind on the th march ; but, having made an erroneous calculation, he was obliged to reject it. he resumed the subject on the th may; and having discovered his former error, he recognised with transport the absolute truth of a principle which for seventeen years had been the object of his incessant labours. the delight which this grand discovery gave him had no bounds. "nothing holds me," says he; "i will indulge in my sacred fury; i will triumph over mankind by the honest confession, that i have stolen the golden vases of the egyptians, to build up a tabernacle for my god, far away from the confines of egypt. if you forgive me, i rejoice; if you are angry, i can bear it. the die is cast; the book is written, to be read either now or by posterity, i care not which. it may well wait a century for a reader, as god has waited six thousand years for an observer." about the same time, in , kepler published, at linz, the _three_ first books of his "epitome of the copernican astronomy," of which the _fourth_ was published at the same place in , and the _fifth_, _sixth_, and _seventh_ at frankfort in the same year. this interesting work is a kind of summary of all his astronomical views, drawn up in the form of a dialogue for the perusal of general readers. immediately after its publication, it was placed by the inquisition in the list of prohibited books; and the moment kepler learned this from his correspondent remus, he was thrown into great alarm, and requested from him some information respecting the terms and consequences of the censure which was then pronounced against him. he was afraid that it might compromise his personal safety if he went to italy; that he would be compelled to retract his opinions; that the censure might extend to austria; that the sale of his work would be ruined; and that he must either abandon his country or his opinions. the reply of his friend remus calmed his agitated mind, by explaining to him the true nature of the prohibition; and he concluded his letter with a piece of seasonable exhortation, "there is no ground for your alarm either in italy or in austria, only keep yourself within bounds, and put a guard upon your own passions." in the year , sir henry wotton, the english ambassador at venice, paid a visit to kepler on his way through germany. it does not appear whether or not this visit was paid at the desire of james i., to whom kepler had dedicated one of his works, but from the nature of the communication which was made to him by the ambassador, there are strong reasons to think that this was the case. sir henry wotton urged kepler to take up his residence in england, where he could assure him of a welcome and an honourable reception; but, notwithstanding the pecuniary difficulties in which he was then involved, he did not accept of the invitation. in referring to this offer in one of his letters, written a year after it was made, he thus balances the difficulties of the question--"the fires of civil war," says he, "are raging in germany. shall i then cross the sea whither wotton invites me? i, a german, a lover of firm land, who dread the confinement of an island, who presage its dangers, and must drag along with me my little wife and flock of children?" as kepler seems to have entertained no doubt of his being well provided for in england, it is the more probable that the british sovereign had made him a distinct offer through his ambassador. a welcome and an honourable reception, in the ordinary sense of these terms, could not have supplied the wants of a starving astronomer, who was called upon to renounce a large though an ill-paid salary in his native land; and kepler had experienced too deeply the faithlessness of royal pledges to trust his fortune to so vague an assurance as that which is implied in the language of the english ambassador. during the two centuries which have elapsed since this invitation was given to kepler, there has been no reign during which the most illustrious foreigner could hope for pecuniary support, either from the sovereign or the government of england. what english science has never been able to command for her indigenous talent, was not likely to be proffered to foreign merit. the generous hearts of individual englishmen, indeed, are always open to the claims of intellectual pre-eminence, and ever ready to welcome the stranger whom it adorns; but through the frozen life-blood of a british minister such sympathies have seldom vibrated; and, amid the struggles of faction and the anxieties of personal and family ambition, he has turned a deaf ear to the demands of genius, whether she appeared in the humble posture of a suppliant, or in the prouder attitude of a national benefactor. if the imperial mathematician, therefore, had no other assurance of a comfortable home in england than that of sir henry wotton, he acted a wise part in distrusting it; and we rejoice that the sacred name of kepler was thus withheld from the long list of distinguished characters whom england has starved and dishonoured. in the year , kepler was exposed to a severe calamity, which continued to harass him for some time. his mother, catherine kepler, to whose peculiarities of temper we have already referred, was arrested on the th april, upon a charge of a very serious nature. one of her friends having some years before suffered a miscarriage, was subsequently attacked with violent headaches, and catherine was charged with having administered poison to her friend. this accusation was indignantly repelled, and a young doctor of the law, whom she consulted, advised her to raise an action against her calumniator. from professional reasons, or probably pecuniary ones, this zealous practitioner continued to delay the lawsuit for five years. the judge who tried it happened to be displaced, and was succeeded by another, who had a personal quarrel with the prosecutor. the defender, who was aware of this favourable change in her case, became the accuser, and, in july , catherine kepler was sent to prison, and condemned to the torture. the moment this event reached the ears of her son, he quitted linz, and arrived in time to save her from punishment. he found that the evidence upon which she was condemned had no other foundation but her own intemperate conduct; and, though his interference was successful, yet she was not finally released from prison till the th november . convinced of her innocence, this bold woman, now in the th year of her age, raised a new action for damages against her opponent; but her death, in april , put an end to her own miseries, as well as to the anxiety of her son. among the virtues of this singular woman, we must number that of generosity. moestlin, the old preceptor of kepler, had generously declined any compensation for his instructions. kepler never forgot this act of kindness, and, in the midst of his poverty, he found means to send to moestlin a handsome silver cup in token of his gratitude. in acknowledging this gift, moestlin remarks, "your mother had taken it into her head that you owed me florins, and had brought florins and a chandelier towards reducing the debt, which i advised her to send to you. i asked her to stay to dinner, which she refused. however, we hanselled your cup, as you know she is of a thirsty temperament." in the same year in which his mother was arrested, the states of styria ordered all the copies of the kalendar for to be publicly burnt. there does not seem to be any reason for supposing that this insult proceeded from his old enemies the catholics. they would, no doubt, take an active share in carrying it into effect; but it would appear that his former patrons were affronted at kepler's giving the precedence in his title page to the states of upper ens, where he then resided, above the states of styria. in , the emperor ferdinand, notwithstanding his own pecuniary difficulties, ordered the whole of kepler's arrears to be paid, even those which had been due by rudolph and mathias; and so great was his anxiety to have the rudolphine tables published, that he supplied the means for their immediate completion. new difficulties, however, sprung up to retard still longer the appearance of this most important work. the wars of the reformation, which were then agitating the whole of germany, interfered with every peaceful pursuit. the library of kepler was sealed up by order of the jesuits, and it was only his position as imperial mathematician that saved him from personal inconvenience. a popular insurrection followed in the train of these disasters. the peasantry blockaded linz, the place of kepler's residence, and it was not till the year , as the title page bears, or , as kepler elsewhere states, that these celebrated tables were given to the world. the rudolphine tables were published at ulm in one volume folio. these tables were calculated by kepler from the observations of tycho, and are founded on his own great discovery of the ellipticity of the planetary orbits. the _first_ and _third_ parts of the work contain logarithmic and other auxiliary tables, for the purpose of facilitating astronomical calculations. the _second_ part contains tables of the sun, moon, and planets; and the _fourth_ a catalogue of stars, as determined by tycho. a nautical map is prefixed to some copies of the tables, and the description of it contains the first notice of the method of determining the longitude by means of occultations. a short time after the publication of these tables, the grand duke of tuscany, instigated no doubt by galileo, sent kepler a gold chain in testimony of his approbation of the great service which he had rendered to astronomy. about this time albert wallenstein, duke of friedland, a great patron of astrology, and one of the most distinguished men of the age, made the most munificent offers to kepler, and invited him to take up his residence at sagan in silesia. the religious dissensions which agitated linz, the love of tranquillity which kepler had so little enjoyed, and the publication of his great work, induced him to accept of this offer. he accordingly removed his family from linz to ratisbon in , and he himself set out for prague, with the double object of presenting the rudolphine tables to the emperor, and of soliciting his permission to go into the service of the duke of friedland. the emperor did not hesitate to grant this request; and would have gladly transferred kepler's arrears as well as himself to the charge of a foreign prince. kepler accordingly set out with his wife and family for sagan, where he arrived in . the duke albert treated him with liberality and distinction. he supplied him with an assistant for his calculations, and also with a printing press; and, by his influence with the duke of mecklenburg, he obtained for him a professorship in the university of rostoch. in this remote situation, kepler found it extremely difficult to obtain payment of the imperial pension which he still retained. the arrears had accumulated to crowns, and he resolved to go to the imperial assembly at ratisbon to make a final effort to obtain them. his attempts, however, were fruitless. the vexation which this occasioned, and the great fatigue which he had undergone, threw him into a violent fever, which is said to have been one of cold, and to have been accompanied with an imposthume in his brain, occasioned by too much study. this disease baffled the skill of his physicians, and carried him off on the th november, o.s. , in the sixtieth year of his age. the remains of this great man were interred in st peter's churchyard at ratisbon, and the following inscription, embodying an epitaph which he had written for himself, was engraven on his tombstone. in hoc quiescit vir nobilissimus, doctissimus et celeberrimus dom. johannes keplerus, trium imperatorum rudolphi ii., mathiÃ�, et ferdinandi ii., per annos xxx, antea vero procerum styriÃ� ab anno usque , postea quoque astriacorum ordinum ab anno usque ad annum , mathematicus toti orbi christiani, per monumenta publica cognitus, ab omnibus doctis, inter principes astronomiÃ� numeratus, qui propria manu assignatum post se reliquit tale epitaphium. mensus eram coelos, nunc terræ metior umbras: mens coelestis erat, corporis umbra jacet. in christo pie obiit anno salutis , die novembris, Ã�tatis suÃ� sexagesimo. this monument was not long preserved. it was destroyed during the wars which desolated germany; and no attempt was made till to mark with honour the spot which contained such venerable remains. this attempt, however, failed, and it was not till that this great duty was paid to the memory of kepler, by the prince bishop of constance, who erected a handsome monumental temple near the place of his interment, and in the botanical garden of the city. the temple is surmounted by a sphere, and in the centre is a bust of kepler in carrara marble. kepler left behind him a wife and seven children--two by his first wife, susanna and louis; and three sons and two daughters by his second wife, viz.--sebald, cordelia, friedman, hildebert, and anna maria. the eldest of these, susanna, was married a few months before her father's death to jacob bartschius, his pupil, who was educated as a physician; and his son louis died in , while practising medicine at konigsberg. the children by his second wife are said to have died young. they were left in very narrow circumstances; and though , florins were due to kepler by the emperor, yet only a part of this sum was received by susanna, in consequence of her refusing to give up tycho's observations till the debt was paid. kepler composed a little work entitled "the dream of john kepler, or lunar astronomy," the object of which was to describe the phenomena seen from the moon; but he died while he and bartschius were engaged in its publication, and bartschius having resumed the task, died also before its completion. louis kepler dreaded to meddle with a work which had proved so fatal to his father and his brother-in-law, but this superstitious feeling was overcome, and the work was published at frankfort in . chapter iv. _number of kepler's published works--his numerous manuscripts in folio volumes--purchased by hevelius, and afterwards by hansch--who publishes kepler's life and correspondence at the expense of charles vi.--the history of the rest of his manuscripts, which are deposited in the library of the academy of sciences at st petersburg--general character of kepler--his candour in acknowledging his errors--his moral and religious character--his astrological writings and opinions considered--his character as an astronomer and a philosopher--the splendour of his discoveries--account of his methods of investigating truth._ although the labours of kepler were frequently interrupted by severe and long-continued indisposition, as well as by the pecuniary embarrassments in which he was constantly involved, yet the ardour and power of his mind enabled him to surmount all the difficulties of his position. not only did he bring to a successful completion the leading inquiries which he had begun, but he found leisure for composing an immense number of works more or less connected with the subject of his studies. between , when he published his kalendar at gratz, and , the year of his death, he published no fewer than _thirty-three_ separate works; and he left behind him _twenty-two_ volumes of manuscripts, _seven_ of which contain his epistolary correspondence. the celebrated astronomer hevelius, who was a cotemporary of louis kepler, purchased all these manuscripts from kepler's representatives. at the death of hevelius they were bought by m. gottlieb hansch, a zealous mathematician, who was desirous of giving them to the world. for this purpose he issued a prospectus in for publishing them by subscription, in volumes folio; but this plan having failed, he was introduced to charles vi., who liberally obtained for him ducats to defray the expense of the publication, and an annual pension of florins. with such encouragement, hansch published in , in one volume folio, the correspondence of kepler, entitled "_epistolæ ad joannem keplerum, insertis ad easdem responsionibus keplerianis, quidquid hactenus reperiri potuerunt, opus novum, et cum jo. kepleri vita._" the expenses of this volume unfortunately exhausted the ducats which had been granted by the emperor, and, instead of being able to publish the rest of the mss., hansch was under the necessity of pledging them for florins. under these difficulties he addressed himself in vain to the celebrated wolfius, to the royal society of london, and to other bodies that were likely to interest themselves in such a subject. in , when m. de murr of nuremberg was in london, he made great exertions to obtain the mss., and dr bradley is said to have been on the eve of purchasing them. the competition probably raised the demands of the proprietor, in whose hands they continued for many years. in they were offered for francs, and sometime afterwards m. de murr purchased them for the imperial academy of sciences at st petersburg, in whose library they still remain. euler, lexell, and kraft undertook the task of examining them, and selecting those that were best fitted for publication, but we believe that no steps have yet been taken for executing this task, nor are we aware that science would derive any advantage from its completion. although, in drawing his own character, kepler describes himself as "troublesome and choleric in politics and domestic matters," yet the general events of his life indicate a more peaceful disposition than might have been expected from the peculiarities of his mind and the ardour of his temperament. on one occasion, indeed, he wrote a violent and reproachful letter to tycho, who had given him no just ground of offence; but the state of kepler's health at that moment, and the necessitous circumstances in which he had been placed, present some palliation of his conduct. but, independent of this apology, his subsequent conduct was so truly noble as to reconcile even tycho to his penitent friend. kepler quickly saw the error which he committed; he lamented it with genuine contrition, and was anxious to remove any unfavourable impression which he might have given of his friend, by the most public confession of his error, and by the warmest acknowledgments of the kindness of tycho. in his relations with the scientific men of his own times, kepler conducted himself with that candour and love of truth which should always distinguish the philosopher. he was never actuated by any mean jealousy of his rivals. he never scrupled to acknowledge their high merits; and when the discoveries made by the telescope established beyond a doubt the errors of some of kepler's views, he willingly avowed his mistake, and never joined in the opposition which was made by many of his friends to the discoveries of galileo. a striking example of this was exhibited in reference to his supposed discovery of mercury on the sun's disc. in the year ,[ ] kepler observed upon the face of the sun a dark spot, which he mistook for mercury; but the day proving cloudy, he had not the means of determining by subsequent observations whether or not this opinion was well founded. as spots on the sun were at that time unknown, kepler did not hesitate to publish the fact in , in his _mercurius in sole visus_; but when galileo, a few years afterwards, discovered a great number of similar spots with the telescope, kepler retracted his opinions, and acknowledged that galileo's discovery afforded an explanation, also, of many similar observations in old writers, which he had found it difficult to reconcile with the actual motions of mercury. [ ] it is said that kepler saw this dark spot _while looking at the sun in a camera obscura_. as a camera obscura is actually a telescope, magnifying objects in proportion to the focal length of the lens employed, he may be said to have first seen these spots with the aid of an optical instrument. kepler was not one of those cold-hearted men who, though continually occupied in the study of the material world, and ambitious of the distinction which a successful examination of it confers, are yet insensible to the goodness and greatness of the being who made and sustains it. his mind was cast in a better mould. the magnificence and harmony of the divine works excited in him not only admiration but love. he felt his own humility the farther he was allowed to penetrate into the mysteries of the universe; and sensible of the incompetency of his unaided powers for such transcendent researches, and recognising himself as but the instrument which the almighty employed to make known his wonders, he never entered upon his inquiries without praying for assistance from above. this frame of mind was by no means inconsistent with that high spirit of delight and triumph with which kepler surveyed his discoveries. his was the unpretending ovation of success, not the ostentatious triumph of ambition; and if a noble pride did occasionally mingle itself with his feelings, it was the pride of being the chosen messenger of physical truth, not that of being the favoured possessor of superior genius. with such a frame of mind, kepler was necessarily a christian. the afflictions with which he was beset confirmed his faith and brightened his hopes: he bore them in all their variety and severity with christian patience; and though he knew that this world was to be the theatre of his intellectual glory, yet he felt that his rest and his reward could be found only in another. it is difficult to form any very intelligible idea of the nature and extent of kepler's astrological opinions, and of the degree of credit which he himself placed in the opinions that he did avow. in his principles of astrology, published in , and in other works, he rails against the vanity and worthlessness of the ordinary astrology. he regards those who professed it as knaves and charlatans; and maintains that the planets and stars exercise no influence whatever over human affairs. he conceives, however, that certain harmonious configurations of suitable planets, like the spur to a horse, or a speech to an audience, have the power of exciting the minds of men to certain general actions or impulses; so that the only effect of these configurations is to operate along with the vital soul in producing results which would not otherwise have taken place. as an example of this, he states that those who are born when many aspects of the planets occur, _generally_ turn out busy and industrious, whether they be occupied in amassing wealth, managing public affairs, or prosecuting scientific studies. kepler himself was born under a triple configuration, and hence, in his opinion, his ardour and activity in study; and he informs us that he knew a lady born under nearly the same configurations, "who not only makes no progress in literature, but troubles her whole family and occasions deplorable misery to herself." this excitement of the faculties of sublunary natures, as he expresses it, by the colours and aspects and conjunctions of the planets, is regarded by kepler as a fact, which he had deduced from observation, and which has "compelled his unwilling belief." "i have been driven to this," says he, "not by studying or admiring plato, but singly and solely by observing seasons, and noting the aspects by which they are produced. i have seen the state of the atmosphere almost uniformly disturbed as often as the planets are in conjunction, or in the other configurations so celebrated among astrologers. i have noticed its tranquil state either when there are none or few such aspects, or when they are transitory and of short duration." had kepler been able to examine these hasty and erroneous deductions by long continued observation, he would soon have found that the coincidence which he did observe was merely accidental, and he would have cheerfully acknowledged it. speculations of this kind, however, are, from their very nature, less subject to a rigorous scrutiny; and a long series of observations is necessary either to establish or to overturn them. the industry of modern observers has now supplied this defect, and there is no point in science more certain than that the sun, moon, and planets do not exercise any influence on the general state of our atmosphere. the philosophers in kepler's day, who had studied the phenomena of the tides, without having any idea of their cause, and who observed that they were clearly related to the daily motions of the two great luminaries, may be excused for the extravagance of their belief in supposing that the planets exercised other influences over "sublunary nature." although kepler, in his commentaries on mars, had considered it probable that the waters of our ocean are attracted by the moon, as iron is by a loadstone, yet this opinion seems to have been a very transient one, as he long afterwards, in his system of harmonies, stated his firm belief that the earth is an enormous living animal, and enumerates even the analogies between its habits and those of known animated beings. he considered the tides as waves produced by the spouting out of water through its gills, and he explains their relation to the solar and lunar motions by supposing that the terrene monster has, like other animals, its daily and nightly alternations of sleeping and waking. from the consideration of kepler's astrological opinions, it is an agreeable transition to proceed to the examination of his high merits as an astronomer and a philosopher. as an experimental philosopher, or as an astronomical observer, kepler does not lay claim to our admiration. he himself acknowledges, "that for observations his sight was dull, and for mechanical operations his hand was awkward." he suffered much from weak eyes, and the delicacy of his constitution did not permit him to expose himself to the night air. notwithstanding these hindrances, however, he added several observations to those of tycho, which he made with two instruments that were presented to him by his friend hoffman, the president of the states of styria. these instruments were an iron sextant, ½ feet in diameter, and a brass azimuthal quadrant ½ feet in diameter, both of which were divided into single minutes of a degree. they were very seldom used, and we must regard the circumstances which disqualified kepler for an observer, as highly favourable to the developement of those great powers which he directed with undivided energy to physical astronomy. even if kepler had never turned his attention to the heavens, his optical labours would have given him a high rank among the original inquirers of his age; but when we consider him also as the discoverer of the three great laws which bear his name, we must assign him a rank next to that of newton. the history of science does not present us with any discoveries more truly original, or which required for their establishment a more powerful and vigorous mind. the speculations of his predecessors afforded him no assistance. from the cumbrous machinery adopted by copernicus, kepler passed, at one step, to an elliptical orbit, with the sun in one of its foci, and from that moment astronomy became a demonstrative science. the splendid discoveries of newton sprung immediately from those of kepler, and completed the great chain of truths which constitute the laws of the planetary system. the eccentricity and boldness of kepler's powers form a striking contrast with the calm intellect and the enduring patience of newton. the bright spark which the genius of the one elicited, was fostered by the sagacity of the other into a steady and a permanent flame. kepler has fortunately left behind him a full account of the methods by which he arrived at his great discoveries. what other philosophers have studiously concealed, kepler has openly avowed, and minutely detailed; and we have no hesitation in considering these details as the most valuable present that has ever been given to science, and as deserving the careful study of all who seek to emulate his immortal achievements. it has been asserted that newton made his discoveries by following a different method; but this is a mere assumption, as newton has never favoured the world with any account of the erroneous speculations and the frequent failures which must have preceded his ultimate success. had kepler done the same, by recording only the final steps of his inquiries, his method of investigation would have obtained the highest celebrity, and would have been held up to future ages as a pattern for their imitation. but such was the candour of his mind, and such his inordinate love of truth, that he not only recorded his wildest fancies, but emblazoned even his greatest errors. if newton had indulged us with the same insight into his physical inquiries, we should have witnessed the same processes which were employed by kepler, modified only by the different characters and intensities of their imaginative powers. when kepler directed his mind to the discovery of a general principle, he set distinctly before him, and never once lost sight of, the explicit object of his search. his imagination, now unreined, indulged itself in the creation and invention of various hypotheses. the most plausible, or perhaps the most fascinating, of these was then submitted to a rigorous scrutiny; and the moment it was found to be incompatible with the results of observation and experiment, it was willingly abandoned, and another hypothesis submitted to the same severe ordeal. by thus gradually excluding erroneous views and assumptions, kepler not only made a decided approximation to the object of his pursuit, but in the trials to which his opinions were submitted, and in the observations or experiments which they called forth, he discovered new facts and arrived at new views which directed his subsequent inquiries. by pursuing this method, he succeeded in his most difficult researches, and discovered those beautiful and profound laws which have been the admiration of succeeding ages. in tracing the route which he followed, it is easy for those who live under the light of modern science to say that his fancies were often wild, and his labour often wasted; but, in judging of kepler's methods, we ought to place ourselves in his times, and invest ourselves with the opinions and the knowledge of his contemporaries. in the infancy of a science there is no speculation so absurd as not to merit examination. the most remote and fanciful explanations of facts have often been found the true ones; and opinions which have in one century been objects of ridicule, have in the next been admitted among the elements of our knowledge. the physical world teems with wonders, and the various forms of matter exhibit to us properties and relations far more extraordinary than the wildest fancy could have conceived. human reason stands appalled before this magnificent display of creative power, and they who have drunk deepest of its wisdom will be the least disposed to limit the excursions of physical speculation. the influence of the imagination as an instrument of research, has, we think, been much overlooked by those who have ventured to give laws to philosophy. this faculty is of the greatest value in physical inquiries. if we use it as a guide, and confide in its indications, it will infallibly deceive us; but if we employ it as an auxiliary, it will afford us the most invaluable aid. its operation is like that of the light troops which are sent out to ascertain the strength and position of an enemy. when the struggle commences, their services terminate; and it is by the solid phalanx of the judgment that the battle must be fought and won. g. s. tullis, printer, cupan. +--------------------------------------------------------------+ | transcriber's notes and errata | | | | the following typographical errors have been corrected: | | | | |error |correction | | | | | | | | |betwen |between | | | |his his |his | | | |secretry |secretary | | | |there sidence |the residence | | | |guaging |gauging | | | | +--------------------------------------------------------------+ _is mars habitable?_ a critical examination of professor percival lowell's book "mars and its canals," with an alternative explanation by alfred russel wallace f.r.s., etc. preface. this small volume was commenced as a review article on professor percival lowell's book, _mars and its canals_, with the object of showing that the large amount of new and interesting facts contained in this work did not invalidate the conclusion i had reached in , and stated in my book on _man's place in the universe_, that mars was not habitable. but the more complete presentation of the opposite view in the volume now under discussion required a more detailed examination of the various physical problems involved, and as the subject is one of great, popular, as well as scientific interest, i determined to undertake the task. this was rendered the more necessary by the fact that in july last professor lowell published in the _philosophical magazine_ an elaborate mathematical article claiming to demonstrate that, notwithstanding its much greater distance from the sun and its excessively thin atmosphere, mars possessed a climate on the average equal to that of the south of england, and in its polar and sub-polar regions even less severe than that of the earth. such a contention of course required to be dealt with, and led me to collect information bearing upon temperature in all its aspects, and so enlarging my criticism that i saw it would be necessary to issue it in book form. two of my mathematical friends have pointed out the chief omission which vitiates professor lowell's mathematical conclusions--that of a failure to recognise the very large conservative and _cumulative_ effect of a dense atmosphere. this very point however i had already myself discussed in chapter vi., and by means of some remarkable researches on the heat of the moon and an investigation of the causes of its very low temperature, i have, i think, demonstrated the incorrectness of mr. lowell's results. in my last chapter, in which i briefly summarise the whole argument, i have further strengthened the case for very severe cold in mars, by adducing the rapid lowering of temperature universally caused by diminution of atmospheric pressure, as manifested in the well-known phenomenon of temperate climates at moderate heights even close to the equator, cold climates at greater heights even on extensive plateaux, culminating in arctic climates and perpetual snow at heights where the air is still far denser than it is on the surface of mars. this argument itself is, in my opinion, conclusive; but it is enforced by two others equally complete, neither of which is adequately met by mr. lowell. the careful examination which i have been led to give to the whole of the phenomena which mars presents, and especially to the discoveries of mr. lowell, has led me to what i hope will be considered a satisfactory physical explanation of them. this explanation, which occupies the whole of my seventh chapter, is founded upon a special mode of origin for mars, derived from the meteoritic hypothesis, now very widely adopted by astronomers and physicists. then, by a comparison with certain well-known and widely spread geological phenomena, i show how the great features of mars--the 'canals' and 'oases'--may have been caused. this chapter will perhaps be the most interesting to the general reader, as furnishing a quite natural explanation of features of the planet which have been termed 'non-natural' by mr. lowell. incidentally, also, i have been led to an explanation of the highly volcanic nature of the moon's surface. this seems to me absolutely to require some such origin as sir george darwin has given it, and thus furnishes corroborative proof of the accuracy of the hypothesis that our moon has had an unique origin among the known satellites, in having been thrown off from the earth itself. i am indebted to professor j. h. poynting, of the university of birmingham, for valuable suggestions on some of the more difficult points of mathematical physics here discussed, and also for the critical note (at the end of chapter v.) on professor lowell's estimate of the temperature of mars. broadstone, dorset, _october_ . table of contents. chapter i. early observers of mars, --mars the only planet the surface of which is distinctly visible --early observation of the snow-caps and seas --the 'canals' seen by schiaparelli in --double canals first seen in --round spots at intersection of canals seen by pickering in --confirmed by lowell in --changes of colour seen in and --existence of seas doubted by pickering and barnard in . chapter ii. mr. lowell's discoveries and theories, --observatory at flagstaff, arizona --illustrated book on his observations of mars --volume on mars and its canals, --non-natural features --the canals as irrigation works of an intelligent race --a challenge to the thinking world --the canals as described and mapped by mr. lowell --the double canals --dimensions of the canals --they cross the supposed seas --circular black spots termed oases --an interesting volume. chapter iii. the climate and physiography of mars, --no permanent water on mars --rarely any clouds and no rain --snow-caps the only source of water --no mountains, hills, or valleys on mars --two-thirds of the surface a desert --water from the snow-caps too scanty to supply the canals --miss clerke's views as to the water-supply --description of some of the chief canals --mr. lowell on the purpose of the canals --remarks on the same --mr. lowell on relation of canals to oases and snow-caps --critical remarks on the same. chapter iv. is animal life possible on mars? --water and air essential for animal life --atmosphere of mars assumed to be like ours --blue tint near melting snow the only evidence of water --fallacy of this argument --dr. johnstone stoney's proof that water-vapour cannot exist on mars --spectroscope gives no evidence of water. chapter v. temperature of mars--mr. lowell's estimate, --problem of terrestrial temperature --ice under recent lava --tropical oceans ice-cold at bottom --earth's surface-heat all from the sun --absolute zero of temperature --complex problem of planetary temperatures --mr. lowell's investigation of the problem --abstract of mr. lowell's paper --critical remarks on mr. lowell's paper. chapter vi. a new estimate of the temperature of mars, --langley's determination of lunar heat --rapid loss of heat by radiation on the earth --rapid loss of heat on moon during eclipse --sir george darwin's theory of the moon's origin --very's researches on the moon's temperature --application of these results to the case of mars --cause of great difference of temperatures of earth and moon --special features of mars influencing its temperature --further criticism of mr. lowell's article --very low temperature of arctic regions on mars. chapter vii. a suggestion as to the 'canals' of mars, --special features of the canals --mr. pickering's suggested explanation --the meteoritic hypotheses of origin of planets --probable mode of origin of mars --structural straight lines on the earth --probable origin of the surface-features of mars --symmetry of basaltic columns --how this applies to mars --suggested explanation of the oases --probable function of the great fissures --suggested origin of blue patches adjacent to snow-caps --the double canals --concluding remarks on the canals. chapter viii. page summary and conclusion, --the canals the origin of mr. lowell's theory --best explained as natural features --evaporation difficulty not met by mr. lowell --how did martians live without the canals --radiation due to scanty atmosphere not taken account of --three independent proofs of low temperature and uninhabitability of mars --conclusion. chapter i. early observers of mars. few persons except astronomers fully realise that of all the planets of the solar system the only one whose solid surface has been seen with certainty is mars; and, very fortunately, that is also the only one which is sufficiently near to us for the physical features of the surface to be determined with any accuracy, even if we could see it in the other planets. of venus we probably see only the upper surface of its cloudy atmosphere.[ ] as regards jupiter and saturn this is still more certain, since their low density will only permit of a comparatively small proportion of their huge bulk being solid. their belts are but the cloud-strata of their upper atmosphere, perhaps thousands of miles above their solid surfaces, and a somewhat similar condition seems to prevail in the far more remote planets uranus and neptune. it has thus happened, that, although as telescopic objects of interest and beauty, the marvellous rings of saturn, the belts and ever-changing aspects of the satellites of jupiter, and the moon-like phases of venus, together with its extreme brilliancy, still remain unsurpassed, yet the greater amount of details of these features when examined with the powerful instruments of the nineteenth century have neither added much to our knowledge of the planets themselves or led to any sensational theories calculated to attract the popular imagination. [footnote : mercury also seems to have a scanty atmosphere, but as its mass is only one-thirtieth that of the earth it can retain only the heavier gases, and its atmosphere may be dust-laden, as is that of mars, according to mr. lowell. its dusky markings, as seen by schiaparelli, seem to be permanent, and they are also for considerable periods unchangeable in position, indicating that the planet keeps the same face towards the sun as does venus. this was confirmed by mr. lowell in . its distance from us and unfavourable position for observation must prevent us from obtaining any detailed knowledge of its actual surface, though its low reflective power indicates that the surface may be really visible.] but in the case of mars the progress of discovery has had a very different result. the most obvious peculiarity of this planet--its polar snow-caps--were seen about years ago, but they were first proved to increase and decrease alternately, in the summer and winter of each hemisphere, by sir william herschell in the latter part of the eighteenth century. this fact gave the impulse to that idea of similarity in the conditions of mars and the earth, which the recognition of many large dusky patches and streaks as water, and the more ruddy and brighter portions as land, further increased. added to this, a day only about half an hour longer than our own, and a succession of seasons of the same character as ours but of nearly double the length owing to its much longer year, seemed to leave little wanting to render this planet a true earth on a smaller scale. it was therefore very natural to suppose that it must be inhabited, and that we should some day obtain evidence of the fact. _the canals discovered by schiaparelli._ hence the great interest excited when schiaparelli, at the milan observatory, during the very favourable opposition of and , observed that the whole of the tropical and temperate regions from ° n. to ° s. lat. were covered with a remarkable network of broader curved and narrower straight lines of a dark colour. at each successive favourable opposition, these strange objects called _canali_ (channels) by their discoverer, but rather misleadingly 'canals' in england and america, were observed by means of all the great telescopes in the world, and their reality and general features became well established. in schiaparelli's first map they were represented as being much broader and less sharply defined than he himself and other observers found by later and equally favourable observations that they really were. _discovery of the double canals._ in another strange feature was discovered by schiaparelli, who found that about twenty canals which had previously been seen single were now distinctly double, that is, that they consisted of two parallel lines, equally distinct and either very close together or a considerable distance apart. this curious appearance was at first thought to be due to some instrumental defect or optical illusion; but as it was soon confirmed by other observers with the best instruments and in widely different localities it became in time accepted as a real phenomenon of the planet's surface. _round spots discovered in_ . at the favourable opposition of , mr. w. h. pickering noticed that besides the 'seas' of various sizes there were numerous very small black spots apparently quite circular and occurring at every intersection or starting-point of the 'canals.' many of these had been seen by schiaparelli as larger and ill-defined dark patches, and were termed seas or lakes; but mr. pickering's observatory was at arequipa in peru, about feet above the sea, and with such perfect atmospheric conditions as were, in his opinion, equal to a doubling of telescopic aperture. they were soon detected by other observers, especially by mr. lowell in , who thus wrote of them: "scattered over the orange-ochre groundwork of the continental regions of the planet, are any number of dark round spots. how many there may be it is not possible to state, as the better the seeing, the more of them there seem to be. in spite, however, of their great number, there is no instance of one unconnected with a canal. what is more, there is apparently none that does not lie at the junction of several canals. reversely, all the junctions appear to be provided with spots. plotted upon a globe they and their connecting canals make a most curious network over all the orange-ochre equatorial parts of the planet, a mass of lines and knots, the one marking being as omnipresent as the other." _changes of colour recognised._ during the oppositions of and it was fully recognised that a regular course of change occurred dependent upon the succession of the seasons, as had been first suggested by schiaparelli. as the polar snows melt the adjacent seas appear to overflow and spread out as far as the tropics, and are often seen to assume a distinctly green colour. these remarkable changes and the extraordinary phenomena of perfect straight lines crossing each other over a large portion of the planet's surface, with the circular spots at their intersections, had such an appearance of artificiality that the idea that they were really 'canals' made by intelligent beings for purposes of irrigation, was first hinted at, and then adopted as the only intelligible explanation, by mr. lowell and a few other persons. this at once seized upon the public imagination and was spread by the newspapers and magazines over the whole civilised world. _existence of seas doubted._ at this time ( ) it began to be doubted whether there were any seas at all on mars. professor pickering thought they were far more limited in size than had been supposed, and even might not exist as true seas. professor barnard, with the lick thirty-six inch telescope, discerned an astonishing wealth of detail on the surface of mars, so intricate, minute, and abundant, that it baffled all attempts to delineate it; and these peculiarities were seen upon the supposed seas as well as on the land-surfaces. in fact, under the best conditions these 'seas' lost all trace of uniformity, their appearance being that of a mountainous country, broken by ridges, rifts, and canyons, seen from a great elevation. as we shall see later on these doubts soon became certainties, and it is now almost universally admitted that mars possesses no permanent bodies of water. chapter ii. mr. percival lowell's discoveries and theories. _the observatory in arizona._ in , after a careful search for the best atmospheric conditions, mr. lowell established his observatory near the town of flagstaff in arizona, in a very dry and uniform climate, and at an elevation of feet above the sea. he then possessed a fine equatorial telescope of inches aperture and feet focal length, besides two smaller ones, all of the best quality. to these he added in a telescope with inch object glass, the last work of the celebrated firm of alvan clark & sons, with which he has made his later discoveries. he thus became perhaps more favourably situated than any astronomer in the northern hemisphere, and during the last twelve years has made a specialty of the study of mars, besides doing much valuable astronomical work on other planets. _mr, lowell's recent books upon mars._ in mr. lowell published an illustrated volume giving a full account of his observations of mars from to , chiefly for the use of astronomers; and he has now given us a popular volume summarising the whole of his work on the planet, and published both in america and england by the macmillan company. this very interesting volume is fully illustrated with twenty plates, four of them coloured, and more than forty figures in the text, showing the great variety of details from which the larger general maps have been constructed. _non-natural features of mars._ but what renders this work especially interesting to all intelligent readers is, that the author has here, for the first time, fully set forth his views both as to the habitability of mars and as to its being actually inhabited by beings comparable with ourselves in intellect. the larger part of the work is in fact devoted to a detailed description of what he terms the 'non-natural features' of the planet's surface, including especially a full account of the 'canals,' single and double; the 'oases,' as he terms the dark spots at their intersections; and the varying visibility of both, depending partly on the martian seasons; while the five concluding chapters deal with the possibility of animal life and the evidence in favour of it. he also upholds the theory of the canals having been constructed for the purpose of 'husbanding' the scanty water-supply that exists; and throughout the whole of this argument he clearly shows that he considers the evidence to be satisfactory, and that the only intelligible explanation of the whole of the phenomena he so clearly sets forth is, that the inhabitants of mars have carried out on their small and naturally inhospitable planet a vast system of irrigation-works, far greater both in its extent, in its utility, and its effect upon their world as a habitation for civilised beings, than anything we have yet done upon our earth, where our destructive agencies are perhaps more prominent than those of an improving and recuperative character. _a challenge to the thinking world._ this volume is therefore in the nature of a challenge, not so much to astronomers as to the educated world at large, to investigate the evidence for so portentous a conclusion. to do this requires only a general acquaintance with modern science, more especially with mechanics and physics, while the main contention (with which i shall chiefly deal) that the features termed 'canals' are really works of art and necessitate the presence of intelligent organic beings, requires only care and judgment in drawing conclusions from admitted facts. as i have already paid some attention to this problem and have expressed the opinion that mars is not habitable,[ ] judging from the evidence then available, and as few men of science have the leisure required for a careful examination of so speculative a subject, i propose here to point out what the facts, as stated by mr. lowell himself, do _not_ render even probable much less prove. incidentally, i may be able to adduce evidence of a more or less weighty character, which seems to negative the possibility of any high form of animal life on mars, and, _a fortiori_, the development of such life as might culminate in a being equal or superior to ourselves. as most popular works on astronomy for the last ten years at least, as well as many scientific periodicals and popular magazines, have reproduced some of the maps of mars by schiaparelli, lowell, and others, the general appearance of its surface will be familiar to most readers, who will thus be fully able to appreciate mr. lowell's account of his own further discoveries which i may have to quote. one of the _best_ of these maps i am able to give as a frontispiece to this volume, and to this i shall mainly refer. [footnote : _man's place in the universe_ p. ( ).] _the canals as described by mr. lowell._ in the clear atmosphere of arizona, mr. lowell has been able on various favourable occasions to detect a network of straight lines, meeting or crossing each other at various angles, and often extending to a thousand or even over two thousand miles in length. they are seen to cross both the light and the dark regions of the planet's surface, often extending up to or starting from the polar snow-caps. most of these lines are so fine as only to be visible on special occasions of atmospheric clearness and steadiness, which hardly ever occur at lowland stations, even with the best instruments, and almost all are seen to be as perfectly straight as if drawn with a ruler. _the double canals._ under exceptionally favourable conditions, many of the lines that have been already seen single appear double--a pair of equally fine lines exactly parallel throughout their whole length, and appearing, as mr. lowell says, "clear cut upon the disc, its twin lines like the rails of a railway track." both schiaparelli and lowell were at first so surprised at this phenomenon that they thought it must be an optical illusion, and it was only after many observations in different years, and by the application of every conceivable test, that they both became convinced that they witnessed a real feature of the planet's surface. mr. lowell says he has now seen them hundreds of times, and that his first view of one was 'the most startlingly impressive' sight he has ever witnessed. _dimensions of the canals._ a few dimensions of these strange objects must be given in order that readers may appreciate their full strangeness and inexplicability. out of more than four hundred canals seen and recorded by mr. lowell, fifty-one, or about one eighth, are either constantly or occasionally seen to be double, the appearance of duplicity being more or less periodical. of 'canals' generally, mr. lowell states that they vary in length from a few hundred to a few thousand miles long, one of the largest being the phison, which he terms 'a typical double canal,' and which is said to be miles long, while the distance between its two constituents is about miles.[ ] the actual width of each canal is from a minimum of about a mile up to several miles, in one case over twenty. a great feature of the doubles is, that they are strictly parallel throughout their whole course, and that in almost all cases they are so truly straight as to form parts of a great circle of the planet's sphere. a few however follow a gradual but very distinct curve, and such of these as are double present the same strict parallelism as those which are straight. [footnote : this is on the opposite side of mars from that shown in the frontispiece.] _canals extend across the seas._ it was only after seventeen years of observation of the canals that it was found that they extended also into and across the dark spots and surfaces which by the earlier observers were termed seas, and which then formed the only clearly distinguishable and permanent marks on the planet's surface. at the present time, professor lowell states that this "curious triangulation has been traced over almost every portion of the planet's surface, whether dark or light, whether greenish, ochre, or brown in colour." in some parts they are much closer together than in others, "forming a perfect network of lines and spots, so that to identify them all was a matter of extreme difficulty." two such portions are figured at pages and of mr. lowell's volume. _the oases._ the curious circular black spots which are seen at the intersections of many of the canals, and which in some parts of the surface are very numerous, are said to be more difficult of detection than even the lines, being often blurred or rendered completely invisible by slight irregularities in our own atmosphere, while the canals themselves continue visible. about of these have now been found, and the more prominent of them are estimated to vary from to miles in diameter. there are however many much smaller, down to minute and barely visible black points. yet they all seem a little larger than the canals which enter them. where the canals are double, the spots (or 'oases' as mr. lowell terms them) lie between the two parallel canals. no one can read this book without admiration for the extreme perseverance in long continued and successful observation, the results of which are here recorded; and i myself accept unreservedly the substantial accuracy of the whole series. it must however always be remembered that the growth of knowledge of the detailed markings has been very gradual, and that much of it has only been seen under very rare and exceptional conditions. it is therefore quite possible that, if at some future time a further considerable advance in instrumental power should be made, or a still more favourable locality be found, the new discoveries might so modify present appearances as to render a satisfactory explanation of them more easy than it is at present. but though i wish to do the fullest justice to mr. lowell's technical skill and long years of persevering work, which have brought to light the most complex and remarkable appearances that any of the heavenly bodies present to us, i am obliged absolutely to part company with him as regards the startling theory of artificial production which he thinks alone adequate to explain them. so much is this the case, that the very phenomena, which to him seem to demonstrate the intervention of intelligent beings working for the improvement of their own environment, are those which seem to me to bear the unmistakable impress of being due to natural forces, while they are wholly unintelligible as being useful works of art. i refer of course to the great system of what are termed 'canals,' whether single or double. of these i shall give my own interpretation later on. chapter iii. the climate and physiography of mars. mr. lowell admits, and indeed urges strongly, that there are no permanent bodies of water on mars; that the dark spaces and spots, thought by the early observers to be seas, are certainly not so now, though they may have been at an earlier period; that true clouds are rare, even if they exist, the appearances that have been taken for them being either dust-storms or a surface haze; that there is consequently no rain, and that large portions (about two-thirds) of the planet's surface have all the characteristics of desert regions. _snow-caps the only source of water._ this state of things is supposed to be ameliorated by the fact of the polar snows, which in the winter cover the arctic and about half the temperate regions of each hemisphere alternately. the maximum of the northern snow-caps is reached at a period of the martian winter corresponding to the end of february with us. about the end of march the cap begins to shrink in size (in the northern hemisphere), and this goes on so rapidly that early in the june of mars it is reduced to its minimum. about the same time changes of colour take place in the adjacent darker portions of the surface, which become at first bluish, and later a decided blue-green; but by far the larger portion, including almost all the equatorial regions of the planet, remain always of a reddish-ochre tint.[ ] [footnote : in at mount wilson, california, mr. w.h. pickering's photographs of mars on april th showed the southern polar cap of moderate dimensions, but with a large dim adjacent area. twenty-four hours later a corresponding plate showed this same area brilliantly white; the result apparently of a great martian snowfall. in the same observer witnessed the steady disappearance of , , square miles of the southern snow-cap, an area nearly one-third of that hemisphere of the planet.] the rapid and comparatively early disappearance of the white covering is, very reasonably, supposed to prove that it is of small thickness, corresponding perhaps to about a foot or two of snow in north-temperate america and europe, and that by the increasing amount of sun-heat it is converted, partly into liquid and partly into vapour. coincident with this disappearance and as a presumed result of the water (or other liquid) producing inundations, the bluish-green tinge which appears on the previously dark portion of the surface is supposed to be due to a rapid growth of vegetation. but the evidence on this point does not seem to be clear or harmonious, for in the four coloured plates showing the planet's surface at successive martian dates from december th to february st, not only is a considerable extent of the south temperate zone shown to change rapidly from bluish-green to chocolate-brown and then again to bluish-green, but the portions furthest from the supposed fertilising overflow are permanently green, as are also considerable portions in the opposite or northern hemisphere, which one would think would then be completely dried up. _no hills upon mars._ the special point to which i here wish to call attention is this. mr. lowell's main contention is, that the surface of mars is wonderfully smooth and level. not only are there no mountains, but there are no hills or valleys or plateaux. this assumption is absolutely essential to support the other great assumption, that the wonderful network of perfectly straight lines over nearly the whole surface of the planet are irrigation canals. it is not alleged that irregularities or undulations of a few hundreds or even one or two thousands of feet could possibly be detected, while certainly all we know of planetary formation or structure point strongly towards _some_ inequalities of surface. mr. lowell admits that the dark portions of the surface, when examined on the terminator (the margin of the illuminated portion), do _look_ like hollows and _may be_ the beds of dried-up seas; yet the supposed canals run across these old sea-beds in perfect straight lines just as they do across the many thousand miles of what are admitted to be deserts--which he describes in these forcible terms: "pitiless as our deserts are, they are but faint forecasts of the state of things existent on mars at the present time." it appears, then, that mr. lowell has to face this dilemma--_only if the whole surface of mars is an almost perfect level could the enormous network of straight canals, each from hundreds to thousands of miles long, have been possibly constructed by intelligent beings for purposes of irrigation; but, if a complete and universal level surface exists no such system would be necessary._ for on a level surface--or on a surface slightly inclined from the poles towards the equator, which would be advantageous in either case--the melting water would of itself spread over the ground and naturally irrigate as much of the surface as it was possible for it to reach. if the surface were not level, but consisted of slight elevations and expressions to the extent of a few scores or a few hundreds of feet, then there would be no possible advantage in cutting straight troughs through these elevations in various directions with water flowing at the bottom of them. in neither case, and in hardly any conceivable case, could these perfectly straight canals, cutting across each other in every direction and at very varying angles, be of any use, or be the work of an intelligent race, if any such race could possibly have been developed under the adverse conditions which exist in mars. _the scanty water-supply._ but further, if there were any superfluity of water derived from the melting snow beyond what was sufficient to moisten the hollows indicated by the darker portions of the surface, which at the time the water reaches them acquire a green tint (a superfluity under the circumstances highly improbable), that superfluity could be best utilised by widening, however little, the borders to which natural overflow had carried it. any attempt to make that scanty surplus, by means of overflowing canals, travel across the equator into the opposite hemisphere, through such a terrible desert region and exposed to such a cloudless sky as mr. lowell describes, would be the work of a body of madmen rather than of intelligent beings. it may be safely asserted that not one drop of water would escape evaporation or insoak at even a hundred miles from its source. [ ] [footnote : what the evaporation is likely to be in mars may be estimated by the fact, stated by professor j.w. gregory in his recent volume on 'australia' in _stanford's compendium_, that in north-west victoria evaporation is at the rate of ten feet per annum, while in central australia it is very much more. the greatly diminished atmospheric pressure in mars will probably more than balance the loss of sun-heat in producing rapid evaporation.] _miss clerke on the scanty water-supply._ on this point i am supported by no less an authority than the historian of modern astronomy, the late miss agnes clerke. in the _edinburgh review_ (of october ) there is an article entitled 'new views about mars,' exhibiting the writer's characteristic fulness of knowledge and charm of style. speaking of mr. lowell's idea of the 'canals' carrying the surplus water across the equator, far into the opposite hemisphere, for purposes of irrigation there (which we see he again states in the present volume), miss clerke writes: "we can hardly imagine so shrewd a people as the irrigators of thule and hellas[ ] wasting labour, and the life-giving fluid, after so unprofitable a fashion. there is every reason to believe that the martian snow-caps are quite flimsy structures. their material might be called snow _soufflé_, since, owing to the small power of gravity on mars, snow is almost three times lighter there than here. consequently, its own weight can have very little effect in rendering it compact. nor, indeed, is there time for much settling down. the calotte does not form until several months after the winter solstice, and it begins to melt, as a rule, shortly after the vernal equinox. (the interval between these two epochs in the southern hemisphere of mars is days.) the snow lies on the ground, at the outside, a couple of months. at times it melts while it is still fresh fallen. thus, at the opposition of - the spreading of the northern snows was delayed until seven weeks after the equinox: and they had, accordingly, no sooner reached their maximum than they began to decline. and professor pickering's photographs of april th and th, , proved that the southern calotte may assume its definitive proportions in a single night. [footnote : areas on mars so named.] "no attempt has yet been made to estimate the quantity of water derivable from the melting of one of these formations; yet the experiment is worth trying as a help towards defining ideas. let us grant that the average depth of snow in them, of the delicate martian kind, is twenty feet, equivalent at the most to one foot of water. the maximum area covered, of , , square miles, is nearly equal to that of the united states, while the whole globe of mars measures , , square miles, of which one-third, on the present hypothesis, is under cultivation, and in need of water. nearly the whole of the dark areas, as we know, are situated in the southern hemisphere, of which they extend over, at the very least, , , square miles; that is to say, they cover an area, in round numbers, seven times that of the snow-cap. only one-seventh of a foot of water, accordingly, could possibly be made available for their fertilisation, supposing them to get the entire advantage of the spring freshet. upon a stint of less than two inches of water these fertile lands are expected to flourish and bear abundant crops; and since they completely enclose the polar area they are necessarily served first. the great emissaries for carrying off the surplus of their aqueous riches, would then appear to be superfluous constructions, nor is it likely that the share in those riches due to the canals and oases, intricately dividing up the wide, dry, continental plains, can ever be realised. "we have assumed, in our little calculation, that the entire contents of a polar hood turn to water; but in actual fact a considerable proportion of them must pass directly into vapour, omitting the intermediate stage. even with us a large quantity of snow is removed aerially; and in the rare atmosphere of mars this cause of waste must be especially effective. thus the polar reservoirs are despoiled in the act of being opened. further objections might be taken to mr. lowell's irrigation scheme, but enough has been said to show that it is hopelessly unworkable." it will be seen that the writer of this article accepted the existence of water on mars, on the testimony of sir w. huggins, which, in view of later observations, he has himself acknowledged to be valueless. dr. johnstone stoney's proof of its absence, derived from the molecular theory of gases, had not then been made public. _description of some of the canals._ at the end of his volume mr. lowell gives a large chart of mars on mercator's projection, showing the canals and other features seen during the opposition of . this contains many canals not shown on the map here reproduced (see frontispiece), and some of the differences between the two are very puzzling. looking at our map, which shows the north-polar snow below, so that the south pole is out of the view at the top of the map, the central feature is the large spot ascraeeus lucus, from which ten canals diverge centrally, and four from the sides, forming wide double canals, fourteen in all. there is also a canal named ulysses, which here passes far to the right of the spot, but in the large chart enters it centrally. looking at our map we see, going downwards a little to the left, the canal udon, which runs through a dark area quite to the outer margin. in the dark area, however, there is shown on the chart a spot aspledon lucus, where five canals meet, and if this is taken as a terminus the udon canal is almost exactly miles long, and another on its right, lapadon, is the same length, while ich, running in a slightly curved line to a large spot (lucus castorius on the chart) is still longer. the ulysses canal, which (on the chart) runs straight from the point of the mare sirenum to the astraeeus lucus is about miles long. others however are even longer, and mr. lowell says: "with them miles is common; while many exceed ; and the eumenides-orcus is miles from the point where it leaves lucus phoeniceus to where it enters the trivium charontis." this last canal is barely visible on our map, its commencement being indicated by the word eumenides. the trivium charontis is situated just beyond the right-hand margin of our map. it is a triangular dark area, the sides about miles long, and it is shown on the chart as being the centre from which radiate thirteen canals. another centre is aquae calidae situated at the point of a dark area running obliquely from ° to ° n. latitude, and, as shown on a map of the opposite hemisphere to our map, has nearly twenty canals radiating from it in almost every direction. here at all events there seems to be no special connection with the polar snow-caps, and the radiating lines seem to have no intelligent purpose whatever, but are such as might result from fractures in a glass globe produced by firing at it with very small shots one at a time. taking the whole series of them, mr. lowell very justly compares them to "a network which triangulates the surface of the planet like a geodetic survey, into polygons of all shapes and sizes." at the very lowest estimate the total length of the canals observed and mapped by mr. lowell must be over a hundred thousand miles, while he assures us that numbers of others have been seen over the whole surface, but so faintly or on such rare occasions as to elude all attempts to fix their position with certainty. but these, being of the same character and evidently forming part of the same system, must also be artificial, and thus we are led to a system of irrigation of almost unimaginable magnitude on a planet which has no mountains, no rivers, and no rain to support it; whose whole water-supply is derived from polar snows, the amount of which is ludicrously inadequate to need any such world-wide system; while the low atmospheric pressure would lead to rapid evaporation, thus greatly diminishing the small amount of moisture that is available. everyone must, i think, agree with miss clerke, that, even admitting the assumption that the polar snows consist of frozen water, the excessively scanty amount of water thus obtained would render any scheme of world-wide distribution of it hopelessly unworkable. the very remarkable phenomena of the duplication of many of the lines, together with the darkspots--the so-called oases--at their intersections, are doubtless all connected in some unknown way with the constitution and past history of the planet; but, on the theory of the whole being works of art, they certainly do _not_ help to remove any of the difficulties which have been shown to attend the theory that the single lines represent artificial canals of irrigation with a strip of verdure on each side of them produced by their overflow. _lowell on the purpose of the canals._ before leaving this subject it will be well to quote mr. lowell's own words as to the supposed perfectly level surface of mars, and his interpretation of the origin and purpose of the 'canals': "a body of planetary size, if unrotating, becomes a sphere, except for solar tidal deformation; if rotating, it takes on a spheroidal form exactly expressive, so far as observation goes, of the so-called centrifugal force at work. mars presents such a figure, being flattened out to correspond to its axial rotation. its surface therefore is in fluid equilibrium, or, in other words, a particle of liquid at any point of its surface at the present time would stay where it was devoid of inclination to move elsewhere. now the water which quickens the verdure of the canals moves from the pole down to the equator as the season advances. this it does then irrespective of gravity. no natural force propels it, and the inference is forthright and inevitable that it is artificially helped to its end. there seems to be no escape from this deduction. water only flows downhill, and there is no such thing as downhill on a surface already in fluid equilibrium. a few canals might presumably be so situated that their flow could, by inequality of terrane, lie equatorward, but not all....now it is not in particular but by general consent that the canal-system of mars develops from pole to equator. from the respective times at which the minima take place, it appears that the canal quickening occupies fifty-two days, as evidenced by the successive vegetal darkenings, to descend from latitude ° north to latitude °, a journey of miles. this gives for the water a speed of fifty-one miles a day, or . miles an hour. the rate of progression is remarkably uniform, and this abets the deduction as to assisted transference. but the fact is more unnatural yet. the growth pays no regard to the equator, but proceeds across it as if it did not exist into the planet's other hemisphere. here is something still more telling than travel to this point. for even if we suppose, for the sake of argument, that natural forces took the water down to the equator, their action must there be certainly reversed, and the equator prove a dead-line, to pass which were impossible" (pp. - ). i think my readers will agree with me that this whole argument is one of the most curious ever put forth seriously by an eminent man of science. because the polar compression of mars is about what calculation shows it ought to be in accordance with its rate of rotation, its surface is in a state of 'fluid equilibrium,' and must therefore be absolutely level throughout. but the polar compression of the earth equally agrees with calculation; therefore its surface is also in 'fluid equilibrium'; therefore it also ought to be as perfectly level on land as it is on the ocean surface! but as we know this is very far from being the case, why must it be so in mars? are we to suppose mars to have been formed in some totally different way from other planets, and that there neither is nor ever has been any reaction between its interior and exterior forces? again, the assumption of perfect flatness is directly opposed to all observation and all analogy with what we see on the earth and moon. it gives no account whatever of the numerous and large dark patches, once termed seas, but now found to be not so, and to be full of detailed markings and varied depths of shadow. to suppose that these are all the same dead-level as the light-coloured portions are assumed to be, implies that the darkness is one of material and colour only, not of diversified contour, which again is contrary to experience, since difference of material with us always leads to differences in rate of degradation, and hence of diversified contour, as these dark spaces actually show themselves under favourable conditions to independent observers. _lowell on the system of canals as a whole._ we will now see what mr. lowell claims to be the plain teaching of the 'canals' as a whole: "but last and all-embracing in its import is the system which the canals form. instead of running at hap-hazard, the canals are interconnected in a most remarkable manner. they seek centres instead of avoiding them. the centres are linked thus perfectly one with another, an arrangement which could not result from centres, whether of explosion or otherwise, which were themselves discrete. furthermore, the system covers the whole surface of the planet, dark areas and light ones alike, a world-wide distribution which exceeds the bounds of natural possibility. any force which could act longitudinally on such a scale must be limited latitudinally in its action, as witness the belts of jupiter and the spots upon the sun. rotational, climatic, or other physical cause could not fail of zonal expression. yet these lines are grandly indifferent to such competing influences. finally, the system, after meshing the surface in its entirety, runs straight into the polar caps. "it is, then, a system whose end and aim is the tapping of the snow-cap for the water there semi-annually let loose; then to distribute it over the planet's face" (p. ). here, again, we have curiously weak arguments adduced to support the view that these numerous straight lines imply works of art rather than of nature, especially in the comparison made with the belts of jupiter and the spots on the sun, both purely atmospheric phenomena, whereas the lines on mars are on the solid surface of the planet. why should there be any resemblance between them? every fact stated in the above quotation, always keeping in mind the physical conditions of the planet--its very tenuous atmosphere and rainless desert-surface--seem wholly in favour of a purely natural as opposed to an artificial origin; and at the close of this discussion i shall suggest one which seems to me to be at least possible, and to explain the whole series of the phenomena set forth and largely discovered by mr. lowell, in a simpler and more probable manner than does his tremendous assumption of their being works of art. readers who may not possess mr. lowell's volume will find three of his most recent maps of the 'canals' reproduced in _nature_ of october th, . chapter iv. is animal life possible on mars? having now shown, that, even admitting the accuracy of all mr. lowell's observations, and provisionally accepting all his chief conclusions as to the climate, the nature of the snow-caps, the vegetation, and the animal life of mars, yet his interpretation of the lines on its surface as being veritably 'canals,' constructed by intelligent beings for the special purpose of carrying water to the more arid regions, is wholly erroneous and rationally inconceivable. i now proceed to discuss his more fundamental position as to the actual habitability of mars by a highly organised and intellectual race of material organic beings. _water and air essential to life._ here, fortunately, the issue is rendered very simple, because mr. lowell fully recognises the identity of the constitution of matter and of physical laws throughout the solar-system, and that for any high form of organic life certain conditions which are absolutely essential on our earth must also exist in mars. he admits, for example, that water is essential, that an atmosphere containing oxygen, nitrogen, aqueous vapour, and carbonic acid gas is essential, and that an abundant vegetation is essential; and these of course involve a surface-temperature through a considerable portion of the year that renders the existence of these--especially of water--possible and available for the purposes of a high and abundant animal life. _blue colour the only evidence of water._ in attempting to show that these essentials actually exist on mars he is not very successful. he adduces evidence of an atmosphere, but of an exceedingly scanty one, since the greatest amount he can give to it is-- "not more than about four inches of barometric pressure as we reckon it";[ ] and he assumes, as he has a fair right to do till disproved, that it consists of oxygen and nitrogen, with carbon-dioxide and water-vapour, in approximately the same proportions as with us. with regard to the last item--the water-vapour--there are however many serious difficulties. the water-vapour of our atmosphere is derived from the enormous area of our seas, oceans, lakes, and rivers, as well as from the evaporation from heated lands and tropical forests of much of the moisture produced by frequent and abundant rains. all these sources of supply are admittedly absent from mars, which has no permanent bodies of water, no rain, and tropical regions which are almost entirely desert. many writers have therefore doubted the existence of water in any form upon this planet, supposing that the snow-caps are not formed of frozen water but of carbon-dioxide, or some other heavy gas, in a frozen state; and mr. lowell evidently feels this to be a difficulty, since the only fact he is able to adduce in favour of the melting snows of the polar caps producing water is, that at the time they are melting a marginal blue band appears which accompanies them in their retreat, and this blue colour is said to prove conclusively that the liquid is not carbonic acid but water. this point he dwells upon repeatedly, stating, of these blue borders: "this excludes the possibility of their being formed by carbon-dioxide, and shows that of all the substances we know the material composing them must be water." [footnote : in a paper written since the book appeared the density of air at the surface of mars is said to be / of the earth's.] this is the only proof of the existence of _water_ he adduces, and it is certainly a most extraordinary and futile one. for it is perfectly well known that although water, in large masses and by transmitted light, is of a blue colour, yet shallow water by reflected light is not so; and in the case of the liquid produced by the snow-caps of mars, which the whole conditions of the planet show must be shallow, and also be more or less turbid, it cannot possibly be the cause of the 'deep blue' tint said to result from the melting of the snow. but there is a very weighty argument depending on the molecular theory of gases against the polar caps of mars being composed of frozen water at all. the mass and elastic force of the several gases is due to the greater or less rapidity of the vibratory motion of their molecules under identical conditions. the speed of these molecular motions has been ascertained for all the chief gases, and it is found to be so great as in certain cases to enable them to overcome the force of gravity and escape from a planet's surface into space. dr. g. johnstone stoney has specially investigated this subject, and he finds that the force of gravity on the earth is sufficient to retain all the gases composing its atmosphere, but not sufficient to retain hydrogen; and as a consequence, although this gas is produced in small quantities by volcanoes and by decomposing vegetation, yet no trace of it is found in our atmosphere. the moon however, having only one-eightieth the mass of the earth, cannot retain any gas, hence its airless and waterless condition. _water vapour cannot exist on mars._ now, dr. stoney finds that in order to retain water vapour permanently a planet must have a mass at least a quarter that of the earth. but the mass of mars is only one-ninth that of the earth; therefore, unless there are some special conditions that prevent its loss, this gas cannot be present in the atmosphere. mr. lowell does not refer to this argument against his view, neither does he claim the evidence of spectroscopy in his favour. this was alleged more than thirty years ago to show the existence of water-vapour in the atmosphere of mars, but of late years it has been doubted, and mr. lowell's complete silence on the subject, while laying stress on such a very weak and inconclusive argument as that from the tinge of colour that is observed a little distance from the edge of the diminishing snow-caps, shows that he himself does not think the fact to be thus proved. if he did he would hardly adduce such an argument for its presence as the following: "the melting of the caps on the one hand and their re-forming on the other affirm the presence of water-vapour in the martian atmosphere, of whatever else that air consists" (p. ). yet absolutely the only proof he gives that the caps are frozen water is the almost frivolous colour-argument above referred to! _no spectroscopic evidence of water vapour._ as sir william huggins is the chief authority quoted for this fact, and is referred to as being almost conclusive in the third edition of miss clerke's _history of astronomy_ in , i have ascertained that his opinion at the present time is that "there is no conclusive proof of the presence of aqueous vapour in the atmosphere of mars, and that observations at the lick observatory (in ), where the conditions and instruments are of the highest order, were negative." he also informs me that marchand at the pic du midi observatory was unable to obtain lines of aqueous vapour in the spectrum of mars; and that in , slipher, at mr. lowell's observatory, was unable to detect any indications of aqueous vapour in the spectrum of mars. it thus appears that spectroscopic observations are quite accordant with the calculations founded on the molecular theory of gases as to the absence of aqueous vapour, and therefore presumably of liquid water, from mars. it is true that the spectroscopic argument is purely negative, and this may be due to the extreme delicacy of the observations required; but that dependent on the inability of the force of gravity to retain it is positive scientific evidence against its presence, and, till shown to be erroneous, must be held to be conclusive. this absence of water is of itself conclusive against the existence of animal life, unless we enter the regions of pure conjecture as to the possibility of some other liquid being able to take its place, and that liquid being as omnipresent there as water is here. mr. lowell however never takes this ground, but bases his whole theory on the fundamental identity of the substance of the bodies of living organisms wherever they may exist in the solar system. in the next two chapters i shall discuss an equally essential condition, that of temperature, which affords a still more conclusive and even crushing argument against the suitability of mars for the existence of organic life. chapter v. the temperature of mars--mr. lowell's estimate. we have now to consider a still more important and fundamental question, and one which mr. lowell does not grapple with in this volume, the actual temperatures on mars due to its distance from the sun and the atmospheric conditions on which he himself lays so much stress. if i am not greatly mistaken we shall arrive at conclusions on this subject which are absolutely fatal to the conception of any high form of organic life being possible on this planet. _the problem of terrestrial temperatures._ in order that the problem may be understood and its importance appreciated, it is necessary to explain the now generally accepted principles as to the causes which determine the temperatures on our earth, and, presumably, on all other planets whose conditions are not wholly unlike ours. the fact of the internal heat of the earth which becomes very perceptible even at the moderate depths reached in mines and deep borings, and in the deepest mines becomes a positive inconvenience, leads many people to suppose that the surface- temperatures of the earth are partly due to this cause. but it is now generally admitted that this is not the case, the reason being that all rocks and soils, in their natural compacted state, are exceedingly bad conductors of heat. a striking illustration of this is the fact, that a stream of lava often continues to be red hot at a few feet depth for years after the surface is consolidated, and is hardly any warmer than that of the surrounding land. a still more remarkable case is that of a glacier on the south-east side of the highest cone of etna underneath a lava stream with an intervening bed of volcanic sand only ten feet thick. this was visited by sir charles lyell in , and a second time thirty years later, when he made a very careful examination of the strata, and was quite satisfied that the sand and the lava stream together had actually preserved this mass of ice, which neither the heat of the lava above it at its first outflow, nor the continued heat rising from the great volcano below it, had been able to melt or perceptibly to diminish in thirty years. another fact that points in the same direction is the existence over the whole floor of the deepest oceans of ice-cold water, which, originating in the polar seas, owing to its greater density sinks and creeps slowly along the ocean bottom to the depths of the atlantic and pacific, and is not perceptibly warmed by the internal heat of the earth. now the solid crust of the earth is estimated as at least about twenty miles in average thickness; and, keeping in mind the other facts just referred to, we can understand that the heat from the interior passes through it with such extreme slowness as not to be detected at the surface, the varying temperatures of which are due entirely to solar heat. a large portion of this heat is stored up in the surface soil, and especially in the surface layer of the oceans and seas, thus partly equalising the temperatures of day and night, of winter and summer, so as greatly to ameliorate the rapid changes of temperature that would otherwise occur. our dense atmosphere is also of immense advantage to us as an equaliser of temperature, charged as it almost always is with a large quantity of water-vapour. this latter gas, when not condensed into cloud, allows the solar heat to pass freely to the earth; but it has the singular and highly beneficial property of absorbing and retaining the dark or lower-grade heat given off by the earth which would otherwise radiate into space much more rapidly. we must therefore always remember that, very nearly if not quite, the _whole_ of _the warmth we experience on the earth is derived from the sun._[ ] [footnote : professor j.h. poynting, in his lecture to the british association at cambridge in , says: "the surface of the earth receives, we know, an amount of heat from the inside almost infinitesimal compared with that which it receives from the sun, and on the sun, therefore, we depend for our temperature."] in order to understand the immense significance of this conclusion we must know what is meant by the _whole_ heat or warmth; as unless we know this we cannot define what half or any other proportion of sun-heat really means. now i feel pretty sure that nine out of ten of the average educated public would answer the following question incorrectly: the mean temperature of the southern half of england is about ° f. supposing the earth received only half the sun-heat it now receives, what would then be the probable mean temperature of the south of england? the majority would, i think, answer at once--about ° f. nearly as many would perhaps say-- ° f. is ° above the freezing point; therefore half the heat received would bring us down to ° above the freezing point, or ° f. very few, i think, would realise that our share of half the amount of sun-heat received by the earth would probably result in reducing our mean temperature to about ° f. below the freezing point, and perhaps even lower. this is about the very lowest temperature yet experienced on the earth's surface. to understand how such results are obtained a few words must be said about the absolute zero of temperature. _the zero of temperature._ heat is now believed to be entirely due to ether-vibration, which produces a correspondingly rapid vibration of the molecules of matter, causing it to expand and producing all the phenomena we term 'heat.' we can conceive this vibration to increase indefinitely, and thus there would appear to be no necessary limit to the amount of heat possible, but we cannot conceive it to decrease indefinitely at the same uniform rate, as it must soon inevitably come to nothing. now it has been found by experiment that gases under uniform pressure expand / of their volume for each degree centigrade of increased temperature, so that in passing from ° c. to ° c. they are doubled in volume. they also decrease in volume at the same rate for each degree below ° c. (the freezing point of water). hence if this goes on to- ° c. a gas will have no volume, or it will undergo some change of nature. hence this is called the zero of temperature, or the temperature to which any matter falls which receives no heat from any other matter. it is also sometimes called the temperature of space, or of the ether in a state of rest, if that is possible. all the gases have now been proved to become, first liquid and then (most of them) solid, at temperatures considerably above this zero. the only way to compare the proportional temperatures of bodies, whether on the earth or in space, is therefore by means of a scale beginning at this natural zero, instead of those scales founded on the artificial zero of the freezing point of water, or, as in fahrenheit's, ° below it. only by using the natural zero and measuring continuously from it can we estimate temperatures in relative proportion to the amount of heat received. this is termed the absolute zero, and so that we start reckoning from that point it does not matter whether the scale adopted is the centigrade or that of fahrenheit. _the complex problem of planetary temperatures._ now if, as is the case with mars, a planet receives only half the amount of solar heat that we receive, owing to its greater distance from the sun, and if the mean temperature of our earth is ° f., this is equal to ° f. on the absolute scale. it would therefore appear very simple to halve this amount and obtain . ° f. as the mean temperature of that planet. but this result is erroneous, because the actual amount of sun heat intercepted by a planet is only one condition out of many that determine its resulting temperature. radiation, that is loss of heat, is going on concurrently with gain, and the rate of loss varies with the temperature according to a law recently discovered, the loss being much greater at high temperatures in proportion to the th power of the absolute temperature. then, again, the whole heat intercepted by a planet does not reach its surface unless it has no atmosphere. when it has one, much is reflected or absorbed according to complex laws dependent on the density and composition of the atmosphere. then, again, the heat that reaches the actual surface is partly reflected and partly absorbed, according to the nature of that surface--land or water, desert or forest or snow-clad--that part which is absorbed being the chief agent in raising the temperature of the surface and of the air in contact with it. very important too is the loss of heat by radiation from these various heated surfaces at different rates; while the atmosphere itself sends back to the surface an ever varying portion of both this radiant and reflected heat according to distinct laws. further difficulties arise from the fact that much of the sun's heat consists of dark or invisible rays, and it cannot therefore be measured by the quantity of light only. from this rough statement it will be seen that the problem is an exceedingly complex one, not to be decided off-hand, or by any simple method. it has in fact been usually considered as (strictly speaking) insoluble, and only to be estimated by a more or less rough approximation, or by the method of general analogy from certain known facts. it will be seen, from what has been said in previous chapters, that mr. lowell, in his book, has used the latter method, and, by taking the presence of water and water-vapour in mars as proved by the behaviour of the snow-caps and the bluish colour that results from their melting, has deduced a temperature above the freezing point of water, as prevalent in the equatorial regions permanently, and in the temperate and arctic zones during a portion of each year. _mr. lowell's mathematical investigation of the problem._ but as this result has been held to be both improbable in itself and founded on no valid evidence, he has now, in the _london, edinburgh, and dublin philosophical magazine_ of july , published an elaborate paper of pages, entitled _a general method for evaluating the surface-temperatures of the planets; with special reference to the temperature of mars_, by professor percival lowell; and in this paper, by what purports to be strict mathematical reasoning based on the most recent discoveries as to the laws of heat, as well as on measurements or estimates of the various elements and constants used in the calculations, he arrives at a conclusion strikingly accordant with that put forward in the recently published volume. having myself neither mathematical nor physical knowledge sufficient to enable me to criticise this elaborate paper, except on a few points, i will here limit myself to giving a short account of it, so as to explain its method of procedure; after which i may add a few notes on what seem to me doubtful points; while i also hope to be able to give the opinions of some more competent critics than myself. _mr. lowell's mode of estimating the surface-temperature of mars._ the author first states, that professor young, in his _general astronomy_ ( ), makes the mean temperature of mars . ° absolute, by using newton's law of heat being radiated in proportion to temperature, and ° abs. (=- ° f.) by dulong and petit's law; but adds, that a closer determination has been made by professor moulton, using stefan's law, that radiation is as the _/ th_ power of the temperature, whence results a mean temperature of- ° f. these estimates assume identity of atmospheric conditions of mars and the earth. but as none of these estimates take account of the many complex factors which interfere with such direct and simple calculations, mr. lowell then proceeds to enunciate them, and work out mathematically the effects they produce: ( ) the whole radiant energy of the sun on striking a planet becomes divided as follows: part is reflected back into space, part absorbed by the atmosphere, part transmitted to the surface of the planet. this surface again reflects a portion and only the balance left goes to warm the planet. ( ) to solve this complex problem we are helped by the _albedoes_ or intrinsic brilliancy of the planets, which depend on the proportion of the visible rays which are reflected and which determines the comparative brightness of their respective surfaces. we also have to find the ratio of the invisible to the visible rays and the heating power of each. ( ) he then refers to the actinometer and pyroheliometer, instruments for measuring the actual heat derived from the sun, and also to the bolometer, an instrument invented by professor langley for measuring the invisible heat rays, which he has proved to extend to more than three times the length of the whole heat-spectrum as previously known, and has also shown that the invisible rays contribute per cent, of the sun's total energy.[ ] [footnote : for a short account of this remarkable instrument, see my _wonderful century_, new ed., pp. - .] ( ) then follows an elaborate estimate of the loss of heat during the passage of the sun's rays through our atmosphere from experiments made at different altitudes and from the estimated reflective power of the various parts of the earth's surface--rocks and soil, ocean, forest and snow--the final result being that three-fourths of the whole sun-heat is reflected back into space, forming our _albedo_, while only one-fourth is absorbed by the soil and goes to warm the air and determine our mean temperature. ( ) we now have another elaborate estimate of the comparative amounts of heat actually received by mars and the earth, dependent on their very different amounts of atmosphere, and this estimate depends almost wholly on the comparative _albedoes_, that of mars, as observed by astronomers being . , while ours has been estimated in a totally different way as being . , whence he concludes that nearly three-fourths of the sun-heat that mars receives reaches the surface and determines its temperature, while we get only one-fourth of our total amount. then by applying stefan's law, that the radiation is as the th power of the surface temperature, he reaches the final result that the actual heating power at the surface of mars is considerably _more_ than on the earth, and would produce a mean temperature of ° f., if it were not for the greater conservative or blanketing power of our denser and more water-laden atmosphere. the difference produced by this latter fact he minimises by dwelling on the probability of a greater proportion of carbonic-acid gas and water-vapour in the martian atmosphere, and thus brings down the mean temperature of mars to ° f., which is almost exactly the same as that of the southern half of england. he has also, as the result of observations, reduced the probable density of the atmosphere of mars to - / inches of mercury, or only one-twelfth of that of the earth. _critical remarks on mr. lowell's paper._ the last part of this paper, indicated under pars. and , is the most elaborate, occupying eight pages, and it contains much that seems uncertain, if not erroneous. in particular, it seems very unlikely that under a clear sky over the whole earth we should only receive at the sea-level . of the solar rays which the earth intercepts (p. ). these data largely depend on observations made in california and other parts of the southern united states, where the lower atmosphere is exceptionally dust-laden. till we have similar observations made in the tropical forest-regions, which cover so large an area, and where the atmosphere is purified by frequent rains, and also on the prairies and the great oceans, we cannot trust these very local observations for general conclusions affecting the whole earth. later, in the same article (p. ), mr. lowell says: "clouds transmit approximately per cent. of the heat reaching them: a clear sky at sea-level per cent. as the sky is half the time cloudy the mean transmission is per cent." these statements seem incompatible with that quoted above. the figure he uses in his calculations for the actual albedo of the earth, . , is also not only improbable, but almost self-contradictory, because the albedo of cloud is . , and that of the great cloud-covered planet, jupiter, is given by lowell as . , while zollner made it only . . again, lowell gives venus an albedo of . , while zollner made it only . and mr. gore . . this shows the extreme uncertainty of these estimates, while the fact that both venus and jupiter are wholly cloud-covered, while we are only half-covered, renders it almost certain that our albedo is far less than mr. lowell makes it. it is evident that mathematical calculations founded upon such uncertain data cannot yield trustworthy results. but this is by no means the only case in which the data employed in this paper are of uncertain value. everywhere we meet with figures of somewhat doubtful accuracy. here we have somebody's 'estimate' quoted, there another person's 'observation,' and these are adopted without further remark and used in the various calculations leading to the result above quoted. it requires a practised mathematician, and one fully acquainted with the extensive literature of this subject, to examine these various data, and track them through the maze of formulae and figures so as to determine to what extent they affect the final result. there is however one curious oversight which i must refer to, as it is a point to which i have given much attention. not only does mr. lowell assume, as in his book, that the 'snows' of mars consist of frozen water, and that therefore there _is_ water on its surface and water-vapour in its atmosphere, not only does he ignore altogether dr. johnstone stoney's calculations with regard to it, which i have already referred to, but he uses terms that imply that water-vapour is one of the heavier components of our atmosphere. the passage is at p. of the _philosophical magazine._ after stating that, owing to the very small barometric pressure in mars, water would boil at ° f., he adds: "the sublimation at lower temperatures would be correspondingly increased. consequently the amount of water-vapour in the martian air must on that score be relatively greater than our own." then follows this remarkable passage: "carbon-dioxide, because of its greater specific gravity, would also be in relatively greater amount so far as this cause is considered. for the planet would part, _caeteris paribus_, with its lighter gases the quickest. whence as regards both water-vapour and carbon-dioxide we have reason to think them in relatively greater quantity than in our own air at corresponding barometric pressure." i cannot understand this passage except as implying that 'water-vapour and carbon-dioxide' are among the heavier and not among the lighter gases of the atmosphere--those which the planet 'parts with quickest.' but this is just what water-vapour _is_, being a little less than two-thirds the weight of air ( . ), and one of those which the planet _would_ part with the quickest, and which, according to dr. johnstone stoney, it loses altogether. * * * * * note on professor lowell's article in the _philosophical magazine_; by j.h. poynting, f.r.s., professor of physics in the university of birmingham. "i think professor lowell's results are erroneous through his neglect of the heat stored in the air by its absorption of radiation both from the sun and from the surface. the air thus heated radiates to the surface and keeps up the temperature. i have sent to the _philosophical magazine_ a paper in which i think it is shown that when the radiation by the atmosphere is taken into account the results are entirely changed. the temperature of mars, with professor lowell's data, still comes out far below the freezing-point--still further below than the increased distance alone would make it. indeed, the lower temperature on elevated regions of the earth's surface would lead us to expect this. i think it is impossible to raise the temperature of mars to anything like the value obtained by professor lowell, unless we assume some quality in his atmosphere entirely different from any found in our own atmosphere." j.h. poynting. october , . chapter vi. a new estimate of the temperature of mars. when we are presented with a complex problem depending on a great number of imperfectly ascertained data, we may often check the results thus obtained by the comparison of cases in which some of the more important of these data are identical, while others are at a maximum or a minimum. in the present case we can do this by a consideration of the moon as compared with the earth and with mars. _langley's determination of the moon's temperature._ in the moon we see the conditions that prevail in mars both exaggerated and simplified. mars has a very scanty atmosphere, the moon none at all, or if there is one it is so excessively scanty that the most refined observations have not detected it. all the complications arising from the possible nature of the atmosphere, and its complex effects upon reflection, absorption, and radiation are thus eliminated. the mean distance of the moon from the sun being identical with that of the earth, the total amount of heat intercepted must also be identical; only in this case the whole of it reaches the surface instead of one-fourth only, according to mr. lowell's estimate for the earth. now, by the most refined observations with his bolometer, mr. langley was able to determine the temperature of the moon's surface exposed to undimmed sunshine for fourteen days together; and he found that, even in that portion of it on which the sun was shining almost vertically, the temperature rarely rose above the freezing point of water. however extraordinary this result may seem, it is really a striking confirmation of the accuracy of the general laws determining temperature which i have endeavoured to explain in the preceding chapter. for the same surface which has had fourteen days of sunshine has also had a preceding fourteen days of darkness, during which the heat which it had accumulated in its surface layers would have been lost by free radiation into stellar space. it thus acquires during its day a maximum temperature of only ° f. absolute, while its minimum, after days' continuous radiation, must be very low, and is, with much reason, supposed to approach the absolute zero. _rapid loss of heat by radiation on the earth._ in order better to comprehend what this minimum may be under extreme conditions, it will be useful to take note of the effects it actually produces on the earth in places where the conditions are nearest to those existing on the moon or on mars, though never quite equalling, or even approaching very near them. it is in our great desert regions, and especially on high plateaux, that extreme aridity prevails, and it is in such districts that the differences between day and night temperatures reach their maximum. it is stated by geographers that in parts of the great sahara the surface temperature is sometimes ° f., while during the night it falls nearly or quite to the freezing point--a difference of degrees in little more than hours.[ ] in the high desert plains of central asia the extremes are said to be even greater.[ ] again, in his _universal geography_, reclus states that in the armenian highlands the thermometer oscillates between ° f. and °f. we may therefore, without any fear of exaggeration, take it as proved that a fall of ° f. in twelve or fifteen hours not infrequently occurs where there is a very dry and clear atmosphere permitting continuous insolation by day and rapid radiation by night. [footnote : keith johnston's 'africa' in _stanford's compendium._] [footnote : _chambers's encyclopaedia_, art. 'deserts.'] now, as it is admitted that our dense atmosphere, however dry and clear, absorbs and reflects some considerable portion of the solar heat, we shall certainly underestimate the radiation from the moon's surface during its long night if we take as the basis of our calculation a lowering of temperature amounting to ° f. during twelve hours, as not unfrequently occurs with us. using these data--with stefan's law of decrease of radiation as the th power of the temperature--a mathematical friend finds that the temperature of the moon's surface would be reduced during the lunar night to nearly ° f. absolute (equal to- ° f.). _more rapid loss of heat by the moon._ although such a calculation as the above may afford us a good approximation to the rate of loss of heat by mars with its very scanty atmosphere, we have now good evidence that in the case of the moon the loss is much more rapid. two independent workers have investigated this subject with very accordant results--dr. boeddicker, with lord rosse's -foot reflector and a thermopile to measure the heat, and mr. frank very, with a glass reflector of inches diameter and the bolometer invented by mr. langley. the very striking and unexpected fact in which these observers agree is the sudden disappearance of much of the stored-up heat during the comparatively short duration of a total eclipse of the moon--less than two hours of complete darkness, and about twice that period of partial obscuration. dr. boeddicker was unable to detect any appreciable heat at the period of greatest obscuration; but, owing to the extreme sensitiveness of the bolometer, mr. very ascertained that those parts of the surface which had been longest in the shadow still emitted heat "to the amount of one per cent. of the heat to be expected from the full moon." this however is the amount of radiation measured by the bolometer, and to get the temperature of the radiating surface we must apply stefan's law of the th power. hence the temperature of the moon's dark surface will be the [fourth root of ( over )] = over . [a] of the highest temperature (which we may take at the freezing-point, ° f. abs.), or ° f. abs., just below the liquefaction point of air. this is about ° lower than the amount found by calculation from our most rapid radiation; and as this amount is produced in a few hours, it is not too much to expect that, when continued for more than two weeks (the lunar night), it might reach a temperature sufficient to liquefy hydrogen ( ° f. abs.), or perhaps even below it. [note a: latex markup $\root \of { \over } = { \over . }$ ] _theory of the moon's origin._ this extremely rapid loss of heat by radiation, at first sight so improbable as to be almost incredible, may perhaps be to some extent explained by the physical constitution of the moon's surface, which, from a theoretical point of view, does not appear to have received the attention it deserves. it is clear that our satellite has been long subjected to volcanic eruptions over its whole visible face, and these have evidently been of an explosive nature, so as to build up the very lofty cones and craters, as well as thousands of smaller ones, which, owing to the absence of any degrading or denuding agencies, have remained piled up as they were first formed. this highly volcanic structure can, i think, be well explained by an origin such as that attributed to it by sir george darwin, and which has been so well described by sir robert ball in his small volume, _time and tide._ these astronomers adduce strong evidence that the earth once rotated so rapidly that the equatorial protuberance was almost at the point of separation from the planet as a ring. before this occurred, however, the tension was so great that one large portion of the protuberance where it was weakest broke away, and began to move around the earth at some considerable distance from it. as about / of the bulk of the earth thus escaped, it must have consisted of a considerable portion of the solid crust and a much larger quantity of the liquid or semi-liquid interior, together with a proportionate amount of the gases which we know formed, and still form, an important part of the earth's substance. as the surface layers of the earth must have been the lightest, they would necessarily, when broken up by this gigantic convulsion, have come together to form the exterior of the new satellite, and be soon adjusted by the forces of gravity and tidal disturbance into a more or less irregular spheroidal form, all whose interstices and cavities would be filled up and connected together by the liquid or semi-liquid mass forced up between them. thence-forward, as the moon increased its distance and reduced its time of rotation, in the way explained by sir robert ball, there would necessarily commence a process of escape of the imprisoned gases at every fissure and at all points and lines of weakness, giving rise to numerous volcanic outlets, which, being subjected only to the small force of lunar gravity (only one-sixth that of the earth), would, in the course of ages, pile up those gigantic cones and ridges which form its great characteristic. but this small gravitative power of the moon would prevent its retaining on its surface any of the gases forming our atmosphere, which would all escape from it and probably be recaptured by the earth. by no process of external aggregation of solid matter to such a relatively small amount as that forming the moon, even if the aggregation was so violent as to produce heat enough to cause liquefaction, could any such long-continued volcanic action arise by gradual cooling, in the absence of internal gases. there might be fissures, and even some outflows of molten rock; but without imprisoned gases, and especially without water and water-vapour producing explosive outbursts, could any such amount of scoriae and ashes be produced as were necessary for the building up of the vast volcanic cones, craters, and craterlets we see upon the moon's surface. i am not aware that either sir robert ball or sir george darwin have adduced this highly volcanic condition of the moon's surface as a phenomenon which can _only_ be explained by our satellite having been thrown off a very much larger body, whose gravitative force was sufficient to acquire and retain the enormous quantity of gases and of water which we possess, and which are _absolutely essential_ for that _special form of cone-building volcanic action_ which the moon exhibits in so pre-eminent a degree. yet it seems to me clear, that some such hypothetical origin for our satellite would have had to be assumed if sir george darwin had not deduced it by means of purely mathematical argument based upon astronomical facts. returning now to the problem of the moon's temperature, i think the phenomena this presents may be in part due to the mode of formation here described. for, its entire surface being the result of long-continued gaseous explosions, all the volcanic products--scoriae, pumice, and ashes--would necessarily be highly porous throughout; and, never having been compacted by water-action, as on the earth, and there having been no winds to carry the finer dust so as to fill up their pores and fissures, the whole of the surface material to a very considerable depth must be loose and porous to a high degree. this condition has been further increased owing to the small power of gravity and the extreme irregularity of the surface, consisting very largely of lofty cones and ridges very loosely piled up to enormous heights. now this condition of the substance of the moon's surface is such as would produce a high specific heat, so that it would absorb a large amount of heat in proportion to the rise of temperature produced, the heat being conducted downwards to a considerable depth. owing, however, to the total absence of atmosphere radiation would very rapidly cool the surface, but afterwards more slowly, both on account of the action of stefan's law and because the heat stored up in the deeper portions could be carried to the surface by conduction only, and with extreme slowness. _very's researches on the moon's heat._ the results of the eclipse observations are supported by the detailed examination of the surface-temperature of the moon by mr. very in his _prize essay on the distribution of the moon's heat_ (published by the utrecht society of arts and sciences in ). he shows, by a diagram of the 'phase-curve,' that at the commencement of the lunar day the surface just within the illuminated limb has acquired about / of its maximum temperature, or about ° f. abs. as the surface exposed to the bolometer at each observation is about / of the moon's surface, and in order to ensure accuracy the instrument has to be directed to a spot lying wholly within the edge of the moon, it is evident that the surface measured has already been for several hours exposed to oblique sunshine. the curve of temperature then rises gradually and afterwards more rapidly, till it attains its maximum (of about + to ° f.) a few hours _before_ noon. this, mr. very thinks, is due to the fact that the half of the moon's face first illuminated for us has, on the average, a darker surface than that of the afternoon, or second quarter, during which the curve descends not quite so rapidly, the temperature near sunset being only a little higher than that near sunrise. this rapid fall while exposed to oblique sunshine is quite in harmony with the rapid loss of heat during the few hours of darkness during an eclipse, both showing the prepotency of radiation over insolation on the moon. two other diagrams show the distribution of heat at the time of full-moon, one half of the curve showing the temperatures along the equator from the edge of the disc to the centre, the other along a meridian from this centre to the pole. this diagram (here reproduced) exhibits the quick rise of temperature of the oblique rim of the moon and the nearly uniform heat of the central half of its surface; the diminution of heat towards the pole, however, is slower for the first half and more rapid for the latter portion. it is an interesting fact that the temperature near the margin of the full-moon increases towards the centre more rapidly than it does when the same parts are observed during the early phases of the first quarter. mr. very explains this difference as being due to the fact that the full-moon to its very edges is fully illuminated, all the shadows of the ridges and mountains being thrown vertically or obliquely _behind them._ we thus measure the heat reflected from the _whole_ visible surface. but at new moon, and somewhat beyond the first quarter, the deep shadows thrown by the smallest cones and ridges, as well as by the loftiest mountains, cover a considerable portion of the visible surface, thus largely reducing the quantity of light and heat reflected or radiated in our direction. it is only at the full, therefore, that the maximum temperature of the whole lunar surface can be measured. it must be considered a proof of the delicacy of the heat-measuring instruments that this difference in the curves of temperature of the different parts of the moon's surface and under different conditions is so clearly shown. _the application of the preceding results to the case of mars._ this somewhat lengthy account of the actual state of the moon's surface and temperature is of very great importance in our present enquiry, because it shows us the extraordinary difference in mean and extreme temperatures of two bodies situated at the same distance from the sun, and therefore receiving exactly the same amount of solar heat per unit of surface. we have learned also what are the main causes of this almost incredible difference, namely: ( ) a remarkably rugged surface with porous and probably cavernous rock-texture, leading to extremely rapid radiation of heat in the one; as compared with a comparatively even and well-compacted surface largely clad with vegetation, leading to comparatively slow and gradual loss by radiation in the other: and ( ), these results being greatly intensified by the total absence of a protecting atmosphere in the former, while a dense and cloudy atmosphere with an ever-present supply of water-vapour, accumulates and equalises the heat received by the latter. the only other essential difference in the two bodies which may possibly aid in the production of this marvellous result, is the fact of our day and night having a mean length of hours, while those of the moon are about - / of our days. but the altogether unexpected fact, in which two independent enquirers agree, that during the few hours' duration of a total eclipse of the moon so large a proportion of the heat is lost by radiation renders it almost certain that the resulting low temperature would be not very much less if the moon had a day and night the same length as our own. the great lesson we learn by this extreme contrast of conditions supplied to us by nature, as if to enable us to solve some of her problems, is, the overwhelming importance, first, of a dense and well-compacted surface, due to water-action and strong gravitative force; secondly, of a more or less general coat of vegetation; and, thirdly, of a dense vapour-laden atmosphere. these three favourable conditions result in a mean temperature of about + ° f. with a range seldom exceeding ° above or below it, while over more than half the land-surface of the earth the temperature rarely falls below the freezing point. on the other hand, we have a globe of the same materials and at the same distance from the sun, with a maximum temperature of freezing water, and a minimum not very far from the absolute zero, the monthly mean being probably much below the freezing point of carbonic-acid gas--a difference entirely due to the absence of these three favourable conditions. _the special features of mars as influencing temperature._ coming now to the special feature of mars and its probable temperature, we find that most writers have arrived at a very different conclusion from that of mr. lowell, who himself quotes mr. moulton as an authority who 'recently, by the application of stefan's law,' has found the mean temperature of this planet to be- ° f. again, professor j.h. poynting, in his lecture on 'radiation in the solar system,' delivered before the british association at cambridge in , gave an estimate of the mean temperature of the planets, arrived at from measurements of the sun's emissive power and the application of stefan's law to the distances of the several planets, and he thus finds the earth to have a mean temperature of ° c. (= - / ° f.) and mars one of- ° c. (=- - / ° f.), a wonderfully close approximation to the mean temperature of the earth as determined by direct measurement, and therefore, presumably, an equally near approximation to that of mars as dependent on distance from the sun, and '_on the supposition that it is earth-like in all its conditions._' but we know that it is far from being earth-like in the very conditions which we have found to be those which determine the extremely different temperatures of the earth, and moon; and, as regards each of these, we shall find that, so far as it differs from the earth, it approximates to the less favourable conditions that prevail in the moon. the first of these conditions which we have found to be essential in regulating the absorption and radiation of heat, and thus raising the mean temperature of a planet, is a compact surface well covered with vegetation, two conditions arising from, and absolutely dependent on, an ample amount of water. but mr. lowell himself assures us, as a fact of which he has no doubt, that there are no permanent bodies of water, great or small, upon mars; that rain, and consequently rivers, are totally wanting; that its sky is almost constantly clear, and that what appear to be clouds are not formed of water-vapour but of dust. he dwells, emphatically, on the terrible desert conditions of the greater part of the surface of the planet. that being the case now, we have no right to assume that it has ever been otherwise; and, taking full account of the fact, neither denied nor disputed by mr. lowell, that the force of gravity on mars is not sufficient to retain water-vapour in its atmosphere, we must conclude that the surface of that planet, like that of the moon, has been moulded by some form of volcanic action modified probably by wind, but not by water. adding to this, that the force of gravity on mars is nearer that of the moon than to that of the earth, and we may r reasonably conclude that its surface is formed of volcanic matter in a light and porous condition, and therefore highly favourable for the rapid loss of surface heat by radiation. the surface-conditions of mars are therefore, presumably, much more like those of the moon than like those of the earth. the next condition favourable to the storing up of heat--a covering of vegetation--is almost certainly absent from mars except, possibly, over limited areas and for short periods. in this feature also the surface of mars approximates much nearer to lunar than to earth-conditions. the third condition--a dense, vapour-laden atmosphere--is also wanting in mars. for although it possesses an atmosphere it is estimated by mr. lowell (in his latest article) to have a pressure equivalent to only - / inches of mercury with us, giving it a density of only one-twelfth part that of ours; while aqueous vapour, the chief accumulator of heat, cannot permanently exist in it, and, notwithstanding repeated spectroscopic observations for the purpose of detecting it, has never been proved to exist. i submit that i have now shown from the statements--and largely as the result of the long-continued observations--of mr. lowell himself, that, so far as the physical conditions of mars are known to differ from those of the earth, the differences are all _unfavourable_ to the conservation and _favourable_ to the dissipation of the scanty heat it receives from the sun--that they point unmistakeably towards the temperature conditions of the moon rather than to those of the earth, and that the cumulative effect of these adverse conditions, acting upon a heat-supply, reduced by solar distance to less than one-half of ours, _must_ result in a mean temperature (as well as in the extremes) nearer to that of our satellite than to that of our own earth. _further criticism of mr. lowell's article._ we are now in a position to test some further conclusions of mr. lowell's _phil. mag._ article by comparison with actual phenomena. we have seen, in the outline i have given of this article, that he endeavours to show how the small amount of solar heat received by mars is counterbalanced, largely by the greater transparency to light and heat of its thin and cloudless atmosphere, and partially also by a greater conservative or 'blanketing' power of its atmosphere due to the presence in it of a large proportion of carbonic acid gas and aqueous vapour. the first of these statements may be admitted as a fact which he is entitled to dwell upon, but the second--the presence of large quantities of carbon-dioxide and aqueous vapour is a pure hypothesis unsupported by any item of scientific evidence, while in the case of aqueous vapour it is directly opposed to admitted results founded upon the molecular theory of gaseous elasticity. but, although mr. lowell refers to the conservative or 'blanketing' effect of the earth's atmosphere, he does not consider or allow for its very great cumulative effect, as is strikingly shown by the comparison with the actual temperature conditions of the moon. this cumulative effect is due to the _continuous_ reflection and radiation of heat from the clouds as well as from the vapour-laden strata of air in our lower atmosphere, which latter, though very transparent to the luminous and accompanying heat rays of the sun, are opaque to the dark heat-rays whether radiated or reflected from the earth's surface. we are therefore in a position strictly comparable with that of the interior of some huge glass house, which not only becomes intensely heated by the direct rays of the sun, but also to a less degree by reflected rays from the sky and those radiated from the clouds, so that even on a cloudy or misty day its temperature rises many degrees above that of the outer air. such a building, if of large size, of suitable form, and well protected at night by blinds or other covering, might be so arranged as to accumulate heat in its soil and walls so as to maintain a tolerably uniform temperature though exposed to a considerable range of external heat and cold. it is to such a power of accumulation of heat in our soil and lower atmosphere that we must impute the overwhelming contrast between our climate and that of the moon. with us, the solar heat that penetrates our vapour-laden and cloudy atmosphere is shut in by that same atmosphere, accumulates there for weeks and months together, and can only slowly escape. it is this great cumulative power which mr. lowell has not taken account of, while he certainly has not estimated the enormous loss of heat by free radiation, which entirely neutralises the effects of increase of sun-heat, however great, when these cumulative agencies are not present.[ ] [footnote : the effects of this 'cumulative' power of a dense atmosphere are further discussed and illustrated in the last chapter of this book, where i show that the universal fact of steadily diminishing temperatures at high altitudes is due solely to the diminution of this cumulative power of our atmosphere, and that from this cause alone the temperature of mars must be that which would be found on a lofty plateau about , feet higher than the average of the peaks of the andes!] _temperature on polar regions of mars._ there is also a further consideration which i think mr. lowell has altogether omitted to discuss. whatever may be the _mean_ temperature of mars, we must take account of the long nights in its polar and high-temperate latitudes, lasting nearly twice as long as ours, with the resulting lowering of temperature by radiation into a constantly clear sky. even in siberia, in lat. - / °n. a cold of- °f. has been attained; while over a large portion of n. asia and america above ° lat. the _mean_ january temperature is from- °f. to- °f., and the whole subsoil is permanently frozen from a depth of or feet to several hundreds. but the winter temperatures, _over the same latitudes_ in mars, must be very much lower; and it must require a proportionally larger amount of its feeble sun-heat to raise the surface even to the freezing-point, and an additional very large amount to melt any considerable depth of snow. but this identical area, from a little below ° to the pole, is that occupied by the snow-caps of mars, and over the whole of it the winter temperature must be far lower than the earth-minimum of- °f. then, as the martian summer comes on, there is less than half the sun-heat available to raise this low temperature after a winter nearly double the length of ours. and when the summer does come with its scanty sun-heat, that heat is not accumulated as it is by our dense and moisture-laden atmosphere, the marvellous effects of which we have already shown. yet with all these adverse conditions, each assisting the other to produce a climate approximating to that which the earth would have if it had no atmosphere (but retaining our superiority over mars in receiving double the amount of sun-heat), we are asked to accept a mean temperature for the more distant planet almost exactly the same as that of mild and equable southern england, and a disappearance of the vast snowfields of its polar regions as rapid and complete as what occurs with us! if the moon, even at its equator, has not its temperature raised above the freezing-point of water, how can the more _distant_ mars, with its _oblique_ noon-day sun falling upon the snow-caps, receive heat enough, first to raise their temperature to ° f., and then to melt with marked rapidity the vast frozen plains of its polar regions? mr. lowell is however so regardless of the ordinary teachings of meteorological science that he actually accounts for the supposed mild climate of the polar regions of mars by the absence of water on its surface and in its atmosphere. he concludes his fifth chapter with the following words: "could our earth but get rid of its oceans, we too might have temperate regions stretching to the poles." here he runs counter to two of the best-established laws of terrestrial climatology-- the wonderful equalising effects of warm ocean-currents which are the chief agents in diminishing polar cold; the equally striking effects of warm moist winds derived from these oceans, and the great storehouse of heat we possess in our vapour-laden atmosphere, its vapour being primarily derived from these same oceans! but, in mr. lowell's opinion, all our meteorologists are quite mistaken. our oceans are our great drawbacks. only get rid of them and we should enjoy the exquisite climate of mars--with its absence of clouds and fog, of rain or rivers, and its delightful expanses of perennial deserts, varied towards the poles by a scanty snow-fall in winter, the melting of which might, with great care, supply us with the necessary moisture to grow wheat and cabbages for about one-tenth, or more likely one-hundredth, of our present population. i hope i may be excused for not treating such an argument seriously. the various considerations now advanced, especially those which show the enormous cumulative and conservative effect of our dense and water-laden atmosphere, and the disastrous effect--judging by the actual condition of the moon--which the loss of it would have upon our temperature, seem to me quite sufficient to demonstrate important errors in the data or fallacies in the complex mathematical argument by which mr. lowell has attempted to uphold his views as to the temperature and consequent climatic conditions of mars. in concluding this portion of my discussion of the problem of mars, i wish to call attention to the fact that my argument, founded upon a comparison of the physical conditions of the earth and moon with those of mars, is dependent upon a small number of generally admitted scientific facts; while the conclusions drawn from those facts are simple and direct, requiring no mathematical knowledge to follow them, or to appreciate their weight and cogency. i claim for them, therefore, that they are in no degree speculative, but in their data and methods exclusively scientific. in the next chapter i will put forward a suggestion as to how the very curious markings upon the surface of mars may possibly be interpreted, so as to be in harmony with the planet's actual physical condition and its not improbable origin and past history. chapter vii. a suggestion as to the 'canals' of mars. the special characteristics of the numerous lines which intersect the whole of the equatorial and temperate regions of mars are, their straightness combined with their enormous length. it is this which has led mr. lowell to term them 'non-natural features.' schiaparelli, in his earlier drawings, showed them curved and of comparatively great width. later, he found them to be straight fine lines when seen under the best conditions, just as mr. lowell has always seen them in the pure atmosphere of his observatory. both of these observers were at first doubtful of their reality, but persistent observation continued at many successive oppositions compelled acceptance of them as actual features of the planet's disc. so many other observers have now seen them that the objection of unreality seems no longer valid. mr. lowell urges, however, that their perfect straightness, their extreme tenuity, their uniformity throughout their whole length, the dual character of many of them, their relation to the 'oases' and the form and position of these round black spots, are all proofs of artificiality and are suggestive of design. and considering that some of them are actually as long as from boston to san francisco, and relatively to their globe as long as from london to bombay, his objection that "no natural phenomena within our knowledge show such regularity on such a scale" seems, at first, a mighty one. it is certainly true that we can point to nothing exactly like them either on the earth or on the moon, and these are the only two planetary bodies we are in a position to compare with mars. yet even these do, i think, afford us some hints towards an interpretation of the mysterious lines. but as our knowledge of the internal structure and past history even of our earth is still imperfect, that of the moon only conjectural, and that of mars a perfect blank, it is not perhaps surprising that the surface-features of the latter do not correspond with those of either of the others. _mr. pickering's suggestion._ the best clue to a natural interpretation of the strange features of the surface of mars is that suggested by the american astronomer mr. w.h. pickering in _popular astronomy_ ( ). briefly it is, that both the 'canals' of mars and the rifts as well as the luminous streaks on the moon are cracks in the volcanic crust, caused by internal stresses due to the action of the heated interior. these cracks he considers to be symmetrically arranged with regard to small 'craterlets' (mr. lowell's 'oases') because they have originated from them, just as the white streaks on the moon radiate from the larger craters as centres. he further supposes that water and carbon-dioxide issue from the interior into these fissures, and, in conjunction with sunlight, promote the growth of vegetation. owing to the very rare atmosphere, the vapours, he thinks, would not ascend but would roll down the outsides of the craterlets and along the borders of the canals, thus irrigating the immediate vicinity and serving to promote the growth of some form of vegetation which renders the canals and oases visible.[ ] [footnote : _nature_, vol. , p. .] this opinion is especially important because, next to mr. lowell, mr. pickering is perhaps the astronomer who has given most attention to mars during the last fifteen years. he was for some time at flagstaff with mr. lowell, and it was he who discovered the oases or craterlets, and who originated the idea that we did not see the 'canals' themselves but only the vegetable growth on their borders. he also observed mars in the southern hemisphere at arequipa; and he has since made an elaborate study of the moon by means of a specially constructed telescope of feet focal length, which produced a direct image on photographic plates nearly inches in diameter.[ ] [footnote : _nature_, vol. , may , p.xi, supplement.] it is clear therefore that mr. lowell's views as to the artificial nature of the 'canals' of mars are not accepted by an astronomer of equal knowledge and still wider experience. yet professor pickering's alternative view is more a suggestion than an explanation, because there is no attempt to account for the enormous length and perfect straightness of the lines on mars, so different from anything that is found either on our earth or on the moon. there must evidently be some great peculiarity of structure or of conditions on mars to account for these features, and i shall now attempt to point out what this peculiarity is and how it may have arisen. _the meteoritic hypothesis._ during the last quarter of a century a considerable change has come over the opinions of astronomers as regards the probable origin of the solar system. the large amount of knowledge of the stellar universe, and especially of nebulae, of comets and of meteor-streams which we now possess, together with many other phenomena, such as the constitution of saturn's rings, the great number and extent of the minor planets, and generally of the vast amount of matter in the form of meteor-rings and meteoric dust in and around our system, have all pointed to a different origin for the planets and their satellites than that formulated by laplace as the nebular hypothesis. it is now seen more clearly than at any earlier period, that most of the planets possess special characteristics which distinguish them from one another, and that such an origin as laplace suggested--the slow cooling and contraction of one vast sun-mist or nebula, besides presenting inherent difficulties--many think them impossibilities--in itself does not afford an adequate explanation of these peculiarities. hence has arisen what is termed the meteoritic theory, which has been ably advocated for many years by sir norman lockyer, and with some unimportant modifications is now becoming widely accepted. briefly, this theory is, that the planets have been formed by the slow aggregation of solid particles around centres of greatest condensation; but as many of my readers may be altogether unacquainted with it, i will here give a very clear statement of what it is, from professor j.w. gregory's presidential address to the geological section of the british association of the present year. he began by saying that these modern views were of far more practical use to men of science than that of laplace, and that they give us a history of the world consistent with the actual records of geology. he then continues: "according to sir norman lockyer's meteoritic hypothesis, nebulae, comets, and many so-called stars consist of swarms of meteorites which, though normally cold and dark, are heated by repeated collisions, and so become luminous. they may even be volatilised into glowing meteoric vapour; but in time this heat is dissipated, and the force of gravity condenses a meteoritic swarm into a single globe. 'some of the swarms are,' says lockyer, 'truly members of the solar system,' and some of these travel round the sun in nearly circular orbits, like planets. they may be regarded as infinitesimal planets, and so chamberlain calls them 'planetismals.' "the planetismal theory is a development of the meteoritic theory, and presents it in an especially attractive guise. it regards meteorites as very sparsely distributed through space, and gravity as powerless to collect them into dense groups. so it assigns the parentage of the solar system to a spiral nebula composed of planetismals, and the planets as formed from knots in the nebula, where many planetismals had been concentrated near the intersections of their orbits. these groups of meteorites, already as dense as a swarm of bees, were then packed closer by the influence of gravity, and the contracting mass was heated by the pressure, even above the normal melting-point of the material, which was kept rigid by the weight of the overlying layers." now, adopting this theory as the last word of science upon the subject of the origin of planets, we see that it affords immense scope for diversity in results depending on the total _amount_ of matter available within the range of attraction of an incipient planetary mass, and the _rates_ at which this matter becomes available. by a special combination of these two quantities (which have almost certainly been different for each planet) i think we may be able to throw some light upon the structure and physical features of mars. _the probable mode of origin of mars._ this planet, lying between two of much greater mass, has evidently had less material from which to be formed by aggregation; and if we assume--as in the absence of evidence to the contrary we have a right to do--that its beginnings were not much later (or earlier) than those of the earth, then its smaller size shows that it has in all probability aggregated very much more slowly. but the internal heat acquired by a planet while forming in this manner will depend upon the rate at which it aggregates and the velocity with which the planetismals' fall into it, and this velocity will increase with its mass and consequent force of gravity. in the early stages of a planet's growth it will probably remain cold, the small amount of heat produced by each impact being lost by radiation before the next one occurs; and with a small and slowly aggregating planet this condition will prevail till it approaches its full size. then only will its gravitative force be sufficient to cause incoming matter to fall upon it with so powerful an impact as to produce intense heat. further, the compressive force of a small planet will be a less effective heat-producing agency than in the case of a larger one. the earth we know has acquired a large amount of internal heat, probably sufficient to liquefy its whole interior; but mars has only one-ninth part the mass of the earth, and it is quite possible, and even probable, that its comparatively small attractive force would never have liquefied or even permanently heated the more central portions of its mass. this being admitted, i suggest the following course of events as quite possible, and not even improbable, in the case of this planet. during the whole of its early growth, and till it acquired nearly its present diameter, its rate of aggregation was so slow that the planetismals falling upon it, though they might have been heated and even partially liquefied by the impact, were never in such quantity as to produce any considerable heating effect on the whole mass, and each local rise of temperature was soon lost by radiation. the planet thus grew as a solid and cold mass, compacted together by the impact of the incoming matter as well as by its slowly increasing gravitative force. but when it had attained to within perhaps , perhaps miles, or less, of its present diameter, a great change occurred in the opportunity for further growth. some large and dense swarm of meteorites, perhaps containing a number of bodies of the size of the asteroids, came within the range of the sun's attraction and were drawn by it into an orbit which crossed that of mars at such a small angle that the planet was able at each revolution to capture a considerable number of them. the result might then be that, as in the case of the earth, the continuous inpour of the fresh matter first heated, and later on liquefied the greater part of it as well perhaps as a thin layer of the planet's original surface; so that when in due course the whole of the meteor-swarm had been captured, mars had acquired its present mass, but would consist of an intensely heated, and either liquid or plastic thin outer shell resting upon a cold and solid interior. the size and position of the two recently discovered satellites of mars, which are believed to be not more than ten miles in diameter, the more remote revolving around its primary very little slower than the planet rotates, while the nearer one, which is considerably less distant from the planet's surface than its own antipodes and revolves around it more than three times during the martian day, may perhaps be looked upon as the remnants of the great meteor-swarm which completed the martian development, and which are perhaps themselves destined at some distant period to fall into the planet. should future astronomers witness the phenomenon the effect produced upon its surface would be full of instruction. as the result of such an origin as that suggested, mars would possess a structure which, in the essential feature of heat-distribution, would be the very opposite of that which is believed to characterise the earth, yet it might have been produced by a very slight modification of the same process. this peculiar heat-distribution, together with a much smaller mass and gravitative force, would lead to a very different development of the surface and an altogether diverse geological history from ours, which has throughout been profoundly influenced by its heated interior, its vast supply of water, and the continuous physical and chemical reactions between the interior and the crust. these reactions have, in our case, been of substantially the same nature, and very nearly of the same degree of intensity throughout the whole vast eons of geological time, and they have resulted in a wonderfully complex succession of rock-formations--volcanic, plutonic, and sedimentary--more or less intermingled throughout the whole series, here remaining horizontal as when first deposited, there upheaved or depressed, fractured or crushed, inclined or contorted; denuded by rain and rivers with the assistance of heat and cold, of frost and ice, in an unceasing series of changes, so that however varied the surface may be, with hill and dale, plains and uplands, mountain ranges and deep intervening valleys, these are as nothing to the diversities of interior structure, as exhibited in the sides of every alpine valley or precipitous escarpment, and made known to us by the work of the miner and the well-borer in every part of the world. _structural straight lines on the earth._ the great characteristic of the earth, both on its surface and in its interior, is thus seen to be extreme diversity both of form and structure, and this is further intensified by the varied texture, constitution, hardness, and density of the various rocks and debris of which it is composed. it is therefore not surprising that, with such a complex outer crust, we should nowhere find examples of those geometrical forms and almost world-wide straight lines that give such a remarkable, and as mr. lowell maintains, 'non-natural' character to the surface of mars, but which, as it seems to me, of themselves afford _prima facie_ evidence of a corresponding simplicity and uniformity in its internal structure. yet we are not ourselves by any means devoid of 'straight lines' structurally produced, in spite of every obstacle of diversity of form and texture, of softness and hardness, of lamination or crystallisation, which are adverse to such developments. examples of these are the numerous 'faults' which occur in the harder rocks, and which often extend for great distances in almost perfect straight lines. in our own country we have the tyneside and craven faults in the north of england, which are miles long and often yards wide; but even more striking is the great cleveland dyke--a wall of volcanic rock dipping slightly towards the south, but sometimes being almost vertical, and stretching across the country, over hill and dale, in an almost perfect straight line from a point on the coast ten miles north of scarborough, in a west-by-north direction, passing about two miles south of stockton and terminating about six miles north-by-east of barnard castle, a distance of very nearly miles. the great fault between the highlands and lowlands of scotland extends across the country from stonehaven to near helensburgh, a distance of miles; and there are very many more of less importance. much more extensive are some of the great continental dislocations, often forming valleys of considerable width and length. the upper rhine flows in one of these great valleys of subsidence for about miles, from mulhausen to frankfort, in a generally straight line, though modified by denudation. vaster still is the valley of the jordan through the sea of galilee to the dead sea, continued by the wady arabah to the gulf of akaba, believed to form one vast geological depression or fracture extending in a straight line for miles. thousands of such faults, dykes, or depressions exist in every part of the world, all believed to be due to the gradual shrinking of the heated interior to which the solid crust has to accommodate itself, and they are especially interesting and instructive for our present purpose as showing the tendency of such fractures of solid rock-material to extend to great lengths in straight lines, notwithstanding the extreme irregularity both in the surface contour as well as in the internal structures of the varied deposits and formations through which they pass. _probable origin of the surface-features of mars._ returning now to mars, let us consider the probable course of events from the point at which we left it. the heat produced by impact and condensation would be likely to release gases which had been in combination with some of the solid matter, or perhaps been itself in a solid state due to intense cold, and these, escaping outwards to the surface, would produce on a small scale a certain amount of upheaval and volcanic disturbance; and as an outer crust rapidly formed, a number of vents might remain as craters or craterlets in a moderate state of activity. owing to the comparatively small force of gravity, the outer crust would become scoriaceous and more or less permeated by the gases, which would continue to escape through it, and this would facilitate the cooling of the whole of the heated outer crust, and allow it to become rather densely compacted. when the greater portion of the gases had thus escaped to the outer surface and assisted to form a scanty atmosphere, such as now exists, there would be no more internal disturbance and the cooling of the heated outer coating would steadily progress, resulting at last in a slightly heated, and later in a cold layer of moderate thickness and great general uniformity. owing to the absence of rain and rivers, denudation such as we experience would be unknown, though the superficial scoriaceous crust might be partially broken up by expansion and contraction, and suffer a certain amount of atmospheric erosion. the final result of this mode of aggregation would be, that the planet would consist of an outer layer of moderate thickness as compared with the central mass, which outer layer would have cooled from a highly heated state to a temperature considerably below the freezing-point, and this would have been all the time _contracting upon a previously cold, and therefore non-contracting nucleus._ the result would be that very early in the process great superficial tensions would be produced, which could only be relieved by cracks or fissures, which would initiate at points of weakness--probably at the craterlets already referred to--from which they would radiate in several directions. each crack thus formed near the surface would, as cooling progressed, develop in length and depth; and owing to the general uniformity of the material, and possibly some amount of crystalline structure due to slow and continuous cooling down to a very low temperature, the cracks would tend to run on in straight lines and to extend vertically downwards, which two circumstances would necessarily result in their forming portions of 'great circles' on the planet's surface--the two great facts which mr. lowell appeals to as being especially 'non-natural.' _symmetry of basaltic columns._ we have however one quite natural fact on our earth which serves to illustrate one of these two features, the direction of the downward fissure. this is, the comparatively common phenomenon of basaltic columns and 'giant's causeways.' the wonderful regularity of these, and especially the not unfrequent upright pillars in serried ranks, as in the palisades of the hudson river, must have always impressed observers with their appearance of artificiality. yet they are undoubtedly the result of the very slow cooling and contraction of melted rocks under compression by strata _below and above them_, so that, when once solidified, the mass was held in position and the tension produced by contraction could only be relieved by numerous very small cracks at short distances from each other in every direction, resulting in five, six, or seven-sided polygons, with sides only a few inches long. this contraction began of course at the coolest surface, generally the upper one; and observation of these columns in various positions has established the rule that their direction lengthways _is always at right angles to the cooling surface_, and thus, whenever this surface was horizontal, the columns became almost exactly vertical. _how this applies to mars._ one of the features of the surface of mars that mr. lowell describes with much confidence is, that it is wonderfully uniform and level, which of course it would be if it had once been in a liquid or plastic state, and not much disturbed since by volcanic or other internal movements. the result would be that cracks formed by contraction of the hardened outer crust would be vertical; and, in a generally uniform material at a very uniform temperature, these cracks would continue almost indefinitely in straight lines. the hardened and contracting surface being free to move laterally on account of there being a more heated and plastic layer below it, the cracks once initiated above would continually widen at the surface as they penetrated deeper and deeper into the slightly heated substratum. now, as basalt begins to soften at about ° f. and the surface of mars has cooled to at least the freezing-point--perhaps very much below it--the contraction would be so great that if the fissures produced were miles apart they might be three miles wide at the surface, and, if only miles apart, then about two-thirds of a mile wide.[ ] but as the production of the fissures might have occupied perhaps millions of years, a considerable amount of atmospheric denudation would result, however slowly it acted. expansion and contraction would wear away the edges and sides of the fissures, fill up many of them with the debris, and widen them at the surfaces to perhaps double their original size.[ ] [footnote : the coefficient of contraction of basalt is . for ° f., which would lead to the results given here.] [footnote : mr. w.h. pickering observed clouds on mars miles high; these are the 'projections' seen on the terminator when the planet is partially illuminated. they were at first thought to be mountains; but during the opposition of , more than of them were seen at flagstaff during nine months' observation. usually they are of rare occurrence. they are seen to change in form and position from day to day, and mr. lowell is strongly of opinion that they are dust-storms, not what we term clouds. they were mostly about miles high, indicating considerable aerial disturbance on the planet, and therefore capable of producing proportional surface denudation.] _suggested explanation of the 'oases.'_ the numerous round dots seen upon the 'canals,' and especially at points from which several canals radiate and where they intersect--termed 'oases' by mr. lowell and 'craterlets' by mr. pickering may be explained in two ways. those from which several canals radiate may be true craters from which the gases imprisoned in the heated surface layers have gradually escaped. they would be situated at points of weakness in the crust, and become centres from which cracks would start during contraction. those dots which occur at the crossing of two straight canals or cracks may have originated from the fact that at such intersections there would be four sharply-projecting angles, which, being exposed to the influence of alternate heat and cold (during day and night) on the two opposite surfaces, would inevitably in time become fractured and crumbled away, resulting in the formation of a roughly circular chasm which would become partly filled up by the debris. those formed by cracks radiating from craterlets would also be subject to the same process of rounding off to an even greater extent; and thus would be produced the 'oases' of various sizes up to miles or more in diameter recorded by mr. lowell and other observers. _probable function of the great fissures._ mr. pickering, as we have seen, supposes that these fissures give out the gases which, overflowing on each side, favour the growth of the supposed vegetation which renders the course of the canals visible, and this no doubt may have been the case during the remote periods when these cracks gave access to the heated portions of the surface layer. but it seems more probable that mars has now cooled down to the almost uniform mean temperature it derives from solar heat, and that the fissures--now for the most part broad shallow valleys--serve merely as channels along which the liquids and heavy gases derived from the melting of the polar snows naturally flow, and, owing to their nearly level surfaces, overflow to a certain distance on each side of them. _suggested origin of the blue patches._ these heavy gases, mainly perhaps, as has been often suggested, carbon-dioxide, would, when in large quantity and of considerable depth, reflect a good deal of light, and, being almost inevitably dust-laden, might produce that blue tinge adjacent to the melting snow-caps which mr. lowell has erroneously assumed to be itself a proof of the presence of liquid water. just as the blue of our sky is undoubtedly due to reflection from the ultra-minute dust particles in our higher atmosphere, similar particles brought down by the 'snow' from the higher martian atmosphere might produce the blue tinge in the great volumes of heavy gas produced by its evaporation or liquefaction. it may be noted that mr. lowell objects to the carbon-dioxide theory of the formation of the snow-caps, that this gas at low pressures does not liquefy, but passes at once from the solid to the gaseous state, and that only water remains liquid sufficiently long to produce the blue colour' which plays so large a part in his argument for the mild climate essential for an inhabited planet. but this argument, as i have already shown, is valueless. for only very deep water can possibly show a blue colour by reflected light, while a dust-laden atmosphere--especially with a layer of very dense gas at the bottom of it, as would be the case with the newly evaporated carbon-dioxide from the diminishing snow-cap --would provide the very conditions likely to produce this blue tinge of colour. it may be considered a support to this view that carbonic-acid gas becomes liquid at-- ° f. and solid at-- ° f., temperatures far higher than we should expect to prevail in the polar and north temperate regions of mars during a considerable part of the year, but such as might be reached there during the summer solstice when the `snows' so rapidly disappear, to be re-formed a few months later. _the double canals._ the curious phenomena of the 'double canals' are undoubtedly the most difficult to explain satisfactorily on any theory that has yet been suggested. they vary in distance apart from about to miles. in many cases they appear perfectly parallel, and mr. lowell gives us the impression that they are almost always so. but his maps show, in some cases, decided differences of width at the two extremities, indicating considerable want of parallelism. a few of the curved canals are also double. there is one drawing in mr. lowell's book (p. ) of the mouths, or starting points, of the euphrates and phison, two widely separated double canals diverging at an angle of about ° from the same two oases, so that the two inner canals cross each other. now this suggests two wide bands of weakness in the planet's crust radiating probably from within the dark tract called the 'mare icarium,' and that some widespread volcanic outburst initiated diverging cracks on either side of these bands. something of this kind may have been the cause of most of the double canals, or they may have been started from two or more craterlets not far apart, the direction being at first decided by some local peculiarity of structure; and where begun continuing in straight lines owing to homogeneity or uniform density of material. this is very vague, but the phenomena are so remarkable, and so very imperfectly known at present, that nothing but suggestion can be attempted. _concluding remarks on the 'canals.'_ in this somewhat detailed exposition of a possible, and, i hope, a probable explanation of the surface-features of mars, i have endeavoured to be guided by known facts or accepted theories both astronomical and geological. i think i may claim to have shown that there are some analogous features of terrestrial rock-structure to serve as guides towards a natural and intelligible explanation of the strange geometric markings discovered during the last thirty years, and which have raised this planet from comparative obscurity into a position of the very first rank both in astronomical and popular interest. this wide-spread interest is very largely due to mr. lowell's devotion to its study, both in seeking out so admirable a position as regards altitude and climate, and in establishing there a first-class observatory; and also in bringing his discoveries before the public in connection with a theory so startling as to compel attention. i venture to think that his merit as one of our first astronomical observers will in no way be diminished by the rejection of his theory, and the substitution of one more in accordance with the actually observed facts. appendix. _a suggested experiment to illustrate the 'canals' of mars._ if my explanation of the 'canals' should be substantially correct--that is, if they were produced by the contraction of a heated outward crust upon a cold, and therefore non-contracting interior, the result of such a condition might be shown experimentally. several baked clay balls might be formed to serve as cores, say of to inches in diameter. these being fixed within moulds of say half an inch to an inch greater diameter, the outer layer would be formed by pouring in some suitable heated liquid material, and releasing it from the mould as soon as consolidation occurs, so that it may cool rapidly from the _outside._ some kinds of impure glass, or the brittle metals bismuth or antimony or alloys of these might be used, in order to see what form the resulting fractures would take. it would be well to have several duplicates of each ball, and, as soon as tension through contraction manifests itself, to try the effect of firing very small charges of small shot to ascertain whether such impacts would start radiating fractures. when taken from the moulds, the balls should be suspended in a slight current of air, and kept rotating, to reproduce the planetary condition as nearly as possible. the exact size and material of the cores, the thickness of the heated outer crust, the material best suited to show fracture by contraction, and the details of their treatment, might be modified in various ways as suggested by the results first obtained. such a series of experiments would probably throw further light on the physical conditions which have produced the gigantic system of fissures or channels we see upon the surface of mars, though it would not, of course, prove that such conditions actually existed there. in such a speculative matter we can only be guided by probabilities, based upon whatever evidence is available. chapter viii. summary and conclusion. this little volume has necessarily touched upon a great variety of subjects, in order to deal in a tolerably complete manner with the very extraordinary theories by which mr. lowell attempts to explain the unique features of the surface of the planet, which, by long-continued study, he has almost made his own. it may therefore be well to sum up the main points of the arguments against his view, introducing a few other facts and considerations which greatly strengthen my argument. the one great feature of mars which led mr. lowell to adopt the view of its being inhabited by a race of highly intelligent beings, and, with ever-increasing discovery to uphold this theory to the present time, is undoubtedly that of the so-called 'canals'--their straightness, their enormous length, their great abundance, and their extension over the planet's whole surface from one polar snow-cap to the other. the very immensity of this system, and its constant growth and extension during fifteen years of persistent observation, have so completely taken possession of his mind, that, after a very hasty glance at analogous facts and possibilities, he has declared them to be 'non-natural'-- therefore to be works of art--therefore to necessitate the presence of highly intelligent beings who have designed and constructed them. this idea has coloured or governed all his writings on the subject. the innumerable difficulties which it raises have been either ignored, or brushed aside on the flimsiest evidence. as examples, he never even discusses the totally inadequate water-supply for such worldwide irrigation, or the extreme irrationality of constructing so vast a canal-system the waste from which, by evaporation, when exposed to such desert conditions as he himself describes, would use up ten times the probable supply. again, he urges the 'purpose' displayed in these 'canals.' their being _all_ so straight, _all_ describing great circles of the 'sphere,' all being so evidently arranged (as he thinks) either to carry water to some 'oasis' miles away, or to reach some arid region far over the equator in the opposite hemisphere! but he never considers the difficulties this implies. everywhere these canals run for thousands of miles across waterless deserts, forming a system and indicating a purpose, the wonderful perfection of which he is never tired of dwelling upon (but which i myself can nowhere perceive). yet he never even attempts to explain how the martians could have lived _before_ this great system was planned and executed, or why they did not _first_ utilise and render fertile the belt of land adjacent to the limits of the polar snows--why the method of irrigation did not, as with all human arts, begin gradually, at home, with terraces and channels to irrigate the land close to the source of the water. how, with such a desert as he describes three-fourths of mars to be, did the inhabitants ever get to _know_ anything of the equatorial regions and its needs, so as to start right away to supply those needs? all this, to my mind, is quite opposed to the idea of their being works of art, and altogether in favour of their being natural features of a globe as peculiar in origin and internal structure as it is in its surface-features. the explanation i have given, though of course hypothetical, is founded on known cosmical and terrestrial facts, and is, i suggest, far more scientific as well as more satisfactory than mr. lowell's wholly unsupported speculation. this view i have explained in some detail in the preceding chapter. mr. lowell never even refers to the important question of loss by evaporation in these enormous open canals, or considers the undoubted fact that the only intelligent and practical way to convey a limited quantity of water such great distances would be by a system of water-tight and air-tight tubes laid _under the ground._ the mere attempt to use open canals for such a purpose shows complete ignorance and stupidity in these alleged very superior beings; while it is certain that, long before half of them were completed their failure to be of any use would have led any rational beings to cease constructing them. he also fails to consider the difficulty, that, if these canals are necessary for existence in mars, how did the inhabitants ever reach a sufficiently large population with surplus food and leisure enabling them to rise from the low condition of savages to one of civilisation, and ultimately to scientific knowledge? here again is a dilemma which is hard to overcome. only a _dense_ population with _ample_ means of subsistence could possibly have constructed such gigantic works; but, given these two conditions, no adequate motive existed for the conception and execution of them--even if they were likely to be of any use, which i have shown they could not be. _further considerations on the climate of mars._ recurring now to the question of climate, which is all-important, mr. lowell never even discusses the essential point--the temperature that must _necessarily_ result from an atmospheric envelope one-twelfth (or at most one-seventh) the density of our own; in either case corresponding to an altitude far greater than that of our highest mountains.[ ] surely this phenomenon, everywhere manifested on the earth even under the equator, of a regular decrease of temperature with altitude, the only cause of which is a less dense atmosphere, should have been fairly grappled with, and some attempt made to show why it should not apply to mars, except the weak remark that on a level surface it will not have the same effect as on exposed mountain heights. but it _does_ have the same effect, or very nearly so, on our lofty plateaux often hundreds of miles in extent, in proportion to their altitude. quito, at ft. above the sea, has a mean temperature of about ° f., giving a lowering of ° from that of manaos at the mouth of the rio negro. this is about a degree for each feet, while the general fall for isolated mountains is about one degree in feet according to humboldt, who notes the above difference between the rate of cooling for altitude of the plains--or more usually sheltered valleys in which the towns are situated--and the exposed mountain sides. it will be seen that this lower rate would bring the temperature of mars at the equator down to ° f. below the freezing point of water from this cause alone. [footnote : a four inches barometer is equivalent to a height of , feet above sea-level with us.] but all enquirers have admitted, that if conditions as to atmosphere were the same as on the earth, its greater distance from the sun would reduce the temperature to- ° f., equal to ° below the freezing point. it is therefore certain that the combined effect of both causes must bring the temperature of mars down to at least ° or °below the freezing point. the cause of this absolute dependence of terrestrial temperatures upon density of the air-envelope is seldom discussed in text-books either of geography or of physics, and there seems to be still some uncertainty about it. some impute it wholly to the thinner air being unable to absorb and retain so much heat as that which is more dense; but if this were the case the soil at great altitudes not having so much of its heat taken up by the air should be warmer than below, since it undoubtedly _receives_ more heat owing to the greater transparency of the air above it; but it certainly does not become warmer. the more correct view seems to be that the loss of heat by radiation is increased so much through the rarity of the air above it as to _more_ than counterbalance the increased insolation, so that though the surface of the earth at a given altitude may receive per cent. more direct sun-heat it loses by direct radiation, combined with diminished air and cloud-radiation, perhaps or per cent. more, whence there is a resultant cooling effect of or per cent. this acts by day as well as by night, so that the greater heat received at high altitudes does not warm the soil so much as a less amount of heat with a denser atmosphere. this effect is further intensified by the fact that a less dense cannot absorb and transmit so much heat as a more dense atmosphere. here then we have an absolute law of nature to be observed operating everywhere on the earth, and the mode of action of which is fairly well understood. this law is, that reduced atmospheric pressure increases radiation, or loss of heat, _more rapidly_ than it increases insolation or gain of heat, so that the result is _always_ a considerable _lowering_ of temperature. what this lowering is can be seen in the universal fact, that even within the tropics perpetual snow covers the higher mountain summits, while on the high plains of the andes, at , or , feet altitude, where there is very little or no snow, travellers are often frozen to death when delayed by storms; yet at this elevation the atmosphere has much more than double the density of that of mars! the error in mr. lowell's argument is, that he claims for the scanty atmosphere of mars that it allows more sun-heat to reach the surface; but he omits to take account of the enormously increased loss of heat by direct radiation, as well as by the diminution of air-radiation, which together necessarily produce a great reduction of temperature. it is this great principle of the prepotency of radiation over absorption with a diminishing atmosphere that explains the excessively low temperature of the moon's surface, a fact which also serves to indicate a very low temperature for mars, as i have shown in chapter vi. these two independent arguments--from alpine temperatures and from those of the moon--support and enforce each other, and afford a conclusive proof (as against anything advanced by mr. lowell) that the temperature of mars must be far too low to support animal life. a third independent argument leading to the same result is dr. johnstone stoney's proof that aqueous vapour cannot exist on mars; and this fact mr. lowell does not attempt to controvert. to put the whole case in the fewest possible words: all physicists are agreed that, owing to the distance of mars from the sun, it would have a mean temperature of about- ° f. (= ° f. abs.) even if it had an atmosphere as dense as ours. ( ) but the very low temperatures on the earth under the equator, at a height where the barometer stands at about three times as high as on mars, proves, that from scantiness of atmosphere alone mars cannot possibly have a temperature as high as the freezing point of water; and this proof is supported by langley's determination of the low _maximum_ temperature of the full moon. the combination of these two results must bring down the temperature of mars to a degree wholly incompatible with the existence of animal life. ( ) the quite independent proof that water-vapour cannot exist on mars, and that therefore, the first essential of organic life--water--is non-existent. the conclusion from these three independent proofs, which enforce each other in the multiple ratio of their respective weights, is therefore irresistible--that animal life, especially in its higher forms, cannot exist on the planet. mars, therefore, is not only uninhabited by intelligent beings such as mr. lowell postulates, but is absolutely uninhabitable. [illustration: kepler] pioneers of progress men of science edited by s. chapman, m.a., d.sc., f.r.s. kepler by walter w. bryant of the royal observatory, greenwich contents. i. astronomy before kepler ii. early life of kepler iii. tycho brahe iv. kepler joins tycho v. kepler's laws vi. closing years appendix i.--list of dates appendix ii.--bibliography glossary chapter i. astronomy before kepler. in order to emphasise the importance of the reforms introduced into astronomy by kepler, it will be well to sketch briefly the history of the theories which he had to overthrow. in very early times it must have been realised that the sun and moon were continually changing their places among the stars. the day, the month, and the year were obvious divisions of time, and longer periods were suggested by the tabulation of eclipses. we can imagine the respect accorded to the chaldaean sages who first discovered that eclipses could be predicted, and how the philosophers of mesopotamia must have sought eagerly for evidence of fresh periodic laws. certain of the stars, which appeared to wander, and were hence called planets, provided an extended field for these speculations. among the chaldaeans and babylonians the knowledge gradually acquired was probably confined to the priests and utilised mainly for astrological prediction or the fixing of religious observances. such speculations as were current among them, and also among the egyptians and others who came to share their knowledge, were almost entirely devoted to mythology, assigning fanciful terrestrial origins to constellations, with occasional controversies as to how the earth is supported in space. the greeks, too, had an elaborate mythology largely adapted from their neighbours, but they were not satisfied with this, and made persistent attempts to reduce the apparent motions of celestial objects to geometrical laws. some of the pythagoreans, if not pythagoras himself, held that the earth is a sphere, and that the apparent daily revolution of the sun and stars is really due to a motion of the earth, though at first this motion of the earth was not supposed to be one of rotation about an axis. these notions, and also that the planets on the whole move round from west to east with reference to the stars, were made known to a larger circle through the writings of plato. to plato moreover is attributed the challenge to astronomers to represent all the motions of the heavenly bodies by uniformly described circles, a challenge generally held responsible for a vast amount of wasted effort, and the postponement, for many centuries, of real progress. eudoxus of cnidus, endeavouring to account for the fact that the planets, during every apparent revolution round the earth, come to rest twice, and in the shorter interval between these "stationary points," move in the opposite direction, found that he could represent the phenomena fairly well by a system of concentric spheres, each rotating with its own velocity, and carrying its own particular planet round its own equator, the outermost sphere carrying the fixed stars. it was necessary to assume that the axes about which the various spheres revolved should have circular motions also, and gradually an increased number of spheres was evolved, the total number required by aristotle reaching fifty-five. it may be regarded as counting in aristotle's favour that he did consider the earth to be a sphere and not a flat disc, but he seems to have thought that the mathematical spheres of eudoxus had a real solid existence, and that not only meteors, shooting stars and aurora, but also comets and the milky way belong to the atmosphere. his really great service to science in collating and criticising all that was known of natural science would have been greater if so much of the discussion had not been on the exact meaning of words used to describe phenomena, instead of on the facts and causes of the phenomena themselves. aristarchus of samos seems to have been the first to suggest that the planets revolved not about the earth but about the sun, but the idea seemed so improbable that it was hardly noticed, especially as aristarchus himself did not expand it into a treatise. about this time the necessity for more accurate places of the sun and moon, and the liberality of the ptolemys who ruled egypt, combined to provide regular observations at alexandria, so that, when hipparchus came upon the scene, there was a considerable amount of material for him to use. his discoveries marked a great advance in the science of astronomy. he noted the irregular motion of the sun, and, to explain it, assumed that it revolved uniformly not exactly about the earth but about a point some distance away, called the "excentric".[ ] the line joining the centre of the earth to the excentric passes through the apses of the sun's orbit, where its distance from the earth is greatest and least. the same result he could obtain by assuming that the sun moved round a small circle, whose centre described a larger circle about the earth; this larger circle carrying the other was called the "deferent": so that the actual motion of the sun was in an epicycle. of the two methods of expression hipparchus ultimately preferred the second. he applied the same process to the moon but found that he could depend upon its being right only at new and full moon. the irregularity at first and third quarters he left to be investigated by his successors. he also considered the planetary observations at his disposal insufficient and so gave up the attempt at a complete planetary theory. he made improved determinations of some of the elements of the motions of the sun and moon, and discovered the precession of the equinoxes, from the alexandrian observations which showed that each year as the sun came to cross the equator at the vernal equinox it did so at a point about fifty seconds of arc earlier on the ecliptic, thus producing in years an unmistakable change of a couple of degrees, or four times the sun's diameter. he also invented trigonometry. his star catalogue was due to the appearance of a new star which caused him to search for possible previous similar phenomena, and also to prepare for checking future ones. no advance was made in theoretical astronomy for years, the interval between hipparchus and ptolemy of alexandria. ptolemy accepted the spherical form of the earth but denied its rotation or any other movement. he made no advance on hipparchus in regard to the sun, though the lapse of time had largely increased the errors of the elements adopted by the latter. in the case of the moon, however, ptolemy traced the variable inequality noticed sometimes by hipparchus at first and last quarter, which vanished when the moon was in apogee or perigee. this he called the evection, and introduced another epicycle to represent it. in his planetary theory he found that the places given by his adopted excentric did not fit, being one way at apogee and the other at perigee; so that the centre of distance must be nearer the earth. he found it best to assume the centre of distance half-way between the centre of the earth and the excentric, thus "bisecting the excentricity". even this did not fit in the case of mercury, and in general the agreement between theory and observation was spoilt by the necessity of making all the orbital planes pass through the centre of the earth, instead of the sun, thus making a good accordance practically impossible. [footnote : see glossary for this and other technical terms.] after ptolemy's time very little was heard for many centuries of any fresh planetary theory, though advances in some points of detail were made, notably by some of the arab philosophers, who obtained improved values for some of the elements by using better instruments. from time to time various modifications of ptolemy's theory were suggested, but none of any real value. the moors in spain did their share of the work carried on by their eastern co-religionists, and the first independent star catalogue since the time of hipparchus was made by another oriental, tamerlane's grandson, ulugh begh, who built a fine observatory at samarcand in the fifteenth century. in spain the work was not monopolised by the moors, for in the thirteenth century alphonso of castile, with the assistance of jewish and christian computers, compiled the alphonsine tables, completed in , in which year he ascended the throne as alphonso x. they were long circulated in ms. and were first printed in , not long before the end of the period of stagnation. copernicus was born in at thorn in polish prussia. in the course of his studies at cracow and at several italian universities, he learnt all that was known of the ptolemaic astronomy and determined to reform it. his maternal uncle, the bishop of ermland, having provided him with a lay canonry in the cathedral of frauenburg, he had leisure to devote himself to science. reviewing the suggestions of the ancient greeks, he was struck by the simplification that would be introduced by reviving the idea that the annual motion should be attributed to the earth itself instead of having a separate annual epicycle for each planet and for the sun. of the seventy odd circles or epicycles required by the latest form of the ptolemaic system, copernicus succeeded in dispensing with rather more than half, but he still required thirty-four, which was the exact number assumed before the time of aristotle. his considerations were almost entirely mathematical, his only invasion into physics being in defence of the "moving earth" against the stock objection that if the earth moved, loose objects would fly off, and towers fall. he did not break sufficiently away from the old tradition of uniform circular motion. ptolemy's efforts at exactness were baulked, as we have seen, by the supposed necessity of all the orbit planes passing through the earth, and if copernicus had simply transferred this responsibility to the sun he would have done better. but he would not sacrifice the old fetish, and so, the orbit of the earth being clearly not circular with respect to the sun, he made all his planetary planes pass through the centre of the earth's orbit, instead of through the sun, thus handicapping himself in the same way though not in the same degree as ptolemy. his thirty-four circles or epicycles comprised four for the earth, three for the moon, seven for mercury (on account of his highly eccentric orbit) and five each for the other planets. it is rather an exaggeration to call the present accepted system the copernican system, as it is really due to kepler, half a century after the death of copernicus, but much credit is due to the latter for his successful attempt to provide a real alternative for the ptolemaic system, instead of tinkering with it. the old geocentric system once shaken, the way was gradually smoothed for the heliocentric system, which copernicus, still hampered by tradition, did not quite reach. he was hardly a practical astronomer in the observational sense. his first recorded observation, of an occultation of aldebaran, was made in , and he is not known to have made as many as fifty astronomical observations, while, of the few he did make and use, at least one was more than half a degree in error, which would have been intolerable to such an observer as hipparchus. copernicus in fact seems to have considered accurate observations unattainable with the instruments at hand. he refused to give any opinion on the projected reform of the calendar, on the ground that the motions of the sun and moon were not known with sufficient accuracy. it is possible that with better data he might have made much more progress. he was in no hurry to publish anything, perhaps on account of possible opposition. certainly luther, with his obstinate conviction of the verbal accuracy of the scriptures, rejected as mere folly the idea of a moving earth, and melanchthon thought such opinions should be prohibited, but rheticus, a professor at the protestant university of wittenberg and an enthusiastic pupil of copernicus, urged publication, and undertook to see the work through the press. this, however, he was unable to complete and another lutheran, osiander, to whom he entrusted it, wrote a preface, with the apparent intention of disarming opposition, in which he stated that the principles laid down were only abstract hypotheses convenient for purposes of calculation. this unauthorised interpolation may have had its share in postponing the prohibition of the book by the church of rome. according to copernicus the earth is only a planet like the others, and not even the biggest one, while the sun is the most important body in the system, and the stars probably too far away for any motion of the earth to affect their apparent places. the earth in fact is very small in comparison with the distance of the stars, as evidenced by the fact that an observer anywhere on the earth appears to be in the middle of the universe. he shows that the revolution of the earth will account for the seasons, and for the stationary points and retrograde motions of the planets. he corrects definitely the order of the planets outwards from the sun, a matter which had been in dispute. a notable defect is due to the idea that a body can only revolve about another body or a point, as if rigidly connected with it, so that, in order to keep the earth's axis in a constant direction in space, he has to invent a third motion. his discussion of precession, which he rightly attributes to a slow motion of the earth's axis, is marred by the idea that the precession is variable. with all its defects, partly due to reliance on bad observations, the work showed a great advance in the interpretation of the motions of the planets; and his determinations of the periods both in relation to the earth and to the stars were adopted by reinhold, professor of astronomy at wittenberg, for the new prutenic or prussian tables, which were to supersede the obsolete alphonsine tables of the thirteenth century. in comparison with the question of the motion of the earth, no other astronomical detail of the time seems to be of much consequence. comets, such as from time to time appeared, bright enough for naked eye observation, were still regarded as atmospheric phenomena, and their principal interest, as well as that of eclipses and planetary conjunctions, was in relation to astrology. reform, however, was obviously in the air. the doctrine of copernicus was destined very soon to divide others besides the lutheran leaders. the leaven of inquiry was working, and not long after the death of copernicus real advances were to come, first in the accuracy of observations, and, as a necessary result of these, in the planetary theory itself. chapter ii. early life of kepler. on st december, , at weil in the duchy of wurtemberg, was born a weak and sickly seven-months' child, to whom his parents henry and catherine kepler gave the name of john. henry kepler was a petty officer in the service of the reigning duke, and in joined the army serving in the netherlands. his wife followed him, leaving her young son in his grandfather's care at leonberg, where he barely recovered from a severe attack of smallpox. it was from this place that john derived the latinised name of leonmontanus, in accordance with the common practice of the time, but he was not known by it to any great extent. he was sent to school in , but in the following year his father returned to germany, almost ruined by the absconding of an acquaintance for whom he had become surety. henry kepler was obliged to sell his house and most of his belongings, and to keep a tavern at elmendingen, withdrawing his son from school to help him with the rough work. in young kepler was sent to the school at elmendingen, and in had another narrow escape from death by a violent illness. in he was sent, at the charges of the duke, to the monastic school of maulbronn; from whence, in accordance with the school regulations, he passed at the end of his first year the examination for the bachelor's degree at tübingen, returning for two more years as a "veteran" to maulbronn before being admitted as a resident student at tübingen. the three years thus spent at maulbronn were marked by recurrences of several of the diseases from which he had suffered in childhood, and also by family troubles at his home. his father went away after a quarrel with his wife catherine, and died abroad. catherine herself, who seems to have been of a very unamiable disposition, next quarrelled with her own relatives. it is not surprising therefore that kepler after taking his m.a. degree in august, , coming out second in the examination lists, was ready to accept the first appointment offered him, even if it should involve leaving home. this happened to be the lectureship in astronomy at gratz, the chief town in styria. kepler's knowledge of astronomy was limited to the compulsory school course, nor had he as yet any particular leaning towards the science; the post, moreover, was a meagre and unimportant one. on the other hand he had frequently expressed disgust at the way in which one after another of his companions had refused "foreign" appointments which had been arranged for them under the duke's scheme of education. his tutors also strongly urged him to accept the lectureship, and he had not the usual reluctance to leave home. he therefore proceeded to gratz, protesting that he did not thereby forfeit his claim to a more promising opening, when such should appear. his astronomical tutor, maestlin, encouraged him to devote himself to his newly adopted science, and the first result of this advice appeared before very long in kepler's "mysterium cosmographicum". the bent of his mind was towards philosophical speculation, to which he had been attracted in his youthful studies of scaliger's "exoteric exercises". he says he devoted much time "to the examination of the nature of heaven, of souls, of genii, of the elements, of the essence of fire, of the cause of fountains, the ebb and flow of the tides, the shape of the continents and inland seas, and things of this sort". following his tutor in his admiration for the copernican theory, he wrote an essay on the primary motion, attributing it to the rotation of the earth, and this not for the mathematical reasons brought forward by copernicus, but, as he himself says, on physical or metaphysical grounds. in , having more leisure from lectures, he turned his speculative mind to the number, size, and motion of the planetary orbits. he first tried simple numerical relations, but none of them appeared to be twice, thrice, or four times as great as another, although he felt convinced that there was some relation between the motions and the distances, seeing that when a gap appeared in one series, there was a corresponding gap in the other. these gaps he attempted to fill by hypothetical planets between mars and jupiter, and between mercury and venus, but this method also failed to provide the regular proportion which he sought, besides being open to the objection that on the same principle there might be many more equally invisible planets at either end of the series. he was nevertheless unwilling to adopt the opinion of rheticus that the number six was sacred, maintaining that the "sacredness" of the number was of much more recent date than the creation of the worlds, and could not therefore account for it. he next tried an ingenious idea, comparing the perpendiculars from different points of a quadrant of a circle on a tangent at its extremity. the greatest of these, the tangent, not being cut by the quadrant, he called the line of the sun, and associated with infinite force. the shortest, being the point at the other end of the quadrant, thus corresponded to the fixed stars or zero force; intermediate ones were to be found proportional to the "forces" of the six planets. after a great amount of unfinished trial calculations, which took nearly a whole summer, he convinced himself that success did not lie that way. in july, , while lecturing on the great planetary conjunctions, he drew quasi-triangles in a circular zodiac showing the slow progression of these points of conjunction at intervals of just over ° or eight signs. the successive chords marked out a smaller circle to which they were tangents, about half the diameter of the zodiacal circle as drawn, and kepler at once saw a similarity to the orbits of saturn and jupiter, the radius of the inscribed circle of an equilateral triangle being half that of the circumscribed circle. his natural sequence of ideas impelled him to try a square, in the hope that the circumscribed and inscribed circles might give him a similar "analogy" for the orbits of jupiter and mars. he next tried a pentagon and so on, but he soon noted that he would never reach the sun that way, nor would he find any such limitation as six, the number of "possibles" being obviously infinite. the actual planets moreover were not even six but only five, so far as he knew, so he next pondered the question of what sort of things these could be of which only five different figures were possible and suddenly thought of the five regular solids.[ ] he immediately pounced upon this idea and ultimately evolved the following scheme. "the earth is the sphere, the measure of all; round it describe a dodecahedron; the sphere including this will be mars. round mars describe a tetrahedron; the sphere including this will be jupiter. describe a cube round jupiter; the sphere including this will be saturn. now, inscribe in the earth an icosahedron, the sphere inscribed in it will be venus: inscribe an octahedron in venus: the circle inscribed in it will be mercury." with this result kepler was inordinately pleased, and regretted not a moment of the time spent in obtaining it, though to us this "mysterium cosmographicum" can only appear useless, even without the more recent additions to the known planets. he admitted that a certain thickness must be assigned to the intervening spheres to cover the greatest and least distances of the several planets from the sun, but even then some of the numbers obtained are not a very close fit for the corresponding planetary orbits. kepler's own suggested explanation of the discordances was that they must be due to erroneous measures of the planetary distances, and this, in those days of crude and infrequent observations, could not easily be disproved. he next thought of a variety of reasons why the five regular solids should occur in precisely the order given and in no other, diverging from this into a subtle and not very intelligible process of reasoning to account for the division of the zodiac into °. the next subject was more important, and dealt with the relation between the distances of the planets and their times of revolution round the sun. it was obvious that the period was not simply proportional to the distance, as the outer planets were all too slow for this, and he concluded "either that the moving intelligences of the planets are weakest in those that are farthest from the sun, or that there is one moving intelligence in the sun, the common centre, forcing them all round, but those most violently which are nearest, and that it languishes in some sort and grows weaker at the most distant, because of the remoteness and the attenuation of the virtue". this is not so near a guess at the theory of gravitation as might be supposed, for kepler imagined that a repulsive force was necessary to account for the planets being sometimes further from the sun, and so laid aside the idea of a constant attractive force. he made several other attempts to find a law connecting the distances and periods of the planets, but without success at that time, and only desisted when by unconsciously arguing in a circle he appeared to get the same result from two totally different hypotheses. he sent copies of his book to several leading astronomers, of whom galileo praised his ingenuity and good faith, while tycho brahe was evidently much struck with the work and advised him to adapt something similar to the tychonic system instead of the copernican. he also intimated that his uraniborg observations would provide more accurate determinations of the planetary orbits, and thus made kepler eager to visit him, a project which as we shall see was more than fulfilled. another copy of the book kepler sent to reymers the imperial astronomer with a most fulsome letter, which tycho, who asserted that reymers had simply plagiarised his work, very strongly resented, thus drawing from kepler a long letter of apology. about the same time kepler had married a lady already twice widowed, and become involved in difficulties with her relatives on financial grounds, and with the styrian authorities in connection with the religious disputes then coming to a head. on account of these latter he thought it expedient, the year after his marriage, to withdraw to hungary, from whence he sent short treatises to tübingen, "on the magnet" (following the ideas of gilbert of colchester), "on the cause of the obliquity of the ecliptic" and "on the divine wisdom as shown in the creation". his next important step makes it desirable to devote a chapter to a short notice of tycho brahe. [footnote : since the sum of the plane angles at a corner of a regular solid must be less than four right angles, it is easily seen that few regular solids are possible. hexagonal faces are clearly impossible, or any polygonal faces with more than five sides. the possible forms are the dodecahedron with twelve pentagonal faces, three meeting at each corner; the cube, six square faces, three meeting at each corner; and three figures with triangular faces, the tetrahedron of four faces, three meeting at each corner; the octahedron of eight faces, four meeting at each corner; and the icosahedron of twenty faces, five meeting at each corner.] chapter iii. tycho brahe. the age following that of copernicus produced three outstanding figures associated with the science of astronomy, then reaching the close of what professor forbes so aptly styles the geometrical period. these three sir david brewster has termed "martyrs of science"; galileo, the great italian philosopher, has his own place among the "pioneers of science"; and invaluable though tycho brahe's work was, the latter can hardly be claimed as a pioneer in the same sense as the other two. nevertheless, kepler, the third member of the trio, could not have made his most valuable discoveries without tycho's observations. of noble family, born a twin on th december, , at knudstrup in scania (the southernmost part of sweden, then forming part of the kingdom of denmark), tycho was kidnapped a year later by a childless uncle. this uncle brought him up as his own son, provided him at the age of seven with a tutor, and sent him in to the university of copenhagen, to study for a political career by taking courses in rhetoric and philosophy. on st august, , however, a solar eclipse took place, total in portugal, and therefore of small proportions in denmark, and tycho's keen interest was awakened, not so much by the phenomenon, as by the fact that it had occurred according to prediction. soon afterwards he purchased an edition of ptolemy in order to read up the subject of astronomy, to which, and to mathematics, he devoted most of the remainder of his three years' course at copenhagen. his uncle next sent him to leipzig to study law, but he managed to continue his astronomical researches. he obtained the alphonsine and the new prutenic tables, but soon found that the latter, though more accurate than the former, failed to represent the true positions of the planets, and grasped the fact that continuous observation was essential in order to determine the true motions. he began by observing a conjunction of jupiter and saturn in august, , and found the prutenic tables several days in error, and the alphonsine a whole month. he provided himself with a cross-staff for determining the angular distance between stars or other objects, and, finding the divisions of the scale inaccurate, constructed a table of corrections, an improvement that seems to have been a decided innovation, the previous practice having been to use the best available instrument and ignore its errors. about this time war broke out between denmark and sweden, and tycho returned to his uncle, who was vice-admiral and attached to the king's suite. the uncle died in the following month, and early in the next year tycho went abroad again, this time to wittenberg. after five months, however, an outbreak of plague drove him away, and he matriculated at rostock, where he found little astronomy but a good deal of astrology. while there he fought a duel in the dark and lost part of his nose, which he replaced by a composition of gold and silver. he carried on regular observations with his cross-staff and persevered with his astronomical studies in spite of the objections and want of sympathy of his fellow-countrymen. the king of denmark, however, having a higher opinion of the value of science, promised tycho the first canonry that should fall vacant in the cathedral chapter of roskilde, so that he might be assured of an income while devoting himself to financially unproductive work. in tycho left rostock, and matriculated at basle, but soon moved on to augsburg, where he found more enthusiasm for astronomy, and induced one of his new friends to order the construction of a large -foot quadrant of heavy oak beams. this was the first of the series of great instruments associated with tycho's name, and it remained in use for five years, being destroyed by a great storm in . tycho meanwhile had left augsburg in and returned to live with his father, now governor of helsingborg castle, until the latter's death in the following year. tycho then joined his mother's brother, steen bille, the only one of his relatives who showed any sympathy with his desire for a scientific career. on th november, , tycho noticed an unfamiliar bright star in the constellation of cassiopeia, and continued to observe it with a sextant. it was a very brilliant object, equal to venus at its brightest for the rest of november, not falling below the first magnitude for another four months, and remaining visible for more than a year afterwards. tycho wrote a little book on the new star, maintaining that it had practically no parallax, and therefore could not be, as some supposed, a comet. deeming authorship beneath the dignity of a noble he was very reluctant to publish, but he was convinced of the importance of increasing the number and accuracy of observations, though he was by no means free from all the erroneous ideas of his time. the little book contained a certain amount of astrology, but tycho evidently did not regard this as of very great importance. he adopted the view that the very rarity of the phenomenon of a new star must prevent the formulation and adoption of definite rules for determining its significance. we gather from lectures which he was persuaded to deliver at the university of copenhagen that, though in agreement with the accepted canons of astrology as to the influence of planetary conjunctions and such phenomena on the course of human events, he did not consider the fate predicted by anyone's horoscope to be unavoidable, but thought the great value of astrology lay in the warnings derived from such computations, which should enable the believer to avoid threatened calamities. in he left denmark once more and made his way to cassel, where he found a kindred spirit in the studious landgrave, william iv. of hesse, whose astronomical pursuits had been interrupted by his accession to the government of hesse, in . tycho observed with him for some time, the two forming a firm friendship, and then visited successively frankfort, basle, and venice, returning by way of augsburg, ratisbon, and saalfeld to wittenberg; on the way he acquired various astronomical manuscripts, made friends among practical astronomers, and examined new instruments. he seemed to have considered the advantages of the several places thus visited and decided on basle, but on his return to denmark to fetch his family with the object of transferring them to basle, he found that his friend the landgrave had written to king frederick on his behalf, urging him to provide the means to enable tycho to pursue his astronomical work, promising that not only should credit result for the king and for denmark but that science itself would be greatly advanced. the ultimate result of this letter was that after refusing various offers, tycho accepted from the king a grant of the small island of hveen, in the sound, with a guaranteed income, in addition to a large sum from the treasury for building an observatory on the island, far removed from the distractions of court life. here tycho built his celebrated observatory of uraniborg and began observations in december, , using the large instruments then found necessary in order to attain the accuracy of observation which within the next half-century was to be so greatly facilitated by the invention of the telescope. here also he built several smaller observing rooms, so that his pupils should be able to observe independently. for more than twenty years he continued his observations at uraniborg, surrounded by his family, and attracting numerous pupils. his constant aim was to accumulate a large store of observations of a high order of accuracy, and thus to provide data for the complete reform of astronomy. as we have seen, few of the danish nobles had any sympathy with tycho's pursuits, and most of them strongly resented the continual expense borne by the king's treasury. tycho moreover was so absorbed in his scientific pursuits that he would not take the trouble to be a good landlord, nor to carry out all the duties laid upon him in return for certain of his grants of income. his buildings included a chemical laboratory, and he was in the habit of making up elixirs for various medical purposes; these were quite popular, particularly as he made no charge for them. he seems to have been something of a homoeopathist, for he recommends sulphur to cure infectious diseases "brought on by the sulphurous vapours of the aurora borealis"! king frederick, in consideration of various grants to tycho, relied upon his assistance in scientific matters, and especially in astrological calculations; such as the horoscope of the heir apparent, prince christian, born in , which has been preserved among tycho's writings. there is, however, no known copy in existence of any of the series of annual almanacs with predictions which he prepared for the king. in november, , appeared a bright comet, which tycho carefully observed with his sextant, proving that it had no perceptible parallax, and must therefore be further off than the moon. he thus definitely overthrew the common belief in the atmospheric origin of comets, which he had himself hitherto shared. with increasing accuracy he observed several other comets, notably one in , when he had a full equipment of instruments and a large staff of assistants. the year , which saw the death of his royal benefactor, saw also the publication of a volume of tycho's great work "introduction to the new astronomy". the first volume, devoted to the new star of , was not ready, because the reduction of the observations involved so much research to correct the star places for refraction, precession, etc.; it was not completed in fact until tycho's death, but the second volume, dealing with the comet of , was printed at uraniborg and some copies were issued in . besides the comet observations it included an account of tycho's system of the world. he would not accept the copernican system, as he considered the earth too heavy and sluggish to move, and also that the authority of scripture was against such an hypothesis. he therefore assumed that the other planets revolved about the sun, while the sun, moon, and stars revolved about the earth as a centre. geometrically this is much the same as the copernican system, but physically it involves the grotesque demand that the whole system of stars revolves round our insignificant little earth every twenty-four hours. since his previous small book on the comet, tycho had evidently considered more fully its possible astrological significance, for he foretold a religious war, giving the date of its commencement, and also the rising of a great protestant champion. these predictions were apparently fulfilled almost to the letter by the great religious wars that broke out towards the end of the sixteenth century, and in the person of gustavus adolphus. king frederick's death did not at first affect tycho's position, for the new king, christian, was only eleven years old, and for some years the council of regents included two of his supporters. after their deaths, however, his emoluments began to be cut down on the plea of economy, and as he took very little trouble to carry out any other than scientific duties it was easy enough for his enemies to find fault. one after another source of income was cut off, but he persevered with his scientific work, including a catalogue of stars. he had obtained plenty of good observations of stars, but thought his catalogue should contain stars, so he hastily observed as many more as he could up to the time of his leaving hveen, though even then he had not completed his programme. about the time that king christian reached the age of eighteen, tycho began to look about for a new patron, and to consider the prospects offered by transferring himself with his instruments and activities to the patronage of the emperor rudolph ii. in , when even his pension from the royal treasury was cut off, he hurriedly packed up his instruments and library, and after a few weeks' sojourn at copenhagen, proceeded to rostock, in mecklenburg, whence he sent an appeal to king christian. it is possible that had he done this before leaving hveen it might have had more effect, but it can be readily seen from the tone of the king's unfavourable reply that his departure was regarded as an aggravation of previous shortcomings. driven from rostock by the plague, tycho settled temporarily at wandsbeck, in holstein, but towards the end of set out to meet the emperor at prague. once more plague intervened and he spent some time at dresden, afterwards going to wittenberg for the winter. he ultimately reached prague in june, . rudolph granted him a salary of at least florins, promising also to settle on him the first hereditary estate that should lapse to the crown. he offered, moreover, the choice between three castles outside prague, of which tycho chose benatek. there he set about altering the buildings in readiness for his instruments, for which he sent to uraniborg. before they reached him, after many vexatious delays, he had given up waiting for the funds promised for his building expenses, and removed from benatek to prague. it was during this interval that after considerable negotiation, kepler, who had been in correspondence with tycho, consented to join him as an assistant. another assistant, longomontanus, who had been with tycho at uraniborg, was finding difficulty with the long series of mars observations, and it was arranged that he should transfer his energies to the lunar observations, leaving those of mars for kepler. before very much could be done with them, however, tycho died at the end of october, . he may have regretted the peaceful island of hveen, considering the troubles in which bohemia was rapidly becoming involved, but there is little doubt that had it not been for his self-imposed exile, his observations would not have come into kepler's hands, and their great value might have been lost. in any case it was at uraniborg that the mass of observations was produced upon which the fame of tycho brahe rests. his own discoveries, though in themselves the most important made in astronomy for many centuries, are far less valuable than those for which his observations furnished the material. he discovered the third and fourth inequalities of the moon in longitude, called respectively the variation and the annual equation, also the variability of the motion of the moon's nodes and the inclination of its orbit to the ecliptic. he obtained an improved value of the constant of precession, and did good service by rejecting the idea that it was variable, an idea which, under the name of trepidation, had for many centuries been accepted. he discovered the effect of refraction, though only approximately its amount, and determined improved values of many other astronomical constants, but singularly enough made no determination of the distance of the sun, adopting instead the ancient and erroneous value given by hipparchus. his magnificent observatory of uraniborg, the finest building for astronomical purposes that the world had hitherto seen, was allowed to fall into decay, and scarcely more than mere indications of the site may now be seen. chapter iv. kepler joins tycho. the association of kepler with tycho was one of the most important landmarks in the history of astronomy. the younger man hoped, by the aid of tycho's planetary observations, to obtain better support for some of his fanciful speculative theories, while the latter, who had certainly not gained in prestige by leaving denmark, was in great need of a competent staff of assistants. of the two it would almost seem that tycho thought himself the greater gainer, for in spite of his reputation for brusqueness and want of consideration, he not only made light of kepler's apology in the matter of reymers, but treated him with uniform kindness in the face of great rudeness and ingratitude. he begged him to come "as a welcome friend," though kepler, very touchy on the subject of his own astronomical powers, was afraid he might be regarded as simply a subordinate assistant. an arrangement had been suggested by which kepler should obtain two years' leave of absence from gratz on full pay, which, because of the higher cost of living in prague, should be supplemented by the emperor; but before this could be concluded, kepler threw up his professorship, and thinking he had thereby also lost the chance of going to prague, applied to maestlin and others of his tübingen friends to make interest for him with the duke of wurtemberg and secure the professorship of medicine. tycho, however, still urged him to come to prague, promising to do his utmost to secure for him a permanent appointment, or in any event to see that he was not the loser by coming. kepler was delayed by illness on the way, but ultimately reached prague, accompanied by his wife, and for some time lived entirely at tycho's expense, writing by way of return essays against reymers and another man, who had claimed the credit of the tychonic system. this kepler could do with a clear conscience, as it was only a question of priority and did not involve any support of the system, which he deemed far inferior to that of copernicus. the following year saw friction between the two astronomers, and we learn from kepler's abject letter of apology that he was entirely in the wrong. it was about money matters, which in one way or another embittered the rest of kepler's life, and it arose during his absence from prague. on his return in september, , tycho presented him to the emperor, who gave him the title of imperial mathematician, on condition of assisting tycho in his calculations, the very thing kepler was most anxious to be allowed to do: for nowhere else in the world was there such a collection of good observations sufficient for his purpose of reforming the whole theory of astronomy. the emperor's interest was still mainly with astrology, but he liked to think that his name would be handed down to posterity in connection with the new planetary tables in the same way as that of alphonso of castile, and he made liberal promises to pay the expenses. tycho's other principal assistant, longomontanus, did not stay long after giving up the mars observations to kepler, but instead of working at the new lunar theory, suddenly left to take up a professorship of astronomy in his native denmark. very shortly afterwards tycho himself died of acute distemper; kepler began to prepare the mass of manuscripts for publication, but, as everything was claimed by the brahe family, he was not allowed to finish the work. he succeeded to tycho's post of principal mathematician to the emperor, at a reduced official salary, which owing to the emptiness of the imperial treasury was almost always in arrear. in order to meet his expenses he had recourse to the casting of nativities, for which he gained considerable reputation and received very good pay. he worked by the conventional rules of astrology, and was quite prepared to take fees for so doing, although he had very little faith in them, preferring his own fanciful ideas. in the constellation of cassiopeia was once more temporarily enriched by the appearance of a new star, said by some to be brighter than tycho's nova, and by others to be twice as bright as jupiter. kepler at once wrote a short account of it, from which may be gathered some idea of his attitude towards astrology. contrasting the two novae, he says: "yonder one chose for its appearance a time no way remarkable, and came into the world quite unexpectedly, like an enemy storming a town and breaking into the market-place before the citizens are aware of his approach; but ours has come exactly in the year of which astrologers have written so much about the fiery trigon that happens in it; just in the month in which (according to cyprian), mars comes up to a very perfect conjunction with the other two superior planets; just in the day when mars has joined jupiter, and just in the region where this conjunction has taken place. therefore the apparition of this star is not like a secret hostile irruption, as was that one of , but the spectacle of a public triumph, or the entry of a mighty potentate; when the couriers ride in some time before to prepare his lodgings, and the crowd of young urchins begin to think the time over long to wait, then roll in, one after another, the ammunition and money, and baggage waggons, and presently the trampling of horse and the rush of people from every side to the streets and windows; and when the crowd have gazed with their jaws all agape at the troops of knights; then at last the trumpeters and archers and lackeys so distinguish the person of the monarch, that there is no occasion to point him out, but every one cries of his own accord--'here we have him'. what it may portend is hard to determine, and this much only is certain, that it comes to tell mankind either nothing at all or high and mighty news, quite beyond human sense and understanding. it will have an important influence on political and social relations; not indeed by its own nature, but as it were accidentally through the disposition of mankind. first, it portends to the booksellers great disturbances and tolerable gains; for almost every _theologus_, _philosophicus_, _medicus_, and _mathematicus_, or whoever else, having no laborious occupation entrusted to him, seeks his pleasure _in studiis_, will make particular remarks upon it, and will wish to bring these remarks to the light. just so will others, learned and unlearned, wish to know its meaning, and they will buy the authors who profess to tell them. i mention these things merely by way of example, because although thus much can be easily predicted without great skill, yet may it happen just as easily, and in the same manner, that the vulgar, or whoever else is of easy faith, or, it may be, crazy, may wish to exalt himself into a great prophet; or it may even happen that some powerful lord, who has good foundation and beginning of great dignities, will be cheered on by this phenomenon to venture on some new scheme, just as if god had set up this star in the darkness merely to enlighten them." he made no secret of his views on conventional astrology, as to which he claimed to speak with the authority of one fully conversant with its principles, but he nevertheless expressed his sincere conviction that the conjunctions and aspects of the planets certainly did affect things on the earth, maintaining that he was driven to this belief against his will by "most unfailing experiences". meanwhile the projected rudolphine tables were continually delayed by the want of money. kepler's nominal salary should have been ample for his expenses, increased though they were by his growing family, but in the depleted state of the treasury there were many who objected to any payment for such "unpractical" purposes. this particular attitude has not been confined to any special epoch or country, but the obvious result in kepler's case was to compel him to apply himself to less expensive matters than the planetary tables, and among these must be included not only the horoscopes or nativities, which owing to his reputation were always in demand, but also other writings which probably did not pay so well. in he published "a supplement to vitellion," containing the earliest known reasonable theory of optics, and especially of dioptrics or vision through lenses. he compared the mechanism of the eye with that of porta's "camera obscura," but made no attempt to explain how the image formed on the retina is understood by the brain. he went carefully into the question of refraction, the importance of which tycho had been the first astronomer to recognise, though he only applied it at low altitudes, and had not arrived at a true theory or accurate values. kepler wasted a good deal of time and ingenuity on trial theories. he would invariably start with some hypothesis, and work out the effect. he would then test it by experiment, and when it failed would at once recognise that his hypothesis was _a priori_ bound to fail. he rarely seems to have noticed the fatal objections in time to save himself trouble. he would then at once start again on a new hypothesis, equally gratuitous and equally unfounded. it never seems to have occurred to him that there might be a better way of approaching a problem. among the lines he followed in this particular investigation were, first, that refraction depends only on the angle of incidence, which, he says, cannot be correct as it would thus be the same for all refracting substances; next, that it depended also on the density of the medium. this was a good shot, but he unfortunately assumed that all rays passing into a denser medium would apparently penetrate it to a depth depending only on the medium, which means that there is a constant ratio between the tangents, instead of the sines, of the inclination of the incident and refracted rays to the normal. experiment proved that this gave too high values for refraction near the vertical compared with those near the horizon, so kepler "went off at a tangent" and tried a totally new set of ideas, which all reduced to the absurdity of a refraction which vanished at the horizon. these were followed by another set, involving either a constant amount of refraction or one becoming infinite. he then came to the conclusion that these geometrical methods must fail because the refracted image is not real, and determined to try by analogy only, comparing the equally unreal image formed by a mirror with that formed by refraction in water. he noticed how the bottom of a vessel containing water appears to rise more and more away from the vertical, and at once jumped to the analogy of a concave mirror, which magnifies the image, while a convex mirror was likened to a rarer medium. this line of attack also failed him, as did various attempts to find relations between his measurements of refraction and conic sections, and he broke off suddenly with a diatribe against tycho's critics, whom he likened to blind men disputing about colours. not many years later snell discovered the true law of refraction, but kepler's contribution to the subject, though he failed to discover the actual law, includes several of the adopted "by-laws". he noted that atmospheric refraction would alter with the height of the atmosphere and with temperature, and also recognised the fact that rainbow colours depend on the angle of refraction, whether seen in the rainbow itself, or in dew, glass, water, or any similar medium. he thus came near to anticipating newton. before leaving the subject of kepler's optics it will be well to recall that a few years later after hearing of galileo's telescope, kepler suggested that for astronomical purposes two convex lenses should be used, so that there should be a real image where measuring wires could be placed for reference. he did not carry out the idea himself, and it was left to the englishman gascoigne to produce the first instrument on this "keplerian" principle, universally known as the astronomical telescope. in came a second treatise on the new star, discussing various theories to account for its appearance, and refusing to accept the notion that it was a "fortuitous concourse of atoms". this was followed in by a treatise on comets, suggested by the comet appearing that year, known as halley's comet after its next return. he regarded comets as "planets" moving in straight lines, never having examined sufficient observations of any comet to convince himself that their paths are curved. if he had not assumed that they were external to the system and so could not be expected to return, he might have anticipated halley's discovery. another suggestive remark of his was to the effect that the planets must be self-luminous, as otherwise mercury and venus, at any rate, ought to show phases. this was put to the test not long afterwards by means of galileo's telescope. in kepler rushed into print with an alleged observation of mercury crossing the sun, but after galileo's discovery of sun-spots, kepler at once cheerfully retracted his observation of "mercury," and so far was he from being annoyed or bigoted in his views, that he warmly adopted galileo's side, in contrast to most of those whose opinions were liable to be overthrown by the new discoveries. maestlin and others of kepler's friends took the opposite view. chapter v. kepler's laws. when gilbert of colchester, in his "new philosophy," founded on his researches in magnetism, was dealing with tides, he did not suggest that the moon attracted the water, but that "subterranean spirits and humours, rising in sympathy with the moon, cause the sea also to rise and flow to the shores and up rivers". it appears that an idea, presented in some such way as this, was more readily received than a plain statement. this so-called philosophical method was, in fact, very generally applied, and kepler, who shared galileo's admiration for gilbert's work, adopted it in his own attempt to extend the idea of magnetic attraction to the planets. the general idea of "gravity" opposed the hypothesis of the rotation of the earth on the ground that loose objects would fly off: moreover, the latest refinements of the old system of planetary motions necessitated their orbits being described about a mere empty point. kepler very strongly combated these notions, pointing out the absurdity of the conclusions to which they tended, and proceeded in set terms to describe his own theory. "every corporeal substance, so far forth as it is corporeal, has a natural fitness for resting in every place where it may be situated by itself beyond the sphere of influence of a body cognate with it. gravity is a mutual affection between cognate bodies towards union or conjunction (similar in kind to the magnetic virtue), so that the earth attracts a stone much rather than the stone seeks the earth. heavy bodies (if we begin by assuming the earth to be in the centre of the world) are not carried to the centre of the world in its quality of centre of the world, but as to the centre of a cognate round body, namely, the earth; so that wheresoever the earth may be placed, or whithersoever it may be carried by its animal faculty, heavy bodies will always be carried towards it. if the earth were not round, heavy bodies would not tend from every side in a straight line towards the centre of the earth, but to different points from different sides. if two stones were placed in any part of the world near each other, and beyond the sphere of influence of a third cognate body, these stones, like two magnetic needles, would come together in the intermediate point, each approaching the other by a space proportional to the comparative mass of the other. if the moon and earth were not retained in their orbits by their animal force or some other equivalent, the earth would mount to the moon by a fifty-fourth part of their distance, and the moon fall towards the earth through the other fifty-three parts, and they would there meet, assuming, however, that the substance of both is of the same density. if the earth should cease to attract its waters to itself all the waters of the sea would he raised and would flow to the body of the moon. the sphere of the attractive virtue which is in the moon extends as far as the earth, and entices up the waters; but as the moon flies rapidly across the zenith, and the waters cannot follow so quickly, a flow of the ocean is occasioned in the torrid zone towards the westward. if the attractive virtue of the moon extends as far as the earth, it follows with greater reason that the attractive virtue of the earth extends as far as the moon and much farther; and, in short, nothing which consists of earthly substance anyhow constituted although thrown up to any height, can ever escape the powerful operation of this attractive virtue. nothing which consists of corporeal matter is absolutely light, but that is comparatively lighter which is rarer, either by its own nature, or by accidental heat. and it is not to be thought that light bodies are escaping to the surface of the universe while they are carried upwards, or that they are not attracted by the earth. they are attracted, but in a less degree, and so are driven outwards by the heavy bodies; which being done, they stop, and are kept by the earth in their own place. but although the attractive virtue of the earth extends upwards, as has been said, so very far, yet if any stone should be at a distance great enough to become sensible compared with the earth's diameter, it is true that on the motion of the earth such a stone would not follow altogether; its own force of resistance would be combined with the attractive force of the earth, and thus it would extricate itself in some degree from the motion of the earth." the above passage from the introduction to kepler's "commentaries on the motion of mars," always regarded as his most valuable work, must have been known to newton, so that no such incident as the fall of an apple was required to provide a necessary and sufficient explanation of the genesis of his theory of universal gravitation. kepler's glimpse at such a theory could have been no more than a glimpse, for he went no further with it. this seems a pity, as it is far less fanciful than many of his ideas, though not free from the "virtues" and "animal faculties," that correspond to gilbert's "spirits and humours". we must, however, proceed to the subject of mars, which was, as before noted, the first important investigation entrusted to kepler on his arrival at prague. the time taken from one opposition of mars to the next is decidedly unequal at different parts of his orbit, so that many oppositions must be used to determine the mean motion. the ancients had noticed that what was called the "second inequality," due as we now know to the orbital motion of the earth, only vanished when earth, sun, and planet were in line, i.e. at the planet's opposition; therefore they used oppositions to determine the mean motion, but deemed it necessary to apply a correction to the true opposition to reduce to mean opposition, thus sacrificing part of the advantage of using oppositions. tycho and longomontanus had followed this method in their calculations from tycho's twenty years' observations. their aim was to find a position of the "equant," such that these observations would show a constant angular motion about it; and that the computed positions would agree in latitude and longitude with the actual observed positions. when kepler arrived he was told that their longitudes agreed within a couple of minutes of arc, but that something was wrong with the latitudes. he found, however, that even in longitude their positions showed discordances ten times as great as they admitted, and so, to clear the ground of assumptions as far as possible, he determined to use true oppositions. to this tycho objected, and kepler had great difficulty in convincing him that the new move would be any improvement, but undertook to prove to him by actual examples that a false position of the orbit could by adjusting the equant be made to fit the longitudes within five minutes of arc, while giving quite erroneous values of the latitudes and second inequalities. to avoid the possibility of further objection he carried out this demonstration separately for each of the systems of ptolemy, copernicus, and tycho. for the new method he noticed that great accuracy was required in the reduction of the observed places of mars to the ecliptic, and for this purpose the value obtained for the parallax by tycho's assistants fell far short of the requisite accuracy. kepler therefore was obliged to recompute the parallax from the original observations, as also the position of the line of nodes and the inclination of the orbit. the last he found to be constant, thus corroborating his theory that the plane of the orbit passed through the sun. he repeated his calculations no fewer than seventy times (and that before the invention of logarithms), and at length adopted values for the mean longitude and longitude of aphelion. he found no discordance greater than two minutes of arc in tycho's observed longitudes in opposition, but the latitudes, and also longitudes in other parts of the orbit were much more discordant, and he found to his chagrin that four years' work was practically wasted. before making a fresh start he looked for some simplification of the labour; and determined to adopt ptolemy's assumption known as the principle of the bisection of the excentricity. hitherto, since ptolemy had given no reason for this assumption, kepler had preferred not to make it, only taking for granted that the centre was at some point on the line called the excentricity (see figs. , ). a marked improvement in residuals was the result of this step, proving, so far, the correctness of ptolemy's principle, but there still remained discordances amounting to eight minutes of arc. copernicus, who had no idea of the accuracy obtainable in observations, would probably have regarded such an agreement as remarkably good; but kepler refused to admit the possibility of an error of eight minutes in any of tycho's observations. he thereupon vowed to construct from these eight minutes a new planetary theory that should account for them all. his repeated failures had by this time convinced him that no uniformly described circle could possibly represent the motion of mars. either the orbit could not be circular, or else the angular velocity could not be constant about any point whatever. he determined to attack the "second inequality," i.e. the optical illusion caused by the earth's annual motion, but first revived an old idea of his own that for the sake of uniformity the sun, or as he preferred to regard it, the earth, should have an equant as well as the planets. from the irregularities of the solar motion he soon found that this was the case, and that the motion was uniform about a point on the line from the sun to the centre of the earth's orbit, such that the centre bisected the distance from the sun to the "equant"; this fully supported ptolemy's principle. clearly then the earth's linear velocity could not be constant, and kepler was encouraged to revive another of his speculations as to a force which was weaker at greater distances. he found the velocity greater at the nearer apse, so that the time over an equal arc at either apse was proportional to the distance. he conjectured that this might prove to be true for arcs at all parts of the orbit, and to test this he divided the orbit into equal parts, and calculated the distances to the points of division. archimedes had obtained an approximation to the area of a circle by dividing it radially into a very large number of triangles, and kepler had this device in mind. he found that the sums of successive distances from his points were approximately proportional to the times from point to point, and was thus enabled to represent much more accurately the annual motion of the earth which produced the second inequality of mars, to whose motion he now returned. three points are sufficient to define a circle, so he took three observed positions of mars and found a circle; he then took three other positions, but obtained a different circle, and a third set gave yet another. it thus began to appear that the orbit could not be a circle. he next tried to divide into equal parts, as he had in the case of the earth, but the sums of distances failed to fit the times, and he realised that the sums of distances were not a good measure of the area of successive triangles. he noted, however, that the errors at the apses were now smaller than with a central circular orbit, and of the opposite sign, so he determined to try whether an oval orbit would fit better, following a suggestion made by purbach in the case of mercury, whose orbit is even more eccentric than that of mars, though observations were too scanty to form the foundation of any theory. kepler gave his fancy play in the choice of an oval, greater at one end than the other, endeavouring to satisfy some ideas about epicyclic motion, but could not find a satisfactory curve. he then had the fortunate idea of trying an ellipse with the same axis as his tentative oval. mars now appeared too slow at the apses instead of too quick, so obviously some intermediate ellipse must be sought between the trial ellipse and the circle on the same axis. at this point the "long arm of coincidence" came into play. half-way between the apses lay the mean distance, and at this position the error was half the distance between the ellipse and the circle, amounting to . of a radius. with these figures in his mind, kepler looked up the greatest optical inequality of mars, the angle between the straight lines from mars to the sun and to the centre of the circle.[ ] the secant of this angle was . , so that he noted that an ellipse reduced from the circle in the ratio of . to would fit the motion of mars at the mean distance as well as the apses. [footnote : this is clearly a maximum at amc in fig. , when its tangent ac/cm = the eccentricity.] it is often said that a coincidence like this only happens to somebody who "deserves his luck," but this simply means that recognition is essential to the coincidence. in the same way the appearance of one of a large number of people mentioned is hailed as a case of the old adage "talk of the devil, etc.," ignoring all the people who failed to appear. no one, however, will consider kepler unduly favoured. his genius, in his case certainly "an infinite capacity for taking pains," enabled him out of his medley of hypotheses, mainly unsound, by dint of enormous labour and patience, to arrive thus at the first two of the laws which established his title of "legislator of the heavens". figures explanatory of kepler's theory of the motion of mars. [illustration: fig. .] _______ / \ / \ | | |___________| q| e c a |p | | \ / \_______/ [illustration: fig. .] ___m___ /___|\__\ // n|\\ \\ |/ | \\ \| |_____|__\\_| q| e c a |p |\ | /| \\___|___// \___|___/ [transcriber's note: approximate renditions of these figures are provided. fig. is a circle. fig. is a circle which contains an ellipse, tangent to the circle at q and p. line segments from m (on the circle) and n (on the ellipse) meet at point a.] fig. .--in ptolemy's excentric theory, a may be taken to represent the earth, c the centre of a planet's orbit, and e the equant, p (perigee) and q (apogee) being the apses of the orbit. ptolemy's idea was that uniform motion in a circle must be provided, and since the motion was not uniform about the earth, a could not coincide with c; and since the motion still failed to be uniform about a or c, some point e must be found about which the motion should be uniform. fig. .--this is not drawn to scale, but is intended to illustrate kepler's modification of ptolemy's excentric. kepler found velocities at p and q proportional not to ap and aq but to aq and ap, or to ep and eq if ec = ca (bisection of the excentricity). the velocity at m was wrong, and am appeared too great. kepler's first ellipse had m moved too near c. the distance ac is much exaggerated in the figure, as also is mn. an = cp, the radius of the circle. mn should be . of the radius, and mc/nc should be . . the velocity at n appeared to be proportional to en ( = an). kepler concluded that mars moved round pnq, so that the area described about a (the sun) was equal in equal times, a being the focus of the ellipse pnq. the angular velocity is not quite constant about e, the equant or empty focus, but the difference could hardly have been detected in kepler's time. kepler's improved determination of the earth's orbit was obtained by plotting the different positions of the earth corresponding to successive rotations of mars, i.e. intervals of days. at each of these the date of the year would give the angle mse (mars-sun-earth), and tycho's observation the angle mes. so the triangle could be solved except for scale, and the ratio of se to sm would give the distance of mars from the sun in terms of that of the earth. measuring from a fixed position of mars (e.g. perihelion), this gave the variation of se, showing the earth's inequality. measuring from a fixed position of the earth, it would give similarly a series of positions of mars, which, though lying not far from the circle whose diameter was the axis of mars' orbit, joining perihelion and aphelion, always fell inside the circle except at those two points. it was a long time before it dawned upon kepler that the simplest figure falling within the circle except at the two extremities of the diameter, was an ellipse, and it is not clear why his first attempt with an ellipse should have been just as much too narrow as the circle was too wide. the fact remains that he recognised suddenly that halving this error was tantamount to reducing the circle to the ellipse whose eccentricity was that of the old theory, i.e. that in which the sun would be in one focus and the equant in the other. having now fitted the ends of both major and minor axes of the ellipse, he leaped to the conclusion that the orbit would fit everywhere. the practical effect of his clearing of the "second inequality" was to refer the orbit of mars directly to the sun, and he found that the area between successive distances of mars from the sun (instead of the sum of the distances) was strictly proportional to the time taken, in short, equal areas were described in equal times ( nd law) when referred to the sun in the focus of the ellipse ( st law). he announced that ( ) the planet describes an ellipse, the sun being in one focus; and ( ) the straight line joining the planet to the sun sweeps out equal areas in any two equal intervals of time. these are kepler's first and second laws though not discovered in that order, and it was at once clear that ptolemy's "bisection of the excentricity" simply amounted to the fact that the centre of an ellipse bisects the distance between the foci, the sun being in one focus and the angular velocity being uniform about the empty focus. for so many centuries had the fetish of circular motion postponed discovery. it was natural that kepler should assume that his laws would apply equally to all the planets, but the proof of this, as well as the reason underlying the laws, was only given by newton, who approached the subject from a totally different standpoint. this commentary on mars was published in , the year of the invention of the telescope, and kepler petitioned the emperor for further funds to enable him to complete the study of the other planets, but once more there was delay; in rudolph died, and his brother matthias who succeeded him, cared very little for astronomy or even astrology, though kepler was reappointed to his post of imperial mathematician. he left prague to take up a permanent professorship at the university of linz. his own account of the circumstances is gloomy enough. he says, "in the first place i could get no money from the court, and my wife, who had for a long time been suffering from low spirits and despondency, was taken violently ill towards the end of , with the hungarian fever, epilepsy and phrenitis. she was scarcely convalescent when all my three children were at once attacked with smallpox. leopold with his army occupied the town beyond the river just as i lost the dearest of my sons, him whose nativity you will find in my book on the new star. the town on this side of the river where i lived was harassed by the bohemian troops, whose new levies were insubordinate and insolent; to complete the whole, the austrian army brought the plague with them into the city. i went into austria and endeavoured to procure the situation which i now hold. returning in june, i found my wife in a decline from her grief at the death of her son, and on the eve of an infectious fever, and i lost her also within eleven days of my return. then came fresh annoyance, of course, and her fortune was to be divided with my step-sisters. the emperor rudolph would not agree to my departure; vain hopes were given me of being paid from saxony; my time and money were wasted together, till on the death of the emperor in , i was named again by his successor, and suffered to depart to linz." being thus left a widower with a ten-year-old daughter susanna, and a boy louis of half her age, he looked for a second wife to take charge of them. he has given an account of eleven ladies whose suitability he considered. the first, an intimate friend of his first wife, ultimately declined; one was too old, another an invalid, another too proud of her birth and quarterings, another could do nothing useful, and so on. number eight kept him guessing for three months, until he tired of her constant indecision, and confided his disappointment to number nine, who was not impressed. number ten, introduced by a friend, kepler found exceedingly ugly and enormously fat, and number eleven apparently too young. kepler then reconsidered one of the earlier ones, disregarding the advice of his friends who objected to her lowly station. she was the orphan daughter of a cabinetmaker, educated for twelve years by favour of the lady of stahrenburg, and kepler writes of her: "her person and manners are suitable to mine; no pride, no extravagance; she can bear to work; she has a tolerable knowledge of how to manage a family; middle-aged and of a disposition and capability to acquire what she still wants". wine from the austrian vineyards was plentiful and cheap at the time of the marriage, and kepler bought a few casks for his household. when the seller came to ascertain the quantity, kepler noticed that no proper allowance was made for the bulging parts, and the upshot of his objections was that he wrote a book on a new method of gauging--one of the earliest specimens of modern analysis, extending the properties of plane figures to segments of cones and cylinders as being "incorporated circles". he was summoned before the diet at ratisbon to give his opinion on the gregorian reform of the calendar, and soon afterwards was excommunicated, having fallen foul of the roman catholic party at linz just as he had previously at gratz, the reason apparently being that he desired to think for himself. meanwhile his salary was not paid any more regularly than before, and he was forced to supplement it by publishing what he called a "vile prophesying almanac which is scarcely more respectable than begging unless it be because it saves the emperor's credit, who abandons me entirely, and with all his frequent and recent orders in council, would suffer me to perish with hunger". in he was invited to italy to succeed magini as professor of mathematics at bologna. galileo urged him to accept the post, but he excused himself on the ground that he was a german and brought up among germans with such liberty of speech as he thought might get him into trouble in italy. in matthias died and was succeeded by ferdinand iii, who again retained kepler in his post. in the same year kepler reprinted his "mysterium cosmographicum," and also published his "harmonics" in five books dedicated to james i of england. "the first geometrical, on the origin and demonstration of the laws of the figures which produce harmonious proportions; the second, architectonical, on figurate geometry and the congruence of plane and solid regular figures; the third, properly harmonic, on the derivation of musical proportions from figures, and on the nature and distinction of things relating to song, in opposition to the old theories; the fourth, metaphysical, psychological, and astrological, on the mental essence of harmonics, and of their kinds in the world, especially on the harmony of rays emanating on the earth from the heavenly bodies, and on their effect in nature and on the sublunary and human soul; the fifth, astronomical and metaphysical, on the very exquisite harmonics of the celestial motions and the origin of the excentricities in harmonious proportions." the extravagance of his fancies does not appear until the fourth book, in which he reiterates the statement that he was forced to adopt his astrological opinions from direct and positive observation. he despises "the common herd of prophesiers who describe the operations of the stars as if they were a sort of deities, the lords of heaven and earth, and producing everything at their pleasure. they never trouble themselves to consider what means the stars have of working any effects among us on the earth whilst they remain in the sky and send down nothing to us which is obvious to the senses, except rays of light." his own notion is "like one who listens to a sweet melodious song, and by the gladness of his countenance, by his voice, and by the beating of his hand or foot attuned to the music, gives token that he perceives and approves the harmony: just so does sublunary nature, with the notable and evident emotion of the bowels of the earth, bear like witness to the same feelings, especially at those times when the rays of the planets form harmonious configurations on the earth," and again "the earth is not an animal like a dog, ready at every nod; but more like a bull or an elephant, slow to become angry, and so much the more furious when incensed." he seems to have believed the earth to be actually a living animal, as witness the following: "if anyone who has climbed the peaks of the highest mountains, throw a stone down their very deep clefts, a sound is heard from them; or if he throw it into one of the mountain lakes, which beyond doubt are bottomless, a storm will immediately arise, just as when you thrust a straw into the ear or nose of a ticklish animal, it shakes its head, or runs shudderingly away. what so like breathing, especially of those fish who draw water into their mouths and spout it out again through their gills, as that wonderful tide! for although it is so regulated according to the course of the moon, that, in the preface to my 'commentaries on mars,' i have mentioned it as probable that the waters are attracted by the moon, as iron by the loadstone, yet if anyone uphold that the earth regulates its breathing according to the motion of the sun and moon, as animals have daily and nightly alternations of sleep and waking, i shall not think his philosophy unworthy of being listened to; especially if any flexible parts should be discovered in the depths of the earth, to supply the functions of lungs or gills." in the same book kepler enlarges again on his views in reference to the basis of astrology as concerned with nativities and the importance of planetary conjunctions. he gives particulars of his own nativity. "jupiter nearest the nonagesimal had passed by four degrees the trine of saturn; the sun and venus in conjunction were moving from the latter towards the former, nearly in sextiles with both: they were also removing from quadratures with mars, to which mercury was closely approaching: the moon drew near to the trine of the same planet, close to the bull's eye even in latitude. the th degree of gemini was rising, and the nd of aquarius culminating. that there was this triple configuration on that day--namely the sextile of saturn and the sun, the sextile of mars and jupiter, and the quadrature of mercury and mars, is proved by the change of weather; for after a frost of some days, that very day became warmer, there was a thaw and a fall of rain." this alleged "proof" is interesting as it relies on the same principle which was held to justify the correction of an uncertain birth-time, by reference to illnesses, etc., met with later. kepler however goes on to say, "if i am to speak of the results of my studies, what, i pray, can i find in the sky, even remotely alluding to it? the learned confess that several not despicable branches of philosophy have been newly extricated or amended or brought to perfection by me: but here my constellations were, not mercury from the east in the angle of the seventh, and in quadratures with mars, but copernicus, but tycho brahe, without whose books of observations everything now set by me in the clearest light must have remained buried in darkness; not saturn predominating mercury, but my lords the emperors rudolph and matthias, not capricorn the house of saturn but upper austria, the house of the emperor, and the ready and unexampled bounty of his nobles to my petition. here is that corner, not the western one of the horoscope, but on the earth whither, by permission of my imperial master, i have betaken myself from a too uneasy court; and whence, during these years of my life, which now tends towards its setting, emanate these harmonics and the other matters on which i am engaged." the fifth book contains a great deal of nonsense about the harmony of the spheres; the notes contributed by the several planets are gravely set down, that of mercury having the greatest resemblance to a melody, though perhaps more reminiscent of a bugle-call. yet the book is not all worthless for it includes kepler's third law, which he had diligently sought for years. in his own words, "the proportion existing between the periodic times of any two planets is exactly the sesquiplicate proportion of the mean distances of the orbits," or as generally given, "the squares of the periodic times are proportional to the cubes of the mean distances." kepler was evidently transported with delight and wrote, "what i prophesied two and twenty years ago, as soon as i discovered the five solids among the heavenly orbits,--what i firmly believed long before i had seen ptolemy's 'harmonics'--what i had promised my friends in the title of this book, which i named before i was sure of my discovery,--what sixteen years ago i urged as a thing to be sought,--that for which i joined tycho brahe, for which i settled in prague, for which i have devoted the best part of my life to astronomical computations, at length i have brought to light, and have recognised its truth beyond my most sanguine expectations. great as is the absolute nature of harmonics, with all its details as set forth in my third book, it is all found among the celestial motions, not indeed in the manner which i imagined (that is not the least part of my delight), but in another very different, and yet most perfect and excellent. it is now eighteen months since i got the first glimpse of light, three months since the dawn, very few days since the unveiled sun, most admirable to gaze on, burst out upon me. nothing holds me; i will indulge in my sacred fury; i will triumph over mankind by the honest confession that i have stolen the golden vases of the egyptians to build up a tabernacle for my god far away from the confines of egypt. if you forgive me, i rejoice, if you are angry, i can bear it; the die is cast, the book is written; to be read either now or by posterity, i care not which; it may well wait a century for a reader, as god has waited six thousand years for an observer." he gives the date th may, , for the completion of his discovery. in his "epitome of the copernican astronomy," he gives his own idea as to the reason for this third law. "four causes concur for lengthening the periodic time. first, the length of the path; secondly, the weight or quantity of matter to be carried; thirdly, the degree of strength of the moving virtue; fourthly, the bulk or space into which is spread out the matter to be moved. the orbital paths of the planets are in the simple ratio of the distances; the weights or quantities of matter in different planets are in the subduplicate ratio of the same distances, as has been already proved; so that with every increase of distance a planet has more matter and therefore is moved more slowly, and accumulates more time in its revolution, requiring already, as it did, more time by reason of the length of the way. the third and fourth causes compensate each other in a comparison of different planets; the simple and subduplicate proportion compound the sesquiplicate proportion, which therefore is the ratio of the periodic times." the only part of this "explanation" that is true is that the paths are in the simple ratio of the distances, the "proof" so confidently claimed being of the circular kind commonly known as "begging the question". it was reserved for newton to establish the laws of motion, to find the law of force that would constrain a planet to obey kepler's first and second laws, and to prove that it must therefore also obey the third. chapter vi. closing years. soon after its publication kepler's "epitome" was placed along with the book of copernicus, on the list of books prohibited by the congregation of the index at rome, and he feared that this might prevent the publication or sale of his books in austria also, but was told that though galileo's violence was getting him into trouble, there would be no difficulty in obtaining permission for learned men to read any prohibited books, and that he (kepler) need fear nothing so long as he remained quiet. in his various works on comets, he adhered to the opinion that they travelled in straight lines with varying velocity. he suggested that comets come from the remotest parts of ether, as whales and monsters from the depth of the sea, and that perhaps they are something of the nature of silkworms, and are wasted and consumed in spinning their own tails. napier's invention of logarithms at once attracted kepler's attention. he must have regretted that the discovery was not made early enough to save him a vast amount of labour in computations, but he managed to find time to compute some logarithm tables for himself, though he does not seem to have understood quite what napier had done, and though with his usual honesty he gave full credit to the scottish baron for his invention. though eugenists may find a difficulty in reconciling napier's brilliancy with the extreme youth of his parents, they may at any rate attribute kepler's occasional fits of bad temper to heredity. his cantankerous mother, catherine kepler, had for some years been carrying on an action for slander against a woman who had accused her of administering a poisonous potion. dame kepler employed a young advocate who for reasons of his own "nursed" the case so long that after five years had elapsed without any conclusion being reached another judge was appointed, who had himself suffered from the caustic tongue of the prosecutrix, and so was already prejudiced against her. the defendant, knowing this, turned the tables on her opponent by bringing an accusation of witchcraft against her, and catherine kepler was imprisoned and condemned to the torture in july, . kepler, hearing of the sentence, hurried back from linz, and succeeded in stopping the completion of the sentence, securing his mother's release the following year, as it was made clear that the only support for the case against her was her own intemperate language. kepler returned to linz, and his mother at once brought another action for costs and damages against her late opponent, but died before the case could be tried. a few months before this sir henry wotton, english ambassador to venice, visited kepler, and finding him as usual, almost penniless, urged him to go to england, promising him a warm welcome there. kepler, however, would not at that time leave germany, giving several reasons, one of which was that he dreaded the confinement of an island. later on he expressed his willingness to go as soon as his rudolphine tables were published, and lecture on them, even in england, if he could not do it in germany, and if a good enough salary were forthcoming. in he went to vienna, and managed to extract from the treasury florins on account of expenses connected with the tables, but, instead of a further grant, was given letters to the states of swabia, which owed money to the imperial treasury. some of this he succeeded in collecting, but the tables were still further delayed by the religious disturbances then becoming violent. the jesuits contrived to have kepler's library sealed up, and, but for the imperial protection, would have imprisoned him also; moreover the peasants revolted and blockaded linz. in , however, the long promised tables, the first to discard the conventional circular motion, were at last published at ulm in four parts. two of these parts consisted of subsidiary tables, of logarithms and other computing devices, another contained tables of the elements of the sun, moon, and planets, and the fourth gave the places of a thousand stars as determined by tycho, with tycho's refraction tables, which had the peculiarity of using different values for the refraction of the sun, moon, and stars. from a map prefixed to some copies of the tables, we may infer that kepler was one of the first, if not actually the first, to suggest the method of determining differences of longitude by occultations of stars at the moon's limb. in an appendix, he showed how his tables could be used by astrologers for their predictions, saying "astronomy is the daughter of astrology, and this modern astrology again is the daughter of astronomy, bearing something of the lineaments of her grandmother; and, as i have already said, this foolish daughter, astrology, supports her wise but needy mother, astronomy, from the profits of a profession not generally considered creditable". there is no doubt that kepler strongly resented having to depend so much for his income on such methods which he certainly did not consider creditable. it was probably galileo whose praise of the new tables induced the grand duke of tuscany to send kepler a gold chain soon after their publication, and we may perhaps regard it as a mark of favour from the emperor ferdinand that he permitted kepler to attach himself to the great wallenstein, now duke of friedland, and a firm believer in astrology. the duke was a better paymaster than either of the three successive emperors. he furnished kepler with an assistant and a printing press; and obtained for him the professorship of astronomy at the university of rostock in mecklenburg. apparently, however, the emperor could not induce wallenstein to take over the responsibility of the crowns, still owing from the imperial treasury on account of the rudolphine tables. kepler made a last attempt to secure payment at ratisbon, but his journey thither brought disappointment and fatigue and left him in such a condition that he rapidly succumbed to an attack of fever, dying in november, , in his fifty-ninth year. his body was buried at ratisbon, but the tombstone was destroyed during the war then raging. his daughter, susanna, the wife of jacob bartsch, a physician who had helped kepler with his ephemeris, lost her husband soon after her father's death, and succeeded in obtaining part of kepler's arrears of salary by threatening to keep tycho's manuscripts, but her stepmother was left almost penniless with five young children. for their benefit louis kepler printed a "dream of lunar astronomy," which first his father and then his brother-in-law had been preparing for publication at the time of their respective deaths. it is a curious mixture of saga and fairy tale with a little science in the way of astronomy studied from the moon, and cast in the form of a dream to overcome the practical difficulties of the hypothesis of visiting the moon. other writings in large numbers were left unpublished. no attempt at a complete edition of kepler's works was made for a long time. one was projected in by his biographer, hantsch, but all that appeared was one volume of letters. after various learned bodies had declined to move in the matter the manuscripts were purchased for the imperial russian library. an edition was at length brought out at frankfort by c. frisch, in eight volumes, appearing at intervals from - . kepler's fame does not rest upon his voluminous works. with his peculiar method of approaching problems there was bound to be an inordinate amount of chaff mixed with the grain, and he used no winnowing machine. his simplicity and transparent honesty induced him to include everything, in fact he seemed to glory in the number of false trails he laboriously followed. he was one who might be expected to find the proverbial "needle in a haystack," but unfortunately the needle was not always there. delambre says, "ardent, restless, burning to distinguish himself by his discoveries he attempted everything, and having once obtained a glimpse of one, no labour was too hard for him in following or verifying it. all his attempts had not the same success, and in fact that was impossible. those which have failed seem to us only fanciful; those which have been more fortunate appear sublime. when in search of that which really existed, he has sometimes found it; when he devoted himself to the pursuit of a chimera, he could not but fail, but even then he unfolded the same qualities, and that obstinate perseverance that must triumph over all difficulties but those which are insurmountable." berry, in his "short history of astronomy," says "as one reads chapter after chapter without a lucid, still less a correct idea, it is impossible to refrain from regrets that the intelligence of kepler should have been so wasted, and it is difficult not to suspect at times that some of the valuable results which lie embedded in this great mass of tedious speculation were arrived at by a mere accident. on the other hand it must not be forgotten that such accidents have a habit of happening only to great men, and that if kepler loved to give reins to his imagination he was equally impressed with the necessity of scrupulously comparing speculative results with observed facts, and of surrendering without demur the most beloved of his fancies if it was unable to stand this test. if kepler had burnt three-quarters of what he printed, we should in all probability have formed a higher opinion of his intellectual grasp and sobriety of judgment, but we should have lost to a great extent the impression of extraordinary enthusiasm and industry, and of almost unequalled intellectual honesty which we now get from a study of his works." professor forbes is more enthusiastic. in his "history of astronomy," he refers to kepler as "the man whose place, as is generally agreed, would have been the most difficult to fill among all those who have contributed to the advance of astronomical knowledge," and again _à propos_ of kepler's great book, "it must be obvious that he had at that time some inkling of the meaning of his laws--universal gravitation. from that moment the idea of universal gravitation was in the air, and hints and guesses were thrown out by many; and in time the law of gravitation would doubtless have been discovered, though probably not by the work of one man, even if newton had not lived. but, if kepler had not lived, who else could have discovered his laws?" appendix i. list of dates. johann kepler, born ; school at maulbronn, ; university of tübingen, ; m.a. of tübingen, ; professor at gratz, ; "prodromus," with "mysterium cosmographicum," published ; first marriage, ; joins tycho brahe at prague, ; death of tycho, ; kepler's optics, ; nova, ; on comets, ; commentary on mars, including first and second laws, ; professor at linz, ; second marriage, ; third law discovered, ; epitome of copernican astronomy, - ; rudolphine tables published, ; died, . appendix ii. bibliography. for a full account of the various systems of kepler and his predecessors the reader cannot do better than consult the "history of the planetary systems, from thales to kepler," by dr. j.l.e. dreyer (cambridge univ. press, ). the same author's "tycho brahe" gives a wealth of detail about that "phoenix of astronomers," as kepler styles him. a great proportion of the literature relating to kepler is german, but he has his place in the histories of astronomy, from delambre and the more modern r. wolfs "geschichte" to those of a. berry, "history of astronomy" (university extension manuals, murray, ), and professor g. forbes, "history of astronomy" (history of science series, watts, ). glossary. apogee: the point in the orbit of a celestial body when it is furthest from the earth. apse: an extremity of the major axis of the orbit of a body; a body is at its greatest and least distances from the body about which it revolves, when at one or other apse. conjunction: when a plane containing the earth's axis and passing through the centre of the sun also passes through that of the moon or a planet, at the same side of the earth, the moon or planet is in conjunction, or if on opposite sides of the earth, the moon or planet is in opposition. mercury and venus cannot be in opposition, but are in inferior or superior conjunction according as they are nearer or further than the sun. deferent: in the epicyclic theory, uneven motion is represented by motion round a circle whose centre travels round another circle, the latter is called the deferent. ecliptic: the plane of the earth's orbital motion about the sun, which cuts the heavens in a great circle. it is so called because obviously eclipses can only occur when the moon is also approximately in this plane, besides being in conjunction or opposition with the sun. epicycle: a point moving on the circumference of a circle whose centre describes another circle, traces an epicycle with reference to the centre of the second circle. equant: in ptolemy's excentric theory, when a planet is describing a circle about a centre which is not the earth, in order to satisfy the convention that the motion must be uniform, a point was found about which the motion was apparently uniform,[ ] and this point was called the equant. [footnote : i.e. the _angular_ motion about the equant was uniform.] equinox: when the sun is in the plane of the earth's equator the lengths of day and night are equal. this happens twice a year, and the times when the sun passes the equator are called the vernal or spring equinox and the autumnal equinox respectively. evection: the second inequality of the moon, which vanishes at new and full moon and is a maximum at first and last quarter. excentric: as an alternative to epicycles, planets whose motion round the earth was not uniform could be represented as moving round a point some distance from the earth called the excentric. geocentric: referred to the centre of the earth; e.g. ptolemy's theory. heliocentric: referred to the centre of the sun; e.g. the theory commonly called copernican. inequality: the difference between the actual position of a planet and its theoretical position on the hypothesis of uniform circular motion. node: the points where the orbit of the moon or a planet intersect the plane of the ecliptic. the ascending node is the one when the planet is moving northwards, and the line of intersection of the orbital plane with the ecliptic is the line of nodes. occultation: usually means when a planet or star is hidden by the moon, but it also includes "occultation" of a star by a planet or of a satellite by a planet or of one planet by another. opposition v. conjunction. parallax: the error introduced by observing from some point other than that required in theory, e.g. in geocentric places because the observations are made from the surface of the earth instead of the centre, or in heliocentric places because observations are made from the earth and not from the sun. perigee: the point in the orbit of a celestial body when it is nearest to the earth. precession: owing to the slow motion of the earth's pole around the pole of the ecliptic, the equator cuts the ecliptic a little earlier every year, so that the equinox each year slightly precedes, with reference to the stars, that of the previous year. jill r. diffendal, barb grow pebareka@iexpress.net.au christine l. hall goleta, ca. usa pamela l. hall pamhall@www.edu great astronomers by sir robert s. ball d.sc. ll.d. f.r.s. lowndean professor of astronomy and geometry in the university of cambridge author of "in starry realms" "in the high heavens" etc. with numerous illustrations [plate: greenwich observatory.] preface. it has been my object in these pages to present the life of each astronomer in such detail as to enable the reader to realise in some degree the man's character and surroundings; and i have endeavoured to indicate as clearly as circumstances would permit the main features of the discoveries by which he has become known. there are many types of astronomers--from the stargazer who merely watches the heavens, to the abstract mathematician who merely works at his desk; it has, consequently, been necessary in the case of some lives to adopt a very different treatment from that which seemed suitable for others. while the work was in progress, some of the sketches appeared in "good words." the chapter on brinkley has been chiefly derived from an article on the "history of dunsink observatory," which was published on the occasion of the tercentenary celebration of the university of dublin in , and the life of sir william rowan hamilton is taken, with a few alterations and omissions, from an article contributed to the "quarterly review" on graves' life of the great mathematician. the remaining chapters now appear for the first time. for many of the facts contained in the sketch of the late professor adams, i am indebted to the obituary notice written by my friend dr. j. w. l. glaisher, for the royal astronomical society; while with regard to the late sir george airy, i have a similar acknowledgment to make to professor h. h. turner. to my friend dr. arthur a. rambaut i owe my hearty thanks for his kindness in aiding me in the revision of the work. r.s.b. the observatory, cambridge. october, contents. introduction. ptolemy. copernicus. tycho brahe. galileo. kepler. isaac newton. flamsteed. halley. bradley. william herschel. laplace. brinkley. john herschel. the earl of rosse. airy. hamilton. le verrier. adams. list of illustrations. the observatory, greenwich. ptolemy. ptolemy's planetary scheme. ptolemy's theory of the movement of mars. thorn, from an old print. copernicus. frauenburg, from an old print. explanation of planetary movements. tycho brahe. tycho's cross staff. tycho's "new star" sextant of . tycho's trigonic sextant. tycho's astronomic sextant. tycho's equatorial armillary. the great augsburg quadrant. tycho's "new scheme of the terrestrial system," . uraniborg and its grounds. ground-plan of the observatory. the observatory of uraniborg, island of hven. effigy on tycho's tomb at prague. by permission of messrs. a. & c. black. tycho's mural quadrant, uraniborg. galileo's pendulum. galileo. the villa arcetri. facsimile sketch of lunar surface by galileo. crest of galileo's family. kepler's system of regular solids. kepler. symbolical representation of the planetary system. the commemoration of the rudolphine tables. woolsthorpe manor. trinity college, cambridge. diagram of a sunbeam. isaac newton. sir isaac newton's little reflector. sir isaac newton's sun-dial. sir isaac newton's telescope. sir isaac newton's astrolabe. sir isaac newton's sun-dial in the royal society. flamsteed's house. flamsteed. halley. greenwich observatory in halley's time. , new king street, bath. from a photograph by john poole, bath. william herschel. caroline herschel. street view, herschel house, slough. from a photograph by hill & saunders, eton. garden view, herschel house, slough. from a photograph by hill & saunders, eton. observatory, herschel house, slough. from a photograph by hill & saunders, eton. the -foot telescope, herschel house, slough. from a photograph by hill & saunders, eton. laplace. the observatory, dunsink. from a photograph by w. lawrence, dublin. astronometer made by sir john herschel. sir john herschel. nebula in southern hemisphere. the cluster in the centaur. observatory at feldhausen. granite column at feldhausen. the earl of rosse. birr castle. from a photograph by w. lawrence, dublin. the mall, parsonstown. from a photograph by w. lawrence, dublin. lord rosse's telescope. from a photograph by w. lawrence, dublin. roman catholic church, parsonstown. from a photograph by w. lawrence, dublin. airy. from a photograph by e.p. adams, greenwich. hamilton. adams. the observatory, cambridge. introduction. of all the natural sciences there is not one which offers such sublime objects to the attention of the inquirer as does the science of astronomy. from the earliest ages the study of the stars has exercised the same fascination as it possesses at the present day. among the most primitive peoples, the movements of the sun, the moon, and the stars commanded attention from their supposed influence on human affairs. the practical utilities of astronomy were also obvious in primeval times. maxims of extreme antiquity show how the avocations of the husbandman are to be guided by the movements of the heavenly bodies. the positions of the stars indicated the time to plough, and the time to sow. to the mariner who was seeking a way across the trackless ocean, the heavenly bodies offered the only reliable marks by which his path could be guided. there was, accordingly, a stimulus both from intellectual curiosity and from practical necessity to follow the movements of the stars. thus began a search for the causes of the ever-varying phenomena which the heavens display. many of the earliest discoveries are indeed prehistoric. the great diurnal movement of the heavens, and the annual revolution of the sun, seem to have been known in times far more ancient than those to which any human monuments can be referred. the acuteness of the early observers enabled them to single out the more important of the wanderers which we now call planets. they saw that the star-like objects, jupiter, saturn, and mars, with the more conspicuous venus, constituted a class of bodies wholly distinct from the fixed stars among which their movements lay, and to which they bear such a superficial resemblance. but the penetration of the early astronomers went even further, for they recognized that mercury also belongs to the same group, though this particular object is seen so rarely. it would seem that eclipses and other phenomena were observed at babylon from a very remote period, while the most ancient records of celestial observations that we possess are to be found in the chinese annals. the study of astronomy, in the sense in which we understand the word, may be said to have commenced under the reign of the ptolemies at alexandria. the most famous name in the science of this period is that of hipparchus who lived and worked at rhodes about the year bc. it was his splendid investigations that first wrought the observed facts into a coherent branch of knowledge. he recognized the primary obligation which lies on the student of the heavens to compile as complete an inventory as possible of the objects which are there to be found. hipparchus accordingly commenced by undertaking, on a small scale, a task exactly similar to that on which modern astronomers, with all available appliances of meridian circles, and photographic telescopes, are constantly engaged at the present day. he compiled a catalogue of the principal fixed stars, which is of special value to astronomers, as being the earliest work of its kind which has been handed down. he also studied the movements of the sun and the moon, and framed theories to account for the incessant changes which he saw in progress. he found a much more difficult problem in his attempt to interpret satisfactorily the complicated movements of the planets. with the view of constructing a theory which should give some coherent account of the subject, he made many observations of the places of these wandering stars. how great were the advances which hipparchus accomplished may be appreciated if we reflect that, as a preliminary task to his more purely astronomical labours, he had to invent that branch of mathematical science by which alone the problems he proposed could be solved. it was for this purpose that he devised the indispensable method of calculation which we now know so well as trigonometry. without the aid rendered by this beautiful art it would have been impossible for any really important advance in astronomical calculation to have been effected. but the discovery which shows, beyond all others, that hipparchus possessed one of the master-minds of all time was the detection of that remarkable celestial movement known as the precession of the equinoxes. the inquiry which conducted to this discovery involved a most profound investigation, especially when it is remembered that in the days of hipparchus the means of observation of the heavenly bodies were only of the rudest description, and the available observations of earlier dates were extremely scanty. we can but look with astonishment on the genius of the man who, in spite of such difficulties, was able to detect such a phenomenon as the precession, and to exhibit its actual magnitude. i shall endeavour to explain the nature of this singular celestial movement, for it may be said to offer the first instance in the history of science in which we find that combination of accurate observation with skilful interpretation, of which, in the subsequent development of astronomy, we have so many splendid examples. the word equinox implies the condition that the night is equal to the day. to a resident on the equator the night is no doubt equal to the day at all times in the year, but to one who lives on any other part of the earth, in either hemisphere, the night and the day are not generally equal. there is, however, one occasion in spring, and another in autumn, on which the day and the night are each twelve hours at all places on the earth. when the night and day are equal in spring, the point which the sun occupies on the heavens is termed the vernal equinox. there is similarly another point in which the sun is situated at the time of the autumnal equinox. in any investigation of the celestial movements the positions of these two equinoxes on the heavens are of primary importance, and hipparchus, with the instinct of genius, perceived their significance, and commenced to study them. it will be understood that we can always define the position of a point on the sky with reference to the surrounding stars. no doubt we do not see the stars near the sun when the sun is shining, but they are there nevertheless. the ingenuity of hipparchus enabled him to determine the positions of each of the two equinoxes relatively to the stars which lie in its immediate vicinity. after examination of the celestial places of these points at different periods, he was led to the conclusion that each equinox was moving relatively to the stars, though that movement was so slow that twenty five thousand years would necessarily elapse before a complete circuit of the heavens was accomplished. hipparchus traced out this phenomenon, and established it on an impregnable basis, so that all astronomers have ever since recognised the precession of the equinoxes as one of the fundamental facts of astronomy. not until nearly two thousand years after hipparchus had made this splendid discovery was the explanation of its cause given by newton. from the days of hipparchus down to the present hour the science of astronomy has steadily grown. one great observer after another has appeared from time to time, to reveal some new phenomenon with regard to the celestial bodies or their movements, while from time to time one commanding intellect after another has arisen to explain the true import of the facts of observations. the history of astronomy thus becomes inseparable from the history of the great men to whose labours its development is due. in the ensuing chapters we have endeavoured to sketch the lives and the work of the great philosophers, by whose labours the science of astronomy has been created. we shall commence with ptolemy, who, after the foundations of the science had been laid by hipparchus, gave to astronomy the form in which it was taught throughout the middle ages. we shall next see the mighty revolution in our conceptions of the universe which are associated with the name of copernicus. we then pass to those periods illumined by the genius of galileo and newton, and afterwards we shall trace the careers of other more recent discoverers, by whose industry and genius the boundaries of human knowledge have been so greatly extended. our history will be brought down late enough to include some of the illustrious astronomers who laboured in the generation which has just passed away. ptolemy. [plate: ptolemy.] the career of the famous man whose name stands at the head of this chapter is one of the most remarkable in the history of human learning. there may have been other discoverers who have done more for science than ever ptolemy accomplished, but there never has been any other discoverer whose authority on the subject of the movements of the heavenly bodies has held sway over the minds of men for so long a period as the fourteen centuries during which his opinions reigned supreme. the doctrines he laid down in his famous book, "the almagest," prevailed throughout those ages. no substantial addition was made in all that time to the undoubted truths which this work contained. no important correction was made of the serious errors with which ptolemy's theories were contaminated. the authority of ptolemy as to all things in the heavens, and as to a good many things on the earth (for the same illustrious man was also a diligent geographer), was invariably final. though every child may now know more of the actual truths of the celestial motions than ever ptolemy knew, yet the fact that his work exercised such an astonishing effect on the human intellect for some sixty generations, shows that it must have been an extraordinary production. we must look into the career of this wonderful man to discover wherein lay the secret of that marvellous success which made him the unchallenged instructor of the human race for such a protracted period. unfortunately, we know very little as to the personal history of ptolemy. he was a native of egypt, and though it has been sometimes conjectured that he belonged to the royal families of the same name, yet there is nothing to support such a belief. the name, ptolemy, appears to have been a common one in egypt in those days. the time at which he lived is fixed by the fact that his first recorded observation was made in ad, and his last in ad. when we add that he seems to have lived in or near alexandria, or to use his own words, "on the parallel of alexandria," we have said everything that can be said so far as his individuality is concerned. ptolemy is, without doubt, the greatest figure in ancient astronomy. he gathered up the wisdom of the philosophers who had preceded him. he incorporated this with the results of his own observations, and illumined it with his theories. his speculations, even when they were, as we now know, quite erroneous, had such an astonishing verisimilitude to the actual facts of nature that they commanded universal assent. even in these modern days we not unfrequently find lovers of paradox who maintain that ptolemy's doctrines not only seem true, but actually are true. in the absence of any accurate knowledge of the science of mechanics, philosophers in early times were forced to fall back on certain principles of more or less validity, which they derived from their imagination as to what the natural fitness of things ought to be. there was no geometrical figure so simple and so symmetrical as a circle, and as it was apparent that the heavenly bodies pursued tracks which were not straight lines, the conclusion obviously followed that their movements ought to be circular. there was no argument in favour of this notion, other than the merely imaginary reflection that circular movement, and circular movement alone, was "perfect," whatever "perfect" may have meant. it was further believed to be impossible that the heavenly bodies could have any other movements save those which were perfect. assuming this, it followed, in ptolemy's opinion, and in that of those who came after him for fourteen centuries, that all the tracks of the heavenly bodies were in some way or other to be reduced to circles. ptolemy succeeded in devising a scheme by which the apparent changes that take place in the heavens could, so far as he knew them, be explained by certain combinations of circular movement. this seemed to reconcile so completely the scheme of things celestial with the geometrical instincts which pointed to the circle as the type of perfect movement, that we can hardly wonder ptolemy's theory met with the astonishing success that attended it. we shall, therefore, set forth with sufficient detail the various steps of this famous doctrine. ptolemy commences with laying down the undoubted truth that the shape of the earth is globular. the proofs which he gives of this fundamental fact are quite satisfactory; they are indeed the same proofs as we give today. there is, first of all, the well-known circumstance of which our books on geography remind us, that when an object is viewed at a distance across the sea, the lower part of the object appears cut off by the interposing curved mass of water. the sagacity of ptolemy enabled him to adduce another argument, which, though not quite so obvious as that just mentioned, demonstrates the curvature of the earth in a very impressive manner to anyone who will take the trouble to understand it. ptolemy mentions that travellers who went to the south reported, that, as they did so, the appearance of the heavens at night underwent a gradual change. stars that they were familiar with in the northern skies gradually sank lower in the heavens. the constellation of the great bear, which in our skies never sets during its revolution round the pole, did set and rise when a sufficient southern latitude had been attained. on the other hand, constellations new to the inhabitants of northern climes were seen to rise above the southern horizon. these circumstances would be quite incompatible with the supposition that the earth was a flat surface. had this been so, a little reflection will show that no such changes in the apparent movements of the stars would be the consequence of a voyage to the south. ptolemy set forth with much insight the significance of this reasoning, and even now, with the resources of modern discoveries to help us, we can hardly improve upon his arguments. ptolemy, like a true philosopher disclosing a new truth to the world, illustrated and enforced his subject by a variety of happy demonstrations. i must add one of them, not only on account of its striking nature, but also because it exemplifies ptolemy's acuteness. if the earth were flat, said this ingenious reasoner, sunset must necessarily take place at the same instant, no matter in what country the observer may happen to be placed. ptolemy, however, proved that the time of sunset did vary greatly as the observer's longitude was altered. to us, of course, this is quite obvious; everybody knows that the hour of sunset may have been reached in great britain while it is still noon on the western coast of america. ptolemy had, however, few of those sources of knowledge which are now accessible. how was he to show that the sun actually did set earlier at alexandria than it would in a city which lay a hundred miles to the west? there was no telegraph wire by which astronomers at the two places could communicate. there was no chronometer or watch which could be transported from place to place; there was not any other reliable contrivance for the keeping of time. ptolemy's ingenuity, however, pointed out a thoroughly satisfactory method by which the times of sunset at two places could be compared. he was acquainted with the fact, which must indeed have been known from the very earliest times, that the illumination of the moon is derived entirely from the sun. he knew that an eclipse of the moon was due to the interposition of the earth which cuts off the light of the sun. it was, therefore, plain that an eclipse of the moon must be a phenomenon which would begin at the same instant from whatever part of the earth the moon could be seen at the time. ptolemy, therefore, brought together from various quarters the local times at which different observers had recorded the beginning of a lunar eclipse. he found that the observers to the west made the time earlier and earlier the further away their stations were from alexandria. on the other hand, the eastern observers set down the hour as later than that at which the phenomenon appeared at alexandria. as these observers all recorded something which indeed appeared to them simultaneously, the only interpretation was, that the more easterly a place the later its time. suppose there were a number of observers along a parallel of latitude, and each noted the hour of sunset to be six o'clock, then, since the eastern times are earlier than western times, p.m. at one station a will correspond to p.m. at a station b sufficiently to the west. if, therefore, it is sunset to the observer at a, the hour of sunset will not yet be reached for the observer at b. this proves conclusively that the time of sunset is not the same all over the earth. we have, however, already seen that the apparent time of sunset would be the same from all stations if the earth were flat. when ptolemy, therefore, demonstrated that the time of sunset was not the same at various places, he showed conclusively that the earth was not flat. as the same arguments applied to all parts of the earth where ptolemy had either been himself, or from which he could gain the necessary information, it followed that the earth, instead of being the flat plain, girdled with an illimitable ocean, as was generally supposed, must be in reality globular. this led at once to a startling consequence. it was obvious that there could be no supports of any kind by which this globe was sustained; it therefore followed that the mighty object must be simply poised in space. this is indeed an astonishing doctrine to anyone who relies on what merely seems the evidence of the senses, without giving to that evidence its due intellectual interpretation. according to our ordinary experience, the very idea of an object poised without support in space, appears preposterous. would it not fall? we are immediately asked. yes, doubtless it could not remain poised in any way in which we try the experiment. we must, however, observe that there are no such ideas as upwards or downwards in relation to open space. to say that a body falls downwards, merely means that it tries to fall as nearly as possible towards the centre of the earth. there is no one direction along which a body will tend to move in space, in preference to any other. this may be illustrated by the fact that a stone let fall at new zealand will, in its approach towards the earth's centre, be actually moving upwards as far as any locality in our hemisphere is concerned. why, then, argued ptolemy, may not the earth remain poised in space, for as all directions are equally upward or equally downward, there seems no reason why the earth should require any support? by this reasoning he arrives at the fundamental conclusion that the earth is a globular body freely lying in space, and surrounded above, below, and on all sides by the glittering stars of heaven. the perception of this sublime truth marks a notable epoch in the history of the gradual development of the human intellect. no doubt, other philosophers, in groping after knowledge, may have set forth certain assertions that are more or less equivalent to this fundamental truth. it is to ptolemy we must give credit, however, not only for announcing this doctrine, but for demonstrating it by clear and logical argument. we cannot easily project our minds back to the conception of an intellectual state in which this truth was unfamiliar. it may, however, be well imagined that, to one who thought the earth was a flat plain of indefinite extent, it would be nothing less than an intellectual convulsion for him to be forced to believe that he stood upon a spherical earth, forming merely a particle relatively to the immense sphere of the heavens. what ptolemy saw in the movements of the stars led him to the conclusion that they were bright points attached to the inside of a tremendous globe. the movements of this globe which carried the stars were only compatible with the supposition that the earth occupied its centre. the imperceptible effect produced by a change in the locality of the observer on the apparent brightness of the stars made it plain that the dimensions of the terrestrial globe must be quite insignificant in comparison with those of the celestial sphere. the earth might, in fact, be regarded as a grain of sand while the stars lay upon a globe many yards in diameter. so tremendous was the revolution in human knowledge implied by this discovery, that we can well imagine how ptolemy, dazzled as it were by the fame which had so justly accrued to him, failed to make one further step. had he made that step, it would have emancipated the human intellect from the bondage of fourteen centuries of servitude to a wholly monstrous notion of this earth's importance in the scheme of the heavens. the obvious fact that the sun, the moon, and the stars rose day by day, moved across the sky in a glorious never-ending procession, and duly set when their appointed courses had been run, demanded some explanation. the circumstance that the fixed stars preserved their mutual distances from year to year, and from age to age, appeared to ptolemy to prove that the sphere which contained those stars, and on whose surface they were believed by him to be fixed, revolved completely around the earth once every day. he would thus account for all the phenomena of rising and setting consistently with the supposition that our globe was stationary. probably this supposition must have appeared monstrous, even to ptolemy. he knew that the earth was a gigantic object, but, large as it may have been, he knew that it was only a particle in comparison with the celestial sphere, yet he apparently believed, and certainly succeeded in persuading other men to believe, that the celestial sphere did actually perform these movements. ptolemy was an excellent geometer. he knew that the rising and the setting of the sun, the moon, and the myriad stars, could have been accounted for in a different way. if the earth turned round uniformly once a day while poised at the centre of the sphere of the heavens, all the phenomena of rising and setting could be completely explained. this is, indeed, obvious after a moment's reflection. consider yourself to be standing on the earth at the centre of the heavens. there are stars over your head, and half the contents of the heavens are visible, while the other half are below your horizon. as the earth turns round, the stars over your head will change, and unless it should happen that you have taken up your position at either of the poles, new stars will pass into your view, and others will disappear, for at no time can you have more than half of the whole sphere visible. the observer on the earth would, therefore, say that some stars were rising, and that some stars were setting. we have, therefore, two totally distinct methods, each of which would completely explain all the observed facts of the diurnal movement. one of these suppositions requires that the celestial sphere, bearing with it the stars and other celestial bodies, turns uniformly around an invisible axis, while the earth remains stationary at the centre. the other supposition would be, that it is the stupendous celestial sphere which remains stationary, while the earth at the centre rotates about the same axis as the celestial sphere did before, but in an opposite direction, and with a uniform velocity which would enable it to complete one turn in twenty-four hours. ptolemy was mathematician enough to know that either of these suppositions would suffice for the explanation of the observed facts. indeed, the phenomena of the movements of the stars, so far as he could observe them, could not be called upon to pronounce which of these views was true, and which was false. ptolemy had, therefore, to resort for guidance to indirect lines of reasoning. one of these suppositions must be true, and yet it appeared that the adoption of either was accompanied by a great difficulty. it is one of his chief merits to have demonstrated that the celestial sphere was so stupendous that the earth itself was absolutely insignificant in comparison therewith. if, then, this stupendous sphere rotated once in twenty-four hours, the speed with which the movement of some of the stars must be executed would be so portentous as to seem well-nigh impossible. it would, therefore, seem much simpler on this ground to adopt the other alternative, and to suppose the diurnal movements were due to the rotation of the earth. here ptolemy saw, or at all events fancied he saw, objections of the weightiest description. the evidence of the senses appeared directly to controvert the supposition that this earth is anything but stationary. ptolemy might, perhaps, have dismissed this objection on the ground that the testimony of the senses on such a matter should be entirely subordinated to the interpretation which our intelligence would place upon the facts to which the senses deposed. another objection, however, appeared to him to possess the gravest moment. it was argued that if the earth were rotating, there is nothing to make the air participate in this motion, mankind would therefore be swept from the earth by the furious blasts which would arise from the movement of the earth through an atmosphere at rest. even if we could imagine that the air were carried round with the earth, the same would not apply, so thought ptolemy, to any object suspended in the air. so long as a bird was perched on a tree, he might very well be carried onward by the moving earth, but the moment he took wing, the ground would slip from under him at a frightful pace, so that when he dropped down again he would find himself at a distance perhaps ten times as great as that which a carrier-pigeon or a swallow could have traversed in the same time. some vague delusion of this description seems even still to crop up occasionally. i remember hearing of a proposition for balloon travelling of a very remarkable kind. the voyager who wanted to reach any other place in the same latitude was simply to ascend in a balloon, and wait there till the rotation of the earth conveyed the locality which happened to be his destination directly beneath him, whereupon he was to let out the gas and drop down! ptolemy knew quite enough natural philosophy to be aware that such a proposal for locomotion would be an utter absurdity; he knew that there was no such relative shift between the air and the earth as this motion would imply. it appeared to him to be necessary that the air should lag behind, if the earth had been animated by a movement of rotation. in this he was, as we know, entirely wrong. there were, however, in his days no accurate notions on the subject of the laws of motion. assiduous as ptolemy may have been in the study of the heavenly bodies, it seems evident that he cannot have devoted much thought to the phenomena of motion of terrestrial objects. simple, indeed, are the experiments which might have convinced a philosopher much less acute than ptolemy, that, if the earth did revolve, the air must necessarily accompany it. if a rider galloping on horseback tosses a ball into the air, it drops again into his hand, just as it would have done had he been remaining at rest during the ball's flight; the ball in fact participates in the horizontal motion, so that though it really describes a curve as any passer-by would observe, yet it appears to the rider himself merely to move up and down in a straight line. this fact, and many others similar to it, demonstrate clearly that if the earth were endowed with a movement of rotation, the atmosphere surrounding it must participate in that movement. ptolemy did not know this, and consequently he came to the conclusion that the earth did not rotate, and that, therefore, notwithstanding the tremendous improbability of so mighty an object as the celestial sphere spinning round once in every twenty-four hours, there was no course open except to believe that this very improbable thing did really happen. thus it came to pass that ptolemy adopted as the cardinal doctrine of his system a stationary earth poised at the centre of the celestial sphere, which stretched around on all sides at a distance so vast that the diameter of the earth was an inappreciable point in comparison therewith. ptolemy having thus deliberately rejected the doctrine of the earth's rotation, had to make certain other entirely erroneous suppositions. it was easily seen that each star required exactly the same period for the performance of a complete revolution of the heavens. ptolemy knew that the stars were at enormous distances from the earth, though no doubt his notions on this point came very far short of what we know to be the reality. if the stars had been at very varied distances, then it would be so wildly improbable that they should all accomplish their revolutions in the same time, that ptolemy came to the conclusion that they must be all at the same distance, that is, that they must be all on the surface of a sphere. this view, however erroneous, was corroborated by the obvious fact that the stars in the constellations preserved their relative places unaltered for centuries. thus it was that ptolemy came to the conclusion that they were all fixed on one spherical surface, though we are not informed as to the material of this marvellous setting which sustained the stars like jewels. nor should we hastily pronounce this doctrine to be absurd. the stars do appear to lie on the surface of a sphere, of which the observer is at the centre; not only is this the aspect which the skies present to the untechnical observer, but it is the aspect in which the skies are presented to the most experienced astronomer of modern days. no doubt he knows well that the stars are at the most varied distances from him; he knows that certain stars are ten times, or a hundred times, or a thousand times, as far as other stars. nevertheless, to his eye the stars appear on the surface of the sphere, it is on that surface that his measurements of the relative places of the stars are made; indeed, it may be said that almost all the accurate observations in the observatory relate to the places of the stars, not as they really are, but as they appear to be projected on that celestial sphere whose conception we owe to the genius of ptolemy. this great philosopher shows very ingeniously that the earth must be at the centre of the sphere. he proves that, unless this were the case, each star would not appear to move with the absolute uniformity which does, as a matter of fact, characterise it. in all these reasonings we cannot but have the most profound admiration for the genius of ptolemy, even though he had made an error so enormous in the fundamental point of the stability of the earth. another error of a somewhat similar kind seemed to ptolemy to be demonstrated. he had shown that the earth was an isolated object in space, and being such was, of course, capable of movement. it could either be turned round, or it could be moved from one place to another. we know that ptolemy deliberately adopted the view that the earth did not turn round; he had then to investigate the other question, as to whether the earth was animated by any movement of translation. he came to the conclusion that to attribute any motion to the earth would be incompatible with the truths at which he had already arrived. the earth, argued ptolemy, lies at the centre of the celestial sphere. if the earth were to be endowed with movement, it would not lie always at this point, it must, therefore, shift to some other part of the sphere. the movements of the stars, however, preclude the possibility of this; and, therefore, the earth must be as devoid of any movement of translation as it is devoid of rotation. thus it was that ptolemy convinced himself that the stability of the earth, as it appeared to the ordinary senses, had a rational philosophical foundation. not unfrequently it is the lot of the philosophers to contend against the doctrines of the vulgar, but when it happens, as in the case of ptolemy's researches, that the doctrines of the vulgar are corroborated by philosophical investigation which bear the stamp of the highest authority, it is not to be wondered at that such doctrines should be deemed well-nigh impregnable. in this way we may, perhaps, account for the remarkable fact that the theories of ptolemy held unchallenged sway over the human intellect for the vast period already mentioned. up to the present we have been speaking only of those primary motions of the heavens, by which the whole sphere appeared to revolve once every twenty-four hours. we have now to discuss the remarkable theories by which ptolemy endeavoured to account for the monthly movement of the moon, for the annual movement of the sun, and for the periodic movements of the planets which had gained for them the titles of the wandering stars. possessed with the idea that these movements must be circular, or must be capable, directly or indirectly, of being explained by circular movements, it seemed obvious to ptolemy, as indeed it had done to previous astronomers, that the track of the moon through the stars was a circle of which the earth is the centre. a similar movement with a yearly period must also be attributed to the sun, for the changes in the positions of the constellations in accordance with the progress of the seasons, placed it beyond doubt that the sun made a circuit of the celestial sphere, even though the bright light of the sun prevented the stars in its vicinity, from being seen in daylight. thus the movements both of the sun and the moon, as well as the diurnal rotation of the celestial sphere, seemed to justify the notion that all celestial movements must be "perfect," that is to say, described uniformly in those circles which were the only perfect curves. the simplest observations, however, show that the movements of the planets cannot be explained in this simple fashion. here the geometrical genius of ptolemy shone forth, and he devised a scheme by which the apparent wanderings of the planets could be accounted for without the introduction of aught save "perfect" movements. to understand his reasoning, let us first set forth clearly those facts of observation which require to be explained. i shall take, in particular, two planets, venus and mars, as these illustrate, in the most striking manner, the peculiarities of the inner and the outer planets respectively. the simplest observations would show that venus did not move round the heavens in the same fashion as the sun or the moon. look at the evening star when brightest, as it appears in the west after sunset. instead of moving towards the east among the stars, like the sun or the moon, we find, week after week, that venus is drawing in towards the sun, until it is lost in the sunbeams. then the planet emerges on the other side, not to be seen as an evening star, but as a morning star. in fact, it was plain that in some ways venus accompanied the sun in its annual movement. now it is found advancing in front of the sun to a certain limited distance, and now it is lagging to an equal extent behind the sun. [fig. . ptolemy's planetary scheme.] these movements were wholly incompatible with the supposition that the journeys of venus were described by a single motion of the kind regarded as perfect. it was obvious that the movement was connected in some strange manner with the revolution of the sun, and here was the ingenious method by which ptolemy sought to render account of it. imagine a fixed arm to extend from the earth to the sun, as shown in the accompanying figure (fig. ), then this arm will move round uniformly, in consequence of the sun's movement. at a point p on this arm let a small circle be described. venus is supposed to revolve uniformly in this small circle, while the circle itself is carried round continuously by the movement of the sun. in this way it was possible to account for the chief peculiarities in the movement of venus. it will be seen that, in consequence of the revolution around p, the spectator on the earth will sometimes see venus on one side of the sun, and sometimes on the other side, so that the planet always remains in the sun's vicinity. by properly proportioning the movements, this little contrivance simulated the transitions from the morning star to the evening star. thus the changes of venus could be accounted for by a combination of the "perfect" movement of p in the circle which it described uniformly round the earth, combined with the "perfect" motion of venus in the circle which it described uniformly around the moving centre. in a precisely similar manner ptolemy rendered an explanation of the fitful apparitions of mercury. now just on one side of the sun, and now just on the other, this rarely-seen planet moved like venus on a circle whereof the centre was also carried by the line joining the sun and the earth. the circle, however, in which mercury actually revolved had to be smaller than that of venus, in order to account for the fact that mercury lies always much closer to the sun than the better-known planet. [fig. . ptolemy's theory of the movement of mars.] the explanation of the movement of an outer planet like mars could also be deduced from the joint effect of two perfect motions. the changes through which mars goes are, however, so different from the movements of venus that quite a different disposition of the circles is necessary. for consider the facts which characterise the movements of an outer planet such as mars. in the first place, mars accomplishes an entire circuit of the heaven. in this respect, no doubt, it may be said to resemble the sun or the moon. a little attention will, however, show that there are extraordinary irregularities in the movement of the planet. generally speaking, it speeds its way from west to east among the stars, but sometimes the attentive observer will note that the speed with which the planet advances is slackening, and then it will seem to become stationary. some days later the direction of the planet's movement will be reversed, and it will be found moving from the east towards the west. at first it proceeds slowly and then quickens its pace, until a certain speed is attained, which afterwards declines until a second stationary position is reached. after a due pause the original motion from west to east is resumed, and is continued until a similar cycle of changes again commences. such movements as these were obviously quite at variance with any perfect movement in a single circle round the earth. here, again, the geometrical sagacity of ptolemy provided him with the means of representing the apparent movements of mars, and, at the same time, restricting the explanation to those perfect movements which he deemed so essential. in fig. we exhibit ptolemy's theory as to the movement of mars. we have, as before, the earth at the centre, and the sun describing its circular orbit around that centre. the path of mars is to be taken as exterior to that of the sun. we are to suppose that at a point marked m there is a fictitious planet, which revolves around the earth uniformly, in a circle called the deferent. this point m, which is thus animated by a perfect movement, is the centre of a circle which is carried onwards with m, and around the circumference of which mars revolves uniformly. it is easy to show that the combined effect of these two perfect movements is to produce exactly that displacement of mars in the heavens which observation discloses. in the position represented in the figure, mars is obviously pursuing a course which will appear to the observer as a movement from west to east. when, however, the planet gets round to such a position as r, it is then moving from east to west in consequence of its revolution in the moving circle, as indicated by the arrow-head. on the other hand, the whole circle is carried forward in the opposite direction. if the latter movement be less rapid than the former, then we shall have the backward movement of mars on the heavens which it was desired to explain. by a proper adjustment of the relative lengths of these arms the movements of the planet as actually observed could be completely accounted for. the other outer planets with which ptolemy was acquainted, namely, jupiter and saturn, had movements of the same general character as those of mars. ptolemy was equally successful in explaining the movements they performed by the supposition that each planet had perfect rotation in a circle of its own, which circle itself had perfect movement around the earth in the centre. it is somewhat strange that ptolemy did not advance one step further, as by so doing he would have given great simplicity to his system. he might, for instance, have represented the movements of venus equally well by putting the centre of the moving circle at the sun itself, and correspondingly enlarging the circle in which venus revolved. he might, too, have arranged that the several circles which the outer planets traversed should also have had their centres at the sun. the planetary system would then have consisted of an earth fixed at the centre, of a sun revolving uniformly around it, and of a system of planets each describing its own circle around a moving centre placed in the sun. perhaps ptolemy had not thought of this, or perhaps he may have seen arguments against it. this important step was, however, taken by tycho. he considered that all the planets revolved around the sun in circles, and that the sun itself, bearing all these orbits, described a mighty circle around the earth. this point having been reached, only one more step would have been necessary to reach the glorious truths that revealed the structure of the solar system. that last step was taken by copernicus. copernicus [plate: thorn, from an old print.] the quaint town of thorn, on the vistula, was more than two centuries old when copernicus was born there on the th of february, . the situation of this town on the frontier between prussia and poland, with the commodious waterway offered by the river, made it a place of considerable trade. a view of the town, as it was at the time of the birth of copernicus, is here given. the walls, with their watch-towers, will be noted, and the strategic importance which the situation of thorn gave to it in the fifteenth century still belongs thereto, so much so that the german government recently constituted the town a fortress of the first class. copernicus, the astronomer, whose discoveries make him the great predecessor of kepler and newton, did not come from a noble family, as certain other early astronomers have done, for his father was a tradesman. chroniclers are, however, careful to tell us that one of his uncles was a bishop. we are not acquainted with any of those details of his childhood or youth which are often of such interest in other cases where men have risen to exalted fame. it would appear that the young nicolaus, for such was his christian name, received his education at home until such time as he was deemed sufficiently advanced to be sent to the university at cracow. the education that he there obtained must have been in those days of a very primitive description, but copernicus seems to have availed himself of it to the utmost. he devoted himself more particularly to the study of medicine, with the view of adopting its practice as the profession of his life. the tendencies of the future astronomer were, however, revealed in the fact that he worked hard at mathematics, and, like one of his illustrious successors, galileo, the practice of the art of painting had for him a very great interest, and in it he obtained some measure of success. by the time he was twenty-seven years old, it would seem that copernicus had given up the notion of becoming a medical practitioner, and had resolved to devote himself to science. he was engaged in teaching mathematics, and appears to have acquired some reputation. his growing fame attracted the notice of his uncle the bishop, at whose suggestion copernicus took holy orders, and he was presently appointed to a canonry in the cathedral of frauenburg, near the mouth of the vistula. to frauenburg, accordingly, this man of varied gifts retired. possessing somewhat of the ascetic spirit, he resolved to devote his life to work of the most serious description. he eschewed all ordinary society, restricting his intimacies to very grave and learned companions, and refusing to engage in conversation of any useless kind. it would seem as if his gifts for painting were condemned as frivolous; at all events, we do not learn that he continued to practise them. in addition to the discharge of his theological duties, his life was occupied partly in ministering medically to the wants of the poor, and partly with his researches in astronomy and mathematics. his equipment in the matter of instruments for the study of the heavens seems to have been of a very meagre description. he arranged apertures in the walls of his house at allenstein, so that he could observe in some fashion the passage of the stars across the meridian. that he possessed some talent for practical mechanics is proved by his construction of a contrivance for raising water from a stream, for the use of the inhabitants of frauenburg. relics of this machine are still to be seen. [plate: copernicus.] the intellectual slumber of the middle ages was destined to be awakened by the revolutionary doctrines of copernicus. it may be noted, as an interesting circumstance, that the time at which he discovered the scheme of the solar system has coincided with a remarkable epoch in the world's history. the great astronomer had just reached manhood at the time when columbus discovered the new world. before the publication of the researches of copernicus, the orthodox scientific creed averred that the earth was stationary, and that the apparent movements of the heavenly bodies were indeed real movements. ptolemy had laid down this doctrine , years before. in his theory this huge error was associated with so much important truth, and the whole presented such a coherent scheme for the explanation of the heavenly movements, that the ptolemaic theory was not seriously questioned until the great work of copernicus appeared. no doubt others, before copernicus, had from time to time in some vague fashion surmised, with more or less plausibility, that the sun, and not the earth, was the centre about which the system really revolved. it is, however, one thing to state a scientific fact; it is quite another thing to be in possession of the train of reasoning, founded on observation or experiment, by which that fact may be established. pythagoras, it appears, had indeed told his disciples that it was the sun, and not the earth, which was the centre of movement, but it does not seem at all certain that pythagoras had any grounds which science could recognise for the belief which is attributed to him. so far as information is available to us, it would seem that pythagoras associated his scheme of things celestial with a number of preposterous notions in natural philosophy. he may certainly have made a correct statement as to which was the most important body in the solar system, but he certainly did not provide any rational demonstration of the fact. copernicus, by a strict train of reasoning, convinced those who would listen to him that the sun was the centre of the system. it is useful for us to consider the arguments which he urged, and by which he effected that intellectual revolution which is always connected with his name. the first of the great discoveries which copernicus made relates to the rotation of the earth on its axis. that general diurnal movement, by which the stars and all other celestial bodies appear to be carried completely round the heavens once every twenty-four hours, had been accounted for by ptolemy on the supposition that the apparent movements were the real movements. as we have already seen, ptolemy himself felt the extraordinary difficulty involved in the supposition that so stupendous a fabric as the celestial sphere should spin in the way supposed. such movements required that many of the stars should travel with almost inconceivable velocity. copernicus also saw that the daily rising and setting of the heavenly bodies could be accounted for either by the supposition that the celestial sphere moved round and that the earth remained at rest, or by the supposition that the celestial sphere was at rest while the earth turned round in the opposite direction. he weighed the arguments on both sides as ptolemy had done, and, as the result of his deliberations, copernicus came to an opposite conclusion from ptolemy. to copernicus it appeared that the difficulties attending the supposition that the celestial sphere revolved, were vastly greater than those which appeared so weighty to ptolemy as to force him to deny the earth's rotation. copernicus shows clearly how the observed phenomena could be accounted for just as completely by a rotation of the earth as by a rotation of the heavens. he alludes to the fact that, to those on board a vessel which is moving through smooth water, the vessel itself appears to be at rest, while the objects on shore seem to be moving past. if, therefore, the earth were rotating uniformly, we dwellers upon the earth, oblivious of our own movement, would wrongly attribute to the stars the displacement which was actually the consequence of our own motion. copernicus saw the futility of the arguments by which ptolemy had endeavoured to demonstrate that a revolution of the earth was impossible. it was plain to him that there was nothing whatever to warrant refusal to believe in the rotation of the earth. in his clear-sightedness on this matter we have specially to admire the sagacity of copernicus as a natural philosopher. it had been urged that, if the earth moved round, its motion would not be imparted to the air, and that therefore the earth would be uninhabitable by the terrific winds which would be the result of our being carried through the air. copernicus convinced himself that this deduction was preposterous. he proved that the air must accompany the earth, just as his coat remains round him, notwithstanding the fact that he is walking down the street. in this way he was able to show that all a priori objections to the earth's movements were absurd, and therefore he was able to compare together the plausibilities of the two rival schemes for explaining the diurnal movement. [plate: frauenburg, from an old print.] once the issue had been placed in this form, the result could not be long in doubt. here is the question: which is it more likely--that the earth, like a grain of sand at the centre of a mighty globe, should turn round once in twenty-four hours, or that the whole of that vast globe should complete a rotation in the opposite direction in the same time? obviously, the former is far the more simple supposition. but the case is really much stronger than this. ptolemy had supposed that all the stars were attached to the surface of a sphere. he had no ground whatever for this supposition, except that otherwise it would have been well-nigh impossible to have devised a scheme by which the rotation of the heavens around a fixed earth could have been arranged. copernicus, however, with the just instinct of a philosopher, considered that the celestial sphere, however convenient from a geometrical point of view, as a means of representing apparent phenomena, could not actually have a material existence. in the first place, the existence of a material celestial sphere would require that all the myriad stars should be at exactly the same distances from the earth. of course, no one will say that this or any other arbitrary disposition of the stars is actually impossible, but as there was no conceivable physical reason why the distances of all the stars from the earth should be identical, it seemed in the very highest degree improbable that the stars should be so placed. doubtless, also, copernicus felt a considerable difficulty as to the nature of the materials from which ptolemy's wonderful sphere was to be constructed. nor could a philosopher of his penetration have failed to observe that, unless that sphere were infinitely large, there must have been space outside it, a consideration which would open up other difficult questions. whether infinite or not, it was obvious that the celestial sphere must have a diameter at least many thousands of times as great as that of the earth. from these considerations copernicus deduced the important fact that the stars and the other celestial bodies must all be vast objects. he was thus enabled to put the question in such a form that it could hardly receive any answer but the correct one. which is it more rational to suppose, that the earth should turn round on its axis once in twenty-four hours, or that thousands of mighty stars should circle round the earth in the same time, many of them having to describe circles many thousands of times greater in circumference than the circuit of the earth at the equator? the obvious answer pressed upon copernicus with so much force that he was compelled to reject ptolemy's theory of the stationary earth, and to attribute the diurnal rotation of the heavens to the revolution of the earth on its axis. once this tremendous step had been taken, the great difficulties which beset the monstrous conception of the celestial sphere vanished, for the stars need no longer be regarded as situated at equal distances from the earth. copernicus saw that they might lie at the most varied degrees of remoteness, some being hundreds or thousands of times farther away than others. the complicated structure of the celestial sphere as a material object disappeared altogether; it remained only as a geometrical conception, whereon we find it convenient to indicate the places of the stars. once the copernican doctrine had been fully set forth, it was impossible for anyone, who had both the inclination and the capacity to understand it, to withhold acceptance of its truth. the doctrine of a stationary earth had gone for ever. copernicus having established a theory of the celestial movements which deliberately set aside the stability of the earth, it seemed natural that he should inquire whether the doctrine of a moving earth might not remove the difficulties presented in other celestial phenomena. it had been universally admitted that the earth lay unsupported in space. copernicus had further shown that it possessed a movement of rotation. its want of stability being thus recognised, it seemed reasonable to suppose that the earth might also have some other kinds of movements as well. in this, copernicus essayed to solve a problem far more difficult than that which had hitherto occupied his attention. it was a comparatively easy task to show how the diurnal rising and setting could be accounted for by the rotation of the earth. it was a much more difficult undertaking to demonstrate that the planetary movements, which ptolemy had represented with so much success, could be completely explained by the supposition that each of those planets revolved uniformly round the sun, and that the earth was also a planet, accomplishing a complete circuit of the sun once in the course of a year. [plate: explanation of planetary movements.] it would be impossible in a sketch like the present to enter into any detail as to the geometrical propositions on which this beautiful investigation of copernicus depended. we can only mention a few of the leading principles. it may be laid down in general that, if an observer is in movement, he will, if unconscious of the fact, attribute to the fixed objects around him a movement equal and opposite to that which he actually possesses. a passenger on a canal-boat sees the objects on the banks apparently moving backward with a speed equal to that by which he is himself advancing forwards. by an application of this principle, we can account for all the phenomena of the movements of the planets, which ptolemy had so ingeniously represented by his circles. let us take, for instance, the most characteristic feature in the irregularities of the outer planets. we have already remarked that mars, though generally advancing from west to east among the stars, occasionally pauses, retraces his steps for awhile, again pauses, and then resumes his ordinary onward progress. copernicus showed clearly how this effect was produced by the real motion of the earth, combined with the real motion of mars. in the adjoining figure we represent a portion of the circular tracks in which the earth and mars move in accordance with the copernican doctrine. i show particularly the case where the earth comes directly between the planet and the sun, because it is on such occasions that the retrograde movement (for so this backward movement of mars is termed) is at its highest. mars is then advancing in the direction shown by the arrow-head, and the earth is also advancing in the same direction. we, on the earth, however, being unconscious of our own motion, attribute, by the principle i have already explained, an equal and opposite motion to mars. the visible effect upon the planet is, that mars has two movements, a real onward movement in one direction, and an apparent movement in the opposite direction. if it so happened that the earth was moving with the same speed as mars, then the apparent movement would exactly neutralise the real movement, and mars would seem to be at rest relatively to the surrounding stars. under the actual circumstances represented, however, the earth is moving faster than mars, and the consequence is, that the apparent movement of the planet backwards exceeds the real movement forwards, the net result being an apparent retrograde movement. with consummate skill, copernicus showed how the applications of the same principles could account for the characteristic movements of the planets. his reasoning in due time bore down all opposition. the supreme importance of the earth in the system vanished. it had now merely to take rank as one of the planets. the same great astronomer now, for the first time, rendered something like a rational account of the changes of the seasons. nor did certain of the more obscure astronomical phenomena escape his attention. he delayed publishing his wonderful discoveries to the world until he was quite an old man. he had a well-founded apprehension of the storm of opposition which they would arouse. however, he yielded at last to the entreaties of his friends, and his book was sent to the press. but ere it made its appearance to the world, copernicus was seized by mortal illness. a copy of the book was brought to him on may , . we are told that he was able to see it and to touch it, but no more, and he died a few hours afterwards. he was buried in that cathedral of frauenburg, with which his life had been so closely associated. tycho brahe. the most picturesque figure in the history of astronomy is undoubtedly that of the famous old danish astronomer whose name stands at the head of this chapter. tycho brahe was alike notable for his astronomical genius and for the extraordinary vehemence of a character which was by no means perfect. his romantic career as a philosopher, and his taste for splendour as a danish noble, his ardent friendships and his furious quarrels, make him an ideal subject for a biographer, while the magnificent astronomical work which he accomplished, has given him imperishable fame. the history of tycho brahe has been admirably told by dr. dreyer, the accomplished astronomer who now directs the observatory at armagh, though himself a countryman of tycho. every student of the career of the great dane must necessarily look on dr. dreyer's work as the chief authority on the subject. tycho sprang from an illustrious stock. his family had flourished for centuries, both in sweden and in denmark, where his descendants are to be met with at the present day. the astronomer's father was a privy councillor, and having filled important positions in the danish government, he was ultimately promoted to be governor of helsingborg castle, where he spent the last years of his life. his illustrious son tycho was born in , and was the second child and eldest boy in a family of ten. it appears that otto, the father of tycho, had a brother named george, who was childless. george, however, desired to adopt a boy on whom he could lavish his affection and to whom he could bequeath his wealth. a somewhat singular arrangement was accordingly entered into by the brothers at the time when otto was married. it was agreed that the first son who might be born to otto should be forthwith handed over by the parents to george to be reared and adopted by him. in due time little tycho appeared, and was immediately claimed by george in pursuance of the compact. but it was not unnatural that the parental instinct, which had been dormant when the agreement was made, should here interpose. tycho's father and mother receded from the bargain, and refused to part with their son. george thought he was badly treated. however, he took no violent steps until a year later, when a brother was born to tycho. the uncle then felt no scruple in asserting what he believed to be his rights by the simple process of stealing the first-born nephew, which the original bargain had promised him. after a little time it would seem that the parents acquiesced in the loss, and thus it was in uncle george's home that the future astronomer passed his childhood. when we read that tycho was no more than thirteen years old at the time he entered the university of copenhagen, it might be at first supposed that even in his boyish years he must have exhibited some of those remarkable talents with which he was afterwards to astonish the world. such an inference should not, however, be drawn. the fact is that in those days it was customary for students to enter the universities at a much earlier age than is now the case. not, indeed, that the boys of thirteen knew more then than the boys of thirteen know now. but the education imparted in the universities at that time was of a much more rudimentary kind than that which we understand by university education at present. in illustration of this dr. dreyer tells us how, in the university of wittenberg, one of the professors, in his opening address, was accustomed to point out that even the processes of multiplication and division in arithmetic might be learned by any student who possessed the necessary diligence. it was the wish and the intention of his uncle that tycho's education should be specially directed to those branches of rhetoric and philosophy which were then supposed to be a necessary preparation for the career of a statesman. tycho, however, speedily made it plain to his teachers that though he was an ardent student, yet the things which interested him were the movements of the heavenly bodies and not the subtleties of metaphysics. [plate: tycho brahe.] on the st october, , an eclipse of the sun occurred, which was partially visible at copenhagen. tycho, boy though he was, took the utmost interest in this event. his ardour and astonishment in connection with the circumstance were chiefly excited by the fact that the time of the occurrence of the phenomenon could be predicted with so much accuracy. urged by his desire to understand the matter thoroughly, tycho sought to procure some book which might explain what he so greatly wanted to know. in those days books of any kind were but few and scarce, and scientific books were especially unattainable. it so happened, however, that a latin version of ptolemy's astronomical works had appeared a few years before the eclipse took place, and tycho managed to buy a copy of this book, which was then the chief authority on celestial matters. young as the boy astronomer was, he studied hard, although perhaps not always successfully, to understand ptolemy, and to this day his copy of the great work, copiously annotated and marked by the schoolboy hand, is preserved as one of the chief treasures in the library of the university at prague. after tycho had studied for about three years at the university of copenhagen, his uncle thought it would be better to send him, as was usual in those days, to complete his education by a course of study in some foreign university. the uncle cherished the hope that in this way the attention of the young astronomer might be withdrawn from the study of the stars and directed in what appeared to him a more useful way. indeed, to the wise heads of those days, the pursuit of natural science seemed so much waste of good time which might otherwise be devoted to logic or rhetoric or some other branch of study more in vogue at that time. to assist in this attempt to wean tycho from his scientific tastes, his uncle chose as a tutor to accompany him an intelligent and upright young man named vedel, who was four years senior to his pupil, and accordingly, in , we find the pair taking up their abode at the university of leipzig. the tutor, however, soon found that he had undertaken a most hopeless task. he could not succeed in imbuing tycho with the slightest taste for the study of the law or the other branches of knowledge which were then thought so desirable. the stars, and nothing but the stars, engrossed the attention of his pupil. we are told that all the money he could obtain was spent secretly in buying astronomical books and instruments. he learned the name of the stars from a little globe, which he kept hidden from vedel, and only ventured to use during the latter's absence. no little friction was at first caused by all this, but in after years a fast and enduring friendship grew up between tycho and his tutor, each of whom learned to respect and to love the other. before tycho was seventeen he had commenced the difficult task of calculating the movements of the planets and the places which they occupied on the sky from time to time. he was not a little surprised to find that the actual positions of the planets differed very widely from those which were assigned to them by calculations from the best existing works of astronomers. with the insight of genius he saw that the only true method of investigating the movements of the heavenly bodies would be to carry on a protracted series of measurements of their places. this, which now seems to us so obvious, was then entirely new doctrine. tycho at once commenced regular observations in such fashion as he could. his first instrument was, indeed, a very primitive one, consisting of a simple pair of compasses, which he used in this way. he placed his eye at the hinge, and then opened the legs of the compass so that one leg pointed to one star and the other leg to the other star. the compass was then brought down to a divided circle, by which means the number of degrees in the apparent angular distance of the two stars was determined. his next advance in instrumental equipment was to provide himself with the contrivance known as the "cross-staff," which he used to observe the stars whenever opportunity offered. it must, of course, be remembered that in those days there were no telescopes. in the absence of optical aid, such as lenses afford the modern observers, astronomers had to rely on mechanical appliances alone to measure the places of the stars. of such appliances, perhaps the most ingenious was one known before tycho's time, which we have represented in the adjoining figure. [plate: tycho's cross staff.] let us suppose that it be desired to measure the angle between two stars, then if the angle be not too large it can be determined in the following manner. let the rod ab be divided into inches and parts of an inch, and let another rod, cd, slide up and down along ab in such a way that the two always remain perpendicular to each other. "sights," like those on a rifle, are placed at a and c, and there is a pin at d. it will easily be seen that, by sliding the movable bar along the fixed one, it must always be possible when the stars are not too far apart to bring the sights into such positions that one star can be seen along dc and the other along da. this having been accomplished, the length from a to the cross-bar is read off on the scale, and then, by means of a table previously prepared, the value of the required angular distance is obtained. if the angle between the two stars were greater than it would be possible to measure in the way already described, then there was a provision by which the pin at d might be moved along cd into some other position, so as to bring the angular distance of the stars within the range of the instrument. [plate: tycho's "new star" sextant of . (the arms, of walnut wood, are about / ft. long.)] no doubt the cross-staff is a very primitive contrivance, but when handled by one so skilful as tycho it afforded results of considerable accuracy. i would recommend any reader who may have a taste for such pursuits to construct a cross-staff for himself, and see what measurements he can accomplish with its aid. to employ this little instrument tycho had to evade the vigilance of his conscientious tutor, who felt it his duty to interdict all such occupations as being a frivolous waste of time. it was when vedel was asleep that tycho managed to escape with his cross staff and measure the places of the heavenly bodies. even at this early age tycho used to conduct his observations on those thoroughly sound principles which lie at the foundation of all accurate modern astronomy. recognising the inevitable errors of workmanship in his little instrument, he ascertained their amount and allowed for their influence on the results which he deduced. this principle, employed by the boy with his cross-staff in , is employed at the present day by the astronomer royal at greenwich with the most superb instruments that the skill of modern opticians has been able to construct. [plate: tycho's trigonic sextant. (the arms, ab and ac, are about / ft. long.)] after the death of his uncle, when tycho was nineteen years of age, it appears that the young philosopher was no longer interfered with in so far as the line which his studies were to take was concerned. always of a somewhat restless temperament, we now find that he shifted his abode to the university of rostock, where he speedily made himself notable in connection with an eclipse of the moon on th october, . like every other astronomer of those days, tycho had always associated astronomy with astrology. he considered that the phenomena of the heavenly bodies always had some significance in connection with human affairs. tycho was also a poet, and in the united capacity of poet, astrologer, and astronomer, he posted up some verses in the college at rostock announcing that the lunar eclipse was a prognostication of the death of the great turkish sultan, whose mighty deeds at that time filled men's minds. presently news did arrive of the death of the sultan, and tycho was accordingly triumphant; but a little later it appeared that the decease had taken place before the eclipse, a circumstance which caused many a laugh at tycho's expense. [plate: tycho's astronomic sextant. (made of steel: the arms, ab, ac, measure ft.) plate: tycho's equatorial armillary. (the meridian circle, e b c a d, made of solid steel, is nearly ft. in diameter.)] tycho being of a somewhat turbulent disposition, it appears that, while at the university of rostock, he had a serious quarrel with another danish nobleman. we are not told for certain what was the cause of the dispute. it does not, however, seem to have had any more romantic origin than a difference of opinion as to which of them knew the more mathematics. they fought, as perhaps it was becoming for two astronomers to fight, under the canopy of heaven in utter darkness at the dead of night, and the duel was honourably terminated when a slice was taken off tycho's nose by the insinuating sword of his antagonist. for the repair of this injury the ingenuity of the great instrument-maker was here again useful, and he made a substitute for his nose "with a composition of gold and silver." the imitation was so good that it is declared to have been quite equal to the original. dr. lodge, however, pointedly observes that it does not appear whether this remark was made by a friend or an enemy. [plate: the great augsburg quadrant. (built of heart of oak; the radii about ft.) plate: tycho's "new scheme of the terrestrial system," .] the next few years tycho spent in various places ardently pursuing somewhat varied branches of scientific study. at one time we hear of him assisting an astronomical alderman, in the ancient city of augsburg, to erect a tremendous wooden machine--a quadrant of -feet radius--to be used in observing the heavens. at another time we learn that the king of denmark had recognised the talents of his illustrious subject, and promised to confer on him a pleasant sinecure in the shape of a canonry, which would assist him with the means for indulging his scientific pursuits. again we are told that tycho is pursuing experiments in chemistry with the greatest energy, nor is this so incompatible as might at first be thought with his devotion to astronomy. in those early days of knowledge the different sciences seemed bound together by mysterious bonds. alchemists and astrologers taught that the several planets were correlated in some mysterious manner with the several metals. it was, therefore hardly surprising that tycho should have included a study of the properties of the metals in the programme of his astronomical work. [plate: uraniborg and its grounds. plate: ground-plan of the observatory.] an event, however, occurred in which stimulated tycho's astronomical labours, and started him on his life's work. on the th of november in that year, he was returning home to supper after a day's work in his laboratory, when he happened to lift his face to the sky, and there he beheld a brilliant new star. it was in the constellation of cassiopeia, and occupied a position in which there had certainly been no bright star visible when his attention had last been directed to that part of the heavens. such a phenomenon was so startling that he found it hard to trust the evidence of his senses. he thought he must be the subject of some hallucination. he therefore called to the servants who were accompanying him, and asked them whether they, too, could see a brilliant object in the direction in which he pointed. they certainly could, and thus he became convinced that this marvellous object was no mere creation of the fancy, but a veritable celestial body--a new star of surpassing splendour which had suddenly burst forth. in these days of careful scrutiny of the heavens, we are accustomed to the occasional outbreak of new stars. it is not, however, believed that any new star which has ever appeared has displayed the same phenomenal brilliance as was exhibited by the star of . this object has a value in astronomy far greater than it might at first appear. it is true, in one sense, that tycho discovered the new star, but it is equally true, in a different sense, that it was the new star which discovered tycho. had it not been for this opportune apparition, it is quite possible that tycho might have found a career in some direction less beneficial to science than that which he ultimately pursued. [plate: the observatory of uraniborg, island of hven.] when he reached his home on this memorable evening, tycho immediately applied his great quadrant to the measurement of the place of the new star. his observations were specially directed to the determination of the distance of the object. he rightly conjectured that if it were very much nearer to us than the stars in its vicinity, the distance of the brilliant body might be determined in a short time by the apparent changes in its distance from the surrounding points. it was speedily demonstrated that the new star could not be as near as the moon, by the simple fact that its apparent place, as compared with the stars in its neighbourhood, was not appreciably altered when it was observed below the pole, and again above the pole at an interval of twelve hours. such observations were possible, inasmuch as the star was bright enough to be seen in full daylight. tycho thus showed conclusively that the body was so remote that the diameter of the earth bore an insignificant ratio to the star's distance. his success in this respect is the more noteworthy when we find that many other observers, who studied the same object, came to the erroneous conclusion that the new star was quite as near as the moon, or even much nearer. in fact, it may be said, that with regard to this object tycho discovered everything which could possibly have been discovered in the days before telescopes were invented. he not only proved that the star's distance was too great for measurement, but he showed that it had no proper motion on the heavens. he recorded the successive changes in its brightness from week to week, as well as the fluctuations in hue with which the alterations in lustre were accompanied. it seems, nowadays, strange to find that such thoroughly scientific observations of the new star as those which tycho made, possessed, even in the eyes of the great astronomer himself, a profound astrological significance. we learn from dr. dreyer that, in tycho's opinion, "the star was at first like venus and jupiter, and its effects will therefore, first, be pleasant; but as it then became like mars, there will next come a period of wars, seditions, captivity, and death of princes, and destruction of cities, together with dryness and fiery meteors in the air, pestilence, and venomous snakes. lastly, the star became like saturn, and thus will finally come a time of want, death, imprisonment, and all kinds of sad things!" ideas of this kind were, however, universally entertained. it seemed, indeed, obvious to learned men of that period that such an apparition must forebode startling events. one of the chief theories then held was, that just as the star of bethlehem announced the first coming of christ, so the second coming, and the end of the world, was heralded by the new star of . the researches of tycho on this object were the occasion of his first appearance as an author. the publication of his book was however, for some time delayed by the urgent remonstrances of his friends, who thought it was beneath the dignity of a nobleman to condescend to write a book. happily, tycho determined to brave the opinion of his order; the book appeared, and was the first of a series of great astronomical productions from the same pen. [plate: effigy on tycho's tomb at prague.] the fame of the noble dane being now widespread, the king of denmark entreated him to return to his native country, and to deliver a course of lectures on astronomy in the university of copenhagen. with some reluctance he consented, and his introductory oration has been preserved. he dwells, in fervent language, upon the beauty and the interest of the celestial phenomena. he points out the imperative necessity of continuous and systematic observation of the heavenly bodies in order to extend our knowledge. he appeals to the practical utility of the science, for what civilised nation could exist without having the means of measuring time? he sets forth how the study of these beautiful objects "exalts the mind from earthly and trivial things to heavenly ones;" and then he winds up by assuring them that "a special use of astronomy is that it enables us to draw conclusions from the movements in the celestial regions as to human fate." an interesting event, which occurred in , distracted tycho's attention from astronomical matters. he fell in love. the young girl on whom his affections were set appears to have sprung from humble origin. here again his august family friends sought to dissuade him from a match they thought unsuitable for a nobleman. but tycho never gave way in anything. it is suggested that he did not seek a wife among the highborn dames of his own rank from the dread that the demands of a fashionable lady would make too great an inroad on the time that he wished to devote to science. at all events, tycho's union seems to have been a happy one, and he had a large family of children; none of whom, however, inherited their father's talents. [plate: tycho's mural quadrant picture, uraniborg.] tycho had many scientific friends in germany, among whom his work was held in high esteem. the treatment that he there met with seemed to him so much more encouraging than that which he received in denmark that he formed the notion of emigrating to basle and making it his permanent abode. a whisper of this intention was conveyed to the large-hearted king of denmark, frederick ii. he wisely realised how great would be the fame which would accrue to his realm if he could induce tycho to remain within danish territory and carry on there the great work of his life. a resolution to make a splendid proposal to tycho was immediately formed. a noble youth was forthwith despatched as a messenger, and ordered to travel day and night until he reached tycho, whom he was to summon to the king. the astronomer was in bed on the morning of th february, , when the message was delivered. tycho, of course, set off at once and had an audience of the king at copenhagen. the astronomer explained that what he wanted was the means to pursue his studies unmolested, whereupon the king offered him the island of hven, in the sound near elsinore. there he would enjoy all the seclusion that he could desire. the king further promised that he would provide the funds necessary for building a house and for founding the greatest observatory that had ever yet been reared for the study of the heavens. after due deliberation and consultation with his friends, tycho accepted the king's offer. he was forthwith granted a pension, and a deed was drawn up formally assigning the island of hven to his use all the days of his life. the foundation of the famous castle of uraniborg was laid on th august, . the ceremony was a formal and imposing one, in accordance with tycho's ideas of splendour. a party of scientific friends had assembled, and the time had been chosen so that the heavenly bodies were auspiciously placed. libations of costly wines were poured forth, and the stone was placed with due solemnity. the picturesque character of this wonderful temple for the study of the stars may be seen in the figures with which this chapter is illustrated. one of the most remarkable instruments that has ever been employed in studying the heavens was the mural quadrant which tycho erected in one of the apartments of uraniborg. by its means the altitudes of the celestial bodies could be observed with much greater accuracy than had been previously attainable. this wonderful contrivance is represented on the preceding page. it will be observed that the walls of the room are adorned by pictures with a lavishness of decoration not usually to be found in scientific establishments. a few years later, when the fame of the observatory at hven became more widely spread, a number of young men flocked to tycho to study under his direction. he therefore built another observatory for their use in which the instruments were placed in subterranean rooms of which only the roofs appeared above the ground. there was a wonderful poetical inscription over the entrance to this underground observatory, expressing the astonishment of urania at finding, even in the interior of the earth, a cavern devoted to the study of the heavens. tycho was indeed always fond of versifying, and he lost no opportunity of indulging this taste whenever an occasion presented itself. around the walls of the subterranean observatory were the pictures of eight astronomers, each with a suitable inscription--one of these of course represented tycho himself, and beneath were written words to the effect that posterity should judge of his work. the eighth picture depicted an astronomer who has not yet come into existence. tychonides was his name, and the inscription presses the modest hope that when he does appear he will be worthy of his great predecessor. the vast expenses incurred in the erection and the maintenance of this strange establishment were defrayed by a succession of grants from the royal purse. for twenty years tycho laboured hard at uraniborg in the pursuit of science. his work mainly consisted in the determination of the places of the moon, the planets, and the stars on the celestial sphere. the extraordinary pains taken by tycho to have his observations as accurate as his instruments would permit, have justly entitled him to the admiration of all succeeding astronomers. his island home provided the means of recreation as well as a place for work. he was surrounded by his family, troops of friends were not wanting, and a pet dwarf seems to have been an inmate of his curious residence. by way of change from his astronomical labours he used frequently to work with his students in his chemical laboratory. it is not indeed known what particular problems in chemistry occupied his attention. we are told, however, that he engaged largely in the production of medicines, and as these appear to have been dispensed gratuitously there was no lack of patients. tycho's imperious and grasping character frequently brought him into difficulties, which seem to have increased with his advancing years. he had ill-treated one of his tenants on hven, and an adverse decision by the courts seems to have greatly exasperated the astronomer. serious changes also took place in his relations to the court at copenhagen. when the young king was crowned in , he reversed the policy of his predecessor with reference to hven. the liberal allowances to tycho were one after another withdrawn, and finally even his pension was stopped. tycho accordingly abandoned hven in a tumult of rage and mortification. a few years later we find him in bohemia a prematurely aged man, and he died on the th october, . galileo. among the ranks of the great astronomers it would be difficult to find one whose life presents more interesting features and remarkable vicissitudes than does that of galileo. we may consider him as the patient investigator and brilliant discoverer. we may consider him in his private relations, especially to his daughter, sister maria celeste, a woman of very remarkable character; and we have also the pathetic drama at the close of galileo's life, when the philosopher drew down upon himself the thunders of the inquisition. the materials for the sketch of this astonishing man are sufficiently abundant. we make special use in this place of those charming letters which his daughter wrote to him from her convent home. more than a hundred of these have been preserved, and it may well be doubted whether any more beautiful and touching series of letters addressed to a parent by a dearly loved child have ever been written. an admirable account of this correspondence is contained in a little book entitled "the private life of galileo," published anonymously by messrs. macmillan in , and i have been much indebted to the author of that volume for many of the facts contained in this chapter. galileo was born at pisa, on th february, . he was the eldest son of vincenzo de' bonajuti de' galilei, a florentine noble. notwithstanding his illustrious birth and descent, it would seem that the home in which the great philosopher's childhood was spent was an impoverished one. it was obvious at least that the young galileo would have to be provided with some profession by which he might earn a livelihood. from his father he derived both by inheritance and by precept a keen taste for music, and it appears that he became an excellent performer on the lute. he was also endowed with considerable artistic power, which he cultivated diligently. indeed, it would seem that for some time the future astronomer entertained the idea of devoting himself to painting as a profession. his father, however, decided that he should study medicine. accordingly, we find that when galileo was seventeen years of age, and had added a knowledge of greek and latin to his acquaintance with the fine arts, he was duly entered at the university of pisa. here the young philosopher obtained some inkling of mathematics, whereupon he became so much interested in this branch of science, that he begged to be allowed to study geometry. in compliance with his request, his father permitted a tutor to be engaged for this purpose; but he did so with reluctance, fearing that the attention of the young student might thus be withdrawn from that medical work which was regarded as his primary occupation. the event speedily proved that these anxieties were not without some justification. the propositions of euclid proved so engrossing to galileo that it was thought wise to avoid further distraction by terminating the mathematical tutor's engagement. but it was too late for the desired end to be attained. galileo had now made such progress that he was able to continue his geometrical studies by himself. presently he advanced to that famous th proposition which won his lively admiration, and on he went until he had mastered the six books of euclid, which was a considerable achievement for those days. the diligence and brilliance of the young student at pisa did not, however, bring him much credit with the university authorities. in those days the doctrines of aristotle were regarded as the embodiment of all human wisdom in natural science as well as in everything else. it was regarded as the duty of every student to learn aristotle off by heart, and any disposition to doubt or even to question the doctrines of the venerated teacher was regarded as intolerable presumption. but young galileo had the audacity to think for himself about the laws of nature. he would not take any assertion of fact on the authority of aristotle when he had the means of questioning nature directly as to its truth or falsehood. his teachers thus came to regard him as a somewhat misguided youth, though they could not but respect the unflagging industry with which he amassed all the knowledge he could acquire. [plate: galileo's pendulum.] we are so accustomed to the use of pendulums in our clocks that perhaps we do not often realise that the introduction of this method of regulating time-pieces was really a notable invention worthy the fame of the great astronomer to whom it was due. it appears that sitting one day in the cathedral of pisa, galileo's attention became concentrated on the swinging of a chandelier which hung from the ceiling. it struck him as a significant point, that whether the arc through which the pendulum oscillated was a long one or a short one, the time occupied in each vibration was sensibly the same. this suggested to the thoughtful observer that a pendulum would afford the means by which a time-keeper might be controlled, and accordingly galileo constructed for the first time a clock on this principle. the immediate object sought in this apparatus was to provide a means of aiding physicians in counting the pulses of their patients. the talents of galileo having at length extorted due recognition from the authorities, he was appointed, at the age of twenty-five, professor of mathematics at the university of pisa. then came the time when he felt himself strong enough to throw down the gauntlet to the adherents of the old philosophy. as a necessary part of his doctrine on the movement of bodies aristotle had asserted that the time occupied by a stone in falling depends upon its weight, so that the heavier the stone the less time would it require to fall from a certain height to the earth. it might have been thought that a statement so easily confuted by the simplest experiments could never have maintained its position in any accepted scheme of philosophy. but aristotle had said it, and to anyone who ventured to express a doubt the ready sneer was forthcoming, "do you think yourself a cleverer man than aristotle?" galileo determined to demonstrate in the most emphatic manner the absurdity of a doctrine which had for centuries received the sanction of the learned. the summit of the leaning tower of pisa offered a highly dramatic site for the great experiment. the youthful professor let fall from the overhanging top a large heavy body and a small light body simultaneously. according to aristotle the large body ought to have reached the ground much sooner than the small one, but such was found not to be the case. in the sight of a large concourse of people the simple fact was demonstrated that the two bodies fell side by side, and reached the ground at the same time. thus the first great step was taken in the overthrow of that preposterous system of unquestioning adhesion to dogma, which had impeded the development of the knowledge of nature for nearly two thousand years. this revolutionary attitude towards the ancient beliefs was not calculated to render galileo's relations with the university authorities harmonious. he had also the misfortune to make enemies in other quarters. don giovanni de medici, who was then the governor of the port of leghorn, had designed some contrivance by which he proposed to pump out a dock. but galileo showed up the absurdity of this enterprise in such an aggressive manner that don giovanni took mortal offence, nor was he mollified when the truths of galileo's criticisms were abundantly verified by the total failure of his ridiculous invention. in various ways galileo was made to feel his position at pisa so unpleasant that he was at length compelled to abandon his chair in the university. the active exertions of his friends, of whom galileo was so fortunate as to have had throughout his life an abundant supply, then secured his election to the professorship of mathematics at padua, whither he went in . [plate: portrait of galileo.] it was in this new position that galileo entered on that marvellous career of investigation which was destined to revolutionize science. the zeal with which he discharged his professorial duties was indeed of the most unremitting character. he speedily drew such crowds to listen to his discourses on natural philosophy that his lecture-room was filled to overflowing. he also received many private pupils in his house for special instruction. every moment that could be spared from these labours was devoted to his private study and to his incessant experiments. like many another philosopher who has greatly extended our knowledge of nature, galileo had a remarkable aptitude for the invention of instruments designed for philosophical research. to facilitate his practical work, we find that in he had engaged a skilled workman who was to live in his house, and thus be constantly at hand to try the devices for ever springing from galileo's fertile brain. among the earliest of his inventions appears to have been the thermometer, which he constructed in . no doubt this apparatus in its primitive form differed in some respects from the contrivance we call by the same name. galileo at first employed water as the agent, by the expansion of which the temperature was to be measured. he afterwards saw the advantage of using spirits for the same purpose. it was not until about half a century later that mercury came to be recognised as the liquid most generally suitable for the thermometer. the time was now approaching when galileo was to make that mighty step in the advancement of human knowledge which followed on the application of the telescope to astronomy. as to how his idea of such an instrument originated, we had best let him tell us in his own words. the passage is given in a letter which he writes to his brother-in-law, landucci. "i write now because i have a piece of news for you, though whether you will be glad or sorry to hear it i cannot say; for i have now no hope of returning to my own country, though the occurrence which has destroyed that hope has had results both useful and honourable. you must know, then, that two months ago there was a report spread here that in flanders some one had presented to count maurice of nassau a glass manufactured in such a way as to make distant objects appear very near, so that a man at the distance of two miles could be clearly seen. this seemed to me so marvellous that i began to think about it. as it appeared to me to have a foundation in the theory of perspective, i set about contriving how to make it, and at length i found out, and have succeeded so well that the one i have made is far superior to the dutch telescope. it was reported in venice that i had made one, and a week since i was commanded to show it to his serenity and to all the members of the senate, to their infinite amazement. many gentlemen and senators, even the oldest, have ascended at various times the highest bell-towers in venice to spy out ships at sea making sail for the mouth of the harbour, and have seen them clearly, though without my telescope they would have been invisible for more than two hours. the effect of this instrument is to show an object at a distance of say fifty miles, as if it were but five miles." the remarkable properties of the telescope at once commanded universal attention among intellectual men. galileo received applications from several quarters for his new instrument, of which it would seem that he manufactured a large number to be distributed as gifts to various illustrious personages. but it was reserved for galileo himself to make that application of the instrument to the celestial bodies by which its peculiar powers were to inaugurate the new era in astronomy. the first discovery that was made in this direction appears to have been connected with the number of the stars. galileo saw to his amazement that through his little tube he could count ten times as many stars in the sky as his unaided eye could detect. here was, indeed, a surprise. we are now so familiar with the elementary facts of astronomy that it is not always easy to realise how the heavens were interpreted by the observers in those ages prior to the invention of the telescope. we can hardly, indeed, suppose that galileo, like the majority of those who ever thought of such matters, entertained the erroneous belief that the stars were on the surface of a sphere at equal distances from the observer. no one would be likely to have retained his belief in such a doctrine when he saw how the number of visible stars could be increased tenfold by means of galileo's telescope. it would have been almost impossible to refuse to draw the inference that the stars thus brought into view were still more remote objects which the telescope was able to reveal, just in the same way as it showed certain ships to the astonished venetians, when at the time these ships were beyond the reach of unaided vision. galileo's celestial discoveries now succeeded each other rapidly. that beautiful milky way, which has for ages been the object of admiration to all lovers of nature, never disclosed its true nature to the eye of man till the astronomer of padua turned on it his magic tube. the splendid zone of silvery light was then displayed as star-dust scattered over the black background of the sky. it was observed that though the individual stars were too small to be seen severally without optical aid, yet such was their incredible number that the celestial radiance produced that luminosity with which every stargazer was so familiar. but the greatest discovery made by the telescope in these early days, perhaps, indeed, the greatest discovery that the telescope has ever accomplished, was the detection of the system of four satellites revolving around the great planet jupiter. this phenomenon was so wholly unexpected by galileo that, at first, he could hardly believe his eyes. however, the reality of the existence of a system of four moons attending the great planet was soon established beyond all question. numbers of great personages crowded to galileo to see for themselves this beautiful miniature representing the sun with its system of revolving planets. of course there were, as usual, a few incredulous people who refused to believe the assertion that four more moving bodies had to be added to the planetary system. they scoffed at the notion; they said the satellites may have been in the telescope, but that they were not in the sky. one sceptical philosopher is reported to have affirmed, that even if he saw the moons of jupiter himself he would not believe in them, as their existence was contrary to the principles of common-sense! there can be no doubt that a special significance attached to the new discovery at this particular epoch in the history of science. it must be remembered that in those days the doctrine of copernicus, declaring that the sun, and not the earth, was the centre of the system, that the earth revolved on its axis once a day, and that it described a mighty circle round the sun once a year, had only recently been promulgated. this new view of the scheme of nature had been encountered with the most furious opposition. it may possibly have been that galileo himself had not felt quite confident in the soundness of the copernican theory, prior to the discovery of the satellites of jupiter. but when a picture was there exhibited in which a number of relatively small globes were shown to be revolving around a single large globe in the centre, it seemed impossible not to feel that the beautiful spectacle so displayed was an emblem of the relations of the planets to the sun. it was thus made manifest to galileo that the copernican theory of the planetary system must be the true one. the momentous import of this opinion upon the future welfare of the great philosopher will presently appear. it would seem that galileo regarded his residence at padua as a state of undesirable exile from his beloved tuscany. he had always a yearning to go back to his own country and at last the desired opportunity presented itself. for now that galileo's fame had become so great, the grand duke of tuscany desired to have the philosopher resident at florence, in the belief that he would shed lustre on the duke's dominions. overtures were accordingly made to galileo, and the consequence was that in we find him residing at florence, bearing the title of mathematician and philosopher to the grand duke. two daughters, polissena and virginia, and one son, vincenzo, had been born to galileo in padua. it was the custom in those days that as soon as the daughter of an italian gentleman had grown up, her future career was somewhat summarily decided. either a husband was to be forthwith sought out, or she was to enter the convent with the object of taking the veil as a professed nun. it was arranged that the two daughters of galileo, while still scarcely more than children, should both enter the franciscan convent of st. matthew, at arcetri. the elder daughter polissena, took the name of sister maria celeste, while virginia became sister arcangela. the latter seems to have been always delicate and subject to prolonged melancholy, and she is of but little account in the narrative of the life of galileo. but sister maria celeste, though never leaving the convent, managed to preserve a close intimacy with her beloved father. this was maintained only partly by galileo's visits, which were very irregular and were, indeed, often suspended for long intervals. but his letters to this daughter were evidently frequent and affectionate, especially in the latter part of his life. most unfortunately, however, all his letters have been lost. there are grounds for believing that they were deliberately destroyed when galileo was seized by the inquisition, lest they should have been used as evidence against him, or lest they should have compromised the convent where they were received. but sister maria celeste's letters to her father have happily been preserved, and most touching these letters are. we can hardly read them without thinking how the sweet and gentle nun would have shrunk from the idea of their publication. her loving little notes to her "dearest lord and father," as she used affectionately to call galileo, were almost invariably accompanied by some gift, trifling it may be, but always the best the poor nun had to bestow. the tender grace of these endearing communications was all the more precious to him from the fact that the rest of galileo's relatives were of quite a worthless description. he always acknowledged the ties of his kindred in the most generous way, but their follies and their vices, their selfishness and their importunities, were an incessant source of annoyance to him, almost to the last day of his life. on th december, , sister maria celeste writes:-- "i send two baked pears for these days of vigil. but as the greatest treat of all, i send you a rose, which ought to please you extremely, seeing what a rarity it is at this season; and with the rose you must accept its thorns, which represent the bitter passion of our lord, whilst the green leaves represent the hope we may entertain that through the same sacred passion we, having passed through the darkness of the short winter of our mortal life, may attain to the brightness and felicity of an eternal spring in heaven." when the wife and children of galileo's shiftless brother came to take up their abode in the philosopher's home, sister maria celeste feels glad to think that her father has now some one who, however imperfectly, may fulfil the duty of looking after him. a graceful note on christmas eve accompanies her little gifts. she hopes that-- "in these holy days the peace of god may rest on him and all the house. the largest collar and sleeves i mean for albertino, the other two for the two younger boys, the little dog for baby, and the cakes for everybody, except the spice-cakes, which are for you. accept the good-will which would readily do much more." the extraordinary forbearance with which galileo continually placed his time, his purse, and his influence at the service of those who had repeatedly proved themselves utterly unworthy of his countenance, is thus commented on by the good nun.-- "now it seems to me, dearest lord and father, that your lordship is walking in the right path, since you take hold of every occasion that presents itself to shower continual benefits on those who only repay you with ingratitude. this is an action which is all the more virtuous and perfect as it is the more difficult." when the plague was raging in the neighbourhood, the loving daughter's solicitude is thus shown:-- "i send you two pots of electuary as a preventive against the plague. the one without the label consists of dried figs, walnuts, rue, and salt, mixed together with honey. a piece of the size of a walnut to be taken in the morning, fasting, with a little greek wine." the plague increasing still more, sister maria celeste obtained with much difficulty, a small quantity of a renowned liqueur, made by abbess ursula, an exceptionally saintly nun. this she sends to her father with the words:-- "i pray your lordship to have faith in this remedy. for if you have so much faith in my poor miserable prayers, much more may you have in those of such a holy person; indeed, through her merits you may feel sure of escaping all danger from the plague." whether galileo took the remedy we do not know, but at all events he escaped the plague. [plate: the villa arcetri. galileo's residence, where milton visited him.] from galileo's new home in florence the telescope was again directed to the skies, and again did astounding discoveries reward the astronomer's labours. the great success which he had met with in studying jupiter naturally led galileo to look at saturn. here he saw a spectacle which was sufficiently amazing, though he failed to interpret it accurately. it was quite manifest that saturn did not exhibit a simple circular disc like jupiter, or like mars. it seemed to galileo as if the planet consisted of three bodies, a large globe in the centre, and a smaller one on each side. the enigmatical nature of the discovery led galileo to announce it in an enigmatical manner. he published a string of letters which, when duly transposed, made up a sentence which affirmed that the planet saturn was threefold. of course we now know that this remarkable appearance of the planet was due to the two projecting portions of the ring. with the feeble power of galileo's telescope, these seemed merely like small globes or appendages to the large central body. the last of galileo's great astronomical discoveries related to the libration of the moon. i think that the detection of this phenomenon shows his acuteness of observation more remarkably than does any one of his other achievements with the telescope. it is well known that the moon constantly keeps the same face turned towards the earth. when, however, careful measurements have been made with regard to the spots and marks on the lunar surface, it is found that there is a slight periodic variation which permits us to see now a little to the east or to the west, now a little to the north or to the south of the average lunar disc. but the circumstances which make the career of galileo so especially interesting from the biographer's point of view, are hardly so much the triumphs that he won as the sufferings that he endured. the sufferings and the triumphs were, however, closely connected, and it is fitting that we should give due consideration to what was perhaps the greatest drama in the history of science. on the appearance of the immortal work of copernicus, in which it was taught that the earth rotated on its axis, and that the earth, like the other planets, revolved round the sun, orthodoxy stood aghast. the holy roman church submitted this treatise, which bore the name "de revolutionibus orbium coelestium," to the congregation of the index. after due examination it was condemned as heretical in . galileo was suspected, on no doubt excellent grounds, of entertaining the objectionable views of copernicus. he was accordingly privately summoned before cardinal bellarmine on th february , and duly admonished that he was on no account to teach or to defend the obnoxious doctrines. galileo was much distressed by this intimation. he felt it a serious matter to be deprived of the privilege of discoursing with his friends about the copernican system, and of instructing his disciples in the principles of the great theory of whose truth he was perfectly convinced. it pained him, however, still more to think, devout catholic as he was, that such suspicions of his fervent allegiance to his church should ever have existed, as were implied by the words and monitions of cardinal bellarmine. in , galileo had an interview with pope paul v., who received the great astronomer very graciously, and walked up and down with him in conversation for three-quarters of an hour. galileo complained to his holiness of the attempts made by his enemies to embarrass him with the authorities of the church, but the pope bade him be comforted. his holiness had himself no doubts of galileo's orthodoxy, and he assured him that the congregation of the index should give galileo no further trouble so long as paul v. was in the chair of st. peter. on the death of paul v. in , maffeo barberini was elected pope, as urban viii. this new pope, while a cardinal, had been an intimate friend of galileo's, and had indeed written latin verses in praise of the great astronomer and his discoveries. it was therefore not unnatural for galileo to think that the time had arrived when, with the use of due circumspection, he might continue his studies and his writings, without fear of incurring the displeasure of the church. indeed, in , one of galileo's friends writing from rome, urges galileo to visit the city again, and added that-- "under the auspices of this most excellent, learned, and benignant pontiff, science must flourish. your arrival will be welcome to his holiness. he asked me if you were coming, and when, and in short, he seems to love and esteem you more than ever." the visit was duly paid, and when galileo returned to florence, the pope wrote a letter from which the following is an extract, commanding the philosopher to the good offices of the young ferdinand, who had shortly before succeeded his father in the grand duchy of tuscany. "we find in galileo not only literary distinction, but also the love of piety, and he is also strong in those qualities by which the pontifical good-will is easily obtained. and now, when he has been brought to this city to congratulate us on our elevation, we have very lovingly embraced him; nor can we suffer him to return to the country whither your liberality calls him, without an ample provision of pontifical love. and that you may know how dear he is to us, we have willed to give him this honourable testimonial of virtue and piety. and we further signify that every benefit which you shall confer upon him, imitating or even surpassing your father's liberality, will conduce to our gratification." the favourable reception which had been accorded to him by pope urban viii. seems to have led galileo to expect that there might be some corresponding change in the attitude of the papal authorities on the great question of the stability of the earth. he accordingly proceeded with the preparation of the chief work of his life, "the dialogue of the two systems." it was submitted for inspection by the constituted authorities. the pope himself thought that, if a few conditions which he laid down were duly complied with, there could be no objection to the publication of the work. in the first place, the title of the book was to be so carefully worded as to show plainly that the copernican doctrine was merely to be regarded as an hypothesis, and not as a scientific fact. galileo was also instructed to conclude the book with special arguments which had been supplied by the pope himself, and which appeared to his holiness to be quite conclusive against the new doctrine of copernicus. formal leave for the publication of the dialogue was then given to galileo by the inquisitor general, and it was accordingly sent to the press. it might be thought that the anxieties of the astronomer about his book would then have terminated. as a matter of fact, they had not yet seriously begun. riccardi, the master of the sacred palace, having suddenly had some further misgivings, sent to galileo for the manuscript while the work was at the printer's, in order that the doctrine it implied might be once again examined. apparently, riccardi had come to the conclusion that he had not given the matter sufficient attention, when the authority to go to press had been first and, perhaps, hastily given. considerable delay in the issue of the book was the result of these further deliberations. at last, however, in june, , galileo's great work, "the dialogue of the two systems," was produced for the instruction of the world, though the occasion was fraught with ruin to the immortal author. [plate: facsimile sketch of lunar surface by galileo.] the book, on its publication, was received and read with the greatest avidity. but presently the master of the sacred palace found reason to regret that he had given his consent to its appearance. he accordingly issued a peremptory order to sequestrate every copy in italy. this sudden change in the papal attitude towards galileo formed the subject of a strong remonstrance addressed to the roman authorities by the grand duke of tuscany. the pope himself seemed to have become impressed all at once with the belief that the work contained matter of an heretical description. the general interpretation put upon the book seems to have shown the authorities that they had mistaken its true tendency, notwithstanding the fact that it had been examined again and again by theologians deputed for the duty. to the communication from the grand duke the pope returned answer, that he had decided to submit the book to a congregation of "learned, grave, and saintly men," who would weigh every word in it. the views of his holiness personally on the subject were expressed in his belief that the dialogue contained the most perverse matter that could come into a reader's hands. the master of the sacred palace was greatly blamed by the authorities for having given his sanction to its issue. he pleaded that the book had not been printed in the precise terms of the original manuscript which had been submitted to him. it was also alleged that galileo had not adhered to his promise of inserting properly the arguments which the pope himself had given in support of the old and orthodox view. one of these had, no doubt, been introduced, but, so far from mending galileo's case, it had made matters really look worse for the poor philosopher. the pope's argument had been put into the mouth of one of the characters in the dialogue named "simplicio." galileo's enemies maintained that by adopting such a method for the expression of his holiness's opinion, galileo had intended to hold the pope himself up to ridicule. galileo's friends maintained that nothing could have been farther from his intention. it seems, however, highly probable that the suspicions thus aroused had something to say to the sudden change of front on the part of the papal authorities. on st october, , galileo received an order to appear before the inquisition at rome on the grave charge of heresy. galileo, of course, expressed his submission, but pleaded for a respite from compliance with the summons, on the ground of his advanced age and his failing health. the pope was, however, inexorable; he said that he had warned galileo of his danger while he was still his friend. the command could not be disobeyed. galileo might perform the journey as slowly as he pleased, but it was imperatively necessary for him to set forth and at once. on th january, , galileo started on his weary journey to rome, in compliance with this peremptory summons. on th february he was received as the guest of niccolini, the tuscan ambassador, who had acted as his wise and ever-kind friend throughout the whole affair. it seemed plain that the holy office were inclined to treat galileo with as much clemency and consideration as was consistent with the determination that the case against him should be proceeded with to the end. the pope intimated that in consequence of his respect for the grand duke of tuscany he should permit galileo to enjoy the privilege, quite unprecedented for a prisoner charged with heresy, of remaining as an inmate in the ambassador's house. he ought, strictly, to have been placed in the dungeons of the inquisition. when the examination of the accused had actually commenced, galileo was confined, not, indeed, in the dungeons, but in comfortable rooms at the holy office. by the judicious and conciliatory language of submission which niccolini had urged galileo to use before the inquisitors, they were so far satisfied that they interceded with the pope for his release. during the remainder of the trial galileo was accordingly permitted to go back to the ambassador's, where he was most heartily welcomed. sister maria celeste, evidently thinking this meant that the whole case was at an end, thus expresses herself:-- "the joy that your last dear letter brought me, and the having to read it over and over to the nuns, who made quite a jubilee on hearing its contents, put me into such an excited state that at last i got a severe attack of headache." in his defence galileo urged that he had already been acquitted in by cardinal bellarmine, when a charge of heresy was brought against him, and he contended that anything he might now have done, was no more than he had done on the preceding occasion, when the orthodoxy of his doctrines received solemn confirmation. the inquisition seemed certainly inclined to clemency, but the pope was not satisfied. galileo was accordingly summoned again on the st june. he was to be threatened with torture if he did not forthwith give satisfactory explanations as to the reasons which led him to write the dialogue. in this proceeding the pope assured the tuscan ambassador that he was treating galileo with the utmost consideration possible in consequence of his esteem and regard for the grand duke, whose servant galileo was. it was, however, necessary that some exemplary punishment be meted out to the astronomer, inasmuch as by the publication of the dialogue he had distinctly disobeyed the injunction of silence laid upon him by the decree of . nor was it admissible for galileo to plead that his book had been sanctioned by the master of the sacred college, to whose inspection it had been again and again submitted. it was held, that if the master of the sacred college had been unaware of the solemn warning the philosopher had already received sixteen years previously, it was the duty of galileo to have drawn his attention to that fact. on the nd june, , galileo was led to the great hall of the inquisition, and compelled to kneel before the cardinals there assembled and hear his sentence. in a long document, most elaborately drawn up, it is definitely charged against galileo that, in publishing the dialogue, he committed the essentially grave error of treating the doctrine of the earth's motion as open to discussion. galileo knew, so the document affirmed, that the church had emphatically pronounced this notion to be contrary to holy writ, and that for him to consider a doctrine so stigmatized as having any shadow of probability in its favour was an act of disrespect to the authority of the church which could not be overlooked. it was also charged against galileo that in his dialogue he has put the strongest arguments into the mouth, not of those who supported the orthodox doctrine, but of those who held the theory as to the earth's motion which the church had so deliberately condemned. after due consideration of the defence made by the prisoner, it was thereupon decreed that he had rendered himself vehemently suspected of heresy by the holy office, and in consequence had incurred all the censures and penalties of the sacred canons, and other decrees promulgated against such persons. the graver portion of these punishments would be remitted, if galileo would solemnly repudiate the heresies referred to by an abjuration to be pronounced by him in the terms laid down. at the same time it was necessary to mark, in some emphatic manner, the serious offence which had been committed, so that it might serve both as a punishment to galileo and as a warning to others. it was accordingly decreed that he should be condemned to imprisonment in the holy office during the pleasure of the papal authorities, and that he should recite once a week for three years the seven penitential psalms. then followed that ever-memorable scene in the great hall of the inquisition, in which the aged and infirm galileo, the inventor of the telescope and the famous astronomer, knelt down to abjure before the most eminent and reverend lords cardinal, inquisitors general throughout the christian republic against heretical depravity. with his hands on the gospels, galileo was made to curse and detest the false opinion that the sun was the centre of the universe and immovable, and that the earth was not the centre of the same, and that it moved. he swore that for the future he will never say nor write such things as may bring him under suspicion, and that if he does so he submits to all the pains and penalties of the sacred canons. this abjuration was subsequently read in florence before galileo's disciples, who had been specially summoned to attend. it has been noted that neither on the first occasion, in , nor on the second in , did the reigning pope sign the decrees concerning galileo. the contention has accordingly been made that paul v. and urban viii. are both alike vindicated from any technical responsibility for the attitude of the romish church towards the copernican doctrines. the significance of this circumstance has been commented on in connection with the doctrine of the infallibility of the pope. we can judge of the anxiety felt by sister maria celeste about her beloved father during these terrible trials. the wife of the ambassador niccolini, galileo's steadfast friend, most kindly wrote to give the nun whatever quieting assurances the case would permit. there is a renewed flow of these touching epistles from the daughter to her father. thus she sends word-- "the news of your fresh trouble has pierced my soul with grief all the more that it came quite unexpectedly." and again, on hearing that he had been permitted to leave rome, she writes-- "i wish i could describe the rejoicing of all the mothers and sisters on hearing of your happy arrival at siena. it was indeed most extraordinary. on hearing the news the mother abbess and many of the nuns ran to me, embracing me and weeping for joy and tenderness." the sentence of imprisonment was at first interpreted leniently by the pope. galileo was allowed to reside in qualified durance in the archbishop's house at siena. evidently the greatest pain that he endured arose from the forced separation from that daughter, whom he had at last learned to love with an affection almost comparable with that she bore to him. she had often told him that she never had any pleasure equal to that with which she rendered any service to her father. to her joy, she discovers that she can relieve him from the task of reciting the seven penitential psalms which had been imposed as a penance:-- "i began to do this a while ago," she writes, "and it gives me much pleasure. first, because i am persuaded that prayer in obedience to holy church must be efficacious; secondly, in order to save you the trouble of remembering it. if i had been able to do more, most willingly would i have entered a straiter prison than the one i live in now, if by so doing i could have set you at liberty." [plate: crest of galileo's family.] sister maria celeste was gradually failing in health, but the great privilege was accorded to her of being able once again to embrace her beloved lord and master. galileo had, in fact, been permitted to return to his old home; but on the very day when he heard of his daughter's death came the final decree directing him to remain in his own house in perpetual solitude. amid the advancing infirmities of age, the isolation from friends, and the loss of his daughter, galileo once again sought consolation in hard work. he commenced his famous dialogue on motion. gradually, however, his sight began to fail, and blindness was at last added to his other troubles. on january nd, , he writes to diodati:-- "alas, your dear friend and servant, galileo, has been for the last month perfectly blind, so that this heaven, this earth, this universe which i by my marvellous discoveries and clear demonstrations have enlarged a hundred thousand times beyond the belief of the wise men of bygone ages, henceforward is for me shrunk into such a small space as is filled by my own bodily sensations." but the end was approaching--the great philosopher, was attacked by low fever, from which he died on the th january, . kepler. while the illustrious astronomer, tycho brahe, lay on his death-bed, he had an interview which must ever rank as one of the important incidents in the history of science. the life of tycho had been passed, as we have seen, in the accumulation of vast stores of careful observations of the positions of the heavenly bodies. it was not given to him to deduce from his splendid work the results to which they were destined to lead. it was reserved for another astronomer to distil, so to speak, from the volumes in which tycho's figures were recorded, the great truths of the universe which those figures contained. tycho felt that his work required an interpreter, and he recognised in the genius of a young man with whom he was acquainted the agent by whom the world was to be taught some of the great truths of nature. to the bedside of the great danish astronomer the youthful philosopher was summoned, and with his last breath tycho besought of him to spare no labour in the performance of those calculations, by which alone the secrets of the movements of the heavens could be revealed. the solemn trust thus imposed was duly accepted, and the man who accepted it bore the immortal name of kepler. kepler was born on the th december, , at weil, in the duchy of wurtemberg. it would seem that the circumstances of his childhood must have been singularly unhappy. his father, sprung from a well-connected family, was but a shiftless and idle adventurer; nor was the great astronomer much more fortunate in his other parent. his mother was an ignorant and ill-tempered woman; indeed, the ill-assorted union came to an abrupt end through the desertion of the wife by her husband when their eldest son john, the hero of our present sketch, was eighteen years old. the childhood of this lad, destined for such fame, was still further embittered by the circumstance that when he was four years old he had a severe attack of small-pox. not only was his eyesight permanently injured, but even his constitution appears to have been much weakened by this terrible malady. it seems, however, that the bodily infirmities of young john kepler were the immediate cause of his attention being directed to the pursuit of knowledge. had the boy been fitted like other boys for ordinary manual work, there can be hardly any doubt that to manual work his life must have been devoted. but, though his body was feeble, he soon gave indications of the possession of considerable mental power. it was accordingly thought that a suitable sphere for his talents might be found in the church which, in those days, was almost the only profession that afforded an opening for an intellectual career. we thus find that by the time john kepler was seventeen years old he had attained a sufficient standard of knowledge to entitle him to admission on the foundation of the university at tubingen. in the course of his studies at this institution he seems to have divided his attention equally between astronomy and divinity. it not unfrequently happens that when a man has attained considerable proficiency in two branches of knowledge he is not able to see very clearly in which of the two pursuits his true vocation lies. his friends and onlookers are often able to judge more wisely than he himself can do as to which of the two lines it would be better for him to pursue. this incapacity for perceiving the path in which greatness awaited him, existed in the case of kepler. personally, he inclined to enter the ministry, in which a promising career seemed open to him. he yielded, however, to friends, who evidently knew him better than he knew himself, and accepted in , the important professorship of astronomy which had been offered to him in the university of gratz. it is difficult for us in these modern days to realise the somewhat extraordinary duties which were expected from an astronomical professor in the sixteenth century. he was, of course, required to employ his knowledge of the heavens in the prediction of eclipses, and of the movements of the heavenly bodies generally. this seems reasonable enough; but what we are not prepared to accept is the obligation which lay on the astronomers to predict the fates of nations and the destinies of individuals. it must be remembered that it was the almost universal belief in those days, that all the celestial spheres revolved in some mysterious fashion around the earth, which appeared by far the most important body in the universe. it was imagined that the sun, the moon, and the stars indicated, in the vicissitudes of their movements, the careers of nations and of individuals. such being the generally accepted notion, it seemed to follow that a professor who was charged with the duty of expounding the movements of the heavenly bodies must necessarily be looked to for the purpose of deciphering the celestial decrees regarding the fate of man which the heavenly luminaries were designed to announce. kepler threw himself with characteristic ardour into even this fantastic phase of the labours of the astronomical professor; he diligently studied the rules of astrology, which the fancies of antiquity had compiled. believing sincerely as he did in the connection between the aspect of the stars and the state of human affairs, he even thought that he perceived, in the events of his own life, a corroboration of the doctrine which affirmed the influence of the planets upon the fate of individuals. [plate: kepler's system of regular solids.] but quite independently of astrology there seem to have been many other delusions current among the philosophers of kepler's time. it is now almost incomprehensible how the ablest men of a few centuries ago should have entertained such preposterous notions, as they did, with respect to the system of the universe. as an instance of what is here referred to, we may cite the extraordinary notion which, under the designation of a discovery, first brought kepler into fame. geometers had long known that there were five, but no more than five, regular solid figures. there is, for instance, the cube with six sides, which is, of course, the most familiar of these solids. besides the cube there are other figures of four, eight, twelve, and twenty sides respectively. it also happened that there were five planets, but no more than five, known to the ancients, namely, mercury, venus, mars, jupiter, and saturn. to kepler's lively imaginations this coincidence suggested the idea that the five regular solids corresponded to the five planets, and a number of fancied numerical relations were adduced on the subject. the absurdity of this doctrine is obvious enough, especially when we observe that, as is now well known, there are two large planets, and a host of small planets, over and above the magical number of the regular solids. in kepler's time, however, this doctrine was so far from being regarded as absurd, that its announcement was hailed as a great intellectual triumph. kepler was at once regarded with favour. it seems, indeed, to have been the circumstance which brought him into correspondence with tycho brahe. by its means also he became known to galileo. the career of a scientific professor in those early days appears generally to have been marked by rather more striking vicissitudes than usually befall a professor in a modern university. kepler was a protestant, and as such he had been appointed to his professorship at gratz. a change, however, having taken place in the religious belief entertained by the ruling powers of the university, the protestant professors were expelled. it seems that special influence having been exerted in kepler's case on account of his exceptional eminence, he was recalled to gratz and reinstated in the tenure of his chair. but his pupils had vanished, so that the great astronomer was glad to accept a post offered him by tycho brahe in the observatory which the latter had recently established near prague. on tycho's death, which occurred soon after, an opening presented itself which gave kepler the opportunity his genius demanded. he was appointed to succeed tycho in the position of imperial mathematician. but a far more important point, both for kepler and for science, was that to him was confided the use of tycho's observations. it was, indeed, by the discussion of tycho's results that kepler was enabled to make the discoveries which form such an important part of astronomical history. kepler must also be remembered as one of the first great astronomers who ever had the privilege of viewing celestial bodies through a telescope. it was in that he first held in his hands one of those little instruments which had been so recently applied to the heavens by galileo. it should, however, be borne in mind that the epoch-making achievements of kepler did not arise from any telescopic observations that he made, or, indeed, that any one else made. they were all elaborately deduced from tycho's measurements of the positions of the planets, obtained with his great instruments, which were unprovided with telescopic assistance. to realise the tremendous advance which science received from kepler's great work, it is to be understood that all the astronomers who laboured before him at the difficult subject of the celestial motions, took it for granted that the planets must revolve in circles. if it did not appear that a planet moved in a fixed circle, then the ready answer was provided by ptolemy's theory that the circle in which the planet did move was itself in motion, so that its centre described another circle. when kepler had before him that wonderful series of observations of the planet, mars, which had been accumulated by the extraordinary skill of tycho, he proved, after much labour, that the movements of the planet refused to be represented in a circular form. nor would it do to suppose that mars revolved in one circle, the centre of which revolved in another circle. on no such supposition could the movements of the planets be made to tally with those which tycho had actually observed. this led to the astonishing discovery of the true form of a planet's orbit. for the first time in the history of astronomy the principle was laid down that the movement of a planet could not be represented by a circle, nor even by combinations of circles, but that it could be represented by an elliptic path. in this path the sun is situated at one of those two points in the ellipse which are known as its foci. [plate: kepler.] very simple apparatus is needed for the drawing of one of those ellipses which kepler has shown to possess such astonishing astronomical significance. two pins are stuck through a sheet of paper on a board, the point of a pencil is inserted in a loop of string which passes over the pins, and as the pencil is moved round in such a way as to keep the string stretched, that beautiful curve known as the ellipse is delineated, while the positions of the pins indicate the two foci of the curve. if the length of the loop of string is unchanged then the nearer the pins are together, the greater will be the resemblance between the ellipse and the circle, whereas the more the pins are separated the more elongated does the ellipse become. the orbit of a great planet is, in general, one of those ellipses which approaches a nearly circular form. it fortunately happens, however, that the orbit of mars makes a wider departure from the circular form than any of the other important planets. it is, doubtless, to this circumstance that we must attribute the astonishing success of kepler in detecting the true shape of a planetary orbit. tycho's observations would not have been sufficiently accurate to have exhibited the elliptic nature of a planetary orbit which, like that of venus, differed very little from a circle. the more we ponder on this memorable achievement the more striking will it appear. it must be remembered that in these days we know of the physical necessity which requires that a planet shall revolve in an ellipse and not in any other curve. but kepler had no such knowledge. even to the last hour of his life he remained in ignorance of the existence of any natural cause which ordained that planets should follow those particular curves which geometers know so well. kepler's assignment of the ellipse as the true form of the planetary orbit is to be regarded as a brilliant guess, the truth of which tycho's observations enabled him to verify. kepler also succeeded in pointing out the law according to which the velocity of a planet at different points of its path could be accurately specified. here, again, we have to admire the sagacity with which this marvellously acute astronomer guessed the deep truth of nature. in this case also he was quite unprovided with any reason for expecting from physical principles that such a law as he discovered must be obeyed. it is quite true that kepler had some slight knowledge of the existence of what we now know as gravitation. he had even enunciated the remarkable doctrine that the ebb and flow of the tide must be attributed to the attraction of the moon on the waters of the earth. he does not, however, appear to have had any anticipation of those wonderful discoveries which newton was destined to make a little later, in which he demonstrated that the laws detected by kepler's marvellous acumen were necessary consequences of the principle of universal gravitation. [plate: symbolical representation of the planetary system.] to appreciate the relations of kepler and tycho it is necessary to note the very different way in which these illustrious astronomers viewed the system of the heavens. it should be observed that copernicus had already expounded the true system, which located the sun at the centre of the planetary system. but in the days of tycho brahe this doctrine had not as yet commanded universal assent. in fact, the great observer himself did not accept the new views of copernicus. it appeared to tycho that the earth not only appeared to be the centre of things celestial, but that it actually was the centre. it is, indeed, not a little remarkable that a student of the heavens so accurate as tycho should have deliberately rejected the copernican doctrine in favour of the system which now seems so preposterous. throughout his great career, tycho steadily observed the places of the sun, the moon, and the planets, and as steadily maintained that all those bodies revolved around the earth fixed in the centre. kepler, however, had the advantage of belonging to the new school. he utilised the observations of tycho in developing the great copernican theory whose teaching tycho stoutly resisted. perhaps a chapter in modern science may illustrate the intellectual relation of these great men. the revolution produced by copernicus in the doctrine of the heavens has often been likened to the revolution which the darwinian theory produced in the views held by biologists as to life on this earth. the darwinian theory did not at first command universal assent even among those naturalists whose lives had been devoted with the greatest success to the study of organisms. take, for instance, that great naturalist, professor owen, by whose labours vast extension has been given to our knowledge of the fossil animals which dwelt on the earth in past ages. now, though owens researches were intimately connected with the great labours of darwin, and afforded the latter material for his epoch-making generalization, yet owen deliberately refused to accept the new doctrines. like tycho, he kept on rigidly accumulating his facts under the influence of a set of ideas as to the origin of living forms which are now universally admitted to be erroneous. if, therefore, we liken darwin to copernicus, and owen to tycho, we may liken the biologists of the present day to kepler, who interpreted the results of accurate observation upon sound theoretical principles. in reading the works of kepler in the light of our modern knowledge we are often struck by the extent to which his perception of the sublimest truths in nature was associated with the most extravagant errors and absurdities. but, of course, it must be remembered that he wrote in an age in which even the rudiments of science, as we now understand it, were almost entirely unknown. it may well be doubted whether any joy experienced by mortals is more genuine than that which rewards the successful searcher after natural truths. every science-worker, be his efforts ever so humble, will be able to sympathise with the enthusiastic delight of kepler when at last, after years of toil, the glorious light broke forth, and that which he considered to be the greatest of his astonishing laws first dawned upon him. kepler rightly judged that the number of days which a planet required to perform its voyage round the sun must be connected in some manner with the distance from the planet to the sun; that is to say, with the radius of the planet's orbit, inasmuch as we may for our present object regard the planet's orbit as circular. here, again, in his search for the unknown law, kepler had no accurate dynamical principles to guide his steps. of course, we now know not only what the connection between the planet's distance and the planet's periodic time actually is, but we also know that it is a necessary consequence of the law of universal gravitation. kepler, it is true, was not without certain surmises on the subject, but they were of the most fanciful description. his notions of the planets, accurate as they were in certain important respects, were mixed up with vague ideas as to the properties of metals and the geometrical relations of the regular solids. above all, his reasoning was penetrated by the supposed astrological influences of the stars and their significant relation to human fate. under the influence of such a farrago of notions, kepler resolved to make all sorts of trials in his search for the connection between the distance of a planet from the sun and the time in which the revolution of that planet was accomplished. it was quite easily demonstrated that the greater the distance of the planet from the sun the longer was the time required for its journey. it might have been thought that the time would be directly proportional to the distance. it was, however, easy to show that this supposition did not agree with the fact. finding that this simple relation would not do, kepler undertook a vast series of calculations to find out the true method of expressing the connection. at last, after many vain attempts, he found, to his indescribable joy, that the square of the time in which a planet revolves around the sun was proportional to the cube of the average distance of the planet from that body. the extraordinary way in which kepler's views on celestial matters were associated with the wildest speculations, is well illustrated in the work in which he propounded his splendid discovery just referred to. the announcement of the law connecting the distances of the planets from the sun with their periodic times, was then mixed up with a preposterous conception about the properties of the different planets. they were supposed to be associated with some profound music of the spheres inaudible to human ears, and performed only for the benefit of that being whose soul formed the animating spirit of the sun. kepler was also the first astronomer who ever ventured to predict the occurrence of that remarkable phenomenon, the transit of a planet in front of the sun's disc. he published, in , a notice to the curious in things celestial, in which he announced that both of the planets, mercury and venus, were to make a transit across the sun on specified days in the winter of . the transit of mercury was duly observed by gassendi, and the transit of venus also took place, though, as we now know, the circumstances were such that it was not possible for the phenomenon to be witnessed by any european astronomer. in addition to kepler's discoveries already mentioned, with which his name will be for ever associated, his claim on the gratitude of astronomers chiefly depends on the publication of his famous rudolphine tables. in this remarkable work means are provided for finding the places of the planets with far greater accuracy than had previously been attainable. kepler, it must be always remembered, was not an astronomical observer. it was his function to deal with the observations made by tycho, and, from close study and comparison of the results, to work out the movements of the heavenly bodies. it was, in fact, tycho who provided as it were the raw material, while it was the genius of kepler which wrought that material into a beautiful and serviceable form. for more than a century the rudolphine tables were regarded as a standard astronomical work. in these days we are accustomed to find the movements of the heavenly bodies set forth with all desirable exactitude in the nautical almanack, and the similar publication issued by foreign governments. let it be remembered that it was kepler who first imparted the proper impulse in this direction. [plate: the commemoration of the rudolphine tables.] when kepler was twenty-six he married an heiress from styria, who, though only twenty-three years old, had already had some experience in matrimony. her first husband had died; and it was after her second husband had divorced her that she received the addresses of kepler. it will not be surprising to hear that his domestic affairs do not appear to have been particularly happy, and his wife died in . two years later, undeterred by the want of success in his first venture, he sought a second partner, and he evidently determined not to make a mistake this time. indeed, the methodical manner in which he made his choice of the lady to whom he should propose has been duly set forth by him and preserved for our edification. with some self-assurance he asserts that there were no fewer than eleven spinsters desirous of sharing his joys and sorrows. he has carefully estimated and recorded the merits and demerits of each of these would-be brides. the result of his deliberations was that he awarded himself to an orphan girl, destitute even of a portion. success attended his choice, and his second marriage seems to have proved a much more suitable union than his first. he had five children by the first wife and seven by the second. the years of kepler's middle life were sorely distracted by a trouble which, though not uncommon in those days, is one which we find it difficult to realise at the present time. his mother, catherine kepler, had attained undesirable notoriety by the suspicion that she was guilty of witchcraft. years were spent in legal investigations, and it was only after unceasing exertions on the part of the astronomer for upwards of a twelve-month that he was finally able to procure her acquittal and release from prison. it is interesting for us to note that at one time there was a proposal that kepler should forsake his native country and adopt england as a home. it arose in this wise. the great man was distressed throughout the greater part of his life by pecuniary anxieties. finding him in a strait of this description, the english ambassador in venice, sir henry wotton, in the year , besought kepler to come over to england, where he assured him that he would obtain a favourable reception, and where, he was able to add, kepler's great scientific work was already highly esteemed. but his efforts were unavailing; kepler would not leave his own country. he was then forty-nine years of age, and doubtless a home in a foreign land, where people spoke a strange tongue, had not sufficient attraction for him, even when accompanied with the substantial inducements which the ambassador was able to offer. had kepler accepted this invitation, he would, in transferring his home to england, have anticipated the similar change which took place in the career of another great astronomer two centuries later. it will be remembered that herschel, in his younger days, did transfer himself to england, and thus gave to england the imperishable fame of association with his triumphs. the publication of the rudolphine tables of the celestial movements entailed much expense. a considerable part of this was defrayed by the government at venice but the balance occasioned no little trouble and anxiety to kepler. no doubt the authorities of those days were even less willing to spend money on scientific matters than are the governments of more recent times. for several years the imperial treasury was importuned to relieve him from his anxieties. the effects of so much worry, and of the long journeys which were involved, at last broke down kepler's health completely. as we have already mentioned, he had never been strong from infancy, and he finally succumbed to a fever in november, , at the age of fifty-nine. he was interred at st. peter's church at ratisbon. though kepler had not those personal characteristics which have made his great predecessor, tycho brahe, such a romantic figure, yet a picturesque element in kepler's character is not wanting. it was, however, of an intellectual kind. his imagination, as well as his reasoning faculties, always worked together. he was incessantly prompted by the most extraordinary speculations. the great majority of them were in a high degree wild and chimerical, but every now and then one of his fancies struck right to the heart of nature, and an immortal truth was brought to light. i remember visiting the observatory of one of our greatest modern astronomers, and in a large desk he showed me a multitude of photographs which he had attempted but which had not been successful, and then he showed me the few and rare pictures which had succeeded, and by which important truths had been revealed. with a felicity of expression which i have often since thought of, he alluded to the contents of the desk as the "chips." they were useless, but they were necessary incidents in the truly successful work. so it is in all great and good work. even the most skilful man of science pursues many a wrong scent. time after time he goes off on some track that plays him false. the greater the man's genius and intellectual resource, the more numerous will be the ventures which he makes, and the great majority of those ventures are certain to be fruitless. they are in fact, the "chips." in kepler's case the chips were numerous enough. they were of the most extraordinary variety and structure. but every now and then a sublime discovery was made of such a character as to make us regard even the most fantastic of kepler's chips with the greatest veneration and respect. isaac newton. it was just a year after the death of galileo, that an infant came into the world who was christened isaac newton. even the great fame of galileo himself must be relegated to a second place in comparison with that of the philosopher who first expounded the true theory of the universe. isaac newton was born on the th of december (old style), , at woolsthorpe, in lincolnshire, about a half-mile from colsterworth, and eight miles south of grantham. his father, mr. isaac newton, had died a few months after his marriage to harriet ayscough, the daughter of mr. james ayscough, of market overton, in rutlandshire. the little isaac was at first so excessively frail and weakly that his life was despaired of. the watchful mother, however, tended her delicate child with such success that he seems to have thriven better than might have been expected from the circumstances of his infancy, and he ultimately acquired a frame strong enough to outlast the ordinary span of human life. for three years they continued to live at woolsthorpe, the widow's means of livelihood being supplemented by the income from another small estate at sewstern, in a neighbouring part of leicestershire. [plate: woolsthorpe manor. showing solar dial made by newton when a boy.] in , mrs. newton took as a second husband the rev. barnabas smith, and on moving to her new home, about a mile from woolsthorpe, she entrusted little isaac to her mother, mrs. ayscough. in due time we find that the boy was sent to the public school at grantham, the name of the master being stokes. for the purpose of being near his work, the embryo philosopher was boarded at the house of mr. clark, an apothecary at grantham. we learn from newton himself that at first he had a very low place in the class lists of the school, and was by no means one of those model school-boys who find favour in the eyes of the school-master by attention to latin grammar. isaac's first incentive to diligent study seems to have been derived from the circumstance that he was severely kicked by one of the boys who was above him in the class. this indignity had the effect of stimulating young newton's activity to such an extent that he not only attained the desired object of passing over the head of the boy who had maltreated him, but continued to rise until he became the head of the school. the play-hours of the great philosopher were devoted to pursuits very different from those of most school-boys. his chief amusement was found in making mechanical toys and various ingenious contrivances. he watched day by day with great interest the workmen engaged in constructing a windmill in the neighbourhood of the school, the result of which was that the boy made a working model of the windmill and of its machinery, which seems to have been much admired, as indicating his aptitude for mechanics. we are told that isaac also indulged in somewhat higher flights of mechanical enterprise. he constructed a carriage, the wheels of which were to be driven by the hands of the occupant, while the first philosophical instrument he made was a clock, which was actuated by water. he also devoted much attention to the construction of paper kites, and his skill in this respect was highly appreciated by his school-fellows. like a true philosopher, even at this stage he experimented on the best methods of attaching the string, and on the proportions which the tail ought to have. he also made lanthorns of paper to provide himself with light as he walked to school in the dark winter mornings. the only love affair in newton's life appears to have commenced while he was still of tender years. the incidents are thus described in brewster's "life of newton," a work to which i am much indebted in this chapter. "in the house where he lodged there were some female inmates, in whose company he appears to have taken much pleasure. one of these, a miss storey, sister to dr. storey, a physician at buckminster, near colsterworth, was two or three years younger than newton and to great personal attractions she seems to have added more than the usual allotment of female talent. the society of this young lady and her companions was always preferred to that of his own school-fellows, and it was one of his most agreeable occupations to construct for them little tables and cupboards, and other utensils for holding their dolls and their trinkets. he had lived nearly six years in the same house with miss storey, and there is reason to believe that their youthful friendship gradually rose to a higher passion; but the smallness of her portion, and the inadequacy of his own fortune, appear to have prevented the consummation of their happiness. miss storey was afterwards twice married, and under the name of mrs. vincent, dr. stukeley visited her at grantham in , at the age of eighty-two, and obtained from her many particulars respecting the early history of our author. newton's esteem for her continued unabated during his life. he regularly visited her when he went to lincolnshire, and never failed to relieve her from little pecuniary difficulties which seem to have beset her family." the schoolboy at grantham was only fourteen years of age when his mother became a widow for the second time. she then returned to the old family home at woolsthorpe, bringing with her the three children of her second marriage. her means appear to have been somewhat scanty, and it was consequently thought necessary to recall isaac from the school. his recently-born industry had been such that he had already made good progress in his studies, and his mother hoped that he would now lay aside his books, and those silent meditations to which, even at this early age, he had become addicted. it was expected that, instead of such pursuits, which were deemed quite useless, the boy would enter busily into the duties of the farm and the details of a country life. but before long it became manifest that the study of nature and the pursuit of knowledge had such a fascination for the youth that he could give little attention to aught else. it was plain that he would make but an indifferent farmer. he greatly preferred experimenting on his water-wheels to looking after labourers, while he found that working at mathematics behind a hedge was much more interesting than chaffering about the price of bullocks in the market place. fortunately for humanity his mother, like a wise woman, determined to let her boy's genius have the scope which it required. he was accordingly sent back to grantham school, with the object of being trained in the knowledge which would fit him for entering the university of cambridge. [plate: trinity college, cambridge. showing newton's rooms; on the leads of the gateway he placed his telescope.] it was the th of june, , when isaac newton, a youth of eighteen, was enrolled as an undergraduate of trinity college, cambridge. little did those who sent him there dream that this boy was destined to be the most illustrious student who ever entered the portals of that great seat of learning. little could the youth himself have foreseen that the rooms near the gateway which he occupied would acquire a celebrity from the fact that he dwelt in them, or that the ante-chapel of his college was in good time to be adorned by that noble statue, which is regarded as one of the chief art treasures of cambridge university, both on account of its intrinsic beauty and the fact that it commemorates the fame of her most distinguished alumnus, isaac newton, the immortal astronomer. indeed, his advent at the university seemed to have been by no means auspicious or brilliant. his birth was, as we have seen, comparatively obscure, and though he had already given indication of his capacity for reflecting on philosophical matters, yet he seems to have been but ill-equipped with the routine knowledge which youths are generally expected to take with them to the universities. from the outset of his college career, newton's attention seems to have been mainly directed to mathematics. here he began to give evidence of that marvellous insight into the deep secrets of nature which more than a century later led so dispassionate a judge as laplace to pronounce newton's immortal work as pre-eminent above all the productions of the human intellect. but though newton was one of the very greatest mathematicians that ever lived, he was never a mathematician for the mere sake of mathematics. he employed his mathematics as an instrument for discovering the laws of nature. his industry and genius soon brought him under the notice of the university authorities. it is stated in the university records that he obtained a scholarship in . two years later we find that newton, as well as many residents in the university, had to leave cambridge temporarily on account of the breaking out of the plague. the philosopher retired for a season to his old home at woolsthorpe, and there he remained until he was appointed a fellow of trinity college, cambridge, in . from this time onwards, newton's reputation as a mathematician and as a natural philosopher steadily advanced, so that in , while still but twenty-seven years of age, he was appointed to the distinguished position of lucasian professor of mathematics at cambridge. here he found the opportunity to continue and develop that marvellous career of discovery which formed his life's work. the earliest of newton's great achievements in natural philosophy was his detection of the composite character of light. that a beam of ordinary sunlight is, in fact, a mixture of a very great number of different-coloured lights, is a doctrine now familiar to every one who has the slightest education in physical science. we must, however, remember that this discovery was really a tremendous advance in knowledge at the time when newton announced it. [plate: diagram of a sunbeam.] we here give the little diagram originally drawn by newton, to explain the experiment by which he first learned the composition of light. a sunbeam is admitted into a darkened room through an opening, h, in a shutter. this beam when not interfered with will travel in a straight line to the screen, and there reproduce a bright spot of the same shape as the hole in the shutter. if, however, a prism of glass, a b c, be introduced so that the beam traverse it, then it will be seen at once that the light is deflected from its original track. there is, however, a further and most important change which takes place. the spot of light is not alone removed to another part of the screen, but it becomes spread out into a long band beautifully coloured, and exhibiting the hues of the rainbow. at the top are the violet rays, and then in descending order we have the indigo, blue, green, yellow, orange, and red. the circumstance in this phenomenon which appears to have particularly arrested newton's attention, was the elongation which the luminous spot underwent in consequence of its passage through the prism. when the prism was absent the spot was nearly circular, but when the prism was introduced the spot was about five times as long as it was broad. to ascertain the explanation of this was the first problem to be solved. it seemed natural to suppose that it might be due to the thickness of the glass in the prism which the light traversed, or to the angle of incidence at which the light fell upon the prism. he found, however, upon careful trial, that the phenomenon could not be thus accounted for. it was not until after much patient labour that the true explanation dawned upon him. he discovered that though the beam of white light looks so pure and so simple, yet in reality it is composed of differently coloured lights blended together. these are, of course, indistinguishable in the compound beam, but they are separated or disentangled, so to speak, by the action of the prism. the rays at the blue end of the spectrum are more powerfully deflected by the action of the glass than are the rays at the red end. thus, the rays variously coloured red, orange, yellow, green, blue, indigo, violet, are each conducted to a different part of the screen. in this way the prism has the effect of exhibiting the constitution of the composite beam of light. to us this now seems quite obvious, but newton did not adopt it hastily. with characteristic caution he verified the explanation by many different experiments, all of which confirmed his discovery. one of these may be mentioned. he made a hole in the screen at that part on which the violet rays fell. thus a violet ray was allowed to pass through, all the rest of the light being intercepted, and on this beam so isolated he was able to try further experiments. for instance, when he interposed another prism in its path, he found, as he expected, that it was again deflected, and he measured the amount of the deflection. again he tried the same experiment with one of the red rays from the opposite end of the coloured band. he allowed it to pass through the same aperture in the screen, and he tested the amount by which the second prism was capable of producing deflection. he thus found, as he had expected to find, that the second prism was more efficacious in bending the violet rays than in bending the red rays. thus he confirmed the fact that the various hues of the rainbow were each bent by a prism to a different extent, violet being acted upon the most, and red the least. [plate: isaac newton.] not only did newton decompose a white beam into its constituent colours, but conversely by interposing a second prism with its angle turned upwards, he reunited the different colours, and thus reproduced the original beam of white light. in several other ways also he illustrated his famous proposition, which then seemed so startling, that white light was the result of a mixture of all hues of the rainbow. by combining painters' colours in the right proportion he did not indeed succeed in producing a mixture which would ordinarily be called white, but he obtained a grey pigment. some of this he put on the floor of his room for comparison with a piece of white paper. he allowed a beam of bright sunlight to fall upon the paper and the mixed colours side by side, and a friend he called in for his opinion pronounced that under these circumstances the mixed colours looked the whiter of the two. by repeated demonstrations newton thus established his great discovery of the composite character of light. he at once perceived that his researches had an important bearing upon the principles involved in the construction of a telescope. those who employed the telescope for looking at the stars, had been long aware of the imperfections which prevented all the various rays from being conducted to the same focus. but this imperfection had hitherto been erroneously accounted for. it had been supposed that the reason why success had not been attained in the construction of a refracting telescope was due to the fact that the object glass, made as it then was of a single piece, had not been properly shaped. mathematicians had abundantly demonstrated that a single lens, if properly figured, must conduct all rays of light to the same focus, provided all rays experienced equal refraction in passing through the glass. until newton's discovery of the composition of white light, it had been taken for granted that the several rays in a white beam were equally refrangible. no doubt if this had been the case, a perfect telescope could have been produced by properly shaping the object glass. but when newton had demonstrated that light was by no means so simple as had been supposed, it became obvious that a satisfactory refracting telescope was an impossibility when only a single object lens was employed, however carefully that lens might have been wrought. such an objective might, no doubt, be made to conduct any one group of rays of a particular shade to the same focus, but the rays of other colours in the beam of white light must necessarily travel somewhat astray. in this way newton accounted for a great part of the difficulties which had hitherto beset the attempts to construct a perfect refracting telescope. we now know how these difficulties can be, to a great extent, overcome, by employing for the objective a composite lens made of two pieces of glass possessing different qualities. to these achromatic object glasses, as they are called, the great development of astronomical knowledge, since newton's time, is due. but it must be remarked that, although the theoretical possibility of constructing an achromatic lens was investigated by newton, he certainly came to the conclusion that the difficulty could not be removed by employing a composite objective, with two different kinds of glass. in this his marvellous sagacity in the interpretation of nature seems for once to have deserted him. we can, however, hardly regret that newton failed to discover the achromatic objective, when we observe that it was in consequence of his deeming an achromatic objective to be impossible that he was led to the invention of the reflecting telescope. finding, as he believed, that the defects of the telescope could not be remedied by any application of the principle of refraction he was led to look in quite a different direction for the improvement of the tool on which the advancement of astronomy depended. the refraction of light depended as he had found, upon the colour of the light. the laws of reflection were, however, quite independent of the colour. whether rays be red or green, blue or yellow, they are all reflected in precisely the same manner from a mirror. accordingly, newton perceived that if he could construct a telescope the action of which depended upon reflection, instead of upon refraction, the difficulty which had hitherto proved an insuperable obstacle to the improvement of the instrument would be evaded. [plate: sir isaac newton's little reflector.] for this purpose newton fashioned a concave mirror from a mixture of copper and tin, a combination which gives a surface with almost the lustre of silver. when the light of a star fell upon the surface, an image of the star was produced in the focus of this mirror, and then this image was examined by a magnifying eye-piece. such is the principle of the famous reflecting telescope which bears the name of newton. the little reflector which he constructed, represented in the adjoining figure, is still preserved as one of the treasures of the royal society. the telescope tube had the very modest dimension of one inch in diameter. it was, however, the precursor of a whole series of magnificent instruments, each outstripping the other in magnitude, until at last the culminating point was attained in , by the construction of lord rosse's mammoth reflector of six feet in aperture. newton's discovery of the composition of light led to an embittered controversy, which caused no little worry to the great philosopher. some of those who attacked him enjoyed considerable and, it must be admitted, even well-merited repute in the ranks of science. they alleged, however, that the elongation of the coloured band which newton had noticed was due to this, to that, or to the other--to anything, in fact, rather than to the true cause which newton assigned. with characteristic patience and love of truth, newton steadily replied to each such attack. he showed most completely how utterly his adversaries had misunderstood the subject, and how slight indeed was their acquaintance with the natural phenomenon in question. in reply to each point raised, he was ever able to cite fresh experiments and adduce fresh illustrations, until at last his opponents retired worsted from the combat. it has been often a matter for surprise that newton, throughout his whole career, should have taken so much trouble to expose the errors of those who attacked his views. he used even to do this when it plainly appeared that his adversaries did not understand the subject they were discussing. a philosopher might have said, "i know i am right, and whether others think i am right or not may be a matter of concern to them, but it is certainly not a matter about which i need trouble. if after having been told the truth they elect to remain in error, so much the worse for them; my time can be better employed than in seeking to put such people right." this, however, was not newton's method. he spent much valuable time in overthrowing objections which were often of a very futile description. indeed, he suffered a great deal of annoyance from the persistency, and in some cases one might almost say from the rancour, of the attacks which were made upon him. unfortunately for himself, he did not possess that capacity for sublime indifference to what men may say, which is often the happy possession of intellects greatly inferior to his. the subject of optics still continuing to engross newton's attention, he followed up his researches into the structure of the sunbeam by many other valuable investigations in connection with light. every one has noticed the beautiful colours manifested in a soap-bubble. here was a subject which not unnaturally attracted the attention of one who had expounded the colours of the spectrum with such success. he perceived that similar hues were produced by other thin plates of transparent material besides soap-bubbles, and his ingenuity was sufficient to devise a method by which the thicknesses of the different films could be measured. we can hardly, indeed, say that a like success attended his interpretation of these phenomena to that which had been so conspicuous in his explanation of the spectrum. it implies no disparagement to the sublime genius of newton to admit that the doctrines he put forth as to the causes of the colours in the soap-bubbles can be no longer accepted. we must remember that newton was a pioneer in accounting for the physical properties of light. the facts that he established are indeed unquestionable, but the explanations which he was led to offer of some of them are seen to be untenable in the fuller light of our present knowledge. [plate: sir isaac newton's sun-dial.] had newton done nothing beyond making his wonderful discoveries in light, his fame would have gone down to posterity as one of the greatest of nature's interpreters. but it was reserved for him to accomplish other discoveries, which have pushed even his analysis of the sunbeam into the background; it is he who has expounded the system of the universe by the discovery of the law of universal gravitation. the age had indeed become ripe for the advent of the genius of newton. kepler had discovered with marvellous penetration the laws which govern the movements of the planets around the sun, and in various directions it had been more or less vaguely felt that the explanation of kepler's laws, as well as of many other phenomena, must be sought for in connection with the attractive power of matter. but the mathematical analysis which alone could deal with this subject was wanting; it had to be created by newton. at woolsthorpe, in the year , newton's attention appears to have been concentrated upon the subject of gravitation. whatever may be the extent to which we accept the more or less mythical story as to how the fall of an apple first directed the attention of the philosopher to the fact that gravitation must extend through space, it seems, at all events, certain that this is an excellent illustration of the line of reasoning which he followed. he argued in this way. the earth attracts the apple; it would do so, no matter how high might be the tree from which that apple fell. it would then seem to follow that this power which resides in the earth by which it can draw all external bodies towards it, extends far beyond the altitude of the loftiest tree. indeed, we seem to find no limit to it. at the greatest elevation that has ever been attained, the attractive power of the earth is still exerted, and though we cannot by any actual experiment reach an altitude more than a few miles above the earth, yet it is certain that gravitation would extend to elevations far greater. it is plain, thought newton, that an apple let fall from a point a hundred miles above this earth's surface, would be drawn down by the attraction, and would continually gather fresh velocity until it reached the ground. from a hundred miles it was natural to think of what would happen at a thousand miles, or at hundreds of thousands of miles. no doubt the intensity of the attraction becomes weaker with every increase in the altitude, but that action would still exist to some extent, however lofty might be the elevation which had been attained. it then occurred to newton, that though the moon is at a distance of two hundred and forty thousand miles from the earth, yet the attractive power of the earth must extend to the moon. he was particularly led to think of the moon in this connection, not only because the moon is so much closer to the earth than are any other celestial bodies, but also because the moon is an appendage to the earth, always revolving around it. the moon is certainly attracted to the earth, and yet the moon does not fall down; how is this to be accounted for? the explanation was to be found in the character of the moon's present motion. if the moon were left for a moment at rest, there can be no doubt that the attraction of the earth would begin to draw the lunar globe in towards our globe. in the course of a few days our satellite would come down on the earth with a most fearful crash. this catastrophe is averted by the circumstance that the moon has a movement of revolution around the earth. newton was able to calculate from the known laws of mechanics, which he had himself been mainly instrumental in discovering, what the attractive power of the earth must be, so that the moon shall move precisely as we find it to move. it then appeared that the very power which makes an apple fall at the earth's surface is the power which guides the moon in its orbit. [plate: sir isaac newton's telescope.] once this step had been taken, the whole scheme of the universe might almost be said to have become unrolled before the eye of the philosopher. it was natural to suppose that just as the moon was guided and controlled by the attraction of the earth, so the earth itself, in the course of its great annual progress, should be guided and controlled by the supreme attractive power of the sun. if this were so with regard to the earth, then it would be impossible to doubt that in the same way the movements of the planets could be explained to be consequences of solar attraction. it was at this point that the great laws of kepler became especially significant. kepler had shown how each of the planets revolves in an ellipse around the sun, which is situated on one of the foci. this discovery had been arrived at from the interpretation of observations. kepler had himself assigned no reason why the orbit of a planet should be an ellipse rather than any other of the infinite number of closed curves which might be traced around the sun. kepler had also shown, and here again he was merely deducing the results from observation, that when the movements of two planets were compared together, the squares of the periodic times in which each planet revolved were proportional to the cubes of their mean distances from the sun. this also kepler merely knew to be true as a fact, he gave no demonstration of the reason why nature should have adopted this particular relation between the distance and the periodic time rather than any other. then, too, there was the law by which kepler with unparalleled ingenuity, explained the way in which the velocity of a planet varies at the different points of its track, when he showed how the line drawn from the sun to the planet described equal areas around the sun in equal times. these were the materials with which newton set to work. he proposed to infer from these the actual laws regulating the force by which the sun guides the planets. here it was that his sublime mathematical genius came into play. step by step newton advanced until he had completely accounted for all the phenomena. in the first place, he showed that as the planet describes equal areas in equal times about the sun, the attractive force which the sun exerts upon it must necessarily be directed in a straight line towards the sun itself. he also demonstrated the converse truth, that whatever be the nature of the force which emanated from a sun, yet so long as that force was directed through the sun's centre, any body which revolved around it must describe equal areas in equal times, and this it must do, whatever be the actual character of the law according to which the intensity of the force varies at different parts of the planet's journey. thus the first advance was taken in the exposition of the scheme of the universe. the next step was to determine the law according to which the force thus proved to reside in the sun varied with the distance of the planet. newton presently showed by a most superb effort of mathematical reasoning, that if the orbit of a planet were an ellipse and if the sun were at one of the foci of that ellipse, the intensity of the attractive force must vary inversely as the square of the planet's distance. if the law had any other expression than the inverse square of the distance, then the orbit which the planet must follow would not be an ellipse; or if an ellipse, it would, at all events, not have the sun in the focus. hence he was able to show from kepler's laws alone that the force which guided the planets was an attractive power emanating from the sun, and that the intensity of this attractive power varied with the inverse square of the distance between the two bodies. these circumstances being known, it was then easy to show that the last of kepler's three laws must necessarily follow. if a number of planets were revolving around the sun, then supposing the materials of all these bodies were equally affected by gravitation, it can be demonstrated that the square of the periodic time in which each planet completes its orbit is proportional to the cube of the greatest diameter in that orbit. [plate: sir isaac newton's astrolabe.] these superb discoveries were, however, but the starting point from which newton entered on a series of researches, which disclosed many of the profoundest secrets in the scheme of celestial mechanics. his natural insight showed that not only large masses like the sun and the earth, and the moon, attract each other, but that every particle in the universe must attract every other particle with a force which varies inversely as the square of the distance between them. if, for example, the two particles were placed twice as far apart, then the intensity of the force which sought to bring them together would be reduced to one-fourth. if two particles, originally ten miles asunder, attracted each other with a certain force, then, when the distance was reduced to one mile, the intensity of the attraction between the two particles would be increased one-hundred-fold. this fertile principle extends throughout the whole of nature. in some cases, however, the calculation of its effect upon the actual problems of nature would be hardly possible, were it not for another discovery which newton's genius enabled him to accomplish. in the case of two globes like the earth and the moon, we must remember that we are dealing not with particles, but with two mighty masses of matter, each composed of innumerable myriads of particles. every particle in the earth does attract every particle in the moon with a force which varies inversely as the square of their distance. the calculation of such attractions is rendered feasible by the following principle. assuming that the earth consists of materials symmetrically arranged in shells of varying densities, we may then, in calculating its attraction, regard the whole mass of the globe as concentrated at its centre. similarly we may regard the moon as concentrated at the centre of its mass. in this way the earth and the moon can both be regarded as particles in point of size, each particle having, however, the entire mass of the corresponding globe. the attraction of one particle for another is a much more simple matter to investigate than the attraction of the myriad different points of the earth upon the myriad different points of the moon. many great discoveries now crowded in upon newton. he first of all gave the explanation of the tides that ebb and flow around our shores. even in the earliest times the tides had been shown to be related to the moon. it was noticed that the tides were specially high during full moon or during new moon, and this circumstance obviously pointed to the existence of some connection between the moon and these movements of the water, though as to what that connection was no one had any accurate conception until newton announced the law of gravitation. newton then made it plain that the rise and fall of the water was simply a consequence of the attractive power which the moon exerted upon the oceans lying upon our globe. he showed also that to a certain extent the sun produces tides, and he was able to explain how it was that when the sun and the moon both conspire, the joint result was to produce especially high tides, which we call "spring tides"; whereas if the solar tide was low, while the lunar tide was high, then we had the phenomenon of "neap" tides. but perhaps the most signal of newton's applications of the law of gravitation was connected with certain irregularities in the movements of the moon. in its orbit round the earth our satellite is, of course, mainly guided by the great attraction of our globe. if there were no other body in the universe, then the centre of the moon must necessarily perform an ellipse, and the centre of the earth would lie in the focus of that ellipse. nature, however, does not allow the movements to possess the simplicity which this arrangement would imply, for the sun is present as a source of disturbance. the sun attracts the moon, and the sun attracts the earth, but in different degrees, and the consequence is that the moon's movement with regard to the earth is seriously affected by the influence of the sun. it is not allowed to move exactly in an ellipse, nor is the earth exactly in the focus. how great was newton's achievement in the solution of this problem will be appreciated if we realise that he not only had to determine from the law of gravitation the nature of the disturbance of the moon, but he had actually to construct the mathematical tools by which alone such calculations could be effected. the resources of newton's genius seemed, however, to prove equal to almost any demand that could be made upon it. he saw that each planet must disturb the other, and in that way he was able to render a satisfactory account of certain phenomena which had perplexed all preceding investigators. that mysterious movement by which the pole of the earth sways about among the stars had been long an unsolved enigma, but newton showed that the moon grasped with its attraction the protuberant mass at the equatorial regions of the earth, and thus tilted the earth's axis in a way that accounted for the phenomenon which had been known but had never been explained for two thousand years. all these discoveries were brought together in that immortal work, newton's "principia." down to the year , when the "principia" was published, newton had lived the life of a recluse at cambridge, being entirely occupied with those transcendent researches to which we have referred. but in that year he issued from his seclusion under circumstances of considerable historical interest. king james the second attempted an invasion of the rights and privileges of the university of cambridge by issuing a command that father francis, a benedictine monk, should be received as a master of arts in the university, without having taken the oaths of allegiance and supremacy. with this arbitrary command the university sternly refused to comply. the vice-chancellor was accordingly summoned to answer for an act of contempt to the authority of the crown. newton was one of nine delegates who were chosen to defend the independence of the university before the high court. they were able to show that charles the second, who had issued a mandamus under somewhat similar circumstances, had been induced after due consideration to withdraw it. this argument appeared satisfactory, and the university gained their case. newton's next step in public life was his election, by a narrow majority, as member for the university, and during the years and , he seems to have attended to his parliamentary duties with considerable regularity. an incident which happened in was apparently the cause of considerable disturbance in newton's equanimity, if not in his health. he had gone to early morning chapel, leaving a lighted candle among his papers on his desk. tradition asserts that his little dog "diamond" upset the candle; at all events, when newton came back he found that many valuable papers had perished in a conflagration. the loss of these manuscripts seems to have had a serious effect. indeed, it has been asserted that the distress reduced newton to a state of mental aberration for a considerable time. this has, apparently, not been confirmed, but there is no doubt that he experienced considerable disquiet, for in writing on september th, , to mr. pepys, he says: "i am extremely troubled at the embroilment i am in, and have neither ate nor slept well this twelve-month, nor have my former consistency of mind." notwithstanding the fame which newton had achieved, by the publication of his, "principia," and by all his researches, the state had not as yet taken any notice whatever of the most illustrious man of science that this or any other country has ever produced. many of his friends had exerted themselves to procure him some permanent appointment, but without success. it happened, however, that mr. montagu, who had sat with newton in parliament, was appointed chancellor of the exchequer in . ambitious of distinction in his new office, mr. montagu addressed himself to the improvement of the current coin, which was then in a very debased condition. it fortunately happened that an opportunity occurred of appointing a new official in the mint; and mr. montagu on the th of march, , wrote to offer mr. newton the position of warden. the salary was to be five or six hundred a year, and the business would not require more attendance than newton could spare. the lucasian professor accepted this post, and forthwith entered upon his new duties. the knowledge of physics which newton had acquired by his experiments was of much use in connection with his duties at the mint. he carried out the re-coinage with great skill in the course of two years, and as a reward for his exertions, he was appointed, in , to the mastership of the mint, with a salary between , pounds and , pounds per annum. in , his duties at the mint being so engrossing, he resigned his lucasian professorship at cambridge, and at the same time he had to surrender his fellowship at trinity college. this closed his connection with the university of cambridge. it should, however, be remarked that at a somewhat earlier stage in his career he was very nearly being appointed to an office which might have enabled the university to retain the great philosopher within its precincts. some of his friends had almost succeeded in securing his nomination to the provostship of king's college, cambridge; the appointment, however, fell through, inasmuch as the statute could not be evaded, which required that the provost of king's college should be in holy orders. in those days it was often the custom for illustrious mathematicians, when they had discovered a solution for some new and striking problem, to publish that problem as a challenge to the world, while withholding their own solution. a famous instance of this is found in what is known as the brachistochrone problem, which was solved by john bernouilli. the nature of this problem may be mentioned. it was to find the shape of the curve along which a body would slide down from one point (a) to another point (b) in the shortest time. it might at first be thought that the straight line from a to b, as it is undoubtedly the shortest distance between the points, would also be the path of quickest descent; but this is not so. there is a curved line, down which a bead, let us say, would run on a smooth wire from a to b in a shorter time than the same bead would require to run down the straight wire. bernouilli's problem was to find out what that curve must be. newton solved it correctly; he showed that the curve was a part of what is termed a cycloid--that is to say, a curve like that which is described by a point on the rim of a carriage-wheel as the wheel runs along the ground. such was newton's geometrical insight that he was able to transmit a solution of the problem on the day after he had received it, to the president of the royal society. in newton, whose world wide fame was now established, was elected president of the royal society. year after year he was re-elected to this distinguished position, and his tenure, which lasted twenty-five years, only terminated with his life. it was in discharge of his duties as president of the royal society that newton was brought into contact with prince george of denmark. in april, , the queen paid a visit to cambridge as the guest of dr. bentley, the then master of trinity, and in a court held at trinity lodge on april th, , the honour of knighthood was conferred upon the discoverer of gravitation. urged by illustrious friends, who sought the promotion of knowledge, newton gave his attention to the publication of a new edition of the "principia." his duties at the mint, however, added to the supreme duty of carrying on his original investigations, left him but little time for the more ordinary task of the revision. he was accordingly induced to associate with himself for this purpose a distinguished young mathematician, roger coates, a fellow of trinity college, cambridge, who had recently been appointed plumian professor of astronomy. on july th, , newton, by this time a favourite at court, waited on the queen, and presented her with a copy of the new edition of the "principia." throughout his life newton appears to have been greatly interested in theological studies, and he specially devoted his attention to the subject of prophecy. he left behind him a manuscript on the prophecies of daniel and the apocalypse of st. john, and he also wrote various theological papers. many other subjects had from time to time engaged his attention. he studied the laws of heat; he experimented in pursuit of the dreams of the alchymist; while the philosopher who had revealed the mechanism of the heavens found occasional relaxation in trying to interpret hieroglyphics. in the last few years of his life he bore with fortitude a painful ailment, and on monday, march th, , he died in the eighty-fifth year of his age. on tuesday, march th, he was buried in westminster abbey. though newton lived long enough to receive the honour that his astonishing discoveries so justly merited, and though for many years of his life his renown was much greater than that of any of his contemporaries, yet it is not too much to say that, in the years which have since elapsed, newton's fame has been ever steadily advancing, so that it never stood higher than it does at this moment. we hardly know whether to admire more the sublime discoveries at which he arrived, or the extraordinary character of the intellectual processes by which those discoveries were reached. viewed from either standpoint, newton's "principia" is incomparably the greatest work on science that has ever yet been produced. [plate: sir isaac newton's sun-dial in the royal society.] flamsteed. among the manuscripts preserved at greenwich observatory are certain documents in which flamsteed gives an account of his own life. we may commence our sketch by quoting the following passage from this autobiography:--"to keep myself from idleness, and to recreate myself, i have intended here to give some account of my life, in my youth, before the actions thereof, and the providences of god therein, be too far passed out of my memory; and to observe the accidents of all my years, and inclinations of my mind, that whosoever may light upon these papers may see i was not so wholly taken up, either with my father's business or my mathematics, but that i both admitted and found time for other as weighty considerations." the chief interest which attaches to the name of flamsteed arises from the fact that he was the first of the illustrious series of astronomers royal who have presided over greenwich observatory. in that capacity flamsteed was able to render material assistance to newton by providing him with the observations which his lunar theory required. john flamsteed was born at denby, in derbyshire, on the th of august, . his mother died when he was three years old, and the second wife, whom his father took three years later, only lived until flamsteed was eight, there being also two younger sisters. in his boyhood the future astronomer tells us that he was very fond of those romances which affect boy's imagination, but as he writes, "at twelve years of age i left all the wild ones and betook myself to read the better sort of them, which, though they were not probable, yet carried no seeming impossibility in the picturing." by the time flamsteed was fifteen years old he had embarked in still more serious work, for he had read plutarch's "lives," tacitus' "roman history," and many other books of a similar description. in he became ill with some serious rheumatic affection, which obliged him to be withdrawn from school. it was then for the first time that he received the rudiments of a scientific education. he had, however, attained his sixteenth year before he made any progress in arithmetic. he tells us how his father taught him "the doctrine of fractions," and "the golden rule of three"--lessons which he seemed to have learned easily and quickly. one of the books which he read at this time directed his attention to astronomical instruments, and he was thus led to construct for himself a quadrant, by which he could take some simple astronomical observations. he further calculated a table to give the sun's altitudes at different hours, and thus displayed those tastes for practical astronomy which he lived to develop so greatly. it appears that these scientific studies were discountenanced by his father, who designed that his son should follow a business career. flamsteed's natural inclination, however, forced him to prosecute astronomical work, notwithstanding the impediments that lay in his path. unfortunately, his constitutional delicacy seems to have increased, and he had just completed his eighteenth year, "when," to use his own words, "the winter came on and thrust me again into the chimney, whence the heat and the dryness of the preceding summer had happily once before withdrawn me. but, it not being a fit season for physic, it was thought fit to let me alone this winter, and try the skill of another physician on me in the spring." it appears that at this time a quack named valentine greatrackes, was reputed to have effected most astonishing cures in ireland merely by the stroke of his hands, without the application of any medicine whatever. flamsteed's father, despairing of any remedy for his son from the legitimate branch of the profession, despatched him to ireland on august th, , he being then, as recorded with astronomical accuracy, "nineteen years, six days, and eleven hours old." the young astronomer, accompanied by a friend, arrived on a tuesday at liverpool but the wind not being favourable, they remained there till the following friday, when a shift of the wind to the east took place. they embarked accordingly on a vessel called the supply at noon, and on saturday night came in sight of dublin. ere they could land, however, they were nearly being wrecked on lambay island. this peril safely passed, there was a long delay for quarantine before they were at last allowed on shore. on thursday, september th, they set out from dublin, where they had been sojourning at the "ship" hotel, in dame street, towards assaune, where greatrackes received his patients. [plate: flamsteed's house.] flamsteed gives an interesting account of his travels in ireland. they dined at naas on the first day, and on september th they reached carlow, a town which is described as one of the fairest they saw on their journey. by sunday morning, september th, having lost their way several times, they reached castleton, called commonly four mile waters. flamsteed inquired of the host in the inn where they might find a church, but was told that the minister lived twelve miles away, and that they had no sermon except when he came to receive his tithes once a year, and a woman added that "they had plenty enough of everything necessary except the word of god." the travellers accordingly went on to cappoquin, which lies up the river blackwater, on the road to lismore, eight miles from youghal. thence they immediately started on foot to assaune. about a mile from cappoquin, and entering into the house of mr. greatrackes, they saw him touch several patients, "whereof some were nearly cured, others were on the mending hand, and some on whom his strokes had no effect." flamsteed was touched by the famous quack on the afternoon of september th, but we are hardly surprised to hear his remark that "he found not his disease to stir." next morning the astronomer came again to see mr. greatrackes, who had "a kind of majestical yet affable presence, and a composed carriage." even after the third touching had been submitted to, no benefit seems to have been derived. we must, however record, to the credit of mr. greatrackes, that he refused to accept any payment from flamsteed, because he was a stranger. finding it useless to protract his stay any longer, flamsteed and his friend set out on their return to dublin. in the course of his journey he seems to have been much impressed with clonmel, which he describes as an "exceedingly pleasantly seated town." but in those days a journey to ireland was so serious an enterprise that when flamsteed did arrive safely back at derby after an absence of a month, he adds, "for god's providence in this journey, his name be praised, amen." as to the expected benefits to his health from the expedition we may quote his own words: "in the winter following i was indifferent hearty, and my disease was not so violent as it used to be at that time formerly. but whether through god's mercy i received this through mr. greatrackes' touch, or my journey and vomiting at sea, i am uncertain; but, by some circumstances, i guess that i received a benefit from both." it is evident that by this time flamsteed's interest in all astronomical matters had greatly increased. he studied the construction of sun-dials, he formed a catalogue of seventy of the fixed stars, with their places on the heavens, and he computed the circumstances of the solar eclipse which was to happen on june nd, . it is interesting to note that even in those days the doctrines of the astrologers still found a considerable degree of credence, and flamsteed spent a good deal of his time in astrological studies and computations. he investigated the methods of casting a nativity, but a suspicion, or, indeed, rather more than a suspicion, seems to have crossed his mind as to the value of these astrological predictions, for he says in fine, "i found astrology to give generally strong conjectural hints, not perfect declarations." all this time, however, the future astronomer royal was steadily advancing in astronomical inquiries of a recondite nature. he had investigated the obliquity of the ecliptic with extreme care, so far as the circumstances of astronomical observation would at that time permit. he had also sought to discover the sun's distance from the earth in so far as it could be obtained by determining when the moon was exactly half illuminated, and he had measured, with much accuracy, the length of the tropical year. it will thus be seen that, even at the age of twenty, flamsteed had made marked progress, considering how much his time had been interfered with by ill-health. other branches of astronomy began also to claim his attention. we learn that in and he compared the planets jupiter and mars with certain fixed stars near which they passed. his instrumental means, though very imperfect, were still sufficient to enable him to measure the intervals on the celestial sphere between the planets and the stars. as the places of the stars were known, flamsteed was thus able to obtain the places of the planets. this is substantially the way in which astronomers of the present day still proceed when they desire to determine the places of the planets, inasmuch as, directly or indirectly those places are always obtained relatively to the fixed stars. by his observations at this early period, flamsteed was, it is true, not able to obtain any great degree of accuracy; he succeeded, however, in proving that the tables by which the places of the planets were ordinarily given were not to be relied upon. [plate: flamsteed.] flamsteed's labours in astronomy and in the allied branches of science were now becoming generally known, and he gradually came to correspond with many distinguished men of learning. one of the first occasions which brought the talents of the young astronomer into fame was the publication of some calculations concerning certain astronomical phenomena which were to happen in the year . in the monthly revolution of the moon its disc passes over those stars which lie along its track. the disappearance of a star by the interposition of the moon is called an "occultation." owing to the fact that our satellite is comparatively near us, the position which the moon appears to occupy on the heavens varies from different parts of the earth, it consequently happens that a star which would be occulted to an observer in one locality, would often not be occulted to an observer who was situated elsewhere. even when an occultation is visible from both places, the times at which the star disappears from view will, generally speaking, be different. much calculation is therefore necessary to decide the circumstances under which the occultations of stars may be visible from any particular station. having a taste for such computations, flamsteed calculated the occultations which were to happen in the year , it being the case that several remarkable stars would be passed over by the moon during this year. of course at the present time, we find such information duly set forth in the nautical almanac, but a couple of centuries ago there was no such source of astronomical knowledge as is now to be found in that invaluable publication, which astronomers and navigators know so well. flamsteed accordingly sent the results of his work to the president of the royal society. the paper which contained them was received very favourably, and at once brought flamsteed into notice among the most eminent members of that illustrious body, one of whom, mr. collins, became through life his faithful friend and constant correspondent. flamsteed's father was naturally gratified with the remarkable notice which his son was receiving from the great and learned; accordingly he desired him to go to london, that he might make the personal acquaintance of those scientific friends whom he had only known by correspondence previously. flamsteed was indeed glad to avail himself of this opportunity. thus he became acquainted with dr. barrow, and especially with newton, who was then lucasian professor of mathematics at cambridge. it seems to have been in consequence of this visit to london that flamsteed entered himself as a member of jesus college, cambridge. we have but little information as to his university career, but at all events he took his degree of m.a. on june th, . up to this time it would seem that flamsteed had been engaged, to a certain extent, in the business carried on by his father. it is true that he does not give any explicit details, yet there are frequent references to journeys which he had to take on business matters. but the time now approached when flamsteed was to start on an independent career, and it appears that he took his degree in cambridge with the object of entering into holy orders, so that he might settle in a small living near derby, which was in the gift of a friend of his father, and would be at the disposal of the young astronomer. this scheme was, however, not carried out, but flamsteed does not tell us why it failed, his only remark being, that "the good providence of god that had designed me for another station ordered it otherwise." sir jonas moore, one of the influential friends whom flamsteed's talents had attracted, seems to have procured for him the position of king's astronomer, with a salary of pounds per annum. a larger salary appears to have been designed at first for this office, which was now being newly created, but as flamsteed was resolved on taking holy orders, a lesser salary was in his case deemed sufficient. the building of the observatory, in which the first astronomer royal was to be installed, seems to have been brought about, or, at all events, its progress was accelerated, in a somewhat curious manner. a frenchman, named le sieur de s. pierre, came over to london to promulgate a scheme for discovering longitudes, then a question of much importance. he brought with him introductions to distinguished people, and his mission attracted a great deal of attention. the proposals which he made came under flamsteed's notice, who pointed out that the frenchman's projects were quite inapplicable in the present state of astronomical science, inasmuch as the places of the stars were not known with the degree of accuracy which would be necessary if such methods were to be rendered available. flamsteed then goes on to say:--"i heard no more of the frenchman after this; but was told that my letters had been shown king charles. he was startled at the assertion of the fixed stars' places being false in the catalogue, and said, with some vehemence, he must have them anew observed, examined, and corrected, for the use of his seamen." the first question to be settled was the site for the new observatory. hyde park and chelsea college were both mentioned as suitable localities, but, at sir christopher wren's suggestion, greenwich hill was finally resolved upon. the king made a grant of five hundred pounds of money. he gave bricks from tilbury fort, while materials, in the shape of wood, iron, and lead, were available from a gatehouse demolished in the tower. the king also promised whatever further material aid might be shown to be necessary. the first stone of the royal observatory was laid on august th, , and within a few years a building was erected in which the art of modern practical astronomy was to be created. flamsteed strove with extraordinary diligence, and in spite of many difficulties, to obtain a due provision of astronomical instruments, and to arrange for the carrying on of his observations. notwithstanding the king's promises, the astronomer was, however, but scantily provided with means, and he had no assistants to help him in his work. it follows that all the observations, as well as the reductions, and, indeed, all the incidental work of the observatory, had to be carried on by himself alone. flamsteed, as we have seen, had, however, many staunch friends. sir jonas moore in particular at all times rendered him most valuable assistance, and encouraged him by the warm sympathy and keen interest which he showed in astronomy. the work of the first astronomer royal was frequently interrupted by recurrent attacks of the complaints to which we have already referred. he says himself that "his distempers stick so close that that he cannot remove them," and he lost much time by prostration from headaches, as well as from more serious affections. the year found him in the full tide of work in his observatory. he was specially engaged on the problem of the earth's motion, which he sought to derive from observations of the sun and of venus. but this, as well as many other astronomical researches which he undertook, were only subsidiary to that which he made the main task of his life, namely, the formation of a catalogue of fixed stars. at the time when flamsteed commenced his career, the only available catalogue of fixed stars was that of tycho brahe. this work had been published at the commencement of the seventeenth century, and it contained about a thousand stars. the positions assigned to these stars, though obtained with wonderful skill, considering the many difficulties under which tycho laboured, were quite inaccurate when judged by our modern standards. tycho's instruments were necessarily most rudely divided, and he had, of course, no telescopes to aid him. consequently it was merely by a process of sighting that he could obtain the places of the stars. it must further be remembered that tycho had no clocks, and no micrometers. he had, indeed, but little correct knowledge of the motions of the heavenly bodies to guide him. to determine the longitudes of a few principal stars he conceived the ingenious idea of measuring by day the position of venus with respect to the sun, an observation which the exceptional brightness of this planet rendered possible without telescopic aid, and then by night he observed the position of venus with regard to the stars. it has been well remarked by mr. baily, in his introduction to the "british catalogue of stars," that "flamsteed's observations, by a fortunate combination of circumstances, commenced a new and a brilliant era. it happened that, at that period, the powerful mind of newton was directed to this subject; a friendly intercourse then existed between these two distinguished characters; and thus the first observations that could lay any claim to accuracy were at once brought in aid of those deep researches in which our illustrious geometer was then engaged. the first edition of the 'principia' bears testimony to the assistance afforded by flamsteed to newton in these inquiries; although the former considers that the acknowledgment is not so ample as it ought to have been." although flamsteed's observations can hardly be said to possess the accuracy of those made in more recent times, when instruments so much superior to his have been available, yet they possess an interest of a special kind from their very antiquity. this circumstance renders them of particular importance to the astronomer, inasmuch as they are calculated to throw light on the proper motions of the stars. flamsteed's work may, indeed, be regarded as the origin of all subsequent catalogues, and the nomenclature which he adopted, though in some respects it can hardly be said to be very defensible, is, nevertheless, that which has been adopted by all subsequent astronomers. there were also a great many errors, as might be expected in a work of such extent, composed almost entirely of numerical detail. many of these errors have been corrected by baily himself, the assiduous editor of "flamsteed's life and works," for flamsteed was so harassed from various causes in the latter part of his life, and was so subject to infirmities all through his career, that he was unable to revise his computations with the care that would have been necessary. indeed, he observed many additional stars which he never included in the british catalogue. it is, as baily well remarks, "rather a matter of astonishment that he accomplished so much, considering his slender means, his weak frame, and the vexations which he constantly experienced." flamsteed had the misfortune, in the latter part of his life, to become estranged from his most eminent scientific contemporaries. he had supplied newton with places of the moon, at the urgent solicitation of the author of the "principia," in order that the lunar theory should be carefully compared with observation. but flamsteed appears to have thought that in newton's further request for similar information, he appeared to be demanding as a right that which flamsteed considered he was only called upon to render as a favour. a considerable dispute grew out of this matter, and there are many letters and documents, bearing on the difficulties which subsequently arose, that are not, perhaps, very creditable to either party. notwithstanding his feeble constitution, flamsteed lived to the age of seventy-three, his death occurring on the last day of the year . halley. isaac newton was just fourteen years of age when the birth of edmund halley, who was destined in after years to become newton's warmly attached friend, and one of his most illustrious scientific contemporaries, took place. there can be little doubt that the fame as an astronomer which halley ultimately acquired, great as it certainly was, would have been even greater still had it not been somewhat impaired by the misfortune that he had to shine in the same sky as that which was illumined by the unparalleled genius of newton. edmund halley was born at haggerston, in the parish of st. leonard's, shoreditch, on october th, . his father, who bore the same name as his famous son, was a soap-boiler in winchester street, london, and he had conducted his business with such success that he accumulated an ample fortune. i have been unable to obtain more than a very few particulars with respect to the early life of the future astronomer. it would, however, appear that from boyhood he showed considerable aptitude for the acquisition of various kinds of learning, and he also had some capacity for mechanical invention. halley seems to have received a sound education at st. paul's school, then under the care of dr. thomas gale. here, the young philosopher rapidly distanced his competitors in the various branches of ordinary school instruction. his superiority was, however, most conspicuous in mathematical studies, and, as a natural development of such tastes, we learn that by the time he had left school he had already made good progress in astronomy. at the age of seventeen he was entered as a commoner at queen's college, oxford, and the reputation that he brought with him to the university may be inferred from the remark of the writer of "athenae oxonienses," that halley came to oxford "with skill in latin, greek, and hebrew, and such a knowledge of geometry as to make a complete dial." though his studies were thus of a somewhat multifarious nature, yet it is plain that from the first his most favourite pursuit was astronomy. his earliest efforts in practical observation were connected with an eclipse which he observed from his father's house in winchester street. it also appears that he had studied theoretical branches of astronomy so far as to be conversant with the application of mathematics to somewhat abstruse problems. up to the time of kepler, philosophers had assumed almost as an axiom that the heavenly bodies must revolve in circles and that the motion of the planet around the orbit which it described must be uniform. we have already seen how that great philosopher, after very persevering labour, succeeded in proving that the orbits of the planets were not circles, but that they were ellipses of small eccentricity. kepler was, however, unable to shake himself free from the prevailing notion that the angular motion of the planet ought to be of a uniform character around some point. he had indeed proved that the motion round the focus of the ellipse in which the sun lies is not of this description. one of his most important discoveries even related to the fact that at some parts of its orbit a planet swings around the sun with greater angular velocity than at others. but it so happens that in elliptic tracks which differ but little from circles, as is the case with all the more important planetary orbits, the motion round the empty focus of the ellipse is very nearly uniform. it seemed natural to assume, that this was exactly the case, in which event each of the two foci of the ellipse would have had a special significance in relation to the movement of the planet. the youthful halley, however, demonstrated that so far as the empty focus was concerned, the movement of the planet around it, though so nearly uniform, was still not exactly so, and at the age of nineteen, he published a treatise on the subject which at once placed him in the foremost rank amongst theoretical astronomers. but halley had no intention of being merely an astronomer with his pen. he longed to engage in the practical work of observing. he saw that the progress of exact astronomy must depend largely on the determination of the positions of the stars with all attainable accuracy. he accordingly determined to take up this branch of work, which had been so successfully initiated by tycho brahe. at the present day, astronomers of the great national observatories are assiduously engaged in the determination of the places of the stars. a knowledge of the exact positions of these bodies is indeed of the most fundamental importance, not alone for the purposes of scientific astronomy, but also for navigation and for extensive operations of surveying in which accuracy is desired. the fact that halley determined to concentrate himself on this work shows clearly the scientific acumen of the young astronomer. halley, however, found that hevelius, at dantzig, and flamsteed, the astronomer royal at greenwich, were both engaged on work of this character. he accordingly determined to direct his energies in a way that he thought would be more useful to science. he resigned to the two astronomers whom i have named the investigation of the stars in the northern hemisphere, and he sought for himself a field hitherto almost entirely unworked. he determined to go to the southern hemisphere, there to measure and survey those stars which were invisible in europe, so that his work should supplement the labours of the northern astronomers, and that the joint result of his labours and of theirs might be a complete survey of the most important stars on the surface of the heavens. in these days, after so many ardent students everywhere have devoted themselves to the study of nature, it seems difficult for a beginner to find a virgin territory in which to commence his explorations. halley may, however, be said to have enjoyed the privilege of commencing to work in a magnificent region, the contents of which were previously almost entirely unknown. indeed none of the stars which were so situated as to have been invisible from tycho brahe's observatory at uraniborg, in denmark, could be said to have been properly observed. there was, no doubt, a rumour that a dutchman had observed southern stars from the island of sumatra, and certain stars were indicated in the southern heavens on a celestial globe. on examination, however, halley found that no reliance could be placed on the results which had been obtained, so that practically the field before him may be said to have been unworked. at the age of twenty, without having even waited to take that degree at the university which the authorities would have been glad to confer on so promising an undergraduate, this ardent student of nature sought his father's permission to go to the southern hemisphere for the purpose of studying the stars which lie around the southern pole. his father possessed the necessary means, and he had likewise the sagacity to encourage the young astronomer. he was indeed most anxious to make everything as easy as possible for so hopeful a son. he provided him with an allowance of pounds a year, which was regarded as a very munificent provision in those days. halley was also furnished with letters of recommendation from king charles ii., as well as from the directors of the east india company. he accordingly set sail with his instruments in the year , in one of the east india company's ships, for the island of st. helena, which he had selected as the scene of his labours. [plate: halley.] after an uneventful voyage of three months, the astronomer landed on st. helena, with his sextant of five and a half feet radius, and a telescope feet long, and forthwith plunged with ardour into his investigation of the southern skies. he met, however, with one very considerable disappointment. the climate of this island had been represented to him as most favourable for astronomical observation; but instead of the pure blue skies he had been led to expect, he found that they were almost always more or less clouded, and that rain was frequent, so that his observations were very much interrupted. on this account he only remained at st. helena for a single year, having, during that time, and in spite of many difficulties, accomplished a piece of work which earned for him the title of "our southern tycho." thus did halley establish his fame as an astronomer on the same lonely rock in mid-atlantic, which nearly a century and a-half later became the scene of napoleon's imprisonment, when his star, in which he believed so firmly, had irretrievably set. on his return to england, halley prepared a map which showed the result of his labours, and he presented it to the king, in . like his great predecessor tycho, halley did not altogether disdain the arts of the courtier, for he endeavoured to squeeze a new constellation into the group around the southern pole which he styled "the royal oak," adding a description to the effect that the incidents of which "the royal oak" was a symbol were of sufficient importance to be inscribed on the surface of the heavens. there is reason to think that charles ii. duly appreciated the scientific renown which one of his subjects had achieved, and it was probably through the influence of the king that halley was made a master of arts at oxford on november th, . special reference was made on the occasion to his observations at st. helena, as evidence of unusual attainments in mathematics and astronomy. this degree was no small honour to such a young man, who, as we have seen, quitted his university before he had the opportunity of graduating in the ordinary manner. on november th, in the same year, the astronomer received a further distinction in being elected a fellow of the royal society. from this time forward he took a most active part in the affairs of the society, and the numerous papers which he read before it form a very valuable part of that notable series of volumes known as the "philosophical transactions." he was subsequently elected to the important office of secretary to the royal society, and he discharged the duties of his post until his appointment to greenwich necessitated his resignation. within a year of halley's election as a fellow of the royal society, he was chosen by the society to represent them in a discussion which had arisen with hevelius. the nature of this discussion, or rather the fact that any discussion should have been necessary, may seem strange to modern astronomers, for the point is one on which it would now seem impossible for there to be any difference of opinion. we must, however, remember that the days of halley were, comparatively speaking, the days of infancy as regards the art of astronomical observation, and issues that now seem obvious were often, in those early times, the occasions of grave and anxious consideration. the particular question on which halley had to represent the royal society may be simply stated. when tycho brahe made his memorable investigations into the places of the stars, he had no telescopes to help him. the famous instruments at uraniborg were merely provided with sights, by which the telescope was pointed to a star on the same principle as a rifle is sighted for a target. shortly after tycho's time, galileo invented the telescope. of course every one admitted at once the extraordinary advantages which the telescope had to offer, so far as the mere question of the visibility of objects was concerned. but the bearing of galileo's invention upon what we may describe as the measuring part of astronomy was not so immediately obvious. if a star be visible to the unaided eye, we can determine its place by such instruments as those which tycho used, in which no telescope is employed. we can, however, also avail ourselves of an instrument in which we view the star not directly but through the intervention of the telescope. can the place of the star be determined more accurately by the latter method than it can when the telescope is dispensed with? with our present knowledge, of course, there is no doubt about the answer; every one conversant with instruments knows that we can determine the place of a star far more accurately with the telescope than is possible by any mere sighting apparatus. in fact an observer would be as likely to make an error of a minute with the sighting apparatus in tycho's instrument, as he would be to make an error of a second with the modern telescope, or, to express the matter somewhat differently, we may say, speaking quite generally, that the telescopic method of determining the places of the stars does not lead to errors more than one-sixtieth part as great as which are unavoidable when we make use of tycho's method. but though this is so apparent to the modern astronomer, it was not at all apparent in the days of halley, and accordingly he was sent off to discuss the question with the continental astronomers. hevelius, as the representative of the older method, which tycho had employed with such success, maintained that an instrument could be pointed more accurately at a star by the use of sights than by the use of a telescope, and vigorously disputed the claims put forward by those who believed that the latter method was the more suitable. on may th, , halley started for dantzig, and the energetic character of the man may be judged from the fact that on the very night of his arrival he commenced to make the necessary observations. in those days astronomical telescopes had only obtained a fractional part of the perfection possessed by the instruments in our modern observatories, and therefore it may not be surprising that the results of the trial were not immediately conclusive. halley appears to have devoted much time to the investigation; indeed, he remained at dantzig for more than a twelve-month. on his return to england, he spoke highly of the skill which hevelius exhibited in the use of his antiquated methods, but halley was nevertheless too sagacious an observer to be shaken in his preference for the telescopic method of observation. the next year we find our young astronomer starting for a continental tour, and we, who complain if the channel passage lasts more than an hour or two, may note halley's remark in writing to hooke on june th, : "having fallen in with bad weather we took forty hours in the journey from dover to calais." the scientific distinction which he had already attained was such that he was received in paris with marked attention. a great deal of his time seems to have been passed in the paris observatory, where cassini, the presiding genius, himself an astronomer of well-deserved repute, had extended a hearty welcome to his english visitor. they made observations together of the place of the splendid comet which was then attracting universal attention, and halley found the work thus done of much use when he subsequently came to investigate the path pursued by this body. halley was wise enough to spare no pains to derive all possible advantages from his intercourse with the distinguished savants of the french capital. in the further progress of his tour he visited the principal cities of the continent, leaving behind him everywhere the memory of an amiable disposition and of a rare intelligence. after halley's return to england, in , he married a young lady named mary tooke, with whom he lived happily, till her death fifty-five years later. on his marriage, he took up his abode in islington, where he erected his instruments and recommenced his observations. it has often been the good fortune of astronomers to render practical services to humanity by their investigations, and halley's achievements in this respect deserve to be noted. a few years after he had settled in england, he published an important paper on the variation of the magnetic compass, for so the departure of the needle from the true north is termed. this subject had indeed early engaged his attention, and he continued to feel much interest in it up to the end of his life. with respect to his labours in this direction, sir john herschel says: "to halley we owe the first appreciation of the real complexity of the subject of magnetism. it is wonderful indeed, and a striking proof of the penetration and sagacity of this extraordinary man, that with his means of information he should have been able to draw such conclusions, and to take so large and comprehensive a view of the subject as he appears to have done." in , halley explained his theory of terrestrial magnetism, and begged captains of ships to take observations of the variations of the compass in all parts of the world, and to communicate them to the royal society, "in order that all the facts may be readily available to those who are hereafter to complete this difficult and complicated subject." the extent to which halley was in advance of his contemporaries, in the study of terrestrial magnetism, may be judged from the fact that the subject was scarcely touched after his time till the year . the interest which he felt in it was not of a merely theoretical kind, nor was it one which could be cultivated in an easy-chair. like all true investigators, he longed to submit his theory to the test of experiment, and for that purpose halley determined to observe the magnetic variation for himself. he procured from king william iii. the command of a vessel called the "paramour pink," with which he started for the south seas in . this particular enterprise was not, however, successful; for, on crossing the line, some of his men fell sick and one of his lieutenants mutinied, so that he was obliged to return the following year with his mission unaccomplished. the government cashiered the lieutenant, and halley having procured a second smaller vessel to accompany the "paramour pink," started once more in september, . he traversed the atlantic to the nd degree of southern latitude, beyond which his further advance was stopped. "in these latitudes," he writes to say, "we fell in with great islands of ice of so incredible height and magnitude, that i scarce dare write my thoughts of it." on his return in , halley published a general chart, showing the variation of the compass at the different places which he had visited. on these charts he set down lines connecting those localities at which the magnetic variation was identical. he thus set an example of the graphic representation of large masses of complex facts, in such a manner as to appeal at once to the eye, a method of which we make many applications in the present day. but probably the greatest service which halley ever rendered to human knowledge was the share in which he took in bringing newton's "principia" before the world. in fact, as dr. glaisher, writing in , has truly remarked, "but for halley the 'principia' would not have existed." it was a visit from halley in the year which seems to have first suggested to newton the idea of publishing the results of his investigations on gravitation. halley, and other scientific contemporaries, had no doubt some faint glimmering of the great truth which only newton's genius was able fully to reveal. halley had indeed shown how, on the assumptions that the planets move in circular orbits round the sun, and that the squares of their periodic times are proportional to the cubes of their mean distances, it may be proved that the force acting on each planet must vary inversely as the square of its distance from the sun. since, however, each of the planets actually moves in an ellipse, and therefore, at continually varying distances from the sun, it becomes a much more difficult matter to account mathematically for the body's motions on the supposition that the attractive force varies inversely as the square of the distance. this was the question with which halley found himself confronted, but which his mathematical abilities were not adequate to solve. it would seem that both hooke and sir christopher wren were interested in the same problem; in fact, the former claimed to have arrived at a solution, but declined to make known his results, giving as an excuse his desire that others having tried and failed might learn to value his achievements all the more. halley, however, confessed that his attempts at the solution were unsuccessful, and wren, in order to encourage the other two philosophers to pursue the inquiry, offered to present a book of forty shillings value to either of them who should in the space of two months bring him a convincing proof of it. such was the value which sir christopher set on the law of gravitation, upon which the whole fabric of modern astronomy may be said to stand. finding himself unequal to the task, halley went down to cambridge to see newton on the subject, and was delighted to learn that the great mathematician had already completed the investigation. he showed halley that the motions of all the planets could be completely accounted for on the hypothesis of a force of attraction directed towards the sun, which varies inversely as the square of the distance from that body. halley had the genius to perceive the tremendous importance of newton's researches, and he ceased not to urge upon the recluse man of science the necessity for giving his new discoveries publication. he paid another visit to cambridge with the object of learning more with regard to the mathematical methods which had already conducted newton to such sublime truths, and he again encouraged the latter both to pursue his investigations, and to give some account of them to the world. in december of the same year halley had the gratification of announcing to the royal society that newton had promised to send that body a paper containing his researches on gravitation. it seems that at this epoch the finances of the royal society were at a very low ebb. this impecuniosity was due to the fact that a book by willoughby, entitled "de historia piscium," had been recently printed by the society at great expense. in fact, the coffers were so low that they had some difficulty in paying the salaries of their permanent officials. it appears that the public did not care about the history of fishes, or at all events the volume did not meet with the ready demand which was expected for it. indeed, it has been recorded that when halley had undertaken to measure the length of a degree of the earth's surface, at the request of the royal society, it was ordered that his expenses be defrayed either in pounds sterling, or in fifty books of fishes. thus it happened that on june nd, the council, after due consideration of ways and means in connection with the issue of the principia, "ordered that halley should undertake the business of looking after the book and printing it at his own charge," which he engaged to do. it was, as we have elsewhere mentioned, characteristic of newton that he detested controversies, and he was, in fact, inclined to suppress the third book of the "principia" altogether rather than have any conflict with hooke with respect to the discoveries there enunciated. he also thought of changing the name of the work to de motu corporum libri duo, but upon second thoughts, he retained the original title, remarking, as he wrote to halley, "it will help the sale of the book, which i ought not to diminish, now it is yours," a sentence which shows conclusively, if further proof were necessary, that halley had assumed the responsibility of its publication. halley spared no pains in pushing forward the publication of his illustrious friend's great work, so that in the same year he was in a position to present a complete copy to king james ii., with a proper discourse of his own. halley also wrote a set of latin hexameters in praise of newton's genius, which he printed at the beginning of the work. the last line of this specimen of halley's poetic muse may be thus rendered: "nor mortals nearer may approach the gods." the intimate friendship between the two greatest astronomers of the time continued without interruption till the death of newton. it has, indeed, been alleged that some serious cause of estrangement arose between them. there is, however, no satisfactory ground for this statement; indeed, it may be regarded as effectually disposed of by the fact that, in the year , halley took up the defence of his friend, and wrote two learned papers in support of newton's "system of chronology," which had been seriously attacked by a certain ecclesiastic. it is quite evident to any one who has studied these papers that halley's friendship for newton was as ardent as ever. the generous zeal with which halley adopted and defended the doctrines of newton with regard to the movements of the celestial bodies was presently rewarded by a brilliant discovery, which has more than any of his other researches rendered his name a familiar one to astronomers. newton, having explained the movement of the planets, was naturally led to turn his attention to comets. he perceived that their journeyings could be completely accounted for as consequences of the attraction of the sun, and he laid down the principles by which the orbit of a comet could be determined, provided that observations of its positions were obtained at three different dates. the importance of these principles was by no one more quickly recognised than by halley, who saw at once that it provided the means of detecting something like order in the movements of these strange wanderers. the doctrine of gravitation seemed to show that just as the planets revolved around the sun in ellipses, so also must the comets. the orbit, however, in the case of the comet, is so extremely elongated that the very small part of the elliptic path within which the comet is both near enough and bright enough to be seen from the earth, is indistinguishable from a parabola. applying these principles, halley thought it would be instructive to study the movements of certain bright comets, concerning which reliable observations could be obtained. at the expense of much labour, he laid down the paths pursued by twenty-four of these bodies, which had appeared between the years and . amongst them he noticed three, which followed tracks so closely resembling each other, that he was led to conclude the so called three comets could only have been three different appearances of the same body. the first of these occurred in , the second was seen by kepler in , and the third by halley himself in . these dates suggested that the observed phenomena might be due to the successive returns of one and the same comet after intervals of seventy-five or seventy-six years. on the further examination of ancient records, halley found that a comet had been seen in the year , a date, it will be observed, seventy-five years before . another had been observed seventy-six years earlier than , viz., in , and another seventy-five years before that, in . as halley thus found that a comet had been recorded on several occasions at intervals of seventy-five or seventy-six years, he was led to the conclusion that these several apparitions related to one and the same object, which was an obedient vassal of the sun, performing an eccentric journey round that luminary in a period of seventy-five or seventy-six years. to realise the importance of this discovery, it should be remembered that before halley's time a comet, if not regarded merely as a sign of divine displeasure, or as an omen of intending disaster, had at least been regarded as a chance visitor to the solar system, arriving no one knew whence, and going no one knew whither. a supreme test remained to be applied to halley's theory. the question arose as to the date at which this comet would be seen again. we must observe that the question was complicated by the fact that the body, in the course of its voyage around the sun, was exposed to the incessant disturbing action produced by the attraction of the several planets. the comet therefore, does not describe a simple ellipse as it would do if the attraction of the sun were the only force by which its movement were controlled. each of the planets solicits the comet to depart from its track, and though the amount of these attractions may be insignificant in comparison with the supreme controlling force of the sun, yet the departure from the ellipse is quite sufficient to produce appreciable irregularities in the comet's movement. at the time when halley lived, no means existed of calculating with precision the effect of the disturbance a comet might experience from the action of the different planets. halley exhibited his usual astronomical sagacity in deciding that jupiter would retard the return of the comet to some extent. had it not been for this disturbance the comet would apparently have been due in or early in . but the attraction of the great planet would cause delay, so that halley assigned, for the date of its re-appearance, either the end of or the beginning of . halley knew that he could not himself live to witness the fulfilment of his prediction, but he says: "if it should return, according to our predictions, about the year , impartial posterity will not refuse to acknowledge that this was first discovered by an englishman." this was, indeed, a remarkable prediction of an event to occur fifty-three years after it had been uttered. the way in which it was fulfilled forms one of the most striking episodes in the history of astronomy. the comet was first seen on christmas day, , and passed through its nearest point to the sun on march th, . halley had then been lying in his grave for seventeen years, yet the verification of his prophecy reflects a glory on his name which will cause it to live for ever in the annals of astronomy. the comet paid a subsequent visit in , and its next appearance is due about . halley next entered upon a labour which, if less striking to the imagination than his discoveries with regard to comets, is still of inestimable value in astronomy. he undertook a series of investigations with the object of improving our knowledge of the movements of the planets. this task was practically finished in , though the results of it were not published until after his death in . in the course of it he was led to investigate closely the motion of venus, and thus he came to recognise for the first time the peculiar importance which attaches to the phenomenon of the transit of this planet across the sun. halley saw that the transit, which was to take place in the year , would afford a favourable opportunity for determining the distance of the sun, and thus learning the scale of the solar system. he predicted the circumstances of the phenomenon with an astonishing degree of accuracy, considering his means of information, and it is unquestionably to the exertions of halley in urging the importance of the matter upon astronomers that we owe the unexampled degree of interest taken in the event, and the energy which scientific men exhibited in observing it. the illustrious astronomer had no hope of being himself a witness of the event, for it could not happen till many years after his death. this did not, however, diminish his anxiety to impress upon those who would then be alive, the importance of the occurrence, nor did it lead him to neglect anything which might contribute to the success of the observations. as we now know, halley rather over-estimated the value of the transit of venus, as a means of determining the solar distance. the fact is that the circumstances are such that the observation of the time of contact between the edge of the planet and the edge of the sun cannot be made with the accuracy which he had expected. in , halley became a candidate for the savilian professorship of astronomy at oxford. he was not, however, successful, for his candidature was opposed by flamsteed, the astronomer royal of the time, and another was appointed. he received some consolation for this particular disappointment by the fact that, in , owing to newton's friendly influence, he was appointed deputy controller of the mint at chester, an office which he did not retain for long, as it was abolished two years later. at last, in , he received what he had before vainly sought, and he was appointed to the savilian chair. his observations of the eclipse of the sun, which occurred in , added greatly to halley's reputation. this phenomenon excited special attention, inasmuch as it was the first total eclipse of the sun which had been visible in london since the year . halley undertook the necessary calculations, and predicted the various circumstances with a far higher degree of precision than the official announcement. he himself observed the phenomenon from the royal society's rooms, and he minutely describes the outer atmosphere of the sun, now known as the corona; without, however, offering an opinion as to whether it was a solar or a lunar appendage. at last halley was called to the dignified office which he of all men was most competent to fill. on february th, , he was appointed astronomer royal in succession to flamsteed. he found things at the royal observatory in a most unsatisfactory state. indeed, there were no instruments, nor anything else that was movable; for such things, being the property of flamsteed, had been removed by his widow, and though halley attempted to purchase from that lady some of the instruments which his predecessor had employed, the unhappy personal differences which had existed between him and flamsteed, and which, as we have already seen, prevented his election as savilian professor of astronomy, proved a bar to the negotiation. greenwich observatory wore a very different appearance in those days, from that which the modern visitor, who is fortunate enough to gain admission, may now behold. not only did halley find it bereft of instruments, we learn besides that he had no assistants, and was obliged to transact the whole business of the establishment single-handed. in , however, he obtained a grant of pounds from the board of ordnance, and accordingly a transit instrument was erected in the same year. some time afterwards he procured an eight-foot quadrant, and with these instruments, at the age of sixty-four, he commenced a series of observations on the moon. he intended, if his life was spared, to continue his observations for a period of eighteen years, this being, as astronomers know, a very important cycle in connection with lunar movements. the special object of this vast undertaking was to improve the theory of the moon's motion, so that it might serve more accurately to determine longitudes at sea. this self-imposed task halley lived to carry to a successful termination, and the tables deduced from his observations, and published after his death, were adopted almost universally by astronomers, those of the french nation being the only exception. throughout his life halley had been singularly free from illness of every kind, but in he had a stroke of paralysis. notwithstanding this, however, he worked diligently at his telescope till , after which his health began rapidly to give way. he died on january th, , in the eighty-sixth year of his age, retaining his mental faculties to the end. he was buried in the cemetery of the church of lee in kent, in the same grave as his wife, who had died five years previously. we are informed by admiral smyth that pond, a later astronomer royal, was afterwards laid in the same tomb. halley's disposition seems to have been generous and candid, and wholly free from anything like jealousy or rancour. in person he was rather above the middle height, and slight in build; his complexion was fair, and he is said to have always spoken, as well as acted, with uncommon sprightliness. in the eloge pronounced upon him at the paris academie des sciences, of which halley had been made a member in it was said, "he possessed all the qualifications which were necessary to please princes who were desirous of instruction, with a great extent of knowledge and a constant presence of mind; his answers were ready, and at the same time pertinent, judicious, polite and sincere." [plate: greenwich observatory in halley's time.] thus we find that peter the great was one of his most ardent admirers. he consulted the astronomer on matters connected with shipbuilding, and invited him to his own table. but halley possessed nobler qualifications than the capacity of pleasing princes. he was able to excite and to retain the love and admiration of his equals. this was due to the warmth of his attachments, the unselfishness of his devotion to his friends, and to a vein of gaiety and good-humour which pervaded all his conversation. bradley. james bradley was descended from an ancient family in the county of durham. he was born in or , at sherbourne, in gloucestershire, and was educated in the grammar school at northleach. from thence he proceeded in due course to oxford, where he was admitted a commoner at balliol college, on march th, . much of his time, while an undergraduate, was passed in essex with his maternal uncle, the rev. james pound, who was a well-known man of science and a diligent observer of the stars. it was doubtless by intercourse with his uncle that young bradley became so expert in the use of astronomical instruments, but the immortal discoveries he subsequently made show him to have been a born astronomer. the first exhibition of bradley's practical skill seems to be contained in two observations which he made in and . they have been published by halley, whose acuteness had led him to perceive the extraordinary scientific talents of the young astronomer. another illustration of the sagacity which bradley manifested, even at the very commencement of his astronomical career, is contained in a remark of halley's, who says: "dr. pound and his nephew, mr. bradley, did, myself being present, in the last opposition of the sun and mars this way demonstrate the extreme minuteness of the sun's parallax, and that it was not more than twelve seconds nor less than nine seconds." to make the significance of this plain, it should be observed that the determination of the sun's parallax is equivalent to the determination of the distance from the earth to the sun. at the time of which we are now writing, this very important unit of celestial measurement was only very imperfectly known, and the observations of pound and bradley may be interpreted to mean that, from their observations, they had come to the conclusion that the distance from the earth to the sun must be more than millions of miles, and less than millions. we now, of course, know that they were not exactly right, for the true distance of the sun is about millions of miles. we cannot, however, but think that it was a very remarkable approach for the veteran astronomer and his brilliant nephew to make towards the determination of a magnitude which did not become accurately known till fifty years later. among the earliest parts of astronomical work to which bradley's attention was directed, were the eclipses of jupiter's satellites. these phenomena are specially attractive inasmuch as they can be so readily observed, and bradley found it extremely interesting to calculate the times at which the eclipses should take place, and then to compare his observations with the predicted times. from the success that he met with in this work, and from his other labours, bradley's reputation as an astronomer increased so greatly that on november the th, , he was elected a fellow of the royal society. up to this time the astronomical investigations of bradley had been more those of an amateur than of a professional astronomer, and as it did not at first seem likely that scientific work would lead to any permanent provision, it became necessary for the youthful astronomer to choose a profession. it had been all along intended that he should enter the church, though for some reason which is not told us, he did not take orders as soon as his age would have entitled him to do so. in , however, the bishop of hereford offered bradley the vicarage of bridstow, near ross, in monmouthshire, and on july th, , he having then taken priest's orders, was duly instituted in his vicarage. in the beginning of the next year, bradley had some addition to his income from the proceeds of a welsh living, which, being a sinecure, he was able to hold with his appointment at bridstow. it appears, however, that his clerical occupations were not very exacting in their demands upon his time, for he was still able to pay long and often-repeated visits to his uncle at wandsworth, who, being himself a clergyman, seems to have received occasional assistance in his ministerial duties from his astronomical nephew. the time, however, soon arrived when bradley was able to make a choice between continuing to exercise his profession as a divine, or devoting himself to a scientific career. the savilian professorship of astronomy in the university of oxford became vacant by the death of dr. john keill. the statutes forbade that the savilian professor should also hold a clerical appointment, and mr. pound would certainly have been elected to the professorship had he consented to surrender his preferments in the church. but pound was unwilling to sacrifice his clerical position, and though two or three other candidates appeared in the field, yet the talents of bradley were so conspicuous that he was duly elected, his willingness to resign the clerical profession having been first ascertained. there can be no doubt that, with such influential friends as bradley possessed, he would have made great advances had he adhered to his profession as a divine. bishop hoadly, indeed, with other marks of favour, had already made the astronomer his chaplain. the engrossing nature of bradley's interest in astronomy decided him, however, to sacrifice all other prospects in comparison with the opening afforded by the savilian professorship. it was not that bradley found himself devoid of interest in clerical matters, but he felt that the true scope for such abilities as he possessed would be better found in the discharge of the scientific duties of the oxford chair than in the spiritual charge of a parish. on april the th, , bradley read his inaugural lecture in that new position on which he was destined to confer such lustre. it must, of course, be remembered that in those early days the art of constructing the astronomical telescope was very imperfectly understood. the only known method for getting over the peculiar difficulties presented in the construction of the refracting telescope, was to have it of the most portentous length. in fact, bradley made several of his observations with an instrument of two hundred and twelve feet focus. in such a case, no tube could be used, and the object glass was merely fixed at the top of a high pole. notwithstanding the inconvenience and awkwardness of such an instrument, bradley by its means succeeded in making many careful measurements. he observed, for example, the transit of mercury over the sun's disc, on october th, ; he also observed the dimensions of the planet venus, while a comet which halley discovered on october the th, , was assiduously observed at wanstead up to the middle of the ensuing month. the first of bradley's remarkable contributions to the "philosophical transactions" relates to this comet, and the extraordinary amount of work that he went through in connection therewith may be seen from an examination of his book of calculations which is still extant. the time was now approaching when bradley was to make the first of those two great discoveries by which his name has acquired a lustre that has placed him in the very foremost rank of astronomical discoverers. as has been often the case in the history of science, the first of these great successes was attained while he was pursuing a research intended for a wholly different purpose. it had long been recognised that as the earth describes a vast orbit, nearly two hundred million miles in diameter, in its annual journey round the sun, the apparent places of the stars should alter, to some extent, in correspondence with the changes in the earth's position. the nearer the star the greater the shift in its apparent place on the heavens, which must arise from the fact that it was seen from different positions in the earth's orbit. it had been pointed out that these apparent changes in the places of the stars, due to the movement of the earth, would provide the means of measuring the distances of the stars. as, however, these distances are enormously great in comparison with the orbit which the earth describes around the sun, the attempt to determine the distances of the stars by the shift in their positions had hitherto proved ineffectual. bradley determined to enter on this research once again; he thought that by using instruments of greater power, and by making measurements of increased delicacy, he would be able to perceive and to measure displacements which had proved so small as to elude the skill of the other astronomers who had previously made efforts in the same direction. in order to simplify the investigation as much as possible, bradley devoted his attention to one particular star, beta draconis, which happened to pass near his zenith. the object of choosing a star in this position was to avoid the difficulties which would be introduced by refraction had the star occupied any other place in the heavens than that directly overhead. we are still able to identify the very spot on which the telescope stood which was used in this memorable research. it was erected at the house then occupied by molyneux, on the western extremity of kew green. the focal length was feet inches, and the eye-glass was and a half feet above the ground floor. the instrument was first set up on november th, . if there had been any appreciable disturbance in the place of beta draconis in consequence of the movement of the earth around the sun, the star must appear to have the smallest latitude when in conjunction with the sun, and the greatest when in opposition. the star passed the meridian at noon in december, and its position was particularly noticed by molyneux on the third of that month. any perceptible displacement by parallax--for so the apparent change in position, due to the earth's motion, is called--would would have made the star shift towards the north. bradley, however, when observing it on the th, was surprised to find that the apparent place of the star, so far from shifting towards the north, as they had perhaps hoped it would, was found to lie a little more to the south than when it was observed before. he took extreme care to be sure that there was no mistake in his observation, and, true astronomer as he was, he scrutinized with the utmost minuteness all the circumstances of the adjustment of his instruments. still the star went to the south, and it continued so advancing in the same direction until the following march, by which time it had moved no less than twenty seconds south from the place which it occupied when the first observation was made. after a brief pause, in which no apparent movement was perceptible, the star by the middle of april appeared to be returning to the north. early in june it reached the same distance from the zenith which it had in december. by september the star was as much as thirty-nine seconds more to the north than it had been in march, then it returned towards the south, regaining in december the same situation which it had occupied twelve months before. this movement of the star being directly opposite to the movements which would have been the consequence of parallax, seemed to show that even if the star had any parallax its effects upon the apparent place were entirely masked by a much larger motion of a totally different description. various attempts were made to account for the phenomenon, but they were not successful. bradley accordingly determined to investigate the whole subject in a more thorough manner. one of his objects was to try whether the same movements which he had observed in one star were in any similar degree possessed by other stars. for this purpose he set up a new instrument at wanstead, and there he commenced a most diligent scrutiny of the apparent places of several stars which passed at different distances from the zenith. he found in the course of this research that other stars exhibited movements of a similar description to those which had already proved so perplexing. for a long time the cause of these apparent movements seemed a mystery. at last, however, the explanation of these remarkable phenomena dawned upon him, and his great discovery was made. one day when bradley was out sailing he happened to remark that every time the boat was laid on a different tack the vane at the top of the boat's mast shifted a little, as if there had been a slight change in the direction of the wind. after he had noticed this three or four times he made a remark to the sailors to the effect that it was very strange the wind should always happen to change just at the moment when the boat was going about. the sailors, however, said there had been no change in the wind, but that the alteration in the vane was due to the fact that the boat's course had been altered. in fact, the position of the vane was determined both by the course of the boat and the direction of the wind, and if either of these were altered there would be a corresponding change in the direction of the vane. this meant, of course, that the observer in the boat which was moving along would feel the wind coming from a point different from that in which the wind appeared to be blowing when the boat was at rest, or when it was sailing in some different direction. bradley's sagacity saw in this observation the clue to the difficulty which had so long troubled him. it had been discovered before the time of bradley that the passage of light through space is not an instantaneous phenomenon. light requires time for its journey. galileo surmised that the sun may have reached the horizon before we see it there, and it was indeed sufficiently obvious that a physical action, like the transmission of light, could hardly take place without requiring some lapse of time. the speed with which light actually travelled was, however, so rapid that its determination eluded all the means of experimenting which were available in those days. the penetration of roemer had previously detected irregularities in the observed times of the eclipses of jupiter's satellites, which were undoubtedly due to the interval which light required for stretching across the interplanetary spaces. bradley argued that as light can only travel with a certain speed, it may in a measure be regarded like the wind, which he noticed in the boat. if the observer were at rest, that is to say, if the earth were a stationary object, the direction in which the light actually does come would be different from that in which it appears to come when the earth is in motion. it is true that the earth travels but eighteen miles a second, while the velocity with which light is borne along attains to as much as , miles a second. the velocity of light is thus ten thousand times greater than the speed of the earth. but even though the wind blew ten thousand times faster than the speed with which the boat was sailing there would still be some change, though no doubt a very small change, in the position of the vane when the boat was in progress from the position it would have if the boat were at rest. it therefore occurred to this most acute of astronomers that when the telescope was pointed towards a star so as to place it apparently in the centre of the field of view, yet it was not generally the true position of the star. it was not, in fact, the position in which the star would have been observed had the earth been at rest. provided with this suggestion, he explained the apparent movements of the stars by the principle known as the "aberration of light." every circumstance was accounted for as a consequence of the relative movements of the earth and of the light from the star. this beautiful discovery not only established in the most forcible manner the nature of the movement of light; not only did it illustrate the truth of the copernican theory which asserted that the earth revolved around the sun, but it was also of the utmost importance in the improvement of practical astronomy. every observer now knows that, generally speaking, the position which the star appears to have is not exactly the position in which the star does actually lie. the observer is, however, able, by the application of the principles which bradley so clearly laid down, to apply to an observation the correction which is necessary to obtain from it the true place in which the object is actually situated. this memorable achievement at once conferred on bradley the highest astronomical fame. he tested his discovery in every way, but only to confirm its truth in the most complete manner. halley, the astronomer royal, died on the th, january, , and bradley was immediately pointed out as his successor. he was accordingly appointed astronomer royal in february, . on first taking up his abode at greenwich he was unable to conduct his observations owing to the wretched condition in which he found the instruments. he devoted himself, however, assiduously to their repair, and his first transit observation is recorded on the th july, . he worked with such energy that on one day it appears that transit observations were taken by himself alone, and in september, , he had completed the series of observations which established his second great discovery, the nutation of the earth's axis. the way in which he was led to the detection of the nutation is strikingly illustrative of the extreme care with which bradley conducted his observations. he found that in the course of a twelve-month, when the star had completed the movement which was due to aberration, it did not return exactly to the same position which it had previously occupied. at first he thought this must be due to some instrumental error, but after closer examination and repeated study of the effect as manifested by many different stars, he came to the conclusion that its origin must be sought in some quite different source. the fact is that a certain change takes place in the apparent position of the stars which is not due to the movement of the star itself, but is rather to be attributed to changes in the points from which the star's positions are measured. we may explain the matter in this way. as the earth is not a sphere, but has protuberant parts at the equator, the attraction of the moon exercises on those protuberant parts a pulling effect which continually changes the direction of the earth's axis, and consequently the position of the pole must be in a state of incessant fluctuation. the pole to which the earth's axis points on the sky is, therefore, slowly changing. at present it happens to lie near the pole star, but it will not always remain there. it describes a circle around the pole of the ecliptic, requiring about , years for a complete circuit. in the course of its progress the pole will gradually pass now near one star and now near another, so that many stars will in the lapse of ages discharge the various functions which the present pole star does for us. in about , years, for instance, the pole will have come near the bright star, vega. this movement of the pole had been known for ages. but what bradley discovered was that the pole, instead of describing an uniform movement as had been previously supposed, followed a sinuous course now on one side and now on the other of its mean place. this he traced to the fluctuations of the moon's orbit, which undergoes a continuous change in a period of nineteen years. thus the efficiency with which the moon acts on the protuberant mass of the earth varies, and thus the pole is caused to oscillate. this subtle discovery, if perhaps in some ways less impressive than bradley's earlier achievements of the detection of the aberration of light, is regarded by astronomers as testifying even in a higher degree to his astonishing care and skill as an observer, and justly entitles him to a unique place among the astronomers whose discoveries have been effected by consummate practical skill in the use of astronomical instruments. of bradley's private or domestic life there is but little to tell. in , soon after he became astronomer royal, he married a daughter of samuel peach, of chalford, in gloucestershire. there was but one child, a daughter, who became the wife of her cousin, rev. samuel peach, rector of compton, beauchamp, in berkshire. bradley's last two years of life were clouded by a melancholy depression of spirits, due to an apprehension that he should survive his rational faculties. it seems, however, that the ill he dreaded never came upon him, for he retained his mental powers to the close. he died on th july, , aged seventy, and was buried at michinghamton. william herschel. william herschel, one of the greatest astronomers that has ever lived, was born at hanover, on the th november, . his father, isaac herschel, was a man evidently of considerable ability, whose life was devoted to the study and practice of music, by which he earned a somewhat precarious maintenance. he had but few worldly goods to leave to his children, but he more than compensated for this by bequeathing to them a splendid inheritance of genius. touches of genius were, indeed, liberally scattered among the members of isaac's large family, and in the case of his forth child, william, and of a sister several years younger, it was united with that determined perseverance and rigid adherence to principle which enabled genius to fulfil its perfect work. a faithful chronicler has given us an interesting account of the way in which isaac herschel educated his sons; the narrative is taken from the recollections of one who, at the time we are speaking of, was an unnoticed little girl five or six years old. she writes:-- "my brothers were often introduced as solo performers and assistants in the orchestra at the court, and i remember that i was frequently prevented from going to sleep by the lively criticisms on music on coming from a concert. often i would keep myself awake that i might listen to their animating remarks, for it made me so happy to see them so happy. but generally their conversation would branch out on philosophical subjects, when my brother william and my father often argued with such warmth that my mother's interference became necessary, when the names--euler, leibnitz, and newton--sounded rather too loud for the repose of her little ones, who had to be at school by seven in the morning." the child whose reminiscences are here given became afterwards the famous caroline herschel. the narrative of her life, by mrs. john herschel, is a most interesting book, not only for the account it contains of the remarkable woman herself, but also because it provides the best picture we have of the great astronomer to whom caroline devoted her life. this modest family circle was, in a measure, dispersed at the outbreak of the seven years' war in . the french proceeded to invade hanover, which, it will be remembered, belonged at this time to the british dominions. young william herschel had already obtained the position of a regular performer in the regimental band of the hanoverian guards, and it was his fortune to obtain some experience of actual warfare in the disastrous battle of hastenbeck. he was not wounded, but he had to spend the night after the battle in a ditch, and his meditations on the occasion convinced him that soldiering was not the profession exactly adapted to his tastes. we need not attempt to conceal the fact that he left his regiment by the very simple but somewhat risky process of desertion. he had, it would seem, to adopt disguises to effect his escape. at all events, by some means he succeeded in eluding detection and reached england in safety. it is interesting to have learned on good authority that many years after this offence was committed it was solemnly forgiven. when herschel had become the famous astronomer, and as such visited king george at windsor, the king at their first meeting handed to him his pardon for deserting from the army, written out in due form by his majesty himself. it seems that the young musician must have had some difficulty in providing for his maintenance during the first few years of his abode in england. it was not until he had reached the age of twenty-two that he succeeded in obtaining any regular appointment. he was then made instructor of music to the durham militia. shortly afterwards, his talents being more widely recognised, he was appointed as organist at the parish church at halifax, and his prospects in life now being fairly favourable, and the seven years' war being over, he ventured to pay a visit to hanover to see his father. we can imagine the delight with which old isaac herschel welcomed his promising son, as well as his parental pride when a concert was given at which some of william's compositions were performed. if the father was so intensely gratified on this occasion, what would his feelings have been could he have lived to witness his son's future career? but this pleasure was not to be his, for he died many years before william became an astronomer. in , about a couple of years after his return to england from this visit to his old home, we find that herschel had received a further promotion to be organist in the octagon chapel, at bath. bath was then, as now, a highly fashionable resort, and many notable personages patronised the rising musician. herschel had other points in his favour besides his professional skill; his appearance was good, his address was prepossessing, and even his nationality was a distinct advantage, inasmuch as he was a hanoverian in the reign of king george the third. on sundays he played the organ, to the great delight of the congregation, and on week-days he was occupied by giving lessons to private pupils, and in preparation for public performances. he thus came to be busily employed, and seems to have been in the enjoyment of comfortable means. [plate: , new king street, bath, where herschel lived.] from his earliest youth herschel had been endowed with that invaluable characteristic, an eager curiosity for knowledge. he was naturally desirous of perfecting himself in the theory of music, and thus he was led to study mathematics. when he had once tasted the charms of mathematics, he saw vast regions of knowledge unfolded before him, and in this way he was induced to direct his attention to astronomy. more and more this pursuit seems to have engrossed his attention, until at last it had become an absorbing passion. herschel was, however, still obliged, by the exigency of procuring a livelihood, to give up the best part of his time to his profession as a musician; but his heart was eagerly fixed on another science, and every spare moment was steadily devoted to astronomy. for many years, however, he continued to labour at his original calling, nor was it until he had attained middle age and become the most celebrated astronomer of the time, that he was enabled to concentrate his attention exclusively on his favourite pursuit. it was with quite a small telescope which had been lent him by a friend that herschel commenced his career as an observer. however, he speedily discovered that to see all he wanted to see, a telescope of far greater power would be necessary, and he determined to obtain this more powerful instrument by actually making it with his own hands. at first it may seem scarcely likely that one whose occupation had previously been the study and practice of music should meet with success in so technical an operation as the construction of a telescope. it may, however, be mentioned that the kind of instrument which herschel designed to construct was formed on a very different principle from the refracting telescopes with which we are ordinarily familiar. his telescope was to be what is termed a reflector. in this type of instrument the optical power is obtained by the use of a mirror at the bottom of the tube, and the astronomer looks down through the tube towards his mirror and views the reflection of the stars with its aid. its efficiency as a telescope depends entirely on the accuracy with which the requisite form has been imparted to the mirror. the surface has to be hollowed out a little, and this has to be done so truly that the slightest deviation from good workmanship in this essential particular would be fatal to efficient performance of the telescope. [plate: william herschel.] the mirror that herschel employed was composed of a mixture of two parts of copper to one of tin; the alloy thus obtained is an intensely hard material, very difficult to cast into the proper shape, and very difficult to work afterwards. it possesses, however, when polished, a lustre hardly inferior to that of silver itself. herschel has recorded hardly any particulars as to the actual process by which he cast and figured his reflectors. we are however, told that in later years, after his telescopes had become famous, he made a considerable sum of money by the manufacture and sale of great instruments. perhaps this may be the reason why he never found it expedient to publish any very explicit details as to the means by which his remarkable successes were obtained. [plate: caroline herschel.] since herschel's time many other astronomers, notably the late earl of rosse, have experimented in the same direction, and succeeded in making telescopes certainly far greater, and probably more perfect, than any which herschel appears to have constructed. the details of these later methods are now well known, and have been extensively practised. many amateurs have thus been able to make telescopes by following the instructions so clearly laid down by lord rosse and the other authorities. indeed, it would seem that any one who has a little mechanical skill and a good deal of patience ought now to experience no great difficulty in constructing a telescope quite as powerful as that which first brought herschel into fame. i should, however, mention that in these modern days the material generally used for the mirror is of a more tractable description than the metallic substance which was employed by herschel and by lord rosse. a reflecting telescope of the present day would not be fitted with a mirror composed of that alloy known as speculum metal, whose composition i have already mentioned. it has been found more advantageous to employ a glass mirror carefully figured and polished, just as a metallic mirror would have been, and then to impart to the polished glass surface a fine coating of silver laid down by a chemical process. the silver-on-glass mirrors are so much lighter and so much easier to construct that the more old-fashioned metallic mirrors may be said to have fallen into almost total disuse. in one respect however, the metallic mirror may still claim the advantage that, with reasonable care, its surface will last bright and untarnished for a much longer period than can the silver film on the glass. however, the operation of re-silvering a glass has now become such a simple one that the advantage this indicates is not relatively so great as might at first be supposed. [plate: street view, herschel house, slough.] some years elapsed after herschel's attention had been first directed to astronomy, before he reaped the reward of his exertions in the possession of a telescope which would adequately reveal some of the glories of the heavens. it was in , when the astronomer was thirty-six years old, that he obtained his first glimpse of the stars with an instrument of his own construction. night after night, as soon as his musical labours were ended, his telescopes were brought out, sometimes into the small back garden of his house at bath, and sometimes into the street in front of his hall-door. it was characteristic of him that he was always endeavouring to improve his apparatus. he was incessantly making fresh mirrors, or trying new lenses, or combinations of lenses to act as eye-pieces, or projecting alterations in the mounting by which the telescope was supported. such was his enthusiasm that his house, we are told, was incessantly littered with the usual indications of the workman's presence, greatly to the distress of his sister, who, at this time, had come to take up her abode with him and look after his housekeeping. indeed, she complained that in his astronomical ardour he sometimes omitted to take off, before going into his workshop, the beautiful lace ruffles which he wore while conducting a concert, and that consequently they became soiled with the pitch employed in the polishing of his mirrors. this sister, who occupies such a distinct place in scientific history is the same little girl to whom we have already referred. from her earliest days she seems to have cherished a passionate admiration for her brilliant brother william. it was the proudest delight of her childhood as well as of her mature years to render him whatever service she could; no man of science was ever provided with a more capable or energetic helper than william herschel found in this remarkable woman. whatever work had to be done she was willing to bear her share in it, or even to toil at it unassisted if she could be allowed to do so. she not only managed all his domestic affairs, but in the grinding of the lenses and in the polishing of the mirrors she rendered every assistance that was possible. at one stage of the very delicate operation of fashioning a reflector, it is necessary for the workman to remain with his hand on the mirror for many hours in succession. when such labours were in progress, caroline used to sit by her brother, and enliven the time by reading stories aloud, sometimes pausing to feed him with a spoon while his hands were engaged on the task from which he could not desist for a moment. when mathematical work had to be done caroline was ready for it; she had taught herself sufficient to enable her to perform the kind of calculations, not, perhaps, very difficult ones, that herschel's work required; indeed, it is not too much to say that the mighty life-work which this man was enabled to perform could never have been accomplished had it not been for the self-sacrifice of this ever-loving and faithful sister. when herschel was at the telescope at night, caroline sat by him at her desk, pen in hand, ready to write down the notes of the observations as they fell from her brother's lips. this was no insignificant toil. the telescope was, of course, in the open air, and as herschel not unfrequently continued his observations throughout the whole of a long winter's night, there were but few women who could have accomplished the task which caroline so cheerfully executed. from dusk till dawn, when the sky was clear, were herschel's observing hours, and what this sometimes implied we can realise from the fact that caroline assures us she had sometimes to desist because the ink had actually frozen in her pen. the night's work over, a brief rest was taken, and while william had his labours for the day to attend to, caroline carefully transcribed the observations made during the night before, reduced all the figures and prepared everything in readiness for the observations that were to follow on the ensuing evening. but we have here been anticipating a little of the future which lay before the great astronomer; we must now revert to the history of his early work, at bath, in , when herschel's scrutiny of the skies first commenced with an instrument of his own manufacture. for some few years he did not attain any result of importance; no doubt he made a few interesting observations, but the value of the work during those years is to be found, not in any actual discoveries which were accomplished, but in the practice which herschel obtained in the use of his instruments. it was not until that the great achievement took place by which he at once sprang into fame. [plate: garden view, herschel house, slough.] it is sometimes said that discoveries are made by accident, and, no doubt, to a certain extent, but only, i fancy to a very small extent, this statement may be true. it is, at all events, certain that such lucky accidents do not often fall to the lot of people unless those people have done much to deserve them. this was certainly the case with herschel. he appears to have formed a project for making a close examination of all the stars above a certain magnitude. perhaps he intended to confine this research to a limited region of the sky, but, at all events, he seems to have undertaken the work energetically and systematically. star after star was brought to the centre of the field of view of his telescope, and after being carefully examined was then displaced, while another star was brought forward to be submitted to the same process. in the great majority of cases such observations yield really nothing of importance; no doubt even the smallest star in the heavens would, if we could find out all about it, reveal far more than all the astronomers that were ever on the earth have even conjectured. what we actually learn about the great majority of stars is only information of the most meagre description. we see that the star is a little point of light, and we see nothing more. in the great review which herschel undertook he doubtless examined hundreds, or perhaps thousands of stars, allowing them to pass away without note or comment. but on an ever-memorable night in march, , it happened that he was pursuing his task among the stars in the constellation of gemini. doubtless, on that night, as on so many other nights, one star after another was looked at only to be dismissed, as not requiring further attention. on the evening in question, however, one star was noticed which, to herschel's acute vision seemed different from the stars which in so many thousands are strewn over the sky. a star properly so called appears merely as a little point of light, which no increase of magnifying power will ever exhibit with a true disc. but there was something in the star-like object which herschel saw that immediately arrested his attention and made him apply to it a higher magnifying power. this at once disclosed the fact that the object possessed a disc, that is, a definite, measurable size, and that it was thus totally different from any one of the hundreds and thousands of stars which exist elsewhere in space. indeed, we may say at once that this little object was not a star at all; it was a planet. that such was its true nature was confirmed, after a little further observation, by perceiving that the body was shifting its place on the heavens relatively to the stars. the organist at the octagon chapel at bath had, therefore, discovered a new planet with his home-made telescope. i can imagine some one will say, "oh, there was nothing so wonderful in that; are not planets always being discovered? has not m. palisa, for instance, discovered about eighty of such objects, and are there not hundreds of them known nowadays?" this is, to a certain extent, quite true. i have not the least desire to detract from the credit of those industrious and sharp-sighted astronomers who have in modern days brought so many of these little objects within our cognisance. i think, however, it must be admitted that such discoveries have a totally different importance in the history of science from that which belongs to the peerless achievement of herschel. in the first place, it must be observed that the minor planets now brought to light are so minute that if a score of them were rolled to together into one lump it would not be one-thousandth part of the size of the grand planet discovered by herschel. this is, nevertheless, not the most important point. what marks herschel's achievement as one of the great epochs in the history of astronomy is the fact that the detection of uranus was the very first recorded occasion of the discovery of any planet whatever. for uncounted ages those who watched the skies had been aware of the existence of the five old planets--jupiter, mercury, saturn, venus, and mars. it never seems to have occurred to any of the ancient philosophers that there could be other similar objects as yet undetected over and above the well-known five. great then was the astonishment of the scientific world when the bath organist announced his discovery that the five planets which had been known from all antiquity must now admit the company of a sixth. and this sixth planet was, indeed, worthy on every ground to be received into the ranks of the five glorious bodies of antiquity. it was, no doubt, not so large as saturn, it was certainly very much less than jupiter; on the other hand, the new body was very much larger than mercury, than venus, or than mars, and the earth itself seemed quite an insignificant object in comparison with this newly added member of the solar system. in one respect, too, herschel's new planet was a much more imposing object than any one of the older bodies; it swept around the sun in a majestic orbit, far outside that of saturn, which had previously been regarded as the boundary of the solar system, and its stately progress required a period of not less than eighty-one years. king george the third, hearing of the achievements of the hanoverian musician, felt much interest in his discovery, and accordingly herschel was bidden to come to windsor, and to bring with him the famous telescope, in order to exhibit the new planet to the king, and to tell his majesty all about it. the result of the interview was to give herschel the opportunity for which he had so long wished, of being able to devote himself exclusively to science for the rest of his life. [plate: view of the observatory, herschel house, slough.] the king took so great a fancy to the astronomer that he first, as i have already mentioned, duly pardoned his desertion from the army, some twenty-five years previously. as a further mark of his favour the king proposed to confer on herschel the title of his majesty's own astronomer, to assign to him a residence near windsor, to provide him with a salary, and to furnish such funds as might be required for the erection of great telescopes, and for the conduct of that mighty scheme of celestial observation on which herschel was so eager to enter. herschel's capacity for work would have been much impaired if he had been deprived of the aid of his admirable sister, and to her, therefore, the king also assigned a salary, and she was installed as herschel's assistant in his new post. with his usually impulsive determination, herschel immediately cut himself free from all his musical avocations at bath, and at once entered on the task of making and erecting the great telescopes at windsor. there, for more than thirty years, he and his faithful sister prosecuted with unremitting ardour their nightly scrutiny of the sky. paper after paper was sent to the royal society, describing the hundreds, indeed the thousands, of objects such as double stars; nebulae and clusters, which were first revealed to human gaze during those midnight vigils. to the end of his life he still continued at every possible opportunity to devote himself to that beloved pursuit in which he had such unparalleled success. no single discovery of herschel's later years was, however, of the same momentous description as that which first brought him to fame. [plate: the -foot telescope as it was in the year , herschel house, slough.] herschel married when considerably advanced in life and he lived to enjoy the indescribable pleasure of finding that his only son, afterwards sir john herschel, was treading worthily in his footsteps, and attaining renown as an astronomical observer, second only to that of his father. the elder herschel died in , and his illustrious sister caroline then returned to hanover, where she lived for many years to receive the respect and attention which were so justly hers. she died at a very advanced age in . laplace. the author of the "mecanique celeste" was born at beaumont-en-auge, near honfleur, in , just thirteen years later than his renowned friend lagrange. his father was a farmer, but appears to have been in a position to provide a good education for a son who seemed promising. considering the unorthodoxy in religious matters which is generally said to have characterized laplace in later years, it is interesting to note that when he was a boy the subject which first claimed his attention was theology. he was, however, soon introduced to the study of mathematics, in which he presently became so proficient, that while he was still no more than eighteen years old, he obtained employment as a mathematical teacher in his native town. desiring wider opportunities for study and for the acquisition of fame than could be obtained in the narrow associations of provincial life, young laplace started for paris, being provided with letters of introduction to d'alembert, who then occupied the most prominent position as a mathematician in france, if not in the whole of europe. d'alembert's fame was indeed so brilliant that catherine the great wrote to ask him to undertake the education of her son, and promised the splendid income of a hundred thousand francs. he preferred, however, a quiet life of research in paris, although there was but a modest salary attached to his office. the philosopher accordingly declined the alluring offer to go to russia, even though catherine wrote again to say: "i know that your refusal arises from your desire to cultivate your studies and your friendships in quiet. but this is of no consequence: bring all your friends with you, and i promise you that both you and they shall have every accommodation in my power." with equal firmness the illustrious mathematician resisted the manifold attractions with which frederick the great sought to induce him, to take up his residence at berlin. in reading of these invitations we cannot but be struck at the extraordinary respect which was then paid to scientific distinction. it must be remembered that the discoveries of such a man as d'alembert were utterly incapable of being appreciated except by those who possessed a high degree of mathematical culture. we nevertheless find the potentates of russia and prussia entreating and, as it happens, vainly entreating, the most distinguished mathematician in france to accept the positions that they were proud to offer him. it was to d'alembert, the profound mathematician, that young laplace, the son of the country farmer, presented his letters of introduction. but those letters seem to have elicited no reply, whereupon laplace wrote to d'alembert submitting a discussion on some point in dynamics. this letter instantly produced the desired effect. d'alembert thought that such mathematical talent as the young man displayed was in itself the best of introductions to his favour. it could not be overlooked, and accordingly he invited laplace to come and see him. laplace, of course, presented himself, and ere long d'alembert obtained for the rising philosopher a professorship of mathematics in the military school in paris. this gave the brilliant young mathematician the opening for which he sought, and he quickly availed himself of it. laplace was twenty-three years old when his first memoir on a profound mathematical subject appeared in the memoirs of the academy at turin. from this time onwards we find him publishing one memoir after another in which he attacks, and in many cases successfully vanquishes, profound difficulties in the application of the newtonian theory of gravitation to the explanation of the solar system. like his great contemporary lagrange, he loftily attempted problems which demanded consummate analytical skill for their solution. the attention of the scientific world thus became riveted on the splendid discoveries which emanated from these two men, each gifted with extraordinary genius. laplace's most famous work is, of course, the "mecanique celeste," in which he essayed a comprehensive attempt to carry out the principles which newton had laid down, into much greater detail than newton had found practicable. the fact was that newton had not only to construct the theory of gravitation, but he had to invent the mathematical tools, so to speak, by which his theory could be applied to the explanation of the movements of the heavenly bodies. in the course of the century which had elapsed between the time of newton and the time of laplace, mathematics had been extensively developed. in particular, that potent instrument called the infinitesimal calculus, which newton had invented for the investigation of nature, had become so far perfected that laplace, when he attempted to unravel the movements of the heavenly bodies, found himself provided with a calculus far more efficient than that which had been available to newton. the purely geometrical methods which newton employed, though they are admirably adapted for demonstrating in a general way the tendencies of forces and for explaining the more obvious phenomena by which the movements of the heavenly bodies are disturbed, are yet quite inadequate for dealing with the more subtle effects of the law of gravitation. the disturbances which one planet exercises upon the rest can only be fully ascertained by the aid of long calculation, and for these calculations analytical methods are required. with an armament of mathematical methods which had been perfected since the days of newton by the labours of two or three generations of consummate mathematical inventors, laplace essayed in the "mecanique celeste" to unravel the mysteries of the heavens. it will hardly be disputed that the book which he has produced is one of the most difficult books to understand that has ever been written. in great part, of course, this difficulty arises from the very nature of the subject, and is so far unavoidable. no one need attempt to read the "mecanique celeste" who has not been naturally endowed with considerable mathematical aptitude which he has cultivated by years of assiduous study. the critic will also note that there are grave defects in laplace's method of treatment. the style is often extremely obscure, and the author frequently leaves great gaps in his argument, to the sad discomfiture of his reader. nor does it mend matters to say, as laplace often does say, that it is "easy to see" how one step follows from another. such inferences often present great difficulties even to excellent mathematicians. tradition indeed tells us that when laplace had occasion to refer to his own book, it sometimes happened that an argument which he had dismissed with his usual formula, "il est facile a voir," cost the illustrious author himself an hour or two of hard thinking before he could recover the train of reasoning which had been omitted. but there are certain parts of this great work which have always received the enthusiastic admiration of mathematicians. laplace has, in fact, created whole tracts of science, some of which have been subsequently developed with much advantage in the prosecution of the study of nature. judged by a modern code the gravest defect of laplace's great work is rather of a moral than of a mathematical nature. lagrange and he advanced together in their study of the mechanics of the heavens, at one time perhaps along parallel lines, while at other times they pursued the same problem by almost identical methods. sometimes the important result was first reached by lagrange, sometimes it was laplace who had the good fortune to make the discovery. it would doubtless be a difficult matter to draw the line which should exactly separate the contributions to astronomy made by one of these illustrious mathematicians, and the contributions made by the other. but in his great work laplace in the loftiest manner disdained to accord more than the very barest recognition to lagrange, or to any of the other mathematicians, newton alone excepted, who had advanced our knowledge of the mechanism of the heavens. it would be quite impossible for a student who confined his reading to the "mecanique celeste" to gather from any indications that it contains whether the discoveries about which he was reading had been really made by laplace himself or whether they had not been made by lagrange, or by euler, or by clairaut. with our present standard of morality in such matters, any scientific man who now brought forth a work in which he presumed to ignore in this wholesale fashion the contributions of others to the subject on which he was writing, would be justly censured and bitter controversies would undoubtedly arise. perhaps we ought not to judge laplace by the standard of our own time, and in any case i do not doubt that laplace might have made a plausible defence. it is well known that when two investigators are working at the same subjects, and constantly publishing their results, it sometimes becomes difficult for each investigator himself to distinguish exactly between what he has accomplished and that which must be credited to his rival. laplace may probably have said to himself that he was going to devote his energies to a great work on the interpretation of nature, that it would take all his time and all his faculties, and all the resources of knowledge that he could command, to deal justly with the mighty problems before him. he would not allow himself to be distracted by any side issue. he could not tolerate that pages should be wasted in merely discussing to whom we owe each formula, and to whom each deduction from such formula is due. he would rather endeavour to produce as complete a picture as he possibly could of the celestial mechanics, and whether it were by means of his mathematics alone, or whether the discoveries of others may have contributed in any degree to the result, is a matter so infinitesimally insignificant in comparison with the grandeur of his subject that he would altogether neglect it. "if lagrange should think," laplace might say, "that his discoveries had been unduly appropriated, the proper course would be for him to do exactly what i have done. let him also write a "mecanique celeste," let him employ those consummate talents which he possesses in developing his noble subject to the utmost. let him utilise every result that i or any other mathematician have arrived at, but not trouble himself unduly with unimportant historical details as to who discovered this, and who discovered that; let him produce such a work as he could write, and i shall heartily welcome it as a splendid contribution to our science." certain it is that laplace and lagrange continued the best of friends, and on the death of the latter it was laplace who was summoned to deliver the funeral oration at the grave of his great rival. the investigations of laplace are, generally speaking, of too technical a character to make it possible to set forth any account of them in such a work as the present. he did publish, however, one treatise, called the "systeme du monde," in which, without introducing mathematical symbols, he was able to give a general account of the theories of the celestial movements, and of the discoveries to which he and others had been led. in this work the great french astronomer sketched for the first time that remarkable doctrine by which his name is probably most generally known to those readers of astronomical books who are not specially mathematicians. it is in the "systeme du monde" that laplace laid down the principles of the nebular theory which, in modern days, has been generally accepted by those philosophers who are competent to judge, as substantially a correct expression of a great historical fact. [plate: laplace.] the nebular theory gives a physical account of the origin of the solar system, consisting of the sun in the centre, with the planets and their attendant satellites. laplace perceived the significance of the fact that all the planets revolved in the same direction around the sun; he noticed also that the movements of rotation of the planets on their axes were performed in the same direction as that in which a planet revolves around the sun; he saw that the orbits of the satellites, so far at least as he knew them, revolved around their primaries also in the same direction. nor did it escape his attention that the sun itself rotated on its axis in the same sense. his philosophical mind was led to reflect that such a remarkable unanimity in the direction of the movements in the solar system demanded some special explanation. it would have been in the highest degree improbable that there should have been this unanimity unless there had been some physical reason to account for it. to appreciate the argument let us first concentrate our attention on three particular bodies, namely the earth, the sun, and the moon. first the earth revolves around the sun in a certain direction, and the earth also rotates on its axis. the direction in which the earth turns in accordance with this latter movement might have been that in which it revolves around the sun, or it might of course have been opposite thereto. as a matter of fact the two agree. the moon in its monthly revolution around the earth follows also the same direction, and our satellite rotates on its axis in the same period as its monthly revolution, but in doing so is again observing this same law. we have therefore in the earth and moon four movements, all taking place in the same direction, and this is also identical with that in which the sun rotates once every twenty-five days. such a coincidence would be very unlikely unless there were some physical reason for it. just as unlikely would it be that in tossing a coin five heads or five tails should follow each other consecutively. if we toss a coin five times the chances that it will turn up all heads or all tails is but a small one. the probability of such an event is only one-sixteenth. there are, however, in the solar system many other bodies besides the three just mentioned which are animated by this common movement. among them are, of course, the great planets, jupiter, saturn, mars, venus, and mercury, and the satellites which attend on these planets. all these planets rotate on their axes in the same direction as they revolve around the sun, and all their satellites revolve also in the same way. confining our attention merely to the earth, the sun, and the five great planets with which laplace was acquainted, we have no fewer than six motions of revolution and seven motions of rotation, for in the latter we include the rotation of the sun. we have also sixteen satellites of the planets mentioned whose revolutions round their primaries are in the same direction. the rotation of the moon on its axis may also be reckoned, but as to the rotations of the satellites of the other planets we cannot speak with any confidence, as they are too far off to be observed with the necessary accuracy. we have thus thirty circular movements in the solar system connected with the sun and moon and those great planets than which no others were known in the days of laplace. the significant fact is that all these thirty movements take place in the same direction. that this should be the case without some physical reason would be just as unlikely as that in tossing a coin thirty times it should turn up all heads or all tails every time without exception. we can express the argument numerically. calculation proves that such an event would not generally happen oftener than once out of five hundred millions of trials. to a philosopher of laplace's penetration, who had made a special study of the theory of probabilities, it seemed well-nigh inconceivable that there should have been such unanimity in the celestial movements, unless there had been some adequate reason to account for it. we might, indeed, add that if we were to include all the objects which are now known to belong to the solar system, the argument from probability might be enormously increased in strength. to laplace the argument appeared so conclusive that he sought for some physical cause of the remarkable phenomenon which the solar system presented. thus it was that the famous nebular hypothesis took its rise. laplace devised a scheme for the origin of the sun and the planetary system, in which it would be a necessary consequence that all the movements should take place in the same direction as they are actually observed to do. let us suppose that in the beginning there was a gigantic mass of nebulous material, so highly heated that the iron and other substances which now enter into the composition of the earth and planets were then suspended in a state of vapour. there is nothing unreasonable in such a supposition indeed, we know as a matter of fact that there are thousands of such nebulae to be discerned at present through our telescopes. it would be extremely unlikely that any object could exist without possessing some motion of rotation; we may in fact assert that for rotation to be entirety absent from the great primeval nebula would be almost infinitely improbable. as ages rolled on, the nebula gradually dispersed away by radiation its original stores of heat, and, in accordance with well-known physical principles, the materials of which it was formed would tend to coalesce. the greater part of those materials would become concentrated in a mighty mass surrounded by outlying uncondensed vapours. there would, however, also be regions throughout the extent of the nebula, in which subsidiary centres of condensation would be found. in its long course of cooling, the nebula would, therefore, tend ultimately to form a mighty central body with a number of smaller bodies disposed around it. as the nebula was initially endowed with a movement of rotation, the central mass into which it had chiefly condensed would also revolve, and the subsidiary bodies would be animated by movements of revolution around the central body. these movements would be all pursued in one common direction, and it follows, from well-known mechanical principles, that each of the subsidiary masses, besides participating in the general revolution around the central body, would also possess a rotation around its axis, which must likewise be performed in the same direction. around the subsidiary bodies other objects still smaller would be formed, just as they themselves were formed relatively to the great central mass. as the ages sped by, and the heat of these bodies became gradually dissipated, the various objects would coalesce, first into molten liquid masses, and thence, at a further stage of cooling, they would assume the appearance of solid masses, thus producing the planetary bodies such as we now know them. the great central mass, on account of its preponderating dimensions, would still retain, for further uncounted ages, a large quantity of its primeval heat, and would thus display the splendours of a glowing sun. in this way laplace was able to account for the remarkable phenomena presented in the movements of the bodies of the solar system. there are many other points also in which the nebular theory is known to tally with the facts of observation. in fact, each advance in science only seems to make it more certain that the nebular hypothesis substantially represents the way in which our solar system has grown to its present form. not satisfied with a career which should be merely scientific, laplace sought to connect himself with public affairs. napoleon appreciated his genius, and desired to enlist him in the service of the state. accordingly he appointed laplace to be minister of the interior. the experiment was not successful, for he was not by nature a statesman. napoleon was much disappointed at the ineptitude which the great mathematician showed for official life, and, in despair of laplace's capacity as an administrator, declared that he carried the spirit of his infinitesimal calculus into the management of business. indeed, laplace's political conduct hardly admits of much defence. while he accepted the honours which napoleon showered on him in the time of his prosperity, he seems to have forgotten all this when napoleon could no longer render him service. laplace was made a marquis by louis xviii., a rank which he transmitted to his son, who was born in . during the latter part of his life the philosopher lived in a retired country place at arcueile. here he pursued his studies, and by strict abstemiousness, preserved himself from many of the infirmities of old age. he died on march the th, , in his seventy-eighth year, his last words being, "what we know is but little, what we do not know is immense." brinkley. provost baldwin held absolute sway in the university of dublin for forty-one years. his memory is well preserved there. the bursar still dispenses the satisfactory revenues which baldwin left to the college. none of us ever can forget the marble angels round the figure of the dying provost on which we used to gaze during the pangs of the examination hall. baldwin died in , and was succeeded by francis andrews, a fellow of seventeen years' standing. as to the scholastic acquirements of andrews, all i can find is a statement that he was complimented by the polite professors of padua on the elegance and purity with which he discoursed to them in latin. andrews was also reputed to be a skilful lawyer. he was certainly a privy councillor and a prominent member of the irish house of commons, and his social qualities were excellent. perhaps it was baldwin's example that stimulated a desire in andrews to become a benefactor to his college. he accordingly bequeathed a sum of , pounds and an annual income of pounds wherewith to build and endow an astronomical observatory in the university. the figures just stated ought to be qualified by the words of cautious ussher (afterwards the first professor of astronomy), that "this money was to arise from an accumulation of a part of his property, to commence upon a particular contingency happening to his family." the astronomical endowment was soon in jeopardy by litigation. andrews thought he had provided for his relations by leaving to them certain leasehold interests connected with the provost's estate. the law courts, however, held that these interests were not at the disposal of the testator, and handed them over to hely hutchinson, the next provost. the disappointed relations then petitioned the irish parliament to redress this grievance by transferring to them the moneys designed by andrews for the observatory. it would not be right, they contended, that the kindly intentions of the late provost towards his kindred should be frustrated for the sake of maintaining what they described as "a purely ornamental institution." the authorities of the college protested against this claim. counsel were heard, and a committee of the house made a report declaring the situation of the relations to be a hard one. accordingly, a compromise was made, and the dispute terminated. the selection of a site for the new astronomical observatory was made by the board of trinity college. the beautiful neighbourhood of dublin offered a choice of excellent localities. on the north side of the liffey an observatory could have been admirably placed, either on the remarkable promontory of howth or on the elevation of which dunsink is the summit. on the south side of dublin there are several eminences that would have been suitable: the breezy heaths at foxrock combine all necessary conditions; the obelisk hill at killiney would have given one of the most picturesque sites for an observatory in the world; while near delgany two or three other good situations could be mentioned. but the board of those pre-railway days was naturally guided by the question of proximity. dunsink was accordingly chosen as the most suitable site within the distance of a reasonable walk from trinity college. the northern boundary of the phoenix park approaches the little river tolka, which winds through a succession of delightful bits of sylvan scenery, such as may be found in the wide demesne of abbotstown and the classic shades of glasnevin. from the banks of the tolka, on the opposite side of the park, the pastures ascend in a gentle slope to culminate at dunsink, where at a distance of half a mile from the stream, of four miles from dublin, and at a height of feet above the sea, now stands the observatory. from the commanding position of dunsink a magnificent view is obtained. to the east the sea is visible, while the southern prospect over the valley of the liffey is bounded by a range of hills and mountains extending from killiney to bray head, thence to the little sugar loaf, the two rock and the three rock mountains, over the flank of which the summit of the great sugar loaf is just perceptible. directly in front opens the fine valley of glenasmole, with kippure mountain, while the range can be followed to its western extremity at lyons. the climate of dunsink is well suited for astronomical observation. no doubt here, as elsewhere in ireland, clouds are abundant, but mists or haze are comparatively unusual, and fogs are almost unknown. the legal formalities to be observed in assuming occupation exacted a delay of many months; accordingly, it was not until the th december, , that a contract could be made with mr. graham moyers for the erection of a meridian-room and a dome for an equatorial, in conjunction with a becoming residence for the astronomer. before the work was commenced at dunsink, the board thought it expedient to appoint the first professor of astronomy. they met for this purpose on the nd january, , and chose the rev. henry ussher, a senior fellow of trinity college, dublin. the wisdom of the appointment was immediately shown by the assiduity with which ussher engaged in founding the observatory. in three years he had erected the buildings and equipped them with instruments, several of which were of his own invention. on the th of february, , a special grant of pounds was made by the board to dr. ussher as some recompense for his labours. it happened that the observatory was not the only scientific institution which came into being in ireland at this period; the newly-kindled ardour for the pursuit of knowledge led, at the same time, to the foundation of the royal irish academy. by a fitting coincidence, the first memoir published in the "transactions of the royal irish academy," was by the first andrews, professor of astronomy. it was read on the th of june, , and bore the title, "account of the observatory belonging to trinity college," by the rev. h. ussher, d.d., m.r.i.a., f.r.s. this communication shows the extensive design that had been originally intended for dunsink, only a part of which was, however, carried out. for instance, two long corridors, running north and south from the central edifice, which are figured in the paper, never developed into bricks and mortar. we are not told why the original scheme had to be contracted; but perhaps the reason may be not unconnected with a remark of ussher's, that the college had already advanced from its own funds a sum considerably exceeding the original bequest. the picture of the building shows also the dome for the south equatorial, which was erected many years later. ussher died in . during his brief career at the observatory, he observed eclipses, and is stated to have done other scientific work. the minutes of the board declare that the infant institution had already obtained celebrity by his labours, and they urge the claims of his widow to a pension, on the ground that the disease from which he died had been contracted by his nightly vigils. the board also promised a grant of fifty guineas as a help to bring out dr. ussher's sermons. they advanced twenty guineas to his widow towards the publication of his astronomical papers. they ordered his bust to be executed for the observatory, and offered "the death of ussher" as the subject of a prize essay; but, so far as i can find, neither the sermons nor the papers, neither the bust nor the prize essay, ever came into being. there was keen competition for the chair of astronomy which the death of ussher vacated. the two candidates were rev. john brinkley, of caius college, cambridge, a senior wrangler (born at woodbridge, suffolk, in ), and mr. stack, fellow of trinity college, dublin, and author of a book on optics. a majority of the board at first supported stack, while provost hely hutchinson and one or two others supported brinkley. in those days the provost had a veto at elections, so that ultimately stack was withdrawn and brinkley was elected. this took place on the th december, . the national press of the day commented on the preference shown to the young englishman, brinkley, over his irish rival. an animated controversy ensued. the provost himself condescended to enter the lists and to vindicate his policy by a long letter in the "public register" or "freeman's journal," of st december, . this letter was anonymous, but its authorship is obvious. it gives the correspondence with maskelyne and other eminent astronomers, whose advice and guidance had been sought by the provost. it also contends that "the transactions of the board ought not to be canvassed in the newspapers." for this reference, as well as for much other information, i am indebted to my friend, the rev. john stubbs, d.d. [plate: the observatory, dunsink. from a photograph by w. lawrence, upper sackville street, dublin.] the next event in the history of the observatory was the issue of letters patent ( geo. iii., a.d. ), in which it is recited that "we grant and ordain that there shall be forever hereafter a professor of astronomy, on the foundation of dr. andrews, to be called and known by the name of the royal astronomer of ireland." the letters prescribe the various duties of the astronomer and the mode of his election. they lay down regulations as to the conduct of the astronomical work, and as to the choice of an assistant. they direct that the provost and the senior fellows shall make a thorough inspection of the observatory once every year in june or july; and this duty was first undertaken on the th of july, . it may be noted that the date on which the celebration of the tercentenary of the university was held happens to coincide with the centenary of the first visitation of the observatory. the visitors on the first occasion were a. murray, matthew young, george hall, and john barrett. they record that they find the buildings, books and instruments in good condition; but the chief feature in this report, as well as in many which followed it, related to a circumstance to which we have not yet referred. in the original equipment of the observatory, ussher, with the natural ambition of a founder, desired to place in it a telescope of more magnificent proportions than could be found anywhere else. the board gave a spirited support to this enterprise, and negotiations were entered into with the most eminent instrument-maker of those days. this was jesse ramsden ( - ), famous as the improver of the sextant, as the constructor of the great theodolite used by general roy in the english survey, and as the inventor of the dividing engine for graduating astronomical instruments. ramsden had built for sir george schuckburgh the largest and most perfect equatorial ever attempted. he had constructed mural quadrants for padua and verona, which elicited the wonder of astronomers when dr. maskelyne declared he could detect no error in their graduation so large as two seconds and a half. but ramsden maintained that even better results would be obtained by superseding the entire quadrant by the circle. he obtained the means of testing this prediction when he completed a superb circle for palermo of five feet diameter. finding his anticipations were realised, he desired to apply the same principles on a still grander scale. ramsden was in this mood when he met with dr. ussher. the enthusiasm of the astronomer and the instrument-maker communicated itself to the board, and a tremendous circle, to be ten feet in diameter, was forthwith projected. projected, but never carried out. after ramsden had to some extent completed a -foot circle, he found such difficulties that he tried a -foot, and this again he discarded for an -foot, which was ultimately accomplished, though not entirely by himself. notwithstanding the contraction from the vast proportions originally designed, the completed instrument must still be regarded as a colossal piece of astronomical workmanship. even at this day i do not know that any other observatory can show a circle eight feet in diameter graduated all round. i think it is professor piazzi smith who tells us how grateful he was to find a large telescope he had ordered finished by the opticians on the very day they had promised it. the day was perfectly correct; it was only the year that was wrong. a somewhat remarkable experience in this direction is chronicled by the early reports of the visitors to dunsink observatory. i cannot find the date on which the great circle was ordered from ramsden, but it is fixed with sufficient precision by an allusion in ussher's paper to the royal irish academy, which shows that by the th june, , the order had been given, but that the abandonment of the -foot scale had not then been contemplated. it was reasonable that the board should allow ramsden ample time for the completion of a work at once so elaborate and so novel. it could not have been finished in a year, nor would there have been much reason for complaint if the maker had found he required two or even three years more. seven years gone, and still no telescope, was the condition in which the board found matters at their first visitation in . they had, however, assurances from ramsden that the instrument would be completed within the year; but, alas for such promises, another seven years rolled on, and in the place for the great circle was still vacant at dunsink. ramsden had fallen into bad health, and the board considerately directed that "inquiries should be made." next year there was still no progress, so the board were roused to threaten ramsden with a suit at law; but the menace was never executed, for the malady of the great optician grew worse, and he died that year. affairs had now assumed a critical aspect, for the college had advanced much money to ramsden during these fifteen years, and the instrument was still unfinished. an appeal was made by the provost to dr. maskelyne, the astronomer royal of england, for his advice and kindly offices in this emergency. maskelyne responds--in terms calculated to allay the anxiety of the bursar--"mr. ramsden has left property behind him, and the college can be in no danger of losing both their money and the instrument." the business of ramsden was then undertaken by berge, who proceeded to finish the circle quite as deliberately as his predecessor. after four years berge promised the instrument in the following august, but it did not come. two years later ( ) the professor complains that he can get no answer from berge. in , it is stated that berge will send the telescope in a month. he did not; but in the next year ( ), about twenty-three years after the great circle was ordered, it was erected at dunsink, where it is still to be seen. the following circumstances have been authenticated by the signatures of provosts, proctors, bursars, and other college dignitaries:--in the board ordered two of the clocks at the observatory to be sent to mr. crosthwaite for repairs. seven years later, in , mr. crosthwaite was asked if the clocks were ready. this impatience was clearly unreasonable, for even in four more years, , we find the two clocks were still in hand. two years later, in , the board determined to take vigorous action by asking the bursar to call upon crosthwaite. this evidently produced some effect, for in the following year, , the professor had no doubt that the clocks would be speedily returned. after eight years more, in , one of the clocks was still being repaired, and so it was in , which is the last record we have of these interesting time-pieces. astronomers are, however, accustomed to deal with such stupendous periods in their calculations, that even the time taken to repair a clock seems but small in comparison. the long tenure of the chair of astronomy by brinkley is divided into two nearly equal periods by the year in which the great circle was erected. brinkley was eighteen years waiting for his telescope, and he had eighteen years more in which to use it. during the first of these periods brinkley devoted himself to mathematical research; during the latter he became a celebrated astronomer. brinkley's mathematical labours procured for their author some reputation as a mathematician. they appear to be works of considerable mathematical elegance, but not indicating any great power of original thought. perhaps it has been prejudicial to brinkley's fame in this direction, that he was immediately followed in his chair by so mighty a genius as william rowan hamilton. after the great circle had been at last erected, brinkley was able to begin his astronomical work in earnest. nor was there much time to lose. he was already forty-five years old, a year older than was herschel when he commenced his immortal career at slough. stimulated by the consciousness of having the command of an instrument of unique perfection, brinkley loftily attempted the very highest class of astronomical research. he resolved to measure anew with his own eye and with his own hand the constants of aberration and of nutation. he also strove to solve that great problem of the universe, the discovery of the distance of a fixed star. these were noble problems, and they were nobly attacked. but to appraise with justice this work of brinkley, done seventy years ago, we must not apply to it the same criterion as we would think right to apply to similar work were it done now. we do not any longer use brinkley's constant of aberration, nor do we now think that brinkley's determinations of the star distances were reliable. but, nevertheless, his investigations exercised a marked influence on the progress of science; they stimulated the study of the principles on which exact measurements were to be conducted. brinkley had another profession in addition to that of an astronomer. he was a divine. when a man endeavours to pursue two distinct occupations concurrently, it will be equally easy to explain why his career should be successful, or why it should be the reverse. if he succeeds, he will, of course, exemplify the wisdom of having two strings to his bow. should he fail, it is, of course, because he has attempted to sit on two stools at once. in brinkley's case, his two professions must be likened to the two strings rather than to the two stools. it is true that his practical experience of his clerical life was very slender. he had made no attempt to combine the routine of a parish with his labours in the observatory. nor do we associate a special eminence in any department of religious work with his name. if, however, we are to measure brinkley's merits as a divine by the ecclesiastical preferment which he received, his services to theology must have rivalled his services to astronomy. having been raised step by step in the church, he was at last appointed to the see of cloyne, in , as the successor of bishop berkeley. now, though it was permissible for the archdeacon to be also the andrews professor, yet when the archdeacon became a bishop, it was understood that he should transfer his residence from the observatory to the palace. the chair of astronomy accordingly became vacant. brinkley's subsequent career seems to have been devoted entirely to ecclesiastical matters, and for the last ten years of his life he did not contribute a paper to any scientific society. arago, after a characteristic lament that brinkley should have forsaken the pursuit of science for the temporal and spiritual attractions of a bishopric, pays a tribute to the conscientiousness of the quondam astronomer, who would not even allow a telescope to be brought into the palace lest his mind should be distracted from his sacred duties. the good bishop died on the th september, . he was buried in the chapel of trinity college, and a fine monument to his memory is a familiar object at the foot of the noble old staircase of the library. the best memorial of brinkley is his admirable book on the "elements of plane astronomy." it passed through many editions in his lifetime, and even at the present day the same work, revised first by dr. luby, and more recently by the rev. dr. stubbs and dr. brunnow, has a large and well-merited circulation. john herschel. this illustrious son of an illustrious father was born at slough, near windsor, on the th march, . he was the only child of sir william herschel, who had married somewhat late in life, as we have already mentioned. [plate: astronometer made by sir j. herschel to compare the light of certain stars by the intervention of the moon.] the surroundings among which the young astronomer was reared afforded him an excellent training for that career on which he was to enter, and in which he was destined to attain a fame only less brilliant than that of his father. the circumstances of his youth permitted him to enjoy one great advantage which was denied to the elder herschel. he was able, from his childhood, to devote himself almost exclusively to intellectual pursuits. william herschel, in the early part of his career, had only been able to snatch occasional hours for study from his busy life as a professional musician. but the son, having been born with a taste for the student's life, was fortunate enough to have been endowed with the leisure and the means to enjoy it from the commencement. his early years have been so well described by the late professor pritchard in the "report of the council of the royal astronomical society for ," that i venture to make an extract here:-- "a few traits of john herschel's boyhood, mentioned by himself in his maturer life, have been treasured up by those who were dear to him, and the record of some of them may satisfy a curiosity as pardonable as inevitable, which craves to learn through what early steps great men or great nations become illustrious. his home was singular, and singularly calculated to nurture into greatness any child born as john herschel was with natural gifts, capable of wide development. at the head of the house there was the aged, observant, reticent philosopher, and rarely far away his devoted sister, caroline herschel, whose labours and whose fame are still cognisable as a beneficent satellite to the brighter light of her illustrious brother. it was in the companionship of these remarkable persons, and under the shadow of his father's wonderful telescope, that john herschel passed his boyish years. he saw them, in silent but ceaseless industry, busied about things which had no apparent concern with the world outside the walls of that well-known house, but which, at a later period of his life, he, with an unrivalled eloquence, taught his countrymen to appreciate as foremost among those living influences which but satisfy and elevate the noblest instincts of our nature. what sort of intercourse passed between the father and the boy may be gathered from an incident or two which he narrated as having impressed themselves permanently on the memory of his youth. he once asked his father what he thought was the oldest of all things. the father replied, after the socratic method, by putting another question: 'and what do you yourself suppose is the oldest of all things?' the boy was not successful in his answers, thereon the old astronomer took up a small stone from the garden walk: 'there, my child, there is the oldest of all the things that i certainly know.' on another occasion his father is said to have asked the boy, 'what sort of things, do you think, are most alike?' the delicate, blue-eyed boy, after a short pause, replied, 'the leaves of the same tree are most like each other.' 'gather, then, a handful of leaves of that tree,' rejoined the philosopher, 'and choose two that are alike.' the boy failed; but he hid the lesson in his heart, and his thoughts were revealed after many days. these incidents may be trifles; nor should we record them here had not john herschel himself, though singularly reticent about his personal emotions, recorded them as having made a strong impression on his mind. beyond all doubt we can trace therein, first, that grasp and grouping of many things in one, implied in the stone as the oldest of things; and, secondly, that fine and subtle discrimination of each thing out of many like things as forming the main features which characterized the habit of our venerated friend's philosophy." john herschel entered st. john's college, cambridge, when he was seventeen years of age. his university career abundantly fulfilled his father's eager desire, that his only son should develop a capacity for the pursuit of science. after obtaining many lesser distinctions, he finally came out as senior wrangler in . it was, indeed, a notable year in the mathematical annals of the university. second on that list, in which herschel's name was first, appeared that of the illustrious peacock, afterwards dean of ely, who remained throughout life one of herschel's most intimate friends. almost immediately after taking his degree, herschel gave evidence of possessing a special aptitude for original scientific investigation. he sent to the royal society a mathematical paper which was published in the philosophical transactions. doubtless the splendour that attached to the name he bore assisted him in procuring early recognition of his own great powers. certain it is that he was made a fellow of the royal society at the unprecedentedly early age of twenty-one. even after this remarkable encouragement to adopt a scientific career as the business of his life, it does not seem that john herschel at first contemplated devoting himself exclusively to science. he commenced to prepare for the profession of the law by entering as a student at the middle temple, and reading with a practising barrister. but a lawyer john herschel was not destined to become. circumstances brought him into association with some leading scientific men. he presently discovered that his inclinations tended more and more in the direction of purely scientific pursuits. thus it came to pass that the original intention as to the calling which he should follow was gradually abandoned. fortunately for science herschel found its pursuit so attractive that he was led, as his father had been before him, to give up his whole life to the advancement of knowledge. nor was it unnatural that a senior wrangler, who had once tasted the delights of mathematical research, should have been tempted to devote much time to this fascinating pursuit. by the time john herschel was twenty-nine he had published so much mathematical work, and his researches were considered to possess so much merit, that the royal society awarded him the copley medal, which was the highest distinction it was capable of conferring. at the death of his father in , john herschel, with his tastes already formed for a scientific career, found himself in the possession of ample means. to him also passed all his father's great telescopes and apparatus. these material aids, together with a dutiful sense of filial obligation, decided him to make practical astronomy the main work of his life. he decided to continue to its completion that great survey of the heavens which had already been inaugurated, and, indeed, to a large extent accomplished, by his father. the first systematic piece of practical astronomical work which john herschel undertook was connected with the measurement of what are known as "double stars." it should be observed, that there are in the heavens a number of instances in which two stars are seen in very close association. in the case of those objects to which the expression "double stars" is generally applied, the two luminous points are so close together that even though they might each be quite bright enough to be visible to the unaided eye, yet their proximity is such that they cannot be distinguished as two separate objects without optical aid. the two stars seem fused together into one. in the telescope, however, the bodies may be discerned separately, though they are frequently so close together that it taxes the utmost power of the instrument to indicate the division between them. the appearance presented by a double star might arise from the circumstance that the two stars, though really separated from each other by prodigious distances, happened to lie nearly in the same line of vision, as seen from our point of view. no doubt, many of the so-called double stars could be accounted for on this supposition. indeed, in the early days when but few double stars were known, and when telescopes were not powerful enough to exhibit the numerous close doubles which have since been brought to light, there seems to have been a tendency to regard all double stars as merely such perspective effects. it was not at first suggested that there could be any physical connection between the components of each pair. the appearance presented was regarded as merely due to the circumstance that the line joining the two bodies happened to pass near the earth. [plate: sir john herschel.] in the early part of his career, sir william herschel seems to have entertained the view then generally held by other astronomers with regard to the nature of these stellar pairs. the great observer thought that the double stars could therefore be made to afford a means of solving that problem in which so many of the observers of the skies had been engaged, namely, the determination of the distances of the stars from the earth. herschel saw that the displacement of the earth in its annual movement round the sun would produce an apparent shift in the place of the nearer of the two stars relatively to the other, supposed to be much more remote. if this shift could be measured, then the distance of the nearer of the stars could be estimated with some degree of precision. as has not unfrequently happened in the history of science, an effect was perceived of a very different nature from that which had been anticipated. if the relative places of the two stars had been apparently deranged merely in consequence of the motion of the earth, then the phenomenon would be an annual one. after the lapse of a year the two stars would have regained their original relative positions. this was the effect for which william herschel was looking. in certain of the so called double stars, he, no doubt, did find a movement. he detected the remarkable fact that both the apparent distance and the relative positions of the two bodies were changing. but what was his surprise to observe that these alterations were not of an annually periodic character. it became evident then that in some cases one of the component stars was actually revolving around the other, in an orbit which required many years for its completion. here was indeed a remarkable discovery. it was clearly impossible to suppose that movements of this kind could be mere apparent displacements, arising from the annual shift in our point of view, in consequence of the revolution of the earth. herschel's discovery established the interesting fact that, in certain of these double stars, or binary stars, as these particular objects are more expressively designated, there is an actual orbital revolution of a character similar to that which the earth performs around the sun. thus it was demonstrated that in these particular double stars the nearness of the two components was not merely apparent. the objects must actually lie close together at a distance which is small in comparison with the distance at which either of them is separated from the earth. the fact that the heavens contain pairs of twin suns in mutual revolution was thus brought to light. in consequence of this beautiful discovery, the attention of astronomers was directed to the subject of double stars with a degree of interest which these objects had never before excited. it was therefore not unnatural that john herschel should have been attracted to this branch of astronomical work. admiration for his father's discovery alone might have suggested that the son should strive to develop this territory newly opened up to research. but it also happened that the mathematical talents of the younger herschel inclined his inquiries in the same direction. he saw clearly that, when sufficient observations of any particular binary star had been accumulated, it would then be within the power of the mathematician to elicit from those observations the shape and the position in space of the path which each of the revolving stars described around the other. indeed, in some cases he would be able to perform the astonishing feat of determining from his calculations the weight of these distant suns, and thus be enabled to compare them with the mass of our own sun. [plate: nebula in southern hemisphere, drawn by sir john herschel.] but this work must follow the observations, it could not precede them. the first step was therefore to observe and to measure with the utmost care the positions and distances of those particular double stars which appear to offer the greatest promise in this particular research. in , herschel and a friend of his, mr. james south, agreed to work together with this object. south was a medical man with an ardent devotion to science, and possessed of considerable wealth. he procured the best astronomical instruments that money could obtain, and became a most enthusiastic astronomer and a practical observer of tremendous energy. south and john herschel worked together for two years in the observation and measurement of the double stars discovered by sir william herschel. in the course of this time their assiduity was rewarded by the accumulation of so great a mass of careful measurements that when published, they formed quite a volume in the "philosophical transactions." the value and accuracy of the work, when estimated by standards which form proper criteria for that period, is universally recognised. it greatly promoted the progress of sidereal astronomy, and the authors were in consequence awarded medals from the royal society, and the royal astronomical society, as well as similar testimonials from various foreign institutions. this work must, however, be regarded as merely introductory to the main labours of john herschel's life. his father devoted the greater part of his years as an observer to what he called his "sweeps" of the heavens. the great reflecting telescope, twenty feet long, was moved slowly up and down through an arc of about two degrees towards and from the pole, while the celestial panorama passed slowly in the course of the diurnal motion before the keenly watching eye of the astronomer. whenever a double star traversed the field herschel described it to his sister caroline, who, as we have already mentioned, was his invariable assistant in his midnight watches. when a nebula appeared, then he estimated its size and its brightness, he noticed whether it had a nucleus, or whether it had stars disposed in any significant manner with regard to it. he also dictated any other circumstance which he deemed worthy of record. these observations were duly committed to writing by the same faithful and indefatigable scribe, whose business it also was to take a memorandum of the exact position of the object as indicated by a dial placed in front of her desk, and connected with the telescope. john herschel undertook the important task of re-observing the various double stars and nebulae which had been discovered during these memorable vigils. the son, however, lacked one inestimable advantage which had been possessed by the father. john herschel had no assistant to discharge all those duties which caroline had so efficiently accomplished. he had, therefore, to modify the system of sweeping previously adopted in order to enable all the work both of observing and of recording to be done by himself. this, in many ways, was a great drawback to the work of the younger astronomer. the division of labour between the observer and the scribe enables a greatly increased quantity of work to be got through. it is also distinctly disadvantageous to an observer to have to use his eye at the telescope directly after he has been employing it for reading the graduations on a circle, by the light of a lamp, or for entering memoranda in a note book. nebulae, especially, are often so excessively faint that they can only be properly observed by an eye which is in that highly sensitive condition which is obtained by long continuance in darkness. the frequent withdrawal of the eye from the dark field of the telescope, and the application of it to reading by artificial light, is very prejudicial to its use for the more delicate purpose. john herschel, no doubt, availed himself of every precaution to mitigate the ill effects of this inconvenience as much as possible, but it must have told upon his labours as compared with those of his father. but nevertheless john herschel did great work during his "sweeps." he was specially particular to note all the double stars which presented themselves to his observation. of course some little discretion must be allowed in deciding as to what degree of proximity in adjacent stars does actually bring them within the category of "double stars." sir john set down all such objects as seemed to him likely to be of interest, and the results of his discoveries in this branch of astronomy amount to some thousands. six or seven great memoirs in the transactions of the royal astronomical society have been devoted to giving an account of his labours in this department of astronomy. [plate: the cluster in the centaur, drawn by sir john herschel.] one of the achievements by which sir john herschel is best known is his invention of a method by which the orbits of binary stars could be determined. it will be observed that when one star revolves around another in consequence of the law of gravitation, the orbit described must be an ellipse. this ellipse, however, generally speaking, appears to us more or less foreshortened, for it is easily seen that only under highly exceptional circumstances would the plane in which the stars move happen to be directly square to the line of view. it therefore follows that what we observe is not exactly the track of one star around the other; it is rather the projection of that track as seen on the surface of the sky. now it is remarkable that this apparent path is still an ellipse. herschel contrived a very ingenious and simple method by which he could discover from the observations the size and position of the ellipse in which the revolution actually takes place. he showed how, from the study of the apparent orbit of the star, and from certain measurements which could easily be effected upon it, the determination of the true ellipse in which the movement is performed could be arrived at. in other words, herschel solved in a beautiful manner the problem of finding the true orbits of double stars. the importance of this work may be inferred from the fact that it has served as the basis on which scores of other investigators have studied the fascinating subject of the movement of binary stars. the labours, both in the discovery and measurement of the double stars, and in the discussion of the observations with the object of finding the orbits of such stars as are in actual revolution, received due recognition in yet another gold medal awarded by the royal society. an address was delivered on the occasion by the duke of sussex ( th november, ), in the course of which, after stating that the medal had been conferred on sir john herschel, he remarks:-- "it has been said that distance of place confers the same privilege as distance of time, and i should gladly avail myself of the privilege which is thus afforded me by sir john herschel's separation from his country and friends, to express my admiration of his character in stronger terms than i should otherwise venture to use; for the language of panegyric, however sincerely it may flow from the heart, might be mistaken for that of flattery, if it could not thus claim somewhat of an historical character; but his great attainments in almost every department of human knowledge, his fine powers as a philosophical writer, his great services and his distinguished devotion to science, the high principles which have regulated his conduct in every relation of life, and, above all, his engaging modesty, which is the crown of all his other virtues, presenting such a model of an accomplished philosopher as can rarely be found beyond the regions of fiction, demand abler pens than mine to describe them in adequate terms, however much inclined i might feel to undertake the task." the first few lines of the eulogium just quoted allude to herschel's absence from england. this was not merely an episode of interest in the career of herschel, it was the occasion of one of the greatest scientific expeditions in the whole history of astronomy. herschel had, as we have seen, undertaken a revision of his father's "sweeps" for new objects, in those skies which are visible from our latitudes in the northern hemisphere. he had well-nigh completed this task. zone by zone the whole of the heavens which could be observed from windsor had passed under his review. he had added hundreds to the list of nebulae discovered by his father. he had announced thousands of double stars. at last, however, the great survey was accomplished. the contents of the northern hemisphere, so far at least as they could be disclosed by his telescope of twenty feet focal length, had been revealed. [plate: sir john herschel's observatory at feldhausen, cape of good hope.] but herschel felt that this mighty task had to be supplemented by another of almost equal proportions, before it could be said that the twenty-foot telescope had done its work. it was only the northern half of the celestial sphere which had been fully explored. the southern half was almost virgin territory, for no other astronomer was possessed of a telescope of such power as those which the herschels had used. it is true, of course, that as a certain margin of the southern hemisphere was visible from these latitudes, it had been more or less scrutinized by observers in northern skies. and the glimpses which had thus been obtained of the celestial objects in the southern sky, were such as to make an eager astronomer long for a closer acquaintance with the celestial wonders of the south. the most glorious object in the sidereal heavens, the great nebula in orion, lies indeed in that southern hemisphere to which the younger herschel's attention now became directed. it fortunately happens, however, for votaries of astronomy all the world over, that nature has kindly placed her most astounding object, the great nebula in orion, in such a favoured position, near the equator, that from a considerable range of latitudes, both north and south, the wonders of the nebula can be explored. there are grounds for thinking that the southern heavens contain noteworthy objects which, on the whole, are nearer to the solar system than are the noteworthy objects in the northern skies. the nearest star whose distance is known, alpha centauri, lies in the southern hemisphere, and so also does the most splendid cluster of stars. influenced by the desire to examine these objects, sir john herschel determined to take his great telescope to a station in the southern hemisphere, and thus complete his survey of the sidereal heavens. the latitude of the cape of good hope is such that a suitable site could be there found for his purpose. the purity of the skies in south africa promised to provide for the astronomer those clear nights which his delicate task of surveying the nebulae would require. on november , , sir john herschel, who had by this time received the honour of knighthood from william iv., sailed from portsmouth for the cape of good hope, taking with him his gigantic instruments. after a voyage of two months, which was considered to be a fair passage in those days, he landed in table bay, and having duly reconnoitred various localities, he decided to place his observatory at a place called feldhausen, about six miles from cape town, near the base of the table mountain. a commodious residence was there available, and in it he settled with his family. a temporary building was erected to contain the equatorial, but the great twenty-foot telescope was accommodated with no more shelter than is provided by the open canopy of heaven. as in his earlier researches at home, the attention of the great astronomer at the cape of good hope was chiefly directed to the measurement of the relative positions and distances apart of the double stars, and to the close examination of the nebulae. in the delineation of the form of these latter objects herschel found ample employment for his skilful pencil. many of the drawings he has made of the celestial wonders in the southern sky are admirable examples of celestial portraiture. the number of the nebulae and of those kindred objects, the star clusters, which herschel studied in the southern heavens, during four years of delightful labour, amount in all to one thousand seven hundred and seven. his notes on their appearance, and the determinations of their positions, as well as his measurements of double stars, and much other valuable astronomical research, were published in a splendid volume, brought out at the cost of the duke of northumberland. this is, indeed, a monumental work, full of interesting and instructive reading for any one who has a taste for astronomy. herschel had the good fortune to be at the cape on the occasion of the periodical return of halley's great comet in . to the study of this body he gave assiduous attention, and the records of his observations form one of the most interesting chapters in that remarkable volume to which we have just referred. [plate: column at feldhausen, cape town, to commemorate sir john herschel's survey of the southern heavens.] early in sir john herschel returned to england. he had made many friends at the cape, who deeply sympathised with his self- imposed labours while he was resident among them. they desired to preserve the recollection of this visit, which would always, they considered, be a source of gratification in the colony. accordingly, a number of scientific friends in that part of the world raised a monument with a suitable inscription, on the spot which had been occupied by the great twenty-foot reflector at feldhausen. his return to england after five years of absence was naturally an occasion for much rejoicing among the lovers of astronomy. he was entertained at a memorable banquet, and the queen, at her coronation, made him a baronet. his famous aunt caroline, at that time aged eighty, was still in the enjoyment of her faculties, and was able to estimate at its true value the further lustre which was added to the name she bore. but there is reason to believe that her satisfaction was not quite unmixed with other feelings. with whatever favour she might regard her nephew, he was still not the brother to whom her life had been devoted. so jealous was this vigorous old lady of the fame of the great brother william, that she could hardly hear with patience of the achievements of any other astronomer, and this failing existed in some degree even when that other astronomer happened to be her illustrious nephew. with sir john herschel's survey of the southern hemisphere it may be said that his career as an observing astronomer came to a close. he did not again engage in any systematic telescopic research. but it must not be inferred from this statement that he desisted from active astronomical work. it has been well observed that sir john herschel was perhaps the only astronomer who has studied with success, and advanced by original research, every department of the great science with which his name is associated. it was to some other branches of astronomy besides those concerned with looking through telescopes, that the rest of the astronomer's life was to be devoted. to the general student sir john herschel is best known by the volume which he published under the title of "outlines of astronomy." this is, indeed, a masterly work, in which the characteristic difficulties of the subject are resolutely faced and expounded with as much simplicity as their nature will admit. as a literary effort this work is admirable, both on account of its picturesque language and the ennobling conceptions of the universe which it unfolds. the student who desires to become acquainted with those recondite departments of astronomy, in which the effects of the disturbing action of one planet upon the motions of another planet are considered, will turn to the chapters in herschel's famous work on the subject. there he will find this complex matter elucidated, without resort to difficult mathematics. edition after edition of this valuable work has appeared, and though the advances of modern astronomy have left it somewhat out of date in certain departments, yet the expositions it contains of the fundamental parts of the science still remain unrivalled. another great work which sir john undertook after his return from the cape, was a natural climax to those labours on which his father and he had been occupied for so many years. we have already explained how the work of both these observers had been mainly devoted to the study of the nebulae and the star clusters. the results of their discoveries had been announced to the world in numerous isolated memoirs. the disjointed nature of these publications made their use very inconvenient. but still it was necessary for those who desired to study the marvellous objects discovered by the herschels, to have frequent recourse to the original works. to incorporate all the several observations of nebular into one great systematic catalogue, seemed, therefore, to be an indispensable condition of progress in this branch of knowledge. no one could have been so fitted for this task as sir john herschel. he, therefore, attacked and carried through the great undertaking. thus at last a grand catalogue of nebulae and clusters was produced. never before was there so majestic an inventory. if we remember that each of the nebulae is an object so vast, that the whole of the solar system would form an inconsiderable speck by comparison, what are we to think of a collection in which these objects are enumerated in thousands? in this great catalogue we find arranged in systematic order all the nebulae and all the clusters which had been revealed by the diligence of the herschels, father and son, in the northern hemisphere, and of the son alone in the southern hemisphere. nor should we omit to mention that the labours of other astronomers were likewise incorporated. it was unavoidable that the descriptions given to each of the objects should be very slight. abbreviations are used, which indicate that a nebula is bright, or very bright, or extremely bright, or faint, or very faint, or extremely faint. such phrases have certainly but a relative and technical meaning in such a catalogue. the nebulae entered as extremely bright by the experienced astronomer are only so described by way of contrast to the great majority of these delicate telescopic objects. most of the nebulae, indeed, are so difficult to see, that they admit of but very slight description. it should be observed that herschel's catalogue augmented the number of known nebulous objects to more than ten times that collected into any catalogue which had ever been compiled before the days of william herschel's observing began. but the study of these objects still advances, and the great telescopes now in use could probably show at least twice as many of these objects as are contained in the list of herschel, of which a new and enlarged edition has since been brought out by dr. dreyer. one of the best illustrations of sir john herschel's literary powers is to be found in the address which he delivered at the royal astronomical society, on the occasion of presenting a medal to mr. francis baily, in recognition of his catalogue of stars. the passage i shall here cite places in its proper aspect the true merit of the laborious duty involved in such a task as that which mr. baily had carried through with such success:-- "if we ask to what end magnificent establishments are maintained by states and sovereigns, furnished with masterpieces of art, and placed under the direction of men of first-rate talent and high-minded enthusiasm, sought out for those qualities among the foremost in the ranks of science, if we demand qui bono? for what good a bradley has toiled, or a maskelyne or a piazzi has worn out his venerable age in watching, the answer is--not to settle mere speculative points in the doctrine of the universe; not to cater for the pride of man by refined inquiries into the remoter mysteries of nature; not to trace the path of our system through space, or its history through past and future eternities. these, indeed, are noble ends and which i am far from any thought of depreciating; the mind swells in their contemplation, and attains in their pursuit an expansion and a hardihood which fit it for the boldest enterprise. but the direct practical utility of such labours is fully worthy of their speculative grandeur. the stars are the landmarks of the universe; and, amidst the endless and complicated fluctuations of our system, seem placed by its creator as guides and records, not merely to elevate our minds by the contemplation of what is vast, but to teach us to direct our actions by reference to what is immutable in his works. it is, indeed, hardly possible to over-appreciate their value in this point of view. every well-determined star, from the moment its place is registered, becomes to the astronomer, the geographer, the navigator, the surveyor, a point of departure which can never deceive or fail him, the same for ever and in all places, of a delicacy so extreme as to be a test for every instrument yet invented by man, yet equally adapted for the most ordinary purposes; as available for regulating a town clock as for conducting a navy to the indies; as effective for mapping down the intricacies of a petty barony as for adjusting the boundaries of transatlantic empires. when once its place has been thoroughly ascertained and carefully recorded, the brazen circle with which that useful work was done may moulder, the marble pillar may totter on its base, and the astronomer himself survive only in the gratitude of posterity; but the record remains, and transfuses all its own exactness into every determination which takes it for a groundwork, giving to inferior instruments--nay, even to temporary contrivances, and to the observations of a few weeks or days--all the precision attained originally at the cost of so much time, labour, and expense." sir john herschel wrote many other works besides those we have mentioned. his "treatise on meteorology" is, indeed, a standard work on this subject, and numerous articles from the same pen on miscellaneous subjects, which have been collected and reprinted, seemed as a relaxation from his severe scientific studies. like certain other great mathematicians herschel was also a poet, and he published a translation of the iliad into blank verse. in his later years sir john herschel lived a retired life. for a brief period he had, indeed, been induced to accept the office of master of the mint. it was, however, evident that the routine of such an occupation was not in accordance with his tastes, and he gladly resigned it, to return to the seclusion of his study in his beautiful home at collingwood, in kent. his health having gradually failed, he died on the th may, , in the seventy-ninth year of his age. the earl of rosse. the subject of our present sketch occupies quite a distinct position in scientific history. unlike many others who have risen by their scientific discoveries from obscurity to fame, the great earl of rosse was himself born in the purple. his father, who, under the title of sir lawrence parsons, had occupied a distinguished position in the irish parliament, succeeded on the death of his father to the earldom which had been recently created. the subject of our present memoir was, therefore, the third of the earls of rosse, and he was born in york on june , . prior to his father's death in , he was known as lord oxmantown. the university education of the illustrious astronomer was begun in dublin and completed at oxford. we do not hear in his case of any very remarkable university career. lord rosse was, however, a diligent student, and obtained a first-class in mathematics. he always took a great deal of interest in social questions, and was a profound student of political economy. he had a seat in the house of commons, as member for king's county, from to , his ancestral estate being situated in this part of ireland. [plate: the earl of rosse.] lord rosse was endowed by nature with a special taste for mechanical pursuits. not only had he the qualifications of a scientific engineer, but he had the manual dexterity which qualified him personally to carry out many practical arts. lord rosse was, in fact, a skilful mechanic, an experienced founder, and an ingenious optician. his acquaintances were largely among those who were interested in mechanical pursuits, and it was his delight to visit the works or engineering establishments where refined processes in the arts were being carried on. it has often been stated--and as i have been told by members of his family, truly stated--that on one occasion, after he had been shown over some large works in the north of england, the proprietor bluntly said that he was greatly in want of a foreman, and would indeed be pleased if his visitor, who had evinced such extraordinary capacity for mechanical operations, would accept the post. lord rosse produced his card, and gently explained that he was not exactly the right man, but he appreciated the compliment, and this led to a pleasant dinner, and was the basis of a long friendship. i remember on one occasion hearing lord rosse explain how it was that he came to devote his attention to astronomy. it appears that when he found himself in the possession of leisure and of means, he deliberately cast around to think how that means and that leisure could be most usefully employed. nor was it surprising that he should search for a direction which would offer special scope for his mechanical tastes. he came to the conclusion that the building of great telescopes was an art which had received no substantial advance since the great days of william herschel. he saw that to construct mighty instruments for studying the heavens required at once the command of time and the command of wealth, while he also felt that this was a subject the inherent difficulties of which would tax to the uttermost whatever mechanical skill he might possess. thus it was he decided that the construction of great telescopes should become the business of his life. [plate: birr castle. plate: the mall, parsonstown.] in the centre of ireland, seventy miles from dublin, on the border between king's county and tipperary, is a little town whereof we must be cautious before writing the name. the inhabitants of that town frequently insist that its name is birr, * while the official designation is parsonstown, and to this day for every six people who apply one name to the town, there will be half a dozen who use the other. but whichever it may be, birr or parsonstown--and i shall generally call it by the latter name--it is a favourable specimen of an irish county town. the widest street is called the oxmantown mall. it is bordered by the dwelling-houses of the chief residents, and adorned with rows of stately trees. at one end of this distinctly good feature in the town is the parish church, while at the opposite end are the gates leading into birr castle, the ancestral home of the house of parsons. passing through the gates the visitor enters a spacious demesne, possessing much beauty of wood and water, one of the most pleasing features being the junction of the two rivers, which unite at a spot ornamented by beautiful timber. at various points illustrations of the engineering skill of the great earl will be observed. the beauty of the park has been greatly enhanced by the construction of an ample lake, designed with the consummate art by which art is concealed. even in mid-summer it is enlivened by troops of wild ducks preening themselves in that confidence which they enjoy in those happy localities where the sound of a gun is seldom heard. the water is led into the lake by a tube which passes under one of the two rivers just mentioned, while the overflow from the lake turns a water-wheel, which works a pair of elevators ingeniously constructed for draining the low-lying parts of the estate. * considering the fame acquired by parsonstown from lord rosse's mirrors, it may be interesting to note the following extract from "the natural history of ireland," by dr. gerard boate, thomas molyneux m.d., f.r.s., and others, which shows that years ago parsonstown was famous for its glass:-- "we shall conclude this chapter with the glass, there having been several glasshouses set up by the english in ireland, none in dublin or other cities, but all of them in the country; amongst which the principal was that of birre, a market town, otherwise called parsonstown, after one sir lawrence parsons, who, having purchased that lordship, built a goodly house upon it; his son william parsons having succeeded him in the possession of it; which town is situate in queen's county, about fifty miles (irish) to the southwest of dublin, upon the borders of the two provinces of leinster and munster; from this place dublin was furnished with all sorts of window and drinking glasses, and such other as commonly are in use. one part of the materials, viz., the sand, they had out of england; the other, to wit the ashes, they made in the place of ash-tree, and used no other. the chiefest difficulty was to get the clay for the pots to melt the materials in; this they had out of the north."--chap. xxi., sect. viii. "of the glass made in ireland." birr castle itself is a noble mansion with reminiscences from the time of cromwell. it is surrounded by a moat and a drawbridge of modern construction, and from its windows beautiful views can be had over the varied features of the park. but while the visitors to parsonstown will look with great interest on this residence of an irish landlord, whose delight it was to dwell in his own country, and among his own people, yet the feature which they have specially come to observe is not to be found in the castle itself. on an extensive lawn, sweeping down from the moat towards the lake, stand two noble masonry walls. they are turreted and clad with ivy, and considerably loftier than any ordinary house. as the visitor approaches, he will see between those walls what may at first sight appear to him to be the funnel of a steamer lying down horizontally. on closer approach he will find that it is an immense wooden tube, sixty feet long, and upwards of six feet in diameter. it is in fact large enough to admit of a tall man entering into it and walking erect right through from one end to the other. this is indeed the most gigantic instrument which has ever been constructed for the purpose of exploring the heavens. closely adjoining the walls between which the great tube swings, is a little building called "the observatory." in this the smaller instruments are contained, and there are kept the books which are necessary for reference. the observatory also offers shelter to the observers, and provides the bright fire and the cup of warm tea, which are so acceptable in the occasional intervals of a night's observation passed on the top of the walls with no canopy but the winter sky. almost the first point which would strike the visitor to lord rosse's telescope is that the instrument at which he is looking is not only enormously greater than anything of the kind that he has ever seen before, but also that it is something of a totally different nature. in an ordinary telescope he is accustomed to find a tube with lenses of glass at either end, while the large telescopes that we see in our observatories are also in general constructed on the same principle. at one end there is the object-glass, and at the other end the eye-piece, and of course it is obvious that with an instrument of this construction it is to the lower end of the tube that the eye of the observer must be placed when the telescope is pointed to the skies. but in lord rosse's telescope you would look in vain for these glasses, and it is not at the lower end of the instrument that you are to take your station when you are going to make your observations. the astronomer at parsonstown has rather to avail himself of the ingenious system of staircases and galleries, by which he is enabled to obtain access to the mouth of the great tube. the colossal telescope which swings between the great walls, like herschel's great telescope already mentioned, is a reflector, the original invention of which is due of course to newton. the optical work which is accomplished by the lenses in the ordinary telescope is effected in the type of instrument constructed by lord rosse by a reflecting mirror which is placed at the lower end of the vast tube. the mirror in this instrument is made of a metal consisting of two parts of copper to one of tin. as we have already seen, this mixture forms an alloy of a very peculiar nature. the copper and the tin both surrender their distinctive qualities, and unite to form a material of a very different physical character. the copper is tough and brown, the tin is no doubt silvery in hue, but soft and almost fibrous in texture. when the two metals are mixed together in the proportions i have stated, the alloy obtained is intensely hard and quite brittle being in both these respects utterly unlike either of the two ingredients of which it is composed. it does, however, resemble the tin in its whiteness, but it acquires a lustre far brighter than tin; in fact, this alloy hardly falls short of silver itself in its brilliance when polished. [plate: lord rosse's telescope. from a photograph by w. lawrence, upper sackville street, dublin.] the first duty that lord rosse had to undertake was the construction of this tremendous mirror, six feet across, and about four or five inches thick. the dimensions were far in excess of those which had been contemplated in any previous attempt of the same kind. herschel had no doubt fashioned one mirror of four feet in diameter, and many others of smaller dimensions, but the processes which he employed had never been fully published, and it was obvious that, with a large increase in dimensions, great additional difficulties had to be encountered. difficulties began at the very commencement of the process, and were experienced in one form or another at every subsequent stage. in the first place, the mere casting of a great disc of this mixture of tin and copper, weighing something like three or four tons, involved very troublesome problems. no doubt a casting of this size, if the material had been, for example, iron, would have offered no difficulties beyond those with which every practical founder is well acquainted, and which he has to encounter daily in the course of his ordinary work. but speculum metal is a material of a very intractable description. there is, of course, no practical difficulty in melting the copper, nor in adding the proper proportion of tin when the copper has been melted. there may be no great difficulty in arranging an organization by which several crucibles, filled with the molten material, shall be poured simultaneously so as to obtain the requisite mass of metal, but from this point the difficulties begin. for speculum metal when cold is excessively brittle, and were the casting permitted to cool like an ordinary copper or iron casting, the mirror would inevitably fly into pieces. lord rosse, therefore, found it necessary to anneal the casting with extreme care by allowing it to cool very slowly. this was accomplished by drawing the disc of metal as soon as it had entered into the solid state, though still glowing red, into an annealing oven. there the temperature was allowed to subside so gradually, that six weeks elapsed before the mirror had reached the temperature of the external air. the necessity for extreme precaution in the operation of annealing will be manifest if we reflect on one of the accidents which happened. on a certain occasion, after the cooling of a great casting had been completed, it was found, on withdrawing the speculum, that it was cracked into two pieces. this mishap was eventually traced to the fact that one of the walls of the oven had only a single brick in its thickness, and that therefore the heat had escaped more easily through that side than through the other sides which were built of double thickness. the speculum had, consequently, not cooled uniformly, and hence the fracture had resulted. undeterred, however, by this failure, as well as by not a few other difficulties, into a description of which we cannot now enter, lord rosse steadily adhered to his self-imposed task, and at last succeeded in casting two perfect discs on which to commence the tedious processes of grinding and polishing. the magnitude of the operations involved may perhaps be appreciated if i mention that the value of the mere copper and tin entering into the composition of each of the mirrors was about pounds. in no part of his undertaking was lord rosse's mechanical ingenuity more taxed than in the devising of the mechanism for carrying out the delicate operations of grinding and polishing the mirrors, whose casting we have just mentioned. in the ordinary operations of the telescope-maker, such processes had hitherto been generally effected by hand, but, of course, such methods became impossible when dealing with mirrors which were as large as a good-sized dinner table, and whose weight was measured by tons. the rough grinding was effected by means of a tool of cast iron about the same size as the mirror, which was moved by suitable machinery both backwards and forwards, and round and round, plenty of sand and water being supplied between the mirror and the tool to produce the necessary attrition. as the process proceeded and as the surface became smooth, emery was used instead of sand; and when this stage was complete, the grinding tool was removed and the polishing tool was substituted. the essential part of this was a surface of pitch, which, having been temporarily softened by heat, was then placed on the mirror, and accepted from the mirror the proper form. rouge was then introduced as the polishing powder, and the operation was continued about nine hours, by which time the great mirror had acquired the appearance of highly polished silver. when completed, the disc of speculum metal was about six feet across and four inches thick. the depression in the centre was about half an inch. mounted on a little truck, the great speculum was then conveyed to the instrument, to be placed in its receptacle at the bottom of the tube, the length of which was sixty feet, this being the focal distance of the mirror. another small reflector was inserted in the great tube sideways, so as to direct the gaze of the observer down upon the great reflector. thus was completed the most colossal instrument for the exploration of the heavens which the art of man has ever constructed. [plate: roman catholic church at parsonstown.] it was once my privilege to be one of those to whom the illustrious builder of the great telescope entrusted its use. for two seasons in and i had the honour of being lord rosse's astronomer. during that time i passed many a fine night in the observer's gallery, examining different objects in the heavens with the aid of this remarkable instrument. at the time i was there, the objects principally studied were the nebulae, those faint stains of light which lie on the background of the sky. lord rosse's telescope was specially suited for the scrutiny of these objects, inasmuch as their delicacy required all the light-grasping power which could be provided. one of the greatest discoveries made by lord rosse, when his huge instrument was first turned towards the heavens, consisted in the detection of the spiral character of some of the nebulous forms. when the extraordinary structure of these objects was first announced, the discovery was received with some degree of incredulity. other astronomers looked at the same objects, and when they failed to discern--and they frequently did fail to discern--the spiral structure which lord rosse had indicated, they drew the conclusion that this spiral structure did not exist. they thought it must be due possibly to some instrumental defect or to the imagination of the observer. it was, however, hardly possible for any one who was both willing and competent to examine into the evidence, to doubt the reality of lord rosse's discoveries. it happens, however, that they have been recently placed beyond all doubt by testimony which it is impossible to gainsay. a witness never influenced by imagination has now come forward, and the infallible photographic plate has justified lord rosse. among the remarkable discoveries which dr. isaac roberts has recently made in the application of his photographic apparatus to the heavens, there is none more striking than that which declares, not only that the nebulae which lord rosse described as spirals, actually do possess the character so indicated, but that there are many others of the same description. he has even brought to light the astonishingly interesting fact that there are invisible objects of this class which have never been seen by human eye, but whose spiral character is visible to the peculiar delicacy of the photographic telescope. in his earlier years, lord rosse himself used to be a diligent observer of the heavenly bodies with the great telescope which was completed in the year . but i think that those who knew lord rosse well, will agree that it was more the mechanical processes incidental to the making of the telescope which engaged his interest than the actual observations with the telescope when it was completed. indeed one who was well acquainted with him believed lord rosse's special interest in the great telescope ceased when the last nail had been driven into it. but the telescope was never allowed to lie idle, for lord rosse always had associated with him some ardent young astronomer, whose delight it was to employ to the uttermost the advantages of his position in exploring the wonders of the sky. among those who were in this capacity in the early days of the great telescope, i may mention my esteemed friend dr. johnston stoney. such was the renown of lord rosse himself, brought about by his consummate mechanical genius and his astronomical discoveries, and such the interest which gathered around the marvellous workshops at birr castle, wherein his monumental exhibitions of optical skill were constructed, that visitors thronged to see him from all parts of the world. his home at parsonstown became one of the most remarkable scientific centres in great britain; thither assembled from time to time all the leading men of science in the country, as well as many illustrious foreigners. for many years lord rosse filled with marked distinction the exalted position of president of the royal society, and his advice and experience in practical mechanical matters were always at the disposal of those who sought his assistance. personally and socially lord rosse endeared himself to all with whom he came in contact. i remember one of the attendants telling me that on one occasion he had the misfortune to let fall and break one of the small mirrors on which lord rosse had himself expended many hours of hard personal labour. the only remark of his lordship was that "accidents will happen." the latter years of his life lord rosse passed in comparative seclusion; he occasionally went to london for a brief sojourn during the season, and he occasionally went for a cruise in his yacht; but the greater part of the year he spent at birr castle, devoting himself largely to the study of political and social questions, and rarely going outside the walls of his demesne, except to church on sunday mornings. he died on october , . he was succeeded by his eldest son, the present earl of rosse, who has inherited his father's scientific abilities, and done much notable work with the great telescope. airy. in our sketch of the life of flamsteed, we have referred to the circumstances under which the famous observatory that crowns greenwich hill was founded. we have also had occasion to mention that among the illustrious successors of flamsteed both halley and bradley are to be included. but a remarkable development of greenwich observatory from the modest establishment of early days took place under the direction of the distinguished astronomer whose name is at the head of this chapter. by his labours this temple of science was organised to such a degree of perfection that it has served in many respects as a model for other astronomical establishments in various parts of the world. an excellent account of airy's career has been given by professor h. h. turner, in the obituary notice published by the royal astronomical society. to this i am indebted for many of the particulars here to be set down concerning the life of the illustrious astronomer royal. the family from which airy took his origin came from kentmere, in westmoreland. his father, william airy, belonged to a lincolnshire branch of the same stock. his mother's maiden name was ann biddell, and her family resided at playford, near ipswich. william airy held some small government post which necessitated an occasional change of residence to different parts of the country, and thus it was that his son, george biddell, came to be born at alnwick, on th july, . the boy's education, so far as his school life was concerned was partly conducted at hereford and partly at colchester. he does not, however, seem to have derived much benefit from the hours which he passed in the schoolroom. but it was delightful to him to spend his holidays on the farm at playford, where his uncle, arthur biddell, showed him much kindness. the scenes of his early youth remained dear to airy throughout his life, and in subsequent years he himself owned a house at playford, to which it was his special delight to resort for relaxation during the course of his arduous career. in spite of the defects of his school training he seems to have manifested such remarkable abilities that his uncle decided to enter him in cambridge university. he accordingly joined trinity college as a sizar in , and after a brilliant career in mathematical and physical science he graduated as senior wrangler in . it may be noted as an exceptional circumstance that, notwithstanding the demands on his time in studying for his tripos, he was able, after his second term of residence, to support himself entirely by taking private pupils. in the year after he had taken his degree he was elected to a fellowship at trinity college. having thus gained an independent position, airy immediately entered upon that career of scientific work which he prosecuted without intermission almost to the very close of his life. one of his most interesting researches in these early days is on the subject of astigmatism, which defect he had discovered in his own eyes. his investigations led him to suggest a means of correcting this defect by using a pair of spectacles with lenses so shaped as to counteract the derangement which the astigmatic eye impressed upon the rays of light. his researches on this subject were of a very complete character, and the principles he laid down are to the present day practically employed by oculists in the treatment of this malformation. on the th of december, , airy was elected to the lucasian professorship of mathematics in the university of cambridge, the chair which newton's occupancy had rendered so illustrious. his tenure of this office only lasted for two years, when he exchanged it for the plumian professorship. the attraction which led him to desire this change is doubtless to be found in the circumstance that the plumian professorship of astronomy carried with it at that time the appointment of director of the new astronomical observatory, the origin of which must now be described. those most interested in the scientific side of university life decided in that it would be proper to found an astronomical observatory at cambridge. donations were accordingly sought for this purpose, and upwards of , pounds were contributed by members of the university and the public. to this sum , pounds were added by a grant from the university chest, and in further sums amounting altogether to , pounds were given by the university for the same object. the regulations as to the administration of the new observatory placed it under the management of the plumian professor, who was to be provided with two assistants. their duties were to consist in making meridian observations of the sun, moon, and the stars, and the observations made each year were to be printed and published. the observatory was also to be used in the educational work of the university, for it was arranged that smaller instruments were to be provided by which students could be instructed in the practical art of making astronomical observations. the building of the cambridge astronomical observatory was completed in , but in , when airy entered on the discharge of his duties as director, the establishment was still far from completion, in so far as its organisation was concerned. airy commenced his work so energetically that in the next year after his appointment he was able to publish the first volume of "cambridge astronomical observations," notwithstanding that every part of the work, from the making of observations to the revising of the proof-sheets, had to be done by himself. it may here be remarked that these early volumes of the publications of the cambridge observatory contained the first exposition of those systematic methods of astronomical work which airy afterwards developed to such a great extent at greenwich, and which have been subsequently adopted in many other places. no more profitable instruction for the astronomical beginner can be found than that which can be had by the study of these volumes, in which the plumian professor has laid down with admirable clearness the true principles on which meridian work should be conducted. [plate: sir george airy. from a photograph by mr. e.p. adams, greenwich.] airy gradually added to the instruments with which the observatory was originally equipped. a mural circle was mounted in , and in the same year a small equatorial was erected by jones. this was made use of by airy in a well-known series of observations of jupiter's fourth satellite for the determination of the mass of the great planet. his memoir on this subject fully ex pounds the method of finding the weight of a planet from observations of the movements of a satellite by which the planet is attended. this is, indeed, a valuable investigation which no student of astronomy can afford to neglect. the ardour with which airy devoted himself to astronomical studies may be gathered from a remarkable report on the progress of astronomy during the present century, which he communicated to the british association at its second meeting in . in the early years of his life at cambridge his most famous achievement was connected with a research in theoretical astronomy for which consummate mathematical power was required. we can only give a brief account of the subject, for to enter into any full detail with regard to it would be quite out of the question. venus is a planet of about the same size and the same weight as the earth, revolving in an orbit which lies within that described by our globe. venus, consequently, takes less time than the earth to accomplish one revolution round the sun, and it happens that the relative movements of venus and the earth are so proportioned that in the time in which our earth accomplishes eight of her revolutions the other planet will have accomplished almost exactly thirteen. it, therefore, follows that if the earth and venus are in line with the sun at one date, then in eight years later both planets will again be found at the same points in their orbits. in those eight years the earth has gone round eight times, and has, therefore, regained its original position, while in the same period venus has accomplished thirteen complete revolutions, and, therefore, this planet also has reached the same spot where it was at first. venus and the earth, of course, attract each other, and in consequence of these mutual attractions the earth is swayed from the elliptic track which it would otherwise pursue. in like manner venus is also forced by the attraction of the earth to revolve in a track which deviates from that which it would otherwise follow. owing to the fact that the sun is of such preponderating magnitude (being, in fact, upwards of , times as heavy as either venus or the earth), the disturbances induced in the motion of either planet, in consequence of the attraction of the other, are relatively insignificant to the main controlling agency by which each of the movements is governed. it is, however, possible under certain circumstances that the disturbing effects produced upon one planet by the other can become so multiplied as to produce peculiar effects which attain measurable dimensions. suppose that the periodic times in which the earth and venus revolved had no simple relation to each other, then the points of their tracks in which the two planets came into line with the sun would be found at different parts of the orbits, and consequently the disturbances would to a great extent neutralise each other, and produce but little appreciable effect. as, however, venus and the earth come back every eight years to nearly the same positions at the same points of their track, an accumulative effect is produced. for the disturbance of one planet upon the other will, of course, be greatest when those two planets are nearest, that is, when they lie in line with the sun and on the same side of it. every eight years a certain part of the orbit of the earth is, therefore, disturbed by the attraction of venus with peculiar vigour. the consequence is that, owing to the numerical relation between the movements of the planets to which i have referred, disturbing effects become appreciable which would otherwise be too small to permit of recognition. airy proposed to himself to compute the effects which venus would have on the movement of the earth in consequence of the circumstance that eight revolutions of the one planet required almost the same time as thirteen revolutions of the other. this is a mathematical inquiry of the most arduous description, but the plumian professor succeeded in working it out, and he had, accordingly, the gratification of announcing to the royal society that he had detected the influence which venus was thus able to assert on the movement of our earth around the sun. this remarkable investigation gained for its author the gold medal of the royal astronomical society in the year . in consequence of his numerous discoveries, airy's scientific fame had become so well recognised that the government awarded him a special pension, and in , when pond, who was then astronomer royal, resigned, airy was offered the post at greenwich. there was in truth, no scientific inducement to the plumian professor to leave the comparatively easy post he held at cambridge, in which he had ample leisure to devote himself to those researches which specially interested him, and accept that of the much more arduous observatory at greenwich. there were not even pecuniary inducements to make the change; however, he felt it to be his duty to accede to the request which the government had made that he would take up the position which pond had vacated, and accordingly airy went to greenwich as astronomer royal on october st, . he immediately began with his usual energy to organise the systematic conduct of the business of the national observatory. to realise one of the main characteristics of airy's great work at greenwich, it is necessary to explain a point that might not perhaps be understood without a little explanation by those who have no practical experience in an observatory. in the work of an establishment such as greenwich, an observation almost always consists of a measurement of some kind. the observer may, for instance, be making a measurement of the time at which a star passes across a spider line stretched through the field of view; on another occasion his object may be the measurement of an angle which is read off by examining through a microscope the lines of division on a graduated circle when the telescope is so pointed that the star is placed on a certain mark in the field of view. in either case the immediate result of the astronomical observation is a purely numerical one, but it rarely happens, indeed we may say it never happens, that the immediate numerical result which the observation gives expresses directly the quantity which we are really seeking for. no doubt the observation has been so designed that the quantity we want to find can be obtained from the figures which the measurement gives, but the object sought is not those figures, for there are always a multitude of other influences by which those figures are affected. for example, if an observation were to be perfect, then the telescope with which the observation is made should be perfectly placed in the exact position which it ought to occupy; this is, however, never the case, for no mechanic can ever construct or adjust a telescope so perfectly as the wants of the astronomer demand. the clock also by which we determine the time of the observation should be correct, but this is rarely if ever the case. we have to correct our observations for such errors, that is to say, we have to determine the errors in the positions of our telescopes and the errors in the going of our clocks, and then we have to determine what the observations would have been had our telescopes been absolutely perfect, and had our clocks been absolutely correct. there are also many other matters which have to be attended to in order to reduce our observations so as to obtain from the figures as yielded to the observer at the telescope the actual quantities which it is his object to determine. the work of effecting these reductions is generally a very intricate and laborious matter, so that it has not unfrequently happened that while observations have accumulated in an observatory, yet the tedious duty of reducing these observations has been allowed to fall into arrear. when airy entered on his duties at greenwich he found there an enormous mass of observations which, though implicitly containing materials of the greatest value to astronomers, were, in their unreduced form, entirely unavailable for any useful purpose. he, therefore, devoted himself to coping with the reduction of the observations of his predecessors. he framed systematic methods by which the reductions were to be effected, and he so arranged the work that little more than careful attention to numerical accuracy would be required for the conduct of the operations. encouraged by the admiralty, for it is under this department that greenwich observatory is placed, the astronomer royal employed a large force of computers to deal with the work. by his energy and admirable organisation he managed to reduce an extremely valuable series of planetary observations, and to publish the results, which have been of the greatest importance to astronomical investigation. the astronomer royal was a capable, practical engineer as well as an optician, and he presently occupied himself by designing astronomical instruments of improved pattern, which should replace the antiquated instruments he found in the observatory. in the course of years the entire equipment underwent a total transformation. he ordered a great meridian circle, every part of which may be said to have been formed from his own designs. he also designed the mounting for a fine equatorial telescope worked by a driving clock, which he had himself invented. gradually the establishment at greenwich waxed great under his incessant care. it was the custom for the observatory to be inspected every year by a board of visitors, whose chairman was the president of the royal society. at each annual visitation, held on the first saturday in june, the visitors received a report from the astronomer royal, in which he set forth the business which had been accomplished during the past year. it was on these occasions that applications were made to the admiralty, either for new instruments or for developing the work of the observatory in some other way. after the more official business of the inspection was over, the observatory was thrown open to visitors, and hundreds of people enjoyed on that day the privilege of seeing the national observatory. these annual gatherings are happily still continued, and the first saturday in june is known to be the occasion of one of the most interesting reunions of scientific men which takes place in the course of the year. airy's scientific work was, however, by no means confined to the observatory. he interested himself largely in expeditions for the observation of eclipses and in projects for the measurement of arcs on the earth. he devoted much attention to the collection of magnetic observations from various parts of the world. especially will it be remembered that the circumstances of the transits of venus, which occurred in and in , were investigated by him, and under his guidance expeditions were sent forth to observe the transits from those localities in remote parts of the earth where observations most suitable for the determination of the sun's distance from the earth could be obtained. the astronomer royal also studied tidal phenomena, and he rendered great service to the country in the restoration of the standards of length and weight which had been destroyed in the great fire at the house of parliament in october, . in the most practical scientific matters his advice was often sought, and was as cheerfully rendered. now we find him engaged in an investigation of the irregularities of the compass in iron ships, with a view to remedying its defects; now we find him reporting on the best gauge for railways. among the most generally useful developments of the observatory must be mentioned the telegraphic method for the distribution of exact time. by arrangement with the post office, the astronomers at greenwich despatch each morning a signal from the observatory to london at ten o'clock precisely. by special apparatus, this signal is thence distributed automatically over the country, so as to enable the time to be known everywhere accurately to a single second. it was part of the same system that a time ball should be dropped daily at one o'clock at deal, as well as at other places, for the purpose of enabling ship's chronometers to be regulated. airy's writings were most voluminous, and no fewer than forty-eight memoirs by him are mentioned in the "catalogue of scientific memoirs," published by the royal society up to the year , and this only included ten years out of an entire life of most extraordinary activity. many other subjects besides those of a purely scientific character from time to time engaged his attention. he wrote, for instance, a very interesting treatise on the roman invasion of britain, especially with a view of determining the port from which caesar set forth from gaul, and the point at which he landed on the british coast. airy was doubtless led to this investigation by his study of the tidal phenomena in the straits of dover. perhaps the astronomer royal is best known to the general reading public by his excellent lectures on astronomy, delivered at the ipswich museum in . this book has passed through many editions, and it gives a most admirable account of the manner in which the fundamental problems in astronomy have to be attacked. as years rolled by almost every honour and distinction that could be conferred upon a scientific man was awarded to sir george airy. he was, indeed, the recipient of other honours not often awarded for scientific distinction. among these we may mention that in he received the freedom of the city of london, "as a recognition of his indefatigable labours in astronomy, and of his eminent services in the advancement of practical science, whereby he has so materially benefited the cause of commerce and civilisation." until his eightieth year airy continued to discharge his labours at greenwich with unflagging energy. at last, on august th, , he resigned the office which he had held so long with such distinction to himself and such benefit to his country. he had married in the daughter of the rev. richard smith, of edensor. lady airy died in , and three sons and three daughters survived him. one daughter is the wife of dr. routh, of cambridge, and his other daughters were the constant companions of their father during the declining years of his life. up to the age of ninety he enjoyed perfect physical health, but an accidental fall which then occurred was attended with serious results. he died on saturday, january nd, , and was buried in the churchyard at playford. hamilton. william rowan hamilton was born at midnight between the rd and th of august, , at dublin, in the house which was then , but subsequently , dominick street. his father, archibald hamilton, was a solicitor, and william was the fourth of a family of nine. with reference to his descent, it may be sufficient to notice that his ancestors appear to have been chiefly of gentle irish families, but that his maternal grandmother was of scottish birth. when he was about a year old, his father and mother decided to hand over the education of the child to his uncle, james hamilton, a clergyman of trim, in county meath. james hamilton's sister, sydney, resided with him, and it was in their home that the days of william's childhood were passed. in mr. graves' "life of sir william rowan hamilton" a series of letters will be found, in which aunt sydney details the progress of the boy to his mother in dublin. probably there is no record of an infant prodigy more extraordinary than that which these letters contain. at three years old his aunt assured the mother that william is "a hopeful blade," but at that time it was his physical vigour to which she apparently referred; for the proofs of his capacity, which she adduces, related to his prowess in making boys older than himself fly before him. in the second letter, a month later, we hear that william is brought in to read the bible for the purpose of putting to shame other boys double his age who could not read nearly so well. uncle james appears to have taken much pains with william's schooling, but his aunt said that "how he picks up everything is astonishing, for he never stops playing and jumping about." when he was four years and three months old, we hear that he went out to dine at the vicar's, and amused the company by reading for them equally well whether the book was turned upside down or held in any other fashion. his aunt assures the mother that "willie is a most sensible little creature, but at the same time has a great deal of roguery." at four years and five months old he came up to pay his mother a visit in town, and she writes to her sister a description of the boy;-- "his reciting is astonishing, and his clear and accurate knowledge of geography is beyond belief; he even draws the countries with a pencil on paper, and will cut them out, though not perfectly accurate, yet so well that a anybody knowing the countries could not mistake them; but, you will think this nothing when i tell you that he reads latin, greek, and hebrew." aunt sydney recorded that the moment willie got back to trim he was desirous of at once resuming his former pursuits. he would not eat his breakfast till his uncle had heard him his hebrew, and he comments on the importance of proper pronunciation. at five he was taken to see a friend, to whom he repeated long passages from dryden. a gentleman present, who was not unnaturally sceptical about willie's attainments, desired to test him in greek, and took down a copy of homer which happened to have the contracted type, and to his amazement willie went on with the greatest ease. at six years and nine months he was translating homer and virgil; a year later his uncle tells us that william finds so little difficulty in learning french and italian, that he wishes to read homer in french. he is enraptured with the iliad, and carries it about with him, repeating from it whatever particularly pleases him. at eight years and one month the boy was one of a party who visited the scalp in the dublin mountains, and he was so delighted with the scenery that he forthwith delivered an oration in latin. at nine years and six months he is not satisfied until he learns sanscrit; three months later his thirst for the oriental languages is unabated, and at ten years and four months he is studying arabic and persian. when nearly twelve he prepared a manuscript ready for publication. it was a "syriac grammar," in syriac letters and characters compiled from that of buxtorf, by william hamilton, esq., of dublin and trim. when he was fourteen, the persian ambassador, mirza abul hassan khan, paid a visit to dublin, and, as a practical exercise in his oriental languages, the young scholar addressed to his excellency a letter in persian; a translation of which production is given by mr. graves. when william was fourteen he had the misfortune to lose his father; and he had lost his mother two years previously. the boy and his three sisters were kindly provided for by different members of the family on both sides. it was when william was about fifteen that his attention began to be turned towards scientific subjects. these were at first regarded rather as a relaxation from the linguistic studies with which he had been so largely occupied. on november nd, , he notes in his journal that he had begun newton's "principia": he commenced also the study of astronomy by observing eclipses, occultations, and similar phenomena. when he was sixteen we learn that he had read conic sections, and that he was engaged in the study of pendulums. after an attack of illness, he was moved for change to dublin, and in may, , we find him reading the differential calculus and laplace's "mecanique celeste." he criticises an important part of laplace's work relative to the demonstration of the parallelogram of forces. in this same year appeared the first gushes of those poems which afterwards flowed in torrents. his somewhat discursive studies had, however, now to give place to a more definite course of reading in preparation for entrance to the university of dublin. the tutor under whom he entered, charles boyton, was himself a distinguished man, but he frankly told the young william that he could be of little use to him as a tutor, for his pupil was quite as fit to be his tutor. eliza hamilton, by whom this is recorded, adds, "but there is one thing which boyton would promise to be to him, and that was a friend; and that one proof he would give of this should be that, if ever he saw william beginning to be upset by the sensation he would excite, and the notice he would attract, he would tell him of it." at the beginning of his college career he distanced all his competitors in every intellectual pursuit. at his first term examination in the university he was first in classics and first in mathematics, while he received the chancellor's prize for a poem on the ionian islands, and another for his poem on eustace de st. pierre. there is abundant testimony that hamilton had "a heart for friendship formed." among the warmest of the friends whom he made in these early days was the gifted maria edgeworth, who writes to her sister about "young mr. hamilton, an admirable crichton of eighteen, a real prodigy of talents, who dr. brinkley says may be a second newton, quiet, gentle, and simple." his sister eliza, to whom he was affectionately attached, writes to him in :-- "i had been drawing pictures of you in my mind in your study at cumberland street with 'xenophon,' &c., on the table, and you, with your most awfully sublime face of thought, now sitting down, and now walking about, at times rubbing your hands with an air of satisfaction, and at times bursting forth into some very heroical strain of poetry in an unknown language, and in your own internal solemn ventriloquist-like voice, when you address yourself to the silence and solitude of your own room, and indeed, at times, even when your mysterious poetical addresses are not quite unheard." this letter is quoted because it refers to a circumstance which all who ever met with hamilton, even in his latest years, will remember. he was endowed with two distinct voices, one a high treble, the other a deep bass, and he alternately employed these voices not only in ordinary conversation, but when he was delivering an address on the profundities of quaternions to the royal irish academy, or on similar occasions. his friends had long grown so familiar with this peculiarity that they were sometimes rather surprised to find how ludicrous it appeared to strangers. hamilton was fortunate in finding, while still at a very early age, a career open before him which was worthy of his talents. he had not ceased to be an undergraduate before he was called to fill an illustrious chair in his university. the circumstances are briefly as follows. we have already mentioned that, in , brinkley was appointed bishop of cloyne, and the professorship of astronomy thereupon became vacant. such was hamilton's conspicuous eminence that, notwithstanding he was still an undergraduate, and had only just completed his twenty-first year, he was immediately thought of as a suitable successor to the chair. indeed, so remarkable were his talents in almost every direction that had the vacancy been in the professorship of classics or of mathematics, of english literature or of metaphysics, of modern or of oriental languages, it seems difficult to suppose that he would not have occurred to every one as a possible successor. the chief ground, however, on which the friends of hamilton urged his appointment was the earnest of original power which he had already shown in a research on the theory of systems of rays. this profound work created a new branch of optics, and led a few years later to a superb discovery, by which the fame of its author became world-wide. at first hamilton thought it would be presumption for him to apply for so exalted a position; he accordingly retired to the country, and resumed his studies for his degree. other eminent candidates came forward, among them some from cambridge, and a few of the fellows from trinity college, dublin, also sent in their claims. it was not until hamilton received an urgent letter from his tutor boyton, in which he was assured of the favourable disposition of the board towards his candidature, that he consented to come forward, and on june th, , he was unanimously chosen to succeed the bishop of cloyne as professor of astronomy in the university. the appointment met with almost universal approval. it should, however, be noted that brinkley, whom hamilton succeeded, did not concur in the general sentiment. no one could have formed a higher opinion than he had done of hamilton's transcendent powers; indeed, it was on that very ground that he seemed to view the appointment with disapprobation. he considered that it would have been wiser for hamilton to have obtained a fellowship, in which capacity he would have been able to exercise a greater freedom in his choice of intellectual pursuits. the bishop seems to have thought, and not without reason, that hamilton's genius would rather recoil from much of the routine work of an astronomical establishment. now that hamilton's whole life is before us, it is easy to see that the bishop was entirely wrong. it is quite true that hamilton never became a skilled astronomical observer; but the seclusion of the observatory was eminently favourable to those gigantic labours to which his life was devoted, and which have shed so much lustre, not only on hamilton himself, but also on his university and his country. in his early years at dunsink, hamilton did make some attempts at a practical use of the telescopes, but he possessed no natural aptitude for such work, while exposure which it involved seems to have acted injuriously on his health. he, therefore, gradually allowed his attention to be devoted to those mathematical researches in which he had already given such promise of distinction. although it was in pure mathematics that he ultimately won his greatest fame, yet he always maintained and maintained with justice, that he had ample claims to the title of an astronomer. in his later years he set forth this position himself in a rather striking manner. de morgan had written commending to hamilton's notice grant's "history of physical astronomy." after becoming acquainted with the book, hamilton writes to his friend as follows:-- "the book is very valuable, and very creditable to its composer. but your humble servant may be pardoned if he finds himself somewhat amused at the title, 'history of physical astronomy from the earliest ages to the middle of the nineteenth century,' when he fails to observe any notice of the discoveries of sir w. r. hamilton in the theory of the 'dynamics of the heavens.'" the intimacy between the two correspondents will account for the tone of this letter; and, indeed, hamilton supplies in the lines which follow ample grounds for his complaint. he tells how jacobi spoke of him in manchester in as "le lagrange de votre pays," and how donkin had said that, "the analytical theory of dynamics as it exists at present is due mainly to the labours of la grange poisson, sir w. r. hamilton, and jacobi, whose researches on this subject present a series of discoveries hardly paralleled for their elegance and importance in any other branch of mathematics." in the same letter hamilton also alludes to the success which had attended the applications of his methods in other hands than his own to the elucidation of the difficult subject of planetary perturbations. even had his contributions to science amounted to no more than these discoveries, his tenure of the chair would have been an illustrious one. it happens, however, that in the gigantic mass of his intellectual work these researches, though intrinsically of such importance, assume what might almost be described as a relative insignificance. the most famous achievement of hamilton's earlier years at the observatory was the discovery of conical refraction. this was one of those rare events in the history of science, in which a sagacious calculation has predicted a result of an almost startling character, subsequently confirmed by observation. at once this conferred on the young professor a world-wide renown. indeed, though he was still only twenty-seven, he had already lived through an amount of intellectual activity which would have been remarkable for a man of threescore and ten. simultaneously with his growth in fame came the growth of his several friendships. there were, in the first place, his scientific friendships with herschel, robinson, and many others with whom he had copious correspondence. in the excellent biography to which i have referred, hamilton's correspondence with coleridge may be read, as can also the letters to his lady correspondents, among them being maria edgeworth, lady dunraven, and lady campbell. many of these sheets relate to literary matters, but they are largely intermingled with genial pleasantry, and serve at all events to show the affection and esteem with which he was regarded by all who had the privilege of knowing him. there are also the letters to the sisters whom he adored, letters brimming over with such exalted sentiment, that most ordinary sisters would be tempted to receive them with a smile in the excessively improbable event of their still more ordinary brothers attempting to pen such effusions. there are also indications of letters to and from other young ladies who from time to time were the objects of hamilton's tender admiration. we use the plural advisedly, for, as mr. graves has set forth, hamilton's love affairs pursued a rather troubled course. the attention which he lavished on one or two fair ones was not reciprocated, and even the intense charms of mathematical discovery could not assuage the pangs which the disappointed lover experienced. at last he reached the haven of matrimony in , when he was married to miss bayly. of his married life hamilton said, many years later to de morgan, that it was as happy as he expected, and happier than he deserved. he had two sons, william and archibald, and one daughter, helen, who became the wife of archdeacon o'regan. [plate: sir w. rowan hamilton.] the most remarkable of hamilton's friendships in his early years was unquestionably that with wordsworth. it commenced with hamilton's visit to keswick; and on the first evening, when the poet met the young mathematician, an incident occurred which showed the mutual interest that was aroused. hamilton thus describes it in a letter to his sister eliza:-- "he (wordsworth) walked back with our party as far as their lodge, and then, on our bidding mrs. harrison good-night, i offered to walk back with him while my party proceeded to the hotel. this offer he accepted, and our conversation had become so interesting that when we had arrived at his home, a distance of about a mile, he proposed to walk back with me on my way to ambleside, a proposal which you may be sure i did not reject; so far from it that when he came to turn once more towards his home i also turned once more along with him. it was very late when i reached the hotel after all this walking." hamilton also submitted to wordsworth an original poem, entitled "it haunts me yet." the reply of wordsworth is worth repeating:-- "with a safe conscience i can assure you that, in my judgment, your verses are animated with the poetic spirit, as they are evidently the product of strong feeling. the sixth and seventh stanzas affected me much, even to the dimming of my eyes and faltering of my voice while i was reading them aloud. having said this, i have said enough. now for the per contra. you will not, i am sure, be hurt when i tell you that the workmanship (what else could be expected from so young a writer?) is not what it ought to be. . . "my household desire to be remembered to you in no formal way. seldom have i parted--never, i was going to say--with one whom after so short an acquaintance i lost sight of with more regret. i trust we shall meet again." the further affectionate intercourse between hamilton and wordsworth is fully set forth, and to hamilton's latest years a recollection of his "rydal hours" was carefully treasured and frequently referred to. wordsworth visited hamilton at the observatory, where a beautiful shady path in the garden is to the present day spoken of as "wordsworth's walk." it was the practice of hamilton to produce a sonnet on almost every occasion which admitted of poetical treatment, and it was his delight to communicate his verses to his friends all round. when whewell was producing his "bridgewater treatises," he writes to hamilton in :-- "your sonnet which you showed me expressed much better than i could express it the feeling with which i tried to write this book, and i once intended to ask your permission to prefix the sonnet to my book, but my friends persuaded me that i ought to tell my story in my own prose, however much better your verse might be." the first epoch-marking contribution to theoretical dynamics after the time of newton was undoubtedly made by lagrange, in his discovery of the general equations of motion. the next great step in the same direction was that taken by hamilton in his discovery of a still more comprehensive method. of this contribution hamilton writes to whewell, march st, :-- "as to my late paper, a day or two ago sent off to london, it is merely mathematical and deductive. i ventured, indeed, to call it the 'mecanique analytique' of lagrange, 'a scientific poem'; and spoke of dynamics, or the science of force, as treating of 'power acting by law in space and time.' in other respects it is as unpoetical and unmetaphysical as my gravest friends could desire." it may well be doubted whether there is a more beautiful chapter in the whole of mathematical philosophy than that which contains hamilton's dynamical theory. it is disfigured by no tedious complexity of symbols; it condescends not to any particular problems; it is an all embracing theory, which gives an intellectual grasp of the most appropriate method for discovering the result of the application of force to matter. it is the very generality of this doctrine which has somewhat impeded the applications of which it is susceptible. the exigencies of examinations are partly responsible for the fact that the method has not become more familiar to students of the higher mathematics. an eminent professor has complained that hamilton's essay on dynamics was of such an extremely abstract character, that he found himself unable to extract from it problems suitable for his examination papers. the following extract is from a letter of professor sylvester to hamilton, dated th of september, . it will show how his works were appreciated by so consummate a mathematician as the writer:-- "believe me, sir, it is not the least of my regrets in quitting this empire to feel that i forego the casual occasion of meeting those masters of my art, yourself chief amongst the number, whose acquaintance, whose conversation, or even notice, have in themselves the power to inspire, and almost to impart fresh vigour to the understanding, and the courage and faith without which the efforts of invention are in vain. the golden moments i enjoyed under your hospitable roof at dunsink, or moments such as they were, may probably never again fall to my lot. "at a vast distance, and in an humble eminence, i still promise myself the calm satisfaction of observing your blazing course in the elevated regions of discovery. such national honour as you are able to confer on your country is, perhaps, the only species of that luxury for the rich (i mean what is termed one's glory) which is not bought at the expense of the comforts of the million." the study of metaphysics was always a favourite recreation when hamilton sought for a change from the pursuit of mathematics. in the year we find him a diligent student of kant; and, to show the views of the author of quaternions and of algebra as the science of pure time on the "critique of the pure reason," we quote the following letter, dated th of july, , from hamilton to viscount adare:-- "i have read a large part of the 'critique of the pure reason,' and find it wonderfully clear, and generally quite convincing. notwithstanding some previous preparation from berkeley, and from my own thoughts, i seem to have learned much from kant's own statement of his views of 'space and time.' yet, on the whole, a large part of my pleasure consists in recognising through kant's works, opinions, or rather views, which have been long familiar to myself, although far more clearly and systematically expressed and combined by him. . . . kant is, i think, much more indebted than he owns, or, perhaps knows, to berkeley, whom he calls by a sneer, 'gutem berkeley'. . . as it were, 'good soul, well meaning man,' who was able for all that to shake to its centre the world of human thought, and to effect a revolution among the early consequences of which was the growth of kant himself." at several meetings of the british association hamilton was a very conspicuous figure. especially was this the case in , when the association met in dublin, and when hamilton, though then but thirty years old, had attained such celebrity that even among a very brilliant gathering his name was perhaps the most renowned. a banquet was given at trinity college in honour of the meeting. the distinguished visitors assembled in the library of the university. the earl of mulgrave, then lord lieutenant of ireland, made this the opportunity of conferring on hamilton the honour of knighthood, gracefully adding, as he did so: "i but set the royal, and therefore the national mark, on a distinction already acquired by your genius and labours." the banquet followed, writes mr. graves. "it was no little addition to the honour hamilton had already received that, when professor whewell returned thanks for the toast of the university of cambridge, he thought it appropriate to add the words, 'there was one point which strongly pressed upon him at that moment: it was now one hundred and thirty years since a great man in another trinity college knelt down before his sovereign, and rose up sir isaac newton.' the compliment was welcomed by immense applause." a more substantial recognition of the labours of hamilton took place subsequently. he thus describes it in a letter to mr. graves of th of november, :-- "the queen has been pleased--and you will not doubt that it was entirely unsolicited, and even unexpected, on my part--'to express her entire approbation of the grant of a pension of two hundred pounds per annum from the civil list' to me for scientific services. the letters from sir robert peel and from the lord lieutenant of ireland in which this grant has been communicated or referred to have been really more gratifying to my feelings than the addition to my income, however useful, and almost necessary, that may have been." the circumstances we have mentioned might lead to the supposition that hamilton was then at the zenith of his fame but this was not so. it might more truly be said, that his achievements up to this point were rather the preliminary exercises which fitted him for the gigantic task of his life. the name of hamilton is now chiefly associated with his memorable invention of the calculus of quaternions. it was to the creation of this branch of mathematics that the maturer powers of his life were devoted; in fact he gives us himself an illustration of how completely habituated he became to the new modes of thought which quaternions originated. in one of his later years he happened to take up a copy of his famous paper on dynamics, a paper which at the time created such a sensation among mathematicians, and which is at this moment regarded as one of the classics of dynamical literature. he read, he tells us, his paper with considerable interest, and expressed his feelings of gratification that he found himself still able to follow its reasoning without undue effort. but it seemed to him all the time as a work belonging to an age of analysis now entirely superseded. in order to realise the magnitude of the revolution which hamilton has wrought in the application of symbols to mathematical investigation, it is necessary to think of what hamilton did beside the mighty advance made by descartes. to describe the character of the quaternion calculus would be unsuited to the pages of this work, but we may quote an interesting letter, written by hamilton from his death-bed, twenty-two years later, to his son archibald, in which he has recorded the circumstances of the discovery:-- "indeed, i happen to be able to put the finger of memory upon the year and month--october, --when having recently returned from visits to cork and parsonstown, connected with a meeting of the british association, the desire to discover the laws of multiplication referred to, regained with me a certain strength and earnestness which had for years been dormant, but was then on the point of being gratified, and was occasionally talked of with you. every morning in the early part of the above-cited month, on my coming down to breakfast, your (then) little brother william edwin, and yourself, used to ask me, 'well papa, can you multiply triplets?' whereto i was always obliged to reply, with a sad shake of the head: 'no, i can only add and subtract them,' "but on the th day of the same month--which happened to be monday, and a council day of the royal irish academy--i was walking in to attend and preside, and your mother was walking with me along the royal canal, to which she had perhaps driven; and although she talked with me now and then, yet an undercurrent of thought was going on in my mind which gave at last a result, whereof it is not too much to say that i felt at once the importance. an electric circuit seemed to close; and a spark flashed forth the herald (as i foresaw immediately) of many long years to come of definitely directed thought and work by myself, if spared, and, at all events, on the part of others if i should even be allowed to live long enough distinctly to communicate the discovery. nor could i resist the impulse--unphilosophical as it may have been--to cut with a knife on a stone of brougham bridge as we passed it, the fundamental formula which contains the solution of the problem, but, of course, the inscription has long since mouldered away. a more durable notice remains, however, on the council books of the academy for that day (october , ), which records the fact that i then asked for and obtained leave to read a paper on 'quaternions,' at the first general meeting of the session; which reading took place accordingly, on monday, the th of november following." writing to professor tait, hamilton gives further particulars of the same event. and again in a letter to the rev. j. w. stubbs:-- "to-morrow will be the fifteenth birthday of the quaternions. they started into life full-grown on the th october, , as i was walking with lady hamilton to dublin, and came up to brougham bridge--which my boys have since called quaternion bridge. i pulled out a pocketbook which still exists, and made entry, on which at the very moment i felt that it might be worth my while to expend the labour of at least ten or fifteen years to come. but then it is fair to say that this was because i felt a problem to have been at that moment solved, an intellectual want relieved which had haunted me for at least fifteen years before. "but did the thought of establishing such a system, in which geometrically opposite facts--namely, two lines (or areas) which are opposite in space give always a positive product--ever come into anybody's head till i was led to it in october, , by trying to extend my old theory of algebraic couples, and of algebra as the science of pure time? as to my regarding geometrical addition of lines as equivalent to composition of motions (and as performed by the same rules), that is indeed essential in my theory but not peculiar to it; on the contrary, i am only one of many who have been led to this view of addition." pilgrims in future ages will doubtless visit the spot commemorated by the invention of quaternions. perhaps as they look at that by no means graceful structure quaternion bridge, they will regret that the hand of some old mortality had not been occasionally employed in cutting the memorable inscription afresh. it is now irrecoverably lost. it was ten years after the discovery that the great volume appeared under the title of "lectures on quaternions," dublin, . the reception of this work by the scientific world was such as might have been expected from the extraordinary reputation of its author, and the novelty and importance of the new calculus. his valued friend, sir john herschel, writes to him in that style of which he was a master:-- "now, most heartily let me congratulate you on getting out your book--on having found utterance, ore rotundo, for all that labouring and seething mass of thought which has been from time to time sending out sparks, and gleams, and smokes, and shaking the soil about you; but now breaks into a good honest eruption, with a lava stream and a shower of fertilizing ashes. "metaphor and simile apart, there is work for a twelve-month to any man to read such a book, and for half a lifetime to digest it, and i am glad to see it brought to a conclusion." we may also record hamilton's own opinion expressed to humphrey lloyd:-- "in general, although in one sense i hope that i am actually growing modest about the quaternions, from my seeing so many peeps and vistas into future expansions of their principles, i still must assert that this discovery appears to me to be as important for the middle of the nineteenth century as the discovery of fluxions was for the close of the seventeenth." bartholomew lloyd died in . he had been the provost of trinity college, and the president of the royal irish academy. three candidates were put forward by their respective friends for the vacant presidency. one was humphrey lloyd, the son of the late provost, and the two others were hamilton and archbishop whately. lloyd from the first urged strongly the claims of hamilton, and deprecated the putting forward of his own name. hamilton in like manner desired to withdraw in favour of lloyd. the wish was strongly felt by many of the fellows of the college that lloyd should be elected, in consequence of his having a more intimate association with collegiate life than hamilton; while his scientific eminence was world-wide. the election ultimately gave hamilton a considerable majority over lloyd, behind whom the archbishop followed at a considerable distance. all concluded happily, for both lloyd and the archbishop expressed, and no doubt felt, the pre-eminent claims of hamilton, and both of them cordially accepted the office of a vice-president, to which, according to the constitution of the academy, it is the privilege of the incoming president to nominate. in another chapter i have mentioned as a memorable episode in astronomical history, that sir j. herschel went for a prolonged sojourn to the cape of good hope, for the purpose of submitting the southern skies to the same scrutiny with the great telescope that his father had given to the northern skies. the occasion of herschel's return after the brilliant success of his enterprise, was celebrated by a banquet. on june th, , hamilton was assigned the high honour of proposing the health of herschel. this banquet is otherwise memorable in hamilton's career as being one of the two occasions in which he was in the company of his intimate friend de morgan. in the year a scheme was adopted by the royal irish academy for the award of medals to the authors of papers which appeared to possess exceptionally high merit. at the institution of the medal two papers were named in competition for the prize. one was hamilton's "memoir on algebra, as the science of pure time." the other was macullagh's paper on the "laws of crystalline reflection and refraction." hamilton expresses his gratification that, mainly in consequence of his own exertions, he succeeded in having the medal awarded to macullagh rather than to himself. indeed, it would almost appear as if hamilton had procured a letter from sir j. herschel, which indicated the importance of macullagh's memoir in such a way as to decide the issue. it then became hamilton's duty to award the medal from the chair, and to deliver an address in which he expressed his own sense of the excellence of macullagh's scientific work. it is the more necessary to allude to these points, because in the whole of his scientific career it would seem that macullagh was the only man with whom hamilton had ever even an approach to a dispute about priority. the incident referred to took place in connection with the discovery of conical refraction, the fame of which macullagh made a preposterous attempt to wrest from hamilton. this is evidently alluded to in hamilton's letter to the marquis of northampton, dated june th, , in which we read:-- "and though some former circumstances prevented me from applying to the person thus distinguished the sacred name of friend, i had the pleasure of doing justice...to his high intellectual merits... i believe he was not only gratified but touched, and may, perhaps, regard me in future with feelings more like those which i long to entertain towards him." hamilton was in the habit, from time to time, of commencing the keeping of a journal, but it does not appear to have been systematically conducted. whatever difficulties the biographer may have experienced from its imperfections and irregularities, seem to be amply compensated for by the practice which hamilton had of preserving copies of his letters, and even of comparatively insignificant memoranda. in fact, the minuteness with which apparently trivial matters were often noted down appears almost whimsical. he frequently made a memorandum of the name of the person who carried a letter to the post, and of the hour in which it was despatched. on the other hand, the letters which he received were also carefully preserved in a mighty mass of manuscripts, with which his study was encumbered, and with which many other parts of the house were not unfrequently invaded. if a letter was laid aside for a few hours, it would become lost to view amid the seething mass of papers, though occasionally, to use his own expression, it might be seen "eddying" to the surface in some later disturbance. the great volume of "lectures on quaternions" had been issued, and the author had received the honours which the completion of such a task would rightfully bring him. the publication of an immortal work does not, however, necessarily provide the means for paying the printer's bill. the printing of so robust a volume was necessarily costly; and even if all the copies could be sold, which at the time did not seem very likely, they would hardly have met the inevitable expenses. the provision of the necessary funds was, therefore, a matter for consideration. the board of trinity college had already contributed pounds to the printing, but yet another hundred was required. even the discoverer of quaternions found this a source of much anxiety. however, the board, urged by the representation of humphrey lloyd, now one of its members, and, as we have already seen, one of hamilton's staunchest friends, relieved him of all liability. we may here note that, notwithstanding the pension which hamilton enjoyed in addition to the salary of his chair, he seems always to have been in some what straitened circumstances, or, to use his own words in one of his letters to de morgan, "though not an embarrassed man, i am anything rather than a rich one." it appears that, notwithstanding the world-wide fame of hamilton's discoveries, the only profit in a pecuniary sense that he ever obtained from any of his works was by the sale of what he called his icosian game. some enterprising publisher, on the urgent representations of one of hamilton's friends in london, bought the copyright of the icosian game for pounds. even this little speculation proved unfortunate for the purchaser, as the public could not be induced to take the necessary interest in the matter. after the completion of his great book, hamilton appeared for awhile to permit himself a greater indulgence than usual in literary relaxations. he had copious correspondence with his intimate friend, aubrey de vere, and there were multitudes of letters from those troops of friends whom it was hamilton's privilege to possess. he had been greatly affected by the death of his beloved sister eliza, a poetess of much taste and feeling. she left to him her many papers to preserve or to destroy, but he said it was only after the expiration of four years of mourning that he took courage to open her pet box of letters. the religious side of hamilton's character is frequently illustrated in these letters; especially is this brought out in the correspondence with de vere, who had seceded to the church of rome. hamilton writes, august , :-- "if, then, it be painfully evident to both, that under such circumstances there cannot (whatever we may both desire) be now in the nature of things, or of minds, the same degree of intimacy between us as of old; since we could no longer talk with the same degree of unreserve on every subject which happened to present itself, but must, from the simplest instincts of courtesy, be each on his guard not to say what might be offensive, or, at least, painful to the other; yet we were once so intimate, and retain still, and, as i trust, shall always retain, so much of regard and esteem and appreciation for each other, made tender by so many associations of my early youth and your boyhood, which can never be forgotten by either of us, that (as times go) two or three very respectable friendships might easily be carved out from the fragments of our former and ever-to-be-remembered intimacy. it would be no exaggeration to quote the words: 'heu! quanto minus est cum reliquis versari, quam tui meminisse!'" in a correspondence on the subject of quaternions commenced between professor tait and sir william hamilton. it was particularly gratifying to the discoverer that so competent a mathematician as professor tait should have made himself acquainted with the new calculus. it is, of course, well known that professor tait subsequently brought out a most valuable elementary treatise on quaternions, to which those who are anxious to become acquainted with the subject will often turn in preference to the tremendous work of hamilton. in the year gratifying information came to hand of the progress which the study of quaternions was making abroad. especially did the subject attract the attention of that accomplished mathematician, moebius, who had already in his "barycentrische calculus" been led to conceptions which bore more affinity to quaternions than could be found in the writings of any other mathematician. such notices of his work were always pleasing to hamilton, and they served, perhaps, as incentives to that still closer and more engrossing labour by which he became more and more absorbed. during the last few years of his life he was observed to be even more of a recluse than he had hitherto been. his powers of long and continuous study seemed to grow with advancing years, and his intervals of relaxation, such as they were, became more brief and more infrequent. it was not unusual for him to work for twelve hours at a stretch. the dawn would frequently surprise him as he looked up to snuff his candles after a night of fascinating labour at original research. regularity in habits was impossible to a student who had prolonged fits of what he called his mathematical trances. hours for rest and hours for meals could only be snatched in the occasional the lucid intervals between one attack of quaternions and the next. when hungry, he would go to see whether anything could be found on the sideboard; when thirsty, he would visit the locker, and the one blemish in the man's personal character is that these latter visits were sometimes paid too often. as an example of one of hamilton's rare diversions from the all- absorbing pursuit of quaternions, we find that he was seized with curiosity to calculate back to the date of the hegira, which he found on the th july, . he speaks of the satisfaction with which he ascertained subsequently that herschel had assigned precisely the same date. metaphysics remained also, as it had ever been, a favourite subject of hamilton's readings and meditations and of correspondence with his friends. he wrote a very long letter to dr. ingleby on the subject of his "introduction to metaphysics." in it hamilton alludes, as he has done also in other places, to a peculiarity of his own vision. it was habitual to him, by some defect in the correlation of his eyes, to see always a distinct image with each; in fact, he speaks of the remarkable effect which the use of a good stereoscope had on his sensations of vision. it was then, for the first time, that he realised how the two images which he had always seen hitherto would, under normal circumstances, be blended into one. he cites this fact as bearing on the phenomena of binocular vision, and he draws from it the inference that the necessity of binocular vision for the correct appreciation of distance is unfounded. "i am quite sure," he says, "that i see distance with each eye separately." the commencement of , the last year of his life saw hamilton as diligent as ever, and corresponding with salmon and cayley. on april th he writes to a friend to say, that his health has not been good for years past, and that so much work has injured his constitution; and he adds, that it is not conducive to good spirits to find that he is accumulating another heavy bill with the printer for the publication of the "elements." this was, indeed, up to the day of his death, a cause for serious anxiety. it may, however, be mentioned that the whole cost, which amounted to nearly pounds, was, like that of the previous volume, ultimately borne by the college. contrary to anticipation, the enterprise, even in a pecuniary sense, cannot have been a very unprofitable one. the whole edition has long been out of print, and as much as pounds has since been paid for a single copy. it was on the th of may, , that hamilton was in dublin for the last time. a few days later he had a violent attack of gout, and on the th of june he became alarmingly ill, and on the next day had an attack of epileptic convulsions. however, he slightly rallied, so that before the end of the month he was again at work at the "elements." a gratifying incident brightened some of the last days of his life. the national academy of science in america had then been just formed. a list of foreign associates had to be chosen from the whole world, and a discussion took place as to what name should be placed first on the list. hamilton was informed by private communication that this great distinction was awarded to him by a majority of two-thirds. in august he was still at work on the table of contents of the "elements," and one of his very latest efforts was his letter to mr. gould, in america, communicating his acknowledgements of the honour which had been just conferred upon him by the national academy. on the nd of september mr. graves went to the observatory, in response to a summons, and the great mathematician at once admitted to his friend that he felt the end was approaching. he mentioned that he had found in the th psalm a wonderfully suitable expression of his thoughts and feelings, and he wished to testify his faith and thankfulness as a christian by partaking of the lord's supper. he died at half-past two on the afternoon of the nd of september, , aged sixty years and one month. he was buried in mount jerome cemetery on the th of september. many were the letters and other more public manifestations of the feelings awakened by hamilton's death. sir john herschel wrote to the widow:-- "permit me only to add that among the many scientific friends whom time has deprived me of, there has been none whom i more deeply lament, not only for his splendid talents, but for the excellence of his disposition and the perfect simplicity of his manners--so great, and yet devoid of pretensions." de morgan, his old mathematical crony, as hamilton affectionately styled him, also wrote to lady hamilton:-- "i have called him one of my dearest friends, and most truly; for i know not how much longer than twenty-five years we have been in intimate correspondence, of most friendly agreement or disagreement, of most cordial interest in each other. and yet we did not know each other's faces. i met him about at babbage's breakfast table, and there for the only time in our lives we conversed. i saw him, a long way off, at the dinner given to herschel (about ) on his return from the cape and there we were not near enough, nor on that crowded day could we get near enough, to exchange a word. and this is all i ever saw, and, so it has pleased god, all i shall see in this world of a man whose friendly communications were among my greatest social enjoyments, and greatest intellectual treats." there is a very interesting memoir of hamilton written by de morgan, in the "gentleman's magazine" for , in which he produces an excellent sketch of his friend, illustrated by personal reminiscences and anecdotes. he alludes, among other things, to the picturesque confusion of the papers in his study. there was some sort of order in the mass, discernible however, by hamilton alone, and any invasion of the domestics, with a view to tidying up, would throw the mathematician as we are informed, into "a good honest thundering passion." hardly any two men, who were both powerful mathematicians, could have been more dissimilar in every other respect than were hamilton and de morgan. the highly poetical temperament of hamilton was remarkably contrasted with the practical realism of de morgan. hamilton sends sonnets to his friend, who replies by giving the poet advice about making his will. the metaphysical subtleties, with which hamilton often filled his sheets, did not seem to have the same attraction for de morgan that he found in battles about the quantification of the predicate. de morgan was exquisitely witty, and though his jokes were always appreciated by his correspondent, yet hamilton seldom ventured on anything of the same kind in reply; indeed his rare attempts at humour only produced results of the most ponderous description. but never were two scientific correspondents more perfectly in sympathy with each other. hamilton's work on quaternions, his labours in dynamics, his literary tastes, his metaphysics, and his poetry, were all heartily welcomed by his friend, whose letters in reply invariably evince the kindliest interest in all hamilton's concerns. in a similar way de morgan's letters to hamilton always met with a heartfelt response. alike for the memory of hamilton, for the credit of his university, and for the benefit of science, let us hope that a collected edition of his works will ere long appear--a collection which shall show those early achievements in splendid optical theory, those achievements of his more mature powers which made him the lagrange of his country, and finally those creations of the quaternion calculus by which new capabilities have been bestowed on the human intellect. le verrier. the name of le verrier is one that goes down to fame on account of very different discoveries from those which have given renown to several of the other astronomers whom we have mentioned. we are sometimes apt to identify the idea of an astronomer with that of a man who looks through a telescope at the stars; but the word astronomer has really much wider significance. no man who ever lived has been more entitled to be designated an astronomer than le verrier, and yet it is certain that he never made a telescopic discovery of any kind. indeed, so far as his scientific achievements have been concerned, he might never have looked through a telescope at all. for the full interpretation of the movements of the heavenly bodies, mathematical knowledge of the most advanced character is demanded. the mathematician at the outset calls upon the astronomer who uses the instruments in the observatory, to ascertain for him at various times the exact positions occupied by the sun, the moon, and the planets. these observations, obtained with the greatest care, and purified as far as possible from the errors by which they may be affected form, as it were, the raw material on which the mathematician exercises his skill. it is for him to elicit from the observed places the true laws which govern the movements of the heavenly bodies. here is indeed a task in which the highest powers of the human intellect may be worthily employed. among those who have laboured with the greatest success in the interpretation of the observations made with instruments of precision, le verrier holds a highly honoured place. to him it has been given to provide a superb illustration of the success with which the mind of man can penetrate the deep things of nature. the illustrious frenchman, urban jean joseph le verrier, was born on the th march, , at st. lo, in the department of manche. he received his education in that famous school for education in the higher branches of science, the ecole polytechnique, and acquired there considerable fame as a mathematician. on leaving the school le verrier at first purposed to devote himself to the public service, in the department of civil engineering; and it is worthy of note that his earliest scientific work was not in those mathematical researches in which he was ultimately to become so famous. his duties in the engineering department involved practical chemical research in the laboratory. in this he seems to have become very expert, and probably fame as a chemist would have been thus attained, had not destiny led him into another direction. as it was, he did engage in some original chemical research. his first contributions to science were the fruits of his laboratory work; one of his papers was on the combination of phosphorus and hydrogen, and another on the combination of phosphorus and oxygen. his mathematical labours at the ecole polytechnique had, however, revealed to le verrier that he was endowed with the powers requisite for dealing with the subtlest instruments of mathematical analysis. when he was twenty-eight years old, his first great astronomical investigation was brought forth. it will be necessary to enter into some explanation as to the nature of this, inasmuch as it was the commencement of the life-work which he was to pursue. if but a single planet revolved around the sun, then the orbit of that planet would be an ellipse, and the shape and size, as well as the position of the ellipse, would never alter. one revolution after another would be traced out, exactly in the same manner, in compliance with the force continuously exerted by the sun. suppose, however, that a second planet be introduced into the system. the sun will exert its attraction on this second planet also, and it will likewise describe an orbit round the central globe. we can, however, no longer assert that the orbit in which either of the planets moves remains exactly an ellipse. we may, indeed, assume that the mass of the sun is enormously greater than that of either of the planets. in this case the attraction of the sun is a force of such preponderating magnitude, that the actual path of each planet remains nearly the same as if the other planet were absent. but it is impossible for the orbit of each planet not to be affected in some degree by the attraction of the other planet. the general law of nature asserts that every body in space attracts every other body. so long as there is only a single planet, it is the single attraction between the sun and that planet which is the sole controlling principle of the movement, and in consequence of it the ellipse is described. but when a second planet is introduced, each of the two bodies is not only subject to the attraction of the sun, but each one of the planets attracts the other. it is true that this mutual attraction is but small, but, nevertheless, it produces some effect. it "disturbs," as the astronomer says, the elliptic orbit which would otherwise have been pursued. hence it follows that in the actual planetary system where there are several planets disturbing each other, it is not true to say that the orbits are absolutely elliptic. at the same time in any single revolution a planet may for most practical purposes be said to be actually moving in an ellipse. as, however, time goes on, the ellipse gradually varies. it alters its shape, it alters its plane, and it alters its position in that plane. if, therefore, we want to study the movements of the planets, when great intervals of time are concerned, it is necessary to have the means of learning the nature of the movement of the orbit in consequence of the disturbances it has experienced. we may illustrate the matter by supposing the planet to be running like a railway engine on a track which has been laid in a long elliptic path. we may suppose that while the planet is coursing along, the shape of the track is gradually altering. but this alteration may be so slow, that it does not appreciably affect the movement of the engine in a single revolution. we can also suppose that the plane in which the rails have been laid has a slow oscillation in level, and that the whole orbit is with more or less uniformity moved slowly about in the plane. in short periods of time the changes in the shapes and positions of the planetary orbits, in consequence of their mutual attractions, are of no great consequence. when, however, we bring thousands of years into consideration, then the displacements of the planetary orbits attain considerable dimensions, and have, in fact, produced a profound effect on the system. it is of the utmost interest to investigate the extent to which one planet can affect another in virtue of their mutual attractions. such investigations demand the exercise of the highest mathematical gifts. but not alone is intellectual ability necessary for success in such inquiries. it must be united with a patient capacity for calculations of an arduous type, protracted, as they frequently have to be, through many years of labour. le verrier soon found in these profound inquiries adequate scope for the exercise of his peculiar gifts. his first important astronomical publication contained an investigation of the changes which the orbits of several of the planets, including the earth, have undergone in times past, and which they will undergo in times to come. as an illustration of these researches, we may take the case of the planet in which we are, of course, especially interested, namely, the earth, and we can investigate the changes which, in the lapse of time, the earth's orbit has undergone, in consequence of the disturbance to which it has been subjected by the other planets. in a century, or even in a thousand years, there is but little recognisable difference in the shape of the track pursued by the earth. vast periods of time are required for the development of the large consequences of planetary perturbation. le verrier has, however, given us the particulars of what the earth's journey through space has been at intervals of , years back from the present date. his furthest calculation throws our glance back to the state of the earth's track , years ago, while, with a bound forward, he shows us what the earth's orbit is to be in the future, at successive intervals of , years, till a date is reached which is , years in advance of a.d. . the talent which these researches displayed brought le verrier into notice. at that time the paris observatory was presided over by arago, a savant who occupies a distinguished position in french scientific annals. arago at once perceived that le verrier was just the man who possessed the qualifications suitable for undertaking a problem of great importance and difficulty that had begun to force itself on the attention of astronomers. what this great problem was, and how astonishing was the solution it received, must now be considered. ever since herschel brought himself into fame by his superb discovery of the great planet uranus, the movements of this new addition to the solar system were scrutinized with care and attention. the position of uranus was thus accurately determined from time to time. at length, when sufficient observations of this remote planet had been brought together, the route which the newly-discovered body pursued through the heavens was ascertained by those calculations with which astronomers are familiar. it happens, however, that uranus possesses a superficial resemblance to a star. indeed the resemblance is so often deceptive that long ere its detection as a planet by herschel, it had been observed time after time by skilful astronomers, who little thought that the star-like point at which they looked was anything but a star. from these early observations it was possible to determine the track of uranus, and it was found that the great planet takes a period of no less than eighty-four years to accomplish a circuit. calculations were made of the shape of the orbit in which it revolved before its discovery by herschel, and these were compared with the orbit which observations showed the same body to pursue in those later years when its planetary character was known. it could not, of course, be expected that the orbit should remain unaltered; the fact that the great planets jupiter and saturn revolve in the vicinity of uranus must necessarily imply that the orbit of the latter undergoes considerable changes. when, however, due allowance has been made for whatever influence the attraction of jupiter and saturn, and we may add of the earth and all the other planets, could possibly produce, the movements of uranus were still inexplicable. it was perfectly obvious that there must be some other influence at work besides that which could be attributed to the planets already known. astronomers could only recognise one solution of such a difficulty. it was impossible to doubt that there must be some other planet in addition to the bodies at that time known, and that the perturbations of uranus hitherto unaccounted for, were due to the disturbances caused by the action of this unknown planet. arago urged le verrier to undertake the great problem of searching for this body, whose theoretical existence seemed demonstrated. but the conditions of the search were such that it must needs be conducted on principles wholly different from any search which had ever before been undertaken for a celestial object. for this was not a case in which mere survey with a telescope might be expected to lead to the discovery. certain facts might be immediately presumed with reference to the unknown object. there could be no doubt that the unknown disturber of uranus must be a large body with a mass far exceeding that of the earth. it was certain, however, that it must be so distant that it could only appear from our point of view as a very small object. uranus itself lay beyond the range, or almost beyond the range, of unassisted vision. it could be shown that the planet by which the disturbance was produced revolved in an orbit which must lie outside that of uranus. it seemed thus certain that the planet could not be a body visible to the unaided eye. indeed, had it been at all conspicuous its planetary character would doubtless have been detected ages ago. the unknown body must therefore be a planet which would have to be sought for by telescopic aid. there is, of course, a profound physical difference between a planet and a star, for the star is a luminous sun, and the planet is merely a dark body, rendered visible by the sunlight which falls upon it. notwithstanding that a star is a sun thousands of times larger than the planet and millions of times more remote, yet it is a singular fact that telescopic planets possess an illusory resemblance to the stars among which their course happens to lie. so far as actual appearance goes, there is indeed only one criterion by which a planet of this kind can be discriminated from a star. if the planet be large enough the telescope will show that it possesses a disc, and has a visible and measurable circular outline. this feature a star does not exhibit. the stars are indeed so remote that no matter how large they may be intrinsically, they only exhibit radiant points of light, which the utmost powers of the telescope fail to magnify into objects with an appreciable diameter. the older and well-known planets, such as jupiter and mars, possess discs, which, though not visible to the unaided eye, were clearly enough discernible with the slightest telescopic power. but a very remote planet like uranus, though it possessed a disc large enough to be quickly appreciated by the consummate observing skill of herschel, was nevertheless so stellar in its appearance, that it had been observed no fewer than seventeen times by experienced astronomers prior to herschel. in each case the planetary nature of the object had been overlooked, and it had been taken for granted that it was a star. it presented no difference which was sufficient to arrest attention. as the unknown body by which uranus was disturbed was certainly much more remote than uranus, it seemed to be certain that though it might show a disc perceptible to very close inspection, yet that the disc must be so minute as not to be detected except with extreme care. in other words, it seemed probable that the body which was to be sought for could not readily be discriminated from a small star, to which class of object it bore a superficial resemblance, though, as a matter of fact, there was the profoundest difference between the two bodies. there are on the heavens many hundreds of thousands of stars, and the problem of identifying the planet, if indeed it should lie among these stars, seemed a very complex matter. of course it is the abundant presence of the stars which causes the difficulty. if the stars could have been got rid of, a sweep over the heavens would at once disclose all the planets which are bright enough to be visible with the telescopic power employed. it is the fortuitous resemblance of the planet to the stars which enables it to escape detection. to discriminate the planet among stars everywhere in the sky would be almost impossible. if, however, some method could be devised for localizing that precise region in which the planet's existence might be presumed, then the search could be undertaken with some prospect of success. to a certain extent the problem of localizing the region on the sky in which the planet might be expected admitted of an immediate limitation. it is known that all the planets, or perhaps i ought rather to say, all the great planets, confine their movements to a certain zone around the heavens. this zone extends some way on either side of that line called the ecliptic in which the earth pursues its journey around the sun. it was therefore to be inferred that the new planet need not be sought for outside this zone. it is obvious that this consideration at once reduces the area to be scrutinized to a small fraction of the entire heavens. but even within the zone thus defined there are many thousands of stars. it would seem a hopeless task to detect the new planet unless some further limitation to its position could be assigned. it was accordingly suggested to le verrier that he should endeavour to discover in what particular part of the strip of the celestial sphere which we have indicated the search for the unknown planet should be instituted. the materials available to the mathematician for the solution of this problem were to be derived solely from the discrepancies between the calculated places in which uranus should be found, taking into account the known causes of disturbance, and the actual places in which observation had shown the planet to exist. here was indeed an unprecedented problem, and one of extraordinary difficulty. le verrier, however, faced it, and, to the astonishment of the world, succeeded in carrying it through to a brilliant solution. we cannot here attempt to enter into any account of the mathematical investigations that were necessary. all that we can do is to give a general indication of the method which had to be adopted. let us suppose that a planet is revolving outside uranus, at a distance which is suggested by the several distances at which the other planets are dispersed around the sun. let us assume that this outer planet has started on its course, in a prescribed path, and that it has a certain mass. it will, of course, disturb the motion of uranus, and in consequence of that disturbance uranus will follow a path the nature of which can be determined by calculation. it will, however, generally be found that the path so ascertained does not tally with the actual path which observations have indicated for uranus. this demonstrates that the assumed circumstances of the unknown planet must be in some respects erroneous, and the astronomer commences afresh with an amended orbit. at last after many trials, le verrier ascertained that, by assuming a certain size, shape, and position for the unknown planet's orbit, and a certain value for the mass of the hypothetical body, it would be possible to account for the observed disturbances of uranus. gradually it became clear to the perception of this consummate mathematician, not only that the difficulties in the movements of uranus could be thus explained, but that no other explanation need be sought for. it accordingly appeared that a planet possessing the mass which he had assigned, and moving in the orbit which his calculations had indicated, must indeed exist, though no eye had ever beheld any such body. here was, indeed, an astonishing result. the mathematician sitting at his desk, by studying the observations which had been supplied to him of one planet, is able to discover the existence of another planet, and even to assign the very position which it must occupy, ere ever the telescope is invoked for its discovery. thus it was that the calculations of le verrier narrowed greatly the area to be scrutinised in the telescopic search which was presently to be instituted. it was already known, as we have just pointed out, that the planet must lie somewhere on the ecliptic. the french mathematician had now further indicated the spot on the ecliptic at which, according to his calculations, the planet must actually be found. and now for an episode in this history which will be celebrated so long as science shall endure. it is nothing less than the telescopic confirmation of the existence of this new planet, which had previously been indicated only by mathematical calculation. le verrier had not himself the instruments necessary for studying the heavens, nor did he possess the skill of the practical astronomer. he, therefore, wrote to dr. galle, of the observatory at berlin, requesting him to undertake a telescopic search for the new planet in the vicinity which the mathematical calculation had indicated for the whereabouts of the planet at that particular time. le verrier added that he thought the planet ought to admit of being recognised by the possession of a disc sufficiently definite to mark the distinction between it and the surrounding stars. it was the rd september, , when the request from le verrier reached the berlin observatory, and the night was clear, so that the memorable search was made on the same evening. the investigation was facilitated by the circumstance that a diligent observer had recently compiled elaborate star maps for certain tracts of the heavens lying in a sufficiently wide zone on both sides of the equator. these maps were as yet only partially complete, but it happened that hora. xxi., which included the very spot which le verrier's results referred to, had been just issued. dr. galle had thus before his, eyes a chart of all the stars which were visible in that part of the heavens at the time when the map was made. the advantage of such an assistance to the search could hardly be over-estimated. it at once gave the astronomer another method of recognising the planet besides that afforded by its possible possession of a disc. for as the planet was a moving body, it would not have been in the same place relatively to the stars at the time when the map was constructed, as it occupied some years later when the search was being made. if the body should be situated in the spot which le verrier's calculations indicated in the autumn of , then it might be regarded as certain that it would not be found in that same place on a map drawn some years previously. the search to be undertaken consisted in a comparison made point by point between the bodies shown on the map, and those stars in the sky which dr. galle's telescope revealed. in the course of this comparison it presently appeared that a star-like object of the eighth magnitude, which was quite a conspicuous body in the telescope, was not represented in the map. this at once attracted the earnest attention of the astronomer, and raised his hopes that here was indeed the planet. nor were these hopes destined to be disappointed. it could not be supposed that a star of the eighth magnitude would have been overlooked in the preparation of a chart whereon stars of many lower degrees of brightness were set down. one other supposition was of course conceivable. it might have been that this suspicious object belonged to the class of variables, for there are many such stars whose brightness fluctuates, and if it had happened that the map was constructed at a time when the star in question had but feeble brilliance, it might have escaped notice. it is also well known that sometimes new stars suddenly develop, so that the possibility that what dr. galle saw should have been a variable star or should have been a totally new star had to be provided against. fortunately a test was immediately available to decide whether the new object was indeed the long sought for planet, or whether it was a star of one of the two classes to which i have just referred. a star remains fixed, but a planet is in motion. no doubt when a planet lies at the distance at which this new planet was believed to be situated, its apparent motion would be so slow that it would not be easy to detect any change in the course of a single night's observation. dr. galle, however, addressed himself with much skill to the examination of the place of the new body. even in the course of the night he thought he detected slight movements, and he awaited with much anxiety the renewal of his observations on the subsequent evenings. his suspicions as to the movement of the body were then amply confirmed, and the planetary nature of the new object was thus unmistakably detected. great indeed was the admiration of the scientific world at this superb triumph. here was a mighty planet whose very existence was revealed by the indications afforded by refined mathematical calculation. at once the name of le verrier, already known to those conversant with the more profound branches of astronomy, became everywhere celebrated. it soon, however, appeared, that the fame belonging to this great achievement had to be shared between le verrier and another astronomer, j. c. adams, of cambridge. in our chapter on this great english mathematician we shall describe the manner in which he was independently led to the same discovery. directly the planetary nature of the newly-discovered body had been established, the great observatories naturally included this additional member of the solar system in their working lists, so that day after day its place was carefully determined. when sufficient time had elapsed the shape and position of the orbit of the body became known. of course, it need hardly be said that observations applied to the planet itself must necessarily provide a far more accurate method of determining the path which it follows, than would be possible to le verrier, when all he had to base his calculations upon was the influence of the planet reflected, so to speak, from uranus. it may be noted that the true elements of the planet, when revealed by direct observation, showed that there was a considerable discrepancy between the track of the planet which le verrier had announced, and that which the planet was actually found to pursue. the name of the newly-discovered body had next to be considered. as the older members of the system were already known by the same names as great heathen divinities, it was obvious that some similar source should be invoked for a suggestion as to a name for the most recent planet. the fact that this body was so remote in the depths of space, not unnaturally suggested the name "neptune." such is accordingly the accepted designation of that mighty globe which revolves in the track that at present seems to trace out the frontiers of our system. le verrier attained so much fame by this discovery, that when, in , arago's place had to be filled at the head of the great paris observatory, it was universally felt that the discoverer of neptune was the suitable man to assume the office which corresponds in france to that of the astronomer royal in england. it was true that the work of the astronomical mathematician had hitherto been of an abstract character. his discoveries had been made at his desk and not in the observatory, and he had no practical acquaintance with the use of astronomical instruments. however, he threw himself into the technical duties of the observatory with vigour and determination. he endeavoured to inspire the officers of the establishment with enthusiasm for that systematic work which is so necessary for the accomplishment of useful astronomical research. it must, however, be admitted that le verrier was not gifted with those natural qualities which would make him adapted for the successful administration of such an establishment. unfortunately disputes arose between the director and his staff. at last the difficulties of the situation became so great that the only possible solution was to supersede le verrier, and he was accordingly obliged to retire. he was succeeded in his high office by another eminent mathematician, m. delaunay, only less distinguished than le verrier himself. relieved of his official duties, le verrier returned to the mathematics he loved. in his non-official capacity he continued to work with the greatest ardour at his researches on the movements of the planets. after the death of m. delaunay, who was accidentally drowned in , le verrier was restored to the directorship of the observatory, and he continued to hold the office until his death. the nature of the researches to which the life of le verrier was subsequently devoted are not such as admit of description in a general sketch like this, where the language, and still less the symbols, of mathematics could not be suitably introduced. it may, however, be said in general that he was particularly engaged with the study of the effects produced on the movements of the planets by their mutual attractions. the importance of this work to astronomy consists, to a considerable extent, in the fact that by such calculations we are enabled to prepare tables by which the places of the different heavenly bodies can be predicted for our almanacs. to this task le verrier devoted himself, and the amount of work he has accomplished would perhaps have been deemed impossible had it not been actually done. the superb success which had attended le verrier's efforts to explain the cause of the perturbations of uranus, naturally led this wonderful computer to look for a similar explanation of certain other irregularities in planetary movements. to a large extent he succeeded in showing how the movements of each of the great planets could be satisfactorily accounted for by the influence of the attractions of the other bodies of the same class. one circumstance in connection with these investigations is sufficiently noteworthy to require a few words here. just as at the opening of his career, le verrier had discovered that uranus, the outermost planet of the then known system, exhibited the influence of an unknown external body, so now it appeared to him that mercury, the innermost body of our system, was also subjected to some disturbances, which could not be satisfactorily accounted for as consequences of any known agents of attraction. the ellipse in which mercury revolved was animated by a slow movement, which caused it to revolve in its plane. it appeared to le verrier that this displacement was incapable of explanation by the action of any of the known bodies of our system. he was, therefore, induced to try whether he could not determine from the disturbances of mercury the existence of some other planet, at present unknown, which revolved inside the orbit of the known planet. theory seemed to indicate that the observed alteration in the track of the planet could be thus accounted for. he naturally desired to obtain telescopic confirmation which might verify the existence of such a body in the same way as dr. galle verified the existence of neptune. if there were, indeed, an intramercurial planet, then it must occasionally cross between the earth and the sun, and might now and then be expected to be witnessed in the actual act of transit. so confident did le verrier feel in the existence of such a body that an observation of a dark object in transit, by lescarbault on th march, , was believed by the mathematician to be the object which his theory indicated. le verrier also thought it likely that another transit of the same object would be seen in march, . nothing of the kind was, however, witnessed, notwithstanding that an assiduous watch was kept, and the explanation of the change in mercury's orbit must, therefore, be regarded as still to be sought for. le verrier naturally received every honour that could be bestowed upon a man of science. the latter part of his life was passed during the most troubled period of modern french history. he was a supporter of the imperial dynasty, and during the commune he experienced much anxiety; indeed, at one time grave fears were entertained for his personal safety. early in his health, which had been gradually failing for some years, began to give way. he appeared to rally somewhat in the summer, but in september he sank rapidly, and died on sunday, the rd of that month. his remains were borne to the cemetery on mont parnasse in a public funeral. among his pallbearers were leading men of science, from other countries as well as france, and the memorial discourses pronounced at the grave expressed their admiration of his talents and of the greatness of the services he had rendered to science. adams. the illustrious mathematician who, among englishmen, at all events, was second only to newton by his discoveries in theoretical astronomy, was born on june the th, , at the farmhouse of lidcot, seven miles from launceston, in cornwall. his early education was imparted under the guidance of the rev. john couch grylls, a first cousin of his mother. he appears to have received an education of the ordinary school type in classics and mathematics, but his leisure hours were largely devoted to studying what astronomical books he could find in the library of the mechanics' institute at devonport. he was twenty years old when he entered st. john's college, cambridge. his career in the university was one of almost unparalleled distinction, and it is recorded that his answering at the wranglership examination, where he came out at the head of the list in , was so high that he received more than double the marks awarded to the second wrangler. among the papers found after his death was the following memorandum, dated july the rd, : "formed a design at the beginning of this week of investigating, as soon as possible after taking my degree, the irregularities in the motion of uranus, which are as yet unaccounted for, in order to find whether they may be attributed to the action of an undiscovered planet beyond it; and, if possible, thence to determine the elements of its orbit approximately, which would lead probably to its discovery." after he had taken his degree, and had thus obtained a little relaxation from the lines within which his studies had previously been necessarily confined, adams devoted himself to the study of the perturbations of uranus, in accordance with the resolve which we have just seen that he formed while he was still an undergraduate. as a first attempt he made the supposition that there might be a planet exterior to uranus, at a distance which was double that of uranus from the sun. having completed his calculation as to the effect which such a hypothetical planet might exercise upon the movement of uranus, he came to the conclusion that it would be quite possible to account completely for the unexplained difficulties by the action of an exterior planet, if only that planet were of adequate size and had its orbit properly placed. it was necessary, however, to follow up the problem more precisely, and accordingly an application was made through professor challis, the director of the cambridge observatory, to the astronomer royal, with the object of obtaining from the observations made at greenwich observatory more accurate values for the disturbances suffered by uranus. basing his work on the more precise materials thus available, adams undertook his calculations anew, and at last, with his completed results, he called at greenwich observatory on october the st, . he there left for the astronomer royal a paper which contained the results at which he had arrived for the mass and the mean distance of the hypothetical planet as well as the other elements necessary for calculating its exact position. [plate: john couch adams.] as we have seen in the preceding chapter, le verrier had been also investigating the same problem. the place which le verrier assigned to the hypothetical disturbing planet for the beginning of the year , was within a degree of that to which adams's computations pointed, and which he had communicated to the astronomer royal seven months before le verrier's work appeared. on july the th, , professor challis commenced to search for the unknown object with the northumberland telescope belonging to the cambridge observatory. he confined his attention to a limited region in the heavens, extending around that point to which mr. adams' calculations pointed. the relative places of all the stars, or rather star-like objects within this area, were to be carefully measured. when the same observations were repeated a week or two later, then the distances of the several pairs of stars from each other would be found unaltered, but any planet which happened to lie among the objects measured would disclose its existence by the alterations in distance due to its motion in the interval. this method of search, though no doubt it must ultimately have proved successful, was necessarily a very tedious one, but to professor challis, unfortunately, no other method was available. thus it happened that, though challis commenced his search at cambridge two months earlier than galle at berlin, yet, as we have already explained, the possession of accurate star-maps by dr. galle enabled him to discover the planet on the very first night that he looked for it. the rival claims of adams and le verrier to the discovery of neptune, or rather, we should say, the claims put forward by their respective champions, for neither of the illustrious investigators themselves condescended to enter into the personal aspect of the question, need not be further discussed here. the main points of the controversy have been long since settled, and we cannot do better than quote the words of sir john herschel when he addressed the royal astronomical society in :-- "as genius and destiny have joined the names of le verrier and adams, i shall by no means put them asunder; nor will they ever be pronounced apart so long as language shall celebrate the triumphs of science in her sublimest walks. on the great discovery of neptune, which may be said to have surpassed, by intelligible and legitimate means, the wildest pretensions of clairvoyance, it would now be quite superfluous for me to dilate. that glorious event and the steps which led to it, and the various lights in which it has been placed, are already familiar to every one having the least tincture of science. i will only add that as there is not, nor henceforth ever can be, the slightest rivalry on the subject between these two illustrious men--as they have met as brothers, and as such will, i trust, ever regard each other--we have made, we could make, no distinction between then, on this occasion. may they both long adorn and augment our science, and add to their own fame already so high and pure, by fresh achievements." adams was elected a fellow of st. john's college, cambridge, in ; but as he did not take holy orders, his fellowship, in accordance with the rules then existing came to an end in . in the following year he was, however, elected to a fellowship at pembroke college, which he retained until the end of his life. in he was appointed professor of mathematics in the university of st. andrews, but his residence in the north was only a brief one, for in the same year he was recalled to cambridge as lowndean professor of astronomy and geometry, in succession to peacock. in challis retired from the directorship of the cambridge observatory, and adams was appointed to succeed him. the discovery of neptune was a brilliant inauguration of the astronomical career of adams. he worked at, and wrote upon, the theory of the motions of biela's comet; he made important corrections to the theory of saturn; he investigated the mass of uranus, a subject in which he was naturally interested from its importance in the theory of neptune; he also improved the methods of computing the orbits of double stars. but all these must be regarded as his minor labours, for next to the discovery of neptune the fame of adams mainly rests on his researches upon certain movements of the moon, and upon the november meteors. the periodic time of the moon is the interval required for one circuit of its orbit. this interval is known with accuracy at the present day, and by means of the ancient eclipses the period of the moon's revolution two thousand years ago can be also ascertained. it had been discovered by halley that the period which the moon requires to accomplish each of its revolutions around the earth has been steadily, though no doubt slowly, diminishing. the change thus produced is not appreciable when only small intervals of time are considered, but it becomes appreciable when we have to deal with intervals of thousands of years. the actual effect which is produced by the lunar acceleration, for so this phenomenon is called, may be thus estimated. if we suppose that the moon had, throughout the ages, revolved around the earth in precisely the same periodic time which it has at present, and if from this assumption we calculate back to find where the moon must have been about two thousand years ago, we obtain a position which the ancient eclipses show to be different from that in which the moon was actually situated. the interval between the position in which the moon would have been found two thousand years ago if there had been no acceleration, and the position in which the moon was actually placed, amounts to about a degree, that is to say, to an arc on the heavens which is twice the moon's apparent diameter. if no other bodies save the earth and the moon were present in the universe, it seems certain that the motion of the moon would never have exhibited this acceleration. in such a simple case as that which i have supposed the orbit of the moon would have remained for ever absolutely unchanged. it is, however, well known that the presence of the sun exerts a disturbing influence upon the movements of the moon. in each revolution our satellite is continually drawn aside by the action of the sun from the place which it would otherwise have occupied. these irregularities are known as the perturbations of the lunar orbit, they have long been studied, and the majority of them have been satisfactorily accounted for. it seems, however, to those who first investigated the question that the phenomenon of the lunar acceleration could not be explained as a consequence of solar perturbation, and, as no other agent competent to produce such effects was recognised by astronomers, the lunar acceleration presented an unsolved enigma. at the end of the last century the illustrious french mathematician laplace undertook a new investigation of the famous problem, and was rewarded with a success which for a long time appeared to be quite complete. let us suppose that the moon lies directly between the earth and the sun, then both earth and moon are pulled towards the sun by the solar attraction; as, however, the moon is the nearer of the two bodies to the attracting centre it is pulled the more energetically, and consequently there is an increase in the distance between the earth and the moon. similarly when the moon happens to lie on the other side of the earth, so that the earth is interposed directly between the moon and the sun, the solar attraction exerted upon the earth is more powerful than the same influence upon the moon. consequently in this case, also, the distance of the moon from the earth is increased by the solar disturbance. these instances will illustrate the general truth, that, as one of the consequences of the disturbing influence exerted by the sun upon the earth-moon system, there is an increase in the dimensions of the average orbit which the moon describes around the earth. as the time required by the moon to accomplish a journey round the earth depends upon its distance from the earth, it follows that among the influences of the sun upon the moon there must be an enlargement of the periodic time, from what it would have been had there been no solar disturbing action. this was known long before the time of laplace, but it did not directly convey any explanation of the lunar acceleration. it no doubt amounted to the assertion that the moon's periodic time was slightly augmented by the disturbance, but it did not give any grounds for suspecting that there was a continuous change in progress. it was, however, apparent that the periodic time was connected with the solar disturbance, so that, if there were any alteration in the amount of the sun's disturbing effect, there must be a corresponding alteration in the moon's periodic time. laplace, therefore, perceived that, if he could discover any continuous change in the ability of the sun for disturbing the moon, he would then have accounted for a continuous change in the moon's periodic time, and that thus an explanation of the long-vexed question of the lunar acceleration might be forthcoming. the capability of the sun for disturbing the earth-moon system is obviously connected with the distance of the earth from the sun. if the earth moved in an orbit which underwent no change whatever, then the efficiency of the sun as a disturbing agent would not undergo any change of the kind which was sought for. but if there were any alteration in the shape or size of the earth's orbit, then that might involve such changes in the distance between the earth and the sun as would possibly afford the desired agent for producing the observed lunar effect. it is known that the earth revolves in an orbit which, though nearly circular, is strictly an ellipse. if the earth were the only planet revolving around the sun then that ellipse would remain unaltered from age to age. the earth is, however, only one of a large number of planets which circulate around the great luminary, and are guided and controlled by his supreme attracting power. these planets mutually attract each other, and in consequence of their mutual attractions the orbits of the planets are disturbed from the simple elliptic form which they would otherwise possess. the movement of the earth, for instance, is not, strictly speaking, performed in an elliptical orbit. we may, however, regard it as revolving in an ellipse provided we admit that the ellipse is itself in slow motion. it is a remarkable characteristic of the disturbing effects of the planets that the ellipse in which the earth is at any moment moving always retains the same length; that is to say, its longest diameter is invariable. in all other respects the ellipse is continually changing. it alters its position, it changes its plane, and, most important of all, it changes its eccentricity. thus, from age to age the shape of the track which the earth describes may at one time be growing more nearly a circle, or at another time may be departing more widely from a circle. these alterations are very small in amount, and they take place with extreme slowness, but they are in incessant progress, and their amount admits of being accurately calculated. at the present time, and for thousands of years past, as well as for thousands of years to come, the eccentricity of the earth's orbit is diminishing, and consequently the orbit described by the earth each year is becoming more nearly circular. we must, however, remember that under all circumstances the length of the longest axis of the ellipse is unaltered, and consequently the size of the track which the earth describes around the sun is gradually increasing. in other words, it may be said that during the present ages the average distance between the earth and the sun is waxing greater in consequence of the perturbations which the earth experiences from the attraction of the other planets. we have, however, already seen that the efficiency of the solar attraction for disturbing the moon's movement depends on the distance between the earth and the sun. as therefore the average distance between the earth and the sun is increasing, at all events during the thousands of years over which our observations extend, it follows that the ability of the sun for disturbing the moon must be gradually diminishing. [plate: cambridge observatory.] it has been pointed out that, in consequence of the solar disturbance, the orbit of the moon must be some what enlarged. as it now appears that the solar disturbance is on the whole declining, it follows that the orbit of the moon, which has to be adjusted relatively to the average value of the solar disturbance, must also be gradually declining. in other words, the moon must be approaching nearer to the earth in consequence of the alterations in the eccentricity of the earth's orbit produced by the attraction of the other planets. it is true that the change in the moon's position thus arising is an extremely small one, and the consequent effect in accelerating the moon's motion is but very slight. it is in fact almost imperceptible, except when great periods of time are involved. laplace undertook a calculation on this subject. he knew what the efficiency of the planets in altering the dimensions of the earth's orbit amounted to; from this he was able to determine the changes that would be propagated into the motion of the moon. thus he ascertained, or at all events thought he had ascertained, that the acceleration of the moon's motion, as it had been inferred from the observations of the ancient eclipses which have been handed down to us, could be completely accounted for as a consequence of planetary perturbation. this was regarded as a great scientific triumph. our belief in the universality of the law of gravitation would, in fact, have been seriously challenged unless some explanation of the lunar acceleration had been forthcoming. for about fifty years no one questioned the truth of laplace's investigation. when a mathematician of his eminence had rendered an explanation of the remarkable facts of observation which seemed so complete, it is not surprising that there should have been but little temptation to doubt it. on undertaking a new calculation of the same question, professor adams found that laplace had not pursued this approximation sufficiently far, and that consequently there was a considerable error in the result of his analysis. adams, it must be observed, did not impugn the value of the lunar acceleration which halley had deduced from the observations, but what he did show was, that the calculation by which laplace thought he had provided an explanation of this acceleration was erroneous. adams, in fact, proved that the planetary influence which laplace had detected only possessed about half the efficiency which the great french mathematician had attributed to it. there were not wanting illustrious mathematicians who came forward to defend the calculations of laplace. they computed the question anew and arrived at results practically coincident with those he had given. on the other hand certain distinguished mathematicians at home and abroad verified the results of adams. the issue was merely a mathematical one. it had only one correct solution. gradually it appeared that those who opposed adams presented a number of different solutions, all of them discordant with his, and, usually, discordant with each other. adams showed distinctly where each of these investigators had fallen into error, and at last it became universally admitted that the cambridge professor had corrected laplace in a very fundamental point of astronomical theory. though it was desirable to have learned the truth, yet the breach between observation and calculation which laplace was believed to have closed thus became reopened. laplace's investigation, had it been correct, would have exactly explained the observed facts. it was, however, now shown that his solution was not correct, and that the lunar acceleration, when strictly calculated as a consequence of solar perturbations, only produced about half the effect which was wanted to explain the ancient eclipses completely. it now seems certain that there is no means of accounting for the lunar acceleration as a direct consequence of the laws of gravitation, if we suppose, as we have been in the habit of supposing, that the members of the solar system concerned may be regarded as rigid particles. it has, however, been suggested that another explanation of a very interesting kind may be forthcoming, and this we must endeavour to set forth. it will be remembered that we have to explain why the period of revolution of the moon is now shorter than it used to be. if we imagine the length of the period to be expressed in terms of days and fractions of a day, that is to say, in terms of the rotations of the earth around its axis, then the difficulty encountered is, that the moon now requires for each of its revolutions around the earth rather a smaller number of rotations of the earth around its axis than used formerly to be the case. of course this may be explained by the fact that the moon is now moving more swiftly than of yore, but it is obvious that an explanation of quite a different kind might be conceivable. the moon may be moving just at the same pace as ever, but the length of the day may be increasing. if the length of the day is increasing, then, of course, a smaller number of days will be required for the moon to perform each revolution even though the moon's period was itself really unchanged. it would, therefore, seem as if the phenomenon known as the lunar acceleration is the result of the two causes. the first of these is that discovered by laplace, though its value was over-estimated by him, in which the perturbations of the earth by the planets indirectly affect the motion of the moon. the remaining part of the acceleration of our satellite is apparent rather than real, it is not that the moon is moving more quickly, but that our time-piece, the earth, is revolving more slowly, and is thus actually losing time. it is interesting to note that we can detect a physical explanation for the apparent checking of the earth's motion which is thus manifested. the tides which ebb and flow on the earth exert a brake-like action on the revolving globe, and there can be no doubt that they are gradually reducing its speed, and thus lengthening the day. it has accordingly been suggested that it is this action of the tides which produces the supplementary effect necessary to complete the physical explanation of the lunar acceleration, though it would perhaps be a little premature to assert that this has been fully demonstrated. the third of professor adams' most notable achievements was connected with the great shower of november meteors which astonished the world in . this splendid display concentrated the attention of astronomers on the theory of the movements of the little objects by which the display was produced. for the definite discovery of the track in which these bodies revolve, we are indebted to the labours of professor adams, who, by a brilliant piece of mathematical work, completed the edifice whose foundations had been laid by professor newton, of yale, and other astronomers. meteors revolve around the sun in a vast swarm, every individual member of which pursues an orbit in accordance with the well-known laws of kepler. in order to understand the movements of these objects, to account satisfactorily for their periodic recurrence, and to predict the times of their appearance, it became necessary to learn the size and the shape of the track which the swarm followed, as well as the position which it occupied. certain features of the track could no doubt be readily assigned. the fact that the shower recurs on one particular day of the year, viz., november th, defines one point through which the orbit must pass. the position on the heavens of the radiant point from which the meteors appear to diverge, gives another element in the track. the sun must of course be situated at the focus, so that only one further piece of information, namely, the periodic time, will be necessary to complete our knowledge of the movements of the system. professor h. newton, of yale, had shown that the choice of possible orbits for the meteoric swarm is limited to five. there is, first, the great ellipse in which we now know the meteors revolve once every thirty three and one quarter years. there is next an orbit of a nearly circular kind in which the periodic time would be a little more than a year. there is a similar track in which the periodic time would be a few days short of a year, while two other smaller orbits would also be conceivable. professor newton had pointed out a test by which it would be possible to select the true orbit, which we know must be one or other of these five. the mathematical difficulties which attended the application of this test were no doubt great, but they did not baffle professor adams. there is a continuous advance in the date of this meteoric shower. the meteors now cross our track at the point occupied by the earth on november th, but this point is gradually altering. the only influence known to us which could account for the continuous change in the plane of the meteor's orbit arises from the attraction of the various planets. the problem to be solved may therefore be attacked in this manner. a specified amount of change in the plane of the orbit of the meteors is known to arise, and the changes which ought to result from the attraction of the planets can be computed for each of the five possible orbits, in one of which it is certain that the meteors must revolve. professor adams undertook the work. its difficulty principally arises from the high eccentricity of the largest of the orbits, which renders the more ordinary methods of calculation inapplicable. after some months of arduous labour the work was completed, and in april, , adams announced his solution of the problem. he showed that if the meteors revolved in the largest of the five orbits, with the periodic time of thirty three and one quarter years, the perturbations of jupiter would account for a change to the extent of twenty minutes of arc in the point in which the orbit crosses the earth's track. the attraction of saturn would augment this by seven minutes, and uranus would add one minute more, while the influence of the earth and of the other planets would be inappreciable. the accumulated effect is thus twenty-eight minutes, which is practically coincident with the observed value as determined by professor newton from an examination of all the showers of which there is any historical record. having thus showed that the great orbit was a possible path for the meteors, adams next proved that no one of the other four orbits would be disturbed in the same manner. indeed, it appeared that not half the observed amount of change could arise in any orbit except in that one with the long period. thus was brought to completion the interesting research which demonstrated the true relation of the meteor swarm to the solar system. besides those memorable scientific labours with which his attention was so largely engaged, professor adams found time for much other study. he occasionally allowed himself to undertake as a relaxation some pieces of numerical calculation, so tremendously long that we can only look on them with astonishment. he has calculated certain important mathematical constants accurately to more than two hundred places of decimals. he was a diligent reader of works on history, geology, and botany, and his arduous labours were often beguiled by novels, of which, like many other great men, he was very fond. he had also the taste of a collector, and he brought together about eight hundred volumes of early printed works, many of considerable rarity and value. as to his personal character, i may quote the words of dr. glaisher when he says, "strangers who first met him were invariably struck by his simple and unaffected manner. he was a delightful companion, always cheerful and genial, showing in society but few traces of his really shy and retiring disposition. his nature was sympathetic and generous, and in few men have the moral and intellectual qualities been more perfectly balanced." in he married the daughter of haliday bruce, esq., of dublin and up to the close of his life he lived at the cambridge observatory, pursuing his mathematical work and enjoying the society of his friends. he died, after a long illness, on st january, , and was interred in st. giles's cemetery, on the huntingdon road, cambridge. [illustration: messier; messier] the plurality of worlds. on nature's alps i stand, and see a thousand firmaments beneath! a thousand systems, as a thousand grains! so much a stranger, _and so late arrived_, how shall man's curious spirit not inquire what are the natives of this world sublime, of this so distant, unterrestrial sphere, where mortal, untranslated, never strayed? night thoughts. with an introduction by edward hitchcock, d.d., president of amherst college, and professor of theology and geology. boston: gould and lincoln, washington street. . entered, according to act of congress, in the year , by gould and lincoln, in the clerk's office of the district court of the district of massachusetts. preface. although the opinions presented in the following essay are put forwards without claiming for them any value beyond what they may derive from the arguments there offered, they are not published without some fear of giving offence. it will be a curious, but not a very wonderful event, if it should now be deemed as blamable to doubt the existence of inhabitants of the planets and stars, as, three centuries ago, it was held heretical to teach that doctrine. yet probably there are many who will be willing to see the question examined by all the light which modern science can throw upon it; and such an examination can be undertaken to no purpose, except the view which has of late been generally rejected have the arguments in its favor fairly stated and candidly considered. though revealed religion contains no doctrine relative to the inhabitants of planets and stars; and though, till within the last three centuries, no christian thinker deemed such a doctrine to be required, in order to complete our view of the attributes of the creator; yet it is possible that at the present day, when the assumption of such inhabitants is very generally made and assented to, many persons have so mingled this assumption with their religious belief, that they regard it as an essential part of natural religion. if any such persons find their religious convictions interfered with, and their consolatory impressions disturbed, by what is said in this essay, the author will deeply regret to have had any share in troubling any current of pious thought belonging to the time. but, as some excuse, it may be recollected, that if such considerations had prevailed, this very doctrine, of the plurality of worlds, would never have been publicly maintained. and if such considerations are to have weight, it must be recollected, on the other hand, that there are many persons to whom the assumption of an endless multitude of worlds appears difficult to reconcile with the belief of that which, as the christian revelation teaches us, has been done for this our world of earth. in this conflict of religious difficulties, on a point which rather belongs to science than to religion, perhaps philosophical arguments may be patiently listened to, if urged as arguments merely; and in that hope, they are here stated, without reserve and without exaggeration. all speculations on subjects in which science and religion bear upon each other, are liable to one of the two opposite charges;--that the speculator sets philosophy and religion at variance; or that he warps philosophy into a conformity with religion. it is confidently hoped that no candid reader will bring either of these charges against the present essay. with regard to the latter, the arguments must speak for themselves. to the author at least, they appear to be of no small philosophical force; though he is quite ready to weigh carefully and candidly any answers which may be offered to them. with regard to the amount of agreement between our philosophy and religion, it may perhaps be permitted to the author to say, that while it appears to him that some of his philosophical conclusions fall in very remarkably with certain points of religious doctrine, he is well aware that philosophy alone can do little in providing man with the consolations, hopes, supports, and convictions which religion offers; and he acknowledges it as a ground of deep gratitude to the author of all good, that man is not left to philosophy for those blessings; but has a fuller assurance of them, by a more direct communication from him. perhaps, too, the author may be allowed to say, that he has tried to give to the book, not only a moral, but a scientific interest; by collecting his scientific facts from the best authorities, and the most recent discoveries. he would flatter himself, in particular, that the view of the nebulæ and of the solar system, which he has here given, may be not unworthy of some attention on the part of astronomers and observers, as an occasion of future researches in the skies. contents of the plurality of worlds. page introduction. chapter i. astronomical discoveries. chapter ii. astronomical objection to religion. chapter iii. the answer from the microscope. chapter iv. further statement of the difficulty. chapter v. geology. chapter vi. the argument from geology. chapter vii. the nebulæ. chapter viii. the fixed stars. chapter ix. the planets. chapter x. theory of the solar system. chapter xi. the argument from design. chapter xii. the unity of the world. chapter xiii. the future. introductory notice to the american edition. it is an interesting feature in the literature of our day, that so many minds are turning their attention to the bearings of science upon religion. with a few honorable exceptions, christian scholars have regarded this as a most unpromising field, which they have left to the tilting and gladiatorship of scepticism. but we owe it mainly to the disclosures of geology, that the tables are beginning to be turned. for a long time suspected of being in league with infidelity, it was treated as an enemy, and christians thought only of fortifying themselves against its attacks. but they are finding out, that if this science has been seen in the enemy's camp, it was only because of their jealousy that it was compelled to remain there; like captives that are sometimes pushed forwards to cover the front rank and receive the fire of their friends. judging from the number of works, some of them very able, that appear almost monthly from the press, in which illustrations of religion are drawn from geology, we may infer that this science is beginning to be recognized by the friends of religion as an efficient auxiliary. "the plurality of worlds," now republished, is the most recent work of this description that has fallen under our notice. we can see no reason why an essay of so much ability, in which the reasoning is so dispassionate, and opponents are treated so candidly, should appear anonymously. true, the author takes ground against some opinions widely maintained respecting the extent of the inhabited universe, and seems to suppose that he shall meet with little sympathy; and this may be his reason, though in our view quite insufficient, for remaining incognito. we think he will find that there are a secret seven thousand, who never have bowed their understandings to a belief of many of the doctrines which he combats, and he might reasonably calculate that his reasoning will add seven thousand more to the number. we confess, however, that though we have long been of this number to a certain extent, we cannot go as far as this writer has done in his conclusions. all the world is acquainted with dr. chalmers' splendid astronomical discourses. assuming, or rather supposing that he has proved, that the universe contains a vast number of worlds peopled like our own, he imagines the infidel to raise an objection to the mission of the son of god, on the ground that this world is too insignificant to receive such an extraordinary interposition. his replies to this objection, drawn chiefly from our ignorance, are ingenious and convincing. but the author of the plurality of worlds doubts the premises on which the objection is founded. he thinks the facts of science will not sustain the conclusion that many of the heavenly bodies are inhabited; certainly not with moral and intellectual beings like man. nay, by making his appeal to geology, he thinks the evidence strong against such an opinion. this science shows us that this world was once certainly in a molten state, and very probably, at a still earlier date, may have been dissipated into self-luminous vapor, like the nebulæ or the comets. immense periods, then, must have passed before any organic structures, such as have since peopled the earth, could have existed. and during the vast cycles that have elapsed since the first animals and plants appeared upon the globe, it was not in a proper condition to have sustained any other than the inferior races. accordingly, it has been only a few thousand years since man appeared. now, so far as astronomy has revealed the condition of other worlds, almost all of them appear to be passing through those preparatory changes which the earth underwent previous to man's creation. what are the unresolvable nebulæ and most of the comets also, but intensely heated vapor and gas? what is the sun but a molten globe, or perhaps gaseous matter condensed so as to possess almost the density of water? the planets beyond mars, also, (excluding the asteroids,) appear to be in a liquid condition, but not from heat, and therefore may be composed of water, or some fluid perhaps lighter than water; or at least be covered by such fluid. moreover, so great is their distance from the sun, that his light and heat could not sustain organic beings such as exist upon the earth. of the inferior planets, mercury is so near the sun that it would be equally unfit for the residence of such beings. mars, venus, and the moon, then, appear to be the only worlds known to us capable of sustaining a population at all analogous to that upon earth. but of these, the moon appears to be merely a mass of extinguished volcanos, with neither water nor atmosphere. it has proceeded farther in the process of refrigeration than the earth, because it is smaller; and in its present state, is manifestly unfit for the residence either of rational or irrational creatures. so that we are left with only mars and venus in the solar system to which the common arguments in favor of other worlds being inhabited, will apply. but are not the fixed stars the suns of other systems? we will thank those who think so, to read the chapter in this work that treats of the fixed stars, and we presume they will be satisfied that at least many of these bodies exhibit characters quite irreconcilable with such an hypothesis. and if some are not central suns, the presumption that the rest are, is weakened, and we must wait till a greater perfection of instruments shall afford us some positive evidence, before we know whether our solar system is a type of any others. thus far, it seems to us, our author has firm ground, both geological and astronomical, to stand upon. but he does not stop here. he takes the position that probably our earth may be the only body in the solar system, nay in the universe, where an intellectual, moral and immortal being, like man, has an existence. he makes the "earth the domestic hearth of the solar system; adjusted between the hot and fiery haze on one side, and the cold and watery vapor on the other: the only fit region to be a domestic hearth, a seat of habitation." he says that "it is quite agreeable to analogy that the solar system should have borne but one fertile flower. and even if any number of the fixed stars were also found to be barren flowers of the sky, we need not think the powers of creation wasted, or frustrated, thrown away, or perverted." he does not deny that some other worlds may be the abodes of plants and animals such as peopled this earth during the long ages of preadamic history. but he regards the creation of man as the great event of our world. he looks upon the space between man and the highest of the irrational creatures, as a vast one: for though in physical structure they approach one another, in intellectual and moral powers they cannot be compared. he does not think it derogatory to divine wisdom to have created and arranged all the other bodies of the universe to give convenience and elegance to the abode of such a being; especially since this was to be the theatre of the work of redemption. now we sympathize strongly in views that give dignity and exaltation to man, and not at all with that debasing philosophy, so common at this day, that looks upon him as little more than a somewhat improved orang. but we cannot admit that man is the only exalted created being to be found among the vast array of worlds around us. geology does, indeed, teach us, that it is no disparagement of divine wisdom and benevolence to make a world--and if one, why not many--the residence of inferior creatures; nay to leave it without inhabitants through untold ages. but it also shows us, that when such worlds have passed through these preparatory changes, rational and immortal beings may be placed upon them. nay, does not the history of our world show us that this seems to be the grand object of such vast periods of preparation. and is it not incredible, that amid the countless bodies of the universe, a single globe only, and that a small one, should have reached the condition adapted to the residence of beings made in the image of god? of what possible use to man are those numberless worlds visible only through the most powerful telescopes? surely such a view gives us a very narrow idea of the plans and purposes of jehovah, and one not sustained in our opinion by the analogies of science. there is another principle to which our author attaches, as we think, too little importance in this connection. when we see how vast is the variety of organic beings on this globe, and how manifold the conditions of their existence; how exactly adapted they are to the solid, the liquid, and the gaseous states of matter, can we doubt that rational and intelligent beings may be adapted to physical conditions in other worlds widely diverse from those on this globe? may not spirits be connected with bodies much heavier, or much lighter, than on earth; nay, with mere tenuous ether; and those bodies, perhaps, be better adapted to the play of intellect than ours; and be unaffected by temperatures which, on earth, would be fatal? it does seem to us that such conclusions are legitimate inferences from the facts of science; and if so, we can hardly avoid the conclusion that there may be races of intelligent beings upon other worlds where the condition of things is widely different from that on earth. yet there is a limit to this principle; and when we can prove another world to be in a similar condition to our earth, when it was inhabited by preadamic races, or not at all inhabited, the presumption is strong, that such a world has inhabitants of a like character, or none at all. our author makes but a slight allusion to some most important statements of revelation, that seem to us to bear strongly upon the hypothesis which he adopts. we refer to the existence of angels, holy and unholy. in the history of the latter, we learn that _they kept not their first estate, but left their own habitation_. have we not here an example of other rational creatures, more exalted than man, who, like him, have fallen from their first estate; and does not the presumption hence arise, that there may be similar examples in other worlds? and is there not a probability, that holy angels now in heaven, may be rational intelligences who have passed a successful probation in other worlds? it does seem to us, that these biblical facts make the hypothesis of our author respecting man extremely improbable. but though we must demur as to some of the views of this work, we can cordially recommend its perusal to intelligent and reasoning minds. it is an effort in the right direction, and we think will do much to correct some false notions respecting the plurality of worlds. and even the author's peculiar hypothetical views are sustained with much ability. he states the facts of geology and astronomy with great clearness and correctness, and seems quite familiar with mathematical reasoning. nor does he advance opinions that come into collision with natural or revealed religion; though, as already stated, we think his favorite notions narrow our conceptions of the divine plans and purposes. we predict for the work an extended circulation among scientific men and theologians; and commend it with confidence to all readers--and in our country they are numerous--who are fond of tracing out the connection between science and religion. e. h. amherst college, april, . the plurality of worlds. chapter i. astronomical discoveries. "when i consider the heavens, the work of thy fingers, the moon and the stars, which thou hast ordained; what is man, that thou art mindful of him? and the son of man, that thou visitest him?" . these striking words of the hebrew psalmist have been made, by an eloquent and pious writer of our own time, the starting point of a remarkable train of speculation. dr. chalmers, in his _astronomical discourses_, has treated the reflection thus suggested, in connection with such an aspect of the heavens and the stars, the earth and the universe, as modern astronomy presents to us. even from the point of view in which the ancient hebrew looked at the stars; seeing only their number and splendor, their lofty position, and the vast space which they visibly occupy in the sky; compared with the earth, which lies dark, and mean, and perhaps small in extent, far beneath them, and on which man has his habitation; it appeared wonderful, and scarcely credible, that the maker of all that array of luminaries, the lord of that wide and magnificent domain, should occupy himself with the concerns of men: and yet, without a belief in his fatherly care and goodness to us, thoughtful and religious persons, accustomed to turn their minds constantly to a supreme governor and constant benefactor, are left in a desolate and bewildered state of feeling. the notion that while the heavens are the work of god's fingers, the sun, moon, and stars ordained by him, he is _not_ mindful of man, does not regard him, does not visit him, was not tolerable to the thought of the psalmist. while we read, we are sure that he believed that, however insignificant and mean man might be, in comparison with the other works of god,--however difficult it might seem to conceive, that he should be found worthy the regards and the visits of the creator of all,--yet that god _was_ mindful of him, and _did_ visit him. the question, "what is man, that this is so?" implies that there is an answer, whether man can discover it or not. "_what_ is man, that god is mindful of him?" indicates a belief, unshaken, however much perplexed, that man is _something_, of such a kind that god _is_ mindful of him. . but if there was room for this questioning, and cause for this perplexity, to a contemplative person, who looked at the skies, with that belief concerning the stars, which the ancient hebrew possessed, the question recurs with far greater force, and the perplexity is immeasurably increased, by the knowledge, concerning the stars, which is given to us by the discoveries of modern astronomy. the jew probably believed the earth to be a region, upon the whole, level, however diversified with hills and valleys, and the skies to be a vault arched over this level;--a firmament in which the moon and the stars were placed. what magnitude to assign to this vault, he had no means of knowing; and indeed, the very aspect of the nocturnal heavens, with the multitude of stars, of various brightness, which come into view, one set after another, as the light of day dies away, suggests rather the notion of their being scattered through a vast depth of space, at various distances, than of their being so many lights fastened to a single vaulted surface. but however he might judge of this, he regarded them as placed in a space, of which the earth was the central region. the host of heaven all had reference to the earth. the sun and the moon were there, in order to give light to it, by day and by night. and if the stars had not that for their principal office, as indeed the amount of light which they gave was not such as to encourage such a belief,--and perhaps the perception, that the stars must have been created for some other object than to give light to man, was one of the principal circumstances which suggested the train of thought that we are now considering;--yet still, the region of the stars had the earth for its centre and base. perhaps the psalmist, at a subsequent period of his contemplations, when he was pondering the reflections which he has expressed in this passage, might have been led to think that the stars were placed there in order to draw man's thoughts to the greatness of the creator of all things; to give some light to his mental, rather than to his bodily eye; to show how far his mode of working transcends man's faculties; to suggest that there are things in heaven, very different from the things which are on earth. if he thought thus, he was only following a train of thought on which contemplative minds, in all ages and countries, have often dwelt; and which we cannot, even now, pronounce to be either unfounded or exhausted; as we trust hereafter to show. but whether or not this be so, we may be certain that the psalmist regarded the stars, as things having a reference to the earth, and yet not resembling the earth; as works of god's fingers, very different from the earth with its tribes of inhabitants; as luminaries, not worlds. in the feeling of awe and perplexity, which made him ask, "what is man that thou art mindful of him?" there was no mixture of a persuasion that there were, in those luminaries, creatures, like man, the children and subjects of god; and therefore, like man, requiring his care and attention. in asking, "what is man, that thou visitest him?" there was no latent comparison, to make the question imply, "that thou visitest _him_, rather than those who dwell in those abodes?" it was the multitude and magnificence of god's works, which made it seem strange that he should care for a _thing_ so small and mean as man; not the supposed multitude of god's intelligent creatures inhabiting those works, which made it seem strange that he should attend to every _person_ upon this earth. it was not that the psalmist thought that, among a multitude of earths, all peopled like this earth, man might seem to be in danger of being overlooked and neglected by his maker; but that, there being only one earth, occupied by frail, feeble, sinful, short-lived creatures, it might be unworthy the regards of him who dwelt in regions of eternal light and splendor, unsullied by frailty, inaccessible to corruption. . this, we can have no doubt, or something resembling this, was the psalmist's view, when he made the reflection, which we have taken as the basis of our remarks. and even in this view, (which, after all that science has done, is perhaps still the most natural and familiar,) the reflection is extremely striking; and the words cannot be uttered without finding an echo in the breast of every contemplative and religious person. but this view is, as most readers at this time are aware, very different from that presented to us by modern astronomy. the discoveries made by astronomers are supposed by most persons to have proved, or to have made it in the highest degree probable, that this view of the earth, as the sole habitation of intelligent subjects of god's government; and of the stars, as placed in a region of which the earth is the centre, and yet differing in their nature from this lower world; is altogether erroneous. according to astronomers, the earth is not a level space, but a globe. some of the stars which we see in the vault of heaven, are globes, like it; some smaller than the earth, some larger. there are reasons, drawn from analogy, for believing that these globes, the other planets, are inhabited by living creatures, as the earth is. the earth is not at rest, with the celestial luminaries circulating above it, as the ancients believed, but itself moves in a circle about the sun, in the course of every year; and the other planets also move round the sun in like manner, in circles, some within and some without that which the earth describes. this collection of planets, thus circulating about the sun, is the solar system: of which the earth thus forms a very small part. jupiter and saturn are much larger than the earth. mars and venus are nearly as large. if these be inhabited, as the earth is, which the analogy of their form, movements and conditions, seems to suggest, the population of the earth is a very small portion of the population of the solar system. and if the mere number of the subjects of god's government could produce any difficulty in the application of his providence to them, a person to whom this view of the world which we inhabit had been disclosed, might well, and with far more reason than the psalmist, exclaim, "lord, what is man, that thou art mindful of him? the inhabitants of this earth, that thou regardest him?" . but this is only the first step in the asserted revelations of astronomy. some of the stars are, as we have said, planets of the kind just described. but these stars are a few only:--five, or at most six, of those visible to the unassisted eye of man. all the rest, innumerable as they appear, and numerous as they really are, are, it is found, objects of another kind. they are not, as the planets are, opaque globes, deriving their light from a sun, about which they circulate. they shine by a light of their own. they are of the nature of the sun, not of the planets. that they appear mere specks of light, arises from their being at a vast distance from us. at a vast distance they undoubtedly are; for even with our most powerful telescopes, they still appear mere specks of light;--mere luminous points. they do not, as the planets do, when seen through telescopes, exhibit to us a circular face or disk, capable of being magnified and distinguished into parts and features. but this impossibility of magnifying them by means of telescopes, does not at all make us doubt that they may be far larger than the planets. for we know, from other sources of information, that their distance is immensely greater than that of any of the planets. we can measure the bodies of the solar system;--the earth, by absolutely going round a part of it, or in other ways; the other bodies of the system, by comparing their positions, as seen from different parts of the earth. in this manner we find that the earth is a globe , miles in diameter. in this way, again, we find that the circle which the earth describes round the sun has, in round numbers, a radius about , times the earth's radius; that is, nearly a hundred millions of miles. the earth is, at one time, a hundred millions of miles on one side of the sun; and at another time, half a year afterwards, a hundred millions of miles on the other side. of the bright stars which shine by their own light,--the _fixed stars_, as we call them, (to distinguish them from the planets, the _wandering stars_,)--if any one were at any moderate distance from us, we should see it change its apparent place with regard to the others, in consequence of our thus changing our point of view two hundred millions of miles: just as a distant spire changes its apparent place with regard to the more distant mountain, when we move from one window of our house to the other. but no such change of place is discernible in any of the fixed stars: or at least, if we believe the most recent asserted discoveries of astronomers, the change is so small as to imply a distance in the star, of more than two hundred thousand times the radius of the earth's orbit, which is, itself, as we have said, one hundred millions of miles.[ ] this distance is so vastly great, that we can very well believe that the fixed stars, though to our best telescopes they appear only as points of light, are really as large as our sun, and would give as much light as he does, if we could approach as near to them. for since they are thus, the nearest of them, two hundred thousand times as far off as he is, even if we could magnify them a thousand times, which we can hardly do, they would still be only one two-hundredth of the breadth of the sun; and thus, still a mere point. . but if each fixed star be of the nature of the sun, and not smaller than the sun, does not analogy lead us to suppose that they have, some of them at least, planets circulating about them, as our sun has? if the sun is the centre of the solar system, why should not sirius, (one of the brightest of the fixed stars,) be the centre of the _sirian system_? and why should not that system have as many planets, with the same resemblances and differences of the figure, movements, and conditions of the different planets, as this? why should not the sirian system be as great and as varied as the solar system? and this being granted, why should not these planets be inhabited, as men have inferred the other planets of the solar system, as well as the earth, to be? and thus we have, added to the population of the universe of which we have already spoken, a number (so far as we have reason to believe) not inferior to the number of inhabitants of the solar system: this number being, according to all the analogies, very many fold that of the population of the whole earth? and this is the conclusion, when we reason from one star only, from sirius. but the argument is the same, from each of the stars. for we have no reason to think that sirius, though one of the brightest, is more like our sun than any of the others is. the others appear less bright in various degrees, probably because they are further removed from us in various degrees. they may not be all of the same size and brightness; it is very unlikely that they are. but they may as easily be larger than the sun, as smaller. the natural assumption for us to make, having no ground for any other opinion, is, that they are, upon the average, of the size of our sun. on that assumption, we have as many solar systems as we have fixed stars; and, it may be, six or ten, or twenty times as many inhabited globes; inhabited by creatures of whom we must suppose, by analogy, that god is mindful, if he is mindful of us. the question recurs with overwhelming force, if we still follow the same train of reflection: "what is man, that god is mindful of him?" . but we have not yet exhausted the views which thus add to the force of this reflection. the fixed stars, which appear to the eye so numerous, so innumerable, in the clear sky on a moonless night, are not really so numerous as they seem. to the naked eye, there are not visible more than four or five thousand. the astronomers of greece, and of other countries, even in ancient times, counted them, mapped them, and gave them names and designations. but astronomy, who thus began her career by diminishing, in some degree, the supposed numbers of the host of heaven, has ended by immeasurably increasing them. the first application of the telescope to the skies discovered a vast number of fixed stars, previously unseen: and every improvement in that instrument has disclosed myriads of new stars, visibly smaller than those which had before been seen; and smaller and smaller, as the power of vision is more and more strengthened by new aids from art; as if the regions of space contained an inexhaustible supply of such objects; as if infinite space were strewn with stars in every part of it to which vision could reach. the small patch of the sky which forms, at any moment, the field of view of one of the great telescopes of herschel, discloses to him as many stars, and those of as many different magnitudes, as the whole vault of the sky exhibits to the naked eye. but the magnifying power of such an instrument only discloses, it does not make, these stars. there appears to be quite as much reason to believe, that each of these telescopic stars is a sun, surrounded by its special family of planets, as to believe that sirius or arcturus is so. here, then, we have again an extension, indefinite to our apprehension, of the universe, as occupied by material structures; and if so, why not by a living population, such as the material structures which are nearest to us support? . even yet we have not finished the series of successive views which astronomers have had opened to them, extending more and more their spectacle of the fulness and largeness of the universe. not only does the telescope disclose myriads of stars, unseen to the naked eye, and new myriads with each increase of the powers of the instrument; but it discloses also patches of light, which, at first at least, do not appear to consist of stars: _nebulæ_, as they are called; bright specks, it might seem, of stellar matter, thin, diffused, and irregular; not gathered into regular and definite forms, such as we may suppose the stars to be. every one who has noticed the starry skies, may understand what is the general aspect of such nebulæ, by looking at the milky way or galaxy, an irregular band of nebulous light, which runs quite round the sky; "a circling zone, powdered with stars;" as milton calls it. but the nebulæ of which i more especially speak, are minute patches, discovered mainly by the telescope, and in a few instances only discernible by the naked eye. and what i have to remark especially concerning them at present is, that though to visual powers which barely suffice to discern them, they appear like mere bright clouds, patches of diffused starry matter; yet that, when examined by visual powers of a higher order, by more penetrating telescopes, these patches of continuous feeble light are, in many instances at least, distinguishable into definite points: they are found, in fact, to be aggregations of stars; which before appeared as diffused light, only because our telescopes, though strong enough to reveal to our senses the aggregate mass of light of the cluster, were not strong enough to enable us to discern any one of the stars of which the cluster consists. the galaxy, in this way, may, in almost every part, be _resolved_ into separate stars; and thus, the multitude of the stars in the region of the sky occupied by that winding stream of light, is, when examined by a powerful telescope, inconceivably numerous. . the small telescopic nebulæ are of various forms; some of them may be in the shape of flat strata, or cakes, as it were, of stars, of small thickness, compared with the extent of the stratum. now, if our sun were one of the individuals of such a stratum, we, looking at the stars of the stratum from his neighborhood, should see them very numerous and close in the direction of the edge of the stratum, and comparatively few and rare in other parts of the sky. we should, in short, see a galaxy running round the sky, as we see in fact. and hence sir william herschel has inferred, that our sun, with its attendant planets, has its place in such a stratum; and that it thus belongs to a host of stars which are, in a certain way, detached from the other nebulæ which we see. perhaps, he adds, some of those other nebulæ are beds and masses of stars not less numerous than those which compose our galaxy, and which occupy a larger portion of the sky, only because we are immersed in the interior of the crowd. and thus, a minute speck of nebulous light, discernible only by a good telescope, may contain not only as many stars as occupy the sky to ordinary vision, but as many as is the number into which the most powerful telescope resolves the milky light of the galaxy. and of such resolvable nebulæ the number which are discovered in the sky is very great, their forms being of the most various kind; so that many of them may be, for aught we can tell, more amply stocked with stars than the galaxy is. and if all the stars, or a large proportion of the stars, of the galaxy, be suns attended by planets, and these planets peopled with living creatures, what notion must we form of the population of the universe, when we have thus to reckon as many galaxies as there are resolvable nebulæ! the stock of discoverable nebulæ being as yet unexhausted by the powers of our telescopes; and the possibility of resolving them into stars being also an operation which has not yet been pursued to its limit. . for, (and this is the last step which i shall mention in this long series of ascending steps of multitude apparently infinite,) it now begins to be suspected that not some nebulæ only, but _all_, are resolvable into separate stars. when the nebulæ were first carefully studied, it was supposed that they consisted, as they appeared to consist, of some diffused and incoherent matter, not of definite and limited masses. it was conceived that they were not stars, but stellar matter in the course of formation into stars; and it was conceived, further, that by the gradual concentration of such matter, whirling round its centre while it concentrated, not only stars, that is, suns, might be formed, but also systems of planets, circling round these suns; and thus this _nebular hypothesis_, as it has been termed, gave a kind of theory of the origin and formation of systems, such as the solar system. but the great telescope which lord rosse has constructed, and which is much more powerful than any optical instrument yet fabricated, has been directed to many of the nebulæ, whose appearance had given rise to this theory; and the result has been, in a great number of cases, that the nebulæ are proved to consist entirely of distinct stars; and that the diffused nebulous appearance is discovered to have been an illusion, resulting from the accumulated light of a vast number of small stars near to each other. in this manner, we are led to regard every nebula, not as an imperfectly formed star or system, but as a vast multitude of stars, and, for aught we can tell, of systems; for the apparent smallness and nearness of these stars are, it is thought, mere results of the vast distance at which they are placed from us. and thus, perhaps, all the nebulæ are, what some of them seem certainly to be, so many vast armies of stars, each of which stars, we have reason to believe, is of the nature of our sun; and may have, and according to analogy has, an accompaniment of living creatures, such as our sun has, certainly on the earth, probably, it is thought, in the other planets. . it is difficult to grasp, in one view, the effect of the successive steps from number to number, from distance to distance, which we have thus been measuring over. we may, however, state them again briefly, in the way of enumeration. from our own place on the earth, we pass, in thought, as a first step, to the whole globe of the earth; from this, as a second step, to the planets, the other globes which compose the solar system. a third step carries us to the fixed stars, as visible to the naked eye; very numerous and immensely distant. the transition to the telescopic stars makes a fourth step; and in this, the number and the space are increased, almost beyond the power of numbers to express how many there are, and at what distances. but a fifth step:--perhaps all this array of stars, obvious and telescopic, only make up our nebula; while the universe is occupied by other nebulæ innumerable, so distant that, seen from them, our nebula, though including, it may be, stars of the th magnitude, which may be times or , times more remote than sirius, would become a telescopic speck, as their nebulæ are to us. . various images and modes of representation have been employed, in order to convey to the mind some notion of the dimensions of the scheme of the universe to which we are thus introduced. thus, we may reckon that a cannon-ball, moving with its usual original velocity unabated, would describe the interval between the sun and the earth in about one year. and this being so, the same missile would, from what has been said, occupy more, we know not how much more, than , years in going to the nearest fixed star: and perhaps a thousand times as much, in going to other stars belonging to our group; and then again, , times so much, or some number of the like order, in going from one group to another. when we have advanced a step or two in this mode of statement, the velocity of the cannon-ball hardly perceptibly affects the magnitude of the numbers which we have to use. and the same nearly is the case if we have recourse to the swiftest motion with which we are acquainted; that of light. light travels, it is shown by indisputable scientific reasonings, in about eight minutes from the sun to the earth. hence we can easily calculate that it would occupy at least three years to travel as far as sirius, and probably, three thousand years, or a much greater number, to reach to the smallest stars, or to come from them to us. and thus, as sir w. herschel remarked, since light is the only vehicle by which information concerning these distant bodies is conveyed to us, we do, by seeing them, receive information, not what they are at this moment, but what they were, as to visible condition, thousands of years ago. stars may have been created when man was created, and yet their light may not have reached him.[ ] stars may have been extinguished thousands of years ago, and yet may still be visible to our eyes, by means of the light which they emitted previous to their extinction, and which has not yet died away. . so vast then are the distances at which the different bodies of the universe are distributed; and yet so numerous are those bodies. in the vastness of their distances, there is, indeed, nothing which need disturb our minds, or which, after a little reflection, is likely to do so: for when we have said all that can be said, about the largeness of these distances, still there is no difficulty in finding room for them. we necessarily conceive _space_ as being infinite in its extent: however much space the heavenly bodies occupy, there is space beyond them: if they are not there, space is there nevertheless. that the stars and planets are so far from each other, is an arrangement which prevents their disturbing each other with their mutual attractions, to any destructive extent; and is an arrangement which the spacious, the infinite universe, admits of, without any difficulty. . but we are more especially concerned with the _numbers_ of the heavenly bodies. so many planets about our sun: so many suns, each perhaps with its family of planets: and then, all these suns making but one group: and other groups coming into view, one after another, in seemingly endless succession: and all these planets being of the nature of our earth, as all these stars are of the nature of our sun:--all this, presents to us a spectacle of a world--of a countless host of worlds--of which, when we regard them as thus arranged in planetary systems, and as having, according to all probability, years and seasons, days and nights, as we have, we cannot but accept it as at least a likely suggestion, that they have also inhabitants;--intelligent beings who can reckon these days and years; who subsist on the fruits which the season brings forth, and have their daily and yearly occupations, according to their faculties. when we take, as our scheme of the universe, such a scheme as this, we may well be overwhelmed with the number of provinces, besides that in which man dwells, which the empire of the lord of all includes; and, recurring to the words of the psalmist, we may say with a profundity of meaning immeasurably augmented--"lord, what is man?" it was this view, i conceive, which dr. chalmers had in his thoughts, in pursuing the speculations which i have mentioned, in the outset of this essay. footnotes: [ ] it is quite to our purpose to recollect the impression which such discoveries naturally make upon a pious mind. oh! rack me not to such extent, these distances belong to thee; the world's too little for thy tent, a grave too big for me! george herbert. [ ] this thought is, however, older. young expresses it in his _night thoughts_, night ix., (published in ): how distant some of these nocturnal suns! so distant (says the sage) 'twere not absurd to doubt if beams, set out at nature's birth, are yet arrived at this so foreign world. chapter ii. the astronomical objection to religion. . such astronomical views, then, as those just stated, we may suppose to be those to which chalmers had reference, in the argument of his _astronomical discourses_. these real or supposed discoveries of astronomers, or a considerable part of them, were the facts which were present to his mind, and of which he there discusses the bearings upon religious truths. this multiplicity of systems and worlds, which the telescopic scrutiny of the stars is assumed to have disclosed, or to have made probable, is the main feature in the constitution of the universe, as revealed by science, to which his reflections are directed. nor can we say that, in fixing upon this view, he has gone out of his way, to struggle with obscure and latent difficulties, such as the bulk of mankind know and care little about. for in reality, such views are generally diffused in our time and country, are common to all classes of readers, and as we may venture to express it, are the _popular_ views of persons of any degree of intellectual culture, who have, directly or derivatively, accepted the doctrines of modern science. among such persons, expressions which imply that the stars are globes of luminous matter, like the sun; that there are, among them, systems of revolving bodies, seats of life and of intelligence; are so frequent and familiar, that those who so speak, do not seem to be aware that, in using such expressions, they are making any assumption at all; any more than they suppose themselves to be making assumptions, when they speak of the globular form of the earth, or of its motion round the sun, or of its revolution on its axis. it was, therefore, a suitable and laudable purpose, for a writer like chalmers, well instructed in science, of large and comprehensive views with regard both to religion and to philosophy, of deep and pervasive piety, and master of a dignified and persuasive eloquence, to employ himself in correcting any erroneous opinions and impressions respecting the bearing which such scientific doctrines have upon religious truth. it was his lot to labor among men of great intellectual curiosity, acuteness, and boldness: it was his tendency to deal with new views of others on the most various subjects, religious, philosophical, and social; and, on such subjects, to originate new views of his own. it fell especially within his province, therefore, to satisfy the minds of the public who listened to him, with regard to the conflict, if a conflict there was, or seemed to be, between new scientific doctrines, and permanent religious verities. he was, by his culture and his powers, peculiarly fitted, and therefore peculiarly called, to mediate between the scientific and the religious world of his time. . the scientific doctrine which he especially deals with, in the work to which i refer, is the multiplicity of worlds;--the existence of many seats of life, of enjoyment, of intelligence; and it may be, as he suggests also, of moral law, of transgression, of alienation from god, and of the need, and of the means, of reconciliation to him; or of obedience to him and sympathy with him. that if there be many worlds resembling our world in other respects, they may resemble it in some of these, is an obvious, and we may say, an irresistible conjecture, in any speculative mind to which the doctrine itself has been conveyed. nor can it fail to be very interesting, to see how such a writer as i have described deals with such a suggestion; how far he accepts or inclines to accept it; and if so, what aspect such a view leads him to give to truths, either belonging to natural or to revealed theology, which, before the introduction of such a view, were regarded as bearing only upon the world of which man is the inhabitant. . the mode in which chalmers treats this suggestion, is to regard it as the ground of an objection to religion, either natural or revealed. he supposes an objector to take his stand upon the multiplicity of worlds, assumed or granted as true; and to argue that, since there are so many worlds beside this, all alike claiming the care, the government, the goodness, the interposition, of the creator, it is in the highest degree extravagant and absurd, to suppose that he has done, for this world, that which religion, both natural and revealed, represents him as having done, and as doing. when we are told that god has provided, and is constantly providing, for the life, the welfare, the comfort of all the living things which people this earth, we can, by an effort of thought and reflection, bring ourselves to believe that it is so. when we are further told that he has given a moral law to man, the intelligent inhabitant of the earth, and governs him by a moral government, we are able, or at least the great bulk of thoughtful men, on due consideration of all the bearings of the case, are able, to accept the conviction, that this also is so. when we are still farther asked to believe that the imperfect sway of this moral law over man has required to be remedied by a special interposition of the governor of the world, or by a series of special interpositions, to make the law clear, and to remedy the effects of man's transgression of it; this doctrine also,--according to the old and unscientific view, which represents the human race as, in an especial manner, the summit and crown of god's material workmanship, the end of the rest of creation, and the selected theatre of god's dealings with transgression and with obedience,--we can conceive, and, as religious persons hold, we can find ample and satisfactory evidence to believe. but if this world be merely one of innumerable worlds, all, like it, the workmanship of god; all, the seats of life, like it; others, like it, occupied by intelligent creatures, capable of will, of law, of obedience, of disobedience, as man is; to hold that this world has been the scene of god's care and kindness, and still more, of his special interpositions, communications, and personal dealings with its individual inhabitants, in the way which religion teaches, is, the objector is conceived to maintain, extravagant and incredible. it is to select one of the millions of globes which are scattered through the vast domain of space, and to suppose that one to be treated in a special and exceptional manner, without any reason for the assumption of such a peculiarity, except that this globe happens to be the habitation of us, who make this assumption. if religion require us to assume, that one particular corner of the universe has been thus singled out, and made an exception to the general rules by which all other parts of the universe are governed; she makes, it may be said, a demand upon our credulity which cannot fail to be rejected by those who are in the habit of contemplating and admiring those general laws. can the earth be thus the centre of the moral and religious universe, when it has been shown to have no claim to be the centre of the physical universe? is it not as absurd to maintain this, as it would be to hold, at the present day, the old ptolemaic hypothesis, which places the earth in the centre of the heavenly motions, instead of the newer copernican doctrine, which teaches that the earth revolves round the sun? is not religion disproved, by the necessity under which she lies, of making such an assumption as this? . such is, in a general way, the objection to religion with which chalmers deals; and, as i have said, his mode of treating it is highly interesting and instructive. perhaps, however, we shall make our reasonings and speculations apply to a wider class of readers, if we consider the view now spoken of, not as an objection, urged by an opponent of religion, but rather as a difficulty, felt by a friend of religion. it is, i conceive, certain that many of those who are not at all disposed to argue against religion, but who, on the contrary, feel that their whole internal comfort and repose are bound up indissolubly with their religious convictions, are still troubled and dismayed at the doctrines of the vastness of the universe, and the multitude of worlds, which they suppose to be taught and proved by astronomy. they have a profound reverence for the idea of god; they are glad to acknowledge their constant and universal dependence upon his preserving power and goodness; they are ready and desirous to recognize the working of his providence; they receive the moral law, as his law, with reverence and submission; they regard their transgressions of this law as sins against him; and are eager to find the mode of reconciliation to him, when thus estranged from him; they willingly think of god, as near to them. but while they listen to the evidence which science, as we have said, sets before them, of the long array of groups, and hosts, and myriads, of worlds, which are brought to our knowledge, they find themselves perturbed and distressed. they would willingly think of god as near to them; but during the progress of this enumeration, he appears, at every step, to be removed further and further from them. to discover that the earth is so large, the number of its inhabitants so great, its form so different from what man at first imagines it, may perhaps have startled them; but in this view, there is nothing which a pious mind does not easily surmount. but if venus and mars also have their inhabitants; if saturn and jupiter, globes so much larger than the earth, have a proportional amount of population; may not man be neglected or overlooked? is he worthy to be regarded by the creator of all? may not, must not, the most pious mind recur to the exclamation of the psalmist: "lord, what is man, that thou art mindful of him?" and must not this exclamation, under the new aspect of things, be accompanied by an enfeebled and less confident belief that god _is_ mindful of him? and then, this array of planets, which derive their light from the sun, extends much further than even the astronomer at first suspected. the orbit of saturn is ten times as wide as the orbit of the earth; but beyond saturn, and almost twice as far from the sun, herschel discovers uranus, another great planet; and again, beyond uranus, and again at nearly twice _his_ distance, the subtle sagacity of the astronomers of our day, surmises, and then detects, another great planet. in such a system as this, the earth shrinks into insignificance. can its concerns engage the attention of him who made the whole? but again, this whole solar system itself, with all its orbits and planets, shrinks into a mere point, when compared with the nearest fixed star. and again, the distance which lies between us and such stars, shrinks into incalculable smallness, when we journey in thought to other fixed stars. and again, and again, the field of our previous contemplation suffers an immeasurable contraction, as we pass on to other points of view. . and in all these successive moves, we are still within the dominions of the same creator and governor; and at every move, we are brought, we may suppose, to new bodies of his subjects, bearing, in the expansion of their number, some proportion to the expanse of space which they occupy. and if this be so, how shall the earth, and men, its inhabitants, thus repeatedly annihilated, as it were, by the growing magnitude of the known universe, continue to be anything in the regard of him who embraces all? least of all, how shall men continue to receive that special, persevering, providential, judicial, personal care, which religion implies; and without the belief of which, any man who has religious thoughts, must be disturbed and unhappy, desolate and forsaken? . such are, i conceive, the thoughts of many persons, under the influence of the astronomical views which chalmers refers to as being sometimes employed against religious belief. of course, it is natural that the views which are used by unbelievers as arguments against religious belief, should create difficulties and troubles in the minds of believers; at least, till the argument is rebutted. and of course also, the answers to the arguments, considered as infidel arguments, would operate to remove the difficulties which believers entertain on such grounds. chalmers' reasonings against such arguments, therefore, will, so for as they are valid, avail to relieve the mental trouble of believers, who are perplexed and oppressed by the astronomical views of which i have spoken; as well as to confute and convince those who reject religion, on such astronomical grounds. it may, however, as i have said, be of use to deal with these difficulties rather as difficulties of religious men, than as objections of irreligious men; to examine rather how we can quiet the troubled and perplexed believer, than how we can triumph over the dogmatic and self-satisfied infidel. i, at least, should wish to have the former, rather than the latter of these tasks, regarded as that which i propose to myself. i shall hereafter attempt to explain more fully the difficulties which the doctrine of the plurality of worlds appears to some persons to throw in the way of revealed religion; but before i do so, there is one part of chalmers' answer, bearing especially upon natural religion, which it may be proper to attend to. chapter iii. the answer from the microscope. . it is not my business, nor my intention, to criticize the remarkable work of chalmers to which i have so often referred. but i may say, that the arguments there employed by him, so far as they go upon astronomical or philosophical grounds, are of great weight; and upon the whole, such as we may both assent to, as scientifically true, and accept as rationally persuasive. i think, however, that there are other arguments, also drawn from scientific discoveries, which bear, in a very important and striking manner, upon the opinions in question, and which chalmers has not referred to; and i conceive that there are philosophical views of another kind, which, for those who desire and who will venture to regard the universe and its creator in the wider and deeper relations which appear to be open to human speculation, may be a source of satisfaction. when certain positive propositions, maintained as true while they are really highly doubtful, have given rise to difficulties in the minds of religious persons, other positive propositions, combating these, propounded and supported by argument, that they may be accepted according to their evidence, may, at any rate, have force enough to break down and dissipate such loosely founded difficulties. to present to the reader's mind such speculations as i have thus indicated, is the object of the following pages. they can, of course, pretend to no charm, except for persons who are willing to have their minds occupied with such difficulties and such speculations as i have referred to. those who are willing to be so employed, may, perhaps, find in what i have to say something which may interest them. for, of the arguments which i have to expound, some, though they appear to me both very obvious and very forcible, have never, so far as i am aware, been put forth in that religious bearing which seems to belong to them; and others, though aspiring to point out in some degree the relation of the universe and its creator, are of a very simple kind; that is, for minds which are prepared to deal with such subjects at all. . as i have said, the arguments with which we are here concerned refer both to natural religion and to revealed religion; and there is one of chalmers' arguments, bearing especially upon the former branch of the subject, which i may begin by noticing. among the thoughts which, it was stated, might naturally arise in men's minds, when the telescope revealed to them an innumerable multitude of worlds besides the one which we inhabit, was this: that the governor of the universe, who has so many worlds under his management, cannot be conceived as bestowing upon this earth, and its various tribes of inhabitants, that care which, till then, natural religion had taught men that he does employ, to secure to man the possession and use of his faculties of mind and body; and to all animals the requisites of animal existence and animal enjoyment. and upon this chalmers remarks, that just about the time when science gave rise to the suggestion of this difficulty, she also gave occasion to a remarkable reply to it. just about the same time that the invention of the _telescope_ showed that there were innumerable worlds, which might have inhabitants requiring the creator's care as much as the tribes of this earth do,--the invention of the _microscope_ showed that there were, in this world, innumerable tribes of animals, which had been all along enjoying the benefits of the creator's care, as much as those kinds with which man had been familiar from the beginning. the telescope suggested that there might be dwellers in jupiter or in saturn, of giant size and unknown structure, who must share with us the preserving care of god. the microscope showed that there had been, close to us, inhabiting minute crevices and crannies, peopling the leaves of plants, and the bodies of other animals, animalcules of a minuteness hitherto unguessed, and of a structure hitherto unknown, who had been always sharers with us in god's preserving care. the telescope brought into view worlds as numerous as the drops of water which make up the ocean; the microscope brought into view a world in almost every drop of water. infinity in one direction was balanced by infinity in the other. the doubts which men might feel as to what god could do, were balanced by certainties which they discovered, as to what he had always been doing. his care and goodness could not be supposed to be exhausted by the hitherto known population of the earth, for it was proved that they had not hitherto been confined to that population. the discovery of new worlds at vast distances from us, was accompanied by the discovery of new worlds close to us, even in the very substances with which we were best acquainted; and was thus rendered ineffective to disturb the belief of those who had regarded the world as having god for its governor. . this is a striking reflection, and is put by chalmers in a very striking manner; and it is well fitted to remove the scruples to which it is especially addressed. if there be any persons to whom the astronomical discoveries which the telescope has brought to light, suggests doubts or difficulties with regard to such truths of natural religion as god's care for and government of the inhabitants of the earth, the discoveries of the many various forms of animalcular life which the microscope has brought to light are well fitted to remove such doubts, and to solve such difficulties. we may easily believe that the power of god to sustain and provide for animal life, animal sustenance, animal enjoyment, can suffice for innumerable worlds besides this, without being withdrawn or distracted or wearied in this earth; for we find that it does suffice for innumerable more inhabitants of this earth than we were before aware of. if we had imagined before, that, in conceiving god as able and willing to provide for the life and pleasure of all the sentient beings which we knew to exist upon the earth, we had formed an adequate notion of his power and of his goodness, these microscopical discoveries are well adapted to undeceive us. they show us that all the notions which our knowledge, hitherto, had enabled us to form of the powers and attributes of the creator and preserver of all living things, are vastly, are immeasurably below the real truth of the case. they show us that god, as revealed to us in the animal creation, is the author and giver of life, of the organization which life implies, of the contrivances by which it is conducted and sustained, of the enjoyment by which it is accompanied,--to an extent infinitely beyond what the unassisted vision of man could have suggested. the facts which are obvious to man, from which religious minds in all ages have drawn their notions and their evidence of the divine power and goodness, care and wisdom, in providing for its creatures, require, we find, to be indefinitely extended, in virtue of the new tribes of minute creatures, and still new tribes, and still more minute, which we find existing around us. the views of our natural theology must be indefinitely extended on one side; and therefore we need not be startled or disturbed at having to extend them indefinitely on the other side;--at having to believe that there are, in other worlds, creatures whom god has created, whom he sustains in life, for whom he provides the pleasures of life, as he does for the long unsuspected creatures of this world. . this is, i say, a reflection which might quiet the mind of a person, whom astronomical discoveries had led to doubt of the ordinary doctrines of natural religion. but, i think, it may be questioned, whether, to produce such doubts, is a common or probable effect of an acquaintance with astronomical discoveries. undoubtedly, by such discoveries, a person who believes in god, in his wisdom, power, and goodness, on the evidence of the natural world, is required to extend and exalt his conceptions of those divine attributes. he had believed god to be the author of many forms of life;--he finds him to be the author of still more forms of life. he had traced many contrivances in the structure of animals, for their sustentation and well-being; his new discoveries disclose to him (for that is undoubtedly among the effects of microscopic researches) still more nice contrivances. he had seen reason to think that all sentient beings have their enjoyments; he finds new fields of enjoyment of the same kind. but in all this, there is little or nothing to disturb the views and convictions of the natural theologian. he must, even by the evidence of facts patent to ordinary observation, have been led to believe that the divine wisdom and power are not only great, but great in a degree which we cannot fathom or comprehend;--that they are, to our apprehension, infinite: his new discoveries only confirm the impression of this infinite character of the divine attributes. he had before believed the existence of an intelligent and wise creator, on the evidence of the marks of design and contrivance, which the creation exhibited: of such design and contrivance he discovers new marks, new examples. he had believed that god is good, because he found those contrivances invariably had the good of the creature for their object: he finds, still, that this is the general, the universal scheme of the creation, now when his view of it is extended. he has no difficulty in expanding his religious conceptions, to correspond with his scientific discoveries, so far as the microscope is the instrument of discovery; there is no reason why he should have any more difficulty in doing the same, when the telescope is his informant. it is true, that in this case the information is more imperfect. it does not tell him, even that there are living inhabitants in the regions which it reveals; and, consequently, it does not disclose any of those examples of design which belong to the structure of living things. but if we suppose, from analogy, that there are living things in those regions, we have no difficulty in conceiving, from analogy also, that those living things are constructed with a care and wisdom such as appear in the inhabitants of earth. it will not readily or commonly occur to a speculator on such subjects, that there is any source of perplexity or unbelief, in such an assumption of inhabitants of other worlds, even if we make the assumption. it is as easy, it may well and reasonably be thought, for god to create a population for the planets as to make the planets themselves;--as easy to supply jupiter with tenants, as with satellites;--as easy to devise the organization of an inhabitant of saturn, as the structure and equilibrium of saturn's ring. it is no more difficult for the universal creator to extend to those bodies the powers which operate in organized matter, than the powers which operate in brute matter. it is as easy for him to establish circulation and nutrition in material structures, as cohesion and crystallization, which we must suppose the planetary masses to possess; or attraction and inertia, which we know them to possess. no doubt, to our conception, organization appears to be a step beyond cohesion; circulation of living fluids, a step beyond crystallization of dead masses:--but then, it is in tracing such steps, that we discern the peculiar character of the creator's agency. he does not merely work with mechanical and chemical powers, as man to a certain extent can do; but with organic and vital powers, which man cannot command. the creator, therefore, can animate the dust of each planet, as easily as make the dust itself. and when from organic life we rise to sentient life, we have still only another step in the known order of creative power. to create animals, in any province of the universe, cannot be conceived as much more incomprehensible or incredible, than to create vegetables. no doubt, the addition of the living and sentient principle to the material, and even to the organic structure, is a mighty step; and one which may, perhaps, be made the occasion of some speculative suggestions, in a subsequent part of this essay; but still, it is not likely that any one, who had formed his conceptions of the divine mind from its manifestations in the production and sustentation of animal, as well as vegetable life, on this earth, would have his belief in the operation of such a mind, shaken, by any necessity which might be impressed upon him, of granting the existence of animal life on other planets, as well as on the earth, or even on innumerable such planets, and on innumerable systems of planets and worlds, system above system. . the remark of chalmers, therefore, to which i have referred, striking as it is, does not appear to bear directly upon a difficulty of any great force. if astronomy gives birth to scruples which interfere with religion, they must be found in some other quarter than in the possibility of mere animal life existing in other parts of the universe, as well as on our earth. that possibility may require us to enlarge our idea of the deity, but it has little or no tendency to disturb our apprehension of his attributes. chapter iv. further statement of the difficulty. . we have attempted to show that if the discoveries made by the telescope should excite in any one's mind, difficulties respecting those doctrines of natural religion,--the adequacy of the creator to the support and guardianship of all the animal life which may exist in the universe,--the discoveries of the microscope may remove such difficulties; but we have remarked also, that the train of thought which leads men to dwell upon such difficulties does not seem to be common. but what will be the train of thought to which we shall be led, if we suppose that there are, on other planets, and in other systems, not animals only, living things, which, however different from the animals of this earth, are yet in some way analogous to them, according to the difference of circumstances; but also creatures analogous to man;--intellectual creatures, living, we must suppose, under a moral law, responsible for transgression, the subjects of a providential government? if we suppose that, in the other planets of our solar systems, and of other systems, there are creatures of such a kind, and under such conditions as these, how far will the religious opinions which we had previously entertained be disturbed or modified? will any new difficulty be introduced into our views of the government of the world by such a supposition? . i have spoken of man as an intellectual creature; meaning thereby that he has a mind;--powers of thought, by which he can contemplate the relations and properties of things in a general and abstract form; and among other relations, moral relations, the distinction of _right_ and _wrong_ in his actions. those powers of thought lead him to think of a creator and ordainer of all things; and his perception of right and wrong leads him to regard this creator as also the governor and judge of his creatures. the operation of his mind directs him to believe in a supreme mind: his moral nature directs him to believe that the course of human affairs, and the condition of men, both as individuals and as bodies, is determined by the providential government of god. . with regard to the bearing of a merely _intellectual_ nature on such questions, it does not appear that any considerable difficulty would be _at once_ occasioned in our religious views, by supposing such a nature to belong to other creatures, the inhabitants of other planets, as well as to man. the existence of our own minds directs us, as i have said, to a supreme mind; and the nature of mind is conceived to be, in all its manifestations, so much the same, that we can conceive minds to be multiplied indefinitely, without fear of confusion, interference, or exhaustion. there may be, in jupiter, creatures endowed with an intellect which enables them to discover and demonstrate the relations of space; and if so, they cannot have discovered and demonstrated anything of that kind as true, which is not true for us also: their geometry must coincide with ours, as far as each goes:--thus showing how absurdly, as plato long ago observed, we give to the science which deals with the relations of space, a name (_geometry_), borrowed from the art of measuring the earth. the earth with its properties is no more the special basis of geometry, than are jupiter or saturn, or, so far as we can judge, sirius or arcturus and their systems, with their properties. wherever pure intellect is, we are compelled to conceive that, when employed upon the same objects, its results and conclusions are the same. if there be intelligent inhabitants of the moon, they may, like us, have employed their intelligence in reasoning upon the properties of lines and angles and triangles; and must, so far as they have gone, have arrived, in their thoughts, at the same properties of lines and angles and triangles, at which we have arrived. they must, like us, have had to distinguish between right angles and oblique angles. they may have come to know, as some of the inhabitants of the earth came to know, four thousand years ago, that, in a right-angled triangle, the square on the larger side is equal to the sum of the squares on the other two sides. we can conceive occurrences which would give us evidence that the moon, as well as the earth, contains geometers. if we were to see, on the face of the full moon, a figure gradually becoming visible, representing a right-angled triangle with a square constructed on each of its three sides as a base; we should regard it as the work of intelligent creatures there, who might be thus making a signal to the inhabitants of the earth, that they possessed such knowledge, and were desirous of making known to their nearest neighbors in the solar system, their existence and their speculations. in such an event, curious and striking as it would be, we should see nothing but what we could understand and accept, without unsettling our belief in the supreme and divine intelligence. on the contrary, we could hardly fail to receive such a manifestation as a fresh evidence that the divine mind had imparted to the inhabitants of the moon, as he has to us, a power of apprehending, in a very general and abstract form, the relations of that space in which he performs his works. we should judge, that having been led so far in their speculations, they must, in all probability, have been led also to a conception of the universe, as the field of action of a universal and divine mind; that having thus become geometers, they must have ascended to the idea of a god who works by geometry. . but yet, by such a supposition, on further consideration, we find ourselves introduced to views entirely different from those to which we are led by the supposition of mere animal life, existing in other worlds than the earth. for, not to dwell here upon any speculations as to how far the operations of our minds may resemble the operations of the divine mind;--a subject which we shall hereafter endeavor to discuss;--we know that the advance to such truths as those of geometry has been, among the inhabitants of the earth, gradual and progressive. though the human mind have had the same powers and faculties, from the beginning of the existence of the race up to the present time, (as we cannot but suppose,) the results of the exercise of these powers and faculties have been very different in different ages; and have gradually grown up, from small beginnings, to the vast and complex body of knowledge concerning the scheme and relations of the universe, which is at present accessible to the minds of human speculators. it is, as we have said, probably about four thousand years, since the first steps in such knowledge were made. geometry is said to have had its origin in egypt; but it assumed its abstract and speculative character first among the greeks. pythagoras is related to have been the first who saw, in the clear light of demonstration, the property of the right-angled triangle, of which we have spoken. the greeks, from the time of socrates, stimulated especially by plato, pursued, with wonderful success, the investigation of this kind of truths. they saw that such truths had their application in the heavens, far more extensively than on the earth. they were enabled, by such speculations, to unravel, in a great degree, the scheme of the universe, before so seemingly entangled and perplexed. they determined, to a very considerable extent, the relative motions of the planets and of the stars. and in modern times, after a long interval, in which such knowledge was nearly stationary, the progress again began; and further advances were successively made in man's knowledge of the scheme and structure of the visible heavens; till at length the intellect of man was led to those views of the extent of the universe and the nature of the stars, which are the basis of the discussions in which we are now engaged. and thus man, having probably been, in the earliest ages of the existence of the species, entirely ignorant of abstract truth, and of the relations which, by the knowledge of such truth, we can trace in nature, (as the barbarous tribes which occupy the greater part of the earth's surface still are;) has, by a long series of progressive steps, come into the possession of knowledge, which we cannot regard without wonder and admiration; and which seems to elevate him in no inconsiderable degree, towards a community of thought with that divine mind, into the nature and scheme of whose works he is thus permitted to penetrate. . now the knowledge which man is capable, by the nature of his mental faculties, of acquiring, being thus blank and rudimentary at first, and only proceeding gradually, by the steps of a progress, numerous, slow, and often long interrupted, to that stage in which it is the basis of our present speculations; the view which we have just taken, of the nature of intellect, as a faculty always of the same kind, always uniform in its operations, always consistent in its results, appears to require reconsideration; and especially with reference to the application which we made of that view, to the intelligent inhabitants of other planets and other worlds, if such inhabitants there be. for if we suppose that there are, in the moon, or in jupiter, creatures possessing intellectual faculties of the same kind as those of man; capable of apprehending the same abstract and general truths; able, like man, to attain to a knowledge of the scheme of the universe; yet this supposition merely gives the capacity and the ability; and does not include any security, or even high probability, as it would seem, of the exercise of such capacity, or of the successful application of such ability. even if the surface of the moon be inhabited by creatures as intelligent as men, why must we suppose that they know anything more of the geometry and astronomy, than the great bulk of the less cultured inhabitants of the earth, who occupy, really, a space far larger than the surface of the moon; and, all intelligent though they be, and in the full possession of mental faculties, are yet, on the subjects of geometry and astronomy, entirely ignorant;--their minds, as to such a knowledge, a blank? it does not follow, then, that even if there be such inhabitants in the moon, or in the planets, they have any sympathy with us, or any community of knowledge on the subjects of which we are now speaking. the surface of the moon, or of jupiter, or of saturn, even if well peopled, may be peopled only with tribes as barbarous and ignorant as tartars, or esquimaux, or australians; and therefore, by making such a supposition, we do little, even hypothetically, to extend the dominion of that intelligence, by means of which all intelligent beings have some community of thought with each other, and some suggestion of the working of the divine and universal mind. . but, in fact, the view which we have given of the mode of existence of the human species upon the earth, as being a progressive existence, even in the development of the intellectual powers and their results, necessarily fastens down our thoughts and our speculations to the earth, and makes us feel how visionary and gratuitous it is to assume any similar kind of existence in any region occupied by other beings than man. as we have said, we have no insuperable difficulty in conceiving other parts of the universe to be tenanted by animals. animal life implies no progress in the species. such as they are in one century, such are they in another. the conditions of their sustentation and generation being given, which no difference of physical circumstances can render incredible, the race may, so far as we can see, go on forever. but a race which makes a progress in the development of its faculties cannot thus, or at least cannot with the same ease, be conceived as existing through all time, and under all circumstances. progress implies, or at least suggests, a beginning and an end. if the mere existence of a race imply a sustaining and preserving power in the creator, the progress of a race implies a guiding and impelling power; a governor and director, as well as a creator and preserver. and progress, not merely in material conditions, not merely in the exercise of bodily faculties, but in the exercise of mental faculties, in the intellectual condition of a portion of the species, still more implies a special position and character of the race, which cannot, without great license of hypothesis, be extended to other races; and which, if so extended, becomes unmeaning, from the impossibility of our knowing what is progress in any other species;--from what and towards what it tends. the intellectual progress of the human species has been a progress in the use of thought, and in the knowledge which such use procures; it has been a progress from mere matter to mind; from the impressions of sense to ideas; from what in knowledge is casual, partial, temporary, to what is necessary, universal, and eternal. we can conceive no progress, of the nature of this, which is not identical with this; nothing like it, which is not the same. and, therefore, if we will people other planets with creatures, intelligent as man is intelligent, we must not only give to them the intelligence, but the intellectual history of the human species. they must have had their minds unfolded by steps similar to those by which the human mind has been unfolded; or at least, differing from them only as the intellectual history of one nation of the earth differs from that of another. they must have had their pythagoras, their plato, their kepler, their galileo, their newton, if they know what we know. and thus, in order to conceive, on the moon or on jupiter, a race of beings intelligent like man, we must conceive, there, colonies of men, with histories resembling more or less the histories of human colonies; and indeed resembling the history of those nations whose knowledge we inherit, far more closely than the history of any other terrestrial nation resembles that part of terrestrial history. if we do this, we exercise an act of invention and imagination which may be as coherent as a fairy tale, but which, without further proof, must be as purely imaginary and arbitrary. but if we do not do this, we cannot conceive that those regions are occupied at all by intelligent beings. intelligence, as we see in the human race, in order to have those characters which concern our argument, implies a history of intellectual development; and to assume arbitrarily a history of intellectual development for the inhabitants of a remote planet, as a ground of reasoning either for or against religion, is a proceeding which we can hardly be expected either to assent to or to refute. if we are to form any opinions with regard to the condition of such bodies, and to trace any bearing of such opinions upon our religious views, we must proceed upon some ground which has more of reality than such a gratuitous assumption. . thus the condition of man upon the earth, as a condition of intellectual progress, implies such a special guidance and government exercised over the race by the author of his being, as produces progress; and we have not, so far as we yet perceive, any reason for supposing that he exercises a like guidance and government over any of the other bodies with which the researches of astronomers have made us acquainted. the earth and its inhabitants are under the care of god in a special manner; and we are utterly destitute of any reason for believing that other planets and other systems are under the care of god in the same manner. if we regarded merely the existence of unprogressive races of animals upon our globe, we might easily suppose that other globes also are similarly tenanted; and we might infer, that the creator and upholder of animal life was active on those globes, in the same manner as upon ours. but when we come to a progressive creature, whose condition implies a beginning, and therefore suggests an end, we form a peculiar judgment with respect to god's care of that creature, which we have not as yet seen the slightest grounds to extend to other possible fields of existence, where we discern no indication of progress, of beginning, or of end. so far as we can judge, god is mindful of man, and has launched and guided his course in a certain path which makes his lot and state different from that of all other creatures. . now when we have arrived at this result, we have, i conceive, reached one of the points at which the difficulties which astronomical discovery puts in the way of religious conviction begin to appear. the earth and its human inhabitants are, as far as we yet know, in an especial manner the subjects of god's care and government, for the race is progressive. now can this be? is it not difficult to believe that it is so? the earth, so small a speck, only one among so many, so many thousands, so many millions of other bodies, all, probably, of the same nature with itself, wherefore should it draw to it the special regards of the creator of all, and occupy his care in an especial manner? the teaching of the history of the human race, as intellectually progressive, agrees with the teaching of religion, in impressing upon us that god is mindful of man; that he does regard him; but still, there naturally arises in our minds a feeling of perplexity and bewilderment, which expresses itself in the words already so often quoted, what is man, that this should be so? can it be true that this province is thus singled out for a special and peculiar administration by the lord of the universal empire? . before i make any attempt to answer these questions, i must pursue the difficulty somewhat further, and look at it in other forms. as i have said, the history of man has been, in certain nations, a history of intellectual progress, from the earliest times up to our own day. but intellectual progress has been, as i have also said, in a great measure confined to certain nations thus especially favored. the greater part of the earth's inhabitants have shared very scantily in that wealth of knowledge to which the brightest and happiest intellects among men have thus been led. but though the bulk of mankind have thus had little share in the grand treasures of science which are open to the race, their life has still been very different from that of other animals. many nations, though they may not have been conspicuous in the history of intellectual progress, have yet not been without their place in progress of other kinds--in arts, in arms, and, above all, in morals--in the recognition of the distinction of right and wrong in human actions, and in the practical application of this distinction. such a progress as this has been far more extensively aimed at, than a progress in abstract and general knowledge; and, we may venture to say, has been, in many nations and in a very great measure, really effected. no doubt the imperfection of this progress, and the constant recurrence of events which appear to counteract and reverse it, are so obvious and so common as to fill with grief and indignation the minds of those who regard such a progress as the great business of the human race; but yet still, looking at the whole history of the human race, the progress is visible; and even the grief and the indignation of which we have spoken are a part of its evidences. there has been, upon the whole, a moral government of the human race. the moral law, the distinction of right and wrong, has been established in every nation; and penalties have been established for wrong-doing. the notion of right and wrong has been extended, from mere outward acts, to the springs of action, to affection, desire, and will. the course of human affairs has generally been such, that the just, the truthful, the kind, the chaste, the orderly portion of mankind have been happier than the violent and wicked. external wrong has been commonly punished by the act of human society. internal sins, impure and dishonest designs, falsehood, cruelty, have very often led to their own punishment, by their effect upon the guilty mind itself. we do not say that the moral government which has prevailed among men has been such, that we can consider it complete and final in its visible form. we see that the aspect of things is much the contrary; and we think we see reasons why it may be expected to be so. but still, there has existed upon earth a moral government of the human race, exercised, as we must needs hold, by the creator of man; partly through the direct operation of man's faculties, affections, and emotions; and partly through the authorities which, in all ages and nations, the nature of man has led him to establish. now this moral progress and moral government of the human race is one of the leading facts on which natural religion is founded. we are thus led to regard god as the moral governor of man; not only his creator and preserver, but his lawgiver and his judge. and the grounds on which we entertain this belief are peculiarly the human faculties of man, and their operation in history and in society. the belief is derived from the whole complex nature of man--the working of his affections, desires, convictions, reason, conscience, and whatever else enters into the production of human action and its consequences. god is seen to be the moral governor of man by evidence which is especially derived from the character of man, and which we could not attempt to apply to any other creature than man without making our words altogether unmeaning. but would it not be too bold an assumption to speak of the conscience of an inhabitant of jupiter? would it not be a rash philosophy to assume the operation of remorse or self-approval on the planet, in order that we may extend to it the moral government of god? except we can point out something more solid than this to reason from, on such subjects, there is no use in our attempting to reason at all. our doctrines must be mere results of invention and imagination. here then, again, we are brought to the conviction that god is, so far as we yet see, in an especial and peculiar manner, the governor of the earth and of its human inhabitants, in such a way that the like government cannot be conceived to be extended to other planets, and other systems, without arbitrary and fanciful assumptions; assumptions either of unintelligible differences with incomprehensible results, or of beings in all respects human, inhabiting the most remote regions of the universe. and here, again, therefore, we are led to the same difficulty which we have already encountered: can the earth, a small globe among so many millions, have been selected as the scene of this especially divine government? . that when we attempt to extend our sympathies to the inhabitants of other planets and other worlds, and to regard them as living, like us, under a moral government, we are driven to suppose them to be, in all essential respects, human beings like ourselves, we have proof, in all the attempts which have been made, with whatever license of hypothesis and fancy, to present to us descriptions and representations of the inhabitants of other parts of the universe. such representations, though purposely made as unlike human beings as the imagination of man can frame them, still are merely combinations, slightly varied, of the elements of human being; and thus show us that not only our reason, but even our imagination, cannot conceive creatures subjected to the same government to which man is subjected, without conceiving them as being men of one kind or other. a mere animal life, with no interest but animal enjoyment, we may conceive as assuming forms different from those which appear in existing animal races; though even here, there are, as we shall hereafter attempt to show, certain general principles which run through all animal life. but when in addition to mere animal impulses, we assume or suppose moral and intellectual interests, we conceive them as the moral and intellectual interests of man. truth and falsehood, right and wrong, law and transgression, happiness and misery, reward and punishment, are the necessary elements of all that can interest us--of all that we can call _government_. to transfer these to jupiter or to sirius, is merely to imagine those bodies to be a sort of island of formosa, or new atlantis, or utopia, or platonic polity, or something of the like kind. the boldest and most resolute attempts to devise some life different from human life, have not produced anything more different than romance-writers and political theorists have devised _as_ a form of human life. and this being so, there is no more wisdom or philosophy in believing such assemblages of beings to exist in jupiter or sirius, without evidence, than in believing them to exist in the island of formosa, with the like absence of evidence. . any examination of what has been written on this subject would show that, in speculating about moral and intellectual beings in other regions of the universe, we merely make them to be men in another place. with regard to the plants and animals of other planets, fancy has freer play; but man cannot conceive any moral creature who is not man. thus fontenelle, in his _dialogues on the plurality of worlds_, makes the inhabitants of venus possess, in an exaggerated degree, the characteristics of the men of the warm climates of the earth. they are like the moors of grenada; or rather, the moors of grenada would be to them as cold as greenlanders and laplanders to us. and the inhabitants of mercury have so much vivacity, that they would pass with us for insane. "enfin c'est dans mercure que sont les petites-maisons de l'univers." the inhabitants of jupiter and saturn are immensely slow and phlegmatic. and though he and other writers attempt to make these inhabitants of remote regions in some respects superior to man, telling us that instead of only five senses, they may have six, or ten, or a hundred, still these are mere words which convey no meaning; and the great astronomer bessel had reason to say, that those who imagined inhabitants in the moon and planets, supposed them, in spite of all their protestations, as like to men as one egg to another.[ ] . but there is one step more, which we still have to make, in order to bring out this difficulty in its full force. as we have said, the moral law has been, to a certain extent, established, developed, and enforced among men. but, as i have also said, looking carefully at the law, and at the degree of man's obedience to it, and at the operation of the sanctions by which it is supported, we cannot help seeing, that man's knowledge of the law is imperfect, his conviction of its authority feeble, his transgressions habitual, their punishment and consequences obscure. when, therefore, we regard god, as the lawgiver and judge of man, it will not appear strange to us, that he should have taken some mode of promulgating his law, and announcing his judgments, in addition to that ordinary operation of the faculties of man, of which we have spoken. revealed religion teaches us that he has done so: that from the first placing of the race of man upon the earth, it was his purpose to do so: that by his dealing with the race of man in the earlier times, and at various intervals, he made preparation for the mission of a special messenger, whom, in the fulness of time, he sent upon the earth in the form of a man; and who both taught men the law of god in a purer and clearer form than any in which it had yet been given; and revealed his purpose, of rewards for obedience, and punishments for disobedience, to be executed in a state of being to which this human life is only an introduction; and established the means by which the spirit of man, when alienated from god by transgression, may be again reconciled to him. the arrival of this especial messenger of holiness, judgment, and redemption, forms the great event in the history of the earth, considered in a religious view, as the abode of god's servants. it was attended with the sufferings and cruel death of the divine messenger thus sent; was preceded by prophetic announcements of his coming; and the history of the world, for the two thousand years that have since elapsed, has been in a great measure occupied with the consequences of that advent. such a proceeding shows, of course, that god has an especial care for the race of man. the earth, thus selected as the theatre of such a scheme of teaching and of redemption, cannot, in the eyes of any one who accepts this christian faith, be regarded as being on a level with any other domiciles. it is the stage of the great drama of god's mercy and man's salvation; the sanctuary of the universe; the holy land of creation; the royal abode, for a time at least, of the eternal king. this being the character which has thus been conferred upon it, how can we assent to the assertions of astronomers, when they tell us that it is only one among millions of similar habitations, not distinguishable from them, except that it is smaller than most of them that we can measure; confused and rude in its materials like them? or if we believe the astronomers, will not such a belief lead us to doubt the truth of the great scheme of christianity, which thus makes the earth the scene of a special dispensation. . this is the form in which chalmers has taken up the argument. this is the difficulty which he proposes to solve; or rather, (such being as i have said the mode in which he presents the subject,) the objection which he proposes to refute. it is the bearing of the astronomical discoveries of modern times, not upon the doctrines of natural religion, but upon the scheme of christianity, which he discusses. and the question which he supposes his opponent to propound, as an objection to the christian scheme, is:--how is it consistent with the dignity, the impartiality, the comprehensiveness, the analogy of god's proceedings, that he should make so special and pre-eminent a provision for the salvation of the inhabitants of this earth, where there are such myriads of other worlds, all of which may require the like provision, and all of which have an equal claim to their creator's care? . the answer which chalmers gives to this objection, is one drawn, in the first instance, from our ignorance. he urges that, when the objector asserts that other worlds may have the like need with our own, of a special provision for the rescue of their inhabitants from the consequences of the transgression of god's laws, he is really making an assertion without the slightest foundation. not only does science not give us any information on such subjects, but the whole spirit of the scientific procedure, which has led to the knowledge which we possess, concerning other planets and other systems, is utterly opposed to our making such assumptions, respecting other worlds, as the objection involves. modern science, in proportion as she is confident when she has good grounds of proof, however strange may be the doctrines proved, is not only diffident, but is utterly silent, and abstains even from guessing, when she has no grounds of proof. chalmers takes newton's reasoning, as offering a special example of this mixed temper, of courage in following the evidence, and temperance in not advancing when there is no evidence. he puts, in opposition to this, the example of the true philosophical temper,--a supposed rash theorist, who should make unwarranted suppositions and assumptions, concerning matters to which our scientific evidence does not reach;--the animals and plants, for instance, which are to be found in the planet jupiter. no one, he says, would more utterly reject and condemn such speculations than newton, who first rightly explained the motion of jupiter and of his attendant satellites, about which science _can_ pronounce her truths. and thus, nothing can be more opposite to the real spirit of modern science, and astronomy in particular, than arguments, such as we have stated, professing to be drawn from science and from astronomy. since we know nothing about the inhabitants of jupiter, true science requires that we say and suppose nothing about them; still more requires that we should not, on the ground of assumptions made with regard to them, and other supposed groups of living creatures, reject a belief, founded on direct and positive proofs, such as is the belief in the truths of natural and of revealed religion. . to this argument of chalmers, we may not only give our full assent, but we may venture to suggest, in accordance with what we have already said, that the argument, when so put, is not stated in all its legitimate force. the assertion that the inhabitants of jupiter have the same need as we have, of a special dispensation for their preservation from moral ruin, is not only as merely arbitrary an assumption, as any assertion could be, founded on a supposed knowledge of an analogy between the botany of jupiter, and the botany of the earth; but it is a great deal more so. there may be circumstances which may afford some reason to believe that something of the nature of vegetables grows on the surface of jupiter; for instance, if we find that he is a solid globe surrounded by an atmosphere, vapor, clouds, showers. but, as we have already said, there is an immeasurable distance between the existence of unprogressive tribes of organized creatures, plants, or even animals, and the existence of a progressive creature, which can pass through the conditions of receiving, discerning, disobeying, and obeying a moral law; which can be estranged from god, and then reconciled to him. to assume, without further proof, that there are, in jupiter, creatures of such a nature that these descriptions apply to them, is a far bolder and more unphilosophical assumption, than any that the objector could make concerning the botany of jupiter; and therefore, the objection thus supposed to be drawn from our supposed knowledge, is very properly answered by an appeal to our really utter ignorance, as to the points on which the argument rests. . this appeal to our ignorance is the main feature in chalmers' reasonings, so far as the argument on the one side or the other has reference to science. chalmers, indeed, pursues the argument into other fields of speculation. he urges, that not only we have no right to assume that other worlds require a redemption of the same kind as that provided for man, but that the very reverse maybe the case. man maybe the only transgressor; and this, the only world that needed so great a provision for its salvation. we read in scripture, expressions which imply that other beings, besides man, take an interest in the salvation of man. may not this be true of the inhabitants of other worlds, if such inhabitants there be? these speculations he pursues to a considerable length, with great richness of imagination, and great eloquence. but the suppositions on which they proceed are too loosely connected with the results of science, to make it safe for us to dwell upon them here. . i conceive, as i have said, that the argument with which chalmers thus deals admits of answers, also drawn from modern science, which to many persons will seem more complete than that which is thus drawn from our ignorance. but before i proceed to bring forward these answers, which will require several steps of explanation, i have one or two remarks still to make. . undoubtedly they who believe firmly both that the earth has been the scene of a divine plan for the benefit of man, and also that other bodies in the universe are inhabited by creatures who may have an interest in such a plan, are naturally led to conjectures and imaginations as to the nature and extent of that interest. the religious poet, in his night thoughts, interrogates the inhabitants of a distant star, whether their race too has, in its history, events resembling the fall of man, and the redemption of man. enjoy your happy realms their golden age? and had your eden an abstemious eve? or, if your mother fell are you redeemed? and if redeemed, is your redeemer scorned? and such imaginations may be readily allowed to the preacher or the poet, to be employed in order to impress upon man the conviction of his privileges, his thanklessness, his inconsistency, and the like. but every form in which such reflections can be put shows how intimately they depend upon the nature and history of man. and when such reflections are made the source of difficulty or objection in the way of religious thought, and when these difficulties and objections are represented as derived from astronomical discoveries, it cannot be superfluous to inquire whether astronomy has really discovered any ground for such objections. to some persons it may be more grateful to remedy one assumption by another: the assumption of moral agents in other worlds, by the assumption of some operation of the divine plan in other worlds. but since many persons find great difficulty in conceiving such an operation of the divine plan in a satisfactory way; and many persons also think that to make such unauthorized and fanciful assumptions with regard to the divine plans for the government of god's creatures is a violation of the humility, submission of mind, and spirit of reverence which religion requires; it may be useful if we can show that such assumptions, with regard to the divine plans, are called forth by assumptions equally gratuitous on the other side: that astronomy no more reveals to us extra-terrestrial moral agents, than religion reveals to us extra-terrestrial plans of divine government. chalmers has spoken of the _rashness_ of making assumptions on such subjects without proof; leaving it however, to be supposed, that though astronomy does not supply proof of intelligent inhabitants of other parts of the universe, she yet does offer strong analogies in favor of such an opinion. but such a procedure is more than rash: when astronomical doctrines are presented in the form in which they have been already laid before the reader, which is the ordinary and popular mode of apprehending them, the analogies in favor of "other worlds," are (to say the least) greatly exaggerated. and by taking into account what astronomy really teaches us, and what we learn also from other sciences, i shall attempt to reduce such "analogies" to their true value. . the privileges of man, which make the difficulty in assigning him his place in the vast scheme of the universe, we have described as consisting in his being an _intellectual_, _moral_, and _religious_ creature. perhaps the privileges implied in the last term, and their place in our argument, may justify a word more of explanation. religion teaches us that there is opened to man, not only a prospect of a life in the presence of god, after this mortal life, but also the possibility and the duty of spending this life as in the presence of god. this is properly the highest result and manifestation of the effect of religion upon man. precisely because it is this, it is difficult to speak of this effect without seeming to use the language of enthusiasm; and yet again, precisely because it is so, our argument would be incomplete without a reference to it. there is for man, a possibility and a duty of bringing his thoughts, purposes, and affections more and more into continual unison with the will of god. this, even natural religion taught men, was the highest point at which man could aim; and revealed religion has still more clearly enjoined the duty of aiming at such a condition. the means of a progress towards such a state belong to the religion of the heart and mind. they include a constant purification and elevation of the thoughts, affections, and will, wrought by habits of religious reflection and meditation, of prayer and gratitude to god. without entering into further explanation, all religious persons will agree that such a progress is, under happy influences, possible for man, and is the highest condition to which he can attain in this life. whatever names may have been applied at different times to the steps of such a progress;--the cultivation of the divine nature in us; resignation; devotion; holiness; union with god; living in god, and with god in us;--religious persons will not doubt that there is a reality of internal state corresponding to these expressions; and that, to be capable of elevation into the condition which these expressions indicate, is one of the especial privileges of man. man's soul, considered especially as the subject of god's government, is often called his _spirit_; and that man is capable of such conformity to the will of god, and approximation to him, is sometimes expressed by speaking of him as a _spiritual creature_. and though the privilege of being, or of being capable of becoming, in this sense, a spiritual creature, is a part of man's religious privileges; we may sometimes be allowed to use this additional expression, in order to remind the reader, how great those religious privileges are, and how close is the relation between man and god, which they imply. . we have given a view of the peculiar character of man's condition, which seem to claim for him a nature and place unique and incapable of repetition, in the scheme of the universe; and to this view astronomy, exhibiting to us the habitation of man as only one among many similar abodes, offers an objection. we are, therefore, now called upon, i conceive, to proceed to exhibit the answer which a somewhat different view of modern science suggests to this difficulty or objection. for this purpose, we must begin by regarding the earth in another point of view, different from that hitherto considered by us. footnotes: [ ] populäre vorlesungen über wissenschaftliche gegenstände, p. . chapter v. geology. . man, as i trust has been made apparent to the consciousness and conviction of the reader, is an intelligent, moral, religious, and spiritual creature; and we have to discuss the difficulty, or perplexity, or objection, which arises in our minds, when we consider such a creature as occupying an habitation, which is but one among many globes apparently equally fitted to be the dwelling-places of living things--a mere speck in the immensity of creation--an atom among such a vast array of material structures--a world, as we needs must deem it, among millions of other objects which appear to have an equal claim to be regarded as worlds. . the difficulty appears to be great, either way. can the earth alone be the theatre of such intelligent, moral, religious, and spiritual action? on the other hand, can we conceive such action to go on in the other bodies of the universe? if we take the latter alternative, we must people other planets and other systems with men such as we are, even as to their history. for the intellectual and moral condition of man implies a _history_ of the species; and the view of man's condition which religion presents, not only involves a scheme of which the history of the human race is a part, but also asserts a peculiar reference had, in the provisions of god, to the nature of man; and even a peculiar relation and connection between the human and the divine nature. to extend such suppositions to other worlds would be a proceeding so arbitrary and fanciful, that we are led to consider whether the alternative supposition may not be more admissible. the alternative supposition is, that man is, in an especial and eminent manner, the object of god's care; that his place in the creation is, not that he merely occupies one among millions of similar domiciles provided in boundless profusion by the creator of the universe, but that he is the servant, subject, and child of god, in a way unique and peculiar; that his being a spiritual creature, (including his other attributes in the highest for the sake of brevity,) makes him belong to a spiritual world, which is not to be judged of merely by analogies belonging to the material universe. . between these two difficulties the choice is embarrassing, and the decision must be unsatisfactory, except we can find some further ground of judgment. but perhaps this is not hopeless. we have hitherto referred to the evidence and analogies supplied by one science, namely, astronomy. but there are other sciences which give us information concerning the nature and history of the earth. from some of these, perhaps, we may obtain some knowledge of the place of the earth in the scheme of creation--how far it is, in its present condition, a thing unique, or only one thing among many like it. any science which supplies us with evidence or information on this head, will give us aid in forming a judgment upon the question under our consideration. to such sciences, then, we will turn our attention. one science has employed itself in investigating the nature and history of the earth by an examination of the materials of which it is composed; namely, geology. let us call to mind some of the results at which this science has arrived. . a very little attention to what is going on among the materials of which the earth's surface is composed, suffices to show us that there are causes of change constantly and effectually at work. the earth's surface is composed of land and water, hills and valleys, rocks and rivers. but these features undergo change, and produce change in each other. the mountain-rivers cut deeper and deeper into the ravines in which they run; they break up the rocks over which they rush, use the fragments as implements of further destruction, pile them up in sloping mounds where the streams issue from the mountains, spread them over the plains, fill up lakes with sediment, push into the sea great deltas. the sea batters the cliffs and eats away the land, and again, forms banks and islands where there had been deep water. volcanoes pour out streams of lava, which destroy the vegetation over which they flow, and which again, after a series of years, are themselves clothed with vegetation. earthquakes throw down tracts of land beneath the sea, and elevate other tracts from the bottom of the ocean. these agencies are everywhere manifest; and though at a given moment, at a given spot, their effect may seem to us almost imperceptible, too insignificant to be taken account of, yet in a long course of years almost every place has undergone considerable changes. rivers have altered their courses, lakes have become plains, coasts have been swept away or have become inland districts, rich valleys have been ravaged by watery or fiery deluges, the country has in some way or other assumed a new face. the present aspect of the earth is in some degree different from what it was a few thousand years ago. . but yet, in truth, the changes of which we thus speak have not been very considerable. the forms of countries, the lines of coasts, the ranges of mountains, the groups of valleys, the courses of rivers, are much the same now as they were in ancient times. the face of the earth, since man has had any knowledge of it, may have undergone some change, but the changeable has borne a small proportion to the permanent. changes have taken place, and are taking place, but they do not take place rapidly. the ancient earth and the modern earth are, in all their main physical features, identical; and we must go backwards through a considerably larger interval than that which carries us back to what we usually term _antiquity_, before we are led, by the operation of causes now at work, to an aspect of the earth's surface very different from that which it now presents. . for instance, rivers do, no doubt, more or less alter, in the course of years, by natural causes. the rhine, the rhone, the po, the danube, have, certainly, during the last four thousand years, silted up their beds in level places, expanded the deltas at their mouths, changed the channels by which they enter the sea; and very probably, in their upper parts, altered the forms of their waterfalls and of their shingle beds. yet even if we were thus to go backwards ten thousand, or twenty, or thirty thousand years, (setting aside great and violent causes of change, as earthquakes, volcanic eruptions, and the like,) the general form and course of these rivers, and of the ranges of mountains in which they flow, would not be different from what it is now. and the same may be said of coasts and islands, seas and bays. the present geography of the earth may be, and from all the evidence which we have, must be, very ancient, according to any measures of antiquity which can apply to human affairs. . but yet the further examination of the materials of the earth carries us to a view beyond this. though the general forms of the land and the waters of continents and seas, were, several thousand years ago, much the same as they now are; yet it was not always so. we have clear evidence that large tracts which are now dry ground, were formerly the bed of the ocean; and these, not tracts of the shore, where the varying warfare of sea and land is still going on, but the very central parts of great continents; the alps, the pyrenees, the himalayas. for not only are the rocks of which these great mountain-chains consist, of such structure that they appear to have been formed as layers of sediment at the bottom of water; but also, these layers contain vast accumulations of shells, or impressions of shells, and other remains of marine animals. and these appearances are not few, limited, or partial. the existence of such marine remains, in the solid substance of continents and mountains, is a general, predominant, and almost universal fact, in every part of the earth. nor is any other way of accounting for this fact admissible, than that those materials really have, at some time, formed bottoms of seas. the various other conjectures and hypotheses, which were put forward on this subject, when the amount, extent, multiplicity, and coherence of the phenomena were not yet ascertained, and when their natural history was not yet studied, cannot now be considered as worthy of the smallest regard. that many of our highest hills are formed of materials raised from the depths of ocean, is a proposition which cannot be doubted, by any one, who fairly examines the evidence which nature offers. . if we take this proposition only, we cannot immediately connect it with our knowledge respecting the surface of the earth in its present form. we learn that what is now land, has been sea; and we may suppose (since it is natural to assume that the bulk of the sea has not much changed) that what is now sea was formerly land. but, except we can learn something of the manner in which this change took place, we cannot make any use of our knowledge. was the change sudden, or gradual; abrupt, or successive; brief, or long-continuing? . to these questions, the further study of the facts enables us to return answers with great confidence. the change or changes which produced the effects of which we have spoken--the conversion of the bottom of the ocean into the centre of our greatest continents and highest mountains,--were undoubtedly gradual, successive, and long continued. we must state very briefly the grounds on which we make this assertion. . the masses which form our mountain-chains, offer evidence, as i have said, that they were deposited as sediment at the bottom of a sea, and then hardened. they consist of successive layers of such sediment, making up the whole mass of the mountain. these layers are, of course, to a certain extent, a measure of the time during which the deposition of sediment took place. the thicker the mass of sediment, the more numerous and varied its beds, and the longer period must we suppose to have been requisite for its formation. without making any attempt at accurate or definite estimation, which would be to no purpose, it is plain that a mass of sedimentary strata five thousand or ten thousand feet thick, must have required, for its deposit, a long course of years, or rather, a long course of ages. . but again: on further examination it is found, that we have not merely one series of sedimentary deposits, thus forming our mountains. there are a number of different series of such layers or strata, to be found in different ranges of hills, and in the same range, one series resting upon another. these different series of strata are distinguishable from one another by their general structure and appearance, besides more intimate characters, of which we shall shortly have to speak. each such series appears to have a certain consistency of structure within itself; the layers of which it is composed being more or less parallel, but the successive series are not thus always parallel, the lower ones being often highly inclined and irregular, while the upper ones are more level and continuous: as if the lower strata had been broken up and thrown into disorder, and then a new series of strata had been deposited horizontally on their fragments. but in whatever way these different sedimentary series succeeded each other, each series must have required, as we have seen, a long period for its formation; and to estimate the length of the interval between the two series, we have, at the present stage of our exposition, no evidence. . but the mechanical structure of the strata, the result, as it seems, of aqueous sedimentary deposit, is not the only, nor the most important evidence, with regard to the length of time occupied by the formation of the rocky layers which now compose our mountains. as we have said, they contain shells, and other remains of creatures which live in the sea. these they contain, not in small numbers, scattered and detached, but in vast abundance, as they are found in those parts of the ocean which is most alive with them. there are the remains of oysters and other shell-fish in layers, as they live at present in the seas near our shores; of corals, in vast patches and beds, as they now occur in the waters of the pacific; of shoals of fishes, of many different kinds, in immense abundance. each of these beds of shells, of corals, and of fishes, must have required many years, perhaps many centuries, for the growth of the successive individuals and successive generations of which it consists: as long a time, perhaps, as the present inhabitants of the sea have lived therein: or many times longer, if there have been many such successive changes. and thus, while the present condition of the earth extends backwards to a period of vast but unknown antiquity; we have, offered to our notice, the evidence of a series of other periods, each of which, so far as we can judge, may have been as long or longer than that during which the dry land has had its present form. . but the most remarkable feature in the evidence is yet to come. we have spoken in general of the oysters, and corals, and fishes, which occur in the strata of our hills; as if they were creatures of the same kinds which we now designate by those names. but a more exact examination of these remains of organized beings, shows that this is not so. the tribes of animals which are found petrified in our rocks are almost all different, so far as our best natural historians can determine, from those which now live in our existing seas. they are different species; different genera. the creatures which we find thus embedded in our mountains, are not only dead as individuals, but extinct as species. they belonged, not only to a terrestrial period, but to an animal creation, which is now past away. the earth is, it seems, a domicile which has outlasted more than one race of tenants. . it may seem rash and presumptuous in the natural historian to pronounce thus peremptorily that certain forms of life are nowhere to be found at present, even in the unfathomable and inaccessible depths of the ocean. but even if this were so, the proposition that the earth has changed its inhabitants, since the rocks were formed, of which our hills consist, does not depend for its proof on this assumption. for in the organic bodies which our strata contain, we find remains, not only of marine animals, but of animals which inhabit the fresh waters, and the land, and of plants. and the examination of such remains having been pursued with great zeal, and with all the aids which natural history can supply, the result has been, the proofs of a vast series of different tribes of animals and plants, which have successively occupied the earth and the seas; and of which the number, variety, multiplicity, and strangeness, exceed, by far, everything which could have been previously imagined. thus cuvier found, in the limestone strata on which paris stands, animals of the most curious forms, combining in the most wonderful manner the qualities of different species of existing quadrupeds. in another series of strata, the lias, which runs as a band across england from n. e. to s. w., we have the remains of lizards, or lacertine animals, different from those which now exist, of immense size and of extraordinary structure, some approaching to the form of fishes (_ichthyosaurus_); others, with the neck of a serpent; others with wings, like the fabled forms of dragons. then beyond these, that is, anterior to them in the series of time, we have the immense collection of fossil plants, which occur in the coal strata; the shells and corals of the mountain limestone; the peculiar fishes, different altogether from existing fishes, of the old red sandstone; and though, as we descend lower and lower, the traces of organic life appear to be more rare and more limited in kind, yet still we have, beneath these, in slates and in beds of limestone, many fossil remains, still differing from those which occur in the higher, and therefore, newer strata. . we have no intention of instituting any definite calculation with regard to the periods of time which this succession of forms of organic life may have occupied. this, indeed, the boldest geological speculators have not ventured to do. but the scientific discoveries thus made, have a bearing upon the analogies of creation, quite as important as the discoveries of astronomy. and therefore we may state briefly some of the divisions of the series of terrestrial strata which have suggested themselves to geological inquirers. at the outset of such speculations, it was conceived that the lower rocks, composed of granite, slate, and the like, had existed before the earth was peopled with living things; and that these, being broken up into inclined positions, there were deposited upon them, as the sediment of superincumbent waters, strata more horizontal, containing organic remains. the former were then called _primitive_ or _primary_, the latter, _secondary_ rocks. but it was soon found that this was too sweeping and peremptory a division. rocks which had been classed as primary, were found to contain traces of life; and hence, an intermediate class of _transition_ strata was spoken of. but this too was soon seen to be too narrow a scheme of arrangement, to take in the rapidly-accumulating mass of facts, organic and others, which the geological record of the earth's history disclosed. it appeared that among the fossil-bearing strata there might be discerned a long series of formations: the term _formation_ being used to imply a collection of successive strata, which, taking into account all the evidence, of materials, position, relations, and organic remains, appears to have been deposited during some one epoch or period; so as to form a natural group, chronologically and physiologically distinct from the others. in this way it appeared that, taking as the highest part of the secondary series, the beds of chalk, which, marked by characteristic fossils, run through great tracts of europe, with other beds, of sand and clay, which generally accompany these; there was, below this _cretaceous formation_, an _oolitic formation_, still more largely diffused, and still more abundant in its peculiar organic remains. below this, we have, in england, the _new red sandstone formation_, which, in other countries, is accompanied by beds abundant in fossils, as the _muschelkalk_ of germany. below this again we have the _coal formation_, and the _mountain limestone_, with their peculiar fossils. below these, we have the old red sandstone or devonian system, with its peculiar fishes and other fossils. beneath these, occur still numerous series of distinguishable strata; which have been arranged by sir roderick murchison as the members of the _silurian_ formation; the researches by which it was established having been carried on, in the first place, in south wales, the ancient country of the silures. including the lower part of this formation, and descending still lower in order, is the _cambrian_ formation of professor sedgwick. and since the races of organic beings, as we thus descend through successive strata, seem to be fewer and fewer in their general types, till at last they disappear; these lower members of the geological series have been termed, according to their succession, _palæozoic_, _protozoic_, and _hypozoic_ or _azoic_. the general impression on the minds of geologists has been, that, as we descend in this long staircase of natural steps, we are brought in view of a state of the earth in which life was scantily manifested, so as to appear to be near its earliest stages. . each of these formations is of great thickness. several of the members of each formation are hundreds, many of them thousands of feet thick. taken altogether, they afford an astounding record of the time during which they must have been accumulating, and during which these successive groups of animals must have been brought into being, lived, and continued their kinds. . we must add, that over the secondary strata there are found, in patches, generally of more limited extent, another, and of course, newer mass of strata, which have been termed _tertiary formations_. of these, the strata, near and under paris, lying in a hollow of the subjacent strata, and hence termed the _paris basin_, attracted prominent notice in the first place. and these are found to contain an immense quantity of remains of animals, which, being well preserved, and being subjected to a careful and scientific scrutiny by the great naturalist george cuvier, had an eminent share in establishing in the minds of geologists the belief of the extinct character of fossil species, and of the possibility of reconstructing, from such remains, the animals, different from those which now live, which had formerly tenanted the earth. . we have, in this enumeration, a series of groups of strata, each of which, speaking in a general way, has its own population of animals and plants, and is separated, by the peculiarities of these, from the groups below and above it. each group may, in a general manner, be considered as a separate creation of animal and vegetable forms--creatures which have lived and died, as the races now existing upon the earth live and die; and of which the living existence may, and according to all appearance must, have occupied ages, and series of ages, such as have been occupied by the present living generations of the earth. this series of creations, or of successive periods of life, is, no doubt, a very striking and startling fact, very different from anything which the imagination of man, in previous stages of investigation of the earth's condition, had conceived; but still, is established by evidence so complete, drawn from an examination and knowledge of the structures of living things so exact and careful, as to leave no doubt whatever of the reality of the fact, on the minds of those who have attended to the evidence; founded, as it is, upon the analogies, offices, anatomy, and combinations of organic structures. the progress of human knowledge on this subject has been carried on and established by the same alternations of bold conjectures and felicitous confirmations of them,--of minute researches and large generalizations,--which have given reality and solidity to the other most certain portions of human knowledge. that the strata of the earth, as we descend from the highest to the lowest, are distinguished in general by characteristic or organic fossils, and that these forms of organization are different from those which now live on the earth, are truths as clearly and indisputably established in the minds of those who have the requisite knowledge of geology and natural history, as that the planets revolve round the sun, and satellites round the planets. that these epochs of creation are something quite different from anything which we now see taking place on the earth, no more disturbs the belief of those facts, which scientific explorers entertain, than the seemingly obvious difference between the nebulæ which are regarded as yet unformed planetary systems, and the solar system to which our earth belongs, disturbs the belief of astronomers, that such nebulæ, as well as our system, really exist. indeed we may say, as we shall hereafter see, that the fact of our earth having passed through the series of periods of organic life which geologists recognize, is, hitherto, incomparably better established, than the fact that the nebulæ, or any of them, are passing through a series of changes, such as may lead to a system like ours; as some eminent astronomers in modern times have held. in this respect, the history of the world, and its place in the universe, are far more clearly learnt from geology than from astronomy. . but with regard to this series of organic _creations_, if, for the sake of brevity, we may call them so; we may naturally ask, in what manner, by what agencies, at what intervals, they succeeded each other on the earth? now, do the researches of geologists give us any information on these points, which may be brought to bear upon our present speculations? if we ask these questions, we receive, from different classes of geologists, different answers. a little while ago, most geologists held, probably the greater number still hold, that the transitions from one of these periods of organic life to another, were accompanied generally by seasons of violent disruption and mutation of the surface of the earth, exceeding anything which has taken place since the surface assumed its present general form; in the same proportion as the changes of its organic population go beyond any such changes which we can discern to be at present in operation. and there were found to be changes of other kinds, which seemed to show that these epochs of organic transition had also been epochs of mechanical violence, upon a vast and wonderful scale. it appeared that, at some of these epochs at least, the strata previously deposited, as if in comparative tranquillity, had been broken, thrust up from below, or drawn or cast downwards; so that strata which must at first have been nearly level, were thrown into positions highly inclined, fractured, set on edge, contorted, even inverted. over the broken edges of these strata, thus disturbed and fractured, were found vast accumulations of the fragments which such rude treatment might naturally produce; these fragmentary ruins being spread in beds comparatively level, over the bristling edges of the subjacent rocks, as if deposited in the fluid which had overwhelmed the previous structure; and with few or no traces of life appearing in this mass of ruins; while, in the strata which lay over them, and which appeared to have been the result of quieter times, new forms of organic life made their appearance in vast abundance. such is, for example, the relation of the coal strata in a great part of england; broken into innumerable basins, ridges, valleys, strips, and shreds, lying in all positions; and then filled into a sort of level, by the conglomerate of the magnesian limestone, and the superincumbent red sandstone and oolites. in other cases it appeared as if there were the means of tracing, in these dislocations, the agency of igneous stony matter, which had been injected from below, so as to form mountain-chains, or the cores of such; and in which the period of the convulsion could be traced, by the strata to which the disturbance extended; _those_ strata being supposed to have been deposited before the eruption, which were thrust upwards by it into highly-inclined positions; while those strata which, though near to these scenes of mechanical violence, were still comparatively horizontal, as they had been originally deposited, were naturally inferred to have been formed in the waters, after the catastrophe had passed away. by such reasonings as these, m. elie de beaumont has conceived that he can ascertain the relative ages (according to the vast and loose measurements of age which belong to this subject) of the principal ranges of mountains of the earth's surface. . such estimations of age can, indeed, as we have intimated, be only of the widest and loosest kind; yet they all concur in assigning very great and gigantic periods of time, as having been occupied by the events which have formed the earth's strata, and brought them into their present position. for not only must there have been long ages employed, as we have said, while the successive generations of each group of animals lived, and died, and were entombed in the abraded fragments of the then existing earth; but the other operations which intervened between these apparently more tranquil processes, must also have occupied, it would seem, long ages at each interval. the dislocation, disruption, and contortion of the vast masses of previously existing mountains, by which their framework was broken up, and its ruins covered with beds of its own rubbish, many thousand feet thick, and gradually becoming less coarse and smoother, as the higher beds were deposited upon the lower, could hardly take place, it would seem, except in hundreds and thousands of years. and then again, all these processes of deposition, thus arranging loose masses of material into level beds, must have taken place in the bottom of deep oceans; and the beds of these oceans must have been elevated into the position of mountain ridges which they now occupy, by some mighty operation of nature, which must have been comparatively tranquil, since it has not much disturbed those more level beds; and which, therefore, must have been comparatively long continued. if we accept, as so many eminent geologists have done, this evidence of a vast series of successive periods of alternate violence and repose, we must assign to each such period a duration which cannot but be immense, compared with the periods of time with which we are commonly conversant. in the periods of comparative quiet, such as now exist on the earth's surface, and such as seem to be alone consistent with continued life and successive generation, deposits at the bottom of lakes and seas take place, it would seem, only at the rate of a few feet in a year, or perhaps, in a century. when, therefore, we find strata, bearing evidence of such a mode of deposit, and piled up to the amount of thousands and tens of thousands of feet, we are naturally led to regard them as the production of myriads of years; and to add new myriads, as often as, in the prosecution of geological research, we are brought to new masses of strata of the like kind; and again, to interpolate new periods of the same order, to allow for the transition from one such group to another. . nor is there anything which need startle us, in the necessity of assuming such vast intervals of time, when we have once brought ourselves to deal with the question of the antiquity of the earth upon scientific evidence alone. for if geology thus carries us far backwards through thousands, it may be, millions of years, astronomy does not offer the smallest argument to check this regressive supposition. on the contrary, all the most subtle and profound investigations of astronomers have led them to the conviction, that the motions of the earth may have gone on, as they now go on, for an indefinite period of past time. there is no tendency to derangement in the mechanism of the solar system, so for as science has explored it. minute inequalities in the movements exist, too small to produce any perceptible effect on the condition of the earth's surface; and even these inequalities, after growing up through long cycles of ages, to an amount barely capable of being detected by astronomical scrutiny, reach a maximum; and, diminishing by the same slow degrees by which they increased, correct themselves, and disappear. the solar system, and the earth as part of it, constitute, so for as we can discover, a perpetual motion. . there is therefore nothing, in what we know of the cosmical conditions of our globe, to contradict the terrestrial evidence for its vast antiquity, as the seat of organic life. if for the sake of giving definiteness to our notions, we were to assume that the numbers which express the antiquity of these four periods;--the present organic condition of the earth; the tertiary period of geologists, which preceded that; the secondary period, which was anterior to that; and the primary period which preceded the secondary; were on the same scale as the numbers which express these four magnitudes:--the magnitude of the earth; that of the solar system compared with the earth; the distance of the nearest fixed stars compared with the solar system; and the distance of the most remote nebulæ compared with the nearest fixed stars; there is, in the evidence which geological science offers, nothing to contradict such an assumption. . and as the infinite extent which we necessarily ascribe to space, allows us to find room, without any mental difficulty, for the vast distances which astronomy reveals, and even leaves us rather embarrassed with the infinite extent which lies beyond our farthest explorations; so the infinite duration which we, in like manner, necessarily ascribe to past time, makes it easy for us, so far as our powers of intellect are concerned, to go millions of millions of years backwards, in order to trace the beginning of the earth's existence,--the first step of terrestrial creation. it is as easy for the mind of man to reason respecting a system which is billions or trillions of miles in extent, and has endured through the like number of years, or centuries, as it is to reason about a system (the earth, for instance,) which is forty million feet in extent, and has endured for a hundred thousand million of seconds, that is, a few thousand years. . this statement is amply sufficient for the argument which we have to found upon it; but before i proceed to do that, i will give another view which has recently been adopted by some geologists, of the mode in which the successive periods of creation, which geological research discloses to us, have passed into one another. according to this new view, we find no sufficient reason to believe that the history of the earth, as read by us in the organic and mechanical phenomena of its superficial parts, has consisted of such an alternation of periods of violence and of repose, as we have just attempted to describe. according to these theorists, strata have succeeded strata, one group of animals and plants has followed another, through a season of uniform change; with no greater paroxysm or catastrophe, it may be, than has occurred during the time that man has been an observer of the earth. it may be asked, how is this consistent with the phenomena which we have described;--with the vast masses of ruin, which mark the end of one period and the beginning of another, as is the case in passing from the coal measures of england to the superincumbent beds;--with the highly-inclined strata of the central masses, and the level beds of the upper formations which have been described as marking the mountain ranges of europe? to these questions, a reply is furnished, we are told, by a more extensive and careful examination of the strata. it may be, that in certain localities, in certain districts, the transition, from the mountain limestone and the coal, to the superjacent sandstones and oolites, is abrupt and seemingly violent; marked by _unconformable_ positions of the upper upon the lower strata, by beds of conglomerate, by the absence of organic remains in certain of these beds. but if we follow these very strata into other parts of the world, or even into other parts of this island, we find that this abruptness and incongruity between the lower and the higher strata disappears. between the mountain-limestone and the red sandstone which lies over it, certain new beds are found, which fill up the incoherent interval; which offer the same evidence as the strata below and above them, of having been produced tranquilly; and which do not violently differ in position from either group. the appearance of incoherence in the series arose from the occurrence, in the region first examined, of a gap, which is here filled up,--a blank which is here supplied. hence it is inferred, that whatever of violence and extreme disturbance is indicated by the dislocations and ruins there observed, was local and partial only; and that, at the very time when these fragmentary beds, void of organized beings, were forming in one place, there were, at the same time, going on, in another part of the earth's surface, not far removed, the processes of the life, death and imbedding of species, as tranquilly as at any other period. and the same assertion is made with regard to the more general fact, before described, of the stratigraphical constitution of mountain chains. it is asserted that the unconformable relation of the strata which compose the different parts of those chains, is a local occurrence only; and that the same strata, if followed into other regions, are found conformable to each other; or are reduced to a virtually continuous scheme, by the interpolation of other strata, which make a transition, in which no evidence of exceptional violence appears. . we shall not attempt (it is not at all necessary for us to do so) to decide between the doctrines of the two geological schools which thus stand in this opposition to each other. but it will be useful to our argument to state somewhat further the opinions of this latter school on one main point. we must explain the view which these geologists take of the mode of succession of one group of _organized_ beings to another; by which, as we have said, the different successive strata are characterized. such a phenomenon, it would at first seem, cannot be brought within the ordinary rules of the existing state of things. the species of plants and animals which inhabit the earth, do not change from age to age; they are the same in modern times, as they were in the most remote antiquity, of which we have any record. the dogs and horses, sheep and cattle, lions and wolves, eagles and swallows, corn and vines, oaks and cedars, which occupy the earth now, are not, we have the strongest reasons to believe, essentially different now from what they were in the earliest ages. at least, if one or two species have disappeared, no new species have come into existence. we cannot conceive a greater violation of the known laws of nature, than that such an event as the appearance of a new species should have occurred. even those who hold the uniformity of the mechanical changes of the earth, and of the rate of change, from age to age, and from one geological period to another; must still, it would seem, allow that the zoological and phytological changes of which geology gives her testimony, are complete exceptions to what is now taking place. the formation of strata at the bottom of the ocean from the ruin of existing continents, may be going on at present. even the elevation of the bed of the ocean in certain places, as a process imperceptibly slow, may be in action at this moment, as these theorists hold that it is. but still, even when the beds thus formed are elevated into mountain chains, if that should happen, in the course of myriads of years, (according to the supposition it cannot be effected in a less period,) the strata of such mountain chains will still contain only the species of such creatures as now inhabit the waters; and we shall have, even then, no succession of organic epochs, such as geology discovers in the existing mountains of the earth. . the answer which is made to this objection appears to me to involve a license of assumption on the part of the _uniformitarian_ geologist, (as such theorists have been termed,) which goes quite beyond the bounds of natural philosophy: but i wish to state it; partly, in order to show that the most ingenious men, stimulated by the exigencies of a theory, which requires some hypothesis concerning the succession of species, to make it coherent and complete, have still found it impossible to bring the creation of species of plants and animals within the domain of natural science; and partly, to show how easily and readily geological theorists are led to assume periods of time, even of a higher order than those which i have ventured to suggest. . it must, however, be first stated, as a fact on which the assumption is founded which i have to notice, that the organic groups by which these successive strata are characterized, are not so distinct and separate, as it was convenient, for the sake of explanation, to describe them in the first instance. although each body of strata is marked by predominant groups of genera and species, yet it is not true, that all the species of each formation disappear, when we proceed to the next. some species and genera endure through several successive groups of strata; while others disappear, and new forms come into view, as we ascend. and thus, the change from one set of organic forms to another, as we advance in time, is made, not altogether by abrupt transitions, but in part continuously. the uniformitarian, in the case of organic, as in the case of mechanical change, obliterates or weakens the evidence of sudden and catastrophic leaps, by interposing intermediate steps, which involve, partly the phenomena of the preceding, and partly those of the subsequent condition. as he allows no universal transition from one deposit to a succeeding discrepant and unconformable deposit, so he allows no abrupt and complete transition from one collection of organic beings,--one creation, as we may call it,--to another. if creation must needs be an act out of the region of natural science, he will have it to be at least an act not exercised at distant intervals, and on peculiar occasions; but constantly going on, and producing its effects, as much at one time in the geological history of the world, as at another. . and this he holds, not only with regard to the geological periods which have preceded the existing condition of the earth, but also with regard to the transition from those previous periods to that in which we live. the present population of the earth is not one in which all previous forms are extinct. the past population of the earth was not one in which there are found no creatures still living. on the contrary, he finds that there exists a vast mass of strata, superior to the secondary strata, which are characterized by extinct forms, and are yet inferior to those deposits which are now going on by the agency of obvious causes. these masses of strata contain a population of creatures, partly extinct species, and partly such species as are still living on our land and in our waters. the proportion in which the old and the new species occur in such strata, is various; and the strata are so numerous, so rich in organic remains, so different from each other, and have been so well explored, that they have been classified and named according to the proportion of new and of old species which they contain. those which contain the largest proportion of species still living, have been termed _pliocene_, as containing a _greater_ number of _new_ or recent species. below these, are strata which are termed _miocene_, implying a _smaller_ number of _new_ species. below these again, are others which have been termed _eocene_, as containing few new species indeed, but yet enough to mark the _dawn_, the _eos_, of the existing state of the organic world. these strata are, in many places, of very considerable thickness; and their number, their succession, and the great amount of extinct species which they contain, shows, in a manner which cannot be questioned, (if the evidence of geology is accepted at all,) in what a gradual manner, a portion at least, of the existing forms of organic life have taken the place of a different population previously existing on the surface of the globe. . and thus the uniformitarian is led to consider the facts which geology brings to light, as indicating a slow and almost imperceptible, but, upon the whole, constant series of changes, not only in the position of the earth's materials, but in its animal and vegetable population. land becomes sea and sea becomes land; the beds of oceans are elevated into mountain regions, carrying with them the remains of their inhabitants; sheets of lava pour from volcanic vents and overwhelm the seats of life; and these, again, become fields of vegetation; or, it may be, descend to the depths of the sea, and are overgrown with groves of coral; lakes are filled with sediment, imbedding the remains of land animals, and form the museums of future zoologists; the deltas of mighty rivers become the centres of continents, and are excavated as coal-fields by men in remote ages. and yet all this time, so slow is the change, that man is unaware such changes are going on. he knows that the mountains of scandinavia are rising out of the baltic at the rate of a few feet in a century; he knows that the fertile slope of etna has been growing for thousands of years by the addition of lava streams and parasitic volcanos; he knows that the delta of the mississippi accumulates hundreds of miles of vegetable matter every generation; he knows that the shores of europe are yielding to the sea; but all these appear to him minute items, not worth summing; infinitesimal quantities, which he cannot integrate. and so, in truth, they are, for him. his ephemeral existence does not allow him to form a just conception, in any ordinary state of mind, of the effects of this constant agency of change, working through countless thousands of years. but time, inexhausted and unremitting, sums the series, integrates the formula of change; and thus passes, with sure though noiseless progress, from one geological epoch to another. . and in the meanwhile, to complete the view thus taken by the uniformitarian of the geological history of the earth, by some constant but inscrutable law, creative agency is perpetually at work, to introduce, into this progressive system of things, new species of vegetable and animal life. organic forms, ever and ever new ones, are brought into being, and left, visible footsteps, as it were, of the progress which time has made;--marks placed between the rocky leaves of the book of creation; by which man, when his time comes, may turn back and read the past history of his habitation. but the point for us to remark is, the immeasurable, the inconceivable length of time, if any length of time could be inconceivable, which is required of our thoughts, by this new assumption of the constant production of new species, as a law of creation. we might feel ourselves well nigh overwhelmed, when, by looking at processes which we see producing only a few feet of height or breadth or depth during the life of man, we are called upon to imagine the construction of alps and andes,--when we have to imagine a world made a few inches in a century. but there, at least, we had _something_ to start from: the element of change was small, but there _was_ an element of change: we had to expand, but we had not to originate. but in conceiving that all the myriads of successive species, which we find in the earth's strata, have come into being by a law which is now operating, we have _nothing_ to start from. we have seen, and know of, no such change; all sober and skilful naturalists reject it, as a fact not belonging to our time. we have here to build a theory without materials;--to sum a series of which every term, so far as we know, is nothing;--to introduce into our scientific reasonings an assumption contrary to all scientific knowledge. . this appears to me to be the real character of the assumption of the constant creation of new species. but, as i have said, it is not my business here, to pronounce upon the value or truth of this assumption. the only use which i wish to make of it is this:--if any persons, who have adopted the geological view which i have just been explaining, should feel any interest in the speculations here offered to their notice, they must needs be (as i have no doubt they will be) even more willing than other geologists, to grant to our argument a scale of time for geological succession, corresponding in magnitude to the scale of distances which astronomy teaches us, as those which measure the relation of the universe to the earth. this being supposed to be granted, i am prepared to proceed with my argument. chapter vi. the argument from geology. . i have endeavored to explain that, according to the discoveries of geologists, the masses of which the surface of the earth is composed, exhibit indisputable evidence that, at different successive periods, the land and the waters which occupy it, have been inhabited by successive races of plants and animals; which, when taken in large groups, according to the ascending or descending order of the strata, consist of species different from those above and below them. many of these groups of species are of forms so different from any living things which now exist, as to give to the life of those ancient periods an aspect strangely diverse from that which life now displays, and to transfer us, in thought, to a creation remote in its predominant forms from that among which we live. i have shown also, that the life and successive generations of these groups of species, and the events by which the rocks which contain these remains have been brought into their present situation and condition, must have occupied immense intervals of time;--intervals so large that they deserve to be compared, in their numerical expression, with the intervals of space which separate the planets and stars from each other. it has been seen, also, that the best geologists and natural historians have not been able to devise any hypothesis to account for the successive introduction of these new species into the earth's population; except the exercise of a series of acts of creation, by which they have been brought into being; either in groups at once, or in a perpetual succession of one or a few species, which the course of long intervals of time might accumulate into groups of species. it is true, that some speculators have held that by the agency of natural causes, such as operate upon organic forms, one species might be transmuted into another; external conditions of climate, food, and the like, being supposed to conspire with internal impulses and tendencies, so as to produce this effect. this supposition is, however, on a more exact examination of the laws of animal life, found to be destitute of proof; and the doctrine of the successive creation of species remains firmly established among geologists. that the _extinction_ of species, and of groups of species, may be accounted for by natural causes, is a proposition much more plausible, and to a certain extent, probable; for we have good reason to believe that, even within the time of human history, some few species have ceased to exist upon the earth. but whether the extinction of such vast groups of species as the ancient strata present to our notice, can be accounted for in this way, at least without assuming the occurrence of great catastrophes, which must for a time, have destroyed all forms of life in the district in which they occurred, appears to be more doubtful. the decision of these questions, however, is not essential to our purpose. what is important is, that immense numbers of tribes of animals have tenanted the earth for countless ages, before the present state of things began to be. . the present state of things is that to which the existence and the history of man belong; and the remark which i now have to make is, that the existence and the history of man are facts of an entirely different order from any which existed in any of the previous states of the earth; and that this history has occupied a series of years which, compared with geological periods, may be regarded as very brief and limited. . the remains of man are nowhere found in the strata which contain the records of former states of the earth. skeletons of vast varieties of creatures have been disinterred from their rocky tombs; but these cemeteries of nature supply no portion of a human skeleton. in earlier periods of natural science, when comparative anatomy was as yet very imperfectly understood, no doubt, many fossil bones were supposed to be human bones. the remains of giants and of antediluvians were frequent in museums. but a further knowledge of anatomy has made it appear that such bones all belong to animals, of one kind or another; often, to animals utterly different, in their form and skeleton, from man. also some bones, really human, have been found petrified in situations in which petrification has gone on in recent times, and is still going on. human skeletons, imbedded in rocks by this process, have been found in the island of guadaloupe, and elsewhere. but this phenomenon is easily distinguishable from the petrified bones of other animals, which are found in rocks belonging to really geological periods; and does not at all obliterate the distinction between the geological and the historical periods. . indeed not bones only, but objects of art, produced by human workmanship, are found fossilized and petrified by the like processes; and these, of course, belong to the historical period. human bones, and human works, are found in such deposits as morasses, sand-banks, lava-streams, mounds of volcanic ashes; and many of them may be of unknown, and, compared with the duration of a few generations, of very great antiquity; but such deposits are distinguishable, generally without difficulty, from the strata in which the geologist reads the records of former creations. it has been truly said, that the geologist is an _antiquary_; for, like the antiquary, he traces a past condition of things in the remains and effects of it which still subsist; but it has also been truly said, at the same time, that he is an antiquary _of a new order_; for the remains which he studies are those which illustrate the history of the earth, not of man. the geologist's antiquity is not that of ornaments and arms, utensils and habiliments, walls and mounds; but of species and of genera, of seas and of mountains. it is true, that the geologist may have to study the works of man, in order to trace the effects of causes which produce the results which he investigates; as when he examines the pholad-pierced pillars of pateoli, to prove the rise and the fall of the ground on which they stand; or notes the anchoring-rings in the wall of some roman edifice, once a maritime fort, but now a ruin remote from the sea; or when he remarks the streets in the towns of scania, which are now below the level of the baltic,[ ] and therefore show that the land has sunk since these pavements were laid. but in studying such objects, the geologist considers the hand of man as only one among many agencies. man is to him only one of the natural causes of change. . and if, with the illustrious author to whom we have just referred,[ ] we liken the fossil remains, by which the geologist determines the age of his strata, to the medals and coins in which the antiquary finds the record of reigns and dynasties; we must still recollect that a _coin_ really discloses a vast body of characteristics of man, to which there is nothing approaching in the previous condition of the world. for how much does a coin or medal indicate? property; exchange; government; a standard of value; the arts of mining, assaying, coining, drawing, and sculpture; language, writing, and reckoning; historical recollections, and the wish to be remembered by future ages. all this is involved in that small human work, a coin. if the fossil remains of animals may (as has been said) be termed medals struck by nature to record the epochs of her history; medals must be said to be, not merely, like fossil remains, records of material things; they are the records of thought, purpose, society, long continued, long improved, supplied with multiplied aids and helps; they are the permanent results, in a minute compass, of a vast progress, extending through all the ramifications of human life. . not a coin merely, but any, the rudest work of human art, carries us far beyond the domain of mere animal life. there is no transition from man to animals. no doubt, there are races of men very degraded, barbarous, and brutish. no doubt there are kinds of animals which are very intelligent and sagacious; and some which are exceedingly disposed to and adapted to companionship with man. but by elevating the intelligence of the brute, we do not make it become the intelligence of the man. by making man barbarous, we do not make him cease to be a man. animals have their especial capacities, which may be carried very far, and may approach near to human sagacity, or may even go beyond it; but the capacity of man is of a different kind. it is a capacity, not for becoming sagacious, but for becoming rational; or rather it is a capacity which he has in virtue of being rational. it is a capacity of progress. in animals, however sagacious, however well trained, the progress in skill and knowledge is limited, and very narrowly limited. the creature soon reaches a boundary, beyond which it cannot pass; and even if the acquired habits be transmitted by descent to another generation, (which happens in the case of dogs and several other animals,) still the race soon comes to a stand in its accomplishments. but in man, the possible progress from generation to generation, in intelligence and knowledge, and we may also say, in power, is indefinite; or if this be doubted, it is at least so vast, that compared with animals, his capacity is infinite. and this capacity extends to all races of men its characterizing efficacy: for we have good reason to believe that there is no race of human beings who may not, by a due course of culture, continued through generations, be brought into a community of intelligence and power with the most intelligent and the most powerful races. this seems to be well established, for instance, with regard to the african negroes; so long regarded by most, by some probably regarded still, as a race inferior to europeans. it has been found that they are abundantly capable of taking a share in the arts, literature, morality and religion of european peoples. and we cannot doubt that, in the same manner, the native australians, or the bushmen of the cape of good hope, have human faculties and human capacities; however difficult it might be to unfold these, in one or two generations, into a form of intelligence and civilization in any considerable degree resembling our own. . it is not requisite for us, and it might lead to unnecessary difficulties, to fix upon any one attribute of man, as peculiarly characteristic, and distinguishing him from brutes. yet it would not be too much to say that man is, in truth, universally and specifically characterized by the possession of _language_. it will not be questioned that language, in its highest forms, is a wonderful vehicle and a striking evidence of the intelligence of man. his bodily organs can, by a few scarcely perceptible motions, shape the air into sounds which express the kinds, properties, actions and relations of things, under thousands of aspects, in forms infinitely more general and recondite than those in which they present themselves to his senses;--and he can, by means of these forms, aided by the use of his senses, explore the boundless regions of space, the far recesses of past time, the order of nature, the working of the author of nature. this man does, by the exercise of his reason, and by the use of language, a necessary implement of his reason for such purposes. . that language, in such a stage, is a special character of man, will not be doubted. but it may be thought, there is little resemblance between language in this exalted degree of perfection, and the seemingly senseless gibberish of the most barbarous tribes. such an opinion, however, might easily be carried too far. all human language has in it the elements of indefinite intellectual activity, and the germs of indefinite development. even the rudest kind of speech, used by savages, denotes objects by their kinds, their attributes, their relations, with a degree of generality derived from the intellect, not from the senses. the generality may be very limited; the relations which the human intellect is capable of apprehending may be imperfectly conveyed. but to denote kinds and attributes and actions and relations _at all_, is a beginning of generalization and abstraction;--or rather, is far more than a beginning. it is the work of a faculty which can generalize and abstract; and these mental processes once begun, the field of progress which is open to them is indefinite. undoubtedly it may happen that weak and barbarous tribes are, for many generations, so hard pressed by circumstances, and their faculties so entirely absorbed in providing for the bare wants of the poorest life, that their thoughts may never travel to anything beyond these, and their language may not be extended so as to be applicable to any other purposes. but this is not the standard condition of mankind. it is not, by such cases, that man, or that human nature, is to be judged. the normal condition of man is one of an advance beyond the mere means of subsistence, to the arts of life, and the exercise of thought in a general form. to some extent, such an advance has taken place in almost every region of the earth and in every age. . perhaps we may often have a tendency to think more meanly than they deserve, of so-called barbarous tribes, and of those whose intellectual habits differ much from our own. we may be prone to regard ourselves as standing at the summit of civilization; and all other nations and ages, as not only occupying inferior positions, but positions on a slope which descends till it sinks into the nature of brutes. and yet how little does an examination of the history of mankind justify this view! the different stages of civilization, and of intellectual culture, which have prevailed among them, have had no appearance of belonging to one single series, in which the cases differed only as higher or lower. on the contrary, there have been many very different kinds of civilization, accompanied by different forms of art and of thought; showing how universally the human mind tends to such habits, and how rich it is in the modes of manifesting its innate powers. how different have been the forms of civilization among the chinese, the indians, the egyptians, the babylonians, the mexicans, the peruvians! yet in all, how much was displayed of sagacity and skill, of perseverance and progress, of mental activity and grasp, of thoughtfulness and power. are we, in thinking of these manifestations of human capacity, to think of them as only a stage between us and brutes? or are we to think so, even of the stoical red indians of north america, or the energetic new zealanders, and caffres? and if not, why of the african negroes, or the australians, or the bushmen? we may call their language a jargon. very probable it would, in its present form, be unable to express a great deal of what we are in the habit of putting into language. but can we refuse to believe that, with regard to matters with which they are familiar, and on occasions where they are interested, they would be to each other intelligible and clear? and if we suppose cases in which their affections and emotions are strongly excited, (and affections and emotions at least we cannot deny them,) can we not believe that they would be eloquent and impressive? do we not know, in fact, that almost all nations which we call savage, are, on such occasions, eloquent in their own language? and since this is so, must not their language, after all, be a wonderful instrument as well as ours? since it can convey one man's thoughts and emotions to many, clothed in the form which they assume in his mind; giving to things, it may be, an aspect quite different from that which they would have if presented to their own senses; guiding their conviction, warming their hearts, impelling their purposes;--can language, even in such cases, be otherwise than a wonderful produce of man's internal, of his mental, that is, of his peculiarly _human_ faculties? and is not language, therefore, even in what we regard as its lowest forms, an endowment which completely separates man from animals which have no such faculty?--which cannot regard, or which cannot convey, the impressions of the individual in any such general and abstract form? probably we should find, as those who have studied the language of savages always have found, that every such language contains a number of curious and subtle practices,--_contrivances_, we cannot help calling them,--for marking the relations, bearings and connections of words; contrivances quite different from those of the languages which we think of as more perfect; but yet, in the mouths of those who use such speech, answering their purpose with great precision. but without going into such details, the use of any _articulate_ language is, as the oldest greeks spoke of it, a special and complete distinction of man as man. . it would be an obscure and useless labor, to speculate upon the question whether animals have among themselves anything which can properly be called _language_. that they have anything which can be termed language, in the sense in which we here speak of it, as admitting of general expressions, abstractions, address to numbers, eloquence, is utterly at variance with any interpretation which we can put upon their proceedings. the broad distinction of instinct and reason, however obscure it may be, yet seems to be most simply described, by saying, that animals do not apprehend their impressions under general forms, and that man does. resemblance, and consequent association of impressions, may often show like generalization; but yet it is different. there is, in man's mind, a germ of general thoughts, suggested by resemblances, which is evolved and fixed in language; and by the aid of such an addition to the impressions of sense, man has thousands of intellectual pathways from object to object, from effect to cause, from fact to inference. his impressions are projected on a sphere of thought of which the radii can be prolonged into the farthest regions of the universe. animals, on the contrary, are shut up in their sphere of sensation,--passing from one impression to another by various associations, established by circumstances; but still, having access to no wider intellectual region, through which lie lines of transition purely abstract and mental. that they have their modes of communicating their impressions and associations, their affections and emotions, we know; but these modes of communication do not make a language; nor do they disturb the assignment of language as a special character of man; nor the belief that man differs in his kind, and we may say, using a larger phrase, in his order, from all other creatures. . we may sometimes be led to assign much of the development of man's peculiar powers, to the influence of external circumstances. and that the development of those powers is so influenced, we cannot doubt; but their development only, not their existence. we have already said that savages, living a precarious and miserable life, occupied incessantly with providing for their mere bodily wants, are not likely to possess language, or any other characteristic of humanity, in any but a stunted and imperfect form. but, that manhood is debased and degraded under such adverse conditions, does not make man cease to be man. even from such an abject race, if a child be taken and brought up among the comforts and means of development which civilized life supplies, he does not fail to show that he possesses, perhaps in an eminent degree, the powers which specially belong to man. the evidences of human tendencies, human thoughts, human capacities, human affections and sympathies, appear conspicuously, in cases in which there has been no time for external circumstances to operate in any great degree, so as to unfold any difference between the man and the brute; or in which the influence of the most general of external agencies, the impressions of several of the senses, have been intercepted. who that sees a lively child, looking with eager and curious eyes at every object, uttering cries that express every variety of elementary human emotion in the most vivacious manner, exchanging looks and gestures, and inarticulate sounds, with his nurse, can doubt that already he possesses the germs of human feeling, thought and knowledge? that already, before he can form or understand a single articulate word, he has within him the materials of an infinite exuberance of utterance, and an impulse to find the language into which such utterance is to be moulded by the law of his human nature? and perhaps it may have happened to others, as it has to me, to know a child who had been both deaf, dumb, and blind, from a very early age. yet she, as years went on, disclosed a perpetually growing sympathy with the other children of the family in all their actions, with which of course she could only acquaint herself by the sense of touch. she sat, dressed, walked, as they did; even imitated them in holding a book in her hand when they read, and in kneeling when they prayed. no one could look at the change which came over her sightless countenance, when a known hand touched hers, and doubt that there was a human soul within the frame. the human soul seemed not only to be there, but to have been fully developed; though the means by which it could receive such communications as generally constitute human education, were thus cut off. and such modes of communication with her companions as had been taught her, or as she had herself invented, well bore out the belief, that her mind was the constant dwelling-place, not only of human affections, but of human thoughts. so plainly does it appear that human thought is not produced or occasioned by external circumstances only; but has a special and indestructible germ in human nature. . i have been endeavoring to illustrate the doctrine that man's nature is different from the nature of other animals; as subsidiary to the doctrine that the human epoch of the earth's history is different from all the preceding epochs. but in truth, this subsidiary proposition is not by any means necessary to my main purpose. even if barbarous and savage tribes, even if men under unfavorable circumstances, be little better than the brutes, still no one will doubt that the most civilized races of mankind, that man under the most favorable circumstances, is far, is, indeed, immeasurably elevated above the brutes. the history of man includes not only the history of scythians and barbarians, australians and negroes, but of ancient greeks and of modern europeans; and therefore there can be no doubt that the period of the earth's history, which includes the history of man, is very different indeed from any period which preceded that. to illustrate the peculiarity, the elevation, the dignity, the wonderful endowments of man, we might refer to the achievements, the recorded thoughts and actions, of the most eminent among those nations;--to their arts, their poetry, their eloquence; their philosophers, their mathematicians, their astronomers; to the acts of virtue and devotion, of patriotism, generosity, obedience, truthfulness, love, which took place among them;--to their piety, their reverence for the deity, their resignation to his will, their hope of immortality. such characteristic traits of man as man, (which all examples of intelligence, virtue, and religion, are,) might serve to show that man is, in a sense quite different from other creatures, "fearfully and wonderfully made;" but i need not go into such details. it is sufficient for my purpose to sum up the result in the expressions which i have already used; that man is an intellectual, moral, religious, and spiritual being. . but the existence of man upon the earth being thus an event of an order quite different from any previous part of the earth's history, the question occurs, how long has this state of things endured? what period has elapsed since this creature, with these high powers and faculties, was placed upon the earth? how far must we go backward in time, to find the beginning of his wonderful history?--so utterly wonderful compared with anything which had previously occurred. for as to that point, we cannot feel any doubt. the wildest imagination cannot suggest that corals and madrepores, oysters and sepias, fishes and lizards, may have been rational and moral creatures; nor even those creatures which come nearer to human organization; megatheriums and mastodons, extinct deer and elephants. undoubtedly the earth, till the existence of man, was a world of mere brute creatures. how long then has it been otherwise? how long has it been the habitation of a rational, reflective, progressive race? can we by any evidence, geological or other, approximate to the beginning of the human history? . this is a large and curious question, and one on which a precise answer may not be within our reach. but an answer not precise, an approximation, as we have suggested, may suffice for our purpose. if we can determine, in some measure, the order and scale of the period during which man has occupied the earth, the determination may serve to support the analogy which we wish to establish. . the geological evidence with regard to the existence of man is altogether negative. previous to the deposits and changes which we can trace as belonging obviously to the present state of the earth's surface, and the operation of causes now existing, there is no vestige of the existence of man, or of his works. as was long ago observed,[ ] we do not find, among the shells and bones which are so abundant in the older strata, any weapons, medals, implements, structures, which speak to us of the hand of man, the workman. if we look forwards ten or twenty thousand years, and suppose the existing works of man to have been, by that time, ruined and covered up by masses of rubbish, inundations, morasses, lava-streams, earthquakes; still, when the future inhabitant of the earth digs into and explores these coverings, he will discover innumerable monuments that man existed so long ago. the materials of many of his works, and the traces of his own mind, which he stamps upon them, are as indestructible as the shells and bones which give language to the oldest work. indeed, in many cases the oldest fossil remains are the results of objects of seemingly the most frail and perishable material;--of the most delicate and tender animal and vegetable tissues and filaments. that no such remains of textures and forms, moulded by the hand of man, are anywhere found among these, must be accepted as indisputable evidence that man did not exist, so as to be contemporary with the plants and animals thus commemorated. according to geological evidence, the race of man is a novelty upon the earth;--something which has succeeded to all the great geological changes. . and in this, almost all geologists are agreed. even those who hold that, in other ways, the course of change has been uniform;--that even the introduction of man, as a new species of animal, is only an event of the same kind as myriads of like events which have occurred in the history of the earth;--still allow that the introduction of man, as a moral being, is an event entirely different from any which had taken place before; and that event is, geologically speaking, recent. the changes of which we have spoken, as studied by the geologist in connection with the works of man, the destruction of buildings on sea-coasts by the incursions of the ocean, the removal of the shore many miles away from ancient harbors, the overwhelming of cities by earthquakes or volcanic eruptions; however great when compared with the changes which take place in one or two generations; are minute and infinitesimal, when put in comparison with the changes by which ranges of mountains and continents have been brought into being, one after another, each of them filled with the remains of different organic creations. . further than this, geology does not go on this question. she has no chronometer which can tell us when the first buildings were erected, when man first dwelt in cities, first used implements or arms; still less, language and reflection. geology is compelled to give over the question to history. the external evidences of the antiquity of the species fail us, and we must have recourse to the internal. nature can tell us so little of the age of man, that we must inquire what he can tell us himself. . what man can tell us of his own age--what history can say of the beginning of history--is necessarily very obscure and imperfect. we know how difficult it is to trace to its origin the history of any single nation: how much more, the history of all nations! we know that all such particular histories carry us back to periods of the migrations of tribes, confused mixtures of populations, perplexed and contradictory genealogies of races; and as we follow these further and further backwards, they become more and more obscure and uncertain; at least in the histories which remain to us of most nations. still, the obscurity is not such as to lead us to the conviction that research is useless and unprofitable. it is an obscurity such as naturally arises from the lapse of time, and the complexity of the subject. the aspect of the world, however far we go back, is still historical and human; historical and human, in as high a degree, as it is at the present day. men, as described in the records of the oldest times, are of the same nature, act with the same views, are governed by the same motives, as at present. at all points, we see thought, purpose, law, religion, progress. if we do not find a beginning, we find at least evidence that, in approaching the beginning, the condition of man does not, in any way, cease to be that of an intellectual, moral, and religious creature. . there are, indeed, some histories which speak to us of the beginning of man's existence upon earth; and one such history in particular, which comes to us recommended by indisputable evidence of its own great antiquity, by numerous and striking confirmations from other histories, and from facts still current, and by its connection with that religious view of man's condition, which appears to thoughtful men to be absolutely requisite to give a meaning and purpose to man's faculties and endowments. i speak, of course, of the hebrew scriptures. this history professes to inform us how man was placed upon the earth; and how, from one centre, the human family spread itself in various branches into all parts of the world. this genealogy of the human race is accompanied by a chronology, from which it results that the antiquity of the human race does not exceed a few thousand years. even if we accept this history as true and authoritative, it would not be wise to be rigidly tenacious of the chronology, as to its minute exactness. for, in the first place, of three different forms in which this history appears, the chronology is different in all the three: i mean the hebrew, the samaritan, and the septuagint versions of the old testament. and even if this were not so, since this chronology is put in the form of genealogies, of which many of the steps may very probably have a meaning different from the simple succession of generations in a family, (as some of them certainly have,) it would be unwise to consider ourselves bound to the exact number of years stated, in any of the three versions, or even in all. it makes no difference to our argument, nor to any, purpose in which we can suppose this narrative to have a bearing, whether we accept six thousand or ten thousand years, or even a longer period, as the interval which has now elapsed since the creation of man took place, and the peopling of the earth began. . and, in our speculations at least, it will be well for us to take into account the view which is given us of the antiquity of the human race, by other histories as well as by this. a satisfactory result of such an investigation would be attained if, looking at all these histories, weighing their value, interpreting their expressions fairly, discovering their sources of error, and of misrepresentation, we should find them all converge to one point; all give a consistent and harmonious view of the earliest stages of man's history; of the times and places in which he first appeared as man. if all nations of men are branches of the same family, it cannot but interest us, to find all the family traditions tending upwards towards the same quarter; indicating a divergence from the same point; exhibiting a recollection of the original domicile, or of the same original family circle. . to a certain extent at least, this appears to be the result of the historical investigations which have been pursued relative to this subject. a certain group of nations is brought before us by these researches which, a few thousands of years ago, were possessed of arts, and manners, and habits, and belief, which make them conspicuous, and which we can easily believe to have been contemporaneous successors of a common, though, it may be even then, remote stock. such are the jews, egyptians, chaldeans, and assyrians. the histories of these nations are connected with and confirm each other. their languages, or most of them, have certain affinities, which glossologists, on independent grounds, have regarded as affinities implying an original connection. their chronologies, though in many respects discrepant, are not incapable of being reduced into an harmony by very probable suppositions. here we have a very early view of the condition of a portion of the earth as the habitation of man, and perhaps a suggestion of a condition earlier still. . it is true, that there are other nations also, which claim an antiquity for their civilization equal to or greater than that which we can ascribe to these. such are the indians and the chinese. but while we do not question that these nations were at a remote period in possession of arts, knowledge, and regular polity, in a very eminent degree, we are not at all called upon to assent to the immense numbers, tens of thousands and hundreds of thousands of years, by which such nations, in their histories, express their antiquity. for, in the first place, such numbers are easily devised and transferred to the obscure early stages of tradition, when the art of numeration is once become familiar. these vast intervals, applied to series of blank genealogies, or idle fables, gratify the popular appetite for numerical wonders, but have little claim on critical conviction. . and in the next place, we discover that not enumeration only, but a more recondite art, had a great share in the fabrication of these gigantic numbers of years. some of the nations of whom we have thus spoken, the indians, for example, had, at an early period, possessed themselves of a large share of astronomical knowledge. they had observed and examined the motions of the sun, the moon, the planets, and the stars, till they had discovered cycles, in which, after long and seemingly irregular wanderings in the skies, the heavenly bodies came round again to known and regular positions. they had thus detected the order that reigns in the seeming disorder; and had, by this means, enabled themselves to know beforehand when certain astronomical events would occur; certain configurations of the planets, for instance, and eclipses; and knowing how such events would occur in future, they were also able to calculate how the like events had occurred in the past. they could thus determine what eclipses and what planetary configurations had occurred, in thousands and tens of thousands of years of past time; and could, if they were disposed to falsify their early histories, and to confirm the falsification by astronomical evidence, do so with a very near approximation to astronomical truth. such astronomical confirmation of their assertions, so incapable in any common apprehension of being derived from any other source than actual observation of the fact, naturally produced a great effect upon common minds; and still more, on those who examined the astronomical fact, enough only to see that it was, approximately, at least, true. but in recent times the fallacy of this evidence has been shown, and the fabrication detected. for though the astronomical rules which they had devised were approximately true, they were true approximately only. the more exact researches of modern european astronomy discovered that their cycles, though nearly exact, were not quite so. there was in them an error which made the cycle, at every revolution of its period, when it was applied to past ages, more and more wrong; so that the astronomical events which they asserted to have happened, as they had calculated that they would have happened, the better informed astronomer of our day knows would not have happened exactly so, but in a manner differing more and more from their statement, as the event was more and more remote. and thus the fact which they asserted to have been observed, had not really happened; and the confirmation, which it had been supposed to lend to their history, disappeared. and thus, there is not, in the asserted antiquity of indian civilization and indian astronomy, anything which has a well-founded claim to disturb our belief that the nations of the more western regions of asia had a civilization as ancient as theirs. and considerations of nearly the same kind may be applied to the very remote astronomical facts which are recorded as having been observed in the history of some others of the ancient nations above mentioned. . still less need we be disturbed by the long series of dynasties, each occupying a large period of years, which the egyptians are said to have inserted in their early history, so as to carry their origin beyond the earliest times which i have mentioned. if they spoke of the greek nations as children compared with their own long-continued age, as plato says they did, a few thousands of years of previous existence would well entitle them to do so. so far as such a period goes, their monuments and their hieroglyphical inscriptions give a reality to their pretensions, which we may very willingly grant. and even the history of the jews supposes that the egyptians had attained a high point in arts, government, knowledge, when abraham, the father of the jewish nation, was still leading the life of a nomad. but this supposition is not inconsistent with the account which the jewish scriptures give, of the origin of nations; especially if, as we have said, we abstain from any rigid and narrow interpretation of the chronology of those scriptures; as on every ground, it is prudent to do. . it appears then not unreasonable to believe, that a very few thousands, or even a few hundreds of years before the time of abraham, the nations of central and western asia offer to us the oldest aspect of the life of man upon the earth; and that in reasoning concerning the antiquity of the human race, we may suppose that at that period, he was in the earliest stages of his existence. although, in truth, if we were to accept the antiquity claimed by the egyptians, the indians, or the chinese, the nature of our argument would not be materially altered; for ten thousand, or even twenty thousand years, bears a very small proportion to the periods of time which geology requires for the revolutions which she describes; and, as i have said, we have geological evidence also, to show how brief the human period has been, when compared with the period which preceded the existence of man. and if this be so; if such peoples as those who have left to us the monuments of egypt and of assyria, the pyramids and ancient thebes, the walls of nineveh and babylon, were the first nations which lived as nations; or if they were separated from such only by the interval by which the germans of to-day are separated from the germans of tacitus; we may well repeat our remark, that the history of man, in the earliest times, is as truly a history of a wonderful, intellectual, social, political, spiritual creature, as it is at present. we see, in the monuments of those periods, evidences so great and so full of skill, that even now, they amaze us, of arts, government, property, thought, the love of beauty, the recognition of deity; evidences of memory, foresight, power. if london or berlin were now destroyed, overwhelmed, and, four thousand years hence, disinterred, these cities would not afford stronger testimony of those attributes, as existing in modern europeans, than we have of such qualities in the ancient babylonians and egyptians. the history of man, as that of a creature pre-eminent in the creation, is equally such, however far back we carry our researches. . nor is there anything to disturb this view, in the fact of the existence of the uncultured and barbarous tribes which occupy, and always have occupied, a large portion of the earth's surface. for, in the first place, there is not, in the aspect of the fact, or in the information which history gives us, any reason to believe that such tribes exhibit a form of human existence, which, in the natural order of progress, is earlier than the forms of civilized life, of which we have spoken. the opinion that the most savage kind of human life, least acquainted with arts, and least provided with resources, is the state of nature out of which civilized life has everywhere gradually emerged, is an opinion which, though at one time popular, is unsupported by proof, and contrary to probability.[ ] savage tribes do not so grow into civilization; their condition is, far more probably, a condition of civilization degraded and lost, than of civilization incipient and prospective. add to this, that if we were to assume that this were otherwise; if man thus originally and naturally savage, did also naturally tend to become civilized; this _tendency_ is an endowment no less wonderful, than those endowments which civilization exhibits. the capacity is as extraordinary as the developed result; for the capacity involves the result. if savage man be the germ of the most highly civilized man, he differs from all other animal germs, as man differs from brute. and add to this again, that in the tribes which we call savage, and whose condition most differs, in external circumstances, from ours, there are, after all, a vast mass of human attributes: thought, purpose, language, family relations; generally property, law, government, contract, arts, and knowledge, to no small extent; and in almost every case, religion. even uncivilized man is an intellectual, moral, social, religious creature; nor is there, in his condition, any reason why he may not be a spiritual creature, in the highest sense in which the most civilized man can be so. . here then we are brought to the view which, it would seem, offers a complete reply to the difficulty, which astronomical discoveries appeared to place in the way of religion:--the difficulty of the opinion that man, occupying this speck of earth, which is but as an atom in the universe, surrounded by millions of other globes, larger, and, to appearance, nobler than that which he inhabits, should be the object of the peculiar care and guardianship, of the favor and government, of the creator of all, in the way in which religion teaches us that he is. for we find that man, (the human race, from its first origin till now,) has occupied but an atom of time, as he has occupied but an atom of space:--that as he is surrounded by myriads of globes which may, like this, be the habitations of living things, so he has been preceded, on this earth, by myriads of generations of living things, not possibly or probably only, but certainly; and yet that, comparing his history with theirs, he has been, certainly has been fitted to be, the object of the care and guardianship, of the favor and government, of the master and governor of all, in a manner entirely different from anything which it is possible to believe with regard to the countless generations of brute creatures which had gone before him. if we will doubt or overlook the difference between man and brutes, the difficulty of ascribing to man peculiar privileges, is made as great by the revelations of geology, as of astronomy. the scale of man's insignificance is, as we have said, of the same order in reference to time, as to space. there is nothing which at all goes beyond the magnitude which observation and reasoning suggest for geological periods, in supposing that the tertiary strata occupied, in their deposition and elevation, a period as much greater than the period of human history, as the solar system is larger than the earth:--that the secondary strata were as much longer than these in their formation, as the nearest fixed star is more distant than the sun:--that the still earlier masses, call them primary, or protozoic, or what we will, did, in their production, extend through a period of time as vast, compared with the secondary period, as the most distant nebula is remoter than the nearest star. if the earth, as the habitation of man, is a speck in the midst of an infinity of space, the earth, as the habitation of man, is also a speck at the end of an infinity of time. if we are as nothing in the surrounding universe, we are as nothing in the elapsed eternity; or rather, in the elapsed organic antiquity, during which the earth has existed and been the abode of life. if man is but one small family in the midst of innumerable possible households, he is also but one small family, the successor of innumerable tribes of animals, not possible only, but actual. if the planets _may_ be the seats of life, we know that the seas which have given birth to our mountains _were_ the seats of life. if the stars may have hundreds of systems of tenanted planets rolling round them, we know that the secondary group of rocks does contain hundreds of tenanted beds, witnessing of as many systems of organic creation. if the nebulæ may be planetary systems in the course of formation, we know that the primary and transition rocks either show us the earth in the course of formation, as the future seat of life, or exhibit such life as already begun. . how far that which astronomy thus asserts as possible, is probable:--what is the value of these possibilities of life in distant regions of the universe, we shall hereafter consider. but in what geology asserts, the case is clear. it is no possibility, but a certainty. no one will now doubt that shells and skeletons, trunks and leaves, prove animal and vegetable life to have existed. even, therefore, if astronomy could demonstrate all that her most fanciful disciples assume, geology would still have a complete right to claim an equal hearing;--to insist upon having her analogies regarded. she would have a right to answer the questions of astronomy, when she says, how can we believe this? and to have her answers accepted. . astronomy claims a sort of dignity over all other sciences, from her _antiquity_, her _certainty_, and the _vastness_ of her discoveries. but the antiquity of astronomy as a science had no share in such speculations as we are discussing; and if it had had, new truths are better than old conjectures; new discoveries must rectify old errors; new answers must remove old difficulties. the vigorous youth of geology makes her fearless of the age of astronomy. and as to the certainty of astronomy, it has just as little to do with these speculations. the certainty stops, just when these speculations begin. there may, indeed, be some danger of delusion on this subject. men have been so long accustomed to look upon astronomical science as the mother of certainty, that they may confound astronomical discoveries with cosmological conjectures; though these be slightly and illogically connected with those. and then, as to the vastness of astronomical discoveries,--granting that character, inasmuch as it is to a certain degree, a matter of measurement,--we must observe, that the discoveries of geology are no less vast: they extend through time, as those of astronomy do through space. they carry us through millions of years, that is, of the earth's revolutions, as those of astronomy do through millions of the earth's diameters, or of diameters of the earth's orbit. geology fills the regions of duration with events, as astronomy fills the regions of the universe with objects. she carries us backwards by the relation of cause and effect, as astronomy carries us upwards by the relations of geometry. as astronomy steps on from point to point of the universe by a chain of triangles, so geology steps from epoch to epoch of the earth's history by a chain of mechanical and organical laws. if the one depends on the axioms of geometry, the other depends on the axioms of causation. . so far then, geology has no need to regard astronomy as her superior; and least of all, when they apply themselves together to speculations like these. but in truth, in such speculations, geology has an immeasurable superiority. she has the command of an implement, in addition to all that astronomy can use; and one, for the purpose of such speculations, adapted far beyond any astronomical element of discovery. she has, for one of her studies,--one of her means of dealing with her problems,--the knowledge of life, animal and vegetable. vital organization is a subject of attention which has, in modern times, been forced upon her. it is now one of the main parts of her discipline. the geologist must study the traces of life in every form; must learn to decypher its faintest indications and its fullest development. on the question, then, whether there be in this or that quarter, evidence of life, he can speak with the confidence derived from familiar knowledge; while the astronomer, to whom such studies are utterly foreign, because he has no facts which bear upon them, can offer, on such questions, only the loosest and most arbitrary conjectures; which, as we have had to remark, have been rebuked by eminent men, as being altogether inconsistent with the acknowledged maxims of his science. . when, therefore, geology tells us that the earth, which has been the seat of human life for a few thousand years only, has been the seat of animal life for myriads, it may be, millions of years, she has a right to offer this, as an answer to any difficulty which astronomy, or the readers of astronomical books, may suggest, derived from the considerations that the earth, the seat of human life, is but one globe of a few thousand miles in diameter, among millions of other globes, at distances millions of times as great. . let the difficulty be put in any way the objector pleases. is it that it is unworthy of the greatness and majesty of god, according to our conceptions of him, to bestow such peculiar care on so small a part of his creation? but we know, from geology, that he has bestowed upon this small part of his creation, mankind, this special care;--he has made their period, though only a moment in the ages of animal life, the only period of intelligence, morality, religion. if then, to suppose that he has done this, is contrary to our conceptions of his greatness and majesty, it is plain that our conceptions are erroneous; they have taken a wrong direction. god has not judged, as to what is worthy of him, as we have judged. he has found it worthy of him to bestow upon man his special care, though he occupies so small a portion of time; and why not, then, although he occupies so small a portion of space? . or is the objection this; that if we suppose the earth only to be occupied by inhabitants, all the other globes of the universe are wasted;--turned to no purpose? is waste of this kind considered as unsuited to the character of the creator? but here again, we have the like waste, in the occupation of the earth. all its previous ages, its seas and its continents, have been wasted upon mere brute life; often, so far as we can see, for myriads of years, upon the lowest, the least conscious forms of life; upon shell-fish, corals, sponges. why then should not the seas and continents of other planets be occupied at present with a life no higher than this, or with no life at all? will it be said that, so far as material objects are occupied by life, they are not wasted; but that they are wasted, if they are entirely barren and blank of life? this is a very arbitrary saying. why should the life of a sponge, or a coral, or an oyster, be regarded as a good employment of a spot of land and water, so as to save it from being wasted? no doubt, if the coral or the oyster be there, there is a reason why it is so, consistently with the attributes of god. but then, on the same ground, we may say that if it be not there, there is a reason why it is not so. such a mode of regarding the parts of the universe can never give us reasons why they should or should not be inhabited, when we have no other grounds for knowing whether they are. if it be a sufficient employment of a spot of rock or water that it is the seat of organization--of organic powers; why may it not be a sufficient employment of the same spot that it is the seat of attraction, of cohesion, of crystalline powers? all the planets, all parts of the universe, we have good reason to believe, are pervaded by attraction, by forces of aggregation and atomic relation, by light and heat. why may not these be sufficient to prevent the space being wasted, in the eyes of the creator? as, during a great part of the earth's past history, and over large portions of its present mass, they are actually held by him sufficient; for they are all that occupy those portions. this notion, then, of the improbability of there being, in the universe, so vast an amount of waste spaces, or waste bodies, as is implied in the opinion that the earth alone is the seat of life, or of intelligence, is confuted by the fact, that there are vast spaces, waste districts, and especially waste times, to an extent as great as such a notion deems improbable. the avoidance of such waste, according to our notions of waste, is no part of the economy of creation, so far as we can discern that economy, in its most certain exemplifications. . or will the objection be made in this way; that such a peculiar dignity and importance given to the earth is contrary to the analogy of creation;--that since there are so many globes, similar to the earth,--like her, revolving round the sun, like her, revolving on her axes, several of them, like her, accompanied by satellites; it is reasonable to suppose that their destination and office is the same as hers;--that since there are so many stars, each like the sun, a source of light, and probably of heat, it is reasonable to suppose that, like the sun, they are the centres of systems of planets, to which their light and heat are imparted, to uphold life:--is it thought that such a resemblance is a strong ground for believing that the planets of our system, and of other systems, are inhabited as the earth is? if such an astronomical analogy be insisted on, we must again have recourse to geology, to see what such analogy is worth. and then, we are led to reflect, that if we were to follow such analogies, we should be led to suppose that all the successive periods of the earth's history were occupied with life of the same order; that as the earth, in its present condition, is the seat of an intelligent population, so must it have been, in all former conditions. the earth, in its former conditions, was able and fitted to support life; even the life of creatures closely resembling man in their bodily structure. even of monkeys, fossil remains have been found. but yet, in those former conditions, it did not support human life. even those geologists who have dwelt most on the discovery of fossil monkeys, and other animals nearest to man, have not dreamt that there existed, before man, a race of rational, intelligent, and progressive creatures. as we have seen, geology and history alike refute such a fancy. the notion, then, that one period of time in the history of the earth must resemble another, in the character of its population, because it resembles it in physical circumstances, is negatived by the facts which we discover in the history of the earth. and so, the notion that one part of the universe must resemble another in its population, because it resembles it in physical circumstances, is negatived as a law of creation. analogy, further examined, affords no support to such a notion. the analogy of time, the events of which we know, corrects all such guesses founded on a supposed analogy of space, the furniture of which, so far as this point is concerned, we have no sufficient means of examining. . but in truth, we may go further. not only does the analogy of creation not point to any such entire resemblance of similar parts, as is thus assumed, but it points in the opposite direction. not entire resemblance, but universal difference is what we discover; not the repetition of exactly similar cases, but a series of cases perpetually dissimilar, presents itself; not constancy, but change, perhaps advance; not one permanent and pervading scheme, but preparation and completion of successive schemes; not uniformity and a fixed type of existences, but progression and a climax. this may be said to be the case in the geological aspect of the world; for, without occupying ourselves with the question, how far the monuments of animal life, which we find preserved in the earth's strata, exhibited a gradual progression from ruder and more imperfect forms to the types of the present terrestrial population; from sponges and mollusks, to fish and lizards, from cold-blooded to warm blooded animals, and so on, till we come to the most perfect vertebrates;--a doctrine which many eminent geologists have held, and still hold;--without discussing this question, or assuming that the fact is so; this at least cannot be denied or doubted, that man is incomparably the most perfect and highly-endowed creature which ever has existed on the earth. how far previous periods of animal existence were a necessary preparation of the earth, as the habitation of man, or a gradual progression towards the existence of man, we need not now inquire. but this at least we may say; that man, now that he is here, forms a climax to all that has preceded; a term incomparably exceeding in value all the previous parts of the series; a complex and ornate capital to the subjacent column; a personage of vastly greater dignity and importance than all the preceding line of the procession. the analogy of nature, in this case at least, appears to be, that there should be inferior, as well as superior provinces, in the universe; and that the inferior may occupy an immensely larger portion of time than the superior; why not then of space? the intelligent part of creation is thrust into the compass of a few years, in the course of myriads of ages; why not then into the compass of a few miles, in the expanse of systems? the earth was brute and inert, compared with its present condition, dark and chaotic, so far as the light of reason and intelligence are concerned, for countless centuries before man was created. why then may not other parts of creation be still in this brute and inert and chaotic state, while the earth is under the influence of a higher exercise of creative power? if the earth was, for ages, a turbid abyss of lava and of mud, why may not mars or saturn be so still? if the germs of life were, gradually, and at long intervals, inserted in the terrestrial slime, why may they not be just inserted, or not yet inserted, in jupiter? or why should we assume that the condition of those planets resembles ours, even so far as such suppositions imply? why may they not, some or all of them, be barren masses of stone and metal, slag and scoriæ, dust and cinders? that some of them are composed of such materials, we have better reason to believe, than we have to believe anything else respecting their physical constitution, as we shall hereafter endeavor to show. if then, the earth be the sole inhabited spot in the work of creation, the oasis in the desert of our system, there is nothing in this contrary to the analogy of creation. but if, in some way which perhaps we cannot discover, the earth obtained, for accompaniments, mere chaotic and barren masses, as conditions of coming into its present state; as it may have required, for accompaniments, the brute and imperfect races of former animals, as conditions of coming into its present state, as the habitation of man; the analogy is against, and not in favor of, the belief that they too (the other masses, the planets, &c.) are habitations. i may hereafter dwell more fully on such speculations; but the possibility that the planets are such rude masses, is quite as tenable, on astronomical grounds, as the possibility that the planets resemble the earth, in matters of which astronomy can tell us nothing. we say, therefore, that the example of geology refutes the argument drawn from the supposed analogy of one part of the universe with another; and suggests a strong suspicion that the force of analogy, better known, may tend in the opposite direction. . when such possibilities are presented to the reader, he may naturally ask, if we are thus to regard man as the climax of creation, in space, as in time, can we point out any characters belonging to him, which may tend to make it conceivable that the creator should thus distinguish him, and care for him:--should prepare his habitation if it be so, by ages of chaotic and rudimentary life, and by accompanying orbs of brute and barren matter. if man be, thus, the head, the crowned head of the creation, is he worthy to be thus elevated? has he any qualities which make it conceivable that, with such an array of preparation and accompaniment, he should be placed upon the earth, his throne? or rather, if he be thus the chosen subject of god's care, has he any qualities, which make it conceivable that he should be thus selected; taken under such guardianship; admitted to such a dispensation; graced with such favor. the question with which we began again recurs: what is man that god should be thus mindful of him? after the views which have been presented to us, does any answer now occur to us? . the answer which we have to give, is that which we have already repeatedly stated. man is an intellectual, moral, religious, and spiritual creature. if we consider these attributes, we shall see that they are such as to give him a special relation to god, and as we conceive, and must conceive, god to be; and may therefore be, in god, the occasion of special guardianship, special regard, a special dispensation towards man. . as an intellectual creature, he has not only an intelligence which he can apply to practical uses, to minister to the needs of animal and social life; but also an intellect by which he can speculate about the relations of things, in their most general form; for instance, the properties of space and time, the relations of finite and infinite. he can discover truths, to which all things, existing in space and time, must conform. these are conditions of existence to which the creation conforms, that is, to which the creator conforms; and man, capable of seeing that such conditions are true and necessary, is capable, so far, of understanding some of the conditions of the creator's workmanship. in this way, the mind of man has some community with the mind of god; and however remote and imperfect this community may be, it must be real. since, then, man has thus, in his intellect, an element of community with god, it is so far conceivable that he should be, in a special manner, the object of god's care and favor. the human mind, with its wonderful and perhaps illimitable powers, is something of which we can believe god to be "mindful." . again: man is a moral creature. he recognizes, he cannot help recognizing, a distinction of right and wrong in his actions; and in his internal movements which lead to action. this distinction he recognizes as the reason, the highest and ultimate reason, for doing or for not doing. and this law of his own reason, he is, by reflection, led to recognize as a law of the supreme reason; of the supreme mind which has made him what he is. the moral law, he owns and feels as god's law. by the obligation which he feels to obey this law, he feels himself god's subject; placed under his government; compelled to expect his judgment, his rewards, and punishments. by being a moral creature, then, he is, in a special manner, the subject of god; and not only we can believe that, in this capacity, god cares for him; but we cannot believe that he _does not_ care for him. he cares for him, so as to approve of what he does right, and to condemn what he does wrong. and he has given him, in his own breast, an assurance that he will do this; and thus, god cares for man, in a peculiar and special manner. as a moral creature, we have no difficulty in conceiving that god may think him worthy of his regard and government. . the development of man's moral nature, as we have just described it, leads to, and involves the development of his religious nature. by looking within himself, and seeing the moral law, he learns to look upwards to god, the author of the law, and the awarder of the rewards and penalties which follow moral good and evil. but the belief of such a dispensation carries us, or makes us long to be carried, beyond the manifestations of this dispensation, as they appear in the ordinary course of human life. by thinking on such things, man is led to ascribe a wider range to the moral government of god:--to believe in methods of reward and punishment, which do not appear in the natural course of events: to accept events, out of the order of nature, which announce that god has provided such methods: to accept them, when duly authenticated, as messages from god; and thus, when god provides the means, to allow himself to be placed in intercourse with god. since man is capable of this; since, as a religious creature, this is his tendency, his need, the craving of his heart, without which, when his religious nature is fully unfolded, he can feel no comfort nor satisfaction; we cannot be surprised that god should deem him a proper object of a special fatherly care; a fit subject for a special dispensation of his purposes, as to the consequences of human actions. man being this, we can believe that god is not only "mindful of him," but "visits him." . as we have said, the soul of man, regarded as the subject of god's religious government, is especially termed his _spirit_: the course of human being which results from the intercourse with god, which god permits, is a _spiritual_ existence. man is capable, in no small degree, of such an existence, of such an intercourse with god; and, as we are authorized to term it, of such a life with god, and in god, even while he continues in his present human existence. i say _authorized_, because such expressions are used, though reverently, by the most religious men; who are, at any rate, authority as to their own sentiments; which are the basis of our reasoning. whatever, then, may be the imperfection, in this life, of such a union with god, yet since man can, when sufficiently assisted and favored by god, enter upon such a union, we cannot but think it most credible and most natural, that he should be the object of god's special care and regard, even of his love and presence. . that men are, only in a comparatively small number of cases, intellectual, moral, religious, and spiritual, in the degree which i have described, does not, by any means, deprive our argument of its force. the capacity of man is, that he may become this; and such a capacity may well make him a special object in the eyes of him under whose guidance and by whose aid, such a development and elevation of his nature is open to him. however imperfect and degraded, however unintellectual, immoral, irreligious, and unspiritual, a great part of mankind may be, still they all have the germs of such an elevation of their nature; and a large portion of them make, we cannot doubt, no small progress in this career of advancement to a spiritual condition. and with such capacities, and such practical exercise of those capacities, we can have no difficulty in believing, if the evidence directs us to believe, that that part of the creation in which man has his present appointed place, is the special field of god's care and love; by whatever wastes of space, and multitudes of material bodies, it may be surrounded; by whatever races it may have been previously occupied, of brutes that perish, and that, compared with man, can hardly be said to have lived. footnotes: [ ] lyell, ii. . [ th ed.] [ ] cuvier. [ ] by bishop berkeley. see lyell, iii. . [ ] a recent popular writer, who has asserted the self-civilizing tendency of man, has not been able, it would seem, to adduce any example of the operation of this tendency, except a single tribe of north american indians, in whom it operated for a short time, and to a small extent. chapter vii. the nebulÆ . i have attempted to show that, even if we suppose the other bodies of the universe to resemble the earth, so far as to seem, by their materials, forms, and motions, no less fitted than she is to be the abodes of life; yet that, knowing what we do of man, we can believe that the earth is tenanted by a race who are the _special_ objects of god's care. even if the tendency of the analogies of creation were, to incline us to suppose that the other planets are as well suited as our globe, to have inhabitants, still it would require a great amount of evidence, to make us believe that they have such inhabitants as we are; while yet such evidence is altogether wanting. even if we knew that the stars were the centres of revolving systems, we should have an immense difficulty in believing that an earth, with such a population as ours, revolves about any of them. if astronomy made a plurality of worlds probable, we have strong reasonings, drawn from other subjects, to think that the other worlds are not like ours. . the admirers of astronomical triumphs may perhaps be disposed to say, that when so much has been discovered, we may be allowed to complete the scheme by the exercise of fancy. i have attempted to show that we are not in such a state of ignorance, when we look at other relations of the earth and of man, as to allow us to do this. but now we may go a little onwards in our argument; and may ask, whether astronomy really does what is here claimed for her:--whether she carries us so securely to the bounds of the visible universe, that our fancy may take up the task, and people the space thus explored:--whether the bodies which astronomy has examined, be really as fitted as our earth, to sustain a population of living things:--whether the most distant objects in the universe do really seem to be systems, or the beginnings of systems:--whether astronomy herself may not incline in favor of the condition of man, as being the sole creature of his kind? . in making this inquiry, it will of course be understood, that i do so with the highest admiration for the vast discoveries which astronomy has really made; and for the marvellous skill and invention of the great men who have, in all ages of the world, and not least, in our time, been the authors of such discoveries. from the time when galileo first discovered the system of jupiter's satellites, to the last scrutiny of the structure of a nebula by lord rosse's gigantic telescope, the history of the telescopic exploration of the sky, has been a history of genius felicitously employed in revealing wonders. in this history, the noble labors of the first and the second herschel relative to the distribution of the fixed stars, the forms and classes of nebulæ, and the phenomena of double stars, especially bear upon our present speculations; to which we may add, the examination of the aspect of each planet, by various observers, as schroeter, and of the moon by others, from huyghens to mädler and beer. the achievements which are most likely to occur to the reader's mind are those of the earl of rosse; as being the latest addition to our knowledge, and the result of the greatest instrumental powers. by the energy and ingenuity of that eminent person, an eye is directed to the heavens, having a pupil of six feet diameter, with the most complete optical structure, and the power of ranging about for its objects over a great extent of sky; and thus the quantity of light which the eye receives from any point of the heavens is augmented, it may be, fifty thousand times. the rising moon is seen from the observatory in ireland with the same increase of size and light, as if her solid globe, two thousand miles in diameter, retaining all its illumination, really rested upon the summits of the alps, to be gazed at by the naked eye. an object which appears to the naked eye a single star, may, by this telescope, so far as its power of seeing is concerned, be resolved into fifty thousand stars, each of the same brightness as the obvious star. what seems to the unassisted vision a nebula, a patch of diluted light, in which no distinct luminous point can be detected, may, by such an instrument, be discriminated or resolved into a number of bright dots; as the stippled shades of an engraving are resolved into dots by the application of a powerful magnifying glass. similar results of the application of great telescopic power had of course been attained long previously; but, as the nature of scientific research is, each step adds something to our means of knowledge; and the last addition assumes, includes, and augments the knowledge which we possessed before. the discussions in which we are engaged, belong to the very boundary region of science;--to the frontier where knowledge, at least astronomical knowledge, ends, and ignorance begins. such discoveries, therefore, as those made by lord rosse's telescope, require our special notice here. . we may begin, at what appears to us the outskirts of creation, the nebulæ. at one time it was conceived by astronomers in general, that these patches of diffused light, which are seen by them in such profusion in the sky, are not luminous bodies of regular terms and definite boundaries, apparently solid, as the stars are supposed to be; but really, as even to good telescopes many of them seem, masses of luminous cloud or vapor, loosely held together, as clouds and vapors are, and not capable by any powers of vision of being resolved into distinct visible elements. this opinion was for a time so confidentially entertained, that there was founded upon it an hypothesis, that these were gaseous masses, out of which suns and systems might afterwards be formed, by the concentration of these luminous vapors into a solid central sun, more intensely luminous; while detached portions of the mass, flying off, and cooling down so as to be no longer self-luminous, might revolve round the central body, as planets and satellites. this is the _nebular hypothesis_, suggested by the elder herschel, and adopted by the great mathematician laplace. . but the result of the optical scrutiny of the nebulæ by more modern observers, especially by lord rosse in ireland, and mr. bond in america, has been, that many celestial objects which were regarded before as truly nebulous, have been resolved into stars; and this resolution has been extended to so many cases of nebulæ, of such various kinds, as to have produced a strong suspicion in the minds of astronomers that _all_ the nebulæ, however different in their appearance, may really be resolved into stars, if they be attacked with optical powers sufficiently great. . if this were to be assumed as done, and if each of the separate points, into which the nebulæ are thus resolved, were conceived to be a star, which looks so small only because it is so distant, and which really is as likely to have a system of planets revolving about it, as is a star of the first magnitude:--we should then have a view of the immensity of the visible universe, such as i presented to the reader in the beginning of this essay. all the distant nebulæ appear as nebulæ, only because they are so distant; if truly seen, they are groups of stars, of which each may be as important as our sun, being, like it, the centre of a planetary system. and thus, a patch of the heavens, one hundredth or one thousandth part of the visible breadth of our sun, may contain in it more life, not only than exists in the solar system, but in as many such systems as the unassisted eye can see stars in the heavens, on the clearest winter night. . this is a stupendous view of the greatness of the creation; and, to many persons, its very majesty, derived from magnitude and number, will make it so striking and acceptable, that, once apprehended, they will feel as if there were a kind of irreverence in disturbing it. but if this view be really not tenable when more closely examined, it is, after all, not wise to connect our feelings of religious reverence with it, so that they shall suffer a shock when we are obliged to reject it. i may add, that we may entertain an undoubting trust that any view of the creation which is found to be true, will also be found to supply material for reverential contemplation. i venture to hope that we may, by further examination, be led to a reverence of a deeper and more solemn character than a mere wonder at the immensity of space and number. . but whatever the result may be, let us consider the evidence for this view. it assumes that all the nebulæ are resolvable into stars, and that they appear as nebulæ only because they are more distant than the region in which they can appear as stars. are there any facts, any phenomena in the heavens, which may help us to determine whether this is a probable opinion? . it is most satisfactory for us, when we can, in such inquiries, know the thoughts which have suggested themselves to the minds of those who have examined the phenomena with the most complete knowledge, the greatest care, and the best advantages; and have speculated upon these phenomena in a way both profound and unprejudiced. some remarks of sir john herschel, recommended by these precious characters, seem to me to bear strongly upon the question which i have just had to ask:--do all the nebulæ owe their nebulous appearance to their being too distant to be seen as groups of distinct stars, though they really are such groups? . herschel, in the visit which he made to the cape of good hope, for the purpose of erecting to his father the most splendid monument that son ever erected,--the completed survey of the vault of heaven,--had full opportunity of studying a certain pair of remarkable bright spaces of the skies, filled with a cloudy light, which lie near the southern pole; and which, having been unavoidably noticed by the first antarctic voyagers, are called the _magellanic clouds_. when the larger of these two clouds is examined through powerful telescopes, it presents, we are told, a constitution of uncommon complexity: "large patches and tracts of nebulosity in every stage of resolution, from light, irresolvable with eighteen inches of reflecting aperture, up to perfectly separated stars like the milky way, and clustering groups sufficiently insulated and condensed to come under the designation of irregular, and in some cases pretty rich clusters. but besides these, there are also nebulæ in abundance, both regular and irregular; globular clusters in every stage of condensation, and objects of a nebulous character quite peculiar, and which have no analogies in any other region of the heavens."[ ] he goes on to say, that these nebulæ and clusters are far more crowded in this space than they are in any other, even the most crowded parts, of the nebulous heavens. this _nubecula major_, as it is termed, is of a round or oval form, and its diameter is about six degrees, so that it is about twelve times the apparent diameter of the moon. the _nubecula minor_ is a smaller patch of the same kind. if we suppose the space occupied by the various objects which the nubecula major includes, to be, in a general way, spherical, its nearest and most remote parts must (as its angular size proves) differ in their distance from us by little more than a tenth part of our distance from its centre. that the two nubeculæ are thus approximately spherical spaces, is in the highest degree probable; not only from the peculiarity of their contents, which suggests the notion of a peculiar group of objects, collected into a limited space; but from the barrenness, as to such objects, of the sky in the neighborhood of these magellanic clouds. to suppose (the only other possible supposition) that they are two columns of space, with their ends turned towards us, and their lengths hundreds and thousands of times their breadths, would be too fantastical a proceeding to be tolerated; and would, after all, not explain the facts without further altogether arbitrary assumptions. . it appears, then, that, in these groups, there are stars of various magnitudes, clusters of various forms, nebulæ regular and irregular, nebulous tracts and patches of peculiar character; and all so disposed, that the most distant of them, whichever these may be, are not more than one-tenth more distant than the nearest. if the nearest star in this space be at nine times the distance of sirius, the farthest nebulæ, contained in the same space, will not be at more than ten times the distance of sirius. of course, the doctrine that nebulæ are seen as nebulæ, merely because they are so distant, requires us to assume all nebulæ to be hundreds and thousands of times more distant than the smallest stars. if stars of the eighth magnitude (which are hardly visible to the naked eye) be eight times as remote as sirius, a nebula containing a thousand stars, which is invisible to the naked eye, must be more than eight thousand times as remote as sirius. and thus if, in the whole galaxy, we reckon only the stars as far as the eighth magnitude, and suppose all the stars of the galaxy to form a nebula, which is visible to the spectators in a distant nebula, only as their nebula is visible to us; we must place them at eight thousand times two hundred thousand times the distance of the sun; and, even so, we are obviously vastly understating the calculation. these are the gigantic estimates with which some astronomical speculators have been in the habit of overwhelming the minds of their listeners; and these views have given a kind of majesty to the aspect of the nebulæ; and have led some persons to speak of the discovery of every new streak of nebulous light in the starry heavens, as a discovery of new worlds, and still new worlds. but the magellanic clouds show us very clearly that all these calculations are entirely baseless. in those regions of space, there coexists, in a limited compass, and in indiscriminate position, stars, clusters of stars, nebulæ, regular and irregular, and nebulous streaks and patches. these, then, are different kinds of things in themselves, not merely different to us. there are such things as nebulæ side by side with stars, and with clusters of stars. nebulous matter resolvable occurs close to nebulous matter irresolvable. the last and widest step by which the dimensions of the universe have been expanded in the notions of eager speculators, is checked by a completer knowledge and a sager spirit of speculation. whatever inference we may draw from the resolvability of some of the nebulæ, we may not draw this inference;--that they are more distant, and contain a larger array of systems and of worlds, in proportion as they are difficult to resolve. . but indeed, if we consider this process, of the resolution of nebulæ into luminous points, on its own ground, without looking to such facts as i have just adduced, it will be difficult, or impossible, to assign any reason why it should lead to such inferences as have been drawn from it. let us look at this matter more clearly. an astronomer, armed with a powerful telescope, _resolves_ a nebula, discerns that a luminous cloud is composed of shining dots:--but what are these dots? into _what_ does he resolve the nebula? into _stars_, it is commonly said. let us not wrangle about words. by all means let these dots be stars, if we know about what we are speaking: if a _star_ merely mean a luminous dot in the sky. but that these stars shall resemble, in their nature, stars of the first magnitude, and that such stars shall resemble our sun, are surely very bold structures of assumption to build on such a basis. some nebulæ are resolvable; are resolvable into distinct points; certainly a very curious, probably an important discovery. we may hereafter learn that _all_ nebulæ are resolvable into distinct points: that would be a still more curious discovery. but what would it amount to? what would be the simple way of expressing it, without hypothesis, and without assumption? plainly this: that the substance of all nebulæ is not continuous, but discrete;--separable, and separate into distinct luminous elements;--nebulæ are, it would then seem, as it were, of a curdled or granulated texture; they have run into _lumps_ of light, or have been formed originally of such lumps. highly curious. but what are these lumps? how large are they? at what distances? of what structure? of what use? it would seem that he must be a bold man who undertakes to answer these questions. certainly he must appear to ordinary thinkers to be _very_ bold, who, in reply, says, gravely and confidently, as if he had unquestionable authority for his teaching:--"these lumps, o man, are suns; they are distant from each other as far as the dog-star is from us; each has its system of planets, which revolve around it; and each of these planets is the seat of an animal and vegetable creation. among these planets, some, we do not yet know how many, are occupied by rational and responsible creatures, like man; and the only matter which perplexes us, holding this belief on astronomical grounds, is, that we do not quite see how to put our theology into its due place and form in our system." . in discussing such matters as these, where our knowledge and our ignorance are so curiously blended together, and where it is so difficult to make men feel that so much ignorance can lie so close to so much knowledge;--to make them believe that they have been allowed to discover so much, and yet are not allowed to discover more:--we may be permitted to illustrate our meaning, by supposing a case of blended knowledge and ignorance, of real and imaginary discovery. suppose that there were carried from a scientific to a more ignorant nation, excellent maps of the world, finely engraved; the mountain-ranges shaded in the most delicate manner, and the sheet crowded with information of all kinds, in writing large, small, and microscopic. suppose also, that when these maps had been studied with the naked eye, so as to establish a profound respect for the knowledge and skill of the author of them, some of those who perused them should be furnished with good microscopes, so as to carry their examination further than before. they might then find that, in several parts, what before appeared to be merely crooked lines, was really writing, stating, it may be, the amount of population of a province, or the date of foundation of a town. to exhaust all the information thus contained on the maps, might be a work of considerable time and labor. but suppose that, when this was done, a body of resolute microscopists should insist that the information which the map contained was not exhausted: that they should continue peering perseveringly at the lines which formed the shading of the mountains, maintaining that these lines also were writing, if only it might be deciphered; and should go on increasing, with immense labor and ingenuity, the powers of their microscopes, in order to discover the legend contained in these unmeaning lines. we should, perhaps, have here an image of the employment of these astronomers, who now go on looking in nebulæ for worlds. and we may notice in passing, that several of the arguments which are used by such astronomers, might be used, and would be used, by our microscopists:--how improbable it was that a person so full of knowledge, and so able to convey it, as the author of the maps was known to be, should not have a design and purpose in every line that he drew: what a waste of space it would be to leave any part of the sheet blank of information; and the like. to which the reply is to us obvious; that the design of shading the mountains was design enough; and that the information conveyed was all that was necessary or convenient. nor does this illustration at all tend to show that such astronomical scrutiny, directed intelligently, with a right selection of the points examined, may not be highly interesting and important. if the microscopists had examined the map with a view to determine the best way in which mountains can be indicated by shading, they would have employed themselves upon a question which has been the subject of multiplied and instructive discussion in our own day. . but to return to the subject of nebulæ, we may further say, with the most complete confidence, that whether or not nebulous matter be generally resolvable into shining dots, it cannot possibly be true that its being, or not being so resolvable by our telescopes, depends merely upon its smaller or greater distance from the observer. for, in the first place, that there is matter, to the best assisted eye not distinguishable from nebulous matter, which is not so resolvable, is proved by several facts. the tails of comets often resemble nebulæ; so much so that there are several known nebulæ, which are, by the less experienced explorers of the sky, perpetually mistaken for comets, till they are proved not to be so, by their having no cometary motion. such is the nebula in andromeda, which is visible to the naked eye.[ ] but the tails and nebulous appendages of comets, though they alter their appearance very greatly, according to the power of the telescope with which they are examined, have never been resolved into stars, or any kind of dots; and seem, by all investigations, to be sheets or cylinders or cones of luminous vapor, changing their form as they approach to or recede from the sun, and perhaps by the influence of other causes. yet some of them approach very near the earth; all of them come within the limits of our system. here, then, we have (probably, at least,) nebulous matter, which when brought close to the eye, compared with the stellar nebulæ, still appears as nebulous. . again, as another phenomenon, bearing upon the same question, we have the zodiacal light. this is a faint cone of light[ ] which, at certain seasons, may be seen extending from the horizon obliquely upwards, and following the course of the ecliptic, or rather, of the sun's equator. it appears to be a lens-shaped envelope of the sun, extending beyond the orbits of mercury and venus, and nearly attaining that of the earth; and in sir john herschel's view, may be regarded as placing the sun in the list of nebulous stars. no one has ever thought that this nebulous appearance was resolvable into luminous points; but if it were, probably not even the most sanguine of speculators on the multitude of suns would call these points _suns_. . but indeed the nebulæ themselves, and especially the most remote of the nebulæ, or at least those which most especially require the most powerful telescopes, offer far more decisive proofs that their resolvability or non-resolvability,--their apparent constitution as diffused and vaporous masses,--does not depend upon their distance. a remarkable fact in the irregular, and in some of the regular nebulæ[ ] is, that they consist of long patches and streaks, which stretch out in various directions, and of which the form[ ] and extent vary according to the visual power which is applied to them. many of the nebulæ and especially of the fainter ones, entirely change their form with the optical power of the instrument by which they are scrutinized; so that, as seen in the mightier telescopes of modern times, the astronomer scarcely recognizes the figures in which the earlier observers have recorded what they saw in the same place. parts which, before, were separate, are connected by thin bridges of light which are now detected; and where the nebulous space appeared to be bounded, it sends off long tails of faint light into the surrounding space. now, no one can suppose that these newly-seen portions of the nebula are immensely further off than the other parts. however little we know of the nature of the object, we must suppose it to be one connected object, with all its parts, as to sense, at the same distance from us. whether therefore it be resolvable or no, there must be some other reason, besides the difference of distance, why the brighter parts were seen, while the fainter parts were not. the obvious reason is, that the latter were not seen because they were thin films which required more light to see them. we are led, irresistibly as it seems, to regard the whole mass of such a nebula, as an aggregation of vaporous rolls and streaks, assuming such forms as thin volumes of smoke or vapor often assume in our atmosphere, and assuming, like them, different shapes according to the quantity of light which comes to us from them. if, as soon as one of these new filaments or webs of a nebula comes into view, we should say, here we have a new array of suns and of worlds, we should judge as fantastically, as any one who should combine the like imaginations with the varying cloud-work of a summer-sky. to suppose that all the varied streaks by which the patch of nebulous light shades off into the surrounding darkness, and which change their form and extent with every additional polish which we can give to a reflecting or refracting surface, disclose, with every new streak, new worlds, is a wanton indulgence of fancy, to which astronomy gives us no countenance.[ ] . undoubtedly all true astronomers, taught caution and temperance of thought by the discipline of their magnificent science, abstain from founding such assumptions upon their discoveries. they know how necessary it is to be upon their guard against the tricks which fancy plays with the senses; and if they see appearances of which they cannot interpret the meaning, they are content that they should have no meaning for them, till the due explanation comes. we have innumerable examples of this wise and cautious temper, in all periods of astronomy. one has occurred lately. several careful astronomers, observing the stars by day, had been surprised to see globes of light glide across the field of view of their telescopes, often in rapid succession and in great numbers. they did not, as may be supposed, rush to the assumption that these globes were celestial bodies of a new kind, before unseen; and that from the peculiarity of their appearance and movement, they were probably inhabited by beings of a peculiar kind. they proceeded very differently; they altered the focus of their telescopes, looked with other glasses, made various changes and trials, and finally discovered that these globes of light were the winged seeds of certain plants which were wafted through the air; and which, illuminated by the sun, were made globular by being at distances unsuited to the focus of the telescope.[ ] . but perhaps something more may be founded on the ramified and straggling form which belongs to many of the nebulæ. under the powers of lord rosse's telescope, a considerable number of them assume a shape consisting of several spiral films diverging from one centre, and growing broader and fainter as they diverge, so as to resemble a curled feather, or whirlpool of light.[ ] this form, though generally deformed by irregularities, more or less, is traceable in so many of the nebulæ, that we cannot easily divest ourselves of the persuasion that there is some general reason for such a form;--that something, in the mechanical causes which have produced the nebulæ, has tended to give them this shape. now, when this thought has occurred to us, since mathematicians have written a great deal concerning the mechanics of the universe, it is natural to ask, whether any of the problems which they have solved give a result like that thus presented to our eyes. do such spirals as we here see, occur in any of the diagrams which illustrate the possible motions of celestial bodies? and to this, a person acquainted with mathematical literature might reply, that in the second book of newton's _principia_, in the part which has especial reference to the vortices of descartes, such spirals appear upon the page. they represent the path which a body would describe if, acted upon by a central force, it had to move in a medium of which the resistance was considerable;--considerable, that is, in comparison with the other forces which act; as for example, the forces which deflect the motion from a straight line. indeed, that in such a case a body would describe a spiral, of which the general form would be more or less oval, is evident on a little consideration. and in this way, for instance, encke's comet, which, if the resistance to its motion were insensible, would go on describing an ellipse about the sun, always returning upon the same path after every revolution; does really describe a path which, at each revolution, falls a little within the preceding revolution, and thus gradually converges to the centre. and if we suppose the comet to consist of a luminous mass, or a string of masses, which should occupy a considerable arc of such an orbit, the orbit would be marked by a track of light, as an oval spiral. or if such a comet were to separate into two portions, as we have, with our own eyes, recently seen biela's comet do; or into a greater number; then these portions would be distributed along such a spiral. and if we suppose a large mass of cometic matter thus to move in a highly resisting medium, and to consist of patches of different densities, then some would move faster and some more slowly; but all, in spirals such as have been spoken of; and the general aspect produced would be, that of the spiral nebulæ which i have endeavored to describe. the luminous matter would be more diffused in the outer and more condensed in the central parts, because to the centre of attraction all the spirals converge. . this would be so, we say, if the luminous matter moved in a greatly resisting medium. but what is the measure of _great_ resistance? it is, as we have already said, that the resistance which opposes the motion shall bear a considerable proportion to the force which deflects the motion. but what is that force? upon the theory of the universal gravitation of matter, on which theory we here proceed, the force which deflects the motions of the parts of each system into curves, is the mutual attraction of the parts of the system; leaving out of the account the action of other systems, as comparatively insignificant and insensible. the condition, then, for the production of such spiral figures as i have spoken of, amounts really to this; that the mutual attraction of the parts of the luminous matter is slight; or, in other words, that the matter itself is very thin and rare. in that case, indeed, we can easily see that such a result would follow. a cloud of dust, or of smoke, which was thin and light, would make but a little way through the air, and would soon fall downwards; while a metal bullet shot horizontally with the same velocity, might fly for miles. just so, a loose and vaporous mass of cometic matter would be pulled rapidly inwards by the attraction to the centre; and supposing it also drawn into a long train, by the different density of its different parts, it would trace, in lines of light, a circular or elliptical spiral converging to the centre of attraction, and resembling one of the branches of the spiral nebulæ. and if several such cometic masses thus travelled towards the centre, they would exhibit the wheel-like figure with bent spokes, which is seen in the spiral nebulæ. and such a figure would all the more resemble some of these nebulæ, as seen through lord rosse's telescope, if the spirals were accompanied by exterior branches of thinner and fainter light, which nebulous matter of smaller density might naturally form. perhaps too, such matter, when thin, may be supposed to cool down more rapidly from its state of incandescence; and thus to become less luminous. if this were so, a great optical power would of course be required, to make the diverging branches visible at all. . there is one additional remark, which we may make, as to the resemblance of cometary[ ] and nebular matter. that cometary matter is of very small density, we have many reasons to believe:--its transparency, which allows us to see stars through it undimmed;--the absence of any mechanical effect, weight, inertia, impulse, or attraction, in the nearest appulses of comets to planets and satellites:--and the fact that, in the recent remarkable event in the cometic history, the separation of biela's comet into two, the two parts did not appear to exert any perceptible attraction on each other, any more than two volumes of dust or of smoke would do on earth. luminous cometary matter, then, is very light, that is, has very little weight or inertia. and luminous nebulous matter is also very light in this sense: if our account of the cause of spiral nebulæ has in it any truth. but yet, if we suppose the nebulæ to be governed by the law of universal gravitation, the attractive force of the luminous matter upon itself, must be sufficient to bend the spirals into their forms. how are we to reconcile this; that the matter is so loose that it falls to the centre in rapid spirals, and yet that it attracts so strongly that there is a centre, and an energetic central force to curve the spirals thither? to this, the reply which we must make is, that the size of the nebular space is such, that though its rarity is extreme, its whole mass is considerable. one part does not perceptibly attract another, but the whole does perceptibly attract every part. this indeed need the less surprise us, since it is exactly the case with our earth. one stone does not visibly attract another. it is much indeed for man, if he can make perceptible the attraction of a mountain upon a plumb-line; or of a stratum of rock a thousand feet thick upon the going of a pendulum; or of large masses of metal upon a delicate balance. by such experiments men of science have endeavored to measure that minute thing, the attraction of one portion of terrestrial matter upon another; and thus, to weigh the whole mass of the earth. and equally great, at least, may be the disproportion between the mutual attraction of two parts of a nebulous system, and the total central attraction; and thus, though the former be insensible, the latter may be important. . it has been shown by newton, that if any mass of matter be distributed in a uniform sphere, or in uniform concentric spherical shells, the total attraction on a point without the sphere, will be the same as if the whole mass were collected in that single point, the centre. now, proceeding upon the supposition of such a distribution of the matter in a nebula, (which is a reasonable average supposition,) we may say, that if our sun were expanded into a nebula reaching to the extreme bounds of the known solar system, namely, to the newly-discovered planet neptune, or even hundreds of times further; the attraction on an external point would remain the same as it is, while the attraction on points within the sphere of diffusion would be less than it is; according to some law, depending upon the degree of condensation of the nebular matter towards the centre; but still, in the outer regions of the nebula, not differing much from the present solar attraction. if we could discover a mass of luminous matter, descending in a spiral course towards the centre of such a nebula, that is, towards the sun, we should have a sort of element of the spiral nebulæ which have now attracted so much of the attention of astronomers. but, by an extraordinary coincidence, recent discoveries have presented to us such an element. encke's comet, of which we have just spoken, appears to be describing such a spiral curve towards the sun. it is found that its period is, at every revolution, shorter and shorter; the amplitude of its sweep, at every return within the limits of our observation, narrower and narrower; so that in the course of revolutions and ages, however numerous, still, not such as to shake the evidence of the fact, it will fall into the sun. . here then we are irresistibly driven to calculate what degree of resemblance there is, between the comet of encke, and the luminous elements of the spiral nebulæ, which have recently been found to exist in other regions of the universe. can we compare its density with theirs? can we learn whether the luminous matter in such nebulæ is more diffused or less diffused, than that of the comet of encke? can we compare the mechanical power of getting through space, as we may call it, that is, the ratio of the inertia to the resistance, in the one case, and in the other? if we can, the comparison cannot fail, it would seem, to be very curious and instructive. in this comparison, as in most others to which cosmical relations conduct us, we must expect that the numbers to which we are led, will be of very considerable amount. it is not equality in the density of the two luminous masses which we are to expect to find; if we can mark their proportions by thousands of times, we shall have made no small progress in such speculations. . the comet of encke describes a spiral, gradually converging to the sun; but at what rate converging? in how many revolutions will it reach the sun? of how many folds will its spire consist, before it attains the end of its course? the answer is:--of very many. the retardation of encke's comet is very small: so small, that it has tasked the highest powers of modern calculation to detect it. still, however, it is there: detected, and generally acknowledged, and confirmed by every revolution of the comet, which brings it under our notice; that is, commonly, about every three years. and having this fact, we must make what we can of it, in reasoning on the condition of the universe. no accuracy of calculation is necessary for our purpose: it is enough, if we bring into view the kind of scale of numbers to which calculation would lead us. . encke's comet revolves round the sun in , days. the period diminishes at present, by about one-ninth of a day every revolution. this amount of diminution will change, as the orbit narrows; but for our purpose, it will be enough to consider it unchangeable. the orbit therefore will cease to exist in a number of periods expressed by times , ; that is, in something more that , revolutions; and of course sooner than this, in consequence of its coming in contact with the body of the sun. in , years then, it may be, this comet will complete its spiral, and be absorbed by the central mass. this long time, this long series of ten thousand revolutions, are long, because the resistance is so small, compared with the inertia of the moving mass. however thin, and rare, and unsubstantial the comet may be, the medium which resists it is much more so. . but this spiral, converging to its pole so slowly that it reaches it only after , circuits, is very different indeed from the spirals which we see in the nebulæ of which we have spoken. in the most conspicuous of those, there are only at most three or four circular or oval sweeps, in each spiral, or even the spiral reaches the centre before it has completed a single revolution round it. now, what are we to infer from this? how is it, that the comet has a spiral of so many revolutions, and the nebulæ of so few? what difference of the mechanical conditions is indicated by this striking difference of form? why, while the comet thus lingers longer in the outer space, and approaches the sun by almost imperceptible degrees, does the nebular element rush, as it were, headlong to its centre, and show itself unable to circulate even for a few revolutions? . regarding the question as a mechanical problem, the answer must be this:--it is so, because the nebula is so much more rare than the matter of the comet, or the resisting medium so much more dense; or combining the two suppositions, because in the case of the comet, the luminous matter has _much_ more inertia, more mechanical reality and substance, than the medium through which it moves; but in the nebula very _little_ more. . the numbers of revolutions of the spiral, in the two cases, may not exactly represent the difference of the proportions; but, as i have said, they may serve to show the scale of them; and thus we may say, that if encke's comet, approaching the centre by , revolutions, is , times as dense as the surrounding medium, the elements of the nebula, which reach the centre in a single revolution, are only ten times as dense as the medium through which they have to move.[ ] . nor does this result (that the bright element of the nebulæ is so few times denser than the medium in which it moves) offer anything which need surprise us: for, in truth, in a diffused nebula, since we suppose that its parts have mechanical properties, the nebula itself is a resisting medium. the rarer parts, which may very naturally have cooled down in consequence of their rarity, and so, become non-luminous, will resist the motions of the more dense and still-luminous portions. if we recur to the supposition, which we lately made, that the sun were expanded into a nebulous sphere, reaching the orbit of neptune, the diffused matter would offer a far greater resistance to the motions of comets than they now experience. in that case, encke's comet might be brought to the centre after a few revolutions; and if, while it were thus descending, it were to be drawn out into a string of luminous masses, as biela's comet has begun to be, these comets, and any others, would form separate luminous spiral tracks in the solar system; and would convert it into a spiral nebula of many branches, like those which are now the most recent objects of astronomical wonder. . it seems allowable to regard it as one of those coincidences, in the epochs of related yet seeming unconnected discoveries, which have so often occurred in the history of science; that we should, nearly at the same time, have had brought to our notice, the prevalence of spiral nebulæ, and the circumstances, in biela's and in encke's comets, which seem to explain them: the one by showing the origin of luminous broken lines, one part drifting on faster than another, according to its different density, as is usual in incoherent masses;[ ] and the other by showing the origin of the spiral form of those lines, arising from the motion being in a resisting medium. . but though i have made suppositions by which our solar system might become a spiral nebula, undoubtedly it is at present something very different; and the leading points of difference are very important for us to consider. and the main point is, that which has already been cursorily noticed: that instead of consisting of matter all nearly of the same density, and a great deal of it luminous, our solar system consists of kinds of matter immensely different in density, and of large and regular portions which are not luminous. instead of a diffused nebula with vaporous comets trailing spiral tracks through a medium little rarer than themselves; we have a central sun, and the dark globes of the solid planets rolling round him, in a medium so rare, that in thousands of revolutions not a vestige of retardation can be discovered by the most subtle and persevering researches of astronomers. in the solar system, the luminous matter is collected into the body of the sun; the non-luminous matter, into the planets. and the comets and the resisting medium, which offer a small exception to this account, bear a proportion to the rest which the power of numbers scarce suffices to express. . thus with regard to the density of matter in the solar system; we have supposed, as a mode of expression, that the density of a comet, encke's comet for instance, is , times that of the resisting medium. probably this is greatly understated; and probably also we greatly understate the matter, when we suppose that the tail of a comet is , times rarer than the matter of the sun.[ ] and thus the resisting medium would be, at a very low calculation, , millions of times more rare than the substance of the sun. . and thus we are not, i think, going too far, when we say, that our solar system, compared with spiral nebulous systems, is a system completed and finished, while they are mere confused, indiscriminate, incoherent masses. in the nebulæ, we have loose matter of a thin and vaporous constitution, differing as more or less rare, more or less luminous, in a small degree; diffused over enormous spaces, in straggling and irregular forms; moving in devious and brief curves, with no vestige of order or system, or even of separation of different kinds of bodies. in the solar system, we have the luminous separated from the non-luminous, the hot from the cold, the dense from the rare; and all, luminous and non-luminous, formed into globes, impressed with regular and orderly motions, which continue the same for innumerable revolutions and cycles.[ ] the spiral nebulæ, compared with the solar system, cannot be considered as other than a kind of chaos; and not even a chaos, in the sense of a state preceding an orderly and stable system; for there is no indication, in those objects, of any tendency towards such a system. if we were to say that they appear mere shapeless masses, flung off in the work of creating solar systems, we might perhaps disturb those who are resolved to find everywhere worlds like ours; but it seems difficult to suggest any other reason for not saying so. . the same may be said of the other very irregular nebulæ, which spread out patches and paths of various degrees of brightness; and shoot out, into surrounding space, faint branches which are of different form and extent, according to the optical power with which they are seen. these irregular forms are incapable of being permanent according to the laws of mechanics. they are not figures of equilibrium; and, therefore, must change by the attraction of the matter upon itself. but if the tenuity of the matter is extreme, and the resistance of the medium in which it floats considerable, this tendency to change and to condensation may be almost nullified; and the bright specks may long keep their straggling forms, as the most fantastically shaped clouds of a summer-sky often do. it is true, it may be said that the reason why we see no change in the form of such nebulæ, is that our observations have not endured long enough; all visible changes in the stars requiring an immense time, according to the gigantic scale of celestial mechanism. but even this hypothesis (it is no more) tends to establish the extreme tenuity of the nebulæ; for more solid systems, like our solar system, require, for the preservation of their form, motions which are perceptible, and indeed conspicuous, in the course of a month; namely, the motions of the planets. all, therefore, concurs to prove the extreme tenuity of the substance of irregular nebulæ. . nebulæ which assume a regular, for instance, a circular or oval shape, with whatever variation of luminous density from the inner to the outer parts, may have a form of equilibrium, if their parts have a proper gyratory motion. still, we see no reason for supposing that these differ so much from irregular nebulæ, as to be denser bodies, kept in their forms by rapid motions. we are rather led to believe that, though perhaps denser than the spiral nebulæ, they are still of extremely thin and vaporous character. it would seem very unlikely that these vast clouds of luminous vapor should be as dense as the tail of a comet; since a portion of luminous matter so small as such a tail is, must have cooled down from its most luminous condition; and must require to be more dense than nebular matter in order to be visible at all by its own light. . thus we appear to have good reason to believe that nebulæ are vast masses of incoherent or gaseous matter, of immense tenuity, diffused in forms more or less irregular, but all of them destitute of any regular system of solid moving bodies. we seem, therefore, to have made it certain that _these_ celestial objects at least are not inhabited. no speculators have been bold enough to place inhabitants in a comet; except, indeed, some persons who have imagined that such a habitation, carrying its inmates alternately into the close vicinity of the sun's surface, and far beyond the orbit of uranus, and thus exposing them to the fierce extremes of heat and cold, might be the seat of penal inflictions on those who had deserved punishment by acts done in their life on one of the planets. but even to give coherence to this wild imagination, we must further suppose that the tenants of such prison-houses, though still sensible to human suffering from extreme heat and cold, have bodies of the same vaporous and unsubstantial character as the vehicle in which they are thus carried about the system; for no frame of solid structure could be sustained by the incoherent and varying volume of a comet. and probably, to people the nebulæ with such thin and fiery forms, is a mode of providing them with population, that the most ardent advocates of the plurality of worlds are not prepared to adopt. . so far then as the nebulæ are concerned, the improbability of their being inhabited, appears to mount to the highest point that can be conceived. we may, by the indulgence of fancy, people the summer-clouds, or the beams of the aurora borealis, with living beings, of the same kind of substance as those bright appearances themselves; and in doing so, we are not making any bolder assumption than we are, when we stock the nebulæ with inhabitants, and call them in that sense, "distant worlds." footnotes: [ ] herschel, _outl. of astr._ art. . [ ] herschel, _outl. of astr._ art. , and plate , fig. . [ ] ibid. art. . [ ] hersch. . [ ] ibid. - . [ ] at the recent meeting of the british association (sept. ), drawings were exhibited of the same nebulæ, as seen through lord rosse's large telescope, and through a telescope of three feet aperture. with the smaller telescopic power, all the characteristic features were lost. the spiral structure (see next article but one) has been almost entirely brought to light by the large telescope. [ ] see monthly notices of the royal astronomical society, dec. , . [ ] the frontispiece to this volume represents two of these spiral nebulæ; those denominated messier, and messier, as given by lord rosse in the _phil. trans. for _. the former of these two has a lateral focus, besides the principal focus or pole. [ ] i am aware that some astronomers do not consider it as proved that cometary matter is entirely self-luminous. arago found that the light of a comet contained a portion of polarized light, thus proving that it had been reflected (_cosmos_, i. p. , and iii. p. ). but i think the opinion that the greater part of the light is self-luminous, like the nebulæ, generally prevails. any other supposition is scarcely consistent with the rapid changes of brightness which occur in a comet during its motion to and from the sun. [ ] we assume here that the number of revolutions to the centre is greater in proportion as the relative density of the resisting medium is less; which is by no means mechanically true; but the calculation may serve, as we have said, to show the scale of the numbers involved. [ ] humboldt, whom nothing relative to the history of science escapes, quotes from seneca a passage in which mention is made of a comet which divided into two parts; and from the chinese annals, a notice of three "coupled comets," which in the year appeared, and described their paths together. _cosmos_, iii. p. , and the notes. [ ] laplace has proved that the masses of comets are very small. he reckons their mean mass as very much less than - th of the earth's mass. and hence, considering their great size, we see how rare they must be. see _expos. du syst. du monde_. [ ] humboldt repeatedly expresses his conviction that our solar system contains a greater variety of forms than other systems. (_cosmos_, iii. and .) chapter viii. the fixed stars. . we appear, in the last chapter, to have cleared away the supposed inhabitants of the outskirts of creation, so far as the nebulæ are the outskirts of creation. we must now approach a little nearer, in appearance at least, to our own system. we must consider the fixed stars; and examine any evidence which we may be able to discover, as to the probability of their containing, in themselves or in accompanying bodies, as planets, inhabitants of any kind. any special evidence which we can discern on this subject, either way, is indeed slight. on the one side we have the asserted analogy of the parts of the universe; of which point we have spoken, and may have more to say hereafter. each fixed star is conceived to be of the nature of our sun; and therefore, like him, the centre of a planetary system. on the other side, it is extremely difficult to find any special facts relative to the nature of the fixed stars, which may enable us in any degree to judge how far they really are of a like nature with the sun, and how far this resemblance goes. we may, however, notice a few features in the starry heavens, with which, in the absence of any stronger grounds, we may be allowed to connect our speculations on such questions. the assiduous scrutiny of the stars which has been pursued by the most eminent astronomers, and the reflections which their researches have suggested to them, may have a new interest, when discussed under this point of view. . next after the nebulæ, the cases which may most naturally engage our attention, are clusters of stars. the cases, indeed, in which these clusters are the closest, and the stars the smallest, and in which, therefore, it is only by the aid of a good telescope that they are resolved into stars, do not differ from the resolvable nebulæ, except in the degree of optical power which is required to resolve them. we may, therefore, it would seem, apply to such clusters, what we have said of resolvable nebulæ: that when they are thus, by the application of telescopic power, resolved into bright points, it seems to be a very bold assumption to assume, without further proof, that these bright points are suns, distant from each other as far as we are from the nearest stars. the boldness of such an assumption appears to be felt by our wisest astronomers.[ ] that several of the clusters which are visible, some of them appearing as if the component stars were gathered together in a nearly spherical form, are systems bound together by some special force, or some common origin, we may regard, with those astronomers, as in the highest degree probable. with respect to the stability of the form of such a system, a curious remark has been made by sir john herschel,[ ] that if we suppose a globular space filled with equal stars, uniformly dispersed through it, the particular stars might go on forever, describing ellipses about the centre of the globe, in all directions, and of all sizes; and all completing their revolutions in the same time. this follows, because, as newton has shown, in such a case, the compound force which tends to the centre of the sphere would be everywhere proportional to the distance from the centre; and under the action of such a force, ellipses about the centre would be described, all the periods being of the same amount. this kind of symmetrical and simple systematic motion, presented by newton as a mere exemplification of the results of his mechanical principles, is perhaps realized, approximately at least, in some of the globular clusters. the motions will be swift or slow, according to the total mass of the groups. if, for instance, our sun were thus broken into fragments, so as to fill the sphere girdled by the earth's orbit, all the fragments would revolve round the centre in a year. now, there is no symptom, in any cluster, of its parts moving nearly so fast as this; and therefore we have, it would seem, evidence that the groups are much less dense than would be the space so filled with fragments of the sun. the slowness of the motions, in this case, as in the nebulæ, is evidence of the weakness of the forces, and therefore, of the rarity of the mass; and till we have some gyratory motion discovered in these groups, we have nothing to limit our supposition of the extreme tenuity of their total substance. . let us then go on to the cases in which we have proof of such gyratory motions in the stars; for such are not wanting. fifty years ago, herschel the father, had already ascertained that there are certain pairs of stars, very near each other (so near, indeed, that to the unassisted eye they are seen as single stars only,) and which revolve about each other. these binary sidereal systems have since been examined with immense diligence and profound skill by herschel the son, and others; and the number of such binary systems has been found, by such observers, to be very considerable. the periods of their revolutions are of various lengths, from or years to several hundreds of years. some of those pairs which have the shortest periods, have already, since the nature of their movements was discovered, performed more than a complete revolution;[ ] thus leaving no room for doubting that their motions are really of this gyratory kind. not only the fact, but the law of this orbital motion, has been investigated; and the investigations, which naturally were commenced on the hypothesis that these distant bodies were governed by that law of universal gravitation, which prevails throughout the solar system, and so completely explains the minutest features of its motions, have ended in establishing the reality of that law, for several binary systems, with as complete evidence as that which carries its operations to the orbits of uranus and neptune. . being able thus to discern, in distant regions of the universe, bodies revolving about each other, we have the means of determining, as we do in our own solar system, the masses of the bodies so revolving. but for this purpose, we must know their distance from each other; which is, to our vision, exceedingly small, requiring, as we have said, high magnifying powers to make it visible at all. and again, to know what linear distance this small visible distance represents, we must know the distance of the stars from us, which is, for every star, as we know, immensely great; and for most, we are destitute of all means of determining how great it is. there are, however, some of these binary systems, in which astronomers conceive that they have sufficiently ascertained the value of both these elements, (the distance of the two stars from each other, and from us,) to enable them to proceed with the calculation of which i have spoken; the determination of the masses of the revolving bodies. in the case of the star _alpha centauri_, the first star in the constellation of the centaur, the period is reckoned to be years; and as, by the same calculator, the apparent semi-axis of the orbit described is stated at seconds of space, while the annual parallax of each star is about one second, it is evident that the orbit must have a radius about times the radius of the earth's orbit; that is, an orbit greater than that of saturn, and approaching to that of uranus. in the solar system, a revolution in such an orbit would occupy a time greater than that of saturn, which is years, and less than that of uranus, which is about years: it would, in fact, be about years. and since, in the binary star, the period is greater than this, namely years, the attraction which holds together its two elements must be less than that which holds together the sun and a planet at the same distance; and therefore the masses of the two stars together are considerably less than the mass of our sun. . a like conclusion is derived from another of these conspicuous double stars, namely, the one termed by astronomers _ cygni_; of which the annual parallax has lately been ascertained to be one-third of a second of space, while the distance of the two stars is seconds. here therefore we have an orbit times the size of the earth's orbit; larger than that of the newly-discovered planet neptune, whose orbit is times as large as the earth's, and his period nearly years. the period of cygni is however, it appears, probably not short of years; and hence it is calculated that the sum of the masses of the two stars which make up this pair is about one-third of the mass of our sun.[ ] . these results give some countenance to the opinion, that the quantity of luminous matter, in other systems, does not differ very considerably from the mass of our sun. it differs in these cases as to , or thereabouts. in what degree of condensation, however, the matter of these binary systems is, compared with that of our solar system, we have no means whatever of knowing. each of the two stars may have its luminous matter diffused through a globe as large as the earth's orbit; and in that case, would probably not be more dense than the tail of a comet.[ ] it is observed by astronomers, that in the pairs of binary stars which we have mentioned, the two stars of each pair are of different colors; the stars being of a high yellow, approaching to orange color,[ ] but the smaller individual being in each case of a deeper tint. this might suggest to us the conjecture that the smaller mass had cooled further below the point of high luminosity than the larger; but that both these degrees of light belong to a condition still progressive, and probably still gaseous. without attaching any great value to such conjectures, they appear to be at least as well authorized as the supposition that each of these stars, thus different, is nevertheless precisely in the condition of our sun. . but, even granting that each of the individuals of this pair were a sun like ours, in the nature of its material and its state of condensation, is it probable that it resembles our sun also in having planets revolving about it? a system of planets revolving around or among a pair of suns, which are, at the same time, revolving about one another, is so complex a scheme, so impossible to arrange in a stable manner, that the assumption of the existence of such schemes, without a vestige of evidence, can hardly require confutation. no doubt, if we were really required to provide such a binary system of suns with attendant planets, this would be best done by putting the planets so near to one sun, that they should not be sensibly affected by the other; and this is accordingly what has been proposed.[ ] for, as has been well said of the supposed planets, in making this proposal, "unless closely nestled under the protecting wing of their immediate superior, the sweep of the other sun in his perihelion passage round their own, might carry them off, or whirl them into orbits utterly inconsistent with the existence of their inhabitants." to assume the existence of the inhabitants, in spite of such dangers, and to provide against the dangers by placing them so close to one sun as to be out of the reach of the other, though the whole distance of the two may not, and as we have seen, in some cases does not, exceed the dimensions of our solar system, is showing them all the favor which is possible. but in making this provision, it is overlooked that it may not be possible to keep them in permanent orbits so near to the selected centre: their sun may be a vast sphere of luminous vapor; and the planets, plunged into this atmosphere, may, instead of describing regular orbits, plough their way in spiral paths through the nebulous abyss to its central nucleus. . clustered stars, then, and double stars, appear to give us but little promise of inhabitants. we must next turn our attention to the single stars, as the most hopeful cases. indeed, it is certain that no one would have thought of regarding the individual stars of clusters, or of pairs, as the centres of planetary systems, if the view of insulated stars, as the centres of such systems, had not already become familiar, and, we may say, established. what, then, is the probability of that view? is there good evidence that the fixed stars, or some of them, really have planets revolving round them? what is the kind of proof which we have of this? . to this we must reply, that the only proof that the fixed stars are the centres of planetary systems, resides in the assumption that those stars are _like the sun_;--resemble him in their qualities and nature, and therefore, it is inferred, must have the same offices, and the same appendages. they are, as the sun is, independent sources of light, and thence, probably, of heat; and therefore they must have attendant planets, to which they can impart their light and heat; and these planets must have inhabitants, who live under and enjoy those influences. this is, probably, the kind of reasoning on which those rely, who regard the fixed stars as so many worlds, or centres of families of worlds. . everything in this argument, therefore, depends upon this: that the stars are _like the sun_; and we must consider, what evidence we have of the exactness of this likeness. . the stars are like the sun in this, that they shine with an independent light, not with a borrowed light, as the planets shine. in this, however, the stars resemble, not only the sun, but the nebulous patches in the sky, and the tails of comets; for these also, in all probability, shine with an original light. probably it will hardly be urged that we see, by the very appearance of the stars, that they are of the nature of the sun: for the appearance of luminaries in the sky is so far from enabling us to discriminate the nature of their light, that to a common eye, a planet and a fixed star appear alike as stars. there is no obvious distinction between the original light of the stars and the reflected light of the planets. the stars, then, being like the sun in being luminous, does it follow that they are, like the sun, definite dense masses?[ ] or are they, or many of them, luminous masses in a far more diffused state; visually contracted to points, by the immense distance from us at which they are? . we have seen that some of those stars, which we have the best means of examining, are, in mass, one third, or less, of our sun. if such a mass, at the distance of the fixed stars, were diffused through a sphere equal in radius to the earth's orbit, it would still appear to us as a point; as is evident by this, that the fixed stars, for the most part, have no discoverable annual parallax; that is, the earth's orbit appears to them a point. if one of the fixed stars, sirius, for instance, be in this diffused condition, such a circumstance will not, mechanically speaking, prevent his having planets revolving round him; for, as we have said, the attraction of his whole mass, in whatever state of spherical diffusion, will be the same as if it were collected at the centre. but such a state of diffusion will make him so unlike our sun, as much to break the force of the presumption that he must have planets because our sun has. if the luminous matter of the stars gradually cools, grows dark, and solidifies, such diffusion would imply that the time of solidification is not yet begun; and therefore that the solid planets which accompany the luminous central body are not yet brought into being. if there be any truth in this hypothetical account of the changes, through which the matter of the stars successively passes; and if, by such changes, planetary systems are formed; how many of the fixed stars may never yet have reached the planetary state! how many, for want of some necessary mechanical condition, may never give rise to permanent orbits at all! . and that the matter of the stars does go through changes, we have evidence, in many such changes which have actually been observed;[ ] and perhaps in the different colors of different stars; which may, not improbably, arise from their being at different stages of their progress. that planetary systems, once formed, go through mighty changes, we have evidence in the view which geology gives us of the history of this earth; and in that view, we see also, how unique, and how far elevated in its purpose, the last period of this history may be, compared with the preceding periods; and, up to the present time at least, how comparatively brief in its duration. if, therefore, stellar globes can become planetary systems in the progress of ages, it will not be at all inconsistent with what we know of the order of nature, that only a few, or even that only one, should have yet reached that condition. all the others, but the one, may be systems yet unformed, or fragments struck off in the forming of the one. if any one is not satisfied with this account of the degree of resemblance between the fixed stars and the sun, but would make the likeness greater than this; we have only to say, that the proof that it is so lies upon him. such a resemblance as we have supposed, is all that the facts suggest. that the stars are independent luminaries, we see; but whether they are as dense as the sun, or globes a hundred or a thousand times as rare, we have no means whatever of knowing. and, to assume that besides these luminous bodies which we see, there are dark bodies which we do not see, revolving round the others in permanent orbits, which require special mechanical conditions; and to suppose this, in order that we may build upon this assumption a still larger one, that of living inhabitants of these dark bodies; is a hypothetical procedure, which it seems strange that we should have to combat, at the present stage of the history of science, and in dealing with those whose minds have been disciplined by the previous events in the progress of astronomy. . let us consider, however, further, how far astronomy authorizes us to regard the fixed stars as being, like our sun, the centres of systems of planets. those who hold this, consider them as having a permanent condition of brightness, as our sun has had for an indefinite period, so far as we have any knowledge on the subject. yet, as we have said, no small number of the stars undergo changes of brightness; and some of them undergo such changes, in a manner which is not discernibly periodical; and which must therefore be regarded as progressive. this phenomenon countenances the opinion of such a progress from one material condition to another; which, we have seen, is suggested by the analogy of the probable formation of our own solar system. the very star which is so often taken as the probable centre of a system, sirius, has, in the course of the last , years, changed its light from red to white. ptolemy notes it as a red star: in tycho's time it was already, as it is now, a white one.[ ] the star _eta argus_ changes both its degree of light and its color; ranging, in seemingly irregular intervals of time, from the fourth to the first magnitude,[ ] and from yellow to red. several other examples of the like kind have been observed. mr. hind[ ] gives an example in which he has, quite recently, observed in two years a star change its color from very red to bluish. these variable unperiodical stars are probably very numerous. also, some stars, observed of old, are now become invisible. "the lost pleiad," by the loss of which the cluster, called the seven stars, offers now only six to the naked eye, is an example of a change of this kind already noted in ancient times. there are several others, of which the extinction is recognized by astronomers as proved.[ ] in other cases, new stars have appeared, and have then seemed to die away and vanish. the appearance of a new star in the time of the greek astronomer hipparchus, induced him to construct his famous catalogue of the stars. others are recorded to have appeared in the middle ages. the first which was observed by modern astronomers was the celebrated star seen by tycho brahe in . it appeared suddenly in the constellation cassiopeia, was fixed in its place like the neighboring stars, had no nebula or tail, exceeded in splendor all other stars, being as bright as venus when she is nearest the earth. it soon began to diminish in brightness, and passing through various diminishing degrees of magnitude, vanished altogether after seventeen months. this star also passed through various colors; being first white, then yellow, then red. in like manner, in , a new star of great magnitude blazed forth in the constellation serpentarius; and was seen by kepler. and this also, like that of , after a few months, declined and vanished. . these appearances led tycho to frame an hypothesis like that which sir william herschel afterwards proposed, that the stars are formed by the condensation of luminous nebulous matter. nor is it easy to think of such phenomena (of which several others have been observed, though none so conspicuous as these), without regarding them as showing that the matter of the fixed stars, occasionally at least, passes through changes of consistence as great as would be the condensation and extinction of a luminous vapor. and if such changes have been but few within the recorded period of man's observation of the stars, we must recollect how small that period is, compared with the period during which the stars have existed. the stars themselves give us testimony of their having been in being for millions of years. for according to the best estimates we can form of their distances, the time which light would employ in reaching us from the most remote of them, would be millions of years; and, therefore, we now see those remote stars by means of the light emitted from them millions of years ago. and if, in the , years during which such observations are recorded, only stars have undergone such changes in a degree visible to the earth's inhabitants; in a million of years, change going on at the same rate, , stars would exhibit visible progressive change, showing that they had not yet reached a permanent condition. and how much of change may go on in any star without its being in any degree perceptible to the most exact astronomical scrutiny! . the tendency of these considerations is, to lead us to think that the fixed stars are not generally in that permanent condition in which our sun is; and which appears to be alone consistent with the existence of a system such as the solar system.[ ] these views, therefore, fall in with that which we have been led to by this consideration of the nebulæ: that the solar system is in a more complete and advanced state, as a system, than many at least of the stellar systems can be; it may be, than any other. . it has been alleged, as a proof of the likeness of the fixed stars to our sun, that like him, they revolve upon their axes.[ ] this has been supposed to be proved with regard to many of them, by their having periodical recurrences of fainter and brighter lustre; as if they were revolving orbs, with one side darkened by spots. such facts are not very numerous or definite in the heavens. _omicron_[ ] in the constellation _cetus_, is the longest known of them; and is held to revolve in days. from the curious phenomena now spoken of, it has been called _mira ceti_.[ ] _algol_, the second star (_beta_) of _perseus_, called also _caput medusæ_, is another, with a period of days hours; and in this case, the obscuration of the light, and the restoration of it, are so sudden, that from the time when it was first remarked, (by goodricke, in ,) it suggested the hypothesis of an opaque body revolving round the star. the star _delta_, in the constellation _cephus_, is another, with a period of days hours. the star _beta_ in the _lyre_, has a period of days hours, or perhaps days hours, one revolution having been taken for two. another such star is _eta aquilæ_, with a period of days hours. these five are all the periodical stars of which astronomers can speak with precision.[ ] but about thirty more are supposed to be subject to such change, though their periods, epochs, and phases of brightness, cannot at present be given exactly. . that these periodical changes in certain of the fixed stars are a curious and interesting astronomical fact, is indisputable. nothing can be more probable also, than that it indicates, in the stellar masses, a revolution on their axes; which cannot surprise us, seeing that revolution upon an axis is, so far as we know, a universal law of all the large compact masses of matter which exist in the universe; and may be conceived to be a result derived from their origin, and a condition of any permanent or nearly permanent figure. but this can prove little or nothing as to their being like the sun, in any way which implies their having inhabitants, in themselves or in accompanying planets. the rotation of our sun is not, in any intelligible way, connected with its having near it the inhabited earth. . if we were to suppose some of the stars to be centres of planetary systems, we can hardly suppose it likely that these alone rotate, and that the others stand still. probably all the stars rotate, more or less regularly, according as they are permanent or variable in form; but the most regular may still have no planets; and if they have, those planets may be as blank of inhabitants as our moon will be proved to be. . the revolution of algol seems to approach the nearest to a fact in favor of a star being the centre of a revolving system; and from the first, as we have said, the periodical change, and the sudden darkening and brightening of this luminary, suggested the supposition of an opaque body revolving about it. but this body cannot be a planet. the planets which revolve about our sun are not, any of them, nor all of them together, large enough to produce a perceptible obscuration of his light, to a spectator outside the system. but in algol, the phenomena are very different from this.[ ] the star is usually visible as a star of the second magnitude; but during each period of days hours, (or hours,) it suffers a kind of eclipse, which reduces it to a star of the fourth magnitude. during this eclipse, the star diminishes in splendor for - / hours; is at its lowest brightness for a quarter of an hour; and then, in - / hours more, is restored to its original splendor. according to these numbers, if the obscuration be produced by a dark body revolving round a central luminary, and describing a circular orbit, as the regular recurrence of the obscuration implies, the space of the orbit during which the eclipsing body is interposed must be about one-ninth of the circumference; for the obscuration occupies - / hours out of . and therefore the space during which the eclipsing body obscures the central one, must be about one _sixth_ of the _diameter_ of its orbit. but in order that the revolving body may, through this space, obscure the central one, the latter must extend over this space, namely, one sixth of the diameter of the orbit. but we may remark that there is no proof, in the phenomena, that the darkening body is detached from the bright mass. the effect would be the same if the dark mass were a part of the revolving star itself. it may be that the star has not yet assumed a spherical form, but is an oblong nebular mass with one part (perhaps from being thinner in texture) cooled down and become opaque. and the amount of obscuration, reducing the star from the second to the fourth magnitude, implies that the obscuring mass is large (perhaps one half the diameter, or much more) compared with the luminous mass. if this be a probable hypothesis to account for the phenomena, they are much more against than for the supposition of the star being the centre of seats of habitation. and even if we have a planet nearly as large as its sun, revolving at the distance of only six of the sun's radii, how unlike is this to the solar system! . in fact, all these periodical stars, in so far as they are periodical, are proved, not to be like, but to be _unlike_ our sun. it is true that the sun has spots, by means of which his rotation has been determined by astronomers. but these spots, besides being so small that they produce no perceptible alteration in his brightness, and are never, or very rarely, visible to the naked eye, are not permanent. a star with a permanent dark side would be very unlike our sun. the largest known of these stars, _mira_, as the old astronomers called it, becomes invisible to the naked eye for months during a period of months. it must, therefore, have nearly one half its surface quite dark. this is very unlike the condition of the sun; and is a condition, it would seem, very little fitted to make this star the centre of a planetary system like ours. . but there are other remarkable phenomena respecting these periodical stars, which have a bearing on our subject. their periods are not quite regular, but are subject to certain variations. thus it has been supposed that the period of mira is subject to a cyclical fluctuation, embracing of its periods; that is, about years. but this notion of a cycle of so long a duration, requires confirmation; the fact of fluctuation in the period is alone certain. in like manner, algol's periods are not quite uniform. all these facts agree with our suggestion, that the periodical stars are bodies of luminous matter which have not yet assumed a permanent form; and which, therefore, as they revolve about their axes, and turn to us their darker and their brighter parts, do so at intervals, and in an order somewhat variable. and this suggestion appears to be remarkably confirmed, by a result which recent observations have discovered relative to this star, algol; namely, that its periods become shorter and shorter. for if the luminous matter, which is thus revolving, be gradually gathering into a more condensed form;--becoming less rare, or more compact; as, for instance, it would do, if it were collecting itself from an irregular, or elongated, into a more spherical form; such a shortening of the period of revolution would take place; for a mass which contracts while it is revolving, accelerates its rate of revolution, by mechanical principles. and thus we do appear to have, in this observed acceleration of the periods of algol, an evidence that that luminous mass has not yet reached its final and permanent condition. . it is true, it has been conjectured, by high authority,[ ] that this accelerated rapidity of the periods of algol will not continue; but will gradually relax, and then be changed to an increase; like many other cyclical combinations in astronomy. but this conjecture seems to have little to support it. the cases in which an acceleration of motion is retarded, checked, and restored, all belong to our solar system; and to assume that algol, like the solar system, has assumed a permanent and balanced condition, is to take for granted precisely the point in question. we know of no such cycles among the fixed stars, at least with any certainty; for the cycle proposed for mira must be considered as greatly needing confirmation; considering how long is the cycle, and how recent the suggestion of its existence. . and even in the solar system, we have accelerated motions, in which no mathematician or astronomer looks for a check or regress of the acceleration. no one expects that encke's comet will cease to be accelerated, and to revolve in periods continually shorter; though all the other motions hitherto observed in the system are cyclical. in the case of a fixed star, we have much less reason to look for such a cycle, than we have in encke's comet. but further: with regard to the existence of such a cycle of faster and slower motion in the case of algol, the most recent observed facts are strongly against it; for it has been observed by argelander, that not only there is a diminution of the period, but that this diminution proceeds with accelerated rapidity; a course of events which, in no instance, in the whole of the cosmical movements, ends in a regression, retardation, and restoration of the former rate. we are led to believe, therefore, that this remarkable luminary will go on revolving faster and faster, till its extreme point of condensation is attained. and in the meantime, we have very strong reasons to believe that this mutable body is not, like the sun, a permanent centre of a permanent system; and that any argument drawn from its supposed likeness to the sun, in favor of the supposition that the regions which are near it are the seats of habitation, is quite baseless. . there are other phenomena of the fixed stars, and other conjectures of astronomers respecting them, which i need not notice, as they do not appear to have any bearing upon our subject. such are the "proper motions" of the stars, and the explanation which has been suggested of some of them; that they arise from the stars revolving round other stars which are dark, and therefore invisible. such again is the attempt to show that the sun, carrying with it the whole solar system, is in motion; and the further attempt to show the direction of this motion; and again, the hypothesis that the sun itself revolves round some distant body in space. these minute inquiries and bold conjectures, as to the movements of the masses of matter which occupy the universe, do not throw any light on the question whether any part besides the earth is inhabited; any more than the investigation of the movements of the ocean, and of their laws, could prove or disprove the existence of marine plants and animals. they do not on that account cease to be important and interesting subjects of speculation; but they do not belong to our subject. . in fontenelle's _dialogues on the plurality of worlds_, a work which may be considered as having given this subject a place in popular literature, he illustrates his argument by a comparison, which it may be worth while to look at for a moment. the speaker who asserts that the moon, the planets, and the stars, are the seats of habitation, describes the person, who denies this, as resembling a citizen of paris, who, seeing from the towers of notre dame the town of saint denis, (it being supposed that no communication between the two places had ever occurred,) denies that it is inhabited, because he cannot see the inhabitants. of course the conclusion is easy, if we may thus take for granted that what he sees is a town. but we may modify this image, so as to represent our argument more fairly. let it be supposed that we inhabit an island, from which innumerable other islands are visible; but the art of navigation being quite unknown, we are ignorant whether any of them are inhabited. in some of these islands, are seen masses more or less resembling churches; and some of our neighbors assert that these are churches; that churches must be surrounded by houses; and that houses must have inhabitants. others hold that the seeming churches are only peculiar forms of rocks. in this state of the debate, everything depends upon the degree of resemblance to churches which the forms exhibit. but suppose that telescopes are invented, and employed with diligence upon the questionable shapes. in a long course of careful and skilful examination, no house is seen, and the rocks do not at all become more like churches, rather the contrary. so far, it would seem, the probability of inhabitants in the islands is lessened. but there are other reasons brought into view. our island is a long extinct volcano, with a tranquil and fertile soil; but the other islands are apparently somewhat different. some of them are active volcanoes, the volcanic operations covering, so far as we can discern, the whole island; others undergo changes, such as weather or earthquakes may produce; but in none of them can we discover such changes as show the hand of man. for these islands, it would seem the probability of inhabitants is further lessened. and so long as we have no better materials than these for forming a judgment, it would, surely, be accounted rash, to assert that the islands in general are inhabited; and unreasonable, to blame those who deny or doubt it. nor would such blame be justified by adducing theological or _à priori_ arguments; as, that the analogy of island with island makes the assumption allowable; or that it is inconsistent with the plan of the creator of islands to leave them uninhabited. for we know that many islands are, or were long, uninhabited. and if ours were an island occupied by a numerous, well-governed, moral, and religious race, of which the history was known, and of which the relation to the creator was connected with its history; the assumption of a history, more or less similar to ours, for the inhabitants of the other islands, whose existence was utterly unproved, would, probably, be generally deemed a fitter field for the romance-writer than for the philosopher. it could not, at best, rise above the region of vague conjecture. . fontenelle, in the agreeable book just referred to, says, very truly, that the formula by which his view is urged on adversaries is, _pourquoi non_? which he holds to be a powerful figure of logic. it is, however, a figure which has this peculiarity, that it may, in most cases, be used with equal force on either side. when we are asked why the moon, mercury, saturn, the system of sirius, should _not_ be inhabited by intelligent beings; we may ask, why the earth in the ages previous to man might not be so inhabited? the answer would be, that we have proof _how_ it _was_ inhabited. and as to the fact in the other case, i shall shortly attempt to give proof that the moon is certainly not, and mercury and saturn probably not inhabited. with regard to the fixed stars, it is more difficult to reason; because we have the means of knowing so little of their structure. but in this case also, we might easily ask on our side, _pourquoi non_? why should not the solar system be the chief and most complete system in the universe, and the earth the principal planet in that system? so far as we yet know, the sun is the largest sun among the stars; and we shall attempt to show, that the earth is the largest solid opaque globe in the solar system. some system must be the largest and most finished of all; why not ours? some planet must be the largest planet; why not the earth? . it should be recollected that there must be some system which is the most complete of all systems, some planet which is the largest of all planets. and if that largest planet, in the most complete system, be, after being for ages tenanted by irrational creatures, at last, and alone of all, occupied by a rational race, that race must necessarily have the power of asking such questions as these: why they should be alone rational? why their planet should be alone thus favored? if the case be ours, we may hope to be then able to answer these questions, when we can explain the most certain fact which they involve; why the earth was occupied so long by irrational creatures, before the rational race was placed upon it? the mere power of asking such questions can prove or disprove nothing; for it is a power which must equally subsist, whether the human inhabitants of the earth be or be not the only rational population which the universe contains. if there be a race thus favored by the creator, they must, at that stage of their knowledge in which man now is, be able to doubt, as man does, of the extent and greatness of the privilege which they enjoy. . the argument that the fixed stars are like the sun, and therefore the centres of inhabited systems as the sun is, is sometimes called an argument from analogy; and this word _analogy_ is urged, as giving great force to the reasoning. but it must be recollected, that precisely the point in question is, whether there _is_ an analogy. the stars, it is said, are like the sun. in what respects? we know of none, except in being self-luminous; and this they have in common with the nebulæ, which, as we have seen, are not centres of inhabited systems. nor does this quality of being self-luminous at all determine the degree of condensation of a star. sirius may be less than a hundredth or a thousandth of the density of the sun. but the stars, it may be further urged, are like the sun in turning on their axes. to this we reply, that we know this only of those stars in which, the very phenomenon which proves their revolution, proves also that they are unlike the sun, in having one side darker than the other. add to which, their revolution is not connected with the existence of planets, still less of inhabitants of planets, in any intelligible manner. the resemblance, therefore, so far as it bears upon the question, is confined to one single point, in the highest degree ambiguous and inconclusive; and any argument drawn from this one point of resemblance, has little claim to be termed an argument from analogy.[ ] . on a subject on which we know so little, it is difficult to present any view which deserves to be regarded as an analogy. we see, among the stars, nebulæ more or less condensed, which are possibly, in some cases, stages of a connected progress towards a definite star; and it may be, to a star with planets in permanent orbits. we see, in our planet, evidence of successive stages of a connected series of brute animals, preceded perhaps by various stages of lifeless chaos. if the histories of the sun, and of all the stars, are governed by a common analogy, the nebulous condensation, and the stages of animal life, may be parts of the same continued series of events; and different stars may be at different points of that series. but even on this supposition, but a few of the stars may be the seats of conscious life, and none, of intelligence. for among the stars which have condensed to a permanent form, how many have failed in throwing off a permanent planet! how many may be in some stage of lifeless chaos! we must needs suppose a vast number of stages between a nebular chaos and the lowest forms of conscious life. perhaps as many as there are fixed stars; and far more than there are of stars which become fertile of life: so that no two systems may be at the same stage of the planetary progress. and if this be so,--our system being so complicated, that we must suppose it peculiarly developed, having the largest sun that we know of, and our earth being (as we shall hereafter attempt to prove) the largest solid planet that we know of,--this earth may be the sole seat of the highest stage of planetary development. . the assumption that there is anything of the nature of a regular law or order of progress from nebular matter to conscious life,--a law which extends to all the stars, or to many of them,--is in the highest degree precarious and unsupported; but since it is sometimes employed in such speculations as we are pursuing, we may make a remark or two connected with it. if we suppose, on the planets of other systems, a progress in some degree analogous to that which geology shows to have occurred on the earth, there may be, in those planets, creatures in some way analogous to our vegetables and animals; but analogy also requires that they should differ far more from the terrestrial vegetables and animals of any epoch, than those of one epoch do from those of another; since they belong to a different stellar system, and probably exist under very different conditions from any that ever prevailed on the earth. we are forbidden, therefore, by analogy, to suppose that on any other planet there was such an anatomical progression towards the form of man, as we can discern (according to some eminent physiologists) among the tribes which have occupied the earth. are we to conceive that the creatures on the planets of other systems are, like the most perfect terrestrial animals, symmetrical as to right and left, vertebrate, with fore limbs and hind limbs, heads, organs of sense in their heads, and the like? every one can see how rash and fanciful it would be to make such suppositions. those who have, in the play of their invention, imagined inhabitants of other planets, have tried to avoid this servile imitation of terrestrial forms. here is sir humphry davy's account of the inhabitants of saturn. "i saw moving on the surface below me, immense masses, the forms of which i find it impossible to describe. they had systems for locomotion similar to that of the morse or sea-horse, but i saw with great surprise that they moved from place to place by six extremely thin membranes, which they used as wings. i saw numerous convolutions of tubes, more analogous to the trunk of the elephant, than to anything else i can imagine, occupying what i supposed to be the upper parts of the body."[ ] the attendant genius informs the narrator, that though these creatures look like zoophytes, they have a sphere of sensibility and intellectual enjoyment far superior to that of the inhabitants of the earth. if we were to reason upon a work of fancy like this, we might say, that it was just as easy to ascribe superior sensibility and intelligence to zoophyte-formed creatures upon the earth, as in saturn. even fancy cannot aid us in giving consistent form to the inhabitants of other planets. . but even if we could assent to the opinion, as probable, that there may occur, on some other planet, progressions of organized forms analogous in some way to that series of animal forms which has appeared upon the earth, we should still have no ground to assume that this series must terminate in a rational and intelligent creature like man. for the introduction of reason and intelligence upon the earth is no part nor consequence of the series of animal forms. it is a fact of an entirely new kind. the transition from brute to man does not come within the analogy of the transition from brute to brute. the thread of analogy, even if it could lead us so far, would break here. we may conceive analogues to other animals, but we could have no analogue to man, except man. man is not merely a higher kind of animal; he is a creature of a superior order, participating in the attributes of a higher nature; as we have already said, and as we hope hereafter further to show. even, therefore, if we were to assume the general analogy of the stars and of the sun, and were to join to that the information which geology gives us of the history of our own planet; though we might, on this precarious path, be led to think of other planets as peopled with unimagined monsters; we should still find a chasm in our reasoning, if we tried, in this way, to find intelligent and rational creatures in planets which may revolve round sirius or arcturus. . the reasonable view of the matter appears to be this. the assumption that the fixed stars are of exactly the same nature as the sun, was, at the first, when their vast distance and probable great size were newly ascertained, a bold guess; to be confirmed or refuted by subsequent observations and discoveries. any appearances, tending in any degree to confirm this guess, would have deserved the most considerate attention. but there has not been a vestige of any such confirmatory fact. no planet, nor anything which can fairly be regarded as indicating the existence of a planet, revolving about a star, has anywhere been discerned. the discovery of nebulæ, of binary systems, of clusters of stars, of periodical stars, of varying and accelerated periods of such stars, all seem to point the other way. and if all these facts be held to be but small in amount, as to the information which they convey, about the larger, and perhaps nearer stars; still they leave the original assumption a mere guess, unsupported by all that three centuries of most diligent, and in other respects successful research, has been able to bring to light. that copernicus, that galileo, that kepler, should believe the stars to be suns, in every sense of the term, was a natural result of the expansion of thought which their great discoveries produced, in them and in their contemporaries. nor are we yet called upon to withdraw from them our sympathy; or entitled to contradict their conjecture. but all the knowledge that the succeeding times have given us; the extreme tenuity of much of the luminous matter in the skies; the existence of gyratory motion among the stars, quite different from planetary systems; the absence of any observed motions at all resembling such systems; the appearance of changes in stars, quite inconsistent with such permanent systems; the disclosure of the history of our own planet, as one in which changes have constantly been going on; the certainty that by far the greater part of the duration of its existence, it has been tenanted by creatures entirely different from those which give an interest, and thence, a persuasiveness, to the belief of inhabitants in worlds appended to each star; the impossibility, which appears, on the gravest consideration, of transferring to other worlds such interests as belong to our own race in this world; all these considerations should, it would seem, have prevented that old and arbitrary conjecture from growing up, among a generation professing philosophical caution, and scientific discipline, into a settled belief. . some of the moral and theological views which tend to encourage and uphold this belief, may be taken under our more special consideration hereafter: but here, where we are reasoning principally upon astronomical grounds, we may conclude what we have to remark about the fixed stars, as the centres of inhabited systems of worlds, by saying; that it will be time enough to speculate about the inhabitants of the planets which belong to such systems, when we have ascertained that there are such planets, or one such planet. when that is done, we can then apply to them any reasons which may exist, for believing that all, or many planets, are the seats of habitation of living things. what reasons of this kind can be adduced, and what is their force with regard to our own solar system, we must now proceed to discuss.[ ] footnotes: [ ] herschel, . [ ] ibid. . [ ] herschel, . [ ] herschel, . [ ] that these systems have not condensed to _one_ centre, appears to imply a less complete degree of condensation than exists in those systems which have done so. [ ] herschel, . [ ] herschel, . [ ] the density of the sun is about as great as the density of water. [ ] herschel, - . [ ] _cosmos_, iii. , , and . [ ] ibid., iii. and . [ ] _astron. soc. notices_, dec. , . [ ] see grant's _hist. of physical astronomy_, p. . [ ] i am aware of certain speculations, and especially of some recent ones, tending to show that even our sun is wasting away by the emission of light and heat; but these opinions, even if established, do not much affect our argument one way or the other. [ ] chalmers' _astron. disc._ p. . [ ] hersch. . [ ] the periodical character of this star was discovered by david fabricius, a parish priest in east friesland, the father of john fabricius, who discovered the solar spots. (_cosmos_, iii. .) [ ] hersch. . in humboldt's _cosmos_, iii. , argelander, who has most carefully observed and studied these periodical stars, has given a catalogue containing , with the most recent determinations of their periods. [ ] hersch. . humboldt (_cosmos_, iii. and ,) gives the period as hours minutes, and says that it is or hours in its less bright state. if we could suppose the times of the warning, and of the greatest eclipse, given by herschel, to be exactly determined, as - / and / , that is, in the proportion of to , the darkening body must have its effective breadth / of that of the star. but this is on the supposition that the orbit of the darkening body has the spectator's eye in its plane; if this be not so, the darkening body may be much larger. [ ] hersch. _outl. astr._ . another explanation of the variable period of algol, is that the star is moving towards us, and therefore the light occupies less and less time to reach us. [ ] humboldt, very justly, regards the force of analogy as tending in the opposite direction. "after all," he asks, (_cosmos_, iii. ,) "is the assumption of satellites to the fixed stars so absolutely necessary? if we were to begin from the outer planets, jupiter, &c., analogy might seem to require that all planets have satellites. but yet this is not true for mars, venus, mercury." to which we may further add the _twenty-three_ planetoids. in this case there is a much greater number of bodies which have not satellites, than which have them. [ ] _consolations in travel_. dial. . [ ] what is said in art. , that in consequence of the time employed in the transmission of visual impressions, our seeing a star is evidence, not that it exists now, but that it existed, it may be, many thousands of years ago; may seem, to some readers, to throw doubts upon reasonings which we have employed. it may be said that a star which was a mere chaos, when the light, by which we see it, set out from it, may, in the thousands of years which have since elapsed, have grown into an orderly world. to which bare possibility, we may oppose another supposition at least equally possible:--that the distant stars were sparks or fragments struck off in the formation of the solar system, which are really long since extinct; and survive in appearance, only by the light which they at first emitted. chapter ix. the planets. . when it was discovered, by copernicus and galileo, that mercury, venus, mars, jupiter, saturn, which had hitherto been regarded only as "wandering fires, that move in mystic dance," were really, in many circumstances, bodies resembling the earth;--that they and the earth alike, were opaque globes, revolving about the sun in orbits nearly circular, revolving also about their own axes, and some of them accompanied by their satellites, as the earth is by the moon;--it was inevitable that the conjecture should arise, that they too had inhabitants, as the earth has. each of these bodies were seemingly coherent and solid; furnished with an arrangement for producing day and night, summer and winter; and might therefore, it was naturally conceived, have inhabitants moving upon its solid surface, and reckoning their lives and their employment by days, and months, and years. this was an unavoidable guess. it was far less bold and sweeping than the guess that there are inhabitants in the region of the fixed stars, but still, like that, it was, for the time at least, only a guess; and like that, it must depend upon future explorations of these bodies and their conditions, whether the guess was confirmed or discredited. the conjecture could not, by any moderately cautious man, be regarded as so overwhelmingly probable, that it had no need of further proof. its final acceptance or rejection must depend on the subsequent progress of astronomy, and of science in general. . we have to consider then how far subsequent discoveries have given additional value to this conjecture. and, as, in the first place, important among such discoveries, we must note the addition of several new planets to our system. it was found, by the elder herschel, (in ,) that, far beyond saturn, there was another planet, which, for a time, was called by the name of its sagacious discoverer; but more recently, in order to conform the nomenclature of the planets to the mythology with which they had been so long connected, has been termed _uranus_. this was a vast extension of the limits of the solar system. the earth is, as we have already said, nearly a hundred millions of miles from the sun. jupiter is at more than five times, and saturn nearly at ten times this distance: but uranus, it was found, describes an orbit of which the radius is about nineteen times as great as that of the earth. but this did not terminate the extension of the solar system which the progress of astronomy revealed. in , a new planet, still more remote, was discovered: its existence having been divined, before it was seen, by two mathematicians, mr. adams, of cambridge, and m. leverrier, of paris, from the effects of its force upon uranus. this new planet was termed neptune: its distance from the sun is about thirty times the earth's distance. besides these discoveries of large planets, a great number of small planets were detected in the region of the solar system which lies between the orbits of mars and jupiter. this series of discoveries began on the first day of , when ceres was detected by piazzi at palermo; and has gone on up to the present time, when twenty-three of these small bodies have been brought to light; and probably the group is not yet exhausted. . now if we have to discuss the probability that all these bodies are inhabited, we may begin with the outermost of them at present known, namely neptune. how far is it likely that this globe is occupied by living creatures which enjoy, like the creatures on the earth, the light and heat of the sun, about which the planet revolves? it is plain, in the first place, that this light and heat must be very feeble. since neptune is thirty times as far from the sun as the earth is, the diameter of the sun as seen from neptune will only be one-thirtieth as large as it is, seen from the earth. it will, in fact, be reduced to a mere star. it will be about the diameter under which jupiter appears when he is nearest to us. of course its brightness will be much greater than that of jupiter; nearly as much indeed, as the sun is brighter than the moon, both being nearly of the same size: but still, with our full-moonlight reduced to the amount of illumination which we receive from _a full jupiter_, and our sun-light reduced in nearly the same proportion, we should have but a dark, and also a cold world. in fact, the light and the heat which reach neptune, so far as they depend on the distance of the sun, will each be about nine hundred times smaller than they are on the earth. now are we to conceive animals, with their vital powers unfolded, and their vital enjoyments cherished, by this amount of light and heat? of course, we cannot say, with certainty, that any feebleness of light and heat are inconsistent with the existence of animal life: and if we had good reason to believe that neptune is inhabited by animals, we might try to conceive in what manner their vital scheme is accommodated to this scanty supply of heat and light. if it were certain that they were there, we might inquire how they could live there, and what manner of creatures they could be. if there were any general grounds for assuming inhabitants, we might consider what modifications of life their particular conditions would require. . but is there any such general ground!? such a ground we should have, if we could venture to assume that _all_ the bodies of the solar system are inhabited;--if we could proceed upon such a principle, we might reject or postpone the difficulties of particular cases. . but is such an assumption true? is such a principle well founded? the best chance which we have of learning whether it is so, is to endeavor to ascertain the fact, in the body which is nearest to us; and thus, the best placed for our closer scrutiny. this is, of course, the moon; and with regard to the moon, we have, again, this advantage in beginning the inquiry with her:--that she, at least, is in circumstances, as to light and heat, so far as the sun's distance affects them, which we know to be quite consistent with animal and vegetable life. for her distance from the sun is not appreciably different from that of the earth; her revolutions round the earth do not make nearly so great a difference, in her distance from the sun, as does the earth's different distances from the sun in summer and in winter: the fact also being, that the earth is considerably nearer to the sun in the winter of this our northern hemisphere, than in the summer. the moon's distance from the sun then, adapts her for habitation: is she inhabited? . the answer to this question, so far as we can answer it, may involve something more than those mere astronomical conditions, her distance from the sun, and the nature of her motions. but still, if we are compelled to answer it in the negative;--if it appear, by strong evidence, that the moon is not inhabited; then is there an end of the general principle, that, _all_ the bodies of the solar system are inhabited, and that we must begin our speculations about each, with this assumption. if the moon be not inhabited, then, it would seem, the belief that each special body in the system is inhabited, must depend upon reasons specially belonging to that body; and cannot be taken for granted without such reasons. of the two bodies of the solar system which alone we can examine closely, so as to know anything about them, the earth and the moon, if the one be inhabited, and the other blank of inhabitants, we have no right to assume at once, that any other body in the solar system belongs to the former of these classes rather than to the latter. if, even under terrestrial conditions of light and heat, we have a total absence of the phenomenon of life, known to us only as a terrestrial phenomenon; we are surely not entitled to assume that when these conditions fail, we have still the phenomenon, life. we are not entitled to _assume_ it; however it may be capable of being afterwards proved, in any special case, by special reasons; a question afterwards to be discussed. . is, then, the moon inhabited? from the moon's proximity to us, (she is distant only thirty diameters of the earth, less than ten times the earth's circumference; a railroad carriage, at its ordinary rate of travelling, would reach her in a month,) she can be examined by the astronomer with peculiar advantages. the present powers of the telescope enable him to examine her mountains as distinctly as he could the alps at a few hundred miles distance, with the naked eye; with the additional advantage that her mountains are much more brilliantly illuminated by the sun, and much more favorably placed for examination, than the alps are. he can map and model the inequalities of her surface, as faithfully and exactly as he can those of the surface of switzerland. he can trace the streams that seem to have flowed from eruptive orifices over her plains, as he can the streams of lava from the craters of etna or hecla. . now, this minute examination of the moon's surface being possible, and having been made, by many careful and skilful astronomers, what is the conviction which has been conveyed to their minds, with regard to the fact of her being the seat of vegetable or animal life? without exception, it would seem, they have all been led to the belief, that the moon is not inhabited; that she is, so far as life and organization are concerned, waste and barren, like the streams of lava or of volcanic ashes on the earth, before any vestige of vegetation has been impressed upon them: or like the sands of africa, where no blade of grass finds root. it is held, by such observers, that they can discern and examine portions of the moon's surface as small as a square mile;[ ] yet, in their examination, they have never perceived any alteration, such as the cycle of vegetable changes through the revolutions of seasons would produce. sir william herschel did not doubt that if a change had taken place on the visible part of the moon, as great as the growth or the destruction of a great city, as great, for instance, as the destruction of london by the great fire of , it would have been perceptible to his powers of observation. yet nothing of the kind has ever been observed. if there were lunar astronomers, as well provided as terrestrial ones are, with artificial helps of vision, they would undoubtedly be able to perceive the differences which the progress of generations brings about on the surface of our globe; the clearing of the forests of germany or north america; the embankment of holland; the change of the modes of culture which alter the color of the ground in europe; the establishment of great nests of manufactures which shroud portions of the land in smoke, as those which have their centres at birmingham or at manchester. however obscurely they might discern the nature of those changes, they would still see that change was going on. and so should we, if the like changes were going on upon the face of the moon. yet no such changes have ever been noticed. nor even have such changes been remarked, as might occur in a mere brute mass without life;--the formation of new streams of lava, new craters, new crevices, new elevations. the moon exhibits strong evidences, which strike all telescopic observers, of an action resembling, in many respects, volcanic action, by which its present surface has been formed.[ ] but, if it have been produced by such internal fires, the fires seem to be extinguished; the volcanoes to be burned out. it is a mere cinder; a collection of sheets of rigid slag, and inactive craters. and if the moon and the earth were both, at first in a condition in which igneous eruptions from their interior produced the ridges and cones which roughen their surfaces; the earth has had this state succeeded by a series of states of life in innumerable forms, till at last it has become the dwelling-place of man; while the moon, smaller in dimensions, has at an earlier period completely cooled down, as to its exterior at least, without ever being judged fit or worthy by its creator of being the seat of life; and remains, hung in the sky, as an object on which man may gaze, and perhaps, from which he may learn something of the constitution of the universe; and among other lessons this; that he must not take for granted, that all the other globes of the solar system are tenanted, like that on which he has his appointed place. . it is true, that in coming to this conclusion, the astronomers of whom i speak, have been governed by other reasons, besides those which i have mentioned, the absence of any changes, either rapid or slow, discoverable in the moon's face. they have seen reason to believe that water and air, elements so essential to terrestrial life, do not exist in the moon. the dark spaces on her disk, which were called _seas_ by those who first depicted them, have an appearance inconsistent with their being oceans of water. they are not level and smooth, as water would be; nor uniform in their color, but marked with permanent streaks and shades, implying a rigid form. and the absence of an atmosphere of transparent vapor and air, surrounding the moon, as our atmosphere surrounds the earth, is still more clearly proved, by the absence of all the optical effects of such an atmosphere, when stars pass behind the moon's disk, and by the phenomena which are seen in solar eclipses, when her solid mass is masked by the sun.[ ] this absence of moisture and air in the moon, of course, entirely confirms our previous conclusion, of the absence of vegetable and animal life; and leaves us, as we have said, to examine the question for the other bodies, on their special grounds, without any previous presumption that such life exists. undoubtedly the aspect of the case will be different in one feature, when we see reason to believe that other bodies have an atmosphere; and if there be in any planet sufficient light and heat, and clouds and winds, and a due adjustment of the power of gravity, and the strength of the materials of which organized frames consist, there may be, so far as we can judge, life of some kind or other. but yet, even in those cases, we should be led to judge also, by analogy, that the life which they sustain is more different from the terrestrial life of the present period of the earth, than that is from the terrestrial life of any former geological period, in proportion as the conditions of light and heat, and attraction and density, are more different on any other planet, than they can have been on the earth, at any period of its history. . let us then consider the state of these elements of being in the other planets. i have mentioned, among them, the force of gravity, and the density of materials; because these are important elements in the question. it may seem strange, that we are able, not only to measure the planets, but to weigh them; yet so it is. the wonderful discovery of universal gravitation, so firmly established, as the law which embraces every particle of matter in the solar system, enables us to do this, with the most perfect confidence. the revolutions of the satellites round their primary planets, give us a measure of the force by which the planets retain them in their orbits; and in this way, a measure of the quantity of matter of which each planet consists. and other effects of the same universal law, enable us to measure, though less easily and less exactly, the masses, even of those planets which have no satellites. and thus we can, as it were, put the earth, and jupiter or saturn, in the balance against each other; and tell the proportionate number of pounds which they would weigh, if so poised. and again, by another kind of experiment, we can, as we have said, weigh the earth against a known mountain; or even against a small sphere of lead duly adjusted for the purpose. and this has been done; and the results are extremely curious; and very important in our speculations relative to the constitution of the universe. . and in the first place, we may remark that the earth is really much less heavy than we should expect, from what we know of the materials of which it consists. for, measuring the density, or specific gravity, of materials, (that is their comparative weight in the same bulk,) by their proportion to water, which is the usual way, the density of iron is , that of lead , that of gold : the ordinary rocks at the earth's surface have a density of or . moreover, all the substances with which we are acquainted, contract into a smaller space, and have their density increased, by being subjected to pressure. air does this, in an obvious manner; and hence it is, that the lower parts of our atmosphere are denser than the upper parts; being pressed by a greater superincumbent weight, the weight of the superior parts of the atmosphere itself. air is thus obviously and eminently elastic. but all substances, though less obviously and eminently, are still, really, and in some degree, elastic. they all contract by compression. water for instance, if pressed by a column of water feet high, would be reduced to a bulk one-tenth less than before. in the same manner iron, compressed by a column of iron feet high, loses one-tenth of its bulk, and of course gains so much in density. and the like takes place, in different amounts, with all material whatever. this is the rate at which compression produces its effect of increasing the density, in bodies which are in the condition of those which lie around us. but if this law were to go on at the same rate, when the compression is greatly increased, the density of bodies deep down towards the centre of the earth must be immense. the earth's radius is above million feet. at a million feet depth we should have matter subjected to the pressure of a column of a million feet of superincumbent matter, heavier than water; and hence we should have a compression of water times as great as we have mentioned; and, therefore, the bulk of the water would be reduced almost to nothing, its density increased almost indefinitely: and the same would be the case with other materials, as metals and stones. if, therefore, this law of compression were to hold for these great pressures, all materials whatever, contained in the depths of the earth's mass, must be immensely denser, and immensely specifically heavier, than they are at the surface. and thus, the earth consisting of these far denser materials towards the centre, but, nearer the surface, of lighter materials, such as rock, and metals, in their ordinary state, must, we should expect, be, on the whole, much heavier than if it consisted of the heaviest ordinary materials; heavier than iron, or than lead; hundreds of times perhaps heavier than stone. . this, however, is not found to be so. the expectation of the great density of the earth, which we might have derived from the known laws of condensation of terrestrial substances, is not confirmed. the mass of the earth being weighed, by means of such processes as we have already referred to, is found to be only five times heavier than so much water: less heavy than if it were made of iron: less than twice as heavy as if it were made of ordinary rock. this, of course, shows us that the condensation of the interior parts of the earth's mass, is by no means so great as we should have expected it to be, from what we know of the laws of condensation here; and from considering the enormous pressure of superincumbent materials to which those interior parts are subjected. the laws of condensation, it would seem, do not go on operating for these enormous pressures, by the same progression as for smaller pressure. if a mass of a material is compressed into nine-tenths its bulk by the weight of a column of feet high, it does not follow that it will be again compressed into nine-tenths of its condensed bulk, by another column of feet high. the compression and condensation reach, or tend to, a limit; and probably, before they have gone very far. it may be possible to compress a piece of iron by one-thousandth part, even by such forces as we can use; and yet it may not be possible to compress the same piece of iron into one half its bulk, even by the weight of the whole earth, if made to bear upon it. this appears to be probable: and this will explain, how it is, that the materials of the earth are not so violently condensed as we should have supposed; and thus, why, the earth is so light. . we must avoid drawing inferences too boldly, on a subject where our means of knowledge are so obscure as they are with regard to the interior of the earth; but yet, perhaps, we may be allowed to say, that the result which we have just stated, that the earth is so light, suggests to us the belief that the interior consists of the same materials as the exterior, slightly condensed by pressure.[ ] we find no encouragement to believe that there is a nucleus within, of some material, different from what we have on the outside; some metal, for instance, heavier than lead. if the earth were of granite, or of lava, to the centre, it would, so far as we can judge, have much the same weight which it now has. such a central mass, covered with the various layers of stone, which form the upper crust of the earth, would naturally make this globe of at least the weight which it really has. and therefore, if we were to learn that a planet was much lighter than this, as to its materials,--much less dense, taking the whole mass together,--we should be compelled to infer that it was, throughout, or nearly so, formed of less compact matter than metal and stone; or else, that it had internal cavities, or some other complex structure, which it would be absurd to assume, without positive reasons. . now having decided these views from an examination of the earth, let us apply them to other planets, as bearing upon the question of their being inhabited; and in the first place, to jupiter. we can, as we have said, easily compare the mass of jupiter and of the earth; for both of them have satellites. it is ascertained, by this means, that the mass of weight of jupiter is about times the weight of the earth; but as his diameter is also times that of the earth, his bulk is times that of the earth: (the _cube_ of is ); and, therefore, the density of jupiter is to that of the earth, only as to , or about to . thus the density of jupiter, taken as a whole, is about a quarter of the earth's density; less than that of any of the stones which form the crust of the earth; and not much greater than the density of water. indeed, it is tolerably certain, that the density of jupiter is not greater than it would be, if his entire globe were composed of water; making allowance for the compression which the interior parts would suffer by the pressure of those parts superincumbent. we might, therefore, offer it as a conjecture not quite arbitrary, that jupiter is a mere sphere of water. . but is there anything further in the appearance of jupiter, which may serve to contradict, or to confirm, this conjecture? there is one circumstance in jupiter's form, which is, to say the least, perfectly consistent with the supposition, that he is a fluid mass; namely, that he is not an exact sphere, but oblate, like an orange. such a form is produced, in a fluid sphere, by a rotation upon its axis. it is produced, even in a sphere which is (at present at least,) partly solid and partly fluid; and the oblateness of the earth is accounted for in this way. but jupiter, who, while he is much larger than the earth, revolves much more rapidly, is much more oblate than the earth. his polar and equatorial diameters are in the proportion of to . now it is a remarkable circumstance, that this is the amount of oblateness, which, on mechanical principles, would result from his time of revolution, if he were entirely fluid, and of the same density throughout.[ ] so far, then, we have some confirmation at least, of his being composed entirely of some fluid which in its density agrees with water. . but there are other circumstances in the appearances of jupiter, which still further confirm this conjecture of his watery constitution. his belts,--certain bands of darker and lighter color, which run parallel to his equator, and which, in some degree, change their form, and breadth, and place, from time to time,--have been conjectured, by almost all astronomers, to arise from lines of cloud, alternating with tracts comparatively clear, and having their direction determined by currents analogous to our trade-winds, but of a much more steady and decided character, in consequence of the great rotatory velocity.[ ] now vapors, supplying the materials of such masses of cloud, would naturally be raised from such a watery sphere as we have supposed, by the action of the sun; would form such lines; and would change their form from slight causes of irregularity, as the belts are seen to do. the existence of these lines of cloud does of itself show that there is much water on jupiter's surface, and is quite consistent with our conjecture, that his whole mass is water.[ ] . perhaps some persons may be disposed to doubt whether, if jupiter be, as we suppose, merely or principally a mass of water and of vapor, we are entitled to extend to him the law of universal gravitation, which is the basis of our speculations. but this doubt may be easily dismissed. we know that the waters of the earth are affected by gravitation; not only towards the earth, as shown by their weight, but towards those distant bodies, the sun and the moon; for this gravitation produces the tides of the ocean. and our atmosphere also has weight, as we know; and probably has also solar and lunar tides, though these are marked by many other causes of diurnal change. we have, then, the same reason for supposing that air and water, in other parts of the system, are governed by universal gravitation, and exercise themselves the attractive force of gravitation, which we have for making the like suppositions with regard to the most solid bodies. whatever argument proves universal gravitation, proves it for all matter alike; and newton, in the course of his magnificent generalization of the law, took care to demonstrate, by experiment, as well as by reasoning, that it might be so generalized. . as bearing upon the question of life in jupiter, there is another point which requires to be considered; the force of gravity at his surface. though, equal bulk for equal bulk, he is lighter than the earth, yet his bulk is so great that, as we have seen, he is altogether much heavier than the earth. this, his greater mass, makes bodies, at equal distances from the centres, ponderate proportionally more to him than they would do to the earth. and though his surface is times further from his centre than the earth's is, and therefore the gravity at the surface is thereby diminished, yet, even after this deduction, gravity at the surface of jupiter is nearly two and a half times that on the earth.[ ] and thus a man transferred to the surface of jupiter would feel a stone, carried in his hands, and would feel his own limbs also, (for his muscular power would not be altered by the transfer,) become - / times as heavy, as difficult to raise, as they were before. under such circumstances animals of large dimensions would be oppressed with their own weight. in the smaller creatures on the earth, as in insects, the muscular power bears a great proportion to the weight, and they might continue to run and to leap, even if gravity were tripled or quadrupled. but an elephant could not trot with two or three elephants placed upon his back. a lion or tiger could not spring, with twice or thrice his own weight hung about his neck. such an increase of gravity would be inconsistent then, with the present constitution and life of the largest terrestrial animals; and if we are to suppose planets inhabited, in which gravity is much more energetic than it is upon the earth, we must suppose classes of animals which are adapted to such a different mechanical condition. . taking into account then, these circumstances in jupiter's state; his (probably) bottomless waters; his light, if any, solid materials; the strong hand with which gravity presses down such materials as there are; the small amount of light and heat which reaches him, at times the earth's distance from the sun; what kind of inhabitants shall we be led to assign to him? can they have skeletons where no substance so dense as bone is found, at least in large masses? it would seem not probable.[ ] and it would seem they must be dwellers in the waters, for against the existence there of solid land, we have much evidence. they must, with so little of light and heat, have a low degree of vitality. they must then, it would seem, be cartilaginous and glutinous masses; peopling the waters with minute forms: perhaps also with larger monsters; for the weight of a bulky creature, floating in the fluid, would be much more easily sustained than on solid ground. if we are resolved to have such a population, and that they shall live by food, we must suppose that the waters contain at least so much solid matter as is requisite for the sustenance of the lowest classes; for the higher classes of animals will probably find their food in consuming the lower. i do not know whether the advocates of peopled worlds will think such a population as this worth contending for: but i think the only doubt can be, between such a population, and none. if jupiter be a mere mass of water, with perhaps a few cinders at the centre, and an envelope of clouds around it, it seems very possible that he may not be the seat of life at all. but if life be there, it does not seem in any way likely, that the living things can be anything higher in the scale of being, than such boneless, watery, pulpy creatures as i have imagined. . perhaps it may occur to some one to ask, if this planet, which presents so glorious an aspect to our eyes, be thus the abode only of such imperfect and embryotic lumps of vitality as i have described; to what purpose was all that gorgeous array of satellites appended to him, which would present, to intelligent spectators on his surface, a spectacle far more splendid than any that our skies offer to us: four moons, some as great, and others hardly less, than our moon, performing their regular revolutions in the vault of heaven. to which it will suffice, at present, to reply, that the use of those moons, under such a supposition, would be precisely the same, as the use of our moon, during the myriads of years which elapsed while the earth was tenanted by corals and madrepores, shell-fish and belemnites, the cartilaginous fishes of the old red sandstone, or the saurian monsters of the lias; and in short, through all the countless ages which elapsed, before the last few thousand years: before man was placed upon the earth "to eye the blue vault and bless the _useful_ light:" to reckon by it his months and years: to discover by means of it, the structure of the universe, and perhaps, the special care of his creator for him alone of all his creatures. the moons of jupiter, may in this way be of use, as our own moon is. indeed we know that they have been turned to most important purposes, in astronomy and navigation. and knowing this, we may be content not to know how, either the satellites of jupiter, or the satellite of the earth, tend to the advantage of the brute inhabitants of the waters. . there is another point, connected with this doctrine of the watery nature of jupiter, which i may notice, though we have little means of knowledge on the subject. jupiter being thus covered with water, is the water ever converted into ice? the planet is more than times as far from the sun as the earth is: the heat which he receives is, on that account, times less than ours. the veil of clouds which covers a large part of his surface, must diminish the heat still further. what effect the absence of land produces, on the freezing of the ocean, it is not easy to say. we cannot, therefore, pronounce with any confidence whether his waters are ever frozen or not. in the next considerable planet, mars, astronomers conceive that they do trace the effects of frost; but in mars we have also appearances of land. in jupiter, we are left to mere conjecture; whether continents and floating islands of ice still further chill the fluids of the slimy tribes whom we have been led to regard as the only possible inhabitants; or whether the watery globe is converted into a globe of ice; retaining on its surface, of course, as much fluid as is requisite, under the evaporating power of the sun, to supply the currents of vapor which form the belts. in this case, perhaps, we may think it most likely that there are no inhabitants of these shallow pools in a planet of ice: at any rate, it is not worth while to provide any new speculations for such a hypothesis. . we may turn our consideration from jupiter to saturn; for in many respects the two planets are very similar. but in almost every point, which is of force against the hypothesis of inhabitants, the case is much stronger in saturn than it is in jupiter. light and heat, at his distance, are only one ninetieth of those at the earth. none but a very low degree of vitality can be sustained under such sluggish influences. the density of his mass is hardly greater than that of cork; much less than that of water: so that, it does not appear what supposition is left for us, except that a large portion of the globe, which we see as his, is vapor. that the outer part of the globe is vapor, is proved, in saturn as in jupiter, by the existence of several cloudy streaks or belts running round him parallel to his equator. yet his mass, taken altogether, is considerable, on account of his great size; and gravity would be greater, at his outer surface, than it is at the earth's. for such reasons, then, as were urged in the case of jupiter, we must either suppose that he has no inhabitants; or that they are aqueous, gelatinous creatures; too sluggish, almost to be deemed alive, floating on their ice-cold waters, shrouded forever by their humid skies. . whether they have eyes or no, we cannot tell; but probably if they had, they would never see the sun; and therefore we need not commiserate their lot in not seeing the host of saturnian satellites; and the ring, which to an intelligent saturnian spectator, would be so splendid a celestial object. the ring is a glorious object for man's view, and his contemplation; and therefore is not altogether without its use. still less need we (as some appear to do) regard as a serious misfortune to the inhabitants of certain regions of the planet, a solar eclipse of fifteen years' duration, to which they are liable by the interposition of the ring between them and the sun.[ ] . the cases of uranus and neptune are similar to that of saturn, but of course stronger, in proportion to their smaller light and heat. for uranus, this is only - th, for neptune, as we have already said, - th of the light and heat at the earth. moreover, these two new planets agree with jupiter and with saturn, in being of very large size and of very small density; and also we may remark, one of them, probably both, in revolving with great rapidity, and in nearly the same period, namely, about hours: at least, this has been the opinion of astronomers with regard to uranus. the arguments against the hypothesis of these two planets being inhabited, are of course of the same kind as in the case of jupiter and saturn, but much increased in strength; and the supposition of the probably watery nature and low vitality of their inhabitants must be commended to the consideration of those who contend for inhabitants in those remote regions of the solar system. . we may now return towards the sun, and direct our attention to the planet mars. here we have some approximation to the condition of the earth, in circumstances, as in position. it is true, his light and heat, so far as distance from the sun affects them, are less than half those at the earth. his density appears to be nearly equal to that of the earth, but his mass is so much smaller, that gravity at his surface is only one-half of what it is here. then, as to his physical condition, so far as we can determine it, astronomers discern in his face[ ] the outlines of continents and seas. the ruddy color by which he is distinguished, the red and fiery aspect which he presents, arise, they think, from the color of the land, while the seas appear greenish. clouds often seem to intercept the astronomer's view of the globe, which with its continents and oceans thus revolves under his eye; and that there is an atmosphere on which such clouds may float, appears to be further proved, by brilliant white spots at the poles of the planet, which are conjectured to be snow; for they disappear when they have been long exposed to the sun, and are greatest when just emerging from the long night of their polar winter; the snow-line then extending to about six degrees (reckoned upon the meridian of the planet) from the pole. moreover, mars agrees with the earth, in the period of his rotation; which is about hours; and in having his axis inclined to his orbit, so as to produce a cycle of long and short days and nights, a return of summer and winter, in every revolution of the planet. . we have here a number of circumstances which speak far more persuasively for a similarity of condition, in this planet and the earth, than in any of the cases previously discussed. it is true, mars is much smaller than the earth, and has not been judged worthy of the attendance of a satellite, although further from the sun; but still, he may have been judged worthy of inhabitants by his creator. perhaps we are not quite certain about the existence of an atmosphere; and without such an appendage, we can hardly accord him tenants. but if he have inhabitants, let us consider of what kind they must be conceived to be, according to any judgment which we can form. the force of his gravity is so small, that we may allow his animals to be large, without fearing that they will break down by their own weight. in a planet so dense, they may very likely have solid skeletons. the ice about his poles will cumber the seas, cold even for the want of solar heat, as it does in our arctic and antarctic oceans; and we may easily imagine that these seas are tenanted, like those, by huge creatures of the nature of whales and seals, and by other creatures which the existence of these requires and implies. or rather, since, as we have said, we must suppose the population of other planets to be more different from our existing population, than the population of other ages of our own planet, we may suppose the population of the seas and of the land of mars, (if there be any, and if we are not carrying it too high in the scale of vital activity,) to differ from any terrestrial animals, in something of the same way in which the great land and sea saurians, or the iguanodon and dinotherium, differed from the animals which now live on the earth. . that we need not discuss the question, whether there are intelligent beings living on the surface of mars, perhaps the reader will allow, till we have some better evidence that there are living things there at all; if he calls to mind the immense proportion which, on the earth, far better fitted for the habitation of the only intelligent creature which we know or can conceive, the duration of unintelligent life has borne to that of intelligent. here, on this earth, a few thousand years ago, began the life of a creature who can speculate about the past and the future, the near and the absent, the universe and its maker, duty and immortality. this began a few thousand years ago, after ages and myriads of ages, after immense varieties of lives and generations, of corals and mollusks, saurians, iguanodons, and dinotheriums. no doubt the creator might place an intelligent creature upon a planet, without all this preparation, all this preliminary life. he has not chosen to do so on the earth, as we know; and that is by much the best evidence attainable by us, of what his purposes are. it is also possible that he should, on another planet, have established creatures of the nature of corals and mollusks, saurians and iguanodons, without having yet arrived at the period of intelligent creatures: especially if that other planet have longer years, a colder climate, a smaller mass, and perhaps no atmosphere. it is also possible that he should have put that smaller planet near the earth, resembling it in some respects, as the moon does, but without any inhabitants, as she has none; and that mars may be such a planet. the probability against such a belief can hardly be considered as strong, if the arguments already offered be regarded as effective against the opinion of inhabitants in the other planets, and in the moon. . the numerous tribe of small bodies, which revolve between jupiter and mars, do not admit of much of the kind of reasoning, which we have applied to the larger planets. they have, with perhaps one exception (vesta) no disk of visible magnitude; they are mere dots, and we do not even know that their form is spherical. the near coincidence of their orbits has suggested, to astronomers, the conjecture that they have resulted from the explosion of a larger body, and from its fracture into fragments. perhaps the general phenomena of the universe suggest rather the notion of a collapse of portions of sidereal matter, than of a sudden disruption and dispersion of any portion of it; and these small bodies may be the results of some imperfectly effected concentration of the elements of our system; which, if it had gone on more completely and regularly, might have produced another planet, like mars or venus. perhaps they are only the larger masses, among a great number of smaller ones, resulting from such a process: and it is very conceivable, that the meteoric stones which, from time to time, have fallen upon the earth's surface, are other results of the like process:--bits of planets which have failed in the making, and lost their way, till arrested by the resistance of the earth's atmosphere. a remarkable circumstance in these bodies is, that though thus coming apparently from some remote part of the system, they contain no elements but such as had already been found to exist in the mass of the earth; although some substances, as nickel and chrome, which are somewhat rare in the earth's materials, are common parts of the composition of meteoric stones. also they are of crystalline structure, and exhibit some peculiarities in their crystallization. such as these strange visitors are, they seem to show that the other parts of the solar system contain the same elementary substances, and are subject to the same laws of chemical synthesis and crystalline force, which obtain in the terrestrial region. the smallness of these specimens is a necessary condition of their reaching us; for if they had been more massive, they would have followed out the path of their orbits round the sun, however eccentric these might be. the great eccentricity of the smaller planets, their great deviation from the zodiacal path, which is the highway of the large planets, their great number, probably by no means yet exhausted by the discoveries of astronomers; all fall in with the supposition that there are, in the solar system, a vast multitude of such abnormal planetoidal lumps. as i have said, we do not even know that they are approximately spherical; and if they are of the nature of meteoric stones, they are mere crude and irregularly crystallized masses of metal and earth. it will therefore, probably, be deemed unnecessary to give other reasons why these planetoids are not inhabited. but if it be granted that they are not, we have here, in addition to the moon, a large array of examples, to prove how baseless is the assumption, that all the bodies of the solar system are the seats of life. . we have thus performed our journey from the extremest verge of the universe, so far as we have any knowledge of it, to the orbit of our own planet; and have found, till we came into our own most immediate vicinity, strong reasons for rejecting the assumption of inhabited worlds like our own; and indeed, of the habitation of worlds in any sense. and even if mars, in his present condition, may be some image of the earth, in some of its remote geological periods, it is at least equally possible that he may be an image of the earth, in the still remoter geological period before life began. of peculiar fitnesses which make the earth suited to the sustentation of life, as we know that it is, we shall speak hereafter; and at present pass on to the other planets, venus and mercury. but of these, there is, in our point of view, very little to say. venus, which, when nearest to us, fills a larger angle than any other celestial body, except the sun and the moon, might be expected to be the one of which we know most. yet she is really one of the most difficult to scrutinize with our telescopes. astronomers cannot discover in her, as in mars, any traces of continents and seas, mountains and valleys; at least with any certainty.[ ] her illuminated part shines with an intense lustre which dazzles the sight;[ ] yet she is of herself perfectly dark; and it was the discovery, that she presented the phases of the moon, made by the telescope of galileo, which gave the first impulse to planetary research. she is almost as large as the earth; almost as heavy. the light and heat which she receives from the sun must be about double those which come to the earth. we discern no traces of a gaseous or watery atmosphere surrounding her. perhaps if we could see her better, we might find that she had a surface like the moon; or perhaps, in the nearer neighborhood of the sun, she may have cooled more slowly and quietly, like a glass which is annealed in the fire; and hence, may have a smooth surface, instead of the furrowed and pimpled visage which the moon presents to us. with this ignorance of her conditions, it is hard to say what kind of animals we could place in her, if we were disposed to people her surface; except perhaps the microscopic creatures, with siliceous coverings, which, as modern explorers assert, are almost indestructible by heat. to believe that she has a surface like the earth, and tribes of animals, like terrestrial animals, and like man, is an exercise of imagination, which not only is quite gratuitous, but contrary to all the information which the telescope gives us; and with this remark, we may dismiss the hypothesis. . of mercury we know still less. he receives seven times as much light and heat as the earth; is much smaller than the earth, but perhaps more dense; and has not, so far as we can tell, any of the conditions which make animal existence conceivable. if it is so difficult to find suitable inhabitants for venus, the difficulty for mercury is immensely greater. . so far then, we have traversed the solar system, and have found even here, the strongest grounds that there can be no animal existence, like that which alone we can conceive as animal existence, except in the planet next beyond the earth, mars; and there, not without great modifications. but we may make some further remarks on the condition of the several planets, with regard to what appears to us to be the necessary elements of animal life. footnotes: [ ] more recently, at the meeting of the british association in september, , professor phillips has declared, that astronomers can discern the shape of a spot on the moon's surface, which is a few hundred feet in breadth. [ ] a person visiting the eifel, a region of extinct volcanoes, west of the rhine, can hardly fail to be struck with the resemblance of the craters there, to those seen in the moon through a telescope. [ ] bessel has discussed and refuted (it was hardly necessary) the conjecture of some persons (he describes them as "the feeling hearts who would find sympathy even in the moon") that there may be in the moon's valleys air enough to support life, though it does not rise above the hills.--_populäre vorlesungen_, p. . [ ] the doctrine that the interior nucleus of the earth is fluid, whether accepted or rejected, does not materially affect this argument. it appears, that in some cases, at least, the melting of substances is prevented, by their being subjected to extreme pressure; but the density, the element from which we reason, is measured by methods quite independent of such questions. [ ] herschel, . bessel, however, holds that the oblateness of jupiter proves that his interior is somewhat denser than his exterior. _pop. vorles._ p. . [ ] herschel, . [ ] a difficulty may be raised, founded on what we may suppose to be the fact, as to the extreme cold of those regions of the solar system. it may be supposed that water under such a temperature could exist in no other form than ice. and that the cold must there be intense, according to our notion, there is strong reason to believe. even in the outer regions of our atmosphere, the cold is probably very many degrees below freezing, and in the blank and airless void beyond, it may be colder still. it has been calculated by physical philosophers, on grounds which seem to be solid, that the cold of the space beyond our atmosphere is ° below zero. the space near to jupiter, if an absolute vacuum, in which there is no matter to receive and retain heat emitted from the sun, may, perhaps, be no colder than it is nearer the sun. and as to the effect the great cold would produce on jupiter's watery material, we may remark, that if there be a free surface, there will be vapor produced by the sun's heat; and if there be air, there will be clouds. we may add, that so far as we have reason to believe, below the freezing point, no accession of cold produces any material change in ice. even in the expeditions of our arctic navigators, a cold of ° below zero was experienced, and ice was still but ice, and there were vapors and clouds as in our climate. it is quite an arbitrary assumption, to suppose that any cold which may exist in jupiter would prevent the state of things which we suppose. [ ] herschel, . [ ] it may be thought fanciful to suppose that because there is little or no solid matter (of any kind known to us) in jupiter, his animals are not likely to have solid skeletons. the analogy is not very strong; but also, the weight assigned to it in the argument is small. _valeat quantum valere debet._ [ ] herschel, . [ ] herschel, . [ ] according to bessel, schroeter _once_ saw one bright point on the dark ground, near the boundary of light in venus. this was taken as proving a mountain, estimated at , feet high. _pop. vorles._ p. . [ ] herschel, . chapter x. theory of the solar system. . we have given our views respecting the various planets which constitute the solar system;--views established, it would seem, by all that we know, of the laws of heat and moisture, density and attraction, organization and life. we have examined and reasoned upon the cases of the different planets separately. but it may serve to confirm this view, and to establish it in the reader's mind, if we give a description of the system which shall combine and connect the views which we have presented, of the constitution and peculiarities, as to physical circumstances, of each of the planets. it will help us in our speculations, if we can regard the planets not only as a collection, but as a scheme;--if we can give, not an enumeration only, but a theory. now such a scheme, such a theory, appears to offer itself to us. . the planets exterior to mars, jupiter, and saturn especially, as the best known of them, appear, by the best judgment which we can form, to be spheres of water, and of aqueous vapor, combined, it may be, with atmospheric air, in which their cloudy belts float over their deep oceans. mars seems to have some portion at least of aqueous atmosphere; the earth, we know, has a considerable atmosphere of air, and of vapor; but the moon, so near to her mistress, has none. on venus and mercury, we see nothing of a gaseous or aqueous atmosphere; and they, and mars, do not differ much in their density from the earth. now, does not this look as if the water and the vapor, which belong to the solar system, were driven off into the outer regions of its vast circuit; while the solid masses which are nearest to the focus of heat, are all approximately of the same nature? and if this be so, what is the peculiar physical condition which we are led to ascribe to the earth? plainly this: that she is situated just in that region of the system, where the existence of matter, both in a solid, a fluid, and a gaseous condition, is possible. outside the earth's orbit, or at least outside mars and the small planetoids, there is, in the planets, apparently, no solid matter; or rather, if there be, there is a vast preponderance of watery and vaporous matter. inside the earth's orbit, we see, in the planets, no traces of water or vapor, or gas; but solid matter, about the density of terrestrial matter. the earth, alone, is placed at the border where the conditions of life are combined; ground to stand upon; air to breathe; water to nourish vegetables, and thus, animals; and solid matter to supply the materials for their more solid parts; and with this, a due supply of light and heat, a due energy of the force of weight. all these conditions are, in our conception, requisite for life: that all these conditions meet, elsewhere than in the neighborhood of the earth's orbit, we see strong reasons to disbelieve. the earth, then, it would seem, is the abode of life, not because all the globes which revolve round the sun may be assumed to be the abodes of life; but because the earth is fitted to be so, by a curious and complex combination of properties and relations, which do not at all apply to the others. that the earth is inhabited, is not a reason for believing that the other planets are so, but for believing that they are not so. . can we see any physical reason, for the fact which appears to us so probable, that all the water and vapor of the system is gathered in its outward parts? it would seem that we can. water and aqueous vapor are driven from the sun to the outer parts of the solar system, or are allowed to be permanent there only, as they are driven off and retained at a distance by any other source of heat;--to use a homely illustration, as they are driven from wet objects placed near the kitchen-fire: as they are driven from the hot sands of egypt into the upper air: as they are driven from the tropics to the poles. in this latter case, and generally, in all cases, in which vapor is thus driven from a hotter region, when it comes into a colder, it may again be condensed in water, and fall in rain. so the cold of the air in the temperate zone condenses the aqueous vapors which flow from the tropics; and so, we have our clouds and our showers. and as there is this rainy region, indistinctly defined, between the torrid and the frigid zones on the earth; so is there a region of clouds and rain, of air and water, much more precisely defined, in the solar system, between the central torrid zone and the external frigid zone which surrounds the sun at a greater distance. . _the earth's orbit is the temperate zone of the solar system._ in that zone only is the play of hot and cold, of moist and dry, possible. the torrid zone of the earth is not free from moisture; it has its rains, for it has its upper colder atmosphere. but how much hotter are venus and mercury than the torrid zone? there, no vapors can linger; they are expelled by the fierce solar energy; and there is no cool stratum to catch them and return them. if they were there, they must fly to the outer regions; to the cold abodes of jupiter and saturn, if on their way, the earth did not with cold and airy finger outstretched afar, catch a few drops of their treasures, for the use of plant, and beast, and man. the solid stone only, and the metallic ore which can be fused and solidified with little loss of substance, can bear the continual force of the near solar fire, and be the material of permanent solid planets in that region. but the lava pavement of the inner planets bears no superstructure of life; for all life would be scorched away along with water, its first element. on the earth first, can this superstructure be raised; and there, through we know not what graduation of forms, the waters were made to bring forth abundantly things that had life; plants, and animals nourished by plants, and conspiring with them, to feed on their respective appointed elements, in the air which surrounded them. and so, nourished by the influences of air and water, plants and animals lived and died, and were entombed in the scourings of the land, which the descending streams carried to the bottom of the waters. and then, these beds of dead generations were raised into mountain ranges; perhaps by the yet unextinguished forces of subterraneous fires. and then a new creation of plants and animals succeeded; still living under the fostering influence of the united pair, air and water, which never ceased to brood over the world of life, their nurseling; and then, perhaps, a new change of the limits of land and water, and a new creation again: till at last, man was placed upon the earth; with far higher powers, and far different purposes, from any of the preceding tribes of creatures: and with this, for one of his offices;--that there might be an intelligent being to learn how wonderfully the scheme of creation had been carried on, and to admire, and to worship the creator. . but we have a few more remarks to make on the structure of the solar system, in this point of view. when we say that the water and vapor of the system were driven to the outer parts, or retained there, by the central heat of the sun, perhaps it might be supposed to be most simple and natural, that the aqueous vapor, and the water, should assume its place in a distinct circle, or rather a spherical shell, of which the sun was the centre; thus making an elemental sphere about the centre, such as the ancients imagined in their schemes of the universe. nor will we venture to say that such an arrangement of elements might not be; though perhaps it might be shown that no stable equilibrium of the system would be, in this way, mechanically possible. but this at least we may say; that a rotatory motion of all the parts of the universe appears to be a universal law prevalent in it, so far as our observation can reach: and that, by such rotation of the separate masses, the whole is put in a condition which is everywhere one of stable equilibrium. it was, then, agreeable to the general scheme, that the excess of water and vapor, which must necessarily be carried away, or stored up, in the outer regions of the system, should be put into shapes in which it should have a permanent place and form. and thus, it is suitable to the general economy of creation, that this water and vapor should be packed into rotating masses, such as are jupiter and saturn, uranus and neptune. when once collected in such rotating masses, the attraction of its parts would gather it into spheroidal forms; oblate by the effect of rotation, as jupiter, or perhaps into annular forms, like the ring of saturn;[ ] for such also is a mechanically possible form of equilibrium, for a fluid mass. and these spheroids once formed, the water would form a central nucleus, over which would hang a cover of vapor, raised by the evaporating power of the sun, and forming clouds, where the rarity of the upper strata of vapor allowed the cold of the external space to act; and these clouds, spun into belts by the rotation of the sphere. and thus, the vapor, which would otherwise have wandered loose about the atmosphere, was neatly wound into balls; which, again, were kept in their due place, by being made to revolve in nearly circular orbits about the sun. . and thus, according to our view, water and gases, clouds and vapors, form mainly the planets in the outer part of the solar system; while masses such as result from the fusion of the most solid materials, lie nearer the sun, and are found principally within the orbit of jupiter.[ ] to conceive planetary systems as formed by the gradual contraction of a nebular mass, and by the solidification of some of its parts, is a favorite notion of several speculators. if we adopt this notion, we shall, i think, find additional proofs in favor of our view of the system. for, in the first place, we have the zodiacal light, a nebulous appendage to the sun, as herschel conceives, extending beyond the orbits of mercury and venus. these planets, then, have not yet fully emerged from the atmosphere in which they had their origin:--the _mother-light_ and _mother-fire_, in which they began to crystallize, as crystals do in their mother-water. though they are already opaque, they are still immersed in luminous vapor: and bearing such traces of their chaotic state being not yet ended, we need not wonder, if we find no evidence of their having inhabitants, and some evidence to the contrary. they are within a nebular region, which may easily be conceived to be uninhabitable. and where this nebular region, marked by the zodiacal light, terminates, the world of life begins, namely at the earth. . but further, outside this region of the earth, what do we find in the solar system? of solid matter, if our views are right, we find nothing but an immense number of small bodies; namely, first, mars, who, as we have said, is only about one-eighth the earth in mass: the twenty-six small planetoids, (or whatever number may have been discovered when these pages meet the reader's eye,[ ]) between mars and jupiter; the four satellites of jupiter; the eight satellites of saturn; the six (if that be the true number,) satellites of uranus; and the one satellite of neptune, already detected. it is very remarkable, that all this array of small bodies begins to be found just outside the earth's orbit. supposing, as we have found so much reason to suppose, that jupiter, and the other exterior planets, are not solid bodies, but masses of water and of vapor; the existence of great solid planetary masses, such as exist in the region of the earth's orbit, is succeeded externally by the existence of a vast number of smaller bodies. the real quantity of matter in these smaller bodies we cannot in general determine. perhaps the largest of them, (after mars,) may be jupiter's third satellite; which[ ] is reckoned, by laplace, to have a mass less than - , th of that of jupiter himself; and thus, since jupiter, as we have seen, has a mass times that of the earth, the satellite would be above - th of the earth's mass.[ ] that none but masses of this size, and many far below this, are found outside of mars, appears to indicate, that the _planet-making_ powers which were efficacious to this distance from the sun, and which produced the great globe of the earth, were, beyond this point, feebler; so that they could only give birth to smaller masses; to planetoids, to satellites, and to meteoric stones. perhaps we may describe this want of energy in the planet-making power, by saying, that at so great a distance from the central fire, there was not heat enough to melt together these smaller fragments into a larger globe;[ ] or rather, when they existed in a nebular, perhaps in a gaseous state, that there was not heat enough to keep them in that state, till the attraction of the parts of all of them had drawn them into one mass, which might afterwards solidify into a single globe. the tendency of nebular matter to separate into distinct portions, which may afterwards be more and more detached from each other, so as to break the nebulous light into patches and specks, appears to be seen in the structure of the resolvable nebulæ, as we have already had occasion to notice. and according to the view we are now taking, we may conceive such patches, by further cooling and concentration, to remain luminous as comets, and perhaps shooting stars; or to become opaque as planets, planetoids, satellites, or meteoric stones. and here we may call to mind what we have already said, that the meteoric stones consist of the same elements as those of the earth, combined by the same laws; and thus appear to bring us a message from the other solid planets, that they also have the same elements and the same chemical forces as the earth has. . it has already been supposed, by many astronomers, that shooting stars, and meteoric stones, are bodies of connected nature and origin; and that they are cosmical, not terrestrial bodies;--parts of the solar system, not merely appendages to the earth. it has been conceived, that the luminous masses, which appear as shooting stars, when they are without the sphere of terrestrial influences, may, when they reach our atmosphere, collapse into such solid lumps as have from time to time fallen upon the earth's surface: many of them, with such sudden manifestations of light and heat, as implied some rapid change taking place in their chemical constitution and consistence. if shooting stars are of this nature, then, in those cases in which a great number of them appear in close succession, we have evidence that there is a region in which there is a large collection of matter of a nebulous kind, collected already into small clouds, and ready, by any additional touch of the powers that hover round the earth, to be further consolidated into planetary matter. that the earth's orbit carries her through such regions, in her annual course, we have evidence, in the curious fact, now so repeatedly observed, of showers of shooting stars, seen at particular seasons of every year; especially about the th of november, and the th of august. this phenomenon has been held, most reasonably, to imply that at those periods of the year, the earth passes through a crowd of such meteor-planets, which form a ring round the sun; and revolving round him, like the other planets, retain their place in the system from year to year.[ ] it may be that the orbits of these meteor-planets are very elliptical. that they are to a certain extent elliptical, appears to be shown, by our falling in with them only once a year, not every half year, as we should do, if their orbit, being nearly circular, met the earth's orbit in two opposite points. that the shooting stars, thus seen in great numbers when the earth is at certain points of her orbit, are really planetoidal bodies, appears to be further proved by this;--that they all seem to move nearly in the same direction.[ ] they are, each of them, visible for a short time only, (indeed commonly only for a few seconds), while they are nearest the earth; much in the same way in which a comet is visible only for a small portion of its path: and this portion is described in a short time, because they move near the earth. they are so small that a little change of distance removes them beyond our vision. . perhaps these revolving specks of nebulæ are the outriders of the zodiacal light; portions of it, which, being external to the permanently nebulous central mass, have broken into patches, and are seen as stars for the moment that we are near to them. and if this be true, we have to correct, in a certain way, what we have previously said of the zodiacal light;--that no one had thought of resolving it into stars: for it would thus appear, that in its outer region, it resolves itself into stars, visible, though but for a moment, to the naked eye. . and thus, all these phenomena concur in making it appear probable, that the earth is placed in that region of the solar system in which the planet-forming powers are most vigorous and potent;--between the region of permanent nebulous vapor, and the region of mere shreds and specks of planetary matter, such as are the satellites and the planetoidal group. and from these views, finally it follows, that the earth is really the largest planetary body in the solar system. the vast globes of jupiter and saturn, uranus and neptune, which roll far above her, are still only huge masses of cloud and vapor, water and air; which, from their enormous size, are ponderous enough to retain round them a body of small satellites, perhaps, in some degree at least, solid; and which have perhaps a small lump, or a few similar lumps, of planetary matter at the centre of their watery globe. the earth is really the domestic hearth of this solar system; adjusted between the hot and fiery haze on one side, the cold and watery vapor on the other. this region only is fit to be a domestic hearth, a seat of habitation; and in this region is placed the largest solid globe of our system; and on this globe, by a series of creative operations, entirely different from any of those which separated the solid from the vaporous, the cold from the hot, the moist from the dry, have been established, in succession, plants, and animals, and man. so that the habitation has been occupied; the domestic hearth has been surrounded by its family; the fitnesses so wonderfully combined have been employed; and the earth alone, of all the parts of the frame which revolves round the sun, has become a world. . perhaps it may tend still further to illustrate, and to fix in the reader's mind, the view of the constitution of the solar system here given, if we remark an analogy which exists, in this respect, between the earth in particular, and the solar system in general. the earth, like the central parts of the system, is warmed by the sun; and hence, drives off watery vapors into the circumambient space, where they are condensed by the cold. the upper regions of the atmosphere, like the outer regions of the solar system, form the vapors thus raised into clouds, which are really only water in minute drops; while in the solar system, the cold of the outer regions, and the rotation of the masses themselves, maintain the water, and the vapor, in immense spheres. but jupiter and saturn may be regarded as, in many respects, immense clouds; the continuous water being collected at their centres, while the more airy and looser parts circulate above. they are the permanent receptacles of the superfluous water and air of the system. what is not wanted on the earth, is stored up there, and hangs above us, far removed from our atmosphere; but yet, like the clouds in our atmosphere, an example, what glorious objects accumulations of vapor and water, illuminated by the rays of the sun, may become in our eyes. . these views are so different from those hitherto generally entertained, and considered as having a sort of religious dignity belonging to them, that we may fear, at first at least, they will appear to many, rash and fanciful, and almost, as we have said, irreverent. on the question of reverence we may hereafter say a few words; but as to the rashness of these views, we would beg the reader, calmly and dispassionately, to consider the very extraordinary number of points in the solar system, hitherto unexplained, which they account for, or, at least reduce into consistency and connection, in a manner which seems wonderful. the theory, as we may perhaps venture to call it, brings together all these known phenomena;--the great size and small density of the exterior planets;--their belts and streaks;--saturn's ring;--jupiter's oblateness;--the great number of satellites of the exterior planets;--the numerous group of planetoid bodies between jupiter and mars;--the appearance of definite shapes of land and water on mars;--the showers of shooting stars which appear at certain periods of the year;--the zodiacal light;--the appearance of venus as different from mars;--and finally, the material composition of meteoric stones. . perhaps there are other phenomena which more readily find an explanation in this theory, than in any other: for instance, the recent discovery of a dim half-transparent ring, as an appendage to the luminous ring of saturn, which has hitherto alone been observed. perhaps this is the ring of vapor which may naturally be expected to accompany the ring of water. it is the annular atmosphere of the aqueous annulus. but, the discovery of this faint ring being so new, and hitherto not fully unfolded, we shall not further press the argument, which, hereafter, perhaps, may be more confidently derived from its existence. . there are some other facts in the solar system, which, we can hardly doubt, must have a bearing upon the views which we have urged; though we cannot yet undertake to explain that bearing fully. not only do all the planetary bodies of the solar system, as well as the sun himself, revolve upon their axes; but there is a very curious fact relative to these revolutions, which appears to point out a further connection among them. so far as has yet been ascertained, all those which we, in our theory, regard as solid bodies, mercury, venus, the earth, and mars, revolve in very nearly the same time: namely, in about twenty-four hours. all those larger masses, on the other hand, which we, in our theory, hold to be watery planets, jupiter, saturn, uranus, revolve, not in a longer time, as would perhaps have been expected, from their greater size, but in a shorter time; in less than half the time; in about ten hours. the near agreement of the times of revolution in each of these two groups, is an extremely curious fact; and cannot fail to lead our thoughts to the probability of some common original cause of these motions. but no such common cause has been suggested, by any speculator on these subjects. if, in this blank, even of hypotheses, one might be admitted, as at least a mode of connecting the facts, we might say, that the compound collection of solid materials, water, and air, of which the solar system consists, and of which our earth alone, perhaps, retains the combination, being, by whatever means, set a spinning round an axis, at the rate of one revolution in hours, the solid masses which were detached from it, not being liable to much contraction, retained their rate of revolution; while the vaporous masses which were detached from the fluid and airy part, contracting much, when they came into a colder region, increased their rate of revolution on account of their contraction. that such an acceleration of the rate of revolution would be the result of contraction, is known from mechanical principles; and indeed, is evident: for the contraction of a circular ring of such matter into a narrower compass, would not diminish the linear velocity of its elements, while it would give them a smaller path to describe in their revolutions. such an hypothesis would account, therefore, both for the nearly equal times of revolution of all the solid planets, and for the smaller period of rotation, which the larger planets show. . in what manner, however, portions are to be detached from such a rotating mass, so as to form solid planets on the one side, and watery planets on the other, and how these planets, so detached, are to be made to revolve round the sun, in orbits nearly circular, we have no hypothesis ready to explain. and perhaps we may say, that no satisfactory, or even plausible, hypothesis to explain these facts, has been proposed: for the nebular hypothesis, the only one which is likely to be considered as worthy any notice on this subject, is too imperfectly worked out, as yet, to enable us to know, what it will or will not account for. according to that hypothesis, the nebular matter of a system, having originally a rotatory motion, gradually contracts; and separating, at various distances from the centre, forms rings; which again, breaking at some point of their circumference, are, by the mutual attraction of their parts, gathered up into one mass; which, when cooled down, so as to be opaque, becomes a planet; still revolving round the luminous mass which remains at the centre. that such a process, if we suppose the consistency, and other properties, of the nebulous matter to be such as to render it possible, would produce planetary masses revolving round a sun in nearly circular orbits, and rotating about their own axes, seems most likely; though it does not appear that it has been very clearly shown.[ ] but no successful attempt has been made to deduce any laws of the distances from the centre, times of rotation, or other properties of such planets; and therefore, we cannot say that the nebular hypothesis is yet in any degree confirmed. . the theory which we have ventured to propose, of the solar system, agrees with the nebular hypothesis, so far as that hypothesis goes; if we suppose that there is, at the centre of the exterior planets, jupiter, saturn, uranus, and neptune, a solid nucleus, probably small, of the same nature as the other planets. such an addition to our theory is, perhaps, on all accounts, probable: for that circumstance would seem to determine, to particular points, the accumulation of water and vapors, to which we hold that those planets owe the greater part of their bulk. those planets then, jupiter, saturn, and the others, are really small solid planets, with enormous oceans and atmospheres. the nebular hypothesis, in that case, is that part of our hypothesis, which relates to the condensation of luminous nebular matter; while _we_ consider, further, the causes which, scorching the inner planets, and driving the vapors to the outer orbs, would make the region of the earth the only habitable part of the system. . the belief that other planets, as well as our own, are the seats of habitation of living things, has been entertained, in general, not in consequence of physical reasons, but in spite of physical reasons; and because there were conceived to be other reasons, of another kind, theological or philosophical, for such a belief. it was held that venus, or that saturn, was inhabited, not because any one could devise, with any degree of probability, any organized structure which would be suitable to animal existence on the surfaces of those planets; but because it was conceived that the greatness or goodness of the creator, or his wisdom, or some other of his attributes, would be manifestly imperfect, if these planets were not tenanted by living creatures. the evidences of design, of which we can trace so many, and such striking examples, in our own sphere, the sphere of life, must, it was assumed, exist, in the like form, in every other part of the universe. the disposition to regard the universe in this point of view, is very general; the disinclination to accept any change in our belief which seems, for a time, to interfere with this view, is very strong; and the attempt to establish the necessity of new views discrepant from these has, in many eyes, an appearance as if it were unfriendly to the best established doctrines of natural theology. all these apprehensions will, we trust, be shown, in the sequel, to be utterly unfounded: and in order that any such repugnance to the doctrines here urged, may not linger in the reader's mind, we shall next proceed to contemplate the phenomena of the universe in their bearing upon such speculations. footnotes: [ ] other speculators also have regarded saturn's ring as a ring of cloud or water. see _cosmos_, iii. and . [ ] humboldt has already remarked _(cosmos_, i. , and iii. ), that the inner planets as far as mars, and the outer ones beginning with jupiter, form two groups having different properties. also encke. (see humboldt's note.) [ ] printed oct. , . [ ] herschel, . [ ] it is probable, from the small density of jupiter's satellites, that they also consist in a great measure of water and vapor. only one of them is denser than jupiter himself.--_cosmos_. [ ] it has, in our own day, even in the present year, been regarded as a great achievement of man to direct the fiery influences which he can command, so as to cast a colossal statue in a single piece, instead of casting it in several portions. [ ] herschel, - . [ ] herschel, . [ ] besides the curious relation of the times of rotation of the planets, just noticed, there is another curious relation, of their distance from the sun, which any one, wishing to frame an hypothesis on the origin of our solar system, ought by all means to try to account for. the distances from the sun, of the planets, mercury, venus, earth, mars, the planetoids, jupiter, saturn, uranus, are nearly as the numbers, , , , , , , , : now the excesses of each of these numbers above the first are, , , , , , , : a series in which each term (after the first,) is double of the preceding one. hence, the distances of the planets conform to a series following this law, (_bode's law_, as it is termed.) and though the law is by no means exact, yet it was so far considered a probable expression of a general fact, that the deviation from this law, in the interval between mars and jupiter, was the principal cause which led first to the suspicion of a planet interposed in the seemingly vacant space; and thus led to the discovery of the planetoids, which really occupy that region. it is true, that the law is found not to hold, in the case of the newly-discovered planet neptune; for his distance from the sun, which according to this law, should be , is really only , times the earth's distance, instead of times. still, bode's law has a comprehensive approximate reality in the solar system, sufficient to make it a strong recommendation of any hypothesis of the origin of the system, that it shall account for this law. this, however, the nebular hypothesis does not. chapter xi. the argument from design. . there is no more worthy or suitable employment of the human mind, than to trace the evidences of design and purpose in the creator, which are visible in many parts of the creation. the conviction thus obtained, that man was formed by the wisdom, and is governed by the providence, of an intelligent and benevolent being, is the basis of natural religion, and thus, of all religion. we trust that some new lights will be thrown upon the traces of design which the universe offers, even in the work now before the reader; and as our views, regarding the plan of such design, are different, in some respects, and especially as relates to the planets and stars, from those which have of late been generally entertained, it will be proper to make some general remarks, mainly tending to show, that the argument remains undisturbed, though the physical theory is changed. . it cannot surprise any one who has attended to the history of science, to find that the views, even of the most philosophical minds, with regard to the plan of the universe, alter, as man advances from falsehood to truth: or rather, from very imperfect truth to truth less imperfect. but yet such a one will not be disposed to look, with any other feeling than profound respect, upon the reasonings by which the wisest men of former times ascended from their erroneous views of nature to the truth of natural religion. it cannot seem strange to us that man at any point, and perhaps at every point, of his intellectual progress, should have an imperfect insight into the plan of the universe; but, in the most imperfect condition of such knowledge, he has light enough from it, to see vestiges of the wisdom and benevolence of the creating deity; and at the highest point of his scientific progress, he can probably discover little more, by the light which physical science supplies. we can hardly hope, therefore, that any new truths with regard to the material universe, which may now be attainable, will add very much to the evidence of creative design; but we may be confident, also, that they will not, when rightly understood, shake or weaken such evidence. it has indeed happened, in the history of mankind, that new views of the constitution of the universe, brought to the light by scientific researches, and established beyond doubt, in the conviction of impartial persons, have disturbed the thoughts of religious men; because they did not fall in with the view then entertained, of the mode in which god effects his purpose in the universe. but in these cases, it soon came to be seen, after a season of controversy, reproach, and alarm, that the old argument for design was capable of being translated into the language of the new theory, with no loss of force; and the minds of men were gradually tranquillized and pacified. it may be hoped that the world is now so much wiser than it was two or three centuries ago, that if any modification of the current arguments for the divine attributes, drawn from the aspect of the universe, become necessary, in consequence of the rectification of received errors, it will take place without producing pain, fear, or anger. to promote this purpose, we proceed to make a few remarks. . the proof of design, as shown in the works of creation, is seen most clearly, not in mere physical arrangement, but in the structure of organized things;--in the constitution of plants and animals. in those parts of nature, the evidences of intelligent purpose, of wise adaptation, of skilful selection of means to ends, of provident contrivance, are, in many instances, of the most striking kind. such, for example, are the structure of the human eye, so curiously adapted for its office of seeing; the muscles, cords, and pullies by which the limbs of animals are moved, exceeding far the mechanical ingenuity shown in human inventions; the provisions which exist, before the birth of offspring, for its sustenance and well-being when it shall have been born;--these are lucid and convincing proofs of an intelligent creator, to which no ordinary mind can refuse its conviction. nor is the evidence, which we here recognize, deprived of its force, when we see that many parts of the structure of animals, though adapted for particular purposes, are yet framed as a portion of a system which does not seem, in its general form, to have any bearing on such purposes.[ ] the beautiful contrivances which exist in the skeleton of man, and the contrivances, possessing the same kind of beauty, in the skeleton of a sparrow, do not appear to any reasonable person less beautiful, because the skeleton of a man, and of a sparrow, have an agreement, bone for bone, for which we see no reason, and which appears to us to answer no purpose. the way in which the human hand and arm are made capable of their infinite variety of use, by the play of the radius and ulna, the bones of the wrist and the fingers, is not the less admirable, because we can trace the representatives and rudiments of each of these bones, in cases where they answer no such ends;--in the foreleg of the pig, the ox, the horse, or the seal. the provision for feeding the young creature, which is made, with such bounteous liberality, and such opportune punctuality, by the breasts of the mother, has not any doubt thrown upon its reality, by the teats of male animals and the paps of man, which answer no such purpose. that in these cases there is manifested a wider plan, which does not show any reference to the needs of particular cases; as well as peculiar contrivances for the particular cases, does not disturb our impression of design in each case. why should so large a portion of the animal kingdom, intended, as it seems, for such different fields of life and modes of living;--beasts, birds, fishes;--still have a skeleton of the same plan, and even of the same parts, bone for bone; though many of the parts, in special cases, appear to be altogether useless (namely, the vertebrate plan)? we cannot tell. our naturalists and comparative anatomists, it would seem, cannot point out any definite end, which is answered by making so many classes of animals on this one vertebrate plan. and since they cannot do this, and since we cannot tell why animals are so made, we must be content to say that we do not know; and therefore, to leave this feature in the structure of animals out of our argument for design. hence we do not say that the making of beasts, birds, and fishes, on the same vertebrate plan, proves design in the creator, in any way in which we can understand design. that plan is not of itself a proof of design; it is something in addition to the proofs of design; a general law of the animal creation, established, it may be, for some other reason. but this common plan being given, we can discern and admire, in every kind of animal, the manner in which the common plan is adapted to the particular purpose which the animal's kind of life involves.[ ] the general law is not all; there is also, in every instance, a special care for the species. the general law may seem, in many cases, to remove further from us the proof of providential care; by showing that the elements of the benevolent contrivance are not provided in the cases alone where they are needed, but in others also. but yet this seeming, this obscuration of the evidence of design, by interposing the form of general law, cannot last long. if the general law supplies the elements, still a special adaptation is needed to make the elements answer such a purpose; and what is this adaptation, but design? the radius and ulna, the carpal and metacarpal bones, are all in the general type of the vertebrate skeleton. but does this fact make it the less wonderful, that man's arm and hand and fingers should be constructed so that he can make and use the spade, the plow, the loom, the pen, the pencil, the chisel, the lute, the telescope, the microscope, and all other instruments? is it not, rather, very wonderful that the bones which are to be found rudimentally, in the leg-bone of a horse, or the hoof of an ox, should be capable of such a curious and fertile development and modification? and is not such development and modification a work, and a proof, of design and intention in the creator? and so in other cases. the teats of male animals, the nipples of man, may arise from this, that the general plan of the animal frame includes paps, as portions of it; and that the frame is so far moulded in the embryo, before the sex of the offspring is determined. be it so. yet still this provision of paps in the animal form in general, has reference to offspring; and the development of that part of the frame, when the sex is determined, is evidence of design, as clear as it is possible to conceive in the works of nature. the general law is moulded to the special purpose, at the proper stage; and this play of general laws, and special contrivances, into each other's provinces, though it may make the phenomena a little more complex, and modify our notion as to the mode of the creator's working, will not, in philosophical minds, disturb the conviction that there is design in the special adaptations: besides which, some other feature of the operation of the creative mind may be suggested by the prevalence of general laws in the creation. . there is, however, one caution suggested by this view. since, besides, and mixed with the examples of design which the creation offers, there are also results of general laws, in which we cannot trace the purpose and object of the law; we may fall into error, if we fasten upon something which is a result of such mere general laws, and imagine that we can discern its object and purpose. thus, for instance, we might possibly persuade ourselves that we had discovered the use and purpose of the teats of male animals; or of the trace of separation into parts which the leg-bone of a horse offers; or of the false toes of a pig: all which are, as we have seen, the rudiments of a plan more general than is developed in the particular case. and if, when we had made such a fancied discovery, it were found that the uses and purposes which we had imagined to belong to these parts or features, were not really served by them; at first, perhaps, we might be somewhat disturbed, as having lost one of the evidences of the design of the creator, all which are, precious to a reverent mind. but it is not likely that any disturbance of a reverent mind on such grounds as this, would continue long, or go far. we should soon come to recollect, how light and precarious, perhaps how arbitrary and ill-supported by our real knowledge, were the grounds on which we had assigned such uses to such parts. we should turn back from them to the more solid and certain evidences, not shaken, nor likely to be shaken, by any change in prevalent zoological or anatomical doctrines, which those who love to contemplate such subjects habitually dwell upon; and, holding ourselves ready to entertain any speculations by which the bearing of those general laws upon natural religion could be shown, in such a way as to convince our reason, we should rest in the confident and tranquil persuasion that no success or failure in such speculations could vitally affect our belief in a wise and benevolent deity:--that though additional illustrations of his attributes might be interesting and welcome, no change of our scientific point of view could make his being or action doubtful. . this is, it would seem, the manner in which a reasonable and reverent man would regard the proof of a supreme creator and governor, which is derived from design, as seen in the organic creation; and the mode in which such proof would be affected by changes in the knowledge which we may acquire of the general laws by which the organic creation is constituted and governed. and hence, if it should be found to be established by the researches of the most comprehensive and exact philosophy, that there are, in any province of the universe, resemblances, gradations, general laws, indications of the mode in which one form approaches to another, and seems to pass into and generate another, which tend to obliterate distinctions which at first appeared broad and conspicuous; still the argument, from the design which appears in the parts of which we most clearly see the purpose, would not lose its force. if, for instance, it should be made apparent, by geological investigations of the extinct fossil creation, that the animal forms which have inhabited the earth, have gradually approached to that type in which the human form is included, passing from the rudest and most imperfect animal organizations, mollusks, or even organic monads, to vertebrate animals, to warm-blooded animals, to monkeys, and to men; still, the evidences of design in the anatomy of man are not less striking than they were, when no such gradation was thought of. and what is more to the purpose of our argument, the evidences of the peculiar nature and destination of man, as shown in other characters than his anatomy,--his moral and intellectual nature, his history and capacities,--stand where they stood before; nor is the vast chasm which separates man, as a being with such characters as these latter, from all other animals, at all filled up or bridged over. . the evidence of design in the inorganic world,--in the relation of earth, air, water, heat and light,--is, to most persons, less striking and impressive, than it is in the organic creation. but even among these mere physical elements of the world, when we consider them with reference to living things, we find many arrangements which, on a reflective view, excite our admiration, by the beneficial effect, and seemingly beneficent purpose. our condition is furnished with the solid earth, on which we stand, and in which we find the materials of man's handiworks; stone and metal, clay and sand;--with the atmosphere which we breathe, and which is the vehicle of oral intercourse between man and man;--with revolutions of the sun, by which are brought round the successions of day and night, through all their varying lengths, and of summer and winter;--with the clouds above us, which pour upon the earth their fertilizing showers. all this furniture of the earth, so marvellously adapting it for the abode of living creatures, and especially of man, may well be regarded as a collection of provisions for his benefit:--as _intended_ to do him the good, which they do. nor would this impression be removed, or even weakened, if we were to discover that some of these arrangements, instead of being produced by a machinery confined to that single purpose, were only partial results of a more general plan. for instance; we learn that the varying lengths of days and nights through the year, and the varying declination of the sun, are produced, not, as was at first supposed, by the sun moving round the earth, in a complex diurnal and annual path, but by the earth revolving in an annual orbit round the sun; while at the same time she has a diurnal rotation about her own axis, which axis, by the laws of mechanics, remains always parallel to itself. when we learn that this is so, we see that the effect is produced by a mechanical arrangement far more simple than any which the imagination of man had devised; but in this case, the effect is plainly rather an increased admiration at the simplicity of the mechanism, than a wavering belief in the reality of the purpose. in like manner when, instead of supposing water to exist in a continuous reservoir in a firmament above the earth, and to fall in the earlier and in the latter rain, by some special agency for that purpose; men learnt to see that the water in the upper regions of the air must exist in clouds and in vapors only, and must fall in showers by the condensing influence of cold currents of air; they needed not to cease to admire the kindness of the creator, in providing the rain to water the earth, and the wind to dry it; although the mechanism by which the effect was produced was of a larger kind than they had before imagined. and even if this mechanism extend through the solar system: if the arrangement by which the earth's atmosphere is the special region in which there are winds hot and cold, clouds compact or dissolving,--be an arrangement which extends its influence to other planets, as well as to ours;--if this mixed atmosphere be placed, not only at the meeting point of clear aqueous vapor above, and warmer airs below, but also at the meeting point of a hot central region surrounding the sun, and a cold exterior zone in which water and vapor can exist in immense collected masses, such as are jupiter and saturn;--still it would not appear, to a reasonable view, that this larger expansion of the machinery by which the effect is produced, makes the machinery less remarkable; or can at all tend to diminish the belief that it was _intended_ to produce the effect which it does produce. hot and cold, moist and dry, are constantly mixed together for the support of vegetable and animal life; and not the less so, if we believe that, though elements of this kind pervade the whole solar system, it is only at the earth that they are combined so as to foster and nourish living things. . but it will perhaps be said, that to suppose the whole solar system to be a machine merely operating for the benefit of the earth and its population, is to give to the earth and its population an importance in the scheme of creation which is quite extravagant and improbable:--it is to make the greater orbs, jupiter and saturn, minister to the less; instead of having their own purpose, and their own population, which their size naturally leads us to expect. to this we reply, that, in the first place, we have shown good reason for believing that the earth is really the largest dense solid globe which exists in the solar system, and that the size of jupiter and saturn arises from their being composed mainly of water and vapor. and with regard to the difficulty of the greater ministering to the less;--if by _greater_, mere size and extent be understood, it appears to be the universal law of creation, that the greater, in that sense, _should_ minister to the less, when the less includes living things. even if the planets be all inhabited, the sun, which is greater far than all of them together, ministers light and heat to all of them. even on this supposition, the vast spaces by which the planets are separated have no use, that we can discern, except to place them at suitable distances from the sun. even on this supposition, their solid globes within, their atmospheres without are all merely subservient to the benefit of a thin and scattered population on the surface. the space occupied by men and animals on the earth's surface, even taking into account the highest buildings and the deepest seas, is only a few hundreds, or a thousand feet. the benefit of this minute shell, interrupted in many places for vast distances, everywhere loosely and sparsely filled, is ministered to by the solidity and attraction of a mass below it millions of feet deep; by the influence of an atmosphere above it thousand feet high at least, and it may be, much more. and this being so, if we increase the depth of the centre thousand times; if we carry the extreme verge of air and vapor to thirty times the radius of the earth's orbit from us, how does the construction of the machine become more improbable, or the disproportion of its size to its purpose more incongruous? is mere size,--extent of brute matter or blank space,--so majestic a thing? is not infinite space large enough to admit of machines of any size without grudging? but if we thus move the centre of the earth's peopled surface thousand times further off, we reach the sun. if we carry the limit of air and vapor to the distance of times the radius of the earth's orbit we arrive at neptune. are these new numbers monstrous, while the old ones were accepted without scruple? is number such an alarming feature in the description of the universe? does not the description of every part and every aspect of it, present us with numbers so large, that wonder and repugnance, on that ground are long ago exhausted? surely this is so: and if the evidence really tend to prove to us that all the solar system ministers to the earth's population; the mere size of the system, compared with the space occupied by the population, will not long stand in the way of the reception of such a doctrine. . but the objection will perhaps be urged in another form. it will be said that the other planets have so many points of resemblance with the earth, that we must suppose their nature and purpose the same. they, like the earth, revolve in circles round the sun, rotate on their own axes, have, several of them, satellites, are opaque bodies, deriving light and probably heat from the sun. to an external spectator of the solar system, they would not be distinguishable from the earth. such a spectator would never be tempted to guess that the earth alone, of all these, neither the greatest nor the least, neither the one with the most satellites, nor the fewest, neither the innermost nor the outermost of the planets, is the only one inhabited; or at any rate the only one inhabited by an intelligent population. and to this we reply; that the largest of the other planets, if we judge rightly, are _not_ like the earth in one most essential respect, their density; and none of them, in having a surface consisting of land and water; except perhaps mars: that if the supposed external spectator could see that this was so, he might see that the earth was different from the rest; and he might be able to see the vaporous nature of the outer planets, so that he would no more think of peopling them, than we do, of peopling the grand alpine ridges and vallies which we see in the clouds of a summer-sky. . but even if the supposed spectator attended only to the obvious and superficial resemblances between one of the planets and another, he might still, if he were acquainted with the general economy of the universe, have great hesitation in inferring that, if one of them were inhabited, the others also must be inhabited. for, as we have said, in the plan of creation, we have a profusion of examples, where similar visible structures do not answer a similar purpose; where, so far as we can see, the structure answers no purpose in many cases; but exists, as we may say, for the sake of similarity: the similarity being a general law, the result, it would seem, of a creative energy, which is wider in its operation than the particular purpose. such examples are, as we have said, the finger-bones which are packed into the hoofs of a horse, or the paps and nipples of a male animal. now the spectator, recollecting such cases might say: i know that the earth is inhabited; no doubt mars and jupiter are a good deal like the earth; but are they inhabited? they look like the terrestrial breast of nature: but are they really nursing breasts? do they, like that, give food to living offspring? or are they mere images of such breasts? male teats, dry of all nutritive power? sports, or rather overworks of nature; marks of a wider law than the needs of mother earth require? many sketches of a design, of which only one was to be executed? many specimens of the preparatory process of making a planet, of which only one was to be carried out into the making of a world? such questions might naturally occur to a person acquainted with the course of creation in general; even before he remarked the features which tend to show that jupiter and saturn, that venus and mercury, have not been developed into peopled worlds, like our earth. . perhaps it may be said, that to hold this, is to make nature work in vain; to waste her powers; to suppose her to produce the frame work, and not to build; to make the skeleton, and not to clothe it with living flesh; to delude us with appearances of analogy and promises of fertility, which are fallacious. what can we reply to this? . we reply, that to work in vain, in the sense of producing means of life which are not used, embryos which are never vivified, germs which are not developed; is so far from being contrary to the usual proceedings of nature, that it is an operation which is constantly going on, in every part of nature. of the vegetable seeds which are produced, what an infinitely small proportion ever grow into plants! of animal ova, how exceedingly few become animals, in proportion to those that do not; and that are wasted, if this be waste! it is an old calculation, which used to be repeated as a wonderful thing, that a single female fish contains in its body millions of ova, and thus, might, of itself alone, replenish the seas, if all these were fostered into life. but in truth, this, though it may excite wonder, cannot excite wonder as anything uncommon. it is only one example of what occurs everywhere. every tree, every plant, produces innumerable flowers, the flowers innumerable seeds, which drop to the earth, or are carried abroad by the winds, and perish, without having their powers unfolded. when we see a field of thistles shed its downy seeds upon the wind, so that they roll away like a cloud, what a vast host of possible thistles are there! yet very probably none of them become actual thistles. few are able to take hold of the ground at all; and those that do, die for lack of congenial nutriment, or are crushed by external causes before they are grown. the like is the case with every tribe of plants.[ ] the like with every tribe of animals. the possible fertility of some kinds of insects is as portentous as anything of this kind can be. if allowed to proceed unchecked, if the possible life were not perpetually extinguished, the multiplying energies perpetually frustrated, they would gain dominion over the largest animals, and occupy the earth. and the same is the case, in different degrees, in the larger animals. the female is stocked with innumerable ovules, capable of becoming living things: of which incomparably the greatest number end as they began, mere ovules;--marks of mere possibility, of vitality frustrated. the universe is so full of such rudiments of things, that they far outnumber the things which outgrow their rudiments. the marks of possibility are much more numerous than the tale of actuality. the vitality which is frustrated is far more copious than the vitality which is consummated. so far, then, as this analogy goes, if the earth alone, of all the planetary harvest, has been a fertile seed of creation;--if the terrestrial embryo have alone been evolved into life, while all the other masses have remained barren and dead:--we have, in this, nothing which we need regard as an unprecedented waste, an improbable prodigality, an unusual failure in the operations of nature: but on the contrary, such a single case of success among many of failure, is exactly the order of nature in the production of life. it is quite agreeable to analogy, that the solar system, of which the _flowers_ are not many, should have borne but one _fertile_ flower. one in eight, or in twice eight, reared into such wondrous fertility as belongs to the earth, is an abundant produce, compared with the result in the most fertile provinces of nature. and even if any number of the fixed stars were also found to be barren flowers of the sky; objects, however beautiful, yet not sources of life or development, we need not think the powers of creation wasted or frustrated, thrown away or perverted. one such fertile result as the earth, with all its hosts of plants and animals, and especially with man, an intelligent being, to stand at the head of those hosts, is a worthy and sufficient produce, so far as we can judge of the creator's ways by analogy, of all the universal scheme. . but when we follow this analogy, so far as to speak of the mere material mass of a planet as an _embryo world_;--a barren flower;--a seed which has never been developed into a plant;--we are in danger of allowing the analogy to mislead us. for a planet, as to its brute mass, has really nothing in common with a seed or an embryo. it has no organization, or tendency to organization; no principle of life, however obscure. so far as we can judge, no progress of time, or operation of mere natural influence, would clothe a brute mass with vegetables, or stock it with animals. no species of living thing would have its place upon the surface; by the mere order of unintelligent nature. so much is this so, according to all that our best knowledge teaches, that those geologists who must most have desired, for the sake of giving completeness and consistency to their systems, to make the production of vegetable and animal species from brute matter, a part of the order of nature, (inasmuch as they have explained everything else by the order of nature,) have not ventured to do so. they allow, generally at least, each separate species to require a special act of creative power, to bring it into being. they make the peopling of the earth, with its successive races of inhabitants, a series of events altogether different from the operation of physical laws in the sustentation of existing species. the creation of life is, they allow, something out of the range of the ordinary laws of nature. and therefore, when we speak of uninhabited planets, as cases in which vital tendencies have been defeated; in which their apparent destiny, as worlds of life, has been frustrated; we really do injustice to our argument. the planets had no vital tendencies: they could have had such given, only by an additional act, or a series of additional acts, of creative power. as mere inert globes, they had no settled destiny to be seats of life: they could have such a destiny, only by the appointment of him who creates living things, and puts them in the places which he chooses for them. if, when a planetary mass had come into being, (in virtue of the same general physical law, suppose, which produced the earth,) the creator placed a host of living things upon the earth, and none upon the other planet; there was still no violation of analogy, no seeming change of purpose, no unfinished plan. in the solar system, we can see what seem to be good reasons why he did this; but if we could not see such reasons, still we should be yet further from being able to see reasons why he necessarily must place inhabitants upon the other planet. . it is sometimes said, that it is agreeable to the goodness of god, that all parts of the creation should swarm with life; that life is enjoyment; and that the benevolence of the supreme being is shown in the diffusion of such enjoyment into every quarter of the universe. to leave a planet without inhabitants, would, it is thought, be to throw away an opportunity of producing happiness. now we shall not here dwell upon the consideration, that the enjoyment thus spoken of, is, in a great degree, the enjoyment which the mere life of the lower tribes of animals implies;--the enjoyment of madrepores and oysters, cuttle-fish and sharks, tortoises and serpents; but we reply more broadly, that it is not the rule followed by the creator, to fill all places with living things. to say nothing of the vast intervals between planet and planet, which, it is presumed, no one supposes to be occupied by living things; how large a portion of the surface of the earth is uninhabited, or inhabited only in the scantiest manner. vast desert tracts exist in africa and in asia, where the barren sand nourishes neither animal nor vegetable life. the highest regions of mountain-ranges, clothed with perpetual snow, and with far-reaching sheets of glacier ice, are untenanted, except by the chamois at their outskirts. there are many uninhabited islands; and were formerly many more. the ocean, covering nearly three-fourths of the globe, is no seat of habitation for land animals or for man; and though it has a large population of the fishy tribes, is probably peopled in smaller numbers than if it were land, as well as by inferior orders. we see, in the earth then, which is the only seat of life of which we really know anything, nothing to support the belief that every field in the material universe is tenanted by living inhabitants. . that vegetables and animals, being once placed upon the earth, have multiplied or are multiplying, so as to occupy every part of the land and water which is suited for their habitation, we can see much reason to believe. philosophical natural-historians have been generally led to the conviction that each species has had an original centre of dispersion, where it was first native, and that from this centre it has been diffused in all directions, as far as the circumstances of climate and soil were favorable to its production. but we can see also much reason to believe that this general diffusion of vegetable and animal life from centres, is a part of the order of nature which may often be made to give way to other and higher purposes;--to the diffusion, over the whole surface of the earth, of a race of intelligent, moral agents. this process may often interfere with the general law of diffusion: as for instance, when man exterminates noxious animals. and whatever may be the laws which tend to replenish the earth, on which such centres of the diffusion of life exist for animals and plants; according to all analogy, these laws can have no force on any other planet, till such origins and centres of life are established on their surfaces. and even if any of the species which have ever tenanted the earth were so established on any other planet, we have the strongest reason to believe that they could not survive to a second generation. . perhaps it may be said that we unjustifiably limit the power and skill of the supreme creator, if we deny that he could frame creatures fitted to live on any of the other planets, as well as in the earth:--that the wonderful variety, and unexpected resource, of the ways in which animals are adapted for all kinds of climates, habitations, and conditions, upon the earth, may give us confidence that, under conditions still more extended, in habitations still further removed, in climates going beyond the terrestrial extremes, still the same wisdom and skill may well be supposed to have devised possible modes of animal life. . to this we reply, that we are so far from saying that the creator could not place inhabitants in the other planets, that we have attempted to show what kind of inhabitants would be most likely to be placed there, by considering the way in which animals are accommodated to special conditions in their habitation. in judging of such modes of accommodating animals to an abode on other planets, as well as the earth, we have reasoned from what we know, of the mode in which animals are accommodated to their different habitations on the earth. we believe this to be the only safe and philosophical way of treating the question. if we are to reason at all about the possibility of animal life, we must suppose that heat and light, gravity and buoyancy, materials and affinities, air and moisture, produce the same effect, require the same adaptations, in jupiter or in venus, as they do on the earth. if we do not suppose this, we run into the error which so long prevented many from accepting the newtonian system:--the error of thinking that matter in the heavens is governed by quite different laws from matter on the earth. we must adopt that belief, if we hold that animals may live under relations of heat and moisture, materials and affinities, in jupiter or venus, under which they could not live on our planet. and that belief, as we have said, appears to us contrary to all the teaching which the history of science offers us. . and not only is it contrary to the teaching of the history of science, to suppose the laws, which connect elemental and organic nature, to be different in the other planets from what they are on ours; but moreover the supposition would not at all answer the purpose, of making it probable that the planets are inhabited. for if we begin to imagine new and unknown laws of nature for those abodes, what is there to limit or determine our assumptions in any degree? what extravagant mixtures of the attributes and properties of mind and matter may we not then accept as probable truths? we know how difficult the poets have found it to describe, with any degree of consistency, the actions and events of a world of angels, or of evil spirits, souls or shades, embodied in forms so as to admit of description, and yet not subject to the laws of human bodies. virgil, tasso, milton, klopstock, and many others, have struggled with this difficulty:--no one of them, it will be probably agreed, with any great success; at least, regarding his representation as a hypothesis of a possible form of life, different from all the forms which we know. yet if we are to reject the laws which govern the known forms of life, in order that we may be able to maintain the possibility of some unknown form in a different planet, we must accept some of these hypotheses, or find a better. we must suppose that weight and cohesion, wounds and mutilations, wings and plumage, would have, either the effect which the poets represent them as having, or some different effect: and in either case it will be impossible to give any sufficient reason why we should confine the population to the surface of a planet. if gravity have not, upon any set of beings, the effect which it has upon us, such beings may live upon the surface of saturn, though it be mere vapor: but then, on that supposition, they may equally well live in the vast space between saturn and jupiter, without needing any planet for their mansion. if we are ready to suppose that there are, in the solar system, conscious beings, not subject to the ordinary laws of life, we may go on to imagine creatures constituted of vaporous elements, floating in the fiery haze of a nebula, or close to the body of a sun; and cloudy forms which soar as vapors in the region of vapor. but such imaginations, besides being rather fitted for the employment of poets than of philosophers, will not, as we have said, find a population for the planets; since such forms may just as easily be conceived swimming round the sun in empty space, or darting from star to star, as confining themselves to the neighborhood of any of the solid globes which revolve about the central sun. . we should not, then add anything to the probability of inhabitants on the other planets of our system, even if we were arbitrarily to assume unlimited changes in the laws of nature, when we pass from our region to theirs. but probably, all readers will be of opinion that such assumptions are contrary to the whole scheme and spirit of such speculations as we are here presuming:--that if we speculate on such subjects at all, it must be done by supposing that the same laws of nature operate in the same manner, in planetary, as in terrestrial spaces;--and that as we suppose, and prove, gravity and attraction, inertia and momentum, to follow the same rules, and produce the same effects, on brute matter there, which they do here; so, both these forces, and others, as light and heat, moisture and air, if, in the planets, they go beyond the extremes which limit them here, yet must imply, in any organized beings which exist in the planets, changes, though greater in amount, of the same kind as those which occur in approaching the terrestrial extremes of those elementary agents. and what kind of a population that would lead us to suppose in jupiter or saturn, mars or venus, the reader has already seen our attempt to determine; and may thence judge whether, when we go so far beyond the terrestrial extremes of heat and cold, light and dimness, vapor and water, air and airlessness, any population at all is probable. . perhaps some persons, even if they cannot resist the force of these reasons, may still yield to them with regret; and may feel as if, having hitherto believed that the planets were inhabited, and having now to give up that belief, their view of the solar system, as one of the provinces of god's creation, were made narrower and poorer than it was before. and this feeling may be still further increased, if they are led to believe also that many of the fixed stars are not the centres of inhabited systems; or that very few, or none are. it may seem to them, as if, by such a change of belief, the field of god's greatness, benevolence, and government, were narrowed and impoverished, to an extent painful and shocking;--as if, instead of being the maker and governor of innumerable worlds, of the most varied constitution, we were called upon to regard him as merely the master of the single world in which we live:--as if, instead of being the object of reverence and adoration to the intelligent population of these thousand spheres, he was recognized and worshipped on one only, and on that, how scantily and imperfectly! . it is not to be denied that there may be such a regret and disturbance naturally felt at having to give up our belief that the planets and the stars probably contain servants and worshippers of god. it must always be a matter of pain and trouble, to be urged with tenderness, and to be performed in time, to untwine our reverential religious sentiments from erroneous views of the constitution of the universe with which they have been involved. but the change once made, it is found that religion is uninjured, and reverence undiminished. and therefore we trust that the reader will receive with candor and patience the argument which we have to offer with reference to this view, or rather, this sentiment. . we remark, in the first place, that however repugnant it may be to us to believe a state of any part of the universe in which there are not creatures who can know, obey and worship god; we are compelled, by geological evidence, to admit that such a state of things has existed upon the earth, during a far longer period than the whole duration of man's race. if we suppose that the human race, if not by their actual knowledge, obedience, and worship of god, yet at least by their faculties for knowing, obeying, and worshipping, are a sufficient reason why there should be such a province in god's empire; still in fact, this race has existed only for a few thousand years, out of the, perhaps, millions of years of the earth's existence; and during all the previous period, the earth, if tenanted, was tenanted by brute creatures, fishes and lizards, beasts and birds, of which none had any faculty, intellectual, moral, or religious. by the same analogy, therefore, on which we have already insisted, we may argue that there is reason to believe, that if other planets, and other stars, are the seats of habitation, it is rather of such habitation as has prevailed upon the earth during the millions, than during the six thousand years; and that if we have, in consequence of physical reasons, to give up the belief of a population in the other planets, or in the stars; we are giving up, not anything with which we might dwell with religious pleasure--hosts of fellow-servants and fellow-worshippers of the divine author of all:--but the mere brute tribes, of the land and of the water, things that creep and crawl, prowl and spring;--none that can lift its visage to the sky, with a feeling that it is looking for its maker and master. there have not existed upon the earth, during the immense ages of its præhuman existence, beings who could recognize and think of the creator of the world: and if astronomy introduces us, as geology has done, to a new order of material structures, thus barren of an intelligent and religious population, we must learn to accept the prospect, in the one case, as in the other. nor need we fear that on a further contemplation of the universe, we shall find every part of it ministering, though perhaps not in the way our first thoughts had guessed, to sentiments of reverence and adoration towards the maker of the universe. . the truth is, as the slightest recollection of the course of opinion about the stars may satisfy us, that men have had repeatedly to give up the notions which they had adopted, of the manner in which the material heavens, the stars and the skies, are to minister to man's feeling of reverence for the creator. it was long ago said, that the heavens declare the glory of god, and the firmament showeth his handiwork: that day and night, sun and moon, clouds and stars, unite in impressing upon us this sentiment. and this language still finds a sympathetic echo, in the breasts of all religious persons. nor will it ever cease to do so, however our opinions of the structure and nature of the heavenly bodies may alter. when the new aspects of things become familiar, they will show us the handiwork of god, and declare his glory, as plainly as the old ones. but in the progress of opinions, man has often had to resign what seemed to him, at the time, visions so beautiful, sublime, and glorious, that they could not be dismissed without regret. the universal lord was at one time conceived as directing the motions of all the spheres by means of ruling angels, appointed to preside over each. the prevalence of proportion and number, in the dimensions of these spheres, was assumed to point to the existence of harmonious sounds, accompanying their movements, though unheard by man; as proportion and number had been found to be the accompaniments and conditions of harmony upon earth. the time came, when these opinions were no longer consistent with man's knowledge of the heavenly motions, and of the wide-spreading causes by which they are produced. then "ruling angels from their spheres were hurled," as a matter of belief; though still the poets loved to refer to imagery in which so many lofty and reverent thoughts had so long been clothed. the aspect of the stars was most naturally turned to a lesson of cheerful and thoughtful piety, by the adoption of such a view of their nature and office; and thus, the midnight contemplator of an italian sky teaches his companion concerning the starry host; sit, jessica; look how the floor of heav'n is thick inlaid with patterns of bright gold. there's not the meanest orb, which thou behold'st, but in his motion like an angel sings, still quiring to the young-eyed cherubims; such harmony is in immortal souls. meaning, apparently, the harmony between the immortal spirits that govern each star, and the cherubims that sing before the throne of god. but however beautiful and sublime may be this representation, the philosopher has had to abandon it in its literal sense. he may have adopted, instead, the opinion that each of the stars is the seat, or the centre of a group of seats, of choirs of worshippers; but this again, is still to suppose the nature of those orbs to be entirely different from that of this earth; though in many respects, we know that they are governed by the same laws. and if he will be content to know no more than he has the means of knowing, or even to know only according to his best means of knowing, he must be prepared, if the force of proof so requires, to give up this belief also; at least for the present. . indeed, those who have not been content with this, and have sought to combine with the visible splendor of the skies, some scheme, founded upon astronomical views, which shall people them with intelligent beings and worshippers, have drawn upon their fancy quite as much as lorenzo in his lesson to jessica; or rather, they have done what he and those from whom his love was derived, had done before. they have taken the truths which astronomers have discovered and taught, and made the objects and regions so revealed, the scenes and occasions of such sentiments of piety as they themselves have, or feel that they ought to have. even in shakspeare, the stars are already _orbs_, each orb has his _motion_, and in his motion produces the music of the spheres. more recent preachers, following sounder views of the nature of these orbs and motions, have been equally poetical when they come to their religious reflection. when the poet of the _night thoughts_ says, "each of these stars is a religious house; i saw their altars smoke, their incense rise, and heard hosannas ring through every sphere." he is no less imaginative than the poet of that _midsummer night's dream_, which we have in the _merchant of venice_. and we are compelled, by all the evidence which we can discern, to say the same of the preacher who speaks, from the pulpit, of these orbs of worlds, and tells us of the stars which "give animation to other systems[ ];" when he says[ ] "worlds roll in these distant regions; and these worlds must be the centres of life and intelligence;" when he speaks of the earth[ ] as "the humblest of the provinces of god's empire." but then we must recollect that these thoughts still prove the religious nature of man; they show how he is impelled to endeavor to elevate his mind to god by every part of the universe; and it is not too much to say, that through the faculties of man, thus regarding the starry heavens, every star does really testify to the greatness of god, and minister to his worship. . we may trust that this mere material magnificence does not require inhabitants, to make it lift man's heart towards the universal creator, and to make him accept it as a sublime evidence of his greatness. the grandest objects in nature are blank and void of life;--the mountain-peaks that stand, ridge beyond ridge, serene in the region of perpetual snow;--the summer-clouds, images of such mountain tracts, even upon a grander scale, and tinted with more gorgeous colors;--the thunder-cloud with its dazzling bolt;--the stormy ocean with its mountainous waves;--the aurora borealis, with its mysterious pillars of fire;--all these are sublime; all these elevate the soul, and make it acknowledge a mighty worker in the elements, in spite of any teaching of a material philosophy. and if we have to regard the planets as merely parts of the same great spectacle of nature, we shall not the less regard them with an admiration which ministers to pious awe. even merely as a spectacle, saturn made visible in his real shape, only by a vast exertion of human skill, yet shining like a star, in form so curiously complex, symmetrical and seemingly artificial, will never cease to be an object of the ardent and contemplative gaze of all who catch a sight of him. and however much the philosopher may teach that he is merely a mass of water and vapor, ice and snow, he must be far more interesting to the eye than the alps, or the clouds that crown them, or the ocean with its icebergs; where the same elements occur in forms comparatively shapeless and lawless, irregular and chaotic. . but perhaps there is in the minds of many persons, a sentiment connected with this regular and symmetrical form of the heavenly bodies; that being thus beautifully formed and finished they must have been the objects of especial care to the creator. these regular globes, these nearly circular orbits, these families of satellites, they too so regular in their movements; this ring of saturn; all the adjustments by which the planetary motions are secured from going wrong, as the profoundest researches into the mechanics of the universe show;--all these things seem to indicate a peculiar attention bestowed by the maker on each part of the machine. so much of law and order, of symmetry and beauty in every part, implies, it may be thought, that every part has been framed with a view to some use;--that its symmetry and its beauty are the marks of some noble purpose. . to reply to this argument, so far as it is requisite for us to do so, we must recur to what we have already said; that though we see in many parts of the universe, inorganic as well as organic, marks which we cannot mistake, of design and purpose; yet that this design and purpose are often effected by laws which are of a much wider sweep than the design, so far as we can trace its bearing. these laws, besides answering the purpose, produce many other effects, in which we can see no purpose. we have now to observe further that these laws, thus ranging widely through the universe, and working everywhere, as if the creator delighted in the generality of the law, independently of its special application, do often produce innumerable results of beauty and symmetry, as if the creator delighted in beauty and symmetry, independently of the purpose answered. . thus, to exemplify this reflection: the powers of aggregation and cohesion, which hold together the parts of solid bodies, as metals and stones, salts and ice,--which solidify matter, in short,--we can easily see, to be necessary, in order to the formation and preservation of solid terrestrial bodies. they are requisite, in order that man may have the firm earth to stand upon, and firm materials to use. but let us observe, what a wonderful and beautiful variety of phenomena grows out of this law, with no apparent bearing upon that which seems to us its main purpose. the power of aggregation of solid bodies is, in fact, the force of crystallization. it binds together the particles of bodies by molecular forces, which not only hold the particles together, but are exerted in special directions, which form triangles, squares, hexagons, and the like. and hence we have all the variety of crystalline forms which sparkle in gems, ores, earths, pyrites, blendes; and which, when examined by the crystallographers, are found to be an inexhaustible field of the play of symmetrical complexity. the diamond, the emerald, the topaz, have got each its peculiar kind of symmetry. gold and other metals have, for the basis of their forms, the cube, but run from this into a vastly greater variety of regular solids than ever geometer dreamt of. some single species of minerals, as calc-spar, present hundreds of forms, all rigorously regular, and have been alone the subject of volumes. ice crystallizes by the same laws as other solid bodies; and our arctic voyagers have sometimes relieved the weariness of their sojourn in those regions, by collecting some of the innumerable forms, resembling an endless collection of hexagonal flowers, sporting into different shapes, which are assumed by flakes of snow[ ]. in these and many other ways, the power of crystallization produces an inexhaustible supply of examples of symmetrical beauty. and what are we to conceive to be the object and purpose of this? as we have said, that part of the purpose which is intelligible to us is, that we have here a force holding together the particles of bodies, so as to make them solid. but all these pretty shapes add nothing to this intelligible use. why then are they there? they are there, it would seem, for their own sake;--because they are pretty;--symmetry and beauty are there on their own account; or because they are universal adjuncts of the general laws by which the creator works. or rather we may say, combining different branches of our knowledge, that crystallization is the mark and accompaniment of chemical composition: and that as chemical composition takes place according to definite numbers, so crystalline aggregation takes place according to definite forms. the symmetrical relations of space in crystals correspond to the simple relations of number in synthesis; and thus, because there is rule, there is regularity, and regularity assumes the form of beauty. . this, which thus shows itself throughout the mineral kingdom, or, speaking more widely and truly, throughout the whole range of chemical composition, is still more manifest in the vegetable domain. all the vast array of flowers, so infinitely various, and so beautiful in their variety, are the results of a few general laws; and show, in the degree of their symmetry, the alternate operation of one law and another. the rose, the lily, the cowslip, the violet, differ in something of the same way, in which the crystalline forms of the several gems differ. their parts are arranged in fives or in threes, in pentagons or in hexagons, and in these regular forms, one part or another is expanded or contracted, rendered conspicuous by color or by shape, so as to produce all the multiplicity of beauty which the florist admires. or rather, in the eye of the philosophical botanist, the whole of the structure of plants, with all their array of stems and leaves, blossoms and fruits, is but the manifestation of one law; and all these members of the vegetable form, are, in their natures, the same, developed more or less in this way or in that. the daisy consists of a close cluster of flowers of which each has, in its form, the rudiments of the valerian. the peablossom is a rose, with some of its petals expanded into butterfly-like wings. even without changing the species, this general law leads to endless changes. the garden-rose is the common hedge-rose with innumerable filaments changed into glowing petals. by the addition of whorl to whorl, of vegetable coronet over coronet, green and colored, broad and narrow, filmy and rigid, every plant is generated, and the glory of the field and of the garden, of the jungle and of the forest, is brought forth in all its magnificence. here, then, we have an immeasurable wealth of beauty and regularity, brought to view by the operation of a single law. and to what use? what purpose do these beauties answer? what is the object for which the lilies of the field are clothed so gaily and gorgeously? some plants, indeed, are subservient to the use of animals and of man: but how small is the number in which we can trace this, as an intelligent purpose of their existence! and does it not, in fact, better express the impression which the survey of this province of nature suggests to us, to say, that they grow because the creator willed that they should grow? their vegetable life was an object of his care and contrivance, as well as animal and human life. and they are beautiful, also because he willed that they should be so:--because he delights in producing beauty;--and, as we have further tried to make it appear, because he acts by general law, and law produces beauty. is not such a tendency here apparent, as a part of the general scheme of creation? . we have already attempted to show, that in the structure of animals, especially that large class best known to us, vertebrate animals, there is also a general plan which, so far as we can see, goes beyond the circuit of the special adaptation of each animal to its mode of living: and is a rule of creative action, in addition to the rule that the parts shall be subservient to an intelligible purpose of animal life. we have noticed several phenomena in the animal kingdom, where parts and features appear, rudimentary and inert, discharging no office in their economy, and speaking to us, not of purpose, but of law:--consistent with an end which is visible, but seemingly the results of a rule whose end is in itself. . and do we not, in innumerable cases, see beauties of color and form, texture and lustre, which suggests to us irresistibly the belief that beauty and regular form are rules of the creative agency, even when they seem to us, looking at the creation for uses only, idle and wanton expenditure of beauty and regularity. to what purpose are the host of splendid circles which decorate the tail of the peacock, more beautiful, each of them, than saturn with his rings? to what purpose the exquisite textures of microscopic objects, more curiously regular than anything which the telescope discloses? to what purpose the gorgeous colors of tropical birds and insects, that live and die where human eye never approaches to admire them? to what purpose the thousands of species of butterflies with the gay and varied embroidery of their microscopic plumage, of which one in millions, if seen at all, only draws the admiration of the wandering schoolboy? to what purpose the delicate and brilliant markings of shells, which live, generation after generation, in the sunless and sightless depths of the ocean? do not all these examples, to which we might add countless others, (for the world, so far as human eye has scanned it, is full of them,) prove that beauty and regularity are universal features of the work of creation, in all its parts, small and great: and that we judge in a way contrary to a vast range of analogy, which runs through the whole of the universe, when we infer that, because the objects which are presented to our contemplation are beautiful in aspect and regular in form, they must, in each case, be means for some special end, of those which we commonly fix upon, as the main ends of the creation, the support and advantage of animals or of man? . if this be so, then the beautiful and regular objects which the telescope reveals to us; jupiter and his moons, saturn and his rings, the most regular of the double stars, clusters and nebulæ; cannot reasonably be inferred, because they are beautiful and regular, to be also fields of life, or scenes of thought. they may be, as to the poet's eye they often appear, the gems of the robe of night, the flowers of the celestial fields. like gems and like flowers, they are beautiful and regular, because they are brought into being by vast and general laws. these laws, although, in the mind of the creator, they have their sufficient reason, as far as they extend, may have, in no other region than that which we inhabit, the reason which we seek to discover everywhere, the sustentation of a life like ours. that we should connect with the existence of such laws, the existence of mind like our own mind, is most natural; and, as we might easily show, is justifiable, reasonable, even necessary. but that we should suppose the result of such laws are so connected with mind, that wherever the laws gather matter into globes, and whirl it round the central body, _there_ is also a local seat of minds like ours; is an assumption altogether unwarranted; and is, without strong evidence, of which we have as yet no particle, quite visionary. . but finally, it may be said that by this our view of the universe, we diminish the greatness of the work of creation, and the majesty of the creator. such a view appears to represent the other planets as mere fragments, which have flown off in the fabrication of this our earth, and of the mechanism by which it answers its purpose. instead of a vast array of completed worlds, we have one world, surrounded by abortive worlds and inert masses. instead of perfection everywhere, we have imperfection everywhere, except at one spot; if even there the workmanship be perfect. . to this, the reply is contained in what we have already said: but we may add, that it cannot be wise or right, to prop up our notions of god's greatness, by physical doctrines which will not bear discussion. god's greatness has no need of man's inventions for its support. the very conviction that the creation must be such as to confirm our belief in the greatness of god, shows that such a belief is more deeply seated than any special views of the structure of the universe, and will triumphantly survive the removal of error in such views. we may add, that till within a few thousand years, this earth, compared with what it now is, having upon it no intelligent beings, might be regarded as an abortive world; that all the parts of the solar system which we can best scrutinize, the moon, and meteoric stones, are inert masses; and further, that there is everywhere the perfection which results from the operation of law, and that _that_ seems to be the perfection with which the creator is contented. . and perhaps, when the view of the universe which we here present has become familiar, we may be led to think that the aspect which it gives to the mode of working of the creator, is sufficiently grand and majestic. instead of manufacturing a multitude of worlds on patterns more or less similar, he has been employed in one great work, which we cannot call imperfect, since it includes and suggests all that we can conceive of perfection. it may be that all the other bodies, which we can discover in the universe, show the greatness of this work, and are rolled into forms of symmetry and order, into masses of light and splendor, by the vast whirl which the original creative energy imparted to the luminous element. the planets and the stars are the lumps which have flown from the potter's wheel of the great worker;--the shred-coils which, in the working, sprang from his mighty lathe:--the sparks which darted from his awful anvil when the solar system lay incandescent thereon;--the curls of vapor which rose from the great cauldron of creation when its elements were separated. if even these superfluous portions of the material are marked with universal traces of regularity and order, this shows that universal rules are his implements, and that order is the first and universal law of the heavenly work. . and, that we may see the full dignity of this work, we must always recollect that man is a part of it, and the crowning part. the workmanship which is employed on mere matter is, after all, of small account, in the eyes of intellectual and moral creatures, when compared with the creation and government of intellectual and moral creatures. the majesty of god does not reside in planets and stars, in orbs and systems; which are, after all, only stone and vapor, materials and means. if, as we believe, god has not only made the material world, but has made and governs man, we need not regret to have to depress any portion of the material world below the place which we had previously assigned to it; for, when all is done, the material world _must_ be put in an inferior place, compared with the world of mind. if there be a world of mind, _that_, according to all that we can conceive, must have been better worth creating, must be more worthy to exist, as an object of care in the eyes of the creator, than thousands and millions of stars and planets, even if they were occupied by a myriad times as many species of brute animals as have lived upon the earth since its vivification. in saying this, we are only echoing the common voice of mankind, uttered, as so often it is, by the tongues of poets. one such speaks thus of stellar systems: behold this midnight splendor, worlds on worlds; ten thousand add and twice ten thousand more, then weigh the whole: one soul outweighs them all, and calls the seeming vast magnificence of unintelligent creation, poor. and as this is true of intelligence, with the suggestion which that faculty so naturally offers, of the inextinguishable nature of mind, so is it true of the moral nature of man. no accumulation of material grandeur, even if it fill the universe, has any dignity in our eyes, compared with moral grandeur: as poetry has also expressed: look then abroad through nature, to the range of planets, suns, and adamantine spheres, wheeling unshaken through the void immense, and speak, o man! can this capacious scene with half that kindling majesty exalt thy strong conception, as when brutus rose refulgent from the stroke of cæsar's fate amid the band of patriots; and his arm aloft extending, like eternal jove when guilt calls down the thunder, call'd aloud on tully's name, and shook his crimson steel, and bade the father of his country, hail! for lo! the tyrant prostrate in the dust, and rome again is free. this action being taken, as it is here meant to be conceived, for one of the highest examples of moral greatness. and however we may judge of this action, we must allow that the characters which are implied in this praise of it,--the loftiest kinds of moral excellence,--are more suitable to the highest idea of the object and purpose of a deity creating worlds, than would be any mere material structure of planets and suns, whether kept in their places by adamantine spheres, wheeling unshaken through the void immense, or themselves wheeling unshaken by the power of a universal law. the thoughts of rights and obligations, duty and virtue, of law and liberty, of country and constitution, of the glory of our ancestors, the elevation of our fellow-citizens, the freedom and happiness and dignity of posterity,--are thoughts which belong to a world, a race, a body of beings, of which any one individual, with the capacities which such thoughts imply, is more worthy of account, than millions of millions of mollusks and belemnites, lizards and fishes, sloths and pachyderms, diffused through myriads of worlds. . we might illustrate this argument further, by taking actions of the moral character of which there will be less doubt. if we look at the great acts which render greece illustrious and interesting in our eyes,--such as the death of socrates, for instance, the triumph of a reverence for law and a love of country;--can we think it any real diminution of the glory of the universe, if we are reduced to the necessity of rejecting the belief in a multitude of worlds, which though, it may be, peopled with lower animals, contain none endowed with any higher principle than hunger and thirst? . that the human race possesses a worth in the eyes of reason beyond that which any material structure, or any brute population can possess, might be maintained on still higher and stronger grounds; namely, on religious grounds: but we do not intend here to dwell on that part of the subject. if man be, not merely (and he alone of all animals) capable of virtue and duty, of universal love and self-devotion, but be also immortal; if his being be of infinite duration, his soul created never to die; then, indeed, we may well say that one soul outweighs the whole unintelligent creation. and if the earth have been the scene of an action of love and self-devotion for the incalculable benefit of the whole human race, in comparison with which the death of socrates fades into a mere act of cheerful resignation to the common lot of humanity; and if this action, and its consequences to the whole race of man, in his temporal and eternal destiny, and in his history on earth before and after it, were the main object for which man was created, the cardinal point round which the capacities and the fortunes of the race were to turn; then indeed we see that the earth has a pre-eminence in the scheme of creation, which may well reconcile us to regard all the material splendor which surrounds it, all the array of mere visible luminaries and masses which accompany it, as no unfitting appendages to such a drama. the elevation of millions of intellectual, moral, religious, spiritual creatures, to a destiny so prepared, consummated, and developed, is no unworthy occupation of all the capacities of space, time, and matter. and, so far as any one has yet shown, to regard this great scheme as other than the central point of the divine plan; to consider it as one part among other parts, similar, co-ordinate, or superior; involves those who so speculate, in difficulties, even with regard to the plan itself, which they strive in vain to reconcile; while the assumption of the subjects of such a plan, in other regions of the universe, is at variance with all which we, looking at the analogies of space and time, of earth and stars, of life in brutes and in man, have found reason to deem in any degree probable. . and thus that conjecture of the plurality of worlds, to which a wide and careful examination of the physical constitution of the universe supplied no confirmation, derives also little support from a contemplation of the design which the creator may be supposed to have had in the work of the creation; when such design is regarded in a comprehensive manner, and in all its bearings. such a survey seems to speak rather in favor of the unity of the world, than of a plurality of worlds. a further consideration of the intellectual, moral, and religious nature of man may still further illustrate this view; and with that object, we shall make a few additional remarks. footnotes: [ ] the greatest anatomists, and especially mr. owen, have recently expressed their conviction, that researches on the structure of animals must be guided by the principle of _unity of composition_ as well as the principle of _final causes_. see owen _on the nature of limbs_. [ ] this has been termed by physiologists _the law of the development from the general to the special_. [ ] every reader of physiological works knows how easy it would be to multiply examples of this kind to any extent. thus it is held by physiologists, that the sporules of fungi are universally diffused through the atmosphere, ready to vegetate whenever an opportunity presents itself: and that a single individual produces not less than ten millions of germs. it is held also that innumerable seeds of plants still capable of vegetation, lie in strata far below the earth's surface, finding the occasion to vegetate only by the rarest and most exceptional occurrences.--carpenter, _manual of physiology_. , art. . [ ] chalmers, p. . [ ] ibid. p. [ ] ibid. p. . [ ] dr. scoresby, in his _account of the arctic regions_ ( ) vol. ii. has given figures of such forms, selected for their eminent regularity from many more. chapter xii. the unity of the world. . the two doctrines which we have here to weigh against each other are the plurality of worlds, and the unity of the world. in so saying, we include in our present view, a necessary part of the conception of a _world_, a collection of intelligent creatures: for even if the suppositions to which we have been led, respecting the kind of unintelligent living things which may inhabit other parts of the universe, be conceived to be probable; such a belief will have little interest for most persons, compared with the belief of other worlds, where reside intelligence, perception of truth, recognition of moral law, and reverence for a divine creator and governor. in looking outwards at the universe, there are certain aspects which suggest to man, at first sight, a conjecture that there may be other bodies like the earth, tenanted by other creatures like man. this conjecture, however, receives no confirmation from a closer inquiry, with increased means of observation. let us now look inwards, at the constitution of man; and consider some characters of his nature, which seem to remove or lessen the difficulties which we may at first feel, in regarding the earth as, in a unique and special manner, the field of god's providence and government. . in the first place, the earth, as the abode of man, the intellectual creature, contains a being, whose mind is, in some measure, of the same nature as the divine mind of the creator. the laws which man discovers in the creation must be laws known to god. the truths,--for instance the truths of geometry,--which man sees to be true, god also must see to be true. that there were, from the beginning, in the creative mind, creative thoughts, is a doctrine involved in every intelligent view of creation. . this doctrine was presented by the ancients in various forms; and the most recent scientific discoveries have supplied new illustrations of it. the mode in which plato expressed the doctrine which we are here urging was, that there were in the divine mind, before or during the work of creation, certain archetypal ideas, certain exemplars or patterns of the world and its parts, according to which the work was performed: so that these ideas or exemplars existed in the objects around us being in so many cases discernible by man, and being the proper objects of human reason. if a mere metaphysician were to attempt to revive this mode of expressing the doctrine, probably his speculations would be disregarded, or treated as a pedantic resuscitation of obsolete platonic dreams. but the adoption of such language must needs be received in a very different manner, when it proceeds from a great discoverer in the field of natural knowledge: when it is, as it were, forced upon _him_, as the obvious and appropriate expression of the result of the most profound and comprehensive researches into the frame of the whole animal creation. the recent works of mr. owen, and especially one work, _on the nature of limbs_, are full of the most energetic and striking passages, inculcating the doctrine which we have been endeavoring to maintain. we may take the liberty of enriching our pages with one passage bearing upon the present part of the subject. "if the world were made by any antecedent mind or understanding, that is by a deity, then there must needs be an idea and exemplar of the whole world before it was made, and consequently actual knowledge, both in the order of time and nature, before things. but conceiving of knowledge as it was got by their own finite minds, and ignorant of any evidence of an ideal archetype for the world or any part of it, they [the democritic philosophers who denied a divine creative mind] affirmed that there was none, and concluded that there could be no knowledge or mind before the world was, as its cause." plato's assertion of archetypal ideas was a protest against this doctrine, but was rather a guess, suggested by the nature of mathematical demonstration, than a doctrine derived from a contemplation of the external world. "now however," mr. owen continues, "the recognition of an ideal exemplar for the vertebrated animals proves that the knowledge of such a being as man must have existed before man appeared. for the divine mind which planned the archetypal also foreknew all its modifications. the archetypal idea was manifested in the flesh under divers modifications upon this planet, long prior to the existence of those animal species which actually exemplify it. to what natural or secondary causes the orderly succession and progression of such organic phenomena may have been committed, we are as yet ignorant. but if without derogation to the divine power, we may conceive such ministers and personify them by the term _nature_, we learn from the past history of our globe that she has advanced with slow and stately steps, guided by the archetypal light amidst the wreck of worlds, from the first embodiment of the vertebrate idea, under its old ichthyic vestment, until it became arrayed in the glorious garb of the human form." . law implies a lawgiver, even when we do not see the object of the law; even as design implies a designer, when we do not see the object of the design. the laws of nature are the indications of the operation of the divine mind; and are revealed to us, as such, by the operations of our minds, by which we come to discover them. they are the utterances of the creator, delivered in language which we can understand; and being thus language, they are the utterances of an intelligent spirit. . it may seem to some persons too bold a view, to identify, so far as we thus do, certain truths as seen by man, and as seen by god:[ ]--to make the divine mind thus cognizant of the truths of geometry, for instance. if any one has such a scruple, we may remark that truth, when of so luminous and stable a kind as are the truths of geometry, must be alike _truth_ for all minds, even for the highest. the mode of arriving at the knowledge of such truths, may be very different, even for different human minds;--deduction for some;--intuition for others. but the intuitive apprehension of necessary truth is an act so purely intellectual, that even in the supreme intellect, we may suppose that it has its place. can we conceive otherwise, than that god does contemplate the universe as existing in space, since it really does so;--and subject to the relations of space, since these are as real as space itself? we are well aware that the supreme being must contemplate the world under many other aspects than this;--even man does so. but that does not prevent the truths, which belong to the aspect of the world, contemplated as existing in space, from being truths, regarded as such, even by the divine mind. . if these reflections are well founded, as we trust they will, on consideration, be seen to be, we may adopt many of the expressions by which philosophers heretofore have attempted to convey similar views; for in fact, this view, in its general bearing at least, is by no means new. the mind of man is a partaker of the thoughts of the divine mind. the intellect of man is a spark of the light by which the world was created. the ideas according to which man builds up his knowledge, are emanations of the archetypal ideas according to which the work of creation was planned and executed. these, and many the like expressions, have been often used; and we now see, we may trust, that there is a great philosophical truth, which they all tend to convey; and this truth shows at the same time, how man may have some knowledge respecting the laws of nature, and how this knowledge may, in some cases, seem to be a knowledge of necessary relations, as in the case of space.[ ] . now, the views to which we have been led, bear very strongly upon that argument. for if man, when he attains to a knowledge of such laws, is really admitted, in some degree, to the view with which the creator himself beholds his creation;--if we can gather, from the conditions of such knowledge, that his intellect partakes of the nature of the supreme intellect;--if his mind, in its clearest and largest contemplations, harmonizes with the divine mind;--we have, in this, a reason which may well seem to us very powerful, why, even if the earth alone be the habitation of intelligent beings, still, the great work of creation is not wasted. if god have placed upon the earth a creature who can so far sympathize with him, if we may venture upon the expression;--who can raise his intellect into some accordance with the creative intellect; and that, not once only, nor by few steps, but through an indefinite gradation of discoveries, more and more comprehensive, more and more profound; each, an advance, however slight, towards a divine insight;--then, so far as intellect alone (and we are here speaking of intellect alone) can make man a worthy object of all the vast magnificence of creative power, we can hardly shrink from believing that he is so. . we may remark further, that this view of god, as the author of the laws of the universe, leads to a view of all the phenomena and objects of the world, as the work of god; not a work made, and laid out of hand, but a field of his present activity and energy. and such a view cannot fail to give an aspect of dignity to all that is great in creation, and of beauty to all that is symmetrical, which otherwise they could not have. accordingly, it is by calling to their thoughts the presence of god as suggested by scenes of grandeur or splendor, that poets often reach the sympathies of their readers. and this dignity and sublimity appear especially to belong to the larger objects, which are destitute of conscious life; as the mountain, the glacier, the pine-forest, the ocean; since in these, we are, as it were, alone with god, and the only present witnesses of his mysterious working. . now if this reflection be true, the vast bodies which hang in the sky, at such immense distances from us, and roll on their courses, and spin round their axles with such exceeding rapidity; jupiter and his array of moons, saturn with his still larger host of satellites, and with his wonderful ring, and the other large and distant planets, will lose nothing of their majesty, in our eyes, by being uninhabited; any more than the summer-clouds, which perhaps are formed of the same materials, lose their dignity from the same cause;--any more than our moon, one of the tribe of satellites, loses her soft and tender beauty, when we have ascertained that she is more barren of inhabitants than the top of mount blanc. however destitute the planets and moons and rings may be of inhabitants, they are _at least vast scenes of god's presence,_ and of the activity with which he carries into effect, everywhere, the laws of nature. the light which comes to us from them is transmitted according to laws which he has established, by an energy which he maintains. the remotest planet is not devoid of life, for god lives there. at each stage which we make, from planet to planet, from star to star, into the regions of infinity, we may say, with the patriarch, "surely god is here, and i knew it not." and when those who question the habitability of the remote planets and stars are reproached as presenting a view of the universe, which takes something from the magnificence hitherto ascribed to it, as the scene of god's glory, shown in the things which he has created; they may reply, that they do not at all disturb that glory of the creation which arises from its being, not only the product, but the constant field of god's activity and thought, wisdom and power; and they may perhaps ask, in return, whether the dignity of the moon would be greatly augmented if her surface were ascertained to be abundantly peopled with lizards; or whether mount blanc would be more sublime, if millions of frogs were known to live in the crevasses of its glaciers. . again: the earth is a scene of moral trial. man is subject to a moral law; and this moral law is a law of which god is the legislator. it is a law which man has the power of discovering, by the use of the faculties which god has given him. by considering the nature and consequences of actions, man is able to discern, in a great measure, what is right and what is wrong;--what he ought and what he ought not to do;--what his duty and virtue, what his crime and vice. man has a law on such subjects, written on his heart, as the apostle paul says. he has a conscience which accuses or excuses him; and thus, recognizes his acts as worthy of condemnation or approval. and thus, man is, and knows himself to be, the subject of divine law, commanding and prohibiting; and is here, in a state of probation, as to how far he will obey or disobey this law. he has impulses, springs of action, which urge him to the violation of this law. appetite, desire, anger, lust, greediness, envy, malice, impel him to courses which are vicious. but these impulses he is capable of resisting and controlling;--of avoiding the vices and practising the opposite virtues;--and of rising from one stage of virtue to another, by a gradual and successive purification and elevation of the desires, affections and habits, in a degree, so far as we know, without limit. . now in considering the bearing of this view upon our original subject, we have, in the first place, to make this remark: that the existence of a body of creatures, capable of such a law, of such a trial, and of such an elevation as this, is, according to all that we can conceive, an object infinitely more worthy of the exertion of the divine power and wisdom, in the creation of the universe, than any number of planets occupied by creatures having no such lot, no such law, no such capacities, and no such responsibilities. however imperfectly the moral law be obeyed; however ill the greater part of mankind may respond to the appointment which places them here in a state of moral probation; however few those may be who use the capacities and means of their moral purification and elevation;--still, that there is such a plan in the creation, and that any respond to its appointments,--is really a view of the universe which we can conceive to be suitable to the nature of god, because we can approve of it, in virtue of the moral nature which he has given us. one school of moral discipline, one theatre of moral action, one arena of moral contests for the highest prizes, is a sufficient centre for innumerable hosts of stars and planets, globes of fire and earth, water and air, whether or not tenanted by corals and madrepores, fishes and creeping things. so great and majestic are those names of _right_ and _good_, _duty_ and _virtue_, that all mere material or animal existence is worthless in the comparison. . but further: let us consider what is this moral progress of which we have spoken;--this purification and elevation of man's inner being. man's intellectual progress, his advance in the knowledge of the general laws of the universe, we found reason to believe that we were not describing unfitly, when we spoke of it as bringing us nearer to god;--as making our thoughts, in some degree, resemble his thoughts;--as enabling us to see things as he sees them. and on that account, we held that the placing man, with his intellectual powers, in a condition in which he was impelled, and enabled, to seek such knowledge, was of itself a great thing, and tended much to give to the creation a worthy end. now the moral elevation of man's being is the elevation of his sentiments and affections towards a standard or idea, which god, by his law, has indicated as that point towards which man ought to tend. we do not ascribe _virtue_ to god, adapting to him our notions taken from man's attributes, as we do when we ascribe knowledge to god: for virtue implies the control and direction of human springs of action;--implies human efforts and human habits. but we ascribe to god infinite goodness, justice, and truth, as well as infinite wisdom and power; and goodness, justice, truth, form elements of the character at which man also is, by the moral law, directed to aim. so far, therefore, man's moral progress is a progress towards a likeness with god; and such a progress, even more than a progress towards an intellectual likeness with god, may be conceived as making the soul of man fit to endure forever with god; and therefore, as making this earth a prefatory stage of human souls, to fit them for eternity;--a nursery of plants which are to be fully unfolded in a celestial garden. . and to this, we must add that, on other accounts also, as well as on account of the capacity of the human soul for moral and intellectual progress, thoughtful men have always been disposed, on grounds supplied by the light of nature, to believe in the existence of human souls after this present earthly life is past. such a belief has been cherished in all ages and nations, as the mode in which we naturally conceive that which is apparently imperfect and deficient in the moral government of the world, to be completed and perfected. and if this mortal life be thus really only the commencement of an infinite divine plan, beginning upon earth and destined to endure for endless ages after our earthly life; we need no array of other worlds in the universe to give sufficient dignity and majesty to the scheme of the creation. . we may make another remark which may have an important bearing upon our estimate of the value of the moral scheme of the world which occupies the earth. if, by any act of the divine government, the number of those men should be much increased, who raise themselves towards the moral standard which god has appointed, and thus, towards a likeness to god, and a prospect of a future eternal union with him;--such an act of divine government would do far more towards making the universe a scene in which god's goodness and greatness were largely displayed, than could be done by any amount of peopling of planets with creatures who were incapable of moral agency; or with creatures whose capacity for the development of their moral faculties was small, and would continue to be small till such an act of divine government were performed. the interposition of god, in the history of man, to remedy man's feebleness in moral and spiritual tasks, and to enable those who profit by the interposition, to ascend towards a union with god, is an event entirely out of the range of those natural courses of events which belong to our subject; and to such an interposition, therefore, we must refer with great reserve; using great caution that we do not mix up speculations and conjectures of our own, with what has been revealed to man concerning such an interposition. but this, it would seem, we may say:--that such a divine interposition for the moral and spiritual elevation of the human race, and for the encouragement and aid of those who seek the purification and elevation of their nature, and an eternal union with god, is far more suitable to the idea of a god of infinite goodness, purity, and greatness, than any supposed multiplication of a population, (on our planet or on any other,) not provided with such means of moral and spiritual progress. . and if we were, instead of such a supposition, to imagine to ourselves, in other regions of the universe, a moral population purified and elevated without the aid or need of any such divine interposition; the supposed possibility of such a moral race would make the sin and misery, which deform and sadden the aspect of our earth, appear more dark and dismal still. we should therefore, it would seem, find no theological congruity, and no religious consolation, in the assumption of a plurality of worlds of moral beings: while, to place the seats of such worlds in the stars and the planets, would be, as we have already shown, a step discountenanced by physical reasons; and discountenanced the more, the more the light of science is thrown upon it. . perhaps it may be said, that all which we have urged to show that other animals, in comparison with man, are less worthy objects of creative design, may be used as an argument to prove that other planets are tenanted by men, or by moral and intellectual creatures like man; since, if the creation of _one_ world of such creatures exalts so highly our views of the dignity and importance of the plan of creation, the belief in _many_ such worlds must elevate still more our sentiments of admiration and reverence of the greatness and goodness of the creator; and must be a belief, on that account, to be accepted and cherished by pious minds. . to this we reply, that we cannot think ourselves authorized to assert cosmological doctrines, selected arbitrarily by ourselves, on the ground of their exalting our sentiments of admiration and reverence for the deity, _when the weight of all the evidence which we can obtain respecting the constitution of the universe is against them_. it appears to us, that to discern one great scheme of moral and religious government, which is the spiritual centre of the universe, may well suffice for the religious sentiments of men in the present age; as in former ages such a view of creation was sufficient to overwhelm men with feelings of awe, and gratitude, and love; and to make them confess, in the most emphatic language, that all such feelings were an inadequate response to the view of the scheme of providence which was revealed to them. the thousands of millions of inhabitants of the earth to whom the effects of the divine plan extend, will not seem, to the greater part of religious persons, to need the addition of more, to fill our minds with sufficiently vast and affecting contemplations, so far as we are capable of pursuing such contemplations. the possible extension of god's spiritual kingdom upon the earth will probably appear to them a far more interesting field of devout meditation, than the possible addition to it of the inhabitants of distant stars, connected in some inscrutable manner with the divine plan. . to justify our saying that the weight of the evidence is against such cosmological doctrines, we must recall to the reader's recollection the whole course of the argument which we have been pursuing. it is a possible conjecture, at first, that there may be other worlds, having, as this has, their moral and intellectual attributes, and their relations to the creator. it is also a possible conjecture, that this world, having such attributes, and such relations, may, on that account, be necessarily unique and incapable of repetition, peculiar, and spiritually central. these two opposite possibilities may be placed, at first, front to front, as balancing each other. we must then weigh such evidence and such analogies as we can find on the one side or on the other. we see much in the intellectual and moral nature of man, and in his history, to confirm the opinion that the human race is thus unique, peculiar and central. in the views which religion presents, we find much more, tending the same way, and involving the opposite supposition in great difficulties. we find, in our knowledge of what we ourselves are, reasons to believe that if there be, in any other planet, intellectual and moral beings, they must not only be _like_ men, but must _be_ men, in all the attributes which we can conceive as belonging to such beings. and yet to suppose other groups of the human species, in other parts of the universe, must be allowed to be a very bold hypothesis, to be justified only by some positive evidence in its favor. when from these views, drawn from the attributes and relations of man, we turn to the evidence drawn from physical conditions, we find very strong reason to believe that, so far as the solar system is concerned, the earth _is_, with regard to the conditions of life, in a peculiar and central position; so that the conditions of any life approaching at all to human life, exist on the earth alone. as to other systems which may circle other suns, the possibility of their being inhabited by men, remains, as at first, a mere conjecture, without any trace of confirmatory evidence. it was suggested at first by the supposed analogy of other stars to our sun; but this analogy has not been verified in any instance; and has been, we conceive, shown in many cases, to vanish altogether. and that there may be such a plan of creation,--one in which the moral and intelligent race of man is the climax and central point to which innumerable races of mere unintelligent species tend,--we have the most striking evidence, in the history of our own earth, as disclosed by geology. we are left, therefore, with nothing to cling to, on one side, but the bare possibility that some of the stars are the centres of systems like the solar system;--an opinion founded upon the single fact, shown to be highly ambiguous, of those stars being self-luminous; and to this possibility, we oppose all the considerations, flowing from moral, historical, and religious views, which represent the human race as unique and peculiar. the force of these considerations will, of course, be different in different minds, according to the importance which each person attaches to such moral, historical, and religious views; but whatever the weight of them may be deemed, it is to be recollected that we have on the other side a bare possibility, a mere conjecture; which, though suggested at first by astronomical discoveries, all more recent astronomical researches have failed to confirm in the smallest degree. in this state of our knowledge, and with such grounds of belief, to dwell upon the plurality of worlds of intellectual and moral creatures, as a highly probable doctrine, must, we think, be held to be eminently rash and unphilosophical. . on such a subject, where the evidences are so imperfect, and our power of estimating analogies so small, far be it from us to speak positively and dogmatically. and if any one holds the opinion, on whatever evidence, that there are other spheres of the divine government than this earth,--other regions in which god has subjects and servants,--other beings who do his will, and who, it may be, are connected with the moral and religious interests of man;--we do not breathe a syllable against such a belief; but, on the contrary, regard it with a ready and respectful sympathy. it is a belief which finds an echo in pious and reverent hearts;[ ] and it is, of itself, an evidence of that religious and spiritual character in man, which is one of the points of our argument. but the discussion of such a belief does not belong to the present occasion, any further than to observe, that it would be very rash and unadvised,--a proceeding unwarranted, we think, by religion, and certainly at variance with all that science teaches,--to place those other, extra-human spheres of divine government, in the planets and in the stars. with regard to the planets and the stars, if we reason at all, we must reason on physical grounds; we must suppose, as to a great extent we can prove that the laws and properties of terrestrial matter and motion apply to them also. on such grounds, it is as improbable that visitants from jupiter or from sirius can come to the earth, as that men can pass to those stars: as unlikely that inhabitants of those stars know and take an interest in human affairs, as that we can learn what they are doing. a belief in the divine government of other races of spiritual creatures besides the human race, and in divine ministrations committed to such beings, cannot be connected with our physical and astronomical views of the nature of the stars and the planets, without making a mixture altogether incongruous and incoherent; a mixture of what is material and what is spiritual, adverse alike to sound religion and to sound philosophy. . perhaps again, it may be said, that in speaking of the shortness of the time during which man has occupied the earth, in comparison with the previous ages of irrational life, and of blank matter, we are taking man at his present period of existence on the earth:--that we do not know that the race may not be destined to continue upon the earth for as many ages as preceded the creation of man. and to this we reply, that in reasoning, as we must do, at the present period, we can only proceed upon that which has happened up to the present period. if we do not know how long man will continue to inhabit the earth, we cannot reason as if we did know that he will inhabit it longer than any other species has done. we may not dwell upon a mere possibility, which, it is assumed, may at some indefinitely future period, alter the aspect of the facts now before us. for it would be as easy to assume possibilities which may come hereafter to alter the aspect of the facts, in favor of the one side, as of the other.[ ] what the future destinies of our race, and of the earth, may be, is a subject which is, for us, shrouded in deep darkness. it would be very rash to assume that they will be such as to alter the impression derived from what we now know, and to alter it in a certain preconceived manner. but yet it is natural to form conjectures on this subject; and perhaps we may be allowed to consider for a moment what kind of conjectures the existing stage of our knowledge suggests, when we allow ourselves the license of conjecturing. the next chapter contains some remarks bearing upon such conjectures. footnotes: [ ] among the most recent expositors of this doctrine we may place m. henri martin, whose _philosophie spiritualiste de la nature_ is full of striking views of the universe in its relation to god. (paris. .) [ ] most readers who have given any attention to speculations of this kind, will recollect newton's remarkable expressions concerning the deity: "Æternus est et infinitus, omnipotens et omnisciens; id est, durat ab æterno in æternum, et adest ab infinito in infinitum.... non est æternitas et infinitas, sed æternus et infinitus; non est duratio et spatium, sed durat et adest. durat semper et adest ubique, et existendo semper et ubique durationem et spatium constituit." to say that god by existing always and everywhere _constitutes duration and space_, appears to be a form of expression better avoided. besides that it approaches too near to the opinion, which the writer rejects, that he _is_ duration and space, it assumes a knowledge of the nature of the divine existence, beyond our means of knowing, and therefore rashly. it appears to be safer, and more in conformity with what we really know, to say, not that the existence of god constitutes time and space; but that god has constituted _man_, so that _he_ can apprehend the works of creation, only as existing in time and space. that god has constituted time and space as conditions of man's knowledge of the creation, is certain: that god has constituted time and space as results of his own existence in any other way, _we_ cannot know. [ ] "for doubt not that in other worlds above there must be other offices of love, that other tasks and ministries there are, since it is promised that his servants, there, shall serve him still."--trench. [ ] for instance, we may assume that in two or three hundred years, by the improvement of telescopes, or by other means, it may be ascertained that the other planets of the solar system are not inhabited, and that the other stars are not the centres of regular systems. chapter xiii. the future. . we proceed then to a few reflections to which we cannot but feel ourselves invited by the views which we have already presented in these pages. what will be the future history of the human race, and what the future destination of each individual, most persons will, and most wisely, judge on far other grounds than the analogies which physical science can supply. analogies derived from such a quarter can throw little light on those grave and lofty questions. yet perhaps a few thoughts on this subject, even if they serve only to show how little the light thus attainable really is, may not be an unfit conclusion to what has been said; and the more so, if these analogies of science, so far as they have any specific tendency, tend to confirm some of the convictions, with regard to those weighty and solemn points,--the destiny of man, and of mankind,--which we derive from other and higher sources of knowledge. . man is capable of looking back upon the past history of himself, his race, the earth, and the universe. so far as he has the means of doing so, and so far as his reflective powers are unfolded, he cannot refrain from such a retrospect. as we have seen, man has occupied his thoughts with such contemplations, and has been led to convictions thereupon, of the most remarkable and striking kind. man is also capable of looking forwards to the future probable or possible history of himself, his race, the earth, and the universe. he is irresistibly tempted to do this, and to endeavor to shape his conjectures on the future, by what he knows of the past. he attempts to discern what future change and progress may be imagined or expected, by the analogy of past change and progress, which have been ascertained. such analogies may be necessarily very vague and loose; but they are the peculiar ground of speculation, with which we have here to deal. perhaps man cannot discover with certainty any fixed and permanent laws which have regulated those past changes which have modified the surface and population of the earth; still less, any laws which have produced a visible progression in the constitution of the rest of the universe. he cannot, therefore, avail himself of any close analogies, to help him to conjecture the future course of events, on the earth or in the universe; still less can he apply any known laws, which may enable him to predict the future configurations of the elements of the world; as he can predict the future configurations of the planets for indefinite periods. he can foresee the astronomical revolutions of the heavens, so long as the known laws subsist. he cannot foresee the future geological revolutions of the earth, even if they are to be produced by the same causes which have produced the past revolutions, of which he has learnt the series and order. still less can he foresee the future revolutions which may take place in the condition of man, of society, of philosophy, of religion; still less, again, the course which the divine government of the world will take, or the state of things to which, even as now conducted, it will lead. . all these subjects are covered with a veil of mystery, which science and philosophy can do little in raising. yet these are subjects to which the mind turns, with a far more eager curiosity, than that which it feels with regard to mere geological or astronomical revolutions. man is naturally, and reasonably, the greatest object of interest to man. what shall happen to the human race, after thousands of years, is a far dearer concern to him, than what shall happen to jupiter or sirius; and even, than what shall happen to the continents and oceans of the globe on which he lives, except so far as the changes of his domicile affect himself. if our knowledge of the earth and of the heavens, of animals and of man, of the past condition and present laws of the world, is quite barren of all suggestion of what may or may not hereafter be the lot of man, such knowledge will lose the charm which would have made it most precious and attractive in the eyes of mankind in general. and if, on such subjects, any conjectures, however dubious,--any analogies, however loose,--can be collected from what we know, they will probably be received as acceptable, in spite of their insecurity; and will be deemed a fit offering from the scientific faculty, to those hopes and expectations,--to that curiosity and desire of all knowledge,--which gladly receive their nutriment and gratification from every province of man's being. . now if we ask, what is likely to be the future condition of the population of the earth as compared with the present; we are naturally led to recollect, what has been the past condition of that population as compared with the present. and here, our thoughts are at once struck by that great fact, to which we have so often referred; which we conceive to be established by irrefragable geological evidence, and of which the importance cannot be overrated:--namely, the fact that the existence of man upon the earth has been for only a few thousand years:--that for thousands, and myriads, and it may be for millions of years, previous to that period, the earth was tenanted, entirely and solely, by brute creatures, destitute of reason, incapable of progress, and guided merely by animal instincts, in the preservation and continuation of their races. after this period of mere brute existence, in innumerable forms, had endured for a vast series of cycles, there appeared upon the earth a creature, even in his organization, superior far to all; but still more superior, in his possession of peculiar endowments;--reason, language, the power of indefinite progress, and of raising his thoughts towards his creator and governor: in short, to use terms already employed, an intellectual, moral, religious, and spiritual creature. after the ages of intellectual darkness, there took place this creation of intellectual light. after the long-continued play of mere appetite and sensual life, there came the operation of thought, reflection, invention, art, science, moral sentiments, religious belief and hope; and thus, life and being, in a far higher sense than had ever existed, even in the highest degree, in the long ages of the earth's previous existence. . now, this great and capital fact cannot fail to excite in us many reflections, which, however vaguely and dimly, carry us to the prospect of the future. the present being _so_ related to the past, how may we suppose that the future will be related to the present? in the first place, _this_ is a natural reflection. the terrestrial world having made this advance from brute to human life, can we think it at all likely, that the present condition of the earth's inhabitants is a final condition? has the vast step from animal to human life, exhausted the progressive powers of nature? or to speak more reverently and justly, has it completed the progressive plan of the creator? after the great revolution by which man became what he is, can and will nothing be done, to bring into being something better than now is; however that future creature may be related to man? we leave out of consideration any supposed progression, which may have taken place in the animal creation previous to man's existence; any progression by which the animal organization was made to approximate, gradually or by sudden steps, to the human organization; partly, because such successive approximation is questioned by some geologists; and is, at any rate, obscure and perplexed: but much more, because it is not really to our purpose. similarity of organization is not the point in question. the endowments and capacities of man, by which he is man, are the great distinction, which places all other animals at an immeasurable distance below him. the closest approximation of form or organs, does nothing to obliterate this distinction. it does not bring the monkey nearer to man, that his tongue has the same muscular apparatus as man's, so long as he cannot talk; and so long as he has not the thought and idea which language implies, and which are unfolded indefinitely in the use of language. the step, then, by which the earth became, a _human_ habitation, was an immeasurable advance on all that existed before; and therefore there is a question which we are, it seems, irresistibly prompted to ask, is this the last such step? is there nothing beyond it? man is the head of creation, in his present condition; but is that condition the final result and ultimate goal of the progress of creation in the plan of the creator? as there was found and produced something so far beyond animals, as man is, may there not also, in some course of the revolutions of the world, be produced something far beyond what man is? the question is put, as implying a difficulty in believing that it should be so; and this difficulty must be very generally felt. considering how vast the resources of the creative power have been shown to be, it is difficult to suppose they are exhausted. considering how great things have been done, in the progress of the work of creation, we naturally think that even greater things than these, still remain to be done. . but then, on the other hand, there is an immense difficulty in supposing, even in imagining, any further change, at all commensurate in kind and degree, with the step which carried the world from a mere brute population, to a human population. in a proportion in which the two first terms are _brute_ and _man_, what can be the third term? in the progress from mere instinct to reason, we have a progress from blindness to sight; and what can we do more than see? when pure intellect is evolved in man, he approaches to the nature of the supreme mind: how can a creature rise higher? when mere impulse, appetite, and passion are placed under the control and direction of duty and virtue, man is put under divine government: what greater lot can any created being have? . and the difficulty of conceiving any ulterior step at all analogous to the last and most wonderful of the revolutions which have taken place in the condition of the earth's inhabitants, will be found to grow upon us, as it is more closely examined. for it may truly be said, the change which occurred when man was placed on the earth, was not one which could have been imagined and constructed beforehand, by a speculator merely looking at the endowments and capacities of the creatures which were previously living. even in the way of organization, could any intelligent spectator, contemplating anything which then existed in the animal world, have guessed the wonderful new and powerful purposes to which it was to be made subservient in man? could such a spectator, from seeing the _rudiments of a hand_, in the horse or the cow, or even from seeing the hand of a quadrumanous animal, have conjectured, that the hand was, in man, to be made an instrument by which infinite numbers of new instruments were to be constructed, subduing the elements to man's uses, giving him a command over nature which might seem supernatural, taming or conquering all other animals, enabling him to scrutinize the farthest regions of the universe, and the subtlest combinations of material things? . or again; could such a spectator, by dissecting the tongues of animals, have divined that the tongue, in man, was to be the means of communicating the finest movements of thought and feeling; of giving one man, weak and feeble, an unbounded ascendency over robust and angry multitudes; and, assisted by the (writing) hand, of influencing the intimate thoughts, laws, and habits of the most remote posterity? . and again, could such a spectator, seeing animals entirely occupied by their appetites and desires, and the objects subservient to their individual gratification, have ever dreamt that there should appear on earth a creature who should desire to know, and should know, the distances and motions of the stars, future as well as present; the causes of their motions, the history of the earth, and his own history; and even should know truths by which all possible objects and events not only are, but must be regulated? . and yet again, could such a spectator, seeing that animals obeyed their appetites with no restraint but external fear, and knew of no difference of good and bad except the sensual difference, ever have imagined that there should be a creature acknowledging a difference of right and wrong, as a distinction supreme over what was good or bad to the sense; and a rule of duty which might forbid and prevent gratification by an internal prohibition? . and finally, could such a spectator, seeing nothing but animals with all their faculties thus entirely immersed in the elements of their bodily being, have supposed that a creature should come, who should raise his thoughts to his creator, acknowledge him as his master and governor, look to his judgment, and aspire to live eternally in his presence? . if it would have been impossible for a spectator of the præhuman creation, however intelligent, imaginative, bold and inventive, to have conjectured beforehand the endowments of such a creature as man, taking only those which we have thus indicated; it may well be thought, that if there is to be a creature which is to succeed man, as man has succeeded the animals, it must be equally impossible for us to conjecture beforehand, what kind of creature _that_ must be, and what will be _his_ endowments and privileges. . thus a spectator who should thus have studied the præhuman creation, and who should have had nothing else to help him in his conjectures and conceptions, (of course, by the supposition of a præhuman period, not any knowledge of the operation of intelligence, though a most active intelligence would be necessary for such speculations,) would not have been able to divine the future appearance of a creature, so excellent as man; or to guess at his endowments and privileges, or his relation to the previous animal creation; and just as little able may we be, even if there is to exist at some time, a creature more excellent and glorious than man, to divine what kind of creature he will be, and how related to man. and here, therefore, it would perhaps be best, that we should quit the subject; and not offer conjectures which we thus acknowledge to have no value. perhaps, however, the few brief remarks which we have still to make, put forwards, as they are, merely as suggestions to be weighed by others, can not reasonably give offence, or trouble even the most reverent thinker. . to suppose a higher development of endowments which already exist in man, is a natural mode of rising to the imagination of a being nobler than man is; but we shall find that such hypotheses do not lead us to any satisfactory result. looking at the first of those features of the superiority of man over brutes, which we have just pointed out, the human hand, we can imagine this superiority carried further. indeed, in the course of human progress, and especially in recent times, and in our own country, man employs instead of, or in addition to the hand, innumerable instruments to make nature serve his needs and do his will. he works by tools and machinery, derivative hands, which increase a hundred-fold the power of the natural hand. shall we try to ascend to a new period, to imagine a new creature, by supposing this power increased hundreds and thousands of times more, so that nature should obey man, and minister to his needs, in an incomparably greater degree than she now does? we may imagine this carried so far, that all need for manual labor shall be superseded; and thus, abundant time shall be left to the creature thus gifted, for developing the intellectual and moral powers which must be the higher part of its nature. but still, that higher nature of the creature itself, and not its command over external material nature, must be the quarter in which we are to find anything which shall elevate the creature above man, as man is elevated above brutes. . or, looking at the second of the features of human superiority, shall we suppose that the means of communication of their thoughts to each other, which exist for the human race, are to be immensely increased, and that this is to be the leading feature of a new period? already, in addition to the use of the tongue, other means of communication have vastly multiplied man's original means of carrying on the intercourse of thought:--writing, employed in epistles, books, newspapers; roads, horses and posting establishments; ships; railways; and, as the last and most notable step, made in our time, electric telegraphs, extending across continents and even oceans. we can imagine this facility and activity of communication, in which man so immeasurably exceeds all animals, still further increased, and more widely extended. but yet so long as what is thus communicated is nothing greater or better than what is now communicated among men;--such news, such thoughts, such questions and answers, as now dart along our roads;--we could hardly think that the creature, whatever wonderful means of intercourse with its fellow-creatures it might possess, was elevated above man, so as to be of a higher nature than man is. . thus, such improved endowments as we have now spoken of, increased power over materials, and increased means of motion and communication, arising from improved mechanism, do little, and we may say, nothing, to satisfy our idea of a more excellent condition than that of man. for such extensions of man's present powers are consistent with the absence of all intellectual and moral improvement. men might be able to dart from place to place, and even from planet to planet, and from star to star, on wings, such as we ascribe to angels in our imagination: they might be able to make the elements obey them at a beck; and yet they might not be better, nor even wiser, than they are. it is not found generally, that the improvement of machinery, and of means of locomotion, among men, produces an improvement in morality, nor even an improvement in intelligence, except as to particular points. we must therefore look somewhat further, in order to find possible characters, which may enable us to imagine a creature more excellent than man. . among the distinctions which elevate man above brutes, there is one which we have not mentioned, but which is really one of the most eminent. we mean, his faculty and habit of forming himself into societies, united by laws and language for some common object, the furtherance of which requires such union. the most general and primary kind of such societies, is that civil society which is bound together by law and government, and which secures to men the rights of property, person, family, external peace, and the like. that this kind of society may be conceived, as taking a more excellent character than it now possesses, we can easily see: for not only does it often very imperfectly attain its direct object, the preservation of rights, but it becomes the means and source of wrong. not only does it often fail to secure peace with strangers, but it acts as if its main object were to enable men to make wars with strangers. if we were to conceive a universal and perpetual peace to be established among the nations of the earth; (for instance by some general agreement for that purpose;) and if we were to suppose, further, that those nations should employ all their powers and means in fully unfolding the intellectual and moral capacities of their members, by early education, constant teaching, and ready help in all ways; we might then, perhaps, look forwards to a state of the earth in which it should be inhabited, not indeed by a being exalted above man, but by man exalted above himself as he now is. . that by such combinations of communities of men, even with their present powers, results may be obtained, which at present appear impossible, or inconceivable, we may find good reason to believe; looking at what has already been done, or planned as attainable by such means, in the promotion of knowledge, and the extension of man's intellectual empire. the greatest discovery ever made, the discovery, by newton, of the laws which regulate the motions of the cosmical system, has been earned to its present state of completeness, only by the united efforts of all the most intellectual nations upon earth; in addition to vast labors of individuals, and of smaller societies, voluntarily associated for the purpose. astronomical observatories have been established in every land; scientific voyages, and expeditions for the purpose of observation, wherever they could throw light upon the theory, have been sent forth; costly instruments have been constructed, achievements of discovery have been rewarded; and all nations have shown a ready sympathy with every attempt to forward this part of knowledge. yet the largest and wisest plans for the extension of human knowledge in other provinces of science by the like means, have remained hitherto almost entirely unexecuted, and have been treated as mere dreams. the exhortations of francis bacon to men, to seek, by such means, an elevation of their intellectual condition, have been assented to in words; but his plans of a methodical and organized combination of society for this purpose, it has never been even attempted to realize. if the nations of the earth were to employ, for the promotion of human knowledge, a small fraction only of the means, the wealth, the ingenuity, the energy, the combination, which they have employed in every age, for the destruction of human life and of human means of enjoyment; we might soon find that what we hitherto knew, is little compared with what man has the power of knowing. . but there is another kind of society, or another object of society among men, which in a still more important manner aims at the elevation of their nature. man sympathizes with man, not only in his intellectual aspirations, but in his moral sentiments, in his religious beliefs and hopes, in his efforts after spiritual life. society, even civil society, has generally recognized this sympathy, in a greater or less degree; and has included morality and religion, among the objects which it endeavored to uphold and promote. but any one who has any deep and comprehensive perception of man's capacities and aspirations, on such subjects, must feel that what has commonly, or indeed ever, been done by nations for such a purpose, has been far below that which the full development of man's moral, religious, and spiritual nature requires. can we not conceive a society among men, which should have for its purpose, to promote this development, far more than any human society has yet done?--a body selected from all nations, or rather, including all nations, the purpose of which should be to bind men together by a universal feeling of kindness and mutual regard, to associate them in the acknowledgment of a common divine lawgiver, governor, and father;--to unite them in their efforts to divest themselves of the evil of their human nature, and to bring themselves nearer and nearer to a conformity with the divine idea; and finally, a society which should unite them in the hope of such a union with god that the parts of their nature which seem to claim immortality, the mind, the soul, and the spirit, should endure forever in a state of happiness arising from their exalted and perfected condition? and if we can suppose such a society; fully established and fully operative, would not this be a condition, as far elevated above the ordinary earthly condition of man, as that of man is elevated above the beasts that perish? . yet one more question; though we hesitate to mix such suggestions from analogy, with trains of thought and belief, which have their proper nutriment from other quarters. we know, even from the evidence of natural science, that god _has_ interposed in the history of this earth, in order to place man upon it. in that case, there was a clear, and, in the strongest sense of the term, a _supernatural interposition_ of the divine creative power. god interposed to place upon the earth, man, the social and rational being. god thus directly instituted human society; gave man his privileges and his prospects in such society; placed him far above the previously existing creation; and endowed him with the means of an elevation of nature entirely unlike anything which had previously appeared. would it then be a violation of analogy, if god were to interpose again, to institute a divine society, such as we have attempted to describe; to give to its members their privileges; to assure to them their prospects; to supply to them his aid in pursuing the objects of such a union with each other; and thus, to draw them, as they aspire to be drawn, to a spiritual union with him? it would seem that those who believe, as the records of the earth's history seem to show, that the establishment of man, and of human society, or of the germ of human society, upon the earth, was an interposition of creative power beyond the ordinary course of nature; may also readily believe that another supernatural interposition of divine power might take place, in order to plant upon the earth the germ of a more divine society; and to introduce a period in which the earth should be tenanted by a more excellent creature than at present. . but though we may thus prepare ourselves to assent to the possibility, or even probability, of such a divine interposition, exercised for the purpose of establishing upon earth a divine society: it would be a rash and unauthorized step,--especially taking into account the vast differences between material and spiritual things,--to assume that such an interposition would have any resemblance to the commencement of a new period in the earth's history, analogous to the periods by which that history has already been marked. what the manner and the operation of such a divine interposition would be, philosophy would attempt in vain to conjecture. it is conceivable that such an event should produce its effect, not at once, by a general and simultaneous change in the aspect of terrestrial things, but gradually, by an almost imperceptible progression. it is possible also that there may be such an interposition, which is only one step in the divine plan;--a preparation for some other subsequent interposition, by which the change in the earth's inhabitants is to be consummated. or it is possible that such a divine interposition in the history of man, as we have hinted at, may be a preparation, not for a new form of terrestrial life, but for a new form of human life;--not for a new peopling of the earth, but for a new existence of man. these possibilities are so vague and doubtful, so far as any scientific analogies lead, that it would be most unwise to attempt to claim for them any value, as points in which science supplies support to religion. those persons who most deeply feel the value of religion, and are most strongly convinced of its truths, will be the most willing to declare, that religious belief is, and ought to be, independent of any such support, and must be, and may be, firmly established on its own proper basis. . we find no encouragement, then, for any attempt to obtain, from science, by the light of the analogy of the past, any definite view of a future condition of the creation. and that this is so, we cannot, for reasons which have been given, feel any surprise. yet the reasonings which we have, in various parts of this essay, pursued, will not have been without profit, even in their influence upon our religious thoughts, if they have left upon our minds these convictions:--that if the analogy of science proves anything, it proves that the creator of man can make a creator as far superior to man, as man, when most intellectual, moral, religious, and spiritual, is superior to the brutes:--and again, that man's intellect is of a divine, and therefore of an immortal nature. those persons who can, on any basis of belief, combine these two convictions, so as to feel that they have a personal interest in both of them;--those who have such grounds as religion, happily appealed to, can furnish, for hoping that their imperishable element may, hereafter, be clothed with a new and more glorious apparel by the hand of its almighty maker;--may be well content to acknowledge that science and philosophy could not give them this combined conviction, in any manner in which it could minister that consolation, and that trust in the divine power and goodness, which human nature, in its present condition, requires. the end. transcriber's notes. spelling irregularities where there was no obviously preferred version were left as is. variants include: "embedded" and "imbedded;" "a hypothesis" and "an hypothesis;" "inexhausted" and "unexhausted;" "volcanos" and "volcanoes." changed "intelligencies" to "intelligences" on page xvi: "may be rational intelligences." changed "familar" to "familiar" on page : "had been familiar." changed "chalmer's" to "chalmers'" on page : "chalmers' reasonings." inserted missing period after "live in the sea" on page . changed "disapear" to "disappear" on page : "at last they disappear." changed "natturally" to "naturally" on page : "we may naturally ask." changed "planets" to "plants" on page : "plants and animals." changed "intelligenee" to "intelligence" on page : "intelligence, morality, religion." changed "crystaline" to "crystalline" on page : "of crystalline powers." changed "dissimiliar" to "dissimilar" on page : "perpetually dissimilar." changed "words" to "worlds" on page : "plurality of worlds." changed "insignificent" to "insignificant" on page : "insignificant and insensible." changed "tales" to "tails" on page : "tails of comets." changed "chambers'" to "chalmers'" in the footnote on page : "chalmers' astron. disc." in the footnote on page , "the times of the warning" might be a typographic error for "the times of the waning," but was not changed. changed "disaprove" to "disprove" on page : "prove or disprove." changed "one-thirteenth" to "one-thirtieth" on page : "be one-thirtieth as large." changed "skeletous" to "skeletons" on page : "can they have skeletons." in the footnote from page , "schroeter" appears with the oe-ligature; elsewhere it does not. the ligature was replaced by the two separate characters in the footnote. changed "how-however" to "however" in the footnote from page : "this, however." changed "hisorians" to "historians" on page : "natural-historians." changed "meaning" to "meaning" at the beginning of page , since it's not a new sentence. changed "crystalizes" to "crystallizes" and "crystaline" to "crystalline" on page : "ice crystallizes;" "crystalline aggregation." changed "artic" to "arctic" in the footnote from page : "account of the arctic regions." changed "kingdon" to "kingdom" on page : "the animal kingdom." changed "splendour" to "splendor" on page : "the material splendor." changed "hightest" to "highest" on page : "the highest degree." changed "deely" to "deeply" on page : "who most deeply feel." international conference held at washington for the purpose of fixing a prime meridian and a universal day. october, . protocols of the proceedings. washington, d. c. gibson bros., printers and bookbinders. . table of contents. page i. protocol, october , ii. protocol, october , iii. protocol, october , iv. protocol, october , v. protocol, october , vi. protocol, october , vii. protocol, october , viii. protocol, november , final act act of congress authorizing the president of the united states to invite the conference (annex i) act of congress making appropriation for expenses (annex ii) circular to united states representatives abroad bringing the subject to the attention of foreign governments (annex iii) circular to united states ministers extending invitation to foreign governments (annex iv) international meridian conference held in the city of washington. i. session of october , . the delegates to the international meridian conference, who assembled in washington upon invitation addressed by the government of the united states to all nations holding diplomatic relations with it, "for the purpose of fixing upon a meridian proper to be employed as a common zero of longitude and standard of time-reckoning throughout the globe," held their first conference to-day, october , , in the diplomatic hall of the department of state. the following delegates were present: on behalf of austria-hungary-- baron ignatz von schÆffer, _envoy extraordinary and minister plenipotentiary_. on behalf of brazil-- dr. luiz cruls, _director of the imperial observatory of rio janeiro_. on behalf of colombia-- commodore s. r. franklin, _u. s. navy_, _superintendent u. s. naval observatory_. on behalf of costa rica-- mr. juan francisco echeverria, _civil engineer_. on behalf of france-- mr. a. lefaivre, _minister plenipotentiary and consul-general_. mr. janssen, _of the institute_, _director of the physical observatory of paris_. on behalf of germany-- baron h. von alvensleben, _envoy extraordinary and minister plenipotentiary_. on behalf of great britain-- captain sir f. j. o. evans, _royal navy_. prof. j. c. adams, _director of the cambridge observatory_. lieut.-general strachey, _member of the council of india_. mr. sandford fleming, _representing the dominion of canada_. on behalf of guatemala-- m. miles rock, _president of the boundary commission_. on behalf of hawaii-- hon. w. d. alexander, _surveyor-general_. hon. luther aholo, _privy counsellor_. on behalf of italy-- count albert de foresta, _first secretary of legation_. on behalf of japan-- professor kikuchi, _dean of the scientific dep't of the university of tokio_. on behalf of mexico-- mr. leandro fernandez, _civil engineer_. mr. angel anguiano, _director of the national observatory of mexico_. on behalf of paraguay-- captain john stewart, _consul-general_. on behalf of russia-- mr. c. de struve, _envoy extraordinary and minister plenipotentiary_. major-general stebnitzki, _imperial russian staff_. mr. j. de kologrivoff, _conseiller d'État actuel_. on behalf of san domingo-- mr. m. de j. galvan, _envoy extraordinary and minister plenipotentiary_. on behalf of salvador-- mr. antonio batres, _envoy extraordinary and minister plenipotentiary_. on behalf of spain, mr. juan valera, _envoy extraordinary and minister plenipotentiary_. mr. emilio ruiz del arbol, _naval attaché to the spanish legation_. mr. juan pastorin, _officer of the navy_. on behalf of sweden-- count carl lewenhaupt, _envoy extraordinary and minister plenipotentiary_. on behalf of switzerland-- colonel emile frey, _envoy extraordinary and minister plenipotentiary_. on behalf of the united states-- rear-admiral c. r. p. rodgers, _u. s. navy_. mr. lewis m. rutherfurd. mr. w. f. allen, _secretary railway time conventions_. commander w. t. sampson, _u. s. navy_. professor cleveland abbe, _u. s. signal office_. on behalf of venezuela-- señor dr. a. m. soteldo, _chargé d'affaires_. the following delegates were not present: on behalf of chili-- mr. francisco vidal gormas, _director of the hydrographic office_. mr. alvaro bianchi tupper, _assistant director_. on behalf of denmark-- mr. carl steen andersen de bille, _minister resident and consul-general_. on behalf of germany-- mr. hinckeldeyn, _attaché of the german legation_. on behalf of liberia-- mr. william coppinger, _consul-general_. on behalf of the netherlands-- mr. g. de weckherlin, _envoy extraordinary and minister plenipotentiary_. on behalf of turkey-- rustem effendi, _secretary of legation_. the delegates were formally presented to the secretary of state of the united states, the honorable frederick t. frelinghuysen, in his office at o'clock. upon assembling in the diplomatic hall, he called the conference to order, and spoke as follows: gentlemen: it gives me pleasure, in the name of the president of the united states, to welcome you to this congress, where most of the nations of the earth are represented. you have met to discuss and consider the important question of a prime meridian for all nations. it will rest with you to give a definite result to the preparatory labors of other scientific associations and special congresses, and thus make those labors available. wishing you all success in your important deliberations, and not doubting that you will reach a conclusion satisfactory to the civilized world, i, before leaving you, take the liberty to nominate, for the purpose of a temporary organization, count lewenhaupt. it will afford this department pleasure to do all in its power to promote the convenience of the congress and to facilitate its proceedings. by the unanimous voice of the conference the delegate of sweden, count lewenhaupt, took the chair, and said that, for the purpose of proceeding to a permanent organization, it was necessary to elect a president, and that he had the honor to propose for that office the chairman of the delegation of the united states of america, admiral c. r. p. rodgers. the conference agreed unanimously to the proposition thus made, whereupon admiral rodgers took the chair as president of the conference, and made the following address: gentlemen: i beg you to receive my thanks for the high honor you have conferred upon me in calling me, as the chairman of the delegation from the united states, to preside at this congress. to it have come from widely-separated portions of the globe, delegates renowned in diplomacy and science, seeking to create a new accord among the nations by agreeing upon a meridian proper to be employed as a common zero of longitude and standard of time reckoning throughout the world. happy shall we be, if, throwing aside national preferences and inclinations, we seek only the common good of mankind, and gain for science and for commerce a prime meridian acceptable to all countries, and secured with the least possible inconvenience. having this object at heart, the government of the united states has invited all nations with which it has diplomatic relations to send delegates to a congress to assemble at washington to-day, to discuss the question i have indicated. the invitation has been graciously received, and we are here this morning to enter upon the agreeable duty assigned to us by our respective governments. broad as is the area of the united states, covering a hundred degrees of longitude, extending from ° ' west from greenwich to ° ' at our extreme limit in alaska, not including the aleutian islands; traversed, as it is, by railway and telegraph lines, and dotted with observatories; long as is its sea coast, of more than twelve thousand miles; vast as must be its foreign and domestic commerce, its delegation to this congress has no desire to urge that a prime meridian shall be found within its confines. in my own profession, that of a seaman, the embarrassment arising from the many prime meridians now in use is very conspicuous, and in the valuable interchange of longitudes by passing ships at sea, often difficult and hurried, sometimes only possible by figures written on a black-board, much confusion arises, and at times grave danger. in the use of charts, too, this trouble is also annoying, and to us who live upon the sea a common prime meridian will be a great advantage. within the last two years we have been given reason to hope that this great desideratum may be obtained, and within a year a learned conference, in which many nations were represented, expressed opinions upon it with singular unanimity, and in a very broad and catholic spirit. i need not trespass further upon your attention, except to lay before you the subject we are invited to discuss: the choice of "a meridian to be employed as a common zero of longitude and standard of time reckoning throughout the world;" and i shall beg you to complete our organization by the election of a vice-president, and the proper secretaries necessary to the verification of our proceedings. mr. lefaivre, delegate from france, stated that on behalf of his colleague he would suggest that all motions and addresses made in english should be translated into french. the president inquired whether the proposition made by the delegate for france met with the approval of the conference, when it was unanimously agreed to. the president thereupon said that he was ready to lay before the conference the subject of the election of vice-president. count lewenhaupt, the delegate of sweden, stated that elections in such large bodies were always difficult, and inquired whether it was necessary to have a vice-president. he further said that for his part he had every reason to hope and to expect that the services of a vice-president would not be required. it was thereupon agreed that a vice-president should be dispensed with. the president then stated that the next business was the election of secretaries; but suggested, in view of the proceedings already had, and of the necessity of some consultation in regard to the matter, that the election might be postponed till to-morrow. mr. valera, delegate of spain, stated that he saw no reason why the nomination of secretaries could not be made just as well at present as at any future time. mr. lefaivre, delegate of france, inquired what would be the functions of the secretaries. the president in reply said that an acting secretary had been appointed by the secretary of state, who was at the same time a stenographer, and that the principal labor of keeping the records of the conference would devolve upon him; that nevertheless regular secretaries of the conference had to be appointed, for the purpose of examining and verifying the protocols from day to day, which would be the more important in the event of the records of the conference being made in two or three different languages, and that these secretaries ought no doubt to be members of the conference, in order to give the requisite authenticity to the acts thereof, and, in view of the character of the proceedings, should be specialists and informed as to the subjects under discussion. mr. soteldo, delegate of venezuela, said that he thought the conference should adjourn until to-morrow, as they had done already enough to-day in settling its organization; that by adjourning over it would give an opportunity to the delegates to consult as to the functions of the secretaries, and who would be most likely to be qualified for those functions; that there were gentlemen from different countries who were not familiar with the english language, and by to-morrow the conference could determine as to the languages in which the proceedings should be had, although, as it seemed to him, that the proceedings should be recorded in french and english. he then moved that the conference adjourn until to-morrow. mr. lefaivre, delegate of france, stated that he agreed with what had been said by the president, that the conference should have secretaries who were specialists, and that the proceedings should be recorded in two languages. by adjourning till to-morrow he thought that the delegates would have an opportunity to reflect upon the subject, and to come back prepared to vote upon it. the president then stated that if any delegates wished to make propositions in regard to the proceedings to-morrow it would be in the power of the conference to proceed to the consideration of those subjects after the election of the secretaries, and he suggested to the delegate of venezuela (mr. soteldo) that the motion to adjourn be withdrawn for the present. the delegate of venezuela thereupon withdrew his motion. mr. frey, delegate of switzerland, said that, in his opinion, the order of proceedings to-morrow should be first a general discussion. mr. valera, delegate of spain, stated that he thought the proceedings should be recorded in two languages at least, and that secretaries conversant with these languages and specially acquainted with the subject matter pending before the conference should be selected; that, in order to have the record of the proceedings accurate, officers qualified in this way were requisite, and that it would be preferable to elect these officers after consultation among the members of the conference, which could be had between now and the meeting to-morrow. count lewenhaupt, delegate of sweden, said that he saw no difficulty in deciding now that the order of proceedings to-morrow would be first the election of the secretaries and then a general discussion, and he moved that this proposition be adopted. the conference then unanimously agreed to the proposition. professor abbe, delegate of the united states, inquired whether it would not facilitate the action of the conference to-morrow if the president appointed a committee now who could nominate the secretaries. the president replied by asking whether it would not be better to select this committee at a subsequent meeting, rather than at the first meeting, which was held to-day. commander sampson, delegate of the united states, then gave notice that at the session to-morrow he would bring before the conference the question whether the meetings shall be open to the public or not, and that he would, at the proper time, also make a motion for the purpose of determining the sense of the conference as to the propriety of inviting distinguished scientists, some of whom are now in washington, and who may desire to be present at the meetings of this conference, to take part in the discussion of the questions pending. mr. lefaivre, delegate of france, stated that in regard to the first proposition--that is, as to making the proceedings public, he would object, inasmuch as he thought that by opening the doors of this conference to the public nothing could be gained, while the proceedings might be embarrassed or delayed by such a course. professor adams, delegate of england, stated that he did not favor the first proposition to make the proceedings of this conference public, but he did agree with the second proposition, and thought it was a very important and valuable one. the president remarked that the propositions made by the delegate of the united states of america were merely in the nature of a notice, and that they were not before the conference at the present time, and, consequently, were not the subject of discussion; still he thought that much good could be elicited from this interchange of opinions in a preliminary way. captain stewart, delegate of paraguay, said that he thought that it would be a very good thing, in view of the proposition to make the meetings public, to invite all the world to the capitol for the discussion of these subjects. professor abbe, delegate of the united states, stated that it would be perfectly practicable to have the discussions of the conference printed in full from day to day for our own official use, and that the public might thereby be made familiar with the proceedings if it were necessary. the president announced that arrangements had been made by the state department whereby the proceedings of each day would be printed and furnished in time for the examination of the members of the conference before the next meeting, and that they would be printed in two languages, french and english; but that these records or protocols could not be regularly verified until the conference shall have appointed duly authorized secretaries. baron von schÆffer, delegate of austro-hungary, asked that a list of the delegates be presented to each of the members of the conference. the president replied that he would instruct the acting secretary (mr. peddrick) to have the list prepared. upon the motion of mr. de struve, delegate of russia, the conference then adjourned until to-morrow, (thursday,) the second instant, at one o'clock p. m. ii. session of october , . the conference met pursuant to adjournment in the diplomatic hall of the department of state, at one o'clock p. m. present: austria-hungary: baron ignatz von schÆffer. brazil: dr. luiz cruls. colombia: commodore s. r. franklin. costa rica: mr. juan francisco echeverria. france: mr. a. lefaivre, mr. janssen. germany: baron h. von alvensleben, mr. hinckeldeyn. great britain: sir f. j. o. evans, prof. j. c. adams, lieut.-general strachey, mr. sandford fleming. guatemala: mr. miles rock. hawaii: hon. w. d. alexander, hon. luther aholo. italy: count albert de foresta. japan: professor kikuchi. mexico: mr. leandro fernandez, mr. ansel anguiano. paraguay: capt. john stewart. russia: mr. c. de struve, major-general stebnitzki, mr. kologrivoff. san domingo: mr. de j. galvan. salvador: mr. antonio batres. spain: mr. juan valera, mr. emilio ruiz del arbol, and mr. juan pastorin. sweden: count carl lewenhaupt. switzerland: col. emile frey, professor hirsch. united states: rear-admiral c. r. p. rodgers, mr. lewis m. rutherford, mr. w. f. allen, commander w. t. sampson, professor cleveland abbe. venezuela: señor dr. a. m. soteldo. absent: chili: mr. f. v. gormas and mr. a. b. tupper. denmark: mr. o. s. a. de bille. liberia: mr. wm. coppinger. netherlands: mr. g. de weckherlin. turkey: rustem effendi. the president stated that the first business before the conference was the election of secretaries. mr. de struve, delegate of russia, stated that it was his opinion that it would be very difficult to elect secretaries by a direct vote, and he proposed that the selection of the secretaries be left to a committee to be appointed by the president; that the committee present the names of the officers selected to the conference, and that these secretaries be four in number. count lewenhaupt, delegate of sweden, stated that it was generally understood among the delegates that mr. hirsch, one of the delegates from switzerland, should be elected a secretary, as he was a secretary of the conference held at rome, but as he has not yet arrived, he proposed that the conference elect only three secretaries to-day. mr. de struve, delegate of russia, stated that he believed that mr. hirsch would soon arrive, and he accepted the amendment just offered. the original motion, as modified by the amendment, was thereupon unanimously agreed to. the chair appointed the delegate of russia, mr. de struve, the delegate from spain, mr. valera, the delegate from france, mr. lefaivre, and the delegate from sweden, count lewenhaupt, as the committee to select the secretaries. the conference thereupon took a recess, to enable the committee to consult and report. upon the reassembling of the conference, the delegate of sweden, count lewenhaupt, announced that the committee had selected for secretaries the delegate from great britain, lieut.-general strachey, the delegate of france, mr. janssen, and the delegate from brazil, dr. cruls. the report of the committee was then unanimously adopted by the conference, and the delegates named as secretaries signified their acceptance of the office. mr. de struve, delegate of russia, moved that the president direct the acting secretary to arrange the seats of the delegates according to the alphabetical order of the countries represented. he added that it would be a great convenience to the members to have their seats permanently fixed. the motion was unanimously agreed to. commander sampson, delegate of the united states, then presented the following resolution: _resolved_, that the congress invite prof. newcomb, superintendent of the united states nautical almanac; prof. hildgard, superintendent of the united states coast and geodetic surveys; professor a. hall; professor de valentiner, director of the observatory at karlsruhe; and sir william thomson, to attend the meetings of this congress. general strachey, delegate of england, stated that, as he understood this resolution, it would not necessarily authorize the parties invited to take any part in the discussions. the president stated that the resolution seems merely to invite the gentlemen to be present. general strachey, delegate of great britain, stated that he thought it necessary to clear up this matter a little; that if the gentlemen invited could not address the conference, it seemed very little use to have them invited; that it was not for their own advantage but for that of the conference that the invitations were extended to those scientific gentlemen, and therefore he thought it was the intention in inviting them to have the benefit of any information which they might desire from time to time to express on the subjects before the congress. he thought that if any remarks on the part of these gentlemen were presented to the conference, with the assent of the congress, through the president, that would doubtless meet all the requirements of the case. the president inquired whether the delegate of great britain meant that the remarks should be presented in writing. general strachey, delegate of great britain, replied that that would not necessarily be the case. prof. abbe, delegate of the united states, inquired whether the persons named in the resolution were the only ones to be invited. the president replied that it was so, so far as the chair was informed, but that it would be in order at any time to add new names in the same way. prof. abbe, delegate of the united states, stated that this was a matter which he had very much at heart, and he would like to observe that some of the nations which were invited to send delegates to this conference had failed to do so, and that it would be a courtesy to invite persons of those nations to be present. commander sampson, delegate of the united states, stated that after consulting with a number of the delegates he drew the resolution, and that it was suggested to him this very morning that possibly there might be a difference of opinion as to whether these gentlemen should take part in the discussion, and that that was the reason why the first resolution merely proposed to invite them to be present. he stated that he proposed subsequently to submit another resolution authorizing these gentlemen to take part in the discussion; that he thought that the original intention was to confer an honor on certain distinguished scientists, and that it would be well for the conference to limit the invitation to gentlemen of that character. mr. lefaivre, delegate of france, stated that he was opposed to the proposition to admit to the deliberations of this conference gentlemen, no matter how distinguished or eminent they might be, who were not specially delegated by their governments as members of this body. he questioned the power of the conference to admit to its discussions persons who were not regularly appointed to vote upon the subject at issue; that this was an international conference created for the purpose of obtaining an interchange of views from the representatives of the different governments; that it would extend the scope of the work before this body to entertain the views and opinions of persons not authorized to speak for the governments whose delegates are here; that there would be a great divergence of opinion among such men, and the result would be rather to embarrass than to help this conference to an accord. he insisted that the matter was exclusively governmental, and, while he would be happy to extend any courtesy to men distinguished in science, such as the gentlemen who are proposed to be invited, he felt constrained to oppose the proposition under the circumstances. the president stated that he understood that the resolution did not propose to confer a vote upon the gentlemen invited, but simply to enable them to lay any information before the conference which they might have upon the matter at issue. mr. lefaivre, delegate of france, contended that the resolution was intended to authorize these gentlemen to deliberate, and he thought that the inconvenience would be very great of extending this privilege to persons not authorized to represent their governments. he did not think it was reasonable or fair that his opinions should be questioned or opposed by the opinions of men not authorized to speak for their governments. gen. strachey, delegate of great britain, said that as he had taken upon himself to make some remarks both as to the manner in which the gentlemen should be invited and the extent of their rights when invited, he wished to say that while he agreed with much that had been said by the delegate of france, he held that these gentlemen should have an opportunity of expressing their views; that they were not to come here merely to listen to the proceedings, but that they should themselves be heard. the president directed that the resolution be read in french, and then put it to the vote, when it was unanimously adopted. commander sampson. delegate of the united states, then offered the following resolution: "_resolved_, that the gentlemen who have just been invited to attend the meetings of the conference be permitted to take part in the discussion of all scientific questions." mr. lefaivre, delegate for france, then stated that it was not in accordance with the object of this conference that private individuals, not authorized by their respective governments, should be permitted to influence the decision of this body, and that, while it was very proper to extend courtesy to such learned gentlemen as were invited, it surely was never intended that they should participate in our proceedings. gen. strachey, delegate of great britain, said that it would, perhaps, save trouble if he stated his views on the point under discussion, which he apprehended were generally in accordance with those of the representative from france. he said that, if he were permitted, he would read a resolution, which he suggested might be accepted as a substitute for that pending before the conference, and it was as follows: "_resolved_, that the president be authorized, with the concurrence of the delegates, to request an expression of the opinions of the gentlemen invited to attend the congress on any subject on which their opinion may be likely to be valuable." the president inquired in what way they would express it. gen. strachey, delegate of great britain, stated that it would be orally. the president replied that the resolution undoubtedly read that way. gen. strachey, delegate of great britain, stated that the language, "to take part in the discussion," employed in the resolution of commander sampson, would mean that the persons invited would be in a position, of their own motion, either to reply to remarks made, or to state their own views, or to take part in the discussion just as the delegates are entitled to do. mr. lefaivre, delegate of france, stated that he hoped that the proposition of the delegate of great britain would not be pressed until a vote was had upon the original resolution. the president then put the resolution to a vote; but, being unable to determine from the _viva voce_ vote whether it was carried or not, he stated that the roll would be called. mr. frey, delegate of switzerland, stated that he thought before the vote was taken a decision should be had upon the question, how the delegates were to vote--whether as nations or as individuals. the president announced that it had been the custom in all such conferences to vote as nations, each nation casting one vote, and that no other way seemed practicable; and that in conformity with this ruling the roll would be called and the vote taken by nations. the roll was then called, when the following states voted in the affirmative: costa rica, guatemala, italy, mexico, san domingo, salvador, switzerland, venezuela. and the following in the negative: austria-hungary, brazil, colombia, france, germany, great britain, hawaii, japan, paraguay, russia, spain, sweden. united states, the president then announced that the ayes were and the noes , and that the resolution was lost. gen. strachey, delegate of great britain, then renewed his resolution, which was as follows: "_resolved_, that the president be authorized, with the concurrence of the delegates, to request an expression of the opinions of the gentlemen invited to attend the congress on any subject on which their opinion may be likely to be valuable." no discussion arose upon this resolution, and it was adopted. commander sampson, delegate of the united states, then offered the following resolution: "_resolved_, that the meetings of this congress be open to interested visitors." mr. lefaivre, delegate of france, stated that he considered this a subject of grave importance; that this was an official and confidential body; scientific, it was true, but also diplomatic; that it was empowered to confer about matters with which the general public have now nothing to do; that to admit the public to the meetings would destroy their privacy and subject the conference to the influence of an outside pressure which might prove very prejudicial to its proceedings, and that he would object to this resolution absolutely. no further discussion being had, the president, after a _viva voce_ vote of doubtful result, ordered the roll to be called, when the following states voted in the affirmative: colombia, costa rica, guatemala, paraguay, salvador, spain. venezuela, and the following states in the negative. austria-hungary, brazil, france, germany, great britain, hawaii, italy, japan, mexico, russia, san domingo, sweden, switzerland, united states. the president then announced that the ayes were and the noes , and that the resolution was therefore lost. the president then said that there would doubtless be some preliminary general discussion on the subject before the conference, and suggested that if delegates desired to be heard upon the subject it would be expedient to give an intimation to the secretary. prof. abbe, delegate of the united states, then said: i have been requested to present to the conference the communication that i hold in my hand, and in doing so wish to offer the following resolution: "whereas several persons desire to submit to this conference inventions, devices, and systems of universal time: therefore, "_resolved_, that the conference will acknowledge the receipt of such communications, but will abstain from any expression of opinion as to their respective merits." professor adams, delegate of great britain, said that the conference should be very cautious in admitting the devices and schemes of people who have no connection with this body; that there are, no doubt, many inventors and many people who have plans and schemes which they wish to press upon the conference, and that it was probable that the conference would be subjected to very great inconvenience if they took upon themselves even the burden of acknowledging the receipt of these communications. the president stated that he had received several communions of this character, one proposing that jerusalem should be taken as the prime meridian. mr. lefaivre, delegate of france, proposed that the conference should appoint a committee to examine the different papers submitted by outside parties, and to make such suggestions as they might deem proper after examining the papers. mr. valera, delegate of spain, said that it seemed to him the proper course of proceeding for the conference was to take up the subject article by article, and treat it in that order; that there were presented to the conference certain well-defined propositions, and that besides these there were the resolutions which had been adopted by the conference at rome, which could be used as a basis for the discussions of this conference; that in that way the delegates would have before them some precise subject-matter, and after discussion, if any proposition needed to be altered or amended it would be in the power of the conference to do so, but that unless some regular method of proceeding were adopted the sessions would be prolonged indefinitely, and the conference would be confused by a multitude of irrelevant propositions that might be presented to them. mr. rutherfurd, delegate of the united states, stated that it seemed to him that to invite a general discussion upon the subject, which has undoubtedly a great many heads, the best method would be the one just suggested; that by having a well-defined course much time would be saved, and there would be a precision in the proceedings, which undoubtedly is always valuable; that in this way the discussion could be kept within bounds, but unless there is some proposition pending before the conference it is impossible to say whether any discussion is in order or out of order; that it seemed to him there should be some well-defined propositions laid before the conference, and those propositions could easily be gathered, not only from what has gone before, not only from the conference which has been held in rome, but from the acts of congress and the circulars of the secretary of state, under which this body has been organized. the president stated that if these communications from outside parties were brought before the conference it would entail a great deal of labor. the resolution of the delegate of the united states, prof. abbe, was then put to the vote, and was negatived. mr. rutherfurd, delegate of the united states, then presented the following resolution: "_resolved_, that the conference proposes to the governments represented the adoption as a standard meridian that of greenwich passing through the centre of the transit instrument at the observatory of greenwich." mr. lefaivre, delegate of france, remarked that the proposed resolution seemed to him out of order, and that his colleague, mr. janssen, desired to address the conference on the subject. he went on to say: the competence of the conference can give rise to no long debate among us. let us remark, in the first place, that no previous engagement exists, on the part of the governments, to adopt the results of our discussions, and that consequently our decisions cannot be compared to those of a deliberative congress or an international commission acting according to definite powers. we have no definite powers, or rather, we have no executive power, since our decisions cannot be invoked executively by one government towards others. does this mean that our decisions will be wholly unauthoritative? an assembly which numbers so many eminent delegates, and in which there is so much scientific knowledge, must certainly be regarded with profound respect by all the powers of the world. its powers, however, must be of a wholly moral character, and will have to be balanced against rights and interests no less worthy of consideration, leaving absolutely intact the independence of each individual state. under these circumstances, gentlemen, it seems to me that our course is already marked out for us. from our conference is to be elicited the expression of a collective wish, a draft of a resolution, which is to be adopted by the majority of this assembly, and afterwards submitted to the approval of our respective governments. this is our mission. it is a great one, and has a lofty international bearing. we must, however, realize its extent from the very outset, and not go beyond its limits. an appeal has been made to the decisions of the conference held at rome. but, gentlemen, i beg leave to remark that that conference was composed entirely of specialists, and that it did not meet for the purpose of examining the question in an international point of view. this conference is composed of various elements, among which are scientists of the highest standing, but also functionaries of high rank, who are not familiar with scientific subjects, and who are charged with an examination of this question from a political stand-point. it is, moreover, our privilege to be philosophers and cosmopolitans, and to contemplate the interests of mankind not only for the present, but for the most distant future. you see, gentlemen, that we enjoy absolute freedom, and that we are in nowise bound by the decisions of the conference held at rome. it is even desirable that those precedents should be appealed to as little as possible, inasmuch as we have scientists among us who are regarded as authorities in both the old and the new world, and who are perfectly capable of directing us in technical matters, and of furnishing all the information that we can desire. i will say even more than this: the results of the conference held at rome are by no means regarded as possessing official authority by the governments that have accredited us; for if those results had been taken as a starting point, there would be no occasion for our conference, and our governments would simply have to decide with regard to the acceptance or rejection of the resolutions adopted by the geodetic congress at rome. everything, however, is intact, even the scientific side of the question, and that is the reason why we have so many delegates possessing technical knowledge among us. the president stated that he considered the resolution entirely in order, and likely to bring about a discussion upon the very point for which this conference was called together; that the resolution was open to any amendment that might be offered, could be altered from time to time if necessary, and, if it did not meet the sense of the conference, could be defeated. mr. lefaivre, delegate of france, inquired whether this proposition did not demand an immediate solution. mr. rutherfurd, delegate of the united states, replied that no such thing was contemplated. prof. janssen, delegate of france, then spoke as follows: gentlemen: i formally request that the resolution just proposed by my eminent colleague and friend, mr. rutherford, be held in reserve, and that it may not now be pressed for discussion. it is wholly undesirable that a proposition of so grave a character, which forestalls one of the most important resolutions that we shall be called upon to adopt, should be put to the vote while our meeting has scarcely been organized, and before any discussion relative to the true merits of the questions to be considered has taken place. this would be inverting the proper order of things and reaching a conclusion before having examined the subject before us. before discussing the question of the selection of a meridian which is to serve as a common zero of longitude for all the nations of the world, (if the congress shall think proper to discuss that point,) it is evident that we must first decide the question of principle which is to govern all our proceedings; that is to say, whether it is desirable to fix upon a common zero of longitude for all nations. i therefore formally ask for the withdrawal of mr. rutherford's proposition. the president stated that as something had been said about the conference at rome, he desired to say that he had carefully abstained from any allusion to it, and that the delegation of the united states found no allusions to it in their instructions; that, so far as the chair understood the resolution offered by the delegate of the united states, it was simply to bring before the conference the consideration of the subject of a prime meridian; that he did not understand that even the delegate who presented the motion offered it as an expression of his own opinion on the subject, but that he had carefully stated, when he had brought the resolution before the conference, that it was for the purpose of enabling the delegates to proceed to an immediate discussion. he added, further, that the resolution was quite open to amendment in case the delegates from france desired to amend it. commander sampson, delegate of the united states, stated that he wished to offer the following as a substitute for the resolution already pending: "_resolved_, that it is the opinion of this congress that it is desirable to adopt a single prime meridian for all nations in place of the multiplicity of initial meridians which now exist." mr. rutherfurd, delegate of the united states, then announced that he accepted this substitution in place of the first resolution. general strachey, delegate of england, stated that if he rightly understood the remarks made by the delegate of france, mr. lefaivre, he thought that it was intended to call attention to the ultimate form in which the resolutions of this congress should be recorded. he referred to the address which the secretary of state of the united states (mr. frelinghuysen) made to the delegates on their assembling, in which he said: "you have met to discuss and consider the important question of a prime meridian for all nations. it will rest with you to give a definite result to the preparatory labors of other scientific associations and special congresses, and thus make those labors available." he added that the object at which they should aim was to put together a series of resolutions which could be presented to the various governments whose representatives are here present, with a view to inducing them to accept the decision which may be arrived at by this conference, and, finally, to put that decision in a diplomatic form--a form which shall be more definite and precise than the mere resolutions which would be adopted by a purely scientific body; this he understood to be the position to be adopted by the delegates to this conference. he then said that it seemed to him that it would be necessary, after settling the original shape of the resolutions, that they should be reconsidered and afterwards put together in an orderly way, in a manner which would give a regular and satisfactory record of the proceedings; that it appeared almost certain to him that the discussions would be desultory in their nature, but that ultimately a revision would be had after the rough-hewing of the blocks out of which the edifice was to be formed; that he had no wish, at the present stage of the discussion, to go into the merits of the question presented; that, for his part, he thought it more prudent to abstain, but that with reference to the remarks of his honorable friends from france, he could not agree that they should set aside what occurred at rome; that the discussions at rome were most valuable; they went thoroughly into the whole question, and he apprehended that every gentleman in the conference was possessed of the records of what occurred there. he continued by saying that he thought that the delegate from france, mr. lefaivre, went a little beyond what was strictly right in saying that we should shut our eyes to what occurred there; that, for his own part, he was obliged to pay attention to what occurred there; that some of the most eminent scientific men to be found in any country met there and fully discussed the questions now before us, and that the delegates here present were now called upon to revise what occurred there. mr. rutherfurd, delegate of the united states, said that the delegate from france, mr. lefaivre, in his remarks, insisted that we should first establish for what purpose the delegates were here assembled; that he wished to refer to the circulars sent out by the government of the united states, under which this conference was called together. he said that he could assert, without fear of contradiction, that in those communications the president stated that it was believed to be a foregone conclusion that a prime meridian was desirable; that that was the basis on which the president acted in giving his invitation; that how he came to that conclusion he does not state--whether or not the proceedings at rome had anything to do with it, but he thought that they had a great influence on the mind of the president; that, doubtless, his action was not determined solely by that, and, therefore, that the secretary of state first made a tentative application to see whether a proposition for another conference was acceptable, and that he found all countries here represented answering the circular in the affirmative; that they agreed with him that a conference for this purpose was desirable. he continued by saying that the secretary of state then sent a second invitation to the different nations to send delegates, who were to assemble here on the first of october, , for the purpose of establishing a prime meridian and a universal time. he added that it seemed to him a great loss of time to go over the question whether a prime meridian was or was not desirable; that the delegates were sent here for the purpose of agreeing upon a prime meridian. he then asked why this conference should lose time in discussing that question. the resolution offered by the delegate of the united states, commander sampson, was then unanimously adopted as follows: "_resolved_, that it is the opinion of this congress that it is desirable to adopt a single prime meridian for all nations in place of the multiplicity of initial meridians which now exist." mr. rutherfurd, delegate of the united states, then renewed his original resolution, as follows: "_resolved_, that the conference proposes to the governments represented the adoption as a standard meridian that of greenwich, passing through the centre of the transit instrument at the observatory of greenwich." mr. janssen, delegate of france, stated that he wished to reiterate the objections that he had already offered to the first resolution, and spoke as follows: gentlemen: mr. lefaivre, my honorable colleague, and i are of the opinion that the mission of this congress is chiefly to examine questions of principle. i consider that we shall do a very important thing if we proclaim the principle of the adoption of a meridian which shall be the same for all nations. the advantages of such a meridian have been felt by the geographers and navigators of all ages. france might claim the honor of having sought to accomplish this reform as early as the seventeenth century. it is not to be expected, therefore, that france, at this late day, will seek to place any obstacles in the way of the adoption of an improvement which would by this time have been adopted if the use of the meridian which she proposed, and which she had caused to be generally accepted, had been continued. we therefore fully agree with you, gentlemen, as to the principle of a common international meridian, impartially defined and wisely applied, and we think that if the congress should cause a useful reform, which has been so long expected, to be finally adopted, it would render a great service to the world, and one that would do us the highest honor. this point being gained, is it proper for us to proceed to the adoption of such a meridian? we think not, unless we are assured by a previous declaration as to the principle which is to govern the selection of that meridian. without such a declaration, we should have no power to begin a discussion on an undefined subject, and we are not authorized to pledge ourselves. i must even add that our acquiescence in the principle of an international meridian could not be maintained if the congress proceeded to a choice at variance with the exclusively scientific principles which we are instructed to maintain. thus, in the very interest of the great principle which we all desire to see adopted, it would, to my way of thinking, be wiser to confine ourselves to a general declaration which, by uniting the opinions of all, would sustain the principle with all the authority possible. the principle having once been adopted, our governments would subsequently convoke a conference of a more technical character than this, at which questions of application would be more thoroughly examined. mr. valera, delegate of spain, stated that it seemed to him the order of proceeding for this conference was very well laid down in the invitations addressed by the president of the united states to the different countries and in the articles which were formulated at rome; that if these were taken up one after the other and discussed there would be a clearly-defined line of action for the delegates; that if an article was not satisfactory it could be altered or amended, or could be rejected; but if the propositions were taken up one at a time and the discussions directed to these propositions, the conference would be more likely to reach a definite result than in any general discussion. the president stated that, so far as he understood the proposition, there was no desire to press it to an immediate vote; that it was quite proper for the delegate from france to offer any other proposition, as suggested by the delegate of spain, in lieu of the motion now pending; that so far as the chair was concerned it seemed to him that the conference could at once proceed to the discussion of the general subject of a prime meridian under the pending resolution; that if the delegate from france desires to make any other proposition, or offer anything else in a distinct form, he will be listened to with great attention and with profound respect. mr. rutherfurd, delegate of the united states, remarked that the delegate from france, his learned friend, mr. janssen, had expressed the opinion that the delegates had not the power to decide upon any particular meridian, but that they were sent here merely to discuss this principle, namely, whether a general meridian was desirable. he added that he was, of course, not in possession of the instructions which the delegates from france received from their own government, but that he found among the instructions received by the delegates of the united states from their government a copy of one of the communications made by the president of the united states to france, as well as to the other nations, through the secretary of state, in which was this language: "i am accordingly directed by the president to request you to bring the matter to the attention of the government of ----, through the minister for foreign affairs, with a view to learning, whether its appreciation of the benefits to accrue to the intimate intercourse of civilized peoples from the consideration and adoption of the suggested common standard of time, so far coincides with that of this government as to lead it to accept an invitation to participate in an international conference at a date to be designated in the near future." the delegate of the united states continued by saying that the whole object of this conference was not to establish the principle that it is desirable to have a prime meridian, but to fix that prime meridian; that that was the object of the meeting, and that it seemed to him that there must be some misapprehension on the part of the learned gentleman from france in thinking that this conference has not the power to fix upon a prime meridian; that as to our organization, the delegate of france (mr. lefaivre) spoke of its not being sufficiently complete to take up this subject at present, but that it seemed to him that the delegates undoubtedly were ready to hear and express arguments _pro_ and _con_ in regard to that question; that he supposed that every delegate had studied this matter before coming here, and that he did not think that any delegate would be likely to come here unless he knew, or thought he knew, some thing about this matter. mr. valera, delegate from spain, announced that he had no power to pledge his country on this subject; that his authority merely extended to the power of recommending to his government such resolutions as this conference might adopt. count lewenhaupt, delegate of sweden, then said: "i desire to state in the protocol that i have no power to engage my government by my votes on the different questions which will be submitted to this conference, and that, therefore, these votes must only be considered as an engagement on my part to recommend to my government the decisions for which i vote." general strachey, delegate of great britain, said that in the name of the delegates of great britain he wished to state that they were in the same position, but that would not prevent them or this conference from forming an opinion and expressing it. the president stated that on behalf of the delegates from the united states they had no power except that of discussion and recommendation. mr. de struve made, on behalf of the delegates of russia, a declaration identical with that made by the delegate of sweden. baron von alvensleben, delegate from germany, made the same announcement on behalf of his government. mr. fernandez, delegate from mexico, made the same announcement. mr. valera, delegate of spain, remarked that this conference was called together not merely to discuss the subject of a prime meridian, but to determine, so far as these delegates were concerned, the propriety of adopting a particular prime meridian, and that his government would decide afterwards whether it would accept what this conference should recommend. dr. cruls, delegate of brazil, stated that his government authorized him to take part in the discussion, but not to commit his government to the adoption of any particular proposition. mr. fleming, delegate of great britain, said that he would like to call the attention of the conference to the language of the act of congress calling this conference together, and that language runs as follows: "that the president of the united states be authorized and requested to extend to the governments of all nations in diplomatic relations with our own an invitation to appoint delegates to meet delegates from the united states in the city of washington, at such time as he may see fit to designate, for the purpose of fixing upon a meridian proper to be employed as a common zero of longitude and standard of time-reckoning throughout the globe." he added that he thought the object of the conference clearly was to determine and to recommend; that although the word "recommend" was not used in the body of the resolution, it was certainly understood, and, as a matter of fact, the title of the joint resolution passed by congress contains the word "recommend." it reads as follows: "an act to authorize the president of the united states to call an international conference to fix on and recommend for universal adoption a common prime meridian, to be used in the reckoning of longitude and in the regulation of time throughout the world." baron von schæffer, delegate of austria-hungary, then moved that the conference adjourn until monday, the th instant, at one o'clock, to enable delegates to confer on this subject. the proposition of the delegate of austria-hungary was then agreed to, and the conference adjourned to monday, october , , at o'clock, p. m. iii. session of october , . the conference met pursuant to adjournment in the diplomatic hall of the department of state, at one o'clock p. m. present: austro-hungary: baron ignatz von schÆffer. brazil: dr. luiz cruls. colombia: commodore s. r. franklin. costa rica: mr. juan francisco echeverria. france: mr. a. lefaivre, mr. janssen. germany: baron h. von alvensleben, mr. hinckeldeyn. great britain: capt. sir f. j. o. evans, prof. j. c. adams, lieut.-general strachey, mr. sandford fleming. guatemala: mr. miles rook. hawaii: hon. w. d. alexander, hon. luther aholo. italy: count albert de foresta. japan: professor kikuchi. mexico: mr. leandro fernandez, mr. angel arguiano. paraguay: capt. john stewart. russia: mr. c. de struve, major-general stebnitzki, mr. kologrivoff. san domingo: mr. de j. galvan. salvador: mr. antonio batres. spain: mr. juan valera, mr. emilio ruiz del arbol, mr. juan pastorin. sweden: count carl lewenhaupt. turkey: rustem effendi. united states: rear-admiral c. r. p. rodgers, mr. lewis m. rutherfurd, mr. w. f. allen, commander w. t. sampson, professor cleveland abbe. venezuela: dr. a. m. soteldo. mr. rutherfurd, delegate of the united states, said that the resolution offered by him at the last meeting omitted to state that the proposed meridian was for longitude, and he would offer the following as a substitute therefor: "_resolved_, that the conference proposes to the governments here represented the adoption of the meridian passing through the centre of the transit instrument at the observatory of greenwich as the standard meridian for longitude." the president then asked if the conference would permit the substitution to be made, and it was unanimously agreed to. mr. rutherfurd, delegate of the united states, stated that he did not propose to press the resolution to an early vote, but that it was offered simply to elicit the opinions of delegates on the subject. he further stated that, having heard that the delegates of france, mr. lefaivre and mr. janssen, desired to present certain propositions, he would, for that purpose, move to withdraw for the time being the resolution offered by him. no objection being made, the resolution was temporarily withdrawn. mr. lefaivre, delegate of france, then made the following statement: our colleague, mr. rutherfurd, having withdrawn his motion for the adoption of the meridian of greenwich, we, the delegates of france, after consultation with him, submit the following motion: "_resolved_, that the initial meridian should have a character of absolute neutrality. it should be chosen exclusively so as to secure to science and to international commerce all possible advantages, and in particular especially should cut no great continent--neither europe nor america." sir f. j. o. evans, delegate of great britain, then stated that he presumed the conference could hardly pass by the important meeting held at rome, where twelve of the thirty-eight delegates were directors of national observatories, and where the subject of the conditions which should attach to a prime meridian were discussed without reference to any particular nationality; that these learned gentlemen came to the conclusion (which he thought was a very wise one) that the necessity existed for a prime meridian that it should pass through an astronomical observatory of the first order; that modern science demanded such precision, and therefore they excluded all ideas of a meridian being established on an island, in a strait, on the summit of a mountain, or as indicated by a monumental building. looking at the subject in its various aspects, they came to the conclusion that there were only four great observatories which in their minds combined all the conditions, and this decision was unanimously received by that conference. those great observatories were paris, berlin, greenwich, and washington. he stated further that, having this in view, he thought this conference should be particularly guarded, looking at the question from a scientific point of view, not to depart from the conditions laid down by the conference at rome; that he had no desire to advocate any one of the places enumerated, but merely mentioned them as satisfying all the conditions of science, which was so brilliantly represented at rome. commander sampson, delegate of the united states, then said: i can only attempt to anticipate the arguments which may be advanced by the learned delegate from france in support of his resolution to adopt a neutral meridian. but it is our simple duty, in our present judicial capacity, to examine the question of a prime meridian from all points of view. with the object, then, of considering the question from another stand-point, i ask your attention for one moment. this congress, at its last meeting, by a unanimous vote, declared its opinion that it was desirable to adopt a single prime meridian for the purpose of reckoning longitude. further, it is fair to assume that the delegates here assembled, in answer to a specific invitation from the government of the united states, and for a stated purpose, have come empowered by their respective governments to act upon the questions submitted for their consideration in the invitation. at the last meeting, the delegates from france left us somewhat in doubt regarding their views upon this important question of the powers of the delegates, or at least of their own delegation. but as they have to-day advocated the adoption of a neutral meridian, we may conclude that they have the necessary delegated power to fully consider and determine the main question before us--the selection of a prime meridian. in the absence of any declared opinion to the contrary, we may take it for granted that the delegates from all states here represented are deputed to "fix upon a meridian proper to be employed as a common zero of longitude throughout the globe," and to recommend the same for adoption to their respective governments. if, then, we are of one mind as to the desirability of a single prime meridian, and if we are fully empowered to make the selection, which may be taken as another way of saying that we are directed by our respective governments to make the selection, we may proceed directly to the performance of this duty. in the choice of a prime meridian, there is no physical feature of our earth which commends itself above others as the best starting point; nor does the form of the earth itself present any peculiarity which might be used as an initial point. if the refinements of geodesy should finally lead to the conclusion that the figure of the earth is an ellipsoid with three axes, yet the question of the direction of either of the equatorial axes must remain to such a degree uncertain that the extremity of the axis could not be assumed as the point of departure for counting longitude. indeed, as an initial meridian must above all things be fixed in position, it would not answer to make its position depend upon any physical constant which is itself in the slightest degree uncertain; for in these days, when refinements in physical measurements are constantly leading to more and more accurate results, each advance in accuracy would necessitate an annoying change in the initial meridian, or, what would more probably result, the retention of the first chosen meridian, which would thus lose its dependence upon the original definition, and become as arbitrary as if taken by chance in the first instance. we may then say that, from a purely scientific point of view, any meridian may be taken as the prime meridian. but from the standpoint of convenience and economy there is undoubtedly much room for a choice. considering this question of convenience in connection with the necessary condition of fixity already referred to, the prime meridian should pass through some well-established national observatory. in making the choice of a prime meridian which is to serve for a great period of time, it is important to so fix and define it that the natural changes of time may not render it in the least degree uncertain. to this end, the nation within whose borders the chosen point may fall should engage to establish it in the most enduring manner, and protect it against all possible causes of change or destruction. when taken in connection with other requirements, to be mentioned hereafter, this character of permanence will be best secured by making the adopted meridian pass through an observatory which is under the control of the government. such observatory should be in telegraphic communication with the whole world, in order that the differences of longitude from the prime meridian may be determined for any point. these conditions of convenience are so important that they may fairly be considered imperative. to fulfil them one of the national meridians now in use should be selected. to select any other than one of these meridians, or a meridian directly dependent upon one of them, and defined simply by its angular distance from one of these national meridians, would be to introduce endless confusion into all charts and maps now in use. to select as a prime meridian one which shall be a defined angular distance from one of the national meridians, must have for its object either to remove some inconvenience which results from the use of the national meridian itself, or it must be to satisfy a desire to deprive the selected meridian of any nationality. the inconvenience of east and west longitudes, which results from having the prime meridian pass through a thickly populated portion of the world, will be removed by reckoning the longitude continuously from o° to °. at the same time an important advantage is secured by having the prime meridian occupy a central position with regard to the most densely populated part of the earth; because the distances which will then separate the various points from the central observatory marking the initial meridian will be a minimum, and consequently less liable to error in determination. the selection of a meridian by calculation, defined as a certain number of degrees east or west of one of the national meridians, would not thereby deprive the meridian thus selected of a national character; for though we may reckon longitude from a meridian passing through the atlantic or pacific ocean, yet the initial point from which all measurements of longitude must be made would still remain one of the national meridians. again, if any other than one of the national meridians were selected, or a meridian dependent upon one of them, as, for example, a neutral meridian in the atlantic or pacific ocean, it would necessitate a change in all charts and maps. it is hardly necessary to say that no scientific or practical advantage is to be secured by adopting the meridian of the great pyramid, or by attempting to establish permanent meridian marks over a great length of the selected meridian, for even in the present advanced condition of astronomical and geodetic science it is not practicable to establish two points on the same meridian at a considerable distance from each other with such a degree of accuracy as would warrant the use of them indifferently as the initial point. as a matter of economy as well as convenience that meridian should be selected which is now in most general use. this additional consideration of economy would limit our choice to the meridian of greenwich, for it may fairly be stated upon the authority of the distinguished delegate from canada that more than per cent. of all the shipping of the world uses this meridian for purposes of navigation. the charts constructed upon this meridian cover the whole navigable globe. the cost of the plates from which these charts are printed is probably per cent. of the cost of all plates in the world for printing mariners' charts, and is probably not less than ten millions of dollars. as a matter of economy, then, to the world at large, it would be better to permit those plates to remain unchanged which are engraved for the meridian of greenwich and to make the necessary changes in all plates engraved for other meridians. a very natural pride has led the great nations to establish by law their own prime meridian within their own borders, and into this error the united states was led about years ago. should any of us now hesitate in the adoption of a particular meridian, or should any nation covet the honor of having the selected meridian within its own borders, it is to be remembered that when the prime meridian is once adopted by all it loses its specific name and nationality, and becomes simply the prime meridian. mr. rutherfurd, delegate of the united states, stated that he did not propose to take up much of the time of the conference; that he had listened with great pleasure to the exhaustive speech of his colleague, commander sampson, but that he wished to say a few words about the conditions of permanence in the prime meridian to which allusion had just been made. he said that he would call attention to the fact that the observatory at paris stands within the heart of a large and populous city; that it has already been thought by many of the principal french astronomers that it should no longer remain there; that it has been, interfered with by the tremors of the earth and emanations in the air, which prevent it from fulfilling its usefulness; that for several years past strenuous efforts have been made to remove the observatory from paris to some other place where it may be free to follow out its course of usefulness, and that the only thing which keeps it there is the remembrance of the honorable career of that observatory in times past. he added that he was sure that there was no one here who failed to recognize its claims to distinction; that there was no one here acquainted with the past history of astronomy but looks with pride upon the achievements of the human intellect effected there. at the same time, however, if a change is to be made, if sentiment should give way to practical reason, a locality, no doubt, will be found which may be calculated to fulfil the requirements of a prime meridian better than that one. as to the fitness of greenwich, he said that the observatory was placed in the middle of a large park under the control of the government, so that no nuisance can come near it without their consent, and that it was in a position which speaks for itself; that he would only add one word more in regard to this matter, and that is, that the adoption of the meridian of greenwich as the prime meridian has not been sought after by great britain; that it was not her proposition, but that she consented to it after it had been proposed by other portions of the civilized world. mr. janssen, delegate of france, said: we do not put forward the meridian of the observatory of paris as that to be chosen for the prime meridian; but if it were chosen, and we wished to compare it with that of greenwich as to the accuracy with which it is actually connected with the other observatories of europe, it would not lose by the comparison. the latest observations of the differences of longitude made by electricity by the bureau of longitudes of france and our officers have given very remarkable results of great accuracy. it is well known that what is important for a starting point in reckoning longitude is, above all things, that it should be accurately connected with points whose positions have been precisely fixed, such as the great observatories. there is, therefore, a slight confusion on the part of my eminent colleague, namely, that of not distinguishing between the conditions which require the exact connection of the starting point of longitudes with observatories, and the merits of the position of such a point in an astronomical aspect, which is here a matter of secondary importance. mr. lefaivre, delegate of france, said that he did not not know if his observation was well founded, but it seemed to him that what the delegates of france had proposed had not been contested, but that the arguments used had rather been those in favor of the adoption of the meridian of greenwich. mr. rutherfurd, delegate of the united states, said that the observations which he had made were merely to be regarded as a negative of the proposition made by the delegates of france, and not as a statement of the arguments in favor of the adoption of greenwich. the president said that the remarks of the delegate of the united states were not out of order, inasmuch as they were intended to combat the proposition brought forward by the delegate of france. mr. janssen, delegate of france, then spoke as follows: gentlemen: at the last session, when a proposition was made by my eminent colleague and friend, mr. rutherfurd, to discuss and vote upon the adoption of the meridian of greenwich as the common prime meridian, i thought it necessary to say that the proposal appeared to me prematurely made, and that we could not agree to the discussion proceeding in that manner. mr. rutherfurd has informed me that he would withdraw his proposition for the present, in order to permit me to direct the discussion, in the first place, to the principle which should direct the choice of a common prime meridian. i here take the opportunity of thanking mr. rutherfurd for his courtesy, and i no longer object to proceeding with the debate. what we ask is, that after the general declaration of the second session as to the utility of a common prime meridian, the congress should discuss the question of the principle which should guide the choice of that meridian. being charged to maintain before you, gentlemen, the principle of the neutrality of the prime meridian, it is evident that if that principle was rejected by the congress it would be useless for us to take part in the further discussion of the choice of the meridian to be adopted as the point of departure in reckoning longitude. we think, gentlemen, that if this question of the unification of longitude is again taken up after so many unsuccessful attempts to settle it as are recorded in history, there will be no chance of its final solution unless it be treated upon an exclusively geographical basis, and that at any cost all national competition should be set aside. we do not advocate any particular meridian. we put ourselves completely aside in the debate, and thus place ourselves in a position of far greater freedom for expressing our opinion, and discussing the question exclusively in view of the interests affected by the proposed reform. the history of geography shows us a great number of attempts to establish a uniformity of longitude, and when we look for the reasons which have caused those attempts (many of which were very happily conceived) to fail, we are struck with the fact that it appears due to two principal causes--one of a scientific and the other of a moral nature. the scientific cause was the incapacity of the ancients to determine exactly the relative positions of different points on the globe, especially if it was a question of an island far from a continent, and which consequently could not be connected with that continent by itinerary measurements. for example, the first meridian of marinus of tyre and ptolemy, placed on the fortunate isles, in spite of its being so well chosen at the western extremity of the then known world, could not continue to be used on account of the uncertainty of the point of departure. that much to be regretted obstacle caused the method to be changed. it became necessary to fall back on the continent. but then, in place of a single common origin of longitude indicated by nature, the first meridians were fixed at capitals of countries, at remarkable places, at observatories. the second cause to which i just now alluded, the cause of a moral nature--national pride--has led to the multiplication of geographical starting-points where the nature of things would have required, on the contrary, their reduction to a single one. in the seventeenth century, cardinal richelieu, in view of this confusion, desired to take up again the conception of marinus of tyre, and assembled at paris french and foreign men of science, and the famous meridian of the island of ferro was the result of their discussions. here, gentlemen, we find a lesson which should not be lost sight of. this meridian of ferro, which at first had the purely geographical and neutral character which could alone establish and maintain it as an international first meridian, was deprived of its original characteristic by the geographer delisle, who, to simplify the figures, placed it at degrees in round numbers west of paris. this unfortunate simplification abandoned entirely the principle of impersonality. it was no longer then an independent meridian; it was the meridian of paris disguised. the consequences were soon felt. the meridian of ferro, which has subsequently been considered as a purely french meridian, aroused national susceptibilities, and thus lost the future which was certainly in store for it if it had remained as at first defined. this was a real misfortune for geography. our maps, while being perfected, would have preserved a common unit of origin, which, on the contrary, has altered more and more. if, as soon as astronomical methods had been far enough advanced to permit the establishment of relative positions with that moderate accuracy which is sufficient for ordinary geography, (and that could have been done at the end of the th century,) we had again taken up the just and geographical conception of marinus of tyre, the reform would have been accomplished two centuries sooner, and to-day we should have been in the full enjoyment of it. but the fault was committed of losing sight of the essential principles of the question, and the establishment of numerous observatories greatly contributed to this. furnishing naturally very accurate relative positions, each one of these establishments was chosen by the nation to which it belonged as a point of departure for longitude, so that the intervention of astronomy in these questions of a geographical nature, an intervention which, if properly understood, should have been so useful, led us further away from the object to be attained. in fact, gentlemen, the study of these questions tends to show that there is an essential distinction between meridians of a geographical or hydrographical nature and meridians of observatories. the meridians of observatories should be considered essentially national. their function is to permit observatories to connect themselves one with another for the unification of the observations made at them. they serve also as bases for geodetic and topographical operations carried on around them. but their function is of a very special kind, and should be generally limited to the country to which they belong. on the contrary, initial meridians for geography need not be fixed with quite such a high degree of accuracy as is required by astronomy; but, in compensation, their operation must be far reaching, and while it is useful to increase as much as possible the number of meridians of observatories, it is necessary to reduce as much as we can the starting points for longitudes in geography. further, it may be said that as the position of an observatory should be chosen with reference to astronomical considerations, so an initial meridian in geography should only be fixed for geographical reasons. gentlemen, have these two very different functions been always well understood, and has this necessary distinction been preserved? in no wise. as observatories, on account of the great accuracy of their operations, furnish admirable points of reference, each nation which was in a condition to do it connected with its principal observatory not only the geodetic or topographical work which was done at home--a very natural thing--but also general geographical or hydrographical work which was executed abroad, a practice which contained the germ of all the difficulties with which we are troubled to-day. thus, as maps accumulated, the need of uniformity, especially in those that referred to general geography, was felt more and more. this explains why this question of a single meridian as a starting point has been so often raised of late. among the assemblies which have occupied themselves with this question, the one which principally calls for our attention is that which was held at rome last year; indeed, for many of our colleagues the conclusions adopted by the congress of rome settle the whole matter. these conclusions must, therefore, receive our special attention. in reading the reports of the discussions of that congress, i was struck with the fact that in an assembly of so many learned men and eminent theorists it was the practical side of the question that was chiefly considered, and which finally determined the character of the resolutions adopted. thus, instead of laying down the great principle that the meridian to be offered to the world as the starting-point for all terrestrial longitudes should, have above all things, an essentially geographical and impersonal character, the question was simply asked, which one of the meridians in use among the different observatories has (if i may be allowed to use the expression) the largest number of clients? in a matter which interests geography much more than hydrography, as most sailors acknowledge, because there exist really but two initial hydrographic meridians, greenwich and paris, a prime meridian has been taken, the reign (practical influence) of which is principally over the sea; and this meridian, instead of being chosen with reference to the configuration of the continents, is borrowed from an observatory; that is to say, that it is placed on the globe in a hap-hazard manner, and is very inconveniently situated for the function that it is to perform. finally, instead of profiting by the lessons of the past, national rivalries are introduced in a question that should rally the good-will of all. well, gentlemen, i say that considerations of economy and of established custom should not make us lose sight of the principles which must be paramount in this question, and which alone can lead to the universal acceptance and permanence of its settlement. furthermore, gentlemen, these motives of economy and of established custom, which have been appealed to as a decisive argument, exist, it is true, for the majority in behalf of which they have been put forward, but exist for them only, and leave to us the whole burden of change in customs, publications, and material. since the report considers us of so little weight in the scales, allow me, gentlemen, to recall briefly the past and the present of our hydrography, and for that purpose i can do no better than to quote from a work that has been communicated to me, and which emanates from one of our most learned hydrographers. "france," he says, "created more than two centuries ago the most ancient nautical ephemerides in existence. she was the first to conceive and execute the great geodetic operations which had for their object the construction of civil and military maps and the measurement of arcs of the meridian in europe, america, and africa. all these operations were and are based on the paris meridian. nearly all the astronomical tables used at the present time by the astronomers and the navies of the whole world are french, and calculated for the paris meridian. as to what most particularly concerns shipping, the accurate methods now used by all nations for hydrographic surveys are of french origin, and our charts, all reckoned from the meridian of paris, bear such names as those of bougainville, la pérouse, fleurieu, borda, d'entrecasteaux, beautemps, beaupré, duperrey, dumont d'urville, daussy, to quote only a few among those who are not living. "our actual hydrographic collections amount to more than , charts. by striking off those which the progress of explorations have rendered useless, there still remain about , charts in use. of this number more than half represent original french surveys, a large part of which foreign nations have reproduced. amongst the remainder, the general charts are the result of discussions undertaken in the bureau of the marine, by utilizing all known documents, french as well as foreign, and there are relatively few which are mere translations of foreign works. our surveys are not confined to the coasts of france and of its colonies; there is scarcely a region of the globe for which we do not possess original work--newfoundland, the coasts of guiana, of brazil, and of la plata, madagascar, numerous points of japan and of china, original charts relative to the pacific. we must not omit the excellent work of our hydrographic engineers on the west coast of italy, which was honored by the international jury with the great medal of honor at the universal exhibition of . the exclusive use of the paris meridian by our sailors is justified by reference to a past of two centuries, which we have thus briefly recalled. "if another initial meridian had to be adopted, it would be necessary to change the graduation of our , hydrographic plates; it would be necessary to do the same thing for our nautical instructions, (sailing directions,) which exceed in number. the change would also necessarily involve a corresponding change in the _connaissance des temps_." these are titles to consideration of some importance. well, if under these circumstances the projected reform, instead of being directed by the higher principles which ought to govern the subject, should take solely for its base the respect due to the established customs of the largest number and the absence on their part of all sacrifice, reserving to us alone the burden of the change and the abandonment of a valued and glorious past, are we not justified in saying that a proposition thus made would not be acceptable? when france, at the end of the last century, instituted the metre, did she proceed thus? did she, as a measure of economy and in order to change nothing in her customs, propose to the world the "pied de roi" as a unit of measure? you know the facts. the truth is, everything with us was overthrown--both the established methods and instruments for measurement; and the measure adopted being proportioned only to the dimensions of the earth, is so entirely detached from everything french that in future centuries the traveller who may search the ruins of our cities may inquire what people invented the metrical measure that chance may bring under his eyes. permit me to say that it is thus a reform should be made and becomes acceptable. it is by setting the example of self-sacrifice; it is by complete self-effacement in any undertaking, that opposition is disarmed and true love of progress is proved. i now hasten to say that i am persuaded that the proposition voted for at rome was neither made nor suggested by england, but i doubt whether it would render a true service to the english nation if it be agreed to. an immense majority of the navies of the world navigate with english charts; that is true, and it is a practical compliment to the great maritime activity of that nation. when this freely admitted supremacy shall be transformed into an official and compulsory supremacy, it will suffer the vicissitudes of all human power, and that institution, (the common meridian,) which by its nature is of a purely scientific nature, and to which we would assure a long and certain future, will become the object of burning competition and jealousy among nations. all this shows, gentlemen, how much wiser it would be to take for the origin of terrestrial longitude a point chosen from geographical considerations only. upon the globe, nature has so sharply separated the continent on which the great american nation has arisen, that there are only two solutions possible from a geographical point of view, both of them very natural. the first solution would consist in returning, with some small modification, to the solution of the ancients, by placing our meridian near the azores; the second by throwing it back to that immense expanse of water which separates america from asia, where on its northern shores the new world abuts on the old. these two solutions may be discussed; this has been often done, and again quite recently, by one of our ablest geologists, m. de chancourtois. each of these meridians combine the fundamental conditions which geography demands and upon which there has always been an agreement when national meridians are set aside from the discussion. as to the determination of the position of the point which may be adopted, the present excellent astronomical methods will give it with a degree of exactness as great as that which geography requires. but what is the necessity for a special and costly determination of the longitude of a point which can be fixed arbitrarily, provided this be done within certain limits, as for instance by satisfying the conditions of passing through a strait or an island. we may be content with fixing the position of the point adopted in an approximate manner. the position thus obtained would be connected with certain of the great observatories selected for the purpose from their being accurately connected one with another, and the relative positions thus ascertained would supply the definition of the first meridian. as to any material mark on the globe, if one be desired, though it is in no manner necessary, it would be established in conformity with this definition, and its position should be changed until it exactly complied with it. as to the question of the changes to be introduced in existing maps and charts which, by our proposition, would be imposed upon everybody, they could be very much reduced, especially if it were agreed--which would be sufficient at first--to draw upon existing charts only a subsidiary additional scale of graduation which would permit immediate use of the international meridian. later, and as new charts were engraved, a more complete scale of graduation would be given; but i think that it would always be desirable to preserve in the manner now done in many atlases both systems of reckoning longitude--the national and international. if it be necessary at the present time to facilitate the external relations of all nations, it is also well to preserve among them all manifestations of personal life, and to respect the symbols which represent their traditions and past history. gentlemen, i do not propose to dwell upon the details of the establishment of such a meridian. we have only to advocate before you the principle of its acceptance. if this principle be admitted by the congress, we are instructed to say that you will find in it a ground for agreement with france. without doubt, on account of our long and glorious past, of our great publications, of our important hydrographic works, a change of meridian would cause us heavy sacrifices. nevertheless, if we are approached with offers of self-sacrifice, and thus receive proofs of a sincere desire for the general good, france has given sufficient proofs of her love of progress to make her co-operation certain. but we shall have to regret that we are not able to join a combination which to protect the interests of one portion of the contracting parties would sacrifice the more weighty scientific character of the meridian to be adopted, a character which in our eyes is indispensable to justify its imposition upon all, and to assure it permanent success. prof. j. c. adams, delegate of great britain, stated that if he were allowed to offer a few observations upon the eloquent address made by his colleague, the representative of france, mr. janssen, he would remark that, so far as he could follow that discourse, it seemed to him to turn almost entirely on sentimental considerations; that it appeared to him that the delegate of france had overlooked one great point which was correctly laid down by the president in his opening address, viz., that one of the main objects to be kept in view in the deliberations of this conference would be, how best to secure the aggregate convenience of the world at large--how we should choose a prime meridian which would cause the least inconvenience by the change that would take place. of course, any change would necessarily be accompanied by a certain amount of inconvenience, but our object, as he understood it, was to take care that that inconvenience should be as small in its aggregate amount as possible. he stated that if that were taken as the ground of consideration by this conference, it appeared to him that the question was narrowed to one of fact rather than to be one of sentiment, which latter would admit of no solution whatever; for it was quite clear that if all the delegates here present were guided by merely sentimental considerations, or by considerations of _amour propre_, the conference would never arrive at any conclusion, because each nation would put its own interests on a level with those of every other. he added that if the conference should be able to agree in the opinion that the adoption of one meridian (for his part he did not undertake to say what meridian) would be accompanied by a greater amount of convenience in the aggregate than the adoption of any other, he thought that this should be the predominant consideration in guiding the decision of this conference, on the question referred to them, and it appeared to him that this is a consideration which the delegate of france has not put before this conference, at least not in a prominent way. it is clear that the inconvenience caused to any one nation by the adoption of a new neutral meridian would not be lessened by the fact that all other nations would suffer the same inconvenience. with respect to the question of a neutral meridian, professor adams wished to call the attention of the congress to the fact that the delegates here present are not a collection of representatives of belligerents; that they are all neutral as men should be in a matter purely scientific, or in any other matter which affects the convenience of the world at large, and that this conference is not met here at the end of a war to see how territory should be divided, but in a friendly way, representing friendly nations. he stated that he hoped the delegates would be guided in their decision by the main consideration, which was, what will tend to the greatest practical convenience of the world? that he need not address a word to the other part of the argument which he thought at first of commenting upon a little, for the delegate of the united states, commander sampson, who spoke first, had put his views so clearly before the conference that he (professor adams) would not detain it longer. he would add, however, that if the conference is to take a neutral meridian they must either erect an observatory on the point selected, which might be very inconvenient if they should choose such a point as is alluded to by the delegate of france, or if some such place was not selected, we should merely have a zero of longitude by a legal fiction, and that would not be a real zero at all; that they would have to select their zero with reference to a known observatory, and that, for instance, supposing they took a point for zero twenty degrees west of paris, of course that would be really adopting paris as the prime meridian; that it would not be so nominally, but in reality it would be, and he thought that we now-a-days should get rid of legal fictions as much as possible, and call things by their right names. mr. janssen, delegate of france, said: my eminent colleague, whose presence is an honor to this congress, professor adams, thinks that i overlook too much the practical side of the question; namely, how a prime meridian can be established so as to cause the least inconvenience. he says that i pay too much attention to what he calls a question of sentiment, and he concludes by expressing the hope that all nations will lay aside their national pride and only be guided by this consideration: what meridian offers the greatest practical advantages? my reply is that i intend no more than professor adams to place the question upon the ground of national pride; but it is one thing to speak in the name of national pride and another to foresee that this sentiment common to all men, may show itself, and that we should avoid conclusions likely to arouse it, or we may compromise our success. that is all our argument; and the history of the great nation to which professor adams belongs furnishes us with examples of considerable significance, for the french meridian of ferro was never adopted by the english, notwithstanding its happy geographical situation, and we all still awaiting the honor of seeing the adoption of the metrical system for common use in england. but let us put aside these questions which i would not have been the first to touch upon, and place ourselves upon the true ground of the importance of the proposed reform, which is the only one worthy of ourselves or of this discussion. we do not refuse to enter into an agreement on account of a mere question, of national pride, and the statement of the changes and expenses to which we should have to submit in order to accomplish the agreement is a sufficient proof of this. but we consider that a reform which consists in giving to a geographical question one of the worst solutions possible, simply on the ground of practical convenience, that is to say, the advantage to yourselves and those you represent, of having nothing to change, either in your maps, customs, or traditions--such a solution, i say, can have no future before it, and we refuse to take part in it. prof. abbe, delegate of the united states, stated that the delegate of france, mr. janssen, had made a very important proposition to the conference: that the meridian adopted should be a neutral one. he said that he had endeavored to determine what a neutral meridian is. on what principle shall the conference fix upon a neutral meridian, and what is a neutral meridian? shall it be historical, geographical, scientific, or arithmetical? in what way shall it be fixed upon? he looked back a little into the history of an important system adopted some years ago. france determined to give us a neutral system of weights and measures, and the world now thanks her for it. she determined that the base of this neutral system should be the ten-millionth part of a quadrant of the meridian. she fixed it by measurement, and to-day we use the metre as the standard in all important scientific work; but is that metre part of a neutral system? is our metric system neutral? it was intended to be, but it is not; we are using a french system. had the english, or the germans, or the americans taken the ten-millionth part of the quadrant of the meridian, they would have arrived at a slightly different measure, and there would have been an english, a german, and an american measure. we are using the french metric system. it was intended to be a neutral system, but it is a french system. we adopt it because it deserves our admiration, but it is not a neutral system. the various nations of the world might meet and agree upon some slight modification of this metric system which would agree with the results of all scientific investigations, and thus make it international instead of french; but we do not care to do that, and are willing to adopt one system, taking the standard of paris as our standard. how shall we determine a neutral system of longitude? the expression "neutral system of longitude" is a myth, a fancy, a piece of poetry, unless you can tell precisely how to do it. he would vote for a neutral system if the french representatives can tell the conference clearly how to decide that it is neutral, and satisfy them that it is not national in any way. mr. janssen, delegate of france, said: i perfectly understand the objection of my honorable colleague, prof. abbe. he asks what is a neutral meridian, and adds that the metre itself does not appear to him to be a neutral measure, but to be a french measure. he relies upon the consideration that if the english, the americans, and germans, in adopting a definition of the metre, had measured it for themselves, they would have arrived each at a slightly different result, which would have given us an english, american, and german metre; nevertheless, he adds, we use the french metre, because we find it so admirable. i would answer, first, that the metre, as far as the measure is derived from the dimensions of the earth, is not french, and it was precisely to take away this character of nationality that those who fixed on the metre sought to establish it on the dimensions of the earth itself. what is french is the particular metre of our national archives, which exhibits a very slight difference from that which our actual geodesy would have given us. also, i think that if, at the time of the adoption of the convention du mêtre, in which the nations of europe participated, we had slightly changed the length of our standard to make it agree with the result of actual geodetic measurements, we should have done an excellent thing in depriving this measure of any shadow of nationality. i agree with my honorable colleague that if a few slight changes adopted by common accord could perfect the metrical system, we french ought to have no motive for opposing it. we have the honor of having invented a system of measures which, being based upon considerations of a purely scientific nature, has been accepted by all. therefore if it can be said with truth that the metre of the archives of paris is french, (not intentionally, but because it bears the mark of an error of french origin,) it is an international metre, by the same title that the discovery of the satellites of mars made by my friend, prof. asaph hall, whom i have the pleasure of seeing here, is scientific and of a universal nature. the metre--equal to the ten-millionth part of the distance from the equator to the pole--is no more french than that distance itself, and, nevertheless, if the americans, english, or germans had measured it, they would each have arrived at a slightly different metre. now, my honorable colleague adds that a neutral meridian appears to him a myth, a fancy, a piece of poetry, so long as we have not exactly settled the method of determining it. i shall disregard the expressions which my honorable colleague has thus introduced into the discussion, because this discussion should be serious. it is plain that prof. abbe did not thoroughly apprehend the explanations which i gave of the proper methods of fixing the initial meridian, and of the conditions which make a meridian neutral; but i return to them, since i am invited to do so. our meridian will be neutral if, in place of taking one of those which are fixed by the existing great observatories, to which, consequently, the name of a nation is attached, and which by long usage is identified with that nation, we choose a meridian based only upon geographical considerations, and upon the uses for which we propose to adopt it. do you want a striking example of what differentiates a neutral meridian from a national meridian? in order to avoid the confusion which existed in geography at the beginning of the seventeenth century, on account of the multiplicity of initial meridians then in use, a congress of learned men, assembled in paris at the instance of richelieu to select a new common meridian, fixed its choice on the most eastern point of the island of ferro. this was a purely geographical meridian, being attached to no capital, to no national observatory, and consequently neutral, or, if you please, purely geographical. later, le père feuillet, sent in by the academy of sciences to determine the exact longitude of the initial point, having given the figure ° ' " west of paris, the geographer, delisle, for the sake of simplicity, adopted the round number °; and, as i stated a little while ago, this alteration completely changed the character of this prime meridian. it ceased to be neutral, and became merely the meridian of paris disguised, as has been truly said, and the english, notably, never adopted it. here is the difference, gentlemen, between a neutral meridian and a national meridian. and, parenthetically, you see, gentlemen, how dangerous it is to awaken national susceptibilities on a subject of a purely scientific nature. now allow me to add that, if in it was possible to find a neutral meridian, a purely geographical meridian, an independent meridian, it may easily be done in if we wish to do so; and that a point chosen on purely geographical considerations, either in behring's strait or in the azores, could be much better determined now than was possible to father feuillet in , and could take the position which the meridian of ferro would not have lost had it not been confounded with the meridian of paris. professor j. c. adams, delegate of great britain, stated that he merely desired to refer to one subject touched on by the delegate of france, mr. janssen, whose opinion he thought could hardly be supported, and that was that the question of longitude was purely one of geography. he desired to controvert that, and to hold that the question of longitude was purely one of astronomical observation. the difference of longitude between two places could not be determined by geodetic observations, because to do this you must take hypothesis as to the figure of the earth, and the figure of the earth is not a simple figure. you may take as hypothesis that the figure of the earth is spheroidal, and that the ratio of the axes is exactly defined. now, in the first place, we are not agreed as to the exact ratio of the axes, nor are we agreed as to the exact figure of the earth. if an attempt is made to measure the difference of longitude between two points on the earth's surface, especially when they are a considerable distance from each other, it is necessary to depend upon astronomical observations. in attempting to deduce the difference of longitude from geodetic measures, you must assume that the true figure and dimensions of the earth are known, which is far from being the case. the theory that the prime meridian is a matter purely of a geographical nature is liable to the fatal objection that the determination of the difference of longitude between one place and the other is really the determination of the difference of time of the passage of a star across the meridian of the two places concerned. that is very definite. you observe the transit of the star at one place, and you observe the transit of the star at the other place, and by means of telegraphic communications you are able to determine their difference of longitude independent of the figure of the earth. he said, in conclusion, that he thought the honorable delegate of france was mistaken upon the main point which he had just referred to, if, indeed, he had rightly understood him. m. janssen, delegate of france, replied as follows: i think that m. adams entirely misunderstands me. i agree with him absolutely in thinking that longitudes cannot be determined, especially of places far apart, except by astronomical methods. geodesy can only furnish it for short distances; in such cases, it is true, it supplies it with a degree of accuracy which meridianal observations cannot attain. so, if the question be to determine rigorously the difference of longitude in time between two places on the earth at considerable distances apart, it becomes one of astronomy, because here it is astronomy which gives the quickest and most accurate solution. for these reasons if, for instance, we should wish to connect a given observatory with a point situated on the other side of the ocean which had been chosen as the starting point of longitudes, it would become a question of astronomy. astronomy here is an admirable instrument for the solution, but it should only be the instrument. on the contrary, the question becomes geographical, if it be that of determining where it will be most convenient to fix the origin of terrestrial longitudes. if the question be, for instance, to select one or another point, in some one or other ocean, astronomy has nothing to do with it, and when it wishes to impose upon us one of its observatories to fulfil such a function it tends to give an inaccurate solution. at first sight it may seem that any point might become a starting point for terrestial longitudes, but when we study the question a little more we see there may be great advantages in choosing some one point in preference to some other. hence it is that all geographers have agreed to place initial meridians, when possible, in the oceans. the president stated that, in accordance with the decision of the conference, he had sent to the scientists named by them invitations to a seat upon this floor. the chair sees several of these gentlemen here to-day, notably one of the most eminent astronomers of this country, to whom his countrymen are always ready to do homage, professor newcomb, superintendent of the united states nautical almanac. if it be the pleasure of the congress, the chair will now request professor newcomb to give us his views upon the resolution now under discussion. no objection being made to the proposition of the president, professor newcomb arose and said: that in reference to the remarks of the distinguished delegate of france, professor janssen, he would prefer, if the conference would consent, to study his arguments more carefully when they should be in print. he remarked that some points raised by that argument have been already replied to, and he wished now more particularly to request that professor janssen would define precisely what he meant by "a neutral meridian;" that he had partially answered this question in reply to professor abeÉ; but that there was a more fundamental point, one of practice, which must be brought in and kept in mind at every step, and which was raised by commander sampson's paper, to which he had listened with great interest. commander sampson held that it would be necessary to have a fixed observatory on the chosen prime meridian, but he (professor newcomb) did not concur in that view, but rather agreed to a limited extent with what professor janssen had said on that question. in choosing a meridian from which to count longitude, you meet a difficult problem. you have a point on the globe defined as the first meridian. this would be taken as the initial point of departure, and you are to determine the longitude of a certain place from that point. now, doubtless, there is no other way to do this than to have an astronomical instrument and telegraphic communication. and if they chose the azores or behring's strait, in neither case could they mount a transit instrument or have a system of telegraphic communication. nor could we make a determination of longitude from a single fixed observatory in any case. he then stated that it was impracticable under any circumstances to have an absolutely neutral prime meridian; that the definition of the prime meridian must practically depend upon subsidiary considerations, no matter where it might be located. in the practical work of determining longitudes a connection with the prime meridian cannot be made in each case. what is really determined is the longitude from some intermediate point, generally in the same country, and in telegraphic communication with the place whose longitude we wish to know. this intermediate point would, for the time, be the practical prime meridian. but the longitude of this point itself must always be uncertain. science is continually advancing in accuracy, and we find that we continually need to correct the longitude of our intermediate meridian, and hence of all points determined from it. how can this difficulty of constantly changing longitudes be avoided? he replied that each system of connected longitudes must rest upon its own basis. it must be referred to an assumed prime meridian, and the measurements must be made from that, even if it be found to be somewhat in error. if some such system had been adopted thirty or forty years ago, we would have avoided the confusion arising from the fact that the longitudes given on many maps do not refer at all to any absolute meridian. all that is known is that the astronomers determined the longitude of the place, and then the maps had to be corrected accordingly. the longitude of one place would be determined from cambridge, and perhaps in the neighborhood is another place determined from the observatory at washington. in either case we know nothing of the longitude of cambridge or washington which the observer assumed in his calculations. generally, in determining longitude, the country adopts the principal place within its confines as a subsidiary prime meridian, and the assumed longitude of this place is necessarily selected somewhat arbitrarily. the longitude, for instance, of washington was, thirty years ago, known to be nearly hours minutes and seconds west from greenwich. had we adopted this difference by law, it would have amounted to choosing for our prime meridian a point hours minutes and seconds east of washington, whether we happened to strike the transit instrument at greenwich or not. this would have fixed an assumed longitude for the cambridge observatory and for all points within our telegraphic net-work. we should have had a practical system, which might, however, require to be corrected from time to time, if some slight error were found in the assumed longitude of washington. in the present state of astronomical observation these little errors are of no consequence except in some very refined astronomical discussions. for all geographical and perhaps geodetical purposes the error may be regarded as zero, and it may be said, in regard to astronomical work, that it will always be independent of any meridian that might be chosen. but even if this difficulty were avoided, he could not see how they could have any place which would come within the definition of a neutral meridian. supposing they took the azores, they belong to portugal; then certainly they would have a portuguese prime meridian, belonging to the portuguese nation. thus they would no longer have a neutral point, if he (professor newcomb) rightly understood the meaning of professor janssen. he said that the delegate of great britain, professor adams, had expressed very clearly his (professor newcomb's) ideas, and the difficulty we have in meeting the propositions of the french delegates; that what he had said would apply very properly to any neutral meridian that might be chosen in accordance with the plans of professor janssen. whatever that meridian might be, we must always assume for it a certain number of degrees from the capital of the country, where the place to be determined is located, and then take that imaginary meridian instead of a real point on the surface of the globe. it is true that this is perfectly practicable, and on that theory there might not be any necessity of having an astronomical observatory. but why we should go to this trouble and expense mr. janssen did not make very clear; his considerations were purely sentimental, as was remarked by the delegate of great britain, professor adams, and he (prof. newcomb) did not see what advantage would be gained by a neutral meridian in preference to one fixed by convenience. in order that a discussion may proceed, it is necessary to agree on a given basis from which to start, and it is extremely difficult to agree upon a basis if there are considerations of sentiment introduced, because such considerations are peculiar to each person. he therefore wished to propose this question again to the delegate of france, namely, what advantages can we derive from fixing upon a neutral meridian? mr. janssen, delegate of france, said: professor newcomb asks me to point out the advantages of a neutral meridian. these advantages are of two kinds--they are of a geographical nature and a moral nature. let us examine the first. by placing the initial meridian between asia and america, we get away from the centres of population, which is almost indispensable in view of the change of dates. we divide the world into two parts, the old world and the new. the advantage of drawing the prime meridian through the ocean has always been understood, and it was precisely for this reason that marinus of tyre, during the first century, placed it at the fortunate isles, west of the african continent. it is idle to urge the difficulty of fixing such a meridian as an objection. astronomy is so far advanced in our day as to enable us to make this calculation with all desirable accuracy. as to the methods of obtaining this meridian exactly, there are several. i have already spoken of them, but i return to the subject, since more details are desired. these methods fall under two principal heads. we can, and that is the ancient idea, choose some remarkable physical point--as, for instance, the extremity of an island, a strait, the summit of a mountain--and determine approximately the distance in longitude of this point from the points of reference, which are at present the observatories. this method, if all the precision that science can now attain is required, would be costly in certain cases. for the azores the expense would be small, because of the proximity of the telegraphic cables; it would be much greater for behring straits. on the hypothesis of the employment of this method, it would evidently be necessary to place our meridian at the azores. according to the other method, it is not the physical point which is fixed, but simply the distance of the assumed origin from the points of comparison. for example, admit that the general definition of our prime meridian was that it should pass through the middle of behring straits. to obtain its theoretical definition, we should obtain a position of this point, either by summary observations of the nature of hydrographic surveys, or by the aid of existing information, and the longitude thus obtained would be connected with the observatories best connected with each other. a list of the differences of longitude would become the definition of our meridian, and not the physical point in the sea which marks the exact middle of the strait. if, now, we absolutely wished for a physical point, we have the island of st. lawrence, which is cut towards its eastern part by such a meridian, and we could put a point of reference there, subject to the condition that the position of this point should conform to the definition, and that it should be removed, in one direction or the other, until it did conform to it. as to the very slight errors which might still affect the relative positions of the great observatories actually connected by electricity, they do not concern geography. if i am not mistaken, the eminent superintendent of the american nautical almanac acknowledges that we could thus avoid the difficulties which might result from the changes to which the perfecting of science would in the course of time give rise in the statement of longitudes. in this manner the expense would be nothing or small. thus, also, the meridian would be truly neutral, both by reason of its position in the ocean between the continents, and by reason of its definition, since the zero of longitude would then be so placed as to occupy a point not identified with any nation. this illustration appears to me to answer the demands of professor newcomb. i have taken it only for that reason, for i maintain no particular method, but only the principle of neutrality. finally, i must return again to those sentimental reasons which my eminent and friendly opponents so often call to my attention. if i do not err, the very warmth of these interesting discussions shows me that the honor of being personally connected with a great reform touches us more than we are willing to admit, or than practical interests alone could effect. professor adams himself supplies an illustration of this. he should remember the lively discussions of the english and french press on the occasion of the magnificent discovery of neptune, and on the claims of the two illustrious competitors who were then the objects of universal admiration. if we go back in history, do we not see the friends of newton and of leibnitz equally contesting with asperity the discovery of the infinitesimal calculus. the love of glory is one of the noblest motives of men; we must bow before it, but we must also be careful not to permit it to produce bad fruits. when our men of science sought, a hundred years ago, to determine a new measure of length, some one proposed the length of the seconds pendulum at paris. this measure was rejected, because it introduced the idea of time in a measure of length, and also because it was peculiar to paris, and because a measure acceptable to the whole world was desired. it is important not to introduce questions of national rivalries into a scientific reform intended to be accepted by all, and history shows us precisely on this question of prime meridians what active rivalries there are. there was a time when almost every nation which had a large observatory had a meridian, and that meridian was considered an object of national pride. there were the meridians of paris, of rome, of florence, of london, and so on, and no nation was willing to abandon its meridian for that of another. if you please to adopt either the meridian of greenwich, washington, paris, berlin, pulkowa, vienna, or rome, our reform may be accepted for the moment, especially if it offers immediate advantages in economy; but it will contain within it a vice which will prevent its becoming definitive, and we are not willing to participate in action which will not be definitive. whatever we may do, the common prime meridian will always be a crown to which there will be a hundred pretenders. let us place the crown on the brow of science, and all will bow before it. commander sampson, delegate of the united states, said that he thought that the delegate of france, professor janssen, had explained very fully the advantages of a neutral meridian, but he thought that he had not explained how we are to determine the neutral meridian. he added that he quite agreed with professor adams and professor newcomb, that to establish a prime meridian it is necessary to refer its position to an astronomical observatory. he stated further that if a meridian were selected passing through the atlantic or pacific ocean, it must be referred to some initial point whose longitude is known, and the consequence of that would be, it seemed to him, that the prime meridian selected would still be dependent upon some national observatory, and that to select a meridian at random without reference to any observatory would lead to the utmost confusion, and, he had no doubt, would not be entertained by any one. prof. janssen, delegate of france. when my honorable colleague, commander sampson, reads the remarks which i have just made, he will see that i have very fully shown what characterizes a neutral or geographical meridian, as contradistinguished from those meridians which, passing through capitals and observatories of different countries, bear the names of nations, whilst geographical meridians bear geographical names, such as the meridian of ferro, of the azores, behring's strait, &c. of course it would be necessary to connect the places selected with observatories, either by calculation or in some other effective manner. i said all this a few moments ago. mr. rutherfurd, delegate of the united states, then remarked that in addition to what had been said he would merely call attention to the fact that after that neutral point had been established it would cease to be a neutral meridian; that if the azores be chosen they belong to portugal, and he did not know any island in the pacific which would serve the purpose, and at the same time not be subject to this objection; that perhaps behring's strait, mentioned by the french delegate, might be less objectionable than any other place. he added that it is absolutely necessary that there should be some means of determining the difference between this adopted place and the other places, or else no use could be made of it. we must know how far other places are from the prime meridian, and for that reason it is necessary that it should be on land. now, that land must belong to some country, and after we have fixed upon it it would cease to be a neutral meridian, and it would have to be connected by telegraphic wires with all the great observatories in the world. prof. janssen, delegate of france. my honorable friend, mr. rutherfurd, says that from the time the prime meridian was chosen it would cease to be neutral. i reply that he confounds a scientific principle with a question of property in the soil. if, for reasons of a geographical nature, we should fix upon a point in the azores, that meridian would be neutral, because it would have been chosen on scientific grounds alone. the equator is neutral because geographical conditions give it that character; and, nevertheless, the countries along it belong to various nations, do they not? as to the manner of connecting the prime meridian with the system of observatories, i have already explained how this may be done in my former speech. general strachey, delegate of england, remarked that he had rather hesitated about saying anything on the subject, after the expression of so many opinions of persons better qualified to speak than himself, but he felt that he ought to make a few remarks as to the distinction which prof. janssen had attempted to establish between astronomical and geographical longitude. it appeared to him that longitude was longitude. it would never do if, for geographic purposes, we are to have a second or third-class longitude and for astronomical purposes a first-class longitude. he said that as a geographer he repudiated any such idea. when you come to the practical application of the determination of longitude at sea for maritime purposes, it is true that a much less accurate determination suffices than would suffice for the determination of longitude for astronomical observatories; but, for all that, what is the object of a ship desiring to know what its place at sea is? obviously to arrive at the port to which it is destined, and the object to be obtained is such a determination of the longitude as to enable that ship to arrive at its port without danger. you obtain a comparatively imperfect determination of longitude, but it is sufficiently accurate to prevent you from striking on the solid earth. but how is the longitude of the port to be determined? certainly, as has been properly said, by astronomical observations, which can only be made with certainty on the earth. consequently, it seemed to him that it is absolutely essential for fixing an initial meridian for the determination of longitude that it should be placed at an astronomical observatory which can be connected with other places by astronomical observations and by telegraph wires, and that the idea of fixing a neutral meridian is nothing more than the establishment of an ideal meridian really based upon some point at which there is located an observatory. this has been repeated once or twice before, and i need not enlarge upon it. prof. janssen, delegate of france. my honorable colleague, general strachey, thinks that longitude is longitude, and that there is not an astronomical longitude and a geographical longitude. i answer, that this is, nevertheless, what the nature of things indicates. the longitude of observatories, or rather the difference of longitude between those establishments, must be fixed with an accuracy which is never sufficiently great. in the bureau of longitude of france we are occupied with the differences of longitude of european observatories, and we adopt for these calculations all the latest scientific improvements, and especially the employment of electricity. geography, especially for general purposes, does not require this great accuracy, which could not be expressed on maps. all geographers agree upon that subject. a statement of the longitude is like the statement of a weight, of a measure, or of anything, and its precision must vary according to the purpose to which it is applied. is not a weighing necessary to determine a chemical equivalent of an entirely different kind from that of a commercial weighing? yet it is still a weight. is it necessary to insist on this further? it is entirely a secondary question. if general strachey, whom i had the pleasure of meeting in india, demands that the prime meridian should be connected with observatories with rigorous accuracy, this can be done if it be desired; the astronomical and electrical methods at our disposal will permit of it. prof. abbe, delegate of the united states, said that he was quite interested in the determination, if possible, of what is a neutral meridian. we are precisely in the condition in which we were years ago, when the french institute determined that the basis of the metric system should be the one ten-millionth of the quadrant of the globe. having settled upon that ideal basis, they spent years of labor, and finally legalized a standard metre, which is still preserved at paris. we have now the same problem to solve. we have before us the idea of a neutral meridian, and, if it be adopted, we must see that there be embodied in the system the distance of certain other important places with reference to it. the only suggestion given as to the location of this neutral meridian is behring's strait. this is said to be a neutral meridian, because it lies between russia and america; but how long will it remain so? perhaps a year or two, or perhaps fifty years. who knows when russia will step over and reconquer the country on this side of behring's strait? who knows when america will step over and purchase half of siberia? at any rate, that point is not cosmopolitan; something must be found which is fixed, either within the sphere of the earth or in the stars above the earth--something that is above all human considerations--otherwise we shall fail in securing a neutral meridian. commander sampson, delegate of the united states, said that he would like to ask the delegate from france, mr. janssen, where he would place the neutral meridian. the president said that the delegate of the united states, commander sampson, puts a question which seems to be somewhat categorical. at this point in the proceedings the president stated that it would be convenient if the conference would take a short recess to enable the secretaries, with himself, to consult upon the subject of the preparation and approval of the protocols. a recess was thereupon taken. after the recess, the delegate from france, prof. janssen, presented the following resolution: "_resolved_, that the decision upon the motion of the french delegates, in regard to the choice of a neutral meridian, be postponed to the next meeting of the conference." he said that as he must speak french, and as several of his colleagues could, perhaps, not entirely grasp the meaning of the discussion, he asked for the adjournment of the vote until the next meeting, so that the protocol of this meeting may be printed and distributed to the members of the conference. the president stated that as far as he understood this resolution it merely amounted to this: that no vote shall be taken upon the original resolution of the french delegate--namely, as to the adoption of a neutral meridian--until the next meeting of the conference, when the protocols in both languages will have been printed and distributed. commander sampson, delegate of the united states, inquired whether, if this resolution were adopted, it would be necessary to vote upon the original question at the next meeting. the president replied that was not necessarily the case. the delegate of france simply desires that no vote shall be taken to-day. the original subject will come up and be open for debate at the next meeting, but it seemed to the chair that it should be as far as possible exhausted to-day, so that the delegates could have the whole matter before them at the next meeting. mr. lefaivre, delegate from france, said that the arguments already presented will require time for careful consideration. consequently he asked for the adjournment of the vote, and he hoped that none of his colleagues would object to it. the president stated that he would venture to suggest, for the purpose of preventing delay, that so far as was possible any arguments that are to be offered should be made now, so that in the protocol of this day's proceedings, which will be of considerable length, these arguments may be incorporated. mr. rustem effendi, delegate of turkey, stated that it would be impossible to prepare a proper protocol of this conference without the assistance of a french stenographer, and he therefore suggested that such a stenographer be secured as early as possible. the president stated that efforts had been made to obtain a french stenographer, but without success, and that if any delegate knows of such a stenographer and will communicate with the chair it will be happy to take the necessary steps to secure his services. count lewenhaupt, delegate of sweden, then made the following statement: i beg to propose that the conference adjourn at the call of the president, that the time and hour for the next meeting be communicated to the delegates hours before the meeting, and that at the same time a proof-copy of the protocols of the present meeting be forwarded. he added that by giving the delegates hours after the protocols are printed time would be allowed them to revise the protocols and make such corrections as they thought necessary, and those corrections could be reported to the secretaries and made in the printed text. the protocol can then be finally and definitively printed and approved at the beginning of the next meeting of the conference. the proposition of the delegate of sweden was then adopted. the conference then adjourned at o'clock p. m., subject to the call of the president. iv. session of october , . the conference met pursuant to adjournment in the diplomatic hall, in the state department, at one o'clock p. m. present: austria-hungary: baron i. von schÆffer. brazil: dr. luiz cruls. chili: mr. f. v. gormas and mr. a. b. tupper. colombia: commodore franklin. costa rica: mr. j. f. echeverria. france: mr. a. lefaivre and mr. janssen. germany: baron h. von alvensleben and mr. hinckeldeyn. great britain: sir f. j. o. evans, prof. j. c. adams, lieut. general strachey, and mr. sandford fleming. guatemala: mr. miles rock. hawaii: hon. w. d. alexander and hon. luther aholo. italy: count albert de foresta. japan: professor kikuchi. liberia: mr. william coppinger. mexico: mr. leandro fernandez and mr. angel anguiano. netherlands: mr. g. de weckherlin. paraguay: capt. john stewart. russia: mr. c. de struve, major-general stebnitzki, and mr. j. de kologrivoff. san domingo: mr. m. de j. galvan. spain: mr. juan valera, mr. emilio ruiz del arbol, and mr. juan pastorin. sweden: count carl lewenhaupt. switzerland: col. emile frey. turkey: mr. rustem effendi. venezuela: dr. a. m. soteldo. united states: rear-admiral c. r. p. rodgers, mr. lewis m. rutherfurd, mr. w. f. allen, commander w. t. sampson, and prof. cleveland abbe. absent: denmark: mr. c. s. a. de bille. salvador: mr. a. batres. the president. in view of the many communications addressed to the president of this conference, having reference to the business before it, presenting statements and arguments in relation thereto, the chair asks that a committee be appointed, to which shall be referred all such communications, and that the committee be instructed to make such report upon them as it may deem advisable. count lewenhaupt, delegate of sweden. i beg leave to propose to the conference that the appointment of this committee be left to the president. mr. soteldo, delegate of venezuela. i second the motion of the delegate of sweden. mr. de struve, delegate of russia. i entertain the same opinion, and i support the motion. the motion was then unanimously adopted. the president. i will name as the members of the committee the delegate of great britain, professor adams; the delegate of germany, mr. hinckeldeyn; the delegate of the united states, professor abbe; the delegate of japan, mr. kikuchi; and the delegate of costa rica, mr. echeverria. president. alter a discussion of only three hours this conference adjourned a week ago to-day, subject to the call of its president. owing to the want of a french stenographer to report the words that were spoken in french, there has been much delay in preparing the protocol, which has not yet been completed. fortunately, an experienced french stenographer has been procured through the kind intervention of mr. sandford fleming, of the delegation from great britain, and mr. william smith, deputy minister of marine for the dominion of canada. we may now hope to have a fairly accurate report of what is said, both in french and english, needing only slight verbal corrections, and the chair trusts that delegates may find it convenient to make the corrections very promptly, so that the protocols may be printed and verified as speedily as possible. should any delegate, who has not yet spoken, desire to address the conference upon the resolution of the delegate from france, his remarks will now be received, and when the mover of the resolution shall close the debate, the vote will be taken, if such be the pleasure of the conference. mr. sandford fleming, delegate of great britain. i have listened with great attention and deep interest to the remarks which have fallen from the several gentlemen who have spoken, and i desire your kind indulgence for a few moments while i explain the views i have formed on the motion of the distinguished delegates from france. i feel that the important question which this conference has to consider must be approached in no narrow spirit. it is one which affects every nationality, and we should endeavor, in the common interest, to set aside any national or individual prejudices we possess, and view the subject as members of one community--in fact, as citizens of the world. acting in this broad spirit, we cannot fail to arrive at conclusions which will promote the common good of mankind. in deliberating on the important subject before us, it seems to me there are two essential points which we should constantly bear in mind. . we should consider what will best promote the general advantage, not now only, but for all future years, while causing at the present time as little individual and national inconvenience as possible. . we should, in coming to a determination on the main question for which this conference is called, leave nothing undone to avoid offence, now or hereafter, to the sensitiveness of individual nations. the motion is, that the initial meridian to be chosen should be selected on account of its neutrality. this undoubtedly involves the selection of an entirely new meridian, one which has never previously been used by any nation, as all initial meridians in use are more or less national, and, as such, would not be considered neutral in the sense intended by the honorable delegates from france. let us suppose that this conference adopted the motion. let us suppose, further, that we found a meridian quite independent of and unrelated to any existing initial meridian. would we then have accomplished the task for which we are met? i ask, would the twenty-six nations here represented accept our recommendation to adopt the neutral meridian? i greatly fear that the passing of the resolution would not in the least promote the settlement of the important question before the conference. the world has already at least eleven different first meridians. the adoption of the new meridian contemplated by the delegates from france would, i apprehend, simply increase the number and proportionately increase the difficulty which so many delegates from all parts of the earth are assembled here to remove. this would be the practical effect of the passing of the resolution. if it had any effect, it would increase the difficulty, and i need not say that is not the object which the different governments had in view when they sent delegates to this conference. the president has well pointed out in his opening address the advantages which would be gained, and the great dangers which, at times, would be avoided by seafaring vessels having one common zero of longitude. besides the benefits which would accrue to navigation, there are advantages of equal importance in connection with the regulation of time, to spring, i trust, from our conclusions. it does not appear to me that the adoption of the motion would in any way advance these objects. i do not say that the principle of a neutral meridian is wrong, but to attempt to establish one would, i feel satisfied, be productive of no good result. a neutral meridian is excellent in theory, but i fear it is entirely beyond the domain of practicability. if such be the case, it becomes necessary to consider how far it would be practicable to secure the desired advantages by adopting as a zero some other meridian which, while related to some existing first meridian, would not be national in fact, and would have the same effect as a perfectly neutral meridian in allaying national susceptibilities. the selection of an initial meridian related to meridians now in use gives us a sufficiently wide choice. allow me to read the following list, showing the number and the total tonnage of vessels using the several meridians named, in ascertaining their longitude. ====================================================================== | ships of all kinds. | per cent. initial meridians. +---------------------+-------------------- | number. | tonnage. | ships. | tonnage. ---------------------------+---------+-----------+--------+----------- greenwich..................| , | , , | | paris......................| , | , , | | cadiz......................| , | , | | naples.....................| , | , | | christiana.................| , | , | | ferro......................| , | , | | pulkova....................| | , | / | / stockholm..................| | , | / | lisbon.....................| | , | | copenhagen.................| | , | | / rio de janeiro.............| | , | / | / miscellaneous..............| , | , | / | / |---------+-----------+--------+----------- total ...............| , | , , | | ---------------------------+---------+-----------+--------+----------- it thus appears that one of these meridians, that of greenwich, is used by per cent. of the whole floating commerce of the world, while the remaining per cent. is divided among ten different initial meridians. if, then, the convenience of the greatest number alone should predominate, there can be no difficulty in a choice; but greenwich is a national meridian, and its use as an international zero awakens national susceptibilities. it is possible, however, to a great extent, to remove this objection by taking, for a zero of longitude and time, the meridian farthest distant from greenwich. this being on the same great circle as greenwich, it would not require the establishment of a new observatory; its adoption would produce no change in charts or nautical tables, beyond the notation of longitude. it would possess all the advantage claimed for the greenwich meridian in connection with navigation, and as a zero for regulating time it would be greatly to be preferred to the greenwich meridian. this pacific meridian being accepted as the common zero, and longitude being reckoned continuously in one direction, there would be an end to the necessity of any nation engraving on its charts the words "longitude east or west of greenwich." the one word "longitude" would suffice. the zero meridian would be international and in no respect national. even on british charts all reference to greenwich would disappear. this view of the question is sustained by many distinguished men. i shall only ask permission to read the opinion of mr. otto struvé, director of the imperial observatory at pulkova, than whom there is no higher authority. "the preference given to the greenwich meridian was based, on one side, on the historical right of the royal observatory of england, acquired by eminent services rendered by this establishment during the course of two centuries, to mathematical geography and navigation; on the other side, considering that the great majority of charts now in use upon all the seas are made according to this meridian, and about per cent. of the navigators of long standing are accustomed to take their longitude from this meridian. however, an objection against this proposition is, that the meridian of greenwich passes through two countries of europe, and thus the longitude would be reckoned by different signs in different portions of our own continent and also of africa. "moreover, the close proximity of the meridian of paris, to which, perhaps, some french geographers and navigators of other nations would still hold to, from custom, from a spirit of contradiction or from national rivalry, might easily cause sad disaster. to obviate these inconveniences, i have proposed to choose as prime meridian another meridian, situated at an integral number of hours east or west of greenwich, and among the meridians meeting this condition, i have indicated, in the first place, the meridian proposed to-day by scientific americans, as that which would combine the most favorable conditions for its adoption. thus the meridian situated ° from greenwich presents the following advantages:-- " . it does not cross any continent but the eastern extremity of the north of asia, inhabited by people very few in number and little civilized, called tschouktschis. " . it coincides exactly with that line where, after the custom introduced by a historical succession of maritime discoveries, the navigator makes a change of one unit in the date, a difference which is made near a number of small islands in the pacific ocean, discovered during the voyages made to the east and west. thus the commencement of a new date would be identical with that of the hours of cosmopolitan time. " . it makes no change to the great majority of navigators and hydrographers, except the very simple addition of twelve hours, or of ° to all longitudes. " . it does not involve any change in the calculations of the ephemerides most in use amongst navigators, viz., the english nautical almanac, except turning mid-day into midnight, and _vice versa_. in the american nautical almanac there would be no other change to introduce. with a cosmopolitan spirit, and in the just appreciation of a general want, the excellent ephemerides published at washington, record all data useful to navigators calculated from the meridian of greenwich. "for universal adoption, as proposed by the canadian institute, it recommends itself to the inhabitants of all civilized countries, by reason of the great difference in longitude, thus removing all the misunderstandings and uncertainties concerning the question, as to whether, in any case, cosmopolitan or local time was used. "in answer to the first question offered by the institute at toronto, i would, therefore, recommend the academy to pronounce without hestation in favor of the universal adoption of the meridian situated ° from greenwich, as prime meridian of the globe." i quote from the report of m. otto struvé to the imperial academy of sciences of st. petersburg, th sept., . i respectfully submit, we have thus the means of solving the problem presented to us, without attempting to find such a meridian as that contemplated in the motion of the honorable delegates. whatever its origin, the pacific meridian referred to would soon be recognized as being as much neutral as any meridian could possibly be. if, on the other hand, we adopt the motion, i very greatly fear that the great object of this conference will be defeated, and the settlement of a question so pregnant with advantages to the world will be indefinitely postponed. dr. cruls, delegate of brazil. gentlemen. since the opening of this discussion more authoritative voices than mine--among others that of the honorable mr. sandford fleming, delegate of great britain, who has just expressed his opinion upon the question--have been heard upon the important subject which we are now called upon to discuss, and of which we should endeavor to find a full and final solution. the various aspects of the projected reform--viz., the unification of longitude, which numerous international interests recommend to our care--appear to me to have been examined, and that relieves me of the task of taking up again the question in its details, and permits me to abridge very much the considerations which i think it is my duty to present in order to explain my vote. upon to the present moment we have settled one point, gentlemen, and it is one of great importance; that is, the necessity of adopting a common prime meridian. this point has obtained the support of all the delegates present at the conference. this necessity being recognized, it is proper to take another step towards the solution of the problem presented to us, and to decide what that meridian shall be. it is this choice, gentlemen, which at this moment forms the subject of our discussion, and upon which we have to decide. my honorable colleague, mr. rutherfurd, the delegate of the united states, has presented a motion proposing the adoption of the meridian of greenwich, a motion which is again made, having been withdrawn temporarily from our discussion with the consent of its proposer. the motion which was presented at the last session, and which has formed the subject of numerous interesting discussions is that made by my honorable colleague, mr. janssen, delegate of france, who proposes that the meridian adopted should have a neutral character, and should not cross either of the great continents of europe or america. this proposition, gentlemen, has been strongly resisted by the delegates of great britain and the united states, and firmly maintained by the delegates of france, and the debates which followed gave us an opportunity of being present at a scientific tournament of the highest interest. the speakers whom we have had the honor of hearing seem to me to have exhausted all the arguments for and against, and at the present stage of the discussion i presume that these debates have permitted each one of us to form, with a full knowledge of the case, an opinion upon the question on which we are called to vote. for my part, gentlemen, i desire to state clearly the attitude that brazil, in my opinion, must take in this conference. that attitude is one of absolute neutrality, inasmuch as the question is whether or not to choose a national meridian which may provoke among certain nations very legitimate rivalries. from the point of view only of the interests of brazil, the choice of one meridian rather than any other is recommended to me by no consideration. our local charts are referred to the nearest meridian, that of the observatory of rio janeiro, which is the point of departure in the geodetic or hydrographic operations in course of execution in brazil, and which all are connected with that same meridian. the marine charts of the coast most in use are the result of the hydrographic works executed by the commandant mouchez, now admiral and director of the observatory of paris. as to the telegraphic determination of the longitude of the observatory of rio, we owe it to the american commission, directed by commandant green, of the united states navy. now, gentlemen, up to the day on which the conference met for the first time, i had hoped that these discussions entered upon under the influence of a generous rivalry, and having for their only purpose the establishment of a measure, the necessity of which is strongly sought by many interests of a diverse nature, would lead to a complete and final solution of the problem. unfortunately, and i regret to be obliged to add it, the differences of opinion which have manifested themselves in this congress permit scarcely a hope of this result. for my part, gentlemen, i cannot lose sight of the fact that it is indispensable that the question for which this congress is assembled should receive a complete settlement; if not, the purpose of the congress will not be attained. since the delegates of france have manifested from the begining of our discussions their opposition to the adoption of any meridian which had a national character, which has given rise to the motion presented by mr. janssen, it follows that every measure voted by the congress tending to the adoption of a national meridian, will be, by the very fact of the abstention of france, an incomplete measure, and which will not answer the purpose sought by the conference. i hasten to add, in order to avoid all erroneous interpretations which could be given to my words, that it would be the same, if, for instance, the meridian of paris was proposed, and any great maritime nation, such as england, the united states, or any other, should abstain from voting for its adoption. in that case, also, the measure adopted would not be complete, and in that case, also, my line of conduct would be the same. to resume, i would say that the great benefits that the whole world will receive from the adoption of a common prime meridian will not be fully produced unless the measure is unanimously accepted by all the most important maritime nations. in any other event, i am, for my part, absolutely convinced that the measure adopted will be partly inefficacious, its adoption not being general, and everything will have to be done over again in the not distant future. the discussions at which we have been present abundantly prove to me that it will always be so, as long as the meridian of some great nation is proposed. in the face of this difficulty, which appears to me insurmountable, the only solution which, by its very nature, will not raise exciting questions of national pride is that of a meridian having a character of absolute neutrality. if the adoption of such a meridian was admitted in principle, i am certain that a discussion based upon pure science, and following the best conditions which it should realize, would conduct us rapidly to a practical settlement of the question. in such a discussion the arguments which ought to prevail should be, before everything, drawn from science, the only source of truth which alone can enlighten us, so as to permit us to form a sound judgment, and to decide solely upon considerations of a purely scientific nature. in addition to these considerations, i am not ignorant that there are others. i refer to questions of economy of which it is necessary to take count. as to political interests, if there are any, our eminent colleagues who represent so worthily the diplomatic element in this assembly would see that they had due weight, and, thanks to this assembly of men distinguished, some in science and others in diplomacy, there was every reason to hope that the final practical solution of the question which we are seeking would not be long in being made clear to us all by the discussions. moreover, this practical solution appears to me already to follow from what our honorable colleague, m. janssen, has told us on that subject. the principle of the neutral meridian once adopted, there would still to be discussed the conditions which it should fulfil and the determination of its position. two things must be considered, either the meridian will be exclusively over the ocean, and then, by its very nature, it will be neutral, or it will cut some island, and in that case nothing would prevent an international diplomatic convention making neutral the plot of land on which it was desirable to establish an observatory, which would in reality be a very small matter. of these two solutions, both of which satisfy the conditions which the meridian ought to fulfil in its character of neutrality and by the requirements of science, i prefer the second. i wish merely to suggest by what i have said how it would be possible to arrive at a practical solution of the question, since now i am only speaking of the adoption of the principle of the neutral meridian. i conclude, gentlemen, by declaring that i shall vote in favor of the adoption of a meridian with a character of absolute neutrality, and in doing so i hope to contribute my share to giving our resolutions such a character of independence as is necessary to make them generally acceptable in the future, and to unite in their support, at present, scientific men without distinction of nationality who are now awaiting our decision. professor janssen, delegate of france. gentlemen, i have listened with a great deal of attention to the discourse of the delegate of england, mr. fleming, and if we had not had such an exhaustive discussion last session, at which, i believe, all the reasons for and against were given, i would certainly have asked permission to answer it. but i believe that on all sides we are sufficiently enlightened on the question, and i desire above all to declare that it is not our intention of making this debate eternal. it is now for you, gentlemen, to decide. i am the more inclined to act thus, as my honorable colleague, the delegate of brazil, dr. l. cruls, who is an astronomer like myself, appears to me to have recapitulated the question with a loftiness of views, and in such happy language, that, in truth, we may take his arguments as our own. before concluding, i wish to thank my colleagues for the kind attention that they have been good enough to accord me. the president. the question recurs upon the resolution offered by the delegates of france. the resolution is as follows: "_resolved_, that the initial meridian should have a character of absolute neutrality. it should be chosen exclusively so as to secure to science and to international commerce all possible advantages, and especially should cut no great continent--neither europe nor america." the president. is the conference ready for the question? no objection being made, the roll was called, with the following result: _ayes_. brazil, san domingo. france, _noes_. austria, germany, chili, great britain, colombia, guatemala, costa rica, hawaii, italy, spain, japan, sweden, liberia, switzerland, mexico, turkey, netherlands, united states, paraguay, venezuela. russia, twenty-one noes and three ayes. the president. the resolution is, therefore, lost. mr. rutherfurd, delegate of the united states. mr. president, in presenting again the resolution which was withdrawn by me to give place to the resolution offered by our colleagues from france, having taken the advice from several members of the conference with whom i consulted, it was thought best to offer a system of resolutions which should be responsive to the mandate under which we act. with the view of bringing the subject to the notice of all the members of the conference, i caused copies of the resolutions which i hold in my hand to be sent to them. i have since heard that is has been held that these resolutions had been irregularly so communicated; that is, that the communication was made in a semi-official manner. i beg to express an entire disclaimer of anything of that sort. it was merely my individual action, and i desired to give notice of certain resolutions, with the sole view of having them fully understood before we met and to save time. i hope, therefore, that this excuse and explanation will be understood and accepted. these resolutions are founded, as far as may be, upon those adopted at rome. they differ from them only in two points. in the counting of longitude the conference at rome proposed that it should take place around the globe in one direction. this counting was to be in the direction from west to east. very singularly, i find in the report of the proceedings of the roman conference no discussion on that subject. no questions were asked, nor were any reasons given, why it should be so counted, and yet it was an entire divergence from the usage of the world at that time. the wording of the resolution of the conference at rome is substantially this: that the counting of longitude should take place from the meridian of greenwich in the single direction of west to east. it being my desire to avail myself, as far as possible, of the work of the conference at rome, i consulted with my colleagues here, and found that there was a great diversity of opinion. in the first place, some said we have always counted longitude both ways, east to west and west to east. shall we cease to do that? those who claimed that it was a more scientific way to count all around the globe immediately differed on the direction in which the longitude should be counted. without going into any argument as to which of these methods would be the best or most convenient, i propose, by the second resolution, that we should go on in the old way, and count longitude from the initial meridian in each direction. one of the objects of the third resolution is to make the new universal day coincide with the civil day rather than with the astronomical day. in the conference at rome the universal day was made to coincide with the astronomical day. it seems to me that the inconvenience of that system would be so great that we ought to hesitate before adopting it. for us in america, perhaps the inconvenience would not be so very great, but for such countries as france and england, and those lying about the initial meridian, the inconvenience would be very great, for the morning hours would be one day, and the afternoon hours would be another day. that seems to me to be a very great objection. it was simply, therefore, to obviate this difficulty that this resolution was offered. i hope, notwithstanding, that some day, not far distant, all these conflicting days, the local, the universal, the nautical, and the astronomical, may start from some one point. this hope i have the greater reason to cherish since i have communicated with the distinguished gentlemen who are here present, and it was with that hope before me that i framed the resolution so that the beginning of the day should be the midnight at the initial meridian, and not the mid-day. with this explanation, i now again move the adoption of the first resolution, which is as follows: "_resolved_, that the conference proposes to the governments here represented the adoption of the meridian passing through the centre of the transit instrument at the observatory of greenwich as the initial meridian for longitude." the president. the conference has heard the resolution. any remarks are now in order. mr. sandford fleming, delegate of great britain. i think, sir, the resolution goes a little too far at a single leap. i beg leave, therefore, to move an amendment in harmony with the resolution, at the same time leaving it to be settled by a subsequent resolution, whether the zero be at greenwich or at the other side of the globe. "that a meridian proper, to be employed as a common zero in the reckoning of longitude and the regulation of time throughout the world, should be a great circle passing through the poles and the centre of the transit instrument at the observatory of greenwich." prof. adams, delegate of great britain. mr. president, i desire merely to state, in reference to the amendment brought forward by one of our delegates, that the remaining delegates of great britain are by no means of the opinion expressed in that amendment, and that it is their intention, if it should come to a vote, to vote against it. the proposition to count longitude from a point degrees from the meridian of greenwich appears to them not to be accompanied by any advantage whatever. on the contrary, it must lead to inconvenience. you do not, by adopting the meridian opposite greenwich, get rid of the nationality of the meridian. if there is objection to the meridian of greenwich on account of its nationality, the meridian of degrees from greenwich is subject to the same objection. the one half is just as national as the other half. the president. the chair would say that no specific meridian is mentioned in the amendment. prof. adams, delegate of great britain. that is true, but, at the same time, it should be said that the meridian described is ambiguous. it is the meridian that passes through the poles and the centre of the transit instrument of the observatory of greenwich. that is the language of the amendment. but it is intended to apply to only one-half of the great circle passing through the poles, that is to the distant half of the meridian rather than to the nearer half. unless it defines which half it is intended to take, the amendment is ambiguous, and it is not proper to be voted on. mr. miles rock, delegate of guatemala. mr. president, it may be well to hear the words of the original resolution, in order that we can clearly see the relation of the amendment to that resolution. the original resolution of the delegate of the united states was then read. baron von alvensleben, delegate of germany. mr. president, i think that in this amendment offered by the delegate of great britain two questions are mixed up together. the first thing for us to do is to fix upon a prime meridian; the second thing to settle is the question whether the adoption of a universal day is desirable or not. if we adopt this amendment, these two questions are involved in one vote. therefore, i think that they should be divided, for they are not appropriate in the form in which they are presented. mr. valera, delegate of spain. i ask permission to speak, in order to explain my vote. the government which i represent here has told me to accept the greenwich meridian as the international meridian for longitudes, but i think it my duty to say that, though the question does not arise in this debate, that spain accepts this in the hope that england and the united states will accept on their part the metric system as she has done herself. i only wish to state this, and i have no intention of making it a subject of discussion. i shall only add that i believe italy is similarly situated with spain in this matter. the president. the chair would say with great deference to the distinguished delegate from spain that the question of weights and measures is beyond the scope of this conference. the invitation given by the government of the united states to the nations here represented was for a distinct and specific purpose, the selection of a prime meridian, a zero of longitude throughout the world and a standard of time-reckoning. so far as the chair is informed, it would not be in order at this conference to discuss a question of metric system. mr. juan valera, delegate of spain. my only intention in making these remarks was to verify a fact. i know very well that we have not to discuss that question. besides, the government which i represent expresses only a hope, and i know we do not insert any hopes in our protocols; but i thought it my duty to make this declaration. mr. lefaivre, delegate of france. i desire to make some remarks on the question when it is put to a vote; for the time being i shall only say a few words on the remarks of my honorable colleague, the delegate of spain, mr. valera. i believe that though the question of weights and measures is not before the conference, it is allowable for a member to state, in the name of his government, the conditions to which his vote has been subordinated. even though the question is not under discussion, it may appear from such an explanation that the vote is conditional, instead of being a simple affirmation. if my honorable colleague has received from his government instructions to subordinate his vote to such or such a condition, even when the question to which it is subordinated is not submitted to the conference, it follows from it, according to me, and everybody will admit it, that the consequences of that vote are at least conditional. mr. valera, delegate of spain. my government has charged me to express here its hopes and desires, but the vote which i have given is not, in my opinion, conditional; for i have received instructions to pronounce in favor of the greenwich meridian to measure the degrees of longitude. however, it was necessary for me to say at the same time that it was with the hope that england and the united states would adopt the french weights and measure. general strachey, delegate of great britain. while i entirely agree with the view which the chair has taken of the question whether the adoption of metrical weights and measures is before this conference--namely, that it is beyond our competence to discuss it--yet i am glad to have the opportunity of saying that i am authorized to state that great britain, after considering the opinions which were expressed at rome, has desired that it may be allowed to join the convention du mètre. the arrangements for that purpose, when i left my country, were either completed, or were in course of completion, so that, as a matter of fact, great britain henceforth will be, as regards its system of weights and measures, exactly in the same position as the united states. in great britain the use of metrical weights and measures is authorized by law. contracts can be made in which they are used, and the department which regulates the weights and measures of great britain is charged, consequently, with the duty of providing properly authenticated standard metric weights and measures for purposes of verification. it is quite true that the government of england does not hold out any expectation that she will adopt the compulsory use of the metric system, either at the present time, or, so far as that goes, at any future time; but it is a well known fact--and in saying this i shall be supported, i have no doubt, by the views of the eminent scientific men of my own country who are here present--that there is a strong feeling on the part of scientific men of england that, sooner or later, she will be likely to join in the use of that system, which, no doubt, is an extremely good one, and which, so far as purely scientific purposes are concerned, is largely in use at the present time. mr. valera, delegate of spain. i desire to thank the honorable delegate of england, general strachey, for the friendly words which he has just pronounced, and to felicitate myself for having manifested the desire and hope of my government that england should accept the weights and measures which have been accepted in spain and in other parts of the european continent. mr. lefaivre, delegate of france. mr. chairman, i cannot pretend to make any suggestion of any technical value on the question now before us. i only rise to add a few words to the views which have been so authoritatively expounded to you by prof. janssen, in order to explain clearly the situation of the french government in this important discussion. it is henceforth evident, after the instructive debate at which we have just assisted, that the meridian of greenwich is not a scientific one, and that its adoption implies no progress for astronomy, geodesy, or navigation; that is to say, for all the branches and pursuits of human activity interested in the unification at which we aim. thus, science is absolutely disinterested in the selection which we are now discussing and that fact i wish to emphasize particularly, as we are about to take a vote which we can easily anticipate by the one we had a few minutes ago, in order that the opponents of the resolution may not be accused of obstructing progress and the great aims of science for private interests. if, on the contrary, any conclusion is to be drawn from the instructive debate at which we have assisted, it is that the principal, i will say more, the only merit of the greenwich meridian--and our colleague from great britain just now reminded us of it by enumerating with complacency the tonnage of british and american shipping--is that there are grouped around it, interests to be respected, i will acknowledge it willingly, by their magnitude, their energy, and their power of increasing, but entirely devoid of any claim on the impartial solicitude of science. to strengthen my assertion, gentlemen, i fall back upon the arguments brought forward by mr. hirsch in his remarkable report to the geodetic conference at rome, arguments that evidently carried the vote of that assembly. the greenwich meridian, says that report, corresponds to an empire that embraces twenty million square kilometres and a population of two hundred and fifty millions. her merchant marine, which counts , ships of a tonnage from six to nine million tons, and crews of , men, surpasses in importance all the other marines put together. other states, equally important by their merchant marine, especially the united states, make use of the greenwich meridian. well, gentlemen, if we weigh these reasons--the only ones that have been set forth, the only ones that at present militate for the greenwich meridian--is it not evident that these are material superiorities, commercial preponderances that are going to influence your choice? science appears here only as the humble vassal of the powers of the day to consecrate and crown their success. but, gentlemen, nothing is so transitory and fugitive as power and riches. all the great empires of the world, all financial, industrial, and commercial prosperities of the world, have given us a proof of it, each in turn. so long as there are not in polities or commerce any scientific means by which to fix, to enchain fortune, i see no reason to fix, to enchain, to subordinate, so to say, science to their fate. the character of the proposed determination of the initial meridian is so evident, that the reporter of the conference at rome, mr. hirsch, admits it implicitly, for recognizing that the adoption of the meridian of greenwich is a sacrifice for france, he asks that england should respond by a similar concession, by favoring the definitive adoption of the metric system, and by acceding to the convention of the metre which furnishes to all states metric standards rigorously compared. thus, mr. hirsch, in a spirit of justice, wished to make for each a balance of profit and loss--evident proof that the question was of a commercial, and of no scientific advantage. i am not aware, and my mission is not to discover, whether the bargain might have been accepted by france. however, it is with great pleasure that i heard our colleague from england declare that his government was ready to join the international metric convention, but i notice, with sorrow, that our situation in this congress is not as favorable as that of rome, since the total abandonment of our meridian is proposed without any compensation. at rome the adoption of the metric system of weights and measures, of which france had the glorious initiative, was held out to us, but here we are simply invited to sacrifice traditions dear to our navy, to national science, by adding to that immolation pecuniary sacrifices. we are assuredly very much flattered that there should be attributed to us sufficient abnegation to elevate us to that double heroism. we wish that we were able to justify such a flattering opinion, and especially we should like to be encouraged by examples. there are at this very moment magnificent transformations to be realized for the progress of science, and of the friendly relations of nations--unification of weights and measures, adoption of a common standard of moneys, and many other innovations of a well recognized utility, infinitely more pressing and more practical than that of meridians. when the discussion of these great questions is begun, let each nation come and bring its share of sacrifices for this international progress. france, according to her usage, i may say so without vain glory as without false modesty, france will not remain behind. for the present we decline the honor of immolating ourselves alone for progress of a problematic, and eminently secondary order; and it is with perfect tranquillity of conscience that we declare that we do not concur in the adoption of the meridian of greenwich, persuaded as we are that france does not incur the reproach of retarding and of obstructing the march of science by abstaining from participating in this decision. the president. unless some other delegate desires to speak, the question will be put upon the amendment of the delegate of great britain, mr. fleming. the question was then put, and the amendment was lost. the president. the chair sees upon the floor to-day, as the guest of this conference, one of the most distinguished scientists, who was invited to be present at our meetings, sir william thomson, whose name is known the world over in connection with subjects kindred to this we are now discussing. if it be the pleasure of the conference to ask sir william thomson briefly to express his views, the chair would be very happy to make the invitation. the chair, hearing no dissent, takes pleasure in introducing sir william thomson. sir william thomson. mr. president and gentlemen, i thank you for permitting me to be present on this occasion, and i thank you also for giving me the opportunity of expressing myself in reference to the subject under discussion. i only wish that the permission which you have so kindly given me may conduce to the objects of this conference more than i can hope any words of mine can do. the question immediately under discussion is, i understand, the proposal that the meridian passing through the centre of the instrument at the observatory of greenwich shall be adopted as the initial meridian of longitude, and it does seem to me that this is a practical question; that this resolution expresses a practical conclusion that it is expected by the world the present conference may reach. it is expected that the resolutions adopted will be for the general convenience, and not for the decision of a scientific question. it is the settlement of a question which is a matter of business arrangement. the question is, what will be most convenient, on the whole, for the whole world. it cannot be said that one meridian is more scientific than another, but it can be said that one meridian is more convenient for practical purposes than another, and i think that this may be said pre-eminently of the meridian of greenwich. i do most sincerely and fervently hope that the delegates from france and from the other nations who voted for the preceding resolution will see their way to adopt the resolution that is now before the conference. it does seem to me that it is a question of sacrifice, and i do trust that the honorable delegate from france who spoke last, mr. lefaivre, will see that france is not being asked to make any sacrifice that it was not prepared to make. in the admirable and interesting addresses which mr. janssen has given to this conference, (which i had not the pleasure or satisfaction of hearing, but which i have read with great interest,) the readiness of france to make a much greater sacrifice than that which is now proposed was announced. the amount of sacrifice involved in making any change from an existing usage must always be more or less great, because it cannot be said that it is a matter of no trouble to make such a change; but what i may be allowed to suggest is that the sacrifice which france was ready to make would be very much greater than that which would be made by adopting the resolution now pending. if the resolution for a neutral meridian had been adopted, all nations would have to make the sacrifice necessary for a change to a meridian not actually determined, and the relations of which could not be so convenient with those meridians already adopted as are the relations between the meridians now in use with that of greenwich. it does seem to me that if the delegates of france could see their way to adopt this resolution, they would have no occasion whatever to regret it. i sympathize deeply with what has been said in regard to a common metrical system. i have a very strong opinion upon this subject, which i will not express, however, if it meets any objection from the chair; but it seems to me that england is making a sacrifice in not adopting the metrical system. the question, however, cannot be put in that way. we are not here to consider whether england would gain or lose by adopting the metrical system. that is not the way to view this question at all, because whether england should adopt the metrical system is a matter for its own convenience and use, and whether it adopts it or not, other nations are not affected by its course. it would not at all be for the benefit or the reverse of other nations. the president. the chair would be very glad to hear sir wm. thomson's views on this subject if it were before the conference for discussion, but it is not. sir william thomson. i beg pardon for having mentioned it. i would repeat that the adoption of the meridian of greenwich is one of convenience. the difference of other meridians from it is readily ascertained, and therefore it seems to me that the minimum of trouble will be entailed on the world by the general adoption of the meridian of greenwich. this would require the minimum of change, and, furthermore, the changes which would be necessary are already wholly ascertained. i would inquire of the chair whether it would be in order for me to allude to the resolutions number and , which have been read? the president. i think that we must confine ourselves to the subject immediately under discussion--the adoption of a prime meridian. sir william thomson. then i have only to thank you and the delegates for allowing me to speak, and to express my very strong approbation of the resolution that has been proposed. sir f. j. o. evans, delegate of great britain, then made the following remarks: in view of the interesting information furnished to the congress by m. janssen on the hydrographic labors of france, past and present, and of the results as represented by the number of government charts; it has appeared to myself--as having held the office of hydrographer to the admiralty of great britain for many years--in which opinion i am supported by my colleagues, that i should place at the disposal of the congress certain statistical facts bearing on the great interests of navigation and commerce, as illustrated by the number of marine charts, of sailing directions, and of nautical almanacs annually produced under the authority of the british government, and of their distribution. i would wish to disclaim any comparison in this respect with the labors of other countries. from personal knowledge i am aware that all nations--with only one or two exceptions--are, and especially so in the last few years, diligent in the development of hydrography, and that a cordial interchange of the results unfettered by any conditions is steadily being pursued. with this preface i would lay before you the following statements, observing that the shores of the whole navigable parts of the globe are embraced in the series of admiralty charts referred to: the number of copper chart plates in constant use is between , and , . this number keeps up steadily. about new plates are added every year. average number of copper plates annually receiving correction amount to , . total number of charts annually printed for the daily use of the ships of her majesty's fleet in commission, and for sale to the general public, has for some years ranged between , and , . the sale of admiralty charts to the public through an authorized agent, both in london and at other commercial ports in the kingdom, has been for the last seven years as follows: ................................ , ................................ , ................................ , ................................ , ................................ , ................................ , ................................ , of these numbers, about one-fifth have been purchased by the governments or agents of austria, france, germany, italy, russia, turkey, and the united states. the appended list, which was furnished to me by the admiralty chart agent during the present year, gives the more precise particulars. +-------+-------+------+-------+------+-------+-------+--------+-------+ | | |ger- |united | | | | | | |years. |france.|many. |states.|italy.|russia.|turkey.|austria.|total. | +-------+-------+------+-------+------+-------+-------+--------+-------+ | ..| , | , | , | , | , | | | , | | ..| , | , | , | , | , | | | , | | ..| , | , | , | | , | | | , | | ..| , | , | , | | , | | | , | | ..| , | , | , | , | , | , | | , | | ..| , | , | , | , | , | | , | , | | ..| , | , | , | , | , | , | , | , | | | | | | | | | | | |( st | | | | | | | | | |quar.) | , | , | , | | , | | | , | | +-------+------+-------+------+-------+-------+--------+-------+ | | , | , | , | , | , | , | , | , | +-------+-------+------+-------+------+-------+-------+--------+-------+ but the chart resources of the british admiralty, great as they are, do not suffice to meet the requirements of the smaller class ships of the mercantile marine of great britain. there are three commercial firms in london who publish special charts, based, however, on admiralty documents, to satisfy this demand. on inquiry i found that these firms publish charts, which, from their large size, require about copper plates. i am not able to furnish the number of charts sold by these firms, but it is large. supplementary to the admiralty charts, there are volumes of sailing directions. several of these volumes exceed pages, and have passed through several editions. private commercial firms also, in addition to their charts, publish directions for many parts of the globe. these include regions with which the admiralty have not yet, notwithstanding great diligence, been able to deal. the annual sales of nautical almanacs for the past seven years have been: ................................ , ................................ , ................................ , ................................ , ................................ , ................................ , ................................ , i think, sir, that these are salient points, which will assist the conference in coming to a clearer view of the great interest which navigation and commerce have in the charts of a particular country. the question was then put on the adoption of the resolution offered by the delegate of the united states, mr. rutherfurd, as follows: "that the conference proposes to the governments here represented the adoption of the meridian passing through the transit instrument at the observatory of greenwich as the initial meridian for longitude." the roll was called, and the different states voted as follows: in the affirmative-- austria, mexico, chili, netherlands, costa rica, paraguay, columbia, russia, germany, spain, great britain, sweden, guatemala, switzerland, hawaii, turkey, italy, venezuela, japan, united states. liberia, in the negative-- san domingo. abstaining from voting-- brazil, france. the result was then announced, as follows: ayes, ; noes, ; abstaining from voting, . the president then announced that the resolution was passed. mr. de struve, delegate of russia. in the name of the delegates for russia i have now, at this point of the discussion, to say a few words. if we had to consider the scientific side alone of the questions, which have already been discussed and resolved by the prominent scientists of the different countries at the general conference of the international geodetical association at rome, in , we might as well simply adhere to the resolutions of the roman conference, and limit our work to the shaping of these resolutions into the form of a draft of an international convention, to be submitted for approbation to our respective governments. but, as we have, besides, to consider the application of the intended reform to practical life, we beg to submit the following suggestions to the kind attention of the conference. it is important to find for the more densely populated countries the simplest mode possible of transition from local to universal time, and _vice versa_; and we believe, therefore, that it would be convenient for the practical purposes of the question to adopt for the beginning of the universal day the midnight of greenwich, and not the noon, as was deemed advisable by the conference of rome. this modification would offer for the whole of europe and for the greatest part of america the advantage of avoiding the double date in local and universal time during the principal business hours of the day, and would afford great facilities in the transition from local time to universal. in adopting the universal time for the astronomical almanacs and for astronomical ephemerides, and in counting the beginning of the day from the midnight of greenwich, there would be, it is true, a modification of the astronomical chronology, as heretofore used; but we think it easier for the astronomers to change the starting point, and to make allowance for these hours of difference in their calculations, than it would be for the public and for the business men, if the date for the universal time began at noon, and not at midnight. the conference at rome proposes to count the longitudes from o° to ° in the direction from west to east. it seems to us that this system can lead to misunderstanding in the local and universal chronology for the countries beyond the ° east of greenwich. we believe that a more practical result of the reform could be easily obtained by modifying the clause iv of the resolutions of the roman conference, and by maintaining the system already in use for a long time, which is to count the longitudes from ° to ° to east and west, adopting the sign + for eastern longitudes, and the sign - for western longitudes thus the transition from universal to local time could be exactly expressed by the formula: universal time = local time - longitude. the adoption of this modification would necessitate that the change of the day of the week, historically established on or about the anti-meridian of greenwich, should henceforth take place exactly on that meridian. we are in favor of the adoption of the universal time (clause v of the resolutions of the roman conference) side by side with the local time, for international telegraphic correspondence, and for through international lines by railroads and steamers. we fully accept the resolution of the roman conference concerning the introduction of the system of counting the hours of the universal day from to ; and we think it desirable that the same system should be introduced for counting the hours in ordinary life. this would greatly contribute to the disappearance of the arbitrary division of the day into two parts, a. m. and p. m., and to an easier transition from local to universal time. we think it advisable to mark on all general maps the meridians in time as well as in degrees of longitude, which would render the reform familiar to the public, and facilitate its introduction in the education of the young. on maritime charts the longitudes ought to be given in degrees, as these are necessary for the determination of distances in maritime miles. the topographical maps may maintain temporarily their national meridian, in consequence of the difficulties of the modification of the co-ordinates for plates already engraved; but it would be necessary to mark on every sheet the difference between the national and the initial universal meridian in degrees of longitude. it would be most desirable to have in all new geographical catalogues of astronomical and geodetical points the longitudes given in degrees as well as in time, and that in these new catalogues the new initial meridian be taken as the starting point for the longitudes. the president. the chair has listened with great interest and pleasure to the paper which has just been read by the delegate of russia, mr. de struve, but the chair begs to state that there is no resolution before the conference. the president. the chair will now direct the second resolution to be read. the resolution was read, as follows: "from this meridian" (_i.e._, the meridian passing through the centre of the transit instrument at the observatory at greenwich) "longitude shall be counted in two directions up to degrees, east longitude being plus and west longitude minus." mr. rutherfurd, delegate of the united states. mr. president, in submitting this resolution to the conference, i wish to say that the remarks of the delegate of russia have increased my confidence in the belief of its propriety. mr. w. f. allen, delegate of the united states. mr. president, the establishment of a prime meridian has, from the force of circumstances, become of practical importance to certain interests entrusted with vast responsibilities for the safety of life and property. these interests bear an important relation to the commerce of the world, and especially to the internal commerce of an extent of country embracing within its limits about sixty-five degrees of longitude. exactness of time reckoning is an imperative necessity in the conduct of business. on november , , the several railway companies of the united states and the dominion of canada united in the adoption of the mean local times of the seventy-fifth, ninetieth, one hundred and fifth, and one hundred and twentieth meridians, west from greenwich, as the standards of time for the operation of their roads. the system under which they have since been working has proved satisfactory. they have no desire to make any further change. a large majority of the people in the several sections of the country through which the railways pass have either by mutual consent or special legislation adopted for their local use, for all purposes, the standards of time employed by the adjacent roads. upon the public and working railway time-tables generally the fact has been published that the trains are run by the time of the seventy-fifth or ninetieth, etc., meridians, as the case may be. the same standards are used by the railway mail service of the united states post-office department, which had previously used washington time exclusively for through schedules. it will at once be apparent how undesirable any action would be to the transportation interests of this country, which should so locate the prime meridian as to require these time-standard meridians to be designated by other than exact degrees of longitude. that these standard meridians should continue to be designated as even multiples of fifteen degrees from greenwich is regarded as decidedly preferable. to change to different standards, based upon exact degrees of some other prime meridian, would require an amount of legislation very difficult to obtain. at a convention of the managers of many important railway lines which control through their connections fully three-fourths of the entire railway system of this country, held in philadelphia on october , , certain action was taken, of which i have the honor to present a duly attested copy. "at a meeting of the _general railway time convention_, held in _philadelphia, october th, _, the following minute was unanimously adopted: "_whereas_, an international conference is now in session at washington, d. c., for the purpose of fixing upon a prime meridian and standard of time-reckoning; and "_whereas_, the railway companies of the united states and canada have adopted a system of time standards based, respectively, upon the mean local times of the th, th, th, and th meridians west from greenwich, and this system has proved so satisfactory in its working as to render any further change inexpedient and unnecessary; therefore "_resolved_, that it is the opinion of this convention that the selection of any prime meridian which would change the denomination of these governing meridians from even degrees and make them fractional in their character would be disturbing in no small measure to the transportation lines of the united states and canada. "_resolved_, that a duly attested copy of these resolution be presented to the conference." p. p. wright, _chairman._ attest: henry b. stone, _secretary pro tempore_. count lewenhaupt, delegate of sweden. mr. president, i propose as an amendment to the resolution just offered the fourth resolution adopted by the congress at rome: "it is proper to count longitude from the meridian of greenwich in one direction from west to east." baron h. von alvensleben, delegate of germany. mr. president, i beg to state that i think that this is only a question of detail; and, if the question is put to the conference, i shall not be able to vote, and i shall abstain from voting. the president. may i ask the delegate from germany whether his remark applies to the amendment? baron h. von alvensleben, delegate of germany. yes, sir; to the amendment, and to the resolution, also. prof. adams, delegate of england. mr. president, i must say that i am very much inclined to agree with the delegate of germany in the opinion that this is only a question of detail. it is a mere matter of convenience whether we count longitudes in one direction only, or in two opposite directions, considering longitudes measured in one direction as positive and in the opposite direction as negative. these two methods are nominally different from each other, but in reality there is no contradiction between them. in the mathematical reckoning of angles we may agree to begin at zero, and reckon in one direction round the entire circumference of degrees, but this does not prevent a mathematician, if he finds it convenient for any purpose, from reckoning angles as positive when measured in one direction, and negative when measured in the opposite direction. if angles be considered positive when reckoned towards the east, it is quite consistent with this usage that they should be considered negative when reckoned towards the west. it is much more convenient to consider all angles as positive in astronomical tables, but for other purposes it may be more convenient to employ negative angles also, especially when, by so doing, you avoid the use of large numbers. in comparatively small countries, like great britain for instance, it is more convenient when giving the longitude of a place in the west of england to consider it as being a few degrees west of greenwich, rather than and some degrees to the east of that meridian. commander sampson, delegate of the united states. mr. president, while i think the question of reckoning longitude is a matter of detail, i think it devolves upon us to decide it one way or the other. navigators are more interested in the question than mathematicians, and the longitudes must be engraved upon our hydrographic charts. now, as the learned delegate of great britain, prof. adams, who has just spoken, has stated, the principle involved is the same, whether we reckon east or west, or reckon continuously in the same direction. it seems to me, however, that when we come to consider the reckoning of longitude in connection with the adoption of a universal day, we should then make a decided choice in favor of counting longitude from zero to degrees. if we adopt the resolution which my friend, the delegate of the united states, mr. rutherfurd, has offered, it will be in perfect conformity with the habits of the world. for that reason, and it is a very strong reason, i think it might be adopted; but a little consideration will show that if we reckon the longitude from zero to degrees, east to west, then we will change the existing practice of reckoning longitude; but, of course, only in one hemisphere, and that will be eastward of the prime meridian; but, as we shall all remember, to the eastward of the prime meridian we have the main portions of the continents of asia, europe, and africa, and in all the navigable water lying in the other hemisphere the longitude will continue to be reckoned as now. to navigators of the water lying to the eastward of the prime meridian there will be a change in the method of counting longitude both ways, it would be necessary to adopt two different rules for converting local into universal time. prof. adams, delegate of great britain. oh! no; by no means. commander sampson, delegate of the united states. for although one rule would answer, by having regard to the algebraical sign affecting the longitude, it must be remembered that this rule is to be applied by many who are not accustomed to distinguishing east and west longitudes by a difference of sign, and who would therefore require one rule when the longitude is east and another when it is west. if, however, we adopt the method of reckoning from zero to degrees, from east to west, the relation existing between the local and the universal time becomes the simplest possible. to obtain the universal date and hour, under these circumstances, it only becomes necessary to add the longitude to the local time, understanding by local time the local date as well as the local hour. i think, for this reason, it will be preferable to reckon the longitude in one direction from east to west, instead of west to east. sir frederick evans, delegate of great britain. i would like to present a few words on behalf of seamen. there is clearly an important change proposed by the amendment. in the resolution before us it is simply a question of the reckoning of longitude as now employed by seamen of all nations, and i think it would be well to keep that fact separate from the reckoning of time. the president. the chair begs to state that the discussion is now upon the amendment of the delegate of sweden, count lewenhaupt, to adopt the fourth resolution of the congress at rome. sir frederick evans, delegate of great britain. then i consider that, in the interest of seamen, it would be very undesirable to accept the amendment. we must recollect that an immense deal of the world's traffic is carried around the world entirely by sea, and that this proposed dislocation of the methods of seamen by reckoning longitude in one direction only would, to say the least, be extremely inconvenient, and it would require considerable time for them to get into the habit of doing so. i think, however, that as to the question of time, there would be no difference of opinion; doubtless, it is the easier method; but, as we have to look at the practical side of this calculation of longitude, i must certainly disagree with the amendment and vote for the original resolution. mr. juan pastorin, delegate of spain, then presented the following amendment: "_resolved_, that the conference proposes to the governments here represented that longitude shall be counted from the prime meridian westward, in the direction opposite to the terrestrial rotation, and reckoned from zero degrees to degrees, and from zero hours to hours." the president. the question before the conference now is the amendment of the delegate of sweden. if the delegate of spain desires to offer his resolution as an amendment to the amendment already offered, the chair will place it before the conference. mr. juan pastorin, delegate of spain. i am in accord with the views expressed by our colleague, commander sampson, and i propose the resolution which i have just presented. mr. valera, the delegate of spain. i believe the amendment proposed by my colleague, mr. pastorin, delegate of spain, does not apply to the amendment of the delegate of sweden, but to the original resolution. in order to avoid all ambiguity it would be much better to discuss them one after the other. therefore let us decide the question whether it is better to count up to ° in each direction or up to ° continuously. then we can go on to something else. the president. in order to meet the views expressed by mr. valera, the delegate of spain, mr. pastorin will withdraw his amendment, and the delegate of sweden, count lewenhaupt, will propose the substance of his original resolution so modified in form that its details may be considered separately. mr. juan pastorin, delegate of spain. in conformity with the statement of the president, i now withdraw my amendment. count lewenhaupt, delegate of sweden. i beg to offer the following propositions in the form of amendments to the original resolution offered by the delegate of the united states; these may be discussed in succession: " . that from this prime meridian (the greenwich meridian) longitude shall be counted in one direction." " . that such longitude shall be counted from west to east." or, in place of no. -- " . that such longitude shall be counted from east to west." the president. the delegates from sweden and spain have agreed as to the first part of the resolution, that longitude shall be counted in one direction--that is, from zero to degrees. the question before the conference is now upon the first clause of the resolution, and the other two will be subsequently discussed. general strachey, delegate of great britain. i think it is impossible to proceed to a vote upon these propositions without bearing in mind what is to be decided as to the universal day. that day, as it appears to me, will have to be determined with reference to the initial meridian in such manner as to prevent, as far as possible, inconvenience from discontinuity of local time and date in passing around the world. no matter how longitude is calculated, you must necessarily arrive at discontinuity at some point in passing around the great circle of the earth. it seems to me that the most convenient way of counting both longitude and time is that the discontinuity in both shall take place on the same point on the earth. now, certainly, as was observed at rome, it will be far less inconvenient if the discontinuity of date takes place on the meridian of degrees from greenwich. then the reckoning of local time all around the world, going from west to east in the direction of the earth's rotation, will be continuous. in any other way, as far as i can see, there will be a discontinuity at some point on the inhabited part of the earth. if the discontinuity were to take place on the meridian of greenwich, as has been proposed by the conference at rome, the dates will change there during the daytime. that, as it appears to me, will be extremely inconvenient. in order to harmonize what i have called the discontinuity of date with the discontinuity in the reckoning of longitude, it appears to me that it will be best to reckon the longitude in both directions. there will be no discontinuity then except on the th meridian. it would be very inconvenient for a great part of the civilized world if the resolution which has been offered should be adopted, if, as i presume it would do, it caused discontinuity both in longitude and local time in europe. after all, what are we here to endeavor to do? notwithstanding what has been said in the other direction, for my part i must say that the great object before us is to secure the greatest convenience of the whole civilized world, and it seems to me that we should try to obtain it. if there is no very strong reason for altering the existing system of counting longitudes, it appears to me that this is a very excellent reason in favor of maintaining it. i do not see myself that, for any practical purpose, anything would be gained by reckoning longitude from zero to degrees. there may be some special scientific purposes for which it may be convenient, but the object which this resolution is intended to meet is of another character. what we want is longitude for ordinary purposes, and on that hangs the reckoning of universal time, which, of course, should be for the general use of the whole world. professor adams, delegate of great britain. mr. president, i doubt whether i should trouble the conference in reference to this point. i think, however, that it is a matter of little importance whether we consider longitude as positive, when reckoned toward the east, and negative, when reckoned to the west, or go on in one direction from zero to degrees; it amounts, mathematically speaking, to the same thing. we never can consider mathematical lines or angles as positive in one direction, without implying that in the opposite direction they are negative. one of these is merely the complement of the other. for myself, i would say that there is no use in the conference resolving that we should count longitude only in the eastwardly direction. the conference may say that if longitude is reckoned towards the east, it shall be considered positive, and, if reckoned towards the west, negative; and that is all we should say. i do not think it is within the competence of the conference to say that mathematicians shall reckon longitude only in one direction. whether you choose to reckon right through to degrees or not is a matter of detail, and of no importance in a scientific point of view. you can adopt one style or the other, according to which is found the more convenient in practice. mr. sandford fleming, delegate of great britain. i would suggest that this matter of detail can very well be discussed and arranged by a committee, otherwise, it may take up the whole time of the conference. i move, therefore, that a committee be appointed to take up this matter and report upon it at the next meeting. the president. the chair desires only to carry out the wish of the conference, but it does not see clearly what we should gain by a committee. still, if it be the desire of the conference to order a committee, then the question will arise as to the organization of that committee, and the chair would feel some hesitation in appointing it. mr. rutherfurd, delegate of the united states. mr. president, if this was a new question, in regard to which we had heard no discussion, it would be eminently proper that we should put it into the hands of a committee to formalize and thereby to shorten our deliberations; but it seems to me that the appointment of a committee now would not help us at all. when the report of that committee came in, we should have to proceed exactly as we do now. there are only three questions before the conference, and they come within very narrow limits. first, shall we count longitude both ways? second, shall we count it all around the degrees? third, if so, in which direction is the counting to take place? these are the only three questions, and, after all, they are questions of convenience. we are just as capable of voting upon these propositions now as we should be after the appointment of a committee. baron von schÆffer, delegate of austria-hungary. mr. president, i move that we adjourn until to-morrow at one o'clock p.m. the question upon the motion to adjourn was then put and adopted, and the conference accordingly adjourned at . p.m. until tuesday, the th inst., at one o'clock p.m. v. session of october , . the conference met, pursuant to adjournment, in the diplomatic hall of the department of state, at one o'clock p. m. present: austro-hungary: baron ignatz von schÆffer. brazil: dr. luiz cruls. chili: mr. f. v. gormas and mr. s. r. franklin. costa rica: mr. juan francisco echeverria. france: mr. a. lefaivre, mr. janssen. germany: baron h. von alvensleben, mr. hinckeldeyn. great britain: sir f. j. o. evans, prof. j. o. adams, lieut.-general strachey, mr. sandford fleming. guatemala: mr. miles rock. hawaii: hon. w. d. alexander, hon. luther aholo. italy: count albert de foresta. japan: professor kikuchi. liberia: mr. wm. coppinger. mexico: mr. leandro fernandez, mr. angel anguiano. netherlands: mr. g. de weckherlin. paraguay: capt. john stewart. russia: mr. c. de struve, major-general stebnitzki, mr. kologrivoff. san domingo: mr. de j. galvan. salvador: mr. atonio batres. spain: mr. juan valera, mr. emilo ruiz del arbol, mr. juan pastorin. sweden: count carl lewenhaupt. switzerland: mr. emile frey. turkey: rustem effendi. united states: rear-admiral c. r. p. rodgers, mr. lewis m. rutherfurd, mr. w. f. allen, commander w. t. sampson, professor cleveland abbe. venezuela: señor dr. a. m. soteldo. absent: denmark: mr. c. s. a. de bille. the president: the chair begs leave to announce that, in the regular order of business, the first matter before the conference to-day would have been the proposition of the delegate of great britain, mr. sandford fleming, that a committee be appointed to consider a report upon the resolution offered by him yesterday. the chair understood, however, from mr. fleming this morning that he had no desire to press that proposition, and, therefore, it may be considered as withdrawn. the question then would be upon the amendment offered by the delegate of spain, mr. juan pastorin, and if that amendment be withdrawn upon the amendment offered by the delegate of sweden, count lewenhaupt. the chair understands that both of those gentlemen desire to withdraw their propositions temporarily, and, in that event, the first action to be taken will be upon the resolution offered by the delegate of the united states, mr. rutherfurd. mr. rustem effendi, delegate of turkey. in voting yesterday in favor of the resolutions proposed by the hon. delegate of the united states, i wish to have it well understood that my vote does not bind my government. i am, indeed, obliged to vote against any proposition which would tend to bind it in any way, for i desire to leave it free to act in the matter. i engage to submit to my government the result of our deliberations and to recommend their adoption, but that is all. in other words, i have only voted "_ad referendum_," and i ask that my statement be entered in the protocol. the president. the chair would inform the delegate who has just spoken that the same statement was made by several delegates at a former meeting of the conference. m. janssen, delegate of france. i believe that the very correct doctrine just enunciated by the delegate of turkey, mr. rustem effendi, is the one adopted by all the members of the congress, and that we have all voted "_ad referendum_." the president. the chair so understood the general sense of the conference as expressed at one of our former meetings, when many of the delegates made the same declaration. mr. antonio batres, delegate of salvador. mr. president, i could not be present yesterday, on account of illness, and i now request permission to register my name in favor of the resolution adopting the meridian of greenwich as the prime meridian. the president. the delegate of salvador, mr. batres, informs the chair that he was not able to be present yesterday, on account of illness, and he desires that his name may be recorded as voting for the meridian of greenwich. if there be no objection to the request of the delegate to salvador, his vote will be so entered. no objection being made, the president instructed the secretary to make the proper entry in the protocol. the president. the delegate of spain, mr. pastorin, has withdrawn his amendment, and the delegate of sweden, count lewenhaupt, has also withdrawn the amendment which he offered to the resolution of the delegate of the united states, mr. rutherfurd. the resolution originally offered will now be read. the secretary then read the resolution, as follows: "_resolved_, that from this meridian [_i.e._, the meridian of greenwich] longitude shall be counted in two directions up to degrees, east longitude being plus, and west longitude minus." mr. sandford fleming, delegate of great britain, representing the dominion of canada. i wish to offer some observations on the resolution before the conference, but i am unable to separate the particular question from the general question. to my mind, longitude and time are so related that they are practically inseparable, and when i consider longitude, my thoughts naturally revert to time, by which it is measured. i trust, therefore, i may be permitted to extend my remarks somewhat beyond the immediate scope of the resolution. i agree with those who think that longitude should be reckoned in one direction only, and i am disposed to favor a mode of notation differing in other respects from that commonly followed. if a system of universal time be brought into use, advantages would result from having the system of time and the system of terrestrial longitude in complete harmony. the passage of time is continuous, and, therefore, i think longitude should be reckoned continuously. to convey my meaning fully, however, it is necessary that i should enter into explanations at some length. ten days back i ventured informally to place my views, with a series of recommendations on this subject, before the delegates. i hope i may now be permitted to submit them to the conference. the president. the chair would inquire of the conference whether the recommendations and remarks which were sent in print to the delegates a few days ago by mr. sandford fleming, the delegate of great britain, may be entered upon the protocol as presented to-day. each member was, it is understood, furnished with a copy of these papers. mr. tupper, delegate of chili. the delegates of chili have not received them. the president. the chair will take care that they are sent. no objection was made to the request of the delegate of great britain, mr. sandford fleming, who continued as follows: the adoption of a prime meridian, common to all nations, admits of the establishment of a system of reckoning time equally satisfactory to our reason and our necessities. at present we are without such a system. the mode of notation followed by common usage from time immemorial, whatever its applicability to limited areas, when extended to a vast continent, with a net-work of lines of railway and telegraph, has led to confusion and created many difficulties. further, it is insufficient for the purposes of scientific investigation, so marked a feature of modern inquiry. taking the globe as a whole, it is not now possible precisely to define when a year or a month or a week begins. there is no such interval of time as the commonly defined day everywhere and invariable. by our accepted definition, a day is local; it is limited to a single meridian. at some point on the earth's surface one day is always at its commencement and another always ending. thus, while the earth makes one diurnal revolution, we have continually many days in different stages of progress on our planet. necessarily the hours and minutes partake of this normal irregularity. clocks, the most perfect in mechanism, disagree if they differ in longitude. indeed, if clocks are set to true time, as it is now designated, they must, at least in theory, vary not only in the same state or county, but to some extent in the same city. as we contemplate the general advance in knowledge, we cannot but feel surprised that these ambiguities and anomalies should be found, especially as they have been so long known and felt. in the early conditions of the human race, when existence was free from the complications which civilization has led to; in the days when tribes followed pastoral pursuits and each community was isolated from the other; when commerce was confined to few cities, and intercommunication between distant countries rare and difficult; in those days there was no requirement for a common system of uniform time. no inconvenience was felt in each locality having its own separate and distinct reckoning. but the conditions under which we live are no longer the same. the application of science to the means of locomotion and to the instantaneous transmission of thought and speech have gradually contracted space and annihilated distance. the whole world is drawn into immediate neighborhood and near relationship, and we have now become sensible to inconveniences and to many disturbing influences in our reckoning of time utterly unknown and even unthought of a few generations back. it is also quite manifest that, as civilization advances, such evils must greatly increase rather than be lessened, and that the true remedy lies in changing our traditional usages in respect to the notation of days and hours, whatever shock it may give to old customs and the prejudices engendered by them. in countries of limited extent, the difficulty is easily grappled with. by general understanding, an arrangement affecting the particular community may be observed, and the false principles which have led to the differences and disagreements can be set aside. in great britain the time of the observatory at greenwich is adopted for general use. but this involves a departure from the principles by which time is locally determined, and hence, if these principles be not wrong, every clock in the united kingdom, except those on a line due north and south from greenwich, must of necessity be in error. on the continent of north america efforts have recently been made to adjust the difficulty. the steps taken have been in a high degree successful in providing a remedy for the disturbing influences referred to, and, at the same time, they are in harmony with principles, the soundness of which is indisputable. when we examine into time in the abstract, the conviction is forced upon us that it bears no resemblance to any sort of matter which comes before our senses; it is immaterial, without form, without substance, without spiritual essence. it is neither solid, liquid, nor gaseous. yet it is capable of measurement with the closest precision. nevertheless, it may be doubted if anything measurable could be computed on principles more erroneous than those which now prevail with regard to it. what course do we follow in reckoning time? our system implies that there are innumerable conceptions designated "time." we speak of solar, astronomical, nautical, and civil time, of apparent and mean time. moreover, we assign to every individual point around the surface of the earth separate and distinct times in equal variety. the usages inherited by us imply that there is an infinite number of times. is not all this inconsistent with reason, and at variance with the cardinal truth, that there is one time only? time may be compared to a great stream forever flowing onward. to us, nature, in its widest amplitude, is a unity. we have but one earth, but one universe, whatever its myriad component parts. that there is also but one flow of time is consistent with the plain dictates of our understanding. that there can be more than one passage of time is inconceivable. from every consideration, it is evident that the day has arrived when our method of time-reckoning should be reformed. the conditions of modern civilization demand that a comprehensive system should be established, embodying the principle that time is one abstract conception, and that all definite portions of it should be based on, or be related to, one unit measure. on these grounds i feel justified in respectfully asking the consideration of the conference to the series of recommendations which i venture to submit. the matter is undoubtedly one in which every civilized nation is interested. indeed, it may be said that, more or less, every human being is concerned in it. the problem is of universal importance, and its solution can alone be found in the general adoption of a system grounded on principles recognized as incontrovertible. such principles are embodied in the recommendations which i am permitted to place before the conference. they involve, as an essential requirement, the determination of a unit of measurement, and it is obvious that such a unit must have its origin in the motion of the heavenly bodies. no motion is more uniform than the motion of the earth on its axis. this diurnal revolution admits of the most delicate measurement, and, in all respects, is the most available for a unit measure. it furnishes a division of time definite and precise, and one which, without difficulty, can be made plain and manifest. a revolution of the earth, denoted by the mean solar passage at the prime or anti-prime meridian, will be recognizable by the whole world as a period of time common to all. by general agreement this period may be regarded as the common unit by which time may be everywhere measured for every purpose in science, in commerce, and in every-day life. the scheme set forth in the recommendations has in view three principal objects, viz: . to define and establish an universal day for securing chronological accuracy in dates common to the whole world. . to obtain a system of universal time on a basis acceptable to all nations, by which, everywhere, at the same time, the same instant may be observed. . to establish a sound and rational system of reckoning time which may eventually be adopted for civil purposes everywhere, and thus secure uniformity and accuracy throughout the globe. but, in the inauguration of a scheme affecting so many individuals, it is desirable not to interfere with prevailing customs more than necessary. such influences as arise from habit are powerful and cannot be ignored. the fact must be recognized that it will be difficult to change immediately the usages to which the mass of men have been accustomed. in daily life we are in the habit of eating, sleeping, and following the routine of our existence at certain periods of the day. we are familiar with the numbers of the hours by which these periods are known, and, doubtless, there will be many who will see little reason in any attempt to alter their nomenclature, especially those who take little note of cause and effect, and who, with difficulty, understand the necessity of a remedy to some marked irregularity which, however generally objectionable, does not bear heavily upon them individually. for the present, therefore, we must adapt a new system, as best we are able, to the habits of men and women as we find them. provision for such adaptation is made in the recommendations by which, while local reckoning would be based on the principles laid down, the hours and their numbers need not appreciably vary from those with which we are familiar. thus, time-reckoning in all ordinary affairs in every locality may be made to harmonize with the general system. standard time throughout the united states and canada has been established in accord with this principle. its adoption has proved the advantages which may be attained generally by the same means. on all sides these advantages have been widely appreciated, and no change intimately bearing upon common life was ever so unanimously accepted. certainly, it is an important step towards the establishment of one system of universal time, or, as it is designated in the recommendations, cosmic time. the alacrity and unanimity with which the change has been accepted in north america encourages the belief that the introduction of cosmic time in every-day life is not unattainable. the intelligence of the people will not fail to discover, before long, that the adoption of correct principles of time-reckoning will in no way change or seriously affect the habits they have been accustomed to. it will certainly sweep away nothing valuable to them. the sun will rise and set to regulate their social affairs. all classes will soon learn to understand the hour of noon, whatever the number on the dial, whether six, as in scriptural times, or twelve, or eighteen, or any other number. people will get up and retire to bed, begin and end work, take breakfast and dinner at the same periods of the day as at present, and our social habits and customs will remain without a change, depending, as now, on the daily returning phenomena of light and darkness. the one alteration will be in the notation of the hours, so as to secure uniformity in every longitude. it is to be expected that this change will at first create some bewilderment, and that it will be somewhat difficult to be understood by the masses. the causes for such a change to many will appear insufficient or fanciful. in a few years, however, this feeling must pass away, and the advantages to be gained will become so manifest that i do not doubt so desirable a reform will eventually commend itself to general favor, and be adopted in all the affairs of life. be that as it may, it seems to me highly important that a comprehensive time system should be initiated to facilitate scientific observations, and definitely to establish chronological dates; that it should be designed for general use in connection with railways and telegraphs, and for such other purposes for which it may be found convenient. the cosmic day set forth in the recommendations would be the date for the world recognizable by all nations. it would theoretically and practically be the mean of all local days, and the common standard to which all local reckoning would be referable. with regard to the reckoning of longitude, i submit that longitude and time are so intimately related that they may be expressed by a common notation. longitude is simply the angle formed by two planes passing through the earth's axis, while time is the period occupied by the earth in rotating through that angle. if we adopt the system of measuring time by the revolution of the earth from a recognized zero, one of these planes--that through the zero--may be considered fixed; the other--that through the meridian of the place--being movable, the longitudinal angle is variable. obviously the variable angle ought to be measured from the fixed plane as zero, and as the motion of the earth by which the equivalent time of the angle is measured is continuous, the longitude ought to be reckoned continuously in one direction. the direction is determined by the notation of the hour meridians, viz., from east to west. if longitude be so reckoned and denoted by the terms used in the notation of cosmic time, the time of day everywhere throughout the globe would invariably denote the precise longitude of the place directly under the mean sun. conversely, at the epoch of mean solar passage at any place, the longitude being known, cosmic time would be one and the same with the longitude of the place. the advantages of such a system of reckoning and nomenclature, as suggested in the recommendations which i now submit, will be, i think, self-evident. recommendations for the regulation of time and the reckoning of longitude . _that a system of universal time be established, with the view of facilitating synchronous scientific observations, for chronological reckonings, for the purpose of trade and commerce by sea and land, and for all such uses to which it is applicable._ . _that the system be established for the common observance of all peoples, and of such a character that it may be adopted by each separate community, as may be found expedient._ . _that the system be based on the principle that for all terrestrial time reckonings there be one recognized unit of measurement only, and that all measured intervals of time be directly related to the one unit measure._ . _that the unit measure be the period occupied by the diurnal revolution of the earth, defined by the mean solar passage at the meridian twelve hours from the prime meridian established through greenwich._ . _that the unit measure defined as above be held to be a day absolute, and designated a cosmic day._ . _that such cosmic day be held as the chronological date of the earth, changing with the mean solar passage at the anti-meridian of greenwich._ . _that all divisions and multiples of the cosmic day be known as cosmic time._ . _that the cosmic day be divided into hours, numbered in a single series, one to twenty-four, ( to ,) and that the hours be subdivided, as ordinary hours, into minutes and seconds. note.--as an alternative means of distinguishing the cosmic hours from the hours in local reckonings, they may be denoted by the letters of the alphabet, which, omitting i and v, are twenty-four in number._ . _that until cosmic time be admitted as the recognized means of reckoning in the ordinary affairs of life, it is advisable to assimilate the system to present usages and to provide for the easy translation of local reckonings into cosmic time, and vice versa; that, therefore, in theory, and as closely as possible in practice, local reckonings be based on a known interval in advance or behind cosmic time._ . _that the surface of the globe be divided by twenty-four equidistant hour meridians, corresponding with the hours of the cosmic day._ . _that, as far as practicable, the several hour meridians be taken according to the longitude of the locality, to regulate local reckonings, in a manner similar to the system in use throughout north america._ . _that, in all cases where an hour meridian is adopted as the standard for regulating local reckonings, in a particular section or district, the civil day shall be held to commence twelve hours before and end twelve hours after the mean solar passage of such hour meridian._ . _that the civil day, based on the prime meridian of greenwich, shall coincide and be one with the cosmic day. that civil days on meridians east of greenwich shall be (according to the longitude) a known number of hours, or hours and minutes in advance of cosmic time, and to the west of greenwich the contrary._ . _that the surface of the globe being divided by twenty-four equidistant meridians (fifteen degrees apart) corresponding with the hours of the cosmic day, it is advisable that longitude be reckoned according to these hour meridians._ . _that divisions of longitude less than an hour (fifteen degrees) be reckoned in minutes and seconds and parts of seconds._ . _that longitude be reckoned continuously towards the west, beginning with zero at the anti-prime meridian, twelve hours from greenwich._ . _that longitude, generally, be denoted by the same terms as those applied to cosmic time._ i submit these recommendations suggestively, and without any desire unduly to press them. i shall be content if the leading principles laid down be recognized by the conference. with regard to the more immediate question, i have come to the firm conviction that extreme simplicity of reckoning and corresponding benefits would result if longitude be notated in the same manner, and denoted by the same terms as universal time. if, therefore, the conference adopts the motion of the distinguished delegate of the united states, which, i apprehend, is designed to cause as little change as possible in the practices of sea-faring men, i trust the claims of other important interests will not be overlooked. i refer to all those interests, so deeply concerned in securing accurate time on land, and in having easy means provided for translating any one local reckoning into any other local reckoning, or into the standard universal time. in this view i trust the conference will give some expression of opinion in favor of extending around the globe the system of hour meridians which has proved so advantageous in north america. in an educational aspect alone it seems to me important that the hour meridians, one to twenty-four, numbered from the anti-prime meridian continuously toward the west, should be conspicuously marked on our maps and charts. prof. adams, delegate of great britain. i wish, mr. president, to express my entire adhesion to the proposition which has been made by the delegate of the united states, mr. rutherfurd. it seems to me to satisfy one of the principal conditions that we have had before us to guide our decision; that is, that we should pursue a course which will produce the least possible inconvenience. now, i think if we keep that in mind, we shall have very little difficulty in coming to the conclusion that we should reckon longitude eastward, as positive or plus, and westward as negative or minus. this mode of reckoning would be attended with the least inconvenience; in fact, it will not be attended with any inconvenience at all, because it will keep to the present mode of reckoning. for my part, i see no adequate reason for changing that. there is no scientific reason, and certainly there is no practical reason. there is no scientific reason, because, as i stated yesterday, if in mathematics you measure from the zero a distance in one direction and consider that positive, you must, by the very nature of the case, consider the distance measured in the opposite direction from the same zero as negative. one follows mathematically and necessarily from the other, and by adopting this resolution you thus include both in one general formula. it seems to me quite as scientific, to say the least, to start from zero and go in both directions, distinguishing the longitudes by the signs plus and minus, according as the directions are taken east or west, as to reckon longitudes in one direction only from zero to degrees. it is, i say, just as scientific to do this, and practically it is more convenient. because if you go on reckoning from zero to degrees continuously, you have to make a break at degrees. you do not count on after you have completed one revolution, but have to drop the degrees and start again at zero. but this is attended with great inconvenience, because this break in counting occurs in countries which are thickly inhabited. the longitude would be a little less than degrees on one side of the prime meridian, and on the other side the longitude would be a small angle. this seems to me very inconvenient. on the other hand, if you count longitudes in one direction from zero to degrees as positive, and in the opposite direction from zero to degrees as negative, you are, no doubt, obliged to make a break in passing abruptly from plus degrees to minus degrees. but the break would then occur where it would cause the least inconvenience, viz., in mid-ocean, where there is very little land and very few inhabitants, and where we are accustomed to make the break now. this will require no change in the habits and customs of the people, and no inconvenience whatever would be caused by the action of the conference if it decides on this method, which also has the minor advantage of not requiring the use of such large numbers as the other. but to adopt the reckoning of longitude from zero to degrees would involve a very considerable change, and i think it may be doubted whether it would be generally accepted. under the circumstances, i think the resolution contains the most expedient course for us to adopt. i do not object to anybody who chooses to do so reckoning on, for certain purposes, from zero to degrees, but i do not think it would be well to make it compulsory. with regard to the proposal of the delegate of great britain, mr. fleming, i would say that it would be attended with great inconvenience, because it departs from the usages and habits now existing. that, to my mind, is a very great and insuperable objection, and i do not see any countervailing advantage. with regard to the subject of time that mr. fleming is anxious to take into consideration, i think that nothing can be simpler, if i may be allowed to deal with the question of time, than the relation between time and longitude which is proposed to be created by the resolution of mr. rutherfurd. by that resolution the longitude indicates the relation between the local time and the universal time in the simplest possible way. what can be easier than the method involved in the resolution of mr. rutherfurd? it is this: local time at any place is equal to universal time plus the longitude of the place, plus being understood always in a mathematical sense. the longitude is to be added to the universal time if it is positive, and subtracted if it is negative. that is very simple, the whole being involved in one general formula. now, i think it is perfectly impossible for mr. fleming to make a more simple formula than that. the formula laid down in the proceedings of the roman conference was far less simple, as it involved an odd twelve hours. you got the universal time equal to the local time, minus the longitude, plus twelve hours. this is far from simple. it makes the calculation more complicated, and it seems to me that for other reasons it is objectionable. mr. rutherfurd, delegate of the united states. mr. president, i do not propose to take up the time of the conference in reiterating the very conclusive remarks in favor of this resolution made by the delegate of great britain. i wish, however, to allude, for a moment, to another view of this question. suppose we do not adopt this resolution. what is the course before the conference? we shall then be called upon, no doubt, to decide that longitude shall be counted all around the world from zero to degrees. that general proposition is one which would not probably meet with violent opposition, but the next point is one that will divide us very materially, and perhaps disastrously. which way shall we count? shall it be towards the east or towards the west? my conversations with the gentlemen here present have lead me to know that there is a very great difference of opinion upon this point, and i believe that if we should not adopt this resolution and should decide to count longitude from zero to degrees, a preference to count it in one direction rather than the other would be established only by a very close vote, nearly annulling the whole moral influence of the conference, and we should go back to our governments without much, if any, authority on the point in question. and i doubt whether our resolutions would be accepted by these governments if we show ourselves to be divided upon a question of so much practical importance. it is simply a question of practice--of convenience. we all bowed to the rule of convenience in selecting the meridian of greenwich. and why? because seven-tenths of the civilized nations of the world use this meridian, not that it was intrinsically better than the meridian of paris, or washington, or berlin, or st. petersburg. nobody claimed any scientific preference among these meridians. it was simply because seven-tenths of the civilized world were already using the meridian of greenwich. if we accept this argument in favor of the first resolution for selecting the initial meridian, why should we not be equally inclined to recognize the fact that all the civilized world count longitude in both ways? there is no difference of opinion on that point. there is no difference of usage. shall we break that usage? shall we introduce a new system, which may or may not be found practical or agreeable? shall we not rather adopt the rule of all nations, already in use among their practised astronomers and navigators, by saying continue to do as you have already done? sir frederick evans, delegate of great britain. having for many years mixed among the practical seamen of more than one nation, i confess i look with some dismay on any other system for the notation of longitude being adopted than the one proposed in this resolution. my colleague, mr. fleming, made the remark that he could not disassociate longitude from time. if he had mixed with seamen, he would have found out that there is very frequently a well-defined difference between the two in their minds. longitude with seamen means, independently of time, space, distance. it indicates so many miles run in an east or west direction. consequently, i am not able to look upon longitude and time as being identical. under these circumstances, this resolution also, as i understand it, should be considered on practical grounds. the question of universal time will come on for consideration hereafter, and how that may be settled seems to me a matter of indifference compared with the decision on this resolution. i question, for myself, whether any other plan than that it proposes would be generally accepted. that is what i am afraid of. whatever respect nations may have for this conference, public opinion would be very strong upon the point now at issue. when you further recollect that all around the globe, in all these various seas, there are colonies with histories; that their geographical positions and boundaries were originally recorded by longitude according to the notation of which i have spoken, i think it is to be over sanguine to expect that those colonies will accept a new notation of longitude without greater proof of the positive necessity of the change. it would not be the fiat of this conference, or the fiat of any government, that would bring about the change. i say this with all deference to the opinions of those who have advocated a change. general strachey, delegate of great britain. at the risk of repeating somewhat my remarks made to the congress when we last met, i would add a few words to what has now been said. it is our wish that the points of real difference should, as far as possible, be clearly brought out before the conference comes to a vote. as regards the counting of longitude in two directions, and the degree of advantage or disadvantage that may arise in starting from zero and treating east longitude as positive or plus, and west longitude as negative or minus, let me ask the attention of the congress to the fact that longitude is already counted in these two directions, and that, as a matter of fact also, latitude is counted in the same way, in both directions from the equator, north latitude being plus and south latitude minus. nobody, so far as i have heard, has ever proposed that we should abolish this method of reckoning latitude, and substitute for it north or south polar distance, to be counted right round the earth; and yet there is the same _quasi_ scientific objection to the present method of counting in the one case as in the other. as already stated, it seems to me that, for purposes of practical convenience, it is extremely difficult, if not impossible, to separate the ideas on which the reckoning of longitude must be based, from those which must regulate the reckoning of time, and especially the reckoning of time in the sense of adopting a universal day over the whole world. now, it appears to me that, as regards the acceptance of the universal day, it certainly will be anything but convenient, if it begins and ends otherwise than when the sun passes the th meridian. on the contrary, i think it will be extremely inconvenient. i think that if the world were to adopt the meridian of greenwich as the origin of longitude, the natural thing for it to do would be to have the international day, the universal day, begin from the th meridian from greenwich--that is, to coincide with the greenwich civil day. that meridian passes, as i said before, outside of new zealand, and outside of the fijee islands; it goes over only a very small portion of inhabited country. it appears to me, therefore, that inasmuch as there must be an absolute break or discontinuity in time in passing round the earth--a break of twenty-four hours--it is much more convenient that this break should take place in the uninhabited part of the earth than in the very centre of civilization. if we adopt the universal day which coincides with the civil day at greenwich, then you will be able to have complete continuity of local time over the whole earth, in harmonious relation with the universal day, except at the break which necessarily takes place on the th meridian. otherwise this will not be possible. for instance, according to the system proposed by the resolution, the local time corresponding, say, to hours of monday at greenwich, would, in passing round the earth to the eastward from the th meridian, gradually change from hours of sunday to hours of monday; and, on returning to that meridian, the break of time would occur, and one day would appear to be lost. but complete continuity both in the days and hours, and harmony with the universal day, that is, the greenwich civil day, would be preserved for the whole earth, excepting on crossing the th meridian. the result of the system which was proposed at rome would be to cause the break of dates to take place at greenwich at noon, so that the morning hours of the civil day would have a different universal date from the afternoon hours, and this would be the case all over europe. but if the universal day be made to correspond to the civil day of greenwich, and the longitude is counted east in one direction and west in another direction to the th meridian, these difficulties would be overcome, and a perfectly simple rule would suffice for converting local into universal time. as regards what was said upon the subject of longitude being plus or minus, according as you move to the east or west, it appears to me that there is a positive, clear, and rational reason for calling longitude eastward plus and longitude westward minus. the time is later to the east, and therefore the hour is indicated by a higher number. in converting universal into local time, if the place is east of greenwich, you add the longitude to the universal time, and therefore increase the number of the hour; if the place be west of greenwich, you subtract the longitude, and therefore diminish the number of the hour. it is natural, therefore, to call east longitude positive and the other negative. it appears to me also that the passage of the sun over the meridian is, in reality, what may be called the index of the day, the day consisting of hours, distributed equally on either side of the meridian. noon of the universal day would thus coincide with the time of the sun passing the initial meridian. there is perfect consistency, therefore, in adopting the reckoning of longitude and time that is proposed in the resolution before us. it is a rational and symmetrical method. mr. juan pastorin, the delegate of spain. i listened with great pleasure to the observations which our honorable colleague, the delegate of england, general strachey, has just made. i am not sufficiently acquainted with the english tongue to make a speech, though i know it well enough to follow the debate. moreover, as i had beforehand studied the subject which is now before us, i have quite well understood all that has been said on this point. i proposed an amendment yesterday, in order to obtain what i consider the most simple formula for converting local time into cosmical time. this formula is not, perhaps, the most suitable for astronomers and sailors, but they form the minority, and it is, i am sure, the easiest for the mass of the people. this formula would be based on the considerations which are now under discussion. i am not sufficiently familiar with the language to give the reasons upon which i based my amendment, but, as i demonstrated in the pamphlet which i had the honor of addressing to my learned colleagues, the means, in my opinion, of obtaining the simplest and the most suitable formula is to make the beginning of civil time and of dates on the first meridian coincide with the cosmical time and date, and to count longitude continuously in the same direction from the initial meridian. this is what i proposed to obtain by my amendment. count lewenhaupt, delegate of sweden. mr. president, i now propose that the conference take a recess for a few moments before a vote is taken upon the resolution. no objection being made to the motion, the president announced that a recess would be taken until the chair called the conference to order. the president, having called the conference to order, said. the recess has given an opportunity for an interchange of opinion upon the subject pending, and if the conference be ready the vote will now be taken. commander sampson, delegate of the united states. mr. president, i think that the informal discussion which we have had upon this question of the method of counting longitude must lead to the conclusion that there is a great difference of opinion. so far as i have been able to learn, many of the delegates have come here instructed to favor the resolution adopted by the roman conference. it is my own opinion that the recommendation to count longitude continuously from the prime meridian from west to east, as recommended by the conference at rome, is not so good as the proposition now before us. personally, however, i would prefer to see it counted continuously from east to west, as being more in conformity with present usage among astronomers. but, as it appears that so many delegates are instructed by their governments to favor counting in the opposite direction, and as, if this congress adopts any other plan than that proposed by the conference at rome, they will have to lay before their governments as the action of this congress something that will be opposed to the recommendation of the roman conference, and as these two recommendations would naturally tend to neutralize each other, i would favor the proposition which is now before us as being the most expedient. i would suggest, however, that, instead of making a positive declaration upon the question, we leave it as it now stands; that is to say, that longitude shall be counted east and west from the prime meridian, without specifying which direction shall be considered positive, and declare it to be the opinion of this congress that it is not expedient to change the present method of counting longitude both ways from the prime meridian. count lewenhaupt, delegate from sweden. in my opinion the delegates have not undertaken to recommend the resolutions adopted by a majority of the conference, but only the resolutions for which they have themselves voted. as regards the fact that there may be great differences of opinion concerning the questions which remain for our consideration, i am unable to see in it any reason for our not proceeding to vote upon them. on the contrary it will be of great interest to our governments to know the exact position taken by each of the delegates, and even if any delegate should abstain from voting, such abstention would be of interest in the event of future negotiations on the subject. i am therefore of opinion that we should proceed to vote on the remaining resolutions. the vote was then taken upon the resolution of the delegate of the united states, mr. rutherfurd, which is as follows: "_resolved_, that from this meridian (_id est_, greenwich) longitude shall be counted in two directions up to degrees, east longitude being plus and west longitude minus." the following states voted in the affirmative: chili, liberia, colombia, mexico, costa rica, paraguay, great britain, russia, guatemala, salvador, hawaii, united states, japan, venezuela. the following states voted in the negative: italy, sweden, netherlands, switzerland. spain, the following states abstained from voting: austria-hungary, germany, brazil, san domingo, france, turkey. ayes, ; noes, ; abstaining, . the president then announced that the resolution was adopted. mr. rutherfurd, delegate of the united states. mr. president, i now propose to read the third resolution from the printed circular which has been furnished to the delegates. it is as follows: "_resolved_, that the conference proposes the adoption of a universal day for all purposes for which it may be found convenient, and which shall not interfere with the use of local time where desirable. this universal day is to be a mean solar day; is to begin for all the world at the moment of midnight of the initial meridian coinciding with the beginning of the civil day and date of that meridian, and is to be counted from zero up to twenty-four hours." this resolution is somewhat complex, and in order to facilitate debate, i propose that we first occupy ourselves only with the first clause, namely: "_resolved_, that the conference proposes the adoption of a universal day for all purposes for which it may be found convenient, and which shall not interfere with the use of local time where desirable." after having disposed of that clause we can proceed to dispose of the other parts of the resolution. the president. you propose, then, to divide the resolution as printed in the circular into two resolutions, and you now offer the first part for consideration. mr. rutherfurd, delegate of the united states. if that is the more convenient form of putting it, it meets my views. it will be more easy to discuss the subject, more easy to arrive at a decision, in that form. m. le comte albert de foresta, delegate of italy. i propose as an amendment the fifth resolution of the roman conference, which reads as follows: "the conference recognizes, for certain scientific needs and for the internal service of great administrations of ways of communications, such as those of railroads, lines of steamships, telegraphic and postal lines, the utility of adopting a universal time, in connection with local or national times, which will necessarily continue to be employed in civil life." the president. the question is now upon the amendment offered by the delegate of italy. professor abbe, delegate of the united states. i would like to ask whether this amendment adds anything substantially to the resolution. i think it does not. it simply specifies the details of the resolution pending before us. that resolution "proposes the adoption of a universal day for all purposes for which it may be found convenient." that is general. the amendment merely specifies certain of these purposes. that is a matter of detail. mr. allen, delegate of the united states. mr. president, i desire to offer an amendment to the amendment, as follows: "civil or local time is to be understood as the mean time of the approximately central meridian of a section of the earth's surface, in which a single standard of time may be conveniently used." mr. rutherfurd, delegate of the united states. mr. president, it does not seem to me that it is within the competence of this conference to define what is local time. that is a thing beyond us. mr. w. f. allen, delegate of the united states, then said: mr. president and gentlemen, all efforts to arrive at uniformity in scientific or every-day usage originate in a desire to attain greater convenience in practice. the multiplicity of coins of which the relative value can only be expressed by fractions, the various common standards of weights and of measures, are inconvenient both to the business man and the scientist. alike inconvenient to both are the diverse standards of time by which the cities of the world are governed, differing, as they do, by all possible fractions of hours. all coins have a relative and interchangeable value based upon their weight and fineness. weights and measures remain the same by whatever unit they may be expressed; but, primarily, time can only be measured by a standard actually or apparently in motion. absolutely accurate mean local time, varying, as it does, by infinitesimal differences at every point in the circuit of the earth, may be shown on a stationary object, but cannot in general be kept by an individual or object in motion. the mean local time of some fixed point in each locality must be taken as the standard for practical use. the important question to be determined is, over what extent of territory, measuring east and west from such fixed point, its mean time may be employed for all ordinary purposes without inconvenience. this can be absolutely determined only by practical experience. careful study of this phase of this subject led, perhaps, more directly than any one single cause, to the proposal of the detailed system of standard time which now satisfactorily controls the operations of one hundred and twenty thousand miles of railway in the united states and canada, and governs the movements of fifty millions of people. before the recent change there were a number of localities where standards of time were exclusively employed which varied as much as thirty minutes, both on the east and the west, from mean local time, without appreciable inconvenience to those using them. from this fact the conclusion was inevitable that within those limits a single standard might be employed. the result has proved this conclusion to have been well founded. no public reform can be accomplished unless the evil to be remedied can be made plainly apparent. that an improvement will be effected must be clearly demonstrated, or the new status of affairs which will exist after the change, must be shown to have been already successfully tried. here, as in law, custom and precedent are all powerful. it would be a difficult task to secure the general adoption of any system of time-reckoning which cannot be employed by all classes of the community. business men would refuse to regard as a reform any proposition which introduced diversity where uniformity now exists, nor would railway managers consent to adopt for their own use a standard of time not coinciding with or bearing a ready relation to the standard employed in other business circles. to adopt the time of a universal day for all transportation purposes throughout the world, and to use it collaterally with local time, would simply restore, and possibly still more complicate, the very condition of things in this country which the movement of last year was intended to and did to a great extent obviate. railway managers desire that the time used in their service shall be either precisely the same as that used by the public, or shall differ from it at as few points as possible, and then by the most readily calculated differences. the public, on the other hand, have little use for absolutely accurate time, except in connection with matters of transportation, but will refuse to adopt a standard which would materially alter their accustomed habits of thought and of language in every-day life. that this position is absurd may be argued, and, perhaps, admitted, but it is a fact, and one which cannot be disregarded. the adoption of the universal day or any system of time-reckoning based upon infrequent--such as the great quadrant--meridians, to be used by transportation lines collaterally with local time, is, therefore, practically impossible. shall it, then, be concluded that there is no hope of securing uniformity in time-reckoning for practical purposes? or does the proposition for the general division of the earth's surface into specified sections, governed by standards based upon meridians fifteen degrees or one hour apart, supply the remedy? objections have been urged against this proposition on account of difficulties encountered, or supposed to be encountered, in the vicinity of the boundary lines between the sections. it is argued that the contact of two sections with standards of time differing by one hour will cause numerous and insuperable difficulties. in railway business, in which time is more largely referred to than in any other, the experience of the past year has proved this fear to be groundless. it is true that the approximate local time of a number of cities near the boundary lines between the eastern and central sections in the united states is still retained. a curious chapter of incidents could be related which led to this retention, not affecting, however, the merits of the case; but the fact serves to show that changes much greater than thirty minutes from local time would not be acceptable. adjacent to and on either side of all national boundary lines the inhabitants become accustomed to the standards of weights, measures, and money of both countries, and constantly refer to and use them without material inconvenience. in the readjustment of a boundary upon new lines of demarcation it must be expected that some temporary difficulties in business transactions will be encountered, but all history shows that such difficulties soon adjust themselves. legal enactments will finally determine the precise boundaries of the several sections. if different laws respecting many other affairs of life may exist on either side of a state or national boundary line, with positive advantage or without material inconvenience, why should laws respecting time-reckoning be an exception? coins and measures are distinguished by their names. so, also, may standards of time be distinguished. the adoption of standard time for all purposes of daily life, based upon meridians fifteen degrees apart, would practically abolish the use of exact local time, except upon those meridians. numerous circumstances might be related demonstrating how very inaccurate and undetermined was the local time used in many cities in this country before the recent change. except for certain philosophical purposes, does the inherent advantage claimed in the use of even approximately accurate local time really exist? would the proposed change affect any custom of undoubted value to the community? these questions have been answered in the negative by the experience of great britain since january , , of sweden since january , , and of the united states and canada since november , . greenwich time is exclusively used in great britain, and differs from mean local time about eight minutes on the east and about twenty-two and a half minutes on the west. in sweden the time of the fifteenth degree of east longitude is the standard for all purposes. it differs from mean local time about thirty-six and a half minutes on the east and about sixteen minutes on the west. in the united states the standards recently adopted are used exclusively in cities like portland, me., ( , inhabitants,) and atlanta, ga., ( , inhabitants,) of which the local times are, respectively, nineteen minutes and twenty two minutes faster than the standard, and at omaha, neb., ( , inhabitants,) and houston, tex., ( , inhabitants,) each twenty-four minutes slower. at ellsworth, me., a city of six thousand inhabitants, a change of twenty-six minutes has been made. nearly eighty-five per cent. of the total number of cities in the united states of over ten thousand inhabitants have adopted the new standard time for all purposes, and it is used upon ninety-seven and a half per cent. of all the miles of railway lines. let us now consider whether insuperable practical difficulties owing to geographical peculiarities will prevent the adoption of this system throughout the world. a table has been prepared, and accompanies this paper, upon which are designated the several governing meridians and names suggested for the corresponding sectional times. for the use of this table i am indebted to mr. e. b. elliott, of this city. on the north american continent, in the united states and canada, the th, th, th, and th west greenwich meridians now govern time. in mexico the th west meridian is approximately central, except for yucatan, which is traversed by the th. for guatemala, salvador, and costa rica, the th west meridian is approximately central. san domingo closely approaches and cuba touches the th. in south america--the united states of columbia, ecuador, peru, the western portion of bolivia, and chili would use the time of the th west meridian, while venezuela, guiana, western brazil, including the amazon river region, eastern bolivia, paraguay, uruguay, and the argentine republic, would be governed by the time of the th meridian. in eastern brazil the th west meridian would govern. passing to europe, we find great britain already governed by the zero meridian time, which can also be used in the netherlands, belgium, france, spain, and portugal. the th east meridian, which is about as far east of berlin as west of vienna, and no more distant from rome than from stockholm, now governs all time in sweden. this time could also be advantageously used in denmark, germany, austria-hungary, switzerland, italy, and servia. the time of the th east meridian, which is nearly the mean between constantinople and st. petersburg times, could be used in western russia, turkey, roumania, bulgaria, east roumelia, and greece. when the development of eastern russia in europe shall require it, the division of that great country between the times of the th and th east meridians, upon lines of convenience similar to those employed in the united states, can doubtless be arranged. the governing meridians for africa appear to present some advantages, especially for egypt, and no insuperable difficulties; but for continents where the boundaries of countries are so loosely defined, the limits of time-reckoning cannot well and need not now be shown. they would ultimately adjust themselves. in asia the th east meridian passes through khiva. bombay would use the th and calcutta the th. the th east meridian touches siam, the th is near shanghai, and the th passes through japan and near corea. the th meridian of west longitude is sufficiently near hawaii. in australia the th, th, and th meridians of east longitude are admirably located for governing, respectively, the time of the eastern, central, and western divisions of that continent. in none of the localities defined or mentioned, would the standards proposed vary more from mean local time than has already been demonstrated to be practicable without detriment to any material interest. convenience of use, based largely upon the direction of greater commercial intercourse, would determine the action of communities other than those mentioned, and probably somewhat modify the schedule proposed. that no practical difficulty of usage would prevent the universal adoption of the hour-section system of time-reckoning is apparent. its convenience has been abundantly realized. in adopting it, practically no expense whatever is incurred. the alteration of the works or faces of watches or clocks is not required. their hands are simply set to the new standard, and the desired result is accomplished. by the adoption of this system, the exact hours of time-reckoning, although called by different names in the several sections for every-day life, but specifically designated, if desired, for scientific purposes, would be indicated at the same moment of time at all points. the minutes and seconds would everywhere agree. the absolute time of the occurrence of any event could, therefore, be readily determined. the counting of the hour meridians should begin where the day begins at the transition line. it would then be one of the possibilities of the powers of electricity that the pendulum of a single centrally located clock, beating seconds, could regulate the local time-reckoning of every city on the face of the earth. _table of standards governing the hour-section system of time-reckoning._ ====================================================================== longitude | hour meridians. |simultaneous from |----------------------------------------------| hours in greenwich.| | | the several |proposed names of sectional times. | numbers. | sections. ----------+-----------------------------------+----------+------------ _degrees._| | | ----------| | | |transition time | or th| midnight west |alaskan | st......| a. m. |hawaii | d ......| |sitka | d ......| |pacific (adopted in u.s. and can.)| th......| |mountain " " | th......| |central (american) time " " | th......| |eastern (or coastwise) " " | th......| |la plata | th......| |brazilian | th......| |central atlantic | th......| |west african | th......| |int'l or unvs'l (used in gt. brit.)| th......| noon. east |continental (used in sweden.) | th......| p. m. |bosporus | th......| |caucasus | th......| |ural | th......| |bombay | th......| |central asian | th......| |siam | th......| |east asian | th......| |japan | st......| |east australian | d.......| |new caledonian | d.......| ----------------------------------------------------------------------- i have no desire, however, to press on the conference the consideration of the question of local time reckoning. but, as the system adopted in the united states and canada has proved successful, and is now firmly established, i have deemed it proper that a statement of this fact and of the possibilities of the application of the system to other parts of the world should be made to the congress. i will now, therefore, withdraw my amendment. mr. rutherfurd, delegate of the united states. the delegate of italy has moved, as an amendment to the first part of the resolution offered by me, the fifth resolution adopted in the conference at rome. really, in spirit and in substance, there is little or no difference between them, except that the conference at rome has specified that the objects they had in view as suitable for regulation by universal time were these, namely: "for the internal service of the great administrations of means of communication, such as railways, steamships, telegraphs, and post-offices." now, i submit that in the words used in my resolution all this is embraced, and a good deal more, for this universal day is to be adopted "for all purposes for which it may be found convenient." if it were desirable that every purpose for which the universal day may be found convenient should be specified, it would make a very long resolution. on the other hand, however, we might find in the end that we had omitted some of the purposes for which it was eminently convenient. it appears, also, that in this same fifth roman resolution all questions of chronology of universal date, etc., are omitted, although they are brought forward and appear in the sixth resolution. it seems to me, mr. president, that nothing would be gained by the adoption of this amendment, for everything that is embraced there is more comprehensively embraced in the original resolution. general strachey, delegate of great britain. in explanation of the amendment offered by the delegate of italy, let me call attention to what really passed at the roman conference. i find, first of all, in the report of the roman conference, in the abstract of the discussion before the special committee, these words, (p. of the reprint:) "the fourth resolution, in favor of a universal hour for certain scientific and practical purposes, is unanimously adopted." there appears no discussion whatever upon it; not a word seems to have been said as to how it should be defined or acted upon. i then turn back to the report of the committee which prepared the resolutions, and there we see what, in reality, they had in their minds when they drew up that resolution. it is perfectly evident that they had no intention of tying the hands of anybody. this is what they say on page of the report: "the administrations of railroads, of the great steamship lines, telegraph lines, and postal routes, which would thus secure for their relations with each other a uniform time, excluding all complication and error, could nevertheless not entirely avoid the use of local time in their relations with the public. they would probably use the universal time only in their internal service, for the rules of the road, for the time-tables of their engineers and conductors, for the connection of trains at frontiers, etc.; but the time-tables for the use of the public could hardly be expressed otherwise than in local or national time. the depots or stations of the railroads, post-offices, and telegraph offices, and the waiting-rooms, could exhibit outwardly clocks showing local or national time, while within the offices there would be, besides, clocks indicating universal time. telegraphic dispatches could show in future the time of despatch and of receipt, both in local and universal time." now, i think that the subject of universal time is dealt with in a better manner in the proposition offered by mr. rutherfurd than in the proposition which emanated from the congress at rome. this conference cannot designate positively the manner in which local time may be best reckoned. we are concerned now only with universal time. it may, however, be proper that the resolution offered by mr. rutherfurd in regard to the employment of universal time should be supplemented by something more specific--something, for instance, of this sort: the conference will not designate the system on which local time may best be reckoned so as to conform, as far as possible, to universal time; this should be determined by each nation to suit its convenience. the arrangements for adopting universal time for the use of international telegraphs will be left for regulation by the telegraph international congress. this last idea was expressed, i forget now by whom, but by one of the delegates since the conference met, and it appears to me that inasmuch as there is an international congress specially appointed to regulate all matters of international telegraphy, this subject can be left to them with the firm belief that it will be regulated satisfactorily. the question was then put to the vote; and upon the amendment offered by the delegate of italy the following states voted in the affirmative: colombia, paraguay, italy, spain, netherlands, sweden. the following in the negative: brazil, liberia, chili, mexico, costa rica, russia, france, salvador, germany, san domingo, great britain, switzerland, guatemala, turkey, hawaii, united states, japan, venezuela. austria-hungary abstained from voting. ayes, ; noes, ; abstaining, . so the amendment was lost. the question then recurred upon the original resolution. mr. rutherfurd, delegate of the united states. mr. president, it has been represented to me that it may, perhaps, be found advantageous in different countries and different localities to use a time that would not be accurately described as local time. in one place the standard of time may be strictly local time; in another place it may be national time; in another place it may be railroad time. in order to meet this condition of things, i propose to alter the phraseology of the original resolution in this way: by inserting the words "or other," so that it shall read "which shall not interfere with the use of local or _other_ time where desirable." professor adams, delegate of great britain. may it not be better to put it in this way: "which shall not interfere with the use of local or other _standard_ time where desirable." mr. rutherfurd, delegate of the united states. i accept the amendment offered by the delegate of great britain. mr. jean valera, delegate of spain. as i consider that both the amendment which was just rejected and the present proposition really signify the same thing, i shall vote for the proposition, as i before did for the amendment. the president. the question is now upon the resolution, as modified. it will be read. the resolution was then read, as follows: "_resolved_, that the conference proposes the adoption of a universal day for all purposes for which it may be found convenient, and which shall not interfere with the use of local or other standard time where desirable." the following states voted in the affirmative: austria-hungary, mexico, brazil, netherlands, chili, paraguay, colombia, russia, costa rica, salvador, france, spain, great britain, sweden, guatemala, switzerland, hawaii, turkey, italy, united states, japan, venezuela. liberia, there were no negative votes. germany and san domingo abstained from voting. ayes, ; noes, ; abstaining, . so the resolution was carried. mr. rutherfurd, delegate of the united states. mr. president, i now propose to offer the other portion of the resolution, or rather i propose to offer the other portion in the form of a distinct resolution. it will run as follows: "_resolved_, that this universal day is to be a mean solar day; is to begin for all the world at the moment of midnight of the initial meridian, coinciding with the beginning of the civil day and date of that meridian; and is to be counted from zero up to twenty-four hours." this is, in substance, the resolution adopted by the conference at rome, with the exception that the conference at rome proposed that the universal day should coincide with the astronomical day instead of the civil day, and begin at greenwich noon, instead of greenwich midnight. professor adams, delegate of great britain. i desire to make one remark merely. would it not be a little more correct if we said "at the moment of mean midnight?" i think i have mentioned this before, but, to be clear, i think it should be made. mr. rutherfurd accepted professor adams's suggestion. mr. juan valera, delegate of spain. mr. president, i wish to call special attention to the proposition now before us, on which we are called upon to vote, as it is of very great importance. as for me, i acknowledge that my mission is already fulfilled. the government of spain had directed me to admit the necessity or the usefulness of a common prime meridian, and also to accept the meridian of greenwich as the universal meridian. i have attended to these directions. we have now to deal with a scientific question on which i cannot well express an opinion, as i do not feel that i am competent in such matters; besides, i am not authorized to do so. this may be due to my ignorance in matters of this kind, but i fear that extraordinary difficulties may arise in the adoption of this proposition, and if we proceed with too great haste, we run the risk of placing ourselves in contradiction to common sense. all the popular ideas of men for thousands of years past will, perhaps, be overturned. it may happen that when the day begins at greenwich it will be hours later at berlin. the east will be confounded with the west, and the west with the east. if we made the day begin at the anti-meridian these questions would be avoided, and we should at one be with the rest of the human race. i believe that it would be better to adjourn till to-morrow to give us time to reflect; in this way we shall not risk by our devotion to science drawing upon ourselves popular criticism. i propose, therefore, that the vote on this question be put off till to-morrow. m. lefaivre, delegate of france. not to-morrow. count lewenhaupt, delegate of sweden. i beg to propose as an amendment the sixth resolution adopted by the conference at rome, which is as follows: the conference recommends as initial point for the universal hour and the cosmic day the mean midday of greenwich, coinciding with the moment of midnight or the beginning of the civic day at the meridian hours or ° from greenwich. the universal hours are to be counted from up to hours. the president. the chair quite concurs with the delegate of spain in thinking that it would be very proper for us to take some time to consider this matter. a motion to adjourn would be in order, but before that motion is made, the chair would like to read a communication which he has just received from the assistant secretary of state. it is this: "the president of the united states will receive the members of the conference on thursday, the th instant, at o'clock, at the white house." the assistant secretary of state proposes that we shall meet here at a quarter before , and go to the white house from this hall. the president. if the delegate of spain will withdraw his motion to adjourn for one moment, the delegate of sweden desires to offer a resolution. count lewenhaupt, delegate of sweden, then read the following proposal: hereafter the reports of the speeches, whether in english or french, will be sent as soon as possible to the delegates who made them, and the proofs should be corrected and returned by them without delay to the secretary. no correction will be allowed afterward, except such as are considered necessary by the secretaries, who will meet as soon as possible after the first corrections shall have been printed to prepare the protocols for the approval of the conference. the motion being put to a vote by the president, was unanimously carried. the president. the chair would very informally state that he has received to-day a letter from sir william thomson, the distinguished scientist who addressed the conference yesterday, expressing his regret that he did not then say something which he had in his mind and which he wished to say, namely, that the meridian of greenwich passes directly through the great commercial port of havre. mr. janssen, delegate of france. since the chairman refers to this subject, i may state to my colleagues that i have received a telegram from sir william thomson, in which he makes certain propositions of the nature described. yet it is not possible to make out precisely, by this telegram, what are sir william thomson's ideas. all that i can say is, that whatever proceeds from such an eminent man should be treated with great consideration, and that is a reason for asking sir w. thomson to be good enough to explain to me his ideas more fully. if we could adjourn to monday, i think that it would be better. the preparation of the protocols is very much behind-hand, and it is desirable that the members of the conference be kept fully acquainted with all the discussions. i would, therefore, suggest that we adjourn till monday. the president. there are several propositions to adjourn to different days. the chair will take them up in order and will first put the question upon the motion to adjourn until monday. the motion was carried, and at four o'clock the conference adjourned until monday, the th instant, at one o'clock p. m. vi. session of october , . the conference met, pursuant to adjournment, in the diplomatic hall of the department of state, at one o'clock p. m. present: austro-hungary: baron ignatz von schÆffer. brazil: dr. luiz cruls. chili: mr. f. v. gormas and mr. a. b. tupper. colombia: commodore s. r. franklin. costa rica: mr. juan francisco echeverria. france: mr. a. lefaivre, mr. janssen. germany: baron h. von alvensleben, mr. hinckeldeyn. great britain: sir f. j. o. evans, prof. j. c. adams, lieut.-general strachey, mr. sandford fleming. gautemala: mr. miles rock. hawaii: hon. w. d. alexander. italy: count albert de foresta. japan: professor kikuchi. liberia: mr. wm. coppinger. mexico: mr. leandro fernandez, mr. angel anguiano. netherlands: mr. g. de weckherlin. paraguay: capt. john stewart. russia: mr. c. de struve, major-general stebnitzki, mr. j. de kologrivoff. san domingo: mr. de j. galvan. spain: mr. juan valera, mr. emilo ruiz del arbol, mr. juan pastorin. sweden: count carl lewenhaupt. switzerland: col. emile frey. turkey: rustem effendi. united states: rear-admiral c. r. p. rodgers, mr. lewis m. rutherfurd, mr. w. f. allen, commander w. t. sampson, professor cleveland abbe. venezuela: dr. a. m. soteldo. absent: denmark: mr. c. s. a. de bille. hawaii: hon. luther aholo. salvador: mr. antonio batres. the president. some days ago a committee was appointed to report on communications addressed to the conference through the chair. all communications that have been received from time to time, and they have been numerous, have been referred to this committee, of which the delegate from england, prof. adams, is the chairman. he now informs the chair that he is prepared to make a report. the delegate of england, prof. adams, then read the following report: _letter from the president of the conference._ international meridian conference, department of state, washington, _oct. , _. sir: i have the honor to submit to the committee of which you are the chairman the following communications: no. . letters from mr. roumanet du cailland, through mr. hunter, ass't sec. of state. no. . letter and communication from mr. c. m. raffensparger. no. . letter from mr. a. s. de chancourtois, accompanying books from paris. no. . letter from mr. a. w. spofford, enclosing letter of mr. j. w. stolting, of dobbs' ferry. no. . letter from mr. b. aycrigg, passaic, n. j. no. . letter from j. t. field, st. louis, mo. no. . letter and two enclosures from mr. theodor pæsche. no. . description of the universal time-piece of dr. a. m. cory. no. . letter and enclosure from mr. e. r. knorr. no. . letter from mr. j. e. hilgard, of the u. s. coast survey and geodetic survey. no. . arguments by committee of new york and new jersey branch, and other papers relating to weights and measures. no. . letter from lt. c. a. s. totten, u.s.a., in relation to a standard meridian. no. . letter from mr. j. p. merritt, in relation to the metric system. no. . postal card from w. h. yates, in relation to the mercator projection. no. . a new system of mensuration, by lawrence s. benson. no. . letter of t. c. octman, of hope mills, n. c., calling attention to the fact that the meridian of greenwich passes through havre. no. . letter from dr. h. k. whitner, explaining his notation of hours. i am, sir, with great respect, your obedient servant, c. r. p. rodgers, _president international meridian conference_. prof. j. c. adams. _report of the committee._ the committee on communications respectfully reports as follows: we have carefully examined all of the communications referred to us, as enumerated in the letter of president rodgers, with the following results: no. recommends that the meridian of bethlehem be adopted as the initial meridian. this question has been already disposed of by the conference; therefore further consideration of the proposition is unnecessary. no. refers to an invention, the author of which states that "a patent has been applied for," consequently your committee does not feel called upon to express any opinion upon it. no. is a letter from m. de chancourtois, accompanying a work by him which contains an elaborate program of a system of geography based on decimal measures, both of time and of angles, and on the adoption of an international meridian. the work also contains copious historical notices on the metric system and on the initial meridian. a copy of this work was presented to each of the delegates prior to the discussions of the conference with regard to the choice of an initial meridian, and therefore no special report of the author's views on this subject appears to your committee to be necessary. these views are nearly identical with those which were so ably laid before the conference by professor janssen, but which failed to meet with their approval. the author further proposes to supersede the present mode of measuring both angles and time by a system in which the entire circumference and the length of the day should each be first divided into four equal parts, and then each of these parts should be subdivided decimally. however deserving of consideration these proposals may be, in the abstract, your committee are clearly of the opinion that they do not fall within the limits indicated by the instructions which we have received from our respective governments, and that, therefore, any discussion of them would only be of a purely academical character, and could lead to no practical result. such a discussion would be sure to elicit great differences of opinion, and would, therefore, occupy a considerable time. hence, your committee think that it would be very undesirable for the conference to enter upon it. no. is a letter from mr. spofford, librarian of congress, including a communication of mr. j. w. stolting, dobbs' ferry, n. y. the author recommends the adoption of the meridian ° w. from greenwich as the prime meridian; he proposes further, not to say east or west, but first or second half, and also recommends the adoption of a universal time, not to interfere with local or other standard time, and to reckon from " to ." he expresses no opinion as to whether the day should begin at noon or midnight. there seems to be nothing in the communication to influence the decisions of the conference. no. . see report as to letter no. . no. suggests that the prime meridian should be ° from greenwich, and that longitude should be reckoned from ° to °. this proposition has been already considered and rejected by the conference. no. . this communication proposes "to adopt as the prime meridian the frontier line between russia and the united states, as defined in the treaty of march , ." as the initial meridian has already been agreed to by the conference, this proposition needs no further notice. no. . this communication refers to an invention which has no bearing on the question before the conference. the committee therefore abstain from expressing an opinion as to its merits. no. . two letters from mr. e. r. knorr, of washington, d.c., advocating the advisability of reckoning longitude "westward from ° to °," and marking them on charts by time instead of by degrees. the conference has already taken action on the question involved. no. . a letter from prof. hilgard, enclosing a pamphlet by lt. c. a. s. totten on the metrology of the great pyramid, a subject which does not fall within the scope of the subjects presented for the consideration of this conference. in the enclosing letter prof. hilgard says: "i am purely and squarely for greenwich midnight as the beginning of the universal day, and an east and west count of longitude; that is, ° each way." no. advocates the preservation of the anglo-saxon system of weights and measures. this subject being foreign to the questions under consideration by this conference, the committee deems further comment unnecessary. no. . a letter from lieut. c. a. s. totten, u.s.a., advocating a prime meridian through the great pyramid. the proposition involved has already been decided by the conference. no. recommends redistribution of time according to the decimal system. as already remarked under no. , this proposition is clearly not within the limits indicated by the instructions which we have received from our respective governments. no. states that the author has a plan by which "chronometers will record the longitude equably." this proposition is foreign to the subjects under consideration by the conference. no. proposes a new system of mensuration; and, therefore, this does not fall within the subjects for consideration by the conference. no. . this communication suggests that as the prime meridian passes through havre, it should be allowable to call it by that name. this committee recommends that the prime meridian be not named after the localities through which it passes, but be called simply "the prime meridian." no. is the subject of a patent. the committee does not feel called upon to express an opinion respecting it. this report is respectfully submitted to the conference. j. c. adams, _chairman committee on communications._ washington, _oct. th, _. the president. the report of the committee is before the conference. mr. rutherfurd, the delegate of the united states. i move that the report be accepted, and its conclusions adopted. there being no objection, the report was adopted. the president. in the regular order of business to-day, the first subject before the conference is the resolution offered on saturday by the delegate of the united states, mr. rutherfurd, with the amendment offered by the delegate of sweden, count lewenhaupt. the resolution is as follows: "_resolved_, that this universal day is to be a mean solar day, is to begin for all the world at the moment of mean midnight of the initial meridian coinciding with the beginning of the civil day and date of that meridian, and is to be counted from zero up to twenty-four hours." the amendment offered is as follows: "the conference recommends as initial point for the universal hour and the cosmic day the mean mid-day of greenwich, coinciding with the moment of midnight or the beginning of the civil day at the meridian hours or ° from greenwich. "the universal hours are to be counted from up to hours." mr. valera, the delegate of spain, said that he thought that the amendment of the delegate of sweden should be first discussed. mr. janssen, the delegate of france. at the last session i informed the congress that i had received a telegram from sir william thomson upon the question of the meridian. since then, that illustrious foreign member of the institute of france has written me a very kind letter upon the subject, in which he expresses his complete appreciation of the disinterested attitude taken by france in this congress. i thank sir william thomson for his sentiments towards france, and i am persuaded that, with such excellent feelings, we should arrive at an understanding, upon scientific bases, in which the moral and material interests of all would be equitably adjusted, as we have always understood them. but the question is not open now, and this congress would, doubtless, not be disposed to reopen it. sir william thomson will understand, therefore, that in the present condition of affairs we have only to maintain the attitude which we have taken and the votes which we have given. the president. the chair will simply say to the conference that he very informally alluded to the letter that he had received from sir william thomson, and the chair would also say in answer to the spanish minister that the rule in this conference, a simple one, is to discuss the last amendment offered and dispose of it, instead, as suggested by the delegate of spain, of taking up the one most important in its character. it would be somewhat difficult for the chair to decide on all occasions which amendment is the most important. i think, therefore, as chairman, that i will pursue the rule in force in this country, and, unless the conference order otherwise, shall present the amendment which is the last offered. mr. ruiz del arbol, delegate of spain. mr. chairman, the spanish minister has not referred to the most important amendment, but to the most radical. for instance, here there are several propositions to select a meridian; one of them must be considered, and it seems to me that my amendment, which is the most radical, is the one to be first presented to the conference. the president. unless the conference shall direct otherwise, the chair must pursue the principle on which it has acted hitherto, taking the amendments in the order in which they are offered, and presenting them inversely for the action of the conference. the proposition before the conference, therefore, is the amendment offered by the delegate of spain, mr. arbol, which is as follows: "having accepted the meridian of greenwich to account the longitudes, as a general need for practical purposes, but thinking that the introduction of any new system of time-reckoning is far more scientific and important, and liable to great difficulties and confusion in the future, we propose the following resolution: "_resolved_, the congress, taking in consideration that there is already a meridian tacitly accepted by almost all the civilized nations as the origin of dates, the anti-meridian of rome, abstains from designating any other meridian to reckon the universal time." mr. ruiz del arbol, delegate of spain. it is proposed to introduce an absolute universal or cosmopolitan system of time-reckoning, which, it is hoped, will, at a more or less distant day, be generally adopted, not only for scientific purposes, but for all the ordinary purposes of life for which it can possibly be used; and it is further proposed to designate a meridian at which this cosmopolitan time-reckoning is to begin. what i have to state is, that this method of absolute time-reckoning already exists, (although we do not use it,) as does this universal meridian which has been tacitly chosen by almost all civilized nations--that is to say, by all such as have adopted the julian calendar, with or without the gregorian correction. thus it is that anything involving even a slight modification of our present system is nothing more than a chronological reform, which i do not feel certain that it will be well for us to introduce or recommend, and with regard to which i have my doubts whether it will be received with unanimous or hearty approval. in fact, gentlemen, all nations that have adopted the julian and gregorian systems of time-reckoning have necessarily accepted their consequences, and these consequences are, as rome told us in the time of caesar and in that of gregory xiii, that we must reckon our days according to certain fixed dates; some part of the world had to reckon their dates before all the rest, and as rome consented that countries situated to the east of it should reckon their date before it and countries situated to the west after it, it is evident that both reckonings had to meet at some point on some meridian, which was and could be no other than the anti-meridian of rome. nature itself seems to have lent its sanction to this, since the anti-meridian of rome crosses no continent, and, probably, no land whatever. let us suppose, for the sake of illustration, that it were agreed to abandon the gregorian system of reckoning at a given moment, and to adopt another; that it were agreed to abandon it at all points on the globe when the hour should be twelve o'clock at noon at greenwich, on the first day of january, ; and let us suppose that for historical or scientific purposes we were interested in knowing exactly how long the gregorian system had been in use. is it possible to ascertain this? it is; and very easily. using that system of universal time-reckoning which it is proposed to establish, but logically referring it to the origin of that cosmopolitan reckoning which really exists, that is to say, to the anti-meridian of rome, we shall find that years have been reckoned according to the gregorian system, plus the difference of longitude between the anti-meridians of greenwich and rome. nothing is more certain than this, and there is no other way of solving the problem. as i have already shown, when the gregorian correction was made, the day which, according to the old mode of reckoning, would have been the th of october, was called the th of october, ; the countries situated to the east of rome had, however, previously begun to reckon according to the new system (previously in absolute time i mean,) and the countries situated to the west adopted it successively afterwards. now, then, as that portion of the globe which lies to the east of any given point or meridian is nothing more or less than one hemisphere, and as that which lies to the west is another hemisphere, it is evident that, at the anti-meridian of rome, the two meridians, which constantly differ by one day in their dates, are confounded, and that the anti-meridian of rome, being the first one in the world that adopted the julian and the gregorian systems of reckoning, is the prime meridian of the world, the meridian by which we now reckon, and ought to reckon universal time, until the establishment of a different system. if we had, at the present time, to settle any question depending on dates, in the region where there is some confusion in regard to them, we should have to do so on this principle. if we desired to compel the entire world to keep a regular and logical account of dates, we should have to do so by compelling all the nations to the west of the anti-meridian of rome to go on reckoning their dates uninterruptedly after they have begun to be reckoned at the said anti-meridian, and by forbidding all the nations to the east of it to reckon any date until it has been reckoned at the anti-meridian of rome. for this reason i say that the express designation, for the reckoning of universal time, of the meridian of greenwich or of any other than the anti-meridian of rome, involves a chronological reform, inasmuch as it will involve the abandonment of the system to which we now adhere, and which we now use by common consent. this reform will cause a change of nearly hours--that is to say, hours plus the difference of longitude between rome and greenwich, if the meridian of greenwich is designated as the new initial point of the universal date. i do not believe, however, that you will adopt this choice irrevocably, since its curious and strange consequences may be shown by one example, which i will adduce: this table is of about sufficient extent to allow the difference between the geographical longitude of its two ends to be observed and appreciated. let us suppose that these sessions were held at greenwich, and that the table were placed east and west, so that the meridian intersected it lengthwise; let us further suppose that we had agreed to reckon the new universal time by this meridian--that is to say, by that of greenwich--and that, in signing the protocol, we wished to set an example to the world by using the universal date, the present civil date and the future civil date, which, by the daily use of the universal date, the nations will or may finally accept, to the exclusion of all others, for the ordinary purposes of life. well, now, gentlemen, we should bring our own choice into discredit. we could not sign, according to these three dates. as regards the last, we should find that half the table and half the congress were under one date, and the other half under another; even our chairman, if seated in the middle, would find that he had been presiding over our sessions with his right side in one day and his left in the next. i may be told that this would happen, whatever might be the meridian chosen, but we could afford to allow it to happen at sea, or in some isolated and uninhabited region where congresses never sit, and where no ray of civilization ever penetrates. but to return to the reform, what are you going to do? i will say that if, instead of the meridian of greenwich, you designate the anti-meridian for the reckoning of universal time and for the initial point of cosmopolitan dates for the present, but for the future as the initial point also of local dates, the reform will amount to about an hour only, but it will still be a reform. in a word, the anti-meridian of rome is the one which now furnishes dates to the entire world, and you propose to make the meridian of greenwich or the anti-meridian do so in future. i therefore tell you, if you desire a common hour for postal and commercial purposes, designate no meridian at all; let the railway and telegraph companies, the postal authorities and the governments make an arrangement and select an artificial hour, so to speak, whatever it be the hour of rome, london, paris, or even that of greenwich, but do not make a premature declaration which will be an authoritative one as emanating from this congress, an apparently insignificant reform, but in reality one of very great importance, since, giving the preference to determinate localities in the face of what is scientific, historical, and logical, you render difficult, in the future, the adoption of that very reform, which will, perhaps, then be more necessary, and which can perhaps then be introduced more intelligently. you see that i am not speaking in behalf of any special meridian, not even that of rome, since i admit that the reform may be necessary. you see, and i assure you, that i have not the slightest wish that the meridian which is to be the initial point of universal time should bear the name of any observatory or place in spain, although that nation discovered the new world in which this congress is holding its sessions, and although it may be said of that nation that it discovered those very meridians concerning which we are now speaking, inasmuch as terrestial meridians were indefinite and unknown lines, and were even without form until one was given them by sebastian elcano. i therefore hope that if you do not honor my proposition by accepting it, you will at least do justice to my intentions. prof. adams, delegate of great britain. mr. president, i shall be very short in any remarks which i may make upon the proposition before us. as far as i understand it, it is that, although we have adopted the meridian of greenwich as a prime meridian from which to count longitudes, we should begin to count our time according to the meridian at rome. i cannot consent to that proposition. it appears to me to be wanting in every element of simplicity, which should be our chief aim in this conference. to count longitude from one meridian and time from another, is something that will never be adopted. i do not understand that that was at all the proposition recommended by the roman conference. on the contrary, i think that it was quite a different one. mr. ruiz del arbol, delegate of spain. mr. president, i do not in reality propose to adopt the meridian or anti-meridian of rome. what i have been contending for is that we should abstain at present from adopting any meridian as a point of departure for the calculation of time; otherwise, we introduce a new element of confusion for the future. we should change the chronological reckoning which is now in vogue, and i contend that we have no right, scientific or historical, to make that change now. according to my views, the meridian of longitude is relatively an unimportant affair. it is a practical one; it cannot be changed in twenty years, probably, and it will take that time to correct all existing charts. but if you adopt a meridian for time, it will be very difficult to alter it in the future. i cannot now clearly see what the difficulties will be, but i apprehend that the application of this new principle to the various details of scientific and civil matters will necessarily be attended with great inconvenience, and may result in proving to be quite impracticable. i understand it very well that it is proposed to confine this principle to certain subjects, and that it is adopted for the purpose of avoiding dangers in communications, in navigation, in railways, and in transmitting telegrams, &c.; but this is purely an administrative matter, and can be left for settlement to other bodies. the president. the chair would remind the delegate of spain, mr. ruiz del arbol, that at its last session the conference resolved, with singular unanimity, that it was expedient to adopt "a universal day for all purposes for which it may be found convenient, and which shall not interfere with the local or other standard time where desirable." the chair would politely suggest that the subject now under consideration is the adoption of the proposition recommended by the conference at rome, and which has been presented here by the delegate of sweden, count lewenhaupt. mr. ruiz del arbol, delegate of spain. my proposition is to abstain from the adoption of any one meridian, and that we leave the matter to some other congress, organized with the special object of regulating this question. commander sampson, delegate of the united states. mr. president, as near as i can follow the delegate of spain, he seems to be under the apprehension that by the adoption of the universal day, which has been proposed here, we should either gain or lose time in our chronology; that we should skip hours, more or less. but, of course, that is not the case. any event which has occurred, or which will occur, at the time of the adoption of the universal day will be expressed just as exactly with reference to time as if the time had been calculated from the beginning of the christian era. there will not only be no confusion, but it seems to me the adoption of the universal day will tend to avoid confusion hereafter, because confusion must exist where we have so many standards of time. now, if any event which is taking place, or has taken place at any past time in the history of the world, is referred to the prime meridian, or is expressed in the time of any locality or of several localities, these times will all be different. the adoption of the universal day is to avoid any difficulty of that sort, and any event which has transpired will, when expressed in the time of the universal day--that is, according to the universal method--represent exactly the interval of time which has elapsed since the beginning of the christian era. nothing is gained or lost. general strachey, delegate of great britain. it seems to me that the congress having accepted the resolution to which reference was made a little while ago, adopting the universal day, it is incumbent upon us, in the nature of things, to determine when that universal day shall begin. the resolution presented by the delegate of the united states proposes to define how that universal day shall be reckoned; that is, when it shall begin and how its hours shall be counted. it was explained by him that the difference between his proposition and the proposition made at rome consisted in altering the time of the commencement of the so-called universal day from noon at greenwich to the commencement of the civil day. certainly what commander sampson just said is perfectly true. the adoption of this so-called universal day will not interfere in the smallest degree with any purpose for which time is employed in civil life. the two objects are entirely distinct. it is obvious that the conception of the necessity of having a universal day has arisen from the more clear conception of the fact that time on the globe is essentially local; that the time upon any given line (supposing it to be a meridian) is not the time at the same moment on either side of that line, however small the departure from it may be; and for scientific accuracy it has, therefore, been thought desirable to have some absolute standard to which days and hours can be referred. up to the present time it has been the practice to say, in an indefinite way, that an event happened, say, on the st of january at o'clock in the morning, and such a statement of the time has been considered sufficient; but, in truth, this does not completely describe a definite epoch of time, for if the event occurred at madrid and was so reported, that report would not designate the same moment as a report of an event which was described to have occurred at precisely the same date and hour at greenwich, or rome, or washington. what is required and desired is that we should have an absolute and definite standard for reckoning events of a certain description, for which complete precision is desirable. i consider, therefore, that the delegate of spain leads us astray in the proposition which he has offered, by which he virtually proposes to nullify the resolution already adopted. we have already decided that a universal day was expedient, and it is for the conference to settle now when that universal day shall begin. mr. ruiz del arbol, delegate of spain. i understand that the consequences, perhaps, would not be troublesome at first; but who can look into the future and say, if we take the meridian of greenwich as the standard of time, what difficulties we may be driven into? every country will be obliged to count both ways. they will have to use civil time and universal time. perhaps all countries may get accustomed to this radical change sooner or later, but we cannot foresee the difficulty now. i have here a treatise (a book) on "analytic chronology," showing the rules by which to bring into accord different dates of different calendars and eras, and i do not know how they would be affected by this universal time; but it is unnecessary for me to speak of that, as i think you are acquainted with the subject. mr. juan pastorin, delegate of spain. the congress has already come to very important decisions on the subject of the reckoning of longitude, and it will also certainly approve to-day those which have just been submitted on the subject of the universal day. i say certainly, because the result of the former votes being already known, it cannot be doubted on which side the majority will be, and because, from a scientific point of view, having chosen greenwich as the prime meridian for the calculation of longitude, and having decided to reckon longitude in two directions from zero hours to twenty-four hours, with the sign plus towards the east and minus towards the west, it will be advantageous to make the civil day of greenwich coincide with the universal day, if we would have an easy formula for passing from local to cosmic time. so many of the resolutions submitted to the congress by mr. rutherfurd having been approved one after another, the plan that our colleague has carefully studied will be accepted in its entirety; but it will be impossible for the conference to know in all their details other plans which, perhaps, would not be less worthy of attention. is the resolution adopted by a majority of the congress the best? should we reach the end of the reform in complete harmony with the hopes of all the governments represented here? on the contrary hypothesis, it seems to me, that the sessions of this congress will only be another step towards that reform, but not the reform itself. if the majority of the congress, in accordance with the logical consequence of its work, adopts as the cosmic time the civil time of greenwich, that decision will be contrary to the most ancient ideas of the human race. for many centuries the day has been reckoned as starting from the east, and the world will not easily abandon the traditions of its predecessors. the civil day of the world commences near the anti-meridian of rome, greenwich, or paris. therefore it is not natural that one of these meridians should be chosen as the point of departure of dates. really, one phenomenon cannot be the commencement of a series of phenomena if there is another which precedes it periodically. if the majority, as is logical, adopts the formula, "cosmic time=local time-longitude," and applies in the calculation longitude with the signs plus and minus, according as the longitude is east and west, the system will be source of frequent mistakes, and those, in their turn, will be the cause of disastrous accidents, especially on railroads. let us take the st of december, for instance. it is three o'clock at a point nine hours east of greenwich; at the same moment they will count at greenwich eighteen civil hours of the th of the same month, after the actual manner of reckoning the civil day, and that civil time of greenwich will be the cosmic time. apply to the proposed example the formula which i suppose the majority of the congress will adopt, and the result will be a negative quantity, minus six hours--a result not sufficiently comprehensible in itself, and one that could not be easily applied by the general public. can a majority prevail in questions, such as those we are speaking of, simply by the force of numbers? the whole world for several centuries thought that the earth was the centre of our planetary system; in fact, until an insignificant minority rose against this theory, for a long time considered by their ancestors indisputable. i will conclude by expressing my opinion upon the subject with which the congress is occupied. my opinion is not new, in spite of its having been modified in the course of our sitting. the works of our eminent colleague and indefatigable propagandist, mr. sandford fleming, the resolution of the conference at rome, the valuable opinions of messrs. faye, otto struve, beaumont de boutiller, hugo gyldén, the scientific work of monsieur chancourtois, and the report which m. gaspari has just presented to the academy of sciences of paris are the text upon which i base the simplest and most practical method of solving the problem, namely, to adopt as the prime meridian for cosmic time and longitude a meridian near the point at which our dates change, and to reckon longitude from zero hours to twenty-four hours towards the west, contrary to the movement of the earth. the formula would be then: cosmic time = local time + longitude. i think that the best way of finding cosmic time in relation to local time and longitude is to add a quantity to the civil hour of each point of the globe. but as the majority of this congress, so worthy of respect, admits no modifications of the system which we may call greenwich, let us lay aside the question of longitude and consider cosmic time separately. i have the honor, therefore, to present the following resolutions, and i ask the congress to consider them, and to accept them as a means of compromise: i. we agree to choose as the prime meridian for cosmic time that meridian near which the civil day of the world commences, namely, the anti-meridian of rome, greenwich, or havre. ii. the cosmic day consists of twenty-four hours, and commences at midnight of the prime meridian. iii. the earth is divided from the initial meridian into twenty-four hour-spaces, counted in a direction contrary to the movement of the earth from _ h._ to _ h_. we shall, then, have the following formula: t = t + r, where r represents the difference reckoned from _ h._ to _ h_. between the local time of the prime meridian and the local time of each point of the globe; t the cosmic time and t the local time. the president. the chair would ask the delegate of spain, mr. pastorin, whether he offers his resolution as an amendment to that offered by his colleague, mr. ruiz del arbol. mr. ruiz del arbol, delegate of spain. mr. chairman, the amendment last offered is not intended to interfere with my proposition. the president then put the question to the conference upon the amendment offered by the delegate of spain, mr. ruiz del arbol. upon a vote being taken, the amendment was lost. the president. the question now recurs upon the amendment offered by the delegate of spain, mr. pastorin. that amendment runs as follows: "i. we agree to choose as the prime meridian for cosmic time that meridian near which the civil day of the world commences, namely, the anti-meridian of greenwich or havre. "ii. the cosmic day consists of twenty-four hours, and commences at midnight of the prime meridian. "iii. the earth is divided from the initial meridian into twenty-four hour spaces, counted in a direction contrary to the movement of the earth. "we shall, then, have the following formula: f = a + r where r represents the difference reckoned from h. to h. between the local time of the prime meridian and the local time of each point of the globe; f the cosmic time, and a the local time." the president. in order that this amendment may be presented more clearly to the conference, i would propose a recess for a few minutes. if there be no objection, a recess will be taken. no objection being made, the conference took a recess. * * * * * the president having called the conference to order stated that, unless further remarks were presented, the vote would be taken upon the resolution offered by the delegate of spain, mr. pastorin. no objection being made, the vote was then taken upon the amendment, and it was lost. the president. the question now recurs upon the resolution offered by the delegate of sweden, count lewenhaupt, which will again be read. the resolution is as follows: "the conference recommends as initial point for the universal hour and the cosmic day the mean mid-day of greenwich, coinciding with the moment of midnight or the beginning of the civil day at the meridian hours or ° from greenwich. the universal hours are to be counted from up to hours." professor adams, delegate of great britain. mr. president, i intended to speak on the resolution offered by the delegate of the united states, mr. rutherfurd, but the remarks which i have put together apply equally well to the amendment to that resolution now offered by the delegate of sweden, which is identical with one of the recommendations of the conference at rome, because, in fact, in my remarks i discuss these propositions alternatively. therefore, with your permission, i will lay before you the observations which i wish to make. i beg leave to express my entire approval of the resolution which has been laid before the conference by mr. rutherfurd. there is only one point involved in the resolution which seems to call for or even to admit of any discussion. it appears evident that the universal day and date should coincide with the day and date of the initial meridian. the only question, therefore, which we have now to decide is, when shall this day of the initial meridian be considered to commence? and the proper answer to be given to this question does not appear to me in any degree doubtful. in modern times it is the universal practice to reckon dates by _days_ and not by _nights_. the word "day" is used in two different significations, being sometimes applied to the period of daylight and sometimes to the period of hours, including both day and night; but in whichever of these senses the word _day_ is employed, the term mid-day has one and the same signification, viz., the instant of noon or of the sun's passage over the meridian. in the present case, where we are concerned with mean time, mid-day means the instant of mean noon, or of the passage of the mean sun over the meridian. accordingly, the civil day, by which all the ordinary affairs of life are regulated, begins and ends at midnight, and has its middle or mid-day at noon. it appears, then, most natural that the universal day should follow this example, and should begin and end at the instant of mean midnight on the initial meridian, and should have its middle at the instant of mean noon on the same meridian. i fail, therefore, to see the force of the reasons which induced the conference at rome to recommend that the universal day should commence at _noon_ on the initial meridian. the only ground for making this recommendation is that astronomers, instead of adopting the use of the civil day, like the rest of the world, are accustomed to employ a so-called astronomical day, which begins at noon. the advantage thus gained is that they avoid the necessity of changing the date in the course of the night, which is the time of their greatest activity; but this advantage is surely very small when compared with the inconvenience of having two conflicting methods of reckoning dates, and of being obliged to specify, in giving any date, which mode of reckoning is adopted. if this diversity is to disappear, it is plain that it is the astronomers who will have to yield. they are few in number compared with the rest of the world. they are intelligent, and could make the required change without any difficulty, and with very slight or no inconvenience. the requisite changes in the astronomical and nautical ephemerides would be easily made. as these ephemerides are published several years in advance, there would be plenty of time for navigators to become familiar with the proposed change in time-reckoning before they were called upon to employ it in their calculations. i believe that they would soon come to think it more convenient and natural to reckon according to civil time than according to the present astronomical time. i am told that this practice is already universally adopted in keeping the log on board ship. to avoid any chance of mistake, it should be prominently stated on each page of the ephemerides that mean time reckoned from mean _midnight_ is kept throughout. whether or not astronomers agree to adopt the civil reckoning, i think we ought to adopt the instant of midnight on the initial meridian as the commencement of the universal day. the relation between the local time at any place and the universal time would then be expressed by the simple formula: local time = universal time + longitude. whereas, if the proposition of the roman conference were adopted, we should have to employ the less simple formula: local time = universal time + longitude - hours. in recommending the mean noon at greenwich as the commencement of the universal day and of cosmopolitan dates, the roman conference refers to this instant as coinciding with the instant of midnight, or with the commencement of the civil day, under the meridian situated at h. or ° from greenwich. now, this reference to the civil day and date on the meridian opposite to greenwich appears not only to be unnecessary and to be wanting in simplicity, but it may also lead to ambiguity in the date, as expressed in universal days, unless this ambiguity be avoided by making an arbitrary assumption. no doubt the greenwich mean noon of january coincides with midnight on the meridian h. from greenwich, but with what midnight. what shall be its designation and the corresponding date given to the universal day? shall we call the instant above defined the commencement of the universal day denoted by january or by january ? each of these dates has equal claims to be chosen, and the choice between them must clearly be an arbitrary one, and may, therefore, lead to ambiguity. by adopting greenwich mean midnight as the commencement of the universal day, bearing the same designation as the corresponding greenwich civil day, all ambiguity is avoided, and there is no need to refer to the opposite meridian at all. those are the ideas i wish to express with regard to the commencement of the universal day. i may mention in connection with this subject that professor valentiner is one of the gentlemen who were invited, a week or two ago, to attend the meetings of this conference, in order that, if requested, they might express their opinions from a scientific standpoint upon the questions before it; but as professor valentiner had to leave washington before our sessions were at an end, i thought it would be expedient to ask him for his opinion in writing upon the matter which is now pending before this conference. he has written a letter in german, expressing his opinion. i have caused that letter to be translated into english, and if the conference allows me i will read it. the president. if there be no objection to the proposition of the delegate of great britain the letter will be read. no objection being made, professor adams continued: it is well known that professor valentiner is an eminent practical astronomer, and i think that any opinion coming from him on this subject, which interests astronomers very much, will be considered of great weight. the letter runs as follows: charlottesville, va., _october th, _. honored sir: you had the kindness to ask me for my views as to the choice of the moment for the beginning of the day. as i cannot remain longer in washington, i allow myself thus briefly to write to you. when, as in the present case, the object is to introduce uniformity in the time-reckoning of the astronomical and the civil world, i am of the opinion that it is the astronomer only that must give way. for all purposes of civil life one cannot begin the day in the middle of the day-light--that is to say, in the middle of that interval during which work is prosecuted. in general it appears to me natural that the middle of the day, and not the beginning of the day, should be indicated by the highest position of the sun which governs all civil life. in fact, it would in civil life be simply impossible to bring about a change of date in the middle of the daylight. for the astronomer there certainly exist difficulties. his activity occurs mostly in the civil night, and he, therefore, has to make the change of date in the midst of his observations; and this difficulty is increased, since he almost exclusively observes according to sidereal time, so that often a computation must be made in order to ascertain whether the observations were made before or after the midnight or moment of change of date. however, this difficulty can be overcome by habit, and i believe that scarcely any doubt will occur as soon as a uniformnity of expression has established itself through the astronomical world. as regards the ephemerides, we already employ, in fact, the beginning of the date at midnight, since the places of planets and comets, are generally computed for o'clock midnight of berlin or greenwich or other places. but these are points that have themselves long since been discussed. i scarcely need to say anything further. i would not hesitate for a moment to give the preference to making the change of date take place at midnight, according to civil reckoning, in order to establish a uniformity with the customs of civil life. it, perhaps, may be important to remark that we could not introduce this change immediately, since the ephemerides are already computed and published for three or four years in advance. it would, therefore, be well to fix the epoch of change of normal dates to some distant time, such as . i remain, very respectfully yours, w. valentiner. i may also mention that the practice that prevails among astronomers at the present time of reckoning the day from noon is by no means without exceptions. there are very important astronomical tables which reckon the day from midnight; for instance, in delambre's tables of the sun; in burg's, burckhardt's and damoiseau's tables of the moon; in bouvard's tables of jupiter, saturn, and uranus, and in damoiseau's tables of jupiter's satellites, mean midnight is employed as the epoch of the tables. i may also mention that laplace, in his mécanique celeste, adopts the mean midnight of paris as the origin from which his day is reckoned. hence there are great authorities, even among astronomers, in favor of commencing the day at midnight. general strachey, delegate of great britain. sir, i observe that a very eminent american authority is present in this room, i mean professor hilgard. as he was invited to attend the meeting of this conference, i suggest that the views of the conference may be taken, whether he may not be invited to express his opinion on the point now under consideration. the president. with the concurrence of the conference, the chair will be most happy to ask professor hilgard to do us the favor to give us his opinion upon the question now before the conference. no objection was made to the proposition of the president. professor hilgard arose and said. i thank you and the conference very much for this invitation, and general strachey for having proposed it to the conference, but my opinion has been squarely expressed both in french and english in the report of a certain committee, that i am in favor of midnight at greenwich as the beginning of the universal day, and of longitude being calculated both ways from greenwich. i really cannot add anything to what has been said in the arguments already presented by professor adams, and i do not think that i ought to detain this conference a moment by repeating the opinion he has expressed to all the experts in this matter. i beg you will excuse me for not further ventilating my views. absence from the city, i regret, has prevented me from availing myself of the invitation earlier. sir frederick evans, delegate of great britain. i have the honor to address the conference once more upon the practical aspect of the subject before us as affecting the large body of navigators. i wish to say upon this point that there appears to me, in the address of my colleague, professor adams, somewhat of a mixing together of two subjects. the question immediately before us, as i understand it, is whether the commencement of the universal day shall be midnight or noon of the initial meridian. that is what we practically have to decide. now, i gather from professor adams' remarks that upon this question the ephemerides which we now employ have some important bearing. i do not think that that should influence us, for this reason, that the next resolution which will come before the conference "expresses the hope that as soon as may be practicable the astronomical and nautical days will be arranged everywhere to begin at midnight." this resolution, so far as i understand it, will be the warning to astronomers to begin to make the changes growing out of this resolution which may be necessary for seamen. therefore, i consider that we may at once proceed to vote upon the question whether the day is to commence at midnight or noon, without any reference to the practice or interests of navigation. in reality, it does not appear to me to affect that subject at all. i have given some consideration to the practical bearings of this question--whether it should be midnight or noon. what we ought to decide is what will be the least inconvenience to the world at large. i have ascertained from two of my colleagues, who have given this matter the greatest consideration, that the adoption of midnight will really cause less confusion than noon, for this reason, that all the great colonies of the world would be less affected; that is to say, that the times they are using now would be less affected by midnight than by noon. that being so, it appears to me to be an essential point in coming to a settlement of this question. mr. ruiz del arbol, delegate of spain. i have only to say that i have listened to the remarks about navigators changing the reckoning of time. i do not know whether there are many navigators here, but it is a fact that seamen reckon the day from noon. the president. i beg the pardon of the delegate of spain; but, in the united states navy, we reckon the day from midnight. mr. ruiz del arbol, delegate of spain. i am speaking generally. now, there is some reason for this rule among seamen, for the only way to find out the position of a ship is to observe the meridian altitude of the sun; and everybody requires to know, at sea, what has taken place in the course of every day, from the beginning to the last moment of the day; and i think that whatever the rule may be in the united states navy, navigators generally will count their time as they count it now. i think that navigators will not change the rule now in force, no matter what we may adopt in this conference. commander sampson, delegate of the united states. i think, mr. president and gentlemen, that the change to the adoption of the universal day, beginning at midnight, would be a very decided advantage to navigators. the quantities as now given in the nautical ephemerides are for noon of the meridian for which they are computed, as washington, greenwich, &c. it is very evident that every navigator, in making use of the quantities given in the nautical almanac, must find the corresponding time at greenwich, wherever he may be on the surface of the earth. consequently, if we suppose that navigators are pretty equally distributed, one-half on one side of the earth and one-half on the other side, the greenwich day for one portion would be the local night for the other. the usual observations made by navigators at sea consist in a meridian observation of the sun for latitude, and a morning and possibly afternoon observation of the sun near the prime vertical for longitude. consequently all navigators, when in the vicinity of the initial meridian, might have their day's work occurring in two astronomical days. on the other hand, those navigators who were in the neighborhood of the th meridian would have all their work of one day occurring in the same astronomical day. the first would have the advantage of interpolating for short intervals only, while the second would be obliged to interpolate for much larger intervals. consequently, on the whole, it would make no difference to navigators whether the quantities given in the nautical almanacs were for noon or midnight of the initial meridian. another consideration, however, would make it very advantageous to have the quantities given for midnight. that consideration is this: if midnight were chosen, then the universal day would be identical with the nautical almanac day, and navigators would have only ship time and universal time to deal with, while, if the quantities were given for noon, they would have astronomical time, in addition to the other two. this consideration i think a very important one. the president. the question will be on the amendment offered by the delegate of sweden, count lewenhaupt, which has been read. the vote was then taken, as follows: states voting in the affirmative: austria, sweden, italy, switzerland, netherlands, turkey. in the negative: brazil, japan, chili, liberia, colombia, mexico, costa rica, paraguay, great britain, russia, guatemala, united states, hawaii, venezuela. abstaining from voting: france, san domingo, germany, spain. ayes, ; noes, ; abstaining from voting, . the president then announced that the amendment was lost. the question then recurred on the original resolution offered by the delegate of the united states. rustem effendi, delegate of turkey. mr. president, i have listened with a great deal of interest and attention to the learned arguments bearing upon the proposition under discussion offered by the hon. mr. rutherfurd, the delegate of the united states for the adoption of a universal hour. this question is of such high importance, and of such interest to every one, that i consider it my duty to make a few remarks upon the subject, as i wish to state clearly the position my government proposes to take in the matter. i do not pretend to discuss scientifically this subject, which has already been so ably treated by several of the gentlemen present. my task is of a different and inferior order. i merely propose to briefly examine the manner in which the proposition ought to be made, in order that it may be adopted by our respective governments. the question of a universal hour is not of equal interest and importance to all. the united states of america, although comparatively a young nation, have done so much in the pursuit of science and scientific investigation that they must have more than a common interest on the subject. the vast expanse of their country, stretching over sixty degrees of longitude, with a difference of time of more than four hours, almost compels them to adopt a universal hour. the thousands of miles of railroad tracts covering this continent, facilitating the intercourse between distant places, necessitate a uniform system to avoid confusion. it was, therefore, natural that the united states and canada should have taken the lead in proposing such a reform, which would likewise benefit other countries, as, for instance, the british empire, russia, and germany. but there are, at the same time, other countries, like france, spain, italy, scandinavia, etc., that may content themselves with a national hour, owing to the small difference in time within their dominion. for them, the adoption of a universal hour would only be of secondary importance, because it would only affect their international relations. i hope i may be permitted to remind you of the conclusions arrived at by a commission consisting of scientists, railroad and telegraph officials, &c., appointed by the french government to express their opinion upon this subject. if i am not mistaken, they recommended a universal hour, stating, however, at the same time, that the benefit to be derived from such an hour would be only of secondary importance for their country. the learned delegate from france, professor janssen, will probably be kind enough to inform us whether i am right or not. the few remarks i have made bring me to the point i wanted to consider more specially. i mean that the originators of the pending proposition, and those directly interested in it, should be induced to modify their proposition somewhat if they wish it to be adopted by other countries. in other words, to leave to each country the greatest latitude possible in adopting a universal hour. with regard to the ottoman empire, i must state that it is placed in a somewhat exceptional position in this respect, and is, therefore, obliged to ask for more latitude even than the other countries concerned. in our country we have two modes of reckoning time: one from noon to noon, or from midnight to midnight, as everywhere else, (heure à la franque), the other (heure à la turque) from sundown to sundown. in this latter case the hours count from the moment when the disk of the sun is bisected by the horizon, and we count twice from _ h._ to _ h._, instead of counting without any interruption from _ h._ to _ h._ we are well aware of the inconveniences this system of counting produces, because _ h._ necessarily varies from day to day, for the interval of time between one sunset and the one following is not exactly hours. according to the season the sun will set earlier or later, and our watches and clocks at constantinople will be at most about three minutes fast or slow from day to day, according to the season. reasons of a national and religious character prevent us, however, from abandoning this mode of counting our time. the majority of our population is agricultural, working in the fields, and prefer to count to sunset; besides, the hours for the moslem prayers are counted from sundown to sundown. therefore it is impossible for us to abandon our old system of time, although in our navy we generally use the customary reckoning or "heure à la franque." finally, permit me to state that i am ready to cast my vote in favor of a universal hour, with the precise understanding that the universal hour will have to be limited to international transactions, and that will not interfere with the rules up to now in force in my own country. before resuming my seat i wish to thank the president and the members of the conference for their kind indulgence in having listened to my remarks. the president, the chair would remind the delegate of turkey that the following resolution was passed at our last session: "_resolved_, that the conference propose the adoption of a universal day for all purposes for which it may be found convenient, and which shall not interfere with the use of local or other standard time where desirable." the very difficulty which the delegate of turkey anticipates was thus carefully provided for in the resolution just read. mr. sandford fleming, delegate of great britain. to my mind it is of very great importance that this resolution should be adopted. i have already given generally my views on this question, and therefore i do not intend to trespass on the attention of the conference beyond saying a very few words. from what i have already ventured to submit, it will be obvious that i hold that all our usages in respect to the reckoning of time are arbitrary. of one thing there can be no doubt. there is only one, and there can only be one flow of time, although our inherited usages have given us a chaotic number of arbitrary reckonings of this one conception. there can be no doubt of another matter; the progress of civilization requires a simple and more rational system than we now have. we have, it seems to me, reached a stage when a unification of the infinite number of time-reckonings is demanded. this unification will be, to a large extent, accomplished if the resolution be adopted, and by adopting it, it seems to me to be in the power of the conference to confer lasting benefits on the world. universal time will in no way interfere with local time. each separate community may continue the usages of the past in respect to local time, or may accept whatever change the peculiar conditions in each case may call for. but the use of universal time will not necessarily involve a change; it will rather be something added to what all now possess. it will be a boon to those who avail themselves of it. to the east of the prime meridian all possible local days will be in advance; to the west all possible days will be behind the universal day. the universal day, as defined by the resolution, will at once be the mean of all possible local days, and the standard to which they will all be related by a certain known interval, that interval being determined by the longitude. in my judgment, the resolution is an exceedingly proper one, and the conference will act wisely in passing it. the president. in taking the vote upon the resolution, it is requested that the roll be called. the following states voted in the affirmative: brazil, liberia, chili, mexico, colombia, netherlands, costa rica, paraguay, great britain, turkey, guatemala, united states, hawaii, venezuela. japan, states voting in the negative: austria-hungary, spain. abstaining from voting: france, san domingo, germany, sweden, italy, switzerland. netherlands, ayes, ; noes, ; abstained, . the president then announced that the resolution was passed. mr. rutherfurd, delegate of the united states. mr. president, i now present for the consideration of the conference the following resolution: "_resolved_, that the conference expresses the hope that as soon as may be practicable the astronomical and nautical days will be arranged everywhere to begin at midnight." before action is taken upon this resolution, i would make a verbal correction. i think that the word "_mean_" ought to be introduced before the word "_midnight_" and i therefore alter my resolution in that way. the vote was then taken upon the resolution just offered, and it was carried without division. the president. the chair begs leave to state that the protocols in french and in english of the first and second sessions of the conference, have been examined, and are now before the conference for adoption. if any delegate wishes to make any correction in these protocols, he can submit it to the conference, and, if approved, it can be immediately made. no objection was raised, and the president put the question to the conference on the adoption of the protocols of the first and second sessions in french and english, and they were unanimously adopted. m. janssen, delegate of france. mr. president, we have been directed to present for the approval of the congress the desire that studies relative to the application of the decimal system to the division of angular space and of time should be resumed in order that this application may be extended to all cases--and they are numerous and important--where it presents real advantages. i would say that a similar desire upon the same subject was expressed by the conference at rome. you are aware, gentlemen, that at the time of the establishment of the metrical system the decimal division had been extended to the measurement of angular space and of time. numerous instruments were even made according to the new system. as to time, the reform was introduced too abruptly, and, we might say, without enough discretion, and it came into conflict with old habits and was quickly abandoned; but as to the division of angular space, in which the decimal division presented many advantages, the reform sustained itself much better, and is still used for certain purposes. so, the division of the circumference into parts was adopted by laplace, and we find it constantly employed in the mécanique celeste. delambre and mechain used, for the measurement of the are of the meridian from which the metre was derived, repeating circles divided into "_grades_." finally, in our own time, colonel perrier, chief of the geographical division of our department of war, has used instruments decimally divided, and at the present time logarithmic tables appropriate to that method of division are in course of calculation. but it is especially when it is a question of making long calculations of angular space that the decimal system presents great advantages. in this respect we find, so to speak, only one opinion expressed by scientists. the conference at rome, which brought together so many astronomers, geodetists, eminent topographers--that is to say, the men most competent and most interested in the question--expressed in respect to it a desire, the high authority of which it is impossible to mistake. it is, therefore, now evident that the decimal system, which has already done such good service in the measurements of length, volume, and weight, is called upon to render analagous services in the domain of angular dimensions and of time. i know that this question of the decimal division encounters legitimate doubts, principally as to its application to the measurement of time. it is feared that we want to destroy habits fixed for centuries, and upset established usages. in this respect, gentlemen, i think that we ought to be fully satisfied. the teachings of the past will be respected. it will be perceived that if we failed at the time of the revolution, it is because we put forward a reform which was not limited to the domain of science, but which did violence to the habits of daily life. it is necessary to take the question up again, but with due regard to the limits which common sense and experience would prescribe to wise and well-informed men. i think that the character of the reform would be well defined by saying that it is intended especially to make a new effort towards the application of the decimal system in scientific matters. but, gentlemen, i have not to discuss here the bearing of the reforms which the study of this question will lead to. it is sufficient for me to show that there is in that direction an indispensable step to be made, and to ask you to express the desire that the question should be studied. i do not think that there is anybody here who would desire to oppose a request which does not in truth commit us to any specific solution of the question, and which appears so opportune at the present time. i would ask the president to be so kind as to submit the following proposition to the conference: "_resolved_, that the conference expresses the hope that the studies designed to regulate and extend the application of the decimal system to the division of angular space and of time shall be resumed, so as to permit the extension of this application to all cases where it presents real advantages." the president. the chair is of opinion that the conference was called for a special and somewhat narrow purpose, and the consideration of the decimal system, proposed by the delegate of france, seems to it foreign to that purpose and beyond the scope of the conference. the president, however, simply acts for the conference, and if the conference shall decide to take the matter up, he will acquiesce, but it strikes the chair that the resolution is out of order. gen. strachey, delegate of great britain. sir, i desire to express my personal views on this subject. i should be very happy to join the delegate of france in voting for such a resolution, but i fear that there is a feeling among many of the delegates that it is not within our competence to discuss it. if that is so, i would suggest whether it might not be better that it should not be pressed to a vote. it would be a pity if there should be on the records of the proceedings of this conference anything in the shape of a vote against the subject-matter of this resolution. i consequently think that if delegates have formed any decided opinion on the subject, they might express their opinion without voting; but i repeat that it would be a great pity if a negative vote should be taken on the subject of the decimal system of dividing the circle and time, particularly as it was received with unanimity in the conference at rome. prof. adams, delegate of great britain. mr. president, i may say that while i agree with gen. strachey in thinking that i should not like to vote against the proposition brought forward by our eminent colleague, mr. janssen, yet i feel it is somewhat beyond the scope of the subjects which we have to discuss, and, therefore, i should abstain from voting. i quite recognize that, for certain purposes, the decimal division of the circle is very valuable. the president. unless the conference decides to entertain this proposition, the chair suggests that no discussion shall take place. if any member present desires to bring the matter up, he can do so by taking an appeal from the decision just made. gen. strachey, delegate of great britain. do i understand, sir, that the subject is dropped? the president. the chair has decided that the resolution offered by the delegate of france is out of order, and unless a difference of opinion is expressed by the conference, the subject will be dropped. the chair wishes to treat with the most distinguished deference the delegate of france, because we are all most happy to do honor to him in every way. does the chair understand that the delegate of france appeals from its decision, and wishes to take the sense of the conference upon it? mr. janssen, delegate of france, replied in the affirmative. commodore franklin, delegate of colombia. mr. president, i would like hear the resolution read again. if it be merely a suggestion to consider the subject of the decimal system, i should like to know it. the vote was then taken upon the appeal of the delegate of france from the decision of the chair. states voting in favor of the appeal: austria-hungary, netherlands, brazil, san domingo, chili, spain, france, switzerland, italy, turkey, japan, venezuela. mexico, states voting against the appeal: colombia, hawaii, costa rica, liberia, germany, paraguay, great britain, united states. guatemala, abstaining from voting: russia, sweden. ayes, ; noes, ; abstained, . the president. the appeal from the decision of the chair is sustained, and the proposition offered by the delegate of france is now before the conference. if no delegate wishes to speak upon the resolution, the vote will be taken. mr. janssen, delegate of france. mr. president, before the definitive vote i desire to again call my colleague's attention to the fact that it is a question here of the much-needed extension of the decimal system, an extension desired by a large number of the highest scientific authorities and of the most distinguished observers. as i said only a moment ago, the congress at rome, whose high authority in the matters which have occupied us is acknowledged, was a still higher authority as to astronomy, geodesy, topography; that is to say, in the domain to which our proposition relates. at rome a wish, similar to that which we ask you to formulate, was expressed. besides, if we observe that it is a question here only of expressing the desire that studies should be resumed upon the matter in question, is there anyone among us who would wish to oppose the liberal proposition which prejudges nothing in the solution of the question, but which will surely lead to important progress. i do not doubt, then, that all our colleagues will desire to unite in a resolution, which by its object and by the manner in which it is expressed, ought, it appears to me, to unite the suffrages of all. no further remarks were made upon the resolution, and the vote was accordingly taken on the question whether it should be adopted. states voting in the affirmative: austria-hungary, mexico, brazil, netherlands, chili, paraguay, colombia, russia, costa rica, san domingo, france, spain, great britain, switzerland, hawaii, turkey, italy, united states, japan, venezuela. liberia, states voting in the negative: none. abstained from voting: germany, sweden. guatemala, ayes, ; noes, ; abstained, . the president. the resolution of the delegate of france is, therefore, adopted. general strachey, delegate of great britain. sir, before concluding the session to-day, i hope that the delegates will be in a position to listen to the two resolutions which i now desire to propose, and which i think will tend to clear up a good deal of the discussion which we have had. the first of these resolutions is as follows: "the conference adopts the opinion that, for the purposes of civil life, it will be convenient to reckon time, according to the local civil time at successive meridians destributed round the earth, at time-intervals of either ten minutes, or some integral multiple of ten minutes, from the prime meridian; but that the application of this principle be left to the various nations or communities concerned by it." this resolution, as it stands, embraces all the practical suggestions which have been made on the subject up to the present time. the only limitation it proposes to put upon the adoption of what may be called local standard time is that the breaks shall be at definite intervals of ten minutes or more. the second resolution which i propose is a very simple one. it is this: "the arrangements for adopting the universal day in international telegraphy should be left for the consideration of the international telegraph congress." there has been established by an international arrangement a congress which meets every two years to settle questions of international telegraphy, and i think that the precise manner in which universal time may be adapted to telegraphy would very properly be left to that congress. mr. de struve, delegate of russia. on behalf of the delegates of russia, i beg to make the following remarks: we have already expressed the opinion that the universal time could be properly used for international postal, railway, and telegraphic communications. but it is to be understood that local or any other standard time, which is intimately connected with daily life, will necessarily be used side by side with the universal time. it has been proposed, in order to establish an easier connection between local and universal time, to accept twenty-four meridians at equal distances of hour or °, or to divide the whole circumference of the earth by meridians at distances of minutes of time or / °. this question not yet having been made the subject of special and thorough investigation by the respective governments, and not having been discussed at the international conference at rome, we believe that it would as yet be difficult to express, in regard to europe, any positive opinion on the practical convenience of the above mentioned or other possible methods of dividing the globe into equal time-zones. we would suggest to recommend that the system of counting the hours of the universal day from to , which probably will be adopted for the universal day, might also be introduced for counting the local time side by side with the old method of counting the hours of to a. m. and to p. m. count lewenhaupt, delegate of sweden. i have had the honor to transmit to the members of the conference a résumé of a report on this subject made by professor gyldén, an eminent swedish astronomer, whose name, no doubt, is familiar to many of the delegates. the system proposed by mr. gyldén is similar to the one now proposed by the delegate for great britain. the only difference is that mr. gyldén, in explaining the system, recommends the adoption of equidistant meridians, separated by intervals of / °, or minutes of time, while the proposition of the delegate for great britain is so worded that this distance may be greater than minutes. this difference is, however, only a question of detail. the basis of mr. gyldén's system is that time meridians should be separated from the standard initial meridian by either or some integral multiple of minutes. therefore, i shall, with pleasure, vote for the resolution of the delegate from great britain. i beg only permission of the conference to insert mr. gyldén's report as part of my remarks: _rÉsumÉ of a report read before the swedish geographical society by hugo gyldén, professor of astronomy and member of the academy of sciences in stockholm, concerning the use of equidistant meridians for the fixation of the hour._ if we suppose the meridian passing through the observatory of greenwich extended round the globe, this grand circle will cut the equator, at ° from greenwich, at some place a little east of new zealand. this meridian falls almost entirely in the ocean, and cuts, in any case, not more than a few small islands in the pacific. if we suppose, further, another great circle at ° from the meridian of greenwich, the western half touches very nearly new orleans, and the eastern half passes a few minutes from calcutta. if, now, the hour is fixed according to these four meridians, we have four cardinal times--one european, one american, one asiatic, and one oceanic. it will, however, be necessary to fix much more than one civil time for europe. therefore i suppose for europe a whole system of meridians, which, however, ought not to be closer together than / °. the difference of time between these meridians is then only minutes, which, in general, can be considered as an insignificant difference between the civil and the true solar time. the starting point of this system is the meridian of greenwich. to the west the system ought to extend minutes; to the east / hours, or to a meridian passing near moscow. i suppose as time zero the meridian of greenwich. the next meridian to the east is meridian . this meridian will not pass far from the observatory of paris, because the difference between this meridian and the meridian of paris is only seconds, an insignificant difference in civil life. the meridian can be called the meridian of paris, or french meridian. the second meridian (to the east of greenwich) does not touch utrecht, but will pass so close that the time of this city could, without the least inconvenience, be regulated as if the difference of time between greenwich and utrecht were exactly minutes. the second meridian would also pass almost as close to amsterdam, ( s.,) and would not be far from marseilles, ( m. s.) in the vicinity of the third meridian we have, first, bern, ( s.;) next, a little further, turin, ( s.) the fourth meridian is close to hamburg, altona, and gottingen, (respectively s. and s.) not far from the same meridian is christiania, although at a distance of a little over minutes. the fifth meridian passes also close to three large cities--rome, ( s.,) leipzig, ( s.,) and copenhagen, ( s.) the sixth meridian does not touch any city of importance, but it coincides very nearly with the meridian adopted for the normal civil time in sweden; the difference amounts only to seconds. the seventh meridian touches the little town of brieg, in the vicinity of breslau, and königsberg is situated two minutes from the eighth. the ninth meridian passes less than one minute to the west of abo, and is situated at a distance of only a few seconds from mistra, a town in greece. the tenth meridian almost touches helsingfors in finland. as regards the eleventh meridian, i have not been able to find any locality of importance exactly so situated that it merits a place in this list, but i can, however, mention the cities of minsk and jassy. the twelfth meridian is situated m. s. to the west of the academy of sciences, in st. petersburg, and the distance from kiew is about the same. it is not necessary to continue the enumeration of the other meridians to the east by intervals of minutes, but i will mention that moscow is situated _ h. m. s._ to the east of greenwich, and in consequence the system would be convenient with regard to this city. if we pass to the west of greenwich, we will find that the first meridian west touches the little town of almeria, in the south of spain, which country extends to equal distances on both sides of this meridian, east and west, and the situation of portugal is the same with regard to the third meridian west. then, in all the towns and localities given above, of which the greater part are of some importance, the local time coincides so closely with times differing from the greenwich time, by whole multiples of minutes, that there is no reason to fear any real inconvenience if these times were taken to regulate local reckonings. if the different countries in europe should decide to adopt the system which i have explained, the following system of normal times would, perhaps, be found convenient: east of greenwich. st meridian, france. d " holland and belgium. d " switzerland. th " norway, (and western germany.) th " denmark, germany, and italy. th " sweden and austria. th " eastern germany. th " hungary. th " poland and greece. th " finland, roumania, and bulgaria, th " european turkey. th " western russia. west of greenwich. st meridian, spain. d " portugal. it is, however, not at all necessary that each country should adopt a single civil time for the whole of its territory. if several normal times should be adopted, it is still possible to use the system, provided only the several times differ from greenwich time by minutes, minutes, &c.; but it would be necessary that the clocks should indicate the times adopted with great precision, and that the difference did not amount to even a few seconds, because otherwise the advantages of the adoption of the system would be materially reduced. this circumstance, that it is possible for each country to adopt the system, and at the same time to maintain a certain independence with regard to the adoption of the most convenient normal times, is of considerable importance with regard to the possibility of introducing a system of this kind. in fact, it is possible to arrive at the application of the system in such a way that the transition would hardly be observed by the great majority of the population. as regards railroads and telegraphs, the advantages would be the same as if the local times were everywhere identical, because it is easy to remember the multiple of minutes which ought to be added to the time of a given country for translation into the time of another country. the difference of time between sweden and denmark would, for instance, be minutes--a circumstance which everybody would soon learn to remember. a traveller leaving sweden would then know that his watch, if correct, shows exactly minutes more than the clocks of the danish railroad stations, and if he continued his voyage to paris, he would know that the clocks of paris are exactly minutes behind the clocks in sweden. i have tried to explain the advantages of this system for the countries in europe. i am not able to judge if similar systems can be considered necessary in america and asia. it is possible that north america could be satisfied with one single normal time, which, if america connects this time with the european system, ought to be fixed exactly hours behind greenwich. while starting from this normal meridian, it is possible to establish a more or less elaborate system of equidistant times analogous to the system which has been proposed for europe. the same can be said of the civil times of asia, which ought to be connected with a normal time hours in advance of the time of greenwich. africa ought to belong to the european system. the french civil time could be adopted for algeria and tunis; the time of denmark, germany, and italy for tripoli; for egypt the time of russia; the spanish time for morocco; at the mouth of the congo where, no doubt, sooner or later, an important centre of civilization will rise, the meridian of sweden and austria could be used; the meridian of hungary could be adopted for the cape of good hope. it will not be possible to connect south america and australia with any of the four cardinal times mentioned, but some other combination, into which it is not necessary to enter on this occasion, can easily be found. the president. if the chair hears no objection, the pamphlet referred by the delegate of sweden will be printed as proposed. mr. lefaivre, delegate of france. mr. president, i move that the conference adjourn until wednesday, at one o'clock p. m. the motion was put and agreed to, and the conference thereupon adjourned at : p. m. until wednesday, the d inst., at one o'clock p. m. vii. session of october , . the conference met pursuant to adjournment in the diplomatic hall of the department of state, at one o'clock p. m. present: austria-hungary: baron ignatz von schÆffer. brazil: dr. luiz cruls. chili: mr. f. y. gormas and mr. a. b. tupper. colombia: commodore s. e. franklin. costa rica: mr. juan francisco echeverria. france: mr. a. lefaivre, mr. janssen. germany: baron h. von alvensleben, mr. hinckeldeyn. great britain: sir f. j. o. evans, prof. j. c. adams, lieut.-general strachey, mr. sandford fleming. guatemala: mr. miles book. hawaii: hon. w. d. alexander, hon. luther aholo. italy: count albert de foresta. japan: professor kikuchi. liberia: mr. wm. coppinger. mexico: mr. leandro fernandez, mr. angel anguiano. netherlands: mr. g. de weckherlin. paraguay: capt. john stewart. russia: mr. c. de struve, major-general stebnitzki, mr. j. de kologrivoff. san domingo: mr. de j. galvan. spain: mr. juan valera, mr. emilio ruiz del arbol, and mr. juan pastorin. sweden: count carl lewenhaupt. switzerland: col. emile frey. turkey: rustem effendi. united states: rear-admiral c. r. p. rodgers, mr. lewis m. rutherfurd, mr. w. f. allen, commander w. t. sampson, professor cleveland abbe. venezuela: dr. a. m. soteldo. absent: denmark: mr. c. s. a. de bille. salvador: mr. antonio batres. the president. the first business before the conference to-day is the resolutions offered by the delegate of great britain, general strachey; but before we proceed the delegate of san domingo, mr. galvan, asks permission, as a matter of privilege, to read a communication to the conference. mr. galvan, the delegate of san domingo. before the sessions of the conference come to a close, i feel compelled to make a declaration which will be a tribute to the illustrious scientists who have directed the decisions of the majority of the conference, and at the same time a reservation of future freedom of action to the country which i have the honor to represent. the negative vote of san domingo on the principal question was entirely in consequence of the proposal by the delegates of france of a neutral international meridian, which was rejected by the conference. san domingo, which had no part in the various important interests connected with the meridian of greenwich, was bound to regard equity alone on the occurrence of the disagreement produced by the proposal of the delegates of france, a nation renowned for being one of the first in intellectual progress. at the last session i was glad that another proposal of the delegates of france was accepted almost unanimously by the conference. that fact should be considered as a good omen of a more complete and unanimous agreement at some future time in behalf of the general interest of science. that day will be saluted with a cordial _hosanna_ by the republic of san domingo, which is always ready freely to give its assent to the progress of civilization. the president. the resolutions offered by the delegate of great britain, general strachey, are now before the conference, and will be read. the resolutions were then read, as follows: " . the conference adopts the opinion that, for the purposes of civil life, it will be convenient to reckon time according to the local civil time at successive meridians distributed round the earth, at time-intervals of either ten minutes, or some integral multiple of ten minutes, from the prime meridian; but that the application of this principle be left to the various nations or communities concerned by it." " . the arrangements for the use of the universal day in international telegraphy should be left for the consideration of the international telegraph congress." general strachey, delegate of great britain. in consequence of the opinions i have heard expressed regarding the resolutions which i brought forward at our last meeting, i feel constrained to say that i am not disposed to ask the congress to proceed to a vote upon them. i find that, although i had reason to think that those resolutions, in substance, that is in their main features, would be acceptable, still there is extreme difficulty in finding precise expressions that shall meet the views of everybody, and there are divisions of opinion as to the exact manner in which these resolutions should be modified. my object in bringing forward the resolutions was mainly to obtain a decided expression of opinion on the part of the congress, that the method of counting local time, so as to harmonize as far as possible with universal time, should be left for settlement locally; and that, at the utmost, all the congress could do would be to suggest some general principle such as that embodied in my resolution. there was, of course, never any intention of employing the universal day so as to interfere with the use of local standard time; and as i shall, no doubt, elicit a further clear expression of opinion on the part of the delegates, that there is no intention of bringing about this interference, i will now, with the permission of the conference, withdraw the resolutions. mr. rutherfurd, delegate of the united states. mr. president, i think that all of us appreciate the desire which moved the delegate of great britain to present these resolutions. there is a wish on his part that we should not seem, in any way, by our action here, to interfere with the convenience of the world in the use of its present civil time, or any other time which it may be found convenient to adopt, while he recognizes that some of the proposals made as to local time are such as could not be objected to. still, i cannot refrain from expressing my satisfaction that he has come to the conclusion that these resolutions are not necessary. i think the whole question is covered by the resolutions already adopted by this congress; that our universal day is for those purposes only for which it may be found convenient, and that it is not to interfere in any way with the use of civil or other standard time where that may be found convenient. this seems to me to be so fully embodied in our resolutions that it is unnecessary to enunciate again in a negative form the same idea, and i therefore express my satisfaction that the resolutions are withdrawn. mr. sandford fleming, delegate of great britain. mr. president, i have a few words bearing on the subject before the conference which i wish to express before any action is taken. the president. there will be no subject before the congress if the resolutions of general strachey are withdrawn, and the chair understands that the object of general strachey in withdrawing these resolutions was to avoid a discussion upon a subject that could hardly lead to any satisfactory conclusion. if, however, mr. fleming desires to address the conference, he will be at liberty to do so. mr. fleming, delegate of great britain. i do not wish to intrude any new matter upon the conference. what i had to say had a bearing upon the subject, but, if the resolutions are withdrawn and the conference desires to end the matter, i shall not insist upon speaking. no objection being made, the resolutions offered by general strachey at the last session of the conference were then withdrawn. count lewenhaupt, delegate for sweden, then proposed that the resolutions passed by the conference should be formally recorded in a final act, stating the votes on each resolution that was adopted. the conference took a recess, in order to allow the delegates to examine the draft of the final act. after the recess the final act was unanimously adopted, as follows: final act. the president of the united states of america, in pursuance of a special provision of congress, having extended to the governments of all nations in diplomatic relations with his own, an invitation to send delegates to meet delegates from the united states in the city of washington on the first of october, , for the purpose of discussing, and, if possible, fixing upon a meridian proper to be employed as a common zero of longitude and standard of time-reckoning throughout the whole world, this international meridian conference assembled at the time and place designated; and, after careful and patient discussion, has passed the following resolutions: i. "that it is the opinion of this congress that it is desirable to adopt a single prime meridian for all nations, in place of the multiplicity of initial meridians which now exist." this resolution was unanimously adopted. ii. "that the conference proposes to the governments here represented the adoption of the meridian passing through the centre of the transit instrument at the observatory of greenwich as the initial meridian for longitude." the above resolution was adopted by the following vote: in the affirmative: austria-hungary, mexico, chili, netherlands, colombia, paraguay, costa rica, russia, germany, salvador, great britain, spain, guatemala, sweden, hawaii, switzerland, italy, turkey, japan, united states, liberia, venezuela. in the negative: san domingo. abstaining from voting: brazil, france. ayes, ; noes, ; abstaining, . iii. "that from this meridian longitude shall be counted in two directions up to degrees, east longitude being plus and west longitude minus." this resolution was adopted by the following vote: in the affirmative: chili, liberia, colombia, mexico, costa rica, paraguay, great britain, russia, guatemala, salvador, hawaii, united states, japan, venezuela. in the negative: italy, sweden, netherlands, switzerland. spain, abstaining from voting: austria-hungary, germany, brazil, san domingo, france, turkey. ayes, ; noes, ; abstaining, . iv. "that the conference proposes the adoption of a universal day for all purposes for which it may be found convenient, and which shall not interfere with the use of local or other standard time where desirable." this resolution was adopted by the following vote: in the affirmative: austria-hungary, mexico, brazil, netherlands, chili, paraguay, colombia, russia, costa rica, salvador, france, spain, great britain, sweden, guatemala, switzerland, hawaii, turkey, italy, united states, japan, venezuela. liberia, abstaining from voting: germany, san domingo. ayes, ; abstaining, . v. "that this universal day is to be a mean solar day; is to begin for all the world at the moment of mean midnight of the initial meridian, coinciding with the beginning of the civil day and date of that meridian; and is to be counted from zero up to twenty-four hours." this resolution was adopted by the following vote: in the affirmative: brazil, liberia, chili, mexico, colombia, paraguay, costa rica, russia, great britain, turkey, guatemala, united states, hawaii, venezuela. japan, in the negative: austria-hungary, spain. abstaining from voting: france, san domingo, germany, sweden, italy, switzerland. netherlands, ayes, ; noes, ; abstaining, . vi. "that the conference expresses the hope that as soon as may be practicable the astronomical and nautical days will be arranged everywhere to begin at mean midnight." this resolution was carried without division. vii. "that the conference expresses the hope that the technical studies designed to regulate and extend the application of the decimal system to the division of angular space and of time shall be resumed, so as to permit the extension of this application to all cases in which it presents real advantages." the motion was adopted by the following vote: in the affirmative: austria-hungary, mexico brazil, netherlands, chili, paraguay, colombia, russia, costa rica, san domingo, france, spain, great britain, turkey, hawaii, united states, italy, venezuela. japan, abstaining from voting: germany, sweden. guatemala, ayes, ; abstaining, . done at washington, the d of october, . c. r. p. rodgers, _president_. r. strachey, j. janssen, l. cruls, _secretaries._ the following resolution was then adopted unanimously: "that a copy of the resolutions passed by this conference shall be communicated to the government of the united states of america, at whose instance and within whose territory the conference has been convened." mr. rutherfurd, delegate of the united states, then presented the following resolution: "_resolved_, that the conference adjourn, to meet upon the call of the president, for the purpose of verifying the protocols." this resolution was then unanimously carried, and the conference adjourned at half past three, to meet upon the call of the president. viii. session of november , . the conference met at the call of the president for the approval of the protocols, as arranged at the last meeting, in the diplomatic hall of the department of state, at o'clock p. m. the president having called the conference to order, said: the protocols in french and english, having been examined by the secretaries of the conference, have been submitted to all of the delegates for perusal. if any delegate should desire to make any observation on them the opportunity is now given for his doing so. rustem effendi, delegate of turkey, stated that he desired to change his vote on the fifth resolution of the final act, providing for the commencement of the universal day, from the affirmative to the negative. no objection being made, the change was ordered to be made. the president then said: no further observations having been made on the protocols, they will now be signed by the secretaries and the president. mr. de struve, delegate of russia. before the conference terminates, i beg to express, in the name of my colleagues, our sincere gratitude for the hospitality extended to the conference by the government of the united states, and i beg to express our heartiest thanks to you, mr. president, for the able and impartial manner in which you have presided over our deliberations. when we elected you, we unanimously elected the first delegate of the united states. if we had to begin again, the personal feelings of all the delegates would supply powerful additional reasons for making the election equally unanimous. mr. de struve's observation met with the unanimous approval of the delegates. the president. gentlemen, i am greatly honored by the kind expression of your good feeling towards me as the president of this conference, and i thank you very heartily for it. the duty assigned to us all has not been free from difficulty, but our meetings and discussions have been characterized by great courtesy and kindness, and by a conciliatory spirit. with patience and devotion the delegates to this congress have sought to discharge the trust committed to them, and, as your chairman, i beg you to receive my most cordial thanks for the courteous consideration i have received at your hands. the president of the united states and the secretary of state desire me to renew to you their thanks for your presence here, and their best wishes for your safe and happy return each to his own home. i shall esteem myself very happy hereafter whenever i shall have the good fortune to meet any of my colleagues of the international meridian conference. mr. rutherfurd, the delegate of the united states. mr. president and gentlemen, i am sure that you will all unite with me in passing the resolution which i now propose to read: "_resolved_, that the thanks of the conference be presented to the secretaries for the able manner in which they have discharged their arduous duties." the resolution was unanimously adopted. general strachey, delegate of great britain. i wish, sir, as one of the secretaries, to express my thanks for the manner in which my labors have been esteemed by the delegates present. all that i can say on the subject is, that however troublesome the duties of the secretaries have been, i have not the least doubt that anybody else named instead of myself would equally have bestowed his best attention on the discharge of those duties. mr. janssen, delegate of france, then said: before the dissolution of the conference, mr. cruls and i desire specially to thank our colleagues for the honor they have done us by entrusting to us the revision of the french version of the protocols. in order that we might fully respond to that honor, we have examined with all possible care the french translations of the remarks of our colleagues. our only regret is that, in consequence of the desire of several of them to quit washington, we have been obliged to leave portions of the translations, particularly of the last protocols, much in the state in which we received them from the official translators, not having had the time to correct these translations as we would have desired. upon motion of mr. janssen, delegate of france, the conference passed a vote of thanks to the delegate of turkey for the aid he has rendered the secretaries in the revision of the protocols. the president then said: before our final adjournment i desire to express a very high appreciation of the ability, fidelity, and zeal with which mr. w. f. peddrick, the secretary attached by the department of state to this conference, has performed his difficult duties, and to thank him for his services. the conference expressed its cordial assent to these observations. the president then declared that the business of the conference having been concluded, it would adjourn _sine die_. c. r. p. rodgers, _president._ r. strachey, j. janssen, l. cruls, _secretaries._ annex i. an act to authorize the president of the united states to call an international conference to fix on and recommend for universal adoption a common prime meridian, to be used in the reckoning of longitude and in the regulation of time throughout the world. _be it enacted by the senate and house of representatives of the united states of america in congress assembled_, that the president of the united states be authorized and requested to extend to the governments of all nations in diplomatic relations with our own an invitation to appoint delegates to meet delegates from the united states in the city of washington, at such time as he may see fit to designate, for the purpose of fixing upon a meridian proper to be employed as a common zero of longitude and standard of time-reckoning throughout the globe, and that the president be authorized to appoint delegates, not exceeding three in number, to represent the united states in such international conference. approved august , . * * * * * annex ii. an act making appropriations for sundry civil expenses of the government for the fiscal year ending june thirtieth, eighteen hundred and eighty-five, and for other purposes. _be it enacted by the senate and house of representatives of the united states of america in congress assembled_, that the following sums be, and the same are hereby, appropriated for the objects hereinafter expressed for the fiscal year ending june thirtieth, eighteen hundred and eighty-five, namely: under the state department: for expenses of the international conference for fixing a common zero of longitude and standard of time-reckoning, including cost of printing and translations, to be expended under the direction of the secretary of state, five thousand dollars; and the president is hereby authorized to appoint two delegates to represent the united states at said international conference, in addition to the number authorized by the act approved august third, eighteen hundred and eighty-two, and who shall serve without compensation. approved july , . annex iii. circular.] department of state washington, _october , _. sir: on the d of august last the president approved an act of congress, in the following words: "_be it enacted by the senate and house of representatives of the united states of america in congress assembled_, that the president of the united states be authorized and requested to extend to the governments of all nations in diplomatic relations with our own an invitation to appoint delegates to meet delegates from the united states in the city of washington, at such time as he may see fit to designate, for the purpose of fixing upon a meridian proper to be employed as a common zero of longitude and standard of time-reckoning throughout the globe, and that the president be authorized to appoint delegates, not exceeding three in number, to represent the united states in such international conference." it may be well to state that, in the absence of a common and accepted standard for the computation of time for other than astronomical purposes, embarrassments are experienced in the ordinary affairs of modern commerce; that this embarrassment is especially felt since the extension of telegraphic and railway communications has joined states and continents possessing independent and widely separated meridional standards of time; that the subject of a common meridian has been for several years past discussed in this country and in europe by commercial and scientific bodies, and the need of a general agreement upon a single standard recognized; and that, in recent european conferences especially, favor was shown to the suggestion that, as the united states possesses the greatest longitudinal extension of any country traversed by railway and telegraph lines, the initiatory measures for holding an international convention to consider so important a subject should be taken by this government. the president, while convinced of the good to flow eventually from the adoption of a common time unit, applicable throughout the globe, thinks, however, that the effort now to be made should be to reach by consultation a conclusion as to the advisability of assembling an international congress with the object of finally adopting a common meridian. he, therefore, abstains from extending an invitation for a meeting at an assigned day, until he has ascertained the views of the leading governments of the world as to whether such international conference is deemed desirable. i am accordingly directed by the president to request you to bring the matter to the attention of the government of ----, through the minister of foreign affairs, with a view to learning whether its appreciation of the benefits to accrue to the intimate intercourse of civilized peoples from the consideration and adoption of the suggested common standard of time so far coincides with that of this government as to lead it to accept an invitation to participate in an international conference at a date to be designated in the near future. you may leave a copy of this instruction with the minister for foreign affairs, and request the views of his government thereon, at as early a day as may be conveniently practicable. i am, sir, your obedient servant, fred'k t. frelinghuysen. * * * * * annex iv. circular.] department of state, washington, _december , _. sir: by a circular instruction of october , , you were made acquainted with (the language of) an act of congress, approved august , , authorizing and requesting the president to extend to other governments an invitation to appoint delegates to meet in the city of washington for the purpose of fixing upon a meridian proper to be employed as a common zero of longitude and standard of time-reckoning throughout the world; and you were instructed to bring the matter to the attention of the government to which you are accredited and to inform it that the president deemed it advisable to abstain from the issuance of the formal invitation contemplated, until through preliminary consultation the views of the leading governments of the world as to the desirability of holding such an international conference could be ascertained. in the year that has since elapsed this government has received from most of those in diplomatic relations with the united states the approval of the project, while many have in terms signified their acceptance and even named their delegates. besides this generally favorable reception of the suggestion so put forth, interest in the proposed reform has been shown by the geographical conference held at rome in october last, which very decisively expressed its opinion in favor of the adoption of the meridian of greenwich as the common zero of time longitude, and adjourned, leaving the discussion and final adoption of this or other equivalent unit, and the framing of practical rules for such adoption, to the international conference to be held at washington. the president therefore thinks the time has come to call the convention referred to in my instruction of october , . i am accordingly directed by the president to instruct you to tender to the government of ----, through its minister for foreign affairs, an invitation to be represented by one or more delegates (not exceeding three) to meet delegates from the united states and other nations in an international conference to be held in the city of washington on the first day of october next, , for the purpose of discussing and, if possible, fixing upon a meridian proper to be employed as a common zero of longitude and standard of time-reckoning throughout the globe. you will seek the earliest convenient occasion to bring this invitation to the attention of the minister of foreign affairs of ---- by handing him a copy hereof and requesting that the answer of his government may be made known to you. i am, sir, your obedient servant, fred'k t. frelinghuysen. astronomical discovery [illustration: astronomers royal.] astronomical discovery by herbert hall turner, d.sc., f.r.s. savilian professor of astronomy in the university of oxford _with plates_ london edward arnold & maddox street, w. (all rights reserved) to edward emerson barnard astronomical discoverer these pages are inscribed in memory of never-to-be-forgotten days spent with him at the yerkes observatory of the university of chicago preface the aim of the following pages is to illustrate, by the study of a few examples chosen almost at random, the variety in character of astronomical discoveries. an attempt has indeed been made to arrange the half-dozen examples, once selected, into a rough sequence according to the amount of "chance" associated with the discovery, though from this point of view chapter iv. should come first; but i do not lay much stress upon it. there is undoubtedly an element of "luck" in most discoveries. "the biggest strokes are all luck," writes a brother astronomer who had done me the honour to glance at a few pages, "but a man must not drop his catches. have you ever read montaigne's essay 'of glory'? it is worth reading. change war and glory to discovery and it is exactly the same theme. if you are looking for a motto you will find a score in it." indeed even in cases such as those in chapters v. and vi., where a discovery is made by turning over a heap of rubbish--declared such by experts and abandoned accordingly--we instinctively feel that the finding of something valuable was especially "fortunate." we should scarcely recommend such waste material as the best hunting ground for gems. the chapters correspond approximately to a series of six lectures delivered at the university of chicago in august , at the hospitable invitation of president harper. they afforded me the opportunity of seeing something of this wonderful university, only a dozen years old and yet so amazingly vigorous; and especially of its observatory (the yerkes observatory, situated eighty miles away on lake geneva), which is only eight years old and yet has taken its place in the foremost rank. for these opportunities i venture here to put on record my grateful thanks. in a portion of the first chapter it will be obvious that i am indebted to miss clerke's "history of astronomy in the nineteenth century"; in the second to professor r. a. sampson's memoir on the adams mss.; in the third to rigaud's "life of bradley." there are other debts which i hope are duly acknowledged in the text. my grateful thanks are due to mr. f. a. bellamy for the care with which he has read the proofs; and i am indebted for permission to publish illustrations to the royal astronomical society, the astronomer royal, the editors of _the observatory_, the cambridge university press, the harvard college observatory, the yerkes observatory, and the living representatives of two portraits. h. h. turner. university observatory, oxford, _november , _. contents page chapter i uranus and eros chapter ii the discovery of neptune chapter iii bradley's discoveries of the aberration of light and of the nutation of the earth's axis chapter iv accidental discoveries chapter v schwabe and the sun-spot period chapter vi the variation of latitude index list of plates plate i. portrait of j. c. adams _to face page_ ii. portrait of a. graham " " iii. portrait of u. j. le verrier " " iv. portrait of j. g. galle " " v. corner of the berlin map by the use of which galle found neptune " " vi. astronomers royal _frontispiece_ vii. great comet of nov. , _to face page_ viii. the oxford new star " " ix. nebulosity round nova persei " " x. sun-spots at greenwich, feb. and , " " xi. sun-spots at greenwich, feb. and , " " xii. number of sun-spots compared with daily range of magnetic declination and daily range of magnetic horizontal force " " xiii. greenwich magnetic curves, - " " xiv. greenwich magnetic curves, - " " xv. sun-spots and turns of vane " " errata page , line , _for_ " stars" _read_ " stars per hour." " , see note on page . " , bottom of page. this nebulosity was first discovered by dr. max wolf of heidelberg. see _astr. nachr._ . " , line , _for_ "observation" _read_ "aberration." astronomical discovery chapter i uranus and eros [sidenote: popular view of discovery.] discovery is expected from an astronomer. the lay mind scarcely thinks of a naturalist nowadays discovering new animals, or of a chemist as finding new elements save on rare occasions; but it does think of the astronomer as making discoveries. the popular imagination pictures him spending the whole night in watching the skies from a high tower through a long telescope, occasionally rewarded by the finding of something new, without much mental effort. i propose to compare with this romantic picture some of the actual facts, some of the ways in which discoveries are really made; and if we find that the image and the reality differ, i hope that the romance will nevertheless not be thereby destroyed, but may adapt itself to conditions more closely resembling the facts. [sidenote: keats' lines.] the popular conception finds expression in the lines of keats:-- then felt i like some watcher of the skies when a new planet swims into his ken. keats was born in , published his first volume of poems in , and died in . at the time when he wrote the discovery of planets was comparatively novel in human experience. uranus had been found by william herschel in , and in the years to followed the first four minor planets, a number destined to remain without additions for nearly forty years. it would be absurd to read any exact allusion into the words quoted, when we remember the whole circumstances under which they were written; but perhaps i may be forgiven if i compare them especially with the actual discovery of the planet uranus, for the reason that this was by far the largest of the five--far larger than any other planet known except jupiter and saturn, while the others were far smaller--and that keats is using throughout the poem metaphors drawn from the first glimpses of "vast expanses" of land or water. perhaps i may reproduce the whole sonnet. his friend c. c. clarke had put before him chapman's "paraphrase" of homer, and they sat up till daylight to read it, "keats shouting with delight as some passage of especial energy struck his imagination. at ten o'clock the next morning mr. clarke found the sonnet on his breakfast-table." sonnet xi _on first looking into chapman's "homer"_ much have i travell'd in the realms of gold, and many goodly states and kingdoms seen; round many western islands have i been which bards in fealty to apollo hold. oft of one wide expanse had i been told that deep-brow'd homer ruled as his demesne; yet did i never breathe its pure serene till i heard chapman speak out loud and bold: then felt i like some watcher of the skies when a new planet swims into his ken; or like stout cortez when with eagle eyes he star'd at the pacific--and all his men look'd at each other with a wild surmise-- silent, upon a peak in darien. [sidenote: comparison with discovery of uranus.] let us then, as our first example of the way in which astronomical discoveries are made, turn to the discovery of the planet uranus, and see how it corresponds with the popular conception as voiced by keats. in one respect his words are true to the life or the letter. if ever there was a "watcher of the skies," william herschel was entitled to the name. it was his custom to watch them the whole night through, from the earliest possible moment to daybreak; and the fruits of his labours were many and various almost beyond belief. but did the planet "swim into his ken"? let us turn to the original announcement of his discovery as given in the philosophical transactions for . philosophical transactions, xxxii.--account of a comet by mr. herschel, f.r.s. (communicated by dr. watson, jun., of bath, f.r.s.) _read april , _ [sidenote: original announcement.] "on tuesday the th of march, between ten and eleven in the evening, while i was examining the small stars in the neighbourhood of h geminorum, i perceived one that appeared visibly larger than the rest; being struck with its uncommon magnitude, i compared it to h geminorum and the small star in the quartile between auriga and gemini, and finding it to be so much larger than either of them, suspected it to be a comet. "i was then engaged in a series of observations on the parallax of the fixed stars, which i hope soon to have the honour of laying before the royal society; and those observations requiring very high powers, i had ready at hand the several magnifiers of , , , , , &c., all which i have successfully used upon that occasion. the power i had on when i first saw the comet was . from experience i knew that the diameters of the fixed stars are not proportionally magnified with higher powers as the planets are; therefore i now put on the powers of and , and found the diameter of the comet increased in proportion to the power, as it ought to be, on a supposition of its not being a fixed star, while the diameters of the stars to which i compared it were not increased in the same ratio. moreover, the comet being magnified much beyond what its light would admit of, appeared hazy and ill-defined with these great powers, while the stars preserved that lustre and distinctness which from many thousand observations i knew they would retain. the sequel has shown that my surmises were well founded, this proving to be the comet we have lately observed. "i have reduced all my observations upon this comet to the following tables. the first contains the measures of the gradual increase of the comet's diameter. the micrometers i used, when every circumstance is favourable, will measure extremely small angles, such as do not exceed a few seconds, true to , , or thirds at most; and in the worst situations true to or thirds; i have therefore given the measures of the comet's diameter in seconds and thirds. and the parts of my micrometer being thus reduced, i have also given all the rest of the measures in the same manner; though in large distances, such as one, two, or three minutes, so great an exactness, for several reasons, is not pretended to." [sidenote: called first a comet.] [sidenote: other observers would not have found it at all.] at first sight this seems to be the wrong reference, for it speaks of a new comet, not a new planet. but it is indeed of uranus that herschel is speaking; and so little did he realise the full magnitude of his discovery at once, that he announced it as that of a comet; and a comet the object was called for some months. attempts were made to calculate its orbit as a comet, and broke down; and it was only after much work of this kind had been done that the real nature of the object began to be suspected. but far more striking than this misconception is the display of skill necessary to detect any peculiarity in the object at all. among a number of stars one seemed somewhat exceptional in size, but the difference was only just sufficient to awaken suspicion in a keen-eyed herschel. would any other observer have noticed the difference at all? certainly several good observers had looked at the object before, and looked at it with the care necessary to record its position, without noting any peculiarity. their observations were recovered subsequently and used to fix the orbit of the new planet more accurately. i shall remind you in the next chapter that uranus had been observed in this way no less than seventeen times by first-rate observers without exciting their attention to anything remarkable. the first occasion was in , nearly a century before herschel's grand discovery, and these chance observations, which lay so long unnoticed as in some way erroneous, subsequently proved to be of the utmost value in fixing the orbit of the new planet. but there is even more striking testimony than this to the exceptional nature of herschel's achievement. it is a common experience in astronomy that an observer may fail to notice in a general scrutiny some phenomenon which he can see perfectly well when his attention is directed to it: when a man has made a discovery and others are told what to look for, they often see it so easily that they are filled with amazement and chagrin that they never saw it before. not so in the case of uranus. at least two great astronomers, lalande and messier, have left on record their astonishment that herschel could differentiate it from an ordinary star at all; for even when instructed where to look and what to look for, they had the greatest difficulty in finding it. i give a translation of messier's words, which herschel records in the paper already quoted announcing the discovery:-- "nothing was more difficult than to recognise it; and i cannot conceive how you have been able to return several times to this star or comet; for absolutely it has been necessary to observe it for several consecutive days to perceive that it was in motion." [sidenote: no "swimming into ken."] we cannot, therefore, fit the facts to keats' version of them. the planet did not majestically reveal itself to a merely passive observer: rather did it, assuming the disguise of an ordinary star, evade detection to the utmost of its power; so that the keenest eye, the most alert attention, the most determined following up of a mere hint, were all needed to unmask it. but is the romance necessarily gone? if another keats could arise and know the facts, could he not coin a newer and a truer phrase for us which would still sound as sweetly in our ears? [sidenote: though this may happen at times.] [sidenote: name of new planet.] i must guard against a possible misconception. i do not mean to convey that astronomical discoveries are not occasionally made somewhat in the manner so beautifully pictured by keats. three years ago a persistent "watcher of the skies," dr. anderson of edinburgh, suddenly caught sight of a brilliant new star in perseus; though here "flashed into his ken" would perhaps be a more suitable phrase than "swam." and comets have been detected by a mere glance at the heavens without sensible effort or care on the part of the discoverer. but these may be fairly called exceptions; in the vast majority of cases hard work and a keen eye are necessary to make the discovery. the relative importance of these two factors of course varies in different cases; for the detection of uranus perhaps the keen eye may be put in the first place, though we must not forget the diligent watching which gave it opportunity. other cases of planetary discovery may be attributed more completely to diligence alone, as we shall presently see. but before leaving uranus for them i should like to recall the circumstances attending the naming of the planet. herschel proposed to call it _georgium sidus_ in honour of his patron, king george iii., and as the best way of making his wishes known, wrote the following letter to the president of the royal society, which is printed at the beginning of the philosophical transactions for . _a letter from_ william herschel, esq., f.r.s., _to_ sir joseph banks, bart., p.r.s. "sir,--by the observations of the most eminent astronomers in europe it appears that the new star, which i had the honour of pointing out to them in march , is a primary planet of our solar system. a body so nearly related to us by its similar condition and situation in the unbounded expanse of the starry heavens, must often be the subject of conversation, not only of astronomers, but of every lover of science in general. this consideration then makes it necessary to give it a name whereby it may be distinguished from the rest of the planets and fixed stars. [sidenote: _georgium sidus._] "in the fabulous ages of ancient times, the appellations of mercury, venus, mars, jupiter, and saturn were given to the planets as being the names of their principal heroes and divinities. in the present more philosophical era, it would hardly be allowable to have recourse to the same method, and call on juno, pallas, apollo, or minerva for a name to our new heavenly body. the first consideration in any particular event, or remarkable incident, seems to be its chronology: if in any future age it should be asked, _when_ this last found planet was discovered? it would be a very satisfactory answer to say, 'in the reign of king george the third.' as a philosopher then, the name georgium sidus presents itself to me, as an appellation which will conveniently convey the information of the time and country where and when it was brought to view. but as a subject of the best of kings, who is the liberal protector of every art and science; as a native of the country from whence this illustrious family was called to the british throne; as a member of that society which flourishes by the distinguished liberality of its royal patron; and, last of all, as a person now more immediately under the protection of this excellent monarch, and owing everything to his unlimited bounty;--i cannot but wish to take this opportunity of expressing my sense of gratitude by giving the name _georgium sidus_, _georgium sidus ----jam nunc assuesce vocari,_ _virg. georg._ to a star which (with respect to us) first began to shine under his auspicious reign. "by addressing this letter to you, sir, as president of the royal society, i take the most effectual method of communicating that name to the literati of europe, which i hope they will receive with pleasure.--i have the honour to be, with the greatest respect, sir, your most humble and most obedient servant, w. herschel." [sidenote: herschel.] this letter reminds us how long it was since a new name had been required for a new planet,--to find a similar occasion herschel had to go to the almost prehistoric past, when the names of heroes and divinities were given to the planets. it is, perhaps, not unnatural that he should have considered an entirely new departure appropriate for a discovery separated by so great a length of time from the others; but his views were not generally accepted, especially on the continent. lalande courteously proposed the name of herschel for the new planet, in honour of the discoverer, and this name was used in france; but bode, on the other hand, was in favour of retaining the old practice simply, and calling the new planet uranus. all three names seem to have been used for many years. only the other day i was interested to see an old pack of cards, used for playing a parlour game of astronomy, in which the name herschel is used. the owner told me that they had belonged to his grandfather; and the date of publication was , and the place london, so that this name was in common use in england nearly half a century after the actual discovery; though in the "english nautical almanac" the name "the georgian" (apparently preferred to herschel's _georgium sidus_) was being used officially after , and did not disappear from that work until (published in .) [sidenote: uranus finally adopted.] it would appear to have been the discovery of neptune, with which we shall deal in the next chapter, which led to this official change; for in the volume for is included adams' account of his discovery with the title-- "on the perturbations of uranus," and there was thus a definite reason for avoiding two names for the same planet in the same work. but le verrier's paper on the same topic at the same date still uses the name "herschel" for the planet. [sidenote: bode's law.] the discovery of neptune, as we shall see, was totally different in character from that of uranus. the latter may be described as the finding of something by an observer who was looking for anything; neptune was the finding of something definitely sought for, and definitely pointed out by a most successful and brilliant piece of methodical work. but before that time several planets had been found, as the practical result of a definite search, although the guiding principle was such as cannot command our admiration to quite the same extent as in the case of neptune. to explain it i must say something of the relative sizes of the orbits in which planets move round the sun. these orbits are, as we know, ellipses; but they are very nearly circles, and, excluding refinements, we may consider them as circles, with the sun at the centre of each, so that we may talk of the distance of any planet from the sun as a constant quantity without serious error. now if we arrange the planetary distances in order, we shall notice a remarkable connection between the terms of the series. here is a table showing this connection. table of the distances of the planets from the sun, showing "bode's law." +----------------------------------------------------+ | name of | distance from | "bode's law" | | planet. | sun, taking | (originally formulated | | | that of earth | by titius, but brought | | | as . | into notice by bode). | |----------------------------------------------------| | mercury | | + = | | venus | | + = | | the earth | | + = | | mars | | + = | | ( ) | ( ) | + = | | jupiter | | + = | | saturn | | + = | | uranus | | + = | +----------------------------------------------------+ [sidenote: gap in the series suggesting unknown planet.] [sidenote: search for it.] [sidenote: accidental discovery.] if we write down a series of 's, and then add the numbers , , , and so on, each formed by doubling the last, we get numbers representing very nearly the planetary distances, which are shown approximately in the second column. but three points call for notice. firstly, the number before should be - / , and not zero, to agree with the rest. secondly, there is a gap, or rather was a gap, after the discovery of uranus, between mars and jupiter; and thirdly, we see that when uranus was discovered, and its distance from the sun determined, this distance was found to fall in satisfactorily with this law, which was first stated by titius of wittenberg. this third fact naturally attracted attention. no explanation of the so-called "law" was known at the time; nor is any known even yet, though we may be said to have some glimmerings of a possible cause; and in the absence of such explanation it must be regarded as merely a curious coincidence. but the chances that we are in the presence of a mere coincidence diminish very quickly with each new term added to the series, and when it was found that herschel's new planet fitted in so well at the end of the arrangement, the question arose whether the gap above noticed was real, or whether there was perhaps another planet which had hitherto escaped notice, revolving in an orbit represented by this blank term. this question had indeed been asked even before the discovery of uranus, by bode, a young astronomer of berlin; and for fifteen years he kept steadily in view this idea of finding a planet to fill the vacant interval. the search would be a very arduous one, involving a careful scrutiny, not perhaps of the whole heavens, but of a considerable portion of it along the zodiac; too great for one would-be discoverer single-handed; but in september bode succeeded in organising a band of six german astronomers (including himself) for the purpose of conducting this search. they divided the zodiac into twenty-four zones, and were assigning the zones to the different observers, when they were startled by the news that the missing planet had been accidentally found by piazzi in the constellation taurus. the discovery was made somewhat dramatically on the first evening of the nineteenth century (january , ). piazzi was not looking for a planet at all, but examining an error made by another astronomer; and in the course of this work he recorded the position of a star of the eighth magnitude. returning to it on the next night, it seemed to him that it had slightly moved westwards, and on the following night this suspicion was confirmed. remark that in this case no peculiar appearance in the star suggested that it might be a comet or planet, as in the case of the discovery of uranus. we are not unfair in ascribing the discovery to pure accident, although we must not forget that a careless observer might easily have missed it. piazzi was anything but careless, and watched the new object assiduously till february th, when he became dangerously ill; but he had written, on january rd, to oriani of milan, and to bode at berlin on the following day. these letters, however, did not reach the recipients (in those days of leisurely postal service) until april th and march th respectively; and we can imagine the mixed feelings with which bode heard that the discovery which he had contemplated for fifteen years, and for which he was just about to organise a diligent search, was thus curiously snatched from him. [sidenote: hegel's forecast.] more curious still must have seemed the intelligence to a young philosopher of jena named hegel, who has since become famous, but who had just imperilled his future reputation by publishing a dissertation proving conclusively that the number of the planets could not be greater than seven, and pouring scorn on the projected search of the half-dozen enthusiasts who were proposing to find a new planet merely to fill up a gap in a numerical series. [sidenote: the planet lost again.] the sensation caused by the news of the discovery was intensified by anxiety lest the new planet should already have been lost; for it had meanwhile travelled too close to the sun for further observation, and the only material available for calculating its orbit, and so predicting its place in the heavens at future dates, was afforded by the few observations made by piazzi. was it possible to calculate the orbit from such slender material? it would take too long to explain fully the enormous difficulty of this problem, but some notion of it may be obtained, by those unacquainted with mathematics, from a rough analogy. if we are given a portion of a circle, we can, with the help of a pair of compasses, complete the circle: we can find the centre from which the arc is struck, either by geometrical methods, or by a few experimental trials, and then fill in the rest of the circumference. if the arc given is large we can do this with certainty and accuracy; but if the arc is small it is difficult to make quite sure of the centre, and our drawing may not be quite accurate. now the arc which had been described by the tiny planet during piazzi's observations was only three degrees; and if any one will kindly take out his watch and look at the minute marks round the dial, three degrees is just _half_ a single minute space. if the rest of the dial were obliterated, and only this small arc left, would he feel much confidence in restoring the obliterated portion? this problem gives some idea of the difficulties to be encountered, but only even then a very imperfect one. [sidenote: gauss shows how to find it.] briefly, the solution demanded a new mathematical method in astronomy. but difficulties are sometimes the opportunities of great men, and this particular difficulty attracted to astronomy the great mathematician gauss, who set himself to make the best of the observation available, and produced his classical work, the _theoria motus_, which is the standard work for such calculations to the present day. may we look for a few moments at what he himself says in the preface to his great work? i venture to reproduce the following rough translation (the book being written in latin, according to the scientific usage of the time):-- extract from the preface to the _theoria motus_. [sidenote: the _theoria motus_.] "some ideas had occurred to me on this subject in september , at a time when i was occupied on something quite different; ideas which seemed to contribute to the solution of the great problem of which i have spoken. in such cases it often happens that, lest we be too much distracted from the attractive investigation on which we are engaged, we allow associations of ideas which, if more closely examined, might prove extraordinarily fruitful, to perish from neglect. perchance these same idea-lets of mine would have met with this fate, if they had not most fortunately lighted upon a time than which none could have been chosen more favourable for their preservation and development. for about the same time a rumour began to be spread abroad concerning a new planet which had been detected on january st of that year at the observatory of palermo; and shortly afterwards the actual observations which had been made between january st and february th by the renowned philosopher piazzi were published. nowhere in all the annals of astronomy do we find such an important occasion; and scarcely is it possible to imagine a more important opportunity for pointing out, as emphatically as possible, the importance of that problem, as at the moment when every hope of re-discovering, among the innumerable little stars of heaven, that mite of a planet which had been lost to sight for nearly a year, depended entirely on an approximate knowledge of its orbit, which must be deduced from those scanty observations. could i ever have had a better opportunity for trying whether those idea-lets of mine were of any practical value than if i then were to use them for the determination of the orbit of ceres, a planet which, in the course of those forty-one days, had described around the earth an arc of no more than three degrees? and, after a year had passed, required to be tracked out in a region of the sky far removed from its original position? the first application of this method was made in the month of october , and the first clear night, when the planet was looked for by the help of the ephemeris i had made, revealed the truant to the observer. three new planets found since then have supplied fresh opportunities for examining and proving the efficacy and universality of this method. "now a good many astronomers, immediately after the rediscovery of ceres, desired me to publish the methods which had been used in my calculations. there were, however, not a few objections which prevented me from gratifying at that moment these friendly solicitations, viz. other business, the desire of treating the matter more fully, and more especially the expectation that, by continuing to devote myself to this research, i should bring the different portions of the solution of the problem to a more perfect pitch of universality, simplicity, and elegance. as my hopes have been justified, i do not think there is any reason for repenting of my delay. for the methods which i had repeatedly applied from the beginning admitted of so many and such important variations, that scarcely a vestige of resemblance remains between the method by which formerly i had arrived at the orbit of ceres and the practice which i deal with in this work. although indeed it would be alien to my intention to write a complete history about all these researches which i have gradually brought to even greater perfection, yet on many occasions, especially whenever i was confronted by some particularly serious problem, i thought that the first methods which i employed ought not to be entirely suppressed. nay, rather, in addition to the solutions of the principal problems, i have in this work followed out many questions which presented themselves to me, in the course of a long study of the motions of the heavenly bodies in conic sections, as being particularly worthy of attention, whether on account of the neatness of the analysis, or more especially by reason of their practical utility. yet i have always given the greater care to subjects which i have made my own, merely noticing by the way well-known facts where connection of thought seemed to demand it." [sidenote: rediscovery of ceres.] [sidenote: another planet found.] these words do not explain in any way the methods introduced by gauss, but they give us some notion of the flavour of the work. aided by these brilliant researches, the little planet was found on the last day of the year by von zach at gotha, and on the next night, independently, by olbers at bremen. but, before this success, there had been an arduous search, which led to a curious consequence. olbers had made himself so familiar with all the small stars along the track which was being searched for the missing body, that he was at once struck by the appearance of a stranger near the spot where he had just identified ceres. at first he thought this must be some star which had blazed up to brightness; but he soon found that it also was moving, and, to the great bewilderment of the astronomical world, it proved to be another planet revolving round the sun at a distance nearly the same as the former. this was an extraordinary and totally unforeseen occurrence. the world had been prepared for _one_ planet; but here were _two_! [sidenote: hypothesis of many fragments.] the thought occurred to olbers that they were perhaps fragments of a single body which had been blown to pieces by some explosion, and that there might be more of the pieces; and he therefore suggested as a guide for finding others that, since by the known laws of gravitation, bodies which circle round the sun return periodically to their starting-point, therefore all these fragments would in due course return to the point in the heavens where the original planet had exploded. hence the search might be most profitably conducted in the neighbourhood of the spot where the two first fragments (which had been named ceres and pallas) had already been found. we now have good reason to believe that this view is a mistaken one, but nevertheless it was apparently confirmed by the discovery of two more bodies of the same kind, which were called juno and vesta; the second of these being found by olbers himself after three years' patient work in . hence, although the idea of searching for a more or less definitely imagined planet was not new, although bode had conceived it as early as , and organised a search on this plan, three planets were actually found before the first success attending a definite search. ceres, as already remarked, was found by a pure accident; and the same may be said of pallas and juno, though it may fairly be added that pallas was actually contrary to expectation. minor planets, to . +---------------------------------------+ |number| name. | discoverer. | date.| |---------------------------------------| | | ceres | piazzi | | | | pallas | olbers | | | | juno | harding | | | | vesta | olbers | | |------|-----------|-------------|------| | | astraea | hencke | | | | hebe | hencke | | | | iris | hind | | | | flora | hind | | | | metis | graham | | | | hygeia | de gasparis | | | | parthenope| de gasparis | | | | victoria | hind | | | | egeria | de gasparis | | +---------------------------------------+ [sidenote: hencke's long search.] here now is a table showing how other bodies were gradually added to this first list of four, but you will see that no addition was made for a long time. not that the search was immediately abandoned; but being rewarded by no success for some years, it was gradually dropped, and the belief gained ground that the number of the planets was at last complete. the discoverers of uranus and of these first four minor planets all died before any further addition was made; and it was not until the end of that astraea was found by an ex-postmaster of the prussian town of driessen, by name hencke, who, in spite of the general disbelief in the existence of any more planets, set himself diligently to search for them, and toiled for fifteen long years before at length reaping his reward. others then resumed the search; hind, the observer of an english amateur astronomer near london, found iris a few weeks after hencke had been rewarded by a second discovery in , and in the following year mr. graham at markree in ireland (who is still living, and has only just retired from active work at the cambridge observatory) found metis; and from that time new discoveries have been added year by year, until the number of planets now known exceeds , and is steadily increasing. [illustration: _by permission of messrs. macmillan & co._ i.--j. c. adams.] [illustration: ii.--a. graham. discoverer of the ninth minor planet (metis).] [sidenote: the photographic method.] you will see the great variety characterising these discoveries; some of them are the result of deliberate search, others have come accidentally, and some even contrary to expectation. of the great majority of the earlier ones it may be said that enormous diligence was required for each discovery; to identify a planet it is necessary to have either a good map of the stars or to know them thoroughly, so that the map practically exists in the brain. we need only remember hencke's fifteen years of search before success to recognise what vast stores of patience and diligence were required in carrying out the search. but of late years photography has effected a great revolution in this respect. it is no longer necessary to do more than set what sir robert ball has called a "star-trap," or rather planet-trap. if a photograph be taken of a region of the heavens, by the methods familiar to astronomers, so that each star makes a round dot on the photographic plate, any sufficiently bright object moving relatively to the stars will make a small line or trail, and thus betray its planetary character. in this way most of the recent discoveries have been made, and although diligence is still required in taking the photographs, and again in identifying the objects thus found (which are now very often the images of already known members of the system), the tedious scrutiny with the eye has become a thing of the past. table showing the number of minor planets discovered in each decade since . to --altogether discoveries. to -- " " to -- " " to -- " " to -- " " to -- " announcements in " " " " " " " " --- total [_n.b._--many of the more recent announcements turned out to refer to old discoveries.] [sidenote: scarcity of names.] the known number of these bodies has accordingly increased so rapidly as to become almost an embarrassment; and in one respect the embarrassment is definite, for it has become quite difficult to find _names_ for the new discoveries. we remember with amusement at the present time that for the early discoveries there was sometimes a controversy (of the same kind as in the case of uranus) about the exact name which a planet should have. thus when it was proposed to call no. (discovered in , in london, by mr. hind) "victoria," there was an outcry by foreign astronomers that by a subterfuge the name of a reigning monarch was again being proposed for a planet, and considerable opposition was manifested, especially in america. but it became clear, as other discoveries were added, that the list of goddesses, or even humbler mythological people, would not be large enough to go round if we were so severely critical, and must sooner or later be supplemented from sources hitherto considered unsuitable; so, ultimately, the opposition to the name victoria was withdrawn. later still the restriction to feminine names has been broken through; one planet has been named endymion, and another, of which we shall presently speak more particularly, has been called eros. but before passing to him you may care to look at some of the names selected for others:-- no. name. lameia bettina prymno libussa phaëtusa eduarda california ornamenta ninina ilmatar industria ingeborg eros photographica alleghenia eriphyla ocllo pittsburghia evelyn [sidenote: bettina.] [sidenote: the provisional letters.] in connection with no. there is an interesting little history. in the _observatory_ for , page , appeared the following advertisement:--"herr palisa being desirous to raise funds for his intended expedition to observe the total solar eclipse of august , will sell the right of naming the minor planet no. for £ ." the bright idea seems to have struck herr palisa, who had already discovered many planets and begun to find difficulties in assigning suitable names, that he might turn his difficulty into a source of profit in a good cause. the offer was not responded to immediately, nor until herr palisa had discovered two more planets, nos. and . he found names for two, leaving, however, the last discovered always open for a patron, and on page of the same magazine for the following note informs us how his patience was ultimately rewarded:--"minor planet no. has been named 'bettina' by baron albert de rothschild." i have not heard, however, that this precedent has been followed in other cases, and the ingenuity of discoverers was so much overtaxed towards the end of last century that the naming of their planets fell into arrears. recently a commission, which has been established to look after these small bodies generally, issued a notice that unless the naming was accomplished before a certain date it would be ruthlessly taken out of the hands of the negligent discoverers. perhaps we may notice, before passing on, the provisional system which was adopted to fill up the interval required for finding a suitable name, and required also for making sure that the planet was in fact a new one, and not merely an old one rediscovered. there was a system of _numbering_ in existence as well as of _naming_, but it was unadvisable to attach even a number to a planet until it was quite certain that the discovery was new, for otherwise there might be gaps created in what should be a continuous series by spurious discoveries being struck out. accordingly it was decided to attach at first to the object merely a _letter of the alphabet_, with the year of discovery, as a provisional name. the alphabet was, however, run through so quickly, and confusion was so likely to ensue if it was merely repeated, that on recommencing it the letter a was prefixed, and the symbols adopted were therefore aa, ab, ac, &c.; after completing the alphabet again, the letter b was prefixed, and so on; and astronomers began to fear that they had before them a monotonous prospect of continually adding new planets, varied by no incident more exciting than starting the alphabet over again after every score. [sidenote: eros.] fortunately, however, on running through it for the fifth time, an object of particular interest was discovered. most of these bodies revolve at a distance from the sun intermediate between that of mars and that of jupiter, but the little planet which took the symbol dq, and afterwards the name of eros, was found to have a mean distance actually less than that of mars, and this gave it an extraordinary importance with respect to the great problem of determining the sun's distance. to explain this importance we must make a small digression. [sidenote: transit of venus.] about the middle of the last century our knowledge of the sun's distance was very rough, as may be seen from the table on p. ; but there were in prospect two transits of venus, in and , and it was hoped that these would give opportunities of a special kind for the measurement of this important quantity, which lies at the root of all our knowledge of the exact masses and dimensions of not only the sun, but of the planets as well. [illustration: fig. .] [sidenote: the "black drop."] the method may be briefly summarised thus: an observer in one part of the earth would see venus cross the disc of the sun along a different path from that seen by another observer, as will be clear from the diagram. if the size of the earth, the distance of the sun, and the _relative_ distance of venus be known, it can be calculated what this difference in path will be. now the relative distance of venus _is_ known with great accuracy, from observing the time of her revolution round the sun; the size of the earth we can measure by a survey; there remains, therefore, only one unknown quantity, the sun's distance. and since from a knowledge of this we could calculate the difference in path, it is easy to invert the problem, and calculate the sun's distance from the knowledge of the observed difference in path. accordingly, observers were to be scattered, not merely to two, but to many stations over the face of the earth, to observe the exact path taken by venus in transit over the sun's disc as seen from their station; and especially to observe the exact times of beginning and ending of the transit; and, by comparison of their results, it was hoped to determine this very important quantity, the sun's distance. it was known from previous experience that there were certain difficulties in observing very exactly the beginning and end of the transit. there was an appearance called the "black drop," which had caused trouble on previous occasions; an appearance as though the round black spot which can be seen when venus has advanced some distance over the sun's disc was reluctant to make the entry and clung to the edge or "limb" of the sun as it is called, somewhat as a drop of ink clings to a pen which is slowly withdrawn from an inkpot. similarly, at the end of the transit or egress, instead of approaching the limb steadily the planet seems at the last moment to burst out towards it, rendering the estimation of the exact moment when the transit is over extremely doubtful. [sidenote: failure.] these difficulties, as already stated, were known to exist; but there is a long interval between transits of venus, or rather between every pair of such transits. after those of and there will be no more until and , so that we shall never see another; similarly, before that pair of the last century, there had not been any such occasion since and , and no one was alive who remembered at first hand the trouble which was known to exist. it was proposed to obviate the anticipated difficulties by careful practice beforehand; models were prepared to resemble as nearly as possible the expected appearances, and the times recorded by different observers were compared with the true time, which could, in this case of a model, be determined. in this way it was hoped that the habit of each observer, his "personal equation" as it is called, could be determined beforehand, and allowed for as a correction when he came to observe the actual transit. the result, however, was a great disappointment. the actual appearances were found to be totally different in character from those shown by the model; chiefly, perhaps, because it had been impossible to imitate with a model the effect of the atmosphere which surrounds the planet venus. observers trained beforehand, using similar instruments, and standing within a few feet of each other, were expected, after making due allowance for personal equation, to give the same instant for contact; but their observations when made were found to differ by nearly a minute of time, and after an exhaustive review of the whole material it was felt that all hope of determining accurately the sun's distance by this method must be given up. the following table will show how much was learned from the transits of venus, and how much remained to be settled. they left the result in doubt over a range of about two million miles. sun's distance, in millions of miles, as found by different observers =before the transits of venus= estimates varied between = = million miles (gilliss and gould, ) and = = million (winneche, ), a range of million miles. =the transits of and = gave results lying between = - / = million (airy, from british observations of ), = - / = million (stone, from british observations of ), and = - / = million (puiseux, from french observations), a range of - / millions. =gill's heliometer results= all lie very near = = millions. the observations of mars in give about , miles over this figure: but the observations of victoria, iris, and sappho, which are more trustworthy, all agree in giving about , miles _less_ than the millions. it became necessary, therefore, to look to other methods; and before the second transit of was observed, an energetic astronomer, dr. david gill, had already put into operation the method which may be now regarded as the standard one. [sidenote: modern method for sun's distance.] [sidenote: photography.] [sidenote: dr. gill's expedition to ascension.] we have said that the _relative_ distance of venus from the sun is accurately known from observations of the exact time of revolution. it is easy to see that these times of revolution can be measured accurately by mere accumulation. we may make an error of a few seconds in noting the time of return; but if the whole interval comprises revolutions, this error is divided by , if revolutions by , and so on; and by this time a great number of revolutions of all the planets (except those just discovered) have been recorded. hence we know their relative distances with great precision; and if we can find the distance in miles of any one of them, we can find that of the sun itself, or of any other planet, by a simple rule-of-three sum. by making use of this principle many of the difficulties attending the direct determination of the sun's distance can be avoided; for instance, since the sun's light overpowers that of the stars, it is not easy to directly observe the place of the sun among the stars; but this is not so for the planets. we can photograph a planet and the stars surrounding it on the same plate, and then by careful measurement determine its exact position among the stars; and since this position differs slightly according to the situation of the observer on the earth's surface, by comparing two photographs taken at stations a known distance apart we can find the distance of the planet from the earth; and hence, as above remarked, the distance of the sun and all the other members of the solar system. or, instead of taking photographs from two different stations, we can take from the same station two photographs at times separated by a known interval. for in that interval the station will have been carried by the earth's rotation some thousands of miles away from its former position, and becomes virtually a second station separated from the first by a distance which is known accurately when we know the elapsed time. again, instead of taking photographs, and from them measuring the position of the planet among the stars, we may make the measurements on the planet and stars in the sky itself; and since in , when dr. gill set out on his enterprise of determining the sun's distance, photography was in its infancy as applied to astronomy, he naturally made his observations on the sky with an instrument known as a heliometer. he made them in the little island of ascension, which is suitably situated for the purpose; because, being near the earth's equator, it is carried by the earth's rotation a longer distance in a given time than places nearer the poles, and in these observations for "parallax," as they are called, it is important to have the displacement of the station as large as possible. for a similar reason the object selected among the planets must be as near the earth as possible; and hence the planet mars, which at favourable times comes nearer to us than any other superior planet[ ] then known, was selected for observation with the heliometer. and now it will be seen why the discovery of the little planet eros was important, for mars was no longer the known planet capable of coming nearest to us; it had been replaced by this new arrival. [sidenote: victoria, iris, and sappho.] [sidenote: eros.] further, a small planet which is in appearance just like an ordinary star has, irrespective of this great proximity, some distinct advantages over a planet like mars, which appears as a round disc, and is, moreover, of a somewhat reddish colour. when the distance of an object of this kind from a point of line such as a star is measured with the heliometer it is found that a certain bias, somewhat difficult to allow for with certainty, is introduced into the measures; and our confidence in the final results suffers accordingly. after his observations of mars in , dr. david gill was sufficiently impressed with this source of error to make three new determinations of the sun's distance, using three of the minor planets instead of mars, in spite of the fact that they were sensibly farther away; and his choice was justified by finding that the results from these three different sets of observations agreed well among themselves, and differed slightly from that given by the observations of mars. hence it seems conclusively proved that one of these bodies is a better selection than mars in any case, and the discovery of eros, which offered the advantage of greater proximity in addition, was hailed as a new opportunity of a most welcome kind. it was seen by a little calculation that in the winter of - the planet would come very near the earth; not the nearest possible (for it was also realised that a still better opportunity had occurred in , though it was lost because the planet had not yet been discovered), but still the nearest approach which would occur for some thirty years; and extensive, though somewhat hasty, preparations were made to use it to the fullest advantage. photography had now become established as an accurate method of making measurements of the kind required; and all the photographic telescopes which could be spared were pressed into the service, and diligently photographed the planet and surrounding stars every fine night during the favourable period. the work of measuring and reducing these photographs involves an enormous amount of labour, and is even yet far from completed, but we know enough to expect a result of the greatest value. more than this we have not time to say here about this great problem, but it will have been made clear that just when astronomers were beginning to wonder whether it was worth while continuing the monotonous discovery of new minor planets by the handful, the rd discovery also turned out to be one of the greatest importance. to canons for the advantageous prosecution of research, if we care to make them, we may therefore add this--that there is no line of research, however apparently unimportant or monotonous, which we can afford to neglect. just when we are on the point of relinquishing it under the impression that the mine is exhausted, we may be about to find a nugget worth all our previous and future labour. this rule will not, perhaps, help us very much in choosing what to work at; indeed, it is no rule at all, for it leaves us the whole field of choice unlimited. but this negative result will recur again and again as we examine the lessons taught by discoveries: there seem to be no rules at all. whenever we seem to be able to deduce one from an experience, some other experience will flatly contradict it. thus we might think that the discovery of eros taught us to proceed patiently with a monotonous duty, and not turn aside to more novel and attractive work; yet it is often by leaving what is in hand and apparently has first claim on our attention that we shall do best, and we shall learn in the next chapter how a failure thus to turn flexibly aside was repented. chapter ii the discovery of neptune [sidenote: search for definite objects.] in the last chapter we saw that the circumstances under which planets were discovered varied considerably. sometimes the discoveries were not previously expected, occurring during a general examination of the heavens, or a search for other objects; and, on one occasion at least, the discovery may be said to have been even contrary to expectation, though, as the existence of a number of minor planets began to be realised, there have also been many cases where the discovery has been made as the result of a definite and deliberate search. but the search cannot be said to have been inspired by any very clear or certain principle: for the law of bode, successful though it has been in indicating the possible existence of new planets, cannot, as yet, be said to be founded upon a formulated law of nature. we now come, however, to a discovery made in direct interpretation of newton's great law of gravitation--the discovery of neptune from its observed disturbance of uranus. i will first briefly recall the main facts relating to the actual discovery. [sidenote: disturbance of uranus.] after uranus had been discovered and observed sufficiently long for its orbit to be calculated, it was found that the subsequent position of the planet did not always agree with this orbit; and, more serious than this, some early observations were found which could not be reconciled with the later ones at all. it is a wonderful testimony to the care and sagacity of sir william herschel, as was remarked in the last chapter, that uranus was found to have been observed, under the mistaken impression that it was an ordinary star, by flamsteed, lemonnier, bradley, and mayer, all observers of considerable ability. flamsteed's five observations dated as far back as , , and ; observations by others were in , , , , and so on up to , and the body of testimony was so considerable that there was no room for doubt as to the irreconcilability of the observations with the orbit, such as might have been the case had there been only one or two, possibly affected with some errors. [sidenote: suspicion of perturbing planet.] it is difficult to mention an exact date for the conversion into certainty of the suspicion that no single orbit could be found to satisfy all the observations; but we may certainly regard this fact as established in , when alexis bouvard published some tables of the planet, and showed fully in the introduction that when every correction for the disturbing action of other planets had been applied, it was still impossible to reconcile the old observations with the orbit calculated from the new ones. the idea accordingly grew up that there might be some other body or bodies attracting the planet and causing these discrepancies. here again it is not easy to say exactly when this notion arose, but it was certainly existent in , as the following letter to the astronomer royal will show. i take it from his well-known "account of some circumstances historically connected with the discovery of the planet exterior to uranus," which he gave to the royal astronomical society at its first meeting after that famous discovery (monthly notices of the r.a.s., vol. iii., and memoirs, vol. xvi.). no. .--_the_ rev. t. j. hussey _to_ g. b. airy. [_extract._] "'hayes, kent, _ th november _. "'with m. alexis bouvard i had some conversation upon a subject i had often meditated, which will probably interest you, and your opinion may determine mine. having taken great pains last year with some observations of _uranus_, i was led to examine closely bouvard's tables of that planet. the apparently inexplicable discrepancies between the ancient and modern observations suggested to me the possibility of some disturbing body beyond _uranus_, not taken into account because unknown. my first idea was to ascertain some approximate place of this supposed body empirically, and then with my large reflector set to work to examine all the minute stars thereabouts: but i found myself totally inadequate to the former part of the task. if i could have done it formerly, it was beyond me now, even supposing i had the time, which was not the case. i therefore relinquished the matter altogether; but subsequently, in conversation with bouvard, i inquired if the above might not be the case: his answer was, that, as might have been expected, it had occurred to him, and some correspondence had taken place between hansen and himself respecting it. hansen's opinion was, that one disturbing body would not satisfy the phenomena; but that he conjectured there were two planets beyond _uranus_. upon my speaking of obtaining the places empirically, and then sweeping closely for the bodies, he fully acquiesced in the propriety of it, intimating that the previous calculations would be more laborious than difficult; that if he had leisure he would undertake them and transmit the results to me, as the basis of a very close and accurate sweep. i have not heard from him since on the subject, and have been too ill to write. what is your opinion on the subject? if you consider the idea as possible, can you give me the limits, roughly, between which this body or those bodies may probably be found during the ensuing winter? as we might expect an eccentricity [inclination?] approaching rather to that of the old planets than of the new, the breadth of the zone to be examined will be comparatively inconsiderable. i may be wrong, but i am disposed to think that, such is the perfection of my equatoreal's object-glass, i could distinguish, almost at once, the difference of light of a small planet and a star. my plan of proceeding, however, would be very different: i should accurately map the whole space within the required limits, down to the minutest star i could discern; the interval of a single week would then enable me to ascertain any change. if the whole of this matter do not appear to you a chimæra, which, until my conversation with bouvard, i was afraid it might, i shall be very glad of any sort of hint respecting it.' "my answer was in the following terms:-- [sidenote: airy's scepticism.] no. .--g. b. airy _to the_ rev. t. j. hussey. [_extract._] "'observatory, cambridge, _ , nov. _. "'i have often thought of the irregularity of _uranus_, and since the receipt of your letter have looked more carefully to it. it is a puzzling subject, but i give it as my opinion, without hesitation, that it is not yet in such a state as to give the smallest hope of making out the nature of any external action on the planet ... if it were certain that there were any extraneous action, i doubt much the possibility of determining the place of a planet which produced it. i am sure it could not be done till the nature of the irregularity was well determined from several successive revolutions.'" [sidenote: le verrier's papers.] [sidenote: planet to be detected by disc.] [sidenote: galle's discovery of the planet.] although only a sentence or two have been selected from airy's reply (he was not yet astronomer royal), they are sufficient to show that the problem of finding the place of such a possible disturbing body was regarded at that time as one of extreme difficulty; and no one appears seriously to have contemplated embarking upon its solution. it was not until many years later that the solution was attempted. of the first attempt we shall speak presently, putting it aside for the moment because it had no actual bearing on the discovery of the planet, for reasons which form an extraordinary episode of this history. the attempt which led to success dates from november . the great french astronomer le verrier, on november , , read to the french academy a paper on the orbit of uranus, considering specially the disturbances produced by jupiter and saturn, and showing clearly that with no possible orbit could the observations be satisfied. on june , , followed a second paper by the same author, in which he considers all the possible explanations of the discordance, and concludes that none is admissible except that of a disturbing planet exterior to uranus. and assuming, in accordance with bode's law, that the distance of this new planet from the sun would be about double that of uranus (and it is important to note this assumption), he proceeds to investigate the orbit of such a planet, and to calculate the place where it must be looked for in the heavens. this was followed by a third paper on august st, giving a rather completer discussion, and arriving at the conclusion that the planet should be recognisable from its disc. this again is an important point. we remember that in the discovery of uranus it needed considerable skill on the part of sir william herschel to detect the disc, to see in fact any difference between it and surrounding stars; and that other observers, even when their attention had been called to the planet, found it difficult to see this difference. it might be expected, therefore, that with a planet twice as far away (as had been assumed for the new planet) the disc would be practically unrecognisable, and as we shall presently see, this assumption was made in some searches for the planet which had been commenced even before the publication of this third paper. le verrier's courageous announcement, which he deduced from a consideration of the mass of the planet, that the disc should be recognisable, led immediately to the discovery of the suspected body. he wrote to a german astronomer, dr. galle (still, i am glad to say, alive and well, though now a very old man), telling him the spot in the heavens to search, and stating that he might expect to detect the planet by its appearance in this way; and the same night dr. galle, by comparing a star map with the heavens, found the planet. [sidenote: adams' work publicly announced.] to two points to which i have specially called attention in this brief summary--namely, the preliminary assumption that the planet would be, according to bode's law, twice as far away as uranus; secondly, the confident assertion that it would have a visible disc--i will ask you to add, thirdly, that it was found by the aid of a star map, for this map played an important part in the further history to which we shall now proceed. it may naturally be supposed that the announcement of the finding of a planet in this way, the calculation of its place from a belief in the universal action of the great law of gravitation, the direction to an eminent observer to look in that place for a particular thing, and his immediate success,--this extraordinary combination of circumstances caused a profound sensation throughout not only the astronomical, but the whole world; and this sensation was greatly enhanced by the rumour which had begun to gather strength that, but for some unfortunate circumstances, the discovery might have been made even earlier and as a consequence of totally independent calculations made by a young cambridge mathematician, j. c. adams. some of you are doubtless already familiar with the story in its abridged form, for it has been scattered broadcast through literature. in england it generally takes the form of emphasising the wickedness or laziness of the astronomer royal who, when told where to look for a planet, neglected his obvious duty, so that in consequence another astronomer who made the calculation much later and gave a more virtuous observer the same directions where to look, obtained for france the glory of a discovery which ought to have been retained in england. there is no doubt that airy's conduct received a large amount of what he called "savage abuse." when the facts are clearly stated i think it will be evident that many of the harsh things said of him were scarcely just, though at the same time it is also difficult to understand his conduct at two or three points of the history, even as explained by himself. [sidenote: facts undoubted.] there is fortunately no doubt whatever about any of the _facts_. airy himself gave a very clear and straightforward account of them at the time, for which more credit is due to him than he commonly receives; and since the death of the chief actors in this sensational drama they have been naturally again ransacked, with the satisfactory result that there is practically no doubt about any of the facts. as to the proper interpretations of them there certainly may be wide differences of opinion, nor does this circumstance detract from their interest. it is almost impossible to make a perfectly colourless recital of them, nor is it perhaps necessary to do so. i will therefore ask you to remember in what i now say that there is almost necessarily an element of personal bias, and that another writer would probably give a different colouring. having said this, i hope i may speak quite freely as the matter appears in my personal estimation. [sidenote: airy's "account."] [sidenote: "a movement of the age."] airy's account was, as above stated, given to the royal astronomical society at their first meeting (after the startling announcement of the discovery of the new planet), on november , , and i have already quoted an extract from it. he opens with a tribute to the sensational character of the discovery, and then states that although clearly due to two individuals (namely, le verrier and galle), it might also be regarded as to some extent the consequence of a movement of the age. his actual words are these: "the principal steps in the theoretical investigations have been made by one individual, and the published discovery of the planet was necessarily made by one individual. to these persons the public attention has been principally directed; and well do they deserve the honours which they have received, and which they will continue to receive. yet we should do wrong if we considered that these two persons alone are to be regarded as the authors of the discovery of this planet. i am confident that it will be found that the discovery is a consequence of what may properly be called a movement of the age; that it has been urged by the feeling of the scientific world in general, and has been nearly perfected by the collateral, but independent labours, of various persons possessing the talents or powers best suited to the different parts of the researches." [sidenote: airy under-estimated adams' work.] i have quoted these words as the first point at which it is difficult to understand airy's conduct in excluding from them all specific mention of adams, knowing as he did the special claims which entitled him to such mention; claims indeed which he proceeded immediately to make clear. it seems almost certain that airy entirely under-estimated the value of adams' work throughout. but this will become clearer as we proceed. the "account" takes the form of the publication of a series of letters with occasional comments. airy was a most methodical person, and filed all his correspondence with great regularity. it was jestingly said of him once that if he wiped his pen on a piece of blotting-paper, he would date the blotting-paper and file it for reference. the letters reproduced in this "account" are still in the observatory at greenwich, pinned together just as airy left them; and in preparing his "account" it was necessary to do little else than to have them copied out and interpolate comments. from two of them i have already quoted to show how difficult the enterprise of finding an exterior planet from its action on uranus was considered in . to these may be added the following sentence from no. , dated . "if it be the effect of any unseen body," writes airy to bouvard, "it will be nearly impossible ever to find out its place." but the first letter which need concern us is no. , and it is only necessary to explain that professor challis was the professor of astronomy at cambridge, and in charge of the cambridge observatory, in which offices he had succeeded airy himself on his leaving cambridge for greenwich some eight years earlier. no. .--professor challis _to_ g. b. airy. [_extract._] "'cambridge observatory, _feb. , _. [sidenote: challis mentions adams to airy, and suggests adams' visit to greenwich.] "'a young friend of mine, mr. adams of st. john's college, is working at the theory of _uranus_, and is desirous of obtaining errors of the tabular geocentric longitudes of this planet, when near opposition, in the years - , with the factors for reducing them to errors of heliocentric longitude. are your reductions of the planetary observations so far advanced that you could furnish these data? and is the request one which you have any objection to comply with? if mr. adams may be favoured in this respect, he is further desirous of knowing, whether in the calculation of the tabular errors any alterations have been made in bouvard's _tables of uranus_ besides that of _jupiter's_ mass.' "my answer to him was as follows:-- no. .--g. b. airy _to_ professor challis. [_extract._] "'royal observatory, greenwich, _ , feb. _. "'i send all the results of the observations of _uranus_ made with both instruments (that is, the heliocentric errors of _uranus_ in longitude and latitude from to , for all those days on which there were observations, both of right ascension and of polar distance). no alteration is made in bouvard's _tables of uranus_ except in increasing the two equations which depend on _jupiter_ by / part. as constants have been added (in the printed tables) to make the equations positive, and as / part of the numbers in the tables has been added, / part of the constants has been subtracted from the final results.' "professor challis in acknowledging the receipt of these, used the following expressions:-- no. .--professor challis _to_ g. b. airy. [_extract._] "'cambridge observatory, _feb. , _. "'i am exceedingly obliged by your sending so complete a series of tabular errors of _uranus_.... the list you have sent will give mr. adams the means of carrying on in the most effective manner the inquiry in which he is engaged.' "the next letter shows that mr. adams has derived results from these errors. no. .--professor challis _to_ g. b. airy. "'cambridge observatory, _sept. , _. "'my friend mr. adams (who will probably deliver this note to you) has completed his calculations respecting the perturbation of the orbit of _uranus_ by a supposed ulterior planet, and has arrived at results which he would be glad to communicate to you personally, if you could spare him a few moments of your valuable time. his calculations are founded on the observations you were so good as to furnish him with some time ago; and from his character as a mathematician, and his practice in calculation, i should consider the deductions from his premises to be made in a trustworthy manner. if he should not have the good fortune to see you at greenwich, he hopes to be allowed to write to you on this subject.' "on the day on which this letter was dated, i was present at a meeting of the french institute. i acknowledged it by the following letter:-- no. .--g. b. airy _to_ professor challis. "'royal observatory, greenwich, _ , sept. _. "'i was, i suppose, on my way from france, when mr. adams called here; at all events, i had not reached home, and therefore, to my regret, i have not seen him. would you mention to mr. adams that i am very much interested with the subject of his investigations, and that i should be delighted to hear of them by letter from him?' "on one of the last days of october , mr. adams called at the royal observatory, greenwich, in my absence and left the following important paper:-- no. .--j. c. adams, esq., _to_ g. b. airy. [sidenote: adams' announcement of the new planet.] "'according to my calculations, the observed irregularities in the motion of _uranus_ may be accounted for by supposing the existence of an exterior planet, the mass and orbit of which are as follows:-- mean distance (assumed nearly in accordance with bode's law) . mean sidereal motion in . days ° '. mean longitude, st october longitude of perihelion eccentricity . . mass (that of the sun being unity) . . for the modern observations i have used the method of normal places, taking the mean of the tabular errors, as given by observations near three consecutive oppositions, to correspond with the mean of the times; and the greenwich observations have been used down to : since which, the cambridge and greenwich observations, and those given in the _astronomische nachrichten_, have been made use of. the following are the remaining errors of mean longitude:-- _observation--theory._ " + . - . - . + . - . + . - . - . + . - . + . - . - . - . + . + . + . - . - . - . + . the error for is concluded from that for given by observation, compared with those of four or five following years, and also with lemonnier's observations in and . "'for the ancient observations, the following are the remaining errors:-- _observation--theory._ " + . + . - . - . + . - . - . + . + . the errors are small, except for flamsteed's observation of . this being an isolated observation, very distant from the rest, i thought it best not to use it in forming the equations of condition. it is not improbable, however, that this error might be destroyed by a small change in the assumed mean motion of the planet.' "i acknowledged the receipt of this paper in the following terms:-- no. .--g. b. airy _to_ j. c. adams, esq. "'royal observatory, greenwich, _ , nov. _. [sidenote: airy's inquiry about the "radius vector."] "'i am very much obliged by the paper of results which you left here a few days since, showing the perturbations on the place of _uranus_ produced by a planet with certain assumed elements. the latter numbers are all extremely satisfactory: i am not enough acquainted with flamsteed's observations about to say whether they bear such an error, but i think it extremely probable. "'but i should be very glad to know whether this assumed perturbation will explain the error of the radius vector of _uranus_. this error is now very considerable, as you will be able to ascertain by comparing the normal equations, given in the greenwich observations for each year, for the times _before_ opposition with the times _after_ opposition.' "i have before stated that i considered the establishment of this error of the radius vector of _uranus_ to be a very important determination. i therefore considered that the trial, whether the error of radius vector would be explained by the same theory which explained the error of longitude, would be truly an _experimentum crucis_. and i waited with much anxiety for mr. adams' answer to my query. had it been in the affirmative, i should at once have exerted all the influence which i might possess, either directly, or indirectly through my friend professor challis, to procure the publication of mr. adams' theory. "from some cause with which i am unacquainted, probably an accidental one, i received no immediate answer to this inquiry. i regret this deeply, for many reasons." [sidenote: adams' silence.] here we may leave airy's "account" for a few moments to consider the reason why he received no answer. adams was a very shy and retiring young man, and very sensitive; though capable of a great resolution, and of enormous perseverance in carrying it out. we know (what is not indicated in the above account), how steadily he had kept in view the idea of solving this great problem. it was characteristic of him that as early as he had formed a resolution to undertake it, although at the time he was not able to enter upon its accomplishment. the following memorandum, which is still in existence, having been found among his papers after his death, records these facts: " , july . formed a design, in the beginning of this week, of investigating, as soon as possible after taking my degree, the irregularities in the motion of uranus, which were as yet unaccounted for: in order to find whether they may be attributed to the action of an undiscovered planet beyond it, and if possible thence to determine the elements of its orbit, &c., approximately, which would probably lead to its discovery." accordingly, "as soon as possible after taking his degree" he embarked upon the enterprise, and the first solution was made in the long vacation of , assuming the orbit of the unknown planet to be a circle with a radius equal to twice the mean distance of uranus from the sun (an assumption which, as we have seen, was also made by le verrier). having satisfied himself that there was a good general agreement between his results and the observations, adams began a more complete solution; indeed from first to last he made no less than six separate solutions, the one which he announced to airy in the above letter being the fourth. hence he had already done an enormous amount of work on the problem, and was in his own mind so justly convinced of the correctness and value of his results that he was liable to forget that others had not had the same opportunity of judging of their completeness; and he was grievously disappointed when his announcement was not received with full confidence. [sidenote: his disappointment at greenwich, and at airy's question.] but perhaps it should first be stated that by a series of mischances adams had been already much disappointed at the failure of his attempts to see the astronomer royal on his visits to greenwich. this does not seem to have been exactly airy's fault; he was, as may well be supposed, an extremely busy man, and was much occupied at the time on a question of great practical importance, at the direct request of the government, namely, the settling of the proper gauge for railways throughout the country. the first time adams called to see him, he was actually in london sitting on the committee which dealt with this question, and adams was asked to call later; when the visit was repeated, airy was unfortunately at dinner (and it may be added that his hours for dinner were somewhat peculiar), and the butler, acting somewhat in the manner of his kind, protected his master's dinner by sending away one whom he doubtless regarded as a troublesome visitor. there is, as i have said, little doubt about any of the facts, and it seems well established that airy himself did not learn of adams' visits until afterwards, and it would scarcely be just to blame him for a servant's oversight. but adams had left the paper above reproduced, and airy with his business-like habits ultimately proceeded to deal with it; he wrote the answer given above asking adams a definite question, filed a copy of it with the original letter, and then dismissed the matter from his thoughts until the reply from adams, which he confidently expected should again bring it under notice. this further disappointment was, however, too much for adams; he regarded the question put by airy as having so obvious an answer that it was intended as an evasion, though this was far from being the case. airy was thoroughly in earnest about his question, though it must be admitted that a more careful study of the problem would have shown him that it was unnecessary. later, when he learnt of le verrier's researches, he put the same question to him, and received a polite but very clear answer, showing that the suggested test was not an _experimentum crucis_ as he supposed. but adams did not feel equal to making this reply; he shrank into his shell and solaced himself only by commencing afresh another solution of the problem which had so engrossed his life at that time. [sidenote: the merits of airy's question.] [sidenote: the range of possibilities.] i have heard severe or contemptuous things said about this question by those who most blame airy. some of them have no hesitation in accusing him of intellectual incompetence: they say that it was the question of a stupid man. i think that in the first place they forget the difference between a deliberate error of judgement and a mere consequence of insufficient attention. but there is even more than this to be said in defence of the question. the "error of radius vector" came before airy in an entirely independent way, and as an entirely independent phenomenon, from the "error of longitude," and there was nothing unnatural in regarding it as requiring independent explanation. it is true that, _as the event proved_, a mere readjustment of the orbit of uranus got rid of this error of radius vector (this was substantially le verrier's answer to airy's question); but we must not judge of what was possible before the event in the light of what we now know. the original possibilities were far wider, though we have forgotten their former extent now that they have been narrowed down by the discovery. if a sentry during war time hears a noise in a certain direction, he may be compelled to make the assumption that it is the movement of an enemy; and if he fires in that direction and kills him, and thus saves his own army from destruction, he is deservedly applauded for the success which attends his action. but it does not follow that the assumption on which he acted was the only possible one. or, to take a more peaceful illustration, in playing whist it sometimes becomes apparent that the game can only be won if the cards lie in a certain way; and a good player will thereupon assume that this is the fact, and play accordingly. adams and le verrier played to win the game on the particular assumption that the disturbance of uranus was due to an external planet revolving at a distance from the sun about twice that of uranus; _and won it_; and we applaud them for doing so. but it is easy to imagine a rearrangement of the cards with which they would have lost it; and airy's question simply meant that he was alive to these wider possibilities, and did not see the need for attempting to win the game in that particular way. one such alternative possibility has already been mentioned. "hansen's opinion was, that one disturbing body would not satisfy the phenomena; but he conjectured that there were two planets beyond _uranus_." another conceivable alternative is that there was some change in the law of gravitation at the distance of uranus, which, it must be remembered, is twice as great as that of any planet previously known. or some wandering body might have passed close enough to uranus to change its orbit somewhat suddenly. we now know, for instance, that the swarm of meteorites which gives rise to the well-known "november meteors" must have passed very close to uranus in a.d. , assuming that neither the planet nor the swarm have been disturbed in any unknown manner in the meantime. it is to this encounter that we owe the introduction of this swarm to our solar system: wandering through space, they met uranus, and were swept by his attraction into an orbit round the sun. was there no reaction upon uranus himself? the probabilities are that the total mass of the swarm was so small as to affect the huge planet inappreciably; but who was to say that some other swarm of larger mass, or other body, might not have approached near uranus at some date between and , and been responsible at any rate in part for the observed errors? these are two or three suppositions from our familiar experience; and there are, of course, limitless possibilities beyond. which is the true scientific attitude, to be alive to them all, or to concentrate attention upon one? but we are perhaps wandering too far from the main theme. it is easy to do so in reviewing this extraordinary piece of history, for at almost every point new possibilities are suggested. [illustration: iii--u. j. le verrier. (_from a print in the possession of the royal astronomical society._)] [illustration: iv--j. g. galle. who first saw the planet neptune] [sidenote: airy receives le verrier's memoir.] we must return, however, to airy's "account." we reached the point where he had written to adams (on november , ), asking his question about the radius vector, and received no reply; and there the matter remained, so far as he was concerned, until the following june, when le verrier's memoir reached him; and we will let him give his own version of the result. "this memoir reached me about the rd or th of june. i cannot sufficiently express the feeling of delight and satisfaction which i received from it. the place which it assigned to the disturbing planet was the same, to one degree, as that given by mr. adams' calculations, which i had perused seven months earlier. to this time i had considered that there was still room for doubt of the accuracy of mr. adams' investigations; for i think that the results of algebraic and numerical computations, so long and so complicated as those of an inverse problem of perturbations, are liable to many risks of error in the details of the process: i know that there are important numerical errors in the _mécanique céleste_ of laplace; in the _théorie de la lune_ of plana; above all, in bouvard's first tables of _jupiter_ and _saturn_; and to express it in a word, i have always considered the correctness of a distant mathematical result to be a subject rather of moral than of mathematical evidence. but now i felt no doubt of the accuracy of both calculations, as applied to the perturbation in longitude. i was, however, still desirous, as before, of learning whether the perturbation in radius vector was fully explained. i therefore addressed to m. le verrier the following letter:-- no. .--g. b. airy _to_ m. le verrier. "'royal observatory, greenwich, _ , june _. [sidenote: he puts the "radius-vector" question to le verrier, but makes no mention of adams.] "'i have read, with very great interest, the account of your investigations on the probable place of a planet disturbing the motions of _uranus_, which is contained in the _compte rendu de l'académie_ of june ; and i now beg leave to trouble you with the following question. it appears, from all the later observations of _uranus_ made at greenwich (which are most completely reduced in the _greenwich observations_ of each year, so as to exhibit the effect of an error either in the tabular heliocentric longitude, or the tabular radius vector), that the tabular radius vector is considerably too small. and i wish to inquire of you whether this would be a consequence of the disturbance produced by an exterior planet, now in the position which you have indicated?'" there is more of the letter, but this will suffice to show that he wrote to le verrier in the same way as to adams, and, as already stated, received a reply dated three or four days later. but the rest of the letter contains no mention of adams, and thus arises a second difficulty in understanding airy's conduct. it seems extraordinary that when he wrote to le verrier he made no mention of the computations which he had previously received from adams; or that he should not have written to adams, and made some attempt to understand his long silence, now that, as he himself states, he "felt no doubt of the accuracy of both calculations." the omission may have been, and probably was, mere carelessness or forgetfulness; but he could hardly be surprised if others mistook it for deliberate action. [sidenote: airy announces the likelihood of a new planet, and suggests a search for it at cambridge not having suitable telescope at greenwich] however, attention had now been thoroughly attracted to the near possibility of finding the planet. on june , , there was a special meeting of the board of visitors of greenwich observatory, and airy incidentally mentioned to them this possibility. the impression produced must have been definite and deep; for sir john herschel, who was present, was bold enough to say on september th following to the british association assembled at southampton: "we see it (the probable new planet) as columbus saw america from the shores of spain. its movements have been felt trembling along the far-reaching line of our analysis with a certainty hardly inferior to that of ocular demonstration." airy discussed the matter with professor challis (who, it will be remembered, had originally written to him on behalf of adams), suggesting that he should immediately commence a search for the supposed planet at cambridge. it may be asked why airy did not commence this search himself at greenwich, and the answer is that he had no telescope which he regarded as large enough for the purpose. the royal observatory at greenwich has always been, and is now, better equipped in some respects than any other observatory, as might be expected from its deservedly great reputation; but to possess the largest existing telescope has never been one of its ambitions. the instruments in which it takes most pride are remarkable for their steadiness and accuracy rather than for their size; and at that time the best telescope possessed by the observatory was not, in airy's opinion, large enough to detect the planet with certainty. in this opinion we now know that he was mistaken; but, again, we must not judge his conduct before the event in the light of what we have since discovered. it may be recalled here that it was not until le verrier's third paper, published on august , that he (le verrier) emphatically pointed out that the new planet might be of such a size as to have a sensible disc; and it was this remark which led immediately to its discovery. until this was so decisively stated, it must have seemed exceptionally improbable; for we saw in the last chapter how diligently the zodiac had been swept in the search for minor planets,--how, for instance, hencke had searched for fifteen years without success; and it might fairly be considered that if there were a fairly bright object (such as neptune has since been found to be) it would have been discovered earlier. hence airy not unreasonably considered it necessary to spread his net for very small objects. on july he wrote to professor challis as follows:-- no. .--g. b. airy _to_ professor challis. "the deanery, ely, _ , july _. "you know that i attach importance to the examination of that part of the heavens in which there is ... reason for suspecting the existence of a planet exterior to _uranus_. i have thought about the way of making such examination, but i am convinced that (for various reasons, of declination, latitude of place, feebleness of light, and regularity of superintendence) there is no prospect whatever of its being made with any chance of success, except with the northumberland telescope. "now, i should be glad to ask you, in the first place, whether you could make such an examination? "presuming that your answer would be in the negative, i would ask, secondly, whether, supposing that an assistant were supplied to you for this purpose, you would superintend the examination? "you will readily perceive that all this is in a most unformed state at present, and that i am asking these questions almost at a venture, in the hope of rescuing the matter from a state which is, without the assistance that you and your instruments can give, almost desperate. therefore i should be glad to have your answer, not only responding simply to my questions, but also entering into any other considerations which you think likely to bear on the matter. "the time for the said examination is approaching near." [sidenote: challis undertakes the search.] [sidenote: he finds too late that he had observed the planet.] professor challis did not require an assistant, but determined to undertake the work himself, and devised his own plan of procedure; but he also set out on the undertaking with the expectation of a long and arduous search. no such idea as that of finding the planet on the first night ever entered his head. for one thing, he had no map of the region to be examined, for although the map used by galle had been published, no copy of it had as yet reached cambridge, and professor challis had practically to construct a map for himself. in these days of photography to make such a map is a simple matter, but at that time the process was terribly laborious. "i get over the ground very slowly," he wrote on september nd to airy, "thinking it right to include all stars to - magnitude; and i find that to scrutinise thoroughly in this way the proposed portion of the heavens will require many more observations than i can take this year." with such a prospect, it is not surprising that one night's observations were not even compared with the next; there would be a certain economy in waiting until a large amount of material had been accumulated, and then making the comparisons all together, and this was the course adopted. but when le verrier's third paper, with the decided opinion that the planet would be bright enough to be seen by its disc, ultimately reached professor challis, it naturally gave him an entirely different view of the possibilities; he immediately began to compare the observations already made, and found that he had observed the planet early in august. but it was now too late to be first in the field, for galle had already made his announcement of discovery. writing to airy on october , challis could only lament that after four days' observing the planet was in his grasp, _if_ only he had examined or mapped the observations, and _if_ he had not delayed doing so until he had more observations to reduce, and _if_ he had not been very busy with some comet observations. oh! these terrible _ifs_ which come so often between a man and success! the third of them is a peculiarly distressing one, for it represents that eternal conflict between one duty and another, which is so constantly recurring in scientific work. shall we finish one piece of work now well under way, or shall we attend to something more novel and more attractive? challis thought his duty lay in steadily completing the comet observations already begun. we saw in the last lecture how the steady pursuit of the discovery of minor planets, a duty which had become tedious and apparently led nowhere, suddenly resulted in the important discovery of eros. but challis was not so fortunate in electing to plod along the beaten track; he would have done _better_ to leave it. there is no golden rule for the answer; we must be guided in each case by the special circumstances, and the dilemma is consequently a new one on every occasion, and perhaps the more trying with each repetition. [sidenote: sensation caused by the discovery.] [sidenote: not all _national_ jealousy.] such are briefly the events which led to the discovery of neptune, which was made in germany by direction from france, when it might have been made in cambridge alone. the incidents created a great stir at the time. the "account" of them, as read by airy to the royal astronomical society on november , , straightforward and interesting though it was, making clear where he had himself been at fault, nevertheless stirred up angry passions in many quarters, and chiefly directed against airy himself. cambridge was furious at airy's negligence, which it considered responsible for costing the university a great discovery; and others were equally irate at his attempting to claim for adams some of that glory which they considered should go wholly to le verrier. but it may be remarked that feeling was not purely national. some foreigners were cordial in their recognition of the work of adams, while some of those most eager to oppose his claims were found in this country. in their anxiety to show that they were free from national jealousy, scientific men went almost too far in the opposite direction. [sidenote: the position of cambridge in the matter.] [sidenote: challis the weakest point.] airy's conduct was certainly strange at several points, as has already been remarked. one cannot understand his writing to le verrier in june without any mention of adams. he could not even momentarily have forgotten adams' work; for he tells us himself how he noticed the close correspondence of his result with that of le verrier: and had he even casually mentioned this fact in writing to the latter, it would have prepared the way for his later statement. but we can easily understand the unfavourable impression produced by this statement after the discovery had been made, when there had been no previous hint on the subject at all. of those who abused him cambridge had the least excuse; for there is no doubt that with a reasonably competent professor of astronomy in cambridge, she need not have referred to airy at all. it would not seem to require any great amount of intelligence to undertake to look in a certain region for a strange object if one is in possession of a proper instrument. we have seen that challis had the instrument, and when urged to do so was equal to the task of finding the planet; but he was a man of no initiative, and the idea of doing so unless directed by some authority never entered his head. he had been accustomed for many years to lean rather helplessly upon airy, who had preceded him in office at cambridge. for instance, when appointed to succeed him, and confronted with the necessity of lecturing to students, he was so helpless that he wrote to implore airy to come back to cambridge and lecture for him; and this was actually done, airy obtaining leave from the government to leave his duties at greenwich for a time in order to return to cambridge, and show challis how to lecture. now it seems to me that this helplessness was the very root of all the mischief of which cambridge so bitterly complained. i claimed at the outset the privilege of stating my own views, with which others may not agree: and of all the mistakes and omissions made in this little piece of history, the most unpardonable and the one which had most serious consequences seems to me to be this: that challis never made the most casual inquiry as to the result of the visit to greenwich which he himself had directed adams to make. i am judging him to some extent by default; because i assume the facts from lack of evidence to the contrary: but it seems practically certain that after sending this young man to see airy on this important topic, challis thereupon washed his hands of all responsibility so completely that he never even took the trouble to inquire on his return, "well! how did you get on? what did the astronomer royal say?" had he put this simple question, which scarcely required the initiative of a machine, and learnt in consequence, as he must have done, that the sensitive young man thought airy's question trivial, and did not propose to answer it, i think we might have trusted events to right themselves. even challis might have been trusted to reply, "oh! but you must answer the astronomer royal's question: you may think it stupid, but you had better answer it politely, and show him that you know what you are about." it is unprofitable to pursue speculation further; this did _not_ happen, and something else did. but i have always felt that my old university made a scapegoat of the wrong man in venting its fury upon airy, when the real culprit was among themselves, and was the man they had themselves chosen to represent astronomy. he was presumably the best they had; but if they had no one better than this, they should not have been surprised, and must not complain, if things went wrong. if a university is ambitious of doing great things, it must take care to see that there are men of ability and initiative in the right places. this is a most difficult task in any case, and we require all possible incentives towards it. to blink the facts when a weak spot is mercilessly exposed by the loss of a great opportunity is to lose one kind of incentive, and perhaps not the least valuable. [sidenote: curious difference between actual and supposed planet.] [sidenote: professor peirce's contention that the discovery was a mere accident.] [sidenote: the explanation.] let us now turn to some curious circumstances attending this remarkable discovery of a planet by mathematical investigation, of which there are several. the first is, that although neptune was found so near the place where it was predicted, its orbit, after discovery, proved to be very different from that which adams and le verrier had supposed. you will remember that both calculators assumed the distance from the sun, in accordance with bode's law, to be nearly twice that of uranus. the actual planet was found to have a mean distance less than this by per cent., an enormous quantity in such a case. for instance, if the supposed planet and the real were started round the sun together, the real planet would soon be a long way ahead of the other, and the ultimate disturbing effect of the two on uranus would be very different. to explain the difference, we must first recall a curious property of such disturbances. when two planets are revolving, so that one takes just twice or three times, or any exact number of times, as long to revolve round the sun as the other, the usual mathematical expressions for the disturbing action of one planet on the other would assign an _infinite_ disturbance, which, translated into ordinary language, means that we must start with a fresh assumption, for this state of things cannot persist. if the period of one were a little _longer_ than this critical value, some of the mathematical expressions would be of contrary sign from those corresponding to a period a little _shorter_. now it is curious that the supposed planet and the real had orbits on opposite sides of a critical value of this kind, namely, that which would assign a period of revolution for neptune exactly half that of uranus; and it was pointed out in america by professor peirce that the effect of the planet imagined by adams and le verrier was thus totally different from that of neptune. he therefore declared that the mathematical work had not really led to the discovery at all; but that it had resulted from mere coincidence, and this opinion--somewhat paradoxical though it was--found considerable support. it was not replied to by adams until some thirty years later, when a short reply was printed in _liouville's journal_. the explanation is this: the expressions considered by professor peirce are those representing the action of the planet throughout an indefinite past, and did not enter into the problem, which would have been precisely the same if neptune had been suddenly created in ; while, on the other hand, if neptune had existed up till (the time when uranus was first observed, although unknowingly), and then had been destroyed, there would have been no means of tracing its previous existence. in past ages it had no doubt been perturbing the orbit of uranus, and had effected large changes in it; but if it had then been suddenly destroyed, we should have had no means of identifying these changes. there might have been instead of neptune another planet, such as that supposed by adams and le verrier; and its action in all past time would have been very different from that of neptune, as is properly represented in the mathematical expressions which professor peirce considered. in consequence the orbit of uranus in would have been very different from the orbit as it was actually found; but in either case the mathematicians adams and le verrier would have had to take it as they found it; and the disturbing action which they considered in their calculations was the comparatively small disturbance which began in and ended in . during this limited number of years the disturbance of the planet they imagined, although not precisely the same as that of neptune, was sufficiently like it to give them the approximate place of the planet. still it is somewhat bewildering to look at the mathematical expressions for the disturbances as used by adams and le verrier, when we can now compare with them the actual expressions to which they ought to correspond; and one may say frankly that there seems to be no sort of resemblance. recently a memorial of adams' work has been published by the royal astronomical society; they have reproduced in their memoirs a facsimile of adams' ms. containing the "first solution," which he made in in the long vacation after he had taken his degree, and which would have given the place of neptune at that time with an error of °. in an introduction describing the whole of the mss., written by professor r. a. sampson of durham, it is shown how different the actual expressions for neptune's influence are from those used by adams, and it is one of the curiosities of this remarkable piece of history that some of them seem to be actually _in the wrong direction_; and others are so little alike that it is only by fixing our attention resolutely on the considerations above mentioned that we can realise that the analytical work did indeed lead to the discovery of the planet. [sidenote: suggested elementary method for finding neptune illusory.] a second curiosity is that a mistaken idea should have been held by at least one eminent man (sir j. herschel), to the effect that it would have been possible to find the place of the planet by a much simpler mathematical calculation than that actually employed by adams or le verrier. in his famous "outlines of astronomy" sir john herschel describes a simple graphical method, which he declares would have indicated the place of the planet without much trouble. concerning it i will here merely quote professor sampson's words:-- "the conclusion is drawn that _uranus_ arrived at a conjunction with the disturbing planet about ; and this was the case. plausible as this argument may seem, it is entirely baseless. for the maximum of perturbations depending on the eccentricities has no relation to conjunction, and the others which depend upon the differences of the mean motions alone are of the nature of forced oscillations, and conjunction is not their maximum or stationary position, but their position of most rapid change." professor sampson goes on to show that a more elaborate discussion seems quite as unpromising; and he concludes that the refinements employed were not superfluous, although it seems _now_ clear that a different mode of procedure might have led more certainly to the required conclusion. [sidenote: the evil influence of bode's law.] for the third curious point is that both calculators should have adhered so closely to bode's law. if they had not had this guiding principle it seems almost certain that they would have made a better approximation to the place of the planet, for instead of helping them it really led them astray. we have already remarked that if two planets are at different distances from the sun, however slight, and if they are started in their revolution together, they must inevitably separate in course of time, and the amount of separation will ultimately become serious. thus by assuming a distance for the planet which was in error, however slight, the calculators immediately rendered it impossible for themselves to obtain a place for the planet which should be correct for more than a very brief period. professor sampson has given the following interesting lists of the dates at which adams' six solutions gave the true place of the planet and the intervals during which the error was within ° either way. i. ii. iii. iv. v. vi. correct within ± ° { { now the date at which it was most important to obtain the correct place was or thereabouts when it was proposed to look for the planet; but no special precaution seems to have been taken by either investigator to secure any advantage for this particular date. criticising the procedure after the event (and of course this is a very unsatisfactory method of criticism), we should say that it would have been better to make several assumptions as regards the distance instead of relying upon bode's law; but no one, so far as i know, has ever taken the trouble to write out a satisfactory solution of the problem as it might have been conducted. such a solution would be full of interest, though it could only have a small weight in forming our estimation of the skill with which the problem was solved in the first instance. [sidenote: le verrier's erroneous limits.] fourthly, we may notice a very curious point. le verrier went to some trouble not only to point out the most likely place for the planet, but to indicate limits outside which it was not necessary to look. this part of his work is specially commented upon with enthusiasm by airy, and i will reproduce what he says. it is rather technical perhaps, but those who cannot follow the mathematics will be able to appreciate the tone of admiration. [sidenote: the visible disc.] "m. le verrier then enters into a most ingenious computation of the limits between which the planet must be sought. the principle is this: assuming a time of revolution, all the other unknown quantities may be varied in such a manner that though the observations will not be so well represented as before, yet the errors of observation will be tolerable. at last, on continuing the variation of elements, one error of observation will be intolerably great. then, by varying the elements in another way, we may at length make another error of observation intolerably great; and so on. if we compute, for all these different varieties of elements, the place of the planet for , its _locus_ will evidently be a discontinuous curve or curvilinear polygon. if we do the same thing with different periodic times, we shall get different polygons; and the extreme periodic times that can be allowed will be indicated by the polygons becoming points. these extreme periodic times are and years. if now we draw one grand curve, circumscribing all the polygons, it is certain that the planet must be within that curve. in one direction, m. le verrier found no difficulty in assigning a limit; in the other he was obliged to restrict it, by assuming a limit to the eccentricity. thus he found that the longitude of the planet was certainly not less than °, and not greater than ° or °, according as we limit the eccentricity to . or . . and if we adopt . as the limit, then the mass will be included between the limits . and . ; either of which exceeds that of _uranus_. from this circumstance, combined with a probable hypothesis as to the density, m. le verrier concluded that the planet would have a visible disk, and sufficient light to make it conspicuous in ordinary telescopes. "m. le verrier then remarks, as one of the strong proofs of the correctness of the general theory, that the error of radius vector is explained as accurately as the error of longitude. and finally, he gives his opinion that the latitude of the disturbing planet must be small. "my analysis of this paper has necessarily been exceedingly imperfect, as regards the astronomical and mathematical parts of it; but i am sensible that, in regard to another part, it fails totally. i cannot attempt to convey to you the impression which was made on me by the author's undoubting confidence in the general truth of his theory, by the calmness and clearness with which he limited the field of observation, and by the firmness with which he proclaimed to observing astronomers, 'look in the place which i have indicated, and you will see the planet well.' since copernicus declared that, when means should be discovered for improving the vision, it would be found that _venus_ had phases like the moon, nothing (in my opinion) so bold, and so justifiably bold, has been uttered in astronomical prediction. it is here, if i mistake not, that we see a character far superior to that of the able, or enterprising, or industrious mathematician; it is here that we see the philosopher." [sidenote: peirce's views of the limits.] but now this process of limitation was faulty and actually misleading. let us compare what is said about it by professor peirce a little later. "guided by this principle, well established, and legitimate, if confined within proper limits, m. le verrier narrowed with consummate skill the field of research, and arrived at two fundamental propositions, namely:-- " st. that the mean distance of the planet cannot be less than or more than . . the corresponding limits of the time of sidereal revolution are about and years. " nd. 'that there is only one region in which the disturbing planet can be placed in order to account for the motions of uranus; that the mean longitude of this planet must have been, on january , , between ° and °.' "'neither of these propositions is of itself necessarily opposed to the observations which have been made upon neptune, but the two combined are decidedly inconsistent with observation. it is impossible to find an orbit, which, satisfying the observed distance and motion, is subject to them. if, for instance, a mean longitude and time of revolution are adopted according with the first, the corresponding mean longitude in must have been at least ° distant from the limits of the second proposition. and again, if the planet is assumed to have had in a mean longitude near the limits of the second proposition, the corresponding time of revolution with which its motions satisfy the present observations cannot exceed years, and must therefore be about years less than the limits of the first proposition.' "neptune cannot, then, be the planet of m. le verrier's theory, and cannot account for the observed perturbations of uranus under the form of the inequalities involved in his analysis"--(_proc. amer. acad. i._, - , _p._ ). [sidenote: newcomb's criticism.] at the time when professor peirce wrote, the orbit of neptune was not sufficiently well determined to decide whether one of the two limitations might not be correct, though he could see that they could not both be right, and we now know that they are _both wrong_. the mean distance of neptune is , which does _not_ lie between and . ; and the longitude in was °, which does _not_ lie between ° and °. the ingenious process which airy admired and which peirce himself calls "consummately skilful" was wrong in principle. as professor newcomb has said, "the error was the elementary one that, instead of considering all the elements simultaneously variable, le verrier took them one at a time, considering the others as fixed, and determining the limits between which each could be contained on this hypothesis. no solver of least square equations at the present day ought to make such a blunder. of course one trouble in le verrier's demonstration, had he attempted a rigorous one, would have been the impossibility of forming the simultaneous equations expressive of possible variations of all the elements." [sidenote: element of good fortune.] [sidenote: the map used by galle.] the account of le verrier's limits by professor peirce, though it exhibits the error with special clearness, is a little unfair to le verrier in one point. if, instead of taking the limits for the date , we take them for (when the search for neptune was actually made), we shall find that they do include the actual place of the planet, as airy found. the erroneous mean motion of le verrier's planet allowed of his being right at one time and wrong at another; and airy examined the limits under favourable conditions, which explains his enthusiasm. but we can scarcely wonder that professor peirce came to the conclusion that the planet discovered was not the one pointed out by le verrier, and had been found by mere accident. and all these circumstances inevitably contribute to a general impression that the calculators had a large element of good fortune to thank for their success. nor need we hesitate to make this admission, for there is an element of good fortune in all discoveries. to look no further than this--if a man had not been doing a particular thing at a particular time, as he might easily not have been, most discoveries would never have been made. if sir william herschel had not been looking at certain small stars for a totally different purpose he would never have found uranus; and no one need hesitate to admit the element of chance in the finding of neptune. it is well illustrated by a glance at the map which, as has been remarked, galle used to compare with the sky on the night when he made the actual discovery. the planet was found down near the bottom corner of the map, and since the limits assigned for its place might easily have varied a few degrees one way or the other, it might easily have been off the map; in which case, it is probable that the search would not have been successful, or at any rate that success would have been delayed. [illustration: v.--corner of the berlin map, by the use of which galle found neptune.] [sidenote: every one made mistakes.] thus, it is a most remarkable feature of the discovery of neptune that mistakes were made by almost every one concerned, however eminent. airy made a mistake in regarding the question of the radius vector as of fundamental importance; sir j. herschel was wrong in describing an elementary method which he considered might have found the planet; professor peirce was wrong in supposing that the actual and the supposed planet were essentially different in their action on uranus; le verrier was wrong in assigning limits outside which it was not necessary to look when the actual planet was outside them; adams was more or less wrong in thinking that the eccentricity of the new planet could be found from the material already at disposal of man. both adams and le verrier gave far too much importance to bode's law. to review a piece of history of this kind and note the mistakes of such men is certainly comforting, and need not in any way lessen our admiration. in the case of the investigators themselves, much may be set down to excitement in the presence of a possible discovery. professor sampson has provided us with a small but typical instance of this fact. when adams had carried through all his computations for finding neptune, and was approaching the actual place of the planet, he, "who could carry through fabulous computations without error," for the first time wrote down a wrong figure. the mistake was corrected upon the ms., "probably as soon as made," but no doubt betrays the excitement which the great worker could not repress at this critical moment. there is a tradition that, similarly, when the mighty newton was approaching the completion of his calculations to verify the law of gravitation, his excitement was so great that he was compelled to assign to a friend the task of finishing them. finally, we may remark how the history of the discovery of neptune again illustrates the difficulty of formulating any general principles for guiding scientific work. sometimes it is well to follow the slightest clue, however imperfectly understood; at other times we shall do better to refuse such guidance. bode's law pointed to the existence of minor planets, and might conceivably have helped in finding uranus: but by trusting to it in the case of neptune, the investigators were perilously near going astray. sometimes it is better to follow resolutely the work in hand whatever it may be, shutting one's ears to other calls; but airy and challis lost their opportunities by just this course of action. the history of science is full of such contradictory experiences; and the only safe conclusion seems to be that there are no general rules of conduct for discovery. chapter iii bradley's discoveries of the aberration of light and of the nutation of the earth's axis [sidenote: biographical method adopted.] in examining different types of astronomical discovery, we shall find certain advantages in varying to some extent the method of presentation. in the two previous chapters our opportunities for learning anything of the life and character of those who made the discoveries have been slight; but i propose to adopt a more directly biographical method in dealing with bradley's discoveries, which are so bound up with the simple earnestness of his character that we could scarcely appreciate their essential features properly without some biographical study. but the record of his life apart from his astronomical work is not in any way sensational; indeed it is singularly devoid of incident. he had not even a scientific quarrel. there was scarcely a man of science of that period who had not at least one violent quarrel with some one, save only bradley, whose gentle nature seems to have kept him clear of them all. judged by ordinary standards his life was uneventful: and yet it may be doubted whether, to him who lived it, that life contained one dull moment. incident came for him in his scientific work: in the preparation of apparatus, the making of observations, above all in the hard-thinking which he did to get at the clue which would explain them; and after reviewing his biography,[ ] i think we shall be inclined to admit that if ever there was a happy life, albeit one of unremitting toil, it was that of james bradley. [sidenote: bradley's birth and early life.] [sidenote: brief clerical career.] he was born at sherbourn, in gloucestershire, in . we know little of his boyhood except that he went to the grammar school at northleach, and that the memory of this fact was preserved at the school in when rigaud was writing his memoir. [the school is at present shut up for want of funds to carry it on; and all inquiries i have made have failed to elicit any trace of this memory.] similarly we know little of his undergraduate days at oxford, except that he entered as a commoner at balliol in , took his b.a. in the regular course in , and his m.a. in . as a career he chose the church, being ordained in , and presented to the vicarage of bridstow in monmouthshire; but he only discharged the duties of vicar for a couple of years, for in he returned to oxford as professor of astronomy, an appointment which involved the resignation of his livings; and so slight was this interruption to his career as an astronomer that we may almost disregard it, and consider him as an astronomer from the first. but to guard against a possible misconception, let me say that bradley entered on a clerical career in a thoroughly earnest spirit; to do otherwise would have been quite foreign to his nature. when vicar of bridstow he discharged his duties faithfully towards that tiny parish, and moreover was so active in his uncle's parish of wansted that he left the reputation of having been curate there, although he held no actual appointment. and thirty years later, when he was astronomer royal and resident at greenwich, and when the valuable vicarage of greenwich was offered to him by the chancellor of the exchequer, he honourably refused the preferment, "because the duty of a pastor was incompatible with his other studies and necessary engagements." [sidenote: learnt astronomy _not_ at oxford, but from his uncle, james pound.] [sidenote: pound a first-rate observer.] but now let us turn to bradley's astronomical education. i must admit, with deep regret, that we cannot allow any of the credit of it to oxford. there was a great astronomer in oxford when bradley was an undergraduate, for edmund halley had been appointed savilian professor of geometry in , and had immediately set to work to compute the orbits of comets, which led to his immortal discovery that some of these bodies return to us again and again, especially the one which bears his name--halley's comet--and returns every seventy-five years, being next expected about . but there is no record that bradley came under halley's teaching or influence as an undergraduate. in later years the two men knew each other well, and it was halley's one desire towards the close of his life that bradley should succeed him as astronomer royal at greenwich; a desire which was fulfilled in rather melancholy fashion, for halley died without any assurance that his wish would be gratified. but bradley got no astronomical teaching at oxford either from halley or others. the art of astronomical observation he learnt from his maternal uncle, the rev. james pound, rector of wansted, in essex. he is the man to whom we owe bradley's training and the great discoveries which came out of it. he was, i am glad to say, an oxford man too; very much an oxford man; for he seems to have spent some thirteen years there migrating from one hall to another. his record indeed was such as good tutors of colleges frown upon; for it was seven years before he managed to take a degree at all; and he could not settle to anything. after ten years at oxford he thought he would try medicine; after three years more he gave it up and went out in as chaplain to the east indies. but he seems to have been a thoroughly lovable man, for news was brought of him four years later that he had a mind to come home, but was dissuaded by the governor saying that "if dr. pound goes, i and the rest of the company will not stay behind." soon afterwards the settlement was attacked in an insurrection, and pound was one of the few who escaped with his life, losing however all the property he had gradually acquired. he returned to england in , and was presented to the living of wansted; married twice, and ended his days in peace and fair prosperity in . such are briefly the facts about bradley's uncle, james pound; but the most important of all remains to be told--that somehow or other he had learnt to make first-rate astronomical observations, how or when is not recorded; but in he was already so skilled that sir isaac newton made him a present of fifty guineas for some observations; and repeated the gift in the following year; and even three years before this we find halley writing to ask for certain observations from mr. pound. [sidenote: bradley worked with him.] with this excellent man bradley used frequently to stay. to his nephew he seems to have been more like a father than an uncle. when his nephew had smallpox in , he nursed him through it; and he supplemented from his own pocket the scanty allowance which was all that bradley's own father could afford. but what concerns us most is that he fostered, if he did not actually implant, a love of astronomical observation in his nephew. the two worked together, entering their observations one after the other on the same paper; and it was to the pair of them together, rather than to the uncle alone, that newton made his princely presents, and halley wrote for help in his observations. there seems to be no doubt that the uncle and nephew were about this time the best astronomical observers in the world. there was no rivalry between them, and therefore there is no need to discuss whether the partnership was one of equal merit on both sides; but it is interesting to note that it probably was. the ability of pound was undoubted; many were keenly desirous that he, and not his nephew, should be elected to the oxford chair in , but he felt unequal to the duties at his advanced age. on the other hand, when bradley lost his uncle's help, there was no trace of faltering in his steps to betray previous dependence on a supporting or guiding hand. he walked erect and firm, and trod paths where even his uncle might not have been able to follow. [sidenote: the work done by pound and bradley.] [sidenote: use of very long telescopes.] [sidenote: reason for great length.] a few instances will suffice to show the kind of observations made by this notable firm of pound and bradley. they observed the positions of the fixed stars and nebulæ: these being generally the results required by halley and newton. they also observed the places of the planets among the stars, and especially the planet mars, and determined its distance from the earth by the method of parallax, thus anticipating the modern standard method of finding the sun's distance; and though with their imperfect instruments they did not obtain a greater accuracy than in , still this was a great advance on what had been done before, and excited the wonder and admiration of halley. they also paid some attention to double stars, and did a great deal of work on jupiter's satellites. we might profitably linger over the records of these early years, which are full of interest, but we must press on to the time of the great discoveries, and we will dismiss them with brief illustrations of three points: bradley's assiduity, his skill in calculation, and his wonderful skill in the management of instruments. of his assiduity an example is afforded by his calculations of the orbits of two comets which are still extant. one of them fills thirty-two pages of foolscap, and the other sixty; and it must be remembered that the calculations themselves were quite novel at that time. of his _skill_ in calculation, apart from his assiduity, we have a proof in a paper communicated to the royal society rather later ( ), where he determines the longitudes of lisbon and new york from the eclipses of jupiter's satellites, using observations which were not simultaneous, and had therefore to be corrected by an ingenious process which bradley devised expressly for this purpose. and finally, his skill in the management of instruments is shown by his measuring the diameter of the planet venus with a telescope actually - / feet in length. it is difficult for us to realise in these days what this means; even the longest telescope of modern times does not exceed feet in length, and it is mounted so conveniently with all the resources of modern engineering, in the shape of rising floors, &c., that the management of it is no more difficult than that of a -foot telescope. but bradley had no engineering appliances beyond a pole to hold up one end of the telescope and his own clever fingers to work the other; and he managed to point the unwieldy weapon accurately to the planet, and measure the diameter with an exactness which would do credit to modern times. a few words of explanation may be given why such long telescopes were used at all. the reason lay in the difficulty of getting rid of coloured images, due to the composite character of white light. whenever we use a _single_ lens to form an image, coloured fringes appear. nowadays we know that by making two lenses of different kinds of glass and putting them together, we can practically get rid of these coloured fringes; but this discovery had not been made in bradley's time. the only known ways of dealing with the evil then were to use a reflecting telescope like newton and gregory, or if a lens was used, to make one of very great focal length; and hence the primary necessity for these very long telescopes. they had another advantage in producing a large image, or they would probably have given way to the reflector. this advantage is gradually bringing them back into use, and perhaps in the eclipse of we may use a telescope as long as bradley's; but we shall not use it as he did in any case. it will be laid comfortably flat on the ground, and the rays of light reflected into it by a coelostat. [sidenote: bradley appointed at oxford, but continues to work at wansted.] in bradley was appointed to the savilian professorship of astronomy at oxford, vacant by the death of dr. john keill. once it became clear that there was no chance of securing his uncle for this position, bradley himself was supported enthusiastically by all those whose support was worth having, especially by the earl of macclesfield, who was then lord chancellor; by martin foulkes, who was afterwards the president of the royal society; and by sir isaac newton himself. he was accordingly elected on october , , and forthwith resigned his livings. his resignation of the livings was necessitated by a definite statute of the university relating to the professorship, and not by the existence of any very onerous duties attaching to it; indeed such duties seem to have been conspicuously absent, and after bradley's election he passed more time than ever with his uncle in wansted, making the astronomical observations which both loved; for there was not the vestige of an observatory in oxford. his uncle's death in interrupted the continuity of these joint observations, and by an odd accident prepared the way for bradley's great discovery. he was fain to seek elsewhere that companionship in his work which had become so essential to him, and his new friend gave a new bent to his observations. [sidenote: samuel molyneux.] [sidenote: attempts to find stellar parallax.] samuel molyneux was a gentleman of fortune much attached to science, and particularly to astronomy, who was living about this time at kew. he was one of the few, moreover, who are not content merely to amuse themselves with a telescope, but had the ambition to do some real earnest work, and the courage to choose a problem which had baffled the human race for more than a century. the theory of copernicus, that the earth moved round the sun, necessitated a corresponding apparent change in the places of the stars, one relatively to another; and it was a standing difficulty in the way of accepting this theory that no such change could be detected. in the old days before the telescope it was perhaps easy to understand that the change might be too small to be noticed, but the telescope had made it possible to measure changes of position at least a hundred times as small as before, and still no "parallax," as the astronomical term goes, could be found for the stars. the observations of galileo, and the measures of tycho brahé, as reduced to systematic laws by kepler, and finally by the great newton, made it clear that the copernican theory was _true_: but no one had succeeded in proving its truth in this particular way. samuel molyneux must have been a man of great courage to set himself to try to crack this hard nut; and we can understand the attraction which his enterprise must have had for bradley, who had just lost the beloved colleague of many courageous astronomical undertakings. his co-operation seems to have been welcomed from the first; his help was invited and freely given in setting up the instrument, and he fortunately had the leisure to spend considerable time at kew making the observations with molyneux, just as he had been wont to observe with his uncle. i must now briefly explain what these observations were. there is a bright star [gamma] draconis, which passes almost directly overhead in the latitude of london. its position is slowly changing owing to the precession of the equinoxes, but for two centuries it has been, and is still, under constant observation by london astronomers owing to this circumstance, that it passes directly overhead, and so its position is practically undisturbed by the refraction of our atmosphere. [sidenote: the instrument.] [sidenote: expected results.] it was therefore thought at the time that, there being no disturbance from refraction, the disturbance from precession being accurately known, and there being nothing else to disturb the position but "parallax" (the apparent shift due to the earth's motion which it was desirable to find), this star ought to be a specially favourable object for the determination of parallax. indeed it had been announced many years before by hooke that its parallax had been found; but his observations were not altogether satisfactory, and it was with a view of either confirming them or seeing what was wrong with them that molyneux and bradley started their search. they set up a much more delicate piece of apparatus than hooke had employed. it was a telescope feet long pointed upwards to the star, and firmly attached to a large stack of brick chimneys within the house. the telescope was not absolutely fixed, for the lower end could be moved by a screw so as to make it point accurately to the star, and a plumb-line showed how far it was from the vertical when so pointing. hence if the star changed its position, however slightly, the reading of this screw would show the change. now, before setting out on the observations, the observers knew what to expect if the star had a real parallax; that is to say, they knew that the star would seem to be farthest south in december, farthest north in june, and at intermediate positions in march and september; though they did not know _how much_ farther south it would appear in december than in june--this was exactly the point to be decided. [illustration: fig. .] [sidenote: unexpected results.] the reason of this will be clear from fig. . [remark, however, that this figure and the corresponding figure do not represent the case of bradley's star, [gamma] draconis: another star has been chosen which simplifies the diagram, though the principle is essentially the same.] let a b c d represent the earth's orbit, the earth being at a in june, at b in september, and so on, and let k represent the position of the star on the line d b. then in march and september it will be seen from the earth in the same direction, namely, d b k; but the directions in which it is seen in june and december, viz. a k and c k, are inclined in opposite ways to this line. the farther away the star is, the less will this inclination or "parallax" be; and the star is actually so far away that the inclination can only be detected with the utmost difficulty: the lines c k and a k are sensibly parallel to d b k. but bradley did not know this; it was just this point which he was to examine, and he expected the greatest inclination in one direction to be in december. accordingly when a few observations had been made on december , , , and it was thought that the star had been caught at its most southerly apparent position, and might be expected thereafter to move northwards, if at all. but when bradley repeated the observation on december , he found to his great surprise that the star was still moving southwards. here was something quite new and unexpected, and such a keen observer as bradley was at once on the alert. he soon found that the changes in the position of the star were of a totally unexpected character. instead of the extreme positions being occupied in june and december, they were occupied in march and september, just midway between these. and the range in position was quite large, about "--not a quantity which could have been detected in the days before telescopes, but one which was unmistakable with an instrument of the most moderate measuring capacity. [sidenote: tentative explanations.] what, then, was the cause of this quite unforeseen behaviour on the part of the star? the first thought of the observers was that something might be wrong with their instrument, and it was carefully examined, but without result. the next was that the apparent movement was in the plumb-line, the line of reference. if the whole earth, instead of carrying its axis round the sun in a constant direction, were to be executing an oscillation, then all our plumb-lines would oscillate, and when the direction of a star like [gamma] draconis was compared with that of the plumb-line it would seem to vary, owing actually to the variation in the plumb-line. the earth might have a motion of this kind in two ways, which it will be necessary for us to distinguish, and the adopted names for them are "nutation of the axis" and "variation of latitude" respectively. in the case of nutation the north pole remains in the same geographical position, but points to a different part of the heavens. the "variation of latitude," on the other hand, means that the north pole wanders about on the earth itself. we shall refer to the second phenomenon more particularly in the sixth chapter. [sidenote: nutation?] [sidenote: anomalous refraction.] but it was the first kind of change, the nutation, which bradley suspected; and very early in the series of observations he had already begun to test this hypothesis. if it was not the star, but the earth and the plumb-line, which were in motion, then other stars ought to be affected. the telescope had been deliberately restricted in its position to suit [gamma] draconis; but since the stars circle round the pole, if we draw a narrow belt in the heavens with the pole as centre, and including [gamma] draconis, the other stars included would make the same circuit, preceding or following [gamma] draconis by a constant interval. most of them would be too faint for observation with bradley's telescope; but there was one bright enough to be observed, which also came within its limited range, and it was promptly put under _surveillance_ when a nutation of the earth's axis was suspected. careful watching showed that it was not affected in the same way as [gamma] draconis, and hence the movement could not be in the plumb-line. was there, then, after all, some effect of the earth's atmosphere which had been overlooked? we have already remarked that since the star passes directly overhead there should be practically no refraction; and this assumption was made by molyneux and bradley in choosing this particular star for observation. it follows at once, if we assume that the atmosphere surrounds the earth in spherical layers. but perhaps this was not so? perhaps, on the contrary, the atmosphere was deformed by the motion of the earth, streaming out behind her like the smoke of a moving engine? no possibility must be overlooked if the explanation of this puzzling fact was to be got at. [illustration: fig. .] the way in which a deformation of the atmosphere might explain the phenomenon is best seen by a diagram. first, it must be remarked that rays of light are only bent by the earth's atmosphere, or "refracted," if they enter it obliquely. if the atmosphere were of the same density throughout, like a piece of glass, then a vertical ray of light, a b (see fig. ), entering the atmosphere at b would suffer no bending or refraction, and a star shining from the direction a b would be seen truly in that direction from c. but an oblique ray, d e, would be bent on entering the atmosphere at e along the path ef, and a star shining along d e would appear from f to be shining along the dotted line g e f. the atmosphere is not of the same density throughout, but thins out as we go upwards from the earth; and in consequence there is no clear-cut surface, b e, and no sudden bending of the rays as at e: they are gradually bent at an infinite succession of imaginary surfaces. but it still remains true that there is no bending at all for vertical rays; and of oblique rays those most oblique are most bent. [illustration: fig. .] now, suppose the atmosphere of the earth took up, owing to its revolution round the sun, an elongated shape like that indicated in diagram , and suppose the star to be at a great distance away to the right of the diagram. when the earth is in the position labelled "june," the light would fall vertically on the nose of the atmosphere at a, and there would be no refraction. similarly in "december" the light would fall at c on the stern, also vertically, and there would be no refraction. [the rays from the distant star in december are to be taken as sensibly parallel to those received in june, notwithstanding that the earth is on the opposite side of the sun, as was remarked on p. .] but in march and september the rays would strike obliquely on the sides of the supposed figure, and thus be bent in opposite directions, as indicated by the dotted lines; and the extreme positions would thus occur in march and september, as had been observed. the explanation thus far seems satisfactory enough. but we have assumed the star to lie in the plane of the earth's orbit; and the stars under observation by bradley did not lie in this plane, nor did they lie in directions equally inclined to it. making the proper allowance for their directions, it was found impossible to fit in the facts with this hypothesis, which had ultimately to be abandoned. [sidenote: delay in finding real explanation.] [sidenote: bradley sets up another instrument at wansted.] [sidenote: finds the right clue.] [sidenote: a wind-vane on a boat.] it is remarkable to find that two or three years went by before the real explanation of this new phenomenon occurred to bradley, and during this time he must have done some hard thinking. we have all had experience of the _kind_ of thinking if only in the guessing of conundrums. we know the apparent hopelessness of the quest at the outset: the racking of our brains for a clue, the too frequent despair and "giving it up," and the simplicity of the answer when once it is declared. but with scientific conundrums the expedient of "giving it up" is not available. we must find the answer for ourselves or remain in ignorance; and though we may feel sure that the answer when found will be as simple as that to the best conundrum, this expected simplicity does not seem to aid us in the search. bradley was not content with sitting down to think: he set to work to accumulate more facts. molyneux's instrument only allowed of the observation of two stars, [gamma] draconis and the small star above mentioned. bradley determined to have an instrument of his own which should command a wider range of stars; and by this time he was able to return to his uncle's house at wansted for this purpose. his uncle had been dead for two or three years, and the memory of the loss was becoming mellowed with time. his uncle's widow was only too glad to welcome back her nephew, though no longer to the old rectory, and she allowed him to set up a long telescope, even though he cut holes in her floor to pass it through. the object-glass end was out on the roof and the eye end down in the coal cellar; and accordingly in this coal cellar bradley made the observations which led to his immortal discovery. he had a list of seventy stars to observe, fifty of which he observed pretty regularly. it may seem odd that he did not set up this new instrument at oxford, but we find from an old memorandum that his professorship was not bringing him in quite £ a year, and probably he was glad to accept his aunt's hospitality for reasons of economy. by watching these different stars he gradually got a clear conception of the laws of aberration. the real solution of the problem, according to a well-authenticated account, occurred to him almost accidentally. we all know the story of the apple falling and setting newton to think about the causes of gravitation. it was a similarly trivial circumstance which suggested to bradley the explanation which he had been seeking for two or three years in vain. in his own words, "at last, when he despaired of being able to account for the phenomena which he had observed, a satisfactory explanation of them occurred to him all at once when he was not in search of it." he accompanied a pleasure party in a sail upon the river thames. the boat in which they were was provided with a mast which had a vane at the top of it. it blew a moderate wind, and the party sailed up and down the river for a considerable time. dr. bradley remarked that every time the boat put about the vane at the top of the boat's mast shifted a little, as if there had been a slight change in the direction of the wind. he observed this three or four times without speaking; at last he mentioned it to the sailors, and expressed his surprise that the wind should shift so regularly every time they put about. the sailors told him that the wind had not shifted, but that the apparent change was owing to the change in the direction of the boat, and assured him that the same thing invariably happened in all cases. this accidental observation led him to conclude that the phenomenon which had puzzled him so much was owing to the combined motion of light and of the earth. to explain exactly what is meant we must again have recourse to a diagram; and we may also make use of an illustration which has become classical. [illustration: fig. .] [sidenote: analogy of rain.] if rain is falling vertically, as represented by the direction a b; and if a pedestrian is walking horizontally in the direction c d, the rain will appear to him to be coming in an inclined direction, e f, and he will find it better to tilt his umbrella forwards. the quicker his pace the more he will find it advisable to tilt the umbrella. this analogy was stated by lalande before the days of umbrellas in the following words: "je suppose que, dans un temps calme, la pluie tombe perpendiculairement, et qu'on soit dans une voiture ouverte sur le devant; si la voiture est en repos, on ne reçoit pas la moindre goutte de pluie; si la voiture avance avec rapidité, la pluie entre sensiblement, comme si elle avoit pris une direction oblique." lalande's example, modified to suit modern conditions, has been generally adopted by teachers, and in examinations candidates produce graphic pictures of the stationary, the moderate-paced, and the flying, possessors of umbrellas. [sidenote: aberration.] applying it to the phenomenon which it is intended to illustrate, if light is being received from a star by an earth, travelling across the direction of the ray, the telescope (which in this case represents the umbrella) must be tilted forward to catch the light. now on reference to fig. it will be seen that the earth is travelling across the direction of rays from the star in march and september; and in opposite directions in the two cases. hence the telescope must be tilted a little, in opposite directions, to catch the light; or, in other words, the star will appear to be farthest south in march, farthest north in september. and so at last the puzzle was solved, and the solution was found, as so often happens, to be of the simplest kind; so simple when once we know, and so terribly hard to imagine when we don't! it may comfort us in our struggles with minor problems to reflect that bradley manfully stuck to his problem for two or three years. it was probably never out of his thoughts, waking or sleeping; when at work it was the chief object of his labours, and when on a pleasure party he was ready to catch at the slightest clue, in the motion of a wind-vane on a boat, which might help him to the solution. [sidenote: results of discovery.] the discovery of aberration made bradley famous at a bound. oxford might well be proud of her two savilian professors at this time, for they had both made world-famous discoveries--halley that of the periodicity of comets, and bradley of the aberration of light. how different their tastes were and how difficult it would have been for either to do the work of the other! bradley was no great mathematician, and though he was quite able to calculate the orbit of a comet, and carried on such work when halley left it, it was probably not congenial to him. halley, on the other hand, almost despised accurate observations as finicking. "be sure you are correct to a minute," he was wont to say, "and the fractions do not so much matter." with such a precept bradley would never have made his discoveries. no quantity was too small in his eyes, and no sooner was the explanation of aberration satisfactorily established than he perceived that though it would account for the main facts, it would not explain all. there was something left. this is often the case in the history of science. a few years ago it was thought that we knew the constitution of our air completely--oxygen, nitrogen, water vapour, and carbonic acid gas; but a great physicist, lord rayleigh, found that after extracting all the water and carbonic acid gas, all the oxygen and all the nitrogen, there was something left--a very minute residuum, which a careless experimenter would have overlooked or neglected, but which a true investigator like lord rayleigh saw the immense importance of. he kept his eye on that something left, and presently discovered a new gas which we now know as argon. had he repeated the process, extracting all the argon after the nitrogen, he might have found by a scrutiny much more accurate still yet another gas, helium, which we now know to exist in extremely minute quantities in the air. but meantime this discovery was made in another way. [sidenote: still something to be explained.] [sidenote: probably nutation.] [sidenote: his nineteen years' campaign.] when bradley had extracted all the aberration from his observations he found that there was something left, another problem to be solved and some more thinking to be done to solve it. but he was now able to profit by his previous labours, and the second step was made more easily than the first. the residuum was not the parallax of which he had originally been in search, for it did not complete a cycle within the year; it was rather a progressive change from year to year. but there was an important clue of another kind. he saw that the apparent movements of all stars were in this case the same; and he knew that a movement of this kind can be referred, not to the stars themselves, but to the plumb-line from which their directions are measured. he had thought out the possible causes of such a movement of the plumb-line or of the earth itself, and had realised that there might be a _nutation_ which would go through a cycle in about nineteen years, the period in which the moon's nodes revolve. he was not mathematician enough to work out the cause completely, but he saw clearly that to trace the whole effect he must continue the observations for nineteen years; and accordingly he entered on this long campaign without any hesitation. his instrument was still that in his aunt's house at wansted, where he continued to live and make the observations for a few years, but in he removed to oxford, as we shall see, and he must have made many journeys between wansted and oxford in the course of the remaining fifteen years during which he continued to trace out the effects of nutation. his aunt too left wansted to accompany bradley to oxford, and the house passed into other hands. it is to the lasting credit of the new occupant, mrs. elizabeth williams, that the great astronomer was allowed to go on and complete the valuable series of observations which he had commenced. bradley was not lodged in her house; he stayed with a friend close by on his visits to wansted, but came freely in and out of his aunt's old home to make his observations. how many of us are there who would cheerfully allow an astronomer to enter our house at any hour of the night to make observations in the coal-cellar! it says much, not only for bradley's fame, but for his personal attractiveness, that he should have secured this permission, and that there should be no record of any friction during these fifteen years. at the end of the whole series of nineteen years his conclusions were abundantly verified, and his second great discovery of nutation was established. honours were showered upon him, and no doubt the gentle heart of mrs. elizabeth williams was uplifted at the glorious outcome of her long forbearance. [sidenote: residence at oxford.] but we may now turn for a few moments from bradley's scientific work to his daily life. we have said that in , after holding his professorship for eleven years, he first went definitely to reside in oxford. he actually had not been able to afford it previously. his income was only £ a year, and the statutes prevented him from holding a living: so that he was fain to accept mrs. pound's hospitable shelter. but in an opportunity of adding to his income presented itself, by giving lectures in "experimental philosophy." the observations on nutation were not like those on aberration: he was not occupied day and night trying to find the solution: he had practically made up his mind about the solution, and the actual observations were to go on in a quiet methodical manner for nineteen years, so that he now had leisure to look about him for other employment. dr. keill, who had been professor of astronomy before bradley, had attracted large classes to lectures, not on astronomy, but on experimental philosophy: but had sold his apparatus and goodwill to mr. whiteside, of christ church, one of the candidates who were disappointed by bradley's election. in bradley purchased the apparatus from whiteside, and began to give lectures in experimental philosophy. his discovery of aberration had made him famous, so that his classes were large from the first, and paid him considerable fees. suddenly therefore he changed his poverty for a comfortable income, and he was able to live in oxford in one of two red brick houses in new college lane, which were in those days assigned to the savilian professors (now inhabited by new college undergraduates). his aunt, mrs. pound, to whom he was devotedly attached, came with him, and two of her nephews. in his time of prosperity bradley was thus able to return the hospitality which had been so generously afforded him in times of stress. [sidenote: astronomer royal at greenwich.] [sidenote: letter from earl of macclesfield.] before he completed his observations for nutation another great change in his fortunes took place. in he was elected to succeed halley as astronomer royal. it was halley's dying wish that bradley should succeed him, and it is said that he was even willing to resign in his favour, for his right hand had been attacked by paralysis, and the disease was gradually spreading. but he died without any positive assurance that his wish would be fulfilled. the chief difficulty in securing the appointment of bradley seems to have been that he was the obvious man for the post in universal opinion. "it is not only my friendship for mr. bradley that makes me so ardently wish to see him possessed of the position," wrote the earl of macclesfield to the lord chancellor; "it is my real concern for the honour of the nation with regard to science. for as our credit and reputation have hitherto not been inconsiderable amongst the astronomical part of the world, i should be extremely sorry we should forfeit it all at once by bestowing upon a man of inferior skill and abilities the most honourable, though not the most lucrative, post in the profession (a post so well filled by dr. halley and his predecessor), when at the same time we have amongst us a man known by all the foreign, as well as our own astronomers, not to be inferior to either of them, and one whom sir isaac newton was pleased to call the best astronomer in europe." and again, "as mr. bradley's abilities in astronomical learning are allowed and confessed by all, so his character in every respect is so well established, and so unblemished, that i may defy the worst of his enemies (if so good and worthy a man have any) to make even the lowest or most trifling objection to it." "after all," the letter goes on, "it may be said if mr. bradley's skill is so universally acknowledged, and his character so established, there is little danger of opposition, since no competitor can entertain the least hope of success against him. but, my lord, we live in an age when most men how little soever their merit may be, seem to think themselves fit for whatever they can get, and often meet with some people, who by their recommendations of them appear to entertain the same opinion of them, and it is for this reason that i am so pressing with your lordship not to lose any time." such recommendations had, however, their effect: the dreaded possibility of a miscarriage of justice was averted, and bradley became the third astronomer royal, though he did not resign his professorship at oxford. halley, bradley, and bliss, who were astronomers royal in succession, all held the appointment along with one of the savilian professorships at oxford; but since the death of bliss in , the appointment has always gone to a cambridge man. [sidenote: instruments very defective.] when bradley went to greenwich, in june , he was at first unable to do much from the wretched state in which he found the instruments. halley was not a good observer: his heart was not in the work, and he had not taken the trouble to set the instruments right when they went wrong. the counterpoises of that instrument which ought to have been the best in the world at the time rubbed against the roof so that the telescope could scarcely be moved in some positions: and some of the screws were broken. there was no proper means of illuminating the cross-wires, and so on. with care and patience bradley set all this right, and began observations. he had the good fortune to secure the help of his nephew, john bradley, as assistant, and the companionship seems to have been as happy as that previous one of james bradley and his uncle pound. john bradley was able to carry on the observations when his uncle was absent in oxford, and the work the two got through together in the first year is (in the words of bradley's biographer rigaud) "scarcely to be credited." the transit observations occupy folio pages, and no less than observations were taken on one night. and at the same time, it must be remembered, bradley was still carrying on his nutation observations at wansted, still lecturing at oxford, and not content with all this, began a course of experiments on the length of the seconds' pendulum. truly a giant for hard work! [sidenote: new instruments.] but, in spite of his care in setting them right, the instruments in the observatory were found to be hopelessly defective. the history of the instruments at the royal observatory is a curious one. when flamsteed was appointed the first astronomer royal he was given the magnificent salary of £ a year, and no instruments to observe with. he purchased some instruments with his own money, and at his death they were claimed by his executors. hence halley, the second astronomer royal, found the observatory totally unprovided in this respect. he managed to persuade the nation to furnish the funds for an equipment; but halley, though a man of great ability in other ways, did not know a good instrument from a bad one; so that bradley's first few years at the observatory were wasted owing to the imperfection of the equipment. when this was fully realised he asked for funds to buy new instruments, and such was the confidence felt in him that he got what he asked for without much difficulty. more than £ , a large sum for those days, was spent under his direction, the principal purchases being two quadrants for observation of the position of the stars, one to the north and the other to the south. with these quadrants, which represented the perfection of such apparatus at that time, bradley made that long and wonderful series of observations which is the starting-point of our knowledge of the movements of the stars. the instruments are still in the royal observatory, the more important of the two in its original position as bradley mounted it and left it. [sidenote: work at greenwich.] it seems needless to mention his work as astronomer royal, but i will give quite briefly a summary of what he accomplished, and then recall a particular incident, which shows how far ahead of his generation his genius for observation placed him. the summary may be given as follows. we owe to bradley-- . a better knowledge of the movements of jupiter's satellites. . the orbits of several comets calculated directly from his own observations, when such work was new and difficult. . experiments on the length of the pendulum. . the foundation of our knowledge of the refraction of our atmosphere. . considerable improvements in the tables of the moon, and the promotion of the method for finding the longitude by lunar distances. . the proper equipment of our national observatory with instruments, and the use of these to form the basis of our present knowledge of the positions and motions of the stars. many men would consider any one of these six achievements by itself a sufficient title to fame. bradley accomplished them all in addition to his great discoveries of aberration and nutation. [sidenote: might have found variation of latitude.] and with a little more opportunity he might have added another great discovery which has shed lustre on the work of the last decade. we said earlier in this chapter that the axis of the earth may move in one or two ways. either it may point to a different star, remaining fixed relatively to the earth, as in the nutation which bradley discovered; or it may actually change its position in the earth. this second kind of movement was believed until twenty years ago not to exist appreciably; but the work of küstner and chandler led to the discovery that it did exist, and its complexities have been unravelled, and will be considered in the sixth chapter. now a century and a half ago bradley was on the track of this "variation of latitude." his careful observations actually showed the motion of the pole, as mr. chandler has recently demonstrated; and, moreover, bradley himself noticed that there was something unexplained. once again there was a _residuum_ after (first) aberration and (next) nutation had been extracted from the observations; and with longer life he might have explained this residuum, and added a third great discovery to the previous two. or another coming after him might have found it; but after the giant came men who could not tread in his footsteps, and the world waited years before the discrepancy was explained. [sidenote: oxford's tardy recognition of bradley.] the attitude of our leading universities towards science and scientific men is of sufficient importance to justify another glance at the relations between bradley and oxford. we have seen that oxford's treatment of bradley was not altogether satisfactory. she left him to learn astronomy as he best could, and he owes no teaching to her. she made him professor of astronomy, but gave him no observatory and a beggarly income which he had to supplement by giving lectures on a different subject. but when he had disregarded these discouragements and made a name for himself, oxford took her share in recognition. he was created d.d. by diploma in ; and when his discovery of nutation was announced in , and produced distinctions and honours of all kinds from over the world, we are told that "amidst all these distinctions, wide as the range of modern science, and permanent as its history, there was one which probably came nearer his heart, and was still more gratifying to his feeling than all. lowth (afterwards bishop of london), a popular man, an elegant scholar, and possessed of considerable eloquence, had in to make his last speech in the sheldonian theatre at oxford as professor of poetry. in recording the benefits for which the university was indebted to its benefactors, he mentioned the names of those whom sir henry savile's foundation had established there: 'what men of learning! what mathematicians! we owe to savile, briggs, wallis, halley; to savile we owe greaves, ward, wren, gregory, keill, and one whom i will not name, for posterity will ever have his name on its lips.' bradley was himself present; there was no one in the crowded assembly on whom the allusion was lost, or who did not feel the truth and justice of it; all eyes were turned to him, while the walls rung with shouts of heartfelt affection and admiration; it was like the triumph of themistocles at the olympic games." [sidenote: the study of "residual phenomena."] these words of rigaud indicate the fame deservedly acquired by an earnest and simple-minded devotion to science: but can we learn anything from the study of bradley's work to guide us in further research? the chief lessons would seem to be that if we make a series of careful observations, we shall probably find some deviation from expectation: that we must follow up this clue until we have found some explanation which fits the facts, not being discouraged if we cannot hit upon the explanation at once, since bradley himself was puzzled for several years: that after finding one _vera causa_, and allowing for the effect of it, the observations may show traces of another which must again be patiently hunted, even though we spend nineteen years in the chase: and that again we may have to leave the complete rectification of the observations to posterity. but though we may admit the general helpfulness of these directions, and that this patient dealing with residual phenomena seems to be a method capable of frequent application, we cannot deduce any universal principle of procedure from them: witness the difficulty of dealing with meteorological observations, for instance. it is not always possible to find any orderly arrangement of the residuals which will give us a clue to start with. when such an arrangement is manifested, we must certainly follow up the clue; it would almost seem that no expense should be prohibitive, since it is impossible to foresee the importance of the result. chapter iv accidental discoveries [sidenote: the oxford new star found during work on astrographic chart.] in reviewing various types of astronomical discovery i have laid some stress upon the fact that they are, generally speaking, far from being accidental in character. a new planet does not "swim into our ken," at any rate not usually, but is found only after diligent search, and then only by an investigator of acute vision, or other special qualifications. but this is, of course, not always the case. some discoveries are made by the merest accident, as we have had occasion to remark incidentally in the case of the minor planets; and for the sake of completeness it is desirable to include among our types at least one case of such accidental discovery. as, however, the selection is a little invidious, i may perhaps be pardoned for taking the instance from my own experience, which happens to include a case where one of those remarkable objects called "new stars" walked deliberately into a net spread for totally different objects. there is the further reason for choosing this instance: that it will afford me the opportunity of saying something about the special research in which we were actually engaged, the work of mapping out the heavens by photography, or, as it has been called, the astrographic chart--a great scheme of international co-operation by which it is hoped to leave as a legacy for future centuries a record of the state of the sky in our age. such a record cannot be complete; for however faint the stars included, we know that there are fainter stars which might have been included had we given longer exposures to the plates. nor can it be in other ways final or perfect; however large the scale, for instance, on which the map is made, we can imagine the scale doubled or increased many-fold. but the map will be a great advance on anything that has hitherto been made, and some account of it will therefore no doubt be of interest. [sidenote: origin of the chart.] we may perhaps begin with a brief historical account of the enterprise. photographs of the stars were taken many years ago, but only by a few enthusiasts, and with no serious hope of competing with eye observations of the sky. the old wet-plate photography was, in fact, somewhat unsuited to astronomical purposes; to photograph faint objects a long exposure is necessary, and the wet plate may dry up before the exposure is concluded--nay, even before it is commenced, if the observer has to wait for passing clouds--and therefore it may be said that the successful application of photography to astronomy dates from the time when the dry plate was invented; when it became possible to expose a plate in the telescope for hours, or by accumulation even for days. the dry plate remains sensitive for a long period, and if it is desired to extend an exposure beyond the limits of one night, it is quite easy to close up the telescope and return to the operations again on the next fine night; and so on, if not perhaps indefinitely, at any rate so long as to transcend the limits of human patience up to the present. [illustration: vii.--great comet of nov. th, (_from a photograph taken at the royal observatory, cape of good hope._)] [sidenote: comet of .] [sidenote: stars shown on the pictures.] but to consider our particular project. we may assign, perhaps, the date as that in which it first began to take shape. in that year there was a magnificent bright comet, the last really large comet which we, in the northern hemisphere, have had the good fortune to see. some of us, of course, were not born at that time, and perhaps others who were alive may nevertheless not have seen that comet; for it kept somewhat uncomfortably early morning hours, and i can well remember myself feeling rather more resentment than gratitude to the man who waked me up about four o'clock to see it. many observations were of course made of this interesting visitor, and what specially concerns us is that at the cape of good hope some enterprising photographers tried to photograph it. they tried in the first instance with ordinary cameras, and soon found--what any astronomer could have told them--that the movement of the earth, causing an apparent movement of the comet and the stars in the opposite direction, frustrated their efforts. the difficulties of obtaining pictures of moving objects are familiar to all photographers. a "snap-shot" might have met the difficulty, but the comet was scarcely bright enough to affect the plate with a short exposure. ultimately dr. david gill, the astronomer at the cape observatory, invited one of the photographers to strap his camera to one of the telescopes at the observatory, a telescope which could be carried round by clockwork in the usual way, so as to counteract the earth's motion, and in effect to keep the comet steadily in view, as though it were at rest. as a consequence, some very beautiful and successful pictures of the comet were obtained, and on them a large number of stars were also shown. they were, as i have said, not by any means the first pictures of stars obtained by photography, but they represented in facility and in success so great an advance upon what had been formerly obtained that they attracted considerable attention. they were sent to europe and stimulated various workers to further experiments. [sidenote: the brothers henry begin work.] [sidenote: conference of .] the late dr. common in england, an amateur astronomer, began that magnificent pioneer work in astronomical photography which soon brought him the gold medal of the royal astronomical society for his photographs of nebulæ. but the most important result for our purpose was produced in france. there had been started many years before by the french astronomer chacornac a series of star maps round the zodiac similar in intention to the berlin maps which figured in the history of the discovery of neptune. chacornac died before his enterprise was very far advanced, and the work was taken up by two brothers, paul and prosper henry, who followed chacornac in adopting for the work the laborious method of eye observation of each individual star. they proceeded patiently with the work on these lines; but when they came to the region where the zodiac is crossed by the milky way, and the number of stars in a given area increases enormously, they found the labour so great as to be practically prohibitive, and were in doubt how to deal with the difficulty. it was at this critical moment that these comet photographs, showing the stars so beautifully, suggested the alternative of mapping the stars photographically. they immediately set to work with a trial lens, and obtained such encouraging results that they proceeded themselves to make a larger lens of the same type; this again was satisfactory, and the idea naturally arose of extending to the whole heavens the scheme which they had hitherto intended only for the zodiac, a mere belt of the heavens. but this rendered the enterprise too large for a single observatory. it became necessary to obtain the co-operation of other observatories, and with this end in view an international conference was summoned to meet in paris in to consider the whole project. there were delegates from, if not all nations, at any rate a considerable number:-- france british empire germany russia holland u.s. america austria sweden denmark belgium italy spain switzerland portugal brazil argentine republic [sidenote: choice of instrument.] [sidenote: expense of "doublet."] [sidenote: advantages of reflector.] [sidenote: refractor chosen.] the conference had a number of very important questions to discuss, for knowledge of the photographic method and its possibilities was at that time in its infancy. there was, for instance, the question whether all the instruments need be of the same pattern, and if so what that pattern should be. the first of these questions was settled in the affirmative, as we might expect; in the interests of uniformity it was desirable that the maps should be as nearly similar as possible. the second question was not so easy; there were at least three different types of instruments which might be used. first of all, there was the photographic lens, such as is familiar to all who have used an ordinary camera, consisting of two lenses with a space between; though since each of these lenses is itself made up of two, we should more correctly say four lenses in all. it was with a lens of this kind that the comet pictures had been taken at the cape of good hope, and it might seem the safest plan to adopt what had been shown to be capable of such good work. but there was this difficulty; the pictures of the comet were on a very small scale, and taken with a small lens; a much larger lens was required for the scheme now under contemplation, and when there are four separate lenses to be made, each with two surfaces to polish, and each requiring a perfectly sound clear piece of glass, it will be obvious that the difficulties of making a large compound lens of this kind are much greater, and the expense much more serious than in the case of a single lens, or even a pair. it was this question of expense which had led the brothers henry to experiment with a different kind of instrument, in which only one pair of lenses was used instead of two. their instrument was, in fact, very similar to the ordinary telescope, excepting that they were bound to make their lenses somewhat different in shape in order to bring to focus the rays of light suitable for photography, which are not the same as those suitable for eye observation with the ordinary telescope. dr. common, again, had used a third kind of instrument, mainly with the view of reducing the necessary expense still further, or, perhaps, of increasing the size of the instrument for the same expense. his telescope had no lens at all, but a curved mirror instead, the mirror being made of glass silvered on the face (not on the back as in the ordinary looking-glass). in this case there is only one surface to polish instead of four, as in the henrys' telescope, or eight, as in the case of the photographic doublet; and, moreover, since the rays of light are reflected from the surface of the glass, and do not pass _through_ it at all, the internal structure of the glass is not so strictly important as in the other cases. hence the reflector is a very cheap instrument, and it is, moreover, quite free from some difficulties attached to the other instruments. no correction for rays of light of different colours is required, since all rays of whatever colour come to the same focus automatically. these advantages of the reflector were so considerable as to almost outweigh one well-known disadvantage, which is, however, not very easily expressed in words. the reflector might be described as an instrument with a temper; sometimes it gives excellent results, but at others _something_ seems to be wrong, though the worried observer does not exactly know what. long experience and patience are requisite to humour the instrument and get the best results from it, and it was felt that this uncertainty was sufficient to disqualify the instrument for the serious piece of routine work contemplated in mapping the heavens. accordingly the handier and more amiable instrument with which the brothers henry had done such good work was selected as the pattern to be adopted. [sidenote: doublet would have been better.] it is curious that at the conference of nothing at all was said about the type of instrument first mentioned (the "doublet lens"), although a letter was written in its favour by professor pickering of harvard college observatory. since that time we have learnt much of its advantages, and it is probable that if the conference were to meet now they might arrive at a different decision; but at that time they were, to put it briefly, somewhat afraid of an instrument which seemed to promise, if anything, too well, especially in one respect. with the reflector and the refractor it had been found that the field of good images was strictly limited. the henrys' telescope would not photograph an area of the sky greater in extent than ° in diameter at any one time, and the reflector was more limited still; within this area the images of the stars were good, and it had been found that their places were accurately represented. now the "doublet" seemed to be able to show much larger areas than this with accuracy, but no one had been able to test the accuracy to see whether it was sufficient for astronomical purposes; and although no such feeling was openly expressed or is on record, i think there is no doubt that a feeling existed of general mistrust of an instrument which seemed to offer such specious promises. whatever the reason, its claims were passed over in silence at the conference, and the safer line (as it was then thought) of adopting as the type the henrys' instrument, was taken. [sidenote: the eighteen observatories.] this was perhaps the most important question settled at the conference, and the answers to many of the others naturally followed. the size of the plates, for instance, was settled automatically. the question down to what degree of faintness should stars be included, resolved itself into the equivalent question, what should be the length of time during which the plates were exposed? then, again, the question, what observatories should take part in the work? became simply this: what observatories could afford to acquire the instruments of this new pattern and get other funds for carrying out the work specified? it was ultimately found that eighteen observatories were able to obtain the apparatus and funds, though unfortunately three of the eighteen have since found it impossible to proceed. the following is the original list, and in brackets are added the names of three other observatories which in undertook to fill the places of the defaulters. observatories co-operating for the astrographic chart. +----------------------+------------+----------+ | observatory. | zones of | number | | |declination.|of plates.| +----------------------+------------+----------+ |greenwich |+ ° to + °| | |rome |+ ° " + °| | |catania |+ ° " + °| | |helsingfors |+ ° " + °| | |potsdam |+ ° " + °| | |oxford |+ ° " + °| | |paris |+ ° " + °| | |bordeaux |+ ° " + °| | |toulouse |+ ° " + °| | |algiers |+ ° " - °| | |san fernando |- ° " - °| | |tacubaya |- ° " - °| | |santiago (monte video)|- ° " - °| | |la plata (cordoba) |- ° " - °| | |rio (perth, australia)|- ° " - °| | |cape of good hope |- ° " - °| | |sydney |- ° " - °| | |melbourne |- ° " - °| | +----------------------+------------+----------+ [sidenote: sky covered twice.] in the list is also shown the total number of plates that were to be taken by each observatory. when once the size of the plates had been settled, it was a straightforward matter to divide up the sky into the proper number of regions necessary to cover it completely, not only without gaps between the plates, but with actually a small overlap of contiguous plates. and more than this, it was decided that the whole sky should be completely covered _twice over_. it was conceivable that a question might arise whether an apparent star image on a plate was, on the one hand, a dust speck, or, on the other hand, a planet, or perhaps a variable or new star. by taking two different plates at slightly different times, questions of this kind could be settled; and to make the check more independent it was decided that the plates should not be exactly repeated on the same portion of sky, but that in the second series the centre of a plate should occupy the point assigned to the corner of a plate in the first series. [sidenote: times of exposure.] then there came the important question of time of exposure, which involved a long debate between those who desired the most modest programme possible consistent with efficiency, and those enthusiasts who were anxious to strain the programme to the utmost limits attainable. ultimately it was resolved to take two series of plates; one series of long exposure which was set in the first instance at minutes, then became , then , then , and has by some enterprising observers been extended to - / hours; the other a series of short exposures which have been generally fixed at minutes. thus instead of covering the sky twice, it was decided to cover it in all four times, and the number of plates assigned to each observatory in the above list must be regarded as doubled by this new decision. and further still, on the series of short-exposure plates it was decided to add to the exposure of six minutes another one of three minutes, having slightly shifted the telescope between the two so that they should not be superimposed; and later still, a third exposure of twenty seconds was added to these. it would take too long to explain here the reasons for these details, but it will be clear that the general result of the discussion was to extend the original programme considerably, and render the work even more laborious than it had appeared at the outset. [sidenote: measurement of plates.] [sidenote: the réseau.] [sidenote: the microscope.] [sidenote: reversal of plates.] [sidenote: personal equation.] when all these plates have been taken, the work is by no means finished; indeed, it is only just commencing. there remains the task of measuring accurately on each of the short-exposure plates the positions of the stars which it represents, numbering on the average some or ; so that for instance at oxford the total number of stars measured on the twelve hundred plates is nearly half a million. these are not all separate stars; for the sky is represented twice over, and there is also the slight overlap of contiguous plates; but the number of actual separate stars measured at this one observatory is not far short of a quarter of a million, and it has taken nearly ten years to make the measurements, with the help of three or four measurers trained for the purpose. to render the measures easy, a network or réseau of cross lines is photographed on each plate by artificial light after it has been exposed to the stars, so that on development these cross lines and the stars both appear. we can see at a glance the approximate position of a star by counting the number of the space from left to right and from top to bottom in which it occurs; and we can also estimate the fraction of a space in addition to the whole number; but it is necessary for astronomical purposes to estimate this fraction with the greatest exactness. the whole numbers are already given with great exactness by the careful ruling of the cross lines, which can be spaced with extraordinary perfection. to measure the fraction, we place the plate under a microscope in the eye-piece of which there is a finally divided cross scale; the centre of the cross is placed over a star image, and then it is noted where the lines of the réseau cut the cross scale. in this way the position of the image of a star is read off with accuracy, and after a little practice with considerable rapidity. it has been found at oxford that under favourable conditions the places of nearly stars per hour can be recorded in this way by a single measurer, if he has some one to write down for him the numbers he calls out. this is only one form of measuring apparatus; there are others in which, instead of a scale in the eye-piece, micrometer screws are used to measure the fractions; but the general principle in all these instruments is much the same, and the rate of work is not very different; while to the minor advantages and disadvantages of the different types there seems no need here to refer. one particular point, however, is worth noting. after a plate has been measured, it is turned round completely, so that left is now right, and top is now bottom, and the measurements are repeated. this repetition has the advantage first of all of checking any mistakes. when a long piece of measuring or numerical work of any kind is undertaken there are invariably moments when the attention seems to wander, and some small error is the result. but there are also certain errors of a systematic character similar to those denoted by the term "personal equation," which has found its way into other walks of life. in the operation of placing a cross exactly over the image of a star, different observers would show slight differences of habit; one might place it a little more to the right than another. but when the plate is turned round the effect of this habit on the measure is exactly reversed, and hence if we take the mean of the two measures any personal habit of this kind is eliminated. it has been found by experience that such personal habits are much smaller for measures of this kind than for those to which we have long been accustomed in observations made by eye on the stars themselves. the troubles from "personal equation" have been much diminished by the photographic method, and certain peculiarities of the former method have been clearly exhibited by the comparison. for instance, it has gradually become clear that with eye observations personal equation is not a constant quantity, but is different for stars of different brightness. when observing the transit of a bright star the observer apparently records an instant definitely earlier than in recording the transit of a faint one; and this peculiarity seems to be common to the large majority of observers, which is perhaps the reason why it was not noticed earlier. but when positions of the stars determined in this way are compared with their positions measured on the photographic plates, the peculiarity is made clearly manifest. for example, at oxford, our first business after making measurements is to compare them with visual observations on a limited number of the brighter stars made at cambridge about twenty years ago. (about , stars were observed at cambridge, and we are dealing with ten times that number.) the comparison shows that the cambridge observations are affected with the following systematic errors:-- if stars of magnitude are observed correctly, then " " " . secs. too early " " " . " " " " . " " " " . " " " " . " [sidenote: main object of the work.] this may serve as an illustration of various incidental results which are already flowing from the enormous and laborious piece of work which, as far as the university observatory at oxford is concerned, we have just completed, though some of the other colleagues are not so far advanced. but the main results will not appear just yet. the work must be repeated, and the positions of the stars just obtained must be compared with those which they will be found to occupy at some future date, in order to see what kind of changes are going on in the heavens. whether this future date shall be one hundred years hence, or fifty, or ten, or whether we should begin immediately to repeat what has been done, is a matter not yet decided, and one which requires some little consideration. [sidenote: the concluding year.] i have said perhaps enough to give you a general idea of the work on which we have been engaged at oxford for the last ten years. ten years ago it seemed to stretch out in front of us rather hopelessly; the pace we were able to make seemed so slow in view of the distance to be covered. we felt rather like the schoolboy who has just returned to school and sees the next holidays as a very remote prospect, and we solaced ourselves much in the same way as he does, by making a diagram representing the total number of plates to be dealt with and crossing off each one as it was finished, just as he sometimes crosses off the days still remaining between him and the prospective holidays. it was pleasant to watch the growth of the number of crosses on this diagram, and by the end of the year we had the satisfaction of seeing very little blank space remaining. now, up to this point it had not much mattered whether any particular plate was secured in any particular year, or in a subsequent year, so long as there were always sufficient plates to keep us occupied in measuring them. but it then became a matter of importance to secure each plate at the proper time of year; for the sun, as we know, travels round the zodiac among the stars, obliterating by his radiance a large section of the sky for a period of some months, and in this way a particular region of the heavens is apt to "run into daylight," as the observatory phrase goes, and ceases to be available for photography during several months, until the sun is again far enough away to allow of the particular region being seen at night. [sidenote: a disappointment.] [sidenote: a curious plate.] [sidenote: a strange object.] [sidenote: a new star?] roughly speaking then, if a plate which should be taken in february is not secured in this month owing to bad weather, the proper time for taking it will not occur again until the following february; and when there was a fair prospect of finishing our work in , it became important to secure each plate at the proper time in that year. hence we were making special efforts to utilise to the full any fine night that providence sent in our way, and on such occasions it is clearly an economy, if not exactly to "make hay while the sun shines," at any rate to take plates vigorously while the sun is _not_ shining and the night is fine; leaving the development of them until the daytime. there is, of course, the risk that the whole night's work may in this way be lost owing to some fault in the plates, which might have been detected if some of them were immediately developed. perhaps in the early days of our work it would have been reckless or foolish to neglect this little precaution; but we had for years been accustomed to rely upon the excellence of the plates without finding our trust betrayed; and the sensitiveness of the plates had increased rather than diminished as time went on. hence it will be readily understood that when one fatal morning we developed a series of some thirty plates, and found that owing to some unexplained lack of sensitiveness they were all unsuitable for our purpose, it came as a most unwelcome and startling surprise. it was, of course, necessary to make certain that there was no oversight, that the developer was not at fault, and that the weather had not been treacherous. all such possibilities were carefully considered before communication with the makers of the plates, but it ultimately became clear that there had been some unfortunate failure in sensitiveness, and that it would be necessary to repeat the work with opportunities restricted by the intervening lapse of time. however, disappointments from this or similar causes are not unknown in astronomical work; and we set about this repetition with as little loss of time and cheerfulness as was possible. under the circumstances, however, it seemed desirable to examine carefully whether anything could be saved from the wreck--whether any of the plates could be admitted as _just_ coming up to the minimum requirements. and i devoted a morning to this inquiry. in the course of it i came across one plate which certainly seemed worth an inclusion among our series from the point of view of the number of stars shown upon it. it seemed quite rich in stars, perhaps even a little richer than might have been expected. on inquiry i was told that this was not one of the originally condemned plates, but one which had been taken since the failure in sensitiveness of the plates had been detected; was from a new and specially sensitive batch with which the courteous makers had supplied us; but though there were certainly a sufficient number of stars upon the plate, owing to some unexplained cause the telescope had been erroneously pointed, and the region taken did not correspond to the region required. to investigate the cause of the discrepancy i thereupon took down from our store of plates the other one of the same region which had been rejected for insufficiency of stars, and on comparing the two it was at once evident that there was a strange object on the plate taken later of the two, a bright star or other heavenly body, which was not on the former plate. i have explained that by repeating the exposure more than once, it is easily possible to recognise whether a mark upon the plate is really a celestial body or is an accidental blot or dust speck, and there was no doubt that this was the image of some strange celestial body. it might, of course, be a new planet, or even an old one which had wandered into the region; but a few measures soon showed that it was not in movement. the measures consisted in comparing the separation of the three exposures with the separation of the corresponding exposures of obvious stars, for the exposures were not, of course, simultaneous, and if the body were a planet and had moved in the interval between them, this would be made manifest on measuring the separations. no such movements could be detected; and the possibilities were thus restricted to two. so far as we knew the object was a star, but might be either a star of the class known as _variable_ or of that known as _new_. in the former case it would become bright and faint at more or less regular intervals, and might possibly have been already catalogued; for the number of these bodies already known amounts to some hundreds. search being made in the catalogues, no entry of it was found, though it still might be one of this class which had hitherto escaped detection. or it might be a "new star," one of those curious bodies which blaze up quite suddenly to brightness and then die away gradually until they become practically invisible. the most famous perhaps of these is the star which appeared in , and was so carefully observed by tycho brahé; but such apparitions are rare, and altogether we have not records as yet of a score altogether; so that in this latter case the discovery would be of much greater interest than in the former. in either event it was desirable to inform other observers as soon as possible of the existence of a strange body; already some time had elapsed since the plate had been taken, march th, for the examination of which i have spoken was not made until march th. accordingly, a telegram was at once despatched to the central office at kiel, which undertakes to distribute such information all over the world, and a few post-cards were sent to observers close at hand who might be able to observe the star the same night. certain observations with the spectroscope soon made it clear that the object was really a "new star." [sidenote: the discovery accidental.] [sidenote: mrs. fleming's discoveries.] this, therefore, is the discovery which we made at oxford: as you will see, in an entirely accidental manner, during the course of a piece of work in which it was certainly never contemplated. its purely accidental nature is sufficiently illustrated by the fact that if the plates originally supplied by the makers had been of the proper quality, the plate which led to the discovery would never have been taken. if the plates exposed in february had been satisfactory, we should have been content, and should not have repeated the exposure on march th. again i can testify personally how purely accidental it was that the examination was made on march th to see whether anything could be saved, as i have said, from the wreck. the idea came casually into my mind as i was walking through the room and saw the neat pile of rejected plates; and one may fairly call it an accidental impulse. this new star is not, however, the first of such objects to have been discovered "accidentally"; many of the others were found just as much by chance, though a notable exception must be made of those discovered at the harvard observatory, which are the result of a deliberate search for such bodies by the careful examination of photographic plates. mrs. fleming, who spends her life in such work, has had the good fortune to detect no less than six of these wonderful objects as the reward of her laborious scrutiny; and she is the _only_ person who has thus found new stars by photography until this accidental discovery at oxford. the following is a complete list of new stars discovered to date:-- list of new stars. +----------------------------------------------+ |ref. no.| constellation. | year.| discoverer. | +----------------------------------------------+ | | cassiopeia | | tycho brahé.| | | cygnus | | janson. | | | ophiuchus | | kepler. | | | vulpecula | | anthelm. | | | ophiuchus | | hind. | | | scorpio | | auwers. | | | corona borealis| | birmingham. | | | cygnus | | schmidt. | | | andromeda | | hartwig. | | | perseus | | fleming. | | | auriga | | anderson. | | | norma | | fleming. | | | carina | | fleming. | | | centaurus | | fleming. | | | sagittarius | | fleming. | | | aquila | | fleming. | | | perseus | | anderson. | | | gemini | | at oxford. | +----------------------------------------------+ [illustration: march , march , viii.--the oxford new star. a pair of photographs taken at the harvard college observatory before and after its appearance (_the arrow indicates the place of the new star. it will be seen that the left-hand picture though it shews fainter stars than the other, has not a trace of the new star._)] [sidenote: dr. anderson.] [sidenote: nova persei.] generally these stars have been noted by eye observation, as in the case of the two found by dr. anderson of edinburgh. in these cases also we may say that deliberate search was rewarded; for dr. anderson is probably the most assiduous "watcher of the skies" living, though he seldom uses a telescope; sometimes he uses an opera-glass, but usually the naked eye. he describes himself as an "astrophil" rather than as an astronomer. "i love the stars," he says; "and whenever they are shining, i must be looking." and so on every fine night he stands or sits at his open study window gazing at the heavens. i believe he was just about to leave them for his bed, near a.m. on the night of february , , when, throwing a last glance upward, he suddenly saw a brilliant star in the constellation perseus. his first feeling was actually one of disappointment, for he felt sure that this object must have been there for some time past without his knowing of it, and he grudged the time lost when he might have been regarding it. more in a spirit of complaint than of inquiry, he made his way to the royal observatory at edinburgh next day to hear what they had to say about it, though he found it difficult to approach the subject. he first talked about the weather, and the crops, and similar topics of general interest; and only after some time dared he venture a casual reference to the "new portent in the heavens." seeing his interlocutor look somewhat blank, he ventured a little farther, and made a direct reference to the new star in perseus; and then found to his astonishment, as also to his great delight, that he was the first to bring news of it. the news was soon communicated to other observers; all the telescopes of the world were soon trained upon it; and this wonderful "new star of the new century" has taught us more of the nature of these extraordinary bodies than all we knew before. [sidenote: records previous to discovery.] [sidenote: was nova geminorum previously shining faintly?] [sidenote: the suspicion negatived.] perhaps i may add a few remarks on one or two features of these bodies. firstly, let us note that professor pickering of harvard is now able to make a most important contribution to the _former_ history of these objects--that is to say, their history preceding their actual detection. we remember that, after uranus had been discovered, it was found that several observers had long before recorded its place unknowingly; and similarly professor pickering and his staff have usually photographed other new objects unknowingly. there are on the shelves at harvard vast stores of photographs, so many that they are unable to examine them when they have been taken; but once any object of interest has been discovered, it is easy to turn over the store and examine the particular plates which may possibly show it at an earlier date. in this way it was found that dr. anderson's new star had been visible only for a few days before its discovery, there being no trace of it on earlier plates. similarly, in the case of the new star found at oxford, plates taken on march st and th, fifteen days and ten days respectively before the discovery-plate of march th, showed the star. but, in this particular instance, greater interest attaches to two still earlier plates taken elsewhere, and with exposures much longer than any available at harvard. one had been obtained at heidelberg by dr. max wolf, and another at the yerkes observatory of chicago university, by mr. parkhurst; and on both there appeared to be a faint star of about the fourteenth or fifteenth magnitude, in the place subsequently occupied by the nova; and the question naturally arose, was this the object which ultimately blazed up and became the new star? to settle this point, it was necessary to measure its position, with reference to neighbouring stars, with extreme precision; and here it was unfortunate that the photographs did not help us as much as they might, for they were scarcely capable of being measured with the requisite precision. the point was an important one, because if the identity of the nova with this faint star could be established, it would be the second instance of the kind; but so far as they went, measurements of the photographs were distinctly against the identity. such was the conclusion of mr. parkhurst from his photograph alone; and it was confirmed by measures made at oxford on copies of both plates, which were kindly sent there for the purpose. the conclusion seemed to be that there was a faint star _very near_, but _not at_, the place of the new star; and it was therefore probable that, although this faint star was temporarily invisible from the brightness of the adjacent nova, as the latter became fainter (in the way with which we have become familiar in the case of new stars), it might be possible to see the two stars alongside each other. this critical observation was ultimately made by the sharp eyes of professor barnard, aided by the giant telescope of the yerkes observatory; and it seems clear therefore that the object which blazed up to become the nova of could not have previously been so bright as a faint star of the fourteenth magnitude. although this is merely a negative conclusion, it is an important one in the history of these bodies. [sidenote: nebula round nova persei.] [sidenote: its changes.] [sidenote: due to travelling illumination.] the second point to which i will draw your attention is from the history of the other nova just mentioned--dr. anderson's new star of . in this instance it is not the history previous to discovery, but what followed many months after discovery, that was of engrossing interest; and again yerkes observatory, with its magnificent equipment, played an important part in the drama. when, with its giant reflecting telescope, photographs were taken of the region of nova persei after it had become comparatively faint, it was found that there was an extraordinarily faint nebulosity surrounding the star. repeating the photographs at intervals, it was found that this nebulosity was rapidly changing in shape. "rapidly" is, of course, a relative term, and a casual inspection of two of the photographs might not convey any impression of rapidity; it is only when we come to consider the enormous distance at which the movements, or apparent movements, of the nebulæ must be taking place that it becomes clear how rapid the changes must be. it was not possible to determine this distance with any exactness, but limits to it could be set, and it seemed probable that the velocity of the movement was comparable with that of light. the conclusion suggested itself that the velocity might actually be identical with that of light, in which case what we saw was not the movement of actual matter, but merely that of illumination, travelling from point to point of matter already existing. [illustration: sept. , nov. , ix--nebulosity round nova persei (_from photographs taken at the yerkes observatory by g. w. ritchey._)] [sidenote: when did it all happen?] an analogy from the familiar case of sound may make clearer what is meant. if a loud noise is made in a large hall, we hear echoes from the walls. the sound travels with a velocity of about feet per second, reaches the walls, is reflected back from them, and returns to us with the same velocity. from the interval occupied in going and returning we could calculate the distance of the walls. the velocity of light is so enormous compared with that of sound that we are usually quite unable to observe any similar phenomenon in the case of light. if we strike a match in the largest hall, all parts of it are illuminated so immediately that we cannot possibly realise that there was really an interval between the striking of the match, the travelling of the light to the walls, and its return to our eyes. the scale of our terrestrial phenomenon is far too small to render this interval perceptible. but those who accept the theory above mentioned regarding the appearances round nova persei (although there are some who discredit it) believe that we have in this case an illustration of just this phenomenon of light echoes, on a scale large enough to be easily visible. they think that, surrounding the central star which blazed up so brightly in february , there was a vast dark nebula, of which we had no previous knowledge, because it was not shining with any light of its own. when the star blazed up, the illumination travelled from point to point of this dark nebula and lighted it up; but the size of the nebula was so vast that, although the light was travelling with the enormous velocity of , miles per second, it was not until months afterwards that it reached different portions of this nebula; and we accordingly got news of the existence of this nebula some months after the news reached us of the central conflagration, whatever it was. remark that all we can say is that the news of the nebula reached us _some months later_ than that of the outburst. the actual date when either of the actual things happened, we have as yet no means of knowing; it may have been hundreds or even thousands of years ago that the conflagration actually occurred of which we got news in february , the light having taken all that time to reach us from that distant part of space; and the light reflected from the nebula was following it on its way to us all these years at that same interval of a few months. [sidenote: an objection.] now, let me refer before leaving this point to the chief objection which has been urged against this theory. it has been maintained that the illumination would necessarily appear to travel outwards from the centre with an approach to uniformity, whereas the observed rate of travel is not uniform, and has been even towards the centre instead of away from it; which would seem as though portions of the nebula more distant from the centre were lighted up sooner than those closer to it. by a simple illustration from our solar system, we shall see that these curious anomalies may easily be explained. let us consider for simplicity two planets only, say the earth and saturn. we know that saturn travels round the sun in an orbit which is ten times larger than the orbit of the earth. suppose now that the sun were suddenly to be extinguished; light takes about eight minutes to travel from the sun to the earth, and consequently we should not get news of the extinction for some eight minutes; the sun would appear to us to still go on shining for eight minutes after he had really been extinguished. saturn being about ten times as far away from the sun, the news would take eighty minutes to reach saturn; and from the earth we should see saturn shining more[ ] than eighty minutes after the sun had been extinguished, although we ourselves should have lost the sun's light after eight minutes. i think we already begin to see possibilities of curious anomalies; but they can be made clearer than this. instead of imagining an observer on the earth, let us suppose him removed to a great distance away in the plane of the two orbits; and let us suppose that the sun is now lighted up again as suddenly as the new star blazed up in february . then such an observer would first see this blaze in the centre; eight minutes afterwards the illumination would reach the earth, a little speck of light near the sun would be illuminated, just as we saw a portion of the dark nebula round nova persei illuminated; eighty minutes later another speck, namely, saturn, would begin to shine. but now, would saturn necessarily appear to the distant observer to be farther away from the sun than the earth was? looking at the diagram, we can see that if saturn were at s{ } then it would present this natural appearance of being farther away from the sun than the earth; but it might be at s{ } or s{ }, in which case it would seem to be nearer the sun, and the illumination would seem to travel inwards towards the central body instead of outwards. without considering other cases in detail, it will be tolerably clear that almost any anomalous appearance might be explained by choosing a suitable arrangement of the nebulous matter which we suppose lighted up by the explosion of nova persei. another objection urged against the theory i have sketched is that the light reflected from such a nebula would be so feeble that it would not affect our photographic plates. this depends upon various assumptions which we have no time to notice here; but i think we may say that there is certainly room for the acceptance of the theory. [illustration: fig. .] [sidenote: did the nebula cause the outburst?] now, if this dark nebula was previously existing in this way all round the star which blazed up, the question naturally arises whether the nebula had anything to do with the conflagration. was there previously a star, either so cold or so distant as not to be shining with appreciable light, which, travelling through space, encountered this vast nebula, and by the friction of the encounter was suddenly rendered so luminous as to outshine a star of the first magnitude? the case of meteoric stones striking our own atmosphere seems to suggest such a possibility. these little stones are previously quite cold and invisible, and are travelling in some way through space, many of them probably circling round our sun. if they happen in their journey to encounter our earth, even the extremely tenuous atmosphere, so thin that it will scarcely bend the rays of light appreciably, even this is sufficient by its friction to raise the stones to a white heat, so that they blaze up into the falling stars with which we are familiar. this analogy is suggested, but we must be cautious in accepting it; for we know so very little of the nature of nebulæ such as that of which we have been speaking. but in any case, a totally new series of phenomena have been laid open to our study by those wonderful photographs taken at the yerkes observatory and the lick observatory in the few years which the present century has as yet run. [sidenote: importance of new stars] one thing is quite certain: we must lose no opportunity of studying such stars as may appear, and no diligence spent in discovering them at the earliest possible moment is thrown away. we have only known up to the present, as already stated, less than a score of them, and of these many have told us but little; partly because they were only discovered too late (after they had become faint), and partly because the earlier ones could not be observed with the spectroscope, which had not then been invented. it seems clear that in the future we must not allow accident to play so large a part in the discovery of these objects; more must be done in the way of deliberate search. although we know beforehand that this will involve a vast amount of apparently useless labour, that months and years must be spent in comparing photographic plates, or portions of the sky itself, with one another without detecting anything remarkable, it will not be the first time that years have been cheerfully spent in such searches without result. we need only recall hencke's fifteen years of fruitless search, before finding a minor planet, to realise this fact. [sidenote: superposition of plates.] [sidenote: the stereo-comparator.] one thing of importance may be done; we may improve our methods of making the search, so as to economise labour, and several successful attempts have already been made in this direction. the simplest plan is to superpose two photographs taken at different dates, so that the stars on one lie very close to those on the other; then if an image is seen to be unpaired we _may_ have found a new star, though of course the object may be merely a planet or a variable. the superposition of the plates may be either actual or virtual. a beautiful instrument has been devised on the principle of the stereoscope for examining two plates placed side by side, one with each eye. we know that in this way two photographs of the same object from different points of view will appear to coalesce, and at the same time to give an appearance of solidity to the object or landscape, portions of which will seem to stand out in front of the background. applying this principle to two photographs of stars, what happens is this: if the stars have all remained in the same positions exactly, the two pictures will seem to us to coalesce, and the images all to lie on a flat background; but if in the interval between the exposures of the two plates one of the stars has appreciably moved or disappeared, it will seem, when looked at with this instrument, to stand out in front of this background, and is accordingly detected with comparatively little trouble. this new instrument, to which the name stereo-comparator has been given, promises to be of immense value in dredging the sky for strange bodies in the future. i am glad to say that a generous friend has kindly presented the university observatory at oxford with one of these beautiful instruments, which have been constructed by messrs. zeiss of jena after the skilful designs of dr. pulfrich. whether we shall be able to repeat by deliberate search the success which mere accident threw in our way remains to be seen. chapter v schwabe and the sun-spot period [sidenote: discoveries contrary to expectation.] in preceding chapters we have reviewed discoveries, some of which have been made as a result of a deliberate search, and others accidentally in the course of work directed to a totally different end; but so far we have not considered a case in which the discoverer entered upon an enterprise from which he was positively dissuaded. [sidenote: nothing expected from spots.] in the next chapter we shall come across a very striking instance of this type; but even in the discovery that there was a periodicity in the solar spots, with which i propose to deal now, herr schwabe began his work in the face of deterrent opinions from eminent men. his definite announcement was first made in , though he had himself been convinced some years earlier. in the royal astronomical society awarded him their gold medal for the discovery; and in the address delivered on the occasion the president commenced by drawing attention to this very fact, that astronomers who had expressed any opinions on the subject had been uniformly and decidedly against the likelihood of there being anything profitable in the study of the solar spots. i will quote the exact words of the president, mr. manuel johnson, then radcliffe observer at oxford. "it was in that heinrich schwabe, a gentleman resident in dessau, entered upon those researches which are now to engage our attention. i am not aware of the motive that induced him--whether any particular views had suggested themselves to his own mind--or whether it was a general desire of investigating, more thoroughly than his predecessors had done, the laws of a remarkable phenomenon, which it had long been the fashion to neglect. he could hardly have anticipated the kind of result at which he has arrived; at the same time we cannot imagine a course of proceeding better calculated for its detection, even if his mind had been prepared for it, than that which he has pursued from the very commencement of his career. assuredly if he entertained such an idea, it was not borrowed from the authorities of the last century, to whom the solar spots were objects of more attention than they have been of late years. "'nulla constanti temporum lege apparent aut evanescunt,' says keill in .--_introduct. ad physic. astronom._, p. . "'il est manifest par ce que nous venons de rapporter qu'il n'y a point de règle certaine de leur formation, ni de leur nombre et de leur figure,' says cassini ii. in .--_elém d'astron._, vol. i. p. . "'il semble qu'elles ne suivent aucune loi dans leur apparitions,' says le monnier in .--_instit. astron._, p. . "'solar spots observe no regularity in their shape, magnitude, number, or in the time of their appearance or continuance,' says long in .--_astron._, vol. ii. p. . "'les apparitions des tâches du soleil n'ont rien de regulier,' says lalande in .--_astron._, vol. iii. § , nd edit. "and delambre's opinion may be inferred from a well-known passage in the third volume of his 'astronomy' (p. ), published in , where treating of the solar spots he says, 'il est vrai qu'elles sont plus curieuses que vraiment utiles.'"[ ] it will thus be evident that herr schwabe had the courage to enter upon a line of investigation which others had practically condemned as likely to lead nowhere, and that his discovery was quite contrary to expectation. it is a lesson to us that not even the most unlikely line of work is to be despised; for the outcome of schwabe's work was the first step in the whole series of discoveries which have gradually built up the modern science of solar physics, which occupies so deservedly large a part of the energies of, for instance, the great observatory attached to the university of chicago. [sidenote: schwabe's announcement.] it has been our practice to recall the actual words in which the discoverer himself stated his discovery, and i will give the original modest announcement of schwabe, though for convenience of those who do not read german i will attempt a rough translation. he had communicated year by year the results of his daily counting of the solar spots to the _astronomische nachrichten_, and after he had given ten years' results in this way he collected them together, but he made no remark on the curious sequence which they undoubtedly showed at that time. waiting patiently six years for further material, in he ventured to make his definite announcement as follows:--"from my earlier observations, which i have communicated annually to this journal, there was manifest already a certain periodicity of sun-spots; and the probability of this being really the case is confirmed by this year's results. although i gave in volume the total numbers of groups for the years - , nevertheless i will repeat here a complete series of all my observations of sun-spots, giving not only the number of groups, but also the number of days of observation, and further the days when the sun was free from spots. the number of groups alone will not in itself give sufficient accuracy for determination of a period, since i have convinced myself that when there are a large number of sun-spots the number will be reckoned somewhat too small, and when few sun-spots, the number somewhat too large; in the first case several groups are often counted together in one, and in the second it is easy to divide a group made up of two component parts into two separate groups. this must be my excuse for repeating the early catalogue, as follows:-- +---------------------------------------------+ | year.| number of | days free | days of | | | groups. | from spots.| observation.| |---------------------------------------------| | | | | | | | | | | | | | | | | | | | | | | | | | |---------------------------------------------| | | | | | | | | | | | | | | | | | | | | | | | | | |---------------------------------------------| | | | | | | | | | | | | | | | | | | | | | | | | | |---------------------------------------------| | | | | | | | | | | | | | | | |( )| ( ) | ( ) | ( ) | +---------------------------------------------+ "if we now compare together the number of groups, and the days free from spots, we find that the sun-spots have a period of about ten years, and that for about five years they are so numerous that during this period few days, if any, are free from spots. the sequel must show whether this period is constant, whether the minimum activity of the sun in producing spots lasts for one or two years, and whether this activity increases more quickly than it decreases." [illustration: feb. , . feb. , . x.--photographs of the sun taken at the royal observatory, greenwich, shewing sunspots.] [sidenote: attracted little attention, until eight years later.] this brief announcement is all that the discoverer says upon the subject; and it is perhaps not remarkable that it attracted very little attention, especially when we remember that it related to a matter which the astronomical world had agreed to put aside as unprofitable and not worth attention. next year, in giving his usual paper on the spots for he recurs to the subject in the following sentence: "the periodicity of spots of about ten years which was indicated in my summary published last year, is confirmed by this year's observations." i have added in brackets to the table above reproduced the numbers for subsequently given, and it will be seen how nearly they might have been predicted. [sidenote: other phenomena sympathetic and others not.] still the subject attracted little attention. turning over the leaves of the journal at random, i came across the annual report of the astronomer royal of england, printed at length. but in it there is no reference to this discovery, which opened up a line of work now strongly represented in the annual programme of the royal observatory at greenwich. mr. johnson remarks that the only person who had taken it up was julius schmidt, who then resided near hamburg. but schwabe went on patiently accumulating facts; and in the great von humboldt in the third volume of his _cosmos_, drew attention to the discovery, which was accordingly for the first time brought into general notice. it will be seen that there are not many facts of general interest relating to the actual discovery beyond the courage with which the work was commenced in a totally unpromising direction, and the scant attention it received after being made for us. we may admit that interest centres chiefly in the tremendous consequences which flowed from it. we now recognise that many other phenomena are bound up with this waxing and waning of the solar spots. we might be prepared for a sympathy in phenomena obviously connected with the sun itself; but it was an unexpected and startling discovery that magnetic phenomena on the earth had also a sympathetic relation with the changes in sun-spots, and it is perhaps not surprising that when once this connection of solar and terrestrial phenomena was realised, various attempts have been made to extend it into regions where we cannot as yet allow that it has earned a legitimate right of entry. we have heard of the weather and of indian famines occurring in cycles identical with the sun-spot cycle; and it is obvious how tremendously important it would be for us if this were found to be actually the case. for we might in this way predict years of possible famine and guard against them; or if we could even partially foretell the kind of weather likely to occur some years hence, we might take agricultural measures accordingly. the importance of the connection, if only it could be established, is no doubt the reason which has misled investigators into laying undue stress on evidence which will not bear close scrutiny. for the present we must say decidedly that no case has been made out for paying serious attention to the influence of sun-spots on weather. nevertheless, putting all this aside, there is quite enough of first-rate importance in the sequel to schwabe's discovery. [sidenote: greenwich sun records.] [sidenote: the sun's rotation.] let us review the facts in order. most of us, though we may not have had the advantage of seeing an actual sun-spot through a telescope, have seen drawings or photographs of spots. there is a famous drawing made by james nasmyth (of steam-hammer fame), in july, , which is of particular interest, because at that time nasmyth was convinced--and he convinced many others with him--that the solar surface was made up of a miscellaneous heap of solid bodies in shape like willow leaves, or grains of rice, thrown together almost at random, and the drawing was made by him to illustrate this idea. comparing a modern photograph with it, we see that there is something to be said for nasmyth's view, which attracted much attention at the time and occasioned a somewhat heated controversy. but since the invention of the spectroscope it has become quite obsolete; it probably does not correspond in any way to the real facts. but instead of looking at pictures which have been enlarged to show the detailed structure in and near a spot, we will look at a series of pictures of the whole sun taken on successive days at greenwich in which the spots are necessarily much smaller, but which show the behaviour of the spots from day to day. (see plates x. and xi.) from the date at the foot of each it will be seen that they gradually cross the disc of the sun (a fact first discovered by galileo in ), showing that the sun rotates on an axis once in about every twenty-five days. there are many interesting facts connected with this rotation; especially that the sun does not rotate as a solid body, the parts near the (sun's) equator flowing quicker than those nearer the poles; but for the present we cannot stop to dwell upon them. what interests us particularly is the history, not from day to day, but from year to year, as schwabe has already given it for a series of years. [illustration: feb. , . feb. , . xi.--photographs of the sun taken at the royal observatory, greenwich, shewing sunspots] [sidenote: wolf's numbers.] [sidenote: greenwich areas.] [sidenote: magnetic fluctuations.] when it became generally established that this periodicity existed, rudolf wolf of zurich collected the facts about sun-spots from the earliest possible date, and represented this history by a series of numbers which are still called wolf's sun-spot numbers. you will see from the diagram the obvious rise and fall for eleven years,--not ten years, as schwabe thought, but just a little over eleven years. the facts are, however, given more completely by the work done at the royal observatory at greenwich. it is part of the regular daily work of that observatory to photograph the sun at least twice. many days are of course cloudy or wet, so that photographs cannot be obtained; but there are available photographs similarly taken in india or in mauritius, where the weather is more favourable, and from these the gaps are so well filled up that very few days, if any, during the whole year are left without some photograph of the sun's surface. on these photographs the positions and the areas of the spots are carefully measured under a microscope, and the results when submitted to certain necessary calculations are published year by year. it is clearly a more accurate estimate of the spottedness of the sun to take the total _area_ of all the spots rather than their mere _number_, for in the latter case a large spot and a small one count equally. hence the greenwich records will perhaps give us an even better idea of the periodicity than wolf's numbers. now, at the same observatory magnetic observations are also made continuously. if a magnet be suspended freely we are accustomed to say that it will point to the north pole; but this is only very roughly true. in the first place, it is probably well known to you that there is a considerable deviation from due north owing to the fact that the magnetic north pole is not the same as the geographical north pole; but this for the present need not concern us. what does concern us is, that if the needle is hung up and left long enough to come to rest, it does not then remain steadily at rest, but executes slow and small oscillations backwards and forwards, up and down, throughout the day; repeating nearly the same oscillations on the following day, but at the same time gradually changing its behaviour so as to oscillate differently in summer and winter. these changes are very small, and would pass unnoticed by the naked eye; but when carefully watched through a telescope, or better still, when photographed by some apparatus which will at the same time magnify them, they can be rendered easily visible. when the history of these changes is traced it is seen at once that there is a manifest connection with the cycle of sun-spot changes; for instance, if we measure the range of swing backwards and forwards during the day and take the average for all the days in the year, and then compare this with the average number of sun-spots, we shall see that the averages rise and fall together. similarly we may take the up and down swing, find the average amount of it throughout the year, and again we shall find that this corresponds very closely with the average number of sun-spots. [illustration: plate xii. number of sunspots (wolf) compared with daily range of magnetic declination & daily range of magnetic horzl. force (as observed at greenwich.)] [sidenote: daily curves.] [sidenote: difference between summer and winter, and between sun-spot maximum and minimum.] [sidenote: cause unknown.] but perhaps the most striking way to exhibit the sympathy is to combine different variations of the needle into one picture. and first we must remark that there is another important variation of the earth's magnetic action which we have not yet considered. we have mentioned the swing of the needle to and fro, and the swing up and down, and these correspond to changes in the _direction_ of the force of attraction on the needle. but there may be also changes in _intensity_ of this action; the pull may be a little stronger or a little weaker than before, and these variations are not represented by any actual movement of the needle, though they can be measured by proper experiments. we can, however, imagine them represented by a movement of the end of the needle if we suppose it made of elastic material, so that it would lengthen when the force was greater and contract slightly when the force was less. if a pencil were attached to the end of such an elastic needle so as to make a mark on a sheet of paper, and if for a moment we exclude the up and down movements, the pencil would describe during the day a curve on the paper, as the end of the needle swung backwards and forwards with the change in direction, and moved across the direction of swing with the change in intensity. now when curves of this kind are described for a day in each month of the year, there is a striking difference between the forms of them. during the summer months they are, generally speaking, comparatively large and open, and during the winter months they are small and close. this change in form is seen by a glance at plate xiii., which gives the curves throughout the whole of one year. let us now, instead of forming a curve of this kind for each month, form a single average curve for the whole year; and let us further do this for a series of years. (plate xiv.) we see that the curves change from year to year in a manner very similar to that in which they change from month to month in any particular year, and the law of change is such that in years when there are many sun-spots we get a large open curve similar to those found in the summer, while for years when there are few sun-spots we get small close curves very like those in the winter. hence we have two definite conclusions suggested: firstly, that the changes of force are sympathetic with the changes in the sun-spots; and secondly, that times of maximum sun-spots correspond to summer, and times of minimum to winter. and here i must admit that this is about as far as we have got at present in the investigation of this relationship. _why_ the needle behaves in this way we have as yet only the very vaguest ideas; suggestions of different kinds have certainly been put forward, but none of them as yet can be said to have much evidence in favour of its being the true one. for our present purpose, however, it is sufficient to note that there is this very real connection, and that consequently schwabe's sun-spot period may have a very real importance with regard to changes in our earth itself. [illustration: greenwich magnetic curves - plate xiii. greenwich magnetic curves for april - ] [sidenote: illustration of spurious connection.] but i may perhaps repeat the word of caution already uttered against extending without sufficient evidence this notion of the influence of sun-spots to other phenomena, such as weather. a simple illustration will perhaps serve better than a long argument to show both the way in which mistakes have been made and the way in which they can be seen to be mistakes. there is at the royal observatory at greenwich an instrument for noting the direction of the wind, the essential part being an ordinary wind-vane, the movements of which are automatically recorded on a sheet of paper. as the wind shifts from north to east the pencil moves in one direction, and when it shifts back again towards the north the pencil moves in the reverse way. but sometimes the wind shifts continuously from north to east, south, west, and back to north again, the vane making a complete revolution; and this causes the pencil to move continuously in one direction, until when the vane has come to north again, the pencil is far away from the convenient place of record; on such occasions it is often necessary to replace it by hand. then again, the vane may turn in the opposite direction, sending the pencil inconveniently to the other side of the record. during the year it is easy to count the number of complete changes of wind in either direction, and subtracting one number from the other, we get the excess of complete revolutions of the vane in one direction over that in the other. now if these rather arbitrary numbers are set down year by year, or plotted in the shape of a diagram, we get a curve which may be compared with the sun-spot curve, and during a period of no less than sixteen years--from to --there was a remarkable similarity between the two diagrams. from this evidence _alone_ it might fairly be inferred that the sun-spots had some curious effect upon the weather at greenwich, traceable in this extraordinary way in the changes of the wind. but the particular way in which these changes are recorded is so arbitrary that we should naturally feel surprise if there was a real connection between the two phenomena; and fortunately there were other records preceding these years and following them which enabled us to test the connection further, and it was found, as we might naturally expect, that it was not confirmed. on looking at diagrams (plate xv.) for the periods before and after, no similarity can be traced between the sun-spot curve and the wind-vane curve, and we infer that the similarity during the period first mentioned was entirely accidental. this shows that we must be cautious in accepting, from a limited amount of evidence, a connection between two phenomena as real and established; for it may be purely fortuitous. we may particularly remark that it is desirable to have repetitions through several complete periods instead of one alone. it is possible to reduce to mathematical laws the rules for caution in this matter; and much useful work has already been done in this direction by professor schuster of manchester and others, though as yet too little attention has been paid to their rules by investigators naturally eager to discover some hitherto unthought-of connection between phenomena. [sidenote: faculæ follow spots and the chromosphere.] with this example of the need for caution, we may return to phenomena of which we can certainly say that they vary sympathetically with the sun-spots. roughly speaking, the whole history of the sun seems to be bound up with them. besides these dark patches which we call spots (which, by the way, are not really dark but only less bright than the surrounding part of the disc), there are patches brighter than the rest which have been called faculæ. with ordinary telescopes, either visual or photographic, these can generally only be detected near the edge of the sun's disc; but even with this limitation it can easily be established that the faculæ vary in number and size from year to year much in the same way as the spots, and this conclusion is amply confirmed by the beautiful method of observing the faculæ with the new instrument designed by professor hale of the yerkes observatory. with this instrument, called a spectroheliograph, it is possible to photograph the faculæ in all parts of the sun's disc, and thus to obtain a much more complete history of them, and there is no doubt whatever of their variation sympathetically with the spots. nor is there any doubt about similar variations in other parts of the sun which we cannot see _at all_ with ordinary telescopes, except on the occasions when the sun is totally eclipsed. roughly speaking, these outlying portions of the sun consist of two kinds, the chromosphere and the corona, the former looking like an irregular close coating of the ordinary sun, and the latter like a pearly halo of light extending to many diameters of the sun's disc, but not with any very regular form. [illustration: plate xv. smoothed sunspot curve (wolf) compared with the number of turns made in each year by the osler anemometer vane of the royal observatory, greenwich (the excess of the direct turns (d) over the retrograde turns (r) or _vice versa_.) the upper curve is in each case the sunspot curve, the lower the vane curve. the break in in the vane curve is due to the omission of evidently accidental turns from that date.] the chromosphere, from which shoot out the prominences or "red flames," can now be observed without an eclipse if we employ the beautiful instrument above-mentioned, the spectroheliograph; and professor hale has succeeded in photographing spots, faculæ, and prominences all on the same plate. but although many have made the attempt (and professor hale, perhaps, a more determined attempt than any man living), no one has yet succeeded in obtaining any picture or evidence of the existence of the corona excepting on the occasion of a total solar eclipse. [sidenote: eclipses of sun.] [sidenote: total eclipses rare.] now these occasions are very rare. there are two or three eclipses of the sun every year, but they are generally of the kind known as partial; when the moon does indeed come between us and the sun to some extent, but only cuts off a portion of his light--a clean-cut black disc is seen to encroach more or less on the surface of the sun. most of us have had an opportunity of seeing a partial eclipse, probably more than once; but few have seen a total eclipse. for this the moon must come with great exactness centrally between us and the sun; and the spot where this condition is fulfilled completely only covers a few hundred miles of the earth's surface at one moment. as the earth turns round, and as the moon revolves in its orbit, this patch from which the sun is totally eclipsed travels over the earth's surface, marking out a track some thousands of miles in length possibly, but still not more than miles wide; and in order to see the sun totally eclipsed even on the rare occasions when it is possible at all (for, as already remarked, in the majority of cases the eclipse is only partial), we must occupy some station in this narrow belt or track, which often tantalisingly passes over either the ocean or some regions not easily accessible to civilised man. moreover, if we travel to such favoured spots the whole time during which the sun is totally eclipsed cannot exceed a few minutes, and hence observations are made under rather hurried and trying conditions. in these modern days of photography it is easier to take advantage of these precious moments than it used to be when there was only the eye and memory of an excited observer to rely upon. it is perhaps not surprising that some of the evidence collected on these earlier occasions was conflicting; but nowadays the observers, generally speaking, direct their energies in the first place to mounting accurately in position photographic apparatus of different kinds, each item of it specially designed to settle some particular problem in the most feasible way; secondly, to rehearsing very carefully the exact programme of exposures necessary during the critical few minutes; and finally, to securing these photographs with as few mistakes as possible when the precious moments actually arrive. even then the whole of their efforts are quite likely to be rendered unavailing by a passing cloud; and bitter is the disappointment when, after travelling thousands of miles, and spending months in preparation, the whole enterprise ends in nothing owing to some caprice of the weather. [sidenote: corona follows spots.] hence it will easily be imagined that our knowledge of the corona, the part of the sun which we can still only study on occasions of a total solar eclipse, advances but slowly. during the last twenty years there has been altogether scarcely half-an-hour available for this research, though it may fairly be said that the very best possible use has been made of that half-hour. and, what is of importance for our immediate purpose, it has gradually been established by comparing the photographs of one eclipse with those of another, that the corona itself undergoes distinct changes in form in the same period which governs the changes of sun-spots. when there are many sun-spots the corona spreads out in all directions from the edge of the sun's disc; when there are few sun-spots the corona extends very much further in the direction of the sun's equator, so that at sun-spot minimum there is an appearance of two huge wings. although the evidence is necessarily collected in a scrappy manner, by this time there is sufficient to remove this relationship out of the region of mere suspicion, and to give it a well-established place in our knowledge of the sun's surroundings. [sidenote: corona may influence magnets.] now the corona of the sun may be compared to some rare animal which we only see by paying a visit to some distant land, and may consider ourselves even then fortunate to get a glimpse of; and it might be thought that the habits of such an animal are not likely to be of any great importance in our everyday life. but so far from this being the case in regard to the corona, it is more than possible that the knowledge of its changes may be of vital interest to us. i have already said that, as yet, we have no satisfactory account of the reason why changes in sun-spots seem to influence changes in our magnets on the earth; but one of the theories put forward in explanation, and one by no means the least plausible, is that this influence may come, not from the sun-spots themselves, but from some other solar phenomenon which varies in sympathy with them; and in particular that it may come from the corona. these wings which reach out at sun-spot minimum can be seen to extend a considerable distance, and there is no reason to suppose that they actually cease at the point where they become too faint for us to detect them further; they may extend quite as far as the earth itself and even beyond; and they may be of such a nature as to influence our magnets. as the earth revolves round the sun it may sometime plunge into them, to emerge later and pass above or below them; as again the wings spread themselves at sun-spot minimum and seem to shrink at maximum, so our magnets may respond by sympathetic though very small vibrations. hence it is quite possible that the corona is directly influencing the magnetic changes on the earth. [sidenote: possible importance of corona.] but it may be urged that these changes are so slight as to be merely of scientific interest. that may be true to-day, but who will be bold enough to say that it will be true to-morrow? if we are thinking of practical utility alone, we may remember that two great forces of nature which we have chained into the service of man, steam and electricity, put forth originally the most feeble manifestations, which might readily have been despised as valueless; but by careful attention to proper conditions results of overwhelming practical importance have been obtained from these forces, which might have been, and for many centuries were, neglected as too trivial to be worth attention. recently the world has been startled by the discovery of new elements, such as radium, whose very existence was only detected by a triumph of scientific acuteness in investigation, and yet which promise to yield influences on our lives which may overwhelm in importance all that has gone before. and similarly it may be that these magnetic changes, when properly interpreted or developed, may become of an importance in the future out of all proportion to the attention which they have hitherto attracted. hence, although perhaps sufficient has already been established to show the immense consequences which flow from schwabe's remarkable discovery of the periodicity in solar spots, we may be as yet only on the threshold of its real value. from what little causes great events spring! how little can schwabe have realised, when he began to point his modest little telescope at the sun, and to count the number of spots--the despised spots which he had been assured were of no interest and exhibited no laws, and were generally unprofitable--that he was taking the first step in the invention of the great science of solar physics!--a science which is, i am glad to say, occupying at the present moment so much of the attention, not only of the great yerkes observatory, but of many other observatories scattered over the globe. chapter vi the variation of latitude if we should desire to classify discoveries in order of merit, we must undoubtedly give a high place to those which are made under direct discouragements. in the last chapter we saw that schwabe entered upon his work under conditions of this kind, it being the opinion of experienced astronomers who had looked at the facts that there was nothing of interest to be got by watching sun-spots. in the present chapter i propose to deal with a discovery made in the very teeth of the unanimous opinion of the astronomical world by an american amateur, mr. s. c. chandler of cambridge (massachusetts). it is my purpose to allow him to himself explain the steps of this discovery by giving extracts from the magnificent series of papers which he contributed to the _astronomical journal_ on the subject in the years - , but it may help in the understanding of these extracts if i give a brief summary of the facts. and i will first explain what is meant by the "variation of latitude." [sidenote: latitude.] [sidenote: precession.] we are all familiar with the existence of a certain star in the heavens called the pole star, and we know that at any particular place it is seen constantly in the north at a definite height above the horizon, which is the latitude of the place. when watched carefully with a telescope it is found to be not absolutely stationary, but to describe a small circle in the heavens day by day, or rather night by night. these simple facts are bound up with the phenomenon of the earth's rotation in this way: the axis about which it is rotating points to the centre of that little circle, and any change in the position of the axis can therefore be determined by observing these motions of the pole star. such changes may be of two kinds: firstly, we might find that the size of the circle increased or diminished, and this would mean that the earth's axis was pointing farther away from the pole star or nearer to it--pointing, that is to say, in a different direction in space. this actually happens (as has been known for some thousands of years) owing to the phenomenon called "precession"; the circle described by our pole star is at present getting a little smaller, but it will ultimately increase in size, and after thousands of years become so large that the pole star will entirely lose its character as a steady guide to the north. [sidenote: change of latitude.] [sidenote: twenty years ago disbelieved.] secondly (and this is what more immediately concerns us), the centre of the circle may alter its position and be no longer at the same height above the horizon of any given place. this would mean that the earth's axis was shifting _in the earth itself_--that the north pole which our explorers go to seek is not remaining in the same place. that it does not change appreciably in position we know from familiar experience; our climates, for instance, would suffer considerably if there were any large changes. but astronomers are concerned with minute changes which would not have any appreciable effect on climate, and the question has long been before them whether, putting aside large movements, there were any minute variations in position of the north pole. twenty years ago the answer to this question would have been given decidedly in the negative; it was considered as certain that the north pole did not move at all within the limits of our most refined astronomical observations. accepted theory seemed to indicate that any movements must in any case recur after a period of ten months, and careful discussion of the observations showed that there was no oscillation in such a period. now we know that the theory itself was wrong, or rather was founded upon a mistaken assumption; and that the facts when properly examined show clearly a distinct movement of the north pole, not a very large one, for all its movements take place within the area occupied by a moderate-sized room, but still a movement easily measurable by astronomical observations, and mr. chandler was the first to point out the law of these movements, and very possibly the first to suspect them. [sidenote: chandler's papers.] with these few words of explanation i will let mr. chandler tell his own story. his first paper appeared in the _astronomical journal_ in november , and is courageously headed, "on the variation of latitude"--i say courageously, because at that time it was believed that the latitude did _not_ vary, and mr. chandler himself was only in possession of a small portion of the facts. they unravelled themselves as he went forward; but he felt that he had firm hold of the end of the thread, and he faced the world confidently in that belief. he begins thus:-- [sidenote: first signs of change.] "in the determination of the latitude of cambridge[ ] with the almucantar, about six years and a half ago, it was shown that the observed values, arranged according to nights of observation, exhibited a decided and curious progression throughout the series, the earlier values being small, the later ones large, and the range from november to april being about four-tenths of a second. there was no known or imaginable instrumental or personal cause for this phenomenon, yet the only alternative seemed to be an inference that the latitude had actually changed. this seemed at the time too bold an inference to place upon record, and i therefore left the results to speak for themselves. the subsequent continuation of the series of observations to the end of june gave a maximum about may , while the discussion of the previous observations from may to november gave a minimum about september , indicating a range of ". within a half-period of about seven months." mr. chandler then gives some figures in support of these statements, presenting them with the clearness which is so well marked a feature of the whole series of papers, and concludes this introductory paper as follows:-- "it thus appears that the apparent change in the latitude of cambridge is verified by this discussion of more abundant material. the presumption that it is real, on this determination alone, would justify further inquiry. [sidenote: confirmed in europe.] "curiously enough dr. küstner, in his determination of the aberration from a series of observations coincident in time with those of the almucantar, came upon similar anomalies, and his results, published in , furnish a counterpart to those which i had pointed out in . the verification afforded by the recent parallel determinations at berlin, prague, potsdam, and pulkowa, which show a most surprising and satisfactory accordance, as to the character of the change, in range and periodicity, with the almucantar results, has led me to make further investigations on the subject. they seem to establish the nature of the law of those changes, and i will proceed to present them in due order." the second paper appeared on november , and opens with the following brief statement of his general results at that time:-- [sidenote: days' period.] "before entering upon the details of the investigations spoken of in the preceding number, it is convenient to say that the general result of a preliminary discussion is to show a revolution of the earth's pole in a period of days, from west to east, with a radius of thirty feet, measured at the earth's surface. assuming provisionally, for the purpose of statement, that this is a motion of the north pole of the principal axis of inertia about that of the axis of rotation, the direction of the former from the latter lay towards the greenwich meridian about the beginning of the year . this, with the period of days, will serve to fix approximately the relative positions of these axes at any other time, for any given meridian. it is not possible at this stage of the investigation to be more precise, as there are facts which appear to show that the rotation is not a perfectly uniform one, but is subject to secular change, and perhaps irregularities within brief spaces of time." [sidenote: contrary to received views.] it is almost impossible, now that we have become familiar with the ideas conveyed in this paragraph, to understand, or even fully to remember, the impression produced by them at the time; the sensation caused in some quarters, and the ridicule excited in others. they were in flat contradiction to all accepted views; and it was believed that these views were not only theoretically sound, but had been matured by a thorough examination of observational evidence. the only period in which the earth's pole could revolve was believed to be ten mouths; and here was mr. chandler proclaiming, apparently without any idea that he was contradicting the laws of dynamics, that it was revolving in fourteen months! the radius of its path had been found to be insensible by careful discussion of observations, and now he proclaimed a sensible radius o£ thirty feet. finally, he had the audacity to announce a _variable_ period, to which there was nothing at all corresponding in the mathematical possibilities. this was the bitterest pill of all. even after professor newcomb had shown us how to swallow the other two, he could not recommend any attempt at the third, as we shall presently see; and mr. chandler was fain ultimately to gild it a little before it could be gulped. [sidenote: pulkowa puzzle solved, also washington.] but this is anticipating, and it is our intention to follow patiently the evidence adduced in support of the above statements, made with such splendid confidence to a totally disbelieving world. mr. chandler first examines the observations of dr. küstner of berlin, quoted at the end of his last paper, and shows how well they are suited by the existence of a variation in the latitude of days; and that this new fact is added--when the cambridge (u.s.a.) latitudes were the smallest those of berlin were the largest, and _vice versâ_, as would clearly be the case if the phenomenon was due to a motion of the earth's pole; for if it moved nearer america it must move further from europe. he then examines a long series of observations made in the years - at pulkowa, near st. petersburg, and again finds satisfactory confirmation of his law of variation. now it had long been known that there was something curious about these observations, but no one could tell what it was. the key offered by mr. chandler fitted the lock exactly, and the anomalies which had been a puzzle were removed. this was in itself a great triumph; but there was another to come, which we may let mr. chandler describe in his own words:-- "in professor hubbard began a series of observations of [a] lyræ at the washington observatory with the prime vertical transit instrument, for the purpose of determining the constants of aberration and nutation and the parallax of the star. the methods of observation and reduction were conformed to those used with such success by w. struve. after hubbard's death the series was continued by professors newcomb, hall, and harkness until the beginning of . professor hall describes these observations as the most accurate determinations of declination ever made at the naval observatory. the probable error of a declination from a single transit was ± ". , and judging from the accidental errors, the series ought to give trustworthy results. upon reducing them, however, it was found that some abnormal source of error existed, which resulted in anomalous values of the aberration-constant in the different years, and a negative parallax in all. a careful verification of the processes of reduction failed to discover the cause of the trouble, and professor hall says that the results must stand as printed, and that probably some annual disturbance in the observations or the instrument occurred, which will never be explained, and which renders all deductions from them uncertain. the trouble could not be connected with personal equation, the anomalies remaining when the observations of the four observers who took part were separately treated. nor, as professor hall points out, will the theoretical ten-month period in the latitude furnish the explanation. "it is manifest, however, that if the -day period exists, its effect ought to appear distinctly in declination-measurements of such high degree of excellence as these presumably were, and, as i hope satisfactorily to show, actually are. when this variation is taken into account the observations will unquestionably vindicate the high expectations entertained with regard to them by the accomplished and skilful astronomers who designed and carried them out." [sidenote: direction of revolution of pole.] [sidenote: example of results.] from this general account i am excluding technical details and figures, and unfortunately a great deal is thereby lost. we lose the sense of conviction which the long rows of accordant figures force upon us, and we lose the opportunities of admiring both the astonishing amount of work done and the beautiful way in which the material is handled by a master. but i am tempted to give one very small illustration of the numerical results from near the end of the paper. after discussing the washington results, and amply fulfilling the promise made in the preceding extract, mr. chandler compares them with the pulkowa results, and shows that the earth's pole must be revolving from west to east, and not from east to west. and then he writes down a simple formula representing this motion, and compares his formula with the observations. he gives the results in seconds of arc, but for the benefit of those not familiar with astronomical measurements we may readily convert these into feet; and in the following tables are shown the distances of the earth's pole _in feet_ from its average position,[ ] as observed at washington and at pulkowa, and the same distances calculated according to the formula which mr. chandler was able to write down at this early stage. the signs + and - of course indicate opposite directions of displacement:-- washington. _deviation of pole._ +-------------------------------------+ | date. | observed.| formula. | |-------------------------------------| | , dec. | - feet | - feet | | , mar. | - " | - " | | " june | + " | + " | | " aug. | + " | + " | | " oct. | + " | + " | | " dec. | - " | - " | +-------------------------------------+ pulkowa. _deviation of pole._ +-------------------------------------+ | date. | observed.| formula. | |-------------------------------------| | , july | - feet | - feet | | " sept. | + " | + " | | " nov. | + " | + " | | , feb. | + " | + " | | " june | - " | - " | | " july | - " | - " | +-------------------------------------+ of course the figures are not exact in every case, but they are never many feet wrong; and it may well be imagined that it is a difficult thing to deduce, even from the most refined observations, the position of the earth's pole to within a foot. the difficulty is exactly the same as that of measuring the length of an object miles away to within an inch! mr. chandler winds up his second paper thus:-- "we thus find that the comparison of the simultaneous series at pulkowa and washington, - , leads to the same conclusion as that already drawn from the simultaneous series at berlin and cambridge, - . the direction of the polar motion may therefore be looked upon as established with a large degree of probability. "in the next paper i will present the results derived from peters, struve, bradley, and various other series of observations, after which the results of all will be brought to bear upon the determination of the best numerical values of the constants involved." [sidenote: bradley's observations.] [sidenote: latitude varied in twelve months then.] the results were not, however, presented in this order. in the next paper, which appeared on december , , mr. chandler begins, with the work of bradley, the very series of observations at kew and wansted which led to the discoveries of aberration and nutation, and which we considered in the third chapter. he first shows that, notwithstanding the obvious accuracy of the observations, there is some unexplained discordance. the very constant of aberration which bradley discovered from them differs by half-a-second of arc from our best modern determinations. attempts have been made to ascribe the discordance to changes in the instrument, but mr. chandler shows that such changes, setting aside the fact that bradley would almost certainly have discovered them, will not fit in with the facts. the facts, when analysed with the skill to which we have become accustomed, are that there is a periodic swing in the results _with a period of about a year_, and not fourteen months, as before, "a result so curious," as he admits, that "if we found no further support, it might lead us to distrust the above reasoning, and throw us back to the possibility that, after all, bradley's observations may have been vitiated by some kind of annual instrumental error. but it will abundantly appear, when i have had the opportunity to print the deductions from all the other series of observations down to the present time, that the inference of an increase in the period of polar revolution is firmly established by their concurrent testimony." we shall presently return to this curious result, which might well have dismayed a less determined researcher than mr. chandler, but which only led him on to renewed exertions. the results obtained from bradley's observations may be put in the form of a diagram thus:-- [illustration: fig. .] it will be seen that the maxima and minima fall in the spring and autumn, and this fact alone seemed to show that the effect could not be due to temperature, for we should expect the greatest effect in that case in winter and summer. it could not be due to the parallax of the stars for which bradley began his search, for stars in different quarters of the heavens would then be differently affected, and this was not the case. "there remains," concluded mr. chandler after full discussion, "the only natural conclusion of an actual displacement of the zenith, in other words, a change of latitude." and he concludes this paper with the following fine passage:-- "so far, then, as the results of this incomparable series of observations at kew and wansted, considered by themselves alone, can now be stated, the period of the polar rotation at that epoch appears to have been probably somewhat over a year, and certainly shorter by about two months than it is at the present time. the range of the variation was apparently in the neighbourhood of a second of arc, or considerably larger than that shown by the best modern observations. [sidenote: bradley's greatness.] "before taking leave of these observations for the present i cannot forbear to speak of the profound impression which a study of them leaves upon the mind, and the satisfaction which all astronomers must feel in recognising that, besides its first fruits of the phenomena of aberration and nutation, we now owe also our first knowledge of the polar motion to this same immortal work of bradley. its excellence, highly appreciated as it has been, has still been hitherto obscured by the presence of this unsuspected phenomenon. when divested of its effects, the wonderful accuracy of this work must appear in a finer light, and our admiration must be raised to higher pitch. going back to it after one hundred and sixty years seems indeed like advancing into an era of practical astronomy more refined than that from which we pass. and this leads to a suggestion worthy of serious practical consideration--whether we can do better in the future study of the polar rotation, than again to avail ourselves of bradley's method, without endangering its elegant simplicity and effectiveness by attempts at improvement, other than supplying certain means of instrumental control which would without doubt commend themselves to his sagacious mind. [sidenote: other puzzles explained.] "in the next article bradley's later observations at greenwich, the results of which are not so distinct, will be discussed; and also those of brinkley at dublin, - and - . this will bring again to the surface one of the most interesting episodes in astronomical history, the spirited and almost acrimonious dispute between brinkley and pond with regard to stellar parallaxes. i hope to show that the hitherto unsolved enigma of brinkley's singular results finds its easy solution in the fact of the polar motion. the period of his epoch appears to have been about a year, and its range more than a second. afterwards will follow various discussions already more or less advanced towards completion. these include bessel's observations at königsberg, - , with the reichenbach circle, and in - with the repsold circle; the latitudes derived from the polar-point determinations of struve and mädler with the dorpat circle, - ; struve's observations for the determination of the aberration; peters' observations of _polaris_, - , with the vertical-circle; the results obtained from the reflex zenith-tube at greenwich, - , whose singular anomalies can be referred in large part to our present phenomenon, complicated with instrumental error, to which until now they have been exclusively attributed; the greenwich transit-circle results, - , in which case, however, a similar complication and the large accidental errors of observation seem to frustrate efforts to get any pertinent results; the berlin prime-vertical observations of weyer and brünnow, - , in which i hope to show that the parallax of [beta] _draconis_ derived from them is simply a record of the change of latitude; the conflicting latitude determinations at cambridge, england; the washington observation of _polaris_ and other close polars, - , with the transit-circle; also those at melbourne, - , a portion of which have already been drawn upon in the last number of the _journal_, and some others. while the list is a considerable one, i shall be able to compress the statement of results for many of the series into a short space. [sidenote: provisional nature of results.] "in connection with this synopsis of the scope of the investigations, one or two particulars may be of interest, which at the present writing seem to foreshadow the probable outcome. i beg, however, that the statement will be regarded merely as a provisional one. first, while the period is manifestly subject to change, as has already once or twice been intimated, i have hitherto failed in tracing the variations to any regular law, expressible in a numerical formula. indeed, the general impression produced by a study of these changes in the length of the period is that the cause which produces them operates capriciously to a certain degree, although the average effect for a century has been to diminish the velocity of the revolution of the pole. how far this impression is due to the uncertainty of the observations, and to the complication of the phenomenon with other periodical changes of a purely instrumental kind, i cannot say. almost all of the series of any extent which have been examined, have the peculiarity that they manifest the periodicity quite uniformly and distinctly for a number of years, then for a while obscurely. in some cases, however, what at first appears to be an objective irregularity proves not to be so by comparison with overlapping series at other observatories. "another characteristic which has struck my attention, although somewhat vaguely, is that the variations in the length of the period seem to go hand in hand with simultaneous alterations in the amplitude of the rotation; the shorter periods being apparently associated with the larger coefficients for the latter. the verification of these surmises awaits a closer comparative scrutiny, the opportunity for which will come when the computations are in a more forward state. if confirmed, these observations will afford a valuable touchstone, in seeking for the cause of a phenomenon which now seems to be at variance with the accepted laws of terrestrial rotation." [sidenote: reception of discovery.] let us now for a few moments turn aside from the actual research to see how the announcement was received. it would be ungracious to reprint here any of the early statements of incredulity which found their way into print, especially in germany. but the first note of welcome came from simon newcomb, in the same number of the _astronomical journal_ as the paper just dealt with, and the following extract will indicate both the difficulties felt in receiving mr. chandler's results and the way in which newcomb struck at the root of them. [sidenote: newcomb's explanation.] "mr. chandler's remarkable discovery, that the apparent variations in terrestrial latitudes may be accounted for by supposing a revolution of the axis of rotation of the earth around that of figure, in a period of days, is in such disaccord with the received theory of the earth's rotation that at first i was disposed to doubt its possibility. but i am now able to point out a _vera causa_ which affords a complete explanation of this period. up to the present time the treatment of this subject has been this: the ratio of the moment of inertia of the earth around its principal axis to the mean of the other two principal moments, admits of very accurate determination from the amount of precession and nutation. this ratio involves what we might call, in a general way, the solid ellipticity of the earth, or the ellipticity of a homogeneous spheroid having the same moments of inertia as the earth. "when the differential equations of the earth's rotation are integrated, there appear two arbitrary constants, representing the position of any assigned epoch of the axis of rotation relative to that of figure. theory then shows that the axis of rotation will revolve round that of figure, in a period of days, and in a direction from west toward east. the attempts to determine the value of these constants have seemed to show that both are zero, or that the axes of rotation and figure are coincident. several years since, sir william thomson published the result of a brief computation from the washington prime-vertical observations of [alpha] lyrae which i made at his request and which showed a coefficient ". . this coefficient did not exceed the possible error of the result; i therefore regarded it as unreal. [sidenote: the forgotten assumption.] "the question now arises whether mr. chandler's result can be reconciled with dynamic theory. i answer that it can, because the theory which assigns days as the time of revolution is based on the hypothesis that the earth is an absolutely rigid body. but, as a matter of fact, the fluidity of the ocean plays an important part in the phenomenon, as does also the elasticity of the earth. the combined effect of this fluidity and elasticity is that if the axis of rotation is displaced by a certain amount, the axis of figure will, by the changed action of the centrifugal force, be moved toward coincidence with the new axis of rotation. the result is, that the motion of the latter will be diminished in a corresponding ratio, and thus the time of revolution will be lengthened. an exact computation of the effect is not possible without a knowledge of the earth's modulus of elasticity. but i think the result of investigation will be that the rigidity derived from mr. chandler's period is as great as that claimed by sir william thomson from the phenomena of the tides." [sidenote: but chandler's work still mistrusted.] this was very satisfactory. professor newcomb put his finger on the assumption which had been made so long ago that it had been forgotten: and the lesson is well worth taking to heart, for it is not the first time that mistaken confidence in a supposed fact has been traced to some forgotten preliminary assumption: and we must be ever ready to cast our eyes backward over all our assumptions, when some new fact seems to challenge our conclusions. it might further be expected that this discovery of the way in which theory had been defective would as a secondary consequence inspire confidence in the other conclusions which mr. chandler had arrived at in apparent contradiction to theory; or at least suggest the suspension of judgment. but professor newcomb did not feel that this was possible in respect of the _change_ of period, from about twelve months in bradley's time to fourteen months in ours. we have seen that mr. chandler himself regarded this as a "curious result" requiring confirmation: but since the confirmation was forthcoming, he stated it with full confidence, and drew the following remarks from professor newcomb in july , :-- "the fact of a periodic variation of terrestrial latitudes, and the general law of that variation, have been established beyond reasonable doubt by the observations collected by mr. chandler. but two of his minor conclusions, as enumerated in no. of this volume, do not seem to me well founded. they are-- " . that the period of the inequality is a variable quantity. " . that the amplitude of the inequality has remained constant for the last half century." professor newcomb proceeds to give his reasons for scepticism, which are too technical in character to reproduce here. but i will quote the following further sentence from his paper:-- "the question now arises how far we are entitled to assume that the period must be invariable. i reply that, perturbations aside, any variation of the period is in such direct conflict with the laws of dynamics that we are entitled to pronounce it impossible. but we know that there are perturbations, and i do not see how one can doubt that they have so acted as to increase the amplitude of the variation since ." [sidenote: chandler's reply.] in other words, while recognising that there may be a way of reconciling one of the "minor" conclusions with theory, professor newcomb considers that in this case the other must go. mr. chandler's answer will speak for itself. it was delayed a little in order that he might present an immense mass of evidence in support of his conclusions, and was ultimately printed on august , . "the material utilised in the foregoing forty-five series aggregates more than thirty-three thousand observations. of these more than one-third were made in the southern hemisphere, a fact which we owe principally to cordoba. it comprises the work of seventeen observatories (four of them in the southern hemisphere) with twenty-one different instruments, and by nine distinct methods of observation. only three of the series (xxi., xxv., and xxxv.), and these among the least precise intrinsically, give results contradictory of the general law developed in no. . this degree of general harmony is indeed surprising when the evanescent character of the phenomenon under investigation is considered. "the reader has now before him the means for independent scrutiny of the material on which the conclusions already drawn, and those which are to follow, are based. the space taken in the printing may seem unconscionable, but i hope this will be charged to the extent of the evidence collected, and not to diffuseness or the presentation of needless detail; for i have studiously sought to compress the form of statement without omitting anything essential for searching criticism. that it was important to do this is manifest, since the conclusions, if established, overthrow the existing theory of the earth's rotation, as i have pointed out on p. . i am neither surprised nor disconcerted, therefore, that professor newcomb should hesitate to accept some of these conclusions on the ground (_a. j._, no. ) that they are in such conflict with the laws of dynamics that we are entitled to pronounce them impossible. he has been so considerate and courteous in his treatment of my work thus far, that i am sure he will not deem presumptuous the following argument in rebuttal. [sidenote: he "put aside all teachings of theory," and "is not dismayed."] "it should be said, first, that in beginning these investigations last year, i deliberately put aside all teachings of theory, because it seemed to me high time that the facts should be examined by a purely inductive process; that the nugatory results of all attempts to detect the existence of the eulerian period probably arose from a defect of the theory itself; and that the entangled condition of the whole subject required that it should be examined afresh by processes unfettered by any preconceived notions whatever. the problem which i therefore proposed to myself was to see whether it would not be possible to lay the numerous ghosts--in the shape of numerous discordant residual phenomena pertaining to determinations of aberration, parallaxes, latitudes, and the like--which had heretofore flitted elusively about the astronomy of precision during the century; or to reduce them to tangible form by some simple consistent hypothesis. it was thought that if this could be done, a study of the nature of the forces, as thus indicated, by which the earth's rotation is influenced, might lead to a physical explanation of them. "naturally, then, i am not much dismayed by the argument of conflict with dynamic laws, since all that such a phrase means must refer merely to the existent state of the theory at any given time. when the -day period was propounded, it was as inconsistent with known dynamic law as the variation of it now appears to be. professor newcomb's own happy explanation has already set aside the first difficulty, as it would appear, and advanced the theory by an important step. are we so sure yet of a complete knowledge of all the forces at work as to exclude the chance of a _vera causa_ for the second?" [sidenote: faraday's words.] there is a splendid ring of resolution about these words. let us compare them with a notable utterance of faraday:-- "the philosopher should be a man willing to listen to every suggestion, but determined to judge for himself. he should not be biassed by appearances; have no favourite hypothesis; be of no school; and in doctrine have no master. he should not be a respecter of persons, but of things. truth should be his primary object. if to these qualities be added industry, he may indeed hope to walk within the veil of the temple of nature." [sidenote: chandler's other work at this time.] [sidenote: his ultimate satisfactory solution.] [sidenote: interference of two waves.] tested by this severe standard, mr. chandler fails in no particular, least of all in that of industry. the amount of work he got through about this time was enormous, for besides the main line of investigation, of which we have only had after all a mere glimpse, he had been able to turn aside to discuss a subsidiary question with professor comstock; he had examined with great care some puzzling characteristics in the variability of stars; he computed some comet ephemerides; and he was preparing a new catalogue of variable stars--a piece of work involving the collection and arrangement of great masses of miscellaneous material. yet within a few months after replying as above to professor newcomb's criticism, he was able to announce that he had found the key to the new puzzle, and that "theory and observation were again brought into complete accord." we will as before listen to the account of this new step in his own words, but a slight preliminary explanation may help those unaccustomed to the terminology. the polar motion was found to be compounded of _two_ independent motions, both periodic, but having different periods. now, the general results of such a composition are well known in several different branches of physics, especially in the theory of sound. if two notes of nearly the same pitch be struck at the same time, we hear the resultant sound alternately swell and die away, because the vibrations caused by the two notes are sometimes going in the same direction, and after an interval are going exactly in opposite directions. diagrammatically we should represent the vibrations by two waves, as below; the upper wave goes through its period seven and a half times between a and d, the lower only six times; and it is easily seen that at a and c the waves are sympathetic, at b and d antipathetic. at a and c the compound vibration would be doubled; at b and d reduced to insensibility. the point is so important that perhaps a numerical illustration of it will not be superfluous. the waves are now represented by rows of figures as below. the first series recurs after every , the second after every . [illustration: fig. .] first wave second wave ------------------------------------------------------------- combined effect great disturbance. calm. ----------------------------------------------------------------------------- first wave second wave ------------------------------------------------------------- combined effect great disturbance. [sidenote: illustration from ocean travel.] adding the two rows together, the oscillations at first reinforce one another and we get numbers ranging from to instead of from to ; but one wave gains on the other, until it is rising when the other is falling, and the numbers add up to a steady series of 's. it will be seen that there are no less than seven consecutive 's, and all the variation seems to have disappeared. but presently the waves separate again, and the period of great disturbance recurs; it will be seen that in the "combined effect" the numbers repeat exactly after the nd term. now those unfamiliar with the subject may not be prepared for the addition of one physical wave to another, as though they were numbers, but the analogy is perfect. travellers by some of the fast twin-screw steamers have had unpleasant occasion to notice this phenomenon, when the engineer does not run the two screws precisely at the same speed; there come times when the ship vibrates violently, separated by periods of comparative stillness. instances from other walks of life may recur to the memory when once attention is called to the general facts; but enough has been said to explain the point numbered ( ) in the subjoined statement. to understand the rest, we must remember that if the two waves are not equal in "amplitude," _i.e._ if the backward and forward motion is not the same in both, they cannot annul one another, but the greater will always predominate. those interested in following the matter further should have no difficulty in constructing simple examples to illustrate such points. we will proceed to give mr. chandler's statements:-- [sidenote: chandler's final formulæ.] "we now come upon a new line of investigation. heretofore, as has been seen, the method has been to condense the results of each series of observations into the interval comprised by a single period, then to determine the mean epoch of minimum and the mean range for each series, and, finally, by a discussion of these quantities, to establish the general character of the law of the rotation of the pole. it is now requisite to analyse the observations in a different way, and discover whether the deviations from the general provisional law, in the last column of table ii., are real, and also in what manner the variation of the period is brought about. the outcome of this discussion, which is to be presented in the present paper, is extremely satisfactory. the real nature of the phenomenon is most distinctly revealed, and may be described as follows:-- " . the observed variation of the latitude is the resultant curve arising from two periodic fluctuations superposed upon each other. the first of these, and in general the more considerable, has a period of about days, and a semi-amplitude of about ". . the second has an annual period with a range variable between ". and ". during the last half-century. during the middle portion of this interval, roughly characterised as between and , the value represented by the lower limit has prevailed, but before and after those dates, the higher one. the minimum and maximum of this annual component of the variation occur at the meridian of greenwich, about ten days before the vernal and autumnal equinoxes respectively, and it becomes zero just before the solstices. " . as the resultant of these two motions, the effective variation of the latitude is subject to a systematic alternation in a cycle of seven years' duration, resulting from the commensurability of the two terms. according as they conspire or interfere, the total range varies between two-thirds of a second as a maximum, to but a few hundredths of a second, generally speaking, as a minimum. " . in consequence of the variability of the coefficient of the annual term above mentioned, the apparent average period between and approximated to or days; widely fluctuated from to ; from to about was very nearly days, with minor fluctuations; afterwards increased to near days, and very recently fell to somewhat below days. the general course of these fluctuations is quite faithfully represented by the law of eq. ( ), (no. ), and accurately, even down to the minor oscillations of individual periods, by the law of eq. ( ), hereafter given, and verbally interpreted above. this law also gives a similarly accurate account of the corresponding oscillations in the amplitude. the closeness of the accordance between observation and the numerical theory, in both particulars, places the reality of the law beyond reasonable doubt." those who cannot follow the details of the above statement will nevertheless catch the general purport--that the difficulties felt by professor newcomb have been surmounted; and this is made clearer by a later extract:-- "a very important conclusion necessarily follows from the agreement of the values of the -day term, deduced from the intervals between the consecutive values of t in table xii., namely, that there has been no discontinuity in the revolution, such as professor newcomb regarded as so probable that he doubted the possibility of drawing any conclusions from the comparison of observations before and after (_a. j._, , p. ). [sidenote: theory must go, if it will not fit observation.] "the present investigation demonstrates that the way out of the apparently irreconcilable contradiction of theory and observation in this matter does not lie in the direction of discrediting the observations, as he is inclined to do. on the contrary, the result is a beautiful vindication of the trustworthiness of the latter, and, at the same time, of the theory that demands an invariable rate of motion; providing a perfectly fitting key to the riddle by showing that another cause has intervened to produce the variability of the period. i feel confident that professor newcomb will agree with the reality of the explanation here set forth, and will reconsider his view that the perturbations in the position of the pole must be of the nature of chance accumulations of motion, a view which he then considered necessary to the maintenance of the constancy in the period of latitude-variation." [sidenote: the final paper.] the paper from which these words are taken appeared on november , . the next paper on the main theme did not appear till a year later, though much work was being done in the meantime on the constant of aberration and other matters arising immediately after the discovery. on november , , mr. chandler winds up the series of eight papers "on the variation of latitude," which he had commenced just two years before. his work was by no means done; rather was it only beginning, for the torch he had lit illuminated many dark corners. but he rightly regarded his discovery as now so firmly established that the series of papers dealing with it as still under consideration might be terminated. in this final paper he first devotes the most careful attention to one point of detail. he had shown earlier in the series that the north pole must be revolving from west to east, and not from east to west; but this was when the motion was supposed to be simple and not complex, and it was necessary to re-examine the question of direction for each of the components. after establishing conclusively that the original direction holds for each of the components, he almost apologises for the trouble he has taken, thus:-- "it is therefore proved beyond reasonable doubt that the directions of the rotations is from west to east in both elements; whence the general form of the equation for the variation of latitude adopted in _a. j._, , p. , eq. ( ). it may be thought that too much pains have been here bestowed upon a point which might be trusted to theory to decide. i cannot think so. one of the most salient results of these articles has been the proof of the fact that theory has been a blind guide with regard to the velocity of the polar rotation, obscuring truth and misleading investigators for a half a century. and even if we were certain, which we are not, that the fourteen months' term is the eulerian period in a modified form. it would still be necessary to settle by observation the direction of the annual motion, with regard to which theory is powerless to inform us. to save repetition of argument, i must refer to the statement in _a. j._, , pp. , , of the principles adopted in beginning these inquiries in ." finally, he answers one of the few objectors of eminence who still lingered, the great french physicist cornu:-- [sidenote: cornu answered.] "the ground is now cleared for examination of the only topic remaining to be covered, to establish, upon the foundation of fact, every point in the present theory of these remarkable movements of the earth's axis. this is the question of the possibility that these movements are not real, but merely misinterpretations of the observed phenomena; being in whole or in part an illusory effect of instrumental error due to the influence of temperature. such a possibility has been a nightmare in practical astronomy from the first, frightening us in every series of unexplained residuals, brought to light continually in nearly all attempts at delicate instrumental research. a source of danger so subtile could not fail to be ever present in the mind of every astronomer and physicist who has given even a superficial attention to the question of the latitude variations, and there is no doubt that some are even now thus deterred from accepting these variations as proved facts. perhaps the most explicit and forcible statement of the doubts that may arise on this subject has been given very recently by mr. cornu. the views of so distinguished a physicist, and of others who are inclined to agree with him, call for careful attention, and cannot be neglected in the present closing argument upon the theory presented in these articles. it is unnecessary, for the purpose of disposing of objections of the sort raised by cornu, to insist that it is not sufficient to show that the observed variations, attributed to the unsteadiness of the earth's pole, are near the limit of precision attainable in linear differential measures, and in the indication of the direction of gravity by means of the air bubble of the level; or to show that there are known variations in divided circles and in levels, dependent on temperature and seasons. nor need we require of objectors the difficult, although essential, task--which they have not distinctly attempted--of showing that these errors are not eliminated, as they appear to be, by the modes in which astronomers use their instruments. neither need we even urge the fact that a large portion of the data which have been utilised in the present researches on the latitude were derived by methods which dispense with levels, or with circles, a part of them indeed with both, and yet that the results of all are harmonious. on the contrary, let us admit, although merely for argument's sake, that all the known means of determining the direction of gravity--including the plumb-line, the level, and a fluid at rest, whether used for a reflecting surface or as a support for a floating instrument--are subject to a common law of periodical error which vitiates the result of astronomical observation, obtained by whatever methods, and in precisely the same manner. now, the observed law of latitude variation includes two terms, with periods of fourteen and twelve months respectively. since the phases of the first term are repeated at intervals of two months in successive years, and hence in a series of years come into all possible relations to conditions of temperature dependent on season, the argument against the reality of this term, on this ground, absolutely fails, and needs no further notice. as to the second, or annual term, while the phases, as observed in any given longitude, are indeed synchronical with the seasons, they are not so as regards different longitudes. if, therefore, the times of any given phase, as observed in the same latitude, but in successively increasing longitudes, occurred at the same date in all of them, there would be a fatal presumption against the existence of an annual period in the polar motion. if, on the contrary, they occur at times successively corresponding to the differences of longitude, the presumption is equally fatal to the hypothesis that they can possibly be due to temperature variation as affecting instrumental measurement. but the facts given in the foregoing section correspond most distinctly to the latter condition. therefore, unless additional facts can be brought to disprove successively these observed results, we may dismiss for ever the bugbear which has undoubtedly led many to distrust the reality of the annual component of the latitude-variation, while they admit the existence of the -day term." [sidenote: consequences of the discovery.] [sidenote: suspected observers acquitted.] at this point we must leave the fascinating account of the manner in which this great discovery was established, in the teeth of opposition such as might have dismayed and dissuaded a less clear-sighted or courageous man. it is my purpose to lay more stress upon the method of making the discovery than upon its results; but we may afford a brief glance at some of the consequences which have already begun to flow from this step in advance. some of them have indeed already come before us, especially that large class represented by the explanation of anomalies in series of observations which had been put aside as inexplicable. we have seen how the observations made in russia, or in washington, or at greenwich, in all of which there was some puzzling error, were immediately straightened out when chandler applied his new rule to them. we in england have special cause to be grateful to chandler; not only has he demonstrated more clearly than ever the greatness of bradley, but he has rehabilitated pond, the astronomer royal of the beginning of the nineteenth century; showing that his observations, which had been condemned as in some way erroneous, were really far more accurate than might have been expected; and further he has shown that the beautiful instrument designed by airy, and called the reflex zenith tube, which seemed to have unaccountably failed in the purpose for which it was designed, was really all the time accumulating observations of this new phenomenon, the variation of latitude. instead of airy having failed in his design, he had in chandler's words "builded better than he knew." [sidenote: constant of aberration improved.] secondly, there is the modifying influence of this new phenomenon on other phenomena already known, such, for instance, as that of "aberration." we saw in the third chapter how bradley discovered this effect of the velocity of light, and how the measure of it is obtained by comparing the velocity of light with that of the earth. this comparison can be effected in a variety of ways, and we should expect all the results to agree within certain limits; but this agreement was not obtained, and chandler has been able to show one reason why, and to remove some of the more troublesome differences. it is impossible to give here an idea of the far-reaching consequences which such work as this may have; so long as there are differences of this kind we cannot trust any part of the chain of evidence, and there is in prospect the enormous labour of examining each separate link until the error is found. the velocity of light, for instance, may be measured by a terrestrial experiment; was there anything wrong in the apparatus? the velocity of the earth in its journey round the sun depends directly upon the distance of the sun: have we measured this distance wrongly, and if so what was the error in the observations made? these are some of the questions which may arise so long as the values for the _constant of aberration_ are still conflicting; but it requires considerable knowledge of astronomy to appreciate them fully. [sidenote: latitude variation tide.] [sidenote: earthquakes.] another example will, perhaps, be of more general interest. if the axis of the earth is executing small oscillations of this kind, there should be an effect upon the tides; the liquid ocean should feel the wobble of the earth's axis in some way; and an examination of tidal registers showed that there was in fact a distinct effect. it may cause some amusement when i say that the rise and fall are only a few inches in any case; but they are unmistakable evidences that the earth is not spinning smoothly, but has this kind of unbalanced vibration, which i have compared to the vibrations felt by passengers on an imperfectly engineered twin-screw steamer. a more sensational effect is that apparently earthquakes are more numerous at the time when the vibration is greatest. we remarked that the vibration waxes and wanes, much as that of the steamer waxes and wanes if the twin-screws are not running quite together. now the passengers on the steamer would be prepared to find that breakages would be more numerous during the times of vigorous oscillation; and it seems probable that in a similar way the little cracks of the earth's skin which we call great earthquakes are more numerous when these unbalanced vibrations are at their maximum; that is to say, about once every seven years. this result is scarcely yet worthy of complete confidence, for our observations of earthquakes have only very recently been reduced to proper order; but if it should turn out to be true, it is scarcely necessary to add any words of mine to demonstrate the importance of this rather unexpected result of the latitude variation. [sidenote: the kimura phenomenon.] finally i will mention another phenomenon which seems to be at present more of a curiosity than anything else, but which may lead to some future great discovery. it is the outcome of observations which have been recently made to watch these motions of the pole; for although there seems good reason to accept mr. chandler's laws of variation as accurate, it is necessary to establish their accuracy and complete the details by making observations for some time yet to come; and there could be no better proof of this necessity than the discovery recently made by mr. kimura, one of those engaged in this watch of the pole in japan. perhaps i can give the best idea of it by mentioning one possible explanation, which, however, i must caution you may not be by any means the right one. we are accustomed to think of this great earth as being sufficiently constant in shape; if asked, for instance, whether its centre of gravity remains constantly in the same place inside it, we should almost certainly answer in the affirmative, just as only twenty years ago we thought that the north pole remained in the same place. but it seems possible that the centre of gravity moves a few feet backwards and forwards each year--this would at any rate explain certain curious features in the observations to which mr. kimura has drawn attention. whatever the explanation of them may be, or to settle whether this explanation is correct, we want more observations, especially observations in the southern hemisphere; and it is a project under consideration by astronomers at the present moment whether three stations can be established in the southern hemisphere for the further observation of this curious phenomenon. the question resolves itself chiefly into a question of money; indeed, most astronomical projects do ultimately resolve themselves into questions of money; and i fear the world looks upon scientific men as insatiable in this respect. one can only hope that on the whole the money is expended so as to give a satisfactory return. in this instance i have no hesitation in saying that an immediate return of value for a comparatively modest expenditure is practically certain, if only in some way we can get the means of making the observations. it would be natural, at the conclusion of this brief review of some types of astronomical discovery, to summarise the lessons indicated: but there is the important difficulty that there appear to be none. it has been pointed out as we proceeded that what seemed to be a safe deduction from one piece of history has been flatly contradicted by another; no sooner have we learnt that important results may be obtained by pursuing steadily a line of work in spite of the fact that it seems to have become tedious and unprofitable (as in the search for minor planets) than we are confronted with the possibility that by such simple devotion to the day's work we may be losing a great opportunity, as challis did. we can scarcely go wrong in following up the study of residual phenomena in the wake of bradley; but there is the important difficulty that we may be wholly unable to find a clue for the arrangement of our residuals, as is at present largely the case in meteorology. and, in general, human expectations are likely to be quite misleading, as has been shown in the last two chapters; the discoveries we desire may lie in the direction precisely opposite to that indicated by the best opinion at present available. there is no royal road to discovery, and though this statement may meet with such ready acceptance that it seems scarcely worth making, it is hoped that there may be sufficient of interest in the illustrations of its truth. the one positive conclusion which we may derive from the examples studied is that discoveries are seldom made without both hard work and conspicuous ability. a new planet, even as large as uranus, does not reveal itself to a passive observer: thirteen times it may appear to such a one without fear of detection, until at last it encounters an alert herschel, who suspects, tests, and verifies, and even then announces a comet--so little did he realise the whole truth. fifteen years of unrequited labour before astræa was found, nineteen years of observation before the discovery of nutation could be announced: how seldom do these years of toil present themselves to our imaginations when we glibly say that "bradley discovered nutation," or "hencke discovered astræa"! that the necessary labour is so often forgotten must be my excuse for recalling attention to it somewhat persistently in these examples. but beyond the fact that he must work hard, it would seem as though there were little of value to tell the would-be discoverer. the situation has been well summarised by jevons in his chapter on induction in the "principles of science;" and his words will form a fitting conclusion to these chapters:-- "it would seem as if the mind of the great discoverer must combine contradictory attributes. he must be fertile in theories and hypotheses, and yet full of facts and precise results of experience. he must entertain the feeblest analogies, and the merest guesses at truth, and yet he must hold them as worthless till they are verified in experiment. when there are any grounds of probability he must hold tenaciously to an old opinion, and yet he must be prepared at any moment to relinquish it when a clearly contradictory fact is encountered." index aberration, - , , , , , , , , , accidental discovery, , , - adams, , - ; resolution, airy, , - , algiers, alleghenia, almucantar, , alphabet used for planets, anderson, dr. t. c., , , , , anthelm, apollo, argon, ascension, assumption, forgotten, astræa, , , astrographic chart, , , _astronomical journal_, - _astronomische nachrichten_, , astrophil, auwers, ball, sir r., balliol college, banks, sir j., barnard, e. e., , berlin, , , , , berlin star-map, , , , bessel, bettina, , birmingham, "black drop" (in transit of venus), bliss, board of visitors of greenwich observatory, bode, , , , bode's law, , , , , , , , , , bourdeaux, bouvard, , , , , , , bradley, , - , - , , , , bradley, john, bremen, bridstow, , , briggs, brinkley, british association, brünnow, california, cambridge (mass.), , , cambridge observatory, , , , , , , , , cambridge university, - , cape observatory, , , cards, cassini ii., catania, ceres, - chacornac, challis, - , - , , , chandler, s. c., , - chapman's "homer," chicago, chromosphere, clarke, c. c., coelostat, columbus, comet, - , , , , , commission, planetary, common, a. a., , _compte rendu_, comstock, conference, astrographic, - copernicus, , cordoba, , cornu, - corona, - _cosmos_ (humboldt's), delambre, deviation of pole, disc of neptune, , , disc of uranus, - dorpat, doublet (photographic), - draconis, [gamma], - draconis, [beta], driessen, dry plate, dublin, earthquakes, earth's pole, - eccentricity, , eclipses, - edinburgh, eduarda, egeria, endymion, eriphyla, eros, , , , , , eulerian, , evelyn, exposure, times of, , faculæ, faraday, flamsteed, , , fleming, mrs., flora, foulkes, martin, french academy, , , galileo, , galle, , , , , , gasparis, gauge (railways), gauss, - geminorum, h., george iii., , "georgian," _georgium sidus_, , , gill, sir d., , , , gilliss, gotha, gould, graham, , gravitation, law of, , , , , greaves, greenwich observatory, - , , , - , , - , , , , , gregory, , hale, g. e., , hall, a., , halley, - , , - , hansen, , harkness, hartwig, harvard college observatory, , , , hebe, hegel, heidelberg, heliometer, , helium, helsingfors, hencke, , , , , henry brothers, - herschel, sir john, , , herschel, sir william, - , , , , herschel (uranus), , hind, , , , hooke, , hubbard, humboldt, hussey, rev. t. j., , hygeia, ilmata, industria, ingeborg, instruments at greenwich, - iris, , , , janson, jevons, johnson, m., , juno, , , jupiter, , , , , , ; satellites, , keats, - , , keill, , , , kelvin, lord, , kepler, , kew, , , , kiel, kimura, königsberg, küstner, , , lalande, , , , lameia, laplace, la plata, latitude variation, , , , , - lemonnier, , , le verrier, , - libussa, lick observatory, _liouville's journal_, lisbon, longitude of, london, , , long, longitude, , lowth, bishop, lyrae, [alpha], , macclesfield, earl of, , mädler, magnetic observations, , , magnitude equation, markree, mars, , , , , , mayer, measurement of plates, - _mécanique céleste_, melbourne, , memorandum (adams), mercury, messier, meteorites, meteors (november), metis, , micrometer, , milky way, minerva, minor planets, - minor planets tables, , , mistakes, - molyneux, samuel, - , , monte video, moon, tables of, names of minor planets, - nasmyth, "nautical almanac," nebula, , - neptune, , , - , new college lane, newcomb, simon, , , , - , , new stars, , - newton, , , - , , new york, longitude, ninina, northleach, northumberland, nova geminorum, , , nova persei, , - nutation, , , , , , , , _observatory_ (magazine), ocllo, olbers, - olympic games, oriani, ornamenta, oxford university, - , , - oxford university observatory, , , , , , , palermo, observatory of, palisa, pallas, , , parallax, , , - , , paris, parkhurst, j. a., parthenope, peirce, , - pendulum, perseus, , personal equation, , , , perth, perturbations of uranus, , , , , , , peters, , phaëtusa, philosopher, , _philosophical transactions_, , , photographica, photographic methods, , , , - ; lenses, , photographs of sun, , - piazzi, - , pickering, e. c., , pittsburghia, plana, planetary distances, ; commission, ; numbering, planets by photography, pole star (_polaris_), , , , pond, , potsdam, , pound, mrs., , - pound, rev. james, - , , prague, precession, , prymno, puiseux, pulfrich, pulkowa, - , quadrants at greenwich, radium, radius vector, - , - , , rayleigh, lord, records before discovery, reflector, , , reflex zenith tube, , refraction, , - , refractor, , réseau, residual phenomena, - , , , rigaud, s. p., , , rome, rothschild, royal astronomical society, , , , , , , royal society, , , , , sampson, r. a., - , san fernando, santiago, sappho, , saturn, , , , , savile, sir h., savilian professorship, - , - schmidt, julius, , schuster, a., schwabe, - , , sheldonian theatre, sherbourn, solar eclipse, , - spectro-heliograph, , star-maps, , , , "star-trap," stereo-comparator, stone, e. j., struve, , , sun's distance, - sun-spots, - sydney observatory, tacubaya observatory, telescopes, , - thames river, themistocles, _theoria motus_, theory and observation, thomson, sir w., , tides, titius, toulouse observatory, tycho brahé, , , uranus, - , , - , , variable stars, variation of latitude, , , , , - venus, , ; diameter of, ; transit of, - , vesta, , victoria, , , , von zach, wallace, wansted, - , , , , , ward, washington observatory, - , , , weather and sun-spots, , - weyer, whiteside, williams, mrs. e., , wind-vane, revolutions, - winnecke, wolf, dr. max, wolf, rudolf, wren, sir c., yerkes observatory, , , , , , zeiss, zodiac, , , the end printed by ballantyne, hanson & co. edinburgh & london mr. edward arnold's list of scientific and technical books. =lectures on diseases of children.= by robert hutchison, m.d. 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and f. c. clarke, a.r.c.sc., b.sc. =the evolution theory.= by august weismann, professor of zoology in the university of freiburg-im-breisgau. translated by professor j. arthur thomson. with numerous illustrations and coloured plates. two vols. royal vo., s. net. the importance of this work is twofold. in the first place, it sums up the teaching of one of darwin's greatest successors, who has been for many years a leader in biological progress. as professor weismann has from time to time during the last quarter of a century frankly altered some of his positions, this deliberate summing up of his mature conclusions is very valuable. in the second place, as the volumes discuss all the chief problems of organic evolution, they form a reliable guide to the whole subject, and may be regarded as furnishing--what is much needed--a text-book of evolution theory. =animal behaviour.= by c. lloyd morgan, ll.d., f.r.s., principal of university college bristol, author of 'animal life and intelligence,' etc. with numerous illustrations. large crown vo., s. d. =habit and instinct.= by c. lloyd morgan, ll.d., f.r.s. with photogravure frontispiece. viii + pages. demy vo., cloth, s. =a text-book of zoology.= by g. p. mudge, a.r.c.sc. lond., lecturer on biology at the london school of medicine for women, and the polytechnic institute, regent street. with about original illustrations. crown vo., cloth, s. d. =elementary natural philosophy.= by alfred earl, m.a., assistant master at tonbridge school. with numerous illustrations and diagrams. crown vo., cloth, s. d. =a class-book of botany.= by g. p. mudge, a.r.c.sc. lond., f.z.s., and a. j. maslen, f.l.s., lecturer on botany at the woolwich polytechnic. with over illustrations. crown vo., s. d. =the becquerel rays and the properties of radium.= by the hon. r. j. strutt, fellow of trinity college, cambridge. with diagrams. demy vo. s. d. net. =a manual of alcoholic fermentation and the allied industries.= by charles g. matthews, f.i.c., f.c.s., etc. fully illustrated. crown vo., cloth, s. d. net. =wood.= a manual of the natural history and industrial applications of the timbers of commerce. by g. s. boulger, f.l.s., f.g.s. fully illustrated. crown vo., s. d. net. =psychology for teachers.= by c. lloyd morgan, ll.d., f.r.s. xii + pages. crown vo., s. d. =animal sketches.= by c. lloyd morgan, ll.d., f.r.s. viii + pages, with illustrations (many of them full-page). crown vo., cloth, s. d. _london: edward arnold, & maddox street, w._ footnotes: [ ] the inferior planet venus comes closer, but is not visible throughout the night. [ ] the facts were collected with great care and ability by s. p. rigaud, and published by the oxford university press in as "miscellaneous works and correspondence of the rev. james bradley." [ ] since the light must travel from the sun to saturn _and back again to the earth_, the interval would be more nearly minutes. [ ] monthly notices of the royal astronomical society, vol. xvii. p. . [ ] this should be cambridge, _mass._ [ ] the distances do not represent the _total_ displacement, but only the displacement towards washington in one case and towards pulkowa in the other. transcriber's notes: passages in italics are indicated by _italics_. passages in bold are indicated by =bold=. subscripted letters are indicated by {subscript}. the original text includes the greek a, b, and g. for this text version these letters are presented as [alpha], [beta], and [gamma]. all side notes belonging to a single paragraph have been moved to the beginning of the paragraph. sidenotes split across pages have been joined together. punctuation has been corrected without note. corrections in the "errata" have been made in this text version. the following misprints have been corrected: "hencke'" corrected to "hencke's" (page sidenote) "annouced" corrected to "announced" (page sidenote) "are are" corrected to "are" (page ) "konigsberg" corrected to "königsberg" (index) other than the corrections listed above, inconsistencies in spelling and hyphenation have been retained from the original. the children's book of stars the children's book of stars mitton a.&c. black [illustration: the moon-child must keep on running round her. p. .] the children's book of stars +-------------------------------------------------+ | by the same author | | | | children's book of london | | | | containing full-page illustrations | | in colour by john williamson | | price = s.= | | | | 'the stories are told in a way that is bound | | to rivet attention, and the historical sketches| | will leave a lasting impression on the minds | | of young readers which will be very useful | | when their studies in history become more | | advanced.'--_scotsman._ | | | | | | animal autobiographies | | | | the dog | | | | with full-page illustrations in | | colour by j. williamson | | | | price = s.= | | | | 'a true life history, written "out of the | | fulness of first-hand knowledge" by an author | | who is thoroughly acquainted with all the | | ways of "the friend of man."'--_glasgow | | herald._ | | | | 'the story is admirably told in clear and | | fascinating language.'--_freeman's journal._ | | | | a. & c. black. soho square. london, w. | | | +-------------------------------------------------+ agents america the macmillan company & fifth avenue, new york canada the macmillan company of canada, ltd. richmond street west, toronto india macmillan & company, ltd. macmillan building, bombay bow bazaar street, calcutta the children's book of stars by g. e. mitton author of 'the children's book of london,' 'animal autobiographies: the dog,' etc. london adam and charles black _published september, _ preface it was the intention of the late agnes clerke to write the preface to this 'children's book of stars.' miss clerke took a warm and sympathetic interest in the authoress and her work, but her lamented death occurred before this kindly intention could be fulfilled. i cannot pretend to write adequately as her substitute, but i could not resist the appeal made to me by the author, in the name and for the sake of her dear friend and mine, to write a few words of introduction. i am in no way responsible either for the plan or for any portion of this work, but i can commend it as a book, written in a simple and pleasant style, calculated to awaken the interest of intelligent children, and to enable parents otherwise ignorant or astronomy to answer many of those puzzling questions which such children often put. david gill. author's note this little work is the outcome of many suggestions on the part of friends who were anxious to teach their small children something of the marvels of the heavens, but found it exceedingly difficult to get hold of a book wherein the intense fascination of the subject was not lost in conventional phraseology--a book in which the stupendous facts were stated in language simple enough to be read aloud to a child without paraphrase. whatever merit there may be in the present work is due entirely to my friend agnes clerke, the well-known writer on astronomy; the faults are all my own. she gave me the impetus to begin by her warm encouragement, and she helped me to continue by hearing every chapter read as it was written, and by discussing its successor and making suggestions for it. thus she heard the whole book in ms. a week after the last chapter had been read to her i started on a journey lasting many months, and while i was in the far east the news reached me of her death, by which the world is the poorer. for her sake, as he has stated, her friend sir david gill, k.c.b., kindly undertook to supply the missing preface. g. e. mitton. contents chapter i page the earth chapter ii hanging in space chapter iii the shining moon chapter iv the earth's brothers and sister chapter v four small worlds chapter vi four large worlds chapter vii the sun chapter viii shining visitors chapter ix shooting stars and fiery balls chapter x the glittering heavens chapter xi the constellations chapter xii what the stars are made of chapter xiii restless stars chapter xiv the colours of the stars chapter xv temporary and variable stars chapter xvi star clusters and nebulÆ illustrations printed in colour the moon-child must keep on running round her _frontispiece_ facing page the earth and moon hanging in space the english summer and winter jupiter and one of his moons the planet saturn and two of his moons flames from the sun the comet in the bayeux tapestry a stick thrust into the water appears crooked constellations near the pole star orion and his neighbours the spectrum of the sun and sirius illustrations in black and white page the moon _facing_ an eclipse of the moon an eclipse of the sun the moon raising the tides comparative sizes of the planets different phases of venus orbits of mars, the earth, venus, and mercury map of mars _facing_ orbits of the earth and mars jupiter and his principal moons sun-spots _facing_ a great comet " the great nebula in andromeda " the children's book of stars chapter i the earth it is a curious fact that when we are used to things, we often do not notice them, and things which we do every day cease to attract our attention. we find an instance of this in the curious change that comes over objects the further they are removed from us. they grow smaller and smaller, so that at a distance a grown-up person looks no larger than a doll; and a short stick planted in the ground only a few feet away appears as long as a much longer one at ten times the distance. this process is going on all round us every minute: houses, trees, buildings, animals, all seem larger or smaller in proportion to their distance from us. sometimes i have seen a row of raindrops hanging on a bar by the window. when the sun catches one of them, it shines so brilliantly that it is as dazzling as a star; but my sense tells me it is a raindrop, and not a star at all. it is only because it is so near it seems as bright and important as a mighty star very, very far away. we are so much accustomed to this fact that we get into a habit of judging the distance of things by their size. if we see two lights shining on a dark night, and one is much larger than the other, we think that the bright one must be nearer to us; yet it need not necessarily be so, for the two lights might possibly be at the same distance from us, and one be large and the other small. there is no way in which we can tell the truth by just looking at them. now, if we go out on any fine moonlight night and look up at the sky, we shall see one object there apparently much larger than any other, and that is the moon, so the question that occurs to us at once is, is the moon really very much larger than any of the stars, or does it only seem so because it is very much nearer to us? as a matter of fact, the moon is one of the smallest objects in view, only, as it is our nearest neighbour, it appears very conspicuous. having learned this, we shall probably look about to see what else there is to attract attention, and we may notice one star shining very brilliantly, almost like a little lamp, rather low down in the sky, in that part of it where the sun has lately set. it is so beautifully bright that it makes all the others look insignificant in comparison, yet it is not really large compared with the others, only, as it comes nearer to us than anything else in the sky except the moon, it looks larger than it has any right to do in comparison with the others. after this we might jump to the conclusion that all the bright large stars are really small and near to us, and all the faintly shining ones large and far away. but that would not be true at all, for some bright ones are very far away and some faint ones comparatively near, so that all we can do is to learn about them from the people who have studied them and found out about them, and then we shall know of our own knowledge which of them seem bright only because they are nearer than the others, and which are really very, very brilliant, and so still shine brightly, though set in space at an almost infinite distance from us. the sun, as we all know, appears to cross the sky every day; he gets up in the east and drops down in the west, and the moon does the same, only the moon is unlike the sun in this, that it changes its shape continually. we see a crescent moon growing every night larger and larger, until it becomes full and fat and round, and then it grows thinner and thinner, until it dies away; and after a little while it begins again, and goes through all the same changes once more. i will tell you why this is so further on, when we have a chapter all about the moon. if you watch the stars quietly for at least five minutes, you will see that they too are moving steadily on in the same way as the sun and moon. watch one bright star coming out from behind a chimney-pot, and after about five minutes you will see that it has changed its place. yet this is not true of all, for if we watch carefully we shall find that some, fairly high up in the sky, do not appear to move at all. the few which are moving so slowly that they seem to us to stand still are at a part of the sky close to the pole star, so called because it is always above the north pole of the earth. i will explain to you how to find it in the sky for yourselves later on, but now you can ask anyone to point it out. watch it. it appears to be fixed in one place, while the other stars are swinging round it in circles. in fact, it is as if we on the earth were inside a great hollow globe or ball, which continually turned round, with the pole star near the top of the globe; and you know that if you put your finger on the spot at the top of a spinning globe or ball, you can hold it there while all the rest of the ball runs round. now, if you had to explain things to yourself, you would naturally think: 'here is the great solid earth standing still, and the sun and moon go round it; the stars are all turning round it too, just as if they were fixed on to the inside of a hollow globe; we on the earth are in the middle looking up at them; and this great globe is slowly wheeling round us night by night.' in the childhood of the world men believed that this was really true--that the earth was the centre of the universe, that the sun and moon and all the hosts of heaven were there solely to light and benefit us; but as the world grew wiser the wonders of creation were fathomed little by little. some men devoted their whole lives to watching the heavens, and the real state of things was gradually revealed to them. the first great discovery was that of the daily movement of the earth, its rotation on its own axis, which makes it appear as if all these shining things went round it. it is indeed a very difficult matter to judge which of two objects is moving unless we can compare them both with something outside. you must have noticed this when you are sitting in a train at a station, and there is another train on the other side of yours. for if one of the trains moves gently, either yours or the other, you cannot tell which one it is unless you look at the station platform; and if your position remains the same in regard to that, you know that your train is still standing, while the other one beside it has begun to move. and i am quite sure that there is no one of us who has not, at one time or another, stood on a bridge and watched the water running away underneath until we felt quite dizzy, and it seemed as if the water were standing still and the bridge, with ourselves on it, was flying swiftly away backwards. it is only when we turn to the banks and find them standing still, that we realize the bridge is not moving, and that it is the running water that makes it seem to do so. these everyday instances show us how difficult it is to judge whether we are moving or an outside object unless we have something else to compare with it. and the marvellous truth is that, instead of the sun and moon and stars rolling round the earth, it is the earth that is spinning round day by day, while the sun and the stars are comparatively still; and, though the moon does move, yet when we see her get up in the east and go down in the west that is due to our own movement and not to hers. the earth turns completely round once in a day and night. if you take an orange and stick a knitting-needle through it, and hold it so that the needle is not quite straight up but a little slanting, and then twirl it round, you will get quite a good idea of the earth, though of course there is no great pole like a gigantic needle stuck through it, that is only to make it easy for you to hold it by. in spinning the orange you are turning it as the earth turns day by day, or, as astronomers express it, as it rotates on its axis. there is a story of a cruel eastern king who told a prisoner that he must die if he did not answer three questions correctly, and the questions were very difficult; this is one of them: 'how long would it take a man to go round the earth if he never stopped to eat or drink on the way?' and the prisoner answered promptly: 'if he rose with the sun and kept pace with it all day, and never stopped for a moment to eat or drink, he would take just twenty-four hours, your royal highness.' for in those days it was supposed that the sun went round the earth. everyone is so remarkably clever nowadays that i am sure there will be someone clever enough to object that, if what i have said is true, there would be a great draught, for the air would be rushing past us. but, as a matter of fact, the air goes with us too. if you are inside a railway carriage with the windows shut you do not feel the rush of air, because the air in the carriage travels with you; and it is the same thing on the earth. the air which surrounds the earth clings to it and goes round with it, so there is no continuous breeze from this cause. but the spinning round on its own axis is not the earth's only movement, for all the time it is also moving on round the sun, and once in a whole year it completes its journey and comes back to the place from whence it started. thus the turning round like a top or rotating on its axis makes the day and night, and the going in a great ring or revolving round the sun makes the years. our time is divided into other sections besides days and years. we have, for instance, weeks and months. the weeks have nothing to do with the earth's movements; they are only made by man to break up the months; but the months are really decided by something over which we have no control. they are due to the moon, and, as i have said already, the moon must have a chapter to herself, so we won't say any more about the months here. if any friend of ours goes to india or new zealand or america, we look upon him as a great traveller; yet every baby who has lived one year on the earth has travelled millions of miles without the slightest effort. every day of our lives we are all flung through space without knowing it or thinking of it. it is as if we were all shut up in a comfortable travelling car, and were provided with so many books and pictures and companions that we never cared to look out of the windows, so that hour by hour as we were carried along over miles of space we never gave them a thought. even the most wonderful car ever made by man rumbles and creaks and shakes, so that we cannot help knowing it is moving; but this beautiful travelling carriage of ours called the earth makes never a creak or groan as she spins in her age-long journey. it is always astonishing to me that so few people care to look out of the window as we fly along; most of them are far too much absorbed in their little petty daily concerns ever to lift their eyes from them. it is true that sometimes the blinds are down, for the sky is thickly covered with clouds, and we cannot see anything even if we want to. it is true also that we cannot see much of the scenery in the daytime, for the sun shining on the air makes a veil of blue glory, which hides the stars; but on clear nights we can see on every side numbers of stars quite as interesting and beautiful as any landscape; and yet millions of people never look up, never give a thought to the wonderful scenery through which their car is rushing. by reason of the onward rush of the earth in space we are carried over a distance of at least eighteen miles every second. think of it: as we draw a breath we are eighteen miles away in space from the point we were at before, and this goes on unceasingly day and night. these astonishing facts make us feel how small and feeble we are, but we can take comfort in the thought that though our bodies are insignificant, the brain of man, which has discovered these startling facts, must in itself be regarded as one of the most marvellous of all the mysteries amid which we live. well, we have arrived at some idea of our earth's position; we know that the earth is turning round day by day, and progressing round the sun year by year, and that all around lie the sentinel stars, scattered on a background of infinite space. if you take an older boy or girl and let him or her stand in the middle to represent the sun, then a smaller one would be the earth, and the smallest of all the moon; only in truth we could never get anyone large enough to represent the sun fairly, for the biggest giant that ever lived would be much too small in proportion. the one representing the sun must stand in the middle, and turn slowly round and round. then let the earth-child turn too, and all the time she is spinning like a top she must be also hastening on in a big ring round the sun; but she must not go too fast, for the little moon-child must keep on running round her all the time. and the moon-child must keep her face turned always to the earth, so that the earth never sees her back. that is an odd thing, isn't it? we have never seen the other side of the moon, which goes round us, always presenting the same face to us. the earth is not the only world going round the sun; she has many brothers and a sister; some are nearer to the sun than she is, and some are further away, but all circle round the great central light-giver in rings lying one outside the other. these worlds are called planets, and the earth is one of them, and one of the smaller ones, too, nothing so great and important as we might have imagined. chapter ii hanging in space if you are holding something in your hand and you let it go, what happens? it falls to the ground, of course. now, why should it do so? you will say: 'how could it do anything else?' but that is only because you are hampered by custom. try to shake yourself free, and think, why should it go down instead of up or any other way? the first man who was clever enough to find some sort of an answer to this question was the great philosopher sir isaac newton, though he was not quite the first to be puzzled by it. after years of study he discovered that every thing attracts every other thing in proportion to their masses (which is what you know as weight) and their distance from each other. in more scientific language, we should say every _body_ instead of every _thing_, for the word body does not only mean a living body, but every lump or mass of matter in the universe. the earth is a body in this sense, and so is the table or anything else you could name. now as the earth is immeasurably heavier than anything that is on it, it pulls everything toward itself with such force that the little pulls of other things upon each other are not noticed. the earth draws us all toward it. it is holding us down to it every minute of the day. if we want to move we have to exert another force in order to overcome this attraction of the earth, so we exert our own muscles and lift first one foot and then the other away from the earth, and the effort we make in doing this tires us. all the while you are walking or running you are exercising force to lift your feet away from the ground. the pull of the earth is called gravitation. just remember that, while we go on to something else which is almost as astonishing. we know that nothing here on earth continues to move for ever; everything has to be kept going. anything left to itself has a tendency to stop. why is this? this is because here in the world there is something that fights against the moving thing and tries to stop it, whether it be sent along the ground or thrown up in the air. you know what friction is, of course. if you rub your hands along any rough substance you will quickly feel it, but on a smooth substance you feel it less. that is why if you send a stone spinning along a carpet or a rough road it stops comparatively soon, whereas if you use the same amount of force and send it along a sheet of ice it goes on moving much longer. this kind of resistance, which we call friction, is one of the causes which is at work to bring things to a standstill; and another cause is the resistance of the air, which is friction in another form. it may be a perfectly still day, yet if you are bicycling you are breaking through the air all the time, just as you would be through water in swimming, only the resistance of the air is less than that of water. as the friction or the resistance of the air, or both combined, gradually lessens the pace of the stone you sent off with such force, the gravitation of the earth begins to be felt. when the stone first started the force you gave to it was enough to overcome the gravitation force, but as the stone moves more slowly the earth-pull asserts itself, and the stone drops down to the ground and lies still upon the surface. now, if there were no friction, and therefore no resistance, there would be no reason why anything once set moving should not go on moving for ever. the force you give to any object you throw is enough to overcome gravitation; and it is only when the first force has been diminished by friction that the earth asserts its authority and pulls the moving object toward it. if it were possible to get outside the air and out of reach of the pull of the earth, we might fling a ball off into space, and it would go on in a straight line until something pulled it to itself by the force of gravity. gravitation affects everything connected with the earth; even our air is held to the earth by gravitation. it grows thinner and thinner as we get further away from the earth. at the top of a high mountain the air is so thin that men have difficulty in breathing, and at a certain height they could not breathe at all. as they cannot breathe in very fine air, it is impossible for them to tell by personal experiment exactly where the air ends; but they have tried to find out in other ways, and though different men have come to different conclusions on the subject, it is safe to say that at about two hundred miles above the earth there is nothing that could be called air. thus we can now picture our spinning earth clothed in a garment of air that clings closely about her, and grows thinner and thinner until it melts away altogether, for there is no air in space. [illustration: the earth and moon hanging in space] now in the beginning god made the world, and set it off by a first impulse. we know nothing about the details, though further on you shall hear what is generally supposed to have taken place; we only know that, at some remote age, this world, probably very different from what it is now, together with the other planets, was sent spinning off into space on its age-long journey. these planets were not sent off at random, but must have had some particular connection with each other and with the sun, for they all belong to one system or family, and act and react on each other. now, if they had been at rest and not in movement, they would have fallen right into the sun, drawn by the force of gravitation; then they would have been burned up, and there would have been an end of them. but the first force had imparted to them the impulse to go on in a straight line, so when the sun pulled the result was a movement between the two: the planets did not continue to move in a straight line, neither did they fall on to the sun, but they went on a course between the two--that is, a circle--for the sun never let them get right away from him, but compelled them to move in circles round him. there is a very common instance of this kind of thing which we can see, or perhaps feel, every day. if you try to sit still on a bicycle you tumble off, because the earth pulls you down to itself; but if, by using the force of your own muscles, you give the bicycle a forward movement this resists the earth-pull, and the result is the bicycle runs along the ground. it does not get right away from the earth, not even two or three feet above ground; it is held to the earth, but still it goes forward and does not fall over, for the movement is made up of the earth-pull, which holds it to the ground, and the forward movement, which propels it along. then again, as another instance, if you tie a ball to a string and whirl it round you, so long as you keep on whirling it will not fall to the ground, but the moment you stop down it drops, for there is nothing to fight against the pull of gravitation. thus we can picture the earth and all the planets as if they were swinging round the sun, held by invisible strings. it is the combination of two forces that keeps them in their places--the first force and the sun's pull. it is very wonderful to think of. here we are swinging in space on a ball that seems only large to us because we are so much smaller ourselves; there is nothing above or below it but space, yet it travels on day by day and year by year, held by invisible forces that the brain of man has discovered and measured. of course, every planet gives a pull at every other planet too, but these pulls are so small compared with that of the sun that we need not at present notice them. then we come to another point. we said that every body pulled every other body in proportion to their weights and their distance. now, gravity acts much more strongly when things are near together than when they are far away from each other; so that if a smaller body is near to another somewhat larger than itself, it is pulled by it much more strongly than by a very much larger one at a considerably greater distance. we have an instance of this in the case of the earth and moon: as the earth responds to the pull of the sun, so the moon responds to the pull of the earth. the moon is so comparatively near to the earth that the earth-pull forces her to keep on going round and round, instead of leaving her free to circle round the sun by herself; and yet if you think of it the moon does go round the sun too. recall that game we had when the sun was in the middle, and the two smaller girls, representing the earth and moon, went round it. the moon-child turned round the earth-child, but all the while the earth-child was going round the sun, so that in a year's time the moon had been all round the sun too, only not in a straight line. the moon is something like a dog who keeps on dancing round and round you when you go for a walk. he does go for the walk too, but he does much more than that in the same time. thus we have further completed our idea of our world. we see it now hanging in space, with no visible support, held in its place by two mighty forces; spinning on year after year, attended by its satellite the moon, while we run, and walk, and cry, and laugh, and play about on its surface--little atoms who, except for the brain that god has given them, would never even have known that they are continually moving on through endless space. chapter iii the shining moon 'once upon a time,' long, long ago, the earth was not a compact, round, hard body such as she is now, but much larger and softer, and as she rotated a fragment broke off from her; it did not go right away from her, but still went on circling round with the motion it had inherited from her. as the ages passed on both the earth and this fragment, which had been very hot, cooled down, and in cooling became smaller, so that the distance between them was greater than it had been before they shrank. and there were other causes also that tended to thrust the two further from each other. yet, compared with the other heavenly bodies, they are still near, and by looking up into the sky at night you can generally see this mighty fragment, which is a quarter the diameter of the earth--that is to say, a quarter the width of the earth measured from side to side through the middle. it is--as, of course, you have guessed--the moon. the moon is the nearest body to us in all space, and so vast is the distance that separates us from the stars that we speak as if she were not very far off, yet compared with the size of the earth the space lying between us and her is very great. if you went right round the world at the thickest part--that is to say, in the region of the equator--and when you arrived at your starting-point went off once again, and so on until you had been round ten times, you would only then have travelled about as far as from the earth to the moon! the earth is not the only planet which has a moon, or as it is called, a satellite, in attendance. some of the larger planets have several, but there is not one to compare with our moon. which would you prefer if you had the choice, three or four small moons, some of them not much larger than a very big bright star, or an interesting large body like our own moon? i know which i should say. 'you say that the moon broke off from the earth, so perhaps there may be some people living on her,' i hear someone exclaim. if there is one thing we have found out certainly about the moon, it is that no life, as we know it, could exist there, for there is neither air nor water. whether she ever had any air or water, and if so, why they disappeared, are questions we cannot answer. we only know that now she is a dead world. bright and beautiful as she is, shedding on us a pale, pure light, in vivid contrast with the fiery yellow rays of the sun, yet she is dead and lifeless and still. we can examine her surface with the telescope, and see it all very plainly. even with a large opera-glass those markings which, to the naked eye, seem to be like a queer distorted face are changed, and show up as the shadows of great mountains. we can only see one side of the moon, because as i have said, she keeps always the same face turned to the earth; but as she sways slightly in her orbit, we catch a glimpse of sometimes a little more on one side and sometimes a little more on the other, and so we can judge that the unseen part is very much the same as that turned toward us. at first it is difficult to realize what it means to have no air. besides supporting life in every breath that is drawn by living creatures, the air does numerous other kind offices for us--for instance, it carries sound. supposing the most terrific volcano exploded in an airless world, it could not be heard. the air serves as a screen by day to keep off the burning heat of the sun's rays, and as a blanket by night to keep in the heat and not let it escape too quickly. if there were no air there could be no water, for all water would evaporate and vanish at once. imagine the world deprived of air; then the sun's rays would fall with such fierceness that even the strongest tropical sun we know would be as nothing in comparison with it, and every green thing would shrivel up and die; this scorching sun would shine out of a black sky in which the stars would all be visible in the daytime, not hidden by the soft blue veil of air, as they are now. at night the instant the sun disappeared below the horizon black darkness would set in, for our lingering twilight is due to the reflection of the sun in the upper layers of air, and a bitterness of deathly cold would fall upon the earth--cold fiercer than that of the arctic regions--and everything would be frozen solid. it would need but a short time to reduce the earth to the condition of the moon, where there is nothing to shrivel up, nothing to freeze. her surface is made up of barren, arid rocks, and her scenery consists of icy black shadows and scorching white plains. [illustration: _paris observatory._ the moon.] the black shadows define the mountains, and tremendous mountains they are. most of them have craters. a crater is like a cup, and generally has a little peak in the middle of it. this is the summit of a volcano, and when the volcano has burst up and vomited out floods of lava and débris, this has fallen down in a ring a little distance away from it, leaving a clear space next to the peak, so that, as the mountain ceases vomiting and the lava cools down, the ring hardens and forms a circular ridge. the craters on the moon are immense, not only in proportion to her size, but immense even according to our ideas on the earth. one of the largest craters in our own world is in japan, and this measures seven miles across, while in the moon craters of fifty, sixty, and even a hundred miles are by no means uncommon, though there are also hundreds and thousands of smaller ones. we can see the surface of the moon very plainly with the magnificent telescopes that have now been made, and with the best of these anything the size of a large town would be plainly visible. needless to say, no town ever has been or ever will be seen upon the moon! all these mountains and craters show that at one time the moon must have been convulsed with terrific disturbances, far worse than anything that we have any knowledge of on our earth; but this must have been ages ago, while the moon still probably had an atmosphere of its own. now it has long been quiet. nothing changes there; even the forces that are always at work on the earth--namely, damp and mould and water--altering the surface and breaking up the rocks, do not act there, where there is no moisture of any sort. so far as we can see, the purpose of the moon is to be the servant of the earth, to give her light by night and to raise the tides. beautiful light it is, soft and mysterious--light that children do not often have a chance of seeing, for they are generally in bed before the moon rises when she is at the full. we know that the moon has no heat of her own--she parted with all that long ago; she cannot give us glowing light from brilliant flames, as the sun does; she shines only by the reflection of the sun on her surface, and this is the reason why she appears to change her shape so constantly. she does not really change; the whole round moon is always there, only part of it is in shadow. sometimes you can see the dark part as well as the bright. when there is a crescent moon it looks as if it were encircling the rest; some people call it, 'seeing the old moon in the new moon's arms.' i don't know if you would guess why it is we can see the dark part then, or how it is lighted up. it is by reason of our own shining, for we give light to the moon, as she does to us. the sun's rays strike on the earth, and are reflected on to the moon, so that the moon is lighted by earthshine as we are lighted by moonshine, and it is these reflected earth-rays that light up the dark part of the moon and enable us to see it. what a journey these rays have had! they travel from the sun to the earth, and the earth to the moon, and then back to the earth again! from the moon the earth must appear a much bigger and more glorious spectacle than she does to us--four times wider across and probably brighter--for the sun's light strikes often on our clouds, which shine more brilliantly than her surface. once again we must use an illustration to explain the subject. set a lamp in the middle of a dark room, and let that be the sun, then take a small ball to represent the earth and a smaller one for the moon. place the moon-ball between the lamp and the earth-ball. you will see that the side turned to the earth-ball is dark, but if you move the moon to one side of the earth, then from the earth half of it appears light and half dark; if you put it right away from the lamp, on the outer side of the earth, it is all gloriously lit up, unless it happens to be exactly behind the earth, when the earth's shadow will darken it. this is the full explanation of all the changes of the moon. [illustration: an eclipse of the moon.] does it ever fall within the earth's shadow? yes, it does; for as it passes round the earth it is not always at the same level, but sometimes a little higher and sometimes a little lower, and when it chances to pass exactly behind it enters the shadow and disappears. that is what we call an eclipse of the moon. it is nothing more than the earth's shadow thrown on to the moon, and as the shadow is round that is one of the proofs that the earth is round too. but there is another kind of eclipse--the eclipse of the sun; and this is caused by the moon herself. for when she is nearest to the sun, at new moon--that is to say, when her dark side is toward us, and she happens to get exactly between us and the sun--she shuts out the face of the sun from us; for though she is tiny compared with him, she is so much nearer to us that she appears almost the same size, and can blot him right out. thus the eclipses of both sun and moon are not difficult to understand: that of the moon can only happen at full moon, when she is furthest from the sun, and it is caused by the earth's shadow falling upon the moon; and that of the sun at new moon, when she is nearest to him, and it is caused by the solid body of the moon coming between us and the sun. [illustration: an eclipse of the sun.] besides giving us light by night, the moon serves other important purposes, and the most important of all is the raising of the tides. without the rising of the sea twice in every day and night our coasts would become foul and unwholesome, for all the dead fish and rotting stuff lying on the beach would poison the air. the sea tides scour our coasts day by day with never-ceasing energy, and they send a great breath of freshness up our large rivers to delight many people far inland. the moon does most of this work, though she is a little helped by the sun. the reason of this is that the moon is so near to the earth that, though her pull is a comparatively small one, it is very strongly felt. she cannot displace the actual surface to any great extent, as it is so solid; but when it comes to the water she can and does displace that, so that the water rises up in answer to her pull, and as the earth turns round the raised-up water lags behind, reaching backward toward the moon, and is drawn up on the beach, and makes high tide. but it is stopped there, and meantime, by reason of the earth's movement, the moon is left far behind, and pulls the water to itself further on, when the first high tide relapses and falls down again. at length the moon gets round to quite the opposite side of the earth to that where she began, and there she makes a high tide too; but as she draws the water to herself she draws also the solid earth beneath the water to her in some degree, and so pulls it away from the place where the first high tide occurred, leaving the water there deeper than before, and so causing a secondary high tide. [illustration: the moon raising the tides.] the sun has some influence on the tides too, and when moon and sun are in the same line, as at full and new moon, then the tides are highest, and are called spring tides; but when they pull in different directions, as when it is half-moon, then the tides are lowest and are called neap tides. chapter iv the earth's brothers and sister the earth is not the only world that, poised in space, swings around the sun. it is one of a family called the solar system, which means the system controlled and governed by the sun. when we look up at the glorious sky, star-studded night by night, it might seem to us that the stars move only by reason of the earth's rotation; but when men first began to study the heavens attentively--and this is so long ago that the record of it is not to be found--they noticed that, while every shining object in the sky was apparently moving round us, there were a few which also had another movement, a proper motion of their own, like the moon. these curious stars, which appeared to wander about among the other stars, they called planets, or wanderers. and the reason, which was presently discovered, of our being able to see these movements was that these planets are very much nearer to us than any of the real stars, and in fact form part of our own solar system, while the stars are at immeasurable distances away. of all the objects in the heavens the planets are the most intensely interesting to us; for though removed from us by millions of miles, the far-reaching telescope brings some of them within such range that we can see their surfaces and discover their movements in a way quite impossible with the stars. and here, if anywhere, might we expect to find traces of other living beings like ourselves; for, after all the earth is but a planet, not a very large nor a very small one, and in no very striking position compared with the other planets; and thus, arguing by what seems common-sense, we say, if this one planet has living beings on its surface, may not the other planets prove to be homes for living beings also? counting our own earth, there are eight of these worlds in our solar system, and also a number of tiny planets, called asteroids; these likewise go round the sun, but are very much smaller than any of the first eight, and stand in a class by themselves, so that when the planets are mentioned it is generally the eight large well-known planets which are referred to. if we go back for a moment to the illustration of the large lamp representing our sun, we shall now be able to fill in the picture with much more detail. the orbits of the planets, as their paths round the sun are called, lie like great circles one outside another at various distances, and do not touch or cut each other. where do you suppose our own place to be? will it be the nearest to the sun or the furthest away from him? as a matter of fact, it is neither, we come third in order from the sun, for two smaller planets, one very small and the other nearly as large as the earth, circle round and round the sun in orbits lying inside ours. now if we want to place objects around our lamp-sun which will represent these planets in size, and to put them in places corresponding to their real positions, we should find no room large enough to give us the space we ought to have. we must take the lamp out into a great open field, where we shall not be limited by walls. then the smallest planet, named mercury, which lies nearest of all to the sun, would have to be represented by a pea comparatively close to the sun; venus, the next, would be a greengage plum, and would be about twice as far away; then would come the earth, a slightly larger plum, about half as far again as venus. after this there would be a lesser planet, called mars, like a marble. these are the first four, all comparatively small; beyond them there is a vast gap, in which we find the asteroids, and after this we come to four larger planets, mighty indeed as regards ourselves, for if our earth were a greengage plum, the first of these, jupiter, would have to be the size of a football at least, and the next, saturn, a smaller football, while uranus and neptune, the two furthest out, would be about the size of the toy balloons children play with. the outermost one, neptune, would be thirty times as far from the sun as we are. [illustration: comparative sizes of the planets.] this is the solar system, and in it the only thing that shines by its own light is the sun; all the rest, the planets and their moons, shine only because the rays of light from the sun strike on their surfaces and are reflected off again. our earth shines like that, and from the nearer planets must appear as a brilliant star. the little solar system is separated by distances beyond the realm of thought from the rest of the universe. vast as are the intervals between ourselves and our planetary neighbours, they are as nothing to the space that separates us from the nearest of the steady shining fixed stars. why, removed as far from us as the stars, the sun himself would have sunk to a point of light; and as for the planets, the largest of them, jupiter, could not possibly be seen. thus, when we look at those stars across the great gulf of space, we know that though we see them they cannot see us, and that to them our sun must seem only a star; consequently we argue that perhaps these stars themselves are suns with families of planets attached to them; and though there are reasons for thinking that this is not the case with all, it may be with some. now if, after learning this, we look again at the sky, we do so with very different eyes, for we realize that some of these shining bodies are like ourselves in many things, and are shining only with a light borrowed from the sun, while others are mighty glowing suns themselves, shining by their own light, some greater and brighter, some less than our sun. the next thing to do is to learn which are stars and which are planets. of the planets you will soon learn to pick out one or two, and will recognize them even if they do change their places--for instance, venus is at times very conspicuous, shining as an evening star in the west soon after the sun goes down, or us a morning star before he gets up, though you are not so likely to see her then; anyway, she is never found very far from the sun. jupiter is the only other planet that compares with her in brilliancy, and he shines most beautifully. he is, of course, much further away from us than venus, but so much larger that he rivals her in brightness. saturn can be quite easily seen as a conspicuous object, too, if you know where to look for him, and mars is sometimes very bright with a reddish glow. the others you would not be able to distinguish. it is to our earth's family of these eight large planets going steadily round the same sun that we must give our attention first, before going on to the distant stars. many of the planets are accompanied by satellites or moons, which circle round them. we may say that the sun is our parent--father, mother, what you will--and that the planets are the family of children, and that the moons are _their_ children. our earth, you see, has only one child, but that a very fine one, of which she may well be proud. when i say that the planets go round the sun in circles i am only speaking generally; as a matter of fact, the orbits of the planets are not perfect circles, though some are more circular than others. instead of this they are as a circle might look if it were pressed in from two sides, and this is called an ellipse. the path of our own earth round the sun is one of the most nearly circular of them all, and yet even in her orbit she is a good deal nearer to the sun at one time than another. would you be surprised to hear that she is nearer in our winter and further away in our summer? yet that is the case. and for the first moment it seems absurd; for what then makes the summer hotter than the winter? that is due to an altogether different cause; it depends on the position of the earth's axis. if that axis were quite straight up and down in reference to the earth's path round the sun we should have equal days and nights all the year round, but it is not; it leans over a little, so that at one time the north pole points towards the sun and at another time away from it, while the south pole is pointing first away from it and then toward it in exactly the reverse way. when the north pole points to the sun we in the northern hemisphere have our summer. to understand this you must look at the picture, which will make it much clearer than any words of mine can do. the dark part is the night, and the light part the day. when we are having summer any particular spot on the northern hemisphere has quite a long way to travel in the light, and only a very short bit in the dark, and the further north you go the longer the day and shorter the night, until right up near the north pole, within the arctic circle, it is daylight all the time. you have, perhaps, heard of the 'midnight sun' that people go to see in the north, and what the expression means is that at what should be midnight the sun is still there. he seems just to circle round the horizon, never very far above, but never dipping below it. when the sun is high overhead, his rays strike down with much more force than when he is low. it is, for instance, hotter at mid-day than in the evening. now, when the north pole is bowed toward the sun, the sun appears to us to be higher in the sky. in the british isles he never climbs quite to the zenith, as we call the point straight above our heads; he always keeps on the southern side of that, so that our shadows are thrown northward at mid-day, but yet he gets nearer to it than he does in winter. look at the picture of the earth as it is in winter. then we have long nights and short days, and the sun never appears to climb very high, because we are turned away from him. during the short days we do not receive a great deal of heat, and during the long night the heat we have received has time to evaporate to a great extent. these two reasons--the greater or less height of the sun in the sky and the length of the days--are quite enough to account for the difference between our summer and winter. there is one rather interesting point to remember, and that is that in the northern hemisphere, whether it is winter or summer, the sun is south at mid-day, so that you can always find the north then, for your shadow will point northwards. [illustration: the english summer (left) and winter (right).] new zealand and australia and other countries placed in the southern hemisphere, as we are in the northern, have their summer while we have winter, and winter while we have summer, and their summer is warmer than ours, because it comes when the earth in its journey is three million miles nearer to the sun than in our summer. all this seems to refer to the earth alone, and this chapter should be about the planets; but, after all, what applies to one planet applies to another in some degree, and we can turn to the others with much more interest now to see if their axes are bowed toward the sun as ours is. it is believed that in the case of mercury, in regard to its path round the sun, the axis is straight up and down; if it is the changes of the seasons must depend on the nearness of mercury to the sun and nothing else, and as he is a great deal nearer at one time than another, this might make a very considerable difference. some of the planets are like the earth in regard to the position of their axes, but the two outermost ones, uranus and neptune, are very peculiar, for one pole is turned right toward the sun and the other right away from it, so that in one hemisphere there is continuous day all the summer, in the other there is continuous night, and then the process is reversed. but these little peculiarities we shall have to note more particularly in the account of the planets separately. there is a curious fact in regard to the distances of the planets from the sun. each one, after the first, is, very roughly, about double the distance from the sun of the one inside it. this holds good for all the first four, then there is a great gap where we might expect to find another planet, after which follow the four large planets. now, this gap puzzled astronomers greatly; for though there seemed to be no reason why the planets should be at regular distances one outside the other, yet there the fact was, and that the series should be broken by a missing planet was annoying. so very careful search was made, and a thrill of excitement went all through the scientific world when it was known that a tiny planet had been discovered in the right place. but this was not the end of it, for within a few years three or four more tiny planets were observed not far from the first one, and, as years rolled on, one after another was discovered until now the number amounts to over six hundred and others are perpetually being added to the list! here was a new feature in the solar system, a band of tiny planets not one of which was to be compared in size with the least of those already known. the largest may be about as large as europe, and others perhaps about the size of wales, while there may be many that have only a few square miles of surface altogether, and are too small for us to see. to account for this strange discovery many theories were advanced. one was that there had been a planet--it might be about the size of mars--which had burst up in a great explosion, and that these were the pieces--a very interesting and exciting idea, but one which proved to be impossible. the explanation now generally accepted is a little complicated, and to understand it we must go back for a bit. when we were talking of the earth and the moon we realized that once long ago the moon must have been a part of the earth, at a time when the earth was much larger and softer than she now is; to put it in the correct way, we should say when she was less dense. there is no need to explain the word 'dense,' for in its ordinary sense we use it every day, but in an astronomical sense it does not mean exactly the same thing. everything is made up of minute particles or atoms, and when these atoms are not very close together the body they compose is loose in texture, while if they are closer together the body is firmer. for instance, air is less dense than water, and water than earth, and earth than steel. you see at once by this that the more density a thing has the heavier it is; for as a body is attracted to another body by every atom or particle in it, so if it has more particles it will be more strongly attracted. thus on the earth the denser things are really heavier. but 'weight' is only a word we use in connection with the earth; it means the earth's pulling power toward any particular thing at the surface, and if we were right out in space away from the earth, the pulling power of the earth would be less, and so the weight would be less; and as it would be impossible always to state just how far away a thing was from the earth, astronomers talk about density, which means the number of particles a body contains in proportion to other bodies. thus the planet jupiter is very much larger than the earth, but his density is less. that does not mean to say that if jupiter were in one scale and the earth in the other he would weigh less, because he is so very much bigger he would outweigh the earth still; his total _mass_ would be greater than that of the earth, but it means that a piece of jupiter the same size as a piece of the earth would weigh less under the same conditions. now, before there were any planets at all or any sun, in the place of our solar system was a vast gaseous cloud called a nebula, which slowly rotated, and this rotation was the first impulse or force which god gave it. it was not at all dense, and as it rotated a part broke off, and inheriting the first impulse, went on rotating too. the impulse would have sent it off in a straight line, but the pull of gravity from the nebula held it in place, and it circled round; then the nebula, as it rotated, contracted a little, and occupied less space and grew denser, and presently a second piece was thrown off, to become in time another planet. the same process was repeated with saturn, and then with the huge jupiter. the nebula was always rotating and always contracting. and as it behaved, so did the planets in their turn; they spun round and cooled and contracted, and the moons were flung off from them, just as they--the planets--had been flung off from the parent nebula. now, after the original nebula had parted with the mighty mass of jupiter, it never again made an effort so great, and for a long time the fragments that were detached were so small as hardly to be worth calling planets; they were the asteroids, little lumps and fragments that the nebula left behind. but as it still contracted in time there came mars; and having recovered a little, the nebula with more energy got rid of the earth, and next venus, and lastly little mercury, the smallest of the eight planets. then it contracted further, and perhaps you can guess what the remainder of it is--the sun; and by spinning in a plastic state the sun, like the earth, has become a globe, round and comparatively smooth; and its density is now too great to allow of its losing any more fragments, so, as far as we can see, the solar system is complete. this theory of the origin of the planets is called the nebula theory. we cannot prove it, but there are so many facts that can only be explained by it, we have strong reason for believing that something of the kind must have happened. when we come to speak of the starry heavens we shall see that there are many masses of glowing gas which are nebulæ of the same sort, and which form an object-lesson in our own history. we have spoken rather lightly of the nebula rotating and throwing off planets; but we must not think of all this as having happened in a short time. it is almost as impossible for the human mind to conceive the ages required for such slow changes as to grasp the great gulfs of space that separate us from the stars. we can only do it by comparison. you know what a second is, and how the seconds race past without ceasing day and night. it makes one giddy to picture the seconds there are in a year; yet if each one of those seconds was a year in itself, what then? that seems a stupendous time, but it is nothing compared with the time needed to form a nebula into a planetary system. if we had five thousand of such years, with every second in them a year, we should then only have counted one billion real years, and billions must have passed since the sun was a gaseous nebula filling the outermost bounds of our system! chapter v four small worlds what must the sun appear to mercury, who is so much nearer to him than we are? to understand that we should have to imagine our sun increased to eight or nine times his apparent size, and pouring out far greater heat and light than anything that we have here, even in the tropics. it was at first supposed that mercury must have an extra thick covering of clouds to protect him from this tremendous glare; but recent observations tend to prove that, far from this, he is singularly free from cloud. as this is so, no life as we know it could possibly exist on mercury. his year--the time he takes to go round the sun and come back to the same place again--is eighty-eight days, or about one-quarter of ours. as his orbit is much more like an ellipse than a circle, it follows that he is much nearer to the sun at one time than at another--in fact, when he is nearest, the size of the sun must seem three and a half times greater than when he is furthest away from it! even at the best mercury is very difficult to observe, and what we can learn about him is not much; but, as we have heard, his axis is supposed to be upright. if so his seasons cannot depend on the bend toward or away from the sun, but must be influenced solely by the changes in his distance from the sun, which are much greater than in our own ease. there is some reason to believe, too, that mercury's day and year are the same length. this means that as the planet circles round the sun he turns once. if this is so the sun will shine on one half of the planet, producing an accumulated heat terrific to think of; while the other side is plunged in blackness. the side which faces the sun must be heated to a pitch inconceivable to us during the nearer half of the orbit--a pitch at which every substance must be at boiling-point, and which no life as we know it could possibly endure. seen from our point of view, mercury goes through all the phases of the moon, as he shines by the reflected light of the sun; but this point we shall consider more particularly in regard to venus, as venus is nearer to us and easier to study. for a long time astronomers had a fancy that there might be another planet even nearer to the sun than mercury, perhaps hidden from us by the great glare of the sun. they even named this imaginary planet vulcan, and some thought they had seen it, but it is tolerably certain that vulcan existed only in imagination. mercury is the nearest planet to the sun, and also the smallest, of course excepting the asteroids. it is about three thousand miles in diameter, and as our moon is two thousand miles, it is not so much bigger than that. so far as we are concerned, it is improbable we shall ever know very much more about this little planet. but next we come to venus, our beautiful bright neighbour, who approaches nearer to us than any other heavenly body except the moon. alas! when she is nearest, she like mercury, turns her dark side toward us, coming in between us and the sun, so that we cannot observe her at all. everyone must have noticed venus, however carelessly they have looked at the sky; but it is likely that far more people have seen her as an evening than a morning star, for most people are in bed when the sun rises, and it is only before sunrise or after sunset we can see venus well. she is at her best from our point of view when she seems to us to be furthest from the sun, for then we can study her best, and at these times she appears like a half or three-quarter moon, as we only see a part of the side from which the sunlight is reflected. she shines like a little silver lamp, excelling every other planet, even jupiter, the largest of all. if we look at her even with the naked eye, we can see that she is elongated or drawn out, but her brilliance prevents us from seeing her shape exactly; to do this we must use a telescope. [illustration: different phases of venus.] it is a curious fact that some planets shine much more brightly than others, without regard to their size--that is to say, the surface on which the sun's rays strike is of greater reflecting power in some than in others. one of the brightest things in nature that we can imagine is a bank of snow in sunlight; it is so dazzling that we have to look away or wink hard at the sight; and the reflective power of the surface of venus is as dazzling as if she were made of snow. this is probably because the light strikes on the upper surface of the clouds which surround her. in great contrast to this is the surface of mercury, which reflects as dully as a mass of lead. our own moon has not a high reflecting power, as will be easily understood if we imagine what the world would be if condemned to perpetual moonlight only. it would, indeed, be a sad deprivation if the mournful cold light of the moon, welcome enough as a change from sunlight, were to take the place of sunlight in the daytime. for a very long time astronomers could not discover what time venus took in rotating on her own axis--that is to say, what the length of her day was. she is difficult to observe, and in order to find out the rotation it is necessary to note some fixed object on the surface which turns round with the planet and comes back to the same place again, so that the time it takes in its journey can be measured. but the surface of venus is always changing, so that it is impossible to judge at all certainly. opinions differ greatly, some astronomers holding that venus's day is not much longer than an earthly day, while others believe that the planet's day is equal to her year, just as in the case of mercury. venus's year is days, or about seven and a half of our months, and if, indeed, her day and year are the same length, very peculiar effects would follow. for instance, terrible heat would be absorbed by the side of the planet facing the sun in the perpetual summer; and the cold which would be felt in the dreary winter's night would far exceed our bitterest arctic climate. we cannot but fancy that any beings who might live on a planet of this kind must be different altogether from ourselves. then, there is another point: even here on earth very strong winds are caused by the heating of the tropics; the hot air, being lighter than the cold air, rises, and the colder air from the poles rushes in to supply its place. this causes wind, but the winds which would be raised on venus by the rush of air from the icy side of the planet to the hot one would be tornadoes such as we could but faintly dream of. it is, of course, useless to speculate when we know so little, but in a subject so intensely interesting we cannot help guessing a little. venus is only slightly smaller than the earth, and her density is not very unlike ours; therefore the pull of gravity must be pretty much there what it is here--that is to say, things will weigh at her surface about the same as they do here. her orbit is nearly a circle, so that her distance from the sun does not vary much, and the heat will not be much greater from this cause at one time of the year than another. as her orbit is tilted up a little she does not pass between us and the sun at each revolution, but occasionally she does so, and this passing is called a transit. many important facts have been learned by watching these transits. mercury also has transits across the sun, but as she is so much smaller than venus they are not of such great importance. it was by the close observation of venus during her transits that the distance from the earth to the sun was first measured. not until the year will another transit of venus occur. it is not difficult to imagine that the earth must appear a splendid spectacle from venus, whence she is seen to great advantage. when nearest to us she must see us like a little moon, with markings as the continents and seas rotate, and these will change as they are obscured by the clouds rolling over them. at the north and south poles will be glittering ice-caps, growing larger and smaller as they turn toward or away from the sun. a brilliant spectacle! [illustration: orbits of mars, the earth, venus, and mercury.] we might say with a sigh, 'if only we could see such a world!' well, we can see a world--not indeed, so large as venus, yet a world that comes almost as near to us as venus does, and which, unlike her, is outside us in order from the sun, so that when it is nearest to us the full sunlight is on it. this is mars, our neighbour on the other side, and of all the fascinating objects in the sky mars is the most fascinating, for there, if anywhere, should we be likely to discover beings like ourselves! mars takes rather more than half an hour longer to rotate than we do, and as he is so much smaller than the earth, this means that he moves round more slowly. his axis is bent at nearly the same angle as ours is. mars is much smaller than the earth, his diameter is about twice that of the moon, and his density is about three-quarters that of the earth, so that altogether, with his smaller size and less density, anything weighing a hundred pounds here would only weigh some forty pounds on mars; and if, by some miraculous agency, you were suddenly transported there, you would find yourself so light that you could jump enormous distances with little effort, and skip and hop as if you were on springs. [illustration: _memoirs of the british astronomical association._ map of mars.] look at the map of mars, in which the surface appears to be cut up into land and water, continents and oceans. the men who first observed mars with accuracy saw that some parts were of a reddish colour and others greenish, and arguing from our own world, they called the greenish parts seas and the reddish land. for a long while no one doubted that we actually looked on a world like our own, more especially as there was supposed to be a covering of atmosphere. the so-called land and water are much more cut up and mixed together than ours, it is true. here and there is a large sea, like that marked 'mare australe,' but otherwise the water and the land are strangely intermingled. the red colour of the part they named land puzzled astronomers a good deal, for our land seen at the same distance would not appear so red, and they came at last to the conclusion that vegetation on mars must be red instead of green! but after a while another disturbing fact turned up to upset their theories, and that was that they saw canals, or what they called canals, on mars. these were long, straight, dark markings, such as you see on the map. it is true that some people never saw these markings at all, and disbelieved in their existence; but others saw them clearly, and watched them change--first go fainter and then darker again. and quite recently a photograph has been obtained which shows them plainly, so they must have an existence, and cannot be only in the eye of the observer, as the most sceptical people were wont to suggest. but further than this, one astronomer announced that some of these lines appeared to be double, yet when he looked at them again they had grown single. it was like a conjuring trick. great excitement was aroused by this, for if the canals were altered so greatly it really did look as if there were intelligent beings on mars capable of working at them. in any case, if these are really canals, to make them would be a stupendous feat, and if they are artificial--that is, made by beings and not natural--they show a very high power of engineering. imagine anyone on earth making a canal many miles wide and two thousand miles long! it is inconceivable, but that is the feat attributed to the martians. the supposed doubling of the canals, as i say, caused a great deal of talk, and very few people could see that they were double at all. even now the fact is doubted, yet there seems every reason to believe it is true. they do not all appear to be double, and those that do are always the same ones, while others undoubtedly remain single all the time. but the canals do not exhaust the wonders of mars. at each pole there is an ice-cap resembling those found at our own poles, and this tells us pretty plainly something about the climate of mars, and that there is water there. this ice-cap melts when the pole which it surrounds is directed toward the sun, and sometimes in a hot summer it dwindles down almost to nothing, in a way that the ice-caps at the poles of the earth never do. a curious appearance has been noticed when it is melting: a dark shadow seems to grow underneath the edge of it and extends gradually, and as it extends the canals near it appear much darker and clearer than they did before, and then the canals further south undergo the same change. this looks as if the melting of the snow filled up the canals with water, and was a means of watering the planet by a system totally different from anything we know here, where our poles are surrounded by oceans, and the ice-caps do not in the least affect our water-supply. but, then, another strange fact had to be taken into consideration. these straight lines called canals ran out over the seas occasionally, and it was impossible to believe that if they were canals they could do that. other things began to be discussed, such as the fact that the green parts of mars did not always remain green. in what is the springtime of mars they are so, but afterwards they become yellow, and still later in the season parts near the pole turn brown. thus the idea that the greenish parts are seas had to be quite given up, though it appeared so attractive. the idea now generally believed is that the greenish parts are vegetation--trees and bushes and so on, and that the red parts are deserts of reddish sand, which require irrigation--that is to say, watering--before anything can be grown on them. the apparent doubling of the canals may be due to the green vegetation springing up along the banks. this might form two broad lines, while the canal itself would not be seen, and when the vegetation dies down, we should see only the trench of the canal, which would possibly appear faint and single. therefore the arrangements on mars appear to be a rich and a barren season on each hemisphere, the growth being caused by the melting of the polar ice-cap, which sends floods down even beyond the equator. if we could imagine the same thing on earth we should have to think of pieces of land lying drear and dry and dead in winter between straight canal-like ditches of vast size. a little water might remain in these ditches possibly, but not enough to water the surrounding land. then, as summer progressed, we should hear, 'the floods are coming,' and each deep, huge canal would be filled up with a tide of water, penetrating further and further. the water drawn up into the air would fall in dew or rain. vegetation would spring up, especially near the canal banks, and instead of dreary wastes rich growths would cover the land, gradually dying down again in the winter. so far mars seems in some important respects very different from the earth. he is also less favourably placed than we are, for being so much further from the sun, he receives very much less heat and light. his years are of our days, or one year and ten and a half months, and his atmosphere is not so dense as ours. with this greater distance from the sun and less air we might suppose the temperature would be very cold indeed, and that the surface would be frost-bound, not only at the poles, but far down towards the equator. instead of this being so, as we have seen, the polar caps melt more than those on the earth. we can only surmise there must be some compensation we do not know of that softens down the rigour of the seasons, and makes them milder than we should suppose possible. of course, the one absorbing question is, are there people on mars? to this it is at present impossible to reply. we can only say the planet seems in every way fitted to support life, even if it is a little different from our earth. it is most certainly a living world, not a dead one like the moon, and as our knowledge increases we may some day be able to answer the question which so thrills us. our opportunities for the observation of mars vary very greatly, for as the earth's orbit lies inside that of mars, we can best see him when we are between him and the sun. of course, it must be remembered that the earth and the other planets are so infinitely small in regard to the space between them that there is no possibility of any one of them getting in such a position that it would throw a shadow on any other or eclipse it. the planets are like specks in space, and could not interfere with one another in this way. when mars, therefore, is in a line with us and the sun we can see him best, but some of these times are better than others, for this reason--the earth's orbit is nearly a circle, and that of mars more of an ellipse. [illustration: orbits of the earth and mars.] look at the illustration and remember that mars' year is not quite two of ours--that is to say, every time we swing round our orbit we catch him up in a different place, for he will have progressed less than half his orbit while we go right round ours. sometimes when we overtake him he may be at that part which is furthest away from us, or he may be at that part which is nearest to us, and if he is in the latter position we can see him best. now at these, the most favourable times of all, he is still more than thirty-five millions of miles away--that is to say, one hundred and forty times as far as the moon, yet comparatively we can see him very well. he is coming nearer and nearer to us, and very soon will be nearer than he has been since , or fifteen years ago. then many telescopes will be directed on him, and much may be learned about him. for a long time it was supposed that mars had no moons, and when dean swift wrote 'gulliver's travels' he wanted to make the laputans do something very clever, so he described their discovery of two moons attending mars, and to make it quite absurd he said that when they observed these moons they found that one of them went round the planet in about ten hours. now, as mars takes more than twenty-four hours to rotate, this was considered ridiculous, for no moon known then took less time to go round its primary world than the primary world took to turn on its own axis. our own moon, of course, takes thirty times as long--that is a month contains thirty days. then one hundred and fifty years later this jest of dean swift's came true, for two moons were really discovered revolving round mars, and one of them does actually take less time to complete its orbit than the planet does to rotate--namely, a little more than seven hours! so the absurdity in 'gulliver's travels' was a kind of prophecy! these two moons are very small, the outer one perhaps five or six miles in diameter, and the inner one about seven; therefore from mars the outer one, deimos, cannot look much more than a brilliant star, and the inner one would be but a fifth part the apparent width of our own moon. so mars is not very well off, after all. still, there is great variety, for it must be odd to see the same moon appearing three times in the day, showing all the different phases as it goes from new to full, even though it is small! such wonderful discoveries have already been made that it is not too much to say that perhaps some day we may be able to establish some sort of communication with mars, and if it be inhabited by any intelligent beings, we may be able to signal to them; but it is almost impossible that any contrivance could bridge the gulf of airless space that separates us, and it is not likely that holiday trips to mars will ever become fashionable! chapter vi four large worlds i have told you about the four lesser worlds of which our earth is one, and you know that beyond mars, the last of them, there lies a vast space, in which are found the asteroids, those strange small planets circling near to each other, like a swarm of bees. after this there comes jupiter, who is so enormous, so superb in size compared with us, that he might well serve as the sun of a little system of his own. you remember that we represented him by a football, while the earth was only a greengage plum. but jupiter himself is far less in comparison with the sun than we are in comparison with him. he differs from the planets we have heard about up to the present in that he seems to glow with some heat that he does not receive from the sun. the illumination which makes him appear as a star to us is, of course, merely reflected sunlight, and what we see is the external covering, his envelope of cloud. there is every reason to believe that the great bulk of jupiter is still at a high temperature. we know that in the depths of the earth there is still plenty of heat, which every now and then makes its presence felt by bursting up through the vents we call volcanoes, the weak spots in the earth's crust; but our surface long ago cooled, for the outside of any body gets cool before the inside, as you may have found if ever you were trying to eat hot porridge, and circled round the edge of the plate with a spoon. a large body cools more slowly than a small one, and it is possible that jupiter, being so much larger than we are, has taken longer to cool. one reason we have for thinking this is that he is so very light compared with his size--in other words, his density is so small that it is not possible he could be made of materials such as the earth is made of. as i said, when we study him through telescopes we see just the exterior, the outer envelope of cloud, and as we should expect, this changes continually, and appears as a series of belts, owing to the rotation of the planet. jupiter's rotation is very rapid; though he is so much greater than the earth, he takes less than half the time the earth does to turn round--that is to say, only ten hours. his days and nights of five hours each seem short to us, accustomed to measure things by our own estimates. but we must remember that everything is relative; that is to say, there is really no such thing as fast or slow; it is all by comparison. a spider runs fast compared with a snail, but either is terribly slow compared with an express train; and the speed of an express train itself is nothing to the velocity of light. in the same way there is nothing absolutely great or small; it is all by comparison. we say how marvellous it is that a little insect has all the mechanism of life in its body when it is so tiny, but if we imagine that insect magnified by a powerful microscope until it appears quite large, the marvel ceases. again, imagine a man walking on the surface of the earth as seen from a great distance through a telescope: he would seem less than an insect, and we might ask how could the mechanism of life be compressed into anything so small? thus, when we say enormous or tiny we must always remember we are only speaking by the measurements of our own standards. there is nothing very striking about jupiter's orbit. he takes between eleven and twelve of our years to get round the sun, so you see, though his day is shorter, his year is longer than ours. and this is not only because his path is much larger, but because by the law of gravity the more distant a planet is from the sun the more slowly it travels, so that while the earth speeds over eighteen miles jupiter has only done eight. of course, we must be careful to remember the difference between rotation and revolution. jupiter rotates much quicker than the earth--that is to say, he turns round more quickly--but he actually gets over the ground more slowly. the sun appears much smaller to him than it does to us, and he receives considerably less light and heat. there are various spots on his surface, and one remarkable feature is a dark mark, which is called the 'great red spot.' if as we suppose what we see of the planet is merely the cloudy upper atmosphere, we should not expect to find anything permanent there, for the markings would change from day to day, and this they do with this exception--that this spot, dark red in colour, has been seen for many years, turning as the planet turned. it was first noticed in , and was supposed to be some great mountain or excrescence peeping up through the clouds. it grew stronger and darker for several years, and then seemed to fade, and was not so easily seen, and though still remaining it is now pale. but, most startling to say, it has shifted its position a little--that is, it takes a few seconds longer to get round the planet than it did at first. a few seconds, you will say, but that is nothing! it does not seem much, but it shows how marvellously accurate astronomers are. discoveries of vast importance have been made from observing a few seconds' discrepancy in the time the heavenly bodies take in their journeys, and the fact that this spot takes a little longer in its rotation than it did at first shows that it cannot be attached to the body of the planet. it is impossible for it to be the summit of a mountain or anything of that sort. what can it be? no one has yet answered that question. [illustration: jupiter and one of his moons] when we get to the chapter on the sun, we shall find curiosities respecting the spots there as well. jupiter has seven moons, and four of these are comparatively large. they have the honour of having been the first heavenly bodies ever actually discovered, for the six large planets nearest the sun have been known so long that there is no record of their first discovery, and of course our own moon has always been known. galileo, who invented the telescope, turned it on to the sky in , when our king charles i. was on the throne, and he saw these curious bodies which at first he could not believe to be moons. the four which he saw vary in size from two thousand one hundred miles in diameter to nearly three thousand six hundred. you remember our own moon is two thousand miles across, so even the smallest is larger than she. they go round at about the same level as the planet's equator, and therefore they cross right in front of him, and go behind him once in every revolution. since then the other three have been discovered in the band of jupiter's satellites--one a small moon closer to him than any of the first set, and two others further out. it was by observation of the first four, however, that very interesting results were obtained. mathematicians calculated the time that these satellites ought to disappear behind jupiter and reappear again, but they found that this did not happen exactly at the time predicted; sometimes the moons disappeared sooner than they should have done, and sometimes later. then this was discovered to have some relation to the distance of our earth from jupiter. when he was at the far side of his immense orbit he was much more distant from us than when he was on the nearer side--in fact, the difference may amount to more than three hundred millions of miles. and it occurred to some clever man that the irregularities in time we noticed in the eclipses of the satellites corresponded with the distance of jupiter from us. the further he drew away from us, the later were the eclipses, and as he came nearer they grew earlier. by a brilliant inspiration, this was attributed to the time light took to travel from them to us, and this was the first time anyone had been able to measure the velocity or speed of light. for all practical purposes, on the earth's surface we hold light to be instantaneous, and well we may, for light could travel more than eight times round the world in one second. it makes one's brain reel to think of such a thing. then think how far jupiter must be away from us at the furthest, when you hear that sometimes these eclipses were delayed seventeen minutes--minutes, not seconds--because it took that time for light to cross the gulf to us! [illustration: jupiter and his principal moons.] sound is very slow compared with light, and that is why, if you watch a man hammering at a distance, the stroke he gives the nail does not coincide with the bang that reaches you, for light gets to you practically at once, and the sound comes after it. no sound can travel without air, as we have heard, therefore no sound reaches us across space. if the moon were to blow up into a million pieces we should see the amazing spectacle, but should hear nothing of it. light travels everywhere throughout the universe, and by the use of this universal carrier we have learnt all that we know about the stars and planets. when the time that light takes to travel had been ascertained by means of jupiter's satellites, a still more important problem could be solved--that was our own distance from the sun, which before had only been known approximately, and this was calculated to be ninety-two millions seven hundred thousand miles, though sometimes we are a little nearer and sometimes a little further away. jupiter is marvellous, but beyond him lies the most wonderful body in the whole solar system. we have found curiosities on our way out: we have studied the problem of the asteroids, of the little moon that goes round mars in less time than mars himself rotates; we have considered the 'great red spot' on jupiter, which apparently moves independently of the planet; but nothing have we found as yet to compare with the rings of saturn. may you see this amazing sight through a telescope one day! look at the picture of this wonderful system, and think what it would be like if the earth were surrounded with similar rings! the first question which occurs to all of us is what must the sky look like from saturn? what must it be to look up overhead and see several great hoops or arches extending from one horizon to another, reflecting light in different degrees of intensity? it would be as if we saw several immense rainbows, far larger than any earthly rainbow, and of pure light, not split into colours, extending permanently across the sky, and now and then broken by the black shadow of the planet itself as it came between them and the sun. however, we must begin at the beginning, and find out about saturn himself before we puzzle ourselves over his rings. saturn is not a very great deal less than jupiter, though, so small are the other planets in comparison, that if saturn and all the rest were rolled together, they would not make one mass so bulky as jupiter! saturn is so light--in other words, his density is so small--that he is actually lighter than water. he is the lightest, in comparison with his size, of any of the planets. therefore he cannot be made largely of solid land, as our earth is, but must be to a great extent, composed of air and gaseous vapour, like his mighty neighbour. he approaches at times as near to jupiter as jupiter does to us, and on these occasions he must present a splendid spectacle to jupiter. he takes no less than twenty-nine and a half of our years to complete his stately march around the sun, and his axis is a little more bent than ours; but, of course, at his great distance from the sun, this cannot have the same effect on the seasons that it does with us. saturn turns fast on his axis, but not so fast as jupiter, and in turning his face, or what we call his surface, presents much the same appearance to us that we might expect, for it changes very frequently and looks like cloud belts. the marvellous feature about saturn is, of course, the rings. there are three of these, lying one within the other, and separated by a fine line from each other. the middle one is much the broadest, probably about ten thousand miles in width, and the inner one, which is the darkest, was not discovered until some time after the others. as the planet swings in his orbit the rings naturally appear very different to us at different times. sometimes we can only see them edgewise, and then even in the largest telescope they are only like a streak of light, and this shows that they cannot be more than fifty or sixty miles in thickness. the one which is nearest to saturn's surface does not approach him within ten thousand miles. saturn has no less than ten satellites, in addition to the rings, so that his midnight sky must present a magnificent spectacle. the rings, which do not shine by their own light but by reflected sunlight, are solid enough to throw a shadow on the body of the planet, and themselves receive his shadow. sometimes for days together a large part of saturn must suffer eclipse beneath the encircling rings, but at other times, at night, when the rings are clear of the planet's body, so that the light is not cut off from them, they must appear as radiant arches of glory spanning the sky. the subject of these rings is so complicated by the variety of their changes that it is difficult for us even to think about it. it is one of the most marvellous of all the features of our planetary system. what are these rings? what are they made of? it has been positively proved that they cannot be made of continuous matter, either liquid or solid, for the force of gravity acting on them from the planet would tear them to pieces. what, then, can they be? it is now pretty generally believed that they are composed of multitudes of tiny bodies, each separate, and circling separately round the great planet, as the asteroids circle round the sun. as each one is detached from its neighbour and obeys its own impulses, there is none of the strain and wrench there would be were they all connected. according to the laws which govern planetary bodies, those which are nearest to the planet will travel more quickly than those which are further away. of course, as we look at them from so great a distance, and as they are moving, they appear to us to be continuous. it is conjectured that the comparative darkness of the inside ring is caused by the fact that there are fewer of the bodies there to reflect the sunlight. then, in addition to the rings, enough themselves to distinguish him from all other planets, there are the ten moons of richly-endowed saturn to be considered. it is difficult to gather much about these moons, on account of our great distance from them. the largest is probably twice the diameter of our own moon. one of them seems to be much brighter--that is to say, of higher reflecting power--on one side than the other, and by distinguishing the sides and watching carefully, astronomers have come to the conclusion that it presents always the same face to saturn in the same way as our own moon does to us; in fact, there is reason to think that all the moons of large planets do this. [illustration: the planet saturn with two of his moons.] all the moons lie outside the rings, and some at a very great distance from saturn, so that they can only appear small as seen from him. yet at the worst they must be brighter than ordinary stars, and add greatly to the variations in the sky scenery of this beautiful planet. in connection with saturn's moons there is another of those astonishing facts that are continually cropping up to remind us that, however much we know, there is such a vast deal of which we are still ignorant. so far in dealing with all the planets and moons in the solar system we have made no remark on the way they rotate or revolve, because they all go in the same direction, and that direction is called counter-clockwise, which means that if you stand facing a clock and turn your hand slowly round the opposite direction to that in which the hands go, you will be turning it in the same way that the earth rotates on its axis and revolves in its orbit. it is, perhaps, just as well to give here a word of caution. rotating of course means a planet's turning on its own axis, revolving means its course in its orbit round the sun. mercury, venus, earth, mars, jupiter, and all their moons, as well as saturn himself, rotate on their axes in this one direction--counter-clockwise--and revolve in the same direction as they rotate. even the queer little moon of mars, which runs round him quicker than he rotates, obeys this same rule. nine of saturn's moons follow this example, but one independent little one, which has been named phoebe, and is far out from the planet, actually revolves in the opposite way. we cannot see how it rotates, but if, as we said just now, it turns the same face always to saturn, then of course it rotates the wrong way too. a theory has been suggested to account for this curious fact, but it could not be made intelligible to anyone who has not studied rather high mathematics, so there we must just leave it, and put it in the cabinet of curiosities we have already collected on our way out to saturn. for ages past men have known and watched the planets lying within the orbit of saturn, and they had made up their minds that this was the limit of our system. but in a great astronomer named herschel was watching the heavens through a telescope when he noticed one strange object that he was certain was no star. the vast distance of the stars prevents their having any definite outline, or what is called a disc. the rays dart out from them in all directions and there is no 'edge' to them, but in the case of the planets it is possible to see a disc with a telescope, and this object which attracted herschel's attention had certainly a disc. he did not imagine he had discovered a new planet, because at that time the asteroids had not been found, and no one thought that there could be any more planets. yet herschel knew that this was not a star, so he called it a comet! he was actually the first who discovered it, for he knew it was not a fixed star, but it was after his announcement of this fact that some one else, observing it carefully, found it to be a real planet with an orbit lying outside that of saturn, then the furthest boundary of the solar system. herschel suggested calling it georgius sidus, in honour of george iii., then king; but luckily this ponderous name was not adopted, and as the other planets had been called after the olympian deities, and uranus was the father of saturn, it was called uranus. it was subsequently found that this new planet had already been observed by other astronomers and catalogued as a star no less than seventeen times, but until herschel's clear sight had detected the difference between it and the fixed stars no one had paid any attention to it. uranus is very far away from the sun, and can only sometimes be seen as a small star by people who know exactly where to look for him. in fact, his distance from the sun is nineteen times that of the earth. yet to show at all he must be of great size, and that size has actually been found out by the most delicate experiments. if we go back to our former comparison, we shall remember that if the earth were like a greengage plum, then uranus would be in comparison about the size of one of those coloured balloons children play with; therefore he is much larger than the earth. in this far distant orbit the huge planet takes eighty-four of our years to complete one of his own. a man on the earth will have grown from babyhood to boyhood, from boyhood to the prime of life, and lived longer than most men, while uranus has only once circled in his path. but in dealing with uranus we come to another of those startling problems of which astronomy is full. so far we have dealt with planets which are more or less upright, which rotate with a rotation like that of a top. now take a top and lay it on one side on the table, with one of its poles pointing toward the great lamp we used for the sun and the other pointing away. that is the way uranus gets round his path, on his side! he rotates the wrong way round compared with the planets we have already spoken of, but he revolves the same way round the sun that all the others do. it seems wonderful that even so much can be found out about a body so far from us, but we know more: we have discovered that uranus is made of lighter material than the earth; his density is less. how can that be known? well, you remember every body attracts every other body in proportion to the atoms it contains. if, therefore, there were any bodies near to uranus, it could be calculated by his influence on them what was his own mass, which, as you remember, is the word we use to express what would be weight were it at the earth's surface; and far away as uranus is, the bodies from which such calculations may be made have been discovered, for he has no less than four satellites, or moons. considering now the peculiar position of the planet, we might expect to find these moons revolving in a very different way from others, and this is indeed the case. they turn round the planet at about its equator--that is to say, if you hold the top representing uranus as was suggested just now, these moons would go above and below the planet in passing round it. only we must remember there is really no such thing as above and below absolutely. we who are on one side of the world point up to the sky and down to the earth, while the people on the other side of the earth, say at new zealand, also point up to the sky and down to the earth, but their pointings are directly the opposite of ours. so when we speak of moons going above and below that is only because, for the moment, we are representing uranus as a top we hold in our hands, and so we speak of above and below as they are to us. it was herschel who discovered these satellites, as well as the planet, and for these great achievements he occupies one of the grandest places in the rôle of names of which england is proud. but he did much more than this: his improvements in the construction of telescopes, and his devotion to astronomy in many other ways, would have caused him to be remembered without anything else. of uranus's satellites one, the nearest, goes round in about two and a half days, and the one that is furthest away takes about thirteen and a half days, so both have a shorter period than our moon. the discovery of uranus filled the whole civilized world with wonder. the astronomers who had seen him, but missed finding out that he was a planet, must have felt bitterly mortified, and when he was discovered he was observed with the utmost accuracy and care. the calculations made to determine his path in the sky were the easier because he had been noted as a star in several catalogues previously, so that his position for some time past was known. everybody who worked at astronomy began to observe him. from these facts mathematicians set to work, and, by abstruse calculations, worked out exactly the orbit in which he ought to move; then his movements were again watched, and behold he followed the path predicted for him; but there was a small difference here and there: he did not follow it exactly. now, in the heavens there is a reason for everything, though we may not always be clever enough to find it out, and it was easily guessed that it was not by accident that uranus did not precisely follow the path calculated for him. the planets all act and react on one another, as we know, according to their mass and their distance, and in the calculations the pull of jupiter on saturn and of saturn on uranus were known and allowed for. but uranus was pulled by some unseen influence also. a young englishman named adams, by some abstruse and difficult mathematical work far beyond the power of ordinary brains, found out not only the fact that there must be another planet nearly as large as uranus in an orbit outside his, but actually predicted where such a planet might be seen if anyone would look for it. he gave his results to a professor of astronomy at cambridge. now, it seems an easy thing to say to anyone, 'look out for a planet in such and such a part of the sky,' but in reality, when the telescope is turned to that part of the sky, stars are seen in such numbers that, without very careful comparison with a star chart, it is impossible to say which are fixed stars and which, if any, is an intruder. there happened to be no star chart of this kind for the particular part of the sky wanted, and thus a long time elapsed and the planet was not identified. meantime a young frenchman named leverrier had also taken up the same investigation, and, without knowing anything of adams' work, had come to the same conclusion. he sent his results to the berlin observatory, where a star chart such as was wanted was actually just being made. by the use of this the berlin astronomers at once identified this new member of our system, and announced to the astonished world that another large planet, making eight altogether, had been discovered. then the english astronomers remembered that they too held in their hands the means for making this wonderful discovery, but, by having allowed so much time to elapse, they had let the honour go to france. however, the names of adams and leverrier will always be coupled together as the discoverers of the new planet, which was called neptune. the marvel is that by pure reasoning the mind of man could have achieved such results. if the observation of uranus is difficult, how much more that of neptune, which is still further plunged in space! yet by patience a few facts have been gleaned about him. he is not very different in size from uranus. he also is of very slight density. his year includes one hundred and sixty-five of ours, so that since his discovery in he has only had time to get round less than a third of his path. his axis is even more tilted over than that of uranus, so that if we compare uranus to a top held horizontally, neptune will be like a top with one end pointing downwards. he rotates in this extraordinary position, in the same manner as uranus--namely, the other way over from all the other planets, but he revolves, as they all do, counter-clockwise. seen from neptune the sun can only appear about as large as venus appears to us at her best, and the light and heat received are but one nine-hundreth part of what he sends us. yet so brilliant is sunshine that even then the light that falls on neptune must be very considerable, much more than that which we receive from venus, for the sun itself glows, and from venus the light is only reflected. the sun, small as it must appear, will shine with the radiance of a glowing electric light. to get some idea of the brilliance of sunlight, sit near a screen of leaves on some sunny day when the sun is high overhead, and note the intense radiance of even the tiny rays which shine through the small holes in the leaves. the scintillating light is more glorious than any diamond, shooting out coloured rays in all directions. a small sun the apparent size of venus would, therefore, give enough light for practical purposes to such a world as neptune, even though to us a world so illuminated would seem to be condemned to a perpetual twilight. chapter vii the sun so far we have referred to the sun just so much as was necessary to show the planets rotating round him, and to acknowledge him as the source of all our light and heat; but we have not examined in detail this marvellous furnace that nourishes all the life on our planet and burns on with undiminished splendour from year to year, without thought or effort on our part. to sustain a fire on the earth much time and care and expense are necessary; fuel has to be constantly supplied, and men have to stoke the fire to keep it burning. considering that the sun is not only vastly larger than all the fires on the earth put together, but also than the earth itself, the question very naturally occurs to us, who supplies the fuel, and who does the stoking on the sun? before we answer this we must try to get some idea of the size of this stupendous body. it is not the least use attempting to understand it by plain figures, for the figures would be too great to make any impression on us--they would be practically meaningless; we must turn to some other method. suppose, for instance, that the sun were a hollow ball; then, if the earth were set at the centre, the moon could revolve round her at the same distance she is now, and there would be as great a distance between the moon and the shell of the sun as there is between the moon and the earth. this gives us a little idea of the size of the sun. again, if we go back to that solar system in which we represented the planets by various objects from a pea to a football, and set a lamp in the centre to do duty for the sun, what size do you suppose that lamp would have to be really to represent the sun in proportion to the planets? well, if our greengage plum which did duty for the earth were about three-quarters of an inch in diameter we should want a lamp with a flame as tall as the tallest man you know, and even then it would not give a correct idea unless you imagined that man extending his arms widely, and you drew round him a circle and filled in all the circle with flame! if this glorious flame burnt clear and fair and bright, radiating beams of light all around, the little greengage plum would not have to be too near, or it would be shrivelled up as in the blast of a furnace. to place it at anything resembling the distance it is from the sun in reality you would have to walk away from the flaming light for about three hundred steps, and set it down there; then, after having done all this, you would have some little idea of the relative sizes of the sun and the earth, and of the distance between them. of course, all the other planets would have to be at corresponding distances. on this same scale, neptune, the furthest out, would be three miles from our artificial sun! it seems preposterous to think that some specks so small as to be quite invisible, specks that crawl about on that plum, have dared to weigh and measure the gigantic sun; but yet they have done it, and they have even decided what he is made of. the result of the experiments is that we know the sun to be a ball of glowing gas at a temperature so high that nothing we have on earth could even compare with it. of his radiating beams extending in all directions few indeed fall on our little plum, but those that do are the source of all life, whether animal or vegetable. if the sun's rays were cut off from us, we should die at once. even the coal we use to keep us warm is but sun's heat stored up ages ago, when the luxuriant tropical vegetation sprang up in the warmth and then fell down and was buried in the earth. at night we are still enjoying the benefit of the sun's rays--that is, of those which are retained by our atmosphere; for if none remained even the very air itself would freeze, and by the next morning not one inhabitant would be left alive to tell the awful tale. yet all this life and growth and heat we receive on the whole earth is but one part in two thousand two hundred millions of parts that go out in all directions into space. it has been calculated that the heat which falls on to all the planets together cannot be more than one part in one hundred millions and the other millions of parts seem to us to be simply wasted. for untold ages the sun has been pouring out this prodigal profusion of glory, and as we know that this cannot go on without some sort of compensation, we want to understand what keeps up the fires in the sun. it is true that the sun is so enormous that he might go on burning for a very long time without burning right away; but, then, even if he is huge, his expenditure is also huge. if he had been made of solid coal he would have been all used up in about six thousand years, burning at the pace he does. now, we know that the ancient egyptians kept careful note of the heavenly bodies, and if the sun were really burning away he must have been very much larger in their time; but we have no record of this; on the contrary, all records of the sun even to five thousand years ago show that he was much the same as at present. it is evident that we must search elsewhere for an explanation. it has been suggested that his furnace is supplied by the number of meteors that fall into him. meteors are small bodies of the same materials as the planets, and may be likened to the dust of the solar system. it is not difficult to calculate the amount of matter he would require on this assumption to keep him going, and the amount required is so great as to make it practically impossible that this is the source of his supply. we have seen that all matter influences all other matter, and the quantity of meteoric stuff that would be required to support the sun's expenditure would be enough to have a serious effect on mercury, an effect that would certainly have been noticed. there can, therefore, be no such mass of matter near the sun, and though there is no doubt a certain number of meteors do fall into his furnaces day by day, it is not nearly enough to account for his continuous radiation. it seems after this as if nothing else could be suggested; but yet an answer has been found, an answer so wonderful that it is more like a fairy tale than reality. to begin at the beginning, we must go back to the time when the sun was only a great gaseous nebula filling all the space included in the orbit of neptune. this nebula was not in itself hot, but as it rotated it contracted. now, heat is really only a form of energy, and energy and heat can be interchanged easily. this is a very startling thing when heard for the first time, but it is known as surely as we know anything and has been proved again and again. when a savage wants to make a fire he turns a piece of hard wood very very quickly between his palms--twiddles it, we should say expressively--into a hole in another piece of wood, until a spark bursts out. what is the spark? it is the energy of the savage's work turned to heat. when a horse strikes his iron-shod hoofs hard on the pavement you see sparks fly; that is caused by the energy of the horse's leg. when you pump hard at your bicycle you feel your pump getting quite hot, for part of the energy you are putting into your work is transformed into heat; and so on in numberless instances. no energetic action of any kind in this world takes place without some of the energy being turned into heat, though in many instances the amount is so small as to be unnoticeable. nothing falls to the ground without some heat being generated. now, when this great nebula first began its remarkable career, by the action of gravity all the particles in it were drawn toward the centre; little by little they fell in, and the nebula became smaller. we are not now concerned with the origin of the planets--we leave that aside; we are only contemplating the part of the nebula which remained to become the sun. now these particles being drawn inward each generated some heat, so as the nebula contracted its temperature rose. throughout the ages, over the space of millions and millions of miles, it contracted and grew hotter. it still remained gaseous, but at last it got to an immense temperature, and is the sun as we know it. what then keeps it shining? it is still contracting, but slowly, so slowly that it is quite imperceptible to our finest instruments. it has been calculated that if it contracts two hundred and fifty feet in diameter in a year, the energy thus gained and turned into heat is quite sufficient to account for its whole yearly output. this is indeed marvellous. in comparison with the sun's size two hundred and fifty feet is nothing. it would take nine thousand years at this rate before any diminution could be noticed by our finest instruments! here is a source of heat which can continue for countless ages without exhaustion. thus to all intents and purposes we may say the sun's shining is inexhaustible. yet we must follow out the train of reasoning, and see what will happen in the end, in eras and eras of time, if nothing intervenes. well, some gaseous bodies are far finer and more tenuous than others, and when a gaseous body contracts it is all the time getting denser; as it grows denser and denser it at last becomes liquid, and then solid, and then it ceases to contract, as of course the particles of a solid body cannot fall freely toward the centre, as those of a gaseous body can. our earth has long ago reached this stage. when solid the action ceases, and the heat is no more kept up by this source of energy, therefore the body begins to cool--surface first, and lastly the interior; it cools more quickly the smaller it is. our moon has parted with all her heat long ago, while the earth still retains some internally. in the sun, therefore, we have an object-lesson of the stages through which all the planets must have passed. they have all once been glowing hot, and some may be still hot even on the surface, as we have seen there is reason to believe is the case with jupiter. by this marvellous arrangement for the continued heat of the sun we can see that the warmth of our planets is assured for untold ages. there is no need to fear that the sun will wear out by burning. his brightness will continue for ages beyond the thoughts of man. besides this, a few other things have been discovered about him. he is, of course, exceptionally difficult to observe; for though he is so large, which should make it easy, he is so brilliant that anyone regarding him through a telescope without the precaution of prepared glasses to keep off a great part of the light would be blinded at once. one most remarkable fact about the sun is that his surface is flecked with spots, which appear sometimes in greater numbers and sometimes in less, and the reason and shape of these spots have greatly exercised men's minds. sometimes they are large enough to be seen without a telescope at all, merely by looking through a piece of smoked or coloured glass, which cuts off the most overpowering rays. when they are visible like this they are enormous, large enough to swallow many earths in their depths. at other times they may be observed by the telescope, then they may be about five thousand miles across. sometimes one spot can be followed by an astronomer as it passes all across the sun, disappears at the edge, and after a lapse of time comes back again round the other edge. this first showed men that the sun, like all the planets, rotated on his axis, and gave them the means of finding out how long he took in doing so. but the spots showed a most surprising result, for they took slightly different times in making their journey round the sun, times which differed according to their position. for instance, a spot near the equator of the sun took twenty-five days to make the circuit, while one higher up or lower down took twenty-six days, and one further out twenty-seven; so that if these spots are, as certainly believed, actually on the surface, the conclusion is that the sun does not rotate all in one piece, but that some parts go faster than others. no one can really explain how this could be, but it is certainly more easily understood in the case of a body of gas than of a solid body, when it would be simply impossible to conceive. the spots seem to keep principally a little north and a little south of the equator; there are very few actually at it, and none found near the poles, but no reason for this distribution has been discovered. it has been noted that about every eleven years the greatest number of spots appears, and that they become fewer again, mounting up in number to the next eleven years, and so on. all these curious facts show there is much yet to be solved about the sun. the spots were supposed for long to be eruptions bursting up above the surface, but now they are generally held to be deep depressions like saucers, probably caused by violent tempests, and it is thought that the inrush of cooler matter from above makes them look darker than the other parts of the sun's surface. but when we use the words 'cooler' and 'darker,' we mean only by comparison, for in reality the dark parts of the spots are brighter than electric light. [illustration: _royal observatory, greenwich._ sun-spots.] the fact that the spots are in reality depressions or holes is shown by their change of appearance as they pass over the face of the sun toward the edge; for the change of shape is exactly that which would be caused by foreshortening. it sounds odd to say that the best time for observing the sun is during a total eclipse, for then the sun's body is hidden by the moon. but yet to a certain extent this is true, and the reason is that the sun's own brilliance is our greatest hindrance in observing him, his rays are so dazzling that they light up our own atmosphere, which prevents us seeing the edges. now, during a total eclipse, when nearly all the rays are cut off, we can see marvellous things, which are invisible at other times. but total eclipses are few and far between, and so when one is approaching astronomers make great preparations beforehand. a total eclipse is not visible from all parts of the world, but only from that small part on which the shadow of the moon falls, and as the earth travels, this shadow, which is really a round spot, passes along, making a dark band. in this band astronomers choose the best observatories, and there they take up their stations. the dark body of the moon first appears to cut a little piece out of the side of the sun, and as it sails on, gradually blotting out more and more, eager telescopes follow it; at last it covers up the whole sun, and then a marvellous spectacle appears, for all round the edges of the black moon are seen glorious red streamers and arches and filaments of marvellous shapes, continually changing. these are thrown against a background of pale green light that surrounds the black moon and the hidden sun. in early days astronomers thought these wonderful coloured streamers belonged to the moon; but it was soon proved that they really are part of the sun, and are only invisible at ordinary times, because our atmosphere is too bright to allow them to be seen. an instrument has now been invented to cut off most of the light of the sun, and when this is attached to a telescope these prominences, as they are called, can be seen at any time, so that there is no need to wait for an eclipse. [illustration: the earth as it would appear in comparison with the flames shooting out from the sun.] what are these marvellous streamers and filaments? they are what they seem, eruptions of fiery matter discharged from the ever-palpitating sun thousands of miles into surrounding space. they are for ever shooting out and bursting and falling back, fireworks on a scale too enormous for us to conceive. some of these brilliant flames extend for three hundred thousand miles, so that in comparison with one of them the whole world would be but a tiny ball, and this is going on day and night without cessation. look at the picture where the artist has made a little black ball to represent the earth as she would appear if she could be seen in the midst of the flames shooting out from the sun. do not make a mistake and think the earth really could be in this position; she is only shown there so that you may see how tiny she is in comparison with the sun. all the time you have lived and your father, and grandfather, and right back to the beginnings of english history, and far, far further into the dim ages, this stupendous exhibition of energy and power has continued, and only of late years has anyone known anything about it; even now a mere handful of people do know, and the rest, who are warmed and fed and kept alive by the gracious beams of this great revolving glowing fireball, never give it a thought. i said just now a pale green halo surrounded the sun, extending far beyond the prominences; this is called the corona and can only be seen during an eclipse. it surrounds the sun in a kind of shell, and there is reason to believe that it too is made of luminous stuff ejected by the sun in its burning fury. it is composed of large streamers or filaments, which seem to shoot out in all directions; generally these are not much larger than the apparent width of the sun, but sometimes they extend much further. the puzzle is, this corona cannot be an atmosphere in any way resembling that of our earth; for the gravitational force of the sun, owing to its enormous size, is so great that it would make any such atmosphere cling to it much more densely near to the surface, while it would be thinner higher up, and the corona is not dense in any way, but thin and tenuous throughout. this makes it very difficult to explain; it is supposed that some kind of electrical force enters into the problem, but what it is exactly we are far from knowing yet. chapter viii shining visitors our solar system is set by itself in the midst of a great space, and so far as we have learnt about it in this book everything in it seems orderly: the planets go round the sun and the satellites go round the planets, in orbits more or less regular; there seems no place for anything else. but when we have considered the planets and the satellites, we have not exhausted all the bodies which own allegiance to the sun. there is another class, made up of strange and weird members, which flash in and out of the system, coming and going in all directions and at all times--sometimes appearing without warning, sometimes returning with a certain regularity, sometimes retiring to infinite depths of space, where no human eye will ever see them more. these strange visitors are called comets, and are of all shapes and sizes and never twice alike. even as we watch them they grow and change, and then diminish in splendour. some are so vast that men see them as flaming signs in the sky, and regard them with awe and wonder; some cannot be seen at all without the help of the telescope. from the very earliest ages those that were large enough to be seen without glasses have been regarded with astonishment. men used to think that they were signs from heaven foretelling great events in the world. timid people predicted that the end of the world would come by collision with one of them. others, again, fancifully likened them to fishes in that sea of space in which we swim--fishes gigantic and terrifying, endowed with sense and will. it is perhaps unnecessary to say that comets are no more alive than is our own earth, and as for causing the end of the world by collision, there is every reason to believe the earth has been more than once right through a comet's tail, and yet no one except scientific men even discovered it. these mysterious visitors from the outer regions of space were called comets from a greek word signifying hair, for they often leave a long luminous trail behind, which resembles the filaments of a woman's hair. it is not often that one appears large and bright enough to be seen by the naked eye, and when it does it is not likely to be soon forgotten. in the year such a comet is expected, a comet which at its former appearance compelled universal attention by its brilliancy and strangeness. at the time of the norman conquest of england a comet believed to be the very same one was stretching its glorious tail half across the sky, and the normans seeing it, took it as a good omen, fancying that it foretold their success. the history of the norman conquest was worked in tapestry--that is to say, in what we should call crewels on a strip of linen--and in this record the comet duly appears. look at him in the picture as the normans fancied him. he has a red head with blue flames starting from it, and several tails. the little group of men on the left are pointing and chattering about him. we can judge what an impression this comet must have made to be recorded in such an important piece of work. [illustration: the comet in the bayeux tapestry.] but we are getting on too fast. we have yet to learn how anyone can know that the comet which appeared at the time of the norman conquest is the same as that which has come back again at different times, and above all, how anyone can tell that it will come again in the year . all this involves a long story. before the invention of telescopes of course only those comets could be seen which were of great size and fine appearance. in those days men did not realize that our world was but one of a number and of no great importance except to ourselves, and they always took these blazing appearances in the heavens as a particular warning to the human race. but when astronomers, by the aid of the telescope, found that for one comet seen by the eye there were hundreds which no mortal eye unaided could see, this idea seemed, to say the least of it, unlikely. yet even then comets were looked upon as capricious visitors from outer space; odd creatures drawn into our system by the attraction of the sun, who disappeared, never to return. it was newton, the same genius who disclosed to us the laws of gravity, who first declared that comets moved in orbits, only that these orbits were far more erratic than any of those followed by the planets. so far we have supposed that the planets were all on what we should call a level--that is to say, we have regarded them as if they were floating in a sea of water around the sun; but this is only approximately correct, for the orbits of the planets are not all at one level. if you had a number of slender hoops or rings to represent the planetary orbits, you would have to tilt one a little this way and another a little that way, only never so far but that a line through the centre of the hoop from one side to another could pass through the sun. the way in which the planetary orbits are tilted is slight in comparison with that of the orbits of comets, for these are at all sorts of angles--some turned almost sideways, and others slanting, and all of them are ellipses long drawn out and much more irregular than the planetary orbits; but erratic as they are, in every case a line drawn through the sun and extended both ways would touch each side of the orbits. a great astronomer called halley, who was born in the time of the commonwealth, was lucky enough to see a very brilliant comet, and the sight interested him so much that he made all the calculations necessary to find out just in what direction it was travelling in the heavens. he found out that it followed an ellipse which brought it very near to the sun at one part of its journey, and carried it far beyond the orbit of the earth, right out to that of neptune, at the other. then he began to search the records for other comets which had been observed before his time. he found that two particularly bright ones had been carefully noted--one about seventy-five years before that which he had seen, and the other seventy-five years before that again. both these comets had been watched so scientifically that the paths in which they had travelled could be computed. a brilliant inspiration came to halley. he believed that instead of these three, his own and the other two, being different comets, they were the same one, which returned to the sun about every seventy-five years. this could be proved, for if this idea were correct, of course the comet would return again in another seventy-five years, unless something unforeseen occurred. but halley was in the prime of life: he could not hope to live to see his forecast verified. the only thing he could do was to note down exact particulars, by means of which others who lived after him might recognize his comet. and so when the time came for its return, though halley was in his grave, numbers of astronomers were watching eagerly to see the fulfilment of his prediction. the comet did indeed appear, and since then it has been seen once again, and now we expect it to come back in the year , when you and i may see it for ourselves. when the identity of the comet was fully established men began to search further back still, to compare the records of other previous brilliant comets, and found that this one had been noticed many times before, and once as i said, at the time of the norman conquest. halley's comet is peculiar in many ways. for instance, it is unusual that so large and interesting a comet should return within a comparatively limited time. it is the smaller comets, those that can only be seen telescopically, that usually run in small orbits. the smallest orbits take about three and a half years to traverse, and some of the largest orbits known require a period of one hundred and ten thousand years. between these two limits lies every possible variety of period. one comet, seen about the time napoleon was born, was calculated to take two thousand years to complete its journey, and another, a very brilliant one seen in , must journey for eight hundred years before it again comes near to the sun. but we never know what might happen, for at any moment a comet which has traversed a long solitary pathway in outer darkness may flash suddenly into our ken, and be for the first time noted and recorded, before flying off at an angle which must take it for ever further and further from the sun. everything connected with comets is mysterious and most fascinating. from out of the icy regions of space a body appears; what it is we know not, but it is seen at first as a hairy or softly-glowing star, and it was thus that herschel mistook uranus for a comet when he first discovered it. as it draws nearer the comet sends out some fan-like projections toward the sun, enclosing its nucleus in filmy wrappings like a cocoon of light, and it travels faster and faster. from its head shoots out a tail--it may be more than one--growing in splendour and width, and always pointing away from the sun. so enormous are some of these tails that when the comet's head is close to the sun the tail extends far beyond the orbit of the earth. faster still and faster flies the comet, for as we have seen it is a consequence of the law of gravitation that the nearer planets are to the sun the faster they move in their orbits, and the same rule applies to comets too. as the comet dashes up to the sun his pace becomes something indescribable; it has been reckoned for some comets at three hundred miles a second! but behold, as the head flies round the sun the tail is always projected outwards. the nucleus or head may be so near to the sun that the heat it receives would be sufficient to reduce molten iron to vapour; but this does not seem to affect it: only the tail expands. sometimes it becomes two or more tails, and as it sweeps round behind the head it has to cover a much greater space in the same time, and therefore it must travel even faster than the head. the pace is such that no calculations can account for it, if the tail is composed of matter in any sense as we know it. then when the sun is passed the comet sinks away again, and as it goes the tail dies down and finally disappears. the comet itself dwindles to a hairy star once more and goes--whither? into space so remote that we cannot even dream of it--far away into cold more appalling than anything we could measure, the cold of absolute space. more and more slowly it travels, always away and away, until the sun, a short time back a huge furnace covering all the sky, is now but a faint star. thus on its lonely journey unseen and unknown the comet goes. this comet which we have taken as an illustration is a typical one, but all are not the same. some have no tails at all, and never develop any; some change utterly even as they are watched. the same comet is so different at different times that the only possible way of identifying it is by knowing its path, and even this is not a certain method, for some comets appear to travel at intervals along the same path! now we come to the question that must have been in the mind of everyone from the beginning of this chapter, what are comets? this question no one can answer definitely, for there are many things so puzzling about these strange appearances that it is difficult even to suggest an explanation. yet a good deal is known. in the first place, we are certain that comets have very little density--that is to say, they are indescribably thin, thinner than the thinnest kind of gas; and air, which we always think so thin, would be almost like a blanket compared with the material of comets. this we judge because they exercise no sort of influence on any of the planetary bodies they draw near to, which they certainly would do if they were made of any kind of solid matter. they come sometimes very close to some of the planets. a comet was so near to jupiter that it was actually in among his moons. the comet was violently agitated; he was pulled in fact right out of his old path, and has been going on a new one ever since; but he did not exercise the smallest effect on jupiter, or even on the moons. and, as i said earlier in this chapter, we on the earth have been actually in the folds of a comet's tail. this astonishing fact happened in june, . one evening after the sun had set a golden-yellow disc, surrounded with filmy wrappings, appeared in the sky. the sun's light, diffused throughout our atmosphere, had prevented its being seen sooner. this was apparently the comet's head. it is described as 'though a number of light, hazy clouds were floating around a miniature full moon.' from this a cone of light extended far up into the sky, and when the head disappeared below the horizon this tail was seen to reach to the zenith. but that was not all. strange shafts of light seemed to hang right overhead, and could only be accounted for by supposing that they were caused by another tail hanging straight above us, so that we looked up at it foreshortened by perspective. the comet's head lay between the earth and the sun, and its tail, which extended over many millions of miles, stretched out behind in such a way that the earth must have gone right through it. the fact that the comet exercised no perceptible influence on the earth at all, and that there were not even any unaccountable magnetic storms or displays of electricity, may reassure us so that if ever we do again come in contact with one of these extremely fine, thin bodies, we need not be afraid. there is another way in which we can judge of the wonderful tenuity or thinness of comets--that is, that the smallest stars can be seen through their tails, even though those tails must be many thousands of miles in thickness. now, if the tails were anything approaching the density of our own atmosphere, the stars when seen through them would appear to be moved out of their places. this sounds odd, and requires a word of explanation. the fact is that anything seen through any transparent medium like water or air is what is called refracted--that is to say, the rays coming from it look bent. everyone is quite familiar with this in everyday life, though perhaps they may not have noticed it. you cannot thrust a stick into the water without seeing that it looks crooked. air being less dense than water has not quite so strong a refracting power, but still it has some. we cannot prove it in just the same way, because we are all inside the atmosphere ourselves, and there is no possibility of thrusting a stick into it from the outside! the only way we know it is by looking at something which is 'outside' already, and we find plenty of objects in the sky. as a matter of fact, the stars are all a little pulled out of their places by being seen through the air, and though of course we do not notice this, astronomers know it and have to make allowance for it. the effect is most noticeable in the case of the sun when he is going down, for the atmosphere bends his rays up, and though we see him a great glowing red ball on the horizon, and watch him, as we think, drop gradually out of sight, we are really looking at him for the last moment or two when he has already gone, for the rays are bent up by the air and his image lingers when the real sun has disappeared. [illustration: a stick thrust into the water appears crooked.] therefore in looking through the luminous stuff that forms a comet's tail astronomers might well expect to see the stars displaced, but not a sign of this appears. it is difficult to imagine, therefore, what the tail can be made of. the idea is that the sun exercises a sort of repulsive effect on certain elements found in the comet's head--that is to say, it pushes them away, and that as the head approaches the sun, these elements are driven out of it away from the sun in vapour. this action may have something to do with electricity, which is yet little understood; anyway, the effect is that, instead of attracting the matter toward itself, in which case we should see the comet's tails stretching toward the sun, the sun drives it away! in the chapter on the sun we had to imagine something of the same kind to account for the corona, and the corona and the comet's tails may be really akin to each other, and could perhaps be explained in the same way. now we come to a stranger fact still. some comets go right through the sun's corona, and yet do not seem to be influenced by it in the smallest degree. this may not seem very wonderful at first perhaps, but if you remember that a dash through anything so dense as our atmosphere, at a pace much less than that at which a comet goes, is enough to heat iron to a white heat, and then make it fly off in vapour, we get a glimpse of the extreme fineness of the materials which make the corona. here is herschel's account of a comet that went very near the sun: 'the comet's distance from the sun's centre was about the th part of our distance from it. all the heat we enjoy on this earth comes from the sun. imagine the heat we should have to endure if the sun were to approach us, or we the sun, to one th part of its present distance. it would not be merely as if suns were shining on us all at once, but, times , according to a rule which is well known to all who are conversant with such matters. now, that is , . only imagine a glare , times fiercer than that of the equatorial sunshine at noon day with the sun vertical. in such a heat there is no substance we know of which would not run like water, boil, and be converted into smoke or vapour. no wonder the comet gave evidence of violent excitement, coming from the cold region outside the planetary system torpid and ice-bound. already when arrived even in our temperate region it began to show signs of internal activity; the head had begun to develop, and the tail to elongate, till the comet was for a time lost sight of--not for days afterwards was it seen; and its tail, whose direction was reversed, and which could not possibly be the same tail it had before, had already lengthened to an extent of about ninety millions of miles, so that it must have been shot out with immense force in a direction away from the sun.' we remember that comets have sometimes more than one tail, and a theory has been advanced to account for this too. it is supposed that perhaps different elements are thrust away by the sun at different angles, and one tail may be due to one element and another to another. but if the comet goes on tail-making to a large extent every time it returns to the sun, what happens eventually? do the tails fall back again into the head when out of reach of the sun's action? such an idea is inconceivable; but if not, then every time a comet approaches the sun he loses something, and that something is made up of the elements which were formerly in the head and have been violently ejected. if this be so we may well expect to see comets which have returned many times to the sun without tails at all, for all the tail-making stuff that was in the head will have been used up, and as this is exactly what we do see, the theory is probably true. where do the comets come from? that also is a very large question. it used to be supposed they were merely wanderers in space who happened to have been attracted by our sun and drawn into his system, but there are facts which go very strongly against this, and astronomers now generally believe that comets really belong to the solar system, that their proper orbits are ellipses, and that in the case of those which fly off at such an angle that they can never return they must at some time have been pulled out of their original orbit by the influence of one of the planets. [illustration: _royal observatory, cape of good hope._ a great comet.] to get a good idea of a really fine comet, until we have the opportunity of seeing one for ourselves, we cannot do better than look at this picture of a comet photographed in at the cape of good hope. it is only comparatively recently that photography has been applied to comets. when halley's comet appeared last time such a thing was not thought of, but when he comes again numbers of cameras, fitted up with all the latest scientific appliances, will be waiting to get good impressions of him. chapter ix shooting stars and fiery balls all the substances which we are accustomed to see and handle in our daily lives belong to our world. there are vegetables which grow in the earth, minerals which are dug out of it, and elementary things, such as air and water, which have always made up a part of this planet since man knew it. these are obvious, but there are other things not quite so obvious which also help to form our world. among these we may class all the elements known to chemists, many of which have difficult names, such as oxygen and hydrogen. these two are the elements which make up water, and oxygen is an important element in air, which has nitrogen in it too. there are numbers and numbers of other elements perfectly familiar to chemists, of which many people never even hear the names. we live in the midst of these things, and we take them for granted and pay little attention to them; but when we begin to learn about other worlds we at once want to know if these substances and elements which enter so largely into our daily lives are to be found elsewhere in the universe or are quite peculiar to our own world. this question might be answered in several ways, but one of the most practical tests would be if we could get hold of something which had not been always on the earth, but had fallen upon it from space. then, if this body were made up of elements corresponding with those we find here, we might judge that these elements are very generally diffused throughout the bodies in the solar system. it sounds in the highest degree improbable that anything should come hurling through the air and alight on our little planet, which we know is a mere speck in a great ocean of space; but we must not forget that the power of gravity increases the chances greatly, for anything coming within a certain range of the earth, anything small enough, that is, and not travelling at too great a pace, is bound to fall on to it. and, however improbable it seems, it is undoubtedly true that masses of matter do crash down upon the earth from time to time, and these are called meteorites. when we think of the great expanse of the oceans, of the ice round the poles, and of the desert wastes, we know that for every one of such bodies seen to fall many more must have fallen unseen by any human being. meteors large enough to reach the earth are not very frequent, which is perhaps as well, and as yet there is no record of anyone's having been killed by them. most of them consist of masses of stone, and a few are of iron, while various substances resembling those that we know here have been found in them. chemists in analyzing them have also come across certain elements so far unknown upon earth, though of course there is no saying that these may not exist at depths to which man has not penetrated. a really large meteor is a grand sight. if it is seen at night it appears as a red star, growing rapidly bigger and leaving a trail of luminous vapour behind as it passes across the sky. in the daytime this vapour looks like a cloud. as the meteor hurls itself along there may be a deep continuous roar, ending in one supreme explosion, or perhaps in several explosions, and finally the meteor may come to the earth in one mass, with a force so great that it buries itself some feet deep in the soil, or it may burst into numbers of tiny fragments, which are scattered over a large area. when a meteor is found soon after its fall it is very hot, and all its surface has 'run,' having been fused by heat. the heat is caused by the friction of our atmosphere. the meteor gets entangled in the atmosphere, and, being drawn by the attraction of the earth, dashes through it. part of the energy of its motion is turned to heat, which grows greater and greater as the denser air nearer to the earth is encountered; so that in time all the surface of the meteor runs like liquid, and this liquid, rising to a still higher temperature, is blown off in vapour, leaving a new surface exposed. the vapour makes the trail of fire or cloud seen to follow the meteor. if the process went on for long the meteor would be all dissipated in vapour, and in any case it must reach the earth considerably reduced in size. numbers and numbers of comparatively small ones disappear, and for every one that manages to come to earth there must be hundreds seen only as shooting stars, which vanish and 'leave not a wrack behind.' when a meteor is seen to fall it is traced, and, whenever possible, it is found and placed in a museum. men have sometimes come across large masses of stone and iron with their surfaces fused with heat. these are in every way like the recognized meteorites, except that no eye has noted their advent. as there can be no reasonable doubt that they are of the same origin as the others, they too are collected and placed in museums, and in any large museum you would be able to see both kinds--those which have been seen to come to earth and those which have been found accidentally. the meteors which appear very brilliant in their course across the sky are sometimes called fire-balls, which is only another name for the same thing. some of these are brighter than the full moon, so bright that they cause objects on earth to cast a shadow. in a fiery ball was noticed above a small town in normandy; it burst and scattered stones far and wide, but luckily no one was hurt. the largest meteorites that have been found on the earth are a ton or more in weight; others are mere stones; and others again just dust that floats about in the atmosphere before gently settling. of course, meteors of this last kind could not be seen to fall like the larger ones, yet they do fall in such numbers that calculations have been made showing that the earth must catch about a hundred millions of meteors daily, having altogether a total weight of about a hundred tons. this sounds enormous, but compared with the weight of the earth it is very small indeed. now that we have arrived at the fact that strange bodies do come hurtling down upon us out of space, and that we can actually handle and examine them, the next question is, where do they come from? at one time it was thought that they were fragments which had been flung off by the earth herself when she was subject to violent explosions, and that they had been thrown far enough to resist the impulse to drop down upon her again, and had been circling round the sun ever since, until the earth came in contact with them again and they had fallen back upon her. it is not difficult to imagine a force which would be powerful enough to achieve the feat of speeding something off at such a velocity that it passed beyond the earth's power to pull it back, but nothing that we have on earth would be nearly strong enough to achieve such a feat. imaginative writers have pictured a projectile hurled from a cannon's mouth with such tremendous force that it not only passed beyond the range of the earth's power to pull it back, but so that it fell within the influence of the moon and was precipitated on to her surface! such things must remain achievements in imagination only; it is not possible for them to be carried out. other ideas as to the origin of meteors were that they had been expelled from the moon or from the sun. it would need a much less force to send a projectile away from the moon than from the earth on account of its smaller size and less density, but the distance from the earth to the moon is not very great, and any projectile hurled forth from the moon would cross it in a comparatively short time. therefore if the meteorites come from the moon, the moon must be expelling them still, and we might expect to see some evidence of it; but we know that the moon is a dead world, so this explanation is not possible. the sun, for its part, is torn by such gigantic disturbances that, notwithstanding its vast size, there is no doubt sufficient force there to send meteors even so far as the earth, but the chances of their encountering the earth would be small. both these theories are now discarded. it is believed that the meteors are merely lesser fragments of the same kind of materials as the planets, circling independently round the sun; and a proof of this is that far more meteorites fall on that part of the earth which is facing forward in its journey than on that behind, and this is what we should expect if the meteors were scattered independently through space and it was by reason of our movements that we came in contact with them. there is no need to explain this further. everyone knows that in cycling or driving along a road where there is a good deal of traffic both ways the people we meet are more in number than those who overtake us, and the same result would follow with the meteors; that is to say, in travelling through space where they were fairly evenly distributed we should meet more than we should be overtaken by. you remember that it was suggested the sun's fuel might be obtained from meteors, and this was proved to be not possible, even though there are no doubt unknown millions of these strange bodies circling throughout the solar system. there are so many names for these flashing bodies that we may get a little confused: when they are seen in the sky they are meteors, or fire-balls; when they reach the earth they are called meteorites, and also aerolites. then there is another class of the same bodies called shooting stars, and these are in reality only meteors on a smaller scale; but there ought to be no confusion in our thoughts, for all these objects are small bodies travelling round the sun, and caught by the earth's influence. when you watch the sky for some time on a clear night, you will seldom fail to see at least one star flash out suddenly in a path of thrilling light and disappear, and you cannot be certain whether that star had been shining in the sky a minute before, or if it had appeared suddenly only in order to go out. the last idea is right. we must get rid at once of the notion that it would be possible for any fixed star to behave in this manner. to begin with, the fixed stars are many of them actually travelling at a great velocity at present, yet so immeasurably distant are they that their movement makes no perceptible difference to us. for one of them to appear to dash across the heavens as a meteor does would mean a velocity entirely unknown to us, even comparing it with the speed of light. no, these shooting stars are not stars at all, though they were so named, long before the real motions of the fixed stars were even dimly guessed at. as we have seen, they belong to the same class as meteors. i remember being told by a clergyman, years ago, that one night in november he had gone up to bed very late, and as he pulled up his blind to look at the sky, to his amazement he saw a perfect hail of shooting stars, some appearing every minute, and all darting in vivid trails of light, longer or shorter, though all seemed to come from one point. so marvellous was the sight that he dashed across the village street, unlocked the church door, and himself pulled the bell with all his might. the people in that quiet country village had long been in bed, but they huddled on their clothes and ran out of their pretty thatched cottages, thinking there must be a great fire, and when they saw the wonder in the sky they were amazed and cried out that the world must be coming to an end. the clergyman knew better than that, and was able to reassure them, and tell them he had only taken the most effectual means of waking them so that they might not miss the display, for he was sure as long as they lived they would never see such another sight. a star shower of this kind is certainly well worth getting up to see, but though uncommon it is not unique. there are many records of such showers having occurred in times gone by, and when men put together and examined the records they found that the showers came at regular intervals. for instance, every year about the same time in november there is a star shower, not comparable, it is true, with the brilliant one the clergyman saw, but still noticeable, for more shooting stars are seen then than at other times, and once in every thirty-three years there is a specially fine one. it happened in fact to be one of these that the village people were wakened up to see. not all at once, but gradually, the mystery of these shower displays was solved. it was realized that the meteors need not necessarily come from one fixed place in the sky because they seemed to us to do so, for that was only an effect of perspective. if you were looking down a long, perfectly straight avenue of tree-trunks, the avenue would seem to close in, to get narrower and narrower at the far end until it became a point; but it would not really do so, for you would know that the trees at the far end were just the same distance from each other as those between which you were standing. now, two meteors starting from the same direction at a distance from each other, and keeping parallel, would seem to us to start from a point and to open out wider and wider as they approached, but they would not really do so; it would only be, as in the case of the avenue, an effect of perspective. if a great many meteors did the same thing, they would appear to us all to start from one point, whereas really they would be on parallel lines, only as they rushed to meet us or we rushed to meet them this effect would be produced. therefore the first discovery was that these meteors were thousands and thousands of little bodies travelling in lines parallel to each other, like a swarm of little planets. to judge that their path was not a straight line but a circle or ellipse was the next step, and this was found to be the case. from taking exact measurements of their paths in the sky an astronomer computed they were really travelling round the sun in a lengthened orbit, an ellipse more like a comet's orbit than that of a planet. but next came the puzzling question, why did the earth apparently hit them every year to some extent, and once in thirty-three years seem to run right into the middle of them? this also was answered. one has only to imagine a swarm of such meteors at first hastening busily along their orbit, a great cluster all together, then, by the near neighbourhood of some planet, or by some other disturbing causes, being drawn out, leaving stragglers lagging behind, until at last there might be some all round the path, but only thinly scattered, while the busy, important cluster that formed the nucleus was still much thicker than any other part. now, if the orbit that the meteors followed cut the orbit or path of the earth at one point, then every time the earth came to what we may call the level crossing she must run into some of the stragglers, and if the chief part of the swarm took thirty-three years to get round, then once in about thirty-three years the earth must strike right into it. this would account for the wonderful display. so long drawn-out is the thickest part of the swarm that it takes a year to pass the points at the level crossing. if the earth strikes it near the front one year, she may come right round in time to strike into the rear part of the swarm next year, so that we may get fine displays two years running about every thirty-three years. the last time we passed through the swarm was in , and then the show was very disappointing. here in england thick clouds prevented our seeing much, and there will not be another chance for us to see it at its best until . these november meteors are called leonids, because they _seem_ to come from a group of stars named leo, and though the most noticeable they are not the only ones. a shower of the same kind occurs in august too, but the august meteors, called perseids, because they seem to come from perseus, revolve in an orbit which takes a hundred and forty-two years to traverse! so that only every one hundred and forty-second year could we hope to see a good display. when all these facts had been gathered up, it seemed without doubt that certain groups of meteors travelled in company along an elliptical orbit. but there remained still something more--a bold and ingenious theory to be advanced. it was found that a comet, a small one, only to be seen with the telescope, revolved in exactly the same orbit as the november meteors, and another one, larger, in exactly the same orbit as the august ones; hence it could hardly be doubted that comets and meteors had some connection with each other, though what that connection is exactly no one knows. anyway, we can have no shadow of doubt when we find the comet following a marked path, and the meteors pursuing the same path in his wake, that the two have some mysterious affinity. there are other smaller showers besides these of november and august, and a remarkable fact is known about one of them. this particular stream was found to be connected with a comet named biela's comet, that had been many times observed, and which returned about every seven years to the sun. after it had been seen several times, this astonishing comet split in two and appeared as two comets, both of which returned at the end of the next seven years. but on the next two occasions when they were expected they never came at all, and the third time there came instead a fine display of shooting stars, so it really seemed as if these meteors must be the fragments of the lost comet. it is very curious and interesting to notice that in these star showers there is no certain record of any large meteorite reaching the earth; they seem to be made up of such small bodies that they are all dissipated in vapour as they traverse our air. chapter x the glittering heavens on a clear moonless night the stars appear uncountable. you see them twinkling through the leafless trees, and covering all the sky from the zenith, the highest point above your head, down to the horizon. it seems as if someone had taken a gigantic pepper-pot and scattered them far and wide so that some had fallen in all directions. if you were asked to make a guess as to how many you can see at one time, no doubt you would answer 'millions!' but you would be quite wrong, for the number of stars that can be seen at once without a telescope does not exceed two thousand, and this, after the large figures we have been dealing with, appears a mere trifle. with a telescope, even of small power, many more are revealed, and every increase in the size of the telescope shows more still; so that it might be supposed the universe is indeed illimitable, and that we are only prevented from seeing beyond a certain point by our limited resources. but in reality we know that this cannot be so. if the whole sky were one mass of stars, as it must be if the number of them were infinite, then, even though we could not distinguish the separate items, we should see it bright with a pervading and diffused light. as this is not so, we judge that the universe is not unending, though, with all our inventions, we may never be able to probe to the end of it. we need not, indeed, cry for infinity, for the distances of the fixed stars from us are so immeasurable that to atoms like ourselves they may well seem unlimited. our solar system is set by itself, like a little island in space, and far, far away on all sides are other great light-giving suns resembling our own more or less, but dwindled to the size of tiny stars, by reason of the great void of space lying between us and them. our sun is, indeed, just a star, and by no means large compared with the average of the stars either. but, then, he is our own; he is comparatively near to us, and so to us he appears magnificent and unique. judging from the solar system, we might expect to find that these other great suns which we call stars have also planets circling round them, looking to them for light and heat as we do to our sun. there is no reason to doubt that in some instances the conjecture is right, and that there may be other suns with attendant planets. it is however a great mistake to suppose that because our particular family in the solar system is built on certain lines, all the other families must be made on the same pattern. why, even in our own system we can see how very much the planets differ from each other: there are no two the same size; some have moons and some have not; saturn's rings are quite peculiar to himself, and uranus and neptune indulge in strange vagaries. so why should we expect other systems to be less varied? as science has advanced, the idea that these faraway suns must have planetary attendants as our sun has been discarded. the more we know the more is disclosed to us the infinite variety of the universe. for instance, so much accustomed are we to a yellow sun that we never think of the possibility of there being one of another colour. what would you say then to a ruby sun, or a blue one; or to two suns of different colours, perhaps red and green, circling round each other; or to two such suns each going round a dark companion? for there are dark bodies as well as shining bodies in the sky. these are some of the marvels of the starry sky, marvels quite as absorbing as anything we have found in the solar system. it requires great care and patience and infinite labour before the very delicate observations which alone can reveal to us anything of the nature of the fixed stars can be accomplished. it is only since the improvement in large telescopes that this kind of work has become possible, and so it is but recently men have begun to study the stars intimately, and even now they are baffled by indescribable difficulties. one of these is our inability to tell the distance of a thing by merely looking at it unless we also know its size. on earth we are used to seeing things appear smaller the further they are from us, and by long habit can generally tell the real size; but when we turn to the stars, which appear so much alike, how are we to judge how far off they are? two stars apparently the same size and close together in the sky may really be as far one from another as the earth is from the nearest; for if the further one were very much larger than the nearer, they would then appear the same size. at first it was natural enough to suppose that the big bright stars of what we call the first magnitude were the nearest to us, and the less bright the next nearest, and so on down to the tiny ones, only revealed by the telescope, which would be the furthest away of all; but research has shown that this is not correct. some of the brightest stars may be comparatively near, and some of the smallest may be near also. the size is no test of distance. so far as we have been able to discover, the star which seems nearest _is_ a first magnitude one, but some of the others which outshine it must be among the infinitely distant ones. thus we lie in the centre of a jewelled universe, and cannot tell even the size of the jewels which cover its radiant robe. i say 'lie,' but that is really not the correct word. so far as we have been able to find out, there is no such thing as absolute rest in the universe--in fact, it is impossible; for even supposing any body could be motionless at first, it would be drawn by the attraction of its nearest neighbours in space, and gradually gain a greater and greater velocity as it fell toward them. even the stars we call 'fixed' are all hurrying along at a great pace, and though their distance prevents us from seeing any change in their positions, it can be measured by suitable instruments. our sun is no exception to this universal rule. like all his compeers, he is hurrying busily along somewhere in obedience to some impulse of which we do not know the nature; and as he goes he carries with him his whole cortège of planets and their satellites, and even the comets. yes, we are racing through space with another motion, too, besides those of rotation and revolution, for our earth keeps up with its master attractor, the sun. it is difficult, no doubt, to follow this, but if you think for a moment you will remember that when you are in a railway-carriage everything in that carriage is really travelling along with it, though it does not appear to move. and the whole solar system may be looked at as if it were one block in movement. as in a carriage, the different bodies in it continue their own movements all the time, while sharing in the common movement. you can get up and change your seat in the train, and when you sit down again you have not only moved that little way of which you are conscious, but a great way of which you are not conscious unless you look out of the window. now in the case of the earth's own motion we found it necessary to look for something which does not share in that motion for purposes of comparison, and we found that something in the sun, who shows us very clearly we are turning on our axis. but in the case of the motion of the solar system the sun is moving himself, so we have to look beyond him again and turn to the stars for confirmation. then we find that the stars have motions of their own, so that it is very difficult to judge by them at all. it is as if you were bicycling swiftly towards a number of people all walking about in different directions on a wide lawn. they have their movements, but they all also have an apparent movement, really caused by you as you advance toward them; and what astronomers had to do was to separate the true movements of the stars from the false apparent movement made by the advance of the sun. this great problem was attacked and overcome, and it is now known with tolerable certainty that the sun is sweeping onward at a pace of about twelve miles a second toward a fixed point. it really matters very little to us where he is going, for the distances are so vast that hundreds of years must elapse before his movement makes the slightest difference in regard to the stars. but there is one thing which we can judge, and that is that though his course appears to be in a straight line, it is most probably only a part of a great curve so huge that the little bit we know seems straight. when we speak of the stars, we ought to keep quite clearly in our minds the fact that they lie at such an incredible distance from us that it is probable we shall never learn a great deal about them. why, men have not even yet been able to communicate with the planet mars, at its nearest only some thirty-five million miles from us, and this is a mere nothing in measuring the space between us and the stars. to express the distances of the stars in figures is really a waste of time, so astronomers have invented another way. you know that light can go round the world eight times in a second; that is a speed quite beyond our comprehension, but we just accept it. then think what a distance it could travel in an hour, in a day; and what about a year? the distance that light can travel in a year is taken as a convenient measure by astronomers for sounding the depths of space. measured in this way light takes four years and four months to reach us from the nearest star we know of, and there are others so much more distant that hundreds--nay, thousands--of years would have to be used to convey it. light which has been travelling along with a velocity quite beyond thought, silently, unresting, from the time when the britons lived and ran half naked on this island of ours, has only reached us now, and there is no limit to the time we may go back in our imaginings. we see the stars, not as they are, but as they were. if some gigantic conflagration had happened a hundred years ago in one of them situated a hundred light-years away from us, only now would that messenger, swifter than any messenger we know, have brought the news of it to us. to put the matter in figures, we are sure that no star can lie nearer to us than twenty-five billions of miles. a billion is a million millions, and is represented by a figure with twelve noughts behind it, so-- , , , , ; and twenty-five such billions is the least distance within which any star can lie. how much farther away stars may be we know not, but it is something to have found out even that. on the same scale as that we took in our first example, we might express it thus: if the earth were a greengage plum at a distance of about three hundred of your steps from the sun, and neptune were, on the same scale, about three miles away, the nearest fixed star could not be nearer than the distance measured round the whole earth at the equator! all this must provoke the question, how can anyone find out these things? well, for a long time the problem of the distances of the stars was thought to be too difficult for anyone to attempt to solve it, but at last an ingenious method was devised, a method which shows once more the triumph of man's mind over difficulties. in practice this method is extremely difficult to carry out, for it is complicated by so many other things which must be made allowance for; but in theory, roughly explained, it is not too hard for anyone to grasp. the way of it is this: if you hold up your finger so as to cover exactly some object a few feet distant from you, and shut first one eye and then the other, you will find that the finger has apparently shifted very considerably against the background. the finger has not really moved, but as seen from one eye or the other, it is thrown on a different part of the background, and so appears to jump; then if you draw two imaginary lines, one from each eye to the finger, and another between the two eyes, you will have made a triangle. now, all of you who have done a little euclid know that if you can ascertain the length of one side of a triangle, and the angles at each end of it, you can form the rest of the triangle; that is to say, you can tell the length of the other two sides. in this instance the base line, as it is called--that is to say the line lying between the two eyes--can easily be measured, and the angles at each end can be found by an instrument called a sextant, so that by simple calculation anyone could find out what distance the finger was from the eye. now, some ingenious man decided to apply this method to the stars. he knew that it is only objects quite near to us that will appear to shift with so small a base line as that between the eyes, and that the further away anything is the longer must the base line be before it makes any difference. but this clever man thought that if he could only get a base line long enough he could easily compute the distance of the stars from the amount that they appeared to shift against their background. he knew that the longest base line he could get on earth would be about eight thousand miles, as that is the diameter of the earth from one side to the other; so he carefully observed a star from one end of this immense base line and then from the other, quite confident that this plan would answer. but what happened? after careful observations he discovered that no star moved at all with this base line, and that it must be ever so much longer in order to make any impression. then indeed the case seemed hopeless, for here we are tied to the earth and we cannot get away into space. but the astronomer was nothing daunted. he knew that in its journey round the sun the earth travels in an orbit which measures about one hundred and eighty-five millions of miles across, so he resolved to take observations of the stars when the earth was at one side of this great circle, and again, six months later, when she had travelled to the other side. then indeed he would have a magnificent base line, one of one hundred and eighty-five millions of miles in length. what was the result? even with this mighty line the stars are found to be so distant that many do not move at all, not even when measured with the finest instruments, and others move, it may be, the breadth of a hair at a distance of several feet! but even this delicate measure, a hair's-breadth, tells its own tale; it lays down a limit of twenty-five billion miles within which no star can lie! this system which i have explained to you is called finding the star's parallax, and perhaps it is easier to understand when we put it the other way round and say that the hair's-breadth is what the whole orbit of the earth would appear to have shrunk to if it were seen from the distance of these stars! many, many stars have now been examined, and of them all our nearest neighbour seems to be a bright star seen in the southern hemisphere. it is in the constellation or star group called centaurus, and is the brightest star in it. in order to designate the stars when it is necessary to refer to them, astronomers have invented a system. to only the very brightest are proper names attached; others are noted according to the degree of their brightness, and called after the letters of the greek alphabet: alpha, beta, gamma, delta, etc. our own word 'alphabet' comes, you know, from the first two letters of this greek series. as this particular star is the brightest in the constellation centaurus, it is called alpha centauri; and if ever you travel into the southern hemisphere and see it, you may greet it as our nearest neighbour in the starry universe, so far as we know at present. chapter xi the constellations from the very earliest times men have watched the stars, felt their mysterious influence, tried to discover what they were, and noted their rising and setting. they classified them into groups, called constellations, and gave such groups the names of figures and animals, according to the positions of the stars composing them. some of these imaginary figures seem to us so wildly ridiculous that we cannot conceive how anyone could have gone so far out of their way as to invent them. but they have been long sanctioned by custom, so now, though we find it difficult to recognize in scattered groups of stars any likeness to a fish or a ram or a bear; we still call the constellations by their old names for convenience in referring to them. supposing the axis of the earth were quite upright, straight up and down in regard to the plane at which the earth goes round the sun, then we should always see the same set of stars from the northern and the same set of stars from the southern hemispheres all the year round. but as the axis is tilted slightly, we can, during our nights in the winter in the northern hemisphere, see more of the sky to the south than we can in the summer; and in the southern hemisphere just the reverse is the case, far more stars to the north can be seen in the winter than in the summer. but always, whether it is winter or summer, there is one fixed point in each hemisphere round which all the other stars seem to swing, and this is the point immediately over the north or the south poles. there is, luckily, a bright star almost at the point at which the north pole would seem to strike the sky were it infinitely lengthened. this is not one of the brightest stars in the sky, but quite bright enough to serve the purpose, and if we stand with our faces towards it, we can be sure we are looking due north. how can we discover this star for ourselves in the sky? go out on any starlight night when the sky is clear, and see if you can find a very conspicuous set of seven stars called the great bear. i shall not describe the great bear, because every child ought to know it already, and if they don't, they can ask the first grown-up person they meet, and they will certainly be told. (see map.) [illustration: constellations near the pole star.] having found the great bear, you have only to draw an imaginary line between the two last stars forming the square on the side away from the tail, and carry it on about three times as far as the distance between those two stars, and you will come straight to the pole star. the two stars in the great bear which help one to find it are called the pointers, because they point to it. the great bear is one of the constellations known from the oldest times; it is also sometimes called charles's wain, the dipper, or the plough. it is always easily seen in england, and seems to swing round the pole star as if held by an invisible rope tied to the pointers. besides the great bear there is, not far from it, the little bear, which is really very like it, only smaller and harder to find. the pole star is the last star in its tail; from it two small stars lead away parallel to the great bear, and they bring the eye to a small pair which form one side of a square just like that in the great bear. but the whole of the little bear is turned the opposite way from the great bear, and the tail points in the opposite direction. and when you come to think of it, it is very ridiculous to have called these groups bears at all, or to talk about tails, for bears have no tails! so it would have been better to have called them foxes or dogs, or almost any other animal rather than bears. now, if you look at the sky on the opposite side of the pole star from the great bear, you will see a clearly marked capital w made up of five or six bright stars. this is called cassiopeia, or the lady's chair. in looking at cassiopeia you cannot help noticing that there is a zone or broad band of very many stars, some exceedingly small, which apparently runs right across the sky like a ragged hoop, and cassiopeia seems to be set in or on it. this band is called the milky way, and crosses not only our northern sky, but the southern sky too, thus making a broad girdle round the whole universe. it is very wonderful, and no one has yet been able to explain it. the belt is not uniform and even, but it is here and there broken up into streamers and chips, having the same appearance as a piece of ribbon which has been snipped about by scissors in pure mischief; or it may be compared to a great river broken up into many channels by rocks and obstacles in its course. the milky way is mainly made up of thousands and thousands of small stars, and many more are revealed by the telescope; but, as we see in cassiopeia, there are large bright stars in it too, though, of course, these may be infinitely nearer to us, and may only appear to us to be in the milky way because they are between us and it. now, besides the few constellations that i have mentioned, there are numbers of others, some of which are difficult to discover, as they contain no bright stars. but there are certain constellations which every one should know, because in them may be found some of the brightest stars, those of the first magnitude. magnitude means size, and it is really absurd for us to say a star is of the first magnitude simply because it appears to us to be large, for, as i have explained already, a small star comparatively near to us might appear larger than a greater one further away. but the word 'magnitude' was used when men really thought stars were large or small according to their appearance, and so it is used to this day. they called the biggest and brightest first magnitude stars. of these there are not many, only some twenty, in all the sky. the next brightest--about the brightness of the pole star and the stars in the great bear--are of the second magnitude, and so on, each magnitude containing stars less and less bright. when we come to stars of the sixth magnitude we have reached the limit of our sight, for seventh magnitude stars can only be seen with a telescope. now that we understand what is meant by the magnitude, we can go back to the constellations and try to find some more. if you draw an imaginary line across the two stars forming the backbone of the bear, starting from the end nearest the tail, and continue it onward for a good distance, you will come to a very bright star called capella, which you will know, because near it are three little ones in a triangle. now, capella means a goat, so the small ones are called the kids. in winter capella gets high up into the sky, and then there is to be seen below her a little cluster called the pleiades. there is nothing else like this in the whole sky. it is formed of six stars, as it appears to persons of ordinary sight, and these stars are of the sixth magnitude, the lowest that can be seen by the naked eye. but though small, they are set so close together, and appear so brilliant, twinkling like diamonds, that they are one of the most noticeable objects in the heavens. a legend tells that there were once seven stars in the pleiades clearly visible, and that one has now disappeared. this is sometimes spoken of as 'the lost pleiad,' but there does not seem to be any foundation for the story. in old days people attached particular holiness or luck to the number seven, and possibly, when they found that there were only six stars in this wonderful group, they invented the story about the seventh. as the pleiades rise, a beautiful reddish star of the first magnitude rises beneath them. it is called aldebaran, and it, as well as the pleiades, forms a part of the constellation of taurus the bull. in england we can see in winter below aldebaran the whole of the constellation of orion, one of the finest of all the constellations, both for the number of the bright stars it contains and for the extent of the sky it covers. four bright stars at wide distances enclose an irregular four-sided space in which are set three others close together and slanting downwards. below these, again, are another three which seem to fall from them, but are not so bright. the figure of orion as drawn in the old representations of the constellations is a very magnificent one. the three bright stars form his belt, and the three smaller ones the hilt of his sword hanging from it. [illustration: orion and his neighbours.] if you draw an imaginary line through the stars forming the belt and prolong it downwards slantingly, you will see, in the very height of winter, the brightest star in all the sky, either in the northern or southern hemisphere. this is sirius, who stands in a class quite by himself, for he is many times brighter than any other first magnitude star. he never rises very high above the horizon here, but on crisp, frosty nights may be seen gleaming like a big diamond between the leafless twigs and boughs of the rime-encrusted trees. sirius is the dog star, and it is perhaps fortunate that, as he is placed, he can be seen sometimes in the southern and sometimes in the northern skies, so that many more people have a chance of looking at his wonderful brilliancy, than if he had been placed near the pole star. in speaking of the supreme brightness of sirius among the stars, we must remember that venus and jupiter, which outrival him, are not stars, but planets, and that they are much nearer to us. sirius is so distant that the measures for parallax make hardly any impression on him, but, by repeated experiments, it has now been proved that light takes more than eight years to travel from him to us. so that, if you are eight years old, you are looking at sirius as he was when you were a baby! not far from the pleiades, to the left as you face them, are to be found two bright stars nearly the same size; these are the heavenly twins, or gemini. returning now to the great bear, we find, if we draw a line through the middle and last stars of his tail, and carry it on for a little distance, we come fairly near to a cluster of stars in the form of a horseshoe; there is only one fairly bright one in it, and some of the others are quite small, but yet the horseshoe is distinct and very beautiful to look at. this is the northern crown. the very bright star not far from it is another first-class star called arcturus. to the left of the northern crown lies hercules, which is only mentioned because near it is the point to which the sun with all his system appears at present to be speeding. for other fascinating constellations, such as leo or the lion, andromeda and perseus, and the three bright stars by which we recognize aquila the eagle, you must wait awhile, unless you can get some one to point them out. those which you have noted already are enough to lead you on to search for more. perhaps some of you who live in towns and can see only a little strip of sky from the nursery or schoolroom windows have already found this chapter dull, and if so you may skip the rest of it and go on to the next. for the others, however, there is one more thing to know before leaving the subject, and that is the names of the string of constellations forming what is called the zodiac. you may have heard the rhyme: 'the ram, the bull, the heavenly twins, and next the crab, the lion shines, the virgin and the scales; the scorpion, archer, and he-goat, the man that holds the watering-pot, the fish with glittering tails.' this puts in a form easy to remember the signs of the constellations which lie in the zodiac, an imaginary belt across the whole heavens. it is very difficult to explain the zodiac, but i must try. imagine for a moment the earth moving round its orbit with the sun in the middle. now, as the earth moves the sun will be seen continually against a different background--that is to say, he will appear to us to move not only across our sky in a day by reason of our rotation, but also along the sky, changing his position among the stars by reason of our revolution. you will say at once that we cannot see the stars when the sun is there, and no more we can. but the stars are there all the same, and every month the sun seems to have moved on into a new constellation, according to astronomers' reckoning. if you count up the names of the constellations in the rhyme, you will find that there are just twelve, one for each month, and at the end of the year the sun has come round to the first one again. the first one is aries the ram, and the sun is seen projected or thrown against that part of the sky where aries is, in april, when we begin spring; this is the first month to astronomers, and not january, as you might suppose. perhaps you will learn to recognize all the constellations in the zodiac one day; a few of them, such as the bull and the heavenly twins, you know already if you have followed this chapter. chapter xii what the stars are made of how can we possibly tell what the stars are made of? if we think of the vast oceans of space lying between them and us, and realize that we can never cross those oceans, for in them there is no air, it would seem to be a hopeless task to find out anything about the stars at all. but even though we cannot traverse space ourselves, there is a messenger that can, a messenger that needs no air to sustain him, that moves more swiftly than our feeble minds can comprehend, and this messenger brings us tidings of the stars--his name is light. light tells us many marvellous things, and not the least marvellous is the news he gives us of the workings of another force, the force of gravitation. in some ways gravitation is perhaps more wonderful than light, for though light speeds across airless space, it is stopped at once by any opaque substance--that is to say, any substance not transparent, as you know very well by your own shadows, which are caused by your bodies stopping the light of the sun. light striking on one side of the earth does not penetrate through to the other, whereas gravitation does. you remember, of course, what the force of gravitation is, for we read about that very early in this book. it is a mysterious attraction existing between all matter. every atom pulls every other atom towards itself, more or less strongly according to distance. now, solid matter itself makes no difference to the force of gravitation, which acts through it as though it were not there. the sun is pulling the earth toward itself, and it pulls the atoms on the far side of the earth just as strongly as it would if there were nothing lying between it and them. therefore, unlike light, gravitation takes no heed of obstacles in the way, but acts in spite of them. the gravitation of the earth holds you down just the same, though you are on the upper floor of a house, with many layers of wood and plaster between you and it. it cannot pull you down, for the floor holds you up, but it is gravitation that keeps your feet on the ground all the same. a clever man made up a story about some one who invented a kind of stuff which stopped the force of gravitation going through it, just as a solid body stops light; when this stuff was made, of course, it went right away off into space, carrying with it anyone who stood on it, as there was nothing to hold it to the earth! that was only a story, and it is not likely anyone could invent such stuff, but it serves to make clear the working of gravitation. these two tireless forces, light and gravitation, run throughout the whole universe, and carry messages of tremendous importance for those who have minds to grasp them. without light we could know nothing of these distant worlds, and without understanding the laws of gravity we should not be able to interpret much that light tells us. to begin with light, what can we learn from it? we turn at once to our own great light-giver, the sun, to whom we owe not only all life, but also all the colour and beauty on earth. it is well known to men of science that colour lies in the light itself, and not in any particular object. that brilliant blue cloak of yours is not blue of itself, but because of the light that falls on it. if you cannot believe this, go into a room lighted only by gas, and hey, presto! the colour is changed as if it were a conjuring trick. you cannot tell now by looking at the cloak whether it is blue or green! therefore you must admit that as the colour changes with the change of light it must be due to light, and not to any quality belonging to the material of the cloak. but, you may protest, if the colour is solely due to light, and light falls on everything alike, why are there so many colours? that is a very fair question. if the light that comes from the sun were of only one colour--say blue or red--then everything would be blue or red all the world over. some doors in houses are made with a strip of red or blue glass running down the sides. if you have one in your house like that, go and look through it, and you will see an astonishing world made up of different tones of the same colour. everything is red or blue, according to the colour of the glass, and the only difference in the appearance of objects lies in the different shades, whether things are light or dark. this is a world as it might appear if the sun's rays were only blue or only red. but the sun's light is not of one colour only, fortunately for us; it is of all the colours mixed together, which, seen in a mass, make the effect of white light. now, objects on earth are only either seen by the reflected light of the sun or by some artificial light. they have no light of their own. put them in the dark and they do not shine at all; you cannot see them. it is the sun's light striking on them that makes them visible. but all objects do not reflect the light equally, and this is because they have the power of absorbing some of the rays that strike on them and not giving them back at all, and only those rays that are given back show to the eye. a white thing gives back all the rays, and so looks white, for we have the whole of the sun's light returned to us again. but how about a blue thing? it absorbs all the rays except the blue, so that the blue rays are the only ones that come back or rebound from it again to meet our eyes, and this makes us see the object blue; and this is the case with all the other colours. a red object retains all rays except the red, which it sends back to us; a yellow object gives back only the yellow rays, and so on. what an extraordinary and mysterious fact! imagine a brilliant flower-garden in autumn. here we have tall yellow sunflowers with velvety brown centres, clustering pink and crimson hollyhocks, deep red and bright yellow peonies, slender fairy-like japanese anemones, great bunches of mauve michaelmas daisies, and countless others, and mingled with all these are many shades of green. yet it is the light of the sun alone that falling on all these varied objects, makes that glorious blaze of colour; it seems incredible. it may be difficult to believe, but it is true beyond all doubt. each delicate velvety petal has some quality in it which causes it to absorb certain of the sun's rays and send back the others, and its colour is determined by those it sends back. well then how infinitely varied must be the colours hidden in the sun's light, colours which, mixed all together, make white light! yes, this is so, for all colours that we know are to be found there. in fact, the colours that make up sunlight are the colours to be seen in the rainbow, and they run in the same order. have you ever looked carefully at a rainbow? if not, do so at the next chance. you will see it begins by being dark blue at one end, and passes through all colours until it gets to red at the other. we cannot see a rainbow every day just when we want to, but we can see miniature rainbows which contain just the same colours as the real ones in a number of things any time the sun shines. for instance, in the cut-glass edge of an inkstand or a decanter, or in one of those old-fashioned hanging pieces of cut-glass that dangle from the chandelier or candle-brackets. of course you have often seen these colours reflected on the wall, and tried to get them to shine upon your face. or you have caught sight of a brilliant patch of colour on the wall and looked around to see what caused it, finally tracing it to some thick edge of shining glass standing in the sunlight. now, the cut-glass edge shows these colours to you because it breaks up the light that falls upon it into the colours it is made of, and lets each one come out separately, so that they form a band of bright colours instead of just one ray of white light. this is perhaps a little difficult to understand, but i will try to explain. when a ray of white light falls on such a piece of glass, which is known as a prism, it goes in as white light at one side, but the three-cornered shape of the glass breaks it up into the colours it is made of, and each colour comes out separately at the other side--namely, from blue to red--like a little rainbow, and instead of one ray of white light, we have a broad band of all the colours that light is made of. who would ever have thought a pretty plaything like this could have told us what we so much wanted to know--namely, what the sun and the stars are made of? it seems too marvellous to be true, yet true it is that for ages and ages light has been carrying its silent messages to our eyes, and only recently men have learnt to interpret them. it is as if some telegraph operator had been going steadily on, click, click, click, for years and years, and no one had noticed him until someone learnt the code of dot and dash in which he worked, and then all at once what he was saying became clear. the chief instrument in translating the message that the light brings is simply a prism, a three-cornered wedge of glass, just the same as those hanging lustres belonging to the chandeliers. when a piece of glass like this is fixed in a telescope in such a way that the sun's rays fall on it, then there is thrown on to a piece of paper or any other suitable background a broad coloured band of lovely light like a little rainbow, and this is called the sun's spectrum, and the instrument by which it is seen is called a spectroscope. but this in itself could tell us little; the message it brings lies in the fact that when it has passed through the telescope, so that it is magnified, it is crossed by hundreds of minute black lines, not placed evenly at all, but scattered up and down. there may be two so close together that they look like one, and then three far apart, and then some more at different distances. when this remarkable appearance was examined carefully it was found that in sunlight the lines that appeared were always exactly the same, in the same places, and this seemed so curious that men began to seek for an explanation. someone thought of an experiment which might teach us something about the matter, and instead of letting sunlight fall on the prism, he made an artificial light by burning some stuff called sodium, and then allowed the band of coloured light to pass through the telescope; when he examined the spectrum that resulted, he found that, though numbers of lines to be found in the sun's spectrum were missing, there were a few lines here exactly matching a few of the lines in the sun's spectrum; and this could not be the result of chance only, for the lines are so mathematically exact, and are in themselves so peculiarly distributed, that it could only mean that they were due to the same cause. what could this signify, then, but that away up there in the sun, among other things, stuff called sodium, very well known to chemists on earth, is burning? after this many other substances were heated white-hot so as to give out light, in order to discover if the lines to be seen in their spectra were also to be found in the sun's spectrum. one of these was iron, and, astonishing to say, all the many little thread-like lines that appeared in its spectrum were reproduced to a hair's-breadth, among others, in the sun's spectrum. so we have found out beyond all possibility of doubt some of the materials of which the sun is made. we know that iron, sodium, hydrogen, and numerous other substances and elements, are all burning away there in a terrific furnace, to which any furnace we have on earth is but as the flicker of a match. it was not, of course, much use applying this method to the planets, for we know that the light which comes from them to us is only reflected sunlight, and this, indeed, was proved by means of the spectroscope. but the stars shine by their own light, and this opened up a wide field for inquiry. the difficulty was, of course, to get the light of one star separated from all the rest, because the light of one star is very faint and feeble to cast a spectrum at all. yet by infinite patience difficulties were overcome. one star alone was allowed to throw its light into the telescope; the light passed through a prism, and showed a faint band of many colours, with the expected little black lines cutting across it more or less thickly. examinations have thus been made of hundreds of stars. in the course of them some substances as yet unknown to us on earth have been encountered, and in some stars one element--hydrogen--is much stronger than in others; but, on the whole, speaking broadly, it has been satisfactorily shown that the stars are made on the same principles as our own sun, so that the reasoning of astronomers which had argued them to be suns was proved. [illustration: the spectrum of the sun and sirius.] we have here in the picture the spectrum of the sun and the spectrum of arcturus. you can see that the lines which appear in the band of light belonging to sirius are also in the band of light belonging to the sun, together with many others. this means that the substances flaming out and sending us light from the far away star are also giving out light from our own sun, and that the sun and sirius both contain the same elements in their compositions. this, indeed, seems enough for the spectroscope to have accomplished; it has interpreted for us the message light brings from the stars, so that we know beyond all possibility of mistake that these glowing, twinkling points of light are brilliant suns in a state of intense heat, and that in them are burning elements with which we ourselves are quite familiar. but when the spectroscope had done that, its work was not finished, for it has not only told us what the stars are made of, but another thing which we could never have known without it--namely, if they are moving toward us or going away from us. chapter xiii restless stars you remember we have already remarked upon the difficulty of telling how far one star lies behind another, as we do not know their sizes. it is, to take another similar case, easy enough to tell if a star moves to one side or the other, but very difficult by ordinary observation to tell if it is advancing toward us or running away from us, for the only means we have of judging is if it gets larger or smaller, and at that enormous distance the fact whether it advances or recedes makes no difference in its size. now, the spectroscope has changed all this, and we can tell quite as certainly if a star is coming toward us as we can if it moves to one side. i will try to explain this. you know, perhaps, that sound is caused by vibration in the air. the noise, whatever it is, jars the air and the vibrations strike on our ears. it is rather the same thing as the result of throwing a stone into a pond: from the centre of the splash little wavelets run out in ever-widening circles; so through the air run ever-widening vibrations from every sound. the more vibrations there are in a second the shriller is the note they make. in a high note the air-vibrations follow one another fast, pouring into one's ear at a terrific speed, so that the apparatus in the ear which receives them itself vibrates fiercely and records a high note, while a lower note brings fewer and slower vibrations in a second, and the ear is not so much disturbed. have you ever noticed that if a railway engine is sweeping-toward you and screaming all the time, its note seems to get shriller and shriller? that is because the engine, in advancing, sends the vibrations out nearer to you, so more of them come in a second, and thus they are crowded up closer together, and are higher and higher. now, light is also caused by waves, but they are not the same as sound waves. light travels without air, whereas sound we know cannot travel without air, and is ever so much slower, and altogether a grosser, clumsier thing than light. but yet the waves or rays which make light correspond in some ways to the vibrations of sound. what corresponds to the treble on the piano is the blue end of the spectrum in light, and the bass is the red end. now, when we are looking at the spectrum of any body which is advancing swiftly toward us, something of the same effect is observed as in the case of the shrieking engine. take any star and imagine that that star is hastening toward us at a pace of three hundred miles a second, which is not at all an unusual rate for a star; then, if we examine the band of light, the spectrum, of such a star, we shall observe an extraordinary fact--all these little lines we have spoken of are shoved up toward the treble or blue end of the spectrum. they still remain just the same distances from each other, and are in twos and threes or single, so that the whole set of lines is unaltered as a set, but everyone of them is shifted a tiny fraction up toward the blue end of the spectrum, just a little displaced. now if, instead of advancing toward us, this same star had been rushing away from us at a similar pace, all these lines would have been moved a tiny bit toward the red or bass end of the spectrum. this is known to be certainly true, so that by means of the spectroscope we can tell that some of these great sun-stars are advancing toward us and some receding from us, according to whether the multitudes of little lines in the spectrum are shifted slightly to the blue or the red end. you remember that it has been surmised that the pace the sun moves with his system is about twelve miles a second. this seems fast enough to us, who think that one mile a minute is good time for an express train, but it is slow compared with the pace of many of the stars. as i have said, some are travelling at a rate of between two hundred and three hundred miles a second; and it is due to the spectroscope that we know not only whether a star is advancing toward us or receding from us, but also whether the pace is great or not; it even tells us what the pace is, up to about half a mile a second, which is very marvellous. it is a curious fact that many of the small stars show greater movement than the large ones, which mayor may not mean that they are nearer to us. it may be taken as established that there is no such thing as absolute rest in the universe: everything, stars and nebulæ alike, are moving somewhere; in an infinite variety of directions, with an infinite variety of speed they hasten this way and that. it would be impossible for any to remain still, for even supposing it had been so 'in the beginning,' the vast forces at work in the universe would not let it remain so. out of space would come the persistent call of gravitation: atoms would cry silently to atoms. there could be no perfect equality of pull on all sides; from one side or another the pull would be the stronger. slowly the inert mass would obey and begin falling toward it; it might be an inch at a time, but with rapid increase, until at last it also was hastening some whither in this universe which appears to us to be infinite. it must be remembered that these stars, even when moving at an enormous pace, do not change their places in the sky when regarded by ordinary observers. it would take thousands of years for any of the constellations to appear at all different from what they are now, even though the stars that compose them are moving in different directions with a great velocity, for a space of many millions of miles, at the distance of most of the stars, would be but as the breadth of a fine hair as seen by us on earth. so thousands of years ago men looked up at the great bear, and saw it apparently the same as we see it now; yet for all that length of time the stars composing it have been rushing in this direction and that at an enormous speed, but do not appear to us on the earth to alter their positions in regard to each other. i know of nothing that gives one a more overwhelming sense of the mightiness of the universe and the smallness of ourselves than this fact. from age to age men look on changeless heavens, yet this apparently stable universe is fuller of flux and reflux than is the restless ocean itself, and the very wavelets on the sea are not more numerous nor more restless than the stars that bestrew the sky. chapter xiv the colours of the stars has it ever occurred to you that the stars are not all of the same colour? it is true that, just glancing at them casually, you might say they are all white; but if you examine them more carefully you cannot help seeing that some shine with a steely blue light, while others are reddish or yellowish. these colours are not easy to distinguish with the naked eye, and might not attract any attention at all unless they were pointed out; yet when attention is drawn to the fact, it is impossible to deny the redness of some, such as aldebaran. but though we may admit this, we might add that the colours are so very faint and inconspicuous, that they might be, after all, only the result of imagination. to prove that the colours are constant and real we must use a telescope, and then we need have no further doubt of their reality, for instead of disappearing, the colours of some stars stand out quite vividly beyond the possibility of mistake. red stars are a bright red, and they are the most easily seen of all, though the other colours, blue and yellow and green, are seen very decidedly by some people. the red stars have been described by various observers as resembling 'a drop of blood on a black field,' 'most magnificent copper-red,' 'most intense blood-red,' and 'glowing like a live coal out of the darkness of space.' some people see them as a shining red, like that of a glowing cloud at sunset. therefore there can be no doubt that the colours are genuine enough, and are telling us some message. this message we are able to read, for we have begun to understand the language the stars speak to us by their light since the invention of the spectroscope. the spectroscope tells us that these colours indicate different stages in the development of the stars, or differences of constitution--that is to say, in the elements of which they are made. our own sun is a yellow star, and other yellow stars are akin to him; while red and blue and green stars contain different elements, or elements in different proportions. stars do not always remain the same colours for an indefinite time; one star may change slowly from yellow to white, and another from red to yellow; and there are instances of notable changes, such as that of the brilliant white sirius, who was stated in old times by many different observers to be a red star. all this makes us think, and year by year thought leads us on to knowledge, and knowledge about these distant suns increases. but though we know a good deal now, there are still many questions we should like to ask which we cannot expect to have answered for a long time yet, if ever. the star colours have some meanings which we cannot even guess; we can only notice the facts regarding them. for instance, blue stars are never known to be solitary--they always have a companion, but why this should be so passes our comprehension. what is it in the constitution of a blue star which holds or attracts another? whatever it may be, it is established by repeated instances that blue stars do not stand alone. in the constellation of cygnus there are two stars, a blue and a yellow one, which are near enough to each other to be seen in the same telescope at the same time, and yet in reality are separated by an almost incredible number of billions of miles. but as we know that a blue star is never seen alone, and that it has often as its companion a yellowish or reddish star, it is probable that these two, situated at an enormous distance from one another, are yet in some mysterious way dependent on each other, and are not merely seen together because they happen to fall in the same field of view. many double stars show most beautifully contrasted colours: among them are pairs of yellow and rose-red, golden and azure, orange and purple, orange and lilac, copper-colour and blue, apple-green and cherry-red, and so on. in the southern hemisphere there is a cluster containing so many stars of brilliant colours that sir john herschel named it 'the jewelled cluster.' i expect most of you have seen an advertisement of pear's soap, in which you are asked to stare at some red letters, and then look away to some white surface, such as a ceiling, when you will see the same letters in green. this is because green is the complementary or contrasting colour to red, and the same thing is the case with blue and yellow. when any one colour of either of these pairs is seen, it tends to make the other appear by reaction, and if the eye gazed hard at blue instead of red, it would next see yellow, and not green. now, many people to whom this curious fact is known argue that perhaps the colours of the double stars are not real, but the effect of contrast only; for instance, they say a red star near a companion white one would tend to make the companion appear green, and so, of course, it would. but this does not account for the star colours, which are really inherent in the stars themselves, as may be proved by cutting off the light of one star, and looking only at the other, when its colour still appears unchanged. another argument equally strong against the contrast theory is that the colours of stars in pairs are by no means always those which would appear if the effect was only due to complementary colours. it is not always blue and yellow or red and green pairs that we see, though these are frequent, but many others of various kinds, such as copper and blue, and ruddy and blue. we have therefore come to the conclusion that there are in this astonishing universe numbers of gloriously coloured suns, some of which apparently lie close together. what follows? why, we want to know, of course, if these stars are really pairs connected with each other, or if they only appear so by being in the same line of sight, though one is infinitely more distant than the other. and that question also has been answered. there are now known thousands of cases in which stars, hitherto regarded as single, have been separated into two, or even more, by the use of a telescope. of these thousands, some hundreds have been carefully investigated, and the result is that, though there are undoubtedly some in which the connexion is merely accidental, yet in by far the greater number of cases the two stars thus seen together have really some connexion which binds them to one another; they are dependent on one another. this has been made known to us by the working of the wonderful law of gravitation, which is obeyed throughout the whole universe. we know that by the operation of this law two mighty suns will be drawn toward each other with a certain pull, just as surely as we know that a stone let loose from the hand will fall upon the earth; so by noting the effect of two mighty suns upon each other many facts about them may be found out. by the most minute and careful measurements, by the use of the spectroscope, and by every resource known to science, astronomers have, indeed, actually found out with a near approach to exactness how far some of these great suns lie from each other, and how large they are in comparison with one another. the very first double star ever discovered was one which you have already seen, the middle one in the tail of the great bear. if you look at it you will be delighted to find that you can see a wee star close to it, and you will think you are looking at an example of a double star with your very own eyes; but you will be wrong, for that wee star is separated by untold distances from the large one to which it seems so near. in fact, any stars which can be seen to be separate by the naked eye must lie immeasurably far apart, however tiny seems the space between them. such stars may possibly have some connexion with each other, but, at any rate in this case, such a connexion has not been proved. no, the larger star itself is made up of two others, which can only be seen apart in a telescope. since this discovery double stars have been plentifully found in every part of the sky. the average space between such double stars as seen from our earth is--what do you think? it is the width of a single hair held up thirty-six feet from our eyes! this could not, of course, be seen without the use of a telescope or opera-glasses. it serves to give some impression of star distances when we think that the millions and millions of miles lying between those stars have shrunk to that hair's-breadth seen from our point of view. twin stars circle together round a common centre of gravity, and are bound by the laws of gravitation just as the planets are. our sun is a solitary star, with no companion, and therefore such a state of things seems to us to be incredible. fancy two gigantic suns, one topaz-yellow and the other azure-blue, circling around in endless movement! where in such a system would there be room for the planets? how could planets exist under the pull of two suns in opposite directions? still more wonders are unfolded as the inquiry proceeds. certain irregularities in the motions of some of these twin systems led astronomers to infer that they were acted upon by another body, though this other body was not discernible. in fact, though they could not see it, they knew it must be there, just as adams and leverrier knew of the existence of neptune, before ever they had seen him, by the irregularities in the movements of uranus. as the results showed, it was there, and was comparable in size to the twin suns it influenced, and yet they could not see it. so they concluded this third body must be dark, not light-giving like its companions. we are thus led to the strange conclusion that some of these systems are very complicated, and are formed not only of shining suns, but of huge dark bodies which cannot be called suns. what are they, then? can they be immense planets? is it possible that life may there exist? no fairy tale could stir the imagination so powerfully as the thought of such systems including a planetary body as large or larger than its sun or suns. if indeed life exists there, what a varied scene must be presented day by day! at one time both suns mingling their flashing rays may be together in the sky; at another time only one appears, a yellow or blue sun, as the case may be. the surface of such planets must undergo weird transformations, the foliage showing one day green, the next yellow, and the next blue; shadows of azure and orange will alternate! but fascinating as such thoughts are, we can get no further along that path. to turn from fancy to facts, we find that telescope and spectroscope have supplied us with quite enough matter for wonder without calling upon imagination. we have discovered that many of the stars which seem to shine with a pure single light are double, and many more consist not only of two stars, but of several, some of which may be dark bodies. the pole star was long known to be double, and is now discovered to have a third member in its system. these multiple systems vary from one another in almost every case. some are made up of a mighty star and a comparatively small one; others are composed of stars equal in light-giving power--twin suns. some progress swiftly round their orbits, some go slowly; indeed, so slowly that during the century they have been under observation only the very faintest sign of movement has been detected; and in other systems, which we are bound to suppose double, the stars are so slow in their movements that no progress seems to have been made at all. the star we know as the nearest to us in the heavens, alpha centauri, is composed of two very bright partners, which take about eighty-seven years to traverse their orbit. they sometimes come as near to each other as saturn is to the sun. in the case of sirius astronomers found out that he had a companion by reason of his irregularities of movement before they discovered that companion, which is apparently a very small star, only to be discerned with good telescopes. but here, again, it would be unwise to judge only by what we see. though the star appears small, we know by the influence it exercises on sirius that it is very nearly the same size as he is. thus we judge that it is poor in light-giving property; in fact, its shining power is much less than that of its companion, though its size is so nearly equal. this is not wonderful, for sirius's marvellous light-giving power is one of the wonders of the universe; he shines as brilliantly as twenty-nine or thirty of our suns! in some cases the dark body which we cannot see may even be larger than the shining one, through which alone we can know anything of it. here we have a new idea, a hint that in some of these systems there may be a mighty earth with a smaller sun going round it, as men imagined our sun went around the earth before the real truth was found out. so we see that, when we speak of the stars as suns comparable with our sun, we cannot think of them all as being exactly on the same model. there are endless varieties in the systems; there are solitary suns like ours which may have a number of small planets going round them, as in the solar system; but there are also double suns going round each other, suns with mighty dark bodies revolving round them which may be planets, and huge dark bodies with small suns too. every increase of knowledge opens up new wonders, and the world in which we live is but one kind of world amid an infinite number. in this chapter we have learnt an altogether new fact--the fact that the hosts of heaven comprise not only those shining stars we are accustomed to see, but also dark bodies equally massive, and probably equally numerous, which we cannot see. in fact, the regions of space may be strewn with such dark bodies, and we could have no possible means of discovering them unless they were near enough to some shining body to exert an influence upon it. it is not with his eyes alone, or with his senses, man knows of the existence of these great worlds, but often solely by the use of the powers of his mind. chapter xv temporary and variable stars it is a clear night, nearly all the world is asleep, when an astronomer crosses his lawn on his way to his observatory to spend the dark hours in making investigations into profound space. his brilliant mind, following the rays of light which shoot from the furthest star, will traverse immeasurable distances, while the body is forgotten. just before entering the observatory he pauses and looks up; his eye catches sight of something that arrests him, and he stops involuntarily. yet any stranger standing beside him, and gazing where he gazes, would see nothing unusual. there is no fiery comet with its tail stretching across from zenith to horizon, no flaming meteor dashing across the darkened sky. but that there is something unusual to be seen is evident, for the astronomer breathes quickly, and after another earnest scrutiny of the object which has attracted him, he rushes into the observatory, searches for a star-chart, and examines attentively that part of the sky at which he has been gazing. he runs his finger over the chart: here and there are the well-known stars that mark that constellation, but here? in that part there is no star marked, yet he knows, for his own eyes have told him but a few moments ago, that here there is actually blazing a star, not large, perhaps, but clear enough to be seen without a telescope--a star, maybe, which no eye but his has yet observed! he hurries to his telescope, and adjusts it so as to bring the stranger into the field of view. a new star! whence has it come? what does it mean? by the next day at the latest the news has flown over the wires, and all the scientific world is aware that a new star has been detected where no star ever was seen before. hundreds of telescopes are turned on to it; its spectrum is noted, and it stands revealed as being in a state of conflagration, having blazed up from obscurity to conspicuousness. night after night its brilliance grows, until it ranks with the brightest stars in heaven, and then it dies down and grows dim, gradually sinking--sinking into the obscurity from whence it emerged so briefly, and its place in the sky knows it no more. it may be there still, but so infinitely faint and far away that no power at our command can reveal it to us. and the amazing part of it is that this huge disaster, this mighty conflagration, is not actually happening as it is seen, but has happened many hundreds of years ago, though the message brought by the light carrier has but reached us now. there have not been a great many such outbursts recorded, though many may have taken place unrecorded, for even in these days, when trained observers are ceaselessly watching the sky, 'new' stars are not always noticed at once. in a new star appeared, and shone for two months before anyone noticed it. this particular one never rose to any very brilliant size. i twas situated in the constellation of auriga, and was noticed on february . it remained fairly bright until march , when it began to die down; but it has now sunk so low that it can only be seen in the very largest telescopes. photography has been most useful in recording these stars, for when one is noticed it has sometimes been found that it has been recorded on a photographic plate taken some time previously, and this shows us how long it has been visible. more and more photography becomes the useful handmaid of astronomers, for the photographic prepared plate is more sensitive to rays of light than the human eye, and, what is more useful still, such plates retain the rays that fall upon them, and fix the impression. also on a plate these rays are cumulative--that is to say, if a very faint star shines continuously on a plate, the longer the plate is exposed, within certain limits, the clearer will the image of that star become, for the light rays fall one on the top of the other, and tend to enforce each other, and so emphasize the impression, whereas with our eyes it is not the same thing at all, for if we do not see an object clearly because it is too faint, we do not see it any better, however much we may stare at the place where it ought to be. this is because each light ray that reaches our eye makes its own impression, and passes on; they do not become heaped on each other, as they do on a photographic plate. one variable star in perseus, discovered in , rose to such brilliancy that for one night it was queen of the northern hemisphere, outshining all the other first-class stars. it rose into prominence with wonderful quickness, and sank equally fast. at its height it outshone our sun eight thousand times! this star was so far from us that it was reckoned its light must take about three hundred years to reach us, consequently the great conflagration, or whatever caused the outburst, must have taken place in the reign of james i., though, as it was only seen here in , it was called the new star of the new century. when these new stars die down they sometimes continue to shine faintly for a long time, so that they are visible with a telescope, but in other cases they may die out altogether. we know very little about them, and have but small opportunity for observing them, and so it is not safe to hazard any theories to account for their peculiarities. at first men supposed that the great flame was made by a violent collision between two bodies coming together with great velocity so that both flared up, but this speculation has been shown by the spectroscope to be improbable, and now it is supposed by some people that two stars journeying through space may pass through a nebulous region, and thus may flare up, and such a theory is backed up by the fact that a very great number of such stars do seem to be mixed up in some strange way with a nebulous haze. all these new stars that we have been discussing so far have only blazed up once and then died down, but there is another class of stars quite as peculiar, and even more difficult to explain, and these are called variable stars. they get brighter and brighter up to a certain point, and then die down, only to become bright once more, and these changes occur with the utmost regularity, so that they are known and can be predicted beforehand. this is even more unaccountable than a sudden and unrepeated outburst, for one can understand a great flare-up, but that a star should flare and die down with regularity is almost beyond comprehension. clearly we must look further than before for an explanation. let us first examine the facts we know. variable stars differ greatly from each other. some are generally of a low magnitude, and only become bright for a short time, while others are bright most of the time and die down only for a short time. others become very bright, then sink a little bit, but not so low as at first; then they become bright again, and, lastly, go right down to the lowest point, and they keep on always through this regular cycle of changes. some go through the whole of these changes in three days, and others take much longer. the periods, as the intervals between the complete round of changes are called, vary, in fact, between three days and six hundred! it may seem impossible that changes covering so long as six hundred days could be known and followed, but there is nothing that the patience of astronomers will not compass. one very well-known variable star you can see for yourselves, and as an ounce of observation is worth a pound of hearsay, you might take a little trouble to find it. go out on any clear starlight night and look. not very far from cassiopeia (w.), to the left as you face it, are three bright stars running down in a great curve. these are in the constellation called perseus, and a little to the right of the middle and lowest one is the only variable star we can see in the sky without a telescope. this is algol. for the greater part of three days he is a bright star of about the second magnitude, then he begins to fade, and for four and a half hours grows steadily dimmer. at the dimmest he remains for about twenty minutes, and then rises again to his ordinary brightness in three and a half hours. how can we explain this? you may possibly be able to suggest a reason. what do you say to a dark body revolving round algol, or, rather, revolving with him round a common centre of gravity? if such a thing were indeed true, and if such a body happened to pass between us and algol at each revolution, the light of algol would be cut off or eclipsed in proportion to the size of such a body. if the dark body were the full size of algol and passed right between him and us, it would cut off all the light, but if it were not quite the same size, a little would still be seen. and this is really the explanation of the strange changes in the brightness of algol, for such a dark body as we are imagining does in reality exist. it is a large dark body, very nearly as large as algol himself, and if, as we may conjecture, it is a mighty planet, we have the extraordinary example of a planet and its sun being nearly the same size. we have seen that the eclipse happens every three days, and this means, of course, that the planetary body must go round its sun in that time, so as to return again to its position between us and him, but the thing is difficult to believe. why, the nearest of all our planets to the sun, the wee mercury, takes eighty-seven days to complete its orbit, and here is a mighty body hastening round its sun in three! to do this in the time the large dark planet must be very near to algol; indeed, astronomers have calculated that the surfaces of the two bodies are not more than about two million miles apart, and this is a trifle when we consider that we ourselves are more than forty-six times as far as that from the sun. at this distance algol, as observed from the planet, will fill half the sky, and the heat he gives out must be something stupendous. also the effects of gravitation must be queer indeed, acting on two such huge bodies so close together. if any beings live in such a strange world, the pull which draws them to their mighty sun must be very nearly equal to the pull which holds them to their own globe; the two together may counteract each other, but the effect must be strange! from irregularities in the movements of algol it has been judged that there may be also in the same system another dark body, but of it nothing has been definitely ascertained. but all variable stars need not necessarily be due to the light being intercepted by a dark body. there are cases where two bright stars in revolving round each other produce the same effect; for when seen side by side the two stars give twice as much light as when one is hidden behind the other, and as they are seen alternately side by side and in line, they seem to alter regularly in lustre. chapter xvi star clusters and nebulÆ could you point out any star cluster in the sky? you could if you would only think for a minute, for one has been mentioned already. this is the cluster known as the pleiades, and it is so peculiar and so different from anything else, that many people recognize the group and know where to look for it even before they know the great bear, the favourite constellation in the northern sky, itself. the pleiades is a real star cluster, and the chief stars in it are at such enormous distances from one another that they can be seen separately by the eye unaided, whereas in most clusters the stars appear to be so close together that without a telescope they make a mere blur of brightness. for a long time it was supposed that the stars composing the pleiades could not really be connected because of the great distances between them; for, as you know, even a hair's-breadth apparently between stars signifies in reality many millions of miles. light travelling from the pleiades to us, at that incomprehensible pace of which you already know, takes a hundred and ninety years to reach us! at this incredibly remote distance lies the main part of the cluster from us; but it is more marvellous still that we have every reason to believe that the outlying stars of this cluster are as far from the central ones as the nearest star we know of, alpha centauri, is from us! little wonder was it, then, that men hesitated to ascribe to the pleiades any real connection with each other, and supposed them to be merely an assemblage of stars which seemed to us to lie together. with the unaided eye we see comparatively few stars in the pleiades. six is the usual number to be counted, though people with very good sight have made out fourteen. viewed through the telescope, however, the scene changes: into this part of space stars are crowded in astonishing profusion; it is impossible to count them, and with every increase in the power of the telescope still more are revealed. well over a thousand in this small space seems no exaggerated estimate. now, it is impossible to say how many of these really belong to the group, and how many are seen there accidentally, but observations of the most prominent ones have shown that they are all moving in exactly the same direction at the same pace. it would be against probability to conceive that such a thing could be the result of mere chance, considering the infinite variety of star movements in general, and so we are bound to believe that this wonderful collection of stars is a real group, and not only an apparent one. so splendid are the great suns that illuminate this mighty system, that at least fifty or sixty of them far surpass our own sun in brilliancy. therefore when we look at that tiny sparkling group we must in imagination picture it as a vast cluster of mighty stars, all controlled and swayed by some dominant impulse, though separated by spaces enough to make the brain reel in thinking of them. if these suns possess also attendant planets, what a galaxy of worlds, what a universe within a universe is here! other star clusters there are, not so conspicuous as the pleiades, and most of these can only be seen through a telescope, so we may be thankful that we have one example so splendid within our own vision. there are some clusters so far and faintly shining that they were at first thought to be nebulæ, and not stars at all; but the telescope gradually revealed the fact that many of these are made up of stars, and so people began to think that all faint shining patches of nebulous light were really star clusters, which would be resolved into stars if only we had better telescopes. since the invention of the spectroscope, however, fresh light has been thrown on the matter, for the spectrum which is shown by some of the nebulous patches is not the same as that shown by stars, and we know that many of these strange appearances are not made up of infinitely distant stars. we are talking here quite freely about nebulæ because we have met one long ago when we discussed the gradual evolution of our own system, and we know quite well that a nebula is composed of luminous faintly-glowing gas of extreme fineness and thinness. we see in the sky at the present time what we may take to be object-lessons in our own history, for we see nebulæ of all sorts and sizes, and in some stars are mixed up, and in others stars are but dimly seen, so that it does not require a great stretch of the imagination to picture these stars as being born, emerging from the swaddling bands of filmy webs that have enwrapped them; and other nebulæ seem to be gas only, thin and glowing, with no stars at all to be found in it. we still know very little about these mysterious appearances, but the work of classifying and resolving them is going on apace. nebulæ are divided into several classes, but the easiest distinction to remember is that between white nebulæ and green nebulæ. this is not to say that we can see some coloured green, but that green appears in the spectrum of some of the nebulæ, while the spectrum of a white nebula is more like that of a star. it is fortunate for us that in the sky we can see without a telescope one instance of each of the several objects of interest that we have referred to. we have been able to see one very vivid example of a variable star; we have seen one very beautiful example of a star cluster; and it remains to look for one very good example of a white nebula. just as in finding algol you were doing a little bit of practical work, proving something of which you had read, so by seeing this nebula you will remember more about nebulæ in general than by reading many chapters on the subject. this particular nebula is in andromeda, and is not far from algol; and it is not difficult to find. it is the only one that can be well seen without a telescope, and was known to the ancients; it is believed to have been mentioned in a book of the tenth century! if you take an imaginary line down from the two left-hand stars of cassiopeia, and follow it carefully, you will come before long to a rather faint star, and close to it is the nebula. when you catch sight of it you will, perhaps, at first be disappointed, for all you will see is a soft blur of white, as if someone had laid a dab of luminous paint on the sky with a finger; but as you gaze at it night after night and realize its unchangeableness, realize also that it is a mass of glowing gas, an island in space, infinitely distant, unsupported and inexplicable, something of the wonder of it will creep over you. [illustration: _dr. max wolf._ the great nebula in andromeda.] thousands of telescopic nebulæ are now known, and have been examined, and they are of all shapes. roughly, they have been divided up into several classes--those that seem to us to be round and those that are long ovals, like this one in andromeda; but these may, of course, be only round ones seen edgewise by us; others are very irregular, and spread over an enormous part of the sky. the most remarkable of these is that in orion, and if you look very hard at the middle star in the sword-hilt of orion, you may be able to make out a faint mistiness. this, when seen through a telescope, becomes a wonderful and far-spreading nebula, with brighter and darker parts like gulfs in it, and dark channels. it has been sometimes called the fish-mouth nebula, from a fanciful idea as to its shape. indeed, so extraordinarily varied are these curious structures, that they have been compared with numbers of different objects. we have some like brushes, others resembling fans, rings, spindles, keyholes; others like animals--a fish, a crab, an owl, and so on; but these suggestions are imaginative, and have nothing to do with the real problem. in _the system of the stars_ miss clerke says: 'in regarding these singular structures we seem to see surges and spray-flakes of a nebulous ocean, bewitched into sudden immobility; or a rack of tempest-driven clouds hanging in the sky, momentarily awaiting the transforming violence of a fresh onset. sometimes continents of pale light are separated by narrow straits of comparative darkness; elsewhere obscure spaces are hemmed in by luminous inlets and channels.' one curious point about the orion nebula is that the star which seems to be in the midst of it resolves itself under the telescope into not one but six, of various sizes. nebulæ are in most cases too enormously remote from the earth for us to have any possible means of computing the distance; but we may take it that light must journey at least a thousand years to reach us from them, and in many cases much more. therefore, if at the time of the norman conquest a nebula had begun to grow dim and fade away, it would, for all intents and purposes, still be there for us, and for those that come after us for several generations, though all that existed of it in reality would be its pale image fleeting onward through space in all directions in ever-widening circles. that nebulæ do sometimes change we have evidence: there are cases in which some have grown indisputably brighter during the years they have been under observation, and some nebulæ that have been recorded by careful observers seem to have vanished. when we consider that these strange bodies fill many, many times the area of our whole solar system to the outermost bounds of neptune's orbit, it is difficult to imagine what force it is that acts on them to revive or quench their light. that that light is not the direct result of heat has long been known; it is probably some form of electric excitement causing luminosity, very much as it is caused in the comets. indeed, many people have been tempted to think of the nebulæ as the comets of the universe, and in some points there are, no doubt, strong resemblances between the two. both shine in the same way, both are so faint and thin that stars can be seen through them; but the spectroscope shows us that to carry the idea too far would be wrong, as there are many differences in constitution. we have seen that there are dark stars as well as light stars; if so, may there not be dark nebulæ as well as light ones? it may very well be so. we have seen that there are reasons for supposing our own system to have been at first a cool dark nebula rotating slowly. the heavens may be full of such bodies, but we could not discern them. their thinness would prevent their hiding any stars that happened to be behind them. no evidence of their existence could possibly be brought to us by any channel that we know. it is true that, besides the dark rifts in the bright nebulæ, which may themselves be caused by a darker and non-luminous gas, there are also strange rifts in the milky way, which at one time were conjectured to be due to a dark body intervening between us and the starry background. this idea is now quite discarded; whatever may cause them, it is not that. one of the most startling of these rifts is that called the coal-sack, in the southern hemisphere, and it occurs in a part of the sky otherwise so bright that it is the more noticeable. no possible explanation has yet been suggested to account for it. thus it may be seen that, though much has been discovered, much remains to be discovered. by the patient work of generations of astronomers we have gained a clear idea of our own position in the universe. here are we on a small globe, swinging round a far mightier and a self-luminous globe, in company with seven other planets, many of which, including ourselves, are attended by satellites or moons. between the orbits of these planets is a ring or zone of tiny bodies, also going round the sun. into this system flash every now and then strange luminous bodies--some coming but once, never to return; others returning again and again. far out in space lies this island of a system, and beyond the gulfs of space are other suns, with other systems: some may be akin to ours and some quite different. strewn about at infinite distances are star clusters, nebulæ, and other mysterious objects. the whole implies design, creation, and the working of a mighty intelligence; and yet there are small, weak creatures here on this little globe who refuse to believe in a god, or who, while acknowledging him, would believe themselves to know better than he. the end billing and sons, ltd., printers, guildford astronomical myths. [illustration] [illustration: the cliffs of flamanville.] astronomical myths, based on flammarion's "history of the heavens." by john f. blake. [illustration] london: macmillan and co. . london: r. clay, sons, and taylor, printers, bread street hill, queen victoria street. [illustration] preface. the book which is here presented to the public is founded upon a french work by m. flammarion which has enjoyed considerable popularity. it contained a number of interesting accounts of the various ideas, sometimes mythical, sometimes intended to be serious, that had been entertained concerning the heavenly bodies and our own earth; with a popular history of the earliest commencement of astronomy among several ancient peoples. it was originally written in the form of conversations between the members of an imaginary party at the seaside. it was thought that this style would hardly be so much appreciated by english as by french readers, and therefore in presenting the materials of the french author in an english dress the conversational form has been abandoned. several facts of extreme interest in relation to the early astronomical myths and the development of the science among the ancients having been brought to light, especially by the researches of mr. haliburton, a considerable amount of new matter, including the whole chapter on the pleiades, has been introduced, which makes the present issue not exactly a translation, but rather a book founded on the french author's work. it is hoped that it may be found of interest to those who care to know about the early days of the oldest of our sciences, which is now attracting general attention again by the magnitude of its recent advances. astronomy also, in early days, as will be seen by a perusal of this book, was so mixed up with all the affairs of life, and contributed so much even to religion, that a history of its beginnings is found to reveal the origin of several of our ideas and habits, now apparently quite unconnected with the science. there is matter of interest here, therefore, for those who wish to know only the history of the general ideas of mankind. [illustration: the annual revolution of the earth round the sun, with the signs of the zodiac and the constellations.] list of illustrations. the cliffs of flamanville _frontispiece._ the annual revolution of the earth round the sun, with the signs of the zodiac and the constellations page ix the earth's year, and the months " xiv an astronomer at work to face page the northern constellations " the constellations from the sea-shore " the zodiac of denderah " i. babylonian astronomers ii. druidical worship iii. chaldean astronomers iv. the zodiac and the dead in egypt v. the legends of the druids vi. the nemÆan lion vii. heavens of the fathers viii. death of copernicus ix. the solar system x. the discovery of the telescope xi. the foundation of the paris observatory xii. the legend of owen xiii. christopher columbus and the eclipse of the moon xiv. prodigies in the middle ages xv. an astrologer at work xvi. the end of the world . the earliest (aryan) representation of the earth . ancient gaulish medals, bearing astronomical signs . ancient celestial sphere . positions of the great bear on september . constellation of the bear . constellation of orion . chart of constellations in sixteenth and seventeenth centuries . flamsteed's chart . arabian sphere of the eleventh century . ancient chinese pieces of money, bearing representations of the zodiac . the zodiac . diagram illustrating the position of certain stars, b.c. . curious fifteenth century figure, representing eleven different heavens . ptolemy's astronomical system . the epicycles of ptolemy . heavens of the middle ages . emblematic drawing from ancient astronomical work . egyptian system . capella's system . the copernican system . tycho brahe's system . descartes' theory of vortices . vortices of the stars . variation of descartes' theory . the earth floating . the earth with roots . the earth of the vedic priests . hindoo earth . the earth of anaximander . plato's cubical earth . egyptian representation of the earth . homeric cosmography . the earth of the later greeks . pomponius mela's cosmography . the earth's shadow . ditto . ditto . ditto . the cosmography of cosmas . the square earth . explanation of sunrise . the earth as an egg . the earth as a floating egg . eighth-century map of the world . tenth-century maps . the map of andrea bianco . from the map in hereford cathedral . ditto . cosmography of st. denis . the map of marco polo . map on a medal of charles v . dante's infernal regions . paradise of fra mauro . the paradise of the fifteenth century . representation of a comet, sixteenth century . an egg marked with a comet . the roman calendar . diagram illustrating the order of the days of the week [illustration: the earth's year, and the months.] contents. page chapter i. the first beginnings of astronomy chapter ii. astronomy of the celts chapter iii. origin of the constellations chapter iv. the zodiac chapter v. the pleiades chapter vi. the nature and structure of the heavens according to the ancients chapter vii. the celestial harmony chapter viii. astronomical systems chapter ix. the terrestrial world of the ancients.--cosmography and geography chapter x. cosmography and geography of the church chapter xi. legendary worlds of the middle ages chapter xii. eclipses and comets chapter xiii. the greatness and the fall of astrology chapter xiv. time and the calendar chapter xv. the end of the world [illustration: an astronomer at work.] [illustration] history of the heavens. chapter i. the first beginnings of astronomy. astronomy is an ancient science; and though of late it has made a fresh start in new regions, and we are opening on the era of fresh and unlooked-for discoveries which will soon reveal our present ignorance, our advance upon primitive ideas has been so great that it is difficult for us to realize what they were without an attentive and not uninstructive study of them. no other science, not even geology, can compare with astronomy for the complete revolution which it has effected in popular notions, or for the change it has brought about in men's estimate of their place in creation. it is probable that there will always be men who believe that the whole universe was made for their benefit; but, however this may be, we have already learned from astronomy that our habitation is not that central spot men once deemed it, but only an ordinary planet circulating round an ordinary star, just as we are likely also to learn from biology, that we occupy the position, as animals, of an ordinary family in an ordinary class. that we may more perfectly realize this strange revolution of ideas, we must throw ourselves as far as possible into the feeling and spirit of our ancestors, when, without the knowledge we now possess, they contemplated, as they could not fail to do, the marvellous and awe-inspiring phenomena of the heavens by night. to them, for many an age, the sun and moon and stars, with all the planets, seemed absolutely to rise, to shine, and to set; the constellations to burst out by night in the east, and travel slowly and in silence to the west; the ocean waves to rise and fall and beat against the rock-bound shore as if endowed with life; and even in the infancy of the intellect they must have longed to pierce the secrets of this mysterious heavenly vault, and to know the nature of the starry firmament as it seemed to them, and the condition of the earth which appeared in the centre of these universal movements. the simplest hypothesis was for them the truth, and they believed that the sky was in reality a lofty and extended canopy bestudded with stars, and the earth a vast plain, the solid basis of the universe, on which dwelt man, sole creature that lifted his eyes and thoughts above. two distinct regions thus appeared to compose the whole system--the upper one, or the air, in which were the moving stars, the lights of heaven, and the firmament over all; and the lower one, or earth and sea, adorned on the surface with the products of life, and below with the minerals, metals, and stones. for a long time the various theories of the universe, grotesque and changing as they might be, were but modifications of this one central idea, the earth below, the heavens above, and on this was based every religious system that was promulgated--the very phrases founded upon it remaining to this day for a testimony to the intimate relation thus manifested between the infant ideas in astronomy and theology. no wonder that early revolutions in the conceptions in one science were thought to militate against the other. it is only when the thoughts on both are enlarged that it is seen that their connection is not necessary, but accidental, or, at least, inevitable only in the infancy of both. it is scarcely possible to estimate fully the enormous change from these ideas representing the appearances to those which now represent the reality; or to picture to ourselves the total revolution in men's minds before they could transform the picture of a vast terrestrial surface, to which the sun and all the heavenly bodies were but accessories for various purposes, to one in which the earth is but a planet like mars, moving in appearance among the stars, as it does, and rotating with a rapidity that brings a whole hemisphere of the heavens into view through the course of a single day and night. at first sight, what a loss of dignity! but, on closer thought, what a gain of grandeur! no longer some little neighbouring lights shine down upon us from a solid vault; but we find ourselves launched into the sea of infinity; with power to gaze into its almost immeasurable depths. to appreciate rightly our position, we have to plant ourselves, in imagination, in some spot removed from the surface of the earth, where we may be uninfluenced by her motion, and picture to ourselves what we should see. were we placed in some spot far enough removed from the earth, we should find ourselves in eternal day; the sun would ever shine, for no great globe would interpose itself between it and our eyes; there would be no night there. were we in the neighbourhood of the earth's orbit, and within it, most wonderful phenomena would present themselves. at one time the earth would appear but an ordinary planet, smaller than venus, but, as time wore on, unmeasured by recurring days or changing seasons, it would gradually be seen to increase in size--now appearing like the moon at the full, and shining like her with a silver light. as it came nearer, and its magnitude increased, the features of the surface would be distinguished; the brighter sea and the darker shining continents, with the brilliant ice-caps at the poles; but, unlike what we see in the moon, these features would appear to move, and, one after another, every part of the earth would be visible. the actual time required for all to pass before us would be what we here call a day and night. and still, as it rotates, the earth passes nearer to us, assumes its largest apparent size, and so gradually decreasing again, becomes once more, after the interval we here call a year, an ordinary-looking star-like planet. to us, in these days, this description is easy of imagination; we find no difficulty in picturing it to ourselves; but, if we will think for a moment what such an idea would have been to the earliest observers of astronomy, we shall better appreciate the vast change that has taken place--how we are removed from them, as we may say, _toto coelo_. but not only as to the importance of the earth in the universe, but on other matters connected with astronomy, we perceive the immensity of the change in our ideas--in that of distance, for instance. this celestial vault of the ancients was near enough for things to pass from it to us; it was in close connection with the earth, supported by it, and therefore of less diameter; but now, when our distance from the sun is expressed by numbers that we may write, indeed, but must totally fail to adequately appreciate, and the distance from the _next_ nearest star is such, that with the velocity of light--a velocity we are accustomed to regard as instantaneous--we should only reach it after a three years' journey, we are reminded of the pathetic lines of thomas hood: "i remember, i remember, the fir trees straight and high, and how i thought their slender tops were close against the sky; it was a childish fantasy, but now 'tis little joy, to know i'm further off from heaven than when i was a boy." the astronomer's answer to the last line would be that as far as the material heaven goes, we are just as much in it as the stars or as any other member of the universe; we cannot, therefore, be far off or near to it. it is probable that we are even yet but little awake to true cosmical ideas in other respects;--as to velocity, for instance. we know indeed, of light and electricity and the motions of the earth, but revelations are now being made to us of motions of material substances in the sun with such velocities that in comparison with them any motions on the earth appear infinitesimally small. our progress to our present notions, and appreciations of the truth of nature in the heavens, will thus occupy much of our thoughts; but we must also recount the history of the acquirement of those facts which have ultimately become the basis for our changes of idea. our rustic forefathers, whatever their nation, were not so enamoured of the "wonders of science"--that their astronomy was greatly a collection of theories, though theories, and wild ones, they had; it was a more practical matter, and was believed too by them to be more practical than we now find reason to believe to be the case. they noticed the various seasons, and they marked the changes in the appearances of the heavens that accompanied them; they connected the two together, and conceived the latter to be the cause of the former, and so, with other apparently uncertain events. the celestial phenomena thus acquired a fictitious importance which rendered their study of primary necessity, but gave no occasion for a theory. that we may better appreciate the earliest observations on astronomy, it may be well to mention briefly what are the varying phenomena which may most easily be noticed. if we except the phases of the moon, which almost without observation would force their recognition on people who had no other than lunar light by night, and which must therefore, from the earliest periods of human history have divided time into lunar months; there are three different sets of phenomena which depend on the arrangement of our planetary system, and which were early observed. the first of these depends upon the earth's rotation on its axis, the result of which is that the stars appear to revolve with a uniform motion from east to west; the velocity increasing with the distance from the pole star, which remains nearly fixed. this circumstance is almost as easy of observation as the phases of the moon, and was used from the earliest ages to mark the passage of time during the night. the next arises from the motion of the earth in her orbit about the sun, by which it happens that the earth is in a different position with respect to the sun every night, and, therefore, a different set of stars are seen in his neighbourhood; these are setting with him, and therefore also a different set are just rising at sunset every evening. these changes, which would go through the cycle in a year, are, of course, less obvious, but of great importance as marking the approach of the various seasons during ages in which the hour of the sun's rising could not be noted by a clock. the last depends on the proper motions of the moon and planets about the earth and sun respectively, by reason of which those heavenly bodies occupy varying positions among the stars. only a careful and continuous scrutiny of the heavens would detect these changes, except, perhaps, in the case of the moon, and but little of importance really depends on them; nevertheless, they were very early the subject of observation, as imagination lent them a false value, and in some cases because their connection with eclipses was perceived. the practical cultivation of astronomy amongst the earliest people had always reference to one or other of these three sets of appearances, and the various terms and signs that were invented were intended for the clearer exposition of the results of their observations on these points. in looking therefore into extreme antiquity we shall find in many instances our only guide to what their knowledge was is the way in which they expressed these results. we do not find, and perhaps we should scarcely expect to find, any one man or even one nation who laid the foundation of astronomy--for it was an equal necessity for all, and was probably antecedent to the practice of remembering men by their names. we cannot, either, conjecture the antiquity of ideas and observations met with among races who are themselves the only record of their past; and if we are to find any origins of the science, it is only amongst those nations which have been cultivators of arts by which their ancient doings are recorded. amongst the earliest cultivators of astronomy we may refer to the primitive greeks, the chinese, the egyptians, the babylonians, and the aryans, and also to certain traditions met with amongst many savage as well as less barbarous races, the very universality of which proclaims as loudly as possible their extreme antiquity. each of the four above-mentioned races have names with which are associated the beginnings of astronomy--uranus and atlas amongst the greeks; folic amongst the chinese; thaut or mercury in egypt; zoroaster and bel in persia and babylonia. names such as these, if those of individuals, are not necessarily those of the earliest astronomers--but only the earliest that have come down to us. indeed it is very far from certain whether these ancient celebrities have any real historical existence. the acts and labours of the earliest investigators are so wrapped in obscurity, there is such a mixture of fable with tradition, that we can have no reliance that any of them, or that others mentioned in ancient mythology, are not far more emblematical than personal. some, such as uranus, are certainly symbolical; but the very existence of the name handed down to us, if it prove nothing else, proves that the science was early cultivated amongst those who have preserved or invented them. if we attempt to name in years the date of the commencement--not of astronomy itself--for that probably in some form was coeval with the race of man itself, but of recorded observations, we are met with a new difficulty arising from the various ways in which they reckoned time. this was in every case by the occurrence of the phases of one or other of the above-mentioned phenomena; sometimes however they selected the apparent rotation of the sun in twenty-four hours, sometimes that of the moon in a month, sometimes the interval from one solstice to the next, and yet they apparently gave to each and all of these the same title--such as _annus_--obviously representing a cycle only, but without reference to its length. by these different methods of counting, hopeless confusion has often been introduced into chronology; and the moderns have in many instances unjustly accused the ancients of vanity and falsehood. bailly attempted to reconcile all these various methods and consequent dates with each other, and to prove that practical astronomy commenced "about , years before the deluge, or that it is about , years old;" but we shall see reason in the sequel for suspecting any such attempt, and shall endeavour to arrive at more reliable dates from independent evidence. perhaps the remotest antiquity to which we can possibly mount is that of the aryans, amongst whom the hymns of the _rig veda_ were composed. the short history of hebrew and greco-roman civilization seems to be lost in comparison with this the earliest work of human imagination. when seeking for words to express their thoughts, these primitive men by the banks of the oxus personified the phenomena of the heavens and earth, the storm, the wind, the rain, the stars and meteors. here, of course, it is not practical but theoretical astronomy we find. we trace the first figuring of that primitive idea alluded to before--the heaven above, the earth below. here, as we see, is the earth represented as an indefinite plane surface and passive being forming the foundation of the world; and above it the sky, a luminous and variable vault beneath which shines out the fertile and life-giving light. thus to the earth they gave the name p'rthovi, "the wide expanse;" the blue and star-bespangled heavens they called varuna, "the vault;" and beneath it in the region of the clouds they enthroned the light dyaus, _i.e._ "the luminous air." [illustration: fig. .] from hence, it would appear, or on this model, the early ideas of all peoples have been formed. among the greeks the name for heaven expresses the same idea of a hollow vault ([greek: koilos], hollow, concave) and the earth is called [greek: gê], or mother. among the latins the name _coelum_ has the same signification, while the earth _terra_ comes from the participle _tersa_ (the dry element) in contradistinction to _mare_ the wet. in this original aryan notion, however, as represented by the figure, we have more than this, the origin of the names _jupiter_ and _deus_ comes out. for it is easy to trace the connection between _dyaus_ (the luminiferous air) and the greek word _zeus_ from whence _dios_, [greek: _theos_], _deus_, and the french word _dieu_, and then by adding _pater_ or father we get _deuspater_, _zeuspater_, jupiter. these etymologies are not however matters beyond dispute, and there are at least two other modes of deriving the same words. thus we are told the earliest name for the deity was jehovah, the word _jehov_ meaning father of life; and that the greeks translated this into _dis_ or _zeus_, a word having, according to this theory, the same sense, being derived from [greek: zaô] to live. of course there can be no question of the later word _deus_ being the direct translation of _dios_. a third theory is that there exists in one of the dialects which formed the basis of the old languages of asia, a word _yahouh_, a participle of the verb _nîh_, to exist, to be; which therefore signifies the self-existent, the principle of life, the origin of all motion, and this is supposed to be the allusion of diodorus, who explaining the theology of the greeks, says that the egyptians according to manetho, priest of memphis, in giving names to the five elements have called the spirit or ether youpiter in the _proper sense_ of the word, for the spirit is the source of life, the author of the vital principle in animals, and is hence regarded as the father or generator of all beings. the people of the homeric ages thought the lightning-bearing jupiter was the commencement, origin, end, and middle of all things, a single and universal power, governing the heavens, the earth, fire, water, day and night, and all things. porphyry says that when the philosophers discoursed on the nature and parts of the deity, they could not imagine any single figure that should represent all his attributes, though they presented him under the appearance of a man, who was _seated_ to represent his immovable essence; uncovered in his upper part, because the upper parts of the universe or region of the stars manifest most of his nature; but clothed below the loins, because he is more hidden in terrestrial things; and holding a sceptre in his left hand, because his heart is the ruler of all things. there are, besides, the etymologies which assert that jupiter is derived from _juvare_ to help, meaning the assisting father; or again that he is _dies pater_--the god of the day--in which case no doubt the sun would be alluded to. it appears then that the ancient aryan scheme, though _possibly_ supplying us with the origin of one of the widest spread of our words, is not universally allowed to do so. this origin, however, appears to derive support from the apparent occurrence of the original of another well-known ancient classical word in the same scheme, that is varuna, obviously the same word as [greek: ouranos], and uranus, signifying the heavens. less clearly too perhaps we may trace other such words to the same source. thus the sun, which according to these primitive conceptions is the husband of the earth, which it nourishes and makes fruitful, was called _savitr_ and _surya_, from which the passage to the gothic _sauil_ is within the limits of known etymological changes, and so comes the lithuanian _saull_, the cymric _haul_, the greek _heilos_, the latin _sol_, and the english _solar_. so from their _nakt_, the destructive, we get _nux_, _nacht_, _night_. from _glu_, the shining, whence the participle _glucina_, and so to _lucina_, _luena_, _luna_, _lune_. turning from the ancient aryans, whose astronomy we know only from poems and fables, and so learn but little of their actual advance in the science of observation, we come to the babylonians, concerning whose astronomical acquirements we have lately been put in possession of valuable evidence by the tablets obtained by mr. smith from kouyunjik, an account the contents of which has been given by mr. sayce (_nature_, vol. xii. p. ). as the knowledge thus obtained is more certain, being derived from their actual records, than any that we previously possessed, it will be well to give as full an account of it as we are able. the originators of babylonian astronomy were not the chaldæans, but another race from the mountains of elam, who are generally called acadians. of the astronomy of this race we have no complete records, but can only judge of their progress by the words and names left by them to the science, as afterwards cultivated by the semitic babylonians. these last were a subsequent race, who entering the country from the east, conquered the original inhabitants about b.c., and borrowed their civilization, and with it their language in the arts and sciences. but even this latter race is one of considerable antiquity, and when we see, as we shortly shall, the great advances they had made in observations of the sun and moon, and consider the probable slowness of development in those early ages, we have some idea of the remoteness of the date at which astronomical science was there commenced. our chief source of information is an extremely ancient work called the _observations of bell_, supposed to have been written before b.c., which was compiled for a certain king saigou, of agave in babylonia. this work is in seventy books or parts, and is composed of numerous small earthen tablets having impressed upon them the cuneiform character in which they printed, and which we are now able to read. we generally date the art of printing from caxton, in , because it took the place of manuscript that had been previously in use in the west; but that method of writing, if in some respects an improvement on previous methods of recording ideas as more easily executed, was in others a retrogression as being less durable: while the manuscripts have perished the impressions on stone have remained to this day, and will no doubt last longer than even our printed books. these little tablets represented so many leaves, and in large libraries, such as that from which those known have been derived, they were numbered as our own are now, so that any particular one could be asked for by those who might wish to consult it. the great difficulty of interpreting these records, which are written in two different dialects, and deal often with very technical matters, may well be imagined. these difficulties however have been overcome, and a good approach to the knowledge of their contents has been made. the chaldæans, as is well known, were much given to astronomy and many of their writings deal with this subject; but they did practical work as well, and did not indulge so much in theory as the aryans. we shall have future occasion in this book to refer to their observations on various points, as they did not by any means confine themselves to the simplest matters; much, in fact, of that with which modern astronomy deals, the dates and duration of eclipses of the sun and moon, the accurate measurement of time, the existence of cycles in lunar and solar phenomena, was studied and recorded by them. we can make some approach to the probable dates of the invention of some part of their system, by means of the signs of the zodiac, which were invented by them and which we will discuss more at length hereafter. we need only say at present that what is now the sign of spring, was not reckoned so with them, and that we can calculate how long ago it is that the sign they reckoned the spring sign was so. semiramis also raised in the centre of babylon a temple consecrated to jupiter, whom the babylonians called bel. it was of an extraordinary height and served for an observatory. the whole edifice was constructed with great art in asphalte and brick. on its summit were placed the statues of jupiter, juno, and rhea, covered with gold. the egyptians have always been named as the earliest cultivators of astronomy by the grecian writers, by whom the science has been handed down to us, and the chaldæans have even been said to have borrowed from them. the testimony of such writers however is not to be received implicitly, but to be weighed with the knowledge we may now obtain, as we have noticed above with respect to the babylonians, from the actual records they have left us, whether by actual records, or by words and customs remaining to the present day. [illustration: plate i.--babylonian astronomers.] herodotus declares that the egyptians had made observations for , years and had seen the course of the sun change four times, and the ecliptic placed perpendicular to the equator. this is the style of statement on which opinions of the antiquity of egyptian astronomy have been founded, and it is obviously unworthy of credit. diodorus says that there is no country in which the positions and motions of the stars have been so accurately observed as in egypt (_i.e._ to his knowledge). they have preserved, he says, for a great number of years registers in which their observations are recorded. expositions are found in these registers of the motions of the planets, their revolutions and their stations, and, moreover, the relation which each bears to the birthdays of animals, and its good or evil influence. they often predicted the future with success. the earthquakes, inundations, the appearance of comets, and many other phenomena which it is impossible for the vulgar to know beforehand, were foreseen by them by means of the observations they had made over a long series of years. on the occasion of the french expedition to egypt, a long passage was discovered leading from karnak to lucksor. this passage was adorned on each side of the way with a range of sphinxes with the body of a lion and the head of a ram. now in egyptian architecture, the ornaments are never the result of caprice or chance; on the contrary, all is done with intention, and what often appears at first sight strange, appears, after having been carefully examined and studied, to present allegories full of sense and reason, founded on a profound knowledge of natural phenomena, that the ornaments are intended to record. these sphinxes and rams of the passage were probably the emblems of the different signs of the zodiac along the route of the sun. the date of the avenue is not known; but it would doubtless lead us to a high antiquity for the egyptian observations. the like may be said of the great pyramid, which according to piazzi smyth was built about b.c. certainly there are no carvings about it exhibiting any astronomical designs; but the exact way in which it is executed would seem to indicate that the builders had a very clear conception of the importance of the meridian line. it should, however, be stated that piazzi smyth does not consider it to have been built by the egyptians for themselves; but under the command of some older race. there seem, however, to be indications in various festivals and observances, which are met with widely over the earth's surface, as will be indicated more in detail in the chapter on the pleiades, that some astronomical observations, though of the rudest, were made by races anterior even to those whose history we partially possess; and that not merely because of its naturalness, but because of positive evidence, we must trace back astronomy to a source from whence egyptians, indians, and perhaps babylonians themselves derived it. the chinese astronomy is totally removed from these and stands on its own basis. with them it was a matter concerning the government, and stringent laws were enforced on the state astronomers. the advance, however, that they made would appear to be small; but if we are to believe their writers, they made observations nearly three thousand years before our era. under the reign of hoangti, yuchi recorded that there was a large star near the poles of the heavens. by a method which we shall enlarge upon further on, it can be astronomically ascertained that about the epoch this observation was said to be made there was a star ([greek: a] draconis) so near the pole as to appear immovable, which is so far a confirmation of his statement. in the first of a series of eclipses was recorded by them; but the value of their astronomy seems to be doubtful when we learn that calculation proves that not one of them previous to the age of ptolemy can be identified with the dates given. amongst all nations except the chinese, where it was political, and the greeks, where it was purely speculative, astronomy has been intimately mixed with religious ideas, and we consequently find it to have taken considerable hold on the mind. just as we have seen among the indians that the basis of their astronomical ideas was the two-fold division into heaven and earth, so among other nations this duality has formed the basis of their religion. two aspects of things have been noticed by men in the constitution of things--that which remains always, and that which is merely transitory, causes and effects. the heaven and the earth have presented the image of this to their minds--one being the eternal existence, the other the passing form. in heaven nothing seems to be born, increase, decrease, or die above the sphere of the moon. that alone showed the traces of alteration in its phases; while on the other hand there was an image of perpetuity in its proper substance, in its motion, and the invariable succession of the same phases. from another point of view, the heavens were regarded as the father, and the earth as the mother of all things. for the principle of fertility in the rains, the dew and the warmth, came from above; while the earth brought forth abundantly of the products of nature. such is the idea of plutarch, of hesiod, and of virgil. from hence have arisen the fictions which have formed the basis of theogony. uranus is said to have espoused ghe, or the heavens took the earth to wife, and from their marriage was born the god of time or saturn. another partly religious, and partly astronomical antagonism has been drawn between light and darkness, associated respectively with good and evil. in the days when artificial lights, beyond those of the flickering fire, were unknown, and with the setting of the sun all the world was enveloped in darkness and seemed for a time to be without life, or at least cut off entirely from man, it would seem that the sun and its light was the entire origin of life. hence it naturally became the earliest divinity whose brilliant light leaping out of the bosom of chaos, had brought with it man and all the universe, as we see it represented in the theologies of orpheus and of moses; whence the god bel of the chaldeans, the oromaza of the persians, whom they invoke as the source of all that is good in nature, while they place the origin of all evil in darkness and its god ahrinam. we find the glories of the sun celebrated by all the poets, and painted and represented by numerous emblems and different names by the artists and sculptors who have adorned the temples raised to nature or the great first cause. among the jews there are traditions of a very high antiquity for their astronomy. josephus assures us that it was cultivated before the mosaic deluge. according to him it is to the public spirit and the labour of the antediluvians that we owe the science of astrology: "and since they had learnt from adam that the world should perish by water and by fire, the fear that their science should be lost, made them erect two columns, one of brick the other of stone, on which they engraved the knowledge they had acquired, so that if a deluge should wash away the column of brick, the stone one might remain to preserve for posterity the memory of what they had written. the prescience was rewarded, and the column of stone is still to be seen in syria." whatever we may think of this statement it would certainly be interesting if we could find in syria or anywhere else a monument that recorded the ancient astronomical observations of the jews. ricard and others believe that they were very far advanced in the science, and that we owe a great part of our present astronomy to them; but such a conjecture must remain without proof unless we could prove them anterior to the other nations, whom, we have seen, cultivated astronomy in very remote times. one observation seems peculiar to them, if indeed it be a veritable observation. josephus says, "god prolonged the life of the patriarchs that preceded the deluge, both on account of their virtues, and to give them the opportunity of perfecting the sciences of geometry and astronomy which they had discovered; which they could not have done if they had not lived for years, because it is only after the lapse of years that the _great year_ is accomplished." now what is this great year or cycle of years? m. cassini, the director of the observatory of paris, has discussed it astronomically. he considers it as a testimony of the high antiquity of their astronomy. "this period," he says, "is one of the most remarkable that have been discovered; for, if we take the lunar month to be days h. m. s. we find that , - / days make , lunar months, and that this number of days gives solar years of days h. m. s. if this year was in use before the deluge, it appears very probable it must be acknowledged that the patriarchs were already acquainted to a considerable degree of accuracy with the motions of the stars, for this lunar month agrees to a second almost with that which has been determined by modern astronomers." a very similar argument has been used by prof. piazzi smyth to prove that the great pyramids were built by the descendants of abraham near the time of noah; namely, that measures of two different elements in the measurement of time or space when multiplied or divided produce a number which may be found to represent some proportion of the edifice, and hence to assume that the two numbers were known to the builders. we need scarcely point out that numbers have always been capable of great manipulation, and the mere fact of one number being so much greater than another, is no proof that _both_ were known, unless we knew that _one_ of them was known independently, or that they are intimately connected. in the case of josephus' number the cycle during which the lunar months and solar years are commensurable has been long discussed and if the number had been instead of , we should have had little doubt of its reference; yet is a very simple number and might refer to many other cycles than the complicated one pointed out by m. cassini. a similar case may be quoted with regard to the indians, which, according to our temperament, may be either considered a proof that these reasonings are correct, or that they are easy to make. they say that there are two stars diametrically opposite which pass through the zodiac in years; nothing can be made of this period, nor yet of another equally problematical one of years; but if we multiply the two together we obtain , , which is very nearly the length of the cycle for the precession of the equinoxes. in this review of the ancient ideas of different peoples, we have followed the most probable order in considering that the observation of nature came first, and the different parts of it were afterwards individualized and named. it is proper to add that according to some ancient authors--such as diodorus siculus--the process was considered to have been the other way. that uranus was an actual individual, that atlas and saturn were his sons or descendants or followers, and that because atlas was a great astronomer he was said to support the heavens, and that his seven daughters were real, and being very spiritual they were regarded as goddesses after death and placed in heaven under the name of the pleiades. however, the universality of the ideas seems to forbid this interpretation, which is also in itself much less natural. these various opinions lead us to remark, in conclusion, that the fables of ancient mythological astronomy must be interpreted by means of various keys. allegory is the first--the allegory employed by philosophers and poets who have spoken in figurative language. their words taken in the letter are quite unnatural, but many of the fables are simply the description or explanation of physical facts. hieroglyphics are another key. having become obscure by the lapse of time they sometimes, however, present ideas different from those which they originally expressed. it is pretty certain that hieroglyphics have been the source of the men with dogs' heads, or feet of goats, &c. fables also arise from the adoption of strange words whose sound is something like another word in the borrowing language connected with other ideas, and the connection between the two has to be made by fable. chapter ii. astronomy of the celts. the numerous stone monuments that are to be found scattered over this country, and over the neighbouring parts of normandy, have given rise to many controversies as to their origin and use. by some they have been supposed to be mere sepulchral monuments erected in late times since the roman occupation of great britain. such an idea has little to rest upon, and we prefer to regard them, as they have always been regarded, as relics of the druidical worship of the celtic or gaulish races that preceded us in this part of europe. if we were to believe the accounts of ordinary historians, we might believe that the druids were nothing more than a kind of savage race, hidden, like the fallow-deer in the recesses of their woods. thought to be sanguinary, brutal, superstitious, we have learned nothing of them beyond their human sacrifices, their worship of the oak, their raised stones; without inquiring whether these characteristics which scandalize our tastes, are not simply the legacy of a primitive era, to which, by the side of the tattered religions of the old paganism, druidism remained faithful. nevertheless the druids were not without merit in the order of thought. for the celts, as for all primitive people, astronomy and religion were intimately associated. they considered that the soul was eternal, and the stars were worlds successively inhabited by the spiritual emigrants. they considered that the stars were as much the abodes of human life as our own earth, and this image of the future life constituted their power and their grandeur. they repelled entirely the idea of the destruction of life, and preferred to see in the phenomena of death, a voyage to a region already peopled by friends. under what form did druidical science represent the universe? their scientific contemplation of the heavens was at the same time a religious contemplation. it is therefore impossible to separate in our history their astronomical and theological heavens. in their theological astronomy, or astronomical theology, the druids considered the totality of all living beings as divided into three circles. the first of these circles, the circle of immensity, _ceugant_, corresponding to incommunicable, infinite attributes, belonged to god alone; it was properly the absolute, and none, save the ineffable being, had a right there. the second circle, that of blessedness, _gwyn-fyd_, united in it the beings that have arrived at the superior degrees of existence; this was heaven. the third, the circle of voyages, _abred_, comprised all the noviciate; it was there, at the bottom of the abysses, in the great oceans, as taliesin says, that the first breath of man commenced. the object proposed to men's perseverance and courage was to attain to what the bards called the point of liberty, very probably the point at which, being suitably fortified against the assaults of the lower passions, they were not exposed to be troubled, against their wills, in their celestial aspirations; and when they arrived at such a point--so worthy of the ambition of every soul that would be its own master--they quitted the circle of abred and entered that of gwyn-fyd; the hour of their recompense had come. demetrius, cited by plutarch, relates that the druids believed that these souls of the elect were so intimately connected with our circle that they could not emerge from it without disturbing its equilibrium. this writer states, that being in the suite of the emperor claudius, in some part of the british isles, he heard suddenly a terrible hurricane, and the priests, who alone inhabited these sacred islands, immediately explained the phenomenon, by telling him that a vacuum had been produced on the earth, by the departure of an important soul. "the great men," he said, "while they live are like torches whose light is always beneficent and never harms any one, but when they are extinguished their death generally occasions, as you have just seen, winds, storm, and derangements of the atmosphere." the palingenetic system of the druids is complete in itself, and takes the being at his origin, and conducts him to the ultimate heaven. at the moment of his creation, as henry martyn says in his commentary, the being has no conscience of the gifts that are latent in him. he is created in the lowest stage of life, in _annwfn_, the shadowy abyss at the base of _abred_. there, surrounded by nature, submitted to necessity, he rises obscurely through the successive degrees of inorganic matter, and then through the organic. his conscience at last awakes. he is man. "three things are primarily contemporaneous--man, liberty, and light." before man there was nothing in creation but fatal obedience to physical laws; with man commences the great battle between liberty and necessity, good and evil. the good and the evil present themselves to man in equilibrium, "and he can at his pleasure attach himself to one or the other of them." it might appear at first sight that it was carrying things too far to attribute to the druids the knowledge, not indeed of the true system of the world, but the general idea on which it was constructed. but, on closer examination, this opinion seems to have some consistency. if it was from the druids that pythagoras derived the basis of his theology, why should it not be from them that he derived also that of his astronomy? why, if there is no difficulty in seeing that the principle of the subordination of the earth might arise from the meditations of an isolated spirit, should there be any more difficulty in thinking that the principles of astronomy should take birth in the midst of a corporation of theologians embued with the same ideas as the philosophers on the circulation of life, and applied with continued diligence to the study of celestial phenomena. the druid, not having to receive mythological errors, might be led by that circumstance to imagine in space other worlds similar to our own. independently of its intrinsic value, this supposition rests also upon the testimony of historians. a singular statement made by hecatæus with regard to the religious rites of great britain exhibits this in a striking manner. this historian relates that the moon, seen in this island, appears much larger than it does anywhere else, and that it is possible to distinguish mountains on its surface, such as there are on the earth. now, how had the druids made an observation of this kind? it is of not much consequence whether they had actually seen the lunar mountains or had only imagined them, the curious thing is that they were persuaded that that body was like the earth, and had mountains and other features similar to our own. plutarch, in his treatise _de facie in orbe lunæ_, tells us that, according to the druids, and conformably to an idea which had long been held in science, the surface of the moon is furrowed with several mediterraneans, which the grecian philosophers compare to the red and caspian seas. it was also thought that immense abysses were seen, which were supposed to be in communication with the hemisphere that is turned away from the earth. lastly, the dimensions of this sky-borne country were estimated; (ideas very different to those that were current in greece): its size and its breadth, says the traveller depicted by the writer, are not at all such as the geometers say, but much larger. it is through the same author, who is in accordance in this respect with all the bards, that we know that this celestial earth was considered by the theologians of the west as the residence of happy souls. they rose and approached it in proportion as their preparation had been complete, but, in the agitation of the whirlwind, many reached the moon that it would not receive. "the moon repelled a great number, and rejected them by its fluctuations, at the moment they reached it; but those that had better success fixed themselves there for good; their soul is like the flame, which, raising itself in the ether of the moon, as fire raises itself on that of the earth receives force and solidity in the same way that red-hot iron does when plunged into the water." they thus traced an analogy between the moon and the earth, which they doubtless carried out to its full development, and made the moon an image of what they knew here, picturing there the lunar fields and brooks and breezes and perfumes. what a charm such a belief must have given to the heavens at night. the moon was the place and visible pledge of immortality. on this account it was placed in high position in their religion; the order of all the festivals was arranged after that which was dedicated to it; its presence was sought in all their ceremonies, and its rays were invoked. the druids are always therefore represented as having the crescent in their hands. astronomy and theology being so intimately connected in the spirit of the druids, we can easily understand that the two studies were brought to the front together in their colleges. from certain points of view we may say that the druids were nothing more than astronomers. this quality was not less striking to the ancients in them than in the chaldæans. the observation of the stars was one of their official functions. cæsar tells us, without entering more into particulars, that they taught many things about _the form and dimensions of the earth, the size and arrangements of the different parts of heaven, and the motions of the stars_, which includes the greater part of the essential problems of celestial geometry, which we see they had already proposed to themselves. we can see the same fact in the magnificent passage of taliesin. "i will ask the bards," he says in his _hymn of the world_, "and why will not the bards answer me? i will ask of them what sustains the earth, since having no support it does not fall? or if it falls which way does it go? but what can serve for its support? is the world a great traveller? although it moves without ceasing, it remains tranquil in its route; and how admirable is that route, seeing that the world moves not in any direction." this suffices to show that the ideas of the druids on material phenomena were not at all inferior to their conceptions of the destiny of the soul, and that they had scientific views of quite another origin from the alexandrian greeks, the latins, their disciples, or the middle ages. an anecdote of the eighth century furnishes another proof in favour of druidical science. every one knows that virgilius, bishop of salzburg, was accused of heresy by boniface before the pope zacharias, because he had asserted that there were antipodes. now virgilius was educated in one of the learned monasteries of ireland, which were fed by the christian bards, who had preserved the scientific traditions of druidism. [illustration: plate ii.--druidical worship.] the fundamental alliance between the doctrine of the plurality of worlds and of the eternity of the soul is perhaps the most memorable character in the thoughts of this ancient race. the death upon earth was for them only a psychological and astronomical fact, not more grave than that which happened to the moon when it was eclipsed, nor the fall of the verdant clothing of the oak under the breath of the autumnal breeze. we see these conceptions and manners, at first sight so extraordinary, clothe themselves with a simple and natural aspect. the druids were so convinced of the future life in the stars, that they used _to lend money to be repaid in the other world_. such a custom must have made a profound impression on the minds of those who daily practised it. pomponius mela and valerius maximus both tell us of this custom. the latter says, "after having left marseilles i found that ancient custom of the gauls still in force, namely, of lending one another money to be paid back in the infernal regions, for they are persuaded that the souls of men are immortal." in passing to the other world they lost neither their personality, their memory, nor their friends; they there re-encountered the business, the laws, the magistrates of this world. they had capitals and everything the same as here. they gave one another rendezvous as emigrants might who were going to america. this superstition, so laudable as far as it had the effect of pressing on the minds of men the firm sentiment of immortality, led them to burn, along with the dead, all the objects which had been dear to them, or of which they thought they might still wish to make use. "the gauls," says pomponius mela, "burn and bury with the dead that which had belonged to the living." they had another custom prompted by the same spirit, but far more touching. when any one bade farewell to the earth, each one charged him to take letters to his absent friends, who should receive him on his arrival and doubtless load him with questions as to things below. it is to diodorus that we owe the preservation of the remembrance of this custom. "at their funerals," he says, "they place letters with the dead which are written to those already dead by their parents, so that they may be read by them." they followed the soul in thought in its passage to the other planets, and the survivors often regretted that they could not accomplish the voyage in their company; sometimes, indeed, they could not resist the temptation. "there are some," says mela, "who burn themselves with their friends in order that they may continue to live together." they entertained another idea also, which led even to worse practices than this, namely, that death was a sort of recruiting that was commanded by the laws of the universe for the sustenance of the army of existences. in certain cases they would replace one death by another. posidonius, who visited gaul at an epoch when it had not been broken up, and who knew it far better than cæsar, has left us some very curious information on this subject. if a man felt himself seriously warned by his disease that he must hold himself in readiness for departure, but who, nevertheless, had, for the moment, some important business on hand, or the needs of his family chained him to this life, or even that death was disagreeable to him; if no member of his family or his clients were willing to offer himself instead, he looked out for a substitute; such a one would soon arrive accompanied by a troop of friends, and stipulating for his price a certain sum of money, he distributed it himself as remembrances among his companions,--often even he would only ask for a barrel of wine. then they would erect a stage, improvise a sort of festival, and finally, after the banquet was over, our hero would lie down on the shield, and driving a sword into his bosom, would take his departure for the other world. such a custom, indeed, shows anything but what we should rightly call civilization, however admirable may have been their opinions; but it receives its only palliation from the fact that their indifference to death did not arise from their undervaluing life here, but that they had so firm a belief in the existence and the happiness of a life hereafter. that these beliefs were not separated from their astronomical ideas is seen from the fact that they peopled the firmament with the departed. the milky way was called the town of gwyon (coër or ker gwydion, ker in breton, caer in gaulish, kohair in gaelic); certain bardic legends gave to gwyon as father a genius called don, who resides in the constellation of cassiopeia, and who figures as "the king of the fairies" in the popular myths of ireland. the empyrean is thus divided between various heavenly spirits. arthur had for residence the great bear, called by the druids "arthur's chariot." we are not, however, entirely limited to tradition and the reports of former travellers for our information as to the astronomy of the druids, but we have also at our service numerous coins belonging to the old gauls, who were of one family with those who cultivated druidism in our island, which have been discovered buried in the soil of france. the importance which was given to astronomy in that race becomes immediately evident upon the discovery of the fact that these coins are marked with figures having reference to the heavenly bodies, in other words are astronomical coins. if we examine, from a general point of view, a large collection of gaulish medals such as that preserved in the national museum of paris, we observe that among the essential symbols that occupy the fields are types of the horse, the bull, the boar, the eagle, the lion, the horseman, and the bear. we remark next a great number of signs, most often astronomical, ordinarily accessory, but occasionally the chief, such as the sign [symbol: rotated mirrored s], globules surrounded by concentric circles, stars of five, six, or eight points, radiated and flaming bodies, crescents, triangles, wheels with four spokes, the sign [symbol: infinity], the lunar crescent, the zigzag, &c. lastly, we remark other accessory types represented by images of real objects or imaginary figures, such as the lyre, the diota, the serpent, the hatchet, the human eye, the sword, the bough, the lamp, the jewel, the bird, the arrow, the ear of corn, the fishes, &c. on a great number of medals, on the stateres of vercingetorix, on the reverses of the coins of several epochs, we recognize principally the sign of the waterer, which appears to symbolize for one part of antiquity the knowledge of the heavenly sphere. on the gaulish types this sign (an amphora with two handles) bears the name of diota, and represents amongst the druids as amongst the magi the sciences of astronomy and astrology. some of these coins are represented in the woodcut below. [illustration: fig. .] the first of these represents the course of the sun-horse reaching the tropic of cancer (summer solstice), and brought back to the tropic of capricorn (winter solstice). on the second is seen the symbol of the year between the south (represented by the sun [symbol: sun]) and the north (represented by the northern bear). in the third the calendar (or course of the year) between the sun [symbol: sun] and the moon [symbol: moon]. time the sun, and the bear are visible on the fourth. the diurnal motion of the heavens is represented on the fifth; and lastly, on the sixth, appears the watering-pot, the sun-horse, and the sign of the course of the heavenly bodies. on other groups of money the presence of the zodiac may be made out. these medals would seem to show that some part of the astronomical knowledge of the druids was not invented by themselves, but borrowed from the chaldeans or others who in other lands invented them in previous ages, and from whom they may have possibly derived them from the phenicians. we may certainly expect, however, from these pieces of money, if found in sufficient number and carefully studied, to discover a good many positive facts now wanting to us, of the religion, sciences, manners, language, commercial relation, &c. which belonged to the celtic civilization. it was far from being so barbarous as is ordinarily supposed, and we shall do more justice to it when we know it better. m. fillioux, the curator of the museum of guéret, who has studied these coins with care, after having sought for a long time for a clear and concise method of determining exactly the symbolic and religious character of the gaulish money, has been able to give the following general statements. the coins have for their ordinary field the heavens. on the right side they present almost universally the ideal heads of gods or goddesses, or in default of these, the symbols that are representative of them. on the reverse for the most part, they reproduce, either by direct types or by emblems artfully combined, the principal celestial bodies, the divers aspects of the constellations, and probably the laws, which, according to their ancient science, presided over their course; in a smaller proportion they denote the religious myths which form the base of the national belief of the gauls. as we have seen above, for them the present life was but a transitory state of the soul, only a prodrome of the future life, which should develop itself in heaven and the astronomical worlds with which it is filled. borrowed from an elevated spiritualism, incessantly tending towards the celestial worlds, these ideas were singularly appropriate to a nation at once warlike and commercial. these circumstances explain the existence of these strange types, founded at the same time on those of other nations, and on the symbolism which was the soul of the druidical religion. to this religious caste, indeed, we must give the merit of this ingenious and original conception, of turning the reverses of the coins into regular charts of the heavens. nothing indeed could be better calculated to inspire the people with respect and confidence than these mysterious and learned symbols, representing the phenomena of the heavens. not making use of writing to teach their dogmas, which they wished to maintain as part of the mysteries of their caste, the druids availed themselves of this method of placing on the money that celestial symbolism of which they alone possessed the key. the religious ideas founded on astronomical observations were not peculiar to, or originated by, the druids, any more than their zodiac. there seems reason to believe that they had come down from a remote antiquity, and been widely spread over many nations, as we shall see in the chapter on the pleiades; but we can certainly trace them to the east, where they first prevailed in persia and egypt, and were afterwards brought to greece, where they disappeared before the new creations of anthropomorphism, though they were not forgotten in the days of the poet anacreon, who says, "do not represent for me, around this vase" (a vase he had ordered of the worker in silver), "either the heavenly bodies, or the chariot, or the melancholy orion; i have nothing to do with the pleiades or the herdsman." he only wanted mythological subjects which were more to his taste. the characters which are made use of in these astronomical moneys of the druids would appear to have a more ancient origin than we are able to trace directly, since they are most of them found on the arms and implements of the bronze age. some of them, such as the concentric pointed circles, the crescent with a globule or a star, the line in zigzag, were used in egypt; where they served to mark the sun, the month, the year, the fluid element; and they appear to have had among the druids the same signification. the other signs, such as the [symbol: wave], and its multiple combinations, the centred circles, grouped in one or two, the little rings, the alphabetical characters recalling the form of a constellation, the wheel with rays, the radiating discs, &c. are all represented on the bronze arms found in the celtic, germanic, breton, and scandinavian lands. from this remote period, which was strongly impressed with the oriental genius, we must date the origin of the celtic symbolism. it has been supposed, and not without reason, that this epoch, besides being contemporaneous with the phenician establishments on the borders of the ocean, was an age of civilization and progress in gaul, and that the ideas of the druids became modified at the same time that they acquired just notions in astronomy and in the art of casting metals. at a far later period, the druidic theocracy having, with religious care, preserved the symbols of its ancient traditions, had them stamped on the coins which they caused to be struck. this remarkable fact is shown in an incontestable manner in the rougher attempts in gaulish money, and this same state of things was perpetuated even into the epoch of the high arts, since we find on the imitation statues of macedonia the old celtic symbols associated with emblems of a grecian origin. in italy a different result was arrived at, because the warlike element of the nobles soon predominated over the religious. nevertheless the most ancient roman coins, those which are known to us under the name of consular, have not escaped the common law which seems to have presided, among all nations, over the origin of money. the two commonest types, one in bronze of _janus bifrons_ with the _palus_; the other in silver, the _dioscures_ with their stars, have an eminently astronomical aspect. the comparison between the gaulish and roman coins may be followed in a series of analogies which are very remarkable from an astronomical point of view. to cite only a few examples, we may observe on a large number of pennies of different families, the impression of auriga "the coachman" conducting a quadriga; or the sun under another form (with his head radiated and drawn in profile); or diana with her lunar attributes; or the five planets well characterised; for example, venus by a double star, as that of the morning or of the evening; or the constellations of the dog, hercules, the kid, the lyre, and almost all those of the zodiac and of the circumpolar region and the seven-kine (septemtriones). in later times, under the cæsars, in the villa of borghèse, is found a calendar whose arrangements very much recall the ancient gaulish coin. the head of the twelve great gods and the twelve signs of the zodiac are represented, and the drawing of the constellations establishes a correspondence between their rising and the position of the sun in the zodiac. it may therefore be affirmed that in the coinage and works of art in italy and greece, the characteristic influence of astronomical worship is found as strongly as among the druids. nor have the western nations alone had the curious habit of impressing their astronomical ideas upon their coinage, for in china and japan coins of a similar description have been met with, containing on their reverse all the signs of the zodiac admitted by them. in conclusion, we may say, that it was cosmography, that constructed the dogmas of the druidical religion, which was, in its essential elements, the same as that of the old oriental theocracies. the outward ceremonies were addressed to the sun, the moon, the stars, and other visible phenomena; but, above nature, there was the great generating and moving principle, which the celts placed, at a later period perhaps, among the attributes of their supreme deities. [illustration: the northern constellations. the lyre--cassiopeia--the little bear--the dragon--andromeda--the great bear--capella--algol, or medusa's head.] chapter iii. origin of the constellations. when we look upon the multitude of heavenly bodies with which the celestial vault is strewed, our attention is naturally arrested by certain groupings of brilliant stars, apparently associated together on account of their great proximity; and also by certain remarkable single stars which have excessive brilliancy or are completely isolated from the rest. these natural groups seem to have some obscure connection with or dependence on each other. they have always been noticed, even by the most savage races. the languages of several such races contain different names for the same identical groups, and these names, mostly borrowed from terrestrial beings, give an imaginary life to the solitude and silence of the skies. a celestial globe, as we know, presents us with a singular menagerie, rich in curious monsters placed in inconceivable positions. how these constellations, as they are called, were first invented, and by whom, is an interesting question which by the aid of comparative philology we must endeavour now to answer. among these constellations there are twelve which have a more than ordinary importance, and to which more attention has always been paid. they are those through which the sun appears to pass in his annual journey round the ecliptic, entering one region each month. at least, this is what they were when first invented. they were called the zodiacal constellations or signs of the zodiac--the name being derived from their being mostly named after living beasts. in our own days the zodiacal constellations are no longer the signs of the zodiac. when they were arranged the sun entered each one on a certain date. he now is no longer at the same point in the heavens at that date, nevertheless he is still said to enter the same sign of the zodiac--which therefore no longer coincides with the zodiacal constellation it was named from--but merely stands for a certain twelfth part of the ecliptic, which varies from time to time. it will be of course of great interest to discover the origin of these particular constellations, the date of their invention, &c.; and we shall hope to do so after having discussed the origin of those seen in the northern hemisphere which may be more familiar even than those. we have represented in the frontispiece the two halves of the grecian celestial sphere--the northern and the southern, with the various constellations they contain. this sphere was not invented by the greeks, but was received by them from more ancient peoples, and corrected and augmented. it was used by hipparchus two thousand years ago; and ptolemy has given us a description of it. it contained constellations, of which belonged to the northern, to the southern hemisphere, and the remaining twelve were those of the zodiac, situated along the ecliptic. the constellations reckoned by ptolemy contained altogether , stars, whose relative positions were determined by hipparchus; with reference to which accomplishment pliny says, "hipparchus, with a height of audacity too great even for a god, has ventured to transmit to posterity the number of the stars!" ptolemy's catalogue contains:-- for the northern constellations stars for the zodiacal " for the southern " or ----- for all the constellations , " or, since of these are named twice , " of course this number is not to be supposed to represent the whole of the stars visible even to the naked eye; there are twice as many in the northern hemisphere alone, while there are about , in the whole sky. the number visible in a telescope completely dwarfs this, so that more than , are now catalogued; while the number visible in a large telescope may be reckoned at not less than millions. the principal northern constellations named by ptolemy are contained in the following list, with the stars of the first magnitude that occur in each:-- the great bear, or david's chariot, near the centre. the little bear, with the pole star at the end of the tail. the dragon. cepheus, situated to the right of the pole. the herdsman, or the keeper of the bear, with the star arcturus. the northern crown to the right. hercules, or the man who kneels. the lyre, or falling vulture, with the beautiful star vega. the swan, or bird, or cross. cassiopeia, or the chair, or the throne. perseus. the carter, or the charioteer, with capella ophiuchus, or serpentarius, or esculapius. the serpent. the bow and arrow, or the dart. the eagle, or the flying vulture, with altaïr. the dolphin. the little horse, or the bust of the horse. pegasus, or the winged horse, or the great cross. andromeda, or the woman with the girdle. the northern triangle, or the delta. the fifteen constellations on the south of the ecliptic were:-- the whale. orion, with the beautiful stars rigel and betelgeuse. the river endanus, or the river orion, with the brilliant achernar. the hare. the great dog, with the magnificent sirius. the little dog, or the dog which runs before, with procyon. the ship argo, with its fine alpha (canopus) and eta. the female hydra, or the water snake. the cup, or the urn, or the vase. the raven. the altar, or the perfuming pot. the centaur, whose star alpha is the nearest to the earth. the wolf, or the centaur's lance, or the panther, or the beast. the southern crown, or the wand of mercury, or uraniscus. the southern fish, with fomalhaut. the twelve zodiacal constellations, which are of more importance than the rest, are generally named in the order in which the sun passes through them in its passage along the ecliptic, and both latins and english have endeavoured to impress their names on the vulgar by embodying them in verses. the poet ausonius thus catalogues them:-- "sunt: aries, taurus, gemini, cancer, leo, virgo, libraque, scorpius, arcitenens, caper, amphora, pisces." and the english effusion is as follows:-- "the ram, the bull, the heavenly twins, and next the crab the lion shines, the virgin and the scales. the scorpion, archer, and he goat, the man that holds the watering-pot, and fish with glittering scales." these twelve have hieroglyphics assigned to them, by which they are referred to in calendars and astronomical works, some of the marks being easily traced to their origin. thus [symbol: aries] refers to the horns of the ram; [symbol: taurus] to the head of the bull; [symbol: scorpion] to the joints and tail-sting of the scorpion; [symbol: saggitarius] is very clearly connected with an archer; [symbol: capricorn] is formed by the junction of the first two letters [greek: t] and [greek: r] in [greek: tragos], the sea-goat, or capricorn; [symbol: libra] for the balance, is suggestive of its shape; [symbol: aquarius] refers to the water in the watering-pot; and perhaps [symbol: pisces] to the two fishes; [symbol: gemini] for twins may denote two sides alike; [symbol: cancer] for the crab, has something of its side-walking appearance; while [symbol: leo] for the lion, and [symbol: virgo] for the virgin, seem to have no reference that is traceable. these constellations contain the following stars of the first magnitude--aldebaran, antares, and spica. to these constellations admitted by the greeks should be added the locks of berenice, although it is not named by ptolemy. it was invented indeed by the astronomer conon. the story is that berenice was the spouse and the sister of ptolemy euergetes, and that she made a vow to cut off her locks and devote them to venus if her husband returned victorious; to console the king the astronomer placed her locks among the stars. if this is a true account arago must be mistaken in asserting that the constellation was created by tycho brahe in . the one he did add to the former ones was that of antinöus, by collecting into one figure some unappropriated stars near the eagle. at about the same time j. bayer, from the information of vespuccius and the sailors, added twelve to the southern constellations of ptolemy; among which may be mentioned the peacock, the toucan, the phoenix, the crane, the fly, the chameleon, the bird of paradise, the southern triangle, and the indian. augustus royer, in , formed five new groups, among which we may name the great cloud, the fleur-de-lis, and the southern cross. hevelius, in , added ; the most important being the giraffe, the unicorn, the little lion, the lynx, the little triangle. among these newer-named constellations none is more interesting than the southern cross, which is by some considered as the most brilliant of all that are known. some account of it, possibly from the arabs, seems to have reached dante, who evidently refers to it, before it had been named by royer, in a celebrated passage in his "purgatory." some have thought that his reference to such stars was only accidental, and that he really referred only to the four cardinal virtues of theology, chiefly on account of the difficulty of knowing how he could have heard of them; but as the arabs had establishments along the entire coast of africa, there is no difficulty in understanding how the information might reach italy. americus vespuccius, who in his third voyage refers to these verses of dante, does not mention the name of the southern cross. he simply says that the four stars form a rhomboidal figure. as voyages round the cape multiplied, however, the constellation became rapidly more celebrated, and it is mentioned as forming a brilliant cross by the florentine andrea corsali, in , and a little later by pigafetta, in . all these constellations have not been considered sufficient, and many subsequent additions have been made. thus lacaille, in , created fourteen new ones, mostly characterized by modern names--as the sculptor's studio, the chemical furnace, the clock, the compass, the telescope, the microscope, and others. lemonnier, in , added the reindeer, the solitaire, and the indian bird, and lalande the harvestman. poczobut, in , added one more, and p. hell another. finally, in the charts drawn by bode, eight more appear, among which the aerostat, and the electrical and printing machines. we thus arrive at a total of constellations. to which we may add that the following groups are generally recognized. the head of medusa, near perseus; the pleiades, on the back, and the hyades on the forehead of the bull; the club of hercules; the shield of orion, sometimes called the rake; the three kings; the staff of s. james; the sword of orion; the two asses in the crab, having between them the star cluster, called the stall, or the manger; and the kids, near capella, in the constellation of the coachman. this brings the list of the constellations to , which is the total number now admitted. a curious episode with respect to these star arrangements may here be mentioned. about the eighth century bede and certain other theologians and astronomers wished to depose the olympian gods. they proposed, therefore, to change the names and arrangements of the constellations; they put s. peter in the place of the ram; s. andrew instead of the bull; and so on. in more recent calendars david, solomon, the magi, and other new and old testament characters were placed in the heavens instead of the former constellations; but these changes of name were not generally adopted. as an example of these celestial spheres we figure a portion of one named _coeli stellati christiani hemisphericum prius_. we here see the great bear replaced by the barque of s. peter, the little bear by s. michael, the dragon by the innocents, the coachman by s. jerome, perseus by s. paul, cassiopeia by the magdalene, andromache by s. sepulchre, and the triangle by s. peter's mitre; while for the zodiac were substituted the twelve apostles. [illustration: fig. .] in the seventeenth century a proposal was made by weigel, a professor in the university of jena, to form a series of heraldic constellations, and to use for the zodiac the arms of the twelve most illustrious families in europe; but these attempts at change have been in vain, the old names are still kept. having now explained the origin in modern times of out of the constellations, there remain the which were acknowledged by the greeks, whose origin is involved in more obscurity. one of the first to be noticed and named, as it is now the most easily recognized and most widely known, is the _great bear_, which attracts all the more attention that it is one of those that never sets, being at a less distance from the pole than the latter is from the horizon. every one knows the seven brilliant stars that form this constellation. the four in the rectangle and the three in a curved line at once call to mind the form of a chariot, especially one of antique build. it is this resemblance, no doubt, that has obtained for the constellation the name of "the chariot" that it bears among many people. among the ancient gauls it was "arthur's chariot." in france it is "david's chariot," and in england it goes by the name of "king charles' wain," and by that of the "plough." the latter name was in vogue, too, among the latins (_plaustrum_), and the three stars were three oxen, from whence it would appear that they extended the idea to all the seven stars, and at last called them the _seven_ oxen, _septem-triones_, from whence the name sometimes used for the north--septentrional. the greeks also called it the chariot ([greek: hamaxa]), and the same word seems to have stood sometimes for a plough. it certainly has some resemblance to this instrument. if we take the seven stars as representing the characteristic points of a chariot, the four stars of the quadrilateral will represent the four wheels, and the three others will represent the three horses. above the centre of the three horses any one with clear sight may perceive a small star of the fifth or sixth magnitude, called the cavalier. each of these several stars is indicated, as is usual with all the constellations, by a greek letter, the largest being denoted by the first letter. thus the stars in the quadrilateral are [greek: a], [greek: b], [greek: g], [greek: d], and the tail stars [greek: e], [greek: x], [greek: ê]. the arabs give to each star its special name, which in this case are as follows:--dubhé and mérak are the stars at the back; phegda and megrez those of the front; alioth, mizat, and ackïar the other three, while the little one over mizat is alcor. another name for it is saidak, or the tester, the being able to see it being a mark of clear vision. there is some little interest in the great bear on account of the possibility of its being used as a kind of celestial time-keeper, and its easy recognition makes it all the more available. the line through [greek: a] and [greek: b] passes almost exactly through the pole. now this line revolves of course with the constellation round the pole in hours; in every such interval being once, vertical above the pole, and once vertical below, taking the intermediate positions to right and left between these times. the instant at which this line is vertical over the pole is not the same on any two consecutive nights, since the stars advance each day minutes on the sun. on the st of march the superior passage takes place at minutes to at night; on the following night four minutes earlier, or at minutes to . in three months the culmination takes place hours earlier, or at minutes to . in six months, _i.e._ on sept. , it culminates at . in the morning, being vertically below the pole at the same hour in the evening. the following woodcut exhibits the positions of the great bear at the various hours of september th. it is plain from this that, knowing the day of the month, the hour of the night may be told by observing what angle the line joining [greek: a] and [greek: b] of this constellation makes with the vertical. [illustration: fig. .] we have used the name _great bear_, by which the constellation is best known. it is one of the oldest names also, being derived from the greeks, who called it arctos megale ([greek: arktos megalê]), whence the name arctic; and singularly enough the iroquois, when america was discovered, called it okouari, their name for a bear. the explanation of this name is certainly not to be found in the resemblance of the constellation to the animal. the three stars are indeed in the tail, but the four are in the middle of the back; and even if we take in the smaller stars that stand in the feet and head, no ingenuity can make it in this or any other way resemble a bear. it would appear, as aristotle observes, that the name is derived from the fact, that of all known animals the bear was thought to be the only one that dared to venture into the frozen regions of the north and tempt the solitude and cold. [illustration: fig. .] other origins of the name, and other names, have been suggested, of which we may mention a few. for example, "ursa" is said to be derived from _versus_, because the constellation is seen to _turn_ about the pole. it has been called the screw ([greek: elikê]), or helix, which has plainly reference to its turning. another name is callisto, in reference to its beauty; and lastly, among the arabs the great and little bears were known as the great and little coffins in reference to their slow and solemn motion. these names referred to the four stars of each constellation, the other three being the mourners following the bearers. the christian arabs made it into the grave of lazarus and the three weepers, mary, martha, and their maid. next as to the little bear. this constellation has evidently received its name from the similarity of its form to that of the great bear. in fact, it is composed of seven stars arranged in the same way, only in an inverse order. if we follow the line from [greek: b] to [greek: a] of the great bear to a distance of five times as great as that between these stars we reach the brightest star of the little bear, called the pole star. all the names of the one constellation have been applied to the other, only at a later date. the new constellations were added one by one to the celestial sphere by the greeks before they arranged certain of them as parts of the zodiac. the successive introduction of the constellations is proved completely by a long passage of strabo, which has been often misunderstood. "it is wrong," he says, "to accuse homer of ignorance because he speaks only of one of the two celestial bears. the second was probably not formed at that time. the phenicians were the first to form them and to use them for navigation. they came later to the greeks." [illustration: the constellations from the sea-shore. the swan--the lyre--hercules--the crown--the herdsman--the eagle--the serpent--the balance--the scorpion--sagittarius.] all the commentators on homer, hygin and diogenes laertes, attribute to thales the introduction of this constellation. pseudo-eratosthenes called the little bear [greek: phoinikê], to indicate that it was a guide to the phenicians. a century later, about the seventeenth olympiad, cleostrates of tenedos enriched the sphere with the archer ([greek: toxotês], sagittarius) and the ram ([greek: krios], aries), and about the same time the zodiac was introduced into the grecian sphere. with regard to the little bear there is another passage of strabo which it will be interesting to quote. he says--"the position of the people under the parallel of cinnamomophore, _i.e._ , stadia south of meroe and , stadia north of the equator, represents about the middle of the interval between the equator and the tropic, which passes by syene, which is , stadia north of meroe. these same people are the first for whom the little bear is comprised entirely in the arctic circle and remains always visible; the most southern star of the constellation, the brilliant one that ends the tail being placed on the circumference of the arctic circle, so as just to touch the horizon." the remarkable thing in this passage is that it refers to an epoch anterior to strabo, when the star [greek: a] of the little bear, which now appears almost immovable, owing to its extreme proximity to the pole, was then more to the south than the other stars of the constellation, and moved in the arctic circle so as to touch the horizon of places of certain latitudes, and to set for latitudes nearer the equator. in those days it was not the _pole_ star--if that word has any relation to [greek: poleô], i turn--for the heavens did not turn about it then as they do now. the grecian geographer speaks in this passage of a period when the most brilliant star in the neighbourhood of the pole was [greek: a] of the dragon. this was more than three thousand years ago. at that time the little bear was nearer to the pole than what we now call the polar star, for this latter was "the most southern star in the constellation." if we could alight upon documents dating back fourteen thousand years, we should find the star vega ([greek: a] lyra) referred to as occupying the pole of the world, although it now is at a distance of degrees from it, the whole cycle of changes occupying a period of about twenty-six thousand years. before leaving these two constellations we may notice the origin of the names according to plutarch. he would have it that the names are derived from the use that they were put to in navigation. he says that the phenicians called that constellation that guided them in their route the _dobebe_, or _doube_, that is, the speaking constellation, and that this same word happens to mean also in that language a bear; and so the name was confounded. certainly there is still a word _dubbeh_ in arabic having this signification. next as to the herdsman. the name of its characteristic star and of itself, arcturus ([greek: arktos], bear; [greek: ouros], guardian), is explained without difficulty by its position near the bears. there are six small stars of the third magnitude in the constellation round its chief one--three of its stars forming an equilateral triangle. arcturus is in the continuation of the curved line through the three tail stars of the great bear. the constellation has also been called atlas, from its nearness to the pole--as if it held up the heavens, as the fable goes. beyond this triangle, in the direction of the line continued straight from the great bear, is the northern crown, whose form immediately suggests its name. among the stars that compose it one, of the second magnitude, is called the pearl of the crown. it was in this point of the heavens that a temporary star appeared in may, , and disappeared again in the course of a few weeks. among the circumpolar constellations we must now speak of cassiopeia, or the chair--or throne--which is situated on the opposite side of the pole from the great bear; and which is easily found by joining its star [greek: d] to the pole and continuing it. the chair is composed principally of five stars, of the third magnitude, arranged in the form of an m. a smaller star of the fourth magnitude completes the square formed by the three [greek: b], [greek: a], and [greek: g]. the figure thus formed has a fair resemblance to a chair or throne, [greek: d] and [greek: e] forming the back; and hence the justification for its popular name. the other name cassiopeia has its connection and meaning unknown. we may suitably remark in this place, with arago, that no precise drawing of the ancient constellations has come down to us. we only know their forms by written descriptions, and these often very short and meagre. a verbal description can never take the place of a drawing, especially if it is a complex figure, so that there is a certain amount of doubt as to the true form, position, and arrangement of the figures of men, beasts, and inanimate objects which composed the star-groups of the grecian astronomers--so that unexpected difficulties attend the attempt to reproduce them on our modern spheres. add to this that alterations have been avowedly introduced by the ancient astronomers themselves, among others by ptolemy, especially in those given by hipparchus. ptolemy says he determined to make these changes because it was necessary to give a better proportion to the figures, and to adapt them better to the real positions of the stars. thus in the constellation of the virgin, as drawn by hipparchus, certain stars corresponded to the shoulders; but ptolemy placed them in the sides, so as to make the figure a more beautiful one. the result is that modern designers give scope to their imagination rather than consult the descriptions of the greeks. _cassiopeia_, _cepheus_, _andromeda_, and _perseus_ holding in his hand the _head of medusa_, appear to have been established at the same epoch, no doubt subsequently to the great bear. they form one family, placed together in one part of the heavens, and associated in one drama; the ardent perseus delivering the unfortunate andromeda, daughter of cepheus and cassiopeia. we can never be sure, however, whether the constellations suggested the fable, or the fable the constellations: the former may only mean that perseus, rising before andromeda, seems to deliver it from the night and from the constellation of the whale. the head of medusa, a celebrated woman, that perseus cut off and holds in his hand, is said by volney to be only the head of the constellation virgo, which passes beneath the horizon precisely as the perseus rises, and the serpents which surround it are ophiucus and the polar dragon, which then occupies the zenith. either way, we have no account of the origin of the _names_, and it is possible that we may have to seek it, if ever we find it, from other sources--for it would appear that similar names were used for the same constellations by the indians. this seems inevitably proved by what is related by wilford (_asiatic researches_, iii.) of his conversation with his pundit, an astronomer, on the names of the indian constellations. "asking him," he says, "to show me in the heavens the constellation of antarmada, he immediately pointed to andromeda, though i had not given him any information about it beforehand. he afterwards brought me a very rare and curious work in sanscrit, which contained a chapter devoted to _upanacchatras_, or extra-zodiacal constellations, with drawings of _capuja_ (cepheus), and of _casyapi_ (cassiopeia) seated and holding a lotus flower in her hand, of _antarmada_ charmed with the fish beside her, and last of _parasiea_ (perseus) who, according to the explanation of the book, held the head of a monster which he had slain in combat; blood was dropping from it, and for hair it had snakes." as the stars composing a constellation have often very little connection with the figure they are supposed to form, when we find the same set of stars called by the same name by two different nations, as was the case, for instance, in some of the indian names of constellations among the americans, it is a proof that one of the nations copied it from the other, or that both have copied from a common source. so in the case before us, we cannot think these similar names have arisen independently, but must conclude that the grecian was borrowed from the indian. another well-known constellation in this neighbourhood, forming an isosceles triangle with arcturus and the pole star, is the lyre. lucian of samosatus says that the greeks gave this name to the constellation to do honour to the lyre of orpheus. another possible explanation is this. the word for lyre in greek [greek: chelys] and in latin (_testudo_) means also a tortoise. now at the time when this name was imposed the chief star in the lyre may have been very near to the pole of the heavens and therefore have had a very slow motion, and hence it might have been named the tortoise, and this in greek would easily be interpreted into lyre instead. indeed this double meaning of the word seems certainly to have given rise to the fable of mercury having constructed a lyre out of the back of a tortoise. circling round the pole of the ecliptic, and formed by a sinuous line of stars passing round from the great bear to the lyre, is the dragon, which owes its name to its form. its importance is derived from its relation to the ecliptic, the pole of which is determined by reference to the stars of the first coil of the body. the centre of the zodiacal circle is a very important point, that circle being traced on the most ancient spheres, and probably being noticed even before the pole of the heavens. closely associated with the dragon both in mythology and in the celestial sphere is hercules. he is always drawn kneeling; in fact, the constellation is rather a man in a kneeling posture than any particular man. the poets called it engonasis with reference to this, which is too melancholy or lowly a position than would agree well with the valiant hero of mythology. there is a story related by Æschylus about the stones in the champ des cailloux, between marseilles and the embouchure of the rhône, to the effect that hercules, being amongst the ligurians, found it necessary to fight with them; but he had no more missiles to throw; when jupiter, touched by the danger of his son, sent a rain of round stones, with which hercules repulsed his enemies. the engonasis is thus considered by some to represent him bending down to pick up the stones. posidonius remarks that it was a pity jupiter did not rain the stones on the ligurians at once, without giving hercules the trouble to pick them up. ophiucus, which comes close by, simply means the man that holds the serpent [greek: ophi-ouchos]. it is obviously impossible to know the origins of all the names, as those we now use are only the surviving ones of several that from time to time have been applied to the various constellations according to their temporary association with the local legends. the prominent ones are favoured with quite a crowd of names. we need only cite a few. hercules, for instance, has been called [greek: okalzôn korynêtês], engonasis, ingeniculus, nessus, thamyris, desanes, maceris, almannus, al-chete, &c. the swan has the names of [greek: kyknos], [greek: iktin], [greek: ornis], olar, helenæ genitor, ales jovis, ledæus, milvus, gallina, the cross, while the coachman has been [greek: ippilatês], [greek: elastippos], [greek: airôêlatês], [greek: Êniochos], auriga, acator, hemochus, erichthonus, mamsek, alánat, athaiot, alatod, &c. with respect to the coachman, in some old maps he is drawn with a whip in his left hand turned towards the chariot, and is called the charioteer. no doubt its proximity to the former constellation has acquired for it its name. the last we need mention, as of any celebrity, is that of orion, which is situated on the equator, which runs exactly through its midst. regel forms its left foot, and the hare serves for a footstool to the right foot of the hero. three magnificent stars in the centre of the quadrilateral, which lie in one straight line are called the rake, or the three kings, or the staff of jacob, or the belt. these names have an obvious origin; but the meaning of orion itself is more doubtful. in the grecian sphere it is written [greek: Ôriôn], which also means a kind of bird. the allied word [greek: ôros] has very numerous meanings, the only one of which that could be conjectured to be connected with the constellations is a "guardian." the word [greek: horion], on the contrary, the diminutive of [greek: hôros], means a limit, and has been assigned to jupiter; and in this case may have reference to the constellation being situated on the confines of the two hemispheres. in mythology orion was an intrepid hunter of enormous size. he was the same personage as orus, arion, the minotaur, and nimrod, and afterwards became saturn. orion is called _tsan_ in chinese, which signifies three, and corresponds to the three kings. [illustration: fig. .] the asiatics used not to trace the images of their constellations, but simply joined the component stars by straight lines, and placed at the side the hieroglyphic characters that represented the object they wished to name. thus joining by five lines the principal stars in orion, they placed at the side the hieroglyphics representing a man and a sword, from whence the greeks derived the figure they afterwards drew of a giant armed with a sword. we must include in this series that brightest of all stars, sirius. it forms part of the constellation of the great dog, and lies to the south of orion near the extreme limit of our vision into the southern hemisphere in our latitudes. this star seems to have been intimately connected with egypt, and to have derived its name--as well as the name of the otherwise unimportant constellation it forms part of--from that country, and in this way:-- the overflowing of the nile was always preceded by an etesian wind, which, blowing from north to south about the time of the passage of the sun beneath the stars of the crab, drove the mists to the south, and accumulated them over the country whence the nile takes its source, causing abundant rains, and hence the flood. the greatest importance attached to the foretelling the time of this event, so that people might be ready with their provisions and their places of security. the moon was no use for this purpose, but the stars were, for the inundation commenced when the sun was in the stars of the lion. at this time the stars of the crab just appeared in the morning, but with them, at some distance from the ecliptic, the bright star sirius also rose. the morning rising of this star was a sure precursor of the inundation. it seemed to them to be the warning star, by whose first appearance they were to be ready to move to safer spots, and thus acted for each family the part of a faithful dog. whence they gave it the name of the dog, or monitor, in egyptian _anubis_, in phenician _hannobeach_, and it is still the dog-star--_caniculus_, and its rising commences our _dog-days_. the intimate connection between the rising of this star and the rising of the nile led people to call it also the nile star, or simply the nile; in egyptian and hebrew, _sihor_; in greek, [greek: sothis]; in latin, _sirius_. in the same way the egyptians and others characterised the different days of the year by the stars which first appeared in the evening--as we shall see more particularly with reference to the pleiades--and in this way certain stars came to be associated in their calendar with variations of temperature and operations of agriculture. they soon took for the cause what was originally but the sign, and thus they came to talk of moist stars, whose rising brought rain, and arid stars, which brought drought. some made certain plants to grow, and others had influence over animals. in the case of egypt, no other so great event could occur as that which the dog-star foretold, and its appearance was consequently made the commencement of the year. instead, therefore, of painting it as a simple star, in which case it would be indistinguishable from others, they gave it shape according to its function and name. when they wished to signify that it opened the year, it was represented as a porter bearing keys, or else they gave it two heads, one of an old man, to represent the passing year, the other of a younger, to denote the succeeding year. when they would represent it as giving warning of the inundation they painted it as a dog. to illustrate what they were to do when it appeared, anubis had in his arms a stew-pot, wings to his feet, a large feather under his arm, and two reptiles behind him, a tortoise and a duck. there is also in the celestial sphere a constellation called the little dog and procyon; the latter name has an obvious meaning, as appearing _before_ the dog-star. we cannot follow any farther the various constellations of the northern sphere, nor of the southern. the zodiacal constellations we must reserve for the present, while we conclude by referring to some of the changes in form and position that some of the above-mentioned have undergone in the course of their various representations. these changes are sometimes very curious, as, for example, in a coloured chart, printed at paris in , we have the charioteer drawn in the costume of adam, with his knees on the milky way, and turning his back to the public; the she-goat appears to be climbing over his neck, and two little she-goats seem to be running towards their mother. cassiopeia is more like king solomon than a woman. compare this with the _phenomena of aratus_, published , where cassiopeia is represented sitting on an oak chair with a ducal back, holding the holy palm in her left hand, while the coachman, "erichthon," is in the costume of a minion of henry the third of france. now compare the cassiopeia of the greeks with that drawn in the sixteenth and seventeenth centuries, or the coachman of the same periods, and we can easily see the fancies of the painters have been one of the most fertile sources of change. they seem, too, to have had the fancy in the middle ages to draw them all hideous and turning their backs. compare, for instance, the two pictures of andromeda and hercules, as given below, where those on one side are as heavy and gross as the others are artistic and pretty. unfortunately for the truth of andromeda's beauty, as depicted in these designs, she was supposed to be a negress, being the daughter of the ethiopians, cepheus and cassiopeia. not one of the drawings indicates this; indeed they all take after their local beauties. [illustration: fig. .] in flamsteed's chart, as drawn above, the coachman is a female; and instead of the she-goat being on the back, she holds it in her arms. no one, indeed, from any of the figures of this constellation would ever dream it was intended to represent a coachman. [illustration: fig. .] one more fundamental cause of changes has been the confusion of names derived by one nation from another, these having sometimes followed their signification, but at others being translated phonetically. thus the latins, in deriving names from the greek [greek: arktos], have partly translated it by ursa, and partly have copied it in the form arcticus. so also with reference to the three stars in the head of the bull, called by the greeks hyades. the romans thought it was derived from [greek: hyes], sows, so they called them _suculæ_, or little sows; whereas the original name was derived from [greek: hyein], to rain, and signified stars whose appearance indicated the approach of the rainy season. more curious still is the transformation of the pearl of the northern crown (margarita coronæ) in a saint--s. marguerite. the names may have had many origins whose signification is lost, owing to their being misunderstood. thus figurative language may have been interpreted as real, as when a conjunction is called a marriage; a disappearance, death; and a reappearance, a resurrection; and then stories must be invented to fit these words; or the stars that have in one country given notice of certain events lose the meaning of their names when these are used elsewhere; as when a boat painted near the stars that accompany an inundation, becomes the ship argo; or when, to represent the wind, the bird's wing is drawn; or those stars that mark a season are associated with the bird of passage, the insect or the animal that appears at that time: such as these would soon lose their original signification. the celestial sphere, therefore, as we now possess it, is not simply a collection of unmeaning names, associated with a group of stars in no way connected with them, which have been imposed at various epochs by capricious imagination, but in most instances, if not in all, they embody a history, which, if we could trace it, would probably lead us to astronomical facts, indicating the where and the when of their first introduction; and the story of their changes, so far as we can trace it, gives us some clue to the mental characteristics or astronomical progress of the people who introduced the alterations. we shall find, indeed, in a subsequent chapter, that many of our conclusions as to the birth and growth of astronomy are derived from considerations connected with the various constellations, more especially those of the zodiac. with regard to the date when and the country where the constellations of the sphere were invented, we will here give what evidence we possess, independent of the origin of the zodiac. in the first place it seems capable of certain proof that they were not invented by the greeks, from whom we have received them, but adopted from an older source, and it is possible to give limits to the date of introduction among them. newton, who attributes its introduction to musæus, a contemporary of chiron, remarks, that it must have been settled _after_ the expedition of the argonauts, and _before_ the destruction of troy; because the greeks gave to the constellation names that were derived from their history and fables, and devoted several to celebrate the memory of the famous adventurers known as the argonauts, and they would certainly have dedicated some to the heroes of troy, if the siege of that place had happened at the time. we remark that at this time astronomy was in too infant a state in greece for them to have fixed with so much accuracy the position of the stars, and that we have in this a proof they must have borrowed their knowledge from older cultivators of the science. the various statements we meet with about the invention of the sphere may be equally well interpreted of its introduction only into greece. such, for instance, as that eudoxus first constructed it in the thirteenth or fourteenth century b.c., or that by clement of alexandria, that chiron was the originator. the oldest direct account of the names of the constellations and their component stars is that of hesiod, who cites by name in his _works and days_ the pleiades, arcturus, orion, and sirius. he lived, according to herodotus, about years before christ. the knowledge of all the constellations did not reach the greeks at the same time, as we have seen from the omission by homer of any mention of the little bear, when if he had known it, he could hardly have failed to speak of it. for in his description of the shield of achilles, he mentions the pleiades, the hyades, orion and the bear, "which alone does not bathe in the ocean." he could never have said this last if he had known of the dragon and little bear. we may then safely conclude that the greeks received the idea of the constellations from some older source, probably the chaldeans. they received it doubtless as a sphere, with figured, but nameless constellations; and the greeks by slight changes adapted them to represent the various real or imaginary heroes of their history. it would be a gracious task, for their countrymen would glory in having their great men established in the heavens. when they saw a ship represented, what more suitable than to name it the ship argo? the swan must be jupiter transformed, the lyre is that of orpheus, the eagle is that which carried away ganymede, and so on. this would be no more than what other nations have done, as, for example, the chinese, who made greater changes still, unless we consider theirs to have had an entirely independent origin. [illustration: fig. .] that the celestial sphere was a conception known to others than the greeks is easily proved. the arabians, for instance, certainly did not borrow it from them; yet they have the same things represented. above is a figure of a portion of an arabian sphere drawn in the eleventh century, where we get represented plainly enough the great and little bears, the dragon, cassiopeia, andromeda, perseus, with the triple head of medusa; the triangle, one of the fishes, auriga, the ram, the bull obscurely, and the twins. there is also the famous so-called zodiac of denderah, brought from egypt to paris. this in reality contains more constellations than those of the zodiac. most of the northern ones can be traced, with certain modifications. its construction is supposed to belong to the eighth century b.c. most conspicuous on it is the lion, in a kind of barque, recalling the shape of the hydra. below it is the calf isis, with sirius, or the dog-star, on the forehead; above it is the crab, to the right the twins, over these along instrument, the plough, and above that a small animal, the little bear, and so we may go on:--all the zodiacal constellations, especially the balance, the scorpion, and the fishes being very clear. this sphere is indeed of later date than that supposed for the grecian, but it certainly appears to be independent. the remains we possess of older spheres are more particularly connected with the zodiac, and will be discussed hereafter. from what people the greeks received the celestial sphere, is a question on which more than one opinion has been formed. one is that it was originated in the tropical latitudes of egypt. the other, that it came from the chaldeans, and a third that it came from more temperate latitudes further to the east. the arguments for the last of these are as follow: there is an empty space of about °, formed by the last constellations of the sphere, towards the south pole, that is by the centaur, the altar, the archer, the southern fish, the whale, and the ship. now in a systematic plan, if the author were situated near the equator there would be no vacant space left in this way, for in this case the southern stars, attracting as much attention as the northern, would be inevitably inserted in the system of constellations which would be extended to the horizon on all sides. but a country of sufficiently high latitude to be unable to see at any time the stars about the southern pole must be north both of egypt and chaldea. this empty space remained unfilled until the discovery of the cape of good hope, except that the star canopus was included in the constellation argo, and the river eridan had an arbitrary extension given to it, instead of terminating in latitude °. another less cogent argument is derived from the interpretation of the fable of the phoenix. this is supposed to represent the course of the sun, which commences its growth at the time of its death. a similar fable is found among the swedes. now a tropical nation would find the difference of days too little to lead it to invent such a fable to represent it. it must needs have arisen where the days of winter were very much shorter than those of summer. the book of zoroaster, in which some of the earliest notices of astronomy are recorded, states that the length of a summer day is twice as long as that of winter. this fixes the latitude in which that book must have been composed, and makes it °. whence it follows, that to such a place must we look for the origin of these spheres, and not to egypt or chaldea. [illustration: plate iii.--chaldean astronomers.] diodorus siculus speaks of a nation in that part of the world, whom he calls hyperboreans, who had a tradition that their country is the nearest to the moon, on which they discovered mountains like those on the earth, and that apollo comes there once every nineteen years. this period being that of the metonic cycle of the moon, shows that if this could have really been discovered by them, they must have had a long acquaintance with astronomy. the babylonian tablets lead us to the belief that astronomy, and with it the sphere, and the zodiac were introduced by a nation coming from the east, from the mountains of elam, called the accadians, before b.c., and these may have been the nation to whom the whole is due. on the other hand, the arguments for the egyptians, or chaldeans being the originators depend solely on the tradition handed down by many, that one or other are the oldest people in the world, with the oldest civilization, and they have long cultivated astronomy. more precise information, however, seems to render these traditions, to say the least, doubtful, and certainly incapable of overthrowing the arguments adduced above. chapter iv. the zodiac. the zodiac, as already stated, is the course in the heavens apparently pursued by the sun in his annual journey through the stars. let us consider for a moment, however, the series of observations and reflections that must have been necessary to trace this zone as representing such a course. first, the diurnal motion of the whole heavens from east to west must have been noticed during the night, and the fact that certain stars never set, but turn in a circle round a fixed point. what becomes then, the next question would be, of those stars that do descend beneath the horizon, since they rise in the same relative positions as those in which they set. they could not be thought to be destroyed, but must complete the part of the circle that is invisible _beneath the earth_. the possibility of any stars finding a path beneath the earth must have led inevitably to the conception of the earth as a body suspended in the centre with nothing to support it. but leaving this alone, it would also be concluded that the sun went with the stars, and was in a certain position among them, even when both they and it were invisible. the next observations necessary would be that the zodiacal constellations visible during the nights of winter were not the same as those seen in summer, that such and such a group of stars passed the meridian at midnight at a certain time, and that six months afterwards the group exactly opposite in the heavens passed at the same hour. now since at midnight the sun will be exactly opposite the meridian, if it continues uniformly on its course, it will be among that group of stars that is opposite the group that culminates at midnight, and so the sign of the zodiac the sun occupies would be determined. this method would be checked by comparisons made in the morning and evening with the constellations visible nearest to the sun at its rising and setting. the difficulty and indirectness of these observations would make it probable that originally the zodiac would be determined rather by the path of the moon, which follows nearly the same path as the sun, and which could be observed at the same time as, and actually associated with, the constellations. now the moon is found each night so far to the east of its position on the previous night that it accomplishes the whole circumference in twenty-seven days eight hours. the two nearest whole number of days have generally been reckoned, some taking twenty-eight, and others twenty-seven. the zodiac, or, as the chinese called it, the yellow way, was thus divided into twenty-eight parts, which were called _nakshatras_ (mansions, or hotels), because the moon remains in each of them for a period of twenty-four hours. these mansions were named after the brightest stars in each, though sometimes they went a long way off to fix upon a characteristic star, as in the sixteenth indian constellation, _vichaca_, which was named after the northern crown, in latitude °. this arose from the brightness of the moon extinguishing the light of those that lie nearest to it. this method of dividing the zodiac was very widely spread, and was common to almost all ancient nations. the chinese have twenty-eight constellations, but the word _siou_ does not mean a group of stars, but simply a mansion or hotel. in the coptic and ancient egyptian the word for constellation has the same meaning. they also had twenty-eight, and the same number is found among the arabians, persians, and indians. among the chaldeans, or accadians, we find no sign of the number twenty-eight. the ecliptic or "yoke of the sky," with them, as we see in the newly-discovered tablets, was divided into twelve divisions as now, and the only connection that can be imagined between this and the twenty-eight is the opinion of m. biot, who thinks that the chinese had originally only twenty-four mansions, four more being added by chenkung (b.c. ), and that they corresponded with the twenty-four stars, twelve to the north and twelve to the south, that marked the twelve signs of the zodiac among the chaldeans. but under this supposition the twenty-eight has no reference to the moon, whereas we have every reason to believe that it has. the siamese only reckoned twenty-seven, and occasionally inserted an extra one, called _abigitten_, or intercalary moon. they made use, moreover, of the constellations to tell the hour of the night by their position in the heavens, and their method of doing this appears to have involved their having twenty-eight constellations. the names of the twenty-eight divisions among the arabs were derived from parts of the larger constellations that made the twelve signs, the first being the horns, and the second the belly, of the ram. the twenty-eight divisions among the persians, of which we may notice that the second was formed by the pleiades, and called _pervis_, soon gave way to the twelve, the names of which, recorded in the works of zoroaster, and therefore not less ancient than he, were not quite the same as those now used. they were the lamb, the bull, the twins, the crab, the lion, the ear of corn, the balance, the scorpion, the bow, the sea-goat, the watering-pot, and the fishes. nor were the chinese continually bound to the number twenty-eight. they, too, had a zodiac for the sun as well as the moon, as may be seen on some very curious pieces of money, of which those figured below are specimens. [illustration: fig. .] on some of these the various constellations of the northern hemisphere are engraved, especially the great bear--under innumerable disguises--and on others the twelve signs of the zodiac. these are very different, however, from the grecian set--they are the mouse, the bull, the tiger, the hare, the dragon, the serpent, the horse, the ram, the monkey, the cock, the dog, and the pig. the japanese series were the same. the mongolians had a series of zodiacal coins struck in the reign of jehanjir shah ( ). he had pieces of gold stamped, representing the sun in the constellation of the lion; and some years afterwards other coins were made, with one side having the impress of the particular sign in which the sun happened to be when the coin was struck. in this way a series is preserved having all the twelve signs. tavernier tells the story that one of the wives of the sultan, wishing to immortalise herself, asked jehanjir to be allowed to reign for four-and-twenty hours, and took the opportunity to have a large quantity of new gold and silver zodiacal coins struck and distributed among the people. the twenty-eight divisions are less known now, simply from the fact that the greeks did not adopt them; but they were much used by the early asian peoples, who distinguished them, like the twelve, by a series of animals, and they are still used by the arabs. so far for the nature of the zodiac, as used in various countries, and as adopted from more ancient sources by the greeks and handed on to us. it is very remarkable that the arrangement of it, and its relation to the pole of the equator, carries with it some indication of the age in which it must have been invented, as we now proceed to show. we may remark, in the first place, that from very early times the centre of the zodiacal circle has been marked in the celestial sphere, though there is no remarkable star near the spot; and the centre of the equatorial circle, or pole, has been even less noticed, though much more obvious. we cannot perhaps conclude that the instability of the pole was known, but that the necessity for drawing the zodiac led to attention being paid to its centre. both the persians and the chinese noted in addition four bright stars, which they said watched over the rest, _taschter_ over the east, _sateris_ over the west, _venaud_ over the south, and _hastorang_ over the north. now we must understand these points to refer to the sun, the east being the spring equinox, the west the autumnal, and the north and south the summer and winter solstices. there are no stars of any brilliancy that we could now suppose referred to in these positions; but if we turn the zodiac through ° we shall find aldebaran, the antares, regulus, and fomalhaut, four stars of the first magnitude, pretty nearly in the right places. does the zodiac then turn in this way? the answer is, it does. the effect of the attraction of the sun and moon upon the equatorial protuberance of the earth is to draw it round from west to east by a very slow motion, and make the ecliptic cross the equator each year about one minute of arc to the east of where it crossed it the year before. so, then, the sidereal year, or interval between the times at which the sun is in a certain position amongst the stars, is longer than the solar year, or interval between the times at which the sun crosses the equator at the vernal equinox. now the sun's position in the zodiac refers to the former, his appearance at the equinox to the latter kind of year. each solar year then--and these are the years we usually reckon by--the equinox is at a point fifty seconds of arc to the east on the zodiac, an effect which is known by the name of the precession of the equinoxes. now it is plain that if it keeps moving continuously to the east it will at last come round to the same point again, and the whole period of its revolution can easily be calculated from the distance it moves each year. the result of such a calculation shows that the whole revolution is completed in , years, after which time all will be again as it is now in this respect. [illustration: fig. .] if we draw a figure of the zodiac, as below, and know that at this time the vernal equinox takes place when the sun is in the fishes, then, the constellation of the ram being to the west of this, the date at which the equinox was there must be before our present date, while at some time in the future it will be in aquarius. now if in any old description we find that the equinox is referred to as being in the ram or in the bull, it tells us at once how long ago such a description was a true one, and, therefore, when it was written. this is the way in which the zodiac carries with it an intimation of its date. thus in the example lately referred to of the persians and their four stars, it must have been about , years ago, according to the above calculation, that these were in the positions assigned, which is therefore the date of this part of persian astronomy, if we have rightly conjectured the stars referred to. we have already said that the signs of the zodiac are not now the same as the zodiacal constellations, and this is now easily understood. it is not worth while to say that the sun enters such and such a part of the fishes at the equinox, and changes every year. so the part of the heavens it _does_ then enter--be it fishes, or aquarius, or the ram--is called by the same name--and is called a _sign_; the name chosen is the ram or aries, which coincided with the constellation of that name when the matter was arranged. there is another equally important and instructive result of this precession of the equinox. for the earth's axis is always perpendicular to the plane of the equator, and if the latter moves, the former must too, and change its position with respect to the axis of the ecliptic, which remains immovable. and the ends of these axes, or the points they occupy among the stars, called their poles, will change in the same way; the pole of the equator, round which the heavens appear to move, describing a curve about the pole of the ecliptic; and since the ecliptic and equator are always _nearly_ at the same angle, this curve will be very nearly a circle, as represented on preceding page. [illustration: fig. .] now the pole of the equator is a very marked point in the heavens, because the star nearest to it appears to have no motion. if then we draw such a figure as above, so as to see where this pole would be at any given date, and then read in any old record that such and such a star had no motion, we know at once at what date such a statement must have been made. this means of estimating dates is less certain than the other, because any star that is nearer to the pole than any other will appear to have no motion _relatively_ to the rest, unless accurate measurements were made. nevertheless, when we have any reason to believe that observations were carefully made, and there is any evidence that some particular star was considered the pole star, we have some confidence in concluding the date, examples of which will appear in the sequel; and we may give one illustration now, though not a very satisfactory one. hipparchus cites a passage from the sphere of eudoxus, in which he says, _est vero stella quædam in eodem consistens loco, quæ quidem polus est mundi._ (there is a certain immovable star, which is the pole of the world.) now referring to our figure, we find that about b.c. the two stars, [greek: b] ursæ minoris and [greek: k] draconis were fairly near the pole, and this fact leads us to date the invention of this sphere at about this epoch, rather than a little before or a little after, although, of course, there is nothing in _this_ argument (though there may be in others), to prevent us dating it when [greek: a] draconis was near the pole, b.c. this star was indeed said by the chinese astronomers in the reign of hoangti to mark the pole, which gives a date to their observations. the chief use of this latter method is to _confirm_ our conclusions from the former, rather than to originate any. let us now apply our knowledge to the facts. in the first place we may notice that in the time of hipparchus the vernal equinox was in the first degree of the ram, from which our own arrangement has originated. hipparchus lived years b.c., or nearly , years ago, at which time the equinox was exactly at [greek: b] aries. secondly, there are many reasons for believing that at the time of the invention of the zodiac, indeed in the first dawning of astronomy, the bull was the first sign into which the sun entered at the vernal equinox. now it takes , years to retrograde through a sign, and therefore the bull might occupy this position any time between and b.c., and any nearer approximation must depend on our ability to fix on any particular _part_ of the constellation as the original equinoctial point. we may say that whoever invented the zodiac would no doubt make this point the _beginning_ of a sign, and therefore date its invention b.c.; or on the other hand, if it can be proved that the constellations were known and observed before this, we may have to put back the date to near the end of the sign, and make its last remarkable stars the equinoctial ones, say those in the horns of taurus. compare the line of virgil, "candidus auratis aperit cum cornibus annum taurus." the date in this case would be about b.c.--or once more some remarkable part of the constellation may give proof that its appearance with the sun commenced the year--and our date would be intermediate between these two. in fact, the remarkable group of stars known as the _pleiades_ actually does play this part. so much interest clusters, however, round this group, so much light is thrown by it on the past history of astronomical ideas--and so much new information has recently been obtained about it--that it requires a chapter to itself, and we shall therefore pass over its discussion here. let us now review some of the indications that some part of the constellation of the bull was originally the first sign of the zodiac. we need perhaps only mention the astrological books of the jews--the cabal--in which the bull is dealt with as the first zodiacal sign. among the persians, who designate the successive signs by the letters of the alphabet, _a_ stands for taurus, _b_ for the twins, and so on. the chinese attribute the commencement of the sun's apparent motion to the stars of taurus. in thebes is a sepulchral chamber with zodiacal signs, and taurus at the head of them. the zodiac of the pagoda of elephanta (salsette) commences with the same constellation. however, reasons have been given for assigning to the zodiac a still earlier date than this would involve. thus laplace writes:--"the names of the constellations of the zodiac have not been given to them by chance--they embody the results of a large number of researches and of astronomical systems. some of the names appear to have reference to the motion of the sun. the crab, for instance, and the he-goat, indicate its retrogression at the solstices. the balance marks the equality of the days and nights at the equinoxes, and the other names seem to refer to agriculture and to the climate of the country in which the zodiac was invented. the he-goat appears better placed at the highest point of the sun's course than the lowest. in this position, which it occupied fifteen thousand years ago, the balance was at the vernal equinox, and the zodiacal constellations match well with the climate and agriculture of egypt." if we examine this, however, we see that all that is probable in it is satisfied by the ram being at the vernal, and the balance at the autumnal equinox, which corresponds much better with other evidence. [illustration: the zodiac of denderah.] in the first instance, no doubt, the names of the zodiacal constellations would depend on the principal star or stars in each, and these stars and the portion of the ecliptic assigned to each may have been noticed before the stars round them were grouped into constellations with different names. in any case, the introduction of the zodiac into greece seems to have been subsequent to that of the celestial sphere, and not to have taken place more than five or six centuries before our era. eudemus, of rhodes, one of the most distinguished of the pupils of aristotle, and author of a history of astronomy, attributes the introduction of the zodiac to oenopides of chio, a contemporary of anaxagoras. they did not receive it complete, as at first it had only eleven constellations, one of them, the scorpion, being afterwards divided, to complete the necessary number. their zodiacal divisions too would have been more regular had they derived them directly from the east, and would not have stretched in some instances over ° to °, like the lion, the bull, the fishes, or the virgin--while the crab, the ram, and the he-goat, have only ° to °. nor would their constellations be disposed so irregularly, some to the north and some to the south of the ecliptic, nor some spreading out widely and others crammed close together, so that we see that they only borrowed the idea from the easterns, and filled it out with their ancient constellations. such is the opinion of humboldt. with regard to the origin of the names of the signs of the zodiac, we must remember that a certain portion of the zodiacal circle, and not any definite group of stars, forms each sign, and that the constellations may have been formed separately, and have received independent names, though afterwards receiving those of the sign in which they were. the only rational suggestion for the origin of the names is that they were connected with some events which took place, or some character of the sun's motion observed, when it was in each sign. thus we have seen that the balance may refer to equal nights and days (though only introduced among the _greeks_ in the time of hipparchus), and the crab to the retrogression or stopping of the sun at the solstice. the various pursuits of husbandry, having all their necessary times, which in the primeval days were determined by the positions of the stars, would give rise to more important names. thus the ethiopian, at thebes, would call the stars that by their rising at a particular time indicated the inundation, aquarius, or the waterer; those beneath which it was necessary to put the plough to the earth, the bull stars. the lion stars would be those at whose appearance this formidable animal, driven from the deserts by thirst, showed himself on the borders of the river. those of the ear of corn, or the virgin of harvest, those beneath which the harvest was to be gathered in; and the sign of the goat, that in which the sun was when these animals were born. there can be but little doubt but that such was the origin of the names imposed, and for a time they would be understood in that sense. but afterwards, when time was more accurately kept, and calendars regulated, without each man studying the stars for himself, when the precession of the equinoxes made the periods not exactly coincide, the original meaning would be lost, the stars would be associated with the animals, as though there was a real bull, a real lion, &c., in the heavens; and then the step would be easy to represent these by living animals, whom they would endow with the heavenly attributes of what they represented; and so the people came at last to pray to and worship the several creatures for the sake of their supposed influence. they asked of the ram from their flocks the influences they thought depended on the constellation. they prayed the scorpion not to spread his evil venom on the world; they revered the crab, the scarabæus, and the fish, without perceiving the absurdity of it. it is certain at least that the gods of many nations are connected or are identical with the signs of the zodiac, and it seems at least more reasonable to suppose the former derived from the latter than _vice versâ_. among the greeks indeed, who had, so to speak, their gods ready made before they borrowed the idea of the zodiac, the process appears to have been the reverse, they made the signs to represent as far as they could their gods. in the more pastoral peoples, however, of the east, and in egypt, this process can be very clearly traced. among the jews there seems to be some remarkable connection between their patriarchs and these signs, though the history of that connection may not well be made out. the twelve signs are mentioned as being worshipped, along with the sun and moon, in the book of kings. but what is more remarkable is the dream of joseph, in which the sun and moon and the other eleven stars worshipped him, coupled with the various designations or descriptions given to each son in the blessing of jacob. in reuben we have the man who is said to be "unstable as water," in which we may recognise aquarius. in simeon and levi "the brethren," we trace the twins. judah is the "lion." zebulun, "that dwells at the haven of the sea," represents fishes. issachar is the bull, or "strong ass couching down between two burdens." dan, "the serpent by the way, the adder in the path," represents the scorpion. gad is the ram, the leader to a flock or troop of sheep. asher the balance, as the weigher of bread. naphtali, "the hind let loose," is the capricorn, joseph the archer, whose bow abode in strength. brujanin the crab, changing from morning to evening, and dinah, the only daughter, represents the virgin. there is doubtless something far-fetched in some of these comparisons, but when we consider the care with which the number twelve was retained, and that the four chief tribes carried on their sacred standards these very signs--namely, judah a lion, reuben a man, ephraim a bull, and dan a scorpion--and notice the numerous traces of astronomical culture in the jewish ceremonies, the seven lights of the candlestick, the twelve stones of the high priest, the feasts at the two equinoxes, the ceremonies connected with a ram and a bull, we cannot doubt that there is something more than chance in the matter, but rather conclude that we have an example of the process by which, in the hands of the egyptians themselves, astronomical representations became at last actually deified. it has been thought possible indeed to assign definitely each god of the egyptians to one of the twelve zodiacal signs. the ram was consecrated to jupiter ammon, who was represented with a ram's head and horns. the bull became the god apis, who was worshipped under that similitude. the twins correspond to horus and harpocrates, two sons of osiris. the crab was consecrated to anubis or mercury. the lion belonged to the summer sun, osiris; the virgin to isis. the balance and the scorpion were included together under the name of scorpion, which animal belongs to typhon, as did all dangerous animals. the archer was the image of hercules, for whom the egyptians had great veneration. the capricorn was consecrated to pan or mendes. the waterer--or man carrying a water-pot--is found on many egyptian monuments. this process of deification was rendered easier by the custom they had of celebrating a festival each month, under the name _neomenia_. they characterised the neomenias of the various months by making the animal whose sign the sun was entering accompany the isis which announced the _fête_. they were not content with a representation only, but had the animal itself. the dog, being the symbol of cannulus, with which the year commenced, a living dog was made to head the ceremonial of the first neomenia. diodorus testifies to this as an eye-witness. these neomenias thus came to be called the festival of the bull, of the ram, the dog, or the lion. that of the ram would be the most solemn and important in places where they dealt much in sheep. that of the bull in the fat pasture-lands of memphis and lower egypt. that of capella would be brilliant at mendes, where they bred goats more than elsewhere. we may fortify these opinions by a quotation from lucian, who gives expression to them very clearly. "it is from the divisions of the zodiac," he says, "that the crowd of animals worshipped in egypt have had their origin. some employed one constellation, and some another. those who used to consult that of the ram came to adore a ram. those who took their presages from the fishes would not eat fish. the goat was not killed in places were they observed capricornus, and so on, according to the stars whose influence they cared most for. if they adored a bull it was certainly to do honour to the celestial bull. the apis, which was a sacred object with them, and wandered at liberty through the country, and for which they founded an oracle, was the astrological symbol of the bull that shone in the heavens." [illustration: plate iv.--the zodiac and the dead in egypt.] their use of the zodiac is illustrated in an interesting manner by a mummy found some years ago in egypt. at the bottom of the coffin was found painted a zodiac, something like that of denderah; underneath the lid, along the body of a great goddess, were drawn eleven signs, but with that of _capricornus_ left out. the inscription showed that the mummy was that of a young man, aged years, months, and days, who died the th year of trajan, on the th of the month pazni, which corresponds to the nd of june, a.d. . the embalmed was therefore born on the th of january, a.d. , at which time the sun was in the constellation of capricornus. this shows that the zodiac was the representation of the astrological theories about the person embalmed, who was doubtless a person of some importance. (see plate iv.) any such use as this, however, must have been long subsequent to the invention of the signs themselves, as it involves a much more complicated idea. chapter v. the pleiades. among the most remarkable of the constellations is a group of seven stars arranged in a kind of triangular cluster, and known as the pleiades. it is not, strictly speaking, one of the constellations, as it forms only part of one. we have seen that one of the ancient signs of the zodiac is the bull, or taurus; the group of stars we are now speaking of forms part of this, lying towards the eastern part in the shoulders of the bull. the pleiades scarcely escape anybody's observation now, and we shall not be, therefore, surprised that they have always attracted great attention. so great indeed has been the attention paid to them that festivals and seasons, calendars and years, have by many nations been regulated by their rising or culmination, and they have been thus more mixed up with the early history of astronomy, and have left more marks on the records of past nations, than any other celestial object, except the sun and moon. the interesting details of the history of the pleiades have been very carefully worked out by r. g. haliburton, f.s.a., to whom we owe the greater part of the information we possess on the subject.[ ] let us first explain what may be observed with respect to the pleiades. it is a group possessing peculiar advantages for observation; it is a compact group, the whole will appear at once; and it is an unmistakable group and it is near the equator, and is therefore visible to observers in either hemisphere. now suppose the sun to be in the same latitude as the pleiades on some particular day; owing to the proximity of the group to the ecliptic, it will be then very near the sun, and it will set with it and be invisible during the night. if the sun were to the east of the pleiades they would have already set, and the first view of the heavens at sunset would not contain this constellation; and so it would be so long as the sun was to the east, or for nearly half a year; though during some portion of this time it would rise later on in the night. during the other half year, while the sun was to the west, the pleiades would be visible at sunset, and we immediately see how they are thus led to divide the whole year into two portions, one of which might be called _the pleiades below_, and the other _the pleiades above_. it is plain that the pleiades first become visible at sunset, when they are then just rising, in which case they will culminate a little after midnight (not at midnight, on account of the twilight) and be visible all night. this will occur when the sun is about half a circle removed from them--that is, at this time, about the beginning of november; which would thus be the commencement of one half of the year, the other half commencing in may. the culmination of the pleiades at midnight takes place a few days later, when they rise at the time that the sun is really on the horizon, in which case they are exactly opposite to it; and this will happen on the same day all over the earth. the opposite effect to this would be when the sun was close to the pleiades--a few days before which the latter would be just setting after sunset, and a few days after would be just rising before sunrise. [ ] mr. haliburton's observations are contained in an interesting pamphlet, entitled _new materials for the history of man_, which is quoted by prof. piazzi smyth, but which is not easy to obtain. it may be seen, however, in the british museum. we have thus the following observations, that might be made with respect to this, or any other well-marked constellation. first, the period during which it was visible at sunset; secondly, the date of its culmination at midnight; thirdly, its setting in the evening; and fourthly, its rising in the morning: the last two dates being nearly six months removed from the second. there are also the dates of its culmination at sunrise and sunset, which would divide these intervals into two equal halves. on account of the precession of the equinoxes, as explained in the last chapter, the time at which the sun has any particular position with respect to the stars, grows later year by year in relation to the equinoctial points. and as we regulate our year by the date of the sun's entrance on the northern hemisphere, the sidereal dates, as we may call them, keep advancing on the months. as, however, the change is slow, it has not prevented years being commenced and husbandry being regulated by the dates above mentioned. any date that is regulated by the stars we might expect to be nearly the same all over the world, and the customs observed to be universal, though the date itself might alter, and in this way. so long as the date was directly obtained from the position of the star, all would agree; but as soon as a solar calendar was arranged, and it was found that at that time this position coincided with a certain day, say the pleiades culminating at midnight on november , then some would keep on the date november as the important day, even when the pleiades no longer culminated at midnight then, and others would keep reckoning by the stars, and so have a different date. with these explanations we shall be able to recognise how much the configurations of the pleiades have had to do with the festivals and calendars of nations, and have even left their traces on customs and names in use among ourselves to the present day. we have evidences from two very different quarters of the universality of the division of the year into two parts by means of the pleiades. on the one hand we learn from hesiod that the greeks commenced their winter seasons in his days by the setting of the pleiades in the morning, and the summer season by their rising at that time. and mr. ellis, in his _polynesian researches_, tells us that "the society islanders divided the year into two seasons of the pleiades, or _matarii_. the first they called _matarii i nia_, or the _pleiades above_. it commenced when, in the evening, these stars appeared at or near the horizon, and the half year during which, immediately after sunset, they were seen above the horizon was called _matarii i nia_. the other season commenced when at sunset these stars are invisible, and continued until at that time they appeared again above the horizon. this season was called _matarii i raro_, i.e. _the pleiades below_." besides these direct evidences we shall find that many semi-annual festivals connected with these stars indicate the commencement of the two seasons among other nations. one of these festivals was of course always taken for the commencement of the year, and much was made of it as new-year's day. a new-year's festival connected with and determined by the pleiades appears to be one of the most universal of all customs; and though some little difficulty arises, as we have already pointed out, in fixing the date with reference to solar calendars, and differences and coincidences in this respect among different nations may be to a certain extent accidental, yet the fact of the wide-spread observance of such a festival is certain and most interesting. the actual observance at the present day of this festival is to be found among the australian savages. at their midnight culmination in november, they still hold a new-year's _corroboree_, in honour of the _mormodellick_, as they call the pleiades, which they say are "very good to the black fellows." with them november is somewhat after the beginning of spring, but in former days it would mark the actual commencement, and the new year would be regulated by the seasons. in the northern hemisphere this culmination of the pleiades has the same relation to the autumnal equinox, which would never be taken as the commencement of the year; and we must therefore look to the southern hemisphere for the origin of the custom; especially as we find the very pleiades themselves called _vergiliæ_, or stars of spring. of course we might suppose that the rising of the constellation in the _morning_ had been observed in the northern hemisphere, which would certainly have taken place in the beginning of spring some , years ago; but this seems improbable, first, because it is unlikely that different phenomena of the pleiades should have been most noticed, and secondly, because neither april nor may are among any nations connected with this constellation by name. whereas in india the year commenced in the month they called _cartiguey_, which means the pleiades. among the ancient egyptians we find the same connection between _athar-aye_, the name of the pleiades, with the chaldeans and hebrews, and _athor_ in the egyptian name of november. the arabs also call the constellation _atauria_. we shall have more to say on this etymology presently, but in the meantime we learn that it was the phenomenon connected with the pleiades at or about november that was noticed by all ancient nations, from which we must conclude that the origin of the new-year's spring festival came from the southern hemisphere. there is some corroboration of this in the ancient traditions as to the stars having changed their courses. in the southern hemisphere a man standing facing the position of the sun at noon would see the stars rise on his right hand and move towards his left. in the northern hemisphere, if he also looked in the direction of the sun at noon, he would see them rise on his left hand. now one of a race migrating from one side to the other of the equator would take his position from the sun, and fancy he was facing the same way when he looked at it at noon, and so would think the motion of the stars to have altered, instead of his having turned round. such a tradition, then, seems to have arisen from such a migration, the fact of which seems to be confirmed by the calling the pleiades stars of spring, and commencing the year with their culmination at midnight. in order to trace this new-year's festival into other countries, and by this means to show its connection with the pleiades, we must remark that every festival has its peculiar features and rites, and it is by these that we must recognise it, where the actual date of its occurrence has slightly changed; bearing, of course, in mind that the actual change of date must not be too great to be accounted for by the precession of the equinoxes, or about seventy-one years for each day of change, since the institution of the festival, and that the change is in the right direction. now we find that everywhere this festival of the pleiades' culmination at midnight (or it may be of the slightly earlier one of their first appearance at the horizon at apparent sunset) was always connected with the memory of the dead. it was a "feast of ancestors." among the australians themselves, the _corroborees_ of the natives are connected with a worship of the dead. they paint a white stripe over their arms, legs, and ribs, and, dancing by the light of their fires by night, appear like so many skeletons rejoicing. what is also to be remarked, the festival lasts three days, and commences in the evening; the latter a natural result of the date depending on the appearance of the pleiades on the horizon at that time. the society islanders, who, as we have seen, divided their year by the appearance of the pleiades at sunset, commenced their year on the first day of the appearance, about november, and also celebrated the closing of one and the opening of a new year by a "usage resembling much the popish custom of mass for souls in purgatory," each man returning to his home to offer special prayers for the spirits of departed relatives. in the tonga islands, which belong to the fiji group, the festival of _inachi_, a vernal first-fruits' celebration, and also a commemoration of the dead takes place towards the end of october, and commences at sunset. in peru the new-year's festival occurs in the beginning of november, and is "called _ayamarca_ from _aya_, a corpse, and _marca_, carrying in arms, because they celebrated the solemn festival of the dead, with tears, lugubrious songs, and plaintive music; and it was customary to visit the tombs of relations, and to leave in them food and drink." the fact that this took place at the time of the discovery of peru on the very same day as a similar ceremony takes place in europe, was only an accidental coincidence, which is all the more remarkable because the two appear, as will be seen in the sequel, to have had the same origin, and therefore at first the same date, and to have altered from it by exactly the same amount. these instances from races south of the equator prove clearly that there exists a very general connection with new-year's day, as determined by the rising of the pleiades at sunset, and a festival of the dead; and in some instances with an offering of first-fruits. what the origin of this connection may be is a more difficult matter. at first sight one might conjecture that with the year that was passed it was natural to connect the men that had passed away; and this may indeed be the true interpretation: but there are traditions and observances which may be thought by some to point to some ancient wide-spread catastrophe which happened at this particular season, which they yearly commemorated, and reckoned a new year from each commemoration. such traditions and observances we shall notice as we trace the spread of this new-year's festival of the dead among various nations, and its connection, with the pleiades. we have seen that in india november is called the month of the pleiades. now on the th day of that month is celebrated the hindoo durga, a festival of the dead, and said by greswell to have been a new-year's commemoration at the earliest time to which indian calendars can be carried back. among the ancient egyptians the same day was very noticeable, and they took care to regulate their solar calendars that it might remain unchanged. numerous altered calendars have been discovered, but they are all regulated by this one day. this was determined by the culmination of the pleiades at midnight. on this day commenced the solemn festival of the isia, which, like the _corroborees_ of the australians, lasted three days, and was celebrated in honour of the dead, and of osiris, the lord of tombs. now the month athyr was undoubtedly connected with the pleiades, being that "in which the pleiades are most distinct"--that is, in which they rise near and before sunset. among the egyptians, however, more attention was paid to astronomy than amongst the savage races with which the year of the pleiades would appear to have originated, and they studied very carefully the connection between the positions of the stars and the entrance of the sun into the northern hemisphere, and regulated their calendar accordingly; as we shall see shortly in speaking of the pyramid builders. the persians formerly called the month of november _mordâd_, the angel of death, and the feast of the dead took place at the same time as in peru, and was considered a new-year's festival. it commenced also in the evening. in ceylon a combined festival of agriculture and of the dead takes place at the beginning of november. among the better known of the ancient nations of the northern hemispheres, such as the greeks and romans, the anomaly of having the beginning of the year at the autumnal equinox seems to have induced them to make a change to that of spring, and with this change has followed the festival of the dead, although some traces of it were left in november. the commemoration of the dead was connected among the egyptians with a deluge, which was typified by the priest placing the image of osiris in a sacred coffer or ark, and launching it out into the sea till it was borne out of sight. now when we connect this fact, and the celebration taking place on the th day of athyr, with the date on which the mosaic account of the deluge of noah states it to have commenced, "in the second month (of the jewish year, which corresponds to november), the th day of the month," it must be acknowledged that this is no chance coincidence, and that the precise date here stated must have been regulated by the pleiades, as was the egyptian date. this coincidence is rendered even stronger by the similiarity of traditions among the two nations concerning the dove and the tree as connected with the deluge. we find, however, no festival of the dead among the hebrews; their better form of faith having prevented it. we have not as yet learnt anything of the importance of the pleiades among the ancient babylonian astronomers, but as through their tablets we have lately become acquainted with their version of the story of the deluge, we may be led in this way to further information about their astronomical appreciation of this constellation. from whatever source derived, it is certain that the celtic races were partakers in this general culture, we might almost call it, of the pleiades, as shown by the time and character of their festival of the dead. this is especially interesting to ourselves, as it points to the origin of the superstitions of the druids, and accounts for customs remaining even to this day amongst us. [illustration: plate v.--the legends of the druids.] the first of november was with the druids a night full of mystery, in which they annually celebrated the reconstruction of the world. a terrible rite was connected with this; for the druidess nuns were obliged at this time to pull down and rebuild each year the roof of their temple, as a symbol of the destruction and renovation of the world. if one of them, in bringing the materials for the new roof, let fall her sacred burden, she was lost. her companions, seized with a fanatic transport, rushed upon her and tore her to pieces, and scarcely a year is said to have passed without there being one or more victims. on this same night the druids extinguished the sacred fire, which was kept continually burning in the sacred precincts, and at that signal all the fires in the island were one by one put out, and a primitive night reigned throughout the land. then passed along to the west the phantoms of those who had died during the preceding year, and were carried away by boats to the judgment-seat of the god of the dead. (plate v.) although druidism is now extinct, the relics of it remain to this day, for in our calendar we still find november marked as all saints' day, and in the pre-reformation calendars the last day of october was marked all hallow eve, and the nd of november as all souls'; indicating clearly a three days' festival of the dead, commencing in the evening, and originally regulated by the pleiades--an emphatic testimony how much astronomy has been mixed up with the rites and customs even of the english of to-day. in former days the relics were more numerous, in the hallowe'en torches of the irish, the bonfires of the scotch, the _coel-coeth_ fires of the welsh, and the _tindle_ fires of cornwall, all lighted on hallowe'en. in france it still lingers more than here, for to this very day the parisians at this festival repair to the cemeteries, and lunch at the graves of their ancestors. if the extreme antiquity of a rite can be gathered from the remoteness of the races that still perform it, the fact related to us by prescott in his _history of the conquest of mexico_ cannot fail to have great interest. there we find that the great festival of the mexican cycle was held in november, at the time of the midnight culmination of the pleiades. it began at sunset, and at midnight as that constellation approached the zenith, a human victim, was offered up, to avert the dread calamity which they believed impended over the human race. they had a tradition that the world had been previously destroyed at this time, and they were filled with gloom and dismay, and were not at rest until the pleiades were seen to culminate, and a new cycle had begun; this great cycle, however, was only accomplished in fifty-two years. it is possible that the festival of lanthorns among the japanese, which is celebrated about november, may be also connected with this same day, as it is certain that that nation does reckon days by the pleiades. these instances of a similar festival at approximately the same period of the year, and regulated (until fixed to a particular day in a solar calendar) by the midnight culmination of the pleiades, show conclusively how great an influence that constellation has had on the manners and customs of the world, and throw some light on the history of man. even where we find no festival connected with the particular position of the pleiades which is the basis of the above, they still are used for the regulation of the seasons--as amongst the dyaks of borneo. this race of men are guided in their farming operations by this constellation. "when it is low in the east at early morning, before sunrise, the elders know it is time to cut down the jungle; when it approaches mid-heaven, then it is time to burn what they have cut down; when it is declining towards the west, then they plant; and when in the early evening it is seen thus declining, then they may reap in safety and in peace;" the latter period is also that of their feast of _nycapian_, or first-fruits. we find the same regulations amongst the ancient greeks in the days of hesiod, who tells us that the corn is to be cut when the pleiades rise, and ploughing is to be done when they set. also that they are invisible for forty days, and reappear again at harvest. when the pleiades rise, the care of the vine must cease; and when, fleeing from orion, they are lost in the waves, sailing commences to be dangerous. the name, indeed, by which we now know these stars is supposed to be derived from the word [greek: plein], to sail--because sailing was safe after they had risen; though others derive it from [greek: peleiai], a flight of doves. any year that is regulated by the pleiades, or by any other group of stars, must, as we have seen before, be what is called a sidereal, and not a solar year. now a year in uncivilised countries can only mean a succession of seasons, as is illustrated by the use of the expression "a person of so many summers." it is difficult of course to say when any particular season begins by noticing its characteristics as to weather; even the most regular phenomena are not certain enough for that; we cannot say that when the days and nights become exactly equal any marked change takes place in the temperature or humidity of the atmosphere, or in any other easily-noticed phenomena. the day therefore on which spring commences is arbitrary, except that, inasmuch as spring depends on the position of the sun, its commencement, ought to be regulated by that luminary, and not by some star-group which has no influence in the matter. nevertheless the position of such a group is much more easily observed, and in early ages could almost alone be observed; and so long as the midnight culmination of the pleiades--judged of, it must be noticed, by their appearance _on the horizon_ at sunset--fairly coincided with that state of weather which might be reckoned the commencement of spring conditions, no error would be detected, because the change in their position is so slow. the solar spring is probably a later discovery, which now, from its greater reasonableness and constancy, has superseded the old one. but since the time of the sun's crossing the equator is the natural commencement of spring, whether discovered or not, it is plain that no group of stars could be taken as a guide instead, if their indication did not approximately coincide with this. if then we can determine the exact date at which the pleiades indicated by their midnight culmination the sun's passage across the equator, we can be sure that the spring could only have been regulated by this during, say, a thousand years at most, on either side of this date. it is very certain that if the method of reckoning spring by the stars had been invented at a more remote date, some other set of stars would have been chosen instead. now when was this date? it is a matter admitting of certain calculation, depending only on numbers derived from observation in our own days and records of the past few centuries, and the answer is that this date is about b.c. we have seen that, though it was probably brought from the southern hemisphere, the egyptians adopted the year of the pleiades, and celebrated the new-year's festival of the dead; but they were also advanced astronomers, and would soon find out the change that took place in the seasons when regulated by the stars. and to such persons the date at which the two periods coincided, or at least were exactly half a year apart, would be one of great importance and interest, and there seems to be evidence that they did commemorate it in a very remarkable manner. the evidence, however, is all circumstantial, and the conclusion therefore can only claim probability. the evidence is as follows:--the most remarkable buildings of egypt are the pyramids. these are of various sizes and importance, but are built very much after the same plan. they seem, however, to be all copies from one, the largest, namely, the pyramid of gizeh, and to be of subsequent date to this. their object has long been a puzzle, and the best conclusion has been supposed to be that they were for sepulchral purposes, as in some of them coffins have been found. the large one, however, shows far more than the rest of the structure, and cannot have been meant for a funeral pile alone. its peculiarities come out on a careful examination and measurement such as it has been subjected to at the devoted hands of piazzi smyth, the astronomer royal for scotland. he has shown that it is not built at random, as a tomb might be, but it is adjusted with exquisite design, and with surprising accuracy. in the first place it lies due north, south, east, and west, and the careful ascertainment of the meridian of the place, by modern astronomical instruments, could not suggest any improvement in its position in this respect. the outside of it is now, so to speak, pealed, that is to say, there was originally, covering the whole, another layer of stones which have been taken away. these stones, which were of a different material, were beautifully polished, as some of the remaining ones, now covered and concealed, can testify. the angle at which they are cut, and which of course gives the angle and elevation of the whole pyramid, is such that the height of it is in the same proportion to its circumference or perimeter, as the radius of a circle is to its circumference approximately. the height, in fact, is proved by measurement and observation to be ft., and the four sides together to be , ft., or about - / times the height. it does not seem improbable that, considering their advancement, the egyptians might have calculated approximately how much larger the circumference of the circle is than its diameter, and it is a curious coincidence that the pyramid expresses it. professor piazzi smyth goes much further and believes that they knew, or were divinely taught, the shape and size of the earth, and by a little manipulation of the length of their unit, or as he expresses it the "pyramid inch," he makes the base of the pyramid express the number of miles in the diameter of the earth. now in the interior of the apparently solid structure, besides the usual slanting passage down to a kind of cellar or vault beneath the middle of the base, which may have been used for a sepulchral resting-place, there are two slanting passages, one running north and the other running south, and slanting up at different angles. part of that which leads south is much enlarged, and is known as the grand gallery. it is of a very remarkable shape, being perfectly smooth and polished along its ascending base, as indeed it is in every part, and having a number of steps or projections, pointing also upwards at certain angles, very carefully maintained. whether we understand its use or not, it is very plain that it has been made with a very particular design, and one not easily comprehended. this leads into a chamber known as the king's chamber, whose walls are exquisitely polished and which contains a coffer known as _cheops' coffin_. this coffer has been villainously treated by travellers, who have chipped and damaged it, but originally it was very carefully made and polished. it is too large to have been brought in by the only entrance into the chamber after it was finished, and therefore is obviously no coffin at all, as is proved also by the elaborateness of the means of approach. professor piazzi smyth has made the happy suggestion that it represents their standard of length and capacity, and points out the remarkable fact that it contains exactly as much as four quarters of our dry measure. as no one has ever suggested what our "quarters" are quarters of, professor smyth very naturally supplies the answer--"of the contents of the pyramid coffer." there are various other measurements that have been made by the same worker, and their meaning suggested in his interesting book, _our inheritance in the great pyramid_, which we may follow or agree to as we can; but from all that has been said above, it will appear probable that this pyramid was built with a definite design to mark various natural phenomena or artificial measures, which is all we require for our present purpose. now we come to the question, what is the meaning of the particular angles at which the north-looking and south-looking passages rise, if, as we now believe, they must have _some_ meaning. the exits of these passages were closed, and they could not therefore have been for observation, but they may have been so arranged as to be a memorial of any remarkable phenomena to be seen in those directions. to ascertain if there be any such to which they point, we must throw back the heavens to their position in the days of the egyptians, because, as we have seen, the precession of the equinoxes alters the meridian altitude of every star. as the passages point north and south, if they refer to any star at all, it must be to their passing the meridian. now let us take the heavens as they were b.c., the date at which the pleiades _really_ commenced the spring, by their midnight culmination, and ask how high they would be then. the answer of astronomy is remarkable--"_exactly at that height that they could be seen in the direction of the southward-pointing passage of the pyramid._" and would any star then be in a position to be seen in the direction of the other or northward-looking passage? yes, the largest star in the constellation of the dragon, which would be so near the pole ( ° ´) as to be taken as the pole star in those days. these are such remarkable coincidences in a structure admittedly made with mathematical accuracy and design, and truly executed, that we cannot take them to be accidental, but must endeavour to account for them. the simplest explanation seems to be, that everything in the pyramid is intended to represent some standard or measure, and that these passages have to do with their year. they had received the year of the pleiades from a remoter antiquity than their own, they had discovered the true commencement of solar spring, as determined from the solar autumnal equinox, and they commemorated by the building of the pyramid the coincidence of the two dates, making passages in it which would have no meaning except at that particular time. whether the pyramid was built _at that time_, or whether their astronomical knowledge was sufficient to enable them to predict it and build accordingly, just as we calculate back to it, we have no means of knowing. it is very possible that the pyramid may have been built by some immigrating race more learned in astronomy, like the accadians among the babylonians. either the whole of the conclusions respecting the pyramid is founded on pure imagination and the whole work upon it thrown away, or we have here another very remarkable proof of the influence of the pleiades on the reckoning of the year, and a very interesting chapter in the history of the heavens. following the guidance of mr. haliburton, we shall find still more customs, and names depending in all probability on the influence the pleiades once exerted, and the observances connected with the feasts in their honour. the name by which the pleiades are known among the polynesians is the "tau," which means a season, and they speak of the years of the tau, that is of the pleiades. now we have seen that the egyptians had similar feasts at similar times, in relation to this constellation, and argued that they did not arise independently. this seems still further proved by their name for these stars--the atauria. now the egyptians do not appear to have derived their signs of the zodiac from the same source; these had a babylonian origin, and the constellation in which the pleiades were placed by the latter people was the bull, by whatever name he went. the egyptians, we may make the fair surmise, adopted from both sources; they took the pleiades to indicate the bull, and they called this animal after the atauria. from thence we got the latin taurus, and the german thier. it is possible that this somehow got connected with the letter "tau" in greek, which seems itself connected with the sacred scarabæus or tau-beetle of egypt; but the nature of the connection is by no means obvious. mr. haliburton even suggests that the "tors" and "arthur's seat," which are names given to british hill-tops, may be connected with the "high places," of the worship of the pleiades, but of this we have no proof. among the customs possibly derived from the ancients, through the phoenicians, though now adopted as conveying a different meaning in a christian sense, is that of the "hot cross bun," or "bull cake." it is found on egyptian monuments, signifying the four quarters of the year, and sometimes stamped with the head and horns of the bull. it is found among ourselves too, essentially connected with the dead, and something similar to it appears in the "soul cake" connected originally with all souls' day. among the scotch it was traditionally thought that on new year's eve the candlemas bull can be seen, rising at twilight and sailing over the heavens--a very near approach to a matter-of-fact statement. we have seen that among the ancient indians there was some notice taken of the pleiades, and that they in all probability guided their year by them or by some other stars: it would therefore behove them to know something of the precession of the equinoxes. it seems very well proved that their days of brahma and other periods were meant to represent some astronomical cycles, and among these we find one that is applicable to the above. they said that in every thousand divine ages, or in every day of brahma, fourteen menus are successively invested with the sovereignty of the earth. each menu transmits his empire to his sons during seventy-one divine ages. we may find a meaning for this by putting it that the equinox goes forward fourteen days in each thousand years, and each day takes up seventy-one years. these may not be the only ones among the various customs, sayings, and names that are due in one way or other to this primitive method of arranging the seasons by the positions of the stars, especially of those most remarkable and conspicuous ones the pleiades, but they are those that are best authenticated. if the connection between the pleiades and the festival of the dead, the new year and a deluge, can be clearly made out; if the tradition of the latter be found as universal as that of the former, and be connected with it in the mosaic narrative; if we can trace all these traditions to the south of the equator, and find numerous further traditions connected with islands, we may find some reason for believing in their theory who suggest that the early progenitors of the human race (? all of them) were inhabitants of some fortunate islands of even temperature in the southern hemisphere, where they made some progress in civilisation, but that their island was swallowed up by the sea, and that they only escaped by making huge vessels, and, being carried by the waves, they landed on continental shores, where they commemorated yearly the great catastrophe that had happened to them, notifying its time by the position of the pleiades, making it a feast of the dead whom they had left behind, and opening the year with the day, whether it were spring or not, and handing down to their descendants and to those among whom they came, the traditions and customs which such events had impressed upon them. whether such an account be probable, mythical, or unnatural, there are certainly some strange things to account for in connection with the pleiades. chapter vi. the nature and structure of the heavens according to the ancients. many and various have been the ideas entertained by reflecting men in former times on the nature and construction of the heavenly vault, wherein appeared those stars and constellations whose history we have already traced. is it solid? or liquid? or gaseous? each of these and many other suppositions have been duly formulated by the ancient philosophers and sages, although, as we are told by modern astronomy, it does not exist at all. in our study of the ancient ideas about the structure of the universe, we will commence with that early and curious system which considered the heavenly vault to be material and solid. the theory of a solid sky received the assent of all the most ancient philosophers. in his commentary on aristotle's work on the heavens, simplicius reveals the repugnance the ancient philosophers felt in admitting that a star could stand alone in space, or have a free motion of its own. it must have a support, and they therefore conceived that the sky must be solid. however strange this idea may now appear, it formed for many centuries the basis of all astronomical theories. thus anaximenas (in the sixth century b.c.) is related by plutarch to have said that "the outer sky is solid and crystalline," and that the stars are "fixed to its surface like studs," but he does not say on what this opinion was founded, though it is probable that, like his master anaximander, he could not understand how the stars could move without being supported. pythagoras, who lived about the same epoch, is also supposed by some to have held the same views, and it is possible that they all borrowed these ideas from the persians, whose earliest astronomers are said in the _zend avesta_ to have believed in concentric solid skies. eudoxus of cnidus, in the fifth century b.c., is said by his commentator aratus to have also believed in the solidity of the heavens, but his reasons are not assigned. notwithstanding these previously expressed opinions, aristotle (fourth century, b.c.) has for a long time been generally supposed to be the inventor of solid skies, but in fact he only gave the idea his valuable and entire support. the sphere of the stars was his eighth heaven. the less elevated heavens, in which he also believed, were invented to explain as well as they might, the proper motions of the sun, moon, and planets. the philosopher of stagira said that the motion of his eighth or outermost solid sky was uniform, nor ever troubled by any perturbation. "within the universe there is," he says, "a fixed and immovable centre, the earth; and without there is a bounding surface enclosing it on all sides. the outermost part of the universe is the sky. it is filled with heavenly bodies which we know as stars, and it has a perpetual motion, carrying round with it these immortal bodies in its unaltering and unending revolution." euclid, to whom we may assign a date of about before our present era, also considered the stars to be set in a solid sphere, having the eye of the observer as centre; though for him this conception was simply a deduction from exact and fundamental observations, namely, that their revolution took place as a whole, the shape and size of the constellation being never altered. cicero, in the last century before christ, declared himself a believer in the solidity of the sky. according to him the ether was too rarefied to enable it to move the stars, which must therefore require to be fixed to a sphere of their own, independent of the ether. in the time of seneca there seem to have been difficulties already raised about the solidity of the heavens, for he only mentions it in the form of a question--"is the sky solid and of a firm and compact substance?" (_questions_, book ii.) in the fifth century the idea of the star sphere still lingered, and in the eyes of simplicius, the commentator of aristotle, it was not merely an artifice suitable for the representation of the apparent motions, but a firm and solid reality; while mahomet and most of the fathers of the christian church had the same conception of these concentric spheres. it appears then from this review that the phrases "starry vault," and especially "fixed stars," have been used in two very distinct senses. when we meet with them in aristotle or ptolemy, it is obvious that they have reference to the crystal sphere of anaximenas, to which they were supposed to be affixed, and to move with it; but that later the word "fixed" carried with it the sense of immovable, and the stars were conceived as fixed in this sense, independently of the sphere to which they were originally thought to be attached. thus seneca speaks of them as the _fixum et immobilem populum_. if we would inquire a little further into the supposed nature of this solid sphere, we find that empedocles considered it to be a solid mass, formed of a portion of the ether which the elementary fire has converted into crystal, and his ideas of the connection between cold and solidification being not very precise, he described it by names that give the best idea of transparence, and, like lactantius, called it _vitreum cælum_, or said _cælum ærem glaciatum esse_, though we cannot suppose that he made any allusion to what we now call glass, but simply meant some body eminently transparent into which the fire had transformed the air; while so far from having any idea of cold, as we might imagine possible from observations of the snowy tops of mountains, they actually believed in a warm region above the lower atmosphere. thus aristotle considers that the spheres heat by their motion the air below them, without being heated themselves, and that there is thus a production of heat. "the motion of the sphere of fixed stars," he says, "is the most rapid, as it moves in a circle with all the bodies attached to it, and the spaces immediately below are strongly heated by the motion, and the heat, thus engendered, is propagated downwards to the earth." this however, strangely enough, does not appear to have prevented their supposing an eternal cold to reign in the regions next below, for macrobius, in his commentary on cicero, speaks of the decrease of temperature with the height, and concludes that the extreme zones of the heavens where saturn moves must be eternally cold; but this they reckoned as part of the atmosphere, beyond whose limits alone was to be found the fiery ether. it is to the fathers of the church that we owe the transmission during the middle ages of the idea of a crystal vault. they conceived a heaven of glass composed of eight or ten superposed layers, something like so many skins in an onion. this idea seems to have lingered on in certain cloisters of southern europe even into the nineteenth century, for a venerable prince of the church told humboldt in , that a large aërolite lately fallen, which was covered with a vitrified crust, must be a fragment of the crystalline sky. on these various spheres, one enveloping without touching another, they supposed the several planets to be fixed, as we shall see in a subsequent chapter. whether the greater minds of antiquity, such as plato, plutarch, eudoxus, aristotle, apollonius, believed in the reality of these concentric spheres to carry the planets, or whether this conception was not rather with them an imaginary one, serving only to simplify calculation and assist the mind in the solution of the difficult problem of their motion, is a point on which even humboldt cannot decide. it is certain, however, that in the middle of the sixteenth century, when the theory involved no less than seventy-seven concentric spheres, and later, when the adversaries of copernicus brought them all into prominence to defend the system of ptolemy, the belief in the existence of these solid spheres, circles and epicycles, which was under the especial patronage of the church, was very widespread. tycho brahe expressly boasts of having been the first, by considerations concerning the orbits of the comets, to have demonstrated the impossibility of solid spheres, and to have upset this ingenious scaffolding. he supposed the spaces of our system to be filled with air, and that this medium, disturbed by the motion of the heavenly bodies, opposed a resistance which gave rise to the harmonic sounds. it should be added also that the grecian philosophers, though little fond of observation, but rejoicing rather in framing systems for the explanation of phenomena of which they possessed but the faintest glimpse, have left us some ideas about the nature of shooting stars and aërolites that come very close to those that are now accepted. "some philosophers think," says plutarch in his life of lysander, "that shooting stars are not detached particles of ether which are extinguished by the atmosphere soon after being ignited, nor do they arise from the combustion of the rarefied air in the upper regions, but that they are rather heavenly bodies which fall, that is to say, which escaping in some way from the general force of rotation are precipitated in an irregular manner, sometimes on inhabited portions of the earth, but sometimes also in the ocean, where of course they cannot be found." diogenes of apollonius expresses himself still more clearly: "amongst the stars that are visible move others that are invisible, to which in consequence we are unable to give any name. these latter often fall to the earth and take fire like that star-stone which fell all on fire near Ægos potamos." these ideas were no doubt borrowed from some more ancient source, as he believed that all the stars were made of something like pumice-stone. anaxagoras, in fact, thought that all the heavenly bodies were fragments of rocks which the ether, by the force of its circular motion, had detached from the earth, set fire to, and turned into stars. thus the ionic school, with diogenes of apollonius, placed the aërolites and the stars in one class, and assigned to all of them a terrestrial origin, though in this sense only, that the earth, being the central body, had furnished the matter for all those that surround it. plutarch speaks thus of this curious combination:--"anaxagoras teaches that the ambient ether is of an igneous nature, and by the force of its gyratory motion it tears off blocks of stone, renders them incandescent, and transforms them into stars." it appears that he explained also by an analogous effect of the circular motion the descent of the nemæan lion, which, according to an old tradition, fell out of the moon upon the peloponnesus. according to boeckh, this ancient myth of the nemæan lion had an astronomical origin, and was symbolically connected in chronology with the cycle of intercalation of the lunar year, with the worship of the moon in nemaea, and the games by which it was accompanied. [illustration: plate vi.--the nemÆan lion.] anaxagoras explains the apparent motion of the celestial sphere from east to west by the hypothesis of a general revolution, the interruption of which, as we have just seen, caused the fall of meteoric stones. this hypothesis is the point of departure of the theory of vortices, which more than two thousand years later, by the labours of descartes, huyghens, and hooke, took so prominent a place among the theories of the world. it may be worth adding with regard to the famous aërolite of Ægos potamus, alluded to above, that when the heavens were no longer believed to be solid, the faith in the celestial origin of this, as of other aërolites, was for a long time destroyed. thus bailly the astronomer, alluding to it, says, "if the fact be true, this stone must have been thrown out by a volcano." indeed it is only within the last century that it has been finally accepted for fact that stones do fall from the sky. laplace thought it probable that they came from the moon; but it has now been demonstrated that aërolites, meteors, and shooting stars belong all to one class of heavenly bodies, that they are fragments scattered through space, and circulate like the planets round the sun. when the earth in its motion crosses this heavenly host, those which come near enough to touch its atmosphere leave a luminous train behind them by their heating by friction with the air: these are the _shooting stars_. sometimes they come so close as to appear larger than the moon, then they are _meteors;_ and sometimes too the attraction of the earth makes them fall to it, and these become _aërolites_. but to return to our ancient astronomers:-- they believed the heavens to be in motion, not only because they saw the motion with their eyes, but because they believed them to be animated, and regarded motion as the essence of life. they judged of the rapidity of the stars' motion by a very ingenious means. they perceived that it was greater than that of a horse, a bird, an arrow, or even of the voice, and cleomenas endeavoured to estimate it in the following way. he remarks that when the king of persia made war upon greece he placed men at certain intervals, so as to lie in hearing of each other, and thus passed on the news from athens to susa. now this news took two days and nights to pass over this distance. the voice therefore only accomplished a fraction of the distance that the stars had accomplished twice in the same time. the heavens, as we have seen, were not supposed to consist of a single sphere, but of several concentric ones, the arrangement and names of which we must now inquire into. the early chaldeans established three. the first was the empyreal heaven, which was the most remote. this, which they called also the solid firmament, was made of fire, but of fire of so rare and penetrating a nature, that it easily passed through the other heavens, and became universally diffused, and in this way reached the earth. the second was the ethereal heaven, containing the stars, which were simply formed of the more compact and denser parts of this substance; and the third heaven was that of the planets. the persians, however, gave a separate heaven to the sun, and another to the moon. the system which has enjoyed the longest and most widely-spread reign is that which places above, or rather round, the solid firmament a heaven of water--(the nature of which is not accurately defined), and round this a _primum mobile_, prime mover, or originator of all the motions, and round all this the empyreal heaven, or abode of the blessed. in the most anciently printed scientific encyclopædia known, the _magarita philosophica_, edited in the fifteenth century, that is, two centuries before the adoption of the true system of the world, we have the curious figure represented on the next page, in which we find no less than eleven different heavens. we here see on the exterior the solid empyreal heaven, which is stated in the body of the work to be the abode of the blessed and to be immovable, while the next heaven gives motion to all within, and is followed by the aqueous heaven, then the crystal firmament, and lastly by the several heavens of the planets, sun, and moon. the revolution of these spheres was not supposed to take place, like the motion of the earth in modern astronomy, round an imaginary axis, but round one which had a material existence, which was provided with pivots moving in fixed sockets. thus vitruvius, architect to augustus, teaches it expressly in these words:-- "the heaven turns continually round the earth and sea upon an axis, where two extremities are like two pivots that sustain it: for there are two places in which the governor of nature has fashioned and set these pivots as two centres; one is above the earth among the northern stars; the other is at the opposite end beneath the earth to the south; and around these pivots, as round two centres, he has placed little naves, like those of a wheel upon which the heaven turns continually." [illustration: fig. .] similarly curious ideas we shall find to have prevailed with respect to the meaning of everything that they observed in the heavens: thus what a number of opinions have been hazarded on the nature of the "milky way" alone! some of which we may learn from plutarch. the milky way, he says, is a nebulous circle, which constantly appears in the sky, and which owes its name to its white appearance. certain pythagoreans assert that when phaeton lit up the universe, one star, which escaped from its proper place, set light to the whole space it passed over in its circular course, and so formed the milky way. others thought that this circle was where the sun had been moving at the beginning of the world. according to others it is but an optical phenomenon produced by the reflection of the sun's rays from the vault of the sky as from a mirror, and comparable with the effects seen in the rainbow and illuminated clouds. metrodorus says it is the mark of the sun's passage which moves along this circle. parmenidas pretends that the milky colour arises from a mixture of dense and rare air. anaxagoras thinks it an effect of the earth's shadow projected on this part of the heavens, when the sun is below. democritus says that it is the lustre of several little stars which are very near together, and which reciprocally illuminate each other. aristotle believes it to be a vast mass of arid vapours, which takes fire from a glowing tress, above the region of the ether, and far below that of the planets. posidonius says that the circle is a compound of fire less dense than that of the stars, but more luminous. all such opinions, except that of democritus, are of little value, because founded on nothing; perhaps the worst is that of theophrastus, who said it was the junction between the two hemispheres, which together formed the vault of heaven: and that it was so badly made that it let through some of the light that he supposed to exist everywhere behind the solid sky. we now know that the milky way, like many of the nebulæ, is an immense agglomeration of suns. the milky way is itself a nebula, a mass of sidereal systems, with our own among them, since our sun is a single star in this vast archipelago of eighteen million orbs. the greeks called it the galaxy. the chinese and arabians call it the river of heaven. it is the path of souls among the north american indians, and the road of s. jacques de compostelle among french peasants. in tracing the history of ideas concerning the structure of the heavens among the greek philosophers, we meet with other modifications which it will be interesting to recount. thus eudoxus, who paid greater attention than others to the variations of the motions of the planets, gave more than one sphere to each of them to represent these observed changes. each planet, according to him, has a separate part of the heaven to itself, which is composed of several concentric spheres, whose movements, modifying each other, produce that of the planet. he gave three spheres to the sun: one which turned from east to west in twenty-four hours, to represent the diurnal rotation; a second, which turned about the pole of the ecliptic in - / days, and produced its annual movement; and a third was added to account for a certain supposed motion, by which the sun was drawn out of the ecliptic, and turned about an axis, making such an angle with that of the ecliptic, as represented the supposed aberration. the moon also had three spheres to produce its motions in longitude and latitude, and its diurnal motion. each of the other planets had four, the extra one being added to account for their stations and retrogressions. it should be added that these concentric spheres were supposed to fit each other, so that the different planets were only separated by the thicknesses of these crystal zones. polemarch, the disciple of eudoxus, who went to athens with his pupil calippus for the express purpose of consulting aristotle on these subjects, was not satisfied with the exactness with which these spheres represented the planetary motions, and made changes in the direction of still greater complication. instead of the twenty-six spheres which represented eudoxus' system, calippus established thirty-three, and by adding also intermediary spheres to prevent the motion of one planet interfering with that of the adjacent ones, the number was increased to fifty-six. there is extant a small work, ascribed to aristotle, entitled "letter of aristotle to alexander on the system of the world," which gives so clear an account of the ideas entertained in his epoch that we shall venture to give a somewhat long extract from it. the work, it should be said, is not by all considered genuine, but is ascribed by some to nicolas of damas, by others to anaximenas of lampsacus, a contemporary of alexander's, and by others to the stoic posidonius. it is certain, however, that aristotle paid some attention to astronomy, for he records the rare phenomena of an eclipse of mars by the moon, and the occultation of one of the gemini by the planet jupiter, and the work may well be genuine. it contains the following:-- "there is a fixed and immovable centre to the universe. this is occupied by the earth, the fruitful mother, the common focus of every kind of living thing. immediately surrounding it on all sides is the air. above this in the highest region is the dwelling-place of the gods, which is called the heavens. the heavens and the universe being spherical and in continual motion, there must be two points on opposite sides, as in a globe which turns about an axis, and these points must be immovable, and have the sphere between them, since the universe turns about them. they are called the poles. if a line be drawn from one of these points to the other it will be the diameter of the universe, having the earth in the centre and the two poles at the extremities; of these two poles the northern one is always visible above our horizon, and is called the arctic pole; the other, to the south, is always invisible to us--it is called the antarctic pole. "the substance of the heavens and of the stars is called ether; not that it is composed of flame, as pretended by some who have not considered its nature, which is very different from that of fire, but it is so called because it has an eternal circular motion, being a divine and incorruptible element, altogether different from the other four. "of the stars contained in the heavens some are fixed, and turn with the heavens, constantly maintaining their relative positions. in their middle portion is the circle called the _zoophore_, which stretches obliquely from one tropic to the other, and is divided into twelve parts, which are the twelve signs (of the zodiac). the others are wandering stars, and move neither with the same velocity as the fixed stars, nor with a uniform velocity among themselves, but all in different circles, and with velocities depending on the distances of these circles from the earth. "although all the fixed stars move on the same surface of the heavens, their number cannot be determined. of the movable stars there are seven, which circulate in as many concentric circles, so arranged that the lower circle is smaller than the higher, and that the seven so placed one within the other are all within the spheres of the fixed stars. "on the nearer, that is inner, side of this ethereal, immovable, unalterable, impassible nature is placed our movable, corruptible, and mortal nature. of this there are several kinds, the first of which is fire, a subtle inflammable essence, which is kindled by the great pressure and rapid motion of the ether. it is in this region of air, when any disturbance takes place in it, that we see kindled shooting-stars, streaks of light, and shining motes, and it is there that comets are lighted and extinguished. "below the fire comes the air, by nature cold and dark, but which is warmed and enflamed, and becomes luminous by its motion. it is in the region of the air, which is passive and changeable in any manner, that the clouds condense, and rain, snow, frost, and hail are formed and fall to the earth. it is the abode of stormy winds, of whirlwinds, thunder, lightning, and many other phenomena. "the cause of the heaven's motion is god. he is not in the centre, where the earth is a region of agitation and trouble, but he is above the outermost circumference, which is the purest of all regions, a place which we call rightly _ouranos_, because it is the highest part of the universe, and _olympos_, that is, perfectly bright, because it is altogether separated from everything like the shadow and disordered movements which occur in the lower regions." we notice in this extract a curious etymology of the word ether, namely, as signifying perpetual motion ([greek: aei teein]), though it is more probable that its true, as its more generally accepted derivation is from [greek: aithein], to burn or shine, a meaning doubtless alluded to in a remarkable passage of hippocrates, [greek: peri sarkôn]. "it appears to me," he says, "that what we call the principle of heat is immortal, that it knows all, sees all, hears all, perceives all, both in the past and in the future. at the time when all was in confusion, the greater part of this principle rose to the circumference of the universe; it is this that the ancients have called _ether_." the first greek that can be called an astronomer was thales, born at miletus b.c., who introduced into greece the elements of astronomy. his opinions were these: that the stars were of the same substance as the earth, but that they were on fire; that the moon borrowed its light from the sun, and caused the eclipses of the latter, while it was itself eclipsed when it entered the earth's shadow; that the earth was round, and divisible into five zones, by means of five circles, _i.e._ the arctic and antarctic, the two tropics, and the equator; that the latter circle is cut obliquely by the ecliptic, and perpendicularly by the meridian. up to his time no division of the sphere had been made beyond the description of the constellations. these opinions do not appear to have been rapidly spread, since herodotus, one of the finest intellects of greece, who lived two centuries later, was still so ill-instructed as to say, in speaking of an eclipse, "the sun abandoned its place, and night took the place of day." anaxagoras, of whom we have spoken before, asserted that the sun was a mass of fire larger than the peloponnesus. plutarch says he regarded it as a burning stone, and diogenes laertius looked upon it as hot iron. for this bold idea he was persecuted. they considered it a crime that he taught the causes of the eclipses of the moon, and pretended that the sun is larger than it looks. he first taught the existence of one god, and he was taxed with impiety and treason against his country. when he was condemned to death, "nature," he said, "has long ago condemned me to the same; and as to my children, when i gave them birth i had no doubt but they would have to die some day." his disciple pericles, however, defended him so eloquently that his life was spared, and he was sent into exile. pythagoras, who belonged to the school of thales, and who travelled in phoenicia, chaldea, judæa, and egypt, to learn their ideas, ventured, in spite of the warnings of the priests, to submit to the rites of initiation at heliopolis, and thence returned to samos, but meeting with poor reception there, he went to italy to teach. from him arose the _italian school_, and his disciples took the name of philosophers (lovers of wisdom) instead of that of sages. we shall learn more about him in the chapter on the harmony of the spheres. his first disciple, empedocles, famous for the curiosity which led him to his death in the crater of Ætna, as the story goes, thought that the true sun, the fire that is in the centre of the universe, illuminated the other hemisphere, and that what we see is only the reflected image of that, which is invisible to us, and all of whose movements it follows. his disciple, philolaus, also taught that the sun was a mass of glass, which sent us by reflection all the light that it scattered through the universe. we must not, however, forget that these opinions are recorded by historians who probably did not understand them, and who took in the letter what was only intended for a comparison or figure. if we are to believe plutarch, xenophanes, who flourished about b.c., was very wild in his opinions. he thought the stars were lighted every night and extinguished every morning; that the sun is a fiery cloud; that eclipses take place by the sun being extinguished and afterwards rekindled; that the moon is inhabited, but is eighteen times larger than the earth; that there are several suns and several moons for giving light to different countries. this can only be matched by those who said the sun went every night through a hole in the earth round again to the east; or that it went above ground, and if we did not see it going back it was because it accomplished the journey in the night. parmenidas was the disciple of xenophanes. he divided the earth, like thales, into zones; and he added that it was suspended in the centre of the universe, and that it did not fall because there was no reason why it should move in one direction rather than another. this argument is perfectly philosophical, and illustrates a principle employed since the time of archimedes, and of which leibnitz made so much use. such are some of the general ideas which were held by the greeks and others on the nature of the heavens, omitting that of ptolemy, of which we shall give a fuller account hereafter. we see that they were all affected by the dominant idea of the superiority of the earth over the rest of the universe, and were spoiled for want of the grand conception of the immensity of space. the universe was for them a closed space, outside of which there was _nothing_; and they busied themselves with metaphysical questions as to the possibility of space being infinite. in the meantime their conceptions of the distances separating us from other visible parts of the universe were excessively cramped. hesiod, for instance, thinks to give a grand idea of the size of the universe by saying that vulcan's anvil took seven days to fall from heaven to earth, when in reality, as now calculated, it would take no less than seventy-two years for the light, even travelling at a far greater rate, to reach us from one of the nearest of the fixed stars. chapter vii. the celestial harmony. nature presents herself to us under various aspects. at times, it may be, she presents to us the appearance of discord, and we fail to perceive the unity that pervades the whole of her actions. at others, however, and most often to an instructed mind, there is a concord between her various powers, a harmony even in her sounds, that will not escape us. even the wild notes of the tempest and the bass roll of the thunder form themselves into part of the grand chorus which in the great opera are succeeded by the solos of the evening breeze, the songs of birds, or the ripple of the waves. these are ideas that would most naturally present themselves to contemplative minds, and such must have been the students of the silent, but to them harmonious and tuneful, star-lit sky, under the clear atmosphere of greece. the various motions they observed became indissolubly connected in their minds with music, and they did not doubt that the heavenly spheres made harmony, if imperceptible to human ears. but their ideas were more precise than this. they discovered that harmony depended on number, and they attempted to prove that whether the music they might make were audible or not, the celestial spheres had motions which were connected together in the same way as the numbers belonging to a harmony. the study of their opinions on this point reveals some very curious as well as very interesting ideas. we may commence by referring to an ancient treatise by timæus of locris on the soul of the universe. to him we owe the first serious exposition of the complete harmonic cosmography of pythagoras. we must premise that, according to this school, god employed all existing matter in the formation of the universe--so that it comprehends all things, and all is in it. "it is a unique, perfect, and spherical production, since the sphere is the most perfect of figures; animated and endowed with reason, since that which is animated and endowed with reason is better than that which is not." so begins timæus, and then follows, as a quotation from plato, a comparison of the earth to what would appear to us nowadays to be a very singular animal. not only, says plato, is the earth a sphere, but this sphere is perfect, and its maker took care that its surface should be perfectly uniform for many reasons. the universe in fact has no need of eyes, since there is nothing outside of it to see; nor yet of ears, since there is nothing but what is part of itself to make a sound; nor of breathing organs, as it is not surrounded by air: any organ that should serve to take in nourishment, or to reject the grosser parts, would be absolutely useless, for there being nothing outside it, it could not receive or reject anything. for the same reason it needs no hands with which to defend itself, nor yet of feet with which to walk. of the seven kinds of motion, its author has given it that which is most suitable for its figure in making it turn about its axis, and since for the execution of this rotatory motion no arms or legs are wanted, its maker gave it none. with regard to the soul of the universe, plato, according to timæus, says that god composed it "of a mixture of the divisible and indivisible essences, so that the two together might be united into one, uniting two forces, the principles of two kinds of motion, one that which is _always the same_, and the other that which is _always changing_. the mixture of these two essences was difficult, and was not accomplished without considerable skill and pains. the proportions of the mixture were according to harmonic numbers, so chosen that it is possible to know of what, and by what rule, the soul of the universe is compounded." by harmonic numbers timæus means those that are proportional to those representing the consonances of the musical scale. the consonances known to the ancients were three in number: the diapason, or octave, in the proportion of to , the diapent, or fifth, in that of to , and the diatessaron, or fourth, in that of to ; when to these are joined the tones which fill the intervals of the consonances, and are in the proportion of to , and the semitones in that of to , all the degrees of the musical scale is complete. the discovery of these harmonic numbers is due to pythagoras. it is stated that when passing one day near a forge, he noticed that the hammers gave out very accurate musical concords. he had them weighed, and found that of those which sounded the octave, one weighed twice as much as the other; that of those which made a perfect fifth, one weighed one third more than the other, and in the case of a fourth, one quarter more. after having tried the hammers, he took a musical string stretched with weights, and found that when he had applied a given weight in the first instance to make any particular note, he had to double the weight to obtain the octave, to add one third extra only to obtain a fifth, a quarter for the fourth, and eight for one tone, and about an eighteenth for a half-tone; or more simply still, he stretched a cord once for all, and then when the whole length sounded any note, when stopped in the middle it gave the octave, at the third it gave the fifth, at the quarter the fourth, at the eighth the tone, and at the eighteenth the semi-tone. since the ancients conceived of the soul by means of motion, the quantity of motion developed in anything was their measure of the quantity of its soul. now the motion of the heavenly bodies seemed to them to depend on their distance from the centre of the universe, the fastest being those at the circumference of the whole. to determine the relative degrees of velocity, they imagined a straight line drawn outwards from the centre of the earth, as far as the empyreal heaven, and divided it according to the proportions of the musical scale, and these divisions they called the harmonic degrees of the soul of the universe. taking the earth's radius for the first number, and calling it unity, or, in order to avoid fractions, denoting it by , the second degree, which is at the distance of an harmonic third, will be represented by plus its eighth part, or . the third degree will be , plus its eighth part, or . the fourth, being a semitone, will be as to , which will give ; and so on. the eighth degree will in this way be the double of or , and represents the first octave. they continued this series to degrees, as in the following table:-- the earth. mi + / = re + / = ut : : : : si + / = la + / = sol + / = fa : : : : mi + / = re + / = ut : : : : si + / = la + / = sol + / = fa : : : : mi + / = re + / = ut : : : si + = si : : : : la + / = sol + / = fa : : : : mi + / = re + / = ut + / = si : : : : la + / = sol + / = fa : : : : mi + = mi : : : : re + / = ut + / = si : : : : la + / = sol = + the empyreal heaven. sum of all the terms, , . this series they considered a complete one, because by taking the terms in their proper intervals, the last becomes times the original number, and in the school of pythagoras this had a mystic signification, and was considered as the perfect number. the reason for considering a perfect number was curious. it is the sum of the first linear, square, and cubic numbers added to unity. first there is , which represents the point, then and , the first linear numbers, even and uneven, then and , the first square or surface numbers, even and uneven, and the last and , the first solid or cubic numbers, even and uneven, and is the sum of all the former. whence, taking the number as the symbol of the universe, and the numbers which compose it as the elements, it appeared right that the soul of the universe should be composed of the same elements. on this scale of distances, with corresponding velocities, they arranged the various planets, and the universe comprehended all these spheres, from that of the fixed stars (which was excluded) to the centre of the earth. the sphere of the fixed stars was the common envelope, or circumference of the universe, and saturn, immediately below it, corresponded to the thirty-sixth tone, and the earth to the first, and the other planets with the sun and moon at the various harmonic distances. they reckoned one tone from the earth to the moon, half a tone from the moon to mercury, another half-tone to venus, one tone and a half from venus to the sun, one from the sun to mars, a semitone from mars to jupiter, half a tone from jupiter to saturn, and a tone and a half from saturn to the fixed stars; but these distances were not, as we shall see, universally agreed upon. according to timæus, the sphere of the fixed stars, which contains within it no principle of contrariety, being entirely divine and pure, always moves with an equal motion in the same direction from east to west. but the stars which are within it, being animated by the mixed principle, whose composition has been just explained, and thus containing two contrary forces, yield on account of one of these forces to the motion of the sphere of fixed stars from east to west, and by the other they resist it, and move in a contrary direction, in proportion to the degree with which they are endowed with each; that is to say, that the greater the proportion of the material to the divine force that they possess, the greater is their motion from west to east, and the sooner they accomplish their periodic course. now the amount of this force depends on the matter they contain. thus, according to this system, the planets turn each day by the common motion with all the heavens about the earth from east to west, but they also retrograde towards the east, and accomplish their periods according to their component parts. the additions which plato made to this theory have always been a proverb of obscurity, and none of his commentators have been able to make anything of them, and very possibly they were never intended to. so far the harmony of the heavenly bodies has been explained with reference to numbers only, and we may add to this that they reckoned , stadia, or , miles, to represent a tone, which was thus the distance of the earth to the moon, and the same measurement made it , from the earth to the sun, and the same distance from the sun to the fixed stars. but plato teaches in his _republic_ that there is actual musical, harmony between the planets. each of the spheres, he said, carried with it a siren, and each of these sounding a different note, they formed by their union a perfect concert, and being themselves delighted with their own harmony, they sang divine songs, and accompanied them by a sacred dance. the ancients said there were nine muses, eight of whom, according to plato, presided over celestial, and the ninth over terrestrial things, to protect them from disorder and irregularity. cicero and macrobius also express opinions on this harmonious concert. such great motions, says cicero, cannot take place in silence, and it is natural that the two extremes should have related sounds as in the octave. the fixed stars must execute the upper note, and the moon the base. kepler has improved on this, and says jupiter and saturn sing bass, mars takes the tenor, the earth and venus are contralto, and mercury is soprano! true, no one has ever heard these sounds, but pythagoras himself may answer this objection. we are always surrounded, he says, by this melody, and our ears are accustomed to it from our birth, so that, having nothing different to compare it with, we cannot perceive it. we may here recall the further development of the idea of the soul of the universe, which was the source of this harmony, and endeavour to find a rational interpretation of their meaning. they said that nature had made the animals mortal and ephemeral, and had infused their souls into them, as they had been extracts from the sun or moon, or even from one of the planets. a portion of the unchangeable essence was added to the reasoning part of man, to form a germ of wisdom in privileged individuals. for the human soul there is one part which possesses intelligence and reason, and another part which has neither the one nor the other. the various portions of the general soul of the universe resided, according to timæus, in the different planets, and depended on their various characters. some portions were in the moon, others in mercury, venus, or mars, and so on, and thus they give rise to the various characters and dispositions that are seen among men. but to these parts of the human soul that are taken from the planets is joined a spark of the supreme divinity, which is above them all, and this makes man a more holy animal than all the rest, and enables him to have immediate converse with the deity himself. all the different substances in nature were supposed to be endowed with more or less of this soul, according to their material nature or subtilty, and were placed in the same order along the line, from the centre to the circumference, on which the planets were situated, as we have seen above. in the centre was the earth, the heaviest and grossest of all, which had but little if any soul at all. between the earth and the moon, timæus placed first water, then the air, and lastly elementary fire, which he considered to be principles, which were less material in proportion as they were more remote and partook of a larger quantity of the soul of the universe. beyond the moon came all the planets, and thus were filled up the greater number of the harmonic degrees, the motions of the various bodies being guided by the principle enunciated above. when we carefully consider this theory we find that by a slight change of name we may bring it more into harmony with modern ideas. it would appear indeed that the ancients called that "soul" which we now call "force," and while we say that this force of attraction is in proportion to the masses and the inverse square of the distance, they put it that it was proportional to the matter, and to the divine substance on which the distance depended. so that we may interpret timæus as stating this proposition: _the distances of the stars and their forces are proportional among themselves to their periodic times._ "some people," says plutarch, "seek the proportions of the soul of the universe in the velocities (or periodic times), others in the distances from the centre; some in the masses of the heavenly bodies, and others more acute in the ratios of the diameters of their orbits. it is probable that the mass of each planet, the intervals between the spheres and the velocities of their motions, are like well-tuned musical instruments, all proportional harmonically with each other and with all other parts of the universe, and by necessary consequence that there are the same relative proportions in the soul of the universe by which they were formed by the deity." it is marvellous how deeply occupied were all the best minds in greece and italy on this subject, both poets and philosophers; ocellus, democritus, timæus, aristotle, and lucretius have all left treatises on the same subject, and almost with the same title, "the nature of the universe." though somewhat similar to that of timæus, it will be interesting to give an account of the ideas of one of these, ocellus of lucania. ocellus represents the universe as having a spherical form. this sphere is divided into concentric layers; above that of the moon they were called celestial spheres, while below it and inwards as far as the centre of the earth they were called the elementary spheres, and the earth was the centre of them all. in the celestial spheres all the stars were situated, which were so many gods, and among them the sun, the largest and most powerful of all. in these spheres is never any disturbance, storm, or destruction, and consequently no reparation, no reproduction, no action of any kind was required on the part of the gods. below the moon all is at war, all is destroyed and reconstructed, and here therefore it is that generations are possible. but these take place under the influence of the stars, and particularly that of the sun, which in its course acts in different ways on the elementary spheres, and produces continual variations in them, from whence arises the replenishing and diversifying of nature. it is the sun that lights up the region of fire, that dilates the air, melts the water, and renders fertile the earth, in its daily course from east to west, as well as in this annual journey into the two tropics. but to what does the earth owe its germs and its species? according to some philosophers these germs were celestial ideas which both gods and demons scattered from above over every part of nature, but according to ocellus they arise continually under the influence of the heavenly bodies. the divisions of the heavens were supposed to separate the portion that is unalterable from that which is in ceaseless change. the line dividing the mortal from the immortal is that described by the moon: all that lies above that, inclusive, is the habitation of the gods; all that lies below is the abode of nature and discord; the latter tending constantly to destruction, the former to the reconstruction of all created things. ideas such as these, of which we could give other examples more remotely connected with harmony, whatever amount of truth we may discover in them, prove themselves to have been made before the sciences of observation had enabled men to make anything better than empty theories, and to support them with false logic. no better example of the latter can perhaps be mentioned here than the way in which ocellus pretends to prove that the world is eternal. "the universe," he says, "_having_ always existed, it follows that everything in it and every arrangement of it must always have been as it is now. the several parts of the universe _having_ always existed with it, we may say the same of the parts of these parts; thus the sun, the moon, the fixed stars, and the planets have always existed with the heavens; animals, vegetables, gold, and silver with the earth; the currents of air, winds, and changes from hot to cold, from cold to hot, with the air. _therefore_ the heaven, with all that it now contains; the earth, with all that it produces and supports; and lastly, the whole aërial region, with all its phenomena, have always existed." when this system of argument passed away, and exact observation took its place, it was soon found that so far from what the ancients had argued _must be_ really being the case, no such relation as they indicated between the distances or velocities of the planets could be traced, and therefore no harmony in the heavens in this sense. it is not indeed that we can say no sounds exist because we hear none; but considering harmony really to consist of the relations of numbers, no such relations exist between the planets' distances, as measured now of course from the sun, instead of being, as then, imagined from the earth. the gamut is nothing else than the series of numbers:-- do re mi fa sol la si do / / / / / / and is independent of our perception of the corresponding notes. a concert played before a deaf assembly would be a concert still. if one note is made by , vibrations per second, and another by , , we should hear them as an octave, but if one had only and the other , they would still be an octave, though inaudible as notes to us; so too we may speak even of the harmony of luminous vibrations of ether, though they do not affect our ears. the velocities of the planets do not coincide with the terms of this series. the nearer they are to the sun the faster is their motion, mercury travelling at the mean rate of , metres a second, venus, , , the earth , , mars , , jupiter , , saturn , , uranus , , and neptune , , numbers which are in the proportion roundly of , , , , , , , , which have no sufficient relation to the terms of an harmonic series, to make any harmony obvious. returning, however, to the ancient philosophers, we are led by their ideas about the soul of the universe to discover the origin of their gods and natural religion. they were persuaded that only living things could move, and consequently that the moving stars must be endowed with superior intelligence. it may very well be that from the number seven of the planets, including the sun and moon, which were their earliest gods, arose the respect and superstition with which all nations, and especially the orientals, regarded that number. from these arose the seven superior angels that are found in the theologies of the chaldeans, persians, and arabians; the seven gates of mithra, through which all souls must pass to reach the abode of bliss; the seven worlds of purification of the indians, and all the other applications of the number seven which so largely figure in judaism, and have descended from it to our own time. on the other hand, as we have seen, this number seven may have been derived from the number of the stars in the pleiades. we have noticed in our chapter on the history of the zodiac how the various signs as they came round and were thought to influence the weather and other natural phenomena, came at last to be worshipped. not less, of course, were the sun and moon deified, and that by nations who had no zodiac. among the egyptians the sun was painted in different forms according to the time of year, very much as he is represented in our own days in pictures of the old and new years. at the winter solstice with them he was an infant, at the spring equinox he was a young man, in summer a man in full age with flowing beard, and in the autumn an old man. their fable of osiris was founded on the same idea. they represented the sun by the hawk, and the moon by the ibis, and to these two, worshipped under the names of osiris and isis they attributed the government of the world, and built a city, heliopolis, to the former, in the temple of which they placed his statue. the phenicians in the same way, who were much influenced by ideas of religion, attributed divinity to the sun, moon, and stars, and regarded them as the sole causes of the production and destruction of all things. the sun, under the name of hercules, was their great divinity. the ethiopians worshipped the same, and erected the famous table of the sun. those who lived above meroë, admitted the existence of eternal and incorruptible gods, among which they included the sun, moon, and the universe. like the incas of peru, they called themselves the children of the sun, whom they regarded as their common father. the moon was the great divinity of the arabs. the saracens called it cabar, or the great, and its crescent still adorns the religious monuments of the turks. each of their tribes was under the protection of some particular star. sabeism was the principal religion of the east. the heavens and the stars were its first object. in reading the sacred books of the ancient persians contained in the _zendavesta_, we find on every page invocations addressed to mithra, to the moon, the stars, the elements, the mountains, the trees, and every part of nature. the ethereal fire circulating through all the universe, and of which the sun is the principal focus, was represented among the fire-worshippers by the sacred and perpetual fire of their priests. each planet had its own particular temple, where incense was burnt in its honour. these ancient peoples embodied in their religious systems the ideas which, as we have seen, led among the greeks to the representation of the harmony of heaven. all the world seemed to them animated by a principle of life which circulated through all parts, and which preserved it in an eternal activity. they thought that the universe lived like man and the other animals, or rather that these latter only lived because the universe was essentially alive, and communicated to them for an instant an infinitely small portion of its own immortality. they were not wise, it may be, in this, but they appear to have caught some of the ideas that lie at the basis of religious thought, and to have traced harmony where we have almost lost the perception of it. chapter viii. astronomical systems. in our former chapters we have gained some idea of the general structure of the heavens as represented by ancient philosophers, and we no longer require to know what was thought in the infancy of astronomy, when any ideas promulgated were more or less random ones; but in this chapter we hope to discuss those arrangements of the heavenly bodies which have been promulgated by men as complete systems, and were supposed to represent the totality of the facts. the earliest thoroughly-established system is that of ptolemy. it was not indeed invented by him. the main ideas had been entertained long before his time, but he gave it consistence and a name. we obtain an excellent view of the general nature of this system from cicero. he writes:-- "the universe is composed of nine circles, or rather of nine moving globes. the outermost sphere is that of the heavens which surrounds all the others, and on which are fixed the stars. beneath this revolve seven other globes, carried round by a motion in a direction contrary to that of the heavens. on the first circle revolves the star which men call saturn; on the second jupiter shines, that beneficent and propitious star to human eyes; then follows mars, ruddy and awful. below, and occupying the middle region, revolves the sun, the chief, prince, and moderator of the other stars, the soul of the world, whose immense globe spreads its light through space. after him come, like two companions, venus and mercury. lastly, the lowest globe is occupied by the moon, which borrows its light from the star of day. below this last celestial circle, there is nothing but what is mortal and corruptible, except the souls given by a beneficent divinity to the race of men. above the moon all is eternal. the earth, situated in the centre of the world, and separated from heaven on all sides, forms the ninth sphere; it remains immovable, and all heavy bodies are drawn to it by their own weight." the earth, we should add, is surrounded by the sphere of air, and then by that of fire, and by that of ether and the meteors. with respect to the motions of these spheres. the first circle described about the terrestrial system, namely, that of the moon, was accomplished in days, hours, and minutes. next to the moon, mercury in the second, and venus in the third, and the sun in the fourth circle, all turned about the earth in the same time, days, hours, and minutes. but these planets, in addition to the general movement, which carried them in hours round from east to west and west to east, and the annual revolution, which made them run through the zodiacal circle, had a third motion by which they described a circle about each point of their orbit taken as a centre. [illustration: fig. .--ptolemy's astronomical system.] the fifth sphere, carrying mars, accomplished its revolution in two years. jupiter took years, days, and hours to complete his orbit, and saturn in the seventh sphere took years and days. above all the planets came the sphere of the fixed stars, or firmament, turning from east to west in hours with inconceivable rapidity, and endued also with a proper motion from west to east, which was measured by hipparchus, and which we now call the precession of the equinoxes, and know that it has a period of , years. above all these spheres, a _primum mobile_ gave motion to the whole machine, making it turn from east to west, but each planet and each fixed star made an effort against this motion, by means of which each of them accomplished their revolution about the earth in greater or less time, according to its distance, or the magnitude of the orbit it had to accomplish. one immense difficulty attended this system. the apparent motions of the planets is not uniform, for sometimes they are seen to advance from west to east, when their motion is called _direct_, sometimes they are seen for several nights in succession at the same point in the heavens, when they are called _stationary_, and sometimes they return from east to west, and then their motion is called _retrograde_. we know now that this apparent variation in the motion of the planets is simply due to the annual motion of the earth in its orbit round the sun. for example, saturn describes its vast orbit in about thirty years, and the earth describes in one year a much smaller one inside. now if the earth goes faster in the same direction as saturn, it is plain that saturn will be left behind and appear to go backwards, while if the earth is going in the same direction the velocity of saturn will appear to be decreased, but his direction of motion will appear unaltered. to explain these variations, however, according to his system, ptolemy supposed that the planets did not move exactly in the circumference of their respective orbits, but about an _ideal centre_, which itself moved along this circumference. instead therefore of describing a circle, they described parts of a series of small circles, which would combine, as is easy to see, into a series of uninterrupted waves, and these he called _epicycles_. another objection, which even this arrangement did not overcome, was the variation of the size of the planets. to overcome this hipparchus gave to the sphere of each planet a considerable thickness, and saw that the planet did not turn centrally round the earth, but round a centre of motion placed outside the earth. its revolution took place in such a manner, that at one time it reached the inner boundary, at another time the outer boundary of its spherical heaven. but this reply was not satisfactory, for the differences in the apparent sizes proved by the laws of optics such a prodigious difference between their distances from the earth at the times of conjunction and opposition, that it would be extremely difficult to imagine spheres thick enough to allow of it. it was a gigantic and formidable piece of machinery to which it was necessary to be continually adding fresh pieces to make observation accord with theory. in the thirteenth century, in the times of the king-astronomer, alphonso x. of castile, there were already seventy-five circles, one within the other. it is said that one day he exclaimed, in a full assemblage of bishops, that if the deity had done him the honour to ask his advice before creating the world, he could have told him how to make it a little better, or at all events more simply. he meant to express how unworthy this complication was of the dignity of nature. [illustration: fig. .--the epicycles of ptolemy.] fracastor, in his _homocentrics_, says that nothing is more monstrous or absurd than all the excentrics and epicycles of ptolemy, and proposes to explain the difference of velocity in the planets at different parts of their orbits by the medium offering greater or less resistance, and their alteration in apparent size by the effect of refraction. the essential element of this system was that it took appearances for realities, and was founded on the assumption that the earth is fixed in the centre of the universe, and of course therefore neglected all the appearances produced by its motion, or had to explain them by some peculiarity in the other planets. although it was corrected from time to time to make it accord better with observation, it was the same essentially that was taught officially everywhere. it reigned supreme in egypt, greece, italy, and arabia, and in the great school of alexandria, which consolidated it and enriched it by its own observations. but though the same in essence, the details, and especially the means of overcoming the difficulties raised by increased observations, have much varied, and it will be interesting and instructive to record some of the chief of them. one of the most important influences in modifying the astronomical systems taught to the world has been that of the fathers of the christian church. when, after five centuries of patient toil, of hopes, ambitions, and discussions, the christian church took possession of the thrones and consciences of men, they founded their physical edifice on the ancient system, which they adapted to their special wants. with them aristotle and ptolemy reigned supreme. they decreed that the earth constituted the universe, that the heavens were made for it, that god, the angels, and the saints inhabited an eternal abode of joy situated above the azure sphere of the fixed stars, and they embodied this gratifying illusion in all their illuminated manuscripts, their calendars, and their church windows. the doctors of the church all acknowledged a plurality of heavens, but they differed as to the number. st. hilary of poitiers would not fix it, and the same doubt held st. basil back; but the rest, for the most part borrowing their ideas from paganism, said there were six or seven, or up to ten. they considered these heavens to be so many hemispheres supported on the earth, and gave to each a different name. in the system of bede, which had many adherents, they were the air, ether, fiery space, firmament, heaven of the angels, and heaven of the trinity. the two chief varieties in the systems of the middle ages may be represented as follows:-- those who wished to have everything as complete as possible combined the system of ptolemy with that of the fathers of the church, and placed in the centre of the earth the infernal regions which they surrounded by a circle. another circle marked the earth itself, and after that the surrounding ocean, marked as water, then the circle of air, and lastly that of fire. enveloping these, and following one after the other, were the seven circles of the seven planets; the eighth represented the sphere of the fixed stars on the firmament, then came the ninth heaven, then a tenth, the _coelum cristallinum_, and lastly an eleventh and outermost, which was the empyreal heaven, where dwelt the cherubim and seraphim, and above all the spheres was a throne on which sat the father, as jupiter olympus. the others who wished for more simplicity, represented the earth in the centre of the universe, with a circle to indicate the ocean, the second sphere was that of the moon; the third was that of the sun; on the fourth were placed the four planets, jupiter, mars, venus and mercury; there was a fifth for the space outside the planets, and the last outside one was the firmament; altogether seven spheres instead of eleven. as a specimen of the style of representation of the astronomical systems of the middle ages, we may take the figure on the following page:-- here we see the earth placed immovable in the centre of the universe, and represented by a disc traversed by the mediterranean, and surrounded by the ocean. round this are circumscribed the celestial spheres. that of the moon first, then that of mercury, in which several constellations, as the lyre, cassiopeia, the crown, and others, are roughly indicated, then comes the sphere of venus with sagittarius and the swan. after this comes the _celestis_ _paradisus_, and the legend that, "the paradise to which paul was raised is in this third locality; some of these must reach to us, since in them repose the souls of the prophets." in the other circles are yet other constellations: for example pegasus, andromeda, the dog, argo, the he-goat, aquarius, the fishes, and canopus, figured by a star of the first magnitude. to the north is seen near the constellation of the swan a large star with seven rays, meant to represent the brightest of those which compose the great bear. the stars of cassiopeia are not only misplaced, but roughly represented. the lyre is curiously drawn. the positions of the constellations just named are all wrong in this figure, just as we find those of towns in maps of the earth. the cartographers of the middle ages, with incredible ignorance, misplaced in general every locality. they did the same for the constellations in the celestial hemispheres. in the heaven of jupiter, and in that of saturn we read the words--seraphim, dominationes, potestates, archangeli, virtutes coelorum, principatus, throni, cherubim, all derived from their theology. a veritable muddle! the angels placed with the heroes of mythology, the immortal virgins with venus and andromeda, and the saints with the great bear, the hydra, and the scorpion! [illustration: fig. .--heavens of the middle ages.] another such richly illuminated manuscript in the library at ghent, entitled liber floridus, contains a drawing similar to this under the title _astrologia secundum bedum_. only, instead of the earth, there is a serpent in the centre with the name great bear, and the twins are represented by a man and woman, andromeda in a chasuble, and venus as a nun! several similar ones might be quoted, varying more or less from this; one, executed in a geographical manuscript of the fifteenth century, has the tenth sphere, being that of the fixed stars, then the crystalline heaven, and then the immovable heaven, "which," it says, "according to sacred and certain theology, is the dwelling-place of the blessed, where may we live for ever and ever, amen;" "this is also called the empyreal heaven." near each planet the author marks the time of its revolution, but not at all correctly. [illustration: plate vii.--heavens of the fathers.] the constructors of these systems were not in the least doubt as to their reality, for they actually measured the distance between one sphere and another, though in every case their numbers were far from the truth as we now know it. we may cite as an example an italian system whose spheres were as follows:--terra, aqua, aria, fuoco, luna, mercurio, venus, sol, marte, giove, saturno, stelle fixe, sfera nona, cielo empyreo. attached to the design is the following table of dimensions which we may copy:-- miles. from the centre of the earth to the surface , " " " " inner side of the heaven of the moon , diameter of moon , from the centre of the earth to mercury , diameter of mercury from the centre of the earth to venus , diameter of venus , from the centre of the earth to the sun , , diameter of the sun , from the centre of the earth to mars , , diameter of mars , from the centre of the earth to jupiter , , diameter of jupiter , from the centre of the earth to outside of saturn's heaven , , diameter of saturn , from the centre of the earth to the fixed stars , , the author states that he cannot pursue his calculations further, and condescends to acknowledge that it is very difficult to know accurately what is the thickness of the ninth and of the crystalline heavens! perhaps, however, these reckonings are better than those of the egyptians, who came to the conclusion that saturn was only distant miles, the sun only , and the moon . these numerous variations and adaptations of the ptolemaic system, prove what a firm hold it had taken, and how it reigned supreme over all minds. nor are we merely left to gather this. they consciously looked to ptolemy as their great light, if we may judge from an emblematic drawing taken from an authoritative astronomical work, the _margarita philosophica_, which we give on the opposite page. [illustration: fig. .] in all the systems derived from ptolemy, the order of the planets remained the same, and mercury and venus were placed nearer to the earth than the sun is. according to many authors, however, plato made a variation in this respect, by putting them outside the sun, on the ground that they never were seen to pass across its surface. he had obviously never heard of the "transit of venus." this arrangement was adopted by theon, in his commentary on the _almagesta_ of ptolemy, and afterwards by geber, who alone among the arabians departed from the strict ptolemaic system. [illustration: fig. .--egyptian system.] the egyptians improved upon this idea, and made the first step towards the true system, by representing these two planets, mercury and venus, as revolving round the sun instead of the earth. all the rest of their system was the same as that of ptolemy, for the sun itself, and the other planets and the fixed stars all revolved round the earth in the centre. this system of course accounted accurately for the motions of the two inferior planets, whose nearness to the sun may have suggested their connection with it. this system was in vogue at the same time as ptolemy's, and numbers vitruvius amongst its supporters. [illustration: fig. .--capella's system.] in the fifth century of our era martian capella taught a variation on the egyptian system, in which he made mercury and venus revolve in the same orbit round the sun. in the treatise entitled _quod tellus non sit centrum omnibus planetis_, he explains that when mercury is on this side of the orbit it is nearer to us than venus, and farther off from us than that planet when it is on the other side. this hypothesis was also adopted in the middle ages. we have here indicated the time of the revolution of the various planets, and notice that the firmament is said to move round from west to east in , years; the second heaven in , , while the _primum mobile_ outside moved in the contrary direction in twenty-four hours. these egyptian systems survived in some places the true one, as they were thought to overcome the chief difficulties of the ptolemaic without interfering with the stability of the earth, and they were known as the _common system_, _i.e._ containing the elements of both. such were the astronomical systems in vogue before the time of copernicus--all of them based upon the principle of the earth being the immovable centre of the universe. we must now turn to trace the history of the introduction of that system which has completely thrown over all these former ones, and which every one knows now to be the true one--the copernican. no revolution is accomplished, whether in science or politics, without having been long in preparation. the theory of the motion of the earth had been conceived, discussed, and even taught many ages before the birth of copernicus. and the best proof of this is the acknowledgment of copernicus himself in his great work _de revolutionibus orbium cælestium_, in which he laid down the principles of his system. we will quote the passage in which it is contained. "i have been at the trouble," he writes, "to read over all the works of philosophers that i could procure, to see if i could find in them any different opinion to that which is now taught in the schools respecting the motions of the celestial spheres. and i saw first in cicero that mætas had put forth the opinion that the earth moves. (mætam sensisse terram moveri.) afterwards i found in plutarch that others had entertained the same idea." here copernicus quotes the original as far as it relates to the system of philolaus, to the effect "that the earth turns round the region of fire (ethereal region), and runs through the zodiac like the sun and the moon." the principal pythagoreans, such as archytas of tarentum, heraclides of pontium, taught also the same doctrine, saying that "the earth is not immovable in the centre of the universe, but revolves in a circle, and is far from occupying the chief place among the celestial bodies." pythagoras learnt this doctrine, it is said, from the egyptians, who in their hieroglyphics represented the symbol of the sun by the stercoral beetle, because this insect forms a ball with the excrement of the oxen, and lying down on its back, turns it round and round with its legs. timæus of locris was more precise than the other pythagoreans in calling "the five planets the organs of time, on account of their revolutions," adding that we must conclude that the earth is not immovable in one place, but that it turns, on the contrary, about itself, and travels also through space. plutarch records that plato, who had always taught that the sun turned round the earth, had changed his opinion towards the end of his life, regretting that he had not placed the sun in the centre of the universe, which was the only place, he then thought, that was suitable for that star. three centuries before jesus christ, aristarchus of samos is said by aristotle to have composed a special work to defend the motion of the earth against the contrary opinions of philosophers. in this work, which is now lost, he laid down in the most positive manner that "the sun remains immovable, and that the earth moves round it in a circular curve, of which that star is the centre." it would be impossible to state this in clearer terms; and what makes his meaning more clear, if possible, is that he was persecuted for it, being accused of irreligion and of troubling the repose of vesta--"because," says plutarch, "in order to explain the phenomena, he taught that the heavens were immovable, and that the earth accomplished a motion of translation in an oblique line, at the same time that it turned round its own axis." this is exactly the opinion that copernicus took up, after an interval of eighteen centuries--and he too was accused of irreligion. in passing from the greeks to the romans, and from them to the middle ages, the doctrine of aristarchus underwent a curious modification, assimilating it to the system of tycho brahe, which we shall hereafter consider, rather than to that of copernicus. this consisted in making the planets move round the sun, while the sun itself revolved round the earth, and carried them with him, and the heavens revolved round all. vitruvius and macrobius both taught this doctrine. although cicero and seneca, with aristotle and the stoics, taught the immobility of the earth in the centre of the universe, the question seemed undecided, to seneca at least, who writes:--"it would be well to examine whether it is the universe that turns about the immovable earth, or the earth that moves, while the universe remains at rest. indeed some men have taught that the earth is carried along, unknown to ourselves, that it is not the motion of the heavens that produces the rising and setting of the stars, but that it is we who rise and set relatively to them. it is a matter worthy of contemplation, to know in what state we are--whether we are assigned an immovable or rapidly-moving home--whether god makes all things revolve round us, or we round them." the double motion of the earth, then, is an idea revived from the grecian philosophers. the theory was known indeed to ptolemy, who devotes a whole chapter in his celebrated _almagesta_ to combat it. from his point of view it seemed very absurd, and he did not hesitate to call it so; and it was in reality only when fresh discoveries had altered the method of examining the question that the absurdities disappeared, and were transferred to the other side. not until it was discovered that the earth was no larger and no heavier than the other planets could the idea of its revolution and translation have appeared anything else than absurd. we are apt to laugh at the errors of former great men, while we forget the scantiness of the knowledge they then possessed. so it will be instructive to draw attention to ptolemy's arguments, that we may see where it is that new knowledge and ideas have led us, as they would doubtless have led him, had he possessed them, to a different conclusion. his argument depends essentially on the observed effects of weight. "light bodies," he says, "are carried towards the circumference, they appear to us to go _up_; because we so speak of the space that is over our heads, as far as the surface which appears to surround us. heavy bodies tend, on the contrary, towards the middle, as towards a centre, and they appear to us to fall _down_, because we so speak of whatever is under our feet, in the direction of the centre of the earth. these bodies are piled up round the centre by the opposed forces of their impetus and friction. we can easily see that the whole mass of the earth, being so large compared with the bodies that fall upon it, can receive them without their weight or their velocity communicating to it any perceptible oscillation. now if the earth had a motion in common with all the other heavy bodies, it would not be long, on account of its weight, in leaving the animals and other bodies behind it, and without support, and it would soon itself fall out of heaven. such would be the consequences of its motion, which are most ridiculous even to imagine." against the idea of the earth's diurnal rotation he argued as follows:--"there are some who pretend that nothing prevents us from supposing that the heaven remains immovable, and the earth turns round upon its axis from west to east, accomplishing the rotation each day. it is true that, as far as the stars are concerned, there is nothing against our supposing this, if guided only by appearances, and for greater simplicity; but those who do so forget how thoroughly ridiculous it is when we consider what happens near us and in the air. for even if we admit, which is not the case, that the lighter bodies have no motion, or only move as bodies of a contrary nature, although we see that aërial bodies move with greater velocity than terrestrial--if we admit that very dense and heavy bodies have a rapid and constant motion of their own, whereas in reality they obey but with difficulty the impulses communicated to them--we should then be obliged to assert that the earth, by its rotation, has a more rapid motion than any of the bodies that are round it, as it makes so large a circuit in so short a time. in this case the bodies which are not supported by it would appear to have a motion contrary to it, and no cloud or any flying bird could ever appear to go to the east, since the earth would always move faster than it in that direction." the _almagesta_ was for a long time the gospel of astronomers; to believe in the motion of the earth was to them more than an innovation, it was simply folly. copernicus himself well expresses the state of opinion in which he found the question, and the process of his own change, in the following words:--"and i too, taking occasion by these testimonies, commenced to cogitate on the motion of the earth, and although that opinion appeared absurd, i thought that as others before me had invented an assemblage of circles to explain the motion of the stars, i might also try if, by supposing the earth to move, i could not find a better account of the motions of the heavenly bodies than that with which we are at present contented. after long researches, i am at last convinced that if we assign to the circulation of the earth the motions of the other planets, calculation and observation will agree better together. and i have no doubt that mathematicians will be of my opinion, if they will take the trouble to consider carefully and not superficially the demonstrations i shall give in this work." although the opinions of copernicus had been held before, it is very just that his should be the name by which they are known; for during the time that elapsed before he wrote, the adherents of such views became fewer and fewer, until at last the very remembrance of them was almost forgotten, and it required research to know who had held them and taught them. it took him thirty years' work to establish them on a firm basis. we shall make no excuse for quoting further from his book, that we may know exactly the circumstances, as far as he tells us, of his giving this system to the world. "i hesitated for a long time whether i should publish my commentaries on the motions of the heavenly bodies, or whether it would not be better to follow the example of certain pythagoreans, who left no writings, but communicated the mysteries of their philosophy orally from man to man among their adepts and friends, as is proved by the letter of lysidas to hipparchus. they did not do this, as some suppose, from a spirit of jealousy, but in order that weighty questions, studied with great care by illustrious men, might not be disparaged by the idle, who do not care to undertake serious study, unless it be lucrative, or by shallow-minded men, who, though devoting themselves to science, are of so indolent a spirit that they only intrude among philosophers, like drones among bees. "when i hesitated and held back, my friends pressed me on. the first was nicolas schonberg, cardinal of capua, a man of great learning. the other was my best friend, tideman gysius, bishop of culm, who was as well versed in the holy scriptures as in the sciences. the latter pressed me so much that he decided me at last to give to the public the work i had kept for more than twenty-seven years. many illustrious men urged me, in the interest of mathematics, to overcome my repugnance and to let the fruit of my labours see the light. they assured me that the more my theory of the motion of the earth appeared absurd, the more it would be admired when the publication of my work had dissipated doubts by the clearest demonstrations. yielding to these entreaties, and buoying myself with the same hope, i consented to the printing of my work." he tried to guard himself against the attacks of dogmatists by saying, "if any evil-advised person should quote against me any texts of scripture, i deprecate such a rash attempt. mathematical truths can only be judged by mathematicians." notwithstanding this, however, his work, after his death, was condemned by the index in , under paul v. on examining the ancient systems, copernicus was struck by the want of harmony in the arrangements proposed, and by the arbitrary manner in which new principles were introduced and old ones neglected, comparing the system to a collection of legs and arms not united to any trunk, and it was the simplicity and harmony which the one idea of the motion of the earth introduced into the whole system that convinced him most thoroughly of its truth. he knew well that new views and truths would appear as paradoxes, and be rejected by men who were wedded to old doctrines, and on this account he took such pains to show that these views had been held before, and thus to disarm them of their apparent novelty. [illustration: fig. .--the copernican system.] copernicus dealt only with the six planets then known and the sun and moon. as to the stars, he had no idea that they were suns like our own, at immense and various distances from us. the knowledge of the magnitude of the sidereal universe was reserved for our own century, when it was discovered by the method of parallaxes. we will give copernicus's own sketch of the planetary system:-- "in the highest place is the sphere of the fixed stars, an immovable sphere, which surrounds the whole of the universe. among the movable planets the first is saturn, which requires thirty years to make its revolution. after it jupiter accomplishes its journey in twelve years; mars follows, requiring two years. in the fourth line come the earth and the moon which in the course of one year return to their original position. the fifth place is occupied by venus, which requires nine months for its journey. mercury occupies the sixth place, whose orbit is accomplished in eighty days. in the midst of all is the sun. what man is there, who in this majestic temple could choose another and better place for that brilliant lamp which illuminates all the planets with their satellites? it is not without reason that the sun is called the lantern of the world, the soul and thought of the universe. in placing it in the centre of the planets, as on a regal throne, we give it the government of the great family of celestial bodies." the hypothesis of the motion of the earth in its orbit appeared simply to copernicus as a good basis for the exact determination of the ratios of the distances of the several planets about the sun. but he did not give up the excentrics and epicycles for the explanation of the irregular motions of the planets, and certain imaginary variations in the precession of the equinoxes and the obliquity of the ecliptic. according to him the earth was endowed with three different motions, the first about its axis, the second along the ecliptic, and a third, which he called the declination, moving it backwards along the signs of the zodiac from east to west. this last motion was invented to explain the phenomena of the seasons. he thought, like many other ancient philosophers, that a body could not turn about another without being fixed in some way to it--by a crystal sphere, or something--and in this case that the same surface would each day be presented to the sun, and so it requires a third rotation, by which its axis may remain constantly parallel to itself. galileo, however, afterwards demonstrated the independence of the two motions in question, and proved that the third was unnecessary. copernicus was born in the polish village of thorn, in , and died in , at warmia, of which he was canon, and where he built an observatory. the voyages of his youth, his labours, adversities, and old age at last broke him down, and in the winter of he took to his bed, and was incapable of further work. his work, which was just finished printing at nuremberg, was brought to him by his friends before he died. he soon after completely failed in strength, and passed away tranquilly on the rd of may, . [illustration: plate viii.--death of copernicus.] the copernican system required, however, establishing in the minds of astronomers generally before it took the place it now holds, and this work was done by galileo--a name as celebrated as that of copernicus himself, if not more so. this perhaps is due not only to his demonstration of the motion of the earth, but to his introduction of experimental philosophy, and his observational method in astronomy. the next advance was made by kepler, who overthrew at one blow all the excentrics and epicycles of the ancients, when by his laborious calculations he proved the ellipticity of the orbit of mars. the grecian hypotheses were the logical consequences of two propositions which were universally admitted as axioms in the early and middle ages. first, that the motions of the heavenly bodies were uniform; second, that their orbits were perfect circles. nothing appeared more natural than this belief, though false. so then when kepler, in , recognised the fact, by incontestable geometrical measurements, that mars described an oval orbit round the sun, in which its velocity varied periodically, he could not believe either his observation or his calculation, and he puzzled his brain to discover what secret principle it was that forced the planet to approach and depart from the sun by turns. fortunately for him, in this inquietude he came across a treatise by gilbert, _de magnate_, which had been published in london nine years before. in this remarkable work gilbert proved by experiment that the earth acts on magnetized needles and on bars of iron placed near its surface just as a magnet does--and by a conjectural extension of this fact, which was a vague presentiment of the truth, he supposed that the earth itself might be retained in its constant orbit round the sun by a magnetic attraction. this idea was a ray of light to kepler. it led him to see the secret cause of the alternating motions that had troubled him so much, and in the joy of that discovery he said, "if we find it impossible to attribute the vibration to a magnetic power residing in the sun, acting on the planet without any material medium between, we must conclude that the planet is itself endowed with a kind of intelligent perception which gives it power to know at each instant the proper angles and distances for its motion." in the result kepler was led to enunciate to the world his three celebrated laws:-- st. that the planets move in ellipses, of which the sun is in one of the foci. nd. the spaces described by the ideal radius which joins each planet to the sun are proportional to the times of their description. in other words, the nearer a planet is to the sun, the faster it moves. rd. the squares of the times of revolution are as the cubes of the major axes of the orbits. such were the laws of kepler, the basis of modern astronomy, which led in the hands of newton to the simple explanation by universal gravitation, which itself is now asking to be explained. we are not to suppose that the system of copernicus was universally accepted even by astronomers of note. by some an attempt was made to invent a system which should have all the advantages of this, and yet if possible save the immobility of the earth. such was that of tycho brahe, who was born three years after the death of copernicus, and died in . he was one of the most laborious and painstaking observers of his time, although by the peculiarity of fate he is known generally only by his false system. [illustration: fig. .--tycho brahe's system.] in , tycho brahe wrote a little treatise, _tychonis brahe, dani, de mundi Ætherei recentioribus phenomenis, à propos_ of a comet that had lately appeared. he speaks at length of his system as follows:--"i have remarked that the ancient system of ptolemy is not at all natural, and too complicated. but neither can i approve of the new one introduced by the great copernicus after the example of aristarchus of samos. this heavy mass of earth, so little fit for motion, could not be displaced in this manner, and moved in three ways, like the celestial bodies, without a shock to the principles of physics. besides, it is opposed to scripture! i think then," he adds, "that we must decidedly and without doubt place the earth immovable in the centre of world, according to the belief of the ancients and the testimony of scripture. in my opinion the celestial motions are arranged in such a way that the sun, the moon, and the sphere of the fixed stars, which incloses all, have the earth for their centre. the five planets turn about the sun as about their chief and king, the sun being constantly in the centre of their orbits, and accompany it in its annual motion round the earth." this system perfectly accounts for the apparent motions of the planets as seen from the earth, and is essentially a variation on the copernican, rather than on the ptolemaic system, but it lent itself less readily to future discoveries. it simply amounts, as far as the solar system is concerned, to impressing upon all the rest of it the motions of the earth, so as to leave the latter at rest; and were the sun only as large with respect to the earth as it seems, were the planets really smaller than the moon, and the stars only at a short distance, and smaller than the planets, it might seem more natural that they should move than the earth; but when all these suppositions were disproved, the very argument of tycho brahe for the stability of the earth turned the other way, and proved as incontestably that it moved. in the copernican system, however, these questions are of no consequence; if the sun be at rest, this mass makes no difference; if the earth moves like the planets, their relative size does not alter anything; and if stars are immovable they may be at any distance and of any magnitude. the objections of tycho brahe to the earth's motion were: first, that it was too heavy--we know now, however, that some other planets are heavier--and that the sun, which he would make move instead, is , times as heavy. secondly, that if the earth moved, all loose things would be carried from east to west; but we have experience of many loose things being kept by friction on moving bodies, and can conceive how, all things may be kept by the attraction of the earth under the influence of its own motion. thirdly, that he could not imagine that the earth was turned upside down every day, and that for twelve hours our heads are downwards. but the existence of the antipodes overcomes this objection, and shows that there is no up and down in the universe, but each man calls that _down_ which is nearer to the centre of the earth than himself. a variation on tycho brahe's system was attempted by one longomontanus, who had lived with him for ten years. it consisted in admitting the diurnal rotation, but not the annual revolution, of the earth; but it made no progress, and was soon forgotten. more remarkable than this was the attempt by descartes in the same direction, namely, to hold the principles of copernicus, and yet to teach the immobility of the earth. his idea of immobility was however very different from that of tycho brahe, or of any one else, and would only be called so by those who were bound to believe it at all costs. his theory of vortices, as it is called, will be best given in his own words as contained in his _les principes de la philosophie_, third part, chap. xxvi., entitled, "that the earth is at rest in its heaven, which does not prevent its being carried along with it, and that it is the same with all the planets." "i adhere," he says, "to the hypothesis of copernicus, because it seems to me the simplest and clearest. there is no vacuum anywhere in space.... the heavens are full of a universal liquid substance. this is an opinion now commonly received among astronomers, because they cannot see how the phenomena can be explained without it. the substance of the heavens has the common property of all liquids, that its minutest particles are easily moved in any direction, and when it happens that they all move in one way, they necessarily carry with them all the bodies they surround, and which are not prevented from moving by any external cause. the matter of the heaven in which the planets are turns round continually like a vortex, which has the sun for its centre. the parts that are nearest the sun move faster than those that are at a greater distance; and all the planets, including the earth, remain always suspended in the same place in the matter of the heaven. and just as in the turns of rivers, when the water turns back on itself and twists round in circles, if any twig or light body floats on it, we see it carry them round, and make them move with it, and even among these twigs we may see some turning on their own centre, and those that are nearest to the middle of the vortex moving quicker than those on the outside; so we may easily imagine it to be with the planets, and this is all that is necessary to explain the phenomena. the matter that is round saturn takes about thirty years to run its circle; that which surrounds jupiter carries it and its satellites round in twelve years, and so on.... the satellites are carried round their primaries by smaller vortices.... the earth is not sustained by columns, nor suspended in the air by ropes, but it is environed on all sides by a very liquid heaven. it is at rest, and has no propulsion or motion, since we do not perceive any in it. this does not prevent it being carried round by its heaven, and following its motion without moving itself, just as a vessel which is not moved by winds or oars, and is not retained by anchors, remains in repose in the middle of the sea, although the flood of the great mass of water carries it insensibly with it. like the earth, the planets remain at rest in the region of heaven where each one is found. copernicus made no difficulty in allowing that the earth moves. tycho, to whom this opinion seemed absurd and unworthy of common sense, wished to correct him, but the earth has far more motion in his hypothesis than in that of copernicus." [illustration: fig. .--descartes' theory of vortices.] such is the celebrated theory of vortices. the comparison of the rotation of the earth and planets and their revolution round the sun to the turning of small portions of a rapid stream, may contain an idea yet destined to be developed to account for these motions; but as used by descartes it is a mere playing upon words admirably adapted to secure the concurrence of all parties; those who believed in the motion of the earth seeing that it did not interfere with their ideas in the least, and those who believed in its stability being gratified to find some way by which they might still cling to that belief and yet adopt the new ideas. this was its purpose, and that purpose it well served; but as a philosophical speculation it was worthless. when former astronomers declared that any planet moved, whether it were the earth or any other, they had no idea of attraction, but supposed the planet fixed to a sphere; this sphere moving and carrying the planet with it was what they meant by the planet moving: the theory of vortices merely substituted a liquid for a solid sphere, with this disadvantage, that if the planet were fixed to a solid moving sphere, it _must_ move; if only placed in a liquid one, that liquid might pass it if it did not have motion of its own. [illustration: fig. .--vortices of the stars.] a variation on descartes' system of vortices was proposed in the eighteenth century, which supposed that the sun, instead of being fixed in the centre of the system, itself circulated round another centre, carrying mercury with it. this motion of the sun was intented to explain the changes of magnitude of its disc as seen from the earth, and the diurnal and annual variations in its motion, without discarding its circular path. [illustration: fig. .--variation of descartes' theory.] we have thus noticed all the chief astronomical systems that have at any time been entertained by astronomers. they one and all have given way before the universally acknowledged truth about which there is no longer any dispute. systems are not now matters of opinion or theory. we speak of facts as certain as any that can be ascertained in any branch of knowledge. we have much to learn, but what we have settled as the basis of our knowledge will never more be altered as far as we can see. of course there have been always fantastic fancies put forth about the solar system, but they are more amusing than instructive. some have said that there is no sun, moon, or stars, but that they are reflections from an immense light under the earth. some savage races say that the moon when decreasing breaks up into stars, and is renewed each month by a creative act. the indians used to say that it was full of nectar which the gods ate up when it waned, and which grew again when it waxed. the brahmins placed the earth in the centre, and said that the stars moved like fishes in a sea of liquid. they counted nine planets, of which two are invisible dragons which cause eclipses; which, since they happen in various parts of the zodiac, show that these dragons revolve like the rest. they said the sun was nearer than the moon, perhaps because it is hotter and brighter. berosus the chaldean gave a very original explanation of the phases and eclipses of the moon. he said it had one side bright, and the other side just the colour of the sky, and in turning it represented the different colours to us. before concluding this chapter we may notice what information we possess as to the origin of the names by which the planets are known. these names have not always been given to them, and date only from the time when the poets began to associate the grecian mythology with astronomy. the earlier names had reference rather to their several characters, although there appear to have been among every people two sets of names applied to them. the earliest greek names referred to their various degrees of brilliancy: thus saturn, which is not easily distinguished, was called phenon, or _that which appears_; jupiter was named phaëton, _the brilliant_; mars was pysoïs, or _flame-coloured_; mercury, stilbon, _the sparkling_; venus, phosphorus; and lucifer, _the light-bearer_. they called the latter also calliste, _the most beautiful_. it was also known then as now under the appellations of the morning star and evening star, indicating its special position. with the ancient accadians, the planets had similar names, among others. thus, "mars was sometimes called _the vanishing star_, in allusion to its recession from the earth, and jupiter the _planet of the ecliptic_, from its neighbourhood to the latter" (sayce). the name of mars raises the interesting question as to whether they had noticed its phases as well as its movements--especially when, with reference to venus, it is recorded in the "observations of bel," that "it rises, and in its orbit duly grows in size." they had also a rather confusing system of nomenclature by naming each planet after the star that it happened to be the nearest to at any point of its course round the ecliptic. among less cultivated nations also the same practice held, as with the natives of south america, whose name for the sun is a word meaning _it brings the day_; for the moon, _it brings the night_; and for venus, _it announces the day_. but even among the eastern nations, from whom the greeks and romans borrowed their astronomical systems, it soon became a practice to associate these planets with the names of the several divinities they worshipped. this was perhaps natural from the adoration they paid to the celestial luminaries themselves on account of their real or supposed influence on terrestrial affairs; and, moreover, as time went on, and heroes had appeared, and they had to find them dwelling-places in the heavens, they would naturally associate them with one or other of the most brilliant and remarkable luminaries, to which they might suppose them translated. beyond these general remarks, only conjectures can be made why any particular divinity should among the greeks be connected with the several planets as we now know them. such conjectures as the following we may make. thus jupiter, the largest, would take first rank, and be called after the name of the chief divinity. the soft and sympathising venus--appearing at the twilight--would well denote the evening star. mars would receive its name from its red appearance, naturally suggesting carnage and the god of war. saturn, or kronos, the god of time, is personified by the slow and almost imperceptible motion of that remote planet. while mercury, the fiery and quick god of thieves and commerce, is well matched with the hide-and-seek planet which so seldom can be seen, and moves so rapidly. these were the only planets known to the ancients, and were indeed all that could be discovered without a telescope. if the ancient babylonians possessed telescopes, as has been conjectured from their speaking, as we have noticed above, of the increase of the size of venus, and from the finding a crystal lens among the ruins of nineveh, they did not use them for this purpose. the other planets now known have a far shorter history. uranus was discovered by sir william herschel on the th of march, , and was at first taken for a comet. herschel proposed to call it georgium sidus, after king george iii. lalande suggested it should be named herschel, after its discoverer, and it bore this name for some time. afterwards the names, neptune, astroea, cybele, and uranus were successively proposed, and the latter, the suggestion of bode, was ultimately adopted. it is the name of the most ancient of the gods, connected with the then most modern of planets in point of discovery, though also most ancient in formation, if recent theories be correct. neptune, as everybody knows, was calculated into existence, if one may so speak, by adams and leverrier independently, and was first seen, in the quarter indicated, by dr. galle at berlin, in september, , and by universal consent it received the name it now bears. there are now also known a long series of what are called minor planets, all circulating between mars and jupiter, with their irregular orbits inextricably mingled together. their discovery was led to in a remarkable manner. it was observed that the distances of the several planets might approximately be expressed by the terms of a certain mathematical series, if one term was supplied between mars and jupiter--a fact known by the name of bode's law. when the new planet, uranus, was found to obey this law, the feeling was so strong that there must be something to represent this missing term, that strong efforts were made to discover it, which led to success, and several, whose names are derived from the minor gods and goddesses, are now well known. all these planets, like the signs of the zodiac, are indicated by astronomers by certain symbols, which, as they derive their form from the names or nature of the planets, may properly here be explained. the sign of neptune is [symbol: neptune], representing the trident of the sea; for uranus [symbol: uranus], which is the first letter of herschel with a little globe below; [symbol: saturn] is the sickle of time, or saturn; [symbol: jupiter] is the representation of the first letter of zeus or jupiter; [symbol: mars] is the lance and buckler of mars; [symbol: venus] the mirror of venus; [symbol: mercury] the wand of mercury; [symbol: sun] the sun's disc; and [symbol: moon] the crescent of the moon. [illustration: plate ix.--the solar system.] the more modern discoveries have, of course, been all made by means of the telescope, and a few words on the history of its discovery may fitly close this chapter. according to olbers, a concave and convex lens were first used in combination, to render objects less distant in appearance, in the year . in that year the children of one jean lippershey, an optician of middelburg, in zealand, were playing with his lenses, and happened to hold one before the other to look at a distant clock. their great surprise in seeing how near it seemed attracted their father's attention, and he made several experiments with them, at last fixing them as in the modern telescope--in draw tubes. on the nd of october, , he made a petition to the states-general of holland for a patent. the aldermen, however, saw no advantage in it, as you could only look with one eye instead of two. they refused the patent, and though the discovery was soon found of value, lippershey reaped no benefit. galileo was the first to apply the telescope to astronomical observations. he did not have it made in holland, but constructed it himself on lippershey's principle. this was in . its magnifying power was at first , and he afterwards increased it to , and then to . with this he discovered the phases of venus, the spots on the sun, the four satellites of jupiter, and the mountains of the moon. [illustration: plate x.--the discovery of the telescope.] kepler, in , made the first astronomical telescope with two concave glasses. huyghens increased the magnifying power successively to , , and , and discovered saturn's ring and his satellite no. . cassini, the first director of the paris observatory, brought it to , aided by auzout campani of rome, and rives of london. he observed the rotation of jupiter ( ), that of venus and mars ( ), the fifth and third satellites of saturn ( ), and afterwards the two nearer ones ( ); the other satellites of this planet were discovered, the sixth and seventh, by sir william herschel ( ), and the eighth by bond and lasel ( ). we may add here that the satellites of uranus were discovered, six by herschel from to , and two by lassel in , the latter also discovering neptune's satellite in . the rotation of saturn was discovered by herschel in , and that of mercury by schroeter in . the earliest telescopes which were reflectors were made by gregory in and newton in . the greatest instruments of our century are that of herschel, which magnifies , times, and lord rosse's, magnifying , times, the foucault telescope at marseilles, of , , the reflector at melbourne, of , , and the newall refractor. [illustration: plate xi.--the foundation of paris observatory.] the exact knowledge of the heavens, which makes so grand a feature in modern science, is due, however, not only to the existence of instruments, but also to the establishment of observatories especially devoted to their use. the first astronomical observatory that was constructed was that at paris. in colbert submitted the designs of it to louis xiv., and four years afterwards it was completed. the greenwich observatory was established in , that of berlin in , and that of st. petersburg in . since then numerous others have been erected, private as well as public, in all parts of the world, and no night passes without numerous observations being taken as part of the ordinary duty of the astronomers attached to them. chapter ix. the terrestrial world of the ancients.--cosmography and geography. with respect to the shape and position of the earth itself in the material universe, and the question of its motion or immobility, we cannot go so far back as in the case of the heavens, since it obviously requires more observation, and is not so pressing for an answer. amongst the greeks several authors appear to have undertaken the subject, but only one complete work has come down to us which undertakes it directly. this is a work attributed to aristotle, _de mundo_. it is addressed to alexander, and by some is considered to be spurious, because it lacks the majestic obscurity that in his acknowledged works repels the reader. although, however, it is not as obscure as it might be, for the writer, it is quite bad enough, and its dryness and vagueness, its mixture of metaphysical and physical reasoning, logic and observation, and the change that has naturally passed over the meanings of many common words since they were written, render it very tedious and unpleasant reading. nevertheless, as presenting us with the first recorded ideas on these questions of the nature and properties of the earth, it deserves attentive study. it is not a system of observations like those of ptolemy and the alexandrian school, but an entirely theoretical work. it is founded entirely on logic; but unfortunately, if the premisses are bad, the better the syllogism the more erroneous will be the conclusion; and it is just this which we find here. thus if he be asked whether the earth turns or the heavens, he will reply that the earth is _evidently_ in repose, and that this is the case not only because we observe it to be so, but because it is a necessity that it should be; because repose is _natural_ to the earth, and it is _naturally_ in equilibrium. this idea of "natural" leads very often astray. he is guided to his idea of what is natural by seeing what is, and then argues that what is, or appears to be, must be, because it is natural--thus arguing in a circle. another example may be given in his answer to the question, why must the stars move round the earth? he says it is natural, because a circle is a more perfect line, and must therefore be described by the perfect stars, and a circle is perfect because it has no ends! unfortunately there are other curves that have no ends; but the circle was considered, without more reason, the most perfect curve, and therefore the planets must move in circles--an idea which had to wait till kepler's time to be exploded. one more specimen of this style may be quoted, namely, his proof that every part of heaven must be eternally moving, while the earth must be in the centre and at rest. the proof is this. everything which performs any act has been made for the purpose of that act. now the work of god is immortality, from which it follows that all that is divine must have an eternal motion. but the heavens have a divine quality, and for this reason they have a spherical shape and move eternally in a circle. now when a body has a circular motion, one part of it must remain at rest in its place, namely, that which is in the centre; the earth is in the centre--therefore it is at rest. aristotle says in this work that there are two kinds of simple motion, that in a circle and that in a straight line. the latter belongs to the elements, which either go up or down, and the former to the celestial bodies, whose nature is more divine, and which have never been known to change; and the earth and world must be the only bodies in existence, for if there were another, it must be the contrary to this, and there is no contrary to a circle; and again, if there were any other body, the earth would be attracted towards it, and move, which it does not. such is the style of argument which was in those days thought conclusive, and which with a little development and inflation of language appeared intensely profound. but what brings these speculations to the subject we have now in hand is this: that when aristotle thus proves the earth to be immovable in the centre of the universe, he is led on to inquire how it is possible for it to remain in one fixed place. he observed that even a small fragment of earth, when it is raised into the air and then let go, immediately falls without ever stopping in one place--falling, as he supposed, all the quicker according to its weight; and he was therefore puzzled to know why the whole mass of the earth, notwithstanding its weight, could be kept from falling. aristotle examines one by one the answers that have been given to this question. thus xenophanes gave to the earth infinitely extended roots, against which empedocles uses such arguments as we should use now. thales of miletus makes the earth rest upon water, without finding anything on which the water itself can rest, or answering the question how it is that the heavier earth can be supported on the lighter water. anaxemenes, anaxagoras, and democritus, who make the earth flat, consider it to be sustained by the air, which is accumulated below it, and also presses down upon it like a great coverlet. aristotle himself says that he agrees with those philosophers who think that the earth is brought to the centre by the primitive rotation of things, and that we may compare it, as empedocles does, to the water in glasses which are made to turn rapidly, and which does not fall out or move, even though upside down. he also quotes with approval another opinion somewhat similar to this, namely, that of anaximander, which states that the earth is in repose, on account of its own equilibrium. placed in the centre and at an equal distance from its extremities, there is no reason why it should move in one direction rather than the other, and rests immovable in the centre without being able to leave it. the result of all is that aristotle concludes that the earth is immovable, in the centre of the universe, and that it is not a star circulating in space like other stars, and that it does not rotate upon its axis; and he completes the system by stating that the earth is spherical, which is proved by the different aspects of the heavens to a voyager to the north or to the south. such was the aristotelian system, containing far more error than truth, which was the first of any completeness. scattered ideas, however, on the shape and method of support of the earth and the cause of various phenomena, such as the circulation of the stars, are met with besides in abundance. the original ideas of the earth were naturally tinged by the prepossessions of each race, every one thinking his own country to be situated in the centre. thus among the hindoos, who lived near the equator, and among the scandinavians, inhabiting regions nearer the pole, the same meaning attaches to the words by which they express their own country, _medpiama_ and _medgard_, both meaning the central habitation. olympus among the greeks was made the centre of the earth, and afterwards the temple of delphi. for the egyptians the central point was thebes; for the assyrians it was babylon; for the indians it was the mountain mero; for the hebrews jerusalem. the chinese always called their country the central empire. it was then the custom to denote the world by a large disc, surrounded on all sides by a marvellous and inaccessible ocean. at the extremities of the earth were placed imaginary regions and fortunate isles, inhabited by giants or pigmies. the vault of the sky was supposed to be supported by enormous mountains and mysterious columns. numerous variations have been suggested on the earliest supposed form of the earth, which was, as we have seen in a former chapter, originally supposed to be an immense flat of infinite depth, and giving support to the heavens. as travels extended and geography began to be a science, it was remarked that an immense area of water circumscribed the solid earth by irregular boundaries--whence the idea of a universal ocean. when, however, it was perceived that the horizon at sea was always circular, it was supposed that the ocean was bounded, and the whole earth came to be represented as contained in a circle, beneath which were roots reaching downwards without end, but with no imagined support. [illustration: fig. .--the earth floating.] [illustration: fig. .--the earth with roots.] the vedic priests asserted that the earth was supported on twelve columns, which they very ingeniously turned to their own account by asserting that these columns were supported by virtue of the sacrifices that were made to the gods, so that if these were not made the earth would collapse. [illustration: fig. .--the earth of the vedic priests.] these pillars were invented in order to account for the passing of the sun beneath the earth after his setting, for which at first they were obliged to imagine a system of tunnels, which gradually became enlarged to the intervals between the pillars. the hindoos made the hemispherical earth to be supported upon four elephants, and the four elephants to stand on the back of an immense tortoise, which itself floated on the surface of a universal ocean. we are not however to laugh at this as intended to be literal; the elephants symbolised, it may be, the four elements, or the four directions of the compass, and the tortoise was the symbol for strength and for eternity, which was also sometimes represented by a serpent. [illustration: fig. .--hindoo earth.] the floating of the earth on water or some other liquid long held ground. it was adopted by thales, and six centuries later seneca adopts the same opinion, saying that the humid element that supports the earth's disc like a vessel may be either the ocean or some liquid more simple than water. diodorus tells us that the chaldeans considered the earth hollow and boat-shaped--perhaps turned upside down--and this doctrine was introduced into greece by heraclitus of ephesus. [illustration: fig. .--the earth of anaximander.] anaximander represents the earth as a cylinder, the upper face of which alone is inhabited. this cylinder, he states, is one-third as high as its diameter, and it floats freely in the centre of the celestial vault, because there is no reason why it should move to one side rather than the other. leucippus, democritus, heraclitus, and anaxagoras all adopted this purely imaginary form. europe made the northern half, and lybia (africa) and asia the southern, while delphi was in the centre. anaximenes, without giving a precise opinion as to the form of the earth, made it out to be supported on compressed air, though he gave no idea as to how the air was to be compressed. plato thought to improve upon these ideas by making the earth cubical. the cube, which is bound by six equal faces, appeared to him the most perfect of solids, and therefore most suitable for the earth, which was to stand in the centre of the universe. [illustration: fig. .--plato's cubical earth.] eudoxus, who in his long voyages throughout greece and egypt had seen new constellations appear as he went south, while others to the north disappeared, deduced the sphericity of the earth, in which opinion he was followed by archimedes, and, as we have seen, by aristotle. according to achilles tatius, xenophanes gave to the earth the shape of an immense inclined plane, which stretched out to infinity. he drew it in the form of a vast mountain. the summit only was inhabited by men, and round it circulated the stars, and the base was at an infinite depth. hesiod had before this obscurely said: "the abyss is surrounded by a brazen barrier; above it rest the roots of the earth." epicurus and his school were well pleased with this representation. if such were the foundations of the earth, then it was impossible that the sun, and moon, and stars should complete their revolutions beneath it. a solid and indefinite support being once admitted, the epicurean ideas about the stars were a necessary consequence; the stars must inevitably be put out each day in the west, since they are not seen to return to the place whence they started, and they must be rekindled some hours afterwards in the east. in the days of augustus, cleomedes still finds himself obliged to combat these epicurean ideas about the setting and rising of the sun and stars. "these stupid ideas," he says, "have no other foundation than an old woman's story--that the iberians hear each night the hissing noise made by the burning sun as it is extinguished, like a hot iron in the waters of the ocean." modern travellers have shown us that similar ideas about the support of the earth have been entertained by more remote people. thus, in the opinion of the greenlanders, handed down from antiquity to our own days, the earth is supported on pillars, which are so consumed by time that they often crack, and were it not that they are supported by the incantations of the magicians, they would long since have broken down. this idea of the breaking of the pillars may possibly have originated in the known sinking of the land beneath the sea, which is still going on even at the present day. [illustration: fig. .--egyptian representation of the earth.] an ancient egyptian papyrus in the library of paris gives a very curious hieroglyphical representation of the universe. the earth is here figured under the form of a reclining figure, and is covered with leaves. the heavens are personified by a goddess, which forms the vault by her star-bespangled body, which is elongated in a very peculiar manner. two boats, carrying, one the rising sun, and the other the setting sun, are represented as moving along the heavens over the body of the goddess. in the centre of the picture is the god, maon, a divine intelligence, which presides over the equilibrium of the universe. we will now pass on from the early ideas of the general shape and situation of the world to inquire into the first outlines of geographical knowledge of details. of all the ancient writings which deal with such questions, the hebrew scriptures have the greatest antiquity, and in them are laid down many details of known countries, from which a fair map of the world as known to them might be made out. the prophet esdras believed that six-sevenths of the earth was dry land--an idea which could not well be exploded till the great oceans had been traversed and america discovered. more interesting, as being more complete, and written to a certain extent for the very purpose of relating what was known of the geography of the earth, are the writings of the oldest grecian poets. the first elements of grecian geography are contained in the two national and almost sacred poems, the _iliad_ and _odyssey_. so important have these writings been considered in regard to ancient geography, that for many centuries discussions have been carried on with regard to the details, though evidently fictitious, of the voyage of ulysses, and twenty lines of the _iliad_ have furnished matter for a book of thirty volumes. the shield of achilles, forged by vulcan and described in the eighteenth book of the _iliad_, gives us an authentic representation of the primitive cosmographical ideas of the age. the earth is there figured as a disc, surrounded on all sides by the _river ocean_. however strange it may appear to us, to apply the term _river_ to the ocean, it occurs too often in homer and the other ancient poets to admit of a doubt of its being literally understood by them. hesiod even describes the sources of the ocean at the western extremity of the world, and the representation of these sources was preserved from age to age amongst authors posterior to homer by nearly a thousand years. herodotus says plainly that the geographers of his time drew their maps of the world according to the same ideas; the earth was figured with them as a round disc, and the ocean as a river, which washed it on all sides. the earth's disc, the _orbis terrarum_, was covered according to homer by a solid vault or firmament, beneath which the stars of the day and night were carried by chariots supported by the clouds. in the morning the sun rose from the eastern ocean, and in the evening it declined into the western; and a vessel of gold, the mysterious work of vulcan, carried it quickly back by the north, to the east again. beneath the earth homer places, not the habitation of the dead, the caverns of hades, but a vault called tartarus, corresponding to the firmament. here lived the titans, the enemies of the gods, and no breath of wind, no ray of light, ever penetrated to this subterranean world. writers subsequent to homer by a century determined even the height of the firmament and the depth of tartarus. an anvil, they said, would take nine days to fall from heaven to earth, and as many more to fall from earth to the bottom of tartarus. this estimate of the height of heaven was of course far too small. if a body were to fall for nine days and nights, or , seconds under the attraction of the earth, it would only pass over , miles, that is not much more than half as far again as the moon. a ray of light would only take two seconds to pass over that distance, whereas it takes eight minutes to reach us from the sun, and four hours to come from neptune--to say nothing of the distance of the stars. the limits of the world in the homeric cosmography were surrounded by obscurity. the columns of which atlas was the guardian were supported on unknown foundations, and disappeared in the systems subsequent to homer. beyond the mysterious boundary where the earth ended and the heavens began an indefinite chaos spread out--a confused medley of life and inanity, a gulf where all the elements of heaven, tartarus, and earth and sea are mixed together, a gulf of which the gods themselves are afraid. ideas such as these prevailed long after geometers and astronomers had proved the spherical form of the globe, and they were revived by the early christian geographers and have left their trace even on the common language of to-day. [illustration: fig. .--homeric cosmography.] the centre of the terrestrial disc was occupied by the continent and isles of greece, which in the time of homer possessed no general name. the centre of greece passed therefore for the centre of the whole world; and in homer's system it was reckoned to be olympus in thessaly, but the priests of the celebrated temple of apollo at delphi (known then under the name of python) gave out a tradition that that sacred place was the real centre of the habitable world. the straits which separate italy from sicily were so to speak the vestibule of the fabulous world of homer. the threefold ebb and flow, the howling of the monster scylla, the whirlpools of charybdis, the floating rocks--all tell us that we are quitting here the region of truth. sicily itself, although already known under the name of _trinacria_, was filled with marvels; here the flocks of the sun wandered in a charming solitude under the guardianship of nymphs; here the cyclops, with one eye only, and the anthropophagous lestrigons scared away the traveller from a land that was otherwise fertile in corn and wine. two historical races were placed by homer in sicily, namely the _sicani_, and the _siceli_, or _siculi_. to the west of sicily we find ourselves in the midst of a region of fables. the enchanted islands of circe and calypso, and the floating island of eolus can no longer be found, unless we imagine them to have originated, like graham's island in this century, from volcanic eruptions or elevations, and to have disappeared again by the action of the sea. the homeric map of the world terminated towards the west by two fabulous countries which have given rise to many traditions among the ancients, and to many discussions among moderns. near to the entrance of the ocean, and not far from the sombre caverns where the dead are congregated, ulysses found the _cimmerians_, "an unhappy people, who, constantly surrounded by thick shadows, never enjoyed the rays of the sun, neither when it mounted the skies, nor when it descended below the earth." still farther away, and in the ocean itself, and therefore beyond the limits of the earth, beyond the region of winds and seasons, the poet paints for us a fortunate land, which he calls _elysium_, a country where tempests and winter are unknown, where a soft zephyr always blows, and where the elect of jupiter, snatched from the common lot of mortals, enjoy a perpetual felicity. whether these fictions had an allegory for their basis, or were founded on the mistaken notions of voyagers--whether they arose in greece, or, as the hebrew etymology of the name cimmerian might seem to indicate, in the east, or in phenicia, it is certain that the images they present, transferred to the world of reality, and applied successively to various lands, and confused by contradictory explanations, have singularly embarrassed the progress of geography through many centuries. the roman travellers thought they recognised the fortunate isles in a group to the west of africa, now known as the canaries. the philosophical fictions of plato and theopompus about atlantes and meropis have been long perpetuated in historical theories; though of course it is possible that in the numerous changes that have taken place in the surface of the earth, some ancient vast and populous island may have descended beneath the level of the sea. on the other side, the poetic imagination created the _hyperboreans_, beyond the regions where the northern winds were generated, and according to a singular kind of meteorology, they believed them for that reason to be protected from the cold winds. herodotus regrets that he has not been able to discover the least trace of them; he took the trouble to ask for information about them from their neighbours, the _arimaspes_, a very clear-sighted race, though having but a single eye; but they could not inform him where the hyperboreans dwelt. the enchanted isles, where the hesperides used to guard the golden fruit, and which the whole of antiquity placed in the west, not far from the fortunate isles, are sometimes called hyperborean by authors well versed in the ancient traditions. it is also in this sense that sophocles speaks of the garden of phoebus, near the vault of heaven, and not far from the _sources of the night_, _i.e._ of the setting of the sun. avienus explains the mild temperature of the hyperborean country by the temporary proximity of the sun, since, according to the homeric ideas, it passes during the night by the northern ocean to return to its palace in the east. this ancient tradition was not entirely exploded in the time of tacitus, who states that on the confines of germany might be seen the veritable setting of apollo beyond the water, and he believes that as in the east the sun gives rise to incense and balm by its great proximity to the earth, so in the regions where it sets it makes the most precious of juices to transude from the earth and form amber. it is this idea that is embedded in the fables of amber being the tears of gold that apollo shed when he went to the hyperborean land to mourn the loss of his son Æsculapius, or by the sisters of phaëton, changed into poplars; and it is denoted by the greek name for amber, _electron_--a sun-stone. the grecian sages, long before the time of tacitus, said that this very precious material was an exhalation from the earth that was produced and hardened by the rays of the sun, which they thought came nearer to the earth in the west and in the north. florus, in relating the expedition of decimus brutus along the coast of spain, gives great effect to the epicurean views about the sun, by declaring that brutus only stopped his conquests after having witnessed the actual descent of the sun into the ocean, and having heard with horror the terrible noise occasioned by its extinction. the ancients also believed that the sun and the other heavenly bodies were nourished by the waters--partly the fresh water of the rivers, and partly the salt water of the sea. cleanthes gave the reason for the sun returning towards the equator on reaching the solstices, that it could not go too far away from the source of its nourishment. pytheas relates that in the island of thule, six days' journey north of great britain, and in all that neighbourhood, there was no land nor sea nor air, but a compound of all three, on which the earth and the sea were suspended, and which served to unite together all the parts of the universe, though it was not possible to go into these places, neither on foot nor in ships. perhaps the ice floating in the frozen seas and the hazy northern atmosphere had been seen by some navigator, and thus gave rise to this idea. as it stands, the history may be perhaps matched by that of the amusing monk who said he had been to the end of the world and had to stoop down, as there was not room to stand between heaven and earth at their junction. homer lived in the tenth century before our era. herodotus, who lived in the fifth, developed the homeric chart to three times its size. he remarks at the commencement of his book that for several centuries the world has been divided into three parts--europe, asia, and libya; the names given to them being female. the exterior limits of these countries remained in obscurity notwithstanding that those boundaries of them that lay nearest to greece were clearly defined. one of the greatest writers on ancient geography was strabo, whose ideas we will now give an account of. he seems to have been a disciple of hipparchus in astronomy, though he criticises and contradicts him several times in his geography. he had a just idea of the sphericity of the earth; but considered it as the centre of the universe, and immovable. he takes pains to prove that there is only one inhabited earth--not in this refuting the notion that the moon and stars might have inhabitants, for these he considered to be insignificant meteors nourished by the exhalations of the ocean; but he fought against the fact of there being on this globe any other inhabited part than that known to the ancients. it is remarkable to notice that the proofs then used by geographers of the sphericity of the earth are just those which we should use now. thus strabo says, "the indirect proof is drawn from the centripetal force in general, and the tendency that all bodies have in particular towards a centre of gravity. the direct proof results from the phenomena observed on the sea and in the sky. it is evident, for example, that it is the curvature of the earth that alone prevents the sailor from seeing at a distance the lights that are placed at the ordinary height of the eye, and which must be placed a little higher to become visible even at a greater distance; in the same way, if the eye is a little raised it will see things which previously were hidden." homer had already made the same remark. on this globe, representing the world, strabo and the cosmographers of his time placed the habitable world in a surface which he describes in the following way: "suppose a great circle, perpendicular to the equator, and passing through the poles to be described about the sphere. it is plain that the surface will be divided by this circle, and by the equator into four equal parts. the northern and southern hemispheres contain, each of them, two of these parts. now on any one of these quarters of the sphere let us trace a quadrilateral which shall have for its southern boundary the half of the equator, for northern boundary a circle marking the commencement of polar cold, and for the other sides two equal and opposite segments of the circle that passes through the poles. it is on one such quadrilateral that the habitable world is placed." he figures it as an island, because it is surrounded on all sides by the sea. it is plain that strabo had a good idea of the nature of gravity, because he does not distinguish in any way an upper or a lower hemisphere, and declares that the quadrilateral on which the habitable world is situated may be any one of the four formed in this way. the form of the habitable world is that of a "chlamys," or cloak. this follows, he says, both from geometry and the great spread of the sea, which, enveloping the land, covers it both to the east and to the west and reduces it to a shortened and truncated form of such a figure that its greatest breadth preserved has only a third of its length. as to the actual length and breadth, he says, "it measures seventy thousand stadia in length, and is bounded by a sea whose immensity and solitude renders it impassable; while the breadth is less than thirty thousand stadia, and has for boundaries the double region where the excess of heat on one side and the excess of cold on the other render it uninhabitable." the habitable world was thus much longer from east to west than it was broad from north to south; from whence come our terms _longitude_, whose degrees are counted in the former direction, and _latitude_, reckoned in the latter direction. eratosthenes, and after him hipparchus, while he gives larger numbers than the preceding for the dimensions of the inhabited part of the earth, namely, thirty-eight thousand stadia of breadth and eighty thousand of length, declares that physical laws accord with calculations to prove that the length of the habitable earth must be taken from the rising to the setting of the sun. this length extends from the extremity of india to that of iberia, and the breadth from the parallel of ethiopia to that of ierne. that the earth is an island, strabo considers to be proved by the testimony of our senses. for wherever men have reached to the extremities of the earth they have found the sea, and for regions where this has not been verified it is established by reasoning. those who have retraced their steps have not done so because their passage was barred by any continent, but because their supplies have run short, and they were afraid of the solitude; the water always ran freely in front of them. it is extraordinary that strabo and the astronomers of that age, who recognised so clearly the sphericity of the earth and the real insignificance of mountains, should yet have supposed the stars to have played so humble a part, but so it was; and we find strabo arguing in what we may call quite the contrary direction. he says, "the larger the mass of water that is spread round the earth, so much more easy is it to conceive how the vapours arising from it are sufficient to nourish the heavenly bodies." [illustration: fig. .--the earth of the later greeks.] among the latin cosmographers we may here cite one who flourished in the first century after christ, pomponius mela, who wrote a treatise, called _de situ orbis_. from whatever source, whether traditional or otherwise, he arrived at the conclusion, he divided the earth into two continents, our own and that of the antichthones, which reached to our antipodes. this map was in use till the time of christopher columbus, who modified it in the matter of the position of this second continent, which till then remained a matter of mystery. [illustration: fig. .--pomponius mela's cosmography.] of those who in ancient times added to the knowledge then possessed of cosmography, we should not omit to mention the name of pytheas, of marseilles, who flourished in the fourth century before our era. his chief observations, however, were not so closely related to geography as to the relation of the earth with the heavenly bodies. by the observation of the gnomon at mid-day on the day of the solstice he determined the obliquity of the ecliptic in his epoch. by the observation of the height of the pole, he discovered that in his time it was not marked by any star, but formed a quadrilateral with three neighbouring stars, [greek: b] of the little bear and [greek: k] and [greek: a] of the dragon. chapter x. cosmography and geography of the church. after the writers mentioned in the last chapter a long interval elapsed without any progress being made in the knowledge of the shape or configuration of the earth. from the fall of the roman empire, whose colonies themselves gave a certain knowledge of geography, down to the fifteenth century, when the great impetus was given to discovery by the adventurous voyagers of spain and portugal, there was nothing but servile copying from ancient authors, who were even misrepresented when they were not understood. even the peninsula of india was only known by the accounts of orientals and the writings of the ancients until the beginning of the fifteenth century. vague notions, too, were held as to the limits of africa, and even of europe and asia--while of course they knew nothing of america, in spite of their marking on their maps an antichthonal continent to the south. denys, the traveller, a greek writer of the first century, and priscian, his latin commentator of the fourth, still maintained the old errors with regard to the earth. according to them the earth is not round, but leaf-shaped; its boundaries are not so arranged as to form everywhere a regular circle. macrobius, in his system of the world, proves clearly that he had no notion that africa was continued to the south of ethiopia, that is of the tenth degree of n. latitude. he thought, like cleanthus and crates and other ancient authors, that the regions that lay nearest the tropics, and were burnt by the sun, could not be inhabited; and that the equatorial regions were occupied by the ocean. he divided the hemisphere into five zones, of which only two were habitable. "one of them," he said, "is occupied by us, and the other by men of whose nature we are ignorant." orosus, writing in the same century (fourth), and whose work exercised so great an influence on the cosmographers of the middle ages and on those who made the maps of the world during that long period, was ignorant of the form or boundaries of africa, and of the contours of the peninsulas of southern asia. he made the heavens rest upon the earth. s. basil, also of the fourth century, placed the firmament on the earth, and on this heaven a second, whose upper surface was flat, notwithstanding that the inner surface which is turned towards us is in the form of a vault; and he explains in this way how the waters can be held there. s. cyril shows how useful this reservoir of water is to the life of men and of plants. diodorus, bishop of tarsus, in the same century, also divided the world into two stages, and compared it to a tent. severianus, bishop of gabala, about the same time, compared the world to a house of which the earth is the ground floor, the lower heavens the ceiling, and the upper, or heaven of heavens, the roof. this double heaven was also admitted by eusebius of cæsaræa. in the fifth, sixth, and seventh centuries science made no progress whatever. it was still taught that there were limits to the ocean. thus lactantius asserted that there could not be inhabitants beyond the line of the tropics. this father of the church considered it a monstrous opinion that the earth is round, that the heavens turn about it, and that all parts of the earth are inhabited. "there are some people," he says, "so extravagant as to persuade themselves that there are men who have their heads downwards and their feet upwards; that all that lies down here is hung up there; that the trees and herbs grow downwards; and that the snow and hail fall upwards.... those people who maintain such opinions do so for no other purpose than to amuse themselves by disputation, and to show their spirit; otherwise it would be easy to prove by invincible argument that it is impossible for the heavens to be underneath the earth." (divine institution). saint augustin also, in his _city of god_, says: "there is no reason to believe in that fabulous hypothesis of the antipodes, that is to say, of men who inhabit the other side of the earth--where the sun rises when it sets with us, and who have their feet opposed to ours." ... "but even if it were demonstrated by any argument that the earth and world have a spherical form, it would be too absurd to pretend that any hardy voyagers, after having traversed the immensity of the ocean, had been able to reach that part of the world and there implant a detached branch of the primæval human family." in the same strain wrote s. basil, s. ambrose, s. justin martyr, s. chrysostom, procopius of gaza, severianus, diodorus bishop of tarsus, and the greater number of the thinkers of that epoch. eusebius of cæsaræa was bold enough on one occasion to write in his commentaries on the psalms, that, "according to the opinion of some the earth is round;" but he draws back in another work from so rash an assertion. even in the fifteenth century the monks of salamanca and alcala opposed the old arguments against the antipodes to all the theories of columbus. [illustration: fig. .--the earth's shadow.] in the middle of the sixteenth century gregory of tours adopted also the opinion that the intertropical zone was uninhabitable, and, like other historians, he taught that the nile came from the unknown land in the east, descended to the south, crossed the ocean which separated the antichthone from africa, and then alone became: visible. the geographical and cosmographical ideas that were then prevalent may also be judged of by what s. avitus, a latin poet of the sixth century and nephew of the emperor flavius avitus, says in his poem on the creation, where he describes the terrestrial paradise. "beyond india," he writes, "_where the world commences_, where the confines of heaven and earth are joined, is an exalted asylum, inaccessible to mortals, and closed by eternal barriers, since the first sin was committed." in a treatise on astronomy, published a little after this in , by apian and gemma frison, they very distinctly state their belief in a round earth, though they do not go into details of its surface. the argument is the old one from eclipses, but the figures they give in illustration are very amusing, with three or four men of the size of the moon disporting themselves on the earth's surface. as, however, they all have their feet to the globe representing the earth, and consequently have their feet in opposite directions at the antipodes, the idea is very clearly shown. [illustration: fig. .] "if," they say, "the earth were square, its shadow on the moon would be square also. "if the earth were triangular, its shadow, during an eclipse of the moon, would also be triangular. [illustration: fig. .] "if the earth had six sides, its shadow would have the same figure. [illustration: fig. .] "since, then, the shadow of the earth is round, it is a proof that the earth is round also." this of course is one of the proofs that would be employed in the present day for the same purpose. the most remarkable of all the fantastical systems, however, the _chef d'oeuvre_ of the cosmography of that age, was the famous system of the square earth, with solid walls for supporting the heavens. its author was _cosmas_, surnamed _indicopleustes_ after his voyage to india and ethiopia. he was at first a merchant, and afterwards a monk. he died in . his manuscript was entitled "christian topography," and was written in . it was with the object of refuting the opinions of those who gave a spherical form to the earth that cosmas composed his work after the systems of the church fathers, and in opposition to the cosmography of the gentiles. he reduced to a systematic form the opinions of the fathers, and undertook to explain all the phenomena of the heavens in accordance with the scriptures. in his first book he refutes the opinion of the sphericity of the earth, which he regarded as a heresy. in the second he expounds his own system, and the fifth to the ninth he devotes to the courses of the stars. this mongrel composition is a singular mixture of the doctrines of the indians, chaldeans, greeks, and christian fathers. with respect to his opponents he says, "there are on all sides vigorous attacks against the church," and accuses them of misunderstanding scripture, being misled by the eclipses of the sun and moon. he makes great fun of the idea of rain falling upwards, and yet accuses his opponents of making the earth at the same time the centre and the base of the universe. the zeal with which these pretended refutations are used proves, no doubt, that in the sixth century there were some men, more sensible and better instructed than others, who preserved the deposit of progress accomplished by the grecian genius in the alexandrian school, and defended the labours of hipparchus and ptolemy; while it is manifest that the greater number of their contemporaries kept the old indian and homeric traditions, which were easier to understand, and more accessible to the false witness of the senses, and not improved by combination with texts of scripture misinterpreted. in fact, cosmographical science in the general opinion retrograded instead of advancing. according to cosmas and his map of the world, the habitable earth is a plane surface. but instead of being supposed, as in the time of thales, to be a disc, he represented it in the form of a parallelogram, whose long sides are twice the shorter ones, so that man is on the earth like a bird in a cage. this parallelogram is surrounded by the ocean, which breaks in in four great gulfs, namely, the mediterranean and caspian seas, and the persian and arabian gulfs. beyond the ocean in every direction there exists another continent which cannot be reached by man, but of which one part was once inhabited by him before the deluge. to the east, just as in other maps of the world, and in later systems, he placed the _terrestrial paradise_, and the four rivers that watered eden, which come by subterranean channels to water the post-diluvian earth. after the fall, adam was driven from paradise; but he and his descendants remained on its coasts until the deluge carried the ark of noah to our present earth. on the four outsides of the earth rise four perpendicular walls, which surround it, and join together at the top in a vault, the heavens forming the cupola of this singular edifice. the world, according to cosmas, was therefore a large oblong box, and it was divided into two parts; the first, the abode of men, reaches from the earth to the firmament, above which the stars accomplish their revolutions; there dwell the angels, who cannot go any higher. the second reaches upwards from the firmament to the upper vault, which crowns and terminates the world. on this firmament rest the waters of the heavens. cosmas justifies this system by declaring that, according to the doctrine of the fathers and the commentators on the bible, the earth has the form of the tabernacle that moses erected in the desert; which was like an oblong box, twice as long as broad. but we may find other similarities,--for this land beyond the ocean recalls the atlantic of the ancients, and the mahomedans, and orientals in general, say that the earth is surrounded by a high mountain, which is a similar idea to the walls of cosmas. [illustration: fig. .--the cosmography of cosmas.] "god," he says, "in creating the earth, rested it on nothing. the earth is therefore sustained by the power of god, the creator of all things, supporting all things by the word of his power. if below the earth, or outside of it, anything existed, it would fall of its own accord. so god made the earth the base of the universe, and ordained that it should sustain itself by its own proper gravity." after having made a great square box of the universe, it remained for him to explain the celestial phenomena, such as the succession of days and nights and the vicissitudes of the seasons. [illustration: fig. .--the square earth.] this is the remarkable explanation he gives. he says that the earth, that is, the oblong table circumscribed on all sides by high walls, is divided into three parts; first the habitable earth, which occupies the middle; secondly, the ocean which surrounds this on all sides; and thirdly, another dry land which surrounds the ocean, terminated itself by these high walls on which the firmament rests. according to him the habitable earth is always higher as we go north, so that southern countries are always much lower than northern. for this reason, he says, the tigris and euphrates, which run towards the south, are much more rapid than the nile, which runs northwards. at the extreme north there is a large conical mountain, behind which the sun, moon, planets, and comets all set. these stars never pass below the earth, they only pass behind this great mountain, which hides them for a longer or shorter time from our observation. according as the sun departs from or approaches the north, and consequently is lower or higher in the heavens, he disappears at a point nearer to or further from the base of the mountain, and so is behind it a longer or shorter time, whence the inequality of the days and nights, the vicissitudes of the seasons, eclipses, and other phenomena. this idea is not peculiar to cosmas, for according to the indians, the mountain of someirat is in the centre of the earth, and when the sun appears to set, he is really only hiding behind this mountain. his idea, too, of the manner in which the motions are performed is strange, but may be matched elsewhere. "all the stars are created," he says, "to regulate the days and nights, the months and the years, and they move, not at all by the motion of the heaven itself, but by the action of certain divine beings, or _lampadophores_. god made the angels for his service, and he has charged some of them with the motion of the air, others with that of the sun, or the moon, or the other stars, and others again with the collecting of clouds and preparing the rain." similar to this were the ideas of other doctors of the [illustration: fig. .--explanation of sunrise.] church, such as s. hilary and theodorus, some of whom supposed that the angels carried the stars on their shoulders like the _omophores_ of the manichees; others that they rolled them in front of them or drew them behind; while the jesuit riccioli, who made astronomical observations, remarks that each angel that pushes a star takes great care to observe what the others are doing, so that the relative distances between the stars may always remain what they ought to be. the abbot trithemus gives the exact succession of the seven angels or spirits of the planets, who take it in turns during a cycle of three hundred and fifty-four years to govern the celestial motions from the creation to the year . the system thus introduced seems to have been spread abroad, and to have lingered even into the nineteenth century among the arabs. a guide of that nationality hired at cairo in , remarked to two travellers how the earth had been made square and covered with stones, but the stones had been thrown into the four corners, now called france, italy, england, and russia, while the centre, forming a circle round mount sinai, had been given to the arabians. alongside of this system of the square was another equally curious--that of the egg. its author was the famous venerable bede, one of the most enlightened men of his time, who was educated at the university of armagh, which produced alfred and alcuin. he says: "the earth is an element placed in the middle of the world, as the yolk is in the middle of an egg; around it is the water, like the white surrounding the yolk; outside that is the air, like the membrane of the egg; and round all is the fire which closes it in as the shell does. the earth being thus in the centre receives every weight upon itself, and though by its nature it is cold and dry in its different parts, it acquires accidentally different qualities; for the portion which is exposed to the torrid action of the air is burnt by the sun, and is uninhabitable; its two extremities are too cold to be inhabited, but the portion that lies in the temperate region of the atmosphere is habitable. the ocean, which surrounds it by its waves as far as the horizon, divides it into two parts, the upper of which is inhabited by us, while the lower is inhabited by our antipodes; although not one of them can come to us, nor one of us to them." [illustration: fig. .--the earth as an egg.] this last sentence shows that however far he may have been from the truth, he did not, like so many of his contemporaries, stumble over the idea of up and down in the universe, and so consider the notion of antipodes absurd. [illustration: fig. .--the earth as a floating egg.] a great number of the maps of the world of the period followed this idea, and drew the world in the shape of an egg at rest. it was broached, however, in another form by edrisi, an arabian geographer of the eleventh century, who, with many others, considered the earth to be like an egg with one half plunged into the water. the regularity of the surface is only interrupted by valleys and mountains. he adopted the system of the ancients, who supposed that the torrid zone was uninhabited. according to him the known world only forms a single half of the egg, the greater part of the water belonging to the surrounding ocean, in the midst of which earth floats like an egg in a basin. several artists and map-makers adopted this theory in the geographical representations, and so, whether in this way or the last, the egg has had the privilege of representing the form of the earth for nearly a thousand years. the celebrated raban maur, of mayence, composed in the ninth century a treatise, entitled _de universo_, divided into twenty-two books. it is a kind of encyclopædia, in which he gives an abridged view of all the sciences. according to his cosmographic system the earth is in the form of a wheel, and is placed in the middle of the universe, being surrounded by the ocean; on the north it is bounded by the caucasus, which he supposes to be mountains of gold, which no one can reach because of dragons, and griffins, and men of monstrous shape that dwell there. he also places jerusalem in the centre of the earth. the treatise of honorus, entitled _imago mundi_, and many other authors of the same kind, represent, st, the terrestrial paradise in the most easterly portion of the world, in a locality inaccessible to man; nd, the four rivers which had their sources in paradise; rd, the torrid zone, uninhabited; th, fantastic islands, transformed from the atlantis into _antillia_. [illustration: fig .--eighth century map of the world.] in a manuscript commentary on the apocalypse, which is in the library of turin, is a very curious chart, referred to the tenth, but belonging possibly to the eighth century. it represents the earth as a circular planisphere. the four sides of the earth are each accompanied by a figure of a wind, as a horse on a bellows, from which air is poured out, as well as from a shell in his mouth. above, or to the east, are adam, and eve with the serpent. to their right is asia with two very elevated mountains--cappadocia and caucasus. from thence comes the river _eusis_, and the sea into which it falls forms an arm of the ocean which surrounds the earth. this arm joins the mediterranean, and separates europe from asia. towards the middle is jerusalem, with two curious arms of the sea running past it; while to the south there is a long and straight sea in an east and west direction. the various islands of the mediterranean are put in a square patch, and rome, france, and germany are indicated, while thula, britannia, and scotia are marked as islands in the north-west of the ocean that surrounds the whole world. [illustration: fig. .--tenth century maps.] we figure below two very curious maps of the world of the tenth century--one of which is round, the other square. the first is divided into three triangles; that of the east, or asia, is marked with the name of _shem_; that of the north, or europe, with that of _japhet_; that of the south, or africa, with that of _cham_. the second is also divided between the three sons of noah; the ocean surrounds it, the mediterranean forms the upright portion of a cross of water which divides the adamic world. omons, the author of a geographical poem entitled _the image of the world_, composed in , who was called the lucretius of the thirteenth century, was not more advanced than the cosmographers of the former centuries of which we have hitherto spoken. the cosmographical part of his poem is borrowed from the system of pythagoras and the venerable bede. he maintains that the earth is enveloped in the heavens, as the yoke in the white of an egg, and that it is in the middle as the centre is within the circle, and he speaks like pythagoras of the harmony of the celestial spheres. omons supposed also that in his time the terrestrial paradise was still existing in the east, with its tree of life, its four rivers, and its angel with a flaming sword. he appears to have confounded hecla with the purgatory of st. patrick, and he places the latter in iceland, saying that it never ceases to burn. the volcanoes were only, according to him, the breathing places or mouths of the infernal regions. the latter he placed with other cosmographers in the centre of the earth. another author, nicephorus blemmyde, a monk who lived during the same century, composed three cosmographical works, among them the following: _on the heavens and the earth, on the sun and moon, the stars, and times and days_. according to his system the earth is flat, and he adopts the homeric theory of the ocean surrounding the world, and that of the seven climates. nicolas of oresmus, a celebrated cosmographer of the fourteenth century, although his celebrity as a mathematician attracted the attention of king john of france, who made him tutor to his son charles v., was not wiser than those we have enumerated above. he composed among other works a _treatise on the sphere_. he rejected the theory of an antichthonal continent as contrary to the faith. a map of the world, prepared by him about the year , represents the earth as round, with one hemisphere only inhabited, the other, or lower one, being plunged in the water. he seems to have been led by various borrowed ideas, as, for instance, theological ones, such as the statement in the psalms that god had founded the earth upon the waters, and grecian ones borrowed from the school of thales, and the theories of the arabian geographers. in fact we have seen that edrisi thought that half of the earth was in the water, and aboulfeda thought the same. the earth was placed by nicolas in the centre of the universe, which he represented by painting the sky blue, and dotting it over with stars in gold. leonardo dati, who composed a geographical poem entitled _della spera_, during this century, advanced no further. a coloured planisphere showed the earth in the centre of the universe surrounded by the ocean, then the air, then the circles of the planets after the ptolemaic system, and in another representation of the same kind he figures the infernal regions in the centre of the earth, and gives its diameter as seven thousand miles. he proves himself not to have known one half of the globe by his statement of the shape of the earth--that it is like a t inside an o. this is a comparison given in many maps of the world in the middle ages, the mean parallel being about the th degree of north latitude, that is to say at the straits of gibraltar; the mediterranean is thus placed so as to divide the earth into two equal parts. john beauvau, bishop of angiers under louis xl, expresses his ideas as follows:-- "the earth is situated and rests in the middle of the firmament, as the centre or point is in the middle of a circle. of the whole earth mentioned above only one quarter is inhabited. the earth is divided into four parts, as an apple is divided through the centre by cutting it lengthways and across. if one part of such an apple is taken and peeled, and the peel is spread out over anything flat, such as the palm of the hand, then it resembles the habitable earth, one side of which is called the east, and the other the west." the arabians adopted not only the ideas of the ancients, but also the fundamental notions of the cosmographical system of the greeks. some of them, as _bakouy_, regarded the earth as a flat surface, like a table, others as a ball, of which one half is cut off, others as a complete revolving ball, and others that it was hollow within. others again went as far as to say that there were several suns and moons for the several parts of the earth. in a map, preserved in the library at cambridge, by henry, canon of st. marie of mayence, the form of the world is given after herodotus. the four cardinal points are indicated, and the orientation is that of nearly all the cartographic monuments of the middle ages, namely, the east at the top of the map. the four cardinal points are four angels, one foot placed on the disc of the earth; the colours of their vestments are symbolical. the angel placed at the boreal extremity of the earth, or to the north of the scythians, points with his finger to people enclosed in the ramparts of gog and magog, _gens immunda_ as the legend says. in his left hand he holds a die to indicate, no doubt, that there are shut up the jews who cast lots for the clothes of christ. his vestments are green, his mantle and his wings are red. the angel placed to the left of paradise has a green mantle and wings, and red vestments. in his left hand he holds a kind of palm, and by the right he seems to mark the way to paradise. the position of the other angels placed at the west of the world is different. they seem occupied in stopping the passage beyond the _columns_ (that is, the entrance to the atlantic ocean). all of them have golden aureolas. the surrounding ocean is painted of a clear green. another remarkable map of the world is that of andrea bianco. in it we see eden at the top, which represents the east, and the four rivers are running out of it. much of europe is indicated, including spain, paris, sweden, norway, ireland, which are named, england, iceland, spitzbergen, &c., which are not named. the portion round the north pole to the left is indicated as "cold beneath the pole star." in these maps the systematic theories of the ancient geographers seem mixed with the doctrines of the fathers of the church. they place generally in the red sea some mark denoting the passage of the hebrews, the terrestrial paradise at the extreme east, and jerusalem in the centre. the towns are figured often by edifices, as in the list of theodosius, but without any regard to their respective positions. each town is ordinarily represented by two towers, but the principal ones are distinguished by a little wall that appears between these two towers, on which are painted several windows, or else they may be known by the size of the edifices. st. james of compostella in gallicia and rome are represented by edifices of considerable size, as are nazareth, troy, antioch, damascus, babylon, and nineveh. [illustration: fig. .--the map of andrea bianco.] one of the most remarkable monuments of the geography of the last centuries of the middle ages is the map in hereford cathedral, by richard of haldingham, not only on account of its numerous legends, but because of its large dimensions, being several square yards in area. on the upper part of this map is represented the last judgment; jesus christ, with raised arms, holding in his hands a scroll with these words, _ecce testimonium meum_. at his side two angels carry in their hands the instruments of his passion. on the right hand stands an angel with a trumpet to his mouth, out of which come these words, _levez si vendres vous par_. an angel brings forward a bishop by the hand, behind whom is a king, followed by other personages; the angel introduces them by a door formed of two columns, which seems to serve as an entrance to an edifice. the virgin is kneeling at the feet, of her son. behind her is another woman kneeling, who holds a crown, which she seems ready to place on the head of the mother of christ, and by the side of the woman is a kneeling angel, who appears to be supporting the maternal intercessor. the virgin uncovers her breast and pronounces the words of a scroll which is held by an angel kneeling in front of her, _vei i b' fiz mon piz de deuiz lauele chare preistes--eles mame lettes dont leit de virgin qui estes--syes merci de tous si com nos mesmes deistes.--r ... em ... ont servi kaut sauveresse me feistes_. to the left another angel, also with a trumpet to his mouth, gives out the following words, which are written on a scroll, _leves si alles all fu de enfer estable_. a gate, drawn like that of the entrance, represents probably the passage by which those must go out who are condemned to eternal pains. in fact the devil is seen dragging after him a crowd of men, who are tied by a cord which he holds in his hand. [illustration: fig. .--from the map in hereford cathedral.] the map itself commences at its upper part, that is, the east, by the terrestrial paradise. it is a circle, in the centre of which is represented the tree of the knowledge of good and evil. adam and eve are there in company with the serpent that beguiled them. the four legendary rivers come out of the base of the tree, and they are seen below crossing the map. outside eden the flight of the first couple, and the angel that drove them away, are represented. at this extreme eastern portion is the region of giants with the heads of beasts. there, too, is seen the first human habitation, or town, built by enoch. below appears the tower of babel. near this are two men seated on a hill close to the river jaxartes; one of them is eating a human leg and the other an arm, which the legend explains thus:--"here live the essedons, whose custom it is to sing at the funerals of their parents; they tear the corpses with their teeth, and prepare their food with these fragments of flesh, mixed with that of animals. in their opinion it is more honourable to the dead to be enclosed in the bodies of their relations than in those of worms." [illustration: fig. .--from the map in hereford cathedral. tower of babel. essedons. dragons. pigmies. the monoceros. the mantichore. a sphinx. the king of the cyclops. blemmye. parasol lip. monocle.] below are seen dragons and pigmies, always to the east of asia, and a little further away in the midst of a strange country, _the king of the cyclops_. this extraordinary geography shows us in india the "mantichore, who has a triple range of teeth, the face of a man, blue eyes, the red colour of blood, the body of a lion, and the tail of a scorpion; its voice is a whistle." on the north of the ganges is represented a man with one leg, shading his head with his foot, which is explained by the following legend:--"in india dwell the monocles, who have only one leg, but who nevertheless move with surprising velocity; when they wish to protect themselves from the heat of the sun they make a shadow with the sole of their foot, which is very large." the blemmys have their mouth and eyes in their chest; others have their mouths and eyes on their shoulders. the parvini are ethiopians that have four eyes. to the east of syene is a man seated who is covering his head with his lip, "people who with their prominent lip shade their faces from the sun." above is drawn a little sun, with the word _sol_. then comes an animal of human form, having the feet of a horse and the head and beak of a bird; he rests on a stick, and the legend tells us it is a satyr; the fauns, half men and half horses; the cynocephales--men with the head of a dog; and the cyanthropes--dogs with the heads of men. the sphinx has the wings of a bird, the tail of a serpent, the head of a woman. it is placed in the midst of the cordilleras, which are joined to a great chain of mountains. here lastly is seen the _monoceros_, a terrible animal; but here is the marvel: "when one shows to this _monoceros_ a young girl, who, when the animal approaches, uncovers her breast, the monster, forgetting his ferocity, lays his head there, and when he is asleep may be taken defenceless." near to the lake meotides is a man clothed in oriental style, with a hat that terminates in a point, and holding by the bridle a horse whose harness is a human skin, and which is explained thus by the latin legend: "here live the griffins, very wicked men, for among other crimes they proceed so far as to make clothes for themselves and their horses out of the skins of their enemies." more to the south is a large bird, the ostrich; according to the legend, "the ostrich has the head of a goose, the body of a crane, the feet of a calf; it eats iron." not far from the riphean mountains two men with long tunics and round bonnets are represented in the attitude of fighting; one brandishes a sword, the other a kind of club, and the legend tells us, "the customs of the people of the interior of scythia are somewhat wild; they inhabit caves; they drink the blood of the slain by sucking their wounds; they pride themselves on the number of people they have slain--not to have slain any one in combat is reckoned disgraceful." near the river that empties itself into the caspian sea it is written: "this river comes from the infernal regions; it enters the sea after having descended from mountains covered with wood, and it is there, they say, that the mouth of hell opens." to the south of this river, and to the north of hyrcania, is represented a monster having the body of a man, the head, tail, and feet of a bull: this is the minotaur. further on are the mountains of armenia, and the ark of noah on one of its plateaux. here, too, is seen a large tiger, and we read: "the tiger, when he sees that he has been deprived of his young, pursues the ravisher precipitately; but the latter, hastening away on his swift horse, throws a mirror to him and is safe." elsewhere appears lot's wife changed into a pillar of salt; the lynx who can see through a stone wall; the river lethe; so called because all who drink of it forget everything. numerous other details might be mentioned, but enough has been said to show the curious nature and exceeding interest of this map, in which matters of observation and imagination are strangely mixed. another very curious geographical document of that epoch is the map of the world of the _grandes chroniques de saint-denis_. this belongs to the fourteenth century. the capitals here too are represented by edifices. the mediterranean is a vertical canal, which goes from the columns of hercules to jerusalem. the caspian sea communicates with it to the north, and the red sea to the south-east, by the nile. it preserves the same position for paradise and for the land of gog and magog that we have seen before. the geography of europe is very defective. britannia and anglia figure as two separate islands, being represented off the west coast of spain, with allemania and germania, also two distinct countries, to the north. the ocean is represented as round the whole, and the various points of the compass are represented by different kinds of winds on the outside. [illustration: fig. .--cosmography of st. denis.] this was the general style of the maps of the world at that period, as we may perceive from the various illustrations we have been able to give, and it curiously initiates us into the mediæval ideas. sometimes they are surrounded by laughable figures of the winds with inflated cheeks, sometimes there are drawn light children of eolus seated on leathern bottles, rotating the liquid within; at other times, saints, angels, adam and eve, or other people, adorn the circumference of the map. within are shown a profusion of animals, trees, populations, monuments, tents, draperies, and monarchs seated on their thrones--an idea which was useful, no doubt, and which gave the reader some knowledge of the local riches, the ethnography, the local forms of government and of architecture in the various countries represented; but the drawings were for the most part childish, and more fantastic than real. the language, too, in which they were written was as mixed as the drawings; no regularity was preserved in the orthography of a name, which on the same map may be written in ten different ways, being expressed in barbarous latin, roman, or old french, catalan, italian, castilian, or portuguese! during the same epoch other forms of maps in less detail and of smaller size show the characters that we have seen in the maps of earlier centuries. marco polo, the traveller, at the end of the fourteenth century, has preserved in his writings all the ancient traditions, and united them in a singular manner with the results of his own observations. he had not seen paradise, but he had seen the ark of noah resting on the top of ararat. his map of the world, preserved in the library at stockholm, is oval, and represents two continents. in that which we inhabit, the only seas indicated are the mediterranean and the black sea. asia appears at the east, europe to the north, and africa to the south. the other continent to the south of the equator, which is not marked, is antichthonia. in a map of the world engraved on a medal of the fifteenth century during the reign of charles v. there is still a reminiscence of the ideas of the concealed earth and meropides, as described by theopompus. we see the winds as cherubim; europe more accurately represented than usual; but africa still unknown, and a second continent, called brumæ, instead of antichthonia, with imaginary details upon it. [illustration: fig. --the map of marco polo.] if such were the ideas entertained amongst the most enlightened nations, what may we expect among those who were less advanced? it would take us too long to describe all that more eastern nations have done upon this point since the commencement of our present era, but we may give an example or two from the arabians. [illustration: fig. .--map on a medal of charles v.] in the ancient arabian chronicle of tabari is a system founded on the earth being the solid foundation of all things; we read: "the prophet says, the all-powerful and inimitable deity has created the mountain of kaf round about the earth; it has been called the foundation pile of the earth, as it is said in the koran, 'the mountains are the piles.' this world is in the midst of the mountain of kaf, just as the finger is in the midst of the ring. this mountain is emerald, and blue in colour; no man can go to it, because he would have to pass four months in darkness to do so. there is in that mountain neither sun, nor moon, nor stars; it is so blue that the azure colour you see in the heavens comes from the brilliancy of the mountain of kaf, which is reflected in the sky. if this were not so the sky would not be blue. all the mountains that you see are supported by kaf; if it did not exist, all the earth would be in a continual tremble, and not a creature could live upon its surface. the heavens rest upon it like a tent." another arabian author, benakaty, writing in , says: "know that the earth has the form of a globe suspended in the centre of the heavens. it is divided by the two great circles of the meridian and equator, which cut each other at light angles, into four equal parts, namely, those of the north-west, north-east, south-west, and south-east. the inhabited portion of the earth is situated in the southern hemisphere, of which one half is inhabited." ibn-wardy, who lived in the same century, adopted the idea of the ocean surrounding all the earth, and said we knew neither its depth nor its extent. this ocean was also acknowledged by the author of the kaf mountain; he says it lies between the earth and that mountain, and calls it bahr-al-mohith. the end of the fifteenth century saw the dawn of a new era in knowledge and science. the discoveries of columbus changed entirely the aspect of matters, the imagination was excited to fresh enterprises, and the hardihood of the adventurers through good or bad success was such as want of liberty could not destroy. nevertheless, as we have seen, columbus imagined the earth to have the shape of a pear. not that he obtained this idea from his own observations, but rather retained it as a relic of past traditions. it is probable that it really dates from the seventh century. we may read in several cosmographical manuscripts of that epoch, that the earth has the form of a cone or a top, its surface rising from south to north. these ideas were considerably spread by the compilations of john of beauvais in , from whom probably columbus derived his notion. although columbus is generally and rightly known as the discoverer of the new world, a very curious suit was brought by pinzon against his heirs in . pinzon pretended that the discovery was due to him alone, as columbus had only followed his advice in making it. pinzon told the admiral himself that the required route was intimated by an inspiration, or revelation. the truth was that this "revelation" was due to a flock of parrots, flying in the evening towards the south-west, which pinzon concluded must be going in the direction of an invisible coast to pass the night in the bushes. certainly the consequences of columbus resisting the advice of pinzon would have been most remarkable; for had he continued to sail due west he would have been caught by the gulf stream and carried to florida, or possibly to virginia, and in this case the united states would have received a spanish and catholic population, instead of an english and protestant one. the discoveries of those days were often commemorated by the formation of heraldic devices for the authors of them, and we have in this way some curious coats of arms on record. that, for instance, of sebastian cano was a globe, with the legend, _primus circumdedisti me_. the arms given to columbus in consisted of the first map of america, with a range of islands in a gulf. charles v. gave to diego of ordaz the figure of the peak of orizaba as his arms, to commemorate his having ascended it; and to the historian oviedo, who passed thirty-four years without interruption ( - ) in tropical america, the four beautiful stars of the southern cross. we have arrived at the close of our history of the attempts that preceded the actual discovery of the form and constitution of the globe; since these were established our further progress has been in matters of detail. there now remains briefly to notice the attempts at discovering the size of the earth on the supposition, and afterwards certainty, of its being a globe. the earliest attempt at this was made by eratosthenes, years before our era, and it was founded on the following reasoning. the sun illuminates the bottom of pits at syene at the summer solstice; on the same day, instead of being vertical over the heads of the inhabitants of alexandria, it is - / degrees from the zenith. seven-and-a-quarter degrees is the fiftieth part of an entire circumference; and the distance between the two towns is five thousand stadia; hence the circumference of the earth is fifty times this distance, or thousand stadia. a century before our era posidonius arrived at an analogous result by remarking that the star canopus touched the horizon at rhodes when it was degrees minutes above that of alexandria. these measurements, which, though rough, were ingenious, were, followed in the eighth century by similar ones by the arabian caliph, almamoun, who did not greatly modify them. the first men who actually went round the world were the crew of the ship under magellan, who started to the west in ; he was slain by the philippine islanders in , but his ship, under his lieutenant, sebastian cano, returned by the east in . the first attempt at the actual measurement of a part of the earth's surface along the meridian was made by fernel in . his process was a singular, but simple one, namely, by counting the number of the turns made by the wheels of his carriage between paris and amiens. he made the number , , and accurate measurements of the distance many years after showed he had not made an error of more than four turns. the astronomer picard attempted it again under louis xiv. by triangulation. the french astronomers have always been forward in this inquiry, and to them we owe the systematic attempts to arrive at a truer knowledge of the length of an arc of the meridian which were made in - in lapland and in peru; and later under mechain and delambre, by order of the national assembly, for the basis of the metrical system. observations of this kind have also been made by the english, as at lough foyle in ireland, and in india. the review which has here been made of the various ideas on what now seems so simple a matter cannot but impress us with the vast contrast there is between the wild attempts of the earlier philosophers and our modern affirmations. what progress has been made in the last two thousand years! and all of this is due to hard work. the true revelation of nature is that which we form ourselves, by our persevering efforts. we now know that the earth is approximately spherical, but flattened by about / at the poles, is three-quarters covered with water, and enveloped everywhere by a light atmospheric mantle. the distance from the centre of the earth to its surface is , miles, its area is million square miles, its volume is , millions of cubic miles, its weight is six thousand trillion tons. so, thanks to the bold measurements of its inhabitants, we know as much about it as we are likely to know for a long time to come. chapter xi. legendary worlds of the middle ages. the legends that were for so many ages prevalent in europe had their foundation in the attempt to make the accounts of scripture and the ideas and dogmas of the fathers of the church fit into the few and insignificant facts that were known with respect to the earth, and the system of which it forms a part, and the far more numerous imaginations that were entertained about it. we are therefore led on to examine some of these legends, that we may appreciate how far a knowledge of astronomy will effect the eradication of errors and fantasies which, under the aspect of truth, have so long enslaved the people. no doubt the authors of the legendary stories knew well enough their allegorical nature; but those who received them supposed that they gave true indications of the nature of the earth and world, and therefore accepted them as facts. some indeed considered that the whole physical constitution of the world was a scaffold or a model, and that there was a real theological universe hidden beneath this semblance. no one omitted from his system the spiritual heaven in which the angels and just men might spend their existence; but in addition to this there were places whose reality was believed in, but whose locality is more difficult to settle, and which therefore were moved from one place to another by various writers, viz., the infernal regions, purgatory, and the terrestrial paradise. we will here recount some of those legends, which wielded sufficient sway over men's minds as to gain their belief in the veritable existence of the places described, and in this way to influence their astronomical and cosmographical ideas. and for the first we will descend to the infernal regions with plutarch and thespesius. this thespesius relates his adventures in the other world. having fallen head-first from an elevated place, he found himself unwounded, but was contused in such a way as to be insensible. he was supposed to be dead, but, after three days, as they were about to bury him, he came to life again. in a few days he recovered his former powers of mind and body; but made a marvellous change for the better in his life. he said that at the moment that he lost consciousness he found himself like a sailor at the bottom of the sea; but afterwards, having recovered himself a little, he was able to breathe perfectly, and seeing only with the eyes of his soul, he looked round on all that was about him. he saw no longer the accustomed sights, but stars of prodigious magnitude, separated from each other by immense distances. they were of dazzling brightness and splendid colour. his soul, carried like a vessel on the luminous ocean, sailed along freely and smoothly, and moved everywhere with rapidity. passing over in silence a large number of the sights that met his eye, he stated that the souls of the dead, taking the form of bubbles of fire, rise through the air, which opens a passage above them; at last the bubbles, breaking without noise, let out the souls in a human form and of a smaller size, and moving in different ways. some, rising with astonishing lightness, mounted in a straight line; others, running round like a whipping-top, went up and down by turns with a confused and irregular motion, making small advance by long and painful efforts. among this number he saw one of his parents, whom he recognised with difficulty, as she had died in his infancy; but she approached him, and said, "good day, thespesius." surprised to hear himself called by this name, he told her that he was called arideus, and not thespesius. "that was once your name," she replied, "but in future you will bear that of thespesius, for you are not dead, only the intelligent part of your soul has come here by the particular will of the gods; your other faculties are still united to your body, which keeps them like an anchor. the proof i will give you is that the souls of the dead do not cast any shadow, and they cannot move their eyes." further on, in traversing a luminous region, he heard, as he was passing, the shrill voice of a female speaking in verse, who presided over the time thespesius should die. his genius told him that it was the voice of the sibyl, who, turning on the orbit of the moon, foretold the future. thespesius would willingly have heard more, but, driven off by a rapid whirlwind, he could make out but little of her predictions. in another place he remarked several parallel lakes, one filled with melted and boiling gold, another with lead colder than ice, and a third with very rough iron. they were kept by genii, who, armed with tongs like those used in forges, plunged into these lakes, and then withdrew by turns, the souls of those whom avarice or an insatiable cupidity had led into crime; after they had been plunged into the lake of gold, where the fire made them red and transparent, they were thrown into the lake of lead. then, frozen by the cold, and made as hard as hail, they were put into the lake of iron, where they became horribly black. broken and bruised on account of their hardness, they changed their form, and passed once more into the lake of gold, and suffered in these changes inexpressible pain. in another place he saw the souls of those who had to return to life and be violently forced to take the form of all sorts of animals. among the number he saw the soul of nero, which had already suffered many torments, and was bound with red-hot chains of iron. the workmen were seizing him to give him the form of a viper, under which he was destined to live, after having devoured the womb that bore him. the locality of these infernal regions was never exactly determined. the ancients were divided upon the point. in the poems of homer the infernal regions appear under two different forms: thus, in the _iliad_, it is a vast subterranean cavity; while in the _odyssey_, it is a distant and mysterious country at the extremity of the earth, beyond the ocean, in the neighbourhood of the cimmerians. the description which homer gives of the infernal region proves that in his time the greeks imagined it to be a copy of the terrestrial world, but one which had a special character. according to the philosophers it was equally remote from all parts of the earth. thus cicero, in order to show that it was of no consequence where one died, said, wherever we die there is just as long a journey to be made to reach the "infernal regions." the poets fixed upon certain localities as the entrance to this dismal empire: such was the river lethe, on the borders of the scythians; the cavern acherusia in epirus, the mouth of pluto, in laodicoea, the cave of zenarus near lacedæmon. in the map of the world in the _polychronicon_ of ranulphus uygden, now in the british museum, it is stated: "the island of sicily was once a part of italy. there is mount etna, containing the infernal regions and purgatory, and it has scylla and charybdis, two whirlpools." ulysses was said to reach the place of the dead by crossing the ocean to the cimmerian land, Æneas to have entered it by the lake of avernus. xenophon says that hercules went there by the peninsula of arechusiade. much of this, no doubt, depends on the exaggeration and misinterpretation of the accounts of voyagers; as when the phoenicians related that, after passing the columns of hercules, to seek tin in thule and amber in the baltic, they came, at the extremity of the world, to the fortunate isles, the abode of eternal spring, and further on to the hyperborean regions, where a perpetual night enveloped the country--the imagination of the people developed from this the elysian fields, as the places of delight in the lower regions, having their own sun, moon, and stars, and tartarus, a place of shades and desolation. in every case, however, both among pagans and christians, the locality was somewhere in the centre of the earth. the poets and philosophers of greece and rome made very detailed and circumstantial maps of the subterranean regions. they enumerated its rivers, its lakes, and woods, and mountains, and the places where the furies perpetually tormented the wicked souls who were condemned to eternal punishment. these ideas passed naturally into the creeds of christians through the sect of the essenes, of whom josephus writes as follows:--"they thought that the souls of the just go beyond the ocean to a place of repose and delight, where they were troubled by no inconvenience, no change of seasons. those of the wicked, on the contrary, were relegated to places exposed to all the inclemencies of the weather, and suffered eternal torments. the essenes," adds the same author, "have similar ideas about these torments to those of the greeks about tartarus and the kingdom of pluto. the greater part of the gnostic sects, on the contrary, considered the lower regions as simply a place of purgatory, where the soul is purified by fire." amongst all the writings of christian ages in which matters such as we are now passing in review are described, there is one that stands out beyond all others as a masterpiece, and that is the magnificent poem of dante, his _divine comedy_, wherein he described the infernal regions as they presented themselves to his lively and fertile imagination. we have in it a picture of mediæval ideas, painted for us in indelible lines, before the remembrance of them was lost in the past. the poem is at once a tomb and a cradle--the tomb of a world that was passing, the cradle of the world that was to come: a portico between two temples, that of the past and that of the future. in it are deposited the traditions, the ideas, the sciences of the past, as the egyptians deposited their kings and symbolic gods in the sepulchres of thebes and memphis. the future brings into it its aspirations and its germs enveloped in the swaddling clothes of a rising language and a splendid poetry--a mysterious infant that is nourished by the two teats of sacred tradition and profane fiction, moses and st. paul, homer and virgil. the theology of dante, strictly orthodox, was that of st. thomas and the other doctors of the church. natural philosophy, properly so called, was not yet in existence. in astronomy, ptolemy reigned supreme, and in the explanation of celestial phenomena no one dreamt or dared to dream of departing in any way from the traditionally sacred system. in those days astronomy was indissolubly linked with a complete series of philosophical and theological ideas, and included the physics of the world, the science of life in every being, of their organisation, and the causes on which depended the aptitudes, inclinations, and even in part the actions, of men, the destinies of individuals, and the events of history. in this theological, astronomical, and terrestrial universe everything emanated from god; he had created everything, and the creation embraced two orders of beings, the immaterial and the corporeal. the pure spirits composed the nine choirs of the celestial hierarchy. like so many circles, they were ranged round a fixed point, the eternal being, in an order determined by their relative perfection. first the seraphim, then the cherubim, and afterwards the simple angels. those of the first circle received immediately from the central point the light and the virtue which they communicated to those of the second; and so on from circle to circle, like mirrors which reflect, with an ever-lessening light, the brilliancy of a single luminous point. the nine choirs, supported by love, turned without ceasing round their centre in larger and larger circles according to their distance; and it was by their means that the motion and the divine inflatus was communicated to the material creation. this latter had in the upper part of it the empyreal, or heaven of pure light. below that, was the _primum mobile_, the greatest body in the heavens, as dante calls it, because it surrounds all the rest of the circle, and bounds the material world. then came the heaven of the fixed stars; then, continuing to descend, the heavens of saturn, jupiter, mars, the sun, venus, mercury, the moon, and lastly, the earth, whose solid and compact nucleus is surrounded by the spheres of water, air, and fire. as the choirs of angels turn about a fixed point, so the nine material circles turn also about another fixed point, and are moved by the pure spirits. let us now descend to the geography of the interior of the earth. within the earth is a large cone, whose layers are the frightful abodes of the condemned, and which ends in the centre, where the divine justice keeps bound up to his chest in ice the prince of the rebellious angels, the emperor of the kingdom of woe. such are the infernal regions which dante describes according to ideas generally admitted in the middle ages. the form of the infernal regions was that of a funnel or reversed cone. all its circles were concentric, and continually diminished; the principal ones were nine in number. virgil also admitted nine divisions--three times three, a number sacred _par excellence_. the seventh, eighth, and ninth circles were divided into several regions; and the space between the entrance to the infernal regions and the river acheron, where the resting-place of the damned really commenced, was divided into two parts. dante, guided by virgil, traversed all these circles. it was in that the poet, "in the midst of the course of life," at the age of thirty-five, passed in spirit through the three regions of the dead. lost in a lonely, wild, and dismal forest, he reached the base of a hill, which he attempted to climb. but three animals, a panther, a lion, and a thin and famished wolf, prevented his passage; so, returning again where the sun was powerless, into the shades of the depths of the valley, there met him a shadow of the dead. this human form, whom a long silence had deprived of speech, was virgil, who was sent to guide and succour him by a celestial dame, beatrice, the object of his love, who was at the same time a real and a mystically ideal being. virgil and dante arrived at the gate of the infernal regions; they read the terrible inscription placed over the gate; they entered and found first those unhappy souls who had lived without virtue and without vice. they reached the banks of acheron and saw charon, who carried over the souls in his bark to the other side; and dante was surprised by a profound sleep. he woke beyond the river, and he descended into the limbo which is the first circle of the infernal regions. he found there the souls of those who had died without baptism, or who had been indifferent to religion. they descended next to the second circle, where minos, the judge of those below, is enthroned. here the luxurious are punished. the poet here met with francesca of rimini and paul, her friend. he completely recovered the use of his senses, and passed through the third circle, where the gourmands are punished. in the fourth he found plutus, who guards it. here are tormented the prodigal and the avaricious. in the fifth are punished those who yield to anger. dante and virgil there saw a bark approaching, conducted by phlegias; they entered it, crossed a river, and arrived thus at the base of the red-hot iron walls of the infernal town of dite. the demons that guarded the gates refused them admittance, but an angel opened them, and the two travellers there saw the heretics that were enclosed in tombs surrounded by flames. the travellers then visited the circles of violence, fraud, and usury, when they came to a river of blood guarded by a troop of centaurs; suddenly they saw coming to them geryon, who represents fraud, and this beast took them behind him to carry them across the rest of the infernal space. [illustration: fig. .--dante's infernal regions.] the eighth circle was divided into ten valleys, comprising: the flatterers; the simoniacal; the astrologers; the sorcerers; the false judges; the hypocrites who walked about clothed with heavy leaden garments; the thieves, eternally stung by venomous serpents; the heresiarchs; the charlatans, and the forgers. at last the poets descended into the ninth circle, divided into four regions, where are punished four kinds of traitors. here is recounted the admirable episode of count ugolin. in the last region, called the region of judas, lucifer is enchained. there is the centre of the earth, and dante, hearing the noise of a little brook, reascended to the other hemisphere, on the surface of which he found, surrounded by the southern ocean, the mountain of purgatory. such was the famous _inferno_ of dante. not only was the geography of the infernal regions attempted in the middle ages, but even their size. dexelius calculated that the number of the damned was a hundred millions, and that their abode need not measure more than one german mile in every direction. cyrano of bergerac amusingly said that it was the damned that kept turning the earth, by hanging on the ceiling like bats, and trying to get away. in an english clergyman, dr. swinden, published a book entitled, _researches on the nature of the fire of hell and the place where it is situated_. he places it in the sun. according to him the christians of the first century had placed it beneath the earth on account of a false interpretation of the descent of jesus into hell after his crucifixion, and by false ideas of cosmography. he attempted to show, st, that the terrestrial globe is too small to contain even the angels that fell from heaven after their battle; nd, that the fire of hell is real, and that the closed globe of earth could not support it a sufficiently long period; rd, that the sun alone presents itself as the necessary place, being a well-sustained fire, and directly opposite in situation to heaven, since the empyreal is round the outside of the universe, and the sun in the centre. what a change to the present ideas, even of doctors of divinity, in a hundred years! so far, then, for mediæval ideas on the position and character of hell. next as to purgatory. the voyage to purgatory that has met with most success is certainly the celebrated irish legend of st. patrick, which for several centuries was admitted as authentic, and the account of which was composed certainly a century before the poem of dante. this purgatory, the entrance to which is drawn in more than one illuminated manuscript, is situated in ireland, on one of the islands of lough derg, county donegal, where there are still two chapels and a shrine, at which annual ceremonies are performed. a knight, called owen, resolved to visit it for penance; and the chronicle gives us an account of his adventures. first he had his obsequial rites performed, as if he had been dead, and then he advanced boldly into the deep ravine; he marched on courageously, and entered into the semi-shadows; he marched on, and even this funereal twilight abandoned him, and "when he had gone for a long time in this obscurity, there appeared to him a little light as it were from a glimmer of day." he arrived at a house, built with much care, an imposing mansion of grief and hope, a marvellous edifice, but similar nevertheless to a monkish cloister, where there was no more light than there is in this world in winter at vesper-time. the knight was in dreadful suspense. suddenly he heard a terrible noise, as if the universe was in a riot; for it seemed certainly to him as if every kind of beast and every man in the world were together, and each gave utterance to their own cry, at one time and with one voice, so that they could not make a more frightful noise. then commenced his trials, and discourse with the infernal beings; the demons yelled with delight or with fury round him. "miserable wretch," said some, "you are come here to suffer." "fly," said others, "for you have not behaved well in the time that is passed: if you will take our advice, and will go back again to the world, we will take it as a great favour and courtesy." [illustration: plate xii.--the legend of owen.] owen was thrown on the dark shadowy earth, where the demons creep like hideous serpents. a mysterious wind, which he scarcely heard, passed over the mud, and it seemed to the knight as if he had been pierced by a spear-head. after a while the demons lifted him up; they took him straight off to the east, where the sun rises, as if they were going to the place where the universe ends. "now, after they had journeyed for a long time here and there over divers countries, they brought him to an open field, very long and very full of griefs and chastisements; he could not see the end of the field, it was so long; there were men and women of various ages, who lay down all naked on the ground with their bellies downwards, who had hot nails driven into their hands and feet; and there was a fiery dragon, who sat upon them and drove his teeth into their flesh, and seemed as if he would eat them; hence they suffered great agony, and bit the earth in spite of its hardness, and from time to time they cried most piteously 'mercy, mercy;' but there was no one there who had pity or mercy, for the devils ran among them and over them, and beat them most cruelly." the devils brought the knight towards a house of punishment, so broad and long that one could not see the end. this house is the house of baths, like those of the infernal regions, and the souls that are bathed in ignominy are there heaped in large vats. "now so it was, that each of these vats was filled with some kind of metal, hot and boiling, and there they plunged and bathed many people of various ages, some of whom were plunged in over their heads, others up to the eyebrows, others up to the eyes, and others up to the mouth. now all in truth of these people cried out with a loud voice and wept most piteously." scarcely had the knight passed this terrible place, and left behind in his mysterious voyage that column of fire which rose like a lighthouse in the shades, and which shone so sadly betwixt hope and eternal despair, than a vast and magnificent spectacle displayed itself in the subterranean space. this luminous and odorescent region, where one might see so many archbishops, bishops, and monks of every order, was the terrestrial paradise; man does not stay there always; they told the knight that he could not taste too long its rapid delights; it is a place of transition between purgatory and the abodes of heaven, just as the dark places which he had traversed were made by the creator between the world and the infernal regions. "in spite of our joys," said the souls, "we shall pass away from here." then they took him to a mountain, and told him to look, and asked of him what colour the heavens seemed to be there where he was standing, and he replied it was the colour of burning gold, such as is in the furnace; and then they said to him, "that which you see is the entrance to heaven and the gate of paradise." the attempts at identification of hell and purgatory have not been so numerous, perhaps because the subjects were not very attractive, except as the spite of men might think of them in reference to other people; but when we come to the terrestrial paradise, quite a crowd of attempts by every kind of writer to fix its position in any and every part of the globe is met with on every side. in the seventeenth century, under louis xiv., daniel huet, bishop of avranches, gave great attention to the question, and collected every opinion that had been expressed upon it, with a view to arriving at some definite conclusion for himself. he was astonished at the number of writings and the diversity of the opinions they expressed. "nothing," he says, "could show me better how little is really known about the situation of the terrestrial paradise than the differences in the opinions of those who have occupied themselves about the question. some have placed it in the third heaven, some in the fourth, in the heaven of the moon, in the moon itself, on a mountain near the lunar heaven, in the middle region of the air, out of the earth, upon the earth, beneath the earth, in a place that is hidden and separated from man. it has been placed under the north pole, in tartary, or in the place now occupied by the caspian sea. others placed it in the extreme south, in the land of fire. others in the levant, or on the borders of the ganges, or in the island of ceylon, making the name india to be derived from eden, the land where the paradise was situated. it has been placed in china, or in an inaccessible place beyond the black sea; by others in america, in africa, beneath the equator, in the east, &c. &c." notwithstanding this formidable array, the good bishop was bold enough to make his choice between them all. his opinion was that the dwelling-place of the first man was situated between the tigris and euphrates, above the place where they separate before falling into the persian gulf; and, founding this opinion on very extensive reading, he declared that of all his predecessors, calvin had come nearest to the truth. among the other authors of greater or less celebrity that have occupied themselves in this question, we may instance the following:-- raban maur (ninth century) believed that the terrestrial paradise was at the eastern extremity of the earth. he described the tree of life, and added that there was neither heat nor cold in that garden; that immense rivers of water nourished all the forest; and that the paradise was surrounded by a wall of fire, and its four rivers watered the earth. james of vitry supposed pison to come out of the terrestrial paradise. he describes also the garden of eden; and, like all the cosmographers of the middle ages, he placed it in the most easterly portion of the world in an inaccessible place, and surrounded by a wall of fire, which rose up to heaven. dati placed also the terrestrial paradise in asia, like the cosmographers that preceded him, and made the nile come from the east. stenchus, the librarian of st. siége, who lived in the sixteenth century, devoted several years to the problem, but discovered nothing. the celebrated orientalist and missionary bochart wrote a treatise on this subject in . thévenot published also in the seventeenth century a map representing the country of the lybians, and adds that "several great doctors place the terrestrial paradise there." an armenian writer who translated and borrowed from st. epiphanius (eighth century) produced a _memorial on the four rivers of the terrestrial paradise_. he supposes they rise in the unknown land of the amazons, whence also arise the danube and the hellespont, and they deliver their waters into that great sea that is the source of all seas, and which surrounds the four quarters of the globe. he afterwards says, following up the same theory, that the rivers of paradise surround the world and enter again into the sea, which is the universal ocean." gervais and robert of st. marien d'auxerre taught that the terrestrial paradise was on the eastern border of the _square_ which formed the world. alain de lille, who lived in the thirteenth century, maintained in his _anticlaudianus_ that the earth is circular, and the garden of eden is in the east of asia. joinville, the friend of st. louis, gives us a curious notion of his geographical ideas, since, with regard to paradise, he assures us that the four great rivers of the south come out of it, as do the spices. "here," he says, referring to the nile, "it is advisable to speak of the river which passes by the countries of egypt, and comes from the terrestrial paradise. where this river enters egypt there are people very expert and experienced, as thieves are here, at stealing from the river, who in the evening throw their nets on the streams and rivers, and in the morning they often find and carry off the spices which are sold here in europe as coming from egypt at a good rate, and by weight, such as cinnamon, ginger, rhubarb, cloves, lignum, aloes, and several other good things, and they say that these good things come _from the terrestrial paradise_, and that the wind blows them off the trees that are growing there." and he says that near the end of the world are the peoples of gog and magog, who will come at the end of the world with antichrist. we find, however, more than descriptions--we have representations of the terrestrial paradise by cartographers of the middle ages, some of which we have seen in speaking of their general ideas of geography, and we will now introduce others. [illustration: fig. .--paradise of fra mauro.] fra mauro, a religious cosmographer of the fifteenth century, gives on the east side of a map of the world a representation which shows us that at that epoch the "garden of delights" had become very barren. it is a vast plain, on which we see jehovah and the first human couple, with a circular rampart surrounding it. the four rivers flow out of it by bifurcating. an angel protects the principal gate, which cannot be reached but by crossing barren mountains. the cosmographical map of gervais, dedicated to the emperor otho iv., shows the terrestrial paradise in the centre of the earth, which is square, and is situated in the midst of the seas. adam and eve appear in consultation. the map of the world prepared by andreas bianco, in the fifteenth century, represents eden, adam and eve, and the tree of life. on the left, on a peninsula, are seen the reprobated people of gog and magog, who are to accompany antichrist. alexander is also represented there, but without apparent reason. the paradisaical peninsula has a building on it with this inscription, "ospitius macarii." formalconi says, on this subject, that a certain macarius lives near paradise, who is a witness to all that the author states, and as bianco has indicated, his cell was close to the gates of paradise. this legend has reference to the pilgrims of st. macarius, a tradition that was spread on the return of the crusaders, of three monks who undertook a voyage to discover the point where the earth and heaven meet, that is to say, the place of the terrestrial paradise. the map of rudimentum, a vast compilation published at lübeck in by the dominican brocard, represents the terrestrial paradise surrounded by walls, but it is less sterile that in the last picture, as may be seen on the next page. in the year , when varthema, the adventurous bolognian, went to the indies by the route of palestine and syria, he was shown the evil-reputed house which cain dwelt in, which was not far from the terrestrial paradise. master gilius, the learned naturalist who travelled at the expense of francis i., had the same satisfaction. the simple faith of our ancestors had no hesitation in accepting such archæology. [illustration: fig. .--the paradise of the fifteenth century.] the most curious and interesting of all attempts to discover the situation of paradise was that made half unconsciously by columbus when he first found the american shore. in his third voyage, when for the first time he reached the main land, he was persuaded not only that he had arrived at the extremity of asia, but that he could not be far from the position of paradise. the orinoco seemed to be one of those four great rivers which, according to tradition, came out of the garden inhabited by our first parents, and his hopes were supported by the fragrant breezes that blew from the beautiful forests on its banks. this, he thought, was but the entrance to the celestial dwelling-place, and if he had dared--if a religious fear had not held back him who had risked everything amidst the elements and amongst men, he would have liked to push forward to where he might hope to find the celestial boundaries of the world, and, a little further, to have bathed his eyes, with profound humility, in the light of the flaming swords which were wielded by two seraphim before the gate of eden. he thus expresses himself on this subject in his letter to one of the monarchs of spain, dated hayti, october, . "the holy scriptures attest that the lord created paradise, and placed in it the tree of life, and made the four great rivers of the earth to pass out of it, the ganges of india, the tigris, the euphrates (passing from the mountains to form mesopotamia, and ending in persia), and the nile, which rises in ethiopia and goes to the sea of alexander. i cannot, nor have been ever able to find in the books of the latins or greeks anything authentic on the site of this terrestrial paradise, nor do i see anything more certain in the maps of the world. some place it at the source of the nile, in ethiopia; but the travellers who have passed through those countries have not found either in the mildness of the climate or in the elevation of the site towards heaven anything that could lead to the presumption that paradise was there, and that the waters of the deluge were unable to reach it or cover it. several pagans have written for the purpose of proving it was in the fortunate isles, which are the canaries. st. isidore, bede, and strabo, st. ambrosius, scotus, and all judicious theologians affirm with one accord that paradise was in the east. it is from thence only that the enormous quantity of water can come, seeing that the course of the rivers is extremely long; and these waters (of paradise) arrive here, where i am, and form a lake. there are great signs here of the neighbourhood of the terrestrial paradise, for the site is entirely conformable to the opinion of the saints and judicious theologians. the climate is of admirable mildness. i believe that if i passed beneath the equinoctial line, and arrived at the highest point of which i have spoken, i should find a milder temperature, and a change in the stars and the waters; not that i believe that the point where the greatest height is situated is navigable, or even that there is water there, or that one could reach it, but i am convinced that _there_ is the terrestrial paradise, where no one can come except by the will of god." in the opinion of this illustrious navigator the earth had the form of a pear, and its surface kept rising towards the east, indicated by the point of the fruit. it was there that he supposed might be found the garden where ancient tradition imagined the creation of the first human couple was accomplished. we can scarcely think without astonishment of the great amount of darkness that obscured scientific knowledge, when this great man appeared on the scene of the world, nor of the rapidity with which the obscurity and vagueness of ideas were dissipated almost immediately after his marvellous discoveries. scarcely had a half century elapsed after his death, than all the geographical fables of the middle ages did no more than excite smiles of incredulity, although during his life the universal opinion was not much advanced upon the times of the famous knight john of mandeville, who wrote gravely as follows:-- "no mortal man can go to or approach this paradise. by land no one can go there on account of savage beasts which are in the deserts, and because of mountains and rocks that cannot be passed over, and dark places without number; nor can one go there any better by sea; the water rushes so wildly, it comes in so great waves, that no vessel dare sail against them. the water is so rapid, and makes so great a noise and tempest, that no one can hear however loud he is spoken to, and so when some great men with good courage have attempted several times to go by this river to paradise, in large companies, they have never been able to accomplish their journey. on the contrary, many have died with fatigue in swimming against the watery waves. many others have become blind, others have become deaf by the noise of the water, and others have been suffocated and lost in the waves, so that no mortal man can approach it except by the special grace of god." with one notable exception, no attempts have been made of late years to solve such a question. that exception is by the noble and indefatigable livingstone, who declared his conviction to sir roderick murchison, in a letter published in the _athenæum_, that paradise was situated somewhere near the sources of the nile. those generally who now seek an answer to the question of the birthplace of the human race do not call it paradise. since man is here, and there was a time quite recent, geologically speaking, when he was not, there must have been some actual locality on the earth's surface where he was first a man. whether we have, or even can hope to have, enough information to indicate where that locality was situated, is a matter of doubt. we have not at present. those who have attended most to the subject appear to think some island the most probable locality, but it is quite conjectural. the name "paradise" appears to have been derived from the persian, in which it means a garden; similarly derived words express the same idea in other languages; as in the hebrew _pardês_, in the arabian _firdaus_, in the syriac _pardiso_, and in the armenian _partes_. it has been thought that the persian word itself is derived from the sanscrit _pradesa_, or _paradesa_, which means a circle, a country, or strange region; which, though near enough as to sound, does not quite agree as to meaning. "eden" is from a hebrew root meaning delights. chapter xii. eclipses and comets. we have seen in the earlier chapters on the systems of the ancients and their ideas of the world how everything was once supposed to have exclusive reference to man, and how he considered himself not only chief of animate objects, but that his own city was the centre of the material world, and his own world the centre of the material universe; that the sun was made to shine, as well as the moon and stars, for his benefit; and that, were it not for him they would have no reason for existence. and we have seen how, step by step, these illusions have been dispelled, and he has learnt to appreciate his own littleness in proportion as he has realised the immensity of the universe of which he forms part. if such has been his history, and such his former ideas on the regular parts, as we may call them, of nature, much more have similar ideas been developed in relation to those other phenomena which, coming at such long intervals, have not been recognised by him as periodic, but have seemed to have some relation to mundane affairs, often of the smallest consequence. such are eclipses of the sun and moon, comets, shooting-stars, and meteors. among the less instructed of men, even when astronomers of the same age and nation knew their real nature, eclipses have always been looked upon as something ominous of evil. among the ancient nations people used to come to the assistance of the moon, by making a confused noise with all kinds of instruments, when it was eclipsed. it is even done now in persia and some parts of china, where they fancy that the moon is fighting with a great dragon, and they think the noise will make him loose his hold and take to flight. among the east indians they have the same belief that when the sun and the moon are eclipsed, a dragon is seizing them, and astronomers who go there to observe eclipses are troubled by the fears of their native attendants, and by their endeavours to get into the water as the best place under the circumstances. in america the idea is that the sun and moon are tired when they are eclipsed. but the more refined greeks believed for a long time that the moon was bewitched, and that the magicians made it descend from heaven, to put into the herbs a certain maleficent froth. perhaps the idea of the dragon arose from the ancient custom of calling the places in the heavens at which the eclipses of the moon took place the head and tail of the dragon. in ancient history we have many curious instances of the very critical influence that eclipses have had, especially in the case of events in a campaign, where it was thought unfavourable to some projected attempt. thus an eclipse of the moon was the original cause of the death of the athenian general nicias. just at a critical juncture, when he was about to depart from the harbour of syracuse, the eclipse filled him and his whole army with dismay. the result of his terror was that he delayed the departure of his fleet, and the athenian army was cut in pieces and destroyed, and nicias lost his liberty and life. plutarch says they could understand well enough the cause of the eclipse of the sun by the interposition of the moon, but they could not imagine by the opposition of what body the moon itself could be eclipsed. one of the most famous eclipses of antiquity was that of thales, recorded by herodotus, who says:--"the lydians and the medes were at war for five consecutive years. now while the war was sustained on both sides with equal chance, in the sixth year, one day when the armies were in battle array, it happened that in the midst of the combat the day suddenly changed into night. thales of miletus had predicted this phenomenon to the ionians, and had pointed out precisely that very year as the one in which it would take place. the lydians and medes, seeing the night succeeding suddenly to the day, put an end to the combat, and only cared to establish peace." another notable eclipse is that related by diodorus siculus. it was a total eclipse of the sun, which took place while agathocles, fleeing from the port of syracuse, where he was blockaded by the carthaginians, was hastening to gain the coast of africa. "when agathocles was already surrounded by the enemy, night came on, and he escaped contrary to all hope. on the day following so complete an eclipse of the sun took place that it seemed altogether night, for the stars shone out in all places. the soldiers therefore of agathocles, persuaded that the gods were intending them some misfortune, were in the greatest perturbation about the future. agathocles was equal to the occasion. when disembarked in africa, where, in spite of all his fine words, he was unable to reassure his soldiers, whom the eclipse of the sun had frightened, he changed his tactics, and pretending to understand the prodigy, "i grant, comrades," he said, "that had we perceived this, eclipse before our embarkation we should indeed have been in a critical situation, but now that we have seen it after our departure, and as it always signifies a change in the present state of affairs, it follows that our circumstances, which were very bad in sicily, are about to amend, while we shall indubitably ruin those of the carthaginians, which have been hitherto so flourishing." we are reminded by this of the story of pericles, who, when ready to set sail with his fleet on a great expedition, saw himself stopped by a similar phenomenon. he spread his mantle over the eyes of the pilot, whom fear had prevented acting, and asked him if that was any sign of misfortune, when the pilot answered in the negative. "what misfortune then do you suppose," said he, "is presaged by the body that hides the sun, which differs from this in nothing but being larger?" with reference to these eclipses, when their locality and approximate date is known, astronomy comes to the assistance of history, and can supply the exact day, and even hour, of the occurrence. for the eclipses depend on the motions of the moon, and just as astronomers can calculate both the time and the path of a solar eclipse in the future, so they can for the past. if then the eclipses are calculated back to the epoch when the particular one is recorded, it can be easily ascertained which one it was that about that time passed over the spot at which it was observed, and as soon as the particular eclipse is fixed upon, it may be told at what hour it would be seen. thus the eclipse of thales has been assigned by different authors to various dates, between the st of october, b.c., and the rd of february, b.c. the only eclipse of the sun that is suitable between those dates has been found by the astronomer-royal to be that which would happen in lydia on the th of may, b.c., which must therefore be the date of the event. so of the eclipse of agathocles, m. delaunay has fixed its date to the th august, b.c. in later days, when christopher columbus had to deal with the ignorant people of america, the same kind of story was repeated. he found himself reduced to famine by the inhabitants of the country, who kept him and his companions prisoners; and being aware of the approach of the eclipse, he menaced them with bringing upon them great misfortunes, and depriving them of the light of the moon, if they did not instantly bring him provisions. they cared little for his menaces at first; but as soon as they saw the moon disappear, they ran to him with abundance of victuals, and implored pardon of the conqueror. this was on the st of march, , a date which may be tested by the modern tables of the moon, and columbus's account proved to be correct. the eclipse was indeed recorded in other places by various observers. eclipses in their natural aspect have thus had considerable influence on the vulgar, who knew nothing of their cause. this of course was the state with all in the early ages, and it is interesting to trace the gradual progress from their being quite unexpected to their being predicted. it is very probable, if not certain, that their recurrence in the case of the moon at least was recognised long before their nature was understood. [illustration: plate xiii.--christopher columbus and the eclipse of the moon.] among the chinese they were long calculated, and, in fact, it is thought by some that they have pretended to a greater antiquity by calculating backwards, and recording as observed eclipses those which happened before they understood or noticed them. it seems, however, authenticated that they did in the year b.c. observe an eclipse of the sun, and that at that date they were in the habit of predicting them. for this particular eclipse is said to have cost several of the astronomers their lives, as they had not calculated it rightly. as the lives of princes were supposed to be dependent on these eclipses, it became high treason to expose them to such a danger without forewarning them. they paid more attention to the eclipses of the sun than of the moon. among the babylonians the eclipses of the moon were observed from a very early date, and numerous records of them are contained in the observations of bel in sargon's library, the tablets of which have lately been discovered. in the older portion they only record that on the th day of such and such a (lunar) month an eclipse takes place, and state in what watch it begins, and when it ends. in a later portion the observations were more precise, and the descriptions of the eclipse more accurate. long before b.c. the discovery of the lunar cycle of lunar months had been made, and by means of it they were able to state of each lunar eclipse, that it was either "according to calculation" or "contrary to calculation." they dealt also with solar eclipses, and tried to trace on a sphere the path they would take on the earth. accordingly, like the eclipses of the moon, these too were spoken of as happening either "according to calculation" or "contrary to calculation." "in a report sent in to one of the later kings of assyria by the state astronomer, abil islar states that a watch had been kept on the th, th, and th of sivan, or may, for an eclipse of the sun, which did not, however, take place after all. the shadow, it is clear, must have fallen outside the field of observation." besides the more ordinary kind of solar eclipses, mention is made in the observations of bel of annular eclipses which, strangely enough, are seldom alluded to by classical writers. a record of a later eclipse has been found by sir henry rawlinson on one of the nineveh tablets. this occurred near that city in b.c. , and from the character of the inscription it may be inferred that it was a rare occurrence with them, indeed that it was nearly, if not quite, a total eclipse. this has an especial interest as being the earliest that we have any approximate date for. it is possible that the remarkable phenomenon, alluded to by the prophet isaiah, of the shadow going backwards ten degrees on the dial of ahaz, may be really a record of an eclipse of the sun, such as astronomy proves to have occurred at jerusalem in the year b.c. we have very little notice of the calculation of eclipses by the egyptians; all that is told us is more or less fabulous. thus diogenes laertius says that they reckoned that during a period of , years, eclipses of the sun and eclipses of the moon had occurred, which is far fewer than the right number for so long a time, and which, of course, has no basis in fact. among the greeks, anaxagoras was the first who entertained clear ideas about the nature of eclipses; and it was from him that pericles learnt their harmlessness. plutarch relates that helicon of cyzicus predicted an eclipse of the sun to dionysius of syracuse, and received as a reward a talent of silver. livy records an eclipse of the sun as having taken place on the th of quintilis, which corresponds to the th of july. it happened during the appollinarian games, b.c. the same author tells us of an eclipse of the moon that was predicted by one gallus, a tribune of the second legion, on the eve of the battle of pydna--a prediction which was duly fulfilled on the following night. the fact of its having been foretold quieted the superstitious fears of the soldiers, and gave them a very high opinion of gallus. other authors, among them cicero, do not give so flattering a story, but state that gallus's part consisted only in explaining the cause of the eclipse after it had happened. the date of this eclipse was the rd of september, b.c. ennius, writing towards the end of the second century b.c., describes an eclipse which was said to have happened nearly two hundred years before ( , b.c.), in the following remarkable words:--"on the nones of july the moon passed over the sun, and there was night." aristarchus, three centuries before christ, understood and explained the nature of eclipses; but the chief of the ancient authors upon this subject was hipparchus. he and his disciples were able to predict eclipses with considerable accuracy, both as to their time and duration. geminus and cleomedes were two other writers, somewhat later, who explained and predicted eclipses. in later times regular tables were drawn up, showing when the eclipses would happen. one that ptolemy was the author of was founded on data derived from ancient observers--callipus, democritus, eudoxus, hipparchus--aided by his own calculations. after the days of ptolemy the knowledge of the eclipses advanced _pari passu_ with the advance of astronomy generally. so long as astronomy itself was empirical, the time of the return of an eclipse was only reckoned by the intervals that had elapsed during the same portion of previous cycles; but after the discovery of elliptic orbits and the force of gravitation the whole motion of the moon could be calculated with as great accuracy as any other astronomical phenomenon. in point of fact, if the new moon is in the plane of the ecliptic there must be an eclipse of the sun; if the full moon is there, there must be an eclipse of the moon; and if it should in these cases be only partially in that plane, the eclipses also will be partial. the cycle of changes that the position of the moon can undergo when new and full occupies a period of eighteen years and eleven days, in which period there are forty-one eclipses of the sun and twenty-nine of the moon. each year there are at most seven and at least two eclipses; if only two, they are eclipses of the sun. although more numerous in reality for the whole earth, eclipses of the sun are more rarely observed in any particular place, because they are not seen everywhere, but only where the shadow of the moon passes; while all that part of the earth that sees the moon at all at the time sees it eclipsed. we now come to comets. the ancients divided comets into different classes, the chief points of distinction being derived from the shape, length, and brilliancy of the tails. pliny distinguished twelve kinds, which he thus characterised:--"some frighten us by their blood-coloured mane; their bristling hair rises towards the heaven. the bearded ones let their long hair fall down like a majestic beard. the javelin-shaped ones seem to be projected forwards like a dart, as they rapidly attain their shape after their first appearance; if the tail is shorter, and terminates in a point, it is called a sword; this is the palest of all the comets; it has the appearance of a bright sword without any diverging rays. the plate or disc derives its name from its shape, its colour is that of amber, it gives out some diverging rays from its sides, but not in large quantity. the cask has really the form of a cask, which one might suppose to be staved in smoke enveloped in light. the retort imitates the figure of a horn, and the lamp that of a burning flame. the horse-comet represents the mane of a horse which is violently agitated, as by a circular, or rather cylindrical, motion. such a comet appears also of singular whiteness, with hair of a silver hue; it is so bright that one can scarcely look at it. there are bristling comets, they are like the skins of beasts with their hair on, and are surrounded by a nebulosity. lastly, the hair of the comet sometimes takes the form of a lance." pingré, a celebrated historian of comets, tells us that one of the first comets noticed in history is that which appeared over rome forty years before christ, and in which the roman people imagined they saw the soul of cæsar endowed with divine honours. next comes that which threw its light on jerusalem when it was being besieged and remained for a whole year above the city, according to the account of josephus. it was of this kind that pliny said it "is of so great a whiteness that one can scarcely look at it, and _one may see in it the image of god in human form_." diodorus tells us that, a little after the subversion of the towns of helix and bura, there were seen, for several nights in succession, a brilliant light, which was called a beam of fire, but which aristotle says was a true comet. plutarch, in his life of timoleon, says a burning flame preceded the fleet of this general until his arrival at sicily, and that during the consulate of caius servilius a bright shield was seen suspended in the heavens. the historians sazoncenas and socrates relate that in the year a.d. a comet in the form of a sword shone over constantinople, and appeared to touch the town just at the time when great misfortunes were impending through the treachery of gainas. the same phenomenon appeared over rome previous to the arrival of alaric. in fact the ancient chroniclers always associated the appearance of a comet with some terrestrial event, which it was not difficult to do, seeing that critical situations were at all times existing in some one country or other where the comet would be visible, and probably those which could not be connected with any were not thought worthy of being recorded. it is well known that the year a.d. was for a long time predicted to be the end of the world. in this year the astronomers and chroniclers registered the fall of an enormous burning meteor and the appearance of a comet. pingré says: "on the th of the calends of january"--that is the th of december--"the heavens being dark, a kind of burning sword fell to the earth, leaving behind it a long train of light. its brilliancy was such that it frightened not only those who were in the fields, but even those who were shut up in their houses. this great opening in the heavens was gradually closed, and then was seen the figure of a dragon, whose feet were blue, and whose head kept continually increasing. a comet having appeared at the same time as this chasm, or meteor, they were confounded." this relation is given in the chronicles of seigbert in hermann corner, in the chronique de tours, in albert casin, and other historians of the time. bodin, resuscitating an idea of democritus, wrote that the comets were the souls of illustrious personages, who, after having lived on the earth a long series of centuries, and being ready at last to pass away, were carried in a kind of triumph to heaven. for this reason, famine, epidemics, and civil wars followed on the apparition of comets, the towns and their inhabitants finding themselves then deprived of the help of the illustrious souls who had laboured to appease their intestinal feuds. one of the comets of the middle ages which made the greatest impression on the minds of the people was that which appeared during holy week of the year , and frightened louis the debonnaire. the first morning of its appearance he sent for his astrologer. "go," he said, "on to the terrace of the palace, and come back again immediately and tell me what you have seen, for i have not seen that star before, and you have not shown it to me; but i know that this sign is a comet: it announces a change of reign and the death of a prince." the son of charlemagne having taken counsel with his bench of bishops, was convinced that the comet was a notice sent from heaven expressly for him. he passed the nights in prayer, and gave large donations to the monasteries, and finally had a number of masses performed out of fear for himself and forethought for the church committed to his care. the comet, however, was a very inoffensive one, being none other than that known as halley's comet, which returned in . while they were being thus frightened in france, the chinese were observing it astronomically. the historian of merlin the enchanter relates that a few days after the _fêtes_ which were held on the occasion of the erection of the funeral monument of salisbury, a sign appeared in heaven. it was a comet of large size and excessive splendour. it resembled a dragon, out of whose mouth came a long two-forked tongue, one part of which turned towards the north and the other to the east. the people were in a state of fear, each one asking what this sign presaged. uter, in the absence of the king, ambrosius, his brother, who was engaged in pursuing one of the sons of vortigern, consulted all the wise men of britain, but no one could give him any answer. then he thought of merlin the enchanter, and sent for him to the court. "what does this apparition presage?" demanded the king's brother. merlin began to weep. "o son of britain, you have just had a great loss--the king is dead." after a moment of silence he added, "but the britons have still a king. haste thee, uter, attack the enemy. all the island will submit to you, for the figure of the fiery dragon is thyself. the ray that goes towards gaul represents a son who shall be born to thee, who will be great by his achievements, and not less so by his power. the ray that goes towards ireland represents a daughter of whom thou shalt be the father, and her sons and grandsons shall reign over all the britons." these predictions were realised; but it is more than probable that they were made up after the event. the comet of was regarded as a presage of the conquest under william of normandy. in the bayeaux tapestry, on which matilda of flanders had drawn all the most memorable episodes in the transmarine expedition of her husband, the comet appears in one of the corners with the inscription, _isti mirantur stellam_, which proves that the comet was considered a veritable marvel. it is said even to be traditionally reported that one of the jewels of the british crown was taken from the tail of this comet. nevertheless it was no more than halley's comet again in its periodical visit every seventy-six years. in july, , a brilliant comet appeared which was lost to view on the same day as the pope, urban iv., died, _i.e._ the third of october. in june, , a similar body of enormous size, with a very long and extraordinarily bright tail, put all christendom in a fright. the pope, calixtus iii., was engaged in a war at that time with the saracens. he showed the christians that the comet "had the form of a cross," and announced some great event. at the same time mahomet announced to his followers that the comet, "having the form of a yataghan," was a blessing of the prophet's. it is said that the pope afterwards recognised that it had this form, and excommunicated it. nevertheless, the christians obtained the victory under the walls of belgrade. this was another appearance of halley's comet. in the early months of appeared a large comet, which historians agree in saying was very horrible and alarming. belleforest said it was a hideous and frightening comet, which threw its rays from east to west, giving great cause for fear to great people, who were not ignorant that comets are the menacing rods of god, which admonish those who are in authority, that they may be converted. pingré, who has told us of so many of the comets that were seen before his time, wrote of this epoch: "comets became the most efficacious signs of the most important and doubtful events. they were charged to announce wars, seditions, and the internal movements of republics; they presaged famines, pestilence, and epidemics; princes, or even persons of dignity, could not pay the tribute of nature without the previous appearance of that universal oracle, a comet; men could no longer be surprised by any unexpected event; the future might be as easily read in the heavens as the past in history. their effect depended on the place in the heavens where they shone, the countries over which they directly lay, the signs of the zodiac that they measured by their longitude, the constellations they traversed, the form and length of their tails, the place where they went out, and a thousand other circumstances more easily indicated than distinguished; they also announced in general wars, and the death of princes, or some grand personage, but there were few years that passed without something of this kind occurring. the devout astrologers--for there were many of that sort--risked less than the others. according to them, the comet threatened some misfortune; if it did not happen, it was because the prayers of penitence had turned aside the wrath of god; he had returned his sword to the scabbard. but a rule was invented which gave the astrologers free scope, for they said that events announced by a comet might be postponed for one or more periods of forty years, or even as many years as the comet had appeared days; so that one which had appeared for six months need not produce its effect for years." [illustration: fig. .--representation of a comet, th century.] the most frightful of the comets of this period, according to simon goulart, was that of . "it put some into so great a fright that they died; others fell sick. it was seen by several thousand people, and appeared very long, and of the colour of blood. at the summit was seen the representation of a curved arm, holding a large sword in its hand, as if it would strike; at the top of the point of the sword were three stars, but that which touched the point was more brilliant than the others. on the two sides of the rays of this comet were seen large hatchets, poignards, bloody swords, among which were seen a great number of men decapitated, having their heads and beards horribly bristling." a view of this comet is given in the _history of prodigies_. there was another comet remarked in , and another in , like the head of an owl, followed by a mantle of scattered light, with pointed ends. of this comet we read in the same book that recorded the last described: "the comet is an infallible sign of a very evil event. whenever eclipses of the sun or moon, or comets, or earthquakes, conversions of water into blood, and such like prodigies happen, it has always been known that very soon after these miserable portents afflictions, effusion of human blood, massacres, deaths of great monarchs, kings, princes, and rulers, seditions, treacheries, raids, overthrowings of empires, kingdoms, or villages; hunger and scarcity of provisions, burning and overthrowing of towns; pestilences, widespread mortality, both of beasts and men; in fact all sorts of evils and misfortunes take place. nor can it be doubted that all these signs and prodigies give warning that the end of the world is come, and with it the terrible last judgment of god." but even now comets were being observed astronomically, and began to lose their sepulchral aspect. a remarkable comet, however, which appeared in , was not without its fears for the vulgar. we are told that it was recognised as the same which appeared the year of cæsar's death, then in , and afterwards in , having a period of about years. the terror it produced in the towns was great; timid spirits saw in it the sign of a new deluge, as they said water was always announced by fire. while the fearful were making their wills, and, in anticipation of the end of the world, were leaving their money to the monks, who in accepting them showed themselves better physicists than the testators, people in high station were asking what great person it heralded the death of, and it is reported of the brother of louis xiv., who apparently was afraid of becoming too suddenly like cæsar, that he said sharply to the courtiers who were discussing it, "ah, gentlemen, you may talk at your ease, if you please; you are not princes." this same comet gave rise to a curious story of an "extraordinary prodigy, how at rome a hen laid an egg on which was drawn a picture of the comet. "the fact was attested by his holiness, by the queen of sweden, and all the persons of first quality in rome. on the th december, , a hen laid an egg on which was seen the figure of the comet, accompanied by other marks such as are here represented. the cleverest naturalists in rome have seen and examined it, and have never seen such a prodigy before." of this same comet bernouilli wrote, "_that if the body of the comet is not a visible sign of the anger of god, the tail may be_." it was this too that suggested to whiston the idea that he put forward, not as a superstitious, but as a physical speculation, that a comet approaching the earth was the cause of the deluge. [illustration: fig. .--an egg marked with a comet.] the last blow to the superstitious fear of the comets was given by halley, when he proved that they circulated like planets round the sun, and that the comets noticed in , , , , , , were all one, whose period was about years, and which would return in , which prediction was verified, and the comet went afterwards by the name of this astronomer. it returned again in , and will revisit us in . even after the fear arising from the relics of astrology had died away, another totally different alarm was connected with comets--an alarm which has not entirely subsided even in our own times. this is that a comet may come in contact with the earth and destroy it by the collision. the most remarkable panic in this respect was that which arose in paris in . at the previous meeting of the academy of sciences, m. lalande was to have read an interesting paper, but the time failed. it was on the subject of comets that could, by approaching the earth, cause its destruction, with special reference to the one that was soon to come. from the title only of the paper the most dreadful fears were spread abroad, and, increasing day by day, were with great difficulty allayed. the house of m. lalande was filled with those who came to question him on the memoir in question. the fermentation was so great that some devout people, as ignorant as weak, asked the archbishop to make a forty hours' prayer to turn away the enormous deluge that they feared, and the prelate was nearly going to order these prayers, if the members of the academy had not persuaded him how ridiculous it would be. finally, m. lalande, finding it impossible to answer all the questions put to him about his fatal memoir, and wishing to prevent the real evils that might arise from the frightened imaginations of the weak, caused it to be printed, and made it as clear as was possible. when it appeared, it was found that he stated that of the sixty comets known there were eight which could, by coming too near the earth, say within , miles, occasion such a pressure that the sea would leave its bed and cover part of the globe, but that in any case this could not happen till after twenty years. this was too long to make it worth while to make provision for it, and the effervescence subsided. a similar case to this occurred with respect to biela's comet, which was to return in . in calculating its reappearance in this year, damoiseau found that it would pass through the plane of the earth's orbit on the th of october. rushing away with this, the papers made out that a collision was inevitable, and the end of the world was come. but no one thought to inquire where the earth would be when the comet passed through the plane in which it revolved. arago, however, set people's minds at rest by pointing out that at that time the earth would be a month's journey from the spot, which with the rate at which the earth is moving would correspond to a distance of sixty millions of miles. this, like other frights, passed away, but was repeated again in and with like results, and even in a similar end to the world was announced to the public for the th of august, on the supposed authority of a professor at geneva, but who had never said what was supposed. but in reality all cause of fear has now passed away, since it has been proved that the comet is made of gaseous matter in a state of extreme tenuity, so that, though it may make great show in the heavens, the whole mass may not weigh more than a few pounds; and we have in addition the testimony of experience, which might have been relied on on the occasions above referred to, for in lexele's comet was seen to pass through the satellites of jupiter without deranging them in the least, but was itself thrown entirely out of its path, while there is reason to believe that on the th of june, , the earth remained several hours in the tail of a comet without having experienced the slightest inconvenience. as to the nature of comets, the opinions that have been held have been mostly very vague. metrodorus thought they were reflections from the sun; democritus, a concourse of several stars; aristotle, a collection of exhalations which had become dry and inflamed; strabo, that they are the splendour of a star enveloped in a cloud; heracletes of pontus, an elevated cloud which gave out much light; epigenes, some terrestrial matter that had caught fire, and was agitated by the wind; boecius, part of the air, coloured; anaxagoras, sparks fallen from the elementary fire; xenophanes, a motion and spreading out of clouds which caught fire; and descartes, the débris of vortices that had been destroyed, the fragments of which were coming towards us. it is said that the chaldæans held the opinion that they were analogous to planets by their regular course, and that when we ceased to see them, it was because they had gone too far from us; and seneca followed this explanation, since he regarded them as globes turning in the heavens, and which appear and disappear in certain times, and whose periodical motions might be known by regular observation. we have thus traced the particular ideas that have attached themselves to eclipses and comets, as the two most remarkable of the extraordinary phenomena of the heavens, and have seen how the fears and superstitions of mankind have been inevitably linked with them in the earlier days of ignorance and darkness, but they are only part of a system of phenomena, and have been no more connected with superstition than others less remarkable, except in proportion to their remarkableness. other minor appearances that are at all unusual have, on the same belief in the inextricable union of celestial and terrestrial matters, been made the signs of calamities or extra-prosperity; the doleful side of human nature being usually the strongest, the former have been chosen more often than the latter. according to seneca, the tradition of the chaldees announced that a universal deluge would be caused by the conjunction of all the planets in the sign of capricorn, and that a general breaking up of the earth would take place at the moment of their conjunction in cancer. "the general break-up of the world," they said, "will happen when the stars which govern the heaven, penetrated with a quality of heat and dryness, meet one another in a fiery triplicity." [illustration: plate xiv.--prodigies in the middle ages.] everywhere, and in all ages of the past, men have thought that a protecting providence, always watching over them, has taken care to warn them of the destinies which await them; thence the good and evil _presages_ taken from the appearance of certain heavenly bodies, of divers meteors, or even the accidental meeting of certain animate or inanimate objects. the indian of north america dying of famine in his miserable cabin, will not go out to the chase if he sees certain presages in the atmosphere. nor need we be astonished at such ideas in an uncultivated man, when even among europeans, a salt-cellar upset, a glass broken, a knife and fork crossed, the number thirteen at dinner, and such things are regarded as unlucky accidents. the employment of sorcery and divination is closely connected with these superstitions. besides eclipses and comets, meteors were taken as the signs of divine wrath. we learn from s. maximus of turin, that the christians of his time admitted the necessity of making a noise during eclipses, so as to prevent the magicians from hurting the sun or moon, a superstition entirely pagan. they used to fancy they could see celestial armies in the air, coming to bring miraculous assistance to man. they thought the hurricanes and tempests the work of evil spirits, whose rage kept them set against the earth. s. thomas aquinas, the great theologian of the thirteenth century, accepted this opinion, just as he admitted the reality of sorceries. but the full development, as well as the nourishment of these superstitious ideas, was derived from the storehouse of astrology, which dealt with matters of ordinary occurrence, both in the heavens and on the earth--and to the history of which our next chapter is devoted. chapter xiii. the greatness and the fall of astrology. our study of the opinions of the ancients on the various phenomena of astronomy, leads us inevitably to the discussion of their astrology, which has in every age and among every people accompanied it--and though astrology be now no more as a science, or lingers only with those who are ignorant and desirous of taking advantage of the still greater ignorance of others--yet it is not lacking in interest as showing the effect of the phenomena of the heavens on the human mind, when that effect is brought to its most technical and complete development. we must distinguish in the first place two kinds of astrology, viz., natural and judicial. the first proposed to foresee and announce the changes of the seasons, the rains, wind, heat, cold, abundance, or sterility of the ground, diseases, &c., by means of a knowledge of the causes which act on the air and on the atmosphere. the other is occupied with objects which would be still more interesting to men. it traced at the moment of his birth, or at any other period of his life, the line that each must travel according to his destiny. it pretended to determine our characters, our passions, fortune, misfortunes, and perils in reserve for each mortal. we have not here to consider the natural astrology, which is a veritable science of observation and does not deserve the name of astrology. it is rather worthy to be called the meteorological calendar of its cultivators. more rural than their descendants of the nineteenth century, the ancients had recognised the connection between the celestial phenomena and the vicissitudes of the seasons; they observed these phenomena carefully to discover the return of the same inclemencies; and they were able (or thought they were) to state the date of the return of particular kinds of weather with the same positions of the stars. but the very connection with the stars soon led the way to a degeneracy. the autumnal constellations, for example, orion and hercules, were regarded as rainy, because the rains came at the time when these stars rose. the egyptians who observed in the morning, called sirius "the burning," because his appearance in the morning was followed by the great heat of the summer: and it was the same with the other stars. soon they regarded them as the cause of the rain and the heat--although they were but remote witnesses. the star sirius is still connected with heat--since we call it the dog-star--and the hottest days of the year, july nd to august rd, we call dog-days. at the commencement of our era, the morning rising of sirius took place on the earlier of those days--though it does not now rise in the morning till the middle of august--and , years ago it rose about the th of june, and preceded the annual rise of the nile. the belief in the meteorological influence of the stars is one of the causes of judicial astrology. this latter has simply subjected man, like the atmosphere, to the influence of the stars; it has made dependent on them the risings of his passions, the good and ill fortune of his life, as well as the variations of the seasons. indeed, it was very easy to explain. it is the stars, or heavenly bodies in general, that bring the winds, the rains, and the storms; their influences mixed with the action of the rays of the sun modify the cold or heat; the fertility of the fields, health or sickness, depend on these beneficial or injurious influences; not a blade of grass can grow without all the stars having contributed to its increase; man breathes the emanations which escaping from the heavenly bodies fill the air; man is therefore in his entire nature subjected to them; these stars must therefore influence his will and his passions; the good and evil passages in his career, in a word, must direct his life. as soon as it was established that the rising of a certain star or planet, and its aspect with regard to other planets, announced a certain destiny to man, it was natural to believe that the rarer configurations signified extraordinary events, which concerned great empires, nations, and towns. and lastly, since errors grow faster than truth, it was natural to think that the configurations which were still more rare, such as the reunion of all the planets in conjunction with the same star, which can occur only after thousands of centuries, while nations have been renewed an infinity of times, and empires have been ruined, had reference to the earth itself, which had served as the theatre for all these events. joined to these superstitious ideas was the tradition of a deluge, and the belief that the world must one day perish by fire, and so it was announced that the former event took place when all the planets were in conjunction in the sign of the fishes, and the latter would occur when they all met in the sign of the lion. the origin of astrology, like that of the celestial sphere, was in all probability in upper asia. there, the starry heavens, always pure and splendid, invited observation and struck the imagination. we have already seen this with respect to the more matter-of-fact portions of astronomy. the assyrians looked upon the stars as divinities endued with beneficent or maleficent power. the adoration of the heavenly bodies was the earliest form of religion among the pastoral population that came down from the mountains of kurdestan to the plains of babylon. the chaldæans at last set apart a sacerdotal and learned caste devoted to the observation of the heavens; and the temples became regular observatories. such doubtless was the tower of babel--a monument consecrated to the seven planets, and of which the account has come down to us in the ancient book of genesis. a long series of observations put the chaldæans in possession of a theological astronomy, resting on a more or less chimerical theory of the influence of the celestial bodies on the events of nations and private individuals. diodorus siculus, writing towards the commencement of our era, has put us in possession of the most circumstantial details that have reached us with regard to the chaldæan priests. at the head of the gods, the assyrians placed the sun and moon, whose courses and daily positions they had noted in the constellation of the zodiac, in which the sun remained, one month in each. the twelve signs were governed by as many gods, who had the corresponding months under their influence. each of these months were divided into three parts, which made altogether thirty-six subdivisions, over which as many stars presided, called gods of consultation. half of these gods had under their control the things which happen above the earth, and the other half those below. the sun and moon and the five planets occupied the most elevated rank in the divine hierarchy and bore the name of gods of interpretation. among these planets saturn or old bel, which was regarded as the highest star and the most distant from us, was surrounded by the greatest veneration; he was the interpreter _par excellence_--the revealer. each of the other planets had his own particular name. some of them, such as _bel_ (jupiter), _merodaez_ (mars), _nebo_ (mercury), were regarded as male, and the others, as _sin_ (the moon), and _mylitta_ or _baulthis_ (venus), as females; and from their position relative to the zodiacal constellations, which were also called _lords_ or masters of the _gods_, the chaldæans derived the knowledge of the destiny of the men who were born under such and such a conjunction--predictions which the greeks afterwards called horoscopes. the chaldæans invented also relations between each of the planets and meteorological phenomena, an opinion partly founded on fortuitous coincidences which they had more or less frequently observed. in the time of alexander their credit was considerable, and the king of macedonia, either from superstition or policy, was in the habit of consulting them. it is probable that the babylonian priests, who referred every natural property to sidereal influences, imagined there were some mysterious relations between the planets and the metals whose colours were respectively somewhat analogous to theirs. gold corresponded to the sun, silver to the moon, lead to saturn, iron to mars, tin to jupiter, and mercury still retains the name of the planet with which it was associated. it is less than two centuries ago, since the metals have ceased to be designated by the signs of their respective planets. alchemy, the mother of chemistry, was an intimately connected sister of astrology, the mother of astronomy. egyptian civilisation dates back to a no less remote period than that of babylon. not less careful observers than the babylonish astrologers of the meteors and the atmospheric revolutions, they could predict certain phenomena, and they gave it out that they had themselves been the cause of them. diodorus siculus tells us that the egyptian priests pretty generally predicted the years of barrenness or abundance, the contagions, the earthquakes, inundations, and comets. the knowledge of celestial phenomena made an essential part of the theology of the egyptians as it did of the chaldæans. they had colleges of priests specially attached to the study of the stars, at which pythagoras, plato, and eudoxus were instructed. religion was besides completely filled with the symbols relating to the sun or moon. each month, each decade, each day was consecrated to a particular god. these gods, to the number of thirty, were called in the alexandrine astronomy _decans_ ([greek: dekavoi]). the festivals were marked by the periodical return of certain astronomical phenomena, and those heliacal risings to which any mythological ideas were attached, were noted with great care. we find even now proof of this old sacerdotal science in the zodiac sculptured on the ceilings of certain temples, and in the hieroglyphic inscriptions relating to celestial phenomena. according to the egyptians, who were no less aware than the greeks, of the influence of atmospheric changes on our organs, the different stars had a special action on each part of the body. in the funeral rituals which were placed at the bottom of the coffins, constant allusion is made to this theory. each limb of the dead body was placed under the protection of a particular god. the divinities divided between them, so to speak, the spoils of the dead. the head belonged to ra, or the sun, the nose and lips to anubis, and so on. to establish the horoscope of anyone, this theory of specific influences was combined with the state of the heavens at the time of his birth. it seems even to have been the doctrine of the egyptians, that a particular star indicated the coming of each man into the world, and this opinion was held also by the medes, and is alluded to in the gospels. in egypt, as in persia and chaldæa, the science of nature was a sacred doctrine, of which magic and astrology constituted the two branches, and in which the phenomena of the universe were attached very firmly to the divinities or genii with which they believed it filled. it was the same in the primitive religions of greece. the thessalian women had an especially great reputation in the art of enchantments. all the poets rival one another in declaring how they are able, by their magical hymns, to bring down the moon. menander, in his comedy entitled _the thessalian_, represents the mysterious ceremonies by the aid of which these sorcerers force the moon to leave the heavens, a prodigy which so completely became the type of enchantments that nonnus tells us it is done by the brahmins. there was, in addition, another _cultus_ in greece, namely, that of hecate with mysterious rays, the patron of sorcerers. lucian of samosate--if the work on astrology which is ascribed to him be really his--justifies his belief in the influence of the stars in the following terms:--"the stars follow their orbit in the heaven; but independently of their motion, they act upon what passes here below. if you admit that a horse in a gallop, that birds in flying, and men in walking, make the stones jump or drive the little floating particles of dust by the wind of their course, why should you deny that the stars have any effect? the smallest fire sends us its emanations, and although it is not for us that the stars burn, and they care very little about warming us, why should we not receive any emanations from them? astrology, it is true, cannot make that good which is evil. it can effect no change in the course of events; but it renders a service to those who cultivate it by announcing to them good things to come; it procures joy by anticipation at the same time that it fortifies them against the evil. misfortune, in fact, does not take them by surprise, the foreknowledge of it renders it easier and lighter. that is my way of looking at astrology." very different is the opinion of the satirist juvenal, who says that women are the chief cultivators of it. "all that an astrologer predicts to them," he says, "they think to come from the temple of jupiter. avoid meeting with a lady who is always casting up her _ephemerides_, who is so good an astrologer that she has ceased to consult, and is already beginning to be consulted; such a one on the inspection of the stars will refuse to accompany her husband to the army or to his native land. if she only wishes to drive a mile, the hour of departure is taken from her book of astrology. if her eye itches and wants rubbing, she will do nothing till she has run through her conjuring book. if she is ill in bed, she will take her food only at the times fixed in her _petosiris_. women of second-rate condition," he adds, "go round the circus before consulting their destiny, after which they show their hands and face to the diviner." when octavius came into the world a senator versed in astrology, nigidius figulus, predicted the glorious destiny of the future emperor. livia, the wife of tiberius, asked another astrologer, scribius, what would be the destiny of her infant; his reply was, they say, like the other's. the house of poppea, the wife of nero, was always full of astrologers. it was one of the soothsayers attached to her house, ptolemy, who predicted to otho his elevation to the empire, at the time of the expedition into spain, where he accompanied him. the history of astrology under the roman empire supplies some very curious stories, of which we may select an illustrative few. octavius, in company with agrippa, consulted one day the astrologer theagenes. the future husband of julia, more credulous or more curious than the nephew of cæsar, was the first to take the horoscope. theagenes foretold astonishing prosperity for him. octavius, jealous of so happy a destiny, and fearing that the reply would be less favourable to him, instead of following the example of his companion, refused at first to state the day of his birth. but, curiosity getting the better of him, he decided to reply. no sooner had he told the day of his birth than the astrologer threw himself at his feet, and worshipped him as the future master of the empire. octavius was transported with joy, and from that moment was a firm believer in astrology. to commemorate the happy influence of the zodiacal sign under which he was born, he had the picture of it struck on some of the medals that were issued in his reign. the masters of the empire believed in astrological divination, but wished to keep the advantages to themselves. they wanted to know the future without allowing their subjects to do the same. nero would not permit anyone to study philosophy, saying it was a vain and frivolous thing, from which one might take a pretext to divine future events. he feared lest some one should push his curiosity so far as to wish to find out when and how the emperor should die--a sort of indiscreet question, replies to which lead to conspiracies and attempts. this was what the heads of the state were most afraid of. tiberius had been to rhodes, to a soothsayer of renown, to instruct himself in the rules of astrology. he had attached to his person the celebrated astrologer thrasyllus, whose fate-revealing science he proved by one of those pleasantries which are only possible with tyrants. whenever tiberius consulted an astrologer he placed him in the highest part of his palace, and employed for his purpose an ignorant and powerful freedman, who brought by difficult paths, bounded by precipices, the astrologer whose science his majesty wished to prove. on the return journey, if the astrologer was suspected of indiscretion or treachery, the freedman threw him into the sea, to bury the secret. thrasyllus having been brought by the same route across these precipices, struck tiberius with awe while he questioned him, by showing him his sovereign power, and easily disclosing the things of the future. cæsar asked him if he had taken his own horoscope, and with what signs were marked that day and hour for himself. thrasyllus then examined the position and the distance of the stars; he hesitated at first, then he grew pale; then he looked again, and finally, trembling with astonishment and fear, he cried out that the moment was perilous, and he was very near his last hour. tiberius then embraced him and congratulated him on having escaped a danger by foreseeing it; and accepting henceforth all his predictions as oracles, he admitted him to the number of his intimate friends. tiberius had a great number of people put to death who were accused of having taken their horoscope to know what honours were in store for them, although in secret he took the horoscopes of great people, that he might ascertain that he had no rivalry to fear from them. septimus severus was very nearly paying with his head for one of those superstitious curiosities that brought the ambitious of the time to the astrologer. in prosperous times he had gained faith in their predictions, and consulted them about important acts. having lost his wife, and wishing to contract a second marriage, he took the horoscopes of the well-connected ladies who were at the time open to marriage. none of their fortunes, taken by the rules of astrology, were encouraging. he learnt at last that there was living in syria a young woman to whom the chaldæans had predicted that she should be the wife of a king. severus was as yet but a legate. he hastened to demand her in marriage, and he obtained her; julia was the name of the woman who was born under so happy a star; but was he the crowned husband which the stars had promised to the young syrian? this reflection soon began to perplex severus, and to get out of his perplexity he went to sicily to consult an astrologer of renown. the matter came to the ears of the emperor commodus; and judge of his anger! the anger of commodus was rage and frenzy; but the event soon gave the response that severus was seeking in sicily,--commodus was strangled. divination which had the emperor for its object at last came to be a crime of high treason. the rigorous measures resorted to against the indiscreet curiosity of ambition took more terrible proportions under the christian emperors. under constantine, a number of persons who had applied to the oracles were punished with cruel tortures. under valens, a certain palladius was the agent of a terrible persecution. everyone found himself exposed to being denounced for having relations with soothsayers. traitors slipped secretly into houses magic formulæ and charms, which then became so many proofs against the inhabitant. the fear was so great in the east, says ammienus marcellinus, that a great number burned their books, lest matter should be found in them for an accusation of magic or sorcery. one day in anger, vitellius commanded all the astrologers to leave italy by a certain day. they responded by a poster, which impudently commanded the prince to leave the earth before that date, and at the end of the year vitellius was put to death; on the other hand, the confidence accorded to astrologers led sometimes to the greatest extremes. for instance, after having consulted babylus, nero put to death all those whose prophecies promised the elevation of heliogabalus. another instance was that of marcus aurelius and his wife faustina. the latter was struck with the beauty of a gladiator. for a long time she vainly strove in secret with the passion that consumed her, but the passion did nothing but increase. at last faustina revealed the matter to her husband, and asked him for some remedy that should restore peace to her troubled soul. the philosophy of marcus aurelius could not suggest anything. so he decided to consult the chaldæans, who were adepts at the art of mixing philters and composing draughts. the means prescribed were more simple than might have been expected from their complicated science; it was that the gladiator should be cut in pieces. they added that faustina should afterwards be anointed with the blood of the victim. the remedy was applied, the innocent athlete was immolated, and the empress afterwards only dreamed of him with great pleasure. the first christians were as much addicted to astrology as the other sects. the councils of laodicea ( , a.d.), of arles ( ), of agdus ( ), orleans ( ), auxerre ( ), and narbonne ( ), condemned the practice. according to a tradition of the commencement of our era, which appears to have been borrowed from mazdeism, it was the rebel angels who taught men astrology and the use of charms. under constantius the crime of high treason served as a pretext for persecution. a number of people were accused of it, who simply continued to practise the ancient religion. it was pretended that they had recourse to sorceries against the life of the emperor, in order to bring about his fall. those who consulted the oracles were menaced with severe penalties and put to death by torture, under the pretence that by dealing with questions of fate they had criminal intentions. plots without number multiplied the accusations; and the cruelty of the judges aggravated the punishments. the pagans, in their turn had to suffer the martyrdom which they had previously inflicted on the early disciples of christ--or rather, to be truer, it was authority, always intolerable, whether pagan or christian, that showed itself inexorable against those who dared to differ from the accepted faith. libanius and jamblicus were accused of having attempted to discover the name of the successor to the empire. jamblicus, being frightened at the prosecution brought against him, poisoned himself. the name only of philosopher was sufficient to found an accusation upon. the philosopher maximus diogenes alypius, and his son hierocles, were condemned to lose their lives on the most frivolous pretence. an old man was put to death because he was in the habit of driving off the approach of fever by incantations, and a young man who was surprised in the act of putting his hands alternately to a marble and his breast, because he thought that by counting in this way seven times seven, he might cure the stomach-ache, met with the same fate. theodosius prohibited every kind of manifestation or usage connected with pagan belief. whoever should dare to immolate a victim, said his law, or consult the entrails of the animals he had killed, should be regarded as guilty of the crime of high treason. the fact of having recourse to a process of divination was sufficient for an accusation against a man. theodosius ii. thought that the continuation of idolatrous practices had drawn down the wrath of heaven, and brought upon them the recent calamities that had afflicted his empire--the derangement of the seasons and the sterility of the soil--and he thundered out terrible threats when his faith and his anger united themselves into fanaticism. he wrote as follows to florentius, prefect of the prætorium in , the year that preceded his death:-- "are we to suffer any longer from the seasons being upset by the effect of the divine wrath, on account of the atrocious perfidy of the pagans, which disturbs the equilibrium of nature? for what is the cause that now the spring has no longer its ordinary beauty, that the autumn no longer furnishes a harvest to the laborious workman and that the winter, by its rigour, freezes the soil and renders it sterile?" perhaps we are unduly amused with these ideas of theodosius so long as we retain the custom of asking the special intervention of providence for the presence or absence of rain! in the middle ages, when astrology took such a hold on the world, several philosophers went so far as to consider the celestial vault as a book, in which each star, having the value of one of the letters of the alphabet, told in ineffaceable characters the destiny of every empire. the book of _unheard-of curiosities_, by gaffarel, gives us the configuration of these celestial characters, and we find them also in the writings of cornelius agrippa. the middle ages took their astrological ideas from the arabians and jews. the jews themselves at this epoch borrowed their principles from such contaminated sources that we are not able to trace in them the transmission of the ancient ideas. to give an example, simeon ben-jochai, to whom is attributed the famous book called _zohar_, had attained in their opinion such a prodigious acquaintance with celestial mysteries as indicated by the stars, that he could have read the divine law in the heavens before it had been promulgated on the earth. during the whole of the middle ages, whenever they wanted to clear up doubts about geography or astronomy, they always had recourse to this oriental science, as cultivated by the jews and arabians. in the thirteenth century alphonse x. was very importunate with the jews to make them assist him with their advice in his vast astronomical and historical works. nicholas oresmus, when the most enlightened monarch in europe was supplying du guesclin with an astrologer to guide him in his strategical operations, was physician to charles v. of france, who was himself devoted to astrology, and gave him the bishopric of lisieux. he composed the _treatise of the sphere_, of which we have already spoken. a few years later, a learned man, the bishop peter d'ailly, actually dared to take the horoscope of jesus christ, and proved by most certain rules that the great event which inaugurated the new era was marked with very notable signs in astrology. mathias corvin, king of hungary, never undertook anything without first consulting the astrologers. the duke of milan and pope paul also governed themselves by their advice. king louis xi., who so heartily despised the rest of mankind, and had as much malice in him as he had weakness, had a curious adventure with an astrologer. it was told him that an astrologer had had the hardihood to predict the death of a woman of whom the king was very fond. he sent for the wretched prophet, gave him a severe reprimand, and then asked him the question, "you, who know everything, when will _you_ die?" the astrologer, suspecting a trick, replied immediately, "sire, three days before your majesty." fear and superstition overcame the monarch's resentment, and the king took particular care of the adroit impostor. it is well known how much catherine de medicis was under the influence of the astrologers. she had one in her hôtel de soissons in paris, who watched constantly at the top of a tower. this tower is still in existence, by the wool-market, which was built in on the site of the hotel. it is surmounted by a sphere and a solar dial, placed there by the astronomer pingré. one of the most celebrated of the astrologers who was under her patronage was nostradamus. he was a physician of provence, and was born at st. reny in . to medicine he joined astrology, and undertook to predict future events. he was called to paris by catherine in , and attempted to write his oracles in poetry. his little book was much sought after during the whole of the remainder of the sixteenth century, and even in the beginning of the next. according to contemporary writers many imitations were made of it. it was written in verses of four lines, and was called _quatrains astronomiques_. as usual, the prophecies were obscure enough to suit anything, and many believers have thought they could trace in the various verses prophecies of known events, by duly twisting and manipulating the sense. a very amusing prophecy, which happened to be too clear to leave room for mistakes as to its meaning, and which turned out to be most ludicrously wrong, was one contained in a little book published in with this title:--_prognostication touching the marriage of the very honourable and beloved henry, by the grace of god king of navarre, and the very illustrious princess marguerite of france, calculated by master bernard abbatio, doctor in medicine, and astrologer to the very christian king of france._ first he asked if the marriage would be happy, and says:--"having in my library made the figure of the heavens, i found that the lord of the ascendant is joined to the lord of the seventh house, which is for the woman of a trine aspect, from whence i have immediately concluded, according to the opinion of ptolemy, haly, zael, messahala, and many other sovereign astrologers, that they will love one another intensely all their lives." in point of fact they always detested each other. again, "as to length of life, i have prepared another figure, and have found that jupiter and venus are joined to the sun with fortification, and that they will approach a hundred years;" after all henri iv. died before he was sixty. "our good king of navarre will have by his most noble and virtuous queen many children; since, after i had prepared another figure of heaven, i found the ascendant and its lord, together with the moon, all joined to the lord of the fifth house, called that of children, which will be pretty numerous, on account of jupiter and also of venus;" and yet they had no children! "jupiter and venus are found domiciled on the aquatic signs, and since these two planets are found concordant with the lord of the ascendant, all this proves that the children will be upright and good, and that they will love their father and mother, without doing them any injury, nor being the cause of their destruction, as is seen in the fruit of the nut, which breaks, opens, and destroys the stock from which it took its birth. the children will live long, they will be good christians, and with their father will make themselves so benign and favourable towards those of our religion, that at last they will be as beloved as any man of our period, and there will be no more wars among the french, as there would have been but for the present marriage. god grant us grace that so long as we are in this transitory life we may see no other king but charles ix., the present king of france." and yet these words were written in the year of the massacre of st. bartholomew's day! and the marriage was broken off, and henri iv. married to marie de medici. so much for the astrological predictions! the aspect in which astrology was looked upon by the better minds even when it was flourishing may be illustrated by two quotations we may make, from shakespeare and voltaire. our immortal poet puts into the mouth of edmund in _king lear_:--"this is the excellent foppery of the world, that when we are sick in fortune (often the surfeit of our own behaviour) we make guilty of our disasters the sun, the moon, and the stars, as if we were villains by necessity; fools by heavenly compulsion; knaves, thieves, and treacherous, by spherical predominance; drunkards, liars, and adulterers by an enforced obedience of planetary influence; and all that we are evil in, by a divine thrusting on. an admirable evasion of a libertine to lay his goatish disposition to the charge of a star! my father married my mother under the dragon's tail; and my nativity was under _ursa major_; so that it follows i am rough lecherous. tut, i should have been that i am, had the maidenliest star in the firmament twinkled on my birth." voltaire writes thus:--"this error is ancient, and that is enough. the egyptians, the chaldæans, the jews could predict, and therefore we can predict now. if no more predictions are made it is not the fault of the art. so said the alchemists of the philosopher's stone. if you do not find to-day it is because you are not clever enough; but it is certain that it is in the clavicle of solomon, and on that certainty more than two hundred families in germany and france have been ruined. do you wonder either that so many men, otherwise much exalted above the vulgar, such as princes or popes, who knew their interests so well, should be so ridiculously seduced by this impertinence of astrology. they were very proud and very ignorant. there were no stars but for them; the rest of the universe was _canaille_, for whom the stars did not trouble themselves. i have not the honour of being a prince. nevertheless, the celebrated count of boulainvilliers and an italian, called colonne, who had great reputation in paris, both predicted to me that i should infallibly die at the age of thirty-two. i have had the malice already to deceive them by thirty years, for which i humbly beg their pardon." the method by which these predictions were arrived at consisted in making the different stars and planets responsible for different parts of the body, different properties, and different events, and making up stories from the association of ideas thus obtained, which of course admitted of the greatest degree of latitude. the principles are explained by manilius in his great poem entitled _the astronomicals_, written two thousand years ago. according to him the sun presided over the head, the moon over the right arm, venus over the left, jupiter over the stomach, mars the parts below, mercury over the right leg, and saturn over the left. among the constellations, the ram governed the head; the bull the neck; the twins the arms and shoulders; the crab the chest and the heart; the lion the stomach; the abdomen corresponded to the sign of the virgin; the reins to the balance; then came the scorpion; the archer, governing the thighs; the he-goat the knees; the waterer the legs; and the fishes the feet. albert the great assigned to the stars the following influences:--saturn was thought to rule over life, changes, sciences, and buildings; jupiter over honour, wishes, riches, and cleanness; mars over war, prisons, marriages, and hatred; the sun over hope, happiness, gain, and heritages; venus over friendships and amours; mercury over illness, debts, commerce, and fear; the moon over wounds, dreams, and larcenies. each of these stars also presides over particular days of the week, particular colours, and particular metals. the sun governed the sunday; the moon, monday; mars, tuesday; mercury, wednesday; jupiter, thursday; venus, friday; and saturn, saturday; which is partially indicated by our own names of the week, but more particularly in the french names, which are each and all derived from these stars. the sun represented yellow; the moon, white; venus, green; mars, red; jupiter, blue; saturn, black; mercury, shaded colours. we have already indicated the metals that corresponded to each. the sun was reckoned to be beneficent and favourable; saturn to be sad, morose, and cold; jupiter, temperate and benign; mars, vehement; venus, benevolent and fertile; mercury, inconstant; and the moon, melancholy. among the constellations, the ram, the lion, and the archer were hot, dry and vehement. the bull, the virgin, and the he-goat were heavy, cold, and dry; the twins, the balance, and the waterer were light, hot, and moist; the crab, scorpion, and the fishes were moist, soft, and cold. [illustration: plate xv.--an astrologer at work.] in this way the heavens were made to be intimately connected with the affairs of earth; and astrology was in equally intimate connection with astronomy, of which it may in some sense be considered the mother. the drawers of horoscopes were at one time as much in request as lawyers or doctors. one thurneisen, a famous astrologer and an extraordinary man, who lived last century at the electoral court of berlin, was at the same time physician, chemist, drawer of horoscopes, almanack maker, printer, and librarian. his astrological reputation was so widespread that scarcely a birth took place in families of any rank in germany, poland, hungary, or even england without there being sent an immediate envoy to him to announce the precise moment of birth. he received often three and sometimes as many as ten messages a day, and he was at last so pressed with business that he was obliged to take associates and agents. in the days of kepler we know that astrology was more thought of than astronomy, for though on behalf of the world he worked at the latter, for his own daily bread he was in the employ of the former, making almanacks and drawing horoscopes that he might live. chapter xiv. time and the calendar. the opinions of thinkers on the nature of time have been very varied. some have considered time as an absolute reality, which is exactly measured by hours, days, and years, and is as known and real as any other object whose existence is known to us. others maintain that time is only a matter of sensation, or that it is an illusion, or a hallucination of a lively brain. the definitions given of it by different great writers is as various. thus kant calls it "one of the forms of sensibility." schelling declares it is "pure activity with the negation of all being." leibnitz defines it "the order of successions" as he defined space to be the order of co-existences. newton and clarke make space and time two attributes of the deity. a study of the astronomical phenomena of the universe, and a consideration of their teaching, give us authority for saying, that neither space nor time are realities, but that the only things absolute are eternity and infinity. in fact, we give the name of time to the succession of the terrestrial events measured by the motion of the earth. if the earth were not to move, we should have no means of measuring, and consequently no idea of time as we have it now. so long as it was believed that the earth was at rest, and that the sun and all the stars turned round us, this apparent motion was then, as the real motion of the earth is now, the method of generating time. in fact, the fathers said that at the end of the world the diurnal motion would cease, and there would be no more time. but let us examine the fact a little further. suppose for an instant that the earth was, as it was formerly believed to be, an immense flat surface, which was illuminated by a sun which remained always immovable at the zenith, or by an invariable diffused light--such an earth being supposed to be alone by itself in the universe and immovable. now if there were a man created on that earth, would there be such a thing as "time" for him? the light which illumines him is immovable. no moving shadow, no gnomon, no sun-dial would be possible. no day nor night, no morning nor evening, no year. nothing that could be divided into days, hours, minutes, and seconds. in such a case one would have to fall back upon some other terminating events, which would indicate a lapse of time; such for instance as the life of a man. this, however, would be no universal measure, for on one planet the life might be a thousand years, and on another only a hundred. or we may look at it in another way. suppose the earth were to turn twice as fast about itself and about the sun, the persons who lived sixty of such years would only have lived thirty of our present years, but they would have seen sixty revolutions of the earth, and, rigorously speaking, would have lived sixty years. if the earth turned ten times as fast, sixty years would be reduced to ten, but they would still be sixty of those years. we should live just as long; there would be four seasons, days, &c., only everything would be more rapid: but it would be exactly the same thing for us, and the other apparently celestial motions having a similar diminution, there would be no change perceived by us. again, consider the minute animals that are observable under the microscope, which live but for five minutes. during that period, they have time to be born and to grow. from embryos they become adult, marry, so to speak, and have a numerous progeny, which they develop and send into the world. afterwards they die, and all this in a few minutes. the impressions which, in spite of their minuteness, we are justified in presuming them to possess, though rapid and fleeting, may be as profound for them in proportion as ours are to us, and their measure of time would be very different from ours. all is relative. in absolute value, a life completed in a hundred years is not longer than one that is finished in five minutes. it is the same for space. the earth has a diameter of , miles, and we are five or six feet high. now if, by any process, the earth should diminish till it became as small as a marble, and if the different elements of the world underwent a corresponding diminution, our mountains might become as small as grains of sand, the ocean might be but a drop, and we ourselves might be smaller than the microscopic animals adverted to above. but for all that nothing would have changed for us. we should still be our five or six feet high, and the earth would remain exactly the same number of our miles. a value then that can be decreased and diminished at pleasure without change is not a mathematical absolute value. in this sense then it may be said that neither time nor space have any real existence. or once again. suppose that instead of our being on the globe, we were placed in pure space. what time should we find there? no time. we might remain ten years, twenty, a hundred, or a thousand years, but we should never arrive at the next year! in fact each planet makes its own time for its inhabitants, and where there is no planet or anything answering to it there is no time. jupiter makes for its inhabitants a year which is equal to twelve years of ours, and a day of ten of our hours. saturn has a year equal to thirty of ours, and days of ten hours and a quarter. in other solar systems there are two or three suns, so that it is difficult to imagine what sort of time they can have. all this infinite diversity of time takes place in eternity, the only thing that is real. the whole history of the earth and its inhabitants takes place, not in time, but in eternity. before the existence of the earth and our solar system, there was another time, measured by other motions, and having relation to other beings. when the earth shall exist no longer, there may be in the place we now occupy, another time again, for other beings. but they are not realities. a hundred millions of centuries, and a second, have the same real length in eternity. in the middle of space, we could not tell the difference. our finite minds are not capable of grasping the infinite, and it is well to know that our only idea of time is relative, having relation to the regular events that befall this planet in its course, and not a thing which we can in any way compare with that, which is so alarming to the ideas of some--eternity. we have then to deal with the particular form of time that our planet makes for us, for our personal use. it turns about the sun. an entire circuit forms a period, which we can use for a measure in our terrestrial affairs. we call it a year, or in latin _annus_, signifying a circle, whence our word _annual_. a second, shorter revolution, turns the earth upon itself, and brings each meridian directly facing the sun, and then round again to the opposite side. this period we call a _day_, from the latin _dies_, which in italian becomes _giorne_, whence the french _jour_. in sanscrit we have the same word in _dyaus_. the length of time that it takes for the earth to arrive at the same position with respect to the stars, which is called a sidereal year, amounts to · days. but during this time, as we have seen, the equinox is displaced among the stars. this secular retrogression brings it each year a little to the east of its former position, so that the sun arrives there about eleven minutes too soon. by taking this amount from the sidereal we obtain the tropical year, which has reference to the seasons and the calendar. its length is · days, or days, hours, minutes, · seconds. in what way was the primitive year regulated? was it a solar or a sidereal year? there can be no doubt that when there was an absence of all civilisation and a calendar of any sort was unknown, the year meant simply the succession of seasons, and that no attempt would be made to reckon any day as its commencement. and as soon as this was attempted a difficulty would arise from there not being an exact number of days in the year. so that when reckoned as the interval between certain positions of the sun they would be of different lengths, which would introduce some difficulty as to the commencement of the year. be this the case, however, or not, mr. haliburton's researches seem to show that the earliest form of year was the sidereal one, and that it was regulated by the pleiades. in speaking of that constellation we have noticed that among the islanders of the southern hemisphere and others there are two years in one of ours, the first being called the pleiades above and the second the pleiades below; and we have seen how the same new year's day has been recognised in very many parts of the world and among the ancient egyptians and hindoos. this year would begin in november, and from the intimate relation of all the primitive calendars that have been discovered to a particular day, taken as november by the egyptians, it would appear probable that for a long time corrections were made both by the egyptians and others in order to keep the phenomenon of the pleiades just rising at sunset to one particular named day of their year--showing that the year they used was a sidereal one. this can be traced back as far as b.c. among the egyptians, and to b.c. among the hindoos. there seem to have been in use also shorter periods of three months, which, like the two-season year, appear to have been, as they are now among the japanese, regulated by the different positions of the pleiades. among the siamese of the present day, there are both forms of the year existing, one sidereal, beginning in november, and regulated by the fore-named constellation; and the other tropical, beginning in april. whether, however, the year be reckoned by the stars or by the sun, there will always be a difficulty in arranging the length of the year, because in each case there will be about a quarter of a day over. it seems, too, to have been found more convenient in early times to take days as the length of the year, and to add an intercalary month now and then, rather than and add a day. thus among the earliest egyptians the year was of days, which were reckoned in the months, and five days were added each year, between the commencement of one and the end of the other, and called unlucky days. it was the belief of the egyptians that these five days were the birthdays of their principal gods; osiris being born on the first, anieris (or apollo) on the second, typhon on the third, isis on the fourth, nephys (or aphrodite) on the fifth. these appear to have some relation with similar unlucky days among the greeks and romans, and other nations. the days of the egyptian year were represented at acantho, near memphis, in a symbolical way, there being placed a perforated vessel, which each day was filled with water by one of a company of priests, each priest having charge over one day in the year. a similar symbolism was used at the tomb of osiris, around which were placed pitchers, one of which each day was filled with milk. on the other hand, the days were represented by the tomb of osymandyas, at thebes, being surrounded by a circle of gold which was one cubit broad and cubits in circumference. on the side were written the risings and settings of the stars, with the prognostications derived from them by the egyptian astrologers. it was destroyed, however, by cambyses when the persians conquered egypt. they divided their year according to herodotus into twelve months, the names of which have come down to us. even with the days, which their method of reckoning would practically come to, they would still be a quarter of a day each year short; so that in four years it would amount to a whole day, an error which would amount to something perceptible even during the life of a single man, by its bringing the commencement of the civil year out of harmony with the seasons. in fact the first day of the year would gradually go through all the seasons, and at the end of solar years there would have been completed civil years, which would bring back the day to its original position. this period represents a cycle of years in which approximately the sun and the earth come to the same relative position again, as regards the earth's rotation on its axis and revolution round the sun. this cycle was noticed by firmicius. another more accurate cycle of the same kind, noticed by syncellus, is obtained by multiplying by , making , years, which takes into account the defect which the extra hours over have from six. the egyptians, however, did not allow their year to get into so large an error, though it was in error by their using sidereal time, regulating their year, and intercalating days, first according to the risings of the pleiades, and after according to that of sirius, the dog-star, which announced to them the approaching overflowing of the nile, a phenomenon of such great value to egypt that they celebrated it with annual fêtes of the greatest magnificence. among the babylonians, as we are informed by mr. sayce, the year was divided into twelve lunar months and days, an intercalary month being added whenever a certain star, called the "star of stars," or icu, also called dilgan, by the ancient accadians, meaning the "messenger of light," and what is now called aldebaran, which was just in advance of the sun when it crossed the vernal equinox, was not parallel with the moon until the third of nisan, that is, two days after the equinox. they also added shorter months of a few days each when this system became insufficient to keep their calendar correct. they divided their year into four quarters of three months each; the spring quarter not commencing with the beginning of the year when the sun entered the spring equinox, proving that the arrangement of seasons was subsequent to the settling of the calendar. the names of their months were given them from the corresponding signs of the zodiac; which was the same as our own, though the zodiac began with aries and the year with nisan. they too had cycles, but they arose from a very different cause; not from errors of reckoning in the civil year or the revolution of the earth, but from the variations of the weather. every twelve solar years they expected to have the same weather repeated. when we connect this with their observations on the varying brightness of the sun, especially at the commencement of the year on the first of nisan, which they record at one time as "bright yellow" and at another as "spotted," and remember that modern researches have shown that weather is certainly in some way dependent on the solar spots, which have a period _now_ of about eleven years, we cannot help fancying that they were very near to making these discoveries. the year of the ancient persians consisted of days. the extra quarter of a day was not noticed for years, at the end of which they intercalated a month--in the first instance, at the end of the first month, which was thus doubled. at the end of another years they inserted an intercalary month after the second month, and so on through all their twelve months. so that after years the series began again. this period they called the intercalary cycle. the calendar among the greeks was more involved, but more useful. it was _luni-solar_, that is to say, they regulated it at the same time by the revolutions of the moon and the motion about the sun, in the following manner:-- the year commenced with the new moon nearest to the th or st of june, the time of the summer solstice; it was composed in general of twelve months, each of which commenced on the day of the new moon, and which had alternately twenty-nine and thirty days. this arrangement, conformable to the lunar year, only gave days to the civil year, and as this was too short by ten days, twenty-one hours, this difference, by accumulation, produced nearly eighty-seven days at the end of eight years, or three months of twenty-nine days each. to bring the lunar years into accordance with the solstices, it was necessary to add three intercalary months every eight years. the phases of the moon being thus brought into comparison with the rotation of the earth, a cycle was discovered by meton, now known as the metonic cycle, useful also in predicting eclipses, which comprised nineteen years, during which time lunations will have very nearly occurred, and the full moons will return to the same dates. in fact, the year and the lunation are to one another very nearly in the proportion of to . by observing for nineteen years the positions and phases of the moon, they will be found to return again in the same order at the same times, and they can therefore be predicted. this lunar cycle was adopted in the year b.c. to regulate the luni-solar calendar, and it was engraved in letters of gold on the walls of the temple of minerva, from whence comes the name _golden number_, which is given to the number that marks the place of the given year in this period of nineteen. caliphus made a larger and more exact cycle by multiplying by four and taking away one day. thus he made of , days julian years, during which there were lunations. the roman calendar was even more complicated than the greek, and not so good. romulus is said to have given to his subjects a strange arrangement that we can no longer understand. more of a warrior than a philosopher, this founder of rome made the year to consist of ten months, some being of twenty days and others of fifty-five. these unequal lengths were probably regulated by the agricultural works to be done, and by the prevailing religious ideas. after the conclusion of these days they began counting again in the same order; so that the year had only days. the first of these ten months was called _mars_ after the name of the god from whom romulus pretended to have descended. the name of the second, aprilis, was derived from the word _aperire_, to open, because it was at the time that the earth opened; or it may be, from aphrodite, one of the names of venus, the supposed grandmother of Æneas. the third month was consecrated to _maïa_, the mother of mercury. the names of the six others expressed simply their order--quintilis, the fifth; sextilis, the sixth; september, the seventh; and so on. numa added two months to the ten of romulus; one took the name of _januarius_, from _janus_: the name of the other was derived either from the sacrifices (_februalia_), by which the faults committed during the course of the past year were expiated, or from _februo___, the god of the dead, to which the last month was consecrated. the year then had days. these roman months have become our own, and hence a special interest attaches to the consideration of their origin, and the explanation of the manner in which they have been modified and supplemented. each of them was divided into unequal parts, by the days which were known as the calends, nones, and ides. the calends were invariably fixed to the first day of each month; the nones came on the th or th, and the ides the th or th. the romans, looking forward, as children do to festive days, to the fête which came on these particular days, named each day by its distance from the next that was following. immediately after the calends of a month, the dates were referred to the nones, each day being called seven, six, five, and so on days before the nones; on the morrow of the nones they counted to the ides; and so the days at the end of the month always bore the name of the calends of the month following. to complete the confusion the nd day before the fête was called the rd, by counting the fête itself as the st, and so they added one throughout to the number that _we_ should now say expressed our distance from a certain date. since there were thus ten days short in each year, it was soon found necessary to add them on, so a supplementary month was created, which was called mercedonius. this month, by another anomaly, was placed between the rd and th of february. thus, after february rd, came st, nd, rd of mercedonius; and then after the dates of this supplementary month were gone through, the original month was taken up again, and they went on with the th of february. and finally, to complete the medley, the priests who had the charge of regulating this complex calendar, acquitted themselves as badly as they could; by negligence or an arbitrary use of their power they lengthened or shortened the year without any uniform rule. often, indeed, they consulted in this nothing but their own convenience, or the interests of their friends. the disorder which this license had introduced into the calendar proceeded so far that the months had changed from the seasons, those of winter being advanced to the autumn, those of the autumn to the summer. the fêtes were celebrated in seasons different from those in which they were instituted, so that of ceres happened when the wheat was in the blade, and that of bacchus when the raisins were green. julius cæsar, therefore, determined to establish a solar year according to the known period of revolution of the sun, that is days and a quarter. he ordained that each fourth year a day should be intercalated in the place where the month mercedonius used to be inserted, _i.e._ between the rd and th of february. the th of the calends of march in ordinary years was the th of february; it was called _sexto-kalendas_. when an extra day was put in every fourth year before the th, this was a second th day, and was therefore called _bissexto-kalendas_, whence we get the name bissextile, applied to leap year. but it was necessary also to bring back the public fêtes to the seasons they ought to be held in: for this purpose cæsar was obliged to insert in the current year, b.c. (or a.u.c.), two intercalary months beside the month mercedonius. there was, therefore, a year of fifteen months divided into days, and this was called the year of confusion. cæsar gave the strictest injunctions to sosigenes, a celebrated alexandrian astronomer whom he brought to rome for this purpose; and on the same principles flavius was ordered to compose a new calendar, in which all the roman fêtes were entered--following, however, the old method of reckoning the days from the calends, nones, and ides. antonius, after the death of cæsar, changed the name of quintilis, in which julius cæsar was born, into the name _julius_, whence we derive our name july. the name of _augustus_ was given to the month _sextilis_, because the emperor augustus obtained his greatest victories during that month. tiberius, nero, and other imperial monsters attempted to give their names to the other months. but the people had too much independence and sense of justice to accord them such a flattery. the remaining months we have as they were named in the days of numa pompilius. [illustration: fig. .--the roman calendar.] a cubical block of white marble has been found at pompeii which illustrates this very well. each of the four sides is divided into three columns, and on each column is the information about the month. each month is surmounted by the sign of the zodiac through which the sun is passing. beneath the name of the month is inscribed the number of days it contains; the date of the nones, the number of the hours of the day, and of the night; the place of the sun, the divinity under whose protection the month is placed, the agricultural works that are to be done in it, the civil and ecclesiastical ceremonies that are to be performed. these inscriptions are to be seen under the month january to the left of the woodcut. the reform thus introduced by julius cæsar is commonly known as the _julian reform_. the first year in which this calendar was followed was b.c. the julian calendar was in use, without any modification, for a great number of years; nevertheless, the mean value which had been assigned to the civil year being a little different to that of the tropical, a noticeable change at length resulted in the dates in which, each year, the seasons commenced; so that if no remedy had been introduced, the same season would be displaced little by little each year, so as to commence successively in different months. the council of nice, which was held in the year of the christian era, adopted a fixed rule to determine the time at which easter falls. this rule was based on the supposed fact that the spring equinox happened every year on the st of march, as it did at the time of the meeting of the council. this would indeed be the case if the mean value of the civil year of the julian calendar was exactly equal to the tropical year. but while the first is · days, the second is · days; so that the tropical year is too small by minutes and seconds. it follows hence that after the lapse of four julian years the vernal equinox, instead of happening exactly at the same time as it did four years before, will happen minutes seconds too soon; and will gain as much in each succeeding four years. so that at the end of a certain number of years, after the year , the equinox will happen on the th of march, afterwards on the th, and so on. this continual advance notified by the astronomers, determined pope gregory xiii. to introduce a new reform into the calendar. it was in the year that the _gregorian reform_ was put into operation. at that epoch the vernal equinox happened on the th instead of the st of march. to get rid of this advance of ten days that the equinox had made and to bring it back to the original date, pope gregory decided that the day after the th of october, , should be called the th instead of the th. this change only did away with the inconvenience at the time attaching to the julian calendar; it was necessary to make also some modification in the rule which served to determine the lengths of the civil years, in order to avoid the same error for the future. so the pope determined that in each years there should be only bissextile years, instead of , as there used to be in the julian calendar. this made three days taken off the years, and in consequence the mean value of the civil year is reduced to · days, which is not far from the true tropical year. the gregorian year thus obtained is still too great by · of a day; the date of the vernal equinox will still then advance in virtue of this excess, but it is easy to see that the gregorian reform will suffice for a great number of centuries. the method in which it is carried out is as follows:--in the julian calendar each year that divided by four when expressed in its usual way, by a.d., was a leap year, and therefore each year that completed a century was such, as , and so on--but in the gregorian reform, all these century numbers are to be reckoned common years, unless the number without the two cyphers divides by four; thus , will be a common year and , a leap year. it is easy to see that this will leave out three leap years in every years. the gregorian calendar was immediately adopted in france and germany, and a little later in england. now it is in operation in all the christian countries of europe, except russia, where the julian calendar is still followed. it follows that russian dates do not agree with ours. in , the difference was ten days, and this difference remained the same till the end of the seventeenth century, when the year was bissextile in the julian, but not in the gregorian calendar, so the difference increased to eleven days, and now in the same way is twelve days. next to the year, comes the day as the most natural division of time in connection with the earth, though it admits of less difference in its arrangements, as we cannot be mistaken as to its length. it is the natural standard too of our division of time into shorter intervals such as hours, minutes, and seconds. by the word _day_ we mean of course the interval during which the earth makes a complete revolution round itself, while _daytime_ may be used to express the portion of it during which our portion of the earth is towards the sun. the greeks to avoid ambiguity used the word _nyctemere_, meaning night and day. no ancient nation is known that did not divide the day into twenty-four hours, when they divided it at all into such small parts, which seems to show that such a division was comparatively a late institution, and was derived from the invention of a single nation. it would necessarily depend on the possibility of reckoning shorter periods of time than the natural one of the day. in the earliest ages, and even afterwards, the position of the sun in the heavens by day, and the position of the constellations by night, gave approximately the time. instead of asking what "o'clock" is it? the greeks would say, "what star is passing?" the next method of determining time depended on the uniform motion of water from a cistern. it was invented by the egyptians, and was called a clepsydra, and was in use among the babylonians, the greeks, and the romans. the more accurate measurement of time by means of clocks was not introduced till about b.c., when trimalcion had one in his dining chamber. the use of them, however, had been so lost that in a.d. they were considered quite novelties. the clocks, of course, have to be regulated by the sun, an operation which has been the employment of astronomers, among other things, for centuries. each locality had its own time according to the moment when the sun passed the meridian of the place, a moment which was determined by observation. before the introduction of the hour, the day and night appear to have been divided into watches. among the babylonians the night was reckoned from what we call a.m. to p.m., and divided into three watches of four hours each--called the "evening," "middle," and "morning" watch. these were later superseded by the more accurate hour, or rather "double hour" or _casbri_, each of which was divided into sixty minutes and sixty seconds, and the change taking place not earlier than , b.c. whether the babylonians (or accadians) were the inventors of the hour it is difficult to say, though they almost certainly were of other divisions of time. it is remarkable that in the ancient jewish scriptures we find no mention of any such division until the date at which the prophecy of daniel was written, that is, until the jews had come in contact with the babylonians. some nations have counted the twenty-four hours consecutively from one to twenty-four as astronomers do now, but others and the majority have divided the whole period into two of twelve hours each. the time of the commencement of the day has varied much with the different nations. the jews, the ancient athenians, the chinese, and several other peoples, more or less of the past, have commenced the day with the setting of the sun, a custom which perhaps originated with the determination of the commencement of the year, and therefore of the day, by the observation of some stars that were seen at sunset, a custom continued in our memory by the well-known words, "the evening and the morning were the first day." the italians, till recently, counted the hours in a single series, between two settings of the sun. the only gain in such a method would be to sailors, that they might know how many hours they had before night overtook them; the sun always setting at twenty-four o'clock; if the watch marked nineteen or twenty, it would mean they had five or four hours to see by--but such a gain would be very small against the necessity of setting their watches differently every morning, and the inconvenience of never having fixed hours for meals. among the babylonians, syrians, persians, the modern greeks, and inhabitants of the balearic isles, &c., the day commenced with the rising of the sun. nevertheless, among all the astronomical phenomena that may be submitted to observation, none is so liable to uncertainty as the rising and setting of the heavenly bodies, owing among other things to the effects of refraction. among the ancient arabians, followed in this by the author of the _almagesta_, and by ptolemy, the day commenced at noon. modern astronomers adopt this usage. the moment of changing the date is then always marked by a phenomenon easy to observe. lastly, that we may see how every variety possible is sure to be chosen when anything is left to the free choice of men, we know that with the egyptians, hipparchus, the ancient romans, and all the european nations at present, the day begins at midnight. copernicus among the astronomers of our era followed this usage. we may remark that the commencement of the astronomical day commences twelve hours _after_ the civil day. of the various periods composed of several days, the week of seven days is the most widely spread--and of considerable antiquity. yet it is not the universal method of dividing months. among the egyptians the month was divided into periods of ten days each; and we find no sign of the seven days--the several days of the whole month having a god assigned to each. among the hindoos no trace has been found by max müller in their ancient vedic literature of any such division, but the month is divided into two according to the moon; the _clear_ half from the new to the full moon, the _obscure_ half from the full to the new, and a similar division has been found among the aztecs. the chinese divide the month like the egyptians. among the babylonians two methods of dividing the month existed, and both of them from the earliest times. the first method was to separate it into two halves of fifteen days each, and each of these periods into three shorter ones of five days, making six per month. the other method is the week of seven days. the days of the week with them, as they are with many nations now, were named after the sun and moon and the five planets, and the th, th, th, st, and th days of each month--days separated by seven days each omitting the th--were termed "days of rest," on which certain works were forbidden to be done. from this it is plain that we have here all the elements of our modern week. we find it, as is well known, in the earliest of hebrew writings, but without the mark which gives reason for the number seven, that is the names of the seven heavenly bodies. it would seem most probable, then, that we must look to the accadians as the originators of our modern week, from whom the hebrews may have--and, if so, at a very early period--borrowed the idea. it is known that the week was not employed in the ancient calendars of the romans, into which it was afterwards introduced through the medium of the biblical traditions, and became a legal usage under the first christian emperors. from thence it has been propagated together with the julian calendar amongst all the populations that have been subjected to the roman power. we find the period of seven days employed in the astronomical treatises of hindoo writers, but not before the fifth century. dion cassius, in the third century, represents the week as universally spread in his times, and considers it a recent invention which he attributes to the egyptians; meaning thereby, doubtless, the astrologers of the alexandrian school, at that time very eager to spread the abstract speculations of plato and pythagoras. if the names of the days of the week were derived from the planets, the sun and moon, as is easy to see, it is not so clear how they came to have their present order. the original order in which they were supposed to be placed in the various heavens that supported them according to their distance from the earth was thus:--saturn, jupiter, mars, the sun, venus, mercury, the moon. one supposition is that each hour of the day was sacred to one of these, and that each day was named from the god that presided over the first hours. now, as seven goes three times into twenty-four, and leaves three over, it is plain that if saturn began the first hour of saturday, the next day would begin with the planet three further on in the series, which would bring us to the sun for sunday, three more would bring us next day to the moon for monday, and so to mars for tuesday, to mercury for wednesday, to jupiter for thursday, to venus for friday, and so round again to saturn for saturday. the same method is illustrated by putting the symbols in order round the circumference of a circle, and joining them by lines to the one most opposite, following always in the same order as in the following figure. we arrive in this way at the order of the days of the week. [illustration: fig. .] all the nations who have adopted the week have not kept to the same names for them, but have varied them according to taste. thus sunday was changed by the christian church to the "lord's day," a name it still partially retains among ourselves, but which is the regular name among several continental nations, including the corrupted _dimanche_ of the french. the four middle days have also been very largely changed, as they have been among ourselves and most northern nations to commemorate the names of the great scandinavian gods tuesco, woden, thor, and friga. this change was no doubt due to the old mythology of the druids being amalgamated with the new method of collecting the days into weeks. we give below a general table of the names of the days of the week in several different languages. +------------+-----------+------------+------------+------------------+ | english. | french. | italian. | spanish. | portuguese. | +------------+-----------+------------+------------+------------------+ | sunday. | dimanche. | domenica. | domingo. | domingo. | | monday. | lundi. | lunedi. | luneo. | secunda feira. | | tuesday. | mardi. | marteti. | martes. | terça feira. | | wednesday. | mercredi | mercoledi. | miercoles. | quarta feira. | | thursday. | jeudi. | giovedi. | jueves. | quinta feira. | | friday. | vendredi. | venerdi. | viernes. | sexta feira. | | saturday. | samedi. | sabbato. | sabado. | sabbado. | +------------+-----------+------------+------------+------------------+ +------------+--------------+-------------+---------------+-----------+ | german. | anglo-saxon. | ancient | ancient | dutch. | | | | frisian. | northmen. | | +------------+--------------+-------------+---------------+-----------+ | sonntag. | sonnan däg. | sonna dei. | sunnu dagr. | zondag. | | montag. | monan däg. | mona dei. | mâna dagr. | maandag. | | dienstag. | tives däg. | tys dei. | tyrs dagr. | dingsdag. | | mitwoch. | vôdenes däg. | werns dei. | odins dagr. | woensdag. | | donnerstag.| thunores däg.| thunres dei.| thors dagr. | donderdag.| | freitag. | frige däg. | frigen dei. | fria dagr. | vrijdag. | | samstag. | soeternes | sater dei. | laugar dagr | zaturdag. | | | däg. | | (washing day)| | +------------+--------------+-------------+---------------+-----------+ the cycle which must be completed with the present calendar to bring the same day of the year to the same day of the week, is twenty-eight years, since there is one day over every ordinary year, and two every leap year; which will make an overlapping of days which, except at the centuries, will go through all the changes in twenty-eight times, which forms what is called the solar cycle. there is but one more point that will be interesting about the calendar, namely, the date from which we reckon our years. among the jews it was from the creation of the world, as recorded in their sacred books--but no one can determine when that was with sufficient accuracy to make it represent anything but an agreement of the present day. different interpreters do not come within a thousand years of one another for its supposed date; although some of them have determined it very accurately to their own satisfaction--one going so far as to say that creation finished at nine o'clock one sunday morning! in other cases the date has been reckoned from national events--as in the olympiads, the foundation of rome, &c. the word we now use, Æra, points to a particular date from which to reckon, since it is composed of the initials of the words ab exordio regni augusti "from the commencement of the reign of augustus." at the present day the point of departure, both forwards and backwards, is the year of the birth of jesus christ--a date which is itself controverted, and the use of which did not exist among the first christians. they exhibited great indifference, for many centuries, as to the year in which jesus christ entered the world. it was a monk who lived in obscurity at rome, about the year , who was a native of so unknown a country that he has been called a scythian, and whose name was denys, surnamed _exiguus_, or the little, who first attempted to discover by chronological calculations the year of the birth of jesus christ. the era of denys the little was not adopted by his contemporaries. two centuries afterwards, the venerable bede exhorted christians to make use of it--and it only came into general use about the year . among those who adopted the christian era, some made the year commence with march, which was the first month of the year of romulus; others in january, which commences the year of numa; others commenced on christmas day; and others on lady day, march . another form of nominal year was that which commenced with easter day, in which case, the festival being a movable one, some years were shorter than others, and in some years there might be two nd, rd, &c., of april, if easter fell in one year on the nd, and next year a few days later. the st of january was made to begin the year in germany in . an edict of charles ix. prescribes the same in france in . but it was not till that the change was made in england by lord chesterfield's act. the year , as the year that had preceded it, began on march th, and it should have lasted till the next lady day; but according to the act, the months of january, february, and part of march were to be reckoned as part of the year . by this means the unthinking seemed to have grown old suddenly by three months, and popular clamour was raised against the promoter of the bill, and cries raised of "give us our three months." such have been the various changes that our calendar has undergone to bring it to its present state. chapter xv. the end of the world. perhaps the most anxious question that has been asked of the astronomer is when the world is to come to an end. it is a question which, of course, he has no power to answer with truth; but it is also one that has often been answered in good faith. it has perhaps been somewhat natural to ask such a question of an astronomer, partly because his science naturally deals with the structure of the universe, which might give some light as to its future, and partly because of his connection with astrology, whose province it was supposed to be to open the destiny of all things. yet the question has been answered by others than by astronomers, on grounds connected with their faith. in the early ages of the church, the belief in the rapid approach of the end of the world was universally spread amongst christians. the apocalypse of st. john and the acts of the apostles seemed to announce its coming before that generation passed away. afterwards, it was expected at the year ; and though these beliefs did not rest in any way on astronomical grounds, yet to that science was recourse had for encouragement or discouragement of the idea. the middle ages, fall of simple faith and superstitious credulity, were filled with fear of this terrible catastrophe. as the year approached, the warnings became frequent and very pressing. thus, for example, bernard of thuringia, about , began to announce publicly that the world was about to end, declaring that he had had a particular revelation of the fact. he took for his text the enigmatical words of the apocalypse: "at the end of one thousand years, satan shall be loosed from his prison, and shall seduce the people that are in the four quarters of the earth. the book of life shall be open, and the sea shall give up her dead." he fixed the day when the annunciation of the virgin should coincide with good friday as the end of all things. this happened in , but nothing extraordinary happened. during the tenth century the royal proclamations opened by this characteristic phrase: _whereas the end of the world is approaching_.... in the astrologers frightened europe by announcing a conjunction of all the planets. rigord, a writer of that period, says in his _life of philip augustus_: "the astrologers of the east, jews, saracens, and even christians, sent letters all over the world, in which they predicted, with perfect assurance, that in the month of september there would be great tempests, earthquakes, mortality among men, seditions and discords, revolutions in kingdoms, and the destruction of all things. but," he adds, "the event very soon belied their predictions." some years after, in , another alarm of the end of the world was raised, but this time it was not dependent on celestial phenomena. it was said that antichrist was born in babylon, and therefore all the human race would be destroyed. it would be a curious list to make of all the years in which it was said that antichrist was born; they might be counted by hundreds, to say nothing of the future. at the commencement of the fourteenth century, the alchemist arnault of villeneuve announced the end of the world for . in his treatise _de sigillis_ he applies the influence of the stars to alchemy, and expounds the mystical formula by which demons are to be conjured. st. vincent ferrier, a famous spanish preacher, gave to the world as many years' duration as there were verses in the psalms--about . the sixteenth century produced a very plentiful crop of predictions of the final catastrophe. simon goulart, for example, gave the world an appalling account of terrible sights seen in assyria--where a mountain opened and showed a scroll with letters of greek--"the end of the world is coming." this was in ; but after that year had passed in safety, leovitius, a famous astrologer, predicted it again for . louis gayon reports that the fright at this time was great. the churches could not hold those who sought a refuge in them, and a great number made their wills, without reflecting that there was no use in it if the whole world was to finish. one of the most famous mathematicians of europe, named stoffler, who flourished in the th century, and who worked for a long time at the reform of the calendar proposed by the council of constance, predicted a universal deluge for . this deluge was to happen in the month of february, because saturn, jupiter, and mars were then together in the sign of the fishes. everyone in europe, asia, and africa, to whom these tidings came, was in a state of consternation. they expected a deluge, in spite of the rainbow. many contemporary authors report that the inhabitants of the maritime provinces of germany sold their lands for a mere trifle to those who had more money and less credulity. each built himself a boat like an ark. a doctor of toulouse, named auriol, made a very large ark for himself, his family, and his friends, and the same precautions were taken by a great many people in italy. at last the month of february came, and not a drop of rain fell. never was a drier month or a more puzzled set of astrologers. nevertheless they were not discouraged nor neglected for all that, and stoffler himself, associated with the celebrated regiomontanus, predicted once more that the end of the world would come in , or at least that there would be frightful events which would overturn the earth. this new prediction was a new deception; nothing extraordinary occurred in . the year , however, witnessed a strange phenomenon, capable of justifying all their fears. an unknown star came suddenly into view in the constellation of cassiopeia, so brilliant that it was visible even in full daylight, and the astrologers calculated that it was the star of the magi which had returned, and that it announced the second coming of jesus christ. the seventeenth and eighteenth centuries were filled with new predictions of great variety. even our own century has not been without such. a religious work, published in , by the count sallmard montfort, demonstrated perfectly that the world had no more than ten years to exist. "the world," he said, "is old, and its time of ending is near, and i believe that the epoch of that terrible event is not far off. jacob, the chief of the twelve tribes of israel, and consequently of the ancient church, was born in of the world, _i.e._, b.c. the ancient church, which was the figure of the new, lasted years. hence the new one will only last till a.d." similar prophecies by persons of various nations have in like manner been made, without being fulfilled. indeed, we have had our own prophets; but they have proved themselves incredulous of their own predictions, by taking leases that should _commence_ in the year of the world's destruction. but we have one in store for us yet. in , pierre louis of paris calculated that the end would be in , and he calculated in this way:--the apocalypse says the gentiles shall occupy the holy city for forty-two months. the holy city was taken by omar in . forty-two months of years is , which brings the return of the jews to , which will precede by a few years the final catastrophe. daniel also announces the arrival of antichrist , days after the establishment of artaxerxes on the throne of persia, b.c., which again brings us to . some again have put it at a.d., which will make , years, as they think, from the creation; these are the days of work; then comes the , years of millennial sabbath. we are led far away by these vain speculations from the wholesome study of astronomy; they are useful only in showing how by a little latitude that science may wind itself into all the questions that in any way affect the earth. indeed, since the world began, the world will doubtless end, and astronomers are still asked how could it be brought about? certainly it is not an impossible event, and there are only too many ways in which it has been imagined it might occur. the question is one that stands on a very different footing from that it occupied before the days of galileo and copernicus. _then_ the earth was believed to be the centre of the universe, and all the heavens and stars created for it. _then_ the commencement of the world was the commencement of the universe, its destruction would be the destruction of all. _now_, thanks to the revolution in feeling that has been accomplished by the progress of astronomy, we have learned our own insignificance, and that amongst the infinite number of stars, each supporting their own system of inhabited planets, our earth occupies an infinitesimally small portion, and the destruction of it would make no difference whatever--still less its becoming uninhabitable. it is an event which must have happened and be happening to other worlds, without affecting the infinite life of the universe in any marked degree. nevertheless, for ourselves, the question remains as interesting as if we were the all in all, but must be approached in a different manner. numerous hypotheses have been put forth on the question but they may mostly be dismissed as vain. buffon calculated that it had taken , years for the earth to cool down to its present temperature, and that it will take , years more before it would be too cold for men to live upon it. but sir william thomson has shown that the internal heat of the earth, supposed to be due to its cooling from fusion, cannot have seriously modified climate for a long series of years, and that life depends essentially on the heat of the sun. another hypothesis, the most ancient of all, is that which supposes the earth will be destroyed by fire. it comes down from zoroaster and the jews; and on the improbable supposition of the thin crust of the earth over a molten mass, this is thought possible. however, as the tendency in the past has been all the other way, namely, to make the effect of the inner heat of the earth less marked on the surface, we have no reason to expect a reversal. a third theory would make the earth die more gradually and more surely. it is known that by the wearing down of the surface by the rains and rivers, there is a tendency to reduce mountains and all high parts of the earth to a uniform level, a tendency which is only counteracted by some elevating force within the earth. if these elevating forces be supposed to be due to the internal heat--a hypothesis which cannot be proved--then with the cooling of the earth the elevating forces would cease, and, finally, the whole of the continent would be brought beneath the sea and terrestrial life perish. another interesting but groundless hypothesis is that of adhémar on the periodicity of deluges. this theory depends on the fact of the unequal length of the seasons in the two hemispheres. our autumn and our winter last days. in the southern hemisphere they last days. these seven days, or hours, of difference, increase each year the coldness of the pole. during , years the ice accumulates at one pole and melts at the other, thereby displacing the earth's centre of gravity. now a time will arrive when, after the maximum of elevation of temperature on one side, a catastrophe will happen, which will bring back the centre of gravity to the centre of figure, and cause an immense deluge. the deluge of the north pole was , years ago, therefore the next will be , hence. it is very obvious to ask on this--_why_ should there be a _catastrophe_? and why should not the centre of gravity return _gradually_ as it was gradually displaced? another theory has been that it would perish by a comet. that it will not be by the shock we have already seen from the light weight of the comet and from experience; but it has been suggested that the gas may combine with the air, and an explosion take place that would destroy us all; but is not that also contradicted by experience? another idea is that we shall finally fall into the sun by the resistance of the ether to our motion. encke's comet loses in thirty-three years a thousandth part of its velocity. it appears then that we should have to wait millions of centuries before we came too near the sun. in reality, however, we are simply dependent on our sun, and our destiny depends upon that. in the first place, in its voyage through space it might encounter or come within the range of some dark body we at present know nothing of, and the attraction might put out of harmony all our solar system with calamitous results. or since we are aware that the sun is a radiating body giving out its heat on all sides, and therefore growing colder, it may one day happen that it will be too cold to sustain life on the earth. it is, we know, a variable star, and stars have been seen to disappear, or even to have a catastrophe happen to them, as the kindling of enormous quantities of gas. a catastrophe in the sun will be our own end. fontenelle has amusingly described in verse the result of the sun growing cold, which may be thus englished:-- "of this, though, i haven't a doubt, one day when there isn't much light, the poor little sun will go out and bid us politely--good-night. look out from the stars up on high, some other to help you to see; i can't shine any longer, not i, since shining don't benefit me. "then down on our poor habitation what numberless evils will fall, when the heavens demand liquidation, why all will go smash, and then all society come to an end. soon out of the sleepy affair his way will each traveller wend, no testament leaving, nor heir." the cooling of the sun must, however, take place very gradually, as no cooling has been perceived during the existence of man; and the growth of plants in the earliest geological ages, and the life of animals, prove that for so long a time it has been within the limits within which life has been possible--and we may look forward to as long in the future. it is not of course the time when the sun will become a dark ball, surrounded by illuminated planets, that we must reckon as the end of the earth. life would have ceased long before that stage--no man will witness the death of the sun. [illustration: plate xvi.--the end of the world.] the diminution of the sun's heat would have for its natural effect the enlargement of the glacial zones! the sea and the land in those parts of the earth would cease to support life, which would gradually be drawn closer to the equatorial belt. man, who by his nature and his intelligence is best fitted to withstand cold climates, would remain among the last of the inhabitants, reduced to the most miserable nourishment. drawn together round the equator, the last of the sons of earth would wage a last combat with death, and exactly as the shades approached, would the human genius, fortified by all the acquirements of ages past--give out its brightest light, and attempt in vain to throw off the fatal cover that was destined to engulf him. at last, the earth, fading, dry, and sterile, would become an immense cemetery. and it would be the same with the other planets. the sun, already become red, would at last become black, and the planetary system would be an assemblage of black balls revolving round a larger black ball. of course this is all imaginary, and cannot affect ourselves, but the very idea of it is melancholy, and enough to justify the words of campbell:-- "for this hath science searched on weary wing by shore and sea--each mute and living thing, or round the cope her living chariot driven and wheeled in triumph through the signs of heaven. oh, star-eyed science, hast thou wandered there to waft us home the message of despair?" in reality, as we know nothing of the origin, so we know nothing of the end of the world; and where so much has been accomplished, there are obviously infinite possibilities enough to satisfy the hopes of every one. while some stars may be fading, others may be rising into their place, and man need not be identified with one earth alone, but may rest content in the idea that the life universal is eternal. the end. london: p. clay, sons, and taylor, printers. transcriber's notes: . passages in italics are surrounded by _underscores_. . images have been moved from the middle of a paragraph to the closest paragraph break. . the original text includes greek characters. for this text version these letters have been replaced with transliterations, for example, [greek: a] represents first greek letter alpha. . the original text includes certain symbols for planets and zodiac signs. for this text version these symbols are replaced by text name of the corresponding symbol. for example, [symbol: sun] replaces the symbolic representation of sun. . in this text version, fractions are represented using hyphen and forward slash. for example, - / stands for three and a half. . certain words use oe ligature in the original. . obvious errors in punctuation and a few misprints have been silently corrected. . other than the corrections listed above, printer's inconsistencies in hyphenation and ligature usage have been retained. [ transcriber's note: every effort has been made to replicate this text as faithfully as possible, including inconsistencies in spelling and hyphenation. some corrections of spelling and punctuation have been made. they are listed at the end of the text. italic text has been marked with _underscores_. bold text has been marked with =equals signs=. text marked ^{thus} was superscripted. ] [illustration] the sun changes its position in space, therefore it cannot be regarded as being "in a condition of rest." _si concedimus, eos, qui corpora in mundi spatio moveri eademque non moveri posse dicunt, insulsa loqui, praesumere non licet hominem astronomum talem sententiam elocuturum utque eam demonstraret operam daturum esse._ by august tischner. leipzig, gustav fock. . dedicated to all friends of rational astronomy. [illustration: _nicolaus copernicus._ _terrae motor, solis stator._] the system of copernicus is the only possible system; it is the eternal base of all astronomical progress, with this system the science of astronomy stands and falls, and without it we must give up all explication as well as every scientifically founded predication. hence it is clear that an astronomer of the present day cannot enter upon any other system, even by way of trial. dr. _j. h. mädler_. popul. astr. . p.p. . . . _an army of philosophers will not suffice to change the nature of an error and to convert it into truth. ebn-roshd (averrhoës), arabian philosopher of the xii^{th} century._ astronomical science, at the present day insists upon the system of copernicus, which, as is well known, is based upon the theory _of a fixed sun_, and remains convinced of the incontrovertible truth and importance of this system, even after it has become an incontestable fact, that the sun changes its position; endeavouring to explain away this discrepancy by the sophism, that the sun may be considered as _in a condition of rest_. but the smallest movement of the sun overthrows the entire fabric of copernicus. unless we take into account the observations, made for the last years, respecting the movement of the sun in space, it is impossible to comprehend the solar system and its movements. theory must take notice of the phenomena of the sun's own movement and dare not cloak it under imaginary causes; for so long as the motion of the sun is ignored, it is impossible to know thoroughly the motion of the earth which follows it, and if the motion of the earth be not known, it is also impossible to know the motion of the other heavenly bodies, belonging to the solar system, as seen from the earth. in a word, the astronomical theory, as it is now generally accepted and believed to be the only and doubtless true, is wholly untenable, requiring _a total and essential_ reformation; astronomical authors cling to j. h. mädler's assertion, that every body will understand the impossibility for an astronomer of our time to enter upon any other system even by way of trial. if this theory be converted into a _dogma_, stagnation must commence and all progress becomes impossible. in the history of science and its advance, we find that there have been at all times new theories propounded, which had often to be changed later on, or even set aside by others diametrically opposite. the principal circumstance which renders the system of copernicus impossible, is that the orbits of the planets _are considered as closed curves around the sun_. this view has frequently been attacked; but it is maintained by astronomers, as it is requisite for the elucidation of the system. still it is evident that if the centre of attraction moves forward the bodies attracted by it _cannot move around it_. let us examine the system of copernicus. ptolemæus first introduced his system among the ancients. the earth was the fixed centre of the world and around it moved the moon, the sun, the planets and the stars. this system lasted for xv centuries. the ptolemaic system was modified by copernicus, and the system of copernicus was simply the inversion of the ptolemaic. the sun took the place of the earth. in the centre was a fixed point (earth or sun), around which the planets moved in larger or smaller orbits. the main feature of both systems is that one of the heavenly bodies is _stationary, in order that the others may travel round it_. copernicus makes the sun _to be motionless_, and the scientific world bows before his authority. then we have the recurrent curves, _closed orbits_ (or ellipses) with their axes and their _invariable plains_; for the planets _move round the centre of the fixed sun_. whilst however learned men were striving with feverish ardour to confirm the system of copernicus; whilst they were endeavouring to demonstrate in every possible way and by various means clearly, _that the sun is immoveable_: there came the discovery _that the sun moves_. the astronomers of the past century proved that the sun not only has the apparent motion, which every one sees; but that it also has a motion proper to itself. herschel commenced defining the course and direction of it, and now-a-days no one doubts the truth of this fact, it being the general opinion that not only the sun moves itself, but that nothing at all in the world is in a state of rest. astronomers, however, are of opinion that this discovery is of _no consequence whatever as regards the system of copernicus, which is still considered by them to be the most correct of all and the only possible one_. for more than a century there has not been found a single astronomer or scientific man, to whom it has occurred _that the motion proper to the sun, might have, in some way or another, an influence on the present state of theoretical science_. they all seem to regard _this fact_ as an accident, involving no consequences and quite incapable of distracting them from their labours, which they continue to work in the same manner as is indicated in the system of copernicus. if an advancing motion is admitted to be the motion proper to the sun, _the orbits traversed by the planets cannot be closed_. but the question may be asked: is it true that science contradicts itself in this way? we reply: yes! astronomical _observation has overtaken theoretical or explicative science_. _theory has stood still._ in order to set their minds at rest, learned men explain what they wish to explain, and just as heavenly phenomena were accounted for according the systems of ptolemæus, of copernicus and of tycho de brahe, so too there will be no lack of good reasons to account for the motion proper to the sun; only history will tell us that the astronomers of the last but one decennium of the xix^{th} century have taught by writing and speaking in their schools, that the sun is at the same time moving and not moving. a science which cannot make any use of this immense discovery, nor deduce any application from it, does not possess any vital power; it is a dead science, it is strangled by those whose duty is to keep it alive, to lead it onwards to perfection. astronomers assert "_that the sun conducts its system with himself in mundane space_," but in the same breath they add: "_with reference however to the planets it may be regarded as in a state of rest_." hence astronomers have discovered _a motion which is at rest_. if the sun is _not fixed_, the system of copernicus falls to ground. either the sun moves, or does not; a moving sun in a condition of rest, _is an impossibility_. if the sun moves, there is _no fixed centre_, there are _no closed or recurrent curves and no plains of orbits_. if these must be obtained at any price, the sun must be definitively fixed, it cannot be permitted _to move onwards and yet at the same time not to move_. the fact that the sun moves, cannot now be altered and cannot be any longer ignored; and if mathematicians and astronomers do notwithstanding assert, that the sun may with reference to its own planetary system be regarded as fixed, or in a condition of rest, in that the system moves as a whole without any change taking place in the relative position of the planets to each other, or in their relation to the sun; in fact without any alteration taking place in the _configuration_ of the system--we reply, this is one of those meaningless phrases, which should find no place in a scientific discussion. _a body which is in motion cannot be in any way regarded as being motionless_, it would be just as reasonable to say that a locomotive, dragging a train of carriages full of passengers, could with reference to the latter be regarded as motionless. the actual meaning of such an assertion is that the planets are attached to the sun in such a manner, that they can neither approach to, nor recede from it, but must follow it whithersoever it goes. we may in thought pursue a train of hypotheses and suppositions, but they do not thereby acquire reality; still, in a normal condition of the human intellect, it is impossible to conceive that any thing can exist and not exist at the same time. from this confusion of ideas, it might seem as if theoretical astronomy had got into an untenable position which is irreconcilable with science and ought therefore to endeavour to enter upon a better state, as soon as possible. _theory ought therefore, either to have accepted as a fact, the motion proper to the sun with all its inevitable consequences, or else, to have denied this motion altogether._ but the astronomers ignore this alternative, they have decided, once for all and irrevocably _that the sun moves and that at the same time it shall be motionless_. in this manner science loses its reputation and all learnedly technical expressions and formulas are not sufficient to cover the weak part. _the sun cannot be rendered motionless_, and if astronomers and men of science of the present day continue to ignore this fact, they need not be surprised at the inevitable consequences of their own acts. the system of copernicus presupposes the _fixity of the sun_, as a "conditio sine qua non." the most abstruse investigations into the "celestial mechanism" could not be made without this axiom be granted. the mathematician must have a fixed point, a fixed central point of action for his coordinates, he wants fixed invariable plains and closed curves, a radius vector describing plains, he wants axes and poles for the orbits, in order that they may describe certain figures in the heaven, and that the plains of the orbits may move,--one of the other. naturally astronomers and men of science have never asked themselves the question, _how a heavenly body could be fixed in space_. when an astronomer asserts that the copernician system is the only possible, he believes that it is impossible for the sun to have any motion of its own; when he at the same time asserts that all astronomy stands or falls with this system, he believes that no astronomical knowledge existed before the discovery of the copernician system, and with the fall of the system all astronomical knowledge will cease to exist; he believes moreover true astronomy to be _that_, which men of science have imagined to be the truth regarding the heaven and the causes of the phenomena we see. if astronomers had merely presented their ideas and opinions to the world as such, and no more, no one could raise any objection; but they lay down their opinions in words and on paper as a _positive science_, they give their view as _incontroversible truths_, and _this fact_ alters the situation, for we cannot admit that science is a mere barge to be taken in tow by the imagination. the fundamental axiom of astronomical theory, such as the copernician system, kepler's and newton's laws, _are not derived from a knowledge of fact_, they are the opinions, views, ideas and suppositions of individuals, which have been adapted to the heaven, and as they were generally accepted, the question was never raised whether the opinions of an organic creature--however intelligent it might be--are really and truly that which we term penetrating behind the veil of nature and compelling it to yield up its secrets. the fact of no other ideas being at hand which seemed to be better, sufficed to transform these opinions into rules and to cause them to be accepted as the only admissible and correct truths. the opinions set forth by copernicus, kepler and newton are designed by astronomers of the present day under the collective title of the copernician system, and they believe that these three dogmas, systems and laws, distinct as they are from each other, proceed consequentially one from the other, that they mutually supplement each the other, and thus form a harmonious whole. that not one of these things rests upon actual observation or even probable and perceptible facts, and finally, that none of them can be observed or verified, but that they are all three creations of the imagination, must be clearly evident to any one who occupies himself at all with the study of nature and more especially with the study of the heavenly phenomena. when we say that astronomy is an earthly science, we mean to imply that the heaven and the phenomena there apparent cannot be studied otherwise than as seen from the earth. therefore astronomy is not a heavenly science, it consists solely of such ideas as we are able to form, that which we see on the heaven. it is not astronomy that is grand, compared with the vast objects with which it deals it dwindles to insignificance, and we may say that to speak of it as being a science of the "heavenly mechanism," nay more of the "laws of the universe," is sheer nonsense. the _universe_ must be for us a mere term, which does not convey any tangible idea to our minds. as only a very small portion of the heavenly space and its contents is visible to our eyes, astronomy--whatever may be the magnifying power placed at its disposal--must be confined within the limits of our vision and can therefore be no more than a small fragment. in the positive sense of the word, astronomy is more especially a science of _observation_, which is its _only_, but real and successful power. it may be said that astronomy has raised observation to a science, and its immense importance becomes more and more prominent as the explicative science loses in value; which is the more easily accounted for by the fact that observation will finally bring about the overthrow of all untenable theories. we see the heaven as we fly along, the earth whirls us with itself through space, hence astronomy cannot make any drawing room experiment, it cannot reproduce any of the heavenly phenomena, it can do nothing but _observe_. if therefore the science of astronomy be more especially an observative science, that which it does not and cannot observe, must be for it as good as not existent. but astronomy may, in addition, be designated _the science of observation of the apparent things_, things as they seem to be, for it is unable to see or regard the heavenly phenomena otherwise than they present themselves to it. _astronomy is not permitted to observe realities._ if therefore _observation is itself a science_, it must necessarily _be the basis of theory_; observation may be set aside--which is what is actually done--in this way we may plod on, we may term our labour what we please; but whatever is produced in this way is not astronomy. but that glorious science whose sublime object is alone able to unfetter the mind of poor humanity--astronomy--has a future before it. any such as feel themselves called upon to study _seriously_ the phenomena of nature, may set about the task. _the sun is a sure guide._ the great mass of astronomical observations are almost exclusively of european origin, those which in later times have been made in other parts of the earth, are of a special character--they refer for the most part to the stars and are not numerous enough to furnish any general view, but here the question is of establishing a universal astronomy available for the whole earth, which, founded on the actual type of the phenomena, will become the result of science. with respect to astronomical knowledge and its dissemination, the discovery and proving of this type of the phenomena is of the greatest importance, they must be found out not by calculation, but _by actual observation_. when discovered, a large number of important and still undecided problems will be advanced towards solution. it may be asked: how and where shall we however find this _original type_? and the earth itself supplies the answer by means of its--=equator=. no observer, placed either north or south the equator, can see the two poles of the heaven at once, he cannot see the _whole heavenly sphere_; at the equator the entire splendour of the firmament passes before his eyes during the space of-- hours. the _equator of the earth_ is always turned towards the sun, and it thus indicates the direction taken by our planet; therefore we must be able to find this type _at the equator_. either it is there, or it is nowhere else, and it is indispensably necessary that astronomical observations made elsewhere should be repeated at the equator thus as it were confirmed. the erection of small, simple and detached observations along the line of the earth's equator, at certain distances from each other, and the subdivision of the work amongst the various observers, according the objects, would be of incalculable consequence, and would in the course of a few years shed more light upon astronomical knowledge than all that has hitherto been done at hap-hazard and without any plan. an international scientific society could take the matter in hand. instruments of the most excellent kind are to be had in plenty, and there is no lack of young and intelligent men. moreover, ever since there has been established at quito, the "observatorio de collegio nacional," the director of which mr. g. b. menton might superintend the preliminary operations until such time as the work could be prosecuted with greater resources and according to a well considered plan. such men as _lick_, _bischoffsheim_, _remeis_ _etc._, who are willing to make sufficient sacrifices in order to establish this glorious science upon more solid foundations, which do not rest on an imaginary and untenable theory, _but on actual observation_, will surely be found. success cannot be doubtful. would not the americans, who appreciate every thing on a grand scale and are not afraid of any expense in their undertakings, do all in their power to further and promote this splendid work?[ ] if--as is well known--matters are not as they are assumed to be, to what purpose have been and are these laborious works prosecuted and the undying works written? if the imaginary is preferred to reality, we set up an imaginary science, without knowing anything about the heaven, and the science thus set up will become the plaything of fancy. if they inquire, why theory denies reality--_the motion of the sun_--we shall find that it is because it prefers the imaginary. _the sun in motion_ destroys the found illusions of the astronomers, this they will not submit to, their _untenable theory_ must continue to be looked upon as unadulteratest truth, and the consequence is that the manifestations of the grand and sublime nature are put down as lies. this idea _of a fixed sun_ has taken such a firm hold of men's minds that there is no force in nature capable of exercising sufficient power to eradicate it, the sun may move as it pleases, and whilst the whither and rapidity of its motion are diligently studied, men's minds are occupied _with its fixity_, and these "investigations and inquiries" are prosecuted without any consequences being therefrom deduced. directly a theory or a law is to be set up, the sun is at once _very firmly fixed_ on--=ether=. astronomical writers consider that they have done quite enough, when they have accorded honorable mention to the motion of the sun, _but their deductions, conclusions, theories, proofs and laws are all based on the immobility of the sun, according the system of copernicus_. the idea _that the motion of the sun_ does not necessitate any alteration in the system of copernicus leads us to the utmost absurdity. if the earth is to move in the _invariable plain of its recurrent and closed ellipse_, it stands to reason, it cannot follow the sun, and the "circulation around the centre" at once falls to the ground. it is a very remarkable fact, that the astronomers of the by-gone century could, and those of the present century can believe, such as copernicus, kepler and newton, had they been aware of the motion of the sun, would have set up the same system, the same laws and theories, _as they based exclusively on the theory of its being immoveable_. this fact is one of which we are right to be ashamed. the astronomers hug themselves, with great complacency, with the idea--which gradually becomes a delicious certainly--that they have mapped out the heaven very well, and that any change in the arrangement is a thing not to be thought of. if therefore any one of their fellows should get up--which has sometimes occurred--and say: "it is high time that we should clear up the science and subject this untenable theory to a strict examination and test," the immense majority of facultists and authorities proclaim unanimously "=non possumus=," which is after all but a lingual verification of the first law of the nature[ ]. * * * * * why is it that the astronomers of the present day are unwilling to take into consideration and to study the consequences arising from the motion proper to the sun, with reference to its own system? why is it that they are unwilling to recognise or rather to grasp properly and to explain the apperceivable phenomena, which the motion proper to the sun, as seen from the surface of the earth, must produce on the apparently hollow sphere of the heaven? monter d'une échoppe à un palais, c'est rare et beau; monter de l'erreur à la vérité, c'est plus rare et c'est plus beau. _victor hugo._ il arrive fréquemment que la croyance universelle d'un siècle, croyance dont il n'était donné à personne de s'affranchir à moins d'un effort extraordinaire de génie et de courage, devient pour un autre siècle une absurdité si palpable qu'on n'a plus qu'à s'étonner qu'elle ait pu jamais prévaloir. _n. tschernychewsky._ litterature. . =sta, sol, ne moveare.= _august tischner._ leipzig - . gustav fock. . =grösse, entfernung und masse der sonne.= _august tischner._ leipzig . gustav fock. . =die sonne und die astronomie.= _k. nagy._ leipzig . f. a. brockhaus. . =memoire sur le système solaire et sur l'explication des phénomènes célestes.= _charles nagy._ paris . leibner. . =considération sur les comètes, éléments de cométologie.= _charles nagy._ paris . leibner. . =système solaire d'après la marche réelle du soleil.= _e. g. fahrner._ paris ^{me} éd. . . =das wahre sonnensystem.= bewegung und bahnen der gestirne nach einer neuen auffassung über dieselben im himmelsraume, und zwar welche nicht in ellipsen statt hat. _james hermann milberg._ münchen . . =die wahre gestalt der planeten- und kometenbahnen.= _friedrick carl gustav stieber._ dresden . . =die sonne bewegt sich.= folgerungen aus dieser lehre in bezug auf die fixsterne und planeten. _c. r.(ohrbach)._ berlin . . =ueber veranschaulichungsmittel für mathematische geographie.= erläuternde beigabe zu neu construirten veranschaulichungsapparaten für volksschulen und höhere unterrichtsanstalten. _f. a. püschmann_, seminaroberlehrer, grimma. . =der himmels-mechanik gänzliche reform auf grund der inductiven logik= mit der strengberechtigten philosophischen und mathematischen nachweisung. _v. p. kluk-kluczycky._ . g. kreysing, leipzig. [illustration] footnotes: [ ] moreover, other, smaller detached observatories, might be erected on the east and west coasts of america and africa, on the islands of sumatra, borneo, celebes and gilolo, on one of the islands of gilbert's archipelago and upon one of the gallopagos islands, if it be considered worth the effort to acquire some real knowledge as to the movement in space of the leader of our planetary system and the bodies pertaining to it. [ ] inertia is the most simple and most natural (sic) law of nature which can be imagined. laplace i p. . [ the following is a list of changes made to the original. the first line is the original line, the second the changed one. copernicus makes the sun _to be motienless_, copernicus makes the sun _to be motionless_, mauner as is indicated in the system of manner as is indicated in the system of ideas being at hand which seemed be to better, ideas being at hand which seemed to be better, power. if may be said that astronomy has power. it may be said that astronomy has upon to sludy _seriously_ the phenomena of upon to study _seriously_ the phenomena of for the whole earth, which, founded of the for the whole earth, which, founded on the and the subdivision of the work amangst the and the subdivision of the work amongst the if the imaginary is prefered to reality, we if the imaginary is preferred to reality, we celebes and gilolo, on one of the islands ol gilbert's celebes and gilolo, on one of the islands of gilbert's or rather to graph propery and to explain or rather to grasp properly and to explain ] man's place in the universe man's place in the universe a study of the results of scientific research in relation to the unity or plurality of worlds by alfred r. wallace ll.d., d.c.l., f.r.s., etc. 'o, glittering host! o, golden line! i would i had an angel's ken, your deepest secrets to divine, and read your mysteries to men.' _third edition_ london chapman and hall limited 'i said unto my inmost heart, shall i don corslet, helm, and shield, and shall i with a giant strive, and charge a dragon on the field?' j.h. dell. preface this work has been written in consequence of the great interest excited by my article, under the same title, which appeared simultaneously in _the fortnightly review_ and the _new york independent_. two friends who read the manuscript were of opinion that a volume, in which the evidence could be given much more fully, would be desirable, and the result of the publication of the article confirmed their view. i was led to a study of the subject when writing four new chapters on astronomy for a new edition of _the wonderful century_. i then found that almost all writers on general astronomy, from sir john herschel to professor simon newcomb and sir norman lockyer, stated, as an indisputable fact, that our sun is situated _in_ the plane of the great ring of the milky way, and also very nearly in the centre of that ring. the most recent researches also showed that there was little or no proof of there being any stars or nebulæ very far beyond the milky way, which thus seemed to be the limit, in that direction, of the stellar universe. turning to the earth and the other planets of the solar system, i found that the most recent researches led to the conclusion that no other planet was likely to be the seat of organic life, unless perhaps of a very low type. for many years i had paid special attention to the problem of the measurement of geological time, and also that of the mild climates and generally uniform conditions that had prevailed throughout all geological epochs; and on considering the number of concurrent causes and the delicate balance of conditions required to maintain such uniformity, i became still more convinced that the evidence was exceedingly strong against the probability or possibility of any other planet being inhabited. having long been acquainted with most of the works dealing with the question of the supposed _plurality of worlds_, i was quite aware of the very superficial treatment the subject had received, even in the hands of the most able writers, and this made me the more willing to set forth the whole of the available evidence--astronomical, physical, and biological--in such a way as to show both what was proved and what suggested by it. the present work is the result, and i venture to think that those who will read it carefully will admit that it is a book that was worth writing. it is founded almost entirely on the marvellous body of facts and conclusions of the new astronomy together with those reached by modern physicists, chemists, and biologists. its novelty consists in combining the various results of these different branches of science into a connected whole, so as to show their bearing upon a single problem--a problem which is of very great interest to ourselves. this problem is, whether or no the logical inferences to be drawn from the various results of modern science lend support to the view that our earth is the only inhabited planet, not only in the solar system but in the whole stellar universe. of course it is a point as to which absolute demonstration, one way or the other, is impossible. but in the absence of any direct proofs, it is clearly rational to inquire into probabilities; and these probabilities must be determined not by our prepossessions for any particular view, but by an absolutely impartial and unprejudiced examination of the tendency of the evidence. as the book is written for the general, educated body of readers, many of whom may not be acquainted with any aspect of the subject or with the wonderful advance of recent knowledge in that department often termed the new astronomy, a popular account has been given of all those branches of it which bear upon the special subject here discussed. this part of the work occupies the first six chapters. those who are fairly acquainted with modern astronomical literature, as given in popular works, may begin at my seventh chapter, which marks the commencement of the considerable body of evidence and of argument i have been able to adduce. to those of my readers who may have been influenced by any of the adverse criticisms on my views as set forth in the article already referred to, i must again urge, that throughout the whole of this work, neither the facts nor the more obvious conclusions from the facts are given on my own authority, but always on that of the best astronomers, mathematicians, and other men of science to whose works i have had access, and whose names, with exact references, i generally give. what i claim to have done is, to have brought together the various facts and phenomena _they_ have accumulated; to have set forth the hypotheses by which _they_ account for them, or the results to which the evidence clearly points; to have judged between conflicting opinions and theories; and lastly, to have combined the results of the various widely-separated departments of science, and to have shown how they bear upon the great problem which i have here endeavoured, in some slight degree, to elucidate. as such a large body of facts and arguments from distinct sciences have been here brought together, i have given a rather full summary of the whole argument, and have stated my final conclusions in six short sentences. i then briefly discuss the two aspects of the whole problem--those from the materialistic and from the spiritualistic points of view; and i conclude with a few general observations on the almost unthinkable problems raised by ideas of infinity--problems which some of my critics thought i had attempted in some degree to deal with, but which, i here point out, are altogether above and beyond the questions i have discussed, and equally above and beyond the highest powers of the human intellect. broadstone, dorset, _september_ . 'the wilder'd mind is tost and lost, o sea, in thy eternal tide; the reeling brain essays in vain, o stars, to grasp the vastness wide! the terrible tremendous scheme that glimmers in each glancing light, o night, o stars, too rudely jars the finite with the infinite!' j.h. dell. contents chap. page i. early ideas, ii. modern ideas, iii. the new astronomy, iv. the distribution of the stars, v. distances of stars: the sun's motion, vi. unity and evolution of the star-system, vii. are the stars infinite? viii. our relation to the milky way, ix. the uniformity of matter and its laws, x. the essential characters of organisms, xi. physical conditions essential for life, xii. the earth in relation to life, xiii. the atmosphere in relation to life, xiv. the other planets are not habitable, xv. the stars: have they planets? are they useful to us? xvi. stability of the star-system: importance of central position: summary and conclusion, index, _eight diagrams in the text and two star charts at end._ 'who is man, and what his place? anxious asks the heart, perplext in this recklessness of space, worlds with worlds thus intermixt: what has he, this atom creature, in the infinitude of nature?' f.t. palgrave. man's place in the universe chapter i early ideas as to the universe and its relation to man when men attained to sufficient intelligence for speculations as to their own nature and that of the earth on which they lived, they must have been profoundly impressed by the nightly pageant of the starry heavens. the intense sparkling brilliancy of sirius and vega, the more massive and steady luminosity of jupiter and venus, the strange grouping of the brighter stars into constellations to which fantastic names indicating their resemblance to various animals or terrestrial objects seemed appropriate and were soon generally adopted, together with the apparently innumerable stars of less and less brilliancy scattered broadcast over the sky, many only being visible on the clearest nights and to the acutest vision, constituted altogether a scene of marvellous and impressive splendour of which it must have seemed almost impossible to attain any real knowledge, but which afforded an endless field for the imagination of the observer. the relation of the stars to the sun and moon in their respective motions was one of the earliest problems for the astronomer, and it was only solved by careful and continuous observation, which showed that the invisibility of the former during the day was wholly due to the blaze of light, and this is said to have been proved at an early period by the observed fact that from the bottom of very deep wells stars can be seen while the sun is shining. during total eclipses of the sun also the brighter stars become visible, and, taken in connection with the fixity of position of the pole-star, and the course of those circumpolar stars which never set in the latitudes of greece, egypt, and chaldea, it soon became possible to frame a simple hypothesis which supposed the earth to be suspended in space, while at an unknown distance from it a crystal sphere revolved upon an axis indicated by the pole-star, and carried with it the whole host of heavenly bodies. this was the theory of anaximander ( b.c.), and it served as the starting-point for the more complex theory which continued to be held in various forms and with endless modifications down to the end of the sixteenth century. it is believed that the early greeks obtained some knowledge of astronomy from the chaldeans, who appear to have been the first systematic observers of the heavenly bodies by means of instruments, and who are said to have discovered the cycle of eighteen years and ten days after which the sun and moon return to the same relative positions as seen from the earth. the egyptians perhaps derived their knowledge from the same source, but there is no proof that they were great observers, and the accurate orientation, proportions, and angles of the great pyramid and its inner passages may perhaps indicate a chaldean architect. the very obvious dependence of the whole life of the earth upon the sun, as a giver of heat and light, sufficiently explains the origin of the belief that the latter was a mere appanage of the former; and as the moon also illuminates the night, while the stars as a whole also give a very perceptible amount of light, especially in the dry climate and clear atmosphere of the east, and when compared with the pitchy darkness of cloudy nights when the moon is below the horizon, it seemed clear that the whole of these grand luminaries--sun, moon, stars, and planets--were but parts of the terrestrial system, and existed solely for the benefit of its inhabitants. empedocles ( b.c.) is said to have been the first who separated the planets from the fixed stars, by observing their very peculiar motions, while pythagoras and his followers determined correctly the order of their succession from mercury to saturn. no attempt was made to explain these motions till a century later, when eudoxus of cnidos, a contemporary of plato and of aristotle, resided for some time in egypt, where he became a skilful astronomer. he was the first who systematically worked out and explained the various motions of the heavenly bodies on the theory of circular and uniform motion round the earth as a centre, by means of a series of concentric spheres, each revolving at a different rate and on a different axis, but so united that all shared in the motion round the polar axis. the moon, for example, was supposed to be carried by three spheres, the first revolved parallel to the equator and accounted for the diurnal motion--the rising and setting--of the moon; another moved parallel to the ecliptic and explained the monthly changes of the moon; while the third revolved at the same rate but more obliquely, and explained the inclination of the moon's orbit to that of the earth. in the same way each of the five planets had four spheres, two moving like the first two of the moon, another one also moving in the ecliptic was required to explain the retrograde motion of the planets, while a fourth oblique to the ecliptic was needed to explain the diverging motions due to the different obliquity of the orbit of each planet to that of the earth. this was the celebrated ptolemaic system in the simplest form needed to account for the more obvious motions of the heavenly bodies. but in the course of ages the greek and arabian astronomical observers discovered small divergences due to the various degrees of excentricity of the orbits of the moon and planets and their consequent varying rates of motion; and to explain these other spheres were added, together with smaller circles sometimes revolving excentrically, so that at length about sixty of these spheres, epicycles and excentrics were required to account for the various motions observed with the rude instruments, and the rates of motion determined by the very imperfect time-measurers of those early ages. and although a few great philosophers had at different times rejected this cumbrous system and had endeavoured to promulgate more correct ideas, their views had no influence on public opinion even among astronomers and mathematicians, and the ptolemaic system held full sway down to the time of copernicus, and was not finally given up till kepler's _laws_ and galileo's _dialogues_ compelled the adoption of simpler and more intelligible theories. we are now so accustomed to look upon the main facts of astronomy as mere elementary knowledge that it is difficult for us to picture to ourselves the state of almost complete ignorance which prevailed even among the most civilised nations throughout antiquity and the middle ages. the rotundity of the earth was held by a few at a very early period, and was fairly well established in later classical times. the rough determination of the size of our globe followed soon after; and when instrumental observations became more perfect, the distance and size of the moon were measured with sufficient accuracy to show that it was very much smaller than the earth. but this was the farthest limit of the determination of astronomical sizes and distances before the discovery of the telescope. of the sun's real distance and size nothing was known except that it was much farther from us and much larger than the moon; but even in the century before the commencement of the christian era posidonius determined the circumference of the earth to be , stadia, equal to about , miles, a wonderfully close approximation considering the very imperfect data at his command. he is also said to have calculated the sun's distance, making it only one-third less than the true amount, but this must have been a chance coincidence, since he had no means of measuring angles more accurately than to one degree, whereas in the determination of the sun's distance instruments are required which measure to a second of arc. before the discovery of the telescope the sizes of the planets were quite unknown, while the most that could be ascertained about the stars was, that they were at a very great distance from us. this being the extent of the knowledge of the ancients as to the actual dimensions and constitution of the visible universe, of which, be it remembered, the earth was held to be the centre, we cannot be surprised at the almost universal belief that this universe existed solely for the earth and its inhabitants. in classical times it was held to be at once the dwelling-place of the gods and their gift to man, while in christian ages this belief was but slightly, if at all, changed; and in both it would have been considered impious to maintain that the planets and stars did not exist for the service and delight of mankind alone but in all probability had their own inhabitants, who might in some cases be even superior in intellect to man himself. but apparently, during the whole period of which we are now treating, no one was so daring as even to suggest that there were other worlds with other inhabitants, and it was no doubt because of the idea that we occupied _the_ world, the very centre of the whole surrounding universe which existed solely for us, that the discoveries of copernicus, tycho brahé, kepler, and galileo excited so much antagonism and were held to be impious and altogether incredible. they seemed to upset the whole accepted order of nature, and to degrade man by removing his dwelling-place, the earth, from the commanding central position it had always before occupied. chapter ii modern ideas as to man's relation to the universe the beliefs as to the subordinate position held by sun, moon, and stars in relation to the earth, which were almost universal down to the time of copernicus, began to give way when the discoveries of kepler and the revelations of the telescope demonstrated that our earth was not specially distinguished from the other planets by any superiority of size or position. the idea at once arose that the other planets might be inhabited; and when the rapidly increasing power of the telescope, and of astronomical instruments generally, revealed the wonders of the solar system and the ever-increasing numbers of the fixed stars, the belief in other inhabited worlds became as general as the opposite belief had been in all preceding ages, and it is still held in modified forms to the present day. but it may be truly said that the later like the earlier belief is founded more upon religious ideas than upon a scientific and careful examination of the whole of the facts both astronomical, physical, and biological, and we must agree with the late dr. whewell, that the belief that other planets are inhabited has been generally entertained, not in consequence of physical reasons but in spite of them. and he adds:--'it was held that venus, or that saturn was inhabited, not because anyone could devise, with any degree of probability, any organised structure which would be suitable to animal existence on the surfaces of those planets; but because it was conceived that the greatness or goodness of the creator, or his wisdom, or some other of his attributes, would be manifestly imperfect, if these planets were not tenanted by living creatures.' those persons who have only heard that many eminent astronomers down to our own day have upheld the belief in a 'plurality of worlds' will naturally suppose that there must be some very cogent arguments in its favour, and that it must be supported by a considerable body of more or less conclusive facts. they will therefore probably be surprised to hear that any direct evidence which may be held to support the view is almost wholly wanting, and that the greater part of the arguments are weak and flimsy in the extreme. of late years, it is true, some few writers have ventured to point out how many difficulties there are in the way of accepting the belief, but even these have never examined the question from the various points of view which are essential to a proper consideration of it; while, so far as it is still upheld, it is thought sufficient to show, that in the case of some of the planets, there seem to be such conditions as to render life possible. in the millions of planetary systems supposed to exist it is held to be incredible that there are not great numbers as well fitted to be inhabited by animals of all grades, including some as high as man or even higher, and that we must, therefore, believe that they are so inhabited. as in the present work i propose to show, that the probabilities and the weight of direct evidence tend to an exactly opposite conclusion, it will be well to pass briefly in review the various writers on the subject, and to give some indication of the arguments they have used and the facts they have set forth. for the earlier upholders of the theory i am indebted to dr. whewell, who, in his _dialogue on the plurality of worlds_--a supplement to his well-known volume on the subject--refers to all writers of importance known to him. the earliest are the great astronomers kepler and huygens, and the learned bishop wilkins, who all believed that the moon was or might probably be inhabited; and of these whewell considers wilkins to have been by far the most thoughtful and earnest in supporting his views. then we have sir isaac newton himself who, at considerable length, argued that the sun was probably inhabited. but the first regular work devoted to the subject appears to have been written by m. fontenelle, secretary to the academy of sciences in paris, who in published his _conversations on the plurality of worlds_. the book consisted of five chapters, the first explaining the copernican theory; the second maintaining that the moon is a habitable world; the third gives particulars as to the moon, and argues that the other planets are also inhabited; the fourth gives details as to the worlds of the five planets; while the fifth declares that the fixed stars are suns, and that each illuminates a world. this work was so well written, and the subject proved so attractive, that it was translated into all the chief european languages, while the astronomer lalande edited one of the french editions. three english translations were published, and one of these went through six editions down to the year . the influence of this work was very great and no doubt led to that general acceptance of the theory by such men as sir william herschel, sir john herschel, dr. chalmers, dr. dick, dr. isaac taylor, and m. arago, although it was wholly founded on pure speculation, and there was nothing that could be called evidence on one side or the other. this was the state of public opinion when an anonymous work appeared (in ) under the somewhat misleading title of _the plurality of worlds: an essay_. this was written, as already stated, by dr. whewell, who, for the first time, ventured to doubt the generally accepted theory, and showed that all the evidence at our command led to the conclusion that some of the planets were _certainly_ not habitable, that others were _probably_ not so, while in none was there that close correspondence with terrestrial conditions which seemed essential for their habitability by the higher animals or by man. the book was ably written and showed considerable knowledge of the science of the time, but it was very diffuse, and the larger part of it was devoted to showing that his views were not in any way opposed to religion. one of his best arguments was founded on the proposition that '_the earth's orbit is the temperate zone of the solar system_,' that there only is it possible to have those moderate variations of heat and cold, dryness and moisture, which are suitable for animal life. he suggested that the outer planets of the system consisted mainly of water, gases, and vapour, as indicated by their low specific gravity, and were therefore quite unsuitable for terrestrial life; while those near the sun were equally unsuited, because, owing to the great amount of solar heat, water could not exist on their surfaces. he devotes a great deal of space to the evidence that there is no animal life on the moon, and taking this as proved, he uses it as a counter argument against the other side. they always urge that, the earth being inhabited, we must suppose the other planets to be so too; to which he replies:--we know that the moon is not inhabited though it has all the advantage of proximity to the sun that the earth has; why then should not other planets be equally uninhabited? he then comes to mars and admits that this planet is very like the earth so far as we can judge, and that it may therefore be inhabited, or as the author expresses it, 'may have been judged worthy of inhabitants by its maker.' but he urges the small size of mars, its coldness owing to distance from the sun, and that the annual melting of its polar ice-caps will keep it cold all through the summer. if there are animals they are probably of a low type like the saurians and iguanodons of our seas during the wealden epoch; but, he argues, as even on our earth the long process of preparation for man was carried on for countless millions of years, we need not discuss whether there are intelligent beings on mars till we have some better evidence that there are any living creatures at all. several of the early chapters are devoted to an attempt to minimise the difficulties of those religious persons who feel oppressed by the immensity and complexity of the material universe as revealed by modern astronomy; and by the almost infinite insignificance of man and his dwelling-place, the earth, in comparison with it, an insignificance vastly increased if not only the planets of the solar system, but also those which circle around the myriads of suns, are also theatres of life. and these persons are further disquieted because the very same facts are used by sceptics of various kinds in their attacks upon christianity. such writers point out the irrationality and absurdity of supposing that the creator of all this unimaginable vastness of suns and systems, filling for all we know endless space, should take any _special_ interest in so mean and pitiful a creature as man, the imperfectly developed inhabitant of one of the smaller worlds attached to a second or third-rate sun, a being whose whole history is one of war and bloodshed, of tyranny, torture, and death; whose awful record is pictured by himself in such books as josephus' _history of the jews_, the _decline and fall of the roman empire_, and even more forcibly summarised in that terrible picture of human fiendishness and misery, _the martyrdom of man_; while their character is indicated by one of the kindest and simplest of their poets in the restrained but expressive lines:-- 'man's inhumanity to man makes countless thousands mourn.' it is for such a being as this, they say, that god should have specially revealed his will some thousands of years ago, and finding that his commands were not obeyed, his will not fulfilled, yet ordained for their benefit the necessarily unique sacrifice of his son, in order to save a small portion of these 'miserable sinners' from the natural and well-deserved consequence of their stupendous follies, their unimaginable crimes? such a belief they maintain is too absurd, too incredible, to be held by any rational being, and it becomes even less credible and less rational if we maintain that there are countless other inhabited worlds. it is very difficult for the religious man to make any adequate reply to such an attack as this, and as a result many have felt their position to be untenable and have accordingly lost all faith in the special dogmas of orthodox christianity. they feel themselves really to be between the horns of a dilemma. if there are myriads of other worlds, it seems incredible that they should each be the object of a special revelation and a special sacrifice. if, on the other hand, we are the only intelligent beings that exist in the material universe, and are really the highest creative product of a being of infinite wisdom and power, they cannot but wonder at the vast apparent disproportion between the creator and the created, and are sometimes driven to atheism from the hopelessness of comprehending so mean and petty a result as the sole outcome of infinite power. whewell tells us that the great preacher, dr. chalmers, in his astronomical discourses, attempted a reply to these difficulties, but, in his opinion, not a very successful one; and a large part of his own work is devoted to the same purpose. his main point seems to be that we know too little of the universe to arrive at any definite conclusions on the question at issue, and that any ideas that we may have as to the purposes of the creator in forming the vast system we see around us are almost sure to be erroneous. we must therefore be content to remain ignorant, and must rest satisfied in the belief that the creator had a purpose although we are not yet permitted to know what it was. and to those who urge that in other worlds there may be other laws of nature which may render them quite as habitable by intelligent beings as our world is for us, he replies, that if we are to suppose new laws of nature in order to render each planet habitable, there is an end of all rational inquiry on the subject, and we may maintain and believe that animals may live on the moon without air or water, and on the sun exposed to heat which vaporises earths and metals. his concluding argument, and perhaps one of his strongest, is that founded upon the dignity of man, as conferring a pre-eminence upon the planet which has produced him. 'if,' he says, 'man be not merely capable of virtue and duty, of universal love and self-devotion, but be also immortal; if his being be of infinite duration, his soul created never to die; then, indeed, we may well say that one soul outweighs the whole unintelligent creation.' and then, addressing the religious world, he urges that, if, as they believe, god _has_ redeemed man by the sacrifice of his son, and _has_ given to him a revelation of his will, then indeed no other conception is possible than that he is the sole and highest product of the universe. 'the elevation of millions of intellectual, moral, religious, spiritual creatures, to a destiny so prepared, consummated, and developed, is no unworthy occupation of all the capacities of space, time, and matter.' then with a chapter on 'the unity of the world,' and one on 'the future,' neither of which contains anything which adds to the force of his argument, the book ends. the publication of this able if rather vague and diffuse work, contesting popular opinions, was followed by a burst of indignant criticism on the part of a man of considerable eminence in some branches of physics--sir david brewster, but who was very inferior, both in general knowledge of science and in literary skill, to the writer whose views he opposed. the purport of the book in which he set forth his objections is indicated by its title--_more worlds than one, the creed of the philosopher and the hope of the christian_. though written with much force and conviction it appeals mainly to religious prejudices, and assumes throughout that every planet and star is a special creation, and that the peculiarities of each were designed for some special purpose. 'if,' he says, 'the moon had been destined to be merely a lamp to our earth, there was no occasion to variegate its surface with lofty mountains and extinct volcanoes, and cover it with large patches of matter that reflect different quantities of light and give its surface the appearance of continents and seas. it would have been a better lamp had it been a smooth piece of lime or of chalk.' it is, therefore, he thinks, prepared for inhabitants; and then he argues that all the other satellites are also inhabited. again he says that 'when it was found that venus was about the same size as the earth, with mountains and valleys, days and nights, and years analogous to our own, the _absurdity_ of believing that she had no inhabitants, when no other rational purpose could be assigned for her creation, became an argument of a certain amount that she was, like the earth, the seat of animal and vegetable life.' then, when it was found that jupiter was so gigantic 'as to require four moons to give him light, the argument from analogy that _he_ was inhabited became stronger also, because it extended to _two_ planets.' and thus each successive planet having certain points of analogy with the others becomes an additional argument; so that when we take account of all the planets, with atmosphere, and clouds, and arctic snows, and trade-winds, the argument from analogy becomes, he urges, very powerful;--'and the absurdity of the opposite opinion, that planets should have moons and no inhabitants, atmospheres with no creatures to breathe in them, and currents of air without life to be fanned, became a formidable argument which few minds, if any, could resist.' the work is full of such weak and fallacious rhetoric and even, if possible, still weaker. thus after describing double stars, he adds--'but no person can believe that two suns could be placed in the heavens for no other purpose than to revolve round their common centre of gravity'; and he concludes his chapter on the stars thus:--'wherever there is matter there must be life; life physical to enjoy its beauties--life moral to worship its maker, and life intellectual to proclaim his wisdom and his power.' and again--'a house without tenants, a city without citizens, presents to our minds the same idea as a planet without life, and a universe without inhabitants. why the house was built, why the city was founded, why the planet was made, and why the universe was created, it would be difficult even to conjecture.' arguments of this kind, which in almost every case beg the question at issue, are repeated _ad nauseam_. but he also appeals to the old testament to support his views, by quoting the fine passage in the psalms--'when i consider thy heavens the work of thy fingers, the moon and the stars which thou hast ordained; what is man that thou art mindful of him?' on which he remarks--'we cannot doubt that inspiration revealed to him [david] the magnitude, the distances, and the final cause, of the glorious spheres which fixed his admiration.' and after quoting various other passages from the prophets, all as he thinks supporting the same view, he sets forth the extraordinary idea as a confirmatory argument, that the planets or some of them are to be the future abode of man. for, he says--'man in his future state of existence is to consist, as at present, of a spiritual nature residing in a corporeal frame. he must live, therefore, upon a material planet, subject to all the laws of matter.' and he concludes thus:--'if there is not room, then, on our globe for the millions of millions of beings who have lived and died on its surface, we can scarcely doubt that their future abode must be on some of the primary or secondary planets of the solar system, whose inhabitants have ceased to exist, or upon planets which have long been in a state of preparation, as our earth was, for the advent of intellectual life.' it is pleasant to turn from such weak and trivial arguments to the only other modern works which deal at some length with this subject, the late richard a. proctor's _other worlds than ours_, and a volume published five years later under the title--_our place among infinities_. written as these were by one of the most accomplished astronomers of his day, remarkable alike for the acuteness of his reasoning and the clearness of his style, we are always interested and instructed even when we cannot agree with his conclusions. in the first work mentioned above, he assumes, like sir david brewster, the antecedent probability that the planets are inhabited and on much the same theological grounds. so strongly does he feel this that he continually speaks as if the planets _must_ be inhabited unless we can show very good reason that they _cannot_ be so, thus throwing the burden of proving a negative on his opponents, while he does not attempt to prove his positive contention that they are inhabited, except by purely hypothetical considerations as to the creator's purpose in bringing them into existence. but starting from this point he endeavours to show how whewell's various difficulties may be overcome, and here he always appeals to astronomical or physical facts, and reasons well upon them. but he is quite honest; and, coming to the conclusion that jupiter and saturn, uranus and neptune, cannot be habitable, he adduces the evidence and plainly states the result. but then he thinks that the satellites of jupiter and saturn _may_ be habitable, and if they may be, then he concludes that they _must_. one great oversight in his whole argument is, that he is satisfied with showing the possibility that life may exist now, but never deals with the question of whether life could have been developed from its earliest rudiments up to the production of the higher vertebrates and man; and this, as i shall show later, is the _crux_ of the whole problem. with regard to the other planets, after a careful examination of all that is known about them, he arrives at the conclusion that if mercury is protected by a cloud-laden atmosphere of a peculiar kind it may possibly, but not probably, support high forms of animal life. but in the case of venus and mars he finds so much resemblance to and so many analogies with our earth, that he concludes that they almost certainly are so. in the case of the fixed stars, now that we know by spectroscopic observations that they are true suns, many of which closely resemble our sun and give out light and heat as he does, mr. proctor argues, that 'the vast supplies of heat thus emitted by the stars not only suggest the conclusion that there must be worlds around these orbs for which these heat-supplies are intended, but point to the existence of the various forms of force into which heat may be transmuted. we know that the sun's heat poured upon our earth is stored up in vegetable and animal forms of life; is present in all the phenomena of nature--in winds and clouds and rain, in thunder and lightning, storm and hail; and that even the works of man are performed by virtue of the solar heat-supplies. thus the fact that the stars send forth heat to the worlds which circle around them suggests at once the thought that on those worlds there must exist animal and vegetable forms of life.' we may note that in the first part of this passage the presence of worlds or planets is 'suggested,' while later on 'the worlds which circle round them' is spoken of as if it were a proved fact from which the presence of vegetable and animal life may be inferred. a suggestion depending on a preceding suggestion is not a very firm basis for so vast and wide-reaching a conclusion. in the second work referred to above there is one chapter entitled, 'a new theory of life in other worlds,' where the author gives his more matured views of the question, which are briefly stated in the preface as being 'that the weight of evidence favours my theory of the (relative) paucity of worlds.' his views are largely founded on the theory of probabilities, of which subject he had made a special study. taking first our earth, he shows that the period during which life has existed upon it is very small in comparison with that during which it must have been slowly forming and cooling, and its atmosphere condensing so as to form land and water on its surface. and if we consider the time the earth has been occupied by man, that is a very minute part, perhaps not the thousandth part, of the period during which it has existed as a planet. it follows that even if we consider only those planets whose physical condition seems to us to be such as to be able to sustain life, the chances are perhaps hundreds to one against their being at that particular stage when life has begun to be developed, or if it has begun has reached as high a development as on our earth. with regard to the stars, the argument is still stronger, because the epochs required for their formation are altogether unknown, while as to the conditions required for the formation of planetary systems around them we are totally ignorant. to this i would add that we are equally ignorant as to the probability or even possibility of many of these suns producing planets which, by their position, size, atmosphere, or other physical conditions can possibly become life-producing worlds. and, as we shall see later, this point has been overlooked by all writers, including mr. proctor himself. his conclusion is, then, that although the worlds which possess life at all approaching that of our earth may be relatively few in number, yet considering the universe as practically infinite in extent, they may be really very numerous. it has been necessary to give this sketch of the views of those who have written specially on the question of the plurality of worlds, because the works referred to have been very widely read and have influenced educated opinion throughout the world. moreover, mr. proctor, in his last work on the subject, speaks of the theory as being 'identified with modern astronomy'; and in fact popular works still discuss it. but all these follow the same general line of argument as those already referred to, and the curious thing is that while overlooking many of the most essential conditions they often introduce others which are by no means essential--as, for instance, that the atmosphere must have the same proportion of oxygen as our own. they seem to think that if any of our quadrupeds or birds taken to another planet could not live there, no animals of equally high organisation could inhabit it; entirely overlooking the very obvious fact that, supposing, as is almost certain, that oxygen is necessary for life, then, whatever proportion of oxygen within certain limits was present, the forms of life that arose would necessarily be organised in adaptation to that proportion, which might be considerably less or greater than on the earth. the present volume will show how extremely inadequate has been the treatment of this question, which involves a variety of important considerations hitherto altogether overlooked. these are extremely numerous and very varied in their character, and the fact that they all point to one conclusion--a conclusion which so far as i am aware no previous writer has reached--renders it at least worthy of the careful consideration of all unbiassed thinkers. the whole subject is one as to which no direct evidence is obtainable, but i venture to think that the convergence of so many probabilities and indications towards a single definite theory, intimately connected with the nature and destiny of man himself, raises this theory to a very much higher level of probability than the vague possibilities and theological suggestions which are the utmost that have been adduced by previous writers. in order to make every step of my argument clearly intelligible to all educated readers, it will be necessary to refer continually to the marvellous extension of our knowledge of the universe obtained during the last half-century, and constituting what is termed the new astronomy. the next chapter will therefore be devoted to a popular exposition of the new methods of research, so that the results reached, which will have to be referred to in succeeding chapters, may be not only accepted, but clearly understood. chapter iii the new astronomy during the latter half of the nineteenth century discoveries were made which extended the powers of astronomical research into entirely new and unexpected regions, comparable to those which were opened up by the discovery of the telescope more than two centuries before. the older astronomy for more than two thousand years was purely mechanical and mathematical, being limited to observation and measurement of the apparent motions of the heavenly bodies, and the attempts to deduce, from these apparent motions, their real motions, and thus determine the actual structure of the solar system. this was first done when kepler established his three celebrated laws: and later, when newton showed that these laws were necessary consequences of the one law of gravitation, and when succeeding observers and mathematicians proved that each fresh irregularity in the motions of the planets was explicable by a more thorough and minute application of the same laws, this branch of astronomy reached its highest point of efficiency and left very little more to be desired. then, as the telescope became successively improved, the centre of interest was shifted to the surfaces of the planets and their satellites, which were watched and scrutinised with the greatest assiduity in order if possible to attain some amount of knowledge of their physical constitution and past history. a similar minute scrutiny was given to the stars and nebulæ, their distribution and grouping, and the whole heavens were mapped out, and elaborate catalogues constructed by enthusiastic astronomers in every part of the world. others devoted themselves to the immensely difficult problem of determining the distances of the stars, and by the middle of the century a few such distances had been satisfactorily measured. thus, up to the middle of the nineteenth century it appeared likely that the future of astronomy would rest almost entirely on the improvement of the telescope, and of the various instruments of measurement by means of which more accurate determinations of distances might be obtained. indeed, the author of the positive philosophy, auguste comte, felt so sure of this that he deprecated all further attention to the stars as pure waste of time that could never lead to any useful or interesting result. in his _philosophical treatise on popular astronomy_ published in , he wrote very strongly on this point. he there tells us that, as the stars are only accessible to us by sight they must always remain very imperfectly known. we can know little more than their mere existence. even as regards so simple a phenomenon as their temperature this must always be inappreciable to a purely visual examination. our knowledge of the stars is for the most part purely negative, that is, we can determine only that they do _not_ belong to our system. outside that system there exists, in astronomy, only obscurity and confusion, for want of indispensable facts; and he concludes thus:--'it is, then, in vain that for half a century it has been endeavoured to distinguish two astronomies, the one solar the other sidereal. in the eyes of those for whom science consists of real laws and not of incoherent facts, the second exists only in name, and the first alone constitutes a true astronomy; and i am not afraid to assert that it will always be so.' and he adds that--'all efforts directed to this subject for half a century have only produced an accumulation of incoherent empirical facts which can only interest an irrational curiosity.' seldom has a confident assertion of finality in science received so crushing a reply as was given to the above statements of comte by the discovery in (only three years after his death) of the method of spectrum-analysis which, in its application to the stars, has revolutionised astronomy, and has enabled us to obtain that very kind of knowledge which he declared must be for ever beyond our reach. through it we have acquired accurate information as to the physics and chemistry of the stars and nebulæ, so that we now know really more of the nature, constitution, and temperature of the enormously distant suns which we distinguish by the general term stars, than we do of most of the planets of our own system. it has also enabled us to ascertain the existence of numerous invisible stars, and to determine their orbits, their rate of motion, and even, approximately, their mass. the despised stellar astronomy of the early part of the century has now taken rank as the most profoundly interesting department of that grand science, and the branch which offers the greatest promise of future discoveries. as the results obtained by means of this powerful instrument will often be referred to, a short account of its nature and of the principles on which it depends must here be given. the solar spectrum is the band of coloured light seen in the rainbow and, partially, in the dew-drop, but more completely when a ray of sunlight passes through a prism--a piece of glass having a triangular section. the result is, that instead of a spot of white light we have a narrow band of brilliant colours which succeed each other in regular order, from violet at one end through blue, green, and yellow to red at the other. we thus see that light is not a simple and uniform radiation from the sun, but is made up of a large number of separate rays, each of which produces in our eyes the sensation of a distinct colour. light is now explained as being due to vibrations of ether, that mysterious substance which not only permeates all matter, but which fills space at least as far as the remotest of the visible stars and nebulæ. the exceedingly minute waves or vibrations of the ether produce all the phenomena of heat, light, and colour, as well as those chemical actions to which photography owes its wonderful powers. by ingenious experiments the size and rate of vibration of these waves have been measured, and it is found that they vary considerably, those forming the red light, which is least refracted, having a wave-length of about / of an inch, while the violet rays at the other end of the spectrum are only about half that length or / of an inch. the rate at which the vibrations succeed each other is from millions of millions per second for the extreme red rays, to millions of millions for those at the violet end of the spectrum. these figures are given to show the wonderful minuteness and rapidity of these heat and light waves on which the whole life of the world, and all our knowledge of other worlds and other suns, directly depends. but the mere colours of the spectrum are not the most important part of it. very early in the nineteenth century a close examination showed that it was everywhere crossed by black lines of various thicknesses, sometimes single, sometimes grouped together. many observers studied them and made accurate drawings or maps showing their positions and thicknesses, and by combining several prisms, so that the beam of sunlight had to pass through them successively, a spectrum could be produced several feet long, and more than of these dark lines were counted in it. but what they were and how they were caused remained a mystery, till, in the year , the german physicist kirchhoff discovered the secret and gave to chemists and astronomers a new and quite unexpected engine of research. it had already been observed that the chemical elements and various compounds, when heated to incandescence, produced spectra consisting of coloured lines or bands which were constant for each element, so that the elements could at once be recognised by their characteristic spectra; and it had also been noticed that some of these bands, especially the yellow band produced by sodium, corresponded in position with certain black lines in the solar spectrum. kirchhoff's discovery consisted in showing that, when the light from an incandescent body passes through the same substance in a state of vapour or gas, so much of the light is absorbed that the coloured lines or bands become black. the mystery of more than half a century was thus solved; and the thousands of black lines in the solar spectrum were shown to be caused by the light from the incandescent matter of the sun's surface passing through the heated gases or vapours immediately above it, and thereby having the bright coloured lines of their spectra changed, by absorption, to comparative blackness. chemists and physicists immediately set to work examining the spectra of the elements, fixing the position of the several coloured lines or bands by accurate measurement, and comparing them with the dark lines of the solar spectrum. the results were in the highest degree satisfactory. in a large proportion of the elements the coloured bands corresponded exactly with a group of dark lines in the spectrum of the sun, in which, therefore, the same terrestrial elements were proved to exist. among the elements first detected in this manner were hydrogen, sodium, iron, copper, magnesium, zinc, calcium, and many others. nearly forty of the elements have now been found in the sun, and it seems highly probable that all our elements really exist there, but as some are very rare and are present in very minute quantities they cannot be detected. some of the dark lines in the sun were found not to correspond to any known element, and as this was thought to indicate an element peculiar to the sun it was named helium; but quite recently it has been discovered in a rare mineral. many of the elements are represented by a great number of lines, others by very few. thus iron has more than , while lead and potassium have only one each. the value of the spectroscope both to the chemist in discovering new elements and to the astronomer in determining the constitution of the heavenly bodies, is so great, that it became of the highest importance to have the position of all the dark lines in the solar spectrum, as well as the bright lines of all the elements, determined with extreme accuracy, so as to be able to make exact comparisons between different spectra. at first this was done by means of very large-scale drawings showing the exact position of every dark or bright line. but this was found to be both inconvenient and not sufficiently exact; and it was therefore agreed to adopt the natural scale of the wave-lengths of the different parts of the spectrum, which by means of what are termed diffraction-gratings can now be measured with great accuracy. diffraction-gratings are formed of a polished surface of hard metal ruled with excessively fine lines, sometimes as many as , to an inch. when sunlight falls upon one of these gratings it is reflected, and by interference of the rays from the spaces between the fine grooves, it is spread out into a beautiful and well-defined spectrum, which, when the lines are very close, is several yards in length. in these diffraction spectra many dark lines are seen which can be shown in no other way, and they also give a spectrum which is far more uniform than that produced by glass prisms in which minute differences in the composition of the glass cause some rays to be refracted more and others less than the normal amount. the spectra produced by diffraction-gratings are double; that is, they are spread out on both sides of the central line of the ray which remains white, and the several coloured or dark lines are so clearly defined that they can be thrown on a screen at a considerable distance, giving a great length to the spectrum. the data for obtaining the wave-lengths are the distance apart of the lines, the distance of the screen, and the distance apart of the first pair of dark lines on each side of the central bright line. all these can be measured with extreme accuracy by means of telescopes with micrometers and other contrivances, and the result is an accuracy of determination of wave-lengths which can probably not be equalled in any other kind of measurement. as the wave-lengths are so excessively minute, it has been found convenient to fix upon a still smaller unit of measurement, and as the millimetre is the smallest unit of the metric system, the ten-millionth of a millimetre (technically termed 'tenth meter') is the unit adopted for the measurement of wave-lengths, which is equal to about the millionth of an inch. thus the wave-lengths of the red and blue lines characteristic of hydrogen are . and . respectively. this excessively minute scale of wave-lengths, once determined by the most refined measurement, is of very great importance. having the wave-lengths of any two lines of a spectrum so determined, the space between them can be laid down on a diagram of any length, and all the lines that occur in any other spectrum between these two lines can be marked in their exact relative positions. now, as the visible spectrum consists of about , rays of light, each of different wave-lengths and therefore of different refrangibilities, if it is laid down on such a scale as to be of a length of inches ( feet), each wave-length will be / of an inch long, a space easily visible by the naked eye. the possession of an instrument of such wonderful delicacy, and with powers which enable it to penetrate into the inner constitution of the remotest orbs of space, rendered it possible, within the next quarter of a century, to establish what is practically a new science--astrophysics--often popularly termed the new astronomy. a brief outline of the main achievements of this science must now be given. the first great discovery made by spectrum analysis, after the interpretation of the sun's spectrum had been obtained, was, the real nature of the fixed stars. it is true they had long been held by astronomers to be suns, but this was only an opinion of the accuracy of which it did not seem possible to obtain any proof. the opinion was founded on two facts--their enormous distance from us, so great that the whole diameter of the earth's orbit did not lead to any apparent change of their relative positions, and their intense brilliancy which at such distances could only be due to an actual size and splendour comparable with our sun. the spectroscope at once proved the correctness of this opinion. as one after another was examined, they were found to exhibit spectra of the same general type as that of the sun--a band of colours crossed by dark lines. the very first stars examined by sir william huggins showed the existence of nine or ten of our elements. very soon all the chief stars of the heavens were spectroscopically examined, and it was found that they could be classed in three or four groups. the first and largest group contains more than half the visible stars, and a still larger proportion of the most brilliant, such as sirius, vega, regulus, and alpha crucis in the southern hemisphere. they are characterised by a white or bluish light, rich in the ultra-violet rays, and their spectra are distinguished by the breadth and intensity of the four dark bands due to the absorption of hydrogen, while the various black lines which indicate metallic vapours are comparatively few, though hundreds of them can be discovered by careful examination. the next group, to which capella and arcturus belong, is also very numerous, and forms the solar type of stars. their light is of a yellowish colour, and their spectra are crossed throughout by innumerable fine dark lines more or less closely corresponding with those in the solar spectrum. the third group consists of red and variable stars, which are characterised by fluted spectra. such spectra show like a range of doric columns seen in perspective, the red side being that most illuminated. the last group, consisting of few and comparatively small stars, has also fluted spectra, but the light appears to come from the opposite direction. these groups were established by father secchi, the roman astronomer, in , and have been adopted with some modifications by vogel of the astrophysical observatory at potsdam. the exact interpretation of these different spectra is somewhat uncertain, but there can be little doubt that they coincide primarily with differences of temperature and with corresponding differences in the composition and extent of the absorptive atmospheres. stars with fluted spectra indicate the presence of vapours of the metalloids or of compound substances, while the reversed flutings indicate the presence of carbon. these conclusions have been reached by careful laboratory experiments which are now carried on at the same time as the spectral examination of the stars and other heavenly bodies, so that each peculiarity of their spectra, however puzzling and apparently unmeaning, has been usually explained, by being shown to indicate certain conditions of chemical constitution or of temperature. but whatever difficulty there may be in explaining details, there remains no doubt whatever of the fundamental fact that all the stars are true suns, differing no doubt in size, and their stage of development as indicated by the colour or intensity of their light or heat, but all alike possessing a photosphere or light-emitting surface, and absorptive atmospheres of various qualities and density. innumerable other details, such as the often contrasted colours of double stars, the occasional variability of their spectra, their relations to the nebulæ, the various stages of their development and other problems of equal interest, have occupied the continued attention of astronomers, spectroscopists, and chemists; but further reference to these difficult questions would be out of place here. the present sketch of the nature of spectrum-analysis applied to the stars is for the purpose of making its principle and method of observation intelligible to every educated reader, and to illustrate the marvellous precision and accuracy of the results attained by it. so confident are astronomers of this accuracy that nothing less than _perfect correspondence_ of the various bright lines in the spectrum of an element in the laboratory with the dark lines in the spectrum of the sun or of a star is required before the presence of that element is accepted as proved. as miss clerke tersely puts it--'spectroscopic coincidences admit of no compromise. either they are absolute or they are worthless.' measurement of motion in the line of sight we must now describe another and quite distinct application of the spectroscope, which is even more marvellous than that already described. it is the method of measuring the rate of motion of any of the visible heavenly bodies in a direction either directly towards us, or directly away from us, technically described as 'radial motion,' or by the expression--'in the line of sight.' and the extraordinary thing is that this power of measurement is altogether independent of distance, so that the rate of motion in miles per second of the remotest of the fixed stars, if sufficiently bright to show a distinct spectrum, can be measured with as much certainty and accuracy as in the case of a much nearer star or a planet. in order to understand how this is possible we have again to refer to the wave-theory of light; and the analogy of other wave-motions will enable us better to grasp the principle on which these calculations depend. if on a nearly calm day we count the waves that pass each minute by an anchored steamboat, and then travel in the direction the waves come from, we shall find that a larger number pass us in the same time. again, if we are standing near a railway, and an engine comes towards us whistling, we shall notice that it changes its tone as it passes us; and as it recedes the sound will be in a lower key, although the engine may be at exactly the same distance from us as when it was approaching. yet the sound does not change to the ear of the engine driver, the cause of the change being that the sound-waves reach us in quicker succession as the source of the waves is approaching us than when it is retreating from us. now, just as the pitch of a note depends upon the rapidity with which the successive air-vibrations reach our ear, so does the colour of a particular part of the spectrum depend upon the rapidity with which the ethereal waves which produce colour reach our eyes; and as this rapidity is greater when the source of the light is approaching than when it is receding from us, a slight shifting of the position of the coloured bands, and therefore of the dark lines, will occur, as compared with their position in the spectrum of the sun or of any stationary source of light, if there is any motion sufficient in amount to produce a perceptible shift. that such a change of colour would occur was pointed out by professor doppler of prague in , and it is hence usually spoken of as the 'doppler principle'; but as the changes of colour were so minute as to be impossible of measurement it was not at that time of any practical importance in astronomy. but when the dark lines in the spectrum were carefully mapped, and their positions determined with minute accuracy, it was seen that a means of measuring the changes produced by motion in the line of sight existed, since the position of any of the dark or coloured lines in the spectra of the heavenly bodies could be compared with those of the corresponding lines produced artificially in the laboratory. this was first done in by sir william huggins, who, by the use of a very powerful spectroscope constructed for the purpose, found that such a change did occur in the case of many stars, and that their rate of motion towards us or away from us--the radial motion--could be calculated. as the actual distance of some of these stars had been measured, and their change of position annually (their proper motion) determined, the additional factor of the amount of motion in the direction of our line of sight completed the data required to fix their true line of motion among the other stars. the accuracy of this method under favourable conditions and with the best instruments is very great, as has been proved by those cases in which we have independent means of calculating the real motion. the motion of venus towards or away from us can be calculated with great accuracy for any period, being a resultant of the combined motions of the planet and of our earth in their respective orbits. the radial motions of venus were determined at the lick observatory in august and september , by spectroscopic observations, and also by calculation, to be as follows:-- by observation. by calculation. aug. th. . miles per second. . miles per second. " nd. . " " " . " " " " th. . " " " . " " " sep. rd. . " " " . " " " " th. . " " " . " " " showing that the maximum error was only one mile per second, while the mean error was about a quarter of a mile. in the case of the stars the accuracy of the method has been tested by observations of the same star at times when the earth's motion in its orbit is towards or away from the star, whose apparent radial velocity is, therefore, increased or diminished by a known amount. observations of this kind were made by dr. vogel, director of the astrophysical observatory at potsdam, showing, in the case of three stars, of which ten observations were taken, a mean error of about two miles per second; but as the stellar motions are more rapid than those of the planets, the proportionate error is no greater than in the example given above. the great importance of this mode of determining the real motion of the stars is, that it gives us a knowledge of the scale on which such motions are progressing; and when in the course of time we discover whether any of their paths are rectilinear or curved, we shall be in a position to learn something of the nature of the changes that are going on and of the laws on which they depend. invisible stars and imperceptible motions but there is another result of this power of determining radial motion which is even more unexpected and marvellous, and which has extended our knowledge of the stars in quite a new direction. by its means it is possible to determine the existence of invisible stars and to measure the rate of otherwise imperceptible motions; that is of stars which are invisible in the most powerful modern telescopes, and whose motions have such a limited range that no telescope can detect them. double or binary stars forming systems which revolve around their common centre of gravity were discovered by sir william herschel, and very great numbers are known; but in most cases their periods of revolution are long, the shortest being about twelve years, while many extend to several hundred years. these are, of course, all visible binaries, but many are now known of which one star only is visible while the other is either non-luminous or is so close to its companion that they appear as a single star in the most powerful telescopes. many of the variable stars belong to the former class, a good example of which is algol in the constellation perseus, which changes from the second to the fourth magnitude in about four and a half hours, and in about four and a half hours more regains its brilliancy till its next period of obscuration which occurs regularly every two days and twenty-one hours. the name algol is from the arabic _al ghoul_, the familiar 'ghoul' of the arabian nights, so named--'the demon'--from its strange and weird behaviour. it had long been conjectured that this obscuration was due to a dark companion which partially eclipsed the bright star at every revolution, showing that the plane of the orbit of the pair was almost exactly directed towards us. the application of the spectroscope made this conjecture a certainty. at an equal time before and after the obscuration, motion in the line of sight was shown, towards and away from us, at a rate of twenty-six miles per second. from these scanty data and the laws of gravitation which fix the period of revolution of planets at various distances from their centres of revolution, professor pickering of the harvard observatory was able to arrive at the following figures as highly probable, and they may be considered to be certainly not far from the truth. diameter of algol, , , miles. diameter of dark companion, , " distance between their centres, , , " orbital speed of algol, . miles per sec. orbital speed of companion, . " " " mass of algol, / mass of our sun. mass of companion, / " " " when it is considered that these figures relate to a pair of stars only one of which has ever been seen, that the orbital motion even of the visible star cannot be detected in the most powerful telescopes, when, further, we take into account the enormous distance of these objects from us, the great results of spectroscopic observation will be better appreciated. but besides the marvel of such a discovery by such simple means, the facts discovered are themselves in the highest degree marvellous. all that we had known of the stars through telescopic observation indicated that they were at very great distances from each other however thickly they may appear scattered over the sky. this is the case even with close telescopic double stars, owing to their enormous remoteness from us. it is now estimated that even stars of the first magnitude are, on a general average, about eighty millions of millions of miles distant; while the closest double stars that can be distinctly separated by large telescopes are about half a second apart. these, if at the above distance, will be about millions of miles from each other. but in the case of algol and its companion, we have two bodies both larger than our sun, yet with a distance of only - / millions of miles between their surfaces, a distance not much exceeding their combined diameters. we should not have anticipated that such huge bodies could revolve so closely to each other, and as we now know that the neighbourhood of our sun--and probably of all suns--is full of meteoric and cometic matter, it would seem probable that in the case of two suns so near together the quantity of such matter would be very great, and would lead probably by continued collisions to increase of their bulk, and perhaps to their final coalescence into a single giant orb. it is said that a persian astronomer in the tenth century calls algol a red star, while it is now white or somewhat yellowish. this would imply an increase of temperature caused by collisions or friction, and increasing proximity of the pair of stars. a considerable number of double stars with dark companions have been discovered by means of the spectroscope, although their motion is not directly in the line of sight, and therefore there is no obscuration. in order to discover such pairs the spectra of large numbers of stars are taken on photographic plates every night and for considerable periods--for a year or for several years. these plates are then carefully examined with a high magnifying power to discover any periodical displacement of the lines, and it is astonishing in how large a number of cases this has been found to exist and the period of revolution of the pair determined. but besides discovering double stars of which one is dark and one bright, many pairs of bright stars have been discovered by the same means. the method in this case is rather different. each component star, being luminous, will give a separate spectrum, and the best spectroscopes are so powerful that they will separate these spectra when the stars are at their maximum distance although no telescope in existence, or ever likely to be made, can separate the component stars. the separation of the spectra is usually shown by the most prominent lines becoming double and then after a time single, indicating that the plane of revolution is more or less obliquely towards us, so that the two stars if visible would appear to open out and then get nearer together every revolution. then, as each star alternately approaches and recedes from us the radial velocity of each can be determined, and this gives the relative mass. in this way not only doubles, but triple and multiple systems, have been discovered. the stars proved to be double by these two methods are so numerous that it has been estimated by one of the best observers that about one star in every thirteen shows inequality in its radial motion and is therefore really a double star. the nebulÆ one other great result of spectrum-analysis, and in some respects perhaps the greatest, is its demonstration of the fact that true nebulæ exist, and that they are not all star-clusters so remote as to be irresolvable, as was once supposed. they are shown to have gaseous spectra, or sometimes gaseous and stellar spectra combined, and this, in connection with the fact that nebulæ are frequently aggregated around nebulous stars or groups of stars, renders it certain that the nebulæ are in no way separated in space from the stars, but that they constitute essential parts of one vast stellar universe. there is, indeed, good reason to believe that they are really the material out of which stars are made, and that in their forms, aggregations, and condensations, we can trace the very process of evolution of stars and suns. photographic astronomy but there is yet another powerful engine of research which the new astronomy possesses, and which, either alone or in combination with the spectroscope, had produced and will yet produce in the future an amount of knowledge of the stellar universe which could never be attained by any other means. it has already been stated how the discovery of new variable and binary stars has been rendered possible by the preservation of the photographic plates on which the spectra are self-recorded, night after night, with every line, whether dark or coloured, in true position, so as to bear magnification, and, by comparison with others of the series, enabling the most minute changes to be detected and their amount accurately measured. without the preservation of such comparable records, which is in no other way possible, by far the larger portion of spectroscopic discoveries could never have been made. but there are two other uses of photography of quite a different nature which are equally and perhaps in their final outcome may be far more important. the first is, that by the use of the photographic plate the exact positions of scores, hundreds, or even thousands of stars can be self-mapped simultaneously with extreme accuracy, while any number of copies can be made of these star-maps. this entirely obviates the necessity for the old method of fixing the position of each star by repeated measurement by means of very elaborate instruments, and their registration in laborious and expensive catalogues. so important is this now seen to be, that specially constructed cameras are made for stellar photography, and by means of the best kinds of equatorial mounting are made to revolve slowly so that the image of each star remains stationary upon the plate for several hours. arrangements have been now made among all the chief observatories of the world to carry out a photographic survey of the heavens with identical instruments, so as to produce maps of the whole star-system on the same scale. these will serve as fixed data for future astronomers, who will thus be able to determine the movements of stars of all magnitudes with a certainty and accuracy hitherto unattainable. the other important use of photography depends upon the fact that with a longer exposure within certain limits we increase the light-collecting power. it will surprise many persons to learn that an ordinary good portrait-camera with a lens three or four inches in diameter, if properly mounted so that an exposure of several hours can be made, will show stars so minute that they are invisible even in the great lick telescope. in this way the camera will often reveal double-stars or small groups which can be made visible in no other way. such photographs of the stars are now constantly reproduced in works on astronomy and in popular magazine articles, and although some of them are very striking, many persons are disappointed with them, and cannot understand their great value, because each star is represented by a white circle often of considerable size and with a somewhat undefined outline, not by a minute point of light as stars appear in a good telescope. but the essential matter in all such photographs is not so much the smallness, as the roundness, of the star-images, as this proves the extreme precision with which the image of every star has been kept by the clockwork motion of the instrument on the same point of the plate during the whole exposure. for example, in the fine photograph of the great nebula in andromeda, taken th december , by dr. isaac roberts, with an exposure of four hours, there are probably over a thousand stars large and small to be seen, every one represented by an almost exactly circular white dot of a size dependent on the magnitude of the star. these round dots can be bisected by the cross hairs of a micrometer with very great accuracy, and thus the distance between the centres of any of the pairs, as well as the direction of the line joining their centres, can be determined as accurately as if each was represented by a point only. but as a minute white speck would be almost invisible on the maps, and would convey no information as to the approximate magnitude of the star, mistakes would be much more easily made, and it would probably be found necessary to surround each star with a circle to indicate its magnitude, and to enable it to be easily seen. it is probable, therefore, that the supposed defect is really an important advantage. the above-mentioned photograph is beautifully reproduced in proctor's _old and new astronomy_, published after his greatly lamented death. but besides the amount of altogether new knowledge obtained by the methods of research here briefly explained, a great deal of light has been thrown on the distribution of the stars as a whole, and hence on the nature and extent of the stellar universe, by a careful study of the materials obtained by the old methods, and by the application of the doctrine of probabilities to the observed facts. in this way alone some very striking results have been reached, and these have been supported and strengthened by the newer methods, and also by the use of new instruments in the measurement of stellar distances. some of these results bear so closely and directly upon the special subject of the present volume, that our next chapter must be devoted to a consideration of them. chapter iv the distribution of the stars if we look at the heavens on a clear, moonless night in winter, and from a position embracing the entire horizon, the scene is an inexpressibly grand one. the intense sparkling brilliancy of sirius, capella, vega, and other stars of the first magnitude; their striking arrangement in constellations or groups, of which orion, the great bear, cassiopeiæ, and the pleiades, are familiar examples; and the filling up between these by less and less brilliant points down to the limit of vision, so as to cover the whole sky with a scintillating tracery of minute points of light, convey together an idea of such confused scattering and such enormous numbers, that it seems impossible to count them or to reduce them to systematic order. yet this was done for all except the faintest stars by hipparchus, b.c., who catalogued and fixed the positions of more than stars, and this is about the number, down to the fifth magnitude, visible in the latitude of greece. a recent enumeration of all the stars visible to the naked eye, under the most favourable conditions and by the best eyesight, has been made by the american astronomer, pickering. his numbers are--for the northern hemisphere , and for the southern hemisphere , thus showing a somewhat greater richness in the southern celestial hemisphere. but as this difference is due entirely to a preponderance of stars between magnitudes - / and , that is, just on the limits of vision, while those down to magnitude - / are more numerous by in the northern hemisphere, professor newcomb is of opinion that there is no real superiority of numbers of visible stars in one hemisphere over the other. again, the total number of the visible stars by the above enumeration is . but this includes stars down to . magnitude, while it is generally considered that magnitude marks the limit of visibility. on a re-examination of all the materials, the italian astronomer schiaparelli concludes that the total number of stars down to the sixth magnitude is ; and they seem to be about equally divided between the northern and southern skies. the milky way but besides the stars themselves, a most conspicuous object both in the northern and southern hemisphere is that wonderful irregular belt of faintly diffused light termed the milky way or galaxy. this forms a magnificent arch across the sky, best seen in the autumn months in our latitude. this arch, while following the general course of a great circle round the heavens, is extremely irregular in detail, sometimes being single, sometimes double, sending off occasional branches or offshoots, and also containing in its very midst dark rifts, spots, or patches, where the black background of almost starless sky can be seen through it. when examined through an opera-glass or small telescope quantities of stars are seen on the luminous background, and with every increase in the size and power of the telescope more and more stars become visible, till with the largest and best modern instruments the whole of the galaxy seems densely packed with them, though still full of irregularities, wavy streams of stars, and dark rifts and patches, but always showing a faint nebulous background as if there remained other myriads of stars which a still higher optical power would reveal. the relations of this great belt of telescopic stars to the rest of the star-system have long interested astronomers, and many have attempted its solution. by a system of gauging, that is counting all the stars that passed over the field of his telescope in a certain time, sir william herschel was the first who made a systematic effort to determine the shape of the stellar universe. from the fact that the number of stars increased rapidly as the milky way was approached from whatever direction, while in the galaxy itself the numbers visible were at once more than doubled, he formed the idea that the shape of the entire system must be that of a highly compressed very broad mass or ring rather less dense towards the centre where our sun was situated. roughly speaking, the form was likened to a flat disc or grindstone, but of irregular thickness, and split in two on one side where it appears to be double. the immense quantity of the stars which formed it was supposed to be due to the fact that we looked at it edgewise through an immense depth of stars; while at right angles to its direction when looking towards what is termed the pole of the galaxy, and also in a less degree when looking obliquely, we see out into space through a much thinner stratum of stars, which thus seem on the average to be very much farther apart. but, in the latter part of his life, sir william herschel realised that this was not the true explanation of the features presented by the galaxy. the brilliant spots and patches in it, the dark rifts and openings, the narrow streams of light often bounded by equally narrow streams or rifts of darkness, render it quite impossible to conceive that this complex luminous ring has the form of a compressed disc extending in the direction in which we see it to a distance many times greater than its thickness. in one very luminous cluster herschel thought that his telescope had penetrated to regions twenty times as far off as the more brilliant stars forming the nearer portions of the same object. now, in the case of the magellanic clouds, which are two roundish nebular patches of large size some distance from the milky way in the southern hemisphere and looking like detached portions of it, sir john herschel himself has shown that any such interpretation of its form is impossible; because it requires us to suppose that in both these cases we see, not rounded masses of a roughly globular shape, but immensely long cones or cylinders, placed in such a direction that we see only the ends of them. he remarks that one such object so situated would be an extraordinary coincidence, but that there should be two or many such is altogether out of the question. but in the milky way there are hundreds or even thousands of such spots or masses of exceptional brilliancy or exceptional darkness; and, if the form of the galaxy is that of a disc many times broader than thick, and which we see edgewise, then every one of these patches and clusters, and all the narrow winding streams of bright light or intense blackness, must be really excessively long cylinders, or tunnels, or deep curving laminæ, or narrow fissures. and every one of these, which are to be found in every part of this vast circle of luminosity, must be so arranged as to be exactly turned towards our sun. the weight of this argument, which has been most forcibly and clearly set forth by the late mr. r.a. proctor, in his very instructive volume _our place among infinities_, is now generally admitted by astronomers, and the natural conclusion is that the form of the milky way is that of a vast irregular ring, of which the section at any part is, roughly speaking, circular; while the many narrow rifts or lanes or openings where we seem to be able to see completely through it to the darkness of outer space beyond, render it probable that in those directions its thickness is less instead of greater than its apparent width, that is, that we see the broader side rather than the narrow edge of it. before entering on the consideration of the relations which the bulk of the stars we see scattered over the entire vault of heaven bear to this great belt of telescopic stars, it will be advisable to give a somewhat full description of the galaxy itself, both because it is not often delineated on star-maps with sufficient accuracy, or so as to show its wonderful intricacies of structure, and also because it constitutes the fundamental phenomenon upon which the argument set forth in this volume primarily rests. for this purpose i shall use the description of it given by sir john herschel in his _outlines of astronomy_, both because he, of all the astronomers of the last century, had studied it most thoroughly, in the northern and in the southern hemispheres, by eye-observation and with the aid of telescopes of great power and admirable quality; and also because, amid the throng of modern works and the exciting novelties of the last thirty years, his instructive volume is, comparatively speaking, very little known. this precise and careful description will also be of service to any of my readers who may wish to form a closer personal acquaintance with this magnificent and intensely interesting object, by examining its peculiarities of form and beauties of structure either with the naked eye, or with the aid of a good opera-glass, or with a small telescope of good defining power. a description of the milky way sir john herschel's description is as follows:--'the course of the milky way as traced through the heavens by the unaided eye, neglecting occasional deviations and following the line of its greatest brightness as well as its varying breadth and intensity will permit, conforms, as nearly as the indefiniteness of its boundary will allow it to be fixed, to that of a great circle inclined at an angle of about ° to the equinoctial, and cutting that circle in right ascension h. m. and h. m., so that its northern and southern poles respectively are situated in right ascension h. m., north polar distance °, and r.a. h. m., npd. °. throughout the region where it is so remarkably subdivided, this great circle holds an intermediate situation between the two great streams; with a nearer approximation however to the brighter and continuous stream than to the fainter and interrupted one. if we trace its course in order of right ascension, we find it traversing the constellation cassiopeiæ, its brightest part passing about two degrees to the north of the star delta of that constellation. passing thence between gamma and epsilon cassiopeiæ, it sends off a branch to the south-preceding side, towards alpha persei, very conspicuous as far as that star, prolonged faintly towards eta of the same constellation, and possibly traceable towards the hyades and pleiades as remote outliers. the main stream, however (which is here very faint), passes on through auriga, over the three remarkable stars, epsilon, zeta, eta, of that constellation called the hædi, preceding capella, between the feet of gemini and the horns of the bull (where it intersects the ecliptic nearly in the solstitial colure) and thence over the club of orion to the neck of monoceros, intersecting the equinoctial in r.a. h. m. up to this point, from the offset in perseus, its light is feeble and indefinite, but thenceforward it receives a gradual accession of brightness, and where it passes through the shoulder of monoceros and over the head of canis major it presents a broad, moderately bright, very uniform, and to the naked eye, starless stream up to the point where it enters the prow of the ship argo, nearly on the southern tropic. here it again subdivides (about the star _m_ puppis), sending off a narrow and winding branch on the preceding side as far as gamma argûs, where it terminates abruptly. the main stream pursues its southward course to the rd parallel of npd., where it diffuses itself broadly and again subdivides, opening out into a wide fan-like expanse, nearly ° in breadth, formed of interlacing branches, which all terminate abruptly, in a line drawn nearly through lambda and gamma argûs. 'at this place the continuity of the milky way is interrupted by a wide gap, and where it recommences on the opposite side it is by a somewhat similar fan-shaped assemblage of branches which converge upon the bright star eta argûs. thence it crosses the hind feet of the centaur, forming a curious and sharply-defined semicircular concavity of small radius, and enters the cross by a very bright neck or isthmus of not more than three or four degrees in breadth, being the narrowest portion of the milky way. after this it immediately expands into a broad and bright mass, enclosing the stars alpha and beta crucis and beta centauri, and extending almost up to alpha of the latter constellation. in the midst of this bright mass, surrounded by it on all sides, and occupying about half its breadth, occurs a singular dark pear-shaped vacancy, so conspicuous and remarkable as to attract the notice of the most superficial gazer and to have acquired among the early southern navigators the uncouth but expressive appellation of the _coal-sack_. in this vacancy, which is about ° in length and ° broad, only one very small star visible to the naked eye occurs, though it is far from devoid of telescopic stars, so that its striking blackness is simply due to the effect of contrast with the brilliant ground with which it is on all sides surrounded. this is the place of nearest approach of the milky way to the south pole. throughout all this region its brightness is very striking, and when compared with that of its more northern course already traced, conveys strongly the impression of greater proximity, and would almost lead to a belief that our situation as spectators is separated on all sides by a considerable interval from the dense body of stars composing the galaxy, which in this view of the subject would come to be considered as a flat ring or some other re-entering form of immense and irregular breadth and thickness, within which we are excentrically situated, nearer to the southern than to the northern part of its circuit. 'at alpha centauri the milky way again subdivides, sending off a great branch of nearly half its breadth, but which thins off rapidly, at an angle of about ° with its general direction to eta and _d_ lupi, beyond which it loses itself in a narrow and faint streamlet. the main stream passes on increasing in breadth to gamma normæ, where it makes an abrupt elbow and again subdivides into one principal and continuous stream of very irregular breadth and brightness, and a complicated system of interlaced streaks and masses, which covers the tail of scorpio, and terminates in a vast and faint effusion over the whole extensive region occupied by the preceding leg of ophiuchus, extending northward to the parallel of ° npd., beyond which it cannot be traced; a wide interval of °, free from all appearance of nebulous light, separating it from the great branch on the north side of the equinoctial of which it is usually represented as a continuation. 'returning to the point of separation of this great branch from the main stream, let us now pursue the course of the latter. making an abrupt bend to the following side, it passes over the stars iota aræ, theta and iota scorpii, and gamma tubi to gamma sagittarii, where it suddenly collects into a vivid oval mass about ° in length and ° in breadth, so excessively rich in stars that a very moderate calculation makes their number exceed , . northward of this mass, this stream crosses the ecliptic in longitude about °, and proceeding along the bow of sagittarius into antinous has its course rippled by three deep concavities, separated from each other by remarkable protuberances, of which the larger and brighter forms the most conspicuous patch in the southern portion of the milky way visible in our latitudes. 'crossing the equinoctial at the th hour of r.a., it next runs in an irregular, patchy, and winding stream through aquila, sagitta, and vulpecula up to cygnus; at epsilon of which constellation its continuity is interrupted, and a very confused and irregular region commences, marked by a broad dark vacuity, not unlike the southern "coal-sack," occupying the space between epsilon, alpha, and gamma cygni, which serves as a kind of centre for the divergence of three great streams; one, which we have already traced; a second, the continuation of the first (across the interval) from alpha northward, between lacerta and the head of cepheus to the point in cassiopeiæ whence we set out, and a third branching off from gamma cygni, very vivid and conspicuous, running off in a southern direction through beta cygni, and _s_ aquilæ almost to the equinoctial, where it loses itself in a region thinly sprinkled with stars, where in some maps the modern constellation taurus poniatowski is placed. this is the branch which, if continued across the equinoctial, might be supposed to unite with the great southern effusion in ophiuchus already noticed. a considerable offset, or protuberant appendage, is also thrown off by the northern stream from the head of cepheus directly towards the pole, occupying the greater part of the quartile formed by alpha, beta, iota, and delta of that constellation.' to complete this careful, detailed description of the milky way, it will be well to add a few passages from the same work as to its telescopic appearance and structure. 'when examined with powerful telescopes, the constitution of this wonderful zone is found to be no less various than its aspect to the naked eye is irregular. in some regions the stars of which it is composed are scattered with remarkable uniformity over immense tracts, while in others the irregularity of their distribution is quite as striking, exhibiting a rapid succession of closely clustering rich patches separated by comparatively poor intervals, and indeed in some instances by spaces absolutely dark _and completely void of any star_, even of the smallest telescopic magnitude. in some places not more than or stars on an average occur in a gauge-field of ', while in others a similar average gives a result of or . nor is less variety observable in the character of its different regions in respect of the magnitudes of the stars they exhibit, and the proportional numbers of the larger and smaller magnitudes associated together, than in respect of their aggregate numbers. in some, for instance, extremely minute stars occur in numbers so moderate as to lead us irresistibly to the conclusion that in these regions we see _fairly through_ the starry stratum, since it is impossible otherwise that the numbers of the smaller magnitudes should not go on continually increasing ad infinitum. in such cases, moreover, the ground of the heavens is for the most part perfectly dark, which again would not be the case if innumerable multitudes of stars, too minute to be individually discernible, existed beyond. in other regions we are presented with the phænomenon of an almost uniform degree of brightness of the individual stars, accompanied with a very even distribution of them over the ground of the heavens, both the larger and smaller magnitudes being strikingly deficient. in such cases it is equally impossible not to perceive that we are looking _through_ a sheet of stars nearly of a size, and of no great thickness compared with the distance which separates them from us. were it otherwise we should be driven to suppose the more distant stars uniformly the larger, so as to compensate by their greater intrinsic brightness for their greater distance, a supposition contrary to all probability.... 'throughout by far the larger portion of the extent of the milky way in both hemispheres, the general blackness of the ground of the heavens on which its stars are projected, and the absence of that innumerable multitude and excessive crowding of the smallest visible magnitudes, and of glare produced by the aggregate light of multitudes too small to affect the eye singly, must, we think, be considered unequivocal indications that its dimensions in _directions where these conditions obtain_ are not only not infinite, but that the space-penetrating power of our telescopes suffices fairly to pierce through and beyond it.' in the above-quoted passage the italics are those of sir john herschel himself, and we see that he drew the very same conclusions from the facts he describes, and for much the same reasons, as mr. proctor has drawn from the observations of sir william herschel; and, as we shall see, the best astronomers to-day have arrived at a similar result, from the additional facts at their disposal, and in some cases from fresh lines of argument. the stars in relation to the milky way sir john herschel was so impressed with the form, structure, and immensity of the galactic circle, as he sometimes terms it, that he says (in a footnote p. , th ed.), 'this circle is to sidereal what the invariable ecliptic is to planetary astronomy--a plane of ultimate reference, the ground-plane of the sidereal system.' we have now to consider what are the relations of the whole body of the stars to this galactic circle--this plane of ultimate reference for the whole stellar universe. if we look at the heavens on a starry night, the whole vault appears to be thickly strewn with stars of various degrees of brightness, so that we could hardly say that any extensive region--the north, east, south, or west, or the portion vertically above us--is very conspicuously deficient or superior in numbers. in every part there are to be found a fair proportion of stars of the first two or three magnitudes, while where these may seem deficient a crowd of smaller stars takes their place. but an accurate survey of the visible stars shows that there is a large amount of irregularity in their distribution, and that all magnitudes are really more numerous in or near the milky way, than at a distance from it, though not in so large a degree as to be very conspicuous to the naked eye. the area of the whole of the milky way cannot be estimated at more than one-seventh of the whole sphere, while some astronomers reckon it at only one-tenth. if stars of any particular size were uniformly distributed, at most one-seventh of the whole number should be found within its limits. but mr. gore finds that of stars brighter than the second magnitude lie upon the milky way, or considerably more than twice as many as there should be if they were uniformly distributed. and in the case of the stars which are brighter than the third magnitude lie upon the milky way, or one-third instead of one-seventh. mr. gore also counted all the stars in heis's atlas which lie upon the milky way, and finds there are out of a total of , a proportion of between a fourth and a fifth instead of a seventh. the late mr. proctor in laid down on a chart two feet diameter all the stars down to magnitude - / given in agrelander's forty large charts of the stars visible in the northern hemisphere. they were , in number, and they distinctly showed by their greater density not only the whole course of the milky way but also its more luminous portions and many of the curious dark rifts and vacuities, which latter are almost wholly avoided by these stars. later on professor seeliger of munich made an investigation of the relation of more than , stars down to the ninth magnitude to the milky way, by dividing the whole of the heavens into nine regions, one and nine being circles of ° wide (equal to ° diameter) at the two poles of the galaxy; the middle region, five, is a zone ° wide including the milky way itself, and the other six intermediate zones are each ° wide. the following table shows the results as given by professor newcomb, who has made some alterations in the last column of 'density of stars' in order to correct differences in the estimate of magnitudes by the different authorities. regions. area in degree. number of stars. density. i. , . , . ii. , . , . iii. , . , . iv. , . , . v. , . , . vi. , . , . vii. , . , . viii. , . , . ix. . , . _n.b._--the inequality of the n. and s. areas is because the enumeration of the stars only went as far as ° s. decl., and therefore included only a part of regions vii., viii., and ix. [illustration: diagram of star-density from herschel's gauges (as given by professor newcomb, p. ).] upon this table of densities professor newcomb remarks as follows:--'the star-density in the several regions increases continuously from each pole (regions i. and ix.) to the galaxy itself (region v.). if the latter were a simple ring of stars surrounding a spherical system of stars, the star-density would be about the same in regions i., ii., and iii., and also in vii., viii., and ix., but would suddenly increase in iv. and vi. as the boundary of the ring was approached. instead of such being the case, the numbers . , . , and . in the north, and . , . , and . in the south, show a progressive increase from the galactic pole to the galaxy itself. the conclusion to be drawn is a fundamental one. the universe, or at least the denser portion of it, is really flattened between the galactic poles, as supposed by herschel and struve.' but looking at the series of figures in the table, and again as quoted by professor newcomb, they seem to me to show in some measure what he says they do not show. i therefore drew out the above diagram from the figures in the table, and it certainly shows that the density in regions i., ii., and iii., and in regions vii., viii., and ix., may be said to be 'about the same,' that is, they increase very slowly, and that they _do_ 'suddenly increase' in iv. and vi. as the boundary of the galaxy is approached. this may be explained either by a flattening towards the poles of the galaxy, or by the thinning out of stars in that direction. in order to show the enormous difference of star-density in the galaxy and at the galactic poles, professor newcomb gives the following table of the herschelian gauges, on which he only remarks that they show an enormously increased density in the galactic region due to the herschels having counted so many more stars there than any other observers. +-----------+----+----+----+----+-----+-----+-----+-----+-----+ |region, .| i. | ii.|iii.| iv.| v. | vi. | vii.|viii.| ix. | | | | | | | | | | | | |density, .| | | | | , | | | | | +-----------+----+----+----+----+-----+-----+-----+-----+-----+ [illustration: diagram of star-density from a table in _the stars_ (p. ).] but an important characteristic of these figures is, that the herschels alone surveyed the whole of the heavens from the north to the south pole, that they did this with instruments of the same size and quality, and that from almost life-long experience in this particular work they were unrivalled in their power of counting rapidly and accurately the stars that passed over each field of view of their telescopes. their results, therefore, must be held to have a comparative value far above those of any other observer or combination of observers. i have therefore thought it advisable to draw a diagram from their figures, and it will be seen how strikingly it agrees with the former diagram in the very slow increase of star-richness in the first three regions north and south, the sudden increase in regions iv. and vi. as we approach the galaxy, while the only marked difference is in the enormously greater richness of the galaxy itself, which is an undoubtedly real phenomenon, and is brought out here by the unrivalled observing power of the two greatest astronomers in this special department that have ever lived. we shall find later on that professor newcomb himself, as the result of a quite different inquiry arrives at a result in accordance with these diagrams which will then be again referred to. as this is a very interesting subject, it will be well to give another diagram from two tables of star-density in sir john herschel's volume already quoted. the tables are as follows:-- zones of galactic average number of star north polar distance. per field of '. ° to ° . ° to ° . ° to ° . ° to ° . ° to ° . ° to ° . zones of galactic average number of stars south polar distance. per field of '. ° to ° . ° to ° . ° to ° . ° to ° . ° to ° . ° to ° . in these tables the milky way itself is taken as occupying two zones of ° each, instead of one of ° as in professor newcomb's tables, so that the excess in the number of stars over the other zones is not so large. they show also a slight preponderance in all the zones of the southern hemisphere, but this is not great, and may probably be due to the clearer atmosphere of the cape of good hope as compared with that of england. [illustration: diagram of star-density. from table in sir j. herschel's _outlines of astronomy_ ( th ed., pp. - ).] it need only be noted here that this diagram shows the same general features as those already given, of a continuous increase of star-density from the poles of the galaxy, but more rapidly as the galaxy itself is more nearly approached. this fact must, therefore, be accepted as indisputable. clusters and nebulÆ in relation to the galaxy an important factor in the structure of the heavens is afforded by the distribution of the two classes of objects known as clusters and nebulæ. although we can form an almost continuous series from double stars which revolve round their common centre of gravity, through triple and quadruple stars, to groups and aggregations of indefinite extent--of which the pleiades form a good example, since the six stars visible to the naked eye are increased to hundreds by high telescopic powers, while photographs with three hours' exposure show more than stars--yet none of these correspond to the large class known as clusters, whether globular or irregular, which are very numerous, about having been recorded by sir john herschel more than fifty years ago. many of these are among the most beautiful and striking objects in the heavens even with a very small telescope or good opera-glass. such is the luminous spot called praesepe, or the beehive in the constellation cancer, and another in the sword handle of perseus. in the southern hemisphere there is a hazy star of about the fourth magnitude, omega centauri, which with a good telescope is seen to be really a magnificent cluster nearly two-thirds the diameter of the moon, and described by sir john herschel as very gradually increasing in brightness to the centre, and composed of innumerable stars of the thirteenth and fifteenth magnitudes, forming the richest and largest object of the kind in the heavens. he describes it as having rings like lace-work formed of the larger stars. by actual count, on a good photograph, there are more than stars, while other observers consider that there are at least , . in the northern hemisphere one of the finest is that in the constellation hercules, known as messier. it is just visible to the naked eye or with an opera glass as a hazy star of the sixth magnitude, but a good telescope shows it to be a globular cluster, and the great lick telescope resolves even the densest central portion into distinct stars, of which sir john herschel considered there were many thousands. these two fine clusters are figured in many of the modern popular works on astronomy, and they afford an excellent idea of these beautiful and remarkable objects, which, when more thoroughly studied, will probably aid in elucidating some of the obscure problems connected with the constitution and development of the stellar universe. but for the purpose of the present work the most interesting fact connected with star-clusters is their remarkable distribution in the heavens. their special abundance in and near the milky way had often been noted, but the full importance of the fact could not be appreciated till mr. proctor and, later, mr. sidney waters marked down, on maps of the two hemispheres, all the star-clusters and nebulæ in the best catalogues. the result is most interesting. the clusters are seen to be thickly strewn over the entire course of the milky way, and along its margins, while in every other part of the heavens they are thinly scattered at very distant intervals, with the one exception of the magellanic clouds of the southern hemisphere where they are again densely grouped; and if anything were needed to prove the physical connection of these clusters with the galaxy it would be their occurrence in these extensive nebulous patches which seem like outlying portions of the milky way itself. with these two exceptions probably not one-twentieth part of the whole number of star-clusters are found in any part of the heavens remote from the milky way. nebulæ were for a long time confounded with star-clusters, because it was thought that with sufficient telescopic power they could all be resolvable into stars as in the case of the milky way itself. but when the spectroscope showed that many of the nebulæ consisted wholly or mainly of glowing gases, while neither the highest powers of the best telescopes nor the still greater powers of the photographic plate gave any indications of resolvability, although a few stars were often found to be, as it were, entangled in them, and evidently forming part of them, it was seen that they constituted a distinct stellar phenomenon, a view which was enforced and rendered certain by their quite unique mode of distribution. a few of the larger and irregular type, as in the case of the grand orion nebula visible to the naked eye, the great spiral nebula in andromeda, and the wonderful keyhole nebula round eta argûs, are situated in or near the milky way; but with these and a few other exceptions the overwhelming majority of the smaller irresolvable nebulæ appear to avoid it, there being a space almost wholly free from nebulæ along its borders, both in the northern and southern hemispheres; while the great majority are spread over the sky, far away from it in the southern hemisphere, and in the north clustering in a very marked degree around the galactic pole. the distribution of nebulæ is thus seen to be the exact opposite to that of the star-clusters, while both are so distinctly related to the position of the milky way--the ground-plane of the sidereal system, as sir john herschel termed it--that we are compelled to include them all as connected portions of one grand and, to some extent, symmetrical universe, whose remarkable and opposite mode of distribution over the heavens may probably afford a clue to the mode of development of that universe and to the changes that are even now taking place within it. the maps referred to above are of such great importance, and are so essential to a clear comprehension of the nature and constitution of the vast sidereal system which surrounds us, that i have, with the permission of the royal astronomical society, reproduced them here. (see end of volume.) a careful examination of them will give a clearer idea of the very remarkable facts of distribution of star-clusters and nebulæ than can be afforded by any amount of description or of numerical statements. the forms of many of the nebulæ are very curious. some are quite irregular, as the orion nebula, the keyhole nebula in the southern hemisphere, and many others. some show a decidedly spiral form, as those in andromeda and canes venatici; others again are annular or ring-shaped, as those in lyra and cygnus, while a considerable number are termed planetary nebulæ, from their exhibiting a faint circular disc like that of a planet. many have stars or groups of stars evidently forming parts of them, and this is especially the case with those of the largest size. but all these are comparatively few in number and more or less exceptional in type, the great majority being minute cloudy specks only visible with good telescopes, and so faint as to leave much doubt as to their exact shape and nature. sir john herschel catalogued in , and more than were discovered up to ; while the application of the camera has so increased the numbers that it is thought there may really be many hundreds of thousands of them. the spectroscope shows the larger irregular nebulæ to be gaseous, as are the annular and planetary nebulæ as well as many very brilliant white stars; and all these objects are most frequent in or near the milky way. their spectra show a green line not produced by any terrestrial element. with the great lick telescope several of the planetary nebulæ have been found to be irregular and sometimes to be formed of compressed or looped rings and other curious forms. many of the smaller nebulæ are double or triple, but whether they really form revolving systems is not yet known. the great mass of the small nebulæ that occupy large tracts of the heavens remote from the galaxy are often termed irresolvable nebulæ, because the highest powers of the largest telescopes show no indication of their being star-clusters, while they are too faint to give any definite indications of structure in the spectroscope. but many of them resemble comets in their forms, and it is thought not impossible that they may be not very dissimilar in constitution. * * * * * we have now passed in review the main features presented to us in the heavens outside the solar system, so far as regards the numbers and distribution of the lucid stars (those visible to the naked eye) as well as those brought to view by the telescope; the form and chief characteristics of the milky way or galaxy; and lastly, the numbers and distribution of those interesting objects--star-clusters and nebulæ in their special relations to the milky way. this examination has brought clearly before us the unity of the whole visible universe; that everything we can see, or obtain any knowledge of, with all the resources of modern gigantic telescopes, of the photographic plate, and of the even more marvellous spectroscope, forms parts of one vast system which may be shortly and appropriately termed the stellar universe. in our next chapter we shall carry the investigation a step further, by sketching in outline what is known of the motions and distances of the stars, and thus obtain some important information bearing upon our special subject of inquiry. chapter v distance of the stars--the sun's motion through space in early ages, before any approximate idea was reached of the great distances of the stars from us, the simple conception of a crystal sphere to which these luminous points were attached and carried round every day on an axis near which our pole-star is situated, satisfied the demands for an explanation of the phenomena. but when copernicus set forth the true arrangement of the heavenly bodies, earth and planets alike revolving round the sun at distances of many millions of miles, and when this scheme was enforced by the laws of kepler and the telescopic discoveries of galileo, a difficulty arose which astronomers were unable satisfactorily to overcome. if, said they, the earth revolves round the sun at a distance which cannot be less (according to kepler's measurement of the distance of mars at opposition) than - / millions of miles, then how is it that the nearer stars are not seen to shift their apparent places when viewed from opposite sides of this enormous orbit? copernicus, and after him kepler and galileo, stoutly maintained that it was because the stars were at such an enormous distance from us that the earth's orbit was a mere point in comparison. but this seemed wholly incredible, even to the great observer tycho brahé, and hence the copernican theory was not so generally accepted as it otherwise would have been. galileo always declared that the measurement would some day be made, and he even suggested the method of effecting it which is now found to be the most trustworthy. but the sun's distance had to be first measured with greater accuracy, and that was only done in the latter part of the eighteenth century by means of transits of venus; and by later observations with more perfect instruments it is now pretty well fixed at about , , miles, the limits of error being such that - / millions may perhaps be quite as accurate. with such an enormous base-line as twice this distance, which is available by making observations at intervals of about six months when the earth is at opposite points in its orbit, it seemed certain that some parallax or displacement of the nearer stars could be found, and many astronomers with the best instruments devoted themselves to the work. but the difficulties were enormous, and very few really satisfactory results were obtained till the latter half of the nineteenth century. about forty stars have now been measured with tolerable certainty, though of course with a considerable margin of possible or probable error; and about thirty more, which are found to have a parallax of one-tenth of a second or less, must be considered to leave a very large margin of uncertainty. the two nearest fixed stars are alpha centauri and cygni. the former is one of the brightest stars in the southern hemisphere, and is about , times as far from us as the sun. the light from this star will take - / years to reach us, and this 'light-journey,' as it is termed, is generally used by astronomers as an easily remembered mode of recording the distances of the fixed stars, the distance in miles--in this case about millions of millions--being very cumbrous. the other star, cygni, is only of about the fifth magnitude, yet it is the second nearest to us, with a light-journey of about - / years. if we had no other determinations of distance than these two, the facts would be of the highest importance. they teach us, first, that magnitude or brightness of a star is no proof of nearness to us, a fact of which there is much other evidence; and in the second place, they furnish us with a probable minimum distance of independent suns from one another, which, in proportion to their sizes, some being known to be many times larger than our sun, is not more than we might expect. this remoteness may be partly due to those which were once nearer together having coalesced under the influence of gravitation. as this measurement of the distance of the nearer stars should be clearly understood by every one who wishes to obtain some real comprehension of the scale of this vast universe of which we form a part, the method now adopted and found to be most effectual will be briefly explained. everyone who is acquainted with the rudiments of trigonometry or mensuration, knows that an inaccessible distance can be accurately determined if we can measure a base-line from both ends of which the inaccessible object can be seen, and if we have a good instrument with which to measure angles. the accuracy will mainly depend upon our base-line being not excessively short in comparison with the distance to be measured. if it is as much as half or even a quarter as long the measurement may be as accurate as if directly performed over the ground, but if it is only one-hundredth or one-thousandth part as long, a very small error either in the length of the base or in the amount of the angles will produce a large error in the result. in measuring the distance of the moon, the earth's diameter, or a considerable portion of it, has served as a base-line. either two observers at great distances from each other, or the same observer after an interval of nine or ten hours, may examine the moon from positions six or seven thousand miles apart, and by accurate measurements of its angular distance from a star, or by the time of its passage over the meridian of the place as observed with a transit instrument, the angular displacement can be found and the distance determined with very great accuracy, although that distance is more than thirty times the length of the base. the distance of the planet mars when nearest to us has been found in the same way. his distance from us even when at his nearest point during the most favourable oppositions is about million miles, or more than four thousand times the earth's diameter, so that it requires the most delicate observations many times repeated and with the finest instruments to obtain a tolerably approximate result. when this is done, by kepler's law of the fixed proportion between the distances of planets from the sun and their times of revolution, the proportionate distance of all the other planets and that of the sun can be ascertained. this method, however, is not sufficiently accurate to satisfy astronomers, because upon the sun's distance that of every other member of the solar system depends. fortunately there are two other methods by which this important measurement has been made with much greater approach to certainty and precision. [illustration: diagram illustrating the transit of venus.] the first of these methods is by means of the rare occasions when the planet venus passes across the sun's disc as seen from the earth. when this takes place, observations of the transit, as it is termed, are made at remote parts of the earth, the distance between which places can of course easily be calculated from their latitudes and longitudes. the diagram here given illustrates the simplest mode of determining the sun's distance by this observation, and the following description from proctor's _old and new astronomy_ is so clear that i copy it verbally:--'v represents venus passing between the earth e and the sun s; and we see how an observer at e will see venus as at v', while an observer at e' will see her as at v. the measurement of the distance v v', as compared with the diameter of the sun's disc, determines the angle v v v' or e v e'; whence the distance e v can be calculated from the known length of the base-line e e'. for instance, it is known (from the known proportions of the solar system as determined from the times of revolution by kepler's third law) that e v bears to v v the proportion to , or to ; whence e e' bears to v v' the same proportion. suppose, now, that the distance between the two stations is known to be miles, so that v v' is , miles; and that v v' is found by accurate measurement to be / part of the sun's diameter. then the sun's diameter, as determined by this observation, is times , miles, or , miles; whence from his known apparent size, which is that of a globe - / times farther away from us than its own diameter, his distance is found to be , , miles.' of course, there being two observers, the proportion of the distance v v' to the diameter of the sun's disc cannot be measured directly, but each of them can measure the apparent angular distance of the planet from the sun's upper and lower margins as it passes across the disc, and thus the angular distance between the two lines of transit can be obtained. the distance v v' can also be found by accurately noting the times of the upper and lower passage of venus, which, as the line of transit is considerably shorter in one than the other, gives by the known properties of the circle the exact proportion of the distance between them to the sun's diameter; and as this is found to be the most accurate method, it is the one generally adopted. for this purpose the stations of the observers are so chosen that the length of the two chords, v and v', may have a considerable difference, thus rendering the measurement more easy. the other method of determining the sun's distance is by the direct measurement of the velocity of light. this was first done by the french physicist, fizeau, in , by the use of rapidly revolving mirrors, as described in most works on physics. this method has now been brought to such a decree of perfection that the sun's distance so determined is considered to be equally trustworthy with that derived from the transits of venus. the reason that the determination of the velocity of light leads to a determination of the sun's distance is, because the time taken by light to pass from the sun to the earth is independently known to be min. - / sec. this was discovered so long ago as by means of the eclipses of jupiter's satellites. these satellites revolve round the planet in from - / to days, and, owing to their moving very nearly in the plane of the ecliptic and the shadow of jupiter being so large, the three which are nearest to the planet are eclipsed at every revolution. this rapid revolution of the satellites and frequency of the eclipses enabled their periods of recurrence to be determined with extreme accuracy, especially after many years of careful observation. it was then found that when jupiter was at its farthest distance from the earth the eclipses of the satellites took place a little more than eight minutes later than the time calculated from the mean period of revolution, and when the planet was nearest to us the eclipses occurred the same amount earlier. and when further observation showed that there was no difference between calculation and observation when the planet was at its mean distance from us, and that the error arose and increased exactly in proportion to our varying distance from it, then it became clear that the only cause adequate to produce such an effect was, that light had not an infinite velocity but travelled at a certain fixed rate. this however, though a highly probable explanation, was not absolutely proved till nearly two centuries later, by means of two very difficult measurements--that of the actual distance of the sun from the earth, and that of the actual speed of light in miles per second; the latter corresponding almost exactly with the speed deduced from the eclipses of jupiter's satellites and the sun's distance as measured by the transits of venus. [illustration: (a) and the last lines of the next paragraph should be replaced with: one-tenth of an inch long, and from a point (a) - / inches from it (accurately . inches) we draw straight lines to b and c. then the angle at a is one degree.)] but this problem of measuring the sun's distance, and through it the dimensions of the orbits of all the planets of our system, sinks into insignificance when compared with the enormous difficulties in the way of the determination of the distance of the stars. as a great many people, perhaps the majority of the readers of any popular scientific book, have little knowledge of mathematics and cannot realise what an angle of a minute or a second really means, a little explanation and illustration of these terms will not be out of place. an angle of one degree ( °) is the th part of a circle (viewed from its centre), the th part of a right angle, the th part of either of the angles of an equilateral triangle. to see exactly how much is an angle of one degree we draw a short line (b c) one-tenth of an inch long, and from a point we draw straight lines to b and c. then the angle at a is one degree. now, in all astronomical work, one degree is considered to be quite a large angle. even before the invention of the telescope the old observers fixed the position of the stars and planets to half or a quarter of a degree, while mr. proctor thinks that tycho brahé's positions of the stars and planets were correct to about one or two minutes of arc. but a minute of arc is obtained by dividing the line b c into sixty equal parts and seeing the distance between two of these with the naked eye from the point a. but as very long-sighted people can see very minute objects at or inches distance, we may double the distance a b, and then making the line b c one three-hundredth part of an inch long, we shall have the angle of one minute which tycho brahé was perhaps able to measure. how very large an amount a minute is to the modern astronomer is, however, well shown by the fact that the maximum difference between the calculated and observed positions of uranus, which led adams and leverrier to search for and discover neptune, was only - / minutes, a space so small as to be almost invisible to the average eye, so that if there had been two planets, one in the calculated, the other in the observed place, they would have appeared as one to unassisted vision. in order now to realise what one second of arc really means, let us look at the circle here shown, which is as nearly as possible one-tenth of an inch in diameter--(one-o-tenth of an inch). if we remove this circle to a distance of feet inches it will subtend an angle of one minute, and we shall have to place it at a distance of nearly feet--almost one-third of a mile--to reduce the angle to one second. but the very nearest to us of the fixed stars, alpha centauri, has a parallax of only three-fourths of a second; that is, the distance of the earth from the sun--about - / millions of miles--would appear no wider, seen from the nearest star, than does three-fourths of the above small circle at one-third of a mile distance. to see this circle at all at that distance would require a very good telescope with a power of at least , while to see any small part of it and to measure the proportion of that part to the whole would need very brilliant illumination and a large and powerful astronomical telescope. what is a million? but when we have to deal with millions, and even with hundreds and thousands of millions, there is another difficulty--that few people can form any clear conception of what a million is. it has been suggested that in every large school the walls of one room or hall should be devoted to showing a million at one view. for this purpose it would be necessary to have a hundred large sheets of paper each about feet inches square, ruled in quarter inch squares. in each alternate square a round black wafer or circle should be placed a little over-lapping the square, thus leaving an equal amount of white space between the black spots. at each tenth spot a double width should be left so as to separate each hundred spots ( × ). each sheet would then hold ten thousand spots, which would all be distinctly visible from the middle of a room feet wide, each horizontal or vertical row containing a thousand. one hundred such sheets would contain a million spots, and they would occupy a space feet long in one row, or feet long in five rows, so that they would entirely cover the walls of a room, about feet square and feet high, from floor to ceiling, allowing space for doors but not for windows, the hall or gallery being lighted from above. such a hall would be in the highest degree educational in a country where millions are spoken of so glibly and wasted so recklessly; while no one can really appreciate modern science, dealing as it does with the unimaginably great and little, unless he is enabled to realise by actual vision, and summing up, what a vast number is comprised in _one_ of those millions, which, in modern astronomy and physics, he has to deal with not singly only, but by hundreds and thousands or even by millions. in every considerable town, at all events, a hall or gallery should have a _million_ thus shown upon its walls. it would in no way interfere with the walls being covered when required with maps, or ornamental hangings, or pictures; but when these were removed, the visible and countable million would remain as a permanent lesson to all visitors; and i believe that it would have widespread beneficial effects in almost every department of human thought and action. on a small scale any one can do this for himself by getting a hundred sheets of engineer's paper ruled in small squares, and making the spots very small; and even this would be impressive, but not so much so as on the larger scale. in order to enable every reader of this volume at once to form some conception of the number of units in a million, i have made an estimate of the number of _letters_ contained in it, and i find them to amount to about , --considerably less than half a million. try and realise, when reading it, that if every letter were a pound sterling, we waste as many pounds as there are letters in _two_ such volumes whenever we build a battleship. having thus obtained some real conception of the immensity of a million, we can better realise what it must be to have every one of the dots above described, or every one of the letters in two such volumes as this lengthened out so as to be each a mile long, and even then we should have reached little more than a hundredth part of the distance from our earth to the sun. when, by careful consideration of these figures, we have even partially realised this enormous distance, we may take the next step, which is, to compare this distance with that of the nearest fixed star. we have seen that the parallax of that star is three-fourths of a second, an amount which implies that the star is , times as far from us as our sun is. if after _seeing_ what a million is, and knowing that the sun is - / times this distance from us in miles--a distance which itself is almost inconceivable to us--we find that we have to multiply this almost inconceivable distance , times--more than a quarter of a million times--to reach the _nearest_ of the fixed stars, we shall begin to realise, however imperfectly, how vast is the system of suns around us, and on what a scale of immensity the material universe, which we see so gloriously displayed in the starry heavens and the mysterious galaxy, is constructed. this somewhat lengthy preliminary discussion is thought necessary in order that my readers may form some idea of the enormous difficulty of obtaining any measurement whatever of such distances. i now propose to point out what the special difficulties are, and how they have been overcome; and thus i hope to be able to satisfy them that the figures astronomers give us of the distances of the stars are in no way mere guesses or probabilities, but are real measurements which, within certain not very wide limits of error, may be trusted as giving us correct ideas of the magnitude of the visible universe. measurement of stellar distances the fundamental difficulty of this measurement is, of course, that the distances are so vast that the longest available base-line, the diameter of the earth's orbit, only subtends an angle of little more than a second from the nearest star, while for all the rest it is less than one second and often only a small fraction of it. but this difficulty, great as it is, is rendered far greater by the fact that there is no fixed point in the heavens from which to measure, since many of the stars are known to be in motion, and all are believed to be so in varying degrees, while the sun itself is now known to be moving among the stars at a rate which is not yet accurately determined, but in a direction which is fairly well known. as the various motions of the earth while passing round the sun, though extremely complex, are very accurately known, it was first attempted to determine the changed position of stars by observations, many times repeated at six months' intervals, of the moment of their passage over the meridian and their distance from the zenith; and then by allowing for all the known motions of the earth, such as precession of the equinoxes and nutation of the earth's axis, as well as for refraction and for the aberration of light, to determine what residual effect was due to the difference of position from which the star was viewed; and a result was thus obtained in several cases, though almost always a larger one than has been found by later observations and by better methods. these earlier observations, however perfect the instruments and however skilful the observer, are liable to errors which it seems impossible to avoid. the instruments themselves are subject in all their parts to expansion and contraction by changes of temperature; and when these changes are sudden, one part of the instrument may be affected more than another, and this will often lead to minute errors which may seriously affect the amount to be measured when that is so small. another source of error is due to atmospheric refraction, which is subject to changes both from hour to hour and at different seasons. but perhaps most important of all are minute changes in level of the foundations of the instruments even when they are carried down to solid rock. both changes of temperature and changes of moisture of the soil produce minute alterations of level; while earth-tremors and slow movements of elevation or depression are now known to be very frequent. owing to all these causes, actual measurements of differences of position at different times of the year, amounting to small fractions of a second, are found to be too uncertain for the determination of such minute angles with the required accuracy. but there is another method which avoids almost all these sources of error, and this is now generally preferred and adopted for these measurements. it is, that of measuring the distance between two stars situated apparently very near each other, one of which has large proper motion, while the other has none which is measurable. the proper motions of the stars was first suspected by halley in , from finding that several stars, whose places had been given by hipparchus, b.c., were not in the positions where they now ought to be; and other observations by the old astronomers, especially those of occultations of stars by the moon, led to the same result. since the time of halley very accurate observations of the stars have been made, and in many cases it is found that they move perceptibly from year to year, while others move so slowly that it is only after forty or fifty years that the motion can be detected. the greatest proper motions yet determined amount to between " and " in a year, while other stars require twenty, or even fifty or a hundred years to show an equal amount of displacement. at first it was thought that the brightest stars would have the largest proper motion, because it was supposed they were nearest to us, but it was soon found that many small and quite inconspicuous stars moved as rapidly as the most brilliant, while in many very bright stars no proper motion at all can be detected. that which moves most rapidly is a small star of less than the sixth magnitude. it is a matter of common observation that the motion of things at a distance cannot be perceived so well as when near, even though the speed may be the same. if a man is seen on the top of a hill several miles off, we have to observe him closely for some time before we can be sure whether he is walking or standing still. but objects so enormously distant as we now know that the stars are, may be moving at the rate of many miles in a second and yet require years of observation to detect any movement at all. the proper motions of nearly a hundred stars have now been ascertained to be more than one second of arc annually, while a large number have less than this, and the majority have no perceptible motion, presumably due to their enormous distance from us. it is therefore not difficult in most cases to find one or two motionless stars sufficiently close to a star having a large proper motion (anything more than one-tenth of a second is so called) to serve as fixed points of measurement. all that is then required is, to measure with extreme accuracy the angular distance of the moving from the fixed stars at intervals of six months. the measurements can be made, however, on every fine night, each one being compared with one at nearly an interval of six months from it. in this way a hundred or more measurements of the same star may be made in a year, and the mean of the whole, allowance being made for proper motion in the interval, will give a much more accurate result than any single measurement. this kind of measurement can be made with extreme accuracy when the two stars can be seen together in the field of the telescope; either by the use of a micrometer, or by means of an instrument called a heliometer, now often constructed for the purpose. this is an astronomical telescope of rather large size, the object glass of which is cut in two straight across the centre, and the two halves made to slide upon each other by means of an exceedingly fine and accurate screw-motion, so adjusted and tested as to measure the angular distance of two objects with extreme accuracy. this is done by the number of turns of the screw required to bring the two stars into contact with each other, the image of each one being formed by one of the halves of the object glass. but the greatest advantage of this method of determining parallax is, as sir john herschell points out, that it gets rid of all the sources of error which render the older methods so uncertain and inaccurate. no corrections are required for precession, nutation, or aberration, since these affect both stars alike, as is the case also with refraction; while alterations of level of the instrument have no prejudicial effect, since the measures of angular distance taken by this method are quite independent of such movements. a test of the accuracy of the determination of parallax by this instrument is the very close agreement of different observers, and also their agreement with the new and perhaps even superior method by photography. this method was first adopted by professor pritchard of the oxford observatory, with a fine reflector of thirteen inches aperture. its great advantage is, that all the small stars in the vicinity of the star whose parallax is sought are shown in their exact positions upon the plate, and the distances of all of them from it can be very accurately measured, and by comparing plates taken at six months' intervals, each of these stars gives a determination of parallax, so that the mean of the whole will lead to a very accurate result. should, however, the result from any one of these stars differ considerably from that derived from the rest, it will be due in all probability to that star having a proper motion of its own, and it may therefore be rejected. to illustrate the amount of labour bestowed by astronomers on this difficult problem, it may be mentioned that for the photographic measurement of the star cygni, separate plates were taken in - , and on these , measurements of distances of the pairs of star-images were made. the result agreed closely with the best previous determination by sir robert ball, using the micrometer, and the method was at once admitted by astronomers as being of the greatest value. although, as a rule, stars having large proper motions are found to be comparatively near us, there is no regular proportion between these quantities, indicating that the rapidity of the motion of the stars varies greatly. among fifty stars whose distances have been fairly well determined, the rate of actual motion varies from one or two up to more than a hundred miles per second. among six stars with less than a tenth of a second of annual proper motion there is one with a parallax of nearly half a second, and another of one-ninth of a second, so that they are nearer to us than many stars which move several seconds a year. this may be due to actual slowness of motion, but is almost certainly caused in part by their motion being either towards us or away from us, and therefore only measurable by the spectroscope; and this had not been done when the lists of parallaxes and proper motions from which these facts are taken were published. it is evident that the actual direction and rate of motion of a star cannot be known till this radial movement, as it is termed--that is, towards or away from us--has been measured; but as this element always tends to increase the visually observed rate of motion, we cannot, through its absence, exaggerate the actual motions of the stars. the sun's movement through space but there is yet another important factor which affects the apparent motions of all the stars--the movement of our sun, which, being a star itself, has a proper motion of its own. this motion was suspected and sought for by sir william herschel a century ago, and he actually determined the direction of its motion towards a point in the constellation hercules, not very far removed from that fixed upon as the average of the best observations since made. the method of determining this motion is very simple, but at the same time very difficult. when we are travelling in a railway carriage near objects pass rapidly out of sight behind us, while those farther from us remain longer in view, and very distant objects appear almost stationary for a considerable time. for the same reason, if our sun is moving in any direction through space, the nearer stars will appear to travel in an opposite direction to our movement, while the more distant will remain quite stationary. this movement of the nearest stars is detected by an examination and comparison of their proper motions, by which it is found that in one part of the heavens there is a preponderance of the proper motions in one direction and a deficiency in the opposite direction, while in the directions at right angles to these the proper motions are not on the average greater in one direction than in the opposite. but the proper motions of the stars being themselves so minute, and also so irregular, it is only by a most elaborate mathematical investigation of the motions of hundreds or even of thousands of stars, that the direction of the solar motion can be determined. till quite recently astronomers were agreed that the motion was towards a point in hercules near the outstretched arm in the figure of that constellation. but the latest inquiries into this problem, involving the comparison of the motions of several thousand stars in all parts of the heavens, have led to the conclusion that the most probable direction of the 'solar apex' (as the point towards which the sun is moving is termed), is in the adjacent constellation lyra, and not far from the brilliant star vega. this is the position which professor newcomb of washington thinks most probable, though there is still room for further investigation. to determine the rate of the motion is very much more difficult than to fix its direction, because the distances of so few stars have been determined, and very few indeed of these lie in the directions best adapted to give accurate results. the best measurements down to led to a motion of about miles a second. but more recently the american astronomer, campbell, has determined by the spectroscope the motion in the line of sight of a considerable number of stars towards and away from the solar apex, and by comparing the average of these motions, he derives a motion for the sun of about - / miles a second, and this is probably as near as we can yet reach towards the true amount. some numerical results of the above measurements the measurements of distances and proper motions of a considerable number of the stars, of the motion of our sun in space (its proper motion), together with accurate determinations of the comparative brilliancy of the brightest stars as compared with our sun and with each other, have led to some very remarkable numerical results which serve as indications of the scale of magnitude of the stellar universe. the parallaxes of about fifty stars have now been repeatedly measured with such consistent results that professor newcomb considers them to be fairly trustworthy, and these vary from one-hundredth to three-quarters of a second. three more, all stars of the first magnitude--rigel, canopus, and alpha cygni--have no measurable parallax, notwithstanding the long-continued efforts of many astronomers, affording a striking example of the fact that brilliancy alone is no test of proximity. six more stars have a parallax of only one-fiftieth of a second, and five of these are either of the first or second magnitudes. of these nine stars having very small parallax or none, six are situated in or near to the milky way, another indication of exceeding remoteness, which is further shown by the fact that they all have a very small proper motion or none at all. these facts support the conclusion, which had been already reached by astronomers from a careful study of the distribution of the stars, that the larger portion of the stars of all magnitudes scattered throughout the milky way or along its borders really belong to the same great system, and may be said to form a part of it. this is a conclusion of extreme importance because it teaches us that the grandest of the suns, such as rigel and betelgeuse in the constellation orion, antares in the scorpion, deneb in the swan (alpha cygni), and canopus (alpha argus), are in all probability as far removed from us as are the innumerable minute stars which give the nebulous or milky appearance to the galaxy. it is well to consider for a moment what these facts mean. professor s. newcomb, one of the highest authorities on these problems, tells us that the long series of measurements to discover the parallax of canopus, the brightest star in the southern hemisphere, would have shown a parallax of one-hundredth of a second, had such existed. yet the results always seemed to converge to a mean of ". ! suppose, then, we assume the parallax of this star to be somewhat less than the hundredth of a second--let us say / of a second. at the distance this gives, light would take almost exactly years to reach us, so that if we suppose this very brilliant star to be situated a little on this side of the galaxy, we must give to that great luminous circle of stars a distance of about light years. we shall now perceive the advantage of being able to realise what a million really is. a person who had once seen a wall-space more than feet long and feet high completely covered with quarter-inch spots a quarter of an inch apart; and then tried to imagine every spot to be a mile long and to be placed end to end in one row, would form a very different conception of a million miles than those who almost daily _read_ of millions, but are quite unable to visualise even one of them. having really seen one million, we can partially realise the velocity of light, which travels over this million miles in a little less than - / seconds; and yet light takes more than - / years at this inconceivable speed to come to us from the very _nearest_ of the stars. to realise this still more impressively, let us take the _distance_ of this nearest star, which is _millions_ of _millions_ of miles. let us look in imagination at this large and lofty hall covered from floor to ceiling with quarter-inch spots--only _one_ million. let all these be imagined as miles. then repeat this number of miles in a straight line, one after the other, as many times as there are spots in this hall; and even then you have reached only one twenty-sixth part of the distance to the nearest fixed star! this _million_ times a _million_ miles has to be repeated twenty-six times to reach the _nearest_ fixed star; and it seems probable that this gives us a good indication of the distance from each other of at least all the stars down to the sixth magnitude, perhaps even of a large number of the telescopic stars. but as we have found that the bright stars of the milky way must be at least one hundred times farther from us than these nearest stars, we have found what may be termed a minimum distance for that vast star-ring. it may be immensely farther, but it is hardly possible that it should be anything less. the probable size of the stars having thus obtained an inferior limit for the distance of several stars of the first magnitude, and their actual brilliancy or light-emission as compared with our sun having been carefully measured, we have afforded us some indication of size though perhaps an uncertain one. by these means it has been found that rigel gives out about ten thousand times as much light as our sun, so that if its surface is of the same brightness, it must be a hundred times the diameter of the sun. but as it is one of the white or sirian type of stars it is probably very much more luminous, but even if it were twenty times brighter it would still have to be twenty-two and a half times the diameter of the sun; and as the stars of this type are probably wholly gaseous and much less dense than our sun, this enormous size may not be far from the truth. it is believed that the sirian stars generally have a greater surface brilliancy than our sun. beta aurigæ, a star of the second magnitude but of the sirian type, is one of the double stars whose distance has been measured, and this has enabled mr. gore to find the mass of the binary system to be five times that of the sun, and their light one hundred and seventeen times greater. even if the density is much less than the sun's, the intrinsic brilliancy of the surface will be considerably greater. another double star, gamma leonis, has been found to be three hundred times more brilliant than the sun if of the same density, but it would require to be seven times rarer than air to have the extent of surface needed to give the same amount of light if its surface emitted no more light than our sun from equal areas. it is clear, therefore, that many of the stars are much larger than our sun as well as more luminous; but there are also large numbers of small stars whose large proper motions, as well as the actual measurement of some, prove them to be comparatively near to us which yet are only about one-fiftieth part as bright as the sun. these must, therefore, be either comparatively small, or if large must be but slightly luminous. in the case of some double stars it has been proved that the latter is the case; but it seems probable that others are very much smaller than the average. up to the present time no means of determining the size of a star by actual measurement has been discovered, since their distances are so enormous that the most powerful telescopes show only a point of light. but now that we have really measured the distance of a good many stars we are able to determine an upper limit for their actual dimensions. as the nearest fixed star, alpha centauri, has a parallax of ". , this means that if this star has a diameter as great as our distance from the sun (which is not much more than a hundred times the sun's diameter) it would be seen to have a distinct disc about as large as that of jupiter's first satellite. if it were even one-tenth of the size supposed it would probably be seen as a disc in our best modern telescopes. the late mr. ranyard remarks that if the nebular hypothesis is true, and our sun once extended as far as the orbit of neptune, then, among the millions of visible suns there ought to be some now to be found in every stage of development. but any sun having a diameter at all approaching this size, and situated as far off as a hundred times the distance of alpha centauri, would be seen by the lick telescope to have a disc half a second in diameter. hence the fact that there are no stars with visible discs proves that there are no suns of the required size, and adds another argument, though not perhaps a strong one, against the acceptance of the nebular hypothesis. chapter vi the unity and evolution of the star system the very condensed sketch now given of such of the discoveries of recent astronomy as relate to the subject we are discussing will, it is hoped, give some idea both of the work already done and of the number of interesting problems yet remaining to be solved. the most eminent astronomers in every part of the world look forward to the solution of these problems not, perhaps, as of any great value in themselves, but as steps towards a more complete knowledge of our universe as a whole. their aim is to do for the star-system what darwin did for the organic world, to discover the processes of change that are at work in the heavens, and to learn how the mysterious nebulæ, the various types of stars, and the clusters and systems of stars are related to each other. as darwin solved the problem of the origin of organic species from other species, and thus enabled us to understand how the whole of the existing forms of life have been developed out of pre-existing forms, so astronomers hope to be able to solve the problem of the evolution of suns from some earlier stellar types, so as to be able, ultimately, to form some intelligible conception of how the whole stellar universe has come to be what it is. volumes have already been written on this subject, and many ingenious suggestions and hypotheses have been advanced. but the difficulties are very great; the facts to be co-ordinated are excessively numerous, and they are necessarily only a fragment of an unknown whole. yet certain definite conclusions have been reached; and the agreement of many independent observers and thinkers on the fundamental principles of stellar evolution seems to assure us that we are progressing, if slowly yet with some established basis of truth, towards the solution of this, the most stupendous scientific problem with which the human intellect has ever attempted to grapple. the unity of the stellar universe during the latter half of the nineteenth century the opinion of astronomers has been tending more and more to the conception that the whole of the visible universe of stars and nebulæ constitutes one complete and closely-related system; and during the last thirty years especially the vast body of facts accumulated by stellar research has so firmly established this view that it is now hardly questioned by any competent authority. the idea that the nebulæ were far more remote from us than the stars long held sway, even after it had been given up by its chief supporter. when sir william herschel, by means of his then unapproached telescopic power, resolved the milky way more or less completely into stars, and showed that numerous objects which had been classed as nebulæ were really clusters of stars, it was natural to suppose that those which still retained their cloudy appearance under the highest telescopic powers were also clusters or systems of stars, which only needed still higher powers to show their true nature. this idea was supported by the fact that several nebulæ were found to be more or less ring-shaped, thus corresponding on a smaller scale to the form of the milky way; so that when herschel discovered thousands of telescopic nebulæ, he was accustomed to speak of them as so many distinct universes scattered through the immeasurable depths of space. now, although any real conception of the immensity of the one stellar universe, of which the milky way with its associated stars is the fundamental feature, is, as i have shown, almost unattainable, the idea of an unlimited number of other universes, almost infinitely remote from our own and yet distinctly visible in the heavens, so seized upon the imagination that it became almost a commonplace of popular astronomy and was not easily given up even by astronomers themselves. and this was in a large part due to the fact that sir william herschel's voluminous writings, being almost all in the philosophical transactions of the royal society, were very little read, and that he only indicated his change of view by a few brief sentences which might easily be overlooked. the late mr. proctor appears to have been the first astronomer to make a thorough study of the whole of herschel's papers, and he tells us that he read them all over five times before he was able thoroughly to grasp the writer's views at different periods. but the first person to point out the real teaching of the facts as to the distribution of the nebulæ was not an astronomer, but our greatest philosophical student of science in general, herbert spencer. in a remarkable essay on 'the nebular hypothesis' in the _westminster review_ of july, , he maintained that the nebulæ really formed a part of our own galaxy and of our own stellar universe. a single passage from his paper will indicate his line of argument, which, it may be added, had already been partially set forth by sir john herschel in his _outlines of astronomy_. 'if there were but one nebula, it would be a curious coincidence were this one nebula so placed in the distant regions of space as to agree in direction with a starless spot in our own sidereal system. if there were but two nebulæ, and both were so placed, the coincidence would be excessively strange. what, then, shall we say on finding that there are thousands of nebulæ so placed? shall we believe that in thousands of cases these far-removed galaxies happen to agree in their visible positions with the thin places in our own galaxy? such a belief is impossible.' he then applies the same argument to the distribution of the nebulæ as a whole:--'in that zone of celestial space where stars are excessively abundant, nebulæ are rare, while in the two opposite celestial spaces that are farthest removed from this zone, nebulæ are abundant. scarcely any nebulæ lie near the galactic circle (or plane of the milky way); and the great mass of them lie round the galactic poles. can this also be mere coincidence?' and he concludes, from the whole mass of the evidence, that 'the proofs of a physical connection become overwhelming.' nothing could be more clear or more forcible; but spencer not being an astronomer, and writing in a comparatively little read periodical, the astronomical world hardly noticed him; and it was from ten to fifteen years later, when mr. r.a. proctor, by his laborious charts and his various papers read before the royal and royal astronomical societies from to , compelled the attention of the scientific world, and thus did more perhaps than any other man to establish firmly the grand and far-reaching principle of the essential unity of the stellar universe, which is now accepted by almost every astronomical writer of eminence in the civilised world. the evolution of the stellar universe amid the enormous mass of observations and of suggestive speculation upon this great and most interesting problem, it is difficult to select what is most important and most trustworthy. but the attempt must be made, because, unless my readers have some knowledge of the most important facts bearing upon it (besides those already set forth), and also learn something of the difficulties that meet the inquirer into causes at every step of his way, and of the various ideas and suggestions which have been put forth to account for the facts and to overcome the difficulties, they will not be in a position to estimate, however imperfectly, the grandeur, the marvel, and the mystery of the vast and highly complex universe in which we live and of which we are an important, perhaps the most important, if not the only permanent outcome. the sun a typical star it being now a recognised fact that the stars are suns, some knowledge of our own sun is an essential preliminary to an inquiry into their nature, and into the probable changes they have undergone. the fact that the sun's density is only one-fourth that of the earth, or less than one and a half times that of water, demonstrates that it cannot be solid, since the force of gravity at its surface being twenty-six and a half times that at the earth's surface, the materials of a solid globe would be so compressed that the resulting density would be at least twenty times greater instead of four times less than that of the earth. all the evidence goes to show that the body of the sun is really gaseous, but so compressed by its gravitative force as to behave more like a liquid. a few figures as to the vast dimensions of the sun and the amount of light and heat emitted by it will enable us better to understand the phenomena it presents, and the interpretation of those phenomena. proctor estimated that each square inch of the sun's surface emitted as much light as twenty-five electric arcs; and professor langley has shown by experiment that the sun is times brighter, and eighty-seven times hotter than the white-hot metal in a bessemer converter. the actual amount of solar heat received by the earth is sufficient, if wholly utilised, to keep a three-horse-power engine continually at work on every square yard of the surface of our globe. the size of the sun is such, that if the earth were at its centre, not only would there be ample space for the moon's orbit, but sufficient for another satellite , miles beyond the moon, all revolving inside the sun. the mass of matter in the sun is times greater than that of all the planets combined; hence the powerful gravitative force by which they are retained in their distant orbits. what we see as the sun's surface is the photosphere or outer layer of gaseous or partially liquid matter kept at a definite level by the power of gravitation. the photosphere has a granular texture implying some diversity of surface or of luminosity; although the even contour of the sun's margin shows that these irregularities are not on a very large scale. this surface is apparently rent asunder by what are termed sun-spots, which were long supposed to be cavities, showing a dark interior; but are now thought to be due to downpours of cooled materials driven out from the sun, and forming the prominences seen during solar eclipses. they appear to be black, but around their margin is a shaded border or penumbra formed of elongated shining patches crossing and over-lapping, something like heaps of straw. sometimes brilliant portions overhang the dark spots, and often completely bridge them over; and similar patches, called faculæ, accompany spots, and in some cases almost surround them. sun-spots are sometimes numerous on the sun's disc, sometimes very few, and they are of such enormous size that when present they can easily be seen with the naked eye, protected by a piece of smoked glass; or, better still, with an ordinary opera-glass similarly protected. they are found to increase in number for several years, and then to decrease; the maxima recurring after an average period of eleven years, but with no exactness, since the interval between two maxima or minima is sometimes only nine and sometimes as much as thirteen years; while the minima do not occur midway between two maxima, but much nearer to the succeeding than to the preceding one. what is more interesting is, that variations in terrestrial magnetism follow them with great accuracy; while violent commotions in the sun, indicated by the sudden appearance of faculæ, sun-spots, or prominences on the sun's limb, are always accompanied by magnetic disturbances on the earth. what surrounds the sun it has been well said that what we commonly term the sun is really the bright spherical nucleus of a nebulous body. this nucleus consists of matter in the gaseous state, but so compressed as to resemble a liquid or even a viscous fluid. about forty of the elements have been detected in the sun by means of the dark lines in its spectrum, but it is almost certain that all the elements, in some form or other, exist there. this semi-liquid glowing surface is termed the photosphere, since from it are given out the light and heat which reach our earth. immediately above this luminous surface is what is termed the 'reversing layer' or absorbing layer, consisting of dense metallic vapours only a few hundred miles thick, and, though glowing, somewhat cooler than the surface of the photosphere. its spectrum, taken, at the moment when the sun is totally darkened, through a slit which is directed tangentially to the sun's limb, shows a mass of bright lines corresponding in a large degree to the dark lines in the ordinary solar spectrum. it is thus shown to be a vaporous stratum which absorbs the special rays emitted by each element and forming its characteristic coloured lines, changing them into black lines. but as coloured lines are not found in this layer corresponding to all the black lines in the solar spectrum, it is now held that special absorption must also occur in the chromosphere and perhaps in the corona itself. sir norman lockyer, in his volume on _inorganic evolution_, even goes so far as to say, that the true 'reversing layer' of the sun--that which by its absorption produced the dark lines in the solar spectrum--is now shown to be _not_ the chromosphere itself but a layer above it, of lower temperature. above the reversing layer comes the chromosphere, a vast mass of rosy or scarlet emanations surrounding the sun to a depth of about miles. when seen during eclipses it shows a serrated waving outline, but subject to great changes of form, producing the prominences already mentioned. these are of two kinds: the 'quiescent,' which are something like clouds of enormous extent, and which keep their forms for a considerable time; and the 'eruptive,' which shoot out in towering tree-like flames or geyser-like eruptions, and while doing so have been proved to reach velocities of over miles a second, and subside again with almost equal rapidity. the chromosphere and its quiescent prominences appear to be truly gaseous, consisting of hydrogen, helium, and coronium, while the eruptive prominences always show the presence of metallic vapours, especially of calcium. prominences increase in size and number in close accordance with the increase of sun-spots. beyond the red chromosphere and prominences is the marvellous white glory of the corona, which extends to an enormous distance round the sun. like the prominences of the chromosphere, it is subject to periodical changes in form and size, corresponding to the sun-spot period, but in inverse order, a minimum of sun-spots going with a maximum extension of the corona. at the total eclipse of july , when the sun's surface was almost wholly clear, a pair of enormous equatorial streamers stretched east and west of the sun to a distance of ten millions of miles, and less extensions of the corona occurred at the poles. at the eclipses of and , on the other hand, when sun-spots were at a maximum, the corona was regularly stellate with no great extensions, but of high brilliancy. this correspondence has been noted at every eclipse, and there is therefore an undoubted connection between the two phenomena. the light of the corona is believed to be derived from three sources--from incandescent solid or liquid particles thrown out from the sun, from sunlight reflected from these particles, and from gaseous emissions. its spectrum possesses a green ray, which is peculiar to it, and is supposed to indicate a gas named 'coronium'; in other respects the spectrum is more like that of reflected sunlight. the enormous extensions of the corona into great angular streamers seem to indicate electrical repulsive forces analogous to those which produce the tails of comets. connected with the sun's corona is that strange phenomenon, the zodiacal light. this is a delicate nebulosity, which is often seen after sunset in spring and before sunrise in autumn, tapering upwards from the sun's direction along the plane of the ecliptic. under very favourable conditions it has been traced in the eastern sky in spring to ° from the sun's position, indicating that it extends beyond the earth's orbit. long-continued observations from the summit of the pic du midi show that this is really the case, and that it lies almost exactly in the plane of the sun's equator. it is therefore held to be produced by the minute particles thrown off the sun, through those coronal wings and streamers which are visible only during solar eclipses. the careful study of the solar phenomena has very clearly established the fact that none of the sun's envelopes, from the reversing layer to the corona itself, is in any sense an atmosphere. the combination of enormous gravitative force with an amount of heat which turns all the elements into the liquid or gaseous state, leads to consequences which it is difficult for us to follow or comprehend. there is evidently constant internal movement or circulation in the interior of the sun, resulting in the faculæ, the sun-spots, the intensely luminous photosphere, and the chromosphere with its vast flaming coruscations and eruptive protuberances. but it seems impossible that this incessant and violent movement can be kept up without some great and periodical or continuous inrush of fresh materials to renew the heat, keep up the internal circulation, and supply the waste. perhaps the movement of the sun through space may bring him into contact with sufficiently large masses of matter to continually excite that internal movement without which the exterior surface would rapidly become cool and all planetary life cease. the various solar envelopes are the result of this internal agitation, uprushes, and explosions, while the vast white corona is probably of little more density than comets' tails, probably even of less density, since comets not unfrequently rush through its midst without suffering any loss of velocity. the fact that none of the solar envelopes are visible to us until the light of the photosphere is completely shut off, and that they all vanish the very instant the first gleam of direct sunlight reaches us, is another proof of their extreme tenuity, as is also the sharply defined edge of the sun's disc. the envelopes therefore consist partly of liquid or vaporous matter, in a very finely divided state, driven off by explosions or by electrical forces, and this matter, rapidly cooling, becomes solidified into minutest particles, or even physical molecules. much of this matter continually falls back on the sun's surface, but a certain quantity of the very finest dust is continually driven away by electrical repulsion, so as to form the corona and the zodiacal light. the vast coronal streamers and the still more extensive ring of the zodiacal light are therefore in all probability due to the same causes, and have a similar physical constitution with the tails of comets. as the whole of our sunlight must pass through both the reversing layer and the red chromosphere, its colour must be somewhat modified by them. hence it is believed that, if they were absent, not only would the light and heat of the sun be considerably greater, but its colour would be a purer white, tending towards bluish rather than towards the yellowish tinge it actually possesses. the nebular and meteoritic hypotheses as the constitution of the sun, and its agency in producing magnetism and electricity in the matter and orbs around it, afford us our best guide to the constitution of the stars and nebulæ, and to their possible action on each other, and even upon our earth, so the mode of evolution of the sun and solar system, from some pre-existing condition, is likely to help us towards gaining some knowledge of the constitution of the stellar universe and the processes of change going on there. at the very commencement of the nineteenth century the great mathematician laplace published his nebular theory of the origin of the solar system; and although he put it forth merely as a suggestion, and did not support it with any numerical or physical data, or by any mathematical processes, his great reputation, and its apparent probability and simplicity, caused it to be almost universally accepted, and to be extended so as to apply to the evolution of the stellar universe. this theory, very briefly stated, is, that the whole of the matter of the solar system once formed a globular or spheroidal mass of intensely heated gases, extending beyond the orbit of the outermost planet, and having a slow motion of revolution about an axis. as it cooled and contracted, its rate of revolution increased, and this became so great that at successive epochs it threw off rings, which, owing to slight irregularities, broke up, and, gravitating together, formed the planets. the contraction continuing, the sun, as we now see it, was the result. for about half a century this nebular hypothesis was generally accepted, but during the last thirty years so many objections and difficulties have been suggested, that it has been felt impossible to retain it even as a working hypothesis. at the same time another hypothesis has been put forth which seems more in accordance with the facts of nature as we find them in our own solar system, and which is not open to any of the objections against the nebular theory, even if it introduces a few new ones. a fundamental objection to laplace's theory is, that in a gas of such extreme tenuity as the solar nebula must have been, even when it extended only to saturn or uranus, it could not possibly have had any cohesion, and therefore could not have given off whole rings at distant intervals, but only small fragments continuously as condensation went on, and these, rapidly cooling, would form solid particles, a kind of meteoric dust, which might aggregate into numerous small planets, or might persist for indefinite periods, like the rings of saturn or the great ring of the asteroids. another equally vital objection is, that, as the nebula when extending beyond the orbit of neptune could have had a mean density of only about the two-hundred millionth of our air at sea level, it must have been many hundred times less dense than this at and near its outer surface, and would there be exposed to the cold of stellar space--a cold that would solidify hydrogen. it is thus evident that the gases of all the metallic and other solid elements could not possibly exist as such, but would rapidly, perhaps almost instantaneously, become first liquid and then solid, forming meteoric dust even before contraction had gone far enough to produce such increased rotation as would throw off any portion of the gaseous matter. here we have the foundations of the meteoritic hypothesis which is now steadily making its way. it is supported by the fact that we everywhere find proofs of such solid matter in the planetary spaces around us. it falls continually upon the earth. it can be collected on the arctic and alpine snows. it occurs everywhere in the deepest abysses of the ocean where there are not sufficient organic deposits to mask it. it constitutes, as has now been demonstrated, the rings of saturn. thousands of vast rings of solid particles circulate around the sun, and when our earth crosses any of these rings, and their particles enter our atmosphere with planetary velocity, the friction ignites them and we see falling stars. comets' tails, the sun's corona, and the zodiacal light are three strange phenomena, which, though wholly insoluble on any theory of gaseous formation, receive their intelligible explanation by means of excessively minute solid particles--microscopic cosmic dust--driven outward by the tremendous electrical repulsions that emanate from the sun. having these and other proofs that solid matter, ranging in size, perhaps, from the majestic orbs of jupiter and saturn down to the inconceivably minute particles driven millions of miles into space to form a comet's tail, does actually exist everywhere around us, and by collisions between the particles or with planetary atmospheres can produce heat and light and gaseous emanations, we find a basis of fact and observation for the meteoritic hypothesis which laplace's nebular, and essentially gaseous, theory does not possess. during the latter half of the nineteenth century several writers suggested this idea of the possible formation of the solar system, but so far as i am aware, the late r.a. proctor was the first to discuss it in any detail, and to show that it explained many of the peculiarities in the size and arrangement of the planets and their satellites which the nebular hypothesis did not explain. this he does at some length in the chapter on meteors and comets in his _other worlds than ours_, published in . he assumed, instead of the fire-mist of laplace, that the space now occupied by the solar system, and for an unknown distance around it, was occupied by vast quantities of solid particles of all the kinds of matter which we now find in the earth, sun, and stars. this matter was dispersed somewhat irregularly, as we see that all the matter of the universe is now distributed; and he further assumed that it was all in motion, as we now know that all the stars and other cosmical masses are, and must be, in motion towards or around some centre. under these conditions, wherever the matter was most aggregated, there would be a centre of attraction through gravitation, which would necessarily lead to further aggregation, and the continual impacts of such aggregating matter would produce heat. in course of time, if the supply of cosmic matter was ample (as the result shows that it must have been, whatever theory we adopt), our sun, thus formed, would approximate to its present mass and acquire sufficient heat by collision and gravitation to convert its whole body into the liquid or gaseous condition. while this was going on, subordinate centres of aggregation might form, which would capture a certain proportion of the matter flowing in under the attraction of the central mass, while, owing to the nearly uniform direction and velocity with which the whole system was revolving, each subordinate centre would revolve around the central mass, in somewhat different planes, but all in the same direction. mr. proctor shows the probability that the largest outside aggregation would be at a great distance from the central mass, and this having once been formed, any centres farther away from the sun would be both smaller and very remote, while those inside the first would, as a rule, become smaller as they were nearer the centre. the heated condition of the earth's interior would thus be due, not to the primitive heat of matter in a gaseous state out of which it was formed--a condition physically impossible--but would be acquired in the process of aggregation by the collisions of meteoric masses falling on it, and by its own gravitative force producing continuous condensation and heat. on this view jupiter would probably be formed first, and after him at very great distances, saturn, uranus, and neptune; while the inner aggregations would be smaller, as the much greater attractive power of the sun would give them comparatively little opportunity of capturing the meteoric matter that was continuously flowing towards him. the meteoritic nature of the nebulÆ having thus reached the conclusion that wherever apparently nebulous matter exists within the limits of the solar system it is not gaseous but consists of solid particles, or, if heated gases are associated with the solid matter they can be accounted for by the heat due to collisions either with other solid particles or with accumulations of gases at a low temperature, as when meteorites enter our atmosphere, it was an easy step to consider whether the cosmic nebulæ and stars may not have had a similar origin. from this point of view the nebulæ are supposed to be vast aggregations of meteorites or cosmic dust, or of the more persistent gases, revolving with circular or spiral motions, or in irregular streams, and so sparsely scattered that the separate particles of dust may be miles--perhaps hundreds of miles--apart; yet even those nebulæ, only visible by the telescope, may contain as much matter as the whole solar system. from this simple origin, by steps which can be observed in the skies, almost all the forms of suns and systems can be traced by means of the known laws of motion, of heat-production, and of chemical action. the chief english advocate of this view at the present time is sir norman lockyer, who, in numerous papers, and in his works on _the meteoritic hypothesis_ and _inorganic evolution_, has developed it in detail, as the result of many years' continuous research, aided by the contributory work of continental and american astronomers. these views are gradually spreading among astronomers and mathematicians, as will be seen by the very brief outline which will now be given of the explanations they afford of the main groups of phenomena presented by the stellar universe. dr. roberts on spiral nebulÆ dr. isaac roberts, who possesses one of the finest telescopes constructed for photographing stars and nebulæ, has given his views on stellar evolution, in _knowledge_ of february , illustrated by four beautiful photographs of spiral nebulæ. these curious forms were at first thought to be rare, but are now found to be really very numerous when details are brought out by the camera. many of the very large and apparently quite irregular nebulæ, like the magellanic clouds, are found to have faint indications of spiral structure. as more than ten thousand nebulæ are now known, and new ones are continually being discovered, it will be a long time before these can all be carefully studied and photographed, but present indications seem to show that a considerable proportion of them will exhibit spiral forms. dr. roberts tells us that all the spiral nebulæ he has photographed are characterised by having a nucleus surrounded by dense nebulosity, most of them being also studded with stars. these stars are always arranged more or less symmetrically, following the curves of the spiral, while outside the visible nebula are other stars arranged in curves strongly suggesting a former greater extension of the nebulous matter. this is so marked a feature that it at once leads to a possible explanation of the numerous slightly curved lines of stars found in every part of the heavens, as being the result of their origin from spiral nebulæ whose material substance has been absorbed by them. dr. roberts proposes several problems in relation to these bodies: of what materials are spiral nebulæ composed? whence comes the vortical motion which has produced their forms? the material he finds in those faint clouds of nebulous matter, often of vast extent, that exist in many parts of the sky, and these are so numerous that sir william herschel alone recorded the positions of fifty-two such regions, many of which have been confirmed by recent photographs. dr. roberts considers these to be either gaseous or with discrete solid particles intermixed. he also enumerates smaller nebulous masses undergoing condensation and segregation into more regular forms; spiral nebulæ in various stages of condensation and of aggregation; elliptic nebulæ; and globular nebulæ. in the last three classes there is clear evidence, on every photograph that has been taken, that condensation into stars or star like forms is now going on. he adopts sir norman lockyer's view that collisions of meteorites within each swarm or cloud would produce luminous nebulosity; so also would collisions between separate swarms of meteorites produce the conditions required to account for the vortical motions and the peculiar distribution of the nebulosity in the spiral nebulæ. almost any collision between unequal masses of diffused matter would, in the absence of any massive central body round which they would be forced to revolve, lead to spiral motions. it is to be noted that, although the stars formed in the spiral convolutions of the nebulæ follow those curves, and retain them after the nebulous matter has been all absorbed by them, yet, whenever such a nebula is seen by us edgewise, the convolutions with their enclosed stars will appear as straight lines; and thus not only numbers of star groups arranged in curves, but also those which form almost perfect straight lines, may possibly be traced back to an origin from spiral nebulæ. motion being a necessary result of gravitation, we know that every star, planet, comet, or nebula must be in motion through space, and these motions--except in systems physically connected or which have had a common origin--are, apparently, in all directions. how these motions originated and are now regulated we do not know; but there they are, and they furnish the motive power of the collisions, which, when affecting large bodies or masses of diffused matter, lead to the formation of the various kinds of permanent stars; while when smaller masses of matter are concerned those temporary stars are formed which have interested astronomers in all ages. it must be noted that although the motions of the single stars appear to be in straight lines, yet the spaces through which they have been observed to move are so small that they may really be moving in curved orbits around some central body, or the centre of gravity of some aggregation of stars bright and dark, which may itself be comparatively at rest. there may be thousands of such centres around us, and this may sufficiently explain the apparent motions of stars in all directions. a suggestion as to the formation of spiral nebulÆ in a remarkable paper in the astrophysical journal (july ), mr. t.c. chamberlin suggests an origin for the spiral nebulæ, as well as of swarms of meteorites and comets, which seems likely to be a true, although perhaps not the only one. there is a well-known principle which shows that when two bodies in space, of stellar size, pass within a certain distance of each other, the smaller one will be liable to be torn into fragments by the differential attraction of the larger and denser body. this was originally proved in the case of gaseous and liquid bodies, and the distance within which the smaller one will be disrupted (termed the roche limit) is calculated on the supposition that the disrupted body is a liquid mass. mr. chamberlin shows, however, that a solid body will also be disrupted at a lesser distance dependent on its size and cohesive strength; but, as the size of the two bodies increases, the distance at which disruption will occur increases also, till with very large bodies, such as suns, it becomes almost as large as in the case of liquids or gases. the disruption occurs from the well-known law of differential gravitation on the two sides of a body leading to tidal deformation in a liquid, and to unequal strain in a solid. when the changes of gravitative force take place slowly, and are also small in amount, the tides in liquids or strains in solids are very small, as in the case of our earth when acted on by the sun and moon, the result is a small tide in the ocean and atmosphere, and no doubt also in the molten interior, to which the comparatively thin crust may partially adjust itself. but if we suppose two dark or luminous suns whose proper motions are in such a direction as to bring them near each other, then, as they approach, each will be deflected towards the other, and will pass round their common centre of gravity with immense velocity, perhaps hundreds of miles in a second. at a considerable distance they will begin to produce tidal elongation towards and away from each other, but when the disruptive limit is nearly reached, the gravitative forces will be increasing so rapidly that even a liquid mass could not adjust its shape with sufficient quickness and the tremendous internal strains would produce the effects of an explosion, tearing the whole mass (of the smaller of the two) into fragments and dust. but it is also shown that, during the entire process, the two elongated portions of the originally spherical mass would be so acted upon by gravity as to produce increasing rotation, which as the crisis approached would extend the elongation, and aid in the explosive result. this rapid rotation of the elongated mass would, when the disruption occurred, necessarily give to the fragments a whirling or spiral motion, and thus initiate a spiral nebula of a size and character dependent on the size and constitution of the two masses, and on the amount of the explosive forces set up by their approach. there is one very suggestive phenomenon which seems to prove that this _is_ one of the modes of formation of spiral nebulæ. when the explosive disruption occurs the two protuberances or elongations of the body will fly apart, and having also a rapid rotatory movement, the resulting spiral will necessarily be a double one. now, it is the fact that almost all the well-developed spiral nebulæ have two such arms opposite to each other, as beautifully shown in m. comæ, m. canum, and others photographed by dr. i. roberts. it does not seem likely that any other origin of these nebulæ should give rise to a double rather than to a single spiral. the evolution of double stars the advance in knowledge of double and multiple stars has been wonderfully rapid, numerous observers having devoted themselves to this special branch. many thousands were discovered during the first half of the nineteenth century, and as telescopic power increased new ones continued to flow in by hundreds and thousands, and there has been recently published by the yerkes observatory a catalogue of such stars, discovered between and by one observer, mr. s.w. burnham. all these have been found by the use of the telescope, but during the last quarter of a century the spectroscope has opened up a new world of double stars of enormous extent and the highest interest. the telescopic binaries which have been observed for a sufficient time to determine their orbits, range from periods of about eleven years as a minimum up to hundreds and even more than a thousand years. but the spectroscope reveals the fact that the many thousands of telescopic binaries form only a very small part of the binary systems in existence. the overwhelming importance of this discovery is, that it carries the times of revolution from the minimum of the telescopic doubles downward in unbroken series through periods of a few years, to those reckoned by months, by days, and even by hours. and with this reduction of period there necessarily follows a corresponding reduction of distance, so that sometimes the two stars must be in contact, and thus the actual birth or origin of a double star has been observed to occur, even though not actually seen. this mode of origin was indeed anticipated by dr. lee of chicago in , and it has been confirmed by observation in the short space of ten years. in a remarkable communication to _nature_ (september th, ) mr. alexander w. roberts of lovedale, south africa, gives some of the main results of this branch of inquiry. of course all the variable stars are to be found among the spectroscopic binaries. they consist of that portion of the class in which the plane of the orbit is directed towards us, so that during their revolution one of the pair either wholly or partially eclipses the other. in some of these cases there are irregularities, such as double maxima and minima of unequal lengths, which may be due to triple systems or to other causes not yet explained, but as they all have short periods and always appear as one star in the most powerful telescopes, they form a special division of the spectroscopic binary systems. there are known at present twenty-two variables of the algol type, that is, stars having each a dark companion very close to it which obscures it either wholly or partially during every revolution. in these cases the density of the systems can be approximately determined, and they are found to be, on the average, only one-fifth that of water, or one-eighth that of our sun. but as many of them are as large as our sun, or even considerably larger, it is evident that they must be wholly gaseous, and, even if very hot, of a less complex constitution than our luminary. mr. a.w. roberts tells us that five out of these twenty-two variables revolve _in absolute contact_ forming systems of the shape of a dumb-bell. the periods vary from twelve days to less than nine hours; and, starting from these, we now have a continuous series of lengthening periods up to the twin stars of castor which require more than a thousand years to complete their revolution. during his observations of the above five stars, mr. roberts states that one, x carinæ, was found to have parted company, so that instead of being actually united to its companion the two are now at a distance apart equal to one-tenth of their diameters, and he may thus be said to have been almost a witness of the birth of a stellar system. a year later we find the record (in _knowledge_, october ) of professor campbell's researches at the lick observatory. he states that, out of stars observed spectroscopically, one in eight is a spectroscopic binary; and so impressed is he with their abundance that, as accuracy of measurement increases, he believes that _the star that is not a spectroscopic binary will prove to be the rare exception_! professor g. darwin had already shown that the 'dumb-bell' was a figure of equilibrium in a rotating mass of fluid; and we now find proofs that such figures exist, and that they form the starting-point for the enormous and ever-increasing quantities of spectroscopic binary star-systems that are now known. the origin of these binary stars is also of especial interest as giving support to professor darwin's well-known explanation of the origin of the moon by disruption from the earth, owing to the very rapid rotation of the parent planet. it now appears that suns often subdivide in the same manner, but, owing perhaps to their intensely heated gaseous state they seem usually to form nearly equal globes. the evolution of this special form of star-system is therefore now an observed fact; though it by no means follows that all double stars have had the same mode of origin. clusters of stars and variables the clusters of stars, which are tolerably abundant in the heavens and offer so many strange and beautiful forms to the telescopist, are yet among the most puzzling phenomena the philosophic astronomer has to deal with. many of these clusters which are not very crowded and of irregular forms, strongly suggest an origin from the equally irregular and fantastic forms of nebulæ by a process of aggregation like that which dr. roberts describes as developing within the spiral nebulæ. but the dense globular clusters which form such beautiful telescopic objects, and in some of which more than six thousand stars have been counted besides considerable numbers so crowded in the centre as to be uncountable, are more difficult to explain. one of the problems suggested by these clusters is as to their stability. professor simon newcomb remarks on this point as follows: 'where thousands of stars are condensed into a space so small, what prevents them from all falling together into one confused mass? are they really doing so, and will they ultimately form a single body? these are questions which can be satisfactorily answered only by centuries of observation; they must therefore be left to the astronomers of the future.' there are, however, some remarkable features in these clusters which afford possible indications of their origin and essential constitution. when closely examined most of them are seen to be less regular than they at first appear. vacant spaces can be noted in them; even rifts of definite forms. in some there is a radiated structure; in others there are curved appendages; while some have fainter centres. these features are so exactly like what are found, in a more pronounced form, in the larger nebulæ, that we can hardly help thinking that in these clusters we have the result of the condensation of very large nebulæ, which have first aggregated towards numerous centres, while these agglomerations have been slowly drawn towards the common centre of gravity of the whole mass. it is suggestive of this origin that while the smaller telescopic nebulæ are far removed from the milky way, the larger ones are most abundant near its borders; while the star-clusters are excessively abundant on and near the milky way, but very scarce elsewhere, except in or near vast nebulæ like the magellanic clouds. we thus see that the two phenomena may be complementary to each other, the condensation of nebulæ having gone on most rapidly where material was most abundant, resulting in numerous star-clusters where there are now few nebulæ. there is one striking feature of the globular clusters which calls for notice; the presence in some of them of enormous quantities of variable stars, while in others few or none can be found. the harvard observatory has for several years devoted much time to this class of observations, and the results are given in professor newcomb's recent volume on 'the stars.' it appears that twenty-three clusters have been observed spectroscopically, the number of stars examined in each cluster varying from up to , the total number of stars thus minutely tested being , . out of this total number were found to be variable; but the curious fact is, the extreme divergence in the proportion of variables to the whole number examined in the several clusters. in two clusters, though stars were examined, not a single variable was found. in three others the proportion was from one in to one in . five more ranged up to one in , and the remainder showed from that proportion up to one in seven, stars being examined in the last mentioned cluster of which were variable! when we consider that variable stars form only a portion, and necessarily a very small proportion, of binary systems of stars, it follows that in all the clusters which show a large proportion of variables, a very much larger proportion--in some cases perhaps all, must be double or multiple stars revolving round each other. with this remarkable evidence, in addition to that adduced for the prevalence of double stars and variables among the stars in general, we can understand professor newcomb adding his testimony to that of professor campbell already quoted, that 'it is probable that among the stars in general, single stars are the exception rather than the rule. if such be the case, the rule should hold yet more strongly among the stars of a condensed cluster.' the evolution of the stars so long as astronomers were limited to the use of the telescope only, or even the still greater powers of the photographic plate, nothing could be learnt of the actual constitution of the stars or of the process of their evolution. their apparent magnitudes, their movements, and even the distances of a few could be determined; while the diversity of their colours offered the only clue (a very imperfect one) even to their temperature. but the discovery of spectrum analysis has furnished the means of obtaining some definite knowledge of the physics and chemistry of the stars, and has thus established a new branch ofscience--astrophysics--which has already attained large proportions, and which furnishes the materials for a periodical and some important volumes. this branch of the subject is very complex, and as it is not directly connected with our present inquiry, it is only referred to again in order to introduce such of its results as bear upon the question of the classification and evolution of the stars. by a long series of laboratory experiments it has been shown that numerous changes occur in the spectra of the elements when subjected to different temperatures, ranging upwards to the highest attainable by means of a battery producing an electric spark several feet long. these changes are not in the relative position of the bands or dark lines, but in their number, breadth, and intensity. other changes are due to the density of the medium in which the elements are heated, and to their chemical condition as to purity; and from these various modifications and their comparison with the solar spectrum and those of its appendages, it has become possible to determine, from the spectrum of a star, not only its temperature as compared with that of the electric spark and of the sun, but also its place in a developmental series. the first general result obtained by this research is, that the bluish white or pure white stars, having a spectrum extending far towards the violet end, and which exhibits the coloured bands of gases only, usually hydrogen and helium, are the hottest. next come those with a shorter spectrum not extending so far towards the violet end, and whose light is therefore more yellow in tint. to this group our sun belongs; and they are all characterised like it by dark lines due to absorption, and by the presence of metals, especially iron, in a gaseous state. the third group have the shortest spectra and are of a red colour, while their spectra contain lines denoting the presence of carbon. these three groups are often spoken of as 'gaseous stars,' 'metallic stars,' and 'carbon stars.' other astronomers call the first group 'sirian stars,' because sirius, though not the hottest, is a characteristic type; the second being termed 'solar stars'; others again speak of them as stars of class i., class ii., etc., according to the system of classification they have adopted. it was soon perceived, however, that neither the colour nor the temperature of stars gave much information as to their nature and state of development, because, unless we supposed the stars to begin their lives already intensely hot (and all the evidence is against this), there must be a period during which heat increases, then one of maximum heat, followed by one of cooling and final loss of light altogether. the meteoritic theory of the origin of all luminous bodies in the heavens, now very widely adopted, has been used, as we have seen, to explain the development of stars from nebulæ, and its chief exponent in this country, sir norman lockyer, has propounded a complete scheme of stellar evolution and decay which may be here briefly outlined: beginning with nebulæ, we pass on to stars having banded or fluted spectra, indicating comparatively low temperatures and showing bands or lines of iron, manganese, calcium, and other metals. they are more or less red in colour, antares in the scorpion being one of the most brilliant red stars known. these stars are supposed to be in the process of aggregation, to be continually increasing in size and heat, and thus to be subject to great disturbances. alpha cygni has a similar spectrum but with more hydrogen, and is much hotter. the increase of heat goes on through rigel and beta crucis, in which we find mainly hydrogen, helium, oxygen, nitrogen, and also carbon, but only faint traces of metals. reaching the hottest of all--epsilon orionis and two stars in argo--hydrogen is predominant, with traces of a few metals and carbon. the cooling series is indicated by thicker lines of hydrogen and thinner lines of the metallic elements, through sirius, to arcturus and our sun, thence to piscium, which shows chiefly flutings of carbon, with a few faint metallic lines. the process of further cooling brings us to the dark stars. we have here a complete scheme of evolution, carrying us from those ill-defined but enormously diffused masses of gas and cosmic dust we know as nebulæ, through planetary nebulæ, nebulous stars, variable and double-stars, to red and white stars and on to those exhibiting the most intense blue-white lustre. we must remember, however, that the most brilliant of these stars, showing a gaseous spectrum and forming the culminating point of the ascending series, are not necessarily hotter than, or even so hot as, some of those far down on the descending scale; since it is one of the apparent paradoxes of physics that a body may become hotter during the very process of contraction through loss of heat. the reason is that by cooling it contracts and thus becomes denser, that a portion of its mass falls towards its centre, and in doing so produces an amount of heat which, though absolutely less than the heat lost in cooling, will under certain conditions cause the reduced surface to become hotter. the essential point is, that the body in question must be wholly gaseous, allowing of free circulation from surface to centre. the law, as given by professor s. newcomb, is as follows:-- '_when a spherical mass of incandescent gas contracts through the loss of its heat by radiation into space, its temperature continually becomes higher as long as the gaseous condition is retained._' to put it in another way: if the compression was caused by external force and no heat was lost, the globe would get hotter by a calculable amount for each unit of contraction. but the heat lost in causing a similar amount of contraction is so little more than the increase of heat produced by contraction, that the slightly diminished total heat in a smaller bulk causes the temperature of the mass to increase. but if, as there is reason to believe, the various types of stars differ also in chemical constitution, some consisting mainly of the more permanent gases, while in others the various metallic and non-metallic elements are present in very different proportions, there should really be a classification by constitution as well as by temperature, and the course of evolution of the differently constituted groups may be to some extent dissimilar. with this limitation the process of evolution and decay of sun through a cycle of increasing and decreasing temperature, as suggested by sir norman lockyer, is clear and suggestive. during the ascending series the star is growing both in mass and heat, by the continual accretion of meteoritic matter either drawn to it by gravitation or falling towards it through the proper motions of independent masses. this goes on till all the matter for some distance around the star has been utilised, and a maximum of size, heat, and brilliancy attained. then the loss of heat by radiation is no longer compensated by the influx of fresh matter, and a slow contraction occurs accompanied by a slightly increased temperature. but owing to the more stable conditions continuous envelopes of metals in the gaseous state are formed, which check the loss of heat and reduce the brilliancy of colour; whence it follows that bodies like our sun may be really hotter than the most brilliant white stars, though not giving out quite so much heat. the loss of heat is therefore reduced; and this may serve to account for the undoubted fact that during the enormous epochs of geological time there has been very little diminution in the amount of heat we have received from the sun. on the general question of the meteoritic hypothesis one of our first mathematicians, professor george darwin, has thus expressed his views: 'the conception of the growth of the planetary bodies by the aggregation of meteorites is a good one, and perhaps seems more probable than the hypothesis that the whole solar system was gaseous.' i may add, that one of the chief objections made to it, that meteorites are too complex to be supposed to be the primitive matter out of which suns and worlds have been made, does not seem to me valid. the primitive matter, whatever it was, may have been used up again and again, and if collisions of large solid globes ever occur--and it is assumed by most astronomers that they must sometimes occur--then meteoric particles of all sizes would be produced which might exhibit any complexity of mineral constitution. the material universe has probably been in existence long enough for all the primitive elements to have been again and again combined into the minerals found upon the earth and many others. it cannot be too often repeated that no explanation--no theory--can ever take us to the beginning of things, but only one or two steps at a time into the dim past, which may enable us to comprehend, however imperfectly, the processes by which the world, or the universe, as it is, has been developed out of some earlier and simpler condition. chapter vii are the stars infinite in number? most of the critics of my first short discussion of this subject laid great stress upon the impossibility of proving that the universe, a part of which we see, is not infinite; and a well-known astronomer declared that unless it can be demonstrated that our universe is finite the entire argument founded upon our position within it fall to the ground. i had laid myself open to this objection by rather incautiously admitting that if the preponderance of evidence pointed in this direction any inquiry as to our place in the universe would be useless, because as regards infinity there can be no difference of position. but this statement is by no means exact, and even in an infinite universe of matter containing an infinite number of stars, such as those we see, there might well be such infinite diversities of distribution and arrangement as would give to certain positions all the advantages which i submit we actually possess. supposing, for example, that beyond the vast ring of the milky way the stars rapidly decrease in number in all directions for a distance of a hundred or a thousand times the diameter of that ring, and that then for an equal distance they slowly increase again and become aggregated into systems or universes totally distinct from ours in form and structure, and so remote that they can influence us in no way whatever. then, i maintain, our position within our own stellar universe might have exactly the same importance, and be equally suggestive, as if ours were the only material universe in existence--as if the apparent diminution in the number of stars (which is an observed fact) indicated a continuous diminution, leading at some unknown distance to entire absence of luminous--that is, of active, energy-emitting aggregations of matter.[ ] as to whether there are such other material universes or not i offer no opinion, and have no belief one way or the other. i consider all speculations as to what may or may not exist in infinite space to be utterly valueless. i have limited my inquiries strictly to the evidence accumulated by modern astronomers, and to direct inferences and logical deductions from that evidence. yet, to my great surprise, my chief critic declares that 'dr. wallace's underlying error is, indeed, that he has reasoned from the area which we can embrace with our limited perceptions to the infinite beyond our mental or intellectual grasp.' i have distinctly _not_ done this, but many astronomers have done so. the late richard proctor not only continually discussed the question of infinite matter as well as infinite space, but also argued, from the supposed attributes of the deity, for the necessity of holding this material universe to be infinite, and the last chapter of his _other worlds than ours_ is mainly devoted to such speculations. in a later work, _our place among infinities_, he says that 'the teachings of science bring us into the presence of the unquestionable infinities of time and of space, and the presumable infinities of matter and of operation--hence therefore into the presence of infinity of energy. but science teaches us nothing about these infinities as such. they remain none the less inconceivable, however clearly we may be taught to recognise their reality.' all this is very reasonable, and the last sentence is particularly important. nevertheless, many writers allow their reasonings from facts to be influenced by these ideas of infinity. in proctor's posthumous work, _old and new astronomy_, the late mr. ranyard, who edited it, writes: 'if we reject as abhorrent to our minds the supposition that the universe is not infinite, we are thrown back on one of two alternatives--either the ether which transmits the light of the stars to us is not perfectly elastic, or a large proportion of the light of the stars is obliterated by dark bodies.' here we have a well-informed astronomer allowing his abhorrence of the idea of a finite universe to affect his reasoning on the actual phenomena we can observe--doing in fact exactly what my critic erroneously accuses me of doing. but setting aside all ideas and prepossessions of the kind here indicated, let us see what are the actual facts revealed by the best instruments of modern astronomy, and what are the natural and logical inferences from those facts. are the stars infinite in number? the views of those astronomers who have paid attention to this subject are, on the whole, in favour of the view that the stellar universe is limited in extent and the stars therefore limited in number. a few quotations will best exhibit their opinions on this question, with some of the facts and observations on which they are founded. miss a.m. clerke, in her admirable volume, _the system of the stars_, says: 'the sidereal world presents us, to all appearance, with a finite system.... the probability amounts almost to certainty that star-strewn space is of measurable dimensions. for from innumerable stars a limitless sum-total of radiations should be derived, by which darkness would be banished from our skies; and the "intense inane," glowing with the mingled beams of suns individually indistinguishable, would bewilder our feeble senses with its monotonous splendour.... unless, that is to say, light suffer some degree of enfeeblement in space.... but there is not a particle of evidence that any such toll is exacted; contrary indications are strong; and the assertion that its payment is inevitable depends upon analogies which may be wholly visionary. we are then, for the present, entitled to disregard the problematical effect of a more than dubious cause.' professor simon newcomb, one of the first of american mathematicians and astronomers, arrives at a similar conclusion in his most recent volume, _the stars_ ( ). he says, in his conclusions at the end of the work: 'that collection of stars which we call the universe is limited in extent. the smallest stars that we see with the most powerful telescopes are not, for the most part, more distant than those a grade brighter, but are mostly stars of less luminosity situate in the same regions' (p. ). and on page of the same work he gives reasons for this conclusion, as follows: 'there is a law of optics which throws some light on the question. suppose the stars to be scattered through infinite space so that every great portion of space is, in the general average, equally rich in stars. then at some great distance we describe a sphere having its centre in our sun. outside this sphere describe another one of a greater radius, and beyond this other spheres at equal distances apart indefinitely. thus we shall have an endless succession of spherical shells, each of the same thickness. the volume of each of these shells will be nearly proportional to the squares of the diameters of the spheres which bound it. hence each of the regions will contain a number of stars increasing as the square of the radius of the region. since the amount of light we receive from each star is as the inverse square of its distance, it follows that the sum total of the light received from each of these spherical shells will be equal. thus as we add sphere after sphere we add equal amounts of light without limit. the result would be that if the system of stars extended out indefinitely the whole heavens would be filled with a blaze of light as bright as the sun.' but the whole light given us by the stars is variously estimated at from one-fortieth to one-twentieth or, as an extreme limit, to one-tenth of moonlight, while the sun gives as much light as , full moons, so that starlight is only equivalent at a fair estimate to the six-millionth part of sunlight. keeping this in mind, the possible causes of the extinction of almost the whole of the light of the stars (if they are infinite in number and distributed, on the average, as thickly beyond the milky way as they are up to its outer boundary) are absurdly inadequate. these causes are ( ) the loss of light in passing through the ether, and ( ) the stoppage of light by dark stars or diffused meteoritic dust. as to the first, it is generally admitted that there is not a particle of evidence of its existence. there is, however, some distinct evidence that, if it exists, it is so very small in amount that it would not produce a perceptible effect for any distances less remote than hundreds or perhaps thousands of times as far as the farthest limits of the milky way are from us. this is indicated by the fact that the brightest stars are _not_ always, or even generally, the nearest to us, as is shown both by their small proper motions and the absence of measurable parallax. mr. gore states that out of twenty-five stars, with proper motions of more than two seconds annually, only two are above the third magnitude. many first magnitude stars, including canopus, the second brightest star in the heavens, are so remote that no parallax can be found, notwithstanding repeated efforts. they must therefore be much farther off than many small and telescopic stars, and perhaps as far as the milky way, in which so many brilliant stars are found; whereas if any considerable amount of light were lost in passing that distance we should find but few stars of the first two or three magnitudes that were very remote from us. of the twenty-three stars of the first magnitude, only ten have been found to have parallaxes of more than one-twentieth of a second, while five range from that small amount down to one or two hundredths of a second, and there are two with no ascertainable parallax. again, there are stars brighter than magnitude . , yet only thirty-one of these have proper motions of more than " a century, and of these only eighteen have parallaxes of more than one-twentieth of a second. these figures are from tables given in professor newcomb's book, and they have very great significance, since they indicate that the brightest stars are _not_ the nearest to us. more than this, they show that out of the seventy-two stars whose distance has been measured with some approach to certainty, only twenty-three (having a parallax of more than one-fiftieth of a second) are of greater magnitudes than . , while no less than forty-nine are smaller stars down to the eighth or ninth magnitude, and these are on the average much nearer to us than the brighter stars! taking the whole of the stars whose parallaxes are given by professor newcomb, we find that the average parallax of the thirty-one bright stars (from . magnitude up to sirius) is . seconds; while that of the forty-one stars below . magnitude down to about . , is . seconds, showing that they are, on the average, only half as far from us as the brighter stars. the same conclusion was reached by mr. thomas lewis of the greenwich observatory in , namely, that the stars from . magnitude down to about . magnitude have, on the average, double the parallaxes of the brighter stars. this very curious and unexpected fact, however it may be accounted for, is directly opposed to the idea of there being any loss of light by the more distant as compared with the nearer stars; for if there should be such a loss it would render the above phenomenon still more difficult of explanation, because it would tend to exaggerate it. the bright stars being on the whole farther away from us than the less bright down to the eighth and ninth magnitudes, it follows, if there is any loss of light, that the bright stars are really brighter than they appear to us, because, owing to their enormous distance some of their light has been lost before it reached us. of course it may be said that this does not _demonstrate_ that no light is lost in passing through space; but, on the other hand, it is exactly the opposite of what we should expect if the more distant stars were perceptibly dimmed by this cause, and it may be considered to prove that if there is any loss it is exceedingly small, and will not affect the question of the limits of our stellar system, which is all that we are dealing with. this remarkable fact of the enormous remoteness of the majority of the brighter stars is equally effective as an argument against the loss of light by dark stars or cosmic dust, because, if the light is not appreciably diminished for stars which have less than the fiftieth of a second of parallax, it cannot greatly interfere with our estimates of the limits of our universe. both mr. e.w. maunder of the greenwich observatory and professor w.w. turner of oxford lay great stress on these dark bodies, and the former quotes sir robert ball as saying, 'the dark stars are incomparably more numerous than those that we can see ... and to attempt to number the stars of our universe by those whose transitory brightness we can perceive would be like estimating the number of horseshoes in england by those which are red-hot.' but the proportion of dark stars (or nebulæ) to bright ones cannot be determined _a priori_, since it must depend upon the causes that heat the stars, and how frequently those causes come into action as compared with the life of a bright star. we do know, both from the stability of the light of the stars during the historic period and much more precisely by the enormous epochs during which our sun has supported life upon this earth--yet which must have been 'incomparably' less than its whole existence as a light-giver--that the life of most stars must be counted by hundreds or perhaps by thousands of millions of years. but we have no knowledge whatever of the rate at which true stars are born. the so-called 'new stars' which occasionally appear evidently belong to a different category. they blaze out suddenly and almost as suddenly fade away into obscurity or total invisibility. but the true stars probably go through their stages of origin, growth, maturity, and decay, with extreme slowness, so that it is not as yet possible for us to determine by observation when they are born or when they die. in this respect they correspond to species in the organic world. they would probably first be known to us as stars or minute nebulæ: at the extreme limit of telescopic vision or of photographic sensitiveness, and the growth of their luminosity might be so gradual as to require hundreds, perhaps thousands of years to be distinctly recognisable. hence the argument derived from the fact that we have never witnessed the birth of a true permanent star, and that, therefore, such occurrences are very rare, is valueless. new stars may arise every year or every day without our recognising them; and if this is the case, the reservoir of dark bodies, whether in the form of large masses or of clouds of cosmic dust, so far from being incomparably greater than the whole of the visible stars and nebulæ, may quite possibly be only equal to it, or at most a few times greater; and in that case, considering the enormous distances that separate the stars (or star-systems) from each other, they would have no appreciable effect in shutting out from our view any considerable proportion of the luminous bodies constituting our stellar universe. it follows, that professor newcomb's argument as to the very small total light given by the stars has not been even weakened by any of the facts or arguments adduced against it. mr. w.h.s. monck, in a letter to _knowledge_ (may ), puts the case very strongly so as to support my view. he says:--'the highest estimate that i have seen of the total light of the full moon is / of that of the sun. suppose that the dark bodies were a hundred and fifty thousand times as numerous as the bright ones. then the whole sky ought to be as bright as the illuminated portion of the moon. every one knows that this is not so. but it is said that the stars, though infinite, may only extend to infinity in particular directions, _e.g._ in that of the galaxy. be it so. where, in the very brightest portion of the galaxy, will we find a part equal in angular magnitude to the moon which affords us the same quantity of light? in the very brightest spot, the light probably does not amount to one hundredth part that of the full moon.' it follows that, even if dark stars were fifteen million times as numerous as the bright ones, professor newcomb's argument would still apply against an infinite universe of stars of the same average density as the portion we see. telescopic evidence as to the limits of the star system throughout the earlier portion of the nineteenth century every increase of power and of light-giving qualities of telescopes added so greatly to the number of the stars which became visible, that it was generally assumed that this increase would go on indefinitely, and that the stars were really infinite in number and could not be exhausted. but of late years it has been found that the increase in the number of stars visible in the larger telescopes was not so great as might be expected, while in many parts of the heavens a longer exposure of the photographic plate adds comparatively little to the number of stars obtained by a shorter exposure with the same instrument. mr. j.e. gore's testimony on this point is very clear. he says:--'those who do not give the subject sufficient consideration, seem to think that the number of the stars is practically infinite, or at least, that the number is so great that it cannot be estimated. but this idea is totally incorrect, and due to complete ignorance of telescopic revelations. it is certainly true that, to a certain extent, the larger the telescope used in the examination of the heavens, the more the number of the stars seems to increase; but we now know that there is a limit to this increase of telescopic vision. and the evidence clearly shows that we are rapidly approaching this limit. although the number of stars visible in the pleiades rapidly increases at first with increase in the size of the telescope used, and although photography has still further increased the number of stars in this remarkable cluster, it has recently been found that an increased length of exposure--beyond three hours--adds very few stars to the number visible on the photograph taken at the paris observatory in , on which over two thousand stars can be counted. even with this great number on so small an area of the heavens, comparatively large vacant places are visible between the stars, and a glance at the original photograph is sufficient to show that there would be ample room for many times the number actually visible. i find that if the whole heavens were as rich in stars as the pleiades, there would be only thirty-three millions in both hemispheres.' again, referring to the fact that celoria, with a telescope showing stars down to the eleventh magnitude, could see almost exactly the same number of stars near the north pole of the galaxy as sir william herschel found with his much larger and more powerful telescope, he remarks: 'their absence, therefore, seems certain proof that very faint stars do _not_ exist in that direction, and that here, at least, the sidereal universe is limited in extent.' sir john herschel notes the same phenomena, stating that even in the milky way there are found 'spaces absolutely dark _and completely void of any star_, even of the smallest telescopic magnitude'; while in other parts 'extremely minute stars, though never altogether wanting, occur in numbers so moderate as to lead us irresistibly to the conclusion that in these regions we see _fairly through_ the starry stratum, since it is impossible otherwise (supposing their light not intercepted) that the numbers of the smaller magnitudes should not go on continually increasing ad infinitum. in such cases, moreover, the ground of the heavens, as seen between the stars, is for the most part perfectly dark, which again would not be the case if innumerable multitudes of stars, too minute to be individually discernible, existed beyond.' and again he sums up as follows:--'throughout by far the larger portion of the extent of the milky way in both hemispheres, the general blackness of the ground of the heavens on which its stars are projected, and the absence of that innumerable multitude and excessive crowding of the smallest visible magnitudes, and of glare produced by the aggregate light of multitudes too small to affect the eye singly, which the contrary supposition would appear to necessitate, must, we think, be considered unequivocal indications that its dimensions _in directions where these conditions obtain_, are not only not infinite, but that the space-penetrating power of our telescopes suffices fairly to pierce through and beyond it.'[ ] this expression of opinion by the astronomer who, probably beyond any now living, was the most competent authority on this question, to which he devoted a long life of observation and study extending over the whole heavens, cannot be lightly set aside by the opinions or conjectures of those who seem to assume that we must believe in an infinity of stars if the contrary cannot be absolutely proved. but as not a particle of evidence can be adduced to prove infinity, and as all the facts and indications point, as here shown, in a directly opposite direction, we must, if we are to trust to evidence at all in this matter, arrive at the conclusion that the universe of stars is limited in extent. dr. isaac roberts gives similar evidence as regards the use of photographic plates. he writes:--'eleven years ago photographs of the great nebula in _andromeda_ were taken with the -inch reflector, and exposures of the plates during intervals up to four hours; and upon some of them were depicted stars to the faintness of th to th magnitude, and nebulosity to an equal degree of faintness. the films of the plates obtainable in those days were less sensitive than those which have been available during the past five years, and during this period photographs of the nebula with exposures up to four hours have been taken with the -inch reflector. no extensions of the nebulosity, however, nor increase in the number of the stars can be seen on the later rapid plates than were depicted upon the earlier slower ones, though the star-images and the nebulosity have greater density on the later plates.' exactly similar facts are recorded in the cases of the great nebula in _orion_, and the group of the pleiades. in the case of the milky way in _cygnus_ photographs have been taken with the same instrument, but with exposures varying from one hour to two hours and a half, but no fainter stars could be found on one than on the other; and this fact has been confirmed by similar photographs of other areas in the sky. the law of diminishing numbers of stars we will now consider another kind of evidence equally weighty with the two already adduced. this is what may be termed the law of diminishing numbers beyond a certain magnitude, as observed by larger and larger telescopes. for some years past star-magnitudes have been determined very accurately by means of careful photometric comparisons. down to the sixth magnitude stars are visible to the naked eye, and are hence termed lucid stars. all fainter stars are telescopic, and continuing the magnitudes in a series in which the difference in luminosity between each successive magnitude is equal, the seventeenth magnitude is reached and indicates the range of visibility in the largest telescopes now in existence. by the scale now used a star of any magnitude gives nearly two and a half times as much light as one of the next lower magnitude, and for accurate comparison the apparent brightness of each star is given to the tenth of a magnitude which can easily be observed. of course, owing to differences in the colour of stars, these determinations cannot be made with perfect accuracy, but no important error is due to this cause. according to this scale a sixth magnitude star gives about one-hundredth part of the light of an average first magnitude star. sirius is so exceptionally bright that it gives nine times as much light as a standard or average first magnitude star. now it is found that from the first to the sixth magnitude the stars increase in number at the rate of about three and a half times those of the preceding magnitudes. the total number of stars down to the sixth magnitude is given by professor newcomb as . for higher magnitudes the numbers are so great that precision and uniformity are more difficult of attainment; yet there is a wonderful continuance of the same law of increase down to the tenth magnitude, which is estimated to include , , stars, thus conforming very nearly with the ratio of . as determined by the lucid stars. but when we pass beyond the tenth magnitude to those vast numbers of faint stars only to be seen in the best or the largest telescopes, there appears to be a sudden change in the ratio of increased numbers per magnitude. the numbers of these stars are so great that it is impossible to count the whole as with the higher magnitude stars, but numerous counts have been made by many astronomers in small measured areas in different parts of the heavens, so that a fair average has been obtained, and it is possible to make a near approximation to the total number visible down to the seventeenth magnitude. the estimate of these by astronomers who have made a special study of this subject is, that the total number of visible stars does not exceed one hundred millions.[ ] but if we take the number of stars down to the ninth magnitude, which are known with considerable accuracy, and find the numbers in each succeeding magnitude down to the seventeenth, according to the same ratio of increase which has been found to correspond very nearly in the case of the higher magnitudes, mr. j.e. gore finds that the total number should be about millions. of course neither of these estimates makes any pretence to exact accuracy, but they are founded on all the facts at present available, and are generally accepted by astronomers as being the nearest approach that can be made to the true numbers. the discrepancy is, however, so enormous that probably no careful observer of the heavens with very large telescopes doubts that there is a very real and very rapid diminution in the numbers of the fainter as compared with the brighter stars. there is, however, yet one more indication of the decreasing numbers of the faint telescopic stars, which is almost conclusive on this question, and, so far as i am aware, has not yet been used in this relation. i will therefore briefly state it. the light ratio as indicating the number of faint stars professor newcomb points out a remarkable result depending on the fact that, while the average light of successively lower magnitudes diminishes in a ratio of . , their numbers increase at nearly a ratio of . . from this it follows that, so long as this law of increase continues, the total of starlight goes on increasing by about forty per cent. for each successive magnitude, and he gives the following table to illustrate it:-- mag. total light = " " = . " " = . " " = . " " = . " " = . " " = . " " = . " " = . " " = . -------- total light to mag. = . -------- thus the total amount of the light given by all stars down to the tenth magnitude is seventy-four times as great as that from the few first magnitude stars. we also see that the light given by the stars of any magnitude is twice as much as that of the stars two magnitudes higher in the scale, so that we can easily calculate what additional light we ought to receive from each additional magnitude if they continue to increase in numbers below the tenth as they do above that magnitude. now it has been calculated as the result of careful observations, that the total light given by stars down to nine and a half magnitude is one-eightieth of full moonlight, though some make it much more. but if we continue the table of light-ratios from this low starting-point down to magnitude seventeen and a half, we shall find, if the numbers of the stars go on increasing at the same rate as before, that the light of all combined should be at least seven times as great as moonlight; whereas the photometric measurements make it actually about one-twentieth. and as the calculation from light-ratios only includes stars just visible in the largest telescopes, and does not include all those proved to exist by photography, we have in this case a demonstration that the numbers of the stars below the tenth and down to the seventeenth magnitude diminish rapidly. we must remember that the minuter telescopic stars preponderate enormously in and near the milky way. at a distance from it they diminish rapidly, till near its poles they are almost entirely absent. this is shown by the fact (already referred to at p. ) that professor celoria of milan, with a telescope of less than three inches aperture, counted almost as many stars in that region as did herschel with his eighteen-inch reflector. but if the stellar universe extends without limit we can hardly suppose it to do so in one plane only; hence the absence of the minuter stars and of diffused milky light over the larger part of the heavens is now held to prove that the myriads of very minute stars in the milky way really belong to it, and not to the depths of space far beyond. it seems to me that here we have a fairly direct proof that the stars of our universe are really limited in number. there are thus four distinct lines of argument all pointing with more or less force to the conclusion that the stellar universe we see around us, so far from being infinite, is strictly limited in extent and of a definite form and constitution. they may be briefly summarised as follows:-- ( ) professor newcomb shows that, if the stars were infinite in number, and if those we see were approximately a fair sample of the whole, and further, if there were not sufficient dark bodies to shut out almost the whole of their light, then we should receive from them an amount of light theoretically greater than that of sunlight. i have shown, at some length, that neither of these causes of loss of light will account for the enormous disproportion between the theoretical and the actual light received from the stars; and therefore professor newcomb's argument must be held to be a valid one against the infinite extent of our universe. of course, this does not imply that there may not be any number of other universes in space, but as we know absolutely nothing of them--even whether they are material or non-material--all speculation as to their existence is worse than useless. ( ) the next argument depends on the fact that all over the heavens, even in the milky way itself, there are areas of considerable extent, besides rifts, lanes, and circular patches, where stars are either quite absent or very faint and few in number. in many of these areas the largest telescopes show no more stars than those of moderate size, while the few stars seen are projected on an intensely dark background. sir william herschel, humboldt, sir john herschel, r.a. proctor, and many living astronomers hold that, in these dark areas, rifts, and patches, we see completely through our stellar universe into the starless depths of space beyond. ( ) then we have the remarkable fact that the steady increase in the number of stars, down to the ninth or tenth magnitudes, following one constant ratio either gradually or suddenly changes, so that the total number from the tenth down to the seventeenth magnitudes is only about one-tenth of what it would have been had the same ratio of increase continued. the conclusion to be drawn from this fact clearly is, that these faint stars are becoming more and more thinly scattered in space, while the dark background on which they are usually seen shows that, except in the region of the milky way, there are _not_ multitudes of still smaller invisible stars beyond them. ( ) the last indication of a limited stellar universe--the estimate of numbers by the light-ratio of each successive magnitude--powerfully supports the three preceding arguments. the four distinct classes of evidence now adduced must be held to constitute, as nearly as the circumstances permit, a satisfactory proof that the stellar universe, of which our solar system forms a part, has definite limits; and that a full knowledge of its form, structure, and extent, is not beyond the possibility of attainment by the astronomers of the future. footnotes: [ ] in a letter to _knowledge_, june , mr. w.h.t. monck puts the same point in a mathematical form. [ ] _outlines of astronomy_, pp. - . in the passages quoted the italics are sor john herschel's. [ ] mr. j.e. gore in _concise knowledge astronomy_, pp. - . chapter viii our relation to the milky way we now approach what may be termed the very heart of the subject of our inquiry, the determination of how we are actually situated within this vast but finite universe, and how that position is likely to affect our globe as being the theatre of the development of life up to its highest forms. we begin with our relation to the milky way (which we have fully described in our fourth chapter), because it is by far the most important feature in the whole heavens. sir john herschel termed it 'the ground-plane of the sidereal system'; and the more it is studied the more we become convinced that the whole of the stellar universe--stars, clusters of stars, and nebulæ--are in some way connected with it, and are probably dependent on it or controlled by it. not only does it contain a greater number of stars of the higher magnitudes than any other part of the heavens of equal extent, but it also comprises a great preponderance of star-clusters, and a great extent of diffused nebulous matter, besides the innumerable myriads of minute stars which produce its characteristic cloud-like appearance. it is also the region of those strange outbursts forming new stars; while gaseous stars of enormous bulk--some probably a thousand or even ten thousand times that of our sun, and of intense heat and brilliancy--are more abundant there than in any other part of the heavens. it is now almost certain that these enormous stars and the myriads of minute stars just visible with the largest telescopes, are actually intermingled, and together constitute its essential features; in which case the fainter stars are really small and cannot be far apart, forming, as it were, the first aggregations of the nebulous substratum, and perhaps supplying the fuel which keeps up the intense brilliancy of the giant suns. if this is so, then the galaxy must be the theatre of operation of vast forces, and of continuous combinations of matter, which escape our notice owing to its enormous distance from us. among its millions of minute telescopic stars, hundreds or thousands may appear or disappear yearly without being perceived by us, till the photographic charts are completed and can be minutely scrutinised at short intervals. as undoubted changes have occurred in many of the larger nebulæ during the last fifty years, we may anticipate that analogous changes will soon be noted in the stars and the nebulous masses of the milky way. dr. isaac roberts has even observed changes in nebulæ after such a short interval as eight years. the milky way a great circle notwithstanding all its irregularities, its divisions, and its diverging branches, astronomers are generally agreed that the milky way forms a great circle in the heavens. sir john herschel, whose knowledge of it was unrivalled, stated that its course 'conforms, as nearly as the indefiniteness of its boundary will allow it to be fixed, to that of a great circle'; and he gives the right ascension and declination of the points where it crosses the equinoctial, in figures which define those points as being exactly opposite each other. he also defines its northern and southern poles by other figures, so as to show that they are the poles of a great circle. and after referring to struve's view that it was _not_ a great circle, he says, 'i retain my own opinion.' professor newcomb says that its position 'is nearly always near a great circle of the sphere'; and again he says: 'that we are in the galactic plane itself seems to be shown in two ways: ( ) the equality in the counts of stars on the two sides of this plane all the way to its poles; and ( ) the fact that the central line of the galaxy is a great circle, which it would not be if we viewed it from one side of its central plane' (_the stars_, p. ). miss clerke, in her _history of astronomy_, speaks of 'our situation _in_ the galactic plane' as one of the undisputed facts of astronomy; while sir norman lockyer, in a lecture delivered in , said, 'the middle line of the milky way is really not distinguishable from a great circle,' and again in the same lecture--'but the recent work, chiefly of gould in argentina, has shown that it practically is a great circle.'[ ] about this fact, then, there can be no dispute. a great circle is a circle dividing the celestial sphere into two equal portions, as seen from the earth, and therefore the plane of this circle must pass through the earth. of course the whole thing is on such a vast scale, the milky way varying from ten to thirty degrees wide, that the plane of its circular course cannot be determined with minute accuracy. but this is of little importance. when carefully laid down on a chart, as in that of mr. sidney waters (see end of volume), we can see that its central line does follow a very even circular course, conforming 'as nearly as may be' to a great circle. we are therefore certainly well within the space that would be enclosed if its northern and southern margins were connected together across the vast intervening abyss, and in all probability not far from the central plane of that enclosed space. the form of the milky way and our position on its plane although the galaxy forms a great circle in the heavens from our point of view, it by no means follows that it is circular in plan. being unequal in width and irregular in outline, it might be elliptic or even angular in shape without being at all obviously so to us. if we were standing in an open plain or field two or three miles in diameter, and bounded in every direction by woods of very irregular height and density and great diversity of tint, we should find it difficult to judge of the shape of the field, which might be either a true circle, an oval, a hexagon, or quite irregular in outline, without our being able to detect the exact shape unless some parts were very much nearer to us than others. again, just as the woods bounding the field might be either a narrow belt of nearly uniform width, or might in some places be only a few yards wide and in others stretch out for miles, so there have been many opinions as to the width of the milky way in the direction of its plane, that is, in the direction in which we look towards it. lately, however, as the result of long-continued observation and study, astronomers are fairly well agreed as to its general form and extent, as will be seen by the following statements of fact and reasoning. miss clerke, after giving the various views of many astronomers--and as the historian of modern astronomy her opinion has much weight--considers that the most probable view of it is, that it is really very much what it seems to us--an immense ring with streaming appendages extending from the main body in all directions, producing the very complex effect we see. the belief seems to be now spreading that the whole universe of stars is spherical or spheroidal, the milky way being its equator, and therefore in all probability circular or nearly so in plan; and it is also held that it must be rotating--perhaps very slowly--as nothing else can be supposed to have led to the formation of such a vast ring, or can preserve it when formed. professor newcomb considers, from the numbers of the stars in all directions towards the milky way being approximately equal, that there cannot be much difference in our distance from it in various directions. it would follow that its plan is approximately circular or broadly elliptic. the existence of ring-nebulæ may be held to render such a form probable. sir norman lockyer gives facts which tend in the same direction. in an article in _nature_ of november th, , he says: 'we find that the gaseous stars are not only confined to the milky way, but they are the most remote in every direction, in every galactic longitude; all of them have the smallest proper motion.' and again, referring to the hottest stars being equally remote on all sides of us, he says: 'it is because we are in the centre, because the solar system is in the centre, that the observed effect arises.' he also considers that the ring-nebula in lyra nearly represents the form of our whole system; and he adds: 'we practically know that in our system the centre is the region of least disturbance, and therefore cooler conditions.' these various facts and conclusions of some of the most eminent astronomers all point to one definite inference, that our position, or that of the solar system, is not very far from the centre of the vast ring of stars constituting the milky way, while the same facts imply a nearly circular form to this ring. here, more than as regards our position in the plane of the galaxy, there is no possibility of precise determination; but it is quite certain that if we were situated very far away from the centre, say, for instance, one-fourth of its diameter from one side of it and three-fourths from the other, the appearances would not be what they are, and we should easily detect the excentricity of our position. even if we were one-third the diameter from one side and two-thirds from the other, it will, i think, be admitted that this also would have been ascertained by the various methods of research now available. we must, therefore, be somewhere between the actual centre and a circle whose radius is one-third of the distance to the milky way. but if we are about midway between these two positions, we shall only be one-sixth of the radius or one-twelfth of the diameter of the milky way from its exact centre; and if we form part of a cluster or group of stars slowly revolving around that centre, we should probably obtain all the advantages, if any, that may arise from a nearly central position in the entire star-system. this question of our situation within the great circle of the milky way is of considerable importance from the point of view i am here suggesting, so that every fact bearing upon it should be noted; and there is one which has not, i think, been given the full weight due to it. it is generally admitted that the greater brilliancy of some parts of the milky way is no indication of nearness, because surfaces possess equal brilliancy from whatever distance they are seen. thus each planet has its special brilliancy or reflective power, technically termed its 'albedo,' and this remains the same at all distances if the other conditions are similar. but notwithstanding this well-known fact, sir john herschel's remark that the greater brightness of the southern milky way 'conveys strongly the impression of greater proximity,' and therefore, that we are excentrically placed in its plane, has been adopted by many writers as if it were the statement of a fact, or at least a clearly expressed opinion, instead of being a mere 'impression,' and really a misleading one. i therefore wish to adduce a phenomenon which has a real bearing on the question. it is evident that, if the milky way were actually of uniform width throughout, then differences of apparent width would indicate differences of distance. in the parts nearer to us it would appear wider, where more remote, narrower; but in these opposite directions there would not necessarily be any differences in brightness. we should, however, expect that in the parts nearer to us the lucid stars, as well as those within any definite limits of magnitude, would be either more numerous or more wide apart on the average. no such difference as this, however, has been recorded; but there _is_ a peculiar correspondence in the opposite portions of the galaxy which is very suggestive. in the beautiful charts of the nebulæ and star clusters by the late mr. sidney waters, published by the royal astronomical society and here reproduced by their permission (see end of volume), the milky way is delineated in its whole extent with great detail and from the best authorities. these charts show us that, in both hemispheres, it reaches its maximum extension on the right and left margins of the charts, where it is almost equal in extent; while in the centre of each chart, that is at its nearest points to the north and south poles respectively, it is at its narrowest portion; and, although this part in the southern hemisphere is brightest and most strongly defined, yet the actual extent, including the fainter portions, is, again, not very unequal in the opposite segments. here we have a remarkable and significant symmetry in the proportions of the milky way, which, taken in connection with the nearly symmetrical scattering of the stars in all parts of the vast ring, is strongly suggestive of a nearly circular form and of our nearly central position within its plane. there is one other feature in this delineation of the milky way which is worthy of notice. it has been the universal practice to speak of it as being double through a considerable portion of its extent, and all the usual star-maps show the division greatly exaggerated, especially in the northern hemisphere; and this division was considered so important as to lead to the cloven-disc theory of its form, or that it consisted of two separate irregular rings, the nearer one partly hiding the more distant; while various spiral combinations were held by others to be the best way of explaining its complex appearance. but this newer map, reduced from a large one by lord rosse's astronomer, dr. boeddicker, who devoted five years to its delineation, shows us that there is no actual division in any portion of it in the northern hemisphere, but that everywhere, throughout its whole width, it consists of numerous intermingled streams and branches, varying greatly in luminosity, and with many faint or barely distinguishable extensions along its margins, yet forming one unmistakable nebulous belt; and the same general character applies to it in the southern hemisphere as delineated by dr. gould. another feature, which is well shown to the eye by these more accurate maps, is the regular curvature of the central line of the milky way. we can judge of this almost sufficiently by the eye; but if, with a pair of compasses, we find the proper radius and centre of curvature, we shall see that the true circular curve is always in the very centre of the nebulous mass, and the same radius applied in the same manner to the opposite hemisphere gives a similar result. it will be noted that as the milky way is obliquely situated on these charts, the centre of the curve will be about in r.a. h. m. in the map of the southern hemisphere, and in r.a. h. m. in that of the northern hemisphere; while the radius of curvature will be about the length of the chord of eight hours of r.a. as measured on the margin of the maps. this great regularity of curve of the central line of the galaxy strongly suggests rotation as the only means by which it could have originated and be maintained. the solar cluster astronomers are now generally agreed that there is a cluster of stars of which our sun forms a part, though its exact dimensions, form, and limits are still under discussion. sir william herschel long ago arrived at the conclusion that the milky way 'consists of stars very differently scattered from those immediately around us.' dr. gould believed that there were about five hundred bright stars much nearer to us than the milky way, which he termed the solar cluster. and miss clerke observes that the actual existence of such a cluster is indicated by the fact that 'an enumeration of the stars in photometric order discloses a systematic excess of stars brighter than the th magnitude, making it certain that there is an actual condensation in the neighbourhood of the sun--that the average allowance of cubical space per star is smaller within a sphere enclosing him with a radius, say, of light-years, than further away.'[ ] but the most interesting inquiry into this subject is that by professor kapteyn of gröningen, one of the most painstaking students of the distribution of the stars. he founds his conclusions mainly on the proper motions of the stars, this being the best general indication of distance in the absence of actual determination of parallax. he made use of the proper motions and the spectra of more than two thousand stars, and he finds that a considerable body of stars having large proper motions, and also presenting the solar type of spectra, surround our sun in all directions, and show no increased density, as the more distant stars do, towards the milky way. he finds also that towards the centre of this cluster stars are far closer together than near its outer limits (he says there are ninety-eight times as many), that it is roughly spherical in shape, and that the maximum compression is, as nearly as can be ascertained, at the centre of the circle of the milky way, while the sun is at some distance away from this central point.[ ] it is a very suggestive fact that most of the stars belonging to this cluster have spectra of the solar type, which indicates that they are of the same general chemical constitution as our sun, and are also at about the same stage of evolution; and this may well have arisen from their origin in a great nebulous mass situated at or near the centre of the galactic plane, and probably revolving round their common centre of gravity. as kapteyn's result was based on materials which were not so full or reliable as those now available, professor s. newcomb has examined the question himself, using two recent lists of stars, one limited to those having proper motions of " a century, of which there are , and the other of nearly stars with 'appreciable proper motions.' they are situated in two zones, each about ° in breadth and cutting across the milky way in different parts of its course. they afford, therefore, a good test of the distribution of these nearer stars with regard to the galaxy. the result is, that on the average these stars are not more numerous in or near the milky way than elsewhere; and professor newcomb expresses himself on this point as follows:--'the conclusion is interesting and important. if we should blot out from the sky all the stars having no proper motion large enough to be detected, we should find remaining stars of all magnitudes; but they would be scattered almost uniformly over the sky, and show little or no tendency to crowd towards the galaxy, unless, perhaps, in the region near h. of right ascension.'[ ] a little consideration will show that, as the stars of all magnitudes which are, on the average, nearest to us are spread over the sky in 'all directions' and 'almost uniformly,' this necessarily implies that they form a cluster or group, and that our sun is somewhere not very far from the centre of this group. again, professor newcomb refers to 'the remarkable equality in the number of stars in opposite directions from us. we do not detect any marked difference between the numbers lying round the opposite poles of the galaxy, nor, so far as known, between the star-density in different regions at equal distances from the milky way' (_the stars_, p. ). and again he refers to the same question at p. , where he says: 'so far as we can judge from the enumeration of the stars in all directions, and from the aspect of the milky way, our system is near the centre of the stellar universe.' it will, i think, now be clear to my readers that the four main astronomical propositions stated in my article which appeared in the new york _independent_ and in the _fortnightly review_, and which were either denied or declared to be unproved by my astronomical critics, have been shown to be supported by so many converging lines of evidence, that it is no longer possible to deny that they are, at least provisionally, fairly well established. these facts are, ( ) that the stellar universe is not of infinite extent; ( ) that our sun is situated in the central plane of the milky way; ( ) that it is also situated near to the centre of that plane; ( ) that we are surrounded by a group or cluster of stars of unknown extent, which occupy a place not far removed from the centre of the galactic plane, and therefore, near to the centre of our universe of stars. not only are these four propositions each supported by converging lines of evidence, including some which i believe have not before been adduced in their support, but a number of astronomers, admittedly of the first rank, have arrived at the same conclusions as to the bearing of the evidence, and have expressed their convictions in the clearest manner, as quoted by me. it is _their_ conclusions which i appeal to and adopt; yet my two chief astronomical critics positively deny that there is any valid evidence of the finiteness of the stellar universe, which one of them terms 'a myth,' and he even accuses _me_ of having started it. both of them, however, agree in stating very strongly one objection to my main thesis--that our central position (not necessarily at the precise centre) in the stellar universe has a meaning and a purpose, in connection with the development of life and of man upon this earth, and, _so far as we know_, here only. with this one objection, the only one that in my opinion has the slightest weight, i will now proceed to deal. the sun's motion through space the two astronomers who did me the honour to criticise my original article laid the greatest stress on the fact, that even if i had proved that the sun now occupied a nearly central position in the great star-system, it was really of no importance whatever, because, at the rate the sun was travelling, 'five million years ago we were deep in the actual stream of the milky way; five million years hence we shall have completely crossed the gulf which it encircles, and again be a member of one of its constituent groups, but on the opposite side. and ten million years are regarded by geologists and biologists as but a trifle on account to meet their demands upon the bank of time.' thus speaks one of my critics. the other is equally crushing. he says:--'if there is a centre to the visible universe, and if we occupy it to-day, we certainly did not do so yesterday, and shall not do so to-morrow. the solar system is known to be moving among the stars with a velocity which would carry us to sirius within , years, if we happened to be travelling in his direction, as we are not. in the or million years during which, according to geologists, this earth has been a habitable globe, we must have passed by thousands of stars on the right hand and on the left.... in his eagerness to limit the universe in space, dr. wallace has surely forgotten that it is equally important, for his purpose, to limit it in time; but incomparably more difficult in the face of ascertained facts.... indeed, so far from our having tranquilly enjoyed a central position in unbroken continuity for scores or perhaps hundreds of millions of years, we should in that time have traversed the universe from boundary to boundary.'[ ] now the average reader of these two criticisms, taking account of the high official position of both writers, would accept their statements of the case as being demonstrated facts, requiring no qualification whatever, and would conclude that my whole argument had been thereby rendered worthless, and all that i founded upon it a fantastic dream. but if, on the other hand, i can show that their stated facts as to the sun's motion are by no means demonstrated, because founded upon assumptions which may be quite erroneous; and further, that if the facts should turn out to be substantially correct, they have both omitted to state well-known and admitted qualifications which render the conclusions they derive from the facts very doubtful, then the average reader will learn the valuable lesson that official advocacy, whether in medicine, law, or science is never to be accepted till the other side of the case has been heard. let us see, therefore, what the facts really are. professor simon newcomb calculates that, if there are one hundred million stars in the stellar universe each five times the mass of our sun, and spread over a space which light would require thirty thousand years to cross, then any mass traversing such a system with a velocity of more than twenty-five miles a second, would fly off into infinite space never to return. now as there are many stars which have, apparently, very much more than this velocity, it would follow that the visible universe is unstable. it also implies that these great velocities were not acquired in the system itself, but that the bodies which possess them must have entered it from without, thus requiring other universes as the feeders of our universe. for the accuracy of the above statement the authority of professor newcomb is an ample guarantee; but there may be modifications required in the data on which it is founded, and these may greatly alter the result. if i do not mistake, the estimate of a hundred million stars is founded on actual counts or estimates of stars of successive magnitudes in different parts of the heavens, and it does not include either those of the denser star clusters nor the countless millions just beyond the reach of telescopes in the milky way. neither does it make allowance for the dark stars supposed by some astronomers to be many times more numerous than the bright ones, nor for the vast number of the nebulæ, great and small, in calculating the total mass of the stellar system.[ ] in his latest work professor newcomb says, 'the total number of stars is to be counted by hundreds of millions'; and hence the controlling power of the system on bodies within it will be many times greater than that given above, and might even be ample to retain within its bounds such a rapidly moving star as arcturus, which is believed to be travelling at the rate of more than three hundred miles a second. but there is another very important limitation to the conclusions to be drawn from professor newcomb's calculation. it assumes the stars to be nearly uniformly distributed through the whole of the space to which the system extends. but the facts are very different. the existence of clusters, some of which comprise many thousands of stars, is one example of irregularity of distribution, and any one of these larger clusters would probably be able to change the course of even the swiftest stars passing near it. the larger nebulæ might have the same effect, since the late mr. ranyard, taking all his data so as to produce a minimum result, calculated the probable mass of the orion nebula to be four and a half million times that of the sun, and there may be many other nebulæ equally large. but far more important is the fact of the vast ring of the milky way, which is now universally held by astronomers to be, not only apparently but really, more densely crowded with stars and also with vast masses of nebulous matter than any other part of the heavens, so that it may possibly comprise within itself a very large proportion of the whole of the matter of the visible universe. this is rendered more probable by the fact that the great majority of star-clusters lie along its course, most of the huge gaseous stars belong to it, while the occurrence there only of 'new stars' is evidence of a superabundance of matter in various forms leading to frequent heat-producing collisions, just as the frequent occurrence of meteoric showers on our earth is evidence of the superabundance of meteoric matter in the solar system. it is recognised by mathematicians that within any great system of bodies subject to the law of gravitation there can be no such thing as motion of any of them in a straight line; neither can any amount of motion arise within such a system through the action of gravitation alone capable of carrying any of its masses out of the system. the ultimate tendency must be towards concentration rather than towards dispersal. it seems, therefore, only reasonable to consider whatever motions and whatever velocities we find among the stars, as having been produced by the gravitative power of the larger aggregations, modified perhaps by electrical repulsive forces, by collisions, and by the results of those collisions; and we may look to the changes now visibly going on in some of the nebulæ and clusters as indications of the forces that have probably brought about the actual condition of the whole stellar universe. if we examine the beautiful photographs of nebulæ by dr. roberts and other observers, we find that they are of many forms. some are extremely irregular and almost like patches of cirrus clouds, but a large number are either distinctly spiral in form, or show indications of becoming spiral, and this has been found to be the case even with some of the large irregular nebula. then again we have numerous ring-formed nebulæ, usually with a star involved in dense nebulosity in the centre, separated by a dark space of various widths from the outer ring. all these kinds of nebulæ have stars involved in them, and apparently forming part of their structure, while others which do not differ in appearance from ordinary stars are believed by dr. roberts to lie between us and the nebula. in the case of many of the spiral nebulæ, stars are often strung along the coils of the spiral, while other curved lines of stars are seen just outside the nebula, so that it is impossible to avoid the conclusion that both are really connected with it, the outer lines of stars indicating a former greater extension of the nebula whose material has been used up in the growth of these stars. some of these spiral nebulæ show beautifully regular convolutions, and these usually have a large central star like mass, as in m. comæ and i. comæ, in vol. ii. pl. of dr. roberts's photographs. the straight white streaks across the nebula of the pleiades and some others are believed by dr. roberts to be indications of spiral nebulæ seen edgewise. in other cases, clusters of stars are more or less nebulous, and the arrangement of the stars seems to indicate their development from a spiral nebula. it is to be noted that many of the objects classed as planetary nebulæ by sir john herschel are shown by the best photographs to be really of the ring-type, though often with a very narrow division between the ring and the central mass. this form may therefore be of frequent occurrence. but if this annular form with some kind of central nucleus, often very large, is produced under certain conditions by the action of the ordinary laws of motion upon more or less extensive masses of discrete matter, why may not the same laws acting upon similar matter once dispersed over the whole extent of the existing stellar universe, or even beyond what are now its farthest limits, have led to the aggregation of the vast annular formation of the milky way, with all the subordinate centres of concentration or dispersal to be found within or around it? and if this is a reasonable conception, may we not hope that by a concentration of attention upon a few of the best marked and most favourably situated annular and spiral systems, sufficient knowledge of their internal motions may be obtained which may serve as a guide to the kind of motion we may expect to find in the great galactic ring and its subordinate stars? we may then perhaps discover which now seem so erratic, are really all parts of a series of orbital movements limited and controlled by the forces of the great system to which they belong, so that, if not mathematically stable, they may yet be sufficiently so to endure for some thousand millions of years. it is a suggestive fact that the calculated position of the 'solar apex'--the point towards which our sun appears to move--is now found to be much more nearly in the plane of the milky way than the position first assigned to it, and professor newcomb adopts, as most likely to be accurate, a point near the bright star vega in the constellation lyra. other calculators have placed it still farther east, while rancken and otto stumpe assign it a position actually in the milky way; and mr. g.c. bompas concludes that the sun's plane of motion nearly coincides with that of the galaxy. m. rancken found that stars near the milky way showed, in their very small proper motions, a drift along it in a direction from cassiopeiæ towards orion, and this, it is supposed, may be partly due to our sun's motion in an opposite direction. in many other parts of the heavens there are groups of stars which have almost identical proper motions--a phenomenon which the late r.a. proctor termed 'star-drift'; and he especially pointed out that five of the stars of the great bear were all drifting in the same direction; and although this has been denied by later writers, professor newcomb, in his recent book on _the stars_, declares that proctor was right, and explains that the error of his critics was due to not making allowance for the divergence of the circles of right ascension. the pleiades are another group, the stars of which drift in the same direction, and it is a most suggestive fact that photographs now show this cluster to be embedded in a vast nebula, which, therefore, has also a proper motion; but some of the smaller stars do not partake of it. three stars in cassiopeiæ also move together, and no doubt many other similarly connected groups remain to be discovered. these facts have a very important bearing on the question of the motion of our sun in space. for this motion has been determined by comparing the motions of large numbers of stars which are assumed to be wholly independent of each other, and to move, as it were, at random. miss a.m. clerke, in her _system of the stars_, puts this point very clearly, as follows: 'for the assumption that the absolute movements of the stars have no preference for one direction over another, forms the basis of all investigations hitherto conducted into the translatory advance of the solar system. the little fabric of laboriously acquired knowledge regarding it at once crumbles if that basis has to be removed. in all investigations of the sun's movement, the movements of the stars have been regarded as casual irregularities; should they prove to be in any visible degree systematic, the mode of treatment adopted (and there is no other at present open to us) becomes invalid, and its results null and void. the point is then of singular interest, and the evidence bearing upon it deserves our utmost attention.' mr. w.h.s. monck, a well-known astronomer, takes the same view. he says: 'the proof of this motion rests on the assumption that if we take a sufficient number of stars, their real motions in all directions will be equal, and that therefore the apparent preponderances which we observe in particular directions result from the real motion of the sun. but there is no impossibility in a systematic motion of the majority of the stars used in these researches which might reconcile the observed facts with a motionless sun. and, in the second place, if the sun is not in the exact centre of gravity of the universe, we might expect him to be moving in an orbit around this centre of gravity, and our observations on his actual motion are not sufficiently numerous or accurate to enable us to affirm that he is moving in a right line rather than such an orbit.' now this 'systematic motion,' which would render all calculations as to the sun's motion inaccurate or even altogether worthless, is by many astronomers held to be an observed reality. the star-drift, first pointed out by proctor, has been shown to exist in many other groups of stars, while the curious arrangements of stars all over the heavens in straight lines, or regular curves, or spirals, strongly suggests a wide extension of the same kind of relation. but even more extensive systematic movements have been observed or suggested by astronomers. sir d. gill, by an extensive research, believes that he has found indications of a rotation of the brighter fixed stars as a whole in regard to the fainter fixed stars as a whole. mr. maxwell hall has also found indications of a movement of a large group of stars, including our sun, around a common centre, situated in a direction towards epsilon andromedæ, and at a distance of about years of light-travel. these last two motions are not yet established; but they seem to prove two important facts--(_a_) that eminent astronomers believe that _some_ systematic motions must exist among the stars, or they would not devote so much labour to the search for them; and (_b_) that extensive systematic motions of some kind do exist, or even these results would not have been obtained. mr. w.w. campbell, of the lick observatory, thus remarks on the uncertainty of determinations of the sun's motions: 'the motion of the solar system is a purely relative quantity. it refers to specified groups of stars. the results for various groups may differ widely, and all be correct. it would be easy to select a group of stars with reference to which the solar motion would be reversed ° from the values assigned above' (_astrophysical journal_, vol. xiii. p. . ). it must be remembered that, within a uniform cluster of stars, each moving round the common centre of gravity of the whole cluster, kepler's laws do not prevail, the law being that the angular velocities are all identical, so that the more distant stars move faster than those nearer the centre, subject to modifications, however, due to the varying density of the cluster. but if the cluster is nearly globular, there must be stars moving round the centre in every plane, and this would lead to apparent motions in many directions as viewed by us, although those which were moving in the same plane as ourselves would, when compared with remote stars outside the cluster, appear to be all moving in the same direction and at the same rate, forming, in fact, one of those drifting systems of stars already referred to. again, if in the process of formation of our cluster, smaller aggregations already having a rotatory motion were drawn into it, this might lead to their revolving in an opposite direction to those which were formed from the original nebula, thus increasing the diversities of apparent motion. the evidence now briefly set forth fully justifies, i submit, the remarks as to the statements of my astronomical critics at the beginning of this section. they have both given the accepted views as to direction and rate of movement of our sun without any qualification whatever, as if they were astronomical facts of the same certainty and the same degree of accuracy as the sun's distance from the earth; and they will assuredly have been so understood by the great body of non-mathematical readers. it appears, however, if the authorities i have quoted are right, that the whole calculation rests upon certain assumptions, which are certainly to some extent, and may be to a very large extent, erroneous. this is my reply to one part of their criticism. in the next place, they both assert, or imply, not only that the sun's motion is now in a straight line, but that it has been in a straight line from some enormously remote period when it first entered the stellar system on one side, and will so continue to move till it reaches the utmost bounds of that system on the other side. and this is stated by them both, not as a possibility, but as a certainty. they use such terms as 'must' and 'will be,' leaving no room for any doubt whatever. but such a result implies the abrogation of the law of gravitation, since under its action motion in a straight line in the midst of thousands or millions of suns of various sizes is an absolute impossibility; while it also implies that the sun must have been started on its course from some other system outside the milky way, with such a precise determination of direction as not to collide with, or even make a near approach to, any one of the suns or clusters of suns, or vast nebulous masses, during its passage through the very midst of the stellar universe. this is my reply to the main point of their criticism, and i think i am justified in saying that nothing in my whole article is so demonstrably baseless as the statements i have now examined. * * * * * considering then the whole bearing of the evidence, i refuse to accept the unsupported dicta of those who would have us believe that our admitted position not far from the centre of the stellar universe is a mere temporary coincidence of no significance whatever; or that our sun and hosts of other similar orbs near to us have come together by an accident, and are being dispersed into surrounding space, never to meet again. until this is proved by indisputable evidence, it seems to me far more probable that we are moving in an orbit of some kind around the centre of gravity of a vast cluster, as determined by the investigations of kapteyn, newcomb, and other astronomers; and, consequently, that the nearly central position we now occupy may be a permanent one. for even if our sun's orbit should have a diameter a thousand times that of neptune, it would be but a small fraction of the diameter of the milky way; while so vast is the scale of our universe, that it might be even a hundred thousand times as great and still leave us deeply immersed in the solar cluster, and very much nearer to the dense central portion than to its more diffused outer regions. here the subject may be left for the present. after having studied the evidence afforded by the essential conditions of life-development on the earth, and the numerous indications that these conditions do not exist on any of the other planets of the solar system, it may be again touched upon in a general review of the conclusions arrived at. footnotes: [ ] _nature_, october , . [ ] _the system of the stars_, p. . [ ] this account of professor kapteyn's research is taken from an article by miss a.m. clerke in _knowledge_, april . [ ] _the stars_, p. . the region here referred to is that where the milky way has its greatest width (though nearly as wide in the part exactly opposite), and where it may perhaps extend somewhat in our direction. miss a.m. clerke informs me that in april kapteyn withdrew the conclusions arrived at in , as being founded on illegitimate reasoning as to the relation of parallaxes to proper motions. but as this relation is still accepted, under certain limitations, by professor newcomb and other astronomers, who have arrived independently at very similar results, it seems not improbable that, after all, professor kapteyn's conclusions may not require very much modification. professor newcomb also tells us (_the stars_, p. , footnote) that he has seen the latest of professor kapteyn's papers, down to ; but he does not therefore express any doubt as to his own conclusions as here referred to. [ ] see _knowledge_ and _the fortnightly review_ of april . [ ] sir r. ball in an article in _good words_ (april ) says that luminosity is an exceptional phenomenon in nature, and that luminous stars are but the glow-worms and fire-flies of the universe, as compared with the myriads of other animals. chapter ix the uniformity of matter and its laws throughout the stellar universe i have shown in the second chapter of this work that none of the previous writers on the question of the habitability of the other planets have really dealt with the subject in any adequate manner, since not only do they appear to be quite unaware of the delicate balance of conditions which alone renders organic life possible on any planet, but they have altogether omitted any reference to the fact that not only must the conditions be such as to render life possible _now_, but these conditions must have persisted during the long geological epochs needed for the slow development of life from its most rudimentary forms. it will therefore be necessary to enter into some details both as to the physical and chemical essentials for a continuous development of organic life, and also into the combination of mechanical and physical conditions which are required on any planet to render such life possible. the uniformity of matter one of the most important and far-reaching of the discoveries due to the spectroscope is that of the wonderful identity of the elements and material compounds in earth and sun, stars and nebulæ, and also of the identity of the physical and chemical laws that determine the states and forms assumed by matter. more than half the total number of the known elements have been already detected in the sun, including all those which compose the bulk of the earth's solid material, with the one exception of oxygen. this is a very large proportion when we consider the very peculiar conditions which enable us to detect them. for we can only recognise an element in the sun when it exists at its surface in an incandescent state, and also above its surface in the form of a somewhat cooler gas. many of the elements may rarely or never be brought to the surface of so vast a body, or if they do sometimes appear there, it may not be in sufficient quantity or in sufficient purity to produce any bands in the spectroscope, while the cooler gas or vapour may either not be present, or be so dispersed as not to produce sufficient absorption to render its spectral lines visible. again, it is believed that many elements are dissociated by the intense heat of the sun, and may not be recognisable by us, or they may only exist at its surface in a compound form unknown on the earth; and in some such way those lines of the solar spectrum which remain still unrecognised may have been produced. one of these unknown lines was that of helium, a gas found soon afterwards in the rare mineral 'cleveite,' and since detected frequently in many stars. some of the stars have spectra very closely resembling that of the sun. the dark lines are almost as numerous, and most of them correspond accurately with solar lines, so that we cannot doubt their having almost exactly the same chemical constitution, and being also in the same condition as regards heat and stage of development. other stars, as we have already stated, exhibit mainly lines of hydrogen, sometimes combined with fine metallic lines. of the spectra of the nebulæ comparatively little is known, but many are decidedly gaseous, while others show a continuous spectrum indicating a more complex constitution. but we also obtain considerable knowledge of the matter of non-terrestrial bodies by the analysis of the numerous meteorites which fall upon the earth. most of these belong to some of the many meteoric streams which circulate round the sun, and which may be supposed to give us samples of planetary matter. but as it is now believed that many of them are produced by the debris of comets, and the orbits of some of these indicate that they have come from stellar space and have been drawn into our system by the attractive power of the larger planets, it is almost certain that the meteoric stones not infrequently bring us matter from the remoter regions of space, and probably afford us samples of the solid constituents of nebula; or the cooler stars. it is, therefore, a most suggestive fact that none of these meteorites have been found to contain a single non-terrestrial element, although no less than twenty-four elements have been found in them, and it will be of interest to give the list of these, as follows:--_oxygen_, hydrogen, _chlorine_, _sulphur_, _phosphorus_, carbon, silicon, iron, nickel, cobalt, magnesium, chromium, manganese, copper, tin, _antimony_, aluminium, calcium, potassium, sodium, _lithium_, titanium, _arsenic_, and vanadium. seven of the above, printed in italics, have not yet been found in the sun, such as oxygen, chlorine, sulphur, and phosphorus, which form the constituents of many widespread minerals, and they supply important gaps in the series of solar and stellar elements. it may be noted that although meteorites have supplied no new elements, they have furnished examples of some new combinations of these elements forming minerals distinct from any found in our rocks. the fact of the occurrence in meteorites not only of minerals which are peculiar to them or are found on the earth, but also of structures resembling our breccias, veins, and even slicken-side surfaces, has been held to be opposed to the meteoritic theory of the origin of suns and planets, because meteorites seem to be thus proved to be the fragments of suns or worlds, not their primary constituents. but these cases are exceptional, and mr. sorby, who made a special study of meteorites, concluded that their materials have usually been in a state of fusion or even of vapour, as they now exist in the sun, and that they became condensed into minute globular particles, which afterwards collected into larger masses, and may have been broken up by mutual impact, and again and again become aggregated together--thus presenting features which are completely in accordance with the meteoritic theory. but, quite recently, mr. t.c. chamberlin has applied the theory of tidal distortion to showing how solid bodies in space, without ever coming into actual contact, must sometimes be torn apart or disrupted into numerous fragments by passing near to each other. especially when a small body passes near a much larger one, there is a certain distance of approach (termed the roche limit) when the increasing differential force of gravity will be sufficient to tear asunder the smaller body and cause the fragments either to circulate around it or to be dispersed in space.[ ] in this way, therefore, those larger meteorites which exhibit planetary structure may have been produced. of course they would rarely have been true planets attached to a sun, but more frequently some of the smaller dark suns, which may possess many of the physical characteristics of planets, and of which there may be myriads in the stellar spaces. on the whole, then, we have positive knowledge of the existence, in the sun, stars, and planetary and stellar spaces, of such a large proportion of the elements of our globe, and so few indications of any not forming part of it, that we are justified in the statement, that the whole stellar universe is, broadly speaking, constructed of the same series of elementary substances as those we can study upon our earth, and of which the whole realm of nature, animal, vegetable, and mineral, is composed. the evidence of this identity of substance is really far more complete than we could expect, considering the very limited means of inquiry that we possess; and we shall, therefore, not be justified in assuming that any important difference exists. when we pass from the elements of matter to the laws which govern it, we also find the clearest proofs of identity. that the fundamental law of gravitation extends to the whole physical universe is rendered almost certain by the fact that double stars move round their common centre of gravity in elliptical orbits which correspond well with both observation and calculation. that the laws of light are the same both here and in inter-planetary space is indicated by the fact that the actual measurement of the velocity of light on the earth's surface gives a result so completely identical with that prevailing to the limits of the solar system, that the measurement of the sun's distance, by means of the eclipses of jupiter's satellites combined with the measured velocity of light, agrees almost exactly with that obtained by means of the transits of venus, or through our nearest approach to the planets mars or eros. again, the more recondite laws of light are found to be identical in sun and stars with those observed within the narrow bounds of laboratory experiments. the minute change of position of spectral lines caused by the source of light moving towards or away from us enables us to determine this kind of motion in the most distant stars, in the planets, or in the moon, and these results can be tested by the motion of the earth either in its orbit or in its rotation; and these latter tests agree with the theoretical determination of what must occur, dependent on the wave-lengths of the different dark lines of the solar spectrum determined by measurements in the laboratory. in like manner, minute changes in the widening or narrowing of spectral lines, their splitting up, their increase or decrease in number, and their arrangement so as to form flutings, can all be interpreted by experiments in the laboratory, showing that such phenomena are due to alterations of temperature, of pressure, or of the magnetic field, thus proving that the very same physical and chemical laws act in the same way here and in the remotest depths of space. these various discoveries give us the certain conviction that the whole material universe is essentially one, both as regards the action of physical and chemical laws, and also in its mechanical relations of form and structure. it consists throughout of the very same elements with which we are so familiar on our earth; the same ether whose vibrations bring us light and heat, electricity and magnetism, and a whole host of other mysterious and as yet imperfectly known forces; gravitation acts throughout its vast extent; and in whatever direction and by whatever means we obtain a knowledge of the stellar universe, we find the same mechanical, physical, and chemical laws prevailing as upon our earth, so that we have in some cases been actually enabled to reproduce in our laboratories phenomena with which we had first become acquainted in the sun or among the stars. we may therefore feel it to be an almost certain conclusion that--the elements being the same, the laws which act upon, and combine, and modify those elements being the same--organised living beings wherever they may exist in this universe must be, fundamentally, and in essential nature, the same also. the outward forms of life, if they exist elsewhere, may vary almost infinitely, as they do vary on the earth; but, throughout all this variety of form--from fungus or moss to rose-bush, palm or oak; from mollusc, worm, or butterfly to humming-bird, elephant, or man--the biologist recognises a fundamental unity of substance and of structure, dependent on the absolute requirements of the growing, moving, developing, living organism, built up of the same elements, combined in the same proportions, and subject to the same laws. we do not say that organic life _could_ not exist under altogether diverse conditions from those which we know or can conceive, conditions which may prevail in other universes constructed quite differently from ours, where other substances replace the matter and ether of our universe, and where other laws prevail. but, _within_ the universe we know, there is not the slightest reason to suppose organic life to be possible, except under the same general conditions and laws which prevail here. we will, therefore, now proceed to describe, very generally, what are the conditions essential to the existence and the continuous development of vegetable and animal life. footnote: [ ] _the astrophysical journal_, vol. xiv., july , p. . chapter x the essential characters of the living organism before trying to comprehend the physical conditions on any planet which are essential for the development and maintenance of a varied and complex system of organic life comparable to that of our earth, we must obtain some knowledge of what life is, and of the fundamental nature and properties of the living organism. physiologists and philosophers have made many attempts to define 'life,' but in most cases in aiming at absolute generality they have been vague and uninstructive. thus de blainville defined it as 'the twofold internal movement of composition and decomposition, at once general and continuous'; while herbert spencer's latest definition was 'life is the continuous adjustment of internal relations to external relations.' but neither of these is sufficiently precise, explanatory, or distinctive, and they might almost be applied to the changes occurring in a sun or planet, or to the elevation and gradual formation of a continent. one of the oldest definitions, that of aristotle, seems to come nearer the mark: 'life is the assemblage of the operations of nutrition, growth, and destruction.' but these definitions of 'life' are unsatisfactory, because they apply to an abstract idea rather than to the actual living organism. the marvel and mystery of life, as we know it, resides in the body which manifests it, and this living body the definitions ignore. the essential points in the living body, as seen in its higher developments, are, first, that it consists throughout of highly complex but very unstable forms of matter, every particle of which is in a continual state of growth or decay; that it absorbs or appropriates dead matter from without; takes this matter into the interior of its body; acts upon it mechanically and chemically, rejecting what is useless or hurtful; and so transforming the remainder as to renew every atom of its own structure internal and external, at the same time throwing off, particle by particle, all the worn-out or dead portions of its own substance. secondly, in order to be able to do all this, its whole body is permeated throughout by branching vessels or porous tissues, by which liquids and gases can reach every part and carry on the various processes of nutrition and excretion above referred to. as professor burdon sanderson well puts it: 'the most distinctive peculiarity of living matter as compared with non-living is, that it is ever changing while ever the same.' and these changes are the more remarkable because they are accompanied, and even produced, by a very large amount of mechanical work--in animals by means of their normal activities in search of food, in assimilating that food, in continually renewing and building up their whole organism, and in many other ways; in plants by building up their structure, which often involves raising tons of material high into the air, as in forest trees. as a recent writer puts it: 'the most prominent, and perhaps the most fundamental, phenomenon of life is what may be described as the _energy traffic_ or the function of _trading in energy_. the chief physical function of living matter seems to consist in absorbing energy, storing it in a higher potential state, and afterwards partially expending it in the kinetic or active form.'[ ] thirdly--and perhaps most marvellous of all--all living organisms have the power of reproduction or increase, in the lowest forms by a process of self-division or 'fission,' as it is termed, in the higher by means of reproductive cells, which, though in their earliest stage quite indistinguishable physically or chemically in very different species, yet possess the mysterious power of developing a perfect organism, identical with its parents in all its parts, shapes, and organs, and so wonderfully resembling them, that the minutest distinctive details of size, form, and colour, in hair or feathers, in teeth or claws, in scales, spines, or crests, are reproduced with very close accuracy, though often involving metamorphic changes during growth of so strange a nature that, if they were not familiar to us but were narrated as occurring only in some distant and almost inaccessible region, would be treated as travellers' tales, incredible and impossible as those of sindbad the sailor. in order that the substance of living bodies should be able to undergo these constant changes while preserving the same form and structure in minute details--that they should be, as it were, in a constant state of flux while remaining sensibly unchanged, it is necessary that the molecules of which they are built up should be so combined as to be easily separated and as easily united--be, as it is termed, _labile_ or flowing; and this is brought about by their chemical composition, which, while consisting of few elements, is yet highly complex in structure, a large number of chemical atoms being combined in an endless variety of ways. the physical basis of life, as huxley termed it, is protoplasm, a substance which consists essentially of only four common elements, the three gases, nitrogen, hydrogen, and oxygen, with the non-metallic solid, carbon; hence all the special products of plants and animals are termed carbon-compounds, and their study constitutes one of the most extensive and intricate branches of modern chemistry. their complexity is indicated by the fact that the molecule of sugar contains , and that of stearine no less than , constituent atoms. the chemical compounds of carbon are far more numerous than those of all the other chemical elements combined; and it is this wonderful variety and the complexity of its possible combinations which explain the fact, that all the various animal tissues--skin, horn, hair, nails, teeth, muscle, nerve, etc., consist of the same four elements (with occasionally minute quantities of sulphur, phosphorus, lime, or silica, in some of them), as proved by the marvellous fact that these tissues are all produced as well by the grass-eating sheep or ox as by the fish or flesh-eating seal or tiger. and the marvel is still further increased when we consider that the innumerable diverse substances produced by plants and animals are all formed out of the same three or four elements. such are the endless variety of organic acids, from prussic acid to those of the various fruits; the many kinds of sugars, gums, and starches; the number of different kinds of oil, wax, etc.; the variety of essential oils which are mostly forms of turpentines, with such substances as camphor, resins, caoutchouc, and gutta-percha; and the extensive series of vegetable alkaloids, such as nicotine from tobacco, morphine from opium, strychnine, curarine, and other poisons; quinine, belladonna, and similar medicinal alkaloids; together with the essential principles of our refreshing drinks, tea, coffee, and cocoa, and others too numerous to be named here--all alike consisting solely of the four common elements from which almost our whole organism is built up. if this were not indisputably proved, it would scarcely be credited. professor f.j. allen considers that the most important element in protoplasm, and that which confers upon it its most essential properties in the living organism--its extreme mobility and transposibility--is nitrogen. this element, though inert in itself, readily enters into compounds when energy is supplied to it, the most striking illustration of which is the formation of ammonia, a compound of nitrogen and hydrogen, produced by electric discharges through the atmosphere. ammonia, and certain oxides of nitrogen produced in the atmosphere in the same way, are the chief sources of the nitrogen assimilated by plants, and through them by animals; for although plants are continually in contact with the free nitrogen of the atmosphere, they are unable to absorb it. by their leaves they absorb oxygen and carbon-dioxide to build up their woody tissues, while by their roots they absorb water in which ammonia and oxides of nitrogen are dissolved, and from these they produce the protoplasm which builds up the whole substance of the animal world. the energy required to produce these nitrogen-compounds is given up by them when undergoing further changes, and thus the production of ammonia by electricity in the atmosphere, and its being carried by rain into the soil, constitute the first steps in that long series of operations which culminates in the production of the higher forms of life. but the remarkable transformations and combinations continually going on in every living body, which are, in fact, the essential conditions of its life, are themselves dependent on certain physical conditions which must be always present. professor allen remarks: 'the sensitiveness of nitrogen, its proneness to change its state of combination and energy, appear to depend on certain conditions of temperature, pressure, etc., which exist at the surface of this earth. most vital phenomena occur between the temperature of freezing water and ° f. if the general temperature of the earth's surface rose or fell ° f. (a small amount relatively), the whole course of life would be changed, even perchance to extinction.' another important, and even more essential fact, in connection with life, is the existence in the atmosphere of a small but nearly constant proportion of carbonic acid gas, this being the source from which the whole of the carbon in the vegetable and animal kingdoms is primarily derived. the leaves of plants absorb carbonic acid gas from the atmosphere, and the peculiar substance, chlorophyll, from which they derive their green colour, has the power, under the influence of sunlight, to decompose it, using the carbon to build up its own structure and giving out the oxygen. in the laboratory the carbon can only be separated from the oxygen by the application of heat, under which certain metals burn by combining with the oxygen, thus setting free the carbon. chlorophyll has a highly complex chemical structure very imperfectly known, but it is said to be only produced when there is iron in the soil. the leaves of plants, so often looked upon as mere ornamental appendages, are among the most marvellous structures in living organisms, since in decomposing carbonic acid at ordinary temperatures they do what no other agency in nature can perform. in doing this they utilise a special group of ether-waves which alone appear to have this power. the complexity of the processes going on in leaves is well indicated in the following quotation:-- 'we have seen how green leaves are supplied with gases, water, and dissolved salts, and how they can trap special ether-waves. the active energy of these waves is used to transmute the simple inorganic compounds into complex organic ones, which in the process of respiration are reduced to simpler substances again, and the potential energy transformed into kinetic. these metabolic changes take place in living cells full of intense activities. currents course through the protoplasm and cell-sap in every direction, and between the cells which are also united by strands of protoplasm. the gases used and given off in respiration and assimilation are floated in and out, and each protoplasm particle burned or unburned is the centre of an area of disturbance. pure protoplasm is influenced equally by all rays: that associated with chlorophyll is affected by certain red and violet rays in particular. these, especially the red ones, bring about the dissociation of the elements of the carbonic acid, the assimilation of the carbon, and the excretion of the oxygen.'[ ] it is this vigorous life-activity ever at work in the leaves, the roots, and the sap-cells, that builds up the plant, in all its wondrous beauty of bud and foliage, flower and fruit; and at the same time produces, either as useful or waste-products, all that wealth of odours and flavours, of colours and textures, of fibres and varied woods, of roots and tubers, of gums and oils and resins innumerable, that, taken altogether, render the world of vegetable life perhaps more varied, more beautiful, more enjoyable, more indispensable to our higher nature than even that of animals. but there is really no comparison between them. we _could_ have plants without animals; we could _not_ have animals without plants. and all this marvel and mystery of vegetable life, a mystery which we rarely ponder over because its effects are so familiar, is usually held to be sufficiently explained by the statement that it is all due to the special properties of protoplasm. well might huxley say, that protoplasm is not only a substance but a structure or mechanism, a mechanism kept at work by solar heat and light, and capable of producing a thousand times more varied and marvellous results than all the human mechanism ever invented. but besides absorbing carbonic acid from the atmosphere, separating and utilising the carbon and giving out the oxygen, plants as well as animals continually absorb oxygen from the atmosphere, and this is so universally the case that oxygen is said to be the food of protoplasm, without which it cannot continue to live; and it is the peculiar but quite invisible structure of the protoplasm which enables it to do this, and also in plants to absorb an enormous amount of water as well. but although protoplasm is so complex chemically as to defy exact analysis, being an elaborate structure of atoms built up into a molecule in which each atom must occupy its true place (like every carved stone in a gothic cathedral), yet it is, as it were, only the starting-point or material out of which the infinitely varied structures of living bodies are formed. the extreme mobility and changeability of the structure of these molecules enables the protoplasm to be continually modified both in constitution and form, and, by the substitution or addition of other elements, to serve special purposes. thus when sulphur in small quantities is absorbed and built into the molecular structure, proteids are formed. these are most abundant in animal structures, and give the nourishing properties to meat, cheese, eggs, and other animal foods; but they are also found in the vegetable kingdom, especially in nuts and seeds such as grain, peas, etc. these are generally known as nitrogenous foods, and are very nutritious, but not so easily digestible as meat. proteids exist in very varied forms and often contain phosphorus as well as sulphur, but their main characteristic is the large proportion of nitrogen they contain, while many other animal and vegetable products, as most roots, tubers, and grains, and even fats and oils, are mainly composed of starch and sugar. in its chemical and physiological aspects protein is thus described by professor w.d. haliburton:--'proteids are produced only in the living laboratory of animals and plants; proteid matter is the all-important material present in protoplasm. this molecule is the most complex that is known; it always contains five and often six or even seven elements. the task of thoroughly understanding its composition is necessarily vast, and advance slow. but, little by little, the puzzle is being solved, and this final conquest of organic chemistry, when it does arrive, will furnish physiologists with new light on many of the dark places of physiological science.'[ ] what makes protoplasm and its modifications still more marvellous is the power it possesses of absorbing and moulding a number of other elements in various parts of living organisms for special uses. such are silica in the stems of the grass family, lime and magnesia in the bones of animals, iron in blood, and many others. besides the four elements constituting protoplasm, most animals and plants contain also in some parts of their structure sulphur, phosphorus, chlorine, silicon, sodium, potassium, calcium, magnesium, and iron; while, less frequently, fluorine, iodine, bromine, lithium, copper, manganese, and aluminium are also found in special organs or structures; and the molecules of all these are carried by the protoplasmic fluids to the places where they are required and built into the living structure, with the same precision and for similar ends as brick and stone, iron, slate, wood, and glass are each utilised in their proper places in any large building.[ ] the organism, however, is not built, but grows. every organ, every fibre, cell, or tissue is formed from diverse materials, which are first decomposed into their elementary molecules, transformed by the protoplasm or by special solvents formed from it, carried to the places where they are needed by the vital fluids, and there built up atom by atom or molecule by molecule into the special structures of which they are to form a part. but even this marvel of growth and repair of every individual organism is far surpassed by the greater marvel of reproduction. every living thing of the higher orders arises from a single microscopic cell, when fertilised, as it is termed, by the absorption of another microscopic cell derived from a different individual. these cells are often, even under the highest powers of the microscope, hardly distinguishable from other cells which occur in all animals and plants and of which their structure is built up; yet these special cells begin to grow in a totally different manner, and instead of forming one particular part of the organism, develop inevitably into a complete living thing with all the organs, powers, and peculiarities of its parents, so as to be recognisably of the same species. if the simple growth of the fully formed organism is a mystery, what of this growth of thousands of complex organisms each with all its special peculiarities, yet all arising from minute germs or cells the diverse natures of which are wholly indistinguishable by the highest powers of the microscope? this, too, is said to be the work of protoplasm under the influence of heat and moisture, and modern physiologists hope some day to learn 'how it is done.' it may be well here to give the views of a modern writer on this point. referring to a difficulty which had been stated by clerk-maxwell twenty-five years ago, that there was not room in the reproductive cell for the millions of molecules needed to serve as the units of growth for all the different structures in the body of the higher animals, professor m'kendrick says:--'but to-day, it is reasonable from existing data to suppose that the germinal vesicle might contain a million of millions of organic molecules. complex arrangements of these molecules suited for the development of all the parts of a highly complicated organism, might satisfy all the demands of the theory of heredity. doubtless the germ was a material system through and through. the conception of the physicist was, that molecules were in various states of movement; and the thinkers were striving toward a kinetic theory of molecules and of atoms of solid matter, which might be as fruitful as the kinetic theory of gases. there were motions atomic and molecular. it was conceivable that the peculiarities of vital action might be determined by the kind of motion that took place in the molecules of what we call living matter. it might be different in kind from some of the motions dealt with by physicists. life is continually being created from non-living material--such, at least, is the existing view of growth by the assimilation of food. the creation of living matter out of non-living may be the transmission to the dead matter of molecular motions which are _sui generis_ in form.' this is the modern physiological view of 'how it may be done,' and it seems hardly more intelligible than the very old theory of the origin of stone axes, given by adrianus tollius in , and quoted by mr. e.b. tylor, who says:--'he gives drawings of some ordinary stone axes and hammers and tells how naturalists say that they are generated in the sky by a fulgureous exhalation conglobed in a cloud by the circumfixed humour, and are, as it were, baked hard by intense heat, and the weapon becomes pointed by the damp mixed with it flying from the dry part, and leaving the other end denser, but the exhalations press it so hard that it breaks through the cloud and makes thunder and lightning. but--he says--if this is really the way in which they are generated, it is odd they are not round, and that they have holes through them. it is hardly to be believed, he thinks.'[ ] and so, when the physiologists, determined to avoid the assumption of anything beyond matter and motion in the germ, impute the whole development and growth of the elephant or of man from minute cells internally alike, by means of 'kinds of motion' and the 'transmission of motions which are _sui generis_ in form,' many of us will be inclined to say with the old author--'it is hardly to be believed, i think.' this brief statement of the conclusions arrived at by chemists and physiologists as to the composition and structure of organised living things has been thought advisable, because the non-scientific reader has often no conception of the incomparable marvel and mystery of the life-processes he has always seen going on, silently and almost unnoticed, in the world around him. and this is still more the case now that two-thirds of our population are crowded into cities where, removed from all the occupations, the charms, and the interests of country life, they are driven to seek occupation and excitement in the theatre, the music-hall, or the tavern. how little do these know what they lose by being thus shut out from all quiet intercourse with nature; its soothing sights and sounds; its exquisite beauties of form and colour; its endless mysteries of birth, and life, and death. most people give scientific men credit for much greater knowledge than they possess in these matters; and many educated readers will, i feel sure, be surprised to find that even such apparently simple phenomena as the rise of the sap in trees are not yet completely explained. as to the deeper problems of life, and growth, and reproduction, though our physiologists have learned an infinite amount of curious or instructive facts, they can give us no intelligible explanation of them. the endless complexities and confusing amount of detail in all treatises on the physiology of animals and plants are such, that the average reader is overwhelmed with the mass of knowledge presented to him, and concludes that after such elaborate researches everything must be known, and that the almost universal protests against the need of any causes but the mechanical, physical, and chemical laws and forces are well founded. i have, therefore, thought it advisable to present a kind of bird's-eye view of the subject, and to show, in the words of the greatest living authorities on these matters, both how complex are the phenomena and how far our teachers are from being able to give us any adequate explanation of them. i venture to hope that the very brief sketch of the subject i have been able to give will enable my readers to form some faint general conception of the infinite complexity of life and the various problems connected with it; and that they will thus be the better enabled to appreciate the extreme delicacy of those adjustments, those forces, and those complex conditions of the environment, that alone render life, and above all the grand age-long panorama of the development of life, in any way possible. it is to these conditions, as they prevail in the world around us, that we will now direct our attention. footnotes: [ ] professor f.j. allen: _what is life?_ [ ] art. 'vegetable physiology' in _chambers's encyclopædia_. [ ] address to the british association, , section physiology. [ ] this enumeration of the elements that enter into the structure of plants and animals is taken from professor f.j. allen's paper already referred to. [ ] _early history of mankind_, nd ed. p. . chapter xi the physical conditions essential for organic life the physical conditions on the surface of our earth which appear to be necessary for the development and maintenance of living organisms may be dealt with under the following headings:-- . regularity of heat-supply, resulting in a limited range of temperature. . a sufficient amount of solar light and heat. . water in great abundance, and universally distributed. . an atmosphere of sufficient density, and consisting of the gases which are essential for vegetable and animal life. these are oxygen, carbonic-acid gas, aqueous vapour, nitrogen, and ammonia. these must all be present in suitable proportions. . alternations of day and night. small range of temperature required for growth and development vital phenomena for the most part occur between the temperatures of freezing water and ° fahr., and this is supposed to be due mainly to the properties of nitrogen and its compounds, which between these temperatures only can maintain those peculiarities which are essential to life--extreme sensitiveness and lability; facility of change as regards chemical combination and energy; and other properties which alone render nutrition, growth, and continual repair possible. a very small increase or decrease of temperature beyond these limits, if continued for any considerable time, would certainly destroy most existing forms of life, and would not improbably render any further development of life impossible except in some of its lowest forms. as one example of the direct effects of increased temperature, we may adduce the coagulation of albumen. this substance is one of the proteids, and plays an important part in the vital phenomena of both plants and animals, and its fluidity and power of easy combination and change of form are lost by any degree of coagulation which takes place at about ° fahr. the extreme importance to all the higher organisms of a moderate temperature is strikingly shown by the complex and successful arrangements for maintaining a uniform degree of heat in the interior of the body. the normal blood-heat in a man is ° fahr., and this is constantly maintained within one or two degrees though the external temperature may be more than fifty degrees below the freezing-point. high temperatures upon the earth's surface do not range so far from the mean as do the low. in the greater part of the tropics the air-temperature seldom reaches ° fahr., though in arid districts and deserts, which occur chiefly along the margins of the northern and southern tropics, it not unfrequently surpasses ° fahr., and even occasionally rises to ° or ° in australia and central india. yet with suitable food and moderate care the blood-temperature of a healthy man would not rise or fall more than one or at most two degrees. the great importance of this uniformity of temperature in all the vital organs is distinctly shown by the fact that when, during fevers, the temperature of the patient rises six degrees above the normal amount, his condition is critical, while an increase of seven or eight degrees is an almost certain indication of a fatal result. even in the vegetable kingdom seeds will not germinate under a temperature of four or five degrees above the freezing-point. now this extreme sensibility to variations of internal temperature is quite intelligible when we consider the complexity and instability of protoplasm, and of all the proteids in the living organism, and how important it is that the processes of nutrition and growth, involving constant motion of fluids and incessant molecular decompositions and recombinations, should be effected with the greatest regularity. and though a few of the higher animals, including man, are so perfectly organised that they can adapt or protect themselves so as to be able to live under very extreme conditions as regards temperature, yet this is not the case with the great majority, or with the lower types, as evidenced by the almost complete absence of reptiles from the arctic regions. it must also be remembered that extreme cold and extreme heat are nowhere perpetual. there is always some diversity of seasons, and there is no land animal which passes its whole life where the temperature never rises above the freezing point. the necessity of solar light whether the higher animals and man could have been developed upon the earth without solar light, even if all the other essential conditions were present, is doubtful. that, however, is not the point i am at present considering, but one that is much more fundamental. without plant life land animals at all events could never have come into existence, because they have not the power of making protoplasm out of inorganic matter. the plant alone can take the carbon out of the small proportion of carbonic acid in the atmosphere, and with it, and the other necessary elements, as already described, build up those wonderful carbon compounds which are the very foundation of animal life. but it does this solely by the agency of solar light, and even uses a special portion of that light. not only, therefore, is a sun needed to give light and heat, but it is quite possible that _any_ sun would not answer the purpose. a sun is required whose light possesses those special rays which are effective for this operation, and as we know that the stars differ greatly in their spectra, and therefore in the nature of their light, all might not be able to effect this great transformation, which is one of the very first steps in rendering animal life possible on our earth, and therefore probably on all earths. water a first essential of organic life it is hardly necessary to point out the absolute necessity of water, since it actually constitutes a very large proportion of the material of every living organism, and about three-fourths of our own bodies. water, therefore, must be present everywhere, in one form or another, on any globe where life is possible. neither animal nor plant can exist without it. it must also be present in such quantity and so distributed as to be constantly available on every part of a globe where life is to be maintained; and it is equally necessary that it should have persisted in equal profusion throughout those enormous geological epochs during which life has been developing. we shall see later on how very special are the conditions that have secured this continuous distribution of water on our earth, and we shall also learn that this large amount of water, its wide distribution, and its arrangement with regard to the land-surface, is an essential factor in producing that limited range of temperature which, as we have seen, is a primary condition for the development and maintenance of life. the atmosphere must be of sufficient density and composed of suitable gases the atmosphere of any planet on which life can be developed must have several qualities which are unconnected with each other, and the coincidence of which may be a rare phenomenon in the universe. the first of these is a sufficient density, which is required for two purposes--as a storer of heat, and in order to supply the oxygen, carbonic acid, and aqueous vapour in sufficient quantities for the requirements of vegetable and animal life. as a reservoir of heat and a regulator of temperature, a rather dense atmosphere is a first necessity, in co-operation with the large quantity and wide distribution of water referred to in the last section. the very different character of our south-west from our north-east winds is a good illustration of its power of distributing heat and moisture. this it does owing to the peculiar property it possesses of allowing the sun's rays to pass freely through it to the earth which it warms, but acting like a blanket in preventing the rapid escape of the non-luminous heat so produced. but the heat stored up during the day is given out at night, and thus secures a uniformity of temperature which would not otherwise exist. this effect is strikingly seen at high altitudes, where the temperature becomes lower and lower, till at a not very great elevation, even in the tropics, snow lies on the ground all the year round. this is almost wholly due to the rarity of the air, which, on that account, has not so much capacity for heat. it also allows the heat it acquires to radiate more freely than denser air, so that the nights are much colder. at about , feet high our atmosphere is exactly half its density at the sea-level. this is considerably higher than the usual snow-line, even under the equator, whence it follows that if our atmosphere was only half its present density it would render the earth unsuitable for the higher forms of animal life. it is not easy to say exactly what would be the result as regards climate; but it seems likely that, except perhaps in limited areas in the tropics, where conditions were very favourable, the whole land-surface would become buried in snow and ice. this appears inevitable, because evaporation from the oceans by direct sun-heat would be more rapid than now; but as the vapour rose in the rare atmosphere it would rapidly become frozen, and snow would fall almost perpetually, although it might not lie permanently on the ground in the equatorial lowlands. it appears certain, therefore, that with half our present bulk of atmosphere life would be hardly possible on the earth on account of lowered temperature alone. and as there would certainly be an added difficulty in the needful supply of oxygen to animals and carbonic acid to plants, it seems highly probable that a reduction of density of even one-fourth might be sufficient to render a large portion of the globe a snow and ice-clad waste, and the remainder liable to such extremes of climate that only low forms of life could have arisen and been permanently maintained. the gases of the atmosphere coming now to consider the constituent gases of the atmosphere, there is reason to believe that they form a mixture as nicely balanced in regard to animal and vegetable life as are the density and the temperature. at a first view of the subject we might conclude that oxygen is the one great essential for animal life, and that all else is of little importance. but further consideration shows us that nitrogen, although merely a diluent of the oxygen as regards the respiration of animals, is of the first importance to plants, which obtain it from the ammonia formed in the atmosphere and carried down into the soil by the rain. although there is only one part of ammonia to a million of air, yet upon this minute proportion the very existence of the animal world depends, because neither animals nor plants can assimilate the free nitrogen of the air into their tissues. another fundamentally important gas in the atmosphere is carbonic acid, which forms about four parts in ten thousand parts of air, and, as already stated, is the source from which plants build up the great bulk of their tissues, as well as those protoplasms and proteids so absolutely necessary as food for animals. an important fact to notice here is, that carbonic acid, so essential to plants, and to animals through plants, is yet a poison to animals. when present in much more than the normal quantity, as it often is in cities and in badly ventilated buildings, it becomes highly prejudicial to health; but this is believed to be partly due to the various corporeal emanations and other impurities associated with it. pure carbonic acid gas to the amount of even one per cent. in otherwise pure air can, it is said, be breathed for a time without bad effects, but anything more than this proportion will soon produce suffocation. it is probable, therefore, that a very much smaller proportion than one per cent., if constantly present, would be dangerous to life; though no doubt, if this had always been the proportion, life might have been developed in adaptation to it. considering, however, that this poisonous gas is largely given out by the higher animals as a product of respiration, it would evidently be dangerous to the permanence of life if the quantity forming a constant constituent of the atmosphere were much greater than it is. aqueous vapour in the atmosphere this water-gas, although it occurs in the atmosphere in largely varying quantities, is yet, in two distinct ways, essential to organic life. it prevents the too rapid loss of moisture from the leaves of plants when exposed to the sun, and it is also absorbed by the upper surface of the leaf and by the young shoots, which thus obtain both water and minute quantities of ammonia when the supply by the roots is insufficient. but it is of even more vital importance in supplying the hydrogen which, when united with the nitrogen of the atmosphere by electrical discharges, produces the ammonia, which is the main source of all the proteids of the plant, which proteids are the very foundation of animal life. from this brief statement of the purposes served by the various gases forming our atmosphere, we see that they are to some extent antagonistic, and that any considerable increase of one or the other would lead to results that might be injurious either directly or in their ultimate results. and as the elements which constitute the bulk of all living matter possess properties which render them alone suitable for the purpose, we may conclude that the proportions in which they exist in our atmosphere cannot be very widely departed from wherever organic forms are developed. the alternation of day and night although it is difficult to decide positively whether alternations of light and darkness at short intervals are absolutely essential for the development of the various higher forms of life, or whether a world in which light was constant might do as well, yet on the whole it seems probable that day and night are really important factors. all nature is full of rhythmic movements of infinitely varied kinds, degrees, and durations. all the motions and functions of living things are periodic; growth and repair, assimilation and waste, go on alternately. all our organs are subject to fatigue and require rest. all kinds of stimulus must be of short duration or injurious results follow. hence the advantage of darkness, when the stimuli of light and heat are partially removed, and we welcome 'tired nature's sweet restorer, balmy sleep'--giving rest to all the senses and faculties of body and mind, and endowing us with renewed vigour for another period of activity and enjoyment of life. plants as well as animals are invigorated by this nightly repose; and all alike benefit by these longer periods of greater and less amounts of work caused by summer and winter, dry and wet seasons. it is a suggestive fact, that where the influence of heat and light is greatest--within the tropics--the days and nights are of equal length, giving equal periods of activity and rest. but in cold and arctic regions where, during the short summer, light is nearly perpetual, and all the functions of life, in vegetation especially, go on with extreme rapidity, this is followed by the long rest of winter, with its short days and greatly lengthened periods of darkness. of course, all this is rather suggestion than proof. it is possible that in a world of perpetual day or in one of perpetual night, life _might_ have been developed. but on the other hand, considering the great variety of physical conditions which are seen to be necessary for the development and preservation of life in its endless varieties, any prejudicial influences, however slight, might turn the scale, and prevent that harmonious and continuous evolution which we know _must_ have occurred. so far i have only considered the question of day and night as regards the presence or absence of light. but it is probably far more important in its heat aspect; and here its period becomes of great, perhaps vital, importance. with its present duration of twelve hours day and twelve night on the average, there is not time, even between the tropics, for the earth to become so excessively heated as to be inimical to life; while a considerable portion of the heat, stored up in the soil, the water, and the atmosphere, is given out at night, and thus prevents a too sudden and injurious contrast of heat and cold. if the day and night were each very much longer--say or hours--it is quite certain that, during a day of that duration, the heat would become so great as to be inimical, perhaps prohibitive, to most forms of life; while the absence of all sun-heat for an equally long period would result in a temperature far below the freezing point of water. it is doubtful whether any high forms of animal life could have arisen under such great and continual contrasts of temperature. we will now proceed to point out the special features which, in our earth, have combined to bring about and to maintain the various and complex conditions we have seen to be essential for life as it exists around us. chapter xii the earth in its relation to the development and maintenance of life the first circumstance to be considered in relation to the habitability of a planet is its distance from the sun. we know that the heating power of the sun upon our earth is ample for the development of life in an almost infinite variety of forms; and we have a large amount of evidence to show that, were it not for the equalising power of air and water, distributed as they are with us, the heat received from the sun would be sometimes too great and sometimes too little. in some parts of africa, australia, and india, the sandy soil becomes so hot that an egg can be cooked by placing it just below the surface. on the other hand, at an elevation of about , feet in lat. ° it freezes every night, and throughout the day in all places sheltered from the sun. now, both these temperatures are adverse to life, and if either of them persisted over a considerable portion of the earth, the development of life would have been impossible. but the heat derived from the sun is inversely as the square of the distance, so that at half the distance we should have four times as much heat, and at twice the distance only one-fourth of the heat. even at two-thirds of the distance we should receive more than twice as much heat; and, considering the facts as to the extreme sensitiveness of protoplasm and the coagulation of albumen, it seems certain that we are situated in what has been termed the temperate zone of the solar system, and that we could not be removed far from our present position without endangering a considerable portion of the life now existing upon the earth, and in all probability rendering the actual development of life, through all its phases and gradations, impossible. the obliquity of the ecliptic the effect of the obliquity of the earth's equator to its path round the sun, upon which depend our varying seasons and the inequality of day and night throughout all the temperate zones, is very generally known. but it is not usually considered that this obliquity is of any great importance as regards the suitability of the earth for the development and maintenance of life; and it seems to have been passed over as an accident hardly worth notice, because almost any other obliquity or none at all would have been equally advantageous. but if we consider what the direction of the earth's axis might possibly have been, we shall find that it is really a matter of great importance from our present point of view. let us suppose, first, that the earth's axis was, like that of uranus, almost exactly in the plane of its orbit or directed towards the sun. there can be little doubt that such a position would have rendered our world unfitted for the development of life. for the result would be the most tremendous contrasts of the seasons; at mid-winter, on one half the globe, arctic night and more than arctic cold would prevail; while on the other half there would be a midsummer of continuous day with a vertical sun and such an amount of heat as nowhere exists with us. at the two equinoxes the whole globe would enjoy equal day and night, all our present tropics and part of the sub-tropical zone having the sun at noon so near to the zenith as to have the essential of a tropical climate. but the change to about a month of constant sunshine or a month of continuous night would be so rapid, that it seems almost impossible that either vegetable or animal life would ever have developed under such terrible conditions. the other extreme direction of the earth's axis, exactly at right angles to the plane of the orbit, would be much more favourable, but would still have its disadvantages. the whole surface from equator to poles would enjoy equal day and night, and every part would receive the same amount of sun-heat all the year round, so that there would be no change of seasons; but the heat received would vary with the latitude. in our latitude the sun's altitude at noon all the year would be less than °, the same as now occurs at the equinoxes, and we might therefore have a perpetual spring as regards temperature. but the constancy of the heat in the equatorial and tropical regions and of cold towards the poles would lead to a more constant and more rapid circulation of air, and we should probably experience such continuous north-westerly winds as to render our climate always cold and probably very damp. near the poles the sun would always be on, or close to, the horizon, and would give so little heat that the sea might be perpetually frozen and the land deeply snow-buried; and these conditions would probably extend into the temperate zone, and possibly so far south as to render life impossible in our latitudes, since whatever results arose would be due to permanent causes, and we know how powerful are snow and ice to extend their sway over adjacent areas if not counteracted by summer heat or warm moist winds. on the whole, therefore, it seems probable that this position of the earth's axis would result in a much smaller portion of its surface being capable of supporting a luxuriant and varied vegetable and animal life than is now the case; while the extreme uniformity of conditions everywhere present might be so antagonistic to the great law of rhythm that seems to pervade the universe, and be in other ways so unfavourable, that life-development would probably have taken quite a different course from that which it has taken. it appears almost certain, therefore, that some intermediate position of the axis would be the most favourable; and that which actually exists seems to combine the advantage of change of seasons with good climatical conditions over the largest possible area. we know that during the greater part of the epoch of life-development this area was much greater than at present, since a luxuriant vegetation of deciduous and evergreen trees and shrubs extended up to and within the arctic circle, leading to the formation of coal-beds both in palæozoic and tertiary times; the extremely favourable conditions for organic life which then prevailed over so large a portion of the globe's surface, and which persisted down to a comparatively recent epoch, lead to the conclusion that no more favourable degree of obliquity was possible than that which we actually possess. a short account of the evidence on this interesting subject will now be given. persistence of mild climates through geologic time the whole of the geological evidence goes to show that in remote ages the climate of the earth was generally more uniform, though perhaps not warmer, than it is now, and this can be best explained by a slightly different distribution of sea and land, which allowed the warm waters of the tropical oceans to penetrate into various parts of the continents (which were then more broken up than they are now), and also to extend more freely into the arctic regions. so soon as we go back into the tertiary period, we find indications of a warmer climate in the north temperate zone; and when we reach the middle of that period, we find abundant indications, both in plant and animal remains, of mild climates near to the arctic circle, or actually within it. on the west coast of greenland, in ° n. lat., there are found abundance of fossil plants very beautifully preserved, among which are many different species of oaks, beeches, poplars, plane-trees, vines, walnuts, plums, chestnuts, sequoias, and numerous shrubs-- species in all, indicating a vegetation such as now grows in the north temperate parts of america and eastern asia. and even further north, in spitzbergen, in n. lat. ° and °, a somewhat similar flora is found, not quite so varied, but with oaks, poplars, birches, planes, limes, hazels, pines, and many aquatic plants such as may now be found in west norway and in alaska, nearly twenty degrees further south. still more remote, in the cretaceous period, fossil plants have been found in greenland, consisting of ferns, cycads, conifers, and such trees and shrubs as poplars, sassafras, andromedas, magnolias, myrtles, and many others, similar in character and often identical in species with fossils of the same period found in central europe and the united states, indicating a widespread uniformity of climate, such as would be brought about by the great ocean-currents carrying the warm waters of the tropics into the arctic seas. still further back, in the jurassic period, we have proofs of a mild climate in east siberia and at andö in norway just within the arctic circle, in numerous plant remains, and also remains of great reptiles allied to those found in the same strata in all parts of the world. similar phenomena occur in the still earlier triassic period; but we will pass on to the much more remote carboniferous period, during which most of the great coal-beds of the world were formed from a luxuriant vegetation, consisting mostly of ferns, giant horse-tails, and primitive conifers. the luxuriance of these plants, which are often found beautifully preserved and in immense quantities, is supposed to indicate an atmosphere in which carbonic acid gas was much more abundant than now; and this is rendered probable by the small number and low type of terrestrial animals, consisting of a few insects and amphibia. but the interesting point is, that true coal-beds, with similar fossils to those of our own coal-measures, are found at spitzbergen and at bear island in east siberia, both far within the arctic circle, again indicating a great uniformity of climate, and probably a denser and more vapour-laden atmosphere, which would act as a blanket over the earth and preserve the heat brought to the arctic seas by the ocean currents from the warmer regions. the still earlier silurian rocks are also found abundantly in the arctic regions, but their fossils are entirely of marine animals. yet they show the same phenomena as regards climate, since the corals and cephalopodous mollusca found in the arctic beds closely resemble those of all other parts of the earth.[ ] many other facts indicate that throughout the enormous periods required for the development of the varied forms of life upon the earth, the great phenomena of nature were but little different from those that prevail in our own times. the slow and gentle processes by which the various vegetable and animal remains were preserved are shown by the perfect state in which many of the fossils exist. often trunks of trees, cycads, and tree-ferns are found standing erect, with their roots still imbedded in the soil they grew in. large leaves of poplars, maples, oaks, and other trees are often preserved in as perfect a state as if gathered by a botanist and dried between paper for his herbarium, and the same is especially the case with the beautiful ferns of the permian and carboniferous periods. throughout these and most other formations well-preserved ripple-marks are found in the solidified mud or sand of old seashores, differing in no respect from similar marks to be found on almost every coast to-day. equally interesting are the marks of rain-drops preserved in the rocks of almost all ages. sir charles lyell has given illustrations of recent impressions of rain-drops on the extensive mud-flats of nova scotia, and also an illustration of rain-drops on a slab of shale from the carboniferous formation of the same country; and the two are as much alike as the prints of two different showers a few days apart. the general size and form of the drops are almost identical, and imply a great similarity in the general atmospheric conditions. we must not forget that this presence of rain throughout geological time implies, as we have seen in our last chapter, a constant and universal distribution of atmospheric dust. the two chief sources of this dust--the total quantity of which in the atmosphere must be enormous--are volcanoes and deserts, and we are therefore sure that these two great natural phenomena have always been present. of volcanoes we have ample independent evidence in the presence of lavas and volcanic ashes, as well as actual stumps or cores of old volcanoes, through all geological formations; and we can have little doubt that deserts also were present, though perhaps not always so extensive as they are now. it is a very suggestive fact that these two phenomena, usually held to be blots on the fair face of nature, and even to be opposed to belief in a beneficent creator, should now be proved to be really essential to the earth's habitability. notwithstanding this prevalence of warm and uniform conditions, there is also evidence of considerable changes of climate; and at two periods--in the eocene and in the remote permian--there are even indications of ice-action, so that some geologists believe that there were then actual glacial epochs. but it seems more probable that they imply only local glaciation, owing to there having been high land and other suitable conditions for the production of glaciers in certain areas. the whole bearing of the geological evidence indicates the wonderful continuity of conditions favourable for life, and for the most part of climatal conditions more favourable than those now prevailing, since a larger extent of land towards the north pole was available for an abundant vegetation, and in all probability for an equally abundant animal life. we know, too, that there was never any total break in life-development; no epoch of such lowering or raising of temperature as to destroy all life; no such general subsidence as to submerge the whole land-surface. although the geological record is in parts very imperfect, yet it is, on the whole, wonderfully complete; and it presents to our view a continuous progress, from simple to complex, from lower to higher. type after type becomes highly specialised in adaptation to local or climatal conditions, and then dies out, giving room for some other type to arise and be specialised in harmony with the changed conditions. the general character of the inorganic change appears to have been from more insular to more continental conditions, accompanied by a change from more uniform to less uniform climates, from an almost sub-tropical warmth and moisture, extending up to the arctic circle, to that diversity of tropical, temperate, and cold areas, capable of supporting the greatest possible variety in the forms of life, and which seems especially adapted to stimulate mankind to civilisation and social development by means of the necessary struggle against, and utilisation of, the various forces of nature. water, its amount and distribution on the earth although it is generally known that the oceans occupy more than two-thirds of the whole surface of the globe, the enormous bulk of the water in proportion to the land that rises above its surface is hardly ever appreciated. but as this is a matter of the greatest importance, both as regards the geological history of the globe and the special subject we are here discussing, it will be necessary to enter into some details in regard to it. according to the best recent estimates, the land area of the globe is . of the whole surface, and the water area . . but the mean height of the land above the sea-level is found to be feet, while the mean depth of the seas and oceans is , feet; so that though the water area is two and a half times that of the land, the mean depth of the water is more than six times the mean height of the land. this is, of course, due to the fact that lowlands occupy most of the land-area, the plateaus and high mountains a comparatively small portion of it; while, though the greatest depths of the oceans about equal the greatest heights of the mountains, yet over enormous areas the oceans are deep enough to submerge all the mountains of europe and temperate north america, except the extreme summits of one or two of them. hence it follows that the bulk of the oceans, even omitting all the shallow seas, is more than thirteen times that of the land above sea-level; and if all the land-surface and ocean-floors were reduced to one level, that is, if the solid mass of the globe were a true oblate spheroid, the whole would be covered with water about two miles deep. the diagram here given will render this more intelligible and will serve to illustrate what follows. [illustration: _diagram of proportionate mean height of land and depth of oceans_. _land_ _area. . of area of globe._ _ocean_ _area . of area of globe._] in this diagram the lengths of the sections representing land and ocean are proportionate to their areas, while the thickness of each is proportionate to their mean height and mean depth respectively. hence the two sections are in correct proportion to their cubic contents. a mere inspection of this diagram is sufficient to disprove the old idea, still held by a few geologists and by many biologists, that oceans and continents have repeatedly changed places during geological times, or that the great oceans have again and again been bridged over to facilitate the distribution of beetles or birds, reptiles or mammals. we must remember that although the diagram shows the continents and oceans as a whole, yet it also shows, with quite sufficient accuracy, the proportions of each of the great continents to the oceans which are adjacent to them. it must also be borne in mind that there can be no elevation on a large scale without a corresponding subsidence elsewhere; because if there were not a vast unsupported hollow would be left beneath the rising land or in some part adjacent to it. now, looking at the diagram and at a chart or globe, try to imagine the ocean-bottom rising gradually, to form a continent joining africa with south america or with australia (both of which are demanded by many biologists): it is clear that, while such an elevation was going on, either some continental land or some other part of the ocean-bed must sink to a corresponding amount. we shall then see, that if such changes of elevation on a continental scale have taken place again and again at different periods, it would have been almost impossible, on every occasion, to avoid a whole continent being submerged (or even all the continents) in order to equalise subsidence with elevation while new continents were being raised up from the abyssal depths of the ocean. we conclude, therefore, that with the exception of a comparatively narrow belt around the continents, which may be roughly indicated by the thousand fathom line of soundings, the great ocean depths are permanent features of the earth's surface. it is this stability of the general distribution of land and water that has secured the continuity of life upon the earth. had the great oceanic basins, on the other hand, been unstable, changing places with the land at various periods of geological time, they would, almost certainly, again and again have swallowed up the land in their vast abysses, and have thus destroyed all the organic life of the world. there are many confirmatory proofs of this view (which is now widely accepted by geologists and physicists), and a few of them may be briefly stated. . none of the continents present us with marine deposits of any one geological age and occupying a large part of the surface of each, as must have been the case had they ever been sunk deep beneath the ocean and again elevated; neither do any of them contain extensive formations corresponding to the deep oceanic clays and oozes, which again they must have done had they been at any time raised up from the ocean depths. . all the continents present an almost complete and continuous series of rocks of _all_ geological ages, and in each of the great geological periods there are found fresh water and estuarine deposits, and even old land-surfaces, demonstrating continuity of continental or insular conditions. . all the great oceans possess, scattered over them, a few or many islands termed 'oceanic,' and characterised by a volcanic or coralline structure, with no ancient stratified rocks in anyone of them; and in none of these is there found a single indigenous land mammal or amphibian. it is incredible that, if these oceans had ever contained extensive continents, and if these oceanic islands are--as even now they are often alleged to be--parts of these now submerged continents, not one fragment of any of the old stratified rocks, which characterise all existing continents, should remain to show their origin. in the atlantic we find the azores, madeira, and st. helena; in the indian ocean, mauritius, bourbon, and kerguelen island; in the pacific, the fiji, samoan, society, sandwich, and galapagos islands, all without exception telling us the same tale, that they have been built up from the ocean depths by submarine volcanoes and coralline growths, but have never formed part of continental areas. . the contours of the floors of all the great oceans, now fairly well known through the soundings of exploring vessels and for submarine telegraph lines, also give confirmatory evidence that they have never been continental land. for if any part of them were a sunken continent, that part must have retained some impress of its origin. some of the numerous mountain ranges which characterise _every_ continent would have remained. we should find slopes of from ° to ° not uncommon, while valleys bordered by rocky precipices, as in lake lucerne and a hundred others, or isolated rock-walled mountains like roraima, or ranges of precipices as in the ghâts of india or the fiords of norway, would frequently be met with. but not a single feature of this kind has ever been found in the ocean abysses. instead of these we have vast plains, which, if the water were removed, would appear almost exactly level, with no abrupt slopes anywhere. when we consider that deposits from the land never reach these remote ocean depths, and that there is no wave-action below a few hundred feet, these continental features once submerged would be indestructible; and their total absence is, therefore, itself a demonstration that none of the great oceans are on the sites of submerged continents. how ocean depths were produced it is a very difficult problem to determine how the vast basins which are filled by the great oceans, especially that of the pacific, were first produced. when the earth's surface was still in a molten state, it would necessarily take the form of a true oblate spheroid, with a compression at the poles due to its speed of rotation, which is supposed to have been very great. the crust formed by the gradual cooling of such a globe would be of the same general form, and, being thin, would easily be fractured or bent so as to accommodate itself to any unequal stresses from the interior. as the crust thickened and the whole mass slowly cooled and contracted, fissures and crumpling would occur, the former serving as outlets for volcanic activities whose results are found throughout all geological ages; the latter producing mountain chains in which the rocks are almost always curved, folded, or even thrust over each other, indicating the mighty forces due to the adjustments of a solid crust upon a shrinking fluid or semi-fluid interior. but during this whole process there seem to be no forces at work that could lead to the production of such a feature as the pacific, a vast depression covering nearly one-third of the whole surface of the globe. the atlantic ocean, being smaller and nearly opposite to the pacific, but approximately of equal depth, may be looked upon as a complementary phenomenon which will be probably explained as a result of the same causes as the vaster cavity. so far as i am aware, there is only one suggested cause of the formation of these great oceans that seems adequate; and as that cause is to some extent supported by quite independent astronomical evidence, and also directly bears upon the main subject of the present volume, it must be briefly considered. a few years ago, professor george darwin, of cambridge, arrived at a certain conclusion as to the origin of the moon, which is now comparatively well known by sir robert ball's popular account of it in his small volume, _time and tide_. briefly stated, it is as follows. the tides produce friction on the earth and very slowly increase the length of our day, and also cause the moon to recede further from us. the day is lengthened only by a small fraction of a second in a thousand years, and the moon is receding at an equally imperceptible rate. but as these forces are constant, and have always acted on the earth and moon, as we go back and back into the almost infinite past we come to a time when the rotation of the earth was so rapid that gravity at the equator could hardly retain its outer portion, which was spread out so that the form of the whole mass was something like a cheese with rounded edges. and about the same epoch the distance of the moon is found to have been so small that it was actually touching the earth. all this is the result of mathematical calculation from the known laws of gravitation and tidal effects; and as it is difficult to see how so large a body as the moon could have originated in any other way, it is supposed that at a still earlier period the moon and earth were one, and that the moon separated from the parent mass owing to centrifugal force generated by the earth's rapid rotation. whether the earth was liquid or solid at this epoch, and exactly how the separation occurred, is not explained either by professor darwin or sir robert ball; but it is a very suggestive fact that, quite recently, it has been shown, by means of the spectroscope, that double stars of short period _do_ originate in this way from a single star, as already described in our sixth chapter; but in these cases it seems probable that the parent star is in a gaseous state. these investigations of professor g. darwin have been made use of by the rev. osmond fisher (in his very interesting and important work, _physics of the earth's crust_) to account for the basins of the great oceans, the pacific being the chasm left when the larger portion of the mass of the moon parted from the earth. adopting, as i do, the theory of the origin of the earth by meteoric accretion of solid matter, we must consider our planet as having been produced from one of those vast rings of meteorites which in great numbers still circulate round the sun, but which at the much earlier period now contemplated were both more numerous and much more extensive. owing to irregularities of distribution in such a ring and through disturbance by other bodies, aggregations of various size would inevitably occur, and the largest of these would in time draw in to itself all the rest, and thus form a planet. during the early stages of this process the particles would be so small and would come together so gradually, that little heat would be produced, and there would result merely a loose aggregation of cold matter. but as the process went on and the mass of the incipient planet became considerable--perhaps half that of the earth--the rest of the ring would fall in with greater and greater velocity; and this, added to the gravitative compression of the growing mass might, when nearly its present size, have produced sufficient heat to liquefy the outer layers, while the central portion remained solid and to some extent incoherent, with probably large quantities of heavy gases in the interstices. when the amount of the meteoric accretions became so reduced as to be insufficient to keep up the heat to the melting-point, a crust would form, and might have reached about half or three-fourths of its present thickness when the moon became separated. let us now try to picture to ourselves what happened. we should have a globe somewhat larger than our earth is now, both because it then contained the material of the moon and also because it was hotter, revolving so rapidly as to be very greatly flattened at the poles; while the equatorial belt bulged out enormously, and would probably have separated in the form of a ring with a very slight increase of the time of rotation, which is supposed to have been about four hours. this globe would have a comparatively thin crust, beneath which there was molten rock to an unknown depth, perhaps a few hundreds, perhaps more than a thousand miles. at this time the attraction of the sun acting on the molten interior produced tides in it, causing the thin crust to rise and fall every two hours, but to so small an extent--only about a foot or so--as not necessarily to fracture it; but it is calculated that this slight rhythmic undulation coincided with the normal period of undulation due to such a large mass of heavy liquid, and so tended to increase the instability due to rapid rotation. the bulk of the moon is about one-fiftieth part that of the earth, and an easy calculation shows us that, taking the area of the pacific, atlantic, and indian oceans combined as about two-thirds that of the globe, it would require a thickness (or depth) of about forty miles to furnish the material for the moon. we must, of course, assume that there were some inequalities in the thickness of the crust and in its comparative rigidity, so that when the critical moment came and the earth could no longer retain its equatorial protuberance against the centrifugal force due to rotation combined with the tidal undulations caused by the sun, instead of a continuous ring slowly detaching itself, the crust gave way in two or more great masses where it was weakest, and as the tidal wave passed under it and a quantity of the liquid substratum rose with it, the whole would break up and collect into a sub-globular mass a short distance from the earth, and continue revolving with it for some time at about the same rate as the surface had rotated. but as tidal action is always equal on opposite sides of a globe, there would be a similar disruption there, forming, it may be supposed, the atlantic basin, which, as may be seen on a small globe, is almost exactly opposite a part of the central pacific. so soon as these two great masses had separated from the earth, the latter would gradually settle down into a state of equilibrium, and the molten matter of the interior, which would now fill the great oceanic basins up to a level of a few mile below the general surface would soon cool enough to form a thin crust. the larger portion of the nascent moon would gradually attract to itself the one or more smaller portions and form our satellite; and from that time tidal friction by both moon and sun would begin to operate and would gradually lengthen our day and, more rapidly, our month in the way explained in sir robert ball's volume. a very interesting point may now be referred to, because it seems confirmatory of this origin of the great ocean basins. in mr. osmond fisher's work it is explained how the variations in the force of gravity, at numerous points all over the world, have been determined by observations with the pendulum, and also how these variations afford a measure of the thickness of the solid crust, which is of less specific gravity than the molten interior on which it rests. by this means a very interesting result was obtained. the observations on numerous oceanic islands proved that the sub-oceanic crust was considerably more dense than the crust under the continents, but also thinner, the result being to bring the average mass of the sub-oceanic crust and oceans to an equality with that of the continental crust, and this causes the whirling earth to be in a state of balance, or equilibrium. now, both the thinness and the increased density of the crust seem to be well explained by this theory of the origin of the oceanic basins. the new crust would necessarily for a long time be thinner than the older portion, because formed so much later, but it would very soon become cool enough to allow the aqueous vapour of the atmosphere and that given off through fissures from the molten interior to collect in the ocean basins, which would thenceforth be cooled more rapidly and kept at a uniform temperature and also under a uniform pressure, and these conditions would lead to the steady and continuous increase of thickness, with a greater compactness of structure than in the continental areas. it is no doubt to this uniformity of conditions, with a lowering of the bottom temperature throughout the greater part of geological time, till it has become only a few degrees above the freezing-point, that we owe the remarkable persistence of the vast and deep ocean basins on which, as we have seen, the continuity of life on the earth has largely depended. there is one other fact which lends some support to this theory of the origin of the ocean basins--their almost complete symmetry with regard to the equator. both the atlantic and pacific basins extend to an equal distance north and south of the equator, an equality which could hardly have been produced by any cause not directly connected with the earth's rotation. the polar seas which are coterminous with the two great oceans are very much shallower, and cannot, therefore, be considered as forming part of the true oceanic basins. water as an equaliser of temperature the importance of water in regulating the temperature of the earth is so great that, even if we had enough water on the land for all the wants of plants and animals, but had no great oceans, it is almost certain that the earth could not have produced and sustained the various forms of life which it now possesses. the effect of the oceans is twofold. owing to the great specific heat of water, that is, its property of absorbing heat slowly but to a large amount, and giving it out with equal slowness, the surface-waters of the oceans and seas are heated by the sun so that by the evening of a bright day they have become quite warm to a depth of several feet. but air has much less specific heat than water, a pound of water in cooling one degree being capable of warming four pounds of air one degree; but as air is times as light as water, it follows that the heat from one cubic foot of water will warm more than cubic feet of air as much as it cools itself. hence the enormous surface of the seas and oceans, the larger part of which is within the tropics, warms the whole of the lower and denser portions of the air, especially during the night, and this warmth is carried to all parts of the earth by the winds, and thus ameliorates the climate. another quite distinct effect is due to the great ocean currents, like the gulf stream and the japan current, which carry the warm water of the tropics to temperate and arctic regions, and thus render many countries habitable which would otherwise suffer the rigour of an almost arctic winter. these currents are, however, directly due to the winds, and properly belong to the section on the atmosphere. the other equalising action, due primarily to the great area of the seas and oceans, is a result of the vast evaporating surface from which the land derives almost all its water in the form of rain and rivers; and it is quite evident that if there were not sufficient water-surface to produce an ample supply of vapour for this purpose, arid districts would occupy more and more of the earth's surface. how much water-surface is necessary for life we do not know; but if the proportions of water and land-surfaces were reversed, it seems probable that the larger proportion of the earth might be uninhabitable. the vapour thus produced has also a very great effect in equalising temperature; but this also is a point which will come better under our next chapter on the atmosphere. * * * * * there are, however, some matters connected with the water-supply of the earth, and its relation to the development of life, that call for a few remarks here. what has determined the total quantity of water on the earth or on other planets does not appear to be known; but presumably it would depend, partially or wholly, on the mass of the planet being sufficient to enable it to retain by its gravitative force the oxygen and hydrogen of which water is composed. as the two gases are so easily combined to form water, but can only be separated under special conditions, its quantity would be dependent on the supply of hydrogen, which is but rarely found on the earth in a free state. the important fact, however, is, that we do possess so great a quantity of water, that if the whole surface of the globe was as regularly contoured as are the continents, and merely wrinkled with mountain chains, then the existing water would cover the whole globe nearly two miles deep, leaving only the tops of high mountains above its surface as rows of small islands, with a few larger islands formed by what are now the high plateaus of tibet and the southern andes. now there seems no reason why this distribution of the water should not have occurred--in fact it seems probable that it would have occurred, had it not been for the fortunate coincidence of the formation of enormously deep ocean basins. so far as i am aware, no sufficient explanation of the formation of these basins has been given but that of mr. osmond fisher, as here described, and that depends upon three unique circumstances: ( ) the formation of a satellite at a very late period of the planet's development when there was already a rather thick crust; ( ) the satellite being far larger in proportion to its primary than any other in the solar system; and ( ) its having been produced by fission from its primary on account of extremely rapid rotation, combined with solar tides in its molten interior, and a rate of oscillation of that molten interior coinciding with the tidal period.[ ] whether this very remarkable theory of the origin of our moon is the true one, and if so, whether the explanation it seems to afford of the great oceanic basins is correct, i am not mathematician enough to judge. the tidal theory of the origin of the moon, as worked out mathematically by professor g.h. darwin, has been supported by sir robert ball and accepted by many other astronomers; while the researches of the rev. osmond fisher into the _physics of the earth's crust_, together with his mathematical abilities and his practical work as a geologist, entitle his opinion on the question of the mode of origin of the ocean basins to the highest respect. and, as we have seen, the existence of these vast and deep ocean basins, produced by the agency of a series of events so remarkable as to be quite unique in the solar system, played an important part in rendering the earth fit for the development of the higher forms of animal life, while without them it seems not improbable that the conditions would have been such as to render any varied forms of terrestrial life hardly possible. footnotes: [ ] for a fuller account of this arctic fauna and flora see the works of sir c. lyell, sir a. geikie, and other geologists. a full summary of it is also given in the author's _island life_. [ ] professor g.h. darwin states that it is nearly certain that no other satellite nor any of the planets originated in the same way as the moon. chapter xiii the earth in relation to life: atmospheric conditions we have seen in our tenth chapter that the physical basis of life--protoplasm--consists of the four elements, oxygen, nitrogen, hydrogen, and carbon, and that both plants and animals depend largely upon the free oxygen in the air to carry on their vital processes; while the carbonic acid and ammonia in the atmosphere seem to be absolutely essential to plants. whether life could have arisen and have been highly developed with an atmosphere composed of different elements from ours it is, of course, impossible to say; but there are certain physical conditions which seem absolutely essential whatever may be the elements which compose it. the first of these essentials is an atmosphere which shall be of such density at the surface of the planet, and of so great a bulk, as to be not too rare to fulfil its various functions at all altitudes where there is a considerable area of land. what determines the total quantity of gaseous matter on the surface of a planet will be, mainly, its mass, together with the average temperature of its surface. the molecules of gases are in a state of rapid motion in all directions, and the lighter gases have the most rapid motions. the average speed of the motion of the molecules has been roughly determined under varying conditions of pressure and temperature, and also the probable maximum and minimum rates, and from these data, and certain known facts as to planetary atmospheres, mr. g. johnstone stoney, f.r.s., has calculated what gases will escape from the atmospheres of the earth and the other planets. he finds that all the gases which are constituents of air have such comparatively low molecular rates of motion that the force of gravity at the upper limits of the earth's atmosphere is amply sufficient to retain them; hence the stability in its composition. but there are two other gases, hydrogen and helium, which are both known to enter the atmosphere, but never accumulate so as to form any measurable portion of it, and these are found to have sufficient molecular motion to escape from it. with regard to hydrogen, if the earth were much larger and more massive than it is, so as to retain the hydrogen, disastrous consequences might ensue, because, whenever a sufficient quantity of this gas accumulated, it would form an explosive mixture with the oxygen of the atmosphere, and a flash of lightning or even the smallest flame would lead to explosions so violent and destructive as perhaps to render such a planet unsuited for the development of life. we appear, therefore, to be just at the major limit of mass to secure habitability, except in such planets as may have no continuous supply of free hydrogen. * * * * * perhaps the most important mechanical functions of the atmosphere dependent on its density are: ( ) the production of winds, which in many ways bring about an equalisation of temperature, and which also produce surface-currents on the ocean; and ( ) the distribution of moisture over the earth by means of clouds which also have other important functions. winds depend primarily on the local distribution of heat in the air, especially on the great amount of heat constantly present in the equatorial zone, due to the sun being always nearly vertical at noon, and to its being similarly vertical at each tropic once a year, with a longer day, leading to even higher temperatures than at the equator, and producing also that continuous belt of arid lands or deserts which almost encircle the globe in the region of the tropics. heated air being lighter, the colder air from the temperate zones continually flows towards it, lifting it up and causing it to flow over, as it were, to the north and south. but as the inflow comes from an area of less rapid to one of more rapid rotation, the course of the air is diverted, and produces the north-east and south-east trades; while the overflow from the equator going to an area of less rapid rotation, turns westward and produces the south-west winds so prevalent over the north atlantic and the north temperate zone generally, and the north-west in the southern hemisphere. it is outside the zone of the equable trade-winds, and in a region a few degrees on each side of the tropics, that destructive hurricanes and typhoons prevail. these are really enormous whirlwinds due to the intensely heated atmosphere over the arid regions already mentioned, causing an inrush of cool air from various directions, thus setting up a rotatory motion which increases in rapidity till equilibrium is restored. the hurricanes of the west indies and mauritius, and the typhoons of the eastern seas, are thus caused. some of these storms are so violent that no human structures can resist them, while the largest and most vigorous trees are torn to pieces or overturned by them. but if our atmosphere were much denser than it is, its increased weight would give it still greater destructive force; and if to this were added a somewhat greater amount of sun-heat--which might be due either to our greater proximity to the sun or to the sun's greater size or greater heat-intensity, these tempests might be so increased in frequency and violence as to render considerable portions of the earth uninhabitable. the constant and equable trade-winds have a very important function in initiating those far-reaching ocean-currents which are of the greatest importance in equalising temperature. the well known gulf stream is to us the most important of these currents, because it plays the chief part in giving us the mild climate we enjoy in common with the whole of western europe, a mildness which is felt to a considerable distance within the arctic circle; and, in conjunction with the japan current, which does the same for the whole of the temperate regions of the north pacific, renders a large portion of the globe better adapted for life than it would be without these beneficial influences. these equalising currents, however, are almost entirely due to the form and position of the continents, and especially to the fact that they are so situated as to leave vast expanses of ocean along the equatorial zone, and extending north and south to the arctic and antarctic regions. if with the same amount of land the continents had been so grouped as to occupy a considerable portion of the equatorial oceans--such as would have been the case had africa been turned so as to join south america, and asia been brought to the south-east so as to take the place of part of the equatorial pacific, then the great ocean-currents could have been but feeble or have hardly existed. without these currents much of the north and south temperate lands would have been buried in ice, while the largest portion of the continents would have been so intensely heated as perhaps to be unsuited for the development of the higher forms of animal life, since we have shown (in chapters x. and xi.) how delicate is the balance and how narrow the limits of temperature which are required. there seems to be no reason whatever why some such distribution of the sea and land should not have existed, had it not been for the admittedly exceptional conditions which led to the production of our satellite, thus necessarily forming vast chasms along the region of the equator where centrifugal force as well as the internal solar tides were most powerful, and where the thin crust was thus compelled to give way. and as the highest authorities declare that there are no indications of such an origin of satellites in the case of any other planet, the whole series of conditions favourable to life on the earth become all the more remarkable. clouds, their importance and their causes few persons have any adequate conception of the real nature of clouds and of the important part they take in rendering our world a habitable and an enjoyable one. on the average, the rainfall over the oceans is much less than over the land, the whole region of the trade-winds having usually a cloudless sky and very little rain; but in the intervening belt of calms, near to the equator, a cloudy sky and heavy rains are frequent. this arises from the fact that the warm, moist air over the ocean is raised upwards, by the cold and heavy air from north and south, into a cooler region where it cannot hold so much aqueous vapour, which is there condensed and falls as rain. generally, wherever the winds blow over extensive areas of water on to the land, especially if there are mountains or elevated plateaus which cause the moisture-laden air to rise to heights where the temperature is lower, clouds are formed and more or less rain falls. but if the land is of an arid nature and much heated by the sun, the air becomes capable of holding still more aqueous vapour, and even dense rain-clouds disperse without producing any rainfall. from these simple causes, with the large area of sea as compared with the land upon our earth, by far the larger portion of the surface is well supplied with rain, which, falling most abundantly in the elevated and therefore cooler regions, percolates the soil, and gives rise to those innumerable springs and rivulets which moisten and beautify the earth, and which, uniting together, form streams and rivers, which return to the seas and oceans whence they were originally derived. clouds and rain depend upon atmospheric dust the beautiful system of aqueous circulation by means of the atmosphere as sketched above was long thought to explain the whole process, and to require no further elucidation; but about a quarter of a century back a curious experiment was made which indicated that there was another factor in the process which had been entirely overlooked. if a small jet of steam is sent into two large glass receivers, one filled with ordinary air, the other with air which has been filtered by passing through a thick layer of cotton wool so as to keep back all particles of solid matter, the first vessel will be instantly filled with condensed cloudy-looking vapour, while in the other vessel the air and vapour will remain perfectly transparent and invisible. another experiment was then made to imitate more nearly what occurs in nature. the two vessels were prepared as before, but a small quantity of water was placed in each vessel and allowed to evaporate till the air was nearly saturated with vapour, which remained invisible in both. both vessels were then slightly cooled, when instantly a dense cloud was formed in that filled with unfiltered air, while the other remained quite clear. these experiments proved that the mere cooling of air below the dew point will not cause the aqueous vapour in it to condense into drops so as to form mist, fog, or cloud, unless small particles of solid or liquid matter are present to act as nuclei upon which condensation begins. the density of a cloud will therefore depend not only on the quantity of vapour in the air, but on the presence of an abundance of minute dust-particles on which condensation can begin. that such dust exists everywhere in the air, even up to great heights, is not a supposition but a proved fact. by exposing glass plates covered with glycerine in different places and at different altitudes the number of these particles in each cubic foot of air has been determined; and it is found that not only are they present everywhere at low levels, but that there are a considerable number even at the tops of the highest mountains. these solid particles also act in another way. by radiation in the higher atmosphere they become very cold, and thus condense the vapour by contact, just as the points of grass-blades condense it to form dew. when steam is escaping from an engine we see a mass of dense white vapour, a miniature cloud; and if we are near it in cold, damp weather, we feel little drops of rain produced from it. but on a fine, warm day it rises quickly and soon melts away, and entirely disappears. exactly the same thing happens on a larger scale in nature. in fine weather we may have abundant clouds continually passing high overhead, but they never produce rain, because as the minute globules of water slowly fall towards the earth, the warm dry air again turns them into invisible vapour. again, in fine weather, we often see a small cloud on a mountain top which remains there a considerable time, even though a brisk wind is blowing. the mountain top is colder than the surrounding air, and the invisible vapour becomes condensed into cloud by passing over it, but the moment these cloud particles are carried past the summit into the warmer and drier air they are again evaporated and disappear. on table mountain, near cape town, this phenomenon occurs on a large scale, and is termed the table-cloth, the mass of white fleecy cloud seeming to hang over the flat mountain top to some distance down where it remains for several months, while all around there is bright sunshine. another phenomenon that indicates the universal presence of dust to enormous heights in the atmosphere is the blue colour of the sky. this is caused by the presence of such excessively minute particles of dust through an enormous thickness of the higher atmosphere--probably up to a height of twenty or thirty miles, or more--that they reflect only the light of short wave-length from the blue end of the spectrum. this also has been proved by experiment. if a glass cylinder, several feet long, is filled with pure air from which all solid particles have been removed by filtering and passing over red-hot platinum wires, and a ray of electric light is passed through it, the interior, when viewed laterally, appears quite dark, the light passing through in a straight line and not illuminating the air. but if a little more air is passed through the filter so rapidly as to allow only the minutest particles of dust to enter with it, the vessel becomes gradually filled with a blue haze, which gradually deepens into a beautiful blue, comparable with that of the sky. if now some of the unfiltered air is admitted, the blue fades away into the ordinary tint of daylight. since it has been known that liquid oxygen is blue, many people have concluded that this explains the blue colour of the sky. but it has really nothing to do with the point at issue. the blue of the liquid oxygen becomes so excessively faint in the gas, further attenuated as it is by the colourless nitrogen, that it would have no perceptible colour in the whole thickness of our atmosphere. again, if it had a perceptible blue tint we could not see it against the blackness of space behind it; but white objects seen through it, such as the moon and clouds, should all appear blue, which they do not do. the blue we see is from the whole sky, and is therefore reflected light; and as pure air is quite transparent, there must be solid or liquid particles so minute as to reflect blue light only. in the lower atmosphere the rain-producing particles are larger, and reflect all the rays, thus diluting the blue colour near the horizon, and, by refraction and reflection combined, producing the various beautiful hues of sunrise and sunset. this production of exquisite colours by the dust in the atmosphere, though adding greatly to the enjoyment of life, cannot be considered essential to it; but there is another circumstance connected with atmospheric dust which, though little appreciated, might have effects which can hardly be calculated. if there were no dust in the atmosphere, the sky would appear black even at noon, except in the actual direction of the sun; and the stars would be visible in the day as well as at night. this would follow because air does not reflect light, and is not visible. we should therefore receive no light from the sky itself as we do now, and the north side of every hill, house, and other solid objects, would be totally dark, unless there were any surfaces in that direction to reflect the light. the surface of the ground at a little distance would be in sunshine, and this would be the only source of light wherever direct sunlight was cut off. to get a good amount of pleasant light in houses it would be necessary to have them built on nearly level ground, or on ground rising to the north, and with walls of glass all round and down to the floor line, to receive as much as possible of the reflected light from the ground. what effect this kind of light would have on vegetation it is difficult to say, but trees and shrubs would probably grow laterally towards the south, east, and west, so as to get as much direct sunshine as possible. a more important result would be that, as sunshine would be perpetual during the day, so much evaporation would take place that the soil would become arid and almost bare in places that are now covered with vegetation, and plants like the cactuses of arizona and the euphorbias of south africa would occupy a large portion of the surface. returning now from this collateral subject of light and colour to the more important aspect of the question--the absence of cloud and rain--we have to consider what would happen, and in what way the enormous quantity of water which would be evaporated under continual sunshine would be returned to the earth. the first and most obvious means would be by abnormally abundant dews, which would be deposited almost every night on every form of leafy vegetation. not only would all grass and herbage, but all the outer leaves of shrubs and trees, condense so much moisture as to take the place of rain so far as the needs of such vegetation were concerned. but without arrangements for irrigation cultivation would be almost impossible, because the bare soil would become intensely heated during the day, and would retain so much of its heat through the night so as to prevent any dew forming upon it. some more effective mode, therefore, of returning the aqueous vapour of the atmosphere to the earth and ocean, would be required, and this, i believe, would be done by means of hills and mountains of sufficient height to become decidedly colder than the lowlands. the air from over the oceans would be constantly loaded with moisture, and whenever the winds blew on to the land the air would be carried up the slopes of the hills into the colder regions, and there be rapidly condensed upon the vegetation, and also on the bare earth and rocks of northern slopes, and wherever they cooled sufficiently during the afternoon or night to be below the temperature of the air. the quantity of vapour thus condensed would reduce the atmospheric pressure, which would lead to an inrush of air from below, bringing with it more vapour, and this might give rise to perpetual torrents, especially on northern and eastern slopes. but as the evaporation would be much greater than at the present time, owing to perpetual sunshine, so the water returned to the earth would be greater, and as it would not be so uniformly distributed over the land as it is now, the result would perhaps be that extensive mountain sides would become devastated by violent torrents, rendering permanent vegetation almost impossible; while other and more extensive areas, in the absence of rain, would become arid wastes that would support only the few peculiar types of vegetation that are characteristic of such regions. whether such conditions as here supposed would prevent the development of the higher forms of life it is impossible to say, but it is certain that they would be very unfavourable, and might have much more disastrous consequences than any we have here suggested. we can hardly suppose that, with winds and rock-formations at all like what they are now, any world could be wholly free from atmospheric dust. if, however, the atmosphere itself were much less dense than it is, say one-half, which might very easily have been the case, then the winds would have less carrying power, and at the elevations at which clouds are usually formed there would not be enough dust-particles to assist in their formation. hence fogs close to the earth's surface would largely take the place of clouds floating far above it, and these would certainly be less favourable to human life and to that of many of the higher animals than existing conditions. the world-wide distribution of atmospheric dust is a remarkable phenomenon. as the blue colour of the sky is universal, the whole of the higher atmosphere must be pervaded by myriads of ultra-microscopical particles, which, by reflecting the blue rays only, give us not only the azure vault of heaven, but in combination with the coarser dust of lower altitudes, diffused daylight, the grand forms and motions of the fleecy clouds, and the 'gentle rain from heaven' to refresh the parched earth and make it beautiful with foliage and flowers. over every part of the vast pacific ocean, whose islands must produce a minimum of dust, the sky is always blue, and its thousand isles do not suffer for want of rain. over the great forest-plain of the amazon valley, where the production of dust must be very small, there is yet abundance of rain-clouds and of rain. this is due primarily to the two great natural sources of dust--the active volcanoes, together with the deserts and more arid regions of the world; and, in the second place, to the density and wonderful mobility of the atmosphere, which not only carries the finest dust-particles to an enormous height, but distributes them through its whole extent with such wonderful uniformity. every dust-particle is of course much heavier than air, and in a comparatively short time, if the atmosphere were still, would fall to the ground. tyndall found that the air of a cellar under the royal institution in albemarle street, which had not been opened for several months, was so pure that the path of a beam of electric light sent through it was quite invisible. but careful experiments show that not only is the air in continual motion, but the motion is excessively irregular, being hardly ever quite horizontal, but upwards and downwards and in every intermediate direction, as well as in countless whirls and eddies; and this complexity of motion must extend to a vast height, probably to fifty miles or more, in order to provide a sufficient thickness of those minutest particles which produce the blue of the sky. all this complexity of motion is due to the action of the sun in heating the surface of the earth, and the extreme irregularity of that surface both as regards contour and its capacity for heat-absorption. in one area we have sand or rock or bare clay, which, when exposed to bright sunshine, become scorching hot; in another area we have dense vegetation, which, owing to evaporation caused by the sunshine, remains comparatively cool, and also the still cooler surfaces of rivers and alpine lakes. but if the air were much less dense than it is, these movements would be less energetic, while all the dust that was raised to any considerable height would, by its own weight, fall back again to the earth much more rapidly than it does now. there would thus be much less dust permanently in the atmosphere, and this would inevitably lead to diminished rainfall and, partially, to the other injurious effects already described. atmospheric electricity we have already seen that vegetable organisms obtain the chief part of the nitrogen in their tissues from ammonia produced in the atmosphere and carried into the earth by rain. this substance can only be thus produced by the agency of electrical discharges, or lightning, which cause the combination of the hydrogen in the aqueous vapour with the free nitrogen of the air. but clouds are important agents in the accumulation of electricity in sufficient amount to produce the violent discharges we know as lightning, and it is doubtful whether without them there would be any discharges through the atmosphere capable of decomposing the aqueous vapour in it. not only are clouds beneficial in the production of rain, and also in moderating the intensity of continuous sun-heat, but they are also requisite for the formation of chemical compounds in vegetables which are of the highest importance to the whole animal kingdom. so far as we know, animal life could not exist on the earth's surface without this source of nitrogen, and therefore without clouds and lightning; and these, we have just seen, depend primarily on a due proportion of dust in the atmosphere. but this due proportion of dust is mainly supplied by volcanoes and deserts, and its distribution and constant presence in the air depends upon the density of the atmosphere. this again depends on two other factors: the force of gravity due to the mass of the planet, and the absolute quantity of the free gases constituting the atmosphere. we thus find that the vast, invisible ocean of air in which we live, and which is so important to us that deprivation of it for a few minutes is destructive of life, produces also many other beneficial effects of which we usually take little account, except at times when storm or tempest, or excessive heat or cold, remind us how delicate is the balance of conditions on which our comfort, and even our lives, depend. but the sketch i have here attempted to give of its varied functions shows us that it is really a most complex structure, a wonderful piece of machinery, as it were, which in its various component gases, its actions and reactions upon the water and the land, its production of electrical discharges, and its furnishing the elements from which the whole fabric of organic life is composed and perpetually renewed, may be truly considered to be the very source and foundation of life itself. this is seen, not only in the fact of our absolute dependence upon it every minute of our lives, but in the terrible effects produced by even a slight degree of impurity in this vital element. yet it is among those nations that claim to be the most civilised, those that profess to be guided by a knowledge of the laws of nature, those that most glory in the advance of science, that we find the greatest apathy, the greatest recklessness, in continually rendering impure this all-important necessary of life, to such a degree that the health of the larger portion of their populations is injured and their vitality lowered, by conditions which compel them to breathe more or less foul and impure air for the greater part of their lives. the huge and ever-increasing cities, the vast manufacturing towns belching forth smoke and poisonous gases, with the crowded dwellings, where millions are forced to live under the most terrible insanitary conditions, are the witnesses to this criminal apathy, this incredible recklessness and inhumanity. for the last fifty years and more the inevitable results of such conditions have been fully known; yet to this day nothing of importance _has_ been done, nothing is being done. in this beautiful land there is ample space and a superabundance of pure air for every individual. yet our wealthy and our learned classes, our rulers and law-makers, our religious teachers and our men of science, all alike devote their lives and energies to anything or everything but this. yet _this_ is the one great and primary essential of a people's health and well-being, to which _everything_ should, for the time, be subordinate. till this is done, and done thoroughly and completely, our civilisation is naught, our science is naught, our religion is naught, and our politics are less than naught--are utterly despicable; are below contempt. it has been the consideration of our wonderful atmosphere in its various relations to human life, and to all life, which has compelled me to this cry for the children and for outraged humanity. will no body of humane men and women band themselves together, and take no rest till this crying evil is abolished, and with it nine-tenths of all the other evils that now afflict us? let _everything_ give way to this. as in a war of conquest or aggression nothing is allowed to stand in the way of victory, and all private rights are subordinated to the alleged public weal, so, in this war against filth, disease, and misery let nothing stand in the way--neither private interests nor vested rights--and we shall certainly conquer. this is the gospel that should be preached, in season and out of season, till the nation listens and is convinced. let this be our claim: pure air and pure water for every inhabitant of the british isles. vote for no one who says 'it can't be done.' vote only for those who declare 'it shall be done.' it may take five or ten or twenty years, but all petty ameliorations, all piecemeal reforms, must wait till this fundamental reform is effected. then, when we have enabled our people to breathe pure air, and drink pure water, and live upon simple food, and work and play and rest under healthy conditions, they will be in a position to decide (for the first time) what other reforms are really needed. remember! we claim to be a people of high civilisation, of advanced science, of great humanity, of enormous wealth! for very shame do not let us say 'we _cannot_ arrange matters so that our people may all breathe unpolluted, unpoisoned air!' chapter xiv the earth is the only habitable planet in the solar system having shown in the last three chapters how numerous and how complex are the conditions which alone render life possible on our earth, how nicely balanced are opposing forces, and how curious and delicate are the means by which the essential combinations of the elements are brought about, it will be a comparatively easy task to show how totally unfitted are all the other planets either to develop or to preserve the higher forms of life, and, in most cases, any forms above the lowest and most rudimentary. in order to make this clear we will take the most important of the conditions in order, and see how the various planets fulfil them. mass of a planet and its atmosphere the height and density of the atmosphere of a planet is important as regards life in several ways. on its density depends its power of carrying moisture; of holding a sufficient supply of dust-particles for the formation of clouds; of carrying ultra-microscopic particles to such a height and in such quantity as to diffuse the light of the sun by reflection from the whole sky; of raising waves in the ocean and thus aerating its waters, and of producing the ocean currents which so greatly equalise temperature. now this density depends on two factors: the mass of the planet and the quantity of the atmospheric gases. but there is good reason to think that the latter depends directly upon the former, because it is only when a certain mass is attained that any of the lighter permanent gases can be held on the surface of a planet. thus, according to dr. g. johnstone stoney, who has specially studied this subject, the moon cannot retain even such a heavy gas as carbonic acid, or the still heavier carbon disulphide; while no particle of oxygen, nitrogen, or water-vapour can possibly remain on it, owing to the fact of its mass being only about one-eightieth that of the earth. it is believed that there are considerable quantities of gases in the stellar spaces, and probably also within the solar system, but perhaps in the liquid or solid form. in that state they might be attracted by any small mass such as the moon, but the heat of its surface when exposed to the solar rays would quickly restore them to the gaseous condition, when they would at once escape. it is only when a planet attains a mass at least a quarter that of the earth that it is capable of retaining water-vapour, one of the most essential of the gases; but with so small a mass as this, its whole atmosphere would probably be so limited in amount and so rare at the planet's surface that it would be quite unable to fulfil the various purposes for which an atmosphere is required in order to support life. for their adequate fulfilment the mass of a planet cannot be much less than that of the earth. here we come to one of those nice adjustments of which so many have been already pointed out. dr. johnstone stoney arrives at the conclusion that hydrogen escapes from the earth. it is continually produced in small quantities by submarine volcanoes, by fissures in volcanic regions, from decaying vegetation, and from some other sources; yet, though sometimes found in minute quantities, it forms no regular constituent of our atmosphere.[ ] the quantity of hydrogen combined with oxygen to form the mass of water in our vast and deep oceans is enormous. yet if it had been only one-tenth more than it actually is, the present land-surface would have been almost all submerged. how the adjustments occurred so that there was exactly enough hydrogen to fill the vast ocean basins with water to such a depth as to leave enough land-surface for the ample development of vegetable and animal life, and yet not so much as to be injurious to climate, it is difficult to imagine. yet the adjustment stares us in the face. first, we have a satellite unique in size as compared with its primary, and apparently in lateness of origin; then we have a mode of origin for that satellite said to be certainly unique in the solar system; as a consequence of this origin, it is believed, we have enormously deep ocean basins symmetrically placed with regard to the equator--an arrangement which is very important for ocean circulation; then we must have had the right quantity of hydrogen, obtained in some unknown way, which formed water enough to fill these chasms, so as to leave an ample area of dry land, but which one-tenth more water would have ingulfed; and, lastly, we have oxygen enough left to form an atmosphere of sufficient density for all the requirements of life. it could not be that the surplus hydrogen escaped when the water had been produced, because it escapes very slowly, and it combines so easily with free oxygen by means of even a spark, as to make it certain that _all_ the available hydrogen was used up in the oceanic waters, and that the supply from the earth's interior has been since comparatively small in amount. there is yet one more adjustment to be noticed. all the facts now referred to show that the earth's mass is sufficient to bring about the conditions favourable for life. but if our globe had been a little larger, and proportionately denser, in all probability no life would have been possible. between a planet of and one of miles diameter is not a large difference, when compared with the enormous range of size of the other planets. yet this slight increase in diameter would give two-thirds increase in bulk, and, with a corresponding increase of density due to the greater gravitative force, the mass would be about double what it is. but with double the mass the quantity of gases of all sorts attracted and retained by gravity would probably have been double; and in that case there would have been double the quantity of water produced, as no hydrogen could then escape. but the _surface_ of the globe would only be one half greater than at present, in which case the water would have sufficed to cover the whole surface several miles deep. habitability of other planets when we look to the other planets of our system we see everywhere illustrations of the relation of size and mass to habitability. the smaller planets, mercury and mars, have not sufficient mass to retain water-vapour, and, without it, they cannot be habitable. all the larger planets can have very little solid matter, as indicated by their very low density notwithstanding their enormous mass. there is, therefore, very good reason for the belief that the adaptability of a planet for a full development of life is _primarily_ dependent, within very narrow limits, on its size and, more directly, on its mass. but if the earth owes its specially constituted atmosphere and its nicely adjusted quantity of water to such general causes as here indicated, and the same causes apply to the other planets of the solar system, then the only planet on which life can be possible is venus. as, however, it may be urged that exceptional causes may have given other planets an equal advantage in the matter of air and water, we will briefly consider some of the other conditions which we have found to be essential in the case of the earth, but which it is almost impossible to conceive as existing, to the required extent, on any of the other planets of the solar system. a small and definite range of temperature we have already seen within what narrow limits the temperature on a planet's surface must be maintained in order to develop and support life. we have also seen how numerous and how delicate are the conditions, such as density of atmosphere, extent and permanence of oceans, and distribution of sea and land, which are requisite, even with us, in order to render possible the continuous preservation of a sufficiently uniform temperature. slight alterations one way or another might render the earth almost uninhabitable, through its being liable to alternations of too great heat or excessive cold. how then can we suppose that any other of the planets, which have either very much more or very much less sun-heat than we receive, could, by any possible modification of conditions, be rendered capable of producing and supporting a full and varied life-development? mars receives less than half the amount of sun-heat per unit of surface that we do. and as it is almost certain that it contains no water (its polar snows being caused by carbonic acid or some other heavy gas) it follows that, although it may produce vegetable life of some low kinds, it must be quite unsuited for that of the higher animals. its small size and mass, the latter only one-ninth that of the earth, may probably allow it to possess a very rare atmosphere of oxygen and nitrogen, if those gases exist there, and this lack of density would render it unable to retain during the night the very moderate amount of heat it might absorb during the day. this conclusion is supported by its low reflecting power, showing that it has hardly any clouds in its scanty atmosphere. during the greater part of the twenty-four hours, therefore, its surface-temperature would probably be much below the freezing point of water; and this, taken in conjunction with the total absence of aqueous vapour or liquid water, would add still further to its unsuitability for animal life. in venus the conditions are equally adverse in the other direction. it receives from the sun almost double the amount of heat that we receive, and this alone would render necessary some extraordinary combination of modifying agencies in order to reduce and render uniform the excessively high temperature. but it is now known that venus has one peculiarity which is in itself almost prohibitive of animal life, and probably of even the lowest forms of vegetable life. this peculiarity is, that through tidal action caused by the sun, its day has been made to coincide with its year, or, more properly, that it rotates on its axis in the same time that it revolves round the sun. hence it always presents the same face to the sun; and while one half has a perpetual day, the other half has perpetual night, with perpetual twilight through refraction in a narrow belt adjoining the illuminated half. but the side that never receives the direct rays of the sun must be intensely cold, approximating, in the central portions, to the zero of temperature, while the half exposed to perpetual sunshine of double intensity to ours must almost certainly rise to a temperature far too great for the existence of protoplasm, and probably, therefore, of any form of animal life. venus appears to have a dense atmosphere, and its brilliancy suggests that we see the upper surface of a cloud-canopy, and this would no doubt greatly reduce the excessive solar heat. its mass, being a little more than three-fourths that of the earth, would enable it to retain the same gases as we possess. but under the extraordinary conditions that prevail on the surface of this planet, it is hardly possible that the temperature of the illuminated side can be preserved in a sufficient state of uniformity for the development of life in any of its higher forms. mercury possesses the same peculiarity of keeping one face always towards the sun, and as it is so much smaller and so much nearer the sun its contrasts of heat and cold must be still more excessive, and we need hardly discuss the possibility of this planet being habitable. its mass being only one-thirtieth that of the earth, water-vapour will certainly escape from it, and, most probably, nitrogen and oxygen also, so that it can possess very little atmosphere; and this is indicated by its low reflecting power, no less than per cent. of the sun's light being absorbed, and only per cent. reflected, whereas clouds reflect per cent. this planet is therefore intensely heated on one side and frozen on the other; it has no water and hardly any atmosphere, and is therefore, from every point of view, totally unfitted for supporting living organisms. even if it is supposed that, in the case of venus, its perpetual cloud-canopy may keep down the surface temperature within the limits necessary for animal life, the extraordinary turmoil in its atmosphere caused by the excessively contrasted temperatures of its dark and light hemispheres must be extremely inimical to life, if not absolutely prohibitive of it. for on the greater part of the hemisphere that never receives a ray of light or heat from the sun all the water and aqueous vapour must be turned into ice or snow, and it seems almost impossible that the air itself can escape congelation. it could only do so by a very rapid circulation of the whole atmosphere, and this would certainly be produced by the enormous and permanent difference of temperature between the two hemispheres. indications of refraction by a dense atmosphere are visible during the planet's transit over the sun's disc, and also when it is in conjunction with the sun, and the refraction is so great that venus is believed to have an atmosphere much higher than ours. but during the rapid circulation of such an atmosphere, heated on one half the planet and cooled on the other, most of the aqueous vapour must be taken out of it on the dark side as fast as it is produced on the heated side, though sufficient may remain to produce a canopy of very lofty clouds analogous to our cirri. the occasional visibility of the dark side of venus may be caused by an electrical glow due to the friction of the perpetually overflowing and inflowing atmosphere, this being increased by reflection from a vast surface of perpetual snow. if we consider all the exceptional features of this planet, it appears certain that the conditions as regards climate cannot now be such as to maintain a temperature within the narrow limits essential for life, while there is little probability that at any earlier period it can have possessed and maintained the necessary stability during the long epochs which are requisite for its development. before considering the condition of the larger planets, it will be well to refer to an argument which has been supposed to minimise the difficulties already stated as to those planets which approach nearest to the earth in size and distance from the sun. the argument from extreme conditions on the earth in reply to the evidence showing how nice are the adaptations required for life-development, it is often objected that life does _now_ exist under very extreme conditions--under tropic heat and arctic snows; in the burnt-up desert as well as in the moist tropical forest; in the air as well as in the water; on lofty mountains as well as on the level lowlands. this is no doubt true, but it does not prove that life could have been developed in a world where any of these extremes of climate characterised the whole surface. the deserts are inhabited because there are oases where water is attainable, as well as in the surrounding fertile areas. the arctic regions are inhabited because there is a summer, and during that summer there is vegetation. if the surface of the ground were always frozen, there would be no vegetation and no animal life. the late mr. r.a. proctor put this argument of the diversity of conditions under which life actually does exist on the earth as well probably as it can be put. he says: 'when we consider the various conditions under which life is found to prevail, that no difference of climatic relations, or of elevation, of land, or of air, or of water, of soil in land, of freshness or saltness in water, of density in air, appears (so far as our researches have extended) to render life impossible, we are compelled to infer that the power of supporting life is a quality which has an exceedingly wide range in nature.' this is true, but with certain reservations. the only species of animal which does really exist under the most varied conditions of climate is man, and he does so because his intellect renders him to some extent the ruler of nature. none of the lower animals have such a wide range, and the diversity of conditions is not really so great as it appears to be. the strict limits are nowhere permanently overpassed, and there is always the change from winter to summer, and the possibility of migration to less inhospitable areas. the great planets all uninhabitable having already shown that the condition of mars, both as regards water, atmosphere, and temperature, is quite unfitted to maintain life, a view in which both general principles and telescopic examination perfectly agree, we may pass on to the outer planets, which, however, have long been given up as adapted for life even by the most ardent advocates for 'life in other worlds.' their remoteness from the sun--even jupiter being five times as far as the earth, and therefore receiving only one twenty-fifth of the light and heat that we receive per unit of surface--renders it almost impossible, even if other conditions were favourable, that they should possess surface-temperatures adequate to the necessities of organic life. but their very low densities, combined with very large size, renders it certain that they none of them have a solidified surface, or even the elements from which such a surface could be formed. it is supposed that jupiter and saturn, as well as uranus and neptune, retain a considerable amount of internal heat, but certainly not sufficient to keep the metallic and other elements of which the sun and earth consist in a state of vapour, for if so they would be planetary stars and would shine by their own light. and if any considerable portion of their bulk consisted of these elements, whether in a solid or a liquid state, their densities would necessarily be much greater than that of the earth instead of very much less--jupiter is under one-fourth the density of the earth, saturn under an eighth, while uranus and neptune are of intermediate densities, though much less in bulk even than saturn. it thus appears that the solar system consists of two groups of planets which differ widely from each other. the outer group of four very large planets are almost wholly gaseous, and probably consist of the permanent gases--those which can only be liquefied or solidified at a very low temperature. in no other way can their small density combined with enormous bulk be accounted for. the inner group also of four planets are totally unlike the preceding. they are all of small size, the earth being the largest. they are all of a density roughly proportionate to their bulk. the earth is both the largest and the densest of the group; not only is it situated at that distance from the sun which, through solar heat alone, allows water to remain in the liquid state over almost the whole of its surface, but it possesses numerous characteristics which secure a very equable temperature, and which have secured to it very nearly the same temperature during those enormous geological periods in which terrestrial life has existed. we have already shown that no other planet possesses these characteristics now, and it is almost equally certain that they never have possessed them in the past, and never will possess them in the future. a last argument for habitability of the planets although it has been admitted by the late mr. proctor and some other astronomers that most of the planets are not _now_ habitable, yet, it is often urged, they may have been so in the past or may become so in the future. some are now too hot, others are now too cold; some have now no water, others have too much; but all go through their appointed series of stages, and during some of these stages life may be or may have been possible. this argument, although vague, will appeal to some readers, and it may, therefore, be necessary to reply to it. this is the more necessary as it is still made use of by astronomers. in a criticism of my article in _the fortnightly review_, m. camille flammarion, of the paris observatory, dramatically remarks: 'yes, life is universal, and eternal, for time is one of its factors. yesterday the moon, to-day the earth, to-morrow jupiter. in space there are both cradles and tombs.'[ ] it is thus suggested that the moon was once inhabited and that jupiter will be inhabited in some remote future; but no attempt is made to deal with the essential physical conditions of these very diverse objects, rendering them not only _now_, but always, unfitted to develop and to maintain terrestrial or aerial life. this vague supposition--it can hardly be termed an argument--as regards past or future adaptability for life, of all the planets and some of the satellites in the solar system, is, however, rendered invalid by an equally general objection to which its upholders appear never to have given a moment's consideration; and as it is an objection which still further enforces the view as to the unique position of the earth in the solar system, it will be well to submit it to the judgment of our readers. limitation of the sun's heat it is well known that there is, and has been for nearly half a century, a profound difference of opinion between geologists and physicists as to the actual or possible duration in years of life upon the earth. the geologists, being greatly impressed with the vast results produced by the slow processes of the wearing away of the rocks and the deposit of the material in seas or lakes, to be again upheaved to form dry land, and to be again carved out by rain and wind, by heat and cold, by snow and ice, into hills and valleys and grand mountain ranges; and further, by the fact that the highest mountains in every part of the globe very often exhibit on their loftiest summits stratified rocks which contain marine organisms, and were therefore originally laid down beneath the sea; and, yet again, by the fact that the loftiest mountains are often the most recent, and that these grand features of the earth's surface are but the latest examples of the action of forces that have been at work throughout all geological time--studying all their lives the detailed evidences of all these changes, have come to the conclusion that they imply enormous periods only to be measured by scores or hundreds of millions of years. and the collateral study of fossil remains in the long series of rock-formations enforces this view. in the whole epoch of human history, and far back into prehistoric times during which man existed on the earth, although several animals have become extinct, yet there is no proof that any new one has been developed. but this human era, so far as yet known, going back certainly to the glacial epoch and almost certainly to pre-glacial times, cannot be estimated at less than a million, some think even several million years; and as there have certainly been some considerable alterations of level, excavation of valleys, deposits of great beds of gravel, and other superficial changes during this period, some kind of a scale of measurement of geological time has been obtained, by comparison with the very minute changes that have occurred during the historical period. this scale is admittedly a very imperfect one, but it is better than none at all; and it is by comparing these small changes with the far greater ones which have occurred during every successive step backward in geological history that these estimates of geological time have been arrived at. they are also supported by the palæontologists, to whom the vast panorama of successive forms of life is an ever-present reality. directly they pass into the latest stage of the tertiary period--the pliocene of sir charles lyell--all over the world new forms of life appear which are evidently the forerunners of many of our still existing species; and as they go a little further back, into the miocene, there are indications of a warmer climate in europe, and large numbers of mammals resembling those which now inhabit the tropics, but of quite distinct species and often of distinct genera and families. and here, though we have only reached to about the middle of the tertiary period, the changes in the forms of life, in the climate, and in the land-surfaces are so great when compared with the very minute changes during the human epoch, as to require us to multiply the time elapsed many times over. yet the whole of the tertiary period, during which _all_ the great groups of the higher animals were developed from a comparatively few generalised ancestral forms, is yet the shortest by far of the three great geological periods--the mesozoic or secondary, having been much longer, with still vaster changes both in the earth's crust and in the forms of life; while the palæozoic or primary, which carries us back to the earliest forms of life as represented by fossilised remains, is always estimated by geologists to be at least as long as the other two combined, and probably very much longer. from these various considerations most geologists who have made any estimates of geological time from the period of the earliest fossiliferous rocks, have arrived at the conclusion that about millions of years are required. but from the variety of the forms of life at this early period it is concluded that a very much greater duration is needed for the whole epoch of life. speaking of the varied marine fauna of the cambrian period, the late professor ramsay says:--'in this earliest known varied life we find no evidence of its having lived near the beginning of the zoological series. in a broad sense, compared with what must have gone before, both biologically and physically, all the phenomena connected with this old period seem, to my mind, to be of quite a recent description; and the climates of seas and lands were of the very same kind as those the world enjoys at the present day.' and professor huxley held very similar views when he declared: 'if the very small differences which are observable between the crocodiles of the older secondary formations and those of the present day furnish any sort of an approximation towards an estimate of the average rate of change among reptiles, it is almost appalling to reflect how far back in palæozoic times we must go before we can hope to arrive at that common stock from which the crocodiles, lizards, _ornithoscelida_, and _plesiosauria_, which had attained so great a development in the triassic epoch, must have been derived.' now, in opposition to these demands of the geologists, in which they are almost unanimous, the most celebrated physicists, after full consideration of all possible sources of the heat of the sun, and knowing the rate at which it is now expending heat, declare, with complete conviction, that our sun cannot have existed as a heat-giving body for so long a period, and they would therefore reduce the time during which life can possibly have existed on the earth to about one-fourth of that demanded by geologists. in one of his latest articles, lord kelvin says:--'now we have irrefragable dynamics proving that the whole life of our sun as a luminary is a very moderate number of million years, probably less than million, possibly between and ' (_phil. mag._, vol. ii., sixth ser., p. , aug. ). in my _island life_ (chap. x.) i have myself given reasons for thinking that both the stratigraphical and biological changes may have gone on more quickly than has been supposed, and that geological time (meaning thereby the time during which the development of life upon the earth has been going on) may be reduced so as possibly to be brought within the maximum period allowed by physicists; but there will certainly be no time to spare, and any planets dependent on our sun whose period of habitability is either past or to come, cannot possibly have, or have had, sufficient time for the necessarily slow evolution of the higher life-forms. again, all physicists hold that the sun is now cooling, and that its future life will be much less than its past. in a lecture at the royal institution (published in _nature series_, in ), lord kelvin says:--'it would, i think, be exceedingly rash to assume as probable anything more than twenty million years of the sun's light in the past history of the earth, or to reckon more than five or six million years of sunlight for time to come.' these extracts serve to show that, unless either geologists or physicists are very far from any approach to accuracy in their estimates of past or future age of the sun, there is very great difficulty in bringing them into harmony or in accounting for the actual facts of the geological history of the earth and of the whole course of life-development upon it. we are, therefore, again brought to the conclusion that there has been, and is, no time to spare; that the _whole_ of the available past life-period of the sun has been utilised for life-development on the earth, and that the future will be not much more than may be needed for the completion of the grand drama of human history, and the development of the full possibilities of the mental and moral nature of man. we have here, then, a very powerful argument, from a different point of view than any previously considered, for the conclusion that man's place in the solar system is altogether unique, and that no other planet either has developed or can develop such a full and complete life-series as that which the earth has actually developed. even if the conditions had been more favourable than they are seen to be on other planets, mercury, venus, and mars could not possibly have preserved equability of conditions long enough for life-development, since for unknown ages they must have been passing slowly towards their present wholly _unsuitable_ conditions; while jupiter and the planets beyond him, whose epoch of life-development is supposed to be in the remote future when they shall have slowly cooled down to habitability, will then be still more faintly illuminated and scantily warmed by a rapidly cooling sun, and may thus become, at the best, globes of solid ice. this is the teaching of science--of the best science of the twentieth century. yet we find even astronomers who, more than any other exponents of science, should give heed to the teachings of the sister sciences to which they owe so much, indulging in such rhapsodies as the following:--'in our solar system, this little earth has not obtained any special privileges from nature, and it is strange to wish to confine life within the circle of terrestrial chemistry.' and again: 'infinity encompasses us on all sides, life asserts itself, universal and eternal, our existence is but a fleeting moment, the vibration of an atom in a ray of the sun, and our planet is but an island floating in the celestial archipelago, to which no thought will ever place any bounds.'[ ] in place of such 'wild and whirling words,' i have endeavoured to state the sober conclusions of the best workers and thinkers as to the nature and origin of the world in which we live, and of the universe which on all sides surrounds us. i leave it to my readers to decide which is the more trustworthy guide. footnotes: [ ] _transactions of royal dublin society_, vol. vi. (ser. ii.), part xiii. 'of atmospheres upon planets and satellites.' by g. johnstone stoney, f.r.s., etc. etc. [ ] _knowledge_, june . [ ] m. camille flammarion, in _knowledge_, june . chapter xv the stars--have they planetary systems? are they beneficial to us? most of the writers on the plurality of worlds, from fontenelle to proctor, taking into consideration the enormous number of the stars and their apparent uselessness to our world, have assumed that many of them _must_ have systems of planets circling round them, and that some of these planets, at all events, _must_ possess inhabitants, some, perhaps, lower, but others no doubt higher than ourselves. one of our well-known modern astronomers, writing only ten years ago, adopts the same view. he says: 'the suns which we call stars were clearly not created for our benefit. they are of very little practical use to the earth's inhabitants. they give us very little light; an additional small satellite--one considerably smaller than the moon--would have been much more useful in this respect than the millions of stars revealed by the telescope. they must therefore have been formed for some other purpose.... we may therefore conclude, with a high degree of probability, that the stars--at least those with spectra of the solar type--form centres of planetary systems somewhat similar to our own.'[ ] the author then discusses the conditions necessary for life analogous to that of our earth, as regards temperature, rotation, mass, atmosphere, water, etc., and he is the only writer i have met with who has considered these conditions; but he touches on them very briefly, and he arrives at the conclusion that, in the case of the stars of solar type, it is probable that _one_ planet, situated at a proper distance, would be fitted to support life. he estimates roughly that there are about ten million stars of this type, that is, closely resembling our sun, and that if only one in ten of these has a planet at the proper distance and properly constituted in other respects, there will be one million worlds fitted for the support of animal life. he therefore concludes that there are probably many stars having life-bearing planets revolving round them. there are, however, many considerations not taken account of by this writer which tend to reduce very considerably the above estimate. it is now known that immense numbers of the stars of smaller magnitudes are nearer to us than are the majority of the stars of the first and second magnitudes, so that it is probable that these, as well as a considerable proportion of the very faint telescopic stars, are really of small dimensions. we have evidence that many of the brightest stars are much larger than our sun, but there are probably ten times as many that are much smaller. we have seen that the whole of the past light and heat-giving duration of our sun has, according to the best authorities, been only just sufficient for the development of life upon the earth. but the duration of a sun's heat-giving power will depend mainly upon its mass, together with its constituent elements. suns which are much smaller than ours are, therefore, from that cause alone, unsuited to give adequate light and heat for a sufficient time, and with sufficient uniformity, for life-development on planets, even if they possess any at the right distance, and with the extensive series of nicely adjusted conditions which i have shown to be necessary. again, we must, probably, rule out as unfitted for life-development the whole region of the milky way, on account of the excessive forces there in action, as shown by the immense size of many of the stars, their enormous heat-giving power, the crowding of stars and nebulous matter, the great number of star-clusters, and, especially, because it is the region of 'new stars,' which imply collisions of masses of matter sufficiently large to become visible from the immense distance we are from them, but yet excessively small as compared with suns the duration of whose light is to be measured by millions of years. hence the milky way is the theatre of extreme activity and motion; it is comparatively crowded with matter undergoing continual change, and is therefore not sufficiently stable for long periods to be at all likely to possess habitable worlds. we must, therefore, limit our possible planetary systems suitable for life-development, to stars situated inside the circle of the milky way and far removed from it--that is, to those composing the solar cluster. these have been variously estimated to consist of a few hundred or many thousand stars--at all events to a very small number as compared with the 'hundreds of millions' in the whole stellar universe. but even here we find that only a portion are probably suitable. professor newcomb arrives at the conclusion--as have some other astronomers--that the stars in general have a much smaller mass in proportion to the light they give than our sun has; and, after an elaborate discussion, he finally concludes that the brighter stars are, on the average, much less dense than our sun. in all probability, therefore, they cannot give light and heat for so long a period, and as this period in the case of our sun has only been just sufficient, the number of suns of the solar type and of a sufficient mass may be very limited. yet further, even among stars having a similar physical constitution to our sun and of an equal or greater mass, only a portion of their period of luminosity would be suitable for the support of planetary life. while they are in process of formation by accretions of solid or gaseous masses, they would be subject to such fluctuations of temperature, and to such catastrophic outbursts when any larger mass than usual was drawn towards them, that the whole of this period--perhaps by far the longest portion of their existence--must be left out of the account of planet-producing suns. yet all these are to us stars of various degrees of brilliancy. it is almost certain that it is only when the growth of a sun is nearly completed, and its heat has attained a maximum, that the epoch of life-development is likely to begin upon any planets it may possess at the most suitable distance, and upon which all the requisite conditions should be present. it may be said that there are great numbers of stars beyond our solar cluster and yet within the circle of the milky way, as well as others towards the poles of the milky way, which i have not here referred to. but of these regions very little is known, because it is impossible to tell whether stars in these directions are situated in the outer portion of the solar cluster or in the regions beyond it. some astronomers appear to think that these regions may be nearly empty of stars, and i have endeavoured to represent what seems to be the general view on this very difficult subject in the two diagrams of the stellar universe at pp. , . the regions beyond our cluster and above or below the plane of the milky way are those where the small irresolvable nebulæ abound, and these may indicate that sun-formation is not yet active in those regions. the two charts of nebulæ and clusters at the end of the volume illustrate, and perhaps tend to support this view. double and multiple star systems we have already seen, in our sixth chapter, how rapid and extraordinary has been the discovery of what are termed spectroscopic binaries--pairs of stars so close together as to appear like a single star in the most powerful telescopes. the systematic search for such stars has only been carried on for a few years, yet so many have been already found, and their numbers are increasing so rapidly, as to quite startle astronomers. one of the chief workers in this field, professor campbell of the lick observatory, has stated his opinion that, as accuracy of measurement increases, these discoveries will go on till--'the star that is not a spectroscopic binary will prove to be the rare exception,'--and other astronomers of eminence have expressed similar views. but these close revolving star-systems are generally admitted to be out of the category of life-producing suns. the tidal disturbances mutually produced must be enormous, and this must be inimical to the development of planets, unless they were very close to each sun, and thus in the most unfavourable position for life. we thus see that the result of the most recent researches among the stars is entirely opposed to the old idea that the countless myriads of stars _all_ had planets circulating round them, and that the ultimate purpose of their existence was, that they should be supporters of life, as our sun is the supporter of life upon the earth. so far is this from being the case, that vast numbers of stars have to be put aside as wholly unfitted for such a purpose; and when by successive eliminations of this nature we have reduced the numbers which may possibly be available to a few millions, or even to a few thousands, there comes the last startling discovery, that the entire host of stars is found to contain binary systems in such rapidly increasing numbers, as to lead some of the very first astronomers of the day to the conclusion that single stars may someday be found to be the rare exception! but this tremendous generalisation would, at one stroke, sweep away a large proportion of the stars which other successive disqualifications had spared, and thus leave our sun, which is certainly single, and perhaps two or three companion orbs, alone among the starry host as possible supporters of life on some one of the planets which circulate around them. but we do not really _know_ that any such suns exist. if they exist we do not _know_ that they possess planets. if any do possess planets these may not be at the proper distance, or be of the proper mass, to render life possible. if these primary conditions should be fulfilled, and if there should possibly be not only one or two, but a dozen or more that so far fulfil the first few conditions which are essential, what probability is there that all the other conditions, all the other nice adaptations, all the delicate balance of opposing forces that we have found to prevail upon the earth, and whose combination here is due to exceptional conditions which exist in the case of no other known planet--should _all_ be again combined in some of the possible planets of these possibly existing suns? i submit that the probability is _now_ all the other way. so long as we could assume that all the stars might be, in all essentials, like our sun, it seemed almost ludicrous to suppose that our sun alone should be in a position to support life. but when we find that enormous classes like the gaseous stars of small density, the solar stars while increasing in size and temperature, the stars which are much smaller than our sun, the nebulous stars, probably all the stars of the milky way, and lastly that enormous class of spectroscopic doubles--veritable aaron's rods which threaten to swallow up all the rest--that all these are for various reasons unlikely to have attendant planets adapted to develop life, then the probabilities seem to be enormously against there being any considerable number of suns possessing attendant habitable earths. just as the habitability of all the planets and larger satellites, once assumed as so extremely probable as to amount almost to a certainty, is now generally given up, so that in speculating on life in stellar systems mr. gore assumes that only _one_ planet to each sun can be habitable; in like manner it may, and i believe will, turn out, that of all the myriad stars, the more we learn about them, the smaller and smaller will become the scanty residue which, with any probability, we can suppose to illuminate and vivify habitable earths. and when with this scanty probability we combine the still scantier probability that any such planet will possess simultaneously, and for a sufficiently long period, _all_ the highly complex and delicately balanced conditions known to be essential for a full life-development, the conception that on this earth alone has such development been completed will not seem so wildly improbable a conjecture as it has hitherto been held to be. are the stars beneficial to us? when i suggested in my first publication on this subject that some emanations from the stars _might_ be beneficial or injurious, and that a central position _might_ be essential in order to render these emanations equable, one of my astronomical critics laughed the idea to scorn, and declared that 'we might wander into outer space without losing anything more serious than we lose when the night is cloudy and we cannot see the stars.'[ ] how my critic knows that this is so he does not tell us. he states it positively, with no qualification, as if it were an established fact. it may be as well to inquire, therefore, if there is any evidence bearing upon the point at issue. astronomers are so fully occupied with the vast number and variety of the phenomena presented by the stellar universe and the various difficult problems arising therefrom, that many lesser but still interesting inquiries have necessarily received little attention. such a minor problem is the determination of how much heat or other active radiation we receive from the stars; yet a few observations have been made with results that are of considerable interest. in the years and mr. e.f. nichols of the yerkes observatory made a series of experiments with a radiometer of special construction, to determine the heat emitted by certain stars. the result arrived at was, that vega gave about / of the heat of a candle at one metre distance, and arcturus about . times as much. in and mr. g.m. minchin made a series of experiments on the _electrical measurement of starlight_, by means of a photo-electric cell of peculiar construction which is sensitive to the whole of the rays in the spectrum, and also to some of the ultra-red and ultra-violet rays. combined with this was a very delicate electrometer. the telescope employed to concentrate the light was a reflector of two feet aperture. mr. minchin was assisted in the experiments by the late professor g.f. fitzgerald, f.r.s., of trinity college, dublin, which may be considered guarantee of the accuracy of the observations. the following are the chief results obtained:-- ----------------------------------------------------------------------- ---------------------------------+-------------+-----------+----------- | deflection | light | source of light. | in | in | e. m. f. | millimetres | candles. | volts. ---------------------------------+-------------+-----------+----------- candle at feet distance | . | | betelgeuse ( . mag.) | . | . | . aldebaran ( . mag.) | . | . | . procyon ( . mag) | . | . | . alpha cygni ( . mag.) | . | . | . polaris ( . mag.) | . | . | . ---------------------------------+-------------+-----------+----------- volt. | . | | | | | arcturus ( . mag.) | . | . | . vega ( . mag) | . | . | . candle at feet | . | | ---------------------------------+-------------+-----------+----------- n.b.--the standard candle shone directly on the cell, whereas the star's light was concentrated by a -foot mirror. the sensitive surface on which the light of the stars was concentrated was / inch in diameter. we must therefore diminish the amount of candle light in this table in the proportion of the square of the diameter of the mirror (in / of an inch) to one, equal to / . if we make the necessary reduction in the case of vega, and also equalise the distance at which the candle was placed, we find the following result:-- observer. star. candle power at ft. minchin. vega / nichols. " / this enormous difference in the result is no doubt largely due to the fact that mr. nichols's apparatus measured heat alone, whereas mr. minchin's cell measured almost all the rays. and this is further shown by the fact that, whereas mr. nichols found arcturus a red star, hotter than vega a white one, mr. minchin, measuring also the light-giving and some of the chemical rays, found vega considerably more energetic than arcturus. these comparisons also suggest that other modes of measurement might give yet higher results, but it will no doubt be urged that such minute effects must necessarily be quite inoperative upon the organic world. there are, however, some considerations which tend the other way. mr. minchin remarks on the unexpected fact that betelgeuse produces more than double the electrical energy of procyon, a much brighter star. this indicates that many of the stars of smaller visual magnitudes may give out a large amount of energy, and it is this energy, which we now know can take many strange and varied forms, that would be likely to influence organic life. and as to the quantity being too minute to have any effect, we know that the excessively minute amount of light from the very smallest telescopic stars produces such chemical changes on a photographic plate as to form distinct images, with comparatively small lenses or reflectors and with an exposure of two or three hours. and if it were not that the diffused light of the surrounding sky also acts upon the plate and blurs the faint images, much smaller stars could be photographed. we know that not all the rays, but a portion only, are capable of producing these effects; we know also that there are many kinds of radiation from the stars, and probably some yet undiscovered comparable with the x rays and other new forms of radiation. we must also remember the endless variety and the extreme instability of the protoplasmic products in the living organism, many of which are perhaps as sensitive to special rays as is the photographic plate. and we are not here limited to action for a few minutes or a few hours, but throughout the whole night and day, and continued whenever the sky is clear for months or years. thus the cumulative effect of these very weak radiations may become important. it is probable that their action would be most influential on plants, and here we find all the conditions requisite for its accumulation and utilisation in the large amount of leaf-surface exposed to it. a large tree must present some hundreds of superficial feet of receptive surface, while even shrubs and herbs often have a leaf-area of greater superficial extent than the object-glasses of our largest telescopes. some of the highly complex chemical processes that go on in plants may be helped by these radiations, and their action would be increased by the fact that, coming from every direction over the whole surface of the heavens, the rays from the stars would be able to reach and act upon every leaf of the densest masses of foliage. the large amount of growth that takes place at night may be in part due to this agency. of course all this is highly speculative; but i submit, in view of the fact that the light of the very faintest stars _does_ produce distinct chemical changes, that even the very minute heat-effects are measureable, as well as the electro-motive forces caused by them; and further, that when we consider the millions, perhaps hundreds of millions of stars, all acting simultaneously on any organism which may be sensitive to them, the supposition that they do produce some effect, and possibly a very important effect, is not one to be summarily rejected as altogether absurd and not worth inquiring into. it is not, however, these possible direct actions of the stars upon living organisms to which i attach much weight as regards our central position in the stellar universe. further consideration of the subject has convinced me that the fundamental importance of that position is a physical one, as has already been suggested by sir norman lockyer and some other astronomers. briefly, the central position appears to be the only one where suns can be sufficiently stable and long-lived to be capable of maintaining the long process of life-development in any of the planets they may possess. this point will be further developed in the next (and concluding) chapter. footnotes: [ ] _the worlds of space_, by j.e. gore, chapter iii. [ ] _the fortnightly review_, april , p. . chapter xvi stability of the star-system: importance of our central position: summary and conclusion one of the greatest difficulties with regard to the vast system of stars around us is the question of its permanence and stability, if not absolutely and indefinitely, yet for periods sufficiently long to allow for the many millions of years that have certainly been required for our terrestrial life-development. this period, in the case of the earth, as i have sufficiently shown, has been characterised throughout by extreme uniformity, while a continuance of that uniformity for a few millions of years in the future is almost equally certain. but our mathematical astronomers can find no indications of such stability of the stellar universe as a whole, if subject to the law of gravitation alone. in reply to some questions on this point, my friend professor george darwin writes as follows:--'a symmetrical annular system of bodies might revolve in a circle with or without a central body. such a system would be unstable. if the bodies are of unequal masses and not symmetrically disposed, the break-up of the system would probably be more rapid than in the ideal case of symmetry.' this would imply that the great annular system of the milky way is unstable. but if so, its existence at all is a greater mystery than ever. although in detail its structure is very irregular, as a whole it is wonderfully symmetrical; and it seems quite impossible that its generally circular ring-like form can be the result of the chance aggregation of matter from any pre-existing different form. star-clusters are equally unstable, or, rather, nothing is known or can be predicated about their stability or instability, according to professors newcomb and darwin. mr. e.t. whittaker (secretary to the royal astronomical society), to whom professor g. darwin sent my questions, writes:--'i doubt whether the principal phenomena of the stellar universe are consequences of the law of gravitation at all. i have been working myself at spiral nebulæ, and have got a first approximation to an explanation--but it is electro-dynamical and not gravitational. in fact, it may be questioned whether, for bodies of such tremendous extent as the milky way or nebulæ, the effect which we call gravitation is given by newton's law; just as the ordinary formulæ of electrostatic attraction break down when we consider charges moving with very great velocities.' accepting these statements and opinions of two mathematicians who have given special attention to similar problems, we need not limit ourselves to the laws of gravitation as having determined the present form of the stellar universe; and this is the more important because we may thus escape from a conclusion which many astronomers seem to think inevitable, viz. that the observed proper motions of the stars cannot be explained by the gravitative forces of the system itself. in chapter viii. of this work i have quoted professor newcomb's calculation as to the effect of gravitation in a universe of million stars, each five times the mass of our sun, and spread over a sphere which it would take light , years to cross; then, a body falling from its outer limits to the centre could at the utmost acquire a velocity of twenty-five miles a second; and therefore, any body in any part of such a universe having a greater velocity would pass away into infinite space. now, as several stars have, it is believed, much more than this velocity, it follows not only that they will inevitably escape from our universe, but that they do not belong to it, as their great velocity must have been acquired elsewhere. this seems to have been the idea of the astronomer who stated that, even at the very moderate speed of our sun, we should in five million years be deep in the actual stream of the milky way. to this i have already sufficiently replied; but i now wish to bring before my readers an excellent illustration of the importance of the late professor huxley's remark, that the results you got out of the 'mathematical mill' depend entirely on what you put into it. in the _philosophical magazine_ (january ) is a remarkable article by lord kelvin, in which he discusses the very same problem as that which professor newcomb had discussed at a much earlier date, but, starting from different assumptions, equally based on ascertained facts and probabilities deduced from them, brings out a very different result. lord kelvin postulates a sphere of such a radius that a star at its confines would have a parallax of one-thousandth part of a second ( ". ), equivalent to light-years. uniformly distributed through this sphere there is matter equal in mass to million suns like ours. if this matter becomes subject to gravitation, it all begins to move at first with almost infinite slowness, especially near its centre; but nevertheless, in twenty-five million years many of these suns would have acquired velocities of from twelve to twenty miles a second, while some would have less and some probably more than seventy miles a second. now such velocities as these agree generally with the measured velocities of the stars, hence lord kelvin thinks there may be as much matter as million suns within the above-named distance. he then states that if we suppose there to be , million suns within the same sphere, velocities would be produced very much greater than the known star-velocities; hence it is probable that there is very much less matter than , million times the sun's mass. he also states that if the matter were not uniformly distributed within the sphere, then, whatever was the irregularity, the acquired motions would be greater; again indicating that the million suns would be ample to produce the observed effects of stellar motion. he then calculates the average distance apart of each of the million stars, which he finds to be about millions of millions of miles. now the nearest star to our sun is about twenty-six million million of miles distant, and, as the evidence shows, is situated in the denser part of the solar cluster. this gives ample allowance for the comparative emptiness of the space between our cluster and the milky way, as well as of the whole region towards the poles of the milky way (as shown by the diagrams in chapter iv.), while the comparative density of extensive portions of the galaxy itself may serve to make up the average. now, previous writers have come to a different conclusion from the same general line of argument, because they have started with different assumptions. professor newcomb, whose statement made some years back is usually followed, assumed million stars each five times as large as our sun, equal to million suns in all, and he distributed them equally throughout a sphere , light-years in diameter. thus he has half the amount of matter assumed by lord kelvin, but nearly five times the extent, the result being that gravity could only produce a maximum speed of twenty-five miles a second; whereas on lord kelvin's assumption a maximum speed of seventy miles a second would be produced, or even more. by this latter calculation we find no insuperable difficulty in the speed of any of the stars being beyond the power of gravitation to produce, because the rates here given are the direct results of gravitation acting on bodies almost uniformly distributed through space. irregular distribution, such as we see everywhere in the universe, might lead to both greater and less velocities; and if we further take account of collisions and near approaches of large masses resulting in explosive disruptions, we might have almost any amount of motion as the result, but as this motion would be produced by gravitation within the system, it could equally well be controlled by gravitation. [illustration: diagram of stellar universe (plan). . central part of solar cluster. . sun's orbit (black spot). . outer limit of solar cluster. . milky way.] in order that my readers may better understand the calculations of lord kelvin, and also the general conclusions of astronomers as to the form and dimensions of the stellar universe, i have drawn two diagrams, one showing a plan on the central plane of the milky way, the other a section through its poles. both are on the same scale, and they show the total diameter across the milky way as being light-years, or about half that postulated by lord kelvin for his hypothetical universe. i do this because the dimensions given by him are those which are sufficient to lead to motions near the centre such as the stars now possess in a minimum period of twenty-five million years after the initial arrangement he supposes, at which later epoch which we are now supposed to have reached, the whole system would of course be greatly reduced in extent by aggregations towards and near the centre. these dimensions also seem to accord sufficiently with the actual distances of stars as yet measured. the smallest parallax which has been determined with any certainty, according to professor newcomb's list, is that of gamma cassiopeiæ, which is one-hundredth of a second ( ". ), while lord kelvin gives none smaller than ". , and these will all be included within the solar cluster as i have shown it. [illustration: diagram of stellar universe (section). section through poles of milky way.] it must be clearly understood that these two illustrations are merely diagrams to show the main features of the stellar universe according to the best information available, with the proportionate dimensions of these features, so far as the facts of the distribution of the stars and the views of those astronomers who have paid most attention to the subject can be harmonised. of course it is not suggested that the whole arrangement is so regular as here shown, but an attempt has been made by means of the dotted shading to represent the comparative densities of the different portions of space around us, and a few remarks on this point may be needed. the solar cluster is shown very dense at the central portion, occupying one-tenth of its diameter, and it is near the outside of this dense centre that our sun is supposed to be situated. beyond this there seems to be almost a vacuity, beyond which again is the outer portion of the cluster consisting of comparatively thinly scattered stars, thus forming a kind of ring-cluster, resembling in shape the beautiful ring-nebula in lyra, as has been suggested by several astronomers. there is some direct evidence for this ring-form. professor newcomb in his recent book on _the stars_ gives a list of all stars of which the parallax is fairly well known. these are sixty-nine in number; and on arranging them in the order of the amount of their parallax, i find that no less than thirty-five of them have parallaxes between ". and ". of a second, thus showing that they constitute part of the dense central mass; while three others, from ". to ". , indicate those which are our closest companions at the present time, but still at an enormous distance. those which have parallaxes of less than the tenth and down to one-hundredth of a second are only thirty-one in all; but as they are spread over a sphere ten times the diameter, and therefore a thousand times the cubic content of the sphere containing those above one-tenth of a second, they ought to be immensely more numerous even if very much more thinly scattered. the interesting point, however, is, that till we get down to a parallax of ". , there are only three stars as yet measured, whereas those between ". and ". , an equal range of parallax, are twenty-six in number, and as these are scattered in all directions they indicate an almost vacant space followed by a moderately dense outer ring. in the enormous space between our cluster and the milky way, and also above and below its plane to the poles of the galaxy, stars appear to be very thinly scattered, perhaps more densely in the plane of the milky way than above and below it where the irresolvable nebulæ are so numerous; and there may not improbably be an almost vacant space beyond our cluster for a considerable distance, as has been supposed, but this cannot be known till some means are discovered of measuring parallaxes of from one-hundredth to one five-hundredth of a second. these diagrams also serve to indicate another point of considerable importance to the view here advocated. by placing the solar system towards the outer margin of the dense central portion of the solar cluster (which may very possibly include a large proportion of dark stars and thus be much more dense towards the centre than it appears to us), it may very well be supposed to revolve, with the other stars composing it, around the centre of gravity of the cluster, as the force of gravity towards that centre might be perhaps twenty or a hundred times greater than towards the very much less dense and more remote outer portions of the cluster. the sun, as indicated on the diagrams, is about thirty light-years from that centre, corresponding to a parallax of a little more than one-tenth of a second, and an actual distance of millions of millions of miles, equal to about , times the distance of the sun from neptune. yet we see that this position is so little removed from the exact centre of the whole stellar universe, that if any beneficial influences are due to that central position in regard to the galaxy, it will receive them perhaps to as full an extent as if situated at the actual centre. but if it is situated as here shown, there is no further difficulty as to its proper motion carrying it from one side to the other of the milky way in less time than has been required for the development of life upon the earth. and if the solar cluster is really sub-globular, and sufficiently condensed to serve as a centre of gravity for the whole of the stars of the cluster to revolve around, all the component stars which are not situated in the plane of its equator (and that of the milky way) must revolve obliquely at various angles up to an angle of °. these numerous diverging motions, together with the motions of the nearer stars outside the cluster, some of which may revolve round other centres of gravity made up largely of dark bodies, would perhaps sufficiently account for the apparent random motions of so many of the stars. uniform heat-supply due to central position we now come to a point of the greatest interest as regards the problem we are investigating. we have seen how great is the difference in the estimates of geologists and those of physicists as to the time that has elapsed during the whole development of life. but the position we have now found for the sun, in the outer portion of the central star-cluster, may afford a clue to this problem. what we require is, some mode of keeping up the sun's heat during the enormous geological periods in which we have evidence of a wonderful uniformity in the earth's temperature, and therefore in the sun's heat-emission. the great central ring-cluster with its condensed central mass, which presumably has been forming for a much longer period than our sun has been giving heat to the earth, must during all this time have been exerting a powerful attraction on the diffused matter in the spaces around it, now apparently almost void as compared with what they may have been. some scanty remnants of that matter we see in the numerous meteoric swarms which have been drawn into our system. a position towards the outside of this central aggregation of suns would evidently be very favourable for the growth by accretion of any considerable mass. the enormous distance apart of the outer components (the outer ring) of the cluster would allow a large amount of the inflowing meteoritic matter to escape them, and the larger suns situated near the surface of the inner dense cluster would draw to themselves the greater part of this matter.[ ] the various planets of our system were no doubt built up from a portion of the matter that flowed in near the plane of the ecliptic, but much of that which came from all other directions would be drawn towards the sun itself or to its neighbouring suns. some of this would fall directly into it; other masses coming from different directions and colliding with each other would have their motion checked, and thus again fall into the sun; and so long as the matter falling in were not in too large masses, the slow additions to the sun's bulk and increase of its heat would be sufficiently gradual to be in no way prejudicial to a planet at the earth's distance. the main point i wish to suggest here is, that by far the greater portion of the matter of the whole stellar universe has, either through gravitation or in combination with electrical forces, as suggested by mr. whittaker, become drawn together into the vast ring-formed system of the milky way, which is, presumably, slowly revolving, and has thus been checked in its original inflow toward the centre of mass of the stellar universe. it has also probably drawn towards itself the adjacent portions of the scattered material in the spaces around it in all directions. had the vast mass of matter postulated by lord kelvin acquired no motion of revolution, but have fallen continuously towards the centre of mass, the motions developed when the more distant bodies approached that centre would have been extremely rapid; while, as they must have fallen in from every direction, they would have become more and more densely aggregated, and collisions of the most catastrophic nature would frequently have occurred, and this would have rendered the central portion of the universe the _least_ stable and the _least_ fitted to develop life. but, under the conditions that actually prevail, the very reverse is the case. the quantity of matter remaining between our cluster and the milky way being comparatively small, the aggregation into suns has gone on more regularly and more slowly. the motions acquired by our sun and its neighbours have been rendered moderate by two causes: ( ) their nearness to the centre of the very slowly aggregating cluster where the motion due to gravitation is least in amount; and ( ) the slight differential attraction away from the centre by the milky way on the side nearest to us. again, this protective action of the milky way has been repeated, on a smaller scale, by the formation of the outer ring of the solar cluster, which has thus preserved the inner central cluster itself from a too abundant direct inflow of large masses of matter. but although the matter composing the outer portion of the original universe has been to a large extent aggregated into the vast system of the milky way, it seems probable, perhaps even certain, that some portion would escape its attractive forces and would pass through its numerous open spaces--indicated by the dark rifts, channels, and patches, as already described--and thus flow on unchecked towards the centre of mass of the whole system. the quantity of matter thus reaching the central cluster from the enormously remote spaces beyond the milky way might be very small in comparison with what was retained to build up that wonderful star-system; but it might yet be so large in total amount as to play an important part in the formation of the central group of suns. it would probably flow inwards almost continuously, and when it ultimately reached the solar cluster, it would have attained a very high velocity. if, therefore, it were widely diffused, and consisted of masses of small or moderate size as compared with planets or stars, it would furnish the energy requisite for bringing these slowly aggregating stars to the required intensity of heat for forming luminous suns. here, then, i think, we have found an adequate explanation of the very long-continued light and heat-emitting capacity of our sun, and probably of many others in about the same position in the solar cluster. these would at first gradually aggregate into considerable masses from the slowly moving diffused matter of the central portions of the original universe; but at a later period they would be reinforced by a constant and steady inrush of matter from its very outer regions, and therefore possessing such high velocities as to materially aid in producing and maintaining the requisite temperature of a sun such as ours, during the long periods demanded for continuous life-development. the enormous extension and mass of the original universe of diffused matter (as postulated by lord kelvin) is thus seen to be of the greatest importance as regards this ultimate product of evolution, because, without it, the comparatively slow-moving and cool central regions might not have been able to produce and maintain the requisite energy in the form of heat; while the aggregation of by far the larger portion of its matter in the great revolving ring of the galaxy was equally important, in order to prevent the too great and too rapid inflow of matter to these favoured regions. it appears, then, that if we admit as probable some such process of development as i have here indicated, we can dimly see the bearing of all the great features of the stellar universe upon the successful development of life. these are, its vast dimensions; the form it has acquired in the mighty ring of the milky way; and our position near to, but not exactly in, its centre. we know that the star-system _has_ acquired these forms, presumably from some simple and more diffused condition. we know that we _are_ situated near the centre of this vast system. we know that our sun _has_ emitted light and heat, almost uniformly, for periods incompatible with rapid aggregation and the equally rapid cooling which physicists consider inevitable. i have here suggested a mode of development which would lead to a very slow but continuous growth of the more central suns; to an excessively long period of nearly stationary heat-giving power; and lastly, an equally long period of very gradual cooling--a period the commencement of which our sun may have just entered upon. descending now to terrestrial physics, i have shown that, owing to the highly complex nature of the adjustments required to render a world habitable and to retain its habitability during the æons of time requisite for life-development, it is in the highest degree improbable that the required conditions and adaptations should have occurred in any other planets of any other suns, which _might_ occupy an equally favourable position with our own, and which were of the requisite size and heat-giving power. lastly, i submit that the whole of the evidence i have here brought together leads to the conclusion that our earth is almost certainly the only inhabited planet in our solar system; and, further, that there is no inconceivability--no improbability even--in the conception that, in order to produce a world that should be precisely adapted in every detail for the orderly development of organic life culminating in man, such a vast and complex universe as that which we know exists around us, may have been absolutely required. summary of argument as the last ten chapters of this volume embody a connected argument leading to the conclusion above stated, it may be useful to my readers to summarise rather fully the successive steps of this argument, the facts on which it rests, and the various subsidiary conclusions arrived at. ( ) one of the most important results of modern astronomy is to have established the unity of the vast stellar universe which we see around us. this rests upon a great mass of observations, which demonstrate the wonderful complexity in detail of the arrangement and distribution of stars and nebulæ, combined with a no less remarkable general symmetry, indicating throughout a single inter-dependent system, not a number of totally distinct systems so far apart as to have no physical relations with each other, as was once supposed. ( ) this view is supported by numerous converging lines of evidence, all tending to show that the stars are not infinite in number, as was once generally believed, and which view is even now advocated by some astronomers. the very remarkable calculations of lord kelvin, referred to in the early part of this chapter, give a further support to this view, since they show that if the stars extended much beyond those we see or can obtain direct knowledge of, and with no very great change in their average distance apart, then the force of gravitation towards the centre would have produced on the average more rapid motions than the stars generally possess. ( ) an overwhelming consensus of opinion among the best astronomers establishes the fact of our nearly central position in the stellar universe. they all agree that the milky way is nearly circular in form. they all agree that our sun is situated almost exactly in its medial plane. they all agree that our sun, although not situated at the exact centre of the galactic circle, is yet not very far from it, because there are no unmistakable signs of our being nearer to it at any one point and farther away from the opposite point. thus the nearly central position of our sun in the great star-system is almost universally admitted. on the question of the solar-cluster there is more difference of opinion; though here, again, all are agreed that there is such a cluster. its size, form, density, and exact position are somewhat uncertain, but i have, as far as possible, been guided by the best available evidence. if we adopt lord kelvin's general idea of the gradual condensation of an enormous diffused mass of matter towards its common centre of gravity, that centre would be approximately the centre of this cluster. also, as gravitational force at and near this centre would be comparatively small, the motions produced there would be slow, and collisions, being due only to differential motions, when they did occur would be very gentle. we might therefore expect many dark aggregations of matter here, which may explain why we do not find any special crowding of visible stars in the direction of this centre; while, as no star has a sensible disc, the dark stars if at great distances would hardly ever be seen to occult the bright ones. thus, it seems to me, the controlling force may be explained which has retained our sun in approximately the same orbit around the centre of gravity of this central cluster during the whole period of its existence as a sun and our existence as a planet; and has thus saved us from the possibility--perhaps even the certainty--of disastrous collisions or disruptive approaches to which suns, in or near the milky way, and to a less extent elsewhere, are or have been exposed. it seems quite probable that in that region of more rapid and less controlled motions and more crowded masses of matter, no star can remain in a nearly stable condition as regards temperature for sufficiently long periods to allow of a complete system of life-development on any planet it may possess. ( ) the various proofs are next stated that assure us of the almost complete uniformity of matter, and of material physical and chemical laws, throughout our universe. this i believe no one seriously disputes; and it is a point of the greatest importance when we come to consider the conditions required for the development and maintenance of life, since it assures us that very similar, if not identical, conditions must prevail wherever organic life is or can be developed. ( ) this leads us on to the consideration of the essential characteristics of the living organism, consisting as it does of some of the most abundant and most widely distributed of these material elements, and being always subject to the general laws of matter. the best authorities in physiology are quoted, as to the extreme complexity of the chemical compounds which constitute the physical basis for the manifestation of life; as to their great instability; their wonderful mobility combined with permanence of form and structure; and the altogether marvellous powers they possess of bringing about unique chemical transformations and of building up the most complicated structures from simple elements. i have endeavoured to put the broad phenomena of vegetable and animal life in a way that will enable my readers to form some faint conception of the intricacy, the delicacy, and the mystery of the myriad living forms they see everywhere around them. such a conception will enable them to realise how supremely grand is organic life, and to appreciate better, perhaps, the absolute necessity for the numerous, complex and delicate adaptations of inorganic nature, without which it is impossible for life either to exist now, or to have been developed during the immeasurable past. ( ) the general conditions which are absolutely essential for life thus manifested on our planet are then discussed, such as, solar light and heat; water universally distributed on the planet's surface and in the atmosphere; an atmosphere of sufficient density, and composed of the several gases from which alone protoplasm can be formed; some alternations of light and darkness, and a few others. ( ) having treated these conditions broadly, and explained why they are important and even indispensable for life, we next proceed to show how they are fulfilled upon the earth, and how numerous, how complex, and often how exact are the adjustments needed to bring them about, and maintain them almost unchanged throughout the vast æons of time occupied in the development of life. two chapters are devoted to this subject; and it is believed that they contain facts that will be new to many of my readers. the combinations of causes which lead to this result are so varied, and in several cases dependent on such exceptional peculiarities of physical constitution, that it seems in the highest degree improbable that they can _all_ be found again combined either in the solar system or even in the stellar universe. it will be well here just to enumerate these conditions, which are all essential within more or less narrow limits:-- distance of planet from the sun. mass of planet. obliquity of its ecliptic. amount of water as compared with land. surface distribution of land and water. permanence of this distribution, dependent probably on the unique origin of our moon. an atmosphere of sufficient density, and of suitable component gases. an adequate amount of dust in the atmosphere. atmospheric electricity. many of these act and react on each other, and lead to results of great complexity. ( ) passing on to other planets of the solar system, it is shown that none of them combine all the complex conditions which are found to work harmoniously together on the earth; while in most cases there is some one defect which alone removes them from the category of possible life-producing and life-supporting planets. among these are the small size and mass of mars, being such that it cannot retain aqueous vapour; and the fact that venus rotates on its axis in the same time as it takes to revolve round the sun. neither of these facts was known when proctor wrote upon the question of the habitability of the planets. all the other planets are now given up--and were given up by proctor himself--as possible life-bearers in their present stage; but he and others have held that, if not suitable now, some of them may have been the scene of life-development in the past, while others will be so in the future. in order to show the futility of this supposition, the problem of the duration of the sun as a stable heat-giver is discussed; and it is shown that it is only by reducing the periods claimed by geologists and biologists for life-development upon the earth, and by extending the time allowed by physicists to its utmost limits, that the two claims can be harmonised. it follows that the whole period of the sun's duration as a light and heat-giver has been required for the development of life upon the earth; and that it is only upon planets whose phases of development synchronise with that of the earth that the evolution of life is possible. for those whose material evolution has gone on quicker or slower, there has not been, or will not be, time enough for the development of life. ( ) the problem of the stars as possibly having life-supporting planets is next dealt with, and reasons are given why in only a minute portion of the whole is this possible. even in that minute portion, reduced probably to a few of the component suns of the solar-cluster, a large proportion seems likely to be ruled out by being close binary systems, and another large portion by being in process of aggregation. in those remaining, whether they may be reckoned by tens or by hundreds we cannot say, the chances against the same complex combination of conditions as those which we find on the earth occurring on any planet of any other sun are enormously great. ( ) i then refer, briefly, to some recent measurements of star-radiation, and suggest that they may thus possibly have important effects on the development of vegetable and animal life; and, finally, i discuss the problem of the stability of the stellar universe and the special advantage we derive from our central position, suggested by some of the latest researches of our great mathematician and physicist--lord kelvin. conclusions having thus brought together the whole of the available evidence bearing upon the questions treated in this volume, i claim that certain definite conclusions have been reached and proved, and that certain other conclusions have enormous probabilities in their favour. the conclusions reached by modern astronomers are: ( ) that the stellar universe forms one connected whole; and, though of enormous extent, is yet finite, and its extent determinable. ( ) that the solar system is situated in the plane of the milky way, and not far removed from the centre of that plane. the earth is therefore nearly in the centre of the stellar universe. ( ) that this universe consists throughout of the same kinds of matter, and is subjected to the same physical and chemical laws. the conclusions which i claim to have shown to have enormous probabilities in their favour are-- ( ) that no other planet in the solar system than our earth is inhabited or habitable. ( ) that the probabilities are almost as great against any other sun possessing inhabited planets. ( ) that the nearly central position of our sun is probably a permanent one, and has been specially favourable, perhaps absolutely essential, to life-development on the earth. * * * * * these latter conclusions depend upon the combination of a large number of special conditions, each of which must be in definite relation to many of the others, and must all have persisted simultaneously during enormous periods of time. the weight to be given to this kind of reasoning depends upon a full and fair consideration of the _whole_ evidence as i have endeavoured to present it in the last seven chapters of this book. to this evidence i appeal. * * * * * this completes my work as a connected argument, founded wholly on the facts and principles accumulated by modern science; and it leads, if my facts are substantially correct and my reasoning sound, to one great and definite conclusion--that man, the culmination of conscious organic life, has been developed here only in the whole vast material universe we see around us. i claim that this is the logical outcome of the evidence, if we consider and weigh this evidence without any prepossessions whatever. i maintain that it is a question as to which we have no right to form _a priori_ opinions not founded upon evidence. and evidence opposed to this conclusion, or even as to its improbability, we have absolutely none whatever. but, if we admit the conclusion, nothing that need alarm either the scientific or the religious mind necessarily follows, because it can be explained or accounted for in either of two distinct ways. one considerable body, including probably the majority of men of science, will admit that the evidence does apparently lead to this conclusion, but will explain it as due to a fortunate coincidence. there might have been a hundred or a thousand life-bearing planets, had the course of evolution of the universe been a little different, or there might have been none at all. they would probably add, that, as life and man _have_ been produced, that shows that their production was possible; and therefore, if not now then at some other time, if not here then in some other planet of some other sun, we should be sure to have come into existence; or if not precisely the same as we are, then something a little better or a little worse. the other body, and probably much the largest, would be represented by those who, holding that mind is essentially superior to matter and distinct from it, cannot believe that life, consciousness, mind, are products of matter. they hold that the marvellous complexity of forces which appear to control matter, if not actually to constitute it, are and must be mind-products; and when they see life and mind apparently rising out of matter and giving to its myriad forms an added complexity and unfathomable mystery, they see in this development an additional proof of the supremacy of mind. such persons would be inclined to the belief of the great eighteenth century scholar, dr. bentley, that the soul of one virtuous man is of greater worth and excellency than the sun and all his planets and all the stars in the heavens; and when they are shown that there are strong reasons for thinking that man _is_ the unique and supreme product of this vast universe, they will see no difficulty in going a little further, and believing that the universe was actually brought into existence for this very purpose. with infinite space around us and infinite time before and behind us, there is no incongruity in this conception. a universe as large as ours for the purpose of bringing into existence many myriads of living, intellectual, moral, and spiritual beings, with unlimited possibilities of life and happiness, is surely not _more_ out of proportion than is the complex machinery, the life-long labour, the ingenuity and invention which we have bestowed upon the production of the humble, the trivial, _pin_. neither is the apparent waste of energy so great in such a universe, comparatively, as the millions of acorns, produced during its life by an oak, every one of which might grow to be a tree, but of which only _one_ does actually, after several hundred years, produce the _one_ tree which is to replace the parent. and if it is said that the acorns are food for bird and beast, yet the spores of ferns and the seeds of orchids are not so, and countless millions of these go to waste for every one which reproduces the parent form. and all through the animal world, especially among the lower types, the same thing is seen. for the great majority of these entities _we_ can see no use whatever, either of the enormous variety of the species, or the vast hordes of individuals. of beetles alone there are at least a hundred thousand distinct species now living, while in some parts of sub-arctic america mosquitoes are sometimes so excessively abundant that they obscure the sun. and when we think of the myriads that have existed through the vast ages of geological time, the mind reels under the immensity of, to us, apparently useless life. all nature tells us the same strange, mysterious story, of the exuberance of life, of endless variety, of unimaginable quantity. all this life upon our earth has led up to and culminated in that of man. it has been, i believe, a common and not unpopular idea that during the whole process of the rise and growth and extinction of past forms, the earth has been preparing for the ultimate--man. much of the wealth and luxuriance of living things, the infinite variety of form and structure, the exquisite grace and beauty in bird and insect, in foliage and flower, may have been mere by-products of the grand mechanism we call nature--the one and only method of developing humanity. and is it not in perfect harmony with this grandeur of design (if it be design), this vastness of scale, this marvellous process of development through all the ages, that the material universe needed to produce this cradle of organic life, and of a being destined to a higher and a permanent existence, should be on a corresponding scale of vastness, of complexity, of beauty? even if there were no such evidence as i have here adduced for the unique position and the exceptional characteristics which distinguish the earth, the old idea that all the planets were inhabited, and that all the stars existed for the sake of other planets, which planets existed to develop life, would, in the light of our present knowledge, seem utterly improbable and incredible. it would introduce monotony into a universe whose grand character and teaching is endless diversity. it would imply that to produce the living soul in the marvellous and glorious body of man--man with his faculties, his aspirations, his powers for good and evil--that this was an easy matter which could be brought about anywhere, in any world. it would imply that man is an animal and nothing more, is of no importance in the universe, needed no great preparations for his advent, only, perhaps, a second-rate demon, and a third or fourth-rate earth. looking at the long and slow and complex growth of nature that preceded his appearance, the immensity of the stellar universe with its thousand million suns, and the vast æons of time during which it has been developing--all these seem only the appropriate and harmonious surroundings, the necessary supply of material, the sufficiently spacious workshop for the production of that planet which was to produce first, the organic world, and then, man. in one of his finest passages our great world-poet gives us _his_ conception of the grandeur of human nature--'what a piece of work is man! how noble in reason! how infinite in faculty! in form and moving, how express and admirable! in action how like an angel! in apprehension how like a god!' and for the development of such a being what is a universe such as ours? however vast it may seem to our faculties, it is as a mere nothing in the ocean of the infinite. in infinite space there may be infinite universes, but i hardly think they would be all universes of matter. that would indeed be a low conception of infinite power! here, on earth, we see millions of distinct species of animals, millions of different species of plants, and each and every species consisting often of many millions of individuals, no two individuals exactly alike; and when we turn to the heavens, no two planets, no two satellites alike; and outside our system we see the same law prevailing--no two stars, no two clusters, no two nebulæ alike. why then should there be other universes of the _same_ matter and subject to the _same_ laws--as is implied by the conception that the stars are infinite in number, and extend through infinite space? of course there may be, and probably are, other universes, perhaps of other kinds of matter and subject to other laws, perhaps more like our conceptions of the ether, perhaps wholly non-material, and what we can only conceive of as spiritual. but, unless these universes, even though each of them were a million times vaster than our stellar universe, were also infinite in number, they could not fill infinite space, which would extend on all sides beyond them, so that even a million million such universes would shrink to imperceptibility when compared with the vast beyond! of infinity in any of its aspects we can really know nothing, but that it exists and is inconceivable. it is a thought that oppresses and overwhelms. yet many speak of it glibly as if they _knew_ what it contains, and even use that assumed knowledge as an argument against views that are unacceptable to themselves. to me its existence is absolute but unthinkable--that way madness lies. 'o night! o stars, too rudely jars the finite with the infinite!' i will conclude with one of the finest passages relating to the infinite that i am acquainted with, from the pen of the late r.a. proctor: 'inconceivable, doubtless, are these infinities of time and space, of matter, of motion, and of life. inconceivable that the whole universe can be for all time the scene of the operation of infinite power, omnipresent, all-knowing. utterly incomprehensible how infinite purpose can be associated with endless material evolution. but it is no new thought, no modern discovery, that we are thus utterly powerless to conceive or comprehend the idea of an infinite being, almighty, all-knowing, omnipresent, and eternal, of whose inscrutable purpose the material universe is the unexplained manifestation. science is in presence of the old, old mystery; the old, old questions are asked of her--"canst thou by searching find out god? canst thou find out the almighty unto perfection? it is as high as heaven; what canst thou do? deeper than hell; what canst thou know?" and science answers these questions as they were answered of old--"as touching the almighty we cannot find him out."' * * * * * the following beautiful lines--among the latest products of tennyson's genius--so completely harmonise with the subject-matter of the present volume, that no apology is needed for quoting them here:-- (_the question_) will my tiny spark of being wholly vanish in your deeps and heights? must my day be dark by reason, o ye heavens, of your boundless nights, rush of suns and roll of systems, and your fiery clash of meteorites? (_the answer_) 'spirit, nearing yon dark portal at the limit of thy human state, fear not thou the hidden purpose of that power which alone is great, nor the myriad world, his shadow, nor the silent opener of the gate.' footnote: [ ] since writing this chapter i have seen a paper by luigi d'auria dealing mathematically with 'stellar motion, etc.,' and am pleased to see that, from quite different considerations, he has found it necessary to place the solar system at a distance from the centre not very much more remote than the position i have given it. he says: 'we have good reasons to suppose that the solar system is rather near the centre of the milky way, and as this centre would, according to our hypothesis, coincide with the centre of the universe, the distance of light years assumed is not too great, nor can it be very much smaller.'--_journal of the franklin institute_, march . index adrianus tollius on stone axes, . air criminally poisoned by us, . albedo explained, . algol and its companion, ; 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on evolution of stars, ; on milky way, ; on position of solar system, . luigi d'auria on stellar motion, . m'kendrick, prof., on germinal vesicle, . magnetism and sun-spots, . man, shakespeare on, . mars, has no water, ; excessive temperatures on, . matter of universe uniform, . maunder on dark stars, . maxwell hall, mr., on star-motions, . measurement of star-distances, ; difficulty of, . mercury not habitable, . meteorites, elements in, ; not primitive bodies, . meteoritic hypothesis, ; proctor on, ; explains nebulæ, ; dr. roberts on, . milky way, the, ; form of, , ; description of, ; telescopic view of, ; stars in relation to, ; mr. gore on, ; density of stars in, ; clusters and nebulæ in relation to, ; probable distance of, ; forms a great circle, , ; prof. newcomb on, ; probably no life in, ; diagrams of, ; revolution of, important to us, . million, how to appreciate a, . minchin, g.m., on radiation from stars, . monck, mr. w.h.s., on non-infinity of stars, ; on uncertainty of sun's motion, . moon, why no atmosphere, . moon's supposed origin, . motion, in line of sight, . motions, imperceptible, . nebulæ, with gaseous spectra, ; in relation to galaxy, ; distribution of, ; many forms of, ; gaseous, ; meteoritic theory of, ; planetary and annular, ; dr. roberts on spiral, , ; chamberlin on origin of, . nebular hypothesis, , ; objection to, . newcomb, prof. s., on star distribution, ; on parallax of stars, ; on stability of star clusters, ; on scarcity of single stars, ; on limits of star system, ; on milky way, , ; on solar cluster, ; on star velocities, ; on average small mass of stars, ; on star-motions, . newton, sir isaac, on sun's habitability, . nichols, e.f., on heat of stars, . nitrogen, its importance to life, . non-habitability of great planets, . ocean and land, diagram of, . ---- basins, permanence of, . ---- ---- symmetry of, . ---- depths, how produced, . oceans, effect of, on temperature, ; curious relations of, . organic products, diversity of, . photographic astronomy, ; measures of star-distances, . photosphere, the, . physicists on sun's duration, . pickering's measurements of algol, . planets, supposed habitability of, , ; the great, uninhabitable, ; internal heat of great, ; a last argument for habitability of, ; have probably no life, . planets' motions first explained, ; mass and atmosphere, . pleiades, number of stars in, ; a drifting cluster, . plurality of worlds, early writers on, ; proctor on, . posidonius measures the earth, . pritchard's photographic measures of star-distance, . proctor, r.a., on other worlds, ; on form of galaxy, ; on herschel's views, ; on stellar universe, ; on meteoritic theory, ; on infinities, ; on star-drift, ; on life under varied conditions, ; on infinity, . proctor's _old and new astronomy_, ; chart of stars, . prominences of sun, . proteids, formation of, ; prof. haliburton on, . protoplasm, complexity of, ; a mechanism, ; sensibility of, to heat, . ptolemaic system of the heavens, . radial motion, . radiation from stars, . rain in the carboniferous age, ; dependent on dust, . ramsay, prof., on geological climates, . ranyard, on star-discs, ; on infinite universe, ; on mass of orion nebula, . religious bearing of my conclusions, . reproduction, marvel of, . reversing layer of sun, . roberts, a.w., on birth of double stars, . ---- dr. i., on limits of star-system, ; on spiral nebulæ, ; on meteoritic theory, ; photographs of nebulæ, , . roche limit explained, , . sanderson, prof. burdon, on living matter, . scientific and agnostic opinion on my conclusions, . secchi's classification of stars, . single stars perhaps rare, . solar apex, position of, . solar cluster, the, ; diagram showing, ; evidence for, ; importance to us, - , . solar system, position of, . sorby on constitution of meteorites, . spectra, varieties of, ; of elements, changes in, . spectroscopic binaries, abundance of, ; great numbers of, . spectrum analysis, discovery of, . spencer, h., on status of nebulæ, . spiral nebulæ, origin of, . stars, proved to be suns, ; invisible, ; classification of, ; spectroscopic double, ; distribution of the, ; number of visible, ; description of milky way, ; in relation to milky way, ; distances of, ; measurement of distance of, ; mass of binary, ; evolution of double, ; spectroscopic double, ; clusters of, ; evolution of the, ; classification of, ; the hottest, ; when cooling give more heat, ; cycle of evolution and decay, ; supposed infinite number of, ; not infinite, ; law of diminishing numbers of, ; systematic motions of, ; in relation to life, , ; possible use of their emanations, . star-clusters and variables, . star-density, diagram of, . star-drift, proctor on, . starlight, electrical measure of, ; possible uses of, . star-motions, prof. newcomb on, . star-system, limited, ; stability of, ; supposed primitive form of, . stellar motion, luigi d'auria on, . ---- universe, shape of, ; unity of, ; evolution of, ; diagrams of, . stoney, dr., on atmospheres and gravity, . sun, a typical star, ; brightness of, ; heat of, ; surface of, ; surroundings of, - ; corona of, ; colour of, ; elements in, . sun's distance, measure of, . ---- heat, supposed limits of, . ---- life, all required to develop earth-life, . ---- motion through space, , . ---- ---- uncertain, . sun-spots, nature of, . symmetry of oceans, cause of, . temperature, essential for life, ; equalised by water, ; as regards life on planets, . tennyson on man and the universe, . uniformity of matter, . unity of stellar universe, . universe of stars, how its form has affected our sun and earth, . universe not disproportionate if man is its sole product, . venus, radial motions of, ; diagram of transit of, ; life barely possible on, ; adverse climatic conditions of, . water, an essential for life, ; its amount and distribution, ; an equaliser of temperature, . wave-lengths, how measured, . whewell, on plurality of worlds, , ; on man as the highest product of the universe, . whittaker, mr. e.t., on gravitative and electro-dynamical forces, . winds, importance of, to life, . zodiacal light, . printed by t. and a. constable, printers to his majesty, at the edinburgh university press [illustration: the nebulÆ and clusters of the northern heavens. drawn upon an equal surface projection from dr. dreyer's catalogue of the milky way from a dr. boeddicker's drawing by sidney waters] [illustration: the nebulÆ and clusters of the southern heavens. drawn upon an equal surface projection from dr. dreyer's catalogue of the milky way from uranometria argentina by sidney waters] _works by the same author._ travels on the amazon and rio-negro. palm-trees of the amazon, and their uses (_out of print_). the malay archipelago. natural selection and tropical nature. the geographical distribution of animals. _ vols._ island life, or insular faunas and floras. darwinism. australasia. _ vols._ bad times--causes of depression of trade (_out of print_). land nationalisation, its necessity and its aims. vaccination a delusion. miracles and modern spiritualism. the wonderful century (_new and illustrated edition_). the wonderful century reader. studies scientific and social. * * * * * transcriber's notes: obvious punctuation and spelling errors repaired. inconsistent hyphenation has been repaired. the oe and ae ligatures in the text has been left as it appears in the original book. italic text is denoted by _underscores_ and bold text by =equal signs=. in ambiguous cases, the text has been left as it appears in the original book. in particular, many mismatched quotation marks have not been changed. the remaining corrections made: page v, "fort-nightly" changed to "fortnightly". page , "terrestial" changed to "terrestrial". generously made available by the internet archive/american libraries.) transcriber's note the punctuation and spelling from the original text have been faithfully preserved. only obvious typographical errors have been corrected. the advertisement from the beginning of the book has been joined with the other advertisements near the end of the book. greek words are spelled out and represented as [word]. greek letters are represented as [a] "for alpha". astronomy of to-day [illustration: the total eclipse of the sun of august th, . the corona; from a water-colour sketch, made at burgos, in spain, during the total phase, by the french artist, mdlle. andrÉe moch.] astronomy of to-day _a popular introduction in non-technical language_ by cecil g. dolmage, m.a., ll.d., d.c.l. fellow of the royal astronomical society; member of the british astronomical association; member of the astronomical society of the pacific; membre de la société astronomique de france; membre de la société belge d'astronomie with a frontispiece in colour and illustrations & diagrams _third edition_ london seeley and co. limited great russell street preface the object of this book is to give an account of the science of astronomy, as it is known at the present day, in a manner acceptable to the _general reader_. it is too often supposed that it is impossible to acquire any useful knowledge of astronomy without much laborious study, and without adventuring into quite a new world of thought. the reasoning applied to the study of the celestial orbs is, however, of no different order from that which is employed in the affairs of everyday life. the science of mathematics is perhaps responsible for the idea that some kind of difference does exist; but mathematical processes are, in effect, no more than ordinary logic in concentrated form, the _shorthand of reasoning_, so to speak. i have attempted in the following pages to take the main facts and theories of astronomy out of those mathematical forms which repel the general reader, and to present them in the _ordinary language of our workaday world_. the few diagrams introduced are altogether supplementary, and are not connected with the text by any wearying cross-references. each diagram is complete in itself, being intended to serve as a pictorial aid, in case the wording of the text should not have perfectly conveyed the desired meaning. the full page illustrations are also described as adequately as possible at the foot of each. as to the coloured frontispiece, this must be placed in a category by itself. it is the work of the _artist_ as distinct from the scientist. the book itself contains incidentally a good deal of matter concerned with the astronomy of the past, the introduction of which has been found necessary in order to make clearer the astronomy of our time. it would be quite impossible for me to enumerate here the many sources from which information has been drawn. but i acknowledge my especial indebtedness to professor f.r. moulton's _introduction to astronomy_ (macmillan, ), to the works on eclipses of the late rev. s.j. johnson and of mr. w.t. lynn, and to the excellent _journals of the british astronomical association_. further, for those grand questions concerned with the stellar universe at large, i owe a very deep debt to the writings of the famous american astronomer, professor simon newcomb, and of our own countryman, mr. john ellard gore; to the latter of whom i am under an additional obligation for much valuable information privately rendered. in my search for suitable illustrations, i have been greatly aided by the kindly advice of mr. w. h. wesley, the assistant secretary of the royal astronomical society. to those who have been so good as to permit me to reproduce pictures and photographs, i desire to record my best thanks as follows:--to the french artist, mdlle. andrée moch; to the astronomer royal; to sir david gill, k.c.b., ll.d., f.r.s.; to the council of the royal astronomical society; to professor e.b. frost, director of the yerkes observatory; to m.p. puiseux, of the paris observatory; to dr. max wolf, of heidelberg; to professor percival lowell; to the rev. theodore e.r. phillips, m.a., f.r.a.s.; to mr. w.h. wesley; to the warner and swasey co., of cleveland, ohio, u.s.a.; to the publishers of _knowledge_, and to messrs. sampson, low & co. for permission to reproduce the beautiful photograph of the spiral nebula in canes venatici (plate xxii.), i am indebted to the distinguished astronomer, the late dr. w.e. wilson, d.sc., f.r.s., whose untimely death, i regret to state, occurred in the early part of this year. finally, my best thanks are due to mr. john ellard gore, f.r.a.s., m.r.i.a., to mr. w.h. wesley, and to mr. john butler burke, m.a., of cambridge, for their kindness in reading the proof-sheets. cecil g. dolmage. london, s.w., _august , ._ prefatory note to the second edition the author of this book lived only long enough to hear of the favour with which it had been received, and to make a few corrections in view of the second edition which it has so soon reached. _december ._ contents chapter i page the ancient view chapter ii the modern view chapter iii the solar system chapter iv celestial mechanism chapter v celestial distances chapter vi celestial measurement chapter vii eclipses and kindred phenomena chapter viii famous eclipses of the sun chapter ix famous eclipses of the moon chapter x the growth of observation chapter xi spectrum analysis chapter xii the sun chapter xiii the sun--_continued_ chapter xiv the inferior planets chapter xv the earth chapter xvi the moon chapter xvii the superior planets chapter xviii the superior planets--_continued_ chapter xix comets chapter xx remarkable comets chapter xxi meteors or shooting stars chapter xxii the stars chapter xxiii the stars--_continued_ chapter xxiv systems of stars chapter xxv the stellar universe chapter xxvi the stellar universe--_continued_ chapter xxvii the beginning of things chapter xxviii the end of things index list of illustrations list of plates plate the total eclipse of the sun of august , _frontispiece_ i. the total eclipse of the sun of may , _to face page_ ii. great telescope of hevelius " " iii. a tubeless, or "aerial" telescope " " iv. the great yerkes telescope " " v. the sun, showing several groups of spots " " vi. photograph of a sunspot " " vii. forms of the solar corona at the epochs of sunspot maximum and sunspot minimum respectively. (a) the total eclipse of the sun of december , . (b) the total eclipse of the sun of may , " " viii. the moon _to face page_ ix. map of the moon, showing the principal "craters," mountain ranges and "seas" " " x. one of the most interesting regions on the moon " " xi. the moon (showing systems of "rays") " " xii. a map of the planet mars " " xiii. minor planet trails " " xiv. the planet jupiter " " xv. the planet saturn " " xvi. early representations of saturn " " xvii. donati's comet " " xviii. daniel's comet of " " xix. the sky around the north pole " " xx. orion and his neighbours " " xxi. the great globular cluster in the southern constellation of centaurus " " xxii. spiral nebula in the constellation of canes venatici " " xxiii. the great nebula in the constellation of andromeda _to face page_ xxiv. the great nebula in the constellation of orion " " list of diagrams fig. page . the ptolemaic idea of the universe . the copernican theory of the solar system . total and partial eclipses of the moon . total and partial eclipses of the sun . "baily's beads" . map of the world on mercator's projection, showing a portion of the progress of the total solar eclipse of august , , across the surface of the earth . the "ring with wings" . the various types of telescope . the solar spectrum . a section through the sun, showing how the prominences rise from the chromosphere . orbit and phases of an inferior planet . the "black drop" . summer and winter . orbit and phases of the moon . the rotation of the moon on her axis . laplace's "perennial full moon" . illustrating the author's explanation of the apparent enlargement of celestial objects . showing how the tail of a comet is directed away from the sun . the comet of , as represented in the bayeux tapestry . passage of the earth through the thickest portion of a meteor swarm astronomy of to-day chapter i the ancient view it is never safe, as we know, to judge by appearances, and this is perhaps more true of astronomy than of anything else. for instance, the idea which one would most naturally form of the earth and heaven is that the solid earth on which we live and move extends to a great distance in every direction, and that the heaven is an immense dome upon the inner surface of which the stars are fixed. such must needs have been the idea of the universe held by men in the earliest times. in their view the earth was of paramount importance. the sun and moon were mere lamps for the day and for the night; and these, if not gods themselves, were at any rate under the charge of special deities, whose task it was to guide their motions across the vaulted sky. little by little, however, this simple estimate of nature began to be overturned. difficult problems agitated the human mind. on what, for instance, did the solid earth rest, and what prevented the vaulted heaven from falling in upon men and crushing them out of existence? fantastic myths sprang from the vain attempts to solve these riddles. the hindoos, for example, imagined the earth as supported by four elephants which stood upon the back of a gigantic tortoise, which, in its turn, floated on the surface of an elemental ocean. the early western civilisations conceived the fable of the titan atlas, who, as a punishment for revolt against the olympian gods, was condemned to hold up the expanse of sky for ever and ever. later on glimmerings of the true light began to break in upon men. the greek philosophers, who busied themselves much with such matters, gradually became convinced that the earth was spherical in shape, that is to say, round like a ball. in this opinion we now know that they were right; but in their other important belief, viz. that the earth was placed at the centre of all things, they were indeed very far from the truth. by the second century of the christian era, the ideas of the early philosophers had become hardened into a definite theory, which, though it appears very incorrect to us to-day, nevertheless demands exceptional notice from the fact that it was everywhere accepted as the true explanation until so late as some four centuries ago. this theory of the universe is known by the name of the ptolemaic system, because it was first set forth in definite terms by one of the most famous of the astronomers of antiquity, claudius ptolemæus pelusinensis ( - a.d.), better known as ptolemy of alexandria. in his system the earth occupied the centre; while around it circled in order outwards the moon, the planets mercury and venus, the sun, and then the planets mars, jupiter, and saturn. beyond these again revolved the background of the heaven, upon which it was believed that the stars were fixed-- "stellis ardentibus aptum," as virgil puts it (see fig. ). [illustration: fig. .--the ptolemaic idea of the universe.] the ptolemaic system persisted unshaken for about fourteen hundred years after the death of its author. clearly men were flattered by the notion that their earth was the most important body in nature, that it stood still at the centre of the universe, and was the pivot upon which all things revolved. chapter ii the modern view it is still well under four hundred years since the modern, or copernican, theory of the universe supplanted the ptolemaic, which had held sway during so many centuries. in this new theory, propounded towards the middle of the sixteenth century by nicholas copernicus ( - ), a prussian astronomer, the earth was dethroned from its central position and considered merely as one of a number of planetary bodies which revolve around the sun. as it is not a part of our purpose to follow in detail the history of the science, it seems advisable to begin by stating in a broad fashion the conception of the universe as accepted and believed in to-day. the sun, the most important of the celestial bodies so far as we are concerned, occupies the central position; not, however, in the whole universe, but only in that limited portion which is known as the solar system. around it, in the following order outwards, circle the planets mercury, venus, the earth, mars, jupiter, saturn, uranus, and neptune (see fig. , p. ). at an immense distance beyond the solar system, and scattered irregularly through the depth of space, lie the stars. the two first-mentioned members of the solar system, mercury and venus, are known as the inferior planets; and in their courses about the sun, they always keep well inside the path along which our earth moves. the remaining members (exclusive of the earth) are called superior planets, and their paths lie all outside that of the earth. [illustration: fig. .--the copernican theory of the solar system.] the five planets, mercury, venus, mars, jupiter, and saturn, have been known from all antiquity. nothing then can bring home to us more strongly the immense advance which has taken place in astronomy during modern times than the fact that it is only years since observation of the skies first added a planet to that time-honoured number. it was indeed on the th of march , while engaged in observing the constellation of the twins, that the justly famous sir william herschel caught sight of an object which he did not recognise as having met with before. he at first took it for a comet, but observations of its movements during a few days showed it to be a planet. this body, which the power of the telescope alone had thus shown to belong to the solar family, has since become known to science under the name of uranus. by its discovery the hitherto accepted limits of the solar system were at once pushed out to twice their former extent, and the hope naturally arose that other planets would quickly reveal themselves in the immensities beyond. for a number of years prior to herschel's great discovery, it had been noticed that the distances at which the then known planets circulated appeared to be arranged in a somewhat orderly progression outwards from the sun. this seeming plan, known to astronomers by the name of bode's law, was closely confirmed by the distance of the new planet uranus. there still lay, however, a broad gap between the planets mars and jupiter. had another planet indeed circuited there, the solar system would have presented an appearance of almost perfect order. but the void between mars and jupiter was unfilled; the space in which one would reasonably expect to find another planet circling was unaccountably empty. on the first day of the nineteenth century the mystery was however explained, a body being discovered[ ] which revolved in the space that had hitherto been considered planetless. but it was a tiny globe hardly worthy of the name of planet. in the following year a second body was discovered revolving in the same space; but it was even smaller in size than the first. during the ensuing five years two more of these little planets were discovered. then came a pause, no more such bodies being added to the system until half-way through the century, when suddenly the discovery of these so-called "minor planets" began anew. since then additions to this portion of our system have rained thick and fast. the small bodies have received the name of asteroids or planetoids; and up to the present time some six hundred of them are known to exist, all revolving in the previously unfilled space between mars and jupiter. in the year the outer boundary of the solar system was again extended by the discovery that a great planet circulated beyond uranus. the new body, which received the name of neptune, was brought to light as the result of calculations made at the same time, though quite independently, by the cambridge mathematician adams, and the french astronomer le verrier. the discovery of neptune differed, however, from that of uranus in the following respect. uranus was found merely in the course of ordinary telescopic survey of the heavens. the position of neptune, on the other hand, was predicted as the result of rigorous mathematical investigations undertaken with the object of fixing the position of an unseen and still more distant body, the attraction of which, in passing by, was disturbing the position of uranus in its revolution around the sun. adams actually completed his investigation first; but a delay at cambridge in examining that portion of the sky, where he announced that the body ought just then to be, allowed france to snatch the honour of discovery, and the new planet was found by the observer galle at berlin, very near the place in the heavens which le verrier had mathematically predicted for it. nearly fifty years later, that is to say, in our own time, another important planetary discovery was made. one of the recent additions to the numerous and constantly increasing family of the asteroids, a tiny body brought to light in , turned out after all not to be circulating in the customary space between mars and jupiter, but actually in that between our earth and mars. this body is very small, not more than about twenty miles across. it has received the name of eros (the greek equivalent for cupid), in allusion to its insignificant size as compared with the other leading members of the system. this completes the list of the planets which, so far, have revealed themselves to us. whether others exist time alone will show. two or three have been suspected to revolve beyond the path of neptune; and it has even been asserted, on more than one occasion, that a planet circulates nearer to the sun than mercury. this supposed body, to which the name of "vulcan" was provisionally given, is said to have been "discovered" in by a french doctor named lescarbault, of orgères near orleans; but up to the present there has been no sufficient evidence of its existence. the reason why such uncertainty should exist upon this point is easy enough to understand, when we consider the overpowering glare which fills our atmosphere all around the sun's place in the sky. mercury, the nearest known planet to the sun, is for this reason always very difficult to see; and even when, in its course, it gets sufficiently far from the sun to be left for a short time above the horizon after sunset, it is by no means an easy object to observe on account of the mists which usually hang about low down near the earth. one opportunity, however, offers itself from time to time to solve the riddle of an "intra-mercurial" planet, that is to say, of a planet which circulates within the path followed by mercury. the opportunity in question is furnished by a total eclipse of the sun; for when, during an eclipse of that kind, the body of the moon for a few minutes entirely hides the sun's face, and the dazzling glare is thus completely cut off, astronomers are enabled to give an unimpeded, though all too hurried, search to the region close around. a goodly number of total eclipses of the sun have, however, come and gone since the days of lescarbault, and no planet, so far, has revealed itself in the intra-mercurial space. it seems, therefore, quite safe to affirm that no globe of sufficient size to be seen by means of our modern telescopes circulates nearer to the sun than the planet mercury. next in importance to the planets, as permanent members of the solar system, come the relatively small and secondary bodies known by the name of satellites. the name _satellite_ is derived from a latin word signifying _an attendant_; for the bodies so-called move along always in close proximity to their respective "primaries," as the planets which they accompany are technically termed. the satellites cannot be considered as allotted with any particular regularity among the various members of the system; several of the planets, for instance, having a goodly number of these bodies accompanying them, while others have but one or two, and some again have none at all. taking the planets in their order of distance outward from the sun, we find that neither mercury nor venus are provided with satellites; the earth has only one, viz. our neighbour the moon; while mars has but two tiny ones, so small indeed that one might imagine them to be merely asteroids, which had wandered out of their proper region and attached themselves to that planet. for the rest, so far as we at present know, jupiter possesses seven,[ ] saturn ten, uranus four, and neptune one. it is indeed possible, nay more, it is extremely probable, that the two last-named planets have a greater number of these secondary bodies revolving around them; but, unfortunately, the uranian and neptunian systems are at such immense distances from us, that even the magnificent telescopes of to-day can extract very little information concerning them. from the distribution of the satellites, the reader will notice that the planets relatively near to the sun are provided with few or none, while the more distant planets are richly endowed. the conclusion, therefore, seems to be that nearness to the sun is in some way unfavourable either to the production, or to the continued existence, of satellites. a planet and its satellites form a repetition of the solar system on a tiny scale. just as the planets revolve around the sun, so do these secondary bodies revolve around their primaries. when galileo, in , turned his newly invented telescope upon jupiter, he quickly recognised in the four circling moons which met his gaze, a miniature edition of the solar system. besides the planets and their satellites, there are two other classes of bodies which claim membership of the solar system. these are comets and meteors. comets differ from the bodies which we have just been describing in that they appear filmy and transparent, whereas the others are solid and opaque. again, the paths of the planets around the sun and of the satellites around their primaries are not actually circles; they are ovals, but their ovalness is not of a marked degree. the paths of comets on the other hand are usually _very_ oval; so that in their courses many of them pass out as far as the known limits of the solar system, and even far beyond. it should be mentioned that nowadays the tendency is to consider comets as permanent members of the system, though this was formerly not by any means an article of faith with astronomers. meteors are very small bodies, as a rule perhaps no larger than pebbles, which move about unseen in space, and of which we do not become aware until they arrive very close to the earth. they are then made visible to us for a moment or two in consequence of being heated to a white heat by the friction of rushing through the atmosphere, and are thus usually turned into ashes and vapour long before they reach the surface of our globe. though occasionally a meteoric body survives the fiery ordeal, and reaches the earth more or less in a solid state to bury itself deep in the soil, the majority of these celestial visitants constitute no source of danger whatever for us. any one who will take the trouble to gaze at the sky for a short time on a clear night, is fairly certain to be rewarded with the view of a meteor. the impression received is as if one of the stars had suddenly left its accustomed place, and dashed across the heavens, leaving in its course a trail of light. it is for this reason that meteors are popularly known under the name of "shooting stars." [ ] by the italian astronomer, piazzi, at palermo. [ ] probably eight. (see note, page .) chapter iii the solar system we have seen, in the course of the last chapter, that the solar system is composed as follows:--there is a central body, the sun, around which revolve along stated paths a number of important bodies known as planets. certain of these planets, in their courses, carry along in company still smaller bodies called satellites, which revolve around them. with regard, however, to the remaining members of the system, viz. the comets and the meteors, it is not advisable at this stage to add more to what has been said in the preceding chapter. for the time being, therefore, we will devote our attention merely to the sun, the planets, and the satellites. of what shape then are these bodies? of one shape, and that one alone which appears to characterise all solid objects in the celestial spaces: they are spherical, which means _round like a ball_. each of these spherical bodies rotates; that is to say, turns round and round, as a top does when it is spinning. this rotation is said to take place "upon an axis," a statement which may be explained as follows:--imagine a ball with a knitting-needle run right through its centre. then imagine this needle held pointing in one fixed direction while the ball is turned round and round. well, it is the same thing with the earth. as it journeys about the sun, it keeps turning round and round continually as if pivoted upon a mighty knitting needle transfixing it from north pole to south pole. in reality, however, there is no such material axis to regulate the constant direction of the rotation, just as there are no actual supports to uphold the earth itself in space. the causes which keep the celestial spheres poised, and which control their motions, are far more wonderful than any mechanical device. at this juncture it will be well to emphasise the sharp distinction between the terms _rotation_ and _revolution_. the term "rotation" is invariably used by astronomers to signify the motion which a celestial body has upon an axis; the term "revolution," on the other hand, is used for the movement of one celestial body around another. speaking of the earth, for instance, we say, that it _rotates_ on its axis, and that it _revolves_ around the sun. so far, nothing has been said about the sizes of the members of our system. is there any stock size, any pattern according to which they may be judged? none whatever! they vary enormously. very much the largest of all is the sun, which is several hundred times larger than all the planets and satellites of the system rolled together. next comes jupiter and afterwards the other planets in the following order of size:--saturn, uranus, neptune, the earth, venus, mars, and mercury. very much smaller than any of these are the asteroids, of which ceres, the largest, is less than miles in diameter. it is, by the way, a strange fact that the zone of asteroids should mark the separation of the small planets from the giant ones. the following table, giving roughly the various diameters of the sun and the principal planets in miles, will clearly illustrate the great discrepancy in size which prevails in the system. sun , miles mercury , " venus , " earth , " mars , " zone of asteroids jupiter , " saturn , " uranus[ ] , " neptune[ ] , " it does not seem possible to arrive at any generalisation from the above data, except it be to state that there is a continuous increase in size from mercury to the earth, and a similar decrease in size from jupiter outwards. were mars greater than the earth, the planets could then with truth be said to increase in size up to jupiter, and then to decrease. but the zone of asteroids, and the relative smallness of mars, negative any attempt to regard the dimensions of the planets as an orderly sequence. next with respect to relative distance from the sun, venus circulates nearly twice as far from it as mercury, the earth nearly three times as far, and mars nearly four times. after this, just as we found a sudden increase in size, so do we meet with a sudden increase in distance. jupiter, for instance, is about thirteen times as far as mercury, saturn about twenty-five times, uranus about forty-nine times, and neptune about seventy-seven. (see fig. , p. .) it will thus be seen how enormously the solar system was enlarged in extent by the discovery of the outermost planets. the finding of uranus plainly doubled its breadth; the finding of neptune made it more than half as broad again. nothing indeed can better show the import of these great discoveries than to take a pair of compasses and roughly set out the above relative paths in a series of concentric circles upon a large sheet of paper, and then to consider that the path of saturn was the supposed boundary of our solar system prior to the year . we have seen that the usual shape of celestial bodies themselves is spherical. of what form then are their paths, or _orbits_, as these are called? one might be inclined at a venture to answer "circular," but this is not the case. the orbits of the planets cannot be regarded as true circles. they are ovals, or, to speak in technical language, "ellipses." their ovalness or "ellipticity" is, however, in each case not by any means of the same degree. some orbits--for instance, that of the earth--differ only slightly from circles; while others--those of mars or mercury, for example--are markedly elliptic. the orbit of the tiny planet eros is, however, far and away the most elliptic of all, as we shall see when we come to deal with that little planet in detail. it has been stated that the sun and planets are always rotating. the various rates at which they do so will, however, be best appreciated by a comparison with the rate at which the earth itself rotates. but first of all, let us see what ground we have, if any, for asserting that the earth rotates at all? if we carefully watch the heavens we notice that the background of the sky, with all the brilliant objects which sparkle in it, appears to turn once round us in about twenty-four hours; and that the pivot upon which this movement takes place is situated somewhere near what is known to us as the _pole star_. this was one of the earliest facts noted with regard to the sky; and to the men of old it therefore seems as if the heavens and all therein were always revolving around the earth. it was natural enough for them to take this view, for they had not the slightest idea of the immense distance of the celestial bodies, and in the absence of any knowledge of the kind they were inclined to imagine them comparatively near. it was indeed only after the lapse of many centuries, when men had at last realised the enormous gulf which separated them from even the nearest object in the sky, that a more reasonable opinion began to prevail. it was then seen that this revolution of the heavens about the earth could be more easily and more satisfactorily explained by supposing a mere rotation of the solid earth about a fixed axis, pointed in the direction of the polar star. the probability of such a rotation on the part of the earth itself was further strengthened by the observations made with the telescope. when the surfaces of the sun and planets were carefully studied these bodies were seen to be rotating. this being the case, there could not surely be much hesitation in granting that the earth rotated also; particularly when it so simply explained the daily movement of the sky, and saved men from the almost inconceivable notion that the whole stupendous vaulted heaven was turning about them once in every twenty-four hours. if the sun be regularly observed through a telescope, it will gradually be gathered from the slow displacement of sunspots across its face, their disappearance at one edge and their reappearance again at the other edge, that it is rotating on an axis in a period of about twenty-six days. the movement, too, of various well-known markings on the surfaces of the planets mars, jupiter, and saturn proves to us that these bodies are rotating in periods, which are about twenty-four hours for the first, and about ten hours for each of the other two. with regard, however, to uranus and neptune there is much more uncertainty, as these planets are at such great distances that even our best telescopes give but a confused view of the markings which they display; still a period of rotation of from ten to twelve hours appears to be accepted for them. on the other hand the constant blaze of sunlight in the neighbourhood of mercury and venus equally hampers astronomers in this quest. the older telescopic observers considered that the rotation periods of these two planets were about the same as that of the earth; but of recent years the opinion has been gaining ground that they turn round on their axes in exactly the same time as they revolve about the sun. this question is, however, a very doubtful one, and will be again referred to later on; but, putting it on one side, it will be seen from what we have said above, that the rotation periods of the other planets of our system are usually about twenty-four hours, or under. the fact that the rotation period of the sun should run into _days_ need not seem extraordinary when one considers its enormous size. the periods taken by the various planets to revolve around the sun is the next point which has to be considered. here, too, it is well to start with the earth's period of revolution as the standard, and to see how the periods taken by the other planets compare with it. the earth takes about - / days to revolve around the sun. this period of time is known to us as a "year." the following table shows in days and years the periods taken by each of the other planets to make a complete revolution round the sun:-- mercury about days. venus " " mars " year and days. jupiter " years and days. saturn " years and days. uranus " years and days. neptune " years and days. from these periods we gather an important fact, namely, that the nearer a planet is to the sun the faster it revolves. compared with one of our years what a long time does an uranian, or neptunian, "year" seem? for instance, if a "year" had commenced in neptune about the middle of the reign of george ii., that "year" would be only just coming to a close; for the planet is but now arriving back to the position, with regard to the sun, which it then occupied. uranus, too, has only completed a little more than - / of its "years" since herschel discovered it. having accepted the fact that the planets are revolving around the sun, the next point to be inquired into is:--what are the positions of their orbits, or paths, relatively to each other? suppose, for instance, the various planetary orbits to be represented by a set of hoops of different sizes, placed one within the other, and the sun by a small ball in the middle of the whole; in what positions will these hoops have to be arranged so as to imitate exactly the true condition of things? first of all let us suppose the entire arrangement, ball and hoops, to be on one level, so to speak. this may be easily compassed by imagining the hoops as floating, one surrounding the other, with the ball in the middle of all, upon the surface of still water. such a set of objects would be described in astronomical parlance as being _in the same plane_. suppose, on the other hand, that some of these floating hoops are tilted with regard to the others, so that one half of a hoop rises out of the water and the other half consequently sinks beneath the surface. this indeed is the actual case with regard to the planetary orbits. they do not by any means lie all exactly in the same plane. each one of them is tilted, or _inclined_, a little with respect to the plane of the earth's orbit, which astronomers, for convenience, regard as the _level_ of the solar system. this tilting, or "inclination," is, in the larger planets, greatest for the orbit of mercury, least for that of uranus. mercury's orbit is inclined to that of the earth at an angle of about °, that of venus at a little over °, that of saturn - / °; while in those of mars, neptune, and jupiter the inclination is less than °. but greater than any of these is the inclination of the orbit of the tiny planet eros, viz. nearly °. the systems of satellites revolving around their respective planets being, as we have already pointed out, mere miniature editions of the solar system, the considerations so far detailed, which regulate the behaviour of the planets in their relations to the sun, will of necessity apply to the satellites very closely. in one respect, however, a system of satellites differs materially from a system of planets. the central body around which planets are in motion is self-luminous, whereas the planetary body around which a satellite revolves is not. true, planets shine, and shine very brightly too; as, for instance, venus and jupiter. but they do not give forth any light of their own, as the sun does; they merely reflect the sunlight which they receive from him. putting this one fact aside, the analogy between the planetary system and a satellite system is remarkable. the satellites are spherical in form, and differ markedly in size; they rotate, so far as we know, upon their axes in varying times; they revolve around their governing planets in orbits, not circular, but elliptic; and these orbits, furthermore, do not of necessity lie in the same plane. last of all the satellites revolve around their primaries at rates which are directly comparable with those at which the planets revolve around the sun, the rule in fact holding good that the nearer a satellite is to its primary the faster it revolves. [ ] as there seems to be much difference of opinion concerning the diameters of uranus and neptune, it should here be mentioned that the above figures are taken from professor f.r. moulton's _introduction to astronomy_ ( ). they are there stated to be given on the authority of "barnard's many measures at the lick observatory." chapter iv celestial mechanism as soon as we begin to inquire closely into the actual condition of the various members of the solar system we are struck with a certain distinction. we find that there are two quite different points of view from which these bodies can be regarded. for instance, we may make our estimates of them either as regards _volume_--that is to say, the mere room which they take up; or as regards _mass_--that is to say, the amount of matter which they contain. let us imagine two globes of equal volume; in other words, which take up an equal amount of space. one of these globes, however, may be composed of material much more tightly put together than in the other; or of greater _density_, as the term goes. that globe is said to be the greater of the two in mass. were such a pair of globes to be weighed in scales, one globe in each pan, we should see at once, by its weighing down the other, which of the two was composed of the more tightly packed materials; and we should, in astronomical parlance, say of this one that it had the greater mass. volume being merely another word for size, the order of the members of the solar system, with regard to their volumes, will be as follows, beginning with the greatest:--the sun, jupiter, saturn, uranus, neptune, the earth, venus, mars, and mercury. with regard to mass the same order strangely enough holds good. the actual densities of the bodies in question are, however, very different. the densest or closest packed body of all is the earth, which is about five and a half times as dense as if it were composed entirely of water. venus follows next, then mars, and then mercury. the remaining bodies, on the other hand, are relatively loose in structure. saturn is the least dense of all, less so than water. the density of the sun is a little greater than that of water. this method of estimating is, however, subject to a qualification. it must be remembered that in speaking of the sun, for instance, as being only a little denser than water, we are merely treating the question from the point of view of an average. certain parts of it in fact will be ever so much denser than water: those are the parts in the centre. other portions, for instance, the outside portions, will be very much less dense. it will easily be understood that in all such bodies the densest or most compressed portions are to be found towards the centre; while the portions towards the exterior being less pressed upon, will be less dense. we now reach a very important point, the question of gravitation. _gravitation_, or _gravity_, as it is often called, is the attractive force which, for instance, causes objects to fall to the earth. now it seems rather strange that one should say that it is owing to a certain force that things fall towards the earth. all things seem to us to fall so of their own accord, as if it were quite natural, or rather most unnatural if they did not. why then require a "force" to make them fall? the story goes that the great sir isaac newton was set a-thinking on this subject by seeing an apple fall from a tree to the earth. he then carried the train of thought further; and, by studying the movements of the moon, he reached the conclusion that a body even so far off as our satellite would be drawn towards the earth in the same manner. this being the case, one will naturally ask why the moon herself does not fall in upon the earth. the answer is indeed found to be that the moon is travelling round and round the earth at a certain rapid pace, and it is this very same rapid pace which keeps her from falling in upon us. any one can test this simple fact for himself. if we tie a stone to the end of a string, and keep whirling it round and round fast enough, there will be a strong pull from the stone in an outward direction, and the string will remain tight all the time that the stone is being whirled. if, however, we gradually slacken the speed at which we are making the stone whirl, a moment will come at length when the string will become limp, and the stone will fall back towards our hand. it seems, therefore, that there are two causes which maintain the stone at a regular distance all the time it is being steadily whirled. one of these is the continual pull inward towards our hand by means of the string. the other is the continual pull away from us caused by the rate at which the stone is travelling. when the rate of whirling is so regulated that these pulls exactly balance each other, the stone travels comfortably round and round, and shows no tendency either to fall back upon our hand or to break the string and fly away into the air. it is indeed precisely similar with regard to the moon. the continual pull of the earth's gravitation takes the place of the string. if the moon were to go round and round slower than it does, it would tend to fall in towards the earth; if, on the other hand, it were to go faster, it would tend to rush away into space. the same kind of pull which the earth exerts upon the objects at its surface, or upon its satellite, the moon, exists through space so far as we know. every particle of matter in the universe is found in fact to attract every other particle. the moon, for instance, attracts the earth also, but the controlling force is on the side of the much greater mass of the earth. this force of gravity or attraction of gravitation, as it is also called, is perfectly regular in its action. its power depends first of all exactly upon the mass of the body which exerts it. the gravitational pull of the sun, for instance, reaches out to an enormous distance, controlling perhaps, in their courses, unseen planets circling far beyond the orbit of neptune. again, the strength with which the force of gravity acts depends upon distance in a regularly diminishing proportion. thus, the nearer an object is to the earth, for instance, the stronger is the gravitational pull which it gets from it; the farther off it is, the weaker is this pull. if then the moon were to be brought nearer to the earth, the gravitational pull of the latter would become so much stronger that the moon's rate of motion would have also to increase in due proportion to prevent her from being drawn into the earth. last of all, the point in a body from which the attraction of gravitation acts, is not necessarily the centre of the body, but rather what is known as its _centre of gravity_, that is to say, the balancing point of all the matter which the body contains. it should here be noted that the moon does not actually revolve around the centre of gravity of the earth. what really happens is that both orbs revolve around their _common_ centre of gravity, which is a point within the body of the earth, and situated about three thousand miles from its centre. in the same manner the planets and the sun revolve around the centre of gravity of the solar system, which is a point within the body of the sun. the neatly poised movements of the planets around the sun, and of the satellites around their respective planets, will therefore be readily understood to result from a nice balance between gravitation and speed of motion. the mass of the earth is ascertained to be about eighty times that of the moon. our knowledge of the mass of a planet is learned from comparing the revolutions of its satellite or satellites around it, with those of the moon around the earth. we are thus enabled to deduce what the mass of such a planet would be compared to the earth's mass; that is to say, a study, for instance, of jupiter's satellite system shows that jupiter must have a mass nearly three hundred and eighteen times that of our earth. in the same manner we can argue out the mass of the sun from the movements of the planets and other bodies of the system around it. with regard, however, to venus and mercury, the problem is by no means such an easy one, as these bodies have no satellites. for information in this latter case we have to rely upon such uncertain evidence as, for instance, the slight disturbances caused in the motion of the earth by the attraction of these planets when they pass closest to us, or their observed effect upon the motions of such comets as may happen to pass near to them. mass and weight, though often spoken of as one and the same thing, are by no means so. mass, as we have seen, merely means the amount of matter which a body contains. the weight of a body, on the other hand, depends entirely upon the gravitational pull which it receives. the force of gravity at the surface of the earth is, for instance, about six times as great as that at the surface of the moon. all bodies, therefore, weigh about six times as much on the earth as they would upon the moon; or, rather, a body transferred to the moon's surface would weigh only about one-sixth of what it did on the terrestrial surface. it will therefore be seen that if a body of given _mass_ were to be placed upon planet after planet in turn, its _weight_ would regularly alter according to the force of gravity at each planet's surface. gravitation is indeed one of the greatest mysteries of nature. what it is, the means by which it acts, or why such a force should exist at all, are questions to which so far we have not had even the merest hint of an answer. its action across space appears to be instantaneous. the intensity of gravitation is said in mathematical parlance "to vary inversely with the square of the distance." this means that at _twice_ the distance the pull will become only _one-quarter_ as strong, and not one-half as otherwise might be expected. at _four_ times the distance, therefore, it will be _one-sixteenth_ as strong. at the earth's surface a body is pulled by the earth's gravitation, or "falls," as we ordinarily term it, through feet in one _second_ of time; whereas at the distance of the moon the attraction of the earth is so very much weakened that a body would take as long as one _minute_ to fall through the same space. newton's investigations showed that if a body were to be placed _at rest_ in space entirely away from the attraction of any other body it would remain always in a motionless condition, because there would plainly be no reason why it should move in any one direction rather than in another. and, similarly, if a body were to be projected in a certain direction and at a certain speed, it would move always in the same direction and at the same speed so long as it did not come within the gravitational attraction of any other body. the possibility of an interaction between the celestial orbs had occurred to astronomers before the time of newton; for instance, in the ninth century to the arabian musa-ben-shakir, to camillus agrippa in , and to kepler, who suspected its existence from observation of the tides. horrox also, writing in , spoke of the moon as moved by an _emanation_ from the earth. but no one prior to newton attempted to examine the question from a mathematical standpoint. notwithstanding the acknowledged truth and far-reaching scope of the law of gravitation--for we find its effects exemplified in every portion of the universe--there are yet some minor movements which it does not account for. for instance, there are small irregularities in the movement of mercury which cannot be explained by the influence of possible intra-mercurial planets, and similarly there are slight unaccountable deviations in the motions of our neighbour the moon. chapter v celestial distances up to this we have merely taken a general view of the solar system--a bird's-eye view, so to speak, from space. in the course of our inquiry we noted in a rough way the _relative_ distances at which the various planets move around the sun. but we have not yet stated what these distances _actually_ are, and it were therefore well now to turn our attention to this important matter. each of us has a fair idea of what a mile is. it is a quarter of an hour's sharp walk, for instance; or yonder village or building, we know, lies such and such a number of miles away. the measurements which have already been given of the diameters of the various bodies of the solar system appear very great to us, who find that a walk of a few miles at a time taxes our strength; but they are a mere nothing when we consider the distances from the sun at which the various planets revolve in their orbits. the following table gives these distances in round numbers. as here stated they are what are called "mean" distances; for, as the orbits are oval, the planets vary in their distances from the sun, and we are therefore obliged to strike a kind of average for each case:-- mercury about , , miles. venus " , , " earth " , , " mars " , , " jupiter " , , " saturn " , , " uranus " , , , " neptune " , , , " from the above it will be seen at a glance that we have entered upon a still greater scale of distance than in dealing with the diameters of the various bodies of the system. in that case the distances were limited to thousands of miles; in this, however, we have to deal with millions. a million being ten hundred thousand, it will be noticed that even the diameter of the huge sun is well under a million miles. how indeed are we to get a grasp of such distances, when those to which we are ordinarily accustomed--the few miles' walk, the little stretch of sea or land which we gaze upon around us--are so utterly minute in comparison? the fact is, that though men may think that they can picture in their minds such immense distances, they actually can not. in matters like these we unconsciously employ a kind of convention, and we estimate a thing as being two or three or more times the size of another. more than this we are unable to do. for instance, our ordinary experience of a mile enables us to judge, in a way, of a stretch of several miles, such as one can take in with a glance; but in our estimation of a thousand miles, or even of one hundred, we are driven back upon a mental trick, so to speak. in our attempts to realise such immense distances as those in the solar system we are obliged to have recourse to analogies; to comparisons with other and simpler facts, though this is at the best a mere self-cheating device. the analogy which seems most suited to our purpose here, and one which has often been employed by writers, is borrowed from the rate at which an express train travels. let us imagine, for instance, that we possess an express train which is capable of running anywhere, never stops, never requires fuel, and always goes along at sixty miles an hour. suppose we commence by employing it to gauge the size of our own planet, the earth. let us send it on a trip around the equator, the span of which is about , miles. at its sixty-miles-an-hour rate of going, this journey will take nearly days. next let us send it from the earth to the moon. this distance, , miles, being ten times as great as the last, will of course take ten times as long to cover, namely, days; that is to say, nearly half a year. again, let us send it still further afield, to the sun, for example. here, however, it enters upon a journey which is not to be measured in thousands of miles, as the others were, but in millions. the distance from the earth to the sun, as we have seen in the foregoing table, is about millions of miles. our express train would take about _years_ to traverse this. having arrived at the sun, let us suppose that our train makes a tour right round it. this will take more than five years. supposing, finally, that our train were started from the sun, and made to run straight out to the known boundaries of the solar system, that is to say, as far as the orbit of neptune, it would take over years to traverse the distance. that sixty miles an hour is a very great speed any one, i think, will admit who has stood upon the platform of a country station while one of the great mail trains has dashed past. but are not the immensities of space appalling to contemplate, when one realises that a body moving incessantly at such a rate would take so long as , years to traverse merely the breadth of our solar system? ten thousand years! just try to conceive it. why, it is only a little more than half that time since the pyramids were built, and they mark for us the dawn of history. and since then half-a-dozen mighty empires have come and gone! having thus concluded our general survey of the appearance and dimensions of the solar system, let us next inquire into its position and size in relation to what we call the universe. a mere glance at the night sky, when it is free from clouds, shows us that in every direction there are stars; and this holds good, no matter what portion of the globe we visit. the same is really true of the sky by day, though in that case we cannot actually see the stars, for their light is quite overpowered by the dazzling light of the sun. we thus reach the conclusion that our earth, that our solar system in fact, lies plunged within the midst of a great tangle of stars. what position, by the way, do we occupy in this mighty maze? are we at the centre, or anywhere near the centre, or where? it has been indeed amply proved by astronomical research that the stars are bodies giving off a light of their own, just as our sun does; that they are in fact suns, and that our sun is merely one, perhaps indeed a very unimportant member, of this great universe of stars. each of these stars, or suns, besides, may be the centre of a system similar to what we call our solar system, comprising planets and satellites, comets and meteors;--or perchance indeed some further variety of attendant bodies of which we have no example in our tiny corner of space. but as to whether one is right in a conjecture of this kind, there is up to the present no proof whatever. no telescope has yet shown a planet in attendance upon one of these distant suns; for such bodies, even if they do exist, are entirely out of the range of our mightiest instruments. on what then can we ground such an assumption? merely upon analogy; upon the common-sense deduction that as the stars have characteristics similar to our particular star, the sun, it would seem unlikely that ours should be the only such body in the whole of space which is attended by a planetary system. "the stars," using that expression in its most general sense, do not lie at one fixed distance from us, set here and there upon a background of sky. there is in fact no background at all. the brilliant orbs are all around us in space, at different distances from us and from each other; and we can gaze between them out into the blackness of the void which, perhaps, continues to extend unceasingly long after the very outposts of the stellar universe has been left behind. shall we then start our imaginary express train once more, and send it out towards the nearest of the stars? this would, however, be a useless experiment. our express-train method of gauging space would fail miserably in the attempt to bring home to us the mighty gulf by which we are now faced. let us therefore halt for a moment and look back upon the orders of distance with which we have been dealing. first of all we dealt with thousands of miles. next we saw how they shrank into insignificance when we embarked upon millions. we found, indeed, that our sixty-mile-an-hour train, rushing along without ceasing, would consume nearly the whole of historical time in a journey from the sun to neptune. in the spaces beyond the solar system we are faced, however, by a new order of distance. from sun to planets is measured in millions of miles, but from sun to sun is measured in billions. but does the mere stating of this fact convey anything? i fear not. for the word "billion" runs as glibly off the tongue as "million," and both are so wholly unrealisable by us that the actual difference between them might easily pass unnoticed. let us, however, make a careful comparison. what is a million? it is a thousand thousands. but what is a billion? it is a million millions. consider for a moment! a million of millions. that means a million, each unit of which is again a million. in fact every separate " " in this million is itself a million. here is a way of trying to realise this gigantic number. a million seconds make only eleven and a half days and nights. but a billion seconds will make actually more than thirty thousand years! having accepted this, let us try and probe with our express train even a little of the new gulf which now lies before us. at our old rate of going it took almost two years to cover a million miles. to cover a billion miles--that is to say, a million times this distance--would thus take of course nearly two million years. alpha centauri, the nearest star to our earth, is some twenty-five billions of miles away. our express train would thus take about fifty millions of years to reach it! this shows how useless our illustration, appropriate though it seemed for interplanetary space, becomes when applied to the interstellar spaces. it merely gives us millions in return for billions; and so the mind, driven in upon itself, whirls round and round like a squirrel in its revolving cage. there is, however, a useful illustration still left us, and it is the one which astronomers usually employ in dealing with the distances of the stars. the illustration in question is taken from the velocity of light. light travels at the tremendous speed of about , miles a second. it therefore takes only about a second and a quarter to come to us from the moon. it traverses the , , of miles which separate us from the sun in about eight minutes. it travels from the sun out to neptune in about four hours, which means that it would cross the solar system from end to end in eight. to pass, however, across the distance which separates us from alpha centauri it would take so long as about four and a quarter years! astronomers, therefore, agree in estimating the distances of the stars from the point of view of the time which light would take to pass from them to our earth. they speak of that distance which light takes a year to traverse as a "light year." according to this notation, alpha centauri is spoken of as being about four and a quarter light years distant from us. now as the rays of light coming from alpha centauri to us are chasing one another incessantly across the gulf of space, and as each ray left that star some four years before it reaches us, our view of the star itself must therefore be always some four years old. were then this star to be suddenly removed from the universe at any moment, we should continue to see it still in its place in the sky for some four years more, after which it would suddenly disappear. the rays which had already started upon their journey towards our earth must indeed continue travelling, and reaching us in their turn until the last one had arrived; after which no more would come. we have drawn attention to alpha centauri as the nearest of the stars. the majority of the others indeed are ever so much farther. we can only hazard a guess at the time it takes for the rays from many of them to reach our globe. suppose, for instance, we see a sudden change in the light of any of these remote stars, we are inclined to ask ourselves when that change did actually occur. was it in the days of queen elizabeth, or at the time of the norman conquest; or was it when rome was at the height of her glory, or perhaps ages before that when the pyramids of egypt were being built? even the last of these suppositions cannot be treated lightly. we have indeed no real knowledge of the distance from us of those stars which our giant telescopes have brought into view out of the depths of the celestial spaces. chapter vi celestial measurement had the telescope never been invented our knowledge of astronomy would be trifling indeed. prior to the year , when galileo first turned the new instrument upon the sky, all that men knew of the starry realms was gathered from observation with their own eyes unaided by any artificial means. in such researches they had been very much at a disadvantage. the sun and moon, in their opinion, were no doubt the largest bodies in the heavens, for the mere reason that they looked so! the mighty solar disturbances, which are now such common-places to us, were then quite undreamed of. the moon displayed a patchy surface, and that was all; her craters and ring-mountains were surprises as yet in store for men. nothing of course was known about the surfaces of the planets. these objects had indeed no particular characteristics to distinguish them from the great host of the stars, except that they continually changed their positions in the sky while the rest did not. the stars themselves were considered as fixed inalterably upon the vault of heaven. the sun, moon, and planets apparently moved about in the intermediate space, supported in their courses by strange and fanciful devices. the idea of satellites was as yet unknown. comets were regarded as celestial portents, and meteors as small conflagrations taking place in the upper air. in the entire absence of any knowledge with regard to the actual sizes and distances of the various celestial bodies, men naturally considered them as small; and, concluding that they were comparatively near, assigned to them in consequence a permanent connection with terrestrial affairs. thus arose the quaint and erroneous beliefs of astrology, according to which the events which took place upon our earth were considered to depend upon the various positions in which the planets, for instance, found themselves from time to time. it must, however, be acknowledged that the study of astrology, fallacious though its conclusions were, indirectly performed a great service to astronomy by reason of the accurate observations and diligent study of the stars which it entailed. we will now inquire into the means by which the distances and sizes of the celestial orbs have been ascertained, and see how it was that the ancients were so entirely in the dark in this matter. there are two distinct methods of finding out the distance at which any object happens to be situated from us. one method is by actual measurement. the other is by moving oneself a little to the right or left, and observing whether the distant object appears in any degree altered in position by our own change of place. one of the best illustrations of this relative change of position which objects undergo as a result of our own change of place, is to observe the landscape from the window of a moving railway carriage. as we are borne rapidly along we notice that the telegraph posts which are set close to the line appear to fly past us in the contrary direction; the trees, houses, and other things beyond go by too, but not so fast; objects a good way off displace slowly; while some spire, or tall landmark, in the far distance appears to remain unmoved during a comparatively long time. actual change of position on our own part is found indeed to be invariably accompanied by an apparent displacement of the objects about us, such apparent displacement as a result of our own change of position being known as "parallax." the dependence between the two is so mathematically exact, that if we know the amount of our own change of place, and if we observe the amount of the consequent displacement of any object, we are enabled to calculate its precise distance from us. thus it comes to pass that distances can be measured without the necessity of moving over them; and the breadth of a river, for instance, or the distance from us of a ship at sea, can be found merely by such means. it is by the application of this principle to the wider field of the sky that we are able to ascertain the distance of celestial bodies. we have noted that it requires a goodly change of place on our own part to shift the position in which some object in the far distance is seen by us. to two persons separated by, say, a few hundred yards, a ship upon the horizon will appear pretty much in the same direction. they would require, in fact, to be much farther apart in order to displace it sufficiently for the purpose of estimating their distance from it. it is the same with regard to the moon. two observers, standing upon our earth, will require to be some thousands of miles apart in order to see the position of our satellite sufficiently altered with regard to the starry background, to give the necessary data upon which to ground their calculations. the change of position thus offered by one side of the earth's surface at a time is, however, not sufficient to displace any but the nearest celestial bodies. when we have occasion to go farther afield we have to seek a greater change of place. this we can get as a consequence of the earth's movement around the sun. observations, taken several days apart, will show the effect of the earth's change of place during the interval upon the positions of the other bodies of our system. but when we desire to sound the depths of space beyond, and to reach out to measure the distance of the nearest star, we find ourselves at once thrown upon the greatest change of place which we can possibly hope for; and this we get during the long journey of many millions of miles which our earth performs around the sun during the course of each year. but even this last change of place, great as it seems in comparison with terrestrial measurements, is insufficient to show anything more than the tiniest displacements in a paltry forty-three out of the entire host of the stars. we can thus realise at what a disadvantage the ancients were. the measuring instruments at their command were utterly inadequate to detect such small displacements. it was reserved for the telescope to reveal them; and even then it required the great telescopes of recent times to show the slight changes in the position of the nearer stars, which were caused by the earth's being at one time at one end of its orbit, and some six months later at the other end--stations separated from each other by a gulf of about one hundred and eighty-six millions of miles. the actual distances of certain celestial bodies being thus ascertainable, it becomes a matter of no great difficulty to determine the actual sizes of the measurable ones. it is a matter of everyday experience that the size which any object appears to have, depends exactly upon the distance it is from us. the farther off it is the smaller it looks; the nearer it is the bigger. if, then, an object which lies at a known distance from us looks such and such a size, we can of course ascertain its real dimensions. take the moon, for instance. as we have already shown, we are able to ascertain its distance. we observe also that it looks a certain size. it is therefore only a matter of calculation to find what its actual dimensions should be, in order that it may look that size at that distance away. similarly we can ascertain the real dimensions of the sun. the planets, appearing to us as points of light, seem at first to offer a difficulty; but, by means of the telescope, we can bring them, as it were, so much nearer to us, that their broad expanses may be seen. we fail, however, signally with regard to the stars; for they are so very distant, and therefore such tiny points of light, that our mightiest telescopes cannot magnify them sufficiently to show any breadth of surface. instead of saying that an object looks a certain breadth across, such as a yard or a foot, a statement which would really mean nothing, astronomers speak of it as measuring a certain angle. such angles are estimated in what are called "degrees of arc"; each degree being divided into sixty minutes, and each minute again into sixty seconds. popularly considered the moon and sun _look_ about the same size, or, as an astronomer would put it, they measure about the same angle. this is an angle, roughly, of thirty-two minutes of arc; that is to say, slightly more than half a degree. the broad expanse of surface which a celestial body shows to us, whether to the naked eye, as in the case of the sun and moon, or in the telescope, as in the case of other members of our system, is technically known as its "disc." chapter vii eclipses and kindred phenomena since some members of the solar system are nearer to us than others, and all are again much nearer than any of the stars, it must often happen that one celestial body will pass between us and another, and thus intercept its light for a while. the moon, being the nearest object in the universe, will, of course, during its motion across the sky, temporarily blot out every one of the others which happen to lie in its path. when it passes in this manner across the face of the sun, it is said to _eclipse_ it. when it thus hides a planet or star, it is said to _occult_ it. the reason why a separate term is used for what is merely a case of obscuring light in exactly the same way, will be plain when one considers that the disc of the sun is almost of the same apparent size as that of the moon, and so the complete hiding of the sun can last but a few minutes at the most; whereas a planet or a star looks so very small in comparison, that it is always _entirely swallowed up for some time_ when it passes behind the body of our satellite. the sun, of course, occults planets and stars in exactly the same manner as the moon does, but we cannot see these occultations on account of the blaze of sunlight. by reason of the small size which the planets look when viewed with the naked eye, we are not able to note them in the act of passing over stars and so blotting them out; but such occurrences may be seen in the telescope, for the planetary bodies then display broad discs. there is yet another occurrence of the same class which is known as a _transit_. this takes place when an apparently small body passes across the face of an apparently large one, the phenomenon being in fact the exact reverse of an occultation. as there is no appreciable body nearer to us than the moon, we can never see anything in transit across her disc. but since the planets venus and mercury are both nearer to us than the sun, they will occasionally be seen to pass across his face, and thus we get the well-known phenomena called transits of venus and transits of mercury. as the satellites of jupiter are continually revolving around him, they will often pass behind or across his disc. such occultations and transits of satellites can be well observed in the telescope. there is, however, a way in which the light of a celestial body may be obscured without the necessity of its being hidden from us by one nearer. it will no doubt be granted that any opaque object casts a shadow when a strong light falls directly upon it. thus the earth, under the powerful light which is directed upon it from the sun, casts an extensive shadow, though we are not aware of the existence of this shadow until it falls upon something. the shadow which the earth casts is indeed not noticeable to us until some celestial body passes into it. as the sun is very large, and the earth in comparison very small, the shadow thrown by the earth is comparatively short, and reaches out in space for only about a million miles. there is no visible object except the moon, which circulates within that distance from our globe, and therefore she is the only body which can pass into this shadow. whenever such a thing happens, her surface at once becomes dark, for the reason that she never emits any light of her own, but merely reflects that of the sun. as the moon is continually revolving around the earth, one would be inclined to imagine that once in every month, namely at what is called _full moon_, when she is on the other side of the earth with respect to the sun, she ought to pass through the shadow in question. but this does not occur every time, because the moon's orbit is not quite _upon the same plane_ with the earth's. it thus happens that time after time the moon passes clear of the earth's shadow, sometimes above it, and sometimes below it. it is indeed only at intervals of about six months that the moon can be thus obscured. this darkening of her light is known as an _eclipse of the moon_. it seems a great pity that custom should oblige us to employ the one term "eclipse" for this and also for the quite different occurrence, an eclipse of the sun; in which the sun's face is hidden as a consequence of the moon's body coming directly _between_ it and our eyes. the popular mind seems always to have found it more difficult to grasp the causes of an eclipse of the moon than an eclipse of the sun. as mr. j.e. gore[ ] puts it: "the darkening of the sun's light by the interposition of the moon's body seems more obvious than the passing of the moon through the earth's shadow." eclipses of the moon furnish striking spectacles, but really add little to our knowledge. they exhibit, however, one of the most remarkable evidences of the globular shape of our earth; for the outline of its shadow when seen creeping over the moon's surface is always circular. [illustration: fig. .--total and partial eclipses of the moon. the moon is here shown in two positions; i.e. _entirely_ plunged in the earth's shadow and therefore totally eclipsed, and only _partly_ plunged in it or partially eclipsed.] _eclipses of the moon_, or lunar eclipses, as they are also called, are of two kinds--_total_, and _partial_. in a total lunar eclipse the moon passes entirely into the earth's shadow, and the whole of her surface is consequently darkened. this darkening lasts for about two hours. in a partial lunar eclipse, a portion only of the moon passes through the shadow, and so only _part_ of her surface is darkened (see fig. ). a very striking phenomenon during a total eclipse of the moon, is that the darkening of the lunar surface is usually by no means so intense as one would expect, when one considers that the sunlight at that time should be _wholly_ cut off from it. the occasions indeed upon which the moon has completely disappeared from view during the progress of a total lunar eclipse are very rare. on the majority of these occasions she has appeared of a coppery-red colour, while sometimes she has assumed an ashen hue. the explanations of these variations of colour is to be found in the then state of the atmosphere which surrounds our earth. when those portions of our earth's atmosphere through which the sun's rays have to filter on their way towards the moon are free from watery vapour, the lunar surface will be tinged with a reddish light, such as we ordinarily experience at sunset when our air is dry. the ashen colour is the result of our atmosphere being laden with watery vapour, and is similar to what we see at sunset when rain is about. lastly, when the air around the earth is thickly charged with cloud, no light at all can pass; and on such occasions the moon disappears altogether for the time being from the night sky. _eclipses of the sun_, otherwise known as solar eclipses, are divided into _total_, _partial_, and _annular_. a total eclipse of the sun takes place when the moon comes between the sun and the earth, in such a manner that it cuts off the sunlight _entirely_ for the time being from a _portion_ of the earth's surface. a person situated in the region in question will, therefore, at that moment find the sun temporarily blotted out from his view by the body of the moon. since the moon is a very much smaller body than the sun, and also very much the nearer to us of the two, it will readily be understood that the portion of the earth from which the sun is seen thus totally eclipsed will be of small extent. in places not very distant from this region, the moon will appear so much shifted in the sky that the sun will be seen only partially eclipsed. the moon being in constant movement round the earth, the portion of the earth's surface from which an eclipse is seen as total will be always a comparatively narrow band lying roughly from west to east. this band, known as the _track of totality_, can, at the utmost, never be more than about miles in width, and as a rule is very much less. for about miles on either side of it the sun is seen partially eclipsed. outside these limits no eclipse of any kind is visible, as from such regions the moon is not seen to come in the way of the sun (see fig. (i.), p. ). it may occur to the reader that eclipses can also take place in the course of which the positions, where the eclipse would ordinarily be seen as total, will lie outside the surface of the earth. such an eclipse is thus not dignified with the name of total eclipse, but is called a partial eclipse, because from the earth's surface the sun is only seen _partly eclipsed at the utmost_ (see fig. (ii.), p. ). [illustration: (i.) total eclipse of the sun.] [illustration: (ii.) partial eclipse of the sun. fig. .--total and partial eclipses of the sun. from the position a the sun cannot be seen, as it is entirely blotted out by the moon. from b it is seen partially blotted out, because the moon is to a certain degree in the way. from c no eclipse is seen, because the moon does not come in the way. it is to be noted that in a partial eclipse of the sun, the position a lies _outside_ the surface of the earth.] an _annular eclipse_ is an eclipse which just fails to become total for yet another reason. we have pointed out that the orbits of the various members of the solar system are not circular, but oval. such oval figures, it will be remembered, are technically known as ellipses. in an elliptic orbit the controlling body is situated not in the middle of the figure, but rather towards one of the ends; the actual point which it occupies being known as the _focus_. the sun being at the focus of the earth's orbit, it follows that the earth is, at times, a little nearer to him than at others. the sun will therefore appear to us to vary a little in size, looking sometimes slightly larger than at other times. it is so, too, with the moon, at the focus of whose orbit the earth is situated. she therefore also appears to us at times to vary slightly in size. the result is that when the sun is eclipsed by the moon, and the moon at the time appears the larger of the two, she is able to blot out the sun completely, and so we can get a total eclipse. but when, on the other hand, the sun appears the larger, the eclipse will not be quite total, for a portion of the sun's disc will be seen protruding all around the moon like a ring of light. this is what is known as an annular eclipse, from the latin word _annulus_, which means a ring. the term is consecrated by long usage, but it seems an unfortunate one on account of its similarity to the word "annual." the germans speak of this kind of eclipse as "ring-formed," which is certainly much more to the point. there can never be a year without an eclipse of the sun. indeed there must be always two such eclipses _at least_ during that period, though there need be no eclipse of the moon at all. on the other hand, the greatest number of eclipses which can ever take place during a year are seven; that is to say, either five solar eclipses and two lunar, or four solar and three lunar. this general statement refers merely to eclipses in their broadest significance, and informs us in no way whether they will be total or partial. of all the phenomena which arise from the hiding of any celestial body by one nearer coming in the way, a total eclipse of the sun is far the most important. it is, indeed, interesting to consider how much poorer modern astronomy would be but for the extraordinary coincidence which makes a total solar eclipse just possible. the sun is about times farther off from us than the moon, and enormously greater than her in bulk. yet the two are relatively so distanced from us as to look about the same size. the result of this is that the moon, as has been seen, can often blot out the sun entirely from our view for a short time. when this takes place the great blaze of sunlight which ordinarily dazzles our eyes is completely cut off, and we are thus enabled, unimpeded, to note what is going on in the immediate vicinity of the sun itself. in a total solar eclipse, the time which elapses from the moment when the moon's disc first begins to impinge upon that of the sun at his western edge until the eclipse becomes total, lasts about an hour. during all this time the black lunar disc may be watched making its way steadily across the solar face. notwithstanding the gradual obscuration of the sun, one does not notice much diminution of light until about three-quarters of his disc are covered. then a wan, unearthly appearance begins to pervade all things, the temperature falls noticeably, and nature seems to halt in expectation of the coming of something unusual. the decreasing portion of sun becomes more and more narrow, until at length it is reduced to a crescent-shaped strip of exceeding fineness. strange, ill-defined, flickering shadows (known as "shadow bands") may at this moment be seen chasing each other across any white expanse such as a wall, a building, or a sheet stretched upon the ground. the western side of the sky has now assumed an appearance dark and lowering, as if a rainstorm of great violence were approaching. this is caused by the mighty mass of the lunar shadow sweeping rapidly along. it flies onward at the terrific velocity of about half a mile a second. if the gradually diminishing crescent of sun be now watched through a telescope, the observer will notice that it does not eventually vanish all at once, as he might have expected. rather, it breaks up first of all along its length into a series of brilliant dots, known as "baily's beads." the reason of this phenomenon is perhaps not entirely agreed upon, but the majority of astronomers incline to the opinion that the so-called "beads" are merely the last remnants of sunlight peeping between those lunar mountain peaks which happen at the moment to fringe the advancing edge of the moon. the beads are no sooner formed than they rapidly disappear one after the other, after which no portion of the solar surface is left to view, and the eclipse is now total (see fig. ). [illustration: _in a total eclipse_ _in an annular eclipse_ fig. .--"baily's beads."] but with the disappearance of the sun there springs into view a new and strange appearance, ordinarily unseen because of the blaze of sunlight. it is a kind of aureole, or halo, pearly white in colour, which is seen to surround the black disc of the moon. this white radiance is none other than the celebrated phenomenon widely known as the _solar corona_. it was once upon a time thought to belong to the moon, and to be perhaps a lunar atmosphere illuminated by the sunlight shining through it from behind. but the suddenness with which the moon always blots out stars when occulting them, has amply proved that she possesses no atmosphere worth speaking about. it is now, however, satisfactorily determined that the corona belongs to the sun, for during the short time that it remains in view the black body of the moon can be seen creeping across it. all the time that the _total phase_ (as it is called) lasts, the corona glows with its pale unearthly light, shedding upon the earth's surface an illumination somewhat akin to full moonlight. usually the planet venus and a few stars shine out the while in the darkened heaven. meantime around the observer animal and plant life behave as at nightfall. birds go to roost, bats fly out, worms come to the surface of the ground, flowers close up. in the norwegian eclipse of fish were seen rising to the surface of the water. when the total phase at length is over, and the moon in her progress across the sky has allowed the brilliant disc of the sun to spring into view once more at the other side, the corona disappears. there is another famous accompaniment of the sun which partly reveals itself during total solar eclipses. this is a layer of red flame which closely envelops the body of the sun and lies between it and the corona. this layer is known by the name of the _chromosphere_. just as at ordinary times we cannot see the corona on account of the blaze of sunlight, so are we likewise unable to see the chromosphere because of the dazzling white light which shines through from the body of the sun underneath and completely overpowers it. when, however, during a solar eclipse, the lunar disc has entirely hidden the brilliant face of the sun, we are still able for a few moments to see an edgewise portion of the chromosphere in the form of a narrow red strip, fringing the advancing border of the moon. later on, just before the moon begins to uncover the face of the sun from the other side, we may again get a view of a strip of chromosphere. the outer surface of the chromosphere is not by any means even. it is rough and billowy, like the surface of a storm-tossed sea. portions of it, indeed, rise at times to such heights that they may be seen standing out like blood-red points around the black disc of the moon, and remain thus during a good part of the total phase. these projections are known as the _solar prominences_. in the same way as the corona, the chromosphere and prominences were for a time supposed to belong to the moon. this, however, was soon found not to be the case, for the lunar disc was noticed to creep slowly across them also. the total phase, or "totality," as it is also called, lasts for different lengths of time in different eclipses. it is usually of about two or three minutes' duration, and at the utmost it can never last longer than about eight minutes. when totality is over and the corona has faded away, the moon's disc creeps little by little from the face of the sun, light and heat returns once more to the earth, and nature recovers gradually from the gloom in which she has been plunged. about an hour after totality, the last remnant of moon draws away from the solar disc, and the eclipse is entirely at an end. the corona, the chromosphere, and the prominences are the most important of these accompaniments of the sun which a total eclipse reveals to us. our further consideration of them must, however, be reserved for a subsequent chapter, in which the sun will be treated of at length. every one who has had the good fortune to see a total eclipse of the sun will, the writer feels sure, agree with the verdict of sir norman lockyer that it is at once one of the "grandest and most awe-inspiring sights" which man can witness. needless to say, such an occurrence used to cause great consternation in less civilised ages; and that it has not in modern times quite parted with its terrors for some persons, is shown by the fact that in iowa, in the united states, a woman died from fright during the eclipse of . to the serious observer of a total solar eclipse every instant is extremely precious. many distinct observations have to be crowded into a time all too limited, and this in an eclipse-party necessitates constant rehearsals in order that not a moment may be wasted when the longed-for totality arrives. such preparation is very necessary; for the rarity and uncommon nature of a total eclipse of the sun, coupled with its exceeding short duration, tends to flurry the mind, and to render it slow to seize upon salient points of detail. and, even after every precaution has been taken, weather possibilities remain to be reckoned with, so that success is rather a lottery. above all things, therefore, a total solar eclipse is an occurrence for the proper utilisation of which personal experience is of absolute necessity. it was manifestly out of the question that such experience could be gained by any individual in early times, as the imperfection of astronomical theory and geographical knowledge rendered the predicting of the exact position of the track of totality well-nigh impossible. thus chance alone would have enabled one in those days to witness a total phase, and the probabilities, of course, were much against a second such experience in the span of a life-time. and even in more modern times, when the celestial motions had come to be better understood, the difficulties of foreign travel still were in the way; for it is, indeed, a notable fact that during many years following the invention of the telescope the tracks were placed for the most part in far-off regions of the earth, and europe was visited by singularly few total solar eclipses. thus it came to pass that the building up of a body of organised knowledge upon this subject was greatly delayed. nothing perhaps better shows the soundness of modern astronomical theory than the almost exact agreement of the time predicted for an eclipse with its actual occurrence. similarly, by calculating backwards, astronomers have discovered the times and seasons at which many ancient eclipses took place, and valuable opportunities have thus arisen for checking certain disputed dates in history. it should not be omitted here that the ancients were actually able, _in a rough way_, to predict eclipses. the chaldean astronomers had indeed noticed very early a curious circumstance, _i.e._ that eclipses tend to repeat themselves after a lapse of slightly more than eighteen years. in this connection it must, however, be pointed out, in the first instance, that the eclipses which occur in any particular year are in no way associated with those which occurred in the previous year. in other words, the mere fact that an eclipse takes place upon a certain day this year will not bring about a repetition of it at the same time next year. however, the nicely balanced behaviour of the solar system, an equilibrium resulting from æons of orbital ebb and flow, naturally tends to make the members which compose that family repeat their ancient combinations again and again; so that after definite lapses of time the same order of things will _almost exactly_ recur. thus, as a consequence of their beautifully poised motions, the sun, the moon, and the earth tend, after a period of years and - / days,[ ] to occupy very nearly the same positions with regard to each other. the result of this is that, during each recurring period, the eclipses comprised within it will be repeated in their order. to give examples:-- the total solar eclipse of august , , was a repetition of that of august , . the partial solar eclipse of february , , corresponded to that which took place on february , . the annular eclipse of july , , was a recurrence of that of june , . in this way we can go on until the eighteen year cycle has run out, and we come upon a total solar eclipse predicted for september , , which will repeat the above-mentioned ones of and ; and so on too with the others. from mere observation alone, extending no doubt over many ages, those time-honoured watchers of the sky, the early chaldeans, had arrived at this remarkable generalisation; and they used it for the rough prediction of eclipses. to the period of recurrence they give the name of "saros." and here we find ourselves led into one of the most interesting and fascinating by-paths in astronomy, to which writers, as a rule, pay all too little heed. in order not to complicate matters unduly, the recurrence of solar eclipses alone will first be dealt with. this limitation will, however, not affect the arguments in the slightest, and it will be all the more easy in consequence to show their application to the case of eclipses of the moon. the reader will perhaps have noticed that, with regard to the repetition of an eclipse, it has been stated that the conditions which bring it on at each recurrence are reproduced _almost exactly_. here, then, lies the _crux_ of the situation. for it is quite evident that were the conditions _exactly_ reproduced, the recurrences of each eclipse would go on for an indefinite period. for instance, if the lapse of a saros period found the sun, moon, and earth again in the precise relative situations which they had previously occupied, the recurrences of a solar eclipse would tend to duplicate its forerunner with regard to the position of the shadow upon the terrestrial surface. but the conditions _not_ being exactly reproduced, the shadow-track does not pass across the earth in quite the same regions. it is shifted a little, so to speak; and each time the eclipse comes round it is found to be shifted a little farther. every solar eclipse has therefore a definite "life" of its own upon the earth, lasting about years, or saros returns, and working its way little by little across our globe from north to south, or from south to north, as the case may be. let us take an imaginary example. a _partial_ eclipse occurs, say, somewhere near the north pole, the edge of the "partial" shadow just grazing the earth, and the "track of totality" being as yet cast into space. here we have the beginning of a series. at each saros recurrence the partial shadow encroaches upon a greater extent of earth-surface. at length, in its turn, the track of totality begins to impinge upon the earth. this track streaks across our globe at each return of the eclipse, repeating itself every time in a slightly more southerly latitude. south and south it moves, passing in turn the tropic of cancer, the equator, the tropic of capricorn, until it reaches the south pole; after which it touches the earth no longer, but is cast into space. the rear portion of the partial shadow, in its turn, grows less and less in extent; and it too in time finally passes off. our imaginary eclipse series is now no more--its "life" has ended. we have taken, as an example, an eclipse series moving from north to south. we might have taken one moving from south to north, for they progress in either direction. from the description just given the reader might suppose that, if the tracks of totality of an eclipse series were plotted upon a chart of the world, they would lie one beneath another like a set of steps. this is, however, _not_ the case, and the reason is easily found. it depends upon the fact that the saros does not comprise an exact number of days, but includes, as we have seen, one-third of a day in addition. it will be granted, of course, that if the number of days was exact, the _same_ parts of the earth would always be brought round by the axial rotation _to front the sun_ at the moment of the recurrence of the eclipse. but as there is still one-third of a day to complete the saros period, the earth has yet to make one-third of a rotation upon its axis before the eclipse takes place. thus at every recurrence the track of totality finds itself placed one-third of the earth's circumference to the _westward_. three of the recurrences will, of course, complete the circuit of the globe; and so the fourth recurrence will duplicate the one which preceded it, three saros returns, or years and month before. this duplication, as we have already seen, will, however, be situated in a latitude to the south or north of its predecessor, according as the eclipse series is progressing in a southerly or northerly direction. lastly, every eclipse series, after working its way across the earth, will return again to go through the same process after some , years; so that, at the end of that great lapse of time, the entire "life" of every eclipse should repeat itself, provided that the conditions of the solar system have not altered appreciably during the interval. we are now in a position to consider this gradual southerly or northerly progress of eclipse recurrences in its application to the case of eclipses of the moon. it should be evident that, just as in solar eclipses the lunar shadow is lowered or raised (as the case may be) each time it strikes the terrestrial surface, so in lunar eclipses will the body of the moon shift its place at each recurrence relatively to the position of the earth's shadow. every lunar eclipse, therefore, will commence on our satellite's disc as a partial eclipse at the northern or southern extremity, as the case may be. let us take, as an example, an imaginary series of eclipses of the moon progressing from north to south. at each recurrence the partial phase will grow greater, its boundary encroaching more and more to the southward, until eventually the whole disc is enveloped by the shadow, and the eclipse becomes total. it will then repeat itself as total during a number of recurrences, until the entire breadth of the shadow has been passed through, and the northern edge of the moon at length springs out into sunlight. this illuminated portion will grow more and more extensive at each succeeding return, the edge of the shadow appearing to recede from it until it finally passes off at the south. similarly, when a lunar eclipse commences as partial at the south of the moon, the edge of the shadow at each subsequent recurrence finds itself more and more to the northward. in due course the total phase will supervene, and will persist during a number of recurrences until the southerly trend of the moon results in the uncovering of the lunar surface at the south. thus, as the boundary of the shadow is left more and more to the northward, the illuminated portion on the southern side of the moon becomes at each recurrence greater and the darkened portion on the northern side less, until the shadow eventually passes off at the north. the "life" of an eclipse of the moon happens, for certain reasons, to be much shorter than that of an eclipse of the sun. it lasts during only about years, or saros returns. fig. , p. , is a map of the world on mercator's projection, showing a portion of the march of the total solar eclipse of august , , across the surface of the earth. the projection in question has been employed because it is the one with which people are most familiar. this eclipse began by striking the neighbourhood of the north pole in the guise of a partial eclipse during the latter part of the reign of queen elizabeth, and became total on the earth for the first time on the th of june . its next appearance was on the th of july . it has not been possible to show the tracks of totality of these two early visitations on account of the distortion of the polar regions consequent on the _fiction_ of mercator's projection. it is therefore made to commence with the track of its third appearance, viz. on july , . in consequence of those variations in the apparent sizes of the sun and moon, which result, as we have seen, from the variations in their distances from the earth, this eclipse will change from a total into an annular eclipse towards the end of the twenty-first century. by that time the track will have passed to the southern side of the equator. the track will eventually leave the earth near the south pole about the beginning of the twenty-sixth century, and the rear portion of the partial shadow will in its turn be clear of the terrestrial surface by about a.d., when the series comes to an end. [illustration: fig. .--map of the world on mercator's projection, showing a portion of the progress of the total solar eclipse of august , , across the surface of the earth.] [ ] astronomical essays (p. ), london, . [ ] in some cases the periods between the dates of the corresponding eclipses _appear_ to include a greater number of days than ten; but this is easily explained when allowance is made for intervening _leap_ years (in each of which an _extra_ day has of course been added), and also for variations in local time. chapter viii famous eclipses of the sun what is thought to be the earliest reference to an eclipse comes down to us from the ancient chinese records, and is over four thousand years old. the eclipse in question was a solar one, and occurred, so far as can be ascertained, during the twenty-second century b.c. the story runs that the two state astronomers, ho and hi by name, being exceedingly intoxicated, were unable to perform their required duties, which consisted in superintending the customary rites of beating drums, shooting arrows, and the like, in order to frighten away the mighty dragon which it was believed was about to swallow up the lord of day. this eclipse seems to have been only partial; nevertheless a great turmoil ensued, and the two astronomers were put to death, no doubt with the usual _celestial_ cruelty. the next eclipse mentioned in the chinese annals is also a solar eclipse, and appears to have taken place more than a thousand years later, namely in b.c. records of similar eclipses follow from the same source; but as they are mere notes of the events, and do not enter into any detail, they are of little interest. curiously enough the chinese have taken practically no notice of eclipses of the moon, but have left us a comparatively careful record of comets, which has been of value to modern astronomy. the earliest mention of a _total_ eclipse of the sun (for it should be noted that the ancient chinese eclipse above-mentioned was merely partial) was deciphered in , on a very ancient babylonian tablet, by mr. l.w. king of the british museum. this eclipse took place in the year b.c. assyrian tablets record three solar eclipses which occurred between three and four hundred years later than this. the first of these was in b.c.; the total phase being visible near nineveh. the next record of an eclipse of the sun comes to us from a grecian source. this eclipse took place in b.c., and has been the subject of much investigation. herodotus, to whom we are indebted for the account, tells us that it occurred during a battle in a war which had been waging for some years between the lydians and medes. the sudden coming on of darkness led to a termination of the contest, and peace was afterwards made between the combatants. the historian goes on to state that the eclipse had been foretold by thales, who is looked upon as the founder of grecian astronomy. this eclipse is in consequence known as the "eclipse of thales." it would seem as if that philosopher were acquainted with the chaldean saros. the next solar eclipse worthy of note was an annular one, and occurred in b.c., the first year of the peloponnesian war. plutarch relates that the pilot of the ship, which was about to convey pericles to the peloponnesus, was very much frightened by it; but pericles calmed him by holding up a cloak before his eyes, and saying that the only difference between this and the eclipse was that something larger than the cloak prevented his seeing the sun for the time being. an eclipse of great historical interest is that known as the "eclipse of agathocles," which occurred on the morning of the th of august, b.c. agathocles, tyrant of syracuse, had been blockaded in the harbour of that town by the carthaginian fleet, but effected the escape of his squadron under cover of night, and sailed for africa in order to invade the enemy's territory. during the following day he and his vessels experienced a total eclipse, in which "day wholly put on the appearance of night, and the stars were seen in all parts of the sky." a few solar eclipses are supposed to be referred to in early roman history, but their identity is very doubtful in comparison with those which the greeks have recorded. additional doubt is cast upon them by the fact that they are usually associated with famous events. the birth and death of romulus, and the passage of the rubicon by julius cæsar, are stated indeed to have been accompanied by these marks of the approval or disapproval of the gods! reference to our subject in the bible is scanty. amos viii. is thought to refer to the nineveh eclipse of b.c., to which allusion has already been made; while the famous episode of hezekiah and the shadow on the dial of ahaz has been connected with an eclipse which was partial at jerusalem in b.c. the first solar eclipse, recorded during the christian era, is known as the "eclipse of phlegon," from the fact that we are indebted for the account to a pagan writer of that name. this eclipse took place in a.d. , and the total phase was visible a little to the north of palestine. it has sometimes been confounded with the "darkness of the crucifixion," which event took place near the date in question; but it is sufficient here to say that the crucifixion is well known to have occurred during the passover of the jews, which is always celebrated at the _full_ moon, whereas an eclipse of the sun can only take place at _new_ moon. dion cassius, commenting on the emperor claudius about the year a.d. , writes as follows:-- "as there was going to be an eclipse on his birthday, through fear of a disturbance, as there had been other prodigies, he put forth a public notice, not only that the obscuration would take place, and about the time and magnitude of it, but also about the causes that produce such an event." this is a remarkable piece of information; for the romans, an essentially military nation, appear hitherto to have troubled themselves very little about astronomical matters, and were content, as we have seen, to look upon phenomena, like eclipses, as mere celestial prodigies. what is thought to be the first definite mention of the solar corona occurs in a passage of plutarch. the eclipse to which he refers is probably one which took place in a.d. . he says that the obscuration caused by the moon "has no time to last and no extensiveness, but some light shows itself round the sun's circumference, which does not allow the darkness to become deep and complete." no further reference to this phenomenon occurs until near the end of the sixteenth century. it should, however, be here mentioned that mr. e.w. maunder has pointed out the probability[ ] that we have a very ancient symbolic representation of the corona in the "winged circle," "winged disc," or "ring with wings," as it is variously called, which appears so often upon assyrian and egyptian monuments, as the symbol of the deity (fig. ). [illustration: fig. .--the "ring with wings." the upper is the assyrian form of the symbol, the lower the egyptian. (from _knowledge_.) compare the form of the corona on plate vii. (b), p. .] the first solar eclipse recorded to have been seen in england is that of a.d. , mention of which is found in the _anglo-saxon chronicle_. the track of totality did not, however, come near our islands, for only two-thirds of the sun's disc were eclipsed at london. in a great eclipse took place in europe, which was total for more than five minutes across what is now bavaria. terror at this eclipse is said to have hastened the death of louis le debonnaire, emperor of the west, who lay ill at worms. in --_temp._ king alfred--an eclipse of the sun took place which was total at london. from this until no other eclipse was total at london itself; though this does not apply to other portions of england. an eclipse, generally known as the "eclipse of stiklastad," is said to have taken place in , during the sea-fight in which olaf of norway is supposed to have been slain. longfellow, in his _saga of king olaf_, has it that "the sun hung red as a drop of blood," but, as in the case of most poets, the dramatic value of an eclipse seems to have escaped his notice. in the year there occurred a total eclipse of the sun, the last to be visible in england for more than five centuries. indeed there have been only two such since--namely, those of and , to which we shall allude in due course. the eclipse of took place on the th march, and is thus referred to in the _anglo-saxon chronicle_:-- "in the lent, the sun and the day darkened, about the noon-tide of the day, when men were eating, and they lighted candles to eat by. that was the th day before the calends of april. men were very much struck with wonder." several of the older historians speak of a "fearful eclipse" as having taken place on the morning of the battle of crecy, . lingard, for instance, in his _history of england_, has as follows:-- "never, perhaps, were preparations for battle made under circumstances so truly awful. on that very day the sun suffered a partial eclipse: birds, in clouds, the precursors of a storm, flew screaming over the two armies, and the rain fell in torrents, accompanied by incessant thunder and lightning. about five in the afternoon the weather cleared up; the sun in full splendour darted his rays in the eyes of the enemy." calculations, however, show that no eclipse of the sun took place in europe during that year. this error is found to have arisen from the mistranslation of an obsolete french word _esclistre_ (lightning), which is employed by froissart in his description of the battle. in an eclipse was total over scotland and part of north germany. it was observed at torgau by jessenius, an hungarian physician, who noticed a bright light around the moon during the time of totality. this is said to be the first reference to the corona since that of plutarch, to which we have already drawn attention. mention of scotland recalls the fact that an unusual number of eclipses happen to have been visible in that country, and the occult bent natural to the scottish character has traditionalised a few of them in such terms as the "black hour" (an eclipse of ), "black saturday" (the eclipse of which has been alluded to above), and "mirk monday" ( ). the track of the last-named also passed over carrickfergus in ireland, where it was observed by a certain dr. wybord, in whose account the term "corona" is first employed. this eclipse is the last which has been total in scotland, and it is calculated that there will not be another eclipse seen as total there until the twenty-second century. an eclipse of the sun which took place on may , , is recorded as having been seen "through a tube." this probably refers to the then recent invention--the telescope. the eclipses which we have been describing are chiefly interesting from an historical point of view. the old mystery and confusion to the beholders seem to have lingered even into comparatively enlightened times, for we see how late it is before the corona attracts definite attention for the sake of itself alone. it is not a far cry from notice of the corona to that of other accompaniments of a solar eclipse. thus the eclipse of , the total phase of which was visible in switzerland, is of great interest; for it was on this occasion that the famous red prominences seem first to have been noted. a certain captain stannyan observed this eclipse from berne in switzerland, and described it in a letter to flamsteed, the then astronomer royal. he says the sun's "getting out of his eclipse was preceded by a blood-red streak of light from its left limb, which continued not longer than six or seven seconds of time; then part of the sun's disc appeared all of a sudden, as bright as venus was ever seen in the night, nay brighter; and in that very instant gave a light and shadow to things as strong as moonlight uses to do." how little was then expected of the sun is, however, shown by flamsteed's words, when communicating this information to the royal society:-- "the captain is the first man i ever heard of that took notice of a red streak of light preceding the emersion of the sun's body from a total eclipse. and i take notice of it to you because it infers that _the moon has an atmosphere_; and its short continuance of only six or seven seconds of time, tells us that _its height is not more than the five or six hundredth part of her diameter_." what a change has since come over the ideas of men! the sun has proved a veritable mine of discovery, while the moon has yielded up nothing new. the eclipse of , the first total at london since that of , was observed by the famous astronomer, edmund halley, from the rooms of the royal society, then in crane court, fleet street. on this occasion both the corona and a red projection were noted. halley further makes allusion to that curious phenomenon, which later on became celebrated under the name of "baily's beads." it was also on the occasion of this eclipse that the _earliest recorded drawings of the corona_ were made. cambridge happened to be within the track of totality; and a certain professor cotes of that university, who is responsible for one of the drawings in question, forwarded them to sir isaac newton together with a letter describing his observations. in there occurred an eclipse, the total phase of which was visible from the south-west of england, but not from london. the weather was unfavourable, and the eclipse consequently appears to have been seen by only one person, a certain dr. stukeley, who observed it from haraden hill near salisbury plain. this is the last eclipse of which the total phase was seen in any part of england. the next will not be until june , , and will be visible along a line across north wales and lancashire. the discs of the sun and moon will just then be almost of the same apparent size, and so totality will be of extremely short duration; in fact only a few seconds. london itself will not see a totality until the year --a circumstance which need hardly distress any of us personally! it is only from the early part of the nineteenth century that serious scientific attention to eclipses of the sun can be dated. an _annular_ eclipse, visible in in the south of scotland, drew the careful notice of francis baily of jedburgh in roxburghshire to that curious phenomenon which we have already described, and which has ever since been known by the name of "baily's beads." spurred by his observation, the leading astronomers of the day determined to pay particular attention to a total eclipse, which in the year was to be visible in the south of france and the north of italy. the public interest aroused on this occasion was also very great, for the region across which the track of totality was to pass was very populous, and inhabited by races of a high degree of culture. this eclipse occurred on the morning of the th july, and from it may be dated that great enthusiasm with which total eclipses of the sun have ever since been received. airy, our then astronomer royal, observed it from turin; arago, the celebrated director of the paris observatory, from perpignan in the south of france; francis baily from pavia; and sir john herschel from milan. the corona and three large red prominences were not only well observed by the astronomers, but drew tremendous applause from the watching multitudes. the success of the observations made during this eclipse prompted astronomers to pay similar attention to that of july , , the total phase of which was to be visible in the south of norway and sweden, and across the east of prussia. this eclipse was also a success, and it was now ascertained that the red prominences belonged to the sun and not to the moon; for the lunar disc, as it moved onward, was seen to cover and to uncover them in turn. it was also noted that these prominences were merely uprushes from a layer of glowing gaseous matter, which was seen closely to envelop the sun. the total eclipse of july , , was observed in spain, and photography was for the first time _systematically_ employed in its observation.[ ] in the photographs taken the stationary appearance of both the corona and prominences with respect to the moving moon, definitely confirmed the view already put forward that they were actual appendages of the sun. the eclipse of august , , the total phase of which lasted nearly six minutes, was visible in india, and drew thither a large concourse of astronomers. in this eclipse the spectroscope came to the front, and showed that both the prominences, and the chromospheric layer from which they rise, are composed of glowing vapours--chief among which is the vapour of hydrogen. the direct result of the observations made on this occasion was the spectroscopic method of examining prominences at any time in full daylight, and without a total eclipse. this method, which has given such an immense impetus to the study of the sun, was the outcome of independent and simultaneous investigation on the part of the french astronomer, the late m. janssen, and the english astronomer, professor (now sir norman) lockyer, a circumstance strangely reminiscent of the discovery of neptune. the principles on which the method was founded seem, however, to have occurred to dr. (now sir william) huggins some time previously. the eclipse of december , , was total for a little more than two minutes, and its track passed across the mediterranean. m. janssen, of whom mention has just been made, escaped in a balloon from then besieged paris, taking his instruments with him, and made his way to oran, in algeria, in order to observe it; but his expectations were disappointed by cloudy weather. the expedition sent out from england had the misfortune to be shipwrecked off the coast of sicily. but the occasion was redeemed by a memorable observation made by the american astronomer, the late professor young, which revealed the existence of what is now known as the "reversing layer." this is a shallow layer of gases which lies immediately beneath the chromosphere. an illustration of the corona, as it was seen during the above eclipse, will be found on plate vii. (a), p. . in the eclipse of december , , total across southern india, the photographs of the corona obtained by mr. davis, assistant to lord lindsay (now the earl of crawford), displayed a wealth of detail hitherto unapproached. the eclipse of july , , total across the western states of north america, was a remarkable success, and a magnificent view of the corona was obtained by the well-known american astronomer and physicist, the late professor langley, from the summit of pike's peak, colorado, over , feet above the level of the sea. the coronal streamers were seen to extend to a much greater distance at this altitude than at points less elevated, and the corona itself remained visible during more than four minutes after the end of totality. it was, however, not entirely a question of altitude; the coronal streamers were actually very much longer on this occasion than in most of the eclipses which had previously been observed. the eclipse of may , , observed in upper egypt, is notable from the fact that, in one of the photographs taken by dr. schuster at sohag, a bright comet appeared near the outer limit of the corona (see plate i., p. ). the comet in question had not been seen before the eclipse, and was never seen afterwards. this is the third occasion on which attention has been drawn to a comet _merely_ by a total eclipse. the first is mentioned by seneca; and the second by philostorgius, in an account of an eclipse observed at constantinople in a.d. . a fourth case of the kind occurred in , when faint evidences of one of these filmy objects were found on photographs of the corona taken by the american astronomer, professor schaeberle, during the total eclipse of april of that year. the eclipse of may , , had a totality of over five minutes, but the central track unfortunately passed across the pacific ocean, and the sole point of land available for observing it from was one of the marquesas group, caroline island, a coral atoll seven and a half miles long by one and a half broad. nevertheless astronomers did not hesitate to take up their posts upon that little spot, and were rewarded with good weather. the next eclipse of importance was that of april , . it stretched from chili across south america and the atlantic ocean to the west coast of africa, and, as the weather was fine, many good results were obtained. photographs were taken at both ends of the track, and these showed that the appearance of the corona remained unchanged during the interval of time occupied by the passage of the shadow across the earth. it was on the occasion of this eclipse that professor schaeberle found upon his photographs those traces of the presence of a comet, to which allusion has already been made. extensive preparations were made to observe the eclipse of august , . totality lasted from two to three minutes, and the track stretched from norway to japan. bad weather disappointed the observers, with the exception of those taken to nova zembla by sir george baden powell in his yacht _otaria_. the eclipse of january , , across india _viâ_ bombay and benares, was favoured with good weather, and is notable for a photograph obtained by mrs. e.w. maunder, which showed a ray of the corona extending to a most unusual distance. [illustration: plate i. the total eclipse of the sun of may th, a comet is here shown in the immediate neighbourhood of the corona. drawn by mr. w.h. wesley from the photographs. (page )] of very great influence in the growth of our knowledge with regard to the sun, is the remarkable piece of good fortune by which the countries around the mediterranean, so easy of access, have been favoured with a comparatively large number of total eclipses during the past sixty years. tracks of totality have, for instance, traversed the spanish peninsula on no less than five occasions during that period. two of these are among the most notable eclipses of recent years, namely, those of may , , and of august , . in the former the track of totality stretched from the western seaboard of mexico, through the southern states of america, and across the atlantic ocean, after which it passed over portugal and spain into north africa. the total phase lasted for about a minute and a half, and the eclipse was well observed from a great many points along the line. a representation of the corona, as it appeared on this occasion, will be found on plate vii. (b), p. . the track of the other eclipse to which we have alluded, _i.e._ that of august , , crossed spain about miles to the northward of that of . it stretched from winnipeg in canada, through labrador, and over the atlantic; then traversing spain, it passed across the balearic islands, north africa, and egypt, and ended in arabia (see fig. , p. ). much was to be expected from a comparison between the photographs taken in labrador and egypt on the question as to whether the corona would show any alteration in shape during the time that the shadow was traversing the intervening space--some miles. the duration of the total phase in this eclipse was nearly four minutes. bad weather, however, interfered a good deal with the observations. it was not possible, for instance, to do anything at all in labrador. in spain the weather conditions were by no means favourable; though at burgos, where an immense number of people had assembled, the total phase was, fortunately, well seen. on the whole, the best results were obtained at guelma in algeria. the corona on the occasion of this eclipse was a very fine one, and some magnificent groups of prominences were plainly visible to the naked eye (see the frontispiece). the next total eclipse after that of was one which occurred on january , . it passed across central asia and siberia, and had a totality lasting two and a half minutes at most; but it was not observed as the weather was extremely bad, a circumstance not surprising with regard to those regions at that time of year. the eclipse of january , , passed across the pacific ocean. only two small coral islands--hull island in the phoenix group, and flint island about miles north of tahiti--lay in the track. two expeditions set out to observe it, _i.e._ a combined american party from the lick observatory and the smithsonian institution of washington, and a private one from england under mr. f.k. mcclean. as hull island afforded few facilities, both parties installed their instruments on flint island, although it was very little better. the duration of the total phase was fairly long--about four minutes, and the sun very favourably placed, being nearly overhead. heavy rain and clouds, however, marred observation during the first minute of totality, but the remaining three minutes were successfully utilised, good photographs of the corona being obtained. the next few years to come are unfortunately by no means favourable from the point of view of the eclipse observer. an eclipse will take place on june , , the track stretching from greenland across the north polar regions into siberia. the geographical situation is, however, a very awkward one, and totality will be extremely short--only six seconds in greenland and twenty-three seconds in siberia. the eclipse of may , , will be visible in tasmania. totality will last so long as four minutes, but the sun will be at the time much too low in the sky for good observation. the eclipse of the following year, april , , will also be confined, roughly speaking, to the same quarter of the earth, the track passing across the old convict settlement of norfolk island, and then out into the pacific. the eclipse of april , , will stretch from portugal, through france and belgium into north germany. it will, however, be of practically no service to astronomy. totality, for instance, will last for only three seconds in portugal; and, though paris lies in the central track, the eclipse, which begins as barely total, will have changed into an _annular_ one by the time it passes over that city. the first really favourable eclipse in the near future will be that of august , . its track will stretch from greenland across norway, sweden, and russia. this eclipse is a return, after one saros, of the eclipse of august , . the last solar eclipse which we will touch upon is that predicted for june , . it has been already alluded to as the first of those in the future to be _total_ in england. the central line will stretch from wales in a north-easterly direction. stonyhurst observatory, in lancashire, will lie in the track; but totality there will be very short, only about twenty seconds in duration. [ ] _knowledge_, vol. xx. p. , january . [ ] the _first photographic representation of the corona_ had, however, been made during the eclipse of . this was a daguerreotype taken by dr. busch at königsberg in prussia. chapter ix famous eclipses of the moon the earliest lunar eclipse, of which we have any trustworthy information, was a total one which took place on the th march, b.c., and was observed from babylon. for our knowledge of this eclipse we are indebted to ptolemy, the astronomer, who copied it, along with two others, from the records of the reign of the chaldean king, merodach-baladan. the next eclipse of the moon worth noting was a total one, which took place some three hundred years later, namely, in b.c. this eclipse was observed at athens, and is mentioned by aristophanes in his play, _the clouds_. plutarch relates that a total eclipse of the moon, which occurred in b.c., so greatly frightened nicias, the general of the athenians, then warring in sicily, as to cause a delay in his retreat from syracuse which led to the destruction of his whole army. seven years later--namely, in b.c., the twenty-sixth year of the peloponnesian war--there took place another total lunar eclipse of which mention is made by xenophon. omitting a number of other eclipses alluded to by ancient writers, we come to one recorded by josephus as having occurred a little before the death of herod the great. it is probable that the eclipse in question was the total lunar one, which calculation shows to have taken place on the th september b.c., and to have been visible in western asia. this is very important, for we are thus enabled to fix that year as the date of the birth of christ, for herod is known to have died in the early part of the year following the nativity. in those accounts of total lunar eclipses, which have come down to us from the dark and middle ages, the colour of the moon is nearly always likened to "blood." on the other hand, in an account of the eclipse of january , a.d. , our satellite is described as "covered with a horrid black shield." we thus have examples of the two distinct appearances alluded to in chapter vii., _i.e._ when the moon appears of a coppery-red colour, and when it is entirely darkened. it appears, indeed, that, in the majority of lunar eclipses on record, the moon has appeared of a ruddy, or rather of a coppery hue, and the details on its surface have been thus rendered visible. one of the best examples of a _bright_ eclipse of this kind is that of the th march , when the illumination of our satellite was so great that many persons could not believe that an eclipse was actually taking place. a certain mr. foster, who observed this eclipse from bruges, states that the markings on the lunar disc were almost as visible as on an "ordinary dull moonlight night." he goes on to say that the british consul at ghent, not knowing that there had been any eclipse, wrote to him for an explanation of the red colour of the moon on that evening. out of the _dark_ eclipses recorded, perhaps the best example is that of may , , observed by wargentin at stockholm. on this occasion the lunar disc is said to have disappeared so completely, that it could not be discovered even with the telescope. another such instance is the eclipse of june , , observed from london. the summer of that year was particularly wet--a point worthy of notice in connection with the theory that these different appearances are due to the varying state of our earth's atmosphere. sometimes, indeed, it has happened that an eclipse of the moon has partaken of both appearances, part of the disc being visible and part invisible. an instance of this occurred in the eclipse of july , , when the late rev. s.j. johnson, one of the leading authorities on eclipses, who observed it, states that he found one-half the moon's surface quite invisible, both with the naked eye and with the telescope. in addition to the examples given above, there are three total lunar eclipses which deserve especial mention. . a.d. , november . during the progress of this eclipse the moon occulted the star aldebaran in the constellation of taurus. . a.d. , april . this is the celebrated eclipse which is said to have so well served the purposes of christopher columbus. certain natives having refused to supply him with provisions when in sore straits, he announced to them that the moon would be darkened as a sign of the anger of heaven. when the event duly came to pass, the savages were so terrified that they brought him provisions as much as he needed. . a.d. , july . the eclipse in question is notable as having been seen through the telescope, then a recent invention. it was without doubt the first so observed, but unfortunately the name of the observer has not come down to us. chapter x the growth of observation the earliest astronomical observations must have been made in the dawn of historic time by the men who tended their flocks upon the great plains. as they watched the clear night sky they no doubt soon noticed that, with the exception of the moon and those brilliant wandering objects known to us as the planets, the individual stars in the heaven remained apparently fixed with reference to each other. these seemingly changeless points of light came in time to be regarded as sign-posts to guide the wanderer across the trackless desert, or the voyager upon the wide sea. just as when looking into the red coals of a fire, or when watching the clouds, our imagination conjures up strange and grotesque forms, so did the men of old see in the grouping of the stars the outlines of weird and curious shapes. fed with mythological lore, they imagined these to be rough representations of ancient heroes and fabled beasts, whom they supposed to have been elevated to the heavens as a reward for great deeds done upon the earth. we know these groupings of stars to-day under the name of the constellations. looking up at them we find it extremely difficult to fit in the majority with the figures which the ancients believed them to represent. nevertheless, astronomy has accepted the arrangement, for want of a better method of fixing the leading stars in the memory. our early ancestors lived the greater part of their lives in the open air, and so came to pay more attention in general to the heavenly orbs than we do. their clock and their calendar was, so to speak, in the celestial vault. they regulated their hours, their days, and their nights by the changing positions of the sun, the moon, and the stars; and recognised the periods of seed-time and harvest, of calm and stormy weather, by the rising or setting of certain well-known constellations. students of the classics will recall many allusions to this, especially in the odes of horace. as time went on and civilisation progressed, men soon devised measuring instruments, by means of which they could note the positions of the celestial bodies in the sky with respect to each other; and, from observations thus made, they constructed charts of the stars. the earliest complete survey of this kind, of which we have a record, is the great catalogue of stars which was made, in the second century b.c., by the celebrated greek astronomer, hipparchus, and in which he is said to have noted down about stars. it is unnecessary to follow in detail the tedious progress of astronomical discovery prior to the advent of the telescope. certain it is that, as time went on, the measuring instruments to which we have alluded had become greatly improved; but, had they even been perfect, they would have been utterly inadequate to reveal those minute displacements, from which we have learned the actual distance of the nearest of the celestial orbs. from the early times, therefore, until the mediæval period of our own era, astronomy grew up upon a faulty basis, for the earth ever seemed so much the largest body in the universe, that it continued from century to century to be regarded as the very centre of things. to the arabians is due the credit of having kept alive the study of the stars during the dark ages of european history. they erected some fine observatories, notably in spain and in the neighbourhood of bagdad. following them, some of the oriental peoples embraced the science in earnest; ulugh beigh, grandson of the famous tamerlane, founding, for instance, a great observatory at samarcand in central asia. the mongol emperors of india also established large astronomical instruments in the chief cities of their empire. when the revival of learning took place in the west, the europeans came to the front once more in science, and rapidly forged ahead of those who had so assiduously kept alight the lamp of knowledge through the long centuries. the dethronement of the older theories by the copernican system, in which the earth was relegated to its true place, was fortunately soon followed by an invention of immense import, the invention of the telescope. it is to this instrument, indeed, that we are indebted for our knowledge of the actual scale of the celestial distances. it penetrated the depths of space; it brought the distant orbs so near, that men could note the detail on the planets, or measure the small changes in their positions in the sky which resulted from the movement of our own globe. it was in the year that the telescope was first constructed. a year or so previous to this a spectacle-maker of middleburgh in holland, one hans lippershey, had, it appears, hit upon the fact that distant objects, when viewed through certain glass lenses suitably arranged, looked nearer.[ ] news of this discovery reached the ears of galileo galilei, of florence, the foremost philosopher of the day, and he at once applied his great scientific attainments to the construction of an instrument based upon this principle. the result was what was called an "optick tube," which magnified distant objects some few times. it was not much larger than what we nowadays contemptuously refer to as a "spy-glass," yet its employment upon the leading celestial objects instantly sent astronomical science onward with a bound. in rapid succession galileo announced world-moving discoveries; large spots upon the face of the sun; crater-like mountains upon the moon; four subordinate bodies, or satellites, circling around the planet jupiter; and a strange appearance in connection with saturn, which later telescopic observers found to be a broad flat ring encircling that planet. and more important still, the magnified image of venus showed itself in the telescope at certain periods in crescent and other forms; a result which copernicus is said to have announced should of necessity follow if his system were the true one. the discoveries made with the telescope produced, as time went on, a great alteration in the notions of men with regard to the universe at large. it must have been, indeed, a revelation to find that those points of light which they called the planets, were, after all, globes of a size comparable with the earth, and peopled perchance with sentient beings. even to us, who have been accustomed since our early youth to such an idea, it still requires a certain stretch of imagination to enlarge, say, the bright star of eve, into a body similar in size to our earth. the reader will perhaps recollect tennyson's allusion to this in _locksley hall, sixty years after_:-- "hesper--venus--were we native to that splendour or in mars, we should see the globe we groan in, fairest of their evening stars. "could we dream of wars and carnage, craft and madness, lust and spite, roaring london, raving paris, in that point of peaceful light?" the form of instrument as devised by galileo is called the refracting telescope, or "refractor." as we know it to-day it is the same in principle as his "optick tube," but it is not quite the same in construction. the early _object-glass_, or large glass at the end, was a single convex lens (see fig. , p. , "galilean"); the modern one is, on the other hand, composed of two lenses fitted together. the attempts to construct large telescopes of the galilean type met in course of time with a great difficulty. the magnified image of the object observed was not quite pure; its edges, indeed, were fringed with rainbow-like colours. this defect was found to be aggravated with increase in the size of object-glasses. a method was, however, discovered of diminishing this colouration, or _chromatic aberration_ as it is called from the greek word [chrôma] (_chroma_), which means colour, viz. by making telescopes of great length and only a few inches in width. but the remedy was, in a way, worse than the disease; for telescopes thus became of such huge proportions as to be too unwieldy for use. attempts were made to evade this unwieldiness by constructing them with skeleton tubes (see plate ii., p. ), or, indeed, even without tubes at all; the object-glass in the tubeless or "aerial" telescope being fixed at the top of a high post, and the _eye-piece_, that small lens or combination of lenses, which the eye looks directly into, being kept in line with it by means of a string and manoeuvred about near the ground (plate iii., p. ). the idea of a telescope without a tube may appear a contradiction in terms; but it is not really so, for the tube adds nothing to the magnifying power of the instrument, and is, in fact, no more than a mere device for keeping the object-glass and eye-piece in a straight line, and for preventing the observer from being hindered by stray lights in his neighbourhood. it goes without saying, of course, that the image of a celestial object will be more clear and defined when examined in the darkness of a tube. the ancients, though they knew nothing of telescopes, had, however, found out the merit of a tube in this respect; for they employed simple tubes, blackened on the inside, in order to obtain a clearer view of distant objects. it is said that julius cæsar, before crossing the channel, surveyed the opposite coast of britain through a tube of this kind. [illustration: plate ii. great telescope of hevelius this instrument, feet in length, with a _skeleton_ tube, was constructed by the celebrated seventeenth century astronomer, hevelius of danzig. from an illustration in the _machina celestis_. (page )] a few of the most famous of the immensely long telescopes above alluded to are worthy of mention. one of these, feet in length, was presented to the royal society of london by the dutch astronomer huyghens. hevelius of danzig constructed a skeleton one of feet in length (see plate ii., p. ). bradley used a tubeless one feet long to measure the diameter of venus in ; while one of feet is said to have been constructed, but to have proved quite unworkable! such difficulties, however, produced their natural result. they set men at work to devise another kind of telescope. in the new form, called the reflecting telescope, or "reflector," the light coming from the object under observation was _reflected_ into the eye-piece from the surface of a highly polished concave metallic mirror, or _speculum_, as it was called. it is to sir isaac newton that the world is indebted for the reflecting telescope in its best form. that philosopher had set himself to investigate the causes of the rainbow-like, or prismatic colours which for a long time had been such a source of annoyance to telescopic observers; and he pointed out that, as the colours were produced in the passage of the rays of light _through_ the glass, they would be entirely absent if the light were reflected from the _surface_ of a mirror instead. the reflecting telescope, however, had in turn certain drawbacks of its own. a mirror, for instance, can plainly never be polished to such a high degree as to reflect as much light as a piece of transparent glass will let through. further, the position of the eye-piece is by no means so convenient. it cannot, of course, be pointed directly towards the mirror, for the observer would then have to place his head right in the way of the light coming from the celestial object, and would thus, of course, cut it off. in order to obviate this difficulty, the following device was employed by newton in his telescope, of which he constructed his first example in . a small, flat mirror was fixed by thin wires in the centre of the tube of the telescope, and near to its open end. it was set slant-wise, so that it reflected the rays of light directly into the eye-piece, which was screwed into a hole at the side of the tube (see fig. , p. , "newtonian"). although the newtonian form of telescope had the immense advantage of doing away with the prismatic colours, yet it wasted a great deal of light; for the objection in this respect with regard to loss of light by reflection from the large mirror applied, of course, to the small mirror also. in addition, the position of the "flat," as the small mirror is called, had the further effect of excluding from the great mirror a certain proportion of light. but the reflector had the advantage, on the other hand, of costing less to make than the refractor, as it was not necessary to procure flawless glass for the purpose. a disc of a certain metallic composition, an alloy of copper and tin, known in consequence as _speculum metal_, had merely to be cast; and this had to be ground and polished _upon one side only_, whereas a lens has to be thus treated _upon both its sides_. it was, therefore, possible to make a much larger instrument at a great deal less labour and expense. [illustration: plate iii. a tubeless, or "aerial" telescope from an illustration in the _opera varia_ of christian huyghens. (page )] [illustration: fig. .--the various types of telescope. all the above telescopes are _pointed_ in the same direction; that is to say, the rays of light from the object are coming from the left-hand side.] we have given the newtonian form as an example of the principle of the reflecting telescope. a somewhat similar instrument had, however, been projected, though not actually constructed, by james gregory a few years earlier than newton's, _i.e._ in . in this form of reflector, known as the "gregorian" telescope, a hole was made in the big concave mirror; and a small mirror, also concave, which faced it at a certain distance, received the reflected rays, and reflected them back again through the hole in question into the eye-piece, which was fixed just behind (see fig. , p. , "gregorian"). the gregorian had thus the sentimental advantage of being _pointed directly at the object_. the hole in the big mirror did not cause any loss of light, for the central portion in which it was made was anyway unable to receive light through the small mirror being directly in front of it. an adaptation of the gregorian was the "cassegrainian" telescope, devised by cassegrain in , which differed from it chiefly in the small mirror being convex instead of concave (see fig. , p. , "cassegrainian"). these _direct-view_ forms of the reflecting telescope were much in vogue about the middle of the eighteenth century, when many beautiful examples of gregorians were made by the famous optician, james short, of edinburgh. an adaptation of the newtonian type of telescope is known as the "herschelian," from being the kind favoured by sir william herschel. it is, however, only suitable in immense instruments, such as herschel was in the habit of employing. in this form the object-glass is set at a slight slant, so that the light coming from the object is reflected straight into the eye-piece, which is fixed facing it in the side of the tube (see fig. , p. , "herschelian"). this telescope has an advantage over the other forms of reflector through the saving of light consequent on doing away with the _second_ reflection. there is, however, the objection that the slant of the object-glass is productive of some distortion in the appearance of the object observed; but this slant is of necessity slight when the length of the telescope is very great. the principle of this type of telescope had been described to the french academy of sciences as early as by le maire, but no one availed himself of the idea until , when herschel tried it. at first, however, he rejected it; but in he seems to have found that it suited the huge instruments which he was then making. herschel's largest telescope, constructed in , was about four feet in diameter and forty feet in length. it is generally spoken of as the "forty-foot telescope," though all other instruments have been known by their _diameters_, rather than by their lengths. to return to the refracting telescope. a solution of the colour difficulty was arrived at in (two years after newton's death) by an essex gentleman named chester moor hall. he discovered that by making a double object-glass, composed of an outer convex lens and an inner concave lens, made respectively of different kinds of glass, _i.e._ _crown_ glass and _flint_ glass, the troublesome colour effects could be, _to a very great extent_, removed. hall's investigations appear to have been rather of an academic nature; and, although he is believed to have constructed a small telescope upon these lines, yet he seems to have kept the matter so much to himself that it was not until the year that the first example of the new instrument was given to the world. this was done by john dollond, founder of the well-known optical firm of dollond, of ludgate hill, london, who had, quite independently, re-discovered the principle. this "achromatic" telescope, or telescope "free from colour effects," is the kind ordinarily in use at present, whether for astronomical or for terrestrial purposes (see fig. , p. , "achromatic"). the expense of making large instruments of this type is very great, for, in the object-glass alone, no less than _four_ surfaces have to be ground and polished to the required curves; and, usually, the two lenses of which it is composed have to fit quite close together. with the object of evading the expense referred to, and of securing _complete_ freedom from colour effects, telescopes have even been made, the object-glasses of which were composed of various transparent liquids placed between thin lenses; but leakages, and currents set up within them by changes of temperature, have defeated the ingenuity of those who devised these substitutes. the solution of the colour difficulty by means of dollond's achromatic refractor has not, however, ousted the reflecting telescope in its best, or newtonian form, for which great concave mirrors made of glass, covered with a thin coating of silver and highly polished, have been used since about instead of metal mirrors. they are very much lighter in weight and cheaper to make than the old specula; and though the silvering, needless to say, deteriorates with time, it can be renewed at a comparatively trifling cost. also these mirrors reflect much more light, and give a clearer view, than did the old metallic ones. when an object is viewed through the type of astronomical telescope ordinarily in use, it is seen _upside down_. this is, however, a matter of very small moment in dealing with celestial objects; for, as they are usually round, it is really not of much consequence which part we regard as top and which as bottom. such an inversion would, of course, be most inconvenient when viewing terrestrial objects. in order to observe the latter we therefore employ what is called a terrestrial telescope, which is merely a refractor with some extra lenses added in the eye portion for the purpose of turning the inverted image the right way up again. these extra lenses, needless to say, absorb a certain amount of light; wherefore it is better in astronomical observation to save light by doing away with them, and putting up with the slight inconvenience of seeing the object inverted. this inversion of images by the astronomical telescope must be specially borne in mind with regard to the photographs of the moon in chapter xvi. in the year the largest achromatic refractor in existence was one of nine and a half inches in diameter constructed by fraunhofer for the observatory of dorpat in russia. the largest refractors in the world to-day are in the united states, _i.e._ the forty-inch of the yerkes observatory (see plate iv., p. ), and the thirty-six inch of the lick. the object-glasses of these and of the thirty-inch telescope of the observatory of pulkowa, in russia, were made by the great optical house of alvan clark & sons, of cambridge, massachusetts, u.s.a. the tubes and other portions of the yerkes and lick telescopes were, however, constructed by the warner and swasey co., of cleveland, ohio. the largest reflector, and so the largest telescope in the world, is still the six-foot erected by the late lord rosse at parsonstown in ireland, and completed in the year . it is about fifty-six feet in length. next come two of five feet, with mirrors of silver on glass; one of them made by the late dr. common, of ealing, and the other by the american astronomer, professor g.w. ritchey. the latter of these is installed in the solar observatory belonging to carnegie institution of washington, which is situated on mount wilson in california. the former is now at the harvard college observatory, and is considered by professor moulton to be probably the most efficient reflector in use at present. another large reflector is the three-foot made by dr. common. it came into the possession of mr. crossley of halifax, who presented it to the lick observatory, where it is now known as the "crossley reflector." although to the house of clark belongs, as we have seen, the credit of constructing the object-glasses of the largest refracting telescopes of our time, it has nevertheless keen competitors in sir howard grubb, of dublin, and such well-known firms as cooke of york and steinheil of munich. in the four-foot reflector, made in for the observatory of melbourne by the firm of grubb, the cassegrainian principle was employed. with regard to the various merits of refractors and reflectors much might be said. each kind of instrument has, indeed, its special advantages; though perhaps, on the whole, the most perfect type of telescope is the achromatic refractor. [illustration: plate iv. the great yerkes telescope great telescope at the yerkes observatory of the university of chicago, williams bay, wisconsin, u.s.a. it was erected in - , and is the largest refracting telescope in the world. diameter of object-glass, inches; length of telescope, about feet. the object-glass was made by the firm of alvan clark and sons, of cambridge, massachusetts; the other portions of the instrument by the warner and swasey co., of cleveland, ohio. (page )] in connection with telescopes certain devices have from time to time been introduced, but these merely aim at the _convenience_ of the observer and do not supplant the broad principles upon which are based the various types of instrument above described. such, for instance, are the "siderostat," and another form of it called the "coelostat," in which a plane mirror is made to revolve in a certain manner, so as to reflect those portions of the sky which are to be observed, into the tube of a telescope kept fixed. such too are the "equatorial coudé" of the late m. loewy, director of the paris observatory, and the "sheepshanks telescope" of the observatory of cambridge, in which a telescope is separated into two portions, the eye-piece portion being fixed upon a downward slant, and the object-glass portion jointed to it at an angle and pointed up at the sky. in these two instruments (which, by the way, differ materially) an arrangement of slanting mirrors in the tubes directs the journey of the rays of light from the object-glass to the eye-piece. the observer can thus sit at the eye-end of his telescope in the warmth and comfort of his room, and observe the stars in the same unconstrained manner as if he were merely looking down into a microscope. needless to say, devices such as these are subject to the drawback that the mirrors employed sap a certain proportion of the rays of light. it will be remembered that we made allusion to loss of light in this way, when pointing out the advantage in light grasp of the herschelian form of telescope, where only _one_ reflection takes place, over the newtonian in which there are _two_. it is an interesting question as to whether telescopes can be made much larger. the american astronomer, professor g.e. hale, concludes that the limit of refractors is about five feet in diameter, but he thinks that reflectors as large as nine feet in diameter might now be made. as regards refractors there are several strong reasons against augmenting their proportions. first of all comes the great cost. secondly, since the lenses are held in position merely round their rims, they will bend by their weight in the centres if they are made much larger. on the other hand, attempts to obviate this, by making the lenses thicker, would cause a decrease in the amount of light let through. but perhaps the greatest stumbling-block to the construction of larger telescopes is the fact that the unsteadiness of the air will be increasingly magnified. and further, the larger the tubes become, the more difficult will it be to keep the air within them at one constant temperature throughout their lengths. it would, indeed, seem as if telescopes are not destined greatly to increase in size, but that the means of observation will break out in some new direction, as it has already done in the case of photography and the spectroscope. the direct use of the eye is gradually giving place to indirect methods. we are, in fact, now _feeling_ rather than seeing our way about the universe. up to the present, for instance, we have not the slightest proof that life exists elsewhere than upon our earth. but who shall say that the twentieth century has not that in store for us, by which the presence of life in other orbs may be perceived through some form of vibration transmitted across illimitable space? there is no use speaking of the impossible or the inconceivable. after the extraordinary revelations of the spectroscope--nay, after the astounding discovery of röntgen--the word impossible should be cast aside, and inconceivability cease to be regarded as any criterion. [ ] the principle upon which the telescope is based appears to have been known _theoretically_ for a long time previous to this. the monk roger bacon, who lived in the thirteenth century, describes it very clearly; and several writers of the sixteenth century have also dealt with the idea. even lippershey's claims to a practical solution of the question were hotly contested at the time by two of his own countrymen, _i.e._ a certain jacob metius, and another spectacle-maker of middleburgh, named jansen. chapter xi spectrum analysis if white light (that of the sun, for instance) be passed through a glass prism, namely, a piece of glass of triangular shape, it will issue from it in rainbow-tinted colours. it is a common experience with any of us to notice this when the sunlight shines through cut-glass, as in the pendant of a chandelier, or in the stopper of a wine-decanter. the same effect may be produced when light passes through water. the rainbow, which we all know so well, is merely the result of the sunlight passing through drops of falling rain. white light is composed of rays of various colours. red, orange, yellow, green, blue, indigo, and violet, taken all together, go, in fact, to make up that effect which we call white. it is in the course of the _refraction_, or bending of a beam of light, when it passes in certain conditions through a transparent and denser medium, such as glass or water, that the constituent rays are sorted out and spread in a row according to their various colours. this production of colour takes place usually near the edges of a lens; and, as will be recollected, proved very obnoxious to the users of the old form of refracting telescope. it is, indeed, a strange irony of fate that this very same production of colour, which so hindered astronomy in the past, should have aided it in recent years to a remarkable degree. if sunlight, for instance, be admitted through a narrow slit before it falls upon a glass prism, it will issue from the latter in the form of a band of variegated colour, each colour blending insensibly with the next. the colours arrange themselves always in the order which we have mentioned. this seeming band is, in reality, an array of countless coloured images of the original slit ranged side by side; the colour of each image being the slightest possible shade different from that next to it. this strip of colour when produced by sunlight is called the "solar spectrum" (see fig. , p. ). a similar strip, or _spectrum_, will be produced by any other light; but the appearance of the strip, with regard to preponderance of particular colours, will depend upon the character of that light. electric light and gas light yield spectra not unlike that of sunlight; but that of gas is less rich in blue and violet than that of the sun. the spectroscope, an instrument devised for the examination of spectra, is, in its simplest form, composed of a small tube with a narrow slit and prism at one end, and an eye-piece at the other. if we drop ordinary table salt into the flame of a gas light, the flame becomes strongly yellow. if, then, we observe this yellow flame with the spectroscope, we find that its spectrum consists almost entirely of two bright yellow transverse lines. chemically considered ordinary table salt is sodium chloride; that is to say, a compound of the metal sodium and the gas chlorine. now if other compounds of sodium be experimented with in the same manner, it will soon be found that these two yellow lines are characteristic of sodium when turned into vapour by great heat. in the same manner it can be ascertained that every element, when heated to a condition of vapour, gives as its spectrum a set of lines peculiar to itself. thus the spectroscope enables us to find out the composition of substances when they are reduced to vapour in the laboratory. [illustration: fig. .--the solar spectrum.] in order to increase the power of a spectroscope, it is necessary to add to the number of prisms. each extra prism has the effect of lengthening the coloured strip still more, so that lines, which at first appeared to be single merely through being crowded together, are eventually drawn apart and become separately distinguishable. on this principle it has gradually been determined that the sun is composed of elements similar to those which go to make up our earth. further, the composition of the stars can be ascertained in the same manner; and we find them formed on a like pattern, though with certain elements in greater or less proportion as the case may be. it is in consequence of our thus definitely ascertaining that the stars are self-luminous, and of a sun-like character, that we are enabled to speak of them as _suns_, or to call the sun a _star_. in endeavouring to discover the elements of which the planets and satellites of our system are composed, we, however, find ourselves baffled, for the simple reason that these bodies emit no real light of their own. the light which reaches us from them, being merely reflected sunlight, gives only the ordinary solar spectrum when examined with the spectroscope. but in certain cases we find that the solar spectrum thus viewed shows traces of being weakened, or rather of suffering absorption; and it is concluded that this may be due to the sunlight having had to pass through an atmosphere on its way to and from the surface of the planet from which it is reflected to us. since the sun is found to be composed of elements similar to those which go to make up our earth, we need not be disheartened at this failure of the spectroscope to inform us of the composition of the planets and satellites. we are justified, indeed, in assuming that more or less the same constituents run through our solar system; and that the elements of which these bodies are composed are similar to those which are found upon our earth and in the sun. the spectroscope supplies us with even more information. it tells us, indeed, whether the sun-like body which we are observing is moving away from us or towards us. a certain slight shifting of the lines towards the red or violet end of the spectrum respectively, is found to follow such movement. this method of observation is known by the name of _doppler's method_,[ ] and by it we are enabled to confirm the evidence which the sunspots give us of the rotation of the sun; for we find thus that one edge of that body is continually approaching us, and the other edge is continually receding from us. also, we can ascertain in the same manner that certain of the stars are moving towards us, and certain of them away from us. [ ] the idea, initiated by christian doppler at prague in , was originally applied to sound. the approach or recession of a source from which sound is coming is invariably accompanied by alterations of pitch, as the reader has no doubt noticed when a whistling railway-engine has approached him or receded from him. it is to sir william huggins, however, that we are indebted for the application of the principle to spectroscopy. this he gave experimental proof of in the year . chapter xii the sun the sun is the chief member of our system. it controls the motions of the planets by its immense gravitative power. besides this it is the most important body in the entire universe, so far as we are concerned; for it pours out continually that flood of light and heat, without which life, as we know it, would quickly become extinct upon our globe. light and heat, though not precisely the same thing, may be regarded, however, as next-door neighbours. the light rays are those which directly affect the eye and are comprised in the visible spectrum. we _feel_ the heat rays, the chief of which are beyond the red portion of the spectrum. they may be investigated with the _bolometer_, an instrument invented by the late professor langley. chemical rays--for instance, those radiations which affect the photographic plate--are for the most part also outside the visible spectrum. they are, however, at the other end of it, namely, beyond the violet. such a scale of radiations may be compared to the keyboard of an imaginary piano, the sound from only one of whose octaves is audible to us. the brightest light we know on the earth is dull compared with the light of the sun. it would, indeed, look quite dark if held up against it. it is extremely difficult to arrive at a precise notion of the temperature of the body of the sun. however, it is far in excess of any temperature which we can obtain here, even in the most powerful electric furnace. a rough idea of the solar heat may be gathered from the calculation that if the sun's surface were coated all over with a layer of ice feet thick, it would melt through this completely in one hour. the sun cannot be a hot body merely cooling; for the rate at which it is at present giving off heat could not in such circumstances be kept up, according to professor moulton, for more than years. further, it is not a mere burning mass, like a coal fire, for instance; as in that case about a thousand years would show a certain drop in temperature. no perceptible diminution of solar heat having taken place within historic experience, so far as can be ascertained, we are driven to seek some more abstruse explanation. the theory which seems to have received most acceptance is that put forward by helmholtz in . his idea was that gravitation produces continual contraction, or falling in of the outer parts of the sun; and that this falling in, in its turn, generates enough heat to compensate for what is being given off. the calculations of helmholtz showed that a contraction of about feet a year from the surface towards the centre would suffice for the purpose. in recent years, however, this estimate has been extended to about feet. nevertheless, even with this increased figure, the shrinkage required is so slight in comparison with the immense girth of the sun, that it would take a continual contraction at this rate for about years, to show even in our finest telescopes that any change in the size of that body was taking place at all. upon this assumption of continuous contraction, a time should, however, eventually be reached when the sun will have shrunk to such a degree of solidity, that it will not be able to shrink any further. then, the loss of heat not being made up for any longer, the body of the sun should begin to grow cold. but we need not be distressed on this account; for it will take some , , years, according to the above theory, before the solar orb becomes too cold to support life upon our earth. since the discovery of radium it has, on the other hand, been suggested, and not unreasonably, that radio-active matter may possibly play an important part in keeping up the heat of the sun. but the body of scientific opinion appears to consider the theory of contraction as a result of gravitation, which has been outlined above, to be of itself quite a sound explanation. indeed, the late lord kelvin is said to have held to the last that it was amply sufficient to account for the underground heat of the earth, the heat of the sun, and that of all the stars in the universe. one great difficulty in forming theories with regard to the sun, is the fact that the temperature and gravitation there are enormously in excess of anything we meet with upon our earth. the force of gravity at the sun's surface is, indeed, about twenty-seven times that at the surface of our globe. the earth's atmosphere appears to absorb about one-half of the radiations which come to us from the sun. this absorptive effect is very noticeable when the solar orb is low down in our sky, for its light and heat are then clearly much reduced. of the light rays, the blue ones are the most easily absorbed in this way; which explains why the sun looks red when near the horizon. it has then, of course, to shine through a much greater thickness of atmosphere than when high up in the heavens. what astonishes one most about the solar radiation, is the immense amount of it that is apparently wasted into space in comparison with what falls directly upon the bodies of the solar system. only about the one-hundred-millionth is caught by all the planets together. what becomes of the rest we cannot tell. that brilliant white body of the sun, which we see, is enveloped by several layers of gases and vaporous matter, in the same manner as our globe is enveloped by its atmosphere (see fig. , p. ). these are transparent, just as our atmosphere is transparent; and so we see the white bright body of the sun right through them. this white bright portion is called the _photosphere_. from it comes most of that light and heat which we see and feel. we do not know what lies under the photosphere, but, no doubt, the more solid portions of the sun are there situated. just above the photosphere, and lying close upon it, is a veil of smoke-like haze. next upon this is what is known as the _reversing layer_, which is between and miles in thickness. it is cooler than the underlying photosphere, and is composed of glowing gases. many of the elements which go to make up our earth are present in the reversing layer in the form of vapour. the _chromosphere_, of which especial mention has already been made in dealing with eclipses of the sun, is another layer lying immediately upon the last one. it is between and , miles in thickness. like the reversing layer, it is composed of glowing gases, chief among which is the vapour of hydrogen. the colour of the chromosphere is, in reality, a brilliant scarlet; but, as we have already said, the intensely white light of the photosphere shines through it from behind, and entirely overpowers its redness. the upper portion of the chromosphere is in violent agitation, like the waves of a stormy sea, and from it rise those red prominences which, it will be recollected, are such a notable feature in total solar eclipses. [illustration: fig. .--a section through the sun, showing how the prominences rise from the chromosphere.] the _corona_ lies next in order outside the chromosphere, and is, so far as we know, the outermost of the accompaniments of the sun. this halo of pearly-white light is irregular in outline, and fades away into the surrounding sky. it extends outwards from the sun to several millions of miles. as has been stated, we can never see the corona unless, when during a total solar eclipse, the moon has, for the time being, hidden the brilliant photosphere completely from our view. the solar spectrum is really composed of three separate spectra commingled, _i.e._ those of the photosphere, of the reversing layer, and of the chromosphere respectively. if, therefore, the photosphere could be entirely removed, or covered up, we should see only the spectra of those layers which lie upon it. such a state of things actually occurs in a total eclipse of the sun. when the moon's body has crept across the solar disc, and hidden the last piece of photosphere, the solar spectrum suddenly becomes what is technically called "reversed,"--the dark lines crossing it changing into bright lines. this occurs because a strip of those layers which lie immediately upon the photosphere remains still uncovered. the lower of these layers has therefore been called the "reversing layer," for want of a better name. after a second or two this reversed spectrum mostly vanishes, and an altered spectrum is left to view. taking into consideration the rate at which the moon is moving across the face of the sun, and the very short time during which the spectrum of the reversing layer lasts, the thickness of that layer is estimated to be not more than a few hundred miles. in the same way the last of the three spectra--namely, that of the chromosphere--remains visible for such a time as allows us to estimate its depth at about ten times that of the reversing layer, or several thousand miles. when the chromosphere, in its turn during a total eclipse, has been covered by the moon, the corona alone is left. this has a distinct spectrum of its own also; wherein is seen a strange line in the green portion, which does not tally with that of any element we are acquainted with upon the earth. this unknown element has received for the time being the name of "coronium." chapter xiii the sun--_continued_ the various parts of the sun will now be treated of in detail. i. photosphere. the photosphere, or "light-sphere," from the greek [phôs] (_phos_), which means _light_, is, as we have already said, the innermost portion of the sun which can be seen. examined through a good telescope it shows a finely mottled structure, as of brilliant granules, somewhat like rice grains, with small dark spaces lying in between them. it has been supposed that we have here the process of some system of circulation by which the sun keeps sending forth its radiations. in the bright granules we perhaps see masses of intensely heated matter, rising from the interior of the sun. the dark interspaces may represent matter which has become cooled and darkened through having parted with its heat and light, and is falling back again into the solar furnace. the _sun spots_, so familiar to every one nowadays, are dark patches which are often seen to break out in the photosphere (see plate v., p. ). they last during various periods of time; sometimes only for a few days, sometimes so long as a month or more. a spot is usually composed of a dark central portion called the _umbra_, and a less dark fringe around this called the _penumbra_ (see plate vi., p. ). the umbra ordinarily has the appearance of a deep hole in the photosphere; but, that it is a hole at all, has by no means been definitely proved. [illustration: plate v. the sun, showing several groups of spots from a photograph taken at the royal observatory, greenwich. the cross-lines seen on the disc are in no way connected with the sun, but belong to the telescope through which the photograph was taken. (page )] sun spots are, as a rule, some thousands of miles across. the umbra of a good-sized spot could indeed engulf at once many bodies the size of our earth. sun spots do not usually appear singly, but in groups. the total area of a group of this kind may be of immense extent; even so great as to cover the one-hundredth part of the whole surface of the sun. very large spots, when such are present, may be seen without any telescope; either through a piece of smoked glass, or merely with the naked eye when the air is misty, or the sun low on the horizon. the umbra of a spot is not actually dark. it only appears so in contrast with the brilliant photosphere around. spots form, grow to a large size in comparatively short periods of time, and then quickly disappear. they seem to shrink away as a consequence of the photosphere closing in upon them. that the sun is rotating upon an axis, is shown by the continual change of position of all spots in one constant direction across his disc. the time in which a spot is carried completely round depends, however, upon the position which it occupies upon the sun's surface. a spot situated near the equator of the sun goes round once in about twenty-five days. the further a spot is situated from this equator, the longer it takes. about twenty-seven days is the time taken by a spot situated midway between the equator and the solar poles. spots occur to the north of the sun's equator, as well as to the south; though, since regular observations have been made--that is to say, during the past fifty years or so--they appear to have broken out a little more frequently in the southern parts. from these considerations it will be seen that the sun does not rotate as the earth does, but that different portions appear to move at different speeds. whether in the neighbourhood of the solar poles the time of rotation exceeds twenty-seven days we are unable to ascertain, for spots are not seen in those regions. no explanation has yet been given of this peculiar rotation; and the most we can say on the subject is that the sun is not by any means a solid body. _faculæ_ (latin, little torches) are brilliant patches which appear here and there upon the sun's surface, and are in some way associated with spots. their displacement, too, across the solar face confirms the evidence which the spots give us of the sun's rotation. our proofs of this rotation are still further strengthened by the doppler spectroscopic method of observation alluded to in chapter xi. as was then stated, one edge of the sun is thus found to be continually approaching us, and the other side continually receding from us. the varying rates of rotation, which the spots and faculæ give us, are duly confirmed by this method. [illustration: plate vi. photograph of a sunspot this fine picture was taken by the late m. janssen. the granular structure of the sun's surface is here well represented. (from _knowledge_.) (page )] the first attempt to bring some regularity into the question of sunspots was the discovery by schwabe, in , that they were subject to a regular variation. as a matter of fact they wax and wane in their number, and the total area which they cover, in the course of a period, or cycle, of on an average about - / years; being at one part of this period large and abundant, and at another few and small. this period of - / years is known as the sun spot cycle. no explanation has yet been given of the curious round of change, but the period in question seems to govern most of the phenomena connected with the sun. ii. reversing layer. this is a layer of relatively cool gases lying immediately upon the photosphere. we never see it directly; and the only proof we have of its presence is that remarkable reversal of the spectrum already described, when during an instant or two in a total eclipse, the advancing edge of the moon, having just hidden the brilliant photosphere, is moving across the fine strip which the layer then presents edgewise towards us. the fleeting moments during which this reversed spectrum lasts, informs us that the layer is comparatively shallow; little more indeed than about miles in depth. the spectrum of the reversing layer, or "flash spectrum," as it is sometimes called on account of the instantaneous character with which the change takes place, was, as we have seen, first noticed by young in ; and has been successfully photographed since then during several eclipses. the layer itself appears to be in a fairly quiescent state; a marked contrast to the seething photosphere beneath, and the agitated chromosphere above. iii. the chromosphere. the chromosphere--so called from the greek [chrôma] (_chroma_), which signifies _colour_--is a layer of gases lying immediately upon the preceding one. its thickness is, however, plainly much the greater of the two; for whereas the reversing layer is only revealed to us _indirectly_ by the spectroscope, a portion of the chromosphere may clearly be _seen_ in a total eclipse in the form of a strip of scarlet light. the time which the moon's edge takes to traverse it tells us that it must be about ten times as deep as the reversing layer, namely, from to , miles in depth. its spectrum shows that it is composed chiefly of hydrogen, calcium and helium, in the state of vapour. its red colour is mainly due to glowing hydrogen. the element helium, which it also contains, has received its appellation from [hêlios] (_helios_), the greek name for the sun; because, at the time when it first attracted attention, there appeared to be no element corresponding to it upon our earth, and it was consequently imagined to be confined to the sun alone. sir william ramsay, however, discovered it to be also a terrestrial element in , and since then it has come into much prominence as one of the products given off by radium. taking into consideration the excessive force of gravity on the sun, one would expect to find the chromosphere and reversing layer growing gradually thicker in the direction of the photosphere. this, however, is not the case. both these layers are strangely enough of the same densities all through; which makes it suspected that, in these regions, the force of gravity may be counteracted by some other force or forces, exerting a powerful pressure outwards from the sun. iv. the prominences. we have already seen, in dealing with total eclipses, that the exterior surface of the chromosphere is agitated like a stormy sea, and from it billows of flame are tossed up to gigantic heights. these flaming jets are known under the name of prominences, because they were first noticed in the form of brilliant points projecting from behind the rim of the moon when the sun was totally eclipsed. prominences are of two kinds, _eruptive_ and _quiescent_. the eruptive prominences spurt up directly from the chromosphere with immense speeds, and change their shape with great rapidity. quiescent prominences, on the other hand, have a form somewhat like trees, and alter their shape but slowly. in the eruptive prominences glowing masses of gas are shot up to altitudes sometimes as high as , miles,[ ] with velocities even so great as from to miles a second. it has been noticed that the eruptive prominences are mostly found in those portions of the sun where spots usually appear, namely, in the regions near the solar equator. the quiescent prominences, on the other hand, are confined, as a rule, to the neighbourhood of the sun's poles. prominences were at first never visible except during total eclipses of the sun. but in the year , as we have already seen, a method of employing the spectroscope was devised, by means of which they could be observed and studied at any time, without the necessity of waiting for an eclipse. a still further development of the spectroscope, the _spectroheliograph_, an instrument invented almost simultaneously by professor hale and the french astronomer, m. deslandres, permits of photographs being taken of the sun, with the light emanating from _only one_ of its glowing gases at a time. for instance, we can thus obtain a record of what the glowing hydrogen alone is doing on the solar body at any particular moment. with this instrument it is also possible to obtain a series of photographs, showing what is taking place upon the sun at various levels. this is very useful in connection with the study of the spots; for we are, in consequence, enabled to gather more evidence on the subject of their actual form than is given us by their highly foreshortened appearances when observed directly in the telescope. v. corona. (latin, _a crown_.) this marvellous halo of pearly-white light, which displays itself to our view only during the total phase of an eclipse of the sun, is by no means a layer like those other envelopments of the sun of which we have just been treating. it appears, on the other hand, to be composed of filmy matter, radiating outwards in every direction, and fading away gradually into space. its structure is noted to bear a strong resemblance to the tails of comets, or the streamers of the aurora borealis. our knowledge concerning the corona has, however, advanced very slowly. we have not, so far, been as fortunate with regard to it as with regard to the prominences; and, for all we can gather concerning it, we are still entirely dependent upon the changes and chances of total solar eclipses. all attempts, in fact, to apply the spectroscopic method, so as to observe the corona at leisure in full sunlight in the way in which the prominences can be observed, have up to the present met with failure. the general form under which the corona appears to our eyes varies markedly at different eclipses. sometimes its streamers are many, and radiate all round; at other times they are confined only to the middle portions of the sun, and are very elongated, with short feathery-looking wisps adorning the solar poles. it is noticed that this change of shape varies in close accordance with that - / year period during which the sun spots wax and wane; the many-streamered regular type corresponding to the time of great sunspot activity, while the irregular type with the long streamers is present only when the spots are few (see plate vii., p. ). streamers have often been noted to issue from those regions of the sun where active prominences are at the moment in existence; but it cannot be laid down that this is always the case. no hypothesis has yet been formulated which will account for the structure of the corona, or for its variation in shape. the great difficulty with regard to theorising upon this subject, is the fact that we see so much of the corona under conditions of marked foreshortening. assuming, what indeed seems natural, that the rays of which it is composed issue in every direction from the solar body, in a manner which may be roughly imitated by sticking pins all over a ball; it is plainly impossible to form any definite idea concerning streamers, which actually may owe most of the shape they present to us, to the mixing up of multitudes of rays at all kinds of angles to the line of sight. in a word, we have to try and form an opinion concerning an arrangement which, broadly speaking, is _spherical_, but which, on account of its distance, must needs appear to us as absolutely _flat_. the most known about the composition of the corona is that it is made up of particles of matter, mingled with a glowing gas. it is an element in the composition of this gas which, as has been stated, is not found to tally with any known terrestrial element, and has, therefore, received the name of coronium for want of a better designation. one definite conclusion appears to be reached with regard to the corona, _i.e._ that the matter of which it is composed, must be exceedingly rarefied; as it is not found, for instance, to retard appreciably the speed of comets, on occasions when these bodies pass very close to the sun. a calculation has indeed been made which would tend to show that the particles composing the coronal matter, are separated from each other by a distance of perhaps between two and three yards! the density of the corona is found not to increase inwards towards the sun. this is what has already been noted with regard to the layers lying beneath it. powerful forces, acting in opposition to gravity, must hold sway here also. [illustration: (a.) the total eclipse of the sun of december nd, drawn by mr. w.h. wesley from a photograph taken at syracuse by mr. brothers. this is the type of corona seen at the time of _greatest_ sunspot activity. the coronas of (plate i., p. ) and of (frontispiece) are of the same type. (b.) the total eclipse of the sun of may th, drawn by mr. w.h. wesley from photographs taken by mr. e.w. maunder. this is the type of corona seen when the sunspots are _least_ active. compare the "ring with wings," fig. , p. . plate vii. forms of the solar corona at the epochs of sunspot maximum and sunspot minimum, respectively (page )] the - / year period, during which the sun spots vary in number and size, appears to govern the activities of the sun much in the same way that our year does the changing seasonal conditions of our earth. not only, as we have seen, does the corona vary its shape in accordance with the said period, but the activity of the prominences, and of the faculæ, follow suit. further, this constant round of ebb and flow is not confined to the sun itself, but, strangely enough, affects the earth also. the displays of the aurora borealis, which we experience here, coincide closely with it, as does also the varying state of the earth's magnetism. the connection may be still better appreciated when a great spot, or group of spots, has made its appearance upon the sun. it has, for example, often been noted that when the solar rotation carries a spot, or group of spots, across the middle of the visible surface of the sun, our magnetic and electrical arrangements are disturbed for the time being. the magnetic needles in our observatories are, for instance, seen to oscillate violently, telegraphic communication is for a while upset, and magnificent displays of the aurora borealis illumine our night skies. mr. e.w. maunder, of greenwich observatory, who has made a very careful investigation of this subject, suspects that, when elongated coronal streamers are whirled round in our direction by the solar rotation, powerful magnetic impulses may be projected upon us at the moments when such streamers are pointing towards the earth. some interesting investigations with regard to sunspots have recently been published by mrs. e.w. maunder. in an able paper, communicated to the royal astronomical society on may , , she reviews the greenwich observatory statistics dealing with the number and extent of the spots which have appeared during the period from to --a whole sunspot cycle. from a detailed study of the dates in question, she finds that the number of those spots which are formed on the side of the sun turned away from us, and die out upon the side turned towards us, is much greater than the number of those which are formed on the side turned towards us and die out upon the side turned away. it used, for instance, to be considered that the influence of a planet might _produce_ sunspots; but these investigations make it look rather as if some influence on the part of the earth tends, on the contrary, to _extinguish_ them. mrs. maunder, so far, prefers to call the influence thus traced an _apparent_ influence only, for, as she very fairly points out, it seems difficult to attribute a real influence in this matter to the earth, which is so small a thing in comparison not only with the sun, but even with many individual spots. the above investigation was to a certain degree anticipated by mr. henry corder in ; but mrs. maunder's researches cover a much longer period, and the conclusions deduced are of a wider and more defined nature. with regard to its chemical composition, the spectroscope shows us that thirty-nine of the elements which are found upon our earth are also to be found in the sun. of these the best known are hydrogen, oxygen, helium, carbon, calcium, aluminium, iron, copper, zinc, silver, tin, and lead. some elements of the metallic order have, however, not been found there, as, for instance, gold and mercury; while a few of the other class of element, such as nitrogen, chlorine, and sulphur, are also absent. it must not, indeed, be concluded that the elements apparently missing do not exist at all in the solar body. gold and mercury have, in consequence of their great atomic weight, perhaps sunk away into the centre. again, the fact that we cannot find traces of certain other elements, is no real proof of their entire absence. some of them may, for instance, be resolved into even simpler forms, under the unusual conditions which exist in the sun; and so we are unable to trace them with the spectroscope, the experience of which rests on laboratory experiments conducted, at best, in conditions which obtain upon the earth. [ ] on november , , dr. a. rambaut, radcliffe observer at oxford university, noted a prominence which rose to a height of , miles. chapter xiv the inferior planets starting from the centre of the solar system, the first body we meet with is the planet mercury. it circulates at an average distance from the sun of about thirty-six millions of miles. the next body to it is the planet venus, at about sixty-seven millions of miles, namely, about double the distance of mercury from the sun. since our earth comes next again, astronomers call those planets which circulate within its orbit, _i.e._ mercury and venus, the inferior planets, while those which circulate outside it they call the superior planets.[ ] in studying the inferior planets, the circumstances in which we make our observations are so very similar with regard to each, that it is best to take them together. let us begin by considering the various positions of an inferior planet, as seen from the earth, during the course of its journeys round the sun. when furthest from us it is at the other side of the sun, and cannot then be seen owing to the blaze of light. as it continues its journey it passes to the left of the sun, and is then sufficiently away from the glare to be plainly seen. it next draws in again towards the sun, and is once more lost to view in the blaze at the time of its passing nearest to us. then it gradually comes out to view on the right hand, separates from the sun up to a certain distance as before, and again recedes beyond the sun, and is for the time being once more lost to view. to these various positions technical names are given. when the inferior planet is on the far side of the sun from us, it is said to be in _superior conjunction_. when it has drawn as far as it can to the left hand, and is then as east as possible of the sun, it is said to be at its _greatest eastern elongation_. again, when it is passing nearest to us, it is said to be in _inferior conjunction_; and, finally, when it has drawn as far as it can to the right hand, it is spoken of as being at its _greatest western elongation_ (see fig. , p. ). the continual variation in the distance of an interior planet from us, during its revolution around the sun, will of course be productive of great alterations in its apparent size. at superior conjunction it ought, being then farthest away, to show the smallest disc; while at inferior conjunction, being the nearest, it should look much larger. when at greatest elongation, whether eastern or western, it should naturally present an appearance midway in size between the two. [illustration: various positions, and illumination by the sun, of an inferior planet in the course of its orbit. corresponding views of the same situations of an inferior planet as seen from the earth, showing consequent phases and alterations in apparent size. fig. .--orbit and phases of an inferior planet.] from the above considerations one would be inclined to assume that the best time for studying the surface of an interior planet with the telescope is when it is at inferior conjunction, or, nearest to us. but that this is not the case will at once appear if we consider that the sunlight is then falling upon the side away from us, leaving the side which is towards us unillumined. in superior conjunction, on the other hand, the light falls full upon the side of the planet facing us; but the disc is then so small-looking, and our view besides is so dazzled by the proximity of the sun, that observations are of little avail. in the elongations, however, the sunlight comes from the side, and so we see one half of the planet lit up; the right half at eastern elongation, and the left half at western elongation. piecing together the results given us at these more favourable views, we are enabled, bit by bit, to gather some small knowledge concerning the surface of an inferior planet. from these considerations it will be seen at once that the inferior planets show various phases comparable to the waxing and waning of our moon in its monthly round. superior conjunction is, in fact, similar to full moon, and inferior conjunction to new moon; while the eastern and western elongations may be compared respectively to the moon's first and last quarters. it will be recollected how, when these phases were first seen by the early telescopic observers, the copernican theory was felt to be immensely strengthened; for it had been pointed out that if this system were the correct one, the planets venus and mercury, were it possible to see them more distinctly, would of necessity present phases like these when viewed from the earth. it should here be noted that the telescope was not invented until nearly seventy years after the death of copernicus. the apparent swing of an inferior planet from side to side of the sun, at one time on the east side, then passing into and lost in the sun's rays to appear once more on the west side, is the explanation of what is meant when we speak of an _evening_ or a _morning star_. an inferior planet is called an evening star when it is at its eastern elongation, that is to say, on the left-hand of the sun; for, being then on the eastern side, it will set after the sun sets, as both sink in their turn below the western horizon at the close of day. similarly, when such a planet is at its western elongation, that is to say, to the right-hand of the sun, it will go in advance of him, and so will rise above the eastern horizon before the sun rises, receiving therefore the designation of morning star. in very early times, however, before any definite ideas had been come to with regard to the celestial motions, it was generally believed that the morning and evening stars were quite distinct bodies. thus venus, when a morning star, was known to the ancients under the name of phosphorus, or lucifer; whereas they called it hesperus when it was an evening star. since an inferior planet circulates between us and the sun, one would be inclined to expect that such a body, each time it passed on the side nearest to the earth, should be seen as a black spot against the bright solar disc. now this would most certainly be the case were the orbit of an inferior planet in the same plane with the orbit of the earth. but we have already seen how the orbits in the solar system, whether those of planets or of satellites, are by no means in the one plane; and that it is for this very reason that the moon is able to pass time after time in the direction of the sun, at the epoch known as new moon, and yet not to eclipse him save after the lapse of several such passages. transits, then, as the passages of an inferior planet across the sun's disc are called, take place, for the same reason, only after certain regular lapses of time; and, as regards the circumstances of their occurrence, are on a par with eclipses of the sun. the latter, however, happen much more frequently, because the moon passes in the neighbourhood of the sun, roughly speaking, once a month, whereas venus comes to each inferior conjunction at intervals so long apart as a year and a half, and mercury only about every four months. from this it will be further gathered that transits of mercury take place much oftener than transits of venus. until recent years _transits of venus_ were phenomena of great importance to astronomers, for they furnished the best means then available of calculating the distance of the sun from the earth. this was arrived at through comparing the amount of apparent displacement in the planet's path across the solar disc, when the transit was observed from widely separated stations on the earth's surface. the last transit of venus took place in , and there will not be another until the year . _transits of mercury_, on the other hand, are not of much scientific importance. they are of no interest as a popular spectacle; for the dimensions of the planet are so small, that it can be seen only with the aid of a telescope when it is in the act of crossing the sun's disc. the last transit of mercury took place on november , , and there will be another on november , . the first person known to have observed a transit of an inferior planet was the celebrated french philosopher, gassendi. this was the transit of mercury which took place on the th of december . the first time a transit of venus was ever seen, so far as is known, was on the th of november . the observer was a certain jeremiah horrox, curate of hoole, near preston, in lancashire. the transit in question commenced shortly before sunset, and his observations in consequence were limited to only about half-an-hour. horrox happened to have a great friend, one william crabtree, of manchester, whom he had advised by letter to be on the look out for the phenomenon. the weather in crabtree's neighbourhood was cloudy, with the result that he only got a view of the transit for about ten minutes before the sun set. that this transit was observed at all is due entirely to the remarkable ability of horrox. according to the calculations of the great kepler, no transit could take place that year ( ), as the planet would just pass clear of the lower edge of the sun. horrox, however, not being satisfied with this, worked the question out for himself, and came to the conclusion that the planet would _actually_ traverse the lower portion of the sun's disc. the event, as we have seen, proved him to be quite in the right. horrox is said to have been a veritable prodigy of astronomical skill; and had he lived longer would, no doubt, have become very famous. unfortunately he died about two years after his celebrated transit, in his _twenty-second_ year only, according to the accounts. his friend crabtree, who was then also a young man, is said to have been killed at the battle of naseby in . there is an interesting phenomenon in connection with transits which is known as the "black drop." when an inferior planet has just made its way on to the face of the sun, it is usually seen to remain for a short time as if attached to the sun's edge by what looks like a dark ligament (see fig. , p. ). this gives to the planet for the time being an elongated appearance, something like that of a pear; but when the ligament, which all the while keeps getting thinner and thinner, has at last broken, the black body of the planet is seen to stand out round against the solar disc. [illustration: fig. .--the "black drop."] this appearance may be roughly compared to the manner in which a drop of liquid (or, preferably, of some glutinous substance) tends for a while to adhere to an object from which it is falling. when the planet is in turn making its way off the face of the sun, the ligament is again seen to form and to attach it to the sun's edge before its due time. the phenomenon of the black drop, or ligament, is entirely an illusion, and, broadly speaking, of an optical origin. something very similar will be noticed if one brings one's thumb and forefinger _slowly_ together against a very bright background. this peculiar phenomenon has proved one of the greatest drawbacks to the proper observation of transits, for it is quite impossible to note the exact instant of the planet's entrance upon and departure from the solar disc in conditions such as these. the black drop seems to bear a family resemblance, so to speak, to the phenomenon of baily's beads. in the latter instance the lunar peaks, as they approach the sun's edge, appear to lengthen out in a similar manner and bridge the intervening space before their time, thus giving prominence to an effect which otherwise should scarcely be noticeable. the last transit of mercury, which, as has been already stated, took place on november , , was not successfully observed by astronomers in england, on account of the cloudiness of the weather. in france, however, professor moye, of montpellier, saw it under good conditions, and mentions that the black drop remained very conspicuous for fully a minute. the transit was also observed in the united states, the reports from which speak of the black drop as very "troublesome." before leaving the subject of transits it should be mentioned that it was in the capacity of commander of an expedition to otaheite, in the pacific, to observe the transit of venus of june , , that captain cook embarked upon the first of his celebrated voyages. in studying the surfaces of venus and mercury with the telescope, observers are, needless to say, very much hindered by the proximity of the sun. venus, when at the greatest elongations, certainly draws some distance out of the glare; but her surface is, even then, so dazzlingly bright, that the markings upon it are difficult to see. mercury, on the other hand, is much duller in contrast, but the disc it shows in the telescope is exceedingly small; and, in addition, when that planet is left above the horizon for a short time after sunset, as necessarily happens after certain intervals, the mists near the earth's surface render observation of it very difficult. until about twenty-five years ago, it was generally believed that both these planets rotated on their axes in about twenty-four hours, a notion, no doubt, originally founded upon an unconscious desire to bring them into some conformity with our earth. but schiaparelli, observing in italy, and percival lowell, in the clear skies of arizona and mexico, have lately come to the conclusion that both planets rotate upon their axes in the same time as they revolve in their orbits,[ ] the result being that they turn one face ever towards the sun in the same manner that the moon turns one face ever towards the earth--a curious state of things, which will be dealt with more fully when we come to treat of our satellite. the marked difference in the brightness between the two planets has already been alluded to. the surface of venus is, indeed, about five times as bright as that of mercury. the actual brightness of mercury is about equivalent to that of our moon, and astronomers are, therefore, inclined to think that it may resemble her in having a very rugged surface and practically no atmosphere. this probable lack of atmosphere is further corroborated by two circumstances. one of these is that when mercury is just about to transit the face of the sun, no ring of diffused light is seen to encircle its disc as would be the case if it possessed an atmosphere. such a lack of atmosphere is, indeed, only to be expected from what is known as the _kinetic theory of gases_. according to this theory, which is based upon the behaviour of various kinds of gas, it is found that these elements tend to escape into space from the surface of bodies whose force of gravitation is weak. hydrogen gas, for example, tends to fly away from our earth, as any one may see for himself when a balloon rises into the air. the gravitation of the earth seems, however, powerful enough to hold down other gases, as, for instance, those of which the air is chiefly composed, namely, oxygen and nitrogen. in due accordance with the kinetic theory, we find the moon and mercury, which are much about the same size, destitute of atmospheres. mars, too, whose diameter is only about double that of the moon, has very little atmosphere. we find, on the other hand, that venus, which is about the same size as our earth, clearly possesses an atmosphere, as just before the planet is in transit across the sun, the outline of its dark body is seen to be surrounded by a bright ring of light. the results of telescopic observation show that more markings are visible on mercury than on venus. the intense brilliancy of venus is, indeed, about the same as that of our white clouds when the sun is shining directly upon them. it has, therefore, been supposed that the planet is thickly enveloped in cloud, and that we do not ever see any part of its surface, except perchance the summit of some lofty mountain projecting through the fleecy mass. with regard to the great brilliancy of venus, it may be mentioned that she has frequently been seen in england, with the naked eye in full sunshine, when at the time of her greatest brightness. the writer has seen her thus at noonday. needless to say, the sky at the moment was intensely blue and clear. the orbit of mercury is very oval, and much more so than that of any other planet. the consequence is that, when mercury is nearest to the sun, the heat which it receives is twice as great as when it is farthest away. the orbit of venus, on the other hand, is in marked contrast with that of mercury, and is, besides, more nearly of a circular shape than that of any of the other planets. venus, therefore, always keeps about the same distance from the sun, and so the heat which she receives during the course of her year can only be subject to very slight variations. [ ] in employing the terms inferior and superior the writer bows to astronomical custom, though he cannot help feeling that, in the circumstances, interior and exterior would be much more appropriate. [ ] this question is, however, uncertain, for some very recent spectroscopic observations of venus seem to show a rotation period of about twenty-four hours. chapter xv the earth we have already seen (in chapter i.) how, in very early times, men naturally enough considered the earth to be a flat plane extending to a very great distance in every direction; but that, as years went on, certain of the greek philosophers suspected it to be a sphere. one or two of the latter are, indeed, said to have further believed in its rotation about an axis, and even in its revolution around the sun; but, as the ideas in question were founded upon fancy, rather than upon any direct evidence, they did not generally attract attention. the small effect, therefore, which these theories had upon astronomy, may well be gathered from the fact that in the ptolemaic system the earth was considered as fixed and at the centre of things; and this belief, as we have seen, continued unaltered down to the days of copernicus. it was, indeed, quite impossible to be certain of the real shape of the earth or the reality of its motions until knowledge became more extended and scientific instruments much greater in precision. we will now consider in detail a few of the more obvious arguments which can be put forward to show that our earth is a sphere. if, for instance, the earth were a plane surface, a ship sailing away from us over the sea would appear to grow smaller and smaller as it receded into the distance, becoming eventually a tiny speck, and fading gradually from our view. this, however, is not at all what actually takes place. as we watch a vessel receding, its hull appears bit by bit to slip gently down over the horizon, leaving the masts alone visible. then, in their turn, the masts are seen to slip down in the same manner, until eventually every trace of the vessel is gone. on the other hand, when a ship comes into view, the masts are the first portions to appear. they gradually rise up from below the horizon, and the hull follows in its turn, until the whole vessel is visible. again, when one is upon a ship at sea, a set of masts will often be seen sticking up alone above the horizon, and these may shorten and gradually disappear from view without the body of the ship to which they belong becoming visible at all. since one knows from experience that there is no _edge_ at the horizon over which a vessel can drop down, the appearance which we have been describing can only be explained by supposing that the surface of the earth is always curving gradually in every direction. the distance at which what is known as the _horizon_ lies away from us depends entirely upon the height above the earth's surface where we happen at the moment to be. a ship which has appeared to sink below the horizon for a person standing on the beach, will be found to come back again into view if he at once ascends a high hill. experiment shows that the horizon line lies at about three miles away for a person standing at the water's edge. the curving of the earth's surface is found, indeed, to be at the rate of eight inches in every mile. now it can be ascertained, by calculation, that a body curving at this rate in every direction must be a globe about miles in diameter. again, the fact that, if not stopped by such insuperable obstacles as the polar ice and snow, those who travel continually in any one direction upon the earth's surface always find themselves back again at the regions from which they originally set out, is additional ground for concluding that the earth is a globe. we can find still further evidence. for instance, in an eclipse of the moon the earth's shadow, when seen creeping across the moon's face, is noted to be _always_ circular in shape. one cannot imagine how such a thing could take place unless the earth were a sphere. also, it is found from observation that the sun, the planets, and the satellites are, all of them, round. this roundness cannot be the roundness of a flat plate, for instance, for then the objects in question would sometimes present their thin sides to our view. it happens, also, that upon the discs which these bodies show, we see certain markings shifting along continually in one direction, to disappear at one side and to reappear again at the other. such bodies must, indeed, be spheres in rotation. the crescent and other phases, shown by the moon and the inferior planets, should further impress the truth of the matter upon us, as such appearances can only be caused by the sunlight falling from various directions upon the surfaces of spherical bodies. another proof, perhaps indeed the weightiest of all, is the continuous manner in which the stars overhead give place to others as one travels about the surface of the earth. when in northern regions the pole star and its neighbours--the stars composing the plough, for instance--are over our heads. as one journeys south these gradually sink towards the northern horizon, while other stars take their place, and yet others are uncovered to view from the south. the regularity with which these changes occur shows that every point on the earth's surface faces a different direction of the sky, and such an arrangement would only be possible if the earth were a sphere. the celebrated greek philosopher, aristotle, is known to have believed in the globular shape of the earth, and it was by this very argument that he had convinced himself that it was so. the idea of the sphericity of the earth does not appear, however, to have been generally accepted until the voyages of the great navigators showed that it could be sailed round. the next point we have to consider is the rotation of the earth about its axis. from the earliest times men noticed that the sky and everything in it appeared to revolve around the earth in one fixed direction, namely, towards what is called the west, and that it made one complete revolution in the period of time which we know as twenty-four hours. the stars were seen to come up, one after another, from below the eastern horizon, to mount the sky, and then to sink in turn below the western horizon. the sun was seen to perform exactly the same journey, and the moon, too, whenever she was visible. one or two of the ancient greek philosophers perceived that this might be explained, either by a movement of the entire heavens around the earth, or by a turning motion on the part of the earth itself. of these diverse explanations, that which supposed an actual movement of the heavens appealed to them the most, for they could hardly conceive that the earth should continually rotate and men not be aware of its movement. the question may be compared to what we experience when borne along in a railway train. we see the telegraph posts and the trees and buildings near the line fly past us one after another in the contrary direction. either these must be moving, or we must be moving; and as we happen to _know_ that it is, indeed, we who are moving, there can be no question therefore about the matter. but it would not be at all so easy to be sure of this movement were one unable to see the objects close at hand displacing themselves. for instance, if one is shut up in a railway carriage at night with the blinds down, there is really nothing to show that one is moving, except the jolting of the train. and even then it is hard to be sure in which direction one is actually travelling. the way we are situated upon the earth is therefore as follows. there are no other bodies sufficiently near to be seen flying past us in turn; our earth spins without a jolt; we and all things around us, including the atmosphere itself, are borne along together with precisely the same impetus, just as all the objects scattered about a railway carriage share in the forward movement of the train. such being the case, what wonder that we are unconscious of the earth's rotation, of which we should know nothing at all, were it not for that slow displacement of the distant objects in the heavens, as we are borne past them in turn. if the night sky be watched, it will be soon found that its apparent turning movement seems to take place around a certain point, which appears as if fixed. this point is known as the north pole of the heavens; and a rather bright star, which is situated very close to this hub of movement, is in consequence called the pole star. for the dwellers in southern latitudes there is also a point in their sky which appears to remain similarly fixed, and this is known as the south pole of the heavens. since, however, the heavens do not turn round at all, but the earth does, it will easily be seen that these apparently stationary regions in the sky are really the points towards which the axis of the earth is directed. the positions on the earth's surface itself, known as the north and south poles, are merely the places where the earth's axis, if there were actually such a thing, would be expected to jut out. the north pole of the earth will thus be situated exactly beneath the north pole of the heavens, and the south pole of the earth exactly beneath the south pole of the heavens. we have seen that the earth rotates upon its imaginary axis once in about every twenty-four hours. this means that everything upon the surface of the earth is carried round once during that time. the measurement around the earth's equator is about , miles; and, therefore, an object situated at the equator must be carried round through a distance of about , miles in each twenty-four hours. everything at the equator is thus moving along at the rapid rate of about miles an hour, or between sixteen and seventeen times as fast as an express train. if, however, one were to take measurements around the earth parallel to the equator, one would find these measurements becoming less and less, according as the poles were approached. it is plain, therefore, that the speed with which any point moves, in consequence of the earth's rotation, will be greatest at the equator, and less and less in the direction of the poles; while at the poles themselves there will be practically no movement, and objects there situated will merely turn round. the considerations above set forth, with regard to the different speeds at which different portions of a rotating globe will necessarily be moving, is the foundation of an interesting experiment, which gives us further evidence of the rotation of our earth. the measurement around the earth at any distance below the surface, say, for instance, at the depth of a mile, will clearly be less than a similar measurement at the surface itself. the speed of a point at the bottom of a mine, which results from the actual rotation of the earth, must therefore be less than the speed of a point at the surface overhead. this can be definitely proved by dropping a heavy object down a mine shaft. the object, which starts with the greater speed of the surface, will, when it reaches the bottom of the mine, be found, as might be indeed expected, to be a little ahead (_i.e._ to the east) of the point which originally lay exactly underneath it. the distance by which the object gains upon this point is, however, very small. in our latitudes it amounts to about an inch in a fall of feet. the great speed at which, as we have seen, the equatorial regions of the earth are moving, should result in giving to the matter there situated a certain tendency to fly outwards. sir isaac newton was the first to appreciate this point, and he concluded from it that the earth must be _bulged_ a little all round the equator. this is, indeed, found to be the case, the diameter at the equator being nearly twenty-seven miles greater than it is from pole to pole. the reader will, no doubt, be here reminded of the familiar comparison in geographies between the shape of the earth and that of an orange. in this connection it is interesting to consider that, were the earth to rotate seventeen times as fast as it does (_i.e._ in one hour twenty-five minutes, instead of twenty-four hours), bodies at the equator would have such a strong tendency to fly outwards that the force of terrestrial gravity acting upon them would just be counterpoised, and they would virtually have _no weight_. and, further, were the earth to rotate a little faster still, objects lying loose upon its surface would be shot off into space. the earth is, therefore, what is technically known as an _oblate spheroid_; that is, a body of spherical shape flattened at the poles. it follows of course from this, that objects at the polar regions are slightly nearer to the earth's centre than objects at the equatorial regions. we have already seen that gravitation acts from the central parts of a body, and that its force is greater the nearer are those central parts. the result of this upon our earth will plainly be that objects in the polar regions will be pulled with a slightly stronger pull, and will therefore _weigh_ a trifle more than objects in the equatorial regions. this is, indeed, found by actual experiment to be the case. as an example of the difference in question, professor young, in his _manual of astronomy_, points out that a man who weighs pounds at the equator would weigh at the pole. in such an experiment the weighing would, however, have to be made with a _spring balance_, and _not with scales_; for, in the latter case, the "weights" used would alter in their weight in exactly the same degree as the objects to be weighed. it used to be thought that the earth was composed of a relatively thin crust, with a molten interior. scientific men now believe, on the other hand, that such a condition cannot after all prevail, and that the earth must be more or less solid all through, except perhaps in certain isolated places where collections of molten matter may exist. the _atmosphere_, or air which we breathe, is in the form of a layer of limited depth which closely envelops the earth. actually, it is a mixture of several gases, the most important being nitrogen and oxygen, which between them practically make up the air, for the proportion of the other gases, the chief of which is carbonic acid gas, is exceedingly small. it is hard to picture our earth, as we know it, without this atmosphere. deprived of it, men at once would die; but even if they could be made to go on living without it by any miraculous means, they would be like unto deaf beings, for they would never hear any sound. what we call _sounds_ are merely vibrations set up in the air, which travel along and strike upon the drum of the ear. the atmosphere is densest near the surface of the earth, and becomes less and less dense away from it, as a result of diminishing pressure of air from above. the greater portion of it is accumulated within four or five miles of the earth's surface. it is impossible to determine exactly at what distance from the earth's surface the air ceases altogether, for it grows continually more and more rarefied. there are, however, two distinct methods of ascertaining the distance beyond which it can be said practically not to exist. one of these methods we get from twilight. twilight is, in fact, merely light reflected to us from those upper regions of the air, which still continue to be illuminated by the sun after it has disappeared from our view below the horizon. the time during which twilight lasts, shows us that the atmosphere must be at least fifty miles high. but the most satisfactory method of ascertaining the height to which the atmosphere extends is from the observation of meteors. it is found that these bodies become ignited, by the friction of passing into the atmosphere, at a height of about miles above the surface of the earth. we thus gather that the atmosphere has a certain degree of density even at this height. it may, indeed, extend as far as about miles. the layer of atmosphere surrounding our earth acts somewhat in the manner of the glass covering of a greenhouse, bottling in the sun's rays, and thus storing up their warmth for our benefit. were this not so, the heat which we get from the sun would, after falling upon the earth, be quickly radiated again into space. it is owing to the unsteadiness of the air that stars are seen to twinkle. a night when this takes place, though it may please the average person, is worse than useless to the astronomer, for the unsteadiness is greatly magnified in the telescope. this twinkling is, no doubt, in a great measure responsible for the conventional "points" with which art has elected to embellish stars, and which, of course, have no existence in fact. the phenomena of _refraction_,[ ] namely, that bending which rays of light undergo, when passing _slant-wise_ from a rare into a dense transparent medium, are very marked with regard to the atmosphere. the denser the medium into which such rays pass, the greater is this bending found to be. since the layer of air around us becomes denser and denser towards the surface of the earth, it will readily be granted that the rays of light reaching our eyes from a celestial object, will suffer the greater bending the lower the object happens to be in the sky. celestial objects, unless situated directly overhead, are thus not seen in their true places, and when nearest to the horizon are most out of place. the bending alluded to is upwards. thus the sun and the moon, for instance, when we see them resting upon the horizon, are actually _entirely_ beneath it. when the sun, too, is sinking towards the horizon, the lower edge of its disc will, for the above reason, look somewhat more raised than the upper. the result is a certain appearance of flattening; which may plainly be seen by any one who watches the orb at setting. in observations to determine the exact positions of celestial objects correction has to be made for the effects of refraction, according to the apparent elevation of these objects in the sky. such effects are least when the objects in question are directly overhead, for then the rays of light, coming from them to the eye, enter the atmosphere perpendicularly, and not at any slant. a very curious effect, due to refraction, has occasionally been observed during a total eclipse of the moon. to produce an eclipse of this kind, _the earth must, of course, lie directly between the sun and the moon_. therefore, when we see the shadow creeping over the moon's surface, the sun should actually be well below the horizon. but when a lunar eclipse happens to come on just about sunset, the sun, although really sunk below the horizon, appears still above it through refraction, and the eclipsed moon, situated, of course, exactly opposite to it in the sky, is also lifted up above the horizon by the same cause. pliny, writing in the first century of the christian era, describes an eclipse of this kind, and refers to it as a "prodigy." the phenomenon is known as a "horizontal eclipse." it was, no doubt, partly owing to it that the ancients took so long to decide that an eclipse of the moon was really caused by the shadow cast by the earth. plutarch, indeed, remarks that it was easy enough to understand that a solar eclipse was caused by the interposition of the moon, but that one could not imagine by the interposition _of what body_ the moon itself could be eclipsed. in that apparent movement of the heavens about the earth, which men now know to be caused by the mere rotation of the earth itself, a slight change is observed to be continually taking place. the stars, indeed, are always found to be gradually drawing westward, _i.e._ towards the sun, and losing themselves one after the other in the blaze of his light, only to reappear, however, on the other side of him after a certain lapse of time. this is equivalent to saying that the sun itself seems always creeping slowly _eastward_ in the heaven. the rate at which this appears to take place is such that the sun finds itself back again to its original position, with regard to the starry background, at the end of a year's time. in other words, the sun seems to make a complete tour of the heavens in the course of a year. here, however, we have another illusion, just as the daily movement of the sky around the earth was an illusion. the truth indeed is, that this apparent movement of the sun eastward among the stars during a year, arises merely from a _continuous displacement of his position_ caused by an actual motion of the earth itself around him in that very time. in a word, it is the earth which really moves around the sun, and not the sun around the earth. the stress laid upon this fundamental point by copernicus, marks the separation of the modern from the ancient view. not that copernicus, indeed, had obtained any real proof that the earth is merely a planet revolving around the sun; but it seemed to his profound intellect that a movement of this kind on the part of our globe was the more likely explanation of the celestial riddle. the idea was not new; for, as we have already seen, certain of the ancient greeks (aristarchus of samos, for example) had held such a view; but their notions on the subject were very fanciful, and unsupported by any good argument. what copernicus, however, really seems to have done was to _insist_ upon the idea that the sun occupied the _centre_, as being more consonant with common sense. no doubt, he was led to take up this position by the fact that the sun appeared entirely of a different character from the other members of the system. the one body in the scheme, which performed the important function of dispenser of light and heat, would indeed be more likely to occupy a position apart from the rest; and what position more appropriate for its purposes than the centre! but here copernicus only partially solved the difficult question. he unfortunately still clung to an ancient belief, which as yet remained unquestioned; _i.e._ the great virtue, one might almost say, the _divineness_, of circular motion. the ancients had been hag-ridden, so to speak, by the circle; and it appeared to them that such a perfectly formed curve was alone fitted for the celestial motions. ptolemy employed it throughout his system. according to him the "planets" (which included, under the ancient view, both the sun and the moon), moved around the earth in circles; but, as their changing positions in the sky could not be altogether accounted for in this way, it was further supposed that they performed additional circular movements, around peculiarly placed centres, during the course of their orbital revolutions. thus the ptolemaic system grew to be extremely complicated; for astronomers did not hesitate to add new circular movements whenever the celestial positions calculated for the planets were found not to tally with the positions observed. in this manner, indeed, they succeeded in doctoring the theory, so that it fairly satisfied the observations made with the rough instruments of pre-telescopic times. although copernicus performed the immense service to astronomy of boldly directing general attention to the central position of the sun, he unfortunately took over for the new scheme the circular machinery of the ptolemaic system. it therefore remained for the famous kepler, who lived about a century after him, to find the complete solution. just as copernicus, for instance, had broken free from tradition with regard to the place of the sun; so did kepler, in turn, break free from the spell of circular motion, and thus set the coping-stone to the new astronomical edifice. this astronomer showed, in fact, that if the paths of the planets around the sun, and of the moon around the earth, were not circles, but _ellipses_, the movements of these bodies about the sky could be correctly accounted for. the extreme simplicity of such an arrangement was far more acceptable than the bewildering intricacy of movement required by the ptolemaic theory. the copernican system, as amended by kepler, therefore carried the day; and was further strengthened, as we have already seen, by the telescopic observations of galileo and the researches of newton into the effects of gravitation. and here a word on the circle, now fallen from its high estate. the ancients were in error in supposing that it stood entirely apart--the curve of curves. as a matter of fact it is merely _a special kind of ellipse_. to put it paradoxically, it is an ellipse which has no ellipticity, an oval without any ovalness! notwithstanding all this, astronomy had to wait yet a long time for a definite proof of the revolution of the earth around the sun. the leading argument advanced by aristotle, against the reality of any movement of the earth, still held good up to about seventy years ago. that philosopher had pointed out that the earth could not move about in space to any great extent, or the stars would be found to alter their apparent places in the sky, a thing which had never been observed to happen. centuries ran on, and instruments became more and more perfect, yet no displacements of stars were noted. in accepting the copernican theory men were therefore obliged to suppose these objects as immeasurably distant. at length, however, between the years and , it was discovered by the prussian astronomer, bessel, that a star known as cygni--that is to say, the star marked in celestial atlases as no. in the constellation of the swan--appeared, during the course of a year, to perform a tiny circle in the heavens, such as would result from a movement on our own part around the sun. since then about forty-three stars have been found to show minute displacements of a similar kind, which cannot be accounted for upon any other supposition than that of a continuous revolution of the earth around the sun. the triumph of the copernican system is now at last supreme. if the axis of the earth stood "straight up," so to speak, while the earth revolved in its orbit, the sun would plainly keep always on a level with the equator. this is equivalent to stating that, in such circumstances, a person at the equator would see it rise each morning exactly in the east, pass through the _zenith_, that is, the point directly overhead of him, at midday, and set in the evening due in the west. as this would go on unchangingly at the equator every day throughout the year, it should be clear that, at any particular place upon the earth, the sun would in these conditions always be seen to move in an unvarying manner across the sky at a certain altitude depending upon the latitude of the place. thus the more north one went upon the earth's surface, the more southerly in the sky would the sun's path lie; while at the north pole itself, the sun would always run round and round the horizon. similarly, the more south one went from the equator the more northerly would the path of the sun lie, while at the south pole it would be seen to skirt the horizon in the same manner as at the north pole. the result of such an arrangement would be, that each place upon the earth would always have one unvarying climate; in which case there would not exist any of those beneficial changes of season to which we owe so much. the changes of season, which we fortunately experience, are due, however, to the fact that the sun does not appear to move across the sky each day at one unvarying altitude, but is continually altering the position of its path; so that at one period of the year it passes across the sky low down, and remains above the horizon for a short time only, while at another it moves high up across the heavens, and is above the horizon for a much longer time. actually, the sun seems little by little to creep up the sky during one half of the year, namely, from mid-winter to mid-summer, and then, just as gradually, to slip down it again during the other half, namely, from mid-summer to mid-winter. it will therefore be clear that every region of the earth is much more thoroughly warmed during one portion of the year than during another, _i.e._ when the sun's path is high in the heavens than when it is low down. once more we find appearances exactly the contrary from the truth. the earth is in this case the real cause of the deception, just as it was in the other cases. the sun does not actually creep slowly up the sky, and then slowly dip down it again, but, owing to the earth's axis being set aslant, different regions of the earth's surface are presented to the sun at different times. thus, in one portion of its orbit, the northerly regions of the earth are presented to the sun, and in the other portion the southerly. it follows of course from this, that when it is summer in the northern hemisphere it is winter in the southern, and _vice versâ_ (see fig. , p. ). [illustration: fig. .--summer and winter.] the fact that, in consequence of this slant of the earth's axis, the sun is for part of the year on the north side of the equator and part of the year on the south side, leads to a very peculiar result. the path of the moon around the earth is nearly on the same plane with the earth's path around the sun. the moon, therefore, always keeps to the same regions of the sky as the sun. the slant of the earth's axis thus regularly displaces the position of both the sun and the moon to the north and south sides of the equator respectively in the manner we have been describing. were the earth, however, a perfect sphere, such change of position would not produce any effect. we have shown, however, that the earth is not a perfect sphere, but that it is bulged out all round the equator. the result is that this bulged-out portion swings slowly under the pulls of solar and lunar gravitation, in response to the displacements of the sun and moon to the north and to the south of it. this slow swing of the equatorial regions results, of course, in a certain slow change of the direction of the earth's axis, so that the north pole does not go on pointing continually to the same region of the sky. the change in the direction of the axis is, however, so extremely slight, that it shows up only after the lapse of ages. the north pole of the heavens, that is, the region of the sky towards which the north pole of the earth's axis points, displaces therefore extremely slowly, tracing out a wide circle, and arriving back again to the same position in the sky only after a period of about , years. at present the north pole of the heavens is quite close to a bright star in the tail of the constellation of the little bear, which is consequently known as the pole star; but in early greek times it was at least ten times as far away from this star as it is now. after some , years the pole will point to the constellation of lyra, and vega, the most brilliant star in that constellation, will then be considered as the pole star. this slow twisting of the earth's axis is technically known as _precession_, or the _precession of the equinoxes_ (see plate xix., p. ). the slow displacement of the celestial pole appears to have attracted the attention of men in very early times, but it was not until the second century b.c. that precession was established as a fact by the celebrated greek astronomer, hipparchus. for the ancients this strange cyclical movement had a mystic significance; and they looked towards the end of the period as the end, so to speak, of a "dispensation," after which the life of the universe would begin anew:-- "magnus ab integro sæclorum nascitur ordo. jam redit et virgo, redeunt saturnia regna; . . . . . . alter erit tum tiphys, et altera quæ vehat argo delectos heroas; erunt etiam altera bella, atque iterum ad trojam magnus mittetur achilles." we have seen that the orbit of the earth is an ellipse, and that the sun is situated at what is called the _focus_, a point not in the middle of the ellipse, but rather towards one of its ends. therefore, during the course of the year the distance of the earth from the sun varies. the sun, in consequence of this, is about , , miles _nearer_ to us in our northern _winter_ than it is in our northern summer, a statement which sounds somewhat paradoxical. this variation in distance, large as it appears in figures, can, however, not be productive of much alteration in the amount of solar heat which we receive, for during the first week in january, when the distance is least, the sun only looks about _one-eighteenth_ broader than at the commencement of july, when the distance is greatest. the great disparity in temperature between winter and summer depends, as we have seen, upon causes of quite another kind, and varies between such wide limits that the effects of this slight alteration in the distance of the sun from the earth may be neglected for practical purposes. the tides are caused by the gravitational pull of the sun and moon upon the water of the earth's surface. of the two, the moon, being so much the nearer, exerts the stronger pull, and therefore may be regarded as the chief cause of the tides. this pull always draws that portion of the water, which happens to be right underneath the moon at the time, into a heap; and there is also a _second_ heaping of water at the same moment _at the contrary side of the earth_, the reasons for which can be shown mathematically, but cannot be conveniently dealt with here. as the earth rotates on its axis each portion of its surface passes beneath the moon, and is swelled up by this pull; the watery portions being, however, the only ones to yield visibly. a similar swelling up, as we have seen, takes place at the point exactly away from the moon. thus each portion of our globe is borne by the rotation through two "tide-areas" every day, and this is the reason why there are two tides during every twenty-four hours. the crest of the watery swelling is known as high tide. the journey of the moon around the earth takes about a month, and this brings her past each place in turn by about fifty minutes later each day, which is the reason why high tide is usually about twenty-five minutes later each time. the moon is, however, not the sole cause of the tides, but the sun, as we have said, has a part in the matter also. when it is new moon the gravitational attractions of both sun and moon are clearly acting together from precisely the same direction, and, therefore, the tide will be pulled up higher than at other times. at full moon, too, the same thing happens; for, although the bodies are now acting from opposite directions, they do not neutralise each other's pulls as one might imagine, since the sun, in the same manner as the moon, produces a tide both under it and also at the opposite side of the earth. thus both these tides are actually increased in height. the exceptionally high tides which we experience at new and full moons are known as _spring tides_, in contradistinction to the minimum high tides, which are known as _neap tides_. the ancients appear to have had some idea of the cause of the tides. it is said that as early as b.c. the chinese noticed that the moon exerted an influence upon the waters of the sea. the greeks and romans, too, had noticed the same thing; and cæsar tells us that when he was embarking his troops for britain the tide was high _because_ the moon was full. pliny went even further than this, in recognising a similar connection between the waters and the sun. from casual observation one is inclined to suppose that the high tide always rises many feet. but that this is not the case is evidenced by the fact that the tides in the midst of the great oceans are only from three to four feet high. however, in the seas and straits around our isles, for instance, the tides rise very many feet indeed, but this is merely owing to the extra heaping up which the large volumes of water undergo in forcing their passage through narrow channels. as the earth, in rotating, is continually passing through these tide-areas, one might expect that the friction thus set up would tend to slow down the rotation itself. such a slowing down, or "tidal drag," as it is called, is indeed continually going on; but the effects produced are so exceedingly minute that it will take many millions of years to make the rotation appreciably slower, and so to lengthen the day. recently it has been proved that the axis of the earth is subject to a very small displacement, or rather, "wobbling," in the course of a period of somewhat over a year. as a consequence of this, the pole shifts its place through a circle of, roughly, a few yards in width during the time in question. this movement is, perhaps, the combined result of two causes. one of these is the change of place during the year of large masses of material upon our earth; such as occurs, for instance, when ice and snow melt, or when atmospheric and ocean currents transport from place to place great bodies of air and water. the other cause is supposed to be the fact that the earth is not absolutely rigid, and so yields to certain strains upon it. in the course of investigation of this latter point the interesting conclusion has been reached by the famous american astronomer, professor simon newcomb, that our globe as a whole is _a little more rigid than steel_. we will bring this chapter to a close by alluding briefly to two strange appearances which are sometimes seen in our night skies. these are known respectively as the zodiacal light and the gegenschein. the _zodiacal light_ is a faint cone-shaped illumination which is seen to extend upwards from the western horizon after evening twilight has ended, and from the eastern horizon before morning twilight has begun. it appears to rise into the sky from about the position where the sun would be at that time. the proper season of the year for observing it during the evening is in the spring, while in autumn it is best seen in the early morning. in our latitudes its light is not strong enough to render it visible when the moon is full, but in the tropics it is reported to be very bright, and easily seen in full moonlight. one theory regards it as the reflection of light from swarms of meteors revolving round the sun; another supposes it to be a very rarefied extension of the corona. the _gegenschein_ (german for "counter-glow") is a faint oval patch of light, seen in the sky exactly opposite to the place of the sun. it is usually treated of in connection with the zodiacal light, and one theory regards it similarly as of meteoric origin. another theory, however--that of mr. evershed--considers it a sort of _tail_ to the earth (like a comet's tail) composed of hydrogen and helium--the two _lightest_ gases we know--driven off from our planet in the direction contrary to the sun. [ ] every one knows the simple experiment in which a coin lying at the bottom of an empty basin, and hidden from the eye by its side, becomes visible when a certain quantity of water has been poured in. this is an example of refraction. the rays of light coming from the coin ought _not_ to reach the eye, on account of the basin's side being in the way; yet by the action of the water they are _refracted_, or bent over its edge, in such a manner that they do. chapter xvi the moon what we call the moon's "phases" are merely the various ways in which we see the sun shining upon her surface during the course of her monthly revolutions around the earth (see fig. , p. ). when she passes in the neighbourhood of the sun all his light falls upon that side which is turned away from us, and so the side which is turned towards us is unillumined, and therefore invisible. when in this position the moon is spoken of as _new_. as she continues her motion around the earth, she draws gradually to the east of the sun's place in the sky. the sunlight then comes somewhat from the side; and so we see a small portion of the right side of the lunar disc illuminated. this is the phase known as the _crescent_ moon. as she moves on in her orbit more and more of her illuminated surface is brought into view; and so the crescent of light becomes broader and broader, until we get what is called half-moon, or _first quarter_, when we see exactly one-half of her surface lit up by the sun's rays. as she draws still further round yet more of her illuminated surface is brought into view, until three-quarters of the disc appear lighted up. she is then said to be _gibbous_. eventually she moves round so that she faces the sun completely, and the whole of her disc appears illuminated. she is then spoken of as _full_. when in this position it is clear that she is on the contrary side of the earth to the sun, and therefore rises about the same time that he is setting. she is now, in fact, at her furthest from the sun. [illustration: direction from which the sun's rays are coming. various positions and illumination of the mooon by the sun during her revolution around the earth. the corresponding positions as viewed from the earth, showing the consequent phases. fig. .--orbit and phases of the moon.] after this, the motion of the moon in her orbit carries her on back again in the direction of the sun. she thus goes through her phases as before, only these of course are _in the reverse order_. the full phase is seen to give place to the gibbous, and this in turn to the half-moon and to the crescent; after which her motion carries her into the neighbourhood of the sun, and she is once more new, and lost to our sight in the solar glare. following this she draws away to the east of the sun again, and the old order of phases repeat themselves as before. the early babylonians imagined that the moon had a bright and a dark side, and that her phases were caused by the bright side coming more and more into view during her movement around the sky. the greeks, notably aristotle, set to work to examine the question from a mathematical standpoint, and came to the conclusion that the crescent and other appearances were such as would necessarily result if the moon were a dark body of spherical shape illumined merely by the light of the sun. although the true explanation of the moon's phases has thus been known for centuries, it is unfortunately not unusual to see pictures--advertisement posters, for instance--in which stars appear _within_ the horns of a crescent moon! can it be that there are to-day educated persons who believe that the moon is a thing which _grows_ to a certain size and then wastes away again; who, in fact, do not know that the entire body of the moon is there all the while? when the moon shows a very thin crescent, we are able dimly to see her still dark portion standing out against the sky. this appearance is popularly known as the "old moon in the new moon's arms." the dark part of her surface must, indeed, be to some degree illumined, or we should not be able to see it at all. whence then comes the light which illumines it, since it clearly cannot come from the sun? the riddle is easily solved, if we consider what kind of view of our earth an observer situated on this darkened part of the moon would at that moment get. he would, as a matter of fact, just then see nearly the whole disc of the earth brightly lit up by sunlight. the lunar landscape all around would, therefore, be bathed in what to _him_ would be "earthlight," which of course takes the place there of what _we_ call moonlight. if, then, we recollect how much greater in size the earth is than the moon, it should not surprise us that this earthlight will be many times brighter than moonlight. it is considered, indeed, to be some twenty times brighter. it is thus not at all astonishing that we can see the dark portion of the moon illumined merely by sunlight reflected upon it from our earth. the ancients were greatly exercised in their minds to account for this "earthlight," or "earthshine," as it is also called. posidonius ( - b.c.) tried to explain it by supposing that the moon was partially transparent, and that some sunlight consequently filtered through from the other side. it was not, however, until the fifteenth century that the correct solution was arrived at. [illustration: one side of the moon only is ever presented to the earth. this side is here indicated by the letters s.f.e. (side facing earth). by placing the above positions in a row, we can see at once that the moon makes one complete rotation on her axis in exactly the same time as she revolves around the earth. fig. .--the rotation of the moon on her axis.] perhaps the most remarkable thing which one notices about the moon is that she always turns the same side towards us, and so we never see her other side. one might be led from this to jump to the conclusion that she does not rotate upon an axis, as do the other bodies which we see; but, paradoxical as it may appear, the fact that she turns one face always towards the earth, is actually a proof that she _does_ rotate upon an axis. the rotation, however, takes place with such slowness, that she turns round but once during the time in which she revolves around the earth (see fig. ). in order to understand the matter clearly, let the reader place an object in the centre of a room and walk around it once, _keeping his face turned towards it the whole time_, while he is doing this, it is evident that he will face every one of the four walls of the room in succession. now in order to face each of the four walls of a room in succession one would be obliged _to turn oneself entirely round_. therefore, during the act of walking round an object with his face turned directly towards it, a person at the same time turns his body once entirely round. in the long, long past the moon must have turned round much faster than this. her rate of rotation has no doubt been slowed down by the action of some force. it will be recollected how, in the course of the previous chapter, we found that the tides were tending, though exceedingly gradually, to slow down the rotation of the earth upon its axis. but, on account of the earth's much greater mass, the force of gravitation exercised by it upon the surface of the moon is, of course, much more powerful than that which the moon exercises upon the surface of the earth. the tendency to tidal action on the moon itself must, therefore, be much in excess of anything which we here experience. it is, in consequence, probable that such a tidal drag, extending over a very long period of time, has resulted in slowing down the moon's rotation to its present rate. the fact that we never see but one side of the moon has given rise from time to time to fantastic speculations with regard to the other side. some, indeed, have wished to imagine that our satellite is shaped like an egg, the more pointed end being directed away from us. we are here, of course, faced with a riddle, which is all the more tantalising from its appearing for ever insoluble to men, chained as they are to the earth. however, it seems going too far to suppose that any abnormal conditions necessarily exist at the other side of the moon. as a matter of fact, indeed, small portions of that side are brought into our view from time to time in consequence of slight irregularities in the moon's movement; and these portions differ in no way from those which we ordinarily see. on the whole, we obtain a view of about per cent. of the entire lunar surface; that is to say, a good deal more than one-half. the actual diameter of the moon is about miles, which is somewhat more than one-quarter the diameter of the earth. for a satellite, therefore, she seems very large compared with her primary, the earth; when we consider that jupiter's greatest satellite, although nearly twice as broad as our moon, has a diameter only one twenty-fifth that of jupiter. furthermore, the moon moves around the earth comparatively slowly, making only about thirteen revolutions during the entire year. seen from space, therefore, she would not give the impression of a circling body, as other satellites do. her revolutions are, indeed, relatively so very slow that she would appear rather like a smaller planet accompanying the earth in its orbit. in view of all this, some astronomers are inclined to regard the earth and moon rather as a "double planet" than as a system of planet and satellite. when the moon is full she attracts more attention perhaps than in any of her other phases. the moon, in order to be full, must needs be in that region of the heavens exactly opposite to the sun. the sun _appears_ to go once entirely round the sky in the course of a year, and the moon performs the same journey in the space of about a month. the moon, when full, having got half-way round this journey, occupies, therefore, that region of the sky which the sun itself will occupy half a year later. thus in winter the full moon will be found roughly to occupy the sun's summer position in the sky, and in summer the sun's winter position. it therefore follows that the full moon in winter time is high up in the heavens, while in summer time it is low down. we thus get the greatest amount of full moonlight when it is the most needed. the great french astronomer, laplace, being struck by the fact that the "lesser light" did not rule the night to anything like the same extent that the "greater light" ruled the day, set to work to examine the conditions under which it might have been made to do so. the result of his speculations showed that if the moon were removed to such a distance that she took a year instead of a month to revolve around the earth; and if she were started off in her orbit at full moon, she would always continue to remain full--a great advantage for us. whewell, however, pointed out that in order to get the moon to move with the requisite degree of slowness, she would have to revolve so far from the earth that she would only look one-sixteenth as large as she does at present, which rather militates against the advantage laplace had in mind! finally, however, it was shown by m. liouville, in , that the position of a _perennial full moon_, such as laplace dreamed of, would be unstable--that is to say, the body in question could not for long remain undisturbed in the situation suggested (see fig. , p. ). [illustration: various positions of laplace's "moon" with regard to the earth and sun during the course of a year. the same positions of laplace's "moon," arranged around the earth, show that it would make only one revolution in a year. fig. .--laplace's "perennial full moon."] there is a well-known phenomenon called _harvest moon_, concerning the nature of which there seems to be much popular confusion. an idea in fact appears to prevail among a good many people that the moon is a harvest moon when, at rising, it looks bigger and redder than usual. such an appearance has, however, nothing at all to say to the matter; for the moon always _looks_ larger when low down in the sky, and, furthermore, it usually looks red in the later months of the year, when there is more mist and fog about than there is in summer. what astronomers actually term the harvest moon is, indeed, something entirely different from this. about the month of september the slant at which the full moon comes up from below the horizon happens to be such that, during several evenings together, she _rises almost at the same hour_, instead of some fifty minutes later, as is usually the case. as the harvest is being gathered in about that time, it has come to be popularly considered that this is a provision of nature, according to which the sunlight is, during several evenings, replaced without delay by more or less full-moonlight, in order that harvesters may continue their work straight on into the night, and not be obliged to break off after sunset to wait until the moon rises. the same phenomenon is almost exactly repeated a month later, but by reason of the pursuits then carried on it is known as the "hunter's moon." in this connection should be mentioned that curious phenomenon above alluded to, and which seems to attract universal notice, namely, that the moon _looks much larger when near the horizon_--at its rising, for instance, than when higher up in the sky. this seeming enlargement is, however, by no means confined to the moon. that the sun also looks much larger when low down in the sky than when high up, seems to strike even the most casual watcher of a sunset. the same kind of effect will, indeed, be noted if close attention be paid to the stars themselves. a constellation, for instance, appears more spread out when low down in the sky than when high up. this enlargement of celestial objects when in the neighbourhood of the horizon is, however, only _apparent_ and not real. it must be entirely an _illusion_; for the most careful measurements of the discs of the sun and of the moon fail to show that the bodies are any larger when near the horizon than when high up in the sky. in fact, if there be any difference in measurements with regard to the moon, it will be found to be the other way round; for her disc, when carefully measured, is actually the slightest degree _greater_ when _high_ in the sky, than when low down. the reason for this is that, on account of the rotundity of the earth's surface, she is a trifle nearer the observer when overhead of him. this apparent enlargement of celestial objects, when low down in the sky, is granted on all sides to be an illusion; but although the question has been discussed with animation time out of mind, none of the explanations proposed can be said to have received unreserved acceptance. the one which usually figures in text-books is that we unconsciously compare the sun and moon, when low down in the sky, with the terrestrial objects in the same field of view, and are therefore inclined to exaggerate the size of these orbs. some persons, on the other hand, imagine the illusion to have its source in the structure of the human eye; while others, again, put it down to the atmosphere, maintaining that the celestial objects in question _loom_ large in the thickened air near the horizon, in the same way that they do when viewed through fog or mist. the writer[ ] ventures, however, to think that the illusion has its origin in our notion of the shape of the celestial vault. one would be inclined, indeed, to suppose that this vault ought to appear to us as the half of a hollow sphere; but he maintains that it does not so appear, as a consequence of the manner in which the eyes of men are set quite close together in their heads. if one looks, for instance, high up in the sky, the horizon cannot come within the field of view, and so there is nothing to make one think that the expanse then gazed upon is other than quite _flat_--let us say like the ceiling of a room. but, as the eyes are lowered, a portion of the _encircling_ horizon comes gradually into the field of view, and the region of the sky then gazed upon tends in consequence to assume a _hollowed-out_ form. from this it would seem that our idea of the shape of the celestial vault is, that it is _flattened down over our heads and hollowed out all around in the neighbourhood of the horizon_ (see fig. , p. ). now, as a consequence of their very great distance, all the objects in the heavens necessarily appear to us to move as if they were placed on the background of the vault; the result being that the mind is obliged to conceive them as expanded or contracted, in its unconscious attempts to make them always fill their due proportion of space in the various parts of this abnormally shaped sky. from such considerations the writer concludes that the apparent enlargement in question is merely the natural consequence of the idea we have of the shape of the celestial vault--an idea gradually built up in childhood, to become later on what is called "second nature." and in support of this contention, he would point to the fact that the enlargement is not by any means confined to the sun and moon, but is every whit as marked in the case of the constellations. to one who has not noticed this before, it is really quite a revelation to compare the appearance of one of the large constellations (orion, for instance) when high up in the sky and when low down. the widening apart of the various stars composing the group, when in the latter position, is very noticeable indeed. [illustration: fig. .--illustrating the author's explanation of the apparent enlargement of celestial objects.] further, if a person were to stand in the centre of a large dome, he would be exactly situated as if he were beneath the vaulted heaven, and one would consequently expect him to suffer the same illusion as to the shape of the dome. objects fixed upon its background would therefore appear to him under the same conditions as objects in the sky, and the illusions as to their apparent enlargement should hold good here also. some years ago a belgian astronomer, m. stroobant, in an investigation of the matter at issue, chanced to make a series of experiments under the very conditions just detailed. to various portions of the inner surface of a large dome he attached pairs of electric lights; and on placing himself at the centre of the building, he noticed that, in every case, those pairs which were high up appeared closer together than those which were low down! he does not, however, seem to have sought for the cause in the vaulted expanse. on the contrary, he attributed the effect to something connected with our upright stature, to some physiological reason which regularly makes us estimate objects as larger when in front than when overhead. in connection with this matter, it may be noted that it always appears extremely difficult to estimate with the eye the exact height above the horizon at which any object (say a star) happens to be. even skilled observers find themselves in error in attempting to do so. this seems to bear out the writer's contention that the form under which the celestial vault really appears to us is a peculiar one, and tends to give rise to false judgments. before leaving this question, it should also be mentioned that nothing perhaps is more deceptive than the size which objects in the sky appear to present. the full moon looks so like a huge plate, that it astonishes one to find that a threepenny bit held at arm's length will a long way more than cover its disc. [illustration: plate viii. the moon from a photograph taken at the paris observatory by m.p. puiseux. (page )] the moon is just too far off to allow us to see the actual detail on her surface with the naked eye. when thus viewed she merely displays a patchy appearance,[ ] and the imaginary forms which her darker markings suggest to the fancy are popularly expressed by the term "man in the moon." an examination of her surface with very moderate optical aid is, however, quite a revelation, and the view we then get is not easily comparable to what we see with the unaided eye. even with an ordinary opera-glass, an observer will be able to note a good deal of detail upon the lunar disc. if it be his first observation of the kind, he cannot fail to be struck by the fact to which we have just made allusion, namely, the great change which the moon appears to undergo when viewed with magnifying power. "cain and his dog," the "man in the moon gathering sticks," or whatever indeed his fancy was wont to conjure up from the lights and shades upon the shining surface, have now completely disappeared; and he sees instead a silvery globe marked here and there with extensive dark areas, and pitted all over with crater-like formations (see plate viii., p. ). the dark areas retain even to the present day their ancient name of "seas," for galileo and the early telescopic observers believed them to be such, and they are still catalogued under the mystic appellations given to them in the long ago; as, for instance, "sea of showers," "bay of rainbows," "lake of dreams."[ ] the improved telescopes of later times showed, however, that they were not really seas (there is no water on the moon), but merely areas of darker material. the crater-like formations above alluded to are the "lunar mountains." a person examining the moon for the first time with telescopic aid, will perhaps not at once grasp the fact that his view of lunar mountains must needs be what is called a "bird's-eye" one, namely, a view from above, like that from a balloon and that he cannot, of course, expect to see them from the side, as he does the mountains upon the earth. but once he has realised this novel point of view, he will no doubt marvel at the formations which lie scattered as it were at his feet. the type of lunar mountain is indeed in striking contrast to the terrestrial type. on our earth the range-formation is supreme; on the moon the crater-formation is the rule, and is so-called from analogy to our volcanoes. a typical lunar crater may be described as a circular wall, enclosing a central plain, or "floor," which is often much depressed below the level of the surface outside. these so-called "craters," or "ring-mountains," as they are also termed, are often of gigantic proportions. for instance, the central plain of one of them, known as ptolemæus,[ ] is about miles across, while that of plato is about . the walls of craters often rise to great heights; which, in proportion to the small size of the moon, are very much in excess of our highest terrestrial elevations. nevertheless, a person posted at the centre of one of the larger craters might be surprised to find that he could not see the encompassing crater-walls, which would in every direction be below his horizon. this would arise not alone from the great breadth of the crater itself, but also from the fact that the curving of the moon's surface is very sharp compared with that of our earth. [illustration: plate ix. map of the moon, showing the principal "craters," mountain ranges, and "seas" in this, as in the other plates of the moon, the _south_ will be found at the top of the picture; such being the view given by the ordinary astronomical telescope, in which all objects are seen _inverted_. (page )] we have mentioned ptolemæus as among the very large craters, or ring-mountains, on the moon. its encompassing walls rise to nearly , feet, and it has the further distinction of being almost in the centre of the lunar disc. there are, however, several others much wider, but they are by no means in such a conspicuous position. for instance, schickard, close to the south-eastern border, is nearly miles in diameter, and its wall rises in one point to over , feet. grimaldi, almost exactly at the east point, is nearly as large as schickard. another crater, clavius, situated near the south point, is about miles across; while its neighbour bailly--named after a famous french astronomer of the eighteenth century--is , and the largest of those which we can see (see plate ix., p. ). many of the lunar craters encroach upon one another; in fact there is not really room for them all upon the visible hemisphere of the moon. about , have been mapped; but this is only a small portion, for according to the american astronomer, professor w.h. pickering, there are more than , in all. notwithstanding the fact that the crater is the type of mountain associated in the mind with the moon, it must not be imagined that upon our satellite there are no mountains at all of the terrestrial type. there are indeed many isolated peaks, but strangely enough they are nearly always to be found in the centres of craters. some of these peaks are of great altitude, that in the centre of the crater copernicus being over , feet high. a few mountain ranges also exist; the best known of which are styled, the lunar alps and lunar apennines (see plate x., p. ). since the _mass_ of the moon is only about one-eightieth that of the earth, it will be understood that the force of gravity which she exercises is much less. it is calculated that, at her surface, this is only about one-sixth of what we experience. a man transported to the moon would thus be able to jump _six times as high_ as he can here. a building could therefore be six times as tall as upon our earth, without causing any more strain upon its foundations. it should not, then, be any subject for wonder, that the highest peaks in the lunar apennines attain to such heights as , feet. such a height, upon a comparatively small body like the moon, for her _volume_ is only one-fiftieth that of the earth, is relatively very much in excess of the , feet of himalayan structure, mount everest, the boast of our planet, miles across! high as are the lunar apennines, the highest peaks on the moon are yet not found among them. there is, for instance, on the extreme southern edge of the lunar disc, a range known as the leibnitz mountains; several peaks of which rise to a height of nearly , feet, one peak in particular being said to attain to , feet (see plate ix., p. ). [illustration: plate x. one of the most interesting regions on the moon we have here (see "map," plate ix., p. ) the mountain ranges of the apennines, the caucasus and the alps; also the craters plato, aristotle, eudoxus, cassini, aristillus, autolycus, archimedes and linné. the crater linné is the very bright spot in the dark area at the upper left hand side of the picture. from a photograph taken at the paris observatory by m.m. loewy and puiseux. (page )] but the reader will surely ask the question: "how is it possible to determine the actual height of a lunar mountain, if one cannot go upon the moon to measure it?" the answer is, that we can calculate its height from noting the length of the shadow which it casts. any one will allow that the length of a shadow cast by the sun depends upon two things: firstly, upon the height of the object which causes the shadow, and secondly, upon the elevation of the sun at the moment in the sky. the most casual observer of nature upon our earth can scarcely have failed to notice that shadows are shortest at noonday, when the sun is at its highest in the sky; and that they lengthen out as the sun declines towards its setting. here, then, we have the clue. to ascertain, therefore, the height of a lunar mountain, we have first to consider at what elevation the sun is at that moment above the horizon of the place where the mountain in question is situated. then, having measured the actual length in miles of the shadow extended before us, all that is left is to ask ourselves the question: "what height must an object be whose shadow cast by the sun, when at that elevation in the sky, will extend to this length?" there is no trace whatever of water upon the moon. the opinion, indeed, which seems generally held, is that water has never existed upon its surface. erosions, sedimentary deposits, and all those marks which point to a former occupation by water are notably absent. similarly there appears to be no atmosphere on the moon; or, at any rate, such an excessively rare one, as to be quite inappreciable. of this there are several proofs. for instance, in a solar eclipse the moon's disc always stands out quite clear-cut against that of the sun. again during occultations, stars disappear behind the moon with a suddenness, which could not be the case were there any appreciable atmosphere. lastly, we see no traces of twilight upon the lunar surface, nor any softening at the edges of shadows; both which effects would be apparent if there were an atmosphere. the moon's surface is rough and rocky, and displays no marks of the "weathering" that would necessarily follow, had it possessed anything of an atmosphere in the past. this makes us rather inclined to doubt that it ever had one at all. supposing, however, that it did possess an atmosphere in the past, it is interesting to inquire what may have become of it. in the first place it might have gradually disappeared, in consequence of the gases which composed it uniting chemically with the materials of which the lunar body is constructed; or, again, its constituent gases may have escaped into space, in accordance with the principles of that kinetic theory of which we have already spoken. the latter solution seems, indeed, the most reasonable of the two, for the force of gravity at the lunar surface appears too weak to hold down any known gases. this argument seems also to dispose of the question of absence of water; for dr. george johnstone stoney, in a careful investigation of the subject, has shown that the liquid in question, when in the form of vapour, will escape from a planet if its mass is less than _one-fourth_ that of our earth. and the mass of the moon is very much less than this; indeed only the _one-eightieth_, as we have already stated. in consequence of this lack of atmosphere, the condition of things upon the moon will be in marked contrast to what we experience upon the earth. the atmosphere here performs a double service in shielding us from the direct rays of the sun, and in bottling the heat as a glass-house does. on the moon, however, the sun beats down in the day-time with a merciless force; but its rays are reflected away from the surface as quickly as they are received, and so the cold of the lunar night is excessive. it has been calculated that the day temperature on the moon may, indeed, be as high as our boiling-point, while the night temperature may be more than twice as low as the greatest cold known in our arctic regions. that a certain amount of solar heat is reflected to us from the moon is shown by the sharp drop in temperature which certain heat-measuring instruments record when the moon becomes obscured in a lunar eclipse. the solar heat which is thus reflected to us by the moon is, however, on the whole extremely small; more light and heat, indeed, reach us _direct_ from the sun in half a minute than we get by _reflection_ from the moon during the entire course of the year. with regard to the origin of the lunar craters there has been much discussion. some have considered them to be evidence of violent volcanic action in the dim past; others, again, as the result of the impact of meteorites upon the lunar surface, when the moon was still in a plastic condition; while a third theory holds that they were formed by the bursting of huge bubbles during the escape into space of gases from the interior. the question is, indeed, a very difficult one. though volcanic action, such as would result in craters of the size of ptolemæus, is hard for us to picture, and though the lone peaks which adorn the centres of many craters have nothing reminiscent of them in our terrestrial volcanoes, nevertheless the volcanic theory seems to receive more favour than the others. in addition to the craters there are two more features which demand notice, namely, what are known as _rays_ and _rills_. the rays are long, light-coloured streaks which radiate from several of the large craters, and extend to a distance of some hundreds of miles. that they are mere markings on the surface is proved by the fact that they cast no shadows of any kind. one theory is, that they were originally great cracks which have been filled with lighter coloured material, welling up from beneath. the rills, on the other hand, are actually fissures, about a mile or so in width and about a quarter of a mile in depth. the rays are seen to the best advantage in connection with the craters tycho and copernicus (see plate xi., p. ). in consequence of its fairly forward position on the lunar disc, and of the remarkable system of rays which issue from it like spokes from the axle of a wheel, tycho commands especial attention. the late rev. t.w. webb, a famous observer, christened it, very happily, the "metropolitan crater of the moon." [illustration: plate xi. the moon the systems of rays from the craters tycho, copernicus and kepler are well shown here. from a photograph taken at the paris observatory by m.p. puiseux. (page )] a great deal of attention is, and has been, paid by certain astronomers to the moon, in the hope of finding out if any changes are actually in progress at present upon her surface. sir william herschel, indeed, once thought that he saw a lunar volcano in eruption, but this proved to be merely the effect of the sunlight striking the top of the crater aristarchus, while the region around it was still in shadow--sunrise upon aristarchus, in fact! no change of any real importance has, however, been noted, although it is suspected that some minor alterations have from time to time taken place. for instance, slight variations of tint have been noticed in certain areas of the lunar surface. professor w.h. pickering puts forward the conjecture that these may be caused by the growth and decay of some low form of vegetation, brought into existence by vapours of water, or carbonic acid gas, making their way out from the interior through cracks near at hand. again, during the last hundred years one small crater known as linné (linnæus), situated in the mare serenitatis (sea of serenity), has appeared to undergo slight changes, and is even said to have been invisible for a while (see plate x., p. ). it is, however, believed that the changes in question may be due to the varying angles at which the sunlight falls upon the crater; for it is an understood fact that the irregularities of the moon's motion give us views of her surface which always differ slightly. the suggestion has more than once been put forward that the surface of the moon is covered with a thick layer of ice. this is generally considered improbable, and consequently the idea has received very little support. it first originated with the late mr. s.e. peal, an english observer of the moon, and has recently been resuscitated by the german observer, herr fauth. the most unfavourable time for telescopic study of the moon is when she is full. the sunlight is then falling directly upon her visible hemisphere, and so the mountains cast no shadows. we thus do not get that impression of hill and hollow which is so very noticeable in the other phases. the first map of the moon was constructed by galileo. tobias mayer published another in ; while during the nineteenth century greatly improved ones were made by beer and mädler, schmidt, neison and others. in , professor w.h. pickering brought out a complete photographic lunar atlas; and a similar publication has recently appeared, the work of mm. loewy and puiseux of the observatory of paris. the so-called "seas" of the moon are, as we have seen, merely dark areas, and there appears to be no proof that they were ever occupied by any liquid. they are for the most part found in the _northern_ portion of the moon; a striking contrast to our seas and oceans, which take up so much of the _southern_ hemisphere of the earth. there are many erroneous ideas popularly held with regard to certain influences which the moon is supposed to exercise upon the earth. for instance, a change in the weather is widely believed to depend upon a change in the moon. but the word "change" as here used is meaningless, for the moon is continually changing her phase during the whole of her monthly round. besides, the moon is visible over a great portion of the earth _at the same moment_, and certainly all the places from which it can then be seen do not get the same weather! further, careful observations, and records extending over the past one hundred years and more, fail to show any reliable connection between the phases of the moon and the condition of the weather. it has been stated, on very good authority, that no telescope ever shows the surface of the moon as clearly as we could see it with the naked eye were it only miles distant from us. supposing, then, that we were able to approach our satellite, and view it without optical aid at such comparatively close quarters, it is interesting to consider what would be the smallest detail which our eye could take in. the question of the limit of what can be appreciated with the naked eye is somewhat uncertain, but it appears safe to say that at a distance of miles the _minutest speck_ visible would have to be _at least_ some yards across. atmosphere and liquid both wanting, the lunar surface must be the seat of an eternal calm; where no sound breaks the stillness and where change, as we know it, does not exist. the sun beats down upon the arid rocks, and inky shadows lie athwart the valleys. there is no mellowing of the harsh contrasts. we cannot indeed absolutely affirm that life has no place at all upon this airless and waterless globe, since we know not under what strange conditions it may manifest its presence; and our most powerful telescopes, besides, do not bring the lunar surface sufficiently near to us to disprove the existence there of even such large creatures as disport themselves upon our planet. still, we find it hard to rid ourselves of the feeling that we are in the presence of a dead world. on she swings around the earth month after month, with one face ever turned towards us, leaving a certain mystery to hang around that hidden side, the greater part of which men can never hope to see. the rotation of the moon upon her axis--the lunar day--has become, as we have seen, equal to her revolution around the earth. an epoch may likewise eventually be reached in the history of our own planet, when the length of the terrestrial day has been so slowed down by tidal friction that it will be equal to the year. then will the earth revolve around the central orb, with one side plunged in eternal night and the other in eternal sunshine. but such a vista need not immediately distress us. it is millions of years forward in time. [ ] _journal of the british astronomical association_, vol. x. ( - ), nos. and . [ ] certain of the ancient greeks thought the markings on the moon to be merely the reflection of the seas and lands of our earth, as in a badly polished mirror. [ ] mare imbrium, sinus iridum, lacus somniorum. [ ] the lunar craters have, as a rule, received their names from celebrated persons, usually men of science. this system of nomenclature was originated by riccioli, in . chapter xvii the superior planets having, in a previous chapter, noted the various aspects which an inferior planet presents to our view, in consequence of its orbit being nearer to the sun than the orbit of the earth, it will be well here to consider in the same way the case of a superior planet, and to mark carefully the difference. to begin with, it should be quite evident that we cannot ever have a transit of a superior planet. the orbit of such a body being entirely _outside_ that of the earth, the body itself can, of course, never pass between us and the sun. a superior planet will be at its greatest distance from us when on the far side of the sun. it is said then to be in _conjunction_. as it comes round in its orbit it eventually passes, so to speak, at the _back_ of us. it is then at its nearest, or in _opposition_, as this is technically termed, and therefore in the most favourable position for telescopic observation of its surface. being, besides, seen by us at that time in the direction of the heavens exactly opposite to where the sun is, it will thus at midnight be high up in the south side of the sky, a further advantage to the observer. last of all, a superior planet cannot show crescent shapes like an interior; for whether it be on the far side of the sun, or behind us, or again to our right or left, the sunlight must needs appear to fall more or less full upon its face. the planetoid eros the nearest to us of the superior planets is the tiny body, eros, which, as has been already stated, was discovered so late as the year . in point of view, however, of its small size, it can hardly be considered as a true planet, and the name "planetoid" seems much more appropriate to it. eros was not discovered, like uranus, in the course of telescopic examination of the heavens, nor yet, like neptune, as the direct result of difficult calculations, but was revealed by the impress of its light upon a photographic plate, which had been exposed for some length of time to the starry sky. since many of the more recent additions to the asteroids have been discovered in the same manner, we shall have somewhat more to say about this special employment of photography when we come to deal with those bodies later on. the path of eros around the sun is so very elliptical, or, to use the exact technical term, so very "eccentric," that the planetoid does not keep all the time entirely in the space between our orbit and that of mars, which latter happens to be the next body in the order of planetary succession outwards. in portions of its journey eros, indeed, actually goes outside the martian orbit. the paths of the planetoid and of mars are, however, _not upon the same plane_, so the bodies always pass clear of each other, and there is thus as little chance of their dashing together as there would be of trains which run across a bridge at an upper level, colliding with those which pass beneath it at a lower level. when eros is in opposition, it comes within about - / million miles of our earth, and, after the moon, is therefore by a long way our nearest neighbour in space. it is, however, extremely small, not more, perhaps, than twenty miles in diameter, and is subject to marked variations in brightness, which do not appear up to the present to meet with a satisfactory explanation. but, insignificant as is this little body, it is of great importance to astronomy; for it happens to furnish the best method known of calculating the sun's distance from our earth--a method which galle, in , and sir david gill, in , suggested that asteroids might be employed for, and which has in consequence supplanted the old one founded upon transits of venus. the sun's distance is now an ascertained fact to within , miles, or less than half the distance of the moon. the planet mars we next come to the planet mars. this body rotates in a period of slightly more than twenty-four hours. the inclination, or slant, of its axis is about the same as that of the earth, so that, putting aside its greater distance from the sun, the variations of season which it experiences ought to be very much like ours. the first marking detected upon mars was the notable one called the syrtis major, also known, on account of its shape, as the hour-glass sea. this observation was made by the famous huyghens in ; and, from the movement of the marking in question across the disc, he inferred that the planet rotated on its axis in a period of about twenty-four hours. there appears to be very little atmosphere upon mars, the result being that we almost always obtain a clear view of the detail on its surface. indeed, it is only to be expected from the kinetic theory that mars could not retain much of an atmosphere, as the force of gravity at its surface is less than one-half of what we experience upon the earth. it should here be mentioned that recent researches with the spectroscope seem to show that, whatever atmosphere there may be upon mars, its density at the surface of the planet cannot be more than the one-fourth part of the density of the air at the surface of the earth. professor lowell, indeed, thinks it may be more rarefied than that upon our highest mountain-tops. seen with the naked eye, mars appears of a red colour. viewed in the telescope, its surface is found to be in general of a ruddy hue, varied here and there with darker patches of a bluish-green colour. these markings are permanent, and were supposed by the early telescopic observers to imply a distribution of the planet's surface into land and water, the ruddy portions being considered as continental areas (perhaps sandy deserts), and the bluish-green as seas. the similarity to our earth thus suggested was further heightened by the fact that broad white caps, situated at the poles, were seen to vary with the planet's seasons, diminishing greatly in extent during the martian summer (the southern cap in even disappearing altogether), and developing again in the martian winter.[ ] readers of oliver wendell holmes will no doubt recollect that poet's striking lines:-- "the snows that glittered on the disc of mars have melted, and the planet's fiery orb rolls in the crimson summer of its year." a state of things so strongly analogous to what we experience here, naturally fired the imaginations of men, and caused them to look on mars as a world like ours, only upon a much smaller scale. being smaller, it was concluded to have cooled quicker, and to be now long past its prime; and its "inhabitants" were, therefore, pictured as at a later stage of development than the inhabitants of our earth. notwithstanding the strong temptation to assume that the whiteness of the martian polar caps is due to fallen snow, such a solution is, however, by no means so simple as it looks. the deposition of water in the form of snow, or even of hoar frost, would at least imply that the atmosphere of mars should now and then display traces of aqueous vapour, which it does not appear to do.[ ] it has, indeed, been suggested that the whiteness may not after all be due to this cause, but to carbonic acid gas (carbon dioxide), which is known to freeze at a _very low_ temperature. the suggestion is plainly based upon the assumption that, as mars is so much further from the sun than we are, it would receive much less heat, and that the little thus received would be quickly radiated away into space through lack of atmosphere to bottle it in. we now come to those well-known markings, popularly known as the "canals" of mars, which have been the subject of so much discussion since their discovery thirty years ago. it was, in fact, in the year , when mars was in opposition, and thus at its nearest to us, that the famous italian astronomer, schiaparelli, announced to the world that he had found that the ruddy areas, thought to be continents, were intersected by a network of straight dark lines. these lines, he reported, appeared in many cases to be of great length, so long, indeed, as several thousands of miles, and from about twenty to sixty miles in width. he christened the lines _channels_, the italian word for which, "canali," was unfortunately translated into english as "canals." the analogy, thus accidentally suggested, gave rise to the idea that they might be actual waterways.[ ] in the winter of - , when mars was again in opposition, schiaparelli further announced that he had found some of these lines doubled; that is to say, certain of them were accompanied by similar lines running exactly parallel at no great distance away. there was at first a good deal of scepticism on the subject of schiaparelli's discoveries, but gradually other observers found themselves seeing both the lines and their doublings. we have in this a good example of a curious circumstance in astronomical observation, namely, the fact that when fine detail has once been noted by a competent observer, it is not long before other observers see the same detail with ease. an immense amount of close attention has been paid to the planet mars during recent years by the american observer, professor percival lowell, at his famous observatory, feet above the sea, near the town of flagstaff, arizona, u.s.a. his observations have not, like those of most astronomers, been confined merely to "oppositions," but he has systematically kept the planet in view, so far as possible, since the year . the instrumental equipment of his observatory is of the very best, and the "seeing" at flagstaff is described as excellent. in support of the latter statement, mr. lampland, of the lowell observatory, maintains that the faintest stars shown on charts made at the lick observatory with the -inch telescope there, are _perfectly visible_ with the -inch telescope at flagstaff. professor lowell is, indeed, generally at issue with the other observers of mars. he finds the canals extremely narrow and sharply defined, and he attributes the blurred and hazy appearance, which they have presented to other astronomers, to the unsteady and imperfect atmospheric conditions in which their observations have been made. he assigns to the thinnest a width of two or three miles, and from fifteen to twenty to the larger. relatively to their width, however, he finds their length enormous. many of them are miles long, while one is even as much as . such lengths as these are very great in comparison with the smallness of the planet. he considers that the canals stand in some peculiar relation to the polar cap, for they crowd together in its neighbourhood. in place, too, of ill-defined condensations, he sees sharp black spots where the canals meet and intersect, and to these he gives the name of "oases." he further lays particular stress upon a dark band of a blue tint, which is always seen closely to surround the edges of the polar caps all the time that they are disappearing; and this he takes to be a proof that the white material is something which actually _melts_. of all substances which we know, water alone, he affirms, would act in such a manner. the question of melting at all may seem strange in a planet which is situated so far from the sun, and possesses such a rarefied atmosphere. but professor lowell considers that this very thinness of the atmosphere allows the direct solar rays to fall with great intensity upon the planet's surface, and that this heating effect is accentuated by the great length of the martian summer. in consequence he concludes that, although the general climate of mars is decidedly cold, it is above the freezing point of water. the observations at flagstaff appear to do away with the old idea that the darkish areas are seas, for numerous lines belonging to the so-called "canal system" are seen to traverse them. again, there is no star-like image of the sun reflected from them, as there would be, of course, from the surface of a great sheet of water. lastly, they are observed to vary in tone and colour with the changing martian seasons, the blue-green changing into ochre, and later on back again into blue-green. professor lowell regards these areas as great tracts of vegetation, which are brought into activity as the liquid reaches them from the melting snows. [illustration: plate xii. a map of the planet mars we see here the syrtis major (or "hour-glass sea"), the polar caps, several "oases," and a large number of "canals," some of which are double. the south is at the top of the picture, in accordance with the _inverted_ view given by an astronomical telescope. from a drawing by professor percival lowell. (page )] with respect to the canals, the lowell observations further inform us that these are invisible during the martian winter, but begin to appear in the spring when the polar cap is disappearing. professor lowell, therefore, inclines to the view that in the middle of the so-called canals there exist actual waterways which serve the purposes of irrigation, and that what we see is not the waterways themselves, for they are too narrow, but the fringe of vegetation which springs up along the banks as the liquid is borne through them from the melting of the polar snows. he supports this by his observation that the canals begin to appear in the neighbourhood of the polar caps, and gradually grow, as it were, in the direction of the planet's equator. it is the idea of life on mars which has given this planet such a fascination in the eyes of men. a great deal of nonsense has, however, been written in newspapers upon the subject, and many persons have thus been led to think that we have obtained some actual evidence of the existence of living beings upon mars. it must be clearly understood, however, that professor lowell's advocacy of the existence of life upon that planet is by no means of this wild order. at the best he merely indulges in such theories as his remarkable observations naturally call forth. his views are as follows:--he considers that the planet has reached a time when "water" has become so scarce that the "inhabitants" are obliged to employ their utmost skill to make their scanty supply suffice for purposes of irrigation. the changes of tone and colour upon the martian surface, as the irrigation produces its effects, are similar to what a telescopic observer--say, upon venus--would notice on our earth when the harvest ripens over huge tracts of country; that is, of course, if the earth's atmosphere allowed a clear view of the terrestrial surface--a very doubtful point indeed. professor lowell thinks that the perfect straightness of the lines, and the geometrical manner in which they are arranged, are clear evidences of artificiality. on a globe, too, there is plainly no reason why the liquid which results from the melting of the polar caps should trend at all in the direction of the equator. upon our earth, for instance, the transference of water, as in rivers, merely follows the slope of the ground, and nothing else. the lowell observations show, however, that the martian liquid is apparently carried from one pole towards the equator, and then past it to the other pole, where it once more freezes, only to melt again in due season, and to reverse the process towards and across the equator as before. professor lowell therefore holds, and it seems a strong point in favour of his theory, that the liquid must, in some artificial manner, as by pumping, for instance, be _helped_ in its passage across the surface of the planet. a number of attempts have been made to explain the _doubling_ of the canals merely as effects of refraction or reflection; and it has even been suggested that it may arise from the telescope not being accurately focussed. the actual doubling of the canals once having been doubted, it was an easy step to the casting of doubt on the reality of the canals themselves. the idea, indeed, was put forward that the human eye, in dealing with detail so very close to the limit of visibility, may unconsciously treat as an actual line several point-like markings which merely happen to lie in a line. in order to test this theory, experiments were carried out in by mr. e.w. maunder of greenwich observatory, and mr. j.e. evans of the royal hospital school at greenwich, in which certain schoolboys were set to make drawings of a white disc with some faint markings upon it. the boys were placed at various distances from the disc in question; and it was found that the drawings made by those who were just too far off to see distinctly, bore out the above theory in a remarkable manner. recently, however, the plausibility of the _illusion_ view has been shaken by photographs of mars taken during the opposition of by mr. lampland at the lowell observatory, in which a number of the more prominent canals come out as straight dark lines. further still, in some photographs made there quite lately, several canals are said to appear visibly double. following up the idea alluded to in chapter xvi., that the moon may be covered with a layer of ice, mr. w.t. lynn has recently suggested that this may be the case on mars; and that, at certain seasons, the water may break through along definite lines, and even along lines parallel to these. this, he maintains, would account for the canals becoming gradually visible across the disc, without the necessity of professor lowell's "pumping" theory. and now for the views of professor lowell himself with regard to the doubling of the canals. from his observations, he considers that no pairs of railway lines could apparently be laid down with greater parallelism. he draws attention to the fact that the doubling does not take place by any means in every canal; indeed, out of canals seen at flagstaff, only fifty-one--or, roughly, one-eighth--have at any time been seen double. he lays great stress upon this, which he considers points strongly against the duplication being an optical phenomenon. he finds that the distance separating pairs of canals is much less in some doubles than in others, and varies on the whole from to miles. according to him, the double canals appear to be confined to within degrees of the equator: or, to quote his own words, they are "an equatorial feature of the planet, confined to the tropic and temperate belts." finally, he points out that they seem to _avoid_ the blue-green areas. but, strangely enough, professor lowell does not so far attempt to fit in the doubling with his body of theory. he makes the obvious remark that they may be "channels and return channels," and with that he leaves us. the conclusions of professor lowell have recently been subjected to strenuous criticism by professor w.h. pickering and dr. alfred russel wallace. it was professor pickering who discovered the "oases," and who originated the idea that we did not see the so-called "canals" themselves, but only the growth of vegetation along their borders. he holds that the oases are craterlets, and that the canals are cracks which radiate from them, as do the rifts and streaks from craters upon the moon. he goes on to suggest that vapours of water, or of carbonic acid gas, escaping from the interior, find their way out through these cracks, and promote the growth of a low form of vegetation on either side of them. in support of this view he draws attention to the existence of long "steam-cracks," bordered by vegetation, in the deserts of the highly volcanic island of hawaii. we have already seen, in an earlier chapter, how he has applied this idea to the explanation of certain changes which are suspected to be taking place upon the moon. in dealing with the lowell canal system, professor pickering points out that under such a slight atmospheric pressure as exists on mars, the evaporation of the polar caps--supposing them to be formed of snow--would take place with such extraordinary rapidity that the resulting water could never be made to travel along open channels, but that a system of gigantic tubes or water-mains would have to be employed! as will be gathered from his theories regarding vegetation, professor pickering does not deny the existence of a form of life upon mars. but he will not hear of civilisation, or of anything even approaching it. he thinks, however, that as mars is intermediate physically between the moon and earth, the form of life which it supports may be higher than that on the moon and lower than that on the earth. in a small book published in the latter part of , and entitled _is mars habitable?_ dr. alfred russel wallace sets himself, among other things, to combat the idea of a comparatively high temperature, such as professor lowell has allotted to mars. he shows the immense service which the water-vapour in our atmosphere exercises, through keeping the solar heat from escaping from the earth's surface. he then draws attention to the fact that there is no spectroscopic evidence of water-vapour on mars[ ]; and points out that its absence is only to be expected, as dr. george johnstone stoney has shown that it will escape from a body whose mass is less than one-quarter the mass of the earth. the mass of mars is, in fact, much less than this, _i.e._ only one-ninth. dr. wallace considers, therefore, that the temperature of mars ought to be extremely low, unless the constitution of its atmosphere is very different from ours. with regard to the latter statement, it should be mentioned that the swedish physicist, arrhenius, has recently shown that the carbonic acid gas in our atmosphere has an important influence upon climate. the amount of it in our air is, as we have seen, extremely small; but arrhenius shows that, if it were doubled, the temperature would be more uniform and much higher. we thus see how futile it is, with our present knowledge, to dogmatise on the existence or non-existence of life in other celestial orbs. as to the canals dr. wallace puts forward a theory of his own. he contends that after mars had cooled to a state of solidity, a great swarm of meteorites and small asteroids fell in upon it, with the result that a thin molten layer was formed all over the planet. as this layer cooled, the imprisoned gases escaped, producing vents or craterlets; and as it attempted to contract further upon the solid interior, it split in fissures radiating from points of weakness, such, for instance, as the craterlets. and he goes on to suggest that the two tiny martian satellites, with which we shall deal next, are the last survivors of his hypothetical swarm. finally, with regard to the habitability of mars, dr. wallace not only denies it, but asserts that the planet is "absolutely uninhabitable." for a long time it was supposed that mars did not possess any satellites. in , however, during that famous opposition in which schiaparelli first saw the canals, two tiny satellites were discovered at the washington observatory by an american astronomer, professor asaph hall. these satellites are so minute, and so near to the planet, that they can only be seen with very large telescopes; and even then the bright disc of the planet must be shielded off. they have been christened phobos and deimos (fear and dread); these being the names of the two subordinate deities who, according to homer, attended upon mars, the god of war. it is impossible to measure the exact sizes of these satellites, as they are too small to show any discs, but an estimate has been formed from their brightness. the diameter of phobos was at first thought to be six miles, and that of deimos, seven. as later estimates, however, considerably exceed this, it will, perhaps, be not far from the truth to state that they are each roughly about the size of the planetoid eros. phobos revolves around mars in about - / hours, at a distance of about only miles from the planet's surface, and deimos in about hours, at a distance of about , miles. as mars rotates on its axis in about hours, it will be seen that phobos makes more than three revolutions while the planet is rotating once--a very interesting condition of things. a strange foreshadowing of the discovery of the satellites of mars will be familiar to readers of _gulliver's travels_. according to dean swift's hero, the astronomers on the flying island of laputa had found two tiny satellites to mars, one of which revolved around the planet in ten hours. the correctness of this guess is extraordinarily close, though at best it is, of course, nothing more than a pure coincidence. it need not be at all surprising that much uncertainty should exist with regard to the actual condition of the surface of mars. the circumstances in which we are able to see that planet at the best are, indeed, hardly sufficient to warrant us in propounding any hard and fast theories. one of the most experienced of living observers, the american astronomer, professor e.e. barnard, considers that the view we get of mars with the best telescope may be fairly compared with our naked eye view of the moon. since we have seen that a view with quite a small telescope entirely alters our original idea of the lunar surface, a slight magnification revealing features of whose existence we had not previously the slightest conception, it does not seem too much to say that a further improvement in optical power might entirely subvert the present notions with regard to the martian canals. therefore, until we get a still nearer view of these strange markings, it seems somewhat futile to theorise. the lines which we see are perhaps, indeed, a foreshortened and all too dim view of some type of formation entirely novel to us, and possibly peculiar to mars. differences of gravity and other conditions, such as obtain upon different planets, may perhaps produce very diverse results. the earth, the moon, and mars differ greatly from one another in size, gravitation, and other such characteristics. mountain-ranges so far appear typical of our globe, and ring-mountains typical of the moon. may not the so-called "canals" be merely some special formation peculiar to mars, though quite a natural result of its particular conditions and of its past history? the asteroids (or minor planets) we now come to that belt of small planets which are known by the name of asteroids. in the general survey of the solar system given in chapter ii., we saw how it was long ago noticed that the distances of the planetary orbits from the sun would have presented a marked appearance of orderly sequence, were it not for a gap between the orbits of mars and jupiter where no large planet was known to circulate. the suspicion thus aroused that some planet might, after all, be moving in this seemingly empty space, gave rise to the gradual discovery of a great number of small bodies; the largest of which, ceres, is less than miles in diameter. up to the present day some of these bodies have been discovered; the four leading ones, in order of size, being named ceres, pallas, juno, and vesta. all the asteroids are invisible to the naked eye, with the exception of vesta, which, though by no means the largest, happens to be the brightest. it is, however, only just visible to the eye under favourable conditions. no trace of an atmosphere has been noted upon any of the asteroids, but such a state of things is only to be expected from the kinetic theory. for a good many years the discoveries of asteroids were made by means of the telescope. when, in the course of searching the heavens, an object was noticed which did not appear upon any of the recognised star charts, it was kept under observation for several nights to see whether it changed its place in the sky. since asteroids move around the sun in orbits, just as planets do, they, of course, quickly reveal themselves by their change of position against the starry background. the year started a new era in the discovery of asteroids. it occurred to the heidelberg observer, dr. max wolf, one of the most famous of the hunters of these tiny planets, that photography might be employed in the quest with success. this photographic method, to which allusion has already been made in dealing with eros, is an extremely simple one. if a photograph of a portion of the heavens be taken through an "equatorial"--that is, a telescope, moving by machinery, so as to keep the stars, at which it is pointed, always exactly in the field of view during their apparent movement across the sky--the images of these stars will naturally come out in the photograph as sharply defined points. if, however, there happens to be an asteroid, or other planetary body, in the same field of view, its image will come out as a short white streak; because the body has a comparatively rapid motion of its own, and will, during the period of exposure, have moved sufficiently against the background of the stars to leave a short trail, instead of a dot, upon the photographic plate. by this method wolf himself has succeeded in discovering more than a hundred asteroids (see plate xiii., p. ). it was, indeed, a little streak of this kind, appearing upon a photograph taken by the astronomer witt, at berlin, in , which first informed the world of the existence of eros. [illustration: plate xiii. minor planet trails two trails of minor planets (asteroids) imprinted _at the same time_ upon one photographic plate. in the white streak on the left-hand side of the picture we witness the _discovery_ of a new minor planet. the streak on the right was made by a body already known--the minor planet "fiducia." this photograph was taken by dr. max wolf, at heidelberg, on the th of november, , with the aid of a -inch telescope. the time of exposure was two hours. (page )] it has been calculated that the total mass of the asteroids must be much less than one-quarter that of the earth. they circulate as a rule within a space of some , , miles in breadth, lying about midway between the paths of mars and jupiter. two or three, however, of the most recently discovered of these small bodies have been found to pass quite close to jupiter. the orbits of the asteroids are by no means in the one plane, that of pallas being the most inclined to the plane of the earth's orbit. it is actually three times as much inclined as that of eros. two notable theories have been put forward to account for the origin of the asteroids. the first is that of the celebrated german astronomer, olbers, who was the discoverer of pallas and vesta. he suggested that they were the fragments of an exploded planet. this theory was for a time generally accepted, but has now been abandoned in consequence of certain definite objections. the most important of these objections is that, in accordance with the theory of gravitation, the orbits of such fragments would all have to pass through the place where the explosion originally occurred. but the wide area over which the asteroids are spread points rather against the notion that they all set out originally from one particular spot. another objection is that it does not appear possible that, within a planet already formed, forces could originate sufficiently powerful to tear the body asunder. the second theory is that for some reason a planet here failed in the making. possibly the powerful gravitational action of the huge body of jupiter hard by, disturbed this region so much that the matter distributed through it was never able to collect itself into a single mass. [ ] sir william herschel was the first to note these polar changes. [ ] quite recently, however, professor lowell has announced that his observer, mr. e.c. slipher, finds with the spectroscope faint traces of water vapour in the martian atmosphere. [ ] in a somewhat similar manner the term "crater," as applied to the ring-mountain formation on the moon, has evidently given a bias in favour of the volcanic theory as an explanation of that peculiar structure. [ ] mr. slipher's results (see note , page ) were not then known. chapter xviii the superior planets--_continued_ the planets, so far, have been divided into inferior and superior. such a division, however, refers merely to the situation of their orbits with regard to that of our earth. there is, indeed, another manner in which they are often classed, namely, according to size. on this principle they are divided into two groups; one group called the _terrestrial planets_, or those which have characteristics like our earth, and the other called the _major planets_, because they are all of very great size. the terrestrial planets are mercury, venus, the earth, and mars. the major planets are the remainder, namely, jupiter, saturn, uranus, and neptune. as the earth's orbit is the boundary which separates the inferior from the superior planets, so does the asteroidal belt divide the terrestrial from the major planets. we found the division into inferior and superior useful for emphasising the marked difference in aspect which those two classes present as seen from our earth; the inferior planets showing phases like the moon when viewed in the telescope, whereas the superior planets do not. but the division into terrestrial and major planets is the more far-reaching classification of the two, for it includes the whole number of planets, whereas the other arrangement necessarily excludes the earth. the members of each of these classes have many definite characteristics in common. the terrestrial planets are all of them relatively small in size, comparatively near together, and have few or no satellites. they are, moreover, rather dense in structure. the major planets, on the other hand, are huge bodies, circulating at great distances from each other, and are, as a rule, provided with a number of satellites. with respect to structure, they may be fairly described as being loosely put together. further, the markings on the surfaces of the terrestrial planets are permanent, whereas those on the major planets are continually shifting. the planet jupiter jupiter is the greatest of the major planets. it has been justly called the "giant" planet, for both in volume and in mass it exceeds all the other planets put together. when seen through the telescope it exhibits a surface plentifully covered with markings, the most remarkable being a series of broad parallel belts. the chief belt lies in the central parts of the planet, and is at present about , miles wide. it is bounded on either side by a reddish brown belt of about the same width. bright spots also appear upon the surface of the planet, last for a while, and then disappear. the most notable of the latter is one known as the "great red spot." this is situated a little beneath the southern red belt, and appeared for the first time about thirty years ago. it has undergone a good many changes in colour and brightness, and is still faintly visible. this spot is the most permanent marking which has yet been seen upon jupiter. in general, the markings change so often that the surface which we see is evidently not solid, but of a fleeting nature akin to cloud (see plate xiv., p. ). [illustration: plate xiv. the planet jupiter the giant planet as seen at . p.m., on the th of january, , with a - / -inch reflecting telescope. the extensive oval marking in the upper portion of the disc is the "great red spot." the south is at the top of the picture, the view being the _inverted_ one given by an astronomical telescope. from a drawing by the rev. theodore e.r. phillips, m.a., f.r.a.s., director of the jupiter section of the british astronomical association. (page )] observations of jupiter's markings show that on an average the planet rotates on its axis in a period of about hours minutes. the mention here of _an average_ with reference to the rotation will, no doubt, recall to the reader's mind the similar case of the sun, the different portions of which rotate with different velocities. the parts of jupiter which move quickest take hours minutes to go round, while those which move slowest take hours minutes. the middle portions rotate the fastest, a phenomenon which the reader will recollect was also the case with regard to the sun. jupiter is a very loosely packed body. its density is on an average only about - / times that of water, or about one-fourth the density of the earth; but its bulk is so great that the gravitation at that surface which we see is about - / times what it is on the surface of the earth. in accordance, therefore, with the kinetic theory, we may expect the planet to retain an extensive layer of gases around it; and this is confirmed by the spectroscope, which gives evidence of the presence of a dense atmosphere. all things considered, it may be safely inferred that the interior of jupiter is very hot, and that what we call its surface is not the actual body of the planet, but a voluminous layer of clouds and vapours driven upwards from the heated mass underneath. the planet was indeed formerly thought to be self-luminous; but this can hardly be the case, for those portions of the surface which happen to lie at any moment in the shadows cast by the satellites appear to be quite black. again, when a satellite passes into the great shadow cast by the planet it becomes entirely invisible, which would not be the case did the planet emit any perceptible light of its own. in its revolutions around the sun, jupiter is attended, so far as we know, by seven[ ] satellites. four of these were among the first celestial objects which galileo discovered with his "optick tube," and he named them the "medicean stars" in honour of his patron, cosmo de medici. being comparatively large bodies they might indeed just be seen with the naked eye, were it not for the overpowering glare of the planet. it was only in quite recent times, namely, in , that a fifth satellite was added to the system of jupiter. this body, discovered by professor e.e. barnard, is very small. it circulates nearer to the planet than the innermost of galileo's moons; and, on account of the glare, is a most difficult object to obtain a glimpse of, even in the best of telescopes. in december and january respectively, two more moons were added to the system, these being found by _photography_, by the american astronomer, professor c.d. perrine. both the bodies in question revolve at a greater distance from the planet than the outermost of the older known satellites. galileo's moons, though the largest bodies of jupiter's satellite system, are, as we have already pointed out, very small indeed when compared with the planet itself. the diameters of two of them, europa and io, are, however, about the same as that of our moon, while those of the other two, callisto and ganymede, are more than half as large again. the recently discovered satellites are, on the other hand, insignificant; that found by barnard, for example, being only about miles in diameter. of the four original satellites io is the nearest to jupiter, and, seen from the planet, it would show a disc somewhat larger than that of our moon. the others would appear somewhat smaller. however, on account of the great distance of the sun, the entire light reflected to jupiter by all the satellites should be very much less than what we get from our moon. barnard's satellite circles around jupiter at a distance less than our moon is from us, and in a period of about hours. galileo's four satellites revolve in periods of about , - / , , and - / days respectively, at distances lying roughly between a quarter of a million and one million miles. perrine's two satellites are at a distance of about seven million miles, and take about nine months to complete their revolutions. the larger satellites, when viewed in the telescope, exhibit certain defined markings; but the bodies are so far away from us, that only those details which are of great extent can be seen. the satellite io, according to professor barnard, shows a darkish disc, with a broad white belt across its middle regions. mr. douglass, one of the observers at the lowell observatory, has noted upon ganymede a number of markings somewhat resembling those seen on mars, and he concludes, from their movement, that this satellite rotates on its axis in about seven days. professor barnard, on the other hand, does not corroborate this, though he claims to have discovered bright polar caps on both ganymede and callisto. in an earlier chapter we dealt at length with eclipses, occultations, and transits, and endeavoured to make clear the distinction between them. the system of jupiter's satellites furnishes excellent examples of all these phenomena. the planet casts a very extensive shadow, and the satellites are constantly undergoing obscuration by passing through it. such occurrences are plainly comparable to our lunar eclipses. again, the satellites may, at one time, be occulted by the huge disc of the planet, and at another time seen in transit over its face. a fourth phenomenon is what is known as an _eclipse of the planet by a satellite_, which is the exact equivalent of what we style on the earth an eclipse of the sun. in this last case the shadow, cast by the satellite, appears as a round black spot in movement across the planet's surface. in the passages of these attendant bodies behind the planet, into its shadow, or across its face, respectively, it occasionally happens that galileo's four satellites all disappear from view, and the planet is then seen for a while in the unusual condition of being apparently without its customary attendants. an instance of this phenomenon took place on the rd of october . on that occasion, the satellites known as i. and iii. (_i.e._ io and ganymede) were eclipsed, that is to say, obscured by passing into the planet's shadow; satellite iv. (callisto) was occulted by the planet's disc; while satellite ii. (europa), being at the same moment in transit across the planet's face, was invisible against that brilliant background. a number of instances of this kind of occurrence are on record. galileo, for example, noted one on the th of march , while herschel observed another on the rd of may . it was indirectly to jupiter's satellites that the world was first indebted for its knowledge of the velocity of light. when the periods of revolution of the satellites were originally determined, jupiter happened, at the time, to be at his nearest to us. from the periods thus found tables were made for the prediction of the moments at which the eclipses and other phenomena of the satellites should take place. as jupiter, in the course of his orbit, drew further away from the earth, it was noticed that the disappearances of the satellites into the shadow of the planet occurred regularly later than the time predicted. in the year , roemer, a danish astronomer, inferred from this, not that the predictions were faulty, but that light did not travel instantaneously. it appeared, in fact, to take longer to reach us, the greater the distance it had to traverse. thus, when the planet was far from the earth, the last ray given out by the satellite, before its passage into the shadow, took a longer time to cross the intervening space, than when the planet was near. modern experiments in physics have quite confirmed this, and have proved for us that light does not travel across space in the twinkling of an eye, as might hastily be supposed, but actually moves, as has been already stated, at the rate of about , miles per second. the planet saturn seen in the telescope the planet saturn is a wonderful and very beautiful object. it is distinguished from all the other planets, in fact from all known celestial bodies, through being girt around its equator by what looks like a broad, flat ring of exceeding thinness. this, however, upon closer examination, is found to be actually composed of three concentric rings. the outermost of these is nearly of the same brightness as the body of the planet itself. the ring which comes immediately within it is also bright, and is separated from the outer one all the way round by a relatively narrow space, known as "cassini's division," because it was discovered by the celebrated french astronomer, j.d. cassini, in the year . inside the second ring, and merging insensibly into it, is a third one, known as the "crape ring," because it is darker in hue than the others and partly transparent, the body of saturn being visible through it. the inner boundary of this third and last ring does not adjoin the planet, but is everywhere separated from it by a definite space. this ring was discovered _independently_[ ] in by bond in america and dawes in england. [illustration: plate xv. the planet saturn from a drawing made by professor barnard with the great lick telescope. the black band fringing the outer ring, where it crosses the disc, is portion of the _shadow which the rings cast upon the planet_. the black wedge-shaped mark, where the rings disappear behind the disc at the left-hand side, is portion of the _shadow which the planet casts upon the rings_. (page )] as distinguished from the crape ring, the bright rings must have a considerable closeness of texture; for the shadow of the planet may be seen projected upon them, and their shadows in turn projected upon the surface of the planet (see plate xv., p. ). according to professor barnard, the entire breadth of the ring system, that is to say, from one side to the other of the outer ring, is , miles, or somewhat more than double the planet's diameter. in the varying views which we get of saturn, the system of the rings is presented to us at very different angles. sometimes we are enabled to gaze upon its broad expanse; at other times, however, its thin edge is turned exactly towards us, an occurrence which takes place after intervals of about fifteen years. when this happened in the rings are said to have disappeared entirely from view in the great lick telescope. we thus get an idea of their small degree of thickness, which would appear to be only about miles. the last time the system of rings was exactly edgewise to the earth was on the rd of october . the question of the composition of these rings has given rise to a good deal of speculation. it was formerly supposed that they were either solid or liquid, but in it was proved by clerk maxwell that a structure of this kind would not be able to stand. he showed, however, that they could be fully explained by supposing them to consist of an immense number of separate solid particles, or, as one might otherwise put it, extremely small satellites, circling in dense swarms around the middle portions of the planet. it is therefore believed that we have here the materials ready for the formation of a satellite or satellites; but that the powerful gravitative action, arising through the planet's being so near at hand, is too great ever to allow these materials to aggregate themselves into a solid mass. there is, as a matter of fact, a minimum distance from the body of any planet within which it can be shown that a satellite will be unable to form on account of gravitational stress. this is known as "roche's limit," from the name of a french astronomer who specially investigated the question. there thus appears to be a certain degree of analogy between saturn's rings and the asteroids. empty spaces, too, exist in the asteroidal zone, the relative position of one of which bears a striking resemblance to that of "cassini's division." it is suggested, indeed, that this division had its origin in gravitational disturbances produced by the attraction of the larger satellites, just as the empty spaces in the asteroidal zone are supposed to be the result of perturbations caused by the giant planet hard by. it has long been understood that the system of the rings must be rotating around saturn, for if they were not in motion his intense gravitational attraction would quickly tear them in pieces. this was at length proved to be the fact by the late professor keeler, director of the lick observatory, who from spectroscopic observations found that those portions of the rings situated near to the planet rotated faster than those farther from it. this directly supports the view that the rings are composed of satellites; for, as we have already seen, the nearer a satellite is to its primary the faster it will revolve. on the other hand, were the rings solid, their outer portions would move the fastest; as we have seen takes place in the body of the earth, for example. the mass of the ring system, however, must be exceedingly small, for it does not appear to produce any disturbances in the movements of saturn's satellites. from the kinetic theory, therefore, one would not expect to find any atmosphere on the rings, and the absence of it is duly shown by spectroscopic observations. the diameter of saturn, roughly speaking, is about one-fifth less than that of jupiter. the planet is very flattened at the poles, this flattening being quite noticeable in a good telescope. for instance, the diameter across the equator is about , miles, while from pole to pole it is much less, namely, , . the surface of saturn bears a strong resemblance to that of jupiter. its markings, though not so well defined, are of the same belt-like description; and from observation of them it appears that the planet rotates _on an average_ in a little over ten hours. the rotation is in fact of the same peculiar kind as that of the sun and jupiter; but the difference of speed at which the various portions of saturn go round are even more marked than in the case of the giant planet. the density of saturn is less than that of jupiter; so that it must be largely in a condition of vapour, and in all probability at a still earlier stage of planetary evolution. up to the present we know of as many as ten satellites circling around saturn, which is more than any other planet of the solar system can lay claim to. two of these, however, are very recent discoveries; one, phoebe, having been found by photography in august , and the other, themis, in , also by the same means. for both of these we are indebted to professor w.h. pickering. themis is said to be _the faintest object in the solar system_. it cannot be _seen_, even with the largest telescope in existence; a fact which should hardly fail to impress upon one the great advantage the photographic plate possesses in these researches over the human eye. the most important of the whole saturnian family of satellites are the two known as titan and japetus. these were discovered respectively by huyghens in and by cassini in . japetus is about the same size as our moon; while the diameter of titan, the largest of the satellites, is about half as much again. titan takes about sixteen days to revolve around saturn, while japetus takes more than two months and a half. the former is about three-quarters of a million miles distant from the planet, and the latter about two and a quarter millions. to sir william herschel we are indebted for the discovery of two more satellites, one of which he found on the evening that he used his celebrated -foot telescope for the first time. the ninth satellite, phoebe, one of the two discovered by professor pickering, is perhaps the most remarkable body in the solar system, for all the other known members of that system perform their revolutions in one fixed direction, whereas this satellite revolves in the _contrary_ direction. in consequence of the great distance of saturn, the sun, as seen from the planet, would appear so small that it would scarcely show any disc. the planet, indeed, only receives from the sun about one-ninetieth of the heat and light which the earth receives. owing to this diminished intensity of illumination, the combined light reflected to saturn by the whole of its satellites must be very small. with the sole exception of jupiter, not one of the planets circulating nearer to the sun could be seen from saturn, as they would be entirely lost in the solar glare. for an observer upon saturn, jupiter would, therefore, fill much the same position as venus does for us, regularly displaying phases and being alternately a morning and an evening star. it is rather interesting to consider the appearances which would be produced in our skies were the earth embellished with a system of rings similar to those of saturn. in consequence of the curving of the terrestrial surface, they would not be seen at all from within the arctic or antarctic circles, as they would be always below the horizon. from the equator they would be continually seen edgewise, and so would appear merely as line of light stretching right across the heaven and passing through the zenith. but the dwellers in the remaining regions would find them very objectionable, for they would cut off the light of the sun during lengthy periods of time. saturn was a sore puzzle to the early telescopic observers. they did not for a long time grasp the fact that it was surrounded by a ring--so slow is the human mind to seek for explanations out of the ordinary course of things. the protrusions of the ring on either side of the planet, at first looked to galileo like two minor globes placed on opposite sides of it, and slightly overlapping the disc. he therefore informed kepler that "saturn consists of three stars in contact with one another." yet he was genuinely puzzled by the fact that the two attendant bodies (as he thought them) always retained the same position with regard to the planet's disc, and did not appear to revolve around it, nor to be in any wise shifted as a consequence of the movements of our earth. about a year and a half elapsed before he again examined saturn; and, if he was previously puzzled, he was now thoroughly amazed. it happened just then to be one of those periods when the ring is edgewise towards the earth, and of course he only saw a round disc like that of jupiter. what, indeed, had become of the attendant orbs? was some demon mocking him? had saturn devoured his own children? he was, however, fated to be still more puzzled, for soon the minor orbs reappeared, and, becoming larger and larger as time went on, they ended by losing their globular appearance and became like two pairs of arms clasping the planet from each side! (see plate xvi., p. ). galileo went to his grave with the riddle still unsolved, and it remained for the famous dutch astronomer, huyghens, to clear up the matter. it was, however, some little time before he hit upon the real explanation. having noticed that there were dark spaces between the strange appendages and the body of the planet, he imagined saturn to be a globe fitted with handles at each side; "ansæ" these came to be called, from the latin _ansa_, which means a handle. at length, in the year , he solved the problem, and this he did by means of that -foot tubeless telescope, of which mention has already been made. the ring happened then to be at its edgewise period, and a careful study of the behaviour of the ansæ when disappearing and reappearing soon revealed to huyghens the true explanation. [illustration: plate xvi. early representations of saturn from an illustration in the _systema saturnium_ of christian huyghens. (page )] the planets uranus and neptune we have already explained (in chapter ii.) the circumstances in which both uranus and neptune were discovered. it should, however, be added that after the discovery of uranus, that planet was found to have been already noted upon several occasions by different observers, but always without the least suspicion that it was other than a mere faint star. again, with reference to the discovery of neptune, it may here be mentioned that the apparent amount by which that planet had pulled uranus out of its place upon the starry background was exceedingly small--so small, indeed, that no eye could have detected it without the aid of a telescope! of the two predictions of the place of neptune in the sky, that of le verrier was the nearer. indeed, the position calculated by adams was more than twice as far out. but adams was by a long way the first in the field with his results, and only for unfortunate delays the prize would certainly have fallen to him. for instance, there was no star-map at cambridge, and professor challis, the director of the observatory there, was in consequence obliged to make a laborious examination of the stars in the suspected region. on the other hand, all that galle had to do was to compare that part of the sky where le verrier told him to look with the berlin star-chart which he had by him. this he did on september , , with the result that he quickly noted an eighth magnitude star which did not figure in that chart. by the next night this star had altered its position in the sky, thus disclosing the fact that it was really a planet. six days later professor challis succeeded in finding the planet, but of course he was now too late. on reviewing his labours he ascertained that he had actually noted down its place early in august, and had he only been able to sift his observations as he made them, the discovery would have been made then. later on it was found that neptune had only just missed being discovered about fifty years earlier. in certain observations made during , the famous french astronomer, lalande, found that a star, which he had mapped in a certain position on the th of may of that year, was in a different position two days later. the idea of a planet does not appear to have entered his mind, and he merely treated the first observation as an error! the reader will, no doubt, recollect how the discovery of the asteroids was due in effect to an apparent break in the seemingly regular sequence of the planetary orbits outwards from the sun. this curious sequence of relative distances is usually known as "bode's law," because it was first brought into general notice by an astronomer of that name. it had, however, previously been investigated mathematically by titius in . long before this, indeed, the unnecessarily wide space between the orbits of mars and jupiter had attracted the attention of the great kepler to such a degree, that he predicted that a planet would some day be found to fill the void. notwithstanding the service which the so-called law of bode has indirectly rendered to astronomy, it has strangely enough been found after all not to rest upon any scientific foundation. it will not account for the distance from the sun of the orbit of neptune, and the very sequence seems on the whole to be in the nature of a mere coincidence. neptune is invisible to the naked eye; uranus is just at the limit of visibility. both planets are, however, so far from us that we can get but the poorest knowledge of their condition and surroundings. uranus, up to the present, is known to be attended by four satellites, and neptune by one. the planets themselves are about equal in size; their diameters, roughly speaking, being about one-half that of saturn. some markings have, indeed, been seen upon the disc of uranus, but they are very indistinct and fleeting. from observation of them, it is assumed that the planet rotates on its axis in a period of some ten to twelve hours. no definite markings have as yet been seen upon neptune, which body is described by several observers as resembling a faint planetary nebula. with regard to their physical condition, the most that can be said about these two planets is that they are probably in much the same vaporous state as jupiter and saturn. on account of their great distance from the sun they can receive but little solar heat and light. seen from neptune, in fact, the sun would appear only about the size of venus at her best, though of a brightness sufficiently intense to illumine the neptunian landscape with about seven hundred times our full moonlight. [ ] mr. p. melotte, of greenwich observatory, while examining a photograph taken there on february , , discovered upon it a very faint object which it is firmly believed will prove to be an _eighth_ satellite of jupiter. this object was afterwards found on plates exposed as far back as january . it has since been photographed several times at greenwich, and also at heidelberg (by dr. max wolf) and at the lick observatory. its movement is probably _retrograde_, like that of phoebe (p. ). [ ] in the history of astronomy two salient points stand out. the first of these is the number of "independent" discoveries which have taken place; such, for instance, as in the cases of le verrier and adams with regard to neptune, and of lockyer and janssen in the matter of the spectroscopic method of observing solar prominences. the other is the great amount of "anticipation." copernicus, as we have seen, was anticipated by the greeks; kepler was not actually the first who thought of elliptic orbits; others before newton had imagined an attractive force. both these points furnish much food for thought! chapter xix comets the reader has, no doubt, been struck by the marked uniformity which exists among those members of the solar system with which we have dealt up to the present. the sun, the planets, and their satellites are all what we call solid bodies. the planets move around the sun, and the satellites around the planets, in orbits which, though strictly speaking, ellipses, are yet not in any instance of a very oval form. two results naturally follow from these considerations. firstly, the bodies in question hide the light coming to us from those further off, when they pass in front of them. secondly, the planets never get so far from the sun that we lose sight of them altogether. with the objects known as comets it is, however, quite the contrary. these objects do not conform to our notions of solidity. they are so transparent that they can pass across the smallest star without dimming its light in the slightest degree. again, they are only visible to us during a portion of their orbits. a comet may be briefly described as an illuminated filmy-looking object, made up usually of three portions--a head, a nucleus, or brighter central portion within this head, and a tail. the heads of comets vary greatly in size; some, indeed, appear quite small, like stars, while others look even as large as the moon. occasionally the nucleus is wanting, and sometimes the tail also. [illustration: fig. .--showing how the tail of a comet is directed away from the sun.] these mysterious visitors to our skies come up into view out of the immensities beyond, move towards the sun at a rapidly increasing speed, and, having gone around it, dash away again into the depths of space. as a comet approaches the sun, its body appears to grow smaller and smaller, while, at the same time, it gradually throws out behind it an appendage like a tail. as the comet moves round the central orb this tail is always directed _away_ from the sun; and when it departs again into space the tail goes in advance. as the comet's distance from the sun increases, the tail gradually shrinks away and the head once more grows in size (see fig. ). in consequence of these changes, and of the fact that we lose sight of comets comparatively quickly, one is much inclined to wonder what further changes may take place after the bodies have passed beyond our ken. the orbits of comets are, as we have seen, very elliptic. in some instances this ellipticity is so great as to take the bodies out into space to nearly six times the distance of neptune from the sun. for a long time, indeed, it was considered that comets were of two kinds, namely, those which actually _belonged_ to the solar system, and those which were merely _visitors_ to it for the first and only time--rushing in from the depths of space, rapidly circuiting the sun, and finally dashing away into space again, never to return. on the contrary, nowadays, astronomers are generally inclined to regard comets as permanent members of the solar system. the difficulty, however, of deciding absolutely whether the orbits of comets are really always _closed_ curves, that is to say, curves which must sooner or later bring the bodies back again towards the sun, is, indeed, very great. comets, in the first place, are always so diffuse, that it is impossible to determine their exact position, or, rather, the exact position of that important point within them, known as the centre of gravity. secondly, that stretch of its orbit along which we can follow a comet, is such a very small portion of the whole path, that the slightest errors of observation which we make will result in considerably altering our estimate of the actual shape of the orbit. comets have been described as so transparent that they can pass across the sky without dimming the lustre of the smallest stars, which the thinnest fog or mist would do. this is, indeed, true of every portion of a comet except the nucleus, which is, as its name implies, the densest part. and yet, in contrast to this ghostlike character, is the strange fact that when comets are of a certain brightness they may actually be seen in full daylight. as might be gathered from their extreme tenuity, comets are so exceedingly small in mass that they do not appear to exert any gravitational attraction upon the other bodies of our system. it is, indeed, a known fact that in the year a comet passed right amidst the satellites of jupiter without disturbing them in the slightest degree. the attraction of the planet, on the other hand, so altered the comet's orbit, as to cause it to revolve around the sun in a period of seven years, instead of twenty-seven, as had previously been the case. also, in , the comet known as lexell's passed quite close to jupiter, and its orbit was so changed by that planet's attraction that it has never been seen since. the density of comets must, as a rule, be very much less than the one-thousandth part of that of the air at the surface of our globe; for, if the density of the comet were even so small as this, its mass would _not_ be inappreciable. if comets are really undoubted members of the solar system, the circumstances in which they were evolved must have been different from those which produced the planets and satellites. the axial rotations of both the latter, and also their revolutions, take place in one certain direction;[ ] their orbits, too, are ellipses which do not differ much from circles, and which, furthermore, are situated fairly in the one plane. comets, on the other hand, do not necessarily travel round the sun in the same fixed direction as the planets. their orbits, besides, are exceedingly elliptic; and, far from keeping to one plane, or even near it, they approach the sun from all directions. broadly speaking, comets may be divided into two distinct classes, or "families." in the first class, the same orbit appears to be shared in common by a series of comets which travel along it, one following the other. the comets which appeared in the years , , , , and are instances of a number of different bodies pursuing the same path around the sun. the members of a comet family of this kind are observed to have similar characteristics. the idea is that such comets are merely portions of one much larger cometary body, which became broken up by the gravitational action of other bodies in the system, or through violent encounter with the sun's surroundings. the second class is composed of comets which are supposed to have been seized by the gravitative action of certain planets, and thus forced to revolve in short ellipses around the sun, well within the limits of the solar system. these comets are, in consequence, spoken of as "captures." they move around the sun in the same direction as the planets do. jupiter has a fairly large comet family of this kind attached to him. as a result of his overpowering gravitation, it is imagined that during the ages he must have attracted a large number of these bodies on his own account, and, perhaps, have robbed other planets of their captures. his family at present numbers about thirty. of the other planets, so far as we know, saturn possesses a comet family of two, uranus three, and neptune six. there are, indeed, a few comets which appear as if under the influence of some force situated outside the known bounds of the solar system, a circumstance which goes to strengthen the idea that other planets may revolve beyond the orbit of neptune. the terrestrial planets, on the other hand, cannot have comet families; because the enormous gravitative action of the sun in their vicinity entirely overpowers the attractive force which they exert upon those comets which pass close to them. besides this, a comet, when in the inner regions of the solar system, moves with such rapidity, that the gravitational pull of the planets there situated is not powerful enough to deflect it to any extent. it must not be presumed, however, that a comet once captured should always remain a prisoner. further disturbing causes might unsettle its newly acquired orbit, and send it out again into the celestial spaces. with regard to the matter of which comets are composed, the spectroscope shows the presence in them of hydrocarbon compounds (a notable characteristic of these bodies), and at times, also, of sodium and iron. some of the light which we get from comets is, however, merely reflected sunlight. the fact that the tails of comets are always directed away from the sun, has given rise to the idea that this is caused by some repelling action emanating from the sun itself, which is continually driving off the smallest particles. two leading theories have been formulated to account for the tails themselves upon the above assumption. one of these, first suggested by olbers in , and now associated with the name of the russian astronomer, the late professor brédikhine, who carefully worked it out, presumes an electrical action emanating from the sun; the other, that of arrhenius, supposes a pressure exerted by the solar light in its radiation outwards into space. it is possible, indeed, that repelling forces of both these kinds may be at work together. minute particles are probably being continually produced by friction and collisions among the more solid parts in the heads of comets. supposing that such particles are driven off altogether, one may therefore assume that the so-called captured comets are disintegrating at a comparatively rapid rate. kepler long ago maintained that "comets die," and this actually appears to be the case. the ordinary periodic ones, such, for instance, as encke's comet, are very faint, and becoming fainter at each return. certain of these comets have, indeed, failed altogether to reappear. it is notable that the members of jupiter's comet family are not very conspicuous objects. they have small tails, and even in some cases have none at all. the family, too, does not contain many members, and yet one cannot but suppose that jupiter, on account of his great mass, has had many opportunities for making captures adown the ages. of the two theories to which allusion has above been made, that of brédikhine has been worked out so carefully, and with such a show of plausibility, that it here calls for a detailed description. it appears besides to explain the phenomena of comets' tails so much more satisfactorily than that of arrhenius, that astronomers are inclined to accept it the more readily of the two. according to brédikhine's theory the electrical repulsive force, which he assumes for the purposes of his argument, will drive the minutest particles of the comet in a direction away from the sun much more readily than the gravitative action of that body will pull them towards it. this may be compared to the ease with which fine dust may be blown upwards, although the earth's gravitation is acting upon it all the time. the researches of brédikhine, which began seriously with his investigation of coggia's comet of , led him to classify the tails of comets in _three types_. presuming that the repulsive force emanating from the sun did not vary, he came to the conclusion that the different forms assumed by cometary tails must be ascribed to the special action of this force upon the various elements which happen to be present in the comet. the tails which he classes as of the first type, are those which are long and straight and point directly away from the sun. examples of such tails are found in the comets of , , and . tails of this kind, he thinks, are in all probability formed of _hydrogen_. his second type comprises those which are pointed away from the sun, but at the same time are considerably curved, as was seen in the comets of donati and coggia. these tails are formed of _hydrocarbon gas_. the third type of tail is short, brush-like, and strongly bent, and is formed of the _vapour of iron_, mixed with that of sodium and other elements. it should, however, be noted that comets have occasionally been seen which possess several tails of these various types. we will now touch upon a few of the best known comets of modern times. the comet of was the first whose orbit was calculated according to the laws of gravitation. this was accomplished by newton, and he found that the comet in question completed its journey round the sun in a period of about years. in there appeared a great comet, which has become famous under the name of halley's comet, in consequence of the profound investigations made into its motion by the great astronomer, edmund halley. he fixed its period of revolution around the sun at about seventy-five years, and predicted that it would reappear in the early part of . he did not, however, live to see this fulfilled, but the comet duly returned--_the first body of the kind to verify such a prediction_--and was detected on christmas day, , by george palitzch, an amateur observer living near dresden. halley also investigated the past history of the comet, and traced it back to the year . the orbit of halley's comet passes out slightly beyond the orbit of neptune. at its last visit in , this comet passed comparatively close to us, namely, within five million miles of the earth. according to the calculations of messrs p.h. cowell and a.c.d. crommelin of greenwich observatory, its next return will be in the spring of ; the nearest approach to the earth taking place about may . on the th of march, , a great comet appeared, which remained visible for nearly a year and a half. it was a magnificent object; the tail being about millions of miles in length, and the head about , miles in diameter. a detailed study which he gave to this comet prompted olbers to put forward that theory of electrical repulsion which, as we have seen, has since been so carefully worked out by brédikhine. olbers had noticed that the particles expelled from the head appeared to travel to the end of the tail in about eleven minutes, thus showing a velocity per second very similar to that of light. the discovery in of the comet known as encke's, because its orbit was determined by an astronomer of that name, drew attention for the first time to jupiter's comet family, and, indeed, to short-period comets in general. this comet revolves around the sun in the shortest known period of any of these bodies, namely, - / years. encke predicted that it would return in . this duly occurred, the comet passing at its nearest to the sun within three hours of the time indicated; being thus the second instance of the fulfilment of a prediction of the kind. a certain degree of irregularity which encke's comet displays in the dates of its returns to the sun, has been supposed to indicate that it passes in the course of its orbit through some retarding medium, but no definite conclusions have so far been arrived at in this matter. a comet, which appeared in , goes by the name of biela's comet, because of its discovery by an austrian military officer, wilhelm von biela. this comet was found to have a period of between six and seven years. certain calculations made by olbers showed that, at its return in , it would pass _through the earth's orbit_. the announcement of this gave rise to a panic; for people did not wait to inquire whether the earth would be anywhere near that part of its orbit when the comet passed. the panic, however, subsided when the french astronomer, arago, showed that at the moment in question the earth would be some millions of miles away from the point indicated! [illustration: plate xvii. donati's comet from a drawing made on october th, , by g.p. bond, of harvard college observatory, u.s.a. a good illustration of brédikhine's theory: note the straight tails of his _first_ type, and the curved tail of his _second_. (page )] in , shortly after one of its returns, biela's comet divided into two portions. at its next appearance ( ) these portions had separated to a distance of about - / millions of miles from each other. this comet, or rather its constituents, have never since been seen. perhaps the most remarkable comet of recent times was that of , known as donati's, it having been discovered at florence by the italian astronomer, g.b. donati. this comet, a magnificent object, was visible for more than three months with the naked eye. its tail was then millions of miles in length. it was found to revolve around the sun in a period of over years, and to go out in its journey to about - / times the distance of neptune. its motion is retrograde, that is to say, in the contrary direction to the usual movement in the solar system. a number of beautiful drawings of donati's comet were made by the american astronomer, g.p. bond. one of the best of these is reproduced on plate xvii., p. . in there appeared a great comet. on the th of june of that year the earth and moon actually passed through its tail; but no effects were noticed, other than a peculiar luminosity in the sky. in the year there appeared another large comet, known as tebbutt's comet, from the name of its discoverer. this was the _first comet of which a satisfactory photograph was obtained_. the photograph in question was taken by the late m. janssen. the comet of was of vast size and brilliance. it approached so close to the sun that it passed through some , miles of the solar corona. though its orbit was not found to have been altered by this experience, its nucleus displayed signs of breaking up. some very fine photographs of this comet were obtained at the cape of good hope by mr. (now sir david) gill. the comet of was followed with the telescope nearly up to the orbit of saturn, which seems to be the greatest distance at which a comet has ever been seen. the _first discovery of a comet by photographic means_[ ] was made by professor barnard in ; and, since then, photography has been employed with marked success in the detection of small periodic comets. the best comet seen in the northern hemisphere since that of , appears to have been daniel's comet of (see plate xviii., p. ). this comet was discovered on june , , by mr. z. daniel, at princeton observatory, new jersey, u.s.a. it became visible to the naked eye about mid-july of that year, and reached its greatest brilliancy about the end of august. it did not, however, attract much popular attention, as its position in the sky allowed it to be seen only just before dawn. [ ] with the exception, of course, of such an anomaly as the retrograde motion of the ninth satellite of saturn. [ ] if we except the case of the comet which was photographed near the solar corona in the eclipse of . [illustration: plate xviii. daniel's comet of from a photograph taken, on august th, , by dr. max wolf, at the astrophysical observatory, heidelberg. the instrument used was a -inch reflecting telescope, and the time of exposure was fifteen minutes. as the telescope was guided to follow the moving comet, the stars have imprinted themselves upon the photographic plate as short trails. this is clearly the opposite to what is depicted on plate xiii. (page )] chapter xx remarkable comets if eclipses were a cause of terror in past ages, comets appear to have been doubly so. their much longer continuance in the sight of men had no doubt something to say to this, and also the fact that they arrived without warning; it not being then possible to give even a rough prediction of their return, as in the case of eclipses. as both these phenomena were occasional, and out of the ordinary course of things, they drew exceptional attention as unusual events always do; for it must be allowed that quite as wonderful things exist, but they pass unnoticed merely because men have grown accustomed to them. for some reason the ancients elected to class comets along with meteors, the aurora borealis, and other phenomena of the atmosphere, rather than with the planets and the bodies of the spaces beyond. the sudden appearance of these objects led them to be regarded as signs sent by the gods to announce remarkable events, chief among these being the deaths of monarchs. shakespeare has reminded us of this in those celebrated lines in _julius cæsar_:-- "when beggars die there are no comets seen, the heavens themselves blaze forth the death of princes." numbed by fear, the men of old blindly accepted these presages of fate; and did not too closely question whether the threatened danger was to their own nation or to some other, to their ruler or to his enemy. now and then, as in the case of the roman emperor vespasian, there was a cynical attempt to apply some reasoning to the portent. that emperor, in alluding to the comet of a.d. , is reported to have said: "this hairy star does not concern me; it menaces rather the king of the parthians, for he is hairy and i am bald." vespasian, all the same, died shortly afterwards! pliny, in his natural history, gives several instances of the terrible significance which the ancients attached to comets. "a comet," he says, "is ordinarily a very fearful star; it announces no small effusion of blood. we have seen an example of this during the civil commotion of octavius." a very brilliant comet appeared in b.c., and about the same time an earthquake caused helicè and bura, two towns in achaia, to be swallowed up by the sea. the following remark made by seneca concerning it shows that the ancients did not consider comets merely as precursors, but even as actual _causes_ of fatal events: "this comet, so anxiously observed by every one, _because of the great catastrophe which it produced as soon as it appeared_, the submersion of bura and helicè." comets are by no means rare visitors to our skies, and very few years have elapsed in historical times without such objects making their appearance. in the dark and middle ages, when europe was split up into many small kingdoms and principalities, it was, of course, hardly possible for a comet to appear without the death of some ruler occurring near the time. critical situations, too, were continually arising in those disturbed days. the end of louis le debonnaire was hastened, as the reader will, no doubt, recollect, by the great eclipse of ; but it was firmly believed that a comet which had appeared a year or two previously presaged his death. the comet of is reported to have _influenced_ the abdication of the emperor charles v.; but curiously enough, this event had already taken place before the comet made its appearance! such beliefs, no doubt, had a very real effect upon rulers of a superstitious nature, or in a weak state of health. for instance, gian galeazzo visconti, duke of milan, was sick when the comet of appeared. after seeing it, he is said to have exclaimed: "i render thanks to god for having decreed that my death should be announced to men by this celestial sign." his malady then became worse, and he died shortly afterwards. it is indeed not improbable that such superstitious fears in monarchs were fanned by those who would profit by their deaths, and yet did not wish to stain their own hands with blood. evil though its effects may have been, this morbid interest which past ages took in comets has proved of the greatest service to our science. had it not been believed that the appearance of these objects was attended with far-reaching effects, it is very doubtful whether the old chroniclers would have given themselves the trouble of alluding to them at all; and thus the modern investigators of cometary orbits would have lacked a great deal of important material. we will now mention a few of the most notable comets which historians have recorded. a comet which appeared in b.c. was thought to betoken the success of the expedition undertaken in that year by timoleon of corinth against sicily. "the gods by an extraordinary prodigy announced his success and future greatness: a burning torch appeared in the heavens throughout the night and preceded the fleet of timoleon until it arrived off the coast of sicily." the comet of b.c. was generally believed to be the soul of cæsar on its way to heaven. josephus tells us that in a.d. several prodigies, and amongst them a comet in the shape of a sword, announced the destruction of jerusalem. this comet is said to have remained over the city for the space of a year! a comet which appeared in a.d. was considered to have announced the death of the emperor constantine. but perhaps the most celebrated comet of early times was the one which appeared in a.d. . that year was, in more than one way, big with portent, for there had long been a firm belief that the christian era could not possibly run into four figures. men, indeed, steadfastly believed that when the thousand years had ended, the millennium would immediately begin. therefore they did not reap neither did they sow, they toiled not, neither did they spin, and the appearance of the comet strengthened their convictions. the fateful year, however, passed by without anything remarkable taking place; but the neglect of husbandry brought great famine and pestilence over europe in the years which followed. in april , that year fraught with such immense consequences for england, a comet appeared. no one doubted but that it was a presage of the success of the conquest, and perhaps, indeed, it had its due weight in determining the minds and actions of the men who took part in the expedition. _nova stella, novus rex_ ("a new star, a new sovereign") was a favourite proverb of the time. the chroniclers, with one accord, have delighted to relate that the normans, "guided by a comet," invaded england. a representation of this object appears in the bayeux tapestry (see fig. , p. ).[ ] [illustration: fig. .--the comet of , as represented in the bayeux tapestry. (from the _world of comets_.)] we have mentioned halley's comet of , and how it revisits the neighbourhood of the earth at intervals of seventy-six years. the comet of has for many years been supposed to be halley's comet on one of its visits. the identity of these two, however, was only quite recently placed beyond all doubt by the investigations of messrs cowell and crommelin. this comet appeared also in , when john huniades was defending belgrade against the turks led by mahomet ii., the conqueror of constantinople, and is said to have paralysed both armies with fear. the middle ages have left us descriptions of comets, which show only too well how the imagination will run riot under the stimulus of terror. for instance, the historian, nicetas, thus describes the comet of the year : "after the romans were driven from constantinople a prognostic was seen of the excesses and crimes to which andronicus was to abandon himself. a comet appeared in the heavens similar to a writhing serpent; sometimes it extended itself, sometimes it drew itself in; sometimes, to the great terror of the spectators, it opened a huge mouth; it seemed that, as if thirsting for human blood, it was upon the point of satiating itself." and, again, the celebrated ambrose paré, the father of surgery, has left us the following account of the comet of , which appeared in his own time: "this comet," said he, "was so horrible, so frightful, and it produced such great terror in the vulgar, that some died of fear, and others fell sick. it appeared to be of excessive length, and was of the colour of blood. at the summit of it was seen the figure of a bent arm, holding in its hand a great sword, as if about to strike. at the end of the point there were three stars. on both sides of the rays of this comet were seen a great number of axes, knives, blood-coloured swords, among which were a great number of hideous human faces, with beards and bristling hair." paré, it is true, was no astronomer; yet this shows the effect of the phenomenon, even upon a man of great learning, as undoubtedly he was. it should here be mentioned that nothing very remarkable happened at or near the year . concerning the comet of , the extraordinary story got about that, at rome, a hen had laid an egg on which appeared a representation of the comet! but the superstitions with regard to comets were now nearing their end. the last blow was given by halley, who definitely proved that they obeyed the laws of gravitation, and circulated around the sun as planets do; and further announced that the comet of had a period of seventy-six years, which would cause it to reappear in the year . we have seen how this prediction was duly verified. we have seen, too, how this comet appeared again in , and how it is due to return in the early part of . [ ] with regard to the words "isti mirant stella" in the figure, mr. w.t. lynn suggests that they may not, after all, be the grammatically bad latin which they appear, but that the legend is really "isti mirantur stellam," the missing letters being supposed to be hidden by the building and the comet. chapter xxi meteors or shooting stars any one who happens to gaze at the sky for a short time on a clear night is pretty certain to be rewarded with a view of what is popularly known as a "shooting star." such an object, however, is not a star at all, but has received its appellation from an analogy; for the phenomenon gives to the inexperienced in these matters an impression as if one of the many points of light, which glitter in the vaulted heaven, had suddenly become loosened from its place, and was falling towards the earth. in its passage across the sky the moving object leaves behind a trail of light which usually lasts for a few moments. shooting stars, or meteors, as they are technically termed, are for the most part very small bodies, perhaps no larger than peas or pebbles, which, dashing towards our earth from space beyond, are heated to a white heat, and reduced to powder by the friction resulting from their rapid passage into our atmosphere. this they enter at various degrees of speed, in some cases so great as miles a second. the speed, of course, will depend greatly upon whether the earth and the meteors are rushing towards each other, or whether the latter are merely overtaking the earth. in the first of these cases the meteors will naturally collide with the atmosphere with great force; in the other case they will plainly come into it with much less rapidity. as has been already stated, it is from observations of such bodies that we are enabled to estimate, though very imperfectly, the height at which the air around our globe practically ceases, and this height is imagined to be somewhere about miles. fortunate, indeed, is it for us that there is a goodly layer of atmosphere over our heads, for, were this not so, these visitors from space would strike upon the surface of our earth night and day, and render existence still more unendurable than many persons choose to consider it. to what a bombardment must the moon be continually subject, destitute as she is of such an atmospheric shield! it is only in the moment of their dissolution that we really learn anything about meteors, for these bodies are much too small to be seen before they enter our atmosphere. the débris arising from their destruction is wafted over the earth, and, settling down eventually upon its surface, goes to augment the accumulation of that humble domestic commodity which men call dust. this continual addition of material tends, of course, to increase the mass of the earth, though the effect thus produced will be on an exceedingly small scale. the total number of meteors moving about in space must be practically countless. the number which actually dash into the earth's atmosphere during each year is, indeed, very great. professor simon newcomb, the well-known american astronomer, has estimated that, of the latter, those large enough to be seen with the naked eye cannot be in all less than , , , per annum. ten times more numerous still are thought to be those insignificant ones which are seen to pass like mere sparks of light across the field of an observer's telescope. until comparatively recent times, perhaps up to about a hundred years ago, it was thought that meteors were purely terrestrial phenomena which had their origin in the upper regions of the air. it, however, began to be noticed that at certain periods of the year these moving objects appeared to come from definite areas of the sky. considerations, therefore, respecting their observed velocities, directions, and altitudes, gave rise to the theory that they are swarms of small bodies travelling around the sun in elongated elliptical orbits, all along the length of which they are scattered, and that the earth, in its annual revolution, rushing through the midst of such swarms at the same epoch each year, naturally entangles many of them in its atmospheric net. the dates at which the earth is expected to pass through the principal meteor-swarms are now pretty well known. these swarms are distinguished from one another by the direction of the sky from which the meteors seem to arrive. many of the swarms are so wide that the earth takes days, and even weeks, to pass through them. in some of these swarms, or streams, as they are also called, the meteors are distributed with fair evenness along the entire length of their orbits, so that the earth is greeted with a somewhat similar shower at each yearly encounter. in others, the chief portions are bunched together, so that, in certain years, the display is exceptional (see fig. , p. ). that part of the heavens from which a shower of meteors is seen to emanate is called the "radiant," or radiant point, because the foreshortened view we get of the streaks of light makes it appear as if they radiated outwards from this point. in observations of these bodies the attention of astronomers is directed to registering the path and speed of each meteor, and to ascertaining the position of the radiant. it is from data such as these that computations concerning the swarms and their orbits are made. [illustration: fig. .--passage of the earth through the thickest portion of a meteor swarm. the earth and the meteors are here represented as approaching each other from opposite directions.] for the present state of knowledge concerning meteors, astronomy is largely indebted to the researches of mr. w.f. denning, of bristol, and of the late professor a.s. herschel. during the course of each year the earth encounters a goodly number of meteor-swarms. three of these, giving rise to fine displays, are very well known--the "perseids," or august meteors, and the "leonids" and "bielids," which appear in november. of the above three the _leonid_ display is by far the most important, and the high degree of attention paid to it has laid the foundation of meteoric astronomy in much the same way that the study of the fascinating corona has given such an impetus to our knowledge of the sun. the history of this shower of meteors may be traced back as far as a.d. , which was known as the "year of the stars." it is related that in that year, on the night of october th--the shower now comes about a month later--whilst the moorish king, ibrahim ben ahmed, lay dying before cosenza, in calabria, "a multitude of falling stars scattered themselves across the sky like rain," and the beholders shuddered at what they considered a dread celestial portent. we have, however, little knowledge of the subsequent history of the leonids until , since which time the maximum shower has appeared with considerable regularity at intervals of about thirty-three years. but it was not until that they sprang into especial notice. on the th november in that year a splendid display was witnessed at cumana, in south america, by the celebrated travellers, humboldt and bonpland. finer still, and surpassing all displays of the kind ever seen, was that of november , , when the meteors fell thick as snowflakes, , being estimated to have appeared during seven hours. some of them were even so bright as to be seen in full daylight. the radiant from which the meteors seem to diverge was ascertained to be situated in the head of the constellation of the lion, or "sickle of leo," as it is popularly termed, whence their name--leonids. it was from a discussion of the observations then made that the american astronomer, olmsted, concluded that these meteors sprang upon us from interplanetary space, and were not, as had been hitherto thought, born of our atmosphere. later on, in , olbers formulated the theory that the bodies in question travelled around the sun in an elliptical orbit, and at the same time he established the periodicity of the maximum shower. the periodic time of recurrence of this maximum, namely, about thirty-three years, led to eager expectancy as drew near. hopes were then fulfilled, and another splendid display took place, of which sir robert ball, who observed it, has given a graphic description in his _story of the heavens_. the display was repeated upon a smaller scale in the two following years. the leonids were henceforth deemed to hold an anomalous position among meteor swarms. according to theory the earth cut through their orbit at about the same date each year, and so a certain number were then seen to issue from the radiant. but, in addition, after intervals of thirty-three years, as has been seen, an exceptional display always took place; and this state of things was not limited to one year alone, but was repeated at each meeting for about three years running. the further assumption was, therefore, made that the swarm was much denser in one portion of the orbit than elsewhere,[ ] and that this congested part was drawn out to such an extent that the earth could pass through the crossing place during several annual meetings, and still find it going by like a long procession (see fig. , p. ). in accordance with this ascertained period of thirty-three years, the recurrence of the great leonid shower was timed to take place on the th of november . but there was disappointment then, and the displays which occurred during the few years following were not of much importance. a good deal of comment was made at the time, and theories were accordingly put forward to account for the failure of the great shower. the most probable explanation seems to be, that the attraction of one of the larger planets--jupiter perhaps--has diverted the orbit somewhat from its old position, and the earth does not in consequence cut through the swarm in the same manner as it used to do. the other november display alluded to takes place between the rd and th of that month. it is called the _andromedid_ shower, because the meteors appear to issue from the direction of the constellation of andromeda, which at that period of the year is well overhead during the early hours of the night. these meteors are also known by the name of _bielids_, from a connection which the orbit assigned to them appears to have with that of the well-known comet of biela. m. egenitis, director of the observatory of athens, accords to the bielids a high antiquity. he traces the shower back to the days of the emperor justinian. theophanes, the chronicler of that epoch, writing of the famous revolt of nika in the year a.d. , says:--"during the same year a great fall of stars came from the evening till the dawn." m. egenitis notes another early reference to these meteors in a.d. , during the reign of the eastern emperor, constantine copronymous. writing of that year, nicephorus, a patriarch of constantinople, has as follows:--"all the stars appeared to be detached from the sky, and to fall upon the earth." the bielids, however, do not seem to have attracted particular notice until the nineteenth century. attention first began to be riveted upon them on account of their suspected connection with biela's comet. it appeared that the same orbit was shared both by that comet and the bielid swarm. it will be remembered that the comet in question was not seen after its appearance in . since that date, however, the bielid shower has shown an increased activity; which was further noticed to be especially great in those years in which the comet, had it still existed, would be due to pass near the earth. the third of these great showers to which allusion has above been made, namely, the _perseids_, strikes the earth about the th of august; for which reason it is known on the continent under the name of the "tears of st. lawrence," the day in question being sacred to that saint. this shower is traceable back many centuries, even as far as the year a.d. . the name given to these meteors, "perseids," arises from the fact that their radiant point is situated in the constellation of perseus. this shower is, however, not by any means limited to the particular night of august th, for meteors belonging to the swarm may be observed to fall in more or less varying quantities from about july th to august nd. the perseid meteors sometimes fall at the rate of about sixty per hour. they are noted for their great rapidity of motion, and their trails besides often persist for a minute or two before being disseminated. unlike the other well-known showers, the radiants of which are stationary, that of the perseids shifts each night a little in an easterly direction. the orbit of the perseids cuts that of the earth almost perpendicularly. the bodies are generally supposed to be the result of the disintegration of an ancient comet which travelled in the same orbit. tuttle's comet, which passed close to the earth in , also belongs to this orbit; and its period of revolution is calculated to be years. the perseids appear to be disseminated all along this great orbit, for we meet them in considerable quantities each year. the bodies in question are in general particularly small. the swarm has, however, like most others, a somewhat denser portion, and through this the earth passed in . the _aphelion_, or point where the far end of the orbit turns back again towards the sun, is situated right away beyond the path of neptune, at a distance of forty-eight times that of the earth from the sun. the comet of also belongs to the perseid orbit. it revisited the neighbourhood of the earth in , and should have returned in . but we have no record of it in that year; for which omission the then politically disturbed state of europe may account. if not already disintegrated, this comet is due to return in . this supposed connection between comets and meteor-swarms must be also extended to the case of the leonids. these meteors appear to travel along the same track as tempel's comet of . it is considered that the attractions of the various bodies of the solar system upon a meteor swarm must eventually result in breaking up the "bunched" portion, so that in time the individual meteors should become distributed along the whole length of the orbit. upon this assumption the perseid swarm, in which the meteors are fairly well scattered along its path, should be of greater age than the leonid. as to the leonid swarm itself, le verrier held that it was first brought into the solar system in a.d. , having been captured from outer space by the gravitative action of the planet uranus. the acknowledged theory of meteor swarms has naturally given rise to an idea, that the sunlight shining upon such a large collection of particles ought to render a swarm visible before its collision with the earth's atmosphere. several attempts have therefore been made to search for approaching swarms by photography, but, so far, it appears without success. it has also been proposed, by mr. w.h.s. monck, that the stars in those regions from which swarms are due, should be carefully watched, to see if their light exhibits such temporary diminutions as would be likely to arise from the momentary interposition of a cloud of moving particles. between ten and fifteen years ago it happened that several well-known observers, employed in telescopic examination of the sun and moon, reported that from time to time they had seen small dark bodies, sometimes singly, sometimes in numbers, in passage across the discs of the luminaries. it was concluded that these were meteors moving in space beyond the atmosphere of the earth. the bodies were called "dark meteors," to emphasise the fact that they were seen in their natural condition, and not in that momentary one in which they had hitherto been always seen; _i.e._ when heated to white heat, and rapidly vaporised, in the course of their passage through the upper regions of our air. this "discovery" gave promise of such assistance to meteor theories, that calculations were made from the directions in which they had been seen to travel, and the speeds at which they had moved, in the hope that some information concerning their orbits might be revealed. but after a while some doubt began to be thrown upon their being really meteors, and eventually an australian observer solved the mystery. he found that they were merely tiny particles of dust, or of the black coating on the inner part of the tube of the telescope, becoming detached from the sides of the eye-piece and falling across the field of view. he was led to this conclusion by having noted that a gentle tapping of his instrument produced the "dark" bodies in great numbers! thus the opportunity of observing meteors beyond our atmosphere had once more failed. _meteorites_, also known as ærolites and fireballs, are usually placed in quite a separate category from meteors. they greatly exceed the latter in size, are comparatively rare, and do not appear in any way connected with the various showers of meteors. the friction of their passage through the atmosphere causes them to shine with a great light; and if not shattered to pieces by internal explosions, they reach the ground to bury themselves deep in it with a great rushing and noise. when found by uncivilised peoples, or savages, they are, on account of their celestial origin, usually regarded as objects of wonder and of worship, and thus have arisen many mythological legends and deifications of blackened stones. on the other hand, when they get into the possession of the civilised, they are subjected to careful examinations and tests in chemical laboratories. the bodies are, as a rule, composed of stone, in conjunction with iron, nickel, and such elements as exist in abundance upon our earth; though occasionally specimens are found which are practically pure metal. in the museums of the great capitals of both continents are to be seen some fine collections of meteorites. several countries--greenland and mexico, for instance--contain in the soil much meteoric iron, often in masses so large as to baffle all attempts at removal. blocks of this kind have been known to furnish the natives in their vicinity for many years with sources of workable iron. the largest meteorite in the world is one known as the anighito meteorite. it was brought to the united states by the explorer peary, who found it at cape york in greenland. he estimates its weight at from to tons. one found in mexico, called the bacubirito, comes next, with an estimated weight of - / tons. the third in size is the willamette meteorite, found at willamette in oregon in . it measures × - / × - / feet, and weighs about - / tons. [ ] the "gem" of the meteor ring, as it has been termed. chapter xxii the stars in the foregoing chapters we have dealt at length with those celestial bodies whose nearness to us brings them into our especial notice. the entire room, however, taken up by these bodies, is as a mere point in the immensities of star-filled space. the sun, too, is but an ordinary star; perhaps quite an insignificant one[ ] in comparison with the majority of those which stud that background of sky against which the planets are seen to perform their wandering courses. dropping our earth and the solar system behind, let us go afield and explore the depths of space. we have seen how, in very early times, men portioned out the great mass of the so-called "fixed stars" into divisions known as constellations. the various arrangements, into which the brilliant points of light fell as a result of perspective, were noticed and roughly compared with such forms as were familiar to men upon the earth. imagination quickly saw in them the semblances of heroes and of mighty fabled beasts; and, around these monstrous shapes, legends were woven, which told how the great deeds done in the misty dawn of historical time had been enshrined by the gods in the sky as an example and a memorial for men. though the centuries have long outlived such fantasies, yet the constellation figures and their ancient names have been retained to this day, pretty well unaltered for want of any better arrangement. the great and little bears, cassiopeia, perseus, and andromeda, orion and the rest, glitter in our night skies just as they did centuries and centuries ago. many persons seem to despair of gaining any real knowledge of astronomy, merely because they are not versed in recognising the constellations. for instance, they will say:--"what is the use of my reading anything about the subject? why, i believe i couldn't even point out the great bear, were i asked to do so!" but if such persons will only consider for a moment that what we call the great bear has no existence in fact, they need not be at all disheartened. could we but view this familiar constellation from a different position in space, we should perhaps be quite unable to recognise it. mountain masses, for instance, when seen from new directions, are often unrecognisable. it took, as we have seen, a very long time for men to acknowledge the immense distances of the stars from our earth. their seeming unchangeableness of position was, as we have seen, largely responsible for the idea that the earth was immovable in space. it is a wonder that the copernican system ever gained the day in the face of this apparent fixity of the stars. as time went on, it became indeed necessary to accord to these objects an almost inconceivable distance, in order to account for the fact that they remained apparently quite undisplaced, notwithstanding the journey of millions of miles which the earth was now acknowledged to make each year around the sun. in the face of the gradual and immense improvement in telescopes, this apparent immobility of the stars was, however, not destined to last. the first ascertained displacement of a star, namely that of cygni, noted by bessel in the year , definitely proved to men the truth of the copernican system. since then some forty more stars have been found to show similar tiny displacements. we are, therefore, in possession of the fact, that the actual distances of a few out of the great host can be calculated. to mention some of these. the nearest star to the earth, so far as we yet know, is alpha centauri, which is distant from us about billions of miles. the light from this star, travelling at the stupendous rate of about , miles per second, takes about - / years to reach our earth, or, to speak astronomically, alpha centauri is about - / "light years" distant from us. sirius--the brightest star in the whole sky--is at twice this distance, _i.e._ about - / light years. vega is about light years distant from us, capella about , and arcturus about . the displacements, consequent on the earth's movement, have, however, plainly nothing to say to any real movements on the part of the stars themselves. the old idea was that the stars were absolutely fixed; hence arose the term "fixed stars"--a term which, though inaccurate, has not yet been entirely banished from the astronomical vocabulary. but careful observations extending over a number of years have shown slight changes of position among these bodies; and such alterations cannot be ascribed to the revolution of the earth in its orbit, for they appear to take place in every direction. these evidences of movement are known as "proper motions," that is to say, actual motions in space proper to the stars themselves. stars which are comparatively near to us show, as a rule, greater proper motions than those which are farther off. it must not, however, be concluded that these proper motions are of any very noticeable amounts. they are, as a matter of fact, merely upon the same apparently minute scale as other changes in the heavens; and would largely remain unnoticed were it not for the great precision of modern astronomical instruments. one of the swiftest moving of the stars is a star of the sixth magnitude in the constellation of the great bear; which is known as " groombridge," because this was the number assigned to it in a catalogue of stars made by an astronomer of that name. it is popularly known as the "runaway star," a name given to it by professor newcomb. its speed is estimated to be at least miles per second. it may be actually moving at a much greater rate, for it is possible that we see its path somewhat foreshortened. a still greater proper motion--the greatest, in fact, known--is that of an eighth magnitude star in the southern hemisphere, in the constellation of pictor. nothing, indeed, better shows the enormous distance of the stars from us, and the consequent inability of even such rapid movements to alter the appearance of the sky during the course of ages, than the fact that it would take more than two centuries for the star in question to change its position in the sky by a space equal to the apparent diameter of the moon; a statement which is equivalent to saying that, were it possible to see this star with the naked eye, which it is not, at least twenty-five years would have to elapse before one would notice that it had changed its place at all! both the stars just mentioned are very faint. that in pictor is, as has been said, not visible to the naked eye. it appears besides to be a very small body, for sir david gill finds a parallax which makes it only as far off from us as sirius. the groombridge star, too, is just about the limit of ordinary visibility. it is, indeed, a curious fact that the fainter stars seem, on the average, to be moving more rapidly than the brighter. investigations into proper motions lead us to think that every one of the stars must be moving in space in some particular direction. to take a few of the best known. sirius and vega are both approaching our system at a rate of about miles per second, arcturus at about miles per second, while capella is receding from us at about miles per second. of the twin brethren, castor and pollux, castor is moving away from us at about - / miles per second, while pollux is coming towards us at about miles per second. much of our knowledge of proper motions has been obtained indirectly by means of the spectroscope, on the doppler principle already treated of, by which we are enabled to ascertain whether a source from which light is coming is approaching or receding. the sun being, after all, a mere star, it will appear only natural for it also to have a proper motion of its own. this is indeed the case; and it is rushing along in space at a rate of between ten and twelve miles per second, carrying with it its whole family of planets and satellites, of comets and meteors. the direction in which it is advancing is towards a point in the constellation of lyra, not far from its chief star vega. this is shown by the fact that the stars about the region in question appear to be opening out slightly, while those in the contrary portion of the sky appear similarly to be closing together. sir william herschel was the first to discover this motion of the sun through space; though in the idea that such a movement might take place he seems to have been anticipated by mayer in , by michell in , and by lalande in . a suggestion has been made that our solar system, in its motion through the celestial spaces, may occasionally pass through regions where abnormal magnetic conditions prevail, in consequence of which disturbances may manifest themselves throughout the system at the same instant. thus the sun may be getting the credit of _producing_ what it merely reacts to in common with the rest of its family. but this suggestion, plausible though it may seem, will not explain why the magnetic disturbances experienced upon our earth show a certain dependence upon such purely local facts, as the period of the sun's rotation, for instance. one would very much like to know whether the movement of the sun is along a straight line, or in an enormous orbit around some centre. the idea has been put forward that it may be moving around the centre of gravity of the whole visible stellar universe. mädler, indeed, propounded the notion that alcyone--the chief star in the group known as the pleiades--occupied this centre, and that everything revolved around it. he went even further to proclaim that here was the place of the almighty, the mansion of the eternal! but mädler's ideas upon this point have long been shelved. to return to the general question of the proper motion of stars. in several instances these motions appear to take place in groups, as if certain stars were in some way associated together. for example, a large number of the stars composing the pleiades appear to be moving through space in the same direction. also, of the seven stars composing the plough, all but two--the star at the end of its "handle," and that one of the "pointers," as they are called, which is the nearer to the pole star--have a common proper motion, _i.e._ are moving in the same direction and nearly at the same rate. further still, the well-known dutch astronomer, professor kapteyn, of groningen, has lately reached the astonishing conclusion that a great part of the visible universe is occupied by two vast streams of stars travelling in opposite directions. in both these great streams, the individual bodies are found, besides, to be alike in design, alike in chemical constitution, and alike in the stage of their development. a fable related by the persian astronomer, al sufi (tenth century, a.d.) shows well the changes in the face of the sky which proper motions are bound to produce after great lapses of time. according to this fable the stars sirius and procyon were the sisters of the star canopus. canopus married rigel (another star,) but, having murdered her, he fled towards the south pole, fearing the anger of his sisters. the fable goes on to relate, among other things, that sirius followed him across the milky way. mr. j. e. gore, in commenting on the story, thinks that it may be based upon a tradition of sirius having been seen by the men of the stone age on the opposite side of the milky way to that on which it now is. sirius is in that portion of the heavens _from_ which the sun is advancing. its proper motion is such that it is gaining upon the earth at the rate of about ten miles per second, and so it must overtake the sun after the lapse of great ages. vega, on the other hand, is coming towards us from that part of the sky _towards_ which the sun is travelling. it should be about half a million years before the sun and vega pass by one another. those who have specially investigated this question say that, as regards the probability of a near approach, it is much more likely that vega will be then so far to one side of the sun, that her brightness will not be much greater than it is at this moment. considerations like these call up the chances of stellar collisions. such possibilities need not, however, give rise to alarm; for the stars, as a rule, are at such great distances from each other, that the probability of relatively near approaches is slight. we thus see that the constellations do not in effect exist, and that there is in truth no real background to the sky. we find further that the stars are strewn through space at immense distances from each other, and are moving in various directions hither and thither. the sun, which is merely one of them, is moving also in a certain direction, carrying the solar system along with it. it seems, therefore, but natural to suppose that many a star may be surrounded by some planetary system in a way similar to ours, which accompanies it through space in the course of its celestial journeyings. [ ] vega, for instance, shines one hundred times more brightly than the sun would do, were it to be removed to the distance at which that star is from us. chapter xxiii the stars--_continued_ the stars appear to us to be scattered about the sky without any orderly arrangement. further, they are of varying degrees of brightness; some being extremely brilliant, whilst others can but barely be seen. the brightness of a star may arise from either of two causes. on the one hand, the body may be really very bright in itself; on the other hand, it may be situated comparatively near to us. sometimes, indeed, both these circumstances may come into play together. since variation in brightness is the most noticeable characteristic of the stars, men have agreed to class them in divisions called "magnitudes." this term, it must be distinctly understood, is employed in such classification without any reference whatever to actual size, being merely taken to designate roughly the amount of light which we receive from a star. the twenty brightest stars in the sky are usually classed in the first magnitude. in descending the scale, each magnitude will be noticed to contain, broadly speaking, three times as many stars as the one immediately above it. thus the second magnitude contains , the third , the fourth , the fifth , and the sixth . the last of these magnitudes is about the limit of the stars which we are able to see with the naked eye. adding, therefore, the above numbers together, we find that, without the aid of the telescope, we cannot see more than about stars in the entire sky--northern and southern hemispheres included. quite a small telescope will, however, allow us to see down to the ninth magnitude, so that the total number of stars visible to us with such very moderate instrumental means will be well over , . it must not, however, be supposed that the stars included within each magnitude are all of exactly the same brightness. in fact, it would be difficult to say if there exist in the whole sky two stars which send us precisely the same amount of light. in arranging the magnitudes, all that was done was to make certain broad divisions, and to class within them such stars as were much on a par with regard to brightness. it may here be noted that a standard star of the first magnitude gives us about one hundred times as much light as a star of the sixth magnitude, and about one million times as much as one of the sixteenth magnitude--which is near the limit of what we can see with the very best telescope. though the first twenty stars in the sky are popularly considered as being of the first magnitude, yet several of them are much brighter than an average first magnitude star would be. for instance, sirius--the brightest star in the whole sky--is equal to about eleven first magnitude stars, like, say, aldebaran. in consequence of such differences, astronomers are agreed in classifying the brightest of them as _brighter_ than the standard first magnitude star. on this principle sirius would be about two and a half magnitudes _above_ the first. this notation is usefully employed in making comparisons between the amount of light which we receive from the sun, and that which we get from an individual star. thus the sun will be about twenty-seven and a half magnitudes _above_ the first magnitude. the range, therefore, between the light which we receive from the sun (considered merely as a very bright star) and the first magnitude stars is very much greater than that between the latter and the faintest star which can be seen with the telescope, or even registered upon the photographic plate. to classify stars merely by their magnitudes, without some definite note of their relative position in the sky, would be indeed of little avail. we must have some simple method of locating them in the memory, and the constellations of the ancients here happily come to our aid. a system combining magnitudes with constellations was introduced by bayer in , and is still adhered to. according to this the stars in each constellation, beginning with the brightest star, are designated by the letters of the greek alphabet taken in their usual order. for example, in the constellation of canis major, or the greater dog, the brightest star is the well-known sirius, called by the ancients the "dog star"; and this star, in accordance with bayer's method, has received the greek letter [a] (alpha), and is consequently known as alpha canis majoris.[ ] as soon as the greek letters are used up in this way the roman alphabet is brought into requisition, after which recourse is had to ordinary numbers. notwithstanding this convenient arrangement, some of the brightest stars are nearly always referred to by certain proper names given to them in old times. for instance, it is more usual to speak of sirius, arcturus, vega, capella, procyon, aldebaran, regulus, and so on, than of [a] canis majoris, [a] boötis, [a] lyræ, [a] aurigæ, [a] canis minoris, [a] tauri, [a] leonis, &c. &c. in order that future generations might be able to ascertain what changes were taking place in the face of the sky, astronomers have from time to time drawn up catalogues of stars. these lists have included stars of a certain degree of brightness, their positions in the sky being noted with the utmost accuracy possible at the period. the earliest known catalogue of this kind was made, as we have seen, by the celebrated greek astronomer, hipparchus, about the year b.c. it contained stars. it was revised and brought up to date by ptolemy in a.d. . another celebrated list was that drawn up by the persian astronomer, al sufi, about the year a.d. . in it stars were noted down. a catalogue of stars was made in by the famous danish astronomer, tycho brahe. among modern catalogues that of argelander ( - ) contained as many as , stars. it was extended by schönfeld so as to include a portion of the southern hemisphere, in which way , more stars were added. in recent years a project was placed on foot of making a photographic survey of the sky, the work to be portioned out among various nations. a great part of this work has already been brought to a conclusion. about , , stars will appear upon the plates; but, so far, it has been proposed to catalogue only about a million and a quarter of the brightest of them. this idea of surveying the face of the sky by photography sprang indirectly from the fine photographs which sir david gill took, when at the cape of good hope, of the comet of . the immense number of star-images which had appeared upon his plates suggested the idea that photography could be very usefully employed to register the relative positions of the stars. the arrangement of seven stars known as the "plough" is perhaps the most familiar configuration in the sky (see plate xix., p. ). in the united states it is called the "dipper," on account of its likeness to the outline of a saucepan, or ladle. "charles' wain" was the old english name for it, and readers of cæsar will recollect it under _septentriones_, or the "seven stars," a term which that writer uses as a synonym for the north. though identified in most persons' minds with _ursa major_, or the great bear, the plough is actually only a small portion of that famous constellation. six out of the seven stars which go to make up the well-known figure are of the second magnitude, while the remaining one, which is the middle star of the group, is of the third. the greek letters, as borne by the individual stars of the plough, are a plain transgression of bayer's method as above described, for they have certainly not been allotted here in accordance with the proper order of brightness. for instance, the third magnitude star, just alluded to as being in the middle of the group, has been marked with the greek letter [d] (delta); and so is made to take rank _before_ the stars composing what is called the "handle" of the plough, which are all of the second magnitude. sir william herschel long ago drew attention to the irregular manner in which bayer's system had been applied. it is, indeed, a great pity that this notation was not originally worked out with greater care and correctness; for, were it only reliable, it would afford great assistance to astronomers in judging of what changes in relative brightness have taken place among the stars. though we may speak of using the constellations as a method of finding our way about the sky, it is, however, to certain marked groupings in them of the brighter stars that we look for our sign-posts. most of the constellations contain a group or so of noticeable stars, whose accidental arrangement dimly recalls the outline of some familiar geometrical figure and thus arrests the attention.[ ] for instance, in an almost exact line with the two front stars of the plough, or "pointers" as they are called,[ ] and at a distance about five times as far away as the interval between them, there will be found a third star of the second magnitude. this is known as polaris, or the pole star, for it very nearly occupies that point of the heaven towards which the north pole of the earth's axis is _at present_ directed (see plate xix., p. ). thus during the apparently daily rotation of the heavens, this star looks always practically stationary. it will, no doubt, be remembered how shakespeare has put into the mouth of julius cæsar these memorable words:-- "but i am constant as the northern star, of whose true-fix'd and resting quality there is no fellow in the firmament." [illustration: plate xix. the sky around the north pole we see here the plough, the pole star, ursa minor, auriga, cassiopeia's chair, and lyra. also the circle of precession, along which the pole makes a complete revolution in a period of , years, and the temporary star discovered by tycho brahe in the year . (page )] on account of the curvature of the earth's surface, the height at which the pole star is seen above the horizon at any place depends regularly upon the latitude; that is to say, the distance of the place in question from the equator. for instance, at the north pole of the earth, where the latitude is greatest, namely, °, the pole star will appear directly overhead; whereas in england, where the latitude is about °, it will be seen a little more than half way up the northern sky. at the equator, where the latitude is _nil_, the pole star will be on the horizon due north. in consequence of its unique position, the pole star is of very great service in the study of the constellations. it is a kind of centre around which to hang our celestial ideas--a starting point, so to speak, in our voyages about the sky. according to the constellation figures, the pole star is in _ursa minor_, or the little bear, and is situated at the end of the tail of that imaginary figure (see plate xix., p. ). the chief stars of this constellation form a group not unlike the plough, except that the "handle" is turned in the contrary direction. the americans, in consequence, speak of it as the "little dipper." before leaving this region of the sky, it will be well to draw attention to the second magnitude star [z] in the great bear (zeta ursæ majoris), which is the middle star in the "handle" of the plough. this star is usually known as mizar, a name given to it by the arabians. a person with good eyesight can see quite near to it a fifth magnitude star, known under the name of alcor. we have here a very good example of that deception in the estimation of objects in the sky, which has been alluded to in an earlier chapter. alcor is indeed distant from mizar by about one-third the apparent diameter of the moon, yet no one would think so! on the other side of polaris from the plough, and at about an equal apparent distance, will be found a figure in the form of an irregular "w", made up of second and third magnitude stars. this is the well-known "cassiopeia's chair"--portion of the constellation of _cassiopeia_ (see plate xix., p. ). on either side of the pole star, about midway between the plough and cassiopeia's chair, but a little further off from it than these, are the constellations of _auriga_ and _lyra_ (see plate xix., p. ). the former constellation will be easily recognised, because its chief features are a brilliant yellowish first magnitude star, with one of the second magnitude not far from it. the first magnitude star is capella, the other is [b] aurigæ. lyra contains only one first magnitude star--vega, pale blue in colour. this star has a certain interest for us from the fact that, as a consequence of that slow shift of direction of the earth's axis known as precession, it will be very near the north pole of the heavens in some , years, and so will then be considered the pole star (see plate xix., p. ). the constellation of lyra itself, it must also be borne in mind, occupies that region of the heavens towards which the solar system is travelling. the handle of the plough points roughly towards the constellation of _boötes_, in which is the brilliant first magnitude star arcturus. this star is of an orange tint. between boötes and lyra lie the constellations of _corona borealis_ (or the northern crown) and _hercules_. the chief feature of corona borealis, which is a small constellation, is a semicircle of six small stars, the brightest of which is of the second magnitude. the constellation of hercules is very extensive, but contains no star brighter than the third magnitude. near to lyra, on the side away from hercules, are the constellations of _cygnus_ and _aquila_. of the two, the former is the nearer to the pole star, and will be recognised by an arrangement of stars widely set in the form of a cross, or perhaps indeed more like the framework of a boy's kite. the position of aquila will be found through the fact that three of its brightest stars are almost in a line and close together. the middle of these is altair, a yellowish star of the first magnitude. at a little distance from ursa major, on the side away from the pole star, is the constellation of _leo_, or the lion. its chief feature is a series of seven stars, supposed to form the head of that animal. the arrangement of these stars is, however, much more like a sickle, wherefore this portion of the constellation is usually known as the "sickle of leo." at the end of the handle of the sickle is a white first magnitude star--regulus. the reader will, no doubt, recollect that it is from a point in the sickle of leo that the leonid meteors appear to radiate. the star second in brightness in the constellation of leo is known as denebola. this star, now below the second magnitude, seems to have been very much brighter in the past. it is noted, indeed, as a brilliant first magnitude star by al sufi, that famous persian astronomer who lived, as we have seen, in the tenth century. ptolemy also notes it as of the first magnitude. in the neighbourhood of auriga, and further than it from the pole star, are several remarkable constellations--taurus, orion, gemini, canis minor, and canis major (see plate xx., p. ). the first of these, _taurus_ (or the bull), contains two conspicuous star groups--the pleiades and the hyades. the pleiades are six or seven small stars quite close together, the majority of which are of the fourth magnitude. this group is sometimes occulted by the moon. the way in which the stars composing it are arranged is somewhat similar to that in the plough, though of course on a scale ever so much smaller. the impression which the group itself gives to the casual glance is thus admirably pictured in tennyson's _locksley hall_:-- "many a night i saw the pleiads, rising through the mellow shade, glitter like a swarm of fire-flies tangled in a silver braid." [illustration: plate xx. orion and his neighbours we see here that magnificent region of the sky which contains the brightest star of all--sirius. note also especially the milky way, the pleiades, the hyades, and the "belt" and "sword" of orion. (page )] the group of the hyades occupies the "head" of the bull, and is much more spread out than that of the pleiades. it is composed besides of brighter stars, the brightest being one of the first magnitude, aldebaran. this star is of a red colour, and is sometimes known as the "eye of the bull." the constellation of _orion_ is easily recognised as an irregular quadrilateral formed of four bright stars, two of which, betelgeux (reddish) and rigel (brilliant white), are of the first magnitude. in the middle of the quadrilateral is a row of three second magnitude stars, known as the "belt" of orion. jutting off from this is another row of stars called the "sword" of orion. the constellation of _gemini_, or the twins, contains two bright stars--castor and pollux--close to each other. pollux, though marked with the greek letter [b], is the brighter of the two, and nearly of the standard first magnitude. just further from the pole than gemini, is the constellation of _canis minor_, or the lesser dog. its chief star is a white first magnitude one--procyon. still further again from the pole than canis minor is the constellation of _canis major_, or the greater dog. it contains the brightest star in the whole sky, the first magnitude star sirius, bluish-white in colour, also known as the "dog star." this star is almost in line with the stars forming the belt of orion, and is not far from that constellation. taken in the following order, the stars capella, [b] aurigæ, castor, pollux, procyon, and sirius, when they are all above the horizon at the same time, form a beautiful curve stretching across the heaven. the groups of stars visible in the southern skies have by no means the same fascination for us as those in the northern. the ancients were in general unacquainted with the regions beyond the equator, and so their scheme of constellations did not include the sky around the south pole of the heavens. in modern times, however, this part of the celestial expanse was also portioned out into constellations for the purpose of easy reference; but these groupings plainly lack that simplicity of conception and legendary interest which are so characteristic of the older ones. the brightest star in the southern skies is found in the constellation of _argo_, and is known as canopus. in brightness it comes next to sirius, and so is second in that respect in the entire heaven. it does not, however, rise above the english horizon. of the other southern constellations, two call for especial notice, and these adjoin each other. one is _centaurus_ (or the centaur), which contains the two first magnitude stars, [a] and [b] centauri. the first of these, alpha centauri, comes next in brightness to canopus, and is notable as being the nearest of all the stars to our earth. the other constellation is called _crux_, and contains five stars set in the form of a rough cross, known as the "southern cross." the brightest of these, [a] crucis, is of the first magnitude. owing to the precession of the equinoxes, which, as we have seen, gradually shifts the position of the pole among the stars, certain constellations used to be visible in ancient times in more northerly latitudes than at present. for instance, some five thousand years ago the southern cross rose above the english horizon, and was just visible in the latitude of london. it has, however, long ago even ceased to be seen in the south of europe. the constellation of crux happens to be situated in that remarkable region of the southern skies, in which are found the stars canopus and alpha centauri, and also the most brilliant portion of the milky way. it is believed to be to this grand celestial region that allusion is made in the book of job (ix. ), under the title of the "chambers of the south." the "cross" must have been still a notable feature in the sky of palestine in the days when that ancient poem was written. there is no star near enough to the southern pole of the heavens to earn the distinction of south polar star. the galaxy, or _milky way_ (see plate xx., p. ), is a broad band of diffused light which is seen to stretch right around the sky. the telescope, however, shows it to be actually composed of a great host of very faint stars--too faint, indeed, to be separately distinguished with the naked eye. along a goodly stretch of its length it is cleft in two; while near the south pole of the heavens it is entirely cut across by a dark streak. in this rapid survey of the face of the sky, we have not been able to do more than touch in the broadest manner upon some of the most noticeable star groups and a few of the most remarkable stars. to go any further is not a part of our purpose; our object being to deal with celestial bodies as they actually are, and not in those groupings under which they display themselves to us as a mere result of perspective. [ ] attention must here be drawn to the fact that the name of the constellation is always put in the genitive case. [ ] the early peoples, as we have seen, appear to have been attracted by those groupings of the stars which reminded them in a way of the figures of men and animals. we moderns, on the other hand, seek almost instinctively for geometrical arrangements. this is, perhaps, symptomatic of the evolution of the race. in the growth of the individual we find, for example, something analogous. a child, who has been given pencil and paper, is almost certain to produce grotesque drawings of men and animals; whereas the idle and half-conscious scribblings which a man may make upon his blotting-paper are usually of a geometrical character. [ ] because the line joining them _points_ in the direction of the pole star. chapter xxiv systems of stars many stars are seen comparatively close together. this may plainly arise from two reasons. firstly, the stars may happen to be almost in the same line of sight; that is to say, seen in nearly the same direction; and though one star may be ever so much nearer to us than the other, the result will give all the appearance of a related pair. a seeming arrangement of two stars in this way is known as a "double," or double star; or, indeed, to be very precise, an "optical double." secondly, in a pair of stars, both bodies may be about the same distance from us, and actually connected as a system like, for instance, the moon and the earth. a pairing of stars in this way, though often casually alluded to as a double star, is properly termed a "binary," or binary system. but collocations of stars are by no means limited to two. we find, indeed, all over the sky such arrangements in which there are three or more stars; and these are technically known as "triple" or "multiple" stars respectively. further, groups are found in which a great number of stars are closely massed together, such a massing together of stars being known as a "cluster." the pole star (polaris) is a double star, one of the components being of a little below the second magnitude, and the other a little below the ninth. they are so close together that they appear as one star to the naked eye, but they may be seen separate with a moderately sized telescope. the brighter star is yellowish, and the faint one white. this brighter star is found _by means of the spectroscope_ to be actually composed of three stars so very close together that they cannot be seen separately even with a telescope. it is thus a triple star, and the three bodies of which it is composed are in circulation about each other. two of them are darker than the third. the method of detecting binary stars by means of the spectroscope is an application of doppler's principle. it will, no doubt, be remembered that, according to the principle in question, we are enabled, from certain shiftings of the lines in the spectrum of a luminous body, to ascertain whether that body is approaching us or receding from us. now there are certain stars which always appear single even in the largest telescopes, but when the spectroscope is directed to them a spectrum _with two sets of lines_ is seen. such stars must, therefore, be double. further, if the shiftings of the lines, in a spectrum like this, tell us that the component stars are making small movements to and from us which go on continuously, we are therefore justified in concluding that these are the orbital revolutions of a binary system greatly compressed by distance. such connected pairs of stars, since they cannot be seen separately by means of any telescope, no matter how large, are known as "spectroscopic binaries." in observations of spectroscopic binaries we do not always get a double spectrum. indeed, if one of the components be below a certain magnitude, its spectrum will not appear at all; and so we are left in the strange uncertainty as to whether this component is merely faint or actually dark. it is, however, from the shiftings of the lines in the spectrum of the other component that we see that an orbital movement is going on, and are thus enabled to conclude that two bodies are here connected into a system, although one of these bodies resolutely refuses directly to reveal itself even to the all-conquering spectroscope. mizar, that star in the handle of the plough to which we have already drawn attention, will be found with a small telescope to be a fine double, one of the components being white and the other greenish. actually, however, as the american astronomer, professor f.r. moulton, points out, these stars are so far from each other that if we could be transferred to one of them we should see the other merely as an ordinary bright star. the spectroscope shows that the brighter of these stars is again a binary system of two huge suns, the components revolving around each other in a period of about twenty days. this discovery made by professor e.c. pickering, the _first_ of the kind by means of the spectroscope, was announced in from the harvard observatory in the united states. a star close to vega, known as [e] (epsilon) lyræ (see plate xix., p. ), is a double, the components of which may be seen separately with the naked eye by persons with very keen eyesight. if this star, however, be viewed with the telescope, the two companions will be seen far apart; and it will be noticed that each of them is again a double. by means of the spectroscope capella is shown to be really composed of two stars (one about twice as bright as the other) situated very close together and forming a binary system. sirius is also a binary system; but it is what is called a "visual" one, for its component stars may be _seen_ separately in very large telescopes. its double, or rather binary, nature, was discovered in by the celebrated optician alvan g. clark, while in the act of testing the -inch refracting telescope, then just constructed by his firm, and now at the dearborn observatory, illinois, u.s.a. the companion is only of the tenth magnitude, and revolves around sirius in a period of about fifty years, at a mean distance equal to about that of uranus from the sun. seen from sirius, it would shine only something like our full moon. it must be self-luminous and not a mere planet; for mr. gore has shown that if it shone only by the light reflected from sirius, it would be quite invisible even in the great yerkes telescope. procyon is also a binary, its companion having been discovered by professor j.m. schaeberle at the lick observatory in . the period of revolution in this system is about forty years. observations by mr. t. lewis of greenwich seem, however, to point to the companion being a small nebula rather than a star. the star [ê] (eta) cassiopeiæ (see plate xix., p. ), is easily seen as a fine double in telescopes of moderate size. it is a binary system, the component bodies revolving around their common centre of gravity in a period of about two hundred years. this system is comparatively near to us, _i.e._ about nine light years, or a little further off than sirius. in a small telescope the star castor will be found double, the components, one of which is brighter than the other, forming a binary system. the fainter of these was found by belopolsky, with the spectroscope, to be composed of a system of two stars, one bright and the other either dark or not so bright, revolving around each other in a period of about three days. the brighter component of castor is also a spectroscopic binary, with a period of about nine days; so that the whole of what we see with the naked eye as castor, is in reality a remarkable system of four stars in mutual orbital movement. alpha centauri--the nearest star to the earth--is a visual binary, the component bodies revolving around each other in a period of about eighty-one years. the extent of this system is about the same as that of sirius. viewed from each other, the bodies would shine only like our sun as seen from neptune. among the numerous binary stars the orbits of some fifty have been satisfactorily determined. many double stars, for which this has not yet been done, are, however, believed to be, without doubt, binary. in some cases a parallax has been found; so that we are enabled to estimate in miles the actual extent of such systems, and the masses of the bodies in terms of the sun's mass. most of the spectroscopic binaries appear to be upon a smaller scale than the telescopic ones. some are, indeed, comparatively speaking, quite small. for instance, the component stars forming [b] aurigæ are about eight million miles apart, while in [z] geminorum, the distance between the bodies is only a little more than a million miles. spectroscopic binaries are probably very numerous. professor w.w. campbell, director of the lick observatory, estimates, for instance, that, out of about every half-a-dozen stars, one is a spectroscopic binary. it is only in the case of binary systems that we can discover the masses of stars at all. these are ascertained from their movements with regard to each other under the influence of their mutual gravitative attractions. in the case of simple stars we have clearly nothing of the kind to judge by; though, if we can obtain a parallax, we may hazard a guess from their brightness. binary stars were incidentally discovered by sir william herschel. in his researches to get a stellar parallax he had selected a number of double stars for test purposes, on the assumption that, if one of such a pair were much nearer than the other, it might show a displacement with regard to its neighbour as a direct consequence of the earth's orbital movement around the sun. he, however, failed entirely to obtain any parallaxes, the triumph in this being, as we have seen, reserved for bessel. but in some of the double stars which he had selected, he found certain alterations in the relative positions of the bodies, which plainly were not a consequence of the earth's motion, but showed rather that there was an actual circling movement of the bodies themselves under their mutual attractions. it is to be noted that the existence of such connected pairs had been foretold as probable by the rev. john michell, who lived a short time before herschel. the researches into binary systems--both those which can be seen with the eye and those which can be observed by means of the spectroscope, ought to impress upon us very forcibly the wide sway of the law of gravitation. of star clusters about are known, and such systems often contain several thousand stars. they usually cover an area of sky somewhat smaller than the moon appears to fill. in most clusters the stars are very faint, and, as a rule, are between the twelfth and sixteenth magnitudes. it is difficult to say whether these are actually small bodies, or whether their faintness is due merely to their great distance from us, since they are much too far off to show any appreciable parallactic displacement. mr. gore, however, thinks there is good evidence to show that the stars in clusters are really close, and that the clusters themselves fill a comparatively small space. one of the finest examples of a cluster is the great globular one, in the constellation of hercules, discovered by halley in . it contains over stars, and upon a clear, dark night is visible to the naked eye as a patch of light. in the telescope, however, it is a wonderful object. there are also fine clusters in the constellations of auriga, pegasus, and canes venatici. in the southern heavens there are some magnificent examples of globular clusters. this hemisphere seems, indeed, to be richer in such objects than the northern. for instance, there is a great one in the constellation of the centaur, containing some stars (see plate xxi., p. ). [illustration: plate xxi. the great globular cluster in the southern constellation of centaurus from a photograph taken at the cape observatory, on may th, . time of exposure, hour. (page )] certain remarkable groups of stars, of a nature similar to clusters, though not containing such faint or densely packed stars as those we have just alluded to, call for a mention in this connection. the best example of such star groups are the pleiades and the hyades (see plate xx., p. ), coma berenices, and præsepe (or the beehive), the last-named being in the constellation of cancer. stars which alter in their brightness are called _variable stars_, or "variables." the first star whose variability attracted attention is that known as omicron ceti, namely, the star marked with the greek letter [o] (omicron) in the constellation of cetus, or the whale, a constellation situated not far from taurus. this star, the variability of which was discovered by fabricius in , is also known as mira, or the "wonderful," on account of the extraordinary manner in which its light varies from time to time. the star known by the name of algol,[ ] popularly called the "demon star"--whose astronomical designation is [b] (beta) persei, or the star second in brightness in the constellation of perseus--was discovered by goodricke, in the year , to be a variable star. in the following year [b] lyræ, the star in lyra next in order of brightness after vega, was also found by the same observer to be a variable. it may be of interest to the reader to know that goodricke was deaf and dumb, and that he died in at the early age of twenty-one years! it was not, however, until the close of the nineteenth century that much attention was paid to variable stars. now several hundreds of these are known, thanks chiefly to the observations of, amongst others, professor s.c. chandler of boston, u.s.a., mr. john ellard gore of dublin, and dr. a.w. roberts of south africa. this branch of astronomy has not, indeed, attracted as much popular attention as it deserves, no doubt because the nature of the work required does not call for the glamour of an observatory or a large telescope. the chief discoveries with regard to variable stars have been made by the naked eye, or with a small binocular. the amount of variation is estimated by a comparison with other stars. as in many other branches of astronomy, photography is now employed in this quest with marked success; and lately many variable stars have been found to exist in clusters and nebulæ. it was at one time considered that a variable star was in all probability a body, a portion of whose surface had been relatively darkened in some manner akin to that in which sun spots mar the face of the sun; and that when its axial rotation brought the less illuminated portions in turn towards us, we witnessed a consequent diminution in the star's general brightness. herschel, indeed, inclined to this explanation, for his belief was that all the stars bore spots like those of the sun. it appears preferably thought nowadays that disturbances take place periodically in the atmosphere or surroundings of certain stars, perhaps through the escape of imprisoned gases, and that this may be a fruitful cause of changes of brilliancy. the theory in question will, however, apparently account for only one class of variable star, namely, that of which mira ceti is the best-known example. the scale on which it varies in brightness is very great, for it changes from the second to the ninth magnitude. for the other leading type of variable star, algol, of which mention has already been made, is the best instance. the shortness of the period in which the changes of brightness in such stars go their round, is the chief characteristic of this latter class. the period of algol is a little under three days. this star when at its brightest is of about the second magnitude, and when least bright is reduced to below the third magnitude; from which it follows that its light, when at the minimum, is only about one-third of what it is when at the maximum. it seems definitely proved by means of the spectroscope that variables of this kind are merely binary stars, too close to be separated by the telescope, which, as a consequence of their orbits chancing to be edgewise towards us, eclipse each other in turn time after time. if, for instance, both components of such a pair are bright, then when one of them is right behind the other, we will not, of course, get the same amount of light as when they are side by side. if, on the other hand, one of the components happens to be dark or less luminous and the other bright, the manner in which the light of the bright star will be diminished when the darker star crosses its face should easily be understood. it is to the second of these types that algol is supposed to belong. the algol system appears to be composed of a body about as broad as our sun, which regularly eclipses a brighter body which has a diameter about half as great again. since the companion of algol is often spoken of as a _dark_ body, it were well here to point out that we have no evidence at all that it is entirely devoid of light. we have already found, in dealing with spectroscopic binaries, that when one of the component stars is below a certain magnitude[ ] its spectrum will not be seen; so one is left in the glorious uncertainty as to whether the body in question is absolutely dark, or darkish, or faint, or indeed only just out of range of the spectroscope. it is thought probable by good authorities that the companion of algol is not quite dark, but has some inherent light of its own. it is, of course, much too near algol to be seen with the largest telescope. there is in fact a distance of only from two to three millions of miles between the bodies, from which mr. gore infers that they would probably remain unseparated even in the largest telescope which could ever be constructed by man. the number of known variables of the algol type is, so far, small; not much indeed over thirty. in some of them the components are believed to revolve touching each other, or nearly so. an extreme example of this is found in the remarkable star v. puppis, an algol variable of the southern hemisphere. both its components are bright, and the period of light variation is about one and a half days. dr. a. w. roberts finds that the bodies are revolving around each other in actual contact. _temporary stars_ are stars which have suddenly blazed out in regions of the sky where no star was previously seen, and have faded away more or less gradually. it was the appearance of such a star, in the year b.c., which prompted hipparchus to make his celebrated catalogue, with the object of leaving a record by which future observers could note celestial changes. in another star of this kind flashed out in the constellation of cassiopeia (see plate xix., p. ), and was detected by tycho brahe. it became as bright as the planet venus, and eventually was visible in the day-time. two years later, however, it disappeared, and has never since been seen. in kepler recorded a similar star in the constellation of ophiuchus which grew to be as bright as jupiter. it also lasted for about two years, and then faded away, leaving no trace behind. it is rarely, however, that temporary stars attain to such a brilliance; and so possibly in former times a number of them may have appeared, but not have risen to a sufficient magnitude to attract attention. even now, unless such a star becomes clearly visible to the naked eye, it runs a good chance of not being detected. a curious point, worth noting, with regard to temporary stars is that the majority of them have appeared in the milky way. these sudden visitations have in our day received the name of _novæ_; that is to say, "new" stars. two, in recent years, attracted a good deal of attention. the first of these, known as nova aurigæ, or the new star in the constellation of auriga, was discovered by dr. t.d. anderson at edinburgh in january . at its greatest brightness it attained to about the fourth magnitude. by april it had sunk to the twelfth, but during august it recovered to the ninth magnitude. after this last flare-up it gradually faded away. the startling suddenness with which temporary stars usually spring into being is the groundwork upon which theories to account for their origin have been erected. that numbers of dark stars, extinguished suns, so to speak, may exist in space, there is a strong suspicion; and it is just possible that we have an instance of one dark stellar body in the companion of algol. that such dark stars might be in rapid motion is reasonable to assume from the already known movements of bright stars. two dark bodies might, indeed, collide together, or a collision might take place between a dark star and a star too faint to be seen even with the most powerful telescope. the conflagration produced by the impact would thus appear where nothing had been seen previously. again, a similar effect might be produced by a dark body, or a star too faint to be seen, being heated to incandescence by plunging in its course through a nebulous mass of matter, of which there are many examples lying about in space. the last explanation, which is strongly reminiscent of what takes place in shooting stars, appears more probable than the collision theory. the flare-up of new stars continues, indeed, only for a comparatively short time; whereas a collision between two bodies would, on the other hand, produce an enormous nebula which might take even millions of years to cool down. we have, indeed, no record of any such sudden appearance of a lasting nebula. the other temporary star, known as nova persei, or the new star in the constellation of perseus, was discovered early in the morning of february , , also by dr. anderson. a day later it had grown to be brighter than capella. photographs which had been taken, some three days previous to its discovery, of the very region of the sky in which it had burst forth, were carefully examined, and it was not found in these. at the end of two days after its discovery nova persei had lost one-third of its light. during the ensuing six months it passed through a series of remarkable fluctuations, varying in brightness between the third and fifth magnitudes. in the month of august it was seen to be surrounded by luminous matter in the form of a nebula, which appeared to be gradually spreading to some distance around. taking into consideration the great way off at which all this was taking place, it looked as if the new star had ejected matter which was travelling outward with a velocity equivalent to that of light. the remarkable theory was, however, put forward by professor kapteyn and the late dr. w.e. wilson that there might be after all no actual transmission of matter; but that perhaps the real explanation was the gradual _illumination_ of hitherto invisible nebulous matter, as a consequence of the flare-up which had taken place about six months before. it was, therefore, imagined that some dark body moving through space at a very rapid rate had plunged through a mass of invisible nebulous matter, and had consequently become heated to incandescence in its passage, very much like what happens to a meteor when moving through our atmosphere. the illumination thus set up temporarily in one point, being transmitted through the nebulous wastes around with the ordinary velocity of light, had gradually rendered this surrounding matter visible. on the assumptions required to fit in with such a theory, it was shown that nova persei must be at a distance from which light would take about three hundred years in coming to us. the actual outburst of illumination, which gave rise to this temporary star, would therefore have taken place about the beginning of the reign of james i. some recent investigations with regard to nova persei have, however, greatly narrowed down the above estimate of its distance from us. for instance, bergstrand proposes a distance of about ninety-nine light years; while the conclusions of mr. f.w. very would bring it still nearer, _i.e._ about sixty-five light years. the last celestial objects with which we have here to deal are the _nebulæ_. these are masses of diffused shining matter scattered here and there through the depths of space. nebulæ are of several kinds, and have been classified under the various headings of spiral, planetary, ring, and irregular. a typical _spiral_ nebula is composed of a disc-shaped central portion, with long curved arms projecting from opposite sides of it, which give an impression of rapid rotatory movement. the discovery of spiral nebulæ was made by lord rosse with his great -foot reflector. two good examples of these objects will be found in ursa major, while there is another fine one in canes venatici (see plate xxii., p. ), a constellation which lies between ursa major and boötes. but the finest spiral of all, perhaps the most remarkable nebula known to us, is the great nebula in the constellation of andromeda, (see plate xxiii., p. )--a constellation just further from the pole than cassiopeia. when the moon is absent and the night clear this nebula can be easily seen with the naked eye as a small patch of hazy light. it is referred to by al sufi. [illustration: plate xxii. spiral nebula in the constellation of canes venatici from a photograph by the late dr. w.e. wilson, d.sc., f.r.s. (page )] spiral nebulæ are white in colour, whereas the other kinds of nebula have a greenish tinge. they are also by far the most numerous; and the late professor keeler, who considered this the normal type of nebula, estimated that there were at least , of such spirals within the reach of the crossley reflector of the lick observatory. professor perrine has indeed lately raised this estimate to half a million, and thinks that with more sensitive photographic plates and longer exposures the number of spirals would exceed a million. the majority of these objects are very small, and appear to be distributed over the sky in a fairly uniform manner. _planetary_ nebulæ are small faint roundish objects which, when seen in the telescope, recall the appearance of a planet, hence their name. one of these nebulæ, known astronomically as g.c. , has recently been found to be rushing through space towards the earth at a rate of between thirty and forty miles per second. it seems strange, indeed, that any gaseous mass should move at such a speed! what are known as _ring_ nebulæ were until recently believed to form a special class. these objects have the appearance of mere rings of nebulous matter. much doubt has, however, been thrown upon their being rings at all; and the best authorities regard them merely as spiral nebulæ, of which we happen to get a foreshortened view. very few examples are known, the most famous being one in the constellation of lyra, usually known as the annular nebula in lyra. this object is so remote from us as to be entirely invisible to the naked eye. it contains a star of the fifteenth magnitude near to its centre. from photographs taken with the crossley reflector, professor schaeberle finds in this nebula evidences of spiral structure. it may here be mentioned that the great nebula in andromeda, which has now turned out to be a spiral, had in earlier photographs the appearance of a ring. there also exist nebulæ of _irregular_ form, the most notable being the great nebula in the constellation of orion (see plate xxiv., p. ). it is situated in the centre of the "sword" of orion (see plate xx., p. ). in large telescopes it appears as a magnificent object, and in actual dimensions it must be much on the same scale as the andromeda nebula. the spectroscope tells us that it is a mass of glowing gas. the trifid nebula, situated in the constellation of sagittarius, is an object of very strange shape. three dark clefts radiate from its centre, giving it an appearance as if it had been torn into shreds. the dumb-bell nebula, a celebrated object, so called from its likeness to a dumb-bell, turns out, from recent photographs taken by professor schaeberle, which bring additional detail into view, to be after all a great spiral. there is a nest, or rather a cluster of nebulæ in the constellation of coma berenices; over a hundred of these objects being here gathered into a space of sky about the size of our full moon. [illustration: plate xxiii. the great nebula in the constellation of andromeda from a photograph taken at the yerkes observatory. (page )] the spectroscope informs us that spiral nebulæ are composed of partially-cooled matter. their colour, as we have seen, is white. nebulæ of a greenish tint are, on the other hand, found to be entirely in a gaseous condition. just as the solar corona contains an unknown element, which for the time being has been called "coronium," so do the gaseous nebulæ give evidence of the presence of another unknown element. to this sir william huggins has given the provisional name of "nebulium." the _magellanic clouds_ are two patches of nebulous-looking light, more or less circular in form, which are situated in the southern hemisphere of the sky. they bear a certain resemblance to portions of the milky way, but are, however, not connected with it. they have received their name from the celebrated navigator, magellan, who seems to have been one of the first persons to draw attention to them. "nubeculæ" is another name by which they are known, the larger cloud being styled _nubecula major_ and the smaller one _nubecula minor_. they contain within them stars, clusters, and gaseous nebulæ. no parallax has yet been found for any object which forms part of the nubeculæ, so it is very difficult to estimate at what distance from us they may lie. they are, however, considered to be well within our stellar universe. having thus brought to a conclusion our all too brief review of the stars and the nebulæ--of the leading objects in fine which the celestial spaces have revealed to man--we will close this chapter with a recent summation by sir david gill of the relations which appear to obtain between these various bodies. "huggins's spectroscope," he says, "has shown that many nebulæ are not stars at all; that many well-condensed nebulæ, as well as vast patches of nebulous light in the sky, are but inchoate masses of luminous gas. evidence upon evidence has accumulated to show that such nebulæ consist of the matter out of which stars (_i.e._ suns) have been and are being evolved. the different types of star spectra form such a complete and gradual sequence (from simple spectra resembling those of nebulæ onwards through types of gradually increasing complexity) as to suggest that we have before us, written in the cryptograms of these spectra, the complete story of the evolution of suns from the inchoate nebula onwards to the most active sun (like our own), and then downward to the almost heatless and invisible ball. the period during which human life has existed upon our globe is probably too short--even if our first parents had begun the work--to afford observational proof of such a cycle of change in any particular star; but the fact of such evolution, with the evidence before us, can hardly be doubted."[ ] [ ] the name al gûl, meaning the demon, was what the old arabian astronomers called it, which looks very much as if they had already noticed its rapid fluctuations in brightness. [ ] mr. gore thinks that the companion of algol may be a star of the sixth magnitude. [ ] presidential address to the british association for the advancement of science (leicester, ), by sir david gill, k.c.b., ll.d., f.r.s., &c. &c. [illustration: plate xxiv. the great nebula in the constellation of orion from a photograph taken at the yerkes observatory. (page )] chapter xxv the stellar universe the stars appear fairly evenly distributed all around us, except in one portion of the sky where they seem very crowded, and so give one an impression of being very distant. this portion, known as the milky way, stretches, as we have already said, in the form of a broad band right round the entire heavens. in those regions of the sky most distant from the milky way the stars appear to be thinly sown, but become more and more closely massed together as the milky way is approached. this apparent distribution of the stars in space has given rise to a theory which was much favoured by sir william herschel, and which is usually credited to him, although it was really suggested by one thomas wright of durham in ; that is to say, some thirty years or more before herschel propounded it. according to this, which is known as the "disc" or "grindstone" theory, the stars are considered as arranged in space somewhat in the form of a thick disc, or grindstone, close to the _central_ parts of which our solar system is situated.[ ] thus we should see a greater number of stars when we looked out through the _length_ of such a disc in any direction, than when we looked out through its _breadth_. this theory was, for a time, supposed to account quite reasonably for the milky way, and for the gradual increase in the number of stars in its vicinity. it is quite impossible to verify directly such a theory, for we know the actual distance of only about forty-three stars. we are unable, therefore, definitely to assure ourselves whether, as the grindstone theory presupposes, the stellar universe actually reaches out very much further from us in the direction of the milky way than in the other parts of the sky. the theory is clearly founded upon the supposition that the stars are more or less equal in size, and are scattered through space at fairly regular distances from each other. brightness, therefore, had been taken as implying nearness to us, and faintness great distance. but we know to-day that this is not the case, and that the stars around us are, on the other hand, of various degrees of brightness and of all orders of size. some of the faint stars--for instance, the galloping star in pictor--are indeed nearer to us than many of the brighter ones. sirius, on the other hand, is twice as far off from us as [a] centauri, and yet it is very much brighter; while canopus, which in brightness is second only to sirius out of the whole sky, is too far off for its distance to be ascertained! it must be remembered that no parallax had yet been found for any star in the days of herschel, and so his estimations of stellar distances were necessarily of a very circumstantial kind. he did not, however, continue always to build upon such uncertain ground; but, after some further examination of the milky way, he gave up his idea that the stars were equally disposed in space, and eventually abandoned the grindstone theory. since we have no means of satisfactorily testing the matter, through finding out the various distances from us at which the stars are really placed, one might just as well go to the other extreme, and assume that the thickening of stars in the region of the milky way is not an effect of perspective at all, but that the stars in that part of the sky are actually more crowded together than elsewhere--a thing which astronomers now believe to be the case. looked at in this way, the shape of the stellar universe might be that of a globe-shaped aggregation of stars, in which the individuals are set at fairly regular distances from each other; the whole being closely encircled by a belt of densely packed stars. it must, however, be allowed that the gradual increase in the number of stars towards the milky way appears a strong argument in favour of the grindstone theory; yet the belt theory, as above detailed, seems to meet with more acceptance. there is, in fact, one marked circumstance which is remarkably difficult of explanation by means of the grindstone theory. this is the existence of vacant spaces--holes, so to speak, in the groundwork of the milky way. for instance, there is a cleft running for a good distance along its length, and there is also a starless gap in its southern portion. it seems rather improbable that such a great number of stars could have arranged themselves so conveniently, as to give us a clear view right out into empty space through such a system in its greatest thickness; as if, in fact, holes had been bored, and clefts made, from the boundary of the disc clean up to where our solar system lies. sir john herschel long ago drew attention to this point very forcibly. it is plain that such vacant spaces can, on the other hand, be more simply explained as mere holes in a belt; and the best authorities maintain that the appearance of the milky way confirms a view of this kind. whichever theory be indeed the correct one, it appears at any rate that the stars do not stretch out in every direction to an infinite distance; but that _the stellar system is of limited extent_, and has in fact a boundary. in the first place, science has no grounds for supposing that light is in any way absorbed or destroyed merely by its passage through the "ether," that imponderable medium which is believed to transmit the luminous radiations through space. this of course is tantamount to saying that all the direct light from all the stars should reach us, excepting that little which is absorbed in its passage through our own atmosphere. if stars, and stars, and stars existed in every direction outwards without end, it can be proved mathematically that in such circumstances there could not remain the tiniest space in the sky without a star to fill it, and that therefore the heavens would always blaze with light, and the night would be as bright as the noonday.[ ] how very far indeed this is from being the case, may be gathered from an estimate which has been made of the general amount of light which we receive from the stars. according to this estimate the sky is considered as more or less dark, the combined illumination sent to us by all the stars being only about the one-hundreth part of what we get from the full moon.[ ] secondly, it has been suggested that although light may not suffer any extinction or diminution from the ether itself, still a great deal of illumination may be prevented from reaching us through myriads of extinguished suns, or dark meteoric matter lying about in space. the idea of such extinguished suns, dark stars in fact, seems however to be merely founded upon the sole instance of the invisible companion of algol; but, as we have seen, there is no proof whatever that it is a dark body. again, some astronomers have thought that the dark holes in the milky way, "coal sacks," as they are called, are due to masses of cool, or partially cooled matter, which cuts off the light of the stars beyond. the most remarkable of these holes is one in the neighbourhood of the southern cross, known as the "coal sack in crux." but mr. gore thinks that the cause of the holes is to be sought for rather in what sir william herschel termed "clustering power," _i.e._ a tendency on the part of stars to accumulate in certain places, thus leaving others vacant; and the fact that globular and other clusters are to be found very near to such holes certainly seems corroborative of this theory. in summing up the whole question, professor newcomb maintains that there does not appear any evidence of the light from the milky way stars, which are apparently the furthest bodies we see, being intercepted by dark bodies or dark matter. as far as our telescopes can penetrate, he holds that we see the stars _just as they are_. also, if there did exist an infinite number of stars, one would expect to find evidence in some direction of an overpoweringly great force,--the centre of gravity of all these bodies. it is noticed, too, that although the stars increase in number with decrease in magnitude, so that as we descend in the scale we find three times as many stars in each magnitude as in the one immediately above it, yet this progression does not go on after a while. there is, in fact, a rapid falling off in numbers below the twelfth magnitude; which looks as if, at a certain distance from us, the stellar universe were beginning to _thin out_. again, it is estimated, by mr. gore and others, that only about millions of stars are to be seen in the whole of the sky with the best optical aids. this shows well the limited extent of the stellar system, for the number is not really great. for instance, there are from fifteen to sixteen times as many persons alive upon the earth at this moment! last of all, there appears to be strong photographic evidence that our sidereal system is limited in extent. two photographs taken by the late dr. isaac roberts of a region rich in stellar objects in the constellation of cygnus, clearly show what has been so eloquently called the "darkness behind the stars." one of these photographs was taken in , and the other in . on both occasions the state of the atmosphere was practically the same, and the sensitiveness of the films was of the same degree. the exposure in the first case was only one hour; in the second it was about two hours and a half. and yet both photographs show _exactly the same stars, even down to the faintest_. from this one would gather that the region in question, which is one of the most thickly star-strewn in the milky way, is _penetrable right through_ with the means at our command. dr. roberts himself in commenting upon the matter drew attention to the fact, that many astronomers seemed to have tacitly adopted the assumption that the stars extend indefinitely through space. from considerations such as these the foremost astronomical authorities of our time consider themselves justified in believing that the collection of stars around us is _finite_; and that although our best telescopes may not yet be powerful enough to penetrate to the final stars, still the rapid decrease in numbers as space is sounded with increasing telescopic power, points strongly to the conclusion that the boundaries of the stellar system may not lie very far beyond the uttermost to which we can at present see. is it possible then to make an estimate of the extent of this stellar system? whatever estimates we may attempt to form cannot however be regarded as at all exact, for we know the actual distances of such a very few only of the nearest of the stars. but our knowledge of the distances even of these few, permits us to assume that the stars close around us may be situated, on an average, at about eight light-years from each other; and that this holds good of the stellar spaces, with the exception of the encircling girdle of the milky way, where the stars seem actually to be more closely packed together. this girdle further appears to contain the greater number of the stars. arguing along these lines, professor newcomb reaches the conclusion that the farthest stellar bodies which we see are situated at about between and light-years from us. starting our inquiry from another direction, we can try to form an estimate by considering the question of proper motions. it will be noticed that such motions do not depend entirely upon the actual speed of the stars themselves, but that some of the apparent movement arises indirectly from the speed of our own sun. the part in a proper motion which can be ascribed to the movement of our solar system through space is clearly a displacement in the nature of a parallax--sir william herschel called it "_systematic_ parallax"; so that knowing the distance which we move over in a certain lapse of time, we are able to hazard a guess at the distances of a good many of the stars. an inquiry upon such lines must needs be very rough, and is plainly based upon the assumption that the stars whose distances we attempt to estimate are moving at an average speed much like that of our own sun, and that they are not "runaway stars" of the groombridge order. be that as it may, the results arrived at by professor newcomb from this method of reasoning are curiously enough very much on a par with those founded on the few parallaxes which we are really certain about; with the exception that they point to somewhat closer intervals between the individual stars, and so tend to narrow down our previous estimate of the extent of the stellar system. thus far we get, and no farther. our solar system appears to lie somewhere near the centre of a great collection of stars, separated each one from the other, on an average, by some billions of miles; the whole being arranged in the form of a mighty globular cluster. light from the nearest of these stars takes some four years to come to us. it takes about times as long to reach us from the confines of the system. this globe of stars is wrapt around closely by a stellar girdle, the individual stars in which are set together more densely than those in the globe itself. the entire arrangement appears to be constructed upon a very regular plan. here and there, as professor newcomb points out, the aspect of the heavens differs in small detail; but generally it may be laid down that the opposite portions of the sky, whether in the milky way itself, or in those regions distant from it, show a marked degree of symmetry. the proper motions of stars in corresponding portions of the sky reveal the same kind of harmony, a harmony which may even be extended to the various colours of the stars. the stellar system, which we see disposed all around us, appears in fine to bear all the marks of an _organised whole_. the older astronomers, to take sir william herschel as an example, supposed some of the nebulæ to be distant "universes." sir william was led to this conclusion by the idea he had formed that, when his telescopes failed to show the separate stars of which he imagined these objects to be composed, he must put down the failure to their stupendous distance from us. for instance, he thought the orion nebula, which is now known to be made up of glowing gas, to be an external stellar system. later on, however, he changed his mind upon this point, and came to the conclusion that "shining fluid" would better account both for this nebula, and for others which his telescopes had failed to separate into component stars. the old ideas with regard to external systems and distant universes have been shelved as a consequence of recent research. all known clusters and nebulæ are now firmly believed to lie _within_ our stellar system. this view of the universe of stars as a sort of island in the immensities, does not, however, give us the least idea about the actual extent of space itself. whether what is called space is really infinite, that is to say, stretches out unendingly in every direction, or whether it has eventually a boundary somewhere, are alike questions which the human mind seems utterly unable to picture to itself. [ ] the ptolemaic idea dies hard! [ ] even the milky way itself is far from being a blaze of light, which shows that the stars composing it do not extend outwards indefinitely. [ ] mr. gore has recently made some remarkable deductions, with regard to the amount of light which we get from the stars. he considers that most of this light comes from stars below the sixth magnitude; and consequently, if all the stars visible to the naked eye were to be blotted out, the glow of the night sky would remain practically the same as it is at present. going to the other end of the scale, he thinks also that the combined light which we get from all the stars below the seventeenth magnitude is so very small, that it may be neglected in such an estimation. he finds, indeed, that if there are stars so low as the twentieth magnitude, one hundred millions of them would only be equal in brightness to a single first-magnitude star like vega. on the other hand, it is possible that the light of the sky at night is not entirely due to starlight, but that some of it may be caused by phosphorescent glow. chapter xxvi the stellar universe--_continued_ it is very interesting to consider the proper motions of stars with reference to such an isolated stellar system as has been pictured in the previous chapter. these proper motions are so minute as a rule, that we are quite unable to determine whether the stars which show them are moving along in straight lines, or in orbits of immense extent. it would, in fact, take thousands of years of careful observation to determine whether the paths in question showed any degree of curving. in the case of the more distant stars, the accurate observations which have been conducted during the last hundred years have not so far revealed any proper motions with regard to them; but one cannot escape the conclusion that these stars move as the others do. if space outside our stellar system is infinite in extent, and if all the stars within that system are moving unchecked in every conceivable direction, the result must happen that after immense ages these stars will have drawn apart to such a distance from each other, that the system will have entirely disintegrated, and will cease to exist as a connected whole. eventually, indeed, as professor newcomb points out, the stars will have separated so far from each other that each will be left by itself in the midst of a black and starless sky. if, however, a certain proportion of stars have a speed sufficiently slow, they will tend under mutual attraction to be brought to rest by collisions, or forced to move in orbits around each other. but those stars which move at excessive speeds, such, for instance, as groombridge, or the star in the southern constellation of pictor, seem utterly incapable of being held back in their courses by even the entire gravitative force of our stellar system acting as a whole. these stars must, therefore, move eventually right through the system and pass out again into the empty spaces beyond. add to this; certain investigations, made into the speed of groombridge, furnish a remarkable result. it is calculated, indeed, that had this star been _falling through infinite space for ever_, pulled towards us by the combined gravitative force of our entire system of stars, it could not have gathered up anything like the speed with which it is at present moving. no force, therefore, which we can conjure out of our visible universe, seems powerful enough either to have impressed upon this runaway star the motion which it now has, or to stay it in its wild course. what an astounding condition of things! speculations like this call up a suspicion that there may yet exist other universes, other centres of force, notwithstanding the apparent solitude of our stellar system in space. it will be recollected that the idea of this isolation is founded upon such facts as, that the heavens do not blaze with light, and that the stars gradually appear to thin out as we penetrate the system with increasing telescopic power. but perchance there is something which hinders us from seeing out into space beyond our cluster of stars; which prevents light, in fact, from reaching us from other possible systems scattered through the depths beyond. it has, indeed, been suggested by mr. gore[ ] that the light-transmitting ether may be after all merely a kind of "atmosphere" of the stars; and that it may, therefore, thin off and cease a little beyond the confines of our stellar system, just as the air thins off and practically ceases at a comparatively short distance from the earth. a clashing together of solid bodies outside our atmosphere could plainly send us no sound, for there is no air extending the whole way to bear to our ears the vibrations thus set up; so light emitted from any body lying beyond our system of stars, would not be able to come to us if the ether, whose function it is to convey the rays of light, ceased at or near the confines of that system. perchance we have in this suggestion the key to the mystery of how our sun and the other stellar bodies maintain their functions of temperature and illumination. the radiations of heat and light arriving at the limits of this ether, and unable to pass any further, may be thrown back again into the system in some altered form of energy. but these, at best, are mere airy and fascinating speculations. we have, indeed, no evidence whatever that the luminiferous ether ceases at the boundary of the stellar system. if, therefore, it extends outwards infinitely in every direction, and if it has no absorbing or weakening effect on the vibrations which it transmits, we cannot escape from the conclusion that practically all the rays of light ever emitted by all the stars must chase one another eternally through the never-ending abysses of space. [ ] _planetary and stellar studies_, by john ellard gore, f.r.a.s., m.r.i.a., london, . chapter xxvii the beginning of things laplace's nebular hypothesis dwelling upon the fact that all the motions of revolution and rotation in the solar system, as known in his day, took place in the same direction and nearly in the same plane, the great french astronomer, laplace, about the year , put forward a theory to account for the origin and evolution of that system. he conceived that it had come into being as a result of the gradual contraction, through cooling, of an intensely heated gaseous lens-shaped mass, which had originally occupied its place, and had extended outwards beyond the orbit of the furthest planet. he did not, however, attempt to explain how such a mass might have originated! he went on to suppose that this mass, _in some manner_, perhaps by mutual gravitation among its parts, had acquired a motion of rotation in the same direction as the planets now revolve. as this nebulous mass parted with its heat by radiation, it contracted towards the centre. becoming smaller and smaller, it was obliged to rotate faster and faster in order to preserve its equilibrium. meanwhile, in the course of contraction, rings of matter became separated from the nucleus of the mass, and were left behind at various intervals. these rings were swept up into subordinate masses similar to the original nebula. these subordinate masses also contracted in the same manner, leaving rings behind them which, in turn, were swept up to form satellites. saturn's ring was considered, by laplace, as the only portion of the system left which still showed traces of this evolutionary process. it is even probable that it may have suggested the whole of the idea to him. laplace was, however, not the first philosopher who had speculated along these lines concerning the origin of the world. nearly fifty years before, in to be exact, thomas wright, of durham, had put forward a theory to account for the origin of the whole sidereal universe. in his theory, however, the birth of our solar system was treated merely as an incident. shortly afterwards the subject was taken up by the famous german philosopher, kant, who dealt with the question in a still more ambitious manner, and endeavoured to account in detail for the origin of the solar system as well as of the sidereal universe. something of the trend of such theories may be gathered from the remarkable lines in tennyson's _princess_:-- "this world was once a fluid haze of light, till toward the centre set the starry tides, and eddied into suns, that wheeling cast the planets." the theory, as worked out by kant, was, however, at the best merely a _tour de force_ of philosophy. laplace's conception was much less ambitious, for it did not attempt to explain the origin of the entire universe, but only of the solar system. being thus reasonably limited in its scope, it more easily obtained credence. the arguments of laplace were further founded upon a mathematical basis. the great place which he occupied among the astronomers of that time caused his theory to exert a preponderating influence on scientific thought during the century which followed. a modification of laplace's theory is the meteoritic hypothesis of sir norman lockyer. according to the views of that astronomer, the material of which the original nebula was composed is presumed to have been in the meteoric, rather than in the gaseous, state. sir norman lockyer holds, indeed, that nebulæ are, in reality, vast swarms of meteors, and the light they emit results from continual collisions between the constituent particles. the french astronomer, faye, also proposed to modify laplace's theory by assuming that the nebula broke up into rings all at once, and not in detail, as laplace had wished to suppose. the hypothesis of laplace fits in remarkably well with the theory put forward in later times by helmholtz, that the heat of the sun is kept up by the continual contraction of its mass. it could thus have only contracted to its present size from one very much larger. plausible, however, as laplace's great hypothesis appears on the surface, closer examination shows several vital objections, a few of those set forth by professor moulton being here enumerated-- although laplace held that the orbits of the planets were sufficiently near to being in the one plane to support his views, yet later investigators consider that their very deviations from this plane are a strong argument against the hypothesis. again, it is thought that if the theory were the correct explanation, the various orbits of the planets would be much more nearly circular than they are. it is also thought that such interlaced paths, as those in which the asteroids and the little planet eros move, are most unlikely to have been produced as a result of laplace's nebula. further, while each of the rings was sweeping up its matter into a body of respectable dimensions, its gravitative power would have been for the time being so weak, through being thus spread out, that any lighter elements, as, for instance, those of the gaseous order, would have escaped into space in accordance with the principles of the kinetic theory. _the idea that rings would at all be left behind at certain intervals during the contraction of the nebula is, perhaps, one of the weakest points in laplace's hypothesis._ mathematical investigation does not go to show that the rings, presuming they could be left behind during the contraction of the mass, would have aggregated into planetary bodies. indeed, it rather points to the reverse. lastly, such a discovery as that the ninth satellite of saturn revolves in a _retrograde_ direction--that is to say, in a direction contrary to the other revolutions and rotations in our solar system--appears directly to contradict the hypothesis. although laplace's hypothesis seems to break down under the keen criticism to which it has been subjected, yet astronomers have not relinquished the idea that our solar system has probably had its origin from a nebulous mass. but the apparent failure of the laplacian theory is emphasised by the fact, that _not a single example of a nebula, in the course of breaking up into concentric rings, is known to exist in the entire heaven_. indeed, as we saw in chapter xxiv., there seems to be no reliable example of even a "ring" nebula at all. mr. gore has pointed this out very succinctly in his recently published work, _astronomical essays_, where he says:--"to any one who still persists in maintaining the hypothesis of ring formation in nebulæ, it may be said that the whole heavens are against him." the conclusions of keeler already alluded to, that the spiral is the normal type of nebula, has led during the past few years to a new theory by the american astronomers, professors chamberlin and moulton. in the detailed account of it which they have set forth, they show that those anomalies which were stumbling-blocks to laplace's theory do not contradict theirs. to deal at length with this theory, to which the name of "planetesimal hypothesis" has been given, would not be possible in a book of this kind. but it may be of interest to mention that the authors of the theory in question remount the stream of time still further than did laplace, and seek to explain the _origin_ of the spiral nebulæ themselves in the following manner:-- having begun by assuming that the stars are moving apparently in every direction with great velocities, they proceed to point out that sooner or later, although the lapse of time may be extraordinarily long, collisions or near approaches between stars are bound to occur. in the case of collisions the chances are against the bodies striking together centrally, it being very much more likely that they will hit each other rather towards the side. the nebulous mass formed as a result of the disintegration of the bodies through their furious impact would thus come into being with a spinning movement, and a spiral would ensue. again, the stars may not actually collide, but merely approach near to each other. if very close, the interaction of gravitation will give rise to intense strains, or tides, which will entirely disintegrate the bodies, and a spiral nebula will similarly result. as happens upon our earth, two such tides would rise opposite to each other; and, consequently, it is a noticeable fact that spiral nebulæ have almost invariably two opposite branches (see plate xxii., p ). even if not so close, the gravitational strains set up would produce tremendous eruptions of matter; and in this case, a spiral movement would also be generated. on such an assumption the various bodies of the solar system may be regarded as having been ejected from parent masses. the acceptance of the planetesimal hypothesis in the place of the hypothesis of laplace will not, as we have seen, by any means do away with the probability that our solar system, and similar systems, have originated from a nebulous mass. on the contrary it puts that idea on a firmer footing than before. the spiral nebulæ which we see in the heavens are on a vast scale, and may represent the formation of stellar systems and globular clusters. our solar system may have arisen from a small spiral. we will close these speculations concerning the origin of things with a short sketch of certain investigations made in recent years by sir george h. darwin, of cambridge university, into the question of the probable birth of our moon. he comes to the conclusion that at least fifty-four millions of years ago the earth and moon formed one body, which had a diameter of a little over miles. this body rotated on an axis in about five hours, namely, about five times as fast as it does at present. the rapidity of the rotation caused such a tremendous strain that the mass was in a condition of, what is called, unstable equilibrium; very little more, in fact, being required to rend it asunder. the gravitational pull of the sun, which, as we have already seen, is in part the cause of our ordinary tides, supplied this extra strain, and a portion of the mass consequently broke off, which receded gradually from the rest and became what we now know as the moon. sir george darwin holds that the gravitational action of the sun will in time succeed in also disturbing the present apparent harmony of the earth-moon system, and will eventually bring the moon back towards the earth, so that after the lapse of great ages they will re-unite once again. in support of this theory of the terrestrial origin of the moon, professor w.h. pickering has put forward a bold hypothesis that our satellite had its origin in the great basin of the pacific. this ocean is roughly circular, and contains no large land masses, except the australian continent. he supposes that, prior to the moon's birth, our globe was already covered with a slight crust. in the tearing away of that portion which was afterwards destined to become the moon the remaining area of the crust was rent in twain by the shock; and thus were formed the two great continental masses of the old and new worlds. these masses floated apart across the fiery ocean, and at last settled in the positions which they now occupy. in this way professor pickering explains the remarkable parallelism which exists between the opposite shores of the atlantic. the fact of this parallelism had, however, been noticed before; as, for example, by the late rev. s.j. johnson, in his book _eclipses, past and future_, where we find the following passage:-- "if we look at our maps we shall see the parts of one continent that jut out agree with the indented portions of another. the prominent coast of africa would fit in the opposite opening between north and south america, and so in numerous other instances. a general rending asunder of the world would seem to have taken place when the foundations of the great deep were broken up." although professor pickering's theory is to a certain degree anticipated in the above words, still he has worked out the idea much more fully, and given it an additional fascination by connecting it with the birth of the moon. he points out, in fact, that there is a remarkable similarity between the lunar volcanoes and those in the immediate neighbourhood of the pacific ocean. he goes even further to suggest that australia is another portion of the primal crust which was detached out of the region now occupied by the indian ocean, where it was originally connected with the south of india or the east of africa. certain objections to the theory have been put forward, one of which is that the parallelism noticed between the opposite shores of the atlantic is almost too perfect to have remained through some sixty millions of years down to our own day, in the face of all those geological movements of upheaval and submergence, which are perpetually at work upon our globe. professor pickering, however, replies to this objection by stating that many geologists believe that the main divisions of land and water on the earth are permanent, and that the geological alterations which have taken place since these were formed have been merely of a temporary and superficial nature. chapter xxviii the end of things we have been trying to picture the beginning of things. we will now try to picture the end. in attempting this, we find that our theories must of necessity be limited to the earth, or at most to the solar system. the time-honoured expression "end of the world" really applies to very little beyond the end of our own earth. to the people of past ages it, of course, meant very much more. for them, as we have seen, the earth was the centre of everything; and the heavens and all around were merely a kind of minor accompaniment, created, as they no doubt thought, for their especial benefit. in the ancient view, therefore, the beginning of the earth meant the beginning of the universe, and the end of the earth the extinction of all things. the belief, too, was general that this end would be accomplished through fire. in the modern view, however, the birth and death of the earth, or indeed of the solar system, might pass as incidents almost unnoticed in space. they would be but mere links in the chain of cosmic happenings. a number of theories have been forward from time to time prognosticating the end of the earth, and consequently of human life. we will conclude with a recital of a few of them, though which, if any, is the true one, the last men alone can know. just as a living creature may at any moment die in the fulness of strength through sudden malady or accident, or, on the other hand, may meet with death as a mere consequence of old age, so may our globe be destroyed by some sudden cataclysm, or end in slow processes of decay. barring accidents, therefore, it would seem probable that the growing cold of the earth, or the gradual extinction of the sun, should after many millions of years close the chapter of life, as we know it. on the former of these suppositions, the decrease of temperature on our globe might perhaps be accelerated by the thinning of the atmosphere, through the slow escape into space of its constituent gases, or their gradual chemical combination with the materials of the earth. the subterranean heat entirely radiated away, there would no longer remain any of those volcanic elevating forces which so far have counteracted the slow wearing down of the land surface of our planet, and thus what water remained would in time wash over all. if this preceded the growing cold of the sun, certain strange evolutions of marine forms of life would be the last to endure, but these, too, would have to go in the end. should, however, the actual process be the reverse of this, and the sun cool down the quicker, then man would, as a consequence of his scientific knowledge, tend in all probability to outlive the other forms of terrestrial life. in such a vista we can picture the regions of the earth towards the north and south becoming gradually more and more uninhabitable through cold, and human beings withdrawing before the slow march of the icy boundary, until the only regions capable of habitation would lie within the tropics. in such a struggle between man and destiny science would be pressed to the uttermost, in the devising of means to counteract the slow diminution of the solar heat and the gradual disappearance of air and water. by that time the axial rotation of our globe might possibly have been slowed down to such an extent that one side alone of its surface would be turned ever towards the fast dying sun. and the mind's eye can picture the last survivors of the human race, huddled together for warmth in a glass-house somewhere on the equator, waiting for the end to come. the mere idea of the decay and death of the solar system almost brings to one a cold shudder. all that sun's light and heat, which means so much to us, entirely a thing of the past. a dark, cold ball rushing along in space, accompanied by several dark, cold balls circling ceaselessly around it. one of these a mere cemetery, in which there would be no longer any recollection of the mighty empires, the loves and hates, and all that teeming play of life which we call history. tombstones of men and of deeds, whirling along forgotten in the darkness and silence. _sic transit gloria mundi._ in that brilliant flight of scientific fancy, the _time machine_, mr. h.g. wells has pictured the closing years of the earth in some such long-drawn agony as this. he has given us a vision of a desolate beach by a salt and almost motionless sea. foul monsters of crab-like form crawl slowly about, beneath a huge hull of sun, red and fixed in the sky. the rocks around are partly coated with an intensely green vegetation, like the lichen in caves, or the plants which grow in a perpetual twilight. and the air is now of an exceeding thinness. he dips still further into the future, and thus predicts the final form of life:-- "i saw again the moving thing upon the shoal--there was no mistake now that it was a moving thing--against the red water of the sea. it was a round thing, the size of a football perhaps, or it may be bigger, and tentacles trailed down from it; it seemed black against the weltering blood-red water, and it was hopping fitfully about." what a description of the "heir of all the ages!" to picture the end of our world as the result of a cataclysm of some kind, is, on the other hand, a form of speculation as intensely dramatic as that with which we have just been dealing is unutterably sad. it is not so many years ago, for instance, that men feared a sudden catastrophe from the possible collision of a comet with our earth. the unreasoning terror with which the ancients were wont to regard these mysterious visitants to our skies had, indeed, been replaced by an apprehension of quite another kind. for instance, as we have seen, the announcement in that biela's comet, then visible, would cut through the orbit of the earth on a certain date threw many persons into a veritable panic. they did not stop to find out the real facts of the case, namely, that, at the time mentioned, the earth would be nearly a month's journey from the point indicated! it is, indeed, very difficult to say what form of damage the earth would suffer from such a collision. in it passed, as we have seen, through the tail of the comet without any noticeable result. but the head of a comet, on the other hand, may, for aught we know, contain within it elements of peril for us. a collision with this part might, for instance, result in a violent bombardment of meteors. but these meteors could not be bodies of any great size, for the masses of comets are so very minute that one can hardly suppose them to contain any large or dense constituent portions. the danger, however, from a comet's head might after all be a danger to our atmosphere. it might precipitate, into the air, gases which would asphyxiate us or cause a general conflagration. it is scarcely necessary to point out that dire results would follow upon any interference with the balance of our atmosphere. for instance, the well-known french astronomer, m. camille flammarion,[ ] has imagined the absorption of the nitrogen of the air in this way; and has gone on to picture men and animals reduced to breathing only oxygen, first becoming excited, then mad, and finally ending in a perfect saturnalia of delirium. lastly, though we have no proof that stars eventually become dark and cold, for human time has so far been all too short to give us even the smallest evidence as to whether heat and light are diminishing in our own sun, yet it seems natural to suppose that such bodies must at last cease their functions, like everything else which we know of. we may, therefore, reasonably presume that there are dark bodies scattered in the depths of space. we have, indeed, a suspicion of at least one, though perhaps it partakes rather of a planetary nature, namely, that "dark" body which continually eclipses algol, and so causes the temporary diminution of its light. as the sun rushes towards the constellation of lyra such an extinguished sun may chance to find itself in his path; just as a derelict hulk may loom up out of the darkness right beneath the bows of a vessel sailing the great ocean. unfortunately a collision between the sun and a body of this kind could not occur with such merciful suddenness. a tedious warning of its approach would be given from that region of the heavens whither our system is known to be tending. as the dark object would become visible only when sufficiently near our sun to be in some degree illuminated by his rays, it might run the chance at first of being mistaken for a new planet. if such a body were as large, for instance, as our own sun, it should, according to mr. gore's calculations, reveal itself to the telescope some fifteen years before the great catastrophe. steadily its disc would appear to enlarge, so that, about nine years after its discovery, it would become visible to the naked eye. at length the doomed inhabitants of the earth, paralysed with terror, would see their relentless enemy shining like a second moon in the northern skies. rapidly increasing in apparent size, as the gravitational attractions of the solar orb and of itself interacted more powerfully with diminishing distance, it would at last draw quickly in towards the sun and disappear in the glare. it is impossible for us to conceive anything more terrible than these closing days, for no menace of catastrophe which we can picture could bear within it such a certainty of fulfilment. it appears, therefore, useless to speculate on the probable actions of men in their now terrestrial prison. hope, which so far had buoyed them up in the direst calamities, would here have no place. humanity, in the fulness of its strength, would await a wholesale execution from which there could be no chance at all of a reprieve. observations of the approaching body would have enabled astronomers to calculate its path with great exactness, and to predict the instant and character of the impact. eight minutes after the moment allotted for the collision the resulting tide of flame would surge across the earth's orbit, and our globe would quickly pass away in vapour. and what then? a nebula, no doubt; and after untold ages the formation possibly from it of a new system, rising phoenix-like from the vast crematorium and filling the place of the old one. a new central sun, perhaps, with its attendant retinue of planets and satellites. and teeming life, perchance, appearing once more in the fulness of time, when temperature in one or other of these bodies had fallen within certain limits, and other predisposing conditions had supervened. "the world's great age begins anew, the golden years return, the earth doth like a snake renew her winter weeds outworn: heaven smiles, and faiths and empires gleam like wrecks of a dissolving dream. a brighter hellas rears its mountains from waves serener far; a new peneus rolls his fountains against the morning star; where fairer tempes bloom, there sleep young cyclads on a sunnier deep. a loftier argo cleaves the main, fraught with a later prize; another orpheus sings again, and loves, and weeps, and dies; a new ulysses leaves once more calypso for his native shore. * * * * * oh cease! must hate and death return? cease! must men kill and die? cease! drain not to its dregs the urn of bitter prophecy! the world is weary of the past,-- oh might it die or rest at last!" [ ] see his work, _la fin du monde_, wherein the various ways by which our world may come to an end are dealt with at length, and in a profoundly interesting manner. index achromatic telescope, , adams, , , aerial telescopes, , agathocles, eclipse of, agrippa, camillus, ahaz, dial of, air, airy, sir g.b., al gûl, al sufi, , , , alcor, alcyone, aldebaran, , , , algol, , - , , , alpha, centauri, - , , - , , alpha crucis, alps, lunar, altair, altitude of objects in sky, aluminium, amos viii. , anderson, t.d., - andromeda (constellation), , ; great nebula in, , andromedid meteors, anglo-saxon chronicle, - anighito meteorite, annular eclipse, - , , , annular nebula in lyra, - annulus, ansæ, - anticipation in discovery, - apennines, lunar, aphelion, apparent enlargement of celestial objects, - apparent size of celestial objects deceptive, , apparent sizes of sun and moon, variations in, , , aquila (constellation), arabian astronomers, , arago, , arc, degrees minutes and seconds of, arcturus, , , , argelander, argo (constellation), aristarchus of samos, aristarchus (lunar crater), aristophanes, aristotle, , , arrhenius , - assyrian tablet, asteroidal zone, analogy of, to saturn's rings, asteroids (or minor planets), - , - , ; discovery of the, , ; wolf's method of discovering, - astrology, _astronomical essays_, , astronomical society, royal, _astronomy, manual of_, atlantic ocean, parallelism of opposite shores, - atlas, the titan, atmosphere, absorption by earth's, - ; ascertainment of, by spectroscope, - , ; height of earth's, , ; of asteroids, ; of earth, , , - , , , , ; of mars, , , ; of mercury, ; of moon, - , , - ; of jupiter, ; of planets, ; of saturn's rings, "atmosphere" of the stars, atmospheric layer and "glass-house" compared, , august meteors (perseids), auriga (constellation), - , , ; new star in, aurigæ, [b] (beta), , , aurora borealis, , , australia, suggested origin of, axis, - ; of earth, , ; small movement of earth's, - babylonian tablet, babylonian idea of the moon, bacon, roger, bacubirito meteorite, bagdad, baily, francis, "baily's beads," , , - , bailly (lunar crater), ball, sir robert, barnard, e.e., , , - , , "bay of rainbows," bayer's classification of stars, , - bayeux tapestry, bear, great (constellation). _see_ ursa major; little, _see_ ursa minor beehive (præsepe), beer, belopolsky, "belt" of orion, belt theory of milky way, belts of jupiter, bergstrand, berlin star chart, bessel, , , beta ([b]) lyræ, beta ([b]) persei. _see_ algol betelgeux, bible, eclipses in, biela's comet, - , - , bielids, , - billion, - binary stars, spectroscopic, - , ; visual, , - "black drop," - "black hour," "black saturday," blood, moon in eclipse like, blue (rays of light), , bode's law, - , - bolometer, bond, g.p., , bonpland, boötes (constellation), , bradley, brahe, tycho, , brédikhine's theory of comets' tails, - , bright eclipses of moon, , british association for the advancement of science, _british astronomical association, journal of_, british museum, bull (constellation). _see_ taurus; "eye" of the, ; "head" of the, burgos, busch, cæsar, julius, , , , , , , calcium, , callisto, - cambridge, , , , campbell, canali, "canals" of mars, - , - cancer (constellation), canes venatici (constellation), , canis major (constellation), , - ; minor, - canopus, , - , capella, , , , , , , carbon, carbon dioxide. _see_ carbonic acid gas carbonic acid gas, , , - carnegie institution, solar observatory of, cassegrainian telescope, , cassini, j.d., , "cassini's division" in saturn's ring, , cassiopeia (constellation), , , , cassiopeiæ, [ê] (eta), cassiopeia's chair, cassius, dion, castor, , , catalogues of stars, , - , centaur. _see_ centaurus centaurus (constellation), , centre of gravity, , - , ceres, diameter of, , ceti, omicron (or mira), - cetus, or the whale (constellation), chaldean astronomers, , challis, - chamberlin, "chambers of the south," chandler, charles v., "charles' wain," chemical rays, chinese and eclipses, chloride of sodium, chlorine, , christ, birth of, christian era, first recorded solar eclipse in, chromatic aberration, chromosphere, - , - , - , - circle, - clark, alvan, & sons, - , claudius, emperor, clavius (lunar crater), clerk maxwell, "clouds" (of aristophanes), clustering power, clusters of stars, , , , coal sacks. _see_ holes in milky way coelostat, coggia's comet, colour, production of, in telescopes, - , , collision of comet with earth, - ; of dark star with sun, - ; of stars, , columbus, coma berenices (constellation), , comet, first discovery of by photography, ; first orbit calculated, ; first photograph of, - ; furthest distance seen, ; passage of among satellites of jupiter, ; passage of earth and moon through tail of, , comet of a.d., ; , - ; , , ; , - ; , , , ; , - ; , , , ; , ; , comets, - , , chaps. xix. and xx., - ; ancient view of, - ; captured, - ; chinese records of, - ; composition of, ; contrasted with planets, ; families of, - , ; meteor swarms and, ; revealed by solar eclipses, - ; tails of, , , , - common, telescopes of dr. a.a., conjunction, constellations, , - , , contraction theory of sun's heat, - , cook, captain, cooke, copernican system, , , , - , , copernicus, , , , , - , copernicus (lunar crater), , copper, corder, h., corona, - , , - , , - , ; earliest drawing of, ; earliest employment of term, ; earliest mention of, ; earliest photograph of, ; illumination given by, ; possible change in shape of during eclipse, - ; structure of, - ; variations in shape of, corona borealis (constellation), coronal matter, ; streamers, - , - coronium, , , cotes, coudé, equatorial, cowell, p.h., , crabtree, crape ring of saturn, - craterlets on mars, craters (ring-mountains) on moon, - , , ; suggested origin of, - , crawford, earl of, crecy, supposed eclipse at battle of, - crescent moon, , crommelin, a.c.d., , crossley reflector, , - crown glass, crucifixion, darkness of, crucis, [a] (alpha), crux, or "southern cross" (constellation), - , cycle, sunspot, - , , - cygni, , , cygnus, or the swan (constellation), , daniel's comet of , danzig, dark ages, , , dark eclipses of moon, , - dark matter in space, dark meteors, - dark stars, - , , , - "darkness behind the stars," darwin, sir g.h., davis, dawes, dearborn observatory, death from fright at eclipse, debonnaire, louis le, , deimos, deity, symbol of the, "demon star." _see_ algol denebola, denning, w.f., densities of sun and planets, density, deslandres, diameters of sun and planets, disappearance of moon in lunar eclipse, , - disc, "disc" theory. _see_ "grindstone" theory discoveries, independent, discovery, anticipation in, - ; indirect methods of, "dipper," the, ; the "little," distance of a celestial body, how ascertained, - ; of sun from earth, how determined, , distances of planets from sun, distances of sun and moon, relative, dog, the greater. _see_ canis major; the lesser, _see_ canis minor "dog star," , dollond, john, - donati's comet, , doppler's method, , , , - dorpat, double canals of mars, - , - double planet, earth and moon a, double stars, douglass, "dreams, lake of," dumb-bell nebula, earth, , , , , , , chap. xv., ; cooling of, ; diameter of, ; interior of, ; mean distance of from sun, ; rigidity of, ; rotation of, , , - , ; shape of, ; "tail" to, "earthlight," or "earthshine," earth's axis, precessional movement of, - , , - earth's shadow, circular shape of, , eclipse, eclipse knowledge, delay of, eclipse party, work of, eclipse of sun, advance of shadow in total, ; animal and plant life during, ; earliest record of total, ; description of total, - ; duration of total, , ; importance of total, eclipses, ascertainment of dates of past, ; experience a necessity in solar, - ; of moon, - , chap. ix., ; photography in, ; prediction of future, ; recurrence of, - eclipses of sun, , - , chap. viii., - , ; a.d., ; , , ; , , ; , ; , - ; , , ; , ; , ; , ; , ; , ; , - ; , - ; , , ; , , ; , ; , - , - , - ; , ; , ; , ; , , - _eclipses, past and future_, egenitis, electric furnace, electric light, spectrum of, elements composing sun, - ellipses, , , - , - elliptic orbit, , ellipticity, elongation, eastern, , ; western, , encke's comet, , "end of the world," england, solar eclipses visible in, - , - epsilon, ([e]) lyræ, equator, equatorial telescope, equinoxes. _see_ precession of eros, - , , - ; discovery of, , , ; importance of, ; orbit of, , , , eruptive prominences, _esclistre_, ether, - , - europa, , evans, j.e., evening star, - , everest, mount, evershed, eye-piece, fabricius, faculæ, , fauth, faye, _fin du monde_, first quarter, "fixed stars," flagstaff, - , flammarion, camille, flamsteed, "flash spectrum," "flat," flint glass, focus, , "forty-foot telescope," foster, fraunhofer, french academy of sciences, froissart, "full moon" of laplace, galaxy. _see_ milky way. galilean telescope, galileo, , , , , , - , galle, , , ganymede, - gas light, spectrum of, gegenschein, - "gem" of meteor ring, gemini, or the twins (constellation), , - geminorum, [z] (zeta), geometrical groupings of stars, "giant" planet, , - gibbous, , gill, sir david, , , , - gold, goodricke, gore, j.e., , , , - , , - , , , granulated structure of photosphere, gravitation (or gravity), , - , , greek ideas, , , - , , , green (rays of light), greenwich observatory, - , , , gregorian telescope, - grimaldi (lunar crater), "grindstone" theory, - "groombridge, ," - , , groups of stars, - grubb, sir howard, _gulliver's travels_, hale, g.e., , half moon, , hall, asaph, hall, chester moor, halley, edmund, , , - , halley's comet, , - haraden hill, harvard, , harvest moon, - hawaii, heat rays, heidelberg, , height of lunar mountains, how determined, height of objects in sky, estimation of, helium, , , helmholtz, , hercules (constellation), herod the great, - herodotus, herschel, a.s., herschel, sir john, , herschel, sir william, , , - , , , , , , , - , - herschelian telescope, , hesper, hesperus, hevelius, hezekiah, hi, hindoos, hipparchus, , , , ho, holes in milky way, - holmes, oliver wendell, homer, horace, odes of, horizon, horizontal eclipse, horrox, , - hour glass sea, huggins, sir william, , , humboldt, "hunter's moon," huyghens, - , , - hyades, - , hydrocarbon gas, hydrogen, , , , , , , , ibrahim ben ahmed, ice-layer theory: mars, ; moon, , illusion theory of martian canals, imbrium, mare, inclination of orbits, - indigo (rays of light), inferior conjunction, , inferior planets, , , chap. xiv., instruments, pre-telescopic, - , international photographic survey of sky, - intra-mercurial planet, - _introduction to astronomy_, inverted view in astronomical telescope, - io, - iridum, sinus, iron, , _is mars habitable?_ jansen, janssen, , , japetus, jessenius, job, book of, johnson, s.j., , josephus, , juno, jupiter, , - , , , , , - , - , , , ; comet family of, - , ; discovery of eighth satellite, , ; eclipse of, by satellite, ; without satellites, - jupiter, satellites of, , , , , - ; their eclipses, - ; their occultations, , ; their transits, , kant, kapteyn, , keeler, , kelvin, lord, kepler, , , , , , , , kinetic theory, , , , , , , king, l.w., _knowledge_, labrador, lacus somniorum, "lake of dreams," lalande, , lampland, , langley, , laplace, , laputa, le maire, le verrier, , , - , lead, leibnitz mountains (lunar), leo (constellation), , - leonids, - , - lescarbault, lewis, t., lexell's comet, lick observatory, , , - , , , , , ; great telescope of, , , "life" of an eclipse of the moon, ; of the sun, - life on mars, lowell's views, - ; pickering's, ; wallace's, - light, no extinction of, - ; rays of, ; velocity of, , - ; white, "light year," , lindsay, lord, linné (lunar crater), liouville, lippershey, liquid-filled lenses, _locksley hall_, ; _sixty years after_, lockyer, sir norman, , , , loewy, , london, eclipses visible at, - , - longfellow, lowell observatory, , , - lowell, percival, , - , - lucifer, lynn, w.t., , lyra (constellation), , , - , , , mädler, , magellanic clouds, magnetism, disturbances of terrestrial, , magnitudes of stars, - major planets, - "man in the moon," _manual of astronomy_, maps of the moon, mare imbrium, mare serenitatis, mars, , - , - , , , , , - , ; compared with earth and moon, , ; polar caps of, - , ; satellites of, , - ; temperature of, , , - mass, ; of a star, how determined, masses of celestial bodies, how ascertained, ; of earth and moon compared, ; of sun and planets compared, maunder, e.w., , , maunder, mrs., e.w., , maxwell, clerk, mayer, tobias, , mcclean, f.k., mean distance, "medicean stars," mediterranean, eclipse tracks across, , melbourne telescope, melotte, p., mercator's projection, - mercury (the metal), mercury (the planet), , , - , - , , , chap. xiv.; markings on, ; possible planets within orbit of, - ; transit of, , , metals in sun, meteor swarms, - , , - meteors, , , , , chap. xxi. meteors beyond earth's atmosphere, - meteorites, - meteoritic hypothesis, metius, jacob, michell, , middle ages, , , middleburgh, milky way (or galaxy), , , , , - ; penetration of, by photography, million, , - minor planets. _see_ asteroids. mira ceti, - "mirk monday," mirror (speculum), , mizar, , monck, w.h.s., mongol emperors of india, moon, , chap. xvi.; appearance of, in lunar eclipse, , - ; diameter of, ; distance of, how ascertained, ; distance of, from earth, ; full, , , , , , , ; mass of, , ; mountains on, - ; how their height is determined, ; movement of, - ; new, , , , ; origin of, - ; plane of orbit of, ; possible changes on, - , ; "seas" of, , ; smallest detail visible on, ; volume of, morning star, - , moulton, f.r., , , , , , moye, multiple stars, musa-ben-shakir, mythology, neap-tides, nebulæ, - , , , ; evolution of stars from, - nebular hypothesis of laplace, - nebular hypotheses, chap. xxvii. nebulium, neison, neptune, , , , , , - , , , , ; discovery of, - , , , , - ; lalande and, ; possible planets beyond, , ; satellite of, , ; "year" in, - "new" (or temporary), stars, - newcomb, simon, , , , , - , newton, sir isaac, , , , - , , , , , newtonian telescope, , , , nineveh eclipse, - nitrogen, , , , northern crown, nova aurigæ, nova persei, - novæ. _see_ new (or temporary) stars nubeculæ, "oases" of mars, , object-glass, oblate spheroid, occultation, - , , _olaf, saga of king_, olbers, , , , "old moon in new moon's arms," olmsted, omicron (or "mira") ceti, - opposition, "optick tube," - , orange (rays of light), orbit of moon, plane of, orbits, , - , , , oriental astronomy, orion (constellation), , , - , ; great nebula in, , oxford, oxygen, , , , pacific ocean, origin of moon in, palitzch, pallas, , parallax, , , , , , paré, ambrose, - peal, s.e., peary, pegasus (constellation), penumbra of sunspot, perennial full moon of laplace, pericles, perrine, c.d., - , perseids, , - perseus (constellation), , , , phases of an inferior planet, , ; of the moon, , , - phlegon, eclipse of, - phobos, phoebe, retrograde motion of, , , phosphorescent glow in sky, phosphorus (venus), photographic survey of sky, international, - photosphere, - , piazzi, pickering, e.c., pickering, w.h., , - , - , , - pictor, "runaway star" in constellation of, - , , plane of orbit, , planetary nebulæ, , _planetary and stellar studies_, planetesimal hypothesis, - planetoids. _see_ asteroids planets, classification of, ; contrasted with comets, ; in ptolemaic scheme, ; relative distances of, from sun, - plato (lunar crater), pleiades, , - , pliny, , plough, , - , plutarch, , , , "pointers," polaris. _see_ pole star pole of earth, precessional movement of, - , , - pole star, , , , - , - poles, , - ; of earth, speed of point at, pollux, , posidonius, powell, sir george baden, præsepe (the beehive), precession of the equinoxes, , , - pre-telescopic notions, primaries, _princess, the_ (tennyson), princeton observatory, prism, prismatic colours, , procyon, , , , prominences, solar, , , , - , ; first observation of, with spectroscope, , , proper motions of stars, , - , , - ptolemæus (lunar crater), - , ptolemaic idea, ; system, , , , - ptolemy, , , , , puiseux, p., pulkowa telescope, puppis, v., quiescent prominences, radcliffe observer, "radiant," or radiant point, radiation from sun, , radium, , rainbow, "rainbows, bay of," rambaut, a., ramsay, sir william, rays (on moon), recurrence of eclipses, - red (rays of light), , , , red spot, the great, reflecting telescope, - ; future of, reflector. _see_ reflecting telescope refracting and reflecting telescopes contrasted, refracting telescope, - , - ; limits to size of, - refraction, , - refractor, _see_ refracting telescope regulus, , retrograde motion of phoebe, , , "reversing layer," , , , - revival of learning, revolution, ; of earth around sun, - ; periods of sun and planets, riccioli, rice-grain structure of photosphere, rigel, , rills (on moon), ring-mountains of moon. _see_ craters "ring" nebulæ, , "ring with wings," rings of saturn, , - , - , ritchey, g.w., roberts, a.w., , roberts, isaac, "roche's limit," roemer, roman history, eclipses in, - romulus, röntgen, rosse, great telescope of lord, , rotation, ; of earth, , - , ; of sun, , , - , ; periods of sun and planets, royal society of london, - , rubicon, passage of the, "runaway" stars, , , sagittarius (constellation), salt, spectrum of table, samarcand, "saros," chaldean, - , satellites, - , saturn, , , , , , - , ; comet family of, ; a puzzle to the early telescope observers, - ; retrograde motion of satellite phoebe, , , ; ring system of, ; satellites of, , - ; shadows of planet on rings and of rings on planet, schaeberle, - , , schiaparelli, , , schickhard (lunar crater), schmidt, schönfeld, schuster, schwabe, scotland, solar eclipses visible in, - , sea of serenity, "sea of showers," "seas" of moon, , seasons on earth, - ; on mars, secondary bodies, seneca, , _septentriones_, serenitatis, mare, "seven stars," "shadow bands," shadow of earth, circular shape of, - shadows on moon, inky blackness of, shakespeare, , sheepshanks telescope, "shining fluid" of sir w. herschel, "shooting stars." _see_ meteors short (of edinburgh), "showers, sea of," sickle of leo, - , siderostat, silver, silvered mirrors for reflecting telescopes, sinus iridum, sirius, , , - , - , , - , ; companion of, ; stellar magnitude of, size of celestial bodies, how ascertained, skeleton telescopes, sky, international photographic survey of, - ; light of the, slipher, e.c., , smithsonian institution of washington, snow on mars, sodium, , , sohag, solar system, - , - ; centre of gravity of, ; decay and death of, somniorum, lacus, sound, , , south pole of heavens, , , - southern constellations, - southern cross. _see_ crux space, spain, early astronomy in, ; eclipse tracks across , - spectroheliograph, spectroscope, , , - , - , , ; prominences first observed with, , , spectrum of chromosphere, - ; of corona, ; of photosphere, ; of reversing layer, , ; solar, - , , speculum, , ; metal, spherical bodies, spherical shape of earth, proofs of, - spherical shapes of sun, planets, and satellites, spiral nebulæ, - , - spring balance, spring tides, spy-glass, "square of the distance," - stannyan, captain, star, mass of, how determined, ; parallax of, first ascertained, , stars, the, , , , _et seq._; brightness of, , ; distances between, - ; distances of some, , , ; diminution of, below twelfth magnitude, ; evolution of, from nebulæ, - ; faintest magnitude of, ; number of those visible altogether, ; number of those visible to naked eye, "steam cracks," steinheil, stellar system, estimated extent of, - ; an organised whole, ; limited extent of, - , ; possible disintegration of, stiklastad, eclipse of, stone age, stoney, g.j., , stonyhurst observatory, _story of the heavens_, streams of stars, kapteyn's two, stroobant, stukeley, sulphur, summer, , sun, chaps xii. and xiii.; as a star, , , ; as seen from neptune, , ; chemical composition of, - ; distance of, how ascertained, , ; equator of, - , ; gravitation at surface of, , - ; growing cold of, - ; mean distance of, from earth, , ; motion of, through space, - , ; not a solid body, ; poles of, ; radiations from, ; revolution of earth around, - ; stellar magnitude of, - ; variation in distance of, , sunspots, , , - , - , - , ; influence of earth on, suns and possible systems, , superior conjunction, - superior planets, , , - , swan (constellation). _see_ cygnus swift, dean, "sword" of orion, , syrtis major. _see_ hour glass sea "_systematic_ parallax," systems, other possible, , tails of comets, tamerlane, taurus (constellation), , - , "tears of st. lawrence," tebbutt's comet, - telescope, , , - , ; first eclipse of moon seen through, ; of sun, telescopes, direct view reflecting, ; gigantic, ; great constructors of, - ; great modern, - tempel's comet, temperature on moon, ; of sun, temporary (or new) stars, - tennyson, lord, , , terrestrial planets, - terrestrial telescope, thales, eclipse of, themis, "tidal drag," , , , tide areas, - tides, - , - _time machine_, tin, titan, titius, total phase, - totality, ; track of, trail of a minor planet, - transit, , - ; of mercury, , , ; of venus, , - , , trifid nebula, triple stars, tubeless telescopes, - , tubes used by ancients, tuttle's comet, twilight, , twinkling of stars, twins (constellation). _see_ gemini tycho brahe, , tycho (lunar crater), ulugh beigh, umbra of sunspot, - universe, early ideas concerning, - , , , universes, possibility of other, - uranus, - , , , , , ; comet family of, ; discovery of, , , ; rotation period of , ; satellites of, , ; "year" in, - ursa major (constellation), , , , , ; minor, , , - ursæ majoris, ([z]) zeta. _see_ mizar variable stars, - variations in apparent sizes of sun and moon, , , vault, shape of the celestial, - vega, , , , - , , , , , , vegetation on mars, , - ; on moon, venus, , , , , , - , , chap. xiv., , ; rotation period of, , very, f.w., vesta, , violet (rays of light), - , virgil, volcanic theory of lunar craters, - , volume, volumes of sun and planets compared, - "vulcan," wallace, a.r., on mars, - water, lack of, on moon, - water vapour, , , wargentin, warner and swasey co., weather, moon and, - weathering, webb, rev. t.w., weight, , - wells, h.g., whale (constellation). _see_ cetus whewell, willamette meteorite, wilson, mount, wilson, w.e., "winged circle" (or "disc"), winter, , witt, wolf, max, - , wright, thomas, , wybord, xenophon, year, "year" in uranus and neptune, - year, number of eclipses in a, "year of the stars," yellow (rays of light), - , yerkes telescope great, , young, , , zenith, zinc, zodiacal light, zone of asteroids, - , the end printed by ballantyne, hanson & co. edinburgh & london the science of to-day series _with many illustrations. extra crown vo. s. net._ botany of to-day. a popular account of the evolution of modern botany. by prof. g.f. scott elliot, m.a., b.sc., author of "the romance of plant life," _&c. &c._ "one of the books that turn botany from a dryasdust into a fascinating study."--_evening standard._ aerial navigation of to-day. a popular account of the evolution of aeronautics. by charles c. turner. 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"mr. gibson has given us one of the best examples of popular scientific exposition that we remember seeing. his aim has been to produce an account of the chief modern applications of electricity without using technical language or making any statements which are beyond the comprehension of any reader of ordinary intelligence. in this he has succeeded to admiration, and his book may be strongly commended to all who wish to realise what electricity means and does in our daily life."--_the tribune._ seeley & co., ltd., great russell street. the romance of modern electricity describing in non-technical language what is known about electricity & many of its interesting applications by charles r. gibson, a.i.e.e. author or "electricity of to-day," etc. _extra crown vo._ _with illustrations and diagrams._ _s._ "everywhere mr. charles r. gibson makes admirable use of simple analogies which bespeak the practised lecturer, and bring the matter home without technical detail. the attention is further sustained by a series of surprises. the description of electric units, the volt, the ohm, and especially the ampere, is better than we have found in more pretentious works."--_academy._ "mr. gibson's style is very unlike the ordinary text-book. it is fresh, and is non-technical. its facts are strictly scientific, however, and thoroughly up to date. if we wish to gain a thorough knowledge of electricity pleasantly and without too much trouble on our own part, we will read mr. gibson's 'romance.'"--_expository times._ "a book which the merest tyro totally unacquainted with elementary electrical principles can understand, and should therefore especially appeal to the lay reader. especial interest attaches to the chapter on wireless telegraphy, a subject which is apt to 'floor' the uninitiated. the author reduces the subject to its simplest aspect, and describes the fundamental principles underlying the action of the coherer in language so simple that anyone can grasp them."--_electricity._ "contains a clear and concise account of the various forms in which electricity is used at the present day, and the working of the telephone, wireless telegraphy, tramcars, and dynamos is explained with the greatest possible lucidity, while the marvels of the x-rays and of radium receive their due notice. now that electricity plays such an all-important part in our daily life, such a book as this should be in the hands of every boy. indeed, older people would learn much from its pages. for instance, how few people could explain the principles of wireless telegraphy in a few words if suddenly questioned on the subject. the book is well and appropriately illustrated."--_graphic._ "mr. gibson sets out to describe in non-technical language the marvellous discoveries and adaptation of this pervasive and powerful essence, and being a most thorough master of the subject, he leads the reader through its mazes with a sure hand. throughout he preserves a clear and authoritative style of exposition which will be understood by any intelligent reader."--_yorkshire observer._ "a popular and eminently readable manual for those interested in electrical appliances. it describes in simple and non-technical language what is known about electricity and many of its interesting applications. there are a number of capital illustrations and diagrams which will help the reader greatly in the study of the book."--_record._ seeley & co., ltd., great russell street. the romance of savage life describing the habits, customs, everyday life, &c., of primitive man by prof. g.f. scott elliot, m.a., b.sc., &c. _with thirty illustrations._ _extra crown vo._ _s._ "mr. scott elliot has hit upon a good idea in this attempt to set forth the life of the primitive savage. on the whole, too, he has carried it out well and faithfully.... we can recommend the book as filling a gap."--_athenæum._ "a readable contribution to the excellent series of which it forms a part. mr. scott elliot writes pleasantly ... he possesses a sufficiently vivid imagination to grasp the relation of a savage to his environment."--_nature._ "there are things of remarkable interest in this volume, and it makes excellent reading and represents much research."--_spectator._ the romance of plant life describing the curious and interesting in the plant world by prof. g.f. scott elliot, m.a., b.sc., &c. _with thirty-four illustrations._ _extra crown vo._ _s._ "the author has worked skilfully into his book details of the facts and inferences which form the groundwork of modern botany. the illustrations are striking, and cover a wide field of interest, and the style is lively."--_athenæum._ "in twenty-nine fascinating, well-printed, and well-illustrated chapters, prof. scott elliot describes a few of the wonders of plant life. a very charming and interesting volume."--_daily telegraph._ "mr. scott elliot is of course a well-known authority on all that concerns plants, and the number of facts he has brought together will not only surprise but fascinate all his readers."--_westminster gazette._ seeley & co., ltd., great russell street. the romance of insect life describing the curious & interesting in the insect world by edmund selous author of "the romance of the animal world," etc. _with sixteen illustrations._ _extra crown vo._ _s._ "an entertaining volume, one more of a series which seeks with much success to describe the wonders of nature and science in simple, attractive form."--_graphic._ "offers most interesting descriptions of the strange and curious inhabitants of the insect world, sure to excite inquiry and to foster observation. there are ants white and yellow, locusts and cicadas, bees and butterflies, spiders and beetles, scorpions and cockroaches--and especially ants--with a really scientific investigation of their wonderful habits not in dry detail, but in free and charming exposition and narrative. an admirable book to put in the hands of a boy or girl with a turn for natural science--and whether or not."--_educational times._ "both interesting and instructive. such a work as this is genuinely educative. there are numerous illustrations."--_liverpool courier._ "with beautiful original drawings by carton moore park and lancelot speed, and effectively bound in dark blue cloth, blazoned with scarlet and gold."--_lady._ "admirably written and handsomely produced. mr. selous's volume shows careful research, and the illustrations of insects and the results of their powers are well done."--_world._ the romance of modern mechanism interesting descriptions in non-technical language of wonderful machinery, mechanical. devices, & marvellously delicate scientific instruments by archibald williams, b.a., f.r.g.s. author of "the romance of modern exploration," etc. _with twenty-six illustrations._ _extra crown vo._ _s._ "no boy will be able to resist the delights of this book, full to the brim of instructive and wonderful matter."--_british weekly._ "this book has kept your reviewer awake when he reasonably expected to be otherwise engaged. we do not remember coming across a more fascinating volume, even to a somewhat blasé reader whose business it is to read all that comes in his way. the marvels miracles they should be called, of the modern workshop are here exploited by mr. williams for the benefit of readers who have not the opportunity of seeing these wonders or the necessary mathematical knowledge to understand a scientific treatise on their working. only the simplest language is used and every effort is made, by illustration or by analogy, to make sufficiently clear to the non-scientific reader how the particular bit of machinery works and what its work really is. delicate instruments, calculating machines, workshop machinery, portable tools, the pedrail, motors ashore and afloat, fire engines, automatic machines, sculpturing machines--these are a few of the chapters which crowd this splendid volume."--_educational news._ "it is difficult to make descriptions of machinery and mechanism interesting, but mr. williams has the enviable knack of doing so, and it is hardly possible to open this book at any page without turning up something which you feel you must read; and then you cannot stop till you come to the end of the chapter."--_electricity._ "this book is full of interest and instruction, and is a welcome addition to messrs. seeley and company's romance series."--_leeds mercury._ "a book of absorbing interest for the boy with a mechanical turn, and indeed for the general reader."--_educational times._ "an instructive and well-written volume."--_hobbies._ seeley & co., ltd., great russell street. a catalogue of books on art, history, and general literature published by seeley, service & co ltd. great russell st. london _some of the contents_ crown library, the elzevir library, the events of our own times series illuminated series, the miniature library of devotion, the miniature portfolio monographs, the missions, the library of new art library, the portfolio monographs science of to-day series, the seeley's illustrated pocket library seeley's standard library story series, the "things seen" series, the _the publishers will be pleased to post their complete catalogue or their illustrated miniature catalogue on receipt of a post-card_ catalogue of books _arranged alphabetically under the names of authors and series_ abbott, rev. e.a., d.d. how to parse. an english grammar. fcap. vo, s. d. how to tell the parts of speech. an introduction to english grammar. fcap. vo, s. how to write clearly. rules and exercises on english composition. s. d. latin gate, the. a first latin translation book. crown vo, s. d. via latina. a first latin grammar. crown vo, s. d. abbott, rev. e.a., and sir j.r. seeley. english lessons for english people. crown vo, s. d. ady, mrs. _see_ cartwright, julia. À kempis, thomas. of the imitation of christ. with illuminated frontispiece and title page, and illuminated sub-titles to each book. in white or blue cloth, with inset miniatures. gilt top; crown vo, s. nett; also bound in same manner in real classic vellum. each copy in a box, s. d. nett; antique leather with clasps, s. d. nett. "it may well be questioned whether the great work of thomas à kempis has ever been presented to better advantage."--_the guardian._ anderson, prof. w. japanese wood engravings. coloured illustrations. super-royal vo, sewed, s. d. nett; half-linen, s. d. nett; also small to, cloth, s. nett; lambskin, s. nett. armstrong, sir walter. the art of velazquez. illustrated. super-royal vo, s. d. nett. the life of velazquez. illustrated. super-royal vo, s. d. nett. velazquez. a study of his life and art. with eight copper plates and many minor illustrations. super-royal vo, cloth, s. nett. thomas gainsborough. illustrated. super-royal vo, half-linen, s. d. nett. also new edition small to, cloth, s. nett; leather, s. nett and s. nett. the peel collection and the dutch school of painting. with illustrations in photogravure and half-tone. super-royal vo, sewed, s. nett; cloth, s. nett. w.q. orchardson. super-royal vo, sewed, s. d.; half-linen, s. d. nett. augustine, s. confessions of s. augustine. with illuminated pages. in white or blue cloth, gilt top, crown vo, s. nett; also in vellum, s. d. nett. baker, captain b. granville the passing of the turkish empire in europe. with thirty-two illustrations. demy vo, s. nett. baring-gould, rev. s. family names and their story. demy vo, s. d. nett. s. nett. bedford, rev. w.k.r. malta and the knights hospitallers. super-royal vo, sewed, s. d. nett; half-linen, s. d. nett. benham, rev. canon d.d., f.s.a. the tower of london. with four plates in colours and many other illustrations. super-royal vo, sewed, s. nett; cloth, s. nett. mediæval london. with a frontispiece in photogravure, four plates in colour, and many other illustrations. super-royal vo, sewed, s. nett; cloth, gilt top, s. nett. also extra crown vo, s. d. nett. old st. paul's cathedral. with a frontispiece in photogravure, four plates printed in colour, and many other illustrations. super-royal vo, sewed, s. nett, or cloth, gilt top, s. nett. bennett, edward. the post office and its story. an interesting account of the activities of a great government department. with twenty-five illustrations. ex. crn. vo, s. nett. bickersteth, rev. e. family prayers for six weeks. crown vo, s. d. a companion to the holy communion. mo, cloth, s. binyon, laurence. dutch etchers of the seventeenth century. illustrated. super-royal vo, sewed, s. d.; half-linen, s. d. nett. john crome and john sell cotman. illustrated. super-royal vo sewed, s. d. nett. birch, g.h. london on thames in bygone days. with four plates printed in colour and many other illustrations. super-royal vo, sewed, s. nett; cloth, s. nett. bridges, rev. c. an exposition of psalm cxix. crown vo, s. butcher, e.l. things seen in egypt. with fifty illustrations. small to, cloth, s. nett; lambskin, s. nett; velvet leather, in box, s. nett. poems, s. d. nett. cachemaille, rev. e.p., m.a. xxvi present-day papers on prophecy. an explanation of the visions of daniel and of the revelation, on the continuous historic system. with maps and diagrams. pp. s. nett. cartwright, julia. jules bastien-lepage. super-royal vo, sewed, s. d.; cloth, s. d. nett. sacharissa. some account of dorothy sidney, countess of sunderland, her family and friends. with five portraits. demy vo, s. d. raphael in rome. illustrated. super-royal vo, sewed, s. d.; half-linen, s. d. nett; also in small to, cloth, s. nett; leather, s. nett and s. nett. the early work of raphael. illustrated. super-royal vo, sewed s. d.; half-linen, s. d. also new edition, revised, in small to, in cloth, s. nett; leather, s. nett. raphael: a study of his life and work. with eight copper plates and many other illustrations. super-royal vo, s. d. nett. cesaresco, the countess martinengo the liberation of italy. with portraits on copper. crown vo, s. chatterton, e. keble. fore and aft. the story of the fore and aft rig from the earliest times to the present day. sq. ex. royal vo. with illustrations and coloured frontispiece by c. dixon, r.i. s. nett. through holland in the "vivette." the cruise of a -tonner from the solent to the zuyder zee, through the dutch waterways. with sixty illustrations and charts, s. nett. chitty, j.r. things seen in china. with fifty illustrations. small to; cloth, s.; leather, s.; velvet leather in a box, s. nett. choral service-book for parish churches, the. compiled and edited by j.w. elliott, organist and choirmaster of st. mark's, hamilton terrace, london. with some practical counsels taken by permission from "notes on the church service," by bishop walsham how. a. royal vo, sewed, s.; cloth, s. d. b. mo, sewed, d.; cloth, d. _the following portions may be had separately:_-- the ferial and festal responses and the litany. arranged by j.w. elliott. sewed, d. the communion service, kyrie, credo, sanctus, and gloria in excelsis. set to music by dr. j. naylor, organist of york minster. sewed, d. church, sir arthur h., f.r.s. josiah wedgwood, master potter. with many illustrations. super-royal vo, sewed, s. nett; cloth, s. nett; also small to, cloth, s. nett; leather, s. and s. nett. the chemistry of paints and painting. third edition. crown vo, s. church, rev. a.j. nicias, and the sicilian expedition. crown vo, s. d. for other books by professor church see complete catalogue. clark, j.w., m.a. cambridge. with a coloured frontispiece and many other illustrations by a. brunet-debaines and h. toussaint &c. extra crown vo, s.; also crown vo, cloth, s. nett; leather, s.; special leather, in box, s. nett. cody, rev. h.a. an apostle of the north. the biography of the late bishop bompas, first bishop of athabasca, and with an introduction by the archbishop of ruperts-land. with illustrations. demy vo, s. d. nett. s. nett. corbin, t.w. engineering of to-day. with seventy-three illustrations and diagrams. extra crown vo, s. nett. mechanical inventions of to-day. ex. crown vo; with ninety-four illustrations, s. nett. cornish, c.j. animals of to-day: their life and conversation. with illustrations from photographs by c. reid of wishaw. crown vo, s. the isle of wight. illustrated. super-royal vo, sewed, s. d. nett; half-linen, s. d. nett; also a new edition, small to, cloth, s.; leather, s. and s. life at the zoo. notes and traditions of the regent's park gardens. illustrated from photographs by gambier bolton. fifth edition. crown vo, s. the naturalist on the thames. many illustrations. demy vo, s. d. the new forest. super-royal vo, sewed, s. d. nett; half-linen, s. d. nett; also new edition, small to, cloth, s.; leather, s. nett; and special velvet leather, each copy in a box, s. the new forest and the isle of wight. with eight plates and many other illustrations. super-royal vo, s. d. nett. nights with an old gunner, and other studies of wild life. with sixteen illustrations by lancelot speed, charles whymper, and from photographs. crown vo, s. * * * * * the crown library a series of notable copyright books issued in uniform binding. extra crown vo. with many illustrations, s. nett. _just issued. second and cheaper edition._ swann, a.j. fighting the slave hunters in central africa. a record of twenty-six years of travel and adventure round the great lakes, and of the overthrow of tip-pu-tib, rumaliza, and other great slave traders. with illustrations and a map, s. nett. _recently issued._ grubb, w. barbrooke. an unknown people in an unknown land. an account of the life and customs of the lengua indians of the paraguayan chaco, with adventures and experiences met with during twenty years' pioneering and exploration amongst them. with twenty-four illustrations and a map. extra crown vo, s. nett. fraser, sir a.h.l., k.c.s.i., m.a., ll.d., litt.d., ex-lieutenant-governor of bengal. among indian rajahs and ryots. a civil servants' recollections and impressions of thirty-seven years of work and sport in the central provinces and bengal. third edition, s. nett. cody, rev. h.a. an apostle of the north. the story of bishop bompas's life amongst the red indians & eskimo. third edition, s. nett. pennell, t.l., m.d., b.sc. among the wild tribes of the afghan frontier. a record of sixteen years' close intercourse with the natives of afghanistan and the north-west frontier. introduction by earl roberts. extra crown vo. twenty-six illustrations and map. fifth edition, s. net. * * * * * cust, lionel. the engravings of albert dürer. illustrated. super-royal vo, half-linen, s. d. nett. paintings and drawings of albert dürer. illustrated. super-royal vo, sewed, s. d. nett. albrecht dürer. a study of his life and work. with eight copper plates and many other illustrations. super-royal vo, s. d. davenport, cyril. cameos. with examples in colour and many other illustrations. super-royal vo, sewed, s. nett; cloth, s. nett. royal english bookbindings. with coloured plates and many other illustrations. super-royal vo, sewed, s. d.; cloth, s. d. davies, randall, f.s.a. english society of the eighteenth century in contemporary art. with four coloured and many other illustrations. super royal vo, sewed, s. nett; cloth, s. nett. dawson, rev. e.c. the life of bishop hannington. crown vo, paper boards, s. d.; or with map and illustrations, cloth, s. d. destrÉe, o.g. the renaissance of sculpture in belgium. illustrated. super-royal vo, sewed, s. d. nett; half-linen, s. d. nett. dolmage, cecil g., m.a., d.c.l., ll.d., f.r.a.s. astronomy of to-day. a popular account in non-technical language. with forty-six illustrations and diagrams. extra crown vo, s. nett. domville-fife, charles w. submarine engineering of to-day. extra crown vo, s. nett. elzevir library, the. selections from the choicest english writers. exquisitely illustrated, with frontispiece and title-page in colours by h.m. brock, and many other illustrations. half bound in cloth, coloured top, s. nett; full leather, s. d. nett; velvet leather, gilt edges, in a box, s. d. nett. volume i. fancy & humour of lamb. " ii. wit & imagination of disraeli. " iii. vignettes from oliver goldsmith. " iv. wit & sagacity of dr. johnson. " v. insight & imagination of john ruskin. " vi. vignettes of london life from dickens. " vii. xviiith century vignettes from thackeray. " viii. vignettes of country life from dickens. " ix. wisdom & humour of carlyle. "decidedly natty and original in get-up."--_the saturday review._ evans, willmott, m.d. medical science of to-day. ex. crn. vo; illustrations, s. nett. wilmot, eardley, rear-admiral s. our fleet to-day and its development during the last half century. with many illustrations. crown vo, s. events of our own times crown vo. with illustrations, s. each. the war in the crimea. by general sir e. hamley, k.c.b. the indian mutiny. by colonel malleson, c.s.i. the afghan wars, - , and - . by archibald forbes. our fleet to-day and its development during the last half-century. by rear-admiral s. eardley wilmot. the refounding of the german empire. by colonel malleson, c.s.i. the liberation of italy. by the countess martinengo cesaresco. great britain in modern africa. by edgar sanderson, m.a. the war in the peninsula. by a. innes shand. fletcher, w.y. bookbinding in france. coloured plates. super-royal, sewed, s. d. nett; half-linen, s. d. nett. forbes, archibald. the afghan wars of - and - . with four portraits on copper, and maps and plans. crown vo, s. fraser, sir andrew h.l. among indian rajahs and ryots. with illustrations and a map. demy vo, s. nett. third and cheaper edition, s. nett. fraser, donald. winning a primitive people. illustrated. extra crown vo, s. nett. fripp, sir alfred d., k.c.v.o., & r. thompson, f.r.c.s. human anatomy for art students. profusely illustrated with photographs and drawings by innes fripp, a.r.c.a. square extra crown vo, s. d. nett. frobenius, leo. the childhood of man. a popular account of the lives and thoughts of primitive races. translated by prof. a.h. keane, ll.d. with illustrations. demy vo, s. nett. fry, roger. discourses delivered to the students of the royal academy by sir joshua reynolds. with an introduction and notes by roger fry. with thirty-three illustrations. square crown vo, s. d. nett. gardner, j. starkie. armour in england. with eight coloured plates and many other illustrations. super-royal vo, sewed, s. d. nett. foreign armour in england. with eight coloured plates and many other illustrations. super-royal vo, sewed, s. d. nett. armour in england. with sixteen coloured plates and many other illustrations. the two parts in one volume. super-royal vo, cloth, gilt top, s. nett. garnett, r., ll.d. richmond on thames. illustrated. super-royal vo, sewed, s. d. nett. giberne, agnes. beside the waters of comfort. crown vo, s. d. gibson, charles r., f.r.s.e. electricity of to-day. its works and mysteries described in non-technical language. with illustrations. extra crown vo, s. nett. scientific ideas of to-day. a popular account in non-technical language of the nature of matter, electricity, light, heat, &c., &c. with illustrations. extra crown vo, s. nett. how telegraphs and telephones work. with many illustrations. crown vo, s. d. nett. the autobiography of an electron. with illustrations. long vo, s. d. nett. wireless telegraphy. with many illustrations. ex. crn. vo, s. nett. godley, a.d. socrates and athenian society in his day. crown vo, s. d. aspects of modern oxford. with many illustrations. crown vo, cloth, s. nett; lambskin, s. nett; velvet leather, in box, s. nett. golden reciter (_see_ james, prof. cairns.) gomes, edwin h., m.a. seventeen years among the sea dyaks of borneo. with illustrations and a map. demy vo, s. nett. grahame, george. claude lorrain. illustrated. super-royal vo, s. d. nett; half-linen, s. d. nett. griffith, m.e. hume. behind the veil in persia and turkish arabia. an account of an englishwoman's eight years' residence amongst the women of the east. with illustrations and a map. demy vo, s. nett. grindon, leo. lancashire. brief historical and descriptive notes. with many illustrations. crown vo, s. grubb, w. barbrooke (pioneer and explorer of the chaco). an unknown people in an unknown land. with sixty illustrations and a map. demy vo, s. nett. third and cheaper edition, s. a church in the wilds. illustrated. extra crown vo, s. nett. hadow, w.h. a croatian composer. notes toward the study of joseph haydn. crown vo, s. d. nett. studies in modern music. first series. berlioz, schumann, wagner. with an essay on music and musical criticism. with five portraits. crown vo, s. d. studies in modern music. second series. chopin, dvoràk, brahms. with an essay on musical form. with four portraits. crown vo, s. d. hamerton, p.g. the etchings of rembrandt, and dutch etchers of the seventeenth century. by p.g. hamerton and laurence binton. with eight copper plates and many other illustrations. super-royal vo, s. d. nett. the mount. narrative of a visit to the site of a gaulish city on mount beuvray. with a description of the neighbouring city of autun. crown vo, s. d. round my house. notes on rural life in peace and war. crown vo, with illustrations, s. d. nett. cheaper edition, s. nett. paris. illustrated. new edition. cloth, s. nett; leather, s. nett in special leather, full gilt, in box, s. nett. hamley, gen. sir e. the war in the crimea. with copper plates and other illustrations. crown vo, s. hanoum zeyneb (heroine of pierre loti's novel "les désenchantées.") a turkish woman's european impressions. edited by grace ellison. with a portrait by auguste rodin and other illustrations from photographs. crown vo, s. nett. hartley, c. gasquoine. things seen in spain. with fifty illustrations. small to, cloth, s.; leather, s.; velvet leather in a box, s. nett. haywood, capt. a.h.w. through timbuctu & across the great sahara. demy vo, with illustrations and a map. s. nett. henderson, major percy e. a british officer in the balkans. through dalmatia, montenegro, turkey in austria, magyarland, bosnia and herzegovina. with illustrations and a map. gilt top. demy vo, s. nett. herbert, george. the temple. sacred poems and ejaculations. the text reprinted from the first edition. with seventy-six illustrations after albert dÜrer, holbrin, and other masters. crown vo, cloth, s. nett; 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"a more admirable book of its kind could not well be desired."--_liverpool courier._ the golden humorous reciter. edited, and with a practical introduction, by cairns james, professor of elocution at the royal college of music and the guildhall school of music. a volume of recitations and readings selected from the writings of f. anstey, j.m. barrie, s.r. crockett, jerome k. jerome, barry pain, a.w. pinero, owen seaman, g.b. shaw, &c. &c. extra crown vo, over pages, cloth, s. d.; also a thin paper edition, with gilt edges, s. * * * * * the illuminated series new binding. bound in antique leather with metal clasps. with illuminated frontispiece and title-page, and other illuminated pages. finely printed at the ballantyne press, edinburgh. crown vo. each copy in a box, s. d. nett. also in real classic vellum. each copy in a box. s. d. nett. the confessions of s. augustine. of the imitation of christ. by thomas À kempis. the sacred seasons. by the bishop of durham. also cloth, s. and s. d. nett. joy, bedford. a synopsis of roman history. crown vo, s. keane, prof. a.h. 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(this volume is only to be had in this series in leather, s. nett. for other editions of this book, see below.) * * * * * solomon, solomon j., r.a. the practice of oil painting and drawing. with illustrations. s. nett. speed, harold. the practice and science of drawing. with ninety-six illustrations and diagrams. square extra crown vo, s. * * * * * the standard library extra crown vo, with many illustrations. price s. d. nett. lady mary wortley montagu. by a.r. ropes. mrs. thrale. by l.b. seeley. round my house. by p.g. hamerton. fanny burney & her friends. by l.b. seeley. story series, the. extra crown vo, s. nett. the post office and its story. by edward bennett. with illus. family names and their story. by the rev. s. baring gould. the press and its story. by james d. symon. seeley, sir j.r. goethe reviewed after sixty years. with portrait. crown vo, s. d. a short history of napoleon the first. with portrait. crown vo, s. seeley, sir j.r., and dr. abbott. english lessons for english people. crown vo, s. d. seeley, l.b. mrs. thrale, afterwards mrs. piozzi. with eight illustrations. crown vo, s. d nett. fanny burney and her friends. with eight illustrations. crown vo, s. d nett. shand, a. innes. the war in the peninsula. with portraits and plans. s. sharp, william. fair women. illustrated. super-royal vo, sewed, s. d. nett; half-linen, s. d. nett. also new edition, small to, cloth, s. nett; leather, s. and s. nett. shipley, m.e. daily help for daily need. a selection of scripture verses and poetry for every day in the year. crown vo, s. d. stephens, f.g. rossetti, d.g. super-royal vo, sewed, s. d. nett; also small to, cloth, s. nett; leather, s. nett; velvet leather, in a box, s. nett. stevenson, r.l. edinburgh. fcap. vo, with frontispiece, gilt top, cloth, s. nett; leather, s. nett. crown vo, illustrated, cloth, s. d. roxburgh, gilt top, s. library edition. crown vo, buckram, dark blue, gilt top, sixteen full-page illustrations, s. presentation edition. extra crown vo, with sixty-four illustrations, s. nett; also people's edition, demy vo, d. nett. with twenty-four illustrations in colour, by james heron. crown to. printed by messrs. t. & a. constable, of edinburgh. ordinary edition, s. d. nett. edition de luxe, limited to copies, of which only are for sale, printed on unbleached arnold handmade paper, and bound in buckram, with paper label, each copy numbered, s. nett. stevenson, r.a.m. rubens, peter paul. illustrated. super-royal vo, s. d. nett, sewed. also small to, cloth, s. nett; leather, s. nett and s. nett. stigand, captain c.h., f.r.g.s., f.z.s. to abyssinia through an unknown land. with thirty-six illustrations and two maps. demy vo, s. nett. swann, alfred j. fighting the slave hunters in central africa. with forty-five illustrations and a map. demy vo, s. nett. extra crown vo. s. nett. talbot, f.a. the makings of a great canadian railway. demy vo. with forty-one illustrations and a map. s. nett. * * * * * the things seen series each volume with illustrations. small to, cloth, s.; leather, s.; and velvet leather, in a box, s. nett. things seen in oxford. by n.j. davidson, b.a. (oxon.) things seen in russia. by w. barnes steveni. things seen in palestine. by a. goodrich freer. things seen in japan. by clive holland. things seen in china. by j.r. chitty. things seen in egypt. by e.l. butcher. things seen in holland. by c.e. roche. things seen in spain. by c. gasquoine hartley. things seen in northern india. by t.l. pennell, m.d., b.sc. things seen in venice. by lonsdale ragg, b.d. 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cloth, gilt top, s. nett. also extra crown vo, s. d. nett. wicks, mark. to mars via the moon. an astronomical story. with sixteen illustrations and diagrams. extra crown vo, s. the moon a full description and map of its principal physical features by thomas gwyn elger, f.r.a.s. director of the lunar section of the british astronomical association ex-president liverpool astronomical society "altri fiumi, altri laghi, altre campagne sono la su che non son qui tra noi, altri piani, altre valli, altre montagne." orlando furioso, canto xxxii. london george philip & son, fleet street, e.c. liverpool: to south castle street preface this book and the accompanying map is chiefly intended for the use of lunar observers, but it is hoped it may be acceptable to many who, though they cannot strictly be thus described, take a general interest in astronomy. the increasing number of those who possess astronomical telescopes, and devote more or less of their leisure in following some particular line of research, is shown by the great success in recent years of societies, such as the british astronomical association with its several branches, the astronomical society of the pacific, and similar institutions in various parts of the world. these societies are not only doing much in popularising the sublimest of the sciences, but are the means of developing and organising the capabilities of their members by discouraging aimless and desultory observations, and by pointing out how individual effort may be utilised and made of permanent value in almost every department of astronomy. the work of the astronomer, like that of the votary of almost every other science, is becoming every year more and more specialised; and among its manifold subdivisions, the study of the physical features of the moon is undoubtedly increasing in popularity and importance. to those who are pursuing such observations, it is believed that this book will be a useful companion to the telescope, and convenient for reference. great care has been taken in the preparation of the map, which, so far as the positions of the various objects represented are concerned, is based on the last edition of beer and madler's chart, and on the more recent and much larger and elaborate map of schmidt; while as regards the shape and details of most of the formations, the author's drawings and a large number of photographs have been utilised. even on so small a scale as eighteen inches to the moon's diameter, more detail might have been inserted, but this, at the expense of distinctness, would have detracted from the value of the map for handy reference in the usually dim light of the observatory, without adding to its utility in other ways. every named formation is prominently shown; and most other features of interest, including the principal rill-systems, are represented, though, as regards these, no attempt is made to indicate all their manifold details and ramifications, which, to do effectually, would in very many instances require a map on a much larger scale than any that has yet appeared. the insertion of meridian lines and parallels of latitude at every ten degrees, and the substitution of names for reference numbers, will add to the usefulness of the map. with respect to the text, a large proportion of the objects in the catalogue and in the appendix have been observed and drawn by the author many times during the last thirty years, and described in _the observatory_ and other publications. he has had, besides, the advantage of consulting excellent sketches by mr w.h. maw, f.r.a.s., dr. sheldon, f.r.a.s., mr. a. mee, f.r.a.s., mr. g.p. hallowes, f.r.a.s., dr. smart, f.r.a.s., mr. t. gordon, f.r.a.s., mr. g.t. davis, herr brenner, herr krieger, mr. h. corder, and other members of the british astronomical association. through the courtesy of professor holden, director of the lick observatory, and m. prinz, of the royal observatory of brussels, many beautiful photographs and direct photographic enlargements have been available, as have also the exquisite heliogravures received by the author from dr. l. weinek, director of the imperial observatory of prague, and the admirable examples of the photographic work of mm. paul and prosper henry of the paris observatory, which are occasionally published in _knowledge_. the numerous representations of lunar objects which have appeared from time to time in that storehouse of astronomical information, _the english mechanic_, and the invaluable notes in "celestial objects for common telescopes," and in various periodicals, by the late rev. prebendary webb, to whom selenography and astronomy generally owe so much, have also been consulted. as a rule, all the more prominent and important features are described, though very frequently interesting details are referred to which, from their minuteness, could not be shown in the map. the measurements (given in round numbers) are derived in most instances from neison's (nevill) "moon," though occasionally those in the introduction to schmidt's chart are adopted. thomas gywn elger. bedford, . contents introduction maria, or plains, termed "seas" ridges ring-mountains, craters, &c. walled plains mountain rings ring-plains craters crater cones craterlets, crater pits mountain ranges, isolated mountains, &c. clefts, or rills faults valleys bright ray-systems the moon's albedo, surface brightness, &c. temperature of the moon's surface lunar observation progress of selenography, lunar photography catalogue of lunar formations first quadrant-- west longitude deg. to deg. west longitude deg. to deg. west longitude deg. to deg. west longitude deg. to deg. second quadrant-- east longitude deg. to deg. east longitude deg. to deg. east longitude deg. to deg. east longitude deg. to deg. third quadrant-- east longitude deg. to deg. east longitude deg. to deg. east longitude deg. to deg. east longitude deg. to deg. fourth quadrant-- west longitude deg. to deg. west longitude deg. to deg. west longitude deg. to deg. west longitude deg. to deg. map of the moon first quadrant second quadrant third quadrant fourth quadrant appendix description of map list of the maria, or grey plains, termed "seas," &c. list of some of the most prominent mountain ranges, promontories, isolated mountains, and remarkable hills list of the principal ray-systems, light-surrounded craters, and light spots position of the lunar terminator lunar elements alphabetical list of formations introduction we know, both by tradition and published records, that from the earliest times the faint grey and light spots which diversify the face of our satellite excited the wonder and stimulated the curiosity of mankind, giving rise to suppositions more or less crude and erroneous as to their actual nature and significance. it is true that anaxagoras, five centuries before our era, and probably other philosophers preceding him, --certainly plutarch at a much later date--taught that these delicate markings and differences of tint, obvious to every one with normal vision, point to the existence of hills and valleys on her surface; the latter maintaining that the irregularities of outline presented by the "terminator," or line of demarcation between the illumined and unillumined portion of her spherical superficies, are due to mountains and their shadows; but more than fifteen centuries elapsed before the truth of this sagacious conjecture was unquestionably demonstrated. selenography, as a branch of observational astronomy, dates from the spring of , when galileo directed his "optic tube" to the moon, and in the following year, in the _sidereus nuncius_, or "the intelligencer of the stars," gave to an astonished and incredulous world an account of the unsuspected marvels it revealed. in this remarkable little book we have the first attempt to represent the telescopic aspect of the moon's visible surface in the five rude woodcuts representing the curious features he perceived thereon, whose form and arrangement, he tells us, reminded him of the "ocelli" on the feathers of a peacock's tail,--a quaint but not altogether inappropriate simile to describe the appearance of groups of the larger ring-mountains partially illuminated by the sun, when seen in a small telescope. the bright and dusky areas, so obvious to the unaided sight, were found by galileo to be due to a very manifest difference in the character of the lunar surface, a large portion of the northern hemisphere, and no inconsiderable part of the south-eastern quadrant, being seen to consist of large grey monotonous tracts, often bordered by lofty mountains, while the remainder of the superficies was much more conspicuously brilliant, and, moreover, included by far the greater number of those curious ring- mountains and other extraordinary features whose remarkable aspect and peculiar arrangement first attracted his attention. struck by the analogy which these contrasted regions present to the land and water surfaces of our globe, he suspected that the former are represented on the moon by the brighter and more rugged, and the latter by the smoother and more level areas; a view, however, which kepler more distinctly formulated in the dictum, "do maculas esse maria, do lucidas esse terras." besides making a rude lunar chart, he estimated the heights of some of the ring- mountains by measuring the distance from the terminator of their bright summit peaks, when they were either coming into or passing out of sunlight; and though his method was incapable of accuracy, and his results consequently untrustworthy, it served to demonstrate the immense altitude of these circumvallations, and to show how greatly they exceed any mountains on the earth if the relative dimensions of the two globes are taken into consideration. before the close of the century when selenography first became possible, hevel of dantzig, scheiner, langrenus (cosmographer to the king of spain), riccioli, the jesuit astronomer of bologna, and dominic cassini, the celebrated french astronomer, greatly extended the knowledge of the moon's surface, and published drawings of various phases, and charts, which, though very rude and incomplete, were a clear advance upon what galileo, with his inferior optical means, had been able to accomplish. langrenus, and after him hevel, gave distinctive names to the various formations, mainly derived from terrestrial physical features, for which riccioli subsequently substituted those of philosophers, mathematicians, and other celebrities; and cassini determined by actual measurement the relative position of many of the principal objects on the disc, thus laying the foundation of an accurate system of lunar topography; while the labours of t. mayer and schroter in the last century, and of lohrmann, madler, neison (nevill), schmidt, and other observers in the present, have been mainly devoted to the study of the minuter detail of the moon and its physical characteristics. as was manifest to the earliest telescopic observers, its visible surface is clearly divisible into strongly contrasted areas, differing both in colour and structural character. somewhat less than half of what we see of it consists of comparatively level dark tracts, some of them very many thousands of square miles in extent, the monotony of whose dusky superficies is often unrelieved for great distances by any prominent object; while the remainder, everywhere manifestly brighter, is not only more rugged and uneven, but is covered to a much greater extent with numbers of quasi-circular formations, differing widely in size, classed as walled-plains, ring-plains, craters, craterlets, crater-cones, &c. (the latter bearing a great outward resemblance to some terrestrial volcanoes), and mountain ranges of vast proportions, isolated hills, and other features. though nothing resembling sheets of water, either of small or large extent, have ever been detected on the surface, the superficial resemblance, in small telescopes, of the large grey tracts to the appearance which we may suppose our terrestrial lakes and oceans would present to an observer on the moon, naturally induced the early selenographers to term them maria, or "seas"--a convenient name, which is still maintained, without, however, implying that these areas, as we now see them, are, or ever were, covered with water. some, however, regard them as old sea-beds, from which every trace of fluid, owing to some unknown cause, has vanished, and that the folds and wrinkles, the ridges, swellings, and other peculiarities of structure observed upon them, represent some of the results of alluvial action. it is, of course, possible, and even probable, that at a remote epoch in the evolution of our satellite these lower regions were occupied by water, but that their surface, as it now appears, is actually this old sea-bottom, seems to be less likely than that it represents the consolidated crust of some semi- fluid or viscous material (possibly of a basaltic type) which has welled forth from orifices or rents communicating with the interior, and overspread and partially filled up these immense hollows, more or less overwhelming and destroying many formations which stood upon them before this catastrophe took place. though this, like many other speculations of a similar character relating to lunar "geology," must remain, at least for the present, as a mere hypothesis; indications of this partial destruction by some agency or other is almost everywhere apparent in those formations which border the so-called seas, as, for example, fracastorius in the mare nectaris; le monnier in the mare serenitatis; pitatus and hesiodus, on the south side of the mare nubium; doppelmayer in the mare humorum, and in many other situations; while no observer can fail to notice innumerable instances of more or less complete obliteration and ruin among objects within these areas, in the form of obscure rings (mere scars on the surface), dusky craters, circular arrangements of isolated hills, reminding one of the monoliths of a druidical temple; all of which we are justified in concluding were at one time formations of a normal type. it has been held by some selenologists --and schmidt appears to be of the number,--that, seeing the comparative scarcity of large ring-plains and other massive formations on the maria, these grey plains represent, as it were, a picture of the primitive surface of the moon before it was disturbed by the operations of interior forces; but this view affords no explanation of the undoubted existence of the relics of an earlier lunar world beneath their smooth superficies. maria.--leaving, however, these considerations for a more particular description of the maria, it is clearly impossible, in referring to their level relatively to the higher and brighter land surface of the moon, to appeal to any hypsometrical standard. all that is known in this respect is, that they are invariably lower than the latter, and that some sink to a greater depth than others, or, in other words, that they do not all form a part of the same sphere. though they are more or less of a greyish-slaty hue--some of them approximating very closely to that of the pigment known as "payne's grey"--the tone, of course, depends upon the angle at which the solar rays impinge on that particular portion of the surface under observation. speaking generally, they are, as would follow from optical considerations, conspicuously darker when viewed near the terminator, or when the sun is either rising or setting upon them, than under a more vertical angle of illumination. but even when it is possible to compare their colour by eye-estimation under similar solar altitudes, it is found that not only are some of the maria, as a whole, notably darker than others, but nearly all of them exhibit _local_ inequalities of hue, which, under good atmospheric and instrumental conditions, are especially remarkable. under such circumstances i have frequently seen the surface, in many places covered with minute glittering points of light, shining with a silvery lustre, intermingled with darker spots and a network of streaks far too delicate and ethereal to represent in a drawing. in addition to these contrasts and differences in the sombre tone of these extended plains, many observers have remarked traces of a yellow or green tint on the surface of some of them. for example, the mare imbrium and the mare frigoris appear under certain conditions to be of a dirty yellow-green hue, the central parts of the mare humorum dusky green, and part of the mare serenitatis and the mare crisium light green, while the palus somnii has been noted a golden-brown yellow. to these may be added the district round taruntius in the mare foecunditatis, and portions of other regions referred to in the catalogue, where i have remarked a very decided sepia colour under a low sun. it has been attempted to account for these phenomena by supposing the existence of some kind of vegetation; but as this involves the presence of an atmosphere, the idea hardly finds favour at the present time, though perhaps the possibility of plant growth in the low-lying districts, where a gaseous medium may prevail, is not altogether so chimerical a notion as to be unworthy of consideration. nasmyth and others suggest that these tints may be due to broad expanses of coloured volcanic material, an hypothesis which, if we believe the maria to be overspread with such matter, and knowing how it varies in colour in terrestrial volcanic regions, is more probable than the first. anyway, whether we consider these appearances to be objective, or, after all, only due to purely physiological causes, they undoubtedly merit closer study and investigation than they have hitherto received. there are twenty-three of these dusky areas which have received distinctive names; seventeen of them are wholly, or in great part, confined to the northern, and to the south-eastern quarter of the southern hemisphere--the south-western quadrant being to a great extent devoid of them. by far the largest is the vast oceanus procellarum, extending from a high northern latitude to beyond latitude deg. in the south-eastern quadrant, and, according to schmidt, with its bays and inflections, occupying an area of nearly two million square miles, or more than that of all the remaining maria put together. next in order of size come the mare nubium, of about one-fifth the superficies, covering a large portion of the south-eastern quadrant, and extending considerably north of the equator, and the mare imbrium, wholly confined to the northeastern quadrant, and including an area of about , square miles. these are by far the largest lunar "seas." the mare foecunditatis, in the western hemisphere, the greater part of it lying in the south- western quadrant, is scarcely half so big as the mare imbrium; while the maria serenitatis and tranquilitatis, about equal in area (the former situated wholly north of the equator, and the latter only partially extending south of it), are still smaller. the arctic mare frigoris, some , square miles in extent, is the only remaining large sea,--the rest, such as the mare vaporum, the sinus medii, the mare crisium, the mare humorum, and the mare humboldtianum, are of comparatively small dimensions, the mare crisium not greatly exceeding , square miles, the mare humorum (about the size of england) , square miles, while the mare humboldtianum, according to schmidt, includes only about , square miles, an area which is approached by some formations not classed with the maria. this distinction, speaking generally, prevails among the maria,--those of larger size, such as the oceanus procellarum, the mare nubium, and the mare foecunditatis, are less definitely enclosed, and, like terrestrial oceans, communicate with one another; while their borders, or, if the term may be allowed, their coast-line, is often comparatively low and ill-defined, exhibiting many inlets and irregularities in outline. others, again, of considerable area, as, for example, the mare serenitatis and the mare imbrium, are bounded more or less completely by curved borders, consisting of towering mountain ranges, descending with a very steep escarpment to their surface: thus in form and other characteristics they resemble immense wall-surrounded plains. among the best examples of enclosed maria is the mare crisium, which is considered by neison to be the deepest of all, and the mare humboldtianum. though these great plains are described as level, this term must only be taken in a comparative sense. no one who observes them when their surface is thrown into relief by the oblique rays of the rising or setting sun can fail to remark many low bubble-shaped swellings with gently rounded outlines, shallow trough-like hollows, and, in the majority of them, long sinuous ridges, either running concentrically with their borders or traversing them from side to side. though none of these features are of any great altitude or depth, some of the ridges are as much as feet in height, and probably in many instances the other elevations often rise to feet or more above the low-lying parts of the plains on which they stand. hence we may say that the maria are only level in the sense that many districts in the english midland counties are level, and not that their surface is absolutely flat. the same may be said as to their apparent smoothness, which, as is evident when they are viewed close to the terminator, is an expression needing qualification, for under these conditions they often appear to be covered with wrinkles, flexures, and little asperities, which, to be visible at all, must be of considerable size. in fact, were it possible to examine them from a distance of a few miles, instead of from a standpoint which, under the most favourable circumstances, cannot be reckoned at less than , and this through an interposed aerial medium always more or less perturbed, they would probably be described as rugged and uneven, as some modern lava sheets. ridges.--among the maria which exhibit the most remarkable arrangement of ridges is the mare humorum, in the south-eastern quadrant. here, if it be observed under a rising sun, a number of these objects will be seen extending from the region north of the ring-mountain vitello in long undulating lines, roughly concentric with the western border of the "sea," and gradually diminishing in altitude as they spread out, with many ramifications, to a distance of miles or more towards the north. at this stage of illumination they are strikingly beautiful in a good telescope, reminding one of the ripple-marks left by the tide on a soft sandy beach. like most other objects of their class, they are very evanescent, gradually disappearing as the sun rises higher in the lunar firmament, and ultimately leaving nothing to indicate their presence beyond here and there a ghostly streak or vein of a somewhat lighter hue than that of the neighbouring surface. the mare nectaris, again, in the south-western quadrant, presents some fine examples of concentric ridges, which are seen to the best advantage when the morning sun is rising on rosse, a prominent crater north of fracastorius. this "sea" is evidently concave in cross-section, the central portion being considerably lower than the margin, and these ridges appear to mark the successive stages of the change of level from the coast-line to the centre. they suggest the "caving in" of the surface, similar to that observed on a frozen pond or river, where the "cat's ice" at the edge, through the sinking of the water beneath, is rent and tilted to a greater or less degree. the mare serenitatis and the mare imbrium, in the northern hemisphere, are also remarkable for the number of these peculiar features. they are very plentifully distributed round the margin and in other parts of the former, which includes besides one of the longest and loftiest on the moon's visible surface--the great serpentine ridge, first drawn and described nearly a hundred years ago by the famous selenographer, schroter of lilienthal. originating at a little crater under the north- east wall of great ring-plain posidonius, it follows a winding course across the mare toward the south, throwing out many minor branches, and ultimately dies out under a great rocky promontory--the promontory acherusia, at the western termination of the haemus range. a comparatively low power serves to show the curious structural character of this immense ridge, which appears to consist of a number of corrugations and folds massed together, rising in places, according to neison, to a height of feet and more. the mare imbrium also affords an example of a ridge, which, though shorter, is nearly as prominent, in that which runs from the bright little ring-plain piazzi smyth towards the west side of plato. the region round timocharis and other quarters of the mare are likewise traversed by very noteworthy features of a similar class. the oceanus procellarum also presents good instances of ridges in the marvellous ramifications round encke, kepler, and marius, and in the region north of aristarchus and herodotus. perhaps the most perfect examples of surface swellings are those in the mare tranquilitatis, a little east of the ring-plain arago, where there are two nearly equal circular mounds, at least ten miles in diameter, resembling tumuli seen from above. similar, but more irregular, objects of a like kind are very plentiful in many other quarters. it is a suggestive peculiarity of many of the lunar ridges, both on the maria and elsewhere, that they are very generally found in association with craters of every size. illustrations of this fact occur almost everywhere. frequently small craters are found on the summits of these elevations, but more often on their flanks and near their base. where a ridge suddenly changes its direction, a crater of some prominence generally marks the point, often forming a node, or crossing-place of other ridges, which thus appear to radiate from it as a centre. sometimes they intrude within the smaller ring-mountains, passing through gaps in their walls as, for example, in the cases of madler, lassell, &c. various hypotheses have been advanced to account for them. the late professor phillips, the geologist, who devoted much attention to the telescopic examination of the physical features of the moon, compared the lunar ridges to long, low, undulating mounds, of somewhat doubtful origin, called "kames" in scotland, and "eskers" in ireland, where on the low central plain they are commonly found in the form of extended banks (mainly of gravel), with more or less steep sides, rising to heights of from to feet. they are sometimes only a few yards wide at the top, while in other places they spread out into large humps, having circular or oval cavities on their summits, or yards across, and as much as feet deep. like the lunar ridges, they throw out branches and exhibit many breaches of continuity. by some geologists they are supposed to represent old submarine banks formed by tidal currents, like harbour bars, and by others to be glacial deposits; in either case, to be either directly or indirectly due to alluvial action. their outward resemblance to some of the ridges on the moon is unquestionable; and if we could believe that the maria, as we now see them, are dried-up sea-beds, it might be concluded that these ridges had a similar origin; but their close connection with centres of volcanic disturbance, and the numbers of little craters on or near their track, point to the supposition that they consist rather of material exuded from long-extending fissures in the crust of the "seas," and in other surfaces where they are superimposed. this conjecture is rendered still more probable by the fact that we sometimes find the direction of clefts (which are undoubted surface cracks) prolonged in the form of long narrow ridges or of rows of little hillocks. we are, however, not bound to assume that all the manifold corrugations observed on the lunar plains are due to one and the same cause; indeed, it is clear that some are merely the outward indications of sudden drops in the surface, as in the case of the ridges round the western margin of the mare nectaris, and in other situations, where subsidence is manifested by features assuming the outward aspect of ordinary ridges, but which are in reality of a very different structural character. the maria, like almost every other part of the visible surface, abound in craters of a minute type, which are scattered here and there without any apparent law or ascertained principle of arrangement. seeing how imperfect is our acquaintance with even the larger objects of this class, it is rash to insist on the antiquity or permanence of such diminutive objects, or to dogmatise about the cessation of lunar activity in connection with features where the volcanic history of our globe, if it is of any value as an analogue, teaches us it is most likely to prevail. most observers will agree with schmidt, that observations and drawings of objects on the sombre depressed plains of the moon are easier and pleasanter to make than on the dazzling highlands, and that the lunar "sea" is to the working selenographer like an oasis in the desert to the traveller--a relief in this case, however, not to an exhausted body, but to a weary eye. ring-mountains, craters, &c.--it is these objects, in their almost endless variety and bewildering number, which, more than any others, give to our satellite that marvellous appearance in the telescope which since the days of galileo has never failed to evoke the astonishment of the beholder. however familiar we may be with the lunar surface, we can never gaze on these extraordinary formations, whether massed together apparently in inextricable confusion, or standing in isolated grandeur, like copernicus, on the grey surface of the plains, without experiencing, in a scarcely diminished degree, the same sensation of wonder and admiration with which they were beheld for the first time. although the attempt to bring all these _bizarre_ forms under a rigid scheme of classification has not been wholly successful, their structural peculiarities, the hypsometrical relation between their interior and the surrounding district, their size, and the character of their circumvallation, the dimensions of their cavernous opening as compared with that of the more or less truncated conical mass of matter surrounding it, all afford a basis for grouping them under distinctive titles, that are not only convenient to the selenographer, but which undoubtedly represent, as a rule, actual diversities in their origin and physical character. these distinguishing titles, as adopted by schroter, lohrmann, and madler, and accepted by subsequent observers, are walled-plains, mountain rings, ring-plains, craters, crater-cones, craterlets, crater-pits, depressions. walled-plains.--these formations, approximating more or less to the circular form, though frequently deviating considerably from it, are among the largest enclosures on the moon. they vary from upwards of to miles or under in diameter, and are often encircled by a complex rampart of considerable breadth, rising in some instances to a height of , feet or more above the enclosed plain. this rampart is rarely continuous, but is generally interrupted by gaps, crossed by transverse valleys and passes, and broken by more recent craters and depressions. as a rule, the area within the circumvallation (usually termed "the floor") is only slightly, if at all, lower than the region outside: it is very generally of a dusky hue, similar to that of the grey plains or maria, and, like them, is usually variegated by the presence of hills, ridges, and craters, and is sometimes traversed by delicate furrows, termed clefts or rills. _ptolemaeus_, in the third quadrant, and not far removed from the centre of the disc, may be taken as a typical example of the class. here we have a vast plain, miles from side to side, encircled by a massive but much broken wall, which at one peak towers more than feet above a level floor, which includes details of a very remarkable character. the adjoining _alphonsus_ is another, but somewhat smaller, object of the same type, as are also _albategnius_, and _arzachel_; and _plato_, in a high northern latitude, with its noble many-peaked rampart and its variable steel-grey interior. _grimaldi_, near the eastern limb (perhaps the darkest area on the moon), _schickard_, nearly as big, on the south- eastern limb, and _bailly_, larger than either (still farther south in the same quadrant), although they approach some of the smaller "seas" in size, are placed in the same category. the conspicuous central mountain, so frequently associated with other types of ringed enclosures, is by no means invariably found within the walled-plains; though, as in the case of _petavius_, _langrenus_, _gassendi_, and several other noteworthy examples, it is very prominently displayed. the progress of sunrise on all these objects affords a magnificent spectacle. very often when the rays impinge on their apparently level floor at an angle of from deg. to deg., it is seen to be coarse, rough grained, and covered with minute elevations, although an hour or so afterwards it appears as smooth as glass. although it is a distinguishing characteristic that there is no great difference in level between the outside and the inside of a walled-plain, there are some very interesting exceptions to this rule, which are termed by schmidt "transitional forms." among these he places some of the most colossal formations, such as _clavius_, _maurolycus_, _stofler_, _janssen_, and _longomontanus_. the first, which may be taken as representative of the class (well known to observers as one of the grandest of lunar objects), has a deeply sunken floor, fringed with mountains rising some , feet above it, though they scarcely stand a fourth of this height above the plain on the west, which ascends with a very gentle gradient to the summit of the wall. hence the contrast between the shadows of the peaks of the western wall on the floor at sunrise, and of the same peaks on the region west of the border at sunset is very marked. in _gassendi_, _phocylides_, and _wargentin_ we have similar notable departures from the normal type. the floor of the former on the north stands feet _above_ the mare humorum. in _phocylides_, probably through "faulting," one portion of the interior suddenly sinks to a considerable depth below the remainder; while the very abnormal _wargentin_ has such an elevated floor, that, when viewed under favourable conditions, it reminds one of a shallow oval tray or dish filled with fluid to the point of overflowing. these examples, very far from being exhaustive, will be sufficient to show that the walled-plains exhibit noteworthy differences in other respects than size, height of rampart, or included detail. still another peculiarity, confined, it is believed, to a very few, may be mentioned, viz., convexity of floor, prominently displayed in petavius, mersenius, and hevel. mountain rings.--these objects, usually encircled by a low and broken border, seldom more than a few hundred feet in height, are closely allied to the walled-plains. they are more frequently found on the maria than elsewhere. in some cases the ring consists of isolated dark sections, with here and there a bright mass of rock interposed; in others, of low curvilinear ridges, forming a more or less complete circumvallation. they vary in size from or miles to miles and less. the great ring north of flamsteed, miles across, is a notable example; another lies west of it on the north of wichmann; while a third will be found south- east of encke;--indeed, the mare procellarum abounds in objects of this type. the curious formation on the mare imbrium immediately south of plato (called "newton" by schroter), may be placed in this category, as may also many of the low dusky rings of much smaller dimensions found in many quarters of the maria. as has been stated elsewhere, these features have the appearance of having once been formations of a much more prominent and important character, which have suffered destruction, more or less complete, through being partially overwhelmed by the material of the "seas." ring-plains.--these are by far the most numerous of the ramparted enclosures of the moon, and though it is occasionally difficult to decide in which class, walled-plain or ring-plain, some objects should be placed, yet, as a rule, the difference between the structural character of the two is abundantly obvious. the ring-plains vary in diameter from sixty to less than ten miles, and are far more regular in outline than the walled-plains. their ramparts, often very massive, are more continuous, and fall with a steep declivity to a floor almost always greatly depressed below the outside region. the inner slopes generally display subordinate heights, called terraces, arranged more or less concentrically, and often extending in successive stages nearly down to the interior foot of the wall. with the intervening valleys, these features are very striking objects when viewed under good conditions with high powers. in some cases they may possibly represent the effects of the slipping of the upper portions of the wall, from a want of cohesiveness in the material of which it is composed; but this hardly explains why the highest terrace often stands nearly as high as the rampart. nasmyth, in his eruption hypothesis, suggests that in such a case there may have been two eruptions from the same vent; one powerful, which formed the exterior circle, and a second, rather less powerful, which has formed the interior circle. ultimately, however, coming to the conclusion that terraces, as a rule, are not due to any such freaks of the eruption, he ascribes them to landslips. in any case, we can hardly imagine that material standing at such a high angle of inclination as that forming the summit ridge of many of the ring-plains would not frequently slide down in great masses, and thus form irregular plateaus on the lower and flatter portions of the slope; but this fails to explain the symmetrical arrangement of the concentric terraces and intermediate valleys. the inner declivity of the north-eastern wall of plato exhibits what to all appearance is an undoubted landslip, as does also that of hercules on the northern side, and numerous other cases might be adduced; but in all of them the appearance is very different from that of the true terrace. the _glacis_, or outer slope of a ring-plain, is invariably of a much gentler inclination than that which characterises the inner declivity: while the latter very frequently descends at an angle varying from deg. to deg. at the crest of the wall, to from deg. to deg. at the bottom, where it meets the floor; the former extends for a great distance at a very flat gradient before it sinks to the general level of the surrounding country. it differs likewise from the inner descent, in the fact that, though often traversed by valleys, intersected by deep gullies and irregular depressions, and covered with humpy excrescences and craters, it is only rarely that any features comparable to the terraces, usually present on the inner escarpment, can be traced upon it. elongated depressions of irregular outline, and very variable in size and depth, are frequently found on the outer slopes of the border. some of them consist of great elliptical or sub-circular cavities, displaying many expansions and contractions, called "pockets," and suggesting the idea that they were originally distinct cup-shaped hollows, which from some cause or other have coalesced like rows of inosculating craters. while many of these features are so deep that they remain visible for a considerable time under a low sun, others, though perhaps of greater extent, vanish in an hour or so. as in the case of the walled-plains, the ramparts of the ring-plains exhibit gaps and are broken by craters and depressions, but to a much less extent. often the lofty crest, surmounted by _aiguilles_ or by blunter peaks, towering in some cases to nearly double its altitude above the interior, is perfectly continuous (like copernicus), or only interrupted by narrow passes. it is a suggestive circumstance that gaps, other than valleys, are almost invariably found either in the north or south walls, or in both, and seldom in other positions. the buttress, or long-extending spur, is a feature frequently associated with the ring- plain rampart, as are also numbers of what, for the lack of a better name, must be termed little hillocks, which generally radiate in long rows from the outer foot of the slope. the spurs usually abut on the wall, and, either spreading out like the sticks of a fan or running roughly parallel to each other, extend for long distances, gradually diminishing in height and width till they die out on the surrounding surface. they have been compared to lava streams, which those round aristillus, aristoteles, and on the flank of clavius _a_, certainly somewhat resemble, though, in the two former instances, they are rather comparable to immense ridges. in addition to the above, the spurs radiating from the south-eastern rampart of condamine and the long undulating ridges and rows of hillocks running from cyrillus over the eastern _glacis_ of theophilus, may be named as very interesting examples. neison and some other selenographers place in a distinct class certain of the smaller ring-plains which usually have a steeper outer slope, and are supposed to present clearer indications of a volcanic origin than the ring-plains, terming them "crater-plains." craters.--under this generic name is placed a vast number of formations exhibiting a great difference in size and outward characteristics, though generally (under moderate magnification) of a circular or sub-circular shape. their diameter varies from miles or more to , and even less, and their flanks rise much more steeply to the summit, which is seldom very lofty, than those of the ring-plains, and fall more gradually to the floor. there is no portion of the moon in which they do not abound, whether it be on the ramparts, floors, and outer slopes of walled and ring plains, the summits and escarpments of mountain ranges, amid the intricacies of the highlands, or on the grey surface of the maria. in many instances they have a brighter and newer aspect than the larger formations, often being the most brilliant points on their walls, when they are found in this position. very frequently too they are not only very bright themselves, but stand on bright areas, whose borders are generally concentric with them, which shine with a glistening lustre, and form a kind of halo of light around them. euclides and bessarion a, and the craters east of landsberg, are especially interesting examples. it seems not improbable that these areas may represent deposits formed by some kind of matter ejected from the craters, but whether of ancient or modern date, it is, of course, impossible to determine. future observers will perhaps be in a better position to decide the question without cavil, if such eruptions should again take place. like the larger enclosures, these smaller objects frequently encroach upon each other-- crater-ring overlapping crater-ring, as in the case of thebit, where a large crater, which has interfered with the continuity of the east wall, has, in its turn, been disturbed by a smaller crater on its own east wall. the craters in many cases, possibly in the majority if we could detect them, have central mountains, some of them being excellent tests for telescopic definition--as, for example, the central peaks of hortensius, bessarion, and that of the small crater just mentioned on the east wall of thebit a. a tendency to a linear arrangement is often displayed, especially among the smaller class, as is also their occurrence in pairs. crater-cones.--these objects, plentifully distributed on the lunar surface, are especially interesting from their outward resemblance to the parasitic cones found on the flanks of terrestrial volcanoes (etna, for instance). in the larger examples it is occasionally possible to see that the interiors are either inverted cones without a floor, or cup-shaped depressions on the summit of the object. frequently, however, they are so small that the orifice can only be detected under oblique illumination. under a high sun they generally appear as white spots, more or less ill- defined, as on the floors of archimedes, fracastorius, plato, and many other formations, which include a great number, all of which are probably crater cones, although only a few have been seen as such. it is a significant fact that in these situations they are always found to be closely associated with the light streaks which traverse the interior of the formations, standing either on their surface or close to their edges. the instrumental and meteorological requirements necessary for a successful scrutiny of the smallest type of these features, are beyond the reach of the ordinary observer in this country, as they demand direct observation in large telescopes under the best atmospheric conditions. some years ago dr. klein of cologne called attention to some very interesting types of crater-cones, which may be found on certain dark or smoky-grey areas on the moon. these, he considers, may probably represent active volcanic vents, and urges that they should be diligently examined and watched by observers who possess telescopes adequate to the task. the most noteworthy examples of these objects are in the following positions:--( ) west of a prominent ridge running from beaumont to the west side of theophilus, and about midway between these formations; ( ) in the mare vaporum, south of hyginus; ( ) on the floor of werner, near the foot of the north wall; ( ) under the east wall of alphonsus, on the dusky patch in the interior; ( ) on the south side of the floor of atlas. i have frequently described elsewhere with considerable detail the telescopic appearance of these features under various phases, and have pointed out that though large apertures and high powers are needed to see these cones to advantage, the dusky areas, easily traced on photograms, might be usefully studied by observers with smaller instruments, as if they represent the _ejecta_ from the crater-cones which stand upon them, changes in their form and extent could very possibly be detected. in addition to those already referred to, a number of mysterious dark spots were discovered by schmidt in the dusky region about midway between copernicus and gambart, which klein describes as perforated like a sieve with minute craters. a short distance south-west of copernicus stands a bright crater-cone surrounded by a grey nimbus, which may be classed with these objects. it is well seen under a high light, as indeed is the case with most of these features. craterlets, crater-pits.--to a great extent the former term is needless and misleading, as the so-called craters merge by imperceptible gradations into very minute objects, as small as half a mile in diameter, and most probably, if we could more accurately estimate their size, still less. the crater-pit, however, has well-marked peculiarities which distinguish it from all other types, such as the absence of a distinguishable rim and extreme shallowness. they appear to be most numerous on the high-level plains and plateaus in the south-western quadrant, and may be counted by hundreds under good atmospheric conditions on the outer slopes of walter, clavius, and other large enclosures. in these positions they are often so closely aggregated that, as nasmyth remarks, they remind one of an accumulation of froth. even in an / inch reflector i have frequently seen the outer slope of the large ring-plain on the north-western side of vendelinus, so perforated with these objects that it resembled pumice or vesicular lava, many of the little holes being evidently not circular, but square shaped and very irregular. the interior of stadius and the region outside abounds in these minute features, but the well-known crater-row between this formation and copernicus seems rather to consist of a number of inosculating crater-cones, as they stand very evidently on a raised bank of some altitude. mountain ranges, isolated mountains, &c.--the more massive and extended mountain ranges of the moon are found in the northern hemisphere, and (what is significant) in that portion of it which exhibits few indications of other superficial disturbances. the most prominently developed systems, the _alps_, the _caucasus_, and the _apennines_, forming a mighty western rampart to the mare imbrium and giving it all the appearance of a vast walled plain, present few points of resemblance to any terrestrial chain. the former include many hundred peaks, among which, mont blanc rises to a height of , feet, and a second, some distance west of plato, to nearly as great an altitude; while others, ranging from to feet, are common. they extend in a south-west direction from plato to the caucasus, terminating somewhat abruptly, a little west of the central meridian, in about n. lat. deg. one of the most interesting features associated with this range is the so-called great alpine valley, which cuts through it west of plato. the _caucasus_ consist of a massive wedge-shaped mountain land, projecting southwards, and partially dividing the mare imbrium from the mare serenitatis, both of which they flank. though without peaks so lofty as those pertaining to the alps, there is one, immediately east of the ring-plain calippus, which, towering to , feet, surpasses any of which the latter system can boast. the _apennines_, however, are by far the most magnificent range on the visible surface, including as they do some peaks, and extending in an almost continuous curve of more than miles in length from mount hadley, on the north, to the fine ring-plain eratosthenes, which forms a fitting termination, on the south. the great headland mount hadley rises more than , feet, while a neighbouring promontory on the south-east of it is fully , feet, and another, close by, is still higher above the mare. mount huygens, again, in n. lat. deg., and the square-shaped mass mount wolf, near the southern end of the chain, include peaks standing , and , feet respectively above the plain, to which their flanks descend with a steep declivity. the counterscarp of the apennines, in places miles in width from east to west, runs down to the mare vaporum with a comparatively gentle inclination. it is everywhere traversed by winding valleys of a very intricate type, all trending towards the south-west, and includes some bright craters and mountain-rings. the _carpathians_, forming in part the southern border of the mare imbrium, extend for a length of more than miles eastward of e., long. deg., and, embracing the ring-plain gay- lussac, terminate west of mayer. they present a less definite front to the mare than the apennines, and are broken up and divided by irregular valleys and gaps; their loftiest peak, situated on a very projecting promontory north-west of mayer, rising to a height of feet. notwithstanding their comparatively low altitude, the region they occupy forms a fine telescopic picture at lunar sunrise. the _sinus iridum highlands_, bordering the beautiful bay on the north-east side of the mare imbrium, rank among the loftiest and most intricate systems on the moon, and, like the apennines, present a steep face to the grey plain from which they rise, though differing from them in other respects. they include many high peaks, the loftiest, in the neighbourhood of the ring- plain sharp, rising , feet. there are probably some still higher mountains in the vicinity, but the difficulties attending their measurement render it impossible to determine their altitude with any approach to accuracy. _the taurus mountains_ extend from the west side of the mare serenitatis, near le monnier and littrow, in a north-westerly direction towards geminus and berselius, bordering the west side of the lacus somniorum. they are a far less remarkable system than any of the preceding, and consist rather of a wild irregular mountain region than a range. in the neighbourhood of berselius are some peaks which, according to neison, cannot be less than , feet in height. on the north side of the mare imbrium, east of plato, there is a beautiful narrow range of bright outlying heights, called the _teneriffe mountains_, which include many isolated objects of considerable altitude, one of the loftiest rising about feet. farther towards the east lies another group of a very similar character, called the _straight range_, from its linear regularity. it extends from west to east for a distance of about miles, being a few miles shorter than the last, and includes a peak of feet. _the harbinger mountains_.--a remarkable group, north-west of aristarchus, including some peaks as high as feet, and other details noticed in the catalogue. the above comprise all the mountain ranges in the northern hemisphere of any prominence, or which have received distinctive names, except the _hercynian mountains_, on the north-east limb, east of the walled plain otto struve. these are too near the edge to be well observed, but, from what can be seen of them, they appear to abound in lofty peaks, and to bear more resemblance to a terrestrial chain than any which have yet been referred to. the mountain systems of the southern hemisphere, except the ranges visible on the limb, are far less imposing and remarkable than those just described. the _pyrenees_, on the western side of the mare nectaris, extend in a meridional direction for nearly miles, and include a peak east of guttemberg of nearly , feet, and are traversed in many places by fine valleys. _the altai mountains_ form a magnificent chain, miles in length, commencing on the outer eastern slope of piccolomini, and following a tolerably direct north-east course, with a few minor bendings, to the west side of fermat, where they turn more towards the north, ultimately terminating about midway between tacitus and catherina. the region situated on the south-east is a great table-land, without any prominent features, rising gently towards the mountains, which shelve steeply down to an equally barren expanse on the north-west, to which they present a lofty face, having an average altitude of about feet. the loftiest peak, over , feet, rises west of fermat. _the riphaean mountains_, a remarkably bright group, occupying an isolated position in the mare procellarum south of landsberg, and extending for more than miles in a meridional direction. they are most closely aggregated at a point nearly due west of euclides, from which they throw off long-branching arms to the north and south, those on the north bifurcating and gradually sinking to the level of the plain. the loftiest peaks are near the extremity of this section, one of them rising to feet. two bright craters are associated with these mountains, one nearly central, and the other south of it. _the percy mountains_.--this name is given to the bright highlands extending east of gassendi towards mersenius, forming the north-eastern border of the mare humorum. they abound in minute detail--bright little mountains and ridges--and include some clefts pertaining to the mersenius rill-system; but their most noteworthy feature is the long bright mountain-arm, branching out from the eastern wall of gassendi, and extending for more than miles towards the south-east. the principal ranges on the limb are the _leibnitz mountains_, extending from s. lat. deg. on the west to s. lat. deg. on the east limb. they include some giant peaks and plateaus, noteworthy objects in profile, some of which, according to schroter and madler, rise to , feet. the _dorfel mountains_, between s. lat. deg. and deg. on the eastern limb, include, if schroter's estimate is correct, three peaks which exceed , feet. on the eastern limb, between s. lat. deg. and deg., extend the _rook mountains_, which have peaks, according to schroter, as high as , feet. next in order come the _cordilleras_, which extend to s. lat. deg., and the _d'alembert mountains_, lying east of rocca and grimaldi, closely associated with them, and probably part of the same system. some of the peaks approach , feet. in addition to these mountain ranges there are others less prominent on the limb in the northern hemisphere, which have not been named. isolated mountains are very numerous in different parts of the moon, the most remarkable are referred to in the appendix. many remain unnamed. clefts or rills.--though fontenelle, in his _entretiens sur la pluralite des mondes_, informs his pupil, the marchioness, that "m. cassini discovered in the moon something which separates, then reunites, and finally loses itself in a cavity, which from its appearance seemed to be a river," it can hardly be supposed that what the french astronomer saw, or fancied he saw, with the imperfect telescopes of that day, was one of the remarkable and enigmatical furrows termed clefts or rills, first detected by the hanoverian selenographer schroter; who, on october , , discovered the very curious serpentine cleft near herodotus, having a few nights before noted for the first time the great alpine valley west of plato, once classed with the clefts, though it is an object of a very different kind. between and schroter found ten rills; but twenty years elapsed before an addition was made to this number by the discoveries of gruithuisen, and, a short time after, by those of lohrmann, who in twelve months ( - ) detected seventy. kinau, madler, and finally schmidt, followed, till, in , when the latter published his work, _ueber rillen auf dem monde_, the list was thus summarised:-- in the st or n.w. quadrant rills in the nd or n.e. quadrant rills in the rd or s.e. quadrant rills in the th or s.w. quadrant rills or in all. since the date of this book the number of known rills has been more than doubled; in fact, scarcely a lunation passes without new discoveries being made. the significance of the word _rille_ in german, a groove or furrow, describes with considerable accuracy the usual appearance of the objects to which it is applied, consisting as they do of long narrow channels, with sides more or less steep, and sometimes vertical. they often extend for hundreds of miles in approximately straight lines over portions of the moon's surface, frequently traversing in their course ridges, craters, and even more formidable obstacles, without any apparent check or interruption, though their ends are sometimes marked by a mound or crater. their length ranges from ten or twelve to three hundred miles or more (as in the great sirsalis rill), their breadth, which is very variable within certain limits, from less than half a mile to more than two, and their depth (which must necessarily remain to a great extent problematical) from to yards. they exhibit in the telescope a gradation from somewhat coarse grooves, easily visible at suitable times in very moderately sized instruments, to striae so delicate as to require the largest and most perfect optical means and the best atmospheric conditions to be glimpsed at all. viewed under moderate amplification, the majority of rills resemble deep canal-like channels with roughly parallel sides, displaying occasionally local irregularities, and fining off to invisibility at one or both ends. but, if critically scrutinised in the best observing weather with high powers, the apparent evenness of their edges entirely disappears, and we find that the latter exhibit indentations, projections, and little flexures, like the banks of an ordinary stream or rivulet, or, to use a very homely simile, the serrated edges and little jagged irregularities of a biscuit broken across. in some cases we remark crateriform hollows or sudden expansions in their course, and deep sinuous ravines, which render them still more unsymmetrical and variable in breadth. with regard to their distribution on the lunar surface; they are found in almost every region, but perhaps not so frequently on the surface of the maria as elsewhere, though, as in the case of the triesnecker and other systems, they often abound in the neighbourhood of disturbed regions in these plains, and in many cases along their margins, as, for example, the gassendi-mersenius and the sabine-ritter groups. the interior of walled plains are frequently intersected by them, as in gassendi, where nearly forty, more or less delicate examples, have been seen; in hevel, where there is a very interesting system of crossed clefts, and within posidonius. if we study any good modern lunar map, it is evident how constantly they appear near the borders of mountain ranges, walled-plains, and ring-plains; as, for instance, at the foot of the apennines; near archimedes, aristarchus, ramsden, and in many other similar positions. rugged highlands also are often traversed by them, as in the case of those lying west of le monnier and chacornac, and in the region west of the mare humorum. it may be here remarked, however, as a notable fact, that the neighbourhood of the grandest ring-mountain on the moon, copernicus, is, strange to say, devoid of any features which can be classed as true clefts, though it abounds in crater-rows. the intricate network of rills on the west of triesnecker, when observed with a low power, reminds one of the wrinkles on the rind of an orange or on the skin of a withered apple. gruithuisen, describing the rill-traversed region between agrippa and hyginus, says that "it has quite the look of a dutch canal map." in the subjoined catalogue many detailed examples will be given relating to the course of these mysterious furrows; how they occasionally traverse mountain arms, cut through, either completely or partially (as in ramsden), the borders of ring-plains and other enclosures, while not unfrequently a small mound or similar feature appears to have caused them to swerve suddenly from their path, as in the case of the ariadaeus cleft, and in that of one member of the mercator-campanus system. of the actual nature of the lunar rills we are, it must be confessed, supremely ignorant. with some of the early observers it was a very favourite notion that they are artificial works, constructed presumably by kepler's _sub-volvani_, or by other intelligences. there is perhaps some excuse to be made for the freaks of an exuberant fancy in regard to objects which, if we ignore for a moment their enormous dimensions, judged by a terrestrial standard, certainly have, in their apparent absence of any physical relation to neighbouring objects, all the appearance of being works of art rather than of nature. the keen-sighted and very imaginative gruithuisen believed that in some instances they represent roads cut through interminable forests, and in others the dried-up beds of once mighty rivers. his description of the triesnecker rill-system reads like a page from a geographical primer. a portion of it is compared to the river po, and he traces its course mile by mile up to the "delta" at its place of disemboguement into the mare vaporum. from the position of some rills with respect to the contour of the surrounding country, it is evident that if water were now present on the moon, they, being situated at the lowest level, would form natural channels for its reception; but the exceptions to this arrangement are so numerous and obvious, that the idea may be at once dismissed that there is any analogy between them and our rivers. the eminent selenographer, the late w.k. birt, compared many of them to "inverted river-beds" from the fact that, as often as not, they become broader and deeper as they attain a higher level. the branches resemble rivers more frequently than the main channels; for they generally commence as very fine grooves, and, becoming broader and broader, join them at an acute angle. an attempt again has been made to compare the lunar clefts with those vast gorges, the marvellous results of aqueous action, called canyons, which attain their greatest dimensions in north america; such as the great canyon of the colorado, which is at least miles in length, and in places yards in depth, with perpendicular or even overhanging sides; but the analogy, at first sight specious, utterly breaks down under closer examination. some selenographers consider them to consist of long-extending rows of confluent craters, too minute to be separately distinguished, and to be thus due to some kind of volcanic action. this is undoubtedly true in many instances, for almost every lunar region affords examples of crater- rows merging by almost imperceptible gradations into cleft-like features, and crater-rows of considerable size resemble clefts under low powers. still it seems probable that the greater number of these features are immense furrows or cracks in the surface and nothing more; for the higher the magnifying power employed in their examination, the less reason there is to object to this description. dr. klein of cologne believes that rills of this class are due to the shrinkage of parts of the moon's crust, and that they are not as a rule the result of volcanic causes, though he admits that there may be some which have a seismic origin. no good reason has as yet been given for the fact that they so frequently cross small craters and other objects in their course, though it has been suggested that the route followed by a rill from crater to crater in these instances may be a line of least surface resistance, an explanation far from being satisfactory. whether variations in the visibility of lunar details, when observed under apparently similar conditions, actually occur from time to time from some unknown cause, is one of those vexed questions which will only be determined when the moon is systematically studied by experienced observers using the finest instruments at exceptionally good stations; but no one who examines existing records of observations of rills by gruithuisen, lohrmann, madler, schmidt, and other observers, can well avoid the conclusion that the anomalies brought to light therein point strongly to the probability of the existence of some agency which occasionally modifies their appearance or entirely conceals them from view. the following is one illustration out of many which might be quoted. at a point in its course, nearly due north of the ring-plain agrippa, the great ariadaeus cleft sends out a branch which runs into the well-known hyginus cleft, reminding one, as dr. klein remarks, of two rivers connected in the shortest way by a canal. this uniting furrow was detected by gruithuisen, who observed it several times. on some occasions it appeared perfectly straight, at others very irregular; but, what is very remarkable, although two such accurate observers as lohrmann and madler frequently scrutinised the region, neither of them saw a trace of this object; and but for its rediscovery by schmidt in , its existence would certainly have been ignored by selenographers as a mere figment of gruithuisen's too lively imagination. dr. klein has frequently seen this rill with great distinctness, and at other times sought for it in vain; though on each occasion the conditions of illumination, libration, and definition were practically similar. i have sometimes found this cleft an easy object with a inch achromatic. again, many rills described by madler as very delicate and difficult to trace, may now be easily followed in "common telescopes." in short, the more direct telescopic observations accumulate, and the more the study of minute detail is extended, the stronger becomes the conviction, that in spite of the absence of an appreciable atmosphere, there may be something resembling low-lying exhalations from some parts of the surface which from time to time are sufficiently dense to obscure, or even obliterate, the region beneath them. if, as seems most probable, these gigantic cracks are due to contractions of the moon's surface, it is not impossible, in spite of the assertions of the text-books to the effect that our satellite is now "a changeless world," that emanations may proceed from these fissures, even if, under the monthly alternations of extreme temperatures, surface changes do not now occasionally take place from this cause also. should this be so, the appearance of new rills and the extension and modification of those already existing may reasonably be looked for. many instances might be adduced tending to confirm this supposition, to one of which, as coming under my notice, i will briefly refer. on the evening of november , , when examining the interior of the great ring-plain mersenius with a power of on an / inch reflector; in addition to the two closely parallel clefts discovered by schmidt, running from the inner foot of the north-eastern rampart towards the centre, i remarked another distinct cleft crossing the northern part of the floor from side to side. shortly afterwards, m. gaudibert, one of our most experienced selenographers, who has discovered many hitherto unrecorded clefts, having seen my drawing, searched for this object, and, though the night was far from favourable, had distinct though brief glimpses of it with the moderate magnifying power of . mersenius is a formation about miles in diameter, with a prominently convex interior, containing much detail which has received more than ordinary attention from observers. it has, moreover, been specially mapped by schmidt and others, yet no trace of this rill was noted, though objects much more minute and difficult have not been overlooked. does not an instance of this kind raise a well-grounded suspicion of recent change which it is difficult to explain away? to see the lunar clefts to the best advantage, they must be looked for when not very far removed from the terminator, as when so situated the black shadow of one side, contrasted with the usually brightly- illuminated opposite flank, renders them more conspicuous than when they are viewed under a higher sun. though, as a rule, invisible at full moon, some of the coarser clefts--as, for example, a portion of the hyginus furrow, and that north of birt--may be traced as delicate white lines under a nearly vertical light. for properly observing these objects, a power of not less than on telescopes of large aperture is needed; and in studying their minute and delicate details, we are perhaps more dependent on atmospheric conditions than in following up any other branch of observational astronomy. few indeed are the nights, in our climate at any rate, when the rough, irregular character of the steep interior of even the coarser examples of these immense chasms can be steadily seen. we can only hope to obtain a more perfect insight into their actual structural peculiarities when they are scrutinised under more perfect climatic circumstances than they have been hitherto. when observing the hyginus cleft, dr. klein noticed that at one place the declivities of the interior displayed decided differences of tint. at many points the reflected sunlight was of a distinctly yellow hue, while in other places it was white, as if the cliffs were covered with snow. he compares this portion of the rill to the rhine valley between bingen and coblentz, but adds that the latter, if viewed from the moon, would probably not present so fresh an appearance, and would, of course, be frequently obscured by clouds. since the erection of the great lick telescope on mount hamilton, our knowledge of the details of some of the lunar clefts has been greatly increased, as in the case of the ariadaeus cleft, and many others. professor w.h. pickering, also, at arequipa, has made at that ideal astronomical site many observations which, when published, will throw more light upon their peculiar characteristics. a few years ago m.e.l. trouvelot of meudon drew attention to a curious appearance which he noted in connection with certain rills when near the terminator, viz., extremely attenuated threads of light on their sites and their apparent prolongations. he observed it in the ring-plain eudoxus, crossing the southern side of the floor from wall to wall; and also in connection with the prominent cleft running from the north side of burg to the west of alexander, and in some other situations. he terms these phenomena _murs enigmatiques_. apparent prolongations of clefts in the form of rows of hillocks or small mounds are very common. faults.--these sudden drops in the surface, representing local dislocations, are far from unusual: the best examples being the straight wall, or "railroad," west of birt; that which strikes obliquely across plato; another which traverses phocylides; and a fourth that has manifestly modified the mountain arm north of cichus. they differ from the terrestrial phenomena so designated in the fact that the surface indications of these are destroyed by denudation or masked by deposits of subsequent date. in many cases on the moon, though its course cannot be traced in its entirety by its shadow, yet the existence of a fault may be inferred by the displacement and fracture of neighbouring objects. valleys.--features thus designated, differing greatly both in size and character, are met with in almost every part of the surface, except on the grey plains. while the smallest examples, from their delicacy, tenuity, and superficial resemblance to rills, are termed rill-valleys, the larger and more conspicuous assume the appearance of coarse chasms, gorges, or trough-like depressions. between these two extremes, are many objects of moderate dimensions--winding or straight ravines and defiles bounded by steep mountains, and shallow dales flanked by low rounded heights. the rill valleys are very numerous, only differing in many instances from the true rills in size, and are probably due to the same cause. among the most noteworthy valleys of the largest class must, of course, be placed the great valley of the alps, one of the most striking objects in the northern hemisphere, which also includes the great valley south-east of ukert. the rheita valley, the very similar chasm west of reichenbach, and the gorge west of herschel, are also notable examples in the southern hemisphere. the borders of some of the maria (especially that of the mare crisium) and of many of the depressed rimless formations, furnish instances of winding valleys intersecting their borders: the hilly regions likewise often abound in long branching defiles. bright ray-systems.--reference has already been made to the faint light streaks and markings often found on the floors of the ring-mountains and in other situations, and to the brilliant _nimbi_ surrounding some of the smaller craters; but, in addition to these, many objects on the moon's visible surface are associated with a much more remarkable and conspicuous phenomenon--the bright rays which, under a high sun, are seen either to radiate from them as apparent centres to great distances, or, in the form of irregular light areas, to environ them, and to throw out wide-spreading lucid beams, extending occasionally many hundreds of miles from their origin. the more striking of these systems were recognised and drawn at a very early stage of telescopic observation, as may be seen if we consult the quaint old charts of hevel, riccioli, fontana, and other observers of the seventeenth century, where they are always prominently, though very inaccurately, portrayed. the principal ray-systems are those of tycho, copernicus, kepler, anaxagoras, aristarchus, olbers, byrgius a, and zuchius; while autolycus, aristillus, proclus, timocharis, furnerius a, and menelaus are grouped as constituting minor systems. many additional centres exist, a list of which will be found in the appendix. the rays emanating from tycho surpass in extent and interest any of the others. hundreds of distinct light streaks originate round the grey margin of this magnificent object, some of them extending over a greater part of the moon's visible superficies, and "radiating," in the words of professor phillips, "like false meridians, or like meridians true to an earlier pole of rotation." no systematic attempt has yet been made to map them accurately as a whole on a large scale, for their extreme intricacy and delicacy would render the task a very difficult one, and, moreover, would demand a long course of observation with a powerful telescope in an ideal situation; but professor w.h. pickering, observing under these conditions at arequipa, has recently devoted considerable attention both to the tycho and other rays, with especially suggestive and important results, which may be briefly summarised as follows:-- ( .) that the tycho streaks do not radiate from the apparent centre of this formation, but point towards a multitude of minute craterlets on its south-eastern or northern rims. similar craterlets occur on the rims of other great craters, forming ray-centres. ( .) speaking generally, a very minute and brilliant crater is located at the end of the streak nearest the radiant point, the streak spreading out and becoming fainter towards the other end. the majority of the streaks appear to issue from one or more of these minute craters, which rarely exceed a mile in diameter. ( .) the streaks which do not issue from minute craters, usually lie upon or across ridges, or in other similar exposed situations, sometimes apparently coming through notches in the mountain walls. ( .) many of the copernicus streaks start from craterlets within the rim, flow up the inside and down the outside of the walls. kepler includes two such craterlets, but here the flow seems to have been more uniform over the edges of the whole crater, and is not distinctly divided up into separate streams. ( .) though there are similar craters within tycho, the streaks from them do not extend far beyond the walls. all the conspicuous tycho streaks originate outside the rim. ( .) the streaks of copernicus, kepler, and aristarchus are greyish in colour, and much less white than those associated with tycho: some white lines extending south-east from aristarchus do not apparently belong to the system. in the case of craterlets lying between aristarchus and copernicus the streaks point away from the latter. ( .) there are no very long streaks; they vary from ten to fifty miles in length, and are rarely more than a quarter of a mile broad at the crater. from this point they gradually widen out and become fainter. their width, however, at the end farthest from the crater, seldom exceeds five miles. these statements, especially those relating to the length of the streaks, are utterly opposed to prevailing notions, but professor pickering specifies the case of the two familiar parallel rays extending from the north-east of tycho to the region east of bullialdus. his observations show that these streaks, originating at a number of little craters situated from thirty to sixty miles beyond the confines of tycho, "enter a couple of broad slightly depressed valleys. in these valleys are found numerous minute craters of the kind above described, with intensely brilliant interiors. when the streaks issuing from those craters near tycho are nearly exhausted, they are reinforced by streaks from other craters which they encounter upon the way, the streaks becoming more pronounced at these points. these streaks are again reinforced farther out. these parallel rays must therefore not be considered as two streaks, but as two series of streaks, the components of which are placed end to end." thus, according to professor pickering, we must no longer regard the rays emanating from the tycho region and other centres as continuous, but as consisting of a succession of short lengths, diminishing in brilliancy but increasing in width, till they reach the next crater lying in their direction, when they are reinforced; and the same process of gradual diminution in brightness and reinforcement goes on from one end to the other. the following explanation is suggested to account for the origin of the rays:--"the earth and her satellite may differ not so much as regards volcanic action as in the densities of their atmospheres. thus if the craterlets on the rim of tycho were constantly giving out large quantities of gas or steam, which in other regions was being constantly absorbed or condensed, we should have a wind uniformly blowing away from that summit in all directions. should other summits in its vicinity occasionally give out gases, mixed with any fine white powder, such as pumice, this powder would be carried away from tycho, forming streaks." the difficulty surrounding this very ingenious hypothesis is, that though, assuming the existence of pumice-emitting craters and regions of condensation, there might be a more or less lineal and streaky deposition of this white material over large areas of the moon, why should this deposit be so definitely arranged, and why should these active little craters happen to lie on these particular lines? the confused network of streaks round copernicus seem to respond more happily to the requirements of professor pickering's hypothesis, for here there is an absence of that definiteness of direction so manifestly displayed in the case of the tycho rays, and we can well imagine that with an area of condensation surrounding this magnificent object beyond the limits of the streaks, and a number of active little craters on and about its rim, the white material ejected might be drawn outwards in every direction by wind currents, which possibly once existed, and, settling down, assume forms such as we see. nasmyth's well-known hypothesis attributes the radiating streaks to cracks in the lunar globe caused by the action of an upheaving force, and accounts for their whiteness by the outwelling of lava from them which has spread to a greater or less distance on either side. if the moon has been fractured in this way, we can easily suppose that the craters formed on these fissures, being in communication with the interior, might eject some pulverulent white matter long after the rest of the surface with its other types of craters had attained a quiescent stage. the tycho rays, when viewed under ordinary conditions, appear to extend in unbroken bands to immense distances. one of the most remarkable, strikes along the eastern side of fracastorius, across the mare nectaris to guttemberg, while another, more central, extends, with local variations in brightness, through menelaus, over the mare serenitatis nearly to the north-west limb. this is the ray that figures so prominently in rude woodcuts of the moon, in which the mare serenitatis traversed by it is made to resemble the greek letter phi. the kepler, aristarchus, and copernicus systems, though of much smaller extent, are very noteworthy from the crossing and apparent interference of the rays; while those near byrgius, round aristarchus, and the rays from proclus, are equally remarkable. [nichol found that the rays from kepler cut through rays from copernicus and aristarchus, while rays from the latter cut through rays from the former. he therefore inferred that their relative ages stand in the order,--copernicus, aristarchus, kepler.] as no branch of selenography has been more neglected than the observation of these interesting but enigmatical features, one may hope that, in spite of the exacting conditions as to situation and instrumental requirements necessary for their successful scrutiny, the fairly equipped amateur in this less favoured country will not be deterred from attempting to clear up some of the doubts and difficulties which at present exist as to their actual nature. the moon's albedo, surface brightness, &c.--sir john herschel maintained that "the actual illumination of the lunar surface is not much superior to that of weathered sandstone rock in full sunshine." "i have," he says, "frequently compared the moon setting behind the grey perpendicular facade of the table mountain, illuminated by the sun just risen in the opposite quarter of the horizon, when it has been scarcely distinguishable in brightness from the rock in contact with it. the sun and moon being at nearly equal altitudes, and the atmosphere perfectly free from cloud or vapour, its effect is alike on both luminaries." zollner's elaborate researches on this question are closely in accord with the above observational result. though he considers that the brightest parts of the surface are as white as the whitest objects with which we are acquainted, yet, taking the reflected light as a whole, he finds that the moon is more nearly black than white. the most brilliant object on the surface is the central peak of the ring-plain aristarchus, the darkest the floor of grimaldi, or perhaps a portion of that of the neighbouring riccioli. between these extremes, there is every gradation of tone. proctor, discussing this question on the basis of zollner's experiments respecting the light reflected by various substances, concludes that the dark area just mentioned must be notably darker than the dark grey syenite which figures in his tables, while the floor of aristarchus is as white as newly fallen snow. the estimation of lunar tints in the usual way, by eye observations at the telescope, involving as it does physiological errors which cannot be eliminated, is a method far too crude and ambiguous to form the basis of a scientific scale or for the detection of slight variations. an instrument on the principle of dawes' solar eyepiece has been suggested; this, if used with an invariable and absolute scale of tints, would remove many difficulties attending these investigations. the scale which was adopted by schroter, and which has been used by selenographers up to the present time, is as follows:-- deg. = black. deg. = greyish black. deg. = dark grey. deg. = medium grey. deg. = yellowish grey. deg. = pure light grey. deg. = light whitish grey. deg. = greyish white. deg. = pure white. deg. = glittering white. deg. = dazzling white. the following is a list of lunar objects published in the _selenographical journal_, classed in accordance with this scale:-- deg. black shadows. deg. darkest portions of the floors of grimaldi and riccioli. / deg. interiors of boscovich, billy, and zupus. deg. floors of endymion, le monnier, julius caesar, cruger, and fourier _a_. / deg. interiors of azout, vitruvius, pitatus, hippalus, and marius. deg. interiors of taruntius, plinius, theophilus, parrot, flamsteed, and mercator. / deg. interiors of hansen, archimedes, and mersenius. deg. interiors of manilius, ptolemaeus, and guerike. / deg. surface round aristillus, sinus medii. deg. walls of arago, landsberg, and bullialdus. surface round kepler and archimedes. / deg. walls of picard and timocharis. rays from copernicus. deg. walls of macrobius, kant, bessel, mosting, and flamsteed. / deg. walls of langrenus, theaetetus, and lahire. deg. theon, ariadaeus, bode b, wichmann, and kepler. / deg. ukert, hortensius, euclides. deg. walls of godin, bode, and copernicus. / deg. walls of proclus, bode a, and hipparchus c. deg. censorinus, dionysius, mosting a, and mersenius b and c. / deg. interior of aristarchus, la peyrouse delta. deg. central peak of aristarchus. temperature of the moon's surface.--till the subject was undertaken some years ago by lord rosse, no approach was made to a satisfactory determination of the surface temperature of the moon. from his experiments he inferred that the maximum temperature attained, at or near the equator, about three days after full moon, does not exceed deg. c., while the minimum is not much under zero c. subsequent experiments, however, both by himself and professor langley, render these results more than doubtful, without it is admitted that the moon has an atmospheric covering. langley's results make it probable that the temperature never rises above the freezing-point of water, and that at the end of the prolonged lunar night of fourteen days it must sink to at least deg. below zero. mr. f.w. verey of the alleghany observatory has recently conducted, by means of the bolometer, similar researches as to the distribution of the moon's heat and its variation with the phase, by which he has deduced the varying radiation from the surface in different localities of the moon under various solar altitudes. lunar observation.--in observing the moon, we enjoy an advantage of which we cannot boast when most other planetary bodies are scrutinised; for we see the actual surface of another world undimmed by palpable clouds or exhalations, except such as exist in the air above us; and can gaze on the marvellous variety of objects it presents much as we contemplate a relief map of our own globe. but inasmuch as the manifold details of the relief map require to be placed in a certain light to be seen to the best advantage, so the ring-mountains, rugged highlands, and wide-extending plains of our satellite, as they pass in review under the sun, must be observed when suitable conditions of illumination prevail, if we wish to appreciate their true character and significance. as a general rule, lunar objects are best seen when they are at no great distance from "the terminator," or the line dividing the illumined from the unillumined portion of the spherical surface. this line is constantly changing its position with the sun, advancing slowly onwards towards the east at a rate which, roughly speaking, amounts to about . min. in an hour, or passing over deg. of lunar longitude in about hrs. mins. when an object is situated on this line, the sun is either rising or setting on the neighbouring region, and every inequality of the surface is rendered prominent by its shadow; so that trifling variations in level and minor asperities assume for the time being an importance to which they have no claim. if we are observing an object at lunar sunrise, a very short time, often only a few minutes, elapses before the confusion caused by the presence of the shadows of these generally unimportant features ceases to interfere with the observation, and we can distinguish between those details which are really noteworthy and others which are trivial and evanescent. every formation we are studying should be observed, and drawn if possible, under many different conditions of illumination. it ought, in fact, to be examined from the time when its loftiest heights are first illumined by the rising sun till they disappear at sunset. this is, of course, practically impossible in the course of one lunation, but by utilising available opportunities, a number of observations may be obtained under various phases which will be more or less exhaustive. it cannot be said that much is known about any object until an attempt has been made to carry out this plan. features which assume a certain appearance at one phase frequently turn out to be altogether different when viewed under another; important details obscured by shadows, craters masked by those of neighbouring objects, or by the shadows of their own rims, are often only revealed when the sun has attained an altitude of ten degrees or more. in short, there is scarcely a formation on the moon which does not exemplify the necessity of noting its aspect from sunrise to sunset. regard must also be had to libration, which affects to a greater or less degree every object; carrying out of the range of observation regions near the limb at one time, and at another bringing into view others beyond the limits of the maps, which represent the moon in the mean state of libration. the area, in fact, thus brought into view, or taken out of it, is between / th and / th of the entire area of the moon, or about the / th part of the hemisphere turned away from the earth. it is convenient to bear in mind that we see an object under nearly the same conditions every d. h. m., or still more accurately, after the lapse of fifteen lunations, or d. h. many observers avoid the observation of objects under a high light. this, however, should never be neglected when practicable, though in some cases it is not easy to carry out, owing to the difficulty in tracing details under these circumstances. although to observe successfully the minuter features, such as the rills and the smaller craterlets, requires instruments of large aperture located in favourable situations, yet work of permanent value may be accomplished with comparatively humble telescopic means. a inch achromatic, or a silver-on-glass reflector of or / inches aperture, will reveal on a good night many details which have not yet been recorded, and the possessor of instruments of this size will not be long in discovering that the moon, despite of what is often said, has not been so exhaustively surveyed that nothing remains for him to do. only experience and actual trial will teach the observer to choose the particular eyepiece suitable for a given night or a given object. it will be found that it is only on very rare occasions that he can accomplish much with powers which, perhaps only on two or three nights in a year in this climate, tell to great advantage; though it sometimes happens that the employment of an eyepiece, otherwise unsuitable for the night, will, during a short spell of good definition, afford a fleeting glimpse of some difficult feature, and thus solve a doubtful point. it has often been said that the efficiency of a telescope depends to a great extent on "the man at the eye end." this is as true in the case of the moon as it is in other branches of observational astronomy. observers, especially beginners, frequently fall into great error in failing to appreciate the true character of what they see. in this way a shallow surface depression, possibly only a few feet below the general level of the neighbouring country, is often described as a "vast gorge," because, under very oblique light, it is filled with black shadow; or an insignificant hillock is magnified into a mountain when similarly viewed. hence the importance, just insisted on, of studying lunar features under as many conditions as possible before finally attempting to describe them. however indifferent a draughtsman an observer may be, if he endeavours to portray what he sees to the best of his ability, he will ultimately attain sufficient skill to make his work useful for future reference: in any case, it will be of more value than a mere verbal description without a sketch. doubt and uncertainty invariably attend to a greater or less extent written notes unaccompanied by drawings, as some recent controversies, respecting changes in linne and elsewhere, testify. now that photographs are generally available to form the basis of a more complete sketch, much of the difficulty formerly attending the correct representation of the outline and grosser features of a formation has been removed, and the observer can devote his time and attention to the insertion and description of less obvious objects. progress of selenography.--till within recent years, the systematic study of the lunar surface may be said to have been confined, in this country at any rate, to a very limited number of observers, and, except in rare instances, those who possessed astronomical telescopes only directed them to the moon as a show object to excite the wonder of casual visitors. the publication of webb's "celestial objects" in , the supposed physical change in the crater linne, announced in , and the appearance of an unrecorded black spot near hyginus some ten years later, had the effect of awakening a more lively interest in selenography, and undoubtedly combined to bring about a change in this respect, which ultimately resulted in the number of amateurs devoting much of their time to this branch of observational astronomy being notably increased. still, large telescopes, as a rule, held aloof for some unexplained reason, or were only employed in a desultory and spasmodic fashion, without any very definite object. when the council of the british association for the advancement of science, stimulated by the linne controversy, deemed the moon to be worthy of passing attention, observations, directed to objects suspected of change (the phenomena on the floor of plato) were left to three or four observers, under the able direction of mr. birt, the largest instruments available being an / inch reflector and the crossley refractor of inches aperture! during the last decade, however, all this has been changed, and we not only have societies, such as the british astronomical association, setting apart a distinct section for the systematic investigation of lunar detail, but some of the largest and most perfect instruments in the world, among them the noble refractor on mount hamilton, employed in photographing the moon or in scrutinising her manifold features by direct observation. hence, it may be said that selenography has taken a new and more promising departure, which, among other results, must lead to a more accurate knowledge of lunar topography, and settle possibly, ere long, the vexed question of change, without any residuum of doubt. lunar photography as exemplified by the marvellous and beautiful pictures produced at the lick observatory under the auspices of dr. holden, and the exquisite enlargements of them by dr. weinek of prague; at paris by the brothers henry; and at brussels by m. prinz; point to the not far distant time when we shall possess complete photographic maps on a large scale of the whole visible disc under various phases of illumination, which will be of inestimable value as topographical charts. when this is accomplished, the observer will have at his command faithful representations of any formation, or of any given region he may require, to utilise for the study of the smaller details by direct observation. desultory and objectless drawings and notes have hitherto been more or less characteristic of the work done, even by those who have given more than ordinary attention to the moon. though these, if duly recorded, are valuable as illustrating the physical structure, the estimated brightness under various phases, and other peculiarities of lunar features, they do not materially forward investigations relating to the discovery of present lunar activity or to the detection of actual change. it is reiterated _ad nauseam_ in many popular books that the moon is a changeless world, and it is implied that, having attained a state when no further manifestations of internal or external forces are possible, it revolves round the earth in the condition, for the most part, of a globular mass of vesicular lava or slag, possessing no interest except as a notable example of a "burnt-out planet." in answer to these dogmatic assertions, it may be said that, notwithstanding the multiplication of monographs and photographs, the knowledge we possess, even of the larger and more prominent objects, is far too slight to justify us in maintaining that changes, which on earth we should use a strong adjective to describe, have not taken place in connection with some of them in recent years. would the most assiduous observer assert that his knowledge of any one of the great formations, in the south-west quadrant, for example, is so complete that, if a chasm as big as the val del bove was blown out from its flanks, or formed by a landslip, he would detect the change in the appearance of an area (some three miles by four) thus brought about, unless he had previously made a very prolonged and exhaustive study of the object? or, again, among formations of a different class, the craters and crater-cones; might not objects as large as monte nuovo or jorullo come into existence in many regions without any one being the wiser? it would certainly have needed a persistent lunar astronomer, and one furnished with a very perfect telescope, to have noted the changes that have occurred within the old crater-ring of somma or among the santorin group during the past thirty years, or even to have detected the effects resulting from the great catastrophe in a.d. , at vesuvius; yet these objects are no larger than many which, if they were situated on our satellite, would be termed comparatively small, if not insignificant. one of the principal aims of lunar research is to learn as much as possible as to the present condition of the surface. every one qualified to give an opinion will admit that this cannot be accomplished by roaming at large over the whole visible superficies, but only by confining attention to selected areas of limited extent, and recording and describing every object visible thereon, under various conditions of illumination, with the greatest accuracy attainable. this plan was suggested and inaugurated nearly thirty years ago by mr. birt, under the patronage of the british association; but as he proposed to deal with the entire disc in this way, the magnitude and ambitious character of the scheme soon damped the ardour of those who at first supported it, and it was ultimately abandoned. it was, however, based on the only feasible principle which, as it seems to the writer, will not result in doubt and confusion. now that photography has come to the assistance of the observer, mr. birt's proposal, if confined within narrower limits, would be far less arduous an undertaking than before, and might be easily carried out. a complete photographic survey of a few selected regions, as a basis for an equally thorough and exhaustive scrutiny by direct observation, would, it is believed, lead to a much more satisfactory and hopeful method for ultimately furnishing irrefragable testimony as to permanency or change than any that has yet been undertaken. catalogue of lunar formations first quadrant west longitude deg. to deg. schubert.--this ring-plain, about miles in diameter, situated on the n.e. side of the mare smythii, is too near the limb to be well observed. neper.--though still nearer the limb, this walled-plain, miles in diameter, is a much more conspicuous object. it has a lofty border and a prominent central mountain, the highest portion of a range of hills which traverses the interior from n. to s. apollonius.--a ring-plain, miles in diameter, standing in the mountainous region s. of the mare crisium. there is a large crater on the s.w. wall, and another, somewhat smaller, adjoining it on the n. there are many brilliant craters in the vicinity. firmicus.--a somewhat larger, more regular, but, in other respects, very similar ring-plain, n.w. of the last. some distance on the w., madler noted a number of dark-grey streaks which apparently undergo periodical changes, suggestive of something akin to vegetation. they are situated near a prominent mountain situated in a level region. azout.--a small ring-plain, connected with the last by a lofty ridge. it is the apparent centre of many other ridges and valleys which radiate from it towards the n.w. and the mare crisium. there is a central mountain, not an easy telescopic object, on its dusky floor. condorcet.--a very prominent ring-plain, miles in diameter, situated on the mountainous s.w. margin of the mare crisium. it is encircled by a lofty wall about feet in height. the dark interior of this and of the three preceding formations render them easily traceable under a high angle of illumination. hansen.--a ring-plain, miles in diameter, on the w. border of the mare crisium n. of condorcet. schmidt shows a central mountain and a terraced wall. alhazen.--this ring-plain, rather smaller than the last, is the most northerly of the linear chain of formations, associated with the highlands bordering the s.w. and the w. flanks of the mare crisium. it has a central mountain and other minor elevations on the floor. there is a little ring between alhazen and hansen, never very conspicuous in the telescope, which is plainly traceable in good photographs. eimmart.--a conspicuous ring-plain with bright walls on the n.w. margin of the mare crisium. the e. border attains a height of , feet above the interior, which, according to schmidt, has a small central mountain. there is a rill-like valley on the e. of the formation. oriani.--an irregular object, miles in diameter, somewhat difficult to identify, n.w. of the last. there is a conspicuous crater on the n. of it, with which it is connected by a prominent ridge. plutarch.--a fine ring-plain w. of oriani, with regular walls, and, according to neison, with two central mountains, only one of which i have seen. both this formation and the last are beautifully shown in a photograph taken august , , at the lick observatory, when the moon's age was d. hrs. seneca.--rather smaller than plutarch. too near the limb for satisfactory observation. schmidt shows two considerable mountains in the interior. the position of this object in schmidt's chart is not accordant with its place in beer and madler's map, nor in that of neison. hahn.--a ring-plain, miles in diameter, with a fine central mountain and lofty peaks on the border, which is not continuous on the s. there is a large and prominent crater on the e. berosus.--a somewhat smaller object of a similar type, n. of hahn, but with a loftier wall. there is a want of continuity also in the border, the eastern and western sections of which, instead of joining, extend for some distance towards the s., forming a narrow gorge or valley. outside the s.e. wall there is a small crater, and some irregular depressions on the e. the minute central mountain is only seen with difficulty under a low evening sun. the bright region between hahn and berosus and the western flank of cleomedes is an extensive plain, devoid of prominent detail, and which, according to neison, includes an area of , square miles. gauss.--a large, and nearly circular walled-plain, miles in diameter, situated close to the n.w. limb, and consequently always foreshortened into a more or less elongated ellipse. but for this it would be one of the grandest objects in the first quadrant. under the designation of "mercurius falsus" it received great attention from schroter, who gives several representations of it in his _selenotopographische fragmente_, which, though drawn in his usual conventional style, convey a juster idea of its salient features than many subsequent drawings made under far better optical conditions. the border, especially on the w., is very complex, and is discontinuous on the s., where it is intersected by more than one pass, and is prolonged far beyond the apparent limits of the formation. the most noteworthy feature is the magnificent mountain chain which traverses the floor from n. to s. it is interesting to watch the progress of sunset thereon, and see peak after peak disappear, till only the great central boss and a few minute glittering points of light, representing the loftier portions of the chain, remain to indicate its position. madler expatiates on the sublime view which would be obtained by any one standing on the highest peak and observing the setting sun on one side of him and the nearly "full" earth on the other; while beneath him would lie a vast plain, shrouded in darkness, surrounded by the brilliantly illuminated peaks on the lofty border, gradually passing out of sunlight. in addition to the central mountain range, there are some large rings, craters, hillocks, &c., on the floor; and on the inner slope of the w. border there is a very large circular enclosure resembling a ring-plain, not recorded in the maps. schmidt shows a row of large craters on the outer slope of the e. border. of these, one is very conspicuous under a low evening sun, by reason of its brilliant walls and interior. in the region between gauss and berosus is a number of narrow steep ridges which follow the curvature of the e. wall. struve.--a small irregularly-shaped formation, open towards the s., forming one of the curious group of unsymmetrical enclosures associated with messala. its dark floor and a small dusky area on the n. indicate its position under a high sun. carrington.--a small ring-plain, belonging to the messala group, adjoining schumacher on the n.w. mercurius.--this formation is miles in diameter. a small crater stands on the s.e. section of the wall. there is a longitudinal range in the interior, and on the w. and n.w. the remains of two large walled-plains, the more westerly of which is a noteworthy object under suitable conditions. a short distance s. is a large, irregular, and very dark marking. on the n., lies an immense bright plain, extending nearly to the border of endymion. west longitude deg. to deg. taruntius.--notwithstanding its comparatively low walls, this ring-plain, miles in diameter, is a very conspicuous object under a rising sun. like vitello and a few other formations, it has an inner ring on the floor, concentric with the outer rampart, which i have often seen nearly complete under evening illumination. there is a small bright crater on the s.e. wall, and a larger one on the crest of the n.e. wall, with a much more minute depression on the w. of it, the intervening space exhibiting signs of disturbance. the upper portion of the wall is very steep, contrasting in this respect with the very gentle inclination of the _glacis_, which on the s. extends to a distance of at least miles before it sinks to the level of the surrounding country, the gradient probably being as slight as in . two low dusky rings and a long narrow valley with brilliant flanks are prominent objects on the plain e. of taruntius under a low evening sun. secchi.--a partially enclosed little ring-plain s. of taruntius, with a prominent central mountain and bright walls. there is a short cleft running in a n.e. direction from a point near the e. wall. schmidt represents it as a row of inosculating craters. picard.--the largest of the craters on the surface of the mare crisium, miles in diameter. the floor, which includes a central mountain, is depressed about feet below the outer surface, and is surrounded by walls rising some feet above the mare. a small but lofty ring-plain, picard e, on the e., near the border of the mare, is remarkable for its change of aspect under different angles of illumination. a long curved ridge running s. from this, with a lower ridge on the west, sometimes resemble a large enclosure with a central mountain. still farther s., there is another bright deep crater, _a_, with a large low ring adjoining it on the s., abutting on the s.e. border of the mare. schroter bestowed much attention on these and other formations on the mare crisium, and attributed certain changes which he observed to a lunar atmosphere. peirce.--this formation, smaller than picard, is also prominent, its border being very bright. there is a central peak, which, though not an easy object, i once glimpsed with a inch cook achromatic, and have seen it two or three times since with an / inch calver reflector. a small crater, detected by schmidt, which i once saw very distinctly under evening illumination, stands on the floor at the foot of the w. wall. peirce a, a deeper formation, lies a little n. of peirce, and has also, according to neison, a very slight central hill, which is only just perceptible under the most favourable conditions. schmidt appears to have overlooked it. proclus.--one of the most brilliant objects on the moon's visible surface, and hence extremely difficult to observe satisfactorily. it is about miles in diameter, with very steep walls, and, according to schmidt, has a small crater on its east border, where madler shows a break. it is questionable whether there is a central mountain. it is the centre of a number of radiating light streaks which partly traverse the mare crisium, and with those emanating from picard, peirce, and other objects thereon, form a very complicated system. macrobius.--this, with a companion ring on the w., is a very beautiful object under a low sun. it is miles in diameter, and is encircled by a bright, regular, but complex border, some , feet in height above the floor. its crest is broken on the e. by a large brilliant crater, and its continuity is interrupted on the n. by a formation resembling a large double crater, which is associated with a number of low rounded banks and ridges extending some distance towards the n.w., and breaking the continuity of the _glacis_. the w. wall is much terraced, and on the n.w. includes a row of prominent depressions, well seen when the interior is about half illuminated under a rising sun. the central mountain is of the compound type, but not at all prominent. the companion ring, macrobius c, is terraced internally on the w., and the continuity of its n. border broken by two depressions. there is a rill-valley between its n.e. side and macrobius. cleomedes.--a large oblong enclosure, miles in diameter, with massive walls, varying in altitude from to , feet above the interior. the most noteworthy features in connection with the circumvallation are the prominent depressions on the w. wall. under a rising sun, when about one-fourth of the floor is in shadow, three of these can be easily distinguished, each resembling in form the analemma figure. there are two other curious depressions at the s.w. end of the formation. on the dark steel-grey floor are two irregular dusky areas, and a narrow but bright central mountain, on which, according to schmidt, stand two little craters. there are two ring-plains on the s.w. quarter, and a group of three associated craters on the n. side, one of which (a) schroter believed came into existence after he commenced to observe the formation, a supposition that has been shown by birt and others to be very improbable. tralles.--a large irregular crater, one of the deepest on the visible surface of the moon, situated on the n.e. wall of cleomedes. there is a crater on its n. wall, and, according to schmidt, some ridges and three closely associated craters on the floor. burckhardt.--this object, situated on an apparent extension of the w. wall of cleomedes, is miles in diameter, with a lofty border, rising on the e. to an altitude of nearly , feet. it has a prominent central mountain and some low ridges on the floor, which, together with two minute craters on the s.w. wall, i have seen under a low angle of morning illumination. it is flanked both on the e. and w. by deep irregular depressions, which present the appearance of having once been complete formations. geminus.--a fine regular ring-plain, miles in diameter, nearly circular, with bright walls, rising on the e. to a height of more than , feet, and on the opposite side to nearly , feet above the floor. their crest is everywhere very steep, and the inner slope is much terraced. there is a small but conspicuous mountain in the interior; n. of which i have seen a long ridge, where schmidt shows some hillocks. two fine clefts are easily visible within the ring, one running for some distance on the s.e. side of the floor, mounting the inner slope of the s.w. border to the summit ridge (where it is apparently interrupted), and then striking across the plain in a s.w. direction. here it is accompanied for a short distance by a somewhat coarser companion, running parallel to it on the n. the other cleft occupies a very similar position on the n.w. side of the floor at the inner foot of the wall. on several occasions, when observing this formation and the vicinity, i have been struck by its peculiar colour under a low evening sun. at this time the whole region appears to be of a warm light brown or sepia tone. bernouilli.--a very deep ring-plain on the w. side of geminus. under evening illumination its lofty w. wall, which rises to a height of nearly , feet above the floor, is conspicuously brilliant. this formation exhibits a marked departure from the circular type, being bounded by rectilineal sides. the inner slope of the w. wall is slightly terraced. the border on the s. is much lower than elsewhere, as is evident when the formation is on the evening terminator. on the n. is the deep crater messala _a_. newcomb.--the most prominent of a group of formations standing in the midst of the haemus mountains. its crest is nearly , feet above the floor, on which there are some hills. messala.--this fine walled-plain, nearly miles in diameter, is, with its surroundings, an especially interesting object when observed under a low angle of illumination. its complex border, though roughly circular, displays many irregularities in outline, due mainly to rows of depressions. the best view of it is obtained when the w. wall is on the evening terminator. at this phase, if libration is favourable, the manifold details of its very uneven and apparently convex floor are best seen. on the s.w. side is a group of large craters associated with a number of low hills, of which schmidt shows five; but i have seen many more, together with several ridges between them and the e. wall. i noted also a cleft, or it may be a narrow valley, running from the foot of the n.w. wall towards the centre. on the floor, abutting on the n.e. border, is a semicircular ridge of considerable height, and beyond the border on the n.e. there is another curved ridge, completing the circle, the wall forming the diameter. this formation is clearly of more ancient date than messala, as the n.e. wall of the latter has cut through it. where messala joins schumacher there is a break in the border, occupied by three deep depressions. schumacher.--a large irregular ring-plain, miles in diameter, associated with the n. wall of messala, and having other smaller rings adjoining it on the e. and n. the interior seems to be devoid of detail. hooke.--another irregular ring-plain, miles in diameter, on the n.e. of messala. there is a bright crater of considerable size on the s.w., which is said to be more than feet in depth, and, according to neison, is visible as a white spot at full. there is a smaller crater on the slope of the n.w. wall. shuckburgh.--a square-shaped enclosure on the n. of the last, with a comparatively low border. it has a conspicuous crater at its n.w. corner. berzelius.--a considerable ring-plain of regular form, with low walls and dark interior, on which there is a central peak, difficult to detect. franklin.--a ring-plain, miles in diameter, which displays a considerable departure from the circular type, as the border is in great part made up of rectilineal sections. both the w. and n.e. wall is much terraced, and rises about feet above the dark floor, on the s. part of which there is a long ridge. there is a bright little isolated mountain on the plain e. of the formation, and a conspicuous craterlet on the n.w. an incomplete ring, with a very attenuated border, abuts on the s. side of franklin. cepheus.--a peculiarly shaped ring-plain, miles in diameter. the e. border is nearly rectilineal, while on the w., the wall forms a bold curve. there is a very brilliant crater on the summit of this section, and a central mountain on the floor. the w. wall is much terraced. w. of cepheus, close to the brilliant crater, there is a cleft or narrow valley running n. towards oersted. oersted.--an oblong formation with very low walls, scarcely traceable on the s.e., except when near the terminator. there is a conspicuous crater on the n.w. side of the floor, and a curious square enclosure, with a crater on its w. border, abutting on the n.e. wall. chevallier.--an inconspicuous object enclosed by slightly curved ridges. it includes a deep bright crater. on the n. is a low square formation and a long ridge running n. from it. just beyond the n.e. wall is the fine large crater, atlas a, with a much smaller but equally conspicuous crater beyond. a has a central hill, which, in spite of the bright interior, is not a difficult feature. atlas.--this, and its companion hercules on the e., form under oblique illumination a very beautiful pair, scarcely surpassed by any other similar objects on the first quadrant. its lofty rampart, miles in diameter, is surmounted by peaks, which on the n. tower to an altitude of nearly , feet. it exhibits an approach to a polygonal outline, the lineal character of the border being especially well marked on the n. the detail on the somewhat dark interior will repay careful scrutiny with high powers. there is a small but distinct central mountain, south of which stands a number of smaller hills, forming with the first a circular arrangement, suggestive of the idea that they represent the relics of a large central crater. several clefts may be seen on the floor under suitable illumination, among them a forked cleft on the n.e. quarter, and two others, originating at a dusky pit of irregular form situated near the foot of the s.e. wall, one of which runs w. of the central hills, and the other on the opposite side. a ridge, at times resembling a light marking, extends from the central mountain to the n. border. during the years and i bestowed some attention on the dusky pit, and was led to suspect that both it and the surrounding area vary considerably in tone from time to time. professor w.h. pickering, observing the formation in with a inch telescope under the favourable atmospheric conditions which prevail at arequipa, peru, confirmed this supposition, and has discovered some very interesting and suggestive facts relating to these variations, which, it is hoped, will soon be made public. on the plain a short distance beyond the foot of the _glacis_ of the s.e. wall, i have frequently noted a second dusky spot, from which proceeds, towards the e., a long rill-like marking. on the n. there is a large formation enclosed by rectilineal ridges. the outer slopes of the rampart of atlas are very noteworthy under a low sun. hercules.--the eastern companion of atlas, a fine ring-plain, about miles in diameter, with a complex border, rising some , feet above a depressed floor. there are few formations of its class and size which display so much detail in the shape of terraces, apparent landslips, and variation in brightness. in the interior, s.e. of the centre, is a very conspicuous crater, which is visible as a bright spot when the formation itself is hardly traceable, two large craterlets slightly n. of the centre, and several faint little spots on the east of them. the latter, detected some years ago by herr hackel of stuttgart, are arranged in the form of a horse-shoe. there are two small contiguous craters on the s.e. wall, one of which, a difficult object, was recently detected by mr. w.h. maw, f.r.a.s. the well-known wedge-shaped protuberance on the s. wall is due to a large irregular depression. on the bright inner slope of the n. wall are manifest indications of a landslip. endymion.--a large walled-plain, miles in diameter, enclosed by a lofty, broad, bright border, surmounted in places by peaks which attain a height of more than , feet above the interior, one on the w. measuring more than , feet. the walls are much terraced and exhibit two or three breaks. the dark floor appears to be devoid of detail. schmidt, however, draws two large irregular mounds e. of the centre, and shows four narrow light streaks crossing the interior nearly parallel to the longer axis of the formation. de la rue.--notwithstanding its great extent, this formation hardly deserves a distinctive name, as from the lowness of its border it is scarcely traceable in its entirety except under very oblique light. schmidt, nevertheless, draws it with very definite walls, and shows several ridges and small rings in the interior. among these objects, a little e. of the centre, there is a prominent peak. strabo.--a small walled-plain, miles in diameter, connected with the n. border of the last. thales.--a bright formation, also associated with the n. side of de la rue, adjoining strabo on the n.e. schmidt shows a minute hill in the interior. there are several unnamed formations, large and small, between de la rue and the limb, some of which are well worthy of examination. west longitude deg. to deg. maskelyne.--a regular ring-plain, miles in diameter, standing almost isolated in the mare tranquilitatis. the floor, which includes a central mountain, is depressed some feet below the surrounding surface. there are prominent terraces on the inner slope of the walls. schmidt shows no craters upon them, but madler draws a small one on the e., the existence of which i can confirm. manners.--a brilliant little ring-plain, miles in diameter, on the s.e. side of the mare tranquilitatis. there appears to be no detail whatever in connection with its wall. it has a distinct central mountain. about three diameters distant on the s.w. there is a bright crater, omitted by madler and neison. arago.--a much larger formation, miles in diameter, n. of the last, with a small crater on its n. border, and exhibiting two or three spurs from the wall on the opposite side. the inner slopes are terraced, and there is a small central mountain. there are two curious circular protuberances on the mare e. of arago, which are well seen when the w. longitude of the morning terminator is about deg., and a long cleft, passing about midway between them, and extending from the foot of the e. wall to a small crater on the edge of the mare near sosigenes. another cleft, also terminating at this crater, runs towards arago and the more northerly of the protuberances. cauchy.--a bright little crater, not more than or miles in diameter, on the w. side of the mare tranquilitatis, n.e. of taruntius. it has a peak on its w. rim considerably loftier than the rest of the wall, which is visible as a brilliant spot at sunrise long before the rest of the rampart is illuminated. on the s. there are two bright longitudinal ridges ranging from n.e. to s.w. these stand in the position where neison draws two straight clefts. the cauchy cleft, however, lies n. of these, and terminates, as shown by schmidt, among the mountains n.e. of taruntius. i have seen it thus on many occasions, and it is so represented in a drawing by m.e. stuvaert (_dessins de la lune_). there is a number of minute craters and mounds standing on the s. side of this cleft, and many others in the vicinity. jansen.--owing to its comparatively low border, this is not a very conspicuous object. it is chiefly remarkable for the curious arrangement of the mountains and ridges on the s. and w. of it. there is a bright little crater on the s. side of the floor, and many noteworthy objects of the same class in the neighbourhood. the mountain arm running s., and ultimately bending e., forms a large incomplete hook-shaped formation terminating at a ring-plain, jansen b. the ridges in the mare tranquilitatis between jansen b. and the region e. of maskelyne display under a low sun foldings and wrinklings of a very extraordinary kind. maclear.--a conspicuous ring-plain about miles in diameter. the dark floor includes, according to madler, a delicate central hill which schmidt does not show. neison, however, saw a faint greyish mark, and an undoubted peak has been subsequently recorded. i have not succeeded in seeing any detail within the border, which in shape resembles a triangle with curved sides. ross.--a somewhat larger ring-plain of irregular form, on the n.w. of the last. there are gaps on the bright s.w. border and a crater on the s.e. wall. the central mountain is an easy feature. plinius.--this magnificent object reminds one at sunrise of a great fortress or redoubt erected to command the passage between the mare tranquilitatis and the mare serenitatis. it is miles in diameter, and is encompassed by a very massive rampart, rising at one peak on the e. to more than feet above the interior, and displaying, especially on the s.e., and n., many spurs and buttresses. the exterior slopes at sunrise, and even when the sun is more than deg. above the horizon, are seen to be traversed by wide and deep valleys. the s. _glacis_ is especially broad, extending to a distance of or miles before it runs down to the level of the plain. the shape of the circumvallation, when it is fully illuminated, approximates very closely to that of an equilateral triangle with curved sides. there are two bright little craters on the outer slope, just below the summit ridge on the s.e., and another, larger, on the n. wall, in which it makes a prominent gap. the interior is considerably brighter than the surface of the surrounding mare, and, a little s. of the centre, includes two crater-like objects with broken rims. these assume different aspects under different conditions of illumination, and it is only when the floor is lighted by a comparatively low morning sun, that their true character is apparent. on the n.w. quarter of the interior are two smaller distinct craters, and a square arrangement of ridges. on the n.e. there are some hillocks and minor elevations. the plinius rills form an especially interesting system, and under favourable conditions may be seen in their entirety with a good inch refractor, about the time when the morning terminator passes through julius caesar. they consist of three long fissures, originating amid the haemus highlands, on the s. side of the mare serenitatis, and diverging towards the w. the most southerly commences s.s.e. of the acherusian promontory (a great headland, feet high, at the w. termination of the haemus range), and, following a somewhat undulating course, runs up to the n. side of dawes. under a low evening sun, i have remarked many inequalities in the width of that portion of it immediately n. of plinius, which appear to indicate that it is here made up of rows of inosculating craters. the cleft north of this originates very near it, passes a little s. of the promontory, and runs to the e. edge of the plateau surrounding dawes. the third and most northerly cleft begins at a point immediately n. of the promontory, cuts through the s. end of the well-known serpentine ridge on the mare serenitatis, and, after following a course slightly concave to the n., dies out on the n. side of the plateau. this cleft forms the line of demarcation between the dark tone of the mare serenitatis and the light hue of the mare tranquilitatis, traceable under nearly every condition of illumination, and prominent in all good photographs. dawes.--a ring-plain miles in diameter, situated n.w. of plinius, on a nearly circular light area. its bright border rises to a height of feet above the mare, and includes a central mountain, a white marking on the e., and a ridge running from the mountain to the s. wall. there are two closely parallel clefts on the n. side of the plateau running from e. to w., that nearer dawes being the longer, and having a craterlet standing upon it about midway between its extremities. at its w. termination there is a crater-row running at right angles to it. the light area appears to be bounded on the e. by a low curved bank. vitruvius.--a ring-plain miles in diameter with bright but not very lofty walls, situated among the mountains near the s.w. side of the mare serenitatis. it is surrounded by a region remarkable for its great variability in brightness. there is a large bright ring-plain on the w., with a less conspicuous companion on the s. of it. maraldi.--a deep but rather inconspicuous formation, bounded on the w. by a polygonal border. a small ring-plain with a central mountain is connected with the s.w. wall; and, running in a n. direction from this, is a short mountain arm which joins a large circular enclosure with a low broken border standing on the n. side of the mare tranquilitatis. littrow.--a peculiar ring-plain, rather smaller than the last, some distance n. of vitruvius, on the rocky w. border of the mare serenitatis. it is shaped like the letter d, the straight side facing the w. there is a distinct crater on the n. wall. on the n.w. it is flanked by three irregular ring-plains, and on the s.e. by a fourth. neison shows two small mountains on the floor, but schmidt, whose drawing is very true to nature, has no detail whatever. a fine cleft may be traced from near the foot of the e. wall to mount argaeus, passing s. of a bright crater on the mare e. of littrow. it extends towards the plinius system, and is probably connected with it. mount argaeus.--there are few objects on the moon's visible surface which afford a more striking and beautiful picture than this mountain and its surrounding heights with their shadows a few hours after sunrise. it attains an altitude of more than feet above the mare, and at a certain phase resembles a bright spear-head or dagger. there is a well- defined rimmed depression abutting on its southern point. romer.--a prominent formation of irregular outline, miles in diameter, situated in the midst of the taurus highlands. it has a very large central mountain, a crater on the n. side of the floor, and terraced inner slopes. some distance on the n. is another ring, nearly as large, with a crater on its s. rim, and between this and posidonius is another with a wide gap on the s. and a crater on its n. border. one of the most remarkable crater-rills on the moon runs from the e. side of romer through this latter ring, and then northwards on to the plain w. of posidonius. under suitable conditions, it can be seen as such in a inch achromatic. it is easily traceable as a rill in a photograph of the n. polar region of the moon taken by mm. henry at the paris observatory, and recently published in _knowledge_. le monnier.--a great inflection or bay on the w. border of the mare serenitatis s. of posidonius. like many other similar formations on the edges of the maria, it appears at one time or other to have had a continuous rampart, which on the side facing the "sea" has been destroyed. in this, as in most of the other cases, relics of the ruin are traceable under oblique light. a fine crescent-shaped mountain, feet high, stands near the s. side of the gap, and probably represents a portion of a once lofty wall. it will repay the observer to watch the progress of sunrise on the whole of the w. coast-line of the mare up to mount argaeus. posidonius.--this magnificent ring-plain is justly regarded as one of the finest telescopic objects in the first quadrant. its narrow bright wall with its serrated shadow, the conspicuous crater, the clefts and ridges and other details on the floor, together with the beautiful group of objects on the neighbouring plain, and the great serpentine ridge on the e., never fail to excite the interest of the observer. the circumvallation, which is far from being perfectly regular, is about miles in diameter, and, considering its size, is not remarkable for its altitude, as it nowhere exceeds feet above the interior, which is depressed about feet below the surrounding plain. its continuity, especially on the e., is interrupted by gaps. on the n., the wall is notably deformed. it is broader and more regular on the w., where it includes a large longitudinal depression, and on the n.w. section stand two bright little ring-plains. on the floor, which shines with a glittering lustre, are the well-marked remains of a second ring, nearly concentric with the principal rampart, and separated from it by an interval of nine or ten miles. the most prominent object, however, is the bright crater a little e. of the centre. this is partially surrounded on the w. by three or four small bright mountains, through which runs in a meridional direction a rill-valley, not easily traced as a whole, except under a low sun. there is another cleft on the n.e. side of the interior, which is an apparent extension of part of the inner ring, a transverse rill-valley on the n., a fourth _quasi_ rill on the n.w., and a fifth short cleft on the s. part of the floor. between the principal crater and the s.e. wall are two smaller craters, which are easy objects. beyond the border on the n., in addition to daniell, are four conspicuous craters and many ridges. chacornac.--this object, connected with posidonius on the s.w., is remarkable for the brilliancy of its border and the peculiarity of its shape, which is very clearly that of an irregular pentagon with linear sides. i always find the detail within very difficult to make out. two or more low ridges, traversing the floor from n. to s., and a small crater, are, however, clearly visible under oblique illumination. schmidt draws a crater-rill, and neison two parallel rills on the floor,--the former extends in a southerly direction to the w. side of le monnier. daniell.--a bright little ring-plain n. of posidonius. it is connected with a smaller ring-plain on the n.w. wall of the latter by a low ridge. bond, g.p.--a small bright ring-plain miles in diameter, w. of posidonius. neison shows a crater both on the n. and s. rim. schmidt omits these. maury.--a bright deep little ring-plain, about miles in diameter, on the w. border of the lacus somniorum. it is the centre of four prominent hill ranges. grove.--a bright deep ring-plain, miles in diameter, in the lacus somniorum, with a border rising feet above a greatly depressed floor, which includes a prominent mountain. mason.--the more westerly of two remarkable ring-plains, situated in the highlands on the s. side of the lacus mortis. it is miles in diameter, has a distinct crater on its s. wall, and, according to schmidt, a crater on the e. side of the floor. plana.--a formation miles in diameter, closely associated with the last. neison states that the floor is convex and higher than the surrounding region. it has a triangular-shaped central mountain, a crater, and at least three other depressions on the s.w. wall where it joins mason. burg.--a noteworthy formation, miles in diameter, on the mare, n. of plana. the floor is concave, and includes a very large bright mountain, which occupies a great portion of it. the interior slopes are prominently terraced, and there are several spurs associated with the _glacis_ on the s. and n.e. a distinct cleft runs from the n. side of the formation to the s.e. border of the lacus somniorum, which is crossed by another winding cleft running from a crater e. of plana towards the n.e. baily.--a small ring-plain, n. of burg, flanked by mountains, with a large bright crater on the w. the group of mountains standing about midway between it and burg are very noteworthy. gartner.--a very large walled-plain with a low incomplete border on the e., but defined on the w. by a lofty wall. schmidt shows a curved crater- row on the w. side of the floor. democritus.--a deep regular ring-plain, about miles in diameter, with a bright central mountain and lofty terraced walls. arnold.--a great enclosure, bounded, like so many other formations hereabouts, by straight parallel walls. there is a somewhat smaller walled-plain adjoining it on the w. moigno.--a ring-plain with a dark floor, adjoining the last on the n.e. there is a conspicuous little crater in the interior. euctemon.--this object is so close to the limb that very little can be made of its details under the most favourable conditions. according to neison, there is a peak on the n. wall , feet in height. meton.--a peculiarly-shaped walled-plain of great size, exhibiting considerable parallelism. the floor is seen to be very rugged under oblique illumination. west longitude deg. to deg. sabine.--the more westerly of a remarkable pair of ring-plains, of which ritter is the other member, situated on the e. side of the mare tranquilitatis a little n. of the lunar equator. it is about miles in diameter, and has a low continuous border, which includes a central mountain on a bright floor. from a mountain arm extending from the s. wall, run in a westerly direction two nearly parallel clefts skirting the edge of the mare. the more southerly of these terminates near a depression on a rocky headland projecting from the coast-line, and the other stops a few miles short of this. a third cleft, commencing at a point n.e. of the headland, runs in the same direction up to a small crater near the n. end of another cape-like projection. at h. on april , , when the morning terminator bisected sabine, i traced it still farther in the same direction. all these clefts exhibit considerable variations in width, but become narrower as they proceed westwards. ritter.--is very similar in every respect to the last. a curved rill mentioned by neison is on the n.e. side of the floor and is concentric with the wall. on the n. side of this ring-plain are three conspicuous craters, the two nearer being equal in size and the third much smaller. schmidt.--a bright crater at the foot of the s. slope of ritter. dionysius.--this crater, miles in diameter, is one of the brightest spots on the lunar surface. it stands on the e. border of the mare, about miles e.n.e. of ritter. a distinct crater-row runs round its outer border on the w., and ultimately, as a delicate cleft, strikes across the mare to the e. side of ritter. both crater-row and cleft are easy objects in a inch achromatic under morning illumination. ariadaeus.--a bright little crater of polygonal shape, with another crater of about one-third the area adjoining it on the n.w., situated on the rocky e. margin of the mare tranquilitatis, n.e. of ritter. a short cleft runs from it towards the latter, but dies out about midway. a second cleft begins near its termination, and runs up to the n.e. wall of ritter. e. of this pair a third distinct cleft, originating at a point on the coast-line about midway between ariadaeus and dionysius, ends near the same place on the border. there is a fourth cleft extending from the n. side of a little bay n. of ariadaeus across the mare to a point n.w. of the more northerly of the three craters n. of ritter. at a small crater on the s. flank of the mountains bordering the little bay n. of ariadaeus originates one of the longest and most noteworthy clefts on the moon's visible surface, discovered more than a century ago by schroter of lilienthal. it varies considerably in breadth and depth, but throughout its course over the plain, between ariadaeus and silberschlag, it can be followed without difficulty in a very small telescope. e. of the latter formation, towards hyginus (with which rill-system it is connected), it is generally more difficult. a few miles e. of ariadaeus it sends out a short branch, running in a s.w. direction, which can be traced as a fine white line under a moderately high sun. it is interesting to follow the course of the principal cleft across the plain, and to note its progress through the ridges and mountain groups it encounters. in the great lick telescope it is seen to traverse some old crater-rings which have not been revealed in smaller instruments. about midway between ariadaeus and silberschlag it exhibits a duplication for a short distance, first detected by webb. de morgan.--a brilliant little crater, miles in diameter, on the plain s. of the ariadaeus cleft. cayley.--a very deep bright crater, with a dark interior, n. of the last, and more than double its diameter. there is a second crater between this and the cleft. whewell.--another bright little ring, about miles in diameter, some distance to the e. of de morgan and cayley. sosigenes.--a small circular ring-plain, miles in diameter, with narrow walls, a central mountain, and a minute crater outside the wall on the e.; situated on the e. side of the mare tranquilitatis, w. of julius caesar. there is another crater, about half its diameter, on the s., connected with it by a low mound. this has a still smaller crater on the w. of it. julius caesar.--a large incomplete formation of irregular shape. the wall on the e. is much terraced, and forms a flat "s" curve. the summit ridge is especially bright, and has a conspicuous little crater upon it. on the w. is a number of narrow longitudinal valleys trending from n. to s., included by a wide valley which constitutes the boundary on this side. the border on the s. consists of a number of low rounded banks, those immediately e. of sosigenes being traversed by several shallow valleys, which look as if they had been shaped by alluvial action. there is a brilliant little hill at the end of one of these valleys, a few miles e. of sosigenes. the floor of julius caesar is uneven in tone, becoming gradually duskier from s. to n., the northern end ranking among the darkest areas on the lunar surface. there are at least three large circular swellings in the interior. a long low mound, with two or three depressions upon it, bounds the wide valley on the e. side. godin.--a square-shaped ring-plain, miles in diameter, with rounded corners. the bright rampart is everywhere lofty, except on the s., is much terraced, and includes a central mountain. on the s. a curious trumpet-shaped valley, extending some distance towards the s.w., and bounded by bright walls, is a noteworthy feature at sunrise. there are other longitudinal valleys with associated ridges on this side of the formation, all running in the same direction. there is a large bright crater outside the border on the n.e., and, between it and the wall, another, smaller, which is readily seen under a high sun. agrippa.--a ring-plain miles in diameter on the n. of the last, with a terraced border rising to a height of between and feet above the floor, which contains a large bright central mountain and two craters on the s. the shape of this formation deviates very considerably from circularity, the n. wall, on which stands a small crater, being almost lineal. on the w., at a distance of a few miles, runs the prominent mountain range, extending northwards nearly up to the e. flank of julius caesar, which bounds the e. side of the great ariadaeus plain. between this rocky barrier and agrippa is a very noteworthy enclosure containing much minute detail and a long straight ridge resembling a cleft. a few miles n. of agrippa stands a small crater; at a point w. of which the hyginus cleft originates. silberschlag.--a very brilliant crater, or miles in diameter, connected with the great mountain range just referred to. the ariadaeus cleft cuts through the range a few miles n. of it. this neighbourhood at sunrise presents a grand spectacle. with high powers under good atmospheric conditions, the plain e. of the mountains is seen to be traversed by a number of shallow winding valleys, trending towards agrippa, and separated by low rounded hills which have all the appearance of having been moulded by the action of water. boscovich.--this is not a very striking telescopic object under any phase, on account of its broken, irregular, and generally ill-defined border. it is, however, remarkable as being one of the darkest spots on the visible surface: in this respect a fit companion to julius caesar, its neighbour on the w. schmidt shows some ridges within it. rhaeticus.--a very interesting formation, about miles in diameter, situated near the lunar equator, with a border intersected by many passes. a deep rill-like valley winds round its eastern _glacis_, commencing on the s. at a small circular enclosure standing at the end of a spur from the wall; and, after crossing a ridge w. of a bright little crater on the n. of the formation, apparently joins the most easterly cleft of the triesnecker system. a cleft traverses the n. side of the floor of rhaeticus, and extends across the plain on the e. as far as the n. side of reaumur. triesnecker.--apart from being the centre of one of the most remarkable rill-systems on the moon, this ring-plain, though only about miles in diameter, is an object especially worthy of examination under every phase. at sunrise, and for some time afterwards, owing to the superior altitude of the n.w. section of the wall, a considerable portion of the border on the n. and n.e. is masked by its shadow, which thus appears to destroy its continuity. on more than one occasion, friends, to whom i have shown this object under these conditions, have likened it to a breached volcanic cone, a comparison which at a later stage is seen to be very inappropriate. the rampart is terraced within, and exhibits many spurs and buttresses without, especially on the n.w. the central mountain is small and not conspicuous. the rill-system is far too complicated to be intelligibly described in words. it lies on the w. side of the meridian passing through the formation, and extends from the n. side of rhaeticus to the mountain-land lying between ukert and hyginus on the n. birt likened these rills to "an inverted river system," a comparison which will commend itself to most observers who have seen them on a good night, for in many instances they appear to become wider and deeper as they approach higher ground. published maps are all more or less defective in their representations of them, especially as regards that portion of the system lying n. of triesnecker. hyginus.--a deep depression, rather less than miles across, with a low rim of varying altitude, having a crater on its n. edge. this formation is remarkable for the great cleft which traverses it, discovered by schroter in . the coarser parts of this object are easily visible in small telescopes, and may be glimpsed under suitable conditions with a inch achromatic. commencing a little w. of a small crater n. of agrippa, it crosses, as a very delicate object, a plain abounding in low ridges and shallow valleys, and runs nearly parallel to the eastern extension of the ariadaeus rill. as it approaches hyginus it becomes gradually coarser, and exhibits many expansions and contractions, the former in many cases evidently representing craters. when the phase is favourable, it can be followed across the floor of hyginus, and i have frequently seen the banks with which it appears to be bounded (at any rate within the formation), standing out as fine bright parallel lines amid the shadow. on reaching the e. wall, it turns somewhat more to the n., becomes still coarser and more irregular in breadth, and ultimately expands into a wide valley on the n.e. it is connected with the ariadaeus cleft by a branch which leaves the latter at an acute angle on the plain e. of silberschlag, and joins it about midway between its origin n. of agrippa and hyginus. it is also probably joined to the triesnecker system by one or more branches e. of hyginus. on may , , dr. hermann klein of cologne discovered, with a / inch plosel dialyte telescope, a dark apparent depression without a rim in the mare vaporum, a few miles n.w. of hyginus, which, from twelve years' acquaintance with the region, he was certain had not been visible during that period. on the announcement of this discovery in the _wochenschrift fur astronomie_ in march of the following year, the existence of the object described by dr. klein was confirmed, and it was sedulously scrutinised under various solar altitudes. to most observers it appeared as an ill-defined object with a somewhat nebulous border, standing on an irregularly-shaped dusky area, with two or more small dark craters and many low ridges in its vicinity. a little e. of it stands a curious spiral mountain called the schneckenberg. the question as to whether hyginus n. (as the dusky spot is called) is a new object or not, cannot be definitely determined, as, in spite of a strong case in favour of it being so, there remains a residuum of doubt and uncertainty that can never be entirely cleared away. after weighing, however, all that can be said "for and against," the hypothesis of change seems to be the most probable. ukert.--this bright crater, miles in diameter, situated in the region n.e. of triesnecker, is surrounded by a very complicated arrangement of mountains; and on the n. and w. is flanked by other enclosures. it has a distinct central mountain. its most noteworthy feature is the great valley, more than miles long, which extends from n.e. to s.w. on the e. side of it. this gorge is at least six miles in breadth, of great depth, and is only comparable in magnitude with the well-known valley which cuts through the alps, w. of plato. a delicate cleft, not very clearly traceable as a whole, begins near its n. end, and terminates amid the ramifications of the apennines s. of marco polo. taquet.--a conspicuous little crater on the s. border of the mare serenitatis at the foot of the haemus mountains. a branch of the great serpentine ridge, which traverses the w. side of this plain and other lesser elevations, runs towards it. menelaus.--a conspicuously bright regular ring-plain, about miles in diameter, situated on the s. coast-line of the mare serenitatis, and closely associated with the haemus range. it has a brilliant central mountain, but no visible detail on the walls. on the edge of the mare, s.w. of it, there is a curious square formation. the bright streak traversing the mare from n. to s., which is so prominently displayed in old maps of the moon, passes through this formation. sulpicius gallus.--another brilliant object on the south edge of the mare serenitatis, some distance e. of the last. it is a deep circular crater about miles in diameter, rising to a considerable height above the surface. its shadow under a low morning sun is prominently jagged. on the e. are two bright mounds, and s. of that which is nearer the border of the mare, commences a cleft which, following the curvature of the coast- line, terminates at a point in w. long. deg. this object varies considerably in width and depth. another shorter and coarser cleft runs s. of this across an irregularly shaped bay or inflexion in the border of the mare. manilius.--this, one of the most brilliant objects in the first quadrant, is about miles in diameter, with walls nearly feet above the floor, which includes a bright central mountain. the inner slope of the border on the e. is much terraced and contains some depressions. there is a small isolated bright mountain feet high on the mare vaporum, some distance to the e. bessel.--a bright circular crater, miles in diameter, on the s. half of the mare serenitatis, and the largest object of its class thereon. its floor is depressed some feet below the surrounding surface, while the walls, rising nearly feet above the plain, have peaks both on the n. and s. about feet higher. the shadows of these features, noted by schroter in , and by many subsequent observers, are very noteworthy. i have seen the shadow of a third peak about midway between the two. one may faintly imagine the magnificent prospect of the coast- line of the mare with the haemus range, which would be obtained were it possible to stand on the summit of one of these elevations. it is doubtful whether bessel has a central mountain. neither madler nor schmidt have seen one, though webb noted a peak on two occasions. i fail to see anything within the crater. the bright streak crossing the mare from n. to s. passes through bessel. linne.--a formation on the e. side of the mare serenitatis, described by lohrmann and madler as a deep crater, but which in was found by schmidt to have lost all the appearance of one. the announcement of this apparent change led to a critical examination of the object by most of the leading observers, and to a controversy which, if it had no other result, tended to awaken an interest in selenography that has been maintained ever since. according to madler, the crater was more than miles in diameter in his time, and very conspicuous under a low sun, a description to which it certainly did not answer in or at any subsequent epoch. it is anything but an easy object to see well, as there is a want of definiteness about it under the best conditions, though the minute crater, the low ridges, and the nebulous whiteness described by schmidt and noted by webb and others, are traceable at the proper phase. as in the case of hyginus n, there are still many sceptics as regards actual change, despite the records of lohrmann and madler; but the evidence in favour of it seems to preponderate. conon.--a bright little crater, miles in diameter, situated among the intricacies of the apennines, s. of mount bradley. it has a central hill, which is not a difficult object. aratus.--one of the most brilliant objects on the visible surface of the moon, a crater miles in diameter, s. of mount hadley, surrounded by the lofty mountain arms and towering heights of the apennines. a peak close by on the n. is more than , feet, and another farther removed towards the n.w. is over , feet in altitude. autolycus.--a ring-plain miles in diameter, deviating considerably from circularity, w. of archimedes, on the mare imbrium, or rather on that part of it termed the palus putredinis. its floor, which contains an inconspicuous central mountain, is depressed some feet below the surrounding country. with a power of on a / achromatic, dr. sheldon of macclesfield has seen two shallow crateriform depressions in the interior, one nearly central, and the other about midway between it and the n. wall. the wall is terraced within, and has a crater just below its crest on the w., which, when the opposite border is on the morning terminator, is seen as a distinct notch. autolycus is the centre of a minor ray-system. aristillus.--a larger and much more elaborate ring-plain, miles in diameter, n. of autolycus. its complex wall, with its terraces within, and its buttresses, radiating spurs, and gullies without, forms a grand telescopic object under a low sun on a good night. it rises on the east , feet above the mare, and is about feet lower on the w., while the interior is depressed some feet. its massive central mountain, surmounted by many peaks, occupies a considerable area on the floor, and exhibits a digitated outline at the base. on the s. and w. a number of deep valleys radiate from the foot of the border, some of them extending nearly as far as autolycus. shallower but more numerous and regular features of the same class radiate towards the n.e. from the foot of the opposite wall. on the n.w. are several curved ridges, all trending towards theaetetus. on the s.e. the surface is trenched by a number of crossed gullies, well seen when the e. wall is on the morning terminator. just beyond the n. _glacis_ is a large irregular dusky enclosure with a central mound, and another smaller low ring adjoining it on the s.e. the visibility of these objects is very ephemeral, as they disappear soon after sunrise. aristillus is also the centre of a bright ray system. theaetetus.--a conspicuous ring-plain, about miles in diameter, in the palus nebularum, n.w. of aristillus. it is remarkable for its great depth, the floor sinking nearly feet below the surface. its walls, feet high on the w., are devoid of detail. the _glacis_ on the s.w. has a gentle slope, and extends for a great distance before it runs down to the level of the plain. not far from the foot of the wall on the n. is a row of seven or eight bright little hills, near the eastern side of which originates a distinct cleft that crosses the palus in a n.w. direction, and terminates among mountains between cassini and calippus. i have seen this object easily with a inch achromatic. calippus.--a bright ring-plain miles in diameter, situated in the midst of the intricate caucasus mountain range. on the e. is a brilliant peak rising more than , feet above the palus nebularum, and nearer the border, on the n.e., is a second, more than feet higher, with many others nearly as lofty in the vicinity. calippus has not apparently a central peak or any other features on the floor. cassini.--this remarkable ring-plain, about miles in diameter, is very similar in character to posidonius. it has a very narrow wall, nowhere more than feet in height, and falling on the e. to feet. though a prominent and beautiful object under a low sun, its attenuated border and the tone of the floor, which scarcely differs from that of the surrounding surface, render it difficult to trace under a high angle of illumination, and perhaps accounts for the fact that it escaped the notice of hevel and riccioli; though it is certainly strange that a formation which is thrown into such strong relief at sunrise and sunset should have been overlooked, while others hardly more prominent at these times have been drawn and described. the outline of cassini is clearly polygonal, being made up of several rectilineal sections. the interior, nearly at the same level as the outside country, includes a large bright ring-plain, a, miles in diameter and feet in depth, which has a good-sized crater on the s. edge of a great bank which extends from the s.w. side of this ring-plain to the wall. on the e. side of the floor, close to the inner foot of the border, is a bright deep crater about two- thirds of the diameter of a, and between it and the latter brenner has seen three small hills. the outer slope of cassini includes much detail. on the s.w. is a row of shallow depressions just below the crest of the wall, and near the foot of the slope is a large circular shallow depression associated with a valley which runs partly round it. the shape of the _glacis_ on the w. is especially noteworthy, the s.w. and n.w. sides meeting at a slightly acute angle at a point or miles w. of the summit of the ring. on the outer e. slope is a curious elongated depression, and on the n. slope two large dusky rings, well shown by schmidt, but omitted in other maps. most of these details are well within the scope of moderate apertures. perhaps the most striking view of cassini and its surroundings is obtained when the morning terminator is on the central meridian. alexander.--a large irregularly shaped plain, at least miles in longest diameter, enclosed by the caucasus mountains. on the s.w. and n.w. the border is lineal. it has a dark level floor on which there is a great number of low hills. eudoxus.--a bright deep ring-plain, about miles in diameter, in the hilly region between the mare serenitatis and the mare frigoris, with a border much broken by passes, and deviating considerably from circularity. its massive walls, rising more than , feet above the floor on the w., and about , feet on the opposite side, are prominently terraced, and include crater-rows in the intervening valleys, while their outer slopes present a complicated system of spurs and buttresses. there is a bright crater on the n. _glacis_, and some distance beyond the wall on the n.w. is a small ring-plain, and on the s.e. another, with a conspicuous crater between it and the wall. neison draws attention to an area of about square miles on the n.e. which is covered with a great multitude of low hills. e. of eudoxus are two short crossed clefts, and on the n. a long cleft of considerable delicacy running from n.e. to s.w. it was in connection with this formation that trouvelot, on february , , when the terminator passed through aristillus and alphonsus, saw a very narrow thread of light crossing the s. part of the interior and extending from border to border. he noted also similar appearances elsewhere, and termed them _murs enigmatiques_. aristoteles.--a magnificent ring-plain, miles in diameter, with a complex border, surmounted by peaks, rising to nearly , feet above the floor, one of which on the w., pertaining to a terrace, stands out as a brilliant spot in the midst of shadow when the interior is filled with shadow. the formation presents its most striking aspect at sunrise, when the shadow of the w. wall just covers the floor, and the brilliant inner slope of the e. wall with the little crater on its crest is fully illuminated. at this phase the details of the terraces are seen to the best advantage. the arrangement of the parallel ridges and rows of hills on the n.e. and s.w. is likewise better seen at this time than under an evening sun. a bright and deep ring-plain, about miles in diameter, with a distinct central mountain, is connected with the w. wall. egede.--a lozenge-shaped formation, about miles from corner to corner, bounded by walls scarcely more than feet in height. it is consequently only traceable under very oblique illumination. the great alpine valley.--a great wedge-shaped depression, cutting through the alps w. of plato, from w.n.w. to e.s.e. it is more than miles in length, and varies in breadth from miles on the s. to less than miles on the n., where it approaches the s. border of the mare frigoris. for a greater part of its extent it is bounded on the s.w. side by a precipitous linear cliff, which, under a low evening sun, is seen to be fringed by a row of bright little hills. these are traceable up to one of the great mountain masses of the alps, forming the s.w. side of the great oval-shaped expansion of the valley, whose shape has been appropriately compared to that of a florence oil-flask, and which webb terms "a grand amphitheatre." on the opposite or n.e. side, the boundary of the valley is less regular, following a more or less undulating line up to a point opposite, and a little n. of, the great mountain mass, where it abuts on a shallow _quasi_ enclosure with lofty walls, which, projecting westwards, considerably diminish the width of the valley. south of this lies another curved mountain ring, which still farther narrows it. this curtailment in width represents the neck of the flask, and is apparently about or miles in length, and from to miles in breadth, forming a gorge, bordered on the w. by nearly vertical cliffs, towering thousands of feet above the bottom of the valley; and on the e. by many peaked mountains of still greater altitude. at the entrance to the "amphitheatre," the actual distance between the colossal rocks which flank the defile is certainly not much more than miles. from this standpoint the view across the level interior of the elliptical plain would be of extraordinary magnificence. towards the s., but more than miles distant, the outlook of an observer would be limited by some of the loftiest peaks of the alps, whose flanks form the boundary of the enclosure, through which, however, by at least three narrow passes he might perchance get a glimpse of the mare imbrium beyond. the broadest of these aligns with the axis of the valley. it is hardly more than a mile wide at its commencement on the s. border of the "amphitheatre," but expands rapidly into a trumpet-shaped gorge, flanked on either side by the towering heights of the alps as it opens out on to the mare. the bottom, both of the "amphitheatre" and of the long wedge-shaped valley, appears to be perfectly level, and, as regards the central portion of the latter, without visible detail. under morning illumination i have, however, frequently seen something resembling a ridge partially crossing "the neck," and, near sunset, a tongue of rock jutting out from the e. flank of the constriction, and extending nearly from side to side. at the base of the cliff bordering the valley on the s.w., five or six little circular pits have been noted, some of which appear to have rims. they were seen very perfectly with powers of and on an / inch calver reflector at h. on january , , and have been observed, but less perfectly, on subsequent occasions. the most northerly is about miles from the n.w. end of the formation, and the rest occur at nearly regular intervals between it and "the neck." in the neighbourhood of the valley, on either side, there are several bright craters. three stand near the n.e. edge, and one of considerable size near the n.w. end on the opposite side. a winding cleft crosses the valley about midway, which, strange to say, is not shown in the maps, though it may be seen in a inch achromatic. it originates apparently at a bright triangular mountain on the plain s.w. of the valley, and, after crossing the latter somewhat obliquely, is lost amid the mountains on the opposite side. that portion of it on the bottom of the valley is easily traceable under a high light as a white line. the region n. of the alps on the s.w. side of the valley presents many details worthy of examination. among them, parallel rows of little hills, all extending from n.w. to s.e. there is also a number of still smaller objects of the same type on the e. side. the great alpine valley, though first described by schroter, is said to have been discovered on september , , by bianchini, but it is very unlikely that an object which is so prominent when near the terminator was not often remarked before this. archytas.--a bright ring-plain, miles in diameter, on the edge of the mare frigoris, due n. of the alpine valley, with regular walls rising about feet above the interior on the n.w., and about feet on the opposite side. it has a very bright central mountain. several spurs radiate from the wall on the s., and a wide valley, flanked by lofty heights, forming the s.w. boundary of w.c. bond, originates on the n side. there is also a crater-rill running towards the n.w. on the mare, s.w. of archytas, is a somewhat smaller ring-plain, archytas a (called by schmidt, protagoras), with lofty walls and a central hill. christian mayer.--a prominent rhomboidal-shaped ring-plain, miles in diameter, associated on every side, except the n., with a number of irregular inconspicuous enclosures. it has a central peak. madler discovered two delicate short clefts, both running from n.w. to s.e., one on the w. and the other on the e. of this formation. w.c. bond.--a great enclosed plain of rhomboidal shape on the n. of archytas, the bright ring-plain timaeus standing near its e. corner, and another conspicuous but much smaller enclosure with a smaller crater w. of it on the floor at the opposite angle. the interior, which is covered with rows of hillocks, is very noteworthy at sunrise. barrow.--there are few more striking or beautiful objects at sunrise than this, mainly because of the peculiar shape of its brilliant border and the remarkable shadows of the lofty peaks on its western wall. there is a notable narrow gap in the rampart on the w., which appears to extend to the level of the floor. the walls, especially on the s., are very irregular, and include two large deep craters and some minor depressions. if the formation is observed when its e. wall is on the morning terminator, a fine view is obtained of the remarkable crater-row which winds round the n. side of goldschmidt. barrow is about miles in diameter. according to schmidt, there is one crater in the interior, a little s.e. of the centre. scoresby.--a much fore-shortened deep ring-plain, miles in diameter, between barrow and the limb. it has a central mountain with two peaks, which are very difficult to detect. challis.--a ring-plain adjoining scoresby on the n.e. it is of about the same size and shape. main.--a very similar formation, on the n. of the last, much too near the limb to be well observed. second quadrant east longitude deg. to deg. murchison.--a considerable ring-plain about miles across on the e., where it abuts on pallas. it is a pear-shaped formation, bounded on the n. by a mountainous region, and gradually diminishes in width towards the s.e., on which side it is open to the plain. the walls are of no great altitude, but, except on the n.w., are very bright. at the s. termination of the w. wall there is an exceedingly brilliant crater, murchison a, five miles in diameter and some feet deep; adjoining which on the n.w. is an oval depression and a curious forked projection from the border. the only objects visible in the interior are a few low ridges on the e. side, and a number of long spurs running out from the wall on the n. towards the centre of the floor. murchison a is named chladni by lohrmann. pallas.--a fine ring-plain, about miles in diameter, forming with murchison an especially beautiful telescopic object under suitable illumination. its brilliant border, broken by gaps on the w., where it abuts on murchison, has a bright crater on the n.e., from which, following the curvature of the wall, and just below its crest, runs a valley in an easterly direction. there is a large bright central mountain on the floor, with a smaller elevation to the s. of it, and a ridge extending from the n. wall to near the centre. on the w., a section of the border is continued in a n. direction far beyond the limits of the formation; and on the s. it is connected with a small incomplete ring; on the e. of which, near the foot of the wall, is a somewhat smaller and much duskier enclosure. bode.--a brilliant ring-plain, miles in diameter, situated on the n. side of pallas. its walls rise about feet above the interior, which is considerably depressed, and includes, according to schmidt and webb, a mountain or ridge. there are two parallel valleys on the w., which are well worth examination. sommering.--an incomplete ring-plain, miles in diameter, situated on the lunar equator. it has rather low broken walls and a dark interior. schroter.--a somewhat larger formation, with a border wanting on the s. schmidt draws a considerable crater on the s.w. side of the floor. it was in the region north of this object, which abounds in little hills and low ridges, that in the year gruithuisen discovered a very remarkable formation consisting of a number of parallel rows of hills branching out (like the veins of a leaf from the midrib) from a central valley at an angle of deg., represented by a depression between two long ridges running from north to south. the regularly arranged hollows between the hills and the longitudinal valley suggested to his fertile imagination that he had at last found a veritable city in the moon--possibly the metropolis of kepler's _subvolvani_, who were supposed to dwell on that hemisphere of our satellite which faces the earth. at any rate, he was firmly convinced that it was the work of intelligent beings, and not due to natural causes. this curious arrangement of ridges and furrows, which, according to webb, measures about miles both in length and breadth, is, owing to the shallowness of the component hills and valleys, a very difficult object to see in its entirety, as it must be viewed when close to the terminator, and even then the sun's azimuth and good definition do not always combine to afford a satisfactory glimpse of its ramifications. m. gaudibert has given a drawing of it in the _english mechanic_, vol. xviii. p. . gambart.--a regular ring-plain, miles in diameter, with a low border and without visible detail within; situated nearly on the lunar equator, about miles s.s.w. of copernicus, at the n.w. edge of a very hilly region. a prominent pear-shaped mountain, with a small crater upon it, stands a short distance on the s.w., and further in the same direction, a large bright crater with two much smaller craters on the n. of it. the rough hilly district about midway between copernicus and gambart is remarkable for its peculiar dusky tone and for certain small dark spots, first seen by schmidt, and subsequently carefully observed by dr. klein. the noteworthy region where these peculiar features are found represents an area of many thousand square miles, and must resemble a veritable _malpais_, covered probably with an incalculable number of craters, vents, cones, and pits, filled with volcanic _debris_. it is among details of this character that the true analogues of some terrestrial volcanoes must be looked for. under a low angle of illumination the surface presents an extraordinarily rough aspect, well worthy of examination, but the dusky areas and the black spots can only be satisfactorily distinguished under a somewhat high sun. i have, however, seen them fairly well when the w. wall of reinhold was on the morning terminator. marco polo.--a small and very irregularly-shaped enclosure (difficult to see satisfactorily) on the s. flank of the apennines. it is hemmed in on every side by mountains. eratosthenes.--a noble ring-plain, miles in diameter; a worthy termination of the apennines. the best view of it is obtained under morning illumination when the interior is about half-filled with shadow. at this phase the many irregular terraces on the inner slope of the e. wall (which rises at one peak , feet above an interior depressed feet below the mare imbrium) are seen to the best advantage. the central mountain is made up of two principal peaks, nearly central, from which two bright curved hills extend nearly up to the n.w. wall,--the whole forming a v-shaped arrangement. on the s. there is a narrow break in the wall, and the s.w. section of it seems to overlap and extend some distance beyond the s.e. section. the border on the s.w. is remarkable for the great width of its _glacis_. eratosthenes exhibits a marked departure from circularity, especially on the e., where the wall consists of two well-marked linear sections, with an intermediate portion where the crest for miles or more bends inwards or towards the centre. from the s.e. flank of this formation extends towards the w. side of stadius one of the grandest mountain arms on the moon's visible surface, rising at one place feet, and in two others and feet respectively above the mare imbrium. if this magnificent object is observed when the morning terminator falls a little e. of stadius, it affords a spectacle not easily forgotten. i have often seen it at this phase when its broad mass of shadow extended across the well-known crater-row w. of copernicus, some of the component craters appearing between the spires of shade representing the loftiest peaks on the mountain arm. there is a prominent little crater on the crest of the arm between two of the peaks, and another on the plain to the west. stadius.--an inconspicuous though a very interesting formation, miles in diameter, w. of copernicus, with a border scarcely exceeding feet in height. hence it is not surprising that it was for a long time altogether overlooked by madler. except as a known object, it is only traceable under very oblique illumination, and even then some attention is required before its very attenuated wall can be followed all round. it is most prominent on the w., where it apparently consists of a s. extension of the eratosthenes mountain-arm, and is associated with a number of little craters and pits. this is succeeded on the s.w. by a narrow strip of bright wall, and on the s. by a section made up of a piece of straight wall and a strip curving inwards, forming the s. side. on the e. the border assumes a very ghostly character, and appears to be mainly defined by rows of small depressions and mounds. on the n.e., n., and n.w. it is still lower and narrower; so much so, that it is only for an hour or so after sunrise or before sunset that it can be traced at all. on every side, with the exception of the curved piece on the s., the wall consists of linear sections. the interior contains a great number of little craters and very low longitudinal mounds. ten craters are shown in beer and madler's map. schmidt only draws fifteen, though in the text accompanying his chart he says that he once counted fifty. in the monograph published in the _journal_ of the liverpool astronomical society (vol. v. part ), forty-one are represented. they appear to be rather more numerous on the s. half of the floor than elsewhere. just beyond the limits of the border on the n., is a bright crater with a much larger obscure depression on the w. of it. the former is surrounded by a multitude of minute craters and crater-cones, which are easily seen under a low sun. though almost every trace of stadius disappears under a high light, i have had little difficulty in seeing portions of the border and some of the included details when the morning terminator had advanced as far as the e. wall of herodotus, and the site was traversed by innumerable light streaks radiating from copernicus. at this phase the bright crater, just mentioned, on the n. edge of the border was tolerably distinct. copernicus.--this is without question the grandest object, not only on the second quadrant, but on the whole visible superficies of the moon. it undoubtedly owes its supremacy partly to its comparative isolation on the surface of a vast plain, where there are no neighbouring formations to vie with it in size and magnificence, but partly also to its favourable position, which is such, that, though not central, is sufficiently removed from the limb to allow all its manifold details to be critically examined without much foreshortening. there are some other formations, langrenus and petavius, for example, which, if they were equally well situated, would probably be fully as striking; but, as we see it copernicus is _par excellence_ the monarch of the lunar ring-mountains. schmidt remarks that this incomparable object combines nearly all the characteristics of the other ring-plains, and that careful study directed to its unequalled beauties and magnificent form is of much more value than that devoted to a hundred other objects of the same class. it is fully miles in diameter, and, though generally described as nearly circular, exhibits very distinctly under high powers a polygonal outline, approximating very closely to an equilateral hexagon. there are, however, two sections of the crest of the border on the n.e. which are inflected slightly towards the centre, a peculiarity already noticed in the case of eratosthenes. the walls, tolerably uniform in height, are surmounted by a great number of peaks, one of which on the w., according to neison, stands , feet above the floor, and a second on the opposite side is nearly as high. both the inner and outer slopes of this gigantic rampart are very broad, each being fully miles in width. the outer slope, especially on the e., is a fine object at sunrise, when its rugged surface, traversed by deep gullies, is seen to the best advantage. the terraces and other features on the bright inner declivities on this side may be well observed when the sun's altitude is about deg. schmidt, whose measures differ from those of neison, estimates the height of the wall on the e. to be , feet, and states that the interior slopes vary from deg. to deg. above, to from deg. to deg. at the base. the first inclination of deg., and in some cases of deg., is confined to the loftiest steep crests and to the flanks of the terraces. there are apparently five bright little mountains on the floor, the most easterly being rather the largest, and a great number of minute hillocks on the s.e. quarter. s.w. of the centre is a little crater, and on the same side of the interior a curious hook-shaped ridge, projecting from the foot of the wall, and extending nearly halfway across the floor. the region surrounding copernicus is one of the most remarkable on the moon, being everywhere traversed by low ridges, enclosing irregular areas, which are covered with innumerable craterlets, hillocks, and other minute features, and by a labyrinth of bright streaks, extending for hundreds of miles on every side, and varying considerably both in width and brilliancy. the notable crater-row on the w., often utilised by observers for testing the steadiness of the air and the definition of their telescopes, should be examined when it is on the morning terminator, at which time webb's homely comparison, "a mole-run with holes in it," will be appreciated, and its evident connection with the e. side of stadius clearly made out. there is another much more delicate row running closely parallel to this object; it lies a little w. of it, and extends farther in a northerly direction. archimedes.--next to plato the finest object on the mare imbrium. it is about miles in diameter. the average height of its massive border is about feet above the interior, which is only depressed some or feet below the mare, the highest peak (about feet) being on the s.e. the walls are terraced, and include much detail, both within and without. the most noteworthy features in connection with this formation are the crater-cones, craterlets, pits, white spots, and light streaks which figure on the otherwise smooth interior. mr. t.p. gray, f.r.a.s., of bedford, who, with praiseworthy assiduity, has devoted more than ten years to the close scrutiny of these features, mr. stanley williams, and others, have detected four crater-cones on the e. half of the floor, and about fifty minute craters and white spots, also probably volcanic vents, and a very curious and interesting series of light streaks, mostly traversing the formation from e. to w. a little e. of the centre is a dusky oval area about miles across, and s.w. of this is another, much smaller. under some conditions of illumination the two principal light markings may be traced over the w. wall, and for some distance on the plain beyond. on the southern side of archimedes is a very rugged mountain region, extending for more than miles towards the south: on the w. of this originates a remarkable rill-system, best seen under evening illumination. the two principal clefts follow a nearly parallel course up to the face of the apennines near mount bradley, crossing in their way, almost at right angles, other clefts which run at no great distance from the e. foot of this range and ramify among the outlying hills. archimedes a is a brilliant little ring-plain on the s.e. of archimedes. it casts an extraordinary shadow at sunrise, and has a well-marked crater-row on the e. of it, and two long narrow valleys, one of which appears to be a southerly extension of the row. beer.--a very bright little crater, with an unnamed formation of about the same size adjoining it on the n., with which is associated a curious winding ridge that appears to traverse a gap in its n. wall. timocharis.--a fine ring-plain, miles in diameter (the centre of a minor ray-system). it stands isolated on the mare imbrium (below the level of which it is depressed some feet), about midway between archimedes and lambert. its walls, rising about feet above the floor, are conspicuously terraced, and on their w. outer slopes exhibit some remarkable depressions. there is a distinct break on the n., and a bright little crater on the n.w., connected with the foot of the _glacis_ by a prominent ridge. on the bright central mountain, schmidt, in , detected a crater, which is easily seen under a moderately high light. timocharis and the neighbourhood, especially the peculiar shape of the terminator on the e. of the formation, is well worth examination at sunrise. piazzi smyth.--a conspicuous little ring-plain, or miles in diameter, depressed about feet below the mare imbrium, with a border rising about feet above it. with the curious arrangement of ridges, of which it is the apparent centre, it is a striking object under a low sun. kirch.--a rather smaller object of the same class on the s.e. plato.--this beautiful walled-plain, miles in diameter, with its bright border and dark steel-grey floor, has, from the time of hevelius to the present, been one of the most familiar objects to lunar observers. in the rude maps of the seventeenth century it figures as the "lacus niger major," an appellation which not inaptly describes its appearance under a high sun, when the sombre tone of its apparently smooth interior is in striking contrast to that of the isthmus on which the formation stands. it will repay observation under every phase, and though during the last thirty years no portion of the moon has been more diligently scrutinised than the floor; the neighbourhood includes a very great number of objects of every kind, which, not having received so much attention, will afford ample employment to the possessor of a good telescope during many lunations. the border of plato, varying in height from to feet above the interior, is crowned by several lofty peaks, the highest ( feet) standing on the n. side of the curious little triangular formation on the e. wall. those on the w., three in number, reckoning from n. to s., are respectively about , , and feet in altitude above the floor. the circumvallation being very much broken and intersected by passes, exhibits many distinct breaches of continuity, especially on the s. there is a remarkable valley on the s.w., which, cutting through the border at a wide angle, suddenly turns towards the s.e., and descends the slope of the _glacis_ in a more attenuated form. another but shorter valley is traceable at sunrise on the w. on the n.w., the rampart is visibly dislocated, and the gap occupied by an intrusive mountain mass. this dislocation is not confined to the wall, but, under favourable conditions, may be traced across the floor to the broken s.e. border. it is probably a true "fault." on the n.e., the inner slope of the wall is very broad, and affords a fine example of a vast landslip. the spots and faint light markings on the floor are of a particularly interesting character. during the years to they were systematically observed and discussed under the auspices of the lunar committee of the british association. among the forty or more spots recorded, six were found to be crater-cones. the remainder--or at least most of them--are extremely delicate objects, which vary in visibility in a way that is clearly independent of libration or solar altitude; and, what is also very suggestive, they are always found closely associated with the light markings,--standing either upon the surface of these features or close to their edges. recent observations of these spots with a inch telescope by professor w.h. pickering, under the exceptionally good conditions which prevail at arequipa, peru, have revived interest in the subject, for they tend to show that visible changes have taken place in the aspect of the principal crater-cones and of some of the other spots since they were so carefully and zealously scrutinised nearly a quarter of a century ago. the gradual darkening of the floor of plato as the sun's altitude increases from deg. till after full moon may be regarded as an established fact, though no feasible hypothesis has been advanced to account for it. on the n.e. of plato is a large bright crater, a; and, extending in a line from this towards the e., is a number of smaller rings, the whole group being well worth examination. on the n. there is a winding cleft, and some short crossed clefts in the rugged surface just beyond the foot of the wall, which i have seen with a inch achromatic. the region on the w., imperfectly shown in the maps, includes much unrecorded detail. on the mare imbrium s. of plato is a large area enclosed by low ridges, to which schroter gave the name "newton." it suggests the idea that it represents the ruin of a once imposing enclosure, of which the conspicuous mountain pico formed a part. timaeus.--a very bright ring-plain, miles in diameter, with walls about feet in height, on the coast-line of the mare frigoris, and associated with the e. side of the great enclosed plain w.c. bond. schmidt shows a double hill, nearly central, and neison a crater on the s.w. wall. birmingham.--a large rhomboidal-shaped enclosure, defined by mountain chains and traversed by a number of very remarkable parallel ridges. it is situated nearly due n. of plato on the n. edge of the mare frigoris, and lies on the s.e. side of w.c. bond, to which it bears a certain resemblance. this region is characterised by the parallelism displayed by many formations, large and small. it is more apparent hereabouts than in any other part of the moon's visible surface. when favourably placed under a low morning sun, birmingham is a striking telescopic object. fontinelle.--a fine ring-plain, miles in diameter, on the n. margin of the mare frigoris, n.n.e. of plato, with a wall rising on the e., feet above a bright interior. i find its border indistinct and nebulous, excepting under very oblique light, though three of the little craters upon it are bright and prominent. one stands on the s., another on the n.w., and a third on the e. schmidt shows only the first of these, and neison none of them. fontinelle has a low central mountain which is easily distinguished. fontinelle a, an isolated mountain on the s., is more than feet high. on the n. there is a curious mountain group, also of considerable altitude, and on the w. an irregular depression surrounded by a dusky area. north of fontinelle, extending towards goldschmidt and the limb, schroter discovered a very wide irregular valley which he named "j.j. cassini." it is really nothing more than a great plain bounded by ridges. at h. october , , when philolaus was on the morning terminator, i had a fine view of it, and, as regards its general shape, found that it agreed very closely with schroter's drawing. epigenes.--a remarkable ring-plain, about miles in diameter, abutting on a mountain ridge running parallel to the e. flank of w.c. bond. it is a notable object under a low morning sun. there are several elevations on the floor. goldschmidt.--a great abnormally-shaped enclosure with lofty walls between epigenes and the limb. neison mentions only two crater-pits within. i have seen the rimmed crater shown by schmidt on the w. side and three or four other objects of a doubtful kind. anaxagoras.--a brilliant ring-plain of regular form, miles in diameter, adjoining goldschmidt on the e. it is a prominent centre of light streaks, some of which traverse the interior of goldschmidt. on the north a peak rises to the height of , feet. there is a long ridge on the floor, running from e. to w. gioja.--a ring-plain about miles in diameter, near the north pole. east longitude deg. to deg. reinhold.--a prominent ring-plain, miles in diameter, with a lofty border, rising at a peak on the w. to more than feet above the floor. its shape on the w. is clearly polygonal, the wall consisting of three rectilineal sections, and on the e. it is made up of two straight sections connected by a curved section. the inner slope includes a remarkably distinct and regular terrace, the e. portion of which is well seen when the interior is about half illuminated by the rising sun. at this phase also the great extent of the _glacis_ on the s.w., and the deep wide gullies traversing it on the e. are observed to the best advantage. the central mountain, though of considerable size, is not prominent. close to reinhold on the n.w. stands a noteworthy little formation with a low and partially lineal wall, exhibiting a gap on the north. there is a distinct crater on the s. side of its floor. gay-lussac.--a very interesting ring-plain, miles in diameter, situated in the midst of the carpathian highlands n. of copernicus, with a smaller but brighter and deeper formation (gay-lussac a) on the s.w. of it, and a conspicuous little crater, not more than or miles in diameter, between the two. the interior of gay-lussac is traversed by two coarse clefts, lying nearly in a meridional direction. the more easterly runs from the foot of the s. wall, near the little crater just mentioned, across the floor to the low n. border, which it apparently cuts through, and extends for some distance beyond, terminating in a great oval expansion. the other, which is not shown in the maps, is closely parallel to it, and can be traced up to the n. border, but not farther. schmidt represents the first as a crater-row, which it probably is, as it varies considerably in width. from the s.e. side of the formation extends a long cleft, terminating at the end of a prominent spur from the s. side of the carpathians. there are also two remarkable rill-like valleys, commencing on the n. of gay-lussac a, which curve round the w. side of gay-lussac. hortensius.--this brilliant crater, about miles in diameter, is remarkable for its depth, and as being a ray-centre surrounded by a nimbus of light. it has a central mountain, and schmidt shows a minute crater on the outer slope of the s. wall. the former is a test object. milichius.--is situated on the n.n.e. of hortensius. it is fully as bright, but rather smaller. its floor, apparently devoid of detail, is considerably depressed below the surrounding surface. tobias mayer.--like gay-lussac, a noteworthy ring-plain associated with the carpathian mountains. it is miles in diameter, and has a wall which rises on the w. to a height of nearly , feet above the floor; on the latter there is a conspicuous central mountain, and on the e. side a crater, and some little hills. schmidt shows a smaller crater on the w. side, which i have not seen. adjoining the formation on the w. is a ring- plain of about one-fourth its area, which is a bright object. tobias mayer and the neighbouring carpathians form an especially beautiful telescopic picture at sunrise. kunowsky.--an inconspicuous ring-plain, about miles in diameter, standing in a barren region in the mare procellarum, w.s.w. of encke. the central mountain is tolerably distinct. encke.--a regular ring-plain, miles in diameter, with a comparatively low border, nowhere rising more than feet above the interior, which is depressed some feet below the surrounding oceanus procellarum. a lofty ridge traverses the floor from s. to n., bifurcating before it reaches the n. wall. there is a bright crater on the w. wall, and a depression on the opposite wall, neither of which, strange to say, is shown on the maps. encke is encircled by ridges, which, when it is on the morning terminator, combine to make it resemble a large crater surrounded by a vast mountain ring. kepler.--one of the most brilliant objects in the second quadrant,--a ring-plain about miles in diameter, with a lofty border; a peak on the e. attaining an altitude of , feet above the surface. the wall is much terraced, especially the outer slope on the w., where a narrow valley is easily traceable. though omitted from the maps, there is a prominent circular depression on the w. border, which forms a distinct notch thereon at sunrise. on the n., the wall exhibits a conspicuous gap. there is a central hill on the floor. immediately e. of kepler is a bright plateau, bounded on the n. by a very straight border, with two small craters on its edge. both these objects are incomplete on the n., as if they had been deformed by a "fault," which has apparently affected the n. end of kepler also. kepler is the centre of one of the most extended systems of bright streaks on the moon's visible surface. bessarion.--a bright little ring-plain, about miles in diameter, in the oceanus procellarum n. of kepler. there is a smaller and still brighter companion on the n. (bessarion e), standing on a light area. bessarion has a minute central hill, difficult to detect. pytheas.--a small rhomboidal-shaped ring-plain, miles in diameter, standing in an isolated position on the mare imbrium between lambert and gay-lussac. its bright walls, rising about feet above the mare, are much terraced within, especially on the e. there is a bright little crater on the n. outer slope, with a short serpentine ridge running up to it from the region s. of lambert, and another winding ridge extending from the s. wall to the e. of two conspicuous craters, standing about midway between pytheas and gay-lussac. the former bears a great resemblance to the ridge n. of madler, and, like this, appears to traverse the n. border. the interior of pytheas, which is depressed more than feet below the mare, includes a brilliant central peak. lambert.--a ring-plain, miles in diameter, presenting many noteworthy features. the crest of its border stands about feet above the mare imbrium, and more than double this height above the interior. the wall is prominently terraced both within and without; the outer slope on the w. exhibiting at sunrise a nearly continuous valley running round it. when near the morning terminator, the region on the n. is seen to be traversed by some very remarkable ridges and markings; one cutting across the n. wall appears to represent a "fault." on the s. is a large polygonal enclosure formed by low ridges. on the w., towards timocharis, is a brilliant mountain feet high, a beautiful little object under a low sun. leverrier.--the more westerly of a pair of little ring-plains on the n. side of the mare imbrium, and s.w. of the laplace promontory. it is about miles in diameter, with walls rising some feet above the mare, and more than feet above the interior, which seems to be without a central mountain or other features. schmidt shows the crater on the n. rim and another on the s.e. slope, both of which are omitted by neison, though they are easy objects when helicon is on the morning terminator. about miles on the s.e. there is a very bright little crater on a faint light area. helicon.--the companion ring-plain on the e. it is miles in diameter, and is very similar, though not quite so deep. there is a crater on the s.e. wall, and, according to neison, another on the outer slope of the n. border. webb records a central crater. if helicon is observed when on the morning terminator, it will be seen to be traversed by a curved ridge which cuts through the walls, and runs up to a bright crater s.e. of leverrier. it appears to be a "fault," whose "downthrow," though slight, is clearly indicated by an area of lower ground on the e. there is a great number of small craters in the neighbourhood of this formation. euler.--the most easterly of the row of great ring-plains, which, beginning on the w. with autolycus, and followed by archimedes, timocharis, and lambert, extends almost in a great circle from the n.w. to the s.e. side of the mare imbrium. it is about miles in diameter, and is surrounded by terraced walls, which, though of no great height above the mare, rise feet above the floor. there is a distinct little gap in the s. wall, easily glimpsed when it is close to the morning terminator, which probably represents a small crater. euler has a bright central mountain, and is a centre of white silvery streaks. brayley.--a very conspicuous little ring-plain e.s.e. of euler, with two smaller but equally brilliant objects of the same class situated respectively e. and w. of it. diophantus.--forms with delisle, its companion on the n., a noteworthy object. it is about miles in diameter, with a wall, which has a distinct break in its continuity on the n., rising about feet above the mare. a rill-valley runs from the e. side of the ring towards the w. face of a triangular-shaped mountain on the e. of a line joining the formation with delisle. north are three bright little craters in a line, the middle one being much the largest. from the most easterly of these objects a light streak may be traced under a high sun, extending for many miles to another small crater on the n.w. of diophantus, and expanding at a point due n. of the formation into a spindle-shaped marking. at sunrise, the w. portion of the streak has all the appearance of a cleft, with a branch about midway running to the s. side of delisle. under the same phase a broad band of shadow extends from the n.e. wall to the triangular mountain just mentioned, representing a very sudden drop in the surface--resembling on a small scale the well-known "railroad" e. of thebit. diophantus has no central mountain. delisle.--a larger and more irregularly-shaped object than the last, miles in diameter, with loftier and more massive walls, and an extensive but ill-defined central hill. there is an evident break in the northern border. a triangular mountain on the s.e. and a winding ridge running up to the n. wall are prominent features at sunrise, as are also the brilliant summits of a group of hills some distance to the e.n.e. carlini.--a small but prominent and deep little crater about miles in diameter on the mare imbrium about midway between lambert and the sinus iridum. there are many faint light streaks in the vicinity, one of which extends from carlini to bianchini, on the edge of the sinus,--a distance of miles. schmidt shows a central peak. caroline herschel.--a bright and very deep ring-plain about miles in diameter on the mare imbrium, some distance e.n.e. of the last. on the s.e. lies a larger crater, delisle b, which has a small but obvious crater on its n. rim, and casts a very prominent shadow at sunrise. caroline herschel stands on a long curved ridge running n.e. from lambert towards the region e. of helicon, and, according to schmidt, has a central peak. on the e. is a bright mountain with two peaks; some distance n. of which is a large ill-defined white spot, with another spot of a similar kind on the w. of it, nearly due n. of caroline herschel. gruithuisen.--this ring-plain, miles in diameter, is situated on the mare imbrium on the n.e. of delisle. it is associated with a number of ridges trending towards the region n. of aristarchus and herodotus. the laplace promontory.--a magnificent headland marking the extreme western extremity of the finest bay on the moon's visible surface, the sinus iridum; above which it towers to a height of feet or more, projecting considerably in front of the line of massive cliffs which define the border of the sinus, and presenting a long straight face to the s.e. near its summit are two large but shallow depressions, the more easterly having a very bright interior. at a lower level, almost directly below the last, is a third depression. all three are easy objects under a low sun. the best view of the promontory and its surroundings is obtained when the e. side of the bay is on the morning terminator. its prominent shadow is traceable for many days after sunrise. the heraclides promontory.--the less lofty but still very imposing headland at the e. end of the sinus iridum, rising more than feet above it. it consists of a number of distinct mountains, forming a triangular-shaped group running out to a point at the s.w. extremity of the bay, and projecting considerably beyond the shore-line. there is a considerable crater on the e. side of the headland, not visible till a late stage of sunrise. it is among the mountains composing this promontory that some ingenuity and imagination have been expended in endeavouring to trace the lineaments of a female face, termed the "moon- maiden." bianchini.--a fine ring-plain, about miles in diameter, on the n.e. side of the sinus iridum, surrounded by the lofty mountains defining the border of the bay. its walls, which are prominently terraced within, rise about feet on the e., and about feet on the w. above the floor, which includes a prominent ridge and a conspicuous central mountain. there is a distinct crater on the s. wall, not shown in the maps. between this side of the formation and the bay is a number of hills running parallel to the shore-line: these, with the intervening valleys, will repay examination at sunrise. maupertuis.--a great mountain enclosure of irregular shape, about miles in diameter, in the midst of the sinus iridum highlands, n. of laplace. the walls are much broken by passes, and the interior includes many hills and ridges. condamine.--a rhomboidal-shaped ring-plain, about miles in diameter, n. of maupertuis, with lofty walls, especially on the e., where they rise some feet above the interior. there are three large depressions on the outer n.w. slope, and at least three minute craters on the crest of the wall just above. though neither neison nor schmidt draw any detail thereon, there is a prominent ridge on the n. side of the floor, and a low circular hill on the s. on the s.e. four long ridges or spurs radiate from the wall, and on the n.e. are three remarkable square-shaped enclosures. on the edge of the mare frigoris, n.w. of condamine, are many little craters with bright rims and a distinct short cleft, running parallel to the coast-line. the winding valleys in the region bordering the sinus iridum, and other curious details, render this portion of the moon's surface almost unique. bouguer.--a bright regular little ring-plain, about miles in diameter, n. of bianchini. j.f.w. herschel.--a vast enclosed plain, about miles across, bounded on the w. by a mountain range, which here defines the e. side of the mare frigoris, on the s. by massive mountains, and on the other sides by a lofty but much broken wall, intersected by many passes. within is a large ring-plain, nearly central, and a large number of little craters and crater-pits. the floor is traversed longitudinally by many low ridges, lying very close together, which at sunrise resemble fine grooves or scratches of irregular width and depth. horrebow.--a ring-plain of remarkable shape, resembling the analemma figure, standing at the s. end of the mountain range bounding j.f.w. herschel on the w. schmidt shows a crater on the w. wall, near the constriction on this side, and a second at the foot of the slope of the e. wall. philolaus.--a ring-plain miles in diameter, on the n.e. of fontinelle. its bright walls rise on the w. to a height of nearly , feet above the floor (on which there is a conspicuous central mountain), and exhibit many prominent terraces. philolaus is partially encircled, at no great distance, by a curved ridge, on which will be found a number of small craters. anaximines.--a much foreshortened ring-plain, about miles in diameter, on the e. of philolaus. one peak on the e. is nearly feet in height. schmidt shows four craters on the w. side of the floor, and a fifth on the s.e. side. there is a bright streak in the interior, which extends southwards for some distance across the mare frigoris. east longitude deg. to deg. reiner.--a regular ring-plain, miles in diameter, in the mare procellarum, s.s.e. of marius, with a very lofty border terraced without and within, and a minute but conspicuous mountain standing at the n. end of a ridge which traverses the uniformly dark floor in a meridional direction. a long ridge extends some way towards the s. from the foot of the s. wall, and at some distance in the same direction lie six ill- defined white spots of doubtful nature. on the e.n.e. there is a large white marking, resembling a "jew's harp" in shape, and farther on, towards the e., a number of very remarkable ridges. on the w. will be found many bright little craterlets. a ray from kepler extends almost up to the w. wall of reiner. marius.--a very noteworthy ring-plain, miles in diameter, in the oceanus procellarum, e.n.e. of kepler, with a bright border rising about feet above the interior, which is of an uneven tone. the rampart exhibits some breaks, especially on the s. the outer slope on the w. is traversed by a fine deep valley, distinctly marked when the opposite side is on the morning terminator. it originates on the s.w. at a prominent crater situated a little below the crest of the wall, and, following its curvature, runs out on to the plain near a large mountain just beyond the foot of the n. border. in addition to the crater just mentioned, there are two smaller ones below the summit of the s. wall, and a small circular depression on the s.e. wall. mr. w.h. maw, f.r.a.s., has seen, with a inch cooke refractor, a bright marking at the n. extremity of the ring, which, when examined with a dawes' eyepiece, resembled an imperfect crater. the floor includes at least four objects--( ) a crater on the n.w., standing on a circular light area; ( ) a white spot a little s. of the centre; ( ) a smaller white spot s.e. of this; ( ) another, near the inner foot of the s.w. wall. marius is an imposing object under oblique illumination, mainly because of the number of ridges by which it is surrounded. i have frequently remarked at sunrise that the surface on the w., and especially the outer slope of the rampart, is of a decided brown or sepia tint, similar to that which has already been noticed with respect to geminus and its vicinity, viewed under like conditions. schmidt in discovered a long serpentine cleft some distance n. of marius, which has not been seen since. aristarchus.--the brightest object on the moon, forming with herodotus (a companion ring-plain on the e.), and its remarkable surroundings, one of the most striking objects which the telescope has revealed on the visible surface, and one requiring much patient observation before its manifold details can be fully noted and duly appreciated. its border rises feet above the outer surface on the w., but towers to more than double this height above the glistening floor. no lunar object of its moderate dimensions (it is only about miles in diameter) has such conspicuously terraced walls, or a greater number of spurs and buttresses; which are especially prominent on the s. a valley runs round the outer slope of the w. wall, very similar to that found in a similar position round marius. there is also a distinct valley on the brilliant inner slope of the e. wall, below its crest. it originates at a bright little crater, and is traceable round the greater portion of the declivity. under a moderately high sun, an oval area, nearly as large and fully as brilliant as the central mountain, is seen on this inner slope. it is bordered on either side by bands of a duskier hue, which probably represent shallow transverse valleys. from its dazzling brilliancy it is very difficult to observe the interior satisfactorily. in addition, however, to the central mountain, there is a crater on the n.w. side of the floor. on the s. side of aristarchus is a large dusky ring some miles in diameter, connected by ridges with the spurs from the wall, and on the s.e., close to the foot of the slope, is another smaller ring of a like kind. herodotus.--this far less brilliant but equally interesting object is about miles in diameter, and is not so regular in shape as aristarchus. its w. wall rises at one point more than feet above the very dusky floor. except on the s.w. and n.e., the border is devoid of detail. on the s.w. three little notches may be detected on its summit, which probably represent small craters, while on the opposite side, on the inner slope, a little below the crest, is a large crater, easily seen. both the e. and w. sections of the wall are prolonged towards the s. far beyond the limits of the formation. these rocky masses, with an intermediate wall, are very conspicuous under oblique illumination, that on the s.w. being especially brilliant. on the n. there is a gap through which the well-known serpentine cleft passes on to the floor. between the n.w. side of herodotus and aristarchus is a large plateau, seen to the best advantage when the morning terminator lies a little distance e. of the former. it is traversed by a t-shaped cleft which communicates with the great serpentine cleft and extends towards the s. end of aristarchus, till it meets a second cleft (forming the upper part of the t) running from the s.e. side of this formation along the w. side of herodotus. the great serpentine cleft, discovered by schroter, october , , is in many respects the most interesting object of its class. it commences at the n. end of a short wide valley, traversing mountains some distance n.e. of herodotus, as a comparatively delicate cleft. after following a somewhat irregular course towards the n.w. for about miles, and becoming gradually wider and deeper, it makes a sudden turn and runs for about miles in a s.w. direction. it then changes its course as abruptly to the n.w. again for or miles, once more turns to the s.w., and, as a much coarser chasm, maintains this direction for about miles, till it reaches the s.e. edge of a great mountain plateau n. of aristarchus, when it swerves slightly towards the s., becoming wider and wider, up to a place a few miles n. of herodotus, where it expands into a broad valley; and then, somewhat suddenly contracting in width, and becoming less coarse, enters the ring-plain through a gap in the n. wall, as before mentioned. i always find that portion of the valley in the neighbourhood of herodotus more or less indistinct, though it is broad and deep. this part of it, unless it is observed at a late stage of sunrise, is obscured by the shadow of the mountains on the border of the plateau. gruithuisen suspected a cleft crossing the region embraced by the serpentine valley, forming a connection between its coarse southern extremity and the long straight section. this has been often searched for, but never found. it may exist, nevertheless, for in many instances gruithuisen's discoveries, though for a long time discredited, have been confirmed. the mountain plateau n. of aristarchus deserves careful scrutiny, as it abounds in detail and includes many short clefts. harbinger, mountains.--a remarkable group of moderate height, mostly extending from the n.w. towards aristarchus. they include a large incomplete walled-plain about miles in diameter, defined on the w. by a lofty border, forming part of a mountain chain, and open to the south. this curious formation has many depressions in connection with its n.w. edge. on the n. of it there is a crater-row and a very peculiar zig-zag cleft. the region should be observed when the e. longitude of the morning terminator is about deg. schiaparelli.--a conspicuous formation, about miles in diameter, between herodotus and the n.e. limb, with a border rising nearly feet above the mare, and about more above the floor, on which schmidt shows a central hill. wollaston.--a small bright crater on the mare n. of the harbinger mountains, surpassed in interest by a remarkable formation a few miles s. of it, wollaston b, an object of about the same size, but which is associated with a much larger enclosure, resembling a walled-plain, lying on the n. side of it. this formation has a lofty border on the w., surmounted by two small craters. the wall is lower on the e. and exhibits a gap. there is a central hill, only visible under a low sun. about midway between wollaston and this enclosure stands a small isolated triangular mountain. from a hill on the e. runs a rill valley to the more westerly of a pair of craters, connected by a ridge, on the s.e. of wollaston b. mairan.--a bright ring-plain of irregular shape, miles in diameter, on the e. of the heraclides promontory. the border, especially on the e., varies considerably in altitude, as is evident from its shadow at sunrise; at one peak on the w. it is said to attain a height of more than , feet above the interior. there is a very minute crater on the crest of the s. wall, down the inner slope of which runs a rill-like valley. about halfway down the inner face of the e. wall are two other small craters, connected together by a winding valley. these features may be seen under morning illumination, when about one-fourth of the floor is in sunlight. schroter is the only selenographer who gives mairan a central mountain. in this he is right. i have seen without difficulty on several occasions a low hill near the centre. the formation is surrounded by a number of conspicuous craters and crater-pits. on the n. there is a short rill-like valley, and another, much coarser, on the s. sharp.--a ring-plain somewhat smaller than the last, on the e. of the sinus iridum, from the coast-line of which it is separated by lofty mountains. there is a distinct crater at the foot of its n.e. wall, and a bright central mountain on the floor. on the n. is a prominent enclosure, nearly as large as sharp itself; and on the n.e. a brilliant little ring- plain, a, about miles in diameter, connected with sharp, as madler shows, by a wide valley. louville.--a triangular-shaped formation on the e. of a line joining mairan and sharp. it is hemmed in by mountains, one of which towers feet above its dusky floor. foucault.--a bright deep ring-plain, about miles in diameter, lying e. of the mountains fringing the sinus iridum, between bianchini and harpalus. a very lofty peak rises near its n. border, and, according to neison, it has a distinct central mountain, though neither madler or schmidt show any detail within. harpalus.--a conspicuous ring-plain, about miles in diameter, on the n.e. of the last, with a floor sinking , feet below the surrounding surface. as the cubic contents of the border and _glacis_ are quite inadequate to account for it, we may ask, what has become of the material which presumably once filled this vast depression? harpalus has a bright central mountain. south.--on the w. and s., the boundaries of this extensive enclosure are merely indicated by ridges, which nowhere attain the dignity of a wall. on the n., the edge of a tableland intersected by a number of valleys define its limits, and on the e. a border forming also the w. side of babbage. the interior is traversed by a number of longitudinal hills, and includes two bright little heights, drawn by schmidt as craters. babbage.--a still larger enclosed area, adjoining south on the e., and containing a considerable ring-plain near its w. border. it is a fine telescopic object at sunrise, the interior being crossed by a number of transverse markings representing ridges. these are very similar in character (but much coarser) to those found on the floor of j.f.w. herschel. the curious detail on the e. wall is also worth examination at this phase. robinson.--a bright and very deep little ring-plain, about miles in diameter, on a plateau n. of south. schmidt shows a crater on the w. border, and two others at the foot of the n. and e. borders respectively. anaximander.--a fine but much foreshortened ring-plain, miles in diameter, abutting on the e. side of j.f.w. herschel. it has a large crater on its w. border, which is common to the two formations, and a very prominent crater, both on the s. and n. the barrier on the s.w. rises to a height of nearly , feet. schmidt shows a crater and other details on the floor. east longitude deg. to deg. lohrmann.--this ring-plain, with hevel and cavalerius on the n. of it, is a member of a linear group, which, but for its propinquity to the limb, would be one of the most imposing on the moon's surface. lohrmann, about miles in diameter, is surrounded by a bright wall, which, to all appearance, is devoid of detail. two prominent ridges, with a fine intervening valley, connect it with the n. end of grimaldi. it has a large but not conspicuous central mountain. on the rugged surface, between the ring-plain and the e. edge of the oceanus procellarum, lies a very interesting group of crossed clefts, some of which run from n.e. to s.w., and others from n.w. to s.e. three of the latter proceed from different points in a coarse valley extending from w. to e., and cross the ridges just mentioned. they follow a parallel course, and terminate on the s. side of a crater-row, consisting of three bright craters ranging in a line parallel to the coarse valley. on the n. side of these objects, and tangential to them, is another cleft, which traverses the w. and e. walls of lohrmann, and, crossing the region between it and riccioli, terminates apparently at the w. wall of the latter formation. no map shows this cleft, though it is obvious enough; and, when the e. wall of hevel is on the morning terminator, the notches made by it in the border of lohrmann are easily detected. capt. noble, f.r.a.s., aptly compares two of the crossed clefts to a pair of scissors, the craters at which they terminate representing the oval handles. on the grey surface of the mare w. of lohrmann is the bright crater lohrmann a, from which, running n., proceeds a rill-like valley ending at a large white spot, which has a glistening lustre under a high light. hevel.--a great walled-plain, miles in diameter, adjoining lohrmann on the n., with a broad western rampart, rising at one peak to a height above the interior of nearly feet, and presenting a steep bright face to the oceanus procellarum. there are three prominent craters near its crest, and one or two breaks in its continuity. it is not so lofty and is more broken on the e., where three conspicuous craters stand on its inner slope. the floor is slightly convex, and includes a triangular central mountain, on which there is a small crater. the s. half of the interior is crossed by four clefts: (l) running from a little crater n. of the central mountain, on the w. side of it, to a hill at the foot of the s.w. wall; ( ) originating near the most southerly of the three craters on the inner slope of the e. wall, and crossing , terminates at the foot of the w. wall; ( ) has the same origin as , crosses , and, passing over a craterlet w. of the central mountain, also runs up to the w. wall at a point considerably n. of that where joins the latter; ( ) runs from the craterlet just mentioned to the w. end of . cavalerius.--the most northerly member of the linear chain, a ring-plain, miles in diameter, with terraced walls rising about , feet above the floor. within there is a long central mountain with three peaks. under a high light the region on the w. is seen to be crossed by broad light streaks. olbers.--a large ring-plain, miles in diameter, near the limb, n.e. of cavalerius. though a very distinct formation, it is difficult to see its details except under favourable conditions of libration. it has a large crater on its w. wall, a smaller one on the e., and a third on the n. the floor includes a central mountain, and, according to schmidt, four craters. he also shows a crater-rill on the w. wall, n. of the large crater thereon. olbers is the origin of a fine system of light rays. galileo.--a ring-plain, about miles in diameter, n.e. of reiner, associated with ridges, some of which extend to the "jew's harp" marking referred to under this formation. cardanus.--a fine regular ring-plain, about miles in diameter, near the limb n. of olbers. its bright walls, rising about feet above the light grey floor, are clearly terraced, and exhibit, especially on the s.e., several spurs and buttresses. there is a fine valley on the outer w. slope, a large bright crater on the mare just beyond its foot, and a conspicuous mountain in the same position farther north. i have not succeeded in seeing the faint central hill nor the crater n. of it shown by schmidt, but there is a brilliant white circular spot on the floor at the inner foot of the n.e. wall which he does not show. krafft.--a very similar object on the n., of about the same dimensions; with a central peak, and a large crater on the dark floor abutting on the s.w. wall, and another of about half the size on the outer side of the w. border. from this crater a very remarkable cleft runs to the n. wall of cardanus: it is bordered on either side by a bright bank, and cuts through the n.w. border of the latter formation. it is best seen when the e. wall of cardanus is on the morning terminator. vasco de gama.--a bright enclosure, miles in diameter, with a small central mountain. it is associated on the n. with a number of enclosed areas of a similar class, all too near the limb to be well seen. seleucus.--a considerable ring-plain, miles in diameter, with lofty terraced walls, rising , feet above a dark floor which includes an inconspicuous central hill. this formation stands on a ridge extending from near briggs to the w. side of cardanus. otto struve.--an enormous enclosure, bounded on the e. by the hercynian mountains, and on the w. by a mountain chain of considerable altitude, surmounted by three or more bright little rings. on the w. side of the uneven-toned interior, which, according to madler, includes an area of more than , square miles, stand four craters, several little hills, and light spots. on the w. is the much more regular and almost as large formation, otto struve a, the w. border of otto struve forming its e. wall. this enclosure is bounded elsewhere by a very low, broken, and attenuated barrier. at sunrise the e. and w. walls, with the mountain mass at the n. end, which they join, resemble a pair of partially-opened calipers. there is one conspicuous little crater on the w. side of the floor; and, at or near full moon, four or five white spots, nearly central, are prominently visible. briggs.--this bright regular ring-plain, miles in diameter, is situated a short distance n. of otto struve a. a long ridge traverses the interior from n. to s. on the e. is another large enclosure, communicating with otto struve on the s., and really forming a n. extension of this formation. it has a large and very deep crater, miles in diameter, on its w. border. lichtenberg.--a conspicuous little ring-plain, about miles in diameter, in an isolated position on the mare, some distance n. of briggs. it was here that madler records having occasionally noticed a pale reddish tint, which, though often searched for, has not been subsequently seen. ulugh beigh.--a good-sized ring-plain, e. of the last, with a bright border and central mountain. too near the limb for observation. lavoisier.--a small bright walled-plain n. of ulugh beigh. it has a somewhat dark interior. west of it is lavoisier a, a ring-plain about miles in diameter. both are too near the limb for useful observation. gerard.--a large enclosure close to the limb, still farther n., containing a long ridge and a crater. harding.--a small ring-plain w. of gerard, remarkable for the peculiar form of its shadow at sunrise, and for the ridges in its vicinity. repsold.--the largest of a group of walled enclosures, close to the limb, on the e. side of the sinus roris. xenophanes.--but for its position, this deep walled-plain, miles in diameter, would be a fine telescopic object, with its lofty walls, large central mountain, and other details. oenopides.--a large and tolerably regular walled-plain, miles in diameter, on the w. of the last. the depressions on the w. wall are worth examination at sunrise. there is apparently no detail whatever on the floor. cleostratus.--a small ring-plain, n. of xenophanes, surrounded by a number of similar objects, all too near the limb for observation. pythagoras.--a noble walled-plain, miles in diameter, which no one who observes it fails to lament is not nearer the centre of the disc, as it would then undoubtedly rank among the most imposing objects of its class. even under all the disadvantages of position, it is by far the most striking formation in the neighbourhood. its rampart rises, at one point on the n., to a height of nearly , feet above the floor, on which stands a magnificent central mountain, familiar to most observers. third quadrant east longitude deg. to deg. mosting.--a very deep ring-plain, miles in diameter, near the moon's equator, and about deg. e. of the first meridian. there is a crater on the n. side of its otherwise unbroken bright border, an inconspicuous central mountain, and, according to neison, a dark spot on the s. side of the floor. at some distance on the s.s.w., stands the bright crater, mosting a, one of the most brilliant objects on the moon's visible surface. reaumur.--a large pentagonal enclosure, about miles in diameter, with a greatly broken border, exhibiting many wide gaps, situated on the e. side of the sinus medii, n.w. of herschel. the walls are loftiest on the s. and s.w., where several small craters are associated with them. a ridge connects the formation with the great deep crater reaumur a, and a second large enclosure lying on the w. side of the well-known valley w. of herschel. at the end of a spur on the s. side of the great crater originates a cleft, which i have often traced to the n.w. wall of ptolemaeus, and across the n. side of the floor of this formation to a crater on the n.e. quarter of it, ptolemaeus _d_. there is a short cleft on the w. side of the floor of reaumur, running from n. to s. herschel.--a typical ring-plain, situated just outside the n. border of ptolemaeus, with a lofty wall rising nearly , feet above a somewhat dusky floor, which includes a prominent central mountain. its bright border is clearly terraced both within and without, the terraces on the inner slope of the w. wall being beautifully distinct even under a high light, and on the outer slope are some curious irregular depressions. on the s.s.e. is a large oblong deep crater, close to the rocky margin of ptolemaeus, and a little beyond the foot of the wall on the n.w. is a smaller and more regular rimmed depression, _b_, standing near the e. border of the great valley, more than miles long, and in places fully miles wide, which runs from s.s.w. to n.n.e. on the w. side of herschel, and bears a close resemblance to the well-known ukert valley. herschel _d_ is a large but shallow ring-plain on the e. of herschel, with a brilliant but smaller crater on the w. of it. north of herschel, on a plateau concentric with its outline, stands the large polygonal ring-plain herschel _a_, a formation of a very interesting character, with a low broken wall, exhibiting many gaps, and including some craters of a minute class. the largest of these stands on the s.w. wall. mr. w.h. maw has detected some of these objects on the n. side, both in connection with the border and beyond it. flammarion.--a large incomplete walled-plain n.e. of herschel, open towards the n., with a border rising about feet above the floor. the brilliant crater, mosting a, stands just outside the wall on the e. ptolemaeus.--taking its very favourable position into account, this is undoubtedly the most perfect example of a walled-plain on the moon's visible superficies. it is the largest and most northerly component of the fine linear chain of great enclosures, which extend southwards, in a nearly unbroken line, to walter. it exhibits a very marked departure from circularity, the outline of the border approximating in form to a hexagon with nearly straight sides. it includes an area of about square miles, the greatest distance from side to side being about miles. it is, in fact, about equal in size to the counties of york, lancashire, and westmorland combined; and were it possible for one to stand near the centre of its vast floor, he might easily suppose that he was stationed on a boundless plain; for, except towards the west, not a peak, or other indication of the existence of the massive rampart would be discernible; and even in this direction he would only see the upper portion of a great mountain on the wall. the border is much broken by gaps and intersected by passes, especially e. and s., where there are several valleys connecting the interior with that of alphonsus. the loftiest portion of the wall, which includes many crateriform depressions, is on the w., where one peak rises to nearly feet. another on the n.e. is about feet above the interior. on the n.w. is a remarkable crater-row, called, from its discoverer, "webb's furrow," running from a point a little n. of a depression on the border to a larger crateriform depression on the s. of hipparchus k. birt terms it "a very fugitive and delicate lunar feature." as regards the vast superficies enclosed by this irregular border, it is chiefly remarkable for the number of large saucer-shaped hollows which are revealed on its surface under a low sun. they are mostly found on the eastern quarter of the floor. some of them appear to have very slight rims, and in two or three instances small craters may be detected within them. owing to their shallowness, they are very evanescent, and can only be glimpsed for an hour or so about sunrise or sunset. the large bright crater a, about miles in diameter on the n.w. side of the interior, is by far the most conspicuous object upon it. adjoining it on the n. is a large ring with a low border, and n. of this again is another, extending to the wall. mr. maw and mr. mee have seen minute craters on the borders of these obscure formations. in addition to the objects just specified, there is a fairly conspicuous crater, _d_, on the n.e. quarter of the floor, and a very large number of others distributed on its surface, which is also traversed by a network of light streaks, that have recently been carefully recorded by mr. a.s. williams. a cleft, from near reaumur a, traverses the n. side of the floor, and runs up to ptolemaeus _d_. alphonsus.--a large walled-plain, miles in diameter, with a massive irregular border abutting on the s.s.e. side of ptolemaeus, and rising at one place on the n.w. to a height of feet above the interior. the floor presents many features of interest. it includes a bright central peak, forming part of a longitudinal ridge, on either side of which runs a winding cleft, originating at a crater-row on the n. side of the interior. there is a third cleft on the n.w. side, and a fourth near the foot of the e. wall. there are also three peculiar dark areas within the circumvallation; two, some distance apart, abutting on the w. wall, and a third, triangular in shape, at the foot of the e. wall. the last- mentioned cleft traverses this patch. these dusky spots are easily recognised in good photographs of the moon. alpetragius.--a fine object, miles in diameter, closely connected with the s.e. side of alphonsus. it has peaks on the w. towering , feet above the floor, on which there is an immense central mountain, which in extent, complexity, and altitude surpasses many terrestrial mountain systems--as, for example, the snowdonian group. the massive barrier between alpetragius and alphonsus deserves careful scrutiny, and should be examined under a moderately low morning sun. on the e., towards lassell, stands a brilliant light-surrounded crater. arzachel.--another magnificent object, associated on the n. with alphonsus, about miles in diameter, and encircled by a massive complex rampart, rising at one point more than , feet above a depressed floor. it presents some very suggestive examples of terraces and large depressions, the latter especially well seen on the s.e. the bright interior includes a large central mountain with a digitated base on the s.e., some smaller hills on the s. of it, a deep crater w. of it (with small craters n. and s.), and, between the crater and the foot of the w. wall, a very curious winding cleft. lassell.--this ring-plain, some miles in diameter, is irregular both as regards its outline and the width of its rampart. there is a crater on the crest of the n.w. wall, just above a notable break in its continuity through which a ridge from the n.w. passes. there is another crater on the opposite side. the central mountain is small and difficult to see. about miles n.e. of lassell is a remarkable mountain group associated with a bright crater, and further on in the same direction is a light oval area, about miles across, with a crater (alpetragius _d_) on its s. edge. madler described this area as a bright crater, miles in diameter, which now it certainly is not. lalande.--a very deep ring-plain, about miles in diameter, n.e. of ptolemaeus, with bright terraced walls, some feet above the floor, which contains a low central mountain. on the n. is the long cleft running, with some interruptions, in a w.n.w. direction towards reaumur. davy.--a deep irregular ring-plain, miles across, on the mare e. of alphonsus. there is a deep crater with a bright rim on its s.w. wall, and e. of this a notable gap. there is also a wide opening on the n. the e. wall is of the linear type. a cleft crosses the interior. guerike.--the most southerly member of a remarkable group of partially destroyed walled-plains, standing in an isolated position in the mare nubium. its border, on the w. and n. especially, is much broken, and never rises much more than feet above the mare, except at one place on the n., where there is a mountain about feet higher. the e. wall is tolerably continuous, but is of a very abnormal shape. on the s. there is a peculiar lambda-shaped gap (with a bright crater, and another less prominent on the e. side of it), the narrowest part of which opens into a long wide winding valley, bounded by low hills, extending to the w. side of a bright ring-plain, guerike b, on the s.e. a crater-chain occupies the centre of this valley. there is much detail within guerike. a large deep bright crater stands under the e. border on a mound, which, gradually narrowing in width, extends to the n. wall; and a rill-like valley runs from the n. border towards the e. side of the lambda-shaped gap. in addition to these features, there is a shallow rimmed crater, about midway between the extremities of the rill-valley, and several minor elevations on the floor. on the broken n. flank of guerike is a number of incomplete little rings, all open to the n.; and e. of these commences a linear group of lofty isolated mountain masses extending towards the w. side of parry, and prolonged for miles or more towards the north. they are arranged in parallel rows, and remind one of a druidical avenue of gigantic monoliths viewed from above. they terminate on the s. side of a large bright incomplete ring (with a lofty w. wall), connected with the w. side of parry. parry.--a more complete formation than guerike. it is about miles in diameter, and is encompassed by a bright border, which, at a point on the e., is nearly feet in height. it is intersected on the n. by passes communicating with the interior of fra mauro. there is a crater, nearly central, on the dusky interior, which, under a low sun, when the shadows of the serrated crest of the w. wall reach about half-way across the floor, appears to be the centre of three or four concentric ridges, which at this phase are traceable on the e. side of it. there is a conspicuous crater on the e. wall, below which originates a distinct cleft. this object skirts the inner foot of the e. border, and after traversing the n. wall, strikes across the wide expanse of fra mauro, and is ultimately lost in the region n. of this formation. parry a, s. of parry, is a very deep brilliant crater with a central hill and surrounded by a glistening halo. a cleft, originating at a mountain arm connected with the e. side of guerike, runs to the s. flank of this object, and is probably connected with that which skirts the floor of parry on the e. bonpland.--a ruined walled-plain with a low and much broken wall, which on the s.w. appears to be an attenuated prolongation of that of parry. it is of the linear type, the formation approximating in shape to that of a pentagon. the floor is crossed from n. to s. by a fine cleft which originates at a crater beyond the s. wall, and is visible as a light streak under a high light. schmidt shows a short cleft on the w. of this. fra mauro.--a large enclosure of irregular shape, at least miles from side to side, abutting on parry and bonpland. in addition to the cleft which crosses it, the floor is traversed by a great number of ridges, and includes at least seven craters. thebit.--a fine ring-plain, miles in diameter, on the mountainous w. margin of the mare nubium, n.e. of purbach. its irregular rampart is prominently terraced, and its continuity on the n.e. interrupted by a large deep crater (thebit a), at least miles in diameter, which has in its turn a smaller crater, of about half this size, on its margin, and a small central mountain within, which was once considered a good optical test, though it is not a difficult object in a inch achromatic, if it is looked for at a favourable phase. the border of thebit rises at one place on the n.w. to a height of nearly , feet above the interior, which includes much detail. the e. wall of thebit a attains the same height above its floor, which is depressed more than feet below the mare. birt.--this ring-plain, about miles in diameter, is situated on the mare nubium, some distance due e. of thebit. it has a brilliant border, surmounted by peaks rising more than feet above the mare, and a very depressed floor, which does not appear to contain any visible detail. a bright crater adjoins it on the s.w., the wall of which at the point of junction is clearly very low, so that under oblique light the two interiors appear to communicate by a narrow pass or neck filled with shadow. i have frequently seen a break in the n.w. wall of birt, which seems to indicate the presence of a crater. there is a noteworthy cleft on the e., which can be traced from the foot of the e. wall to the hills on the n.e. it is a fine telescopic object, and, under some conditions, the wider portion of it resembles a railway cutting traversing rising ground, seen from above. it is visible as a white line under a high light. the straight wall.--sometimes called "the railroad," is a remarkable and almost unique formation on the w. side of birt, extending for about miles from n.e. to s.w. in a nearly straight line, terminating on the south at a very peculiar mountain group, the shape of which has been compared to a stag's horn, but which perhaps more closely resembles a sword-handle,--the wall representing the blade. when examined under suitable conditions, the latter is seen to be slightly curved, the s. half bending to the west, and the remainder the opposite way. the formation is not a ridge, but is clearly due to a sudden change in the level of the surface, and thus has the outward characteristics of a "fault" along the upper edge of this gigantic cliff (which, though measures differ, cannot be anywhere much less than feet high) i have seen at different times many small craterlets and mounds. near its n. end is a large crater, and on the w. is a row of hillocks, running at right angles to the cliff. no observer should fail to examine the wall under a setting sun when the nearly perpendicular e. face of the cliff is brilliantly illuminated. nicollet.--a conspicuous little ring-plain on the e. of birt, and somewhat smaller. between the two is a still smaller crater, from near which runs a low mountain range, nearly parallel to the straight wall, to the region s.e. of the stag's horn mountains. here will be found three small light-surrounded craters arranged in a triangle, with a somewhat larger crater in the middle. purbach.--an immense enclosure of irregular shape, approximating to that of a rhomboid with slightly curved sides. it is fully miles across, and the walls in places exceed feet in altitude, and include many depressions, large and small. on the e. inner slope are some fine terraces and several craters. the continuity of the circumvallation is broken on the n. by a great ring-plain, on the floor of which i have seen a prominent cleft and a crater near the s. side. there is a large bright crater in the interior of purbach, s. of the centre, two others on the w. half of the floor, and a few ridges. regiomontanus.--a still more irregular walled-plain, of about the same area, closely associated with the s. flank of purbach, having a rampart of a similar complex type, traversed by passes, longitudinal valleys, and other depressions. schmidt alone shows the especially fine example of a crater-row, which is not a difficult object, in connection with the s.e. wall. excepting one crater, nearly central, and some inconspicuous ridges, i have seen no detail on the floor. schmidt, however, records many features. walter.--a great rhomboidal walled-plain, miles in diameter, with a considerably depressed floor, enclosed by a rampart of a very complex kind, crowned by numerous peaks, one of which, on the w., rises , feet above the interior. if the formation is observed when it is close to the morning terminator, say, when the latter lies from l deg. to deg. e. of the centre of the floor, it is one of the most striking and beautiful objects which the lunar observer can scrutinize. the inner slope of the border which abuts on regiomontanus, examined at this phase under a high power, is seen to be pitted with an inconceivable number of minute craters; and the summit ridge, and the region towards werner, scalloped in a very extraordinary way, the engrailing (to use an heraldic term) being due to the presence of a row of big depressions. the floor at this phase is sufficiently illuminated to disclose some of its most noteworthy features. taking its area to be about square miles, at least square miles of it is occupied by the central mountain group and its adjuncts, the highest peak rising to a height of nearly feet (or nearly feet higher than ben nevis), above the interior, and throwing a fine spire of shadow thereon. in the midst of this central boss are two deep craters, one being about miles in diameter, and a number of shallower depressions. in association with the loftiest peak, i noted at h., march , , two brilliant little craters, which presumably are not far from the summit. near the e. corner of the floor there is another large deep crater, and, ranging in a line from the centre to the s.e. wall, three smaller craters. lexell.--on the e. of walter extends an immense plain of irregular outline, which is at least equal to it in area. though no large formation is found thereon; many ridges, short crater-rows, and ordinary craters figure on its rugged superficies; and on its borders stand some very noteworthy objects, among them, on the s., the walled-plain lexell, about miles in diameter, which presents many points of interest. its irregular wall, rising, at one point on the s.w., to a height of nearly feet, is on the n.w. almost completely wanting, only very faint indications of its site being traceable, even under a low morning sun. on the opposite side it is boldly terraced, and has a large crater on its summit. the interior, the tone of which is conspicuously darker than that of the region outside, contains a small central hill, with two craters connected with it. the low n.w. margin is traversed by a delicate valley, which, originating on the n. side of the great plain, crosses the w. quarter of lexell and terminates apparently on the s.w. side of the floor. hell.--a prominent ring-plain, about miles in diameter, on the e. side of the great plain. there is a central mountain and many ridges within. ball.--a somewhat smaller ring-plain on the s.e. edge of the great plain, with a lofty terraced border and a central mountain more than feet high. there are two large irregular depressions on the w. of the formation, a crater on the s., and a smaller one on the n. wall. pitatus.--this remarkable object, miles in diameter, with hesiodus, its companion on the e., situated at the extreme s. end of the mare nubium, afford good examples of a class of formations which exhibit undoubted signs of partial destruction, from some unknown cause, on that side of them which faces the mare. on every side but the n., pitatus is a walled plain of an especially massive type, the border on the s.e. furnishing one of the finest examples of terraces to be found on the visible surface. on the s.w., two parallel rows of large crateriform depressions, perhaps the most remarkable of their kind, extend for miles or more to the w. flank of gauricus. on the n.w., the rampart includes many curious irregular depressions and craters, and gradually diminishes in height, till, for a space of about miles on the n., there can hardly be said to be any border at all, its site being marked by some inconsiderable mounds and shallow hollows. there is a small bright central mountain on the floor, and, s. of it, two larger but lower elevations. a distinct straight cleft traverses the n.w. side of the interior very near the wall, to which it forms an apparent chord, and a second cleft occupies a similar position with respect to the bright n.e. border. a narrow pass forms a communication with the interior of hesiodus. hesiodus.--this walled-plain, little more than half the diameter of the last, has an irregular outline, and for the most part linear walls, which on the s. are massive and lofty ( feet), but on the n. very low, and broken by gaps. there is a fine deep crater on the s. border, and a small but distinct crater on the floor, nearly central, the only object thereon which i have seen, though schmidt draws a smaller one on the w. of it. a mountain abutting on the n.e. side of hesiodus is the w. origin of one of the longest clefts on the moon. running in an e.s.e. direction, it traverses the mare to a crater near the w. face of the cichus mountain arm, reappears on the e. side of this object, and is finally lost amid the hills on the n. of capuanus. the w. section of this cleft is coarser and much more distinct than that lying e. of the mountain arm. gauricus.--a large walled-plain s. of pitatus, about miles in diameter. the border is very irregular, and, according to neison, consists on the e. of a precipitous cliff more than feet high. it is surrounded by a number of large rings on the s., and has several considerable small depressions on its n. border. there is apparently no prominent detail on the floor. schmidt shows some ridges and craterlets. wurzelbauer.--another irregular walled-plain, about miles in diameter, on the s.e. of pitatus, with a very complex border, in connection with which, on the s.w., is a group of fine depressions, and on the s.e. a large crater. there is much detail on the very uneven floor. miller.--one of a group of three moderately large ring-plains, of which nasireddin is a member, near the central meridian in s. latitude deg. its massive border rises nearly , feet above the floor, on which stands a central peak. miller is about miles in diameter. nasireddin.--a somewhat smaller ring-plain on the s. of the last, and of a very similar type. it contains a central peak and several minor elevations. between its n.w. border and the s.w. flank of miller is a smaller ring-plain of about half the size of nasireddin, and on the s.e. a large enclosure named huggins. orontius.--huggins has encroached on the w. side of this irregular ring- plain and overlaps it. it is of considerable size. the floor includes much detail and a prominent crater. sasserides.--a formation of irregular shape, with very lofty walls, situated amid the confusion of ring-plains, craters, crater-pits, &c., in the region n. of tycho, some of which are fully as deserving of a distinct name. heinsius.--a very curious formation on the n.e. of tycho: a fine telescopic object under oblique illumination. it has an irregular but continuous border, except on the s., where two large ring-plains have encroached upon it, and a third, n. of a line joining their centres, occupies no inconsiderable portion of the floor. heinsius is nearly miles across, and the border on the w., is nearly feet above the interior, which includes, at least, three small craters. the walls of the intrusive ring-plains have craters on their summits; the more westerly has two on the w., and its companion, one on the s.w. the ring-plain on the floor has a crater on its e. wall. schmidt shows a small crater between the ring-plains on the s. border. saussure.--a ring-plain w. of tycho, miles in diameter, with bright lofty terraced walls and a somewhat dark interior, on which there is a crater, w. of the centre, and some crater-pits. there are several large depressions on the s.w. wall. it is surrounded by formations which, though nearly as prominent as itself, have not, with the exception of pictet on the e., and one on the n.w., called huggins by schmidt, received distinctive names. the region w. of saussure abounds in craterlets, some of which are of the minutest type. one of the tycho streaks is manifestly deflected from its course by this formation, and another is faintly traceable on the floor. pictet.--a walled-plain of irregular shape, about miles across, between saussure and tycho, with a border broken on the s. by a large conspicuous ring-plain, which is at least miles in diameter, and, according to schmidt, has a central mountain. schmidt draws the s.e. border of pictet as broken by ridges extending on to the floor. he also shows several craters and minor elevations thereon. tycho.--as the centre from which the principal bright ray-system of the moon radiates, and the most conspicuous object in the southern hemisphere, this noble ring-plain may justly claim the pre-eminent title of "the metropolitan crater." it is more than miles in diameter, and its massive border, everywhere traversed by terraces and variegated by depressions within and without, is surmounted by peaks rising both on the e. and w. to a height of about , feet above the bright interior, on which stands a magnificent central mountain at least feet in altitude. were it not somewhat foreshortened, tycho would be seen to deviate considerably from what is deemed to be the normal shape. on the s. and w. especially, the wall approximates to the linear type, no signs of curvature being apparent where these sections meet. the crest on the s. and s.e. exhibits many breaks and irregularities; and it is through a narrow gap on the s. that a rill-like valley, originating at a small depression near the foot of the s.w. _glacis_, passes, and, descending the inner slope of the s.e. wall obliquely, terminates near its foot. there is a distinct crater on the summit ridge on the s.e., and another below the crest on the outer s.w. slope. on the s. inner slope i have often remarked a number of bright oval objects, which, for the lack of a better word, may be termed "mounds" though they represent masses of material many miles in length and breadth. the outer slope of tycho, exhibiting under a high light a grey nimbus encircling the wall, includes--craters, crater-pits, shallow valleys, spurs and buttresses--in short, almost every variety of lunar feature is represented. excepting the central mountain and a crater on the w. of it, i have not seen any object on the floor, which, for some unexplained reason, is never very distinct. schmidt shows several low ridges on the n.e. side. in a paper recently published in the _astronomische nachrichten_, professor w.h. pickering, describing his observations of the tycho streaks made at arequipa, peru, with a inch achromatic, asserts that they do not radiate from the centre of tycho, but from a multitude of minute craters on its s.e. or n. rim. (see introduction.) maginus.--an immense partially ruined enclosure, at least miles from side to side, on the s.w. of tycho, from which it is separated by a region covered with a confused mass of ring-plains and craters. on almost every part of its broken border stand large ring-plains, many of which, if they were isolated, or situated in a less disturbed region, would rank as objects of importance; but among such a multitude of features they pass unnoticed. the largest of them occupies no inconsiderable part of the s.e. wall, and is quite miles in diameter, its own border being also much broken by depressions, as, indeed, are those of almost all the six or more large ring-plains which define the n. limits of maginus. the loftiest portion of what remains of a true border rises at one place to more than , feet. on the floor, which is traversed by some of the tycho rays, there is a mountain group associated with a crater, nearly central, and several large rings on the n. side. though the formation is very difficult to detect under a high sun, madler's dictum that "the full moon knows no maginus" is not strictly true. street.--a walled-plain between tycho and maginus, about miles in diameter, with a border of moderate height, broken by depressions on the n. there are some small craters and ridges within; but the surrounding region, with its almost endless variety of abnormally shaped formations, is far more worthy of the observer's attention. deluc.--the largest and most prominent member of a curious group of ring- plains on the s.w. of maginus. it is about miles in diameter, and is encircled by a wall some feet above the interior, which includes a crater. a large ring with a central mountain encroaches on the n. wall, and a smaller object of the same class on the s. wall. clavius.--there are few lunar observers who have not devoted more or less attention to this beautiful formation, one of the most striking of telescopic objects. however familiar we may consider ourselves to be with its features, there is always something fresh to note and to admire as often as we examine its apparently inexhaustible details. it is miles from side to side, and includes an area of at least , square miles within its irregular circumvallation, which is only comparatively slightly elevated above the bright plateau on the w., though it stands at least , feet above the depressed floor. at a point on the s.w. a peak rises nearly , feet above the interior, while on the e. the cliffs are almost as lofty. there are two remarkable ring-plains, each about miles in diameter, associated, one with the n., and the other with the s. wall, the floors of both abounding in detail. the latter, however, is the most noteworthy on account of the curious corrugations visible soon after sunrise on the outer n. slope of its wall, resembling the ribbed flanks of some of the java volcanoes. there are five large craters on the floor of clavius, following a curve convex to the n., and diminishing in size from w. to e. the most westerly stands nearly midway between the two large ring-plains on the walls, the second (about two- thirds its area) is associated with a complex group of hills and smaller craters. both these objects have central mountains. in addition to this prominent chain, there are innumerable craters of a smaller type on the floor, but they are more plentiful on the s. half than elsewhere. on the s.e. wall are three very large depressions. on the broad massive n.e. border, the bright summit ridge and the many transverse valleys running down from it to the floor, are especially interesting features. there are very clear indications of "faulting" on a vast scale where this broad section of the wall abuts on the n. side of the formation. cysatus.--a regular walled-plain, apparently about miles in diameter, forming the most northerly member of a chain of formations, of which newton, short, and moretus, extending towards the s. limb, form a part. its border rises nearly , feet above a floor devoid of prominent detail. gruemberger.--a much larger and more irregular ring-plain, nearly miles from wall to wall, on the e. side of cysatus. its w. border rises nearly , feet above the interior, which includes an abnormally deep crater, the bottom of which is , feet below the crest of the w. wall, and several small depressions and ridges. the inner e. slope is finely terraced. moretus.--a magnificent object, miles in diameter, but foreshortened into a flat ellipse. its beautifully terraced walls and magnificent central mountain, nearly feet high, are very conspicuous under suitable conditions. the rampart on the e. is more than , feet above the floor, while on the opposite side it is about feet lower. short.--a fine but foreshortened ring-plain of oblong shape, squeezed in between moretus and newton. it is about miles in diameter, and on the s.e., where its border and that of newton are in common, it rises nearly , feet above the interior, which includes, according to neison, a small central hill. schmidt shows a crater on the n. side of the floor. newton.--is situated on the s.e. side of short, and is the deepest walled-plain on the visible surface. it is of irregular form and about miles in extreme length. one gigantic peak on the e. rises to nearly , feet above the floor, the greater part of which is always immersed in shadow, so that neither the earth or sun can at any time be seen from it. malapert.--a ring-plain situated far too near the limb for useful observation. between it and newton is a number of abnormally shaped enclosures. cabeus.--another object out of the range of satisfactory scrutiny. madler considered that it is as deep as newton. according to neison, a central peak and two craters can be seen within under favourable conditions. schmidt draws a long row of great depressions on the n. side of it. east longitude deg. to deg. landsberg.--a ring-plain, about miles in diameter, situated in mare nubium, s.e. of reinhold, which in many respects it resembles. its regular massive border is everywhere continuous. only a solitary crater breaks the uniformity of its crest, that rises on the w. to nearly , feet, and on the e. to about feet above the floor, which is depressed about feet below the surrounding surface. the inner slopes exhibit some fine terraces, and on the broad w. _glacis_ is a curious winding valley, which runs up the slope from the s.w. side to the crater just mentioned, then, bending downwards, joins the plain at the foot of the n. wall. neither this nor the crater is shown in the maps. the large compound central peak is apparently the sole object in the interior. at h. m. on january , , when observing the progress of sunrise on this formation with a / inch calver-reflector charged with different eyepieces, i noticed, when about three-fourths of the floor was in shadow, that the illuminated portion of it was of a dark chocolate hue, strongly contrasting with the grey tone of the surrounding district. this appearance lasted till the interior was more than half illuminated, gradually becoming less pronounced as the sun rose higher on the ring. e. and s.e. of landsberg is a number of ring-plains and craters well worthy of careful examination. five of the largest are surrounded by a glistening halo, and one (that nearest to the formation) and another (the largest of the group) have each a minute crater on their n. wall. euclides.--one of the most brilliant objects on the moon; a crater miles in diameter, standing on a large bright area in the mare procellarum, e. of the riphaean mountains. its e. rim rises nearly feet above the bright depressed floor; on the w. there is a bright little unrecorded crater. wichmann.--this bright crater, about miles in diameter, stands on a light area in oceanus procellarum, n.n.w. of letronne and nearly due e. of euclides. some distance on the n.e. are the relics of what appears once to have been a large enclosure, represented now by a few isolated mountains. herigonius.--a ring-plain, about miles in diameter, in the mare procellarum, n.w. of gassendi. there is a small crater a few miles s.e. of it, among the bright little mountains which flank this formation. herigonius has a small central mountain, which is a good test for moderate apertures. gassendi.--one of the most beautiful telescopic objects on the moon's visible surface, and structurally one of the most interesting and suggestive. it is a walled-plain, miles in diameter, of a distinctly polygonal type, the n.w. and s.w. sections being practically straight, while the intermediate w. section exhibits a slightly convex curvature, or bulging in towards the interior. there is also much angularity about the e. side, which is evident at an early stage of sunrise. the wall on the n. is broken through and almost completely wrecked by the great ring- plain gassendi a. the bright eastern section of the border is in places very lofty, rising at one peak, n. of the well-known triangular depression upon it, to feet, and at other peaks on the same side still higher. it is very low on the s., being only about feet above the surface. the floor, however, on the n. stands feet above the mare humorum. on the w. there is a peak towering feet above the wall, which is here about feet above the floor, and feet above the mare nubium. a very notable feature in connection with this formation is the little bright plain bounding it on the n.w., and separated from it by merely a narrow strip of wall. this enclosure is flanked on the n.e. by gassendi a, and on the s.w. and n.w. by a coarse winding ridge, running from the w. wall and terminating at a large irregular dusky depression. gaudibert has detected a crater near the s.e. edge of this bright plain, which includes also some oval mounds. the interior of gassendi is without question unrivalled for the variety of its details, and, after plato, has perhaps received more attention from observers than any other object. the bright central mountain, or rather mountains, for it consists of a number of grouped masses crowned by peaks, of which the loftiest is about feet, is one of the finest on the moon. it was carefully studied with a / inch cooke-achromatic by the late professor phillips, the geologist, who compared it to the dolomitic or trachytic mountains of the earth. the buttresses and spurs which it throws out give its base a digitated outline, easily seen under suitable illumination. there are between and clefts in the interior, the majority being confined to the s.w. quarter of the floor. those most easily seen pertain to the group which radiates from the central mountain towards the s.w. wall. they are all more or less difficult objects, requiring exceptionally favourable weather and high powers. a fine mountain range, the percy mountains, is connected with the e. flank of gassendi, extending in a s.e. direction towards mersenius, and defining the n.e. side of the mare humorum. bullialdus.--a noble object, miles in diameter, forming with its surroundings by far the most notable formation on the surface of the mare nubium, and one of the most characteristic ring-plains on the moon. it should be observed about the time when the morning terminator lies on the w. border of the mare humorum, as at this phase the best view is obtained of the two deep parallel terrace valleys which run round the bright inner slope of the e. wall, of the crater-row against which they abut on the s.e., and of the massive w. _glacis_, with its spurs and depressions. the s. border of bullialdus has been manifestly modified by the presence of the great ring-plain a, a deep irregular formation with linear walls, which is connected with it by a shallow valley. the rampart of bullialdus rises about feet above a concave floor, which sinks some feet below the mare on the e. with the exception of the fine compound central mountain, feet high, there are few details in the interior. on the s., is the fine ring-plain b, connected with the s.e. wall near the crater-row by a well-marked valley, and nearly due e. of b is another, a square-shaped enclosure, c, with a very lofty little mountain on the e. side of it, and a crater on its s. wall. in addition to these features, there are many ridges and surface inequalities, very prominent under oblique illumination. lubiniezky.--a regular enclosure, about miles in diameter, n.e. of bullialdus, with a low attenuated border, which is nowhere more than feet in height. it is tolerably continuous, except on the s., where there are two or three breaks. its level dark interior presents no details to vary its monotony. close under the n.w. wall is a small crater connected with it by a ridge, and e. of this a very rugged area, traversed in every direction by narrow shallow valleys, which are well worth looking at when close to the morning terminator. a bright spur projects from the n. wall of lubiniezky. kies.--a somewhat similar formation, s. of bullialdus, about miles in diameter, also encircled by a border of insignificant dimensions, attaining an altitude of feet at only one point on the s.e., while elsewhere it is scarcely higher than that of lubiniezky. it is clearly polygonal, approximating to the hexagonal type. on the more distinct s. section a bright spur projects from it. on the n. its continuity is broken by a distinct little crater. it is traversed by a remarkable white streak, extending in a s.w. direction from bullialdus c (where it is very wide), across the interior, to the more westerly of two craters s.w. of mercator. another streak branches out from it near the centre of the floor, and runs to the w. wall. the principal streak, so far as the portion within kies is concerned, represents a cleft. on the mare e. of kies is a curious circular mound, and farther towards campanus two prominent little mountains. on the n.w. is a large obscure ring and a wide shallow valley bordered by ridges. agatharchides.--a very irregular complex ring-plain, about miles in diameter, forming part of the n.w. side of the mare humorum. it must be observed under many phases before one can clearly comprehend its distinctive features. the wall is very deficient on the n., but is represented in places by bright mountain masses. the formation is flanked on the e. by a double rampart, which is at one place more than feet in height, with a deep intervening valley. the s. wall is traversed by a number of parallel valleys, all trending towards hippalus. these are included in a much wider and longer chasm, which, gradually diminishing in breadth, extends up to the n. wall of the latter. hippalus.--a partially ruined walled-plain, about miles in diameter, on the w. side of the mare humorum, s. of agatharchides. at least one- third of the border is wanting on the s.e., but under a low sun its site can be distinguished by a faint marking and the obvious difference in tone between the dark interior and the lighter-coloured plain. the rest of the wall is bright and continuous, except at a place on the w., where what appears to be the segment of a large ring has encroached upon it. there are two craters in the interior of hippalus, and a row of parallel ridges, running obliquely from the s.w. wall up to a cleft which traverses the floor from n. to s. w. of hippalus stands a bright crater, hippalus a, with an incomplete little ring-plain adjoining it on the n.w.; and n.e. of it a much larger obscure ring containing two little hills. the hippalus rill-system is a very interesting one, and the greater part of it can, moreover, be easily traced in a good inch achromatic. it originates in the rugged region e. of campanus, from which five nearly parallel curved clefts extend up to the rocky barrier, connecting the n. side of this formation with the s.w. side of hippalus. the most westerly of these furrows is interrupted by a crater on this wall, but reappears on the n. side of it, and, after making a detour towards the w. to avoid a little mountain in its path, runs partially round the e. flank of hippalus a, and then, continuing its northerly course, terminates amid the mountains w. of agatharchides. (a short parallel cleft runs e. of this from the little mountain to the e. side of a.) the most easterly member of the system, originating n. of ramsden, enters hippalus at the s. side of the great gap in the border, and, after traversing the floor at the w. foot of a ridge thereon, also extends towards the mountains w. of agatharchides. between these clefts are three intermediate furrows, one of which runs n. from the n. side of the encroaching ring already referred to, on the w. wall of hippalus. campanus.--a ring-plain, miles in diameter, on the rocky barrier, extending in nearly a straight line from hippalus to cichus. its terraced walls, which rise on the e. more than feet above the floor, are broken on the s. by a narrow valley, and on the e. by a small crater. a small central mountain is apparently the only object on a very dark interior. mercator.--a more irregular ring-plain of about the same area, adjoining campanus on the s.w. its rampart is somewhat lower, and is partially broken on the n. by two semi-rings, and on the s. by a gap. the e. wall extends on the s. far beyond the limits of the formation, and terminates in a brilliant mountain mass feet in height. there is a bright crater on the crest of both the e. and w. border. on the plain e. of mercator is a remarkable little crater standing on a light area, and, just under the wall, a dusky pit connected with it by a rill-like marking. these objects are of a very doubtful nature, and should be carefully observed. the floor of mercator is much lighter than that of campanus, and appears to be devoid of detail. cichus.--a conspicuous ring-plain, about miles in diameter, with a prominent deep crater about miles across on its e. rim. it is situated on a curious boot-shaped plateau, near the s. end of the rocky mountain barrier associated with the last two formations. its walls rise about feet above a sunken floor, on which there is some faint detail, but apparently nothing deserving the distinction of a central mountain. the plateau on the n. is cut through by a fine broad valley, which has obviously interfered with a large crateriform depression on its southern edge. a cleft runs from a small crater w. of the plateau up to this valley, and extends beyond to the w. wall of capuanus. there is also a delicate cleft crossing the region s. of cichus to the group of complicated formations s.w. of capuanus. as already mentioned, the great hesiodus cleft is associated with the cichus plateau. capuanus.--a large ring-plain, about miles in diameter, e. of cichus, with a border especially remarkable on the e., where it rises more than feet above the outside country, and includes a large brilliant shallow crater. it is broken on the n.w. by a small but noteworthy double crater; and on the s. its continuity is destroyed for many miles by a number of big circular and sub-circular depressions and prominent deep valleys, far too numerous and complicated to describe. the level dusky interior contains only a low mound on the s., but is crossed by some light streaks running from n. to s. ramsden.--this ring-plain, miles in diameter, derives its importance from the remarkable rill-system with which it is so closely associated. its border, about feet on the w. above the outside surface, is slightly terraced within on the e., where there is an unrecorded bright crater on the slope. the two principal clefts on the s. originate among the hills e. of capuanus. the more easterly begins at a crater on the n. edge of these objects, and runs n. to the e. side of ramsden; the other originates at a larger crater, and proceeds in a n. direction up to a bright little mountain s.w. of ramsden; when, swerving to the n.e., it ends at the w. wall of this formation. this mountain is a centre or node from which three other more delicate branches radiate. on the n., three of the shortest clefts pertaining to the system are easily traceable from neighbouring mountains up to the n. wall, which they apparently partially cut through. the e. pair have a common origin, but open out as they approach the border of ramsden. vitello.--a very peculiar ring-plain, miles in diameter, on the s. side of the mare humorum, remarkable for having another nearly concentric ring-plain, of considerably less altitude within it, and a large bright central boss, overlooking the inner wall, feet in height. the outer wall is somewhat irregular, and is broken by gaps and valleys on the s. and n.w. it rises on the e. about feet above the mare, but only about above the interior, which includes a crater on its n. side, and some low ridges. hainzel.--this remarkable formation, which is about miles in greatest length, but is hardly half so broad, derives its abnormal shape from the partial coalescence of two nearly equal ring-plains, the walls of both being very lofty,--more than , feet. it ought to be observed under a morning sun when the floor is about half illuminated. at this phase the extension of the broad bright terraced e. border across a portion of the interior is very apparent, and the true structural character of the formation clearly revealed. the floor abounds in detail, among which, on the s., are some large craters and a bright longitudinal ridge. hainzel is flanked on the w. and s.w. by a broad plateau, w. of which stand two ring-plains about miles in diameter, both having prominent central mountains and bright interiors. wilhelm i.--a large irregular formation, about miles across, s.e. of heinsius, with walls varying very considerably in height, rising more than , feet on the e., but only about feet on the opposite side. the border is everywhere crowded with depressions, large and small. three ring-plains, not less than miles in diameter, stand upon the s. wall, the most westerly overlapping its shallower neighbour on the e., which projects beyond the wall on to the floor. the interior has a very rugged and uneven surface, upon the n. side of which are two very distinct craters, and a short crater-row on the w. of them. it is traversed from w. to e. by three bright streaks from tycho, two on the n. being very prominent under a high light. longomontanus.--a much larger walled-plain, s. of the last. it is miles in diameter, with a border much broken by depressions, especially on the n.e. at one peak on this side it rises to the tremendous altitude of , feet above the floor, and at peaks on the w. more than feet higher. there is a crowd of ring-plains on the n.e. quarter of the interior, and some hills and craterlets in other parts of it. it is also crossed by rays from tycho. schiller.--a fine lozenge-shaped enclosure, with a continuous but somewhat irregular border. it is about miles in extreme length, and rather more than half this in breadth. the loftiest section of the wall is on the w., where it rises , feet above a considerably depressed interior. there is a bright crater on this side and some terraces. on the broad inner slope of the e. border, the summit ridge of which is especially well-marked, there is a large shallow depression. the floor contains scarcely any detail, except some ridges on the n. side and a few craterlets. the great bright plain e. of schiller and the region on the s.e. are especially worthy of scrutiny under a low morning sun. bayer.--this object, miles in diameter, with a terraced border rising on the w. to a height of feet above the floor, is so closely associated with schiller, that it may almost be regarded as forming part of it. a long lofty mountain arm, apparently connected with the w. wall of the latter, runs from the e. side of bayer towards the n.w. there is a crater on the e. side of the interior. rost.--an oblong-shaped ring-plain, miles in diameter, on the s.w. of schiller, with moderately high walls, and, according to neison, a shallow depression within, nearly central. i have seen a crater shown by schmidt on the e. side of the floor. a valley runs from the e. side of rost to the s. of schiller. weigel.--a not very conspicuous ring-plain on the s. of schiller, with a crater on its n.w. rim, and a larger ring adjoining it on the s.e. a prominent curved mountain arm from the e. wall of schiller runs towards the n. side of this formation. blancanus.--a formation, miles in diameter, on the s.e. side of clavius, whose surpassing beauties tend to render the less remarkable features of this magnificent ring-plain and those of its neighbour scheiner less attractive than they otherwise would be. the crest of its finely terraced wall, which at one peak on the e. rises to , feet, is at least , feet above the interior. krieger saw twenty craters on the floor ( , sept. , h.), most of them situated on the s. quarter. scheiner.--a still larger object, being nearly miles in diameter, with a prominently terraced wall, fully as lofty as that of blancanus. there is a large crater, nearly central, two others on the n.e. side of the floor, and a fourth at the inner foot of the e. wall. there is also a shallow ring on the n.e. slope. schmidt shows, but far too prominently, two straight ridges crossing each other on the s. side of the central crater. casatus.--a large walled-plain, about miles in diameter, s.e. of blancanus, near the limb, remarkable for having one of the loftiest ramparts of all known lunar objects; it rises at one peak on the s.w. to the great height of , feet above the floor, while there are other peaks nearly as high on the n. and s. the wall is broken on the e. by a fine crater. there is also a crater on the n.w. side of the very depressed floor, together with some craterlets. klaproth.--casatus partially overlaps this still larger but less massive formation on its s.e. flank. the walls of klaproth are much lower and very irregular and broken, especially on the w. there are some ridges on the floor. the neighbouring region is covered with unnamed objects, large and small. east longitude deg. to deg. flamsteed.--a bright ring-plain, miles in diameter, in a barren region in the oceanus procellarum, n.e. of wichmann. it has a regular border (broken at one place on the n. by a gap, which probably represents a crater), rising to a height of about feet above the surrounding plain. a great enclosure, miles in diameter, lies on the n. of flamsteed. it is defined by low ridges which exhibit many breaks, though under a high light the ring is apparently continuous. within are several small craters and two considerable hills, nearly central. hermann.--a ring-plain, about miles in diameter, in the oceanus procellarum, w. of lohrmann. it is associated with a group of long ridges, running in a meridional direction and roughly parallel to the coast-line. letronne.--a magnificent bay or inflexion in the coast-line of the oceanus procellarum, n.n.e. of gassendi, presenting an opening towards the n. of nearly miles, and bounded on the s. and s.w. by the lofty gassendi highlands. its border on the w., about feet high, is crowned with innumerable small depressions. the interior includes four bright little mountains, nearly central (three of them forming a triangle), a bright crater on the w. side, and several minor elevations and ridges. on the plain n. of the bay, is a large bright crater, from which a fine curved ridge runs to the central mountains. if letronne is observed under oblique illumination, the low mounds and ridges on the mare outside impress one with the idea that they represent the remains of a once complete n. wall. billy.--a ring-plain, miles in diameter, s.e. of letronne, with a very dark floor, depressed about feet below the grey surface on the w., and a regular border, rising more than feet above it. there is a narrow gap on the s., and indications of a crater on the n.w. rim. two small craters stand on the s. half of the interior. the formation is flanked on the s.w. by highlands. hansteen.--a somewhat larger ring-plain, with a lower and more irregular rampart, rising on the w. to nearly feet above the floor, which is depressed to about the same extent as that of billy. both the inner and outer slopes are terraced on the e., where the _glacis_ is traversed by a short, delicate, rill-like valley. there are some bright curved ridges on the floor. on the w. of billy and hansteen is a wide inlet of the oceanus procellarum, bounded by the letronne region on the w., and on the s. by lofty highlands. on the surface, not far from the s.w. border of hansteen, is a curious triangular-shaped mountain mass, with a digitated outline on the s., and including a small bright crater on its area. between this and the ring- plain is a large but somewhat obscure depression, n. of which lies a rill-like object extending from the n. point of the triangular mountain to the w. wall. at the bottom of a gently sloping valley between billy and hansteen is a delicate marking, which seems to represent a cleft connecting the two formations. zupus.--a formation about miles in diameter with a dark floor, situated in the hilly region n.e. of mersenius. fontana.--a noteworthy ring-plain, about miles in diameter, e.n.e. of zupus, with a bright border, exhibiting a narrow gap on the s. and two large contiguous craters on the n.w. the faint central mountain stands on a dusky interior. on the n. is a large peculiar depressed plain with a gently sloping wall, within which are three short rill-like valleys and a crater. mersenius.--with its extensive rill-system and interesting surroundings, one of the most notable ring-plains in the third quadrant. it is miles in diameter, and is encircled by a fine rampart, which on the side fronting the mare humorum rises feet above the floor, which is distinctly convex, and is depressed feet below the region on the e., though it stands considerably above the level of the mare. the prominently terraced border is tolerably regular on the n.w., but on the s. and s.e. is much broken by craters and depressions, the largest and most conspicuous interrupting the continuity of its summit-ridge on the latter side. a fine crater-row traverses the central part of the interior, nearly axially, and a delicate cleft crosses the n. half of the floor from the inner foot of the n.e. wall to a crater not far from the opposite side. i detected another cleft on november , , also crossing the n. side of the floor. south of mersenius is the fine ring-plain mersenius _d_, about miles in diameter, situated on the border of the mare; and, extending in a line from this towards vieta are two others (_a_, and cavendish _d_,), somewhat larger, but otherwise similar; the more easterly being connected with cavendish by a mountain arm. one of the principal clefts of the system (all of which run roughly parallel to the n.e. side of the mare, and extend to the percy mountains e. of gassendi) crosses the floor of _d_, and, i believe, partially cuts into its w. wall. another, the coarsest, abuts on a mountain arm connecting _d_ with mersenius, and, reappearing on the e. side, runs up to the n.w. wall of the other ring- plain, _a_, and, again reappearing on the e. of this, strikes across the rugged ground between _a_ and cavendish _d_, traversing its floor and border, as does also another cleft to the n. of it. cavendish _d_ includes a coarse cleft on its floor, running from n. to s., which i have frequently glimpsed with a inch achromatic. there are two other delicate clefts running from the gassendi region to the s.w. side of mersenius, which are in part crater-rills. cavendish.--a notable ring-plain, miles in diameter, s.e. of mersenius, with a prominently terraced border, rising at one point on the s. to a height of feet above the interior, on which are a few low ridges. a large bright ring-plain (_e_), about miles in diameter, breaks the continuity of the s.e. wall, and adjoining this, but beyond the limits of the formation, is another smaller ring with a central hill. there is also a bright crater on the n.w. border. the w. _glacis_ is very broad, and includes two large shallow depressions. an especially fine valley runs up to the n. wall, to the w. side of _e_. vieta.--one of the finest objects in the third quadrant; a ring-plain miles in diameter, with broad lofty walls, a peak on the west rising to nearly , feet, and another n. of it to considerably more than , feet above the interior. it is bounded by a linear border, approximating very closely to an hexagonal shape, which is broken by many gaps and cross-valleys. on the s., the s.w. and s.e. sections of the wall do not meet, being separated by a wide valley flanked on the w. by a fine crater, which has broken down the rampart at this place. the n. border is likewise intersected by valleys and by a crater-row. the inner slopes are conspicuously terraced. there is a very inconspicuous central mountain and several large craters on the floor, some of them double. ten have been counted on the n. half of the interior. on the s.e. of vieta are two fine overlapping ring-plains, with a crater on the wall common to both. de vico.--a conspicuous little ring-plain, about miles in diameter, with a lofty border, some distance e. of mersenius. lee.--an incomplete walled-plain, about miles in diameter, on the s. side of the mare humorum, e. of vitello, from which it is separated by another partial enclosure, with a striking valley, not shown in the published maps, running round its w. side. if viewed when its e. wall is on the morning terminator, some isolated relics of the wrecked n.w. wall of lee are prominent, in the shape of a number of attenuated bright elevations separated by gaps. within are three or four conspicuous hills. doppelmayer.--under a high sun this large ring-plain, miles in diameter, resembles a great bay open to the n.w., without a trace of detail to break the monotony of the surface on the side facing the mare humorum. when, however, it is viewed under oblique morning illumination, a low broad ridge is easily traceable, extending across the opening, indicating the site of a ruined wall. there is an isolated mountain at the s.w. end of this, which casts a fine spire of shadow across the floor at sunrise. the interior contains a massive bright central mountain and several little hills. the crest of the wall on the e. is much broken. fourier.--a large ring-plain, miles in diameter, s.w. of vieta, with a border rising at a peak on the w. more than feet above the floor, there are two craters on the outer slope of the n.w. wall, a prominent crater on the s. wall, and (according to schmidt) a small central crater on the floor, which i have not seen. in the region between fourier and vieta there are three ring-plains, two (the more westerly) standing side by side, and on the w., towards the mare, are two others much larger, that nearer to fourier being traversed by one cleft, and the other by two clefts, crossing near the centre of the floor. clausius.--a small bright ring-plain in an isolated position n.w. of schickard, with a crater both on its n. and s. rim, and a faint central hill. lacroix.--a ring-plain miles in diameter, n. of schickard. it has a prominent central mountain. schickard.--one of the largest wall-surrounded plains on the visible surface of the moon, extending about miles from n. to s., and about the same from e. to w., enclosing a nearly level area, abounding in detail. its border, to a great extent linear, is very irregular, and much broken by the interposition of small ring-plains and craters, and on the n. by cross-valleys. its general height is about feet, the loftiest peak on the w. wall rising to more than feet above the floor. the inner slopes of this vast rampart are very complex, especially on the e., where many terraces and depressions may be seen under suitable illumination. there are three large ring-plains in the interior, all of them s. of the centre; and at least five smaller ones near the inner foot of the e. wall, which can only be well observed when libration is favourable. the two more easterly of the large ring-plains are connected by a cleft, and there are several short clefts and crater-rows associated with the smaller ring-plains. on the n. side of the area is a number of minute craters. the floor is diversified by two large dark markings--an oblong patch on the s.w. side, abutting on the wall, being the more remarkable; and a dusky area, occupying a great portion of the n. part of the floor, and extending up to the n. border. this is traversed by a light streak running from n. to s., which is the site of a row of minute craters. lehmann.--a ring-plain, about miles in length, on the n. of schickard, with which it is connected by a number of cross-valleys. drebbel.--a bright ring-plain, miles in diameter, on the n.w. of schickard, with a lofty irregular border (especially on the w.), exhibiting a well-marked terrace on the e., a distinct gap on the n., and a small crater on the s.e. rim. on a dusky area between it and schickard stand three prominent deep craters. phocylides.--this extraordinary walled plain, with its neighbouring enclosures, is structurally very remarkable and suggestive. it consists of a large irregular formation, with a lofty wall, flanked on the n. by a smaller and still more irregular enclosure (_b_), the floor of which is feet above that of phocylides, the line of partition being a high cliff, probably representing a "fault," whose shadow under a low sun is very striking. phocylides is about miles in maximum length, or, if we reckon the small enclosure _b_ to form a part of it, more than miles. the loftiest peak, nearly feet, is on the w. border, near the partition wall. the continuity of the rampart is broken on the s. by a large crater. there is a bright ring-plain on the w. side of the floor, and a few small craters. phocylides _b_ has only a solitary crater within it. phocylides c, abutting on the w. flank of phocylides, is about miles in diameter. its somewhat dusky interior is devoid of detail, but the outer slope of its w. wall is crowded with a number of minute craters, which, under good conditions, may be utilised as tests of the defining power of the telescope used. phocylides a, on the bright s.w. plain, is a large deep crater with a fine crater-row flanking it on the w. wargentin.--a most remarkable member of the phocylides group, flanking the s.e. side of schickard. unlike the majority of lunar formations, its floor is raised considerably above the surrounding region, so that it resembles a shallow oval dish turned upside down. it is miles in diameter, and, except on the s.w. (where it abuts on phocylides _b_, and for some distance is bounded by its wall), it has only a border of very moderate dimensions. on the n.e. slope of this ghostly rampart i have seen a distinct little crater, and two much larger depressions on the n.w. slope. there are some low ridges on the floor, radiating from a nearly central point, which have been aptly compared to a bird's foot. segner.--a fine ring-plain, miles in diameter, on the s.e. side of schiller, with a linear border on every side except the n. at a peak on the w., whose shadow is very remarkable, it rises to a height of more than feet above the outer surface. there is a crater on the s.w. wall, another on the n.w. wall, and several depressions on the outer slope on this side. the central mountain is small but conspicuous. a large unnamed enclosure extends n. of segner: it is larger than schiller, and is surrounded by a lofty barrier. the bright plain between this and the latter is worth examination under a low sun. zuchius.--is situated on the s.e. of segner, which it slightly overlaps. it is very similar in size and general character, and has a lofty terraced wall, rising at one place on the w. to nearly , feet above the floor. a very fine chain of craters, well seen when the opposite border is on the morning terminator, runs round the outer w. slope of the wall. there is a bright crater beyond this on the s.w. zuchius has a central peak. bettinus.--another ring-plain of the same type and size, some distance s. of the last, with a massive border, terraced within, and rising on the w. more than , feet above the floor, on which stands a grand central mountain, whose brilliant summit is in sunlight a long time before a ray reaches any part of the deep interior. kircher.--a ring-plain, about miles in diameter, s. of bettinus, remarkable also for its very lofty rampart, which on the s. attains the tremendous height of nearly , feet above the floor, which appears to be devoid of detail. wilson.--the most southerly of the chain of five massive ring-plains, extending in an almost unbroken line from segner and differing only very slightly in size. it is about miles in diameter, and has a somewhat irregular border, both as regards shape and height, rising at one peak on the s.w. to nearly , feet above a level interior, which apparently contains no conspicuous features. east longitude deg. to deg. grimaldi.--this ranks among the largest wall-surrounded plains on the moon, and is perhaps the darkest. it extends miles from n. to s. and miles from e. to w., enclosing an area of some , square miles, or nearly double that of the principality of wales. this vast dusky surface is bounded on the e. by a tolerably regular border, having an average height of about feet, while on the opposite side it is much broken, and in places considerably loftier, rising at one peak on the s.w. to an altitude of feet. about midway, also, this western rampart attains a great height, as may be seen by any one who observes at sunrise the magnificent shadow of it, and its many peaks thrown across the bluish-grey interior. on the s. the wall is broken by a large irregular depression, on the w. of which is a very curious v-shaped rill valley. on the n.w. it is comparatively low, and in places discontinuous; and even to a greater extent than on the s.w., intersected by passes. at the extreme n. end, a number of wide valleys cut through the wall and trend towards lohrmann. there is a considerable ring-plain at the inner foot of the n.e. wall, but, except this and a few longitudinal ridges, just visible under a very low sun, there is apparently no other object to vary the monotony of this great expanse. damoiseau.--consists of a complex arrangement of rings, an enclosure miles in diameter, with a somewhat smaller enclosure placed excentrically within it (the n. side of both abutting on a bright plateau), with two large depressions intervening between their w. borders. this peculiarity, almost unique, renders the formation an especially interesting object. damoiseau is situated on the w. side of grimaldi, on the e. coast-line of the oceanus procellarum, from which the s.w. border rises at a gentle inclination. on the n.w. there is a curious curved inflexion of the mare, bounded by a bright cliff, representing probably the e. side of a destroyed ring, a supposition which is strengthened by the existence of a faint scar on the surface of the sea, extending in a curve from one extremity of the bay to the other, and thus indicating the position of the remainder of the ring. a conspicuous little crater stands at the s. end of it, and two others some distance to the w. the smaller component of damoiseau contains a low central ridge. riccioli.--an immense enclosure, near the limb, n.e. of grimaldi, bounded by a rampart which is very irregular both in form and height, though nowhere of great altitude, and much broken by narrow gaps. it is especially low and attenuated on the n., where a number of ridges with intervening valleys traverse it. on the s. also a wide valley cuts through it. with the exception of a few low rounded hills and ridges, a short crater-row under the s.e. wall, and two small craters on the s.w., there are no details on the floor, which, however, is otherwise remarkable for the dusky tone of its surface, especially on the n. this dark patch occupies the whole of the n.e. side of the interior, and is bounded on the s. by an irregular outline, extending at one point nearly to the centre, and on the w. by a curved edge. the w. side is much darker than the rest. it is, in fact, as dark, if not darker, than any part of the floor of grimaldi. riccioli extends miles from n. to s., and is nearly as broad. it includes an area of square miles. rocca.--an irregular formation, miles in length, near the limb s.e. of grimaldi, consisting of a depression partially enclosed by mountain arms. sirsalis.--the more westerly of a conspicuous pair of ring-plains about miles in diameter, in the disturbed mountain region some distance s.w. of grimaldi. it has lofty bright walls, rising to a great height above a depressed floor, on which there is a prominent central mountain. the e. border encroaches considerably on the somewhat larger companion, which is, however, scarcely a third so deep. one of the longest clefts on the visible surface runs immediately w. of this formation. commencing at a minute crater on the n. of it, it grazes the foot of the w. _glacis_; then, passing a pair of small overlapping craters (resembling sirsalis and its companion in miniature), it runs through a very rugged country to a ring-plain e. of de vico (de vico _a_), which it traverses, and, still following a southerly course, extends towards byrgius, in the neighbourhood of which it is apparently lost at a ridge, though schmidt and gaudibert have traced it still farther in the same direction. it is at least miles in length, and varies much in width and character, consisting in places of distinct crater-rows. cruger.--a regular ring-plain e. of fontana, miles in diameter, with a dark floor, without detail, and comparatively low bright walls. there is a smaller but very conspicuous ring-plain (cruger _a_) on the w. of it, to which runs a branch of the great sirsalis cleft. eichstadt.--a ring-plain, miles in diameter, near the e. limb, s. of rocca. it is the largest and most southerly of three nearly circular enclosures, without central mountains or any other details of interest. on the w. lies a great walled-plain with a very irregular border, containing several ring-plains and craters, and a crater-rill. schmidt has named this formation darwin. byrgius.--a very irregular enclosure, about miles in diameter, between cavendish and the e. limb, with a lofty and discontinuous border, rising at one point on the e. to a height of feet above the floor. there are wide openings both in the n. and s. wall, and some ridges within. the border is broken on the e. by a crater, and on the w. by the well-known crater byrgius a, from which a number of bright streaks radiate, mostly towards the e. one on the w. extends to cavendish, and another to mersenius, traversing the ring-plain cavendish c. north-east of byrgius there is a mountain arm which includes a peak , feet in height. piazzi.--a walled-plain, about miles in length, some distance s.e. of vieta, with a complex broken border, including several depressions on the n.w., rising to about feet above a rather dark interior, on which there is a prominent central mountain. lagrange.--a larger but similar formation, miles in diameter, associated with the last on the n.e., with a complex terraced border, including peaks of feet, a bright crater on the w., and a ring-plain on the n.w. the inner slope of the e. wall is a fine object at sunrise, when libration is favourable. the floor is dark and devoid of detail. bouvard.--a great irregular enclosure, which appears to be still larger than lagrange, s.e. of piazzi, and close to the limb. it is bounded by a very lofty rampart, rising at a peak on the w. to , feet. it has a fine central mountain. inghirami.--a very remarkable ring-plain, miles in diameter, e. of schickard, with a bright, broad, and nearly continuous border, terraced within, and intersected on the n.e. by narrow valleys, one of which is prolonged over the floor and extends to the central mountain. there are two curious dark spots on the n. side of the interior. beyond the foot of the _glacis_ on the s. a distinct cleft runs from a dusky spot to a group of small craters e. of wargentin. there is a fine regular ring-plain with a small central mount w. of inghirami. pingre.--a ring-plain, about miles in diameter, between phocylides and the limb. hausen.--a ring-plain, close to the limb, n. of bailly, which, but for its position, would be a fine object. it is, however, never sufficiently well placed for observation. bailly.--one of the largest wall-surrounded plains on the moon, almost a "sea" in miniature, extending miles from n. to s., and fully as much from w. to e. when caught at a favourable phase, it is, despite its position, especially worthy of scrutiny. the rampart on the w., of the linear type, is broken by several bright craters. on the s.w. two considerable overlapping ring-plains interfere with its continuity. on the s.e. several very remarkable parallel curved valleys traverse the border. the e. wall, which at one point attains a height of nearly , feet, is beautifully terraced. the floor on the eastern side includes several ring-plains (some of which are of a very abnormal type), many ridges, and two delicate dark lines, crossing each other near the s. end, probably representing clefts. legentil.--a large walled-plain, close to the limb, s. of bailly. fourth quadrant west longitude deg. to deg. kastner.--a large walled-plain at the s. end of the mare smythii, too near the limb for satisfactory observation. maclaurin.--the principal member of a group of irregular ring-plains on the w. side of the mare foecunditatis, a little s. of the lunar equator. schmidt shows no details within it, except a small crater on the e. side of the floor. webb.--a ring-plain e. of maclaurin, about miles in diameter, with a dusky floor, enclosed by a bright rim, on the n.e. side of which there is a small crater. schmidt seems to have overlooked the central hill. langrenus.--this noble circumvallation, the most northerly of the meridional chain of immense walled-plains, extending for more than miles from near the equator to s. lat. deg., would, but for its propinquity to the limb, rank with copernicus (which in many respects it resembles) among the most striking objects on the surface of the moon. its length is about miles from n. to s., and its breadth fully as much. in shape it approximates very closely to that of a foreshortened regular hexagon. the walls, which at one point on the e. rise to an altitude of nearly , feet, are continuous, except on this side, where they are broken by the interference of an irregular depression, and on the extreme s., where they are intersected by cross-valleys. within, the terraces are remarkably distinct, and the intervening valleys strongly marked. the brilliant compound central mountain rises at its loftiest peak to a height of more than feet. on the n. of it is an obscure circular ring, which may possibly merely represent a fortuitous combination of ridges, though it has all the appearance of a modified ring-plain. on the mare, some distance n.e. of the formation, is a group of three ring-plains, with two small craters (associated with a ridge) on the n. of them. two of the more westerly of these objects have prominent central mountains, and the third a very dark interior. at least three bright streaks originate on the e. flank of langrenus, which, diverging widely, traverse the mare foecunditatis. [flattenings on the moon's western limb.--about thirty years ago, the rev. henry cooper key drew attention to certain flattenings which he had noted on the w. limb, which are very apparent under favourable conditions of libration. their position cannot be closely defined, but the principal deviation from circularity extends from about s. lat. deg. to the region on the limb opposite the s. border of the mare crisium.] vendelinus.--the second great enclosure pertaining to the meridional chain--a magnificent walled-plain of about the same dimensions as the last. it is bounded by a very irregular rampart, which, under evening illumination, is especially noteworthy, though nowhere approaching the altitude of that of langrenus. its continuity on the w. is broken by the great ring-plain vendelinus c, about miles in diameter, a formation resembling langrenus in miniature. this is hexagonal in shape, and has many rings and depressions on its w. wall. south of vendelinus c, the wall of vendelinus runs up in a bold curve to the fine terraced ring- plain vendelinus b, and is surmounted by a bright serpentine crest, and traversed by several valleys running down the slope to the floor. b has a small crater on its n. wall, and another in the interior. there is a wide gap in the s. border of vendelinus, which is partially occupied by another somewhat smaller ring-plain, bounded by a southerly extension of the e. wall, which includes on its outer slope many craters and other depressions, and abuts near its n. end on the large ring-plain vendelinus a, which has a prominently terraced wall and a large bright central mountain. between a and c extends a plateau that may be regarded as the n. limit of the formation, including, among other minor details, a fine cleft, which traverses it from n. to s., and ultimately extends to a group of craters on the floor. on the s. side of the interior is one large ring-plain, flanked on the w. by two small craters. near the n. end are many bright little craters, many of them unrecorded. vendelinus c is bordered on the e. by two large semicircular formations with low walls extending on to the floor. mr. w.h. maw and others have detected many minute depressions in connection with these curious objects; and n. of them, on the outer slope of c, where it runs out to the level of the plateau, i have seen the surface at sunset riddled like a sieve with craterlets and little pits. there is an irregular ring-plain n. of a, with linear walls, and another, much smaller and brighter, on the n. of this, standing a little beyond the n. limits of langrenus. la peyrouse.--a much foreshortened walled-plain, miles in diameter, close to the limb, s.w. of langrenus. there is a longitudinal ridge on the floor. between it and langrenus are two large ring-plains with central mountains, and on the n.e., la peyrouse a, a bright crater, adjoining which is la peyrouse delta, one of the most brilliant spots on the moon. ansgarius.--a ring-plain, miles in diameter, still nearer to the limb than the last. behaim.--a great ring-plain, miles in diameter, s. of ansgarius, and connected with it by ridges. it has lofty walls and a central mountain. hecataeus.--an immense walled-plain, miles in length, on the s.w. of vendelinus, with a very irregular rampart and a conspicuous central mountain. it is flanked e. and w. by other large enclosures, which can only be seen to advantage when libration is favourable. w. humboldt.--though close to the limb, this enormous wall-surrounded plain, some miles in extreme length, and estimated to have an area of , square miles, is well worth observing under suitable conditions. it ranks among the largest formations of its class, and in many respects resembles bailly on the s.e. limb. at one point on the e. a peak rises to , feet, and on the opposite side there are peaks nearly as high. the floor contains some detail--a crater, nearly central, associated with ridges, and two dark spots, one at the s. and the other at the n. end. phillips.--abuts on the e. side of w. humboldt. it is a walled-plain, about miles in length, with a border much broken on the e., and terraced within on the opposite side. there are many hills and ridges on the floor. legendre.--a fine ring-plain, miles in diameter, on the s.e. of the last. according to schmidt, there is a crater on the s. side of the floor. there is a small ring-plain, adams, on the s. petavius.--the third member of the great meridional chain: a noble walled-plain, with a complex rampart, extending nearly miles from n. to s., which encloses a very rugged convex floor, traversed by many shallow valleys, and includes a massive central mountain and one of the most remarkable clefts on the visible surface. to observe these features to the best advantage, the formation should be viewed when its w. wall is on the evening terminator. at this phase a considerable portion of the interior on the n. is obscured by the shadow of the rampart, but the principal features on the s. half of the floor, and on the broad gently- shelving slope of the w. wall, are seen better than under any other conditions. the border is loftiest on the e., where the ring-plain wrottesley abuts on it. it rises at this point to nearly , feet, while on the opposite side it nowhere greatly exceeds feet above the interior. the terraces, however, on the w. are much more numerous, and, with the associated valleys, render this section of the wall one of the most striking objects of its class. the n. border is conspicuously broken by the many valleys from the region s. of vendelinus, which run up to and traverse it. on the s., also, it is intersected by gaps, and in one place interrupted by a large crater. there is a remarkable bifurcation of the border s. of wrottesley. a lower section separates from the main rampart and, extending to a considerable distance s.e. of it, encloses a wide and comparatively level area which is crossed by two short clefts. the central mountains of petavius, rising at one peak to a height of nearly feet above the floor, form a noble group, exceeding in height those in gassendi by more than feet. the convexity of the interior is such that the centre of it is about feet higher than the margin, under the walls; a protuberance which would, nevertheless, be scarcely remarked _in situ_, as it represents no steeper gradient than about in on any portion of its superficies. the great cleft, extending from the central mountains to the s.e. wall, and perhaps beyond, was discovered by schroter on september , , and can be seen in a inch achromatic. in larger instruments it is found to be in places bordered by raised banks. wrottesley.--a formation, about miles in diameter, closely associated with the e. wall of petavius, the shape of which it has clearly modified. its border on the e., of the linear type, rises nearly feet above a light interior, where there is a small bright central mountain and some mounds. there is a prominent valley running along the inner slope of the w. wall. palitzsch.--if this extraordinary formation is observed when the moon is about three days old, it resembles a great trough, or deep elongated gorge flanking the w. wall of petavius, though it is a true ring-plain, albeit of a very abnormal type, about miles in length and miles in breadth, with a somewhat dusky interior. on the outer slope of its w. wall is a bright ring-plain with a lofty border and a central mountain. hase.--an irregular formation, about miles in diameter, on the s.w. of petavius, with which it is connected by extensions of the w. and e. walls of the latter. its rampart, some feet above the floor, is broken by depressions on the w.; and on the s. is bounded by a smaller ring-plain with still loftier walls. schmidt shows a large crater and three smaller ones on the w. side of the floor. marinus.--a ring-plain on the n.e. side of the mare australe, between furnerius and the limb. furnerius.--the fourth and most southerly component of the great meridional chain of walled-plains, commencing on the n. with langrenus: a fine but irregular enclosure, about miles in extreme length and much more in breadth. its rampart is very lofty, and tolerably continuous on the n. and w., but on the other sides is interrupted by small craters and depressions. at peaks on the e. it attains a height of more than , feet above the interior, and there are other peaks rising nearly as high. there is a ring-plain (furnerius b) with a central hill, on the e. side of the floor, and numerous craters and crater-pits in other parts of it. on the n.w. side of b there is a short cleft, on the w., a well-marked crater-row, and on the e. a long rill-valley. the very brilliant crater (furnerius a) on the n.e. _glacis_ is the origin of two fine light streaks, one extending s. for more than miles, and the other in the opposite direction for a great distance. fraunhofer.--a ring-plain, s. of furnerius, about miles in diameter, with a regular border rising about feet above the floor. a smaller ring-plain abuts on the n.e. side of it, which has slightly disturbed its wall. oken.--a large enclosure in s. lat. deg. with broken irregular walls. it is too near the limb for observation. vega.--schmidt represents this peculiar formation, situated s.e. of oken, as having a regular curved unbroken rampart on the e., while the opposite border is occupied by four large partially overlapping ring-plains, two of which contain small craters. the floor is devoid of detail. pontecoulant.--a great irregular walled plain, about miles in length, near the s.w. limb, with a border rising in places to a height of feet above the floor. hanno.--a smaller and more regular enclosure, adjoining pontecoulant on the n.w., and still nearer the limb. west longitude deg. to deg. messier.--the more westerly of a remarkable pair of bright craters, about miles in diameter, standing in an isolated position in the mare foecunditatis just s. of the equator. madler represents them as similar in every respect, but webb, observing them in and with a / achromatic, found them very distinctly different,--messier, the more westerly, being not only clearly smaller than its companion, but longer from w. to e. than from n. to s., as it undoubtedly is at the present time. messier a, however, as the companion is termed, though larger, is certainly not circular, as sometimes shown, but triangular with curved sides. it is just possible that change may have occurred here, for madler carefully observed these objects more than three hundred times, and, it may be presumed, under very different phases. messier a is the origin of two slightly divergent light streaks, resembling a comet's tail, which extend over the mare towards its e. border n. of lubbock, and are crossed obliquely by a narrower streak. messier and messier a stand near the s. and narrowest end of a tapering curved light area. there is a number of craterlets and minute pits in the neighbourhood, and under a high light two round dusky spots are traceable in connection with the "comet" marking, one just beyond its northern, and the other beyond its southern border, near its e. extremity. lubbock.--a brilliant little crater, about or miles in diameter, near the e. coast-line of the mare foecunditatis. the region e. of this object is particularly well worthy of scrutiny under a low sun, on account of the variety of detail it includes. on the s.e. run three fine parallel clefts, originating near the n. end of the pyrenees. guttemberg.--a very fine ring-plain of peculiar shape, about miles in width, with a lofty wall, broken on the n.w. by another ring-plain some miles in diameter, and on the s.e. by a small but distinct crater. the border presents a wide opening towards the s., which is traversed by a number of longitudinal valleys, both the e. and w. sections of the wall being prolonged in this direction. a fine crater-row runs round the outer slope of the e. wall, from the crater just mentioned to the n. side of the formation. it is best seen when the w. wall is on the evening terminator. there is also a broad valley on the s. prolongation of the w. wall. the central mountain is bright but not large. a cleft crosses the n.w. side of the floor. north of guttemberg there is a curious oblong formation with low walls, connected with the n.e. border by a ridge, and with the n. border by a remarkable row of depressions, situated on a mound; and beyond this object on the e. are three parallel clefts running towards the n.e. on the w. will be found some of the clefts belonging to the goclenius rill-system. in the rugged region s.e. of the formation is a peculiar low ring with a very uneven floor and a large central hill. the e. wall of guttemberg may be regarded as forming a portion of the pyrenees mountains. goclenius.--a ring-plain, about miles in diameter, bearing much resemblance to plinius in form and size, and, like this formation, associated with a fine system of clefts. the lofty rampart, tolerably continuous on the w., is broken on the s.w. by a bright crater, and on the n.w. by a remarkable triangular depression. it is also traversed by a delicate valley extending from the crater on the s.w. to another on the n.w. border; and at a point a little w. of the first crater is dislocated by an intrusive mass of rock. there are several gaps on the e. and many spurs and irregularities in outline both within and without. a great portion of the n. wall is linear, and joins the e. section nearly at right angles. west of the triangular depression it appears to be partially wrecked, indications of the destruction being very evident if it be observed when the e. wall is near the morning terminator. the small bright central mountain is remarkable for its curious oblong shadow. two clefts traverse the interior of goclenius. ( ) originates at the s. wall, e. of the crater, and runs e. of the central mountain to the n. wall; ( ) crosses the _debris_ of the ruined n.w. border, runs parallel to the first, and extends nearly to the centre of the floor, ( ) re-appears at the foot of a mound outside the n. wall, and, after crossing the outer w. slope of the great ring-plain on the n.w. wall of guttemberg, runs to the w. side of an oblong formation n. of it. there are two other clefts, closely parallel and w. of this, traversing the mare, and terminating among the mountains on the n.w. these are crossed at right angles by what appears to be a "fault," running in a n.w. direction from the w. side of guttemberg. macclure.--one of a curious group of formations situated in the mare foecunditatis some distance s.w. of goclenius. it is a bright ring-plain, about miles in diameter, with a narrow gap in the n.e. wall and a small central hill. a prominent ridge runs up to the n. border; and on the s.w. a rill-valley may be traced, extending s. to a bright deep little crater w. of cook. crozier.--a conspicuous ring-plain a few miles n.n.w. of macclure, and of about the same size. it has a faint central hill. neison refers to two long straight streaks extending from crozier towards messier. bellot.--a brilliant little ring-plain n.e. of crozier. cook.--a ring-plain, about miles in diameter, on the e. side of the mare foecunditatis in s. lat. deg., with low and (except on the s.e.) very narrow walls. there is a small circular depression on the s. border, and a prominent crater on the w. side of the dark interior. on the s.s.e. is the curiously shaped enclosure cook _d_, with very bright broad lofty walls and a fine central mountain. on the plain w. of cook is a conspicuous crater-row, consisting of six or seven craters, diminishing in size in both directions from the centre. colombo.--a fine ring-plain, about miles in diameter, situated in the highlands separating the mare foecunditatis and the mare nectaris. the wall, rising at one place to a height of feet above the floor, is very complicated and irregular, being traversed within by many terraces, and almost everywhere by cross-valleys. its shape is greatly distorted by the large ring-plain _a_, which abuts on its n.e. flank. it loses its individuality altogether on the s., its place being occupied by two large depressions, and lofty mountains trending towards the s.e. in the centre there are several distinct bright elevations. magelhaens.--the more northerly and the larger of a pair of ring-plains between colombo and goclenius, with a bright and somewhat irregular though continuous border. the dark interior includes a small central mountain. its companion on the s.w., magelhaens _a_, slightly overlaps it. this also has a central hill, and a crater on the outer slope of its e. wall. santbech.--a very prominent ring-plain, miles in diameter, on the s.e. side of the mare foecunditatis, w. of fracastorius. the continuity of its fine lofty rampart is broken on the w., where it rises nearly , feet above the floor, by a brilliant little crater just below the crest, and by a narrow gap on the s. the wall on the e. towers to a height of , feet above the interior. on its broad outer slope, near the summit, there is a fine crater, and s. of this running obliquely down the slope a distinct valley. on the n.e., where the _glacis_ runs down to the level of the surrounding plain, there is a large crateriform object with a broken n. border, and a small crater opposite the opening. a long coarse valley runs from this latter object in a n.e. direction to the region w. of bohnenberger. santbech contains a prominent central peak. biot.--a brilliant little ring-plain, scarcely more than miles in diameter, standing in an isolated position in the mare foecunditatis n.e. of wrottesley. there is a number of bright streaks in its neighbourhood; and a few miles e. of it, in the hilly region w. of santbech, another conspicuous crater of about the same size. borda.--a ring-plain about miles in diameter, s.s.w. of santbech, with a rampart low on the n. and s., but elsewhere of considerable height, and a very conspicuous central mountain. a wide deep valley flanked by lofty mountains extends from the n. wall for many miles towards the n.w. it is an especially noteworthy object when the w. wall of santbech is on the evening terminator, as its somewhat winding course, indicated by the bright summit-ridges of the bordering mountains, can be followed some hours before either the interior of the valley or the region between it and santbech are in sunlight. among the mountains w. of borda there is a peak more than , feet in height. snellius.--a very fine ring-plain, miles in diameter, s.e. of petavius, with terraced walls, considerably broken on the s.e. by craters, &c. it rises on the e. nearly feet above a dark floor, which contains a central mountain. n.e. of snellius is a smaller ring- plain (snellius _a_), and due e. a curious rough plateau, bordered on the n. and s. by a number of small craters. stevinus.--a somewhat larger ring-plain, s. of snellius, with a border rising on the s. to more than , feet above a dark interior, which includes a bright central mountain. reichenbach.--a very abnormally-shaped ring-plain, about miles in diameter, with a rampart nearly , feet high. the border is broken on the w., s., and e. by craters and depressions, and on the n. is flanked by two overlapping ring-plains, _a_ and _b_. on the s.w. lies a magnificent serpentine valley, fully miles in length and about miles in breadth at the n. end, but gradually diminishing as it runs southwards, till it reaches a depression n. of rheita, where it terminates: here is scarcely more than miles wide. rheita.--a formation, about miles in diameter, s. of reichenbach, with regular lofty walls, rising at a peak on the n.e. to a height of more than , feet above the interior, on which there is a small but prominent central mountain, a smaller elevation w. of the centre, and two adjoining craters at the foot of the s. wall. on the e. originates another fine valley, very similar to that already mentioned in connection with reichenbach. it runs in a s.s.w. direction, is about miles in length, and, in its widest part, is about miles across. like the reichenbach valley, it terminates at a small crater-like object, which has a border broken down on the side facing the valley, and a small central hill. about midway between its extremities, this great gorge is crossed by a wall of rock, like a narrow bridge. janssen.--an immense irregular enclosure, reminding one of the very similar area, bordered by walter, lexell, hell, &c., in the third quadrant. it extends about miles from e. to w., and more than from n. to s., its limits on the n. being rather indefinite. its very rugged humpy surface includes one great central mountain, and innumerable minor hills and ridges, craters, and crater-pits; but the principal feature is the magnificent curved rill-valley running from the s. side of fabricius across the rough expanse to the s. side. this fine object, very coarse on the n., passes the central mountain on the e. side, and becomes gradually narrower as it approaches the border; before reaching which, another finer cleft branches from it on the w., and also runs to the s. side of the plain. lockyer.--a prominent deep ring-plain, miles in diameter, with massive bright lofty walls, standing just outside the s.e. border of janssen. schmidt shows a minute crater on the s. rim. i have seen a crater within, at the inner foot of the w. wall, and a central peak. fabricius.--a ring-plain, miles in diameter, with a lofty terraced border, rising on the s.w. to a height of nearly , feet above the interior. it is partially included by the rampart of janssen, and the great rill-valley on the floor of the latter appears to cut through its s. wall. there is a long central mountain on the floor, with a prominent ridge extending along the e. side of it. w. of fabricius (between it and the border of janssen) lies a very irregular enclosure, with three distinct craters within it; and on the e., running from the wall to the e. side of janssen, is a straight narrow valley. both fabricius and janssen should be viewed under a low morning sun. steinheil.--a double ring-plain, w. of janssen, miles in diameter. the more easterly formation sinks to a depth of nearly , feet below the summit of the border. metius.--this ring-plain, of about the same size as fabricius, but with a still loftier barrier, abuts on the n. wall of this formation, and has caused a very obvious deformation in its contour. it is prominently terraced internally, and on the w. the wall rises at one peak to a height of , feet above the floor, which contains a deep crater on the w. of the centre, and many ridges. biela.--a considerable ring-plain, about miles in diameter, s.w. of janssen, with a wall broken on the n.w., s., and e. by rings and large enclosures. there is a central mountain, but apparently no other details on the floor. rosenberger.--this formation, about miles in diameter, is one of the remarkable group of large rings to which vlacq, hommel, pitiscus, &c., belong. its walls, though of only moderate altitude, are distinctly terraced. in addition to a prominent central mountain (e. of which schmidt shows two craters), there is a large crater on the s. side of the floor, and many smaller craters and crater-pits. hagecius.--the most westerly member of the vlacq group of formations. it is situated on the s.w. of rosenberger, and is about miles in diameter. the rampart on the e. is continuous and of the normal type, but on the opposite side is broken by a number of smaller rings. west longitude deg. to deg. censorinus.--a brilliant little crater, with very bright surroundings, in the mare tranquilitatis, nearly on the moon's equator, in w. long. deg. min. another smaller but less conspicuous crater adjoins it on the w. on the mare to the s. extends a delicate cleft which trends towards the sabine and ritter rill system. capella.--forms with isodorus, its companion on the e. (which it partially overlaps), a very noteworthy object. it is about miles in diameter, with finely terraced walls, broken on the s.w. by broad intrusive rill-valleys. the rampart on the n.e. is also cut through by a magnificent valley, which extends for many miles beyond the limits of the formation. there is a fine central mountain, on which m. gaudibert discovered a crater, the existence of which has been subsequently verified by professor weinek on a lick observatory negative. isodorus.--the rampart of this fine ring-plain, which is of about the same size as capella, rises at a peak on the w. to a height of more than , feet above the interior, which, except a small bright crater at the foot of the e. wall and a smaller one adjoining it on the n., contains no detail. the region between isodorus and the equator includes many interesting objects, among them isodorus _b_, an irregular formation open towards the n., and containing several craters. bohnenberger.--a ring-plain about miles in diameter, situated on the w. side of the mare nectaris, under the precipitous flanks of the pyrenees, whose prominent shadows partially conceal it for many hours after sunrise. the circular border is comparatively low, and, except on the n., continuous. here there is a gap, and on the w. of it an intrusive mass of rock. from its very peculiar shadow at sunrise, the wall on the e. appears to be very irregular. the club-shaped central mountain is of considerable size, but not conspicuous. s. of bohnenberger stands the very attenuated ring, bohnenberger a. it is of about the same diameter, has a large deep crater on its n. rim, and a smaller one, distinguished with difficulty, on its s.e. rim. on the n. of bohnenberger there is a bright little ring-plain connected with the formation by a lofty ridge, under the e. flank of which schmidt shows a crater-chain. an especially fine cleft originates on the e. side of this crater, which, following an undulating course over the mare nectaris, terminates at rosse, n. of fracastorius. torricelli.--a remarkable little formation in the mare tranquilitatis, n. of theophilus, consisting of two unequal contiguous craters ranging from w. to e., whose partition wall has nearly disappeared, so that, under a low sun, when the interior of both is filled with shadow, the pair resemble the head of a javelin. the larger, western, ring is about miles in diameter, and the other about half this size. there is a gap in the w. wall of the first, and a long spur projecting from its s. side; and a minute crater on the s. border of the smaller object. torricelli is partially enclosed on the s. by a circular arrangement of ridges. there is a delicate cleft running in a meridional direction on the mare, e. of the formation, and another on the n., running from w. to e. hypatia.--a ring-plain, about miles in extreme length, of very abnormal shape, on the e. side of the mare, n.n.e. of theophilus, with a wall rising at a peak on the e. to a height of more than feet above a dusky floor, which does not apparently contain any detail. a small crater breaks the uniformity of the border on the w. beyond the wall on the s.e. lies the fine bright crater hypatia a, with another less prominent adjoining it on the s.w. theophilus.--the most northerly of three of the noblest ring-mountains on the visible surface of the moon, situated on the n.e. side of the mare nectaris. it is nearly miles in diameter, and is enclosed by a mighty rampart towering above the floor at one peak on the w. to the height of , feet, and at two other peaks on the opposite side to nearly , and , . the border, though appearing nearly circular with low powers, is seen, under greater magnification, to be made up of several more or less linear sections, which give it a polygonal outline. it is prominently terraced within, the loftier terraces on the w. rising nearly to the height of the crest of the wall, and including several craters and elongated depressions. on the w. _glacis_ is a row of large inosculating craters; and near its foot, s.e. of madler, a short unrecorded rill- valley. the magnificent bright central mountain is composed of many distinct masses surmounted by lofty peaks, one of which is about feet above the floor, and covers an area of at least square miles. except a distinct crater on the s.w. quarter, this appears to be the only object within the ring. cyrillus.--the massive border of theophilus partially overlaps the n.w. side of this great walled-plain, which is even more complex than that of its neighbour, and far more irregular in form, exhibiting many linear sections. its crest on the s.e. is clearly inflected towards the interior, a peculiarity that has already been noticed in connection with copernicus and some other objects. on the inner slope of this wall there is a large bright crater, in connection with which have been detected two delicate rills extending to the summit. i have not seen these, but one of the crater-rows shown by schmidt, between this crater and the crest, has often been noted. the n.e. wall is very remarkable. it appears to be partially wrecked. if observed at an early stage of sunrise, a great number of undulating ridges and rows of hillocks will be seen crossing the region e. of theophilus. they resemble a consolidated stream of "ropy" lava which has flowed through and over the wall and down the _glacis_. the arrangement of the ridges within cyrillus is very noteworthy, as is also the triple mountain near the centre of the floor. the fine curved cleft thereon traverses the w. side, sweeping round the central mountains, and then turning to the south. i have only occasionally seen it in its entirety. there are also two oblong dark patches on the s. side of the interior. the s. wall of cyrillus is broken by a narrow pass opening out into a valley situated on the plateau which bounds the w. side of the oblong formation lying between it and catherina, and overlooking a curious shallow square-shaped enclosure abutting on the s.w. side of cyrillus. catherina.--the largest of the three great formations: a ring-plain with a very irregular outline, extending more than miles in a meridional direction, and of still greater width. the wall is comparatively narrow and low on the n.e. ( feet above the floor), but on the n.w. it rises to more than double this height, and is broken by some large depressions. the inner slope on the s.e. is very gentle, and includes two bright craters, but exhibits only slight indications of terraces. the most remarkable features on an otherwise even interior are the large low narrow ring (with a crater within it), occupying fully a third of the area of the floor, and a large ring-plain on the s. side. madler.--the interest attaching to this formation is not to be measured by its size, for it is only about miles in diameter, but by the remarkable character of its surroundings. its bright regular wall, rising feet on the e. and only about half as much on the w., above a rather dark interior, is everywhere continuous, except at one place on the n. here there is a narrow gap (flanked on the e. by a somewhat obscure little crater) through which a curious bent ridge coming up from the n. passes, and, extending on to the floor, expands into something resembling a central mountain. under a high sun madler has a very peculiar appearance. the lofty e. wall is barely perceptible, while the much lower w. border is conspicuously brilliant; and the e. half of the floor is dark, while the remainder, with two objects representing the loftier portions of the intrusive ridge, is prominently white. under an evening sun, with the terminator lying some distance to the w., a very remarkable obscure ring with a low border, a valley running round it on the w. side, and two large central mounds, may be easily traced. this object is connected with madler by what appears to be under a higher sun a bright elbow-shaped marking, in connection with which i have often suspected a delicate cleft. between the obtuse-angled bend of this object and the w. wall of madler, two large circular dark spots may be seen under a high sun; and on the surface of the mare n. of it, a great number of delicate white spots. beaumont.--a ring-plain about miles in diameter, on the s.e. side of the mare nectaris, midway between theophilus and fracastorius, with the n.e. side of which it is connected by a chain of large depressions. its border is lofty, regular, and continuous on the s. and e., but on the w. it is low, and on the n. sinks to such a very inconsiderable height that it is often scarcely traceable. it exhibits two breaks on the s.w., through one of which passes a coarse valley that ultimately runs on the e. side of the depressions just referred to. the interior is pitted with many craters, one on the w. side being shallow but of considerable size. i once counted twenty with a inch cooke achromatic, and dr. sheldon of macclesfield subsequently noted many more. a ridge, prominent under oblique light, follows a winding course from the n.w. side of beaumont to the w. side of theophilus, and there is another lower ridge e. of it. between them is included a region covered with minute hillocks and asperities. among these objects are certain dusky little crater-cones, which dr. klein of cologne regards as true analogues of some terrestrial volcanoes. they are very similar in character to those, already alluded to, in the dusky area between copernicus and gambart. kant.--a conspicuous ring-plain, miles in diameter, situated in a mountainous district e. of theophilus, with lofty terraced walls and a bright central peak. adjoining it on the w. is a mountain mass, projecting from the coast-line of the mare, on which there is a peak rising to more than , feet above the surface. fracastorius.--this great bay or inflexion at the extreme s. end of the mare nectaris, about miles in diameter, is one of the largest and most suggestive examples of a partially destroyed formation to be found on the visible surface. the w. section of the rampart is practically complete and unbroken, rising at one peak to a height of feet above the interior. it is very broad at its s. end, and its inner slope descends with a gentle gradient to the floor. towards the n., however, it rapidly decreases in width, but apparently not in altitude, till near its bright pointed n. extremity. under a low sun, some long deformed crateriform depressions may be seen on the slope, and a bright little crater on the crest of the border near its n. end. the southern rampart is broken by three large craters, and a fine valley, running some distance in a s. direction, which diminishes gradually in width till it ultimately resembles a cleft, and terminates at a small crater. the e. border is very lofty and irregular, rising at the n. corner of the large triangular formation, which is such a prominent feature upon it, to a height of feet, and at a point on the s.e. to considerably more than feet above the floor. n. of the former peak it becomes much lower and narrower, and is finally only represented by a very attenuated strip of wall, hardly more prominent than the brighter portions of the border of stadius at sunrise, terminating at an obscure semi-ring-plain. between this and the pointed n. termination of the w. border there is a wide gap, open to the north for a space of about miles, appearing, except under very oblique illumination, as smooth and as devoid of detail as the grey surface of the mare nectaris itself. if, however, this interval is observed at sunrise or sunset, it is seen to be not quite so structureless as it appears under different conditions, for a number of mounds and large humpy swellings, with low hills and craterlets, extend across it, and occupy a position which we are justified in regarding as the site of a section of the rampart, which, from some cause or other, has been completely destroyed and overlaid with the material, whatever this may be, of the mare nectaris. the floor of fracastorius is, as regards the light streaks and other features upon it, only second in interest to those of plato and archimedes, and will repay systematic observation. between thirty and forty light spots and craters have been recorded on its surface, most of them, as in these formations, being situated either on or at the edges of the light streaks. on the higher portion of the interior (near the centre) is a curious object consisting apparently of four light spots, arranged in a square, with a craterlet in the middle, all of which undergo (as i have pointed out elsewhere) notable changes of aspect under different phases. there are at least two distinct clefts on the floor, one running from the w. wall towards the centre, and another on the s.e. side of the interior. the last throws out two branches towards the s.w. rosse.--a fine bright deep crater in the mare nectaris, n. of the pointed termination of the w. wall of fracastorius, with which it is connected by a bold curved ridge, with a crater upon it. a ray from tycho, striking along the e. wall of fracastorius passes near this object. a rill from near bohnenberger terminates at this crater. polybius.--a ring-plain, about miles in diameter, in the hilly region s.e. of fracastorius. the border is unbroken, except on the n., where it is interrupted by a group of depressions. there is a long valley on the s.w., at the bottom of which schmidt shows a crater-chain. neander.--this ring-plain, miles in diameter, a short distance w.s.w. of piccolomini, has a somewhat deformed rampart, which, however, except on the n., where there is a narrow gap occupied by a small crater, is continuous. it rises on the e. nearly feet above the floor, on which there is a central mountain about feet high. schmidt shows some minor hills, a large crater on the n.e. side, and three smaller craters in the interior. piccolomini.--a ring-plain of a very massive type, about miles in diameter, s. of fracastorius, with complex and prominently terraced walls, surmounted by very many peaks; one of which on the e. attains a height of , feet, and another, n. of it, on the same side, an altitude of , feet above the interior. the crest of this grand rampart is tolerably continuous, except on the s.w., where, for a distance of twenty miles or more, its character as regards form and brightness is entirely changed. under a low sun, instead of a continuous bright border, we note a wide gap occupied by a dusky rugged plateau, which falls with a gentle gradient to the floor, and is traversed by three or four parallel shallow valleys running towards the s. i can recall no lunar formation which presents an appearance at all like this: one is impressed with the idea that it has resulted from the collapse of the upper portion of the wall, and the flow of some viscous material over the wreck and down the inner slope. the difference between the reflective power of this matter, whatever may be its nature, and the broad bright declivities of the inner slopes, are beautifully displayed at sunset. the cross-valleys are more easily traced under low morning illumination; but to appreciate the actual structure of the wall, it should be observed under both phases. the n.w. section of the border includes many "pockets," or long elliptical depressions, which at an early stage of sunrise give a scalloped appearance to the crest. except the great bright central mountain with its numerous peaks, there does not appear to be any prominent detail on the floor. there is a large ring-plain beyond the foot of the _glacis_ on the w. with two craters on the e. side of it, another on the s., and a fine rill-valley running up to its n. side from near the crest of the w. wall. on the n. side of piccolomini is a remarkable group of deformed and overlapping enclosures, mingled with numberless craters and little depressions. the plain on the n.e. is crossed by a fine cleft. pons.--a complete formation of irregular shape, about miles in greatest diameter, on the s.e. side of the altai range, in w. long. deg. it consists of a crowd of rings and craters enclosed by a narrow wall. stiborius.--an elongated ring-plain, about miles in diameter, s. of piccolomini, with a lofty wall, broken in one place on the n. by a very conspicuous crater. schmidt shows a distinct crater in the centre of the floor. i have only seen a central mountain in this position. there is a large crater on the n.w., a ring-plain on the s.w. side, and a multitude of little craters on the surrounding plain. riccius.--a ring-plain, miles in diameter, of a very irregular type, s.e. of the last. it is enclosed by a complex wall (which is in places double), broken by large rings on the s. the very conspicuous little ring-plain riccius a is situated on the n. of it, and other less prominent features. the interior includes a bright crater and some smaller objects of the same class. zagut.--the most easterly of a group of closely associated irregular walled-plains, of which lindenau and rabbi levi are the other members, all evidently deformed and modified in shape by their proximity. it is about miles in diameter, and is enclosed by a wall which on the s.w. attains a height of about feet, and is much broken on the n. by a number of depressions. a large ring-plain, some miles in diameter, occupies a considerable portion of the w. side of the interior; e. of which, and nearly central, there is a large bright crater, but apparently no other conspicuous details. on the s.e. side of zagut lies an elliptical ring-plain, about miles in diameter, named by schmidt celsius. the border of this is open on the n., the gap being occupied by a large crater, whose s. wall is wanting, so that the interiors of both formations are in communication. lindenau.--this formation, about miles in diameter, is bounded on the w. by a regular unbroken wall nearly feet in height; but which on the e. and n.e. is far loftier and more complex, rising to about , feet above the floor, consisting of four or more distinct ramparts, separated by deep valleys, and extending towards rabbi levi. neison points out that under a high light lindenau appears to have a bright uniform single wall. there is a small central mountain and some minor inequalities in the interior. rabbi levi.--a larger but less obvious formation than either of its neighbours, zagut and lindenau, abutting on the s. side of them. it is about miles in diameter, and is enclosed by a border somewhat difficult to trace in its entirety, except under oblique light. there are some large craters within it, of which one on the n. side of the floor is especially prominent. nicolai.--a tolerably regular ring-plain, miles in diameter, s. of riccius, with a border, rising more than feet above a level floor, on the n. side of which schmidt shows a minute crater. the bright plain surrounding this formation abounds in small craters; and on the w. is a number of curious enclosures, many of them overlapping. vlacq.--a member of a magnificent group of closely associated formations situated on the greatly disturbed area between w. long. deg. and deg. and s. lat. deg. and deg. it is miles in diameter, and is enclosed by terraced walls, rising on the w. about feet, and on the e. more than , feet above the floor. they are broken on the s. by a fine crater. in addition to a conspicuous central peak, there are several small craters, and low short ridges in the interior. hommel.--adjoins vlacq on the s. it is a somewhat larger and a far more irregular formation. on every side except the w., where the border is unbroken, and descends with a gentle slope to the dark interior; ring- plains and smaller depressions encroach on its outline, perhaps the most remarkable being hommel _a_ on the n., which has an especially brilliant wall, that includes a conspicuous central mountain, a large crater, and other details. the best phase for observing hommel and its surroundings is when the w. wall is just within the evening terminator. pitiscus.--the most regular of the vlacq group. it is situated on the n.e. of hommel (a curious oblong-shaped enclosure, hommel _h_, with a very attenuated e. wall, and a large crater on a floor, standing at a higher level than that of pitiscus, intervening). it is miles in diameter, and is surrounded by an apparently continuous rampart, except on the e., where there is a crater, and on the s.w., where it abuts on hommel _h_. here there is a wide gap crossed by what has every appearance of being a "fault," resembling that in phocylides on a smaller scale. there is a fine crater on the n. side of the interior connected with the s. wall by a bright ridge. just beyond the e. border there is a shallow ring-plain of a very extraordinary shape. nearch.--a ring-plain, about miles in diameter, on the s.w. of hommel, forming part of the vlacq group. tannerus.--a ring-plain, about miles in diameter, between mutus and bacon. it has a central mountain. mutus.--a fine but foreshortened walled plain, miles in diameter. there are two ring-plains of about equal size on the floor, one on the n., and the other on the s. side. the wall on the w. rises to nearly , feet above the interior. manzinus.--a walled plain, nearly miles in diameter, with a terraced rampart rising to a height of more than , feet above the interior. schmidt shows three craterlets on the floor, but no traces of the small central peak which is said to stand thereon, but to be only visible in large telescopes. schomberger.--a large walled-plain adjoining simpelius on the s.w. too near the limb for satisfactory observation. west longitude deg. to deg. delambre.--a conspicuous ring-plain, miles in diameter, a little s. of the equator, in w. long. deg. min., with a massive polygonal border, terraced within, rising on the w. to the great height of , feet above the interior, but to little more than half this on the opposite side. its outline approximates to that of a pentagon with slightly curved sides. a section on the s.e. exhibits an inflexion towards the centre. the crest is everywhere continuous except on the n., where it is broken by a deep crater with a bright rim. the north-easterly trend of the ridges and hillocks on the e. is especially noteworthy. the central peak is not prominent, but close under it on the e. is a deep fissure, extending from near the centre, and dying out before it reaches the s. border. at the foot of the n.e. _glacis_ there are traces of a ring with low walls. theon, sen.--a brilliant little ring-plain, e.n.e. of delambre, miles in diameter, and of great depth, with a regular and perfectly unbroken wall. north of it is a bright little crater. theon, jun.--a ring-plain similar in size and in other respects to the last, situated about miles s. of it on a somewhat dusky surface. between the pair is a curious oblong-shaped mountain mass; and on the e. a long cliff (of no great altitude, but falling steeply on the e. side) extending s. towards taylor _a_. just below the escarpment, i find a brilliant little pair of craterlets, of which neison only shows one. alfraganus.--a large bright crater, about miles in diameter, with very steep walls, some distance s.s.w. of delambre, and standing on the w. edge of a large but very shallow and irregular depression w. of taylor. there is a remarkable chain of craters on the w. of it. alfraganus is the centre of a system of light streaks radiating in all directions, one ray extending through cyrillus to fracastorius. taylor.--a deep spindle-shaped ring-plain, s. of delambre, about miles in length. the wall appears to be everywhere continuous, except at the extreme n. and s. ends, where there are small craters. the outer slopes, both on the e. and w., are very broad and prominent, but apparently not terraced. there is an inconspicuous central hill. on the w. is the irregular enclosure, already referred to under alfraganus. three or four short winding valleys traverse the n. edge of this formation, and descend to the dark floor. on the n.e. is the remarkable ring-plain taylor _a_, miles in diameter, rising, at an almost isolated mountain mass on the e. border, to a height of feet above the interior. the more regular and w. section of this formation is not so lofty, and falls with a gentle slope to the dark uneven floor, on which there is some detail in the shape of small bright ridges and mounds. on the surface, n.w. of taylor _a_, is a curious linear row of bright little hills. taylor and the vicinity is better seen under low evening illumination than under morning light. hipparchus.--except under a low sun, this immense walled-plain is by no means so striking an object as a glance at its representation on a chart of the moon would lead one to expect; for the border, in nearly every part of it, bears unmistakable evidence of wreck and ruin, its continuity being interrupted by depressions, transverse valleys, and gaps, and it nowhere attains a great altitude. this imperfect enclosure extends miles from n. to s., and about miles from e. to w., and in shape approximates to that of a rhombus with curved sides. one of the most prominent bright craters on its border is hipparchus g, on the w. another, of about the same size, is hipparchus e, on the n. of horrocks. on the e. there is a moderately bright crater, hipparchus f; and s. of this, on the same side, two others, k and i. the interior is crossed by many ridges, and near the centre includes the relics of a low ring, traversed by a narrow rill-like valley. schmidt shows a cleft running from f across the floor to the s. border. [a valuable monograph of hipparchus, by mr. w.b. birt, was published in .] horrocks.--this fine ring-plain, miles in diameter, stands on the n. side of the interior of hipparchus, close to the border. it has a continuous wall, rising on the e. to a height of nearly feet above the interior, and a distinct central mountain. halley.--a ring-plain, miles in diameter, on the s.w. border of hipparchus, with a bright wall, rising at one point on the e. to a height of feet above the floor, which is depressed about feet below the surface. two craterlets on the floor, one discovered by birt on rutherfurd's photogram of , and the other by gaudibert, raised a suspicion of recent lunar activity within this ring. a magnificent valley, shown in part by schmidt as a crater-row, runs from the s. of halley to the w. side of albategnius. hind.--a ring-plain, miles in diameter, a few miles w. of halley, with a peak on its e. wall , feet above the floor. the border is broken both on the s.e. and n.e. by small craters. [horrocks, halley, and hind may be regarded as strictly belonging to hipparchus.] albategnius.--a magnificent walled-plain, miles in diameter, adjoining hipparchus on the s., surrounded by a massive complex rampart, prominently terraced, including many depressions, and crossed by several valleys. it is surmounted by very lofty peaks, one of which on the n.e. stands nearly , feet above the floor. the great ring-plain albategnius a, miles in diameter, intrudes far within the limits of the formation on the e., and its towering crest rises more than , feet above its floor, on which there is a small central mountain. the central mountain of albategnius is more than feet high, and, with the exception of a few minor elevations, is the only prominent feature in the interior, though there are many small craters. schmidt counted forty with the berlin refractor, among them on the e. side, arranged like a string of pearls. parrot.--an irregularly-shaped formation, miles in diameter, s. of albategnius, with a very discontinuous margin, interrupted on every side by gaps and depressions, large and small; the most considerable of which is the regular ring-plain parrot _a_, on the e. an especially fine valley, shown by schmidt to consist in part of large inosculating craters, cuts through the wall on the s.w., and runs on the e. side of argelander towards airy. the floor of parrot is very rugged. descartes.--this object, about miles in diameter, situated n.w. of abulfeda, is bounded by ill-defined, broken, and comparatively low walls; interrupted on the s.e. by a fine crater, descartes a, and on the s.w. by another, smaller. there is also a brilliant crater outside on the n.w. schmidt shows a crater-row on the floor, which i have seen as a cleft. dollond.--a bright crater, about miles in diameter, on the n.e. side of descartes. between it and the latter there is a rill-valley. tacitus.--a bright ring-plain, about miles in diameter, a few miles e. of catherina, with a lofty wall rising both on the e. and w. to more than , feet above the floor. its continuity is broken on the n. by a gap occupied by a depression, and there is a conspicuous crater below the crest on the s.w. the central mountain is connected with the n. wall by a ridge, recalling the same arrangement within madler. a range of lofty hills, an offshoot of the altai range, extends from tacitus towards fermat. almanon.--this ring-plain, with its companion abulfeda on the n.e., is a very interesting telescopic object. it is about miles in diameter, and is surrounded by an irregular border of polygonal shape, the greatest altitude of which is about feet above the floor on the w. it is slightly terraced, and is broken on the s. by a deep crater pertaining to the bright and large formation tacitus _b_, the e. border of which casts a fine double-peaked shadow at sunrise. on the n.w. there is another bright crater, the largest of the row, running in a w.s.w. direction, and forming a w. extension of the remarkable crater-chain tangential to the borders of almanon and abulfeda. the only objects on the floor are three little hills, in a line, near the centre, a winding ridge on the w. side of it, and two or three other low elevations. abulfeda.--a larger and more massive formation than almanon, miles in diameter, the e. wall rising about , feet above the interior, which is depressed more than feet. it is continuous on the w., but much broken by transverse valleys on the s.e., and by little depressions on the n. on the s.e. originates the very curious bright crater-row which runs in a straight line to the n.w. wall of almanon, crossing for the first few miles the lofty table-land lying on the s.e. side of the border. with the exception of a low central mountain, the interior of abulfeda contains no visible detail. the rampart is finely terraced on the e. and w. the e. _glacis_ is very rugged. argelander.--this conspicuous ring-plain, about miles in diameter, is, if we except two smaller inosculating rings on the s.w. flank of albategnius, the most northerly of a remarkable serpentine chain of seven moderately-sized formations, extending for nearly miles from the s.w. of parrot to the n. side of blanchinus. its border is lofty, slightly terraced within, and includes a central peak. airy.--about miles in diameter, connected with argelander by a depression bounded by linear walls. its border, double on the s.e., is broken on the s. by a prominent crater, with a smaller companion on the w. of it; and again on the n.e. by another not so conspicuous. it has a central peak. the next link in the chain of ring-plains is airy _c_, a very irregular object, somewhat larger, and with, for the most part, linear walls. donati.--a ring-plain on the s. of airy _c_, about miles in greatest length. it is very irregular in outline, with a lofty broken border, especially on the n. and s., where there are wide gaps. there is another ring on the s.e. faye.--the direction of the chain swerves considerably towards the e. at this formation, which resembles donati both in size and in irregularity of outline. the wall, where it is not broken, is slightly terraced. there is a craterlet on the s. rim and a central crater in the interior. delaunay.--adjoins faye on the s.e., and is a larger and more complex object, of irregular form, with very lofty peaks on its border. a prominent ridge of great height traverses the formation from n. to s., abutting on the w. border of lacaille. delaunay is the last link in the chain commencing with argelander. lacaille.--an oblong enclosure situated on the n. side of blanchinus, and apparently about miles in greatest diameter. the border is to a great extent linear and continuous on the n., but elsewhere abounds in depressions. two large inosculating ring-plains are associated with the n.e. wall. blanchinus.--a large walled-plain on the w. of purbach and abutting on the s. side of lacaille. it much resembles purbach in shape, but has lower walls. schmidt shows a crater on the n. side of the floor, which i have seen, and a number of parallel ridges which have not been noted, probably because they are only visible under very oblique light. geber.--a bright ring-plain, miles in diameter, s. of almanon, with a regular border, rising to a height on the w. of nearly feet above the floor. there is a small crater on the crest of the s. wall, and another on the n. a ring-plain about miles in diameter adjoins the formation on the n.e. according to neison, there is a feeble central hill, which, however, is not shown by schmidt. sacrobosco.--this is one of those extremely abnormal formations which are almost peculiar to certain regions in the fourth quadrant. it is about miles in greatest diameter, and is enclosed by a rampart of unequal height, rising on the e. to , feet above the floor, but sinking in places to a very moderate altitude. on the n. its contour is, if possible, rendered still more irregular by the intrusion of a smaller ring-plain. on the n.e. side of the floor stands a very bright little crater and two others on the s. of the centre, each with central mountains. fermat.--an irregular ring-plain miles in diameter on the w. of sacrobosco. its partially terraced wall is broken on the n. by a gap which communicates with the interior of a smaller formation. there are some low hills on the floor, which is depressed feet below the crest of the border. azophi.--a prominent ring-plain, miles in diameter, e.n.e. of sacrobosco, its lofty barrier towering nearly , feet above a somewhat dusky interior, which includes some light spots. a massive curved mountain arm runs from the s. side of this formation to a small ring-plain w. of playfair. abenezra.--when observed near the morning terminator, this noteworthy ring-plain, miles in diameter, seems to be divided into two by a curved ridge which traverses the formation from n. to s., and extends beyond its limits. the irregular border rises on the w. to a height of more than , feet above the deeply-sunken floor, which includes several craters, hills, and ridges. apianus.--a magnificent ring-plain, miles in diameter, n.w. of aliacensis, with lofty terraced walls, rising on the n.e. to about feet above the interior, and crowned on the w. by three large conspicuous craters. the border is broken on the n. by a smaller depression and a large ring with low walls. the dark-grey floor appears to be devoid of conspicuous detail. playfair.--a ring-plain, miles in diameter, with massive walls. it is situated on the n. of apianus, and is connected with it by a mountain arm. the rampart is tolerably continuous, but varies considerably in altitude, rising on the s. to a height of more than feet above the interior. on the e., extending towards blanchinus, is a magnificent unnamed formation, bounded on the e. by a broad lofty rampart flanking blanchinus, lacaille, delaunay, and faye; and on the w. by playfair and the mountain arm just mentioned. it is fully miles in length from n. to s. sunrise on this region affords a fine spectacle to the observer with a large telescope. the best phase is when the morning terminator intersects aliacensis, as at this time the long jagged shadows of the e. wall of playfair and of the mountain arm are very prominent on the smooth, greyish-blue surface of this immense enclosure. pontanus.--an irregular ring-plain, miles in diameter, s.s.w. of azophi, with a low broken border, interrupted on the s.w. by a smaller ring-plain, which forms one of a group extending towards the s.w. the dark floor includes a central mountain. aliacensis.--this ring-plain, miles in diameter, with its neighbour werner on the n.e., are beautiful telescopic objects under a low sun. its lofty terraced border rises at one peak on the e. to the tremendous height of , feet, and at another on the opposite side to nearly , feet above the floor. the wall on the s. is broken by a crater, and on the w. traversed by narrow passes. there is also a prominent crater on the inner slope of the n.e. wall. the floor includes a small mountain, several little hills, and a crater. werner.--a ring-plain, miles in diameter, with a massive rampart crowned by peaks almost as lofty as any on that of aliacensis, and with terraces fully as conspicuous. it has a magnificent central mountain, feet high. at the foot of the n.e. wall madler observed a small area, which he describes as rivalling the central peak of aristarchus in brilliancy. webb, however, was unable to confirm this estimate, though he noted it as very bright, and saw a minute black pit and narrow ravine within it. neison subsequently found that the black pit is a crater-cone. it would perhaps be rash, with our limited knowledge of minute lunar detail, to assert that madler over-estimated the brightness of this area, which may have been due to a _recent_ deposit round the orifice of the crater-cone. poisson.--an irregular formation on the w. of aliacensis, extending about miles from w. to e., but much less in a meridional direction. its n. limits are marked by a number of overlapping ring-plains and craters, and it is much broken elsewhere by smaller depressions. the e. wall is about feet in height. gemma frisius.--a great composite walled-plain, miles or more in length from n. to s., with a wall rising at one place nearly , feet above the floor. it is broken on the n. by two fine ring-plains, each about miles in diameter, and on the e. by a third open to the e. there is a central mountain, and several small craters on the floor, especially on the w. side. busching.--a ring-plain s. of zagut, about miles in diameter, with a moderately high but irregular wall. there are several craterlets within and some low hills. buch.--adjoins busching on the s.e. it is about miles in diameter, and has a less broken barrier. there is a large crater on the e. wall, and another smaller one on the s.w. schmidt shows nothing on the floor, but neison noted two minute crater-cones. maurolycus.--this unquestionably ranks as one of the grandest walled- plains on the moon's visible surface, and when viewed under a low sun presents a spectacle which is not easily effaced from the mind. like so many of the great enclosures in the fourth quadrant, it impresses one with the notion that we have here the result of the crowding together of a number of large rings which, when they were in a semi-fluid or viscous condition, mutually deformed each other. it extends fully miles from e. to w., and more from n. to s.; so it may be taken to include an area on the lunar globe which is, roughly speaking, equal to half the superficies of ireland. this vast space, bounded by one of the loftiest, most massive, and prominently-terraced ramparts, includes ring-plains, craters, crater-rows, and valleys,--in short, almost every type of lunar formation. it towers on the e. to a height of nearly , feet above the interior, and on the w., according to schmidt, to a still greater altitude. a fine rill-valley curves round the outer slope of the w. wall, just below its crest, which is an easy object in a / inch reflector when the opposite border is on the morning terminator, and could doubtless be seen in a smaller instrument; and there is an especially brilliant crater on the s. border, which is not visible till a somewhat later stage of sunrise. the central mountain is of great altitude, its loftiest peaks standing out amid the shadow long before a ray of sunlight has reached the lower slopes of the walls. it is associated with a number of smaller elevations. i have seen three considerable craters and several smaller ones in the interior. barocius.--a massive formation, about miles in diameter, on the s.w. side of maurolycus, whose border it overlaps and considerably deforms. its wall rises on the e. to a height of , feet above the floor, and is broken on the n.w. by two great ring-plains. on the inner slope of the s.e. border is a curious oblong enclosure. there is nothing remarkable in the interior. on the dusky grey plain w. of maurolycus and barocius there is a number of little formations, many of them being of a very abnormal shape, which are well worthy of examination. i have seen two short unrecorded clefts in connection with these objects. stofler.--a grand object, very similar in size and general character to maurolycus, its neighbour on the w. to view it and its surroundings at the most striking phase, it should be observed when the morning terminator lies a little e. of the w. wall. at this time the jagged, clean-cut, shadows of the peaks on faraday and the w. border, the fine terraces, depressions, and other features on the illuminated section of the gigantic rampart, and the smooth bluish-grey floor, combine to make a most beautiful telescopic picture. at a peak on the n.e., the wall attains a height of nearly , feet, but sinks to a little more than a third of this height on the e. it is apparently loftiest on the n. the most conspicuous of the many craters upon it is the bright deep circular depression e. on the s. wall, and another, rather larger and less regular, on the n.w., which has a very low rim on the side facing the floor, and a craterlet on either side of the apparent gap. a large lozenge-shaped enclosure abuts on the wall, near the crater e., with a border crowned by a number of little peaks, which at an early stage of sunrise resemble a chaplet of pearls. the floor of stofler is apparently very level, and in colour recalls the beautiful steel-grey tone of plato seen under certain conditions. i have noted several distinct little craters on its surface, mostly on the n.e. side; and on the e. side a triangular dark patch, close to the foot of the wall, very similar in size and appearance to those within alphonsus. faraday.--a large ring-plain, about miles in diameter, overlapping the s.w. border of stofler; its own rampart being overlapped in its turn by two smaller ring-plains on the s.e., and by two still smaller formations (one of which is square-shaped) on the n.w. the wall is broad and very massive on the e. and n.e., prominently terraced, and includes many brilliant little craters. schmidt shows a ridge and several craters in the interior. licetus.--an irregular formation, about miles in maximum width, on the s. of stofler, with the flanks of which it is connected by a coarse valley. neison points out that it consists of a group of ring-plains united into one, owing to the separating walls having been partially destroyed. this seems to be clearly the case, if licetus is examined under a low sun. on the e. side of the n. portion of the formation, the wall rises to nearly , feet. fernelius.--a ring-plain, about miles in diameter, abutting on the n. wall of stofler. it is overlapped on the e. by another similar formation of about half its size. there are many craters and depressions on the borders of both, and a large crater between the smaller enclosure and the n.e. outer slope of stofler. schmidt shows eight craters on the floor of fernelius. nonius.--a ring-plain, about miles in diameter, abutting on the n. wall of fernelius. there is a prominent bright crater on the w. of it, and another on the n., from which a delicate valley runs towards the w. side of walter. clairaut.--a very peculiar formation, about miles in diameter, s. of maurolycus, affording another good example of interference and overlapping. the continuity of its border, nowhere very regular, has been entirely destroyed on the s. by the subsequent formation of two large rings, some or miles in diameter, the more easterly of which has, in its turn, been partially wrecked on the n. by a smaller object of the same class. there is also a ring-plain n.e. of clairaut, which has very clearly modified the shape of the border on this side. two craters on the floor of clairaut are easy objects. bacon.--a very fine ring-plain, miles in diameter, s.w. of clairaut. at one peak on the e. the terraced wall rises to nearly , feet above the interior. it is broken on the s. by three or four craters. on the w. there is an irregular inconspicuous enclosure, whose contiguity has apparently modified the shape of the border. there are two large rings on the n. (the more easterly having a central peak), and a third on the e. the floor appears to be devoid of prominent detail. cuvier.--a walled-plain, about miles in diameter, on the s.e. of clairaut. the border on the e. rises to , feet; and on the n.w. is much broken by depressions. neison has seen a mound, with a minute crater w. of it, on the otherwise undisturbed interior. jacobi.--a ring-plain s. of cuvier, about miles in diameter, with walls much broken on the n. and s., but rising on the e. to nearly , feet. there is a group of craters (nearly central) on the floor. the region s. of this formation abounds in large unnamed objects. lilius.--an irregular ring-plain, miles in diameter, with a rampart on the e. nearly , feet above the floor. a smaller ring between it and jacobi has considerably inflected the wall towards the interior. it has a conspicuous central mountain. zach.--a massive formation, miles in diameter, on the s. of lilius, with prominently terraced walls, rising on the e. to , feet above the interior. a small ring-plain, whose wall stands feet above the floor, is associated with the n. border. two other rings, on the s.w. and n.e. respectively, have craters on their ramparts and central hills. pentland.--a fine conspicuous formation under a low sun, even in a region abounding in such objects. it is about miles in diameter, with a border exceeding in places , feet in height above the floor, which includes an especially fine central mountain. kinau.--one of the group of remarkable ring-plains extending in a n.w. direction from pentland. simpelius.--another grand circumvallation, almost as large as pentland, but unfortunately much foreshortened. one of its peaks on the e. rises to a height of more than , feet above the floor, on which there is a small central mountain. between simpelius and pentland are several ring- plains, most of which appear to have been squeezed and deformed into abnormal shapes. curtius.--a magnificent formation, about miles in diameter, with one of the loftiest ramparts on the visible surface, rising at a mountain mass on the n.e. to more than , feet, an altitude which is only surpassed by peaks on the walls of newton and casatus. there is a bright crater on the s.e. border and another on the w. the formation is too near the s. limb for satisfactory scrutiny. between curtius and zach is a fine group of unnamed enclosures. appendix description of the map the accompanying map, eighteen inches in diameter, represents the moon under mean libration. meridian lines and parallels of latitude are drawn at every deg., except in the case of the meridians of deg. e. and w. longitude, which are omitted to avoid confusion, and as being practically needless. these lines will enable the observer, with the aid of the tables in the appendix, to find the position of the terminator at any time required. as astronomical telescopes exhibit objects inverted, maps of the moon are always drawn upside down, and with the right and left interchanged, as in the diagram above, which also shows how the quadrants are numbered. this circle [drawing of circle], intended to be . in diameter, represents a circle of one degree in diameter at the centre of the map, and as the length of one selenographical degree is . miles, it represents an area of nearly square miles. the catalogue is so arranged that, beginning with the w. limb, and referring to the lists under the first and fourth, and the second and third quadrants, all the formations falling within the meridians deg. to deg., deg. to deg., deg. to deg., deg. to deg. (the central meridian), and from deg. to deg., and so on, to the e. limb, will be found in convenient proximity in the text. in the catalogue, n. s. e. w. are used as abbreviations for the cardinal points. list of the maria, or grey plains, termed "seas," &c. first quadrant. mare tranquilitatis (nearly the whole), page . ,, foecunditatis (the n. portion), . ,, serenitatis, . ,, crisium, . ,, frigoris (a portion), . ,, vaporum (nearly the whole), . ,, humboldtianum, . ,, smythii (a portion), . lacus mortis, . ,, somniorum. palus somnii. ,, nebularum (a portion), . ,, putredinis, . sinus medii (a portion), . second quadrant. mare imbrium, . ,, nubium (the n. portion), . ,, frigoris (a portion), . ,, vaporum (a portion), . oceanus procellarum (the n. portion), . palus nebularum (a portion), . sinus iridum, . ,, medii (a portion), . ,, roris, . ,, aestuum. third quadrant. mare nubium (the greater portion), . ,, humorum, . oceanus procellarum (the s. portion), . sinus medii (a small portion), . fourth quadrant. mare foecunditatis (the greater portion), . ,, nectaris, . ,, tranquilitatis (a small portion), . ,, australe, . ,, smythii (a portion), . sinus medii (a portion), . list of some of the most prominent mountain ranges, promontories, isolated mountains, and remarkable hills. first quadrant. the alps. the western portion of the range. the apennines. the extreme northern part of the range. the caucasus. the haemus. the taurus. the north polar range. on the limb extending from n. lat. deg. towards the e. the humboldt mountains. on the limb from n. lat. deg. to n. lat. deg. mount argaeus. a mountain mass rising some feet above the mare serenitatis in n. lat. deg., w. long. deg., n.w. of dawes. prom. acherusia. a bright promontory at the w. extremity of the haemus range, rising nearly feet above the mare serenitatis. n. lat. deg., w. long. deg. cape agarum. the n. end of a projecting headland on the s.w. side of the mare crisium, in n. lat. deg., w. long. deg., rising nearly , feet above the mare. le monnier a. an isolated mountain more than feet high, standing about midway between the extremities of the bay: probably a relic of a once complete ring. secchi. south of this formation there is a lofty prominent isolated mountain. manilius a and beta. two conspicuous mountains n. of manilius; a, the more westerly, being more than feet, and beta about feet in height. autolycus a. a mountain of considerable altitude, s. of this formation. mont blanc. principal peak, n. lat. deg., w. long. deg. min., nearly , feet in height. cassini epsilon and delta. two adjoining mountain masses n. of cassini, more than feet high. eudoxus. s.e. of this formation, in n. lat. deg., w. long. deg., are two bright mountain masses, the more southerly rising , and the other feet above the surface. mount hadley. the northern extremity of the apennines, in n. lat. deg. w. long. deg., rising more than , feet above the mare. mount bradley. a promontory of the apennines, in n, lat. deg., w. long. deg., nearly , feet above the mare imbrium. the silberschlag range, running from near the s.e. side of julius caesar to the region w. of agrippa. second quadrant. the alps. the eastern and greater portion. the apennines. nearly the whole of the range. the carpathians. the teneriffe mountains. s.e. of plato. highest peak, feet. the straight range. east of the last, in n. lat. deg., e. long. deg. the harbinger mountains. n.w. of aristarchus. the hercynian mountains. near the n.e. limb, e. of otto struve, n. lat. deg. mount huygens. a mountain mass projecting from the escarpment of the apennines, in n. lat. deg., e. long. deg., one peak rising to , feet above the mare imbrium. mount wolf. a great square-shaped mountain mass, near the s.e. extremity of the apennines, in n. lat. deg., e. long. deg., the loftiest peak rising to nearly , feet above the mare imbrium. eratosthenes i and x. two isolated mountains n. of this formation, in n. lat. deg.; x is feet in height. pico. a magnificent isolated mountain, s. of plato, in n. lat. deg., e. long. deg., rising some feet above the mare imbrium. pico b. a triple-peaked mountain a few miles s. of pico. piton. a bright isolated mountain feet high, in n. lat. deg., e. long. deg. fontinelle a. a conspicuous isolated mountain about feet high, s. of fontinelle. archimedes z. a triangular-shaped group e. of archimedes, in n. lat. deg., e. long. deg., the highest of the peaks rising more than feet. caroline herschel. e. of this formation is a double-peaked mountain rising to feet. gruithuisen delta and gamma. on the n. of this bright crater, in n. lat. deg., e. long. deg., rises a fine mountain, delta, nearly feet in height, and on the n.e. of it the larger mass gamma, almost as lofty. mairan. there is a group of three bright little mountains, the loftiest about feet above the mare, some distance e. of this formation. euler beta. a fine but small mountain group, more than feet high, on the mare imbrium, s.e. of euler. the laplace promontory. a magnificent headland on the n. side of the sinus iridum, rising about feet above the latter, and about feet above the mare imbrium. cape heraclides. a fine but less prominent headland on the opposite side of the bay, rising more than feet above it. lahire. a large bright isolated mountain in the mare imbrium, n.e. of lambert, in n. lat. deg., e. long. deg. it is, according to schroter, nearly feet high. delisle beta. a curious club-shaped mountain on the s.e. of this formation, nearly feet in height. pytheas beta. an isolated mountain, feet high, in n. lat. deg., e. long. deg. kirch. there is a small isolated hill a few miles n. of this formation. kirch gamma. a bright mountain about feet high, in n. lat. deg., e. long. deg. piazzi smyth beta. a small bright isolated mountain on a ridge s. of this, is a noteworthy object under a low sun. lambert gamma. in n. lat. deg., e. long. deg.; a remarkable curved mountain about feet in height, a brilliant object under a low sun. d'alembert mountains. a range on the e. limb running s. from n. lat. deg. wollaston. an isolated triangular mountain about midway between this and wollaston b. third quadrant. the riphaean mountains. an isolated range s. of landsberg in s. lat. deg., e. long. deg. they run in a meridional direction, and rise at one peak to nearly feet above the oceanus procellarum. the percy mountains extend from the eastern flank of gassendi towards mersenius, forming the north-eastern border of the mare humorum. prom. aenarium. a steep bluff situated at the northern end of a plateau, some distance e. of arzachel, in s. lat. deg., e. long. deg. it rises some feet above the mare nubium. euclides zeta and chi. two mountain masses n. of this formation in s. lat. deg.; zeta rises about feet above the mare; both are evidently offshoots from the riphaean range. landsberg h. an isolated hill in s. lat. deg., e. long. deg. nicollet c. s.e. of nicollet, in s. lat. deg., e. long. deg.; is hemmed in by a mountain mass rising to more than feet above the mare nubium. the stag's-horn mountains. at the s. end of the straight wall, or "railroad," in s. lat. deg., e. long. deg., a curious mountain mass rising about feet above the mare nubium. lacroix delta. a mountain more than feet high, n. of lacroix. flamsteed e. a mountain of more than feet in s. lat. deg., e. long. deg. d'alembert mountains. a very lofty range on the e. limb, extending to s. lat. deg. the cordilleras. close to the e. limb; they lie between s. lat. deg. and s. lat. deg. rook mountains. on the e. limb, extending from about s. lat. deg. to s. lat. deg. according to schroter, they attain a height of , feet. dorfel mountains. on the s.e. limb between s. lat. deg. and s. lat. deg. leibnitz mountains. on the s. limb extending w. from s. lat. deg. beyond the pole on to the fourth quadrant. perhaps the loftiest range on the limb. madler's measures give more than , feet as the height of one peak, and there are several others nearly as high. fourth quadrant. the altai mountains. a fine conspicuous serpentine range, extending from the e. side of piccolomini in a north-easterly direction to the region between tacitus and catherina, a length of about miles. the loftiest peak is over , feet. the average height of the southern portion is about feet. the region lying on the s.e. of this range is a vast tableland, devoid of prominent objects, rising gradually towards the mountains, which shelve rapidly down to an equally barren expanse on the n.w. the pyrenees. these mountains, on the e. of guttemberg, border the western side of the mare nectaris. their loftiest peak, rising nearly to , feet, is on the s.e. of guttemberg. list of the principal ray-systems, light-surrounded craters, and light- spots. [in this list, which does claim to be exhaustive, most of the objects noted by schmidt are incorporated.] first quadrant. autolycus. encircled by a delicate nimbus, throwing out four or five prominent rays extending towards archimedes. seen best under evening illumination. aristillus. the centre of a noteworthy system of delicate rays extending w. towards the caucasus; and on the s. disappearing among the rays of autolycus. they are traceable on the mare nubium near kirch. theaetetus. a very brilliant group of little hills e. of this formation. eudoxus a. a light-surrounded crater w. of eudoxus, with distinct long streaks, one of which extends to the s. wall of aristoteles. aristoteles a. a light-surrounded crater in the mare frigoris, n.e. of aristoteles. aratus. a very conspicuously brilliant crater in the apennines, with a smaller light-surrounded crater w. of it. sulpicius gallus. a light spot near. manilius. surrounded by a light halo and streaks. taquet. has a prominent nimbus, and indications of very delicate streaks. plinius a. is surrounded by a well-marked halo. posidonius gamma. among the hills e. of this formation a light spot resembling linne, according to schmidt. he first saw it in , when it had a delicate black spot in the centre. dr. vogel observed and drew it in with the great refractor at bothkamp. these observations were confirmed by schmidt in with the -feet refractor at berlin. littrow. a very bright light-spot with streaks, on the site of a little crater and well-known cleft e. of this ring-plain. romer. a light-surrounded mountain on the e. macrobius. two light-surrounded craters on the e. of this formation, the more northerly being the brighter. cleomedes a. (on the floor.) surrounded by a nimbus and rays. large crater, a, on the e. has also a nimbus and rays. agrippa. exhibits faint rays. godin. exhibits faint rays. proclus. a well-known ray-centre, some of the rays prominent on part of the mare crisium. taruntius. has a very faint nimbus, with rays, on a dark surface. dionysius. a brilliant crater with a prominent, bright, excentrically placed nimbus on a dark surface, on which distinct rays are displayed. hypatia b. a very small bright crater on a dark surface: surrounded by a faint nimbus. apollonius. among the hills s. of this, there is a small bright streak system. eimmart. there is a large white spot n.w. of this. geminus is associated with a system of very delicate rays. menelaus. a brilliant object. it is traversed by a long ray from tycho. second quadrant. anaxagoras. the centre of an important ray-system. timocharis is surrounded by a pale irregular nimbus and faint rays, most prominently developed on the w. side of the formation. copernicus. next to tycho, the most extended ray-centre on the visible surface. some distance on the e., in e. long. deg., n. lat. deg., lies a very small but conspicuous system, and in e. long. deg., n. lat. deg. a bright light spot among little hills. gambart a. a bright crater with large nimbus and rays. landsberg a. a light-surrounded crater on a dark surface, with companions, referred to under the third quadrant. encke. there is a light-surrounded crater s. of this. kepler. a noted ray-centre. it is surrounded by an extensive halo, especially well developed on the e., across the mare procellarum. bessarion. two bright craters: the more northerly is prominently light-surrounded, while its companion is less conspicuously so. aristarchus.--the most conspicuous bright centre on the moon, the origin of a complicated ray-system. delisle. s. of this formation there is a tolerably bright spot on the site of some hills. timaeus. a ray-centre. euler. feeble halo with streaks. galileo. between this and reiner is a curious bright formation with short rays, referred to in the catalogue, under reiner. cavalerius. a light streak originating in the w. wall, and extending on to the oceanus procellarum. olbers. a considerable ray-system, but seldom distinctly visible. lichtenberg. faintly light-surrounded. third quadrant. tycho. the largest and best known system on the visible surface. zuchius. a remarkable ray-system, but one which is only well seen when libration is favourable. bailly. n. of the centre of this great enclosure are two very distinct radiating streaks. schickard. four conspicuous light spots, probably craters, on the s.e. byrgius a. a brilliant ray-centre, most of the rays trending eastward from a nimbus. hainzel. there are several bright spots e. of this formation. mersenius. two or three light-rays originate from a point on the w. rampart. mersenius c. a light-surrounded crater with short rays. grimaldi. there are three bright spots on the w. wall. damoiseau. a light-surrounded crater w. of damoiseau, e. long. deg., s. lat. deg. flamsteed c. a light-surrounded crater on a dark surface. lubieniezky a. crater with halo on a dark surface. lubieniezky f. crater with halo on a dark surface. lubieniezky g. crater with halo on a dark surface. birt _a_. a light-surrounded crater. landsberg. e. of landsberg, four light-surrounded craters, forming with landsberg a (in the second quadrant) an interesting group. lohrmann a. a light-surrounded crater, with a light area a few miles n. of it. s. lat. deg., e. long. deg. euclides. has a conspicuous nimbus with traces of rays, a typical example. guerike. there is a crater, with nimbus, w. of this, in e. long. deg., s. lat. deg. min. parry. a very brilliant light-spot in the s. wall. parry a. surrounded by a bright nimbus. alpetragius b. a conspicuous light-surrounded crater, one of the most remarkable on the moon. alpetragius _d_ (e. long. deg., s. lat. deg. min.). a bright spot, seen by madler as a crater, but which, as schmidt found in , no longer answers to this description. mosting c. a light-surrounded crater. lalande. has a large nimbus and distinct rays. hell. a large ill-defined spot in e. long. deg., s. lat. deg. this is most probably the site of the white cloud seen by cassini. mercator. there is a brilliant crater and light area under e. wall. fourth quadrant. stevinus _a_. a crater e. of stevinus; it is a centre of wide extending rays. furnerius a. prominently light-surrounded, with bright streaks, radiating for a long distance n. and s. messier a. the well-known "comet" rays, extending e. of this. langrenus. has a large but very pale ray-system. it is best seen under a low evening sun. three long streaks radiate towards the e. from the foot of the _glacis_ of the s.e. wall. censorinus. a very brilliant crater with faint rays. theophilus. the central mountain is faintly light-surrounded. madler. this ring-plain and the neighbourhood on the n. and n.w., include many bright areas and curious streaks. almanon. about midway between this and argelander is a very brilliant little crater. beaumont. between this and cyrillus stand three considerable craters with nimbi. cyrillus a. a prominent light-surrounded crater. alfraganus. a light-surrounded crater with rays. position of the lunar terminator though the position of the lunar terminator is given for mean midnight throughout the year in that very useful publication the companion to the observatory, it is frequently important in examining or comparing former drawings and observations to ascertain its position at the times when they were made. for this purpose the subjoined tables (which first appeared in the selenographical journal) will be found useful, as they give for any day between a.d. and a.d. the selenographical longitude of the point where the terminator crosses the moon's equator, which it does very nearly at right angles. [tables and examples] lunar elements moon's mean apparent diameter - min. sec. moon's maximum apparent diameter - min. . sec. moon's minimum apparent diameter - min. . sec. moon's diameter, in miles - miles. volume (earth's = ) - / . or . . mass (earth's = ) - / . or . . density (earth's = ) - . , or . the density of water (water being unity). surface area, about , , square miles (earth's surface area, , , miles) earth's surface area = , moon's - about / or . . action of gravity at surface - . or / . of the earth's. surface of moon never seen - . . surface of moon seen at one time or another - . . synodical revolution, or interval from new moon to new moon (commonly called a lunation) - d. h. m. . s. - . days. sidereal revolution, or time taken in passing from one star to the same star again - d. h. m. . s. - . days. tropical revolution, or time taken in passing from "the first point of aries" to the same point again - d. h. m. . s. - . days. anomalistic revolution, or time taken in passing from perigee to perigee - d. h. m. . s. - . days. nodical revolution, or time taken in passing from rising node to rising node - d. h. m. . s. - . days. distance (mean) in terms of the equatorial radius of the earth - . . distance in miles (mean) - , miles. distance, maximum - , miles. distance, minimum - , miles. mean excentricity of moon's orbit - . . inclination of moon's orbit to the ecliptic (mean) - deg. min. . sec. inclination of moon's axis to the ecliptic - deg. min. sec. inclination of moon's equator to the ecliptic - deg. min. sec. maximum libration in latitude - deg. min. maximum libration in longitude - deg. min. maximum total libration from earth's centre - deg. min. maximum diurnal libration - deg. min. . sec. angle subtended by one degree of selenographical latitude and longitude at the centre of the moon's disc, when at its mean distance - . sec. length of a degree under these conditions - . miles. selenographical arc at the centre of the moon's surface, subtending an angle of one second of arc - min. . sec. miles at the centre of the moon's disc, subtending an angle of one second of arc - . [it must be remembered that this value is _increased_, in departing from the centre, in the proportion of the secants of the angular distance from the centre.] period of similar phase - d. h. m. = lunations. or, more accurately - d. h. = lunations. http://www.archive.org/details/gradualacceptan stim) [transcriber's note: obvious printer errors have been corrected without note. other questionable items are marked with a [transcriber's note].] the gradual acceptance of the copernican theory of the universe dorothy stimson, ph.d. new york copyright by dorothy stimson trade selling agents the baker & taylor co., fourth ave., new york to my father and mother [illustration: the systems of the world in according to father riccioli (reduced facsimile of the frontispiece in riccioli: _almagestum novum_. bologna, .)] explanation "astrea, goddess of the heaven, wearing angel's wings and gleaming everywhere with stars, stands at the right; on the left is argus of the hundred eyes, not tense, but indicating by the position of the telescope at his knee rather than at the eyes in his head, that while observing the work of god's hand, he appears at the same time to be worshipping as in genuflexion." (riccioli: _alm. nov._, _præfatio_, xvii). he points to the cherubs in the heavens who hold the planets, each with its zodiacal sign: above him at the top is mars, then mercury in its crescent form, the sun, and venus also in the crescent phase; on the opposite side are saturn in its "tripartite" form (the ring explanation was yet to be given), the sphere of jupiter encircled by its four satellites, the crescent moon, its imperfections clearly shown, and a comet. thus father riccioli summarized the astronomical knowledge of his day. the scrolls quote psalms : , "day unto day uttereth speech and night unto night showeth knowledge." astrea holds in her right hand a balance in which riccioli's theory of the universe (an adaptation of the tychonic, see p. ) far outweighs the copernican or heliocentric one. at her feet is the ptolemaic sphere, while ptolemy himself half lies, half sits, between her and argus, with the comment issuing from his mouth: "i will arise if only i am corrected." his left hand rests upon the coat of arms of the prince of monaco to whom the _almagestum novum_ is dedicated. at the top is the hebrew _yah-veh_, and the hand of god is stretched forth in reference to the verse in the book of wisdom ( : ): "but thou hast ordered all things in measure, and number and weight." contents illustrations preface part i. an historical sketch of the heliocentric theory of the universe. chapter i. the development of astronomical thought to : preliminary review chapter ii. copernicus and his times chapter iii. later development and scientific defense of the copernican theory part ii. the reception of the copernican theory. chapter i. opinions and arguments in the sixteenth century chapter ii. bruno and galileo chapter iii. the opposition and their arguments chapter iv. the gradual acceptance of the copernican theory chapter v. the church and the new astronomy: conclusion appendices: translations by the writer. a. ptolemy: _almagest_. bk. i, chap. : that the earth has no movement of rotation b. copernicus: _de revolutionibus_, dedication to the pope c. bodin: _universæ naturæ theatrum_, bk. v, sections and in part, and section entire d. fienus: _epistolica quæstio_: is it true that the heavens are moved and the earth is at rest? bibliography index illustrations facsimile of the frontispiece "the systems of the world" in riccioli: _almagestum novum_, _frontispiece_ photographic facsimile (reduced) of a page from a copy of copernicus: _de revolutionibus_, as "corrected" in the th century according to the directions of the congregations of the index in p. photographic facsimile (reduced) of another "corrected" page from the same copy p. preface this study does not belong in the field of astronomy, but in that of the history of thought; for it is an endeavor to trace the changes in people's beliefs and conceptions in regard to the universe as these were wrought by the dissolution of superstition resulting from the scientific and rationalist movements. the opening chapter is intended to do no more than to review briefly the astronomical theories up to the age of copernicus, in order to provide a background for the better comprehension of the work of copernicus and its effects. such a study has been rendered possible only by the generous loan of rare books by professor herbert d. foster of dartmouth college, professor edwin e. slosson of columbia university, doctor george a. plimpton and major george haven putnam, both of new york, and especially by the kindly generosity of professor david eugene smith of teachers college who placed his unique collection of rare mathematical books at the writer's disposal and gave her many valuable suggestions as to available material. professors james t. shotwell and harold jacoby of columbia university have read parts of this study in manuscript. the writer gratefully acknowledges her indebtedness not only to these gentlemen, but to the many others, librarians and their assistants, fellow-students and friends, too numerous to mention individually, whose ready interest and whose suggestions have been of real service, and above all to professor james harvey robinson at whose suggestion and under whose guidance the work was undertaken, and to the reverend doctor henry a. stimson whose advice and criticism have been an unfailing source of help and encouragement. part one an historical sketch of the heliocentric theory of the universe. chapter i. the development of astronomical thought to a.d. _a preliminary sketch of early theories as a background._ the appearances in the heavens have from earliest historic ages filled men with wonder and awe; then they gradually became a source of questioning, and thinkers sought for explanations of the daily and nightly phenomena of sun, moon and stars. scientific astronomy, however, was an impossibility until an exact system of chronology was devised.[ ] meanwhile men puzzled over the shape of the earth, its position in the universe, what the stars were and why the positions of some shifted, and what those fiery comets were that now and again appeared and struck terror to their hearts. [footnote : the earliest observation ptolemy uses is an egyptian one of an eclipse occurring march , b.c. (cumont: ). [in these references, the roman numerals refer to the volume, the arabic to the page, except as stated otherwise. the full title is given in the bibliography at the back under the author's name.]] in answer to such questions, the chaldean thinkers, slightly before the rise of the greek schools of philosophy, developed the idea of the seven heavens in their crystalline spheres encircling the earth as their center.[ ] this conception seems to lie back of both the later egyptian and hebraic cosmologies, as well as of the ptolemaic. through the visits of greek philosophers to egyptian shores this conception helped to shape greek thought and so indirectly affected western civilization. thus our heritage in astronomical thought, as in many other lines, comes from the greeks and the romans reaching europe (in part through arabia and spain), where it was shaped by the influence of the schools down to the close of the middle ages when men began anew to withstand authority in behalf of observation and were not afraid to follow whither their reason led them. [footnote : warren: . see "calendar" in hastings: _ency. of religion and ethics_.] but not all greek philosophers, it seems,[ ] either knew or accepted the babylonian cosmology.[ ] according to plutarch, though thales ( ?- ? b.c.) and later the stoics believed the earth to be spherical in form, anaximander ( - ? b.c.) thought it to be like a "smooth stony pillar," anaximenes ( th cent.) like a "table." beginning with the followers of thales or perhaps parmenides (?- b.c.), as diogenes laërtius claims,[ ] a long line of greek thinkers including plato ( ?- ? b.c.) and aristotle ( - b.c.) placed the earth in the center of the universe. whether plato held that the earth "encircled" or "clung" around the axis is a disputed point;[ ] but aristotle claimed it was the fixed and immovable center around which swung the spherical universe with its heaven of fixed stars and its seven concentric circles of the planets kept in their places by their transparent crystalline spheres.[ ] [footnote : for a summary of recent researches, see the preface of heath: _aristarchus of samos_. for further details, see heath: _op. cit._, and the writings of kugler and schiaparelli.] [footnote : see plutarch: _moralia: de placitas philosophorum_, lib. i et ii, (v. - , - ).] [footnote : diogenes laërtius: _de vitis_, lib. ix, c. ( ).] [footnote : plato: _timæus_, sec. (iii, in jowett's translation).] [footnote : aristotle: _de mundo_, c. et (iii, and ).] the stars were an even greater problem. anaximenes thought they were "fastened like nails" in a crystalline firmament, and others thought them to be "fiery plates of gold resembling pictures."[ ] but if the heavens were solid, how could the brief presence of a comet be explained? [footnote : plutarch: _op. cit._, lib. iii, c. (v, - ).] among the philosophers were some noted as mathematicians whose leader was pythagoras (c. b.c.). he and at least one of the members of his school, eudoxus ( ?- ? b.c.), had visited egypt, according to diogenes laërtius,[ ] and had in all probability been much interested in and influenced by the astronomical observations made by the egyptian priests. on the same authority, pythagoras was the first to declare the earth was round and to discuss the antipodes. he too emphasized the beauty and perfection of the circle and of the sphere in geometry, forms which became fixed for years as the fittest representations of the perfection of the heavenly bodies. [footnote : diogenes laërtius: _de vitis_, lib. viii, c. , et ( , ).] there was some discussion in diogenes' time as to the author of the theory of the earth's motion of axial rotation. diogenes[ ] gives the honor to philolaus ( th cent. b.c.) one of the pythagoreans, though he adds that others attribute it to icetas of syracuse ( th or th cent. b.c.). cicero, however, states[ ] the position of hicetas of syracuse as a belief in the absolute fixedness of all the heavenly bodies except the earth, which alone moves in the whole universe, and that its rapid revolutions upon its own axis cause the heavens apparently to move and the earth to stand still. [footnote : diogenes: _op. cit._, lib. viii, c. ( ).] [footnote : cicero: _academica_, lib. ii, c. ( ).] other thinkers of syracuse may also have felt the egyptian influence; for one of the greatest of them, archimedes (c. - b.c.), stated the theory of the earth's revolution around the sun as enunciated by aristarchus of samos. (perhaps this is the "hearth-fire of the universe" around which philolaus imagined the earth to whirl.[ ]) in _arenarius_, a curious study on the possibility of expressing infinite sums by numerical denominations as in counting the sands of the universe, archimedes writes:[ ] "for you have known that the universe is called a sphere by several astrologers, its center the center of the earth, and its radius equal to a line drawn from the center of the sun to the center of the earth. this was written for the unlearned, as you have known from the astrologers.... [aristarchus of samos][ ] concludes that the world is many times greater than the estimate we have just given. he supposes that the fixed stars and the sun remain motionless, but that the earth following a circular course, revolves around the sun as a center, and that the sphere of the fixed stars having the same sun as a center, is so vast that the circle which he supposes the earth to follow in revolving holds the same ratio to the distance of the fixed stars as the center of a sphere holds to its circumference." [footnote : plutarch: _op. cit._, lib. ii (v. - ).] [footnote : archimedes: _arenarius_, c. . delambre: _astr. anc._, i, .] [footnote : this is the only account of his system. even the age in which he flourished is so little known that there have been many disputes whether he was the original inventor of this system or followed some other. he was probably a contemporary of cleanthes the stoic in the rd century b.c. he is mentioned also by ptolemy, diogenes laërtius and vitruvius. (schiaparelli: _die vorlaufer des copernicus im alterthum_, . see also heath: _op. cit._)] these ancient philosophers realized in some degree the immensity of the universe in which the earth was but a point. they held that the earth was an unsupported sphere the size of which eratosthenes (c. - b.c.) had calculated approximately. they knew the sun was far larger than the earth, and cicero with other thinkers recognized the insignificance of earthly affairs in the face of such cosmic immensity. they knew too about the seven planets, had studied their orbits, and worked out astronomical ways of measuring the passage of time with a fair amount of accuracy. hipparchus and other thinkers had discovered the fact of the precession of the equinoxes, though there was no adequate theory to account for it until copernicus formulated his "motion of declination." the pythagoreans accepted the idea of the earth's turning upon its axis, and some even held the idea of its revolution around the motionless sun. others suggested that comets had orbits which they uniformly followed and therefore their reappearance could be anticipated.[ ] [footnote : plutarch: _op. cit._: bk. iii, c. (v, - ).] why then was the heliocentric theory not definitely accepted? in the first place, such a theory was contrary to the supposed facts of daily existence. a man did not have to be trained in the schools to observe that the earth seemed stable under his feet and that each morning the sun swept from the east to set at night in the west. sometimes it rose more to the north or to the south than at other times. how could that be explained if the sun were stationary? study of the stars was valuable for navigators and for surveyors, perhaps, but such disturbing theories should not be propounded by philosophers. cleanthes,[ ] according to plutarch,[ ] "advised that the greeks ought to have prosecuted aristarchus the samian for blasphemy against religion, as shaking the very foundations of the world, because this man endeavoring to save appearances, supposed that the heavens remained immovable and that the earth moved through an oblique circle, at the same time turning about its own axis." few would care to face their fellows as blasphemers and impious thinkers on behalf of an unsupported theory. eighteen hundred years later galileo would not do so, even though in his day the theory was by no means unsupported by observation. [footnote : the stoic contemporary of aristarchus, author of the famous stoic hymn. see diogenes laërtius: _de vitis_.] [footnote : plutarch: _de facie in orbe lunæ_, (v, ).] furthermore, one of the weaknesses of the greek civilization militated strongly against the acceptance of this hypothesis so contrary to the evidence of the senses. experimentation and the development of applied science was practically an impossibility where the existence of slaves made manual labor degrading and shameful. men might reason indefinitely; but few, if any, were willing to try to improve the instruments of observation or to test their observations by experiments. at the same time another astronomical theory was developing which was an adequate explanation for the phenomena observed up to that time.[ ] this theory of epicycles and eccentrics worked out by apollonius of perga (c. b.c.) and by hipparchus (c. b.c.) and crystallized for posterity in ptolemy's great treatise on astronomy, the _almagest_, (c. a.d.) became the fundamental principle of the science until within the last three hundred years. the theory of the eccentric was based on the idea that heavenly bodies following circular orbits revolved around a center that did not coincide with that of the observer on the earth. that would explain why the sun appeared sometimes nearer the earth and sometimes farther away. the epicycle represented the heavenly body as moving along the circumference of one circle (called the epicycle) the center of which moves on another circle (the deferent). with better observations additional epicycles and eccentric were used to represent the newly observed phenomena till in the later middle ages the universe became a "----sphere with centric and eccentric scribbled o'er, cycle and epicycle, orb in orb"--[ ] [footnote : young: .] [footnote : milton: _paradise lost_, bk. viii, ll. - .] yet the heliocentric theory was not forgotten. vitruvius, a famous roman architect of the augustan age, discussing the system of the universe, declared that mercury and venus, the planets nearest the sun, moved around it as their center, though the earth was the center of the universe.[ ] this same notion recurs in martianus capella's book[ ] in the fifth century a.d. and again, somewhat modified, in the th century in tycho brahe's conception of the universe. [footnote : vitruvius: _de architectura_, lib. ix, c. ( ).] [footnote : martianus capella: _de nuptiis_, lib. viii, ( ).] ptolemy devotes a column or two of his _almagest_[ ] (to use the familiar arabic name for his _syntaxis mathematica_) to the refutation of the heliocentric theory, thereby preserving it for later ages to ponder on and for a copernicus to develop. he admits at the outset that such a theory is only tenable for the stars and their phenomena, and he gives at least three reasons why it is ridiculous. if the earth were not at the center, the observed facts of the seasons and of day and night would be disturbed and even upset. if the earth moves, its vastly greater mass would gain in speed upon other bodies, and soon animals and other lighter bodies would be left behind unsupported in the air--a notion "ridiculous to the last degree," as he comments, "even to imagine it." lastly, if it moves, it would have such tremendous velocity that stones or arrows shot straight up in the air must fall to the ground east of their starting point,--a "laughable supposition" indeed to ptolemy. [footnote : ptolemy: _almagest_, lib. i, c. , ( , - ). translated in appendix b.] this book became the great text of the middle ages; its author's name was given to the geocentric theory it maintained. astronomy for a thousand years was valuable only to determine the time of easter and other festivals of the church, and to serve as a basis for astrology for the mystery-loving people of europe. to the arabians in syria and in spain belongs the credit of preserving for europe during this long period the astronomical works of the greeks, to which they added their own valuable observations of the heavens--valuable because made with greater skill and better instruments,[ ] and because with these observations later scientists could illustrate the permanence or the variability of important elements. they also discovered the so-called "trepidation" or apparent shifting of the fixed stars to explain which they added another sphere to ptolemy's eight. early in the sixth century uranus translated aristotle's works into syrian, and this later was translated into arabic.[ ] albategnius[ ] (c. - [transcriber's note: ] a.d.), the arabian prince who was the greatest of all their astronomers, made his observations from aracte and damascus, checking up and in some cases amending ptolemy's results.[ ] [footnote : whewell: i, .] [footnote : whewell: i, .] [footnote : berry: .] [footnote : his book _de motu stellarum_, translated into latin by plato tiburtinus (fl. ) was published at nuremberg ( ) by melancthon with annotations by regiomontanus. _ency. brit._ th. edit.] then the center of astronomical development shifted from syria to spain and mainly through this channel passed on into western europe. the scientific fame of alphonse x of castile ( - a.d.) called the wise, rests chiefly upon his encouragement of astronomy. with his support the alfonsine tables were calculated. he is said[ ] to have summoned fifty learned men from toledo, cordova and paris to translate into spanish the works of ptolemy and other philosophers. under his patronage the university of salamanca developed rapidly to become within two hundred years one of the four great universities of europe[ ]--a center for students from all over europe and the headquarters for new thought, where columbus was sheltered,[ ] and later the copernican system was accepted and publicly taught at a time when galileo's views were suppressed.[ ] [footnote : vaughan: i, .] [footnote : graux: .] [footnote : graux: .] [footnote : rashdall: ii, pt. i, .] popular interest in astronomy was evidently aroused, for sacrobosco (to give john holywood[ ] his better known latin name) a scotch professor at the sorbonne in paris in the th century, published a small treatise _de sphæri mundo_ that was immensely popular for centuries,[ ] though it was practically only an abstract of the _almagest_. whewell[ ] tells of a french poem of the time of edward i entitled _ymage du monde_, which gave the ptolemaic view and was illustrated in the manuscript in the university of cambridge with a picture of the spherical earth with men upright on it at every point, dropping balls down perforations in the earth to illustrate the tendency of all things toward the center. of the same period ( th century) is an arabian compilation in which there is a reference to another work, the book of hammarmunah the old, stating that "the earth turns upon itself in the form of a circle, and that some are on top, the others below ... and there are countries in which it is constantly day or in which at least the night continues only some instants."[ ] apparently, however, such advanced views were of no influence, and the ptolemaic theory remained unshaken down to the close of the th century. [footnote : _dict. of nat. biog._] [footnote : mss. of it are extremely numerous. it was the second astronomical book to be printed, the first edition appearing at ferrara in . editions appeared before . it was translated into italian, french, german, and spanish, and had many commentators. _dict. of nat. biog._] [footnote : whewell: i, .] [footnote : blavatski: ii, , note.] aside from the adequacy of this explanation of the universe for the times, the attitude of the church fathers on the matter was to a large degree responsible for this acquiescence. early in the first century a.d., philo judæus[ ] emphasized the minor importance of visible objects compared with intellectual matters,--a foundation stone in the church's theory of an homocentric universe. clement of alexandria (c. a.d.) calls the heavens solid since what is solid is capable of being perceived by the senses.[ ] origen (c. -c. .) has recourse to the holy scriptures to support his notion that the sun, moon, and stars are living beings obeying god's commands.[ ] then lactantius thunders against those who discuss the universe as comparable to people discussing "the character of a city they have never seen, and whose name only they know." "such matters cannot be found out by inquiry."[ ] the existence of the antipodes and the rotundity of the earth are "marvelous fictions," and philosophers are "defending one absurd opinion by another"[ ] when in explanation why bodies would not fall off a spherical earth, they claim these are borne to the center. [footnote : philo judæus: _quis rerum divinarum hæres._ (iv, ).] [footnote : clement of alexandria: _stromatum_, lib. v, c. , (iii, ).] [footnote : origen: _de principiis_, lib. i, c. , (xi, ).] [footnote : lactantius: _divinarum institutionum_, lib. iii, c. (vi, ).] [footnote : ibid: lib. iii, c. , (vi, - ).] how clearly even this brief review illustrates what henry osborn taylor calls[ ] the fundamental principles of patristic faith: that the will of god is the one cause of all things (voluntate dei immobilis manet et stat in sæculum terra.[ ] ambrose: _hexæmeron_.) and that this will is unsearchable. he further points out that augustine's and ambrose's sole interest in natural fact is as "confirmatory evidence of scriptural truth." the great augustine therefore denies the existence of antipodes since they could not be peopled by adam's children.[ ] he indifferently remarks elsewhere:[ ] "what concern is it to me whether the heavens as a sphere enclose the earth in the middle of the world or overhang it on either side?" augustine does, however, dispute the claims of astrologers accurately to foretell the future by the stars, since the fates of twins or those born at the same moment are so diverse.[ ] [footnote : taylor: _mediæval mind_, i, .] [footnote : by the will of god the earth remains motionless and stands throughout the age.] [footnote : augustine: _de civitate dei_, lib. xvi, c. , ( , p. ).] [footnote : augustine: _de genesi_, ii, c. , (v. , p. ). (white's translation).] [footnote : augustine: _civitate dei_, lib. v, c. , (v. , p. ).] philastrius (d. before a.d.) dealing with various heresies, denounces those who do not believe the stars are fixed in the heavens as "participants in the vanity of pagans and the foolish opinions of philosophers," and refers to the widespread idea of the part the angels play in guiding and impelling the heavenly bodies in their courses.[ ] [footnote : philastrius: _de hæresibus_, c. , (v. , p. ).] it would take a brave man to face such attitudes of scornful indifference on the one hand and denunciation on the other, in support of a theory the church considered heretical. meanwhile the church was developing the homocentric notion which would, of course, presuppose the central position in the universe for man's abiding place. in the pseudo-dionysius[ ] is an elaborately worked out hierarchy of the beings in the universe that became the accepted plan of later centuries, best known to modern times through dante's blending of it with the ptolemaic theory in the _divine comedy_.[ ] isidore of seville taught that the universe was created to serve man's purposes,[ ] and peter lombard ( th cent.) sums up the situation in the definite statement that man was placed at the center of the universe to be served by that universe and in turn himself to serve god.[ ] supported by the mighty thomas aquinas[ ] this became a fundamental church doctrine. [footnote : pseudo-dionysius: _de coelesti ierarchia_, (v. , p. ).] [footnote : milman: viii, p. - . see the _paradiso_.] [footnote : isidore of seville: _de ordine creaturarum_, c. , sec. , (v. , p. ).] [footnote : lombard: _sententia_, bk. ii, dist. i, sec. , (v. , p. ).] [footnote : aquinas: _summa theologica_, pt. i, qu. , art. . (_op. om. caietani_, v, ).] an adequate explanation of the universe existed. aristotle, augustine, and the other great authorities of the middle ages, all upheld the conception of a central earth encircled by the seven planetary spheres and by the all embracing starry firmament. in view of the phrases used in the bible about the heavens, and in view of the formation of fundamental theological doctrines based on this supposition by the church fathers, is it surprising that any other than a geocentric theory seemed untenable, to be dismissed with a smile when not denounced as heretical? small wonder is it, in the absence of the present day mechanical devices for the exact measurement of time and space as aids to observation, that the ptolemaic, or geocentric, theory of the universe endured through centuries as it did, upheld by the authority both of the church and, in essence at least, by the great philosophers whose works constituted the teachings of the schools. chapter ii. copernicus and his times. during these centuries, one notable scholar at least stood forth in open hostility to the slavish devotion to aristotle's writings and with hearty appreciation for the greater scientific accuracy of "infidel philosophers among the arabians, hebrews and greeks."[ ] in his _opus tertium_ ( ), roger bacon also pointed out how inaccurate were the astronomical tables used by the church, for in , according to these tables "christians will fast the whole week following the true easter, and will eat flesh instead of fasting at quadragesima for a week--which is absurd," and thus christians are made foolish in the eyes of the heathen.[ ] even the rustic, he added, can observe the phases of the moon occurring a week ahead of the date set by the calendar.[ ] bacon's protests were unheeded, however, and the church continued using the old tables which grew increasingly inaccurate with each year. pope sixtus iv sought to reform the calendar two centuries later with the aid of regiomontanus, then the greatest astronomer in europe ( );[ ] the lateran council appealed to copernicus for help ( ), but little could be done, as copernicus replied, till the sun's and the moon's positions had been observed far more precisely;[ ] and the modern scientific calendar was not adopted until under pope gregory xiii. [footnote : roger bacon: _opus tertium_, , - .] [footnote : ibid: .] [footnote : ibid: .] [footnote : delambre: _moyen age_, .] [footnote : prowe: ii, - .] what was the state of astronomy in the century of copernicus's birth? regiomontanus--to use johann müller's latin name--his teacher pürbach, and the great cardinal nicolas of cues were the leading astronomers of this fifteenth century. pürbach[ ] ( - ) died before he had fulfilled the promise of his youth, leaving his _epitome of ptolemy's almagest_ to be completed by his greater pupil. in his _theorica planetarum_ ( ) pürbach sought to explain the motions of the planets by placing each planet between the walls of two curved surfaces with just sufficient space in which the planet could move. as m. delambre remarked:[ ] "these walls might aid the understanding, but one must suppose them transparent; and even if they guided the planet as was their purpose, they hindered the movement of the comets. therefore they had to be abandoned, and in our own modern physics they are absolutely superfluous; they have even been rather harmful, since they interfered with the slight irregularities caused by the force of attraction in planetary movements which observations have disclosed." this scheme gives some indication of the elaborate devices scholars evolved in order to cope with the increasing number of seeming irregularities observed in "the heavens," and perhaps it makes clearer why copernicus was so dissatisfied with the astronomical hypothesis of his day, and longed for some simpler, more harmonious explanation. [footnote : delambre: _moyen age_, - .] [footnote : delambre: _moyen age_, .] regiomontanus[ ] ( - ) after pürbach's death, continued his work, and his astronomical tables (pub. ) were in general use throughout europe till superseded by the vastly more accurate copernican tables a century later. it has been said[ ] that his fame inspired copernicus (born three years before the other's death in ) to become as great an astronomer. m. delambre hails him as the wisest astronomer europe had yet produced[ ] and certainly his renown was approached only by that of the great cardinal. [footnote : it has been claimed that regiomontanus knew of the earth's motion around the sun a hundred years before copernicus; but a german writer has definitely disproved this claim by tracing it to its source in schöner's _opusculum geographicum_ ( ) which states only that he believed in the earth's axial rotation. ziegler: .] [footnote : ibid: .] [footnote : delambre: _op. cit._: .] both janssen,[ ] the catholic historian, and father hagen[ ] of the vatican observatory, together with many other catholic writers, claim that a hundred years before copernicus, cardinal nicolas cusanus[ ] (c. - ) had the courage and independence to uphold the theory of the earth's motion and its rotation on its axis. as father hagen remarked: "had copernicus been aware of these assertions he would probably have been encouraged by them to publish his own monumental work." but the cardinal stated these views of the earth's motions in a mystical, hypothetical way which seems to justify the marginal heading "paradox" (in the edition of ).[ ] and unfortunately for these writers, the jesuit father, riccioli, the official spokesman of that order in the th century after galileo's condemnation, speaking of this paradox, called attention, also, to a passage in one of the cardinal's sermons as indicating that the latter had perhaps "forgotten himself" in the _de docta ignorantia_, or that this paradox "was repugnant to him, or that he had thought better of it."[ ] the passage he referred to is as follows: "prayer is more powerful than all created things. although angels, or some kind of beings, move the spheres, the sun and the stars; prayer is more powerful than they are, since it impedes motion, as when the prayer of joshua made the sun stand still."[ ] this may explain why copernicus apparently disregarded the cardinal's paradox, for he made no reference to it in his book; and the statement itself, to judge by the absence of contemporary comment, aroused no interest at the time. but of late years, the cardinal's position as stated in the _de docta ignorantia_ has been repeatedly cited as an instance of the church's friendly attitude toward scientific thought,[ ] to show that galileo's condemnation was due chiefly to his "contumacy and disobedience." [footnote : janssen: _hist. of ger._, i, .] [footnote : _cath. ency._: "cusanus."] [footnote : from cues near treves.] [footnote : cusanus: _de docta ignorantia_, bk. ii, c. - : "centrum igitur mundi, coincideret cum circumferentiam, nam si centrum haberet et circumferentiam, et sic intra se haberet suum initium et finem et esset ad aliquid aliud ipse mundus terminatus, et extra mundum esset aluid et locus, quæ omnia veritate carent. cum igitur non sit possibile, mundum claudi intra centrum corporale et circumferentiam, non intelligitur mundus, cuius centrum et circumferentia sunt deus: et cum hic non sit mundus infinitus, tamen non potest concipi finitus, cum terminis careat, intra quos claudatur. terra igitur, quæ centrum esse nequit, motu omni carere non potest, nam eam moveri taliter etiam necesse est, quod per infinitum minus moveri posset. sicut igitur terra non est centram mundi.... unde licet terra quasi stella sit, propinquior polo centrali, tamen movetur, et non describit minimum circulum in motu, ut est ostensum.... terræ igitur figura est mobilis et sphærica et eius motus circularis, sed perfectior esse posset. et quia maximum in perfectionibus motibus, et figuris in mundo non est, ut ex iam dictis patent: tunc non est verum quod terra ista sit vilissima et infima, nam quamvis videatur centralior, quo'ad mundum, est tamen etiam, eadem ratione polo propinquior, ut est dictum." (pp. - ).] [footnote : riccioli: _alm. nov._, ii, .] [footnote : cusanus: _opera_, : excitationum, lib. vii, ex sermone: _debitores sumus_: "est enim oratio, omnibus creaturis potentior. nam angeli seu intelligentiæ, movent orbes, solem et stellas: sed oratio potentior, quia impedit motum, sicut oratio josuæ, fecit sistere solem."] [footnote : di bruno: , a; walsh: _an early allusion_, - .] copernicus[ ] himself was born in thorn on february , ,[ ] seven years after that hansa town founded by the teutonic order in had come under the sway of the king of poland by the second peace of thorn.[ ] his father,[ ] niklas koppernigk, was a wholesale merchant of cracow who had removed to thorn before , married barbara watzelrode of an old patrician thorn family, and there had served as town councillor for nineteen years until his death in .[ ] thereupon his mother's brother, lucas watzelrode, later bishop of ermeland, became his guardian, benefactor and close friend.[ ] [footnote : _nicolaus coppernicus_ (berlin, - ; vol.; pt. i, biography, pt. ii, sources), by dr. leopold prowe gives an exhaustive account of all the known details in regard to copernicus collected from earlier biographers and tested most painstakingly by the documentary evidence dr. prowe and his fellow-workers unearthed during a lifetime devoted to this subject. (_allgemeine deutsche biographie._) the manuscript authority dr. prowe cites (prowe: i, - and footnotes), requires the double p in copernicus's name, as copernicus himself invariably used the two p's in the latinized form _coppernic_ without the termination _us_, and usually when this termination was added. also official records and the letters from his friends usually give the double p; though the name is found in many variants--koppernig, copperinck, etc. his signatures in his books, his name in the letter he published in , and the latin form of it used by his friends all bear testimony to his use of the double p. but custom has for so many centuries sanctioned the simpler spelling, that it seems unwise not to conform in this instance to the time-honored usage.] [footnote : prowe: i, .] [footnote : _ency. brit._: "thorn."] [footnote : prowe: i, - .] [footnote : these facts would seem to justify the poles today in claiming copernicus as their fellow-countryman by right of his father's nationality and that of his native city. dr. prowe, however, claims him as a "prussian" both because of his long residence in the prussian-polish bishopric of ermeland, and because of copernicus's own reference to prussia as "unser lieber vaterland." (prowe: ii, .)] [footnote : prowe: i, - .] after the elementary training in the thorn school,[ ] the lad entered the university at cracow, his father's former home, where he studied under the faculty of arts from - .[ ] nowhere else north of the alps at this time were mathematics and astronomy in better standing than at this university.[ ] sixteen teachers taught these subjects there during the years of copernicus's stay, but no record exists of his work under any of them.[ ] that he must have studied these two sciences there, however, is proved by rheticus's remark in the _narratio prima_[ ] that copernicus, after leaving cracow, went to bologna to work with dominicus maria di novara "non tarn discipulus quam adjutor." he left cracow without receiving a degree,[ ] returned to thorn in when he and his family decided he should enter the church after first studying in italy.[ ] consequently he crossed the alps in and was that winter matriculated at bologna in the "german nation."[ ] the following summer he received word of his appointment to fill a vacancy among the canons of the cathedral chapter at ermeland where his uncle had been bishop since .[ ] he remained in italy, however, about ten years altogether, studying civil law at bologna, and canon law and medicine at padua,[ ] yet receiving his degree as doctor of canon law from the university of ferrara in .[ ] he was also in rome for several months during the jubilee year, . [footnote : ibid: i, .] [footnote : ibid: i, - .] [footnote : ibid: i, .] [footnote : ibid: i, - .] [footnote : rheticus: _narratio prima_, (thorn edit.).] [footnote : prowe: i, .] [footnote : ibid: i, .] [footnote : ibid: i, .] [footnote : ibid: i, . this insured him an annual income which amounted to a sum equalling about $ today. later he received a sinecure appointment besides at breslau. (holden in _pop. sci._, .)] [footnote : prowe: i, .] [footnote : ibid: i, .] at this period the professor of astronomy at bologna was the famous teacher dominicus maria di novara ( - ), a man "ingenio et animo liber" who dared to attack the immutability of the ptolemaic system, since his own observations, especially of the pole star, differed by a degree and more from the traditional ones.[ ] he dared to criticise the long accepted system and to emphasize the pythagorean notion of the underlying harmony and simplicity in nature[ ]; and from him copernicus may have acquired these ideas, for whether they lived together or not in bologna, they were closely associated. it was here, too, that copernicus began his study of greek which later was to be the means[ ] of encouraging him in his own theorizing by acquainting him with the ancients who had thought along similar lines. [footnote : ibid: i, and note. little is known about him today, except that he was primarily an observer, and was highly esteemed by his immediate successors; see gilbert: _de magnete_.] [footnote : clerke in _ency. brit._, "novara."] [footnote : prowe: i, .] in the spring of the year ( ) following his visit to rome,[ ] copernicus returned to the chapter at frauenburg to get further leave of absence to study medicine at the university of padua.[ ] whether he received a degree at padua or not and how long he stayed there are uncertain points.[ ] he was back in ermeland early in . [footnote : prowe: i, .] [footnote : ibid, .] [footnote : ibid: i, .] his student days were ended. and now for many years he led a very active life, first as companion and assistant to his uncle the bishop, with whom he stayed at schloss heilsberg till after the bishop's death in ; then as one of the leading canons of the chapter at frauenburg, where he lived most of the rest of his life.[ ] as the chapter representative for five years (at intervals) he had oversight of the spiritual and temporal affairs of two large districts in the care of the chapter.[ ] he went on various diplomatic and other missions to the king of poland,[ ] to duke albrecht of the teutonic order,[ ] and to the councils of the german states.[ ] he wrote a paper of considerable weight upon the much needed reform of the prussian currency.[ ] his skill as a physician was in demand not only in his immediate circle[ ] but in adjoining countries, duke albrecht once summoning him to königsberg to attend one of his courtiers.[ ] he was a humanist as well as a catholic churchman, and though he did not approve of the protestant revolt, he favored reform and toleration.[ ] gassendi claims that he was also a painter, at least in his student days, and that he painted portraits well received by his contemporaries.[ ] but his interest and skill in astronomy must have been recognized early in his life for in the committee of the lateran council in charge of the reform of the calendar summoned him to their aid.[ ] [footnote : prowe: i, - .] [footnote : ibid: ii, - , , .] [footnote : ibid: ii, - .] [footnote : ibid: ii, .] [footnote : ibid: ii, .] [footnote : ibid: ii, .] [footnote : ibid: ii, - .] [footnote : ibid: ii, - .] [footnote : ibid: ii, - .] [footnote : holden in _pop. sci._, .] [footnote : prowe: ii, - .] he was no cloistered monk devoting all his time to the study of the heavens, but a cultivated man of affairs, of recognized ability in business and statesmanship, and a leader among his fellow canons. his mathematical and astronomical pursuits were the occupations of his somewhat rare leisure moments, except perhaps during the six years with his uncle in the comparative freedom of the bishop's castle, and during the last ten or twelve years of his life, after his request for a coadjutor had resulted in lightening his duties. in his masterwork _de revolutionibus_[ ] there are recorded only of his own astronomical observations, and these extend over the years from to . the first was made at bologna, the second at rome in , and seven of the others at frauenburg, where the rest were also probably made. it is believed the greater part of the _de revolutionibus_ was written at heilsburg[ ] where copernicus was free from his chapter duties, for as he himself says[ ] in the dedication to the pope (dated ) his work had been formulated not merely nine years but for "more than three nines of years." it had not been neglected all this time, however, as the original ms. (now in the prague library) with its innumerable changes and corrections shows how continually he worked over it, altering and correcting the tables and verifying his statements.[ ] [footnote : copernicus: _de revolutionibus_, thorn edit., . the last two words of the full title: _de revolutionibus orbium coelestium_ are not on the original ms. and are believed to have been added by osiander. prowe: ii, , note.] [footnote : ibid: ii, - .] [footnote : copernicus: dedication, . (thorn edit.)] [footnote : prowe: ii, - .] copernicus was a philosopher.[ ] he thought out a new explanation of the world machine with relatively little practical work of his own,[ ] though we know he controlled his results by the accumulated observations of the ages.[ ] his instruments were inadequate, inaccurate and out of date even in his time, for much better ones were then being made at nürnberg[ ]; and the cloudy climate of ermeland as well as his own active career prevented him from the long-continued, painstaking observing, which men like tycho brahe were to carry on later. despite such handicaps, because of his dissatisfaction with the complexities and intricacies of the ptolemaic system and because of his conviction that the laws of nature were simple and harmonious, copernicus searched the writings of the classic philosophers, as he himself tells us,[ ] to see what other explanation of the universe had been suggested. "and i found first in cicero that a certain nicetas had thought the earth moved. later in plutarch i found certain others had been of the same opinion." he quoted the greek referring to philolaus the pythagorean, heraclides of pontus, and ecphantes the pythagorean.[ ] as a result he began to consider the mobility of the earth and found that such an explanation seemingly solved many astronomical problems with a simplicity and a harmony utterly lacking in the old traditional scheme. unaided by a telescope, he worked out in part the right theory of the universe and for the first time in history placed all the then known planets in their true positions with the sun at the center. he claimed that the earth turns on its axis as it travels around the sun, and careens slowly as it goes, thus by these three motions explaining many of the apparent movements of the sun and the planets. he retained,[ ] however, the immobile heaven of the fixed stars (though vastly farther off in order to account for the non-observance of any stellar parallax[ ]), the "perfect" and therefore circular orbits of the planets, certain of the old eccentrics, and new epicycles in place of all the old ones which he had cast aside.[ ] he accepted the false notion of trepidation enunciated by the arabs in the th century and later overthrown by tycho brahe.[ ] his calculations were weak.[ ] but his great book is a sane and modern work in an age of astrology and superstition.[ ] his theory is a triumph of reason and imagination and with its almost complete independence of authority is perhaps as original a work as an human being may be expected to produce. [footnote : ibid: ii, .] [footnote : ibid: ii, - .] [footnote : rheticus: _narratio prima_.] [footnote : prowe: ii, .] [footnote : copernicus: dedication, - . see appendix b.] [footnote : for a translation of this dedication in full, see appendix b. in the original ms. occurs a reference (struck out) to aristarchus of samos as holding the theory of the earth's motion. (prowe: ii, , note.) the finding of this passage proves that copernicus had at least heard of aristarchus, but his apparent indifference is the more strange since an account of his teaching occurs in the same book of plutarch from which copernicus learned about philolaus. but the chief source of our knowledge about aristarchus is through archimedes, and the editio princeps of his works did not appear till , a year after the death of copernicus. c.r. eastman in _pop. sci._ : .] [footnote : delambre: _astr. mod._ pp. xi-xii.] [footnote : as the earth moves, the position in the heavens of a fixed star seen from the earth should differ slightly from its position observed six months later when the earth is on the opposite side of its orbit. the distance to the fixed stars is so vast, however, that this final proof of the earth's motion was not attained till when bessel ( - ) observed stellar parallax from königsberg. berry: - .] [footnote : _commentariolus_ in prowe: iii, .] [footnote : holden in _pop. sci._, .] [footnote : delambre: _astr. mod._, p. xi.] [footnote : snyder: .] copernicus was extremely reluctant to publish his book because of the misunderstandings and malicious attacks it would unquestionably arouse.[ ] possibly, too, he was thinking of the hostility already existing between himself and his bishop, dantiscus,[ ] whom he did not wish to antagonize further. but his devoted pupil and friend, rheticus, aided by tiedeman giese, bishop of culm and a lifelong friend, at length ( ) persuaded him.[ ] so he entrusted the matter to giese who passed it on to rheticus, then connected with the university at wittenberg as professor of mathematics.[ ] rheticus, securing leave of absence from melancthon his superior, went to nürnberg to supervise the printing.[ ] this was done by petrejus. upon his return to wittenberg, rheticus left in charge johann schöner, a famous mathematician and astronomer, and andreas osiander, a lutheran preacher interested in astronomy. the printed book[ ] was placed in copernicus's hands at frauenburg on may th, , as he lay dying of paralysis.[ ] [footnote : copernicus: dedication, .] [footnote : prowe: ii, - .] [footnote : ibid: ii, .] [footnote : ibid: ii, .] [footnote : ibid: ii, - .] [footnote : four other editions have since appeared; at basel, , amsterdam , warsaw , and thorn . for further details, see prowe: ii, - , and thorn edition pp. xii-xx. the edition cited in this study is the thorn one of .] [footnote : prowe: ii, - .] copernicus passed away that day in ignorance that his life's work appeared before the world not as a truth but as an hypothesis; for there had been inserted an anonymous preface "ad lectorem de hypothesibus huius opera" stating this was but another hypothesis for the greater convenience of astronomers.[ ] "neque enim necesse est eas hypotheses esse veras, imo ne verisimiles quidem, sed sufficit hoc unum, si calculum observationibus congruentem exhibeant."[ ] for years copernicus was thought to have written this preface to disarm criticism. kepler sixty years later ( ) called attention to this error,[ ] and quoted osiander's letters to copernicus and to rheticus of may, , suggesting that the system be called an hypothesis to avert attacks by theologians and aristotelians. he claimed that osiander had written the preface; but kepler's article never was finished and remained unpublished till .[ ] giese and rheticus of course knew that the preface falsified copernicus's work, and giese, highly indignant at the "impiety" of the printer (who he thought had written it to save himself from blame) wrote rheticus urging him to write another "præfatiunculus" purging the book of this falsehood.[ ] this letter is dated july , , and the book had appeared in april. apparently nothing was done and the preface was accepted without further challenge. [footnote : copernicus: _de revolutionibus_, i. "to the reader on the hypotheses of this book."] [footnote : "for it is not necessary that these hypotheses be true, nor even probable, but this alone is sufficient, if they show reasoning fitting the observations."] [footnote : kepler: _apologia tychonis contra ursum_ in _op. om._: i, - .] [footnote : prowe: ii, , note.] [footnote : ibid: ii, - .] it remains to ask whether people other than copernicus's intimates had known of his theory before . peucer, melancthon's nephew, declared copernicus was famous by ,[ ] and the invitation from the lateran council committee indicates his renown as early as . in vienna in [ ] there was found a _commentariolus_, or summary of his great work,[ ] written by copernicus for the scholars friendly to him. it was probably written soon after , and gives a full statement of his views following a series of seven axioms or theses summing up the new theory. this little book probably occasioned the order from pope clement vii in to widmanstadt to report to him on the new scheme.[ ] this widmanstadt did in the papal gardens before the pope with several of the cardinals and bishops, and was presented with a book as his reward. [footnote : ibid: ii, .] [footnote : ibid: ii, - .] [footnote : a second copy was found at upsala shortly afterwards, though for centuries its existence was unknown save for two slight references to such a book, one by gemma frisius, the other by tycho brahe. prowe: ii, .] [footnote : ibid: ii, - .] in , the cardinal bishop of capua, nicolas von schönberg, apparently with the intent to pave the way for the theory at rome, wrote for a report of it.[ ] it is not known whether the report was sent, and the cardinal died the following year. but that copernicus was pleased by this recognition is evident from the prominence he gave to the cardinal's letter, as he printed it in his book at the beginning, even before the dedication to the pope. [footnote : prowe: ii, , note.] the most widely circulated account at this time, however, was the _narratio prima_, a letter from georg joachim of rhaetia (better known as rheticus), written in october, , from frauenburg to johann schöner at nürnberg.[ ] rheticus,[ ] at twenty-five years of age professor of mathematics at wittenberg, had gone uninvited to frauenburg early that summer to visit copernicus and learn for himself more in detail about this new system. this was rather a daring undertaking, for not only were luther and melancthon outspoken in their condemnation of copernicus, but rheticus was going from wittenberg, the headquarters of the lutheran heresy, into the bishopric of ermeland where to the bishop and the king his overlord, the very name of luther was anathema. nothing daunted, rheticus departed for frauenberg and could not speak too highly of the cordial welcome he received from the old astronomer. he came for a few weeks, and remained two years to return to wittenberg as an avowed believer in the system and its first teacher and promulgator. not only did he write the _narratio prima_ and an _encomium borussæ_, both extolling copernicus, but what is more important, he succeeded in persuading him to allow the publication of the _de revolutionibus_. rheticus returned to his post in , to resign it the next year and become dean of the faculty of arts. in all probability the conflict was too intense between his new scientific beliefs and the statements required of him as professor of the old mathematics and astronomy. [footnote : prowe: ii, - .] [footnote : ibid: ii, - .] his colleague, erasmus reinhold, continued to teach astronomy there, though he, too, accepted the copernican system.[ ] he published a series of tables (_tabulæ prutenicæ_, ) based on the copernican calculations to supersede the inaccurate ones by regiomontanus; and these were in general use throughout europe for the next seventy-odd years. as he himself declared, the series was based in its principles and fundamentals upon the observations of the famous nicolaus copernicus. the almanacs deduced from these calculations probably did more to bring the new system into general recognition and gradual acceptance than did the theoretical works.[ ] [footnote : ibid: ii, .] [footnote : holden in _pop. sci._, .] opposition to the theory had not yet gathered serious headway. there is record[ ] of a play poking fun at the system and its originator, written by the elbing schoolmaster (a dutch refugee from the inquisition) and given in by the villagers at elbing ( miles from frauenburg). elbing and ermeland were hostile to each other, copernicus was well known in elbing though probably from afar, for there seems to have been almost no personal intercourse between canons and people, and the spread of luther's teachings had intensified the hostility of the villagers towards the church and its representatives. but not until giordano bruno made the copernican system the starting-point of his philosophy was the roman catholic church seriously aroused to combat it. possibly osiander's preface turned opposition aside, and certainly the non-acceptance of the system as a whole by tycho brahe, the leading astronomer of europe at that time, made people slow to consider it. [footnote : prowe: ii, - .] chapter iii. the later development and scientific defense of the copernican system. copernicus accomplished much, but even his genius could not far outrun the times in which he lived. when one realizes that not only all the astronomers before him, but he and his immediate successor, tycho brahe, made all their observations and calculations unaided by even the simplest telescope, by logarithms or by pendulum clocks for accurate measurement of time,[ ] one marvels not at their errors, but at the greatness of their genius in rising above such difficulties. this lack of material aids makes the work of tycho brahe,[ ] accounted one of the greatest observers that has ever lived,[ ] as notable in its way perhaps as that of copernicus. [footnote : burckhardt: .] [footnote : the two standard lives of tycho brahe are the _vita tychonis brahei_ by gassendi ( ) till recently the sole source of information, and dreyer's _tycho brahe_ ( ) based not only on gassendi but on the documentary evidence disclosed by the researches of the th century. for tycho's works i have used the _opera omnia_ published at frankfort in . the danish royal scientific society has issued a reprint ( ) of the rare edition of the _de nova stella_.] [footnote : bridges: .] his life[ ] was a somewhat romantic one. born of noble family on december th, , at knudstrup in denmark, tyge brahe, the second of ten children,[ ] was early practically adopted by his father's brother. his family wished him to become a statesman and sent him in to the university at copenhagen to prepare for that career. a partial eclipse of the sun on august st, as foretold by the astronomers thrilled the lad and determined him to study a science that could foretell the future and so affect men's lives.[ ] when he was sent to leipsic with a tutor in to study law, he devoted his time and money to the study of mathematics and astronomy. two years later when eighteen years of age, he resolved to perform anew the task of hipparchos and ptolemy and make a catalogue of the stars more accurate than theirs. his family hotly opposed these plans; and for six years he wandered through the german states, now at wittenberg, now at rostock (where he fought the duel in which he lost part of his nose and had to have it replaced by one of gold and silver)[ ] or at augsburg--everywhere working on his chosen subjects. but upon his return to denmark ( ) he spent two years on chemistry and medicine, till the startling appearance of the new star in the constellation of cassiopæa (november, ) recalled him to what became his life work.[ ] [footnote : dreyer: - .] [footnote : gassendi: .] [footnote : dreyer: .] [footnote : gassendi: - .] [footnote : dreyer: - .] through the interest and favor of king frederick ii, he was given the island of hveen near elsinore, with money to build an observatory and the pledge of an annual income from the state treasury for his support.[ ] there at uraniborg from to he and his pupils made the great catalogue of the stars, and studied comets and the moon. when he was forced to leave hveen by the hostility and the economical tendencies of the young king,[ ] after two years of wandering he accepted the invitation of the emperor rudolphus and established himself at prague in bohemia. among his assistants at prague was young johann kepler who till tycho's death (on october , ) was his chief helper for twenty months, and who afterwards completed his observations, publishing the results in the rudolphine tables of . [footnote : ibid: .] [footnote : ibid: - .] this "phoenix among astronomers"--as kepler calls him,[ ]--was the father of modern practical astronomy.[ ] he also propounded a third system of the universe, a compromise between the ptolemaic and the copernican. in this the tychonic system,[ ] the earth is motionless and is the center of the orbits of the sun, the moon, and the sphere of the fixed stars, while the sun is the center of the orbits of the five planets.[ ] mercury and venus move in orbits with radii shorter than the sun's radius, and the other three planets include the earth within their circuits. this system was in harmony with the bible and accounted as satisfactorily by geometry as either of the other two systems for the observed phenomena.[ ] to tycho brahe, the ptolemaic system was too complex,[ ] and the copernican absurd, the latter because to account for the absence of stellar parallax it left vacant and purposeless a vast space between saturn and the sphere of the fixed stars,[ ] and because tycho's observations did not show any trace of the stellar parallax that must exist if the earth moves.[ ] [footnote : kepler: _tabulæ rudolphinæ_. title page.] [footnote : dreyer: - .] [footnote : as stated in his book on the comet of (pub. ).] [footnote : dreyer: - .] [footnote : schiaparelli in snyder: .] [footnote : brahe: _op. om._, pt. i, p. .] [footnote : ibid: - .] [footnote : the tychonic system has supporters to this day. see chap. viii.] though tycho thus rejected the copernican theory, his own proved to be the stepping stone toward the one he rejected,[ ] for by it and by his study of comets he completely destroyed the ideas of solid crystalline spheres to the discredit of the scholastics; and his promulgation of a third theory of the universe helped to diminish men's confidence in authority and to stimulate independent thinking. [footnote : dreyer: .] copernicus worked out his system by mathematics with but slight aid from his own observations. it was a theory not yet proven true. tycho brahe, though denying its validity, contributed in his mass of painstaking, accurate observations the raw material of facts to be worked up by kepler into the great laws of the planets attesting the fundamental truth of the copernican hypothesis. johann kepler[ ] earned for himself the proud title of "lawmaker for the universe" in defiance of his handicaps of ill-health, family troubles, and straitened finances.[ ] born in weil, wurtemberg, (december , ) of noble but indigent parents, he was a sickly child unable for years to attend school regularly. he finally left the monastery school in mulifontane in and entered the university at tübingen to stay for four and a half years. there he studied philosophy, mathematics, and theology (he was a lutheran) receiving the degree of master of arts in . while at the university he studied under mæstlin, professor of mathematics and astronomy, and a believer in the copernican theory. because of mæstlin's teaching kepler developed into a confirmed and enthusiastic adherent to the new doctrine. [footnote : the authoritative biography is the _vita_ by frisch in vol. viii, pp. - of _op. om. kep._] [footnote : frisch: viii, . [transcriber's note: missing footnote reference in original text has been added above in a logical place.]] in he reluctantly abandoned his favorite study, philosophy, and accepted a professorship in mathematics at grætz in styria. two years later he published his first work: _prodromus dissertationum continens mysterium cosmographicum_ etc. ( ) in which he sought to prove that the creator in arranging the universe had thought of the five regular bodies which can be inscribed in a sphere according to which he had regulated the order, the number and the proportions of the heavens and their movements.[ ] the book is important not only because of its novelty, but because it gave the copernican doctrine public explanation and defense.[ ] kepler himself valued it enough to reprint it with his _harmonia mundi_ twenty-five years later. and it won for him appreciative letters from various scientists, notably from tycho brahe and galileo.[ ] [footnote : delambre: _astr. mod._ - .] [footnote : frisch: viii, .] [footnote : ibid: viii, .] as kepler, a lutheran, was having difficulties in grætz, a catholic city, he finally accepted tycho's urgent invitation to come to prague.[ ] he came early in , and after some adjustments had been made between the two,[ ] he and his family settled with tycho that autumn to remain till the latter's death the following november. kepler himself then held the office of imperial mathematician by appointment for many years thereafter.[ ] [footnote : ibid: viii, - .] [footnote : dreyer: - .] [footnote : frisch: viii, .] with the researches of tycho's lifetime placed at his disposal, kepler worked out two of his three great planetary laws from tycho's observations of the planet mars. yet, as m. bertrand remarks,[ ] it was well for kepler that his material was not too accurate or its variations (due to the then unmeasured force of attraction) might have hindered him from proving his laws; and luckily for him the earth's orbit is so nearly circular that in calculating the orbit of mars to prove its elliptical form, he could base his work on the earth's orbit as a circle without vitiating his results for mars.[ ] that a planet's orbit is an ellipse and not the perfect circle was of course a triumph for the new science over the scholastics and aristotelians. but they had yet to learn what held the planets in their courses. [footnote : bertrand: p. - .] [footnote : the two laws first appeared in in his _physica coelestis tradita commentarius de motu stellæ martis_. (frisch: viii, .) the third he enunciated in his _harmonia mundi_, . (ibid: viii, - .)] from kepler's student days under mæstlin when as the subject of his disputation he upheld the copernican theory, to his death in , he was a staunch supporter of the new teaching.[ ] in his _epitome astronomiæ copernicanæ_ ( ) he answered objections to it at length.[ ] he took infinite pains to convert his friends to the new system. it was in vain that tycho on his deathbed had urged kepler to carry on their work not on the copernican but on the tychonic scheme.[ ] [footnote : "cor et animam meam": kepler's expression in regard to the copernician theory. ibid: viii, .] [footnote : ibid: viii, .] [footnote : ibid: viii, .] kepler had reasoned out according to physics the laws by which the planets moved.[ ] in italy at this same time galileo with his optic tube (invented ) was demonstrating that venus had phases even as copernicus had declared, that jupiter had satellites, and that the moon was scarred and roughened--ocular proof that the old system with its heavenly perfection in number ( planets) and in appearance must be cast aside. within a year after galileo's death newton was born[ ] (january , ). his demonstration of the universal application of the law of gravitation ( ) was perhaps the climax in the development of the copernican system. complete and final proof was adding in the succeeding years by roemer's ( - ) discovery of the velocity of light, by bradley's ( - ) study of its aberration,[ ] by bessel's discovery of stellar parallax in ,[ ] and by foucault's experimental demonstration of the earth's axial motion with a pendulum in .[ ] [footnote : kepler: _op. om._, i, : _præfatio ad lectorem_.] [footnote : berry: .] [footnote : berry: .] [footnote : ibid: .] [footnote : jacoby: .] part two the reception of the copernican theory. chapter i. opinions and arguments in the sixteenth century. during the lifetime of copernicus, roman catholic churchmen had been interested in his work: cardinal schönberg wrote for full information, widmanstadt reported on it to pope clement vii and copernicus had dedicated his book to pope paul iii.[ ] but after his death, the church authorities apparently paid little heed to his theory until some fifty years later when giordano bruno forced it upon their attention in his philosophical teachings. osiander's preface had probably blinded their eyes to its implications. [footnote : see before, p. .] the protestant leaders were not quite so urbane in their attitude. while copernicus was still alive, luther is reported[ ] to have referred to this "new astrologer" who sought to prove that the earth and not the firmament swung around, saying: "the fool will overturn the whole science of astronomy. but as the holy scriptures state, joshua bade the sun stand still and not the earth." melancthon was more interested in this new idea, perhaps because of the influence of rheticus, his colleague in the university of wittenberg and copernicus's great friend and supporter; but he too preferred not to dissent from the accepted opinion of the ages.[ ] informally in a letter to a friend he implies the absurdity of the new teaching,[ ] and in his _initia doctrinæ physicæ_ he goes to some pains to disprove the new assumption not merely by mathematics but by the bible, though with a kind of apology to other physicists for quoting the divine witness.[ ] he refers to the phrase in psalm xix likening the sun in its course "to a strong man about to run a race," proving that the sun moves. another psalm states that the earth was founded not to be moved for eternity, and a similar phrase occurs in the first chapter of ecclesiastes. then there was the miracle when joshua bade the sun stand still. while this is a sufficient witness to the truths there are other proofs: first, in the turning of a circumference, the center remains motionless. next, changes in the length of the day and of the seasons would ensue, were the position of the earth in the universe not central, and it would not be equidistant from the two poles. (he has previously disposed of infinity by stating that the heavens revolve around the pole, which could not happen if a line drawn from the center of the universe were infinitely projected).[ ] furthermore, the earth must be at the center for its shadow to fall upon the moon in an eclipse. he refers next to the aristotelian statement that to a simple body belongs one motion: the earth is a simple body; therefore it can have but one motion. what is true of the parts applies to the whole; all the parts of the earth are borne toward the earth and there rest; therefore the whole earth is at rest. quiet is essential to growth. lastly, if the earth moved as fast as it must if it moves at all, everything would fly to pieces.[ ] [footnote : luther: _tischreden_, iv, ; "der narr will die ganze kunst astronomiæ umkehren. aber wie die heilige schrift anzeigt, so heiss josua die sonne still stehen, und nicht das erdreich."] [footnote : "non est autem hominis bene instituti dissentire a consensu tot sæculorum." præfatio philippi melanthonis, , in sacro-busto: _libellus de sphæra_ (no date).] [footnote : "vidi dialogum et fui dissuassor editionis. fabula per sese paulatim consilescet; sed quidam putant esse egregiam _katorthoma_ rem tam absurdam ornare, sicut ille sarmaticus astronomis qui movet terram et figet solem. profecto sapientes gubernatores deberent ingeniorum petulantia cohercere." _epistola b. mithobio_, oct. . p. melancthon: _opera_: iv, .] [footnote : "quamquam autem rident aliqui physicum testimonia divina citantem, tamen nos honestum esse censemus, philosophiam conferre ad coelestia dicta, et in tanta caligine humanæ mentis autoritatem divinam consulere ubicunque possumus." melancthon: _initia doctrinæ physicæ_: bk. i, .] [footnote : ibid: .] [footnote : ibid: - .] melancthon thus sums up the usual arguments from the scriptures, from aristotle, ptolemy and the then current physics, in opposition to this theory. not only did he publish his own textbook on physics, but he republished sacrobosco's famous introduction to astronomy, writing for it a preface urging diligent study of this little text endorsed by so many generations of scholars. calvin, the great teacher of the protestant revolt, apparently was little touched by this new intellectual current.[ ] he did write a semi-popular tract[ ] against the so called "judicial" astrology, then widely accepted, which he, like luther, condemns as a foolish superstition, though he values "la vraie science d'astrologie" from which men understand not merely the order and place of the stars and planets, but the causes of things. in his _commentaries_, he accepts the miracle of the sun's standing still at joshua's command as proof of the faith christ commended, so strong that it will remove mountains; and he makes reference only to the time-honored ptolemaic theory in his discussion of psalm xix.[ ] [footnote : farrar: _hist. of interpretation_: preface, xviii: "who," asks calvin, "will venture to place the authority of copernicus above that of the holy spirit?"] [footnote : calvin: _oeuvres françois_: _traité ... contre l'astrologie_, - .] [footnote : calvin: _op. om._ in _corpus reformatorum_: vol. , - ; vol. , - .] for the absolute authority of the pope the protestant leaders substituted the absolute authority of the bible. it is not strange, then, that they ignored or derided a theory as yet unsupported by proof and so difficult to harmonize with a literally accepted bible. how widespread among the people generally did this theory become in the years immediately following the publication of the _de revolutionibus_? m. flammarion, in his _vie de copernic_ ( ), refers[ ] to the famous clock in the strasburg cathedral as having been constructed by the university of strasburg in protest against the action taken by the holy office against galileo, (though the clock was constructed in and galileo was not condemned until ). this astronomical clock constructed only thirty years after the death of copernicus, he claims represented the copernican system of the universe with the planets revolving around the sun, and explained clearly in the sight of the people what was the thought of the makers. lest no one should miscomprehend, he adds, the portrait of copernicus was placed there with this inscription: nicolai copernici vera effigies, ex ipsius autographo depicta. [footnote : p. - : "ce planétaire ... represente le système du monde tel qu'il a été expliqué par copernic."] this would be important evidence of the spread of the theory were it true. but m. flammarion must have failed to see a brief description of the strasburg clock written in by charles schwilgué, son of the man who renovated its mechanism in - . he describes the clock as it was before his father made it over and as it is today. originally constructed in , it was replaced in by an astrolabe based on the ptolemaic system; six hands with the zodiacal signs of the planets gave their daily movements and, together with a seventh representing the sun, revolved around a map of the world.[ ] when m. schwilgué repaired the clock in , he changed it to harmonize with the copernican system.[ ] [footnote : schwilgué: p. .] [footnote : ibid: p. .] but within eighteen years after the publication of the _de revolutionibus_, proof of its influence is to be found in such widely separated places as london and the great spanish university of salamanca. in , robert recorde, court physician to edward and to mary and teacher of mathematics, published in london his _castle of knowledge_, an introduction to astronomy and the first book printed in england describing the copernican system.[ ] he evidently did not consider the times quite ripe for a full avowal of his own allegiance to the new doctrine, but the remarks of the _maister_ and the _scholler_ are worth repeating:[ ] "maister: ... howbeit copernicus a man of great learning, of much experience, and of wonderfull diligence in observation, hath renewed the opinion of aristarchus samius, affirming that the earth, not onely moveth circularly about his owne centre, but also may be, yea and is, continually out of the precise centre of the world eight and thirty hundred thousand miles: but because the understanding of that controversie depends of profounder knowledge than in this introduction may be uttered conveniently, i wil let it passe til some other time. "scholler: nay sit, in good faith, i desire not to heare such vaine fantasies, so farre against the common reason, and repugnant to the content of all the learned multitude of writers, and therefore let it passe for ever and a day longer. "maister: you are too yong to be a good judge in so great a matter: it passeth farre your learning, and their's also, that are much better learned than you, to improuve his supposition by good arguments, and therefore you were best condemne nothing that you do not well understand: but an other time, as i saide, i will so declare his supposition, that you shall not onely wonder to heare it, but also peradventure be as earnest then to credite it, as you are now to condemne it: in the meane season let us proceed forward in our former order...." [footnote : _dict. of nat. biog._: "recorde."] [footnote : quoted (p. ), from the edition of in the library of mr. george a. plimpton. see also recorde's _whetstone of witte_ ( ) as cited by berry, .] this little book, reprinted in and in , and one of the most popular of the mathematical writings in england during that century, must have interested the english in the new doctrine even before bruno's emphatic presentation of it to them in the eighties. yet the english did not welcome it cordially. one of the most popular books of this period was sylvester's translation ( ) of dubartas's _the divine weeks_ which appeared in france in , a book loved especially by milton.[ ] dubartas writes:[ ] "those clerks that think--think how absurd a jest! that neither heavens nor stars do turn at all, nor dance around this great, round earthly ball, but the earth itself, this massy globe of our's, turns round about once every twice twelve hours! and we resemble land-bred novices new brought aboard to venture on the seas; who at first launching from the shore suppose the ship stands still and that the firm earth goes." [footnote : dubartas: _the divine weeks_ (sylvester's trans. edited by haight): preface, pp. xx-xxiii and note.] [footnote : _op. cit._: .] quite otherwise was the situation in the sixteenth century at the university of salamanca. a new set of regulations for the university, drawn up at the king's order by bishop covarrubias, was published in . it contained the provision in the curriculum that "mathematics and astrology are to be given in three years, the first, astrology, the second, euclid, ptolemy or copernicus _ad vota audientium_," which also indicates, as vicente de la fuente points out, that at this university "the choice of the subject-matter to be taught lay not with the teachers but with the students, a rare situation."[ ] one wonders what happened there when the professors and students received word[ ] from the cardinal nuncio at madrid in that the congregations of the index had decreed the copernican doctrine was thereafter in no way to be held, taught or defended. [footnote : la fuente: _historia de la universidades ... de españa_: ii, .] [footnote : _doc. _ in favaro: .] one of the graduates of this university, father zuñiga,[ ] (better known as didacus à stunica), wrote a commentary on job that was licensed to be printed in , but was not published until at toledo. another edition appeared at rome seven years later. it evidently was widely read for it was condemned _donec corrigatur_ by the index in and the mathematical literature of the next half century contains many allusions to his remarks on job: ix: ; "who shaketh the earth out of her place, and the pillars thereof tremble." after commenting here upon the greater clarity and simplicity of the copernican theory, didacus à stunica then states that the theory is not contradicted by solomon in ecclesiastes, as that "text signifieth no more but this, that although the succession of ages, and generations of men on earth be various, yet the earth itself is still one and the same, and continueth without any sensible variation" ... and "it hath no coherence with its context (as philosophers show) if it be expounded to speak of the earth's immobility. the motion that belongs to the earth by way of speech is assigned to the sun even by copernicus himself, and those who are his followers.... to conclude, no place can be produced out of holy scriptures which so clearly speaks the earth's immobility as this doth its mobility. therefore this text of which we have spoken is easily reconciled to this opinion. and to set forth the wonderful power and wisdom of god who can indue the frame of the whole earth (it being of monstrous weight by nature) with motion, this our divine pen-man added; 'and the pillars thereof tremble:' as if he would teach us, from the doctrine laid down, that it is moved from its foundations."[ ] [footnote : _diccionario enciclopédico hispano-americano de literatura, ciencias y artes_ (barcelona, ).] [footnote : quoted in salusbury: _math. coll._: i, - ( ), as a work inaccessible to most readers at that time because of its extreme rarity. it remained on the index until the edition of .] french thinkers, like the english, did not encourage the new doctrine at this time. montaigne[ ] was characteristically indifferent: "what shall we reape by it, but only that we neede not care which of the two it be? and who knoweth whether a hundred yeares hence a third opinion will arise which happily shall overthrow these two præcedent?" the famous political theorist, jean bodin, ( - ), was as thoroughly opposed to it as dubartas had been. in the last year of his life, bodin wrote his _universæ naturæ theatrum_[ ] in which he discussed the origin and composition of the universe and of the animal, vegetable, mineral and spiritual kingdoms. these five books (or divisions) reveal his amazing ideas of geology, physics and astronomy while at the same time they show a mind thoroughly at home in hebrew and arabian literature as well as in the classics. his answer to the copernican doctrine is worth quoting to illustrate the attitude of one of the keenest thinkers in a brilliant era: "theorist: since the sun's heat is so intense that we read it has sometimes burned crops, houses and cities in scythia,[ ] would it not be more reasonable that the sun is still and the earth indeed revolves? "mystic: such was the old idea of philolaus, timæus, ecphantes, seleucus, aristarchus of samos, archimedes and eudoxus, which copernicus has renewed in our time. but it can easily be refuted by its shallowness although no one has done it thoroughly. "the.: what arguments do they rely on who hold that the earth is revolved and that the sun forsooth is still? "mys.: because the comprehension of the human mind cannot grasp the incredible speed of the heavenly spheres and especially of the tenth sphere which must be ten times greater than that of the eighth, for in twenty-four hours it must traverse , , miles, so that the earth seems like a dot in the universe. this is the chief argument. besides this, we get rid entirely of epicycles in representing the motions of the planets and what is taught concerning the motion of trepidation in the eighth sphere vanishes. also, there is no need for the ninth and tenth spheres. there is one argument which they have omitted but which seems to me more efficacious than any, viz.: rest is nobler than movement, and that celestial and divine things have a stable nature while elemental things have motion, disturbance and unrest; therefore it seems more probable that the latter move rather than the former. but while serious absurdities result from the idea of eudoxus, far more serious ones result from that of copernicus. "the.: what are these absurdities? "mys.: eudoxus knew nothing of trepidation, so his idea seems to be less in error. but copernicus, in order to uphold his own hypothesis, claims the earth has three motions, its diurnal and annual ones, and trepidation; if we add to these the pull of weight towards the center, we are attributing four natural motions to one and the same body. if this is granted, then the very foundations of physics must fall into ruins; for all are agreed upon this that each natural body has but one motion of its own, and that all others are said to be either violent or voluntary. therefore, since he claims the earth is agitated by four motions, one only can be its own, the others must be confessedly violent; yet nothing violent in nature can endure continuously. furthermore the earth is not moved by water, much less by the motion of air or fire in the way we have stated the heavens are moved by the revolutions of the enveloping heavens. copernicus further does not claim that all the heavens are immobile but that some are moved, that is, the moon, mercury, venus, mars, jupiter and saturn. but why such diversity? no one in his senses, or imbued with the slightest knowledge of physics, will ever think that the earth, heavy and unwieldy from its own weight and mass, staggers up and down around its own center and that of the sun; for at the slightest jar of the earth, we would see cities and fortresses, towns and mountains thrown down. a certain courtier aulicus, when some astrologer in court was upholding copernicus's idea before duke albert of prussia, turning to the servant who was pouring the falernian, said: "take care that the flagon is not spilled."[ ] for if the earth were to be moved, neither an arrow shot straight up, nor a stone dropped from the top of a tower would fall perpendicularly, but either ahead or behind. with this argument ptolemy refuted eudoxus. but if we search into the secrets of the hebrews and penetrate their sacred sanctuaries, all these arguments can easily be confirmed; for when the lord of wisdom said the sun swept in its swift course from the eastern shore to the west, he added this: terra vero stat æternam. lastly, all things on finding places suitable to their natures, remain there, as aristotle writes. since therefore the earth has been alloted a place fitting its nature, it cannot be whirled around by other motion than its own. "the.: i certainly agree to all the rest with you, but aristotle's law i think involves a paralogism, for by this argument the heavens should be immobile since they are in a place fitting their nature. "mys.: you argue subtly indeed, but in truth this argument does not seem necessary to me; for what aristotle admitted, that, while forsooth all the parts of the firmament changed their places, the firmament as a whole did not, is exceedingly absurd. for either the whole heaven is at rest or the whole heaven is moved. the senses themselves disprove that it is at rest; therefore it is moved. for it does not follow that if a body is not moved away from its place, it is not moved in that place. furthermore, since we have the most certain proof of the movement of trepidation, not only all the parts of the firmament, but also the eight spheres, must necessarily leave their places and move up and down, forward and back."[ ] [footnote : montaigne: _essays_: bk. ii, c. : _an apologie of raymonde sebonde_ (ii, ).] [footnote : this book, published at frankfort in , was translated into french by m. fougerolles and printed in lyons that same year. it has become extremely rare since its "atheistic atmosphere" (peignot: _dictionnaire_) caused the roman church to place it upon the index by decree of , where it has remained to this day.] [footnote : cromer in history of poland.] [footnote : cromer in history of poland.[a]] [footnote a: i could not find this reference in either of martin kromer's books; _de origine et rebus gestis polonorum, ad _, or in his _res publicæ sive status regni poloniæ_.] [footnote : bodin: _univ. nat. theatrum_: bk. v, sec. (end).] this was the opinion of a profound thinker and experienced man of affairs living when tycho brahe and bruno were still alive and kepler and galileo were beginning their astronomical investigations. but he was not alone in his views, as we shall see; for at the close of the sixteenth century, the copernican doctrine had few avowed supporters. the roman church was still indifferent; the protestants clinging to the literal interpretation of the bible were openly antagonistic; the professors as a whole were too aristotelian to accept or pay much attention to this novelty, except kepler and his teacher mæstlin (though the latter refused to uphold it in his textbook);[ ] while astronomers and mathematicians who realized the insuperable objections to the ptolemaic conception, welcomed the tychonic system as a _via media_; and the common folk, if they heard of it at all, must have ridiculed it because it was so plainly opposed to what they saw in the heavens every day. in the same way their intellectual superiors exclaimed at the "delirium" of those supporting such a notion.[ ] one thinker, however was to see far more in the doctrine than copernicus himself had conceived, and by giordano bruno the roman church was to be aroused. [footnote : delambre: _astr. mod._: i, .] [footnote : justus-lipsius: _physiologiæ stoicorum_: bk. ii, dissert. (dedication , louvain), (iv, ); "vides deliria, quomodo aliter appellent?"] chapter ii. bruno and galileo. when the roman catholic authorities awoke to the dangers of the new teaching, they struck with force. the first to suffer was the famous monk-philosopher, giordano bruno, whose trial by the holy office was premonitory of trouble to come for galileo.[ ] [footnote : berti: .] after an elementary education at naples near his birth-place, nola,[ ] filippo bruno[ ] entered the dominican monastery in or when about fourteen years old, assuming the name giordano at that time. before , when he entered the priesthood, he had fully accepted the copernican theory which later became the basis of all his philosophical thought. bruno soon showed he was not made for the monastic life. various processes were started against him, and fleeing to rome he abandoned his monk's garments and entered upon the sixteen years of wandering over europe, a peripatetic teacher of the philosophy of an infinite universe as deduced from the copernican doctrine and thus in a way its herald.[ ] he reached geneva in (where he did not accept calvinism as was formerly thought),[ ] but decided before many months had passed that it was wise to depart elsewhere because of the unpleasant position in which he found himself there. he had been brought before the council for printing invectives against one of the professors, pointing out some twenty of his errors. the council sent him to the consistory, the governing body of the church, where a formal sentence of excommunication was passed against him. when he apologized it was withdrawn. probably a certain stigma remained, and he left geneva soon thereafter with a warm dislike for calvinism. after lecturing at the university of toulouse he appeared in paris in , where he held an extraordinary readership. two years later he was in england, for he lectured at oxford during the spring months and defended the copernican theory before the polish prince alasco during the latter's visit there in june.[ ] [footnote : mcintyre: - .] [footnote : four lives of bruno have been written within the last seventy-five years. the first is _jordano bruno_ by christian bartholmèss ( vol., paris ). the next, _vita di giordano bruno da nola_ by domenico berti ( , turin), quotes in full the official documents of his trial. frith's _life of giordano bruno_ (london, ), has been rendered out of date by j.l. mcintyre's _giordano bruno_ (london, ), which includes a critical bibliography. in addition, w.r. thayer's _throne makers_ (new york, ), gives translations of bruno's confessions to the venetian inquisition. bruno's latin works (_opera latina conscripta_), have been republished by fiorentino ( vol., naples, ), and the _opere italiane_ by gentile ( vol., naples, ).] [footnote : bartholmèss: i, .] [footnote : libri: iv, .] [footnote : mcintyre: - .] to bruno belongs the glory of the first public proclamation in england of the new doctrine,[ ] though only gilbert[ ] and possibly wright seem to have accepted it at the time. upon bruno's return to london, he entered the home of the french ambassador as a kind of secretary, and there spent the happiest years of his life till the ambassador's recall in october, . it was during this period that he wrote some of his most famous books. in _la cena de la ceneri_ he defended the copernican theory, incidentally criticising the oxford dons most severely,[ ] for which he apologized in _de la causa, principio et uno_. he developed his philosophy of an infinite universe in _de l'infinito e mondi_, and in the _spaccio de la bestia trionphante_ "attacked all religions of mere credulity as opposed to religions of truth and deeds."[ ] this last book was at once thought to be a biting attack upon the roman church and later became one of the grounds of the inquisition's charges against him. during this time in london also, he came to know sir philip sydney intimately, and fulk greville as well as others of that brilliant period. he may have known bacon;[ ] but it is highly improbable that he and shakespeare met,[ ] or that shakespeare ever was influenced by the other's philosophy.[ ] [footnote : bartholmèss: i, .] [footnote : gilbert: _de magnete_ (london, ).] [footnote : berti: , doc. xiii.] [footnote : mcintyre: - .] [footnote : bartholmèss: i, .] [footnote : beyersdorf: _giordano bruno und shakespear_, - .] [footnote : such passages as _troilus and cressida_: act i, sc. ; _king john_, act iii, sc. ; and _merry wives_, act iii, sc. , indicate that shakespeare accepted fully the ptolemaic conception of a central, immovable earth. see also beyersdorf: _op. cit._] leaving paris soon after his return thither, bruno wandered into southern germany. at marburg he was not permitted to teach, but at wittenberg the lutherans cordially welcomed him into the university. after a stay of a year and a half, he moved on to prague for a few months, then to helmstadt, frankfort and zurich, and back to frankfort again where, in , he received an invitation from a young venetian patrician, moecenigo, to come to venice as his tutor. he re-entered italy, therefore, in august, much to the amazement of his contemporaries. it is probable that moecenigo was acting for the inquisition.[ ] at any rate, he soon denounced bruno to that body and in may, , surrendered him to it.[ ] [footnote : mcintyre: .] [footnote : ibid: - .] in his trial before the venetian inquisition,[ ] bruno told the story of his life and stated his beliefs in answer to the charges against him, based mainly on travesties of his opinions. in this statement as well as in _la cena de le ceneri_, and in _de immenso et innumerabilis_,[ ] bruno shows how completely he had not merely accepted the copernican doctrine, but had expanded it far beyond its author's conception. the universe according to copernicus, though vastly greater than that conceived by aristotle and ptolemy, was still finite because enclosed within the sphere of the fixed stars. bruno declared that not only was the earth only a lesser planet, but "this world itself was merely one of an infinite number of particular worlds similar to this, and that all the planets and other stars are infinite worlds without number composing an infinite universe, so that there is a double infinitude, that of the greatness of the universe, and that of the multitude of worlds."[ ] how important this would be to the church authorities may be realized by recalling the patristic doctrine that the universe was created for man and that his home is its center. of course their cherished belief must be defended from such an attack, and naturally enough, the copernican doctrine as the starting point of bruno's theory of an infinite universe was thus brought into question;[ ] for, as m. berti has said,[ ] bruno's doctrine was equally an astro-theology or a theological astronomy. [footnote : see official documents in berti: - .] [footnote : bruno: _de immenso et innumerabilis_: lib. iii, cap. (vol. , pt. , - ).] [footnote : thayer: .] [footnote : berti: .] [footnote : ibid: .] the roman inquisition was not content to let the venetian court deal with this arch heretic, but wrote in september, , demanding his extradition. the venetian body referred its consent to the state for ratification which the doge and council refused to grant. finally, when the papal nuncio had represented that bruno was not a venetian but a neapolitan, and that cases against him were still outstanding both in naples and in rome, the state consented, and in february of the next year, bruno entered rome, a prisoner of the inquisition. nothing further is known about him until the congregations took up his case on february th, . perhaps pope clement had hoped to win back to the true faith this prince of heretics.[ ] however bruno stood firm, and early in the following year he was degraded, sentenced and handed over to the secular authorities, who burned him at the stake in the campo di fiori, february , .[ ] all his books were put on the index by decree of february , , (where they remain to this day), and as a consequence they became extremely rare. it is well to remember bruno's fate, when considering galileo's case, for galileo[ ] was at that time professor of mathematics in the university of padua and fully cognizant of the event. [footnote : fahie: - .] [footnote : thayer: .] [footnote : the publication of a. favaro's _galileo e l'inquisizione: documenti del processo galileiano ... per la prima volta integralmente pubblicati_, (firenze, ), together with that of the national edition (in vols.) of galileo's works, edited by favaro (firenze, completed ), renders somewhat obsolete all earlier lives of galileo. the more valuable, however, of these books are: martin's _galilée_ (paris, ), a scholarly catholic study containing valuable bibliographical notes; anon. (mrs. olney): _private life of galileo_, based largely on his correspondence with his daughter from which many extracts are given; and von gebler's _galileo galilei and the roman curia_ (trans. by mrs. sturge, london, ), which includes in the appendix the various decrees in the original. fahie's _life of galileo_ (london, ), is based on favaro's researches and is reliable. the documents of the trial have been published in part by de l'epinois, von gebler and berti, but favaro's is the complete and authoritative edition.] galileo's father, though himself a skilled mathematician, had intended that his son (born at pisa, february , ), should be a cloth-dealer, but at length permitted him to study medicine instead at the university of pisa, after an elementary education at the monastery of vallombrosa near florence. at the tuscan court in pisa, galileo received his first lesson in mathematics, which thereupon became his absorbing interest. after nearly four years he withdrew from the university to florence and devoted himself to that science and to physics. his services as a professor at this time were refused by five of the italian universities; finally, in , he obtained the appointment to the chair of physics at pisa. he became so unpopular there, however, through his attacks on the aristotelian physics of the day, that after three years he resigned and accepted a similar position at padua.[ ] he remained here nearly eighteen years till his longing for leisure in which to pursue his researches, and the patronage of his good friend, the grand duke of tuscany, brought him a professorship at the university of pisa again, this time without obligation of residence nor of lecturing. he took up his residence in florence in ; and later ( ), purchased a villa at arcetri outside the city, in order to be near the convent where his favorite daughter "suor maria celeste" was a religious.[ ] [footnote : fahie: - .] [footnote : ibid: .] during the greater part of his lectureship at padua, galileo taught according to the ptolemaic cosmogony out of compliance with popular feeling, though himself a copernican. in a letter to kepler (august , )[ ] he speaks of his entire acceptance of the new system for some years; but not until after the appearance of the new star in the heavens in and , and the controversy that its appearance aroused over the aristotelian notion of the perfect and unchangeable heavens, did he publicly repudiate the old scheme and teach the new. the only information we have as to how he came to adopt the copernican scheme for himself is the account given by "_sagredo_," galileo's spokesman in the famous _dialogue on the two principal systems_ ( ): "being very young and having scarcely finished my course of philosophy which i left off, as being set upon other employments, there chanced to come into these parts a certain foreigner of rostock, whose name as i remember, was christianus vurstitius, a follower of copernicus, who in an academy made two or three lectures upon this point, to whom many flock't as auditors; but i thinking they went more for the novelty of the subject than otherwise, did not go to hear him; for i had concluded with myself that that opinion could be no other than a solemn madnesse. and questioning some of those who had been there, i perceived they all made a jest thereof, except one, who told me that the business was not altogether to be laugh't at, and because this man was reputed by me to be very intelligent and wary, i repented that i was not there, and began from that time forward as oft as i met with anyone of the copernican persuasion, to demand of them, if they had always been of the same judgment; and of as many as i examined, i found not so much as one, who told me not that he had been a long time of the contrary opinion, but to have changed it for this, as convinced by the reasons proving the same: and afterwards questioning them, one by one, to see whether they were well possest of the reasons of the other side, i found them all to be very ready and perfect in them; so that i could not truly say that they had took up this opinion out of ignorance, vanity, or to show the acuteness of their wits. on the contrary, of as many of the peripateticks and ptolemeans as i have asked (and out of curiosity i have talked with many) what pains they had taken in the book of copernicus, i found very few that had so much as superficially perused it: but of those whom, i thought, had understood the same, not one; and moreover, i have enquired amongst the followers of the peripatetick doctrine, if ever any of them had held the contrary opinion, and likewise found that none had. whereupon considering that there was no man who followed the opinion of copernicus that had not been first on the contrary side, and that was not very well acquainted with the reasons of aristotle and ptolemy; and on the contrary, that there is not one of the followers of ptolemy that had ever been of the judgment of copernicus, and that had left that to embrace this of aristotle, considering, i say, these things, i began to think that one, who leaveth an opinion imbued with his milk, and followed by very many, to take up another owned by very few, and denied by all the schools, and that really seems a very great parodox, must needs have been moved, not to say forced, by more powerful reasons. for this cause i am become very curious to dive, as they say, into the bottom of this business ... and bring myself to a certainty in this subject."[ ] [footnote : galileo: _opere_, x, .] [footnote : 'the second day' in salusbury: _math. coll._ i, - .] galileo's brilliant work in mechanics and his great popularity--for his lectures were thronged--combined with his skilled and witty attacks upon the accepted scientific ideas of the age, embittered and antagonized many who were both conservative and jealous.[ ] the jesuits particularly resented his influence and power, for they claimed the leadership in the educational world and were jealous of intruders. furthermore, they were bound by the decree of the fiftieth general congregation of their society in to defend aristotle, a decree strictly enforced.[ ] while a few of the jesuits were friendly disposed to galileo at first, the controversies in which he and they became involved and their bitter attacks upon him made him feel by that they were among his chief enemies.[ ] [footnote : fahie: .] [footnote : conway: - .] [footnote : conway: - .] early in , galileo heard a rumor of a spy-glass having been made in flanders, and proceeded to work one out for himself according to the laws of perspective. the fifth telescope that he made magnified thirty diameters, and it was with such instruments of his own manufacture that he made in the next three years his famous discoveries: jupiter's four satellites (which he named the medicean planets), saturn's "tripartite" character (the rings were not recognized as such for several decades thereafter), the stars of the milky way, the crescent form of venus, the mountains of the moon, many more fixed stars, and the spots on the sun. popular interest waxed with each new discovery and from all sides came requests for telescopes; yet there were those who absolutely refused even to look through a telescope lest they be compelled to admit aristotle was mistaken, and others claimed that jupiter's moons were merely defects in the instrument. the formal announcement of the first of these discoveries was made in the _sidereus nuncius_ ( ), a book that aroused no little opposition. kepler, however, had it reprinted at once in prague with a long appreciative preface of his own.[ ] [footnote : fahie: - .] the following march galileo went to rome to show his discoveries and was received with the utmost distinction by princes and church dignitaries alike. a commission of four scientific members of the roman college had previously examined his claims at cardinal bellarmin's suggestion, and had admitted their truth. now pope paul v gave him long audiences; the academia dei lincei elected him a member, and everywhere he was acclaimed. nevertheless his name appears on the secret books of the holy office as early as may of that year ( ).[ ] already he was a suspect. [footnote : doc. in favaro: .] his _delle macchie solari_ ( ) brought on a sharp contest over the question of priority of discovery between him and the jesuit father, christopher scheiner of ingolstadt, from which galileo emerged victorious and more disliked than before by that order. opposition was becoming active; father castelli, for instance, the professor of mathematics at pisa and galileo's intimate friend, was forbidden to discuss in his lectures the double motion of the earth or even to hint at its probability. this same father wrote to his friend early in december, , to tell him of a dinner-table conversation on this matter at the tuscan court, then wintering at pisa. castelli told how the dowager grand duchess cristina had had her religious scruples aroused by a remark that the earth's motion must be wrong because it contradicted the scriptures, a statement that he had tried to refute.[ ] galileo wrote in reply (december , ), the letter[ ] that was to cause him endless trouble, in which he marked out the boundaries between science and religion and declared it a mistake to take the literal interpretation of passages in scripture that were obviously written according to the understanding of the common people. he pointed out in addition how futile the miracle of the sun's standing still was as an argument against the copernican doctrine for, even according to the ptolemaic system, not the sun but the _primum mobile_ must be stayed for the day to be lengthened. [footnote : fahie: .] [footnote : galileo: _opere_, v, - .] father castelli allowed others to read and to copy this supposedly private letter; copies went from hand to hand in florence and discussion ran high. on the fourth sunday in december, , father caccini of the dominicans preached a sermon in the church of s.m. novella on joshua's miracle, in which he sharply denounced the copernican doctrine taught by galileo as heretical, so he believed.[ ] the copernicans found a neapolitan jesuit who replied to caccini the following sunday from the pulpit of the duomo.[ ] [footnote : doc. in favaro: - .] [footnote : doc. in favaro: .] in february ( ), came the formal denunciation of galileo to the holy office at rome by father lorini, a dominican associate of caccini's at the convent san marco. the father sent with his "friendly warning," a copy of the letter to castelli charging that it contained "many propositions which were either suspect or temerarious," and, he added, "though the _galileisti_ were good christians they were rather stubborn and obstinate in their opinions."[ ] the machinery of the inquisition began secretly to turn. the authorities failed to get the original of the letter, for castelli had returned that to galileo at the latter's request.[ ] pope paul sent word to father caccini to appear before the holy office in rome to depose on this matter of galileo's errors "pro exoneratione suæ conscientiæ."[ ] this he did "freely" in march and was of course sworn to secrecy. he named a certain nobleman, a copernican, as the source of his information about galileo, for he did not know the latter even by sight. this nobleman was by order of the pope examined in november after some delay by the inquisitor at florence. his testimony was to the effect that he considered galileo the best of catholics.[ ] [footnote : ibid: : "amorevole avviso."] [footnote : ibid: , , .] [footnote : ibid: .] [footnote : ibid: .] meanwhile the consultors of the holy office had examined lorini's copy of the letter and reported the finding of only three objectionable places all of which, they stated, could be amended by changing certain doubtful phrases; otherwise it did not deviate from the true faith. it is interesting to note that the copy they had differed in many minor respects from the original letter, and in one place heightened a passage with which the examiners found fault as imputing falsehood to the scriptures although they are infallible.[ ] galileo's own statement ran that there were many passages in the scriptures which according to the literal meaning of the words, "hanno aspetto diverso dal vero...." the copy read, "molte propositioni falso quanto al nudo senso delle parole." [footnote : ibid: - , see original in galileo: _opere_, v, - .] rumors of trouble reached galileo and, urged on by his friends, in he wrote a long formal elaboration of the earlier letter, addressing this one to the dowager grand duchess, but he had only added fuel to the fire. at the end of the year he voluntarily went to rome, regardless of any possible danger to himself, to see if he could not prevent a condemnation of the doctrine.[ ] it came as a decided surprise to him to receive an order to appear before cardinal bellarmin on february , ,[ ] and there to learn that the holy office had already condemned it two days before. he was told that the holy office had declared: first, "that the proposition that the sun is the center of the universe and is immobile is foolish and absurd in philosophy and formally heretical since it contradicts the express words of the scriptures in many places, according to the meaning of the words and the common interpretation and sense of the fathers and the doctors of theology; and, secondly, that the proposition that the earth is not the center of the universe nor immobile receives the same censure in philosophy and in regard to its theological truth, it at least is erroneous in faith."[ ] [footnote : doc. in favaro: .] [footnote : ibid: .] exactly what was said at that meeting between the two men became the crucial point in galileo's trial sixteen years later, hence a somewhat detailed study is important. at the meeting of the congregation on february th, the pope ordered cardinal bellarmin to summon galileo and, in the presence of a notary and witnesses lest he should prove recusant, warn him to abandon the condemned opinion and in every way to abstain from teaching, defending or discussing it; if he did not acquiesce, he was to be imprisoned.[ ] the secret archives of the vatican contain a minute reporting this interview (dated february , ), in which the cardinal is said to have ordered galileo to relinquish this condemned proposition, "nec eam de cætero, quovis modo, teneat, doceat aut defendat, verbo aut scriptis," and that galileo promised to obey.[ ] rumors evidently were rife in rome at the time as to what had happened at this secret interview, for galileo wrote to the cardinal in may asking for a statement of what actually had occurred so that he might silence his enemies. the cardinal replied: "we, robert cardinal bellarmin, having heard that signor galileo was calumniated and charged with having abjured in our hand, and also of being punished by salutary penance, and being requested to give the truth, state that the aforesaid signor galileo has not abjured in our hand nor in the hand of any other person in rome, still less in any other place, so far as we know, any of his opinions and teachings, nor has he received salutary penance nor any other kind; but only was he informed of the declaration made by his holiness and published by the sacred congregation of the index, in which it is stated that the doctrine attributed to copernicus,--that the earth moves around the sun and that the sun stands in the center of the world without moving from the east to the west, is contrary to the holy scriptures and therefore cannot be defended nor held (non si possa difendere né tenere). and in witness of this we have written and signed these presents with our own hand, this th day of may, . robert cardinal bellarmin."[ ] [footnote : ibid: .] [footnote : doc. in favaro: - .] [footnote : ibid: .] galileo's defense sixteen years later[ ] was that he had obeyed the order as given him by the cardinal and that he had not "defended nor held" the doctrine in his _dialoghi_ but had refuted it. the congregation answered that he had been ordered not only not to hold nor defend, but also not to treat in any way (quovis modo) this condemned subject. when galileo disclaimed all recollection of that phrase and produced the cardinal's statement in support of his position, he was told that this document, far from lightening his guilt, greatly aggravated it since he had dared to deal with a subject that he had been informed was contrary to the holy scriptures.[ ] [footnote : ibid: - .] [footnote : ibid: .] to return to . on the third of march the cardinal reported to the congregation in the presence of the pope that he had warned galileo and that galileo had acquiesced.[ ] the congregation then reported its decree suspending "until corrected" "nicolai copernici de revolutionibus orbium coelestium, et didaci astunica in job," and prohibiting "epistola fratris pauli antonii foscarini carmelitæ," together with all other books dealing with this condemned and prohibited doctrine. the pope ordered this decree to be published by the master of the sacred palace, which was done two days later.[ ] but this prohibition could not have been widely known for two or three years; the next year mulier published his edition of the _de revolutionibus_ at amsterdam without a word of reference to it; in thomas feyens, professor at louvain, heard vague rumors of the condemnation and wondered if it could be true;[ ] and the following spring fromundus, also at louvain and later a noted antagonist of the new doctrine, wrote to feyens asking: "what did i hear lately from you about the copernicans? that they have been condemned a year or two ago by our holy father, pope paul v? until now i have known nothing about it; no more have this crowd of german and italian scholars, very learned and, as i think, very catholic, who admit with copernicus that the earth is turned. is it possible that after a lapse of time as considerable as this, we have nothing more than a rumor of such an event? i find it hard to believe, since nothing more definite has come from italy. definitions of this sort ought above all to be published in the universities where the learned men are to whom the danger of such an opinion is very great."[ ] [footnote : ibid: .] [footnote : doc. in favaro: .] [footnote : monchamp: .] [footnote : fromundus: _de cometa anni_ : chap. vii, p. . (from the private library of dr. e.e. slosson. a rare book which lecky could not find. _history of rationalism in europe_, i, , note.)] galileo meanwhile had retired to florence and devoted himself to mechanical science, (of which his work is the foundation) though constantly harassed by much ill health and many family perplexities. at the advice of his friends, he allowed the attacks on the copernican doctrine to go unanswered,[ ] till with the accession to the papacy in of cardinal barberini, as urban viii, a warm admirer and supporter of his, he thought relief was in sight. he was further cheered by a conversation cardinal di zollern reported having had with pope urban, in which his holiness had reminded the cardinal how he (the pope) had defended copernicus in the time of paul v, and asserted that out of just respect owed to the memory of copernicus, if he had been pope then, he would not have permitted his opinion to be declared heretical.[ ] feeling that he now had friends in power, galileo began his great work, _dialogo sopra i due sistemi massimi del mondo_, a dialogue in four "days" in which three interlocutors discuss the arguments for and against the copernican theory, though coming to no definite conclusion. sagredo was an avowed copernican and galileo's spokesman, salviati was openminded, and the peripatetic was simplicio, appropriately named for the famous sicilian sixth century commentator on aristotle.[ ] [footnote : in the congregation issued the changes it required to have made in the _de revolutionibus_. they are nine in all, and consist mainly in changing assertion of the earth's movement to hypothetical statement and in striking out a reference to the earth as a planet. doc. in favaro: - . see illustration, p. .] [footnote : doc. in favaro: .] [footnote : galileo: _dialogo_: to the reader.] [illustration: a "corrected" page from the _de revolutionibus_. a photographic facsimile (reduced) of a page from mulier's edition ( ) of the _de revolutionibus_ as "corrected" according to the _monitum_ of the congregations in . the first writer underlined the passages to be deleted or altered with marginal notes indicating the changes ordered; the second writer scratched out these passages, and wrote out in full the changes the other had given in abbreviated form. the _notæ_ are mulier's own, and so were not affected by the order. the effect of the page is therefore somewhat contradictory!] in he brought the completed manuscript to riccardi, master of the sacred palace, for permission to print it in rome. after much reading and re-reading of it both by riccardi and his associate, father visconti, permission was at length granted on condition that he insert a preface and a conclusion practically dictated by riccardi, emphasizing its hypothetical character.[ ] the pope's own argument was to be used: "god is all-powerful; all things are therefore possible to him; ergo, the tides cannot be adduced as a necessary proof of the double motion of the earth without limiting god's omnipotence--which is absurd."[ ] galileo returned to florence in june with the permission to print his book in rome. meanwhile the plague broke out. he decided to print it in florence instead, and on writing to riccardi for that permission, the latter asked for the book to review it again. the times were too troublesome to risk sending it, so a compromise was finally effected: galileo was to send the preface and conclusion to rome and riccardi agreed to instruct the inquisitor at florence as to his requirements and to authorize him to license the book.[ ] the parts were not returned from rome till july, , and the book did not appear till february of the following year, when it was published at florence with all these licenses, both the roman and the florentine ones. [footnote : doc. in favaro: .] [footnote : fahie: .] [footnote : ibid: .] the _dialogo_ was in italian so that all could read it. it begins with an outline of the aristotelian system, then points out the resemblances between the earth and the planets. the second "day" demonstrates the daily rotation of the earth on its axis. the next claims that the necessary stellar parallax is too minute to be observed and discusses the earth's annual rotation. the last seeks to prove this rotation by the ebb and flow of the tides. it is a brilliant book and received a great reception. the authorities of the inquisition at once examined it and denounced galileo (april , ) because in it he not merely taught and defended the "condemned doctrine but was gravely suspected of firm adherence to this opinion."[ ] other charges made against him were that he had printed the roman licenses without the permission of the congregation, that he had printed the preface in different type so alienating it from the body of the book, and had put the required conclusion into the mouth of a fool (simplicio), that in many places he had abandoned the hypothetical treatment and asserted the forbidden doctrine, and that he had dealt indecisively with the matter though the congregation had specifically condemned the copernican doctrine as contrary to the express words of the scripture.[ ] [footnote : doc. in favaro: - . [transcriber's note: missing footnote reference in original text has been added above in a logical place.]] [footnote : ibid: .] the pope became convinced that galileo had ridiculed him in the character of simplicio to whom galileo had naturally enough assigned the pope's syllogistic argument. on the rd of september, he ordered the inquisitor of florence to notify galileo (in the presence of concealed notary and witnesses in case he were "recusant") to come to rome and appear before the sacred congregation before the end of the next month;[ ] the publication and sale of the _dialogo_ meanwhile being stopped at great financial loss to the printer.[ ] galileo promised to obey; but he was nearly seventy years old and so much broken in health that a long difficult journey in the approaching winter seemed a great and unnecessary hardship, especially as he was loath to believe that the church authorities were really hostile to him. delays were granted him till the pope in december finally ordered him to be in rome within a month.[ ] the florentine inquisitor replied that galileo was in bed so sick that three doctors had certified that he could not travel except at serious risk to his life. this certificate declared that he suffered from an intermittent pulse, from enfeebled vital faculties, from frequent dizziness, from melancholia, weakness of the stomach, insomnia, shooting pains and serious hernia.[ ] the answer the pope made to this was to order the inquisitor to send at galileo's expense a commissary and a doctor out to his villa to see if he were feigning illness; if he were, he was to be sent bound and in chains to rome at once; if [transcriber's note: 'he' missing] were really too ill to travel, then he was to be sent in chains as soon as he was convalescent and could travel safely.[ ] galileo did not delay after that any longer than he could help, and set out for rome in january in a litter supplied by the tuscan grand duke.[ ] the journey was prolonged by quarantine, but upon his arrival (february , ), he was welcomed into the palace of niccolini, the warm-hearted ambassador of the grand duke. [footnote : ibid: - .] [footnote : galileo: _opere_, xv, .] [footnote : doc. in favaro: .] [footnote : ibid: .] [footnote : ibid: .] [footnote : ibid: - .] four times was the old man summoned into the presence of the holy office, though never when the pope was presiding. in his first examination held on the th of april, he told how he thought he had obeyed the decree of as his _dialogo_ did not defend the copernican doctrine but rather confuted it, and that in his desire to do the right, he had personally submitted the book while in manuscript to the censorship of the master of the sacred palace, and had accepted all the changes he and the florentine inquisitor had required. he had not mentioned the affair of because he thought that order did not apply to this book in which he proved the lack of validity and of conclusiveness of the copernican arguments.[ ] with remarkable, in fact unique, consideration, the holy office then assigned galileo to a suite of rooms within the prisons of the holy office, allowed him to have his servant with him and to have his meals sent in by the ambassador. on the th after his examination, they even assigned as his prison, the ambassador's palace, out of consideration for his age and ill-health. [footnote : ibid: - .] in his second appearance (april ), galileo declared he had been thinking matters over after re-reading his book (which he had not read for three years), and freely confessed that there were several passages which would mislead a reader unaware of his real intentions, into believing the worse arguments were the better, and he blamed these slips upon his vain ambition and delight in his own skill in debate.[ ] he thereupon offered to write another "day" or two more for the _dialogo_ in which he would completely refute the two "strong" copernican arguments based on the sun's spots and on the tides.[ ] ten days later, at his third appearance, he presented a written statement of his defence in which he claimed that the phrase _vel quovis modo docere_ was wholly new to him, and that he had obeyed the order given him by cardinal bellarmin over the latter's own signature. however he would make what amends he could and begged the cardinals to "consider his miserable bodily health and his incessant mental trouble for the past ten months, the discomforts of a long hard journey at the worst season, when years old, together with the loss of the greater part of the year, and that therefore such suffering might be adequate punishment for his faults which they might condone to failing old age. also he commended to them his honor and reputation against the calumnies of his ill-wishers who seek to detract from his good name."[ ] to such a plight was the great man brought! but the end was not yet. [footnote : doc. in favaro: .] [footnote : ibid: .] [footnote : ibid: - .] nearly a month later (june ), by order of the pope, galileo was once again interrogated, this time under threat of torture.[ ] once again he declared the opinion of ptolemy true and indubitable and said he did not hold and had not held this doctrine of copernicus after he had been informed of the order to abandon it. "as for the rest," he added, "i am in your hands, do with me as you please." "i am here to obey."[ ] then by the order of the pope, ensued galileo's complete abjuration on his knees in the presence of the full congregation, coupled with his promise to denounce other heretics (i.e., copernicans).[ ] in addition, because he was guilty of the heresy of having held and believed a doctrine declared and defined as contrary to the scriptures, he was sentenced to "formal imprisonment" at the will of the congregation, and to repeat the seven penitential psalms every week for three years.[ ] [footnote : ibid: .] [footnote : doc. in favaro: .] [footnote : doc. in favaro: .] [footnote : ibid: .] at galileo's earnest request, his sentence was commuted almost at once, to imprisonment first in the archiepiscopal palace in siena (from june -december ), then in his own villa at arcetri, outside florence, though under strict orders not to receive visitors but to live in solitude.[ ] in the spring his increasing illness occasioned another request for greater liberty in order to have the necessary visits from the doctor; but on march , , this was denied him with a stern command from the pope to refrain from further petitions lest the sacred congregation be compelled to recall him to their prisons in rome.[ ] [footnote : ibid: , .] [footnote : ibid: .] the rule forbidding visitors seems not to have been rigidly enforced all the time, for milton visited him, "a prisoner of the inquisition" in ;[ ] yet father castelli had to write to rome for permission to visit him to learn his newly invented method of finding longitude at sea.[ ] when in florence on a very brief stay to see his doctor, galileo had to have the especial consent of the inquisitor in order to attend mass at easter. he won approval from the holy congregation, however, by refusing to receive some gifts and letters brought him by some german merchants from the low countries.[ ] he was then totally blind, but he dragged out his existence until january , (the year of newton's birth), when he died. as the pope objected to a public funeral for a man sentenced by the holy office, he was buried without even an epitaph.[ ] the first inscription was made years later, and in , his remains were removed to santa croce after the congregation had first been asked if such action would be unobjectionable.[ ] [footnote : milton: _areopagitica_: .] [footnote : doc. in favaro: .] [footnote : ibid: .] [footnote : fahie: .] [footnote : doc. in favaro: ; and fahie: .] pope urban had no intention of concealing galileo's abjuration and sentence. instead, he ordered copies of both to be sent to all inquisitors and papal nuncios that they might notify all their clergy and especially all the professors of mathematics and philosophy within their districts, particularly those at florence, padua and pisa.[ ] this was done during the summer and fall of . from wilna in poland, cologne, paris, brussels, and madrid, as well as from all italy, came the replies of the papal officials stating that the order had been obeyed.[ ] he evidently intended to leave no ground for a remark like that of fromundus about the earlier condemnation. [footnote : doc. in favaro: , .] [footnote : ibid: - .] galileo was thus brought so low that the famous remark, "eppur si muove," legend reports him to have made as he rose to his feet after his abjuration, is incredible in itself, even if it had appeared in history earlier than its first publication in .[ ] but his discoveries and his fight in defence of the system did much both to strengthen the doctrine itself and to win adherents to it. the appearance of the moon as seen through a telescope destroyed the aristotelian notion of the perfection of heavenly bodies. jupiter's satellites gave proof by analogy of the solar system, though on a smaller scale. the discovery of the phases of venus refuted a hitherto strong objection to the copernican system; and the discovery of the spots on the sun led to his later discovery of the sun's axial rotation, another proof by analogy of the axial rotation of the earth. yet he swore the ptolemaic conception was the true one. [footnote : fahie: , note.] the abjuration of galileo makes a pitiful page in the history of thought and has been a fruitful source of controversy[ ] for nearly three centuries. he was unquestionably a sincere and loyal catholic, and accordingly submitted to the punishment decreed by the authorities. but in his abjuration he plainly perjured himself, however fully he may be pardoned for it because of the extenuating circumstances. had he not submitted and been straitly imprisoned, if not burned, the world would indeed have been the poorer by the loss of his greatest work, the _dialoghi delle nuove scienze_, which he did not publish until .[ ] [footnote : for full statement, see martin: - .] [footnote : gebler: .] even more hotly debated has been the action of the congregations in condemning the copernican doctrine, and sentencing galileo as a heretic for upholding it.[ ] though both paul v and urban viii spurred on these actions, neither signed either the decree or the sentence, nor was the latter present at galileo's examinations. pope urban would prefer not so openly to have changed his position from that of tolerance to his present one of active opposition caused partly by his piqued self-respect[ ] and partly by his belief that this heresy was more dangerous even than that of luther and calvin.[ ] it is a much mooted question whether the infallibility of the church was involved or not. though the issue at stake was not one of faith, nor were the decrees issued by the pope _ex cathedra_, but by a group of cardinals, a fallible body, yet they had the full approbation of the popes, and later were published in the index preceded by a papal bull excommunicating those who did not obey the decrees contained therein.[ ] it seems to be a matter of the letter as opposed to the spirit of the law. de morgan points out that contemporary opinion as represented by fromundus, an ardent opponent of galileo, did not consider the decree of the index or of the inquisition as a declaration of the church,[ ]--a position which galileo himself may have held, thus explaining his practical disregard of the decree of after he was persuaded the authorities were more favorably disposed to him. but m. martin, himself a catholic, thinks[ ] that theoretically the congregations could punish galileo only for disobedience of the secret order,--but even so his book had been examined and passed by the official censors. [footnote : see gebler: - ; white: i, - ; also martin.] [footnote : martin: ; and salusbury: _math. coll._ "to the reader."] [footnote : galileo: _opere_, xv, .] [footnote : putnam: i, .] [footnote : de morgan: i, .] [footnote : martin: .] when the index was revised under pope benedict xiv in , largely through the influence of the jesuit astronomer boscovich, so it is said,[ ] the phrase prohibiting all books teaching the immobility of the sun, and the mobility of the earth was omitted from the decrees.[ ] but in , the master of the sacred palace refused to permit the publication in rome of a textbook on astronomy by canon settele, who thereupon appealed to the congregations. they granted his request in august, and two years later, issued a decree approved by pope pius vii ordering the master of the sacred palace in future "not to refuse license for publication of books dealing with the mobility of the earth and the immobility of the sun according to the common opinion of modern astronomers" on that ground alone.[ ] the next edition of the _index librorum prohibitorum_ ( ) did not contain the works of copernicus, galileo, foscarini, à stunica and kepler which had appeared in every edition up to that time since their condemnation in , (kepler's in ). [footnote : _cath. ency._: "boscovich."] [footnote : doc. in favaro: .] [footnote : ibid: , .] chapter iii. the opposition and their arguments. the protestant leaders had rejected the copernican doctrine as contrary to the scriptures. the roman congregations had now condemned galileo for upholding this doctrine after they had prohibited it for the same reasons. these objections are perhaps best summarized in that open letter foscarini wrote to the general of his order, the carmelites, at naples in january, ,[ ]--the letter that was absolutely prohibited by the index in march, . he gave these arguments and answered them lest, as he said, "whilst otherwise the opinion is favored with much probability, it be found in reality to be extremely repugnant (as at first sight it seems) not only to physical reasons and common principles received on all hands (which cannot do so much harm), but also (which would be of far worse consequence) to many authorities of sacred scripture. upon which account many at first looking into it explode it as the most fond paradox and monstrous _capriccio_ that ever was heard of." "yet many modern authors," he says further on, "are induced to follow it, but with much hesitancy and fear, in regard that it seemeth in their opinion so to contradict the holy scriptures that it cannot possibly be reconciled to them." consequently foscarini argued that the theory is either true or false; if false, it ought not to be divulged; if true, the authority of the sacred scriptures will not oppose it; neither does one truth contradict another. so he turned to the bible. [footnote : in salusbury: _math. coll._: i, - .] he found that six groups of authorities seemed to oppose this doctrine. ( ) those stating that the earth stands fast, as eccles. : . ( ) those stating that the sun moves and revolves; as psalm xix, isaiah xxxviii, and the miracle in josh. x: - . ( ) those speaking of the heaven above and the earth beneath, as in joel ii. also christ came _down_ from heaven. ( ) those authorities who place hell at the center of the world, a "common opinion of divines," because it ought to be in the lowest part of the world, that is, at the center of the sphere. then by the copernican hypothesis, hell must either be in the sun; or, if in the earth, if the earth should move about the sun, then hell within the earth would be in heaven, and nothing could be more absurd. ( ) those authorities opposing heaven to earth and earth to heaven, as in gen. i, mat. vi, etc. since the two are always mutually opposed to each other, and heaven undoubtedly refers to the circumference, earth must necessarily be at the center. ( ) those authorities ("rather of fathers and divines than of the sacred scriptures") who declare that after the day of judgment, the sun shall stand immovable in the east and the moon in west. foscarini then lays down in answer six maxims, the first of which is that things attributed to god must be expounded metaphorically according to our manner of understanding and of common speech. the other maxims are more metaphysical, as that everything in the universe, whether corruptible or incorruptible, obeys a fixed law of its nature; so, for example, fortune is _always_ fickle. in concluding his defense, he claims among other things, that the copernician is a more admirable hypothesis than the ptolemaic, and that it is an easy way into astronomy and philosophy. then he adds that there may be an analogy between the seven-branched candlestick of the old testament and the seven planets around the sun, and possibly the arrangement of the seeds in the "indian figg," in the pomegranate and in grapes is all divine evidence of the solar system. with such an amusing reversion to mediæval analogy his spirited letter ends. some or all of these scriptural arguments appear in most of the attacks on the doctrine even before its condemnation by the index in was widely known. besides these objections, aristotle's and ptolemy's statements were endlessly repeated with implicit faith in their accuracy. even sir francis bacon ( - ) with all his modernity of thought, failed in this instance to recognize the value of the new idea and, despite his interest in galileo's discoveries, harked back to the time-honored objections. at first mild in his opposition, he later became emphatically opposed to it. in the _advancement of learning_[ ] ( ), he speaks of it as a possible explanation of the celestial phenomena according to astronomy but as contrary to natural philosophy. some fifteen years later in the _novum organon_,[ ] he asserts that the assumption of the earth's movement cannot be allowed; for, as he says in his _thema coeli_,[ ] at that time he considered the opinion that the earth is stationary the truer one. finally, in his _de augmentis scientiarum_[ ] ( - ) he speaks of the old notions of the solidity of the heavens, etc., and adds, "it is the absurdity of these opinions that has driven men to the diurnal motion; which i am convinced is most false." he gives his reasons in the _descriptio globi intellectualis_[ ] (ch. - ): "in favor of the earth [as the center of the world] we have the evidence of our sight, and an inveterate opinion; and most of all this, that as dense bodies are contracted into a narrow compass, and rare bodies are widely diffused (and the area of every circle is contracted to the center) it seems to follow almost of necessity that the narrow space about the middle of the world be set down as the proper and peculiar place for dense bodies." the sun's claims to such a situation are satisfied through having two satellites of its own, venus and mercury. copernicus's scheme is inconvenient; it overloads the earth with a triple motion; it creates a difficulty by separating the sun from the number of the planets with which it has much in common; and the "introduction of so much immobility into nature ... and making the moon revolve around the earth in an epicycle, and some other assumptions of his are the speculations of one who cares not what fictions he introduces into nature, provided his calculations answer." the total absence of all reference to the scriptures is the unique and refreshing part of bacon's thought. [footnote : bk. ii: sec. , § .] [footnote : bk. ii, ch. .] [footnote : _phil. works_: .] [footnote : bk. iii.] [footnote : _phil. works_: - .] all the more common arguments against the diurnal rotation of the earth are well stated in an interesting little letter ( ) by thomas feyens, or fienus, a professor at the school of medicine in the university of louvain.[ ] thus catholic, protestant, and unbeliever, feyens, melancthon, bacon and bodin, all had recourse to the same arguments to oppose this seemingly absurd doctrine. [footnote : translated in appendix c. for criticism, see monchamp: - .] froidmont, or fromundus, the good friend and colleague of feyens at louvain, was also much interested in these matters, so much so that some thought he had formerly accepted the copernican doctrine and "only fled back into the camp of aristotle and ptolemy through terror at the decree of the s. congregation of cardinals."[ ] his indignant denial of this imputation of turn-coat in is somewhat weakened by reference to his _saturnalitæ coenæ_[ ] ( ) in which he suggests that, if the copernican doctrine is admitted, then hell may be in the sun at the center of the universe rather than in the earth, in order to be as far as possible from paradise. he also refers in his _de cometa_ ( ) to the remark of justus-lipsius[ ] that this paradox was buried with copernicus, saying "you are mistaken, o noble scholar: it lives, and it is full of vigor even now among many,"[ ] thus apparently not seeing serious objection to it. m. monchamp summarizes froidmont's point of view as against aristotle and ptolemy, half for copernicus and wholly for tycho brahe. [footnote : fromundus: _vesta_: ad lectorem.] [footnote : monchamp: .] [footnote : justus-lipsius: iv, .] [footnote : monchamp: .] froidmont's best known books are the two he wrote in answer to a defense of the copernican position first by philip lansberg, then by his son. the _ant-aristarchus sive orbis terræ immobilis, liber unicus in quo decretum s. congregationis s.r.e. cardinal. an. , adversus pythagorico-copernicanus editum, defenditur_, appeared in before galileo's condemnation. the jesuit cavalieri wrote to galileo in may about it thus:[ ] "i have run it through, and verily it states the copernican theory and the arguments in its favor with so much skill and efficacy that he seems to have understood it very well indeed. but he refutes them with so little force that he seems rather to be of an opinion contrary to that expressed in the title of his book. i have given it to m. césar. if you wish it, i will have it sent to you. the arguments he brings against copernicus are those you have so masterfully stated and answered in your _dialogo_." nearly a year later, galileo wrote to gassendi and diodati that he had received this book a month before and, although he had been unable to read much of it on account of his eye trouble, it seemed to him that of all the opponents of copernicus whom he had seen, fromundus was the most sensible and efficient.[ ] again he wrote in january, , regretting that he had not seen it till six months after he had published his dialogues, for he would have both praised it and commented upon certain points. "as for fromundus (who however shows himself to be a man of great talent) i wish he had not fallen into what seems to me a truly serious error, although a rather common one, in order to refute the copernican opinion, of beginning by poking scorn and ridicule at those who consider it true, and then (what seems to me still less becoming) of basing his attack chiefly on the authority of the scriptures, and finally of deducing from this that in this respect it is an opinion little short of heretical. to argue in this way is clearly not praiseworthy;" for as galileo goes on to show, if the scriptures are the word of god, the heavens themselves are his handiwork. why is the one less noble than the other?[ ] [footnote : ibid: .] [footnote : galileo: _opere_: xv, .] [footnote : ibid: xiv, - .] froidmont replied in to lansberg's reply with his second attack, _vesta, sive ant-aristarchi vindex_, in which he laid even more emphasis upon the theological and scriptural objections. yet, in ignorance of galileo's condemnation, he considers the charge of heresy too strong. "the partisans of this system do not after all disdain the authority of the scriptures, although they appear to interpret it in a way rather in their favor." he also, and rightly, denies the existence at that time of any conclusive proof.[ ] [footnote : monchamp: - .] in spite of froidmont's position, the university of louvain was not cordial in its response to the papal nuncio's announcement in september, , of galileo's abjuration and sentence, in marked contrast to the reply sent by the neighboring university of douay. the latter body, in a letter signed by matthæus kellison (sept. , ), declared the condemned theory "should be discarded and hissed from the schools; and that in the english college there in douay, this paradox never had been approved and never would be, but had always been opposed and always would be."[ ] [footnote : doc. in favaro: - , , .] this opposition in the universities in belgium continued throughout the century to be based not so much on scientific grounds as upon the bible. this may be seen in the manuscript reports of lectures in physics and astronomy given at liège in , and at louvain between - , though one of these does not mention the decree of .[ ] the general congregation of the society of jesus in drew up a list of the propositions proscribed in their teaching, though, according to m. monchamp (himself a catholic) not thereby implying a denial of any probability they might have. the th proposition ran: "terra movetur motu diurno; planetæ, tanquam viventia, moventur ab intrinseco. firmamentum stat."[ ] the jesuit astronomer tacquet in his textbook (antwerp, ) respected this decision, acknowledging that no scientific reason kept him from defending the theory, but solely his respect for the christian faith.[ ] [footnote : monchamp: , .] [footnote : ibid: - .] [footnote : ibid: - .] one of the pupils of the jesuits revolted however. martin van welden, appointed professor of mathematics at louvain in , debated a series of theses in january, . the second read: "indubitum est systhema copernici de planetarum motu circa sole; inter quos merito terra censetur." his refusal to alter the wording except to change _indubitum_ to _certum_ brought on a stormy controversy within the faculty which eventually reached the council of brabant and the papal nuncio at brussels.[ ] the professor finally submitted, though he was not forbidden to teach the copernician system, nor did the faculty affirm its falsity, merely that it was contrary to the roman decree. the professor re-opened the matter with a similar thesis in july, thereby arousing a second controversy that this time reached even the privy council. once more he submitted, but solely with an apology for having caused a disagreement. his new theses in contained no explicit mention of the copernician system; at least he had learned tact.[ ] [footnote : ibid: - .] [footnote : monchamp: .] the absorption of the german states in the thirty years war may account for the apparent absence there of copernican discussion until after the peace of westphalia. a certain georgius ludovicus agricola gave a syllogistic refutation of the doctrine as his disputation at the university of wittenberg in . while he acknowledged its ingenuity, he preferred to it "the noblest, truest, and divinely inspired system" of tycho brahe. the four requirements of an acceptable astronomical hypothesis according to this student are: ( ) that it suit all the observations of all the ages; ( ) that as far as possible, it be simple and clear; ( ) that it be not contrary to the principles of physics and optics; ( ) that it be not contrary to the holy scriptures. as the copernican theory does not meet all these tests, it is unsatisfactory. incidentally, he considers it "ridiculous to include the earth among the planets, because then we would be living in heaven, forsooth, since we would be in a star." he decides finally "that the decree of march, , condemning the copernican opinion was not unjust, nor was galileo unfairly treated."[ ] [footnote : agricola: _disputatio_.] two years later appeared a textbook at nürnberg, by a jesuit father, based on the twelfth century sacrobosco treatise and without a single reference so far as i could find, to copernicus![ ] another publication of the same year was a good deal more up to date. this was a kind of catechism in german by johann-henrich voight[ ] explaining for the common people various scientific and mathematical problems in a hundred questions and answers. he himself, a royal swedish astronomer, obviously preferred the tychonic system, but he left his reader free to choose between that and the copernican one, both of which he carefully explained.[ ] he made an interesting summary in parallel columns of the arguments for and against the earth's motion which it seems worth while to repeat as an instance of what the common people were taught: reasons for asserting the earth is motionless: . david in psalm : god has founded the earth and it shall not be moved. . joshua bade the sun stand still--which would not be notable were it not already at rest. . the earth is the heaviest element, therefore it more probably is at rest. . everything loose on the earth seeks its rest on the earth, why should not the whole earth itself be at rest? . we always see half of the heavens and the fixed stars also in a great half circle, which we could not see if the earth moved, and especially if it declined to the north and south.... . a stone or an arrow shot straight up falls straight down. but if the earth turned under it, from west to east, it must fall west of its starting point. . in such revolutions houses and towers would fall in heaps. . high and low tide could not exist; the flying of birds and the swimming of fish would be hindered and all would be in a state of dizziness. reasons for the belief that the earth is moved: . the sun, the most excellent, the greatest and the midmost star, rightly stands still like a king while all the other stars with the earth swing round it. . that you believe that the heavens revolve is due to ocular deception similar to that of a man on a ship leaving shore. . that joshua bade the sun stand still moses wrote for the people in accordance with the popular misconception. . as the planets are each a special created thing in the heavens, so the earth is a similar creation and similarly revolves. . the sun fitly rests at the center as the heart does in the middle of the human body. . since the earth has in itself its especial _centrum_, a stone or an arrow falls freely out of the air again to its own _centrum_ as do all earthly things. . the earth can move five miles in a second more readily than the sun can go forty miles in the same time. and similarly on both sides.[ ] [footnote : schotto: _organum mathematicum_ ( ).] [footnote : voight: _der kunstgünstigen einfalt mathematischer raritäten erstes hundert_. (hamburg, ).] [footnote : voight: _op. cit._: .] [footnote : ibid: - .] another writer preferring the tychonic scheme was longomontanus, whose _astronomica danica_ (amsterdam, ) upheld this theory because it "obviates the absurdities of the copernican hypothesis and most aptly corresponds to celestial appearances," and also because it is "midway between that and the ptolemaic one."[ ] even though he speaks of the "apparent motion of the sun," he attributed diurnal motion to the heavens, and believed the earth was at the center of the universe because ( ), from the account of the creation, the heaven and the earth were first created, and what could be more likely than that the heavens should fill the space between the center (the earth) and the circumference? ( ) and because of the incredibly enormous interval between the sphere of the fixed stars and the earth necessitated by copernican doctrine.[ ] [footnote : longomontanus: _op. cit._: .] [footnote : longomontanus: _op. cit._: .] the high-water mark of opposition after galileo's condemnation was reached in the _almagestum novum_ (bologna, ) by father riccioli of the society of jesus. it was the authoritative answer of that order, the leaders of the church in matters of education, to the challenges of the literary world for a justification of the condemnation of the copernican doctrine and of galileo for upholding it. father riccioli had been professor of philosophy and of mathematics for six years and of theology for ten when by order of his superiors, he was released from his lectureship to prepare a book containing all the material he could gather together on this great controversy of the age.[ ] he wrote it as he himself said, as "an _apologia_ for the sacred congregation of the cardinals who officially pronounced these condemnations, not so much because i thought such great height and eminence needed this at my hands but especially in behalf of catholics; also out of the love of truth to which every non-catholic, even, should be persuaded and from a certain notable zeal and eagerness for the preservation of the sacred scriptures intact and unimpaired; and lastly because of that reverence and devotion which i owe from my particular position toward the holy, catholic and apostolic church."[ ] [footnote : riccioli: _alm. nov._: præfatio, i, xviii.] [footnote : riccioli: _alm. nov._: ii, .] this monumental work, the most important literary production of the society in the th century,[ ] is abundant witness to riccioli's remarkable erudition and industry. nearly one-fifth of the total bulk of the two huge volumes is devoted to a statement of the copernican controversy. this is prefaced by a brief account of his own theory of the universe--the invention of which is another proof of the ability of the man--for his scientific training prevented his acceptance of the aristotelian-ptolemaic theory in the light of galileo's discoveries; his position as a jesuit and a faithful son of the church precluded him from adopting the system condemned by its representatives; and tycho brahe's scheme was not wholly to his liking. therefor he proposed an adaptation of the last-named, more in accordance, as he thought, with the facts.[ ] where tycho had all the planets except the earth and the moon encircle the sun, and that in turn, together with the moon and the sphere of the fixed stars, sweep around the earth as the center of the universe, riccioli made only mars, mercury and venus encircle the sun,--mars with an orbit the radius of which included the earth within its sweep, the other two planets with orbital radii shorter than that of the sun, and so excluding the earth. this he did, ( ) because both jupiter and saturn have their own kingdoms in the heavens, and mars, mercury and venus are but satellites of the sun; ( ) because there are greater varieties of eccentricity among these three than the other two; ( ) because saturn and jupiter are the greatest planets and with the sphere of the fixed stars move more slowly; ( ) mars belongs with the sun because of their related movements; and ( ) because it is likely that one of the planets would have much in common both with saturn and jupiter and with mercury and venus also.[ ] [footnote : _cath. ency._: "riccioli," and walsh: catholic churchmen in science: . ( nd series, .)] [footnote : riccioli: _alm. nov._: ii, - ; see frontispiece.] [footnote : riccioli: _alm. nov._: ii, - ; see frontispiece.] then he takes up the attack upon the copernican doctrine. m. delambre summarizes and comments upon of his arguments against it,[ ] and riccioli himself claims[ ] to have stated " new arguments in behalf of copernicus and against him." but these sound somewhat familiar to the reader of anti-copernican literature: as, for instance, "which is more natural, straight or circular movement?" or, the copernican argument that movement is easier if the object moved is smaller involves a matter of faith since it implies a question of god's power; for to god all is alike, there is no hard nor easy.[ ] although diurnal movement is useful to the earth alone and so, according to the copernicans, the earth should have the labor of it, riccioli argues that everything was created for man; let the stars revolve around him. the sun may be nobler than the earth, but man is nobler than the sun.[ ] if the earth's movement were admitted, ptolemy's defense would be broken down through the elimination of the epicycles of the superior planets: here, if ever, the copernicans appear to score, as riccioli himself admits,[ ] but he calls to his aid tycho brahe and the bible. "to invoke such aids is to avow his defeat" is m. delambre's comment at this point.[ ] there are many more arguments, of which the foregoing are but instances chosen more or less at random; but no one of them is of especial weight or novelty. [footnote : delambre: _astr. mod._: i, - .] [footnote : riccioli: _apologia_: .] [footnote : riccioli: _alm. nov._: ii, , .] [footnote : riccioli: _alm. nov._: ii, - .] [footnote : ibid: ii, - .] [footnote : delambre: _op. cit._: i, .] to strengthen his case, riccioli listed the supporters of the heliocentric doctrine throughout the ages, with those of the opposite view. if a man's fame adds to the weight of his opinion, the modern reader will be inclined to think the copernicans have the best of it, for omitting the ancients, most of those opposing it are obscure men.[ ] [footnote : ibid: i, .] in favor of the copernican doctrine [references omitted].[ ] copernicus rheticus mæstlin kepler rothman galileo gilbert (diurnal motion) foscarini didacus stunica (_sic_) ismael bullialdus jacob lansberg peter herigonus gassendi,--"but submits his intellect captive to the church decrees." descartes "inclines to this belief." a.l. politianus bruno against the hypothesis of the earth's movement. aristotle ptolemy theon the alexandrine regiomontanus alfraganus macrobius cleomedes petrus aliacensis george buchanan maurolycus clavius barocius michael neander telesius martinengus justus-lipsius scheiner tycho tasso scipio claramontius michael incofer fromundus jacob ascarisius julius cæsar la galla tanner bartholomæus amicus antonio rocce marinus mersennius polacco kircher spinella pineda lorinis mastrius bellutris poncius delphinus elephantutius [footnote : riccioli: _alm. nov._: ii, .] riccioli nevertheless viewed the copernican system with much sympathy. after a full statement of it, he comments: "we have not yet exhausted the full profundities of the copernican hypothesis, for the deeper one digs into it, the more ingenious and valuable subtilties may one unearth." then he adds that "the greatness of copernicus has never been sufficiently appreciated nor will it be,--that man who accomplished what no astronomer before him had scarcely been able even to suggest without an insane machinery of spheres, for by a triple motion of the earth he abolished epicycles and eccentrics. what before so many atlases could not support, this one hercules has dared to carry. would that he had kept himself within the limits of his hypothesis!"[ ] [footnote : riccioli: _op. cit._: ii, , .] his conclusions seem to show that only his position as a jesuit restrained him from being a copernican himself.[ ] "i. if the celestial phenomena alone are considered, they are equally well explained by the two hypotheses [ptolemaic and copernican]. ii. the physical evidence as explained in the two systems with exception of percussion and the speed of bodies driven north or south, and east or west, is all for immobility. iii. one might waver indifferently between the two hypotheses aside from the witness of the scriptures, which settles the question. iv. there are in addition plenty of other motives besides scriptural ones for rejecting this system." (!) but with the scriptural evidence he adduces the decree of the index under paul v against the doctrine, and the sentence of galileo, so that "the sole possible conclusion is that the earth stands by nature immobile in the center of the universe, and the sun moves around it with both a diurnal and an annual motion."[ ] [footnote : delambre: _astr. mod._: i, .] [footnote : riccioli: _op. cit._: ii, (condensed), .] even this great book was as insufficient to stop the criticism of the action of the congregations, as it was to stop the spread of the doctrine. so once again the father took up the cudgels in defense of the church. the full title of his _apologia_ runs: "an apologia in behalf of an argument from physical mathematics against the copernican system, directed against that system by a new argument from the reflex motion of falling weights." (venice, ). he states in this that his _almagestum novum_ had received the approbation of professors of mathematics at bologna, of one at pisa, and of another at padua, and he quotes the conclusion from _nicetas orthodoxus_ ("a diatribe by julius turrinus, doctor of mathematics, philosophy, medicine, law, and greek letters"): "that the sun is revolved by diurnal and by annual motion, and that the earth is at rest i firmly hold, infallibly believe, and openly confess, not because of mathematical reasons, but solely at the command of faith, by the authority of the scriptures, and the nod of approval (_nutu_) of the roman see, whose rules laid down at the dictation of the spirit of truth, may i, as befits everyone, uphold as law."[ ] [footnote : riccioli: _apologia_: .] riccioli further on proceeds to answer his objecters, declaring that "the church did not decide _ex cathedra_ that the scripture concerning movement should be interpreted literally; that the censure was laid by qualified theologians and approved by eminent cardinals, and was not merely provisional, nor for the time being absolute, since the contrary could never be demonstrated; and that while it was the primary intent of the inquisitors to condemn the opinion as heretical and directly contrary to the scriptures ... they added that it was absurd and false also in philosophy, in order, not to avert any objections which could be on the side of philosophy or astronomy, but only lest any one should say that scripture is opposed to philosophy."[ ] these answers are indicative of the type of criticism with which the church had to cope even at that time.[ ] [footnote : ibid: .] [footnote : one bit of contemporary opinion on riccioli and his work has come down to us. a canon at liège, réné-françois sluse, wrote asking a friend (about ) to sound wallis, the english mathematician, as to his opinion of the _almagestum novum_, and of this argument based on the acceleration of movement in falling bodies. wallis himself replied that he thought the argument devoid of all value. the canon at once wrote, "i do not understand how a man as intelligent as riccioli should think he could bring to a close a matter so difficult [the refutation] by a proof as futile as this." monchamp: - . for a full, annotated list of books published against the copernican system between - , see martin: _galilée_: - .] chapter iv. the gradual acceptance of the copernican system. just as tycho brahe's system proved to be for some a good half-way station between the improbable ptolemaic and the heretical copernican system;[ ] so the cartesian philosophy helped others to reconcile their scientific knowledge with their reverence for the scriptures, until newton's work had more fully demonstrated the scientific truth. [footnote : see moxon: _advice, a tutor to astronomy and geography_ ( ): .] its originator, réné descartes[ ] ( - ) was in holland when word of galileo's condemnation reached him in , as he was seeking in the bookshops of amsterdam and leyden for a copy of the _dialogo_.[ ] he at once became alarmed lest he too be accused of trying to establish the movement of the earth, a doctrine which he had understood was then publicly taught even in rome, and which he had made the basis of his own philosophy. if this doctrine were condemned as false, then his philosophy must be also; and, true to his training by the jesuits, rather than go against the church he would not publish his books. he set aside his _cosmos_, and delayed the publication of the _méthode_ for some years in consequence, even starting to translate it into latin as a safeguard.[ ] his conception of the universe, the copernican one modified to meet the requirements of a literally interpreted bible, was not printed until , when it appeared in his _principes_.[ ] [footnote : haldane's _descartes_ ( ) is the most recent and authoritative account based upon descartes's works as published in the adams-tannery edition (paris, . foll.). this edition supersedes that of cousin. [transcriber's note: missing footnote reference in original text has been added above in a logical place.]] [footnote : haldane: .] [footnote : ibid: .] [footnote : descartes: _principes_, pt. iii, chap. .] according to this statement which he made only as a possible explanation of the phenomena and not as an absolute truth, while there was little to choose between the tychonic and the copernican conceptions, he inclined slightly toward the former. he conceived of the earth and the other planets as each borne along in its enveloping heaven like a ship by the tide, or like a man asleep on a ship that was sailing from calais to dover. the earth itself does not move, but it is transported so that its position is changed in relation to the other planets but not visibly so in relation to the fixed stars because of the vast intervening spaces. the laws of the universe affect even the most minute particle, and all alike are swept along in a series of vortices, or whirlpools, of greater or less size. thus the whole planetary system sweeps around the sun in one great vortex, as the satellites sweep around their respective planets in lesser ones. in this way descartes worked out a mechanical explanation of the universe of considerable importance because it was a rational one which anyone could understand. its defects were many, to be sure, as for example, that it did not allow for the elliptical orbits of the planets;[ ] and one critic has claimed that this theory of a motionless earth borne along by an enveloping heaven was comparable to a worm in a dutch cheese sent from amsterdam to batavia,--the worm has travelled about leagues but without changing its place![ ] but this theory fulfilled descartes's aim: to show that the universe was governed by mechanical laws of which we can be absolutely certain and that galileo's discoveries simply indicated this.[ ] [footnote : haldane: .] [footnote : monchamp: , note.] [footnote : haldane: .] this exposition of the copernican doctrine strongly appealed to the literary world of the th and th centuries in western europe, especially in the netherlands, in the paris salons and in the universities.[ ] m. monchamp cites a number of contemporary comments upon its spread, in one of which it is claimed that in , the university of louvain had for the preceding forty years been practically composed of cartesians.[ ] for the time being, this theory was a more or less satisfactory explanation of the universe according to known laws; it answered to galileo's observations; it was in harmony with the scriptures, and its vortices paved the way for the popular acceptance of newton's law of universal gravitation. [footnote : ibid: , .] [footnote : monchamp: - .] protestant england was of course little disturbed by the decree against the copernican doctrine, a fact that makes it possible, perhaps, to see there more clearly the change in people's attitude from antagonism to acceptance, than in catholic europe where fear of the church's power, and respect for its decisions inhibited honest public expression of thought and conviction. while in england also the literal interpretation of the scriptures continued to be with the common people a strong objection against the doctrine, the rationalist movement of the late seventeenth and eighteenth centuries along with newton's great work, helped win acceptance for it among the better educated classes. bruno had failed to win over his english hearers, and in when the _de magnete_ was published, william gilbert, ( - ) was apparently the only supporter of the earth's movement then in england,[ ] and he advocated the diurnal motion only.[ ] not many, however, were as outspoken as bacon in denunciation of the system; they were simply somewhat ironically indifferent. an exception to this was dean wren of windsor (father of the famous architect). he could not speak strongly enough against it in his marginal notes on browne's _pseudodoxia epidemica_. as dr. johnson wrote,[ ] sir thomas browne ( - ) himself in his zeal for the old errors, did not easily admit new positions, for he never mentioned the motion of the earth but with contempt and ridicule. this was not enough for the dean, who wrote in the margin of browne's book, at such a passage,[ ] that there were "eighty-odd expresse places in the bible affirming in plaine and overt terms the naturall and perpetuall motion of sun and moon" and that "a man should be affrighted to follow that audacious and pernicious suggestion which satan used, and thereby undid us all in our first parents, that god hath a double meaning in his commands, in effect condemning god of amphibologye. and all this boldness and overweaning having no other ground but a seeming argument of some phenomena forsooth, which notwithstanding we know the learned tycho, prince of astronomers, who lived fifty-two years since copernicus, hath by admirable and matchlesse instruments and many yeares exact observations proved to bee noe better than a dreame." [footnote : berry quotes (p. ) a passage from thomas digges (d. ) with the date : "but in this our age, one rare witte (seeing the continuall errors that from time to time more and more continually have been discovered, besides the infinite absurdities in their theoricks, which they have been forced to admit that would not confess any mobility in the ball of the earth) hath by long studye, paynfull practise, and rare invention delivered a new theorick or model of the world, shewing that the earth resteth not in the center of the whole world or globe of elements, which encircled or enclosed in the moone's orbit, and together with the whole globe of mortality is carried round about the sunne, which like a king in the middst of all, rayneth and giveth laws of motion to all the rest, sphærically dispersing his glorious beames of light through all this sacred celestiall temple." browne also refers to digges (i, ).] [footnote : gilbert: _de magnete_, bk. vi, c. - ( - ).] [footnote : johnson: _life_, in browne: i, xvii.] [footnote : browne: i, .] richard [transcriber's note: robert] burton ( - ) in _the anatomy of melancholy_ speaks of the doctrine as a "prodigious tenent, or paradox," lately revived by "copernicus, brunus and some others," and calls copernicus in consequence the successor of atlas.[ ] the vast extent of the heavens that this supposition requires, he considers "quite opposite to reason, to natural philosophy, and all out as absurd as disproportional, (so some will) as prodigious, as that of the sun's swift motion of the heavens." if the earth is a planet, then other planets may be inhabited (as christian huygens argued later on); and this involves a possible plurality of worlds. burton laughs at those who, to avoid the church attitude and yet explain the celestial phenomena, invent new hypotheses and new systems of the world, "correcting others, doing worse themselves, reforming some and marring all," as he says of roeslin's endeavors. "in the meantime the world is tossed in a blanket amongst them; they hoyse the earth up and down like a ball, make it stand and goe at their pleasure."[ ] he himself was indifferent. [footnote : burton: _anatomy of melancholy_, i, ; i, . first edition, ; reprinted , , , , - , , .] [footnote : ibid: i, , .] others more sensitive to the implications of this system, might exclaim with george herbert ( - ):[ ] "although there were some fourtie heav'ns, or more, sometimes i peere above them all; sometimes i hardly reach a score, sometimes to hell i fall. "o rack me not to such a vast extent, those distances belong to thee. the world's too little for thy tent, a grave too big for me." [footnote : herbert: ii, .] or they might waver, undecided, like milton who had the archangel answer adam's questions thus:[ ] "but whether thus these things, or whether not, whether the sun predominant in heaven rise on the earth, or earth rise on the sun, hee from the east his flaming robe begin, or shee from west her silent course advance with inoffensive pace that spinning sleeps on her soft axle, while she paces ev'n and bears thee soft with the smooth air along, solicit not thy thoughts with matters hid, leave them to god above, him serve and feare; of other creatures, as him pleases best, wherever plac't, let him dispose; joy thou in what he gives to thee, this paradise and the fair eve: heaven is for thee too high to know what passes there: be lowlie wise." ( ) [footnote : milton: _paradise lost_, bk. viii, lines _et seq._ the great puritan divine, john owen ( - ), accepts the miracle of the sun's standing still without a word of reference to the new astronomy. (_works_: ii, .) farrar states that owen declared newton's discoveries were against the evident testimonies of scripture (farrar: _history of interpretation_: xviii.), but i have been unable to verify this statement. owen died before the _principia_ was published in .] whewell thinks[ ] that at this time the diffusion of the copernican system was due more to the writings of bishop wilkins than to those of any one else, for their very extravagances drew stronger attention to it. the first, "the discovery of a new world: or a discourse tending to prove that there may be another habitable world in the moon," appeared in ; and a third edition was issued only two years later together with the second book; "discourse concerning a new planet--that 'tis probable our earth is one of the planets." in this latter, the bishop stated certain propositions as indubitable; among these were, that the scriptural passages intimating diurnal motion of the sun or of the heavens are fairly capable of another interpretation; that there is no sufficient reason to prove the earth incapable of those motions which copernicus ascribes to it; that it is more probable the earth does move than the heavens, and that this hypothesis is exactly agreeable to common appearances.[ ] and these books appeared when political and constitutional matters, and not astronomical ones, were the burning questions of the day in england. [footnote : whewell: i, .] [footnote : wilkins: _discourse concerning a new planet_.] the spread of the doctrine was also helped by thomas salusbury's translations of the books and passages condemned by the index in and . this collection, "intended for gentlemen," he published by popular subscription immediately after the restoration,[ ] a fact that indicates that not merely mathematicians (whom whewell claims[ ] were by that time all decided copernicans) but the general public were interested and awake.[ ] [footnote : salusbury: _math. coll._: to the reader.] [footnote : whewell: i, .] [footnote : one london bookseller in advertised for sale "spheres according to the ptolmean, tychonean and copernican systems with books for their use." (moxon: .) in in london appeared the third edition of gassendi's _institutio_, the textbook of astronomy in the universities during this period of uncertainty. it too wavers between the tychonic and the copernican systems.] the appearance of newton's _principia_ in with his statement of the universal application of the law of gravitation, soon ended hesitancy for most people. twelve years later, john keill, ( - ), the scotch mathematician and astronomer at oxford, refuted descartes's theory of vortices and opened the first course of lectures delivered at oxford on the new newtonian philosophy.[ ] not only were his lectures thronged, but his books advocating the copernican system in full[ ] went through several editions in relatively few years. [footnote : _dict. of nat. biog._: "keill."] [footnote : keill: _introductio ad veram astronomiam_.] in the colonies, yale university which had hitherto been using gassendi's textbook, adopted the newtonian ideas a few years later, partly through the gift to the university of some books by sir isaac himself, and partly through the enthusiasm of two young instructors there, johnson and brown, who in - widened the mathematical course by including the new theories.[ ] the text they used was by rohault, a cartesian, edited by samuel clarke with critical notes exposing the fallacies of cartesianism. this "disguised newtonian treatise" was used at yale till . the university of pennsylvania used this same text book even later.[ ] [footnote : cajori: - .] [footnote : cajori: .] in pope ( - ) refers to "our copernican system,"[ ] and addison ( - ) in the _spectator_ (july , ) writes this very modern passage: "but among this set of writers, there are none who more gratify and enlarge the imagination, than the authors of the new philosophy, whether we consider their theories of the earth or heavens, the discoveries they have made by glasses, or any other of their contemplations on nature.... but when we survey the whole earth at once, and the several planets that lie within its neighborhood, we are filled with a pleasing astonishment, to see so many worlds hanging one above another, and sliding around their axles in such an amazing pomp and solemnity. if, after this, we contemplate those wide fields of æther, that reach in height as far as from saturn to the fixed stars, and run abroad almost to an infinitude, our imagination finds its capacity filled with so immense a prospect, as puts it upon the stretch to comprehend it. but if we yet rise higher, and consider the fixed stars as so many vast oceans of flame, that are each of them attended with a different set of planets, and still discover new firmaments and new lights, that are sunk farther in those unfathomable depths of æther, so as not to be seen by the strongest of our telescopes, we are lost in such a labyrinth of suns and worlds, and confounded with the immensity and magnificence of nature. "nothing is more pleasant to the fancy, than to enlarge itself by degrees, in its contemplation of the various proportions which its several objects bear to each other, when it compares the body of man to the bulk of the whole earth, the earth to the circle it describes round the sun, that circle to the sphere of the fixed stars, the sphere of the fixed stars to the circuit of the whole creation, the whole creation itself to the infinite space that is everywhere diffused around it; ... but if, after all this, we take the least particle of these animal spirits, and consider its capacity wrought into a world, that shall contain within those narrow dimensions a heaven and earth, stars and planets, and every different species of living creatures, in the same analogy and proportion they bear to each other in our own universe; such a speculation, by reason of its nicety, appears ridiculous to those who have not turned their thoughts that way, though, at the same time, it is founded on no less than the evidence of a demonstration."[ ] [footnote : pope: _works_, vi, .] [footnote : addison: _spectator_, no. , (iv, - ). an interesting contrast to this passage and a good illustration of how the traditional phraseology continued in poetry is found in addison's famous hymn, written a year later: "whilst all the stars that round her [earth] burn and all the planets in their turn, confirm the tidings as they roll, and spread the truth from pole to pole. "what though in solemn silence all move round this dark terrestrial ball; what though no real voice nor sound amidst their radiant orbs be found; "in reason's ear they all rejoice, and utter forth a glorious voice; forever singing, as they shine, 'the hand that made us is divine'."] a little later, cotton mather declared ( ) that the "copernican hypothesis is now generally preferred," and "that there is no objection against the motion of the earth but what has had a full solution."[ ] soon the semi-popular scientific books took up the newtonian astronomy. one such was described as "useful for all sea-faring men, as well as gentlemen, and others."[ ] "newtonianisme pour les dames" was advertised in france in the forties.[ ] by when pope wrote the _universal prayer_: "yet not to earth's contracted span thy goodness let me bound or think thee lord alone of man, when thousand worlds are round," the copernican-newtonian astronomy had become a commonplace to most well-educated people in england. to be sure, the great john wesley ( ) considered the systems of the universe merely "ingenious conjectures," but then, he doubted whether "more than probabilities we shall ever attain in regard to things at so great a distance from us."[ ] [footnote : mather: _christian philosopher_, , .] [footnote : leadbetter: _astronomy_ ( ).] [footnote : in de maupertius: _ouvrages divers_, (at the back).] [footnote : wesley: _compendium of natural philosophy_, i, , .] the old phraseology, however, did recur occasionally, especially in poetry and in hymns. for instance, a hymnal (preface dated ) contains such choice selections as: "before the pondr'ous earthly globe in fluid air was stay'd, before the ocean's mighty springs their liquid stores display'd"-- and: "who led his blest unerring hand or lent his needful aid when on its strong unshaken base the pondr'ous earth was laid?"[ ] [footnote : dobell: _hymns_, no. , no. .] but too much importance should not be attributed to such passages; though poetry and astronomy need not conflict, as keble illustrated:[ ] "ye stars that round the sun of righteousness in glorious order roll...." [footnote : keble: _christian year_, .] by the middle of the th century in england, one could say with horne "that the newtonian system had been in possession of the chair for some years;"[ ] but it had not yet convinced the common people, for as pike wrote in , "many common christians to this day firmly believe that the earth really stands still and that the sun moves all round the earth once a day: neither can they be easily persuaded out of this opinion, because they look upon themselves bound to believe what the scripture asserts."[ ] [footnote : horne: _fair, candid, impartial statement ..._, .] [footnote : pike: _philosophia sacra_, .] there was, however, just at this time a little group of thinkers who objected to newton's scheme, "because of the endless uninterrupted flux of matter from the sun in light, an expense which should destroy that orb."[ ] these hutchinsonians conceived of light as pure ether in motion springing forth from the sun, growing more dense the further it goes till it becomes air, and, striking the circumference of the universe (which is perhaps an immovable solid), is thrown back toward the sun and melted into light again. its force as its tides of motion strike the earth and the other planets produces their constant gyrations.[ ] men like duncan forbes, lord president of the court of sessions, and george horne, president of magdalen college, oxford, as a weapon against rationalism, favored this notion that had been expounded by john hutchinson ( - ) in his _moses's principia_ ( ).[ ] they were also strongly attracted by the scriptural symbolism with which the book abounds. leslie stephen summarizes their doctrines as ( ) extreme dislike for rationalism, ( ) a fanatical respect for the letter of the bible, and ( ) an attempt to enlist the rising powers of scientific enquiry upon the side of orthodoxy.[ ] this "little eddy of thought"[ ] was not of much influence even at that time, but it has a certain interest as indicating the positions men have taken when on the defensive against new ideas. [footnote : forbes: _letter_, ( ).] [footnote : see wesley: i, - .] [footnote : _dict. of nat. biog._: "hutchinson."] [footnote : stephen: _hist. of eng. thought_: i, .] [footnote : ibid: .] chapter v. the church and the new astronomy: conclusion. astronomical thought on the continent was more hampered, in the catholic countries especially, by the restrictive opinions of the church. yet in , when the decree prohibiting all books dealing with the copernican doctrine was removed from the index, that system had already long been adopted by the more celebrated academies of europe, for so mme. de premontval claimed in ; and it was then reaching out to non-scientific readers, through simple accounts for "ladies and others not well versed in these somewhat technical matters."[ ] the great landmark in the development of the doctrine was the publication of newton's _principia_ in , though its effect in europe was of course slower in being felt than it was in england. newton's work and that of the astronomers immediately following him was influential except where the church's prohibitions still held sway. [footnote : de premontval: _le méchaniste philosophe_, , . (the hague, ).] during this period, the books published in free holland were more outspoken in their radical acceptance or in their uncertainty of the truth than were those published in the catholic countries. christian huygens's treatises on the plurality of worlds not only fully accepted the copernican doctrine, but like those of bishop wilkins in england, deduced therefrom the probability that the other planets are inhabited even as the earth is. a writer[ ] on the sphere in stated the different theories of the universe so that his readers might choose the one that to them appeared the most probable. he himself preferred the cartesian explanation as the simplest and most convenient of all, "though it should be held merely as an hypothesis and not as in absolute agreement with the truth." pierre bayle[ ] also explained the different systems, but appears himself to waver between the copernican and the tychonic conceptions. he used, however, the old word "perigee" (nearness to the earth) rather than the newtonian "perihelion" (nearness to the sun). his objections to the copernican doctrine have a familiar ring: it is contrary to the evidence of the senses; a stone would not fall back to its starting-place, nor could a bird return to her nest; the earth would not be equidistant from the horizon and the two poles; and lastly it is contrary to the scriptures. only a few years later, however, de maupertius wrote that no one at that day ( ) doubted any longer the motion of the earth around its axis, and he believed with newton that the laws of gravity applied to the universe as well as to the earth. then he proceeded to explain the copernican system which he favored on the ground of its greater probability.[ ] [footnote : de brisbar: _calendrier historique_, (leyden), - .] [footnote : bayle: _système abregé de philosophie_ (the hague, ), iv, - .] [footnote : de maupertius: _eléments de géographie_, xv, - .] even in , mme. de premontval thought it wiser to publish in holland her little life of her father, _le méchaniste philosophe_. this jean piegeon, she claimed, was the first man in france to make spheres according to the copernican system. an orphan, he was educated by a priest; then took up carpentry and mechanics. when he tried to make a celestial sphere according to the ptolemaic system, he became convinced of its falsity because of its complexities. therefore he plunged into a study of the new system which he adopted. his first copernican sphere was exhibited before louis xiv at versailles in and was bought by the king and presented to the académie des sciences.[ ] the second was taken to canada by one of the royal officials. public interest in his work was keen; even peter the great, who was then in paris, visited his workroom.[ ] m. piegeon also wrote a book on the copernican system.[ ] [footnote : de premontval: .] [footnote : ibid: .] [footnote : ibid: .] it seems, however, as though m. piegeon were slightly in advance of his age, or more daring, perhaps, than his contemporaries, for there was almost no outspoken support of the copernican system at this time in france. even cassini of the french académie des sciences did not explicitly support it, though he spoke favorably of it and remarked that recent observations had demonstrated the revolutions of each planet around the sun in accordance with that supposition.[ ] but the great orator, bossuet, ( - ), clung to the ptolemaic conception as alone orthodox, and scriptural.[ ] abbé fénelon ( - ) writing on the existence of god, asked: "who is it who has hung up this motionless ball of the earth; who has placed the foundations for it," and "who has taught the sun to turn ceasely [transcriber's note: ceaselessly] and regularly in spaces where nothing troubles it?"[ ] and a writer on the history of the heavens as treated by poets, philosophers and moses ( ), tells gassendi, descartes and many other great thinkers that their ideas of the heavens are proved vain and false by daily experience as well as by the account of creation; for the most enlightened experience is wholly and completely in accord with the account of moses. this book was written, the author said, for young people students of philosophy and the humanities, also for teachers.[ ] [footnote : cassini: _de l'origine et du progrès ..._, .] [footnote : shields: . i have failed to find this reference in bossuet's works.] [footnote : fénelon: _oeuvres_, i, and .] [footnote : pluche: _histoire du ciel_: viii, ix, xiii.] the jesuit order, still a power in europe in the early th century, was bound to the support of the traditional view, which led them into some curious positions in connection with the discoveries made in astronomy during this period. thus the famous jesuit astronomer boscovich ( - ) published in rome in a study of the ellipticity of the orbits of planets which necessitated the use of the copernican position; he stated he had assumed it as true merely to facilitate his labors. in the second edition ( ) published some years after the removal from the index of the decree against books teaching the copernican doctrine (at his instigation, it is claimed),[ ] he added a note to this passage asking the reader to remember the time and the place of its former publication.[ ] just at the end of the preceding century, one of the seminary fathers at liège maintained that were the earth to move, being made up of so many and divers combustible materials, it would soon burst into flames and be reduced to ashes![ ] [footnote : _cath. ency._: "boscovich."] [footnote : _opera_: iii ( ).] [footnote : cited in monchamp: note.] during the th century at louvain the copernican doctrine was warmly supported, but as a theory. a ms. of a course given there in has come down to us, in which the professor, while affirming its hypothetical character, described it as a simple, clear and satisfactory explanation of the phenomena, then answered all the objections made against it by theologians, physicists, and astronomers.[ ] a few years earlier, ( ) a jesuit at liège, though well acquainted with newton's work, declared: "for my part i do not doubt the least in the world that the earth is eternally fixed, for god has founded the terrestrial globe, and it will not be shaken."[ ] another priest stated in the first chapter of his astronomy that the sun and the planets daily revolve around the earth; then later on, he explained the copernican and the tychonic schemes and the cartesian theory of motion with evident sympathy.[ ] two others, one a jesuit in at naples,[ ] the other in at verona, frankly preferred the tychonic system, and the latter called the system found by "tommaso copernico" a mere fancy.[ ] still another priest, evidently well acquainted with bradley's work, as late as in declared that there was nothing decisive on either side of the great controversy between the systems.[ ] at this time, however, a father was teaching the copernican system at liège without differentiating between thesis and hypothesis.[ ] and a jesuit, while he denied ( ) universal gravitation, the earth's movement, and the plurality of inhabited worlds, declared that the roman congregation had done wrong in charging these as heretical suggestions. in fact, m. monchamp, himself a catholic priest at louvain, declared that the newtonian proofs were considered by many in the th century virtually to abrogate the condemnation of and ; hence the professors of the seminary at liège had adopted the copernican system.[ ] [footnote : ibid: .] [footnote : ibid: .] [footnote : fontana: _institutio_, ii, - .] [footnote : ferramosca: _positiones ..._: .] [footnote : piccoli: _la scienza_, , .] [footnote : spagnio, _de motu_, .] [footnote : monchamp: .] [footnote : monchamp: .] the famous french astronomer lalande, in rome in when the inquisition first modified its position, tried to persuade the authorities to remove galileo's book also from the index; but his efforts were unavailing, because of the sentence declared against its author.[ ] in canon settele was not allowed by the master of the sacred palace to publish his textbook because it dealt with the forbidden subject. his appeal to the congregation itself resulted, as we have seen, in the decree of removing this as a cause for prohibition. yet as late as in , when a statue to copernicus was being unveiled at warsaw, and a great convocation had met in the church for the celebration of the mass as part of the ceremony, at the last moment the clergy refused in a body to attend a service in honor of a man whose book was on the index.[ ] [footnote : bailly: ii, , note.] [footnote : flammarion: - .] thus the roman catholic church by reason of its organization and of its doctrine requiring obedience to its authority was more conspicuous for its opposition as a body to the copernican doctrine, even though as individuals many of its members favored the new system. but the protestant leaders were quite as emphatic in their denunciations, though less influential because of the protestant idea of the right to individual belief and interpretation. luther, melancthon, calvin, turrettin,[ ] owen, and wesley are some of the notable opponents to it. and when the scientific objections had practically disappeared, those who interpreted the scriptures literally were still troubled and hesitant down to the present day. not many years ago, people flocked to hear a negro preacher of the south, brother jasper, uphold with all his ability that the sun stood still at joshua's command, and that today "the sun do move!" far more surprising is this statement in the new _catholic encyclopedia_ under "faith," written by an english dominican: "if, now, the will moves the intellect to consider some debatable point--_e.g._, the copernican and ptolemaic theories of the relationship between the sun and the earth--it is clear that the intellect can only assent to one of these views in proportion that it is convinced that the particular view is true. but neither view has, as far as we can know, more than probable truth, hence of itself the intellect can only give in its partial adherence to one of these views, it must always be precluded from absolute assent by the possibility that the other may be right. the fact that men hold more tenaciously to one of these than the arguments warrant can only be due to some extrinsic consideration, _e.g._, that it is absurd not to hold to what a vast majority of men hold." [footnote : shields: .] in astronomical thought as in many another field, science and reason have had a hard struggle in men's minds to defeat tradition and the weight of verbal inspiration. within the roman catholic church opposition to this doctrine was officially weakened in , but not completely ended till the publication of the index in --the first edition since the decrees of and which did not contain the works of copernicus, galileo, foscarini, à stunica and kepler. since then, roman catholic writers have been particularly active in defending and explaining the positions of the church in these matters. they have not agreed among themselves as to whether the infallibility of the church had been involved in these condemnations, nor as to the reasons for them. as one writer has summarized these diverse positions,[ ] they first claimed that galileo was condemned not for upholding a heresy, but for attempting to reconcile these ideas with the scriptures,--though in fact he was sentenced specifically for heresy. in their next defense they declared galileo was not condemned for heresy, but for contumacy and want of respect to the pope.[ ] this statement proving untenable, others held that it was the result of a persecution developing out of a quarrel between aristotelian professors and those professors who favored experiment,--a still worse argument for the church itself. then some claimed that the condemnation was merely provisional,--a position hardly warranted by the wording of the decrees themselves and flatly contradicted by father riccioli, the spokesman of the jesuit authorities.[ ] more recently, roman catholics have held that galileo was no more a victim of the roman church than of the protestant--which fails to remove the blame of either. the most recent position is that the condemnation of the doctrine by the popes was not as popes but as men simply, and the church was not committed to their decision since the popes had not signed the decrees. but two noted english catholics, roberts and mivart, publicly stated in that the infallibility of the papacy was fully committed in these condemnations by what they termed incontrovertible evidence.[ ] [footnote : white: i, - .] [footnote : see di bruno: _catholic belief_, a.] [footnote : riccioli: _apologia_, .] [footnote : white: i, . see the answer by wegg-prosser: _galileo and his judges_.] one present-day catholic calls the action of the congregations "a theoretical mistake;"[ ] another admits it was a deplorable mistake, but practically their only serious one;[ ] and a third considers it "providential" since it proved conclusively "that whenever there is apparent contradiction between the truths of science and the truths of faith, either the scientist is declaring as proved what in reality is a mere hypothesis, or the theologian is putting forth his own personal views instead of the teaching of the gospel."[ ] few would accept today, however, the opinion of the anonymous writer in the _dublin review_ in the forties that "to the pontiffs and dignitaries of rome we are mainly indebted for the copernican system" and that the phrases "heretical" and "heresy" in the sentence of were but the _stylus curiæ_, for it was termed heresy only in the technical sense.[ ] [footnote : donat: .] [footnote : walsh: _popes and science_, .] [footnote : conway: .] [footnote : anon.: _galileo--the roman congregation_, , .] the majority of protestants, with the possible exception of the lutherans, were satisfied with the probable truth of the copernican doctrine before the end of the th century. down to the present day, however, there have been isolated protests raised against it, usually on technical grounds supported by reference to the scriptures. de morgan refers to one such, "an inquiry into the copernican system ... wherein it is proved in the clearest manner, that the earth has only her diurnal motion ... with an attempt to point out the only true way whereby mankind can receive any real benefit from the study of the heavenly bodies, by john cunningham, london, ." de morgan adds that "the true way appears to be the treatment of heaven and earth as emblematical of the trinity."[ ] another, by "anglo-american," is entitled "copernicus refuted; or the true solar system" (baltimore, ). it begins thus: "one of these must go, the other stand still, it matters not which, so choose at your will; but when you find one already stuck fast, you've only got hobson's choice left at last." [footnote : de morgan: i, .] this writer admits the earth's axial rotation, but declares the earth is fixed as a pivot in the center of the universe, because the poles of the earth are fixed and immovable, and that the sun as in the tychonic scheme encircles the earth and is itself encircled by five planets.[ ] his account of the origin of the copernican system is noteworthy: it was originated by pythagoras and his deciples but lay neglected because it was held to be untenable in their time; it was "revived when learning was at its lowest ebb by a monk in his cloister, copernicus, who in ransacking the contents of the monastery happened to lay his hands on the ms. and then published it to the world with all its blunders and imperfections!"[ ] one might remark that the anglo-american's own learning was at very low ebb. [footnote : "anglo-american": - .] [footnote : ibid: .] the tychonic scheme was revived also some years later by a dane, zytphen ( ).[ ] three years after, an assembly of lutheran clergy met together at berlin to protest against "science falsely so-called,"[ ] but were brought into ridicule by pastor knap's denunciations of the copernican theory as absolutely incompatible with belief in the bible. a carl schoepffer had taken up the defense of the tychonic scheme in berlin before this ( ) and by his lecture was in its seventh edition. in it he sought to prove that the earth revolves neither upon its own axis nor yet about the sun. he had seen foucault's pendulum demonstration of the earth's movement, but he held that something else, as yet unexplained, caused the deviation of the pendulum, and that the velocity of the heavens would be no more amazing than the almost incredible velocity of light or of electricity.[ ] his lecture, curiously enough, fell into the hands of the late general john watts de peyster of new york, who had it translated and published in together with a supplement by frank allaben.[ ] both these gentlemen accepted its scientific views and deductions, but the general refused to go as far as his colleague in the latter's enthusiastic acceptance of the verbal inspiration of the scriptures as a result of these statements.[ ] a few months later, they published a supplementary pamphlet claiming to prove the possibility of the sun's velocity by the analogy of the velocity of certain comets.[ ] a professor j.r. lange of california (a german), attracted by these documents, sent them his own lucubrations on this subject. he considered newton's doctrine of universal attraction "nonsense," and had "absolute proof" in the fixity of the pole star that the earth does not move.[ ] in a letter to general de peyster, he wrote: "let us hope and pray that the days of the pernicious copernican system may be numbered,"[ ]--but he did not specify why he considered it pernicious. the general was nearly eighty years old when he became interested in these matters, and he did not live long thereafter to defend his position. his biographers make no mention of it. the other men seem almost obsessed, especially lange;--like the italian painter, sindico, who bombarded the director of the paris observatory in with many letters protesting against the copernican system.[ ] [footnote : de morgan: ii, .] [footnote : white: i, .] [footnote : schoepffer: _the earth stands fast_, title-page, - .] [footnote : ibid: supplement by allaben, , .] [footnote : ibid: note by j.w. de p., .] [footnote : de peyster and allaben: _algol_, preface.] [footnote : lange: _the copernican system: the greatest absurdity in the history of human thought_.] [footnote : de peyster and allaben: _algol_, .] [footnote : sindico: _refutation du système de copernic...._] german writers, whether lutherans or not, appear to have opposed the system more often in the last century than have the writers of other nationalities. besides those already mentioned, one proposed an ingenious scheme in which the sun moves through space followed by the planets as a comet is by its tail, the planets revolving in a plane perpendicular to that of the sun's path. a diagram of it would be cone-shaped. he included in this pamphlet, besides a list of his own books, (all published in leipsic), a list of twenty-six titles from to , books and pamphlets evidently opposed in whole or in part to the modern astronomy, and seventeen of these were in german or printed in germany.[ ] in this country at st. louis was issued an _astronomische unterredung_ ( ) by j.c.w.l.; according to the late president white, a bitter attack on modern astronomy and a decision by the scriptures that the earth is the principal body of the universe, that it stands fixed, and that the sun and the moon only serve to light it.[ ] [footnote : tischner: _le système solaire se mouvant_. ( ).] [footnote : white: i, .] such statements are futile in themselves nowadays, and are valuable only to illustrate the advance of modern thought of which these are the little eddies. while modern astronomers know far more than copernicus even dreamed of, much of his work still holds true today. the world was slow to accept his system because of tradition, authority, so-called common sense, and its supposed incompatibility with scriptural passages. catholic and protestant alike opposed it on these grounds; but because of its organization and authority, the roman catholic church had far greater power and could more successfully hinder and delay its acceptance than could the protestants. consequently the system won favor slowly at first through the indifference of the authorities, then later in spite of their active antagonism. scholars believed it long before the universities were permitted to teach it; and the rationalist movement of the th century, the revolt against a superstitious religion, helped to overturn the age-old conception of the heavens and to bring newtonian-copernicanism into general acceptance. the elements of this traditional conception are summarized in the fifth book of bodin's _universæ naturæ theatrum_, a scholar's account of astronomy at the close of the sixteenth century.[ ] man in his terrestrial habitation occupies the center of a universe created solely to serve him, god presides over all from the empyrean above, sending forth his messengers the angels to guide and control the heavenly bodies. such had been the thought of christians for more than a thousand years. then came the influence of a new science. tycho brahe "broke the crystal spheres of aristotle"[ ] by his study of the comet of ; galileo's telescopes revealed many stars hitherto unknown, and partly solved the mysteries of the milky way; kepler's laws explained the courses of the planets, and newton's discovery of the universal application of the forces of attraction relieved the angels of their duties among the heavens. thinkers like bruno proposed the possibility of other systems and universes besides the solar one in which the earth belongs. and thus not only did man shrink in importance in his own eyes; but his conception of the heavens changed from that of a finite place inexplicably controlled by the mystical beings of a supernatural world, to one of vast and infinite spaces traversed by bodies whose density and mass a man could calculate, whose movements he could foretell, and whose very substance he could analyze by the science of today. this dissolution of superstition, especially in regard to comets was notably rapid and complete after the comet of .[ ] thus the rationalist movement with the new science opened men's minds to a universe composed of familiar substances and controlled by known or knowable laws with no tinge remaining of the supernatural. today a man's theological beliefs are not shaken by the discovery of a new satellite or even a new planet, and the appearance of a new comet merely provides the newspaper editor with the subject of a passing jest. [footnote : see translated sections in appendix c.] [footnote : robinson: .] [footnote : ibid: .] yet it was fully one hundred and fifty years after the publication of the _de revolutionibus_ before its system met with the general approval of scholars as well as of mathematicians; then nearly a generation more had to elapse before it was openly taught even at oxford where the roman catholic and lutheran churches had no control. during the latter part of this period, readers were often left free to decide for themselves as to the relative merits of the tychonic and copernican or copernican-cartesian schemes. but it took fully fifty years and more, besides, before these ideas had won general acceptance by the common people, so wedded were they to the traditional view through custom and a superstitious reverence for the bible. briefly then, the _de revolutionibus_ appeared in ; and quietly won some supporters, notably bruno, kepler and galileo; the congregations of the index specifically opposed it in and ; however it continued to spread among scholars and others with the aid of cartesianism for another fifty years till the appearance of newton's _principia_ in . then its acceptance rapidly became general even in catholic europe, till it was almost a commonplace in england by , two hundred years after its first formal promulgation, and had become strong enough in europe to cause the congregations in to modify their stand. thereafter opposition became a curiosity rather than a significant fact. only the roman church officially delayed its recognition of the new astronomy till the absurdity of its obsolete position was brought home to it by canon settele's appeal in . fifteen years later the last trace of official condemnation was removed, a little over two hundred years after the decrees had first been issued, and just before bessel's discovery of stellar parallax at length answered one of the strongest and oldest arguments against the system. since then have come many _apologias_ in explanation and extenuation of the church's decided stand in this matter for so many generations. though galileo himself was forced to his knees, unable to withstand his antagonists, his work lived on after him; he and copernicus, together with kepler and newton stand out both as scientists and as leaders in the advance of intellectual enlightenment. the account of their work and that of their less well-known supporters, compared with that of their antagonists, proves the truth of the ancient greek saying which rheticus used as the motto for the _narratio prima_, the first widely known account of the copernican system: "one who intends to philosophize must be free in mind." appendix a. ptolemy: _syntaxis mathematica (almagest)_ "that the earth has no movement of rotation," in _opera quæ exstant omnia_, edidit heiberg, leipsic, , bk. i, sec. : (i, - ); compared with the translation into french by halma, paris, . by proofs similar to the preceding, it is shown that the earth cannot be transported obliquely nor can it be moved away from the center. for, if that were so, all those things would take place which would happen if it occupied any other point than that of the center. it seems unnecessary to me, therefore, to seek out the cause of attraction towards the center when it is once evident from the phenomena themselves, that the earth occupies the center of the universe and that all heavy bodies are borne towards it; and this will be readily understood if it is remembered that the earth has been demonstrated to have a spherical shape, and according to what we have said, is placed at the center of the universe, for the direction of the fall of heavy bodies (i speak of their own motions) is always and everywhere perpendicular to an uncurved plane drawn tangent to the point of intersection. obviously these bodies would all meet at the center if they were not stopped by the surface, since a straight line drawn to the center is perpendicular to a plane tangent to the sphere at that point. those who consider it a paradox that a mass like the earth is supported on nothing, yet not moved at all, appear to me to argue according to the preconceptions they get from what they see happening to small bodies about them, and not according to what is characteristic of the universe as a whole, and this is the cause of their mistake. for i think that such a thing would not have seemed wonderful to them any longer if they had perceived that the earth, great as it is, is merely a point in comparison to the surrounding body of the heaven. they would find that it is possible for the earth, being infinitely small relative to the universe, to be held in check and fixed by the forces exercised over it equally and following similar directions by the universe, which is infinitely great and composed of similar parts. there is neither up nor down in the universe, for that cannot be imagined in a sphere. as to the bodies which it encloses, by a consequence of their nature it happens that those that are light and subtle are as though blown by the wind to the outside and to the circumference, and seem to appear to us to go _up_, because that is how we speak of the space above our heads that envelops us. it happens on the other hand that heavy bodies and those composed of dense parts are drawn towards the middle as towards a center, and appear to us to fall _down_, because that it is the word we apply to what is beneath our feet in the direction of the center of the earth. but one should believe that they are checked around this center by the retarding effect of shock and of friction. it would be admitted then that the entire mass of the earth, which is considerable in comparison to the bodies falling on it, could receive these in their fall without acquiring the slightest motion from the shock of their weight or of their velocity. but if the earth had a movement which was common to it and to all other heavy bodies, it would soon seemingly outstrip them as a result of its weight, thus leaving the animals and the other heavy bodies without other support than the air, and would soon touch the limits of the heaven itself. all these consequences would seem most ridiculous if one were only even imagining them. there are those who, while they admit these arguments because there is nothing to oppose them, pretend that nothing prevents the supposition, for instance, that if the sky is motionless, the earth might turn on its axis from west to east, making this revolution once a day or in a very little less time, or that, if they both turn, it is around the same axis, as we have said, and in a manner conformable to the relations between them which we have observed. it has escaped these people that in regard to the appearances of the planets themselves, nothing perhaps prevents the earth from having the simpler motion; but they do not realize how very ridiculous their opinion is in view of what takes place around us and in the air. for if we grant them that the lightest things and those composed of the subtlest parts do not move, which would be contrary to nature, while those that are in the air move visibly more swiftly than those that are terrestrial; if we grant them that the most solid and heavy bodies have a swift, steady movement of their own, though it is true however that they obey impelling forces only with difficulty; they would be obliged to admit that the earth by its revolution has a movement more rapid than the movements taking place around it, since it would make so great a circuit in so short a time. thus the bodies which do not rest on it would appear always to have a motion contrary to its own, and neither the clouds, nor any missile or flying bird would appear to go towards the east, for the earth would always outstrip them in this direction, and would anticipate them by its own movement towards the east, with the result that all the rest would appear to move backwards towards the west. if they should say that the atmosphere is carried along by the earth with the same speed as the earth's own revolution, it would be no less true that the bodies contained therein would not have the same velocity. or if they were swept along with the air, no longer would anything seem to precede or to follow, but all would always appear stationary, and neither in flight nor in throwing would any ever advance or retreat. that is, however, what we see happening, since neither the retardation nor the acceleration of anything is traceable to the movement of the earth. appendix b. "to his holiness, paul iii, supreme pontiff, preface by nicholas copernicus to his books on revolutions." (a translation of the _præfatio_ in copernicus: _de revolutionibus_; pp. - .) "i can certainly well believe, most holy father, that, while mayhap a few will accept this my book which i have written concerning the revolutions of the spheres of the world, ascribing certain motions to the sphere of the earth, people will clamor that i ought to be cast out at once for such an opinion. nor are my ideas so pleasing to me that i will not carefully weigh what others decide concerning them. and although i know that the meditations of philosophers are remote from the opinions of the unlearned, because it is their aim to seek truth in all things so far as it is permitted by god to the human reason, nevertheless i think that opinions wholly alien to the right ought to be driven out. thus when i considered with myself what an absurd fairy-tale people brought up in the opinion, sanctioned by many ages, that the earth is motionless in the midst of the heaven, as if it were the center of it, would think it if i were to assert on the contrary that the earth is moved; i hesitated long whether i would give to the light my commentaries composed in proof of this motion, or whether it would indeed be more satisfactory to follow the example of the pythagoreans and various others who were wont to pass down the mysteries of philosophy not by books, but from hand to hand only to their friends and relatives, as the letter of lysis to hipparchus proves.[ ] but verily they seemed to me not to have done this, as some think, from any dislike to spreading their teachings, but lest the most beautiful things and those investigated with much earnestness by great men, should be despised by those to whom spending good work on any book is a trouble unless they make profit by it; or if they are incited to the liberal study of philosophy by the exhortations and the example of others, yet because of the stupidity of their wits they are no more busily engaged among philosophers than drones among bees. when therefore i had pondered these matters, the scorn which was to be feared on account of the novelty and the absurdity of the opinion impelled me for that reason to set aside entirely the book already drawn up. [footnote : see prowe: _nic. cop._: iii, - .] "but friends, in truth, have brought me forth into the light again, though i long hesitated and am still reluctant; among these the foremost was nicholas schönberg, cardinal of capua, celebrated in all fields of scholarship. next to him is that scholar, my very good friend, tiedeman giese, bishop of culm, most learned in all sacred matters, (as he is), and in all good sciences. he has repeatedly urged me and, sometimes even with censure, implored me to publish this book and to suffer it to see the light at last, as it has lain hidden by me not for nine years alone, but also into the fourth 'novenium'. not a few other scholars of eminence also pleaded with me, exhorting me that i should no longer refuse to contribute my book to the common service of mathematicians on account of an imagined dread. they said that however absurd in many ways this my doctrine of the earth's motion might now appear, so much the greater would be the admiration and goodwill after people had seen by the publications of my commentaries the mists of absurdities rolled away by the most lucid demonstrations. brought to this hope, therefore, by these pleaders, i at last permitted my friends, as they had long besought me, to publish this work. "but perhaps your holiness will not be so shocked that i have dared to bring forth into the light these my lucubrations, having spent so much work in elaborating them, that i did not hesitate even to commit to a book my conclusions about the earth's motion, but that you will particularly wish to hear from me how it came into my mind to dare to imagine any motion of the earth, contrary to the accepted opinion of mathematicians and in like manner contrary to common sense. so i do not wish to conceal from your holiness that nothing else moved me to consider some other explanation for the motions of the spheres of the universe than what i knew, namely that mathematicians did not agree among themselves in their examinations of these things. for in the first place, they are so completely undecided concerning the motion of the sun and of the moon that they could not observe and prove the constant length of the great year.[ ] next, in determining the motions of both these and the five other planets, they did not use the same principles and assumptions or even the same demonstrations of the appearances of revolutions and motions. for some used only homocentric circles; others, eccentrics and epicycles, which on being questioned about, they themselves did not fully comprehend. for those who put their trust in homocentrics, although they proved that other diverse motions could be derived from these, nevertheless they could by no means decide on any thing certain which in the least corresponded to the phenomena. but these who devised eccentrics, even though they seem for the most part to have represented apparent motions by a number [of eccentrics] suitable to them, yet in the meantime they have admitted quite a few which appear to contravene the first principles of equality of motion. another notable thing, that there is a definite symmetry between the form of the universe and its parts, they could not devise or construct from these; but it is with them as if a man should take from different places, hands, feet, a head and other members, in the best way possible indeed, but in no way comparable to a single body, and in no respect corresponding to each other, so that a monster rather than a man would be constructed from them. thus in the process of proof, which they call a system, they are found to have passed over some essential, or to have admitted some thing both strange and scarcely relevant. this would have been least likely to have happened to them if they had followed definite principles. for if the hypotheses they assumed were not fallacious, everything which followed out of them would have been verified beyond a doubt. however obscure may be what i now say, nevertheless in its own place it will be made more clear. [footnote : _i.e._, the , solar years in which all the heavenly bodies complete their circuits and return to their original positions.] "when therefore i had long considered this uncertainty of traditional mathematics, it began to weary me that no more definite explanation of the movement of the world machine established in our behalf by the best and most systematic builder of all, existed among the philosophers who had studied so exactly in other respects the minutest details in regard to the sphere. wherefore i took upon myself the task of re-reading the books of all the philosophers which i could obtain, to seek out whether any one had ever conjectured that the motions of the spheres of the universe were other than they supposed who taught mathematics in the schools. and i found first that, according to cicero, nicetas had thought the earth was moved. then later i discovered according to plutarch that certain others had held the same opinion; and in order that this passage may be available to all, i wish to write it down here: "but while some say the earth stands still, philolaus the pythagorean held that it is moved about the element of fire in an oblique circle, after the same manner of motion that the sun and moon have. heraclides of pontus and ecphantus the pythagorean assign a motion to the earth, not progressive, but after the manner of a wheel being carried on its own axis. thus the earth, they say, turns itself upon its own center from west to east."[ ] [footnote : plutarch: _moralia: de placitis philosophorum_, lib. iii, c. (v. ).] when from this, therefore, i had conceived its possibility i myself also began to meditate upon the mobility of the earth. and although the opinion seemed absurd, yet because i knew the liberty had been accorded to others before me of imagining whatsoever circles they pleased to explain the phenomena of the stars, i thought i also might readily be allowed to experiment whether, by supposing the earth to have some motion, stronger demonstrations than those of the others could be found as to the revolution of the celestial sphere. thus, supposing these motions which i attribute to the earth later on in this book, i found at length by much and long observation, that if the motions of the other planets were added to the rotation of the earth and calculated as for the revolution of that planet, not only the phenomena of the others followed from this, but also it so bound together both the order and magnitude of all the planets and the spheres and the heaven itself, that in no single part could one thing be altered without confusion among the other parts and in all the universe. hence, for this reason, in the course of this work i have followed this system, so that in the first book i describe all the positions of the spheres together with the motions i attribute to the earth; thus this book contains a kind of general disposition of the universe. then in the remaining books, i bring together the motions of the other planets and all the spheres with the mobility of the earth, so that it can thence be inferred to what extent the motions and appearances of the other planets and spheres can be solved by attributing motion to the earth. nor do i doubt that skilled and scholarly mathematicians will agree with me if, what philosophy requires from the beginning, they will examine and judge, not casually but deeply, what i have gathered together in this book to prove these things. in order that learned and unlearned may alike see that in no way whatsoever i evade judgment, i prefer to dedicate these my lucubrations to your holiness rather than to any one else; especially because even in this very remote corner of the earth in which i live, you are held so very eminent by reason of the dignity of your position and also for your love of all letters and of mathematics that, by your authority and your decision, you can easily suppress the malicious attacks of calumniators, even though proverbially there is no remedy against the attacks of sycophants. [illustration: a photographic facsimile (reduced) of a page from mulier's edition ( ) as "corrected" according to the _monitum_ of the congregations in . the first writer merely underlined the passage with marginal comment that this was to be deleted by ecclesiastical order. the second writer scratched out the passage and referred to the second volume of riccioli's _almagestum novum_ for the text of the order. the earlier writer was probably the librarian of the florentine convent from which this book came, and wrote this soon after . the later writer did his work after , when riccioli's book was published. this copy of the _de revolutionibus_ is now in the dartmouth college library.] if perchance there should be foolish speakers who, together with those ignorant of all mathematics, will take it upon themselves to decide concerning these things, and because of some place in the scriptures wickedly distorted to their purpose, should dare to assail this my work, they are of no importance to me, to such an extent do i despise their judgment as rash. for it is not unknown that lactantius, the writer celebrated in other ways but very little in mathematics, spoke somewhat childishly of the shape of the earth when he derided those who declared the earth had the shape of a ball.[ ] so it ought not to surprise students if such should laugh at us also. mathematics is written for mathematicians to whom these our labors, if i am not mistaken, will appear to contribute something even to the ecclesiastical state the headship of which your holiness now occupies. for it is not so long ago under leo x when the question arose in the lateran council about correcting the ecclesiastical calendar. it was left unsettled then for this reason alone, that the length of the year and of the months and the movements of the sun and moon had not been satisfactorily determined. from that time on, i have turned my attention to the more accurate observation of these, at the suggestion of that most celebrated scholar, father paul, a bishop from rome, who was the leader then in that matter. what, however, i may have achieved in this, i leave to the decision of your holiness especially, and to all other learned mathematicians. and lest i seem to your holiness to promise more about the value of this work than i can perform, i now pass on to the undertaking. [footnote : these two sentences the congregations in ordered struck out, as part of their "corrections."] appendix c. the drama of universal nature: in which are considered the efficient causes and the ends of all things, discussed in a connected series of five books, by jean bodin, (frankfort, ). _book v_: on the celestial bodies: their number, movement, size, harmony and distances compared with themselves and with the earth. sections and (in part) and (entire). bodin, jean: _universæ naturæ theatrum in quo rerum omnium effectrices causa et fines contemplantur, et continuæ series quinque libris discutiuntur_. frankfort, . book v translated into english by the writer and compared with the french translation by françois de fougerolles, (lyons, ). _section _: on the definition and the number of the spheres. mystagogue: ... now to prove that the heavens have a nature endowed with intelligence i need no other argument than that by which theophrastus and alexander prove they are living, for, they say, if the heavens did not have intelligence, they would be greatly inferior in dignity and excellence to men. that is why aben-ezra,[ ] having interpreted the hebrew of these two words of the psalm: "the heavens declare," has written that the phrase _sapperim_ (declare) in the judgment of all hebrews is appropriate to such great intelligence. also he who said "when the morning stars sang together and shouted for joy,"[ ] indicated a power endowed with intelligence, as did the master of wisdom[ ] also when he said that god created the heavens with intelligence. [footnote : as rabbi david testified on the th psalm [these footnotes are by bodin].] [footnote : job: .] [footnote : proverbs.] theodore. i have learned in the schools that the spheres are not moved of themselves but that they have separate intelligences who incite them to movement. myst. that is the doctrine of aristotle. but theophrastus and alexander,[ ] (when they teach that the spheres are animated bodies) explain adequately that the spheres are agitated by their own coëssential soul. for if the sky were turned by an intelligence external to it, its movement would be accidental with the result that it, and the stars with it, would not be moved otherwise, than as a body without soul. but accidental motion is violent. and nothing violent in nature can be of long duration. on the contrary there is nothing of longer duration, nor more constant, than the movement of the heavens. [footnote : metaphysics: ii. c. , de coelo. i. c. .] theo. what do you call fixed stars? myst. celestial beings who are gifted with intelligence and with light, and who are in continual motion. this is sufficiently indicated by the words of daniel[ ] when he wrote, that the souls of those who have walked justly in this life, and who have brought men back to the path of virtue, all have their seat and dwelling (like the gleaming stars) among the heavens. by these words one can plainly understand the essence and figure of the angels as well as of the celestial beings; for while other beings have their places in this universe assigned to them for their habitation, as the fish the sea, the cattle the fields, and the wild beasts the mountains and forests, even as origen,[ ] eusebius, and diodorus say, so the stars are assigned positions in the heavens. this can also be understood by the curtains of the tabernacle which moses, the great lawgiver, had ornamented with the images of cherubim showing that the heavens were indicated by the angelic faces of the stars. while st. augustine,[ ] jerome,[ ] thomas aquinas[ ] and scotus most fitly called this universe a being, nevertheless albertus, damascenus, and thomas aquinas deny that the heavenly bodies are animated. but thomas aquinas shows himself in this inconsistent and contradictory, for he confesses that spiritual substances are united with the heavenly bodies, which could not be unless they were united in the same hypostasis of an animated body. if this body is animated, it must necessarily be living and either rational or irrational. if, on the other hand, this spiritual substance does not make the same hypostasis with the celestial body, it will necessarily be that the movement of the sky is accidental, as coming from the mover outside to the thing moved, no more nor less than the movement of a wheel comes from the one who turns it: as this is absurd, what follows from it is necessarily absurd also. [footnote : in his last chapter.] [footnote : which is confirmed by pico of mirandola: heptaplus: bk. v.] [footnote : enchiridion: cap. ; gen.: and .] [footnote : on psalm: audite coeli.] [footnote : summa: pt. , art. , ques. .] theo. how many spheres are there? myst. it is difficult to determine their number because of the variety of opinions among the authorities, each differing from the other, and because of the inadequacy of the proofs of such things. for eudoxus has stated that the spheres with their deferents are not more than three and twenty in number. calippus has put it at thirty, and aristotle[ ] at forty-seven, which alexander aphrodisiensis[ ] has amended by adding to it two more on the advice of sosigenes. ptolemy holds that there are celestial spheres not including the bodies of the planets. johan regiomontanus says , an opinion which is followed by nearly all, because in the time of ptolemy they did not yet know that the eighth sphere and all the succeeding ones are carried around by the movement of the trepidation. thus he held that the moon has five orbits, mercury six, venus, mars, jupiter, and saturn each four, aside from the bodies of the planets themselves, for beyond these are still the spheres and deferents of the eighth and ninth spheres. but copernicus, reviving eudoxus' idea, held that the earth moved around the motionless sun; and he has also removed the epicycles with the result that he has greatly reduced their number, so that one can scarcely find eight spheres remaining. [footnote : metaphy. xii.] [footnote : in his commentaries on book xii of metaph. where he gives the opinion of calippus and eudoxus.] theo. what should one do with such a variety of opinions? myst. have recourse to the sacred fountain of the hebrews to search out the mysteries of a thing so deeply hidden from man; for from them we may obtain an absolutely certain decision. the tabernacle which the great lawgiver moses ordered to be made[ ] was like the archetype of the universe, with its ten curtains placed around it each decorated with the figures of cherubim thus representing the ten heavens with the beauty of their resplendent stars. and even though aben-ezra did not know of the movement of trepidation, nevertheless he interpreted this passage, "the heavens are the work of thy fingers" as indicating the number of the ten celestial spheres. the pythagoreans seem also to have agreed upon the same number since, besides the earth and the eight heavens, they imagine a sphere anticthon because they did not then clearly understand the celestial movements. they thought however, all should be embraced in the tenth. [footnote : ex. xviii and following. philo judæus in the allegories.] theo. the authority of such writers has indeed so great weight with me that i place their statements far in advance of the arguments of all others. nevertheless if it can be done, i should wish to have this illustrated and confirmed by argument in order to satisfy those who believe nothing except on absolute proof. myst. it can indeed be proved that there are ten mobile spheres in which the fiery bodies accomplish their regular courses. yet by these arguments that ultimate, motionless sphere which embraces and encircles all from our terrestial abode to its circumference within its crystalline self, encompassing plainly the utmost shores and limits of the universe, cannot be proved. for as it has been shown before [in book i] the elemental world was inundated by celestial waters from above. nor can it apparently be included in the number of the spheres since (as we will point out later) as great a distance exists between it and the nearest sphere as between the ocean and the starry heaven. furthermore it has been said before that the essence of the spheres consists of fire and water which is not fitting for the celestial waters above. theo. by what arguments then can it be proved there are ten spheres? myst. the ancients knew well that there were the seven spheres of the planets, and an eighth sphere of the fixed stars which, down to the time of eudoxus and meto, they thought had but one simple movement. these men were the first who perceived by observation that the fixed stars were carried backward quite contrary to the movement of the primum mobile. after them came timochares, hipparchus, and menelaus, and later ptolemy, who confirmed these observations perceiving that the fixed stars (which people had hitherto thought were fixed in their places) had been separated from their station. for this reason they thought best to add a ninth sphere to the eight inferior ones. much later an arabian and a spanish king, mensor and alphonse, great students of the celestial sciences, in their observations noticed that the eighth sphere with the seven following moved in turning from the north to the east, then towards the south, and so to the west, finally returning to the north, and that such a movement was completed in years. this johannus regiomontanus, a franconian, has proved, with a skill hitherto equalled only by that of those who proved the ninth sphere, which travels from west to east. from this it is necessarily concluded that there are ten spheres. theo. why so? myst. because every natural body[ ] has but one movement which is its own by nature; all others are either voluntary or through violence, contrary to the nature of a mobile object; for just as a stone cannot of its own impulse ascend and descend, so one and the same sphere cannot of itself turn from the east to the west and from the west to the east and still less from the north to the south and south to north. [footnote : aristotle: metaph. ii and xii and de coelo i.] theo. what then? myst. it follows from this that the extremely rapid movement by which all the spheres are revolved in twenty-four hours, belongs to the primum mobile, which we call the tenth sphere, and which carries with it all the nine lesser spheres; that the second or planetary movement, that is, from west to east, is communicated to the lesser spheres and belongs to the ninth sphere; that the third movement, resembling a person staggering, belongs to the eighth sphere with which it affects the other lesser spheres and makes them stagger in a measure outside of the poles, axes and centres of the greater spheres. _section _: on the position of the universe according to its divisions. * * * * theo. does it not also concern physics to discuss those things that lie outside the universe? myst. if there were any natural body beyond the heavens, most assuredly it would concern physics, that is, the observer and student of nature. but in the book of origins,[ ] the master workman is said to have separated the waters and placed the firmament in between them. the hebrew philosophers declare that the crystalline sphere which ezekiel[ ] called the great crystal and upon which he saw god seated, as he wrote, is as far beyond the farthermost heaven as our ocean is far from that heaven, and that this orb is motionless and therefore is called god's throne. for "seat" implies quiet and tranquility which could be proper for none other than the one immobile and immutable god. this is far more probable and likely than aristotle's absurd idea, unworthy the name of a philosopher, by which he placed the eternal god in a moving heaven as if he were its source of motion and in such fashion that he was constrained of necessity to move it. we have already refuted this idea. it has also been shown that these celestial waters full of fertility and productiveness sometimes are spread abroad more widely and sometimes less so, as though obviously restrained, whence the heavens are said to be closed[ ] and roofed[ ] with clouds or that floods burst forth out of the heaven to inundate the earth. finally we read in the holy scriptures that the eternal god is seated upon the flood. [footnote : gen.: .] [footnote : chap. and . exod.: .] [footnote : i kings: . deut.: .] [footnote : psalm .] theo. why then are not eleven spheres counted? myst. because the crystalline sphere is said to have been separated from the inferior waters by the firmament, and it therefore cannot be called a heaven. furthermore motion is proper to all the heavens, but the crystalline one is stationary. that is why rabi akiba called[ ] it a marble counterpart of the universe. this also is signified in the construction of the altar which was covered with a pavilion in addition to its ten curtains for, as it is stated elsewhere,[ ] god covers the heavens with clouds, and the scriptures often make mention of the waters beyond the heavens.[ ] there are those, however, who teach that the hebrew word _scamajim_ may be applied only to a dual number, so that they take it to mean the crystalline sphere and the starry one. but i think those words in solomon's speech[ ] "the heaven of heaven, and the heavens of the heavens" refer in the singular to the crystalline sphere, in the plural to the ten lesser spheres. [footnote : according to maymon: perplexorum, iii.] [footnote : psalm .] [footnote : psalm . gen. and .] [footnote : also in psalm and .] theo. it does not seem so marvelous to me that an aqueous or crystalline sphere exists beyond the ten spheres, as that it is as far beyond the furthermost sphere as the ocean is far this side of it, that is, as astrologists teach, terrestrial diameters. myst. it is written most plainly that the firmament holds the middle place between the two waters. therefore god is called[ ] in hebrew _helion_, the sun, that is, the most high, and under his feet the heaven is spread like a crystal,[ ] although he is neither excluded nor included in any part of the universe, it is however consistent with his majesty to be above all the spheres and to fill heaven and earth with his infinite power as isaiah[ ] indicated when he writes: "his train filled the temple;" it is the purest and simplest act, the others are brought about by forces and powers. he alone is incorporeal, others are corporeal or joined to bodies. he alone is eternal, others according to their nature are transitory and fleeting unless they are strengthened by the creator's might; wherefore the chaldean interpreter is seen everywhere to have used the words, majesty, glory or power in place of the presence of god. [footnote : psalm .] [footnote : exod. . ezek. , .] [footnote : isa. .] theo. nevertheless so vast and limitless a space must be filled with air or fire, since there are no spheres there, nor will nature suffer any vacuum. myst. if then the firmament occupies the middle position between the two waters, then by this hypothesis you must admit that the space beyond the spheres is empty of elemental and celestial bodies; otherwise you would have to admit that the last sphere extends on even to the crystalline orb, which can in no way be reconciled with the holy scriptures and still less with reason because of the incredible velocity of this sphere. therefore it is far more probable that this space is filled with angels. theo. is there some medium between god and the angels which shares in the nature of both? myst. what is incorporeal and indivisible cannot communicate any part of its essence to another; for if a creature had any part of the divine essence, it would be all god, since god neither has parts nor can be divided, therefore he must be separated from all corporeal contact or intermixture. _section _: on guardian angels. theo. what then in corporeal nature is closest to god? myst. the two seraphim, who stand near the eternal creator,[ ] and who are said to have six wings, two wherewith to fly, the others to cover head and feet. by this is signified the admirable swiftness with which they fulfill his commands, yet head and feet are veiled for so the purpose of their origin and its earliest beginning are not known to us. also they have eyes scattered in all parts of their bodies to indicate that nothing is hidden from them. and they also pour oil for lighting through a funnel into the seven-branched candlestick; that is, strength and power are poured forth by the creator to the seven planets, so that we should turn from created things to the worship and love of the creator. [footnote : isa. . ezek. and . zach. . exod. , .] theo. since nothing is more fitting for the divine goodness than to create, to generate, and to pile up good things for all, whence comes the destruction of the world and the ruin of all created things? myst. it is true plato and aristotle attributed the cause of all ills to the imperfection of matter in which they thought was some _kakopoion_,[ ] but that is absurd since it is distinctly written: all that god had made was good, or as the hebrews express it, beautiful,--so evil is nothing-else than the absence[ ] or privation of good. [footnote : maleficium quidam, _i.e._, some evil-power. job .] [footnote : augustine against faustus wrote that vanity is not produced from the dust, nor evil from the earth.] theo. can not wicked angels be defined without privation since they are corporeal essences? myst. anything that exists is said to be good and to be a participant by its existence in the divine goodness; and even as in a well regulated republic, executioners, lictors, and corpse-bearers are no less necessary than magistrates, judges and overseers; so in the republic of this world, for the generation, management and guardianship of things god has gathered together angels as leaders and directors for all the celestial places, for the elements, for living beings, for plants, for minerals, for states, provinces, families and individuals. and not only has he done this, but he has also assigned his servants, lictors, avengers and others to places where they may do nothing without his order, nor inflict any punishment upon wicked men unless the affair has been known fully and so decided. thus god is said[ ] to have made leviathan, which is the outflow of himself, that is, the natural rise and fall of all things. "i have created a killer,"[ ] he said, "to destroy," and so also behemoth, and the demons cleaving to him, which are often called ravens, eagles and lions, and which are said to beg their food of god, that is, the taking of vengeance upon the wicked whose punishment and death they feed upon as upon ordinary fare. from these, therefore, or rather from ourselves, come death, pestilence, famine, war and those things we call ills, and not from the author of all good things except by accident. for so god says of himself:[ ] "i am the god making good and creating evil, making light and creating darkness." for when he withdraws his spirit, evil follows the good; when he takes the light away, darkness is created; as when one removes the pillars of a building, the ruin of a house follows. if he takes the vital spark away, death follows; nor can he be said to do evil[ ] to anyone in taking back what is his own. [footnote : job and . isa. . ezek. .] [footnote : isa. .] [footnote : isa. .] [footnote : job .] theo. when the legislator asked him to disclose his face to his gaze, why did the architect of the universe and the author of all things reply: "my face is to be seen by no mortal man, but only my back?" myst. this fine allegory signifies that god cannot be known from superior or antecedent causes but from behind his back, that is, from results, for a little later he adds, "i will cover thine eyes with my hand." thus the hand signifies those works which he has placed before anyone's eyes, and it indicates that he places man not in an obscure corner but in the center of the universe so that he might better and more easily than in heaven contemplate the universe and all his works through the sight of which, as through spectacles, the sun, that is, god himself, may be disclosed. and therefore we undertook this disputation concerning nature and natural things, so that even if they are but slightly explained, nevertheless we may attain from this disquisition an imperfect knowledge of the creator and may break forth in his praises with all our might, that at length by degrees we may be borne on high and be blessed by the divine reward; for this is indeed the supreme and final good for a man. here endeth the drama of nature which jean bodin wrote while all france was aflame with civil war. finis appendix d. a translation of a letter by thomas feyens on the question: is it true that the heavens are moved and the earth is at rest? (february, ) (_thomæ fieni epistolica quæstio_: an verum sit, coelum moveri et terram quiescere? londini, .) to the eminent and noble scholars, tobias matthias and george gays: it is proved that the heavens are moved and the earth is stationary: first; by authority; for besides the fact that this is asserted by aristotle and ptolemy whom wellnigh all philosophers and mathematicians have followed by unanimous consent, except for copernicus, bernardus patricius[ ] and a very few others, the holy scriptures plainly attest it in at least two places which i have seen. in joshua,[ ] are the words: steteruntque sol et luna donec ulcisceretur gens de inimicis suis. and a little further on: stetit itaque sol in medio coeli, et non festinavit occumbere spatio unius diei, et non fuit antea et postea tam longa dies. the scriptures obviously refer by these words to the motion of the _primum mobile_ by which the sun and the moon are borne along in their diurnal course and the day is defined; and it indicates that the heavens are moved as well as the _primum mobile_. then ecclesiastes, chapter ,[ ] reads: generatio præterit, et generatio advenit, terra autem semper stat, oritur sol et occidit, et ad locum suum revertitur. [footnote : feyens probably refers here to francesco patrizzi, who was an enemy of the peripatetics and a great supporter of platonism. he died in at rome, where clement viii had conferred on him the chair of philosophy.] [footnote : joshua x: - .] [footnote : ecclesiastes i: .] secondly, it is proved by reason. all the heavens and stars were made in man's behalf and, with other terrestrial bodies, are the servants of man to warm, light, and vivify him. this they could not do unless in moving they applied themselves by turns to different parts of the world. and it is more likely that they would apply themselves by their own movement to man and the place in which man lives, than that man should come to them by the movement of his own seat or habitation. for they are the servants of man; man is not their servant; therefore it is more probable that the heavens are moved and the earth is at rest than that the reverse is true. thirdly; no probable argument can be thought out from philosophy to prove that the earth is moved and the heavens are at rest. nor can it be done by mathematics. by saying that the heavens are moved and the earth is at rest, all phenomena of the heavenly bodies can be solved. just as in the same way in optics all can be solved by saying either that sight comes from the thing to the eye, or that rays go from the eye to the thing seen; so is it in astronomy. therefore one ought rather to abide in the ancient and general opinion than in one received recently without justification. fourthly; the earth is the center of the universe; all the heavenly bodies are observed to be moved around it; therefore it itself ought to be motionless, for anything that moves, it seems, should move around or above something that is motionless. fifthly; if the earth is moved in a circle, either it moves that way naturally or by force, either by its own nature or by the nature of another. it is not by its own nature, for straight motion from above downward is natural to it; therefore circular motion could not be natural to it. further, the earth is a simple body; and a simple body can not have two natural motions of distinct kinds or classes. nor is it moved by another body; for by what is it moved? one has to say it is moved either by the sun or by some other celestial body; and this cannot be said, since either the sun or that body is said to be at rest or in motion. if it is said to be at rest, then it cannot impart movement to another. if it is said to be in motion, then it can not move the earth, because it ought to move either by a motion similar to its own or the opposite of it. it is not similar, since thus it would be observed to move neutrally as when two boats moving in the same direction, appear not to move but to be at rest. it is not the opposite motion, since nothing could give motion contrary to its own. and because galileo seems to say, in so far as i have learned from your lordships, that the earth was moved by the sun; i prove anyway that this is not true since the movement of the sun and of the earth ought to be from contrary and distinct poles. the sun, however, can not be the cause of the other's movement because it is moved above different poles. lastly, the earth follows the motion of no other celestial body; since if it is moved, it moves in hours, and all the other celestial bodies require the space of many days, months and years. ergo. finally, if the earth is moved by another, its motion would be violent; but this is absurd, for no violence can be regular and perpetual. sixthly; even so it is declared that the earth is moved. nevertheless, it must be admitted to this that either the planets themselves or their spheres are moved, for in no other way can the diversities of aspects among themselves be solved; nor can a reason be given why the sun does not leave the ecliptic and the moon does; and how a planet can be stationary or retrograde, high or low,--and many other phenomena. for this reason those who said the earth moved, as bernardus patricius and the others said, claimed that the _primum mobile_, forsooth, was stationary and that the earth was moved in its place; yet they could not in the least deny that the planets themselves were moved, but admitted it. that is the reason why both ancient and modern mathematicians, aside from the motion of the _primum mobile_, were forced to admit and consider the peculiar movements of the planets themselves. if therefore it must be acknowledged, and it is certain, that the stars and the celestial bodies are moved; then it is more probable that all movement perceived in the universe belongs rather to the heavenly bodies than to the earth. for if movement were ascribed to all the rest, why for that same reason is not diurnal rotation ascribed rather to the _primum mobile_ than to the earth, particularly when our senses seem to decide thus? although one may well be mistaken, sometimes, concerning other similar movements; yet it is not probable that all ages could be at fault, or should be, about the movements of its most important objects, of course the celestial luminaries. seventhly; it is proved by experience. for if the earth is moved, then an arrow shot straight up on high could never fall back to the place whence it was shot, but should fall somewhere many miles away. but this is not so. ergo. this can be answered and is so customarily in this way: this does not follow because the air is swept along with the earth, and so, since the air which carries the arrow is turning in the same way with the earth, the arrow also is borne along equally with it, and thus returns to the same spot. this in truth is a pure evasion and a worthless answer for many reasons. it is falsely observed that the air is moved and by the same motion as the earth. for what should move the earth? truly, if the air is moved by the same motion as the earth, either it ought to be moved by the earth itself, or by that other which moves the earth, or by itself. it is not moved by itself; since it has another motion, the straight one of course natural to itself, and also since it has a nature, an essence and qualities all different from the nature and the essence of the earth; therefore it could not by its own nature have the same motion as that other, but of necessity ought to have a different one. nor is it moved by any other that may move the earth; as that which moves the earth could not at the same time and with like motion move the air. for since the air is different from the earth in essence, in both active and passive qualities, and in kind of substance, it can not receive the impelling force of the acting body, or that force applied in the same way as the earth, and so could not be moved in the same way. the virtues [of bodies] acting and of moving diversely are received by the recipients according to the diversity of their dispositions. also it can not be moved by the earth; since if it were moved by the earth, it must be said to be moved by force, but such motion appears to be impossible. ergo. the minor premise is proved: for if air is thus moved by the earth by force the air ought to be moved more rapidly than the earth, because air is larger [than the earth]. for what is outside is larger than what is inside. when, however, what is larger and what is outside is driven around equally rapidly with what is less, and what is inside, then the former is moved much more rapidly. thus it is true that the sphere of saturn in its daily course is moved far faster than the sphere of the moon. but it is impossible that the one driven should move more rapidly than the one driving; therefore the air is not moved by the earth's violence. thus would it be if the air were moved with the earth, or by itself, or by force. thus far, then, the force of the original argument remains; since of its own motion, indeed, it could not be in every way conformable to the motion of the earth as i have shown; and this because the air differs from the earth in consistency of substance, in qualities and in essence. but the air ought at all events to move more sluggishly than the earth. it follows from this that an arrow shot straight up could not return to its starting point; for the earth, moving like the air, on account of the other's slower rate leaves it behind, and the arrow also which is carried away from it. besides, if the air does not move so rapidly as the earth, a man living in a very high tower, however quiet the air, ought then always to feel the strongest wind and the greatest disturbance of the air. since mountains and towers are moved with the earth, and the air would not be accompanying them at an equal speed, it would necessarily follow that they would precede the air by cleaving and cutting and ploughing through it which ought to make a great wind perceptible. eighthly; if a person stood in some very high tower or other high place and aimed from that tower at some spot of earth perpendicularly below his eye, and allowed a very heavy stone to fall following that perpendicular line, it is absolutely certain that that stone would land upon the spot aimed at perpendicularly underneath. but if the earth is moved, it would be impossible for the stone to strike that spot. this i prove first: because either the air moves at an unequal rate with the earth; or it moves equally rapidly. if not equally, then it is certain the stone could not land at that spot, since the earth's movement would outstrip the stone borne by the air. if equally rapidly, then again the stone could not land at that spot, since although the air was moving in itself at an equal speed, yet on that account it could not carry the stone thus rapidly with itself and carrying it downward falling by its own weight, for the stone tending by gravity towards the center resists the carrying of the air. you will say: if the earth is moved in a circle, so are all its parts; wherefore that stone in falling not only moves in a circle by the carrying of the air, but also in a circle because of its own nature as being part of the earth and having the same motion with it. verily this answer is worthless. for although the stone is turned in a circle by its own nature like the earth, yet its own natural gravity impeded it so that it is borne along that much the less swiftly, unlike the air or the earth, both of which are in their natural places and which in consequence have no gravity as a stone falling from on high has. lastly; because although the stone is moved in the world by its own nature like the whole earth, yet it is not borne along as swiftly as the whole earth. for as one stone by its own weight falls from the heaven following its own direct motion straight to the center just as a part of the earth, so also the whole earth itself would fall; and yet it would not fall so swiftly as the whole earth, for although the stone would be borne along in its sphere like the whole earth just as a part of it, yet it would not be borne along as swiftly as the whole earth; and so, in whatever way it is said, the motion of the earth ought always to outstrip the stone and leave it a long distance behind. thus a stone could never fall at the point selected or a point perpendicularly beneath it. this is false. ergo. ninthly: if the earth is moved in a circular orbit, it ought to pass from the west through the meridian to the east; consequently the air ought to move by the same path. but if this were so, then if an archer shot toward the east, his arrow ought to fly much farther than if he shot toward the west. for when he shot toward the east, the arrow would fly with the natural movement of the air and would have that supporting it. but when he shot toward the west, he would have the motion of the air against him and then the arrow would struggle against it. but it is certain the arrow ought to go much farther and faster when the movement of the air is favorable to it then when against it, as is obvious in darts sent out with a favoring wind. ergo. similarly not a few other arguments can be worked out, but there are none as valuable for proof as the foregoing ones. though these were written by me with a flying pen far from books and sick in bed with a broken leg, yet they seem to me to have so much value that i do not see any way by which they could rightly be refuted. these i have written for your gracious lordships in gratitude for your goodwill on the occasion of our conversation at your dinner four days ago; 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(bound with _first book_.) "w.r.": _the new astronomer, or astronomy made easy by such instruments that readily shew by observation the stars...._ london, . index addison, j., - . agricola, g.l., . albategnius, . allaben, f., . alphonse x of castile, , . ambrose, . arabian astronomers, , , , . archimedes, . aristarchus of samos, - n., , n., , . aristotle, , , , , , , , , . augustine, , , . bacon, francis, , - . bacon, roger, . bayle, pierre, - . bellarmin, cardinal, , - , . benedict xiv, . bessel, , . bodin, jean, - , - , - . boscovich, , . bossuet, . bradley, , . browne, thomas, - . bruno, , , - , , , , , . burton, richard [transcriber's note: robert], . calvin, , , . cartesian-copernicans, - , , , , . cassini, g.d., - . castelli, , . church fathers, - , . cicero, , , , . cleanthes, . clement of alexandria, . clement viii, n. congregations of the index, , - , - , , , , , , . copernicus, , , , , , , , - , , , , , , , , , . name, n. life, - . theory, , - , , , , - , , - . opponents, , , - , , - , - , , - , , , - . supporters, , , - , , - , - , , - , - , , , - , - , - , - , - . dante, . delambre, , . de maupertius, . de peyster, j.w., . de premontval, mme., . _de revolutionibus_, , , , , , - , - . descartes, , , . didacus à stunica, , , , , . digges, thomas, n. diogenes laërtius, . dominicus maria di novara, , . dubartas, . fénelon, . feyens, thomas, , , - . flammarion, . forbes, duncan, . foscarini, , , - , , . foucault, , . froidmont, see fromundus. fromundus, , , - , . galileo, , , - , , , - , , , , , , , , , , , . gassendi, , , . gilbert, wm., , , . greek philosophers, - , , , . herbert, george, - . hipparchos, , . hicetas, , . horne, george, . hutchinson, john, . huygens, christian, , . index, , , - , , , , . inquisition, , , , - , - , , , . isidore of seville, . jasper, bro., . jesuits, , , , , , , - , . johnson, s., . justus-lipsius, , . keble, j., . keill, j., - . kepler, , , - , , , , , , , , , . knap, . kromer, m., n. lactantius, , . lalande, . lange, j.r., . lansberg, - , . leo x, . liège, univ. of, , - . longomontanus, . louvain, univ. of, , , - , , . luther, , , , . lutherans, , , . mæstlin, , , , . martianus capella, . mather, cotton, . melancthon, , - , . milton, , , . mivart, . montaigne, . _narratio prima_, , . newton, , , , , . nicolas cusanus, , . origen, . osiander, , . owen, j., n., . paul iii, . paul v, - , , , . peter lombard, . peter the great, . philastrius, . philo judæus, . philolaus, , . piegeon, j., . pike, s., . pius vii, . plato, , . plutarch, , , , . pope, alexander, , . pseudo-dionysius, . ptolemy, n., , , , - , , , . theory, , , , , , , , , , , , - . pürbach, . pythagoras, , , . pythagoreans, , . recorde, r., - . regiomontanus, , , , , . reinhold, erasmus, . rheticus, - , , , . riccioli, , , - , , . roberts, . roemer, . sacrobosco, , , . salamanca, univ. of, , . schoepffer, c., . schwilgué, . settele, , . shakespeare, . sindico, . stephen, leslie, . thomas aquinas, . turrettin, . turrinus, j., . tycho brahe, , - , , , . theory, , , , , , , , , , , . urban viii, - . van welden, m., - . vitruvius, . voight, j.h., - . von schönberg, n., , , . wallis, n. wesley, j., , . whewell, , . widmanstadt, , . wilkins, bp., - , . wren, dean, - . yale, univ. of, . zytphen, . plato's doctrine respecting the rotation of the earth, and aristotle's comment upon that doctrine. by george grote, esq. london: john murray, albemarle street. . _the right of translation is reserved._ london: printed by william clowes and sons, stamford street, and charing cross. examination of the three following questions:-- . whether the doctrine of the earth's rotation is affirmed or implied in the platonic timÆus? . if affirmed or implied, in what sense? . what is the cosmical function which plato assigns to the earth in the timÆus? preface. the following paper was originally intended as an explanatory note on the platonic timæus, in the work which i am now preparing on plato and aristotle. interpreting, differently from others, the much debated passage in which plato describes the cosmical function of the earth, i found it indispensable to give my reasons for this new view. but i soon discovered that those reasons could not be comprised within the limits of a note. accordingly i here publish them in a separate dissertation. the manner in which the earth's rotation was conceived, illustrates the scientific character of the platonic and aristotelian age, as contrasted with the subsequent development and improvement of astronomy. plato--on the earth's rotation. in plato, timæus, p. b, we read the following words--[greek: gê=n de\ tropho\n me\n ê(mete/ran, ei(llome/nên de\ peri\ to\n dia\ panto\s po/lon tetame/non phu/laka kai\ dêmiourgo\n nukto/s te kai\ ê(me/ras e)mêchanê/sato, prô/tên kai\ presbuta/tên theô=n, o(/soi e)nto\s ou)ra/nou gego/nasi.] i give the text as it stands in stallbaum's edition. the obscurity of this passage is amply attested by the numerous differences of opinion to which it has given rise, both in ancient and in modern times. various contemporaries of plato ([greek: e)/nioi]--aristot. de coelo, ii. , p. b. ) understood it as asserting or implying the rotatory movement of the earth in the centre of the kosmos, and adhered to this doctrine as their own. aristotle himself alludes to these contemporaries without naming them, and adopts their interpretation of the passage; but dissents from the doctrine, and proceeds to impugn it by arguments. cicero mentions (academic ii. ) that there were persons who believed plato to have indicated the same doctrine obscurely, in his timaeus: this passage must undoubtedly be meant. plutarch devotes a critical chapter to the enquiry, what was plato's real doctrine as to the cosmical function of the earth--its movement or rest (quaestion. platonic. vii. , p. .) there exists a treatise, in doric dialect, entitled [greek: ti/maio** tô= lo/krô peri\ psucha=s ko/smô kai\ phu/sios], which is usually published along with the works of plato. this treatise was supposed in ancient times to be a genuine production of the lokrian timaeus, whom plato introduces as his spokesman in the dialogue so called. as such, it was considered to be of much authority in settling questions of interpretation as to the platonic timaeus. but modern critics hold, i believe unanimously, that it is the work of some later pythagorean or platonist, excerpted or copied from the platonic timaeus. this treatise represents the earth as being in the centre and at rest. but its language, besides being dark and metaphorical, departs widely from the phraseology of the platonic timaeus: especially in this--that it makes no mention of the cosmical axis, nor of the word [greek: i)llome/nên] or [greek: ei(loume/nên]. alexander of aphrodisias (as we learn from simplikius ad aristot. de coelo, fol. ) followed the construction of plato given by aristotle. "it was improbable (he said) that aristotle could be ignorant either what the word signified, or what was plato's purpose" ([greek: a)lla\ tô=| a)ristote/lei, phêsi\n, ou(/tô le/gonti _i)/llesthai_, ou)k eu)/logon a)ntile/gein; ô(s a)lêthô=s ga\r ou)/te tê=s le/xeôs to\ sêmaino/menon ei)ko\s ê)=n a)gnoei=n au)to\n, ou)/te to\n pla/tônos skopo/n.] this passage is not given in the scholia of brandis). alexander therefore construed [greek: i)llome/nên] as meaning or implying rotatory movement, though in so doing he perverted (so simplikius says) the true meaning to make it consonant with his own suppositions. proklus maintains that aristotle has interpreted the passage erroneously,--that [greek: i)llome/nên] is equivalent to [greek: sphiggome/nên] or [greek: xunechome/nên]--and that plato intends by it to affirm the earth as at rest in the centre of the kosmos (ad timaeum, book iv., p. ed. schneider). simplikius himself is greatly perplexed, and scarcely ventures to give a positive opinion of his own. on the whole, he inclines to believe that [greek: i)llome/nên] might possibly be understood, by superficial readers, so as to signify rotation, though such is not its proper and natural sense: that some platonists did so misunderstand it: and that aristotle accepted their sense for the sake of the argument, without intending himself to countenance it (ad aristot. de coelo, p. ). both proklus and simplikius, we must recollect, believed in the genuineness of the doric treatise ascribed to timaeus locrus. reasoning upon this basis, they of course saw, that if aristotle had correctly interpreted plato, plato himself must have interpreted _incorrectly_ the doctrine of timaeus. they had to ascribe wrong construction either to plato or to aristotle: and they could not bear to ascribe it to plato. alkinous, in his eisagôge (c. ) gives the same interpretation as proklus. but it is remarkable that in his paraphrase of the platonic words, he calls the earth [greek: ê(me/ras phu/lax kai\ nukto/s]: omitting the significant epithet [greek: dêmiourgo/s]. in regard to modern comments upon the same disputed point, i need only mention (besides those of m. cousin, in the notes upon his translation of the 'timæus', and of martin in his 'Études sur le timée') the elaborate discussion which it has received in the two recent dissertations 'ueber die kosmischen systeme der griechen,' by gruppe and boeckh. gruppe has endeavoured, upon the evidence of this passage, supported by other collateral proofs, to show that plato, towards the close of his life, arrived at a belief, first, in the rotation of the earth round its own axis, next, at the double movement of the earth, both rotation and translation, round the sun as a centre (that is, the heliocentric or copernican system): that plato was the first to make this discovery, but that he was compelled to announce it in terms intentionally equivocal and obscure, for fear of offending the religious sentiments of his contemporaries ('die kosmischen systeme der griechen, von o. f. gruppe,' berlin, ). to this dissertation m. boeckh--the oldest as well as the ablest of all living philologists--has composed an elaborate reply, with his usual fulness of illustrative matter and sobriety of inference. opinions previously delivered by him (in his early treatises on the platonic and pythagoreian philosophy) had been called in question by gruppe: he has now re-asserted them and defended them at length, maintaining that plato always held the earth to be stationary and the sidereal sphere rotatory--and answering or extenuating the arguments which point to an opposite conclusion ('untersuchungen über das kosmische system des platon, von august boeckh,' berlin, ). gruppe has failed in his purpose of proving that plato adopted either of the two above-mentioned doctrines--either the rotation of the earth round its own axis, or the translation of the earth round the sun as a centre. on both these points i concur with boeckh in the negative view. but though i go along with his reply as to its negative results, i cannot think it satisfactory in its positive aspect as an exposition of the doctrine proclaimed in the platonic timæus: nor can i admit that the main argument of m. boeckh's treatise is sufficient to support the inference which he rests upon it. moreover, he appears to me to set aside or explain away too lightly the authority of aristotle. i agree with alexander of aphrodisias and with gruppe who follows him, in pronouncing aristotle to be a good witness, when he declares what were the doctrines proclaimed in the platonic timæus; though i think that gruppe has not accurately interpreted either timæus or aristotle. the capital argument of boeckh is as follows: "the platonic timæus affirms, in express and unequivocal terms, the rotation of the outer celestial sphere (the sidereal sphere or aplanes) in twenty-four hours, as bringing about and determining the succession of day and night. whoever believes this cannot at the same time believe that the earth revolves round its own axis in twenty-four hours, and that the succession of day and night is determined thereby. the one of these two affirmations excludes the other; and, as the first of the two is proclaimed, beyond all possibility of doubt, in the platonic timæus, so we may be sure that the second of the two cannot be proclaimed in that same discourse. if any passage therein seems to countenance it, we must look for some other mode of interpreting the passage." this is the main argument of m. boeckh, and also of messrs. cousin and martin. the latter protests against the idea of imputing to plato "un mélange monstrueux de deux systêmes incompatibles" (Études sur le timée, vol. ii. p. - ). as applied to any person educated in the modern astronomy, the argument is irresistible. but is it equally irresistible when applied to plato and to plato's time? i think not. the incompatibility which appears so glaring at present, did not suggest itself to him or to his contemporaries. to prove this we have only to look at the reasoning of aristotle, who (in the treatise de coelo, ii. - , p. . b. , . a. ) notices and controverts the doctrine of the rotation of the earth, with express reference to the followers of the platonic timæus--and who (if we follow the view of martin) imputes this doctrine with wilful falsehood to plato, for the purpose of contemptuously refuting it "pour se donner le plaisir de la réfuter avec dédain." granting the view of m. boeckh (still more that of martin) to be correct, we should find aristotle arguing thus:--"plato affirms the diurnal rotation of the earth round the centre of the cosmical axis. this is both incredible, and incompatible with his own distinct affirmation that the sidereal sphere revolves in twenty-four hours. it is a glaring inconsistency that the same author should affirm both the one and the other." such would have been aristotle's reasoning, on the hypothesis which i am considering; but when we turn to his treatise we find that he does not employ this argument at all. he contests the alleged rotation of the earth upon totally different arguments--chiefly on the ground that rotatory motion is not natural to the earth, that the kind of motion natural to the earth is rectilineal, towards the centre; and he adds various corollaries flowing from this doctrine which i shall not now consider. at the close of his refutation, he states in general terms that the celestial appearances, as observed by scientific men, coincided with his doctrine. hence we may plainly see that aristotle probably did not see the incompatibility, supposed to be so glaring, upon which m. boeckh's argument is founded. to say the least, even if he saw it, he did not consider it as glaring and decisive. he would have put it in the foreground of his refutation, if he had detected the gross contradiction upon which m. boeckh insists. but aristotle does not stand alone in this dulness of vision. among the various commentators, ancient and modern, who follow him, discussing the question now before us, not one takes notice of m. boeckh's argument. he himself certifies to us this fact, claiming the argument as his own, and expressing his astonishment that all the previous critics had passed it over, though employing other reasons much weaker to prove the same point. we read in m. boeckh's second 'commentatio de platonico systemate coelestium globorum et de verâ indole astronomiæ philolaicæ,' heidelberg, , p. , the following words:-- "non moveri tellurem, proclus et simplicius ostendunt ex phædone. parum firmum tamen argumentum est ex phædone ductum ad interpretandum timæi locum: nec melius alterum, quod locrus timæus, quem plato sequi putabatur, terram stare affirmat: quia, ut nuper explicuimus, non plato ex locro, sed personatus locrus ex platone, sua compilavit. at omnium firmissionum et certissimum argumentum ex ipso nostro dialogo sumptum, _adhuc, quod jure mirere, nemo reperit_. etenim, quum, paulo supra, orbem stellarum fixarum, quem græci [greek: a)planê=] appellant, dextrorsum ferri quotidiano motu plato statuebat, non poterat ullum terræ motum admittere; quia, _qui hunc admittit, illum non tollere non potest_." (this passage appears again cited by m. boeckh himself in his more recent dissertation 'untersuchungen über das kosmische system des platon,' p. ). the writers named (p. ) as having discussed the question, omitting or disregarding this most cogent argument, are names extending from aristotle down to ruhnken and ideler. it is honourable to the penetration of m. boeckh that he should have pointed out, what so many previous critics had overlooked, that these two opinions are scientifically incompatible. he wonders, and there may be good ground for wondering, how it happened that none of these previous writers were aware of the incompatibility. but the fact that it did not occur to them, is not the less certain, and is of the greatest moment in reference to the question now under debate; for we are not now inquiring what is or is not scientifically true or consistent, but what were the opinions of plato. m. boeckh has called our attention to the fact, that these two opinions are incompatible; but can we safely assume that plato must have perceived such incompatibility between them? surely not. the pythagoreans of his day did not perceive it; their cosmical system included both the revolution of the earth and the revolution of the sidereal sphere round the central fire, ten revolving bodies in all (aristotel. metaphysic. i. , p. a. . de coelo, ii. , p. b. ). they were not aware that the revolutions of the one annulled those of the other as to effect, and that their system thus involved the two contradictory articles, or "mélange monstrueux," of which martin speaks so disdainfully. nay, more, their opponent, aristotle, while producing other arguments against them, never points out the contradiction. since it did not occur to them, we can have no greater difficulty in believing that neither did it occur to plato. indeed, the wonder would rather be if plato _had_ seen an astronomical incompatibility which escaped the notice both of aristotle and of many subsequent writers who wrote at a time when astronomical theories had been developed and compared with greater fulness. even ideler, a good astronomer as well as a good scholar, though he must surely have known that plato asserted the rotation of the sidereal sphere (for no man can read the 'timæus' without knowing it), ascribed to him also the other doctrine inconsistent with it, not noticing such inconsistency until m. boeckh pointed it out. it appears to me, therefore, that m. boeckh has not satisfactorily made good his point--"plato cannot have believed in the diurnal rotation of the earth, because he unquestionably believed in the rotation of the sidereal sphere as causing the succession of night and day." for, though the two doctrines really are incompatible, yet the critics antecedent to m. boeckh took no notice of such incompatibility. we cannot presume that plato saw what aristotle and other authors, even many writing under a more highly developed astronomy, did not see. we ought rather, i think, to presume the contrary, unless plato's words distinctly attest that he did see farther than his successors. now let us examine what plato's words do attest:--[greek: gê=n de\ tropho\n me\n ê(mete/ran, ei(llome/nên] (al. [greek: ei(lome/nên, i)llome/nên]) [greek: de\ peri\ to\n dia\ panto\s po/lon tetame/non phu/laka kai\ dêmiourgo\n nukto/s te kai\ ê(me/ras e)mêchanê/sato, prô/tên kai\ presbuta/tên theô=n, o(/soi e)nto\s ou)ra/nou gego/nasi.] i explain these words as follows:-- in the passage immediately preceding, plato had described the uniform and unchanging rotation of the outer sidereal sphere, or circle of the same, and the erratic movements of the sun, moon, and planets, in the interior circles of the diverse. he now explains the situation and functions of the earth. being the first and most venerable of the intra-kosmic deities, the earth has the most important place in the interior of the kosmos--the centre. it is packed, fastened, or rolled, close round the axis which traverses the entire kosmos; and its function is to watch over and bring about the succession of night and day. _plato conceives the kosmic axis itself as a solid cylinder revolving or turning round, and causing thereby the revolution of the circumference or the sidereal sphere._ the outer circumference of the kosmos not only revolves round its axis, but obeys a rotatory impulse emanating from its axis, like the spinning of a teetotum or the turning of a spindle. plato in the republic illustrates the cosmical axis by comparison with a spindle turned by necessity, and describes it as causing by its own rotation the rotation of all the heavenly bodies (republ. x. p. , c. a). [greek: e)k de\ tô=n a)/krôn tetame/non a)na/gkês a)/trakton, di' ou(= pa/sas e)pistre/phesthai ta\s peri/phoras . . . , kuklei=sthai de\ dê\ strepho/menon to\n a)/trakton o(/lon me\n tê\n au)tê\n phoran . . . . stre/phesthai de\ au)to\n e)n toi=s a)na/gkês go/nasin.][ ] [footnote : proklus in his commentary on the platonic timæus (p. , schn.) notes this passage of the republic as the proper comparison from which to interpret how plato conceived the cosmical axis. in many points he explains this correctly; but he omits to remark that the axis is expressly described as revolving, and as causing the revolution of the peripheral substance:-- ----[greek: to\n de\ a)/xona mi/an theo/têta sunagôgo\n me\n tô=n ke/ntrôn tou= panto\s sunektikê\n de\ tou= o(/lou ko/smou, _kinêtikê\n de\ tô=n thei/ôn periphorô=n_, peri\ ê(\n ê( chorei/a** tô=n o(/lôn, peri\ ê(\n ai( a)nakuklê/seis, a)ne/chousan to\n o(/lon ou)rano\n,** ê(\n kai\ a)/tlanta dia\ tou=to proseirê/kasin, ô(s a)/trepton kai\ a)/truton e)ne/rgeian e)/chousan. kai\ me/ntoi kai\ to\ tetame/non** e)ndei/knutai** titê/nion ei)=nai tê\n mi/an _tau/tên du/namin, tê\n phrourêtikê\n tê=s a)nakuklê/seôs tô=n o(/lôn_.] here proklus recognises the efficacy of the axis in producing and maintaining the revolution of the kosmos, but he does not remark that it initiates this movement by revolving itself. the [greek: theotê\s], which proklus ascribes to the axis, is invested in the earth packed round it, by the platonic timæus.] now the function which plato ascribes to the earth in the passage of the timæus before us is very analogous to that which in the republic he ascribes to necessity--the active guardianship of the axis of the kosmos and the maintenance of its regular rotation. with a view to the exercise of this function, the earth is planted in the centre of the axis, the very root of the kosmic soul (plato, timæus, p. b). it is even "packed close round the axis," in order to make sure that the axis shall not be displaced from its proper situation and direction. the earth is thus not merely active and influential, but is really the chief regulator of the march of the kosmos, being the immediate neighbour and auxiliary of the kosmic soul. such a function is worthy of "the first and eldest of intra-kosmic deities," as plato calls the earth. with perfect propriety he may say that the earth, in the exercise of such a function, "is guardian and artificer of day and night." this is noway inconsistent with that which he says in another passage, that the revolutions of the outer sidereal sphere determine day and night. for these revolutions of the outer sidereal sphere depend upon the revolutions of the axis, which latter is kept in uniform position and movement by the earth grasping it round its centre and revolving with it. the earth does not determine days and nights by means of its own rotations, but by its continued influence upon the rotations of the kosmic axis, and (through this latter) upon those of the outer sidereal sphere. it is important to attend to the circumstance last mentioned, and to understand in what sense plato admitted a rotatory movement of the earth. in my judgment, the conception respecting the earth and its functions, as developed in the platonic timæus, has not been considered with all its points taken together. one point among several, and that too the least important point, has been discussed as if it were the whole, because it falls in with the discussions of subsequent astronomy. thus plato admits the rotation of the earth, but he does not admit it as producing any effects, or as the primary function of the earth: it is only an indirect consequence of the position which the earth occupies in the discharge of its primary function--of keeping the cosmical axis steady, and maintaining the uniformity of its rotations. if the cosmical axis is to revolve, the earth, being closely packed and fastened round it, must revolve along with it. if the earth stood still, and resisted all rotation of its own, it would at the same time arrest the rotations of the cosmical axis, and of course those of the entire kosmos besides. the above is the interpretation which i propose of the passage in the platonic timæus, and which i shall show to coincide with aristotle's comment upon it. messrs. boeckh and martin interpret differently. they do not advert to the sense in which plato conceives the axis of the kosmos--not as an imaginary line, but as a solid revolving cylinder; and moreover they understand the function assigned by the platonic timæus to the earth in a way which i cannot admit. they suppose that the function assigned to the earth is not to keep up and regularize, but to withstand and countervail, the rotation of the kosmos. m. boeckh comments upon gruppe, who had said (after ideler) that when the earth is called [greek: phu/laka _kai\ dêmiourgo\n_ nukto\s kai\ ê(me/ras], plato must have meant to designate some active function ascribed to it, and not any function merely passive or negative. i agree with gruppe in this remark, and i have endeavoured to point out what this active function of the earth is, in the platonic theory. but m. boeckh (untersuchungen, &c., p. - ) controverts gruppe's remark, observing, first, that it is enough if the earth is in any way necessary to the production of the given effect; secondly, that if active force be required, the earth (in the platonic theory) does exercise such, by its purely passive resistance, which is in itself an energetic putting forth of power. m. boeckh's words are:--"es kommt nur darauf an, dass er ein werk, eine wirkung, hervorbringt oder zu einer wirkung beiträgt, die ohne ihn nicht wäre: dann ist er durch seine wirksamkeit ein werkmeister der sache, sey es auch ohne active thätigkeit, durch bloss passiven widerstand, der auch eine mächtige kraft-äusserung ist. die erde ist werkmeisterin der nacht und des tages, wie martin (b. ii. p. ) sehr treffend sagt 'par son énergique existence, c'est à dire, par son immobilité même:' denn sie setzt der täglichen bewegung des himmels beständig eine gleiche kraft in entgegengesetzter richtung entgegen. so _muss_ nach dem zusammenhange ausgelegt werden: so meint es platon klar und ohne verhüllungen: denn wenige zeilen vorher hat er gesagt, nacht und tag, das heisst ein sterntag oder zeittag, sei ein umlauf des kreises des selbigen--_das ist, eine tägliche umkreisung des himmels von osten nach westen, wodurch also die erde in stillstand versetzt ist:_ und diese tägliche bewegung des himmels hat er im vorhergehenden immer und immer gelehrt." . . . . "indem platon die erde nennt [greek: ei(lome/nên], nicht [greek: peri\ to\n e(autê=s po/lon], sondern [greek: peri\ to\n dia\ panto\s po/lon tetame/non], setzt er also die tägliche bewegung des himmels voraus" (p. - ).[ ] [footnote : "we are only required to show, that the earth produces a work or an effect,--or contributes to an effect which would not exist without such help: the earth is then, through such operation, an _artificer_ of what is produced, even without any positive activity, by its simply passive resistance, which indeed is in itself a powerful exercise of force.** the earth is artificer of night and day, according to the striking expression of martin, 'par son énergique existence, c'est-à-dire, par son immobilité même:' for the earth opposes, to the diurnal movement of the heavens, a constant and equal force in the opposite direction. this explanation _must_ be the true one required by the context: this is plato's meaning, plainly and without disguise: for he has said, a few lines before, that night and day (that is, a sidereal day, or day of time) is a diurnal revolution of the heaven from east to west, whereby accordingly the earth is assumed as at rest: and this diurnal movement of the heaven he has taught over and over again in the preceding part of his discourse."--"since therefore plato calls the earth [greek: ei(lome/nên], not [greek: peri\ to\n e(autê=s po/lon], but [greek: peri\ to\n dia\ panto\s po/lon tetame/non], he implies thereby the diurnal movement of the heaven."] i not only admit but put it in the front of my own case, that plato in the timæus assumes the diurnal movement of the celestial sphere; but i contend that he also assumes the diurnal rotation of the earth. m. boeckh founds his contrary interpretation upon the unquestionable truth that these two assumptions are inconsistent; and upon the inference that because the two cannot stand together in fact, therefore they cannot have stood together in the mind of plato. in that inference i have already stated that i cannot acquiesce. but while m. boeckh takes so much pains to vindicate plato from one contradiction, he unconsciously involves plato in another contradiction, for which, in my judgment, there is no foundation whatever. m. boeckh affirms that the function of the earth (in the platonic timæus) is to put forth a great force of passive resistance--"to oppose constantly, against the diurnal movement of the heavens, an equal force in an opposite direction." is it not plain, upon this supposition, that the kosmos would come to a standstill, and that its rotation would cease altogether? as the earth is packed close or fastened round the cosmical axis, so, if the axis endeavours to revolve with a given force, and the earth resists with equal force, the effect will be that the two forces will destroy one another, and that neither the earth nor the axis will move at all. there would be the same nullifying antagonism as if,--reverting to the analogous case of the spindle and the verticilli (already alluded to) in the tenth book of the republic,--as if, while ananké turned the spindle with a given force in one direction, klotho (instead of lending assistance) were to apply her hand to the outermost verticillus with equal force of resistance in the opposite direction (see reipubl. x. p. d). it is plain that the spindle would never turn at all. here, then, is a grave contradiction attaching to the view of boeckh and martin as to the function of the earth. they have not, in my judgment, sufficiently investigated the manner in which plato represents to himself the cosmical axis: nor have they fully appreciated what is affirmed or implied in the debated word [greek: ei(lo/menon--ei(lou/menon--i)llo/menon]. that word has been explained partly by ruhnken in his notes on timæi lexicon, but still more by buttmann in his lexilogus, so accurately and copiously as to leave nothing further wanting. i accept fully the explanation given by buttmann, and have followed it throughout this article. after going over many other examples, buttmann comes to consider this passage of the platonic timæus; and he explains the word [greek: ei(lome/nên] or [greek: i)llo/menên] as meaning--"_sich drängen oder gedrängt werden_ um die axe: d. h. von allen seiten her an die axe. auch lasse man sich das praesens nicht irren: die kräfte, welche den weltbau machen und zusammen halten, sind als fortdauernd thätig gedacht. die erde drängt sich (ununterbrochen) an den pol, _macht, bildet eine kugel um ihn_. welcher gebrauch völlig entspricht dem wonach dasselbe verbum ein _einwickeln_, _einhüllen_, bedeutet. auch hier mengt sich in der vorstellung einiges hinzu, was auf ein _biegen_ _winden_, und mitunter auf ein _drehen_ führt: was aber _überall nur ein durch die sache selbst hinzutretender begriff ist_," p. . and again, p. , he gives the result--that the word has only "die bedeutung _drängen_, _befestigen_, nebst den davon ausgehenden--die von _drehen_, _winden_, aber ihm _gänzlich fremd_ sind, _und nur aus der natur der gegenstände in einigen fällen als nebengedanken hinzutreten_."[ ] [footnote : "to _pack itself_, or to _be packed_, round the axis: that is, upon the axis from all sides. we must not be misled by the present tense: for the forces, which compose and hold together the structure of the universe, are conceived as continuously in active operation. the earth _packs itself_, or _is packed_, on to the axis--_makes or forms a ball round the axis:_ which corresponds fully to that other usage of the word, in the sense of _wrapping up_ or _swathing round_. here too there is a superadded something blended with the idea, which conducts us to _turning_, _winding_, and thus to _revolving_: but this is every where nothing more than an accessory notion, suggested by the circumstances of the case. the word has only the meaning, to _pack_, to _fasten_--the senses, to _wind_, to _revolve_, are altogether foreign to it, and can only be superadded as accessory ideas, in certain particular instances, by the special nature of the case."] in these last words buttmann has exactly distinguished the true, constant, and essential meaning of the word, from the casual accessories which become conjoined with it by the special circumstances of some peculiar cases. the constant and true meaning of the word is, _being packed or fastened close round_, _squeezing or grasping around_. the idea of _rotating_ or _revolving_ is quite foreign to this meaning, but may nevertheless become conjoined with it, in certain particular cases, by accidental circumstances. let us illustrate this. when i say that a body _a_ is [greek: ei(lo/menon] or [greek: i)llo/menon] (packed or fastened close round, squeezing or grasping around), another body _b_, i affirm nothing about revolution or rotation. this is an idea foreign to the proposition _per se_, yet capable of being annexed or implicated with it under some accidental circumstances. whether in any particular case it be so implicated or not depends on the question "what is the nature of the body _b_, round which i affirm _a_ to be fastened?" . it may be an oak tree or a pillar, firmly planted and stationary. . it may be some other body, moving, but moving in a rectilinear direction. . lastly, it may be a body rotating or intended to rotate, like a spindle, a spit, or the rolling cylinder of a machine. in the first supposition, all motion is excluded: in the second, rectilinear motion is implied, but rotatory motion is excluded: in the third, rotatory motion is implied as a certain adjunct. the body which is fastened round another, must share the motion or the rest of that other. if the body _b_ is a revolving cylinder, and if i affirm that _a_ is packed or fastened close round it, i introduce the idea of rotation; though only as an accessory and implied fact, in addition to that which the proposition affirms. the body _a_, being fastened round the cylinder _b_, must either revolve along with it and round it, or it must arrest the rotation of _b_. if the one revolves, so must the other; both must either revolve together, or stand still together. this is a new fact, distinct from what is affirmed in the proposition, yet implied in it or capable of being inferred from it through induction and experience. here we see exactly the position of plato in regard to the rotation of the earth. he does not affirm it in express terms, but he affirms what implies it. for when he says that the earth is packed, or fastened close round the cosmical axis, he conveys to us by implication the knowledge of another and distinct fact--that the earth and the cosmical axis must either revolve together or remain stationary together--that the earth must either revolve along with the axis or arrest the revolutions of the axis. it is manifest that plato does not mean the revolutions of the axis of the kosmos to be arrested: they are absolutely essential to the scheme of the timæus--they are the grand motive-agency of the kosmos. he must, therefore, mean to imply that the earth revolves along with and around the cosmical axis. and thus the word [greek: ei(lo/menon] or [greek: i)llo/menon], according to buttmann's doctrine, becomes accidentally conjoined, through the specialities of this case, with an accessory idea of rotation or revolution; though that idea is foreign to its constant and natural meaning. now if we turn to aristotle, we shall find that he understood the word [greek: ei(lo/menon] or [greek: i)llo/menon], and the proposition of plato, exactly in this sense. here i am compelled to depart from buttmann, who affirms (p. ), with an expression of astonishment, that aristotle misunderstood the proposition of plato, and interpreted [greek: ei(lo/menon] or [greek: i)llo/menon] as if it meant directly as well as incontestably, _rotating_ or _revolving_. proklus, in his commentary on the timæus, had before raised the same controversy with aristotle--[greek: i)llome/nên de\, tê\n sphiggome/nên dêloi= kai\ sunechome/nên ou) ga\r ô(s a)ristote/lês oi)/etai, tê\n kinoume/nên] (procl. p. ). let us, therefore, examine the passages of aristotle out of which this difficulty arises. the passages are two, both of them in the second book de coelo; one in cap. , the other in cap. (p. b. , a. ). . the first stands--[greek: e)/nioi de\ kai\ keime/nên (tê\n gê=n) e)pi\ tou= ke/ntrou phasi\n au)tê\n i)/llesthai peri\ to\n dia\ panto\s tetame/non po/lon, ô(/sper e)n tô=| timai/ô|** ge/graptai.** such is the reading of bekker in the berlin edition: but he gives various readings of two different mss.--the one having [greek: i)/llesthai kai\ kinei=sthai]--the other ei(lei=sthai** kai\ kinei=sthai]. . the second stands, beginning chap. --[greek: ê(mei=s de\ le/gômen prô=ton po/teron] (the earth) [greek: e)/chei ki/nêsin ê)\ me/nei; katha/per ga\r ei)/pomen, oi( me\n au)tê\n e(\n tô=n a)/strôn poiou=sin, oi( d' e)pi\** tou= me/sou the/ntes i)/llesthai kai\ kinei=shai/ phasi peri\ to\n po/lon me/son.] now, in the first of these two passages, where aristotle simply brings the doctrine to view without any comment, he expressly refers to the timæus, and therefore quotes the expression of that dialogue without any enlargement. he undoubtedly understands the affirmation of plato--that the earth was fastened round the cosmical axis--as implying that it rotated along with the rotations of that axis. aristotle thus construes [greek: i)/llesthai], _in that particular proposition_ of the timæus, as implying rotation. but he plainly did not construe [greek: i)/llesthai] as naturally and constantly either denoting or implying rotation. this is proved by his language in the second passage, where he reproduces the very same doctrine with a view to discuss and confute it, and without special reference to the platonic timæus. here we find that he is not satisfied to express the doctrine by the single word [greek: i)/llesthai]. he subjoins another verb--[greek: i)/llesthai kai\ kinei=sthai]: thus bringing into explicit enunciation the fact of rotatory movement, which, while [greek: i)/llesthai] stood alone, was only known by implication and inference from the circumstances of the particular case. if he had supposed [greek: i)/llesthai] by itself to signify _revolving_ the addition of [greek: kinei=sthai] would have been useless, unmeaning, and even impertinent. aristotle, as boeckh remarks, is not given to multiply words unnecessarily. it thus appears, when we examine the passages of aristotle, that he understood [greek: i)/llesthai] quite in conformity with buttmann's explanation. rotatory movement forms no part of the meaning of the word; yet it may accidentally, in a particular case, be implied as an adjunct of the meaning, by virtue of the special circumstances of that case. aristotle describes the doctrine as held by _some persons_. he doubtless has in view various platonists of his time, who adopted and defended what had been originally advanced by plato in the timæus. m. boeckh, in a discussion of some length (untersuch. p. - ), maintains the opinion that the reading in the first passage of aristotle is incorrect; that the two words [greek: i)/llesthai kai\ kinei=sthai] ought to stand in the first as they do in the second,--as he thinks that they stood in the copy of simplikius: that aristotle only made reference to plato with a view to the peculiar word [greek: i)/llesthai], and not to the general doctrine of the rotation of the earth: that he comments upon this doctrine as held by others, but not by plato--who (according to boeckh) was known by everyone not to hold it. m. boeckh gives this only as a conjecture, and i cannot regard his arguments in support of it as convincing. but even if he had convinced me that [greek: i)/llesthai kai\ kinei=sthai] were the true reading in the first passage, as well as in the second, i should merely say that aristotle had not thought himself precluded by the reference to the timæus from bringing out into explicit enunciation what the platonists whom he had in view knew to be implied and intended by the passage. this indeed is a loose mode of citation, which i shall not ascribe to aristotle without good evidence. in the present case such evidence appears to me wanting.[ ] [footnote : exactness of citation is not always to be relied on among ancient commentators. simplikius cites this very passage of the timæus with more than one inaccuracy.--(ad aristot. de coelo, fol. .)] m. martin attributes to aristotle something more than improper citation. he says (Êtudes sur le timée, vol. ii. p. ), "si aristote citait l'opinion de la rotation de la terre comme un titre de gloire pour platon, je dirais--il est probable que la vérité l'y a forcé. mais aristote, qui admettait l'immobilité complète de la terre, attribue à platon l'opinion contraire, _pour se donner le plaisir de la réfuter avec dédain_." a few lines before, m. martin had said that the arguments whereby aristotle combated this opinion ascribed to plato were "very feeble." i am at a loss to imagine in which of aristotle's phrases m. martin finds any trace of disdain or contempt, either for the doctrine or for those who held it. for my part, i find none. the arguments of aristotle against the doctrine, whatever be their probative force, are delivered in that brief, calm, dry manner which is usual with him, without a word of sentiment or rhetoric, or anything [greek: e)/xô tou= pra/gmatos]. indeed, among all philosophers who have written much, i know none who is less open to the reproach of mingling personal sentiment with argumentative debate than aristotle. plato indulges frequently in irony, or sneering, or rhetorical invective; aristotle very rarely. moreover, even apart from the question of contempt, the part which m. martin here assumes aristotle to be playing, is among the strangest anomalies in the history of philosophy. aristotle holds, and is anxious to demonstrate, the doctrine of the earth's immobility; he knows (so we are required to believe) that plato not only holds the same doctrine, but has expressly affirmed it in the timæus: he might have produced plato as an authority in his favour, and the passage of the timæus as an express declaration; yet he prefers to pervert, knowingly and deliberately, the meaning of this passage, and to cite plato as a hostile instead of a friendly authority--simply "to give himself the pleasure of contemptuously refuting plato's opinion!" but this is not all. m. martin tells us that the arguments which aristotle produces against the doctrine are, after all, very feeble. but he farther tells us that there was one argument which might have been produced, and which, if aristotle had produced it, would have convicted plato of "an enormous contradiction" (p. ) in affirming that the earth revolved round the cosmical axis. aristotle might have said to plato--"you have affirmed, and you assume perpetually throughout the timæus, the diurnal revolution of the outer sidereal sphere; you now assert the diurnal revolution of the earth at the centre. here is an enormous contradiction; the two cannot stand together."--yet aristotle, having this triumphant argument in his hands, says not a word about it, but contents himself with various other arguments which m. martin pronounces to be very feeble. perhaps m. martin might say--"the contradiction exists; but aristotle was not sharpsighted enough to perceive it; otherwise he would have advanced it." i am quite of this opinion. if aristotle had perceived the contradiction, he would have brought it forward as the strongest point in his controversy. his silence is to me a proof that he did not perceive it. but this is a part of my case against m. martin. i believe that plato admitted both the two contradictory doctrines without perceiving the contradiction; and it is a strong presumption in favour of this view that aristotle equally failed to perceive it--though in a case where, according to m. martin, he did not scruple to resort to dishonest artifice. it appears to me that the difficulties and anomalies, in which we are involved from supposing that aristotle either misunderstood or perverted the meaning of plato--are far graver than those which would arise from admitting that plato advanced a complicated theory involving two contradictory propositions, in the same dialogue, without perceiving the contradiction; more especially when the like failure of perception is indisputably ascribable to aristotle--upon every view of the case. m. cousin maintains the same interpretation of the platonic passage as boeckh and martin, and defends it by a note on his translation of the timæus (p. ). the five arguments which he produces are considered both by himself and by martin to be unanswerable. as he puts them with great neatness and terseness, i here bestow upon them a separate examination. . "platon a toujours été considéré dans l'antiquité comme partisan de l'immobilité absolue de la terre." m. cousin had before said, "aristote se fonde sur ce passage pour établir que platon a fait tourner la terre sur elle-même: mais aristote est, dans l'antiquité, le seul qui soutienne cette opinion." my reply is, that aristotle is himself a portion and member of antiquity, and that the various platonists, whom he undertakes to refute, are portions of it also. if m. cousin appeals to the authority of antiquity, it must be to antiquity, not merely _minus_ aristotle and these contemporary platonists, but _against_ them. now these are just the witnesses who had the best means of knowledge. besides which, aristotle himself, adopting and anxious to demonstrate the immobility of the earth, had every motive to cite plato as a supporter, if plato was such--and every motive to avoid citing plato as an opponent, unless the truth of the case compelled him to do so. i must here add, that m. cousin represents aristotle as ascribing to plato the doctrine that "la terre tourne sur elle-même." this is not strictly exact. aristotle understands the platonic timæus as saying, "that the earth is packed and moved _round the axis of the kosmos_"--a different proposition. . "dans plusieurs endroits de ses ouvrages où platon parle de l'équilibre de la terre, il ne dit pas un mot de sa rotation." i know of only _one_ such passage--phædon, p. --where undoubtedly plato does not speak of the rotation of the earth; but neither does he speak of the rotation of the sidereal sphere and of the kosmos--nor of the axis of the kosmos. it is the figure and properties of the earth, considered in reference to mankind who inhabit it, that plato sketches in the phædon; he takes little notice of its cosmical relations, and gives no general theory about the kosmos. m. cousin has not adverted to the tenth book of the republic, where plato does propound a cosmical theory, expressly symbolising the axis of the kosmos with its rotatory functions. . "si la _terre suit le mouvement de l'axe du monde_, le mouvement de la huitième sphère, qui est le même, devient nul par rapport à elle, et les étoiles fixes, qui appartiennent à elle, demeurent en apparence dans une immobilité absolue: ce qui est contraire à _l'expérience et au sens commun_, et à l'opinion de platon, exprimée dans ce même passage." this third argument of m. cousin is the same as that which i have already examined in remarking upon m. boeckh. the diurnal rotation of the earth cannot stand in the same astronomical system with the diurnal rotation of the sidereal sphere. incontestably true (i have already said) as a point of science. but the question here is, not what opinions are scientifically consistent, but what opinions were held by plato, and whether he detected the inconsistency between the two. i have shown grounds for believing that he did not--and not he alone, but many others along with him, aristotle among the number. how, indeed, can this be denied, when we find m. boeckh announcing that he is the _first_ among all the critics on the timæus, who has brought forward the inconsistency as a special ground for determining what plato's opinion was--that no other critic before him had noticed it? the first words of this argument deserve particular attention, "si la terre suit le mouvement de l'axe du monde." here we have an exact recital of the doctrine proclaimed by the platonic timæus, and ascribed to him by aristotle (quite different from the doctrine "que la terre tourne sur elle-même"). m. cousin here speaks very distinctly about the cosmical axis, and about its movement; thus implying that plato conceived it as a solid revolving cylinder. this, in my judgment, is the most essential point for clearing up the question in debate. the cosmical axis being of this character, when plato affirms that the earth is _packed or fastened round it_ (_se roule_--cousin: _se serre et s'enroule_--martin: _drängt sich, macht eine kugel um ihn_--buttmann), i maintain that, in the plainest construction of the word, the earth does and must follow the movement of the axis--or arrest the movement of the axis. the word [greek: ei(lome/nên] or [greek: i)llome/nên] has no distinct meaning at all, if it does not mean this. the very synonyms ([greek: sphiggome/nên, peridedeme/nên], &c.), which the commentators produce to prove that plato describes the earth as at rest, do really prove that he describes it as rotating round and with the cosmical axis. we ought not to be driven from this plain meaning of the word, by the assurance of m. cousin and others that plato cannot have meant so, because it would involve him in an astronomical inconsistency. . "les divers mouvemens des huit sphères expliquent toutes les apparences célestes; il n'y a donc aucune raison pour donner un mouvement à la terre." the terms of this fourth argument, if literally construed, would imply that plato had devised a complete and satisfactory astronomical theory. i pass over this point, and construe them as m. cousin probably intended: his argument will then stand thus--"the movement of the earth does not add anything to plato's power of explaining astronomical appearances; therefore plato had no motive to suggest a movement of the earth." i have already specified the sense in which i understand the platonic timæus to affirm, or rather to imply, the rotation of the earth; and that sense is not open to the objections raised in m. cousin's fourth and fifth arguments. the rotation of the earth, as it appears in the platonic timæus, explains nothing, and is not intended to explain anything. it is a consequence, not a cause: it is a consequence arising from the position of the earth, as packed or fastened round the centre of the cosmical axis, whereby the earth participates, of necessity and as a matter of course, in the movements of that axis. the _function_ of the earth, thus planted in the centre of the kosmos, is to uphold and regulate the revolutions of the cosmical axis; and this function explains, in the scheme of the platonic timæus, why the axis revolves uniformly and constantly without change or displacement. now upon these revolutions of the cosmical axis all the revolutions of the exterior sphere depend. this is admitted by m. cousin himself in argument . there is therefore every reason why plato should assign such regulating function to the earth, the "first and oldest of intra-kosmic deities." the movement of the earth (as i before observed) is only an incidental consequence of the position necessary for the earth to occupy in performing such function. . "enfin platon assigne un mouvement aux étoiles fixes, et deux mouvemens aux planètes; puisqu'il ne range la terre ni avec les unes ni avec les autres, il y a lieu de croire qu'elle ne participe à aucun de leurs mouvemens." in so far as this argument is well-founded, it strengthens my case more than that of m. cousin. the earth does not participate in the movements either of the fixed stars or of the planets; but it does participate in the revolutions of the cosmical axis, upon which these movements depend--the movements of the outer sphere, wholly and exclusively--the movements of the planets, to a very great degree, but not exclusively. the earth is not ranked either among the fixed stars or among the planets; it is a body or deity _sui generis_, having a special central function of its own, to regulate that cosmical axis which impels the whole system. the earth has a motion of its own, round and along with the cosmical axis to which it is attached; but this motion of the earth (i will again repeat, to prevent misapprehension) is a fact not important by itself, nor explaining anything. the grand and capital fact is the central position and regulating function of the earth, whereby all the cosmical motions, first those of the axis, next those of the exterior kosmos, are upheld and kept uniform. m. cousin adds, as a sixth argument:-- "on peut ajouter à ces raisons que platon aurait nécessairement insisté sur le mouvement de la terre, s'il l'avait admis; et que ce point étoit trop controversé de son temps et trop important en lui-même, pour qu'il ne fît que l'indiquer en se servant d'une expression équivoque." in the first place, granting plato to have believed in the motion of the earth, can we also assume that he would necessarily have asserted it with distinctness and emphasis, as m. cousin contends? i think not. gruppe maintains exactly the contrary; telling us that plato's language was intentionally obscure and equivocal--from fear of putting himself in open conflict with the pious and orthodox sentiment prevalent around him. i do not carry this part of the case so far as gruppe, but i admit that it rests upon a foundation of reality. when we read (plutarch, de facie in orbe lunæ, p. ) how the motion of the earth, as affirmed by aristarchus of samos (doubtless in a far larger sense than plato ever imagined, including both rotation and translation), was afterwards denounced as glaring impiety, we understand the atmosphere of religious opinion with which plato was surrounded. and we also perceive that he might have reasons for preferring to indicate an astronomical heresy in terms suitable for philosophical hearers, rather than to proclaim it in such emphatic unequivocal words, as might be quoted by some future melêtus in case of an indictment before the dikasts. we must remember that plato had been actually present at the trial of sokrates. he had heard the stress laid by the accusers on astronomical heresies, analogous to those of anaxagoras, which they imputed to sokrates--and the pains taken by the latter to deny that he held such opinions (see the platonic apology). the impression left by such a scene on plato's mind was not likely to pass away: nor can we be surprised that he preferred to use propositions which involved and implied, rather than those which directly and undisguisedly asserted, the heretical doctrine of the earth's rotation. that his phraseology, however indirect, was perfectly understood by contemporary philosophers, both assentient and dissentient, as embodying his belief in the doctrine--is attested by the two passages of aristotle. upon these reasons alone i should dissent from m. cousin's sixth argument. but i have other reasons besides. he rests it upon the two allegations that the doctrine of the earth's motion was the subject of much controversial debate in plato's time, and of great importance in itself. now the first of these two allegations can hardly be proved, as to the time of plato; for aristotle, when he is maintaining the earth's immobility, does not specify any other opponents than the pythagoreians and the followers of the platonic timæus. and the second allegation i believe to be unfounded, speaking with reference to the platonic timæus. in the cosmical system therein embodied, the rotation of the earth round the cosmical axis, though a real part of the system, was in itself a fact of no importance, and determining no results. the capital fact of the system was the position and function of the earth, packed close round the centre of the cosmical axis, and regulating the revolutions of that axis. plato had no motive to bring prominently forward the circumstance that the earth revolved itself along with the cosmical axis, which circumstance was only an incidental accompaniment. i have thus examined all the arguments adduced by m. cousin, and have endeavoured to show that they fail in establishing his conclusion. there is, however, one point of the controversy in which i concur with him more than with boeckh and martin. this point is the proper conception of what plato means by the _cosmical axis_. boeckh and martin seem to assume this upon the analogy of what is now spoken of as the axis of the earth: m. boeckh (p. ) declares the axis of the kosmos to be a prolongation of that axis. but it appears to me (and m. cousin's language indicates the same) that plato's conception was something very different. the axis of the earth (what astronomers speak of as such) is an imaginary line traversing the centre of the earth; a line round which the earth revolves. now the cosmical axis, as plato conceives it, is a solid material cylinder, which not only itself revolves, but causes by this revolution the revolution of the exterior circumference of the kosmos. this is a conception entirely different from that which we mean when we speak of the axis of the earth. it is, however, a conception symbolically enunciated in the tenth book of the republic, where the spindle of necessity is said to be composed of adamant, hard and solid material, and to cause by its own rotation the rotation of all the _verticilli_ packed and fastened around it. what is thus enunciated in the republic is implied in the timæus. for when we read therein that the earth is packed or fastened round the cosmical axis, how can we understand it to be packed or fastened round an imaginary line? i will add that the very same meaning is brought out in the translation of cicero--"_trajecto axe sustinetur_" (terra). the axis, round which the earth is fastened, and which sustains the earth, must be conceived, not as an imaginary line, but as a solid cylinder, itself revolving; while the earth, being fastened round it, revolves round and along with it. the axis, in the sense of an imaginary line, cannot be found in the conception of plato. those contemporaries of plato and aristotle, who all agreed in asserting the revolution of the celestial sphere, did not all agree in their idea of the force whereby such revolution was brought about. some thought that the poles of the celestial sphere exercised a determining force: others symbolised the mythical atlas, as an axis traversing the sphere from pole to pole and turning it round. (aristotel. de motu animal. . p. a. - .) aristotle himself advocated the theory of a _primum movens immobile_ acting upon the sphere from without the sphere. even in the succeeding centuries, when astronomy was more developed, aratus, eratosthenes, and their commentators, differed in their way of conceiving the cosmical axis. most of them considered it as solid: but of these, some thought it was stationary, with the sphere revolving round it--others that it revolved itself: again, among these latter, some believed that the revolutions of the axis determined those of the surrounding sphere--others, that the revolutions of the sphere caused those of the axis within it. again, there were some physical philosophers who looked at the axis as airy or spiritual--[greek: to\ dia\ me/sou tê=s sphai/ras diê=kon pneu=ma]. then there were geometers who conceived it only as an imaginary line. (see the phaenomena of aratus - --with the scholia thereon; achilles tatius ad arati phaenom. apud petavium--uranolog. p. ; also hipparchus ad arat. ib. p. .) i do not go into these dissentient opinions farther than to show, how indispensable it is, when we construe the passage in the platonic timaeus, [greek: peri\ to\n dia\ panto\s po/lon tetame/non], to enquire in what sense plato understood the cosmical axis: and how unsafe it is to assume at once that he must have conceived it as an imaginary line. proklus argues that because the earth is mentioned by plato in the phædon as stationary in the centre of the heaven, we cannot imagine plato to affirm its rotation in the timæus. i agree with m. boeckh in thinking this argument inconclusive; all the more, because, in the phædon, not a word is said either about the axis of the kosmos, or about the rotation of the kosmos; all that sokrates professes to give is [greek: tê\n i)de/an tê=s gê=s kai\ tou\s to/pous au)tê=s**] (p. e). no cosmical system or theory is propounded in that dialogue. when we turn to the phædrus, we find that, in its highly poetical description, the rotation of the heaven occupies a prominent place. the internal circumference of the heavenly sphere, as well as its external circumference or back ([greek: nô=ton]), are mentioned; also its periodical rotations, during which the gods are carried round on the back of the heaven, and contemplate the eternal ideas occupying the super-celestial space (p. , ), or the plain of truth.[ ] but the purpose of this poetical representation appears to be metaphysical and intellectual, to illustrate the antithesis presented by the world of ideas and truth on one side--against that of sense and appearances on the other. astronomically and cosmically considered, no intelligible meaning is conveyed. nor can we even determine whether the rotations of the heaven, alluded to in the phædrus, are intended to be diurnal or not; i incline to believe not ([greek: me/chri tê=s _e(te/ras_ perio/dou]--p. --which can hardly be understood of so short a time as one day). lastly, nothing is said in the phædrus about the cosmical axis; and it is upon this that the rotations of the earth intimated in the timæus depend. [footnote : whether [greek: e)sti/a] in the phædrus, which is said "to remain alone stationary in the house of the gods," can be held to mean the earth, is considered by proklus to be uncertain (p. ).] among the different illustrations, given by plato in his different dialogues respecting the terrestrial and celestial bodies, i select the tenth book of the republic as that which is most suitable for comparison with the timæus, because it is only therein that we learn how plato conceived the axis of the kosmos. m. boeckh (untersuchungen, p. ) wishes us to regard the difference between the view taken in the phædon, and that in the republic, as no way important; he affirms that the adamantine spindle in the republic is altogether mythical or poetical, and that plato conceives the axis as not being material. on this point i dissent from m. boeckh. the mythical illustrations in the tenth book of the republic appear to me quite unsuitable to the theory of an imaginary, stationary, and immaterial axis. here i much more agree with gruppe (p. , - ), who recognises the solid material axis as an essential feature of the cosmical theory in the republic; and recognises also the marked difference between that theory and what we read in the phædon. yet, though gruppe is aware of this important difference between the republic and the phædon, he still wishes to illustrate the timæus by the latter and not by the former. he affirms that the earth in the timæus is conceived as unattached, and freely suspended, the same as in the phædon; but that in the timæus it is conceived, besides, as revolving on its own axis, which we do not find in the phædon (p. , ). here i think gruppe is mistaken. in construing the words of timæus, [greek: ei(lome/nên (i)llome/nên) peri\ to\n dia\ panto\s po/lon tetame/non], as designating "the unattached earth revolving round its own axis," he does violence not less to the text of plato than to the expository comment of aristotle. neither in the one nor the other is anything said about _an axis of the earth_; in both, the cosmical axis is expressly designated; and, if gruppe is right in his interpretation of [greek: ei(lome/nên], we must take plato as affirming, not that the earth is fastened round the cosmical axis, but that it revolves, though unattached, around that axis, which is a proposition both difficult to understand, and leading to none of those astronomical consequences with which gruppe would connect it. again, when gruppe says that [greek: ei(lome/nên peri\] does _not_ mean _packed or fastened round_, but that it _does_ mean _revolving round_, he has both the analogies of the word and the other commentators against him. the main proof, if not the only proof, which he brings, is that aristotle so construed it. upon this point i join issue with him. i maintain that aristotle does _not_ understand [greek: ei(lome/nên] or [greek: i)llome/nên peri\] as naturally meaning _revolving round_, and that he does understand the phrase as meaning _fastened round_. when we find him, in the second passage of the treatise de coelo, not satisfied with the verb [greek: i)/llesthai] alone, but adding to it the second verb [greek: _kai\ kinei=sthai_], we may be sure that he did not consider [greek: i)/llesthai] as naturally and properly denoting _to revolve_ or _move round_. agreeing as i do with gruppe in his view, that the interpretation put by aristotle is the best evidence which we can follow in determining the meaning of this passage in the timæus, i contend that the authority of aristotle contradicts instead of justifying the conclusion at which he arrives. aristotle understands [greek: i)llome/nên] as meaning _packed or fastened round_; he does not understand it as meaning, when taken by itself, _revolving round_. the two meanings here indicated are undoubtedly distinct and independent. but they are not for that reason contradictory and incompatible. it has been the mistake of critics to conceive them as thus incompatible; so that if one of the two were admitted, the other must be rejected. i have endeavoured to show that this is not universally true, and that there are certain circumstances in which the two meanings not only may come together, but must come together. such is the case when we revert to plato's conception of the cosmical axis as a solid revolving cylinder. that which is packed or fastened around the cylinder must revolve around it, and along with it. both m. boeckh and gruppe assume the incompatibility of the two meanings; and we find the same assumption in plutarch's criticisms on the timæus (plutarch. quæst. platon. p. c), where he discusses what plato means by [greek: o)/rgana chro/nou]; and in what sense the earth as well as the moon can be reckoned as [greek: o)/rganon chro/nou] (timæus, p. e, d). plutarch inquires how it is possible that the earth, if stationary and at rest, can be characterised as "among the instruments of time;" and he explains it by saying that this is true in the same sense as we call a gnomon or sun-dial an instrument of time, because, though itself never moves, it marks the successive movements of the shadow. this explanation might be admissible for the phrase [greek: o)/rganon chro/nou]; but i cannot think that the immobility of the earth can be made compatible with the attribute which plato bestows upon it of being [greek: phu/lax kai\ _dêmiourgo\s_ nukto\s te kai\ ê(me/ras]. the difficulty, however, vanishes when we understand the function ascribed by plato to the earth as i have endeavoured to elucidate it. the earth not only is not at rest, but cannot be at rest, precisely because it is packed round the solid revolving cosmical axis, and must revolve along with it. the function of the earth, as the first and oldest of intra-kosmic deities, is to uphold and regulate the revolutions of this axis, upon which depend the revolutions of the sidereal sphere or outer shell of the kosmos. it is by virtue of this regulating function (and not by virtue of its rotation) that the earth is the guardian and artificer of night and day. it is not only "an instrument of time," but the most potent and commanding among all instruments of time. what has just been stated is, in my belief, the theory of the platonic timæus, signified in the words of that dialogue, and embodied in the comment of aristotle. the commentators, subsequent to aristotle, so far as we know them, understood the theory in a sense different from what plato intended. i think we may see how this misconception arose. it arose from the great development and elaboration of astronomical theory during the two or three generations immediately succeeding plato. much was added by eudoxus and others, in their theory of concentric spheres: more still by others of whom we read in cicero (academ. ii. .) "hicetas syracusius, ut ait theophrastus, coelum, solem, lunam, stellas, supera denique omnia, stare censet, neque praeter terram rem ullam in mundo moveri: quae cum circum axem se summâ celeritate convertat et torqueat, eadem effici omnia, quae si stante terrâ coelum moveretur. atque hoc etiam platonem in timaeo dicere quidam arbitrantur, sed paullo obscurius." the same doctrine is said to have been held by herakleides of pontus, the contemporary of aristotle, and by others along with him. (simplikius ad aristot. physic. p. --de coelo, p. --plutarch. plac. phil. iii. .) the doctrine of the rotation of the earth here appears along with another doctrine--the immobility of the sidereal sphere and of the celestial bodies. the two are presented together, as correlative portions of one and the same astronomical theory. there are no celestial revolutions, and therefore there is no solid celestial axis. moreover, even aristarchus of samos (who attained to a theory substantially the same as the copernican, with the double movement of the earth, rotation round its own axis, and translation round the sun as a centre) comes within less than a century after plato's death. though the _quidam_ alluded to by cicero looked upon the obscure sentence in plato's timaeus as a dim indication of the theory of hicetas, yet the two agree only in the supposition of a rotation of the earth, and differ essentially in the pervading cosmical conceptions. hicetas states distinctly that which his theory denies, as well as that which it affirms. the negation of the celestial rotations, is in his theory a point of capital and coordinate importance, on which he contradicts both plato and aristotle as well as the apparent evidence of sense. i cannot suppose that this theory can have been proclaimed or known to aristotle when his works were composed: for the celestial revolutions are the keystone of his system, and he could hardly have abstained from combating a doctrine which denied them altogether. in the hands of hicetas (perhaps in those of herakleides, if we may believe what is said about him) astronomy appears treated as a science by itself, with a view "to provide such hypotheses as may save the phenomena" ([greek: sô/zein ta\ phaino/mena], simpl. ad aristot. de coelo, p. , schol. brandis). it becomes detached from those religious, ethical, poetical, teleological, arithmetical decrees or fancies, in which we see it immersed in the platonic timaeus, and even (though somewhat less) in the aristotelian treatise de coelo. hence the meaning of plato, obscurely announced from the beginning, ceased to be understood: the solid revolving axis of the kosmos, assumed without being expressly affirmed in his timaeus, dropped out of sight: the doctrine of the rotation of the earth was presented in a new point of view, as a substitute for the celestial revolutions. but no proper note was taken of this transition. the doctrine of plato was assumed to be the same as that of hicetas. when we read plutarch's criticism (quæst. plat. p. c) upon the word [greek: i)llome/nên], we see that he puts to himself the question thus--"does plato in the timæus conceive the earth as kept together and stationary--or as turning round and revolving, agreeably to the subsequent theory of aristarchus and seleukus?" here we find that plutarch conceives the alternative thus--either the earth does not revolve at all, or it revolves as aristarchus understood it. one or other of these two positions must have been laid down by plato in the timæus.--so we read in plutarch. but the fact is, that plato meant neither the one nor the other. the rotation of the earth round the solid cosmical axis, which he affirms in the timæus--is a phenomenon utterly different from the rotation of the earth as a free body round the imaginary line called its own axis, which was the doctrine of aristarchus. when expositors in plutarch's day, and since his day, enquired whether or not the platonic timæus affirmed the rotation of the earth, they meant to designate the rotation of the earth in the sense of aristarchus, and in the sense in which modern astronomy understands that capital fact. now speaking the language of modern astronomy, i think it certain that the rotation of the earth is _not_ to be found affirmed in the platonic timæus; and i agree with m. boeckh when he says (untersuch. p. ), "granting that aristotle ascribed to plato the doctrine of the rotation of the earth, he at least did not ascribe to him the doctrine as gruppe assumes, and as now understood." as between gruppe--who holds that the platonic timæus affirms the rotation of the earth, and that aristotle ascribes it to him, in our sense of the words--and m. boeckh, who denies this--i stand with the latter for the negative. but when m. boeckh assumes that the only alternative doctrine is the immobility of the earth, and tries to show that this doctrine is proclaimed in the platonic timæus--nay, that no opposite doctrine _can_ be proclaimed, because the discourse expressly announces the rotation of the sidereal heaven in twenty-four hours--i am compelled to dissent from him as to the conclusion, and to deny the cogency of his proof. m. boeckh has hardly asked himself the question, whether there was not some other sense in which plato might have affirmed it in the timæus. i have endeavoured to show that there was another sense; that there are good analogies in plato to justify the belief that he intended to affirm the doctrine in that other sense; and that the comments of aristotle--while thoroughly pertinent, if we thus understand the passage in the timæus--become either irrelevant, dishonest, or absurd, if we construe the passage as signifying either what is maintained by m. boeckh or what is maintained by gruppe. the eminent critics, whose opinions i here controvert, have been apparently misled by the superior astronomical acquirements of the present age, and have too hastily made the intellectual exigencies of their own minds a standard for all other minds, in different ages as well as in different states of cultivation. the question before us is, not what doctrines are scientifically true or scientifically compatible with each other, but what doctrines were affirmed or implied by plato. in interpreting him, we are required to keep our minds independent of subsequent astronomical theories. we must look, first and chiefly, to what is said by plato himself; next, if that be obscure, to the construction and comments of his contemporaries so far as they are before us. in no case is this more essential than in the doctrine of the rotation of the earth, which in the modern mind has risen to its proper rank in scientific importance, and has become connected with collateral consequences and associations foreign to the ideas of the ancient pythagoreans, or plato, or aristotle. unless we disengage ourselves from these more recent associations, we cannot properly understand the doctrine as it stands in the platonic timæus. this doctrine, as i have endeavoured to explain it, leads to an instructive contrast between the cosmical theories of plato (in the timæus) and aristotle. plato conceives the kosmos as one animated and intelligent being or god, composed of body and soul. its body is moved and governed by its soul, which is fixed or rooted in the centre, but stretches to the circumference on all sides, as well as all round the exterior. it has a perpetual movement of circular rotation in the same unchanged place, which is the sort of movement most worthy of a rational and intelligent being. the revolutions of the exterior or sidereal sphere (circle of the same) depend on and are determined by the revolutions of the solid cylinder or axis, which traverses the kosmos in its whole diameter. besides these, there are various interior spheres or circles (circles of the different), which rotate by distinct and variable impulses in a direction opposite to the sidereal sphere. this latter is so much more powerful than they, that it carries them all round with it; yet they make good, to a certain extent, their own special opposite movement, which causes their positions to be ever changing, and the whole system to be complicated. but the grand capital, uniform, overpowering, movement of the kosmos, consists in the revolution of the solid axis, which determines that of the exterior sidereal sphere. the impulse or stimulus to this movement comes from the cosmical soul, which has its root in the centre. just at this point is situated the earth, "the oldest and most venerable of intra-kosmic deities," packed round the centre of the axis, and having for its function to guard and regulate those revolutions of the axis, and through them those of the outer sphere, on which the succession of day and night depends--as well as to nurse mankind. in all this we see that the ruling principle and force of the kosmos ([greek: to\ ê(gemoniko\n tou= ko/smou]) is made to dwell in and emanate from _its centre_. when we come to aristotle, we find that the ruling principle or force of the kosmos is placed, not in its centre, but in its circumference. he recognises no solid revolving axis traversing the whole diameter of the kosmos the interior of the kosmos is occupied by the four elements--earth, water, air, fire--neither of which can revolve except by violence or under the pressure of extraneous force. to each of them rectilinear motion is _natural_; earth moves naturally towards the centre--fire moves naturally towards the circumference, away from the centre. but the peripheral substance of the kosmos is radically distinct from the four elements: rotatory motion in a circle is _natural_ to it, and is the only variety of motion natural to it. that it is moved at all, it owes to a _primum movens immobile_ impelling it: but the two are coeternal, and the motion has neither beginning nor end. that when moved, its motion is rotatory and not rectilinear, it owes to its own nature. it rotates perpetually, through its own nature and inherent virtue, not by constraining pressure communicated from a centre or from a soul. if constraint were required--if there were any contrary tendency to be overcome--the revolving periphery would become fatigued, and would require periods of repose; but, since in revolving it only obeys its own peculiar nature, it persists for ever without knowing fatigue. this peripheral or fifth essence, perpetually revolving, is the divine, venerable, and commanding portion of the kosmos, more grand and honourable than the interior parts or the centre. aristotle lays this down (de coelo, ii. , p. , b. ) in express antithesis to the pythagoreans, who (like plato) considered the centre as the point of grandeur and command, placing fire in the centre for that reason. the earth has no positive cosmical function in aristotle; it occupies the centre because all its parts have a natural movement towards the centre: and it is unmoved because there _must be_ something in the centre which is always stationary, as a contrary or antithesis to the fifth essence or peripheral substance of the kosmos, which is in perpetual rotation by its own immutable nature. i do not here go farther into the exposition of these ancient cosmical theories. i have adverted to aristotle's doctrine only so far as was necessary to elucidate, by contrast, that which i believe to be the meaning of the platonic timæus about the rotation of the earth. london: printed by w. clowes and sons, stamford street, and charing cross. **************************************************************** transcriber's note the text is based on versions made available by the internet archive. for the greek transcriptions the following conventions have been used: ) is for smooth breathing; ( for hard; + for diaeresis; / for acute accent; \ for grave; = for circumflex; | for iota subscript. ch is used for chi, ph for phi, ps for psi, th for theta; ê for eta and ô for omega; u is used for upsilon in all cases. corrections to the text, indicated with ** text correction ti/ma/iô ti/maio chore/ia chorei/a ou)/ranon ou)rano\n tetamenon tetame/non e)ndeiknutai e)ndei/knutai] forge force tima/iô| timai/ô| gegraptai ge/graptai] e(ilei=sthai ei(lei=sthai e)/pi\ e)pi\ a)utê=s au)tê=s note. in the original negatives of subjects and , there are faint dark rings immediately surrounding some of the stars in the denser parts of the nebulosity. this effect has no doubt been accentuated in the subsequent photographic processes. on the plates of these two subjects in the completed volume, these rings are very distinct and give rise to a suspicion that the effect has been enhanced by the engraver. a critical examination of the prints seems to confirm this view. in the original proofs these rings were inconspicuous and were not noticed. the processes of steel-facing and printing appear to have increased the effect markedly, as it is much stronger on the sheets printed for the edition than in any of the early proofs. inasmuch as these effects were not and could not be discovered until the sheets were assembled in sacramento for binding, it has not been thought desirable to delay the issue of the volume for several weeks additional in order to have new plates and new prints of these subjects made by the distant engraver. lick observatory, mount hamilton, november, . [illustration: _plate _ _the great nebula in orion_] university of california publications publications of the lick observatory printed by authority of the regents of the university volume viii sacramento w. w. shannon superintendent of state printing as a tribute to the memory of james edward keeler and in recognition of his great worth as a man and as an astronomer, the plates for this volume have been provided by mr. william alvord, mr. f. m. smith, mr. robert bruce, miss jennie smith, mr. william h. crocker, miss matilda h. smith, mrs. william h. crocker, mr. benjamin thaw, mr. e. j. de sabla, mrs. william thaw, mr. j. a. donohoe, mr. robert j. tobin, mrs. phoebe a. hearst, the university of california, mr. john b. jackson, the state of california. mr. e. j. molera, organization of the lick observatory. hon. charles w. slack, hon. warren r. porter, hon. william h. crocker, rev. peter c. yorke, _committee of the regents for the lick observatory._ benjamin ide wheeler, _president of the university_. w. w. campbell, _director and astronomer_. r. h. tucker,* _astronomer_. c. d. perrine, _astronomer_. h. d. curtis, _mills acting astronomer_. r. g. aitken, _astronomer_. w. h. wright, _astronomer_. j. h. moore, _assistant astronomer_. sebastian albrecht, _assistant astronomer_. miss a. m. hobe, _carnegie assistant_. g. f. paddock, _mills assistant_. miss l. b. allen, _carnegie assistant_. e. a. fath, _fellow_. j. c. duncan, _fellow_. miss a. e. glancy, _fellow_. miss m. e. french,* _secretary_. miss a. j. van coover, _secretary_. * absent on leave. photographs of nebulÆ and clusters, made with the crossley reflector, by james edward keeler, director of the lick observatory. - . preface. when professor keeler entered upon the duties of director of the lick observatory, on june , , he planned to devote his observing time for several years to photographing the brighter nebulæ and star clusters, with the crossley reflector. the story of his wonderful success with this difficult instrument is familiar to all readers of astronomical literature: this form of telescope was in effect born again; and his contributions to our knowledge of the nebulæ were epoch-making. professor keeler's observing programme included one hundred and four subjects. at the time of his lamented death, on august , , satisfactory negatives of two-thirds of the selected objects had been secured. the unphotographed objects were mainly those which come into observing position in the unfavorable winter and spring months. the completion of the programme was entrusted to assistant astronomer perrine. the observers were assisted chiefly by mr. h. k. palmer, and in smaller degree by messrs. joel stebbins, c. g. dall, r. h. curtiss and sebastian albrecht. professor keeler's photographs enabled him to make two discoveries of prime importance, not to mention several that are scarcely secondary to them. st.--"many thousands of unrecorded nebulæ exist in the sky. a conservative estimate places the number within reach of the crossley reflector at about , . the number of nebulæ in our catalogues is but a small fraction of this." [the number already discovered and catalogued did not exceed , . later observations with the crossley reflector, with longer exposure-times and more sensitive plates, render it probable that the number of nebulæ discoverable with this powerful instrument is of the order of half a million.] d.--"most of these nebulæ have a spiral structure." the photographs of the one hundred and four subjects contain the images of nebulæ not previously observed. a catalogue of these is published in the present volume. their positions, which are thought to be accurate within ´´, were determined by messrs. palmer, curtiss, and albrecht. the main purpose of this volume is to reproduce and make available for study, the larger and more interesting nebulæ and clusters on the programme, sixty-eight in number. the thirty-six subjects not reproduced are for the most part small or apparently not of special interest. the difficulties attending the reproduction of astronomical photographs by mechanical processes are well-known to all who have made the attempt. it seems necessary to recognize, at least at present, that delicate details of structure will be lost, and that contrasts between very bright and very faint regions will be changed, especially if a good sky background is preserved; in other words, that the best obtainable reproductions fall far short of doing justice to the original photographs. technical studies should be based upon the original negatives or upon copies on glass. after considerable experimental work, involving several methods and several firms, the making of the heliogravure plates and the hand-press prints was entrusted to the photogravure and color company of new york city. to this firm's continued interest and willingness to act on constructive criticism is due much of the excellence of the results. the expensive reproductions could hardly have been undertaken without the generous assistance of the donors mentioned on a preceding page. professor keeler's description of the crossley reflector, of his methods of observing, and of the chief results obtained, was written only a short time before his death. it is here republished. other results of his work are described in the several papers to which the footnotes refer. table of contents. the orion nebula, _frontispiece_ the crossley reflector of the lick observatory, page list of nebulæ and clusters photographed, " catalogue of new nebulæ discovered on the negatives, " positions of known nebulæ determined from the crossley negatives, " list of illustrations, " illustrations, following " the crossley reflector of the lick observatory.[ ] by james e. keeler. the crossley reflector, at present the largest instrument of its class in america, was made in by dr. a. a. common, of london, in order to carry out, and test by practical observation, certain ideas of his respecting the design of large reflecting telescopes. for the construction of the instrument embodying these ideas, and for some fine astronomical photographs obtained with it, dr. common was awarded the gold medal of the royal astronomical society in . in , dr. common, wishing to make a larger telescope on a somewhat similar plan, sold the instrument to edward crossley, esq., f. r. a. s., of halifax, england. mr. crossley provided the telescope with a dome of the usual form, in place of the sliding roof used by its former owner, and made observations with it for some years; but the climate of halifax not being suitable for the best use of such a telescope, he consented, at the request of dr. holden, then director of the lick observatory, to present it to this institution. the funds for transporting the telescope and dome to california, and setting them up on mount hamilton, were subscribed by friends of the lick observatory, for the most part citizens of california. the work was completed, and the telescope housed in a suitable observatory building, in .[ ] on taking charge of the lick observatory in , i decided to devote my own observing time to the crossley reflector, although the whole of my previous experience had been with refracting telescopes. i was more particularly desirous of testing the reflector with my own hands, because such preliminary trials of it as had been made had given rise to somewhat conflicting opinions as to its merits.[ ] the result of my experience is given in the following article, which is written chiefly with reference to american readers. if i have taken occasion to point out what i regard as defects in the design or construction of the instrument, i have done so, not from any desire to look a gift horse in the mouth, but in the interest of future improvement, and to make intelligible the circumstances under which the work of the reflector is now being done and will be done hereafter. the most important improvements which have suggested themselves have indeed already been made by dr. common himself, in constructing his five-foot telescope. the three-foot reflector is, in spite of numerous idiosyncracies which make its management very different from the comparatively simple manipulation of a refractor, by far the most effective instrument in the observatory for certain classes of astronomical work. certainly no one has more reason than i to appreciate the great value of mr. crossley's generous gift. [illustration: dome of the crossley reflector.] the crossley dome is about yards from the main observatory, at the end of a long rocky spur which extends from the observatory summit toward the south, and on which are two of the houses occupied by members of the observatory staff. it is below the level of the lowest reservoir, "huyghens," which receives the discharge from the hydraulic machinery of the -inch refractor, and therefore the water engine furnished by mr. crossley for turning the dome can not be used, unless a new water system--overflow reservoir, pump and windmill--is provided. in this respect a better site would have been a point on the south slope of "kepler,"--the middle peak of mount hamilton--just above the huyghens reservoir. no addition to the present water system would then have been needed. the slope of the mountain at this place might cut off the view of the north horizon, but since the telescope can not be turned below the pole, this would be a matter of no consequence. water-power for the dome is not, however, really necessary. the cylindrical walls of the dome, - / feet inside diameter, are double, and provided with ventilators. opening into the dome, on the left of the entrance, are three small rooms, one of which has been fitted up as a photographic dark room, and another, containing a sidereal clock and a telephone, which communicates with the main observatory, as a study, while the third is used for tools and storage. there is also a small room for the water engine, in case it should be used. the dome is at present supplied with water from only the middle reservoir, kepler, which is reserved for domestic purposes and is not allowed to pass through the machinery. the dome itself, feet inches in diameter, is made of sheet-iron plates riveted to iron girders. it also carries the wooden gallery, ladders, and observing platform, which are suspended from it by iron rods. the apparatus for turning the dome consists of a cast-iron circular rack bolted to the lower side of the sole-plate, and a set of gears terminating in a sprocket-wheel, from which hangs an endless rope. as the dome does not turn easily, it has been necessary to multiply the gearing of the mechanism so that one arm's-length pull on the rope moves the dome only about one inch. in some positions of the telescope the dome can not be moved more than six or eight inches at a time without danger of striking the tube, and this slowness of motion is then not disadvantageous. it is only when the dome has to be moved through a considerable angle, as in turning to a fresh object, or in photographing some object which passes nearly through the zenith, that the need for a mechanical means of rotation is felt. the observing slit, feet wide, extends considerably beyond the zenith. it is closed by a double shutter, which is operated by an endless rope. the upper part, within the dome, is also closed by a hood, or shield, which serves to protect the telescope from any water that may find its way through the shutter, and which is rolled back to the north when observations are made near the zenith. i have recently fitted the lower half of the slit with a wind-screen, which has proved to be a most useful addition. it is made of tarpaulin, attached to slats which slide between the two main girders, and is raised or lowered by halliards, which belay to cleats on the north rail of the gallery. a more detailed description of the dome has been given in an article by mr. crossley,[ ] from which the reduced figure in fig. [ ] has been taken. the mounting of the three-foot reflector has been very completely described and illustrated by dr. common,[ ] so that only a very general description need be given here. the most important feature of the mounting is that the telescope tube, instead of being on one side of the polar axis, as in the usual construction, is central, so that the axis of the mirror and the polar axis are in the same line when the telescope is directed to the pole. the declination axis is short, and is supported by a massive goose-neck bolted to the upper end of the polar axis. the mirror is placed just _above_ the declination axis. its weight, and the weight of the whole tube and eye-end, are counterpoised by slabs of lead, placed in two iron boxes, between which the goose-neck of the polar axis passes. the great advantage of this arrangement, and the controlling principle of the design, is that the telescope is perfectly free to pass the meridian at all zenith distances. no reversal of the instrument is needed, or is indeed possible. [illustration: the crossley reflector.] for long-exposure photography, the advantage above referred to is obvious, but it is attended by certain disadvantages. one of these is that a very much larger dome is required than for the usual form of mounting. another is the great amount of dead weight which the axes must carry; for the mirror, instead of helping to counterpoise the upper end of the tube, must itself be counterpoised. when anything is attached to the eye-end (and in astrophysical work one is always attaching things to the eye-end of a telescope), from ten to twenty times as much weight must be placed in the counterpoise boxes below the declination axis. where room is to be found for the weights required to counterpoise the bruce spectrograph, is a problem which i have not yet succeeded in solving. in his five-foot reflector, dr. common has caused the telescope tube to swing between two large ears, which project from the upper end of the boiler-like polar axis, the pivots constituting the declination axis being near, but above, the lower end of the tube. the mirror, therefore, helps to counterpoise the upper end of the tube. this i regard as a distinct improvement. the danger of large masses of metal near the mirror injuring the definition is, in my opinion, imaginary; at least there is no such danger on mount hamilton, where the temperature variations are unusually small. experience with the crossley reflector, as well as with the other instruments of the lick observatory, shows that the definition depends almost entirely on external conditions. my first trials of the reflector, as first mounted at the lick observatory, showed that the center of motion was inconveniently high. among other difficulties arising from this circumstance, the spectroscope projected beyond the top of the dome, so that it had to be removed before the shutter could be closed. in july, , the pier was therefore cut down two feet. this brought the eye-end down nearly to the level of the gallery rail, where it was at a convenient height for the observer when sitting on a camp-stool, and it made all parts of the mounting more accessible. toward the north and south, the range of the telescope, being limited in these directions by the construction of the mounting, was not affected by the change, but the telescope can not now be used at such low altitudes as formerly, near the east and west points of the horizon. the only occasion likely to call for the use of the reflector in these positions is the appearance of a large comet near the sun, and, after some consideration, i decided to sacrifice these chances for the sake of increasing the general usefulness of the instrument. except in rare cases, all observations are made within three hours of the meridian. to adapt the mounting to the latitude of mount hamilton, a wedge-shaped casting, shown in the illustration, had been provided, but through some error, arising probably from the fact that the telescope had been used in two different latitudes in england, the angle of the casting was too great. when the pier was cut down its upper surface was therefore sloped toward the south, in order to compensate the error in the casting. plate vii shows the instrument very nearly as it is at the present time. the polar axis of the crossley reflector is a long, hollow cylinder, separated by a space of about one-eighth of an inch from its concentric casing. the idea was to fill this space with mercury, and float the greater part of the thrust of the axis, the function of a small steel pin at the lower end being merely to steady the axis. but this mercury flotation, as applied to the crossley telescope, is a delusion, as i think mr. crossley had already found. the mercury, it is true, relieves the thrust to some extent, but it greatly increases the already enormous side pressure on the steel pin at the bottom, thus creating a much greater evil than the one it is intended to remedy. the workmen who set up the mounting inform me that the small bearing at the lower end of the polar axis is badly worn, as i should expect it to be. instead of putting mercury into the space intended for it, i have therefore poured in a pint or so of oil, to keep the lower bearing lubricated. for the reasons indicated above, the force required to move the telescope in right ascension is perhaps five times greater than it should be. the lower end of the polar axis ought to be fitted with ball bearings to take the thrust, and with a pair of friction wheels on top; but it would be difficult to make these changes now. it should be observed that the disadvantages of the mercury flotation are considerably greater at mount hamilton than at the latitude for which the telescope was designed. [illustration: the crossley reflector.] as already stated above, the range of the telescope is limited on the south by the construction of the mounting. the greatest southern declination which can be observed is °. in england this would doubtless mark the limit set by atmospheric conditions, but at mount hamilton it would be easy to photograph objects ° farther south, if the telescope could be pointed to them. the original driving-clock having proved to be inefficient, at least without an electric control, a new and powerful driving-clock was made by the observatory instrument maker, from designs by professor hussey. in its general plan it is like that of the -inch refractor. the winding apparatus, contained in the large casting of the original mounting, has no maintaining power, and can not easily be fitted with one. the clock could in no case be wound during a photographic exposure, on account of the tremors attending the operation, but it would be somewhat more convenient to have the stars remain on the plate during the winding. with a little practice, however, one can wind the clock without actually stopping it, though the object must afterwards be brought back to its place by means of the slow motion in right ascension. two finders have recently been fitted to the crossley reflector. one has an object-glass of four inches aperture and eight feet six inches focal length, with a field of about ° ´, which is very nearly the photographic field of the main telescope. its standards are bolted to one of the corner tubes of the reflector. the other finder has a three-inch objective and a large field. it had not been mounted when the photograph for the plate was made. when a telescope is used for photographing objects near the pole, with long exposures, the polar axis must be quite accurately adjusted, for otherwise the centers of motion of the stars and of the telescope will not agree, and the star images will be distorted. it is true that with a double-slide plate-holder, like the one used with the crossley reflector, one star--namely, the guiding star--is forced to remain in a fixed position with respect to the plate; but the differential motion of the other stars causes them to describe short arcs, or trails, around this star as a center. a considerable part of the spring of was spent in efforts to perfect the adjustment of the polar axis, an operation which, on account of the peculiar form of the mounting, offers unusual difficulties. in the first plan which was tried, the reflector was used as a transit instrument. the inclination of the declination axis was determined with a hanging level which had been provided by mr. crossley, the hour circle and polar axis being very firmly clamped. the clock correction being known from the records kept at the observatory, the collimation and azimuth constants were found by the usual formulæ. this method failed to give satisfactory results, and it was found later that the declination and polar axis were not exactly at right angles. there is only one part of the sky on which the telescope can be reversed; namely, the pole. a method which promised well, and on which some time was spent, consists in photographing the pole (the declination axis being horizontal) by allowing the stars near it to trail for ten or fifteen minutes, then turning the polar axis ° and photographing the pole again on the same plate. half the distance between the images gives the error of the polar axis, which, if the plate is properly oriented, is easily resolved into horizontal and vertical components; while the distance of each image from the center of the plate is this error increased or diminished by twice the deviation of the telescope axis. in this case the vertical component depends upon the reading of the declination circle, and the horizontal component gives the error of collimation. this method failed, however, to give consistent results, mainly on account of instability of the mirror, and was abandoned. the use of the large mirror for purposes of adjustment was finally given up, and the axis was adjusted by observations of _polaris_ with the long finder, in the usual manner. in order to reach the star at lower culmination the finder tube had to be thrown out of parallelism with the main telescope. the base-plate having no definite center of rotation in azimuth, and the wedges and crowbars used for moving it being uncertain in their action, a watch telescope, provided with a micrometer eyepiece, was firmly secured to the mounting throughout these operations, in such manner that a mark on the southern horizon could be observed through one of the windows of the dome. the errors of the polar axis were finally reduced to within the limits of error of observation. the movable hour circle and driving wheel of the crossley reflector has two sets of graduations. the driving screw having been thrown out of gear, the circle is turned until the outer vernier indicates the sidereal time, whereupon the driving screw is thrown into gear again. the inner vernier is then set to the right ascension of the object which it is desired to observe. as an inconsistency, of minor importance, in the design of the mounting, i may note that the slow motion in right ascension changes the reading of the outer vernier instead of that of the inner one. in practice, however, no inconvenience is caused by this construction. in the early experiments and photographic work with the crossley telescope, irregularities in driving were a source of great annoyance. dr. roberts, in laying down the conditions which should be fulfilled by a good photographic telescope, says that a star should remain bisected by a thread in the eyepiece for two minutes at a time. the crossley telescope was so far from fulfilling this condition that a star would not keep its place for two consecutive seconds; and the greatest alertness on the part of the observer did not suffice to ensure round star images on a photographic plate. it was obvious that the fault did not lie with the driving clock; in fact, many of the sudden jumps in right ascension, if explained in this way, would have required the clock to run backward; nevertheless the clock was tested by causing its revolutions to be recorded on a chronograph at the main observatory, together with the beats of one of the standard clocks. for this purpose a break-circuit attachment was made by mr. palmer. the errors of the clock were in this way found to be quite small. the principal source of the irregularities was found in the concealed upper differential wheel of the grubb slow motion. this wheel turned with uncertain friction, sometimes rotating on its axis, and sometimes remaining at rest. after it was checked the driving was much better, and was still farther improved by repairing some defective parts of the train. small irregularities still remain. they seem to be partly due to inaccuracies in the cutting of the gears, or of the teeth of the large driving wheel, and partly to the springing of the various parts, due to the very considerable friction of the polar axis in its bearings. the remaining irregularities are so small, however, that they are easily corrected by the screws of the sliding plate-holder, and with reasonable attention on the part of the observer, round star images are obtained with exposures of four hours' duration. the large mirror, the most important part of the telescope, has an aperture of three feet, and a focal length of feet . inches. it was made by mr. calver. its figure is excellent. on cutting off the cone of rays from a star, by a knife-edge at the focus, according to the method of foucault, the illumination of the mirror is very uniform, while the star disks as seen in an ordinary eyepiece are small and almost perfectly round. they are not, i think, quite so good as the images seen with a large refractor; still, they are very good indeed, as the following observations of double stars, made recently for this purpose, will show. several close double stars were examined on the night of april , , with a power of . the seeing was four on a scale of five. the magnitudes and distances of the components, as given in the table, are from recent observations by professor hussey with the -inch refractor. star. mag. _d._ result of obs. [greek: omega sigma] ([greek: phi] _urs. maj._) . , . ´´. not resolved; too bright. [greek: omega sigma] , ab . , . . easily resolved. [greek: omega sigma] . , . . resolved. [greek: omega sigma] . , . . just resolved at best moments. although the theoretical limit of resolution for a three-foot aperture is not reached in these observations, i do not think the mirror can do any better. the small mirror, or flat, at the upper end of the tube, is circular, the diameter being nine inches. its projection on the plane of the photographic plate is therefore elliptical; but the projection of the mirror and its cell on the plane of the great mirror is very nearly circular. the small mirror, acting as a central stop, has the effect of diminishing the size of the central disk of the diffraction pattern, at the expense of an increase in the brightness of the system of rings. to this effect may be due, in part, the inferiority of the reflector for resolving bright doubles, as compared with a refractor of the same aperture. for photographic purposes, it is evident that the mirror is practically perfect. the upper end of the tube can be rotated, carrying with it the flat and the eye-end. whenever the position is changed, the mirrors have to be re-collimated. in practice it is seldom necessary to touch the adjusting screws of the mirrors themselves. the adjustment is effected by means of clamping and butting screws on the eye-end, and a change of the line of collimation, with respect to the finders and the circles, is avoided. the operation is generally referred to, however, as an adjustment of the mirrors. for adjusting the mirrors there are two collimators. one of these is of the form devised by mr. crossley.[ ] it is very convenient in use, and is sufficiently accurate for the adjustment of the eye-end when the telescope is used for photographic purposes, inasmuch as the exact place where the axis of the large mirror cuts the photographic plate is not then a matter of great importance, so long as it is near the center. moreover, as stated farther below, the direction of the axis changes during a long exposure. the other collimator is of a form originally due, i think, to dr. johnstone stoney. it consists of a small telescope, which fits the draw-tube at the eye-end. in the focus of the eyepiece are, instead of cross-wires, two adjustable terminals, between which an electric spark can be passed, generated by a small induction machine, like a replenisher, held in the observer's hand. the terminals are at such a distance inside the principal focus of the objective, that the light from the spark, after reflection from the flat, appears to proceed from the center of curvature of the large mirror. the rays are therefore reflected back normally, and form an image of the spark which, when the mirrors are in perfect adjustment, coincides with the spark itself. the precision of this method is very great. it is in fact out of proportion to the degree of refinement attained in other adjustments of the reflector, for a slight pressure of the hand on the draw-tube, or movement of the telescope to a different altitude, instantly destroys the perfection of the adjustment. i have provided these collimators with an adapter which fits the photographic apparatus, so that one can adjust the mirrors without having to remove this apparatus and substitute for it the ordinary eye-end carrying the eyepieces. for visual observation the crossley telescope is provided with seven eyepieces, with powers ranging from downward. the lowest power is only , and consequently utilizes only inches of the mirror, of which are covered by the central flat. it is therefore of little value, except for finding purposes. the next lowest power utilizes inches of the mirror. the other eyepieces call for no remark. but, while the crossley reflector would doubtless be serviceable for various kinds of visual observations, its photographic applications are regarded as having the most importance, and have been chiefly considered in deciding upon the different changes and improvements which have been made. the interior of the dome is lighted at night by a large lamp, which is enclosed in a suitable box or lantern, fitted with panes of red glass, and mounted on a portable stand. in order to diffuse the light in the lower part of the dome, where most of the assistant's work is done, the walls are painted bright red; while to prevent reflected light from reaching the photographic plate, the inner surface of the dome itself, the mounting, and the ladders and gallery are painted dead black. the observer is therefore in comparative darkness, and not the slightest fogging of the plate, from the red light below, is produced during a four-hours' exposure. on the few occasions when orthochromatic plates are used the lamp need not be lighted. experiments have shown that the fogging of the photographic plate, during a long exposure, is entirely due to diffuse light from the sky, and is therefore unavoidable. for this reason the cloth curtains which lace to the corners of the telescope tube, enclosing it and shutting out light from the lower part of the dome, have not been used, since their only effect would be to catch the wind and cause vibrations of the telescope. they would probably have little effect on the definition, and at any rate could not be expected to improve it. for photographing stars and nebulæ the crossley reflector is provided with a double-slide plate-holder, of the form invented by dr. common.[ ] this apparatus, which had suffered considerably in transportation, and from general wear and tear, was thoroughly overhauled by the observatory instrument-maker. the plates were straightened and the slides refitted. a spring was introduced to oppose the right ascension screw and take up the lost motion--the most annoying defect that such a piece of apparatus can have--and various other improvements were made, as the necessity for them became apparent. they are described in detail farther below. the present appearance of the eye-end is shown in the illustration. the plate-holder is there shown, however, on one side of the tube, and its longer side is parallel to the axis of the telescope. this is not a good position for the eye-end, except for short exposures. in practice, the eye-end is always placed on the north or south side of the tube, according as the object photographed is north or south of the zenith. the right ascension slide is then always at right angles to the telescope axis, and the eye-end can not get into an inaccessible position during a long exposure. as the original wooden plate-holders were warped, and could not be depended upon to remain in the same position for several hours at a time, they were replaced by new ones of metal, and clamping screws were added, to hold them firmly in place. the heads of these screws are shown in the plate, between the springs which press the plate-holder against its bed. to illuminate the cross-wires of the guiding eyepiece, a small electric lamp is used, the current for which is brought down from the storage battery at the main observatory. the coarse wires have been replaced by spider's webs,[ ] and reflectors have been introduced, to illuminate the declination thread. a collimating lens, placed at its principal focal distance from the incandescent filament of the lamp, makes the illumination of the wires nearly independent of their position on the slide, and a piece of red glass, close to the lens, effectually removes all danger of fogging the plate. the light is varied to suit the requirements of observation by rotating the reflector which throws the light in the direction of the eyepiece. [illustration: double-slide plate-holder of the crossley reflector.] in long exposures it is important for the observer to know at any moment the position of the plate with reference to its central or zero position. for this purpose scales with indexes are attached to both slides; but as they can not be seen in the dark, and, even if illuminated with red light, could not be read without removing the eye from the guiding eyepiece, i have added two short pins, one of which is attached to the lower side of the right ascension slide, and the other to its guide, so that the points coincide when the scale reads zero. these pins can be felt by the fingers, and with a little practice the observer can tell very closely how far the plate is from its central position. it would not be a very difficult matter to improve on this contrivance, say by placing an illuminated scale, capable of independent adjustment, in the field of the eyepiece, but the pins answer every purpose. the declination slide is changed so little that no means for indicating its position are necessary. in this apparatus, as originally constructed, the cross-wires of the guiding eyepiece were exactly in the plane of the photographic plate. the earlier observations made with the crossley reflector on mount hamilton showed that this is not the best position of the cross-wires. the image of a star in the guiding eyepiece, which, when in the middle of its slide, is nearly three inches from the axis of the mirror, is not round, and its shape varies as the eyepiece is pushed in or drawn out. in the plane of the photographic plate (assumed to be accurately in focus), it is a crescent, with the convex side directed toward the center of the plate. this form of image is not suitable for accurate guiding. outside this position the image changes to an arrow-head, the point of which is directed toward the axis, and this image can be very accurately bisected by the right ascension thread. as the construction of the apparatus did not allow the plane of the cross-wires to be changed, the wooden bed of the plate-holder was cut down, so as to bring the wires and the plate into the proper relative positions. after some further experience with the instrument, still another change was made in this adjustment. it was found that the focus often changed very perceptibly during a long exposure, and while the arrow-head image above described was suitable for guiding purposes, its form was not greatly affected by changes of focus. between the crescent and the arrow-head images there is a transition form, in which two well-defined caustic curves in the aberration pattern intersect at an acute angle. the intersection of these caustics offers an excellent mark for the cross-wires, and is at the same time very sensitive to changes of focus, which cause it to travel up or down in the general pattern. the bed of the plate-holder was therefore raised, by facing it with a brass plate of the proper thickness. why the focus of the telescope should change during a long exposure is not quite clear. the change is much too great to be accounted for by expansion and contraction of the rods forming the tube, following changes of temperature, while a simple geometrical construction shows that a drooping of the upper end of the tube, increasing the distance of the plate from the (unreflected) axis of the mirror, can not displace the focus in a direction normal to the plate, if it is assumed that the field is flat. the observed effect is probably due to the fact that the focal surface is not flat, but curved. during a long exposure, the observer keeps the guiding star, and therefore, very approximately, all other stars, in the same positions relatively to the plate; but he has no control over the position of the axis of the mirror, which, by changes of flexure, wanders irregularly over the field. the position of maximum curvature, therefore, also varies, and with it the focus of the guiding star relatively to the cross-wires, where the focal surface is considerably inclined to the field of view. it is certain that the focus does change considerably, whatever the cause may be, and that the best photographic star images are obtained by keeping the focus of the guiding star unchanged during the exposures. this is done by turning the focusing screw of the eye-end. in making the photographs of nebulæ for which the crossley telescope is at present regularly employed, it was at first our practice to adjust the driving-clock as accurately as possible to a sidereal rate, and then, when the star had drifted too far from its original position, on account of changes of rate or of flexure, to bring it back by the right-ascension slow motion, the observer either closing the slide of the plate-holder or following the motion of the star as best he could with the right-ascension screw. lately a more satisfactory method, suggested by mr. palmer, has been employed. the slow motion in right ascension is of grubb's form,[ ] and the telescope has two slightly different rates, according to whether the loose wheel is stopped or allowed to turn freely. the driving-clock is adjusted so that one of these rates is too fast, the other too slow. at the beginning of an exposure the wheel is, say, unclamped, and the guiding star begins to drift very slowly toward the left, the observer following it with the screw of the plate-holder. when it has drifted far enough, as indicated by the pins mentioned farther above, the wheel is clamped. the star then reverses its motion and begins to drift toward the right; and so on throughout the exposure. the advantages of this method over the one previously employed are, that the star never has to be moved by the slow motion of the telescope, and that its general drift is in a known direction, so that its movements can be anticipated by the observer. in this way photographs are obtained, with four hours' exposure, on which the smallest star disks are almost perfectly round near the center of the plate, and from ´´ to ´´ in diameter. the star images are practically round over a field at least inch or ´ in diameter. farther from the center they become parabolic, but they are quite good over the entire plate, - / by - / inches. from these statements it will be seen that small irregularities in driving no longer present any difficulties. but certain irregular motions of the image still take place occasionally, and so far it has not been possible entirely to prevent their occurrence. it was found that the declination clamp (the long slow-motion handle attached to which is shown in the illustration) was not sufficiently powerful to hold the telescope firmly during a long exposure. a screw clamp was therefore added, which forces the toothed-declination sector strongly against an iron block just behind it, thus restoring, i think, the original arrangement of the declination clamp as designed by dr. common. this clamp holds the tube very firmly. the irregularities to which i have referred consist in sudden and unexpected jumps of the image, which always occur some time after the telescope has passed the meridian. these jumps are sometimes quite large--as much as one-sixteenth of an inch or . they are due to two causes: flexure of the tube, and sliding of the mirror on its bed. when the jump is due to sudden changes of flexure, the image moves very quickly, and vibrates before it comes to rest in its new position, and at the same time there is often heard a slight ringing sound from the tension rods of the tube. there seems to be no remedy for the sudden motions of this class. the tension rods are set up as tightly as possible without endangering the threads at their ends or buckling the large corner tubes. a round telescope tube, made of spirally-wound steel ribbon riveted at the crossings, would probably be better than the square tube now in use. jumps due to shifting of the mirror are characterized by a gentle, gliding motion. they can be remedied, in part, at least, by tightening the copper bands which pass around the circumference of the mirror within its cell. this will be done the next time the mirror is resilvered. all that the observer can do when a jump occurs is to bring back the image as quickly as possible to the intersection of the cross-wires. if all the stars on the plate are faint, no effect will be produced on the photograph; but stars of the eighth magnitude or brighter will leave short trails. the nebula, if there is one on the plate, will, of course, be unaffected. before beginning an exposure the focus is adjusted by means of a high-power positive eyepiece. an old negative, from which the film has been partially scraped, is placed in one of the plate-holders, and the film is brought into the common focus of the eyepiece and the great mirror. the appearance of the guiding star, which varies somewhat with the position of the guiding eyepiece on its slide, is then carefully noted, and is kept constant during the exposure by turning, when necessary, the focusing screw of the eye-end. for preliminary adjustments a ground-glass screen is often convenient. on it all the _dm._ stars, and even considerably fainter ones, as well as the nebulæ of herschel's class i, are easily visible without a lens. plates are backed, not more than a day or two before use, with carbutt's "columbian backing," which is an excellent preparation for this purpose. during the exposure the observer and assistant exchange places every half hour, thereby greatly relieving the tediousness of the work, though two exposures of four hours each, in one night, have proved to be too fatiguing for general practice. at the end of the first two hours it is necessary to close the slide and wind the clock. the brightness of the guiding star is a matter of some importance. if the star is too bright, its glare is annoying; if it is too faint, the effort to see it strains the eye, and changes of focus are not easily recognized. a star of the ninth magnitude is about right. in most cases a suitable star can be found without difficulty. in such an apparatus as that described above, the amount by which the plate may be allowed to depart from its zero position is subject to a limitation which has not, i think, been pointed out, although it is sufficiently obvious when one's attention has been called to it. it depends upon the fact that the plate necessarily moves as a whole, in a straight line which is tangent to a great circle of the sphere, while the stars move on small circles around the pole. the compensation for drift, when the plate is moved, is therefore exact at the equator only. let the guiding star have the declination [greek: delta]_{ }, and let a star on the upper edge of the plate (which, when the telescope is north of the zenith, and the eye-end is on the north side of the telescope, will be the southern edge) have the declination [greek: delta]_{ }. then if the guiding star is allowed to drift from its zero position through the distance _d_, the other star will drift through the distance _d_ (cos [greek: delta]_{ } / cos [greek: delta]_{ }). if the guiding star is followed by turning the right-ascension screw, the upper edge of the plate, as well as the guiding eyepiece, will be moved through the distance _d_. hence there will be produced an elongation of the upper star, represented by _e_ = _d_ ((cos [greek: delta]_{ } / cos [greek: delta]_{ }) - ) from which _d_ = (_e_ cos [greek: delta]_{ }) / (cos [greek: delta]_{ } - cos [greek: delta_{ }]). now, in the crossley reflector, the upper edge of the plate and the guiding eyepiece are just about - / inches, or °, apart. if _e_ is given, the above formula serves to determine the maximum range of the slide for different positions of the telescope. it has been stated farther above that the smallest star disks, on a good photograph, are sometimes not more than ´´ in diameter, or in a linear measure, about / mm. an elongation of this amount is therefore perceptible. there are many nebulæ in high northern declinations, and there are several particularly fine ones in about + °. if, therefore, we take [greek: delta]_{ } = °, [greek: delta]_{ }, = °, _e_ = . , and substitute these values, we find _d_ = . mm, which is the greatest permissible range of the plate in photographing these nebulæ. before i realized the stringency of this requirement, by making the above simple computation, i spoiled several otherwise fine negatives by allowing the plate to get too far from the center, thus producing elongated star images. there is a corresponding elongation in declination, the amount of which can be determined by an adaptation of the formula for reduction to the meridian, but it is practically insensible. on account of the short focal length of the three-foot mirror, the photographic resolving power of the telescope is much below its optical resolving power. for this reason the photographic images are less sensitive to conditions affecting the seeing than the visual images. on the finest nights the delicate tracery of bright lines or caustic curves in the guiding star is as clear and distinct as in a printed pattern. when the seeing is only fair these delicate details are lost, and only the general form of the image, with its two principal caustics, is seen. a photograph taken on such a night is not, however, perceptibly inferior to one taken when the seeing is perfect. when, however, the image is so blurred that its general form is barely distinguishable, the photographic star disks are likewise blurred and enlarged, and on such nights photographic work is not attempted. the foregoing account of the small changes which have been made in the crossley telescope and its accessories may appear to be unnecessarily detailed, yet these small changes have greatly increased the practical efficiency of the instrument, and, therefore, small as they are, they are important. particularly with an instrument of this character, the difference between poor and good results lies in the observance of just such small details as i have described. at present the crossley reflector is being used for photographing nebulæ, for which purpose it is very effective. some nebulæ and clusters, like the great nebula in _andromeda_ and the _pleiades_, are too large for its plate ( - / × - / in.), but the great majority of nebulæ are very much smaller, having a length of only a few minutes of arc, and a large-scale photograph is required to show them satisfactorily. it is particularly important to have the images of the involved stars as small as they can be made. many nebulæ of herschel's i and ii classes are so bright that fairly good photographs can be obtained with exposures of from one to two hours; but the results obtained with full-light action are so superior to these, that longer exposures of three and one half or four hours are always preferred. in some exceptional cases, exposures of only a few minutes are sufficient. the amount of detail shown, even in the case of very small nebulæ, is surprising. it is an interesting fact that these photographs confirm (in some cases for the first time) many of the visual observations made with the six-foot reflector of the earl of rosse. incidentally, in making these photographs, great numbers of new nebulæ have been discovered. the largest number that i have found on any one plate is thirty-one. eight or ten is not an uncommon number, and few photographs have been obtained which do not reveal the existence of three or four. a catalogue of these new objects will be published in due time. some of the results obtained with the crossley reflector, relating chiefly to particular objects of some special interest, have already been published.[ ] the photographs have also permitted some wider conclusions to be drawn, which are constantly receiving further confirmation as the work progresses. they may be briefly summarized as follows: . many thousands of unrecorded nebulæ exist in the sky. a conservative estimate places the number within reach of the crossley reflector at about , . the number of nebulæ in our catalogues is but a small fraction of this. . these nebulæ exhibit all gradations of apparent size, from the great nebula in _andromeda_ down to an object which is hardly distinguishable from a faint star disk. . most of these nebulæ have a spiral structure. to these conclusions i may add another, of more restricted significance, though the evidence in favor of it is not yet complete. among the objects which have been photographed with the crossley telescope are most of the "double" nebulæ figured in sir john herschel's catalogue (_phil. trans._, , plate xv). the actual nebulæ, as photographed, have almost no resemblance to the figures. they are, in fact, spirals, sometimes of very beautiful and complex structure; and, in any one of the nebulæ, the secondary nucleus of herschel's figure is either a part of the spiral approaching the main nucleus in brightness, or it can not be identified with any real part of the object. the significance of this somewhat destructive conclusion lies in the fact that these figures of herschel have sometimes been regarded as furnishing analogies for the figures which poincaré had deduced, from theoretical considerations, as being among the possible forms assumed by a rotating fluid mass; in other words, they have been regarded as illustrating an early stage in the development of double star systems. the actual conditions of motion in these particular nebulæ, as indicated by the photographs, are obviously very much more complicated than those considered in the theoretical discussion. while i must leave to others an estimate of the importance of these conclusions, it seems to me that they have a very direct bearing on many, if not all, questions concerning the cosmogony. if, for example, the spiral is the form normally assumed by a contracting nebulous mass, the idea at once suggests itself that the solar system has been evolved from a spiral nebula, while the photographs show that the spiral nebula is not, as a rule, characterized by the simplicity attributed to the contracting mass in the nebular hypothesis. this is a question which has already been taken up by professor chamberlin and mr. moulton of the university of chicago. the crossley reflector promises to be useful in a number of fields which are fairly well defined. it is clearly unsuitable for photographing the moon and planets, and for star charting. on the other hand, it has proved to be of value for finding and photographically observing asteroids whose positions are already approximately known. one of the most fruitful fields for this instrument is undoubtedly stellar spectroscopy. little has been done in this field, as yet, with the crossley reflector, but two spectrographs, with which systematic investigations will be made, have nearly been completed by the observatory instrument-maker. one of these, constructed with the aid of a fund given by the late miss c. w. bruce, has a train of three ° prisms and one ° prism, and an aperture of two inches; the other, which has a single quartz prism, will, i have reason to expect, give measurable, though small, spectra of stars nearly at the limit of vision of the telescope. the photogravure[ ] of the trifid nebula, which accompanies this article, was made from a photograph taken with the crossley reflector on july , , with an exposure of three hours. it was not selected as a specimen of the work of the instrument, for the negative was made in the early stages of the experiments that i have described, and the star images are not good, but rather on account of the interest of the subject. at the time the photogravures were ordered no large scale photograph of the trifid nebula had, so far as i am aware, ever been published.[ ] the remarkable branching structure of the nebula is fairly well shown in the photogravure, though less distinctly than in the transparency from which it was made. the enlargement, as compared with the original negative, is . diameters ( mm = ´´). the fainter parts of the nebula would be shown more satisfactorily by a longer exposure. list of nebulÆ and clusters photographed. +----------------------------------------------------------------------+ |n.g.c.| [greek: a]|[greek: d]| remarks. | | no. | . | . | | |----------------------------------------------------------------------| | | h m s | ° ´ | | | | | + . |h ii, | | | | + . |h v, | | | | + . |m | | | | + . |great nebula in _andromeda_ | | | | - . |h v, | | | | - . |h v, i | | | | + . |h i, | | | | + . |m | | | | + |m | | | | + . |m | | | | + . |h v, | | | | + . |h i, | | | | - . |m | | | | - . |h i, | | ... | | + |_pleiades_ in _taurus_ | | | | + |t _tauri_ and hind's variable nebula | | | | + . |h i, | | | | + |crab nebula in _taurus_ | | ... | | - |great nebula in _orion_ | | | | - . |h v, | | | | - . |h v, | | | | + . |m | | | | + . |cluster and nebula in _monoceros_ | | | | + |nebula near _monocerotis_ | | | | - . |m | | ... | | - . |new nebula in _monoceros_ | | | | - . |h v, | | | | + . |h iii, | | - | | + . | h ii, - | | | | + . |h v, | | | | - . |cluster and nebula m | | | | + |_præsepe_ cluster | | | | + . |h i, | | | | + |h i, | | - | | + |h i, - | | | | + . |h v, | | | | + |m | | | | + . |h v, | | | | - . |h i, | | | | + . |h i, | | | | + . |h i, | | | | + . |h i, | | - | | + . |h ii, - | | | | - |h iv, | | ... | | + |new nebula in _ursa major_ (coddington).| | | | + . |h v, | | | | + . |owl nebula, m | | | | + . |m | | | | + |m | | | | + . |h ii, | | | | + . |h v, | | | | + |m | | | | + . |h v, | | | | + . |m | | | | + . |m | | | | + . |m | | - | | + . |h i, - | | | | + . |m | | | | + . |h v, | | | | + . |h i, | | | | + . |h v, | | | | + . |h v, | | - | | + . |h i, - | | | | + |h i, | | | | + . |m | | | | + . |m | | | | + . |m | | - | | + . |m | | | | - . |h ii, | | | | + |m | | - | | + |m | | - | | + . |h ii, - | | | | + . |h i, | | | | + |m | | | | + . |m | | | | - . |m | | | | + . |h vi, | | | | - |trifid nebula in _sagittarius_ | | | | - |m | | | | + |h iv, | | | | - |m omega nebula | | | | - . |m | | | | - . |m | | | | + . |m | | | | + |dumb-bell nebula | | | | + . |h iv, | | | | + . |h iv, | | | | + . | | | | | + . | | | | | + . |h i, | | | | - |h iv, | | | | + . |h iv, | | | | + . |m | | | | - . |m | | | | - . |m | | | | + . |h ii, | | | | + . |h i, | | | | + . |h ii, | | | | + . |h i, | | - | | + . |h ii, - | | | | + . |h iv, | | | | + . |h iii, | | | | + . |h ii, | | | | + . |h ii, | +----------------------------------------------------------------------+ catalogue of new nebulÆ discovered on the negatives. +----------------------------------------------------------------------+ |no.|[greek: a]|precession|[greek: d]|precession| description. | | | . | | . | | | |----------------------------------------------------------------------| | |h m s | s | ° ´ ´´ | ´´ | | | | . | + . |+ | + . |vs eef | | | . | . |+ | . |ef n | | | . | . |+ | . |f vbm e ° | | | . | . |+ | . |ef bm | | | . | . |+ | . |b ve ° | | | . | . |+ | . |ef vs | | | . | . |+ | . |ef vs | | | . | . |- | . | vs r | | | . | . |- | . | vs bm sep. parts | | | . | . |- | . | vs r bm | | | . | . |- | . | vs r | | | . | . |- | . | vs bm e ° | | | . | . |- | . | vs r bsw | | | . | . |- | . | vs bm e ° | | | . | . |- | . | vs n e ° | | | . | . |- | . | s e stell n | | | . | . |- | . | vs spiral bm | | | . | . |- | . | vs ring? | | | . | . |- | . | s spiral n bm | | | . | . |- | . | vs r | | | . | . |- | . | vs r bm | | | . | . |- | . | vs dif | | | . | . |- | . | vs r n | | | . | . |- | . | vs r gbm | | | . | . |- | . | vs r | | | . | . |- | . | vs r bm | | | . | . |+ | . |f s n | | | . | . |+ | . |f vbm spiral? | | | . | . |+ | . |f vbm spiral? | | | . | . |+ | . |f bm e | | | . | . |+ | . |pf e ° bp | | | . | . |+ | . |f r | | | . | . |+ | . |vf l r | | | . | . |+ | . |pf s vf extension °| | | . | . |+ | . |s pb pmb m | | | . | . |+ | . |vvf vs | | | . | . |+ | . |f s e ° | | | . | . |+ | . |pf s r | | | . | . |+ | . |vf s r | | | . | . |+ | . |f l r gbm | | | . | . |+ | . |f l gbm r | | | . | . |+ | . |s pb e ° | | | . | . |+ | . |vf s e ° | | | . | . |+ | . |vf pl | | | . | . |+ | . |vf pl gbm | | | . | . |+ | . |p b r gbm | | | . | . |+ | . |pf e ° | | | . | . |+ | . |pb n r | | | . | . |+ | . |b n | | | . | . |+ | . |f | | | . | . |+ | . |ef vs bm e ° | | | . | . |+ | . |f gbm e ° spiral? | | | . | . |+ | . |f pmbm | | | . | . |+ | . |f b_{*}f | | | . | . |+ | . |ef vs r | | | . | . |+ | . |s f r | | | . | . |+ | . |f e ° bsf | | | . | . |+ | . |b s vbm e ° bnp | | | . | . |+ | . |s f r | | | . | . |+ | . |f s pmbm | | | . | . |+ | . |pb vbm e ° spiral? | | | . | . |+ | . |eef e ° | | | . | . |+ | . |pb pmbm | | | . | . |+ | . |b s pbm | | | . | . |+ | . |pb e ° pmbm | | | . | . |+ | . |vb s mbm | | | . | . |+ | . |f trin npn | | | . | + . |+ | + . |pb bs b_{*}p | +----------------------------------------------------------------------+ +----------------------------------------------------------------------+ |no.|[greek: a]|precession|[greek: d]|precession| description. | | | . | | . | | | +----------------------------------------------------------------------+ | |h m s | s | ° ´ ´´ | ´´ | | | | . | + . |+ | + . |ps pb gbm e ° | | | . | . |+ | . |pf s r | | | . | . |+ | . |vf | | | . | . |+ | . |f vs bnp | | | . | . |+ | . |eef s | | | . | . |+ | . |f vl vmbm | | | . | . |+ | . |s pb bs | | | . | . |+ | . |f bsp | | | . | . |+ | . |b s e ° bm | | | . | . |+ | . |f l bm n b_{*}np | | | . | . |+ | . |pb gbm e ° | | | . | . |+ | . |pb e ° gbm | | | . | . |+ | . |vf pl gbm e ° | | | . | . |+ | . |pf l | | | . | . |+ | . |s b vbm | | | . | . |+ | . |pb e ° spiral | | | . | . |+ | . |eef pl e ° | | | . | . |+ | . |vb e ° | | | . | . |+ | . |f e ° bnf | | | . | . |+ | . |b s gbm | | | . | . |+ | . |vs vf bsp | | | . | . |+ | . |s f bs | | | . | . |+ | . |vf vs | | | . | . |+ | . |f vs n | | | . | . |+ | . |f s bn e ° long n | | | . | . |+ | . |pf s i trin | | | . | . |+ | . |pf vs | | | . | . |+ | . |f l e ° spiral on | | | | | | | edge | | | . | . |+ | . |eeef doubtful | | | . | . |+ | . |pb n e ° s pmbm | | | . | . |+ | . |l f pmbm | | | . | . |+ | . |s f e ° | | | . | . |- | . |vs vf gbm | | | . | . |- | . |vs f m e ° | | | . | . |- | . |f s m e ° | | | . | . |- | . |pb vs e ° | | | . | . |- | . |vf vs mbm | | | . | . |- | . |eef s | | | . | . |- | . | s e ° dif bm | | | . | . |- | . | vs r | | | . | . |- | . | vs r stell | | | . | . |- | . | vs nearly r bm | | | . | . |- | . | vs r (spiral?) | | | . | . |- | . | vs r n | | | . | . |- | . | vs e ° bn | | | . | . |- | . | vs dif | | | . | . |- | . | vs spiral b n | | | | | | | (stell) | | | . | . |+ | + . |bright stell n on | | | | | | | north side | | | . | . |+ | - . | vs bm | | | . | . |+ | . | vs n ring | | | . | . |+ | . | r bm | | | . | . |+ | . | vs | | | . | . |+ | . | vs r | | | . | . |+ | . | vs e ° d? | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs if | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r | | | . | . |+ | . | vs bm r | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs f_{*}inv dif | | | . | . |+ | . | vs e ° bm n | | | | | | | spiral | | | . | . |+ | . | vs dif or n | | | . | . |+ | . | vs if n | | | . | . |+ | . | vs bm | | | . | . |+ | . | vs bm | | | . | . |+ | . | vs if sc | | | . | . |+ | . | vs e ° bm n spiral| | | | | | | on edge | | | . | . |+ | . | vs | | | . | + . |+ | - . | vs r bm n spiral? | +----------------------------------------------------------------------+ +----------------------------------------------------------------------+ |no.|[greek: a]|precession|[greek: d]|precession| description. | | | . | | . | | | |----------------------------------------------------------------------| | |h m s | s | ° ´ ´´ | ´´ | | | | . | + . |+ | - . | vs bm | | | . | . |+ | . | vs neb_{*} | | | . | . |+ | . | vs r bm n spiral? | | | . | . |+ | . | vs n r | | | . | . |+ | . | vs r | | | . | . |+ | . | vs bm n r | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r bm n spiral? | | | . | . |+ | . | vs bm n r spiral? | | | . | . |+ | . |pb e ° bn | | | . | . |+ | . |vf vs | | | . | . |+ | . |pb s gpmbm | | | . | . |+ | . | s e ° stell n m | | | | | | | (spiral on edge?) | | | . | . |+ | . | e ° s dif | | | . | . |+ | . | vs e ° stell n | | | | | | | spiral? | | | . | . |+ | . | s spiral n | | | . | . |+ | . | s r bm n | | | . | . |+ | . |ef e ° | | | . | . |+ | . |vf vs | | | . | . |+ | . |f vs n e ° spiral | | | . | . |+ | . |pb es n r | | | . | . |+ | . |ef es bf | | | . | . |+ | . |eef | | | . | . |+ | . |l m e ° | | | . | . |+ | . | e ° bs s | | | . | . |+ | . | vs ring bs | | | . | . |+ | . | e ° gbm | | | . | . |+ | . | vs e ° stell n | | | . | . |+ | . | e ° vbn spiral? | | | . | . |+ | . | vs n bm | | | . | . |+ | . | vs scnuclei | | | . | . |+ | . | vs r | | | . | . |+ | . | vs bn ring or | | | | | | | spiral | | | . | . |+ | . | s r | | | . | . |+ | . | b bm e ° | | | . | . |+ | . | r s | | | . | . |+ | . | l vf bm | | | . | . |+ | . | r s bs | | | . | . |+ | . |vf vs | | | . | . |+ | . |pb bs s | | | . | . |+ | . |ef e ° | | | . | . |+ | . |pb s r gpmbm n | | | . | . |+ | . |eef vs | | | . | . |+ | . | vs bm e ° | | | . | . |+ | . | vs sbm spiral | | | . | . |+ | . | vs n spiral? | | | . | . |+ | . | vs bm | | | . | . |+ | . | vs sbm n spiral | | | . | . |+ | . | vs bnw r | | | . | . |+ | . | vs bm n spiral | | | . | . |+ | . | vs r n spiral? | | | . | . |+ | . | vs e ° | | | . | . |+ | . |pb vs r gpmbm | | | . | . |+ | . |pf s bf e ° | | | . | . |+ | . |vf dif | | | . | . |+ | . |pf s e ° | | | . | . |+ | . |eef s e ° | | | . | . |+ | . |pb s e ° pmbm spiral | | | . | . |+ | . |ef e ° | | | . | . |+ | . | vs neb_{*} | | | . | . |+ | . | vs r | | | . | . |+ | . | vs e ° bm | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r n | | | . | . |- | . | vs sbn spiral | | | . | . |- | . | vs stell sbn | | | . | . |- | . | s d if gbn bn | | | . | . |- | . | vs stell | | | . | + . |+ | - . | vs d neb_{*} | +----------------------------------------------------------------------+ +----------------------------------------------------------------------+ |no.|[greek: a]|precession|[greek: d]|precession| description. | | | . | | . | | | |----------------------------------------------------------------------| | |h m s | s | ° ´ ´´ | ´´ | | | | . | + . |+ | - . | vs if bm | | | . | . |+ | . | vs bm n spiral e °| | | . | . |+ | . | vs sbm n spiral | | | | | | | e ° | | | . | . |+ | . | s if bm | | | . | . |+ | . | vs r | | | . | . |+ | . | vs sbm n spiral? | | | | | | | e ° | | | . | . |+ | . | e ° ´´ long | | | | | | | small spur follows | | | | | | | e ° | | | . | . |+ | . | vs bm n r | | | . | . |+ | . | vs r | | | . | . |+ | . | vvs stell | | | . | . |+ | . | s e ° | | | . | . |+ | . |vs r stell | | | . | . |+ | . | vvs sbn spiral? | | | . | . |+ | . | vs bm n spiral | | | . | . |+ | . | vs bm spiral n | | | . | . |+ | . | vvs r spiral? n | | | . | . |+ | . | vvs sbn if | | | . | . |+ | . | vvs if | | | . | . |+ | . | vs if | | | . | . |+ | . | vvs bn if | | | . | . |+ | . | vvs spiral sbn | | | . | . |+ | . | vs sbn spiral | | | . | . |+ | . |f s r gbm bf | | | . | . |+ | . | vvs if stell | | | . | . |+ | . | s neb_{*} | | | . | . |+ | . | vs sbn spiral | | | . | . |+ | . | vs sbn spiral | | | . | . |+ | . | vs stell | | | . | . |+ | . | vs e ° spiral? | | | . | . |+ | . |eeef?? | | | . | . |+ | . | vs sbm n | | | . | . |+ | . | vvs bn stell | | | . | . |+ | . |f vs r gbm | | | . | . |+ | . |f s e ° | | | . | . |+ | . | vvs r stell | | | . | . |+ | . | vvs if | | | . | . |+ | . | vs ib n spiral e °| | | . | . |+ | . |b s e ° spiral on | | | | | | | edge | | | . | . |+ | . |b r vm bm | | | . | . |+ | . |ef s r bm | | | . | . |+ | . | vs stell | | | . | . |+ | . | vs spiral stell n | | | . | . |+ | . | vs r gbn | | | . | . |+ | . | vvs gbn spiral n | | | . | . |+ | . |f s e ° spiral? | | | . | . |+ | . | vs sbn | | | . | . |+ | . | vs r gbn | | | . | . |+ | . | vs if gbn | | | . | . |+ | . | vvs if | | | . | . |+ | . |vf vvs r | | | . | . |+ | . | vvs bn spiral | | | . | . |+ | . | vs sbn spiral | | | . | . |- | . | vvs if e ° | | | . | . |+ | . |b s e ° | | | . | . |+ | . |vf vs e ° | | | . | . |+ | . | vs spiral n e ° | | | . | . |+ | . |vvf e ° spindle | | | | | | | shaped | | | . | . |+ | . |vf s r | | | . | . |+ | . |f r s gbm | | | . | . |+ | . |f s r gbm | | | . | . |+ | . |s pb bf | | | . | . |+ | . |f pmbm e ° | | | . | . |+ | . | vs sbm n spiral | | | | | | | e ° | | | . | . |+ | . | vs gbm spiral | | | . | . |+ | . | vs gbn | | | . | . |+ | . | vs if gbm | | | . | . |+ | . | s sbm n spiral | | | | | | | e ° | | | . | . |+ | . | vs sbm n spiral | | | . | . |+ | . | s sbm n spiral | | | . | + . |+ | - . | vs if gbm | +----------------------------------------------------------------------+ +----------------------------------------------------------------------+ |no.|[greek: a]|precession|[greek: d]|precession| description. | | | . | | . | | | |----------------------------------------------------------------------| | |h m s | s | ° ´ ´´ | ´´ | | | | . | + . |+ | - . | vs stell | | | . | . |+ | . |s pf r | | | . | . |+ | . |vs f e ° | | | . | . |+ | . | vs bn if | | | . | . |+ | . | vs neb_{*} | | | . | . |+ | . | s gbm e ° | | | . | . |+ | . | vs stell | | | . | . |+ | . | vs sbm stell n | | | . | . |+ | . | vs n | | | . | . |+ | . | vs sbn r spiral? | | | . | . |+ | . | vs neb_{*} | | | . | . |+ | . | vs stell | | | . | . |+ | . | s r sbm n spiral | | | . | . |+ | . | vs r neb_{*} | | | . | . |+ | . | vs stell | | | . | . |+ | . |two mag. objects, | | | | | | | if, close together | | | . | . |+ | . | vs. uniform | | | | | | | brightness | | | . | . |+ | . | vs if stell | | | . | . |+ | . | vs r gbm n spiral | | | . | . |+ | . | vs r gbm | | | . | . |+ | . | vs sbm n ring | | | . | . |+ | . | vs sbm n spiral? | | | . | . |+ | . |vvf e ° | | | . | . |+ | . |s vf r | | | . | . |+ | . | vs r sbm n spiral | | | . | . |+ | . | vs gbm if | | | . | . |+ | . | s vm e ° | | | . | . |+ | . |vf e ° spindle | | | | | | | shaped | | | . | . |+ | . | vs dif | | | . | . |+ | . |vs if dif | | | . | . |+ | . | vs gbm if | | | . | . |+ | . | vs sbm n spiral | | | . | . |+ | . | vs bm e ° | | | . | . |+ | . | vs dif if | | | . | . |+ | . | vs dif if | | | . | . |+ | . | vs sbm n spiral | | | . | . |+ | . | vs r sbm n spiral | | | . | . |+ | . | s sbm n spiral e °| | | .±| ... |+ ±| ... | vs stell if neb? | | | . | . |+ | . | vs r sbm n spiral | | | . | . |+ | . | vs neb_{*} | | | . | . |+ | . | vs sbm n spiral | | | . | . |+ | . | vs gbm e ° | | | . | . |+ | . | vs neb_{*} | | | . | . |+ | . |pb s r | | | . | . |+ | . | vs sbm | | | . | . |+ | . |s f gbm e ° | | | . | . |+ | . | s gbn be spiral | | | | | | | e ° | | | . | . |+ | . | vs stell | | | . | . |+ | . | vs stell n | | | . | . |+ | . |pb s e ° | | | . | . |+ | . |b irr b_{*}n | | | . | . |+ | . |vs b e ° bm | | | . | . |+ | . |s pf r another | | | | | | | apparently distinct | | | | | | | neb np | | | . | . |+ | . |l b pmbm r | | | . | . |+ | . |vs b e ° spindle | | | | | | | shaped | | | . | . |+ | . |s b e ° gbm | | | . | . |+ | . |s pf e ° companion n| | | . | . |+ | . |vs f e ° bf | | | . | . |+ | . |s b r vmbm | | | . | . |+ | . |s b e ° bsf | | | . | . |+ | . |b spiral | | | . | . |+ | . |vvf s r | | | . | . |+ | . |vb s e e ° | | | . | . |+ | . |b s r neb_{*} | | | . | . |+ | . |s f gbm | | | . | . |+ | . |s pb n | | | . | . |+ | . |vs f | | | . | . |+ | . |vs f | | | . | + . |+ | - . |vs f gbm | +----------------------------------------------------------------------+ +----------------------------------------------------------------------+ |no.|[greek: a]|precession|[greek: d]|precession| description. | | | . | | . | | | |----------------------------------------------------------------------| | |h m s | s | ° ´ ´´ | ´´ | | | | . | + . |+ | - . |vs b vmbm spiral | | | . | . |+ | . |vs b e ° | | | . | . |+ | . |vs f | | | . | . |+ | . |vs b | | | . | . |+ | . |vs vb n e ° | | | . | . |+ | . |ps pf | | | . | . |+ | . |s pb bf | | | . | . |+ | . |s pb e ° | | | . | . |+ | . |s pb bf | | | . | . |+ | . | vs stell | | | . | . |+ | . | vs e ° sbm n | | | | | | | spiral | | | . | . |+ | . | s e ° | | | . | . |+ | . | vs r sbm sn spiral | | | . | . |+ | . |s vf | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r | | | . | . |+ | . | vs vf dif | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs r | | | . | . |+ | . | vs dif vgbm | | | . | . |+ | . | vs vf r | | | . | . |+ | . | vs dif | | | . | . |+ | . |s f r | | | . | . |+ | . | vs if | | | . | . |+ | . | vs r bs | | | . | . |+ | . | vs if dif | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r n | | | . | . |+ | . | vs vf dif | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r n | | | . | . |+ | . |vs vf | | | . | . |+ | . |s f | | | . | . |+ | . |pl vf r | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r n | | | . | . |+ | . | vs r | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs e ° n | | | . | . |+ | . | vs r n | | | . | . |+ | . | vs vf | | | . | . |+ | . | vs r bn | | | . | . |+ | . | vs e ° stell n | | | . | . |+ | . | vs sbm spiral | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs r n | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r n | | | . | . |+ | . | vs vf | | | . | . |+ | . | vs dif | | | . | . |+ | . | vs r two n | | | . | . |+ | . |pf ve ° | | | . | . |+ | . | vs e ° bm | | | . | . |+ | . | vs r | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs r | | | . | . |+ | . |f ps | | | . | . |+ | . |vf s bn | | | . | . |+ | . | s e ° sbm n spiral | | | . | . |+ | . | s sbm stell n r | | | | | | | spiral? | | | . | . |+ | . | vs if | | | . | . |+ | . | vs gbm if | | | . | . |+ | . | s dif if e ° | | | .±| . |+ ±| . | vs sbm spiral n | | | . | . |+ | . | vs bs r | | | . | . |+ | . |f vs l e ° | | | . | + . |+ | . | vs dif gbm r | +----------------------------------------------------------------------+ +----------------------------------------------------------------------+ |no.|[greek: a]|precession|[greek: d]|precession| description. | | | . | | . | | | |----------------------------------------------------------------------| | |h m s | s | ° ´ ´´ | ´´ | | | | . | + . |+ | - . | s sbm n spiral | | | . | . |+ | . |! pb l spiral | | | . | . |+ | . |vf vs | | | . | . |+ | . |vf vs e ° | | | . | . |+ | . | s gbm spiral e ° | | | . | . |+ | . |vvf vs | | | . | . |+ | . | s sbm n spiral? | | | . | . |+ | . | vs stell n spiral | | | . | . |+ | . |eef s | | | . | . |+ | . | vs r diffic | | | . | . |+ | . | vs vf e ´´ | | | . | . |+ | . | vs r gbn spiral | | | . | . |+ | . | vs e ° stell n | | | . | . |+ | . | vs if | | | . | . |+ | . | s gbm n e ° spiral| | | | | | | on edge | | | . | . |+ | . | vs r sbm n spiral | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs if dif | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs vf r | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs r bm | | | . | . |+ | . | s e ° | | | . | . |+ | . | vs dif | | | . | . |+ | . | vs bm if | | | . | . |+ | . | vs gbm | | | . | . |+ | . | s e ° bm | | | . | . |+ | . | vs r | | | . | . |+ | . | vs if bm | | | . | . |+ | . | vs if | | | . | . |+ | . | vs r sbm n | | | . | . |+ | . | neb_{*} | | | . | . |+ | . | vs if gbm n | | | . | . |+ | . | vs gbm n spiral? | | | . | . |+ | . | s bm e ° | | | . | . |+ | . | l m e ° bm n | | | | | | | spiral on edge | | | . | . |+ | . | vs e ° gbm spiral | | | | | | | on edge? | | | . | . |+ | . | l vm e ° small | | | | | | | spur from m | | | . | . |+ | . | vs | | | . | . |+ | . | vs gbm if | | | . | . |+ | . | l bm if sc | | | . | . |+ | . | neb_{*} | | | . | . |+ | . | vs vgbm if | | | . | . |+ | . | vs vgbm | | | . | . |+ | . | vs r (ring?) | | | . | . |+ | . | vs r | | | . | . |+ | . | vs [circle] | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs e ° n | | | . | . |+ | . | vs sbm n spiral | | | | | | | e ° | | | . | . |+ | . | vs gbm | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs vf r | | | . | . |+ | . | vs sbm n ring? | | | . | . |+ | . | vs dif | | | . | . |+ | . | vs e ° n | | | . | . |+ | . | vs r | | | . | . |+ | . | vs sbm n spiral | | | . | . |+ | . | vs stell | | | . | . |+ | . | vs n? spiral? | | | . | . |+ | . | vs dif if | | | . | . |+ | . | vs sbm n spiral | | | . | . |+ | . | vs r stell n | | | . | . |+ | . | vs bm n spiral | | | . | + . |+ | - . | vs r | +----------------------------------------------------------------------+ +----------------------------------------------------------------------+ |no.|[greek: a]|precession|[greek: d]|precession| description. | | | . | | . | | | |----------------------------------------------------------------------| | |h m s | s | ° ´ ´´ | ´´ | | | | . | + . |+ | - . | vs e ° n | | | . | . |+ | . | vs if | | | . | . |+ | . | vs r n | | | . | . |+ | . | vs r n | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r | | | . | . |+ | . | vs bs if | | | . | . |+ | . | vs stell | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r | | | . | . |+ | . | l vm e ° sbm | | | | | | | spiral | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r | | | . | . |+ | . | s e ° n | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs n e ° | | | . | . |+ | . | vs r | | | . | . |+ | . | vs sbm spiral e °| | | . | . |+ | . | vs r | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r n | | | . | . |+ | . | vs r n | | | . | . |+ | . | vs r n | | | . | . |+ | . | vs vf r | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r bm | | | . | . |+ | . | neb_{*} | | | . | . |+ | . | vs vf e ° d | | | . | . |+ | . | vs e ° bm | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs e ° d | | | . | . |+ | . | vs e ° n spiral | | | | | | | on edge | | | . | . |+ | . | vs bm e ° | | | . | . |+ | . | vs r | | | . | . |+ | . | vs e ° bm | | | . | . |+ | . | neb_{*} | | | . | . |+ | . | neb_{*} | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r n | | | . | . |+ | . | vs e ° spiral? | | | . | . |+ | . | vs vr dif | | | . | . |+ | . | vs vf r diffic | | | . | . |+ | . | vs r diffic | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs e ° bm | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r | | | . | . |+ | . | vs vf r diffic | | | . | . |+ | . | vs r | | | . | . |+ | . | vs vf dif d? | | | . | . |+ | . | vs e ° bm spiral? | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs vf e ° bm | | | | | | | spiral on edge | | | . | . |+ | . | vs r | | | . | . |+ | . | vs e ° bs | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r n | | | . | . |+ | . | vs e ° n | +----------------------------------------------------------------------+ +----------------------------------------------------------------------+ |no.|[greek: a]|precession|[greek: d]|precession| description. | | | . | | . | | | |----------------------------------------------------------------------| | |h m s | s | ° ´ ´´ | ´´ | | | | . | + . |+ | - . | vs e ° n | | | . | . |+ | . | vs vf r | | | . | . |+ | . | vs vf r bm | | | . | . |+ | . | vs e ° bs spiral? | | | . | . |+ | . | vs vf r [circle]? | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs e ° bm | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r | | | . | . |+ | . | vs e ° n | | | . | . |+ | . | vs r n | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs dif | | | . | . |+ | . | vs e ° bm | | | . | . |+ | . | vs r | | | . | . |+ | . | vs vf r | | | . | . |+ | . | vs r n | | | . | . |+ | . | vs vf r | | | . | . |+ | . | vs r n | | | . | . |+ | . |s r vf | | | . | . |+ | . |vs vf e ° | | | . | . |+ | . | vs r | | | . | . |+ | . | s e ° four n | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs e ° bm | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs vf r | | | . | . |+ | . | vs e ° bm | | | . | . |+ | . | vs r | | | . | . |+ | . | vs e ° gbm | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r bm neb_{*}? | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r n | | | . | . |+ | . | vs e ° gbm | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r vf | | | . | . |+ | . | vs e ° bm | | | . | . |+ | . | neb_{*} | | | . | . |+ | . | vs vf r | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r | | | . | . |+ | . | vs e ° gbm | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs vf r | | | . | . |+ | . | vs e ° gbm | | | . | . |+ | . | vs e ° bm | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs r bm | | | . | + . |+ | - . | vs vf r | +----------------------------------------------------------------------+ +----------------------------------------------------------------------+ |no.|[greek: a]|precession|[greek: d]|precession| description. | | | . | | . | | | |----------------------------------------------------------------------| | |h m s | s | ° ´ ´´ | ´´ | | | | . | + . |+ | - . | vs r bm | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs e ° bm | | | . | . |+ | . | vs e ° gbm | | | . | . |+ | . | vs r | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs r n | | | . | . |+ | . | vs r | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs e ° gbm | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs e ° gbm | | | . | . |+ | . | vs r | | | . | . |+ | . s | vs r | | | . | . |+ | . | vs r n | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs r bm neb_{*}? | | | . | . |+ | . | vs r | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs vf dif | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r n | | | . | . |+ | . | vs vf r | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r | | | . | . |+ | . | vs r bm | | | . | . |+ | . |vs eef | | | . | . |+ | . |b pl e ° | | | . | . |+ | . |vs ef | | | . | . |+ | . |s pb l e ° | | | . | . |+ | . |s eef | | | . | . |+ | . |f s r | | | . | . |+ | . |vs vf e ° | | | . | . |- | . | vs e ° | | | . | . |- | . | vs e ° bm | | | . | . |- | . | vs r | | | . | . |- | . | vs e ° | | | . | . |- | . | vs bn dif | | | . | . |- | . | vs bm e ° | | | . | . |+ | . |b s e ° neb_{*}? | | | . | . |+ | . |s pf bp | | | . | . |+ | . |pb l i | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs vf e ° | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs r bm | | | . | . |+ | . | vs r bm | | | . | . |+ | . |s r f | | | . | . |+ | . |s pf e ° | | | . | . |+ | . |vs f e ° | | | . | . |+ | . | vs r bm | | | . | . |+ | . |ps f gbm | | | . | . |+ | . | vs r bs | | | . | . |+ | . | neb_{*} | | | . | . |+ | . |vf s r | | | . | . |+ | . | vs n e ° | | | . | . |+ | . |vs f e ° | | | . | . |+ | . |vf pl spiral | | | . | + . |+ | - . | vs e ° | +----------------------------------------------------------------------+ +----------------------------------------------------------------------+ |no.|[greek: a]|precession|[greek: d]|precession| description. | | | . | | . | | | |----------------------------------------------------------------------| | |h m s | s | ° ´ ´´ | ´´ | | | | . | + . |+ | - . | vs bm e ° | | | . | . |+ | . |vs f | | | . | . |+ | . |f pl gbm spiral? | | | . | . |+ | . |pb s e ° | | | . | . |+ | . | vs e ° | | | . | . |+ | . | vs r n | | | . | . |+ | . | vs r n | | | . | . |+ | - . | vs r bm | | | . | . |+ | + . |f s e ° | | | . | . |+ | . |f s r | | | . | . |+ | . |f ps vmbm | | | . | . |+ | . |pb vs m e ° vmbm | | | . | . |+ | . |vf vs m e ° | | | . | . |+ | . |f pl i_{*} inv | | | . | . |+ | . |vf s m e ° | | | . | . |+ | . |pb vs gmbm | | | . | . |+ | . |vf pl gbm | | | . | . |+ | . |d_{*} inv set on p_{*}| | | . | . |+ | . |pb s e ° vmbm | | | . | . |+ | . |vb s l e ° vmbm | | | . | . |+ | . |pf pl l e ° | | | . | . |+ | . |neb_{*} | | | . | . |+ | . |vf ps e ° | | | . | . |+ | . |f s e ° | | | . | . |+ | . |f pl gbm | | | . | . |+ | . |b vs e ° | | | . | . |+ | . |vvf s r | | | . | . |+ | . |b s ve ° | | | . | . |+ | . |b s neb_{*} | | | . | . |+ | . |b neb_{*} | | | . | . |+ | . |ps vf i | | | . | . |+ | . |s l e ° | | | . | . |+ | . |f pl n spiral? | | | . | . |+ | . |vf bn e ° spiral | | | . | . |+ | . |f vs mbm l e ° | | | . | . |+ | . |vvf vs | | | . | . |+ | . |pb vs | | | . | . |+ | . |f vs | | | . | . |+ | . |f vs e ° | | | . | . |+ | . |s vf f_{*} sp | | | . | . |+ | . |vf ps | | | . | . |+ | . |b m e ° n | | | . | . |+ | . |vs vf e ° | | | . | . |+ | . |f vs | | | . | . |+ | . |vvf vs | | | . | + . |+ | + . |s f e ° | +----------------------------------------------------------------------+ abbreviations used in description. the number denotes magnitude,--estimated from the negative. vs very small, < ´´ s small, ´´ to ´ or ´ l large, > ´ or ´ b bright d double e elongated f faint if irregular figure m middle or in the middle n nucleus r round b brighter bn brighter toward the north side bs brighter toward the south side bp brighter toward the preceding side bf brighter toward the following side bsw brightest toward the south-west bm brighter toward the middle dif diffused diffic difficult ef extremely faint g gradually i irregular l little m much p pretty pb pretty bright pf pretty faint sc scattered stell stellar sbm suddenly brighter toward the middle v very vbm very much brighter toward the middle vs very small f_{*}inv faint star involved [circle] planetary positions of known nebulÆ determined from the crossley negatives. +----------------------------------------------------------------------+ |n.g.c. |[greek: a]|precession|[greek: d]|precession| remarks. | | | . | | . | | | |----------------------------------------------------------------------| | | h m s | s | ° ´ ´´ | ´´ | | | | . | + . |+ | + . | | | | . | . |- | . | | | | . | . |- | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | ... | . | . |+ | . |n. g. c. sup. | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |- | . | | | | . | . |- | . | | | | . | . |- | . | | | | . | . |- | . | | | | . | . |+ | + . | | | | . | . |+ | - . | | | - | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | , | . | . |+ | . |n. g. c. and | | | | | | | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |- | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | ... | . | . |+ | . |coddington's neb. | | | | | | | in _ursa major_.| | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | + . |+ | - . | | |----------------------------------------------------------------------| +----------------------------------------------------------------------+ |n.g.c. |[greek: a]|precession|[greek: d]|precession| remarks. | | | . | | . | | | |----------------------------------------------------------------------| | | h m s | s | ° ´ ´´ | ´´ | | | | . | + . |+ | - . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |- | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | - . | | | | . | . |+ | + . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | | | . | . |+ | . | | +----------------------------------------------------------------------+ list of illustrations. +----------------------------------------------------------------------+ |no. | | +---------------------------------------------------------------| | |n.g.c. no. | | | +-------------------------------------------------------| | | |date. | | | | +---------------------------------| | | | |exposure. | | | | | +--------------------------| | | | | |enlargement. | | | | | | +--------------------| | | | | | |_orientation_ top | | | | | | | +----------------| | | | | | | |remarks. | |----------------------------------------------------------------------| | | | |h m | | | | | | | , september | | . |w |great nebula in | | | | | | | | _andromeda_. | | | | , december - | | . |s |h v, . | | | | , september | | . |w |m . | | | | , october | | . |s |m . | | | | , september | | . |s |m . | | | | , november | | . |s |h v, . | | | | , december | | . |s |m . | | | .. | , december | | . |w |_pleiades_. | | | | , december | | . |s |crab nebula. | | | .. | , november | | . |s |great nebula in | | | | | | | | _orion_. | | | .. | , february | | . |s |great nebula in | | | | | | | | _orion_. | | | | , january | | . |s |h v, . | | | | , january | | . |s |h v, . | | | | , november | | . |s |m . | | | | , february | | . |s |nebula near | | | | | | | | _monocerotis_.| | | .. | , february | | . |s |new nebula in | | | | | | | | _monoceros_ | | | | | | | | (roberts). | | | | , february | | . |s |h v, . | | | | , february | | . |s |h i, . | | | | , april | | . |s |h i, . | | | - | , february | | . |s |h i, , . | | | | , march | | . |s |m . | | | | , april | | . |s |h i, . | | | | , march | | . |s |h i, . | | | - | , april | | . |s |h ii, , . | | _{ }| |{ , april | | |s |h iv, . | | | |{ | | | | | | _{ }| |{ , april | | |s |h iv, . | | | | , may | | . |s |h v, . | | | | , march | | . |s |owl nebula. | | | | , april | | . |s |m . | | | | , april | | . |s |m . | | _{a}| | , march | | . |s |h ii, . | | _{b}| | , march | | . |s |h ii, . | | | | , march | | . |s |h v, . | | | | , june | | . |s |m . | | | | , may | | . |s |h v, . | | | | , april | | . |s |m . | | | | , april | | . |s |m . | | | - | , april | | . |s |h i, - . | | | | , june - | | . |s |m . | | | | , may | | . |s |h v, . | | | | , may | | . |s |h i, . | | | | , april | | . |s |h v, . | | | | , june | | . |s |h v, . | | | | , june -july | | . |s |h i, . | | | | , july | | . |s |m . | | | | , july | | . |s |m . | | | | , may | | . |s |m . | | | | , july | | . |s |m . | | | - | , may | | . |s |m . | | | | , may | | . |s |m . | | | - | , june | | . |s |m . | | | - | , may | | . |s |h ii, - . | | | | , july | | . |s |h i, . | | | | , may | | . |s |m . | | | | , june | | . |s | m . | | | | , july | | . |s | m . | | | | , july | | . |s | trifid nebula. | | | | , july | | . |w | m . | | | | , august | | |s | h iv, . | | | | , july | | . |s | omega nebula. | | | | , july | | |s | m . | | | | , july | | . |s | dumb-bell | | | | | | | | nebula. | | | | , august | | . |s | h iv, . | | | | , august | | . |s | h iv, . | | | | , august | | . |s | network nebula | | | | | | | | in _cygnus_. | | | |{ , july | |} |s {| h iv, . | | | |{ , july | |} | {| h iv, . | | | | , august - | | . |s | h iv, . | | | | , august | | . |s | h ii, . | | | | , august | | . |s | h i, . | | | | , august | | . |s | h i, . | | | | | { ^s|} | | | | | | | { ^s|} | | | | | | , september | { ^s|} |s | h, iv, . | | | | | { ^m|} | | | | | | | { ^m|} | | | | | | , september | | . |s | h ii, . | +----------------------------------------------------------------------+ [illustration: _plate _ _the great nebula in andromeda_] [illustration: _plate _ _the spiral nebula h.v.i. ceti_] [illustration: _plate _ _the spiral nebula m. trianguli_] [illustration: _plate _ _the spiral nebula m. piscium_] [illustration: _plate _ _the nebula m. persei_] [illustration: _plate _ _the nebula h.v. andromedae_] [illustration: _plate _ _the spiral nebula m. ceti_] [illustration: _plate _ _the pleiades_] [illustration: _plate _ _the crab nebula in taurus_] [illustration: _plate _ _central portion of the great nebula in orion_] [illustration: _plate _ _the nebula h.v. , orionis_] [illustration: _plate _ _the nebula h.v. orionis_] [illustration: _plate _ _the nebula m. orionis_] [illustration: _plate _ _nebula near monocerotis_] [illustration: _plate _ _new nebula in monoceros (roberts)_] [illustration: _plate _ _the spiral nebula h.v. camelopardi_] [illustration: _plate _ _the nebula h.i. leonis minoris_] [illustration: _plate _ _the spiral nebula h.i. ursae majoris_] [illustration: _plate _ _the spiral nebula h.i. - leonis_] [illustration: _plate _ _the spiral nebula m , ursae majoris_] [illustration: _plate _ _the nebula h.i. , sextantis_] [illustration: _plate _ _the spiral nebula h.i. , ursae majoris_] [illustration: _plate _ _the double nebula h.ii - , leonis_] [illustration: _plate _ _the planetary nebula h.iv , hydrae_] [illustration: _plate _ _the nebula h.v , ursae majoris_] [illustration: _plate _ _the owl nebula, m , ursae majoris_] [illustration: _plate _ _the spiral nebula m , leonis_] [illustration: _plate _ _the spiral nebula m , leonis_] [illustration: _plate _ _the spiral nebula h.ii, , ursae majoris_] [illustration: _plate _ _the nebula h.v , canum venaticorum_] [illustration: _plate _ _the spiral nebula m , comae berenices_] [illustration: _plate _ _the spiral nebula h.v , ursae majoris_] [illustration: _plate _ _the spiral nebula m , virginis_] [illustration: _plate _ _the spiral nebula m , comae berenices_] [illustration: _plate _ _the nebula h.i - , canum venaticorum_] [illustration: _plate _ _the spiral nebula m , comae berenices_] [illustration: _plate _ _the spiral nebula h.v , virginis_] [illustration: _plate _ _the spiral nebula h.i , comae berenices_] [illustration: _plate _ _the nebula h.v , comae berenices_] [illustration: _plate _ _the nebula h.v , comae berenices_] [illustration: _plate _ _the spiral nebula h.i , comae berenices_] [illustration: _plate _ _the spiral nebula m , canum venaticorum_] [illustration: _plate _ _the spiral nebula m canum venaticorum_] [illustration: _plate _ _the spiral nebula m , comae berenices_] [illustration: _plate _ _the spiral nebula m , canum venaticorum_] [illustration: _plate _ _the spiral nebula m , canum venaticorum_] [illustration: _plate _ _the star cluster m , canum venaticorum_] [illustration: _plate _ _the spiral nebula m , ursae majoris_] [illustration: _plate _ _the double nebula h.ii - , bootis_] [illustration: _plate _ _the nebula h.i , draconis_] [illustration: _plate _ _the star cluster m , librae_] [illustration: _plate _ _the star cluster m , herculis_] [illustration: _plate _ _the star cluster m , ophiuchi_] [illustration: _plate _ _the trifid nebula, m , sagittarii_] [illustration: _plate _ _the nebula m , sagittarii_] [illustration: _plate _ _the planetary nebula h.iv , draconis_] [illustration: _plate _ _the horse shoe or omega nebula m , sagittarii_] [illustration: _plate _ _the ring nebula, m. , in lyra_] [illustration: _plate _ _the dumb-bell nebula in vulpecula_] [illustration: _plate _ _the annular nebula h.iv , cygni_] [illustration: _plate _ _the spiral nebula h.iv , cephei_] [illustration: _plate _ _the net-work nebula in cygnus_] [illustration: _plate _ _the planetary nebula h.iv , aquarii_] [illustration: _plate _ _the nebula h.iv , cephei_] [illustration: _plate _ _the nebula h.ii , pegasi_] [illustration: _plate _ _the spiral nebula h.i , pegasi_] [illustration: _plate _ _the spiral nebula h.i , pegasi_] [illustration: _plate _ _the planetary nebula h.iv , andromedae_] [illustration: _plate _ _the nebula h.ii , pegasi_] footnotes: [ ] reprinted from _the astrophysical journal_, = =, , . [ ] for a more complete history of this part of the subject, see dr. holden's articles in _pub. ast. soc. pacific_, = =, _et seq._, . [ ] the difficulties here referred to, about which a good deal has been written, seem to have had their origin in the fact that it was impossible, at the time of the preliminary trials, to provide the observer with an assistant, while the crossley reflector is practically unmanageable by a single person. [ ] _mon. not. r. a. s._, = =, . [ ] kindly lent by the astronomical society of the pacific. [ ] _mem. r. a. s._, = =, . [ ] _mon. not. r. a. s._, = =, , . [ ] _mon. not. r. a. s._, = =, . the construction here described is not followed exactly in the crossley apparatus. the guiding eyepiece slides freely when not held by a clamp. pin-holes for preventing fogging are unnecessary when red light is used. [ ] it so happens that the tension of the vertical thread is such that it begins to slacken when the temperature falls to within about ° of the dew point. the thread thus forms an excellent hygrometer, which is constantly under the eye of the observer. when the thread becomes slack, it is time to cover the mirrors. [ ] _mon. not. r. a. s._, = =, . [ ] the following list includes all papers of interest: "photographic observations of comet i, (brooks), made with the crossley reflector of the lick observatory." a. j. no. , = =, ; see also _ap. j._, = =, . "the small bright nebula near _merope_," _pub. a. s. p._, = =, . "on some photographs of the great nebula in _orion_, taken by means of the less refrangible rays in its spectrum," _ap. j._, = =, . see also _pub. a. s. p._, = =, ; _ap. j._, = =, ; _a. n._, . "small nebulæ discovered with the crossley reflector of the lick observatory," _mon. not. r. a. s._, = =, . "the ring nebula in _lyra_," _ap. j._, = =, . "the annular nebula h. iv. in _cygnus_," _ap. j._, = =, ; see also _pub. a. s. p._, = =, . "on the predominance of spiral forms among the nebulæ," _a. n._, . "the distribution of stars in the cluster _messier _ in _hercules_" (by h. k. palmer), _ap. j._, = =, . "the photographic efficiency of the crossley reflector," _pub. a. s. p._, = =, ; _observatory_, = =, . "new nebulæ discovered photographically with the crossley reflector of the lick observatory," _mon. not. r. a. s._, = =, . "the spiral nebula, h. i., _ pegasi_," _ap. j._, = =, . "photographic observations of hind's variable nebula in _taurus_, made with the crossley reflector of the lick observatory," _mon. not. r. a. s._, = =, . "use of the crossley reflector for photographic measurements of position," _pub. a. s. p._, = =, . "discovery and photographic observations of a new asteroid fd.," _a. n._, . "elements of asteroid fd." (by h. k. palmer), _a. n._ . [ ] footnote added in : this concluding paragraph, retained in the present publication for completeness, loses point in some particulars, because the photogravure referred to is not reproduced here. the heliogravure reproduction of the trifid nebula is no. . [ ] since then a photograph by dr. roberts has appeared in _knowledge_, = =, , february, . transcriber's notes: passages in italics are indicated by _italics_. passages in bold are indicated by =bold=. superscripted characters are indicated by ^x. subscripted characters are indicated by _{x}. the original text utilizes a circle symbol; this is represented in this text version as [circle]. [illustration: photo, mt. wilson solar observatory _an active prominence of the sun, , miles high, photographed july , ._] astronomy the science of the heavenly bodies by david todd director emeritus, amherst college observatory [illustration: harper & brothers logo] new york and london harper & brothers publishers mcmxxii copyright by p. f. collier & son company sir william rowan hamilton, the eminent mathematician of dublin, has, of all writers ancient and modern, most fittingly characterized the ideal science of astronomy as man's golden chain connecting the heavens to the earth, by which we "learn the language and interpret the oracles of the universe." the oldest of the sciences, astronomy is also the broadest in its relations to human knowledge and the interests of mankind. many are the cognate sciences upon which the noble structure of astronomy has been erected: foremost of all, geometry and the higher mathematics, which tell us of motions, magnitudes and distances; physics and chemistry, of the origin, nature, and destinies of planets, sun, and star; meteorology, of the circulation of their atmospheres; geology, of the structure of the moon's surface; mineralogy, of the constitution of meteorites; while, if we attack, even elementally, the fascinating, though perhaps forever unsolvable, problem of life in other worlds, the astronomer must invoke all the resources that his fellow biologists and their many-sided science can afford him. the progress of astronomy from age to age has been far from uniform--rather by leaps and bounds: from the earliest epoch when man's planet earth was the center about which the stupendous cosmos wheeled, for whom it was created, and for whose edification it was maintained--down to the modern age whose discoveries have ascertained that even our stellar universe, the vast region of the solar domain, is but one of the thousands of island universes that tenant the inconceivable immensities of space. such results have been attainable only through the successful construction and operation of monster telescopes that bring to the eye and visualize on photographic plates the faintest of celestial objects which were the despair of astronomers only a few years ago. but the end is not yet; astronomy to-day is but passing from infancy to youth. and with new and greater telescopes, with new photographic processes of higher sensitivity, with the help of modern invention in overcoming the obstacle of the air--that constant foe of the astronomer--who will presume to set down any limit to the leaps and bounds of astronomy in the future? so rapid, indeed, has been the progress of astronomy in very recent years that the present is especially favorable for setting forth its salient features; and this book is an attempt to present the wide range of astronomy in readable fashion, as if a story with a definite plot, from its origin with the shepherds of ancient chaldea down to present-day ascertainment of the actual scale of the universe, and definite measures of the huge volume of supersolar giants among the stars. david todd amherst college observatory november, contents chapter page i. astronomy a living science ii. the first astronomers iii. pyramid, tomb, and temple iv. origin of greek astronomy v. measuring the earth--eratosthenes vi. ptolemy and his great book vii. astronomy of the middle ages viii. copernicus and the new era ix. tycho, the great observer x. kepler, the great calculator xi. galileo, the great experimenter xii. after the great masters xiii. newton and motion xiv. newton and gravitation xv. after newton xvi. halley and his comet xvii. bradley and aberration xviii. the telescope xix. reflectors--mirror telescopes xx. the story of the spectroscope xxi. the story of astronomical photography xxii. mountain observatories xxiii. the program of a great observatory xxiv. our solar system xxv. the sun and observing it xxvi. sun spots and prominences xxvii. the inner planets xxviii. the moon and her surface xxix. eclipses of the moon xxx. total eclipses of the sun xxxi. the solar corona xxxii. the ruddy planet xxxiii. the canals of mars xxxiv. life in other worlds xxxv. the little planets xxxvi. the giant planet xxxvii. the ringed planet xxxviii. the farthest planets xxxix. the trans-neptunian planet xl. comets--the hairy stars xli. where do comets come from? xlii. meteors and shooting stars xliii. meteorites xliv. the universe of stars xlv. star charts and catalogues xlvi. the sun's motion toward lyra xlvii. stars and their spectral type xlviii. star distances xlix. the nearest stars l. actual dimensions of the stars li. the variable stars lii. the novÆ, or new stars liii. the double stars liv. the star clusters lv. moving clusters lvi. the two star streams lvii. the galaxy or milky way lviii. star clouds and nebulÆ lix. the spiral nebulÆ lx. cosmogony lxi. cosmogony in transition list of illustrations active prominence of the sun, , miles high _frontispiece_ facing page nicholas copernicus galileo galilei johann kepler sir isaac newton the hundred-inch reflecting telescope at mount wilson the forty-inch refracting telescope, yerkes observatory -foot tower, mount wilson, a diagram of tower and pit -foot tower--exterior view view looking down into the pit beneath -foot tower mount wilson solar observatory--the -foot dome mount chimborazo, the best site in the world for an observatory lick observatory, mount hamilton, california photographing with the -inch refractor great sunspot group of august , calcium flocculi on the sun eclipse of the moon, with the lunar surface visible moon's surface in the region of copernicus south central portion of the moon, at last quarter corona of the sun during an eclipse venus, in the crescent phase mars, showing bright polar cap jupiter, the giant planet neptune and its satellites saturn, with edge of rings only in view saturn, with rings displayed to fullest extent two views of halley's comet swift's comet, which showed remarkable transformations meteor trail in field with fine nebulÆ ring nebula in lyra dumb-bell nebula star clouds and black holes in sagittarius great nebula in andromeda chapter i astronomy a living science like life itself we do not know when astronomy began; we cannot conceive a time when it was not. man of the early stone age must have begun to observe sun, moon, and stars, because all the bodies of the cosmos were there, then as now. with his intellectual birth astronomy was born. onward through the childhood of the race he began to think on the things he observed, to make crude records of times and seasons; the chaldeans and chinese began each their own system of astronomy, the causes of things and the reasons underlying phenomena began to attract attention, and astronomy was cultivated not for its own sake, but because of its practical utility in supplying the data necessary to accurate astrological prediction. belief in astrology was universal. the earth set in the midst of the wonders of the sky was the reason for it all. clearly the earth was created for humanity; so, too, the heavens were created for the edification of the race. all was subservient to man; naturally all was geocentric, or earth-centered. from the savage who could count only to five, the digits of one hand, civilized man very slowly began to evolve; he noted the progress of the seasons; the old records of eclipses showed thales, an early greek, how to predict their happenings, and true science had its birth when man acquired the power to make forecasts that always came true. few ancient philosophers were greater than pythagoras, and his conceptions of the order of the heavens and the shape and motion of the earth were so near the truth that we sometimes wonder how they could have been rejected for twenty centuries. we must remember, however, that man had not yet learned the art of measuring things, and the world could not be brought into subjection to him until he had. to measure he must have tools--instruments; to have instruments he must learn the art of working in metals, and all this took time; it was a slow and in large part imperceptible process; it is not yet finished. the earliest really sturdy manifestation of astronomical life came with the birth of greek science, culminating with aristarchus, hipparchus and ptolemy. the last of these great philosophers, realizing that only the art of writing prevents man's knowledge from perishing with him, set down all the astronomical knowledge of that day in one of the three greatest books on astronomy ever written, the almagest, a name for it derived through the arabic, and really meaning "the greatest." the system of earth and heaven seemed as if finished, and the authority of ptolemy and his almagest were as holy writ for the unfortunate centuries that followed him. with fatal persistence the fundamental error of his system delayed the evolutionary life of the science through all that period. but man had begun to measure. geometry had been born and eratosthenes had indeed measured the size of the earth. tools in bronze and iron were fashioned closely after the models of tools of stone; astrolabes and armillary spheres were first built on geometric spheres and circles; and science was then laid away for the slumber of the dark ages. nevertheless, through all this dreary period the life of the youthful astronomical giant was maintained. time went on, the heavens revolved; sun, moon, and stars kept their appointed places, and arab and moor and the savage monarchs of the east were there to observe and record, even if the world-mind was lying fallow, and no genius had been born to inspire anew that direction of human intellect on which the later growth of science and civilization depends. with the growth of the collective mind of mankind, from generation to generation, we note that ordered sequence of events which characterizes the development of astronomy from earliest peoples down to the age of newton, herschel, and the present. it is the unfolding of a story as if with a definite plot from the beginning. leaving to philosophical writers the great fundamental reason underlying the intellectual lethargy of the dark ages, we only note that astronomy and its development suffered with every other department of human activity that concerned the intellectual progress of the race. to knowledge of every sort the medieval spirit was hostile. but with the founding and growth of universities, a new era began. the time was ripe for copernicus and a new system of the heavens. the discovery of the new world and the revival of learning through the universities added that stimulus and inspiration which marked the transition from the middle ages to our modern era, and the life of astronomy, long dormant, was quickened to an extraordinary development. it fell to the lot of copernicus to write the second great book on astronomy, "de revolutionibus orbium coelestium." but the new heliocentric or sun-centered system of copernicus, while it was the true system bidding fair to replace the false, could not be firmly established except on the basis of accurate observation. how fortunate was the occurrence of the new star of , that turned the keen intellect of tycho brahe toward the heavens! without the observational labors of tycho's lifetime, what would the mathematical genius of kepler have availed in discovery of his laws of motion of the planets? historians dwell on the destruction and violent conflicts of certain centuries of the middle ages, quite overlooking the constructive work in progress through the entire era. much of this was of a nature absolutely essential to the new life that was to manifest itself in astronomy. the arabs had made important improvements in mathematical processes, european artisans had made great advances in the manufacture of glass and in the tools for working in metals. then came galileo with his telescope revealing anew the universe to mankind. it was the north of italy where the renaissance was most potent, recalling the vigorous life of ancient greece. copernicus had studied here; it was the home of galileo. columbus was a genoese, and the compass which guided him to the western world was a product of deft italian artisans whose skill with that of their successors was now available to construct the instruments necessary for further progress in the accurate science of astronomical observation. even before copernicus, johann müller, better known as regiomontanus, had imbibed the learning of the greeks while studying in italy, and founded an observatory and issued nautical almanacs from nuremberg, the basis of those by which columbus was guided over untraversed seas. about this time, too, the art of printing was invented, and the interrelation of all the movements then in progress led up to a general awakening of the mind of man, and eventually an outburst in science and learning, which has continued to the present day. naturally it put new life into astronomy, and led directly up from galileo and his experimental philosophy to newton and the _principia_, the third in the trinity of great astronomical books of all time. to get to the bottom of things, one must study intimately the history of the intellectual development of europe through the fifteenth and sixteenth centuries. many of the western countries were ruled by sovereigns of extraordinary vigor and force of character, and their activities tended strongly toward that firm basis on which the foundations of modern civilization were securely laid. contemporaneously with this era, and following on through the seventeenth century, came the measurements of the earth by french geodesists, the construction of greater and greater telescopes and the wonderful discoveries with them by huygens, cassini, and many others. most important of all was the application of telescopes to the instruments with which angles are measured. then for the first time man had begun to find out that by accurate measures of the heavenly bodies, their places among the stars, their sizes and distances, he could attain to complete knowledge of them and so conquer the universe. but he soon realized the insufficiency of the mathematical tools with which he worked--how unsuited they were to the solution of the problem of three bodies (sun, earth, and moon) under the newtonian law of gravitation, let alone the problem of n-bodies, mutually attracting each the other; and every one perturbing the motion of every other one. so the invention of new mathematical tools was prosecuted by newton and his rival leibnitz, who, by the way, showed himself as great a man as mathematician: "taking mathematics," wrote leibnitz, "from the beginning of the world to the times when newton lived, what he had done was much the better half." newton was the greatest of astronomers who, since the revival of learning, had observed the motions of the heavenly bodies and sought to find out why they moved. copernicus, tycho brahe, galileo, kepler, newton, all are bound together as in a plot. not one of them can be dissociated from the greatest of all discoveries. but newton, the greatest of them all, revealed his greatness even more by saying: "if i have seen further than other men, it is because i have been standing on the shoulders of giants." elsewhere he says: "all this was in the two plague years of and [he was then but twenty-four], for in those days i was in the prime of my age for invention, and minded mathematics and philosophy more than at any time since." all school children know these as the years of the plague and the fire; but very few, in school or out, connect these years with two other far-reaching events in the world's history, the invention of the infinitesimal calculus and the discovery of the law of gravitation. we have passed over the name of descartes, almost contemporary with galileo, the founder of modern dynamics, but his initiation of one of the greatest improvements of mathematical method cannot be overlooked. this era was the beginning of the golden age of mathematics that embraced the lives of the versatile euler, equally at home in dynamics and optics and the lunar theory; of la grange, author of the elegant "mécanique analytique"; and la place, of the unparalleled "mécanique céleste." with them and a fully elaborated calculus newton's universal law had been extended to all the motions of the cosmos. even the tides and precession of the equinoxes and bradley's nutation were accounted for and explained. mathematical or gravitational astronomy had attained its pinnacle--it seemed to be a finished science: all who were to come after must be but followers. the culmination of one great period, however, proved to be but the inception of another epoch in the development of the living science. the greatest observer of all time, with a telescope built by his own hands, had discovered a great planet far beyond the then confines of the solar system. mathematicians would take care of uranus, and herschel was left free to build bigger telescopes still, and study the construction of the stellar universe. down to his day astronomy had dealt almost wholly with the positions and motions of the celestial bodies--astronomy was a science of _where_. to inquire _what_ the heavenly bodies _are_, seemed to herschel worthy of his keenest attention also. while "a knowledge of the construction of the heavens has always been the ultimate object of my observations," as he said, and his ingenious method of star-gauging was the first practicable attempt to investigate the construction of the sidereal universe, he nevertheless devoted much time to the description of nebulæ and their nature, as well as their distribution in space. he was the founder of double-star astronomy, and his researches on the light of the stars by the simple method of sequences were the inception of the vast fields of stellar photometry and variable stars. the physics of the sun, also, was by no means neglected; and his lifework earned for him the title of father of descriptive astronomy. while progress and discovery in the earlier fields of astronomy were going on, the initial discoveries in the vast group of small planets were made at the beginning of the nineteenth century. the great bessel added new life to the science by revolutionizing the methods and instruments of accurate observation, his work culminating in the measure of the distance of cygni, first of all the stars whose distance from the sun became known. wonderful as was this achievement, however, a greater marvel still was announced just before the middle of the century--a new planet far beyond uranus, whose discovery was made as a direct result of mathematical researches by adams and le verrier, and affording an extraordinary verification of the great newtonian law. these were the days of great discoveries, and about this time the giant of all the astronomical tools of the century was erected by lord rosse, the "leviathan" reflector with a speculum six feet in diameter, which remained for more than half a century the greatest telescope in the world, and whose epochal discovery of spiral nebulæ has greater significance than we yet know or perhaps even surmise. the living science was now at the height of a vigorous development, when a revolutionary discovery was announced by kirchhoff which had been hanging fire nearly half a century--the half century, too, which had witnessed the invention of photography, the steam engine, the railroad, and the telegraph: three simple laws by which the dark absorption lines of a spectrum are interpreted, and the physical and chemical constitution of sun and stars ascertained, no matter what their distance from us. huggins in england and secchi in italy were quick to apply the discovery to the stars, and draper and pickering by masterly organization have photographed and classified the spectra of many hundred thousand stars of both hemispheres, a research of the highest importance which has proved of unique service in studies of stellar movements and the structure of the universe by eddington and shapley, campbell and kapteyn, with many others who are still engaged in pushing our knowledge far beyond the former confines of the universe. few are the branches of astronomy that have not been modified by photography and the spectroscope. it has become a measuring tool of the first order of accuracy; measuring the speed of stars and nebulæ toward and from us; measuring the rotational speed of sun and planets, corona and saturnian ring; measuring the distances of whole classes of stars from the solar system; measuring afresh even the distance of the sun--the yardstick of our immediate universe; measuring the drift of the sun with his entire family of planets twelve miles every second in the direction of alpha lyræ; and discovering and measuring the speed of binary suns too close together for our telescopes, and so making real the astronomy of the invisible. impatient of the handicap of a turbulent atmosphere, the living science has sought out mountain tops and there erected telescopes vastly greater than the "leviathan" of a past century. there the sun in every detail of disk and spectrum is photographed by day, and stars with their spectra and the nebulæ by night. great streams of stars are discovered and the speed and direction of their drift ascertained. the marvels of the spiral nebulæ are unfolded, their multitudinous forms portrayed and deciphered. and their distances? and the distances of the still more wonderful clusters? far, inconceivably far beyond the milky way. and are they "island universes"? and can man, the measurer, measure the distance of the "mainland" beyond? chapter ii the first astronomers who were the first astronomers? and who wrote the first treatise on astronomy, oldest of the sciences? questions not easy to answer in our day. with the progress of archæological research, or inquiry into the civilization and monuments of early peoples, it becomes certain that man has lived on this planet earth for tens of thousands of years in the past as an intelligent, observing, intellectual being; and it is impossible to assign any time so remote that he did not observe and philosophize upon the firmament above. we can hardly imagine a people so primitive that they would fail to regard the sun as "lord of the day," and therefore all important in the scheme of things terrestrial. says anne bradstreet of the sun in her "contemplations": what glory's like to thee? soul of this world, this universe's eye, no wonder some made thee deity. to the babylonians belongs the credit of the oldest known work on astronomy. it was written nearly six thousand years ago, about b. c. , by their monarch sargon the first, king of agade. only the merest fragments of this historic treatise have survived, and they indicate the reverence of the babylonians for the sun. another work by sargon is entitled "omens," which shows the intimate relationship of astronomy to mysticism and superstitious worship at this early date, and which persists even at the present day. as remotely as b. c. , the sun-god shamash and his wife aya are carved upon the historic cylinders of hematite and lapis lazuli, and one of the oldest designs on these cylinders represents the sun-god coming out of the door of sunrise, while a porter is opening the gate of the east. the semitic religion had as its basis a reverence for the bodies of the sky; and samson, hebrew for sun, was probably the sun-god of the hebrews. the phoenician deity, baal, was a sun-god under differing designations; and at the epoch of the shepherd kings, about b. c. , during the hyksos dynasty, the sun-god was represented by a circle or disk with extended rays ending in hands, possibly the precursor of the frequently recurring egyptian design of the winged disk or winged solar globe. hittites, persians, and assyrians, as well as the phoenicians, frequently represented the sun-god in similar fashion in their sacred glyphs or carvings. for a long period in early human history, astronomy and astrology were pretty much the same. we can trace the history of astrology back as far as b. c. in ancient babylonia. the motions of the sun, moon, and the five lucid planets of that time indicated the activity of the various gods who influenced human affairs. so the babylonian priests devised an elaborate system of interpreting the phenomena of the heavens; and attaching the proper significance in human terms to everything that took place in the sky. in babylonia and assyria it was the king and his people for whom the prognostications were made out. it was the same in egypt. later, about the fifth century b. c., astrology spread through greece, where astrologers developed the idea of the influence of planets upon individual concerns. astrology persisted through the dark ages, and the great astronomers copernicus, tycho, kepler, gassendi, and huygens were all astrologers as well. milton makes many references to planetary influence, our language has many words with a direct origin in astrology, and in our great cities to-day are many astrologers who prepare individual horoscopes of more than ordinary interest. it is difficult to assign the antiquity of the chinese astronomy with any approach to definiteness. their earliest records appear to have been total eclipses of the sun, going back nearly , years before the christian era; and nearly a thousand years earlier the hindu astronomy sets down a conjunction of all the planets, concerning which, however, there is doubt whether it was actually observed or merely calculated backward. owing to a colossal misfortune, the burning of all native scientific books by order of the emperor tsin-chi-hwang-ti, in b. c. , excepting only the volumes relating to agriculture, medicine, and astrology, the chinese lost a precious mass of astronomical learning, accumulated through the ages. no less an authority than wells williams credits them with observing solar eclipses between b. c. and a. d. , and there must have been some centuries of eclipses observed and recorded anterior to b. c. , as this is the date assigned to the eclipse which came unheralded by the astronomers royal, hi and ho, who had become intoxicated and forgot to warn the court, in accord with their duty. china was thereby exposed to the anger of the gods, and hi and ho were executed by his majesty's command. it is doubtful if there is an earlier record of any celestial phenomenon. chapter iii pyramid, tomb, and temple inquiry into the beginnings of astronomy in ancient egypt reveals most interesting relations of the origins of the science to the life and work and worship of the people. their astronomers were called the "mystery teachers of heaven"; their monuments indicate a civilization more or less advanced; and their temples were built on astronomical principles and dedicated to purpose of worship. the egyptian records carry us back many thousands of years, and we find that in egypt, as in other early civilizations, observation of the heavenly bodies may be embraced in three pretty distinct stages. awe, fear, wonder and worship were the first. then came utility: a calendar was necessary to tell men when "to plow and sow, to reap and mow," and a calendar necessitated astronomical observations of some sort. following this, the third direction required observations of celestial positions and phenomena also, because astrology, in which the potentates of every ancient realm believed, could only thrive as it was based on astronomy. sun worship was preeminent in early egypt as in india, where the primal antithesis between night and day struck terror in the unformed mind of man. in one of the vedas occurs this significant song to the god of day: "will the sun rise again? will our old friend the dawn come back again? will the power of darkness be conquered by the god of light?" quite different from india, however, is egypt in matters of record: in india, records in papyrus, but no monuments of very great antiquity; in egypt, no papyrus, but monuments of exceeding antiquity in abundance. herodotus and pliny have told us of the great antiquity of these monuments, even in their own day, and research by archæologist and astronomer has made it certain that the pyramids were built by a race possessing great knowledge of astronomy. their temples, too, were constructed in strict relation to stars. not only are the temples, as edfu and denderah, of exceeding interest in themselves, but associated with them are often huge monoliths of syenite, obelisks of many hundred tons in weight, which the astronomer recognizes as having served as observation pillars or gnomons. specimens of these have wandered as far from home as central park and the bank of the thames. but there is an even more remarkable wealth of temple inscriptions, zodiacs especially. next to the sun himself was the worship of the dawn and sunrise, the great revelations of nature. there were numerous hymns to the still more numerous sun-gods and the powers of sunlight. ra was the sun-god in his noontide strength; osiris, the dying sun of sunset. only two gods were associated with the moon, and for the stars a special goddess, sesheta. sacrifices were made at day-break; and the stars that heralded the dawn were the subjects of careful observation by the sacrificial priests, who must therefore have possessed a good knowledge of star places and names, doubtless in belts of stars extending clear around the heavens. these decans, as they were called, are the exact counterparts of the moon stations devised by the arabians, indians, and other peoples for a like purpose. the plane or circle of observation, both in egypt and india, was always the horizon, whether the sun was observed or moon or stars. so the sun was often worshiped by the ancient egyptians as the "lord of the two horizons." it is sometimes difficult to keep in mind the fact, in regard to all temples of the ancients, whether in egypt or elsewhere, that in studying them we must deal with the risings or settings of the heavenly bodies in quite different fashion from that of the astronomer of to-day, who is mainly concerned only with observing them on the meridian. the axis of the temple shows by its direction the place of rising or setting: if the temple faces directly east or west, its amplitude is . now the sun, moon, and planets are, as everyone knows, very erratic as to their amplitudes (i. e., horizon points) of rising and setting; so it must have been the stars that engrossed the attention of the earliest builders of temples. after that, temples were directed to the rising sun, at the equinox or solstices. then came the necessity of finding out about the inclination or obliquity of the ecliptic, and this is where the gnomon was employed. at karnak are many temples of the solstitial order: the wonderful temple of amen-ra is so oriented that its axis stands in amplitude degrees north of west, which is the exact amplitude of the sun at thebes at sunset of the summer solstice. the axis of a lesser temple adjacent points to degrees south of east, which is the exact amplitude of sunrise at the winter solstice. at gizeh we find the temples oriented, not solstitially, but by the equinoxes, that is, they face due east and west. peoples who worshiped the sun at the solstice must have begun their year at the solstice; and sir norman lockyer shows how the rise of the nile, which took place at the summer solstice, dominated not only the industry but the astronomy and religion of egypt. looking into the question of temple orientation in other countries, as china, for example, lockyer finds that the most important temple of that country, the temple of the sun at peking, is oriented to the winter solstice; and stonehenge, as has long been known, is oriented to sunrise at the summer solstice. in like fashion the rising and setting of many stars were utilized by the egyptians, in both temple and pyramid; and no astronomer who has ever seen these ancient structures and studied their orientations can doubt that they were built by astronomers for use by astronomers of that day. the priests were the astronomers, and the temples had a deep religious significance, with a ceremony of exceeding magnificence wherever observations of heavenly bodies were undertaken, whether of sun or stars. hindu and persian astronomy must be passed over very briefly. interesting as their systems are historically, there were few, if any, original contributions of importance, and the indian treatises bear strong evidence of greek origin. chapter iv origin of greek astronomy while the greeks laid the foundations of modern scientific astronomy, they were not as a whole observers: rather philosophers, we should say. the later representatives of the greek school, however, saw the necessity of observation as a basis of true induction; and they discovered that real progress was not possible unless their speculative ideas were sufficiently developed and made definite by the aid of geometry, so that they became capable of detailed comparison with observation. this was the necessary and ultimate test with them, and the same is true to-day. the early greek philosophers were, however, mainly interested, not in observations, but in guessing the causes of phenomena. thales of miletus, founder of the ionian school, introduced the system of egyptian astronomy into greece, about the end of the seventh century b. c. he is universally known as the first astronomer who ever predicted a total eclipse of the sun that happened when he said it would: the eclipse of b. c. . this he did by means of the chaldean eclipse cycle of years known as the saros. aristarchus of samos was the first and most eminent of the alexandrian astronomers, and his treatise "on the magnitudes and distances of the sun and moon" is still extant. this method of ascertaining how many times farther the sun is than the moon is very simple, and geometrically exact. unfortunately it is impossible, even to-day, to observe with accuracy the precise time when the moon "quarters," (an observation essential to his method), because the moon's terminal, or line between day and night, is not a straight line as required by theory, but a jagged one. by his observation, the sun was only twenty times farther away than the moon, a distance which we know to be nearly twenty times too small. his views regarding other astronomical questions were right, although they found little favor among contemporaries. not only was the earth spherical, he said, but it rotated on its axis and also traveled round the sun. aristarchus was, indeed, the true originator of the modern doctrine of motions in the solar system, and not copernicus, seventeen centuries later; but seleucus appears to have been his only follower in these very advanced conceptions. aristarchus made out the apparent diameters of sun and moon as practically equal to one another, and inferred correctly that their real diameters are in proportion to their distances from the earth. also he estimated, from observations during an eclipse of the moon, that the moon's diameter is about one-third that of the earth. aristarchus appears to have been one of the clearest and most accurate thinkers among the ancient astronomers; even his views concerning the distances of the stars were in accord with the fact that they are immeasurably distant as compared with the distances of the sun, moon, and planets. practically contemporary with aristarchus were timocharis and aristillus, who were excellent observers, and left records of position of sun and planets which were exceedingly useful to their successors, hipparchus and ptolemy in particular. indeed their observations of star positions were such that, in a way, they deserve the fame of having made the first catalogue, rather than hipparchus, to whom is universally accorded that honor. spherical astronomy had its origin with the alexandrian school, many famous geometers, and in particular euclid, pointing the way. spherics, or the doctrine of the sphere, was the subject of numerous treatises, and the foundations were securely laid for that department of astronomical research which was absolutely essential to farther advance. the artisans of that day began to build rude mechanical adaptations of the geometric conceptions as concrete constructions in wood and metal, and it became the epoch of the origin of astrolabes and armillary spheres. chapter v measuring the earth--eratosthenes all told, the greek philosophers were probably the keenest minds that ever inhabited the planet, and we cannot suppose them so stupid as to reject the doctrine of a spherical earth. in fact so certain were they that the earth's true figure is a sphere that eratosthenes in the third century b. c. made the first measure of the dimensions of the terrestrial sphere by a method geometrically exact. at syene in upper egypt the sun at the summer solstice was known to pass through the zenith at noon, whereas at alexandria eratosthenes estimated its distance as seven degrees from the zenith at the same time. this difference being about one-fiftieth of the entire circumference of a meridian, eratosthenes correctly inferred that the distance between alexandria and syene must be one-fiftieth of the earth's circumference. so he measured the distance between the two and found it , stadia. this figured out the size of the earth with a percentage of error surprisingly small when we consider the rough means with which eratosthenes measured the sun's zenith distance and the distance between the two stations. greatest of all the greek astronomers and one of the greatest in the history of the science was hipparchus who had an observatory at rhodes in the middle of the second century b. c. his activities covered every department of astronomy; he made extensive series of observations which he diligently compared with those handed down to him by the earlier astronomers, especially aristillus and timocharis. this enabled him to ascertain the motion of the equinoxial points, and his value of the constant of precession of the equinoxes is exceedingly accurate for a first determination. in b. c. a new star blazed out in the constellation scorpio, and this set hipparchus at work on a catalogue of the brighter stars of the firmament, a monumental work of true scientific conception, because it would enable the astronomers of future generations to ascertain what changes, if any, were taking place in the stellar universe. there were , stars in his catalogue, and he referred their positions to the ecliptic and the equinoxes. also he originated the present system of stellar magnitudes or orders of brightness, and his catalogue was in use as a standard for many centuries. hipparchus was a great mathematician as well, and he devoted himself to the improvement of the method of applying numerical calculations to geometrical figures: trigonometry, both plane and spherical, that is; and by some authorities he is regarded as the inventor of original methods in trigonometry. the system of spheres of eudoxus did not satisfy him, so he devised a method of representing the paths of the heavenly bodies by perfectly uniform motion in circles. there is slight evidence that apollonius of perga may have been the originator of the system, but it was reserved for hipparchus to work it out in final form. this enabled him to ascertain the varying length of the seasons, and he fixed the true length of the year as - / days. he had almost equal success in dealing with the irregularities of the moon's motion, although the problem is much more complicated. the distance and size of the moon, by the method of aristarchus, were improved by him, and he worked out, for the distance of the sun, , radii of the earth--a classic for many centuries. hipparchus devoted much attention to eclipses of both sun and moon, and we owe to him the first elucidation of the subject of parallax, or the effect of difference of position of an observer on the earth's surface as affecting the apparent projection of the moon against the sun when a solar eclipse takes place; whereas an eclipse of the moon is unaffected by parallax and can be seen at the same time by observers everywhere, no matter what their location on the earth. indeed, with all that hipparchus achieved, we need not be surprised that astronomy was regarded as a finished science, and made practically no progress whatever for centuries after his time. then came claudius ptolemæus, generally known as ptolemy, the last great name in greek astronomy. he lived in alexandria about the middle of the second century a. d. and wrote many minor astronomical and astrological treatises, also works on geography and optics, in the last of which the atmospheric refraction of rays of light from the heavenly bodies, apparently elevating them toward the zenith, is first dealt with in true form. chapter vi ptolemy and his great book ptolemy was an observer of the heavens, though not of the highest order; but he had all the work of his predecessors, best of all hipparchus, to build upon. ptolemy's greatest work was the "megale syntaxis," generally known as the almagest. it forms a nearly complete compendium of the ancient astronomy, and although it embodies much error, because built on a wrong theory, the almagest nevertheless is competent to follow the motions of all the bodies in the sky with a close approach to accuracy, even at the present day. this marvelous work written at this critical epoch became as authoritative as the philosophy of aristotle, and for many centuries it was the last word in the science. the old astrology held full sway, and the ptolemaic theory of the universe supplied everything necessary: further progress, indeed, was deemed impossible. the almagest comprises in all thirteen books, the first two of which deal with the simpler observations of the celestial sphere, its own motion and the apparent motions of sun, moon, and planets upon it. he discusses, too, the postulates of his system and exhibits great skill as an original geometer and mathematician. in the third book he takes up the length of the year, and in the fourth book similarly the moon and the length of the month. here his mathematical powers are at their best, and he made a discovery of an inequality in the moon's motion known as the evection. book five describes the construction and use of the astrolabe, a combination of graduated circles with which ptolemy made most of his observations. in the sixth book he follows mainly hipparchus in dealing with eclipses of sun and moon. in the seventh and eighth books he discusses the motion of the equinox, and embodies a catalogue of , stars, substantially as in hipparchus. the five remaining books of the almagest deal with the planetary motions, and are the most important of all of ptolemy's original contributions to astronomy. ptolemy's fundamental doctrines were that the heavens are spherical in form, all the heavenly motions being in circles. in his view, the earth too is spherical, and it is located at the center of the universe, being only a point, as it were, in comparison. all was founded on mere appearance combined with the philosophical notion that the circle being the only perfect curve, all motions of heavenly bodies must take place in earth-centered circles. for fourteen or fifteen centuries this false theory persisted, on the authority of ptolemy and the almagest, rendering progress toward the development of the true theory impossible. ptolemy correctly argued that the earth itself is a sphere that is curved from east to west, and from north to south as well, clinching his argument, as we do to-day, by the visibility of objects at sea, the lower portions of which are at first concealed from our view by the curved surface of the water which intervenes. to ptolemy also the earth is at the center of the celestial sphere, and it has no motion of translation from that point; but his argument fails to prove this. truth and error, indeed, are so deftly intermingled that one is led to wonder why the keen intelligence of this great philosopher permitted him to reject the simple doctrine of the earth's rotation on its axis. but if we reflect that there was then no science of natural philosophy or physics proper, and that the age was wholly undeveloped along the lines of practical mechanics, we shall see why the astronomers of ptolemy's time and subsequent centuries were content to accept the doctrines of the heavens as formulated by him. when it came to explaining the movements of the "wandering stars," or planets, as we term them, the ptolemaic theory was very happy in so far as accuracy was concerned, but very unhappy when it had to account for the actual mechanics of the cosmos in space. sun and moon were the only bodies that went steadily onward, easterly: whereas all the others, mercury, venus, mars, jupiter, saturn, although they moved easterly most of the time, nevertheless would at intervals slow down to stationary points, where for a time they did not move at all, and then actually go backward to the west, or retrograde, then become stationary again, finally resuming their regular onward motion to the east. to help out of this difficulty, the worst possible mechanical scheme was invented, that known as the epicycle. each of the five planets was supposed to have a fictitious "double," which traveled eastward with uniformity, attached to the end of a huge but mechanically impossible bar. the earth-centered circle in which this traveled round was called the "deferent." what this bar was made of, what stresses it would be subjected to, or what its size would have to be in order to keep from breaking--none of these questions seems to have agitated the ancient and medieval astronomers, any more than the flat-earth astronomy of the hindu is troubled by the necessity of something to hold up the tortoise that holds up the elephant that holds up the earth. but at the end of this bar is jointed or swiveled another shorter bar, to the revolving end of which is attached the actual planet itself; and the second bar, by swinging once round the end of the primary advancing bar, would account for the backward or retrograde motion of the planet as seen in the sky. for every new irregularity that was found, in the motion of mars, for instance, a new and additional bar was requisitioned, until interplanetary space was hopelessly filled with revolving bars, each producing one of the epicycles, some large, some small, that were needed to take up the vagaries of the several planets. the arabic astronomers who kept the science alive through the middle ages added epicycle to epicycle, until there was every justification for milton's verses descriptive of the sphere: with centric and eccentric scribbled o'er, cycle and epicycle, orb in orb. chapter vii astronomy of the middle ages with the fall of alexandria and the victory of mohammed throughout the west, and a consequent decline in learning, supremacy in science passed to the east and centered round the caliphs of bagdad in the seventh and eighth centuries. they were interested in astronomy only as a practical, and to them useful, science, in adjusting the complicated lunar calendar of the mohammedans, in ascertaining the true direction of mecca which every mohammedan must know, and in the revival of astrology, to which the greeks had not attached any particular significance. harun al-rashid ordered the almagest and many other greek works translated, of which the modern world would otherwise no doubt never have heard, as the greek originals are not extant. splendid observatories were built at damascus and bagdad, and fine instruments patterned after greek models were continuously used in observing. the arab astronomers, although they had no clocks, were nevertheless so fully impressed with the importance of time that they added extreme value to their observations of eclipses, for example, by setting down the altitudes of sun or stars at the same time. on very important occasions the records were certified on oath by a body of barristers and astronomers conjointly--a precedent which fortunately has never been followed. about the middle of the ninth century, the caliph al-mamun directed his astronomers to revise the greek measures of the earth's dimensions, and they had less reverence for the almagest than existed in later centuries: indeed, tabit ben korra invented and applied to the tables of the almagest a theoretical fluctuation in the position of the ecliptic which he called "trepidation," which brought sad confusion into astronomical tables for many succeeding centuries. albategnius was another arab prince whose record in astronomy in the ninth and tenth centuries was perhaps the best: the ptolemaic values of the precession of the equinoxes and of the obliquity of the ecliptic were improved by new observations, and his excellence as mathematician enabled him to make permanent improvements in the astronomical application of trigonometry. abul wefa was the last of the bagdad astronomers in the latter half of the tenth century, and his great treatise on astronomy known as the almagest is sometimes confused with ptolemy's work. following him was ibn yunos of cairo, whose labors culminated in the famous hakemite tables, which became the standard in mathematical and astronomical computations for several centuries. mohammedan astronomy thrived, too, in spain and northern africa. arzachel of toledo published the toledan tables, and his pupils made improvements in instruments and the methods of calculation. the giralda was built by the moors in seville in , the first astronomical observatory on the continent of europe; but within the next half century both seville and cordova became christian again, and arab astronomy was at an end. through many centuries, however, the science had been kept alive, even if no great original advances had been achieved; and arab activities have modified our language very materially, adding many such words as almanac, zenith, and radii, and a wealth of star names, as aldebaran, rigel, betelgeuse, vega, and so on. meanwhile, other schools of astronomy had developed in the east, one at meraga near the modern persia, where nassir eddin, the astronomer of hulagu khan, grandson of the mongol emperor genghis khan, built and used large and carefully constructed instruments, translated all the greek treatises on astronomy, and published a laborious work known as the ilkhanic tables, based on the hakemite tables of ibn yunos. more important still was the tartar school of astronomy under ulugh beg, a grandson of tamerlane, who built an observatory at samarcand in , published new tables of the planets, and made with his excellent instruments the observations for a new catalogue of stars, the first since hipparchus, the star places being recorded with great precision. the european astronomy of the middle ages amounted to very little besides translation from the arabic authors into latin, with commentaries. astronomers under the patronage of alfonso x of leon and castile published in the alfonsine tables, which superseded the toledan tables and were accepted everywhere throughout europe. alfonso published also the "libros del saber," perhaps the first of all astronomical cyclopedias, in which is said to occur the earliest diagram representing a planetary orbit as an ellipse: mercury's supposed path round the earth as a center. purbach of vienna about the middle of the th century began his "epitome of astronomy" based on the "almagest" of ptolemy, which was finished by his collaborator regiomontanus, who was an expert in mathematics and published a treatise on trigonometry with the first table of sines calculated for every minute from ° to °, a most helpful contribution to theoretical astronomy. regiomontanus had a very picturesque career, finally taking up his residence in nuremberg, where a wealthy citizen named walther became his patron, pupil, and collaborator. the artisans of the city were set at work on astronomical instruments of the greatest accuracy, and the comet of was the first to be observed and studied in true scientific fashion. regiomontanus was very progressive and the invention of the new art of printing gave him an opportunity to publish purbach's treatise, which went through several editions and doubtless had much to do in promoting dissatisfaction with the ancient ptolemaic system, and was thus most significant in preparing a background for the coming of the new copernican order. the nuremberg presses popularized astronomy in other important ways, issuing almanacs, the first precursors of our astronomical ephemerides. regiomontanus was practical as well, and invented a new method of getting a ship's position at sea, with tables so accurate that they superseded all others in the great voyages of discovery, and it is probable that they were employed by columbus in his discovery of the american continent. regiomontanus had died several years earlier, in at rome, where he had gone by invitation of the pope to effect a reformation in the calendar. he was only forty, and his patron walther kept on with excellent observations, the first probably to be corrected for the effect of atmospheric refraction, although its influence had been known since ptolemy. the nuremberg school lasted for nearly two centuries. nearly contemporary with regiomontanus were fracastoro and peter apian, whose original observations on comets are worthy of mention because they first noticed that the tails of these bodies always point away from the sun. leonardo da vinci was the first to give the true explanation of earth-shine on the moon, and similarly the moon-illumination of the earth; and this no doubt had great weight in disposing of the popular notion of an essential difference of nature between the earth and celestial bodies--all of which helped to prepare the way for copernicus and the great revolution in astronomical thought. chapter viii copernicus and the new era throughout the middle ages the progress of astronomy was held back by a combination of untoward circumstances. a prolonged reaction from the heights attained by the greek philosophers was to be expected. the uprising of the mohammedan world, and the savage conquerors in the east did not produce conditions favorable to the origin and development of great ideas. at the birth of copernicus, however, in , the time was ripening for fundamental changes from the ancient system, the error of which had helped to hold back the development of the science for centuries. the fifteenth century was most fruitful in a general quickening of intelligence, the invention of printing had much to do with this, as it spread a knowledge of the greek writers, and led to conflict of authorities. even aristotle and ptolemy were not entirely in harmony, yet each was held inviolate. it was the age of the reformation, too, and near the end of the century the discovery of america exerted a powerful stimulus in the advance of thought. copernicus searched the works of the ancient writers and philosophers, and embodied in this new order such of their ideas as commended themselves in the elaboration of his own system. pythagoras alone and his philosophy looked in the true direction. many believe that he taught that the sun, not the earth, is at the center of our solar system; but his views were mingled with the speculative philosophy of the greeks, and none of his writings, barring a few meager fragments, have come down to our modern age. to many philosophers, through all these long centuries, the true theory of the celestial motions must have been obvious, but their views were not formulated, nor have they been preserved in writing. so the fact remains that copernicus alone first proved the truth of the system which is recognized to-day. this he did in his great treatise entitled "de revolutionibus orbium coelestium," the first printed copy of which was dramatically delivered to him on his deathbed, in may, . the seventy years of his life were largely devoted to the preparation of this work, which necessitated many observations as well as intricate calculations based upon them. being a canon in the church, he naturally hesitated about publishing his revolutionary views, his friend rheticus first doing this for him in outline in . so simple are the great principles that they may be embodied in very few words; what appears to us as the daily revolution of the heavens is not a real motion, but only an apparent one; that is, the heavens are at rest, while the earth itself is in motion, turning round an axis which passes through its center. and the second proposition is that the earth is simply one of the six known planets; and they all revolve round the sun as the true center. the solar system, therefore, is "heliocentric," or sun-centered, not "geocentric" or earth-centered, as taught by the ptolemaic theory. copernicus demonstrates clearly how his system explains the retrograde motion of the planets and their stationary points, no matter whether they are within the orbit of the earth, as mercury and venus, or outside of it, as mars, jupiter, and saturn. his system provides also the means of ascertaining with accuracy the proportions of the solar system, or the relative distances of the planets from the sun and from each other. in this respect also his system possessed a vast advantage over that of ptolemy, and the planetary distances which copernicus computed are very close approximations to the measures of the present day. reinhold revised the calculations of copernicus and prepared the "tabulæ prutenicæ," based on the "de revolutionibus," which proved far superior to the alfonsine tables, and were only supplanted by the rudolphine tables of kepler. on the whole we may regard the lifework of copernicus as fundamentally the most significant in the history and progress of astronomy. chapter ix tycho, the great observer clear as copernicus had made the demonstration of the truth of his new system, it nevertheless failed of immediate and universal acceptance. the ptolemaic system was too strongly intrenched, and the motions of all the bodies in the sky were too well represented by it. accurate observations were greatly needed, and the landgrave william iv. of hesse built the cassel observatory, which made a new catalogue of stars, and introduced the use of clocks to carry on the time as measured by the uniform motion of the celestial sphere. three years after the death of copernicus, tycho brahe was born, and when he was the king of denmark built for him the famous observatory of uraniborg, where the great astronomer passed nearly a quarter of a century in critically observing the positions of the stars and planets. tycho was celebrated as a designer and constructor of new types of astronomical instruments, and he printed a large volume of these designs, which form the basis of many in use at the present day. unfortunately for the genius of tycho and the significance of his work, the invention of the telescope had not yet been made, so that his observations had not the modern degree of accuracy. nevertheless, they were destined to play a most important part in the progress of astronomy. tycho was sadly in error in his rejection of the copernican system, although his reasons, in his day, seemed unanswerable. if the outer planets were displaced among the stars by the annual motion of the earth round the sun, he argued, then the fixed stars must be similarly displaced--unless indeed they be at such vast distances that their motions would be too slight to be visible. of course we know now that this is really true, and that no instruments that tycho was able to build could possibly have detected the motions, the effects of which we now recognize in the case of the nearer fixed stars in their annual, or parallactic, orbits. the remarkably accurate instruments devised by tycho brahe and employed by him in improving the observations of the positions of the heavenly bodies were no doubt built after descriptions of astrolabes such as hipparchus used, as described by ptolemy. in his "astronomiæ instauratæ mechanica" we find illustrations and descriptions of many of them. one is a polar astrolabe, mounted somewhat as a modern equatorial telescope is, and the meridian circle is adjustable so that it can be used in any place, no matter what its latitude might be. there is a graduated equatorial ring at right angles to the polar axis, so that the astrolabe could be used for making observations outside the meridian as well as on it. this equatorial circle slides through grooves, and is furnished with movable sights, and a plumb line from the zenith or highest point of the meridian circle makes it possible to give the necessary adjustment in the vertical. screws for adjustment at the bottom are provided, just as in our modern instruments, and two observers were necessary, taking their sights simultaneously; unless, as in one type of the instrument, a clock, or some sort of measure of time, was employed. another early type of instrument is called by tycho the ecliptic astrolabe (_armillæ zodiacales_, or the zodiacal rings). it resembles the equatorial astrolabe somewhat, but has a second ring inclined to the equatorial one at an angle equal to the obliquity of the ecliptic. in observing, the equatorial ring was revolved round till the ecliptic ring came into coincidence with the plane of the ecliptic in the sky. then the observation of a star's longitude and latitude, as referred to the ecliptic plane, could be made, quite as well as that of right ascension and declination on the equatorial plane. but it was necessary to work quickly, as the adjustment on the ecliptic would soon disappear and have to be renewed. tycho is often called the father of the science of astronomical observation, because of the improvements in design and construction of the instruments he used. his largest instrument was a mural quadrant, a quarter-circle of copper, turning parallel to the north-and-south face of a wall, its axis turning on a bearing fixed in the wall. the radius of this quadrant was nine feet, and it was graduated or divided so as to read the very small angle of ten seconds of arc--an extraordinary degree of precision for his day. tycho built also a very large alt-azimuth quadrant, of six feet radius. its operation was very much as if his mural quadrant could be swung round in azimuth. at several of the great observatories of the present day, as greenwich and washington, there are instruments of a similar type, but much more accurate, because the mechanical work in brass and steel is executed by tools that are essentially perfect, and besides this the power of the telescope is superadded to give absolute direction, or pointing on the object under observation. excellent clocks are necessary for precise observation with such an instrument; but neither tycho brahe, nor hevelius was provided with such accessories. hevelius did not avail himself of the telescope as an aid to precision of observation, claiming that pinhole sights gave him more accurate results. it was a dispute concerning this question that halley was sent over from london to danzig to arbitrate. there could be but one way to decide; the telescope with its added power magnifies any displacement of the instrument, and thereby enables the observer to point his instrument more exactly. so he can detect smaller errors and differences of direction than he can without it. and what is of great importance in more modern astronomy, the telescope makes it possible to observe accurately the position of objects so faint that they are wholly invisible to the naked eye. chapter x kepler, the great calculator most fortunate it was for the later development of astronomical theory that tycho brahe not only was a practical or observational astronomer of the highest order, but that he confined himself studiously for years to observations of the places of the planets. of mars he accumulated an especially long and accurate series, and among those who assisted him in his work was a young and brilliant pupil named johann kepler. strongly impressed with the truth of the copernican system, kepler was free to reject the erroneous compromise system devised by tycho brahe, and soon after tycho's death kepler addressed himself seriously to the great problem that no one had ever attempted to solve, viz: to find out what the laws of motion of the planets round the sun really are. of course he took the fullest advantage of all that ptolemy and copernicus had done before him, and he had in addition the splendid observations of tycho brahe as a basis to work upon. copernicus, while he had effected the tremendous advance of substituting the sun for the earth as the center of motion, nevertheless clung to the erroneous notion of ptolemy that all the bodies of the sky must perforce move at uniform speeds, and in circular curves, the circle being the only "perfect curve." kepler was not long in finding out that this could not be so, and he found it out because tycho brahe's observations were much more accurate than any that copernicus had employed. naturally he attempted the nearest planet first, and that was mars--the planet that tycho had assigned to him for research. how fortunate that the orbit of mars was the one, of all the planets, to show practically the greatest divergence from the ancient conditions of uniform motion in a perfectly circular orbit! had the orbit of mars chanced to be as nearly circular as is that of venus, kepler might well have been driven to abandon his search for the true curve of planetary motion. however, the facts of the cosmos were on his side, but the calculations essential in testing his various hypotheses were of the most tedious nature, because logarithms were not yet known in his day. his first discovery was that the orbit of mars is certainly not a circle, but oval or elliptic in figure. and the sun, he soon found, could not be in the center of the ellipse, so he made a series of trial calculations with the sun located in one of the foci of the ellipse instead. then he found he could make his calculated places of mars agree quite perfectly with tycho brahe's observed positions, if only he gave up the other ancient requisite of perfectly uniform motion. on doing this, it soon appeared that mars, when in perihelion, or nearest the sun, always moved swiftest, while at its greatest distance from the sun, or aphelion, its orbital velocity was slowest. kepler did not busy himself to inquire why these revolutionary discoveries of his were as they were; he simply went on making enough trials on mars, and then on the other planets in turn, to satisfy himself that all the planetary orbits are elliptical, not circular in form, and are so located in space that the center of the sun is at one of the two foci of each orbit. this is known as kepler's first law of planetary motion. the second one did not come quite so easy; it concerned the variable speed with which the planet moves at every point of the orbit. we must remember how handicapped he was in solving this problem: only the geometry of euclid to work with, and none of the refinements of the higher mathematics of a later day. but he finally found a very simple relation which represented the velocity of the planet everywhere in its orbit. it was this: if we calculate the area swept, or passed over, by the planet's radius vector (that is, the line joining its center to the sun's center) during a week's time near perihelion, and then calculate the similar area for a week near aphelion, or indeed for a week when mars is in any intermediate part of its orbit, we shall find that these areas are all equal to each other. so kepler formulated his second great law of planetary motion very simply: the radius vector of any planet describes, or sweeps over, equal areas in equal times. and he found this was true for all the planets. but the real genius of the great mathematician was shown in the discovery of his third law, which is more complex and even more significant than the other two--a law connecting the distances of the planets from the sun with their periods of revolution about the sun. this cost kepler many additional years of close calculation, and the resulting law, his third law of planetary motion is this: the cubes of the mean or average distances of the planets from the sun are proportional to the squares of their times of revolution around him. so kepler had not only disposed of the sacred theories of motion of the planets held by the ancients as inviolable, but he had demonstrated the truth of a great law which bound all the bodies of the solar system together. so accurately and completely did these three laws account for all the motions, that the science of astronomy seemed as if finished; and no matter how far in the future a time might be assigned, kepler's laws provided the means of calculating the planet's position for that epoch as accurately as it would be possible to observe it. kepler paused here, and he died in . chapter xi galileo, the great experimenter the fifteenth and sixteenth centuries, containing the lives and work of copernicus, tycho, galileo, kepler, huygens, halley, and newton, were a veritable golden age of astronomy. all these men were truly great and original investigators. none had a career more picturesque and popular than did galileo. born a few years earlier and dying a few years later than kepler, the work of each of these two great astronomers was wholly independent of the other and in entirely different fields. kepler was discovering the laws of planetary motion, while galileo was laying the secure foundations of the new science of dynamics, in particular the laws of falling bodies, that was necessary before kepler's laws could be fully understood. when only eighteen galileo's keen power of observation led to his discovery of the laws of pendulum motion, suggested by the oscillation to and fro of a lamp in the cathedral of pisa. the world-famous leaning tower of this place, where he was born, served as a physical laboratory from the top of which he dropped various objects, and thus was led to formulate the laws of falling bodies. he proved that aristotle was all wrong in saying that a heavy body must fall swifter in proportion to its weight than a lighter one. these and other discoveries rendered him unpopular with his associates, who christened him the "wrangler." the new system of copernicus appealed to him; and when he, first of all men, turned a telescope on the heavenly bodies, there was venus with phases like those of the moon, and jupiter with satellites traveling about it--a copernican system in miniature. nothing could have happened that would have provided a better demonstration of the truth of the new system and the falsity of the old. his marvelous discoveries caused the greatest excitement--consternation even, among the anti-copernicans. galileo published the "sidereus nuncius," with many observations and drawings of the moon, which he showed to be a body not wholly dissimilar to the earth: this, too, was obviously of great moment in corroboration of the copernican order and in contradiction to the ptolemaic, which maintained sharp lines of demarcation between things terrestrial and things celestial. his telescopes, small as they were, revealed to him anomalous appearances on both sides of the planet saturn which he called _ansæ_, or handles. but their subsequent disappearance was unaccountable to him, and later observers, who kept on guessing ineffectively till huygens, nearly a half century after, showed that the true nature of the appendage was a ring. spots on the sun were frequently observed by galileo and led to bitter controversies. he proved, however, that they were objects on the sun itself, not outside it, and by noticing their repeated transits across the sun's disk, he showed that the sun turned round on his axis in a little less than a month--another analogy to the like motion of the earth on the copernican plan. galileo's appointment in as "first philosopher and mathematician" to the grand duke of tuscany gave him abundant time for the pursuit of original investigations and the preparation of books and pamphlets. his first visit to rome the year following was the occasion of a reception with great honor by many cardinals and others of high rank. his lack of sympathy with others whose views differed from his, and his naturally controversial spirit, had begun to lead him headlong into controversies with the jesuits and the church, which culminated in his censure by the authorities of the church and persecution by the inquisition. in three comets appeared, and galileo was again in controversial hot water with the jesuits. but it led to the publication five years later of "il saggiatore" (the assayer), of no great scientific value, but only a brilliant bit of controversial literature dedicated to the newly elevated pope, urban viii. later he wrote through several years a great treatise, more or less controversial in character, entitled a "dialogue on the two chief systems of the world" between three speakers, and extending through four successive days. simplicio argues for the aristotelians, salviati for the copernicans, while sagredo does his best to be neutral. it will always be a very readable book, and we are fortunate to have a recent translation by professor crew of evanston. here we find the first suggestion of the modern method of getting stellar parallaxes, the relative parallax, that is, of two stars in the same field--a method not put into service till bessel's time, two centuries later. but the most important chapters of the "dialogue" deal with galileo's investigations of the laws of motion of bodies in general, which he applied to the problem of the earth's motion. in this he really anticipated newton in the first of his three laws of motion, and in a subsequent work, dealing with the theory of projectiles, he reaches substantially the results of newton's second law of motion, although he gave no general statement of the principle. nevertheless, in the epoch where his life was lived and his work done, his telescopic discoveries, combined with his dynamic researches in untrodden fields, resulted in the complete and final overthrow of the ancient system of error, and the secure establishment of the copernican system beyond further question and discussion. only then could the science of astronomy proceed unhampered to the fullest development by the master minds of succeeding centuries. chapter xii after the great masters following kepler and galileo was a half century of great astronomical progress along many lines laid out by the work of the great masters. the telescope seemed only a toy, but its improvement in size and quality showed almost inconceivable possibilities of celestial discoveries. hevelius of danzig took up the study of the moon, and his "selenographia" was finely illustrated by plates which he not only drew but engraved himself. lunar names of mountains, plains, and craters we owe very largely to him. also he published among other works two on comets, the second of which was published in and called the "cometographia," the first detailed account of all the comets observed and recorded to date. many were the telescopes turned on the planet saturn, and every variety of guess was made as to the actual shape and physical nature of the weird appendages discovered by galileo. the true solution was finally reached by huygens, whose mechanical genius had enabled him to grind and polish larger and better lenses than his contemporaries; in he published the "systema saturnium" interpreting the ring and the cause of its various configurations, and the first discovery of a saturnian satellite is due to him. gascoigne in england about was the first to make the important application of the micrometer to enhance the accuracy of measurement of small angles in the telescopic field; an invention made and applied independently many years later by huygens in holland and auzout and picard in france, where the instrument was first regularly employed as an accessory in the work of an observatory. another englishman, jeremiah horrocks, was the first observer of a transit of venus over the disk of the sun, in . horrocks was possessed of great ability in calculational astronomy also. this was about the time of the invention of the pendulum clock by huygens, which in conjunction with the later invention of the transit instrument by roemer wrought a revolution in the exacting art of practical astronomy. this was because it enabled the time to be carried along continuously, and the revolution of the earth could be utilized in making precise measures of the position of sun, moon, and stars. louis xiv had just founded the new observatory at paris in , and picard was the first to establish regular time-observations there. huygens followed up the motion of the pendulum in theory as well as practice in his "horologium oscillatorium" ( ), showing the way to measure the force of gravity, and his study of circular motion showed the fundamental necessity of some force directed toward the center in planetary motions. the doctrine of the sphericity of the earth being no longer in doubt, the great advance in accuracy of astronomical observation indicated to willebrord snell in holland the best way to measure an arc of meridian by triangulation. picard repeated the measurements near paris with even greater accuracy, and his results were of the utmost significance to newton in establishing his law of gravitation. domenico cassini, an industrious observer, voluminous writer, and a strong personality, devised telescopes of great size, discovered four saturnian satellites and the main division in the ring of saturn, determined the rotation periods of mars and jupiter, and prepared tables of the eclipses of jupiter's satellites. at his suggestion richer undertook an expedition to cayenne in latitude degrees north, where it was found that the intensity of gravity was less than at paris, and his clock therefore lost time, thus indicating that the earth was not a perfect sphere as had been thought, but a spheroid instead. the planet mars passed a near opposition, and richer's observations of it from cayenne, when combined with those of cassini and others in france, gave a new value of the sun's parallax and distance, really the first actual measurement worth the name in the history of astronomy. to close this era of signal advance in astronomy we may cite a discovery by roemer of the first order: no less than that of the velocity of transmission of light through space. at the instigation of picard, roemer in studying the motions of jupiter's satellites found that the intervals between eclipses grew less and less as jupiter and the earth approached each other, and greater and greater than the average as the two planets separated farther and farther. roemer correctly attributed this difference to the progressive motion of light and a rough value of its velocity was calculated, though not accepted by astronomers generally for more than a century. why the laws of kepler should be true, kepler himself was unable to say. nor could anyone else in that day answer these questions: ( ) the planets move in orbits that are elliptical not circular--why should they move in an imperfect curve, rather than the perfect one in which it had always been taught that they moved? ( ) why should our planet vary its velocity at all, and travel now fast, now slow; especially why should the speed so vary that the line of varying length, joining the planet to the sun, always passes over areas proportional to the time of describing them? and ( ) why should there be any definite relation of the distances of planets from the sun to their times of revolution about him? why should it be exactly as the cube of one to the square of the other? we must remember that the copernican system itself was not yet, in the beginning of the seventeenth century, accepted universally; and the great minds of that period were most concerned in overturning the erroneous theory of ptolemy. the next step in logical order was to find a basic explanation of the planetary motions, and descartes and his theory of vortices are worthy of mention, among many unsuccessful attempts in this direction. descartes was a brilliant french philosopher and mathematician, but his hypothesis of a multitude of whirlpools in the ether, while ingenious in theory, was too vague and indefinite to account for the planetary motions with any approach to the precision with which the laws of kepler represented them. another great astronomer whose labors helped immensely in preparing the way for the signal discoveries that were soon to come was huygens, a man of versatility as natural philosopher, mechanician, and astronomical observer. huygens was born thirteen years before the death of galileo, and to the discovery of the laws of motion by the latter huygens added researches on the laws of action of centrifugal forces. neither of them, however, appeared to see the immediate bearing on the great general problem of celestial motions in its true light, and it was reserved for another generation, and an astronomer of another country, to make the one fundamental discovery that should explain the whole by a single simple law. chapter xiii newton and motion "how is it that you are able to make these great discoveries?" was once asked of sir isaac newton, _facile princeps_ of all philosophers, and the discoverer of the great law of universal gravitation. "by perpetually thinking about them," was newton's terse and illuminating reply. he had set for himself the definite problem of kepler's laws: why is it that they are true, and is there not some single, general law that will embody all the circumstances of the planetary motions? newton was born in , the year after the death of galileo. he had a thorough training in the mathematics of his day, and addressed himself first to an investigation and definite formulation of the general laws of motion, which he found to be three in number, and which he was able to put in very simple terms. the first one is: any body, once it is set in motion, will continue to move forward in a straight line with a uniform velocity forever, provided it is acted upon by no force whatever. in other words, a state of motion is as natural as a state of rest (rest in relation to things everywhere adjacent) in which we find all things in general. here on earth where gravity itself pulls all objects downward toward the earth, and where resistance of the air tends to hold a moving body back and bring it to rest, and where friction from contact with whatever material substance may be in its path is perpetually tending to neutralize all motion--with all three of these forces always at work to stop a moving body, the truth of this first and fundamental law of motion was not apparent on the surface. till galileo's time everyone had made the mistake of supposing that some force or other must be acting continually on every moving body to keep it in motion. ptolemy, copernicus, kepler, leonardo da vinci--all failed to see the truth of this law which newton developed in the immortal _principia_. and at the present day it is not always easy to accept at first, although the progress of mechanical science, by reducing friction and resistance, has produced machines in which motion of large masses may be kept up indefinitely with the application of only the merest minimum of force. once a planet is set in motion round the sun, it would go on forever through frictionless, non-resistant space; but there must be a central force, as huygens saw clearly, to hold it in its orbit. otherwise it would at any moment take the direction of a tangent to the orbit. here is where newton's second law of motion comes in, and he formulated it with great definiteness. when any force acts on a moving body, its deviation from a straight line will be in the direction of the force applied and proportional to that force. in accord with this law, newton first began to inquire whether the force of attraction here on earth, which everyone commonly recognizes as gravity, drawing all things down toward the center of the earth, might not extend upward indefinitely. it is found in operation on the summits of mountain peaks, and the clouds above them and the rain falling from them are obviously drawn downward by the same force. may it not extend outward into space, even as far as the moon? this was an audacious question, but newton not only asked, but tried to answer it in the year , when he was only twenty-three. on the surface of the earth this attraction is strong enough to draw a falling body downward through a vertical space of sixteen feet in a second of time. what ought it to be at the distance of the moon. the distance of the moon in newton's time was better known in terms of the earth's size than was the size of the earth itself: the earth's radius was known to be one-sixtieth of the moon's distance, but the earth's diameter was thought to be something under , miles, so that newton's first calculations were most disappointing, and he laid them aside for nearly twenty years. meanwhile the french astronomers led by picard had measured the earth anew, and showed it to be nearly , miles in diameter. as soon as newton learned of this, he revised his calculations, and found that by the law of the inverse square the moon, in one second, should fall away from a tangent to its orbit one thirty-six hundredth of sixteen feet. this accorded exactly with his original supposition that the earth's attraction extended to the moon. so he concluded that the force which makes a stone fall, or an apple, as the story goes, is the same force that holds the moon in its orbit, and that this force diminishes in the exact proportion that the square of the distance from the earth's center increases. the moon, indeed, becomes a falling body; only, as kingdon clifford puts it: "she is going so fast and is so far off that she falls quite around to the other side of the earth, instead of hitting it; and so goes on forever." [illustration: nicholas copernicus] [illustration: galileo galilei] [illustration: johann kepler] [illustration: sir isaac newton] newton goes on in the _principia_ to explain the extension of gravitation to the other bodies of the solar system beyond the earth and moon. clearly the same gravitation that holds the moon in its orbit round the earth, must extend outward from the sun also, and hold all the planets in their orbits centered about him. newton demonstrates by calculation based on kepler's third law that ( ) the forces drawing the planets toward the sun are inversely as the squares of their mean distances from him; and ( ) if the force be constantly directed toward the sun, the radius vector in an elliptic orbit must pass over equal areas in equal times. chapter xiv newton and gravitation so all of kepler's laws could be embodied in a single law of gravitation toward a central body, whose force of attraction decreases outward in exact proportion as the square of the distance increases. only one farther step had to be taken, and this the most complicated of all: he must make all the bodies of the sky conform to his third law of motion. this is: action and reaction are equal, or the mutual actions of any two bodies are always equal and oppositely directed. there must be mutual attractions everywhere: earth for sun as well as sun for earth, moon for sun and sun for moon, earth for venus and venus for earth, jupiter for saturn and saturn for jupiter, and so on. the motions of the planets in the undisturbed ellipses of kepler must be impossible. as observations of the planets became more accurate, it was found that they really did fail to move in exact accord with kepler's laws unmodified. newton was unable, with the imperfect processes of the mathematics of his day to ascertain whether the deviations then known could be accounted for by his law of gravitation; but he nevertheless formulated the law with entire precision, as follows: every particle of matter in the universe attracts every other particle with a force exactly proportioned to the product of their masses, and inversely as the square of the distance between their centers. the centuries of astronomical research since newton's day, however, have verified the great law with the utmost exactness. practically every irregularity of lunar and planetary motion is accounted for; indeed, the intricacies of the problems involved, and the nicety of their solution, have led to the invention of new mathematical processes adequate to the difficulties encountered. and about the middle of the last century, when uranus departed from the path laid out for it by the mathematical astronomers, its orbital deviations were made the basis of an investigation which soon led to the assignment of the position where a great planet could be found that would account for the unexplained irregularities of the motion of uranus. and the immediate discovery of this planet, neptune, became the most striking verification of the newtonian law that the solar system could possibly afford. the astronomers of still later days investigating the statelier motions of stellar systems find the newtonian law regnant everywhere among the stars where our most powerful telescopes have as yet reached. so that newton's law is known as the law of universal gravitation, and its author is everywhere held as the greatest scientist of the ages. newton's _principia_ may be regarded as the culminating research of the inductive method, and further outline of its contents is desirable. it is divided into three books following certain introductory sections. the first book treats of the problems of moving bodies, the solutions being worked out generally and not with special reference to astronomy. the second book deals with the motion of bodies through resistant media, as fluids, and has very little significance in astronomy. the third book is the all important one, and applies his general principles to the case of the actual solar system, providing a full explanation of the motions of all the bodies of the system known in his day. anyone who critically reads the _principia_ of newton will be forced to conclude that its author was a genius in the highest sense of the word. the elegance and thoroughness of the demonstrations, and the completeness of application of the law of gravitation are especially impressive. the universality of his new law was the feature to which he gave particular attention. it was clear to him that the gravitation of a planet, although it acted as if wholly concentrated at the center, was nevertheless resident in every one of the particles of which the planet is composed. indeed, his universal law was so formulated as to make every particle attract every other particle; and an investigation known as the cavendish experiment--a research of great delicacy of manipulation--not only proves this, but leads also to a measurement of the earth's mean density, from which we can calculate approximately how much the earth actually weighs. another way to attack the same problem is by measuring the attraction of mountains, as maskelyne, astronomer royal of scotland did on mount schehallien in scotland, which was selected because of its sheer isolation. the attraction of the mountain deflected the plumb-lines by measurable amounts, the volume of the mountain was carefully ascertained by surveys, and geologists found out what rocks composed it. so the weight of the entire mountain became pretty well known, and combining this with the observed deflection, an independent value of the earth's weight was found. still other methods have been applied to this question, and as an average it is found that the materials composing the earth are about five and a half times as heavy as water, and the total weight of the earth is something like six sextillions of tons. what is the true shape of the earth? and does the earth's turning round on its axis affect this shape? newton saw the answer to these questions in his law of gravitation. a spherical figure followed as a matter of course from the mutual attraction of all materials composing the earth, providing it was at rest, or did not turn round on its axis. but rotation bulges it at the equator and draws it in at the poles, by an amount which calculation shows to be in exact agreement with the amount ascertained by actual measurement of the earth itself. another curious effect, not at first apparent, was that all bodies carried from high latitudes toward the equator would get lighter and lighter, in consequence of the centrifugal force of rotation. this was unexpectedly demonstrated by richer when the french academy sent him south to observe mars in . his clock had been regulated exactly in paris, and he soon found that it lost time when set up at cayenne. the amount of loss was found by observation, and it was exactly equal to the calculated effect that the reduction of gravity by centrifugal action should produce. also newton saw that his law of gravitation would afford an explanation of the rise and fall of the tides. the water on the side of the earth toward the moon, being nearer to the moon, would be more strongly attracted toward it, and therefore raised in a tide. and the water on the farther side of the earth away from the moon, being at a greater distance than the earth itself, the moon would attract the earth more strongly than this mass of water, tending therefore to draw the earth away from the water, and so raising at the same time a high tide on the side of the earth away from the moon. as the earth turns round on its axis, therefore, two tidal waves continually follow each other at intervals of about twelve hours. the sun, too, joins its gravitating force with that of the moon, raising tides nearly half as high as those which the moon produces, because the sun's vaster mass makes up in large part for its much greater distance. at first and third quarters of the moon, the sun acts against the moon, and the difference of their tide-producing forces gives us "neap tides"; while at new moon and full, sun and moon act together, and produce the maximum effect known as "spring tides." newton passed on to explain, by the action of gravitation also, the precession of the equinoxes, a phenomenon of the sky discovered by hipparchus, who pretty well ascertained its amount, although no reason for it had ever been assigned. the plane of the earth's equator extended to the celestial sphere marks out the celestial equator, and the two opposite points where it intersects the plane of the ecliptic, or the earth's path round the sun, are called the equinoctial points, or simply the equinoxes. and precession of the equinoxes is the motion of these points westward or backward, about seconds each year, so that a complete revolution round the ecliptic would take place in about , years. newton saw clearly how to explain this: it is simply due to the attraction of the sun's gravitation upon the protuberant bulge around the earth's equator, acting in conjunction with the earth's rotation on its axis, the effect being very similar to that often seen in a spinning top, or in a gyroscope. the moon moving near the ecliptic produces a precessional effect, as also do the planets to a very slight degree; and the observed value of precession is the same as that calculated from gravitation, to a high degree of precision. newton died in , too early to have witnessed that complete and triumphant verification of his law which ultimately has accounted for practically every inequality in the planetary motions caused by their mutual attractions. the problems involved are far beyond the complexity of those which the mathematical astronomer has to deal with, and the mathematicians of france deserve the highest credit for improving the processes of their science so that obstacles which appeared insuperable were one after another overcome. newton's method of dealing with these problems was mainly geometric, and the insufficiency of this method was apparent. only when the french mathematicians began to apply the higher methods of algebra was progress toward the ultimate goal assured. d'alembert and clairaut for a time were foremost in these researches, but their places were soon taken by lagrange, who wrote the "mécanique analytique," and laplace, whose "mécanique céleste" is the most celebrated work of all. in large part these works are the basis of the researches of subsequent mathematical astronomers who, strictly speaking, cannot as yet be said to have arrived at a complete and rigorous solution of all the problems which the mutual attractions of all the bodies of the solar system have originated. it may well be that even the mathematics of the present day are incompetent to this purpose. when the brilliant genius of sir william hamilton invented quaternion analysis and showed the marvelous facility with which it solved the intricate problems of physics, there was the expectation that its application to the higher problems of mathematical astronomy might effect still greater advances; but nothing in that direction has so far eventuated. some astronomers look for the invention of new functions with numerical tables bearing perhaps somewhat the relation to present tables of logarithms, sines, tangents, and so on, that these tables do to the simple multiplication table of pythagoras. chapter xv after newton we have said that practically all the motions in the solar system have been accounted for by the newtonian law of gravitation. it will be of interest to inquire into the instances that lead to qualification of this absolute statement. one relates to the planet mercury, whose orbit or path round the sun is the most elliptical of all the planetary orbits. this will be explained a little later. the moon has given the mathematical astronomers more trouble than any other of the celestial bodies, for one reason because it is nearest to us and very minute deviations in its motion are therefore detectible. halley it was who ascertained two centuries ago that the moon's motion round the earth was not uniform, but subject to a slight acceleration which greatly puzzled lagrange and laplace, because they had proved exactly this sort of thing to be impossible, unless indeed the body in question should be acted on by some other force than gravitation. but laplace finally traced the cause to the secular or very slow reduction in the eccentricity of the earth's own orbit. the sun's action on the moon was indeed progressively changing from century to century in such manner as to accelerate the moon's own motion in its orbit round the earth. adams, the eminent english astronomer, revised the calculations of laplace, and found the effect in question only half as great as laplace had done; and for years a great mathematical battle was on between the greatest of astronomical experts in this field of research. adams, in conjunction with delaunay, the greatest of the french mathematicians a half century ago, won the battle in so far as the mathematical calculations were concerned; but the moon continues to the present day her slight and perplexing deviation, as if perhaps our standard time-keeper, the earth, by its rotation round its axis, were itself subject to variation. although many investigations have been made of the uniformity of the earth's rotation, no such irregularity has been detected, and this unexplained variation of the moon's motion is one of the unsolved problems of the gravitational astronomer of to-day. but we are passing over the most impressive of all the earlier researches of lagrange and laplace, which concerned the exceedingly slow changes, technically called the secular variations of the elements of the planetary orbits. these elements are geometrical relations which indicate the form of the orbit, the size of the orbit, and its position in space; and it was found that none of these relations or quantities are constant in amount or direction, but that all, with but one exception, are subject to very slow, or secular, change, or oscillation. this question assumed an alarming significance at an early day, particularly as it affected the eccentricity of the earth's orbit round the sun. should it be possible for this element to go on increasing for indefinite ages, clearly the earth's orbit would become more and more elliptical, and the sun would come nearer and nearer at perihelion, and the earth would drift farther and farther from the sun at aphelion, until the extremes of temperature would bring all forms of life on the earth to an end. the refined and powerful analysis of lagrange, however, soon allayed the fears of humanity by accounting for these slow progressive changes as merely part of the regular system of mere oscillations, in entire accord with the operation of the law of gravitation; and extending throughout the entire planetary system. indeed, the periods of these oscillations were so vast that none of them were shorter than , years, while they ranged up to two million years in length--"great clocks of eternity which beat ages as ours beat seconds." about a century ago, an eminent lecturer on astronomy told his audience that the problem of weighing the planets might readily be one that would seem wholly impossible to solve. to measure their sizes and distances might well be done, but actually to ascertain how many tons they weigh--never! yet if a planet is fortunate enough to have one satellite or more, the astronomer's method of weighing the planet is exceedingly simple; and all the major planets have satellites except the two interior ones, mercury and venus. as the satellite travels round its primary, just as the moon does round the earth, two elements of its orbit need to be ascertained, and only two. first, the mean distance of the satellite from its primary, and second the time of revolution round it. now it is simply a case of applying kepler's third law. first take the cube of the satellite's distance and divide it by the square of the time of revolution. similarly take the cube of the planet's distance from the sun and divide by the square of the planet's time of revolution round him. the proportion, then, of the first quotient to the second shows the relation of the mass (that is the weight) of the planet to that of the sun. in the case of jupiter, we should find it to be , , in that of saturn , , and so on. the range of planetary masses, in fact, is very curious, and is doubtless of much significance in the cosmogony, with which we deal later. if we consider the sun and his eight planets, the mass or weight of each of the nine bodies far exceeds the combined mass of all the others which are lighter than itself. to illustrate: suppose we take as our unit of weight the one-billionth part of the sun's weight; then the planets in the order of their masses will be mercury, mars, venus, earth, uranus, neptune, saturn, and jupiter. according to their relative masses, then, mercury being a five-millionth part the weight of the sun will be represented by ; similarly venus, a four hundred and twenty-five thousandth part by , , and so on. then we have mercury mars ------ sum of weights of mercury and mars venus , ------ sum of weights of mercury, mars, and venus , the earth , ------ sum of weights of four inner planets , uranus , ------ sum of weights of five planets , neptune , ------- sum of weights of six planets , saturn , --------- sum of weights of seven planets , jupiter , --------- sum of weights of all the planets , , mass or weight of the sun , , , curious and interesting it is that saturn is nearly three times as heavy as the six lighter planets taken together, jupiter between two and three times heavier than all the other planets combined, while the sun's mass is times that of all the great planets of his system rolled into one. all the foregoing masses, except those of mercury and venus, are pretty accurately known because they were found by the satellite method just indicated. mercury's mass is found by its disturbing effects on encke's comet whenever it approaches very near. the mass of venus is ascertained by the perturbations in the orbital motion of the earth. in such cases the newtonian law of gravitation forms the basis of the intricate and tedious calculations necessary to find out the mass by this indirect method. its inferiority to the satellite method was strikingly shown at the observatory in washington soon after the satellites of mars were discovered in . the inaccurate mass of that planet, as previously known by months of computation based upon years and years of observation, was immediately discarded in favor of the new mass derived from the distance and period of the outer satellite by only a few minutes' calculation. in weighing the planets, astronomers always use the sun as the unit. what then is the sun's own weight? obviously the law of gravitation answers this question, if we compare the sun's attraction with the earth's at equal distances. first we conceive of the sun's mass as if all compressed into a globe the size of the earth, and calculate how far a body at the surface of this globe would fall in one second. the relation of this number to . feet, the distance a body falls in one second on the actual earth, is about , , which is therefore the number of times the sun's weight exceeds that of the earth. a word may be added regarding the force of gravitation and what it really is. as a matter of fact newton did not concern himself in the least with this inquiry, and says so very definitely. what he did was to discover the law according to which gravitation acts everywhere throughout the solar system. and although many physicists have endeavored to find out what gravitation really is, its cause is not yet known. in some manner as yet mysterious it acts instantaneously over distances great and small alike, and no substance has been found which, if we interpose it between two bodies, has in any degree the effect of interrupting their gravitational tendency toward each other. while the newtonian law of gravitation has been accepted as true because it explained and accounted for all the motions of the heavenly bodies, even including such motions of the stars as have been subjected to observation, astronomers have for a long time recognized that quite possibly the law might not be absolutely exact in a mathematical sense, and that deviations from it would surely make their appearance in time. a crude instance of this was suggested about a century ago, when the planet uranus was found to be deviating from the path marked out for it by bouvard's tables based on the newtonian law; and the theory was advocated by many astronomers that this law, while operant at the medium distances from the sun where the planets within jupiter and saturn travel, could not be expected to hold absolutely true at the vast distance of uranus and beyond. the discovery of neptune in , however, put an end to all such speculation, and has universally been regarded as an extraordinary verification of the law, as indeed it is. when, however, le verrier investigated the orbit of mercury he found an excess of motion in the perihelion point of the planet's orbit which neither he nor subsequent investigators have been able to account for by newtonian gravitation, pure and simple. if newton's theory is absolutely true, the excess motion of mercury's perihelion remains a mystery. only one theory has been advanced to account for this discrepancy, and that is the einstein theory of gravitation. this ingenious speculation was first propounded in comprehensive form nearly fifteen years ago, and its author has developed from it mathematical formulæ which appear to yield results even more precise than those based on the newtonian theory. in expressing the difference between the law of gravitation and his own conception, einstein says: "imagine the earth removed, and in its place suspended a box as big as a moon or a whole house and inside a man naturally floating in the center, there being no force whatever pulling him. imagine, further, this box being, by a rope or other contrivance, suddenly jerked to one side, which is scientifically termed 'difform motion,' as opposed to 'uniform motion.' the person would then naturally reach bottom on the opposite side. the result would consequently be the same as if he obeyed newton's law of gravitation, while, in fact, there is no gravitation exerted whatever, which proves that difform motion will in every case produce the same effects as gravitation.... the term relativity refers to time and space. according to galileo and newton, time and space were absolute entities, and the moving systems of the universe were dependent on this absolute time and space. on this conception was built the science of mechanics. the resulting formulas sufficed for all motions of a slow nature; it was found, however, that they would not conform to the rapid motions apparent in electrodynamics.... briefly the theory of special relativity discards absolute time and space, and makes them in every instance relative to moving systems. by this theory all phenomena in electrodynamics, as well as mechanics, hitherto irreducible by the old formulæ, were satisfactorily explained." natural phenomena, then, involving gravitation and inertia, as in the planetary motions, and electro-magnetic phenomena, including the motion of light, are to be regarded as interrelated, and not independent of one another. and the einstein theory would appear to have received a striking verification in both these fields. on this theory the newtonian dynamics fails when the velocities concerned are a near approach to that of light. the newtonian theory, then, is not to be considered as wrong, but in the light of a first approximation. applying the new theory to the case of the motion of mercury's perihelion, it is found to account for the excess quite exactly. on the electro-magnetic side, including also the motion of light, a total eclipse of the sun affords an especially favorable occasion for applying the critical test, whether a huge mass like the sun would or would not deflect toward itself the rays of light from stars passing close to the edge of its disk, or limb. a total eclipse of exceptional duration occurred on may , , and the two eclipse parties sent out by the royal society of london and the royal astronomical society were equipped especially with apparatus for making this test. their stations were one on the east coast of brazil and the other on the west coast of africa. accurate calculation beforehand showed just where the sun would be among the stars at the time of the eclipse; so that star plates of this region were taken in england before the expeditions went out. then, during the total eclipse, the same regions were photographed with the eclipsed sun and the corona projected against them. to make doubly sure, the stars were a third time photographed some weeks after the eclipse, when the sun had moved away from that particular region. measuring up the three sets of plates, it was found that an appreciable deflection of the light of the stars nearest alongside the sun actually exists; and the amount of it is such as to afford a fair though not absolutely exact verification of the theory. the observed deflection may of course be due to other causes, but the english astronomers generally regard the near verification as a triumph for the einstein theory. astronomers are already beginning preparations for a repetition of the eclipse programme with all possible refinement of observation, when the next total eclipse of the sun occurs, september , , visible in australia and the islands of the indian ocean. a third test of the theory is perhaps more critical than either of the others, and this necessitates a displacement of spectral lines in a gravitational field toward the red end of the spectrum; but the experts who have so far made measures for detecting such displacement disagree as to its actual existence. the work of st. john at mt. wilson is unfavorable to the theory, as is that of evershed of kodiakanal, who has made repeated tests on the spectrum of venus, as well as in the cyanogen bands of the sun. the enthusiastic advocates of the einstein theory hold that, as newton proved the three laws of kepler to be special cases of his general law, so the "universal relativity theory" will enable eventually the newtonian law to be deduced from the einstein theory. "this is the way we go on in science, as in everything else," wrote sir george airy, astronomer royal; "we have to make out that something is true; then we find out under certain circumstances that it is not quite true; and then we have to consider and find out how the departure can be explained." meanwhile, the prudent person keeps the open mind. chapter xvi halley and his comet halley is one of the most picturesque characters in all astronomical history. next to newton himself he was most intimately concerned in giving the newtonian law to the world. edmund halley was born ( ) in stirring times. charles i. had just been executed, and it was the era of cromwell's lord protectorate and the wars with spain and holland. then followed ( ) the promising but profligate charles ii. (who nevertheless founded at greenwich the greatest of all observatories when halley was nineteen), the frightful ravages of the black plague, the tyrannies of james ii., and the revolution of --all in the early manhood of halley, whose scientific life and works marched with much of the vigor of the contending personalities of state. the telescope had been invented a half century earlier, and galileo's discoveries of jupiter's moons and the phases of venus had firmly established the sun-centered theory of copernicus. the sun's distance, though, was known but crudely; and why the stars seemed to have no yearly orbits of their own corresponding to that of the earth was a puzzle. newton was well advanced toward his supreme discovery of the law of universal gravitation; and the authority of kepler taught that comets travel helter-skelter through space in straight lines past the earth, a perpetual menace to humanity. "ugly monsters," that comets always were to the ancient world, the medieval church perpetuated this misconception so vigorously that even now these harmless, gauzy visitors from interstellar space possess a certain "wizard hold upon our imagination." this entertaining phase of the subject is excellently treated in president andrew d. white's "history of the doctrine of comets," in the papers of the american historical association. halley's brilliant comet at its earlier apparitions had been no exception. halley's father was a wealthy london soap maker, who took great pride in the growing intellectuality of his son. graduating at queen's college, oxford, the latter began his astronomical labors at twenty by publishing a work on planetary orbits; and the next year he voyaged to st. helena to catalogue the stars of the southern firmament, to measure the force of terrestrial gravity, and observe a transit of mercury over the disk of the sun. while clouds seriously interfered with his observations on that lonely isle, what he saw of the transit led to his invention of "halley's method," which, as applied to the transit of venus, though not till long after his death, helped greatly in the accurate determination of the sun's distance from the earth. halley's researches on the proper motions of the stars of both hemispheres soon made him famous, and it was said of him, "if any star gets displaced on the globe, halley will presently find it out." his return to london and election to the royal society (of which he was many years secretary) added much to his fame, and he was commissioned by the society to visit danzig and arbitrate an astronomical controversy between hooke and hevelius, both his seniors by a generation. on the continent he associated with other great astronomers, especially cassini, who had already found three saturnian moons; and it was then he observed the great comet of , which led up to the most famous event of halley's life. the seerlike seneca may almost be said to have predicted the advent of halley, when he wrote ("quaestiones naturales," vii): "some day there will arise a man who will demonstrate in what region of the heavens comets pursue their way; why they travel apart from the planets; and what their sizes and constitution are. then posterity will be amazed that simple things of this sort were not explained before." to newton it appeared probable that cometary voyagers through space might have orbits of their own; and he proved that the comet of never swerved from such a path. as it could nowhere approach within the moon's orbit, clearly threats of its wrecking the earth and punishing its inhabitants ought to frighten no more. halley then became intensely interested in comets, and gathered whatever data concerning the paths of all these bodies he could find. his first great discovery was that the comets seen in by apian, and in by kepler, traveled round the sun in identical paths with one he had himself observed in . a still earlier appearance of halley's comet ( ) seems to have given rise to a popular and long-reiterated myth of a papal bull excommunicating "the devil, the turk, and the comet." no longer room for doubt: so certain was halley that all three were one and the same comet, completing the round of its orbit in about seventy-six years, that he fearlessly predicted that it would be seen again in or . and with equal confidence he might have foretold its return in and ; for all three predictions have come true to the letter. halley's span of existence did not permit his living to see even the first of these now historic verifications. but we in our day may emphatically term the epoch of the third verified return _annus halleianus_. says turner, halley's successor in the savilian chair at oxford to-day: "there can be no more complete or more sensational proof of a scientific law, than to predict events by means of it. halley was deservedly the first to perform this great service for newton's law of gravitation, and he would have rejoiced to think how conspicuous a part england was to play in the subsequent prediction of the existence of neptune." halley rose rapidly among the chief astronomical figures of his day. but he had little veneration for mere authority, and the significant veering of his religious views toward heterodoxy was for years an obstacle to his advance. still halley the astronomer was great enough to question any contemporary dicta that seemed to rest on authority alone. everyone called the stars "fixed" stars; but halley doubting this, made the first discovery of a star's individual motion--proper motion, as astronomers say. to-day, two hundred years after, every star is considered to be in motion, and astronomers are ascertaining their real motions in the celestial spaces to a nicety undreamed of by even the exacting halley. the moon, of priceless service to the early navigator, was regarded by all astronomers as endowed with an average rate of motion round the earth that did not vary from age to age. but halley questioned this too; and on comparing with the ancient value from chaldean eclipses, he made another discovery--the secular acceleration of the moon's mean motion, as it is technically termed. this was a colossal discovery in celestial dynamics; and the reason underlying it lay hidden in newton's law for yet another century, till the keener mathematics of laplace detected its true origin. * * * * * with newton, halley laid down the firm foundations of celestial mechanics, and they pushed the science as far as the mathematics of their day would permit. halley, however, was not content with elucidating the motion of bodies nearest the earth, and pressed to the utmost confines of the solar system known to him. here, too, he made a signal discovery of that mutual disturbance of the planets in their motion round the sun, called the great inequality of jupiter and saturn. halley's versatile genius attacked all the great problems of the day. his observation of the sun's total eclipse in is the earliest reliable account of such a phenomenon by a trained astronomer. he described the corona minutely and was the first to see that other interesting phenomenon which only an alert observer can detect, which a great astronomer of a later day compared to the "ignition of a fine train of gunpowder," and which has ever since borne the name of "baily's beads." besides being a great astronomer, halley was a man of affairs as well, which newton, although the greater mathematician, was not. without halley, newton's superb discovery might easily have been lost to the age and nation, for the latter was bent merely on making discoveries, and on speculative contemplation of them, with never a thought of publishing to the world. halley, more practical and businesslike, insisted on careful writing out and publication. newton was then only forty-two, and halley fully fourteen years his junior. but the philosophers of that day were keenly alive to the mystery of kepler's laws, and halley was fully conscious of the grandeur and far-reaching significance of newton's great generalization which embodied all three of kepler's laws in one. newton at last yielded, though reluctantly, and the "principia" was given to the world, though wholly at halley's private charges. but halley was far from being completely engrossed with the absorbing problems of the sky; things terrestrial held for years his undivided attention. imagine present-day lords commissioners of the admiralty intrusting a ship of the british navy to civilian command. yet such was their confidence in halley that he was commissioned as captain of h. m.'s pink _paramour_ in , with instructions to proceed to southern seas for geographical discoveries, and for improving knowledge of the longitude problem, and of the variations of the compass. trade winds and monsoons, charts of magnetic variation, tides and surveys of the channel coast, and experiments with diving bells were practical activities that occupied his attention. halley in became astronomer royal. he was the second incumbent of this great office, but the first to supply the royal observatory with instruments of its own, some of which adorn its walls even to-day. his long series of lunar observations and his magnetic researches were of immense practical value in navigation. halley lived to a ripe old age and left the world vastly better than he found it. his rise from humblest obscurity was most remarkable, and he lived to gratify all the ambitions of his early manhood. "of attractive appearance, pleasing manners, and ready wit," says one of his biographers, "loyal, generous, and free from self-seeking, he was one of the most personally engaging men who ever held the office of astronomer royal." he died in office at greenwich in . "halley was buried," says chambers, "in the churchyard of st. margaret's, lee, not far from greenwich, and it has lately been announced that the admiralty have decided to repair his tomb at the public expense, no descendants of his being known." there is no suitable monument in england to the memory of one of her greatest scientific men. in any event the collection and republication of his epoch-making papers would be welcomed by astronomers of every nation. chapter xvii bradley and aberration living at kew in london early in the th century was an enthusiastic young astronomer, james bradley. he is famous chiefly for his accurate observations of star places which have been invaluable to astronomers of later epochs in ascertaining the proper motions of stars. the latitude of bradley's house in kew was very nearly the same as the declination of the bright star gamma draconis, so that it passed through his zenith once every day. bradley had a zenith sector, and with this he observed with the greatest care the zenith distance of gamma draconis at every possible opportunity. this he did by pointing the telescope on the star and then recording the small angle of its inclination to a fine plumb line. so accurate were his measures that he was probably certain of the star's position to the nearest second of arc. what he hoped to find was the star's motion round a very slight orbit once each year, and due to the earth's motion in its orbit round the sun. in other words, he sought to find the star's parallax if it turned out to be a measurable quantity. it is just as well now that his method of observation proved insufficiently delicate to reveal the parallax of gamma draconis; but his assiduity in observation led him to an unexpected discovery of greater moment at that time. what he really found was that the star had a regular annual orbit; but wholly different from what he expected, and very much larger in amount. this result was most puzzling to bradley. the law of relative motion would require that the star's motion in its expected orbit should be opposite to that of the earth in its annual orbit; instead of which the star was all the time at right angles to the earth's motion. bradley was a frequent traveler by boat on the thames, and the apparent change in the direction of the wind when the boat was in motion is said to have suggested to him what caused the displacement of gamma draconis. the progressive motion of light had been roughly ascertained by roemer: let that be the velocity of the wind. and the earth's motion in its orbit round the sun, let that be the speed of the boat. then as the wind (to an observer on the moving boat) always seems to come from a point in advance of the point it actually proceeds from (to an observer at rest), so the star should be constantly thrown forward by an angle given by the relation of the velocity of light to the speed of the earth in orbital revolution round the sun. the apparent places of all stars are affected in this manner, and this displacement is called the aberration of light. astronomers since bradley's discovery of aberration in have devoted a great deal of attention to this astronomical constant, as it is called, and the arc value of it is very nearly ". . this means that light travels more than ten thousand times as fast as the earth in its orbit ( , miles per second as against the earth's . ). and we can ascertain the sun's distance by aberration also because the exact values of the velocity of light and of the constant of aberration when properly combined give the exact orbital speed of the earth; and this furnishes directly by geometry the radius of the earth's orbit, that is the distance of the sun. in fact, this is one of the more accurate modern methods of ascertaining the distance of the sun. as early as it enabled the writer to calculate the sun's parallax equal to ". , a value absolutely identical with that adopted by the paris conference of , and now universally accepted as the standard. in whatever part of the sky we observe, every star is affected by aberration. at the poles of the ecliptic, - / degrees from the earth's poles, the annual aberration orbits of the stars are very small circles, " in diameter. toward the ecliptic the aberration orbits become more and more oval, ellipses in fact of greater and greater eccentricity, but with their major axes all of the same length, until we reach the ecliptic itself; and then the ellipse is flattened into a straight line " in length, in which the star travels forth and back once a year. exact correspondence of the aberration ellipses of the stars with the annual motion of the earth round the sun affords indisputable proof of this motion, and as every star partakes of the movement, this proof of our motion round the sun becomes many million-fold. indeed, if we were to push a little farther the refinement of our analysis of the effect of aberration on stellar positions, we could prove also the rotation of the earth on its axis, because that motion is swift enough to bear an appreciable ratio to the velocity of light. diurnal aberration is the term applied to this slight effect, and as every star partakes of it, demonstration of the earth's turning round on its axis becomes many million-fold also. chapter xviii the telescope had anyone told ptolemy that his earth-centered system of sun, moon, and stars would ultimately be overthrown, not by philosophy but by the overwhelming evidence furnished by a little optical instrument which so aided the human eye that it could actually see systems of bodies in revolution round each other in the sky, he would no doubt have vehemently denied that any such thing was possible. to be sure, it took fourteen centuries to bring this about, and the discovery even then was without much doubt due to accident. through all this long period when astronomy may be said to have merely existed, practically without any forward step or development, its devotees were unequipped with the sort of instruments which were requisite to make the advance possible. there were astrolabes and armillary spheres, with crudely divided circles, and the excellent work done with them only shows the genius of many of the early astronomers who had nothing better to work with. regarding star-places made with instruments fixed in the meridian, bessel, often called the father of practical astronomy, used to say that, even if you provided a bad observer with the best of instruments, a genius could surpass him with a gun barrel and a cart wheel. before the days of telescopes, that is, prior to the seventeenth century, it was not known whether any of the planets except the earth had a moon or not; consequently the masses of these planets were but very imperfectly ascertained; the phases of mercury and venus were merely conjectured; what were the actual dimensions of the planets could only be guessed at; the approximate distances of sun, moon, and planets were little better than guesses; the distances of the stars were wildly inaccurate; and the positions of the stars on the celestial sphere, and of sun, moon, and planets among them were far removed from modern standards of precision--all because the telescope had not yet become available as an optical adjunct to increase the power of the human eye and enable it to see as if distances were in considerable measure annihilated. galileo almost universally is said to have been the inventor of the telescope, but intimate research into the question would appear to give the honor of that original invention to another, in another country. what galileo deserves the highest praise for, however, is the reinvention independently of an "optick tube" by which he could bring distant objects apparently much nearer to him; and being an astronomer, he was by universal acknowledgment first of all men to turn a telescope on the heavenly bodies. this was in the year , and his first discovery was the phase of venus, his second the four medicean moons or satellites of jupiter, discoveries which at that epoch were of the highest significance in establishing the truth of the copernican system beyond the shadow of doubt. but the first telescopes of which we have record were made, so far as can now be ascertained, in holland very early in the th century. metius, a professor of mathematics, and jansen and lipperhey, who were opticians in middelburg--all three are entitled to consideration as claimants of the original invention of the telescope. but that such an instrument was pretty well known would appear to be shown by his government's refusal of a patent to lipperhey in ; while the officials recognizing the value of such an instrument for purposes of war, got him to construct several telescopes and ordered him to keep the invention a secret. within a year galileo heard that an instrument was in use in holland by which it was possible to see distant objects as if near at hand. skilled in optics as he was, the reinvention was a task neither long nor difficult for him. one of his first instruments magnified but three times; still it made a great sensation in venice where he exhibited the little tube to the authorities of that city, in which he first invented it. galileo's telescope was of the simplest type, with but two lenses; the one a double convex lens with which an image of the distant object is formed, the other a double concave lens, much smaller which was the eye-lens for examining the image. it is this simple form of galilean telescope that is still used in opera glasses and field glasses, because of the shorter tube necessary. galileo carried on the construction of telescopes, all the time improving their quality and enlarging their power until he built one that magnified thirty times. what the diameter of the object glass was we do not know, perhaps two inches or possibly a little more. glass of a quality good enough to make a telescope of cannot have been abundant or even obtainable except with great difficulty in those early days. other discoveries by this first of celestial observers were the spots on the sun, the larger mountains of the moon, the separate stars of which the milky way is composed, and, greatest wonder of all, the anomalous "handles" (_ansæ_, he called them) of saturn, which we now know as the planet's ring, the most wonderful of all the bodies in the sky. since galileo's time, only three centuries past, the progress in size and improvement in quality of the telescope have been marvelous. and this advance would not have been possible except for, first, the discoveries still kept in large part secret by the makers of optical glass which have enabled them to make disks of the largest size; second, the consummate skill of modern opticians in fashioning these disks into perfect lenses; and third, the progress in the mechanical arts and engineering, by which telescope tubes of many tons' weight are mounted or poised so delicately that the thrust of a finger readily swerves them from one point of the heavens to another. as the telescope is the most important of all astronomical instruments, it is necessary to understand its construction and adjustment and how the astronomer uses it. telescopes are optical instruments, and nothing but optical parts would be requisite in making them, if only the optical conditions of their perfect working could be obtained without other mechanical accessories. in original principle, all telescopes are as simple as galileo's; first, an object glass to form the image of the distant object; second the eyepiece usually made of two lenses, but really a microscope, to magnify that image, and working in the same way that any microscope magnifies an object close at hand; and third, a tube to hold all the necessary lenses in the true relative positions. [illustration: the -inch hooker telescope, largest reflector in the world, on mt. wilson. (_photo, mt. wilson solar observatory._)] [illustration: the largest refractor, the -inch telescope at yerkes observatory. dome ft. in diameter. (_photo, yerkes observatory._)] [illustration: the -ft. tower at the mt. wilson solar observatory. at the left is a diagram of tower, telescope and pit. at the upper right is an exterior view of the tower; below a view looking down into the pit, ft. deep. (_photo, mt. wilson solar observatory._)] the focal lengths of object glass and eyepiece will determine just what distance apart the lenses must be in order to give perfect vision. but it is quite as important that the axes of all the lenses be adjusted into one and the same straight line, and then held there rigidly and permanently. otherwise vision with the telescope will be very imperfect and wholly unsatisfactory. the distance from the objective, or object glass to its focal point is called its focal length; and if we divide this by the focal length of the eyepiece, we shall have the magnifying power of the telescope. the eyepiece will usually be made of two lenses, or more, and we use its focal length considered as a single lens, in getting the magnifying power. a telescope will generally have many eyepieces of different focal lengths, so that it will have a corresponding range of magnifying powers. the lowest magnifying power will be not less than four or five diameters for each inch of aperture of the objective; otherwise the eye will fail to receive all the light which falls upon the glass. a -inch telescope will therefore have no eyepiece with a lower magnifying power than about diameters. the highest magnifying power advantageous for a glass of this size will be about to , the working rule being about diameters to each inch of aperture, although the theoretical limit is regarded as . the reason for a variety of eyepieces with different magnifying powers soon becomes apparent on using the telescope. comets and nebulæ call for very low powers, while double stars and the planetary surfaces require the higher powers, provided the state of the atmosphere at the moment will allow it. if there is much quivering and unsteadiness, nothing is gained by trying the higher powers, because all the waves of unsteadiness are magnified also in the same proportion, and sharpness of vision, or fine definition, or "good seeing," as it is called, becomes impossible. the vibrations and tremors of the atmosphere are the greatest of all obstacles to astronomical observation, and the search is always in order for regions of the world, in deserts or on high mountains, where the quietest atmosphere is to be found. quite another power of the telescope is dependent on its objective solely: its light-gathering power. light by which we see a star or planet is admitted to the retina of the eye through an adjustable aperture called the pupil. in the dark or at night, the pupil expands to an average diameter of one-fourth of an inch. but the object-glass of a telescope, by focusing the rays from a star, pours into the eye, almost as a funnel acts with water, all the light which falls on its larger surface. and as geometry has settled it for us that areas of surfaces are proportioned to the squares of their diameters, a two-inch object glass focuses upon the retina of the eye times as much light as the unassisted eye would receive. and the great -inch objective of the yerkes telescope would, theoretically, yield , times as much light as the eye alone. but there would be a noticeable percentage of this lost through absorption by the glasses of the telescope and scattering by their surfaces. the first makers of telescopes soon encountered a most discouraging difficulty, because it seemed to them absolutely insuperable. this is known as chromatic aberration, or the scattering of light in a telescope due simply to its color or wave length. when light passes through a prism, red is refracted the least and violet the most. through a lens it is the same, because a lens may be regarded as an indefinite system of prisms. the image of a star or planet, then, formed by a single lens cannot be optically perfect; instead it will be a confused intermingling of images of various colors. with low powers this will not be very troublesome, but great indistinctness results from the use of high magnifying powers. the early makers and users of telescopes in the latter part of the seventeenth century found that the troublesome effects of chromatic aberration could be much reduced by increasing the focal length of the objective. this led to what we term engineering difficulties of a very serious nature, because the tubes of great length were very awkward in pointing toward celestial objects, especially near the zenith, where the air is quietest. and it was next to impossible to hold an object steadily in the field, even after all the troubles of getting it there had been successfully overcome. bianchini and cassini, hevelius and huygens were among the active observers of that epoch who built telescopes of extraordinary length, a hundred feet and upward. one tube is said to have been built feet in length, but quite certainly it could never have been used. so-called aerial telescopes were also constructed, in which the objective was mounted on top of a tower or a pole, and the eyepiece moved along near the ground. but it is difficult to see how anything but fleeting glimpses of the heavenly bodies could have been obtained with such contrivances, even if the lenses had been perfect. newton indeed, who was expert in optics, gave up the problem of improving the refracting telescope, and turned his energies toward the reflector. in , half a century after newton and a century and a quarter after galileo, chester more hall, an englishman, found by experiment that chromatic aberration could be nearly eliminated by making the objective of two lenses instead of one, and the same invention was made independently by dollond, an english optician, who took out letters patent about . so the size of telescopes seemed to be limited only by the skill of the glassmaker and the size of disks that he might find it practicable to produce. what hall and dollond did was to make the outer or crown lens of the objective as before, and place behind it a plano-concave lens of dense flint glass. this had the effect of neutralizing the chromatic effect, or color aberration, while at the same time only part of the refractive effect of the crown lens was destroyed. this ingenious but costly combination prepared the way for the great refracting telescopes of the present day, because it solved, or seemed to solve, the important problem of getting the necessary refraction of light rays without harmful dispersion or decomposition of them. through the th century and the first years of the th many telescopes of a size very great for that day were built, and their success seemed complete. with large increase in the size of the disks, however, a new trouble arose, quite inherent in the glass itself. the two kinds of glass, flint and crown, do not decompose white light with uniformity, so that when the so-called achromatic objective was composed of flint and crown, there was an effect known as irrationality of dispersion, or secondary spectrum, which produced a very troublesome residuum of blue light surrounding the images of bright objects. this is the most serious defect of all the great refractors of the day, and effectively it limits their size to about inches of aperture, with present types of flint and crown. it is expected by present experimenters, however, that further improvements in optical glass will do much to extend this limit; so that a refracting telescope of much greater size than any now in existence will be practicable. improvements in mounting telescopes, too, are still possible. within recent years, hartness, of springfield, vermont, has erected a new and ingenious type of turret telescope which protects the observer from wind and cold while his instrument is outside. it affords exceptional facilities for rapid and convenient observing, as for variable stars, and is adaptable to both refractors and reflectors. the captivating study of the heavens can of course be begun with the naked eye alone, but very moderate optical assistance is a great help and stimulates. an opera-glass affords such assistance; a field-glass does still better, and best of all, for certain purposes, is a modern prism-binocular. chapter xix reflectors--mirror telescopes cherished with the utmost care in the rooms of the royal society of london is a world-famous telescope, a diminutive reflector made by the hands of sir isaac newton. we have already mentioned his connection with the refractor; and how he abandoned that type of telescope in favor of the reflecting mirror, or reflector in which the obstacles to great size appeared to be purely mechanical. by many, indeed, newton is regarded as the inventor of the reflector. by the principles of optics, all the rays from a star that strike a concave mirror will be reflected to the geometric focal point, provided a section of that mirror is a parabola. such a mirror is called a speculum, and is an alloy of tin, copper, and bismuth. its surface takes a very high polish, reflecting when newly polished nearly per cent of the light that falls upon it. but the focus where the eyepiece must be used is in front of the mirror, and if the eye were placed there, the observer's head would intercept all or much of the light that would otherwise reach the mirror. gregory, probably the real inventor of the reflector, was the first to dodge this difficulty by perforating the mirror at the center and applying the eyepiece there, at the back of the speculum; but it was necessary to first send the rays to that point by reflection from a second or smaller mirror, in the optical axis of the speculum. this reflects the rays backward down the tube to the eyepiece, or spectroscope, or camera. another english optician, cassegrain, improved on this design somewhat by placing the secondary mirror inside the focus of the speculum, or nearer to it, so that the tube is shorter. this form is preferable for many kinds of astronomical work, especially photography. herschel sought to do away with the secondary reflector entirely and save the loss of light by tilting the speculum slightly, so as to throw the image at one side of the tube; but this modification introduces bad definition of the image and has never been much used. a better plan is that of newton, who placed a small plane speculum at an angle of degrees in the optical axis where the secondary mirror of the gregory-cassegrainian type is placed. the rays are then received by the eyepiece at the side of the upper end of the tube, the observer looking in at right angles to the axis. and a modern improvement first used by draper is a small rectangular prism in place of the little plane speculum, effecting a saving of five to ten per cent of the light. it is not easy to say which type of telescope, the refractor or the reflector, is the more famous. nor which is the better or more useful, or the more likely to lead in the astronomy of the future. when the successors of dollond had carried the achromatic refractor to the limit enforced by the size of the glass disks they were able to secure, they found these instruments not so great an improvement after all. the single-lens telescopes of great focal length were nearly as good optically, though much more awkward to handle. but the quality of the glass obtainable in that day appeared to set an arbitrary limit to that great amplification of size and power which progress in observational astronomy demanded. then came the elder herschel, best known and perhaps the greatest of all astronomers. at bath, england, music was his profession, especially the organ. but he was dissatisfied with his little gregorian reflector, and being a very clever mechanician he set out to build a reflector for himself. it is said that he cast and polished nearly mirrors, in the course of experiments on the most highly reflective type of alloys, and the sort of mechanism that would enable him to give them the highest polish. in all his work he was ably and enthusiastically aided by his sister, caroline herschel, most famous of all women astronomers. upward in size of his mirrors he advanced, till he had a speculum of two feet diameter with a tube feet long. twelve to fifteen years had elapsed when in , while testing one of these reflectors on stars in the constellation gemini, he made the first discovery of a planet since the invention of the telescope--the great planet now known as uranus. under the patronage of king george, he advanced to telescopes of still greater size, his largest being no less than forty feet in length, with a speculum of four feet in diameter. two new satellites of saturn were discovered with this giant reflector, which was dismantled by sir john herschel with appropriate ceremonies, including the singing of an ode by the herschel family assembled inside of the tube, on new year's eve, - . we have record of but few attempts to improve the size and definition of great reflectors by the continental astronomers during this era. in england and ireland, however, great progress was made. about lassell built a two-foot reflector, with which he discovered two new satellites of uranus, and which he subsequently set up in the island of malta. ten years later thomas grubb and son of dublin constructed a four-foot reflector, now at the observatory in melbourne, australia. calver in conjunction with common of ealing, london, about - built several large reflectors, the largest of five feet diameter, now owned by harvard college observatory; and, rather earlier, martin of paris completed a four-foot reflector. the mirrors of these latter instruments were not made of speculum metal, but of solid glass, which must be very thick (one-seventh their diameter) in order to prevent flexure or bending by their own weight. so sensitive is the optical surface to distortion that unless a complicated series of levers and counterpoises is supplied, to support the under surface of the mirror, the perfection of its optical figure disappears when the telescope is directed to objects at different altitudes in the sky. the upper or outer surface of the glass is the one which receives the optical polish on a heavy coat of silver chemically deposited on the polished glass after its figure has been tested and found satisfactory. but far and away the most famous reflecting telescope of all is the "leviathan" of lord rosse, built at birr castle, parsonstown, ireland, about the middle of the last century. his lordship made many ingenious improvements in grinding the mirror, which was of speculum metal, six feet in diameter and weighed seven tons. it was ground to a focal length of fifty-four feet and mounted between heavy walls of masonry, so that the motion of the great tube was restricted to a few degrees on both sides of the meridian. the huge mechanism was very cumbersome in operation, and photography was not available in those days; nevertheless lord rosse's telescope made the epochal discovery of the spiral nebulæ, which no other telescope of that day could have done. in america the reflector has always kept at least even pace with the refractor. as early as , mason and smith, two students at yale college, enthused by denison olmsted, built a -inch speculum with which they made unsurpassed observations of the nebulæ. dr. henry draper, returning from a visit to lord rosse, began about the construction of two silver-on-glass reflectors, one of inches diameter, the other of inches, with which he did important work for many years in photography and spectroscopy, and his mirrors are now the property of harvard college observatory. alvan clark and sons have in later years built a -inch mirror for the lowell observatory in arizona, and very recently a -foot silver-on-glass mirror has been set up in the dominion of canada astrophysical observatory at victoria, british columbia, where it is doing excellent work in the hands of plaskett, its designer. the huge glass disk for the reflector weighs two tons, and it must be cast so that there are no internal strains; otherwise it is liable to burst in fragments in the process of grinding. it should be free from air-bubbles, too; so the glass is cast in one melting, if possible. this disk was made by the st. gobain plate glass company, whose works have been ruthlessly destroyed by the enemy during the war; but fortunately the great disk had been shipped from antwerp only a week before declaration of hostilities. brashear of allegheny was intrusted with the optical parts, which occupied many months of critical work. the finished mirror is inches in diameter, its focal length is feet, and its thickness inches. a central hole inches in diameter makes possible its use as a gregorian or cassegrainian type, as well as newtonian. the mechanical parts of this great telescope are by warner and swasey of cleveland, after the well-known equatorial mounting of the melbourne reflector by grubb of dublin. friction of the polar and declination axes is reduced by ball bearings. the -foot dome has an opening feet wide and extending six feet beyond the zenith. all motions of the telescope, dome shutters, and observing platform are under complete control by electric motors. spectroscopic binaries form one of the special fields of research with this powerful instrument, and many new binaries have already been detected. the great reflectors designed and constructed by ritchey, formerly of chicago and now of pasadena, deserve especial mention. while connected with the yerkes observatory he constructed a two-foot reflector for that institution, with which he had exceptional success in photography of the stars and nebulæ. later he built a -foot reflector, now at the carnegie observatory on mount wilson, california, with which the spiral nebulæ and many other celestial objects have been especially well photographed. ritchey's later years have been spent on the construction of an even greater mirror, no less than inches in diameter, which was completed in , and has already yielded photographic results dealt with farther on, and far surpassing anything previously obtained. theoretically this huge mirror, if its surface were perfectly reflective so that it would transmit all the rays falling upon it, would gather , times as much light as the unaided eye alone. whether a -inch refractor, should it ever be constructed, would surpass the -inch reflector as an all-round engine for astronomical research, is a question that can only be fully answered by building it and trying the two instruments alongside. probably three-quarters of all the really great astronomical work in the past has been done by refractors. they are always ready and convenient for use, and the optical surfaces rarely require cleaning and readjustment. with increase of size, however, the secondary spectrum becomes very bothersome in the great lenses; and the larger they are, the more light is lost by absorption on account of the increasing thickness of the lenses. with the reflector on the other hand, while there is clearly a greater range of size, the reflective surface retains its high polish only a brief period, so that mere tarnish effectively reduces the aperture; and the great mirror is more or less ineffective in consequence of flexure uncompensated by the lever system that supports the back of the mirror. both types of telescope still have their enthusiastic devotees; and the next great reflector would doubtless be a gratifying success, if mounted in some elevated region of the world, like the andes of northern chile, where the air is exceptionally steady and the sky very clear a large part of the year. the highest magnifying powers suitable for work with such a telescope could then be employed, and new discoveries added as well as important work done in extension of lines already begun on the universe of stars. on the authority of clark, even a six-foot objective would not necessitate a combined thickness of its glasses in excess of six inches. present disks are vastly superior to the early ones in transparency, and there is reason to expect still greater improvement. the engineering troubles incident to execution of the mechanical side of the scheme need not stand in the way; they never have, indeed the astronomer has but just begun to invoke the fertile resources of the modern engineer. not long before his death the younger clark who had just finished the great lenses of the -inch yerkes telescope, ventured this prevision, already in part come true: "the new astronomy, as well as the old, demands more power. problems wait for their solution, and theories to be substantiated or disproved. the horizon of science has been greatly broadened within the last few years, but out upon the borderland i see the glimmer of new lights that await for their interpretation, and the great telescopes of the future must be their interpreters." practically all the great telescopes of the world have in turn signalized the new accession of power by some significant astronomical discovery: to specify, one of herschel's reflectors first revealed the planet uranus; lord rosse's "leviathan" the spiral nebulæ; the -inch cambridge lens the crape, or dusky ring of saturn; the - / -inch chicago refractor the companion of sirius; the washington -inch telescope the satellites of mars; the -inch pulkowa glass the nebulosities of the pleiades; and the -inch lick telescope brought to light a fifth satellite of jupiter. at the time these discoveries were made, each of these great telescopes was the only instrument then in existence with power enough to have made the discovery possible. so we may advance to still farther accessions of power with the expectation that greater discoveries will continue to gratify our confidence. chapter xx the story of the spectroscope sir isaac newton ought really to have been the inventor of the spectroscope, because he began by analyzing light in the rough with prisms, was very expert in optics, and was certainly enough of a philosopher to have laid the foundations of the science. what newton did was to admit sunlight into a darkened room through a small round aperture, then pass the rays through a glass prism and receive the band of color on a screen. he noticed the succession of colors correctly--violet, indigo, blue, green, yellow, orange, red; also that they were not pure colors, but overlapping bands of color. apparently neither he nor any other experimenter for more than a century went any further, when the next essential step was taken by wollaston about in england. he saw that by receiving the light through a narrow slit instead of a round hole, he got a purer spectrum, spectrum being the name given to the succession of colors into which the prism splits up or decomposes the original beam of white sunlight. this seemingly insignificant change, a narrow slit replacing the round hole, made wollaston and not newton the discoverer of the dark lines crossing the spectrum at various irregular intervals, and these singularly neglected lines meant the basis of a new and most important science. even wollaston, however, passed them by, and it was fraunhofer who in - first made a chart of them. consequently they are known as fraunhofer lines, or dark absorption lines. sending the beam of light through a succession of prisms gives greater dispersion and increases the power of the spectroscope. the greater the dispersion the greater the number of absorption lines; and it is the number and intensity of these lines, with their accurate position throughout the range of the spectrum which becomes the basis of spectrum analysis. the half century that saw the invention of the steam engine, photography, the railroad and the telegraph elapsed without any farther developments than mere mapping of the fundamental lines, a, b, c, d, e, f, g, h of the solar spectrum. the moon, too, was examined and its spectrum found the same, as was to be expected from sunlight simply reflected. sir john herschel and other experimenters came near guessing the significance of the dark lines, but the problem of unraveling their mystery was finally solved by bunsen and kirchhoff who ascertained that an incandescent gas emits rays of exactly the same degree of refrangibility which it absorbs when white light is passed through it. this great discovery was at once received as the secure basis of spectrum analysis, and kirchhoff in put in compact and comprehensive form the three following principles underlying the theory of the science: ( ) solid and liquid bodies, also gases under high pressure, give when incandescent a continuous spectrum, that is one with a mere succession of colors, and neither bright nor dark lines; ( ) gases under low pressure give a discontinuous spectrum, crossed by bright lines whose number and position in the spectrum differ according to the substances vaporized; ( ) when white light passes through a gas, this medium absorbs or quenches rays of identical wave-length with those composing its own bright-line spectrum. clearly then it makes no difference where the light originates whether it comes from sun or star. only it must be bright enough so that we can analyze it with the spectroscope. but our analysis of sun and star could not proceed until the chemist had vaporized in the laboratory all the elements, and charted their spectra with accuracy. when this had been done, every substance became at once recognizable by the number and position of its lines, with practical certainty. how then can we be sure of the chemical and physical composition of sun and stars? only by detailed and critical comparison of their spectra with the laboratory spectra of elements which chemical and physical research have supplied. as in the sun, so in the stars, each of which is encircled by a gaseous absorptive layer or atmosphere, the light rays from the self-luminous inner sphere must pass through this reversing layer, which absorbs light of exactly the same wave-length as the lines that make up its own bright line spectrum. whatever substances are here found in gaseous condition, the same will be evident by dark lines in the spectrum of sun or star, and the position of these dark lines will show, by coincidence with the position of the laboratory bright lines, all the substances that are vaporized in the atmospheres of the self-luminous bodies of the sky. here then originated the science of the new astronomy: the old astronomy had concerned itself mainly with positions of the heavenly bodies, _where_ they are; the new astronomy deals with their chemical composition and physical constitution, and _what_ they are. between and the fundamental application of the basic principles was well advanced by the researches of sir william huggins in england, of father angelo secchi in rome, of jules janssen in paris, and of dr. henry draper in new york. in analyzing the spectrum of the sun, many thousands of dark absorption lines are found, and their coincidences with the bright lines of terrestrial elements show that iron, for instance, is most prominently identified, with rather more than , coincidences of bright and dark lines. calcium, too, is indicated by peculiar intensity of its lines, as well as their great number. next in order are hydrogen, nickel and sodium. by prolonged and minute comparison of the solar spectrum with spectra of terrestrial elements, something like forty elemental substances are now known to exist in the sun. rowland's splendid photographs of the solar spectrum have contributed most effectively. about half of these elements, though not in order of certainty, are aluminum, cadmium, calcium, carbon, chromium, cobalt, copper, hydrogen, iron, magnesium, manganese, nickel, scandium, silicon, silver, sodium, titanium, vanadium, yttrium, zinc, and zirconium. oxygen, too, is pretty surely indicated; but certain elements abundant on earth, as nitrogen and chlorine, together with gold, mercury, phosphorus, and sulphur, are not found in the sun. the two brilliant red stars, aldebaran in taurus, and betelgeuse in orion, were the first stars whose chemical constitution was revealed to the eye of man, and sir william huggins of london was the astronomer who achieved this epoch-making result. father secchi of the vatican observatory proceeded at once with the visual examination of the spectra of hundreds of the brighter stars, and he was the first to provide a classification of stellar spectra. there were four types. secchi's type i is characterized chiefly by the breadth and intensity of dark hydrogen lines, together with a faintness or entire absence of metallic lines. these are bluish or white stars and they are very abundant, nearly half of all the stars. vega, altair, and numerous other bright stars belong to this type, and especially sirius, which gives to the type the name "sirians." type ii is characterized by a multitude of fine dark metallic lines, closely resembling the lines of the solar spectrum. these stars are somewhat yellowish in tinge like the sun, and from this similarity of spectra they are called "solars." arcturus and capella are "solars," and on the whole the solars are rather less numerous than the sirians. stars nearest to the solar system are mostly of this type, and, according to kapteyn of groningen, the absolute luminous power of first type stars exceeds that of second type stars seven-fold. secchi's type iii is characterized by many dark bands, well defined on the side toward the blue end of the spectrum, but shading off toward the red--a "colonnaded spectrum", as miss clerke aptly terms it. alpha herculis, antares, and mira, together with orange and reddish stars and most of the variable stars, belong in type iii. type iv is also characterized by dark bands, often called "flutings," similar to those of type iii, but reversed as to shading, that is, well defined on the side toward the red, but fading out toward the blue. their atmospheres contain carbon; but they are not abundant, besides being faint and nearly all blood-red in tint. following up the brilliant researches of draper, who in obtained the first successful photograph of a star's spectrum, that of vega, pickering of harvard supplemented secchi's classification by type v, a spectrum characterized by bright lines. they, too, are not abundant and are all found near the middle of the galaxy. these are usually known as wolf-rayet stars, from the two paris astronomers who first investigated their spectra. type v stars are a class of objects seemingly apart from the rest of the stellar universe, and many of the planetary nebulæ yield the same sort of a spectrum. the late mrs. anna palmer draper, widow of dr. henry draper, established the henry draper memorial at harvard, and investigation of the photographic spectra of all the brighter stars of the entire heavens has been prosecuted on a comprehensive scale, those of the northern hemisphere at cambridge, and of the southern at arequipa, peru. these researches have led to a broad reclassification of the stars into eight distinct groups, a work of exceptional magnitude begun by the late mrs. fleming and recently completed by miss annie cannon, who classified the photographic spectra of more than , stars on the new system, as follows:-- the letters o, b, a, f, g, k, m, n represent a continuous gradation in the supposed order of stellar evolution, and farther subdivision is indicated by tenths, g k meaning a type half way between g and k, and usually written g simply. b would indicate a type between b and a, but nearer to b than a, and so on. on this system, the spectrum of a star in the earliest stages of its evolution is made up of diffuse bright bands on a faint continuous background. as these bands become fewer and narrower, very faint absorption lines begin to appear, first the helium lines, followed by several series of hydrogen lines. on the disappearance of the bright bands, the spectrum becomes wholly absorptive bands and lines. then comes a very great increase in intensity of the true hydrogen spectrum, with wide and much diffused lines, and few if any other lines. then the h and k calcium lines and other lines peculiar to the sun become more and more intense. then the hydrogen lines go through their long decline. the calcium spectrum becomes intense, and later the spectrum becomes quite like that of the sun with a great wealth of lines. following this stage the spectrum shortens from the ultra violet, the hydrogen lines fade out still farther, and bands due to metallic compounds make their appearance, the entire spectrum finally resembling that of sun spots. to designate these types rather more categorically:-- type o--bright bands on a faint continuous background, with five subdivisions, oa, ob, oc, od, oe, according to the varying width and intensity of the bands. type b--the orion type, or helium type, with additional lines of origin unknown as yet, but without any of the bright bands of type o. type a--the sirian type, the regular balmer series of hydrogen lines being very intense, with a few other lines not conspicuously marked. type f--the calcium type, hydrogen lines less strongly marked, but with the narrow calcium lines h and k very intense. type g--the solar type, with multitudes of metallic lines. type k--in some respects similar to g, but with the hydrogen lines fading out, and the metallic lines relatively more prominent. type m--spectrum with peculiar flutings due to titanium oxide, with subdivisions ma and mb, and the variable stars of long period, with a few bright hydrogen lines additional, in a separate class md. type n--similar to m, in that both are pronouncedly reddish, but with characteristic flutings probably indicating carbon compounds. the draper classification being based on photographic spectra, and the original secchi classification being visual, the relation of the two systems is approximately as follows: secchi type i includes draper b & a ii includes draper f, g & k iii includes draper m iv includes draper n pickering's marked success in organization and execution of this great programme was due to his adoption of the "slitless spectroscope," which made it possible to photograph stellar spectra in vast numbers on a single plate. the first observers of stellar spectra placed the spectroscope beyond the focus of the telescope with which it was used, thereby limiting the examination to but one star at a time. in the slitless spectroscope, a large prism is mounted in front of the objective (of short focus), so that the star's rays pass through it first, and then are brought to the same focus on the photographic plate, for all the stars within the field of view, sometimes many thousand in number. this arrangement provides great advantages in the comparison and classification of stellar spectra. when spectroscopic methods were first introduced into astronomy, there was no expectation that the field of the old or so-called exact astronomy would be invaded. physicists were sometimes jocularly greeted among astronomers as "ribbon men," and no one even dreamed that their researches were one day to advance to equal recognition with results derived from micrometer, meridian circle, and heliometer. the first step in this direction was taken in by sir william huggins of london, who noticed small displacements in the lines of spectra of very bright stars. in fact the whole spectrum appeared to be shifted; in the case of sirius it was shifted toward the red, while the whole spectrum of arcturus was shifted by three times this amount toward the violet end of the spectrum. the reason was not difficult to assign. as early as doppler had enunciated the principle that when we are approaching or are approached by a body which is emitting regular vibrations, then the number of waves we receive in a second is increased, and their wave-length correspondingly diminished; and just the reverse of this occurs when the distance of the vibrating body is increasing. it is the same with light as with sound, and everyone has noticed how the pitch of a locomotive whistle suddenly rises as it passes, and falls as suddenly on retreating from us. so huggins drew the immediate inference that the distance between the earth and sirius was increasing at the rate of nearly twenty miles per second, while arcturus was nearing us with a velocity of sixty miles per second. these pioneer observations of motions in the line of sight, or radial velocities as they are now called, led directly to the acceptance of the high value of spectroscopic work as an adjunct of exact astronomy in stellar research. nor has it been found wanting in application to a great variety of exact problems in the solar system which would have been wholly impossible to solve without it. foremost is the sun, of course, because of the overplus of light. young early measured the displacement of lines in the spectra of the prominences, and found velocities sometimes exceeding miles per second. many astronomers, dunér among them, investigated the rotation of the sun by the spectroscopic method. the sun's east limb is coming toward us, while the west is going from us; and by measuring the sum of the displacements, the rate of rotation has been calculated, not only at the sun's equator but at many solar latitudes also, both north and south. as was to be expected, these results agree well with the sun's rotation as found by the transits of sun spots in the lower latitudes where they make their appearance. bélopolsky has applied the same method to the rotation of the planet venus, and keeler, by measuring the displacement of lines in the spectrum of saturn, on opposite sides of the ring, provided a brilliant observational proof of the physical constitution of the rings; because he showed that the inner ring traveled round more swiftly than the outer one, thus demonstrating that the ring could not be solid, but must be composed of multitudes of small particles traveling around the ball of saturn, much as if they were satellites. indeed, keeler ascertained the velocity of their orbital motion and found that in each case it agreed exactly with that required by the keplerian law. even the filmy corona of the sun was investigated in similar fashion by deslandres at the total eclipse of , and he found that it rotates bodily with the sun. but the complete vindication of the spectroscopic method as an adjunct of the old astronomy came with its application to measurement of the distance of the sun. the method is very interesting and was first suggested by campbell in . spectrum-line measurements have become very accurate with the introduction of dry-plate photography, and ecliptic stars were spectrographed, toward and from which the earth is traveling by its orbital motion round the sun. by accurate measurement of these displacements, the orbital velocity of the earth is calculated; and as we know the exact length of the year, or a complete period, the length of the orbit itself in miles becomes known, and thus, by simple mensuration, the length of the radius of the orbit--which is the distance of the sun. if we pass from sun to star, the triumph of the spectroscope has been everywhere complete and significant. as the spectroscopic survey of the stars grew toward completeness, it became evident that the swarming hosts of the stellar universe are in constant motion through space, not only athwart the line of vision as their proper motions had long disclosed, but some stars are swiftly moving toward our solar system and others as swiftly from it. fixed stars, strictly speaking--there are no such. all are in relative motion. exact astronomy by discussion of the proper motions had assigned a region of the sky toward which the sun and planets are moving. spectrography soon verified this direction not only, but gave a determination of the velocity of our motion of twelve miles per second in a direction approximately that of the constellation lyra. from corresponding radial velocities, we draw the ready conclusion that certain groups or clusters of stars are actually connected in space and moving as related systems, as in the pleiades and ursa major. rather more than a quarter century ago, the spectroscope came to the assistance of the telescope in helping to solve the intricate problem of stellar distribution. kapteyn, by combining the proper motions of certain stars with their classification in the draper catalogue of stellar spectra, drew the conclusion that, as stars having very small proper motions show a condensation toward the galaxy, the stars composing this girdle are mostly of the sirian type, and are at vast distances from the solar system. the proper motion of a star near to us will ordinarily be large, and, in the case of solar stars, the larger their proper motion the greater their number. so it would appear that the solar stars are aggregated round the sun himself, and this conclusion is greatly strengthened by the fact that of stars whose distances and spectral type are both ascertained, seven of the eight nearest to us are solar stars. in the spectroscope achieved an unexpected triumph by enabling the late professor pickering to make the first discovery of a spectroscopic double, or binary star, a type of object now quite abundant. unlike the visual binary systems whose periods are years in length, the spectroscopic binaries have short periods, reckoned in some cases in days, or hours even. if the orbit of a very close binary is seen edge on, the light of the two stars will coalesce twice in every revolution. halfway between these points there are two times when the two stars will be moving, one toward the earth and the other from it. at all times the light of the star, in so far as the telescope shows it, proceeds from a single object. now photograph the star's spectrum at each of the four critical points above indicated: in the first pair the lines are sharply defined and single, because at conjunction the stars are simply moving athwart the line of sight, while at the intermediate points the lines are double. doppler's principle completely accounts for this: the light from the receding companion is giving lines displaced toward the red, while the approaching companion yields lines displaced toward the violet. mizar, the double star at the bend of the handle in the great dipper was the first star to yield this peculiar type of spectrum, and the period of its invisible companion is about days. the relative velocity of the components is miles a second, and applying newton's law we find its mass exceeds that of the sun forty-fold. capella has been found to be a spectroscopic binary; also the pole star. spectroscopic binaries have relatively short periods, one of the shortest known being only hours in length. it is in the constellation scorpio. beta aurigæ is another whose lines double on alternate nights, giving a period of four days; and the combined mass of both stars is more than twice that of the sun. the catalogue of spectroscopic binaries is constantly enlarging; but thousands doubtless exist that can never be discovered by this method, as is evident if their orbits are perpendicular to the line of sight or nearly so. the history of the spectroscopic binaries is one of the most interesting chapters in astronomy, and affords a marvelous confirmation of the prediction of bessel who first wrote of "the astronomy of the invisible." find a star's distance by the spectroscope? impossible, everyone would have said, even a very few years ago. now, however, the thing is done, and with increasing accuracy. adams of mount wilson has found, after protracted investigation, that the relative intensity of certain spectral lines varies according to the absolute brightness of a star; indeed, so close is the correspondence that the spectroscopic observations are employed to provide in certain cases a good determination of the absolute magnitude, and therefore of the distance. to test this relation, the spectroscopic parallaxes have been compared with the measured parallaxes in numerous instances, and an excellent agreement is shown. this new method is adding extensively to our knowledge of stellar luminosities and distances, and even the vast distances of globular clusters and spiral nebulæ are becoming known. in fact, but few departments of the old astronomy are left which the new astronomy has not invaded, and this latest triumph of the spectroscope in determining accurately the distances of even the remotest stars is enthusiastically welcomed by advocates of the old and new astronomy alike. chapter xxi the story of astronomical photography the most powerful ally of both telescope and spectroscope is photography. without it the marvelous researches carried on with both these types of instrument would have been essentially impossible. even the great telescopes of herschel and lord rosse, notwithstanding their splendid record as optical instruments, might have achieved vastly more had photography been developed in their time to the point where the astronomer could have employed its wonderful capabilities as he does to-day. and, with the spectroscope, it is hardly too much to say that no investigator ever observes visually with that instrument any more: practically every spectrum is made a matter of photographic record first. the observing, or nowadays the measuring, is all done afterward. all telescopes and cameras are alike, in that each must form or have formed within it an image by means of a lens or mirror. in the telescope the eye sees the fleeting image, in the camera the process of registering the image on a plate or film is known as photography. daguerre first invented the process (silver film on a copper plate) in . the year following it was first employed on the moon, in the first star was photographed, in the first total eclipse of the sun; all by the primitive daguerreotype process, which, notwithstanding its awkwardness and the great length of exposure required, was found to possess many advantages for astronomical work. about the middle of the last century the wet plate process, so called because the sensitized collodion film must be kept moist during exposure, came into general use, and the astronomers of that period were not slow to avail themselves of the advantages of a more sensitive process, which in , in the skillful hands of henry draper, produced the first spectrum of a star. in a nebula was first photographed, and in a comet. before this time, however, the new dry-plate process had been developed to the point where astronomers began to avail of its greater convenience and increased sensitiveness, even in spite of the coarseness of grain of the film. forty years of dry-plate service have brought a wealth of advantages scarcely dreamed of in the beginning, and nearly every department of astronomical research has been enhanced thereby, while many entirely new photographic methods of investigation have been worked out. continued improvement in photographic processes has provided the possibility of pictures of fainter and fainter celestial objects, and all the larger telescopes have photographed stars and nebulæ of such exceeding faintness that the human eye, even if applied to the same instrument, would never be able to see them. this is because the eye, in ten or twelve seconds of keen watching, becomes fatigued and must be rested, whereas the action of very faint light rays is cumulative on the highly sensitive film; so that a continuous exposure of many hours' duration becomes readily visible to the eye on development. so a supersensitive dry plate will often record many thousand stars in a region where the naked eye can see but one. perhaps the greatest amplification of photography has taken place at the harvard observatory under pickering, where a library of many hundred thousand plates has accumulated; and at groningen, holland, where kapteyn has established an astronomical laboratory without instruments except such as are necessary to measure photographic plates, whenever and wherever taken. so it is possible to select the clearest of skies, all over the world, for exposure of the plates, and bring back the photographs for expert discussion. of course the sun was the celestial body first photographed, and its surpassing brilliance necessitates reduction of exposure to a minimum. in moments of exceptional steadiness of the atmosphere, a very high degree of magnification of the solar surface on the photographic plate is permitted, and the details in formation, development, and ending of sun spots are faithfully registered. nevertheless, it cannot be said that photography has yet entirely replaced the eye in this work, and careful drawings of sun spots at critical stages in their life are capable of registering fine detail which the plate has so far been unable to record. janssen of paris took photographs of the solar photosphere so highly magnified that the granulation or willow-leaf structure of the surface was clearly visible, and its variations traceable from hour to hour. the advantages of sun spot photography in ascertaining the sun's rotation, keeping count of the spots, and in a permanent record for measurement of position of the sun's axis and the spot zones, are obvious. in direct portrayal of the sun's corona during total eclipses, photography has offered superior advantages over visual sketching, in the form and exact location of the coronal streamers; but the extraordinary differences of intensity between the inner corona and its outlying extensions are such that halation renders a complete picture on a single plate practically impossible. the filamentous detail of the inner corona, and the faintest outlying extensions or streamers, the eye must still reveal directly. in solar spectrum photography, research has been especially benefited; indeed, exact registry of the multitudinous lines was quite impossible without it. photographic maps of the spectrum by thollon, mcclean and rowland are so complete and accurate that no visual charts can approach them. rowland's great photographic map of the solar spectrum spread out into a band about forty feet in length; and in the infra-red, langley's spectrobolometer extended the invisible heat spectrum photographically to many times that length. at the other end of the spectrum, special photographic processes have extended the ultra-violet spectrum far beyond the ocular limit, to a point where it is abruptly cut off by absorption of the earth's atmosphere. on the same plate with certain regions of the sun's spectrum, the spectra of terrestrial metals are photographed side by side, and exact coincidences of lines show that about forty elemental substances known to terrestrial chemistry are vaporized in the sun. [illustration: a view of the -foot dome in which the largest telescope in the world is housed. (_courtesy, mt. wilson solar observatory._)] [illustration: mount chimborazo, near the equator. an observatory located on this mountain would make it possible to study the phenomena of northern and southern skies from the same point. (_courtesy, pan-american union._)] [illustration: lick observatory, on the summit of mt. hamilton, about twenty-five miles s. w. of san jose, california. it contains the famous lick telescope, a -inch refractor.] [illustration: near view of the eye-end of the yerkes telescope. the eyepiece is removed and its place taken by a photographic plate.] young was the first to photograph a solar prominence in , and twenty years later deslandres of paris and hale of chicago independently invented the spectroheliograph, by which the chromosphere and prominences of the sun, as well as the disk of the sun itself, are all photographed by monochromatic light on a single plate. hale has developed this instrument almost to the limit, first at the yerkes observatory of the university of chicago, and more recently at the mount wilson observatory of the carnegie institution, where spectroheliograms of marvelous perfection are daily taken. it was with this instrument that hale discovered the effect of an electro-magnetic field in sun spots which has revolutionized solar theories, a research impossible to conceive of without the aid of photography. when we apply doppler's principle, photography becomes doubly advantageous, whether we determine, as dunér did and more recently adams, the sun's own rotation and find it to vary in different solar latitudes, the equator going fastest; or apply the method to the sun's corona at the east and west limbs of the sun, which deslandres in proved to be rotating bodily with the sun, because of the measured displacement of spectral lines of the corona in juxtaposition on the photographic plate. in the solar astronomy of measurement, too, photography has been helpfully utilized, as in registering the transits of mercury over the sun's disk, for correcting the tables of the planet's orbital motion; and most prominently in the action taken by the principal governments of the world in sending out expeditions to observe the transits of venus in and , for the purpose of determining the parallax of venus and so the distance of the earth from the sun. in our studies of the moon, photography has almost completely superseded ocular work during the past sixty years. rutherfurd and draper of new york about obtained very excellent lunar photographs with wet plates, which were unexcelled for nearly half a century. the harvard, lick, and paris observatories have published pretty complete photographic atlases of the moon, and the best negatives of these series show nearly everything that the eye can discern, except under unusual circumstances. later lunar photography was taken up at the yerkes observatory, and exceptionally fine photographs on a large scale were obtained with the -inch refractor, using a color screen. more recently the -inch and -inch mirrors of the mount wilson observatory have taken a series of photographs of the moon far surpassing everything previously done, as was to be expected from the unique combination of a tranquil mountain atmosphere with the extraordinary optical power of the instruments, and a special adaptation of photographic methods. during lunar eclipses, pickering has made a photographic search for a possible satellite of the moon, occultations of stars by the moon have been recorded by photography, and russell of princeton has shown how the position of the moon among the stars can be determined by the aid of photography with a high order of precision. the story of planetary photography is on the whole disappointing. much has been done, but there is much that is within reach, or ought to be, that remains undone. from mercury nothing ought perhaps to be expected. on many of the photographs of the transit of venus, especially those taken under the writer's direction at the lick observatory in , we have unmistakable evidence of the planet's atmosphere. here again the wet plate process, although more clumsy, demonstrated its superiority over the dry process used by other expeditions. in spectroscopy, bélopolsky has sought to determine the period of rotation of venus on her axis. at the lowell observatory, douglass succeeded in photographing the faint zodiacal light, and very successful photographs of mars were taken at this institution as early as by slipher. two years later these were much improved upon by the writer's expedition to the andes of chile, when , exposures of mars were made, many of them showing the principal _canali_, and other prominent features of the planet's disk. at subsequent oppositions of the planet, barnard at the yerkes observatory and the mount wilson observers have far surpassed all these photographs. for future oppositions a more sensitive film is highly desired, in connection with instruments possessing greater light-gathering power, so permitting a briefer exposure that will be less influenced by irregularities and defects of the atmosphere. the spectrum of mars is of course that of sunlight, very much reduced, and modified to a slight extent by its passing twice through the atmosphere of mars. what amount of aqueous vapor that atmosphere may contain is a question that can be answered only by critical comparison of the martian spectrum with the spectrum of the moon, and photography affords the only method by which this can be done. many are the ways in which photography has aided research on the asteroid group. since more than of them have been discovered by photography, and it is many times easier to find the new object on the photographic plate than to detect it in the sky as was formerly done by means of star charts. the planet by its motion during the exposure of the plate produces a trail, whereas the surrounding stars are all round dots or images. or by moving the plate slightly during exposure, as in metcalf's ingenious method, we may catch the planet at that point where it will give a nearly circular image, and thus be quite as easy to detect, because all the stars on the same plate will then be trails. photographic photometry of the asteroids has revealed marked variations in their light, due perhaps to irregularities of figure. on account of their faint light, the asteroids are especially suited, as mars is not, to exact photography for ascertaining their parallax, and from this the sun's distance when the asteroid's distance has been found. many asteroids have been utilized in this way, in particular eros ( ). in it approaches the earth within million miles, when the photographic method will doubtless give the sun's distance with the utmost accuracy. photographs of jupiter have been very successfully taken at the yerkes and lowell observatories and elsewhere, but the great depth of the planet's atmosphere is highly absorptive, so that the impression is very weak in the neighborhood of the limb, if the exposure is correctly timed for the center of the disk. the striking detail of the belts, however, is excellently shown. wood of baltimore has obtained excellent results by monochromatic photography of jupiter and saturn with the -inch reflector on mount wilson. jupiter's satellites have not been neglected photographically, and pickering has observed hundreds of the eclipses of the satellites by a sort of cinematographic method of repeated exposures, around the time of disappearance and reappearance by eclipse. the newest outer satellites of jupiter were all discovered by photography, and it is extremely doubtful if they would have been found otherwise. saturn has long been a favorite object with the astronomical photographer, and there are many fine pictures in spite of his yellowish light, relatively weak photographically. the marvelous ring system with the cassini division, the oblateness of the ball, the occasional markings on it--all are well shown in the best photographs; but the call is for more light and a more sensitive photographic process. pickering's ninth satellite (phoebe) was discovered by photography, one of the faintest moons in the solar system. like the faint outer moons of jupiter, few existing telescopes are powerful enough to show it. its orbit has been found from photographic observations, and its position is checked up from time to time by photography. but the crowning achievement of spectrum photography in the saturnian system is keeler's application of doppler's principle in determining the rate of orbital motion of particles in different zones of the rings, thereby establishing the maxwellian theory of the constitution of the rings beyond the possibility of doubt. for uranus and neptune photography has availed but little, except to negative the existence of additional satellites of these planets, which doubtless would have been discovered by the thorough photographic search which has been made for them by w. h. pickering without result. as with the asteroids, so with comets: several of these bodies have been discovered by photography; none more spectacular than the egyptian comet of may th, , which impressed itself on the plates of the corona of that date. withdrawal of the sun's light by total eclipse made the comet visible, and it had never been seen before, nor is it known whether it will ever return. in cometary photography, much the same difficulties are present as in photographing the corona: if the plate is exposed long enough to get the faint extensions of the tail, the fine filaments of the coma or head are obliterated by halation and overexposure. no one has had greater success in this work than barnard, whose photographs of comets, particularly at the lick observatory, are numerous and unexcelled. his photographs of the brooks comet of revealed rapid and violent changes in the tail, as if shattered by encounter with meteors; and the tail of halley's comet in showed the rapid propagation of luminous waves down the tail, similar to phenomena sometimes seen in streamers of the aurora. draper obtained the first photograph of a comet's spectrum in , disclosing an identity with hydrocarbons burning in a bunsen flame, also bands in the violet due to carbon compounds. the photographic spectra of subsequent comets have shown bright lines due to sodium and the vapor of iron and magnesium. even the elusive meteor has been caught by photography, first by wolf in , who was exposing a plate on stars in the milky way. on developing it, he found a fine, dark nearly uniform line crossing it, due to the accidental flight across the field of a meteor of varying brightness. since then meteor trails have been repeatedly photographed, and even the trail spectra of meteors have been registered on the harvard plates. at yale in elkin employed a unique apparatus for securing photographic trails of meteors: six photographic cameras mounted at different angles on a long polar axis driven by clockwork, the whole arranged so as to cover a large area of the sky where meteors were expected. when we pass from the solar system to the stellar universe the advantages of photography and the amplification of research due to its employment as accessory in nearly every line of investigation are enormous. so extensively has photography been introduced that plates, and to a slight extent films, are now almost exclusively used in securing original records. regrettably so in case of the nebulæ, because the numerous photographs of the brighter nebulæ taken since when draper got the first photograph of the nebula of orion, are as a rule not comparable with each other. differences of instruments, of plates, of exposure, and development--all have occasioned differences in portrayal of a nebula which do not exist. when we consider faithful accuracy of portrayal of the nebulæ for purposes of critical comparison from age to age, many of our nebular photographs of the past forty years, fine as they are and marvelous as they are, must fail to serve the purpose of revealing progressive changes in nebular features in the future. roberts and common in england were among the first to obtain nebular photographs with extraordinary detail, also the brothers henry of paris. as early as roberts revealed the true nature of the great nebula in andromeda, which had never been suspected of being spiral; and keeler and perrine at the lick observatory pushed the photographic discovery of spiral nebulæ so far that their estimates fill the sky with many hundred thousands of these objects. in the southern hemisphere the -inch bruce telescope of harvard college observatory has obtained many very remarkable photographs of nebulæ, particularly in the vicinity of eta carinæ. but the great reflectors of the mount wilson observatory, on account of their exceptional location and extraordinary power, have surpassed all others in the photographic portrayal of these objects, especially of the spiral nebulæ which appear to show all stages in transition from nebula to star. no less remarkable are the photographs of such wonderful clusters as omega centauri, a perfect visual representation of which is wholly impossible. intercomparison of the photographs of clusters has afforded bailey of harvard, shapley of mount wilson and others the opportunity of discovery that hundreds of the component stars are variable. what is the longest photographic exposure ever made? at the cape of good hope, under the direction of the late sir david gill, exposures on nebulæ were made, utilizing the best part of several nights, and totaling as high as seventeen, or even twenty-three hours. but the mount wilson observers have far surpassed this duration. to study the rotation and radial velocity of the central part of the nebula of andromeda, an exposure of no less than hours' total duration was made on the exceedingly faint spectrum, and even that record has since been exceeded. the eye cannot be removed from the guiding star for a moment while the exposure is in progress, and this tedious piece of work was rewarded by determining the velocity of the center of the nucleus as a motion of approach at the rate of kilometers per second. but when the stars, their magnitudes and their special peculiarities are to be investigated _en masse_, photography provides the facile means for researches that would scarcely have been dreamed of without it. the international photographic chart of the entire heavens, in progress at twenty observatories since , the photographic charts of the northern heavens at harvard and of the southern sky at cape town, the manifold investigations that have led up to the harvard photometry, and the unparalleled photographic researches of the henry draper memorial, enabling the spectra of many hundred thousand stars to be examined and classified--all this is but a part of the astronomical work in stellar fields that photography has rendered possible. then there are the stellar parallaxes, now observed for many stars at once photographically, when formerly only one star's parallax could be measured at a time and with the eye at the telescope. and photo-electric photometry, measuring smaller differences of light than any other method, and providing more accurate light-curves of the variable stars. and perhaps most remarkable of all, the radial velocity work on both stars and nebulæ, giving us the distance of whole classes of stars, discovering large numbers of spectroscopic binaries and checking up the motion of the solar system toward lyra within a fraction of a mile per second. all told, photography has been the most potent adjunct in astronomical research, and it is impossible to predict the future with more powerful apparatus and photographic processes of higher sensitiveness. the field of research is almost boundless, and the possibilities practically without limit. what would herschel have done with £ , --and photography! chapter xxii mountain observatories the century that has elapsed since the time of sir william herschel, known as the father of the new or descriptive astronomy, has witnessed all the advances of the science that have been made possible by adopting the photographic method of making the record, instead of depending upon the human eye. only one eye can be looking at the eyepiece at a time: the photograph can be studied by a thousand eyes. at mountain elevations telescopes are now extensively employed, and there the camera is of especial and additional value, because the photograph taken on the mountain can be brought down for the expert to study, at ease and in the comfort of a lower elevation. we shall next trace the movement that has led the astronomer to seek the summits of mountains for his observatories, and the photographer to follow him. not only did the genius of newton discover the law of universal gravitation, and make the first experiments in optics essential to the invention of the spectroscope, but he was the real originator also of the modern movement for the occupation of mountain elevations for astronomical observatories. his keen mind followed a ray of light all the way from its celestial source to the eye of the observer, and analyzed the causes of indistinct and imperfect vision. endeavoring to improve on the telescope as galileo and his followers had left it, he found such inherent difficulties in glass itself that he abandoned the refracting type of telescope for the reflector, to the construction of which he devoted many years. but he soon found out, what every astronomer and optician knew to their keen regret, that a telescope, no matter how perfectly the skill of the optician's hand may make it, cannot perform perfectly unless it has an optically perfect atmosphere to look through. so newton conceived the idea of a mountain observatory, on the summit of which, as he thought, the air would be not only cloudless, but so steady and equable that the rays of light from the heavenly bodies might reach the eye undisturbed by atmospheric tremors and quiverings which are almost always present in the lower strata of the great ocean of air that surrounds our planet. this is the way newton puts the question in his treatise on _opticks_--he says: "the air through which we look upon the stars, is in a perpetual tremor; as may be seen by the tremulous motion of shadows cast from high towers, and by the twinkling of the fix'd stars.... the only remedy is a most serene and quiet air, such as may perhaps be found on the tops of the highest mountains above the grosser clouds." newton's suggestion is that the _highest_ mountains may afford the best conditions for tranquillity; and it is an interesting coincidence that the summits of the highest mountains, about , feet in elevation, are at about the same level where the turbulence of the atmosphere most likely ceases, according to the indications of recent meteorological research. these heights are far above any elevations permanently occupied as yet, but a good beginning has been made and results of great value have already been reached. curiously, investigation of mountain peaks and their suitability for this purpose was not undertaken till nearly two centuries after newton, when piazzi smyth in organized his expedition to the summit of a mountain of quite moderate elevation, and published his "teneriffe: an astronomer's experiment." teneriffe is an accessible peak of about , feet, on an island of the canaries off the african coast, where smyth fancied that conditions of equability would exist; and on reaching the summit with his apparatus and spending a few days and nights there, he was not disappointed. could he have reached an elevation of , feet, he would have had fully one-third of all the atmosphere in weight below him, and that the most turbulent portion of all. nevertheless, the gain in steadiness of the atmosphere, providing "better seeing," as the astronomer's expression is, even at , feet, was most encouraging, and led to attempts on other peaks by other astronomers, a few of whom we shall mention. davidson, an observer of the united states coast survey, with a broad experience of many years in mountain observing, investigated the summit of the sierra nevada mountains as early as , at an elevation of , feet. his especial object was to make an accurate comparison between elevated stations at different heights. he found the seeing excellent, especially on the sun; but the excessive snowfall at his station, feet annually, was a condition very adverse to permanent occupation. in the summer of , young spent several weeks at sherman, wyoming, at an elevation exceeding , feet. he carried with him the . -inch telescope of dartmouth college, where he was then professor, and this was the first expedition on which a large glass was used by a very skillful observer at great elevation. he found the number of good days and nights small, but the sky was exceedingly favorable when clear. many th magnitude stars could be detected with the naked eye. young's observations at sherman were mainly spectroscopic, however, and they demonstrated the immense advantage of a high-level station, far above the dust and haze of the lower atmosphere. he pronounced the . -inch glass at , feet the full equivalent of a -inch at sea level. mont blanc of , feet elevation was another summit where the veteran janssen of paris maintained a station for many years; but the continental conditions of atmospheric moisture and circulation were not favorable on the whole. janssen was mainly interested in the sun, and the daylight seeing is rarely benefited, owing to the strong upward currents of warm air set in motion by the sun itself. mountains in the beautiful climate of california were among the earliest investigated, and when in the trustees of mr. james lick's estate were charged with equipping an observatory with the most powerful telescope in existence, they wisely located on the summit of mount hamilton. it is , feet above sea level, and burnham and other astronomers made critical tests of the steadiness of vision there by observing double stars, which afford perhaps the best means of comparing the optical quality of the atmosphere of one region with another. the writer was fortunate in having charge of the observations of the transit of venus in on the mountain, when the observatory was in process of construction, and the quality of the photographs obtained on that occasion demonstrated anew the excellence of the site. particularly at night, for about nine months of the year, the seeing is exceptionally good, especially when fog banks rolling in from the pacific, cover the valleys below like a blanket, preventing harmful radiation from the soil below. the great telescope mounted in , a -inch refractor by alvan clark, has fulfilled every expectation of its projectors, and justified the selection of the site in every particular. the elevation, although moderate, is still high enough to secure very marked advantage in clearness and steadiness of the air, and at the same time not so high that the health and activities of the observers are appreciably affected by the thinner air of the summit. this telescope is known the world over for the monumental contributions to science made by the able astronomers who have worked with it: among them barnard who discovered the fifth satellite of jupiter in ; burnham, hussey, and aitken, who have discovered and measured thousands of close double stars; keeler, who spent many faithful years on the summit; and campbell, the present director, whose spectroscopic researches on stellar movements have added greatly to our knowledge of the structure of the universe. among the many lines of research now in progress at the lick observatory and in the d. o. mills observatory at santiago, chile, are the discoveries of stars whose velocities in space are not constant, but variable with the spectral type of the star. mr. lick's bequest for the observatory was about $ , . so ably has this scientific trust been administered that he might well have endowed it with his entire estate, exceeding $ , , . another california mountain that was early investigated is mount whitney. its summit elevation is nearly , feet, and in langley made its ascent for the purpose of measuring the solar constant. he found conditions much more favorable than on mount etna, sicily--elevation about , feet--which he had visited the year before. but the height of mount whitney was such as to occasion him much inconvenience from mountain sickness, an ailment which is most distressing and due partly to lack of oxygen and partly to mere diminution of mechanical pressure. mount whitney was also visited many years after by campbell for investigating the spectrum of mars in comparison with that of the moon. langley found on mount whitney an excellent station lower down, at about , feet elevation; and by equipping the two stations with like apparatus for measuring the solar heat, he obtained very important data on the selective absorption of the atmosphere. returning from the transit of venus in , copeland of edinburgh visited several sites in the andes of peru, ascending on the railway from mollendo. vincocaya was one of the highest, something over , feet elevation. his report was most enthusiastic, not only as to clearness and transparency of the atmosphere, but also as to its steadiness, which for planetary and double star observations is almost as important. copeland's investigation of this region of the andes has led many other astronomers to make critical tests in the same general region. climatic conditions are particularly favorable, and the sites for high-level research are among the best known, the atmosphere being not only clear a large part of the year, but in certain favored spots exceedingly steady. in the writer ascended the summit of fujiyama, japan, , feet elevation. the early september conditions as to steadiness of atmosphere were extraordinarily fine, but the mountain is covered by cloud many months in each year. there is a saddle on the inside of the crater that would form an ideal location for a high-level observatory. this expedition was undertaken at the request of the late professor pickering, director of harvard college observatory, which had recently received a bequest from uriah a. boyden, amounting to nearly a quarter of a million dollars, to "establish and maintain, in conjunction with others, an astronomical observatory on some mountain peak." great elevations were systematically investigated in colorado and california, the chilean desert of atacama was visited, and a temporary station established at chosica, peru, elevation about , feet. atmospheric conditions becoming unfavorable, a permanent station was established in at arequipa, peru, elevation , feet, which has been maintained as an annex to the harvard observatory ever since. the cloud conditions have been on the whole less favorable than was expected, but the steadiness of the air has been very satisfactory. in addition to planetary researches conducted there in the earlier years by w. h. pickering, many large programs of stellar research have been executed, especially relating to the magnitudes and spectra of the stars. in conjunction with the home observatory in the northern hemisphere, this afforded a vast advantage in embracing all the stars of the entire heavens, on a scale not attempted elsewhere. the bruce photographic telescope of -inch aperture has been employed for many years at arequipa, and with it the plates were taken which enabled pickering to discover the ninth satellite of saturn (phoebe), and the splendid photographs of southern globular clusters in which bailey has found numerous variable stars of very short periods--very faint objects, but none the less interesting, and of much significance in modern study of the evolution and structure of the stellar universe. the crowning research of the observatory is the henry draper catalogue of stellar spectra, now in process of publication, which is of the first order of importance in statistical studies of stellar distribution with reference to spectral type, and in studying the relation of parallax and distance, proper motion, radial velocity and its variation to the spectral characteristics of the stars. perrine of cordova is now establishing on sierra chica about twenty-five miles southwest of cordova, a great reflecting telescope comparable in size with the instruments of the northern hemisphere, for investigation of the southern nebulæ and clusters, and motions of the stars. the elevation of this new argentine observatory will be , feet above sea level. another observatory at mountain elevation and in a highly favorable climate is the lowell observatory, located at about , feet elevation at flagstaff, arizona. many localities were visited and the atmosphere tested especially for steadiness, an optical quality very essential for research on the planetary surfaces. mexico was one of these stations, but local air currents and changes of temperature there were such that good seeing was far from prevalent, as had been expected. at flagstaff, on the other hand, conditions have been pretty uniformly good, and an enormous amount of work on the planet mars has been accumulated and published. the first successful photographs of this planet were taken there in , and jupiter, saturn, the zodiacal light and many other test objects have been photographed, which demonstrates the excellence of the site for astronomical research. within recent years spectrum research by slipher, especially on the nebulæ, has been added to the program, and the rotation and radial velocities of many nebulæ have been determined. on mount wilson, near pasadena, california, at an elevation of nearly , feet, is the carnegie solar observatory, founded and equipped under the direction of professor george e. hale, as a department of the carnegie institution of washington, of which dr. john campbell merriam is president. the climatology of the region was carefully investigated and tests of the seeing made by hussey and others. although equipped primarily for study of the sun, the program of the observatory has been widely amplified to include the stars and nebulæ. the instrumental equipment is unique in many respects. to avoid the harmful effect of unsteadiness of air strata close to the ground a tower feet high was erected, with a dome surmounting it and covering a coelostat with mirror for reflecting the sun's rays vertically downward. underneath the tower a dry well was excavated to a depth equal to / the height of the tower above it. in the subterranean chamber is the spectroheliograph of exceptional size and power. the sun's original image is nearly inches in diameter on the plate, and the solar chromosphere and prominences, together with the photosphere and faculæ, are all recorded by monochromatic light. connected with the observatory on mount wilson are the laboratories, offices and instrument shops in pasadena, miles distant, where the remarkable apparatus for use on the mountain is constructed. a reflecting telescope with silver-on-glass mirror inches in diameter was first built by ritchey and thoroughly tested by stellar photographs. also the northern spiral nebulæ were photographed, exhibiting an extraordinary wealth of detail in apparent star formation. the success of this instrument paved the way for one similar in design, but with a mirror inches in diameter, provided by gift of the late john d. hooker of los angeles. the telescope was completed in . notwithstanding its huge size and enormous weight, the mounting is very successful, as well as the mirror. mercurial bearings counterbalance the weight of the polar axis in large part. this great telescope, by far the largest and most powerful ever constructed, is now employed on a program of research in which its vast light-gathering power will be utilized to the full. under the skillful management of hale and his enthusiastic and capable colleagues, the confines of the stellar heavens will be enormously extended, and secrets of evolution of the universe and of its structure no doubt revealed. in all the mountain stations hitherto established, as the lick observatory at , feet, the mount wilson observatory at , feet, the lowell observatory at , feet, the harvard observatory at , feet; and teneriffe and etna at , , fujiyama at , , pike's peak at , , mont blanc and mount whitney at , , the researches that have been carried on have fully demonstrated the vast advantage of increased elevation in localities where climatological conditions as well as elevation are favorable. nevertheless, only one-half of the extreme altitude contemplated by sir isaac newton has yet been attained. can the greater heights be reached and permanently occupied? geographically and astronomically the most favorably located mountain for a great observatory is mount chimborazo in ecuador. its elevation is , feet, and it was ascended by edward whymper in . situated very nearly on the earth's equator, almost the entire sidereal heavens are visible from this single station, and all the planets are favored by circumzenith conditions when passing the meridian. no other mountain in the world approaches chimborazo in this respect. but the summit is perpetually snow-capped, exceedingly inaccessible, and the defect of barometric pressure would make life impossible up there in the open. only one method of occupation appears to be feasible. the permanent snow line is at about , feet, where excellent water power is available. by tunneling into the mountain at this point, and diagonally upward to the summit, permanent occupation could be accomplished, at a cost not to exceed one million dollars. the rooms of the summit observatory would need to be built as steel caissons, and supplied with compressed air at sea-level tension. the practicability of this plan was demonstrated by the writer in september, , at cerro de pasco, peru. a steel caisson was carried up to an elevation exceeding , feet. patients suffering acutely with mountain sickness were placed inside this caisson, and on restoring the atmospheric pressure within it artificially all unfavorable symptoms--headache, high respiration and accelerated pulse--disappeared. there was every indication that if persons liable to this uncomfortable complaint were brought up to this elevation, or indeed any attainable elevation, under unreduced pressure, the symptoms of mountain sickness would be unknown. comfortable occupation of the highest mountain summits was thereby assured. the working of astronomical instruments from within air-tight compartments does not present any insurmountable difficulties, either mechanical or physical. since the time these experiments were made, the guayaquil-quito railway has been constructed over a saddle of chimborazo, at an elevation of , feet; and only six miles of railway would need to be built from this station to the point where the tunnel would enter the mountain. only by the execution of some such plan as this can astronomers hope to overcome the baleful effects of an ever mobile atmosphere, and secure the advantages contemplated by sir isaac newton in that tranquillity of atmosphere, which he conceived as perpetually surrounding the summits of the highest mountains. in russell's theory of the progressive development of the stars, from the giant class to the dwarf, an element of verification from observation is lacking, because hitherto no certain method of measuring the very minute angular diameters of the stars has been successfully applied. the apparent surface brightness corresponding to each spectral type is pretty well known, and by dividing it into the total apparent brightness, we have the angular area subtended by the star, quite independent of the star's distance. this makes it easy to estimate the angular diameter of a star, and betelgeuse is the one which has the greatest angular diameter of all whose distances we know. antares is next in order of angular diameter, ". , aldebaran ". , arcturus ". , pollux ". , and sirius only ". . can these theoretical estimates be verified by observation? clearly it is of the utmost importance and the exceedingly difficult inquiry has been undertaken with the -inch reflector on mount wilson, employing the method of the interferometer developed by michelson and described later on, an instrument undoubtedly capable of measuring much smaller angles than can be measured by any other known method. unquestionably the interference of atmospheric waves, or in other words what astronomers call "poor seeing," will ultimately set the limit to what can be accomplished. "but even if," says eddington, "we have to send special expeditions to the top of one of the highest mountains in the world, the attack on this far-reaching problem must not be allowed to languish." chapter xxiii the program of a great observatory the mount wilson observatory has now been in operation about fifteen years. the novelty in construction of its instruments, the investigations undertaken with them and the discoveries made, the interpretation of celestial phenomena by laboratory experiment, and the recent addition to its equipment of a telescope inches in diameter, surpassing all others in power, directs especial attention to the extensive activities of this institution, whose budget now exceeds a million dollars annually. results are only achieved by a carefully elaborated program, such as the following, for which the reader is mainly indebted to dr. hale, the director of the observatory, who gives a very clear idea of the trend of present-day research on the magnetic nature of the sun, and the structure and evolution of the sidereal universe. the purpose of the observatory, as defined at its inception, was to undertake a general study of stellar evolution, laying especial emphasis upon the study of the sun, considered as a typical star; physical researches on stars and nebulæ; and the interpretation of solar and stellar phenomena by laboratory experiments. recognizing that the development of new instruments and methods afforded the most promising means of progress, well-equipped machine shops and optical shops were provided with this end in view. the original program of the observatory has been much modified and extended by the independent and striking discovery by campbell and kapteyn of an important relationship between stellar speed and spectral type; the demonstration by hertzsprung and russell of the existence of giant and dwarf stars; the successful application of the -inch reflector by van maanen to the measurement of minute parallaxes of stars and nebulæ; the important developments of shapley's investigation of globular star clusters; the possibilities of research resulting from seares's studies in stellar photometry; and the remarkable means of attack developed by adams through the method of spectroscopic parallaxes. by this method the absolute magnitude, and hence the distance of a star is accurately determined from estimates of the relative intensities of certain lines in stellar spectra. attention was first directed toward lines of this character in , when it was inferred that the weakening of some lines in the spectra of sun spots and the strengthening of others was the result of reduced temperature of the spot vapors. on testing this hypothesis by laboratory experiments, it was fully verified. subsequently adams, who had thus become familiar with these lines and their variability, studied them extensively in the spectra of other stars. in this way was discovered the dependence of their relative intensities on the star's absolute magnitude, so providing the powerful method of spectroscopic parallaxes. this method, giving the absolute magnitude as well as the distance of every star (excepting those of the earliest type) whose spectrum is photographed, is no less important from the evolutional than from the structural point of view. investigations in solar physics which formerly held chief place in the research program have developed along unexpected lines. it could not be foreseen at the outset that solar magnetic phenomena might become a subject of inquiry, demanding special instrumental facilities, and throwing light on the complex question of the nature of the sun spots and other solar problems of long standing. it is obvious that these researches, together with those on the solar rotation and the motions of the solar atmosphere, developed by adams and st. john, must be carried to their logical conclusion, if they are to be utilized to the fullest in interpreting stellar and nebular phenomena. the discovery of solar magnetism, like many other mount wilson results, was the direct outcome of a long series of instrumental developments. the progressive improvement and advance in size of the tools of research was absolutely necessary. hale's first spectroheliograph at kenwood in was attached to a -inch refractor, and the solar image was but two inches in diameter. it was soon found that a larger solar image was essential, and a spectrograph of much greater linear dispersion; in fact, the spectrograph must be made the prime element in the combination, and the telescope so designed as to serve as a necessary auxiliary. accordingly, successive steps have led through spectrographs of and feet dimension to a vertical spectrograph feet in focal length. the telescope is the feet tower telescope, giving a solar image of . inches in diameter. its spectrograph is massive in construction, and by extending deep into the earth, it enjoys the stability and constancy of temperature required for the most exacting work. another direct outgrowth of the work of sun-spot spectra is a study of the spectra of red stars, where the chemistry of these coolest regions of the sun is partially duplicated. the combination of titanium and oxygen, and the significant changes of line intensity already observed in both instances, and also in the electric furnace at reduced temperatures, give indication of what may be expected to result from an attack on the spectra of the red stars with more powerful instrumental means, which is now provided by the -inch telescope and its large stellar spectrograph. other elements in the design of the -inch hooker telescope have the same general object in view--that of developing and applying in astronomical practice the effective research methods suggested by recent advances in physics. fresh possibilities of progress are constantly arising, and these are utilized as rapidly as circumstances permit. the policy of undertaking the interpretations of celestial phenomena by laboratory experiments, an important element in the initial organization of mount wilson, has certainly been justified by its results. indeed, the development of many of the chief solar investigations would have been impossible without the aid of special laboratory studies, going hand in hand with the astronomical observations. so indispensable are such researches, and so great is the promise of their extension, that the time has now come for advancing the laboratory work from an accessory feature to full equality with the major factors in the work of the observatory. accordingly a new instrument now under installation is an extremely powerful electro-magnet, designed by anderson for the extension of researches on the zeeman effect, and for other related investigations. within the large and uniform field of this magnet, which is built in the form of a solenoid, a special electric furnace, designed for this purpose by king, is used for the study of the inverse zeeman effect at various angles with the lines of force. this will provide the means of interpreting certain remarkable anomalies in the magnetic phenomena of sun spots. the -inch telescope is now in regular use. all the tests so far applied show that it greatly surpasses the -inch telescope in every class of work. for many months most of the observations and photographs have been made with the cassegrain combination of mirrors, giving an equivalent focal length of feet and involving three reflections of light. the -inch telescope is found to give nearly . times as much light as the -inch telescope, and therefore extends the scope of the instrument to all the stars an entire magnitude fainter. this is a very important gain for research on the faint globular clusters, as well as the small and faint spiral and planetary nebulæ, providing a much larger scale for these objects and sufficient light at the same time. photographs of the moon and many other less critical tests have been made with very satisfactory results. those of the moon appear to be decidedly superior in definition to any previously taken with other instruments. another investigation is of great importance in the light of recent advances in theoretical dynamics. darwin, in his fundamental researches on the dynamics of rotating masses, dealt with incompressible matter, which assumes the well-known pear-shaped figure, and may ultimately separate into two bodies. roche on the other hand discussed the evolution of a highly compressible mass, which finally acquires a lens-shaped form and ejects matter at its periphery. both of these are extreme cases. jeans has recently dealt with intermediate cases, such as are actually encountered in stars and nebulæ. he finds that when the density is less than about one-fourth that of water, a lens-shaped figure will be produced with sharp edges, as depicted by roche. matter thrown off at opposite points on the periphery, under the influence of small tidal forces from neighboring masses, may take the form of two symmetric filaments, though it is not yet entirely clear how these may attain the characteristic configuration of spiral nebulæ. the preliminary results of van maanen indicate motion outward along the arms, in harmony with jeans's views. jeans further discusses the evolution of the arms, which will break up into nuclei (of the order of mass of the sun) if they are sufficiently massive, but will diffuse away if their gravitational attraction is small. the mass of our solar system is apparently not great enough, according to jeans, to account for its formation in this way. as is apparent, these investigations lead to conclusions very different from those derived by chamberlin and moulton from the planetesimal hypothesis. this is a critical study of spiral nebulæ for which the -inch telescope is of all instruments in existence the best suited. the spectra of the spirals must be studied, as well as the motions of the matter composing the arms. their parallaxes, too, must be ascertained. a photographic campaign including spiral nebulæ of various types will settle the question of internal motions. the large scale of the spiral nebulæ at the principal focus of the hooker telescope, and the experience gained in the measurement of nebular nuclei for parallax determination, will help greatly in this research. a multiple-slit spectrograph, already applied at mount wilson, will be employed, not only on spiral nebulæ whose plane is directed toward us, but also on those whose plane lies at an angle sufficient to permit both components of motion to be measured by the two methods. in dealing with problems of structure and motion in the galactic system, the -inch telescope offers especial advantages, because of its vast light-gathering power. studies of radial velocities of the stars have hitherto been necessarily confined to the brighter stars, for the most part even to those visible to the naked eye. while some of these are very distant, most of the stars whose radial velocities are known belong to a very limited group, perhaps constituting a distinct cluster of which the sun is a member, but in any event of insignificant proportions when contrasted with the galaxy. current spectrographic work with the -inch telescope includes stars of the eighth magnitude, and some even fainter. but while the -inch has enabled adams to measure the distances of many remote stars by his new spectroscopic method, and to double the known extent (so far as spectroscopic evidence is concerned) of the star streams of kapteyn, a much greater advance into space is necessary to find out the community of motion among the stars comprising the galactic system. the hooker telescope will enable us to determine accurate radial velocities to stars of the eleventh magnitude, which doubtless truly represent the galaxy. in order to secure a maximum return within a reasonable period of time, the stars in the selected areas of kapteyn will be given the preference, because of the vast amount of work already done, relating to their positions, proper motions, and visual and photographic magnitudes. such consideration as spectral type, the known directions of star-streaming, and the position of the chosen regions with reference to the plane of the galaxy are given adequate weight, and it is of fundamental importance that the method of spectroscopic parallaxes will permit dwarf stars to be distinguished from stars that are in the giant class, but rendered faint by their much greater distance. in addition to these problems, the stellar spectrograms will provide rich material for study of the relationship between stellar mass and speed, and the nature of giant stars and dwarf stars. shapley's recent studies of globular clusters have indicated the significance of these objects in both evolutional and structural problems, and the possibility of determining their parallaxes by a number of independent methods is of prime importance, both in its bearing on the structure of the universe and because it permits a host of apparent magnitudes to be at once transformed into absolute magnitudes. here the advantage of the hooker telescope is two-fold: at its -foot focus the increased scale of the crowded clusters makes it possible to select separate stars for spectrum photography (which could not be done with the -inch where the images were commingled); and the great gain in light is such that the spectra of stars to the th magnitude have been photographed in less than an hour. faint globular clusters, then, will comprise a large part of the early program with the -inch telescope: the faintest possible stars in them must be detected and their magnitudes and colors measured; spectral types must be determined, and the radial velocities of individual stars and of clusters as a whole; spectroscopic evidence of possible axial rotation of globular clusters must be searched for; and the method of spectroscopic parallaxes, as well as other methods, must be applied to ascertaining the distances of these clusters. the possibility of dealing with many problems relating to the distribution and evolution of the faintest stars depends upon the establishment of photographic and photovisual magnitude scales. below the twelfth magnitude, the only existing scale of standard visual or photovisual magnitudes is the mount wilson sequence, already extended by seares to magnitude . with the -inch telescope. extension of this scale to even fainter magnitudes, and its application to the faintest stars within its range is an important task for this great telescope, as it will doubtless bring within range hundreds of millions of stars that are beyond the reach of the -inch. the giants among them will form for us the outer boundary of the galactic system, while the dwarfs will be of almost equal interest from the evolutional standpoint. the photometric program of the -inch, then, will deal with such questions as the condensation of the fainter stars toward the galactic plane, the color of the most distant stars, and the final settlement of the long inquiry regarding the possible absorption of light in space. [illustration: great sun-spot group, august , . the disk in the lower left corner represents the comparative size of the earth. (_photo, mt. wilson solar observatory._)] [illustration: the sun's disk. the view shows the "rice grain" structure of the photosphere and brilliant calcium flocculi. (_photo, yerkes observatory._)] [illustration: the lunar surface visible during a total eclipse of the moon, february , . (_photo, yerkes observatory._)] another research of exceptional promise will be undertaken, which is of great importance in a general study of stellar evolution; and that is the determination of the spectral-energy curves of stars of various classes, for the purpose of measuring their surface temperatures. a very few of the nebulæ are found to be variable, and their peculiarities need investigation, also special problems of variable stars and temporary stars, and the spectra of the components of close double stars which are beyond the power of all other instruments to photograph. such a program of research conveys an excellent idea of many of the great problems that are under investigation by astronomers to-day, and gives some notion of the instrumental means requisite in executing comprehensive plans of this character. it will not escape notice that the climax of instrumental development attained at mount wilson has only been made possible by an unbroken chain of progress, link by link, each antecedent link being necessary to the successful forging of its following one. in very large part, and certainly indispensable to these instrumental advances, has the art of working in glass and metals been the mainstay of research. as we review the history of astronomical progress, from galileo's time to our own, the consummate genius of the artisan and his deft handiwork compel our admiration almost equally with the keen intelligence of the astronomer who uses these powerful engines of his own devising to wrest the secrets of nature from the heavens. chapter xxiv our solar system now let us go upward in imagination, far, far beyond the tops of the highest mountains, beyond the moon and sun, and outward in space until we reach a point in the northern heavens millions and millions of miles away, directly above and equally distant from all points in the ecliptic, or path in which our earth travels yearly round the sun. then we should have that sort of comprehensive view of the solar system which is necessary if we are to visualize as a whole the working of the vast machine, and the motions, sizes, and distances of all the bodies that comprise it. of such stupendous mechanism our earth is part. or in lieu of this, let us attempt to get in mind a picture of the solar system by means of sir william herschel's apt illustration: "choose any well-leveled field. on it place a globe two feet in diameter. this will represent the sun; mercury will be represented by a grain of mustard seed on the circumference of a circle feet in diameter for its orbit; venus, a pea on a circle of feet in diameter; the earth also a pea, on a circle of feet; mars a rather larger pin's head on a circle of feet; the asteroids, grains of sand in orbits of , to , feet; jupiter, a moderate sized orange in a circle of nearly half a mile across; saturn, a small orange on a circle of four-fifths of a mile; uranus, a full-sized cherry or small plum upon the circumference of a circle more than a mile and a half; and finally neptune, a good-sized plum on a circle about two miles and a half in diameter.... to imitate the motions of the planets in the above mentioned orbits, mercury must describe its own diameter in seconds; venus in minutes, seconds; the earth in minutes; mars in minutes seconds; jupiter in minutes seconds; saturn in minutes seconds; uranus in minutes seconds; and neptune in minutes seconds." now, let us look earthward from our imaginary station near the north pole of the ecliptic. all these planetary bodies would be seen to be traveling eastward round the sun, that is, in a counter-clockwise direction, or contrary to the motions of the hands of a timepiece. their orbits or paths of motion are very nearly circular, and the sun is practically at the center of all of them except mercury and mars; of venus and neptune, almost at the absolute center. the planes of all their orbits are very nearly the same as that of the ecliptic, or plane in which the earth moves. these and many other resemblances and characteristics suggest a uniformity of origin which comports with the idea of a family, and so the whole is spoken of as the solar system, or the sun and his family of planets. in addition to the nine bodies already specified, the solar system comprises a great variety of other and lesser bodies; no less than twenty-six moons or satellites tributary to the planets and traveling round them in various periods as the moon does round our earth. then between the orbits of mars and jupiter are many thousands of asteroids, so called, or minor planets (about , of them have actually been discovered, and their paths accurately calculated). and at all sorts of angles with the planetary orbits are the paths of hundreds of comets, delicate filmy bodies of a wholly different constitution from the planets, and which now and then blaze forth in the sky, their tails appearing much like the beam of a searchlight, and compelling for the time the attention of everybody. connected with the comets and doubtless originally parts of them are uncounted millions of millions of meteors, which for the time become a part of the solar system, their minute masses being attracted to the planets, upon which they fall, those hitting the earth being visible to us as familiar shooting stars. we next follow the story of astronomy through the solar system, beginning with the sun itself and proceeding outward through his family of planets, now much more numerous and vastly more extended than it was to the ancient world, or indeed till within a century and a half of our own day. chapter xxv the sun and observing it as lord of day, king of the heavens, mankind in the ancient world adored the sun. by their researches into the epoch of the assyrians, hittites, phoenicians and other early peoples now passed from earth, archæologists have unearthed many monuments that evidence the veneration in which the early peoples who inhabited egypt and asia minor many thousand years ago held the sun. a striking example is found in the architecture of early egyptian temples, on the lintels of which are carved representations of the winged globe or the winged solar disk, and there is a bare possibility that the wings of the globe were suggested by a type of the solar corona as glimpsed by the ancients. little knew they about the distance and size of the sun; but the effects of his light and heat upon all vegetal and animal life were obvious to them. doubtless this formed the basis for their worship of the sun. occasional huge spots must have been visible to the naked eye, and the sun's corona was seen at rare intervals. plutarch and philostratus describe it very much as we see it to-day. how completely dependent mankind is upon the sun and its powerful radiations, only the science of the present day can tell us. by means of the sun's heat the forests of early geologic ages were enabled to wrest carbon from the atmosphere and store it in forms later converted by nature's chemistry into peat and coal. through processes but imperfectly understood, the varying forms of vegetable life are empowered to conserve, from air and soil, nitrogen and other substances suitable for and essential to the life maintenance of animal creatures. breezes that bring rain and purify the air; the energy of water held under storage in stream and dam and fall; trade winds facilitating commerce between the continents; oceanic currents modifying coastal climates; the violence of tornado, typhoon and water-spout, together with other manifestations of natural forces--all can be traced back to their origin in the tremendous heating power of the solar rays. in everything material the sun is our constant and bountiful benefactor. if his light and heat were withdrawn, practically every form of human activity on this planet would come to an early end. how far away is the sun? what is the size of the sun? these are questions that astronomers of the present day can answer with accuracy. so closely do they know the sun's distance that it is employed as their yardstick of the sky, or unit of celestial measurement. many methods have been utilized in ascertaining the distance of the sun, and the remarkable agreement among them all is very extraordinary. some of them depend upon pure geometry, and the basic measure which we make from the earth is not the distance of the sun directly; but we find out how far away venus is during a transit of venus, for example, or how far away mars is or some of the asteroids are at their closer oppositions. then it is possible to calculate how far away the sun is, because one measurement of distance in the solar system affords us the scale on which the whole structure is built. but perhaps the simplest method of getting the sun's distance is by the velocity of light, , miles a second. from eclipses of jupiter's moons we know that light takes minutes seconds to pass from sun to earth. so that the sun's distance is the simple product of the two, or millions of miles. once this fundamental unit is established, we have a firm basis on which to build up our knowledge of the distances, the sizes and motions of the heavenly bodies, especially those that comprise the solar system. we can at once ascertain the size of the sun, which we do by measuring the angle which it fills, that is, the sun's apparent diameter. finding this to be something over a half a degree in arc, the processes of elementary trigonometry tell us that the sun's globe is , miles in diameter. for nearly a century this has been accurately measured with the greatest care, and diameters taken in every direction are found to be equal and invariably the same. so we conclude that the sun is a perfect sphere, and so far as our instruments can inform us, its actual diameter is not subject to appreciable change. the vastness of the sun's volume commands our attention. as his diameter is times that of the earth, his mere size or volume is × × or , thousand times that of the earth, because the volumes of spheres are in proportion as the cubes of their diameters. if the materials that compose the sun were as heavy as those that make up the earth, it would take , thousand earths to weigh as much as the sun does. but by a method which we need not detail here, the sun's actual weight or mass is found to be only thousand (more nearly , ), times greater than the earth's. so we must infer that, bulk for bulk, the component materials of the sun are about one-fourth lighter than those of the earth, that is, about one and one-half times as dense as water. to look at this in another way: it is known that a body falling freely toward the earth from outer space would acquire a speed of seven miles a second, whereas if it were to fall toward the sun instead, the velocity would be miles a second on reaching his surface. if all the other bodies of the solar system, that is, the earth and moon, all the planets and their satellites, the comets and all were to be fused together in a single globe, it would weigh only one-seven hundred and fiftieth as much as the sun does. at the surface, however, the disproportion of gravity is not so great, because of the sun's vast size: it is only about twenty-eight times greater on the sun than on the earth; and instead of a body falling feet the first second as here, it would fall feet there. pendulums of clocks on the sun would swing five times for every tick here, and an athlete's running high jump would be scaled down to three inches. let us next inquire into the amount of the sun's light and heat, and the enormously high temperature of a body whose heat is so intense even at the vast distance at which we are from it. the intensity of its brightness is such that we have no artificial source of light that we can readily compare it with. in the sky the next object in brightness is the full moon, but that gives less than the half-millionth part as much light as the sun. the standard candle used in physics gives so little light in comparison that we have to use an enormous number to express the quantity of light that the sun gives. a sperm candle burning grains hourly is the standard, and if we compare this with the sun when overhead, and allow for the light absorbed by the atmosphere, we get the number with twenty-four ciphers following it, to express the candlepower of the sun's light. if we interpose the intense calcium light or an electric arc light between the eye and the sun, these artificial sources will look like black spots on the disk. indeed, the sun is nearly four times brighter than the "crater," or brightest part of the electric arc. the late professor langley at a steel works in pennsylvania once compared direct sunlight with the dazzling stream of molten metal from a bessemer converter; but bright as it was, sunlight was found to be five thousand times brighter. equally enormous is the heat of the sun. our intensest sources of artificial heat do not exceed , degrees fahrenheit, but the temperature at the sun's surface is probably not less than , degrees f. one square meter of his surface radiates enough heat to generate , horsepower continuously. at our vast distance of millions of miles, the sun's heat received by the earth is still powerful enough to melt annually a layer of ice on the earth more than a hundred feet in thickness. if the solar heat that strikes the deck of a tropical steamship could be fully utilized in propelling it, the speed would reach at least ten knots. many attempts have been made in tropical and sub-tropical climates to utilize the sun's heat directly for power, and ericsson in sweden, mouchot in france, and shuman in egypt have built successful and efficient solar engines. necessary intermission of their power at night, as well as on cloudy days, will preclude their industrial introduction until present fuels have advanced very greatly in cost. all regions of the sun's disk radiate heat uniformly, and the sun's own atmosphere absorbs so much that we should receive . times more heat if it were removed. so far as is known, solar light and heat are radiated equally in all directions, so that only a very minute fraction of the total amount ever reaches the earth, that is, millionth part of the whole. indeed all the planets and other bodies of the solar system together receive only one one hundred millionth part; the vast remainder is, so far as we know, effectively wasted. it is transformed, but what becomes of it, and whether it ever reappears in any other form, we cannot say. how is this inconceivably vast output of energy maintained practically invariable throughout the centuries? many theories have been advanced, but only one has received nearly universal assent, that of secular contraction of the sun's huge mass upon itself. shrinkage means evolution of heat; and it is found by calculation that if the sun were to contract its diameter by shrinking only two-hundred and fifty feet per year, the entire output of solar heat might thus be accounted for. so distant is the sun and so slow this rate of contraction that centuries must elapse before we could verify the theory by actual measurements. meanwhile, the progress of physical research on the structure and elemental properties of matter has brought to light the existence of highly active internal forces which are doubtless intimately concerned in the enormous output of radiant energy, though the mechanism of its maintenance is as yet known only in part. abbot, from many years' observations of the solar constant, at washington, on mount wilson, and in algeria, finds certain evidence of fluctuation in the solar heat received by the earth. it cannot be a local phenomenon due to disturbances in our atmosphere, but must originate in causes entirely extraneous to the earth. interposition of meteoric dust might conceivably account for it, but there is sufficient evidence to show that the changes must be attributed to the sun itself. the sun, then, is a variable star; and it has not only a period connected with the periodicity of the sun spots, but also an irregular, nonperiodic variation during a cycle of a week or ten days, though sometimes longer, and occasioning irregular fluctuations of two to ten per cent of the total radiation. radiation is found to increase with the spottedness. attempts have been made on the basis of the contraction theory to find out the past history of the sun and to predict its future. probably to millions of years in the past represents the life of the sun much as it is at present; and if solar radiation in the future is maintained substantially as now, the sun will have shrunk to one-half its present diameter in the next five million years. so far then as heat and light from the sun are concerned, the sun may continue to support life on the earth not to exceed ten million years in the future. but the sun's own existence, independently of the orbs of the system dependent upon it, might continue for indefinite millions of aeons before it would ever become a cold dead globe; indeed, in the present state of science, we cannot be sure that it is destined to reach that condition within calculable time. a few words on observing the sun, an object much neglected by amateurs. on account of the intense light, a very slight degree of optical power is sufficient. indeed a piece of window glass, smoked in a candle flame with uniform graduation from end to end, will be found worth while in a beginner's daily observation of the sun. the glass should be smoked densely enough at one end so that the sunlight as seen through it will not dazzle the eye on the clearest days. at the other end of the glass, the degree of smoke film should not be quite so dense, so that the sun can be examined on hazy, foggy or partly cloudy days. an occasional naked-eye spot will reward the patient observer. if a small spyglass, opera glass or field glass is at hand, excellent views of the sun may be had by mounting the glass so that it can be held steadily pointed on the sun, and then viewing the disk by projection on a white card or sheet of paper. care must be taken to get a good focus on the projected image, and then the faculæ, or whitish spots, or mottling nearer the sun's edge will usually be well seen. by moving the card farther away from the eyepiece, a larger disk may be obtained, in effect a higher degree of magnification. but care must be used not to increase it too much. keep direct sunlight outside the tube from falling on the card where the image is being examined. this is conveniently done by cutting a large hole, the size of the brass cell of the object glass, through a sheet of corrugated strawboard, and slipping this on over the cell. in this way the spots on the sun can be examined with ease and safety to the eye. for large instruments a special type of eyepiece is provided known as a helioscope, which disposes of the intense heat rays that are harmful to the eye. frequent examination of the eyepiece should be made and the eyepiece cooled if necessary. that part of the sun's surface under observation is known as the photosphere, that is, the part which radiates light. if the atmosphere admits the use of high magnifying powers, the structure of the photosphere will be found more and more interesting the higher the power employed. it is an irregularly mottled surface showing a species of rice-grain structure under fairly high magnification. these grains are grouped irregularly and are about miles across. under fine conditions of vision they may be subdivided into granules. the faculæ, or white spots, are sometimes elevations above the general solar level; they have occasionally been seen projecting outside the limb, or edge of the disk. chapter xxvi sun spots and prominences dark spots of a deep bluish black will often be seen on the photosphere of the sun. sometimes single, though generally in groups, the larger ones will have a dark center, called the umbra, surrounded by the very irregular penumbra which is darker near its outer edge and much brighter apparently on its inner edge where it joins on the umbra. the penumbra often shows a species of thatch-work structure, and systematic sketches of sun spots by observers skilled in drawing are greatly to be desired, because photography has not yet reached the stage where it is possible to compete with visual observation in the matter of fine detail. the spots themselves nearly always appear like depressions in the photosphere, and on repeated occasions they have been seen as actual notches when on the edge of the sun. many spots, however, are not depressions: some appear to be actual elevations, with the umbra perhaps a central depression, like the crater in the general elevation of a volcano. spots are sometimes of enormous size. the largest on record was seen in ; it was nearly , miles in breadth, and covered a considerable proportion of the whole visible hemisphere of the sun. a spot must be nearly , miles across in order to be seen with the naked eye. in their beginning, development, and end, each spot or group of spots appears to be a law unto itself. sometimes in a few hours they will form, though generally it is a question of days and even weeks. very soon after their formation is complete, tonguelike encroachments of the penumbra appear to force their way across the umbra, and this splitting up of the central spot usually goes on quite rapidly. sun spots in violent disturbance are rarely observed. as the sun turns round on his axis, the spots will often be carried across the disk from the center to the edge, when they become very much foreshortened. the sun's period of rotation is days, so that if a spot lasts more than two weeks without breaking up, it may reappear on the eastern limb of the sun after having disappeared at the western edge. two or three months is an average duration for a spot; the longest on record lasted through months in - . the position of the sun's axis is well known, its equator being tilted about degrees to the ecliptic, and the spots are distributed in zones north and south of the equator, extending as far as degrees of solar latitude. in very high latitudes spots are never seen; they are most abundant in about latitude degrees both north and south, and rather more numerous in the northern than in the southern hemisphere of the sun. recent research at mount wilson makes the sun a great magnet; and its magnetic axis is inclined at an angle of degrees to the axis of rotation, around which it revolves in days. there is a most interesting periodicity of the spots on the sun, for months will sometimes elapse with spots in abundance and visible every day, while at other periods, days and even weeks will elapse without a single spot being seen. there is a well recognized period of eleven and one-tenth years, the reason underlying which is not, however, known. after passing through the minimum of spottedness, they begin to break out again first in latitudes of degrees- degrees, rather suddenly, and on both sides of the equator, and they move toward the equator as their number and individual size decrease. the last observed epoch of maximum spot activity on the sun was passed in . many attempts have been made to ascertain the cause of the periodicity of sun spots, but the real cause is not yet known. if the spots are eruptional in character, the forces held in check during seasons of few spots may well break out in period. the brighter streaks and mottlings known as faculæ are probably elevations above the general photosphere, and seem to be crusts of luminous matter, often incandescent calcium, protruding through from the lower levels. generally the faculæ are numerous around the dark spots, and absorption of the sun's light by his own atmosphere affords a darker background for them, with better visibility nearer the rim of the solar disk. the spectroheliograph reveals vast zones of faculæ otherwise invisible, related to the sun-spot zones proper on both sides of the equator. in some intimate way the magnetism of sun and earth are so related that outbreaks of solar spots are accompanied with disturbances of electrical and other instruments on the earth; also the aurora borealis is seen with greater frequency during periods when many spots are visible. within very recent years the discovery of a magnetic field in sun spots has been made by hale with powerful instruments of his own design. sun spots had never been investigated before with adequate instrumental means. he recognized the necessity of having a spectroscope that would record the widened lines of sun-spot spectra, and the strengthened and weakened lines on a large scale. certain changes in relative intensity were traced to a reduced temperature of the spot vapors by comparison with photographs of the spectrum of iron and other metallic vapors in an electric arc at different temperatures. here the work of the laboratory was essential. sun spots were thus found to be regions of reduced temperature in the solar atmosphere. chemical unions were thus possible, and thousands of faint lines in spot-spectra were measured and identified as band lines due to chemical compounds. thus the chemical changes at work in sun-spot vapors were recognized. then followed the highly significant investigations of solar vortices and magnetic fields. improvements in photographic methods had revealed immense vortices surrounding sun spots in the higher part of the hydrogen atmosphere; and this led to the hypothesis that a sun spot is a solar storm, resembling a terrestrial tornado, and in which the hot vapors whirling at high velocity are cooled by expansion. this would account for the observed intensity changes of the spectrum lines and the presence of chemical compounds. the vortex hypothesis suggested an explanation of the widening of many spot lines, and the doubling or trebling of some of them. as it is known that electrons are emitted by hot bodies, they must be present in vast numbers in the sun; and positive or negative electrons, if caught and whirled in a vortex, would produce a magnetic field. zeeman in had discovered that the lines in the spectrum of a luminous vapor in a magnetic field are widened, or even split into several components if the field is strong enough. characteristic effects of polarization appear also. the new apparatus of the observatory in conjunction with experiments in the laboratory immediately provided evidence that proved the existence of magnetic fields in sun spots, and strengthened the view that the spots are caused by electric vortices. extended investigations have led hale to the conclusion that the sun itself is a magnet, with its poles situated at or near the poles of rotation. in this respect the sun resembles the earth, which has long been known to be a magnet. the sun's axial rotation permits investigation of the magnetic phenomena of all parts of its surface, so that ultimately the exact position of the sun's magnetic poles and the intensity of the field at different levels in the solar atmosphere will be ascertained. schuster is of the opinion that not only the sun and earth, but every star, and perhaps every rotating body, becomes a magnet by virtue of its rotation. hale is confident that the -inch reflector will permit the test for magnetism to be applied to a few of the stars. the sun can be observed at mount wilson on at least nine-tenths of all the days in the year, and a daily record of the polarities of all spots with the -foot tower telescope is a part of the routine. a method has been devised for classifying sun spots on the basis of their magnetic properties, and more than a thousand spots have already been so classified. about per cent of all sun spots are found to be binary groups, the single or multiple members of which are of opposite magnetic polarity. unipolar spots are very seldom observed without some indication of the characteristics of bipolar groups. these are usually exhibited in the form of flocculi following the spot. the bipolar spot seems to be the dominant type, and the unipolar type a variant of it. although devised for quite another purpose, that of photographing the hydrogen prominences on the limb of the sun, the spectroheliograph has contributed very effectively to many departments of solar research. the prominences are dull reddish cloudlets that were first seen during total eclipses of the sun. probably vassenius, a swedish astronomer, during the total eclipse of , made the earliest record of them, as pinkish clouds quite detached from the edge of the moon; and in that day, when it had not yet been proved that the moon was without atmosphere, he naturally thought they belonged to the moon, not the sun. undoubtedly ulloa, a spanish admiral, also saw the prominences in observing the total eclipse of ; but they seem to have attracted little attention till , when a very important total eclipse was central throughout europe, and observed with great care by many of the eminent astronomers of all countries. so different did the prominences appear to different eyes, and so many were the theories as to what they were, that no general consensus of opinion was reached, and some thought them no part of either sun or moon, but a mere mirage or optical illusion. but at the return of this eclipse in , photography was employed so as to demonstrate beyond a shadow of doubt the real existence and true solar character of the prominences. by the slow progress of the moon across the sun and the prominences on the edge, a unique series of photographs by de la rue showed the moon's edge gradually cutting off the prominences piecemeal on one side of the sun, and equally gradually uncovering them on the opposite side. the prominences, then, were known to be real phenomena of the sun, some of them disconnectedly floating in his atmosphere, as if clouds. their forms did not vary rapidly, they were very abundant, and their light was so rich in rays of great photographic intensity that many were caught on the plate which the eye failed to see; they appeared at every part of the sun's limb and their height above it indicated that they must be many thousand miles in actual dimension. what they were, however, remained an entire mystery, and no one even thought it possible to find out what their chemical constitution might be or to measure the speed with which they moved. a few years later came the great indian eclipse (august , ), at that date the longest total eclipse ever observed. janssen of france and many others went out to india to witness it. fortunately the prominences were very brilliant and this led janssen to believe it would be possible for him to see them the day after the eclipse was over. by modifying the adjustment of his apparatus suitably and changing its relation to the sun's edge, he found that hydrogen is the main constituent in the light of the prominences. in addition to this he was able to trace out the shapes of the prominences, and even measure their dimensions. his station in india was at guntoor, many weeks by post from home; so that his account of this important discovery reached the paris academy of sciences for communication with another from the late sir norman lockyer of england, announcing a like discovery, wholly independently. the principle is simply this, and admirably stated by young: "under ordinary circumstances the prominences are invisible, for the same reason as the stars in the daytime: they are hidden by the intense light reflected from the particles of our own atmosphere near the sun's place in the sky; and if we could only sufficiently weaken this aerial illumination, without at the same time weakening _their_ light, the end would be gained. and the spectroscope accomplishes this very thing. since the air-light is reflected sunshine, it of course presents the same spectrum as sunlight, a continuous band of color crossed by dark lines. now, this sort of spectrum is greatly weakened by every increase of dispersive power, because the light is spread out into a longer ribbon and made to cover a more extended area. on the other hand, a spectrum of bright lines undergoes no such weakening by an increase in the dispersive power of the spectroscope. the bright lines are only more widely separated--not in the least diffused or shorn of their brightness." simultaneous announcement of this great discovery, by astronomers of different nations, working in widely separate regions of the earth, led to the striking of a gold medal by the french government in honor of both astronomers and bearing their united effigies. ever since the famous indian eclipse of , it has not been necessary to wait for a total eclipse in order to observe the solar prominences, but every observer provided with suitable apparatus has been able to observe them in full sunlight whenever desired, and the charting of them is part of the daily routine at several observatories in different parts of the world. so vast has been the accumulation of data about them that we know their numbers to fluctuate with the spots on the sun; and their distribution over the sun's surface resembles in a way that of the spots. while the spots and protuberances are most numerous around solar latitude degrees both north and south, the prominences do not disappear above latitude to degrees, as the spots do, but from latitude degrees they increase in number to about degrees, and are occasionally observed even at the sun's poles. faculæ and prominences are more closely related than the sun spots and prominences. there are wide variations in both magnitude and type of the prominences. heights above the sun's limb of a few thousand miles are very common, and they rarely reach elevations as great as , miles, though a very occasional one reaches even greater heights. classification of the prominences divides them into two broad types, the quiescent and the eruptive. the former are for the most part hydrogen, and the latter metallic. the quiescent prominences resemble closely the stratus and cirrus type of terrestrial clouds, and are frequently of enormous extent along the sun's edge. they are relatively long-lived, persisting sometimes for days without much change. the eruptive prominences are more brilliant, changing their form and brightness rapidly. often they appear as brilliant spikes or jets, reaching altitudes that average about , miles. rarely seen near the sun's poles, they are much more numerous nearer the sun spots. speed of motion of their filaments sometimes exceeds one hundred miles a second, and the changing variety of shapes of the eruptive prominences is most interesting. oftentimes they change so rapidly that only photography can do them justice. prominence photography began with young a half century ago, who obtained the first successful impression on a microscope slide with a sensitized film of collodion; as was necessary in the earlier wet-plate process of photography, which required exposures so long that little progress was effected for about twenty years. then it was taken up by deslandres of paris and hale of chicago independently, both of whom succeeded in devising a complex type of apparatus known as the spectroheliograph, by which all the prominences surrounding the entire limb of the sun can be photographed at any time by light of a single wave-length, together with the disk of the sun on the same negative. the prominences appear to be intimately connected with a gaseous envelope surrounding the solar photosphere, in which sodium and magnesium are present as well as hydrogen. the depth of the chromosphere is usually between , and , miles, and its existence was first made out during the total solar eclipses of and , when it appeared as an irregular rose-tinted fringe, though not at the time recognized as belonging to the sun. the constitution of the sun and its envelopes are still under discussion, and no complete theory of the sun has yet been advanced which commands the widest acceptance. of the interior of the sun we can only surmise that it is composed of gases which, because of intense heat and compression, are in a state unfamiliar on earth and impossible to reproduce in our laboratories. their consistency may be that of melted pitch or tar. surrounding the main body of the sun are a series of layers, shells, or atmospheres. outside of all and very irregular in structure, indeed probably not a solar atmosphere at all, is the solar corona, parts of which behave much as if it were an atmosphere, but it appears to be bound up in some way with the sun's radiation. it has streamers that vary with the sun-spot period, but its constitution and function are very imperfectly known, because it has never been seen or photographed except at rare intervals on occasion of total eclipses of the sun. beneath the corona we meet the projecting prominences, to which parts of the corona are certainly related, and beneath them the first true layer or atmosphere of the sun known as the chromosphere, its average depth being about one-hundredth part of the sun's diameter. beneath the chromosphere is the layer of the sun from which emanates the light by which we see it, called the photosphere. it appears to be composed of filaments due to the condensation of metallic vapors, and it is the outer extremities of these filaments which are seen as the granular structures everywhere covering the disk of the sun. their light shines through the chromosphere and the spots are ruptures in this envelope. between photosphere and chromosphere is a very thin envelope, probably not over miles in thickness, called the reversing layer. it is this relatively thin shell that is responsible for the absorption which produces the dark lines in the spectrum of the sun. under normal conditions the filaments of the photosphere are radial, that is vertical on the sun; but whenever eruptions take place, as during the occurrence of spots, the adjacent filaments are violently swept out of their normal vertical lines and these displaced columns then form what we view as the spot's penumbra. from the outer surface of the sun's chromosphere rise in eruptive columns vapors of hydrogen and the various metals of which the sun is composed. these and the spots would naturally occur in periods just as we see them. we have said that the sun is composed of a mass of highly heated or incandescent vapors or gases, whose compression on account of gravity must render their physical condition quite different from any gaseous forms known on the earth or which we can reproduce here. as the result of more than half a century of studious observation of the sun and mapping of its spectrum in every part, and diligent comparison with the spectra of all known chemical elements on the earth, we find that the sun contains no elements not already found here, but that a great preponderance of elements known to earth are found in the sun. the intensity of their spectral lines is one prominent indication of the presence of elements in the sun, and the number of coincidences of spectral lines is another. iron, nickel, calcium, manganese, sodium, cobalt, and carbon are among the elements most strongly identified. a few of the rarer terrestrial elements are of doubtful existence in the sun, and a very few, as gold, bismuth, antimony, and sulphur are not found there, and the existence of oxygen in the sun is regarded by some experts as doubtful. but if the whole earth were vaporized by heat, probably its spectrum would resemble that of the sun very closely. what are the effects of the sun, and sun spots in particular, on our weather? is the influence of their periodicity potent or negligible? if we investigate conditions pertaining to terrestrial magnetism, as fluctuations of the magnetic needle, and the frequency of auroræ, there is no occasion for doubt of the sun's direct influence, although we are not able to say just how that influence becomes potent. if, however, we look into questions of temperature, barometric pressure, rainfall, cyclones, crops, and consequent financial conditions, we find fully as much evidence against solar influence as for it. the slight variations of the sun's light and heat due to the presence or absence of sun spots can scarcely be sensible, and much longer periods of closer observation are necessary before such questions can be finally decided. the slighter such influences are, if they actually exist, and the more veiled they are by other influences more or less powerful, the more difficult it is to discover their effects with certainty. the importance of solar radiation in the prediction of terrestrial weather has long been recognized, but until very recently no practical application has been made. the smithsonian astrophysical observatory at washington, under the direction of dr. abbot, has for many years carried on at a number of stations a series of determinations of the constant of solar radiation by the spectro-bolometric method originated by langley. a new station in calama, chile, has recently been inaugurated, at which the solar constant is worked out each day, and telegraphed to the argentine weather service, where it is employed in forecasting for the day. abbot's new method of solar constant determination is based on the fact that atmospheric transparency varies oppositely to the variations of brightness of the sky. increase of haziness presents more reflecting surface to scatter the solar rays indirectly to the earth. of course it presents also additional surface to obstruct the direct rays from the sun. by measuring the brightness of the sky near the sun, it becomes possible to infer the coefficients of atmospheric transmission at all wave lengths. the direct observations and the complete deduction of the solar constant for the day can all be completed within two or three hours. clayton of buenos aires has now employed these results in the argentine weather predictions for two years, and the introduction of this new element in forecasting has brought about a pronounced gain in the value of the predictions. its adoption by the weather bureaus of other nations will doubtless come in due time, and the new method take a firmly established rank in practical meteorology. abbot's observations many years ago first called attention to the variability of the solar constant through a range of several per cent both from year to year, and in irregular short periods of weeks or even days. abbot considers this the more likely explanation than that atmospheric changes should take place simultaneously all over the earth. the sun is but a star, the stars that are irregularly variable in light and heat are numerous, and the sun itself appears to be one of these. especially important to the agricultural and vineyard interests of argentina is the question of precipitation, and clayton finds this very dependent on solar radiation. at epochs of practically stationary solar intensity, there is little or no precipitation; but quite generally he finds that great decrease of solar radiation is followed in from three to five days by heavy precipitation. direct temperature effects are also traced in buenos aires and other south american cities, lagging from two to three days behind the observed solar fluctuations. the station at calama yields about determinations of the solar constant each year, and the mount wilson station about half that number. they are the only stations of this character at present in existence, and others should be established in widely separated and cloudless regions, as egypt, southern california and australia. uniformity in the methods of observing would be highly desirable, and the smithsonian institution has perfected the details of common control of such stations which it is expected may be established at an early day. chapter xxvii the inner planets vulcan about the middle of the last century, le verrier, a great french astronomer, having added the planet neptune beyond the outside confines of the solar system, sought evidence of a lesser planet traveling round the sun within the orbit of mercury. for many years close watch was kept on the sun in the hope of discovering such a body in the act of passing across the disk, or in transit, as it is technically termed. lescarbault, a french physician, announced that he had actually seen such a planet, vulcan it was called, passing over the sun in . total eclipses of the sun would afford the best opportunity for seeing such a body, and on several such occasions astronomers thought they had found it. but the signal advantages of photography have been applied so often to this search, and always unsuccessfully, that the existence of vulcan, or the intramercurian planet, is now regarded as mythical. mercury this planet is an elusive body that very few, even astronomers, have ever seen. it is not very bright, has a rapid motion and never retreats far from the sun, so that it was a puzzle to the ancients who saw it, sometimes in the twilight after sunset and again in the twilight of dawn. when following the sun down in the west, in march or april, mercury is likely to be best seen; twinkling rather violently and nearly as bright as a star of the first magnitude. very little is to be seen on the minute disk of this planet, except that it goes through all the phases of the moon--crescent, gibbous, full, gibbous, crescent. whether mercury turns round on its axis or not, cannot be said to be known, because the markings that are suspected on its surface are too indefinite to permit exact observation. more than likely the planet presents always the same side or face to the sun, so that it turns round on its axis once, while traveling once around the sun in its orbit. mercury's day and year would therefore be equal in length. nor have we much evidence on the question of an atmosphere surrounding mercury; probably it is very thin, if indeed there is any at all. when mercury comes directly between us and the sun, crossing in transit, the edge of the planet as projected against the sun is very sharply defined, and this would indicate an absence of atmosphere on mercury. transits of mercury can occur in may and november only: there was one on november , , and there will be one on may , . the latter will be nearly eight hours in length, which is almost the limit. mercury's distance from the sun averages million miles, the diameter of the planet is , miles, and his orbital speed is miles per second, the swiftest of all the planets. no moon of mercury is known to exist, although many times diligently searched for, especially during transits of the planet. venus brightest of all the planets, and the most beautiful of all is venus. its path is next outside the orbit of mercury, but within that of the earth, so that it partakes of all the phases of the moon. like mercury it sometimes passes exactly between us and the sun, a rare phenomenon which is known as a transit of venus. being without telescopes, the ancients knew nothing about these occurrences, but they were puzzled for centuries over the appearance of the planet in the west after sunset, when they called it hesperus, and in early dawn in the east when they gave it the name phosphorus. venus is known to be girdled with an atmosphere denser than ours, and it seems to be always filled with dense clouds. it is the reflection of sunlight from this perpetually cloudy exterior which gives venus her singular radiance. so brilliant is she that even full daylight is not strong enough to overpower her rays; and she may often be seen glistening in the clear blue daytime sky, if one knows pretty nearly in what direction to look for her. venus is million miles from the sun, and as our own distance is million miles, this planet can come within million miles of the earth. it is therefore at times our nearest known neighbor in space, excepting only the moon and eros, one of the erratic little planets that travel round the sun between mars and jupiter. also possibly a comet might come much nearer. astronomers always take advantage of this nearness of venus to us, if a transit across the sun takes place; because it affords an excellent method of finding out what the distance of the sun is from the earth. a pair of these transits happens about once a century, there were transits in and , and the next pair occur in and . in actual size, venus is almost as large a planet as our own, being , miles in diameter, as compared with , for the earth. her velocity in her orbit is twenty-two miles per second, and she travels all the way round the sun in seven and one half months or days. venus from her striking brilliancy always leads the novice to expect to see great things on applying the telescope. but aside from a brilliant disk, now a slender crescent, now half full like the moon at quarter, and again gibbous as the moon is between quarter and full, the telescope reveals but little. there is pretty good evidence that the markings thought to have been seen on the planet's surface are illusory, and so it is wholly uncertain in what direction the planet's axis lies; also there is great uncertainty about the length of the day on venus, or the period of turning round on its axis. probably it is the same in length as the planet's year. once when venus passed very close to the sun, just barely escaping a transit, lyman of yale university caught sight of it by hiding the sun behind a tall building or church spire. the dark side of venus was turned toward us and he could not of course see that. but the planet was clearly there, completely encircled by a narrow delicate luminous ring, which was due to sunlight shining through the atmosphere that surrounds the planet. similar ring effects were seen by observers of the transits of venus in and ; and from all their observations it is concluded that venus has an atmosphere probably at least twice as dense and extensive as that which encircles the earth. spurious satellites of venus are many, but no real moon is known to attend this planet. [illustration: the surface of the moon in the region of copernicus. photograph made with the hooker -inch reflecting telescope. (_photo, mt. wilson solar observatory._)] [illustration: a view of the south central portion of the moon at last quarter. (_photo, mt. wilson solar observatory._)] chapter xxviii the moon and her surface as the sun has always reigned as king of day, so is the moon queen of night. observation of her phases, now waxing, now waning, with her stately motion always eastward among the stars, began with the earliest ages. often when near the full she must have been seen herself eclipsed, and much more rarely the occurrence of total eclipses of the sun are certain to have suggested the moon's intervention between earth and sun, shutting off the sunlight completely, because these eclipses never took place except when the moon was in the same part of the sky with the sun. if we watch the nightly march of the moon, we shall find that she travels over her own breadth in about an hour's time. by using a telescope on the stars just eastward or to the left of her, she will now and then be seen to pass between us and a star--on very rare occasions a planet--extinguishing its light with great suddenness, the most nearly instantaneous of all phenomena in nature. draw a line connecting the cusps, or horns of the lunar crescent, and then a line eastward at right angles to this, and it will show the direction of the moon's own motion in its orbit round the earth quite accurately. as the phase advances, note the inside edge of the advancing crescent: this will be quite rough and jagged, compared to the outside edge which is the moon's real contour and relatively very smooth. the position of the inside curve will change from night to night, and it marks the line of sunrise on the moon during the fortnight elapsing between new moon and full; while from full through last quarter and back to new moon, this advancing line marks the region of sunset on the moon. the general shape of this line is never a circle but always elliptical, and astronomers call it the terminator. all along the terminator, sunlight strikes the lunar surface at a small angle, whether near sunrise or sunset; so that owing to the mountains and other high masses of the moon's surface, the terminator is always a more or less jagged and irregular line. onward from new moon toward full the horns of the crescent are always turned upward or eastward. when the general line of the terminator becomes a straight line from cusp to cusp, the moon is said to have reached first quarter or quadrature. onward toward full the terminator will be seen to bend the other way, and in about a week's time it will have merged itself with the moon's limb. the moon is then said to be full. afterward the phase phenomena recur in the reverse order, with third quarter midway between full and new moon again; the phase of the moon being called gibbous all the way from first quarter to third quarter, except when exactly full. as we know that the moon is, like the earth, a nonluminous body, and shines only by virtue of the sunlight falling upon it, clearly an entire half of the moon's globe must be perpetually illumined by sunlight. the varying phases then are due simply to that part of the illuminated hemisphere which is turned toward us. new moon is entirely invisible because the sunward hemisphere is turned wholly away from us, while at full moon we see the lunar disk complete because we are on the same side of the moon that the sun is and practically in line with both sun and moon. if we could visit the moon, we should see the earth in exactly complementary phase. at new moon here we should be enjoying full earth there, and full moon here would be coincident with new or dark earth there. the narrow crescent of new moon here would be the period of gibbous earth there; and it is the reflection of sunlight from this gibbous earth which illuminates the part of the moon but faintly seen at this time, popularly known as the "old moon in the new moon's arms." its greater visibility at some times than at others is due to greater prevalence of clouded area in the reflecting regions of the earth turned toward the moon, and the higher reflective power of clouds than that possessed by mere land and water. as the moon goes all the way round the sky every month, the same as the sun does in a year, and travels in nearly the same path, clearly it must also go north and south every month as the sun does. so in midsummer when the sun runs high upon the meridian, we expect to find full moons running low, and likewise in midwinter the full moon always runs high, as almost everyone has sometimes or other noticed. this eastward or true orbital motion of the moon is responsible for another relation which soon comes to light when we begin to observe the moon; and that is the later hour of rising or setting each night. our clock time is regulated by the sun, which also is moving eastward about ° daily, or twice its own breadth. so the moon's eastward gain on the sun amounts to about degrees daily, and one degree being equal to minutes, the retarded time of moonrise or moonset each day amounts to very nearly minutes on the average; though sometimes the delay will be less than a half hour and at other times it will exceed an hour and a quarter. the season of least retardation of rising of the full moon is in the autumn, and so the moon that falls in late september or october is known as the harvest moon, and the next succeeding full moon is called the hunter's moon. lunation is a term sometimes given to the moon's period from any definite phase round to the same phase again. its length is the true period of the moon's revolution once around the earth, from the sun all the way round till it overtakes the sun again. the synodic period is another name for lunation, and its true length is and one-half days, or very accurately d. h. m. . s. as calculated by astronomers with great exactness from many thousand revolutions of the moon. but if we want the true period of the moon round the earth as referred to a star, it is much shorter than this, amounting to only days and nearly one-third. this is called the moon's sidereal period of revolution, because it is the time elapsed while she is traveling eastward from a given star around to coincidence with the same star again. if we study the moon's path in the sky more critically, we shall find that it does not quite follow the ecliptic, or the sun's path, but that twice each month she deviates from the ecliptic, once to the north and once to the south of it, by roughly ten times her own breadth. more accurately this angle is ° ' ", an almost invariable quantity, and it is therefore known as an astronomical constant, or the inclination of the moon's orbit to the ecliptic. so the moon's orbit must intersect the ecliptic, and as both are great circles in the sky, the points of intersection are known as the moon's nodes, one ascending and the other descending, and the nodes are degrees apart. the figure of the moon's orbit is not circular, although it deviates only slightly from that form. but like the paths of all other satellites round their primary planets, and of the planets themselves round the sun, the moon's orbit is also an ellipse. the distance of the moon's center from the earth's center is therefore perpetually changing; the point of nearest approach is called perigee, and that of farthest recession, apogee. the moon's distance from the earth is easier and simpler to be ascertained than that of any other heavenly body, because it is the nearest. an outline of the method of finding this distance is not difficult to present; and it resembles in every particular the method a surveyor uses to find the distance of some inaccessible point which he cannot measure directly. up and down a stream, for example, he measures the length of a line, and from each end of it he measures the angle between the other end of the line and the object on the opposite side of the stream whose distance he wishes to find out. then he applies the science of trigonometry to these three measures, two of angles and one the length of the side or base included between them, and a few minutes' calculation gives the distance of the inaccessible object from either end of the base line. now in like manner, to transfer the process to the sky, let the two ends of the base be represented by two astronomical observatories, for example, greenwich in the northern hemisphere and cape town in the southern. the base line is the chord or straight line through the earth connecting the two observatories, and we know the length of this line pretty accurately, because we know the size of the earth. the angles measured are somewhat different from those in the terrestrial example, but the process amounts to the same thing because the astronomers at the two observatories measure the angular distance of the center of the moon from the zenith, each using his own zenith at the same time; and the same science of trigonometry enables them to figure out the length of any side of the triangles involved. the side which belongs to both triangles is the distance from the center of the earth to the center of the moon, and the average of many hundred measures of this gives , miles, or about ten times the distance round the equator of the earth. we have said that the orbit in which the moon travels round the earth is practically a circle, but the earth's center is found not at the center of this orbit, but set to one side, or eccentrically, so that the distance spanning the centers of the two bodies is sometimes as small as , miles at perigee, and , miles at apogee. the moon's speed in this orbit averages rather more than half a mile every second of time--more accurately , feet a second, or , miles per hour. once the moon's distance is known, its size or diameter is easy to ascertain. an angular measure is necessary, of course, that of the angle which the disk of the moon fills as seen from the earth. there are many types of astronomical instruments with which this angle can be measured, and its value is something more than half a degree ( ' "). the moon's actual diameter figures out from this , miles; and it would therefore require nearly fifty moons merged in one to make a ball the size of the earth. still, no other planet has a satellite as large in proportion to its primary as the moon is in relation to the earth. but the materials that compose the moon have less than two-thirds the average density of those that make up the earth, so that eighty-one moons fused together would be necessary to equal the mass or weight of the earth. if we figure out the force of attraction of the moon for bodies on its surface, we find it equals about one-sixth that of the earth. athletes could perform some astounding feats there--miracles of high jump and hammer-throw. our interest in the moon's physical characteristics never wanes. her nearness to us has always fascinated astronomer and layman alike. early users of the telescope were readily led into error regarding the general characteristics of the lunar surface; and it is easy to see why they thought the smooth level planes must be seas, and gave them names to that effect which persist to-day, as mare crisium, mare serenitatis and so on. we may be sure that no water exists on the moon's surface, although some astronomers think that solid water, as ice or snow, may still exist there at a temperature too low for appreciable evaporation. perhaps water, seas, and oceans were once there, but their secular dissemination and loss as vapor have gone on through the millions of millions of years till even the moon's atmosphere appears to have vanished completely. at least there is much better evidence of absence of atmosphere on the moon than of its presence--not enough at any rate to equal a thousandth part of the barometric pressure that we have at the earth's surface. frequent observations of stars passing behind the moon in occultation have satisfied astronomers on this point. we often say of the brilliant full moon, it is as bright as day. the photometer or instrument for accurate comparison of lights, their amount and intensity, tells a different story. indeed, if the entire dome of the sky were filled with full moons, we should be receiving only one-eighth of the light the sun gives us, and it would require more than , average full moons to equal the light radiation of the sun. heat from the moon, however, is quite different. early attempts to measure it detected none at all, but with modern instruments there is little trouble in detecting heat from the moon, though measurement of it is not easy. much of the moon's heat is sun heat, directly reflected from the moon, as sunlight is, but most of it is due to radiation of solar heat previously absorbed by the materials of the lunar surface. the actual temperature of the moon's surface suffers great variation. a fortnight's perpetual shining of the sun upon the lunar rocks would certainly heat them above the temperature of boiling water, if the moon had an atmosphere to conserve and store this heat; but the entire absence of such an air blanket probably permits the sun's heat to be radiated away nearly as fast as it is received, leaving the temperature at the surface always very low. what physical influences the moon really has upon the earth must be very slight, barring the tides. but there is little hope of getting people generally to take that view, because the moon appears to be the planet of the people, and opinion that the moon controls the weather, for instance, amounts with them to practical certainty. more than likely all these notions are but legitimate survivals of superstition and astrology. in addition to the tides, our magnetic observatories reveal slight disturbances with the swinging of the moon from apogee to perigee and back; but long series of weather observations have been faithfully interrogated, with negative or contradictory results. if one believes that the moon's changes affect the weather, it is easy to remember coincidences, and pass over the many times when no change has taken place. the moon changes pretty frequently anyhow. as young well puts it: "a change of the moon necessarily occurs about once a week.... _all_ changes, of the weather for instance, must therefore occur within three and three-fourth days of a change of the moon, and fifty per cent of them ought to occur within forty-six hours of a change, even if there were no causal connection whatever." when we turn to the strongly diversified surface of the moon itself, we find much to rivet the attention, even with slender optical aid. everyone wants to know how near the telescope, the biggest possible telescope, brings the moon to us. that will depend on many things, first of all on the magnifying power of the eyepiece employed on the telescope, and eyepieces are changed on telescopes just as they are on microscopes, though not for the same reasons. the theoretical limit of the power of a telescope is usually considered as for each inch of diameter or aperture of the object glass. a -inch telescope, as that of the yerkes observatory, the largest refracting telescope in existence, should bear a magnifying power not to exceed , . but this limit is practically never reached, one-half of it or fifty to the inch of aperture being a good working limit of power, even under exceptional conditions of steadiness of atmosphere. if we reduce the effective distance of the moon from , miles to miles, that is about the utmost that can be expected. but even at that distance we can make out only landscape details, nothing whatever like buildings or the works of intelligence. the larger relations of light and shade, so obvious to the naked eye on the moon, vanish on looking at it with the telescope, but we are at once captivated by the novel character of the surface and the seemingly great variety of detail that is clearly visible. as soon as the new moon comes out in the west, one may begin to gaze with interest and watch the terminator or sunrise line gradually steal over the roughened surface, bringing new and striking craters into view each night. around the time of quarter moon, or a little past it, is one of the best times for telescopic views of the moon, because the huge craters, tycho and copernicus, are then in fine illumination. close to the phase of full moon is never a good time, because there are no shadows of the rough surface then, and its entire structure seems to be quite flat and uninteresting, except for the streaks or rills which radiate from tycho in every direction, and are the only lunar features that are best seen near full. in a broad, general way, the moon's surface, if compared with the earth's, differs in having no water. our extensive oceans are replaced there by smooth, level plains which were at first thought to be seas and so named. there are ten or twelve of them in all. then we find mountain ranges, so numerous on the earth, relatively few on the moon. those that exist are named, in part, for terrestrial mountain ranges, as the alps, caucasus, and the apennines. but the nearly circular crater, a relatively rare formation on the earth, is seen dotted all over the moon in every size, from a fraction of a mile in diameter up to sixty, seventy, and in extreme cases a hundred miles. no mere description of plains and mountains and craters affords an adequate idea of the moon's surface as it actually is; a telescopic view is necessary, or some of the modern photographs which give an even better notion of the moon than any telescopic view. many of the lunar craters are without doubt volcanic in origin, others seem to be ruins of molten lakes. many thousands of the smaller ones appear as if formed by a violent pelting of the surface when semi-plastic, perhaps by enormous showers of meteoric matter. more than , craters cover the half of the lunar surface visible from the earth, and hundreds of them are named for philosophers and astronomers. measurement of the height of lunar mountains has been made in numerous instances, especially when their shadows fall on plains or surfaces that are nearly level, so that the length of the shadow can be measured. in general, the height of lunar peaks is greater than that of terrestrial peaks, owing probably to the lesser surface gravity on the moon. about forty lunar peaks are higher than mont blanc. most astronomers regard it as certain that no changes ever take place on the moon; probably no very conspicuous changes ever do. some, however, have made out a fair case for comparatively recent changes in surface detail. extreme caution is necessary in drawing conclusions, because the varying changes of illumination from one phase to another are themselves sufficient to cause the appearance of change. at intervals of a double lunation, equal to fifty-nine days, one and one-half hours, the terminator goes very nearly through the same objects, so that the circumstances of illumination are comparable. in mare serenitatis the little crater named linné was announced to have disappeared about a half century ago; subsequently it became visible again and other minor changes were reported, perhaps due to falling in of the walls of the crater. if one were to visit the moon, he must needs take air and water along with him, as well as other sustenance. no atmosphere means no diffused light; we could see nothing unless the sun's direct rays were shining upon it. anyone stepping into the shadow of a lunar crag would become wholly invisible. no sound, however loud, could be heard; sound in fact would become impossible. a rock might roll down the wall of a lunar crater, but there would be no noise; though we should know what had happened by the tremor produced. so slight is gravity there that a good ball player might bat a baseball half a mile or more. looking upward, all the stars would be appreciably brighter than here, and visible perpetually in the daytime as well as at night. if one were to go to the opposite side of the moon, he would lose sight of the earth until he came back to the side which is always turned toward the earth. even then the earth would never rise and set at any given place, as the moon does to us, but would remain all the time at about the same height above the lunar horizon. the earth would go through all the phases that the moon shows to us here, full earth occurring there when it is new moon here. our globe would appear to be nearly four times broader than the moon seems to us. its white polar caps of ice and snow, its dark oceans, and the vast cloud areas would be very conspicuous. faint stars, the zodiacal light, and the filmy solar corona would be visible, probably even close up to the sun's edge; but although his rays might shine upon the lunar rocks without intermission for a fortnight, probably they would still be too cold to touch with safety. on the side of the moon turned away from the sun, the temperature of the moon's surface would fall to that of space, or many hundred degrees below zero. chapter xxix eclipses of the moon of all the weird happenings of the nighttime sky, eclipses of the moon are the most impressive. rarely is there a year without one. what is the cause? simply the earth getting in between sun and moon, and thereby shutting off the sunlight which at all other times enables us to see the moon. as the earth is a dark body it must cast a black shadow on the side away from the sun, and it is the moon's passing into this shadow or some part of it that causes a lunar eclipse. sun and earth being so different in size, the earth's shadow must stretch away from it into space, growing smaller and smaller, until at length it comes to an end--the apex of a cone , miles long. if we cut off this shadow at the moon's distance from the earth, we find it about , miles in diameter at that point; and this accounts for the fact that the curvature on the side of the moon, when the eclipse is coming on and where it is dropping into the shadow, is always much less rapid than the curvature of the moon's own disk is. when an eclipse is approaching, the eastern limb will be duskily darkened for half an hour or more, because the moon must first pass through the outer penumbra, or half-shadow which everywhere surrounds the true shadow itself. if the moon hits only the upper or lower part of the shadow, the eclipse will be only partial, and during the progress of the eclipse it will seem as if the uneclipsed part had swung or twisted around in the sky, from the western limb of the moon to the eastern. but when the moon passes through the middle regions of the shadow, the eclipse is always total, and direct sunlight is wholly cut off from every part of the moon's face, for a greater or less length of time, according to the part of the shadow through which it passes. when passing centrally through the shadow, the total eclipse will last about two hours, as the moon's diameter is about one-third of the breadth of the shadow; and the eclipse will be partial about two hours longer, an hour at beginning and an hour at the end, because the moon moves over her own breadth in about an hour. while the moon is wholly immersed in the shadow, her body is nevertheless visible, as a dull tarnished copper disk; and this is caused by the reddish sunlight which grazes the earth all around and is refracted or bent by our atmosphere into the shadow itself. if this belt or ring of terrestrial atmosphere happens to be everywhere filled with dense clouds, as was the case in , even the familiar copper moon of a total lunar eclipse disappears completely in the black sky. quite different from a solar eclipse, all the phases of a lunar eclipse are visible at the same time on the earth wherever the moon is above the horizon. eclipses of the moon are therefore seen with great frequency at any given place as compared with solar eclipses, which are restricted to relatively narrow areas of the earth's surface. nor are lunar eclipses of very much significance to the astronomer, mainly because of the slowness and indefiniteness of the phenomena. it is a good time to observe occultations of faint stars at the moon's edge or limb, and several such programs have been carried out by cooperation of observatories in widely separate regions of the world: the object being improvement in our knowledge of the distance of the moon, and in the accuracy of the mathematical tables of her motion. search by photography for a possible satellite, or moon of the moon, has been made on several occasions, though without success. a lunar eclipse was first observed and photographed from an aeroplane, may , . at the request of the writer, two aviators of the united states navy ascended to a height of , feet above rockaway, and secured many advantages accruing from great elevation in viewing a celestial phenomenon of this character. chapter xxx total eclipses of the sun primitive peoples indulged in every variety of explanation of mysterious happenings in the sky. to the chinese and all through india, a total eclipse of the sun is caused by "a certain dragon with very black claws," who, except for their frightening him away by every conceivable sort of hideous noise, would most certainly "eat up the sun." the eclipse always goes off, the sun has never been eaten yet. can you convince a chinaman that rahu, the dragon, wouldn't have eaten up the sun, if his unearthly din hadn't frightened him away? in japan the eclipse drops poison from the sky into wells, so the japanese cover them up. fontenelle relates that in the middle of the seventeenth century a multitude of people shut themselves up in cellars in paris during a total eclipse. in the shu-king, an ancient chinese work, occurs the earliest record of a total eclipse of the sun, in the year b. c. . the nineveh eclipse of b. c. is perhaps the first of the ancient eclipses of which we possess a really clear description on the assyrian eponym tablets in the british museum. it is the eclipse possibly referred to in the book of amos, viii. but of all the ancient eclipses none perhaps exceeds in interest the famous eclipse of thales, b. c. , may . it is the first eclipse to have been predicted, probably by means of the saros, or -year period of eclipses, which is useful as an approximate method even at the present day. but the accident of a war between the lydians and the medes has added greatly to the historic interest, because the combatants were so terrified by the sudden turning of day into night that they at once concluded a peace cemented by two marriages. very many of the ancient eclipses have been of great use to the historian in verifying dates, and mathematical astronomers have employed them in correcting the lunar tables, or intricate mathematical data by which the motion of the moon is predicted. coming down to the middle of the sixth century, we find the first eclipse recorded in england, in the "saxon chronicle," a. d. . during the epoch of the arabian nights several eclipses were witnessed at bagdad, a. d. to , and many a century later by ibu-jounis, court astronomer of hakem, the caliph of egypt. nothing is more interesting than to search the quaint records of these ancient eclipses. one occurring in , when tycho brahe was but fourteen, had much to do with turning his permanent interest toward mathematics and astronomy. the eclipse of was the first "seen through a tube," the telescope having been invented only a few years before. "paradise lost" was completed about , and the censorship was still in existence; and it is matter of record that the oft-quoted passage, "as when the sun, new risen, looks through the horizontal misty air, shorn of his beams; or from behind the moon, in dim eclipse, disastrous twilight sheds on half the nations, and with fear of change perplexes monarchs." _p. l._, i. was strongly urged as sufficient reason for suppressing the entire epic. london was favored with the outflashing corona, may , , and a pamphlet was issued in prediction, entitled "the black day, or a prospect of doomsday." the first american eclipse expedition was on occasion of the totality of oct. , , sent out by harvard college and the american academy of arts and sciences under professor samuel williams to penobscot. there was a fine total eclipse from albany to boston on june , , and many important observations of it were made in this country. but it was not till the european eclipse of that research got fully under way, because the germ of the new astronomy, particularly as applied to the sun, had begun its development; and the significance of the corona was obvious, if it could be proved a true appendage of the sun. photography had not long been discovered, and the corona of was the first to be automatically registered on a daguerreotype. in it was proved that prominences and corona both belong to the sun and not to the moon. the great indian eclipse of brought the important discovery that the prominences can be observed at any time without an eclipse by means of the spectroscope. in bright lines were found in the spectrum of the corona, one line in the green indicating the presence of an element not then known on the earth and hence called coronium. in the reversing layer or stratum of the sun was discovered. in a vast ecliptic extension of the streams of the corona many millions of miles both east and west of the sun was first seen. this is now known to be the type of corona characteristic of minimum spots on the sun. in the spectrum of the corona was first photographed and in excellent detail photographs of the corona were taken. in it was shown that the corona quite certainly rotates bodily with the sun. in actual spectrum photographs of the reversing layer established its existence beyond doubt--"flash spectrum" it is often called. in the long ecliptic streamers of the corona were successfully photographed for the first time. in the depth of the reversing layer was found to average miles, the heat of the corona was first measured by the bolometer, and many observations showed that the coronal streamers, in part at least, partake of the nature of electric discharges. all subsequent total eclipses have been carefully observed, in whatever part of the world they may happen, and each has added new results of significance to our theories of the corona and its relation to the radiant energy of the sun. in very recent eclipses the cinematograph has been brought into action as an efficient adjunct of observation; in the first successful "movie" of the eclipse was secured in sweden, and in frost of the yerkes observatory first applied the cinematograph to registry of the "flash spectrum," and stebbins tested out his photo-electric cell on the corona, making the brightness . that of the full moon. in (russia) and again in (on the atlantic) the obvious advantages of the aeroplane in ecliptic observation and photography were sought by the writer, though unsuccessfully. the photographic tests, however, conducted in preparation for these expeditions proved the entire practicability of securing eclipse results of much value, independently of clouds below. eclipses in the near future will be total in australia about six minutes on september , ; in california and mexico about four minutes on september , ; and along a line from toronto to nantucket about two minutes on the morning of january , . to all spectators, savage or civilized, scientist or layman, a total eclipse is wonderful and impressive. langley said: "the spectacle is one of which, though the man of science may prosaically state the facts, perhaps only the poet could render the impression." very gradually the moon steals its way across the face of the sun, the lessened light is hardly noticed. if one is near a tree through whose foliage the sunlight filters, an extraordinary sight is seen; the ground all about is covered with luminous crescents, instead of the overlapping disks which were there before the eclipse came on; in both cases they are images of the disk of the sun at the time, and the narrowing crescents will be watched with interest as totality approaches. then the shadow bands may be seen flitting across the landscape, like "visible wind." they are probably related to our atmosphere and the very slender crescent from which true sunlight still comes. then for a few seconds the moon's actual shadow may be caught in its approach, very suddenly the darkness steals over the landscape and--totality is on. how lucky if there are no clouds! every eye is riveted on "the incomparable corona, a silvery, soft, unearthly light, with radiant streamers, stretching at times millions of uncomprehended miles into space, while the rosy flaming protuberances skirt the black rim of the moon in ethereal splendor." then it is now or never with observer and photographer. months of diligent preparations at home followed by weeks of tedious journey abroad, with days of strenuous preparation and rehearsals at the station--all go for naught unless the whole is tuned up to perfect operation the instant totality begins. it may last but a minute, or even less; in , however, total eclipse will last minutes seconds, the longest ever observed, and within half a minute of the longest possible. all is over as suddenly as it came on. the first thing is to complete records, develop plates, and see if everything worked perfectly. there is great utility back of all eclipse research, on account of its wide bearing on meteorology and terrestrial physics, and possibly the direct use of solar energy for industrial purposes. with this purpose in view the astronomer devotes himself unsparingly to the acquisition of every possible fact about the sun and his corona. considering the earth as a whole, the number of total eclipses will average nearly seventy to the century. but at any given place, one may count himself very fortunate if he sees a single total eclipse, although he may see several partial ones without going from home. then, too, there are annular or ring eclipses, averaging seven in eight years. but had one been born in boston or new york in the latter part of the eighteenth century, he might have lived through the entire nineteenth century and a long way into the twentieth without seeing more than one total eclipse of the sun. in london in no total eclipse had been visible for six centuries. however, taking general averages, and recalling the comparatively narrow belt of total eclipse, every part of the earth is likely to come within range of the moon's shadow once in about three and a half centuries. the longest total eclipses always occur near the equator; this is because an observer on the equator is carried eastward by the earth's rotation at a velocity of about , miles per hour, so that he remains longer in the moon's shadow which is passing over him in the same direction with a velocity about twice as great. the general circumstances of total eclipses are readily foretold by means of the ancient chaldean period of eclipses known as the saros. it is years and or days in length (according to the number of leap years intervening). in one complete saros, forty-one solar eclipses will generally happen, but only about one-fourth of them will be total. the saros is a period at the end of which the centers of sun and moon return very nearly to their relative positions at the beginning of the cycle. so, in general, the eclipse of any year will be a repetition of one which took place years before, and another very similar in circumstances will happen years in the future. three periods of the saros, or years and month, will usually bring about a return of any given eclipse to any particular part of the earth, so far as longitude is concerned, though the returning track will lie about miles to the north or south of the one years earlier. paths of total eclipses frequently intersect, if large areas like an entire country are considered; spain, for instance, where total eclipses have occurred in , , , and . besides crossing spain, the tracks of totality on may , , and august , , were unique in intersecting exactly over a large city--tripoli in barbary, on both of which occasions the writer's expeditions to that city were rewarded with perfect observing conditions in that now italian province on the edge of the great desert. kepler was the first astronomer to calculate eclipses with some approach to scientific form, as exemplified in his rudolphine tables. his method was of course geometrical. but la grange, who applied the methods of more refined analysis to the problem, was the first to develop a method by which an eclipse and all its circumstances could be accurately predicted for any part of the earth. to many minds, the prediction of an eclipse affords the best illustration of the superior knowledge of the astronomer: it seems little short of the marvelous. but recalling that the motion of the moon follows the law of gravitation, and that its position in the sky is predictable for years in advance with a high degree of precision, it will readily be seen how the arrival of the moon's shadow, and hence the total eclipses of the sun, can be foretold for any place over which the shadow passes. all these data derived by the mathematician are known as the elements of the eclipse, and they are prepared many years in advance and published in the nautical almanacs and astronomical ephemerides issued by the leading nations. buchanan's "treatise on eclipses" will supply all the technical information regarding the prediction of eclipses that anyone desirous of inquiring into this phase of the problem may desire. so important are total eclipses in the scheme of modern solar research, and so necessary are clear skies in order that expeditions may be favored with success, that every effort is now made to ascertain the weather chances at particular stations along the line of eclipse many years in advance. this method of securing preliminary cloud observations for a series of years has proved especially useful for the eclipses of , , , and ; and had it been employed in russia for totality of , many well-equipped expeditions might have been spared disaster. the california and mexico totality of does not require this forethought, as the regions visited are quite likely to be free from cloud; but observations are now in process of accumulation for the total eclipse of . the out-look for clear skies on that occasion, the total eclipse nearest new york for more than a century, is not very promising. the path of totality passes over marquette, michigan, rochester and poughkeepsie, new york, newport, rhode island, and nantucket about nine in the morning. everyone who saw it will remember the last total eclipse in this part of the world--on june , , visible from oregon to florida. many will recall the last total eclipse that was visible before that in the eastern part of the united states, on may , , visible in a narrow path from new orleans to norfolk. one's father or grandfather will perhaps remember the total eclipse of july , , which passed over the united states from pike's peak to texas (it was the writer's maiden eclipse), and another on august , , which passed southeasterly over iowa and kentucky. on all these occasions the paths of total eclipse were dotted with numerous observing parties, many of them equipped with elaborate apparatus for studying and photographing the solar corona and prominences, together with a multitude of other phenomena which are seen only when total eclipses take place. looking forward rather than backward, a striking series, or family, of eclipses happens in the future: it is the series of may, and , recurring again on june , (over the pacific ocean), june , (through india, siam, and luzon), and june , (visible in sahara, abyssinia, and somali). already in this totality was minutes seconds in duration; in , as already mentioned, it will be minutes seconds, and at the subsequent returns even longer yet, approaching the estimated maximum of minutes seconds which has never been observed. this remarkable series of total eclipses is longer in duration than any others during a thousand years. its next subsequent return is in , occurring with the eclipsed sun practically at noon in the zenith of mount popocatepetl in mexico. whatever may be the progress of solar research during the intervening years, it is impossible to imagine the alert astronomer of that remote day without incentive for further investigation of the sun's corona, in which are concealed no doubt many secrets of the sun's evolution from nebula to star. chapter xxxi the solar corona "and what is the sun's corona?" mildly asked a college professor of a student who might better have answered "not prepared." "i did know, professor, but i have forgotten," was his reply. "what an incalculable loss to science," returned the professor with a twinkle. "the only man who ever knew what the sun's corona is, and he has forgotten!" only in part has the mystery of the corona been cleared by the research of the present day. our knowledge proceeds but slowly, because the corona has never been seen except during total eclipses of the sun; and astronomers, as a matter of fact, have never had a fair chance at it. two total eclipses happen on the average of every three years; their average duration is only two or three minutes; totality can be seen only in a narrow path about a hundred miles wide, though it may be several thousand miles long; there is usually about equal chance of cloud with clear skies; and fully three-fourths of the totality areas of the globe are unavailable because covered by water. so that even if we imagine the tracks of eclipses quite thickly populated with astronomers and telescopes, at least one every hundred miles, how much solid watching of the corona would this permit? only a little more than one week's time in a whole century. the true corona is at least a triple phenomenon and a very complex one. the photographs reveal it much as the eye sees it, with all its complexity of interlacing streamers projected into a flat, or plane, surrounding the disk of the dark moon which hides the true sun completely. but we must keep in mind the fact that the sun is a globe, not a disk, and that the streamers of the corona radiate more or less from all parts of the surface of the solar sphere, much as quills from a porcupine. from the sun's magnetic poles branch out the polar rays, nearly straight throughout their visible extent. gradually as the coronal rays originate at points around the solar disk farther and farther removed from the poles, they are more and more curved. very probably they extend into the equatorial regions, but it is not easy to trace them there because they are projected upon and confused with the filaments having their origin remote from the poles. then there is the inner equatorial corona, apparently connected intimately with truly solar phenomena, quite as the polar rays are. the third element in the composite is the outer ecliptic corona, for the most part made up of long streamers. this is most fully developed at the time of the fewest spots on the sun. it is traceable much farther against the black sky with the naked eye than by photography. without any doubt it is a solar appendage and possibly it may merge into the zodiacal light. naturally this superb spectacle must have been an amazing sight to the beholders of antiquity who were fortunate enough to see it. historical references are rare: perhaps the earliest was by plutarch about a. d. , who wrote of it, "a radiance shone round the rim, and would not suffer darkness to become deep and intense." philostratus a century later mentions the death of the emperor domitian at ephesus as "announced" by a total eclipse. kepler thought the corona was evidence of a lunar atmosphere; indeed, it was not until the middle of the th century that its lack of relation to the moon was finally demonstrated. later observers, wyberd in and ulloa, got the impression that the corona turned round the disk catherine-wheel fashion, "like an ignited wheel in fireworks, turning on its center." but no later observer has reported anything of the sort. quite the contrary, there it stands against the black sky in motionless magnificence a colorless pearly mass of wisps and streamers for the most part nebulous and ill-defined, fading out very irregularly into the black sky beyond, but with a complex interlacing of filaments, sometimes very sharply defined near the solar poles. it defies the skill of artist and draughtsman to sketch it before it is gone. photograph it? yes, but there are troubles. of course the camera work is superior to sketches by hand. as langley used to say, "the camera has no nerves, and what it sets down we may rely on." foremost among the photographic difficulties is the wide variation in intensity of the coronal light in different regions of the corona. if a plate is exposed long enough to get the outer corona, the exceeding brightness of the inner corona overexposes and burns out that part of the plate or film. if the exposure is short, we get certain regions of the inner corona excellently, but the outer regions are a blank because they can be caught only by a long exposure. so the only way is to take a series of pictures with a wide range of exposures, and then by careful and artistic handwork, combine them all into a single drawing. wesley of london has succeeded eminently in work of this character, and his drawings of the sun's corona, visible at total eclipses from onward, in possession of the royal astronomical society, are the finest in existence. they give a vastly better idea of the corona, as the eye sees it, than any single photograph possibly can. the early observers apparently never thought of the corona as being connected with the sun. it was a halo merely, and so drawn. its real structure was neither known, depicted, or investigated. sketches were structureless, as any aureola formed by stray sunlight grazing the moon might naturally be. that the rays are curved and far from radial round the sun was shown for the first time in the sketches of , and in sir francis galton observed that the long arms or streamers "do not radiate strictly from the center." the inner corona had first been recorded photographically on a daguerreotype plate during the eclipse of , but the lens belonged to a heliometer, and was of course uncorrected for the photographic rays. the wet collodion plates of the eclipse of , by de la rue, proved that not only the prominences but the corona were truly solar, because his series of technically perfect pictures revealed the steady and unchanged character of these phenomena while the moon's disk was passing over them as totality progressed. and at the eclipse of , young put the solar theory of the corona beyond the shadow of any further doubt by examination of its light with the spectroscope and discovering a green line in the spectrum due to incandescent vapor of a substance not then identified with anything terrestrial, and therefore called coronium. the total brilliance of the corona was very differently estimated by the earlier observers, though pretty carefully measured at later eclipses. the standard full moon is used for reference, and at one eclipse the corona falls short of, while at another it will exceed the full moon in brightness. variations in brilliancy are quite marked: at one eclipse it was nearly four times as bright as the full moon. much evidence has already accumulated on this question; but whether the observed variations are real, or due mainly to the varying relative sizes of sun and moon at different eclipses, is not yet known. the coronal light is largely bluish in tint, and this is the region of the spectrum most powerfully absorbed by our atmosphere. eclipses are observed by different expeditions located at stations where the eclipsed sun stands at very different altitudes above the horizon; besides this the localities of observation are at varied elevations above sea level; so that the varying amount of absorption of the coronal light renders the problem one of much difficulty. the long ecliptic streamers of the corona were first seen by newcomb and langley during the totality of . on one side of the sun there was a stupendous extension of at least twelve solar diameters, or nearly millions of miles. langley observed from the summit of pike's peak, over , feet high, and was sure that he was witnessing a "real phenomenon heretofore undescribed." the vast advantage of elevation was apparent also from the fact that he held the corona for more than four minutes after true totality had ended. these streamers are characteristic of the epoch of minimum spots on the sun, as ranyard first suggested. it was found that this type of corona had been recorded also in ; and it has reappeared in , and , and will doubtless be visible again in . how rapidly the streamers of the corona vary is not known. occasionally an observer reports having seen the filaments vibrate rapidly as in the aurora borealis, but this is not verified by others who saw the same corona perfectly unmoving. comparisons of photographs taken at widely separate stations during the same eclipse have shown that at least the corona remained stationary for hours at a time. whether it may be unchanged at the end of a day, or a week, or a month, is not known; because no two total eclipses can ever happen nearer each other than within an interval of days, or one-half of the eclipse year. and usually the interval between total eclipses is twice or three times this period. theories of what the solar corona may be are very numerous. the extreme inner corona is perhaps in part a sort of gaseous atmosphere of the sun, due to matter ejected from the sun, and kept in motion by forces of ejection, gravity, and repulsion of some sort. meteoric matter is likely concerned in it, and huggins suggested the débris of disintegrating comets. schuster was in agreement with huggins that the brighter filaments of the corona might be due to electric discharges, but it seems very unlikely that any single hypothesis can completely account for the intricate tracery of so complex a phenomenon. [illustration: solar corona and prominences. photographed during a total eclipse of the sun, june , . (_courtesy, american museum of natural history._)] [illustration: venus, showing crescent phase of the planet. venus is the earth's nearest neighbor on the side toward the sun. (_photo, yerkes observatory._)] [illustration: mars, the planet next beyond the earth. the photograph shows one of the white polar caps. the caps are thought to be snow or ice and may indicate the existence of atmosphere. (_photo, yerkes observatory._)] elaborate spectroscopic programs have been carried out at recent eclipses, affording evidence that certain regions are due to incandescent matter of lower temperature than the sun's surface. a small part of the light of the corona is sunlight reflected from dark particles possibly meteoric, but more likely dust particles or fog of some sort. this accounts for the weakened solar spectrum with fraunhofer absorption lines, and this part of the light is polarized. many have been the attempts to see, or photograph, the corona without an eclipse. none of them has, however, succeeded as yet. huggins got very promising results nearly forty years ago, and success was thought to have been reached; but subsequent experiments on the riffelberg in and later convinced him that his results related only to a spurious corona. in the writer made an unsuccessful attempt to visualize the corona from the summit of fujiyama, and hale tried both optical and photographic methods on pike's peak in without success. he devised later a promising method by which the heat of the corona in different regions can be measured by the bolometer, and an outline corona afterward sketched from these results. still another method of attacking the problem occurred to the writer in , which has not yet been carried out. it would take advantage of recent advances in aeronautics, and contemplates an artificial eclipse in the upper air by means of a black spherical balloon. this would be sent up to an altitude of perhaps , feet, where it would partake of the motion of the air current in which it came to equilibrium. then a snapshot camera would be mounted on an aeroplane, in which the aviator would ascend to such a height that the balloon just covered the sun, as the moon does in a total eclipse. with the center of the balloon in line with the sun's center, he would photograph the regions of the sky immediately surrounding the sun, against which the corona is projected. as the entire apparatus would be above more than an entire half of the earth's atmosphere, the experiment would be well worth the attempt, as pretty much everything else has been tried and found wanting. needless to say, the importance of seeing the corona at regular intervals whenever desired, without waiting for eclipses of the sun, remains as insistent as ever. chapter xxxii the ruddy planet mars is a planet next in order beyond the earth, and its distance from the sun averages - / million miles. it has a relatively rapid motion among the stars, its color is reddish, and, when nearest to us, it is perhaps the most conspicuous object in the sky. mars appeared to the ancients just as it does to us to-day. aristotle recorded an observation of mars, b. c., when the moon passed over the planet, or occulted it, as our expression is. galileo made the first observations of mars with a telescope in , and his little instrument was powerful enough to enable him to discover that the planet had phases, though it did not pass through all the phases that mercury and venus do. this was obvious from the fact that mars is always at a greater distance from the sun than we are, and the phase can only be gibbous, or about like the moon when midway between full and quarter. many observers in the seventeenth century followed up the planet with such feeble optical power as the telescopes of that epoch provided: fontana (who made the first sketch), riccioli and bianchini in italy, cassini in france, huygens in holland, and later sir william herschel in england. it was cassini who first made out the whitish spots or polar caps of mars in , but not until after huygens had noted the fact that mars turned round on an axis in a period but little longer than the earth's. cassini followed it up later with a more accurate value; and observations in our own day, when combined with these early ones, enable us to say that the martian day is equal to hours minutes . seconds, accurate probably to the hundredth part of a second. when we know that a planet turns round on an axis, we know that it has a day. when we know the direction of the axis in space or in relation to the plane of its path round the sun, we know that it has seasons: we can tell their length and when they begin and end. it did not take many years of observation to prove that the axis round which mars turns is tilted to the plane of its path round the sun by an angle practically the same as that at which the earth's axis is tilted. so there is the immediate inference that on mars the order and perhaps the character of the seasons is much the same as here on the earth. at least two things, however, tend to modify them. first, the year of mars is not days like ours, but days. each of the four seasons on mars, therefore, is proportionally longer than our seasons are. then comes the question of atmosphere--how much of an atmosphere does mars really possess in proportion to ours, and how would its lesser amount modify the blending of the seasons into one another? all discussion of mars and the problems of existence of life upon that planet hinge upon the character and extent of martian atmosphere. the planet seems never to be covered, as the earth usually is, with extensive areas of cloud which to an observer in space would completely mask its oceans and continents. nearly all the time mars in his equatorial and temperate zones is quite clear of clouds. a few whitish spots are occasionally seen to change their form and position in both northern and southern latitudes, and they vary with the progress of the day on mars, as clouds naturally would. but schiaparelli, perhaps the best of all observers, thought them to be not low-lying clouds of the nimbus type that would produce rains, but rather a veil of fog, or perhaps a temporary condensation of vapor, as dew or hoar frost. but the strongest argument for an atmosphere is based on the temporary darkening or obscuration of well known and permanent markings on the surface of mars. these are more or less frequently observed and clouds afford the best explanation of their occurrence. so much for evidence supplied by the telescope alone. when, however, we employ the spectroscope in conjunction with the telescope, another sort of evidence is at hand. several astronomers have reached the conclusion that watery vapor exists in the atmosphere of mars, while other astronomers equipped with equal or superior apparatus, and under equally favorable or even better conditions, have reached the remarkable conclusion that the spectra of mars and the moon are identical in every particular. from this we should be led to infer that mars has perhaps no more atmosphere than the moon has, that is to say, none whatever that present instruments and methods of investigation have enabled us to detect. what then, shall we conclude? simply that the atmosphere of mars is neither very dense nor extensive. probably its lower strata close to the planet's surface are about as dense as the earth's atmosphere is at the summits of our highest mountains. this conclusion is not unwelcome, if we keep a few fundamental facts in clear and constant view. mars is a planet of intermediate size between the earth and the moon: twice the moon's diameter ( , miles) very nearly equals the diameter of mars ( , miles), and twice the diameter of mars does not greatly exceed the earth's diameter ( , miles). as to the weights or masses of these bodies, mars is about one-ninth, and the moon one-eightieth of the earth. the atmospheric envelope of the earth is abundant, the moon has none as far as we can ascertain; so it seems safe to infer that mars has an atmosphere of slight density: not dense enough to be detected by spectroscopic methods, but yet dense enough to enable us to explain the varying telescopic phenomena of the planet's disk which we should not know how to account for, if there were no atmosphere whatever. one astronomer has, indeed, gone so far as to calculate that in comparison with our planet mars is entitled to one-twentieth as much atmosphere as we have, and that the mercurial barometer at "sea level" would run about five and a half inches, as against thirty inches on the earth. in general, then, the climate of mars is probably very much like that of a clear season on a very high terrestrial table land or mountain--a climate of wide extremes, with great changes of temperature from day to night. the inequality of martian seasons is such that in his northern hemisphere the winter lasts days and the summer only days. now, the polar caps of mars, which are reasonably assumed to be due to snow or hoar frost, attain their maximum three or four months after the winter solstice, and their minimum about the same length of time after the summer solstice. this lagging should be interpreted as an argument for a martian atmosphere with heat-storing qualities, similar to that possessed by the earth. upon this characteristic, indeed, depends the climate at the surface of mars: whether it is at all similar to our own, and whether fluid water is a possibility on mars or not. while the cosmic relations of the planet in its orbit are quite the same as ours, nevertheless the greater distance of mars diminishes his supply of direct solar heat to about half what we receive. on the other hand, his distance from the sun during his year of motion around it varies much more widely than ours, so that he receives when nearest the sun about one-half more of solar heat than he does when farthest away. southern summers on mars, therefore, must be much hotter, and southern winters colder than the corresponding seasons of his northern hemisphere. indeed, the length of the southern summer, nearly twice that of the terrestrial season, sometimes amply suffices to melt all the polar ice and snow, as in october, , when the southern polar cap of mars dwindled rapidly and finally vanished completely. very interesting in this connection are the researches of stoney on the general conditions affecting planetary atmospheres and their composition. according to the kinetic theory, if the molecules of gases which are continually in motion travel outward from the center of a planet, as they frequently must, and with velocities surpassing the limit that a planet's gravity is capable of controlling, these molecules will effect a permanent escape from the planet, and travel through space in orbits of their own. so the moon is wholly without atmosphere because the moon's gravity is not powerful enough to retain the molecules of its component gases. so also the earth's atmosphere contains no helium or free hydrogen. so, too, mars is possessed of insufficient force of gravity to retain water vapor, and the martian atmosphere may therefore consist mainly of nitrogen, argon, and carbon dioxide. as everyone knows, the axis of the earth if extended to the northern heavens would pass very near the north polar star, which on that account is known as polaris. in a similar manner the axis of mars pierces the northern heavens about midway between the two bright stars alpha cephei and alpha cygni (deneb). the direction of this axis is pretty accurately known, because the measurement of the polar caps of the planet as they turn round from night to night, year in and year out, has enabled astronomers to assign the inclination of the axis with great precision. these caps are a brilliant white, and they are generally supposed to be snow and ice. they wax and wane alternately with the seasons on mars, being largest at the end of the martian winter and smallest near the end of summer. the existence of the polar caps together with their seasonal fluctuations afford a most convincing argument for the reality of a martian atmosphere, sufficiently dense to be capable of diffusing and transporting vapor. the northern cap is centered on the pole almost with geometric exactness, and as far as the th parallel of latitude. on the other hand, the south polar cap is centered about miles from the true pole, and this distance has been observed to vary from one season to another. no suggestion has been made to account for this singular variation. on one occasion it stretched down to martian latitude degrees and was over , miles in diameter. pickering watched the changing conditions of shrinking of the south polar cap in with a large telescope located in the andes of peru. mars was faithfully followed on every night but one from july to september , and the apparent alterations in this cap were very marked, even from night to night. as the snows began to decrease, a long dark line made its appearance near the middle of the cap, and gradually grew until it cut the cap in two. this white polar area (and probably also the northern one in similar fashion) becomes notched on the edge with the progress of its summer season; dark interior spots and fissures form, isolated patches separate from the principal mass, and later seem to dissolve and disappear. possibly if one were located on mars and viewing our earth with a big telescope, the seasonal variation of our north and south polar caps might present somewhat similar phenomena. all the recent oppositions of mars have been critically observed by pickering from an excellent station in jamaica. quite obviously the fluctuations of the polar caps are the key to the physiographic situation on mars, and they are made the subject of the closest scrutiny at every recurring opposition of the planet. several observers, lowell in particular, record a bluish line or a sort of retreating polar sea, following up the diminishing polar cap as it shrinks with the advance of summer. it is said that no such line is visible during the formation of the polar cap with the approach of winter. all such results of critical observation, just on the limit of visibility, have to be repeated over and over again before they become part of the body of accepted scientific fact. and in many instances the only sure way is to fall back on the photographic record, which all astronomers, whether prejudiced or not, may have the opportunity to examine and draw their individual conclusions. already the approaching opposition of , the most favorable since the invention of the telescope, is beginning to attract attention, and preparations are in progress, of new and more powerful instruments, with new and more sensitive photographic processes, by means of which many of the present riddles of mars may be solved. chapter xxxiii the canals of mars then there are the so-called canals of mars, about which so much is written and relatively little known. faint markings which resemble them in character were first drawn in and later in , but schiaparelli, the famous italian astronomer, is probably their original discoverer, when mars was at its least distance from the earth in . he made the first accurate detailed map of mars at this time, and most of the important or more conspicuous canals (_canali_, he called them in italian, that is, channels merely, without any reference whatever to their being watercourses) were accurately charted by him. at all the subsequent close approaches of mars, the canals have been critically studied by a wide range of astronomical observers, and their conclusions as to the nature and visibility of the canals have been equally wide and varied. the most favorable oppositions have occurred in and , also in and . on these occasions a close minimum distance of mars was reached, that is, about millions of miles; but in the planet makes the closest approach in a period of nearly a thousand years. its distance will not much exceed millions of miles. but although this is a minimum distance for mars, it must not be forgotten that it is a really vast distance, absolutely speaking; it is something like times greater than the distance of the moon. with no telescopic power at our command could we possibly see anything on the moon of the size of the largest buildings or other works of human intelligence; so that we seem forever barred from detecting anything of the sort on mars. nevertheless, the closest scrutiny of the ruddy planet by observers of great enthusiasm and intelligence, coupled with imagination and persistence, have built up a system of canals on mars, covering the surface of the planet like spider webs over a printed page, crossing each other at intersecting spots known as "lakes," and embodying a wealth of detail which challenges criticism and explanation. to see the canals at all requires a favorable presentation of mars, a steady atmosphere and a perfect telescope, with a trained eye behind it. not even then are they sure to be visible. the training of the eye has no doubt much to do with it. so photography has been called in, and very excellent pictures of mars have already been taken, some nearly half as large as a dime, showing plainly the lights and shades of the grander divisions of the martian surface, but only in a few instances revealing the actual canals more unmistakably than they are seen at the eyepiece. the appearance and degree of visibility of the canals are variable: possibly clouds temporarily obscure them. but there is a certain capriciousness about their visibility that is little understood. in consequence of the changing physical aspects, as to season, on mars and his orbital position with reference to the earth, some of the canals remain for a long time invisible, adding to the intricacy of the puzzle. for the most part the canals are straight in their course and do not swerve much from a great circle on the planet. but their lengths are very different, some as short as miles, some as long as , miles; and they often join one another like spokes in the hub of a wheel, though at various angles. as depicted by lowell and his corps of observers at flagstaff, arizona, the canal system is a truly marvelous network of fine darkish stripes. their color is represented as a bluish green. each marking maintains its own breadth throughout its entire length, but the breadth of all the canals is by no means the same: the narrowest are perhaps fifteen to twenty miles wide, and the broadest probably ten times that. at least that must be the breadth of the nilosyrtis, which is generally regarded as the most conspicuous of all the canals. the lowell observatory has outstripped all others in the number of canals seen and charted, now about . what may be the true significance of this remarkable system of markings it is impossible to conclude at present. schiaparelli from his long and critical study of them, their changes of width and color, was led to think that they may be a veritable hydrographic system for distributing the liquid from the melting polar snows. in this case it would be difficult to escape the conviction that the canals have, at least in part, been designed and executed with a definite end in view. lowell went even farther and built upon their behavior an elaborate theory of life on the planet, with intelligent beings constructing and opening new canals on mars at the present epoch. pickering propounded the theory that the canals are not water-bearing channels at all, but that they are due to vegetation, starting in the spring when first seen and vitalized by the progress of the season poleward, the intensity of color of the vegetation coinciding with the progress of the season as we observe it. extensive irrigation schemes for conducting agricultural operations on a large scale seem a very plausible explanation of the canals, especially if we regard mars as a world farther advanced in its life history than our own. erosion may have worn the continents down to their minimum elevation, rendering artificial waterways not difficult to build; while with the vanishing martian atmosphere and absence of rains, the necessity of water for the support of animal and vegetal life could only be met by conducting it in artificial channels from one region of the planet to another. interesting as this speculative interpretation is, however, we cannot pass by the fact that many competent astronomers with excellent instruments finely located have been unable to see the canals, and therefore think the astronomers who do see them are deceived in some way. also many other astronomers, perhaps on insufficient grounds, deny their existence _in toto_. many patient years of labor would be required to consult all the literature of investigation of the planet mars, but much of the detail has been critically embodied in maps at different epochs, by kayser, proctor, green, and dreyer. and flammarion in two classic volumes on mars has presented all the observations from the earliest time, together with his own interpretation of them. areography is a term sometimes applied to a description of the surface of mars, and it is scarcely an exaggeration to say that areography is now better known than the geography of immense tracts of the earth. for some reason well recognized, though not at all well understood, mars although the nearest of all the planets, venus alone excepted, is an object by no means easy to observe with the telescope. possibly its unusual tint has something to do with this. with an ordinary opera glass examine the moon very closely, and try to settle precise markings, colors, and the nature of objects on her surface; mars under the best conditions, scrutinized with our largest and best telescope, presents a problem of about the same order of difficulty. there are delicate and changing local colors that add much uncertainty. nevertheless, the planet's leading features are well made out, and their stability since the time of the earliest observers leaves no room to doubt their reality as parts of a permanent planetary crust. the border of the martian disk is brighter than the interior, but this brightness is far from uniform. variations in the color of the markings often depend on the planet's turning round on its axis, and the relation of the surface to our angle of vision. if we keep in mind these obstacles to perfect vision in our own day, it is easy to see why the early users of very imperfect telescopes failed to see very much, and were misled by much that they thought they saw. then, too, they had to contend, as we do, with unsteadiness of atmosphere, which is least troublesome near the zenith. as their telescopes were all located in the northern hemisphere, the northern hemisphere of mars is the one best circumstanced for their investigation; because at the remote oppositions of mars, which always happen in our northern winter with the planet in high north declination, it is always the north pole of mars which is presented to our view. whereas the close oppositions of the planet always come in our northern midsummer, with mars in south declination and therefore passing through the zenith of places in corresponding south latitude. with mars near opposition, high up from the horizon, a fairly steady atmosphere, and a magnifying power of at least diameters, even the most casual observer could not fail to notice the striking difference in brightness of the two hemispheres: the northern chiefly bright and the southern markedly dark. formerly this was thought to indicate that the southern hemisphere of mars was chiefly water and the northern land, much as is the case on the earth: with this difference, however, that water and land on the earth are proportioned about as eleven to four. but mars in its general topography presents no analogy with the present relation of land and water on the earth. there seems no reason to doubt that the northern regions with their prevailing orange tint, in some places a dark red and in others fading to yellow and white, are really continental in character. other vast regions of the martian surface are possibly marshy, the varying depth of water causing the diversity of color. if we could ever catch a reflection of sunlight from any part of the surface of mars, we might conclude that deep water exists on the planet; but the farther research progresses, the more complete becomes the evidence that permanent water areas on mars, if they exist at all, are extremely limited. since mars has been known to possess two satellites, which were discovered in august of that year by hall at washington. moons of this planet had long been suspected to exist and on one or two previous occasions critically looked for, though without success. in the writings of dean swift there is a fanciful allusion to the two moons of mars; and if astronomers had chanced to give serious attention to this, phobos and deimos, as hall named them, might have been discovered long before. they are very small bodies, not only faint in the telescope, but actually of only ten or twenty miles diameter; and from the strange relation that phobos, the inner moon, moves round mars three times while the planet itself is turning round only once on its axis, some astronomers incline to the hypothesis that this moon at least was never part of mars itself, but that it was originally an inner or very eccentric member of the asteroid group, which ventured within the sphere of gravitation of mars, was captured by that planet, and has ever since been tributary to it as a secondary body or satellite. chapter xxxiv life in other worlds popular interest in astronomy is exceedingly wide, but it is very largely confined to the idea of resemblances and differences between our earth and the bodies of the sky. the question most frequently asked the astronomer is, "have any of the stars got people on them?" or more specifically, "is mars inhabited?" the average questioner will not readily be turned off with yes or no for an answer. he may or may not know that it is quite impossible for astronomers to ascertain anything definite in this matter, most interesting as it is. what he wants to find out is the view of the individual astronomer on this absorbing and ever recurring inquiry. we ought first to understand what is meant by the manifestation here on the earth called life, and agree concerning the conditions that render it possible. apparently they are very simple. we may or may not agree that a counterpart of life, or life of a wholly different type from ours, may exist on other planets under conditions wholly diverse from those recognized as essential to its existence here. the problem of the origin of life is, in the present state of knowledge, highly speculative and hardly within the domain of science. here on earth, life is intimately associated with certain chemical compounds, in which carbon is the common element without which life would not exist. also hydrogen, oxygen, and nitrogen are present, with iron, sulphur, phosphorus, magnesium and a few less important elements besides. but carbon is the only substance absolutely essential. protoplasm cannot be built without it, and protoplasm makes up the most of the living cell. closely related to carbon is silica also, as a substitution in certain organic compounds. protoplasm is able to stand very low temperatures, but its properties as a living cell cease when the temperature reaches fahrenheit. animal life as it exists on the earth to-day appears to have been here many million years. the palæontologists agree that all life originated in the waters of the earth. it has passed through evolutionary stages from the lowest to the highest. throughout this vast period the astronomer is able to say that the conditions of the earth which appear to be essential to the maintenance of life have been pretty constantly what they are to-day. the higher the type of life, the narrower the range of conditions under which it thrives. man can exist at the frigid poles even if the temperature is degrees below fahrenheit zero; and in the deserts and the tropics, he swelters under temperatures of degrees, but he still lives. at these extremes, however, he can scarcely be said to thrive. we have, then, a relatively narrow range of temperatures which seems to be essential to his comfortable existence and development: we may call it degrees in extent. had not the surface temperature of the earth been maintained within this range for indefinite ages, in the regions where the human race has developed, quite certainly man would not be here. how this equability of temperature has been maintained does not now matter. clearly the earth must have existed through indefinite ages in the process of cooling down from temperatures of at least , degrees. during this stage the temperature of the surface was earth-controlled. then this period merged very gradually into the stage where life became possible, and the temperature of the surface became, as it now is, sun-controlled. how many years are embraced in this span of periods, or ages, we have no means of knowing. but of the sequence of periods and the secular diminution of temperature, we may be certain. then there is the equally important consideration of water necessary for the origination, support, and development of life. we cannot conceive of life existing without it. on the earth water is superabundant, and has been for indefinite ages in the past. there is little evidence that the oceans are drying up; although the commonly accepted view is that the waters of the earth will very gradually disappear. water can exist in the fluid state, which is essential to life, at all temperatures between degrees and degrees f. air to breathe is essential to life also. the atmosphere which envelops the earth is at least miles in depth, and its own weight compresses it to a tension of nearly pounds to the square inch at sea level. this atmosphere and its physical properties have had everything to do with the development of animal life on the planet. without it and its remarkable property of selective absorption, which imprisons and diffuses the solar heat, it is inconceivable that the necessary equability of surface temperature could be maintained. this appears to be quite independent of the chemical constituents of the atmosphere, and is perhaps the most important single consideration affecting the existence of life on a planet. if the surface of a planet is partly covered with water, it will possess also an atmosphere containing aqueous vapor. heat, water, and air: these three essentials determine whether there is life on a planet or not. of course there must be nutrition suitable to the organism; mineral for the vegetal, and vegetal for the animal. but the narrow range of variation appears to be the striking thing: relatively but a few degrees of temperature, and a narrow margin of atmospheric pressure. if this pressure is doubled or trebled, as in submarine caissons, life becomes insupportable. if, on the other hand, it is reduced even one-third, as on mountains even , feet high, the human mechanism fails to function, partly from lack of oxygen necessary in vitalizing the blood, but mainly because of simple reduction of mechanical pressure. if, then, we conceive of life in other worlds and it is agreed that life there must manifest itself much as it does here, our answer to the question of habitability of the planets must follow upon an investigation of what we know, or can reasonably surmise, about the surface temperatures of these bodies, whether they have water, and what are the probable physical characteristics of their atmospheres. we may inquire about each planet, then, concerning each of these details. the case of mercury is not difficult. at an average distance of only million miles from the sun, and with a large eccentricity of orbit which brings it a fifth part nearer, conditions of temperature alone must be such as to forbid the existence of life. the solar heat received is seven times greater than at the earth, and this is perhaps sufficient reason for a minimum of atmosphere, as indicated by observation. if no air, then quite certainly no water, as evaporation would supply a slight atmosphere. but according to the kinetic theory of gases, the mass of mercury, only a very small fraction of that of the sun, is inadequate to retain an atmospheric envelope. if, however, the planet's day and year are equal, so that it turns a constant face to the sun, surface conditions would be greatly complicated, so that we cannot regard the planet as absolutely uninhabitable on the hemisphere that is always turned away from the sun. venus at millions of miles from the sun presents conditions that are quite different. she receives double the solar heat that we do, but possessing an atmosphere perhaps threefold denser than ours, as reliably indicated by observations of transits of venus, the intensity of the heat and its diffusion may be greatly modified. what the selective absorption of the atmosphere of venus may be, we do not know. nor is the rotation time of the planet definitely ascertained: if equal to her year, as many observations show and as indicated by the theory of tidal evolution, there may well be certain regions on the hemisphere perpetually turned away from the sun where temperature conditions are identical with those on the tropical earth, and where every condition for the origin and development of life is more fully met than anywhere else in the solar system. whether venus has water distributed as on the earth we do not know, as her surface is never seen, owing to dense clouds under which she is always enshrouded. her cloudy condition possibly indicates an overplus of water. is the moon inhabited? quite certainly not: no appreciable air, no water, and a surface temperature unmodified by atmosphere--rising perhaps to degrees f. during the day, which is a fortnight in length, and falling at night to degrees below zero, if not lower. is mars inhabited? the probable surface temperature is much lower than the earth's, because mars receives only half as much solar heat as we do; and more important still, the atmosphere of mars is neither so dense nor so extensive as our own. seasons on mars are established, much the same as here, except that they are nearly twice as long as ours; and alternate shrinking and enlarging of the polar caps keeps even pace with the seasons, thereby indicating a certainty of atmosphere whose equatorial and polar circulation transports the moisture poleward to form the snow and ice of which the polar caps no doubt consist. there is a variety of evidence pointing to an atmosphere on mars of one-third to one-half the density of our own: an atmosphere in which free hydrogen could not exist, although other gases might. the spectroscopic evidence of water vapor in the martian atmosphere is not very strong. it is very doubtful whether water exists on mars in large bodies: quite certainly not as oceans, though the evidence of many small "lakes" is pretty well made out. with very little water, a thin atmosphere and a zero temperature, is mars likely to be inhabited at the present time? the chances are rather against it. if, however, the past development of the planet has progressed in the way usually considered as probable, we may be practically certain that mars has been inhabited in the past, when water was more abundant, and the atmosphere more dense so as to retain and diffuse the solar heat. biologists tell me that they hardly know enough regarding the extreme adaptability of organisms to environment to enable them to say whether life on such a planet as mars would or would not keep on functioning with secular changes of moisture and temperature. the survival of a race might be insured against extremely low temperatures by dwelling in sub-martian caves, and sufficient water might be preserved by conceivable engineering and mechanical schemes; but the secular reduction of the quantity and pressure of atmosphere--it is not easy to see how a race even more advanced than ourselves could maintain itself alive under serious lack of an element so vital to existence. both wallace, the great biologist, and arrhenius, the eminent chemist (but biologist, astronomer, and physicist as well), both reject the habitation theory of mars, regarding the so-called canals as quite like the luminous streaks on the moon; that is, cracks in the volcanic crust caused by internal strains due to the heated interior. wallace, indeed, argues that the planet is absolutely uninhabitable. the asteroids, or minor planets? we may dismiss them with the simple consideration that their individual masses are so insignificant and their gravity so slight that no atmosphere can possibly surround them. their temperatures must be exceedingly low, and water, if present at all, can only exist in the form of ice. jupiter, the giant planet, presents the opposite extreme. his mass is nearly a thousandth part of the sun's, and is sufficient to retain a very high temperature, probably approximating to the condition we call red-hot. this precludes the possibility of life at the outset, although the indications of a very dense atmosphere many thousand miles in depth are unmistakable. of saturn, one thirty-five hundredth the mass of the sun, practically the same may be said. proctor thought it quite likely that saturn might be habitable for living creatures of some sort, but he regarded the planet as on many accounts unsuitable as a habitation for beings constituted like ourselves. mere consideration of surface temperature precludes the possibility of life in the present stage of saturn's development; but the consensus of opinion is to the effect that life may make its appearance on these great planets at some inconceivably remote epoch in the future when the surface temperature is sufficiently reduced for life processes to begin. discoveries of algæ flourishing in hot springs approaching degrees fahrenheit make it possible that these beginnings may take place earlier and at much higher temperatures than have hitherto been thought possible. a century ago, when the ring of saturn was believed to be a continuous plane, this was a favorite corner of the solar system for speculation as to habitability; but now that we know the true constitution of the rings, no one would for a moment consider any such possibility. conditions may, however, be quite different with saturn's huge satellite titan, the giant moon of the solar system. its diameter makes it approximately the size of the planet mars; and although it is much farther removed from the sun, its relative nearness to the highly heated globe of saturn may provide that equability of temperature which is essential to life processes. also the three inner galilean moons of jupiter, especially iii which is about the size of titan, are excellently placed for life possibilities, as far as probable temperature is concerned, but we have of course no basis for surmising what their conditions may be as to air and water, except that their small mass would indicate a probable deficiency of those elements. uranus and neptune are planets so remote, and their apparent disks are so small, that very little is known about their physical condition. they are each about one-third the diameter of jupiter, and the spectrum of uranus shows broad diffused bands, indicating strong absorption by a dense atmosphere very different from that of the earth. indications are that neptune has a similar atmosphere. it is possible that the denser atmospheres of these remote planets may be so conditioned as to selective absorption that the relatively slender supply of solar heat may be conserved, and thus insure a relatively high surface temperature when the sun comes into control. if our theories of origin of the planets are to be trusted, we may rather suppose that uranus and neptune are still in a highly heated condition; that life has not yet made its appearance on them, but that it will begin its development ages before saturn and jupiter have cooled to the requisite temperature. comets? in his _lettres cosmologiques_ ( ) lambert considers the question of habitability of the comets, naturally enough in his day, because he thought them solid bodies surrounded by atmosphere, and related to the planets. the extremes of temperature at perihelia and aphelia to which comets are subjected did not bother him particularly. after calculating that the comet of , "being times nearer to the sun than we are ourselves, must have been subjected to a degree of heat , times as great as we are," lambert goes on to say: "whether this comet was of a more compact substance than our globe, or was protected in some other way, it made its perihelion passage in safety, and we may suppose all its inhabitants also passed safely. no doubt they would have to be of a more vigorous temperament and of a constitution very different from our own. but why should all living beings necessarily be constituted like ourselves? is it not infinitely more probable that amongst the different globes of the universe a variety of organizations exist, adapted to the wants of the people who inhabit them, and fitting them for the places in which they dwell, and the temperatures to which they will be subjected? is man the only inhabitant of the earth itself? and if we had never seen either bird or fish, should we not believe that the air and water were uninhabitable? are we sure that fire has not its invisible inhabitants, whose bodies, made of asbestos, are impenetrable to flame? let us admit that the nature of the beings who inhabit comets is unknown to us; but let us not deny their existence, and still less the possibility of it." little enough is really known about the physical nature of comets even now, but what we do know indicates incessant transformation and instability of conditions that would render life of any type exceedingly difficult of maintenance. a word about sir william herschel's theory of the sun and its habitability. he thought the core of the sun a dark, solid body, quite cold, and surrounded by a double layer, the inner one of which he conceived to act as a sort of fire screen to shield the sun proper against the intense heat of the outer layer, or photosphere by which we see it. viewed in this light, the sun, he says, "appears to be nothing else than a very eminent, large and lucid planet, evidently the first, or, in strictness of speaking, the only primary one of our system.... it is most probably also inhabited, like the rest of the planets, by beings whose organs are adapted to the peculiar circumstances of that vast globe." but physics and biology were undeveloped sciences in herschel's days. herschel knew, however, that the stars are all suns, so that he must have conceived that they are inhabited also, quite independently of the question whether they possess retinues of planets, after the manner of our solar system. this again is a question to which the astronomer of the present day can give no certain answer. so immensely distant are even the nearest of these multitudinous bodies that no telescope can ever be built large enough or powerful enough to reveal a dark planet as large as jupiter, alongside even the nearest fixed star. whatever may be the process of stellar evolution, there doubtless is an era of many hundreds of millions of years in the life of a star when it is passing through a planet-maintaining stage. this would likely depend upon spectral type, or to be indicated by it; and as about half of the stars are of the solar type, it would be a reasonable inference that at least half of the stars may have planets tributary to them. in such a case, the chances must be overwhelmingly in favor of vast numbers of the planets of other stellar systems being favorably circumstanced as to heat and moisture for the maintenance of life at the present time. that is, they are habitable, and if habitable, then thousands of them are no doubt inhabited now. but astronomers know absolutely nothing about this question, nor are they able to conceive at present any way that may lead them to any definite knowledge of it. there is, indeed, one piece of quasi-evidence which might reasonably be interpreted as implying that it is more likely that the stars are not attended by families of planets than that they are. chapter xxxv the little planets along toward the end of the eighteenth century and the beginning of the nineteenth, astronomers were leading a quiet unexcited life. sir william herschel had been knighted by king george for his discovery of the outer planet uranus, and practically everything seemed to be known and discovered in the solar system with a single exception. between mars and jupiter there existed an obvious gap in the planetary brotherhood. could it be possible that some time in the remote cosmic past a planet had actually existed there, and that some celestial cataclysm had blown it to fragments? if so, would they still be traveling round the sun as individual small planets? and might it not be possible to discover some of them among the faint stars that make up the belt of the zodiac in which all the other planets travel? so interesting was this question that the first international association of astronomers banded themselves together to carry on a systematic search round the entire zodiacal heavens in the faint hope of detecting possible fragments of the original planet of mere hypothesis. the astronomers of that day placed much reliance on what is known as bode's law--not a law at all, but a mere arithmetical succession of numbers which represented very well the relative distances of all the planets from the sun. and the distance of the newly found uranus fitted in so well with this law that the utter absence of a planet in the gap between mars and jupiter became very strongly marked. quite by accident a discovery of one of the guessed-at small planetary bodies was made, on january , , in palermo, sicily, by piazzi, who was regularly occupied in making an extensive catalogue of the stars. his observations soon showed that the new object he had seen could not be a fixed star, because it moved from night to night among the stars. he concluded that it was a planet, and named it ceres ( ), for the tutelary goddess of sicily. other astronomers kept up the search, and another companion planet, pallas ( ) was found in the following year. juno ( ) was found in , and vesta ( ), the largest and brightest of all the minor planets, in . vesta is sometimes bright enough when nearest the earth to be seen with the naked eye; but it was the last of the brighter ones, and no more discoveries of the kind were made till the fifth was found in . since then discoveries have been made in great abundance, more and more with every year till the number of little planets at present known is very near , . the early asteroid hunters found the search rather tedious, and the labor increased as it became necessary to examine the increasing thousands of fainter and fainter stars that must be observed in order to detect the undiscovered planets, which naturally grow fainter and fainter as the chase is prolonged. first a chart of the ecliptic sky had to be prepared containing all the stars that the telescope employed in the search would show. some of the most detailed charts of the sky in existence were prepared in connection with this work, particularly by the late dr. peters of hamilton college. once such charts are complete, they are compared with the sky, night after night when the moon is absent. thousands upon thousands of tedious hours are spent in this comparison, with no result whatever except that chart and sky are found to correspond exactly. but now and then the planet hunter is rewarded by finding a new object in the sky that does not appear on his chart. almost certainly this is a small planet, and only a few night's observation will be necessary to enable the discoverer to find out approximately the orbit it is traveling in, and whether it is out-and-out a new planet or only one that had been previously recognized, and then lost track of. nearly all the minor planets so far found have had names assigned to them principally legendary and mythological, and a nearly complete catalogue of them, containing the elements of their orbits (that is, all the mathematical data that tell us about their distance from the sun and the circumstances of their motion around him) is published each year in the "annuaire du bureau des longitudes" at paris. but these little planets require a great deal of care and attention, for some astronomers must accurately observe them every few years, and other astronomers must conduct intricate mathematical computations based on these observations; otherwise they get lost and have to be discovered all over again. professor watson, of the university of michigan and later of the university of wisconsin, endowed the asteroids of his own discovery, leaving to the national academy of sciences a fund for prosecuting this work perpetually, and leuschner is now ably conducting it. [illustration: jupiter, largest of the planets. the irregular belts change their mutual relation and shapes because they do not represent land, but are part of the atmosphere. (_photo, yerkes observatory._)] [illustration: the planet neptune and its satellite. the photograph required an exposure of the plate for one hour. (_photo, yerkes observatory._)] [illustration: saturn, as seen through the -inch refractor, at the time when only the edge of the rings is visible, showing condensations. (_photo, yerkes observatory._)] [illustration: saturn, photographed through the -inch refractor. the rings appear opened to the fullest extent they can be seen from the earth. the picture was made july , . (_photo, yerkes observatory._)] while the number of the asteroids is gratifyingly large, their individual size is so small and their total mass so slight that, even if there are a hundred thousand of them (as is wholly possible), they would not be comparable in magnitude with any one of the great planets. vesta, the largest, is perhaps miles in diameter, and if composed of substances similar to those which make up the earth, its mass may be perhaps one twenty-thousandth of the earth's mass. if we calculate the surface gravity on such a body, we find it about one-thirtieth of what it is here; so that a rifle ball, if fired on vesta with a muzzle velocity of only , feet a second, might overmaster the gravity of the little planet entirely and be projected in space never to return. if, as is likely, some of the smallest asteroids are not more than ten miles in diameter, their gravity must be so feeble a force that it might be overcome by a stone thrown from the hand. there is no reliable evidence that any of the asteroids are surrounded by atmospheric gases of any sort. probably they are for the most part spherical in form, although there is very reliable evidence that a few of the asteroids, being variable in the amount of sunlight that they reflect, are irregular in form, mere angular masses perhaps. the network of orbits of the asteroids is inconceivable complicated. nevertheless, there is a wide variation in their average distance from the sun, and their periods of traveling round him vary in a similar manner, the shortest being only about three years. while the longest is nearly nine years in duration, the average of all their periods is a little over four years. the gap in the zone of asteroids, at a distance from the sun equal to about five-eighths that of jupiter, is due to the excessive disturbing action of jupiter, whose periodic time is just twice as long as that of a theoretical planet at this distance. the average inclination of their orbits to the plane of the ecliptic is not far from degrees. but the orbit of pallas, for example, is inclined degrees, and the eccentricities of the asteroid orbits are equally erratic and excessive. both eccentricity and inclination of orbit at times suggest a possible relation to cometary orbits, but nothing has ever been definitely made out connecting asteroids and comets in a related origin. no comprehensive theory of the origin of the asteroid group has yet been propounded that has met with universal acceptance. according to the nebular hypothesis the original gaseous material, which should have been so concentrated as to form a planet of ordinary type, has in the case of the asteroids collected into a multitude of small masses instead of simply one. that there is a sound physical reason for this can hardly be denied. according to the laplacian hypothesis, the nearness of the huge planetary mass of jupiter just beyond their orbits produced violent perturbations which caused the original ring of gaseous material to collect into fragmentary masses instead of one considerable planet. the theory of a century ago that an original great planet was shattered by internal explosive forces is no longer regarded as tenable. to astronomers engaged upon investigation of distances in the solar system, the asteroid group has proved very useful. the late sir david gill employed a number of them in a geometrical research for finding the sun's distance, and more recently the discovery of eros ( ) has made it possible to apply a similar method for a like purpose when it approaches nearest to the earth in and . then the distance of eros will be less than half that of mars or even venus at their nearest. when the total number of asteroids discovered has reached , , with accurate determination of all their orbits, we shall have sufficient material for a statistical investigation of the group which ought to elucidate the question of its origin, and bear on other problems of the cosmogony yet unsolved. present methods of discovery of the asteroids by photography replace entirely the old method by visual observation alone, with the result that discoveries are made with relatively great ease and rapidity. chapter xxxvi the giant planet i can never forget as a young boy my first glimpse of the planet jupiter and his moons; it was through a bit of a telescope that i had put together with my own hands; a tube of pasteboard, and a pair of old spectacle lenses that chanced to be lying about the house. in the field of view i saw five objects; four of them looking quite alike, and as if they were stars merely (they were jupiter's moons), while the fifth was vastly larger and brighter. it was circular in shape, and i thought i could see a faint darkish line across the middle of it. this experience encouraged me immensely, and i availed myself eagerly of the first chance to see jupiter through a bigger and better glass. then i saw at once that i had observed nothing wrongly, but that i had seen only the merest fraction of what there was to see. in the first place, the planet's disk was not perfectly circular, but slightly oval. inquiring into the cause of this, we must remember that jupiter is actually not a flat disk but a huge ball or globe, more than ten times the diameter of the earth, which turns swiftly round on its axis once every ten hours as against the earth's turning round in twenty-four hours. then it is easy to see how the centrifugal force bulges outward the equatorial regions of jupiter, so that the polar regions are correspondingly drawn inward, thereby making the polar diameter shorter than the equatorial one, which is in line with the moons or satellites. the difference between the two diameters is very marked, as much as one part in fifteen. all the planets are slightly flattened in this way, but jupiter is the most so of all except saturn. the little darkish line across the planet's middle region or equator was found to be replaced by several such lines or irregular belts and spots, often seen highly colored, especially with reflecting telescopes; and they are perpetually changing their mutual relation and shapes, because they are not solid territory or land on jupiter, but merely the outer shapes of atmospheric strata, blown and torn and twisted by atmospheric circulation on this planet, quite the same as clouds in the atmosphere on the earth are. besides this the axial turning of jupiter brings an entirely different part of the planet into view every two or three hours; so that in making a map or chart of the planet, an arbitrary meridian must be selected. even then the process is not an easy one, and it is found that spots on jupiter's equator turn round in hours minutes, while other regions take a few minutes longer, the nearer the poles are approached. the great red spot, about , miles long and a quarter as much in breadth has been visible for about half a century. bolton, an english observer, has made interesting studies of it very recently. the four moons, or satellites, which a small telescope reveals, are exceedingly interesting on many accounts. they were the first heavenly bodies seen by the aid of the telescope, galileo having discovered them in . they travel round jupiter much the same as the moon does round the earth, but faster, the innermost moon about four times per week, the second moon about twice a week, the third or largest moon (larger than the planet mercury) once a week, and the outermost in about sixteen days. the innermost is about , miles from jupiter, and the outermost more than a million miles. from their nearness to the huge and excessively hot globe of jupiter, some astronomers, proctor especially, have inclined to the view that these little bodies may be inhabited. jupiter has other moons; a very small one, close to the planet, which goes round in less than twelve hours, discovered by barnard in . four others are known, very small and faint and remote from the planet, which travel slowly round it in orbits of great magnitude. the ninth, or outermost, is at a distance of fifteen and one-half million miles from jupiter, and requires nearly three years in going round the planet. it was discovered by nicholson at the lick observatory in . the eighth was discovered by melotte at greenwich in , and is peculiar in the great angle of degrees, at which its orbit is inclined to the equator of jupiter. the sixth and seventh satellites revolve round jupiter inside the eighth satellite, but outside the orbit of iv; and they were discovered by photography at the lick observatory in by perrine, now director of the argentine national observatory at cordoba. the ever-changing positions of the medicean moons, as galileo called the four satellites that he discovered--their passing into the shadow in eclipse, their transit in front of the disk, and their occultation behind it--form a succession of phenomena which the telescopist always views with delight. the times when all these events take place are predicted in the "nautical almanac," many thousand of them each year, and the predictions cover two or three years in advance. jupiter, as the naked eye sees him high up in the midnight sky, is the brightest of all the planets except venus; indeed, he is five times brighter than sirius, the brightest of all the fixed stars. his stately motion among the stars will usually be visible by close observation from day to day, and his distance from the earth, at times when he is best seen, is usually about million miles. jupiter travels all the way round the sun in twelve years; his motion in orbit is about eight miles a second. the eclipses of jupiter's moons, caused by passing into the shadow of the planet, would take place at almost perfectly regular intervals, if our distance from jupiter were invariable. but it was early found out that while the earth is approaching jupiter the eclipses take place earlier and earlier, but later and later when the earth is moving away. the acceleration of the earliest eclipse added to the retardation of the latest makes , seconds, which is the time that light takes in crossing a diameter of the earth's orbit round the sun. now the velocity of light is well known to be , miles per second, so we calculate at once and very simply that the sun's distance from the earth, which is half the diameter of the orbit, equals times , , or , , miles. chapter xxxvii the ringed planet saturn is the most remote of all the planets that the ancient peoples knew anything about. these anciently known planets are sometimes called the lucid or naked-eye planets--five in number: mercury, venus, mars, jupiter, and saturn. saturn shines as a first-magnitude star, with a steady straw-colored light, and is at a distance of about million miles from the earth when best seen. saturn travels completely round the sun in a little short of thirty years, and the telescope, when turned to saturn, reveals a unique and astonishing object; a vast globe somewhat similar to jupiter, but surrounded by a system of rings wholly unlike anything else in the universe, as far as at present known; the whole encircled by a family of ten moons or satellites. the saturnian system, therefore, is regarded by many as the most wonderful and most interesting of all the objects that the telescope reveals. at first the flattening of the disk of saturn is not easily made out, but every fifteen years (as and ) the earth comes into a position where we look directly at the thin edge of the rings, causing them to completely disappear. then the remarkable flattening of the poles of saturn is strikingly visible, amounting to as much as one-tenth of the entire diameter. the atmospheric belt system is also best seen at these times. but the rings of saturn are easily the most fascinating features of the system. they can never be seen as if we were directly above or beneath the planet so they never appear circular, as they really are in space, but always oval or elliptical in shape. the minor axis or greatest breadth is about one-half the major axis or length. the latter is the outer ring's actual diameter, and it amounts to , miles, or two and one-half times the diameter of saturn's globe. there are in fact no less than four rings; an outer ring, sometimes seen to be divided near its middle; an inner, broader and brighter ring; and an innermost dusky, or crape ring, as it is often called. this comes within about , miles of the planet itself. after the form and size of the rings were well made out, their thickness, or rather lack of thickness, was a great puzzle. if a model about a foot in diameter were cut out of tissue paper, the relative proportion of size and thickness would be about right. in space the thickness is very nearly miles, so that, when we look at the ring system edge-on, it becomes all but invisible except in very large telescopes. clearly a ring so thin cannot be a continuous solid object and recent observations have proved beyond a doubt that saturn's rings are made up of millions of separate particles moving round the planet, each as if it were an individual satellite. ever since the true theory of the constitution of the saturnian ring has been recognized on theoretic grounds, because clerke-maxwell founded the dynamical demonstration that the rings could be neither fluid nor solid, so that they must be made up of a vast multitude of particles traveling round the planet independently. but the physical demonstration that absolutely verified this conclusion did not come until , when, as we have said in a preceding chapter, keeler, by radial velocity measures on different regions of the ring by means of the spectroscope, proved that the inner parts of the ring travel more swiftly round the planet than the outer regions do. and he further showed that the rates of revolution in different parts of the ring exactly correspond to the periods of revolution which satellites of saturn would have, if at the same distance from the center of the planet. the innermost particles of the dusky ring, for example, travel round saturn in about five hours, while the outermost particles of the outer bright ring take hours to make their revolution. for many years it was thought that the saturnian ring system was a new satellite in process of formation, but this view is no longer entertained; and the system is regarded as a permanent feature of the planet, although astronomers are not in entire agreement as to the evolutionary process by which it came into existence--whether by some cosmic cataclysm, or by gradual development throughout indefinite aeons, as the rest of the solar system is thought to have come to its present state of existence. possibly the planetesimal hypothesis of chamberlin and moulton affords the true explanation, as the result of a rupture due to excessive tidal strain. chapter xxxviii the farthest planets on the th of march, , between and p. m., as sir william herschel was sweeping the constellation gemini with one of his great reflecting telescopes, one star among all that passed through the field of view attracted his attention. removing the eyepiece and applying another with a higher magnifying power, he found that, unlike all the other stars, this one had a small disk and was not a mere point of light, as all the fixed stars seem to be. a few nights' observation showed that the stranger was moving among the stars, so he thought it must be a comet; but a week's observation following showed that he had discovered a new member of the planetary system, far out beyond saturn, which from time immemorial had been assumed to be the outermost planet of all. this, then, was the first real discovery of a planet, as the finding of the satellites of jupiter had been the first of all astronomical discoveries. herschel's discovery occasioned great excitement, and he named the new planet georgium sidus or the georgian, after his king. the king created him a knight and gave him a pension, besides providing the means for building a huge telescope, feet long, with which he subsequently made many other astronomical discoveries. the planet that herschel discovered is now called uranus. uranus is an object not wholly impossible to see with the naked eye, if the sky background is clear and black, and one knows exactly where to look for it. its brightness is about that of a sixth magnitude star or a little fainter. its average distance from the sun is about , million miles and it takes eighty-four years to complete its journey round the sun, traveling only a little more than four miles a second. when we examine uranus closely with a large telescope, we find a small disk slightly greenish in tint, very slightly flattened, and at times faint bands or belts are apparently seen. uranus is about , miles in diameter, and is probably surrounded by a dense atmosphere. its rotation time is h. m. uranus is attended by four moons or satellites, named ariel, umbriel, titania, and oberon, the last being the most remote from the planet. this system of satellites has a remarkable peculiarity: the plane of the orbits in which they travel round uranus is inclined about degrees to the plane of the ecliptic, so that the satellites travel backward, or in a retrograde direction; or we might regard their motion as forward, or direct, if we considered the planes of their orbits inclined at degrees. for many years after the discovery of uranus it was thought that all the great bodies of the solar system had surely been found. least of all was any planet suspected beyond uranus until the mathematical tables of the motion of uranus, although built up and revised with the greatest care and thoroughness, began to show that some outside influence was disturbing it in accordance with newton's law of gravitation. the attraction of a still more distant planet would account for the disturbance, and since no such planet was visible anywhere a mathematical search for it was begun. neptune wholly independently of each other, two young astronomers, adams of england and le verrier of france, undertook to solve the unique problem of finding out the position in the sky where a planet might be found that would exactly account for the irregular motion of uranus. both reached practically identical results. adams was first in point of time, and his announcement led to the earliest observation, without recognition of the new planet (july , ), although it was le verrier's work that led directly to the new planet's being first seen and recognized as such (september , ). figuring backward, it was found that the planet had been accidentally observed in paris in , but its planetary character had been overlooked. neptune is the name finally assigned to this historical planet. it is thirty times farther from the sun than the earth, or , million miles; its velocity in orbit is a little over three miles per second, and it consumes years in going once completely round the sun. so faint is it that a telescope of large size is necessary to show it plainly. the brightness equals that of a star of the eighth magnitude, and with a telescope of sufficient magnifying power, the tiny disk can be seen and measured. the planet is about , miles in diameter, and is not known to possess more than one moon or satellite. if there are others, they are probably too faint to be seen by any telescope at present in existence. chapter xxxix the trans-neptunian planet investigation of the question of a possible trans-neptunian planet was undertaken by the writer in . as neptune requires years to travel completely round the sun, and the period during which it has been carefully observed embraces only half that interval, clearly its orbit cannot be regarded as very well known. any possible deviations from the mathematical orbit could not therefore be traced to the action of a possible unknown planet outside. but the case was different with uranus, which showed very slight disturbances, and these were assumed to be due to a possible planet exterior to both uranus and neptune. as a position for this body in the heavens was indicated by the writer's investigation, that region of the sky was searched by him with great care in - with the twenty-six-inch telescope at washington; and photographs of the same region were afterward taken by others, though only with negative results. in , forbes of edinburgh published his investigation of the problem from an entirely independent angle. families of comets have long been recognized whose aphelion distances correspond so nearly with the distances of the planets that these comet families are now recognized as having been created by the several planets, which have reduced the high original velocities possessed by the comets on first entering the solar system. their orbits have ever since been ellipses with their aphelia in groups corresponding to the distances of the planets concerned. jupiter has a large group of such comets, also saturn. uranus and neptune likewise have their families of comets, and forbes found two groups with average distances far outside of neptune; from which he drew the inference that there are two trans-neptunian planets. the position he assigned to the inner one agreed fairly well with the writer's planet as indicated by unexplained deviations of uranus. the theoretical problem of a trans-neptunian planet has since been taken up by gaillot and lau of paris, the late percival lowell, and w. h. pickering of harvard. the photographic method of search will, it is expected, ultimately lead to its discovery. on account of the probable faintness of the planet, at least the twelfth or thirteenth magnitude, metcalf's method of search is well adapted to this practical problem. when near its opposition the motion of neptune retrograding among the stars amounts to five seconds of arc in an hour; while the trans-neptunian planet would move but three seconds. by shifting the plate this amount hourly during exposure, the suspected object would readily be detected on the photographic plate as a minute and nearly circular disk, all the adjacent stars being represented by short trails. interest in a possible planet or planets outside the orbit of neptune is likely to increase rather than diminish. to the ancients seven was the perfect number, there were seven heavenly bodies already known, so there could be no use whatever in looking for an eighth. the discovery of uranus in proved the futility of such logic, and neptune followed in with further demonstration, if need be. the cosmogony of the present day sets no outer limit to the solar system, and some astronomers advocate the existence of many trans-neptunian planets. chapter xl comets--the hairy stars comets--hairy stars, as the origin of the name would indicate--are the freaks of the heavens. of great variety in shape, some with heads and some without, some with tails and some without, moving very slowly at one time and with exceedingly high velocity at another, in orbits at all possible angles of inclination to the general plane of the planetary paths round the sun, their antics and irregularities were the wonder and terror of the ancient world, and they are keenly dreaded by superstitious people even to the present day. down through the middle ages the advent of a comet was regarded as: threatening the world with famine, plague and war; to princes, death; to kingdoms, many curses; to all estates, inevitable losses; to herdsmen, rot; to plowmen, hapless seasons; to sailors, storms; to cities, civil treasons. comets appeared to be marvelous objects, as well as sinister, chiefly because they bid apparent defiance to all law. kepler had shown that the moon and the planets travel in regular paths--slightly elliptical to be sure, but nevertheless unvarying. none of the comets were known to follow regular paths till the time of halley late in the seventeenth century, when, as we have before told, a fine comet made its appearance, and halley calculated its orbit with much precision. comparing this with the orbits of comets that had previously been seen, he found its path about the sun practically identical with that of at least two comets previously observed in and . so halley ventured to think all these comets were one and the same body, and that it traveled round the sun in a long ellipse in a period of about seventy-five or seventy-six years. we have seen how his prediction of its return in was verified in every particular. on the comet's return in , crowell and crommelin of greenwich made a thorough mathematical investigation of the orbit, indicating that the year will witness its next return to the sun. there is a class of astronomers known as comet-hunters, and they pass hours upon hours of clear, sparkling, moonless nights in search for comets. they are equipped with a peculiar sort of telescope called a comet-seeker, which has an object glass usually about four or five inches in diameter, and a relatively short length of focus, so that a larger field of view may be included. regions near the poles of the heavens are perhaps the most fruitful fields for search, and thence toward the sun till its light renders the sky too bright for the finding of such a faint object as a new comet usually is at the time of discovery. generally when first seen it resembles a small circular patch of faint luminous cloud. when a suspect is found, the first thing to do is to observe its position accurately with relation to the surrounding stars. then, if on the next occasion when it is seen the object has moved, the chances are that it is a comet; and a few days' observation will provide material from which the path of the comet in space can be calculated. by comparing this with the complete lists of comets, now about in number, it is possible to tell whether the comet is a new one, or an old one returning. the total number of comets in the heavens must be very great, and thousands are doubtless passing continually undetected, because their light is wholly overpowered by that of the sun. of those that are known, perhaps one in twelve develops into a naked-eye comet, and in some years six or seven will be discovered. with sufficiently powerful telescopes, there are as a rule not many weeks in the year when no comet is visible. brilliant naked-eye comets are, however, infrequent. comets, except halley's, generally bear the name of their discoverer, as donati ( ), and pons-brooks ( ). pons was a very active discoverer of comets in france early in the nineteenth century: he was a doorkeeper at the observatory of marseilles, and his name is now more famous in astronomy than that of thulis, then the director of the observatory, who taught and encouraged him. messier was another very successful discoverer of comets in france, and in america we have had many: swift, brooks, and barnard the most successful. how bright a comet will be and how long it will be visible depends upon many conditions. so the comets vary much in these respects. the first comet of was under observation for nearly a year and a half, the longest on record till halley's in . in case a comet eludes discovery and observation until it has passed its perihelion, or nearest point to the sun, its period of visibility may be reduced to a few weeks only. the brightest comets on record were visible in and : so brilliant were they that even the effulgence of full daylight did not overpower them. in particular the comet of was not only excessively bright, but at its nearest approach to the earth its tail swept all the way across the sky from one horizon to the other. it must have looked very much like the straight beam of an enormous searchlight, though very much brighter. the tails of comets are to the naked eye the most compelling thing about them, and to the ancient peoples they were naturally most terrifying. their tails are not only curved, but sometimes curved with varying degrees of curvature, and this circumstance adds to their weirdness of appearance. if we examine the tail of a comet with a telescope, it vanishes as if there were nothing to it: as indeed one may almost say there is not. ordinarily, only the head of the comet is of interest in the telescope. when first seen there is usually nothing but the head visible, and that is made up of portions which develop more or less rapidly, presenting a succession of phenomena quite different in different comets. when first discovered a comet is usually at a great distance from the sun, about the distance of jupiter; and we see it, not as we do the planets, by sunlight reflected from them, but by the comet's own light. this is at that time very faint, and nearly all comets at such a distance look alike: small roundish hazy patches of faint, cloudlike light, with very often a concentration toward the center called the nucleus, on the average about , miles in diameter. approach toward the sun brightens up the comet more and more, and the nucleus usually becomes very much brighter and more starlike. then on the sunward side of the nucleus, jetlike streamers or envelopes appear to be thrown off, often as if in parallel curved strata, or concentrically. as they expand and move outward from the nucleus, these envelopes grow fainter and are finally merged in the general nebulosity known as the comet's head, which is anywhere from , to , miles in diameter. as a rule, this is an orderly development which can be watched in the telescope from hour to hour and from night to night; but occasionally a cometary visitor is quite a law to itself in development, presenting a fascinating succession of unpredictable surprises. then follows the development of the comet's tail, perhaps more striking than anything that has preceded it. here a genuine repulsion from the sun appears to come into play. it may be an electrical repulsion. much of the material projected from the comet's nucleus, seems to be driven backward or repelled by the sun, and it is this that goes to form the tail. the particles which form the tail then travel in modified paths which nevertheless can be calculated. the tail is made up of these luminous particles and it expands in space much in the form of a hollow, horn-shaped cone, the nucleus being near the tip of the horn. some comets possess multiple tails with different degrees of curvature, donati's for example. usually there is a nearly straight central dark space, marking the axis of the comet, and following the nucleus. but occasionally this is replaced by a thin light streak very much less in breadth than the diameter of the head. cometary tails are sometimes million miles in length. three different types of cometary tails are recognized. first, the long straight ones, apparently made up of matter repelled by the sun twelve to fifteen times more powerfully than gravitation attracts it. such particles must be brushed away from the comet's head with a velocity of perhaps five miles a second, and their speed is continually increasing. probably these straight tails are due to hydrogen. the second type tails are somewhat curved, or plume-like, and they form the most common type of cometary tail. in them the sun's repulsion is perhaps twice its gravitational attraction, and hydrocarbons in some form appear to be responsible for tails of this character. then there is a third type, much less often seen, short and quickly curving, probably due to heavier vapors, as of chlorine, or iron, or sodium, in which the repulsive force is only a small fraction of that of gravitation. many features of this theory of cometary tails are borne out by examination of their light with the spectroscope, although the investigation is as yet fragmentary. it is evident that the tail of a comet is formed at the expense of the substance of the nucleus and head; so that the matter repelled is forever dissipated through the regions of space which the comet has traveled. comets must lose much of their original substance every time they return to perihelion. comets actually age, therefore, and grow less and less in magnitude of material as well as brightness, until they are at last opaque, nonluminous bodies which it becomes impossible to follow with the telescope. chapter xli where do comets come from? where do comets come from? the answer to this question is not yet fully made out. most likely they have not all had a similar origin, and theories are abundant. apparently they come into the solar system from outer space, from any direction whatsoever. the depths of interstellar space seem to be responsible for most, if not all, of the new ones. whether they have come from other stars or stellar systems we cannot say. while comets are tremendous in size or volume, their mass or the amount of real substance in them is relatively very slight. we know this by the effect they produce on planets that they pass near, or rather by the effect that they fail to produce. the earth's atmosphere weighs about one two hundred and fifty thousandth as much as the earth itself, but a comet's entire mass must be vastly less than this. even if a comet were to collide with the earth head on, there is little reason to believe that dire catastrophe would ensue. at least twice the earth is known to have passed through the tail of a comet, and the only effect noticed was upon the comet itself; its orbit had been modified somewhat by the attraction of the earth. if the comet were a small one, collision with any of the planets would result in absorption and dissipation of the comet into vapor. the whole of a large comet has perhaps as much mass or weight as a sphere of iron a hundred miles in diameter. even this could not wreck the earth, but the effect would depend upon what part of the earth was hit. a comet is very thin and tenuous, because its relatively small mass is distributed through a volume so enormous. so it is probable that the earth's atmosphere could scatter and burn up the invading comet, and we should have only a shower of meteors on an unprecedented scale. diffusion of noxious gases through the atmosphere might vitiate it to some extent, though probably not enough to cause the extinction of animal life. every comet has an interesting history of its own, almost indeed unique. one of the smallest comets and the briefest in its period round the sun is known as encke's comet. it is a telescopic comet with a very short tail, its time of revolution is about three and a half years, and it exhibits a remarkable contraction of volume on approach to the sun. biela's comet has a period about twice as long. at one time it passed within about million miles of the earth, and somewhere about the year this comet divided into two distinct comets, which traveled for months side by side, but later separated and both have since completely disappeared. perhaps the most beautiful of all comets is that discovered by donati of florence in . its coma presented the development of jets and envelopes in remarkable perfection, and its tail was of the secondary or hydrocarbon type, but accompanied by two faint streamer tails, nearly tangential to the main tail and of the hydrogen type. donati's comet moves in an ellipse of extraordinary length, and it will not return to the sun for nearly , years. the most brilliant comet of the last half century is known as the great comet of . in a clear sky it could readily be seen at midday. on september it passed across the disk of the sun and was practically as bright as the surface of the sun itself. the comet had a multiple nucleus and a hydrocarbon tail of the second type, nearly a hundred million miles in length. doubtless this great comet is a member of what is known as a cometary group, which consists of comets having the same orbit and traveling tandem round the sun. the comets of , , , and belong to this particular group, and they all pass within , miles of the sun's surface, at a maximum velocity exceeding miles a second. they must therefore invade the regions of the solar corona, the inference being that the corona as well as the comet is composed of exceedingly rare matter. photography of comets has developed remarkably within recent years, especially under the deft manipulation of barnard, whose plates, in particular during his residence at the lick observatory on mount hamilton, california, show the features of cometary heads and tails in excellent definition. halley's comet, at the apparition, was particularly well photographed at many observatories. the question is often asked, when will the next comet come? if a large bright comet is meant, astronomers cannot tell. at almost any time one may blaze into prominence within only a few days. during the latter half of the last century, bright comets appeared at perihelion at intervals of eight years on the average. several of the lesser and fainter periodic comets return nearly every year, but they are mostly telescopic, and are rarely seen except by astronomers who are particularly interested in observing them. chapter xlii meteors and shooting stars "falling stars," or "shooting stars," have been familiar sights in all ages of the world, but the ancient philosophers thought them scarcely worthy of notice. according to aristotle they were mere nothings of the upper atmosphere, of no more account than the general happenings of the weather. but about the end of the eighteenth century and the beginning of the nineteenth the insufficiency of this view began to be fully recognized, and interplanetary space was conceived as tenanted by shoals of moving bodies exceedingly small in mass and dimension as compared with the planets. millions of these bodies are all the time in collision with the outlying regions of our atmosphere; and by their impact upon it and their friction in passing swiftly through it, they become heated to incandescence, thus creating the luminous appearances commonly known as shooting stars. for the most part they are consumed or dissipated in vapor before reaching the solid surface of the earth; but occasionally a luminous cloud or streak is left glowing in the wake of a large meteor, which sometimes remains visible for half an hour after the passage of the meteor itself. these mistlike clouds projected upon the dark sky have been especially studied by trowbridge of columbia university. many more meteors are seen during the morning hours, say from four to six, than at any other nightly period of equal length, because the visible sky is at that time nearly centered around the general direction toward which the earth is moving in its orbit round the sun; so that the number of meteors that would fall upon the earth if at rest is increased by those which the earth overtakes by its own motion. also from january to july while the earth is traveling from perihelion to aphelion, fewer meteors are seen than in the last half of the year; but this is chiefly because of the rich showers encountered in august and november. although the descent of meteoric bodies from the sky was pretty generally discredited until early in the nineteenth century, such falls had nevertheless been recorded from very early times. they were usually regarded as prodigies or miracles, and such stones were commonly objects of worship among ancient peoples. for example, the phrygian stone, known as the "diana of the ephesians which fell down from jupiter," was a famous stone built into the kaaba at mecca, and even to-day it is revered by mohammedans as a holy relic. perhaps the earliest known meteoric fall is that historically recorded in the parian chronicle as having occurred in the island of crete, b. c. . also in the imperial museum of petrograd is the pallas or krasnoiarsk iron, perhaps three-quarters of a ton in weight, found in by pallas, the famous traveler, at krasnoiarsk, siberia. but a fall of meteoric stones that chanced upon the department of orne, france, in , led to a critical investigation by biot, the distinguished physicist and academician. according to his report a violent explosion in the neighborhood of l'aigle had been heard for a distance of seventy-five miles around, and lasting five or six minutes, about p. m. on tuesday, april . from several adjoining towns a rapidly moving fireball had been seen in a sky generally clear, and there was absolutely no room for doubt that on the same day many stones fell in the neighborhood of l'aigle. biot estimated their number between two and three thousand, and they were scattered over an elliptical area more than six miles long, and two and a half miles broad. thenceforward the descent of meteoric matter from outer space upon the earth has been recognized as an unquestioned fact. the origin of these bodies being cosmic, meteors may be expected to fall upon the earth without reference to latitude, or season, or day and night, or weather. on entering our upper atmosphere their temperature must be that of space, many hundred degrees below zero; and their velocities range from ten miles per second upward. but atmospheric resistance to their flight is so great that their velocity is quickly reduced: at ground impact it does not exceed a few hundred feet per second. on january , , several meteoric stones fell on ice only a few inches thick in sweden, rebounding without either breaking through the ice or being themselves fractured. naturally the flight of a meteor through the atmosphere will be only a few seconds in duration, and owing to the sudden reduction of velocity, it will continue to be luminous throughout only the upper part of its course. visibility generally begins at an elevation of about seventy miles, and ends at perhaps half that altitude. what is the origin of meteors? theories there are in great abundance: that they come from the sun, that they come from the moon, that they come from the earth in past ages as a result of volcanic action, and so on. but there are many difficulties in the way of acceptance of these and several other theories. that all meteors were originally parts of cometary masses is however a theory that may be accepted without much hesitation. comets have been known to disintegrate. biela's comet even disappeared entirely, so that during a shower of biela meteors in november, , an actual fragment of the lost comet fell upon the earth, at mazapil, mexico. and as the bielid meteors encounter the earth with the relatively low velocity of ten miles a second, we may expect to capture other fragments in the future. numerous observers saw the weird disintegration of the nucleus of the great comet of , well recognized as a member of the family of the comet of . as these comets are fellow voyagers through space along the same orbit, probably all five members of the family, with perhaps others, were originally a single comet of unparalleled magnitude. the brooks comet of affords another instance of fragmentary nucleus. the oft-repeated action of solar forces tending to disrupt the mass of a comet more and more, and scatter its material throughout space, the secular dismemberment of all comets becomes an obvious conclusion. during the hundreds of millions of years that these forces are known to have been operant, the original comets have been broken up in great numbers, so that elliptical rings of opaque meteoric bodies now travel round the sun in place of the comets. these bodies in vast numbers are everywhere through space, each too small to reflect an appreciable amount of sunlight, and becoming visible only when they come into collision with our outer atmosphere. the practical identity of several such meteor streams and cometary orbits has already been established, and there is every reason for assigning a similar origin to all meteoric bodies. meteors, then, were originally parts of comets, which have trailed themselves out to such extent that particles of the primal masses are liable to be picked up anywhere along the original cometary paths. the historic records of all countries contain trustworthy accounts of meteoric showers. making due allowances for the flowery imagery of the oriental, it is evident that all have at one time or another seen much the same thing. in a. d. , for instance, the constantinople sky was reported alive with flying stars. in october, , "stars appeared like waves upon the sky; and they flew about like grasshoppers." during the reign of king william ii occurred a very remarkable shower in which "stars seemed to fall like rain from heaven." but the showers of november, and , are easily the most striking of all. the sky was filled with innumerable fiery trails and there was not a space in the heavens a few times the size of the moon that was not ablaze with celestial fireworks. frequently huge meteors blended their dazzling brilliancy with the long and seemingly phosphorescent trails of the shooting stars. the interval of thirty-four years between and appeared to indicate the possibility of a return of the shower in november of or , and all the people of that day were aroused on this subject and made every preparation to witness the spectacle. extemporized observatories were established, watchmen were everywhere on the lookout, and bells were to be rung the minute the shower began. the newspapers of the day did little to allay the fears of the multitude, but the critical days of november, , passed with disappointment in america. in europe, however, a fine shower was seen, though it was not equal to that of . the astronomers at greenwich counted many thousand meteors. in november of , however, american astronomers were gratified by a grand display, which, although failing to match the general expectation, nevertheless was a most striking spectacle, and the careful preparation for observing it afforded data of observation which were of the greatest scientific value. the actual orbits of these bodies in space became known with great exactitude, and it was found that their general path was identical with that of the first comet of , which travels outward somewhat beyond the planet uranus. when the visible paths of these meteors are traced backward, all appear as if they originated from the constellation leo. so they are known as leonids, and a return of the shower was confidently predicted for november, - , which for unknown reasons failed to appear. [illustration: two views of halley's comet. taken with the same camera from the same position, one on may , and the other on may , . (_photo, mt. wilson solar observatory._)] [illustration: swift's comet of . this comet showed extraordinary and rapid transformations, one day having a dozen streamers in its tail, another only two. (_photo by prof. e. e. barnard._)] [illustration: a large meteor trail in the field with fine nebulÆ. (_photo, yerkes observatory._)] during the last half century meteors have been pretty systematically observed, especially by the astronomers of italy and denning of england, so that several hundred distinct showers are now known, their radiant points fall in every part of the heavens, and there is scarcely a clear moonless night when careful watching for meteors will be unrewarded. besides november, the months of august (perseids), april (lyrids), and december (geminids) are favorable. following in tabular form is a fairly comprehensive list of the meteoric showers of the year, with the positions of the radiant points and the epochs of the showers according to denning: radiant point ============================================================ name of shower | r. a. | decl. | date of shower -----------------------+---------+--------+----------------- quadrantids | ° | + ° | jan. - zeta cepheids | ° | + ° | jan. alpha leonids | ° | + ° | feb. -march tau leonids | ° | + ° | march - beta ursids | ° | + ° | march - lyrids | ° | + ° | april - gamma aquarids | ° | - ° | may - zeta herculids | ° | + ° | may - eta pegasids | ° | + ° | may -june theta boötids | ° | + ° | june - alpha capricornids | ° | - ° | july - delta aquarids | ° | - ° | july - perseids | ° | + ° | aug. - omicron draconids | ° | + ° | aug. - zeta draconids | ° | + ° | aug. -sept. piscids | ° | + ° | sept. - alpha andromedids | ° | + ° | sept. epsilon arietids | ° | + ° | oct. - orionids | ° | + ° | oct. - epsilon perseids | ° | + ° | nov. leonids | ° | + ° | nov. - epsilon taurids | ° | + ° | nov. - andromedids | ° | + ° | nov. - beta geminids | ° | + ° | dec. - geminids | ° | + ° | dec. - alpha ursæ majorids | ° | + ° | dec. - kappa draconids | ° | + ° | dec. - ------------------------------------------------------------ the year was exceptional in providing an abundant and previously unknown shower on june , and its stream has nearly the same orbit as that of the pons-winnecke periodic comet. useful observations of meteors are not difficult to make, and they are of service to professional astronomers investigating the orbits of these bodies, among whom are mitchell and olivier of the university of virginia. chapter xliii meteorites meteorites, the name for meteors which have actually gone all the way through our atmosphere, are never regular in form or spherical. as a rule the iron meteorites are covered with pittings or thumb marks, due probably to the resistance and impact of the little columns of air which impede its progress, together with the unequal condition and fusibility of their surface material. the work done by the atmosphere in suddenly checking the meteor's velocity appears in considerable part as heat, fusing the exterior to incandescence. this thin liquid shell is quickly brushed off, making oftentimes a luminous train. but notwithstanding the exceedingly high temperature of the exterior, enforced upon it for the brief time of transit through the atmosphere, it is probable that all large meteorites, if they could be reached at once on striking the earth, would be found to be cold, because the smooth, black, varnishlike crust which always incases them as a result of intense heat is never thick. on one occasion a meteor which was seen to fall in india was dug out of the ground as quickly as possible, and found to be, not hot as was expected, but coated thickly over with ice frozen on it from the moisture in the surrounding soil. as to the composition of shooting stars, and their probable mass, and its effect upon the earth, our data are quite insufficient. the lines of sodium and magnesium have been hurriedly caught in the spectroscope, and, estimating on the basis of the light emitted by them, the largest meteors must weigh ounces rather than pounds. nevertheless, it is interesting to inquire what addition the continual fall of many millions daily upon the earth makes to its weight: somewhere between thirty and fifty thousand tons annually is perhaps a conservative estimate, but even this would not accumulate a layer one inch in thickness over the entire surface of the earth in less than a thousand million years. many hundreds of the meteors actually seen to fall, together with those picked up accidentally, are recovered and prized as specimens of great value in our collections, the richest of which are now in new york, paris, and london. the detailed investigation of them is rather the province of the chemist, the crystallographer and the mineralogist than of the astronomer whose interest is more keen in their life history before they reach the earth. to distinguish a stony meteorite from terrestrial rock substances is not always easy, but there is usually little difficulty in pronouncing upon an iron meteorite. these are most frequently found in deserts, because the dryness of the climate renders their oxidation and gradual disappearance very slow. the surface of a suspected iron meteorite is polished to a high luster and nitric acid is poured upon it. if it quickly becomes etched with a characteristic series of lines, or a sort of cross-hatching, it is almost certain to be a meteorite. occasionally carbon has been found in meteorites, and the existence of diamond has been suspected. the minerals composing meteorites are not unlike terrestrial materials of volcanic origin, though many of them are peculiar to meteorites only. more than one-third of all the known chemical elements have been found by analysis in meteorites, but not any new ones. meteoric iron is a rich alloy containing about ten per cent of nickel, also cobalt, tin, and copper in much smaller amount. calcium, chlorine, sodium, and sulphur likewise are found in meteoric irons. at very high temperatures iron will absorb gases and retain them until again heated to red heat. carbonic oxide, helium, hydrogen, and nitrogen are thus imprisoned, or occluded, in meteoric irons in very small quantities; and in , during a london lecture by graham, a room in the royal institution was for a brief space illuminated by gas brought to earth in a meteorite from interplanetary space. meteorites, too, have been most critically investigated by the biologist, but no trace of germs of organic life of any type has so far been found. farrington of chicago has published a full descriptive catalogue of all the north american meteorites. recent investigations of the radioactivity of meteorites show that the average stone meteorite is much less radioactive than the average rock, and probably less than one-fourth as radioactive as in average granite. the metallic meteorites examined were found about wholly free from radioactivity. from shooting stars, perhaps the chips of the celestial workshop, or more possibly related to the planetesimals which the processes of growth of the universe have swept up into the vastly greater bodies of the universe, transition is natural to the stars themselves, the most numerous of the heavenly bodies, all shining by their own light, and all inconceivably remote from the solar system, which nevertheless appears to be not far removed from the center of the stellar universe. chapter xliv the universe of stars our consideration of the solar system hitherto has kept us quite at home in the universe. the outer known planets, uranus and neptune, are indeed far removed from the sun, and a few of the comets that belong to our family travel to even greater distances before they begin to retrace their steps sunward. when we come to consider the vast majority of the glistening points on the celestial sphere--all in fact except the five great planets, mercury, venus, mars, jupiter, and saturn--we are dealing with bodies that are self-luminous like the sun, but that vary in size quite as the bodies of the solar system do, some stars being smaller than the sun and others many hundred fold larger than he is; some being "giants," and others "dwarfs." but the overwhelming remoteness of all these bodies arrests our attention and even taxes our credulity regarding the methods that astronomers have depended on to ascertain their distances from us. their seeming countlessness, too, is as bewildering as are the distances; though, if we make actual counts of those visible to the naked eye within a certain area, in the body of the "great bear," for example, the great surprise will be that there are so few. and if the entire dome of the sky is counted, at any one time, a clear, moonless sky would reveal perhaps , , so that in the entire sky, northern and southern, we might expect to find , to , lucid stars, or stars visible to the naked eye. but when the telescope is applied, every accession of power increases the myriads of fainter and fainter stars, until the number within optical reach of present instruments is somewhere between and millions. but if we were to push the -inch reflector on mount wilson to its limit by photography with plates of the highest sensitiveness, millions upon millions of excessively faint stars would be plainly visible on the plates which the human eye can never hope to see directly with any telescope present or future, and which would doubtless swell the total number of stars to a thousand millions. recent counts of stars by chapman and melotte of greenwich tend to substantiate this estimate. what have astronomers done to classify or catalogue this vast array of bodies in the sky? even before making any attempt to estimate their number, there is a system of classification simply by the amount of light they send us, or by their apparent stellar magnitudes--not their actual magnitudes, for of those we know as yet very little. we speak of stars of the "first magnitude," of which there are about , sirius being the brightest and regulus the faintest. then there are about of the second, or next fainter, magnitude, stars like polaris, for example, which give an amount of light two and a half times less than the average first magnitude star. stars of the third magnitude are fainter than those of the second in the same ratio, but their number increases to ; fourth magnitude, ; fifth magnitude, , ; sixth magnitude, , , and these are so faint that they are just visible on the best nights without telescopic aid. decimals express all intermediate graduations of magnitude. astronomers carry the telescopic magnitudes much farther, till a magnitude beyond the twentieth is reached, preserving in every case the ratio of two and one-half for each magnitude in relation to that numerically next to it. even jupiter and venus, and the sun and moon, are sometimes calculated on this scale of stellar magnitude, numerically negative, of course, venus sometimes being as bright as magnitude - . , and the sun - . . knowing thus the relation of sun, moon, and stars, and the number of the stars of different magnitudes, it is possible to estimate the total light from the stars. this interesting relation comes out this way: that the stars we cannot see with the naked eye give a greater total of light than those we can because of their vastly greater numbers. and if we calculate the total light of all the brighter stars down to magnitude nine and one-half, we find it equal to / th of the light of the average full moon. many stars show marked differences in color, and strictly speaking the stars are now classified by their colors. the atmosphere affects star colors very considerably, low altitudes, or greater thickness of air, absorbing the bluish rays more strongly and making the stars appear redder than they really are. aldebaran, betelgeuse and antares are well-known red stars, capella and alpha ceti yellowish, vega and sirius blue, and procyon and polaris white. among the telescopic stars are many of a deep blood-red tint, variable stars being numerous among them. double stars, too, are often complementary in color. there is evidence indicating change of color of a very few stars in long periods of time; sirius, for example, two thousand years ago was a red star, now it is blue or bluish white. but the meaning of color, or change of color in a star is as yet only incompletely ascertained. it may be connected with the radiative intensity of the star, or its age, or both. the late professor edward c. pickering was famous for his life-long study and determination of the magnitudes of the stars. standards of comparison have been many, and have led to much unnecessary work. pickering chose polaris as a standard and devised the meridian photometer, an ingenious instrument of high accuracy, in which the light of a star is compared directly with that of the pole star by reflection. all the bright stars of both the northern and the southern skies are worked into a standard system of magnitudes known as hp, or the harvard photometry. astronomers make use of several different kinds of magnitude for the stars: the apparent magnitude, as the eye sees it, often called the visual magnitude; the photographic magnitude, as the photographic plate records it, and these are now determined with the highest accuracy; the photovisual magnitude, quite the same as the visual, but determined photographically on an isochromatic plate with a yellow screen or filter, so that the intensity is nearly the same as it appears to the eye. the difference between the star's visual or photovisual magnitude and its photographic magnitude is called its color-index, and is often used as a measure of the star's color. light of the shorter wave lengths, as blue and violet, affects the photographic plate more rapidly than the reds and yellows of longer wave length by which the eye mainly sees; so that red stars will appear much fainter and blue stars much brighter on the ordinary photographic plate than the eye sees them. so great are the differences of color in the stars that well-known asterisms, with which the eye is perfectly familiar, are sometimes quite unrecognizable on the photographic plate, except by relative positions of the stars composing them. white stars affect the eye and the plate about equally, so that their visual or photovisual and photographic magnitudes are about equal. the studies of the colors of the stars, the different methods of determining them, and the relations of color to constitution have been made the subject of especial investigation by seares of mount wilson and many other astronomers. centuries of the work of astronomers have been faithfully devoted to mapping or charting the stars and cataloguing them. just as we have geographical maps of countries, so the heavens are parceled out in sections, and the stars set down in their true relative positions just as cities are on the map. recent years have added photographic charts, especially of detailed regions of the sky; but owing to spectral differences of the stars, their photographic magnitudes are often quite different from their visual magnitudes. from these maps and charts the positions of the stars can be found with much precision; but if we want the utmost accuracy, we must go to the star catalogues--huge volumes oftentimes, with stellar positions set down therein with the last degree of precision. first there will be the star's name, and in the next column its magnitude, and in a third the star's right ascension. this is its angular distance eastward around the celestial sphere starting from the vernal equinox, and it corresponds quite closely to the longitude of a place which we should get from a gazetteer, if we wished to locate it on the earth. then another column of the catalogue will give the star's declination, north or south of the equator, just as the gazetteer will locate a city by its north or south latitude. chapter xlv star charts and catalogues who made the first star chart or catalogue? there is little doubt that eudoxus (b. c. ) was the first to set down the positions of all the brighter stars on a celestial globe, and he did this from observations with a gnomon and an armillary sphere. later hipparchus (b. c. ) constructed the first known catalogue of stars, so that astronomers of a later day might discover what changes are in progress among the stars, either in their relative positions or caused by old stars disappearing or new stars appearing at times in the heavens. hipparchus was an accurate observer, and he discovered an apparent and perpetual shifting of the vernal equinox westward, by which the right ascensions of the stars are all the time increasing. he determined the amount of it pretty accurately, too. his catalogue contained , stars, and is printed in the "almagest" of ptolemy. centuries elapsed before a second star catalogue was made, by ulugh-beg, an arabian astronomer, a. d. , who was a son of tamerlane, the tartar monarch of samarcand, where the observations for the catalogue were made. the stars were mainly those of ptolemy, and much the same stars were reobserved by tycho brahe (a. d. ) with his greatly improved instruments, thus forming the third and last star catalogue of importance before the invention of the telescope. from the end of the seventeenth century onward, the application of the telescope to all the types of instruments for making observations of star places has increased the accuracy many-fold. the entire heavens has been covered by argelander in the northern hemisphere, and gould in the southern--over , stars in all. many government observatories are still at work cataloguing the stars. the carnegie institution of washington maintains a department of astrometry under boss of albany, which has already issued a preliminary catalogue of more than , stars, and has a great general catalogue in progress, together with investigations of stellar motions and parallaxes. this catalogue of star positions will include proper motions of stars to the seventh magnitude. in on proposal of the late sir david gill, an international congress of astronomers met at paris and arranged for the construction of a photographic chart of the entire heavens, allotting the work to eighteen observatories, equipped with photographic telescopes essentially alike. the total number of plates exceeds , . stars of the fourteenth magnitude are recorded, but only those including the eleventh magnitude will be catalogued, perhaps , , in all. the expense of this comprehensive map of the stars has already exceeded $ , , , and the work is now nearly complete. turner of oxford has conducted many special investigations that have greatly enhanced the progress of this international enterprise. other great photographic star charts have been carried through by the harvard observatory, with the annex at arequipa, peru, employing the bruce photographic telescope, a doublet with -inch lenses; also kapteyn of groningen has catalogued about , stars on plates taken at cape town. charting and cataloguing the stars, both visually and photographically, is a work that will never be entirely finished. improvements in processes will be such that it can be better done in the future than it is now, and the detection of changes in the fainter stars and investigation of their motions will necessitate repetition of the entire work from century to century. the origin of the names of individual stars is a question of much interest. the constellation figures form the basis of the method, and the earliest names were given according to location in the especial figure; as for instance, cor scorpii, the heart of the scorpion, later known as antares or alpha scorpii. the arabians adopted many star names from the greeks, and gave about a hundred special names to other stars. some of these are in common use to-day, by navigators, observers of meteors and of variable stars. sirius, vega, arcturus, and a few other first magnitude stars, are instances. but this method is quite insufficient for the fainter stars whose numbers increase so rapidly. bayer, a contemporary of galileo, originated our present system, which also employs the names of the constellations, the latin genitive in each case, prefixed by the small letters of the greek alphabet, from alpha to omega, in order of decreasing brightness; and followed by the roman letters when the greek alphabet is exhausted. if there were still stars left in a constellation unnamed, numbers were used, first by flamsteed, astronomer royal; and numbers in the order of right ascension in various catalogues are used to designate hundreds of other stars. the vast bulk of the stars are, however, nameless; but about one million are identifiable by their positions (right ascension and declination) on the celestial sphere. chapter xlvi the sun's motion toward lyra if hipparchus or galileo should return to earth to-night and look at the stars and constellations as we see them, there would be no change whatever discernible in either the brightness of the stars or in their relative positions. so the name fixed stars would appear to have been well chosen. halley in the seventeenth century was the first to detect that slow relative change of position of a few stars which is known as proper motion, and all the modern catalogues give the proper motions in both right ascension and declination. these are simply the small annual changes in position athwart the line of vision; and, as a whole, the proper motions of the brighter stars exceed the corresponding motions of the fainter ones because they are nearer to us. the average proper motion of the brightest stars is ". , and of stars of the sixth magnitude only one-sixth as great. a few extreme cases of proper motion have been detected, one as large as ", of an orange yellow star of the eighth magnitude in the southern constellation pictor, and barnard has recently discovered a star with a proper motion exceeding "; several determinations of its parallax give ". , corresponding to a distance of . light years. nevertheless, two centuries would elapse before these stars would be displaced as much as the breadth of the moon among their neighbors in the sky. the proper motions of stars are along perfectly straight lines, so far as yet observed. ultimately we may find a few moving in curved paths or orbits, but this is hardly likely. as for a central sun hypothesis, that pointing out alcyone in particular, there is no reliable evidence whatever. analysis of the proper motions of stars in considerable numbers, first by sir william herschel, showed that they were moving radially from the constellation hercules, and in great numbers also toward the opposite side of the stellar sphere. later investigation places this point, called the sun's goal, or apex of the sun's way, over in the adjacent constellation lyra; and the opposite point, or the sun's quit, is about halfway between sirius and canopus. by means of the radial velocities of stars in these antipodal regions of the sky, it is found that the sun's motion toward lyra, carrying all his planetary family along with him, is taking place at the rate of about miles in every second. while the right ascensions of the solar apex as given by the different investigations have been pretty uniform, the declination of this point has shown a rather wide variation not yet explained. for example, there is a difference of nearly ten degrees between the declination (+ °. ) of the apex as determined by boss from the proper motions of more than , stars, and the declination (+ °. ) found by campbell from the radial velocities of nearly , stars. several investigations tend to show that the fainter the stars are, the greater is the declination of the solar apex. more remarkable is the evidence that this declination varies with the spectral type of the stars, the later types, especially g and k, giving much more northerly values. on the whole the great amount of research that has been devoted to the solar motion relative to the system of the stars for the past hundred years may be said to indicate a point in right ascension h. ( °) and declination ° n. as the direction toward which the sun is moving. this is not very far from the bright star alpha lyræ, and the antipodal point from which the sun is traveling is quite near to beta columbæ. so swift is this motion (nearly twenty kilometers per second) that it has provided a base line of exceptional length, and very great service in determining the average distance of stars in groups or classes. after thousands of years the sun's own motion combined with the proper motions of the stars will displace many stars appreciably from their familiar places. the constellations as we know them will suffer slight distortions, particularly orion, cassiopeia and ursa major. identity or otherwise of spectra often indicates what stars are associated together in groups, and their community of motion is known as star drift. recent investigation of vast numbers of stars by both these methods have led to the epochal discovery of star streaming, which indicates that the stars of our system are drifting by, or rather through, each other, in two stately and interpenetrating streams. the grand primary cause underlying this motion is as yet only surmised. chapter xlvii stars and their spectral type when in dr. henry draper placed a very small wet plate in the camera of his spectroscope and, by careful following, on account of the necessarily long exposure, secured the first photographic spectrum of a star ever taken, he could hardly have anticipated the wealth of the new field of research which he was opening. his wife, anna palmer draper, was his enthusiastic assistant in both laboratory and observatory, and on his death in , she began to devote her resources very considerably to the amplification of stellar spectrum photography. at first with the cooperation of professor young of princeton, and later through extension of the facilities of harvard college observatory, whose director, the late professor edward c. pickering, devoted his energies in very large part to this matter, all the preliminaries of the great enterprise were worked out, and a comprehensive program was embarked upon, which culminated in the "henry draper memorial," a catalogue and classification of the spectra of all the stars brighter than the ninth magnitude, in both the northern and southern hemispheres. one very remarkable result from the investigation of large numbers of stars according to their type is the close correlation between a star's luminosity and its spectral type. but even more remarkable is the connection between spectral type and speed of motion. as early as monck of dublin, later kapteyn, and still later dyson, directed attention to the fact that stars of the secchi type ii had on the average larger proper motions than those of type i. in frost and adams brought out the exceptional character of the orion stars, the radial velocities of twenty of which averaged only seven kilometers per second. soon after, with the introduction of the two-stream hypothesis, a wider generalization was reached by campbell and kapteyn, whose radial velocities showed that the average linear velocity increases continually through the entire series b, a, f, g, k, m, from the earliest types of evolution to the latest. the younger stars of early type have velocities of perhaps five or six kilometers per second, while the older stars of later type have velocities nearly fourfold greater. the great question that occurs at once is: how do the individual stars get their motions? the farther back we go in a star's life history, the smaller we find its velocity to be. when a star reaches the orion stage of development, its velocity is only one-third of what it may be expected to have finally. apparently, then, the stars at birth have no motion, but gradually acquire it in passing through their several types or stages of development. more striking still is the motion of the planetary nebulæ, in excess of kilometers per second, while type a stars move kilometers, type g kilometers, and type m kilometers per second. can the law connecting speed of motion and spectral type be so general that the planetary nebula is to be regarded as the final evolutionary stage? stars have been seen to become nebulæ, and one astronomer at least is strongly of the opinion that a single such instance ought to outweigh all speculation to the contrary, as that stars originate from nebulæ. in his discussion of stellar proper motions, boss has reached a striking confirmation of the relation of speed to type, finding for the cross linear motion of the different types a series of velocities closely paralleling those of kapteyn and campbell. concerning the marked relation of the luminosities of the stars to their spectral types, there is a pronounced tendency toward equality of brightness among stars of a given type; also the brightness diminishes very markedly with advance in the stage of evolution. there has been much discussion as to the order of evolution as related to the type of spectrum, and russell of princeton has put forward the hypothesis of giant stars and dwarf stars, each spectral type having these two divisions, though not closely related. one class embraces intensely luminous stars, the other stars only feebly luminous. when a star is in process of contraction from a diffused gaseous mass, its temperature rises, according to lane's law, until that density is reached where the loss of heat by radiation exceeds the rise in temperature due to conversion of gravitational energy into heat. then the star begins to cool again. so that if the spectrum of a star depends mainly on the effective temperature of the body, clearly the classification of the draper catalogue would group stars together which are nearly alike in temperature, taking no note as to whether their present temperature is rising or falling. another classification of stars by lockyer divides them according to ascending and descending temperatures. russell's theory would assign the succession of evolutionary types in the order, m_{ }, k_{ }, g_{ }, f_{ }, a_{ }, b, a_{ }, f_{ }, g_{ }, k_{ }, m_{ }, the subscript referring to the "giants," and to the dwarf stars. in large part the weight of evidence would appear to favor the order of the harvard classification, independently confirmed as it is by studies of stellar velocities, galactic distribution, and periods of binary stars both spectroscopic and visual, where campbell and aiken find a marked increase in length of period with advance in spectral type. at the same time, a vast amount of evidence is accumulating in support of russell's theory. investigations in progress will doubtless reveal the ground on which both may be harmonized. the publication of the new henry draper catalogue of stellar spectra is in progress, a work of vast magnitude. the great catalogue of thirty years ago embraced the spectra of more than ten thousand stars, and was a huge work for that day; but the new catalogue utterly dwarfs it, with a classification much more detailed than in the earlier work, and with the number of stars increased more than twenty-fold. this work, projected by the late director of the harvard observatory, has been brought to a conclusion by the energy and enthusiasm of miss annie j. cannon through six years of close application, aided by many assistants. the catalogue ranges over the stars of both hemispheres, and is a monument to masterly organization and completed execution which will be of the highest importance and usefulness in all future researches on the bodies of the stellar universe. chapter xlviii star distances so vast are the distances of the stars that all attempts of the early astronomers to ascertain them necessarily proved futile. this led many astronomers after copernicus to reject his doctrine of the earth's motion round the sun, so that they clung rather to the ptolemaic view that the earth was without motion and was the center about which all the celestial motions took place. the geometry of stellar distances was perfectly understood, and many were the attempts made to find the parallaxes and distances of the stars; but the art of instrument making had not yet advanced to a stage where astronomers had the mechanisms that were absolutely necessary to measure very small angles. about , bessel undertook the work of determining stellar parallax in earnest. his instrument was the heliometer, originally designed for measuring the sun's diameter; but as modified for parallax work it is the most accurate of all angle-measuring instruments that the astronomers employ. the star that he selected was cygni, not a bright star, of the sixth magnitude only, but its large proper motion suggested that it might be one of those nearest to us. he measured with the heliometer, at opposite seasons of the year, the distance of cygni from another and very small star in the same field of view, and thus determined the relative parallax of the two stars. the assumption was made that the very faint star was very much more distant than the bright one, and this assumption will usually turn out to be sound. bessel got ". for his parallax of cygni, and struve by applying the same method to alpha lyræ, about the same time, got ". for the parallax of that star. these classic researches of bessel and struve are the most important in the history of star distances, because they were the first to prove that stellar parallax, although minute, could nevertheless be actually measured. about the same time success was achieved in another quarter, and henderson, the british astronomer at the cape of good hope, found a parallax of nearly a whole second for the bright star alpha centauri. although the parallaxes of many hundreds of stars have been measured since, and the parallaxes of other thousands of stars estimated, the measured parallax of alpha centauri, as later investigated by elkin and sir david gill, and found to be ". , is the largest known parallax, and therefore alpha centauri is our nearest neighbor among the stars, so far as we yet know. this star is a binary system and the light of the two components together is about the same as that of capella (alpha aurigæ). but it is never visible from this part of the world, being in degrees of south declination: one might just glimpse it near the southern horizon from key west. how the distances of the stars are found is not difficult to explain, although the method of doing it involves a good deal of complication, interesting to the practical astronomer only. recall the method of getting the moon's distance from the earth: it was done by measuring her displacement among the stars as seen from two widely separated observatories, as near the ends of a diameter of the earth as convenient. this is the base line, and the angle which a radius of the earth as seen from the center of the moon fills, or subtends, is the moon's parallax. so near is the moon that this angle is almost an entire degree, and therefore not at all difficult to measure. but if we go to the distance of even alpha centauri, the nearest of the stars, our earth shrinks to invisibility; so that we must seek a longer base line. fortunately there is one, but although its length is , times the earth's diameter, it is only just long enough to make the star distances measurable. we found that the sun's distance from the earth was million miles; the diameter of the earth's orbit is therefore double that amount. now conceive the diameter of the earth replaced by the diameter of the earth's orbit: by our motion round the sun we are transported from one extremity of this diameter to the opposite one in six month's time; so we may measure the displacement of a star from these two extremities, and half this displacement will be the star's parallax, often called the annual parallax because a year is consumed in traversing its period. and it is this very minute angle which bessel and struve were the first to measure with certainty, and which henderson found to be in the case of alpha centauri the largest yet known. evidently the earth by its motion round the sun makes every star describe, a little parallactic ellipse; the nearer the star is the larger this ellipse will be, and the farther the star the smaller: if the star were at an infinite distance, its ellipse would become a point, that is, if we imagine ourselves occupying the position of the star, even the vast orbit of the earth, million miles across, would shrink to invisibility or become a mathematical point. measurement of stellar parallax is one of many problems of exceeding difficulty that confront the practical astronomer. but the actual research nowadays is greatly simplified by photography, which enables the astronomer to select times when the air is not only clear, but very steady for making the exposures. development and measurement of the plates can then be done at any time. pritchard of oxford, england, was among the earliest to appreciate the advantages of photography in parallax work, and schlesinger, mitchell, miller, slocum and van maanen, with many others in this country, have zealously prosecuted it. how shall we intelligently express the vast distances at which the stars are removed from us? of course we can use miles, and pile up the millions upon millions by adding on ciphers, but that fails to give much notion of the star's distance. let us try with alpha centauri: its parallax of ". means that it is , times farther from the sun than the earth is. multiplying this out, we get trillion miles, that is, millions of million miles--an inconceivable number, and an unthinkable distance. suppose the entire solar system to shrink so that the orbit of neptune, sixty times million miles in diameter, would be a circle the size of the dot over this letter i. on the same scale the sun itself, although nearly a million miles in diameter, could not be seen with the most powerful microscope in existence; and on the same scale also we should have to have a circle ten feet in diameter, if the solar system were imagined at its center and alpha centauri in its circumference. so astronomers do not often use the mile as a yardstick of stellar distance, any more than we state the distance from london to san francisco in feet or inches. by convention of astronomers, the average distance between the centers of sun and earth, or million miles, is the accepted unit of measure in the solar system. so the adopted unit of stellar distance is the distance traveled by a wave of light in a year's time: and this unit is technically called the light-year. this unit of distance, or stellar yardstick, as we may call it, is nearly millions of million miles in length. alpha centauri, then, is four and one-third light-years distant, and cygni seven and one-fifth light-years away. for convenience in their calculations most astronomers now use a longer unit called the parsec, first suggested by turner. its length is equal to the distance of a star whose parallax is one second of arc; that is, one parsec is equal to about three and a quarter light-years. or the light-year is equal to . parsec. also the parsec is equal to , astronomical units, or about millions of million miles. we have, then four distinct methods of stating the distance of a star: sirius, for example, has a parallax of ". or its distance is two and two-thirds parsecs, or eight and a half light-years, or millions of million miles. it is the angle of parallax which is always found first by actual measurement and from this the three other estimates of distance are calculated. so difficult and delicate is the determination of a stellar distance that only a few hundred parallaxes have been ascertained in the past century. the distance of the same star has been many times measured by different astronomers, with much seeming duplication of effort. comprehensive campaigns for determining star parallaxes in large numbers have been undertaken in a few instances, particularly at the suggestion of kapteyn, the eminent astronomer of groningen, holland. his catalogue of star parallaxes is the most complete and accurate yet published, and is the standard in all statistical investigations of the stars. that we find relatively large parallaxes for some of the fainter stars, and almost no measurable parallax for some of the very bright stars is one of the riddles of the stellar universe. we may instance arcturus, in the northern hemisphere and canopus in the southern; the latter almost as bright as sirius. dr. elkin and the late sir david gill determined exhaustively the parallax of canopus, and found it very minute, only ". , making its distance in excess of a hundred light-years. the stupendous brilliancy of this star is apparent if we remember that the intensity of its light must vary inversely as the square of the distance; so that if canopus were to be brought as near us as even cygni is, it would be a hundredfold brighter than sirius, the brightest of all the stars of the firmament. in researches upon the distribution of the more distant stars, the method of measuring parallaxes of individual stars fails completely, and the secular parallax, or parallactic motion of the stars is employed instead. by parallactic motion is meant the apparent displacement in consequence of the solar motion which is now known with great accuracy, and amounts to . kilometers per second. even in a single year, then, the sun's motion is twice the diameter of the earth's orbit, so that in a hundred or more years, a much longer base line is available than in the usual type of observations for stellar parallax. if we ascertain the parallactic motion of a group of stars, then we can find their average distance. it is found, for example, that the mean parallax of stars of the sixth magnitude is ". . also the mean distances of stars thrown into classes according to their spectral type have been investigated by boss, kapteyn, campbell and others. the complete intermingling of the two great star streams has been proved, too, by using the magnitude of the proper motions to measure the average distances of both streams. these come out essentially the same, so that the streaming cannot be due to mere chance relation in the line of sight. most unexpected and highly important is the discovery that the peculiar behavior of certain lines in the spectrum leads to a fixed relation between a star's spectrum and its absolute magnitude, which provides a new and very effective method of ascertaining stellar distances. by absolute magnitudes are meant the magnitudes the stars would appear to have if they were all at the same standard distance from the earth. very satisfactory estimates of the distance of exceedingly remote objects have been made within recent years by this indirect method, which is especially applicable to spiral nebulæ and globular clusters. the absolute magnitude of a star is inferred from the relative intensities of certain lines in its spectrum, so that the observed apparent magnitude at once enables us to calculate the distance of the star. adams and joy have recently determined the luminosities and parallaxes of stars by this spectroscopic method. of these stars have had their parallaxes previously measured; and the average difference between the spectroscopic and the trigonometric values of the parallax is only the very small angle ". , a highly satisfactory verification. an indirect method, but a very simple one, and of the greatest value because it provides the key to stellar distances with the least possible calculation, and we can ascertain also the distances of whole classes of stars too remote to be ascertained in any other way at present known. the problem of spectroscopic determinations of luminosity and parallax has been investigated at mount wilson with great thoroughness from all sides, the separate investigations checking each other. a definitive scale for the spectroscopic determination of absolute magnitudes has now been established, and the parallaxes and absolute magnitudes have already been derived for about , stars. chapter xlix the nearest stars of especial interest are the few stars that we know are the nearest to us, and the following table includes all those whose parallax is ". or greater. there are nineteen in all and nearly half of them are binary systems. the radial motions given are relative to the sun. the transverse velocities are formed by using the measured parallaxes to transform proper motions into linear measures. they are given by eddington in his "stellar movements": column key ==================== a) magnitude b) parallax in seconds of arc c) proper motion in seconds of arc d) linear velocity km. per sec. e) radial velocity km. per sec. f) spectral type g) luminosity (sun= ) h) star stream ==================================================================== star's name | a | b | c | d | e | f | g | h ---------------+-----+------+------+-----+-----+------+-------+----- groombridge | . | . | . | | .. | ma | . | i eta cassiop | . | . | . | | + | f | . | i tau ceti | . | . | . | | - | k | . | ii epsilon erid | . | . | . | | + | k | . | ii cz h | . | . | . | |+ | g-k | . | ii sirius |- . | . | . | | - | a | . | ii procyon | . | . | . | | - | f | . | i ? lal. | . | . | . | | .. | ma | . | ii lal. | . | . | . | | .. | ma | . | i oa (n) | . | . | . | | .. | .. | . | i alpha centauri | . | . | . | | - | g,k |{ . | i | | | | | | |{ . | oa (n) | . | . | . | | .. | f | . | ii pos. med. | . | . | . | | .. | k | . | i sigma draco | . | . | . | | + | k | . | ii alpha aquilæ | . | . | . | | - | a | . | i cygni | . | . | . | | - | k | . | i epsilon indi | . | . | . | | - | k | . | i krüger | . | . | . | | .. | .. | . | ii lacaille | . | . | . | | + | ma | . | i -------------------------------------------------------------------- these stars are distant less than five parsecs (about light-years) from the sun, so they make up the closest fringe of the stellar universe immediately surrounding our system. the large number of binary systems is quite remarkable. why some stars are single and others double is not yet known. by the spectroscopic method the proportion is not so large; campbell finding that about one quarter of , stars examined are spectroscopic binaries, and frost two-fifths to a half. the exceptional number of large velocities is very remarkable; the average transverse motion of the nineteen stars is fifty kilometers per second, whereas thirty is about what would have been expected. as to star streams to which these nearest stars belong, eleven are in stream i and eight in stream ii, in close accord with the ratio : given by the , stars of boss's catalogue. "we are not able," says eddington, "to detect any significant difference between the luminosities, spectra, or speeds of the stars constituting the two streams. the thorough interpenetration of the two star streams is well illustrated, since we find even in this small volume of space that members of both streams are mingled together in just about the average proportion." [illustration: the ring nebula in _lyra_. this is the best example of the annular and elliptic nebulæ, which are not very abundant. (_photo, mt. wilson solar observatory._)] [illustration: the dumb-bell nebula of _vulpecula_. to take the photograph required an exposure of five hours. (_photo, mt. wilson solar observatory._)] chapter l actual dimensions of the stars we have seen that the distances of the stars from the solar system are immense beyond conception, and millions upon millions of them are probably forever beyond our power of ascertaining by direct measurement what their distance really is. after we had found the sun's distance and measured the angle filled by his disk, it was easy to calculate his actual size. this direct method, however, fails when we try to apply it to the stars, because their distances are so vast that no star's disk fills an angle of any appreciable size; and even if we try to get a disk with the highest magnifying powers of a great telescope our efforts end only in failure. there is, indeed, no instrumentally appreciable angle to measure. how then shall we ascertain the actual dimensions of the vast spheres which we know the stars actually are, as they exist in the remotest regions of space? clearly by indirect methods only, and it must be said that astronomers have as yet no general method that yields very satisfactory results for stellar dimensions. the actual magnitude of the variable system of algol, beta persei, is among the best known of all the stars, because the spectroscope measures the rate of approach and recession of algol when its invisible satellite is in opposite parts of the orbit; the law of gravitation gives the mass of the star and the size of its orbit, and so the length of the eclipse gives the actual size of the dark, eclipsing body. this figures out to be practically the same size as that of our sun, while algol's own diameter is rather larger, exceeding a million miles. if we try to estimate sizes of stars by their brightness merely, we are soon astray. differences of brightness are due to difference of dimensions, of course, or of light-giving area; but differences of distance also affect the brightness, inversely as the squares of the distances, while differences of temperature and constitution affect, in very marked degree, the intrinsic brilliance of the light-emitting surface of the star. there are big stars and little stars, stars relatively near to us and stars exceedingly remote, and stars highly incandescent as well as others feebly glowing. we have already shown how the angular diameters subtended by many of the stars have been estimated, through the relation of surface brightness and spectral type. antares and betelgeuse appear to be the most inviting for investigation, because their estimated angular diameters are about one-twentieth of a second of arc. this is the way in which their direct measurement is being attempted. as early as , michelson of chicago suggested the application of interference methods to the accurate measurement of very small angles, such as the diameters of the minor planets, and the satellites of jupiter and saturn, as well as the arc distance between the components of double stars. two portions of the object glass are used, as far apart as possible on the same diameter, and the interference fringes produced at the focus of the objective are then the subject of observation. these fringes form a series of equidistant interference bands, and are most distinct when the light comes from a source subtending an infinitesimal angle. if the object presents an appreciable angle, the visibility is less and may even become zero. michelson tested this method on the satellites of jupiter at the lick observatory in , and showed its accuracy and practicability. nevertheless, the method has not been taken up by astronomers, until very recently at the mount wilson observatory, where anderson has applied it to the measurement of close double stars. it is found that, contrary to general expectation, the method gives excellent results, even if the "seeing" is not the best-- on a scale of , for instance. to simplify the manipulation of the interferometer, a small plate with two apertures in it is placed in the converging beam of light coming from the telescope objective or mirror. the interference fringes formed in the focal plane are then viewed with an eyepiece of very high power, many thousand diameters. the resolving power of the interferometer is found to be somewhat more than double that of a telescope of the same aperture. by applying the interferometer method to capella, arc distances of much less than one-twentieth of a second of arc were measured. more recently the method has been applied to the great star betelgeuse in orion, whose angular diameter was found to be ". , corresponding to an actual diameter of , , miles, if the star's parallax is as small as it appears to be. chapter li the variable stars spectacular as they are to the layman, novæ, or temporary stars, are to the astronomers simply a class among many thousands of stars which they call variables, or variable stars. there are a few objects classified as irregular variables, one of which is very remarkable. we refer to eta argus, an erratic variable in the southern constellation argo and surrounded by a well-known nebula. there is a pretty complete record of this star. halley in when observing at saint helena recorded eta argus as of the fourth magnitude. during the th century, it fluctuated between the fourth magnitude and the second. early in the th it rapidly waxed in brightness, fluctuating between the first and second magnitudes from to . but two years later its light tripled, rivaling all the fixed stars except canopus and sirius. in it was even brighter for a few months, but since then it has declined fairly steadily, reaching a minimum at magnitude seven and a half in , with a slight increase in brightness more recently. a period of half a century has been suggested, but it is very doubtful if eta argus has any regular period of variation. another very interesting class of variables is known as the omicron ceti type. nearly all the time they are very faint, but quite suddenly they brighten through several magnitudes, and then fade away, more or less slowly, to their normal condition of faintness. but the extraordinary thing is that most of these variables go through their fluctuations in regular periods: from six months to two years in length. the type star, omicron ceti, or mira, is the oldest known variable, having been discovered by fabricius in . most of the time it is a relatively faint star of the th magnitude; but once in rather less than a year its brightness runs up to the fourth, third and sometimes even the second magnitude, where it remains for a week or ten days, and afterward it recedes more slowly to its usual faintness, the entire rise and decline in brightness usually requiring about days. the spectrum of omicron ceti contains many very bright lines, and a large proportion of the variable stars are of this type. another class of variables is designated as the beta lyræ type. their periods are quite regular, but there are two or more maxima and minima of light in each period, as if the variation were caused by superposed relations in some way. their spectra show a complexity of helium and hydrogen bands. no wholly satisfactory explanation has yet been offered. probably they are double stars revolving in very small orbits compared with their dimensions, their plane of motion passing nearly through the earth. but the most interesting of all the variables are those of the algol type, their light curves being just the reverse of the omicron ceti type; that is, they are at their maximum brightness most of the time, and then suffer a partial eclipse for a relatively brief interval. algol goes through its variations so frequently that its period is very accurately known; it is d. h. m. . s. for most of this period algol is an easy second magnitude star; then in about four and a half hours it loses nearly five-sixths of its light, receding to the fourth magnitude. here at minimum it remains for fifteen or twenty minutes, and then in the next three and a half hours it regains its full normal brilliancy of the second magnitude. during these fluctuations the star's spectrum undergoes no marked changes. the spectra of all the algol variables are of the first or sirian type. to explain the variation of the algol type of variables is easy: a dark, eclipsing body, somewhat smaller than the primary is supposed to be traveling round it in an orbit lying nearly edgewise to our line of sight. the gravitation of this dark companion displaces algol itself alternately toward and from the earth, because the two bodies revolve round their common center of gravity. with the spectroscope this alternate motion of algol, now advancing and now receding at the rate of miles per second, has been demonstrated; and the period of this motion synchronizes exactly with the period of the star's variability. russell and shapley have made extended studies of the eclipsing binaries, and developed the formulæ by which the investigations of their orbits are conducted. heretofore, visual binaries and spectroscopic binaries afforded the only means of deriving data regarding double systems, but it is now possible to obtain from the orbits of eclipsing variables fully as much information relating to binary systems in general and their bearing on stellar evolution. after an orbit has been determined from the photometric data of the light curve, the addition of spectroscopic data often permits the calculation of the masses, dimensions and densities in terms of the sun. shapley's original investigation included the orbits of ninety eclipsing variables, and with the aid of hypothetical parallaxes, he computed the approximate position of each system in space. the relation to the milky way is interesting, the condensation into the galactic plane being very marked; only thirteen of the ninety systems being found at galactic latitudes exceeding degrees. if we can suppose the variable stars covered with vast areas of spots, perhaps similar to the spots on the sun, and then combine the variation of these spot areas with rotation of the star on its axis, there is a possibility of explanation of many of the observed phenomena, especially where the range of variation is small. but for the omicron ceti type, no better explanation offers than that afforded by sir norman lockyer's collision theory. first he assumes that these stars are not condensed bodies, but still in the condition of meteoric swarms, and the revolution of lesser swarms around larger aggregations, in elliptic orbits of greater or less eccentricity, must produce vast multitudes of collisions; and these collisions, taking place at pretty regular periods, produce the variable maximum light by raising hosts of meteoric particles to a state of incandescence simultaneously. the catalogues of variable stars now contain many thousands of these objects. they are often designated by the letters r, s, t, and so on, followed by the genitive form of the name of the constellation wherein they are found. most of the recently found variables have a range of less than one magnitude. they are so distributed as to be most numerous in a zone inclined about degrees to the celestial equator, and split in two near where the cleft in the galaxy is located. nearly all the temporary stars are in this duplex region. bailey of harvard a quarter century ago began the investigation of variables in close star clusters, where they are very abundant, with marked changes of magnitude within only a few hours. many amateur astronomers afford very great assistance to the professional investigator of variable stars by their cooperation in observing these interesting bodies, in particular the american association of observers of variable stars, organized and directed by william tyler olcott. for a high degree of accuracy in determining stellar magnitudes the photo-electric cell is unsurpassed. stebbins of urbana has been very successful in its application and he discovered the secondary minimum of algol with the selenium cell. his most recent work was done with a potassium cell with walls of fused quartz, perfected after many trial attempts. the stars he has recently investigated are lambda tauri, and pi five orionis. combining results with those reached by the spectroscope, the masses of the two component stars of the former are . and . that of the sun, and the radii are . and . times the sun's. russell of princeton thinks it probable that similar causes are at work in all these variables. in the case of the typical novæ there is evidence that when the outburst takes place a shell of incandescent gas is actually ejected by the star at a very high velocity. what may be the forces that cause such an explosion can only be guessed. repeated outbursts have not, in the case of t pyxidis, destroyed the star, because it has gone through this process three times in the past thirty years. russell inclines to regard it as a standard process occurring somewhere in the stellar universe probably as often as once a year. novæ, then, cannot be due to collisions between two stars, for even if we suppose the stars to be a thousand millions in number, no two should collide except at average intervals of many million years. the idea is gaining ground that the stars are vast storehouses of energy which they are gradually transforming into heat and radiating into space. "under ordinary circumstances, it is probable that the rate of generation of heat is automatically regulated to balance the loss by radiation. but it is quite conceivable that some sudden disturbance in the substance of the star, near the surface, might cause an abrupt liberation of a great amount of energy, sufficient to heat the surface excessively, and drive the hot material off into infinite space, in much the form of a shell of gas, as seems to have been observed in the case of nova aquilæ.... with the rapid advance of our knowledge of the properties of the stars on one hand, and of the very nuclei of atoms on the other, we may, perhaps before many years have passed, find ourselves nearer a solution of the problem." the cepheid variables increase very rapidly in brightness from their least light to their maximum, and then fade out much more slowly, with certain irregularities or roughnesses of their light-curves when declining. their spectral lines also shift in period with their variations of light. in the case of these variables, whose regular fluctuation of light cannot be due to eclipse, and is as a rule embraced within a few days, there is a fluctuation in color also between maximum and minimum, as if there were a periodic change in the star's physical condition. eddington and shapley advocate the theory of a mechanical pulsation of the star as most plausible. knowledge of the internal conditions of the stars make it possible to predict the period of pulsation within narrow limits; and for delta cephei this theoretical period is between four and ten days. its observed period is five and one-third days, and corresponding agreement is found in all the cepheids so far tested. shapley of mount wilson finds that the cepheid variables with periods exceeding a day in length all lie close to the galactic lane. so greatly have the studies of these objects progressed that, as before remarked, when we know the star's period, we can get its absolute magnitude, and from this the star's distance. on all sides of the sun, the distances of the cepheids range up to , parsecs. so they indicate the existence of a galactic system far greater in extent than any previously dealt with. chapter lii the novÆ, or new stars new stars, or temporary stars, we have already mentioned in connection with variables. they are, next to comets, the most dramatic objects in the heavens. they may be variable stars which, in a brief period, increase enormously in brightness, and then slowly wane and disappear entirely, or remain of a very faint stellar magnitude. in the ancient historical records are found accounts of several such stars. for instance, in the chinese annals there is an allusion to such a stellar outburst in the constellation of scorpio, b. c. . this was observed also by hipparchus and, no doubt, it was the immediate incentive which led to his construction of the first known catalogue of stars, so that similar happenings might be detected in the future. in november, , tycho brahe observed the most famous of all new stars, which blazed out in the constellation cassiopeia. in something over a year it had completely disappeared. in - a new star of equal brightness was seen in ophiuchus by kepler; it also faded out to invisibility in . kepler and tycho printed very complete records of these remarkable objects. the eighteenth century passed without any new stars being seen or recorded. there was one of the fifth magnitude in , and another of the seventh magnitude in ; and in may, , a star of the second magnitude suddenly made its appearance in corona borealis; and one of the third magnitude in cygnus in november, . the latter was fully observed by schmidt of athens and became a faint telescopic star within a few weeks. it is now of the fifteenth magnitude. in astronomers were surprised to find suddenly a new star of the sixth magnitude very close to the brightest part of the great nebula in andromeda; it ran its course in about six months, fading with many fluctuations in brightness, and no star is now visible in its position even with the telescope. stars of this class are known to astronomers as novæ, usually with the genitive of the constellation name, as nova andromedæ. in - nova aurigæ made its spectacular appearance and yielded a distinctly double and complex spectrum for more than a month. many pairs of lines indicated a community of origin as to substance, and accurate measurement showed a large displacement with a relative velocity of more than miles per second. for each bright hydrogen line displaced toward the red there was a dark companion line or band about equally displaced toward the violet much as if the weird light of nova aurigæ originated in a solid globe moving swiftly away from us and plunging into an irregular nebulous mass as swiftly approaching us. parallax observations of nova aurigæ made it immensely remote, perhaps within the galaxy, and it still exists as a faint nebulous star. in february, , in the constellation perseus appeared the most brilliant nova of recent years. it was first discovered by dr. anderson, an amateur of glasgow, and at maximum on february it outshone capella. there were many unusual fluctuations in its waning brightness. its spectrum closely resembled that of nova aurigæ, with calcium, helium, and hydrogen lines. in august, , an enveloping nebula was discovered, and a month later certain wisps of this nebulosity appeared to have moved bodily, at a speed seventy-fold greater than ever previously observed in the stellar universe. according to sir norman lockyer's meteoritic hypothesis, a vast nebulous region was invaded, not by one but by many meteor swarms, under conditions such that the effects of collision varied greatly in intensity. the most violent of these collisions gave birth to nova persei itself, and the least violent occurred subsequently in other parts of the disturbed nebula, perhaps immeasurably removed. this explanation would avoid the necessity of supposing actual motion of matter through space at velocities heretofore unobserved and inconceivably high. a recent photograph of nova persei, by ritchey, reveals a nebulous ring of regular structure surrounding the star. the great power of the -inch has made it possible to photograph even the spectra of many of the novæ of years ago which are now very faint. after the lapse of years the characteristic lines of the nebular spectrum generally vanish, as if the star had passed out of the nebula--a plunge into which is generally thought to be the cause of the great and sudden outburst of light. many novæ have recently been found in the spiral nebulæ, especially in the great nebula of andromeda. chapter liii the double stars examining individual stars of the heavens more in detail, thousands of them are found to be double; not the stars that appear double to the naked eye, as theta tauri, mizar, epsilon lyræ, and others; but pairs of stars much closer together, and requiring the power of the telescope to divide or separate them. only a very few seconds apart they are or, in many cases, only the merest fraction of a second of arc. some of them, called binaries, are found to be revolving around a common center, sometimes in only a few years, sometimes in stately periods of hundreds of years. many such binary systems are now known, and the number is constantly increasing. castor is one, gamma virginis another, sirius also is one of these binaries, and a most interesting one, having a period of revolution of about years. aitken, of the lick observatory, in his work on binary stars, directs special attention to the correlation between the elements of known binary orbits and the star's spectral type, and presents a statistical study of the distribution of , visual double stars, of which the spectra of are known. that the masses of binary systems average about twice that of the sun's mass has long been known, and this fact can be employed with confidence in estimates of the probable parallax of these systems. aitken applies the test to fourteen visual systems for which the necessary data are available, and deduces for them a mean mass of . times that of the sun. for the spectroscopic binaries the masses are much greater. triple, quadruple and multiple stars are less frequent; but many exceedingly interesting objects of this class exist. epsilon lyræ is one, a double-double, or four stars as seen with slender telescopic power, and six or seven stars with larger instruments. sigma orionis and lyncis, also theta cancri and mu bootis are good examples of triple stars. chapter liv the star clusters from multiple stars the transition is natural to star clusters although the gap between these types of stellar objects is very broad. the familiar group of the winter sky known as the pleiades is a loose cluster, showing relatively very few stars even in telescopes or on photographic plates. the "beehive," or cluster known as praesepe in cancer, and a double group in the sword-handle of perseus, both just visible to the naked eye, are excellent examples of star clusters of the average type. when the moon is absent, they are easily recognized without a telescope as little patches of nebulous light; but every increase of optical power adds to their magnificence. then we come in regular succession to the truly marvelous globular clusters, that for instance in hercules. messier , a recent photograph of which, taken by ritchey with the -inch reflector on mount wilson, reveals an aggregation of more than , stars. but the finest specimens are in the southern hemisphere. sir john herschel spent much time investigating them nearly a century ago at the cape of good hope. his description of the cluster in the constellation of centaurus is as follows: "the noble globular cluster omega centauri is beyond all comparison the richest and largest object of the kind in the heavens. the stars are literally innumerable, and as their total light when received by the naked eye affects it hardly more than a star of the fifth or fourth to fifth magnitude, the minuteness of each star may be imagined." others of these clusters are so remote that the separate stars are not distinguishable, especially at the center, and their distances are entirely beyond our present powers of direct measurement, although methods of estimating them are in process of development. if gravitation is regnant among the uncounted components of stellar clusters, as doubtless it is, these stars must be in rapid motion, although our photographs of measurements have been made too recently for us to detect even the slightest motion in any of the component stars of a cluster. the only variations are changes of apparent magnitude, of a type first detected in a large number of stars in omega centauri, by bailey of harvard, who by comparison of photographs of the globular clusters was the first to find variable stars quite numerous in these objects. their unexplained variations of magnitude take place with great rapidity, often within a few hours. there are about a hundred of these globular clusters, and the radial velocities of ten of them have been measured by slipher and found to range from a recession of to an approach of kilometers per second. these excessive velocities are comparable with those found for the spiral nebulæ. shapley has estimated the distances of many of these bodies, which contain a large number of variable stars of the cepheid type. by assuming their absolute magnitudes equal to those of similar cepheids at known distances, he finds their distance represented by the inconceivably minute parallax of ". , corresponding to , light-years. this research also places the globular clusters far outside and independent of our galactic system of stars. the distribution of the globular clusters has also been investigated, and these interesting objects are found almost exclusively in but one hemisphere of the sky. its center lies in the rich star clouds of scorpio and sagittarius. success in finding the distances of these objects has made it possible to form a general idea of their distribution in three-dimensional space. the numerous variable stars in any one cluster are remarkable for their uniformity. accepting variables of this type as a constant standard of absolute brightness, and assuming that the differences of average magnitude of the variables in different clusters are entirely due to differences of distance, the relative distances of many clusters were ascertained with considerable accuracy. then it was found that the average absolute magnitude of the twenty-five brightest stars in a cluster is also a uniform standard, or about . magnitudes brighter than the mean magnitude of the variables. this new standard was employed in ascertaining the distances of other clusters not containing many variables. shapley further shows that the linear dimensions of the clusters are nearly uniform, and the proper relative positions in space are charted for sixty-nine of these objects. we can determine the scale of the charts, if we know the absolute brightness of our primary standard--the variable stars; and this is deduced from a knowledge of the distances of variables of the same type in our immediate stellar system. the most striking of all the globular clusters, omega centauri, comes out the nearest; nevertheless it is distant . kiloparsecs. a kiloparsec is a thousand parsecs, and is the equivalent of , light-years. at the inconceivable distance of sixty-seven kiloparsecs, or more than , light-years, is the most remote of the globular clusters, known to astronomers as n.g.c. , from its number in the catalogue which records its position in the sky, the new general catalogue of nebulæ by dreyer of armagh. the clusters are widely scattered, and their center of diffusion is about twenty kiloparsecs on the galactic plane toward the region of scorpio-sagittarius. marked symmetry with reference to this plane makes it evident that the entire system of globular clusters is associated with the galaxy itself. but to conceive of this it is necessary to extend our ideas of the actual dimensions of the galactic system. almost on the circumference of the great system of globular clusters our local stellar system is found, and it contains probably all the naked-eye stars, with millions of fainter ones. its size seems almost diminutive, only about one kiloparsec in diameter. the relative location of our local stellar system shows why the globular clusters appear to be crowded into one hemisphere only. shapley suggests that globular clusters can exist only in empty space, and that when they enter the regions of space tenanted by stars, they dissolve into the well-known loose clusters and the star clouds of the milky way. strangely the radial velocities of the clusters already observed show that most of them are traveling toward this region, and that some will enter the stellar regions within a period of the order of a hundred million years. the actual dimensions of globular clusters are not easy to determine, because the outer stars are much scattered. to a typical cluster, messier , shapley assigns a diameter of parsecs, which makes it comparable with the size of the stellar cluster to which the sun belongs. also on certain likely assumptions, he finds that the diameter of the great cluster in hercules, the finest one in our northern sky, is about parsecs, and its distance no less than , parsecs; in other words, the staggering distance that light would require , , years to travel over. while these distances can never be verified by direct measurement, it lends great weight to the three methods of indirect measurement, or estimation, ( ) from the diameter of the image of the clusters, ( ) from the mean magnitude of the twenty-five brightest stars, and ( ) from the mean magnitude of the short period variables, that they are in excellent agreement. chapter lv moving clusters recent researches on the proper motions of stars have brought to light many groups of stars whose individual members have equal and parallel velocities. eddington calls these moving clusters. the component stars are not exceptionally near to each other, and it often happens that other stars not belonging to the group are actually interspersed among them. they may be likened to double stars which are permanent neighbors, with some orbital motion, though exceedingly slow. the connection is rather one of origin; occurring in the same region of space, perhaps, from a single nebula. they set out with the same motion, and have "shared all the accidents of the journey together." their equality of motion is intact because any possible deflections by the gravitative pull of the stellar system is the same for both. mutual attraction may tend to keep the stars together, but their community of motion persists chiefly because no forces tend to interfere with it. in this way physically connected pairs may be separated by very great distances. so with the moving clusters: their component stars may be widely separate on the celestial sphere, but equality of their motions affords a clue to their association in groups. the hyades, a loose cluster in taurus, is a group of thirty-nine stars, within an area of about degrees square, which has been pretty fully investigated, especially by the late professor lewis boss; and no doubt many fainter stars in the same region will ultimately be found to belong to the same group. if we draw arrows on a chart representing the amount and direction of the proper motions of these stars, these arrows must all converge toward a point. this shows that their motions are parallel in space. it is a relatively compact group, and the close convergence shows that their individual velocities must agree within a small fraction of a kilometer per second. radial velocity measures of six of the component stars are in very satisfactory accord, giving . kilometers per second for the entire group. we can get the transverse velocity, and therefrom the distances of the stars, which are among the best known in the heavens, because the proper motions are very accurately known. the mean parallax of the group by this indirect method comes out ". , agreeing almost exactly with the direct determination by photography, ". , by kapteyn, de sitter, and others. eddington concludes that this taurus group is a globular cluster with a slight central condensation. its entire diameter is about ten parsecs, and its known motion enables us to trace its past and future history. it was nearest the sun , years ago, when it was at about half its present distance. boss calculated that in million years, if the present motion is maintained, this group will have receded so far as to appear like an ordinary globular cluster ' in diameter, its stars ranging from the ninth to the twelfth apparent magnitude. we may infer that the motion will likely continue undisturbed, because there are interspersed among the group many stars not belonging to it, and these have neither scattered its members nor sensibly interfered with the parallelism of their motion. another moving cluster, the similarity of proper motion of whose component stars was first pointed out by proctor, is known as the ursa major system, which embraces primarily beta, gamma, delta, epsilon, and zeta ursæ majoris, or five of the seven stars that mark the familiar dipper. but as many as eight other stars widely scattered are thought to belong to the same system, including sirius and alpha coronæ borealis. the absolute motion amounts to . kilometers per second, and is approximately parallel to the galaxy. turner has made a model of the cluster, which has the form of a flat disk. among stars of the orion type of spectrum are several examples of moving clusters. the pleiades together with many fainter stars form another moving cluster; as also do the brighter stars of orion, together with the faint cloudlike extensions of the great nebula in orion, whose radial velocity agrees with that of the stars in the constellation. still another very remarkable moving cluster is in perseus, first detected by eddington, and embracing eighteen stars, the brightest of which is alpha persei. the further discovery of moving clusters is most important in the future development of stellar astronomy, because with their aid we can find out the relative distribution, luminosity, and distance of very remote stars. so far the stars found associated in groups are of early types of spectrum; but the taurus cluster embraces several members equally advanced in evolution with the sun, and in the more scattered system of ursæ major there are three stars of type f. "some of these systems," eddington concludes, "would thus appear to have existed for a time comparable with the lifetime of an average star. they are wandering through a part of space in which are scattered stars not belonging to their system--interlopers penetrating right among the cluster stars. nevertheless, the equality of motion has not been seriously disturbed. it is scarcely possible to avoid the conclusion that the chance attractions of stars passing in the vicinity have no appreciable effect on stellar motions; and that if the motions change in course of time (as it appears they must do) this change is due, not to the passage of individual stars, but to the central attraction of the whole stellar universe, which is sensibly constant over the volume of space occupied by a moving cluster." chapter lvi the two star streams consider the ships on the atlantic voyaging between europe and america: at any one time there may be a hundred or more, all bound either east or west, some moving in interpenetrating groups, individuals frequently passing each other, but rarely or never colliding. we might say, there are two great streams of ships, one moving east and the other west. now in place of each ship, imagine a hundred ships, and magnify their distances from each other to the vast distances that the stars are from each other, and all in motion in two great streams as before. this will convey some idea of the relatively recent discovery, called by astronomers "star-streaming." early in this century the investigation of moving clusters began to reveal the fact that the motions of the stars were not at random throughout the universe, and about kapteyn was the first to show that the stellar motions considered in great groups are very far from being haphazard, but that the stars tend to travel in two great streams, or favored directions. this was ascertained by analyzing the proper motions of stars in the sky, many thousands of them, and correcting all for the effect which the known motion of the sun would have upon them. the corrected motion, or part that is left over, is known as the star's own motion, or _motus peculiaris_. this important investigation was very greatly facilitated by the general catalogue of , stars well distributed over the entire sky, the work of the late professor boss. it was published by the carnegie institution of washington, and includes all stars down to the sixth magnitude. boss was very critical in the matter of stellar positions and proper motions and his work is the most accurate at present available. excluding stars of the orion type and the known members of moving clusters, kapteyn's investigation was based on , stars, which he divided into seventeen regions of the sky, each northern region having an antipodal one in the southern hemisphere. mathematical analysis of these regions showed them all in substantial agreement, with one exception, and enabled kapteyn to draw the conclusion that the stars of one stream, called drift i, move with a speed of thirty-two kilometers per second, while those of the other, drift ii, travel with a speed of eighteen kilometers per second. their directions are not, like those of east and west bound ships, degrees from each other, but are inclined at an angle of degrees. drift i embraces about three-fifths of the stars, and drift ii the remaining two-fifths. quite as remarkable as the drifts themselves is the fact that the relative motion of the two is very closely parallel to the plane of the milky way. this epochal research has very great significance in all investigations of stellar motions, and it has been verified in various ways, particularly by the astronomer royal, sir frank dyson, who limited the stars under consideration to , in number, but all having very large proper motions. in this way the two streams are even more characteristically marked. but radial velocity determinations afford the ultimate and most satisfactory test, and campbell has this investigation in hand, classifying the stars in their streaming according to the type. type a stars are so far found to be confirmatory. turning to the question of physical differences between the stars of the two streams, eddington inquires into the average magnitude of the stars in both drifts, and their spectral type. also whether they are distributed at the same distance from the sun, and in the same proportion in all parts of the sky. his conclusion is that there is no important difference in the magnitudes of the stars constituting the two drifts. regarding their spectra, stars of early and late types are found in both streams, with a somewhat higher proportion of late types among the stars of drift ii than those of drift i. campbell and moore of the lick observatory have investigated seventy-three planetary nebulæ which exhibit the phenomena of star-streaming, and have motions which are characteristic of the stars. dealing with the very important question whether the two streams are actually intermingled in space, eddington finds them nearly at the same mean distance and thoroughly intermingled, and there is no possible hypothesis of drifts i and ii passing one behind the other in the same line of sight. a third drift, to which all the orion stars belong, is under investigation, together with comprehensive analysis of the drifts according to the spectral type of all the stars included. the farther research on star-streaming is pushed, the more it becomes evident that a third stream, called drift o, is necessary, especially to include b-type stars. the farther we recede from the sun, the more this drift is in evidence. at the average distances of b-type stars, the observed motions are almost completely represented by drift o alone. halm of cape town concludes from recent investigations that the double-drift phenomena (drifts i and ii) is of a distinctly _local_ character, and concerns chiefly the stars in the vicinity of the solar system; while stars at the greatest distances from the sun belong preeminently to drift o. the -inch reflector on mount wilson gathers sufficient light so that the spectra of very faint stars can be photographed, and a discussion of velocities derived in this manner has shown that kapteyn's two star streams extend into space much farther than it was possible to trace them with the nearer stars. star-streaming, then, may be a phenomenon of the widest significance in reference to the entire universe. as to the fundamental causes for the two opposite and nearly equal star streams, it is early perhaps to even theorize upon the subject. eddington, however, finds a possible explanation in the spiral nebulæ, which are so numerous as to indicate the certainty of an almost universal law compelling matter to flow in these forms. why it does so, we cannot be said to know; but obviously matter is either flowing into the nucleus from the branches of the spiral, or it is flowing out from the nucleus into the branches. which of the two directions does not matter, because in either case there would be currents of matter in opposite directions at the points where the arms merge in the central aggregation. the currents continue through the center, because the stars do not interfere with one another's paths. as eddington concludes: "there then we have an explanation of the prevalence of motions to and fro in a particular straight line; it is the line from which the spiral branches start out. the two star streams and the double-branched spirals arise from the same cause." chapter lvii the galaxy or milky way grandest of all the problems that have occupied the mind of man is the distribution of the stars throughout space. to the earliest astronomers who knew nothing about the distances of the stars, it was not much of a problem because they thought all the fixed stars were attached to a revolving sphere, and therefore all at essentially the same distance; a very moderate distance, too. even kepler held the idea that the distances of individual stars from each other are much less than their distances from our sun. thomas wright, of durham, england, seems to have been the first to suggest the modern theory of the structure of the stellar universe, about the middle of the eighteenth century. his idea was taken up by kant who elaborated it more fully. it is founded on the galaxy, the basal plane of stellar distribution, just as the ecliptic is the fundamental circle of reference in the solar system. what is the galaxy or milky way? here is a great poet's view of the most poetic object in all nature: a broad and ample road, whose dust is gold, and pavement stars, as stars to thee appear seen in the galaxy, that milky way which nightly as a circling zone thou seest powder'd with stars. _milton, p. l._ vii, . were the earth transparent as crystal, so that we could see downward through it and outward in all directions to the celestial sphere, the galaxy or milky way would appear as a belt or zone of cloudlike luminosity extending all the way round the heavens. as the horizon cuts the celestial sphere in two, we see at anyone time only one-half of the milky way, spanning the dome of the sky as a cloudlike arch. as the general plane of the galaxy makes a large angle with our equator, the milky way is continually changing its angle with the horizon, so that it rises at different elevations. one-half of the milky way will always be below our horizon, and a small region of it lies so near the south pole of the heavens that it can never be seen from medium northern latitudes. galileo was the first to explain the fundamental mystery of this belt, when he turned his telescope upon it and found that it was not a continuous sheet of faint light, as it seemed to be, but was made up of countless numbers of stars, individually too faint to be visible to the naked eye, but whose vast number, taken in the aggregate, gave the well-known effect which we see in the sky. in some regions, as perseus, the stars are more numerous than in others, and they are gathered in close clusters. the larger the telescope we employ, the greater the number of stars that are seen as we approach the galaxy on either side; and the farther we recede from the galaxy and approach either of its poles fewer and fewer stars are found. indeed, if all the stars visible in a -inch telescope could be conceived as blotted out, nearly all the stars that are left would be found in the galaxy itself. the naked eye readily notes the variations in breadth and brightness of the galactic zone. nearly a third of it, from scorpio to cygnus, is split into two divisions nearly parallel. in many regions its light is interrupted, especially in centaurus, where a dark starless region exists, known as the "coal sack." sir john herschel, who followed up the stellar researches of his father, sir william, in great detail, places the north pole of the galactic plane in declination degrees n., and right ascension h. m. this makes the plane of the milky way lie at an angle of about degrees with the ecliptic, which it intersects not far from the solstices. now kant, in view of the two great facts about the galaxy known in his time, ( ) that it wholly encircles the heavens, and ( ) that it is composed of countless stars too faint to be individually visible to the naked eye, drew the safe conclusions that the system of the stars must extend much farther in the direction of the milky way than in other directions. this theory of kant was next investigated from an observational standpoint by sir william herschel, the ultimate goal of whose researches was always a knowledge of the construction of the heavens. the present conclusion is that we may regard the stellar bodies of the sidereal universe as scattered, without much regard to uniformity, throughout a vast space having in general the shape of a thick watch, its thickness being perhaps one-tenth its diameter. on both sides of this disk of stars, and clustered about the poles of the sidereal system are the regions occupied by vast numbers of nebulæ. the entire visible universe, then, would be spheroidal in general shape. the plane of the milky way passes through the middle of this aggregation of stars and nebulæ, and the solar system is near the center of the milky way. throughout the watch-form space the stars are clustered irregularly, in varied and sometimes fantastic forms, but without approach to order or system. if we except some of the star groups and star clusters and consider only the naked-eye stars, we find them scattered with fair approach to uniformity. [illustration: star clouds and black holes in sagittarius. the dark rifts and lanes resemble those in the nearby milky way. (_photo, yerkes observatory._)] [illustration: the great nebula of andromeda, largest (apparently) of all the spiral nebulÆ. this nebula can be seen very faintly with the naked eye, but no telescope has yet resolved it into separate stars. (_photo, yerkes observatory._)] the watch-shaped disk is not to be understood as representing the actual form of the stellar system, but only in general the limits within which it is for the most part contained. a vigorous attack on the problem of the evolution and structure of the stellar universe as a whole is now being conducted by cooperation of many observatories in both hemispheres. it is known as the kapteyn "plan of selected areas," embracing regions which are distributed regularly over the entire sky. besides this a special plan includes forty-six additional regions, either very rich or extremely poor in stars, or to which other interest attaches. of all investigators kapteyn has gone into the question of our precise location in the milky way most thoroughly, concluding that the solar system lies, not at the center in the exact plane, but somewhat to the north of the galaxy. discussing the sirian stars he finds that if stars of equal brightness are compared, the sirians average nearly three times more distance from the sun than those of the solar type. so, probably, the sirians far exceed the solars in intrinsic brightness. farther, kapteyn concludes that the galaxy has no connection with our solar system, and is composed of a vast encircling annulus or ring of stars, far exceeding in number the stars of the great central solar cluster, and everywhere exceedingly remote from these stars, as well as differing from them in physical type and constitution. so it would be mainly the mere element of distance that makes them appear so faint and crowded thickly together into that gauzy girdle which we call the galaxy. the milky way reveals irregularities of stellar density and star clustering on a large scale, with deep rifts between great clouds of stars. modern photographs, particularly those of barnard in sagittarius, make this very apparent. within the milky way, nearly in its plane and almost central, is what eddington terms the inner stellar system, near the center of which is the sun. surrounding it and near its plane are the masses of star clouds which make up the milky way. whether these star clouds are isolated from the inner system or continuous with it, is not yet ascertained. the vast masses of the milky way stars are very faint, and we know nothing yet as to their proper motions, their radial motions, or their spectra. probably a few stars as bright as the sixth magnitude are actually located in the midst of the milky way clusters, the fainter ninth magnitude stars certainly begin the milky way proper, while the stars of the twelfth or thirteenth magnitude carry us into the very depths of the galaxy. it is now pretty generally believed that many of the dark regions of the milky way are due not to actual absence of stars so much as to the absorption of light by intervening tracts of nebulous matter on the hither side of the galactic aggregations and, probably in fact, within the oblate inner stellar system itself. easton has made many hundred counts of stars in galactic regions of cygnus and aquila where the range of intensity of the light is very marked; in fact, the star density of the bright patches of the galaxy is so far in excess of the density adjacent and just outside the milky way, that the conclusion is inevitable that this excess is due to the star clouds. of the distance of the milky way we have very little knowledge. it is certainly not less than , parsecs, and more likely , parsecs, a distance over which light would travel in about , years. quite certainly all parts of the galaxy are not at the same distance, and probably there are branches in some regions that lie behind one another. while the general regions of the nebulæ are remote from the galactic plane, the large irregular nebulæ, as the trifid, the keyhole, and the omega nebulæ, are found chiefly in the milky way. in addition to the irregular nebulæ many types of stellar objects appear to be strongly condensed toward the milky way, but this may be due to the inner stellar system, rather than a real relation to galactic formations. quite different are the magellanic clouds, which contain many gaseous nebulæ and are unique objects of the sky, having no resemblance to the true spiral nebulæ which, as a rule, avoid the galactic regions. worthy of note also is the theory of easton that the milky way has itself the form of a double-branched spiral, which explains the visible features quite well, but is incapable of either disproof or verification. the central nucleus he locates in the rich galactic region of cygnus, with the sun well outside the nucleus itself. by combining the available photographs of the galaxy, he has produced a chart which indicates in a general way how the stellar aggregations might all be arrayed so as to give the effect of the galaxy as we see it. shapley, at mount wilson, has studied the structure of the galactic system, in which he has been aided by mrs. shapley. an interesting part of this work relates to the distribution of the spiral nebulæ, and to certain properties of their systematic recessional motion, suggesting that the entire galactic system may be rapidly moving through space. apparently the spiral nebulæ are not distant stellar organizations or "island universes," but truly nebular structures of vast volume which in general are actively repelled from stellar systems. a tentative cosmogonic hypothesis has been formulated to account for the motions, distribution, and observed structure of clusters and spiral nebulæ. an additional great problem of the galaxy is a purely dynamical one. doubtless it is in some sort of equilibrium, according to eddington, that is to say, the individual stars do not oscillate to and fro across the stellar system in a period of million years, but remain concentrated in clusters as at present. poincaré has considered the entire milky way as in stately rotation, and on the assumption that the total mass of the inner stellar system is , , , times the sun's mass, and that the distance of the milky way is , parsecs, the angular velocity for equilibrium comes out ". per century. that is to say, a complete revolution would take place in about million years. chapter lviii star clouds and nebulÆ from star clusters to nebulæ, only a century ago, the transition was thought to be easy and immediate. accuracy in determining the distances of stars was just beginning to be reached, the clusters were obviously of all degrees of closeness following to the verge of irresolvability, and it was but natural to jump to the conclusion that the mystery of the nebulæ consisted in nothing but their vaster distance than that of clusters, and it was believed that all nebulæ would prove resolvable into stars whenever telescopes of sufficiently great power could be constructed. but the development of the spectroscope soon showed the error of this hypothesis, by revealing bright lines in the nebular spectra showing that many nebulæ emit light that comes from glowing incandescent gas, not from an infinitude of small stars. in pre-telescope days nothing was known about the nebulæ. the great nebula in andromeda, and possibly the great nebula in orion, are alone visible to the naked eye, but as thus seen they are the merest wisps of light, the same as the larger clusters are. galileo, huygens and other early users of the telescope made observations of nebulæ, but long-focus telescopes were not well adapted to this work. simon mayer has left us the first drawing of a nebula, the orion nebula as he saw it in . the vast light-gathering power of the reflectors built by sir william herschel first afforded glimpses of the structure of the nebulæ, and if his drawings are critically compared with modern ones, no case of motion with reference to the stars or of change in the filaments of the nebulæ themselves has been satisfactorily made out. only very recently has the distance of a nebula been determined, and the few that have been measured seem to indicate that the nebulæ are at distances comparable with the stars. of all celestial objects the nebulæ fill the greatest angles, so that we are forced to conclude, with regard to the actual size of the greater nebulæ as they exist in space, that they far surpass all other objects in bulk. photography invaded the realm of the nebulæ in , when dr. henry draper secured the first photograph of the nebula of orion. theoretically photography ought to help greatly in the study of the nebulæ, and enable us in the lapse of centuries to ascertain the exact nature of the changes which must be going on. the differences of photographic processes, of plates, of exposure and development produce in the finished photograph vastly greater differences than any actual changes that might be going on, so that we must rely rather on optical drawings made with the telescope, or on drawings made by expert artists from photographs with many lengths of exposure on the same object. the great work on nebulæ and star clusters recently concluded by bigourdan of the paris observatory and published in five volumes received the award of the gold medal of the royal astronomical society. while d'arrest measured about , nebulæ, and sir john herschel about double that number in both hemispheres, bigourdan has measured about , . his work forms an invaluable lexicon of information concerning the nebulæ. classification of the nebulæ is not very satisfactory, if made by their shapes alone. there are perhaps fifteen thousand nebulæ in all that have been catalogued, described, and photographed. dreyer's new general catalogue (n.g.c.) is the best and most useful. many of the nebulæ, especially the large ones, can only be classified as irregular nebulæ. the orion nebula is the principal one of this class, revealing an enormous amount of complicated detail, with exceptional brilliancy of many regions and filaments. an extraordinary multiple star, theta orionis, occupies a very prominent position in the nebula, and photographs by pickering have brought to light curved filaments, very faint and optically invisible, in the outlying regions which give the orion nebula in part a spiral character. but the delicate optical wisps of this nebula are well seen, even in very small telescopes. its spectrum yields hydrogen, helium and nitrogen. the orion nebula is receding from the earth about eleven miles in every second. keeler and campbell have shown that nearly every line of the nebular spectrum is a counterpart of a prominent dark line in the spectrum of the brighter stars of the constellation of orion. a recent investigator of the distribution of luminosity in the great nebula of orion finds that radiations from nebulium are confined chiefly to the huygenian region of the nebula and its immediate neighborhood. photography has revealed another extraordinary nebula or group of nebulæ surrounding the stars in the pleiades, which the deft manipulation of barnard has brought to light. all the stars and the nebula are so interrelated that they are obviously bound together physically, as the common proper motion of the stars also appears to show. also in the constellation cygnus, barnard has discovered very extensive nebulosities of a delicate filmy cloudlike nature which are wholly invisible with telescopes, but very obvious on highly sensitive plates with long exposures. another class of these objects are the annular and elliptic nebulæ which are not very abundant. the southern constellation grus, the crane, contains a fine one, but by far the best example is in the constellation lyra. it is a nearly perfect ring, elliptic in figure, exceedingly faint in small telescopes; but large instruments reveal many stars within the annulus, one near the center which, although very faint to the eye, is always an easy object on the photographic plate, because it is rich in blue and violet rays. the parallax of the ring nebula in lyra comes out only one-sixth of that of the planetary nebulæ, and the least greatest diameters of this huge continuous ring are and times the orbit of neptune. planetary nebulæ and nebulous stars are yet another class of nebulæ, for the most part faint and small, resembling in some measure a planetary disk or a star with nebulous outline. practically all are gaseous in composition, and have large radial velocities. probably they are located within our own stellar system. the parallaxes of several of them have been measured by van maanen: one of the very small angle ". , which enables us to calculate the diameter of this faint but interesting object as equal to nineteen times the orbit of neptune. chapter lix the spiral nebulÆ last and most important of all are the spiral nebulæ. the finest example is in the constellation canes venatici, and its spiral configuration was first noted by lord rosse, an epoch-making discovery. the convolutions of its spiral are filled with numerous starlike condensations, themselves engulfed in nebulosity. photography possesses a vast advantage over the eye in revealing the marvelous character of this object, an inconceivably vast celestial whirlpool. naturally the central regions of the whorl would revolve most swiftly, but no comparison of drawings and photographs, separated by intervals of many years, has yet revealed even a trace of any such motion. the number of large spiral nebulæ is not very great; the largest of all is the great nebula of andromeda, whose length stretches over an arc of seven times the breadth of the moon, and its width about half as great. this nebula is a naked-eye object near eta andromedæ, and it is often mistaken for a comet. optically it was always a puzzle, but photographs by roberts of england first revealed the true spiral, with ringlike formations partially distinct, and knots of condensing nebulosity as of companion stars in the making. while its spectrum shows the nongaseous constitution of this nebula, no telescope has yet resolved it into component stars. systematic search for spiral nebulæ by keeler, and later continued by perrine, at the lick observatory, with the -inch crossley reflector, disclosed the existence of vast numbers of these objects, in fact many hundreds of thousands by estimation; so that, next to the stars, the spiral nebulæ are by far the most abundant of all objects in the sky. they present every phase according to the angle of their plane with the line of sight, and the convolutions of the open ones are very perfectly marked. many are filled with stars in all degrees of condensation, and the appearance is strongly as if stars are here caught in every step of the process of making. the vast multitude of the spiral nebulæ indicates clearly their importance in the theory of the cosmogony, or science of the development of the material universe. curtis of the lick observatory has lately extended the estimated number of these objects to , . he has also photographed with the crossley reflector many nebulæ with lanes or dark streaks crossing them longitudinally through or near the center. these remarkable streaks appear as if due to opaque matter between us and the luminous matter of the nebula beyond. perhaps a dark ring of absorptive or occulting matter encircles the nebula in nearly the same plane with the luminous whorls. duncan has employed the -inch mount wilson reflector in photographing bright nebulæ and star clusters in the very interesting regions of sagittarius. one of these shows unmistakable dark rifts or lanes in all parts of the nebula, resembling the dark regions of the neighboring milky way. pease of mount wilson has recently employed the -inch and the -inch reflectors of the mount wilson observatory to good advantage in photographing several hundred of the fainter nebulæ. many of these are spirals, and others present very intricate and irregular forms. a search was made for additional spirals among the smaller nebulæ along the galaxy, but without success. several of the supposedly variable nebulæ are found to be unchanging. many nights in each month when the moon is absent are devoted to a systematic survey of the smaller nebulæ and their spectra by photography. the visible spiral figure of all these objects is a double-branched curve, its two arms joining on the nucleus in opposing points, and coiling round in the same geometrical direction. the spiral nebulæ, as to their distribution, are remote from the galaxy, and the north galactic polar region contains a greater aggregation than the south. the distances of the spiral nebulæ are exceedingly great. they lie far beyond the planetary and irregular gaseous nebulæ, like that of orion, which are closely related to the stars forming part of our own system. possibly the spiral nebulæ are exterior or separate "island universes." if so, they must be inconceivably vast in size, and would develop, not into solar systems, but into stellar clusters. the enormous radial velocities of the spiral nebulæ, averaging to kilometers per second, or twenty-fold that of the stars, tend to sustain the view that they may be "island universes," each comparable in extent with the universe of stars to which our sun belongs. recent spectroscopic observations of the nebulæ applying the principle of doppler have revealed high velocities of rotation. slipher of the lowell observatory made the first discovery of this sort and van maanen of mount wilson has detected in the great ursa major spiral, no. in messier's catalogue, a speed of rotation at five minutes of arc from the center that would correspond to a complete period in , years. as was to be expected, the nebula does not rotate as a rigid body, but the nearer the center the greater the angular velocity, and van maanen finds evidence of motion along the arms and away from the center. these great velocities appear to belong to the spiral nebulæ as a class, and not to other nebulæ. thirteen nebulæ investigated by keeler are as a whole almost at rest relatively to our system, as are the large irregular objects in orion, and the trifid nebula. this would seem to indicate that the spiral nebulæ form systems outside our own and independent of it. quite different from the spirals in their distribution through space are the planetary nebulæ. the spirals follow the early general law of nebulæ arrangement, that is, they are concentrated toward the poles of the galaxy; but the planetary nebulæ, on the other hand, are very few near the poles and show a marked frequency toward the galactic plane. campbell and moore have found spectroscopic evidence of internal rotatory motion in a large proportion of the planetary nebulæ. the distribution of the nebulæ throughout space, like that of the stars, is still under critical investigation, but the location of vast numbers of the more compact nebulæ on the celestial sphere is very extraordinary. the milky way appears to be the determining plane in both cases; the nearer we approach it the more numerous the stars become, whereas this is the general region of fewest nebulæ and they increase in number outward in both directions from the galaxy, and toward both poles of the galactic circle. obviously this relation, or contra-relation of stars and nebulæ on such a vast scale is not accidental, and it also must be duly accounted for in the true theory of the cosmogony. the nebulæ which are found principally in and near the milky way are the large irregular nebulæ, and vast nebulous backgrounds, like those photographed by barnard in scorpio, taurus and elsewhere, as well as the keyhole, omega, and trifid nebulæ. allied to these backgrounds are doubtless some of the dark galactic spaces, radiating little or no intrinsic light, and absorbing the light of the fainter stars beyond them. a peculiar veiled or tinted appearance has been remarked in some cases visually, and examination of the photographs strongly confirms the existence of absorbing nebulosity. the spiral nebulæ are so abundant, and so much attention is now being given to them, both by observers and mathematicians, that their precise relation to the stellar systems must soon be known; that is, whether they are comparatively small objects belonging to the stellar system, or independent systems on the borders of the stellar system, or as seems more likely, vast and exceedingly remote galaxies comparable with that of the milky way itself. our knowledge of the motions of the spirals, both radial and angular, is increasing rapidly, and must soon permit accurate general conclusions to be drawn. chapter lx cosmogony down to the middle of the last century and later, it was commonly believed that in the beginning the cosmos came into being by divine fiat substantially as it is. previously the earth had been "without form and void," as in the scripture. had it not been for the growth and gradual acceptance of the doctrine of evolution, and its reactionary effect upon human thought, it is conceivable that the early view might have persisted to the present day; but now it is universally held that everything in the heavens above and the earth beneath is subject more or less to secular change, and is the result of an orderly development throughout indefinite past ages, a progressive evolution which will continue through indefinite aeons of the future. in the writings of the greek philosophers, and down through the middle ages we find the idea of an original "chaos" prevailing, with no indication whatever of the modern view of the process by which the cosmos came to be what they saw it and as it is to-day. if we go still farther back, there is no glimmer of any ideas that will bear investigation by scientific method, however interesting they may be as purely philosophical conceptions. many ancient philosophers, among them anaxagoras, democritus, and anaximenes, regarded the earth as the product of diffused matter in a state of the original chaos having fallen together haphazard, and they even presumed to predict its future career and ultimate destiny. in anaximander and anaximenes alone do we find any conception of possible progress; their thought was that as the world had taken time to become what it is, so in time it would pass, and as the entire universe had undergone alternate renewal and destruction in the past, that would be its history in the future. aristotle, ptolemy, and others appear to have held the curious notion that although everything terrestrial is evanescent, nevertheless the cosmos beyond the orbit of the moon is imperishable and eternal. by tracing the history of the intellectual development of europe we may find why it was that scientific speculation on the cosmogony was delayed until the th century, and then undertaken quite independently by three philosophers in three different countries. swedenborg, the theologian, set down in due form many of the principles that underlie the modern nebular hypothesis. thomas wright of durham whose early theory of the arrangement of stars in the galaxy we have already mentioned, speculated also on the origin and development of the universe, and his writings were known to kant, who is now regarded as the author of the modern nebular hypothesis. this presents a definite mechanical explanation of the development and formation of the heavenly bodies, and in particular those composing the solar system. kant was illustrious as a metaphysician, but he was a great physicist or natural philosopher as well, and he set down his ideas regarding the cosmogony with precision. learned in the philosophy of the ancients, he did not follow their speculative conceptions, but merely assumed that all the materials from which the bodies of the solar system have been fashioned were resolved into their original elements at the beginning, and filled all that part of space in which they now move. true, this is pretty near the chaos of the greeks, but kant knew of the operation of the newtonian law of gravitation, which the greeks did not. as a natural result of gravitative processes, kant inferred that the denser portions of the original mass would draw upon themselves the less dense portions, whirling motions would be everywhere set up, and the process would continue until many spherical bodies, each with a gaseous exterior in process of condensation, had taken the place of the original elements which filled space. in this manner kant would explain the sameness in direction of motion, both orbital and axial, of all the planets and satellites of our system. but many philosophers are of the opinion that kant's hypothesis would result, not in the formation of such a collection of bodies as the solar system is, but rather in a single central sun formed by common gravitation toward a single center. from quite another viewpoint the work of the elder herschel is important here. no one knew the nebulæ from actual observation better than he did; but, while his ideas about their composition were wrong, he nevertheless conceived of them as gradually condensing into stars or clusters of stars. and it was this speculative aspect of the nebulæ, not as a possible means of accounting for the birth and development of the solar system, which constitutes herschel's chief contribution to the nebular hypothesis. classifying the nebulæ which he had carefully studied with his great telescopes, it seemed obvious to him that they were actually in all the different stages of condensation, and subsequent research has strongly tended to substantiate the herschelian view. then came laplace, who took up the great hypothesis where kant and herschel had left it, added new and important conceptions in the light of his mature labors as mathematician and astronomer, and put the theory in definitive form, such that it has ever since been known under the name of laplacian nebular hypothesis. for reasons like those that prevailed with kant, he began the evolution of the solar system with the sun already formed as the center, but surrounded by a vast incandescent atmosphere that filled all the space which the sun's family of planets now occupy. this entire mass, sun, atmosphere, and all, he conceived to have a stately rotation about its axis. with rotation of the mass and slow reduction of temperature in its outer regions, there would be contraction toward the solar center, and an increase in velocity of rotation until the whole mass had been much reduced in diameter at its poles and proportionately expanded at its equator. when the centrifugal force of the outer equatorial masses finally became equal to the gravitational forces of the central mass, then these conjoined outer portions would be left behind as a ring, still revolving at the velocity it had acquired when detached. the revolution of the entire inner mass goes on, its velocity accelerating until a similar equilibration of forces is again reached, when a second rotating ring is left behind. laplace conceived the process as repeated until as many rings had been detached as there are individual planets, all central about the sun, or nearly so. in all, then, we should have nine gaseous rings; the outer ones preceding the inner in formation, but not all existing as rings at the same time. radiation from the ring on all sides would lead to rapid contraction of its mass, so that many nuclei of condensation would form, of various sizes, all revolving round the central sun in practically the same period. laplace conceived the evolution of the ring to proceed still farther till the largest aggregation in it had drawn to itself all the other separate nuclei in the ring. this, then, was the planet in embryo, in effect a diminutive sun, a secondary incandescent mass endowed with axial rotation in the same direction as the parent nebula. with reduction of temperature by radiation, polar contraction and equatorial expansion go on, and planetary rings are detached from this secondary mass in exactly the same way as from the original sun nebula. and these planetary rings are, in the laplacian hypothesis, the embryo moons or planetary satellites, all revolving round their several planets in the same direction that the planets revolve about the sun. in the case of one of the planetary rings, its formation was so nearly homogeneous throughout that no aggregation into a single satellite was possible; all portions of the ring being of equal density, there was no denser region to attract the less dense regions, and in this manner the rings of saturn were formed, in lieu of condensation into a separate satellite. similarly in the case of the primal solar ring that was detached next after the jovian ring; there was such a nice balancing of masses and densities that, instead of a single major planet, we have the well-known asteroidal ring, composed of innumerable discrete minor planets. this, then, in bare outline, is the laplacian nebular hypothesis, and it accounted very well for the solar system as known in his day; the fairly regular progression of planetary distances; their orbits round the sun all nearly circular and approximately in a single plane; the planetary and satellite revolutions in orbit all in the same direction; the axial rotations of planets in the same direction as their orbital revolutions; and the plane of orbital revolution of the satellites practically coinciding with the plane of the planet's axial rotation. but the principle of conservation of energy was, of course, unknown to laplace, nor had the mechanical equivalence of heat with other forms of energy been established in his day. in , lane of washington first demonstrated the remarkable law that a gaseous sphere, in process of losing heat by radiation and contraction because of its own gravity, actually grows hotter instead of cooler, as long as it continues to be gaseous, and not liquid or solid. so there is no need of postulating with laplace an excessively high temperature of the original nebula. the chief objection to laplace's hypothesis by modern theorists is that the detachment of rings, though possible, would likely be a rare occurrence; protuberances or lumps on the equatorial exterior of a swiftly revolving mass would be more likely, and it is much easier to see how such masses would ultimately become planets than it is to follow the disruption of a possible ring and the necessary steps of the process by which it would condense into a final planet. the continued progress of research in many departments of astronomy has had important bearing on the nebular hypothesis, and we may rest assured that this hypothesis in somewhat modified form can hardly fail of ultimate acceptance, though not in every essential as its great originator left it. lord rosse's discovery of spiral nebulæ, followed up by keeler's photographic search for these bodies, revealing their actual existence in the heavens by the hundreds of thousands, has led to another criticism of the laplacian theory. could laplace have known of the existence of these objects in such vast numbers, his hypothesis would no doubt have been suitably modified to account for their formation and development. it is generally considered that the ring of saturn suggested to laplace the ring feature in his scheme of origin of planets and satellites; so far as we know, the saturnian ring is unique, the only object of its kind in the heavens. whereas, next to the star itself, the spiral nebula is the type object which occurs most frequently. a theory, therefore, which will satisfactorily account for the origin and development of spiral nebulæ must command recognition as of great importance in the cosmogony. such a theory has been set forth by chamberlin and moulton in their planetesimal hypothesis, according to which the genesis of spiral nebulæ happens when two giant suns approach each other so closely that tide-producing effects take place on a vast scale. these suns need not be luminous; they may perhaps belong to the class of dark or extinguished suns. the evidences of the existence of such in vast numbers throughout the universe is thought to be well established. now, on close approach, what happens? there will be huge tides, and the nearer the bodies come to each other, the vaster the scale on which tides will be formed. if the bodies are liquid or gaseous, they will be distorted by the force of gravitation, and the figure of both bodies will become ellipsoidal; and at last under greater stress, the restraining shell of both bodies will burst asunder on opposite sides in streams of matter from the interior. in this manner the arms of the spiral are formed. as chamberlin puts it: "if, with these potent forces thus nearly balanced, the sun closely approaches another sun, or body of like magnitude ... the gravity which restrains this enormous elastic power will be reduced along the line of mutual attraction. at the same time the pressure transverse to this line of relief will be increased. such localized relief and intensified pressure must bring into action corresponding portions of the sun's elastic potency, resulting in protuberances of corresponding mass and high velocity." only a fraction of one per cent of the sun's mass ejected in this fashion would be sufficient to generate the entire planetary system. nuclei or knots in the arms of the spiral gradually grew by accretion, the four interior knots forming mercury, venus, the earth, and mars. the earth knot was a double one, which developed into the earth-moon system. the absence of a dominating nucleus beyond mars accounts for the zone of the asteroids remaining in some sense in the original planetesimal condition. the vaster nuclei beyond mars gradually condensed into jupiter, saturn, uranus, and neptune; and lesser nuclei related to the larger ones form the systems of moons or satellites. the orbits of the planetesimals and the planetary and satellite nuclei would be very eccentric, forming a confusion of ellipses with frequently crossing paths. collisions would occur, and the nuclei would inevitably grow by accretion. each planet, then, would clear up the planetesimals of its zone; and moulton shows that this process would give rise to axial revolution of the planet in the same direction as its orbital revolution. the eccentricities would finally disappear, and the entire mass would revolve in a nearly circular orbit. rotation twists the streams into the spiral form, and the huge amounts of wreckage from the near-collision are thrown into eddies. the fragments or particles (planetesimals) which have given the name to the theory, begin their motion round their central sun in elliptical paths as required by gravitation. the form of the spiral is preserved by the orbital motion of its particles. there is a gradual gathering together of the planetesimals at points or nodes of intersection, and these become aggregations of matter, nuclei that will perhaps become planets, though more likely other stars. the appulse or near approach is but one of the methods by which the spiral nebulæ may have come into existence. the planetesimal hypothesis would seem to account for the formation of many of these objects as we see them in the sky, though perhaps it is hardly competent to replace entirely the laplacian hypothesis of the formation of the solar system, which would appear to be a special case by itself. it will be observed that while the laplacian hypothesis is concerned in the main with the progressive development of the solar system, and systems of a like order surrounding other stellar centers, whose existence is highly probable, the origin and development of the stellar universe is a vaster problem which can only be undertaken and completed in its broadest bearings when the structure of the stellar universe has been ascertained. darwin's important investigations in - on tidal friction may be here related. before his day acceptance of the ring-theory of development of the moon from the earth had scarcely been questioned; but his recondite mathematical researches on the tidal reaction between a central yielding mass and a body revolving round it brought to light the unsuspected effect of tides raised upon both bodies by their mutual attraction. the type of tides here meant is not the usual rise and fall of the waters of the ocean, but primeval tides in the plastic material of which the earth in its early history was composed. the newtonian law of gravitation afforded a complete explanation of the rise and fall of the waters of the oceans, but as applied to the motions of planets and satellites by the lagrangian formulæ, it presupposed that all these bodies are rigid and unyielding. however, mutual tides of phenomenal height in their early plastic substances must have been a necessary consequence of the action of the newtonian law, and they gradually drew upon the earth's rotational moment of momentum. in its very early history, before there was any moon to produce tides, the earth rotated much more rapidly, that is, the day was very much shorter than now, probably about five or six hours long. and with the rapid whirling, it was not a laplacian ring that was detached, but a huge globular mass was separated from the plastic earth's equator. darwin shows that the gravitative interaction of the two bodies immediately began to raise tides of extraordinary height in both, therefore tending to slow down the rotational periods of both bodies. action and reaction being equal, the reaction at once began driving the moon away from the earth and thereby lengthening its period of revolution. so small was the mass of the moon and so near was it to the earth, that its relative rotational energy was in time completely used up, and the moon has ever since turned her constant face toward us. tides of sun and moon in the plastic earth, acting through the ages, slowed down the earth's rotation to its present period, or the length of the day. moulton, however, has investigated the tidal theory of the origin of the moon in the light of the planetesimal hypothesis, concluding that the moon never was part of the earth and separated therefrom by too rapid rotation of the earth, but that the distance of the two bodies has always been the same as now. the more massive earth has in its development throughout time robbed the less massive moon in the gradual process of accretion. so the moon has never acquired either an ocean or atmosphere, and this view is acceptable to geologists who have studied the sheer lunar surface, shaler of harvard among the first, and laid the foundations for a separate science of selenology. tidal friction has also been operant in producing sun-raised tides upon the early plastic substances which composed the planets: more powerfully in the case of planets nearer the sun; less rapidly if the planet's mass is large; also less completely if the planet has solidified earlier on account of its small dimensions. so darwin would account for the present rotation periods of all the planets: both mercury and venus powerfully acted on by the sun on account of their nearness to him, and their rotational energy completely exhausted, so that they now and for all time turn a constant face toward him, as the moon does to the earth; earth and possibly mars even yet undergoing a very slight lengthening of their day; jupiter and saturn, also uranus and probably neptune, still exhibiting relatively swift axial rotation, because of their great mass and great original moment of momentum, and also by reason of their vast distances from the central tide-raising body, the sun. by applying to stellar systems the principles developed by darwin, see accounted for the fact, to which he was the first to direct attention, that the great eccentricity of the binary orbits is a necessary result of the secular action of tidal friction. the double stars, then, were double nebulæ, originally single, but separated by a process allied to that known as "fission" in protozoans. indeed, poincaré proved mathematically that a swiftly revolving nebula, in consequence of contraction, first undergoes distortion into a pear-shaped or hour-glass figure, the two masses ultimately separating entirely; and the observations of the herschels, lord rosse and others, with the recent photographic plates at the lick and mount wilson observatories, afford immediate confirmation in a multitude of double nebulæ, widely scattered throughout the nebular regions of the heavens. jeans of cambridge, england, among the most recent of mathematical investigators of the cosmogony, balances the advantages and disadvantages of the differing cosmogonic systems as follows, in his "problems of cosmogony and stellar dynamics": "some hundreds of millions of years ago all the stars within our galactic universe formed a single mass of excessively tenuous gas in slow rotation. as imagined by laplace, this mass contracted owing to loss of energy by radiation, and so increased its angular velocity until it assumed a lenticular shape.... after this, further contraction was a sheer mathematical impossibility and the system had to expand. the mechanism of expansion was provided by matter being thrown off from the sharp edge of the lenticular figure, the lenticular center now forming the nucleus, and the thrown-off matter forming the arms, of a spiral nebula of the normal type. the long filaments of matter which constituted the arms, being gravitationally unstable, first formed into chains of condensation about nuclei, and ultimately formed detached masses of gas. with continued shrinkage, the temperature of these masses increased until they attained to incandescence, and shone as luminous stars. at the same time their velocity of rotation increased until a large proportion of them broke up by fission into binary systems. the majority of the stars broke away from their neighbors and so formed a cluster of irregularly moving stars--our present galactic universe, in which the flattened shape of the original nebula may still be traced in the concentration about the galactic plane, while the original motion along the nebular arms still persists in the form of 'star-streaming.' in some cases a pair or small group of stars failed to get clear of one another's gravitational attractions and remain describing orbits about one another as wide binaries or multiple stars. the stars which were formed last, the present b-type stars, have been unusually immune from disturbance by their neighbors, partly because they were born when adjacent stars had almost ceased to interfere with one another, partly because their exceptionally large mass minimized the effect of such interference as may have occurred; consequently they remain moving in the plane in which they were formed, many of them still constituting closely associated groups of stars--the moving star clusters. "at intervals it must have happened that two stars passed relatively near to one another in their motion through the universe. we conjecture that something like million years ago our sun experienced an encounter of this kind, a large star passing within a distance of about the sun's diameter from its surface. the effect of this, as we have seen, would be the ejection of a stream of gas toward the passing star. at this epoch the sun is supposed to have been dark and cold, its density being so low that its radius was perhaps comparable with the present radius of neptune's orbit. the ejected stream of matter, becoming still colder by radiation, may have condensed into liquid near its ends and perhaps partially also near its middle. such a jet of matter would be longitudinally unstable and would condense into detached nuclei which would ultimately form planets." chapter lxi cosmogony in transition we have seen how wright in initiated a theory of evolution, not only of the solar system, but of all the stars and nebulæ as well; how kant in by elaborating this theory sought to develop the details of evolution of the solar system on the basis of the newtonian law, though weakened, as we know, by serious errors in applying physical laws; how laplace in put forward his nebular hypothesis of origin and development of the solar system, by contraction from an original gaseous nebula in accord with the newtonian law; how sir william herschel in saw in all nebulæ merely the stuff that stars are made of; how lord rosse in discovered spiral nebulæ; how helmholtz in put forward his contraction theory of maintenance of the solar heat, seemingly reinforcing the laplacian theory; how lane in proved that a contracting gaseous star might rise in temperature; how roche in in attempting to modify the laplacian hypothesis, pointed out the conditions under which a satellite would be broken up by tidal strains; how darwin in showed that the theory of tidal evolution of non-rigid bodies might account for the formation of the moon, and binary stars might originate by fission; how keeler in discovered the vast numbers of spiral nebulæ; how chamberlin and moulton in put forward the planetesimal hypothesis of formation of the spiral nebulæ, showing also how that hypothesis might account for the evolution of the solar system; and how jeans in advocated the median ground in evolution of the arms of the spiral nebulæ, showing that they will break up into nuclei, if sufficiently massive. in all these theories, truth and error, or lack of complete knowledge, appear to be intermingled in varying proportions. is it not early yet to say, either that any one of them must be abandoned as totally wrong, or on the other hand that any one of them, or indeed any single hypothesis, can explain all the evolutionary processes of the universe? clearly the great problems cannot all be solved by the kinetic theory of gases and the law of gravitation alone. recent physical researches into sub-atomic energy and the structure and properties of matter, appear to point in the direction where we must next look for more light on such questions as the origin and maintenance of the sun's heat, the complex phenomena of variable stars and the progressive evolution of the myriad bodies of the stellar universe. because we have actually seen one star turn into a nebula we should not jump to the conclusion that all nebulæ are formed from stars, even if this might seem a direct inference from the high radial velocities of planetary nebulæ. quite as obviously many of the spiral nebulæ are in a stage of transition into local universes of stars--even more obvious from the marvelous photographs in our day than the evolution of stars from nebulæ of all types was to herschel in his day. the physicist must further investigate such questions as the building up of heavy atomic elements by gravitative condensation of such lighter ones as compose the nebulæ; and laboratory investigation must elucidate further the process of development of energy from atomic disintegration under very high pressures. this leads to a reclassification of the stars on a temperature basis. equally important is the inquiry into the mechanism of radiative equilibrium in sun and stars. not impossibly the process of the earth's upper atmosphere in maintaining a terrestrial equilibrium may afford some clue. what this physical mechanism may be is very incompletely known, but it is now open to further research through recent progress of aeronautics, which will afford the investigator a "ceiling" of , feet and probably more. beneath this level, perhaps even below , feet, lie all the strata, including the inversion layer, where the sun's heat is conserved and an equilibrium maintained. even ten years ago, had an astronomer been asked about the physical condition of the interior of the stars, he would have replied that information of this character could only be had on visiting the stars themselves--and perhaps not even then. but at the cardiff meeting of the british association in , eddington, the president of section a, delivered an address on the internal constitution of the stars. he cites the recent investigations of russell and others on truly gaseous stars, like aldebaran, arcturus, antares and canopus, which are in a diffuse state and are the most powerful light-givers, and thus are to be distinguished from the denser stars like our sun. the term _giants_ is applied to the former, and _dwarfs_ to the latter, in accord with russell's theory. as density increases through contraction, these terms represent the progressive stages, from earlier to later, in a star's history. a red or m-type star begins its history as a giant of comparatively low temperature. contracting, according to lane's law, its temperature must rise until its density becomes such that it no longer behaves as a perfect gas. much depends on the star's mass; but after its maximum temperature is attained, the star, which has shrunk to the proportions of a dwarf, goes on cooling and contracts still further. each temperature-level is reached and passed twice, once during the ascending stage and once again in descending--once as a giant, and once as a dwarf. thus there are vast differences in luminosity: the huge giant, having a far larger surface than the shrunken dwarf, radiates an amount of light correspondingly greater. the physicist recognizes heat in two forms--the energy of motion of material atoms, and the energy of ether waves. in hot bodies with which we are familiar, the second form is quite insignificant; but in the giant stars, the two forms are present in about equal proportions. the super-heated conditions of the interior of the stars can only be estimated in millions of degrees; and the problem is not one of convection currents, as formerly thought, bringing hot masses to the surface from the highly heated interior, but how can the heat of the interior be barred against leakage and reduced to the relatively small radiation emitted by the stars. "smaller stars have to manufacture the radiant heat which they emit, living from hand to mouth; the giant stars merely leak radiant heat from their store." so a radioactive type of equilibrium must be established, rather than a convective one. laboratory investigations of the very short waves are now in progress, bearing on the transparency of stellar material to the radiation traversing it; and the penetrating power of the star's radiation is much like that of x-rays. the opacity is remarkably high, explaining why the star is so nearly "heat-tight." opacity being constant, the total radiation of a giant star depends on its mass only, and is quite independent of its temperature or state of diffuseness. so that the total radiation of a star which is measured roughly by its luminosity, may readily remain constant during the entire 'giant' stage of its history. as russell originally pointed out, giant stars of every spectral type have nearly the same luminosity. from the range of luminosity of the giant stars, then, we may infer their range of masses: they come out much alike, agreeing well with results obtained by double-star investigation. these studies of radiation and internal condition of the stars again bring up the question of the original source of that supply of radiant energy continually squandered by all self-luminous bodies. the giant stars are especially prodigal, and radiate at least a hundredfold faster than the sun. "a star is drawing on some vast reservoir of energy," says eddington, "by means unknown to us. this reservoir can scarcely be other than the sub-atomic energy which, it is known, exists abundantly in all matter; we sometimes dream that man will one day learn how to release it and use it for his service. the store is well-nigh inexhaustible, if only it could be tapped. there is sufficient in the sun to maintain its output of heat for fifteen billion years." * * * * * transcriber's notes: obvious punctuation errors repaired. hyphenation and spelling was standardized by using the most prevalent form. the oe ligature was converted to the letters "oe". whole and fractional parts of numbers are displayed as follows: - / . page correction ==== =================== aa => aya ulugh begh => ulugh beg instaurata mecanica => instauratæ mechanica oscillatorium horologium => horologium oscillatorium seceded => succeeded areoplane => aeroplane plate - vulpeculæ => vulpecula text emphasis _text_ - italic =text= - bold the science of the stars by e. walter maunder, f.r.a.s. of the royal observatory, greenwich author of "astronomy without a telescope" "the astronomy of the bible," etc. london: t. c. & e. c. jack long acre, w.c., and edinburgh new york: dodge publishing co. {vii} contents chap. i. astronomy before history ii. astronomy before the telescope iii. the law of gravitation iv. astronomical measurements v. the members of the solar system vi. the system of the stars index { } the science of the stars chapter i astronomy before history the plan of the present series requires each volume to be complete in about eighty small pages. but no adequate account of the achievements of astronomy can possibly be given within limits so narrow, for so small a space would not suffice for a mere catalogue of the results which have been obtained; and in most cases the result alone would be almost meaningless unless some explanation were offered of the way in which it had been reached. all, therefore, that can be done in a work of the present size is to take the student to the starting-point of astronomy, show him the various roads of research which have opened out from it, and give a brief indication of the character and general direction of each. that which distinguishes astronomy from all the other sciences is this: it deals with objects that we cannot touch. the heavenly bodies are beyond our reach; we cannot tamper with them, or subject them to any form of experiment; we cannot bring them into our laboratories to analyse or dissect them. we can only watch them and wait for such indications as their { } own movements may supply. but we are confined to this earth of ours, and they are so remote; we are so short-lived, and they are so long-enduring; that the difficulty of finding out much about them might well seem insuperable. yet these difficulties have been so far overcome that astronomy is the most advanced of all the sciences, the one in which our knowledge is the most definite and certain. all science rests on sight and thought, on ordered observation and reasoned deduction; but both sight and thought were earlier trained to the service of astronomy than of the other physical sciences. it is here that the highest value of astronomy lies; in the discipline that it has afforded to man's powers of observation and reflection; and the real triumphs which it has achieved are not the bringing to light of the beauties or the sensational dimensions and distances of the heavenly bodies, but the vanquishing of difficulties which might well have seemed superhuman. the true spirit of the science can be far better exemplified by the presentation of some of these difficulties, and of the methods by which they have been overcome, than by many volumes of picturesque description or of eloquent rhapsody. there was a time when men knew nothing of astronomy; like every other science it began from zero. but it is not possible to suppose that such a state of things lasted long, we know that there was a time when men had noticed that there were two great lights in the sky--a greater light that shone by day, a lesser light that shone by night--and there were the stars also. and this, the earliest observation of primitive astronomy, is preserved for us, expressed in the simplest possible language, in the first chapter of the first book { } of the sacred writings handed down to us by the hebrews. this observation, that there are bodies above us giving light, and that they are not all equally bright, is so simple, so inevitable, that men must have made it as soon as they possessed any mental power at all. but, once made, a number of questions must have intruded themselves: "what are these lights? where are they? how far are they off?" many different answers were early given to these questions. some were foolish; some, though intelligent, were mistaken; some, though wrong, led eventually to the discovery of the truth. many myths, many legends, some full of beauty and interest, were invented. but in so small a book as this it is only possible to glance at those lines of thought which eventually led to the true solution. as the greater light, the lesser light, and the stars were carefully watched, it was seen not only that they shone, but that they appeared to move; slowly, steadily, and without ceasing. the stars all moved together like a column of soldiers on the march, not altering their positions relative to each other. the lesser light, the moon, moved with the stars, and yet at the same time among them. the greater light, the sun, was not seen with the stars; the brightness of his presence made the day, his absence brought the night, and it was only during his absence that the stars were seen; they faded out of the sky before he came up in the morning, and did not reappear again until after he passed out of sight in the evening. but there came a time when it was realised that there were stars shining in the sky all day long as well as at night, and this discovery was one of the greatest and most important ever made, { } because it was the earliest discovery of something quite unseen. men laid hold of this fact, not from the direct and immediate evidence of their senses, but from reflection and reasoning. we do not know who made this discovery, nor how long ago it was made, but from that time onward the eyes with which men looked upon nature were not only the eyes of the body, but also the eyes of the mind. it followed from this that the sun, like the moon, not only moved with the general host of the stars, but also among them. if an observer looks out from any fixed station and watches the rising of some bright star, night after night, he will notice that it always appears to rise in the same place; so too with its setting. from any given observing station the direction in which any particular star is observed to rise or set is invariable. not so with the sun. we are accustomed to say that the sun rises in the east and sets in the west. but the direction in which the sun rises in midwinter lies far to the south of the east point; the direction in which he rises in midsummer lies as far to the north. the sun is therefore not only moving with the stars, but among them. this gradual change in the position of the sun in the sky was noticed in many ancient nations at an early time. it is referred to in job xxxviii. : "hast thou commanded the morning since thy days; and caused the dayspring to know his place?" and the apparent path of the sun on one day is always parallel to its path on the days preceding and following. when, therefore, the sun rises far to the south of east, he sets correspondingly far to the south of west, and at noon he is low down in the south. his course during the day is a short one, and the daylight { } is much shorter than the night, and the sun at noon, being low down in the sky, has not his full power. the cold and darkness of winter, therefore, follows directly upon this position of the sun. these conditions are reversed when the sun rises in the north-east. the night is short, the daylight prolonged, and the sun, being high in the heavens at noon, his heat is felt to the full. thus the movements of the sun are directly connected with the changes of season upon the earth. but the stars also are connected with those seasons; for if we look out immediately after it has become dark after sunset, we shall notice that the stars seen in the night of winter are only in part those seen in the nights of summer. in the northern part of the sky there are a number of stars which are always visible whenever we look out, no matter at what time of the night nor what part of the year. if we watch throughout the whole night, we see that the whole heavens appear to be slowly turning--turning, as if all were in a single piece--and the pivot about which it is turning is high up in the northern sky. the stars, therefore, are divided into two classes. those near this invisible pivot--the "pole" of the heavens, as we term it--move round it in complete circles; they never pass out of sight, but even when lowest they clear the horizon. the other stars move round the same pivot in curved paths, which are evidently parts of circles, but circles of which we do not see the whole. these stars rise on the eastern side of the heavens and set on the western, and for a greater or less space of time are lost to sight below the horizon. and some of these stars are visible at one time of the year, others at another; some being seen during the { } whole of the long nights of winter, others throughout the short nights of summer. this distinction again, and its connection with the change of the seasons on the earth, was observed many ages ago. it is alluded to in job xxxviii. : "canst thou lead forth the signs of the zodiac in their season, or canst thou guide the bear with her train?" (r.v., margin). the signs of the zodiac are taken as representing the stars which rise and set, and therefore have each their season for being "led forth," while the northern stars, which are always visible, appearing to be "guided" in their continual movement round the pole of the sky in perfect circles, are represented by "the bear with her train." the changes in position of the sun, the greater light, must have attracted attention in the very earliest ages, because these changes are so closely connected with the changes of the seasons upon the earth, which affect men directly. the moon, the lesser light, goes through changes of position like the sun, but these are not of the same direct consequence to men, and probably much less notice was taken of them. but there were changes of the moon which men could not help noticing--her changes of shape and brightness. one evening she may be seen soon after the sun has set, as a thin arch of light, low down in the sunset sky. on the following evenings she is seen higher and higher in the sky, and the bow of light increases, until by the fourteenth day it is a perfect round. then the moon begins to diminish and to disappear, until, on the twenty-ninth or thirtieth day after the first observation, she is again seen in the west after sunset as a narrow crescent. this succession of changes gave men an important measure of time, and, in an age when artificial means of light were difficult to procure, moonlight was of the greatest { } value, and the return of the moonlit portion of the month was eagerly looked for. these early astronomical observations were simple and obvious, and of great practical value. the day, month, and year were convenient measures of time, and the power of determining, from the observation of the sun and of the stars, how far the year had progressed was most important to farmers, as an indication when they should plough and sow their land. such observations had probably been made independently by many men and in many nations, but in one place a greater advance had been made. the sun and moon are both unmistakable, but one star is very like another, and, for the most part, individual stars can only be recognised by their positions relative to others. the stars were therefore grouped together into +constellations+ and associated with certain fancied designs, and twelve of these designs were arranged in a belt round the sky to mark the apparent path of the sun in the course of the year, these twelve being known as the "+signs of the zodiac+"--the ram, bull, twins, crab, lion, virgin, balance, scorpion, archer, goat, water-pourer, and fishes. in the rest of the sky some thirty to thirty-six other groups, or constellations, were formed, the bear being the largest and brightest of the constellations of the northern heavens. but these ancient constellations do not cover the entire heavens; a large area in the south is untouched by them. and this fact affords an indication both of the time when and the place where the old stellar groups were designed, for the region left untouched was the region below the horizon of ° north latitude, about years ago. it is probable, therefore, that the ancient astronomers who carried out this great work { } lived about b.c., and in north latitude ° or °. the indication is only rough, but the amount of uncertainty is not very large; the constellations must be at least years old, they cannot be more than . all this was done by prehistoric astronomers; though no record of the actual carrying out of the work and no names of the men who did it have come down to us. but it is clear from the fact that the signs of the zodiac are arranged so as to mark out the annual path of the sun, and that they are twelve in number--there being twelve months in the year--that those who designed the constellations already knew that there are stars shining near the sun in full daylight, and that they had worked out some means for determining what stars the sun is near at any given time. another great discovery of which the date and the maker are equally unknown is referred to in only one of the ancient records available to us. it was seen that all along the eastern horizon, from north to south, stars rise, and all along the western horizon, from north to south, stars set. that is what was seen; it was the fact observed. there is no hindrance anywhere to the movement of the stars--they have a free passage under the earth; the earth is unsupported in space. that is what was _thought_; it was the inference drawn. or, as it is written in job xxvi. , "he (god) stretcheth out the north over empty space, and hangeth the earth upon nothing." the earth therefore floats unsupported in the centre of an immense star-spangled sphere. and what is the shape of the earth? the natural and correct inference is that it is spherical, and we find in some of the early greek writers the arguments which establish this inference as clearly set forth as they would be to-day. { } the same inference followed, moreover, from the observation of a simple fact, namely, that the stars as observed from any particular place all make the same angle with the horizon as they rise in the east, and all set at the same angle with it in the west; but if we go northward, we find that angle steadily decreasing; if we go southward, we find it increasing. but if the earth is round like a globe, then it must have a definite size, and that size can be measured. the discoveries noted above were made by men whose names have been lost, but the name of the first person whom we know to have measured the size of the earth was eratosthenes. he found that the sun was directly overhead at noon at midsummer at syene (the modern assouan), in egypt, but was ° south of the "zenith"--the point overhead--at alexandria, and from this he computed the earth to be , stadia (a stadium = feet) in circumference. another consequence of the careful watch upon the stars was the discovery that five of them were planets; "wandering" stars; they did not move all in one piece with the rest of the celestial host. in this they resemble the sun and moon, and they further resemble the moon in that, though too small for any change of shape to be detected, they change in brightness from time to time. but their movements are more complicated than those of the other heavenly bodies. the sun moves a little slower than the stars, and so seems to travel amongst them from west to east; the moon moves much slower than the stars, so her motion from west to east is more pronounced than that of the sun. but the five planets sometimes move slower than the stars, sometimes quicker, and sometimes at the same rate. two of the five, which we now know as mercury { } and venus, never move far from the sun, sometimes being seen in the east before he rises in the morning, and sometimes in the west after he has set in the evening. mercury is the closer to the sun, and moves more quickly; venus goes through much the greater changes of brightness. jupiter and saturn move nearly at the same average rate as the stars, saturn taking about thirteen days more than a year to come again to the point of the sky opposite to the sun, and jupiter about thirty-four days. mars, the fifth planet, takes two years and fifty days to accomplish the same journey. these planetary movements were not, like those of the sun and moon and stars, of great and obvious consequence to men. it was important to men to know when they would have moonlight nights, to know when the successive seasons of the year would return. but it was no help to men to know when venus was at her brightest more than when she was invisible. she gave them no useful light, and she and her companion planets returned at no definite seasons. nevertheless, men began to make ordered observations of the planets--observations that required much more patience and perseverance than those of the other celestial lights. and they set themselves with the greatest ingenuity to unravel the secret of their complicated and seemingly capricious movements. this was a yet higher development than anything that had gone before, for men were devoting time, trouble, and patient thought, for long series of years, to an inquiry which did not promise to bring them any profit or advantage. yet the profit which it actually did bring was of the highest order. it developed men's mental powers; it led to the devising of { } instruments of precision for the observations; it led to the foundation of mathematics, and thus lay at the root of all our modern mechanical progress. it brought out, in a higher degree, ordered observation and ordered thought. { } chapter ii astronomy before the telescope there was thus a real science of astronomy before we have any history of it. some important discoveries had been made, and the first step had been taken towards cataloguing the fixed stars. it was certainly known to some of the students of the heavens, though perhaps only to a few, that the earth was a sphere, freely suspended in space, and surrounded on all sides by the starry heavens, amongst which moved the sun, moon, and the five planets. the general character of the sun's movement was also known; namely, that he not only moved day by day from east to west, as the stars do, but also had a second motion inclined at an angle to the first, and in the opposite direction, which he accomplished in the course of a year. to this sum of knowledge, no doubt, several nations had contributed. we do not know to what race we owe the constellations, but there are evidences of an elementary acquaintance with astronomy on the part of the chinese, the babylonians, the egyptians, and the jews. but in the second stage of the development of the science the entire credit for the progress made belongs to the greeks. the greeks, as a race, appear to have been very little apt at originating ideas, but they possessed, beyond all other races, the power of developing and perfecting crude ideas which they had obtained from other sources, { } and when once their attention was drawn to the movements of the heavenly bodies, they devoted themselves with striking ingenuity and success to devising theories to account for the appearances presented, to working out methods of computation, and, last, to devising instruments for observing the places of the luminaries in which they were interested. in the brief space available it is only possible to refer to two or three of the men whose commanding intellects did so much to help on the development of the science. eudoxus of knidus, in asia minor ( - b.c.), was, so far as we know, the first to attempt to represent the movements of the heavenly bodies by a simple mathematical process. his root idea was something like this. the earth was in the centre of the universe, and it was surrounded, at a great distance from us, by a number of invisible transparent shells, or spheres. each of these spheres rotated with perfect uniformity, though the speed of rotation differed for different spheres. one sphere carried the stars, and rotated from east to west in about h. m. the sun was carried by another sphere, which rotated from west to east in a year, but the pivots, or poles, of this sphere were carried by a second, rotating exactly like the sphere of the stars. this explained how it is that the ecliptic--that is to say, the apparent path of the sun amongst the stars--is inclined -½° to the equator of the sky, so that the sun is -½° north of the equator at midsummer and -½° south of the equator at midwinter, for the poles of the sphere peculiar to the sun were supposed to be -½° from the poles of the sphere peculiar to the stars. then the moon had three spheres; that which actually carried the moon having its poles ° from the poles of the sphere peculiar to the { } sun. these poles were carried by a sphere placed like the sphere of the sun, but rotating in days; and this, again, had its poles in the sphere of the stars. the sphere carrying the moon afforded the explanation of the wavy motion of the moon to and fro across the ecliptic in the course of a month, for at one time in the month the moon is ° north of the ecliptic, at another time ° south. the motions of the planets were more difficult to represent, because they not only have a general daily motion from east to west, like the stars, and a general motion from west to east along the ecliptic, like the sun and moon, but from time to time they turn back on their course in the ecliptic, and "retrograde." but the introduction of a third and fourth sphere enabled the motions of most of the planets to be fairly represented. there were thus twenty-seven spheres in all--four for each of the five planets, three for the moon, three for the sun (including one not mentioned in the foregoing summary), and one for the stars. these spheres were not, however, supposed to be solid structures really existing; the theory was simply a means for representing the observed motions of the heavenly bodies by computations based upon a series of uniform movements in concentric circles. but this assumption that each heavenly body moves in its path at a uniform rate was soon seen to be contrary to fact. a reference to the almanac will show at once that the sun's movement is not uniform. thus for the year - the solstices and equinoxes fell as given on the next page: { } _epoch time interval_ winter solstice dec. d. h. m. p.m. d. h. m. spring equinox mar. " " " p.m. " " " summer solstice l june " " " p.m. " " " autumn equinox sept. " " " a.m. " " " winter solstice dec. " " " p.m. so that the winter half of the year is shorter than the summer half; the sun moves more quickly over the half of its orbit which is south of the equator than over the half which is north of it. the motion of the moon is more irregular still, as we can see by taking out from the almanac the times of new and full moon: _new moon interval to full moon_ dec. d. h. . m. p.m. d. h. . m. " " " " . " p.m. " " . " jan. " " . " a.m. " " . " march " " " . " a.m. " " . " " " " " . " p.m. " " . " april " " " . " p.m. " " . " may " " " . " a.m. " " . " june " " " . " p.m. " " . " july " " " . " p.m. " " . " aug. " " " . " a.m. " " . " sept. " " " . " p.m. " " . " oct. " " " . " a.m. " " . " nov. " " " . " p.m. " " . " dec. " " " . " p.m. " " . " { } _full moon interval to new moon_ dec. d h. . m. a.m. d. h. . m. jan. " " . " p.m. " " . " feb. " " " . " a.m. " " . " march " " " . " p.m. " " . " april " " " . " p.m. " " . " may " " " . " a.m. " " . " june " " " . " p.m. " " . " july " " " . " p.m. " " . " aug. " " " . " a.m. " " . " sept. " " " . " p.m. " " . " oct. " " " . " a.m. " " . " nov. " " " . " p.m. " " . " dec. " " " . " a.m. " " . " jan. " " . " p.m. " " . " the astronomer who dealt with this difficulty was hipparchus (about - b.c.), who was born at nicæa, in bithynia, but made most of his astronomical observations in rhodes. he attempted to explain these irregularities in the motions of the sun and moon by supposing that though they really moved uniformly in their orbits, yet the centre of their orbits was not the centre of the earth, but was situated a little distance from it. this point was called "+the excentric+," and the line from the excentric to the earth was called "+the line of apsides+." but when he tried to deal with the movements of the planets, he found that there were not enough good observations available for him to build up any satisfactory theory. he therefore devoted himself to the work of making systematic determinations of the places of the planets that he might put his successors in a better position to deal with the problem than he was. his great successor was claudius ptolemy of { } alexandria, who carried the work of astronomical observation from about a.d. to . he was, however, much greater as a mathematician than as an observer, and he worked out a very elaborate scheme, by which he was able to represent the motions of the planets with considerable accuracy. the system was an extremely complex one, but its principle may be represented as follows: if we suppose that a planet is moving round the earth in a circle at a uniform rate, and we tried to compute the place of the planet on this assumption for regular intervals of time, we should find that the planet gradually got further and further away from the predicted place. then after a certain time the error would reach a maximum, and begin to diminish, until the error vanished and the planet was in the predicted place at the proper time. the error would then begin to fall in the opposite direction, and would increase as before to a maximum, subsequently diminishing again to zero. this state of things might be met by supposing that the planet was not itself carried by the circle round the earth, but by an +epicycle+--_i.e._ a circle travelling upon the first circle--and by judiciously choosing the size of the epicycle and the time of revolution the bulk of the errors in the planet's place might be represented. but still there would be smaller errors going through their own period, and these, again, would have to be met by imagining that the first epicycle carried a second, and it might be that the second carried a third, and so on. the ptolemaic system was more complicated than this brief summary would suggest, but it is not possible here to do more than indicate the general principles upon which it was founded, and the numerous other systems or modifications of them produced in the { } five centuries from eudoxus to ptolemy must be left unnoticed. the point to be borne in mind is that one fundamental assumption underlay them all, an assumption fundamental to all science--the assumption that like causes must always produce like effects. it was apparent to the ancient astronomers that the stars--that is to say, the great majority of the heavenly bodies--do move round the earth in circles, and with a perfect uniformity of motion, and it seemed inevitable that, if one body moved round another, it should thus move. for if the revolving body came nearer to the centre at one time and receded at another, if it moved faster at one time and slower at another, then, the cause remaining the same, the effect seemed to be different. any complexity introduced by superposing one epicycle upon another seemed preferable to abandoning this great fundamental principle of the perfect uniformity of the actings of nature. for more than years the ptolemaic system remained without serious challenge, and the next great name that it is necessary to notice is that of copernicus ( - ). copernicus was a canon of frauenburg, and led the quiet, retired life of a student. the great work which made him immortal, _de revolutionibus_, was the result of many years' meditation and work, and was not printed until he was on his deathbed. in this work copernicus showed that he was one of those great thinkers who are able to look beyond the mere appearance of things and to grasp the reality of the unseen. copernicus realised that the appearance would be just the same whether the whole starry vault rotated every twenty-four hours round an immovable earth from east to west or the earth rotated from west to east in the midst of the starry sphere; and, as the { } stars are at an immeasurable distance, the latter conception was much the simpler. extending the idea of the earth's motion further, the supposition that, instead of the sun revolving round a fixed earth in a year, the earth revolved round a fixed sun, made at once an immense simplification in the planetary motions. the reason became obvious why mercury and venus were seen first on one side of the sun and then on the other, and why neither of them could move very far from the sun; their orbits were within the orbit of the earth. the stationary points and retrogressions of the planets were also explained; for, as the earth was a planet, and as the planets moved in orbits of different sizes, the outer planets taking a longer time to complete a revolution than the inner, it followed, of necessity, that the earth in her motion would from time to time be passed by the two inner planets, and would overtake the three outer. the chief of the ptolemaic epicycles were done away with, and all the planets moved continuously in the same direction round the sun. but no planet's motion could be represented by uniform motion in a single circle, and copernicus had still to make use of systems of epicycles to account for the deviations from regularity in the planetary motions round the sun. the earth having been abandoned as the centre of the universe, a further sacrifice had to be made: the principle of uniform motion in a circle, which had seemed so necessary and inevitable, had also to be given up. for the time came when the instruments for measuring the positions of the stars and planets had been much improved, largely due to tycho brahe ( - ), a dane of noble birth, who was the keenest and most careful observer that astronomy had yet produced. { } his observations enabled his friend and pupil, johann kepler, ( - ), to subject the planetary movements to a far more searching examination than had yet been attempted, and he discovered that the sun is in the plane of the orbit of each of the planets, and also in its +line of apsides+--that is to say, the line joining the two points of the orbit which are respectively nearest and furthest from the sun. copernicus had not been aware of either of these two relations, but their discovery greatly strengthened the copernican theory. then for many years kepler tried one expedient after another in order to find a combination of circular motions which would satisfy the problem before him, until at length he was led to discard the circle and try a different curve--the oval or ellipse. now the property of a circle is that every point of it is situated at the same distance from the centre, but in an ellipse there are two points within it, the "foci," and the sum of the distances of any point on the circumference from these two foci is constant. if the two foci are at a great distance from each other, then the ellipse is very long and narrow; if the foci are close together, the ellipse differs very little from a circle; and if we imagine that the two foci actually coincide, the ellipse becomes a circle. when kepler tried motion in an ellipse instead of motion in a circle, he found that it represented correctly the motions of all the planets without any need for epicycles, and that in each case the sun occupied one of the foci. and though the planet did not move at a uniform speed in the ellipse, yet its motion was governed by a uniform law, for the straight line joining the planet to the sun, the "+radius vector+," passed over equal areas of space in equal periods of time. { } these two discoveries are known as kepler's first and second laws. his third law connects all the planets together. it was known that the outer planets not only take longer to revolve round the sun than the inner, but that their actual motion in space is slower, and kepler found that this actual speed of motion is inversely as the square root of its distance from the sun; or, if the square of the speed of a planet be multiplied by its distance from the sun, we get the same result in each case. this is usually expressed by saying that the cube of the distance is proportional to the square of the time of revolution. thus the varying rate of motion of each planet in its orbit is not only subject to a single law, but the very different speeds of the different planets are also all subject to a law that is the same for all. thus the whole of the complicated machinery of ptolemy had been reduced to three simple laws, which at the same time represented the facts of observation much better than any possible development of the ptolemaic mechanism. on his discovery of his third law kepler had written: "the book is written to be read either now or by posterity--i care not which; it may well wait a century for a reader, as god has waited years for an observer." twelve years after his death, on christmas day (old style), near grantham, in lincolnshire, the predestined "reader" was born. the inner meaning of kepler's three laws was brought to light by isaac newton. { } chapter iii the law of gravitation the fundamental thought which, recognised or not, had lain at the root of the ptolemaic system, as indeed it lies at the root of all science, was that "like causes must always produce like effects." upon this principle there seemed to the ancient astronomers no escape from the inference that each planet must move at a uniform speed in a circle round its centre of motion. for, if there be any force tending to alter the distance of the planet from that centre, it seemed inevitable that sooner or later it should either reach that centre or be indefinitely removed from it. if there be no such force, then the planet's distance from that centre must remain invariable, and if it move at all, it must move in a circle; move uniformly, because there is no force either to hasten or retard it. uniform motion in a circle seemed a necessity of nature. but all this system, logical and inevitable as it had once seemed, had gone down before the assault of observed facts. the great example of uniform circular motion had been the daily revolution of the star sphere; but this was now seen to be only apparent, the result of the rotation of the earth. the planets revolved round the sun, but the sun was not in the centre of their motion; they moved, not in circles, but in ellipses; not at a uniform speed, but at a speed which diminished with the increase of their distance from { } the sun. there was need, therefore, for an entire revision of the principles upon which motion was supposed to take place. the mistake of the ancients had been that they supposed that continued motion demanded fresh applications of force. they noticed that a ball, set rolling, sooner or later came to a stop; that a pendulum, set swinging, might swing for a good time, but eventually came to rest; and, as the forces that were checking the motion--that is to say, the friction exercised by the ground, the atmosphere, and the like--did not obtrude themselves, they were overlooked. newton brought out into clear statement the true conditions of motion. a body once moving, if acted upon by no force whatsoever, must continue to move forward in a straight line at exactly the same speed, and that for ever. it does not require any maintaining force to keep it going. if any change in its speed or in its direction takes place, that change must be due to the introduction of some further force. this principle, that, if no force acts on a body in motion, it will continue to move uniformly in a straight line, is newton's first law of motion. his second lays it down that, if force acts on a body, it produces a change of motion proportionate to the force applied, and in the same direction. and the third law states that when one body exerts force upon another, that second body reacts with equal force upon the first. the problem of the motions of the planets was, therefore, not what kept them moving, but what made them deviate from motion in a straight line, and deviate by different amounts. it was quite clear, from the work of kepler, that the force deflecting the planets from uniform motion in a { } straight line lay in the sun. the facts that the sun lay in the plane of the orbits of all the planets, that the sun was in one of the foci of each of the planetary ellipses, that the straight line joining the sun and planet moved for each planet over equal areas in equal periods of time, established this fact clearly. but the amount of deflection was very different for different planets. thus the orbit of mercury is much smaller than that of the earth, and is travelled over in a much shorter time, so that the distance by which mercury is deflected in a course of an hour from movement in a straight line is much greater than that by which the earth is deflected in the same time, mercury falling towards the sun by about miles, whilst the fall of the earth is only about . miles. the force drawing mercury towards the sun is therefore . times that drawing the earth, but . is the square of . , and the earth is . times as far from the sun as mercury. similarly, the fall in an hour of jupiter towards the sun is about . miles, so that the force drawing the earth is times that drawing jupiter towards the sun. but is the square of . , and jupiter is . times as far from the sun as the earth. similarly with the other planets. the force, therefore, which deflects the planets from motion in a straight line, and compels them to move round the sun, is one which varies inversely as the square of the distance. but the sun is not the only attracting body of which we know. the old ptolemaic system was correct to a small extent; the earth is the centre of motion for the moon, which revolves round it at a mean distance of , miles, and in a period of d. h. m. hence the circumference of her orbit is , , miles, and the length of the straight line which she would travel { } in one second of time, if not deflected by the earth, is feet. in this distance the deviation of a circle from a straight line is one inch divided by . . but we know from experiment that a stone let fall from a height of inches above the earth's surface will reach the ground in exactly one second of time. the force drawing the stone to the earth, therefore, is x . ; _i.e._ times as great as that drawing the moon. but the stone is only / of a mile from the earth's surface, while the moon is , miles away--more than million times as far. the force, therefore, would seem not to be diminished in the proportion that the distance is increased--much less in the proportion of its square. but newton proved that a sphere of uniform density, or made up of any number of concentric shells of uniform density, attracted a body outside itself, just as if its entire mass was concentrated at its centre. the distance of the stone from the earth must therefore be measured, not from the earth's surface, but from its centre; in other words, we must consider the stone as being distant from the earth, not some feet, but miles. this is very nearly one-sixtieth of the moon's distance, and the square of is . the earth's pull upon the moon, therefore, is almost exactly in the inverse square of the distance as compared with its pull on the stone. kepler's book had found its "reader." his three laws were but three particular aspects of newton's great discovery that the planets moved under the influence of a force, lodged in the sun, which varied inversely as the square of their distances from it. but newton's work went far beyond this, for he showed that the same law governed the motion of the moon round the { } earth and the motions of the satellites revolving round the different planets, and also governed the fall of bodies upon the earth itself. it was universal throughout the solar system. the law, therefore, is stated as of universal application. "every particle of matter in the universe attracts every other particle with a force varying inversely as the square of the distance between them, and directly as the product of the masses of the two particles." and newton further proved that if a body, projected in free space and moving with any velocity, became subject to a central force acting, like gravitation, inversely as the square of the distance, it must revolve in an ellipse, or in a closely allied curve. these curves are what are known as the "+conic sections+"--that is, they are the curves found when a cone is cut across in different directions. their relation to each other may be illustrated thus. if we have a very powerful light emerging from a minute hole, then, if we place a screen in the path of the beam of light, and exactly at right angles to its axis, the light falling on the screen will fill an exact circle. if we turn the screen so as to be inclined to the axis of the beam, the circle will lengthen out in one direction, and will become an ellipse. if we turn the screen still further, the ellipse will lengthen and lengthen, until at last, when the screen has become parallel to one of the edges of the beam of light, the ellipse will only have one end; the other will be lost. for it is clear that that edge of the beam of light which is parallel to the screen can never meet it. the curve now shown on the screen is called a +parabola+, and if the screen is turned further yet, the boundaries of the light falling upon it become divergent, and we have a fourth curve, the +hyperbola+. bodies moving under the influence of { } gravitation can move in any of these curves, but only the circle and ellipse are closed orbits. a particle moving in a parabola or hyperbola can only make one approach to its attracting body; after such approach it continually recedes from it. as the circle and parabola are only the two extreme forms of an ellipse, the two foci being at the same point for the circle and at an infinite distance apart for the parabola, we may regard all orbits under gravitation as being ellipses of one form or another. from his great demonstration of the law of gravitation, newton went on to apply it in many directions. he showed that the earth could not be truly spherical in shape, but that there must be a flattening of its poles. he showed also that the moon, which is exposed to the attractions both of the earth and of the sun, and, to a sensible extent, of some of the other planets, must show irregularities in her motion, which at that time had not been noticed. the moon's orbit is inclined to that of the earth, cutting its plane in two opposite points, called the "+nodes+." it had long been observed that the position of the nodes travelled round the ecliptic once in about nineteen years. newton was able to show that this was a consequence of the sun's attraction upon the moon. and he further made a particular application of the principle thus brought out, for, the earth not being a true sphere, but flattened at the poles and bulging at the equator, the equatorial belt might be regarded as a compact ring of satellites revolving round the earth's equator. this, therefore, would tend to retrograde precisely as the nodes of a single satellite would, so that the axis of the equatorial belt of the earth--in other words, the axis of the earth--must revolve round the pole of the ecliptic. { } consequently the pole of the heavens appears to move amongst the stars, and the point where the celestial equator crosses the equator necessarily moves with it. this is what we know as the "+precession of the equinoxes+," and it is from our knowledge of the fact and the amount of precession that we are able to determine roughly the date when the first great work of astronomical observation was accomplished, namely, the grouping of the stars into constellations by the astronomers of the prehistoric age. the publication of newton's great work, the _principia_ (_the mathematical principles of natural philosophy_), in which he developed the laws of motion, the significance of kepler's three planetary laws, and the law of universal gravitation, took place in , and was due to his friend edmund halley, to whom he had confided many of his results. that he was the means of securing the publication of the _principia_ is halley's highest claim to the gratitude of posterity, but his own work in the field which newton had opened was of great importance. newton had treated +comets+ as moving in parabolic orbits, and halley, collecting all the observations of comets that were available to him, worked out the particulars of their orbits on this assumption, and found that the elements of three were very closely similar, and that the interval between their appearances was nearly the same, the comets having been seen in , , and . on further consulting old records he found that comets had been observed in , , and . he concluded that these were different appearances of the same object, and predicted that it would be seen again in , or, according to a later and more careful computation, in . as the time for its return drew near, clairaut { } computed with the utmost care the retardation which would be caused to the comet by the attractions of jupiter and saturn. the comet made its predicted nearest approach to the sun on march , , just one month earlier than clairaut had computed. but in its next return, in , the computations effected by pontÉcoulant were only two days in error, so carefully had the comet been followed during its unseen journey to the confines of the solar system and back again, during a period of seventy-five years. pontécoulant's exploit was outdone at the next return by drs. cowell and crommelin, of greenwich observatory, who not only computed the time of its perihelion passage--that is to say, its nearest approach to the sun--for april , , but followed the comet back in its wanderings during all its returns to the year b.c. halley's comet, therefore, was the first comet that was known to travel in a closed orbit and to return to the neighbourhood of the sun. not a few small or telescopic comets are now known to be "periodic," but halley's is the only one which has made a figure to the naked eye. notices of it occur not a few times in history; it was the comet "like a flaming sword" which josephus described as having been seen over jerusalem not very long before the destruction by titus. it was also the comet seen in the spring of the year when william the conqueror invaded england, and was skilfully used by that leader as an omen of his coming victory. the law of gravitation had therefore enabled men to recognise in halley's comet an addition to the number of the primary bodies in the solar system--the first addition that had been made since prehistoric times. on march , , sir william herschel { } detected a new object, which he at first supposed to be a comet, but afterwards recognised as a planet far beyond the orbit of saturn. this planet, to which the name of uranus was finally given, had a mean distance from the sun nineteen times that of the earth, and a diameter four times as great. this was a second addition to the solar system, but it was a discovery by sight, not by deduction. the first day of the nineteenth century, january , , was signalised by the discovery of a small planet by piazzi. the new object was lost for a time, but it was redetected on december of the same year. this planet lay between the orbits of mars and jupiter--a region in which many hundreds of other small bodies have since been found. the first of these "+minor planets+" was called ceres; the next three to be discovered are known as pallas, juno, and vesta. beside these four, two others are of special interest: one, eros, which comes nearer the sun than the orbit of mars--indeed at some oppositions it approaches the earth within , , miles, and is therefore, next to the moon, our nearest neighbour in space; the other, achilles, moves at a distance from the sun equal to that of jupiter. ceres is much the largest of all the minor planets; indeed is larger than all the others put together. yet the earth exceeds ceres times in volume, and times in mass, and the entire swarm of minor planets, all put together, would not equal in total volume one-fiftieth part of the moon. the search for these small bodies rendered it necessary that much fuller and more accurate maps of the stars should be made than had hitherto been attempted, and this had an important bearing on the next great event in the development of gravitational astronomy. { } the movements of uranus soon gave rise to difficulties. it was found impossible, satisfactorily, to reconcile the earlier and later observations, and in the tables of uranus, published by bouvard in , the earlier observations were rejected. but the discrepancies between the observed and calculated places for the planet soon began to reappear and quickly increase, and the suggestion was made that these discrepancies were due to an attraction exercised by some planet as yet unknown. thus mrs. somerville in a little book on the connection of the physical sciences, published in , wrote, "possibly it (that is, uranus) may be subject to disturbances from some unseen planet revolving about the sun beyond the present boundaries of our system. if, after the lapse of years, the tables formed from a combination of numerous observations should still be inadequate to represent the motions of uranus, the discrepancies may reveal the existence, nay, even the mass and orbit of a body placed for ever beyond the sphere of vision." in john c. adams, who had just graduated as senior wrangler at cambridge, proceeded to attack the problem of determining the position, orbit, and mass of the unknown body by which on this assumption uranus was disturbed, from the irregularities evident in the motion of that planet. the problem was one of extraordinary intricacy, but by september adams had obtained a first solution, which, he submitted to airy, the astronomer royal. as, however, he neglected to reply to some inquiries made by airy, no search for the new planet was instituted in england until the results of a new and independent worker had been published. the same problem had been attacked by a well-known and very gifted french mathematician, u. j. j. leverrier, and { } in june he published his position for the unseen planet, which proved to be in close accord with that which adams had furnished to airy nine months before. on this airy stirred up challis, the director of the cambridge observatory, which then possessed the most powerful telescope in england, to search for the planet, and challis commenced to make charts, which included more than stars, in order to make sure that the stranger should not escape his net. leverrier, on the other hand, communicated his result to the berlin observatory, where they had just received some of the star charts prepared by dr. bremiker in connection with the search for minor planets. the berlin observer, dr. galle, had therefore nothing to do but to compare the stars in the field, upon which he turned his telescope, with those shown on the chart; a star not in the chart would probably be the desired stranger. he found it, therefore, on the very first evening, september , , within less than four diameters of the moon of the predicted place. the same object had been observed by challis at cambridge on august and , but he was deferring the reduction of his observations until he had completed his scrutiny of the zone, and hence had not recognised it as different from an ordinary star. this discovery of the planet now known as neptune, which had been disturbing the movement of uranus, has rightly been regarded as the most brilliant triumph of gravitational astronomy. it was the legitimate crown of that long intellectual struggle which had commenced more than years earlier, when the first greek astronomers set themselves to unravel the apparently aimless wanderings of the planets in the assured faith that they would find them obedient unto law. { } but of what use was all this effort? what is the good of astronomy? the question is often asked, but it is the question of ignorance. the use of astronomy is the development which it has given to the intellectual powers of man. directly the problem of the planetary motions was first attempted, it became necessary to initiate mathematical processes in order to deal with it, and the necessity for the continued development of mathematics has been felt in the same connection right down to the present day. when the greek astronomers first began their inquiries into the planetary movements they hoped for no material gain, and they received none. they laboured; we have entered into their labours. but the whole of our vast advances in mechanical and engineering science--advances which more than anything else differentiate this our present age from all those which have preceded it--are built upon our command of mathematics and our knowledge of the laws of motion--a command and a knowledge which we owe directly to their persevering attempts to advance the science of astronomy, and to follow after knowledge, not for any material rewards which she had to offer, but for her own sake. { } chapter iv astronomical measurements the old proverb has it that "science is measurement," and of none of the sciences is this so true as of the science of astronomy. indeed the measurement of time by observation of the movements of the heavenly bodies was the beginning of astronomy. the movement of the sun gave the day, which was reckoned to begin either at sunrise or at sunset. the changes of the moon gave the month, and in many languages the root meaning of the word for _moon_ is "measurer." the apparent movement of the sun amongst the stars gave a yet longer division of time, the year, which could be determined in a number of different ways, either from the sun alone, or from the sun together with the stars. a very simple and ancient form of instrument for measuring this movement of the sun was the obelisk, a pillar with a pointed top set up on a level pavement. such obelisks were common in egypt, and one of the most celebrated, known as cleopatra's needle, now stands on the thames embankment. as the sun moved in the sky, the shadow of the pillar moved on the pavement, and midday, or noon, was marked when the shadow was shortest. the length of the shadow at noon varied from day to day; it was shortest at mid-summer, and longest at midwinter, _i.e._ at the summer and winter solstices. twice in the year the shadow of the pillar pointed due west at sunrise, and due east at { } sunset--that is to say, the shadow at the beginning of the day was in the same straight line as at its end. these two days marked the two equinoxes of spring and autumn. the obelisk was a simple means of measuring the height and position of the sun, but it had its drawbacks. the length of the shadow and its direction did not vary by equal amounts in equal times, and if the pavement upon which the shadow fell was divided by marks corresponding to equal intervals of time for one day of the year, the marks did not serve for all other days. but if for the pillar a triangular wall was substituted--a wall rising from the pavement at the south and sloping up towards the north at such an angle that it seemed to point to the invisible pivot of the heavens, round which all the stars appeared to revolve--then the shadow of the wall moved on the pavement in the same manner every day, and the pavement if marked to show the hours for one day would show them for any day. the sundials still often found in the gardens of country houses or in churchyards are miniatures of such an instrument. but the greek astronomers devised other and better methods for determining the positions of the heavenly bodies. obelisks or dials were of use only with the sun and moon which cast shadows. to determine the position of a star, "sights" like those of a rifle were employed, and these were fixed to circles which were carefully divided, generally into "degrees." as there are days in a year, and as the sun makes a complete circuit of the zodiac in this time, it moves very nearly a degree in a day. the twelve signs of the zodiac are therefore each ° in length, and each { } takes on the average a double-hour to rise or set. while the sun and moon are each about half a degree in diameter, _i.e._ about one-sixtieth of the length of a sign, and therefore take a double-minute to rise or set. each degree of a circle is therefore divided into minutes, and each minute may be divided into seconds. as the sun or moon are each about half a degree, or, more exactly, minutes in diameter, it is clear that, so long as astronomical observations were made by the unaided sight, a minute of arc (written ') was the smallest division of the circle that could be used. a cord or wire can indeed be detected when seen projected against a moderately bright background if its thickness is a second of arc (written ")--a sixtieth of a minute--but the wire is merely perceived, not properly defined. tycho brahe had achieved the utmost that could be done by the naked eye, and it was the certainty that he could not have made a mistake in an observation in the place of the planet mars amounting to as much as minutes of arc--that is to say, of a quarter the apparent diameter of the moon--that made kepler finally give up all attempts to explain the planetary movements on the doctrine of circular orbits and to try movements in an ellipse. but a contemporary of kepler, as gifted as he was himself, but in a different direction, was the means of increasing the observing power of the astronomer. galileo galilei ( - ), of a noble florentine family, was appointed lecturer in mathematics at the university of pisa. here he soon distinguished himself by his originality of thought, and the ingenuity and decisiveness of his experiments. up to that time it had been taught that of { } two bodies the heavier would fall to the ground more quickly than the lighter. galileo let fall a -lb. weight and a -lb. weight from the top of the leaning tower, and both weights reached the pavement together. by this and other ingenious experiments he laid a firm foundation for the science of mechanics, and he discovered the laws of motion which newton afterwards formulated. he heard that an instrument had been invented in holland which seemed to bring distant objects nearer, and, having himself a considerable knowledge of optics, it was not long before he made himself a little telescope. he fixed two spectacle glasses, one for long and one for short sight, in a little old organ-pipe, and thus made for himself a telescope which magnified three times. before long he had made another which magnified thirty times, and, turning it towards the heavenly bodies, he discovered dark moving spots upon the sun, mountains and valleys on the moon, and four small satellites revolving round jupiter. he also perceived that venus showed "+phases+"--that is to say, she changed her apparent shape just as the moon does--and he found the milky way to be composed of an immense number of small stars. these discoveries were made in the years - . a telescope consists in principle of two parts--an +object-glass+, to form an image of the distant object, and an +eye-piece+, to magnify it. the rays of light from the heavenly body fall on the object-glass, and are so bent out of their course by it as to be brought together in a point called the focus. the "light-gathering power" of the telescope, therefore, depends upon the size of the object-glass, and is proportional to its area. but the size of the image depends upon the focal length of the telescope, _i.e._ upon the distance that the focus { } is from the object-glass. thus a small disc, an inch in diameter--such as a halfpenny--will exactly cover the full moon if held up nine feet away from the eye; and necessarily the image of the full moon made by an object-glass of nine-feet focus will be an inch in diameter. the eye-piece is a magnifying-glass or small microscope applied to this image, and by it the image can be magnified to any desired amount which the quality of the object-glass and the steadiness of the atmosphere may permit. this little image of the moon, planet, or group of stars lent itself to measurement. a young english gentleman, gascoigne, who afterwards fell at the battle of marston moor, devised the "micrometer" for this purpose. the micrometer usually has two frames, each carrying one or more very thin threads--usually spider's threads--and the frames can be moved by very fine screws, the number of turns or parts of a turn of each screw being read off on suitable scales. by placing one thread on the image of one star, and the other on the image of another, the apparent separation of the two can be readily and precisely measured. within the last thirty years photography has immensely increased the ease with which astronomical measurements can be made. the sensitive photographic plate is placed in the focus of the telescope, and the light of sun, moon, or stars, according to the object to which the telescope is directed, makes a permanent impression on the plate. thus a picture is obtained, which can be examined and measured in detail at any convenient time afterwards; a portion of the heavens is, as it were, brought actually down to the astronomer's study. it was long before this great advance was effected. { } the first telescopes were very imperfect, for the rays of different colour proceeding from any planet or star came to different foci, so that the image was coloured, diffused, and ill-defined. the first method by which this difficulty was dealt with was by making telescopes of enormously long focal length; , , or feet were not uncommon, but these were at once cumbersome and unsteady. sir isaac newton therefore discarded the use of object-glasses, and used curved mirrors in order to form the image in the focus, and succeeded in making two telescopes on this principle of reflection. others followed in the same direction, and a century later sir william herschel was most skilful and successful in making "+reflectors+," his largest being feet in focal length, and thus giving an image of the moon in its focus of nearly -½ inches diameter. but in chester moor hall found that by combining two suitable lenses together in the object-glass he could get over most of the colour difficulty, and in the optician dollond began to make object-glasses that were almost free from the colour defect. from that time onward the manufacture of "+refractors+," as object-glass telescopes are called, has improved; the glass has been made more transparent and more perfect in quality, and larger in size, and the figure of the lens improved. the largest refractor now in use is that of the yerkes observatory, wisconsin, u.s.a., and is inches in aperture, with a focal length of feet, so that the image of the moon in its focus has a diameter of more than inches. at present this seems to mark the limit of size for refractors, and the difficulty of getting good enough glass for so large a lens is very great indeed. reflectors have therefore come again into favour, as mirrors can be made larger { } than any object-glass. thus lord rosse's great telescope was feet in diameter; and the most powerful telescope now in action is the great -foot mirror of the mt. wilson observatory, california, with a focal length, as sometimes used, of feet. thus its light-gathering power is about , times that of the unaided eye, and the full moon in its focus is inches in diameter; such is the enormous increase to man's power of sight, and consequently to his power of learning about the heavenly bodies, which the development of the telescope has afforded to him. the measurement of time was the first purpose for which men watched the heavenly bodies; a second purpose was the measurement of the size of the earth. if at one place a star was observed to pass exactly overhead, and if at another, due south of it, the same star was observed to pass the meridian one degree north of the zenith, then by measuring the distance between the two places the circumference of the whole earth would be known, for it would be times that amount. in this way the size of the earth was roughly ascertained years before the invention of the telescope. but with the telescope measures of much greater precision could be made, and hence far more difficult problems could be attacked. one great practical problem was that of finding out the position of a ship when out of sight of land. the ancient phoenician and greek navigators had mostly confined themselves to coasting voyages along the shores of the mediterranean sea, and therefore the quick recognition of landmarks was the first requisite for a good sailor. but when, in , columbus had brought a new continent to light, and long voyages were freely taken across the great oceans, it became an urgent { } necessity for the navigator to find out his position when he had been out of sight of any landmark for weeks. this necessity was especially felt by the nations of western europe, the countries facing the atlantic with the new world on its far-distant other shore. spain, france, england, and holland, all were eager competitors for a grasp on the new lands, and therefore were earnest in seeking a solution of the problem of navigation. the latitude of the ship could be found out by observing the height of the sun at noon, or of the pole star at night, or in several other ways. but the longitude was more difficult. as the earth turns on its axis, different portions of its surface are brought in succession under the sun, and if we take the moment when the sun is on the meridian of any place as its noon, as twelve o'clock for that place, then the difference of longitude between any two places is essentially the difference in their local times. it was possible for the sailor to find out when it was local noon for him, but how could he possibly find out what time it was at that moment at the port from which he had sailed, perhaps several weeks before? the moon and stars supplied eventually the means for giving this information. for the moon moves amongst the stars, as the hand of a clock moves amongst the figures of a dial, and it became possible at length to predict for long in advance exactly where amongst the stars the moon would be, for any given time, of any selected place. when this method was first suggested, however, neither the motion of the moon nor the places of the principal stars were known with sufficient accuracy, and it was to remedy this defect, and put navigation upon { } a sound basis, that charles ii. founded greenwich observatory in the year , and appointed flamsteed the first astronomer royal. in the year maskelyne, the fifth astronomer royal, brought out the first volume of the _nautical almanac_, in which the positions of the moon relative to certain stars were given for regular intervals of greenwich time. much about the same period the problem was solved in another way by the invention of the chronometer, by john harrison, a yorkshire carpenter. the +chronometer+ was a large watch, so constructed that its rate was not greatly altered by heat or cold, so that the navigator had greenwich time with him wherever he went. the new method in the hands of captain cook and other great navigators led to a rapid development of navigation and the discovery of australia and new zealand, and a number of islands in the pacific. the building up of the vast oceanic commerce of great britain and of her great colonial empire, both in north america and in the southern oceans, has arisen out of the work of the royal observatory, greenwich, and has had a real and intimate connection with it. to observe the motions of the moon, sun, and planets, and to determine with the greatest possible precision the places of the stars have been the programme of greenwich observatory from its foundation to the present time. other great national observatories have been copenhagen, founded in ; paris, in ; berlin, in ; st. petersburg, in , superseded by that of pulkowa, in ; and washington, in ; while not a few of the great universities have also efficient observatories connected with them. of the directly practical results of astronomy, the { } promotion of navigation stands in the first rank. but the science has never been limited to merely utilitarian inquiries, and the problem of measuring celestial distances has followed on inevitably from the measurement of the earth. the first distance to be attacked was that of the nearest companion to the earth, _i.e._ the moon. it often happens on our own planet that it is required to find the distance of an object beyond our reach. thus a general on the march may come to a river and need to know exactly how broad it is, that he may prepare the means for bridging it. such problems are usually solved on the following principle. let a be the distant object. then if the direction of a be observed from each of two stations, b and c, and the distance of b from c be measured, it is possible to calculate the distances of a from b and from c. the application of this principle to the measurement of the moon's distance was made by the establishment of an observatory at the cape of good hope, to co-operate with that of greenwich. it is, of course, not possible to see greenwich observatory from the cape, or vice versa, but the stars, being at an almost infinite distance, lie in the same direction from both observatories. what is required then is to measure the apparent distance of the moon from the same stars as seen from greenwich and as seen from the cape, and, the distance apart of the two observatories being known, the distance of the moon can be calculated. this was a comparatively easy problem. the next step in celestial measurement was far harder; it was to find the distance of the sun. the sun is times as far off as the moon, and therefore it seems to be practically in the same direction as seen from each of { } the two observatories, and, being so bright, stars cannot be seen near it in the telescope. but by carefully watching the apparent movements of the planets their _relative_ distances from the sun can be ascertained, and were known long before it was thought possible that we should ever know their real distances. thus venus never appears to travel more than ° ' from the sun. this means that her distance from the sun is a little more than seven-tenths of that of the earth. if, therefore, the distance of one planet from the sun can be measured, or the distance of one planet from the earth, the actual distances of all the planets will follow. we know the proportions of the parts of the solar system, and, if we can fix the scale of one of the parts, we fix the scale of all. it has been found possible to determine the distance of mars, of several of the "minor planets," and especially of eros, a very small minor planet that sometimes comes within , , miles of the earth, or seven times nearer to us than is the sun. from the measures of eros, we have learned that the sun is separated from us by very nearly , , miles--an unimaginable distance. perhaps the nearest way of getting some conception of this vast interval is by remembering that there are only , , seconds of time in a year. if, therefore, an express train, travelling miles an hour--a mile a minute--set out for the sun, and travelled day and night without cease, it would take more than years to accomplish the journey. but this astronomical measure has led on to one more daring still. the earth is on one side of the sun in january, on the other in july. at these two dates, therefore, we are occupying stations , , miles { } apart, and can ascertain the apparent difference in direction of the stars as viewed from the two points but the astonishing result is that this enormous change in the position of the earth makes not the slightest observable difference in the position of most of the stars. a few, a very few, do show a very slight difference. the nearest star to us is about , times as far from us as the sun; this is alpha centauri, the brightest star in the constellation of the centaur and the third brightest star in the sky. sirius, the brightest star, is twice this distance. some forty or fifty stars have had their distances roughly determined; but the stars in general far transcend all our attempts to plumb their distances. but, from certain indirect hints, it is generally supposed that the mass of stars in the milky way are something like , , times as far from us as we are from our sun. thus far, then, astronomy has led us in the direction or measurement. it has enabled us to measure the size of the earth upon which we live, and to find out the position of a ship in the midst of the trackless ocean. it has also enabled us to cast a sounding-line into space, to show how remote and solitary the earth moves through the void, and to what unimaginable lengths the great stellar universe, of which it forms a secluded atom, stretches out towards infinity. { } chapter v the members of the solar system astronomical measurement has not only given us the distances of the various planets from the sun; it has also furnished us, as in the annexed table, with their real diameters, and, as a consequence of the law of gravitation, with their densities and weights, and the force of gravity at their surfaces. and these numerical details are of the first importance in directing us as to the inferences that we ought to draw as to their present physical conditions. the theory of copernicus deprived the earth of its special position as the immovable centre of the universe, but raised it to the rank of a planet. it is therefore a heavenly body, yet needs no telescope to bring it within our ken; bad weather does not hide it from us, but rather shows it to us under new conditions. we find it to be a globe of land and water, covered by an atmosphere in which float changing clouds; we have mapped it, and we find that the land and water are always there, but their relations are not quite fixed; there is give and take between them. we have found of what elements the land and water consist, and how these elements combine with each other or dissociate. in a word, the earth is the heavenly body that we know the best, and with it we must compare and contrast all the others. before the invention of the telescope there were but { } two other heavenly bodies--the sun and the moon--that appeared as orbs showing visible discs, and even in their cases nothing could be satisfactorily made out as to their conditions. now each of the five planets known to the ancients reveals to us in the telescope a measurable disc, and we can detect significant details on their surfaces. the moon is the one object in the heavens which does not disappoint a novice when he first sees it in the telescope. every detail is hard, clear-cut, and sharp; it is manifest that we are looking at a globe, a very rough globe, with hills and mountains, plains and valleys, the whole in such distinct relief that it seems as if it might be touched. no clouds ever conceal its details, no mist ever softens its outlines; there are no half-lights, its shadows are dead black, its high lights are molten silver. certain changes of illumination go on with the advancing age of the moon, as the crescent broadens out to the half, the half to the full, and the full, in its turn, wanes away; but the lunar day is nearly thirty times as long as that of the earth, and these changes proceed but slowly. the full moon, as seen by the naked eye, shows several vague dark spots, which most people agree to fancy as like the eyes, nose, and mouth of a broad, sorrowful face. the ordinary astronomical telescope inverts the image, so the "eyes" of the moon are seen in the lower part of the field of the telescope as a series of dusky plains stretching right across the disc. but in the upper part, near the left-hand corner of the underlip, there is a bright, round spot, from which a number of bright streaks radiate--suggesting a peeled orange with its stalk, and the lines marking the sections radiating from it. this bright spot has been called after the great { } mean distance from sun. period velocity class. name. earth's in millions of revolution. in orbit. eccentricity. distance of miles. in years. miles per = . sec. terrestrial mercury . . . . . planets venus . . . . . earth . . . . . mars . . . . . minor eros . . . . . planets ceres . . . . . achilles . . . . . major jupiter . . . . . planets saturn . . . . . uranus . . . . . neptune . . . . . { } mean diameter. surface. volume. mass. name. symbol. in miles. [earth]= . [earth]= . [earth]= . [earth]= . sun [sun] . . . . moon [moon] . . . . mercury [mercury] . . . . venus [venus] . . . . earth [earth] . . . . mars [mars] . . . . jupiter [jupiter] . . . . saturn [saturn] . . . . uranus [uranus] . . . . neptune [neptune] . . . . { } light gravity. and heat albedo; density. fall in received _i.e._ re- [earth] water [earth] feet per from sun. time of rotation flecting name. = . = . = . sec. [earth]= . on axis. power. d. h. m. sun . . . . ... ± ... moon . . . . . . d. h. m. s. mercury . . . . . (?) . venus . . . . . (?) . earth . . . . . . (?) mars . . . . . . h. m. jupiter . . . . . ± . saturn . . . . . ± . uranus . . . . . (?) . neptune . . . . . (?) . { } danish astronomer, "tycho," and is one of the most conspicuous objects of the full moon. the contrasts of the moon are much more pronounced when she is only partly lit up. then the mountains and valleys stand out in the strongest relief, and it becomes clear that the general type of formation on the moon is that of rings--rings of every conceivable size, from the smallest point that the telescope can detect up to some of the great dusky plains themselves, hundreds of miles in diameter. these rings are so numerous that galileo described the moon as looking as full of "eyes" as a peacock's tail. the "right eye" of the moonface, as we see it in the sky, is formed by a vast dusky plain, nearly as large as france and germany put together, to which has been given the name of the "sea of rains" (_mare imbrium_), and just below this (as seen in the telescope) is one of the most perfect and beautiful of all the lunar rings--a great ring-plain, miles in diameter, called after the thinker who revolutionised men's ideas of the solar system, "copernicus." "copernicus," like "tycho," is the centre of a set of bright streaks; and a neighbouring but smaller ring, bearing the great name of "kepler," stands in a like relation to another set. the most elevated region of the moon is immediately in the neighbourhood of the great "stalk of the orange," "tycho." here the rings are crowded together as closely as they can be packed; more closely in many places, for they intrude upon and overlap each other in the most intricate manner. a long chain of fine rings stretches from this disturbed region nearly to the centre of the disc, where the great alexandrian astronomer is commemorated by a vast walled plain, { } considerably larger than the whole of wales, and known as "ptolemæus." the distinctness of the lunar features shows at once that the moon is in an altogether different condition from that of the earth. here the sky is continually being hidden by cloud, and hence the details of the surface of the earth as viewed from any other planet must often be invisible, and even when actual cloud is absent there is a more permanent veil of dust, which must greatly soften and confuse terrestrial outlines. the clearness, therefore, with which we perceive the lunar formations proves that there is little or no atmosphere there. nor is there any sign upon it of water, either as seas or lakes or running streams. yet the moon shows clearly that in the past it has gone through great and violent changes. the gradation is so complete from the little craterlets, which resemble closely, in form and size, volcanic craters on the earth, up to the great ring-plains, like "copernicus" or "tycho," or formations larger still, that it seems natural to infer not only that the smaller craters were formed by volcanic eruption, like the similar objects with which we are acquainted on our own earth, but that the others, despite their greater sizes, had a like origin. in consequence of the feebler force of gravity on the moon, the same explosive force there would carry the material of an eruption much further than on the earth. the darker, low-lying districts of the moon give token of changes of a different order. it is manifest that the material of which the floors of these plains is composed has invaded, broken down, and almost submerged many of the ring-formations. sometimes half { } of a ring has been washed away; sometimes just the outline of a ring can still be traced upon the floor of the sea; sometimes only a slight breach has been made in the wall. so it is clear that the moon was once richer in the great crater-like formations than it is to-day, but a lava-flood has overflowed at least one-third of its area. more recent still are the bright streaks, or rays, which radiate in all directions from "tycho," and from some of the other ring-plains. it is evident from these different types of structure on the moon, and from the relations which they bear to each other, that the lunar surface has passed through several successive stages, and that its changes tended, on the whole, to diminish in violence as time went on; the minute crater pits with which the surface is stippled having been probably the last to form. but the years during which the moon has been watched with the telescope have afforded no trace of any continuance of these changes. she has had a stormy and fiery past; but nothing like the events of those bygone ages disturbs her serenity to-day. and yet we must believe that change does take place on the moon even now, because during the hours of its long day the sun beats down with full force on the unprotected surface, and during the equally long night that surface is exposed to the cold of outer space. every part of the surface must be exposed in turn to an extreme range of temperature, and must be cracked, torn, and riven by alternate expansion and contraction. apart from this slow, wearing process, and a very few rather doubtful cases in which a minute alteration of some surface detail has been suspected, our sister planet, the moon, shows herself as changeless and inert, without any appreciable trace of air or water or any sign { } of life--a dead world, with all its changes and activities in the past. mars, after the moon, is the planet whose surface we can study to best advantage. its orbit lies outside that of the earth, so that when it is nearest to us it turns the same side to both the sun and earth, and we see it fully illuminated. mercury and venus, on the contrary, when nearest us are between us and the sun, and turn their dark sides to us. when fully illuminated they are at their greatest distance, and appear very small, and, being near the sun, are observed with difficulty. these three are intermediate in size between the moon and the earth. in early telescopic days it was seen that mars was an orange-coloured globe with certain dusky markings upon it, and that these markings slowly changed their place--that, in short, it was a world rotating upon its axis, and in a period not very different from that of the earth. the rotation period of mars has indeed been fixed to the one-hundredth part of a second of time; it is h. m. . s. and this has been possible because some of the dusky spots observed in the seventeenth century can be identified now in the twentieth. some of the markings on mars, like our own continents and seas, and like the craters on the moon, are permanent features; and many charts of the planet have been constructed. other markings are variable. since the planet rotates on its axis, the positions of its poles and equator are known, its equator being inclined to its orbit at an angle of ° ', while the angle in the case of the earth is ° '. the times when its seasons begin and end are therefore known; and it is found that the spring of its northern hemisphere lasts of our { } days, the summer , the autumn , and the winter . round the pole in winter a broad white cap forms, which begins to shrink as spring comes on, and may entirely disappear in summer. no corresponding changes have been observed on the moon, but it is easy to find an analogy to them on the earth. round both our poles a great cap of ice and snow is spread--a cap which increases in size as winter comes on, and diminishes with the advance of summer--and it seems a reasonable inference to suppose that the white polar caps of mars are, like our own, composed of ice and snow. from time to time indications have been observed of the presence on mars of a certain amount of cloud. familiar dark markings have, for a short time, been interrupted, or been entirely hidden, by white bands, and have recovered their ordinary appearance later. with rotation on its axis and succession of seasons, with atmosphere and cloud, with land and water, with ice and snow, mars would seem to be a world very similar to our own. this was the general opinion up to the year , when schiaparelli announced that he had discovered a number of very narrow, straight, dark lines on the planet--lines to which he gave the name of "canali"--that is, "channels." this word was unfortunately rendered into english by the word "+canals+," and, as a canal means a waterway artificially made, this mistranslation gave the idea that mars was inhabited by intelligent beings, who had dug out the surface of the planet into a network of canals of stupendous length and breadth. the chief advocate of this theory is lowell, an american observer, who has given very great attention { } to the study of the planet during the last seventeen years. his argument is that the straight lines, the canals, which he sees on the planet, and the round dots, the "+oases+," which he finds at their intersections, form a system so obviously _un_natural, that it must be the work of design--of intelligent beings. the canals are to him absolutely regular and straight, like lines drawn with ruler and pen-and-ink, and the oases are exactly round. but, on the one hand, the best observers, armed with the most powerful telescopes, have often been able to perceive that markings were really full of irregular detail, which lowell has represented as mere hard straight lines and circular dots, and, on the other hand, the straight line and the round dot are the two geometric forms which all very minute objects must approach in appearance. that we cannot see irregularities in very small and distant objects is no proof at all that irregularities do not exist in them, and it has often happened that a marking which appeared a typical "canal" when mars was at a great distance lost that appearance when the planet was nearer. astronomers, therefore, are almost unanimous that there is no reason for supposing that any of the details that we see on the surface of mars are artificial in their origin. and indeed the numerical facts that we know about the planet render it almost impossible that there should be any life upon it. if we turn to the table, we see that in size, volume, density, and force of gravity at its surface, mars lies between the moon and the earth, but is nearer the moon. this has an important bearing as to the question of the planet's atmosphere. on the earth we pass through half the atmosphere by ascending a mountain { } that is three and a third miles in height; on mars we should have to ascend nearly nine miles. if the atmospheric pressure at the surface of mars were as great as it is at the surface of the earth, his atmosphere would be far deeper than ours and would veil the planet more effectively. but we see the surface of mars with remarkable distinctness, almost as clearly, when its greater distance is allowed for, as we see the moon. it is therefore accepted that the atmospheric pressure at the surface of mars must be very slight, probably much less than at the top of our very highest mountains, where there is eternal snow, and life is completely absent. but mars compares badly with the earth in another respect. it receives less light and heat from the sun in the proportion of three to seven. this we may express by saying that mars, on the whole, is almost as much worse off than the earth as a point on the arctic circle is worse off than a point on the equator. the mean temperature of the earth is taken as about ° of the fahrenheit thermometer (say, ° cent.); the mean temperature of mars must certainly be considerably below freezing-point, probably near ° f. here on our earth the boiling-point of water is °, and, since the mean temperature is ° and water freezes at °, it is normally in the liquid state. on mars it must normally be in the solid state--ice, snow, or frost, or the like. but with so rare an atmosphere water will boil at a low temperature, and it is not impossible that under the direct rays of the sun--that is to say, at midday of the torrid zone of mars--ice may not only melt, but water boil by day, condensing and freezing again during the night. newcomb, the foremost astronomer of his day, concluded "that during { } the night of mars, even in the equatorial regions, the surface of the planet probably falls to a lower temperature than any we ever experienced on our globe. if any water exists, it must not only be frozen, but the temperature of the ice must be far below the freezing point.... the most careful calculation shows that if there are any considerable bodies of water on our neighbouring planet, they exist in the form of ice, and can never be liquid to a depth of more than one or two inches, and that only within the torrid zone and during a few hours each day." with regard to the snow caps of mars, newcomb thought it not possible that any considerable fall of snow could ever take place. he regarded the white caps as simply due to a thin deposit of hoar frost, and it cannot be deemed wonderful that such should gradually disappear, when it is remembered that each of the two poles of mars is in turn presented to the sun for more than consecutive days. newcomb's conclusion was: "thus we have a kind of martian meteorological changes, very slight indeed, and seemingly very different from those of our earth, but yet following similar lines on their small scale. for snowfall substitute frostfall; instead of (the barometer reading) feet or inches say fractions of a millimetre, and instead of storms or wind substitute little motions of an air thinner than that on the top of the himalayas, and we shall have a general description of martian meteorology." we conclude, then, that mars is not so inert a world as the moon, but, though some slight changes of climate or weather take place upon it, it is quite unfitted for the nourishment and development of the different forms of organic life. of mercury we know very little. it is smaller than mars but larger than the moon, but it differs from them { } both in that it is much nearer the sun, and receives, therefore, many times the light and heat, surface for surface. we should expect, therefore, that water on mercury would exist in the gaseous state instead of in the solid state as on mars. the little planet reflects the sunlight only feebly, and shows no evidence of cloud. a few markings have been made out on its surface, and the best observers agree that it appears to turn the same face always to the sun. this would imply that the one hemisphere is in perpetual darkness and cold, the other, exposed to an unimaginable fiery heat. venus is nearly of the same size as the earth, and the conditions as to the arrangement of its atmosphere, the force of gravity at its surface, must be nearly the same as on our own world. but we know almost nothing of the details of its surface; the planet is very bright, reflecting fully seven-tenths of the sunlight that falls upon it. it would seem that, in general, we see nothing of the actual details of the planet, but only the upper surface of a very cloudy atmosphere. owing to the fact that venus shows no fixed definite marking that we can watch, it is still a matter of controversy as to the time in which it rotates upon its axis. schiaparelli and some other observers consider that it rotates in the same time as it revolves round the sun. others believe that it rotates in a little less than twenty-four hours. if this be so, and there is any body in the solar system other than the earth, which is adapted to be the home of life, then the planet venus is that one. the sun, like the moon, presents a visible surface to the naked eye, but one that shows no details. in the telescope the contrast between it and the moon is very great, and still greater is the contrast which is brought { } out by the measurements of its size, volume, and weight. but the really significant difference is that the sun is a body giving out light and heat, not merely reflecting them. without doubt this last difference is connected most closely with the difference in size. the moon is cold, dead, unchanging, because it is a small world; the sun is bright, fervent, and undergoes the most violent change, because it is an exceedingly large world. the two bodies--the sun and moon--appear to the eye as being about the same size, but since the sun is times as far off as the moon it must be times the diameter. that means that it has times , or , times the surface and times times , or , , times the volume. the sun and moon, therefore, stand at the very extremes of the scale. the heat of the sun is so great that there is some difficulty in observing it in the telescope. it is not sufficient to use a dark glass in order to protect the eye, unless the telescope be quite a small one. some means have to be employed to get rid of the greater part of the heat and light. the simplest method of observing is to fix a screen behind the eyepiece of a telescope and let the image of the sun be projected upon the screen, or the sensitive plate may be substituted for the screen, and a photograph obtained, which can be examined at leisure afterwards. as generally seen, the surface of the sun appears to be mottled all over by a fine irregular stippling. this stippling, though everywhere present, is not very strongly marked, and a first hasty glance might overlook it. from time to time, however, dark spots are seen, of ever-changing form and size. by watching these, galileo proved that the sun rotated on its axis in a little more than twenty-five days, and in the { } nineteenth century schwabe proved that the sunspots were not equally large and numerous at all times, but that there was a kind of cycle of a little more than eleven years in average length. at one time the sun would be free from spots; then a few small ones would appear; these would gradually become larger and more numerous; then a decline would follow, and another spotless period would succeed about eleven years after the first. as a rule, the increase in the spots takes place more quickly than the decline. most of the spot-groups last only a very few days, but about one group in four lasts long enough to be brought round by the rotation of the sun a second time; in other words, it continues for about a month. in a very few cases spots have endured for half a year. an ordinary form for a group of spots is a long stream drawn out parallel to the sun's equator, the leading spot being the largest and best defined. it is followed by a number of very small irregular and ill-developed spots, and the train is brought up by a large spot, sometimes even larger than the leader, but by no means so regular in form or so well defined. the leading spot for a short time moves forward much faster than its followers, at a speed of about miles per day. the small middle spots then gradually die out, or rather seem to be overflowed by the bright material of the solar surface, the "+photosphere+," as it is called; the spot in the rear breaks up a little later, and the leader, which is now almost circular, is left alone, and may last in this condition for some weeks. finally, it slowly contracts or breaks up, and the disturbance comes to an end. this is the course of development of many long-lived spot-groups, but all do not conform to the same type. { } the very largest spots are indeed usually quite different in their appearance and history. in size, sunspots vary from the smallest dot that can be discovered in the telescope up to huge rents with areas that are to be counted by thousands of millions of square miles; the great group of february had an area of , , , square miles, a thousand times the area of europe. closely associated with the _maculæ_, as the spots were called by the first observers, are the "+faculæ+"--long, branching lines of bright white light, bright as seen even against the dazzling background of the sun itself, and looking like the long lines of foam of an incoming tide. these are often associated with the spots; the spots are formed between their ridges, and after a spot-group has disappeared the broken waves of faculæ will sometimes persist in the same region for quite a long time. the faculæ clearly rise above the ordinary solar surface; the spots as clearly are depressed a little below it; because from time to time we see the bright material of the surface pour over spots, across them, and sometimes into them. but there is no reason to believe that the spots are deep, in proportion either to the sun itself or even to their own extent. sunspots are not seen in all regions of the sun. it is very seldom that they are noted in a higher solar latitude than °, the great majority of spots lying in the two zones between ° and ° latitude on either side of the equator. faculæ, on the other hand, though most frequent in the spot zones, are observed much nearer the two poles. it is very hard to find analogies on our earth for sunspots and for their peculiarities of behaviour. some { } of the earlier astronomers thought they were like terrestrial volcanoes, or rather like the eruptions from them. but if there were a solid nucleus to the sun, and the spots were eruptions from definite areas of the nucleus, they would all give the same period of rotation. but sunspots move about freely on the solar surface, and the different zones of that surface rotate in different times, the region of the equator rotating the most quickly. this alone is enough to show that the sun is essentially not a solid body. yet far down below the photosphere something approaching to a definite structure must already be forming. for there is a well-marked progression in the zones of sunspots during the eleven-year cycle. at a time when spots are few and small, known as +the sunspot minimum+, they begin to be seen in fairly high latitudes. as they get more numerous, and many of them larger, they frequent the medium zones. when the sun is at its greatest activity, known as +the sunspot maximum+, they are found from the highest zone right down to the equator. then the decline sets in, but it sets in first in the highest zones, and when the time of minimum has come again the spots are close to the equator. before these have all died away, a few small spots, the heralds of a new cycle of activity, begin to appear in high latitudes. this law, called after spÖrer, its discoverer, indicates that the origin and source of sunspot activity lie within the sun. at one time it was thought that sunspots were due to some action of jupiter--for jupiter moves round the sun in . years, a period not very different from the sunspot cycle--or to some meteoric stream. but spörer's law could not be imposed by some influence from without. still sunspots, once formed, may be influenced by the earth, and perhaps by other { } planets also, for mrs. walter maunder has shown that the numbers and areas of spots tend to be smaller on the western half of the disc, as seen from the earth, than on the eastern, while considerably more groups come into view at the east edge of the sun than pass out of view at the west edge, so that it would appear as if the earth had a damping effect upon the spots exposed to it. but the sun is far greater than it ordinarily appears to us. twice every year, and sometimes oftener, the moon, when new, comes between the earth and the sun, and we have an +eclipse of the sun+, the dark body of the moon hiding part, or all, of the greater light. the sun and moon are so nearly of the same apparent size that an eclipse of the sun is total only for a very narrow belt of the earth's surface, and, as the moon moves more quickly than the sun, the eclipse only remains total for a very short time--seven minutes at the outside, more usually only two or three. north or south of that belt the moon is projected, so as to leave uncovered a part of the sun north or south of the moon. a total eclipse, therefore, is rare at any particular place, and if a man were able to put himself in the best possible position on each occasion, it would cost him thirty years to secure an hour's accumulated duration. eclipses of the moon are visible over half the world at one time, for there is a real loss to the moon of her light. her eclipses are brought about when, in her orbit, she passes behind the earth, and the earth, being between the sun and the moon, cuts off from the latter most of the light falling upon her; not quite all; a small portion reaches her after passing through the thickest part of the earth's atmosphere, so that the { } moon in an eclipse looks a deep copper colour, much as she does when rising on a foggy evening. total eclipses of the sun have well repaid all the efforts made to observe them. it is a wonderful sight to watch the blackness of darkness slowly creeping over the very fountain of light until it is wholly and entirely hidden; to watch the colours fade away from the landscape and a deathlike, leaden hue pervade all nature, and then to see a silvery, star-like halo, flecked with bright little rose-coloured flames, flash out round the black disc that has taken the place of the sun. these rose-coloured flames are the solar "+prominences+," and the halo is the "+corona+," and it is to watch these that astronomers have made so many expeditions hither and thither during the last seventy years. the "prominences," or red flames, can be observed, without an eclipse, by means of the spectroscope, but, as the work of the spectroscope is to form the subject of another volume of this series, it is sufficient to add here that the prominences are composed of various glowing gases, chiefly of hydrogen, calcium, and helium. these and other gases form a shell round the sun, about miles in depth, to which the name "+chromosphere+" has been given. it is out of the chromosphere that the prominences arise as vast irregular jets and clouds. ordinarily they do not exceed or thousand miles in height, but occasionally they extend for or even thousand miles from the sun. their changes are as remarkable as their dimensions; huge jets of or thousand miles have been seen to form, rise, and disappear within an hour or less, and movements have been chronicled of or miles in a single second of time. prominences are largest and most frequent when { } sunspots and faculæ are most frequent, and fewest when those are fewest. the corona, too, varies with the sunspots. at the time of maximum the corona sends forth rays and streamers in all directions, and looks like the conventional figure of a star on a gigantic scale. at minimum the corona is simpler in form, and shows two great wings, east and west, in the direction of the sun's equator, and round both of his poles a number of small, beautiful jets like a crest of feathers. some of the streamers or wings of the corona have been traced to an enormous distance from the sun. mrs. walter maunder photographed one ray of the corona of to a distance of millions of miles. langley, in the clear air of pike's peak, traced the wings of the corona of with the naked eye to nearly double this distance. but the rapid changes of sunspots and the violence of some of the prominence eruptions are but feeble indications of the most wonderful fact concerning the sun, _i.e._ the enormous amount of light and heat which it is continually giving off. here we can only put together figures which by their vastness escape our understanding. sunlight is to moonlight as , is to , so that if the entire sky were filled up with full moons, they would not give us a quarter as much light as we derive from the sun. the intensity of sunlight exceeds by far any artificial light; it is times as bright as the calcium light, and three or four times as bright as the brightest part of the electric arc light. the amount of heat radiated by the sun has been expressed in a variety of different ways; c. a. young very graphically by saying that if the sun were encased in a shell of ice feet deep, its heat would melt the shell in one minute, and that if a bridge of ice could be { } formed from the earth to the sun, -½ miles square in section and millions of miles long, and the entire solar radiation concentrated upon it, in one second the ice would be melted, in seven more dissipated into vapour. the earth derives from the sun not merely light and heat, but, by transformation of these, almost every form of energy manifest upon it; the energy of the growth of plants, the vital energy of animals, are only the energy received from the sun, changed in its expression. the question naturally arises, "if the sun, to which the earth is indebted for nearly everything, passes through a change in its activity every eleven years or so, how is the earth affected by it?" it would seem at first sight that the effect should be great and manifest. a sunspot, like that of february , one thousand times as large as europe, into which worlds as large as our earth might be poured, like peas into a saucer, must mean, one might think, an immense falling off of the solar heat. yet it is not so. for even this great sunspot was but small as compared with the sun as a whole. had it been dead black, it would have stopped out much less than per cent. of the sun's heat; and even the darkest sunspot is really very bright. and the more spots there are, the more numerous and brighter are the faculæ; so that we do not know certainly which of the two phases, maximum or minimum, means the greater radiation. if the weather on the earth answers to the sunspot cycle, the connection is not a simple one; as yet no connection has been proved. thus two of the worst and coldest summers experienced in england fell the one in , the other in , _i.e._ at { } maximum and minimum respectively. so, too, the hot summers of and were also, the one at maximum and the other at minimum; and ordinary average years have fallen at both the phases just the same. yet there is an answer on the part of the earth to these solar changes. the earth itself is a kind of magnet, possessing a magnetism of which the intensity and direction is always changing. to watch these changes, very sensitive magnets are set up, and a slight daily to-and-fro swing is noticed in them; this swing is more marked in summer than in winter, but it is also more marked at times of the sunspot maximum than at minimum, showing a dependence upon the solar activity. yet more, from time to time the magnetic needle undergoes more or less violent disturbance; in extreme cases the electric telegraph communication has been disturbed all over the world, as on september , , when the submarine cables ceased to carry messages for several hours. in most cases when such a "magnetic storm" occurs, there is an unusually large or active spot on the sun. the writer was able in to further prove that such "storms" have a marked tendency to recur when the same longitude of the sun is presented again towards the earth. thus in february , when a very large spot was on the sun, a violent magnetic storm broke out. the spot passed out of sight and the storm ceased, but in the following month, when the spot reached exactly the same apparent place on the sun's disc, the storm broke out again. such magnetic disturbances are therefore due to streams of particles driven off from limited areas of the sun, probably in the same way that the long, { } straight rays of the corona are driven off. such streams of particles, shot out into space, do not spread out equally in all directions, like the rays of light and heat, but are limited in direction, and from time to time they overtake the earth in its orbit, and, striking it, cause a magnetic storm, which is felt all over the earth at practically the same moment. jupiter is, after the sun, much the largest member of the solar system, and it is a peculiarly beautiful object in the telescope. even a small instrument shows the little disc striped with many delicately coloured bands or belts, broken by white clouds and dark streaks, like a "windy sky" at sunset. and it changes while being watched, for, though , , miles away from us, it rotates so fast upon its axis that its central markings can actually be seen to move. this rapid rotation, in less than ten hours, is the most significant fact about jupiter. for different spots have different rotation periods, even in the same latitude, proving that we are looking down not upon any solid surface of jupiter, but upon its cloud envelope--an envelope swept by its rapid rotation and by its winds into a vast system of parallel currents. one object on jupiter, the great "+red spot+," has been under observation since , and possibly for years before that. it is a large, oval object fitted in a frame of the same shape. the spot itself has often faded and been lost since , but the frame has remained. the spot is in size and position relative to jupiter much as australia is to the earth, but while australia moves solidly with the rest of the earth in the daily rotation, neither gaining on south america nor losing on africa, the red spot on jupiter sees many other spots and clouds pass it by, and does not even { } retain the same rate of motion itself from one year to another. no other marking on jupiter is so permanent as this. from time to time great round white clouds form in a long series as if shot up from some eruption below, and then drawn into the equatorial current. from time to time the belts themselves change in breadth, in colour, and complexity. jupiter is emphatically the planet of change. and such change means energy, especially energy in the form of heat. if jupiter possessed no heat but that it derived from the sun, it would be colder than mars, and therefore an absolutely frozen globe. but these rushing winds and hurrying clouds are evidences of heat and activity--a native heat much above that of our earth. while mars is probably nearer to the moon than to the earth in its condition, jupiter has probably more analogies with the sun. the one unrivalled distinction of saturn is its ring. nothing like this exists elsewhere in the solar system. everywhere else we see spherical globes; this is a flat disc, but without its central portion. it surrounds the planet, lying in the plane of its equator, but touches it nowhere, a gap of miles intervening. it appears to be circular, and is , miles in breadth. yet it is not, as it appears to be, a flat continuous surface. it is in reality made up of an infinite number of tiny satellites, mere dust or pebbles for the most part, but so numerous as to look from our distance like a continuous ring, or rather like three or four concentric rings, for certain divisions have been noticed in it--an inner broad division called after its discoverer, cassini, and an outer, fainter, narrower one discovered by encke. the innermost part of the ring is dusky, fainter { } than the planet or the rest of the ring, and is known as the "crape-ring." of saturn itself we know little; it is further off and fainter than jupiter, and its details are not so pronounced, but in general they resemble those of jupiter. the planet rotates quickly--in h. m.--its markings run into parallel belts, and are diversified by spots of the same character as on jupiter. saturn is probably possessed of no small amount of native heat. uranus and neptune are much smaller bodies than jupiter and saturn, though far larger than the earth. but their distance from the earth and sun makes their discs small and faint, and they show little in the telescope beyond a hint of "belts" like those of jupiter; so that, as with that planet, the surfaces that they show are almost certainly the upper surfaces of a shell of cloud. in general, therefore, the rule appears to hold good throughout the solar system that a very large body is intensely hot and in a condition of violent activity and rapid change; that smaller bodies are less hot and less active, until we come down to the smallest, which are cold, inert, and dead. our own earth, midway in the series, is itself cold, but is placed at such a distance from the sun as to receive from it a sufficient but not excessive supply of light and heat, and the changes of the earth are such as not to prohibit but to nourish and support the growth and development of the various forms of life. the smallest members of the solar system are known as meteors. these are often no more than pebbles or particles of dust, moving together in associated orbits round the sun. they are too small and too scattered to be seen in open space, and become visible to us only { } when their orbits intersect that of the earth, and the earth actually encounters them. they then rush into our atmosphere at a great speed, and become highly heated and luminous as they compress the air before them; so highly heated that most are vapourised and dissipated, but a few reach the ground. as they are actually moving in parallel paths at the time of one of these encounters, they appear from the effect of perspective to diverge from a point, hence called the "+radiant+." some showers occur on the same date of every year; thus a radiant in the constellation lyra is active about april , giving us meteors, known as the "lyrids"; and another in perseus in august, gives us the "perseids." other radiants are active at intervals of several years; the most famous of all meteoric showers, that of the "leonids," from a radiant in leo, was active for many centuries every thirty-third year; and another falling in the same month, november, came from a radiant in andromeda every thirteen years. in these four cases and in some others the meteors have been found to be travelling along the same path as a comet. it is therefore considered that meteoric swarms are due to the gradual break up of comets; indeed the comet of the andromeda shower, known from one of its observers as "biela's," was actually seen to divide into two in december , and has not been observed as a comet since , though the showers connected with it, giving us the meteors known as the "andromedes," have continued to be frequent and rich. meteors, therefore, are the smallest, most insignificant, of all the celestial bodies; and the shining out of a meteor is the last stage of its history--its death; after death it simply goes to add an infinitesimal trifle to the dust of the earth. { } chapter vi the system of the stars the first step towards our knowledge of the starry heavens was made when the unknown and forgotten astronomers of b.c. arranged the stars into constellations, for it was the first step towards distinguishing one star from another. when one star began to be known as "the star in the eye of the bull," and another as "the star in the shoulder of the giant," the heavens ceased to display an indiscriminate crowd of twinkling lights; each star began to possess individuality. the next step was taken when hipparchus made his catalogue of stars ( b.c.), not only giving its name to each star, but measuring and fixing its place--a catalogue represented to us by that of claudius ptolemy (a.d. ). the third step was taken when bradley, the third astronomer royal, made, at greenwich, a catalogue of more than star-places determined with the telescope. a century later argelander made the great bonn zone catalogue of , stars, and now a great photographic catalogue and chart of the entire heavens have been arranged between eighteen observatories of different countries. this great chart when complete will probably present millions of stars in position and brightness. { } the question naturally arises, "why so many stars? what conceivable use can be served by catalogues of millions or even of stars?" and so far as strictly practical purposes are concerned, the answer must be that there is none. thus maskelyne, the fifth astronomer royal, restricted his observations to some thirty-six stars, which were all that he needed for his _nautical almanac_, and these, with perhaps a few additions, would be sufficient for all purely practical ends. but there is in man a restless, resistless passion for knowledge--for knowledge for its own sake--that is always compelling him to answer the challenge of the unknown. the secret hid behind the hills, or across the seas, has drawn the explorer in all ages; and the secret hid behind the stars has been a magnet not less powerful. so catalogues of stars have been made, and made again, and enlarged and repeated; instruments of ever-increasing delicacy have been built in order to determine the positions of stars, and observations have been made with ever-increasing care and refinement. it is knowledge for its own sake that is longed for, knowledge that can only be won by infinite patience and care. the chief instrument used in making a star catalogue is called a transit circle; two great stone pillars are set up, each carrying one end of an axis, and the axis carries a telescope. the telescope can turn round like a wheel, in one direction only; it points due north or due south. a circle carefully divided into degrees and fractions of a degree is attached to the telescope. in the course of the twenty-four hours every star above the horizon of the observatory must come at least once within the range of this telescope, and at that moment the observer points the telescope to the { } star, and notes the time by his clock when the star crossed the spider's threads, which are fitted in the focus of his eye-piece. he also notes the angle at which the telescope was inclined to the horizon by reading the divisions of his circle. for by these two--the time when the star passed before the telescope and the angle at which the telescope was inclined--he is able to fix the position of the star. "but why should catalogues be repeated? when once the position of a star has been observed, why trouble to observe it again? will not the record serve in perpetuity?" the answers to these questions have been given by star catalogues themselves, or have come out in the process of making them. the earth rotates on its axis and revolves round the sun. but that axis also has a rolling motion of its own, and gives rise to an apparent motion of the stars called +precession+. hipparchus discovered this effect while at work on his catalogue, and our knowledge of the amount of precession enables us to fix the date when the constellations were designed. similarly, bradley discovered two further apparent motions of the stars--+aberration+ and +nutation+. of these, the first arises from the fact that the light coming from the stars moves with an inconceivable speed, but does not cross from star to earth instantly; it takes an appreciable, even a long, time to make the journey. but the earth is travelling round the sun, and therefore continually changing its direction of motion, and in consequence there is an apparent change in the direction in which the star is seen. the change is very small, for though the earth moves -½ miles in a second, light travels , times as fast. stars therefore are deflected from their true positions by aberration, by { } an extreme amount of . " of arc, that being the angle shown by an object that is slightly more distant than , times its diameter. the axis of the earth not only rolls on itself, but it does so with a slight staggering, nodding motion, due to the attractions of the sun and moon, known as +nutation+. and the axis does not remain fixed in the solid substance of the earth, but moves about irregularly in an area of about feet in diameter. the positions of the north and south poles are therefore not precisely fixed, but move, producing what is known as the +variation of latitude+. then star-places have to be corrected for the effect of our own atmosphere, _i.e._ refraction, and for errors of the instruments by which their places are determined. and when all these have been allowed for, the result stands out that different stars have real movement of their own--their +proper motions+. no stars are really "fixed"; the name "+fixed stars+" is a tradition of a time when observation was too rough to detect that any of the heavenly bodies other than the planets were in motion. but nothing is fixed. the earth on which we stand has many different motions; the stars are all in headlong flight. and from this motion of the stars it has been learned that the sun too moves. when copernicus overthrew the ptolemaic theory and showed that the earth moves round the sun, it was natural that men should be satisfied to take this as the centre of all things, fixed and immutable. it is not so. just as a traveller driving through a wood sees the trees in front apparently open out and drift rapidly past him on either hand, and then slowly close together behind him, so sir william herschel showed that the stars in one { } part of the heavens appear to be opening out, or slowly moving apart, while in the opposite part there seems to be a slight tendency for them to come together, and in a belt midway between the two the tendency is for a somewhat quicker motion toward the second point. and the explanation is the same in the one case as in the other--the real movement is with the observer. the sun with all its planets and smaller attendants is rushing onward, onward, towards a point near the borders of the constellations lyra and hercules, at the rate of about twelve miles per second. part of the proper motions of the stars are thus only apparent, being due to the actual motion of the sun--the "+sun's way+," as it is called--but part of the proper motions belong to the stars themselves; they are really in motion, and this not in a haphazard, random manner. for recently kapteyn and other workers in the same field have brought to light the fact of +star-drift+, _i.e._ that many of the stars are travelling in associated companies. this may be illustrated by the seven bright stars that make up the well-known group of the "plough," or "charles's wain," as country people call it. for the two stars of the seven that are furthest apart in the sky are moving together in one direction, and the other five in another. another result of the close study of the heavens involved in the making of star catalogues has been the detection of double stars--stars that not only appear to be near together but are really so. quite a distinct and important department of astronomy has arisen dealing with the continual observation and measurement of these objects. for many double stars are in motion round each other in obedience to the law of gravitation, and their orbits have been computed. { } some of these systems contain three or even four members. but in every case the smaller body shines by its own light; we have no instance in these double stars of a sun attended by a planet; in each case it is a sun with a companion sun. the first double star to be observed as such was one of the seven stars of the plough. it is the middle star in the plough handle, and has a faint star near it that is visible to any ordinarily good sight. star catalogues and the work of preparing them have brought out another class--variable stars. as the places of stars are not fixed, so neither are their brightnesses, and some change their brightness quickly, even as seen by the naked eye. one of these is called +algol+, _i.e._ the demon star, and is in the constellation perseus. the ancient greeks divided all stars visible to the naked eye into six classes, or "+magnitudes+," according to their brightness, the brightest stars being said to be of the first magnitude, those not quite so bright of the second, and so on. algol is then usually classed as a star of the second magnitude, and for two days and a half it retains its brightness unchanged. then it begins to fade, and for four and a half hours its brightness declines, until two-thirds of it has gone. no further change takes place for about twenty minutes, after which the light begins to increase again, and in another four and a half hours it is as bright as ever, to go through the same changes again after another interval of two days and a half. algol is a double star, but, unlike those stars that we know under that name, the companion is dark, but is nearly as large as its sun, and is very close to it, moving round it in a little less than three days. at one point of its orbit it comes between algol and the earth, { } and algol suffers, from our point of view, a partial eclipse. there are many other cases of variable stars of this kind in which the variation is caused by a dark companion moving round the bright star, and eclipsing it once in each revolution; and the diameters and distances of some of these have been computed, showing that in some cases the two stars are almost in contact. in some instances the companion is a dull but not a dark star; it gives a certain amount of light. when this is the case there is a fall of light twice in the period--once when the fainter star partly eclipses the brighter, once when the brighter star partly eclipses the fainter. but not all variable stars are of this kind. there is a star in the constellation cetus which is sometimes of the second magnitude, at which brightness it may remain for about a fortnight. then it will gradually diminish in brightness for nine or ten weeks, until it is lost to the unassisted sight, and after six months of invisibility it reappears and increases during another nine or ten weeks to another maximum. "mira," _i.e._ wonderful star, as this variable is called, is about times as bright at maximum as at minimum, but some maxima are fainter than others; neither is the period of variation always the same. it is clear that variation of this kind cannot be caused by an eclipse, and though many theories have been suggested, the "+long-period variables+," of which mira is the type, as yet remain without a complete explanation. more remarkable still are the "new stars"--stars that suddenly burst out into view, and then quickly fade away, as if a beacon out in the stellar depths had suddenly been fired. one of these suggested to hipparchus the need for a catalogue of the { } stars; another, the so-called "pilgrim star," in the year was the means of fixing the attention of tycho brahe upon astronomy; a third in was observed and fully described by kepler. the real meaning of these "new," or "temporary," stars was not understood until the spectroscope was applied to astronomy. they will therefore be treated in the volume of this series to be devoted to that subject. it need only be mentioned here that their appearance is evidently due to some kind of collision between celestial bodies, producing an enormous and instantaneous development of light and heat. these new stars do not occur in all parts of the heavens. even a hasty glance at the sky will show that the stars are not equally scattered, but that a broad belt apparently made up of an immense number of very small stars divides them into two parts. the milky way, or galaxy, as this belt is called, bridges the heavens at midnight, early in october, like an enormous arch, resting one foot on the horizon in the east, and the other in the west, and passing through the "+zenith+," _i.e._ the point overhead. it is on this belt of small stars--on the milky way--that new stars are most apt to break out. the region of the milky way is richer in stars than are the heavens in general. but it varies itself also in richness in a remarkable degree. in some places the stars, as seen on some of the wonderful photographs taken by e. e. barnard, seem almost to form a continuous wall; in other places, close at hand, barren spots appear that look inky black by contrast. and the +star clusters+, stars evidently crowded together, are frequent in the milky way. and yet again beside the stars the telescope reveals { } to us the nebulÆ. some of these are the irregular nebulæ--wide-stretching, cloudy, diffused masses of filmy light, like the great nebula in orion. others are faint but more defined objects, some of them with small circular discs, and looking like a very dim uranus, or even like saturn--that is to say, like a planet with a ring round its equator. this class are therefore known as "+planetary nebulæ+," and, when bright enough to show traces of colour, appear green or greenish blue. these are, however, comparatively rare. other of these faint, filmy objects are known as the "+white nebulæ+," and are now counted by thousands. they affect the spiral form. sometimes the spiral is seen fully presented; sometimes it is seen edgewise; sometimes more or less foreshortened, but in general the spiral character can be detected. and these white nebulæ appear to shun the galaxy as much as the planetary nebula; and star clusters prefer it; indeed the part of the northern heavens most remote from the milky way is simply crowded with them. it can be by no accident or chance that in the vast edifice of the heavens objects of certain classes should crowd into the belt of the milky way, and other classes avoid it; it points to the whole forming a single growth, an essential unity. for there is but one belt in the heavens, like the milky way, a belt in which small stars, new stars, and planetary nebulæ find their favourite home; and that belt encircles the entire heavens; and similarly that belt is the only region from which the white nebulæ appear to be repelled. the milky way forms the foundation, the strong and buttressed wall of the celestial building; the white nebulæ close in the roof of its dome. { } and how vast may that structure be--how far is it from wall to wall? that, as yet, we can only guess. but the stars whose distances we can measure, the stars whose drifting we can watch, almost infinitely distant as they are, carry us but a small part of the way. still, from little hints gathered here and there, we are able to guess that, though the nearest star to us is nearly , times as far as the sun, yet we must overpass the distance of that star times before we shall have reached the further confines of the galaxy. nor is the end in sight even there. this is, in briefest outline, the story of astronomy. it has led us from a time when men were acquainted with only a few square miles of the earth, and knew nothing of its size and shape, or of its relation to the moving lights which shone down from above, on to our present conception of our place in a universe of suns of which the vastness, glory, and complexity surpass our utmost powers of expression. the science began in the desire to use sun, moon, and stars as timekeepers, but as the exercise of ordered sight and ordered thought brought knowledge, knowledge began to be desired, not for any advantage it might bring, but for its own sake. and the pursuit itself has brought its own reward in that it has increased men's powers, and made them keener in observation, clearer in reasoning, surer in inference. the pursuit indeed knows no ending; the questions to be answered that lie before us are now more numerous than ever they have been, and the call of the heavens grows more insistent: "lift up your eyes on high." { } books to read popular general descriptions:-- sir r. s. ball.--_star-land_. (cassell.) agnes giberne.---sun, moon and stars_. (seeley.) w. t. lynn.--_celestial motions_. (stanford.) a. & w. maunder.---the heavens and their story_. (culley.) simon newcomb.--_astronomy for everybody_. (isbister.) for beginners in observation:-- w. f. denning.--_telescopic work for starlight evenings_. (taylor & francis.) e. w. maunder.--_astronomy without a telescope_. (thacker.) arthur p. norton.--_a star atlas and telescopic handbook_. (gall & inglis.) garrett p. serviss.--_astronomy with an opera-glass_. (appleton.) star-atlases:-- rev. j. gall--_an easy guide to the constellations_. (gall and inglis.) e. m'clure and h. j. klein.--_star-atlas_. (society for promoting christian knowledge.) r. a. proctor.--_new star atlas_. (longmans.) astronomical instruments and methods:-- sir g. b. airy.--_popular astronomy; lectures delivered at ipswich_. (macmillan.) e. w. maunder.--_royal observatory, greenwich; its history and work_. (religious tract society.) { } general text-books:-- clerke, fowler & gore.--concise astronomy. (hutchinson.) simon newcomb.--popular astronomy. (macmillan.) c. a. young.--manual of astronomy. (ginn.) special subjects:-- rev. e. ledger.--_the sun; its planets and satellites_. (stanford.) c. a. young.--_the sun_. (kegan paul.) mrs. todd.--_total eclipses_. (sampson low.) nasmyth and carpenter.--_the moon_. (john murray.) percival lowell.--_mars_. (longmans.) ellen m. clerke.--_jupiter_. (stanford.) e. a. proctor.--_saturn and its system_. (longmans.) w. t. lynn.--_remarkable comets_. (stanford.) e. w. maunder.--_the astronomy of the bible_. (hodder and stoughton.) historical:-- w. w. bryant.--_history of astronomy_. (methuen.) agnes m. clerke.--_history of astronomy in the nineteenth century_. (a. & c. black.) george forbes.--_history of astronomy_. (watts.) biographical:-- sir e. s. ball.--_great astronomers_. (isbister.) agnes m. clerke.--_the herschels and modern astronomy_. (cassell.) sir o. lodge.--_pioneers of science_. (macmillan.) { } index aberration, "achilles" (minor planet), adams, john c., airy, "algol," "andromedes" (meteors), apsides, , argelander, barnard, e. e., "bear," the, biela's comet, bouvard, bradley, , bremiker, catalogues (star), - centauri, alpha, "ceres" (minor planet), challis, charles ii., chromosphere, chronometer, clairaut, columbus, comets, comet, halley's, ---- biela's, conic sections, constellations, the, ---- date of, cook, capt., copernicus, , , "copernicus" (lunar crater), , corona, cowell, crommelin, degrees, dollond, double stars, earth, form of, ---- size of, , eclipses, ecliptic, ellipse, epicycle, eratosthenes, "eros" (minor planet), , eudoxus, excentric, eye-piece, faculÆ, flamsteed, galileo, galle, gascoigne, gravitation, law of, hall, chester moor, halley, halley's comet, harrison, john, herschel, sir w., , , hipparchus, , , , hyperbola, job, book of, , "juno" (minor planet), jupiter, , , - kapteyn, kepler, , , kepler's laws, "kepler" (lunar crater), langley, latitude, variation of, "leonids" (meteors), leverrier, lowell, , "lyrids" (meteors), magnetic storm, magnetism, earth's, magnitudes of stars, "mare imbrium," mars, , , - ---- canals of, maskelyne, , maunder, mrs. walter, , mercury, , , , , - meteors, , micrometer, milky way, , minor planets, , minutes of arc, "mira," moon, , , , , , , - ---- distance of, "_nautical almanac_," , navigation, nebulæ, neptune, , newcomb, new stars, newton, , , newton's laws of motion, nodes, nutation, , "oases of mars," obelisks, object glass, observatories, berlin, ---- copenhagen, ---- greenwich, ---- mt. wilson, ---- paris, ---- pulkowa, ---- st. petersburg, ---- washington, ---- yerkes, "pallas" (minor planet), parabola, "perseids" (meteors), photography, photosphere, "pilgrim" star, piazzi, planets, pole of the heavens, pontécoulant, precession of the equinoxes, , "_principia_," prominences, "ptolemæus" (lunar crater), ptolemy, , radiant points, radius vector, reflectors, refractors, saturn, , - schiaparelli, schwabe, seconds of arc, sirius, solar system, tables of, - somerville, mrs., spheres, planetary, spörer, spörer's law, star catalogues, - ---- clusters, ---- drift, stars, fixed, ---- proper motions of, sun, , , , , , - ---- distance of, ---- dials, sun spots, ---- spot maximum, ---- ---- minimum, "sun's way," telescope, invention of, transit circle, tycho brahe, , , "tycho" (lunar crater), , , uranus, , variable stars, ---- ----, long period, venus, , , "vesta" (minor planet), young, c. a., zenith, , zodiac, signs of, , , , printed by ballantyne, hanson & co. edinburgh & london * * * * * "we have nothing but the highest praise for these little books, and no one who examines them will have anything else."--_westminster gazette_, nd june . the people's books the first ninety volumes the volumes issued are marked with an asterisk science . the foundations of science . . . by w. c. d. whetham, m.a., f.r.s. . embryology--the beginnings of life . . . by prof. gerald leighton, m.d. . biology . . . by prof. w. d. henderson, m.a. . zoology: the study of animal life . . . by prof. e. w. macbride, m.a., f.r.s. . botany; the modern study of plants . . . by m. c. stopes, d.sc., ph.d., f.l.s. . bacteriology . . . by w. e. carnegie dickson, m.d. . the structure of the earth . . . by prof. t. g. bonney, f.r.s. . evolution . . . by e. s. goodrich, m.a., f.r.s. . darwin . . . by prof. w. garstang, m.a., d.sc. . heredity . . . by j. a. s. watson, b.sc. . inorganic chemistry . . . by prof. e. c. c. baly, f.r.s. . organic chemistry . . . by prof. j. b. cohen, b.sc., f.r.s. . the principles of electricity . . . by norman k. campbell, m.a. . radiation . . . by p. phillips, d.sc. . the science of the stars . . . by e. w. maunder, f.r.a.s. . the science of light . . . by p. phillips, d.sc. . weather science . . . by r. g. k. lempfert, m.a. . hypnotism and self-education . . . by a. m. hutchison, m.d. . the baby: a mother's book . . . by a university woman. . youth and sex--dangers and safeguards for boys and girls . . . by mary scharlieb, m.d., m.s., and f. arthur sibly, m.a., ll.d. . marriage and motherhood . . . by h. s. davidson, m.b., f.r.c.s.e. . lord kelvin . . . by a. russell, m.a., d.sc., m.i.e.e. . huxley . . . by professor g. leighton, m.d. . sir william huggins and spectroscopic astronomy . . . by e. w. maunder, f.r.a.s., of the royal observatory, greenwich. . practical astronomy . . . by h. macpherson, jr., f.r.a.s. . aviation . . . by sydney f. walker, r.n. . navigation . . . by william hall, r.n., b.a. . pond life . . . by e. c. ash, m.r.a.c. . dietetics . . . by alex. bryce, m.d., d.p.h. philosophy and religion . the meaning of philosophy . . . by prof. a. e. taylor, m.a., f.b.a. . henri bergson . . . by h. wildon carr, litt.d. . psychology . . . by h. j. watt, m.a., ph.d., d.phil. . ethics . . . by canon rashdall, d.litt., f.b.a. . kant's philosophy . . . by a. d. lindsay, m.a. . the teaching of plato . . . by a. d. lindsay, m.a. . aristotle . . . by prof. a. e. taylor, m.a., f.b.a. . friedrich nietzsche . . . by m. a. mügge. . eucken: a philosophy of life . . . by a. j. jones, m.a., b.sc., ph.d. . the experimental psychology of beauty . . . by c. w. valentine, b.a., d.phil. . the problem of truth . . . by h. wildon carr, litt.d. . buddhism . . . by prof. t. w. rhys davids, m.a., f.b.a. . roman catholicism . . . by h. b. coxon. preface, mgr. r. h. benson. . the oxford movement . . . by wilfrid ward. . the bible and criticism . . . by w. h. bennett, d.d., litt.p., and w. f. adeney, d.d. . cardinal newman . . . by wilfrid meynell. . the church of england . . . by rev. canon masterman. . anglo-catholicism . . . by a. e. manning foster. . the free churches . . . by rev. edward shillito, m.a. . judaism . . . by ephraim levine, m.a. . theosophy . . . by annie besant. history . the growth of freedom . . . by h. w. nevinson. . bismarck and the origin of the german empire . . . by professor f. m. powicke. . oliver cromwell . . . by hilda johnstone, m.a. . mary queen of scots . . . by e. o'neill, m.a. . cecil john rhodes, - 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[transcriber's note: italicized text is indicated with _underscores_. bold text is indicated with +plus signs+. numbers inside curly braces, e.g. { } are page numbers.] [illustration: fig. . the solar system. _frontispiece._] comets and meteors: their phenomena in all ages; their mutual relations; and the theory of their origin. by daniel kirkwood, ll.d., professor of mathematics in indiana university, and author of "meteoric astronomy." [illustration] philadelphia: j. b. lippincott & co. . entered according to act of congress, in the year , by daniel kirkwood, ll.d., in the office of the librarian of congress at washington. preface. the origin of meteoric astronomy, as a science, dates from the memorable star-shower of . soon after that brilliant display it was found that similar phenomena had been witnessed, at nearly regular intervals, in former times. this discovery led at once to another no less important, viz.: that the nebulous masses from which such showers are derived revolve about the sun in paths intersecting the earth's orbit. the theory that these meteor-clouds are but the scattered fragments of disintegrated comets was announced by several astronomers in :--a theory confirmed in a remarkable manner by the shower of meteors from the _débris_ of biela's comet on the th of november, . to gratify the interest awakened in the public mind by the discoveries here named, is the main design of the following work. among the subjects considered are, cometary astronomy; aerolites, with the phenomena attending their fall; the most brilliant star-showers of all ages; and the origin of comets, aerolites, and falling stars. it may be proper to remark that the language used by the writer in a volume[ ] published several years since, and now nearly out of print, has been occasionally adopted in the following treatise. bloomington, indiana, april, . [ ] meteoric astronomy. contents. page. preface chapter i. a general view of the solar system chapter ii. comets comets visible in the day-time periodic comets chapter iii. comets whose elements indicate periodicity, but whose returns have not been recognized chapter iv. other remarkable comets chapter v. the position and arrangement of cometary orbits chapter vi. the disintegration of comets chapter vii. meteoric stones chapter viii. shooting-stars--meteors of november chapter ix. other meteoric streams chapter x. the origin of comets and meteors i. comets. comets and meteors. chapter i. a general view of the solar system. a descriptive treatise on comets and meteors may properly be preceded by a brief general view of the _planetary_ system to which these bodies are related, and by which their motions, in direction and extent, are largely influenced. the solar system consists of the sun, together with the planets, comets, and meteors which revolve around it as the centre of their motions. the sun is the great controlling orb of this system, and the source of light and heat to its various members. its magnitude is one million three hundred thousand times greater than that of the earth, and it contains more than seven hundred times as much matter as all the planets put together. mercury is the nearest planet to the sun; its mean distance being about , , miles. its diameter is miles, and it completes its orbital revolution in days. venus, the next member of the system, is sometimes our morning and sometimes our evening star. its magnitude is almost exactly the same as that of the earth. it revolves round the sun in days. the earth is the third planet from the sun in the order of distance; the radius of its orbit being about , , miles. it is attended by one satellite,--the moon,--the diameter of which is miles. mars is the first planet exterior to the earth's orbit. it is considerably smaller than the earth, and has no satellite. it revolves round the sun in days. _the asteroids._--since the commencement of the present century a remarkable zone of telescopic planets has been discovered immediately exterior to the orbit of mars. these bodies are extremely small; some of them probably containing less matter than the largest mountains on the earth's surface. members of the group are known at present, and the number is annually increasing. jupiter, the first planet exterior to the asteroids, is nearly , , miles from the sun, and revolves round it in a little less than years. this planet is , miles in diameter, and contains more than twice as much matter as all the other planets, primary and secondary, put together. jupiter is attended by four moons or satellites. saturn is the sixth of the principal planets in the order of distance. its orbit is about , , miles beyond that of jupiter. this planet is attended by eight satellites, and is surrounded by three broad flat rings. saturn is , miles in diameter, and its mass or quantity of matter is more than that of all the other planets except jupiter. uranus is at double the distance of saturn, or nineteen times that of the earth. its diameter is about , miles, and its period of revolution years. it is attended by at least four satellites. neptune is the most remote known member of the system; its distance being , , , miles. it is somewhat larger than uranus; has certainly one satellite, and probably several more. its period is about years. a cannon-ball flying outward from the sun at the uniform velocity of miles per hour would not reach the orbit of neptune in less than years. these planets all move round the sun in the same direction,--from west to east. their motions are nearly circular, and also nearly in the same plane. their orbits, except that of neptune, are represented in the frontispiece. it is proper to remark, however, that all representations of the solar system by maps and planetariums must give an exceedingly erroneous view either of the magnitudes or distances of its various members. if the earth, for instance, be denoted by a ball half an inch in diameter, the diameter of the sun, according to the same scale ( , miles to the inch), will be between four and five feet; that of the earth's orbit, about feet; while that of neptune's orbit will be nearly six miles. to give an accurate representation of the solar system at a single view is therefore plainly impracticable. the zodiacal light.--this term was first applied by dominic cassini, in , to a faint nebulous aurora, somewhat resembling the milky way, apparently of a conical or lenticular form, having its base toward the sun and its axis nearly in the direction of the ecliptic. the most favorable time for observing it is when its axis is most nearly perpendicular to the horizon. this, in our latitudes, occurs in march, for the evening, and in october, for the morning. the angular distance of its vertex from the sun is frequently seventy or eighty degrees, while sometimes, though rarely (except within the tropics), it exceeds even one hundred degrees. it was noticed in the latter part of the th century by tycho brahe. the first accurate description of the phenomenon was given, however, by cassini. this astronomer supposed the appearance to be produced by the blended light of innumerable bodies too small to be separately observed,--a theory still very generally accepted. in other words, the zodiacal light is probably a swarm of infinitesimal planets; the greater part of the cluster being interior to mercury's orbit. the distances between the different members of our planetary system, vast as they may seem, sink into insignificance when compared with the intervals which separate us from the so-called fixed stars. _alpha centauri_, the nearest of those twinkling luminaries, is times more distant than neptune from the sun. even light itself, which moves , miles in a second, is more than three years in traversing the mighty interval. chapter ii. comets. the term _comet_--which signifies literally a _hairy star_--may be applied to all bodies that revolve about the sun in very eccentric orbits. the sudden appearance, vast dimensions, and extraordinary aspect of these celestial wanderers, together with their rapid and continually varying motions, have never failed to excite the attention and wonder of all observers. nor is it surprising that in former times, when the nature of their orbits was wholly unknown, they should have been looked upon as omens of impending evil, or messengers of an angry deity. even now, although modern science has reduced their motions to the domain of law, determined approximately their orbits, and assigned in a number of instances their periods, the interest awakened by their appearance is in some respects still unabated. the special points of dissimilarity between planets and comets are the following:--the former are dense, and, so far as we know, solid bodies; the latter are many thousand times rarer than the earth's atmosphere. the planets _all_ move from west to east; many comets revolve in the opposite direction. the planetary orbits are but slightly inclined to the plane of the ecliptic; those of comets may have any inclination whatever. the planets are observed in all parts of their orbits; comets, only in those parts nearest the sun. the larger comets are attended by a _tail_, or train of varying dimensions, extending generally in a direction opposite to that of the sun. the more condensed part, from which the tail proceeds, is called the _nucleus_; and the nebulous envelope immediately surrounding the nucleus is sometimes termed the _coma_. these different parts are seen in fig. , which represents the great comet of . [illustration: fig. . the great comet of .] zeno, democritus, and other greek philosophers held that comets were produced by the collection of several stars into clusters. aristotle taught that they were formed by exhalations, which, rising from the earth's surface, ignited in the upper regions of the atmosphere. this hypothesis, through the great influence of its author, was generally received for almost two thousand years. juster views, however, were entertained by the celebrated seneca, who maintained that comets ought to be ranked among the permanent works of nature, and that their disappearance was not an extinction, but simply a passing beyond the reach of our vision. the observations of tycho brahe first established the fact that comets move through the planetary spaces far beyond the limits of our atmosphere. the illustrious dane, however, supposed them to move in circular orbits. kepler, on the other hand, was no less in error in considering their paths to be rectilinear. james bernoulli supposed comets to be the satellites of a very remote planet, invisible on account of its great distance,--such satellites being seen only in the parts of their orbits nearest the earth. still more extravagant was the hypothesis of descartes, who held that they were originally fixed stars, which, having gradually lost their light, could no longer retain their positions, but were involved in the vortices of the neighboring stars, when such as were thus brought within the sphere of the sun's illuminating power again became visible. _comets visible in the daytime._ comets of extraordinary brilliancy have sometimes been seen during the daytime. at least thirteen authentic instances of this phenomenon have been recorded in history. the first was the comet which appeared about the year b.c., just after the assassination of julius cæsar. the romans called it the _julium sidus_, and regarded it as a celestial chariot sent to convey the soul of cæsar to the skies. it was seen two or three hours before sunset, and continued visible for eight successive days. the great comet of , described as an object of terrific splendor, was seen simultaneously with the sun, and in close proximity to it. dr. halley supposed this and the julian comet to have been previous visits of the great comet of . in the year two comets appeared,--one about the middle of february, the other in june,--both of which were visible while the sun was above the horizon. one was of such magnitude and brilliancy that the nucleus and even the tail could be seen at midday. the comet of , one of the most splendid recorded in history, was visible in full daylight, when nearest the earth, on the st of january. this comet, according to laugier, moves very nearly in the plane of the ecliptic, its inclination being less than two degrees. its least distance from our globe was only , , miles. the comet of , supposed by some to be identical with that of , was also visible in full sunshine. the apparent magnitude of its nucleus was three times greater than that of jupiter. the comet of was seen with the naked eye by tycho brahe before sunset. it was by observations on this body that aristotle's doctrine in regard to the origin, nature, and distance of comets was proved to be erroneous. it was simultaneously observed by tycho at oranienberg, and thaddeus hagecius at prague; the points of observation being more than miles apart, and nearly on the same meridian. the comet was found to have no sensible diurnal parallax; in other words, its apparent place in the heavens was the same to each observer, which could not have been the case had the comet been less distant than the moon. the comet which passed its perihelion on the th of november, , was distinctly seen by marsilius when the sun was above the horizon. the great comet of was seen without the aid of a glass at one o'clock in the afternoon,--only five hours after its perihelion passage. the diameter of this body was nearly equal to that of jupiter. it had _six_ tails, the greatest length of which was about , , miles, or nearly one-third of the distance of the earth from the sun. the spaces between the tails were as dark as the rest of the heavens, while the tails themselves were bordered with a luminous edging of great beauty. the great comet of was distinctly visible to the naked eye, at noon, on the th of february. it appeared as a brilliant body, within less than two degrees from the sun. this comet passed its perihelion on the th of february, at which time its distance from the sun's surface was only about one-fourth of the moon's distance from the earth. this is the nearest approach to the sun ever made by any known comet. the velocity of the body in perihelion was about , , miles an hour, or nearly nineteen times that of the earth in its orbit. the apparent length of its tail was sixty-five degrees, and its true length , , miles. the first comet of , discovered by mr. hind, was also seen near the sun on the day of its perihelion passage. that discovered by klinkerfues on the th of june, , and which passed its perihelion on the st of september, was seen at olmutz in the daytime, august , when only twelve degrees from the sun. after passing its perihelion, it was again observed, _at noon_, on the d, d, and th of september. finally, the great comet of was seen before sunset, on monday evening, july , by rev. henry w. ballantine, of bloomington, indiana. it was again detected on the following evening just as the sun was in the horizon. besides the thirteen comets which we have enumerated, at least four others have been seen in the daytime; all, however, under peculiar circumstances. seneca relates that during a great solar eclipse, years before our era, a large comet was observed not far from the sun. "philostorgius says that on the th of july, a.d. , when the sun was eclipsed and stars were visible, a great comet, in the form of a cone, was discovered near that luminary, and was afterwards observed during the nights."[ ] the comet which passed its perihelion on the th of november, , was observed by both gambart and flaugergues to transit the solar disk,--the least distance of the nucleus from the sun's surface being about , , miles. the second comet of and the comet of are both known in like manner to have passed between the sun and the earth. unfortunately, however, the transits were not observed. [ ] hind. a few cometary orbits are hyperbolas, more ellipses, and a still greater number parabolas. comets moving in ellipses remain permanently within the limits of solar influence. others, however, visit our system but once, and then pass off to wander indefinitely in the sidereal spaces. _comets of known periodicity._ i. halley's comet. as comets are subject to great changes of appearance, one can never be identified by any description of its magnitude, brilliancy, etc., at the time of a previous return. this can be done only by a comparison of orbits. if, for example, we find the elements of an orbit very nearly corresponding in every particular with those of a former comet, there is a degree of probability, amounting almost to certainty, that the two are identical. sir isaac newton, in his _principia_, published shortly after the appearance of the comet of , explained how the periods of those mysterious visitors might thus be ascertained, thus directing the attention of astronomers to the subject. dr. halley soon after undertook a thorough discussion of all the recorded cometary observations within his reach. in the course of his investigations he discovered that the path of the comet observed by kepler in coincided almost exactly with that of the one which passed its perihelion in . hence he concluded that they were the same. he found also that the comet of , whose course had been particularly observed by apian, moved in the same path. the interval between the consecutive appearances being nearly years, halley announced this as the time of the comet's revolution, and boldly predicted its return in or . the law of universal gravitation had at this time just been discovered and announced. but although its application to the determination of planetary and cometary perturbations had not been developed, halley was well aware that the attractive influence of jupiter and saturn might accelerate or retard the motion of the comet, so as to produce a considerable variation in its period. during the interval from to , the application of the higher mathematics to problems in physical astronomy had been studied with eminent success. the disturbing effect of the two large planets, jupiter and saturn, was computed with almost incredible labor by clairaut, lalande, and madame lepaute. the result as announced by clairaut to the academy of sciences in november, , was that the period must be days longer than that immediately preceding, and that the comet accordingly would pass its perihelion about the th of april, . it was stated, however, that, being pressed for want of time, they had neglected certain quantities which might somewhat affect the result. the comet, in fact, passed its perihelion in march, within less than a month of the predicted time. when it is considered that the attraction of the earth was not taken into the account, and that uranus, whose influence must have been sensible, had not then been discovered, this must certainly be regarded as a remarkable approximation. but during the next interval of years the theory of planetary perturbations had been more perfectly developed. the masses of jupiter and saturn had been determined with greater accuracy, and uranus had been added to the known members of the planetary system. a nearer approximation to the exact time of the comet's perihelion passage in was therefore to be expected. prizes were offered by two of the learned societies of europe--the academy of sciences at turin, and the french institute--for the most perfect discussion of its motions. that of the former was awarded to damoiseau,--that of the latter to pontecoulant. the times assigned by these distinguished mathematicians for the comet's perihelion passage were very nearly the same, and differed but a few days from the true time. had the present received mass of jupiter been used in the calculations, pontecoulant, it is believed, would not have been in error as much as hours. it may be proper to remark that, during the entire period from to , the position of neptune was such that it could produce no considerable effect on the motion of the comet. this interesting object will again return about . the visit of was the earliest that halley succeeded in determining with any degree of certainty. peter apian, by whom it was at that time observed, was the first european to ascertain the fact that, as a general thing, the tails of comets are turned from the sun.[ ] to confirm this discovery, he carefully followed the body in its progress through the constellations. by means of his recorded observations halley was enabled to identify this comet with that of and . the great comet of he _conjectured_ to be the same, from the date of its appearance. pingré subsequently confirmed this suspicion by a careful examination of the few trustworthy records that could be collected from the writers of that period. [ ] the chinese, however, as appears from biot's researches, had observed the same fact years earlier. see humboldt's cosmos, vol. iv. (bohn's ed.), p. . from the earlier descriptions of this comet we infer that its brilliancy is gradually diminishing. in its tail, which was slightly curved like a sword or sabre, extended two-thirds of the distance from the horizon to the zenith. the appearance of such an object, in a grossly superstitious age, excited throughout europe the utmost consternation. the moslems had just taken constantinople, and were threatening to advance westward into europe. pope calixtus iii., regarding the comet as confederate with the turk, ordered prayers to be offered three times a day for deliverance from both. the alarm, however, was of short duration. within ten days of its appearance the comet reached its perihelion. receding from the sun, the sword-like form began to diminish in brilliancy and extent; and finally, to the great relief of europe, it entirely disappeared. the perihelion passage of was, until recently, the earliest known. it was shown by laugier, however, in , that among the notices of comets extracted by edward biot from the chinese records, were observations of a body in , which was undoubtedly the comet of halley. further researches among these annals enabled the same astronomer to recognize two ancient returns, one in , the other in . still more recently the distinguished english astronomer, mr. hind, has traced back the returns to the year b.c. he remarks, however, that previous to that epoch, "the chinese descriptions of comets are too vague to aid us in tracing any more ancient appearances," and that "european writers of these remote times render us no assistance." let us now inquire whether the comet had probably made any former approach to the sun in an orbit nearly identical with the present. it is well known that the modern period of this body is considerably less than the ancient. thus, the mean period since a.d. has been . years; while from b.c. to a.d. it was . years. in determining the approximate dates of former returns, the ancient period should evidently be employed. now, it is a remarkable fact that of more than comets,[ ] or objects supposed to be comets, whose appearance was recorded during the six centuries immediately preceding the year b.c., but one--that of b.c.--was observed at a date corresponding nearly to that of a former return of halley's comet. of this object it is merely recorded that "a torch was seen in the heavens." whether this was a comet or some other phenomenon, it is impossible to determine. but as the comet of halley was more brilliant in ancient than in modern times, it seems highly improbable that seven _consecutive_ returns of so conspicuous an object should have been unrecorded, especially as twelve comets per century[ ] were observed during the same period. it would appear, therefore, that the perihelion passage of b.c. was in fact the first ever made by the comet, or at least the first in an orbit nearly the same as the present. [ ] see the catalogues of chambers and williams. [ ] the average number. the motion of halley's comet is retrograde. the point of its nearest approach to the sun is situated within the orbit of venus. its greatest distance from the centre of the system is nearly twice that of uranus, or times that of the earth. the comet is, consequently, subject to great changes of temperature. when nearest the sun its light and heat are almost four times greater than the earth's; when most remote, they are times less. in the former position, the sun would appear much larger than to us; in the latter, his apparent diameter would not greatly exceed that of jupiter, as viewed from the earth. it would be difficult to conjecture what the consequences might be, were our planet transported to either of these extremes of the cometary path. in the perihelion, the waters of the ocean would undoubtedly be reduced to a state of vapor; in the aphelion, they would be solidified by congelation. ii. encke's comet. it was formerly supposed that all comets have their aphelia far beyond the limits of the planetary system. in , however, a small comet was discovered by pons, the orbit of which was subsequently found to be wholly interior to that of jupiter. its elements were presented by bouvard, in , to the board of longitude at paris. the form and position of the orbit were immediately found to correspond with those of a comet observed by several astronomers in . the different appearances were consequently regarded as returns of the same body. its elliptic orbit was calculated by encke, who found its period to be only about three years and four months. its perihelion is within the orbit of mercury; its aphelion, between the asteroids and the orbit of jupiter. encke's comet is invisible to the naked eye, except in very favorable circumstances; it has no tail; its motion, like that of the planets, is from west to east; and its orbit is inclined about ° to the ecliptic. a comparison of the successive periods of this interesting object has led to the discovery that its time of revolution is gradually diminishing; a fact regarded by encke and other astronomers as indicating the existence of an ethereal medium. iii. biela's comet. the discovery of encke's comet of short period was followed, in , by that of another, whose revolution is completed in about six years and eight months. it was observed on the th of february, by m. biela, an austrian officer; accordingly it has since been known as _biela's comet_. on computing its elements and comparing them with those of former comets, it was found to have been observed in and . damoiseau having calculated the dimensions of the comet's elliptic path and the time of its return, announced as the result of his computations the remarkable fact that the orbits of the earth and comet intersect each other, and that the comet would cross the earth's path on the th of october, . this produced no little alarm among the uneducated, especially in france. even some journalists are said to have predicted the destruction of our globe by a collision with the comet. when the latter, however, passed the point of intersection at the predicted time, the earth was at a distance of , , miles. at the return of - , biela's comet exhibited a most remarkable appearance. instead of a single comet, it appeared as two distinct bodies moving together side by side, at a distance from each other somewhat less than that of the moon from the earth. astronomers, anxious to determine whether the cometary fragments had continued separate during an entire revolution, awaited the next return with no ordinary interest. the _two_ bodies appeared at the predicted time (september, ); their distance apart having increased to , , miles. in the comet, on account of its proximity to the sun, entirely escaped detection. at the return in - the position of the object was quite favorable for observation, yet the search of astronomers was again unsuccessful. in the body escaped detection both in europe and america. one fragment was seen, however, at madras, india, on the mornings of the d and d of december,--several weeks after its perihelion passage. the comet's non-appearance in and its greatly diminished magnitude in leave no room to doubt its progressive dissolution. this subject will again be referred to in discussing the phenomena of meteoric showers. iv. faye's comet. on the d of november, , m. faye, of the paris observatory, discovered a comet, which was shown by dr. goldschmidt to revolve in an elliptic orbit, the perihelion of which is exterior to the orbit of mars, and the aphelion immediately beyond that of jupiter. the eccentricity is, therefore, less than that of any other comet previously discovered. its period is about years and months. it is possible that a comet moving in a parabola or hyperbola, with the sun in the focus, may be thrown into an elliptic orbit by the disturbing influence of jupiter or one of the other large planets. the celebrated leverrier undertook to determine whether the comet of faye had in this manner been recently fixed as a permanent member of the solar system. he found that it could not have been so introduced since , and, consequently, that it must have completed at least thirteen revolutions before its discovery. this comet has been observed at each return from to the present time. v. de vico's comet. on the d of august, , de vico, of rome, discovered a comet whose orbit is included between those of the earth and jupiter. its period is days, or about - / years. this body, from some cause,--perhaps a gradual dissolution,--has not been observed at any subsequent return. vi. brorsen's comet. on the th of february, , mr. brorsen, of kiel, discovered a faint comet, the mean distance and period of which are almost identical with those of de vico's. this comet was not observed during the perihelion passage of , on account of its unfavorable position with respect to the sun. it has, however, been subsequently detected. vii. d'arrest's comet. dr. d'arrest discovered a comet on the th of june, , which was soon found to move in an elliptic orbit, with a period of about - / years. it entirely escaped observation, both in europe and america, during its perihelion passage in . it was observed, however, at the cape of good hope. its invisibility in was due to its unfavorable position. at its return in , it was first seen on the st of august, by dr. winnecke, of carlsruhe. viii. tuttle's comet. a faint telescopic comet was discovered at the observatory of harvard college, on the evening of january , , by mr. h. p. tuttle. the same body was independently found one week later by dr. bruhns, of berlin. from observations made at cambridge, massachusetts, and ann arbor, michigan, its elements were soon computed by different astronomers; the result in each case coinciding so closely with the elements of the second comet of , as to place its identity wholly beyond doubt. its period is nearly years and months. it had returned, therefore, without detection, in the years , , , and . on its approach to perihelion in , it was first detected by m. borelly, of marseilles. ix. winnecke's comet. the second comet of was discovered on the th of march, by dr. winnecke, of bonn. this proved to be identical with the third comet of , whose period was computed by encke to be about - / years. it had therefore returned unperceived no less than six times between and . at its return in it again escaped detection. the perihelion passage of was made on the th of june. the comet was seen as early as april , and, after passing the sun, as late as october . "schönfeld states that in part of april and may it appeared to have not one, but several, centres of condensation, and vogel says that, in the beginning of june, it had a much greater resemblance to a star-cluster than to a nebula." this phenomenon, it may be remarked, bore a striking resemblance to the appearances observed in the comets of , , and . x. tempel's comet. on the th of december, , m. tempel, of marseilles, discovered a small comet, which continued visible four weeks, passing its perihelion january , . dr. oppolzer, of vienna, after a careful determination of its elements, announced the interesting fact that its orbit very nearly intersects those of the earth and uranus; the perihelion being situated immediately within the former, and the aphelion a short distance exterior to the latter. the period, according to the same astronomer, is years and days. the identity of this comet with that of was suggested by professor h. a. newton soon after its appearance,--a suggestion which subsequent research has strongly corroborated. it is also highly probable that the comet observed in china, september , , was a former return of the same body. in it was conspicuous to the naked eye, while in it was wholly invisible without a telescope,--a fact indicative of its gradual dissolution. the connection of this comet with the meteors of november will be elsewhere considered. xi. the second comet of . another comet of short period was discovered by m. tempel on the d of april, . its orbit is the least eccentric of all known comets: the perihelion being exterior to the orbit of mars; the aphelion interior to that of jupiter. its motion is direct, and it completes a revolution in years and months. chapter iii. comets whose elements indicate periodicity, but whose returns have not been recognised. i. the group whose periods are nearly equal to that of uranus. since the commencement of the present century five comets have been discovered, which form, with halley's, an interesting and remarkable group. the first of these was detected by pons, on the th of july, ; the second by olbers, on the th of march, ; the third by de vico, on the th of february, ; the fourth by brorsen, on the th of july, ; and the last by westphal, on the th of june, . the periods of these bodies are all nearly equal, ranging from to years; their eccentricities are not greatly different; the motions of all, except that of halley's, are direct; and the distances of their aphelia are somewhat greater than neptune's distance from the sun. of this group, the comets of and seem worthy of special notice. the former became visible to the naked eye shortly after its discovery, and each continued visible about ten weeks. their elements are as follows: perihelion passage. , sept. , mar. _d._ _h._ _d._ _h._ long. of perih'n. ° ´ ° ´ long. of a. node. ° ´ ° ´ incl. ° ´ ° ´ peri'n dist. . . eccentricity. . . period. . _y_ . direction. d d computer. encke. peirce. the wonderful similarity of these elements, except in the longitude of the ascending node, is at once apparent. it will also be noticed that the longitude of the _descending_ node of the latter is very nearly coincident with that of the _ascending_ node of the former. these remarkable coincidences are presented to the eye in the following diagram, where the dotted ellipse represents the orbit of the comet of , and the continuous curve that of the comet of . [illustration: fig. .] it is infinitely improbable that these coincidences should be accidental; they point undoubtedly to a common origin of the two bodies. according to the theory now generally accepted, comets enter the solar system _ab extra_, move in parabolas or hyperbolas around the sun, and, if undisturbed by the planets, pass off beyond the limits of the sun's attraction, to be seen no more. if in their motion, however, they approach very near any of the larger planets, their direction is changed by planetary perturbation,--their orbits being sometimes transformed into ellipses. the new orbits of such bodies would pass very nearly through the points at which their greatest perturbation occurred; and accordingly we find that the aphelia of a large proportion of the periodic comets are near the orbits of the major planets. "i admit," says m. hoek, "that the orbits of comets are by nature parabolas or hyperbolas, and that in the cases when elliptical orbits are met with, these are occasioned by planetary attractions, or derive their character from the uncertainty of our observations. to allow the contrary would be to admit some comets as permanent members of our planetary system, to which they ought to have belonged since its origin, and so to assert the simultaneous birth of that system and of these comets. as for me, i attribute to these a primitive wandering character. traveling through space, they move from one star to another in order to leave it again, provided they do not meet any obstacle that may force them to remain in its vicinity. such an obstacle was jupiter, in the neighborhood of our sun, for the comets of lexell and brorsen, and probably for the greater part of periodical comets; the other part of which may be indebted for their elliptical orbits to the attractions of saturn and the remaining planets. "generally, then, comets come to us from some star or other. the attraction of our sun modifies their orbit, as had been done already by each star through whose sphere of attraction they had passed. we can put the question if they come as single bodies or united in systems." the conclusion of this astronomer's interesting discussion is that-- "_there are systems of comets in space that are broken up by the attraction of our sun, and whose members attain, as isolated bodies, the vicinity of the earth during a course of several years._"[ ] [ ] monthly notices of the r. a. s., vol. xxv., p. . in the researches here referred to, it is shown by professor hoek that the comets of iii., i., and iv. formed a group in space previous to their entrance into our system. the same fact has also been demonstrated in regard to other comets which need not here be specified. now, the comets of and iv. have their aphelia near the orbit of neptune, and hence the original parabolas in which they moved were probably transformed into ellipses by the perturbations of that planet. before entering the solar domain, they were doubtless members of a cometary system. passing neptune near the same time, and at some distance from each other, their different relative positions with regard to the disturbing body may account for the slight differences in the elements of their orbits. _comets of the jovian group._ besides the eight comets enumerated in chapter ii. whose aphelia are in the vicinity of jupiter's orbit, five others have been observed which belong apparently to the same cluster. these are the comets of , i., ii., , and iv. "the fact that these comets have not been re-observed on their successive returns through perihelion may be explained either by the difficulty of observing them, owing to their unfavorable positions, and to the circumstances of observers not expecting their reappearance, their periodic character not being then suspected, or because they may have been thrown by the disturbing action of the larger planets into orbits such as to keep them continually out of the range of view of terrestrial observers."[ ] [ ] dr. lardner. lexell's comet of is the most remarkable instance known of the change produced in the orbits of these bodies by planetary attraction. this comet passed so near jupiter in that the attraction of the latter was times greater than that of the sun. the consequence was that the comet, whose mean distance corresponded to a period of - / years, was thrown into an orbit so entirely different that it has never since been visible. peters' comet. a telescopic comet was discovered by dr. peters on the th of june, , which continued to be observed till the st of july. its period, according to the discoverer, is about years, and its aphelion, like that of tuttle's comet, is in the vicinity of saturn's orbit. it was expected to return in , and again in , but each time escaped detection, owing probably to the fact that its position was unfavorable for observation. stephan's comet ( i.). in january, , m. stephan, of marseilles, discovered a new comet, the elements of which, after two months' observations, were computed by mr. g. m. searle, of cambridge, massachusetts. the perihelion of this body is near the orbit of mars; its aphelion near that of uranus,--the least distance of the orbits being about , , miles. the present form of the cometary path is doubtless due to the disturbing action of uranus. the comet completes its revolution in . years; consequently (as has been pointed out by mr. j. r. hind) five of its periods are almost exactly equal to two periods of uranus. the next approximate appulse of the two bodies will occur in , when the form of the comet's orbit may be sensibly modified. elliptic comets whose aphelia are at a much greater distance than neptune's orbit. in october, , a comet was seen both in europe and china, which was noted for the fact of its having two distinct tails, making with each other an angle of about °. from a discussion of the chinese observations (which extended through a longer period than the european), laugier concluded that this body is identical with the third comet of , which was discovered by galle on the th of march. if, therefore, it has made no intermediate return without being observed, it must have a period of about years. it is also highly probable, from the similarity of elements, that the comet which passed its perihelion on the th of june, , was a reappearance of the comet of ,--the period of revolution being years. the elements of the great comet of are somewhat uncertain. there is a probability, however, of the identity of this body with the comet of . this would make the period years. the third comet of is especially interesting from its connection with the august meteors. its period, according to dr. oppolzer, is - / years. the great comet of was one of the most remarkable in the nineteenth century. it was discovered on the d of june, by donati, of florence, and first became visible to the naked eye about the last of august. the comet attained its greatest brilliancy about the th of october, when its distance from the earth was , , miles. the length of its tail somewhat exceeded this distance. if, therefore, the comet had been at that time directly between the sun and the earth, the latter must have been enveloped for a number of hours in the cometic matter. the observations of this comet during a period of five months enabled astronomers to determine the elements of its orbit within small limits of error. it completes a revolution, according to newcomb, in years, in an orbit somewhat more eccentric than that of halley's comet. it will not return before the th century, and will only reach its aphelion about the year . its motion per second when nearest the sun is miles; when most remote, only yards. chapter iv. other remarkable comets. it remains to describe some of the most remarkable comets of which we have any record, but of which we have no means of determining with certainty whether they move in ellipses, parabolas, or hyperbolas. in the year b.c., a large comet appeared simultaneously with the famous fall of meteoric stones near Ægospotamos. the former was supposed by the ancients to have had some agency in producing the latter phenomenon. another of extraordinary magnitude appeared in the year b.c. this comet was so bright as to throw shadows, and its tail extended one-third of the distance from the horizon to the zenith. the years , , , and , before our era, were also signalized by the appearance of very large comets. the apparent magnitude of the first of these is said to have equaled that of the sun itself; while its light was sufficient to diminish sensibly the darkness of the night. the second is said to have filled a fourth part of the celestial hemisphere. the comet of b.c., sometimes called the comet of mithridates, because of its appearance about the time of his birth, is said to have rivaled the sun in splendor. in a.d. a large comet was visible during a period of nearly three months. its nucleus had a remarkably red or fiery appearance, and the greatest length of its tail exceeded °. the most brilliant comets of the sixth century were probably those of and . the train of the latter, as seen in the west soon after sunset, presented the appearance of a distant conflagration. great comets appeared in the years , , and . of these, the comet of had the greatest apparent magnitude. it was first seen early in july, and attained its greatest brilliancy in the latter part of august, when its tail was ° in length. it disappeared on the d of october, about the time of the death of pope urban iv., of which event the comet, in consequence of this coincidence, was considered the precursor. these comets, on account of the similarity of their elements, were believed by many astronomers to be the same, and to have a period of about years. in the case of identity, however, another reappearance should have occurred soon after the middle of the nineteenth century. as no such return was observed, we may conclude that the comets were not the same, and that their periods are wholly unknown. the comet discovered on the th of november, , was one of the largest in modern times; its tail having attained the extraordinary length of °. the comet of , so carefully observed by hevelius, almost equaled the moon in apparent magnitude. it shone, however, with a lurid, dismal light. the tail of the comet of was ° in length. this body is also remarkable for its near approach to the sun; its least distance from the solar surface having been only , miles. it will always be especially memorable, however, for having furnished newton the data by means of which he first showed that comets in their orbital motions are governed by the same principle that regulates the planetary revolutions. of all the comets which appeared during the eighteenth century, that which passed its perihelion on the th of october, , had the greatest apparent magnitude. it was discovered by messier on the th of august, and continued to be observed till the st of december. on the th of september the length of its tail was °. the comet discovered on the th of march, , is in some respects the most remarkable on record. it was observed during a period of months and days,--the longest period of visibility known. on account of its situation with respect to the earth, the apparent length of its tail was much less than that of some other comets; its true length, however, was at one time , , miles; and sir william herschel found that on the th of october the greatest circular section of the tail was , , miles in diameter. the same astronomer found the diameter of the head of the comet to be , miles, and that of the envelope at least , . as a general thing, the length of a comet-train increases very rapidly as the body approaches the sun. but the perihelion distance of the comet of was considerably greater than the distance of the earth from the sun; while its nearest approach to the earth was , , miles. its true magnitude, therefore, has probably not been surpassed by any other observed; and had its perihelion been very near the sun, it must have exhibited an appearance of terrific grandeur. this comet has an elliptic orbit, and its period, according to argelander, is years. the great comet of was discovered on the th of may, by mr. john tebbut, jr., of new south wales. in this country, as well as in europe, it was first generally observed on the evening of june ,-- days after its perihelion passage. sir john herschel, who observed it in kent, england, remarks that it far exceeded in brilliancy any comets he had ever seen, not excepting those of and . according to father secchi, of the collegio romano, the length of its tail was °. this, with a single exception,[ ] is the greatest on record. the computed orbit is elliptical; the period, years. [ ] the tail of the first comet of (observed in the southern hemisphere) attained the unprecedented length of °.--_m. n. r. a. s._, vol. xxv., p. . chapter v. the position and arrangement of cometary orbits. the cosmical masses from which comets are derived seem to traverse in great numbers the interstellar spaces. in consequence of the sun's progressive motion, these nebulous bodies are sometimes drawn toward the centre of our system. if, in this approach, they are not disturbed by any of the large planets, they again recede in parabolas or hyperbolas. when, however, as must sometimes be the case, they pass near jupiter, saturn, uranus, or neptune, their orbits may be transformed into elongated ellipses. the periodicity of many comets may thus be accounted for. in the present chapter it is proposed to consider the probable consequences of the sun's motion through regions of space in which cometary matter is widely diffused; to compare our theoretical deductions with observed phenomena; and thus refer to their physical cause a variety of facts which have hitherto received no satisfactory explanation.[ ] [ ] this chapter is the substance of a paper read before the american philosophical society, november , . . as comets, at least in many instances, owe their periodicity to the disturbing action of the major planets, and as this planetary influence is sometimes sufficient, especially in the case of jupiter and saturn, to change the _direction_ of cometary motion, the great majority of periodic comets should move in the same direction with the planets. now, of the comets known to be elliptical, per cent. _have direct motion_. in this respect, therefore, theory and observation are in striking harmony. . when the relative positions of a comet and the disturbing planet are such as to give the transformed orbit of the former a small perihelion distance, the comet must return to the point at which it received its greatest perturbation; in other words, to the orbit of the planet. the aphelia of the comets of short period ought therefore to be found, for the most part, _in the vicinity of the orbits of the major planets_. this, as already shown in chapters ii. and iii., is strikingly the case. the actual distances of these aphelia, however, as compared with the respective distances of jupiter, saturn, uranus and neptune, are presented at one view in the following tables: =i.= comets whose aphelion distances are nearly equal to . , the radius of jupiter's orbit. comets. aph. dist. . encke's . . iv . . de vico's . . pigott's ( ) . . ii . . i . . ii . . iii . . brorsen's . . d'arrest's . . faye's . . bicla's . =ii.= comets whose aphelion distances are nearly equal to . , the radius of saturn's orbit. comets. aph. dist. . peters' ( vi.) . . tuttle's ( i.) . =iii.= comets whose aphelion distances are nearly equal to . , the radius of uranus's orbit. comets. aph. dist. . i . . november meteors . . i . =iv.= comets whose aphelion distances are nearly equal to . , the radius of neptune's orbit. comets. aph. dist. . westphal's ( iv.) . . pons' ( ) . . olbers' ( ) . . de vico's ( iv.) . . brorsen's ( v.) . . halley's[ ] . [ ] halley's comet _in aphelio_ is too remote from the plane of the ecliptic to be much disturbed by neptune. has the original position of the orbit been changed by jupiter's influence? the coincidences here pointed out (some of which have been noticed by others) appear, then, to be necessary consequences of the motion of the solar system through spaces occupied by meteoric nebulæ. hence the observed facts receive an obvious explanation. in regard to comets of long period we have only to remark that, for anything we know to the contrary, there may be causes of perturbation far exterior to the orbit of neptune. . from what we observe in regard to the _larger_ bodies of the universe--a clustering tendency being everywhere apparent,--it seems highly improbable that cometic matter should be uniformly distributed in the sidereal spaces. we would expect, on the contrary, to find it collected in groups or clusters. this view is also in remarkable harmony with the facts of observation. in years, from to , comets were visible to the naked eye; of which appeared in the years from to . again, during years, from to , only comets were visible to the naked eye, while in the next years there were double that number. the probable cause of such variations is sufficiently obvious. as the sun in its progressive motion approaches a cometary group, the latter is drawn toward the centre of our system; the nearer members with greater velocity than the more remote. those of the same cluster would enter the solar domain at periods not very distant from each other; the forms of their orbits depending upon their original relative positions with reference to the sun's course, and also on planetary perturbations. it is evident also that the passage of the solar system through a region of space comparatively destitute of cometic clusters would be indicated by a corresponding paucity of comets. . the line of apsides of a large proportion of comets will be approximately coincident with the solar orbit. the point towards which the sun is moving is in longitude about °. the quadrants bisected by this point and that directly opposite extend from ° to °, and from ° to °. the number of cometary perihelia found in these quadrants up to july, (periodic comets being counted but once) was , or per cent.; in the other two quadrants, , or per cent. this tendency of the perihelia to crowd together in two opposite regions has been noticed by different writers. . comets whose positions before entering our system were very remote from the solar orbit must have _overtaken_ the sun in its progressive motion; hence their perihelia must fall, for the most part, in the vicinity of the point towards which the sun is moving; and they must in general have very small perihelion distances. now, what are the observed facts in regard to the longitudes of the perihelia of the comets which have approached within the least distance of the sun's surface? but three have had a perihelion distance less than . . _all_ these, it will be seen by the following table, have their perihelia in close proximity to the point referred to: =i.= comets whose perihelion distances are less than . . perihelion passage. per. dist. long. of per. . , feb. _d._ _h._ . ° ´ . , dec. . . , feb. . in table ii. all but the last have their perihelia in the same quadrant. =ii.= comets whose perihelion distances are greater than . and less than . . perihelion passage. per. dist. long. of per. . , nov _d._ _h._ . ° ´ . , march . . , nov . . , march . . , jan . . the perihelion of the first comet in table iii. is remote from the direction of the sun's motion; that of the second is distant but °, and of the third °. =iii.= comets whose perihelion distances are greater than . and less than . . perihelion passage. per. dist. long. of per. . , july _d._ _h._ . ° ´ . , sept. . . , march . with greater perihelion distances the tendency of the perihelia to crowd together round the point indicated is less distinctly marked. . few comets of small perihelion distance should have their perihelia in the vicinity of longitude °, the point opposite that towards which the sun is moving. accordingly we find, by examining a table of cometary elements, that with a perihelion distance less than . there is not a single perihelion between ° and °; between . and . but ; and between . and . only . chapter vi. the disintegration of comets. the _fact_ that in several instances meteoric streams move in orbits identical with those of certain comets was first established by the researches of signor schiaparelli. the _theory_, however, of an intimate relationship between comets and meteors was advocated by the writer as long since as ,[ ]--several years previous to the publication of schiaparelli's memoirs. in the essay here referred to it was maintained-- [ ] danville quarterly review, december, . . that meteors and meteoric rings "are the _débris_ of ancient but now disintegrated comets whose matter has become distributed around their orbits." . that the separation of biela's comet as it approached the sun in december, , was but one in a series of similar processes which would probably continue until the individual fragments would become invisible. . that certain luminous meteors have entered the solar system from the interstellar spaces.[ ] [ ] others, it was supposed, might have originated within the system,--a view which the writer has not wholly abandoned. . that the orbits of some meteors and periodic comets have been transformed into ellipses by planetary perturbation; and . that numerous facts--some observed in ancient and some in modern times--have been decidedly indicative of cometary disintegration. what was thus proposed as theory has been since confirmed as undoubted facts. when the hypothesis was originally advanced, the data required for its mathematical demonstration were entirely wanting. the evidence, however, by which it was sustained was sufficient to give it a high degree of probability. the existence of a divellent force by which comets near their perihelia have been separated into parts is clearly shown by the following facts. whether this force, as suggested by schiaparelli, is simply the unequal attraction of the sun on different parts of the nebulous mass, or whether, in accordance with the views of other astronomers, it is to be regarded as a cosmical force of repulsion, is a question left for future discussion. historical facts. . seneca informs us that ephoras, a greek writer of the fourth century before christ had recorded the singular fact of a comet's separation into two distinct parts.[ ] this statement was deemed incredible by the roman philosopher, inasmuch as the occurrence was then without a parallel. more recent observations of similar phenomena leave no room to question the historian's veracity. [ ] "quæst. nat.," lib. vii., cap. xvi. . the head of the great comet of a.d. , according to the writers of that period, was "composed of several small stars." (hind's "comets," p. .) . on june , a.d. , two comets appeared in the constellation hercules, and pursued nearly the same apparent path. probably at a former epoch the pair had constituted a single comet.[ ] [ ] chambers' "descr. astr.," p. . . on august , , "a comet was seen which resembled two moons joined together." they subsequently separated, the fragments assuming different forms.[ ] [ ] ibid., p. . . the chinese annals record the appearance of three comets--one large and two smaller ones--at the same time, in the year of our era. "they traveled together for three days. the little ones disappeared first, and then the large one."[ ] the bodies were probably fragments of a large comet which, on approaching the sun, had been separated into parts a short time previous to the date of their discovery. [ ] ibid., p. . . _the third comet of ._--the great comet of exhibited decided symptoms of disintegration. when first observed (on november ), its appearance was that of a lucid and nearly spherical mass. on the eighth day the process of division was distinctly noticed, and on the th of december it resembled a cluster of small stars.[ ] [ ] hevelius, "cometographia," p. . see also grant's "hist. of phys. astr.," p. . . _the comet of ._--the elements of the comets of and have a remarkable resemblance, and previous to the year astronomers regarded the bodies as identical. the similarity of the elements is seen at a glance in the following table: comet of . comet of . longitude of perihelion ° ´ ° ´ longitude of ascending node inclination perihelion distance . . motion direct. direct. the elements of the former are by olbers; those of the latter by mechain. the return of the comet about , though generally expected, was looked for in vain. as a possible explanation of this fact, it is interesting to recur to an almost forgotten statement of hevelius. this astronomer observed in the comet of an apparent breaking up of the body into separate fragments.[ ] the case may be analogous to that of biela's comet. [ ] "cometographia," p. . . the identity of the comets of and , first suggested by professor h. a. newton, is now unquestioned. the existence then of a meteoric swarm, moving in the same track, is not the only evidence of the original comet's partial dissolution. the comet of was invisible to the naked eye; that of , seen under nearly similar circumstances, was a conspicuous object. the statement of the chinese historian that "it appeared nearly as large as a tow measure,"[ ] though somewhat indefinite, certainly justifies the conclusion that its magnitude has greatly diminished during the last years. the meteors moving in the same orbit are doubtless the products of this gradual separation. [ ] williams' "chinese observations of comets," p. . . the repartition of biela's comet in , as well as the non-appearance of the two fragments in and ,[ ] were referred to in a previous chapter. [ ] one of the parts was seen at madras, india, on the mornings of december and , . the comet of halley, if we may credit the descriptions given by ancient writers, has been decreasing in brilliancy from age to age. the same is true in regard to several others believed to be periodic. the comet of a.d. had a tail ° long. at its return, in march, , the length of its tail was only °. the third comet of and the first of are supposed, from the similarity of their elements, to be identical. each perihelion passage occurred in may, yet the tail at the former appearance was ° in length, at the latter but - / °. other instances might be specified of this apparent gradual dissolution. it would seem, indeed, extremely improbable that the particles driven off from comets in their approach to the sun, forming tails extending millions of miles from the principal mass, should again be collected around the same nuclei. the fact, then, that meteors move in the same orbits with comets is but a consequence of that disruptive process so clearly indicated by the phenomena described. in this view of the subject, comets--even such as move in elliptic orbits--are not to be regarded as permanent members of the solar system. their _débris_ becomes gradually scattered around the orbit. some parts of the nebulous ring will be more disturbed than others by planetary perturbation. portions of such streams as nearly intersect the earth's path sometimes penetrate the atmosphere. their rapid motion renders them luminous. if very minute, they are burnt up or dissipated without leaving any solid deposit; we then have the phenomena of _shooting-stars_. when, however, as is sometimes the case, they contain a considerable quantity of solid matter, they reach the earth's surface as _meteoric stones_. ii. meteors. chapter vii. meteoric stones. although numerous instances of the fall of aerolites had been recorded, some of them apparently well authenticated, the occurrence long appeared too marvelous and improbable to gain credence with scientific men. such a shower of rocky fragments occurred, however, on the th of april, , at l'aigle, in france, as forever to dissipate all doubt on the subject. similar displays since that time have been frequently witnessed;--indeed scarcely a year passes without the fall of meteoric stones in some part of the earth, either singly or in clusters. it would not comport with the design of the present treatise to give an extended list of these phenomena. the following account, however, includes the most important instances in which the fall of meteoric stones has been actually observed: ( .) b.c.--according to the celebrated parian chronicle, an aerolite, or _thunder-stone_, as it was called, fell in the island of crete, about years before the christian era. this is undoubtedly the most ancient stone-fall on record. meteoric masses have been _found_, however, the fall of which _probably_ occurred at an epoch still more ancient. ( .) b.c.--a number of stones, which were anciently preserved in orchomenos, a town of boeotia, were said to have fallen from heaven about twelve centuries before our era. ( .) b.c.--a mass of iron, as we learn from the parian chronicle, was seen to descend upon mount ida, in crete. ( .) b.c.--according to livy, a number of meteoric stones fell on the alban hill, near rome, about the year b.c. ( .) b.c., _january_ .--it is related in the chinese annals that on the th of january, b.c., a meteoric stone-fall broke several chariots and killed ten men. ( .) b.c.--a mass of rock, described as "of the size of two millstones," fell at Ægospotamos, in thrace. an attempt to rediscover this meteoric mass, so celebrated in antiquity, was recently made, but without success. notwithstanding this failure, humboldt expressed the hope that, as such a body would be difficult to destroy, it may yet be found, "since the region in which it fell is now become so easy of access to european travelers." ( .) b.c.--the famous stone called the "mother of the gods," and which is described or alluded to by many ancient writers, was said to have fallen from the skies. the poet pindar was seated on a hill at the time of its descent, and the meteorite struck the earth near his feet. the stone, as it fell, was _encircled by fire_. "it is said to have been of moderate dimensions, of a black hue, of an irregular, angular shape, and of a metallic aspect. an oracle had predicted that the romans would continue to increase in prosperity if they were put in possession of this precious deposit; and publius scipio nasico was accordingly deputed to attalus, king of pergamus, to obtain and receive the sacred idol, whose worship was instituted at rome years before the christian era."--_edinburgh encyclopedia._ ( .) a.d. .--an immense aerolite fell into the river (a branch of the tiber) at narni, in italy. it projected three or four feet above the surface of the water. ( .) , _november_ .--an aerolite, weighing pounds, fell at ensisheim, in alsace, penetrating the earth to the depth of three feet. this stone, or the greater part of it, may still be seen at ensisheim. ( .) , _september_ .--at noon an almost total darkening of the heavens occurred at crema. "during this midnight gloom," says a writer of that period, "unheard-of thunders, mingled with awful lightnings, resounded through the heavens.... on the plain of crema, where never before was seen a stone the size of an egg, there fell pieces of rock of enormous dimensions and of immense weight. it is said that ten of these were found, weighing pounds each." a monk was struck dead at crema by one of these rocky fragments. this terrific display is said to have lasted two hours, and aerolites were subsequently found. ( .) , _november_ .--a stone, weighing pounds, fell on mount vaison, in provence. ( .) , _march_ .--a franciscan monk was killed at milan by the fall of a meteoric stone. ( .) .--two swedish sailors were killed on shipboard by the fall of an aerolite. ( .) , _may_ .--two meteoric masses, consisting almost wholly of iron, fell near agram, the capital of croatia. the larger fragment, which weighs pounds, is now in vienna. ( .) , _july_ .--between and o'clock at night a very large meteor was seen near bordeaux, france. over barbotan a loud explosion was heard, which was followed by a shower of meteoric stones of various magnitudes. ( .) , _july_.--a fall of about a dozen aerolites occurred at sienna, tuscany. ( .) , _december_ .--a large meteoric stone fell near wold cottage, in yorkshire, england. "several persons heard the report of an explosion in the air, followed by a hissing sound; and afterward felt a shock, as if a heavy body had fallen to the ground at a little distance from them. one of these, a plowman, saw a huge stone falling toward the earth, eight or nine yards from the place where he stood. it threw up the mould on every side; and after penetrating through the soil, lodged some inches deep in solid chalk-rock. upon being raised, the stone was found to weigh pounds. it fell in the afternoon of a mild, but hazy day, during which there was no thunder or lightning; and the noise of the explosion was heard through a considerable district."--_milner's gallery of nature_, p. . ( .) , _february_ .--a stone of pounds' weight fell in portugal. ( .) , _april_ .--this remarkable shower was referred to on a previous page. at o'clock p.m., the heavens being almost cloudless, a tremendous noise, like that of thunder, was heard, and at the same time an immense fire-ball was seen moving with great rapidity through the atmosphere. this was followed by a violent explosion, which lasted several minutes, and which was heard not only at l'aigle, but in every direction around it to the distance of miles. immediately after, a great number of meteoric stones fell to the earth, generally penetrating to some distance beneath the surface. nearly of these fragments were found and collected, the largest weighing about pounds. the occurrence very naturally excited great attention. m. biot, under the authority of the government, repaired to the place, collected the various facts in regard to the phenomenon, took the testimony of witnesses, etc., and finally embraced the results of his investigations in an elaborate memoir. ( .) , _december_ .--a large meteor exploded over weston, connecticut. the height, direction, velocity and magnitude of this body were discussed by dr. bowditch in a memoir communicated to the american academy of arts and sciences in . the appearance of the meteor occurred about h. m. a.m.,--just after daybreak. its apparent diameter was half that of the full moon; its time of flight, about seconds. within less than a minute from the time of its disappearance three distinct reports, like those of artillery, were heard over an area several miles in diameter. each explosion was followed by the fall of meteoric stones. unlike most aerolites, these bodies when first found were so soft as to be easily pulverized between the fingers. on exposure to the air, however, they gradually hardened. the weight of the largest fragment was pounds. ( .) , _november_ .--between and o'clock in the morning an extraordinary meteor was seen in several of the new england states, new york, new jersey, the district of columbia, and virginia. the apparent diameter of the head was nearly equal to that of the sun, and it had a train, notwithstanding the bright sunshine, several degrees in length. its disappearance on the coast of the atlantic was followed by a series of the most terrific explosions. it is believed to have descended into the water, probably into delaware bay. a highly interesting account of this meteor, by professor loomis, may be found in the _american journal of science and arts_ for january, . ( .) , _may_ .--about minutes before o'clock, p.m., a shower of meteoric stones fell in the southwest corner of guernsey county, ohio. full accounts of the phenomena are given in _silliman's journal_ for july, , and january and july, , by professors e. b. andrews, e. w. evans, j. l. smith, and d. w. johnson. from these interesting papers we learn that the course of the meteor was about ° west of north. its visible track was over washington and noble counties, and the prolongation of its projection, on the earth's surface, passes directly through new concord, in the southeast corner of muskingum county. the meteor when first seen was about miles from the earth's surface. the sky, at the time, was for the most part covered with clouds over northwestern ohio, so that if any portion of the meteoric mass continued on its course it was invisible. the velocity of the meteor, in relation to the earth's surface, was from three to four miles per second; and hence its absolute velocity in the solar system must have been somewhat greater than that of the earth. "at new concord,[ ] muskingum county, where the meteoric stones fell, and in the immediate neighborhood, there were many distinct and loud reports heard. at new concord there was first heard in the sky, a little southeast of the zenith, a loud detonation, which was compared to that of a cannon fired at the distance of half a mile. after an interval of ten seconds, another similar report. after two or three seconds another, and so on with diminishing intervals. twenty-three distinct detonations were heard, after which the sounds became blended together and were compared to the rattling fire of an awkward squad of soldiers, and by others to the roar of a railway train. these sounds, with their reverberations, are thought to have continued for two minutes. the last sounds seemed to come from a point in the southeast ° below the zenith. the result of this cannonading was the falling of a large number of stony meteorites upon an area of about miles long by wide. the sky was cloudy, but some of the stones were seen first as 'black specks', then as 'black birds', and finally falling to the ground. a few were picked up within or minutes. the warmest was no warmer than if it had lain on the ground exposed to the sun's rays. they penetrated the earth from two to three feet. the largest stone, which weighed pounds, struck the earth at the foot of a large oak-tree, and, after cutting off two roots, one five inches in diameter, and grazing a third root, it descended two feet ten inches into hard clay. this stone was found resting under a root that was not cut off. this would seemingly imply that it entered the earth obliquely." [ ] new concord is close to the guernsey county line. nearly all the stones fell in guernsey. over thirty of the stones which fell were discovered, while doubtless many, especially of the smaller, being deeply buried beneath the soil, entirely escaped observation. the weight of the largest ten was pounds. ( .) , _july_ .--about o'clock p.m. on the th of july, , a shower of aerolites fell at dhurmsala, in india. the fall was attended by a tremendous detonation, which greatly terrified the inhabitants of the district. the natives, supposing the stones to have been thrown by some of their deities from the summit of the himalayas, carried off many fragments to be kept as objects of religious veneration. lord canning and mr. j. r. saunders succeeded, however, in obtaining numerous specimens, which they forwarded to the british museum and several european cabinets. they are earthy aerolites, of a specific gravity somewhat greater than that of granite. ( .) , _may_ .--early in the evening a very large and brilliant meteor was seen in france, from paris to the spanish border. at montauban and in the vicinity loud explosions were heard, which were followed by showers of meteoric stones near the villages of orgueil and nohic. the principal facts in regard to the meteor are the following: elevation when first seen, over miles " at the time of its explosion " inclination of its path to the horizon ° or ° velocity per second, about miles, or equal to that of the earth's orbital motion. "this example," says professor newton, "affords the strongest proof that the detonating and stone-producing meteors are phenomena not essentially unlike." ( .) , _january_ .--it is obviously a matter of much importance that the composition and general characteristics of aerolites, together with the phenomena attending their fall, should be carefully noted; as such facts have a direct bearing on the theory of their origin. in this regard the memoirs of professors j. g. galle, of breslau, and g. vom rath, of bonn, on a meteoric fall which occurred at pultusk, poland, on the th of january, , have more than ordinary interest. these memoirs establish the fact that the aerolites of the pultusk shower _entered our atmosphere_ as a swarm or cluster of distinct meteoric masses. it is shown, moreover, by dr. galle that this meteor-group had a proper motion when it entered the solar system of at least from - / to miles per second. the foregoing list contains but a small proportion of the meteoric stones whose fall has been actually observed. but, besides these, other masses have been found so closely similar in structure to aerolites whose descent has been witnessed, as to leave no doubt in regard to their origin. one of these is a mass of iron and nickel, weighing pounds, found by the traveler pallas, in , at abakansk, in siberia. this immense aerolite may be seen in the imperial museum at st. petersburg. on the plain of otumpa, in buenos ayres, is a meteoric mass - / feet in length, partly buried in the ground. its estimated weight is about tons. a specimen of this stone, weighing pounds, has been removed and deposited in one of the rooms of the british museum. a similar block, of meteoric origin, weighing more than six tons, was discovered some years since in the province of bahia, in brazil. general remarks. . a committee on luminous meteors was appointed several years since by the british association for the advancement of science. this committee, consisting at present of james glaisher, f.r.s., robert p. greg, f.r.s., alexander s. herschel, f.r.a.s., and charles brooke, f.r.s., report from year to year not only their own observations on aerolites, fire-balls, and falling stars, but also such facts bearing upon the subject as can be derived from other sources. an analysis of these reports justifies the conclusion that meteoric stone-falls, like star-showers, occur with greater frequency than usual on or about particular days. these epochs, established with more or less certainty, are the following: (_a._) january th. (_b._) " th. (_c._) " th. (_d._) february th. (_e._) " th-- th. (_f._) march th. (_g._) " th. (_h._) april st. (_i._) " th-- th. (_j._) may th-- th. (_k._) " th-- th. (_l._) " th-- th. (_m._) june d. (_n._) " th. (_o._) " th. (_p._) " th. (_q._) july d-- th. (_r._) " th-- th. (_s._) august th-- th. (_t._) " th. (_u._) september th-- th. (_v._) october th. (_w._) november th. (_x._) " th-- th. (_y._) " th-- th. (_z._) december th. (_z´._) " th-- th. (_z´´._) " th. . it is worthy of remark that no new elements have been found in meteoric stones. humboldt, in his "cosmos," called attention to this interesting fact. "i would ask," he remarks, "why the elementary substances that compose one group of cosmical bodies, or one planetary system, may not in a great measure be identical? why should we not adopt this view, since we may conjecture that those planetary bodies, like all the larger or smaller agglomerated masses revolving round the sun, have been thrown off from the once far more expanded solar atmosphere, and have been formed from vaporous rings describing their orbits round the central body?" . but while aerolites contain no elements but such as are found in the earth's crust, the manner in which these elements are combined and arranged is so peculiar that a skillful mineralogist will readily distinguish them from terrestrial substances. . of the eighteen or nineteen elements hitherto observed in meteoric stones, iron is found in the greatest abundance. the specific gravities vary from . to . : the former being that of the stone of alais; the latter that of the meteorite of wayne county, ohio, described by professor j. l. smith in _silliman's journal_ for november, , p. . . the average number of aerolitic falls in a year was estimated by schreibers at . baron reichenbach, however, after a discussion of the data at hand, makes the number much larger. he regards the probable annual average for the entire surface of the earth as not less than . this would give twelve daily falls. they are of every variety as to magnitude, from a weight of less than a single ounce to over fifteen tons. the baron even suspects the meteoric origin of large masses of dolerite which all former geologists had considered native to our planet. . an analysis of any extensive table of meteorites and fire-balls proves that a greater number of aerolitic falls have been observed during the months of june and july, when the earth is near its aphelion, than in december and january, when near its perihelion. it is found, however, that the reverse is true in regard to bolides, or fire-balls. these facts are susceptible of an obvious explanation. the fall of meteoric stones would be more likely to escape observation by night than by day, on account of the relatively small number of observers. but the days are shortest when the earth is in perihelion, and longest when in aphelion; the ratio of their lengths being nearly equal to that of the corresponding numbers of aerolitic falls. on the other hand, it is obvious that fire-balls, unless very large, would not be visible during the day. the _observed_ number will therefore be greatest when the nights are longest; that is, when the earth is near its perihelion. this, it will be found, is precisely in accordance with observation. chapter viii. shooting-stars.--meteors of november . although shooting-stars have doubtless been observed in all ages of the world, it is only within the last half century that they have attracted the special attention of scientific men. a few efforts had been made to determine the height of such meteors, but the first general interest in the subject was excited by the brilliant meteoric display of november , . this shower of fire can never be forgotten by those who witnessed it. the meteors were observed from the west indies to british america, and from ° to ° west longitude from greenwich. as early as o'clock on the evening of the th shooting-stars were observed with unusual frequency; their motions being generally westward. soon after midnight their numbers became so extraordinary as to attract the attention of all who happened to be in the open air. the meteors, however, became more and more numerous till , or half past , o'clock; and the fall did not entirely cease till ten minutes before sunrise. from to o'clock the numbers were so great as to defy all efforts at counting them; while their brilliancy was such that persons sleeping in rooms with uncurtained windows were aroused by their light. the meteors varied in apparent magnitude from the smallest visible points to fire-balls equaling the moon in diameter. occasionally one of the larger class would separate into several parts, and in some instances a luminous train remained visible for three or four minutes. no sound whatever accompanied the display. it was noticed by many observers that all the meteors diverged from a point near the star _gamma leonis_; in other words, their paths if traced backward would intersect each other at a particular locality in the constellation leo. in some parts of the country the inhabitants were completely terror-stricken by the magnificence of the display. in the afternoon of the day on which the shower occurred the writer met with an illiterate farmer who, after describing the phenomena as witnessed by himself, remarked that "the stars continued to fall till none were left," and added, "i am anxious to see how the heavens will appear this evening; i believe we shall see no more stars." a gentleman of south carolina described the effect on the negroes of his plantation as follows:--"i was suddenly awakened by the most distressing cries that ever fell on my ears. shrieks of horror and cries for mercy i could hear from most of the negroes of the three plantations, amounting in all to about or . while earnestly listening for the cause i heard a faint voice near the door, calling my name. i arose, and, taking my sword, stood at the door. at this moment i heard the same voice still beseeching me to arise, and saying, 'o my god, the world is on fire!' i then opened the door, and it is difficult to say which excited me the most,--the awfulness of the scene, or the distressed cries of the negroes. upwards of a hundred lay prostrate on the ground,--some speechless, and some with the bitterest cries, but with their hands raised, imploring god to save the world and them. the scene was truly awful; for never did rain fall much thicker than the meteors fell towards the earth; east, west, north, and south, it was the same." at the time of this wonderful meteoric display captain hammond, of the ship _restitution_, had just arrived at salem, massachusetts, where he observed the phenomenon from midnight till daylight. he recollected with astonishment that precisely one year before, viz., on the th of november, , he had observed a similar appearance (although the meteors were less numerous) at mocha, in arabia. it was found, moreover, as a further and most remarkable coincidence, that an extraordinary fall of meteors had been witnessed on the th of november, . this was seen and described by andrew ellicott, esq., who was then at sea near cape florida. it was also observed by humboldt and bonpland, in cumana, south america. baron humboldt's description of the shower is as follows:--"from half after two, the most extraordinary luminous meteors were seen toward the east. thousands of bolides and falling stars succeeded each other during four hours. they filled a space in the sky extending from the true east ° toward the north and south. in an amplitude of ° the meteors were seen to rise above the horizon at e.n.e. and at e., describe arcs more or less extended, and fall toward the south, after having followed the direction of the meridian. some of them attained a height of °, and all exceeded ° or °. mr. bonpland relates, that from the beginning of the phenomenon there was not a space in the firmament equal in extent to three diameters of the moon, that was not filled at every instant with bolides and falling stars. the guaiqueries in the indian suburb came out and asserted that the firework had begun at one o'clock. the phenomenon ceased by degrees after four o'clock, and the bolides and falling stars became less frequent; but we still distinguished some toward the northeast a quarter of an hour after sunrise." this wonderful correspondence of dates excited a very lively interest throughout the scientific world. it was inferred that a recurrence of the phenomenon might be expected, and accordingly arrangements were made for systematic observations on the th, th, and th of november. the periodicity of the shower was thus, in a very short time, placed wholly beyond question. the facts in regard to the phenomena of november , , were collected and discussed by olmsted, twining, and other astronomers. the inquiry, however, very naturally arose whether any trace of the same meteoric group could be found in ancient times. to determine this question many old historical records were ransacked by the indefatigable scientist, edward c. herrick, in our own country, and by arago, quetelet, and others, in europe. these examinations led to the discovery of ten undoubted returns of the november shower previous to that of . the descriptions of these former meteoric falls are given by professor h. a. newton in the _american journal of science_, for may, . they occurred in the years , , , , , , , , , and . historians represent the meteors of a.d. as innumerable, and as moving like rain in all directions. the exhibition of was scarcely less magnificent. "on the last day of muharrem," says a writer of that period, "stars shot hither and thither in the heavens, eastward and westward, and flew against one another like a scattering swarm of locusts, to the right and left; this phenomenon lasted until daybreak; people were thrown into consternation, and cried to god the most high with confused clamor." the shower of is thus described in a portuguese chronicle, quoted by humboldt: "in the year , twenty-two days of the month of october being past, three months before the death of the king, don pedro (of portugal), there was in the heavens a movement of stars such as men never before saw or heard of. at midnight, and for some time after, all the stars moved from the east to the west; and after being collected together, they began to move, some in one direction and others in another. and afterward they fell from the sky in such numbers, and so thickly together, that as they descended low in the air they seemed large and fiery, and the sky and the air seemed to be in flames, and even the earth appeared as if ready to take fire. that portion of the sky where there were no stars seemed to be divided into many parts, and this lasted for a long time." the showers of - . the fact that all great displays of the november meteors have taken place at intervals of or years, or some multiple of that period, had led to a general expectation of a brilliant shower in . in this country, however, the public curiosity was much disappointed.[ ] the numbers seen were greater than on ordinary nights, but not such as would have attracted any special attention. the greatest number recorded at any one station was seen at new haven by professor newton. on the night of the th were counted in five hours and twenty minutes, and on the following night, in five hours. a more brilliant display was, however, witnessed in europe. meteors began to appear in unusual frequency about o'clock on the night of the th, and their numbers continued to increase with great rapidity for more than two hours; the maximum being reached a little after o'clock. a writer in edinburgh, scotland, thus describes the phenomenon as observed at that city:--"standing on the calton hill, and looking westward,--with the observatory shutting out the lights of princes street,--it was easy for the eye to delude the imagination into fancying some distant enemy bombarding edinburgh castle from long range; and the occasional cessation of the shower for a few seconds, only to break out again with more numerous and more brilliant drops of fire, served to countenance this fancy. again, turning eastward, it was possible now and then to catch broken glimpses of the train of one of the meteors through the grim dark pillars of that ruin of most successful manufacture, the national monument; and in fact from no point in or out of the city was it possible to watch the strange rain of stars, pervading as it did all points of the heavens, without pleased interest and a kindling of the imagination, and often a touch of deeper feeling that bordered on awe." at london about o'clock a single observer counted in two minutes. the whole number seen at greenwich was . the shower was also observed in different countries on the continent. [ ] the first indication of the approaching shower was the appearance of meteors in unusual numbers at malta, on the th of november, . in , as observed at greenwich and other stations, they were still more numerous. in the display was generally observed throughout the united states. from the able and interesting reports of commodore sands and professors newcomb, harkness, and eastman, we derive the following facts in regard to the shower as seen at washington, d. c.: commencement _h._ _m._ a.m. nov. . maximum " " end " " number of meteors per hour at maximum mean height on first appearance miles. " " on disappearance " position of radiant, r. a. °, decl. - / °. the shower of was in some respects quite remarkable, though the number of meteors was less than in or . at new haven the fall commenced about midnight, and from o'clock till daybreak over meteors were counted. the time of maximum could not be accurately determined, as no decrease in the numbers was observable till dawn. the display was also witnessed in england and in cape colony, south africa. the times of maxima in these countries differed so materially as to indicate a decided stratification of the meteoric stream. the entire depth, moreover, where crossed by the earth in , was much greater than at the part traversed either in or . in the shower was observed at port saïd, lower egypt, by g. l. tupman, esq.; in florida, u. s., by commander william gibson, u.s.n.; and at santa barbara, california, by mr. g. davidson and mrs. e. davidson. the first observed meteors in h. m., from h. m. to h. m., alexandria mean time; the numbers during this interval being nearly equal, though slightly decreasing. throughout the morning (november ) the sky was only partly clear. the two observers at santa barbara saw in h. m., ending at h. m. a.m. in florida also the display was quite brilliant, though inferior to that of . it should be remarked that the morning in many parts of the united states was cloudy. no considerable number of the meteors of this stream has been observed in any part of the world since . discussion of the phenomena. since the memorable display of november , , the phenomena of shooting-stars have been observed and discussed with a very lively interest. among the first laborers in this department of research the names of olmsted, herrick, and twining must ever hold a conspicuous place. the fact that the position of the radiant point did not change with the earth's rotation at once placed the cosmical origin of the meteors wholly beyond question. the theory of a ring of nebulous matter revolving round the sun in an elliptic orbit--a theory somewhat different from that proposed by olmsted--was found to afford a simple and satisfactory explanation of the phenomena. this hypothesis of an eccentric stream of meteors intersecting the earth's orbit was adopted by humboldt, arago, and others, shortly after the occurrence of the meteoric shower of . a few years previous to the display of it was shown by professor newton, of yale college, that the distribution of meteoric matter around the ring or orbit is far from uniform; that the motion is retrograde; that the node of the orbit has an annual forward motion of ´´. with respect to the equinox, or of ´´. with respect to the fixed stars; that the periodic time must be limited to five accurately determined periods, viz.: . days, . days, . days, . days, or . years; and that the inclination of the orbit to the ecliptic is about °. professor newton, for reasons assigned, regarded the third period named as the most probable. he remarked, however, that by computing the secular motion of the node for each periodic time, and comparing the result with the known precession, it was possible to determine which of the five periods is the correct one. for the application of this crucial test,--a problem of more than ordinary interest,--we are indebted to professor j. c. adams, of cambridge, england. by an elegant analysis it was first shown that for either of the first four periods designated by professor newton, the annual motion of the node, resulting from planetary perturbation, would be considerably less than one half of the observed motion. it only remained, therefore, to examine whether the period of - / years would give a motion of the node corresponding with observation. professor adams found that in this time the longitude of the node is increased ´ by the action of jupiter, ´ by the action of saturn, and ´ by that of uranus. the effect of the other planets is scarcely perceptible. the _calculated_ motion in - / years is therefore ´. the _observed_ motion in the same time, according to professor newton, as previously stated, is ´. this remarkable accordance was at once accepted by astronomers as satisfactory evidence that the period is about . years. having determined the periodic time, the mean distance, or semi-axis major, is found by kepler's third law to be . . the aphelion is consequently situated at a comparatively short distance beyond the orbit of uranus. the orbit is represented in fig. . [illustration: fig. .] it was stated at the close of chapter vi. that shooting-stars are the dissevered fragments of cometic matter, which, penetrating our atmosphere, are rendered luminous by the resistance so encountered. the discovery that comets and meteors are actually moving in the same orbits was first announced by signor schiaparelli in . the coincidence of the orbits of tempel's comet[ ] as computed by dr. oppolzer, and the meteors of november as determined by schiaparelli, is too close to be regarded as merely accidental. these elements are as follows: nov. meteors. tempel's comet. perihelion passage nov. . , . jan. . , . passage of descending node nov. . , longitude of perihelion ° ´ ° ´ longitude of ascending node ° ´ ° ´ inclination ° ´ ° ´ perihelion distance . . eccentricity . . semi-major axis . . periodic time . _y._ . _y._ motion retrograde. retrograde. [ ] see page . the fact is thus obvious that the meteors of november are the products of the comet's gradual dissolution. it has been stated that the comets of and are probably identical. the interval indicates a period of . years--greater by days than that found by oppolzer. with this value of the periodic time and the known secular variation of the node it is found that the comet and uranus were in close proximity about the beginning of the year b.c. it is therefore not improbable that the former was then thrown into its present orbit by the attraction of the latter. the celebrated leverrier designated the year of our era as the probable epoch of the comet's entrance into our system. this date, however, is incompatible with the period here adopted. it is worthy of remark, moreover, as bearing on this question, that the extension of the cluster in the tenth century, as indicated by the showers of , , and , was too great to have been effected in so short a period as years. with the period of . years it is easy to find that the comet will make a near approach to the earth about the th or th of november, , and to uranus in . at one of these epochs the cometary orbit will probably undergo considerable transformation. we have seen that the comet of , and also the meteoroids following in its path, have their perihelion at the orbit of the earth, and their aphelion at the orbit of uranus. both planets, therefore, at each encounter with the current not only appropriate a portion of the meteoric matter, but entirely change the orbits of many meteoroids. in regard to the devastation produced by the earth in passing through the cluster, it is sufficient to state that, according to weiss, the meteor orbits resulting from the disturbance will have all possible periods from months to years. it may be regarded, therefore, as evidence of the recent[ ] introduction of this meteor-stream into the solar system that the comet of , which constitutes a part of the cluster, has not been deflected from the meteoric orbit by either the earth or uranus. [ ] recent in comparison with the origin of the august meteors, which constitute a continuous ring. chapter ix. other meteoric streams. _the meteors of august - ._--muschenbroek, in his "introduction to natural philosophy," published in , stated as the result of his own observations that shooting-stars are more abundant in august than in any other part of the year. the fact, however, that a maximum occurs on the th or th of the month was first shown by quetelet in . since that time the shower has been regularly observed both in europe and america; the number of meteors at the maximum sometimes amounting to per hour. their tracks when produced backward intersect each other at a particular point in the constellation perseus. of the meteoric displays given in quetelet's catalogue, belong to the august epoch. their dates up to the commencement of the present century are as follows: . a.d. , july th. . , " th- th. . , " th- th. . , " th. . , " th. . , " th. . , " th- th. . , " th- th. . , " th- th. . , " th- th. . , " th- th. . , aug. d. . , " th. . , " th. . , " th- th. . , " th. . , " th- th. . , " th. . , " th. . , " th- th. . , " th. as the earth is about five days in crossing the ring, its breadth in some parts cannot be less than , , miles. in professor schiaparelli, on computing the orbit of this meteoric stream, noticed the remarkable agreement of its elements with those of swift's or tuttle's comet[ ] ( , iii.), as computed by dr. oppolzer. these coincidences are exhibited in the following table: meteors of comet iii. august . of . longitude of perihelion ° ´ ° ´ ascending node ° ´ ° ´ inclination ° ´ ° ´ perihelion distance . . . period years (?) . years. motion retrograde. retrograde. [ ] mr. swift, of marathon, n. y., had two or three days priority in the discovery of this comet, but unfortunately delayed his announcement of the fact. it appears, therefore, that the third comet of is a part of the meteoric stream whose orbit is crossed by the earth on the th of august. the characteristics of different meteor-zones afford interesting indications in regard to their relative age, the magnitude and composition of their corpuscles, etc. thus, if we compare the streams of august and november , we shall find that the former probably entered our system at a comparatively remote epoch. we have seen that at each return to perihelion the meteoric cluster is extended over a greater arc of its orbit. now, tuttle's comet and the august meteors undoubtedly constituted a single group previous to their entering the solar domain. it is evident, however, from the annual return of the shower during the last years, that the ring is at present nearly if not quite continuous. that the meteoric mass had completed many revolutions before the ninth century of our era is manifest from the frequent showers observed between the years and . at the same time, the long interval of years between the last observed display in the ninth century, and the first in the tenth, seems to indicate the existence of a wide chasm in the ring no more than a thousand years since. neither the period of the meteors nor that of the comet can yet be regarded as accurately ascertained. the latter, however, in all probability, exceeds the former by several years. now, at each passage of the earth through the elliptic stream, those meteoroids nearest the disturbing body must be thrown into orbits differing more or less from that of the primitive group. in like manner the near approach of the _comet_ to the earth at an ancient epoch may account for the lengthening of its periodic time. the meteors of november . professor schiaparelli's brilliant discovery of the relation between comets and meteors may now be ranked with the established truths of astronomy. his hypothesis, however, in regard to the _origin_ of meteoric streams has not been generally accepted. comets and meteors, according to his theory, are derived from cosmical clouds existing in great numbers in stellar space. these nebulæ, in consequence of their own motion or that of the sun, are drawn towards the centre of our system. by the unequal influence of the sun's attraction on different parts, such clouds are transformed into currents of great length before reaching the limits of the planetary system. shooting-stars, fire-balls, aerolites, and comets being all of the same nature, differing merely in size, sometimes fall towards the sun as parts of the same current. the views of dr. weiss, of vienna, differ from those of schiaparelli, in that he regards comets as the original bodies by whose disintegration meteor-streams are gradually formed.[ ] "cosmical clouds," he remarks, "undoubtedly appear in the universe, but only of such density that in most cases they possess sufficient coherence to withstand the destructive operation of the sun's attraction, not only up to the boundaries of our solar system, but even within it. such cosmical clouds will always appear to us as comets when they pass near enough to the earth to become visible. approaching the sun, the comet undergoes great physical changes, which finally affect the stability of its structure: it can no longer hold together: parts of it take independent orbits around the sun, having great resemblance to the orbit of the parent comet. with periodical comets, this process is repeated at each successive approach to the sun. gradually the products of disintegration are distributed along the comet's orbit, and if the earth's orbit cuts this, the phenomenon of shooting-stars is produced." [ ] _astr. nach._, nos. , . for a fuller statement of schiaparelli's theory, see silliman's journal for may, . these views of the distinguished astronomer of vienna are confirmed by the star-shower of november , . that the orbits of the earth and biela's comet intersect at the point passed by the former about the last of november, and that in the comet separated into two visible parts, has been stated in a previous chapter. the comet's non-appearance in december, , and in september, , was regarded by astronomers as presumptive evidence of its progressive dissolution. a meteoric shower, resulting from the earth's collision with the cometary _débris_, was accordingly expected about the th of november. the first indication of the approaching display appeared on the evening of november , when meteors in unusual numbers were observed by professor newton, at new haven, connecticut. on wednesday evening, the th, from the close of twilight till o'clock, a decided shower of shooting-stars was noticed in various parts of the united states. at greencastle, indiana, professor joseph tingley counted meteors in minutes, and at princeton, in the same state, mr. d. eckley hunter counted in minutes. the numbers seen at new haven were considerably greater. the fact that the display commenced before daylight had entirely closed seemed to indicate that only the termination of the shower had been observed in this country. accordingly the display was soon found to have been witnessed from ° e. to ° w. of greenwich, or through ° of longitude. in england the first bolide of the swarm was seen by m. m. brinkley, at o'clock, p.m., in full daylight. the meteors were most numerous in the southern part of the continent, particularly in italy. at the observatory of breslau, according to m. faye, were seen from h. m. to h. m. dr. heis reported that at münster per hour were counted by two observers. at naples, signor gasparis observed two meteors per second. at turin, m. denza, director of the observatory, reported , in h. m.; many of various and delicate colors, and followed by long and brilliant trains. at some points the numbers were so great that an accurate enumeration was wholly impossible. in short, the display was decidedly the most brilliant that has occurred since that of november , . but some of the most interesting circumstances in connection with the phenomena of november , , remain to be detailed. astronomers without exception regarded the display as due to the earth's passage through the _débris_ following in the path of biela's comet. in accordance with this view dr. klinkerfues, of gottingen, concluded that the comet itself, or rather its largest portion, ought to be found in the region of the heavens nearly opposite to that from which the meteoroids appeared to radiate.[ ] as this point in the southern hemisphere could not be observed in europe, he conceived the happy idea of detecting the fugitive _by means of the electric telegraph_. the following was accordingly dispatched to mr. pogson, director of the government observatory at madras, in southern india: "_biela touched earth on th; search near theta centauri_." the first two mornings after the receipt of this dispatch were cloudy at madras. on the third, however, the cometary fragment was found, and its motion accurately measured. the observer described it as circular and rather bright, with no traces of a tail. but one fragment could be detected. on the next morning, december , the comet was again observed. its diameter had sensibly increased; it had a bright nucleus, and still presented a circular aspect. a faint tail was also noticed, equal in length to one-fourth of the moon's apparent diameter. the following mornings being again cloudy, no further observations could be obtained. this cometary mass will be in close proximity to the earth about the last of november, . another brilliant meteoric shower may therefore be expected at that epoch. [ ] the radiant of the biela meteors is near _gamma andromedæ_. the meteors of april . meteoric showers have occurred about the th of april in the following years: b.c. a.d. } } } } } } the probability that these meteors are derived from a ring which intersects the earth's orbit, was first suggested by arago in . a comparison of dates led herrick to designate years as the probable period of the cluster. in the _astronomische nachrichten_, no. , dr. weiss called attention to the fact that the orbit of the first comet of very nearly intersects that of the earth, in longitude °--the point passed by the latter at the epoch of the april meteoric shower. a relation between the meteors and the comet, indicating an approximate equality of periods, was thus suggested as probable. but the comet, according to oppolzer, does not complete a revolution in less than years. if, therefore, the meteoric period is nearly the same, the known dates of star-showers indicate a diffusion of meteoroids around one half of the orbit previous to the display of the year b.c. no subsequent perturbation, then, of a particular _part_ could sensibly effect the general orbit of the stream. the infrequency of the display renders, therefore, the hypothesis of a long period extremely improbable. the entire interval between b.c. and a.d. is years, or periods of . years; and the known dates are all satisfied by the following scheme: b.c. to b.c. .. years = periods of . _y._ each. to a.d. .. " = " . " a.d. to .. " = " . " to .. " = " . " to .. " = " . " with a period of years, the perihelion being interior to the earth's orbit, the aphelion distance of the meteors would be very nearly equal to the distance of uranus. the next shower, if the assumed period be correct, ought to occur about . it is worthy of remark that near the time of the last (hypothetical) return mr. du chaillu witnessed the meteors of this epoch, in considerable numbers, in the interior of africa. the meteors of december . meteoric showers have occurred about the th of december in the following years: . a.d. . "the whole hemisphere was filled with those meteors called falling-stars from midnight till morning, to the great surprise of the beholders in egypt." . in a remarkable shower of falling stars was observed in china. . extraordinary meteoric phenomena were observed at zurich at the same epoch in . . on the night of the th and th of december, , a great number of shooting-stars were seen at parma. at the maximum as many as ten were visible at the same time. . (doubtful.) , , and . maximum probably in . the meteors at this return were far from being comparable in numbers with the ancient displays. the shower, however, was distinctly observed. r. p. greg, esq., of manchester, england, says the period of december , , was "exceedingly well defined." these dates indicate a period of about - / years. thus: to period of . years. to periods of . " to periods of . " to period of . " meteors of october - . meteoric showers were observed from the th to the th of october in the years , , , , and . these dates render it somewhat probable that the period is about - / years. thus: a.d. to periods of . years each. to " . " " to " . " " if these periods are correct, it is a remarkable coincidence that the aphelion distances of the meteoric rings of april , october , november , and december , as well as those of the comets i., and i., are all nearly equal to the mean distance of uranus. the meteors of april , may . professor schiaparelli, in his list of meteoric showers whose radiant points are derived from observations made in italy during the years , , and , describes one as occurring on april and may ; the radiant being in the northern crown. the same shower has also been recognized by r. p. greg, f.r.s., of manchester, england. this meteor-stream, it is now proposed to show, is probably derived from one much more conspicuous in ancient times. in quetelet's "physique du globe" we find meteoric displays of the following dates. in each case the corresponding day for is also given,[ ] in order to exhibit the close agreement of the epochs: . a.d. , april th; corresponding to april th, for . . , " th; " april th, " . , " th; " may st, " . , " th; " april th, " . , " th; " may st, " . , " th; " april th, " [ ] making proper allowance for the precession of the equinoxes. the epochs of and suggest as probable the short period of years. it is found accordingly that the entire interval of years--from to --is equal to mean periods of . years each. with this approximate value the six dates are all represented as follows: from a.d. to a.d. , periods of . years. to , " . " to , " . " to , " . " to , " . " this period nearly corresponds to those of several comets whose aphelion distances are somewhat greater than the mean distance of jupiter. so long as the cluster occupied but a small arc of the orbit the displays would evidently be separated by considerable intervals. the comparative paucity of meteors in modern times may be explained by the fact that the ring has been subject to frequent perturbations by jupiter. groups in which the meteoroids are sparsely scattered. by the labors of heis, greg, herschel, schiaparelli, and others, the radiants of more than fifty sparsely strewn meteor-systems have been determined. of these the following, which are well defined, seem worthy of special study: date. position of radiant. r. a. n. decl. january - ° ° january ° ° april ° ° the orbits and periods, except in the few cases previously considered, are entirely unknown. some of the observed clusters are probably the _débris_ of ancient comets whose aphelia were in the vicinity of jupiter's orbit. chapter x. the origin of comets and meteors. the fact that comets and meteors, or at least a large proportion of such bodies, have entered the solar system from stellar space, is now admitted by all astronomers. the question, however, in regard to the origin and nature of these cosmical clouds still remains undecided. the theory that they consist of matter expelled with great velocity from the fixed stars appears to harmonize the greatest number of facts, and is accordingly entitled to respectful consideration. the evidence by which it is sustained may be briefly stated as follows: . the observations of zollner, respighi, and others, have indicated the operation of stupendous eruptive forces beneath the solar surface. the rose-colored prominences, which janssen and lockyer have shown to be masses of incandescent hydrogen, are regarded by professor respighi as phenomena of eruption. "they are the seat of movements of which no terrestrial phenomenon can afford any idea; masses of matter, the volume of which is many hundred times greater than that of the earth, completely changing their position and form in the space of a few minutes." the nature of this eruptive force is not understood. we may assume, however, that it was in active operation long before the sun had contracted to its present dimensions. . with an initial velocity of projection equal to miles per second, the matter thrown off from the sun would be carried beyond the limits of the solar system, never to return. with velocities somewhat less, it would be transported to distances corresponding to those of the aphelia of the periodic comets. . on the th of september, , professor young, of dartmouth college,[ ] witnessed an extraordinary explosion on the sun's surface. the observer, with his telescope, followed the expelled matter to an elevation of over , miles. the mean velocity between the altitudes of , and , miles was miles per second. this rate of motion _in vacuo_ would indicate an initial velocity of about miles per second. but the sun is surrounded by an extensive atmosphere, whose resistance must have greatly retarded the velocity of the outrush before reaching the height of , miles. the original velocity of these hydrogen clouds was therefore sufficient, in all probability, to have carried them, if unresisted, beyond the solar domain. solid or dense matter propelled with equal force would doubtless have been driven off never to return.[ ] [ ] boston journal of chemistry, november, . [ ] see mr. proctor's interesting discussion of this subject in the monthly notices of the r.a.s., vol. xxxii. . this eruptive force, whatever be its nature, is probably common to the sun and the so-called fixed stars. if so, the dispersed fragments of ejected matter ought to be found in the spaces intervening between sidereal systems. accordingly, the phenomena of comets and meteors have demonstrated the existence of such matter, widely diffused, in the portions of space through which the solar system is moving. . according to mr. sorby the microscopic structure of the aerolites he has examined points evidently to the fact that they have been at one time in a state of fusion from intense heat,--a fact in striking harmony with this theory of their origin. . the velocity with which some meteoric bodies have entered the atmosphere has been greater than that which would have been acquired by simply falling toward the sun from any distance, however great. on the theory of their sidereal origin, this excess of velocity has been dependent on the primitive force of expulsion. the shower of aerolites which fell at pultusk, poland, on the th of january, ,[ ] is not only a remarkable illustration of the fact here stated, but also of another which may be accounted for by the same theory, viz.: that meteoric bodies sometimes enter the solar system in groups or clusters. [ ] see chapter vii. . a striking argument in favor of this theory may be derived from the researches of the late professor graham, considered in connection with those of dr. huggins and other eminent spectroscopists. professor graham found large quantities of hydrogen confined in the pores or cavities of certain meteoric masses. now, the spectroscope has shown that the sun's rose-colored prominences consist of immense volumes of incandescent hydrogen; that the same element exists in great abundance in many of the fixed stars, and even in certain nebulæ; and that the star in the northern crown, whose sudden outburst in so astonished the scientific world, afforded decided indications of its presence. the end. _by the author of this volume._ meteoric astronomy: a treatise on shooting stars, fire balls, and aerolites. by daniel kirkwood, ll.d. mo. extra cloth. $ . . for sale by booksellers generally, or will be sent by mail, postpaid, on receipt of the price by j. b. lippincott & co., publishers, _ and market st., philadelphia_. transcriber's note: obvious errors in spelling and punctuation have been corrected. footnotes have been renumbered and moved from the page end to the end of the paragraph. images have been moved from the middle of a paragraph to the closest paragraph break. available by internet archive (https://archive.org) note: project gutenberg also has an html version of this file which includes the original illustrations. see -h.htm or -h.zip: (http://www.gutenberg.org/files/ / -h/ -h.htm) or (http://www.gutenberg.org/files/ / -h.zip) images of the original pages are available through internet archive. see https://archive.org/details/royalobservatory maun transcriber's note: text enclosed by underscores is in italics (_italics_). small capital text has been replaced with all capitals. the carat character (^) indicates that the following letter is superscripted (example: ii^s). if two or more letters are superscripted they are enclosed in curly brackets (example: d^{ni}). [illustration: flamsteed, the first astronomer royal. (_from the portrait in the 'historia coelestis.'_)] the royal observatory greenwich a glance at its history and work by e. walter maunder, f.r.a.s. with many portraits and illustrations from old prints and original photographs london the religious tract society paternoster row, and st. paul's churchyard london: printed by william clowes and sons, limited, stamford street and charing cross. preface i was present on one occasion at a popular lecture delivered in greenwich, when the lecturer referred to the way in which so many english people travel to the ends of the earth in order to see interesting or wonderful places, and yet entirely neglect places of at least equal importance in their own land. 'ten minutes' walk from this hall,' he said, 'is greenwich observatory, the most famous observatory in the world. most of you see it every day of your lives, and yet i dare say that not one in a hundred of you has ever been inside.' whether the lecturer was justified in the general scope of his stricture or not, the particular instance he selected was certainly unfortunate. it was not the fault of the majority of his audience that they had not entered greenwich observatory, since the regulations by which it is governed forbade them doing so. these rules are none too stringent, for the efficiency of the institution would certainly suffer if it were made a 'show' place, like a picture gallery or museum. the work carried on therein is too continuous and important to allow of interruption by daily streams of sightseers. to those who may at some time or other visit the observatory it may be of interest to have at hand a short account of its history, principal instruments, and work. to the far greater number who will never be able to enter it, but who yet feel an interest in it, i would trust that this little book may prove some sort of a substitute for a personal visit. i would wish to take this opportunity of thanking the astronomer royal for his kind permission to reproduce some of the astronomical photographs taken at the observatory and to photograph the domes and instruments. i would also express my thanks to miss airy, for permission to reproduce the photograph of sir g. b. airy; to mr. j. nevil maskelyne, f.r.a.s., for the portrait of dr. maskelyne; to mr. bowyer, for procuring the portraits of bliss and pond; to messrs. edney and lacey, for many photographs of the royal observatory; and to the editor of _engineering_, for permission to copy two engravings of the astrographic telescope. e. w. m. royal observatory, greenwich, _august, _. [illustration: the new building. (_from a photograph by mr. lacey._)] contents chapter page i. introduction ii. flamsteed iii. halley and his successors iv. airy v. the observatory buildings vi. the time department vii. the transit and circle departments viii. the altazimuth department ix. the magnetic and meteorological departments x. the heliographic department xi. the spectroscopic department xii. the astrographic department xiii. the double-star department index list of illustrations page flamsteed, the first astronomer royal _frontispiece_ the new building general view of the observatory buildings from the new dome flamsteed's sextant the royal observatory in flamsteed's time the 'camera stellata' in flamsteed's time edmund halley halley's quadrant james bradley graham's zenith sector nathaniel bliss nevil maskelyne hadley's quadrant john pond george biddell airy, astronomer royal the astronomer royal's room the south-east tower w. h. m. christie, astronomer royal the astronomer royal's house the courtyard plan of observatory at present time the great clock and porter's lodge the chronograph the time-desk harrison's chronometer the chronometer room the chronometer oven the transit pavilion 'lost in the birkenhead' the transit circle the mural circle airy's altazimuth new altazimuth building the new altazimuth the new observatory as seen from flamsteed's observatory the self-registering thermometers the anemometer room, north-west turret the anemometer trace magnetic pavilion--exterior magnetic pavilion--interior the dallmeyer photo-heliograph photograph of a group of sun-spots the great nebula in orion the half-prism spectroscope on the south-east equatorial the workshop the -inch reflector with the new spectroscope attached 'chart plate' of the pleiades the control pendulum and the base of the thompson telescope the astrographic telescope the driving clock of the astrographic telescope the thompson telescope in the new dome the nebulÆ of the pleiades double-star observation with the south-east equatorial the south-east dome with the shutter open [illustration: general view of the observatory buildings from the new dome. (_from a photograph by mr. lacey._)] the royal observatory greenwich chapter i introduction i had parted from a friend one day just as he met an acquaintance of his to whom i was unknown. 'who is that?' said the newcomer, referring to me. my friend replied that i was an astronomer from greenwich observatory. 'indeed; and what does he do there?' this question completely exhausted my friend's information, for as his tastes did not lead him in the direction of astronomy, he had at no time ever concerned himself to inquire as to the nature of my official duties. 'oh--er--why--he _observes_, don't you know?' and the answer, vague as it was, completely slaked the inquirer's thirst for knowledge. it is not every one who has such exceedingly nebulous ideas of an astronomer's duties. more frequently we find that the inquirer has already formed a vivid and highly-coloured picture of the astronomer at his 'soul-entrancing work.' resting on a comfortable couch, fixed at a luxurious angle, the eye-piece of some great and perfect instrument brought most conveniently to his eye, there passes before him, in grand procession, a sight such as the winter nights, when clear and frosty, give to the ordinary gazer, but increased ten thousand times in beauty, brilliance, and wonder by the power of his telescope. for him jupiter reveals his wind-drifted clouds and sunset colours; for him saturn spreads his rings; for him the snows of mars fall and melt, and a thousand lunar plains are ramparted with titanic crags; his are the star-clusters, where suns in their first warm youth swarm thicker than hiving bees; his the faint veils of nebulous smoke, the first hint of shape in worlds about to be, or, perchance, the last relics of worlds for ever dead. and beside the enjoyment of all this entrancing spectacle of celestial beauty, the fortunate astronomer sits at his telescope and _discovers_--always he _discovers_. this, or something like it, is a very popular conception of an astronomer's experiences and duty; and consequently many, when they are told that 'discoveries' are not made at greenwich, are inclined to consider that the observatory has failed in its purpose. an astronomer without 'discoveries' to his record is like an angler who casts all day and comes home without fish--obviously an idle or incompetent person. again, it is considered that astronomy is a most transcendental science. it deals with infinite distances, with numbers beyond all power of human intellect to appreciate, and therefore it is supposed, on the one hand, that it is a most elevating study, keeping the mind continually on the stretch of ecstasy, and, on the other hand, that it is utterly removed from all connection with practical, everyday, ordinary life. these ideas as to the royal observatory, or ideas like them, are very widely current, and they are, in every respect, exactly and wholly wrong. first of all, greenwich observatory was originally founded, and has been maintained to the present day, for a strictly practical purpose. next, instead of leading a life of dreamy ecstasy or transcendental speculation, the astronomer has, perhaps, more than any man, to give the keenest attention to minute practical details. his life, on the one side, approximates to that of the engineer; on the other, to that of the accountant. thirdly, the professional astronomer has hardly anything to do with the show places of the sky. it is quite possible that there are many people whose sole opportunity of looking through a telescope is the penny peep through the instrument of some itinerant showman, who may have seen more of these than an active astronomer in a lifetime; while as to 'discoveries,' these lie no more within the scope of our national observatory than do geographical discoveries within that of the captain and officers of an ocean liner. if it is not to afford the astronomer beautiful spectacles, nor to enable him to make thrilling discoveries, what is the purpose of greenwich observatory? first and foremost, it is to assist navigation. the ease and certainty with which to-day thousands of miles of ocean are navigated have ceased to excite any wonder. we do not even think about it. we go down to the docks and see, it may be, one steamer bound for halifax, another for new york, a third for charleston, a fourth for the west indies, a fifth for rio de janeiro; and we unhesitatingly go on board the one bound for our chosen destination, without the faintest misgiving as to its direction. we have no more doubt about the matter than we have in choosing our train at a railway station. yet, whilst the train is obliged to follow a narrow track already laid for it, from which it cannot swerve an inch, the steamer goes forth to traverse for many days an ocean without a single fixed mark or indication of direction; and it is exposed, moreover, to the full force of winds and currents, which may turn it from its desired path. but for this facility of navigation, great britain could never have obtained her present commercial position and world-wide empire. 'for the lord our god most high, he hath made the deep as dry; he has smote for us a pathway, to the ends of all the earth.' part of this facility is, of course, due to the invention of the steam engine, but much less than is generally supposed. even yet the clippers, with their roods of white canvas, are not entirely superseded; and if we could conceive of all steamships being suddenly annihilated, ere long the sailing vessels would again, as of yore, prove the 'swift shuttles of an empire's loom, that weave us main to main.' but with the art of navigation thrust back into its condition of a hundred and fifty years ago, it is doubtful whether a sufficient tide of commerce could be carried on to keep our home population supplied, or to maintain a sufficiently close political connection between these islands and our colonies. navigation was in a most primitive condition even as late as the middle of last century. then the method of finding a ship's longitude at sea was the insufficient one of dead reckoning. in other words, the direction and speed of the ship were estimated as closely as possible, and so the position was carried on from day to day. the uncertainty of the method was very great, and many terrible stories might be told of the disastrous consequences which might, and often did, follow in the train of this method by guess-work. it will be sufficient, however, to cite the instance of commodore anson. he wanted to make the island of juan fernandez, where he hoped to obtain fresh water and provisions, and to recruit his crew, many of whom were suffering from that scourge of old-time navigators--scurvy. he got into its latitude easily enough, and ran eastward, believing himself to be west of the island. he was, however, really east of it, and therefore made the mainland of america. he had therefore to turn round and sail westwards, losing many days, during which the scurvy increased upon his crew, many of whom died from the terrible disease before he reached the desired island. the necessity for finding out a ship's place when at sea had not been very keenly felt until the end of the fifteenth century. it was always possible for the sailor to ascertain his latitude pretty closely, either by observing the height of the pole-star at night or the height of the sun at noonday; and so long as voyages were chiefly confined to the mediterranean sea, and the navigators were content for the most part to coast from point to point, rarely losing sight of land, the urgency of solving the second problem--the longitude of the ship--was not so keenly felt. but immediately the discoveries of the great portuguese and spanish navigators brought a wider, bolder navigation into vogue, it became a matter of the first necessity. to take, for example, the immortal voyage of christopher columbus. his purpose in setting out into the west was to discover a new way to india. the venetians and genoese practically possessed the overland route across the isthmus of suez and down the red sea. vasco da gama had opened out the route eastward round the cape. firmly convinced that the world was a globe, columbus saw that a third route was possible, namely, one nearly due west; and when, therefore, he reached the bahamas, after traversing some ° of longitude, he believed that he was in the islands of the china sea, some ° from spain. those who followed him still laboured under the same impression, and when they reached the mainland of america, believed that they were close to the shores of india, which was still distant from them by half the circumference of the globe. little by little the intrepid sailors of the sixteenth century forced their way to a true knowledge of the size of the globe, and of the relative position of the great continents. but this knowledge was only attained after many disasters and terrible miseries; and though a new kind of navigation was established--the navigation of the open ocean, far away from any possible landmark, a navigation as different as could be conceived from the old method of coasting--yet it remained terribly risky and uncertain throughout the sixteenth century. therefore many mathematicians endeavoured to solve the problem of determining the position of a ship when at sea. their suggestions, however, remained entirely fruitless at the time, though in several instances they struck upon principles which are being employed at the present day. the first country to profit by the discovery of america was spain, and hence spain was the first to feel keenly the pinch of the problem. in , therefore, philip iii. offered a prize of , crowns to any one who would devise a method by which a captain of a vessel could determine his position when out of sight of land. holland, which had recently started on its national existence, and which was challenging the colonial empire of spain, followed very shortly after with the offer of a reward of , florins. not very long after the offer of these rewards, a master mind did work out a simple method for determining the longitude, a method theoretically complete, though practically it proved inapplicable. this was galileo, who, with his newly invented telescope, had discovered that jupiter was attended by four satellites. at first sight such a discovery, however interesting, would seem to have not the slightest bearing upon the sailor's craft, or upon the commercial progress of one nation or another. but galileo quickly saw in it the promise of great practical usefulness. the question of the determination of the place of a ship when in the open ocean really resolved itself into this: how could the navigator ascertain at any time what was the true time, say at the port from which he sailed? as already pointed out, it was possible, by observing the height of the sun at noon, or of the pole-star at night, to infer the latitude of the ship. the longitude was the point of difficulty. now, the longitude may be expressed as the difference between the local time of the place of observation and the local time at the place chosen as the standard meridian. the sailor could, indeed, obtain his own local time by observations of the height of the sun. the sun reached its greatest height at local noon, and a number of observations before and after noon would enable him to determine this with sufficient nicety. but how was he to determine when he, perhaps, was half-way across the atlantic, what was the local time at genoa, cadiz, lisbon, bristol, or amsterdam, or whatever was the port from which he sailed? galileo thought out a way by which the satellites of jupiter could give him this information. for as they circle round their primary, they pass in turn into its shadow, and are eclipsed by it. it needed, then, only that the satellites should be so carefully watched, that their motions, and, consequently, the times of their eclipses could be foretold. it would follow, then, that if the mariner had in his almanac the local time of the standard city at which a given satellite would enter into eclipse, and he were able to note from the deck of his vessel the disappearance of the tiny point, he would ascertain the difference between the local times of the two places, or, in other words, the difference of their longitudes. the plan was simplicity itself, but there were difficulties in carrying it out, the greatest being the impossibility of satisfactorily making telescopic observations from the moving deck of a ship at sea. nor were the observations sufficiently sharp to be of much help. the entry of a satellite into the shadow of jupiter is in most cases a somewhat slow process, and the moment of complete disappearance would vary according to the size of the telescope, the keenness of the observer's sight, and the transparency of the air. as the power and commerce of spain declined, two other nations entered into the contest for the sovereignty of the seas, and with that sovereignty predominance in the new world of america--france and england. the problem of the longitude at sea, or, as already pointed out, what amounts to the same thing, the problem how to determine when at sea the local time at some standard place, became, in consequence, of greater necessity to them. the standard time would be easily known, if a thoroughly good chronometer which did not change its rate, and which was set to the standard time before starting, was carried on board the ship. this plan had been proposed by gemma frisius as early as , but at the time was a mere suggestion, as there were no chronometers or watches sufficiently good for the purpose. there was, however, another method of ascertaining the standard time. the moon moves pretty quickly amongst the stars, and at the present time, when its motions are well known, it is possible to draw up a table of its distances from a number of given stars at definite times for long periods in advance. this is actually done to-day in the _nautical almanac_, the moon's distance from certain stars being given for every three hours of greenwich time. it is possible, then, by measuring these distances, and making, as in the case of the latitude, certain corrections, to find out the time at greenwich. in short, the whole sky may be considered as a vast clock set to greenwich time, the stars being the numbers on the dial face, and the moon the hand (for this clock has only one hand) moving amongst them. the local apparent time--that is, the time at the place at which the ship itself was--is a simpler matter. it is noon at any place when the sun is due south--or, as we may put it a little differently, when it culminates--that is, when it reaches its highest point. to find the longitude at sea, therefore, it was necessary to be able to predict precisely the apparent position of the moon in the sky for any time throughout the entire year, and it was also necessary that the places of the stars themselves should be very accurately known. it was therefore to gather the materials for a better knowledge of the motions of the moon and the position of the stars that greenwich observatory was founded, whilst the _nautical almanac_ was instituted to convey this information to mariners in a convenient form. this proposal was actually made in the reign of charles ii. by a frenchman, le sieur de saint-pierre, who, having secured an introduction to the duchess of portsmouth, endeavoured to obtain a reward for his scheme. it would appear that he had simply borrowed the idea from a book which an eminent french mathematician brought out forty years before, without having himself any real knowledge of the subject. but when the matter was brought before the king's notice, he desired some of the leading scientific men of the day to report upon its practicability, and the rev. john flamsteed was the man selected for the task. he reported that the scheme in itself was a good one, but impracticable in the then state of science. the king, who, in spite of the evil reputation which he has earned for himself, took a real interest in science, was startled when this was reported to him, and commanded the man who had drawn his attention to these deficiencies 'to apply himself,' as the king's astronomer, 'with the most exact care and diligence to the rectifying the tables of the motions of the heavens and the places of the fixed stars, in order to find out the so much desired longitude at sea, for the perfecting the art of navigation.' this man, the rev. john flamsteed, was accordingly appointed first astronomer royal at the meagre salary of £ a year, with full permission to provide himself with the instruments he might require, at his own expense. he followed out the task assigned to him with extreme devotion, amidst many difficulties and annoyances, until his death in . he has been succeeded by seven astronomers royal, each of whom has made it his first object to carry out the original scheme of the institution; and the chief purpose of greenwich observatory to-day, as when it was founded in , is to observe the motions of the sun, moon, and planets, and to issue accurate star catalogues. it will be seen, therefore, that the establishment of greenwich observatory arose from the actual necessity of the nation. it was an essential step in its progress towards its present position as the first commercial nation. no thoughts of abstract science were in the minds of its founders; there was no desire to watch the cloud-changes on jupiter, or to find out what sirius was made of. the observatory was founded for the benefit of the royal navy and of the general commerce of the realm; and, in essence, that which was the sole object of its foundation at the beginning has continued to be its first object down to the present time. it was impossible that the work of the observatory should be always confined within the above limits, and it will be my purpose, in the pages which follow, to describe when and how the chief expansions of its programme have taken place. but assistance to navigation is now, and has always been, the dominant note in its management. chapter ii flamsteed for the first century of its existence, the lives of its astronomers royal formed practically the history of the royal observatory. during this period, the observatory was itself so small that the astronomer royal, with a single assistant, sufficed for the entire work. everything, therefore, depended upon the ability, energy, and character of the actual director. there was no large organized staff, established routine, or official tradition, to keep the institution moving on certain lines, irrespective of the personal qualities of the chief. it was specially fortunate, therefore, that the first four astronomers royal, flamsteed, halley, bradley, and maskelyne (for bliss, the immediate successor of bradley, reigned for so short a time that he may be practically left out of the count), were all men of the most conspicuous ability. it will be convenient to divide the history of the first seven astronomers royal into three sections. in the first, we have the founder, john flamsteed, a pathetic and interesting figure, whom we seem to know with especial clearness, from the fulness of the memorials which he has left to us. he was succeeded by the man who was, indeed, best fitted to succeed him, but whom he most hated. the second to the sixth astronomers royal formed what we might almost speak of as a dynasty, each in turn nominating his successor, who had entered into more or less close connection with the observatory during the lifetime of the previous director; and the lives of these five may well form the second section. the line was interrupted after the resignation of the sixth astronomer royal, and the third section will be devoted to the seventh director, airy, under whom the observatory entered upon its modern period of expansion. 'god suffers not man to be idle, although he swim in the midst of delights; for when he had placed his own image (adam) in a paradise so replenished (of his goodness) with varieties of all things, conducing as well to his pleasure as sustenance, that the earth produced of itself things convenient for both,--he yet (to keep him out of idleness) commands him to till, prune, and dress his pleasant, verdant habitation; and to add (if it might be) some lustre, grace, or conveniency to that place, which, as well as he, derived its original from his creator.' in these words john flamsteed begins the first of several autobiographies which he has handed down to us; this particular one being written before he attained his majority, 'to keep myself from idleness and to recreate myself.' 'i was born,' he goes on, 'at denby, in derbyshire, in the year , on the th day of august, at hours minutes after noon. my father, named stephen, was the third son of mr. william flamsteed, of little hallam; my mother, mary, was the daughter of mr. john spateman, of derby, ironmonger. from these two i derived my beginning, whose parents were of known integrity, honesty, and fortune, as they [were] of equal extraction and ingenuity; betwixt whom i [was] tenderly educated (by reason of my natural weakness, which required more than ordinary care) till i was aged three years and a fortnight; when my mother departed, leaving my father a daughter, then not a month old, with me, then weak, to his fatherly care and provision.' the weakly, motherless boy became at an early age a voracious reader. at first, he says-- 'i began to affect the volubility and ranting stories of romances; and at twelve years of age i first left off the wild ones, and betook myself to read the better sort of them, which, though they were not probable, yet carried no seeming impossibility in the fiction. afterwards, as my reason increased, i gathered other real histories; and by the time i was fifteen years old i had read, of the ancients, plutarch's _lives_, appian's and tacitus's _roman histories_, holingshed's _history of the kings of england_, davies's _life of queen elizabeth_, saunderson's of _king charles the first_, heyling's _geography_, and many others of the moderns; besides a company of romances and other stories, of which i scarce remember a tenth at present.' flamsteed received his education at the free school at derby, where he continued until the whitsuntide of , when he was nearly sixteen years of age. two years earlier than this, however, a great misfortune fell upon him. 'at fourteen years of age,' he writes, 'when i was nearly arrived to be the head of the free-school, [i was] visited with a fit of sickness, that was followed with a consumption and other distempers, which yet did not so much hinder me in my learning, but that i still kept my station till the form broke up, and some of my fellows went to the universities; for which, though i was designed, my father thought it not advisable to send me, by reason of my distemper.' this was a keen disappointment to him, but seems to have really been the means of determining his career. the sickly, suffering boy could not be idle, though 'a day's short reading caused so violent a headache;' and a month or two after he had left school, he had a book lent to him--sacrobosco's _de sphæra_, in latin--which was the beginning of his mathematical studies. a partial eclipse of the sun in september of the same year seems to have first drawn his attention to astronomical observation, and during the winter his father, who had himself a strong passion for arithmetic, instructed him in that science. it was astonishing how quickly his appetite for his new subjects grew. the _art of dialling_, the calculation of tables of the sun's altitudes for all hours of the day, and for different latitudes, and the construction of a quadrant--'of which i was not meanly joyful'--were the occupations of this winter of illness. in he made the acquaintanceship of two friends, mr. george linacre and mr. william litchford; the former of whom taught him to recognize many of the fixed stars, whilst the latter was the means of his introduction to a knowledge of the motions of the planets. 'i had now completed eighteen years, when the winter came on, and thrust me again into the chimney; whence the heat and dryness of the preceding summer had happily once before withdrawn me.' the following year, , was memorable to him 'for the appearance of the comet,' and for a journey which he made to ireland to be 'stroked' for his rheumatic disorder by valentine greatrackes, a kind of mesmerist, who had the repute of effecting wonderful cures. the journey, of which he gives a full and vivid account, occupied a month; but though he was a little better, the following winter brought him no permanent benefit. but, ill or well, he pressed on his astronomical studies. a large partial eclipse of the sun was due the following june; he computed the particulars of it for derby, and observed the eclipse itself to the best of his ability. he argued out for himself 'the equation of time'; the difference, that is, between time as given by the actual sun, or 'apparent time,' and that given by a perfect clock, or 'mean time.' he drew up a catalogue of seventy stars, computing their right ascensions, declinations, longitudes, and latitudes for the year ; he attempted to determine the inclination of the ecliptic, the mean length of the tropical year, and the actual distance of the earth from the sun. and these were the recreations of a sickly, suffering young man, not yet twenty-one years of age, and who had only begun the study of arithmetic, such as fractions and the rule of three, four years previously! his next attempt was almanac-making, in the which he improved considerably upon those current at the time. his almanac for was rejected, however, and returned to him, and, not to lose his whole labour, he sent his calculations of an eclipse of the sun, and of five occultations of stars by the moon, which he had undertaken for the almanac, to the royal society. he sent the paper anonymously, or, rather, signed it with an anagram, 'in mathesi a sole fundes,' for 'johannes flamsteedius.' his covering letter ends thus:-- 'excuse, i pray you, this juvenile heat for the concerns of science and want of better language, from one who, from the sixteenth year of his age to this instant, hath only served one bare apprenticeship in these arts, under the discouragement of friends, the want of health, and all other instructors except his better genius.' this letter was dated november , , and on january , mr. oldenburg, the secretary of the society, replied to him in a letter which the young man cannot but have felt encouraging and flattering to the highest degree. 'though you did what you could to hide your name from us,' he writes, 'yet your ingenious and useful labours for the advancement of astronomy addressed to the noble president of the royal society, and some others of that illustrious body, did soon discover you to us, upon our solicitous inquiries after their worthy author.' and after congratulating him upon his skill, and encouraging him to furnish further similar papers, he signs himself, 'your very affectionate friend and real servant'--no unmeaning phrase, for the friendship then commenced ceased only with oldenburg's life. the following june, his father, pleased with the notice that some of the leading scientific men of the day were taking of his son, sent him up to london, that he might be personally acquainted with them; and he then was introduced to sir jonas moore, the surveyor of the ordnance, who made him a present of townley's micrometer, and promised to furnish him with object-glasses for telescopes at moderate rates. on his return journey he called at cambridge, where he visited dr. barrow and newton, and entered his name in jesus college. it was not until the following year, , that he was enabled to complete his own observatory, as he had had to wait long for the lenses which sir jonas moore and collins had promised to procure for him. he still laboured under several difficulties, in that he had no good means for measuring time, pendulum clocks not then being common. he, therefore, with a practical good sense which was characteristic, refrained from attempting anything which lay out of his power to do well, and he devoted himself to such observations as did not require any very accurate knowledge of the time. at the same time, he was careful to ascertain the time of his observations as closely as possible, by taking the altitudes of the stars. the next four years seem to have passed exceedingly pleasantly to him. the notes of ill-health are few. he was making rapid progress in his acquaintanceship with the work of other astronomers, particularly with those of the three marvellously gifted young men--horrox, crabtree, and gascoigne--who had passed away shortly before his own birth. he was making new friends in scientific circles, and, in particular, sir jonas moore was evidently esteeming him more and more highly. in he became more intimate with newton, the occasion which led to this acquaintanceship being the amusing one, that his assistance was asked by newton, who had found himself unable to adjust a microscope, having forgotten its object-glass--not the only instance of the great mathematician's absent-mindedness. the same year he took his degree of a.m. at cambridge, designing to enter the church; but sir jonas moore was extremely anxious to give him official charge of an observatory, and was urging the royal society to build an astronomical observatory at chelsea college, which then belonged to that body. he therefore came up to london, and resided some months with sir jonas moore at the tower. but shortly after his coming up to london, 'an accident happened,' to use his own expression, that hastened, if it did not occasion, the building of greenwich observatory. 'a frenchman that called himself le sieur de st. pierre, having some small skill in astronomy, and made an interest with a french lady, then in favour at court, proposed no less than the discovery of the longitude, and had procured a kind of commission from the king to the lord brouncker, dr. ward (bishop of salisbury), sir christopher wren, sir charles scarborough, sir jonas moore, colonel titus, dr. pell, sir robert murray, mr. hook, and some other ingenious gentlemen about the town and court, to receive his proposals, with power to elect, and to receive into their number, any other skilful persons; and having heard them, to give the king an account of them, with their opinion whether or no they were practicable, and would show what he pretended. sir jonas moore carried me with him to one of their meetings, where i was chosen into their number; and, after, the frenchman's proposals were read, which were: '( ) to have the year and day of the observations. '( ) the height of two stars, and on which side of the meridian they appeared. '( ) the height of the moon's two limbs. '( ) the height of the pole--all to degrees and minutes. 'it was easy to perceive, from these demands, that the sieur understood not that the best lunar tables differed from the heavens; and that, therefore, his demands were not sufficient for determining the longitude of the place where such observations were, or should be, made, from that to which the lunar tables were fitted, which i represented immediately to the company. but they, considering the interests of his patroness at court, desired to have him furnished according to his demands. i undertook it; and having gained the moon's true place by observations made at derby, february , , and november , , gave him observations such as he demanded. the half-skilled man did not think they could have been given him, and cunningly answered "_they were feigned_." i delivered them to dr. pell, february , - , who, returning me his answer some time after, i wrote a letter in english to the commissioners, and another in latin to the sieur, to assure him they were not feigned, and to show them that, if they had been, yet if we had astronomical tables that would give us the two places of the fixed stars and the moon's true places, both in longitude and latitude, nearer than to half a minute, we might hope to find the longitude of places by lunar observations, but not by such as he demanded. but that we were so far from having the places of the fixed stars true, that the tychonic catalogues often erred ten minutes or more; that they were uncertain to three or four minutes, by reason that tycho assumed a faulty obliquity of the ecliptic, and had employed only plain sights in his observations: and that the best lunar tables differ one-quarter, if not one-third, of a degree from the heavens; and lastly, that he might have learnt better methods than he proposed, from his countryman morin, whom he had best consult before he made any more demands of this nature.' this was in effect to tell st. pierre that his proposal was neither original nor practicable. if st. pierre had but consulted morin's writings (morin himself had died more than eighteen years before), he would have known that practically the same proposal had been laid before cardinal richelieu in , and had been rejected, as quite impracticable in the then state of astronomical knowledge. possibly flamsteed meant further to intimate that st. pierre had simply stolen his method from morin, hoping to trade it off upon the government of another country; in which case he would no doubt regard flamsteed's letter as a warning that he had been found out. flamsteed continues:-- 'i heard no more of the frenchman after this; but was told that, my letters being shown king charles, he startled at the assertion of the fixed stars' places being false in the catalogue; said, with some vehemence, "he must have them anew observed, examined, and corrected, for the use of his seamen;" and further (when it was urged to him how necessary it was to have a good stock of observations taken for correcting the motions of the moon and planets), with the same earnestness, "he must have it done." and when he was asked who could, or who should do it? "the person (says he) that informs you of them." whereupon i was appointed to it, with the incompetent allowance aforementioned; but with assurances, at the same time, of such further additions as thereafter should be found requisite for carrying on the work.' [illustration: flamsteed's sextant. (_from an engraving in the 'historia coelestis.'_)] thus, in his twenty-ninth year, john flamsteed became the first astronomer royal. in many ways he was an ideal man for the post. in the twelve years which had passed since he left school he had accomplished an amazing amount of work. despite his constant ill-health and severe sufferings, and the circumstance--which may be inferred from many expressions in his autobiographies--that he assisted his father in his business, he had made himself master, perhaps more thoroughly than any of his contemporaries, of the entire work of a practical astronomer as it was then understood. he was an indefatigable computer; the calculation of tables of the motions of the moon and planets, which should as faithfully as possible represent their observed positions, had had an especial attraction for him, and, as has been already mentioned, some years before his appointment he had drawn up a catalogue of stars, based upon the observations of tycho brahe. more than that, he had not been a merely theoretical worker, he had been a practical observer of very considerable skill, and, in the dearth of suitable instruments, had already made one or two for himself, and had contemplated the making of others. in his first letter to sir jonas moore he asks for instruction as to the making of object-glasses for telescopes, for he was quite prepared to set about the task of making his own. in addition to his tireless industry, which neither illness nor suffering could abate, he was a man of singularly exact and business-like habits. the precision with which he preserves and records the dates of all letters received or sent is an illustration of this. on the other hand, he had the defects of his circumstances and character. his numerous autobiographical sketches betray him, not indeed as a conceited man, in the ordinary sense of the word, but as an exceedingly self-conscious one. devout and high-principled he most assuredly was, but, on the other hand, he shows in almost every line he wrote that he was one who could not brook anything like criticism or opposition. such a man, however efficient, was little likely to be happy as the first incumbent of a new and important government post; but there was another circumstance which was destined to cause him greater unhappiness still. if we believe, as surely we must, that not only the moral and the physical progress of mankind is watched over and controlled by god's good providence, but its intellectual progress as well, then there can be no doubt that john flamsteed was raised up at this particular time, not merely to found greenwich observatory, and to assist the solution of the problem of the longitude at sea, but also, and chiefly, to become the auxiliary to a far greater mind, the journeyman to a true master-builder. but for the founding of greenwich observatory, and for john flamsteed's observations made therein, the working out of newton's grand theory of gravitation must have been hindered, and its acceptance by the men of science of his time immensely delayed. we cannot regard as accidental the combination, so fortunate for us, of newton, the great world-genius, to work out the problem, of flamsteed, the painstaking observer, to supply him with the materials for his work, and of the newly-founded institution, greenwich observatory, where flamsteed was able to gather those materials together. this is the true debt that we owe to flamsteed, that, little as he understood the position in which he had been placed from the standpoint from which we see it to-day, yet, to the extent of his ability, and as far as he conceived it in accordance with his duty, he gave newton such assistance as he could. this is how we see the matter to-day. it wore a very different aspect in flamsteed's eyes; and the two following documents, the one, the warrant founding the observatory and making him astronomer royal; the other, the warrant granting him a salary, will go far to explain his position in the matter. he had a high-sounding, official position, which could not fail to impress him with a sense of importance; whilst his salary was so insufficient that he naturally regarded himself as absolute owner of his own work. _'warrant for the payment of mr. flamsteed's salary._ 'charles rex. 'whereas, we have appointed our trusty and well-beloved john flamsteed, master of arts, our astronomical observator, forthwith to apply himself with the most exact care and diligence to the rectifying the tables of the motions of the heavens, and the places of the fixed stars, so as to find out the so-much-desired longitude of places for the perfecting the art of navigation, our will and pleasure is, and we do hereby require and authorize you, for the support and maintenance of the said john flamsteed, of whose abilities in astronomy we have very good testimony, and are well satisfied, that from time to time you pay, or cause to be paid, unto him, the said john flamsteed, or his assigns, the yearly salary or allowance of one hundred pounds per annum; the same to be charged and borne upon the quarter-books of the office of the ordnance, and paid to him quarterly, by even and equal portions, by the treasurer of our said office, the first quarter to begin and be accompted from the feast of st. michael the archangel last past, and so to continue during our pleasure. and for so doing, this shall be as well unto you, as to the auditors of the exchequer, for allowing the same, and all other our officers and ministers whom it may concern, a full and sufficient warrant. 'given at our court at whitehall, the th day of march, - . 'by his majesty's command, 'j. williamson. 'to our right-trusty and well-beloved counsellor, sir thomas chichely, knt., master of our ordnance, and to the lieutenant-general of our ordnance, and to the rest of the officers of our ordnance, now and for the time being, and to all and every of them.' _'warrant for building the observatory._ 'charles rex. 'whereas, in order to the finding out of the longitude of places for perfecting navigation and astronomy, we have resolved to build a small observatory within our park at greenwich, upon the highest ground, at or near the place where the castle stood, with lodging-rooms for our astronomical observator and assistant, our will and pleasure is, that according to such plot and design as shall be given you by our trusty and well-beloved sir christopher wren, knight, our surveyor-general of the place and scite of the said observatory, you cause the same to be fenced in, built and finished with all convenient speed, by such artificers and workmen as you shall appoint thereto, and that you give order unto our treasurer of the ordnance for the paying of such materials and workmen as shall be used and employed therein, out of such monies as shall come to your hands for old and decayed powder, which hath or shall be sold by our order of the st of january last, provided that the whole sum, so to be expended or paid, shall not exceed five hundred pounds; and our pleasure is, that all our officers and servants belonging to our said park be assisting to those that you shall appoint, for the doing thereof, and for so doing, this shall be to you, and to all others whom it may concern, a sufficient warrant. 'given at our court at whitehall, the nd day of june, , in the th year of our reign. 'by his majesty's command, 'j. williamson. 'to our right-trusty and well-beloved counsellor, sir thomas chichely, knt., master-general of our ordnance.' the first question that arose, when it had been determined to found the new observatory, was where it was to be placed. hyde park was suggested, and sir jonas moore recommended chelsea college, where he had already thought of establishing flamsteed in a private observatory. fortunately, both these localities were set aside in favour of one recommended by sir christopher wren. there was a small building on the top of the hill in the royal park of greenwich belonging to the crown, and which was now of little or no use. visible from the city, and easily accessible by that which was then the best and most convenient roadway, the river thames, it was yet so completely out of town as to be entirely safe from the smoke of london. in greenwich park, too, but on the more easterly hill, charles i. had contemplated setting up an observatory, but the pressure of events had prevented him carrying out his intention. a further practical advantage was that materials could be easily transported thither. the management of public affairs under charles ii. left much to be desired in the matter of efficiency and economy, and it was not very easy to procure what was wanted for the erection of a purely scientific building. however, the matter was arranged. a gate-house demolished in the tower supplied wood; iron, and lead, and bricks were supplied from tilbury fort, and these could be easily brought by water to the selected site. the sum of £ , actually £ , was further allotted from the results of a sale of spoilt gunpowder; and with these limited resources greenwich observatory was built. the foundation-stone was laid august , , and flamsteed amused himself by drawing the horoscope of the observatory, a fact which--in spite of his having written across the face of the horoscope _risum teneatis amici?_ (can you keep from laughter, my friends?), and his having two or three years before written very severely against the imposture of astrology--has led some modern astrologers to claim him as a believer in their cult. he actually entered into residence july , . [illustration: the royal observatory in flamsteed's day. (_from an engraving in the 'historia coelestis.'_)] his position was not a bright one. the government had, indeed, provided him with a building for his observatory, and a small house for his own residence, but he had no instrument and no assistant. the first difficulty was partly overcome for the moment by gifts or loans from sir jonas moore, and by one or two small loans from the royal society. the death of this great friend and patron, four years after the founding of the observatory, and only three years after his entering into residence, deprived him of several of these; it was with difficulty that he maintained against sir jonas' heirs his claim to the instruments which sir jonas had given him. there was nothing for him to do but to make his instruments himself, and in he built a mural quadrant of fifty inches radius. his circumstances improved the following year, when lord north gave him the living of burstow, near horley, surrey, flamsteed having received ordination almost at the time of his appointment to the astronomer royalship. we have little or no account of the way in which he fulfilled his duties as a clergyman. evidently he considered that his position as astronomer royal had the first claim upon him. at the same time, comparatively early in life he had expressed his desire to fill the clerical office, and he was a man too conscientious to neglect any duty that lay upon him. that in spite of his feeble health he often journeyed to and fro between burstow and greenwich we know; and we may take it as certain that at a time when the standard of clerical efficiency was extremely low, he was not one of those who 'for their bellies' sake, creep and intrude and climb into the fold.' his chief source of income, however, seems to have been the private pupils whom he took in mathematics and astronomy. these numbered in the years to no fewer than ; and as many of them were of the very first and wealthiest families in the kingdom, the gain to flamsteed in money and influence must have been considerable. but it was most distasteful work. it was in no sense that which he felt to be his duty, and which he had at heart. it was undertaken from sheer, hard necessity, and he grudged bitterly the time and strength which it diverted from his proper calling. how faithfully he followed that, one single circumstance will show. in the thirteen years ending , he made , observations, and had revised single-handed the whole of the theories and tables of the heavenly bodies then in use. in the death of his father brought him a considerable accession of means, and, far more important, the assistance of abraham sharp,[ ] the first and most distinguished of the long list of greenwich assistants, men who, though far less well known than the astronomers royal, have contributed scarcely less in their own field to the high reputation of the observatory. [ ] abraham sharp had been with flamsteed earlier than this--in and . sharp was not only a most careful and indefatigable calculator, he was what was even more essential for flamsteed--a most skilful instrument-maker; and he divided for him a new mural arc of ° and seven feet radius, with which he commenced operations on december , . above all, sharp became his faithful and devoted friend and adherent, and no doubt his sympathy strengthened flamsteed to endure the trouble which was at hand. that trouble began in , when newton visited the royal observatory. at that time flamsteed, though he had done so much, had published nothing, and newton, who had made his discovery of the laws of gravitation some few years before, was then employed in deducing from them a complete theory of the moon's motion. this work was one of absolutely first importance. in the first place and chiefly, upon the success with which it could be carried out, depended undoubtedly the acceptance of the greatest discovery which has yet been made in physical science. secondarily--and this should, and no doubt did, appeal to flamsteed--the perfecting of our knowledge of the movements of the moon was a primary part of the very work which he was commissioned to do as astronomer royal. newton was, therefore, anxious beyond everything to receive the best possible observations of the moon's places, and he came to flamsteed, as to the man from whom he had a right to expect to receive a supply of them. at first flamsteed seems to have given these as fully as he was able; but it is evident that newton chafed at the necessity for these frequent applications to flamsteed, and to the constant need of putting pressure upon him. flamsteed, on the other hand, as clearly evidently resented this continual demand. feeling, as he keenly did, that, though he had been named astronomer royal, he had been left practically entirely without support; his instruments were entirely his own, either made or purchased by himself; his nominal salary of £ was difficult to get, and did not nearly cover the actual current expenses of his position, he not unnaturally regarded his observations as his own exclusive property. he had a most natural dislike for his observations to be published, except after such reduction as he himself had carried through, and in the manner which he himself had chosen. the idea which was ever before him was that of carrying out a single great work that should not only be a monument to his own industry and skill, but should also raise the name of england amongst scientific nations. he complained of it, therefore, both as a personal wrong and an injury to the country when some observations of cassini's were combined with some observations of his own in order to deduce a better orbit for a comet. unknown to himself, therefore, he was called upon to decide a question that has proved fundamental to the policy of greenwich observatory, and he decided it wrongly--the question of publication. newton had urged upon him as early as that he should not wait until he had formed an exhaustive catalogue of all the brighter stars, but that he should publish at once a catalogue of a few, which might serve as standards; but flamsteed would not hear of it. he failed to see that his office had been created for a definite practical purpose, not for the execution of some great scheme, however important to science. all his work of thirty years had done nothing to forward navigation so long as he published nothing. but if, year by year, he had published the places of the moon and of a few standard stars, he would have advanced the art immensely and yet have not hindered himself from eventually bringing out a great catalogue. no doubt the little incident of newton's difficulty with the microscope, of which he had forgotten the object-glass, had given flamsteed a low opinion of newton's qualifications as a practical astronomer. if so, he was wrong, for newton's insight into practical matters was greater than flamsteed's own, and his practical skill was no less, though his absent-mindedness might occasionally lead him into an absurd mistake. the following extract from flamsteed's own 'brief history of the observatory' gives an account of his view of newton's action towards him in desiring the publication of his star catalogue, and at the same time it illustrates flamsteed's touchy and suspicious nature. 'whilst mr. flamsteed was busied in the laborious work of the catalogue of the fixed stars, and forced often to watch and labour by night, to fetch the materials for it from the heavens, that were to be employed by day, he often, on sir isaac newton's instances, furnished him with observations of the moon's places, in order to carry on his correction of the lunar theory. a civil correspondence was carried on between them; only mr. flamsteed could not but take notice that as sir isaac was advanced in place, so he raised himself in his conversation and became more magisterial. at last, finding that mr. flamsteed had advanced far in his designed catalogue by the help of his country calculators, that he had made new lunar tables, and was daily advancing on the other planets, sir isaac newton came to see him (tuesday, april , ); and desiring, after dinner, to be shown in what forwardness his work was, had so much of the catalogue of the fixed stars laid before him as was then finished; together with the maps of the constellations, both those drawn by t. weston and p. van somer, as also his collation of the observed places of saturn and jupiter, with the rudolphine numbers. having viewed them well, he told mr. flamsteed he would (_i.e._ he was desirous to) recommend them to the prince _privately_. mr. flamsteed (who had long been sensible of his partiality, and heard how his two flatterers cried sir isaac's performances up, was sensible of the snare in the word _privately_) answered that would not do; and (upon sir isaac's demanding "why not?") that then the prince's attendants would tell him these were but curiosities of no great use, and persuade him to save that expense, that there might be the more for them to beg of him: and that the recommendation must be made _publicly_, to prevent any such suggestions. sir isaac apprehended right, that he was understood, and his designs defeated: and so took his leave not well satisfied with the refusal. 'it was november following ere mr. flamsteed heard from him any more: when, considering with himself that what he had done was not well understood, he set himself to examine how many folio pages his work when printed would fill; and found upon an easy computation that they would at least take up . being amazed at this, he set himself to consider them more seriously; drew up an estimate of them; and, to obviate the misrepresentations of dr. s[loane] and some others, who had given out that what he had was inconsiderable, he delivered a copy of the estimate to mr. hodgson, then lately chosen a member of the royal society, with directions to deliver it to a friend, who he knew would do him justice; and, on this fair account, obviate those unjust reports which had been studiously spread to his prejudice. it happened soon after, mr. hodgson being at a meeting, spied this person there, at the other side of the room; and therefore gave the paper to one that stood in some company betwixt them, to be handed to him. but the gentleman, mistaking his request, handed to the secretary [dr. sloane], who, being a physician, and not acquainted with astronomical terms, did not read it readily. whereupon another in the company took it out of his hands; and, having read it distinctly, desired that the works therein mentioned might be recommended to the prince; the charge of printing them being too great either for the author or the royal society. sir isaac closed in with this.' [illustration: the 'camera stellata' in flamsteed's time. (_from an engraving in the 'historia coelestis.'_)] the work was in consequence recommended to prince george of denmark, the queen's consort; but it was not till november , , that the contract for the printing was signed. two years later, the observations which he had made with his sextant in his first thirteen years of office were printed. then came the difficulty of the catalogue. it was not complete to flamsteed's satisfaction, and he was most unwilling to let it pass out of his hands. however, two manuscripts, comprising some three-quarters of the whole, were deposited with referees, the first of these being sealed up. the seal was broken with flamsteed's concurrence; but the fact that it had been so broken was made by him the subject of bitter complaint later. at this critical juncture prince george died, and a stop was put to the progress of the printing. two years more elapsed without any advance being made, and then, in order to check any further obstruction, a committee of the royal society was appointed as a board of visitors to visit and inspect the observatory, and so maintain a control over the astronomer royal. this was naturally felt by so sensitive a man as flamsteed as a most intolerable wrong, and when he found that the printing of his catalogue had been placed in the hands of halley as editor, a man for whom he had conceived the most violent distrust, he absolutely refused to furnish the visitors with any further material. this led to, perhaps, the most painful scene in the lives either of newton or flamsteed. flamsteed was summoned to meet the council of the royal society at their rooms in crane court. a quorum was not present, and so the interview was not official, and no record of it is preserved in the archives. flamsteed has himself described it with great particularity in more than one document, and it is only too easy to understand the scene that took place. newton was a man who had an absolutely morbid dread of anything like controversy, and over and over again would have preferred to have buried his choicest researches, rather than to have encountered the smallest conflict of the kind. he was perhaps, therefore, the worst man to deal with a high-principled, sensitive, and obstinate man who was in the wrong, and yet who had been so hardly dealt with that it was most natural for him to think himself wholly in the right. flamsteed adhered absolutely to his position, from which it is clear it would have been extremely difficult for the greatest tact and consideration to have dislodged him. newton, on his part, simply exerted his authority, and, that failing, was reduced to the miserable extremity of calling names. the scene is described by flamsteed himself, in a letter to abraham sharp, as follows:-- 'i have had another contest with the president[ ] of the royal society, who had formed a plot to make my instruments theirs; and sent for me to a committee, where only himself and two physicians (dr. sloane, and another as little skilful as himself) were present. the president ran himself into a great heat, and very indecent passion. i had resolved aforehand his kn--sh talk should not move me; showed him that all the instruments in the observatory were my own; the mural arch and voluble quadrant having been made at my own charge, the rest purchased with my own money, except the sextant and two clocks, which were given me by sir jonas moore, with mr. towneley's micrometer, his gift, some years before i came to greenwich. this nettled him; for he has got a letter from the secretary of state for the royal society to be visitors of the observatory, and he said, "_as good have no observatory as no instruments_." i complained then of my catalogue being printed by raymer, without my knowledge, and that i was _robbed of the fruit of my labours_. at this he fired, and called me all the ill names, puppy, etc., that he could think of. all i returned was, i put him in mind of his passion, desired him to govern it, and keep his temper: this made him rage worse, and he told me how much i had received from the government in thirty-six years i had served. i asked what he had done for the £ per annum that he had received ever since he had settled in london. this made him calmer; but finding him going to burst out again, i only told him my catalogue, half finished, was delivered into his hands, on his own request, sealed up. he could not deny it, but said dr. arbuthnott had procured the queen's order for opening it. this, i am persuaded, was false; or it was got after it had been opened. i said nothing to him in return; but, with a little more spirit than i had hitherto showed, told them that god (who was seldom spoken of with due reverence in that meeting) had hitherto prospered all my labours, and i doubted not would do so to a happy conclusion; took my leave and left them. dr. sloane had said nothing all this while; the other doctor told me i was proud, and insulted the president, and ran into the same passion with the president. at my going out, i called to dr. sloane, told him he had behaved himself civilly, and thanked him for it. i saw raymer after, drank a dish of coffee with him, and told him, still calmly, of the villany of his conduct, and called it _blockish_. since then they let me be quiet; but how long they will do so i know not, nor am i solicitous.' [ ] sir isaac newton. the visitors continued the printing, halley being the editor, and the work appeared in under the title of _historia coelestis_. this seemed to flamsteed the greatest wrong of all. the work as it appeared seemed to him so full of errors, wilfully or accidentally inserted, as to be the greatest blot upon his fair fame, and he set himself, though now an old man, to work it out _de novo_ and at his own expense. to that purpose he devoted the remaining seven years of his life. few things can be more pathetic than the letters which he wrote in that period referring to it. he was subject to the attacks of one of the cruelest of all diseases--the stone; he was at all times liable to distracting headaches. he had been, from his boyhood, a great sufferer from rheumatism, and yet, in spite of all, he resolutely pushed on his self-appointed task. the following extract from one of his letters will give a more vivid idea of the brave old man than much description:-- 'i can still, i praise god for it, walk from my door to the blackheath gate and back, with a little resting at some benches i have caused to be set up betwixt them. but i found myself so tired with getting up the hill when i return from church, that at last i have bought a sedan, and am carried thither in state on sunday mornings and back; i hope i may employ it in the afternoons, though i have not hitherto, by reason of the weather is too cold for me.' after the death of queen anne, a change in the ministry enabled him to secure that three hundred copies of the total impression of four hundred of the _historia coelestis_ were handed over to him. these, except the first volume, containing his sextant observations (which had received his own approval), he burned, 'as a sacrifice to heavenly truth.' his own great work had advanced so far that the first volume was printed, and much of the second, when he himself died, on the last day of . he was buried in the chancel of burstow church. the completion of his work took ten years more; a work of piety and regard on the part of his assistant, joseph crosthwait. when compared with the catalogues that have gone before, it was a work of wonderful accuracy. nevertheless, as caroline herschel showed, nearly a century later, not a few errors had crept into it. some of the stars are non-existent, others have been catalogued in more than one constellation; important stars have been altogether omitted. perhaps the most serious fault arises from the neglect of flamsteed to accept from newton a practical hint, namely, to read the barometer and thermometer at the time of his observations. nevertheless, the work accomplished was not only wonderful under the untoward conditions in which flamsteed was placed; it was wonderful in itself, winning from airy the following high encomium:-- 'in regard not only to accuracy of observation, and to detail in publication of the methods of observing, but also to steadiness of system followed through many years, and to completeness of calculation of the useful results deduced from the observations, this work may shame any other collection of observations in this or any other country.' this catalogue was not flamsteed's only achievement. he had determined the latitude of the observatory, the obliquity of the ecliptic, and the position of the equinoctial points. he thought out an original method of obtaining the absolute right ascensions of stars by differential observations of the places of the stars and the sun near to both equinoxes. he had revised and improved horrox's theory of the lunar motions, which was by far the best existing in flamsteed's day. he showed the existence of the long inequality of jupiter and saturn; that is to say, the periodic influence which they exercise upon each other. he determined the time in which the sun rotates on its axis, and the position of that axis. he observed an apparent movement of the stars in the course of a year, which he ascribed, though erroneously, to the stellar parallax, and which was explained by the third astronomer royal, bradley. flamsteed not only met with harsh treatment during his lifetime; he has not yet received, except from a few, anything like the meed of appreciation which is his just due; but, at least, his successors in the office have not forgotten him. they have been proud that their official residence should be known as flamsteed house, and his name is inscribed over the main entrance of the latest and finest of the observatory buildings, and his bust looks forth from its front towards the home where he laboured so devotedly for nearly fifty years. but he has received little honour, save at greenwich, and--in spite of the proverb--in his other home, the village of burstow, in surrey, of which he was for many years the rector. here a stained glass window representing, appropriately, the adoration of the magi, has been recently set up to his memory, largely through the interest taken in his history by an amateur astronomer of the neighbourhood, mr. w. tebb, f.r.a.s. no instrument of flamsteed's remains in the observatory, his wife removing them after his death. but we may consider his principal instrument, the mural quadrant made for him by abraham sharp, as represented by the remains of a quadrant by the same artist, which was presented to the observatory by the rev. n. s. heineken, in , and now hangs over the door of the transit room. chapter iii halley and his successors there is no need to give the lives of the succeeding astronomers royal so fully as that of flamsteed. not that they were inferior men to him; on the contrary, there can be little doubt that we ought to reckon some of them as his superiors, but, in the case of several, their best work was done apart from greenwich observatory, and before they came to it. this was particularly the case with edmund halley. born on october , , he was ten years the junior of flamsteed. like flamsteed, he came of a derbyshire family, though he was born at haggerston, in the parish of st. leonard's, shoreditch. he was educated at st. paul's school, where he made very rapid progress, and already showed the bent of his mind. he learnt to make dials; he made himself so thoroughly acquainted with the heavens that it is said, 'if a star were displaced in the globe he would presently find it out,' and he observed the changes in the direction of the mariner's compass. in he went to queen's college, oxford, where he observed a sunspot in july and august, , and an occultation of mars. this was not his first astronomical observation, as, in june, , he had observed an eclipse of the moon from his father's house in winchester street. [illustration: edmund halley. (_from an old print._)] a much wider scheme of work than such merely casual observations now entered his mind, possibly suggested to him by flamsteed's appointment to the direction of the new royal observatory. this was to make a catalogue of the southern stars. tycho's places for the northern stars were defective enough, but there was no catalogue at all of stars below the horizon of tycho's observatory. here, then, was a field entirely unworked, and young halley was so eager to enter upon it that he would not wait at oxford to obtain his degree, but was anxious to start at once for the southern hemisphere. his father, who was wealthy and proud of his gifted son, strongly supported him in his project. the station he selected was st. helena, an unfortunate choice, as the skies there were almost always more or less clouded, and rain was frequent during his stay. however, he remained there a year and a half, and succeeded in making a catalogue of stars. this catalogue was finally reduced by sharp, and included in the third volume of flamsteed's _historia coelestis_. in he was elected fellow of the royal society, and the following year he was chosen to represent that society in a discussion with hevelius. the question at issue was as to whether more accurate observations of the place of a star could be obtained by the use of sights without optical assistance, or by the use of a telescope. the next year he visited the paris observatory, and, later in the same tour, the principal cities of the continent. not long after his return from this tour, halley was led to that undertaking for which we owe him the greatest debt of gratitude, and which must be regarded as his greatest achievement. some fifty years before, the great kepler had brought out the third of his well-known laws of planetary motion. these laws stated that the planets move round the sun in ellipses, of which the sun occupies one of the foci; that the straight line joining any planet with the sun moves over equal areas of space in equal periods of time; and, lastly, that the squares of the times in which the several planets complete a revolution round the sun are proportional to the cubes of their mean distances from it. these three laws were deduced from actual examination of the movements of the planets. kepler did not work out any underlying cause of which these three laws were the consequence. but the desire to find such an underlying cause was keen amongst astronomers, and had given rise to many researches. amongst those at work on the subject was halley himself. he had seen, and been able to prove, that if the planets moved in circles round the sun, with the sun in the centre, then the law of the relation between the times of revolution and the distances of the planets would show that the attractive force of the sun varied inversely as the square of the distance. the actual case, however, of motion in an ellipse was too hard for him, and he could not deal with it. halley therefore went up to cambridge to consult newton, and, to his wonder and delight, found that the latter had already completely solved the problem, and had proved that kepler's three laws of planetary motion were summed up in one, namely, that the sun attracted the planets to it with a force inversely proportional to the square of the distance. halley was most enthusiastic over this great discovery, and he at once strongly urged newton to publish it. newton's unwillingness to do so was great, but at length halley overcame his reluctance; and the royal society not being able at the time to afford the expense, halley took the charges upon himself, although his own resources had been recently seriously damaged by the death of his father. the publication of newton's _principia_, which, but for him, might never have seen the light, and most certainly would have been long delayed, is halley's highest claim to our gratitude. but, apart from this, his record of scientific achievement is indeed a noble one. always, from boyhood, he had taken a great interest in the behaviour of the magnetic compass, and he now followed up the study of its variations with the greatest energy. for this purpose it was necessary that he should travel, in view of the great importance of the subject to navigation. king william iii. gave him a captain's commission in the royal navy--a curious and interesting illustration of the close connection between astronomy and the welfare of our navy--and placed him in command of a 'pink,' that is to say, a small vessel with pointed stern, named the paramour, in which he proceeded to the southern ocean. his first voyage was unfortunate, but the paramour was recommissioned in , and he sailed in it as far as south latitude °. in and the succeeding year he made further voyages in the paramour, surveying the tides and coasts of the british channel and of the adriatic, and helping in the fortification of trieste. he became savilian professor of geometry at oxford in , having failed twelve years previously to secure the savilian professorship of astronomy, mainly through the opposition of flamsteed, who had already formed a strong prejudice against him, which some writers have traced to halley's detection of several errors in one of flamsteed's tide-tables, others to halley's supposed materialistic views. probably the difference was innate in the two men. there was likely to be but little sympathy between the strong, masterful man of action and society and the secluded, self-conscious, suffering invalid. at any rate, in the contest between newton and flamsteed, which has been already described, halley took warmly the side of the former, and was appointed to edit the publication of flamsteed's results, and, on the death of the latter, to succeed him at the royal observatory. the condition of things at greenwich when halley succeeded to the post of astronomer royal in was most discouraging. the instruments there had all belonged to flamsteed, and therefore, most naturally, had been removed by his widow. the observatory had practically to be begun _de novo_, and halley had now almost attained the age at which in the present day an astronomer royal would have to retire. more fortunate, however, than his predecessor, he was able to get a grant for instruments, and he equipped the observatory as well as the resources of the time permitted, and his transit instrument and great eight-foot quadrant still hang upon the observatory walls. as astronomer royal his great work was the systematic observation of the positions of the moon through an entire _saros_. as is well known, a period of eighteen years and ten or eleven days brings the sun and moon very nearly into the same positions relatively to the earth which they occupied at the commencement of the period. this period was well known to the ancient chaldeans, who gave it its name, since they had noticed that eclipses of the sun or eclipses of the moon recurred at intervals of the above length. it was halley's desire to obtain such a set of observations of the moon through an entire _saros_ period as to be able to deduce therefrom an improved set of tables of the moon's motion. it was an ambitious scheme for a man so much over sixty to undertake, nevertheless he carried it through successfully. his desire to complete this scheme, and to found upon it improved lunar tables, hindered him from publishing his observations, for he feared that others might make use of them before he was in a position to complete his work himself. this omission to publish troubled newton, who, as president of the royal society--the greenwich board of visitors having lapsed at queen anne's death--drew attention at a meeting of the royal society, march , , to halley's disobedience of the order issued under queen anne, for the prompt communication of the observatory results. that newton should thus have put public pressure upon halley, the man to whom he was so much indebted, and with whom there was so close an affection, is sufficient proof that his similar attitude towards flamsteed was one of principle and not of arbitrariness. halley, on his side, stood firm, as flamsteed had done, urging the danger that, by publishing before he had completed his task, he might give an opportunity to others to forestall his results. it is said--probably without sufficient ground--that this refusal broke newton's heart and caused his death. certainly halley's writings in that very year show his reverence and affection for newton to have been as keen and lively as ever. halley's work at the observatory went on smoothly, on the lines he had laid down for himself, for ten years after newton's death; but in he had a stroke of paralysis, and his health, which had been remarkably robust up to that time, began to give way. he died january , , and was buried in the cemetery of lee church. as an astronomer, his services to the science rank higher than those of his predecessor; but as astronomer royal, as director, that is to say, of greenwich observatory, he by no means accomplished as much as flamsteed had done. professor grant, in his _history of physical astronomy_, says that he seems to have undervalued those habits of minute attention which are indispensable to the attainment of a high degree of excellence in the practice of astronomical observation. he was far from being sufficiently careful as to the adjustment of his instruments, the going of his clocks, or the recording of his own observations. the important feature of his administration was that under him the observatory was first supplied with instruments which belonged to it. [illustration: halley's quadrant. (_from an old print._)] his astronomical work apart from the observatory was of the first importance. he practically inaugurated the study of terrestrial magnetism, and his map giving the results of his observations during his voyage in the paramour introduced a new and most useful style of recording observations. he joined together by smooth curves places of equal variation, the result being that the chart shows at a glance, not merely the general course of the variation over the earth's surface, but its value at any spot within the limits of the chart. another work which has justly made his name immortal was the prediction of the return of the comet which is called by his name, to which reference will be made later. another great scheme, and one destined to bear much fruit, was the working out of a plan to determine the distance of the sun by observations of the transit of venus. of attractive appearance, pleasing manners, and ready wit, loyal, generous, and free from self-seeking, he probably was one of the most personally engaging men who ever held the office. the salary of the astronomer royal remained under halley at the same inadequate rate which it had done under flamsteed--£ , without provision for an assistant. but in queen caroline, learning that halley had actually had a captain's commission in the royal navy, secured for him a post-captain's pay. [illustration: james bradley. (_from the painting by hudson._)] halley's work is represented at the observatory by two of his instruments which are still preserved there, and which hang on the west wall of the present transit room: the iron quadrant afterwards made famous by the observations of bradley, and 'halley's transit,' the first of the great series of instruments upon which the fame of greenwich chiefly rests. this transit instrument seems to have been set up in a small room at the west end of what is now known as the north terrace. his quadrant was mounted on the pier which is now the base of the pier of the astrographic telescope. this pier was the first extension which the observatory received from the original building. on the breakdown of his health halley nominated as his successor, james bradley; indeed, it is stated that he offered to resign in his favour. he had known him then for over twenty years, and that keen and generous appreciation of merit in others which was characteristic of halley had led him very early to recognize bradley's singular ability. * * * * * james bradley was born in or , of an old north of england family. his birthplace was sherbourne, in gloucestershire, and he was educated at north leach grammar school and at baliol college, oxford. during the years of his undergraduateship he resided much with his uncle, the rev. james pound, rector of wanstead, essex, an ardent amateur astronomer, a frequent visitor at the observatory in flamsteed's time, and one of the most accurate observers in the country. from him, no doubt, he derived his love of the science, and possibly some of his skill in observation. bradley's earliest observations seem to have been devoted to the phenomena of jupiter's satellites and to the measures of double stars. the accuracy with which he followed up the first drew the attention of halley, and so began a friendship which lasted through life. his observations of double stars, particularly of castor, only just failed to show him the orbital movement of the pair, because his attention was drawn to other subjects before it had become sufficiently obvious. in bradley and his uncle made an attempt to determine the distance of the sun through observations of mars when in opposition, observations which were so accurate that they sufficed to show that the distance of the sun could not be greater than millions of miles, nor less than about millions. the lower limit which they thus found has proved to be almost exactly correct, our best modern determinations giving it as millions. the instrument with which the observations were made was a novel one, being 'moved by a machine that made it to keep pace with the stars;' in other words, it was the first, or nearly the first, example of what we should now call a clock-driven equatorial. that same year he was offered the vicarage of bridstow, near ross, in monmouthshire, where, having by that time taken priest's orders, he was duly installed, july, . to this was added the sinecure rectory of llandewi-velgry; but he held both livings only a very short time. in the death of dr. john keill rendered vacant the savilian professorship of astronomy at oxford, for which bradley became a candidate, and was duly elected, and resigned his livings in consequence. it was whilst he was savilian professor that bradley made that great discovery which will always be associated with his name. though professor at oxford, he had continued to assist his uncle, mr. pound, at his observations at wanstead, and after the death of the latter he still lived there as much as possible, and continued his astronomical work. but in he was invited by mr. samuel molyneux, who had set up a twenty-four-foot telescope made by graham as a zenith tube at his house on kew green, to verify some observations which he was making. these were of the star gamma draconis, a star which passes through the zenith of london, and which, therefore, had been much observed both by flamsteed and hooke, inasmuch as by fixing a telescope in an absolutely vertical position--a position which could be easily verified--it was easy to ascertain if there was any minute change in the apparent position of the star. dr. hooke had declared that there was such a change, a change due to the motion of the earth in its orbit, which would prove that the star was not an infinite distance from the earth, the seeming change of its place in the sky corresponding to the change in the place of the earth from which the observer was viewing it. bradley found at once that there was such a change--a marked one. it amounted to as much as ´´ of arc in three days; but it was not in the direction in which the parallax of the star would have moved it, but in the opposite. whether, therefore, the star was near enough to show any parallax or not, some other cause was giving rise to an apparent displacement of the star, which entirely masked and overcame the effect of parallax. so far, bradley had but come to the same point which flamsteed had reached. flamsteed had detected precisely the same apparent displacement of stars, and, like hooke, had ascribed it to parallax. cassini had shown that this could not be the case, as the displacement was in the wrong direction; and there the matter had rested. bradley now set to follow the question up. other stars beside gamma draconis were found to show a displacement of the same general character, but the amount varied with their distance from the plane of the ecliptic, the earth's orbit. the first explanation suggested was that the axis of the earth, which moves very nearly parallel to itself as the earth moves round the sun, underwent a slight regular 'wobble' in the course of a year. to check this, a star was observed on the opposite side of the pole from gamma draconis; then bradley investigated as to whether refraction might explain the difficulty, but again without success. he now was most keenly interested in the problem, and he purchased a zenith telescope of his own, made, like that of molyneux, by graham, and mounted it in his aunt's house at wanstead, and observed continuously with it. the solution of the problem came at last to him as he was boating on the thames. watching a vane at the top of the mast, he saw with surprise that it shifted its direction every time that the boat was put about. remarking to the boatmen that it was very odd that the wind should change just at the same moment that there was a shift in the boat's course, they replied that there was no change in the wind at all, and that the apparent change of the vane was simply due to the change of direction of the motion of the boat. [illustration: graham's zenith sector. (_from an old print._)] this supplied bradley with a key to the solution of the mystery that had troubled him so long. it had been discovered long before this that light does not travel instantaneously from place to place, but takes an appreciable time to pass from one member of the solar system to another. this had been discovered by römer from observations of the satellites of jupiter. he had noted that the eclipses of the satellites always fell late of the computed time, when jupiter was at his greatest distance from the earth; and bradley's own work in the observation of those satellites had brought the fact most intimately under his own acquaintance. the result of the boating incident taught him, then, that he might look upon light as analogous to the wind blowing on the boat. as the wind, so long as it was steady, would seem to blow from one fixed quarter so long as the boat was also in rest, but as it seemed to shift its direction when the boat was moving and changed its direction, so he saw that the light coming from a particular star must seem to slightly change the direction in which it came, or, in other words, the apparent position of the star, to correspond with the movement of the earth in its orbit round the sun. this was the celebrated discovery of the aberration of light, a triumph of exact observation and of clear insight. as to the exactness of bradley's observations, it is sufficient to say that his determination of the value of the 'constant of aberration' gave it as · ´´; the value adopted to-day is · ´´. on the death of halley, in , bradley was appointed to succeed him. he found the observatory in as utterly disheartening a condition as his predecessors had done. as already mentioned, halley had not the same qualifications as an observer that flamsteed had. he was, further, an old man when appointed to the post, he had no assistant provided for him, and the last five years of his life his health and strength had entirely given way. under these circumstances, it was no wonder that bradley found the instruments of the observatory in a deplorable state. nevertheless, he set to work most energetically, and in the year of his appointment he made observations in the last five months of the year. he was particularly earnest in examining the condition and the errors of his instruments; and as their defects became known to him, he was more and more anxious for a better equipment. he moved the royal society, therefore, to apply on his behalf for the instruments he required; and a petition from that body, in , obtained what in those days must be considered the generous grant of £ , the proceeds of the sale of old admiralty stores. the principal instruments purchased therewith were a mural quadrant and a transit instrument, both eight feet in focal length, still preserved on the walls of the transit-room. it is interesting also to note that, following in the steps of halley, and forecasting, as it were, the magnetic observatory which airy would found, he devoted £ of the grant to purchasing magnetic instruments. meantime he had continued his observations on aberration, and had discovered that the aberration theory was not sufficient entirely to account for the apparent changes in places of stars which he had discovered. a second cause was at work, a movement of the earth's axis, a 'wobble' in its inclination, technically known as nutation, which is due to the action of the moon, and goes through its course in a period of nineteen years. beside these two great discoveries of aberration and nutation, bradley's reputation rests upon his magnificent observations of the places of more than three thousand stars. this part of his work was done with such thoroughness, that the star-places deduced from them form the basis of most of our knowledge as to the actual movements of individual stars. in particular, he was careful to investigate and to correct for the errors of his instrument, and to determine the laws of refraction, introducing corrections for changes in the readings of thermometer and barometer. his tables of refraction were used, indeed, for seventy years after his death. of his other labours it may be sufficient to refer to his determination of the longitudes of lisbon and of new york, and to his effort to ascertain the parallax of the sun and moon, in combination with la caille, who was observing at the cape of good hope. as astronomer royal, bradley's great achievement was the high standard to which he raised the practical work of observation. from his day onwards, also, there was always at least one assistant. his first assistant was his own nephew, john bradley, who received the munificent salary of ten shillings a week. still, this was not out of proportion to the then salary of the astronomer royal, which practically amounted only to £ . however, in , bradley was awarded a crown pension of £ a year. he refused the living of greenwich, which was offered him in order to increase his emoluments, on the ground that he could not suitably fulfil the double office. bradley's later assistants were charles mason and charles green. bradley's last work was the preparation for the observations of the transit of venus of , according to the lines laid down by his predecessor, halley. his health gave way, and he became subject to melancholia, so that the actual observations were taken by the rev. nathaniel bliss, who succeeded him in his office after his death, in . he was buried at minchinhampton. so far as we know bradley's character, he seems to have been a gentle, modest, unassuming man, entirely free from self-seeking, and indifferent to personal gain. he was in many ways an ideal astronomer, exact, methodical, and conscientious to the last degree. his skill as an observer was his chief characteristic; and though his abilities were not equal as a mathematician or a mechanician, yet, on the one hand, he had a very clear insight into the meaning of his observations, and, on the other, he was skilful enough to himself adjust, repair, and improve his instruments. of bradley's instruments, there are still preserved his famous twelve-and-a-half-foot zenith sector, with which he made his two great discoveries; his brass quadrant, which in he substituted for halley's iron quadrant; his transit instrument, and equatorial sector. bradley added to the buildings of the observatory that portion which is now represented by the upper and lower computing rooms, and the chronometer room, which adjoins the latter. this room--the chronometer room--was his transit room, and the position of the shutters is still marked by the window in the roof. * * * * * the rev. nathaniel bliss, who succeeded bradley, only held the office for a couple of years, and during that time was much at oxford. he, therefore, has left no special mark behind him as astronomer royal. he was born november , . his father, like himself, nathaniel bliss, was a gentleman, of bisley, gloucestershire. [illustration: nathaniel bliss. (_from an engraving on an old pewter flagon._)] bliss graduated at pembroke college, oxford, as b.a. in , and m.a. in . he became the rector of st. ebb's, oxford, in , and on halley's death succeeded him as savilian professor of geometry. he supplied bradley with his observations of jupiter's satellites, and from time to time, at his request, rendered him some assistance at the royal observatory. this was particularly the case, as has been already mentioned, with respect to the transit of venus of , the observations of which were carried out by bliss, owing to bradley's ill-health. it was natural, therefore, that on bradley's death he should succeed to the vacant post; but he held it too short a time to do any distinctive work. such observations as he made seem to have been entirely in continuation of bradley's. he took a great interest, however, in the improvement of clocks, a department in which so much was being done at this time by graham, ellicott, and others. * * * * * nevil maskelyne, the fifth astronomer royal, was, like bliss, a close friend of bradley's. he was the third son of a wealthy country gentleman, edmund maskelyne, of purton, in wiltshire. maskelyne was born in london, october , , and was educated at westminster school. thence he proceeded to cambridge, where he graduated seventh wrangler in . he was ordained to the curacy of barnet in , and, twenty years later, was presented by his nephew, lord clive, to the living of shrawardine, in shropshire. in he was presented by his college to the rectory of north runcton, norfolk. the event which turned his thoughts in the direction of astronomy was the solar eclipse of july , ; and about the time that he was appointed to the curacy of barnet he became acquainted with bradley, then the astronomer royal, to whom he gave great assistance in the preparation of his table of refractions. like halley before him, he made an astronomical expedition to the island of st. helena. this was for the special purpose of observing the transit of venus of june , , bradley having induced the royal society to send him out for that purpose. here he stayed ten months, and made many observations. but though the transit of venus was his special object, it was not the chief result of the expedition: not because clouds hindered his observations, but because the voyage gave him the especial bent of his life. halley had actually held a captain's commission in the royal navy, and commanded a ship; maskelyne, more than any of the astronomers royal before or since, made the improvement of the practical business of navigation his chief aim. none of all the incumbents of the office kept its original charter--'to find the so much desired longitude at sea, for the perfecting the art of navigation,' so closely before him. the solution of the problem was at hand at this time--its solution in two different ways. on the one hand, the offer by the government of a reward of £ , for a clock or watch which should go so perfectly at sea, notwithstanding the tossing of the ship and the wide changes of temperature to which it might be exposed, that the navigator might at any moment learn the true greenwich time from it, had brought out the invention of harrison's time-keeper; on the other hand, the great improvement that had now taken place in the computation of tables of the moon's motion, and the more accurate star-catalogues now procurable, had made the method of 'lunars,' suggested a hundred and thirty years before by the frenchman, morin, and others, a practicable one. [illustration: nevil maskelyne.] in principle, the method of finding the longitude from 'lunars,' that is to say, from measurements of the distances between the moon and certain stars, is an exceedingly simple one. in actual practice, it involves a very toilsome calculation, beside exact and careful observation. the principle, as already mentioned, is simply this: the moon travels round the sky, making a complete circuit of the heavens in between twenty-seven and twenty-eight days. it thus moves amongst the stars, roughly speaking, its own diameter, in about an hour. when once its movements were sufficiently well known to be exactly predicted, almanacs could be drawn up in which the greenwich time of its reaching any definite point of the sky could be predicted long beforehand; or, what comes to the same thing, its distances from a number of suitable stars could be given for definite intervals of greenwich time. it is only necessary, then, to measure the distances between the moon and some of these stars, and by comparing them with the distances given in the almanac, the exact time at greenwich can be inferred. as has been already pointed out, the determination of the latitude of the ship and of the local time at any place where the ship is, is not by any means so difficult a matter; but the local time being known and the greenwich time, the difference between these gives the longitude; and the latitude having been also ascertained, the exact position of the ship is known. there are, of course, difficulties in the way of working out this method. one is, that whilst it takes the sun but twenty-four hours to move round the sky from one noon to the next, and consequently its movements, from which the local time is inferred, are fairly rapid, the moon takes nearly twenty-eight days to move amongst the stars from the neighbourhood of one particular star round to that particular star again. consequently, it is much easier to determine the local time with a given degree of exactness than the greenwich time; it is something like the difference of reading a clock from both hands and from the hour hand alone. there are other difficulties in the case which make the computation a long and laborious one, and difficult in that sense; but they do not otherwise affect its practicability. during this voyage to st. helena, both when outward bound and when returning, maskelyne gave the method of 'lunars' a very thorough testing, and convinced himself that it was capable of giving the information required. for by this time the improvement of the sextant, or quadrant as it then was, by the introduction of a second mirror, by hadley, had rendered the actual observation at sea of lunar distances, and of altitudes generally, a much more exact operation. this conclusion he put at once to practical effect, and, in , he published the _british mariner's guide_, a handbook for the determination of the longitude at sea by the method of lunars. at the same time, the other method, that by the time-keeper or chronometer, was practically tested by him. the time-keeper constructed by john harrison had been tested by a voyage to jamaica in , and now, in , another time-keeper was tested in a voyage to barbadoes. charles green, the assistant at greenwich observatory, was sent in charge of the chronometer, and maskelyne went with him to test its performance, in the capacity of chaplain to his majesty's ship louisa. [illustration: hadley's quadrant. (_from an old print._)] the position which maskelyne had already won for himself as a practical astronomer, and the intimate relations into which he had entered with bradley and bliss, made his appointment to the astronomer royalship, on the death of the latter, most suitable. at once he bent his mind to the completion of the revolution in nautical astronomy which his _british mariner's guide_ had inaugurated, and in the year after his appointment he published the first number of the _nautical almanac_, together with a volume entitled, _tables requisite to be used with the nautical ephemeris_, the value of which was so instantly appreciated, that , copies were sold at once. the _nautical almanac_ was maskelyne's greatest work, and it must be remembered that he carried it on from this time up to the day of his death--truly a formidable addition to the routine labours of an astronomer royal who had but a single assistant on his staff. the _nautical almanac_ was, however, in the main not computed at the observatory; the calculations were effected by computers living in different parts of the country, the work being done in duplicate, on the principle which flamsteed had inaugurated in the preparation of his _historia coelestis_. maskelyne's next service to science was almost as important. he arranged that the regular and systematic publication of the observations made at greenwich should be a distinct part of the duties of an astronomer royal, and he procured an arrangement by which a special fund was set apart by the royal society for printing them. his observations covering the years to fill four large folio volumes, and though, as already stated, he had but one assistant, they are , in number. thus it was maskelyne who first rendered effective the design which charles ii. had in the establishment of the observatory. flamsteed and halley had been too jealous of their own observations to publish; bradley's observations--though he himself was entirely free from this jealousy--were made, after his death, the subject of litigation by his heirs and representatives, who claimed an absolute property in them, a claim which the government finally allowed. none of the three, however much their work ultimately tended to the improvement of the art of navigation, made that their first object. whereas maskelyne set this most eminently practical object in the forefront, and so gave to the royal observatory, which under his predecessors somewhat resembled a private observatory, its distinctive characteristics of a public institution. it fell to maskelyne to have to advise the government as to the assignment of their great reward of £ , for the discovery of the longitude at sea. maskelyne, while reporting favourably of the behaviour of harrison's time-keeper, considered that the method of 'lunars' was far too important to be ignored, and he therefore recommended that half the sum should be given to harrison for his watch, whilst the other half was awarded for the lunar tables which mayer, before his death, had sent to the board of longitude. this decision, though there can be no doubt it was the right one, led to much dissatisfaction on the part of harrison, who urged his claim for the whole grant very vigorously; and eventually the whole £ , was paid him. the whole question of rewards to chronometer-makers must have been one which caused maskelyne much vexation. he was made the subject of a bitter and most voluminous attack by thomas mudge, for having preferred the work of arnold and earnshaw to his own. otherwise his reign at the observatory seems to have been a singularly peaceful one, and there is little to record about it beyond the patient prosecution, year by year, of an immense amount of sober, practical work. to maskelyne, however, we owe the practice of taking a transit of a star over five wires instead of over one, and he provided the transit instrument with a sliding eye-piece, to get over the difficulty of the displacement which might ensue if the star were observed askew when out of the centre of the field. to maskelyne, too, we owe in a pre-eminent degree the orderly form of recording, reducing, and printing the observations. much of the work in this direction which is generally ascribed to airy was really due to maskelyne. indeed, without a wonderful gift of organization, it would have been impossible to plan and to carry the _nautical almanac_. beside the editing of various works intended for use in nautical astronomy or in general computation, the chief events of his long reign at greenwich were the transit of venus in , which he himself observed, and for which he issued instructions in the _nautical almanac_; and his expedition in to scotland, where he measured the deviation of the plumb-line from the vertical caused by the attraction of the mountain schiehallion, deducing therefrom the mean density of the earth to be four and a half times that of water. [illustration: john pond. (_from an old engraving._)] he died at the observatory, february , , aged , leaving but one child, a daughter, who married mr. anthony mervin story, to whom she brought the family estates in wiltshire, inherited by maskelyne on the deaths of his elder brothers, and, in consequence, mr. story added the name of maskelyne to his own. maskelyne's character and policy as astronomer royal have been sufficiently dwelt upon. his private character was mild, amiable, and generous. 'every astronomer, every man of learning, found in him a brother;' and, in particular, when the french revolution drove some french astronomers to this country to find a refuge, they received from the astronomer royal the kindest reception and most delicate assistance. maskelyne added no instrument to the observatory during his reign, though he improved bradley's transit materially. he designed the mural circle, but it was not completed until after his death. his additions to the observatory buildings consisted of three new rooms in the astronomer royal's house, and the present transit circle room. * * * * * john pond was recommended by maskelyne as his successor at greenwich. at the time of his succession he was forty-four years of age, having been born in . he was educated at trinity college, cambridge, and then spent some considerable time travelling in the south of europe and egypt. on his return home he settled at westbury, where he erected an altazimuth by troughton, with a two-and-a-half-foot circle. a born observer, his observations of the declinations of some of the principal fixed stars showed that the instrument which maskelyne was using at greenwich--the quadrant by bird--could no longer be trusted. maskelyne, in consequence, ordered a six-foot mural circle from troughton, but did not live to see it installed, and in this was supplemented by troughton's transit instrument of five inches aperture and ten feet focal length. the introduction of these two important instruments, and of other new instruments, together with new methods of observation, form one of the chief characteristics of pond's administration. under this head must be specially mentioned the introduction of the mercury trough, both for determining the position of the vertical, and for obtaining a check upon the flexure of the mural circle in different positions; and the use in combination of a pair of mural circles for determining the declinations of stars. another characteristic of his reign was that under him there was the first attempt to give the astronomer royal a salary somewhat higher than that of a mechanic, and to support him with an adequate staff of assistants. his salary was fixed at £ a year, and the single assistant of maskelyne was increased to six. this multiplication of assistants was for the purpose of multiplying observations, for pond was the first astronomer to recognize the importance of greatly increasing the number of all observations upon which the fundamental data of astronomy were to be based. in he finished his standard catalogue of stars, at that time the fullest of any catalogue prepared on the same scale of accuracy. 'it is not too much to say,' was the verdict of the royal astronomical society, 'that meridian sidereal observation owes more to him than to all his countrymen put together since the time of bradley.' a yet higher testimony to the exactness of his work is given by his successor, airy. 'the points upon which, in my opinion, mr. pond's claims to the gratitude of astronomers are founded, are principally the following. _first_ and chief, the accuracy which he introduced into all the principal observations. this is a thing which, from its nature, it is extremely difficult to estimate now, so long after the change has been made; and i can only say that, so far as i can ascertain from books, the change is one of very great extent; for certainty and accuracy, astronomy is quite a different thing from what it was, and this is mainly due to mr. pond.' the same authority eulogizes him further for his laborious working out of every conceivable cause or indication of error in his declination instruments, for the system which he introduced in the observation of transits, for the thoroughness with which he determined all his fundamental data, and for the regularity which he infused into the greenwich observations. one result of this great increase of accuracy was that pond was able at once authoritatively to discard the erroneous stellar parallaxes that had been announced by brinkley, royal astronomer for ireland. but pond's administration was open, in several particulars, to serious censure, and the board of visitors, which had been for many years but a committee of the royal society, but which had recently been reconstituted, proved its value and efficiency by the remonstrances which it addressed to him, and which eventually brought about his resignation. his personal skill and insight as an observer were of the highest order; but either from lack of interest or failing health, he absented himself almost entirely from the observatory in later years, visiting it only every ninth or tenth day. he had caused the staff of assistants to be increased from one to six, but had stipulated that the men supplied to him should be 'drudges.' his minute on the subject ran-- 'i want indefatigable, hard-working, and, above all, obedient drudges (for so i must call them, although they are drudges of a superior order), men who will be contented to pass half their day in using their hands and eyes in the mechanical act of observing, and the remainder of it in the dull process of calculation.' this was a fatal mistake, and one which it is very hard to understand how any one with a real interest in the science could have made. men who had the spirit of 'drudges,' to whom observation was a mere 'mechanical act,' and calculation a 'dull process,' were not likely to maintain the honour of the observatory, particularly under an absentee astronomer royal. pond tried to overcome the difficulty by devising rules for their guidance of iron rigidity. the result was that after his resignation, in , the first lord and the secretary of the admiralty expressed their feeling to airy, pond's successor, 'that the observatory had fallen into such a state of disrepute that the whole establishment should be cleared out.' a further evil was the excessive development of chronometer business, so as practically to swamp the real work of the observatory, whilst the prices paid for the chronometers at this time were often much larger than would have been the case under a more business-like administration. with all his merits, therefore, as an observer, the administration of pond was, in some respects, the least satisfactory of all that the observatory has known, and he alone of all the astronomers royal retired under pressure. he did not long survive his resignation, dying in september, . he was buried by the side of halley, in the churchyard at lee. of pond's instruments, the observatory retains the fine transit instrument which was constructed by troughton at his direction, and the mural circle, designed by maskelyne, but which pond was the first to use. both of these have, of course, long been obsolete, and now hang on the walls of the transit room. the small equatorial, called, after its donor, the shuckburgh equatorial, was also added in pond's day, and though practically never used, still remains mounted in its special dome. chapter iv airy one hundred and sixty years from the day when flamsteed laid the foundation stone of the observatory, the royal warrant under the sign manual was issued, appointing the seventh and strongest of the astronomers royal, august , . he actually entered on his office in the following october, but did not remove to the observatory until the end of the year. george biddell airy was born at alnwick, in northumberland, on july , . his father was william airy, of luddington, in lincolnshire, a collector of excise; his mother was the daughter of george biddell, a well-to-do farmer, of playford, near ipswich. he was educated at the grammar school, colchester, and so distinguished himself there that although his father was at this time very straitened in his circumstances, it was resolved that young airy should go to cambridge. here he was entered as sizar at trinity college, and his robust, self-reliant character was seen in the promptness with which he rendered himself independent of all pecuniary help from his relatives. in he graduated as bachelor of arts, being senior wrangler and smith's prizeman, entirely distancing all other men of his year. he had already begun to pay attention to astronomy, at first from the side of optics, to the study of which he had been very early attracted; a paper of his on the achromatism of eye-pieces and microscopes, written in , being one of especial value. in he attempted to determine 'the diminution of gravity in a deep mine'--that of dolcoath, in cornwall. in the winter of - he was invited to london by mr. (afterwards sir) james south, who took him, amongst other places, to greenwich observatory, and gave him his first introduction to practical astronomy. in he was appointed lucasian professor at cambridge, and in , plumian professor, with the charge of the new university observatory. prior to his election he had definitely told the electors that the salary proposed was not sufficient for him to undertake the responsibility of the observatory. he followed this up by a formal application for an increase, which created not a little commotion at the time, the action being so unprecedented; and after a delay of a little over a year he obtained what he had asked for. the delay gave rise, however, to the remark of a local wit, that the university had given 'to airy, nothing, a local habitation and a name.' [illustration: george biddell airy.] the seven years which he spent in the cambridge observatory were the best possible preparation for that greater charge which he was to assume later. when he entered on his duties the observatory had been completed four years, but no observations had been published; there was no assistant, and the only instruments were a couple of good clocks and a transit instrument. but airy set to work at once with so much energy that the observations for were published early in the following year, and he had very quickly worked out the best methods for correcting and reducing his observations. in an assistant was granted to him, in a second, and in the latter year mr. baldrey, the senior assistant, observed about transits, and mr. glaisher, the junior, about the same number of zenith distances. a syndicate had been appointed at cambridge for the purpose of visiting the observatory once in each term, and making an annual report to the senate. a smaller-minded and less acute man than airy might have resented such an arrangement. he, on the contrary, threw himself heartily into it, and made such formal written reports to the syndicate as best helped them in the performance of their duty, and at the same time secured for the observatory the support and assistance which from time to time it required. on his appointment to greenwich, he at once entered into the same relations to the board of visitors of that observatory, and from that time forth the friction that had occasionally existed between the board and the astronomer royal in the past entirely ceased. the board was henceforth no longer a body whose chief function was to reprove, to check, or to quicken the astronomer royal, but rather a company of experts, before whom he might lay the necessities of the observatory, that they in turn might present them to the government. such representations were not likely to be in vain. for, as mr. sheepshanks has left on record-- 'when mr. airy wants to carry anything into effect by government assistance, he states, clearly and briefly, why he wants it; what advantages he expects from it; and what is the probable expense. he also engages to direct and superintend the execution, making himself personally responsible, and giving his labour gratis. when he has obtained permission (which is very seldom refused), he arranges everything with extraordinary promptitude and foresight, conquers his difficulties by storm, and presents his results and his accounts in perfect order, before men like ... or myself would have made up our minds about the preliminaries. now, men in office naturally like persons of this stamp. there is no trouble, no responsibility, no delay, no inquiries in the house; the matter is done, paid for, and published, before the seekers of a grievance can find an opportunity to be heard. this mode of proceeding is better relished by busy statesmen than recommendations from influential noblemen or fashionable ladies.' his first action towards the board was, however, a very bold and independent one. he made strong representations on the subject of the growth of the chronometer business, which proved displeasing to the hydrographer, captain beaufort, who was one of the official visitors, and by his influence the report was not printed. airy 'kept it, and succeeding reports, safe for three years, and then the board of visitors agreed to print them, and four reports were printed together, and bound with the greenwich observations of .' with the completion of arrangements which put the chronometer business in proper subordination to the scientific charge of the observatory, airy was free to push forward its development on the lines which he had already marked out for himself. to go through these in detail is simply to describe the observatory as he left it. little by little he entirely renovated the equipment. greatly as pond had improved the instruments of the observatory, airy carried that work much further still. though he did not observe much himself, and was not pond's equal in the actual handling of a telescope, he had a great mechanical gift, and the detail in its minutest degree of every telescope set up during his long reign was his own design. in the work of reduction he introduced the use of printed skeleton forms, to which pond had been a stranger. the publication of the greenwich results was carried on with the utmost regularity; and, in striking contrast to the reluctance of flamsteed and halley, he was always most prompt in communicating any observations to every applicant who could show cause for his request for them. it is most difficult to give any adequate impression of his far-reaching ability and measureless activity. perhaps the best idea of these qualities may be obtained from a study of his autobiography, edited and published some four years after his death by his son. the book, to any one who was not personally acquainted with airy, is heavy and monotonous, chiefly for the reason that its pages are little but a mere catalogue of the works which he undertook and carried through; and catalogues, except to the specialist, are the dullest of reading. to enter into the details of his work might fill a library. [illustration: the astronomer royal's room.] as astronomer royal he seems to have inherited and summed up all the great qualities of his predecessors: flamsteed's methodical habits and unflagging industry; halley's interest in the lunar theory; bradley's devotion to star observation and catalogue making; maskelyne's promptitude in publishing, and keen interest in practical navigation; pond's refinement of observation. nor did he allow this inheritance to be merely metaphorical; he made it an actual reality. he discussed, reduced, and published, in forms suitable for use and comparison to-day, the whole vast mass of planetary and lunar observations made at the royal observatory from the year to his own accession, a work of prodigious labour, but of proportionate importance. airy has been accused--and with some reason--of being a strong, selfish, aggressive man; yet nothing can show more clearly than this great work how thoroughly he placed the fame and usefulness of the observatory before all personal considerations. with far less labour he could have carried on a dozen investigations that would have brought him more fame than this great enterprise, the purpose of which was to render the work of his predecessors of the highest possible use. the light in which he regarded his office may best be expressed in his own words:-- 'the observatory was expressly built for the aid of astronomy and navigation, for promoting methods of determining longitude at sea, and (as the circumstances that led to its foundation show) more especially for determination of the moon's motions. all these imply, as their first step, the formation of accurate catalogues of stars, and the determination of the fundamental elements of the solar system. these objects have been steadily pursued from the foundation of the observatory; in one way by flamsteed; in another way by halley, and by bradley in the earlier part of his career; in a third form by bradley in his later years; by maskelyne (who contributed most powerfully both to lunar and to chronometric nautical astronomy), and for a time by pond; then with improved instruments by pond, and by myself for some years; and subsequently, with the instruments now in use. it has been invariably my own intention to maintain the principles of the long-established system in perfect integrity; varying the instruments, the modes of employing them, and the modes of utilizing the observations of calculation and publication, as the progress of science might seem to require.' the result of this keen appreciation of the essential continuity of the astronomer royalship has been that it is to airy, more than to any of his predecessors, or than to all of them put together, that the high reputation of greenwich observatory is due. professor newcomb, the greatest living authority on the subject outside our own land--and other great foreign astronomers have independently pronounced the same verdict--has said:-- 'the most useful branch of astronomy has hitherto been that which, treating of the positions and motions of the heavenly bodies, is practically applied to the determination of geographical positions on land and at sea. the greenwich observatory has, during the past century, been so far the largest contributor in this direction as to give rise to the remark that, if this branch of astronomy were entirely lost, it could be reconstructed from the greenwich observations alone.' early in airy proposed to the board of visitors the creation of the magnetic and meteorological department of the observatory, and in a system of regular two-hourly observations was set on foot. this was the first great enlargement of programme for the observatory beyond the original one expressed in flamsteed's warrant. it was followed in with the formation of the solar photographic department, to which the spectroscope was added a little later. though he had objected strongly on his first coming to the observatory to the excessive time devoted to the merely commercial side of the care of chronometers, yet the perfecting of these instruments was one that he had much at heart, and many recent appliances are either of his own invention or are due to suggestions which he threw out. much work lying outside the observatory, and yet intimately connected with it, was carried out either by him or in accordance with his directions. the transit of venus expeditions of , the delimitation of the boundary line between canada and the united states, and, later, that of the oregon boundary; the determination of the longitudes of valencia, cambridge, edinburgh, brussels, and paris; assistance in the determination of the longitude of altona--all came under airy's direction. nor did he neglect expeditions in connection with what we would now call the physical side of astronomy. on three occasions, , , and , he himself personally took part in successful eclipse expeditions. the determination of the increase of gravity observable in the descent of a deep mine was also the subject of another expedition, to the harton colliery, near south shields. but with all these, and many other inquiries--for he was the confidential adviser of the government in a vast number of subjects: lighthouses, railways, standard weights and measures, drainage, bridges--he yet always kept the original objects of the observatory in the very first place. it was in order to get more frequent observations of the moon that he had the altazimuth erected, which was completed in may, . this was followed, in , by the transit circle, as he had long felt the need for more powerful light grasp in the fundamental instrument of the observatory. the transit circle took the place both of the old transit instrument and of the mural circle. above all, he arranged for the observations of moon and stars to be carried out with practical continuity. the observations were made and reduced at once, and published in such a way that any one wishing to discuss them afresh could for himself go over every step of the reduction from the commencement, and could see precisely what had been done. the greatest addition made to the equipment of the observatory in airy's day was the erection of the - / -inch merz equatorial, which proved of great service when spectroscopy became a department of the observatory. [illustration: the south-east tower. (_from a photograph by mr. lacey._)] so strong and gifted a man as airy was bound to make enemies, and at different times of his life bitter attacks were made on him from one quarter or another. one of these, curiously enough, was from sir james south, the man who, as he said, first introduced him to practical astronomy. later came the discovery of neptune, and airy was subjected to much bitter criticism, since, as it appeared on the surface, it was owing to his supineness that adams missed being held the sole discoverer of the new planet, and narrowly missed all credit for it altogether. last of all was the vehement attack made upon him by richard anthony proctor, in connection with his preparations for the transit of venus. all such attacks, however, simply realized the old fable of the viper and the file. attacks which would have agonized flamsteed's every nerve, and have called forth full and dignified rejoinders from maskelyne, were absolutely and entirely disregarded by airy. he had done his duty, and in his own estimation--and, it should be added, in the estimation of those best qualified to judge--had done it well. he was perfectly satisfied with himself, and what other people thought or said about him influenced him no more than the opinions of the inhabitants of saturn. but great as airy was, he had the defects of his qualities, and some of these were serious. his love of method and order was often carried to an absurd extreme, and much of the time of one of the greatest intellects of the century was often devoted to doing what a boy at fifteen shillings a week could have done as well, or better. the story has often been told, and it is exactly typical of him, that on one occasion he devoted an entire afternoon to himself labelling a number of wooden cases 'empty,' it so happening that the routine of the establishment kept every one else engaged at the time. his friend dr. morgan jocularly said that if airy wiped his pen on a piece of blotting-paper he would duly endorse the blotting-paper with the date and particulars of its use, and file it away amongst his papers. his mind had that consummate grasp of detail which is characteristic of great organizers, but the details acquired for him an importance almost equal to the great principles, and the statement that he had put a new pane of glass into a window would figure as prominently in his annual report to the board of visitors as the construction of the new transit circle. his son remarks of him that 'in his last days he seemed to be more anxious to put letters which he received into their proper place for reference than even to master their contents,' his system having grown with him from being a means to an end, to becoming the end itself. so, too, his regulation of his subordinates was, especially in his earlier days, despotic in the extreme--despotic to an extent which would scarcely be tolerated in the present day, and which was the cause of not a little serious suffering to some of his staff, whom, at that time, he looked upon in the true spirit of pond, as mere mechanical 'drudges.' for thirty-five years of his administration the salaries of his assistants remained discreditably low, and his treatment of the supernumerary members of his staff would now probably be characterized as 'remorseless sweating.' the unfortunate boys who carried out the computations of the great lunar reductions were kept at their desks from eight in the morning till eight at night, without the slightest intermission, except an hour at midday. as an example of the extreme detail of the oversight which he exercised over his assistants, it may be mentioned that he drew up for each one of those who took part in the harton colliery experiment, instructions, telling them by what trains to travel, where to change, and so forth, with the same minuteness that one might for a child who was taking his first journey alone; and he himself packed up soap and towels with the instruments, lest his astronomers should find themselves, in co. durham, out of reach of these necessaries of civilization. a regime so essentially personal may indeed have been necessary after pond's administration, and to give the observatory a fresh start. but it would not have been to the advantage of the observatory, had it become a permanent feature of its administration, as it militated--was almost avowedly intended to militate--against the growth of real zeal and intelligence in the staff, and necessarily occasioned labour and discomfort out of proportion to the results obtained. fortunately, in airy's later years, the extension of the work of the observatory, a slight failing in his own powers, and the efforts he was devoting to the working out of the lunar theory, compelled him to relax something of that microscopic imperiousness which had been the chief characteristic of his rule for so long. airy had, in the fullest degree, the true spirit of the public servant; his sense of duty to the state was very high. he was always ready to undertake any duty which he felt to be of public usefulness, and many of these he discharged without fee or reward. so great an astronomer was necessarily most highly esteemed by astronomers. he was president of the royal society for two years; he was five times president of the royal astronomical society, and twice received its gold medal, beside a special testimonial for his reduction of the greenwich lunar observations. from the royal society he received the copley medal and the royal medal, beside honorary titles from the universities of oxford, cambridge, and edinburgh. so invaluable a public servant, he received the distinction of a knight commandership of the bath in . he had been repeatedly offered knighthood before, but had not thought it well to receive it. he was in the receipt of decorations also from a great number of foreign countries; for, for many years, he was looked up to, not only by english astronomers, but by scientific men in all countries, as the very head and representative of his science. and he also received a more popular appreciation--and most justly so. for whilst no one could have less of the arts of the ordinary popularizer about him, no one has ever given popular lectures on astronomy which more fully corresponded to the ideal of what such should be than airy's six lectures to working men, delivered at ipswich. and we may count the bestowal upon him of the honorary freedom of the city of london, in , as one of the tokens that his services in this direction had not been unappreciated. during the last seven years of his official career he undertook the working out of a lunar theory, and, to allow himself more leisure for its completion, he resigned his position august , , after forty-six years of office. he was now eighty years of age, and he took up his residence at the white house, just outside greenwich park. he resided there till his death, more than ten years later--january , . * * * * * airy was succeeded in the astronomer royalship by the present and eighth holder of the office, w. h. m. christie. he was born at woolwich, in , his father having been professor samuel hunter christie, f.r.s. he was educated at king's college, london, and trinity college, cambridge, graduating as fourth wrangler in . in he was appointed chief assistant at greenwich, in succession to mr. stone, who had become her majesty's astronomer at the cape, and in he succeeded airy as astronomer royal. [illustration: w. h. m. christie, astronomer royal. (_from a photograph by elliott and fry._)] during mr. christie's office, the two new departments of the astrographic chart and double-star observations have come into being. the following buildings have been erected under his administration: the great new observatory in the south ground, the new altazimuth, the new library, nearly opposite to it, the transit pavilion, the porter's lodge, and the magnetic pavilion out in the park. whilst in the old buildings the astrographic dome has been added, and the upper and lower computing rooms have been rebuilt and enlarged. as to the instruments, the -inch refractor, the astrographic twin telescope, the new altazimuth, the -inch and -inch thompson photographic refractors, and the -inch reflector are all additions during the present reign. roughly speaking, therefore, we may say that three-fourths of the present observatory has been added during the nineteen years of the present astronomer royal. one exceedingly important improvement should not be overlooked. airy observed little himself whilst at greenwich, and had an inadequate idea of the necessity for room in a dome and breadth in a shutter-opening. with the sole exception, perhaps, of the transit circle, every instrument set up by airy was crammed into too small a dome or looked out through too narrow an opening. the increase of shutter-opening of the newer domes may be well seen by contrasting, say, the old altazimuth or the sheepshanks dome with that of the astrographic. this reform has had much to do with the success of later work. chapter v the observatory buildings like a living organism, greenwich observatory bears the record of its life-history in its structure. it was not one of those favoured institutions that have sprung complete and fully equipped from the liberality of some great king or private millionaire. as we have seen, it was originally established on the most modest--not to say meagre--scale, and has been enlarged just as it has been absolutely necessary. to quote again from professor newcomb-- 'whenever any part of it was found insufficient for its purpose, new rooms were built for the special object in view, and thus it has been growing from the beginning by a process as natural and simple as that of the growth of a tree. even now the very value of its structure is less than that of several other public observatories, though it eclipses them all in the results of its work.' entering the courtyard--an enclosure some eighty feet deep by ninety feet in extreme breadth--by the great gate, we see before us flamsteed house, the original building of the observatory. flamsteed's little domain was only some twenty-seven yards wide by fifty deep, and for buildings comprised little beyond a small dwelling-house on the ground floor, and one fine room above it. this room--the original greenwich observatory--still remains, and is used as a council room by the official board of visitors, who come down to the observatory on the first saturday in june, to examine into its condition and to receive the astronomer royal's report. the room is called, from its shape, the octagon room, and is well known to londoners from the great north window which looks out straight over the river between the twin domes of the hospital. in bradley's time, about , the first extension of the domains of the observatory took place to the south and east of the original building, the direction in which, on the whole, all subsequent extensions have taken place, owing to the fact that the original building was constructed at the extremity of what sir george airy was accustomed to call a 'peninsula'--a projecting spur of the blackheath plateau, from which the ground falls away very sharply on three sides and on part of the fourth. the observatory domain at present is fully two hundred yards in greatest length, with an average breadth of about sixty. nearly the whole of this accession took place under the directorates of pond and airy. the present instruments are, therefore, as a rule, the more modern in direct proportion to their distance from the octagon room--the old original observatory. there is one notable exception. the very first extension of the observatory buildings, made in the time of halley, the second astronomer royal, consisted in the setting up of a strong pier, to carry two quadrant telescopes. the pier still remains, but now forms the base of the support of the twin telescopes devoted to the photographic survey of the heavens for the international chart. standing just within the gate of the courtyard, and looking westward, that is toward flamsteed house, we have immediately on our right hand the porter's lodge; a little farther forward, also on the right, the transit pavilion, a small building sheltering a portable transit instrument; and farther forward, still on the right, the entrance to the chronograph room. above the chronograph room is a little, inconveniently-placed dome, containing a small equatorially-mounted telescope, known as the shuckburgh. beyond the chronograph room a door opens on to the north terrace, over which is seen the great north window of the octagon room. close by the door of the chronograph room a great wooden staircase rises to the roof of the main building. it is not an attractive-looking ascent, as the steps overlap inconveniently. still, there is no record of an accident upon them, and those who venture on the climb to the roof, where are placed the anemometers and the turret carrying the time-ball, which is dropped daily at p.m., will be well repaid by the splendid view of the river which is there afforded to them. passing under this staircase, on the wall by its side is seen the following inscription:-- carolus ii^s rex optimus astronomiÆ et nauticÆ artis patronus maximus speculam hanc in utriusque commodum fecit anno d^{ni} mdclxxvi. regni sui xxviii. curante iona moore milite r. t. s. g. [illustration: the astronomer royal's house. (_from a photograph by mr. lacey._)] in the extreme angle of the courtyard is the entrance to the mean solar clock cupboard, and to the staircase leading up to the octagon room. at the head of this staircase in a small closet is the winch for winding up the time-ball. coming back into the courtyard, and crossing the face of the astronomer royal's private house, the range of buildings is reached which form the left hand or south side of the enclosure. entering the first of these, we find ourselves in the lower computing room, which is devoted to the 'time department.' the next room which opens out of it, as we turn eastwards, was bradley's transit room, but is now used for the storage of chronometers. passing through bradley's transit room, we come to the present transit room, which brings us close to the great gate. the range of buildings is, however, continued somewhat farther, containing on the ground floor some small sitting-rooms and a fire-proof room for records. [illustration: the courtyard. (_from a photograph by mr. lacey._)] turning back to the lower computing room, we notice in it the stone pier, already alluded to, which was set up by halley, and formed the first addition to the original observatory of flamsteed. the lower computing room itself and bradley's transit room were due to the astronomer after which the latter is named. an iron spiral staircase in the middle of the lower computing room leads up to the upper computing room, and above that to the astrographic dome, so called because the twin telescope housed therein is devoted to the work of the astrographic chart--a chart of the entire sky to be made by eighteen co-operating observatories by means of photography. in this way it is intended to secure a record of the places of far more stars than could be done by the ordinary methods, and in this project greenwich has necessarily taken a premier place. this is a work which, whilst it is the legitimate and natural outcome of the original purpose of the observatory, is yet pushed beyond what is necessary for any mere utilitarian assistance to navigation. for the sailor it will always be sufficient to know the places of a mere handful of the brightest stars, and the vast majority of those in the great photographic map will never be visible in the little portable telescope of the sailor's sextant. but it will be freely admitted that in the case of an enterprise of this nature, in which the observatories of so many different nations were uniting, and which was so precisely on the lines of its original charter, though an extension of it, it was impossible for greenwich to hold back on the plea that the work was not entirely utilitarian. descending again to the lower computing room, and passing through it, not to the east, into bradley's transit room, but through a little lobby to the south, we come upon an inconvenient wooden staircase winding round a great stone pillar with three rays. this pillar is the support of airy's altazimuth, and very nearly marks the place where flamsteed set up his original sextant. returning again to the lower computing room, and passing out to the east, just in front of the time superintendent's desk, we enter a small passage running along the back of bradley's transit room, and from this passage enter the present transit room near its south end. just before reaching the transit room, however, we pass the reflex zenith tube, a telescope of a very special kind. immediately outside the transit room is a staircase leading on the first floor to two rooms long used as libraries, and to the leads above them, on which is a small dome containing the sheepshanks equatorial. these libraries are over the small sitting-rooms already referred to. the fire-proof record rooms, two stories in height, terminate this range of buildings. beyond the record rooms the boundary turns sharply south, where stands a large octagonal building surmounted by a dome of oriental appearance, a 'circular versatile roof,' as the visitors would have called it a hundred years ago. this dome--which has been likened, according to the school of æsthetics in which its critics have been severally trained, to the taj at agra, a collapsed balloon, or a mammoth spanish onion--houses the largest refractor in england, the 'south-east equatorial' of twenty-eight inches aperture. but, though the largest that england possesses, it would appear but as a pigmy beside some of the great telescopes for which america is famous. beyond this dome the hollow devoted to the astronomer royal's private garden reduces the observatory ground to a mere 'wasp's waist,' a narrow, inconvenient passage from the old and north observatory to the younger southern one. the first building, as the grounds begin to widen out to the south, contains the new altazimuth, a transit instrument which can be turned into any meridian. a library of white brick and a low wooden cruciform building--the magnetic observatory--follow it closely. this latter building houses the magnetic department, a department which, though it lies aside from the original purposes of the observatory, as defined in the warrant given to flamsteed, is yet intimately connected with navigation, and was founded by airy very early in his period of office. this deals with the observation of the changes in the force and direction of the earth's magnetism, an inquiry which the greater delicacy of modern compasses, and, in more recent times, the use of iron instead of wood in the construction of ships, has rendered imperative. closely associated with the magnetic department is the meteorological. weather forecasts, so necessary for the safety of shipping round our coasts, are not issued from greenwich observatory, any more than the _nautical almanac_ is now issued from it. but just as the observatory furnishes the astronomical data upon which the almanac is based, so also a considerable department is set apart for furnishing observations to be used by the meteorological office at westminster for their daily predictions. so far, the development of the observatory had been along the central line of assistance to navigation. but the 'magnetic department' led on to a new one, which had but a secondary connection with it. it had been discovered that the extent of the daily range of the magnetic needle, and the amount of the disturbances to which it was subjected, were in close connection with the numbers and size of the spots on the sun's surface. this led to the institution of a daily photographic record of the state of the sun's surface, a record of which greenwich has now the complete monopoly. [illustration: plan of observatory at present time. (for key to plan, see p. .)] key to the plan of the observatory on page . . chronograph room. . old altazimuth dome. . safe room. . computing room. . bradley's transit room. . transit circle room. . assistants' room. . chief assistant's room. . computers' room. . record rooms. . chronometer rooms and south-east dome. . greenhouse and outbuildings. . new library. . magnetic observatory. . offices. . sheds. . winch room for time-ball. . porter's lodge. . new transit pavilion. . new altazimuth pavilion. . museum: new building. . south wing " . north wing " . west wing " . east wing " f. rooms built for flamsteed. h. added by halley. b. " bradley. m. " maskelyne. a. " airy. f'f'. flamsteed's boundaries. m'm'. maskelyne's " . p'p'. pond's " . a'a'. airy's " . a"a". airy's " . beyond the magnetic observatory the ground widens out into an area about equal to that of the northern part, and the new building just completed, and which is now emphatically 'the observatory,' stands clear before us. the transfer to this stately building of the computing rooms, libraries, and store rooms has been aptly described as a shift in the latitude of greenwich observatory, which still preserves its longitude. it may be noted that the only two buildings of any architectural pretensions in the whole range are--flamsteed's original observatory, built by sir christopher wren, and containing little beyond the octagon room, in the extreme north; and this newest building in the extreme south. this 'new observatory,' like the old, and like the great south-eastern tower, is an octagon in its central portion. but whilst the two other great buildings are simply octagonal, here the octagon serves only as the centre from which radiate four great wings to the four points of the compass. the building is by far the largest on the ground, but in little accord with the popular idea of an astronomer as perpetually looking through a telescope, carries but a single dome; its best rooms being set apart as 'computing rooms,' for the use of those members of the staff who are employed in the calculations and other clerical work, which form, after all, much the greater portion of the observatory routine. an observer with the transit instrument, for instance, will take only three or four minutes to make a complete determination of the place of a single star. but that observation will furnish work to the computers for many hours afterwards. or, to take a photograph of the sun will occupy about five minutes in setting the instrument, whilst the actual exposure will take but the one-thousandth part of a second. but the plate, once exposed, will have to be developed, fixed, and washed; then measured, and the measures reduced, and, _on the average_, will provide one person with work for four days before the final results have been printed and published. it is easy to see, then, that observing, though the first duty of the observatory, makes the smallest demand on its time. the visitor who comes to the observatory by day (and none are permitted to do so by night) finds the official rooms not unlike those of somerset house or whitehall, and its occupants for the most part similarly engaged in what is, apparently, merely clerical work. an examination of the big folios would of course show that instead of being ledgers of sales of stamps, or income-tax schedules, they referred to stars, planets, and sun-spots; but for one person actively engaged at a telescope, the visitor would see a dozen writing or computing at a desk. the staff, like the building, is the result of a gradual development, and bears traces of its life history in its composition. first comes the astronomer royal, the representative and successor of the original 'king's astronomer,' the rev. john flamsteed. but the 'single surly and clumsy labourer,' which was all that the 'merry monarch' could grant for his assistance, is now represented by a large and complex body of workers; each varied class and rank of which is a relic of some stage in the progress of the observatory to its present condition. the following extract from the annual report of the astronomer royal to the board of visitors, june, , describes the present _personnel_ of the establishment:-- 'the staff at the present time is thus constituted, the names in each class being arranged in alphabetical order:-- 'chief assistants--mr. cowell, mr. dyson. 'assistants--mr. hollis, mr. lewis, mr. maunder, mr. nash, mr. thackeray. 'second-class assistants--mr. bryant, mr. crommelin. 'clerical assistant--mr. outhwaite. 'established computers--mr. bowyer, mr. davidson, mr. edney, mr. furner, mr. rendell, and one vacancy. 'the two second-class assistants will be replaced by higher grade established computers as vacancies occur. 'mr. dyson and mr. cowell have the general superintendence of all the work of the observatory. mr. maunder is charged with the heliographic photography and reductions, and with the preparation of the library catalogue. mr. lewis has charge of the time-signals and chronometers, and of the -inch equatorial. mr. thackeray superintends the miscellaneous astronomical computations, including the preparation of the new ten-year catalogue. mr. hollis has charge of the photographic mapping of the heavens, the measurement of the plates, and the computations for the astrographic catalogue. mr. crommelin undertakes the altazimuth and sheepshanks equatorial reductions, and mr. bryant the transit and meridian zenith distance reductions and time-determinations. in the magnetic and meteorological branch, mr. nash has charge of the whole of the work. mr. outhwaite acts as responsible accountant officer; has charge of the library, records, manuscripts, and stores, and conducts the official correspondence. as regards the established computers, mr. bowyer, mr. furner, mr. davidson, and mr. rendell assist mr. lewis, mr. thackeray, mr. hollis, and mr. bryant respectively, and mr. edney assists mr. nash. 'there are at the present time twenty-four supernumerary computers employed at the observatory, ten being attached to the astronomical branch, two the chronometer branch, six to the astrographic, one to the heliographic, four to the magnetic and meteorological, and one to the clerical. 'a foreman of works, with two carpenters, and two labourers; a skilled mechanic with an assistant; a gate porter, two messengers, a watchman, a gardener, and a charwoman, are also attached to the observatory. 'the whole number of persons regularly employed at the observatory is fifty-three.' the day work, as said before, is by far the greatest in amount, the 'office hours' being from nine till half-past four, with an hour's interval. the arrangements for the night watches present some complications. for many years the instruments in regular use were two only, the transit circle and the altazimuth. the arrangements for observing were simple. four assistants divided the work between them thus: an assistant was on duty with the transit circle one day, his watch beginning about six a.m. or a little later, and ending about three the following morning; a watch of twenty-one hours in maximum length. the second day his duties were entirely computational, and were only two or three hours in length. the third day he had a full day's work on the calculations, followed by a night duty with the altazimuth. the latter instrument might give him a very easy watch or a terribly severe one. if the moon were a young one it was easy, especially if the night was clear, as in that case an hour was enough to secure the observations required. very different was the case with a full moon, especially in the long, often cloudy, nights of winter. then a vigilant watch had to be kept from sunset to sunrise, so that in case of a short break in the clouds the moon might yet be observed. such a watch was the severest (with one exception) that an assistant had to undergo. his fourth day would then resemble his second, and with the fifth day a second cycle of his quartan fever would commence, the symptoms following each other in the same sequence as before. such a routine carried on with iron inflexibility was exceedingly trying, as it was absolutely impossible for an observer to keep any regularity in his hours of rest or times for meals. this routine has been considerably modified by the present astronomer royal, partly because the instruments now in regular daily use are five instead of two, and partly because a less stringent system has proved not merely far less wearing to the observers, but also much more prolific of results. it was impossible for a man to be at his best for long under the old _régime_, and from forty-six to forty-seven has been an ordinary age for an assistant to break down under the strain. one point in which the observing work has been lightened has been in the discontinuance of the altazimuth observations at the full of the moon, another in the shortening of the hours of the transit circle watch; and a further and most important one in the arrangement that the observers with the larger instruments should have help at their work. the net result of these changes has been a most striking increase in the amount of work achieved. thus, whilst in the year ending may , , transits were taken with the transit circle, and determinations of north polar distance; in that ending may , , the numbers had risen to , and , respectively, the telescope remaining precisely the same. one principle of airy's rule still remains. so far as possible no observer is on duty for two consecutive days, but a long day of desk work and observing is followed by a short day of desk work without observing. it will be readily understood that with five principal telescopes in constant work and one or two minor ones, some demanding two observers, others only one, each telescope having its special programme and its special hours of work, whilst by no means every member of the staff is authorized to observe with all instruments indifferently, it becomes a somewhat intricate matter to arrange the weekly _rota_ in strict accordance with the foregoing principle, and with the further one, that whilst a considerable amount of sunday observing is inevitable, the average duty of an observer should be three days a week, not seven days a fortnight. there is a story, received with much reserve at cambridge, that there was once a man at that university who had mastered all the colours and combinations of shades and colours of the various colleges and clubs. if so gifted a being ever existed, he may be paralleled by the greenwich assistant who can predict for any future epoch the sequence of duties throughout the entire establishment. at any rate, one of the first items in the week's programme is the preparation of the _rota_ for the week, or rather, to use an ecclesiastical term, for the 'octave,' _i.e._ from the monday to the monday following. the special work to be carried out on any telescope is likewise a matter of programme. for the transit circle a list of the most important objects to be observed is supplied for the observer's use, and the general lines upon which the other stars are to be selected from a huge 'working catalogue' are well understood. with some of the other telescopes the principles upon which the objects are to be selected are laid down, but the actual choice is left to the discretion of the observer at the time. there is no time for the watcher to spend in what the outsider would regard as 'discovery'; such as sweeping for comets or asteroids, hunting for variable stars, sketching planets, and so forth. indeed, there is a story current in the observatory that some fifty years ago, when the tide of asteroid discovery first set in, airy found an assistant, since famous, working with a telescope on his 'off-duty' night. that stern disciplinarian asked what business the assistant had to be there on his free night, and on being told he was 'searching for new planets,' he was severely reprimanded and ordered to discontinue at once. a similar energy would not meet so gruff a discouragement to-day; but the routine work so fully occupies both staff and telescopes that an assistant may be most thoroughly devoted to his science, and yet pass a decade at the observatory without ever seeing those 'show places' of the sky which an amateur would have run over in the first week after receiving his telescope. for example, there is no refractor in the british isles so competent to bring out the vivid green light of the great orion nebula--that marvellous mass of glowing, curdling, emerald cloud--or the indescribable magnificence of the myriad suns that cluster like swarming bees or the grapes of eshcol in the constellation of hercules; yet probably most of the staff have never seen either spectacle through it. the professional astronomer who is worth his salt will find abundance of charm and interest in his work, but he will not, 'like a girl, valuing the giddy pleasures of the eyes,' consider the charm to lie mainly in the occasional sight of wonderful beauty which his work may bring to him, nor the interest in some chance phenomenon which may make his name known. it is not every field of astronomy that is cultivated at greenwich. the search for comets and for 'pocket planets' forms no part of its programme; and the occupation so fascinating to those who take it up, of drawing the details on the surfaces of the moon, mars, jupiter, or saturn, has been but little followed. such work is here incidental, not fundamental, and the same may be said of certain spectroscopic observations of new or variable stars, and of many similar subjects. work such as this is most interesting to the general public, and is followed with much devotion by many amateur astronomers. for that very reason it does not form an integral part of the programme of our state observatory. but work which is necessary for the general good, or for the advancement of the science, and which demands observations carried on continuously for many years, and strict unity of instruments and methods, cannot possibly be left to chance individual zeal, and is therefore rightly made the first object at greenwich. those striking discoveries which from time to time appeal strongly to the popular imagination, and which have rendered so justly famous some of the great observatories of the sister continent, have not often been made here. its work has, none the less, been not only useful but essential. a century ago, when we were engaged in the hand-to-hand struggle with napoleon, by far the most brilliant part of that naval war which we waged against the french, and the most productive of prize-money, was carried on by our cruisers, who captured valuable prizes in every sea. but a much greater service, indeed an absolutely vital one, was rendered to the state by those line-of-battle ships which were told off to watch the harbours wherein the french fleet was taking refuge. this was a work void of the excitement, interest, and profit of cruising. it was monotonous, wearing, and almost inglorious, but absolutely necessary to the very existence of england. so the continuance for more than two centuries of daily observations of places of moon, stars, and planets is likewise 'monotonous, wearing, and almost inglorious;' the one compensation is that it is essential to the life of astronomy. the eight astronomers royal have, as already said, kept the observatory strictly on the lines originally laid down for it, subject, of course, to that enlargement which the growth of the science has inevitably brought. but had they been inclined to change its course, the board of visitors has been specially appointed to bring them back to the right way. as already mentioned in the account of flamsteed, the board dates from , when it practically consisted of the president and council of the royal society. its royal warrant lapsed on the death of queen anne, and was not renewed at the accession of the two following sovereigns; but in the reign of george iii. a new warrant was issued under date february , ; and this was renewed at the accession of george iv. when william iv. came to the throne, the constitution of the board was extended, so as to give a representation to the new royal astronomical society, founded in . the president of the royal society is still chairman of the board, but the admiralty, of which the observatory is a department, the two universities of oxford and cambridge, and the royal astronomical society are all represented on it by _ex officio_ members, and twelve other members are contributed by the royal and royal astronomical societies respectively, six by each. the first saturday in june is the appointed day for the annual inspection by the board, and for the presentation to it of the astronomer royal's report. to this all-important business meeting has been added something of a social function, by the invitation of many well-known astronomers and the leading men of the allied sciences to inspect the results of the year, and to partake of the chocolate and cracknels, which have been the traditional refreshments offered on these occasions for a period 'whereof the memory of man runneth not to the contrary.' chapter vi the time department one day two scotchmen stood just outside the main entrance of greenwich observatory, looking intently at the great twenty-four-hour clock, which is such an object of attention to the passers through the park. 'jock,' said one of them to the other, 'd'ye ken whaur ye are?' jock admitted his ignorance. 'ye are at the vara ceentre of the airth.' geographers tell us that there is a sense in which this statement as it stands may be accepted as true. for if the surface of the globe be divided into two hemispheres, so related to each other that the one contains as much land as possible, and the other as little, then london will occupy the centre or thereabouts of the hemisphere with most land. this was not, however, what the scotchman meant. he meant to tell his companion that he was standing on the prime meridian of the world, the imaginary base line from which all distances, east or west, are reckoned; in short, that he was on 'longitude nought.' he was not absolutely correct, however, for the great twenty-four-hour clock does not mark the exact meridian of greenwich. to find the instrument which marks it out and defines it we must step inside the observatory precincts, and just within the gate we see before us on the left hand a door which leads through a little lobby straight into the most important room of the whole observatory--the transit room. [illustration: the great clock and porter's lodge. (_from a photograph by mr. lacey._)] this room is not well adapted for representation by artist or photographer. four broad stone pillars occupy the greater part of the space, and leave little more than mere passage room beside. two of these pillars are tall, as well as broad and massive, and stand east and west of the centre of the room, carrying between them the fundamental instrument of the observatory, the transit circle. the optical axis of this telescope marks 'longitude nought,' which is further continued by a pair of telescopes, one to the north of it, the other to the south, mounted on the third and fourth of the pillars alluded to above. this room has not always marked the meridian of greenwich, for it stands outside the original boundary of the observatory. but it is only a few feet to the east of the first transit instrument which was set up by halley, the second astronomer royal, in the extreme n.-w. corner of the observatory domain, a distance equivalent to very much less than one-tenth of a second of time, an utterly insensible quantity with the instruments of two hundred years ago. it would be a long story to tell in detail how the greenwich transit room has come to define one of the two fundamental lines that encircle the earth. the other, the equator, is fixed for us by the earth itself, and is independent of any political considerations, or of any effort or enterprise of man. but of all the infinite number of great circles which could be drawn at right angles to the equator, and passing through the north and south poles, it was not easy to select one with such an overwhelming amount of argument in its favour as to obtain a practically universal acceptance. the meridians of jerusalem and of rome have both been urged, upon what we may call religious or sentimental grounds; that of the great pyramid at ghizeh has been pressed in accordance with the fantastic delusion that the pyramid was erected under divine inspiration and direction; that of ferro, in the canaries, as being an oceanic station, well to the west of the old world, and as giving a base line without preference or distinction for one nation rather than another. the actual decision has been made upon no such grounds as these. it has been one of pure practical convenience, and has resulted from the amazing growth of great britain as a naval and commercial power. like tyre of old, she is 'situate at the entry of the sea, a merchant of the people for many isles,' and 'her merchants are the great men of the earth.' to tell in full, therefore, the steps by which the greenwich meridian has overcome all others is practically to tell again, from a different standpoint, the story of the 'expansion of england.' the need for a supreme navy, the development of our empire beyond the seven seas, the vast increase of our carrying trade--these have made it necessary that englishmen should be well supplied with maps and charts. the hydrographic and geographic surveys carried on, either officially by this country, or by englishmen in their own private capacity, have been so numerous, complete, and far-reaching as not only to outweigh those of all other countries put together, but to induce the surveyors and explorers of not a few other countries to adopt in their work the same prime meridian as that which they found in the british charts of regions bordering on those which they were themselves studying. naturally, the meridian of greenwich has not only been adopted for great britain, but also for the british possessions over-sea, and, from these, for a large number of foreign countries; whilst our american cousins retain it, an historic relic of their former political connection with us. the victories of clive at arcot and plassy, of nelson at the nile and trafalgar, the voyages and surveys of cook and flinders, and many more; the explorations of bruce, park, livingstone, speke, cameron, and stanley; these are some of the agencies which have tended to fix 'longitude nought' in the greenwich transit room. there are two somewhat different senses in which the meridian of greenwich is the standard meridian for nearly the entire world. the first is the sense about which we have already been speaking; it constitutes the fundamental line whence distances east and west are measured, just as distances north and south are measured from the equator. but there is another, though related sense, in which it has become the standard. _it gives the time to the world._ there are few questions more frequently put than, 'what time is it?' 'can you tell me the true time?' a stickler for exactitude might reply, 'what kind of time do you mean?' 'do you mean solar or sidereal time?' 'apparent time or mean time?' 'local time or standard time?' there are all these six kinds of time, but it is only within the last two generations, within, indeed, the reign of our sovereign, queen victoria, that the subject of the differences of most of these kinds of time has become of pressing importance to any but theorists. in one of the public gardens of paris a little cannon is set up with a burning-glass attached to it in such a manner that the sun itself fires the cannon as it reaches the meridian. this, of course, is the time of paris noon--apparent noon--but it would be exceedingly imprudent of any traveller through paris who wished, say, to catch the one o'clock express, to set his watch by the gun. for if it happened to be in february, he would find when he reached the railway station that the station clock was faster than the sun by nearly a full quarter of an hour, and that his train had gone; whilst towards the end of october or the beginning of november, he would find himself as much too soon. until machines for accurately measuring time were invented, apparent time--time, that is to say, given by the sun itself, as by a sun-dial--was the only time about which men knew or cared. but when reasonably good clocks and watches were made, it was very soon seen that at different times in the year there was a marked difference between sun-dial time and that shown by the clock, the reason being simply that the apparent rate of motion of the sun across the sky was not always quite the same, whilst the movement of the clock was, of course, as regular as it could be made. this difference between time as shown by the actual sun and by a perfect clock is known as the 'equation of time.' it is least about april , june , august , and december . it is greatest, the sun being after the clock, about february ; and the sun being before the clock, about november . flamsteed, before he became astronomer royal, investigated the question, and so clearly demonstrated the existence, cause, and amount of the equation of time as entirely to put an end to controversy on the subject. we had thus, early in the century, the two kinds of time in common use, apparent time and mean time, or clock time. but as the sun can only be on one particular meridian at any given instant, the time as shown by the clocks in one particular town will differ from that of another town several miles to the east or west of it. it is thus noon at moscow hr. min. before it is noon at berlin, and noon at berlin min. before it is noon in london. this was all well enough known, but occasioned no inconvenience until the introduction of railway travelling; then a curious difficulty arose. suppose an express train was running at the rate of sixty miles an hour from london to bristol. the guard of the train sets his watch to london time before he leaves paddington, but if the various towns through which the train passes, reading, swindon, etc., each keep their own local time, he will find his watch apparently fast at each place he reaches; but on his return journey, if he sets to bristol time before starting, he will in a similar way find it apparently slow by the swindon, reading, and paddington clocks as he reaches them in succession. it became at once necessary to settle upon one uniform system of time for use in the railway guides. apart from this, a passenger taking train, say, at swindon, might have been very troubled to know whether the advertised time of his train was that of exeter, the place whence it started, or swindon, the station where he was getting in, or london, its destination. 'railway time,' therefore, was very early fixed for the whole of great britain to be the same as london time, which is, of course, time as determined at greenwich observatory. at first it was the custom to keep at the various stations two clocks, one showing local time, the other 'railway,' or greenwich time, or else the clocks would be provided with a double minute hand, one branch of which pointed to the time of the place, the other to the time of greenwich. it was soon found, however, that there was no sufficient reason for keeping up local time. even in the extreme west of england the difference between the two only amounted to twenty-three minutes, and it was found that no practical inconvenience resulted from saying that the sun rose at twenty-three minutes past six on march , rather than at six o'clock. the hours of work and business were practically put twenty-three minutes earlier in the day, a change of which very few people took any notice. other countries besides england felt the same difficulty, and solved it in the same way, each country as a rule taking as its standard time the time of its own chief city. there were two countries for which this expedient was not sufficient--the united states and canada. the question was of no importance until the iron road had linked the atlantic to the pacific in both countries. then it became pressing. no fewer than seventy different standards prevailed in the united states only some twenty years ago. the case was a very different one here from that of england, where east and west differed in local time by only a little over twenty minutes. in north america, in the extreme case, the difference amounted to four hours, and it seemed asking too much of men to call eight o'clock in their morning, or it might be four o'clock in their afternoon, their noonday. the device was therefore adopted of keeping the minutes and seconds the same for all places right across the continent, but of changing the hour at every ° of longitude. the question then arose what longitude should be adopted as the standard. the americans might very naturally have taken their standard time from their great national observatory at washington, or from that of their chief city, new york, or of their principal central city, chicago. but, guided partly no doubt by a desire to have their standard times correspond directly to the longitudes of their maps, and partly from a desire to fall in, if possible, with some universal time scheme, if such could be brought forward, they fixed upon the meridian of greenwich as their ultimate reference line, and defined their various hour standards as being exactly so many hours slow of greenwich mean time. the decision of the united states and of canada brought with it later a similar decision on the part of all the principal states of europe; and greenwich is not only 'longitude nought' for the bulk of the civilized world, but greenwich mean time, increased or decreased by an exact number of hours or half-hours, is the standard time all over the planet. no; the statement requires correction. two countries hold out, both close to our own doors. france, instead of adopting greenwich time as such, adopts _paris time less_ m. s. (that being the precise difference in longitude between the two national observatories). ireland disdains even such a veiled surrender, and dublin time is the only one recognized from the hill of howth to far valentia. so the distressful country preserves her old grievance, that she does not even get her time until after england has been served. the alteration in national habits following on the adoption of this european system has had a very perceptible effect in some cases. thus, switzerland has adopted mid-european time, one hour fast of greenwich; the true local time for berne being just half an hour later. the result of putting the working hours this thirty minutes earlier in the day has had such a noticeable effect on the consumption of gas, as to lead the gas company to contemplate agitating for a return to the old system. thus, greenwich time, as well as the greenwich meridian, has practically been adopted the world over. it follows, then, that the determination of time is the most important duty of the royal observatory; and the time department, the one to which is entrusted the duty of determining, keeping, and distributing the time, calls for the first attention. entering the transit room, the first thing that strikes the visitor is the extreme solidity with which the great telescope is mounted. it turns but in one plane, that of 'longitude nought,' and its pivots are supported by the pair of great stone pillars which we have already spoken of as occupying the principal part of the transit-room area, and the foundations of which go deep down under the surface of the hill. on the west side of the telescope, and rigidly connected with it, is a large wheel some six feet in diameter, and with a number of wooden handles attached to it, resembling the steering-wheel of a large steamer. this wheel carries the setting circle, which is engraved upon a band of silver let into its face near its circumference, a similar circle being at the back of the wheel nearer the pillar. eleven microscopes, of which only seven are ordinarily used, penetrate through the pier, and are directed on to this second circle. the present transit is the fourth which the observatory has possessed, and its three predecessors, known as halley's, bradley's, and troughton's, respectively, are still preserved and hang on the walls of the transit room, affording by their comparison an interesting object-lesson in the evolution of a modern astronomical instrument. the watcher who wishes to observe the passing of a star must note two things: he must know in what direction to point his telescope, and at what time to look for the star. then, about two minutes before the appointed time, he takes his place at the eyepiece. as he looks in he sees a number of vertical lines across his field of view. these are spider-threads placed in the focus of the eye-piece. presently, as he looks, a bright point of silver light, often surrounded by little flashing, vibrating rays of colour, comes moving quickly, steadily onward--'swims into his ken,' as the poet has it. the watcher's hand seeks the side of the telescope till his finger finds a little button, over which it poises itself to strike. on comes the star, 'without haste, without rest,' till it reaches one of the gleaming threads. tap! the watcher's finger falls sharply on the button. some three or four seconds later and the star has reached another 'wire,' as the spider-threads are commonly called. tap! again the button is struck. another brief interval and the third wire is reached, and so on, until ten wires have been passed, and the transit is over. the intervals are not, however, all the same, the ten wires being grouped into three sets, two of three apiece, and the third of four. [illustration: the chronograph.] each tap of the observer's finger completed for an instant an electric circuit, and recorded a mark on the 'chronograph.' this is a large metal cylinder covered with paper, and turned by a carefully-regulated clock once in every two minutes. once in every two seconds a similar mark was made by a current sent by means of the standard sidereal clock of the observatory. the paper cover of the chronograph after an hour's work shows a spiral trace of little dots encircling it some thirty times. these dots are at regular intervals, about an inch apart, and are the marks made by the clock. interspersed between them are certain other dots, in sets of ten; and these are the signals sent from the telescope by the transit observer. if, then, one of the clock dots and one of the observer's dots come exactly side by side, we know that the star was on one of the wires at a given precise second. if the observer's dot comes between two clock dots, it is easy, by measuring its distance from them with a divided scale, to tell the instant the star was on the wire to the tenth of a second, or even to a smaller fraction. whilst, since the transit was taken over ten wires, and the distance of each wire from the centre of the field of view is known, we have practically ten separate observations, and the average of these will give a much better determination of the time of transit than a single one would. but let the watcher be ever so little too slow in setting his telescope, or ever so little late in placing himself at his eye-piece, and the star will have passed the wire, and as it smoothly, resistlessly moves on its inexorable way, will tell the tardy watcher in a language there is no mistaking, 'lost moments can never be recalled.' the opportunity let slip, not until twenty-four hours have gone by will another chance come of observing that same star. it is the stars that are chiefly used in this determination, partly because the stars are so many, whilst there is but one sun. if, therefore, clouds cover the sun at the important moment of transit, the astronomer may well exclaim, so far as this observation is concerned, 'i have lost a day!' the chance will not be offered him again until the following noon. but if one star is lost by cloud, there are many others, and the chance is by no means utterly gone. beside, the sun enables us to tell the time only at noon; the stars enable us to find it at various times throughout the entire night; indeed, throughout both day and night, since the brighter stars can be observed in a large telescope even during the day. there are two great standard clocks at the observatory: the mean solar clock and the sidereal clock. the latter registers twenty-four hours in the precise time that the earth rotates on its axis. a 'day' in our ordinary use of the term is somewhat longer than this; it is the average time from one noon to the next, and as the earth whilst turning round on its axis is also travelling round the sun, it has to rather more than complete a rotation in order to bring the sun again on to the same meridian. a solar day is therefore some four minutes longer than an actual rotation of the earth, _i.e._ a sidereal day, as it is called, since such rotation brings a star back again to the same meridian. the sidereal clock can therefore be readily checked by the observation of star transits, for the time when the star ought to be on the meridian is known. if, therefore, the comparison of the transit taps on the chronograph with the taps of the sidereal clock show that the clock was not indicating this time at the instant of the transit, we know the clock must be so much fast or slow. similarly, the difference which should be shown between the sidereal and solar clocks at any moment is known; and hence when the error of the sidereal clock is known, that of the solar can be readily found. it is often quite sufficient to know how much a clock is wrong without actually setting its hands right; but it is not possible to treat the greenwich clock so, for it controls a number of other clocks continually, and sends hourly signals out over the whole country, by which the clocks and watches all over the kingdom are set right. in the lower computing room, below the south window, we find the time-desk, the head-quarters of the time department. this is a very convenient place for the department, since one of the chronometer rooms, formerly bradley's transit room, opens out of the lower computing room; the transit instrument is just beyond; it is close to the main gate of the observatory, and so convenient for chronometer makers or naval officers bringing chronometers or coming for them, whilst just across the courtyard is the chronograph room, with the battery basement, in which the batteries for the electric currents are kept, and the mean solar clock lobby, with the winch for the winding of the time-ball at the head of the stairs above it. these rooms do not exhaust the territory of the department, since it owns two other chronometer rooms on the ground floor and first floor respectively of the s.-e. tower. at the time-desk means are provided for setting the clock right very easily and exactly. just above the desk are a range of little dials and bright brass knobs, that almost suggest the stops of a great organ. two of these little dials are clock faces, electrically connected with the solar and sidereal standard clocks, so that, though these clocks are themselves a good way off, in entirely different parts of the observatory, the time superintendent, seated here at the time-desk, can see at once what they are indicating. between the two is a dial labelled 'commutator.' from this dial a little handle usually hangs vertically downwards, but it can be turned either to the right or to the left, and when thus switched hard over, an electric current is sent through to the mean solar clock. if now we leave the computing room and cross the courtyard to the extreme north-west corner, we find the mean solar clock in a little lobby, carefully guarded by double doors and double windows against rapid changes of temperature. opening the door of the clock case, we see that the pendulum carries on its side a long steel bar, and that this bar as the pendulum swings passes just over the upper end of an electro-magnet. when the current is switched on at the commutator, this electro-magnet attracts or repels the steel bar according to the direction of the current, and the action of the clock is accordingly quickened or retarded. to put the commutator in action for one minute will alter the clock by the tenth of a second. as the error of the clock is determined twice a day, shortly before ten o'clock in the morning, and shortly before one o'clock in the afternoon, its error is always small, usually only one or two tenths. these two times are chosen because, though time-signals are sent over the metropolitan area every hour from the greenwich clock through the medium of the post office, at ten and at one o'clock signals are also sent to all the great provincial centres. further, at one o'clock the time balls at greenwich and at deal are dropped, so that the captains of ships in the docks, on the river, or in the downs may check their chronometers. the time-ball is dropped directly by the mean solar clock itself. it is raised by means of a windlass turned by hand-power to the top of its mast just before one o'clock. connected with it is a piston working in a stout cylinder. when the ball has reached the top of the mast, the piston is lightly supported by a pair of catches. these catches are pulled back by the hourly signal current, and the piston at once falls sharply, bringing the ball with it. but after a fall of a few feet, the air compressed by the piston acts as a cushion and checks the fall, the ball then gently and slowly finishing its descent. the instant of the beginning of the fall is, of course, the true moment to be noted. the other dials on the time-desk are for various purposes connected with the signals. one little needle in a continual state of agitation shows that the electric current connecting the various sympathetic clocks of the observatory is in full action. another receives a return signal from various places after the despatch of the time-signal from greenwich, and shows that the signal has been properly received at the distant station, whilst all the many electric wires within the observatory or radiating from it are made to pass through the great key-board, where they can be at once tested, disconnected, or joined up, as may be required. [illustration: the time-desk.] the distribution of greenwich time over the island in this way is thus a simple matter. the far more important one of the distribution of greenwich time to ships at sea is more difficult. the difficulty lay in the construction of a clock or watch, the rate of which would not be altered by the uneasy motion of a ship, or by the changes of temperature which are inevitable on a voyage. two hundred years ago it was not deemed possible to construct a watch of anything like sufficient accuracy. they would not even keep going whilst they were being wound, and would lose or gain as much as a minute in the day for a fall or rise of ° in temperature. this was owing to the extreme sensitiveness of the balance spring--which takes the place in a watch of a pendulum in a clock--to the effects of temperature. the british government, therefore, in offered a prize of the amount of £ , for a means of finding the longitude at sea within half a degree, or, in other words, for a watch that would keep greenwich time correct to two minutes in a voyage across the atlantic. in , james harrison, the son of a yorkshire carpenter, succeeded in solving the problem. his method was to attach a sort of automatic regulator to the spring which should push the regulator over in one direction as the temperature rose, and bring it back as it fell. this he effected by fastening together two strips of brass and steel. the brass expanded with heat more rapidly than the steel, and hence with a rise of temperature the strip bent over on the steel side. this was the first germ of the idea of making watches 'compensated for temperature;' watches, that is, which maintain practically the same rate whether they are in heat or cold, an idea now brought to great perfection in the modern chronometer. [illustration: harrison's chronometer.] the great reward the government had offered stimulated many men to endeavour to solve the problem. of these, dr. halley, the second astronomer royal, and graham, the inventor of the astronomical clock, were the most celebrated. but when harrison, then poor and unknown, came to london in , and laid his invention before them, with an utter absence of self-seeking, and in the true scientific spirit, they gave him every assistance. harrison's first four time-keepers are still preserved at the royal observatory. he did not, however, receive his reward until a facsimile of the fourth had been made by his apprentice, larcum kendall. the latter is preserved at the royal observatory. there is a larcum kendall at the royal institution which is said to have been used by captain cook. harrison's chronometer was sent on a trial voyage to jamaica in , and on its return to portsmouth in the following year it was found that its complete variation was under the two minutes for which the government had stipulated. since harrison's day the improvement of the chronometer has been carried on almost to perfection, and now the care and rating of chronometers for the royal navy is one of the most important duties of the observatory. [illustration: the chronometer room.] a visitor who should make the attempt to compare a single chronometer with a standard clock would probably feel very disheartened when, after many minutes of comparison, he had got out its error to the nearest second, were he told that it was his duty to compare the entire army here collected, some five hundred or more, and to do it not to the second, but to the nearest tenth of a second. practice and system make, however, the impossible easy, and one assistant will quietly walk round the room calling out the error of each chronometer as he passes it, as fast as a second assistant seated at the table can enter it at his dictation in the chronometer ledgers. the seconds beat of a clock sympathetic with the solar standard, rings out loud and clear above the insect-like chatter of the ticking of the hundreds of chronometers, and wherever the assistant stands, he has but to lift his eyes to see straight before him, if not a complete clock-face, at least a seconds dial moving in exact accordance with the solar standard. the test to which chronometers are subjected is not merely one of rate, but one of rate under carefully altered conditions. thus they may be tried with the xii pointing in succession to the four points of the compass, or, in the case of chronometer watches, they may be laid flat down on the table or hung from the ring or pendant, or with the ring right or left, as it would be likely to be when carried in the waistcoat pocket. but the chief test is the performance of a chronometer when subjected to considerable heat for a long period. this is a matter of great consequence, since a chronometer travelling from england to india, australia, or the cape, would necessarily be subjected to very different conditions of temperature from those to which it would be exposed in england. they are therefore kept for eight weeks in a closed stove at a temperature of about ° or °. at one time a cold test was also applied, and sir george airy, the late astronomer royal, in one of his popular lectures, drew a humorous comparison between the unhappy chronometers thus doomed to trial, now in heat and now in frost, and the lost spirits whom dante describes as alternately plunged in flame and ice. the cold test has, however, been done away with. it is perfectly easy on the modern ship to keep the chronometer comfortably warm even on an arctic expedition. the elaborate cold testing applied to sir george nares' chronometers before he started on his polar journey was found to have been practically quite superfluous; the chronometers were, if anything, kept rather too warm. the exposure of the chronometer in the cooling box, moreover, was found to be attended with a risk of rusting its springs. [illustration: the chronometer oven.] once the determination of the longitude at sea became possible, it was clearly the next duty to fix with precision the position of the principal places, cities, ports, capes, islands, the world over. of all the work done in this department none has ever been done better, in proportion to the means at command, than that accomplished by captain cook in his celebrated three voyages. as has already been pointed out, it is the extent and thoroughness of the hydrographic surveys of the british admiralty which have largely contributed to the honour done to england by the international selection of the english meridian, and of english standard time, as in principle those for the whole civilized world. the generosity and public spirit therefore which led the second astronomer royal to help forward and support his rival, has almost directly led to this great distinction accruing to the observatory of which he was the head. three different methods have successively been used in the determination of longitudes of distant places. in each case the problem required was to ascertain the time at the standard place, say greenwich, at the same time that it was being determined in the ordinary way at the given station. one method of ascertaining greenwich time when at a distance from it was, as stated in chapter i., to use the moon, as it were, as the hand of a vast clock, of which the sky was the face and the stars the dial figures. this is the method of 'lunar distances,' the distances of the moon from a certain number of bright stars being given in the _nautical almanac_ for every three hours of greenwich time. as chronometers were brought to a greater point of perfection, it was found easier and better in many cases to use 'chronometer runs,' that is, to carry backwards and forwards between the two stations a number of good chronometers, and by constant comparison and re-comparison to get over the errors which might attach to any one of them. [illustration: the transit pavilion. (_from a photograph by mr. lacey._)] but of late years another method has proved available. distant nations are now woven together across thousands of miles of ocean by the submarine telegraph. the american reads in his morning paper a summary of the debates of the previous night in the house of commons at westminster. the londoner watches with interest the scores of the english cricket team in australia. it is now therefore possible for an astronomer in england to record, should he so desire, the time of the transit of a star across the wires of his instrument, not only on his own chronograph, but upon that of another observatory, it may be miles away. or, much more conveniently, each observer may independently determine the error of his own clock, and then bring his clock into the current, so that it may send a signal to the chronograph of the other station. in one way or another this work of the determination of geographical longitudes has been an important part of the extra-routine work at greenwich, part of the work which has built up and sustained its claim to define 'longitude nought'; and many distinguished astronomers, especially from the leading observatories of the continent, have come here from time to time to obtain more accurately the longitude of their own cities. the traces of their visits may be seen here and there about the observatory grounds in flat stones which lie level with the surface, and bear a name and date like the gravestones in some old country churchyard. these are not, as one might suppose, to mark the burial-places of deceased astronomers, but record the sites where, on their visits for longitude purposes, different foreign astronomers have set up their transit instruments. now, however, a permanent pier has been erected in the courtyard, and a neat house--the transit pavilion--built over it, so that in all probability no fresh additions will be made to these sepulchral-looking little monuments. it might be asked, what reason is there for a foreign observer to come over to england for such a purpose? would it not be sufficient for the clock signals to be exchanged? but a curious little fact has come out with the increase of accuracy of transit observation, and that is, that each observer has his own particular habit or method of observation. a hundred years ago, maskelyne, the fifth astronomer royal, was greatly disturbed to find that his assistant, david kinnebrook, constantly and regularly observed a star-transit a little later than he did himself. the offender was scolded, warned, exhorted, and finally, when all proved useless to bring his observations into exact agreement with the astronomer royal's, dismissed as an incompetent observer. as a matter of fact, poor kinnebrook has a right to be regarded as one of the martyrs of science, and maskelyne, by this most natural but mistaken judgment, missed the chance of making an important discovery, which was not made until some thirty years later. astronomers now would be more cautious of concluding that observations were bad simply because they differed from what had been expected. they have learnt by experience that these unexpected differences are the most likely hunting-ground in which to look for new discoveries. in a modern transit observation with the use of the chronograph it will be seen at once that before the observer can register a star-transit on the chronograph, he has to perceive with his eye that the star has reached the wire, he has to mentally recognize the fact, and consciously or unconsciously to exert the effort of will necessary to bring his finger down on the button. a very slight knowledge of character will show that this will require different periods of time for different people. it will be but a fraction of a second in any case, but there will be a distinct difference, a constant difference, between the eager, quick, impulsive man who habitually anticipates, as it were, the instant when he sees star and wire together, and the phlegmatic, slow-and-sure man who carefully waits till he is quite sure that the contact has taken place, and then deliberately and firmly records it. these differences are so truly personal to the observer that it is quite possible to correct for them, and after a given observer's habit has become known, to reduce his transit times to those of some standard observer. it must, of course, be remembered that this 'personal equation' is an exceedingly minute quantity, and in most cases is rather a question of hundredths of seconds than of tenths. it will be seen from the foregoing description how little of what may be termed the picturesque or sensational side of astronomy enters into the routine of the time department, the most important of all the departments of the observatory. the daily observation of sun and of many stars--selected from a carefully chosen list of some hundreds, and known as 'clock stars'--the determination of the error of the standard clock to the hundredth of a second if possible, and its correction twice a day, the sending out of time signals to the general post office and other places, whence they are distributed all over the country; the care, winding, and rating of hundreds of chronometers and chronometer watches, and from time to time the determination of the longitude of foreign or colonial cities, make up a heavy, ceaseless routine in which there is little opportunity for the realization of an astronomer's life as it is apt to be popularly conceived. yet there is interest enough in the work. there is the charm which always attaches to work of precision, the delight of using delicate and exact instruments, and of obtaining results of steadily increasing perfection. it may be akin to the sporting passion for record-breaking, but surely it is a noble form of it which has led the assistants, in recent years, to steadily increase the number of observations in a normal night's work up to the very limit, taking care the while that their accuracy has in no degree suffered. in longitude work also 'the better is the enemy of the good,' and there is the ambition that each fresh determination shall be markedly more precise than all that have preceded it. the constant care of chronometers soon reveals a kind of individuality in them which forms a fresh source of interest, whilst if a man has but a spark of imagination, how easily he will wrap them round with a halo of romance! glance through the ledgers, and you will see how some of them have heard the guns at the siege of alexandria, others have been carried far into the frozen north, others have wandered with livingstone or cameron in the trackless forests of equatorial africa. more striking still are those pages across which the closing line has been drawn; never again will the time-keeper there scheduled return to the kindly inquisition of flamsteed hill. this sailed away in the wasp, and was swallowed up in the eastern typhoon; that went down in the sudden squall that smote the eurydice off the isle of wight; these foundered with the captain. the last fatal journey of sir john franklin to find the north-west passage leaves its record here; the chronometers of the erebus and terror will never again appear on the greenwich muster roll. land exploration claims its victims too. sturt's ill-fated expedition across australia, and livingstone's last wandering, are represented. [illustration: 'lost in the birkenhead.'] sometimes an amusing entry interrupts the silent pathos of these closed pages. 'lost by mr. smith on the coast of africa,' reads at first sight like a rather thin attempt of some one to shift the responsibility of his own carelessness on to the broad shoulders of mr. nobody. in reality it probably gives a hint of the necessary, dangerous, and exciting work of slave-dhow chasing which gives employment to our ships on the african coast. 'mr. smith' was no doubt a petty officer who was told off to carry the chronometer for a boat's crew sent to search for a slave-dhow up some equatorial estuary. probably the dhow was found, and the arabs who manned it gave so stout a resistance that 'mr. smith' and his men had other things to do than take care of chronometers before they could overcome them. we may take it that the real story outlined here was one of courage and hard fighting, not of carelessness and shirking. stories of higher valour and nobler courage yet are also hinted: the calm discipline of the crew of the victoria as she sank from the ram of the camperdown, the yet nobler devotion of the men of the birkenhead, as they formed up in line on deck and cheered the boats that bore away the women and children to safety, whilst they themselves went down with the ship into the shark-crowded sea. 'there rose no murmur from the ranks, no thought by shameful strength, unhonoured life to seek; our post to quit we were not trained, nor taught to trample down the weak. 'what followed, why recall? the brave who died died without flinching in that bloody surf. they sleep as well beneath that purple tide as others under turf.' chapter vii the transit and circle departments the determination of time is a duty the importance of which readily commends itself to the general public. it is easy to see that in any civilized country it is very necessary to have an accurate standard of time. our railways and telegraphs make it quite impossible for us to be content with the rough-and-ready sun-dial which satisfied our forefathers. but it should be remembered that it was neither to establish a 'longitude nought,' nor to create a system of standard time, that greenwich observatory was founded in . it was for 'the rectifying the tables of the motions of the heavens and the places of the fixed stars, in order to find out the so-much-desired longitude at sea for the perfecting the art of navigation.' the two related departments, therefore, those of the transit and the circle, which are concerned in the work of making star-catalogues, come next in order to the time department. though both departments deal with the same instrument, the transit circle, they are at present placed at opposite ends of the observatory domain; the circle department being lodged in the upper computing room of the old building; the transit department in the south wing of the new observatory in the south ground. it may be asked why, if this were the purpose of the observatory at its foundation, two and a quarter centuries ago; if, as was the case, the work was set on foot from the beginning and was carried out with every possible care, how comes it that it is still the fundamental work of the observatory, and, instead of being completed, has assumed greater proportions at the present day than ever before? the answer to this inquiry may be found in a special application of the old proverb, originally directed against the discontent of man: 'the more he has, the more he wants.' for, however paradoxical it may seem, it is true that the fuller a star-catalogue is, and the more accurate the places of the stars that it contains, the greater is the need for a yet fuller catalogue, with places more accurate still. it is worth while following up this paradox in some detail, as it affords a very instructive example of the way in which a work started on purely utilitarian grounds extends itself till it crosses the undefined boundary and enters the region of pure science. we have no idea who made the earliest census of the sky. it is written for us in no book; it is not even engraved on any monument. and yet no small portion of it is in our hands to-day, and, strangest of all, we are able to fix fairly closely the time at which it was made, and the latitude in which its compiler lived. the catalogue is very unlike our star-catalogues of to-day. the places of the stars are very roughly indicated; and yet this catalogue has left a more enduring mark than all those that have succeeded it. the catalogue simply consists of the star names. an old lady who had attended a university extension lecture on astronomy was heard to exclaim: 'what wonderful men these astronomers are! i can understand how they can find out how far off the stars are, how big they are, and what they weigh--that is all easy enough; and i think i can see how they find out what they are made of. but there is one thing that i can't understand--i don't know how they can find out what are their names!' this same difficulty, though with a much deeper meaning than the old lady in her simplicity was able to grasp, has occurred to many students of astronomy. many have wished to know what was the meaning of, and whence were derived, the sonorous names which are found attached to all the brighter stars on our celestial globes: adhara, alderamin, betelgeuse, denebola, schedar, zubeneschamal, and many more. the explanation lies here. some years ago, a man, or college of men, living in latitude ° north, in order that they might better remember the stars, associated certain groups of them with certain fancied figures, and the individual star names are simply arabic words designed to indicate whereabouts in its peculiar figure or constellation that special star was situated. thus adhara means 'back,' and is the name of the bright star in the back of the great dog. alderamin means 'right arm,' and is the brightest star in the right arm of cepheus, the king. betelgeuse is 'giant's shoulder,' the giant being orion; denebola is 'lion's tail.' schedar is the star on the 'breast' of cassiopeia, and zubeneschamal is 'northern claw,' that is, of the scorpion. so far is clear enough. the names of the stars for the most part explain themselves; but whence the constellations derived their names, how it was that so many snakes and fishes and centaurs were pictured out in the sky, is a much more difficult problem, and one which does not concern us here. one point, however, these old constellations do tell us, and tell us plainly. they show that the axis of the earth, which, as the earth travels round the sun, moves parallel with itself, yet, in the course of ages, itself rotates so as in a period of some , years to trace out a circle amongst the stars. this is the cause of what is called 'precession,' and explains how it is that the star we call the pole-star to-day was not always the pole-star, nor will always remain so. we learn this fact from the circumstance that the old constellations do not cover the entire celestial sphere. they leave a great circular space of ° radius unmapped in the southern heavens. this simply means that the originators of the constellations lived in ° north latitude, and stars within ° of their south pole never rose above their horizon, and consequently were never seen, and could not be mapped, by them. in like manner, the star census taken at greenwich observatory does not include the whole sky, but leaves a space some ° in radius round our south pole. since the latitude of greenwich is nearly ° north, stars within that distance of the south pole do not rise above our horizon, and are never seen here. but if we compare the vacant space left by the old original constellations with the vacant space left by a greenwich catalogue of to-day, we see that the centre of the first space, which must have been the south pole of that time, is a long way from the centre of the second space--our south pole of to-day. the difference tells us how far the pole has moved since those old forgotten astronomers did their work. we know the rate at which the pole appears to move, by comparing our more modern catalogues one with another; and so we are able to fix pretty nearly the time when lived those old first census-takers of the stars, whose names have perished so completely, but whose work has proved so immortal. these old workers gave us the constellation groupings and names which still remain to us, and are still in common, every-day use. their work affords us the most striking illustration of the result of precession, but precession itself was not recognized till nearly years after their day, when a marvellous genius, hipparchus, established the fact, and 'built himself an everlasting name' by the creation of a catalogue of over stars prepared on modern principles. that catalogue formed the basis of one which survives to us at the present time, and was made some years ago by claudius ptolemy, the great astronomer of alexandria, whose work, which still bears the proud name of _almagest_, 'the greatest,' remained for fourteen centuries the one universal astronomical text-book. a modern catalogue contains, like that of ptolemy, four columns of entry. the first gives the star's designation; the second an indication of its brightness; the third and fourth the determinations of its place. these are expressed in two directions, which, in modern catalogues, not in ptolemy's, correspond on the celestial sphere to longitude and latitude on the terrestrial. distance north or south of the celestial equator is termed 'declination,' corresponding to terrestrial latitude. distance in a direction parallel to the equator is termed 'right ascension,' corresponding to terrestrial longitude. for geographical purposes we conceive the earth to be encircled by two imaginary lines at right angles to each other--the one, the equator, marked out for us by the earth itself; the other, 'longitude nought,' the meridian of greenwich, fixed for us by general consent, after the lapse of centuries, by a kind of historical evolution. on the celestial globe in like manner we have two fundamental lines--one, the celestial equator, marked out for us by nature; the other at right angles to it, and passing through the poles of the sky, adopted as a matter of convenience. but a difficulty at once confronts us--where can we fix our 'right ascension nought'? what star has the right to be considered the greenwich of the sky? the difficulty is met in the following manner: for six months of the year, the summer months, the sun is north of the celestial equator; for the other six months of the year, the months of winter, it is south of it. it crosses the equator, therefore, twice in the year--once when moving northward at the spring equinox; once when moving southward at the equinox of autumn. the point where it crosses the equator at the first of these times is taken as the fundamental point of the heavens, and the first sign of the zodiac, aries the ram, is said to begin here, and it is called, therefore, 'the first point of aries.' one of the very first facts noticed in the very early days of astronomy was that, as the stars seemed to move across the sky night by night, they seemed to move in one solid piece, as if they were lamps rigidly fixed in one and the same solid vault. of course it has long been perfectly understood that this apparent movement was not in the least due to any motion of the stars, but simply to the rotation of the earth on its axis. this rotation is the smoothest, most constant, and regular movement of which we know. it follows, therefore, that the interval of time between the passage of one star across the meridian of greenwich and that of any other given star is always the same. this interval of time is simply the difference of their right ascension. if we are able, then, to turn our transit instrument to the sun, and to a number of stars, each in its proper turn, and by pressing the tapping-piece on the instrument as the sun or star comes up to each of the ten wires in succession, to record the times of its transit on the chronograph, we shall have practically determined their right ascensions--one of the two elements of their places. the other element, that of declination, is found in a different manner. the celestial equator, like the terrestrial, is ° from the pole. the bright star polaris is not exactly at the north pole, but describes a small circle round it. twice in the twenty-four hours it transits across the meridian--once when going from east to west it passes above the pole, once when going from west to east below the pole. the mean between these two altitudes of polaris above the horizon gives the position of the true pole. [illustration: the transit circle.] a complete transit observation of a star consists therefore of two operations. the observer, as we have already described, sees a star entering the field of the telescope, and as it swims forward, he presses the galvanic button, which sends a signal to the chronograph as the star comes up to each of the ten vertical wires in succession. but, beside the ten wires, there are others. two vertical wires lie outside the ten of which we have already spoken, and there is also a horizontal wire. the latter can be moved by a graduated screw-head just above the eye-piece, and as the star comes in succession to these two vertical wires, this horizontal wire is moved by the screw-head, so as to meet the star at the moment it is crossing the vertical wire, and the observer presses a second little button, which records the position of the horizontal wire on a small paper-covered drum. then, the transit over, the observer leaves the telescope and comes round to the outside of the west pier. here he finds seven large microscopes, which pierce the whole thickness of the pier, and are directed towards the circumference of a large wheel which is rigidly attached to the telescope and revolves with it. this wheel is six feet in diameter, and has a silver circle upon both faces. each circle is divided extremely carefully into divisions--these divisions, therefore, being about the one-twentieth of an inch apart. there are, therefore, twelve divisions to every degree ( × = ), and each division equals five minutes of arc. the lowest microscope is the least powerful, and shows a large part of the circle, enabling the observer to see at once to what degree and division of a degree the microscope is pointing. the other six microscopes are very carefully placed ° apart--as equally placed as they possibly can be. these microscopes are all fitted with movable wires--wires moved by a very fine and delicate screw; the screw-head having divisions upon it so that the exact amount of its movement can be told. each of the six screw-heads will read to the one five-thousandth part of a division of the circle; in other words, to the one hundred thousandth part of an inch. using all six microscopes, and taking their mean, we are able to _read_ to the one-hundredth of a second of arc. if, therefore, the observations could be made with perfect certainty down to the extremest nicety of reading which the instrument supplies, we should be able to read the declination of a star to this degree of refinement. it may be added that a halfpenny, at a distance of three miles, appears to be one second of arc in diameter; at three hundred miles it would be one-hundredth of a second. it need scarcely be said that we cannot observe with quite such refinement of exactness as this would indicate. nevertheless, this exactness is one after which the observer is constantly striving, and tenths, even hundredths, of seconds of arc are quantities which the astronomer cannot now neglect. the observer has then to read the heads of all these seven microscopes on the pier side, and also two positions of the horizontal wire on the screw-head at the eye-piece. the following morning he will also read off from the chronograph-sheet the times when he made the ten taps as the star passed each of the ten vertical wires. there are, therefore, nine entries to make for one position of a star in declination, and ten for one position of a star in right ascension. the observer will also have to read the barometer to get the pressure of the air at the time of observation, and one thermometer inside the transit room, and another outside, to get the temperature of the air. in some cases thermometers at different heights in the room are also read. a complete observation of a single star means, therefore, the entry of two-and-twenty different numbers. it may be asked, what is the use of reading the barometer and thermometer? the answer to the question can only be given by contradicting a statement made above, that the true pole lay midway between the position of the telescope when pointing to the pole-star at its upper transit, and its position when pointing to it at its lower transit. the pole being very high in the heavens in this country, there are a great number of stars that, like the pole-star, cross the meridian twice in the twenty-four hours--once when they pass above the pole, moving from east to west, once when they pass below it, moving from west to east as the real distance of a star from the true pole does not alter, it follows that we ought to get the position of the pole from the mean of the two transits of any of these stars, and they ought all to exactly agree with each other. but they do not. so, too, i said that the stars all appeared to move as in a single piece. if, then, we constructed an instrument with its axis parallel to the axis of the earth, and fixed a telescope to it, pointing to any particular star, if we turn the telescope round as fast from east to west as the earth itself is turning from west to east--if we built an equatorial, that is to say--we ought to find that the star once in the centre of the field would remain there. as a matter of fact, when the star got near the horizon it would soon be a long way from the centre of the field. sir george airy, the seventh astronomer royal, makes, with reference to this very point, the following remarks: 'perhaps you may be surprised to hear me say the rule is established as true, and yet there is a departure from it. this is the way we go on in science, as in everything else; we have to make out that something is true, then we find out under certain circumstances that it is not quite true; and then we have to consider and find out how the departure can be explained.' in this particular case, the disturbing cause is found in the action of our own atmosphere. the rays of light from the star are bent out of a perfectly straight course as they pass through the various layers of that atmosphere, layers which necessarily become denser the closer we get to the actual surface of the earth. every celestial body therefore appears to be a little higher in the sky than it really is. this action is most noticeable at the horizon, where it amounts to about half a degree. as both sun and moon are about half a degree in diameter, it follows that when they have really just entirely sunk below the horizon they appear to be just entirely above it. it happens, in consequence, on rare occasions, that an eclipse of the moon will take place when both sun and moon are together seen above the horizon. it was a great matter to discover this effect of refraction. it was soon seen that it was not constant, that it varied with both temperature and pressure. it is, indeed, the most troublesome of all the hindrances to exact observation with which the astronomer has to contend; partly because of its large amount--half a degree, as has been already said, in the extreme case--and partly because it is difficult in many cases to determine its exact effect. the double observation with the transit circle gives us, then, the place in the sky where the star _appeared_ to be at the moment of observation, not its true place; to find that true place we have to calculate how much refraction had displaced the star at the particular height in the sky, and at the particular temperature and atmospheric pressure at which the observation was made. [illustration: the mural circle.] the transit circle is a comparatively recent instrument. in earlier times the two observations of right ascension and declination were entrusted to perfectly separate instruments. the transit instrument was mounted as the transit circle is, between two solid stone piers, and moved in precisely the same way. but the great six-foot wheel, which was made as stiff as it possibly could be, was mounted on the face of a great stone pier or wall, from which circumstance it was called the 'mural circle,' and a light telescope was attached to it which turned about its centre. this arrangement had a double disadvantage--that the two observations had to be made separately, and the mural circle, not being a symmetrical instrument, was liable to small errors which it was difficult to detect. thus, being supported on one side only, a flexure or bending outwards of either telescope or circle, or both, might be feared. it was for this reason that pond set up a pair of mural circles, one on the east side of its supporting pier and the other on the west.[ ] his plan was not only to have each star observed simultaneously in the two instruments, a plan by which, at the cost of some additional labour, he would have got rid, to a large extent, of the individual errors of the two separate instruments, inasmuch as, on the whole, it might have been expected that the errors of the two instruments would have been very nearly equal in amount, but of opposite character. the differences, too, between the two instruments would have afforded the means for tracing these small errors to their respective causes, and so ascertaining the laws to which they were subject. [ ] the second circle was intended for the cape observatory, but pond obtained leave to retain it. in it was transferred to the observatory of queen's college, belfast. pond went further still. he added to the mural circle a simple instrument, the extreme value of which every astronomer recognizes to-day--the mercury trough. not only was the star to be observed by both circles when the two telescopes were pointing directly to it, it was also to be observed by reflection; the telescopes were to be pointed down towards a basin of mercury, in which the image of the star would be seen reflected. the mercury being a liquid, its surface is perfectly horizontal; and, since the law of reflection is that the angle of incidence is equal to the angle of reflection, it follows that the telescope, when pointed down toward the mercury trough, points just at as great an angle below the horizon as, when it is set directly on the star, it points above it. if the circle, therefore, be carefully read at both settings, half the difference between the two readings will give the angular elevation of the star above the horizon. a combination, therefore, of all four observations, that is to say, one reflection and one direct with each of the telescopes, would give an exceedingly exact value for the star's altitude. the conception of this method gives a striking idea of pond's thoroughness and skill as a practical observer, and it is a distinct blot upon airy's justly high reputation in the same line that he discontinued the system upon his accession to office. however, in , as already mentioned, airy substituted for the two separate instruments--the transit and mural circle--the transit circle, which, unlike the mural circle, is equally supported on both sides. this, however, does not free it from the liability to some minute flexure in the direction of its length, from the weight of its two ends, and the mercury trough is used for the detection of such bending, should it exist. the present practice is to observe a star both by reflection and directly in the course of the same transit. the observer sets the telescope carefully before ever the star comes into the field of view, and reads his seven microscopes. then he climbs up a narrow wooden staircase and watches the star transit nearly half across the field. then comes a rush, the observer swings himself down the ladder, unclamps the telescope, turns it rapidly up to the star itself, clamps it again, flings himself on his back on a bench below the telescope, and does it so quickly that he is able to observe the star across the second half of the field. there is no time for dawdling, no room for making any mistakes; the stars never forgive; 'they haste not, they rest not;' and if the unfortunate observer is too slow, or makes some slip in his second setting, the star, cold and inexorable, takes no pity, and moves regardless on. it will be seen that a considerable amount of work is involved in taking a single observation of a star-place. but in making a star-catalogue it is always deemed necessary to obtain at least three observations of each star; and many are observed much more frequently. a modern star-catalogue contains, like ptolemy's, four columns. it contains also several more. of these the principal are devoted to the effect of precession. as precession is caused by a movement of the earth's axis making the pole of the sky seem to describe a circle in the heavens, it follows that the celestial poles, and the celestial equator with them are slowly, but continually, changing their place with respect to the stars, and therefore that the declinations of the stars are always undergoing change, and as the equator changes, the point where the sun crosses it in spring--the first point of aries--changes also, and with it the stars' right ascensions. to make one determination of a star's place comparable with another made at another time, it is clear that we must correct for the effects of precession in the interval of time between the two observations, and for the effects of refraction. but observations made with the transit circle must also be corrected for errors in the instrument itself. the astronomer will see to it that his instrument is made and is set up as perfectly as possible. the pivots on which it turns must be exactly on the same level; they must point exactly east and west, and the axis of the telescope must be exactly at right angles to the line joining the pivots in all positions of the instrument. these conditions are very nearly fulfilled, but never absolutely. day by day, therefore, the astronomer has to ascertain just how much his instrument is in error in each of these three matters. were his instrument absolutely without error to-day, he could not assume that it would remain so, nor, if he had measured the amount of its errors yesterday, would it be safe to assume that those errors would not change to-day. in the examination of these sources of error the mercury trough comes again into use. the transit circle is turned directly downwards, and the mercury trough brought below it. a light is so arranged as to illuminate the field of the telescope, and the observer, looking in, sees the ten transit wires and the one declination wire, and also sees their images reflected from the surface of the mercury. if the telescope be pointing _exactly_ down towards the surface of the mercury, then the image of the declination wire will fall exactly on the declination wire itself, and by reading the circle we can tell where the zenith point of the circle is. similarly, if the pivots of the telescope are precisely on the same level, the centre wire of the right ascension series would coincide with its reflected image. a third point is determined by looking through the eye-piece of the north collimator telescope--that is to say, the telescope mounted in a horizontal position at the north end of the room--at the spider lines in the focus of the south collimator. in order to get this view, the transit telescope has either to be lifted up out of its usual position, or else the middle of the tube has to be opened. the spider lines in the north collimator are then made to coincide with the image of the wires of the south collimator. the transit telescope is then turned first to one collimator, then to the other, and the central wire of the right ascension series is turned till it coincides with the wire of the collimator; the amount by which it has to be moved giving an index of the error of collimation; that is to say, of the deviation of the optical axis of the telescope from perpendicularity to the line joining the pivots. i have said enough to show that the making of an observation is a small matter as compared with those corrections which have to be made to it afterwards, before it is available for use. but i have only mentioned some of the reductions and corrections which have to be made. there are several more, and it is a just pride of greenwich that her third ruler, bradley, as has been already told in the notice of his life, discovered two of the most important. the one, aberration, is due to the fact that light, though it moves so swiftly-- , miles per second--yet does not move with an infinitely greater velocity than that of the earth. the other, nutation, might be called a correction to precession, inasmuch as, moved by the moon's attraction, the earth's axis does not swing round smoothly, but with a slight nodding or staggering motion. but when these observations of the places of a star have been made, and have been properly 'reduced,' even then we do not find an exact correspondence between two different determinations. little differences still remain. some of these are to be accounted for by changes in the actual crust of the earth, which, solid and stable as we think it, is yet always in motion. professor milne, our greatest authority on earth movements, says, 'the earth is so elastic that a comparatively small impetus will set it vibrating; why, even two hills tip together when there is a heavy load of moisture in the valley between them. and then, when the moisture evaporates in a hot sun, they tip away from each other.' so there is a perceptible rocking to and fro even of the huge stone piers of a transit circle, as seasons of rain and drought, heat and cold, follow each other. more than that, the earth is so sensitive to pressure that it was found, at the oxford university observatory, that there was a distinct swaying shown by a horizontal pendulum when the whole of a party of seventy-six undergraduates stood on one side of the instrument and close up to it, from the position it had when the party stood ninety feet away. more wonderful still, a comparison of the star-places, obtained at a number of observatories, including greenwich, has shown that the earth is continually changing her axis of rotation. and so the star-places determined at greenwich have shown that the north pole of the earth, miles away, moves about in an irregular curve about thirty feet in radius. nothing is stable, nothing is immovable, nothing is constant. the astronomer even finds that his own presence near the instrument is sufficient to disturb it. the great interest attaching to transit-circle work is this striving after ever greater and greater precision, with the result of bringing out fresh little discordances, which, at first sight, appear purely accidental, but which, under further scrutiny, show themselves to be subject to some law. then comes the hunt for this new unknown law. its discovery follows. it explains much, but when it is allowed for, though the observations now come much closer together, little deviations still remain, to form the subject of a fresh inquiry. astronomy has well been called the exact science, and yet exactitude ever eludes its pursuer. if it be asked, 'what is the use of this ever-increasing refinement of observation?' no better answer can be given than the words of sir john herschel in one of his presidential addresses to the royal astronomical society:-- 'if we ask to what end magnificent establishments are maintained by states and sovereigns, furnished with masterpieces of art, and placed under the direction of men of first-rate talent and high-minded enthusiasm, sought out for those qualities among the foremost in the ranks of science, if we demand, _cui bono?_ for what good a bradley has toiled, or a maskelyne or a piazzi has worn out his venerable age in watching?--the answer is, not to settle mere speculative points in the doctrine of the universe; not to cater for the pride of man by refined inquiries into the remoter mysteries of nature; not to trace the path of our system through space, or its history through past and future eternities. these, indeed, are noble ends, and which i am far from any thought of depreciating; the mind swells in their contemplation, and attains in their pursuit an expansion and a hardihood which fit it for the boldest enterprise. but the direct practical utility of such labours is fully worthy of their speculative grandeur. the stars are the landmarks of the universe; and, amidst the endless and complicated fluctuations of our system, seem placed by its creator as guides and records, not merely to elevate our minds by the contemplation of what is vast, but to teach us to direct our actions by reference to what is immutable in his works. it is, indeed, hardly possible to over-appreciate their value in this point of view. every well-determined star, from the moment its place is registered, becomes to the astronomer, the geographer, the navigator, the surveyor, a point of departure which can never deceive or fail him, the same for ever and in all places; of a delicacy so extreme as to be a test for every instrument yet invented by man, yet equally adapted for the most ordinary purposes; as available for regulating a town clock as for conducting a navy to the indies; as effective for mapping down the intricacies of a petty barony as for adjusting the boundaries of transatlantic empires. when once its place has been thoroughly ascertained and carefully recorded, the brazen circle with which that useful work was done may moulder, the marble pillar may totter on its base, and the astronomer himself survive only in the gratitude of posterity; but the record remains, and transfuses all its own exactness into every determination which takes it for a groundwork, giving to inferior instruments--nay, even to temporary contrivances, and to the observations of a few weeks or days--all the precision attained originally at the cost of so much time, labour, and expense.' but for these strictly utilitarian purposes a comparatively small number of stars would meet our every requisite. in the narrow sense of which sir john herschel is here speaking, we have no use for anything beyond the smallest of catalogues; and if the question before us is, 'why are we continually extending our catalogues?' the following words of a more recent writer[ ] on the subject will set forth the real explanation:-- 'a word in conclusion, suggested by the history of star-catalogues. we have no difficulty in understanding that it is necessary to study the planets, and a reasonable number of the brighter stars, for the purpose of determining the figure of the earth, and the positions of points upon its surface; but the use for a catalogue of ten thousand stars, such as la caille compiled, is not just so apparent. nay, what did ptolemy want with a thousand stars, or tamerlane's grandson, born, reared, and destined to die amidst a horde of savages, however splendid in their trappings? there is not, and there never was, any real, practical use for the great volumes of star-catalogues that weigh down the shelves of our libraries. the navigator and explorer need never see them at all. why, then, were these pages compiled? why have astronomers, from hipparchus's time to the present, spent their lives in the weary routine-work of observing the places of tiny points in the stellar depths? does it not seem that there is something in the mind of man that impels him to seek after knowledge--truly--for its own sake? something heaven-born, heaven-nurtured, god-given ... that there is something in man common to him and his creator, and therefore eternal ... in beautiful accord with the plain statement that "god made man in his own image?"' [ ] mr. thomas lindsay, _transactions of the astronomical and physical society of toronto_, , p. . chapter viii the altazimuth department the determining of the places of the fixed stars which flamsteed carried out so efficiently in his _british catalogue of stars_--the first 'census of the sky' made with the aid of a telescope--was but half of the work imposed upon him. the other half, equally necessary for the solution of the problem of the longitude at sea, was the 'rectifying the tables of the motions of the heavens.' this second duty was not less necessary than the other, for, if we may again use the old simile of the clock-face, the fixed stars may be taken to represent the figures on the vast dial of the sky, whilst the moon, as it moves amongst them, corresponds to the moving hand of the timepiece. to know the places of the stars, then, without being able to predict the place of the moon, would be much like having a clock without its hands. but if not less necessary, it was certainly more difficult. the secret of the movements of the moon and planets had not then been grasped, and the only tables which had been calculated were based upon observations made before the days of telescopes. it is one of the most fortunate and remarkable coincidences in the whole history of science, that at the very time that greenwich observatory was being called into existence, the greatest of all astronomers was working out his demonstration of the great fundamental law of the material universe--the law that every particle of matter attracts every other particle with a force which varies directly with the mass and inversely with the square of the distance. several other of the great minds of that time, in particular dr. hooke, the gresham professor of astronomy, had seen that it was possible that some such law might supply the secret of planetary motion; but it is one thing to make a suggestion, and a very different matter indeed to be able to demonstrate it; and the latter was in newton's power alone. he did much more than demonstrate it--he brought out a whole series of most important and far-reaching consequences. he showed that the ebb and flow of the tides was due to the attraction of both sun and moon, especially the latter, upon the waters of our oceans. he pointed out certain irregularities which must take place in the motion of our moon, due to the influence of the sun upon it. he showed, too, what was the cause of that swinging of the axis of the earth which gives rise to precession. he deduced the relative weights of the earth, the sun, and of jupiter and saturn, the planets with satellites. he proved also that comets, which had seemed hitherto to men as perfectly lawless wanderers, obeyed in their orbits the self-same law which governed the moon and planets. the whole vast system of celestial movements, which had long seemed to men irregular and uncontrolled, now fell, every one of them, into its place, as but the necessary manifestations of one grand, simple order. this great discovery gave a new and additional importance to the regular observation of the moon and planets. they were needed now, not only to assist in the practical work of navigation, but for the development of questions of pure science. halley, the second astronomer royal, and maskelyne, the fifth, devoted themselves chiefly to this department of work, to the partial neglect of the observation of the places of stars. airy, the seventh, whilst making catalogue-work a part of the regular routine of the observatory, developed the observation of the members of the solar system, and especially of the moon, in a most marked degree, and collected and completely reduced the vast mass of material which the industry of his predecessors had gathered. it is pre-eminently of the work of airy that the memorable words quoted before of professor newcomb, the great american mathematician and astronomer, are applicable: 'that if this branch of astronomy were entirely lost, it could be reconstructed from the greenwich observations alone.' a most important step taken by airy was the construction of an altazimuth. an altazimuth is practically a theodolite on a large scale. its purpose is to determine, not the declination and right ascension of some celestial body, as is the case with the transit circle, but its altitude, _i.e._ its height above the horizon, and its azimuth, _i.e._ the angle measured on the horizontal plane from the north point. the altazimuth, then, like the transit circle, consists of a telescope revolving on a horizontal axis, but, unlike the transit circle, both the telescope and the piers which carry its pivots can be rotated so as to point not merely due north and south, but in any direction whatsoever. [illustration: airy's altazimuth.] the observations with the altazimuth are rather more complicated than those with the transit circle. looking in the telescope, the observer sees a double set of spider threads or 'wires'; and when a star or other heavenly body enters the field, it will generally be observed to move obliquely across both sets of wires. the observer usually determines to make an observation either in altitude or azimuth. in the former case he presses the little contact button, which, as in the transit circle, is provided close to the eyepiece, as the star reaches each of the horizontal wires in succession. if in azimuth, it is the times of crossing the vertical wires that are in like manner telegraphed to the chronograph. the transit over, the appropriate circle is read; for the telescope itself is rigidly attached to a vertical wheel having a carefully engraved circle on its face and read by four microscopes, whilst the entire instrument carries another set of microscopes, pointing to a fixed horizontal circle, and upon which the azimuth can be read. a complete observation involves four such transits and sets of circle readings, two of altitude, and two of azimuth; for after one of altitude and one of azimuth the telescope is turned round, and a second observation is taken in each element. the observation gives us the altitude and azimuth of the star. these particulars are of no direct value to us. but it is a mere matter of computation, though a long and laborious one, to convert these elements into right ascension and declination. the usefulness of the altazimuth will be seen at once. it will be remembered that with the transit circle any particular object can only be observed as it crosses the meridian. if the weather should be cloudy, or the observer late, the chance of observation is lost for four and twenty hours, and in the case of the moon, for which the altazimuth is specially used, it is on the meridian only in broad daylight during that part of the month which immediately precedes and follows new moon. at such times it is practically impossible to observe it with the transit circle; with the altazimuth it may be caught in the twilight before sunrise or after sunset; and at other times in the month, if lost on the meridian in the transit circle, the altazimuth still gives the observer a chance of catching it any time before it sets. but for this instrument, our observations of the moon would have been practically impossible over at least one-fourth of its orbit. airy's altazimuth was but a small instrument of three and three-quarter inches aperture, mounted in a high tower built on the site of flamsteed's mural arc; and, after a life history of about half a century, has been succeeded by a far more powerful instrument. the 'new altazimuth' has an aperture of eight inches, and is housed in a very solidly constructed building of striking appearance, the connection of the observatory with navigation being suggested by a row of circular lights which strongly recall a ship's portholes. this building is at the southern end of the narrow passage, 'the wasp's waist,' which connects the older observatory domain with the newer. it is the first building we come to in the south ground. the computations of the department are carried on in the south wing of the new observatory. it will be seen from the photograph that the instrument is much larger, heavier, and less easy to move in azimuth than the old altazimuth. it is, therefore, not often moved in azimuth, but is set in some particular direction, not necessarily north and south, in which it is used practically as a transit circle. [illustration: new altazimuth building.] there is quite another way of determining the place of the moon, which is sometimes available, and which offers one of the prettiest of observations to the astronomer. as the moon travels across the sky, moving amongst the stars from west to east, it necessarily passes in front of some of them, and hides them from us for a time. such a passage, or 'occultation,' offers two observations: the 'disappearance,' as the moon comes up to the star and covers it; the 'reappearance,' as it leaves it again, and so discloses it. [illustration: the new altazimuth. (_from a photograph by mr. lacey._)] except at the exact time of full moon, we do not see the entire face of our satellite; one edge or 'limb' is in darkness. as the moon therefore passes over the star, either the limb at which the star disappears, or that at which it reappears, is invisible to us. to watch an occultation at the bright limb is pretty; the moon, with its shining craters and black hollows, its mountain ranges in bright relief, like a model in frosted silver, slowly, surely, inevitably comes nearer and nearer to the little brilliant which it is going to eclipse. the movement is most regular, most smooth, yet not rapid. the observer glances at his clock, and marks the minute as the two heavenly bodies come closer and closer to each other. then he counts the clock beats: 'five, six, seven,' it may be, as the star has been all but reached by the advancing moon. 'eight,' it is still clear; ere the beat of the clock rings to the 'nine,' perhaps the little diamond point has been touched by the wide arch of the moon's limb, and has gone! less easy to exactly time is a reappearance at the bright limb. in this case the observer must ascertain from the _nautical almanac_ precisely where the star will reappear; then a little before the predicted time he takes his place at the telescope, watches intently the moon's circumference at the point indicated, and, listening for the clock-beats, counts the seconds as they fly. suddenly, without warning, a pin-point of light flashes out at the edge of the moon, and at once draws away from it. the star has 'reappeared.' far more striking is a disappearance or reappearance at the 'dark limb.' in this case the limb of the moon is absolutely invisible, and it may be that no part of the moon is visible in the field of the telescope. in this case the observer sees a star shining brightly and alone in the middle of the field of his telescope. he takes the time from his faithful clock, counting beat after beat, when suddenly the star is gone! so sudden is the disappearance that the novice feels almost as astonished as if he had received a slap in the face, and not unfrequently he loses all count or recollection of the clock beats. the reappearance at the dark limb is quite as startling; with a bright star it is almost as if a shell had burst in his very face, and it would require no very great imagination to make him think that he had heard the explosion. one moment nothing was visible; now a great star is shining down serenely on the watcher. a little practice soon enables the observer to accustom himself to these effects, and an old hand finds no more difficulty in observing an occultation of any kind than in taking a transit. such an observation is useful for more purposes than one. if the position of the star occulted is known--and it can be determined at leisure afterwards--we necessarily know where the limb of the moon was at the time of the observation. then the time which the moon took to pass over the star enables us to calculate the diameter of our satellite; the different positions of the moon relative to the star, as seen from different observatories, enable us to calculate its distance. but if the disappearance takes place at the bright limb, the reappearance usually takes place at the dark, and _vice versâ_; and the two observations are not quite comparable. there is one occasion, however, when both observations are made under similar circumstances, namely, at the full. and if the moon happens also to be totally eclipsed, the occultations of quite faint stars can be successfully observed, much fainter than can ordinarily be seen close up to the moon. total eclipses of the moon, therefore, have recently come to be looked upon as important events for the astronomer, and observatories the world over usually co-operate in watching them. october , , was the first occasion when such an organised observation was made; there have been several since, and on these nights every available telescope and observer at greenwich is called into action. it may be asked why these different modes of observing the moon are still kept up, year in and year out. 'do we not know the moon's orbit sufficiently well, especially since the discovery of gravitation?' no; we do not. this simple and beautiful law--simple enough in itself, gives rise to the most amazing complexity of calculation. if the earth and moon were the only two bodies in the universe, the problem would be a simple one. but the earth, sun, and moon are members of a triple system, each of which is always acting on both of the others. more, the planets, too, have an appreciable influence, and the net result is a problem so intricate that our very greatest mathematicians have not thoroughly worked it out. our calculations of the moon's motions need, therefore, to be continually compared with observation, need even to be continually corrected by it. there is a further reason for this continual observation, not only in the case of the sun, which is our great standard star, since from it we derive the right ascensions of the stars, and it is also our great timekeeper; not only in that of the moon, but also in the case of the planets. their places as computed need continually to be compared with their places as observed, and the discordances, if any, inquired into. the great triumph which resulted to science from following this course--to pure science, since uranus is too faint a planet to be any help to the sailor in navigation--is well known. the observed movements of uranus proved not to be in accord with computation, and from the discordances between calculation and observation adams and leverrier were able to predicate the existence of a hitherto unseen planet beyond-- 'to see it, as columbus saw america from spain. its movements were felt by them trembling along the far-reaching line of their analysis, with a certainty hardly inferior to that of ocular demonstration.'[ ] [ ] from sir john herschel's address to the british association, september , , thirteen days before galle's first observation of the planet. the discovery of neptune was not made at greenwich, and airy has been often and bitterly attacked because he did not start on the search for the predicted planet the moment adams addressed his first communication to him, and so allowed the french astronomer to engross so much of the honour of the exploit. the controversy has been argued over and over again, and we may be content to leave it alone here. there is one point, however, which is hardly ever mentioned, which must have had much effect in determining airy's conduct. in , the year in which adams sent his provisional elements of the unseen disturbing planet to airy, the largest telescope available for the search at greenwich was an equatorial of only six and three-quarter inches aperture, provided with small and insufficient circles for determining positions, and housed in a very small and inconvenient dome; whilst at cambridge, within a mile or so of adams' own college, was the 'northumberland' equatorial, of nearly twelve inches aperture, under the charge of the university professor of astronomy, professor challis, and which was then much the largest, best mounted and housed equatorial in the entire country. the 'northumberland' had been begun from airy's designs and under his own superintendence, when he was professor of astronomy at cambridge. naturally, then, knowing how much superior the cambridge telescope was to any which he had under his care, he thought the search should be made with it. he had no reason to believe that his own instrument was competent for the work. [illustration: the new observatory as seen from flamsteed's observatory.] on the other hand, it is hard for the ordinary man to understand how it was that adams not only left unnoticed and unanswered for three-quarters of a year, an inquiry of airy's with respect to his calculations, but also never took the trouble to visit challis, whom he knew well, and who was so near at hand, to stir him up to the search. but, in truth, the whole interest of the matter for adams rested in the mathematical problem. the irregularities in the motion of uranus were interesting to him simply for the splendid opportunity which they gave him for their analysis. a purely imaginary case would have served his purpose nearly as well. the actuality of the planet which he predicted was of very little moment; the _éclat_ and popular reputation of the discovery was less than nothing; the problem itself and the mental exercise in its solution, were what he prized. but it was not creditable to the nation that the royal observatory should have been so ill-provided with powerful telescopes; and a few years later airy obtained the sanction of the government for the erection of an equatorial larger than the 'northumberland,' but on the same general plan and in a much more ample dome. this was for thirty-four years the 'great' or 'south-east' equatorial, and the mounting still remains and bears the old name, though the original telescope has been removed elsewhere. the object-glass had an aperture of twelve and three-quarter inches and a focal length of eighteen feet, and was made by merz of munich, the engineering work by ransomes and sims of ipswich, and the graduations and general optical work by simms, now of charlton, kent. the mounting was so massive and stable that the present astronomer royal has found it quite practicable and safe to place upon it a telescope (with its counterpoises) of many times the weight, one made by sir howard grubb, of dublin, of twenty-eight inches aperture and twenty-eight feet focal length, the largest refractor in the british empire, though surpassed by several american and continental instruments. the stability of the mounting was intended to render the telescope suitable for a special work. this was the observation of 'minor planets.' on the first day of the present century the first of these little bodies was discovered by piazzi at palermo. three more were discovered at no great interval afterwards, and then there was an interval of thirty-eight years without any addition to their number. but from december , , up to the present time, the work of picking up fresh individuals of these 'pocket planets' has gone on without interruption, until now more than are known. most of these are of no interest to us, but a few come sufficiently near to the earth for their distance to be very accurately determined; and when the distance of one member of the solar system is determined, those of all the others can be calculated from the relations which the law of gravitation reveals to us. it is a matter of importance, therefore, to continue the work of discovery, since we may at any time come across an interesting or useful member of the family; and that we may be able to distinguish between minor planets already discovered and new ones, their orbits must be determined as they are discovered, and some sort of watch kept on their movements. a striking example of the scientific prizes which we may light upon in the process of the rather dreary and most laborious work which the minor planets cause, has been recently supplied by the discovery of eros. on august , , herr witt, of the urania observatory, berlin, discovered a very small planet that was moving much faster in the sky than is common with these small bodies. the great majority are very much farther from the sun than the planet mars, many of them twice as far, and hence, since the time of a planet's revolution round the sun increases, in accordance with kepler's law, more rapidly than does its distance, it follows that they move much more slowly than mars. but this new object was moving at almost the same speed as mars; it must, therefore, be most unusually near to us. further observations soon proved that this was the case, and eros, as the little stranger has been called, comes nearer to us than any other body of which we are aware except the moon. venus when in transit is - / millions of miles from us, mars at its nearest is - / millions, eros at its nearest approach is but little over millions. the use of such a body to us is, of course, quite apart from any purpose of navigation, except very indirectly. but it promises to be of the greatest value in the solution of a question in which astronomers must always feel an interest, the determination of the distance of the earth from the sun. we know the _relative_ distances of the different planets, and, consequently if we could determine the absolute distance of any one, we should know the distances of all. as it is practically impossible to measure our distance from the sun directly, several attempts have been made to determine the distances of venus, mars, or such of the minor planets as come the nearest to us. three of these in particular, the little planets iris, victoria, and sappho, have given us the most accurate determinations of the sun's distance ( , , miles) which we have yet obtained; but eros at its nearest approach will be six times as near to us as either of the three mentioned above, and therefore should give us a value with only one-sixth of the uncertainty attaching to that just mentioned. the discovery of minor planets has lain outside the scope of greenwich work, but their observation has formed an integral part of it. the general public is apt to lay stress rather on the first than on the second, and to think it rather a reproach to greenwich that it has taken no part in such explorations. experience has, however, shown that they may be safely left to amateur activity, whilst the monotonous drudgery of the observation of minor planets can only be properly carried out in a permanent institution. the observation of these minute bodies with the transit circle and altazimuth is attended with some difficulties; but precise observations of various objects may be made with an equatorial; indeed, comets are usually observed by its means. the most ordinary way of observing a comet with an equatorial is as follows: two bars are placed in the eye-piece of the telescope, at right angles to each other, and at an angle of forty-five degrees to the direction of the apparent daily motion of the stars. the telescope is turned to the neighbourhood of the comet, and moved about until it is detected. the telescope is then put a little in front of the comet, and very firmly fixed. the observer soon sees the comet entering his field, and by pressing the contact button he telegraphs to the chronograph the time when the comet is exactly bisected by each of the bars successively. he then waits until a bright star, or it may be two or three, have entered the telescope and been observed in like manner. the telescope is then unclamped, and moved forward until it is again ahead of the comet, and the observations are repeated; and this is done as often as is thought desirable. the places of the stars have, of course, to be found out from catalogues, or have to be observed with the transit circle, but when they are known the position of the comet or minor planet can easily be inferred. next to the glory of having been the means of bringing about the publication of newton's _principia_, the greatest achievement of halley, the second astronomer royal, was that he was the first to predict the return of a comet. newton had shown that comets were no lawless wanderers, but were as obedient to gravitation as were the planets themselves, and he also showed how the orbit of a comet could be determined from observations on three different dates. following these principles, halley computed the orbits of no fewer than twenty-four comets, and found that three of them, visible at intervals of about seventy-five years, pursued practically the same path. he concluded, therefore, that these were really different appearances of the same object, and, searching old records, he found reason to believe that it had been observed frequently earlier still. it seems, in fact, to have been the comet which is recorded to have been seen in in england at the time of the norman invasion; in a.d. , shortly before the commencement of that war which ended in the destruction of jerusalem by titus; and earlier still, so far back as b.c. . halley, however, experienced a difficulty in his investigation. the period of the comet's revolution was not always the same. this, he concluded, must be due to the attraction of the planets near which the comet might chance to travel. in the summer of it had passed very close to jupiter, for instance, and in consequence he expected that instead of returning in august , seventy-five years after its last appearance, it would not return until the end of or the beginning of . it has returned twice since halley's day, a triumphant verification of the law of gravitation; and we are looking for it now for a third return some ten years hence, in . halley's comet, therefore, is an integral member of our solar system, as much so as the earth or neptune, though it is utterly unlike them in appearance and constitution, and though its path is so utterly unlike theirs that it approaches the sun nearer than our earth, and recedes farther than neptune. but there are other comets, which are not permanent members of our system, but only passing visitors. from the unfathomed depths of space they come, to those depths they go. they obey the law of gravitation so far as our sight can follow them, but what happens to them beyond? do they come under some other law, or, perchance, in outermost space is there still a region reserved to primeval chaos, the 'anarch old,' where no law at all prevails? gravitation is the bond of the solar system; is it also the bond of the universe? chapter ix the magnetic and meteorological departments passing out of the south door of the new altazimuth building, we come to a white cruciform erection, constructed entirely of wood. this is the magnet house or magnetic observatory, the home of a double department, the magnetic and meteorological. this department does not, indeed, lie within the original purpose of the observatory as that was defined in the warrant given to flamsteed, and yet is so intimately connected with it, through its bearing on navigation, that there can be no question as to its suitability at greenwich. indeed, its creation is a striking example of the thorough grasp which airy had upon the essential principles which should govern the great national observatory of an essentially naval race, and of the keen insight with which he watched the new development of science. the magnetic observatory, therefore, the purpose of which was to deal with the observation of the changes in the force and direction of the earth's magnetism--an inquiry which the greater delicacy of modern compasses, and, in more recent times, the use of iron instead of wood in the construction of ships has rendered imperative--was suggested by airy on the first possible occasion after he entered on his office, and was sanctioned in . the meteorological department has a double bearing on the purpose of the observatory. on the one side, a knowledge of the temperature and pressure of the atmosphere is, as we have already seen, necessary in order to correct astronomical observations for the effect of refraction. on the other hand, meteorology proper, the study of the movements of the atmosphere, the elucidation of the laws which regulate those movements, leading to accurate forecasts of storms, are of the very first necessity for the safety of our shipping. it is true that weather forecasts are not issued from greenwich observatory, any more than the _nautical almanac_ is now issued from it; but just as the observatory furnishes the astronomical data upon which the almanac is based, so also it takes its part in furnishing observations to be used by the meteorological office at westminster for its daily predictions. those predictions are often made the subject of much cheap ridicule; but, however far short they may fall of the exact and accurate predictions which we would like to have, yet they mark an enormous advance upon the weather-lore of our immediate forefathers. 'he that is weather wise is seldom other wise,' says the proverb, and the saying is not without a shrewd amount of truth. for perhaps nowhere can we find a more striking combination of imperfect observation and inconsequent deduction than in the saws which form the stock-in-trade of the ordinary would-be weather prophet. how common it is to find men full of the conviction that the weather must change at the co-called 'changes of the moon,' forgetful that 'if we'd no moon at all-- and that may seem strange-- we still should have weather that's subject to change.' they will say, truly enough, no doubt, that they have known the weather to change at 'new' or 'full,' as the case may be, and they argue that it, therefore, must always do so. but, in fact, they have only noted a few chance coincidences, and have let the great number of discordances pass by unnoticed. but observations of this kind seem scientific and respectable compared with those numerous weather proverbs which are based upon the mere jingle of a rhyme, as 'if the ash is out before the oak, you may expect a thorough soak'-- a proverb which is deftly inverted in some districts by making 'oak' rhyme to 'choke.' others, again, are based upon a mere childish fancy, as, for example, when the young moon 'lying on her back' is supposed to bode a spell of dry weather, because it looks like a cup, and so might be thought of as able to hold the water. during the present reign, however, a very different method of weather study has come into action, and the foundations of a true weather wisdom have been laid. these have been based, not on fancied analogies or old wives' rhymes, or a few forechosen coincidences, but upon observations carried on for long periods of time and over wide areas of country, and discussed in their entirety without selection and bias. above all, mathematical analysis has been applied to the motions of the air, and ideas, ever gaining in precision and exactness, have been formulated of the general circulation of the atmosphere. as compared with its sister science, astronomy, meteorology appears to be still in a very undeveloped state. there is such a difference between the power of the astronomer to foretell the precise position of sun, moon, and planets for years, even for centuries, beforehand, and the failure of the meteorologist to predict the weather for a single season ahead, that the impression has been widely spread that there is yet no true meteorological science at all. it is forgotten that astronomy offered us, in the movements of the heavenly bodies, the very simplest and easiest problem of related motion. yet for how many thousands of years did men watch the planets, and speculate concerning their motions, before the labours of tycho, kepler, and newton culminated in the revelation of their meaning? for countless generations it was supposed that their movements regulated the lives, characters, and private fortunes of individual men; just as quite recently it was fancied that a new moon falling on a saturday, or two full moons coming within the same calendar month, brought bad weather! it is still impossible to foresee the course of weather change for long ahead; but the difference between the modern navigator, surely and confidently making a 'bee-line' across thousands of miles of ocean to his destination, and the timid sailor of old, creeping from point to point of land, is hardly greater than the contrast between the same two men, the one watching his barometer, the other trusting in the old wives' rhymes which afforded him his only indication as to coming storms. it is still impossible to foresee the weather change for long ahead, but in our own and in many other countries, especially the united states, it has been found possible to predict the weather of the coming four-and-twenty hours with very considerable exactness, and often to forecast the coming of a great storm several days ahead. this is the chief purpose of the two great observatories of the storm-swept indian and chinese seas, hong kong and mauritius; and the value of the work which they have done in preventing the loss of ships, and the consequent loss of lives and property, has been beyond all estimate. the royal observatory, greenwich, is a meteorological as well as an astronomical observatory, but, as remarked above, it does not itself issue any weather forecasts. just as the greenwich observations of the places of the moon and stars are sent to the _nautical almanac_ office, for use in the preparation of that ephemeris; just as the greenwich determinations of time are used for the issue of signals to the post office, whence they are distributed over the kingdom, so the greenwich observations of weather are sent to the meteorological office, there to be combined with similar records from every part of the british isles, to form the basis of the daily forecasts which the latter office publishes. to each of these three offices, therefore, the royal observatory, greenwich, stands in the relation of purveyor. it supplies them with the original observations more or less in reduced and corrected form, without which they could not carry on most important portions of their work. let it be noted how closely these three several departments, the _nautical almanac_ office, the time department, and the meteorological office, are related to practical navigation. whatever questions of pure science--of knowledge, that is, apart from its useful applications--may arise out of the following up of these several inquiries, yet the first thought, the first principle of each, is to render navigation more sure and more safe. the first of all meteorological instruments is the barometer, which, under its two chief forms of mercurial and aneroid, is simply a means of measuring the pressure exerted by the atmosphere. there are two important corrections to which its readings are subject. the first is for the height of the station above the level of the sea; the second is for the effect of temperature upon the mercury in the barometer itself, lengthening the column. to overcome these, the height of the standard barometer at greenwich above sea-level has been most carefully ascertained, and the heights relative to it of the other barometers of the observatory, particularly those in rooms occupied by fundamental telescopes, have also been determined, whilst the self-recording barometer is mounted in a basement, where it is almost completely protected from changes of temperature. next in importance to the barometer as a meteorological instrument comes the thermometer. the great difficulty in the observatory use of the thermometer is to secure a perfectly unexceptionable exposure, so that the thermometer may be in free and perfect contact with the air, and yet completely sheltered from any direct ray from the sun. this is secured in the great thermometer shed at greenwich by a double series of 'louvre' boards, on the east, south, and west sides of the shed, the north side being open. the shed itself is made a very roomy one, in order to give access to a greater body of air. a most important use of the thermometer is in the measurement of the amount of moisture in the air. to obtain this, a pair of thermometers are mounted close together, the bulb of one being covered by damp muslin, and the other being freely exposed. if the air is completely saturated with moisture, no evaporation can take place from the damp muslin, and consequently the two thermometers will read the same. but if the air be comparatively dry, more or less evaporation will take place from the wet bulb, and its temperature will sink to that at which the air would be fully saturated with the moisture which it already contained. for the higher the temperature, the greater is its power of containing moisture. the difference of the reading of the two thermometers is, therefore, an index of humidity. the greater the difference, the greater the power of absorbing moisture, or, in other words, the dryness of the air. the great shed already alluded to is devoted to these companion thermometers. [illustration: the self-registering thermometers.] very closely connected with atmospheric pressure, as shown us by the barometer, is the study of the direction of winds. if we take a map of the british isles and the neighbouring countries, and put down upon them the barometer readings from a great number of observing stations, and then join together the different places which show the same barometric pressure, we shall find that these lines of equal pressure--technically called 'isobars'--are apt to run much nearer together in some places than in others. clearly, where the isobars are close together it means that in a very short distance of country we have a great difference of atmospheric pressure. in this case we are likely to get a very strong wind blowing from the region of high pressure to the region of low pressure, in order to restore the balance. if, further, we had information from these various observing stations of the direction in which the wind was blowing, we should soon perceive other relationships. for instance, if we found that the barometer read about the same in a line across the country from east to west, but that it was higher in the north of the islands than in the south, we should then have a general set of winds from the east, and a similar relation would hold good if the barometer were highest in some other quarter; that is, the prevailing wind will come from a quarter at right angles to the region of highest barometer, or, as it is expressed in what is known as 'buys ballot's law,' 'stand with your back to the wind, and the barometer will be lower on your left hand than on your right.' this law holds good for the northern hemisphere generally, except near to the equator; in the southern hemisphere the right hand is the side of low barometer. the instruments for wind observation are of two classes: vanes to show its direction, and anemometers to show its speed and its pressure. these may be regarded as two different modes in which the strength of the wind manifests itself. pressure anemometers are usually of two forms: one in which a heavy plate is allowed to swing by its upper edge in a position fronting the wind, the amount of its deviation from the vertical being measured; and the other in which the plate is supported by springs, the degree of compression of the springs being the quantity registered in that case. of the speed anemometers, the best known form is the 'robinson,' in which four hemispherical cups are carried at the extremities of a couple of cross bars. for the mounting of these wind instruments the old original observatory, known as the octagon room, has proved an excellent site, with its flat roof surmounted by two turrets in the north-east and north-west corners, and raised some two hundred feet above high-water mark. [illustration: the anemometer room, north-west turret.] the two chief remaining instruments are those for measuring the amount of rainfall and of full sunshine. the rain gauge consists essentially of a funnel to collect the rain, and a graduated glass to measure it. the sunshine recorder usually consists of a large glass globe arranged to throw an image of the sun on a piece of specially prepared paper. this image, as the sun moves in the sky, moves along the paper, charring it as it moves, and at the end of the day it is easy to see, from the broken, burnt trace, at what hours the sun was shining clear, and when it was hidden by cloud. an amusing difficulty was encountered in an attempt to set on foot another inquiry. the superintendent of the meteorological department at the time wished to have a measure of the rate at which evaporation took place, and therefore exposed carefully measured quantities of water in the open air in a shallow vessel. for a few days the record seemed quite satisfactory. then the evaporation showed a sudden increase, and developed in the most erratic and inexplicable manner, until it was found that some sparrows had come to the conclusion that the saucer full of water was a kindly provision for their morning 'tub,' and had made use of it accordingly. a large proportion of the meteorological instruments at greenwich and other first-class observatories are arranged to be self-recording. it was early felt that it was necessary that the records of the barometer and thermometer should be as nearly as possible continuous; and at one time, within the memory of members of the staff still living, it was the duty of the observer to read a certain set of instruments at regular two-hour intervals during the whole of the day and night--a work probably the most monotonous, trying, and distasteful of any that the observatory had to show. the two-hour record was no doubt practically equivalent to a continuous one, but it entailed a heavy amount of labour. automatic registers were, therefore, introduced whenever they were available. the earliest of these were mechanical, and several still make their records in this manner. on the roof of the octagon room we find, beside the two turrets already referred to, a small wooden cabin built on a platform several feet above the roof level. this cabin and the north-western turret contain the wind-registering instruments. opening the turret door, we find ourselves in a tiny room which is nearly filled by a small table. upon this table lies a graduated sheet of paper in a metal frame, and as we look at it, we see that a clock set up close to the table is slowly drawing the paper across it. three little pencils rest lightly on the face of the paper at different points. one of these, and usually the most restless, is connected with a spindle which comes down into the turret from the roof, and which is, in fact, the spindle of the wind vane. the gearing is so contrived that the motion on a pivot of the vane is turned into motion in a straight line at right angles to the direction in which the paper is drawn by the clock. a second pencil is connected with the wind-pressure anemometer. the third pencil indicates the amount of rain that has fallen since the last setting, the pencil being moved by a float in the receiver of the rain gauge. [illustration: the anemometer trace.] an objection to all the mechanical methods of continuous registration is that, however carefully the gearing between the instrument itself and the pencil is contrived, however lightly the pencil moves over the paper, yet some friction enters in and affects the record: this is of no great moment in wind registration, when we are dealing with so powerful an agent as the wind, but it becomes a serious matter when the barometer is considered, since its variations require to be registered with the greatest minuteness. when photography, therefore, was invented, meteorologists were very prompt to take advantage of this new ally. a beam of light passing over the head of the column of mercury in a thermometer or barometer could easily be made to fall upon a drum revolving once in the twenty-four hours, and covered with a sheet of photographic paper. in this case, when the sensitive paper is developed, we find its upper half blackened, the lower edge of the blackened part showing an irregular curve according as the mercury in the thermometer or barometer rose or fell, and admitted less or more light through the space above it. here we have a very perfect means of registration: the passage of the light exercises no friction or check on the free motion of the mercury in the tube, or on the turning of the cylinder covered by the sensitive paper, whilst it is easy to obtain a time scale on the register by cutting off the light for an instant--say at each hour. in this way the wet and dry bulb thermometers in the great shed make their registers. the supply of material to the meteorological office is not the only use of the greenwich meteorological observations. two elements of meteorology, the temperature and the pressure of the atmosphere, have the very directest bearing upon astronomical work. and this in two ways. an instrument is sensible to heat and cold, and undergoes changes of form, size, or scale, which, however absolutely minute, yet become, with the increased delicacy of modern work, not merely appreciable, but important. so too with the density of the atmosphere: the light from a distant star, entering our atmosphere, suffers refraction; and being thus bent out of its path, the star appears higher in the heavens than it really is. the amount of this bending varies with the density of the layers of air through which the light has to pass. the two great meteorological instruments, the thermometer and barometer, are therefore astronomical instruments as well. in the arrangements at greenwich the magnetic department is closely connected with the meteorological, and it is because the two departments have been associated together that the building devoted to both is constructed of wood, not brick, since ordinary bricks are made of clay which is apt to be more or less ferruginous. copper nails have alone been employed in the construction of the buildings. the fire-grates, coal-scuttles, and fire-irons are all of the same metal. the growth of the observatory has, however, made it necessary to set up some of the new telescopes, into the mounting of which much iron enters, very close to the magnetic building. the present astronomer-royal has therefore erected a magnetic pavilion right out in the park at an ample distance from these disturbing causes. the double department is, therefore, the most widely scattered in the whole observatory. it is located for computing purposes in the west wing of the new observatory; many of its magnetic instruments are in the old magnet house, others are across the park in the new magnetic pavilion; the anemometers are on the roof of the octagon room, flamsteed's original observatory, and the self-registering thermometers are in the south ground between the old magnet house and the new observatory. [illustration: magnetic pavilion--exterior. (_from a photograph by mr. lacey._)] the object of the magnetic observatory is to study the movements of the magnetic needle. the quaintest answer that i ever received in an examination was in reply to the question, 'what is meant by magnetic inclination and declination?' the examinee replied: 'to make a magnet, you take a needle, and rub it on a lodestone. if it refuses or _declines_ to become a magnet, that is magnetic declination; if it is easily made a magnet, or is _inclined_ to become one, that is magnetic inclination.' one greatly regretted that it was necessary to mark the reply according to its ignorance, and not, as one would have wished, in proportion to its ingenuity. magnetic declination, however, as everybody knows, measures the deviation of the 'needle' from the true geographical north and south direction; the inclination or dip is the angle which a 'needle' makes with the horizon. at one time the only method of watching the movements of the magnetic needles was by direct observation, just precisely as it was wont to be in the case of the barometer and thermometer. but the same agent that has been called in to help in their case has enabled the magnets also to give us a direct and continuous record of their movements. in principle the arrangement is as follows: a small light mirror is attached to the magnetic needle, and a beam of light is arranged to fall upon the mirror, and is reflected away from it to a drum covered with sensitive paper. if, then, the needle is perfectly at rest, a spot of light falls on the drum and blackens the paper at one particular point. the drum is made to revolve by clockwork once in twenty-four hours, and the black dot is therefore lengthened out into a straight line encircling the drum. if, however, the needle moves, then the spot of light travels up or down, as the case may be. now, if we look at one of these sheets of photographic paper after it has been taken from the drum, we shall see that the north pole of the magnet has moved a little, a very little, towards the west in the early part of the day, say from sunrise to p.m., and has swung backwards from that hour till about p.m., remaining fairly quiet during the night. the extent of this daily swing is but small, but it is greater in summer than in winter, and it varies also from year to year. [illustration: magnetic pavilion--interior. (_from a photograph by mr. lacey._)] besides this daily swing, there occasionally happen what are called 'magnetic storms;' great convulsive twitchings of the needle, as if some unseen operator were endeavouring, whilst in a state of intense excitement, to telegraph a message of vast importance, so rapid and so sharp are the movements of the needle to and fro. these great storms are felt, so far as we know, simultaneously over the whole earth, and the more characteristic begin with a single sharp twitch of the needle towards the east. besides the movements of the magnetic needle, the intensity of the currents of electricity which are always passing through the crust of the earth are also determined at greenwich; but this work has been rendered practically useless for the last few years by the construction of the electric railway from stockwell to the city. since it was opened, the photographic register of earth currents has shown a broad blurring from the moment of the starting of the first train in the morning to the stopping of the last train at night. as an indication of the delicacy of modern instruments, it may be mentioned that distinct indications of the current from this railway have been detected as far off as north walsham, in norfolk, a distance of more than a hundred miles. a further illustration of the delicacy of the magnetic needles was afforded shortly after the opening of the railway referred to. on one occasion the then superintendent of the magnetic department visited the generating station at stockwell, and on his return it was noticed day after day that the traces from the magnets showed a curious deflection from a.m. to p.m., the hours of his attendance. this gave rise to some speculation, as it did not seem possible that the gentleman could himself have become magnetized. eventually, the happy accident of a fine day solved the mystery. that morning the superintendent left his umbrella at home, and the magnets were undisturbed. the secret was out. the umbrella had become a permanent magnet, and its presence in the lobby of the magnetic house had been sufficient to influence the needles. chapter x the heliographic department so far the development of the observatory had been along the central line of assistance to navigation. but the magnetic department led on to one which had but a very secondary connection with it. a greatly enhanced interest was given to the observations of earth magnetism, when it was found that the intensity and frequency of its disturbances were in close accord with changes that were in progress many millions of miles away. that the surface of the sun was occasionally diversified by the presence of dark spots, had been known almost from the first invention of the telescope; but it was not until the middle of the present century that any connection was established between these solar changes and the changes which took place in the magnetism of the earth. then two observers, the one interesting himself entirely with the spots on the sun, the other as wholly devoted to the study of the movements of the magnetic needle, independently found that the particular phenomenon which each was watching was one which varied in a more or less regular cycle. and further, when the cycles were compared, they proved to be the same. whatever the secret of the connection, it is now beyond dispute that as the spots on the sun become more and more numerous, so the daily swing of the magnetic needle becomes stronger; and, on the other hand, as the spots diminish, so the magnetic needle moves more and more feebly. this discovery has given a greatly increased significance to the study of the earth's magnetism. the daily swing, the occasional 'storms,' are seen to be something more than matters of merely local interest; they have the closest connection with changes going on in the vast universe beyond; they have an astronomical importance. and it was soon felt to be necessary to supplement the magnetic observatory at greenwich by one devoted to the direct study of the solar surface; and here again that invaluable servant of modern science, photography, was ready to lend its help. just as, by the means of photography, the magnets recorded their own movements, so even more directly the sun himself makes register of his changes by the same agency, and gives us at once his portrait and his autograph. this new department was again due to airy, and in the 'kew' photo-heliograph, which had been designed by de la rue for this work, was installed at greenwich. [illustration: the dallmeyer photo-heliograph.] in order to photograph so bright a body as the sun, it is not in the least necessary to have a very large telescope. the one in common use at greenwich from to , is only four inches in aperture and even that is usually diminished by a cap to three inches, and its focal length is but five feet. this is not very much larger than what is commonly called a 'student's telescope,' but it is amply sufficient for its work. this 'dallmeyer' telescope, so called from the name of its maker, is one of five identical instruments which were made for use in the observation of the transit of venus of , and which, since they are designed for photographing the sun, are called 'photo-heliographs.' the image of the sun in the principal focus of this telescope is about six-tenths of an inch in diameter; but a magnifying lens is used, so that the photograph actually obtained is about eight inches. even with this great enlargement, the light of the sun is so intense that with the slowest photographic plates that are made the exposure has to be for only a very small fraction of a second. this is managed by arranging a very narrow slit in a strip of brass. the strip is made to run in a groove across the principal focus. before the exposure, it is fastened up so as to cut off all light from entering the camera part of the telescope. when all is ready, it is released and drawn down very rapidly by a powerful spring, and the slit, flying across the image of the sun, gives exposure to the plate for a very minute fraction of a second--in midsummer for less than a thousandth of a second. two of these photographs are taken every fine day at greenwich; occasionally more, if anything specially interesting appears to be going on. but in our cloudy climate at least one day in three gives no good opportunity for taking photographs of the sun, and in the winter time long weeks may pass without a chance. the present astronomer-royal, mr. christie, has therefore arranged that photographs with precisely similar instruments should be taken in india and in the mauritius, and these are sent over to greenwich as they are required, to fill up the gaps in the greenwich series. we have therefore at greenwich, from one source or another, practically a daily record of the state of the sun's surface. more recently the 'dallmeyer' photo-heliograph, though still retained for occasional use, has been superseded generally by the 'thompson'; a photographic refractor of nine inches aperture, and nearly nine feet focal length, presented to the observatory by sir henry thompson. the image of the sun obtained after enlargement in the telescope, with this instrument, is seven and a half inches in diameter. the 'thompson' is mounted below the great twenty-six-inch photographic refractor,--also presented to the observatory by sir henry thompson,--in the dome which crowns the centre of the new observatory. a photograph of the sun taken, it has next to be measured, the four following particulars being determined for each spot: first, its distance from the centre of the image of the sun; next, the angle between it and the north point; thirdly, the size of the spot; and fourthly, the size of the umbra of the spot, that is to say, of its dark central portion. the size or area of the spot is measured by placing a thin piece of glass, on which a number of cross-lines have been ruled one-hundredth of an inch apart, in contact with the photograph. these cross-lines make up a number of small squares, each the ten-thousandth ( / in.) part of a square inch in area. when the photograph and the little engraved glass plate are nearly in contact, the photograph is examined with a magnifying glass, and the number of little squares covered by a given spot are counted. it will give some idea of the vast scale of the sun when it is stated that a tiny spot, so small that it only just covers one of these little squares, and which is only one-millionth of the visible hemisphere of the sun in area, yet covers in actual extent considerably more than one million of square miles. the dark spots are not the only objects on the sun's surface. here and there, and especially near the edge of the sun, are bright marks, generally in long branching lines, so bright as to appear bright even against the dazzling background of the sun itself. these are called 'faculæ,' and they, like the spots, have their times of great abundance and of scarcity, changing on the whole at the same time as the spots. after the solar photographs have been measured, the measures must be 'reduced,' and the positions of the spots as expressed in longitude and latitude on the sun computed. there is no difficulty in doing this, for the position of the sun's equator and poles have long been known approximately, the sun revolving on its axis in a little more than twenty-five days, and carrying of course the spots and faculæ round with him. there are few studies in astronomy more engrossing than the watch on the growth and changes of the solar spots. their strange shapes, their rapid movements, and striking alterations afford an unfailing interest. for example, the amazing spectacle is continually being afforded of a spot, some two, three, or four hundred millions of square miles in area, moving over the solar surface at a speed of three hundred miles an hour, whilst other spots in the same group are remaining stationary. but a higher interest attaches to the behaviour of the sun as a whole than to the changes of any particular single spot; and the curious fact has been brought to light, that not only do the spots increase and diminish in a regular cycle of about eleven years in length, but they also affect different regions of the sun at different points of the cycle. at the time when spots are most numerous and largest, they are found occupying two broad belts, the one with its centre about ° north of the equator, the other about as far south, the equator itself being very nearly free from them. but as the spots begin to diminish, so they appear continually in lower and lower latitudes, until instead of having two zones of spots there is only one, and this one lies along the equator. by this time the spots have become both few and small. the next stage is that a very few small spots are seen from time to time in one hemisphere or the other at a great distance from the equator, much farther than any were seen at the time of greatest activity. there are then for a little time three sun-spot belts, but the equatorial one soon dies out. the two belts in high latitude, on the other hand, continually increase; but as they increase, so do they move downwards in latitude, until at length they are again found in about latitude ° north or south, when the spots have attained their greatest development. [illustration: photograph of a group of sun-spots. (_from a photograph taken at the royal observatory, greenwich, april, , d. h. m._)] the clearest connection between the magnetic movements and the sun-spot changes is seen when we take the mean values of either for considerable periods of time, as, for instance, year by year. but occasionally we have much more special instances of this connection. some three or four times within the last twenty years an enormous spot has broken out on the sun, a spot so vast that worlds as great as our own could lie in it like peas in a breakfast saucer, and in each case there has been an immediate and a threefold answer from the earth. one of the most remarkable of these occurred in november, . a great spot was then seen covering an area of more than three thousand millions of square miles. the weather in london happened to be somewhat foggy, and the sun loomed, a dull red ball, through the haze, a ball it was perfectly easy to look at without specially shading the eyes. so large a spot under such circumstances was quite visible to the naked eye, and it caught the attention of a great number of people, many of whom knew nothing about the existence of spots on the sun. this great disturbance, evidently something of the nature of a storm in the solar atmosphere, stretched over one hundred thousand miles on the surface of the sun. the disturbance extended farther still, even to nearly one hundred millions of miles. for simultaneously with the appearance of the spot the magnetic needles at greenwich began to suffer from a strange excitement, an excitement which grew from day to day until it had passed half-way across the sun's disc. as the twitchings of the magnetic needle increased in frequency and violence, other symptoms were noticed throughout the length of the british isles. telegraphic communication was greatly interfered with. the telegraph lines had other messages to carry more urgent than those of men. the needles in the telegraph instruments twitched to and fro. the signal bells on many of the railway lines were rung, and some of the operators received shocks from their instruments. lastly, on november , a superb aurora was witnessed, the culminating feature of which was the appearance, at about six o'clock in the evening, of a mysterious beam of greenish light, in shape something like a cigar, and many degrees in length, which rose in the east and crossed the sky at a pace much quicker than but nearly as even as that of sun, moon, or stars, till it set in the west two minutes after its rising. so far we have been dealing only with effects. their causes still rest hidden from us. there is clearly a connection between the solar activity as shown by the spots and the agitation of the magnetic needles. but many great spots find no answer in any magnetic vibration, and not a few considerable magnetic storms occur when we can detect no great solar changes to correspond. thus even in the simplest case before us we have still very much to explain. two far more difficult problems are still offered us for solution. what is the cause of these mysterious solar spots? and have they any traceable connection with the fitful vagaries of earthly weather? it was early suggested that probably the first problem might find an answer in the ever-varying combinations and configurations of the various planets, and that the sun-spots in their turn might hold the key of our meteorology. both ideas were eagerly followed up--not that there was much to support either, but because they seemed to offer the only possible hope of our being able to foretell the general current of weather change for any long period in advance. so far, however, the first idea may be considered as completely discredited. as to the second, there would appear to be, in the case of certain great tropical and continental countries like india, some slight but by no means conclusive evidence of a connection between the changes in the annual rainfall and the changes in the spotted surface of the sun. dr. meldrum, the late veteran director of the great meteorological observatory in mauritius, has expressed himself as confident that the years of most spots are the years of most violent cyclones in the indian ocean. but this is about as far as real progress has been made, and it may be taken as certain that many years more of observation will be required, and the labours of many skilful investigators, before we can hope to carry much farther our knowledge as to any connection between storm and sun. a further relation of great interest has come to light within the last few years. the year opened a new epoch in the study of eclipses of the sun. these, perhaps, scarcely lie within the scope of a book on the royal observatory, since greenwich has seen but one in all its history. that fell in the year ; for the next it must wait many centuries. yet, as the late astronomer royal conducted three expeditions to see total eclipses, and as the present astronomer royal has undertaken a like number, and members of the staff have been sent on other occasions, it may not be deemed quite a digression to refer to one feature which they have brought to light. when the dark body of the moon has entirely hidden the sun, we have revealed to us, there and then only, that strange and beautiful surrounding of the sun which we call the corona. the earlier observations of the corona seem to reveal it as a body of the most weird and intricate form, a form which seemed to change quite lawlessly from one eclipse to another. but latterly it has been abundantly clear that the forms which it assumes may be grouped under a few well-defined types. in the corona was of a particularly simple and striking character. two great wings shot out east and west in the direction of the sun's equator; round either pole was a cluster of beautiful radiating 'plumes.' it was then recollected that the corona of had been of precisely the same character, both years being years when sun-spots were at their fewest. the coronæ, on the other hand, seen at times when sun-spots are more abundant, were of an altogether different character, the streamers being irregularly distributed all round the sun. other types also have been recognized, and it is perfectly apparent that the corona changes its shape in close accordance with the eleven-year period. the eclipses of and , for example, showed coronæ that bore the very closest resemblance to those of and , the interval of eleven years bringing a return to the same form. the further problem, therefore, now confronts us: does the corona produce the sun-spots, or do the sun-spots produce the corona, or are both the result of some mysterious magnetic action of the sun, an action powerful enough on occasion to thrill through and through this world of ours, ninety-three millions of miles away? chapter xi the spectroscopic department another department was set on foot by airy at the same time as the heliographic department, and in connection with it; and it is the department which has the greatest of interest for the general public. this deals with astronomical physics, or astrophysics, as it is sometimes more shortly called; the astronomy, that is, which treats of the constitution and condition of the heavenly bodies, not with their movements. the older astronomy, on the other hand, confined itself to the movements of the heavens so entirely that bessel, the man whose practical genius revolutionized the science of observation, and whose influence may be traced throughout in airy's great reconstitution of greenwich observatory, denied that anything but the study of the celestial movements had a right to the title of astronomy at all. hardly more than sixty years ago he wrote: 'what astronomy is expected to accomplish is evidently at all times the same. it may lay down rules by which the movements of the celestial bodies, as they appear to us upon the earth, can be computed. all else which we may learn respecting these bodies, as, for example, their appearance, and the character of their surfaces, is, indeed, not undeserving of attention, but possesses no proper astronomical interest. whether the mountains of the moon are arranged in this way or in that is no further an object of interest to astronomers than is a knowledge of the mountains of the earth to others. whether jupiter appears with dark stripes upon its surface, or is uniformly illuminated, pertains as little to the inquiries of the astronomer; and its four moons are interesting to him only for the motions they have. to learn so perfectly the motions of the celestial bodies that for any specified time an accurate computation of these can be given--that was, and is, the problem which astronomy has to solve.' there is a curious irony of progress which seems to delight in falsifying the predictions of even master minds as to the limits beyond which it cannot advance. bessel laid down his dictum as to the true subjects of astronomical inquiry, comte declared that we could never learn what were the elements of which the stars were composed, at the very time that the first steps were being taken towards the creation of a research which should begin by demonstrating the existence in the heavenly bodies of the elements with which we are familiar on the earth, and should go on to prove itself a true astronomy, even in bessel's restricted sense, by supplying the means for determining motion in a direction which he would have thought impossible--that is to say, directly to or from us. the years that followed kirchhoff's application of the spectroscope to the study of the sun, and his demonstration that sodium and iron existed in the solar atmosphere, were crowded with a succession of brilliant discoveries in the same field. kirchhoff, bunsen, angström, thalèn, added element after element to the list of those recognized in the sun. huggins and miller carried the same research into a far more difficult field, and showed us the same elements in the stars. rutherfurd and secchi grouped the stars according to the types of their spectra, and so laid the foundations of what may be termed stellar comparative anatomy. huggins discovered true gaseous nebulæ, and so revived the nebular theory, which had been supposed crushed when the great telescope of lord rosse appeared to have resolved several portions of the orion nebula into separate stars. the great riddle of 'new stars'--which still remains a riddle--was at least attacked, and glowing hydrogen was seen to be a feature in their constitution. glowing hydrogen, again, was, in the observation of total eclipses, seen to be a principal constituent of those surroundings of our own sun which we now call prominences and chromosphere. then the method was discovered of observing the prominences without an eclipse, and they were found to wax and wane in more or less sympathy with the solar spots. sun-spots, planets, comets, meteors, variable stars, all were studied with the new instrument, and all yielded to it fresh and valuable, and often unexpected, information. [illustration: the great nebula in orion. (_from a photograph taken at the royal observatory greenwich, december , , with an exposure of - / hours._)] in this activity greenwich observatory practically took no part. airy, ever mindful of the original purpose of the observatory, and deeply imbued with views similar to those which we have quoted from bessel, considered that the new science lay outside the scope of his duties, until in mr., now sir william, huggins's skilful hands the spectroscope showed itself not only as a means for determining the condition and constitution of the stars, but also their movements--until, in short, it had shown itself as an astronomical instrument even within bessel's narrow definition. the principle of this inquiry is as follows: if a source of light is approaching us very rapidly, then the waves of light coming from it necessarily appear a little shorter than they really are, or, in other words, that light appears to be slightly more blue--the blue waves being shorter than the red--than it really is. a similar thing with regard to the waves of sound is often noticed in connection with a railway train. if an express train, the whistle of which is blowing the whole time, dashes past us at full speed, there is a perceptible drop in the note of the whistle after it has gone by. the sound waves as it was coming were a little shortened, and the whistle therefore appeared to have a sharper note than it had in reality. and in the same way, when it had gone by, the sound waves were a little lengthened, making the note of the whistle appear a very little flatter. such a change of colour in a star could never have been detected without the spectroscope; but since when light passes through a prism the shorter waves are refracted more strongly, that is to say, are more turned out of their course than the longer, the spectroscope affords us the means of detecting and measuring this change. let us suppose that the lines of hydrogen are recognized in a given star. if we compare the spectrum of this star with the spectrum of a tube containing hydrogen and through which the electric spark is passing, we shall be able to see whether any particular hydrogen line occupies the same place as shown by the two spectra. if the line from the star is a little to the red of the line from the tube, the star must be receding from us; if to the blue, approaching us. the amount of displacement may be measured by a delicate micrometer, and the rate of motion concluded from it. [illustration: the half-prism spectroscope on the south-east equatorial.] the principle is clear enough. the actual working out of the observation was one of very great difficulty. the movements of the stars towards us, or away from us, are, in general, extremely slow as compared with the speed of light itself; and hence the apparent shift in the position of a line is only perceptible when a very powerful spectroscope is used. this means that the feeble light of a star has to be spread out into a great length of spectrum, and a very powerful telescope is necessary. the work of observing the motions of stars in the line of sight was started at greenwich in , the 'great equatorial' being devoted to it. this telescope, of - / inches aperture, was not powerful enough to do much more than afford a general indication of the direction in which the principal stars were moving, and to confirm in a general way the inference which various astronomers had found, from discussing the proper motions of stars, that the sun and the solar system were moving towards that part of the heavens where the constellations hercules and lyra are placed. in , therefore, the work was discontinued, and as already mentioned, the - / telescope by merz was removed to make room for the present much larger instrument by sir howard grubb, upon the same mounting. the new telescope being much larger than the one for which mounting and observing room were originally built, it was not possible to put the spectroscope in the usual position, in the same straight line as the great telescope. it was therefore mounted under it, and parallel to it, and the light of the star was brought into it after two reflections. the observer therefore stood with his back to the object and looked down into the spectroscope. it had, however, become apparent by this time that this most delicate field of work was one for which photography possessed several advantages, and as sir henry thompson had made the munificent gift to the observatory of a great photographic equatorial, it was resolved to devote the -inch telescope chiefly to double-star work, and to transfer the spectroscope to the 'new building.' the 'new observatory' in the south ground is crowned indeed with the dome devoted to the great thompson photographic refractor, but this is not its chief purpose. its principal floor contains four fine rooms which are used as 'computing rooms'--for the office work, that is to say, of the observatory. of these the principal is in the north wing, where the main entrance is placed, and is occupied by the astronomer royal and the two chief assistants. the basement contains the libraries and the workshops of the mechanics and carpenters. the upper floor will eventually be used for the storage of photographs and manuscripts, and the terrace roofs of the four wings will be exceedingly convenient for occasional observations, as, for example, of meteor showers. the central dome, which rises high above the level of the terraces, is the only room in the building devoted to telescopic work. as in the new altazimuth building, a ring of circular lights just below the coping of the wall recalls the portholes of a ship, and again reminds us of the connection of the observatory with navigation. [illustration: the workshop.] here the spectroscope is now placed, but not, as it happens, on the thompson refractor. the equatorial mounting in this new dome is a modification of what is usually called the 'german' form of mounting--that is to say, there is but one pier to support the telescope, and the telescope rides on one side of the pier and a counterpoise balances it on the other the 'great equatorial,' on the other hand, is an example of the english mounting, and has two piers, one north and the other south, whilst the telescope swings in a frame between them. in the new dome three telescopes are found rigidly connected with each other on one side of the pier, the telescopes being ( ) the great thompson photographic telescope, double the aperture and double the focal length of the standard astrographic telescope used for the international photographic survey; ( ) the - / telescope by merz, that used to be in the great south-east dome, but which is now rigidly connected with the thompson refractor as a guide telescope; and ( ) a photographic telescope of inches aperture, already described as the 'thompson' photo-heliograph, and used for photographing the sun or in eclipse expeditions. the counterpoise to this collection of instruments is not a mere mass of lead, but a powerful reflector of inches' aperture, and it is to this telescope that the spectroscope is now attached. at the present time, however (august, ), regular work has not been commenced with it. [illustration: the -inch reflector with the new spectroscope attached.] beside this attempt to determine the motions of the stars as they approach us or retreat from us, on rare occasions the spectroscope has been turned on the planets. as these shine by reflected light, their spectra are normally the same as that of the sun. mars appeared to the writer, as to huggins and others, to show some slight indication of the presence of water vapour in its atmosphere. jupiter and saturn show that their atmospheres contain some absorbing vapour unknown to ours. and uranus and neptune, faint and distant as they are, not only show the same dark band given by the two nearer planets, but several others. more attractive has been the examination of the spectra of the brighter comets that have visited us. the years and were especially rich in these. the two principal comets of were called after their respective discoverers, tebbutt's and schaeberle's. they were not bright enough to attract popular attention, though they could be seen with the naked eye, and both gave clear indications of the presence of carbon, their spectra closely resembling that of the blue part of a gas or candle flame. there was nothing particularly novel in these observations, since comets usually show this carbon spectrum, though why they should is still a matter for inquiry; but the two comets of the following year were much more interesting. both comets came very near indeed to the sun. the earlier one, called from its discoverer comet wells, as it drew near to the sun, began to grow more and more yellow, until in the first week of june it looked as full an orange as even the so-called red planet, mars. the spectroscope showed the reason of this at a glance. the comet had been rich in sodium. so long as it was far from the sun the sodium made no sign, but as it came close to it the sodium was turned into glowing vapour under the fierce solar heat. and as the writer saw it in the early dawn of june , the comet itself was a disc of much the same colour as mars, whilst its spectrum resembled that of a spirit lamp that has been plentifully fed with carbonate of soda or common salt. the 'great comet' of the autumn of the same year, and which was so brilliant an object in the early morning, came yet nearer to the sun, and the heating process went on further. the sodium lines blazed up as they had done with comet wells, but under the fiercer stress of heat to which the great comet was subjected, the lines of iron also flashed out, a significant indication of the tremendous temperature to which it was exposed. there are two other departments of spectroscopic work which it was attempted for a time to carry on as part of the greenwich routine. these were the daily mapping of the prominences round the sun, and the detailed examination of the spectra of sun-spots. both are almost necessary complements of the work done in the heliographic department--that is to say, the work of photographing the appearance of the sun day by day, and of measuring the positions and areas of the spots. for the spots afford but one index out of several, of the changes in the sun's activity. the prominences afford another, nor can we at the present moment say authoritatively which is the more significant. then again, with regard to the spots themselves, it is not certain that either their extent or their changes of appearance are the features which it is most important for us to study. we want, if possible, to get down to the soul of the spot, to find out what makes one spot differ from another; and here the spectroscope can help us. great sun-spots are often connected with violent agitation of the magnetic needles, and with displays of auroræ. but they are not always so, and the inquiry, 'what makes them to differ?' has been made again and again, without as yet receiving any unmistakable answer. the great spot of november, , which was connected with so remarkable an aurora and so violent a magnetic storm, was as singular in its spectrum as in its earthly effects. the sun was only seen through much fog, and the spectrum was therefore very faint, but shooting up from almost every part of its area, except the very darkest, were great masses of intensely brilliant hydrogen, evidently under great pressure. the sodium lines were extremely broadened, and on november a broad bright flame of hydrogen was seen shooting up at an immense speed from one edge of the nucleus. a similar effect--an outburst of intensely luminous hydrogen--has often been observed in spots which have been accompanied by great magnetic storms; and it may even be that it is this violent eruption of intensely heated gas which has the directest connection with the magnetic and auroral disturbances here upon earth. this sun-spot work was not carried on for very long, as only one assistant could be spared for the entire solar work of whatever character. yet in that time an interesting discovery was made by the writer--namely, that in the green part of the spectrum of certain spots a number of broad diffused lines or narrow bands made their appearance from time to time, and especially when sun-spots were increasing in number, or were at their greatest development. the prominence work had also to be dropped, partly for the same reason, but chiefly because the atmospheric conditions at greenwich are not suitable for these delicate astrophysical researches. when the observatory was founded 'in the golden days' of charles ii., greenwich was a little country town far enough removed from the great capital, and no interference from its smoke and dust had to be feared or was dreamt of. now the 'great wen,' as cobbett called it, has spread far around and beyond it, and the days when the sky is sufficiently pure round the sun for successful spectrum work on the spots or prominences are few indeed. whether in the future it will be thought advisable for the royal observatory to enter into serious competition in inquiries of this description with the great 'astrophysical' observatories of the continent and of america--potsdam, meudon, the lick, and the yerkes--we cannot say. that would involve a very considerable departure from its original programme, and probably also a departure from its original site. for the conditions at greenwich tend to become steadily less favourable for such work, and it would most probably be found that full efficiency could only be secured by setting up a branch or branches far from the monster town. with the older work it is otherwise. so long as greenwich park and blackheath are kept--as it is to be hoped they always will be--sacred from the invasion of the builder; so long as no new railways burrow their tunnels in the neighbourhood of the observatory, so long the fundamental duties laid upon flamsteed, 'of rectifying the tables of the motions of the heavens and the places of the fixed stars,' will be carried out by his successors on flamsteed hill. chapter xii the astrographic department the two last departments mentioned, the heliographic and spectroscopic, lie clearly and unmistakably outside the terms of the original warrant of the observatory, though the progress of science has led naturally and inevitably to their being included in the greenwich programme. but the astrographic department, though it could no more have been conceived in the days of charles ii. than the spectroscopic, does come within the terms of the warrant, and is but an expansion of that work of 'rectifying the places of the fixed stars,' which formed part of the programme enjoined upon flamsteed, the first astronomer royal, at the first foundation of the observatory, and which was so diligently carried out by him, the first greenwich catalogue, containing about stars, being due to his labours. [illustration: 'chart plate' of the pleiades. (_from a photograph taken at the royal observatory, greenwich, with an exposure of forty minutes._)] his immediate successors did much less in this field, though bradley's observations were published, long after his death, as a catalogue of stars, in some aspects the most important ever issued. pond, the sixth astronomer royal, restored catalogue-making to a prominent place in the greenwich routine, and his precedent is sedulously followed to-day. but each of these was confined to about stars. the necessity has long been felt for a much ampler census, and argelander, at the bonn observatory, brought out a catalogue of , stars north of south declination °, a work which has been completed by schönfeld, who carried the census down to south declination °, and by the two great astronomers of cordoba, south america, dr. gould and dr. thome, by whom it was extended to the south pole. these last three catalogues embrace stars of all magnitudes down to the th or th; but certain astronomers had endeavoured to go much lower, and to make charts of limited portions of the sky down to even the th magnitude. from the very earliest days that men observed the stars, they could not help noticing that 'one star differeth from another star in glory,' and consequently they divided them into six classes, according to their brightness--classes which are commonly spoken of now as magnitudes. the ordinary th magnitude star is one which can be clearly seen by average sight on a good night, and it gives us about one-hundredth the light of an average st magnitude star. sirius, the brightest of all the fixed stars, is called a st magnitude star, but is really some six or seven times as bright as the average. it would take, therefore, more than two and a half million stars of the th magnitude to give as much light as sirius. it is evident that so searching a census as to embrace stars of the th magnitude would involve a most gigantic chart. but the work went on in more than one observatory for a considerable time, until at last the observers entered on to the region of the milky way. here the numbers of the stars presented to them were so great as to baffle all ordinary means of observation. what could be done? just at this time immense interest was caused in the astronomical world by the appearance of the great comet of . it was watched and observed and sketched by countless admirers, but more important still, it was photographed, and some of its photographs, taken at the royal observatory, cape of good hope, showed not only the comet with marvellous beauty of detail, but also thousands of stars, and the success of these photographs suggested to her majesty's astronomer at the cape, dr. gill, that in photography we possessed the means for making a complete sky census even to the th magnitude. the project was thought over in all its bearings, and in a great conference of astronomers at paris resolved upon an international scheme for photographing the entire heavens. the work was to be divided between eighteen observatories of different nationalities. it was to result in a photographic chart extending to the th magnitude, and probably embracing some forty million stars, and a catalogue made from measures of the photographs down to the th magnitude, which would probably include between two and three million stars. [illustration: the control pendulum and the base of the thompson telescope.] the eighteen observatories all undertook to use instruments of the same capacity. this was to be a photographic refractor, with an object-glass of inches aperture and feet focus. at greenwich this telescope is mounted equatorially--that is, so as to follow the stars in their courses--and is mounted on the top of the pier that once supported halley's quadrant. the telescope is driven by a most efficient clock, whose motive power is a heavy weight. the rate of the weight in falling is regulated by an ingenious governor, which brings its speed very nearly indeed to that of the star, and any little irregularities in its motion are corrected by the following device. a seconds pendulum is mounted in a glass case on the wall of the observatory, and a needle at the lower end of the pendulum passes at each swing through a globule of mercury. on one of the wheels of the clock are arranged a number of little brass points, at such intervals apart that the wheel, when going at the proper rate, takes exactly one second to move through the distance between any pair. a little spring is arranged above the wheel, so that these points touch it as they pass. if this occurs exactly as the pendulum point passes through the mercury nothing happens, but if the clock is ever so little late or early, the electric current from the pendulum brings into action a second wheel, which accelerates or retards the driving of the clock, as the case may be. the total motion, therefore, is most beautifully even. [illustration: the astrographic telescope. (_reproduced from 'engineering' by permission._)] but even this is not quite sufficient, especially as the plates for the great chart have to be exposed for at least forty minutes. rigidly united with the -inch refractor, so that the two look like the two barrels of a huge double-barrelled gun, is a second telescope for the use of the observer. in its eyepiece are fixed two pairs of cross spider lines, commonly called wires, and a bright star, as near as possible to the centre of the field to be photographed, is brought to the junction of two wires. should the star appear to move away from the wire, the observer has but to press one of two buttons on a little plate which he carries in his hand, and which is connected by an electric wire with the driving clock, to bring it back to its position. the photographs taken with this instrument are of two kinds. those for the great chart have but a single exposure, but this lasts for forty minutes. those for the great catalogue have three exposures on them, the three images of a star being some seconds of arc apart. these exposures are of six minutes', three minutes', and twenty seconds' duration, and the last exposure is given as a test, since, if stars of the th magnitude are visible with an exposure of twenty seconds, stars of the th magnitude should be visible with three minutes' exposure. thus it will be seen that in three minutes an impression is got of many scores of stars, whose places it would require many hours to determine at the transit instrument. but the positions of these stars on the plate still remain to be measured. for this purpose a net-work of lines, at right angles to each other, is printed on the photograph before its development, and, after it has been developed, washed and dried, the distances of the stars from their nearest cross-lines are measured in the measuring machine. [illustration: the driving clock of the astrographic telescope. (_reproduced from 'engineering' by permission._)] the measuring machine is constructed to hold two plates, one half its breadth higher than the other. in fact, in each of the two series of photographs the whole sky is taken twice, but the two photographs of any region are not simply duplicates of each other. the centre of each plate is at a corner of four other plates, and in the micrometer the stars on the quarter common to two plates are measured simultaneously. in this way will be carried out a great census of the sky that will exceed flamsteed's ten thousand fold. and just as flamsteed's was but the first of many similar catalogues, so, no doubt, will this be followed by others--not superseded, for its value will increase with its age and the number of those that follow it, by comparison with which it will prove an inexhaustible mine of information concerning the motions of the stars and the structure of the universe. there is a great difference between the work of the observer with the 'astrographic telescope,' as this great twin photographic instrument is called, and the work of the transit observer. the latter sees the star gliding past him, and telegraphs the instant that the star threads itself on each of the ten vertical wires in succession. the astrographic observer, on the other hand, sees his star shining almost immovably in the centre of his field, threaded on the two cross wires placed there, for the driving-clock moves the telescope so as to almost exactly compensate for the rotation movement of the earth. the observer's duty in this case is to telegraph to his driving-clock, when it has in the least come short of or exceeded its duty, and so to bring back the 'guiding star' to its exact proper place on the cross wires. so far, the work of the astrographic department has been, as mentioned above, a development on an extraordinary scale, but a development still, of the original programme of the observatory. but the munificent gift of sir henry thompson has put it within the power of the astronomer royal to push this work of sidereal photography a stage further. sir henry thompson gave to the observatory, not merely the photographic refractor of inches' aperture, now used for solar photography, and known as the 'thompson photo-heliograph,' but also one of inches' aperture and - / feet focal length. this instrument was specially designed of exactly double the dimensions of the standard astrographic telescope used for the international photographic survey, the idea being that, in the case of a field of special interest and importance, a photograph could be obtained with the larger instrument on exactly double the scale given by the smaller. it has rather, however, found its usefulness in a slightly different field. the observation of the satellites of jupiter was suggested by galileo as a means of determining the longitude at sea. as already pointed out, the suggestion did not prove to be a practical one for that purpose, but observations of the satellites have been made none the less with a view simply to improving our knowledge of their movements, and of the mass of jupiter. the utilitarian motive for the work having fallen through, it has been carried on as a matter of pure science. and the work has not stopped with the satellites of jupiter; eight satellites were in due time discovered to saturn, four to uranus, and two to mars; and though these could give not the remotest assistance to navigation, they too have been made the subjects of observation for precisely the same reason as those of jupiter have been. [illustration: the thompson telescope in the new dome.] in just the same way, when the discovery of neptune was followed by that of a solitary companion to it, this also had to be followed. the difficulties in the way of observing the fainter of all these satellites were considerable, and the work has been mostly confined to two or three observatories possessing very large telescopes. as the largest telescope at greenwich was only inches in aperture up to , and only - / inches up to , it is only very recently that it has been able to take any very substantial part in satellite measures. but since the thompson photographic telescope was set up, it has been found that a photograph of neptune and its satellite can be taken in considerably less time than a complete set of direct measures can be made, whilst the photograph, which can be measured at leisure during the day, gives distinctly the more accurate results. so, too, the places of the minor planets can be got more accurately and quickly by means of photographs with this great telescope than by direct observation, and photographs of the most interesting of them all, the little planet eros, have been very successfully obtained. so that, though doing nothing directly to improve the art of navigation, or to find the longitude at sea, the great photographic refractor takes its share in the work of 'rectifying the tables of the planets.' [illustration: the nebulÆ of the pleiades. (_from a photograph taken at the royal observatory, greenwich, december , , with an exposure of three hours._)] the reflector of inches' aperture, which acts as a counterpoise to the sheaf of telescopes of the thompson, is intended for use with the spectroscope, the quality which mirrors possess of bringing all rays, whatever their colour, to the same focus being of great importance for spectroscopic work. but the experiments which have been made with it in celestial photography have proved so extremely successful as to cause the postponement of the recommencement of the spectroscopic researches. chief amongst these photographs are some good ones of the moon, and more recently some exceedingly fine photographs of the principal nebulæ. in no department of astronomy has photography brought us such striking results as in regard to the nebulæ. dr. roberts' photograph of the great nebula in andromeda converted the two or three meaningless rifts--which some of the best drawings had shown--into the divisions between concentric rings; and what had appeared a mere shapeless cloud was seen to be a vast symmetrical structure, a great sidereal system in the making. the great nebula in orion has grown in successive photographs in detail and extent, until we have a large part of the constellation bound together in the convolutions of a single nebula of the most exquisite detail and most amazing complexity. the group of the pleiades has had a more wonderful record still. manifestly a single system even to the naked eye, and showing some faint indications of nebulosity in the telescope, the photographs have revealed its principal stars shining out from nebulous masses, in appearance like carded wool, and have shown smaller stars threaded on nebulous lines like pearls upon a string. such photographs are, of course, of no utilitarian value, and at present they lead us to no definite scientific conclusions. they lie, therefore, doubly outside the limits of the purely practical, but they attract us by their extreme beauty, and by the amazing difficulty of the problems they suggest. how are these weird masses of gas retained in such complex form over distances which must be reckoned by millions of millions of miles? by what agency are they made to glow so as to be visible to us here? what conceivable condition threads together suns on a line of nebula? what universes are here in the making, or perhaps it may be falling into ruin and decay? chapter xiii the double-star department the foregoing chapters will have shown that though the original purpose of the observatory has always been kept in view, yet the progress of science has caused many researches to be undertaken which overstep its boundaries. thus in the present transit room, beside the successive transit instruments we find upon the wall two long thin tubes, labelled respectively alpha aquilæ and alpha cygni. these were two telescopes set up by pond for a special purpose. dr. brinkley, royal astronomer for ireland, had announced that he had found that several stars shifted their apparent place in the sky in the course of a year, due to the change in the position of the earth from which we view them, by an amount which would show that they were only about six to nine billions of miles distant from us; or, in other words, they showed a parallax of from two to three seconds of arc. pond was not able to confirm these parallaxes from his observations, and to decide the point he set up these two telescopes, the alpha aquilæ telescope being rigidly fixed on the west side of the pier of troughton's mural circles; the alpha cygni telescope on another pier, the one which now forms the base of the pier of the astrographic telescope. pond's method was to compare the position of these two stars with that of a star almost exactly the same distance from the pole, but at a great distance from it in time of crossing the meridian; in other words, of almost the same declination, but widely different right ascension. the result proved that brinkley was wrong, and vindicated the delicacy and accuracy of pond's observations. these two telescopes, therefore, had their day and ceased to be. others have followed them. an ingenious telescope was set up by sir george airy in order to ascertain if the speed of light were different when passing through water than when passing through air. or, in other words, if the aberration of light would give the same value as at present if we observed through water. the water telescope, as it was called, is kept on the ground floor of the central octagon of the new observatory. the observations obtained with it were hardly quite satisfactory, but gave on the whole a negative result. turning back to the transit room, and leaving it by the south-west door, we come into the little passage which leads at the back of bradley's transit room into the lower computing room. just inside this passage, on the left-hand side, there is a little room of a most curious shape, the 'reflex zenith room.' here is fixed a telescope pointing straight upwards, the eye-piece being fixed by the side of the object-glass. the light from a star--the star gamma draconis--which passes exactly over the zenith of greenwich, enters the object-glass, passes downwards to a basin of mercury, and is reflected upwards from the surface of the mercury to a little prism placed over the centre of the object-glass, from which it is reflected again into the eye-piece. by means of this telescope the distance of the star gamma draconis from the zenith could be measured very exactly, and, consequently, the changes in the apparent position of the star due to aberration, parallax, and other causes could be very exactly followed, and the corrections to be applied on account of these causes precisely determined. this particular telescope was devised by airy, and the observations with it were continued to the end of his reign. the germ of the idea may be traced back, however, to the time of flamsteed, who would seem to have occasionally observed gamma draconis from the bottom of a deep well; the precise position of the well is not, however, now known. later, bradley set up his celebrated - / -foot zenith sector, still preserved in the transit room, first at wanstead and then at greenwich, for the determination of the amount of aberration. later, a zenith tube by troughton, of feet focus, was used by pond in conjunction with the mural circle for observations of gamma draconis in order to determine the zenith point of the latter instrument. these telescopes for special purposes have passed out of use. observations with the spectroscope have been suspended for some years. the work of the astrographic department will come to an end, in the ordinary course of events, when the programme assigned to greenwich in the international scheme is completed. within the last few years a new department has come into being at greenwich--a department which has been steadily worked at many foreign public observatories, but only recently here. this is the department of double-star observation. the first double star, zeta ursæ majoris, was discovered years ago. bradley discovered two exceedingly famous double stars whilst still a young man observing with his uncle at wanstead--gamma virginis and castor. bradley made also other discoveries of double stars after his appointment to greenwich, and maskelyne succeeded him in the same line, but the great foundation of double-star astronomy was laid by sir william herschel. at first it was supposed that double stars were double only in appearance; one star comparatively near us 'happened' to lie in almost exactly the same direction as another star much further off. it was, indeed, in the very expectation that this would prove to be the case, that the elder herschel first took up their study. but he was soon convinced that many of the objects were true double stars--members of the same system of which the smaller revolved round the larger--not merely apparently double, one star appearing by chance to be close to another with which it had no connection--but real double stars. the discovery of these has led to the establishment of a new department of astronomy, again scientific rather than utilitarian. [illustration: double-star observation with the south-east equatorial. (_from a photograph by mr. edney._)] as mentioned above, it is only recently that greenwich has taken any appreciable part in this work. under airy, the largest equatorial of the time had been furnished with a good micrometer, and observations of one or two double stars been made now and again; but airy's programme of work was far too rigid, and kept the staff too closely engaged for such observations to be anything but extremely rare. and, indeed, when the micrometers of the equatorials were brought into use, they were far more generally devoted to the satellites of saturn than to the companions of stars. in the main, double-star astronomy has been in the hands of amateurs, at least in england. but the discovery in recent years of many pairs so close that a telescope of the largest size is required for their successful observation, has put an important section of double stars beyond the reach of most private observers, and therefore the great telescope at greenwich is now mainly devoted to their study. the astronomer royal, therefore, soon after the completion of the great equatorial of -inches aperture placed in the south-east dome, added this work to the observatory programme. the -inch equatorial is a remarkable-looking instrument, its mounting being of an entirely different kind to that of the other equatorials in the observatory, with the solitary exception of the shuckburgh, which is set up in a little dome over the chronograph room. the shuckburgh was presented to the observatory in the year , by sir g. shuckburgh. it was first intended to be mounted as an altazimuth, but proved to be unsteady in that position, and was then converted into an equatorial without clockwork, and mounted in its present position. the position is about as hopelessly bad a one as a telescope could well have, completely overshadowed as it is by the trees and buildings close at hand. the dome is a small one, and the arrangements for the shutters and for turning the dome are as bad as they could possibly be. it has practically been useless for the last forty years. its only interest is that the method of mounting employed is a small scale model of that of the great telescope in the s.-e. dome. in the german or fraunhofer form of mounting for an equatorial there is but a single pillar, which carries a comparatively short polar axis. at the upper end of the polar axis we find the declination axis, and at one end of the declination axis is the telescope, whilst at the other end is a heavy weight to counterpoise it. the german mounting has the advantage that the telescope can easily point to the pole of the heavens; its drawbacks are that, except in certain special forms, the telescope cannot travel very far when it is on the same side of the meridian as the star to which it is pointed, the end of the telescope coming into contact under such circumstances with the central pier, whilst the introduction of mere deadweight as the necessary counterpoise, is not economical. it has been already pointed out that the present astronomer royal has not only considerably modified the german mounting in the great collection of telescopes in the thompson dome, but has used a powerful reflector as a counterpoise to the sheaf of refractors at the other end of the declination axis. the english equatorial requires two piers. between these two piers is a long polar axis. both in the little shuckburgh and in the great -inch equatorial the frame of the polar axis consists of six parallel rods disposed in two equilateral triangles, with their bases parallel to each other, the telescope swinging in the space between the two bases. the construction of this form of equatorial, therefore, is expensive, as it requires two piers. it takes much more room than the german form, and the telescope cannot be directed precisely to the pole. but the instrument is symmetrical, there is no deadweight, and the telescope can follow a star from rising to setting without having to be reversed on crossing the meridian. the great stability of the english form of mounting, therefore, commended it very highly to airy, and he designed the great northumberland equatorial of the cambridge observatory on that plan, as well as one for the liverpool observatory at bidston, and in the s.-e. equatorial at greenwich. the telescope at first mounted upon it had an object-glass of - / inches' aperture, and feet focal length. that was dismounted in , and is now used as the guiding telescope of the thompson -inch photographic refractor. its place was taken by an immensely heavier instrument, the present refractor of inches' aperture, and feet focal length; and that this change was effected safely was an eloquent testimony to the solidity of the original mounting. the clock that drives this great instrument, so that it can follow a star or other celestial object in its apparent daily motion across the sky, is in the basement of the s.-e. tower. it is a very simple looking instrument, a conical pendulum in a glass case. the pendulum makes a complete revolution once in two seconds. below it in a closed case is a water turbine. a cistern on the roof of the staircase supplies this turbine with water, having a fall of about thirty feet. the water rushing out of the arms of the turbine forces it backward, and the turbine spins rapidly round, driving a spindle which runs up into the dome, and gears through one or two intermediate wheels with the great circle of the telescope; the extremely rapid rotation of the spindle, four times in a second, being converted by these intermediate wheels into the exceedingly slow one of once in twenty-four hours. just above the centre of motion of the turbine is a set of three small wheels, all of exactly the same size, and of the same number of teeth. of these the bottom wheel is horizontal, and is turned by the turbine. the top wheel is also horizontal, and is turned by the pendulum. the third wheel gears into both these, and is vertical. if the top and bottom wheels are moving exactly at the same rate, the intermediate wheel simply turns on its axis, but does not travel; but if the turbine and pendulum are moving at different rates, then the vertical wheel is forced to run in one direction or the other, and, doing so, it opens or closes a throttle valve, which controls the supply of water to the turbine, and so speedily brings the turbine into accord with the pendulum. the control of the motion of the great telescope is therefore almost as perfect as that of the astrographic and thompson equatorials, though the principle employed is very different. and the control needs to be perfect, for, as said above, the great telescope is mostly devoted to the observation of double stars, and there can be no greater hindrance to this work than a telescope which does not move accurately with the star. there is a striking contrast between the great telescope and all the massive machinery for its direction and movement, and the objects on which it is directed--two little points of light separated by a delicate hair of darkness. the observation is very unlike those of which we have hitherto spoken. the object is not to ascertain the actual position in the sky of the two stars, but their relative position to each other. a spider's thread of the finest strands is moved from one star to the other by turning an exquisitely fine screw; this enables us to measure their distance apart. another spider thread at right angles to the first is laid through the centres of both stars, and a divided circle enables us to read the angle which this line makes to the true east and west direction. such observations repeated year after year on many stars have enabled the orbits of not a few to be laid down with remarkable precision; and we find that their movements are completely consistent with the law of gravitation. further, just as neptune was pre-recognized and discovered from noting the irregularities in the motion of uranus, so the discordances in the place of sirius led to the belief that it was attracted by a then unseen companion, whose position with respect to the brighter star was predicted and afterwards seen. [illustration: the south-east dome with the shutter open.] gravitation thus appears, indeed, to be the bond of the universe, yet it leaves us with several weighty problems. the observation of the positions of stars shows that though we call them fixed they really have motions of their own. of these motions, a great part consists of a drift away from one portion of the heavens towards a point diametrically opposite to it, a drift such as must be due, not to a true motion of the individual stars, but to a motion through space of our sun and its attendant system. the elder herschel was the first to discover this mysterious solar motion. sir george airy and mr. edwin dunkin, for forty-six years a member of the greenwich staff, and from - the chief assistant, contributed important determinations of its direction. what is the cause of this motion, what is the law of this motion, is at present beyond our power to find out. many years ago a german astronomer made the random suggestion that possibly we were revolving in an orbit round the pleiades as a centre. the suggestion was entirely baseless, but unfortunately has found its way into many popular works, and still sometimes is brought forward as if it were one of the established truths of astronomy. we can at present only say that this solar motion is a mystery. there is a greater mystery still. the stars have their own individual motions, and in the case of a few these are of the most amazing swiftness. the earth in its motion round the sun travels nearly nineteen miles in a second, say one thousand times faster than the quickest rush of an express train. the sun's rate of motion is probably not quite so swift, but arcturus, a sun far larger than our own, has a pace some twenty times as swift as the orbital motion of the earth. this is not a motion that we can conceive of as being brought about by gravitation, for if there were some unseen body so vast as to draw arcturus with this swiftness, other stars too would be hurtling across the sky as quickly. such 'runaway stars' afford a problem to which we have as yet no key, and, like job of old, we are speechless when the question comes to us from heaven, 'canst thou guide arcturus and his sons?' it will be seen then that, fundamentally, greenwich observatory was founded and has been maintained for distinctly practical purposes, chiefly for the improvement of the eminently practical science of navigation. other inquiries relating to navigation, as, for instance, terrestrial magnetism and meteorology, have been added since. the pursuit of these objects has of necessity meant that the observatory was equipped with powerful and accurate instruments, and the possession of these again has led to their use in fields which lay outside the domain of the purely utilitarian, fields from which the only harvest that could be reaped was that of the increase of our knowledge. so we have been led step by step from the mere desire to help the mariner to find his way across the trackless ocean, to the establishment of the secret law which rules the movements of every body of the universe, till at length we stand face to face with the mysteries of vast systems in the making, with the intimate structure of the stellar universe, with the apparently aimless, causeless wanderings of vast suns in lightning flight; with problems that we cannot solve, nor hope to solve, yet cannot cease from attempting, problems to which the only answer we can give is the confession of the magicians of egypt--'this is the finger of god.' index aberration of light, adams, john c., his discovery of neptune, adhara, airy, george biddell, seventh astronomer royal, his early life, ; his work at cambridge, ; comes to greenwich, ; his relations with the visitors, ; his autobiography, ; his character, ; his labours, ; attacks on, ; his distinctions, ; his resignation, ; his death, ; anecdote of, ; his conduct _re_ adams, ; his water telescope, alderamin, _almagest_, almanac making, alpha aquilæ, telescope for, ---- cygni, telescope for, altazimuth the, ; description and work of, , _et seq._ altazimuth department, , _et seq._ american time, andromeda nebula, anemometer, use of, ; trace of, angström, anson, commodore, apparent time, arcturus, motion of, argelander, star catalogue of, _art of dialling_, the, assistants, position of the, , , , astrographic chart, ---- department, , _et seq._ ---- dome, ---- telescope, , _et seq._ astronomers royal, the, astrophysical researches, auroræ, automatic register, axis of the earth, precession of, ball, time, barometer, use of the, , battery basement, beaufort, captain, bessel quoted, betelgeuse, birkenhead, wreck of the, bliss, nathaniel, fourth astronomer royal, history of, bradley, james, third astronomer royal, his life, ; his ordination, ; vicar of bridstow, ; savilian professor of astronomy, ; discovers aberration of light, , _et seq._; becomes astronomer royal, ; labours of, ; character of, bradley's transit room, brinkley, dr., _british mariner's guide_, the, bunsen, buys ballot's law, canadian time, castor, , catalogues, star, , , _et seq._, , cepheus, charles ii., warrants of, , christie, w. h. m., eighth astronomer royal, work of, chromosphere of the sun, chronograph, the, ---- room, chronometer business, , chronometers, harrison's improvements in, , _et seq._; tests of, ; 'runs' of, ; romance of, circle department, , _et seq._ clock, astrographic driving, ; driving -inch telescope, clocks, standard, columbus, aim of voyage of, comet, appearance of a, ---- wells, comets, observation of, ; spectra of, commutator, the, comte, assertion of, constant of aberration, cook, captain, work of, copper, use of in observatory, corona of the sun, crabtree, james, crosthwait, joseph, dallmeyer telescope, declination, , _et seq._ denebola, distances of planets, ; of sun, double-star department, , _et seq._ double stars, dublin time, dunkin, edwin, earth, the, movements of, eclipses of the moon, ; of the sun, july , ... ; other eclipses of the sun, , _et seq._ electric railway, influence of, equation of time, the, , equatorial, shuckburgh's, ----, the great -inch, ----, the merz, - / -inch, ----, -inch, driving clock of, ; use of, ----, clock-driven, eros, discovery of, ; photographs of, errors in observations, noting of, , _et seq._ evaporation, faculæ of the sun, flamsteed, john, his report on saint-pierre's proposal, , ; appointed first astronomer royal, , ; his autobiography, ; his studies, ; his almanac, ; sent to london, ; enters jesus college, cambridge, ; completes his observatory, ; acquaintance with newton, ; takes his degree, ; his work, ; warrant for his salary, ; position of, ; his ordination, ; his pupils, ; his trouble with newton, , _et seq._; his catalogue, ; his letter to sharp, ; his death, ; his labours, flamsteed house, fraunhofer mounting, french time, galileo, his discovery of jupiter's satellites, gamma draconis, , ---- virginis, gascoigne, william, gemma frisius, plan of, george of denmark, prince, german mounting, , gould, dr., graham, gravitation, the bond of the universe, great comet of , the, , greatrackes, valentine, green, charles, greenwich time, ; distribution of, halley, edmund, his life, ; his early work, ; his catalogue of stars, ; elected f.r.s., ; his work on kepler's laws, ; becomes captain, ; savilian professor of geometry, ; astronomer royal, ; observations on saros of the moon, ; pressed by newton, ; his death, ; his services to science, ; his pay, ; nominates his successor, ; his transit instrument, halley's comet, harrison, james, timekeepers of, , , , heineken, rev. n. s., heineken quadrant, heliographic department, , _et seq._ herschel, caroline, hipparchus, catalogue of, hodgson, mr., hooke, robert, , horrox, jeremiah, huggins, sir w., his use of spectroscope, inscription, an, international photographic survey, ireis, iron quadrant, isobars, jupiter, satellites of, , ; atmosphere of, keill, john, kendall, larcum, kepler, laws of, kew, photo-heliograph, the, kinnebrook, david, kirchhoff's use of spectroscope, latitude, finding the, ledgers, chronometer, romance of, leverrier, his discovery of neptune, libraries, linacre, g., lindsay, thomas, quoted, litchford, w., local apparent time, longitude, finding the, ; at sea, problem of, ; determination of, longitude nought, lower computing room, lunars, method of, magnetic department, work of, ; description of, , _et seq._ magnetic inclination and declination, ---- needles, movements of, , ---- observatory, ---- pavilion, ---- storms, , mars, distance of, ; atmosphere of, ; satellites of, maskelyne, nevil, fifth astronomer royal, ; practical work of, ; astronomer royal, ; his work, ; his publications, ; his observations and work, , _et seq._; his death, ; his character, ; recommends his successor, ; his mural circle, mean solar clock, mean time, meldrum, dr., on sun spots, meridian, the, merz telescope, meteorological department, work of, ; description of, , _et seq._ micrometers, use of, microscopes, use of, milky way, miller, professor, milne, professor, on earth movements, minor planets, molyneux, samuel, moon, observation of the, , _et seq._; eclipses of, moore, sir jonas, ; death of, morin, mounting telescopes, modes of, mudge, thomas, mural arc, -feet, mural circles, , names of stars, origin of, nares, sir george, _nautical almanac_, the, , , navigation, state of primitive, neptune, discovery of, ; atmosphere of, ; satellite of, new altazimuth, the, , new observatory, the, , new stars, newcomb, professor, on growth of observatory, ; on greenwich observations, newton, sir i., his absent-mindedness, ; his trouble with flamsteed, , _et seq._; on kepler's laws, ; his _principia_, ; his pressure on halley, ; his discovery of gravitation, north terrace, the, northumberland equatorial, nutation of the earth, observation, modes of, , , ; by reflection, ; of comets, observatory, greenwich, work of, ; foundation of, ; warrant for building, ; position of, ; foundation stone laid, ; condition of, ; enlargement of, ; recent extensions of, ; description of, , _et seq._; staff of, ; work of, , _et seq._; visitors to, ; new altazimuth building, ; magnet house, ; magnetic pavilion, ; new observatory, ; future of, ; reflex zenith room, ; objects of, occultations by the moon, , _et seq._ octagon room, , , oldenburg, mr., orion nebula, , parallax of stars, paramour, the, paris, conference at, ----, noon at, philip iii., offer of, photographic registration, , , , ; refractors, photographs, star, photo-heliographs, , _et seq._, piazzi, discovery of, pleiades, the, polar plumes of the corona, polaris, pole-star, variation of, pond, john, sixth astronomer royal, his life, ; his reign, ; his salary, ; his assistants, ; his observations, ; censured by visitors, ; his observations of stars, pound, james, precession of earth's axis, _principia_, publication of, proctor, r. a., attack of, ptolemy, claudius, catalogue of, publication, the problem of, , quadrant, heineken, ----, the iron, railway time, rain gauge, record rooms, reflection, observation by, reflex zenith room, ---- ---- tube, refraction, effects of, right ascension, , _et seq._ roberts, dr. isaac, römer, discovery of, rosse, lord, royal society and flamsteed, , _et seq._ saint-pierre, le sieur de, proposal of, , sappho, saros of the moon, satellites, discovery of, saturn, atmosphere of, ; satellites of, schaeberle's comet, schedar, schiehallion, attraction of, schönfeld, scotchmen, anecdote of, sharp, abraham, sheepshanks, rev. james, on airy, shuckburgh equatorial, sidereal clock, sirius, sloane, dr., 'smith, mr.,' his chronometer, solar photographs, ---- storms, , sound waves, south, sir james, , south-east equatorial, the, , spectroscope, use of, spectroscopic department, , _et seq._ spots, sun, , _et seq._, staff of observatory, ; work of, , _et seq._ standard time, stars, observations of, , , ; origin of names of, ; movements of, ; catalogues of, , , _et seq._; composition of, , _et seq._; colour of, ; classes of, ; census of, ; photographs of, , _et seq._; motions of, , story, mr. a. m., sun, distance of the, , ; spots on, , _et seq._, ; eclipses of, , _et seq._; chromosphere of, ; motions of, sunshine recorder, swiss time, tebb, mr. w., tebbutt's comet, telescope, the great transit, ----, -inch, ----, astrographic, ----, shuckburgh, ----, thompson, , , thalèn, thermometer, use of, , thome, dr., thompson photo-heliograph, , , time ball, ---- department, the, , _et seq._ ---- desk, ----, foreign, ---- signals, ---- standard, transit, halley's, transit circle, the, ; mode of observation with, , _et seq._ transit circle, troughton's, ---- department, , _et seq._ ---- observations, number of, ---- pavilion, , ---- room, , troughton's transit circle, uranus, discovery of, ; atmosphere of, ; satellites of, vanes, use of, venus, distance of, victoria, visitors, the board of, ; censures pond, ; work of, ; constitution of, visitors to observatory, warrant for flamsteed's salary, water telescope, weather predictions, , _et seq._ winds, study of, witt, herr, discovery of, working catalogue, the, zenith sector, , ---- tube, , zeta ursæ majoris, zubeneschamal, the end london: printed by william clowes and sons, limited, stamford street and charing cross. * * * * * transcriber's note: minor typographical errors have been corrected without note. irregularities and inconsistencies in the text have been retained as printed. the illustrations have been moved so that they do not break up paragraphs. mismatched quotation marks were not corrected if it was not clear where the missing quotation mark should be placed. [illustration: plate i. maps i.-iv.] half-hours with the telescope; being a popular guide to the use of the telescope as a means of amusement and instruction. by richard a. proctor, b.a., f.r.a.s., author of "saturn and its system," etc. with illustrations on stone and wood. * * * * * an undevout astronomer is mad: true, all things speak a god; but, in the small men trace out him: in great he seizes man. young. * * * * * new york: g.p. putnam's sons. . london: printed by william clowes and sons, stamford street and charing cross. preface. the object which the author and publisher of this little work have proposed to themselves, has been the production, at a moderate price, of a useful and reliable guide to the amateur telescopist. among the celestial phenomena described or figured in this treatise, by far the larger number may be profitably examined with small telescopes, and there are none which are beyond the range of a good -inch achromatic. the work also treats of the construction of telescopes, the nature and use of star-maps, and other subjects connected with the requirements of amateur observers. r.a.p. _january_, . contents. chapter i. page a half-hour on the structure of the telescope chapter ii. a half-hour with orion, lepus, taurus, etc. chapter iii. a half-hour with lyra, hercules, corvus, crater, etc. chapter iv. a half-hour with bootes, scorpio, ophiuchus, etc. chapter v. a half-hour with andromeda, cygnus, etc. chapter vi. half-hours with the planets chapter vii. half-hours with the sun and moon description of plates. plate i.--_frontispiece._ this plate presents the aspect of the heavens at the four seasons, dealt with in chapters ii., iii., iv., and v. in each map of this plate the central point represents the point vertically over the observer's head, and the circumference represents his horizon. the plan of each map is such that the direction of a star or constellation, as respects the compass-points, and its elevation, also, above the horizon, at the given season, can be at once determined. two illustrations of the use of the maps will serve to explain their nature better than any detailed description. suppose first, that--at one of the hours named under map i.--the observer wishes to find castor and pollux:--turning to map i. he sees that these stars lie in the lower left-hand quadrant, and very nearly towards the point marked s.e.; that is, they are to be looked for on the sky towards the south-east. also, it is seen that the two stars lie about one-fourth of the way from the centre towards the circumference. hence, on the sky, the stars will be found about one-fourth of the way from the zenith towards the horizon: castor will be seen immediately above pollux. next, suppose that at one of the hours named the observer wishes to learn what stars are visible towards the west and north-west:--turning the map until the portion of the circumference marked w ... n.w. is lowermost, he sees that in the direction named the square of pegasus lies not very high above the horizon, one diagonal of the square being vertical, the other nearly horizontal. above the square is andromeda, to the right of which lies cassiopeia, the stars [beta] and [epsilon] of this constellation lying directly towards the north-west, while the star [alpha] lies almost exactly midway between the zenith and the horizon. above andromeda, a little towards the left, lies perseus, algol being almost exactly towards the west and one-third of the way from the zenith towards the horizon (because one-third of the way from the centre towards the circumference of the map). almost exactly in the zenith is the star [delta] aurigæ. the four maps are miniatures of maps i., iv., vii., and x. of my 'constellation seasons,' fourth-magnitude stars, however, being omitted. plates ii., iii., iv., and v., illustrating chapters ii., iii., iv., and v. plates ii. and iv. contain four star-maps. they not only serve to indicate the configuration of certain important star-groups, but they illustrate the construction of maps, such as the observer should make for himself when he wishes to obtain an accurate knowledge of particular regions of the sky. they are all made to one scale, and on the conical projection--the simplest and best of all projections for maps of this sort. the way in which the meridians and parallels for this projection are laid down is described in my 'handbook of the stars.' with a little practice a few minutes will suffice for sweeping out the equidistant circular arcs which mark the parallels and ruling in the straight meridians. the dotted line across three of the maps represents a portion of the horizontal circle midway between the zenith and the horizon at the hour at which the map is supposed to be used. at other hours, of course, this line would be differently situated. plates iii. and v. represent fifty-two of the objects mentioned in the above-named chapters. as reference is made to these figures in the text, little comment is here required. it is to be remarked, however, that the circles, and especially the small circles, do not represent the whole of the telescope's field of view, only a small portion of it. the object of these figures is to enable the observer to know what to expect when he turns his telescope towards a difficult double star. many of the objects depicted are very easy doubles: these are given as objects of reference. the observer having seen the correspondence between an easy double and its picture, as respects the relation between the line joining the components and the apparent path of the double across the telescope's field of view, will know how to interpret the picture of a difficult double in this respect. and as all the small figures are drawn to one scale, he will also know how far apart he may expect to find the components of a difficult double. thus he will have an exact conception of the sort of duplicity he is to look for, and this is--_crede experto_--a great step towards the detection of the star's duplicity. plates vi. and vii., illustrating chapters vi. and vii. the views of mercury, venus, and mars in these plates (except the smaller view of jupiter in plate vii.) are supposed to be seen with the same "power." the observer must not expect to see the details presented in the views of mars with anything like the distinctness i have here given to them. if he place the plate at a distance of six or seven yards he will see the views more nearly as mars is likely to appear in a good three-inch aperture. the chart of mars is a reduction of one i have constructed from views by mr. dawes. i believe that nearly all the features included in the chart are permanent, though not always visible. i take this opportunity of noting that the eighteen orthographic pictures of mars presented with my shilling chart are to be looked on rather as maps than as representing telescopic views. they illustrate usefully the varying presentation of mars towards the earth. the observer can obtain other such illustrations for himself by filling in outlines, traced from those given at the foot of plate vi., with details from the chart. it is to be noted that mars varies in presentation, not only as respects the greater or less opening out of his equator towards the north or south, but as respects the apparent slope of his polar axis to the right or left. the four projections as shown, or inverted, or seen from the back of the plate (held up to the light) give presentations of mars towards the sun at twelve periods of the martial year,--viz., at the autumnal and vernal equinoxes, at the two solstices, and at intermediate periods corresponding to our terrestrial months. in fact, by means of these projections one might readily form a series of sun-views of mars resembling my 'sun-views of the earth.' in the first view of jupiter it is to be remarked that the three satellites outside the disc are supposed to be moving in directions appreciably parallel to the belts on the disc--the upper satellites from right to left, the lower one from left to right. in general the satellites, when so near to the disc, are not seen in a straight line, as the three shown in the figure happen to be. of the three spots on the disc, the faintest is a satellite, the neighbouring dark spot its shadow, the other dark spot the shadow of the satellite close to the planet's disc. half-hours with the telescope. chapter i. a half-hour on the structure of the telescope. there are few instruments which yield more pleasure and instruction than the telescope. even a small telescope--only an inch and a half or two inches, perhaps, in aperture--will serve to supply profitable amusement to those who know how to apply its powers. i have often seen with pleasure the surprise with which the performance even of an opera-glass, well steadied, and directed towards certain parts of the heavens, has been witnessed by those who have supposed that nothing but an expensive and colossal telescope could afford any views of interest. but a well-constructed achromatic of two or three inches in aperture will not merely supply amusement and instruction,--it may be made to do useful work. the student of astronomy is often deterred from telescopic observation by the thought that in a field wherein so many have laboured, with abilities and means perhaps far surpassing those he may possess, he is little likely to reap results of any utility. he argues that, since the planets, stars, and nebulæ have been scanned by herschel and rosse, with their gigantic mirrors, and at pulkova and greenwich with refractors whose construction has taxed to the utmost the ingenuity of the optician and mechanic, it must be utterly useless for an unpractised observer to direct a telescope of moderate power to the examination of these objects. now, passing over the consideration that a small telescope may afford its possessor much pleasure of an intellectual and elevated character, even if he is never able by its means to effect original discoveries, two arguments may be urged in favour of independent telescopic observation. in the first place, the student who wishes to appreciate the facts and theories of astronomy should familiarize himself with the nature of that instrument to which astronomers have been most largely indebted. in the second place, some of the most important discoveries in astronomy have been effected by means of telescopes of moderate power used skilfully and systematically. one instance may suffice to show what can be done in this way. the well-known telescopist goldschmidt (who commenced astronomical observation at the age of forty-eight, in ) added fourteen asteroids to the solar system, not to speak of important discoveries of nebulæ and variable stars, by means of a telescope only five feet in focal length, mounted on a movable tripod stand. the feeling experienced by those who look through a telescope for the first time,--especially if it is directed upon a planet or nebula--is commonly one of disappointment. they have been told that such and such powers will exhibit jupiter's belts, saturn's rings, and the continent-outlines on mars; yet, though perhaps a higher power is applied, they fail to detect these appearances, and can hardly believe that they are perfectly distinct to the practised eye. the expectations of the beginner are especially liable to disappointment in one particular. he forms an estimate of the view he is to obtain of a planet by multiplying the apparent diameter of the planet by the magnifying power of his telescope, and comparing the result with the apparent diameter of the sun or moon. let us suppose, for instance, that on the day of observation jupiter's apparent diameter is ", and that the telescopic power applied is , then in the telescope jupiter should appear to have a diameter of ", or half a degree, which is about the same as the moon's apparent diameter. but when the observer looks through the telescope he obtains a view--interesting, indeed, and instructive--but very different from what the above calculation would lead him to expect. he sees a disc apparently much smaller than the moon's, and not nearly so well-defined in outline; in a line with the disc's centre there appear three or four minute dots of light, the satellites of the planet; and, perhaps, if the weather is favourable and the observer watchful, he will be able to detect faint traces of belts across the planet's disc. yet in such a case the telescope is not in fault. the planet really appears of the estimated size. in fact, it is often possible to prove this in a very simple manner. if the observer wait until the planet and the moon are pretty near together, he will find that it is possible to view the planet with one eye through the telescope and the moon with the unaided eye, in such a manner that the two discs may coincide, and thus their relative apparent dimensions be at once recognised. nor should the indistinctness and incompleteness of the view be attributed to imperfection of the telescope; they are partly due to the nature of the observation and the low power employed, and partly to the inexperience of the beginner. it is to such a beginner that the following pages are specially addressed, with the hope of affording him aid and encouragement in the use of one of the most enchanting of scientific instruments,--an instrument that has created for astronomers a new sense, so to speak, by which, in the words of the ancient poet: subjecere oculis distantia sidera nostris, Ætheraque ingenio supposuere suo. in the first place, it is necessary that the beginner should rightly know what is the nature of the instrument he is to use. and this is the more necessary because, while it is perfectly easy to obtain such knowledge without any profound acquaintance with the science of optics, yet in many popular works on this subject the really important points are omitted, and even in scientific works such points are too often left to be gathered from a formula. when the observer has learnt what it is that his instrument is actually to do for him, he will know how to estimate its performance, and how to vary the application of its powers--whether illuminating or magnifying--according to the nature of the object to be observed. let us consider what it is that limits the range of _natural_ vision applied to distant objects. what causes an object to become invisible as its distance increases? two things are necessary that an object should be visible. it must be _large_ enough to be appreciated by the eye, and it must _send light_ enough. thus increase of distance may render an object invisible, either through diminution of its apparent size, or through diminution in the quantity of light it sends to the eye, or through both these causes combined. a telescope, therefore, or (as its name implies) an instrument to render distant objects visible, must be both a magnifying and an illuminating instrument. [illustration: _fig. ._] let ef, fig. , be an object, not near to ab as in the figure, but so far off that the bounding lines from a and b would meet at the point corresponding to the point p. then if a large convex glass ab (called an _object-glass_) be interposed between the object and the eye, all those rays which, proceeding from p, fall on ab, will be caused to converge nearly to a point _p_. the same is true for every point of the object emf, and thus a small image, _emf_, will be formed. this image will not lie exactly on a flat surface, but will be curved about the point midway between a and b as a centre. now if the lens ab is removed, and an eye is placed at _m_ to view the distant object emf, those rays only from each point of the object which fall on the pupil of the eye (whose diameter is about equal to _mp_ suppose) will serve to render the object visible. on the other hand, every point of the image _emf_ has received the whole of the light gathered up by the large glass ab. if then we can only make this light _available_, it is clear that we shall have acquired a large increase of _light_ from the distant object. now it will be noticed that the light which has converged to _p_, diverges from _p_ so that an eye, placed that this diverging pencil of rays may fall upon it, would be too small to receive the whole of the pencil. or, if it did receive the whole of this pencil, it clearly could not receive the whole of the pencils proceeding from other parts of the image _emf_. _something_ would be gained, though, even in this case, since it is clear that an eye thus placed at a distance of ten inches from _emf_ (which is about the average distance of distinct vision) would not only receive much more light from the image _emf_, than it would from the object emf, but see the image much larger than the object. it is in this way that a simple object-glass forms a telescope, a circumstance we shall presently have to notice more at length. but we want to gain the full benefit of the light which has been gathered up for us by our object-glass. we therefore interpose a small convex glass _ab_ (called an eye-glass) between the image and the eye, at such a distance from the image that the divergent pencil of rays is converted into a pencil of parallel or nearly parallel rays. call this an emergent pencil. then all the emergent pencils now converge to a point on the axial line _m_m (produced beyond _m_), and an eye suitably placed can take in all of them at once. thus the whole, or a large part, of the image is seen at once. but the image is seen inverted as shown. this is the telescope, as it was first discovered, and such an arrangement would now be called a _simple astronomical telescope_. let us clearly understand what each part of the astronomical telescope does for us:-- the object-glass ab gives us an illuminated image, the amount of illumination depending on the size of the object-glass. the eye-glass enables us to examine the image microscopically. we may apply eye-glasses of different focal length. it is clear that the shorter the focal length of _ab_, the nearer must _ab_ be placed to the image, and the smaller will the emergent pencils be, but the greater the magnifying power of the eye-glass. if the emergent pencils are severally larger than the pupil of the eye, light is wasted at the expense of magnifying power. therefore the eye-glass should never be of greater focal length than that which makes the emergent pencils about equal in diameter to the pupil of the eye. on the other hand, the eye-glass must not be of such small focal length that the image appears indistinct and contorted, or dull for want of light. [illustration: _fig. ._] let us compare with the arrangement exhibited in fig. that adopted by galileo. surprise is sometimes expressed that this instrument, which in the hands of the great florentine astronomer effected so much, should now be known as the _non-astronomical telescope_. i think this will be readily understood when we compare the two arrangements. in the galilean telescope a small concave eye-glass, _ab_ (fig. ), is placed between the object-glass and the image. in fact, no image is allowed to be formed in this arrangement, but the convergent pencils are intercepted by the concave eye-glass, and converted into parallel emergent pencils. now in fig. the concave eye-glass is so placed as to receive only a part of the convergent pencil a _p_ b, and this is the arrangement usually adopted. by using a concave glass of shorter focus, which would therefore be placed nearer to _m p_, the whole of the convergent pencil might be received in this as in the former case. but then the axis of the emergent pencil, instead of returning (as we see it in fig. ) _towards_ the axis of the telescope, would depart as much _from_ that axis. thus there would be no point on the axis at which the eye could be so placed as to receive emergent pencils showing any considerable part of the object. the difference may be compared to that between looking through the small end of a cone-shaped roll of paper and looking through the large end; in the former case the eye sees at once all that is to be seen through the roll (supposed fixed in position), in the latter the eye may be moved about so as to command the same range of view, but _at any instant_ sees over a much smaller range. to return to the arrangement actually employed, which is illustrated by the common opera-glass. we see that the full illuminating power of the telescope is not brought into play. but this is not the only objection to the galilean telescope. it is obvious that if the part c d of the object-glass were covered, the point p would not be visible, whereas, in the astronomical arrangement no other effect is produced on the visibility of an object, by covering part of the object-glass, than a small loss of illumination. in other words, the dimensions of the field of view of a galilean telescope depend on the size of the object-glass, whereas in the astronomical telescope the field of view is independent of the size of the object-glass. the difference may be readily tested. if we direct an opera-glass upon any object, we shall find that any covering placed over a part of the object-glass _becomes visible_ when we look through the instrument, interfering therefore _pro tanto_ with the range of view. a covering similarly placed on any part of the object-glass of an astronomical telescope does not become visible when we look through the instrument. the distinction has a very important bearing on the theory of telescopic vision. in considering the application of the telescope to practical observation, the circumstance that in the galilean telescope no real image is formed, is yet more important. a real image admits of measurement, linear or angular, while to a _virtual_ image (such an image, for instance, as is formed by a common looking-glass) no such process can be applied. in simple observation the only noticeable effect of this difference is that, whereas in the astronomical telescope a _stop_ or diaphragm can be inserted in the tube so as to cut off what is called the _ragged edge_ of the field of view (which includes all the part not reached by _full pencils of light_ from the object-glass), there is no means of remedying the corresponding defect in the galilean telescope. it would be a very annoying defect in a telescope intended for astronomical observation, since in general the edge of the field of view is not perceptible at night. the unpleasant nature of the defect may be seen by looking through an opera-glass, and noticing the gradual fading away of light round the circumference of the field of view. the properties of reflection as well as of refraction have been enlisted into the service of the astronomical observer. the formation of an image by means of a concave mirror is exhibited in fig. . as the observer's head would be placed between the object and the mirror, if the image, formed as in fig. , were to be microscopically examined, various devices are employed in the construction of reflecting telescopes to avoid the loss of light which would result--a loss which would be important even with the largest mirrors yet constructed. thus, in gregory's telescope, a small mirror, having its concavity towards the great one, is placed in the axis of the tube and forms an image which is viewed through an aperture in the middle of the great mirror. a similar plan is adopted in cassegrain's telescope, a small convex mirror replacing the concave one. in newton's telescope a small inclined-plane reflector is used, which sends the pencil of light off at right-angles to the axis of the tube. in herschel's telescope the great mirror is inclined so that the image is formed at a slight distance from the axis of the telescope. in the two first cases the object is viewed in the usual or direct way, the image being erect in gregory's and inverted in cassegrain's. in the third the observer looks through the side of the telescope, seeing an inverted image of the object. in the last the observer sees the object inverted, but not altered as respects right and left. the last-mentioned method of viewing objects is the only one in which the observer's back is turned towards the object, yet this method is called the _front view_--apparently _quasi lucus a non lucendo_. [illustration: _fig. ._] it appears, then, that in all astronomical telescopes, reflecting or refracting, a _real image_ of an object is submitted to microscopical examination. of this fact the possessor of a telescope may easily assure himself; for if the eye-glass be removed, and a small screen be placed at the focus of the object-glass, there will appear upon the screen a small picture of any object towards which the tube is turned. but the image may be viewed in another way which requires to be noticed. if the eye, placed at a distance of five or six inches from the image, be directed down the tube, the image will be seen as before; in fact, just as a single convex lens of short focus is the simplest microscope, so a simple convex lens of long focus is the simplest telescope.[ ] but a singular circumstance will immediately attract the observer's notice. a real picture, or the image formed on the screen as in the former case, can be viewed at varying distances; but when we view the image directly, it will be found that for distinct vision the eye must be placed almost exactly at a fixed distance from the image. this peculiarity is more important than it might be thought at first sight. in fact, it is essential that the observer who would rightly apply the powers of his telescope, or fairly test its performance, should understand in what respect an image formed by an object-glass or object-mirror differs from a real object. the peculiarities to be noted are the _curvature_, _indistinctness_, and _false colouring_ of the image. the curvature of the image is the least important of the three defects named--a fortunate circumstance, since this defect admits neither of remedy nor modification. the image of a distant object, instead of lying in a plane, that is, forming what is technically called a _flat field_, forms part of a spherical surface whose centre is at the centre of the object-glass. hence the centre of the field of view is somewhat nearer to the eye than are the outer parts of the field. the amount of curvature clearly depends on the extent of the field of view, and therefore is not great in powerful telescopes. thus, if we suppose that the angular extent of the field is about half a degree (a large or low-power field), the centre is nearer than the boundary of the field by about - th part only of the field's diameter. the indistinctness of the image is partly due to the obliquity of the pencils which form parts of the image, and partly to what is termed _spherical aberration_. the first cause cannot be modified by the optician's skill, and is not important when the field of view is small. spherical aberration causes those parts of a pencil which fall near the boundary of a convex lens to converge to a nearer (_i.e._ shorter) focus than those which fall near the centre. this may be corrected by a proper selection of the forms of the two lenses which replace, in all modern telescopes, the single lens hitherto considered. the false colouring of the image is due to _chromatic aberration_. the pencil of light proceeding from a point, converges, not to one point, but to a short line of varying colour. thus a series of coloured images is formed, at different distances from the object-glass. so that, if a screen were placed to receive the mean image _in focus_, a coloured fringe due to the other images (_out of focus, and therefore too large_) would surround the mean image. newton supposed that it was impossible to get rid of this defect, and therefore turned his attention to the construction of reflectors. but the discovery that the _dispersive_ powers of different glasses are not proportional to their reflective powers, supplied opticians with the means of remedying the defect. let us clearly understand what is the discovery referred to. if with a glass prism of a certain form we produce a spectrum of the sun, this spectrum will be thrown a certain distance away from the point on which the sun's rays would fall if not interfered with. this distance depends on the _refractive_ power of the glass. the spectrum will have a certain length, depending on the _dispersive_ power of the glass. now, if we change our prism for another of exactly the same shape, but made of a different kind of glass, we shall find the spectrum thrown to a different spot. if it appeared that the length of the new spectrum was increased or diminished in exactly the same proportion as its distance from the line of the sun's direct light, it would have been hopeless to attempt to remedy chromatic aberration. newton took it for granted that this was so. but the experiments of hall and the dollonds showed that there is no such strict proportionality between the dispersive and refractive powers of different kinds of glass. it accordingly becomes possible to correct the chromatic aberration of one glass by superadding that of another. [illustration: _fig. ._] this is effected by combining, as shown in fig. , a convex lens of _crown_ glass with a concave lens of _flint_ glass, the convex lens being placed nearest to the object. a little colour still remains, but not enough to interfere seriously with the distinctness of the image. but even if the image formed by the object-glass were perfect, yet this image, viewed through a single convex lens of short focus placed as in fig. , would appear curved, indistinct, coloured, and also _distorted_, because viewed by pencils of light which do not pass through the centre of the eye-glass. these effects can be diminished (but not entirely removed _together_) by using an _eye-piece_ consisting of two lenses instead of a single eye-glass. the two forms of eye-piece most commonly employed are exhibited in figs. and . fig. is huyghens' eye-piece, called also the _negative_ eye-piece, because a real image is formed _behind_ the _field-glass_ (the lens which lies nearest to the object-glass). fig. represents ramsden's eye-piece, called also the _positive_ eye-piece, because the real image formed by the object-glass lies _in front of_ the field-glass. [illustration: _fig. ._] [illustration: _fig. ._] the course of a slightly oblique pencil through either eye-piece is exhibited in the figures. the lenses are usually plano-convex, the convexities being turned towards the object-glass in the negative eye-piece, and towards each other in the positive eye-piece. coddington has shown, however, that the best forms for the lenses of the negative eye-piece are those shown in fig. . the negative eye-piece, being achromatic, is commonly employed in all observations requiring distinct vision only. but as it is clearly unfit for observations requiring micrometrical measurement, or reference to fixed lines at the focus of the object-glass, the positive eye-piece is used for these purposes. for observing objects at great elevations the diagonal eye-tube is often convenient. its construction is shown in fig. . abc is a totally reflecting prism of glass. the rays from the object-glass fall on the face ab, are totally reflected on the face bc, and emerge through the face ac. in using this eye-piece, it must be remembered that it lengthens the sliding eye-tube, which must therefore be thrust further in, or the object will not be seen in focus. there is an arrangement by which the change of direction is made to take place between the two glasses of the eye-piece. with this arrangement (known as the _diagonal eye-piece_) no adjustment of the eye-tube is required. however, for amateurs' telescopes the more convenient arrangement is the diagonal eye-tube, since it enables the observer to apply any eye-piece he chooses, just as with the simple sliding eye-tube. [illustration: _fig. ._] we come next to the important question of the _mounting_ of our telescope. the best known, and, in some respects, the simplest method of mounting a telescope for general observation is that known as the _altitude-and-azimuth_ mounting. in this method the telescope is pointed towards an object by two motions,--one giving the tube the required _altitude_ (or elevation), the other giving it the required _azimuth_ (or direction as respects the compass points). for small alt-azimuths the ordinary pillar-and-claw stand is sufficiently steady. for larger instruments other arrangements are needed, both to give the telescope steadiness, and to supply slow movements in altitude and azimuth. the student will find no difficulty in understanding the arrangement of sliding-tubes and rack-work commonly adopted. this arrangement seems to me to be in many respects defective, however. the slow movement in altitude is not uniform, but varies in effect according to the elevation of the object observed. it is also limited in range; and quite a little series of operations has to be gone through when it is required to direct the telescope towards a new quarter of the heavens. however expert the observer may become by practice in effecting these operations, they necessarily take up some time (performed as they must be in the dark, or by the light of a small lantern), and during this time it often happens that a favourable opportunity for observation is lost. these disadvantages are obviated when the telescope is mounted in such a manner as is exhibited in fig. , which represents a telescope of my own construction. the slow movement in altitude is given by rotating the rod _he_, the endless screw in which turns the small wheel at _b_, whose axle in turn bears a pinion-wheel working in the teeth of the quadrant _a_. the slow movement in azimuth is given in like manner by rotating the rod _h'e'_, the lantern-wheel at the end of which turns a crown-wheel on whose axle is a pinion-wheel working in the teeth of the circle _c_. the casings at _e_ and _e'_, in which the rods _he_ and _h'e'_ respectively work, are so fastened by elastic cords that an upward pressure on the handle _h_, or a downward pressure on the handle _h'_, at once releases the endless screw or the crown-wheel respectively, so that the telescope can be swept at once through any desired angle in altitude or azimuth. this method of mounting has other advantages; the handles are conveniently situated and constant in position; also, as they do not work directly on the telescope, they can be turned without setting the tube in vibration. [illustration: _fig. ._] i do not recommend the mounting to be exactly as shown in fig. . that method is much too expensive for an alt-azimuth. but a simple arrangement of belted wheels in place of the toothed wheels _a_ and _c_ might very readily be prepared by the ingenious amateur telescopist; and i feel certain that the comfort and convenience of the arrangement would amply repay him for the labour it would cost him. my own telescope--though the large toothed-wheel and the quadrant were made inconveniently heavy (through a mistake of the workman who constructed the instrument)--worked as easily and almost as conveniently as an equatorial. still, it is well for the observer who wishes systematically to survey the heavens--and who can afford the expense--to obtain a well-mounted _equatorial_. in this method of mounting, the main axis is directed to the pole of the heavens; the other axis, at right angles to the first, carries the telescope-tube. one of the many methods adopted for mounting equatorials is that exhibited--with the omission of some minor details--in fig. . _a_ is the polar axis, _b_ is the axis (called the declination axis) which bears the telescope. the circles _c_ and _d_ serve to indicate, by means of verniers revolving with the axes, the motion of the telescope in right ascension and declination, respectively. the weight _w_ serves to counterpoise the telescope, and the screws _s_, _s_, _s_, _s_, serve to adjust the instrument so that the polar axis shall be in its proper position. the advantage gained by the equatorial method of mounting is that only one motion is required to follow a star. owing to the diurnal rotation of the earth, the stars appear to move uniformly in circles parallel to the celestial equator; and it is clear that a star so moving will be kept in the field of view, if the telescope, once directed to the star, be made to revolve uniformly and at a proper rate round the polar axis. [illustration: _fig. ._] the equatorial can be directed by means of the circles _c_ and _d_ to any celestial object whose right ascension and declination are known. on the other hand, to bring an object into the field of view of an alt-azimuth, it is necessary, either that the object itself should be visible to the naked eye, or else that the position of the object should be pretty accurately learned from star-maps, so that it may be picked up by the alt-azimuth after a little searching. a small telescope called a _finder_ is usually attached to all powerful telescopes intended for general observation. the finder has a large field of view, and is adjusted so as to have its axis parallel to that of the large telescope. thus a star brought to the centre of the large field of the finder (indicated by the intersection of two lines placed at the focus of the eye-glass) is at, or very near, the centre of the small field of the large telescope. if a telescope has no finder, it will be easy for the student to construct one for himself, and will be a useful exercise in optics. two convex lenses not very different in size from those shown in fig. , and placed as there shown--the distance between them being the sum of the focal lengths of the two glasses--in a small tube of card, wood, or tin, will serve the purpose of a finder for a small telescope. it can be attached by wires to the telescope-tube, and adjusted each night before commencing observation. the adjustment is thus managed:--a low power being applied to the telescope, the tube is turned towards a bright star; this is easily effected with a low power; then the finder is to be fixed, by means of its wires, in such a position that the star shall be in the centre of the field of the finder when also in the centre of the telescope's field. when this has been done, the finder will greatly help the observations of the evening; since with high powers much time would be wasted in bringing an object into the field of view of the telescope without the aid of a finder. yet more time would be wasted in the case of an object not visible to the naked eye, but whose position with reference to several visible stars is known; since, while it is easy to bring the point required to the centre of the _finder's_ field, in which the guiding stars are visible, it is very difficult to direct the _telescope's_ tube on a point of this sort. a card tube with wire fastenings, such as we have described, may appear a very insignificant contrivance to the regular observer, with his well-mounted equatorial and carefully-adjusted finder. but to the first attempts of the amateur observer it affords no insignificant assistance, as i can aver from my own experience. without it--a superior finder being wanting--our "half-hours" would soon be wasted away in that most wearisome and annoying of all employments, trying to "pick up" celestial objects. it behoves me at this point to speak of star-maps. such maps are of many different kinds. there are the observatory maps, in which the places of thousands of stars are recorded with an amazing accuracy. our beginner is not likely to make use of, or to want, such maps as these. then there are maps merely intended to give a good general idea of the appearance of the heavens at different hours and seasons. plate i. presents four maps of this sort; but a more complete series of eight maps has been published by messrs. walton and maberly in an octavo work; and my own 'constellation-seasons' give, at the same price, twelve quarto maps (of four of which those in plate i. are miniatures), showing the appearance of the sky at any hour from month to month, or on any night, at successive intervals of two hours. but maps intermediate in character to these and to observatory maps are required by the amateur observer. such are the society's six gnomonic maps, the set of six gnomonic maps in johnstone's 'atlas of astronomy,' and my own set of twelve gnomonic maps. the society's maps are a remarkably good set, containing on the scale of a ten-inch globe all the stars in the catalogue of the astronomical society (down to the fifth magnitude). the distortion, however, is necessarily enormous when the celestial sphere is presented in only six gnomonic maps. in my maps all the stars of the british association catalogue down to the fifth magnitude are included on the scale of a six-inch globe. the distortion is scarcely a fourth of that in the society's maps. the maps are so arranged that the relative positions of all the stars in each hemisphere can be readily gathered from a single view; and black duplicate-maps serve to show the appearance of the constellations. it is often convenient to make small maps of a part of the heavens we may wish to study closely. my 'handbook of the stars' has been prepared to aid the student in the construction of such maps. in selecting maps it is well to be able to recognise the amount of distortion and scale-variation. this may be done by examining the spaces included between successive parallels and meridians, near the edges and angles of the maps, and comparing these either with those in the centre of the map, or with the known figures and dimensions of the corresponding spaces on a globe. we may now proceed to discuss the different tests which the intending purchaser of a telescope should apply to the instrument. the excellence of an object-glass can be satisfactorily determined only by testing the performance of the telescope in the manner presently to be described. but it is well to examine the quality of the glass as respects transparency and uniformity of texture. bubbles, scratches, and other such defects, are not very important, since they do not affect the distinctness of the field as they would in a galilean telescope,--a little light is lost, and that is all. the same remark applies to dust upon the glass. the glass should be kept as free as possible from dirt, damp, or dust, but it is not advisable to remove every speck which, despite such precaution, may accidentally fall upon the object-glass. when it becomes necessary to clean the glass, it is to be noted that the substance used should be soft, perfectly dry, and free from dust. silk is often recommended, but some silk is exceedingly objectionable in texture,--old silk, perfectly soft to the touch, is perhaps as good as anything. if the dust which has fallen on the glass is at all gritty, the glass will suffer by the method of cleaning commonly adopted, in which the dust is _gathered up_ by pressure. the proper method is to clean a small space near the edge of the glass, and to _sweep_ from that space as centre. in this way the dust is _pushed before_ the silk or wash-leather, and does not cut the glass. it is well always to suspect the presence of gritty dust, and adopt this cautious method of cleaning. the two glasses should on no account be separated. in examining an eye-piece, the quality of the glass should be noted, and care taken that both glasses (but especially the field-glass) are free from the least speck, scratch, or blemish of any kind, for these defects will be exhibited in a magnified state in the field of view. hence the eye-pieces require to be as carefully preserved from damp and dust as the object-glass, and to be more frequently cleaned. the tube of the telescope should be light, but strong, and free from vibration. its quality in the last respect can be tested by lightly striking it when mounted; the sound given out should be dead or non-resonant. the inside of the tube must absorb extraneous light, and should therefore be coloured a dull black; and stops of varying radius should be placed along its length with the same object. sliding tubes, rack-work, etc., should work closely, yet easily. the telescope should be well balanced for vision with the small astronomical eye-pieces. but as there is often occasion to use appliances which disturb the balance, it is well to have the means of at once restoring equilibrium. a cord ring running round the tube (pretty tightly, so as to rest still when the tube is inclined), and bearing a small weight, will be all that is required for this purpose; it must be slipped along the tube until the tube is found to be perfectly balanced. nothing is more annoying than, after getting a star well in the field, to see the tube shift its position through defective balance, and thus to have to search again for the star. even with such an arrangement as is shown in fig. , though the tube cannot readily shift its position, it is better to have it well balanced. the quality of the stand has a very important influence on the performance of a telescope. in fact, a moderately good telescope, mounted on a steady stand, working easily and conveniently, will not only enable the observer to pass his time much more pleasantly, but will absolutely exhibit more difficult objects than a finer instrument on a rickety, ill-arranged stand. a good observing-chair is also a matter of some importance, the least constraint or awkwardness of position detracting considerably from the power of distinct vision. such, at least, is my own experience. but the mere examination of the glasses, tube, mounting, &c., is only the first step in the series of tests which should be applied to a telescope, since the excellence of the instrument depends, not on its size, the beauty of its mounting, or any extraneous circumstances, but on its performance. the observer should first determine whether the chromatic aberration is corrected. to ascertain this the telescope should be directed to the moon, or (better) to jupiter, and accurately focussed for distinct vision. if, then, on moving the eye-piece towards the object-glass, a ring of purple appears round the margin of the object, and on moving the eye-glass in the contrary direction a ring of green, the chromatic aberration is corrected, since these are the colours of the secondary spectrum. to determine whether the spherical aberration is corrected, the telescope should be directed towards a star of the third or fourth magnitude, and focussed for distinct vision. a cap with an aperture of about one-half its diameter should then be placed over the object-glass. if no new adjustment is required for distinct vision, the spherical aberration is corrected, since the mean focal length and the focal length of the central rays are equal. if, when the cap is on, the eye-piece has to be pulled out for distinct vision, the spherical aberration has not been fully corrected; if the eye-piece has to be pushed in, the aberration has been over-corrected. as a further test, we may cut off the central rays, by means of a circular card covering the middle of the object-glass, and compare the focal length for distinct vision with the focal length when the cap is applied. the extent of the spherical aberration may be thus determined; but if the first experiment gives a satisfactory result, no other is required. a star of the first magnitude should next be brought into the field of view. if an irradiation from one side is perceived, part of the object-glass has not the same refractive power as the rest; and the part which is defective can be determined by applying in different positions a cap which hides half the object-glass. if the irradiation is double, it will probably be found that the object-glass has been too tightly screwed, and the defect will disappear when the glass is freed from such undue pressure. if the object-glass is not quite at right angles to the axis of the tube, or if the eye-tube is at all inclined, a like irradiation will appear when a bright star is in the field. the former defect is not easily detected or remedied; nor is it commonly met with in the work of a careful optician. the latter defect may be detected by cutting out three circular cards of suitable size with a small aperture at the centre of each, and inserting one at each end of the eye-tube, and one over the object-glass. if the tube is rightly placed the apertures will of course lie in a right line, so that it will be possible to look through all three at once. if not, it will be easy to determine towards what part of the object-glass the eye-tube is directed, and to correct the position of the tube accordingly. the best tests for determining the defining power of a telescope are close double or multiple stars, the components of which are not very unequal. the illuminating power should be tested by directing the telescope towards double or multiple stars having one or more minute components. many of the nebulæ serve as tests both for illumination and defining power. as we proceed we shall meet with proper objects for testing different telescopes. for the present, let the following list suffice. it is selected from admiral smyth's tests, obtained by diminishing the aperture of a -in. telescope having a focal length of - / feet: a two-inch aperture, with powers of from to , should exhibit [alpha] piscium ( "· ). | [delta] cassiopeiæ ( "· ), | mag. ( and - / ) [gamma] leonis ( "· ). | polaris ( "· ), mag. ( - / | and - / ) a four-inch, powers to , should exhibit [xi] ursæ majoris ( "· ). | [sigma] cassiopeiæ ( "· ), | mag. ( and ). [gamma] ceti ( "· ). | [delta] geminorum ( "· ), | mag. ( and ). the tests in the first column are for definition, those in the second for illumination. it will be noticed that, though in the case of polaris the smaller aperture may be expected to show the small star of less than the th magnitude, a larger aperture is required to show the th magnitude component of [sigma] cassiopeiæ, on account of the greater closeness of this double. in favourable weather the following is a good general test of the performance of a telescope:--a star of the rd or th magnitude at a considerable elevation above the horizon should exhibit a small well defined disc, surrounded by two or three fine rings of light. a telescope should not be mounted within doors, if it can be conveniently erected on solid ground, as every movement in the house will cause the instrument to vibrate unpleasantly. further, if the telescope is placed in a warm room, currents of cold air from without will render observed objects hazy and indistinct. in fact, sir w. herschel considered that a telescope should not even be erected near a house or elevation of any kind round which currents of air are likely to be produced. if a telescope is used in a room, the temperature of the room should be made as nearly equal as possible to that of the outer air. when a telescope is used out of doors a 'dew-cap,' that is, a tube of tin or pasteboard, some ten or twelve inches long, should be placed on the end of the instrument, so as to project beyond the object-glass. for glass is a good radiator of heat, so that dew falls heavily upon it, unless the radiation is in some way checked. the dew-cap does this effectually. it should be blackened within, especially if made of metal. "after use," says old kitchener, "the telescope should be kept in a warm place long enough for any moisture on the object-glass to evaporate." if damp gets between the glasses it produces a fog (which opticians call a sweat) or even a seaweed-like vegetation, by which a valuable glass may be completely ruined. the observer should not leave to the precious hours of the night the study of the bearing and position of the objects he proposes to examine. this should be done by day--an arrangement which has a twofold advantage,--the time available for observation is lengthened, and the eyes are spared sudden changes from darkness to light, and _vice versâ_. besides, the eye is ill-fitted to examine difficult objects, after searching by candle-light amongst the minute details recorded in maps or globes. of the effect of rest to the eye we have an instance in sir j. herschel's rediscovery of the satellites of uranus, which he effected after keeping his eyes in darkness for a quarter of an hour. kitchener, indeed, goes so far as to recommend (with a _crede experto_) an _interval of sleep_ in the darkness of the observing-room before commencing operations. i have never tried the experiment, but i should expect it to have a bad rather than a good effect on the eyesight, as one commonly sees the eyes of a person who has been sleeping in his day-clothes look heavy and bloodshot. the object or the part of an object to be observed should be brought as nearly as possible to the centre of the field of view. when there is no apparatus for keeping the telescope pointed upon an object, the best plan is so to direct the telescope by means of the finder, that the object shall be just out of the field of view, and be brought (by the earth's motion) across the centre of the field. thus the vibrations which always follow the adjustment of the tube will have subsided before the object appears. the object should then be intently watched during the whole interval of its passage across the field of view. it is important that the student should recognise the fact that the highest powers do not necessarily give the best views of celestial objects. high powers in all cases increase the difficulty of observation, since they diminish the field of view and the illumination of the object, increase the motion with which (owing to the earth's motion) the image moves across the field, and magnify all defects due to instability of the stand, imperfection of the object-glass, or undulation of the atmosphere. a good object-glass of three inches aperture will in very favourable weather bear a power of about , when applied to the observation of close double or multiple stars, but for all other observations much lower powers should be used. nothing but failure and annoyance can follow the attempt to employ the highest powers on unsuitable objects or in unfavourable weather. the greatest care should be taken in focussing the telescope. when high powers are used this is a matter of some delicacy. it would be well if the eye-pieces intended for a telescope were so constructed that when the telescope is focussed for one, this might be replaced by any other without necessitating any use of the focussing rack-work. this could be readily effected by suitably placing the shoulder which limits the insertion of the eye-piece. it will be found that, even in the worst weather for observation, there are instants of distinct vision (with moderate powers) during which the careful observer may catch sight of important details; and, similarly, in the best observing weather, there are moments of unusually distinct vision well worth patient waiting for, since in such weather alone the full powers of the telescope can be employed. the telescopist should not be deterred from observation by the presence of fog or haze, since with a hazy sky definition is often singularly good. the observer must not expect distinct vision of objects near the horizon. objects near the eastern horizon during the time of morning twilight are especially confused by atmospheric undulations; in fact, early morning is a very unfavourable time for the observation of all objects. the same rules which we have been applying to refractors, serve for reflectors. the performance of a reflector will be found to differ in some respects, however, from that of a refractor. mr. dawes is, we believe, now engaged in testing reflectors, and his unequalled experience of refractors will enable him to pronounce decisively on the relative merits of the two classes of telescopes. we have little to say respecting the construction of telescopes. whether it is advisable or not for an amateur observer to attempt the construction of his own telescope is a question depending entirely on his mechanical ability and ingenuity. my own experience of telescope construction is confined to the conversion of a -feet into a - / -feet telescope. this operation involved some difficulties, since the aperture had to be increased by about an inch. i found a tubing made of alternate layers of card and calico well pasted together, to be both light and strong. but for the full length of tube i think a core of metal is wanted. a learned and ingenious friend, mr. sharp, fellow of st. john's college, informs me that a tube of tin, covered with layers of brown paper, well pasted and thicker near the middle of the tube, forms a light and strong telescope-tube, almost wholly free from vibration. suffer no inexperienced person to deal with your object-glass. i knew a valuable glass ruined by the proceedings of a workman who had been told to attach three pieces of brass round the cell of the double lens. what he had done remained unknown, but ever after a wretched glare of light surrounded all objects of any brilliancy. one word about the inversion of objects by the astronomical telescope. it is singular that any difficulty should be felt about so simple a matter, yet i have seen in the writings of more than one distinguished astronomer, wholly incorrect views as to the nature of the inversion. one tells us that to obtain the correct presentation from a picture taken with a telescope, the view should be inverted, held up to the light, and looked at from the back of the paper. another tells us to invert the picture and hold it opposite a looking-glass. neither method is correct. the simple correction wanted is to hold the picture upside down--the same change which brings the top to the bottom brings the right to the left, _i.e._, fully corrects the inversion. in the case, however, of a picture taken by an herschelian reflector, the inversion not being complete, a different method must be adopted. in fact, either of the above-named processes, incorrect for the ordinary astronomical, would be correct for the herschelian telescope. the latter inverts but does not reverse right and left; therefore after inverting our picture we must interchange right and left because they have been reversed by the inversion. this is effected either by looking at the picture from behind, or by holding it up to a mirror. [illustration: plate ii.] chapter ii. a half-hour with orion, lepus taurus, etc. any of the half-hours here assigned to the constellation-seasons may be taken first, and the rest in seasonal or cyclic order. the following introductory remarks are applicable to each:-- if we stand on an open space, on any clear night, we see above us the celestial dome spangled with stars, apparently fixed in position. but after a little time it becomes clear that these orbs are slowly shifting their position. those near the eastern horizon are rising, those near the western setting. careful and continuous observation would show that the stars are all moving in the same way, precisely, as they would if they were fixed to the concave surface of a vast hollow sphere, and this sphere rotated about an axis. this axis, in our latitude, is inclined about - / ° to the horizon. of course only one end of this imaginary axis can be above our horizon. this end lies very near a star which it will be well for us to become acquainted with at the beginning of our operations. it lies almost exactly towards the north, and is raised from ° to ° (according to the season and hour) above the horizon. there is an easy method of finding it. we must first find the greater bear. it will be seen from plate , that on a spring evening the seven conspicuous stars of this constellation are to be looked for towards the north-east, about half way between the horizon and the point overhead (or _zenith_), the length of the set of stars being vertical. on a summer's evening the great bear is nearly overhead. on an autumn evening he is towards the north-west, the length of the set of seven being somewhat inclined to the horizon. finally, on a winter's evening, he is low down towards the north, the length of the set of seven stars being nearly in a horizontal direction. having found the seven stars, we make use of the pointers [alpha] and [beta] (shown in plate ) to indicate the place of the pole-star, whose distance from the pointer [alpha] is rather more than three times the distance of [alpha] from [beta]. now stand facing the pole-star. then all the stars are travelling round that star _in a direction contrary to that in which the hands of a watch move_. thus the stars below the pole are moving _towards the right_, those above the pole _towards the left_, those to the right of the pole _upwards_, those to the left of the pole _downwards_. next face the south. then all the stars on our left, that is, towards the east, are rising slantingly towards the south; those due south are moving horizontally to the right, that is, towards the west; and those on our right are passing slantingly downwards towards the west. it is important to familiarise ourselves with these motions, because it is through them that objects pass out of the field of view of the telescope, and by moving the tube in a proper direction we can easily pick up an object that has thus passed away, whereas if we are not familiar with the varying motions in different parts of the celestial sphere, we may fail in the attempt to immediately recover an object, and waste time in the search for it. the consideration of the celestial motions shows how advantageous it is, when using an alt-azimuth, to observe objects as nearly as possible due south. of course in many cases this is impracticable, because a phenomenon we wish to watch may occur when an object is not situated near the meridian. but in examining double stars there is in general no reason for selecting objects inconveniently situated. we can wait till they come round to the meridian, and then observe them more comfortably. besides, most objects are higher, and therefore better seen, when due south. northern objects, and especially those within the circle of perpetual apparition, often culminate (that is, cross the meridian, or north and south line) at too great a height for comfortable vision. in this case we should observe them towards the east or west, and remember that in the first case they are rising, and in the latter they are setting, and that in both cases they have also a motion from left to right. if we allow an object to pass right across the field of view (the telescope being fixed), the apparent direction of its motion is the exact reverse of the true direction of the star's motion. this will serve as a guide in shifting the alt-azimuth after a star has passed out of the field of view. the following technical terms must be explained. that part of the field of view towards which the star appears to move is called the _preceding_ part of the field, the opposite being termed the _following_ part. the motion for all stars, except those lying in an oval space extending from the zenith to the pole of the heavens, is more or less from right to left (in the inverted field). now, if we suppose a star to move along a diameter of the field so as to divide the field into two semicircles, then in all cases in which this motion takes places from right to left, that semicircle which contains the lowest point (apparently) of the field is the _northern_ half, the other is the _southern_ half. over the oval space just mentioned the reverse holds. thus the field is divided into four quadrants, and these are termed _north following_ (_n.f._) and _south following_ (_s.f._); _north preceding_ (_n.p._), and _south preceding_ (_s.p._). the student can have no difficulty in interpreting these terms, since he knows which is the following and which the preceding _semicircle_, which the northern and which the southern. in the figures of plates and , the letters _n.f._, _n.p._, &c., are affixed to the proper quadrants. it is to be remembered that the quadrants thus indicated are measured either way from the point and feather of the diametral arrows. next, of the apparent annual motion of the stars. this takes place in exactly the same manner as the daily motion. if we view the sky at eight o'clock on any day, and again at the same hour one month later, we shall find that at the latter observation (as compared with the former) the heavens appear to have rotated by the _twelfth part_ of a complete circumference, and the appearance presented is precisely the same as we should have observed had we waited for two hours (the _twelfth part_ of a day) on the day of the first observation. * * * * * our survey of the heavens is supposed to be commenced during the first quarter of the year, at ten o'clock on the th of january, or at nine on the th of february, or at eight on the th of february, or at seven on the th of march, or at hours intermediate to these on intermediate days. we look first for the great bear towards the north-east, as already described, and thence find the pole-star; turning towards which we see, towards the right and downwards, the two guardians of the pole ([beta] and [gamma] ursæ minoris). immediately under the pole-star is the dragon's head, a conspicuous diamond of stars. just on the horizon is vega, scintillating brilliantly. overhead is the brilliant capella, near which the milky way is seen passing down to the horizon on either side towards the quarters s.s.e. and n.n.w. for the present our business is with the southern heavens, however. facing the south, we see a brilliant array of stars, sirius unmistakeably overshining the rest. orion is shining in full glory, his leading brilliant, betelgeuse[ ] being almost exactly on the meridian, and also almost exactly half way between the horizon and the zenith. in plate is given a map of this constellation and its neighbourhood. let us first turn the tube on sirius. it is easy to get him in the field without the aid of a finder. the search will serve to illustrate a method often useful when a telescope has no finder. having taking out the eye-piece--a low-power one, suppose--direct the tube nearly towards sirius. on looking through it, a glare of light will be seen within the tube. now, if the tube be slightly moved about, the light will be seen to wax and wane, according as the tube is more or less accurately directed. following these indications, it will be found easy to direct the tube, so that the object-glass shall appear _full of light_. when this is done, insert the eye-piece, and the star will be seen in the field. but the telescope is out of focus, therefore we must turn the small focussing screw. observe the charming chromatic changes--green, and red, and blue light, purer than the hues of the rainbow, scintillating and coruscating with wonderful brilliancy. as we get the focus, the excursions of these light flashes diminish until--if the weather is favourable--the star is seen, still scintillating, and much brighter than to the naked eye, but reduced to a small disc of light, surrounded (in the case of so bright a star as sirius) with a slight glare. if after obtaining the focus the focussing rack work be still turned, we see a coruscating image as before. in the case of a very brilliant star these coruscations are so charming that we may be excused for calling the observer's attention to them. the subject is not without interest and difficulty as an optical one. but the astronomer's object is to get rid of all these flames and sprays of coloured light, so that he has very little sympathy with the admiration which wordsworth is said to have expressed for out-of-focus views of the stars. we pass to more legitimate observations, noticing in passing that sirius is a double star, the companion being of the tenth magnitude, and distant about ten seconds from the primary. but our beginner is not likely to see the companion, which is a very difficult object, vowing to the overpowering brilliancy of the primary. orion affords the observer a splendid field of research. it is a constellation rich in double and multiple stars, clusters, and nebulæ. we will begin with an easy object. the star [delta] (plate ), or _mintaka_, the uppermost of the three stars forming the belt, is a wide double. the primary is of the second magnitude, the secondary of the seventh, both being white. the star [alpha] (_betelgeuse_) is an interesting object, on account of its colour and brilliance, and as one of the most remarkable variables in the heavens. it was first observed to be variable by sir john herschel in . at this period its variations were "most marked and striking." this continued until , when the changes became "much less conspicuous. in january, , they had recommenced, and on december th, , mr. fletcher observed [alpha] orionis brighter than capella, and actually the largest star in the northern hemisphere." that a star so conspicuous, and presumably so large, should present such remarkable variations, is a circumstance which adds an additional interest to the results which have rewarded the spectrum-analysis of this star by mr. huggins and professor miller. it appears that there is decisive evidence of the presence in this luminary of many elements known to exist in our own sun; amongst others are found sodium, magnesium, calcium, iron, and bismuth. hydrogen appears to be absent, or, more correctly, there are no lines in the star's spectrum corresponding to those of hydrogen in the solar spectrum. secchi considers that there is evidence of an actual change in the spectrum of the star, an opinion in which mr. huggins does not coincide. in the telescope betelgeuse appears as "a rich and brilliant gem," says lassell, "a rich topaz, in hue and brilliancy differing from any that i have seen." turn next to [beta] (rigel), the brightest star below the belt. this is a very noted double, and will severely test our observer's telescope, if small. the components are well separated (see plate ), compared with many easier doubles; the secondary is also of the seventh magnitude, so that neither as respects closeness nor smallness of the secondary, is rigel a difficult object. it is the combination of the two features which makes it a test-object. kitchener says a - / -inch object-glass should show rigel double; in earlier editions of his work he gave - / -inches as the necessary aperture. smyth mentions rigel as a test for a -inch aperture, with powers of from to . a -inch aperture, however, will certainly show the companion. rigel is an orange star, the companion blue. turn next to [lambda] the northernmost of the set of three stars in the head of orion. this is a triple star, though an aperture of inches will show it only as a double. the components are " apart, the colours pale white and violet. with the full powers of a - / -inch glass a faint companion may be seen above [lambda]. the star [zeta], the lowest in the belt, may be tried with a - / -inch glass. it is a close double, the components being nearly equal, and about - / " apart (see plate ). for a change we will now try our telescope on a nebula, selecting the great nebula in the sword. the place of this object is indicated in plate . there can be no difficulty in finding it since it is clearly visible to the naked eye on a moonless night--the only sort of night on which an observer would care to look at nebulæ. a low power should be employed. the nebula is shown in plate as i have seen it with a -inch aperture. we see nothing of those complex streams of light which are portrayed in the drawings of herschel, bond, and lassell, but enough to excite our interest and wonder. what is this marvellous light-cloud? one could almost imagine that there was a strange prophetic meaning in the words which have been translated "canst thou loose the bands of orion?" telescope after telescope had been turned on this wonderful object with the hope of resolving its light into stars. but it proved intractable to herschel's great reflector, to lassell's -feet reflector, to lord rosse's -feet reflector, and even partially to the great -feet reflector. then we hear of its supposed resolution into stars, lord rosse himself writing to professor nichol, in , "i may safely say there can be little, if any, doubt as to the resolvability of the nebula;--all about the trapezium is a mass of stars, the rest of the nebula also abounding with stars, and exhibiting the characteristics of resolvability strongly marked." it was decided, therefore, that assuredly the great nebula is a congeries of stars, and not a mass of nebulous matter as had been surmised by sir w. herschel. and therefore astronomers were not a little surprised when it was proved by mr. huggins' spectrum-analysis that the nebula consists of gaseous matter. how widely extended this gaseous universe may be we cannot say. the general opinion is that the nebulæ are removed far beyond the fixed stars. if this were so, the dimensions of the orion nebula would be indeed enormous, far larger probably than those of the whole system whereof our sun is a member. i believe this view is founded on insufficient evidence, but this would not be the place to discuss the subject. i shall merely point out that the nebula occurs in a region rich in stars, and if it is not, like the great nebula in argo, clustered around a remarkable star, it is found associated in a manner which i cannot look upon as accidental with a set of small-magnitude stars, and notably with the trapezium which surrounds that very remarkable black gap within the nebula. the fact that the nebula shares the proper motion of the trapezium appears inexplicable if the nebula is really far out in space beyond the trapezium. a very small proper motion of the trapezium (alone) would long since have destroyed the remarkable agreement in the position of the dark gap and the trapezium which has been noticed for so many years. but whether belonging to our system or far beyond it, the great nebula must have enormous dimensions. a vast gaseous system it is, sustained by what arrangements or forces we cannot tell, nor can we know what purposes it subserves. mr. huggins' discovery that comets have gaseous nuclei, (so far as the two he has yet examined show) may suggest the speculation that in the orion nebula we see a vast system of comets travelling in extensive orbits around nuclear stars, and so slowly as to exhibit for long intervals of time an unchanged figure. "but of such speculations" we may say with sir j. herschel "there is no end." to return to our telescopic observations:--the trapezium affords a useful test for the light-gathering power of the telescope. large instruments exhibit nine stars. but our observer may be well satisfied with his instrument and his eye-sight if he can see five with a - / -inch aperture.[ ] a good -inch glass shows four distinctly. but with smaller apertures only three are visible. the whole neighbourhood of the great nebula will well repay research. the observer may sweep over it carefully on any dark night with profit. above the nebula is the star-cluster h. the star [iota] (double as shown in plate ) below the nebula is involved in a strong nebulosity. and in searching over this region we meet with delicate double, triple, and multiple stars, which make the survey interesting with almost any power that may be applied. above the nebula is the star [sigma], a multiple star. to an observer with a good - / -inch glass [sigma] appears as an octuple star. it is well seen, however, as a fine multiple star with a smaller aperture. some of the stars of this group appear to be variable. the star [rho] orionis is an unequal, easy double, the components being separated by nearly seven seconds. the primary is orange, the smaller star smalt-blue (see plate ). the middle star of the belt ([epsilon]) has a distant blue companion. this star, like [iota], is nebulous. in fact, the whole region within the triangle formed by stars [gamma], [kappa] and [beta] is full of nebulous double and multiple stars, whose aggregation in this region i do not consider wholly accidental. we have not explored half the wealth of orion, but leave much for future observation. we must turn, however, to other constellations. below orion is lepus, the hare, a small constellation containing some remarkable doubles. among these we may note [xi], a white star with a scarlet companion; [gamma], a yellow and garnet double; and [iota], a double star, white and pale violet, with a distant red companion. the star [kappa] leporis is a rather close double, white with a small green companion. the intensely red star r leporis (a variable) will be found in the position indicated in the map. still keeping within the boundary of our map, we may next turn to the fine cluster h (vii.) in monoceros. this cluster is visible to the naked eye, and will be easily found. the nebula h (iv.) is a remarkable one with a powerful telescope. the star monocerotis is a fine triple star described by the elder herschel as one of the finest sights in the heavens. our observer, however, will see it as a double (see plate ). [delta] monocerotis is an easy double, yellow and lavender. we may now leave the region covered by the map and take a survey of the heavens for some objects well seen at this season. towards the south-east, high up above the horizon, we see the twin-stars castor and pollux. the upper is castor, the finest double star visible in the northern heavens. the components are nearly equal and rather more than " apart (see plate ). both are white according to the best observers, but the smaller is thought by some to be slightly greenish. pollux is a coarse but fine triple star (in large instruments multiple). the components orange, grey, and lilac. there are many other fine objects in gemini, but we pass to cancer. the fine cluster præsepe in cancer may easily be found as it is distinctly visible to the naked eye in the position shown in plate , map i. in the telescope it is seen as shown in plate . the star [iota] cancri is a wide double, the colours orange and blue. procyon, the first-magnitude star between præsepe and sirius, is finely coloured--yellow with a distant orange companion, which appears to be variable. below the twins, almost in a line with them, is the star [alpha] hydræ, called al fard, or "the solitary one." it is a nd magnitude variable. i mention it, however, not on its own account, but as a guide to the fine double [epsilon] hydræ. this star is the middle one of a group of three, lying between pollux and al fard rather nearer the latter. the components of [epsilon] hydræ are separated by about - / " (see plate ). the primary is of the fourth, the companion of the eighth magnitude; the former is yellow, the latter a ruddy purple. the period of [epsilon] hydræ is about years. the constellation leo minor, now due east and about midway between the horizon and the zenith, is well worth sweeping over. it contains several fine fields. let us next turn to the western heavens. here there are some noteworthy objects. to begin with, there are the pleiades, showing to the naked eye only six or seven stars. in the telescope the pleiades appear as shown in plate . the hyades also show some fine fields with low powers. aldebaran, the principal star of the hyades, as also of the constellation taurus, is a noted red star. it is chiefly remarkable for the close spectroscopic analysis to which it has been subjected by messrs. huggins and miller. unlike betelgeuse, the spectrum of aldebaran exhibits the lines corresponding to hydrogen, and no less than eight metals--sodium, magnesium, calcium, iron, bismuth, tellurium, antimony, and mercury, are proved to exist in the constitution of this brilliant red star. on the right of aldebaran, in the position indicated in plate , map i., are the stars [zeta] and [beta] tauri. if with a low power the observer sweep from [zeta] towards [beta], he will soon find--not far from [zeta] (at a distance of about one-sixth of the distance separating [beta] from [zeta]), the celebrated crab nebula, known as m. this was the first nebula discovered by messier, and its discovery led to the formation of his catalogue of nebulæ. in a small telescope this object appears as a nebulous light of oval form, no traces being seen of the wisps and sprays of light presented in lord rosse's well known picture of the nebula. here i shall conclude the labours of our first half-hour among the stars, noticing that the examination of plate will show what other constellations besides those here considered are well situated for observation at this season. it will be remarked that many constellations well seen in the third half-hour (chapter iv.) are favourably seen in the first also, and _vice versâ_. for instance, the constellation ursa major well-placed towards the north-east in the first quarter of the year, is equally well-placed towards the north-west in the third, and similarly of the constellation cassiopeia. the same relation connects the second and fourth quarters of the year. [illustration: plate iii.] chapter iii. a half-hour with lyra, hercules, corvus, crater, etc. the observations now to be commenced are supposed to take place during the second quarter of the year,--at ten o'clock on the th of april, or at nine on the th of may, or at eight on the st of may, or at seven on the th of june, or at hours intermediate to these on intermediate days. we again look first for the great bear, now near the zenith, and thence find the pole-star. turning towards the north, we see cassiopeia between the pole-star and the horizon. towards the north-west is the brilliant capella, and towards the north-east the equally brilliant vega, beneath which, and somewhat northerly, is the cross in cygnus. the milky way passes from the eastern horizon towards the north (low down), and so round to the western horizon. in selecting a region for special observation, we shall adopt a different plan from that used in the preceding "half-hour." the region on the equator and towards the south is indeed particularly interesting, since it includes the nebular region in virgo. within this space nebulæ are clustered more closely than over any corresponding space in the heavens, save only the greater magellanic cloud. but to the observer with telescopes of moderate power these nebulæ present few features of special interest; and there are regions of the sky now well situated for observation, which, at most other epochs are either low down towards the horizon or inconveniently near to the zenith. we shall therefore select one of these, the region included in the second map of plate , and the neighbouring part of the celestial sphere. at any of the hours above named, the constellation hercules lies towards the east. a quadrant taken from the zenith to the eastern horizon passes close to the last star ([eta]) of the great bear's tail, through [beta], a star in bootes' head, near [beta] herculis, between the two "alphas" which mark the heads of hercules and ophiuchus, and so past [beta] ophiuchi, a third-magnitude star near the horizon. and here we may turn aside for a moment to notice the remarkable vertical row of six conspicuous stars towards the east-south-east; these are, counting them in order from the horizon, [zeta], [epsilon], and [delta] ophiuchi, [epsilon], [alpha], and [delta] serpentis. let the telescope first be directed towards vega. this orb presents a brilliant appearance in the telescope. its colour is a bluish-white. in an ordinary telescope vega appears as a single star, but with a large object-glass two distant small companions are seen. a nine-inch glass shows also two small companions within a few seconds of vega. in the great harvard refractor vega is seen with no less than thirty-five companions. i imagine that all these stars, and others which can be seen in neighbouring fields, indicate the association of vega with the neighbouring stream of the milky way. let our observer now direct his telescope to the star [epsilon] lyræ. or rather, let him first closely examine this star with the naked eye. the star is easily identified, since it lies to the left of vega, forming with [zeta] a small equilateral triangle. a careful scrutiny suffices to indicate a peculiarity in this star. if our observer possesses very good eye-sight, he will distinctly recognise it as a "naked-eye double"; but more probably he will only notice that it appears lengthened in a north and south direction.[ ] in the finder the star is easily divided. applying a low power to the telescope itself, we see [epsilon] lyræ as a wide double, the line joining the components lying nearly north and south. the southernmost component (the upper in the figure) is called [epsilon]^{ }, the other [epsilon]^{ }. seen as a double, both components appear white. now, if the observer's telescope is sufficiently powerful, each of the components may be seen to be itself double. first try [epsilon]^{ }, the northern pair. the line joining the components is directed as shown in plate . the distance between them is "· , their magnitudes and - / , and their colours yellow and ruddy. if the observer succeeds in seeing [epsilon]^{ } fairly divided, he will probably not fail in detecting the duplicity of [epsilon]^{ }, though this is a rather closer pair, the distance between the components being only "· . the magnitudes are and - / , both being white. between [epsilon]^{ } and [epsilon]^{ } are three faint stars, possibly forming with the quadruple a single system. let us next turn to the third star of the equilateral triangle mentioned above--viz. to the star [zeta] lyræ. this is a splendid but easy double. it is figured in plate , but it must be noticed that the figure of [zeta] and the other nine small figures are not drawn on the same scale as [epsilon] lyræ. the actual distance between the components of [zeta] lyra is ", or about one-fourth of the distance separating [epsilon]^{ } from [epsilon]^{ }. the components of [zeta] are very nearly equal in magnitude, in colour topaz and green, the topaz component being estimated as of the fifth magnitude, the green component intermediate between the fifth and sixth magnitudes. we may now turn to a star not figured in the map, but readily found. it will be noticed that the stars [epsilon], [alpha], [beta], and [gamma] form, with two small stars towards the left, a somewhat regular hexagonal figure--a hexagon, in fact, having three equal long sides and three nearly equal short sides alternating with the others. the star [eta] lyræ forms the angle next to [epsilon]. it is a wide and unequal double, as figured in plate . the larger component is azure blue; the smaller is violet, and, being only of the ninth magnitude, is somewhat difficult to catch with apertures under inches. the star [delta]^{ } lyræ is orange, [delta]^{ } blue. in the same field with these are seen many other stars. the stars [gamma]^{ } and [gamma]^{ } may also be seen in a single field, the distance between them being about half the moon's mean diameter. the former is quadruple, the components being yellow, bluish, pale blue, and blue. turn next to the stars [beta] and [gamma] lyræ, the former a multiple, the latter an unequal double star. it is not, however, in these respects that these stars are chiefly interesting, but for their variability. the variability of [gamma] has not indeed been fully established, though it is certain that, having once been less bright, [gamma] is now considerably brighter than [beta]. the change, however, may be due to the variation of [beta] alone. this star is one of the most remarkable variables known. its period is d. h. m. s. in this time it passes from a maximum brilliancy--that of a star of the · magnitude--to a minimum lustre equal to that of a star of the · magnitude, thence to the same maximum brilliancy as before, thence to another minimum of lustre--that of a star of the · magnitude--and so to its maximum lustre again, when the cycle of changes recommences. these remarkable changes seem to point to the existence of two unequal dark satellites, whose dimensions bear a much greater proportion to those of the bright components of [beta] lyræ than the dimensions of the members of the solar system bear to those of the sun. in this case, at any rate, the conjecture hazarded about algol, that the star revolves around a dark central orb, would be insufficient to account for the observed variation. nearly midway between [beta] and [gamma] lies the wonderful ring-nebula m, of which an imperfect idea will be conveyed by the last figure of plate . this nebula was discovered in , by darquier, at toulouse. it is seen as a ring of light with very moderate telescopic power. in a good - / -inch telescope the nebula exhibits a mottled appearance and a sparkling light. larger instruments exhibit a faint light within the ring; and in lord rosse's great telescope "wisps of stars" are seen within, and faint streaks of light stream from the outer border of the ring. this nebula has been subjected to spectrum-analysis by mr. huggins. it turns out to be a gaseous nebula! in fact, ring-nebulæ--of which only seven have been detected--seem to belong to the same class as the planetary nebulæ, all of which exhibit the line-spectrum indicative of gaseity. the brightest of the three lines seen in the spectrum of the ring-nebula in lyra presents a rather peculiar appearance, "since it consists," says mr. huggins, "of two bright dots, corresponding to sections of the ring, and between these there is not darkness, but an excessively faint line joining them. this observation makes it probable that the faint nebulous matter occupying the central portion is similar in constitution to that of the ring." the constellation hercules also contains many very interesting objects. let us first inspect a nebula presenting a remarkable contrast with that just described. i refer to the nebula m, known as halley's nebula (plate ). this nebula is visible to the naked eye, and in a good telescope it is a most wonderful object: "perhaps no one ever saw it for the first time without uttering a shout of wonder." it requires a very powerful telescope completely to resolve this fine nebula, but the outlying streamers may be resolved with a good -inch telescope. sir w. herschel considered that the number of the stars composing this wonderful object was at least , . the accepted views respecting nebulæ would place this and other clusters far beyond the limits of our sidereal system, and would make the component stars not very unequal (on the average) to our own sun. it seems to me far more probable, on the contrary, that the cluster belongs to our own system, and that its components are very much smaller than the average of separate stars. perhaps the whole mass of the cluster does not exceed that of an average first-magnitude star. the nebulæ m and h may be found, after a little searching, from the positions indicated in the map. they are both well worthy of study, the former being a very bright globular cluster, the latter a bright and large round nebula. the spectra of these, as of the great cluster, resemble the solar spectrum, being continuous, though, of course, very much fainter. the star [delta] herculis (seen at the bottom of the map) is a wide and easy double--a beautiful object. the components, situated as shown in plate , are of the fourth and eighth magnitude, and coloured respectively greenish-white and grape-red. the star [kappa] herculis is not shown in the map, but may be very readily found, lying between the two gammas, [gamma] herculis and [gamma] serpentis (_see_ frontispiece, map ), rather nearer the latter. it is a wide double, the components of fifth and seventh magnitude, the larger yellowish-white, the smaller ruddy yellow.[ ] ras algethi, or [alpha] herculis, is also beyond the limits of the map, but may be easily found by means of map , frontispiece. it is, properly speaking, a multiple star. considered as a double, the arrangement of the components is that shown in plate . the larger is of magnitude - / , the smaller of magnitude - / ; the former orange, the latter emerald. the companion stars are small, and require a good telescope to be well seen. ras algethi is a variable, changing from magnitude to magnitude - / in a period of - / days. the star [rho] herculis is a closer double. the components are "· apart, and situated as shown in plate . the larger is of magnitude , the smaller - / ; the former bluish-white, the latter pale emerald. there are other objects within the range of our map which are well worthy of study. such are [mu] draconis, a beautiful miniature of castor; [gamma]^{ } and [gamma]^{ } draconis, a wide double, the distance between the components being nearly " (both grey); and [gamma]^{ } and [gamma]^{ } coronæ, a naked-eye double, the components being ' apart, and each double with a good -inch telescope. we turn, however, to another region of the sky. low down, towards the south is seen the small constellation corvus, recognised by its irregular quadrilateral of stars. of the two upper stars, the left-hand one is algorab, a wide double, the components placed as in plate , "· apart, the larger of magnitude , the smaller - / , the colours pale yellow and purple. there is a red star in this neighbourhood which is well worth looking for. to the right of corvus is the constellation crater, easily recognised as forming a tolerably well-marked small group. the star alkes, or [alpha] crateris, must first be found. it is far from being the brightest star in the constellation, and may be assumed to have diminished considerably in brilliancy since it was entitled [alpha] by bayer. it will easily be found, however, by means of the observer's star maps. if now the telescope be directed to alkes, there will be found, following him at a distance of · s, and about one minute southerly, a small red star, r. crateris. like most red stars, this one is a variable. a somewhat smaller blue star may be seen in the same field. there is another red star which may be found pretty easily at this season. first find the stars [eta] and [omicron] leonis, the former forming with regulus (now lying towards the south-west, and almost exactly midway between the zenith and the horizon) the handle of the sickle in leo, the other farther off from regulus towards the right, but lower down. now sweep from [omicron] towards [eta] with a low power.[ ] there will be found a sixth-magnitude star about one-fourth of the way from [omicron] to [eta]. south, following this, will be found a group of four stars, of which one is crimson. this is the star r leonis. like r crateris and r leporis it is variable. next, let the observer turn towards the south again. above corvus, in the position shown in the frontispiece, there are to be seen five stars, forming a sort of wide v with somewhat bowed legs. at the angle is the star [gamma] virginis, a noted double. in the components were - / seconds apart. they gradually approached till, in , they could not be separated by the largest telescopes. since then they have been separating, and they are now - / seconds apart, situated as shown in plate . they are nearly equal in magnitude ( ), and both pale yellow. the star [gamma] leonis is a closer and more beautiful double. it will be found above regulus, and is the brightest star on the blade of the sickle. the components are separated by about - / seconds, the larger of the second, the smaller of the fourth magnitude; the former yellow-orange, the latter greenish-yellow. lastly, the star [iota] leonis may be tried. it will be a pretty severe test for our observer's telescope, the components being only "· apart, and the smaller scarcely exceeding the eighth magnitude. the brighter (fourth magnitude) is pale yellow, the other light blue. chapter iv. a half-hour with bootes, scorpio, ophiuchus, etc. we now commence a series of observations suited to the third quarter of the year, and to the following hours:--ten o'clock on the nd of july; nine on the th of august; eight on the rd of august; seven on the th of october; and intermediate hours on days intermediate to these. we look first for the great bear towards the north-west, and thence find the pole-star. turning towards the north we see capella and [beta] aurigæ low down and slightly towards the left of the exact north point. the milky way crosses the horizon towards the north-north-east and passes to the opposite point of the compass, attaining its highest point above the horizon towards east-south-east. this part of the milky way is well worth observing, being marked by singular variations of brilliancy. near arided (the principal star of cygnus, and now lying due east--some twenty-five degrees from the zenith) there is seen a straight dark rift, and near this space is another larger cavity, which has been termed the northern coal-sack. the space between [gamma], [delta], and [beta] cygni is covered by a large oval mass, exceedingly rich and brilliant. the neighbouring branch, extending from [epsilon] cygni, is far less conspicuous here, but near sagitta becomes brighter than the other, which in this neighbourhood suddenly loses its brilliancy and fading gradually beyond this point becomes invisible near [beta] ophiuchi. the continuous stream becomes patchy--in parts very brilliant--where it crosses aquila and clypeus. in this neighbourhood the other stream reappears, passing over a region very rich in stars. we see now the greatest extent of the milky way, towards this part of its length, ever visible in our latitudes--just as in spring we see its greatest extent towards monoceros and argo. [illustration: plate iv.] i may note here in passing that sir john herschel's delineation of the northern portion of the milky way, though a great improvement on the views given in former works, seems to require revision, and especially as respects the very remarkable patches and streaks which characterise the portion extending over cepheus and cygnus. it seems to me, also, that the evidence on which it has been urged that the stars composing the milky way are (on an average) comparable in magnitude to our own sun, or to stars of the leading magnitudes, is imperfect. i believe, for instance, that the brilliant oval of milky light in cygnus comes from stars intimately associated with the leading stars in that constellation, and not far removed in space (proportionately) beyond them. of course, if this be the case, the stars, whose combined light forms the patch of milky light, must be far smaller than the leading brilliants of cygnus. however, this is not the place to enter on speculations of this sort; i return therefore to the business we have more immediately in hand. towards the east is the square of pegasus low down towards the horizon. towards the south is scorpio, distinguished by the red and brilliant antares, and by a train of conspicuous stars. towards the west is bootes, his leading brilliant--the ruddy arcturus--lying somewhat nearer the horizon than the zenith, and slightly south of west. bootes as a constellation is easily found if we remember that he is delineated as chasing away the greater bear. thus at present he is seen in a slightly inclined position, his head (marked by the third-magnitude star [beta]) lying due west, some thirty degrees from the zenith. it has always appeared to me, by the way, that bootes originally had nobler proportions than astronomers now assign to him. it is known that canes venatici now occupy the place of an upraised arm of bootes, and i imagine that corona borealis, though undoubtedly a very ancient constellation, occupies the place of his other arm. giving to the constellation the extent thus implied, it exhibits (better than most constellations) the character assigned to it. one can readily picture to oneself the figure of a herdsman with upraised arms driving ursa major before him. this view is confirmed, i think, by the fact that the arabs called this constellation the vociferator. bootes contains many beautiful objects. partly on this account, and partly because this is a constellation with which the observer should be specially familiar, a map of it is given in plate . arcturus has a distant pale lilac companion, and is in other respects a remarkable and interesting object. it is of a ruddy yellow colour. schmidt, indeed, considers that the star has changed colour of late years, and that whereas it was once very red it is now a yellow star. this opinion does not seem well grounded, however. the star _may_ have been more ruddy once than now, though no other observer has noticed such a peculiarity; but it is certainly not a pure yellow star at present (at any rate as seen in our latitude). owing probably to the difference of colour between vega, capella and arcturus, photometricians have not been perfectly agreed as to the relative brilliancy of these objects. some consider vega the most brilliant star in the northern heavens, while others assign the superiority to capella. the majority, however, consider arcturus the leading northern brilliant, and in the whole heavens place three only before him, viz., sirius, canopus, and [alpha] centauri. arcturus is remarkable in other respects. his proper motion is very considerable, so great in fact that since the time of ptolemy the southerly motion (alone) of arcturus has carried him over a space nearly half as great again as the moon's apparent diameter. one might expect that so brilliant a star, apparently travelling at a rate so great compared with the average proper motions of the stars, must be comparatively near to us. this, however, has not been found to be the case. arcturus is, indeed, one of the stars whose distance it has been found possible to estimate roughly. but he is found to be some three times as far from us as the small star cygni, and more than seven times as far from us as [alpha] centauri. the star [delta] bootis is a wide and unequal double, the smaller component being only of the ninth magnitude. above alkaid the last star in the tail of the greater bear, there will be noticed three small stars. these are [theta], [iota], and [kappa] bootis, and are usually placed in star-maps near the upraised hand of the herdsman. the two which lie next to alkaid, [iota] and [kappa], are interesting doubles. the former is a wide double (see plate ), the magnitudes of components and , their colours yellow and white. the larger star of this pair is itself double. the star [kappa] bootis is not so wide a double (see plate ), the magnitudes of the components and , their colours white and faint blue--a beautiful object. the star [xi] bootis is an exceedingly interesting object. it is double, the colours of the components being orange-yellow and ruddy purple, their magnitudes - / and - / . when this star was first observed by herschel in the position of the components was quite different from that presented in plate . they were also much closer, being separated by a distance of less than - / seconds. since that time the smaller component has traversed nearly a full quadrant, its distance from its primary first increasing, till in the stars were nearly - / seconds apart, and thence slowly diminishing, so that at present the stars are less than seconds apart. the period usually assigned to the revolution of this binary system is years, and the period of peri-astral passage is said to be . it appears to me, however, that the period should be about years, the epoch of last peri-astral passage and of next peri-astral passage, therefore, . the angular motion of the secondary round the primary is now rapidly increasing, and the distance between the components is rapidly diminishing, so that in a few years a powerful telescope will be required to separate the pair. not far from [xi] is [pi] bootis, represented in plate as a somewhat closer double, but in reality--now at any rate--a slightly wider pair, since the distance between the components of [xi] has greatly diminished of late. both the components of [pi] are white, and their magnitudes are - / and . we shall next turn to an exceedingly beautiful and delicate object, the double star [epsilon] bootis, known also as mirac and, on account of its extreme beauty, called pulcherrima by admiral smyth. the components of this beautiful double are of the third and seventh magnitude, the primary orange, the secondary sea-green. the distance separating the components is about seconds, perhaps more; it appears to have been slowly increasing during the past ten or twelve years. smyth assigns to this system a period of revolution of years, but there can be little doubt that the true period is largely in excess of this estimate. observers in southern latitudes consider that the colours of the components are yellow and blue, not orange and green as most of our northern observers have estimated them. a little beyond the lower left-hand corner of the map is the star [delta] serpentis, in the position shown in the frontispiece, map . this is the star which at the hour and season depicted in map formed the uppermost of a vertical row of stars, which has now assumed an almost horizontal position. the components of [delta] serpentis are about - / seconds apart, their magnitudes and , both white. the stars [theta]^{ } and [theta]^{ } serpentis form a wide double, the distance between the components being - / seconds. they are nearly equal in magnitude, the primary being - / , the secondary . both are yellow, the primary being of a paler yellow colour than the smaller star. but the observer may not know where to look for [theta] serpentis, since it falls in a part of the constellation quite separated from that part in which [delta] serpentis lies. in fact [theta] lies on the extreme easterly verge of the eastern half of the constellation. it is to be looked for at about the same elevation as the brilliant altair, and (as to azimuth) about midway between altair and the south. the stars [alpha]^{ } and [alpha]^{ } libræ form a wide double, perhaps just separable by the naked eye in very favourable weather. the larger component is of the third, the smaller of the sixth magnitude, the former yellow the latter light grey. the star [beta] libræ is a beautiful light-green star to the naked eye; in the telescope a wide double, pale emerald and light blue. in scorpio there are several very beautiful objects:-- the star antares or cor scorpionis is one of the most beautiful of the red stars. it has been termed the sirius of red stars, a term better merited perhaps by aldebaran, save for this that, in our latitude, antares is, like sirius, always seen as a brilliant "scintillator" (because always low down), whereas aldebaran rises high above the horizon. antares is a double star, its companion being a minute green star. in southern latitudes the companion of antares may be seen with a good -inch, but in our latitudes a larger opening is wanted. mr. dawes once saw the companion of antares shining alone for seven seconds, the primary being hidden by the moon. he found that the colour of the secondary is not merely the effect of contrast, but that this small star is really a green sun. the star [beta] scorpionis is a fine double, the components "· apart, their magnitudes and - / , colours white and lilac. it has been supposed that this pair is only an optical double, but a long time must elapse before a decisive opinion can be pronounced on such a point. the star [sigma] scorpionis is a wider but much more difficult double, the smaller component being below the th magnitude. the colour of the primary ( ) is white, that of the secondary maroon. the star [xi] scorpionis is a neat double, the components "· apart, their magnitudes - / and - / , their colours white and grey. this star is really triple, a fifth-magnitude star lying close to the primary. in ophiuchus, a constellation covering a wide space immediately above scorpio, there are several fine doubles. among others-- ophiuchi, distance between components "· , their magnitudes - / and - / , their colours orange and blue. the star ophiuchi, a fourth-magnitude star on the right shoulder of ophiuchus, is a noted double. the distance between the components about - / ", their magnitudes - / and , the colours yellow and red. the pair form a system whose period of revolution is about years. ophiuchi (variable), distance "· , magnitudes - / and - / , colours red and yellow. [rho] opiuchi, distance ", colours yellow and blue, magnitudes and . between [alpha] and [beta] scorpionis the fine nebula m may be looked for. (or more closely thus:--below [beta] is the wide double [omega]^{ } and [omega]^{ } scorpionis; about as far to the right of antares is the star [sigma] scorpionis, and immediately above this is the fifth-magnitude star .) the nebula we seek lies between and [omega], nearer to (about two-fifths of the way towards [omega]). this nebula is described by sir w. herschel as "the richest and most condensed mass of stars which the firmament offers to the contemplation of astronomers." there are two other objects conveniently situated for observation, which the observer may now turn to. the first is the great cluster in the sword-hand of perseus (see plate ), now lying about ° above the horizon between n.e. and n.n.e. the stars [gamma] and [delta] cassiopeiæ (see map of frontispiece) point towards this cluster, which is rather farther from [delta] than [delta] from [gamma], and a little south of the produced line from these stars. the cluster is well seen with the naked eye, even in nearly full moonlight. in a telescope of moderate power this cluster is a magnificent object, and no telescope has yet revealed its full glory. the view in plate gives but the faintest conception of the glories of [chi] persei. sir w. herschel tried in vain to gauge the depths of this cluster with his most powerful telescope. he spoke of the most distant parts as sending light to us which must have started or years ago. but it appears improbable that the cluster has in reality so enormous a longitudinal extension compared with its transverse section as this view would imply. on the contrary, i think we may gather from the appearance of this cluster, that stars are far less uniform in size than has been commonly supposed, and that the mere irresolvability of a cluster is no proof of excessive distance. it is unlikely that the faintest component of the cluster is farther off than the brightest (a seventh-magnitude star) in the proportion of more than about to , while the ordinary estimate of star magnitudes, applied by herschel, gave a proportion of or to at least. i can no more believe that the components of this cluster are stars greatly varying in distance, but accidentally seen in nearly the same direction, (or that they form an _enormously long system_ turned by accident directly towards the earth), than i could look on the association of several thousand persons in the form of a procession as a fortuitous arrangement. next there is the great nebula in andromeda--known as "the transcendantly beautiful queen of the nebulæ." it will not be difficult to find this object. the stars [epsilon] and [delta] cassiopeiæ (map , frontispiece) point to the star [beta] andromedæ. almost in a vertical line above this star are two fourth-magnitude stars [mu] and [gamma], and close above [nu], a little to the right, is the object we seek--visible to the naked eye as a faint misty spot. to tell the truth, the transcendantly beautiful queen of the nebulæ is rather a disappointing object in an ordinary telescope. there is seen a long oval or lenticular spot of light, very bright near the centre, especially with low powers. but there is a want of the interest attaching to the strange figure of the great orion nebula. the andromeda nebula has been partially resolved by lord rosse's great reflector, and (it is said) more satisfactorily by the great refractor of harvard college. in the spectroscope, mr. huggins informs us, the spectrum is peculiar. continuous from the blue to the orange, the light there "appears to cease very abruptly;" there is no indication of gaseity. lastly, the observer may turn to the pair mizar and alcor, the former the middle star in the great bear's tail, the latter ' off. it seems quite clear, by the way, that alcor has increased in brilliancy of late, since among the arabians it was considered an evidence of very good eyesight to detect alcor, whereas this star may now be easily seen even in nearly full moonlight. mizar is a double star, and a fourth star is seen in the same field of view with the others (see plate ). the distance between mizar and its companion is "· ; the magnitude of mizar , of the companion ; their colours white and pale green, respectively. chapter v. a half-hour with andromeda, cygnus, etc. our last half-hour with the double stars, &c., must be a short one, as we have already nearly filled the space allotted to these objects. the observations now to be made are supposed to take place during the fourth quarter of the year,--at ten o'clock on october rd; or at nine on november th; or at eight on november nd; or at seven on december th; or at hours intermediate to these on intermediate days. we look first, as in former cases, for the great bear, now lying low down towards the north. towards the north-east, a few degrees easterly, are the twin-stars castor and pollux, in a vertical position, castor uppermost. above these, a little towards the right, we see the brilliant capella; and between capella and the zenith is seen the festoon of perseus. cassiopeia lies near the zenith, towards the north, and the milky way extends from the eastern horizon across the zenith to the western horizon. low down in the east is orion, half risen above horizon. turning to the south, we see high up above the horizon the square of pegasus. low down towards the south-south-west is fomalhaut, pointed to by [beta] and [alpha] pegasi. towards the west, about half-way between the zenith and the horizon, is the noble cross in cygnus; below which, towards the left, we see altair, and his companions [beta] and [gamma] aquilæ: while towards the right we see the brilliant vega. during this half-hour we shall not confine ourselves to any particular region of the heavens, but sweep the most conveniently situated constellations. [illustration: plate v.] first, however, we should recommend the observer to try and get a good view of the great nebula in andromeda, which is _not_ conveniently situated for observation, but is so high that after a little trouble the observer may expect a more distinct view than in the previous quarter. he will see [beta] andromedæ towards the south-east, about ° from the zenith, [mu] and [nu] nearly in a line towards the zenith, and the nebula about half-way between [beta] and the zenith. with a similar object it will be well to take another view of the great cluster in perseus, about ° from the zenith towards the east-north-east (_see_ the pointers [gamma] and [delta] cassiopeiæ in map , frontispiece), the cluster being between [delta] cassiopeiæ and [alpha] persei. not very far off is the wonderful variable algol, now due east, and about ° above the horizon. the variability of this celebrated object was doubtless discovered in very ancient times, since the name al-gol, or "the demon" seems to point to a knowledge of the peculiarity of this "slowly winking eye." to goodricke, however, is due the rediscovery of algol's variability. the period of variation is d. h. m.; during h. m. algol appears of the second magnitude; the remaining - / hours are occupied by the gradual decline of the star to the fourth magnitude, and its equally gradual return to the second. it will be found easy to watch the variations of this singular object, though, of course, many of the minima are attained in the daytime. the following may help the observer:-- on october th, , at about half-past eleven in the evening, i noticed that algol had reached its minimum of brilliancy. hence the next minimum was attained at about a quarter-past eight on the evening of october th; the next at about five on the evening of october th, and so on. now, if this process be carried on, it will be found that the next evening minimum occurred at about h. (_circiter_) on the evening of october st, the next at about h. m. on the evening of november th. thus at whatever hour any minimum occurs, another occurs _six weeks and a day later_, at about the same hour. this would be exact enough if the period of variation were _exactly_ d. m. s., but the period is nearly a minute greater, and as there are fifteen periods in six weeks and a day, it results that there is a difference of about m. in the time at which the successive recurrences at nearly the same hour take place. hence we are able to draw up the two following tables, which will suffice to give all the minima conveniently observable during the next two years. starting from a minimum at about h. m. on november th, , and noticing that the next -day period (with the m. added) gives us an observation at midnight on january nd, , and that successive periods would make the hour later yet, we take the minimum next after that of january nd, viz. that of january th, , h. m., and taking -day periods (with m. added to each), we get the series-- h. m. jan. , , p.m. feb. , ----, ---- mar. , ----, ---- may , ----, ---- june , ----, ---- aug. , ----, ---- sept. , ----, ---- nov. ----, ---- dec. , ----, ---- jan. , , ---- mar. , ----, ---- mar. , ----, ----[ ] apr. , ----, ---- june , ----, ---- july , ----, ---- sept. , ----, ---- oct. , ----, ---- nov. , ----, ---- jan. , , ---- feb. , ----, ---- from the minimum at about p.m. on october st, , we get in like manner the series-- h. m. dec. , , p.m. jan. , , ---- mar. , ----, ---- apr. , ----, ---- june , ----, ---- june , ----, ----[ ] july , ----, ---- aug. , ----, ---- oct. , ----, ---- nov. , ----, ---- jan. , , ---- feb. , ----, ---- apr. , ----, ---- may , ----, ---- june , ----, ---- aug. , ----, ---- sept. , ----, ---- nov. , ----, ---- dec. , ----, ---- jan. , , ---- from one or other of these tables every observable minimum can be obtained. thus, suppose the observer wants to look for a minimum during the last fortnight in august, . the first table gives him no information, the latter gives him a minimum at h. m. p.m. on august ; hence of course there is a minimum at h. m. p.m. on august ; and there are no other conveniently observable minima during the fortnight in question. the cause of the remarkable variation in this star's brilliancy has been assigned by some astronomers to the presence of an opaque secondary, which transits algol at regular intervals; others have adopted the view that algol is a luminous secondary, revolving around an opaque primary. of these views the former seems the most natural and satisfactory. it points to a secondary whose mass bears a far greater proportion to that of the primary, than the mass even of jupiter bears to the sun; the shortness of the period is also remarkable. it may be noticed that observation points to a gradual diminution in the period of algol's variation, and the diminution seems to be proceeding more and more rapidly. hence (assuming the existence of a dark secondary) we must suppose that either it travels in a resisting medium which is gradually destroying its motion, or that there are other dependent orbs whose attractions affect the period of this secondary. in the latter case the decrease in the period will attain a limit and be followed by an increase. however, interesting as the subject may be, it is a digression from telescopic work, to which we now return. within the confines of the second map in plate is seen the fine star [gamma] andromedæ. at the hour of our observations it lies high up towards e.s.e. it is seen as a double star with very moderate telescopic power, the distance between the components being upwards of "; their magnitudes and - / , their colours orange and green. perhaps there is no more interesting double visible with low powers. the smaller star is again double in first-class telescopes, the components being yellow and blue according to some observers, but according to others, both green. below [gamma] andromedæ lie the stars [beta] and [gamma] triangulorum, [gamma] a fine naked-eye triple (the companions being [delta] and [eta] triangulorum), a fine object with a very low power. to the right is [alpha] triangulorum, certainly less brilliant than [beta]. below [alpha] are the three stars [alpha], [beta], and [gamma] arietis, the first an unequal and difficult double, the companion being purple, and only just visible (under favourable circumstances) with a good -inch telescope; the last an easy double, interesting as being the first ever discovered (by hook, in ), the colours of components white and grey. immediately below [alpha] arietis is the star [alpha] ceti, towards the right of which (a little lower) is mira, a wonderful variable. this star has a period of - / days; during a fortnight it appears as a star of the nd magnitude,--on each side of this fortnight there is a period of three months during one of which the star is increasing, while during the other it is diminishing in brightness: during the remaining five months of the period the star is invisible to the naked eye. there are many peculiarities and changes in the variation of this star, into which space will not permit me to enter. immediately above mira is the star [alpha] piscium at the knot of the fishes' connecting band. this is a fine double, the distance between the components being about - / ", their magnitudes and , their colours pale green and blue (see plate ). close to [gamma] aquarii (see frontispiece, map ), above and to the left of it, is the interesting double [zeta] aquarii; the distance between the components is about - / ", their magnitudes and - / , both whitish yellow. the period of this binary seems to be about years. turning next towards the south-west we see the second-magnitude star [epsilon] pegasi, some ° above the horizon. this star is a wide but not easy double, the secondary being only of the ninth magnitude; its colour is lilac, that of the primary being yellow. towards the right of [epsilon] pegasi and lower down are seen the three fourth-magnitude stars which mark the constellation equuleus. of these the lowest is [alpha], to the right of which lies [epsilon] equulei, a fifth-magnitude star, really triple, but seen as a double star with ordinary telescopes (plate ). the distance between the components is nearly ", their colours white and blue, their magnitudes - / and - / . the primary is a very close double, which appears, however, to be opening out rather rapidly. immediately below equuleus are the stars [alpha]^{ } and [alpha]^ capricorni, seen as a naked-eye double to the right of and above [beta]. both [alpha]^ and [alpha]^ are yellow; [alpha]^ is of the rd, [alpha]^ of the th magnitude; in a good telescope five stars are seen, the other three being blue, ash-coloured, and lilac. the star [beta] capricorni is also a wide double, the components yellow and blue, with many telescopic companions. to the right of equuleus, towards the west-south-west is the constellation delphinus. the upper left-hand star of the rhombus of stars forming the head of the delphinus is the star [gamma] delphini, a rather easy double (see plate ), the components being nearly " apart, their magnitudes and , their colours golden yellow and flushed grey. turn we next to the charming double albireo, on the beak of cygnus, about ° above the horizon towards the west. the components are - / " apart, their magnitudes and , their colours orange-yellow, and blue. it has been supposed (perhaps on insufficient evidence) that this star is merely an optical double. it must always be remembered that a certain proportion of stars (amongst those separated by so considerable a distance) _must_ be optically combined only. the star [chi] cygni is a wide double (variable) star. the components are separated by nearly ", their magnitudes and , their colours yellow and light blue. [chi] may be found by noticing that there is a cluster of small stars in the middle of the triangle formed by the stars [gamma], [delta], and [beta] cygni (see map , frontispiece), and that [chi] is the nearest star _of the cluster_ to [beta]. the star [phi] cygni, which is just above and very close to [beta] (albireo), does not belong to the cluster. [chi] is about half as far again from [phi] as [phi] from albireo. but as [chi] descends to the th magnitude at its minimum the observer must not always expect to find it very easily. it has been known to be invisible at the epoch when it should have been most conspicuous. the period of this variable is days. the star cygni is an interesting one. so far as observation has yet extended, it would seem to be the nearest to us of all stars visible in the northern hemisphere. it is a fine double, the components nearly equal ( - / and ), both yellow, and nearly " apart. the period of this binary appears to be about years. to find cygni note that [epsilon] and [delta] cygni form the diameter of a semicircle divided into two quadrants by [alpha] cygni (arided). on this semicircle, on either side of [alpha], lie the stars [nu] and [alpha] cygni, [nu] towards [epsilon]. now a line from [alpha] to [nu] produced passes very near to cygni at a distance from [nu] somewhat greater than half the distance of [nu] from [alpha]. the star [mu] cygni lies in a corner of the constellation, rather farther from [zeta] than [zeta] from [epsilon] cygni. a line from [epsilon] to [zeta] produced meets [kappa] pegasi, a fourth-magnitude star; and [mu] cygni, a fifth-magnitude star, lies close above [kappa] pegasi. the distance between the components is about - / ", their magnitudes and , their colours white and pale blue. the star [psi] cygni may next be looked for, but for this a good map of cygnus will be wanted, as [psi] is not pointed to by any well-marked stars. a line from [alpha], parallel to the line joining [gamma] and [delta], and about one-third longer than that line, would about mark the position of [psi] cygni. the distance between the components of this double is about - / ", their magnitudes - / and , their colours white and lilac. lastly, the observer may turn to the stars [gamma]_{ } and [gamma]_{ } draconis towards the north-west about ° above the horizon (they are included in the second map of plate ). they form a wide double, having equal (fifth-magnitude) components, both grey. (see plate .) chapter vi. half-hours with the planets. in observing the stars, we can select a part of the heavens which may be conveniently observed; and in this way in the course of a year we can observe every part of the heavens visible in our northern hemisphere. but with the planets the case is not quite so simple. they come into view at no fixed season of the year: some of them can never be seen _by night_ on the meridian; and they all shift their place among the stars, so that we require some method of determining where to look for them on any particular night, and of recognising them from neighbouring fixed stars. the regular observer will of course make use of the 'nautical almanac'; but 'dietrichsen and hannay's almanac' will serve every purpose of the amateur telescopist. i will briefly describe those parts of the almanac which are useful to the observer. it will be found that three pages are assigned to each month, each page giving different information. if we call these pages i. ii. iii., then in order that page i. for each month may fall to the left of the open double page, and also that i. and ii. may be open together, the pages are arranged in the following order: i. ii. iii.; iii. i. ii.; i. ii. iii.; and so on. now page iii. for any month does not concern the amateur observer. it gives information concerning the moon's motions, which is valuable to the sailor, and interesting to the student of astronomy, but not applicable to amateur observation. [illustration: plate vi.] we have then only pages i. and ii. to consider:-- across the top of both pages the right ascension and declination of the planets venus, jupiter, mars, saturn, mercury, and uranus are given, accompanied by those of two conspicuous stars. this information is very valuable to the telescopist. in the first place, as we shall presently see, it shows him what planets are well situated for observation, and secondly it enables him to map down the path of any planet from day to day among the fixed stars. this is a very useful exercise, by the way, and also a very instructive one. the student may either make use of the regular maps and mark down the planet's path in pencil, taking a light curve through the points given by the data in his almanac, or he may lay down a set of meridians suited to the part of the heavens traversed by the planet, and then proceed to mark in the planet's path and the stars, taking the latter either from his maps or from a convenient list of stars.[ ] my 'handbook of the stars' has been constructed to aid the student in these processes. it must be noticed that old maps are not suited for the work, because, through precession, the stars are all out of place as respects r.a. and dec. even the society's maps, constructed so as to be right for , are beginning to be out of date. but a matter of or years either way is not important.[ ] my maps, handbook and zodiac-chart have been constructed for the year , so as to be serviceable for the next fifty years or so. next, below the table of the planets, we have a set of vertical columns. these are, in order, the days of the month, the calendar--in which are included some astronomical notices, amongst others the diameter of saturn on different dates, the hours at which the sun rises and sets, the sun's right ascension, declination, diameter, and longitude; then eight columns which do not concern the observer; after which come the hours at which the moon rises and sets, the moon's age; and lastly (so far as the observer is concerned) an important column about jupiter's system of satellites. next, we have, at the foot of the first page, the hours at which the planets rise, south, and set; and at the foot of the second page we have the dates of conjunctions, oppositions, and of other phenomena, the diameters of venus, jupiter, mars, and mercury, and finally a few words respecting the visibility of these four planets. after the thirty-six pages assigned to the months follow four (pp. - ) in which much important astronomical information is contained; but the points which most concern our observer are (i.) a small table showing the appearance of saturn's rings, and (ii.) a table giving the hours at which jupiter's satellites are occulted or eclipsed, re-appear, &c. we will now take the planets in the order of their distance from the sun: we shall see that the information given by the almanac is very important to the observer. mercury is so close to the sun as to be rarely seen with the naked eye, since he never sets much more than two hours and a few minutes after the sun, or rises by more than that interval before the sun. it must not be supposed that at each successive epoch of most favourable appearance mercury sets so long after the sun or rises so long before him. it would occupy too much of our space to enter into the circumstances which affect the length of these intervals. the question, in fact, is not a very simple one. all the necessary information is given in the almanac. we merely notice that the planet is most favourably seen as an evening star in spring, and as a morning star in autumn.[ ] the observer with an equatorial has of course no difficulty in finding mercury, since he can at once direct his telescope to the proper point of the heavens. but the observer with an alt-azimuth might fail for years together in obtaining a sight of this interesting planet, if he trusted to unaided naked-eye observations in looking for him. copernicus never saw mercury, though he often looked for him; and mr. hind tells me he has seen the planet but once with the naked eye--though this perhaps is not a very remarkable circumstance, since the systematic worker in an observatory seldom has occasion to observe objects with the unaided eye. by the following method the observer can easily pick up the planet. across two uprights (fig. ) nail a straight rod, so that when looked at from some fixed point of view the rod may correspond to the sun's path near the time of observation. the rod should be at right-angles to the line of sight to its centre. fasten another rod at right angles to the first. from the point at which the rods cross measure off and mark on both rods spaces each subtending a degree as seen from the point of view. thus, if the point of view is - / feet off, these spaces must each be inches long, and they must be proportionately less or greater as the eye is nearer or farther. [illustration: _fig. ._] now suppose the observer wishes to view mercury on some day, whereon mercury is an evening star. take, for instance, june th, . we find from 'dietrichsen' that on this day (at noon) mercury's r.a. is h. m. s.: and the sun's h. m. s. we need not trouble ourselves about the odd hours after noon, and thus we have mercury's r.a. greater than the sun's by h. m. s. now we will suppose that the observer has so fixed his uprights and the two rods, that the sun, seen from the fixed point of view, appears to pass the point of crossing of the rods at half-past seven, then mercury will pass the cross-rod at m. s. past nine. but where? to learn this we must take out mercury's declination, which is ° ' " n., and the sun's, which is ° ' " n. the difference, ° ' " n. gives us mercury's place, which it appears is rather less than - / degree north of the sun. thus, about h. m. after the sun has passed the cross-rod, mercury will pass it between the first and second divisions above the point of fastening. the sun will have set about an hour, and mercury will be easily found when the telescope is directed towards the place indicated. it will be noticed that this method does not require the time to be exactly known. all we have to do is to note the moment at which the sun passes the point of fastening of the two rods, and to take our h. m. from that moment. this method, it may be noticed in passing, may be applied to give naked-eye observations of mercury at proper seasons (given in the almanac). by a little ingenuity it may be applied as well to morning as to evening observations, the sun's passage of the cross-rod being taken on one morning and mercury's on the next, so many minutes _before_ the hour of the first observation. in this way several views of mercury may be obtained during the year. such methods may appear very insignificant to the systematic observer with the equatorial, but that they are effective i can assert from my own experience. similar methods may be applied to determine from the position of a known object, that of any neighbouring unknown object even at night. the cross-rod must be shifted (or else two cross-rods used) when the unknown _precedes_ the known object. if two cross-rods are used, account must be taken of the gradual diminution in the length of a degree of right ascension as we leave the equator. even simpler methods carefully applied may serve to give a view of mercury. to show this, i may describe how i obtained my first view of this planet. on june st, , i noticed, that at five minutes past seven the sun, as seen from my study window, appeared from behind the gable-end of mr. st. aubyn's house at stoke, devon. i estimated the effect of mercury's northerly declination (different of course for a vertical wall, than for the cross-rod in fig. , which, in fact, agrees with a declination-circle), and found that he would pass out opposite a particular point of the wall a certain time after the sun. i then turned the telescope towards that point, and focussed for distinct vision of distant objects, so that the outline of the house was seen out of focus. as the calculated time of apparition approached, i moved the telescope up and down so that the field swept the neighbourhood of the estimated point of apparition. i need hardly say that mercury did not appear exactly at the assigned point, nor did i see him make his first appearance; but i picked him up so soon after emergence that the outline of the house was in the field of view with him. he appeared as a half-disc. i followed him with the telescope until the sun had set, and soon after i was able to see him very distinctly with the naked eye. he shone with a peculiar brilliance on the still bright sky; but although perfectly distinct to the view when his place was indicated, he escaped detection by the undirected eye.[ ] mercury does not present any features of great interest in ordinary telescopes; though he usually appears better defined than venus, at least as the latter is seen on a dark sky. the phases are pleasingly seen (as shown in plate ) with a telescope of moderate power. for their proper observation, however, the planet must be looked for with the telescope in the manner above indicated, as he always shows a nearly semi-circular disc when he is visible to the naked eye. we come next to venus, the most splendid of all the planets to the eye. in the telescope venus disappoints the observer, however. her intense lustre brings out every defect of the instrument, and especially the chromatic aberration. a dark glass often improves the view, but not always. besides, an interposed glass has an unpleasant effect on the field of view. perhaps the best method of observing venus is to search for her when she is still high above the horizon, and when therefore the background of the sky is bright enough to take off the planet's glare. the method i have described for the observation of mercury will prove very useful in the search for venus when the sun is above the horizon or but just set. of course, when an object is to be looked for high above the horizon, the two rods which support the cross-rods must not be upright, but square to the line of view to that part of the sky. but the observer must not expect to see much during his observation of venus. in fact, he can scarcely do more than note her varying phases (see plate ) and the somewhat uneven boundary of the terminator. our leading observers have done so little with this fascinating but disappointing planet, that amateurs must not be surprised at their own failure. i suppose the question whether venus has a satellite, or at any rate whether the object supposed to have been seen by cassini and other old observers were a satellite, must be considered as decided in the negative. that cassini should have seen an object which dawes and webb have failed to see must be considered utterly improbable. leaving the inferior planets, we come to a series of important and interesting objects. first we have the planet mars, nearly the last in the scale of planetary magnitude, but far from being the least interesting of the planets. it is in fact quite certain that we obtain a better view of mars than of any object in the heavens, save the moon alone. he may present a less distinguished appearance than jupiter or saturn, but we see his surface on a larger scale than that of either of those giant orbs, even if we assume that we ever obtain a fair view of their real surface. nor need the moderately armed observer despair of obtaining interesting views of mars. the telescope with which beer and mädler made their celebrated series of views was only a -inch one, so that with a -inch or even a -inch aperture the attentive observer may expect interesting views. in fact, more depends on the observer than on the instrument. a patient and attentive scrutiny will reveal features which at the first view wholly escape notice. in plate i have given a series of views of mars much more distinct than an observer may expect to obtain with moderate powers. i add a chart of mars, a miniature of one i have prepared from a charming series of tracings supplied me by mr. dawes. the views taken by this celebrated observer in , , , , and , are far better than any others i have seen. the views by beer and mädler are good, as are some of secchi's (though they appear badly drawn), nasmyth's and phillips'; delarue's two views are also admirable; and lockyer has given a better set of views than any of the others. but there is an amount of detail in mr. dawes' views which renders them superior to any yet taken. i must confess i failed at a first view to see the full value of mr. dawes' tracings. faint marks appeared, which i supposed to be merely intended to represent shadings scarcely seen. a more careful study shewed me that every mark is to be taken as the representative of what mr. dawes actually saw. the consistency of the views is perfectly wonderful, when compared with the vagueness and inconsistency observable in nearly all other views. and this consistency is not shown by mere resemblance, which might have been an effect rather of memory (unconsciously exerted) than observation. the same feature changes so much in figure, as it appears on different parts of the disc, that it was sometimes only on a careful projection of different views that i could determine what certain features near the limb represented. but when this had been done, and the distortion through the effect of foreshortening corrected, the feature was found to be as true in shape as if it had been seen in the centre of the planet's disc. in examining mr. dawes' drawings it was necessary that the position of mars' axis should be known. the data for determining this were taken from dr. oudemann's determinations given in a valuable paper on mars issued from mr. bishop's observatory. but instead of calculating mars' presentation by the formulæ there given, i found it convenient rather to make use of geometrical constructions applied to my 'charts of the terrestrial planets.' taking mädler's start-point for martial longitudes, that is the longitude-line passing near dawes' forked bay, i found that my results agreed pretty fairly with those in prof. phillips' map, so far as the latter went; but there are many details in my charts not found in prof. phillips' nor in mädler's earlier charts. i have applied to the different features the names of those observers who have studied the physical peculiarities presented by mars. mr. dawes' name naturally occurs more frequently than others. indeed, if i had followed the rule of giving to each feature the name of its discoverer, mr. dawes' name would have occurred much more frequently than it actually does. on account of the eccentricity of his orbit, mars is seen much better in some oppositions than in others. when best seen the southern hemisphere is brought more into view than the northern because the summer of his northern hemisphere occurs when he is nearly in aphelion (as is the case with the earth by the way). the relative dimensions and presentation of mars, as seen in opposition in perihelion, and in opposition in aphelion, are shown in the two rows of figures. in and near quadrature mars is perceptibly gibbous. he is seen thus about two months before or after opposition. in the former case, he rises late and comes to the meridian six hours or so after midnight. in the latter case, he is well seen in the evening, coming to the meridian at six. his appearance and relative dimensions as he passes from opposition to quadrature are shown in the last three figures of the upper row. mars' polar caps may be seen with very moderate powers. i add four sets of meridians (plate ), by filling in which from the charts the observer may obtain any number of views of the planet as it appears at different times. passing over the asteroids, which are not very interesting objects to the amateur telescopist, we come to jupiter, the giant of the solar system, surpassing our earth more than times in volume, and overweighing all the planets taken together twice over. jupiter is one of the easiest of all objects of telescopic observation. no one can mistake this orb when it shines on a dark sky, and only venus can be mistaken for it when seen as a morning or evening star. sometimes both are seen together on the twilight sky, and then venus is generally the brighter. seen, however, at her brightest and at her greatest elongation from the sun, her splendour scarcely exceeds that with which jupiter shines when high above the southern horizon at midnight. jupiter's satellites may be seen with very low powers; indeed the outer ones have been seen with the naked eye, and all are visible in a good opera-glass. their dimensions relatively to the disc are shown in plate . their greatest elongations are compared with the disc in the low-power view. jupiter's belts may also be well seen with moderate telescopic power. the outer parts of his disc are perceptibly less bright than the centre. more difficult of observation are the transits of the satellites and of their shadows. still the attentive observer can see the shadows with an aperture of two inches, and the satellites themselves with an aperture of three inches. the minute at which the satellites enter on the disc, or pass off, is given in 'dietrichsen's almanac.' the 'nautical almanac' also gives the corresponding data for the shadows. the eclipses of the satellites in jupiter's shadow, and their occultations by his disc, are also given in 'dietrichsen's almanac.' in the inverting telescope the satellites move from right to left in the nearer parts of their orbit, and therefore transit jupiter's disc in that direction, and from left to right in the farther parts. also note that _before_ opposition, (i.) the shadows travel in front of the satellites in transiting the disc; (ii.) the satellites are eclipsed in jupiter's _shadow_; (iii.) they reappear from behind his _disc_. on the other hand, _after_ opposition, (i.) the shadows travel _behind_ the satellites in transiting the disc; (ii.) the satellites are occulted by the _disc_; (iii.) they reappear from eclipse in jupiter's _shadow_. conjunctions of the satellites are common phenomena, and may be waited for by the observer who sees the chance. an eclipse of one satellite by the shadow of another is not a common phenomenon; in fact, i have never heard of such an eclipse being seen. that a satellite should be quite extinguished by another's shadow is a phenomenon not absolutely impossible, but which cannot happen save at long intervals. the shadows are not _black spots_ as is erroneously stated in nearly all popular works on astronomy. the shadow of the fourth, for instance, is nearly all penumbra, the really black part being quite minute by comparison. the shadow of the third has a considerable penumbra, and even that of the first is not wholly black. these penumbras may not be perceptible, but they affect the appearance of the shadows. for instance, the shadow of the fourth is perceptibly larger but less black than that of the third, though the third is the larger satellite. in transit the first satellite moves fastest, the fourth slowest, the others in their order. the shadow moves just as fast (appreciably) as the satellite it belongs to. sometimes the shadow of the satellite may be seen to overtake (apparently) the disc of another. in such a case the shadow does not pass over the disc, but the disc conceals the shadow. this is explained by the fact that the shadow, if visible throughout its length, would be a line reaching slantwise from the satellite it belongs to, and the end of the shadow (that is, the point where it meets the disc) is _not_ the point where the shadow crosses the orbit of any inner satellite. thus the latter may be interposed between the end of the shadow--the only part of the shadow really visible--and the eye; but the end of the shadow _cannot_ be interposed between the satellite and the eye. if a satellite _on the disc_ were eclipsed by another satellite, the black spot thus formed would be in another place from the black spot on the planet's body. i mention all this because, simple as the question may seem, i have known careful observers to make mistakes on this subject. a shadow is seen crossing the disc and overtaking, apparently, a satellite in transit. it seems therefore, on a first view, that the shadow will hide the satellite, and observers have even said that they have _seen_ this happen. but they are deceived. it is obvious that _if one satellite eclipse another, the shadows of both must occupy the same point on jupiter's body_. thus it is the overtaking of one _shadow_ by another on the disc, and not the overtaking of a _satellite_ by a shadow, which determines the occurrence of that as yet unrecorded phenomenon, the eclipse of one satellite by another.[ ] the satellites when far from jupiter seem to lie in a straight line through his centre. but as a matter of fact they do not in general lie in an exact straight line. if their orbits could be seen as lines of light, they would appear, in general, as very long ellipses. the orbit of the fourth would frequently be seen to be _quite clear_ of jupiter's disc, and the orbit of the third might in some very exceptional instances pass _just_ clear of the disc. the satellites move most nearly in a straight line (apparently) when jupiter comes to opposition in the beginning of february or august, and they appear to depart most from rectilinear motion when opposition occurs in the beginning of may and november. at these epochs the fourth satellite may be seen to pass above and below jupiter's disc at a distance equal to about one-sixth of the disc's radius. the shadows do not travel in the same apparent paths as the satellites themselves across the disc, but (in an inverting telescope) _below_ from august to january, and _above_ from february to july. we come now to the most charming telescopic object in the heavens--the planet saturn. inferior only to jupiter in mass and volume, this planet surpasses him in the magnificence of his system. seen in a telescope of adequate power, saturn is an object of surpassing loveliness. he must be an unimaginative man who can see saturn for the first time in such a telescope, without a feeling of awe and amazement. if there is any object in the heavens--i except not even the sun--calculated to impress one with a sense of the wisdom and omnipotence of the creator it is this. "his fashioning hand" is indeed visible throughout space, but in saturn's system it is most impressively manifest. saturn, to be satisfactorily seen, requires a much more powerful telescope than jupiter. a good -inch telescope will do much, however, in exhibiting his rings and belts. i have never seen him satisfactorily myself with such an aperture, but mr. grover has not only seen the above-named features, but even a penumbra to the shadow on the rings with a -inch telescope. saturn revolving round the sun in a long period--nearly thirty years--presents slowly varying changes of appearance (see plate ). at one time the edge of his ring is turned nearly towards the earth; seven or eight years later his rings are as much open as they can ever be; then they gradually close up during a corresponding interval; open out again, exhibiting a different face; and finally close up as first seen. the last epoch of greatest opening occurred in , the next occurs in : the last epoch of disappearance occurred in - , the next occurs in . the successive views obtained are as in plate in order from right to left, then back to the right-hand figure (but sloped the other way); inverting the page we have this figure thus sloped, and the following changes are now indicated by the other figures in order back to the first (but sloped the other way and still inverted), thus returning to the right-hand figure as seen without inversion. the division in the ring can be seen in a good -inch aperture in favourable weather. the dark ring requires a good -inch and good weather. saturn's satellites do not, like jupiter's, form a system of nearly equal bodies. titan, the sixth, is probably larger than any of jupiter's satellites. the eighth also (japetus) is a large body, probably at least equal to jupiter's third satellite. but rhea, dione, and tethys are much less conspicuous, and the other three cannot be seen without more powerful telescopes than those we are here dealing with. so far as my own experience goes, i consider that the five larger satellites may be seen distinctly in good weather with a good - / -inch aperture. i have never seen them with such an aperture, but i judge from the distinctness with which these satellites may be seen with a -inch aperture. titan is generally to be looked for at a considerable distance from saturn--_always_ when the ring is widely open. japetus is to be looked for yet farther from the disc. in fact, when saturn comes to opposition in perihelion (in winter only this can happen) japetus may be as far from saturn as one-third of the apparent diameter of the moon. i believe that under these circumstances, or even under less favourable circumstances, japetus could be seen with a good opera-glass. so also might titan. transits, eclipses, and occulations of saturn's satellites can only be seen when the ring is turned nearly edgewise towards the earth. for the orbits of the seven inner satellites lying nearly in the plane of the rings would (if visible throughout their extent) then only appear as straight lines, or as long ellipses cutting the planet's disc. the belts on saturn are not very conspicuous. a good - / -inch is required (so far as my experience extends) to show them satisfactorily. the rings when turned edgewise either towards the earth or sun, are not visible in ordinary telescopes, neither can they be seen when the earth and sun are on opposite sides of the rings. in powerful telescopes the rings seem never entirely to disappear. the shadow of the planet on the rings may be well seen with a good -inch telescope, which will also show ball's division in the rings. the shadow of the rings on the planet is a somewhat more difficult feature. the shadow of the planet on the rings is best seen when the rings are well open and the planet is in or near quadrature. it is to be looked for to the left of the ball (in an inverting telescope) at quadrature preceding opposition, and to the right at quadrature following opposition. saturn is more likely to be studied at the latter than at the former quadrature, as in quadrature preceding opposition he is a morning star. the shadow of the rings on the planet is best seen when the rings are but moderately open, and saturn is in or near quadrature. when the shadow lies outside the rings it is best seen, as the dark ring takes off from the sharpness of the contrast when the shadow lies within the ring. it would take more space than i can spare here to show how it is to be determined (independently) whether the shadow lies within or without the ring. but the 'nautical almanac' gives the means of determining this point. when, in the table for assigning the appearance of the rings, _l_ is less than _l'_ the shadow lies outside the ring, when _l_ is greater than _l'_ the shadow lies within the ring. uranus is just visible to the naked eye when he is in opposition, and his place accurately known. but he presents no phenomena of interest. i have seen him under powers which made his disc nearly equal to that of the moon, yet could see nothing but a faint bluish disc. neptune also is easily found if his place be accurately noted on a map, and a good finder used. we have only to turn the telescope to a few stars seen in the finder nearly in the place marked in our map, and presently we shall recognise the one we want by the peculiarity of its light. what is the lowest power which will exhibit neptune as a disc i do not know, but i am certain no observer can mistake him for a fixed star with a -inch aperture and a few minutes' patient scrutiny in favourable weather. [illustration: plate vii.] chapter vii. half-hours with the sun and moon. the moon perhaps is the easiest of all objects of telescopic observation. a very moderate telescope will show her most striking features, while each increase of power is repaid by a view of new details. yet in one sense the moon is a disappointing object even to the possessor of a first-class instrument. for the most careful and persistent scrutiny, carried on for a long series of years, too often fails to reward the observer by any new discoveries of interest. our observer must therefore rather be prepared to enjoy the observation of recognised features than expect to add by his labours to our knowledge of the earth's nearest neighbour. although the moon is a pleasing and surprising telescopic object when full, the most interesting views of her features are obtained at other seasons. if we follow the moon as she waxes or wanes, we see the true nature of that rough and bleak mountain scenery, which when the moon is full is partially softened through the want of sharp contrasts of light and shadow. if we watch, even for half an hour only, the changing form of the ragged line separating light from darkness on the moon's disc, we cannot fail to be interested. "the outlying and isolated peak of some great mountain-chain becomes gradually larger, and is finally merged in the general luminous surface; great circular spaces, enclosed with rough and rocky walls many miles in diameter, become apparent; some with flat and perfectly smooth floors, variegated with streaks; others in which the flat floor is dotted with numerous pits or covered with broken fragments of rock. occasionally a regularly-formed and unusually symmetrical circular formation makes its appearance; the exterior surface of the wall bristling with terraces rising gradually from the plain, the interior one much more steep, and instead of a flat floor, the inner space is concave or cup-shaped, with a solitary peak rising in the centre. solitary peaks rise from the level plains and cast their long narrow shadows athwart the smooth surface. vast plains of a dusky tint become visible, not perfectly level, but covered with ripples, pits, and projections. circular wells, which have no surrounding wall dip below the plain, and are met with even in the interior of the circular mountains and on the tops of their walls. from some of the mountains great streams of a brilliant white radiate in all directions and can be traced for hundreds of miles. we see, again, great fissures, almost perfectly straight and of great length, although very narrow, which appear like the cracks in moist clayey soil when dried by the sun."[ ] but interesting as these views may be, it was not for such discoveries as these that astronomers examined the surface of the moon. the examination of mere peculiarities of physical condition is, after all, but barren labour, if it lead to no discovery of physical variation. the principal charm of astronomy, as indeed of all observational science, lies in the study of change--of progress, development, and decay, and specially of systematic variations taking place in regularly-recurring cycles. and it is in this relation that the moon has been so disappointing an object of astronomical observation. for two centuries and a half her face has been scanned with the closest possible scrutiny; her features have been portrayed in elaborate maps; many an astronomer has given a large portion of his life to the work of examining craters, plains, mountains, and valleys, for the signs of change; but until lately no certain evidence--or rather, no evidence save of the most doubtful character--has been afforded that the moon is other than "a dead and useless waste of extinct volcanoes." whether the examination of the remarkable spot called linné--where lately signs were supposed to have been seen of a process of volcanic eruption--will prove an exception to this rule, remains to be seen. the evidence seems to me strongly to favour the supposition of a change of some sort having taken place in this neighbourhood. the sort of scrutiny required for the discovery of changes, or for the determination of their extent, is far too close and laborious to be attractive to the general observer. yet the kind of observation which avails best for the purpose is perhaps also the most interesting which he can apply to the lunar details. the peculiarities presented by a spot upon the moon are to be observed from hour to hour (or from day to day, according to the size of the spot) as the sun's light gradually sweeps across it, until the spot is fully lighted; then as the moon wanes and the sun's light gradually passes from the spot, the series of observations is to be renewed. a comparison of them is likely--especially if the observer is a good artist and has executed several faithful delineations of the region under observation, to throw much light upon the real contour of the moon's surface at this point. in the two lunar views in plate some of the peculiarities i have described are illustrated. but the patient observer will easily be able to construct for himself a set of interesting views of different regions. it may be noticed that for observation of the waning moon there is no occasion to wait for those hours in which only the waning moon is visible _during the night_. of course for the observation of a particular region under a particular illumination, the observer has no choice as to hour. but for generally interesting observations of the waning moon he can wait till morning and observe by daylight. the moon is, of course, very easily found by the unaided eye (in the day time) when not very near to the sun; and the methods described in chapter v. will enable the observer to find the moon when she is so near to the sun as to present the narrowest possible sickle of light. one of the most interesting features of the moon, when she is observed with a good telescope, is the variety of colour presented by different parts of her surface. we see regions of the purest white--regions which one would be apt to speak of as _snow-covered_, if one could conceive the possibility that snow should have fallen where (now, at least) there is neither air nor water. then there are the so-called seas, large grey or neutral-tinted regions, differing from the former not merely in colour and in tone, but in the photographic quality of the light they reflect towards the earth. some of the seas exhibit a greenish tint, as the sea of serenity and the sea of humours. where there is a central mountain within a circular depression, the surrounding plain is generally of a bluish steel-grey colour. there is a region called the marsh of sleep, which exhibits a pale red tint, a colour seen also near the hyrcinian mountains, within a circumvallation called lichtenburg. the brightest portion of the whole lunar disc is aristarchus, the peaks of which shine often like stars, when the mountain is within the unillumined portion of the moon. the darkest regions are grimaldi and endymion and the great plain called plato by modern astronomers--but, by hevelius, the greater black lake. the sun.--observation of the sun is perhaps on the whole the most interesting work to which the possessor of a moderately good telescope can apply his instrument. those wonderful varieties in the appearance of the solar surface which have so long perplexed astronomers, not only supply in themselves interesting subjects of observation and examination, but gain an enhanced meaning from the consideration that they speak meaningly to us of the structure of an orb which is the source of light and heat enjoyed by a series of dependent worlds whereof our earth is--in size at least--a comparatively insignificant member. swayed by the attraction of this giant globe, jupiter and saturn, uranus and neptune, as well as the four minor planets, and the host of asteroids, sweep continuously in their appointed orbits, in ever new but ever safe and orderly relations amongst each other. if the sun's light and heat were lost, all life and work among the denizens of these orbs would at once cease; if his attractive energy were destroyed, these orbs would cease to form a _system_. the sun may be observed conveniently in many ways, some more suited to the general observer who has not time or opportunity for systematic observation; others more instructive, though involving more of preparation and arrangement. the simplest method of observing the sun is to use the telescope in the ordinary manner, protecting the eye by means of dark-green or neutral-tinted glasses. some of the most interesting views i have ever obtained of the sun, have resulted from the use of the ordinary terrestrial or erecting eye-piece, capped with a dark glass. the magnifying power of such an eye-piece is, in general, much lower than that available with astronomical eye-pieces. but vision is very pleasant and distinct when the sun is thus observed, and a patient scrutiny reveals almost every feature which the highest astronomical power applicable could exhibit. then, owing to the greater number of intervening lenses, there is not the same necessity for great darkness or thickness in the coloured glass, so that the colours of the solar features are seen much more satisfactorily than when astronomical eye-pieces are employed. in using astronomical eye-pieces it is convenient to have a rotating wheel attached, by which darkening glasses of different power may be brought into use as the varying illumination may require. those who wish to observe carefully and closely a minute portion of the solar disc, should employ dawes' eye-piece: in this a metallic screen placed in the focus keeps away all light but such as passes through a minute hole in the diaphragm. another convenient method of diminishing the light is to use a glass prism, light being partially reflected from one of the exterior surfaces, while the refracted portion is thrown out at another. very beautiful and interesting views may be obtained by using such a pyramidal box as is depicted in fig. . [illustration: _fig. ._] this box should be made of black cloth or calico fastened over a light framework of wire or cane. the base of the pyramid should be covered on the inside with a sheet of white glazed paper, or with some other uniform white surface. captain noble, i believe, makes use of a surface of plaster of paris, smoothed while wet with plate-glass. the door _b c_ enables the observer to "change power" without removing the box, while larger doors, _d e_ and _g f_, enable him to examine the image; a dark cloth, such as photographers use, being employed, if necessary, to keep out extraneous light. the image may also be examined from without, if the bottom of the pyramid be formed of a sheet of cut-glass or oiled tissue-paper. when making use of the method just described, it is very necessary that the telescope-tube should be well balanced. a method by which this may be conveniently accomplished has been already described in chapter i. but, undoubtedly, for the possessor of a moderately good telescope there is no way of viewing the sun's features comparable to that now to be described, which has been systematically and successfully applied for a long series of years by the rev. f. howlett. to use his own words: "any one possessing a good achromatic of not more than three inches' aperture, who has a little dexterity with his pencil, and a little time at his disposal (all the better if it be at a somewhat early hour of the morning)" may by this method "deliberately and satisfactorily view, measure, and (if skill suffice) delineate most of those interesting and grand solar phenomena of which he may have read, or which he may have seen depicted, in various works on physical astronomy."[ ] the method in question depends on the same property which is involved in the use of the pyramidal box just described, supplemented (where exact and systematic observation is required) by the fact that objects lying on or between the lenses of the eye-piece are to be seen faithfully projected on the white surface on which the sun's image is received. in place, however, of a box carried upon the telescope-tube, a darkened room (or true _camera obscura_) contains the receiving sheet. a chamber is to be selected, having a window looking towards the south--a little easterly, if possible, so as to admit of morning observation. all windows are to be completely darkened save one, through which the telescope is directed towards the sun. an arrangement is to be adopted for preventing all light from entering by this window except such light as passes down the tube of the telescope. this can readily be managed with a little ingenuity. mr. howlett describes an excellent method. the following, perhaps, will sufficiently serve the purposes of the general observer:--a plain frame (portable) is to be constructed to fit into the window: to the four sides of this frame triangular pieces of cloth (impervious to light) are to be attached, their shape being such that when their adjacent edges are sewn together and the flaps stretched out, they form a rectangular pyramid of which the frame is the base. through the vertex of this pyramid (near which, of course, the cloth flaps are not sewn together) the telescope tube is to be passed, and an elastic cord so placed round the ends of the flaps as to prevent any light from penetrating between them and the telescope. it will now be possible, without disturbing the screen (fixed in the window), to move the telescope so as to follow the sun during the time of observation. and the same arrangement will serve for all seasons, if so managed that the elastic cord is not far from the middle of the telescope-tube; for in this case the range of motion is small compared to the range of the tube's extremity. a large screen of good drawing-paper should next be prepared. this should be stretched on a light frame of wood, and placed on an easel, the legs of which should be furnished with holes and pegs that the screen may be set at any required height, and be brought square to the tube's axis. a large t-square of light wood will be useful to enable the observer to judge whether the screen is properly situated in the last respect. we wish now to direct the tube towards the sun, and this "without dazzling the eyes as by the ordinary method." this may be done in two ways. we may either, before commencing work--that is, before fastening our elastic cord so as to exclude all light--direct the tube so that its shadow shall be a perfect circle (when of course it is truly directed), then fasten the cord and afterwards we can easily keep the sun in the field by slightly shifting the tube as occasion requires. or (if the elastic cord has already been fastened) we may remove the eye-tube and shift the telescope-tube about--the direction in which the sun lies being roughly known--until we see the spot of light received down the telescope's axis grow brighter and brighter and finally become a _spot of sun-light_. if a card be held near the focus of the telescope there will be seen in fact an image of the sun. the telescope being now properly directed, the eye-tube may be slipped in again, and the sun may be kept in the field as before. there will now be seen upon the screen a picture of the sun very brilliant and pleasing, but perhaps a little out of focus. the focusing should therefore next be attended to, the increase of clearness in the image being the test of approach to the true focus. and again, it will be well to try the effect of slight changes of distance between the screen and the telescope's eye-piece. mr. howlett considers one yard as a convenient distance for producing an excellent effect with almost any eye-piece that the state of the atmosphere will admit of. of course, the image becomes more sharply defined if we bring the screen nearer to the telescope, while all the details are enlarged when we move the screen away. the enlargement has no limits save those depending on the amount of light in the image. but, of course, the observer must not expect enlargement to bring with it a view of new details, after a certain magnitude of image has been attained. still there is something instructive, i think, in occasionally getting a very magnified view of some remarkable spot. i have often looked with enhanced feelings of awe and wonder on the gigantic image of a solar spot thrown by means of the diagonal eye-piece upon the ceiling of the observing-room. blurred and indistinct through over-magnifying, yet with a new meaning to me, _there_ the vast abysm lies pictured; vague imaginings of the vast and incomprehensible agencies at work in the great centre of our system crowd unbidden into my mind; and i seem to _feel_--not merely think about--the stupendous grandeur of that life-emitting orb. to return, however, to observation:--by slightly shifting the tube, different parts of the solar disc can be brought successively upon the screen and scrutinized as readily as if they were drawn upon a chart. "with a power of--say about or linear--the most minute solar spot, properly so called, that is capable of formation" (mr. howlett believes "they are never less than three seconds in length or breadth) will be more readily detected than by any other method," see plate ; "as also will any faculæ, mottling, or in short, any other phenomena that may then be existing on the disc." "drifting clouds frequently sweep by, to vary the scene, and occasionally an aërial hail- or snow-storm." mr. howlett has more than once seen a distant flight of rooks pass slowly across the disc with wonderful distinctness, when the sun has been at a low altitude, and likewise, much more frequently, the rapid dash of starlings, which, very much closer at hand, frequent his church-tower." an eclipse of the sun, or a transit of an inferior planet, is also much better seen in this way than by any other method of observing the solar disc. in plate are presented several solar spots as they have appeared to mr. howlett, with an instrument of moderate power. the grotesque forms of some of these are remarkable; and the variations the spots undergo from day to day are particularly interesting to the thoughtful observer. a method of measuring the spots may now be described. it is not likely indeed that the ordinary observer will care to enter upon any systematic series of measurements. but even in his case, the means of forming a general comparison between the spots he sees at different times cannot fail to be valuable. also the knowledge--which a simple method of measurement supplies--of the actual dimensions of a spot in miles (roughly) is calculated to enhance our estimate of the importance of these features of the solar disc. i give mr. howlett's method in his own words:-- "cause your optician to rule for you on a circular piece of glass a number of fine graduations, the th part of an inch apart, each fifth and tenth line being of a different length in order to assist the eye in their enumeration. insert this between the anterior and posterior lenses of a huygenian eye-piece of moderate power, say linear. direct your telescope upon the sun, and having so arranged it that the whole disc of the sun may be projected on the screen, count carefully the number of graduations that are seen to exactly occupy the solar diameter.... it matters not in which direction you measure your diameter, provided only the sun has risen some ° or ° above the horizon, and so escaped the distortion occasioned by refraction.[ ] "next let us suppose that our observer has been observing the sun on any day of the year, say, if you choose, at the time of its mean apparent diameter, namely about the first of april or first of october, and has ascertained that" (as is the case with mr. howlett's instrument) "sixty-four graduations occupy the diameter of the projected image. now the semi-diameter of the sun, at the epochs above mentioned, according to the tables given for every day of the year in the 'nautical almanac' (the same as in dietrichsen and hannay's very useful compilation) is ' ", and consequently his mean total diameter is ' " or ". if now we divide " by " this will, of course, award as nearly as possible " as the value in celestial arc of each graduation, either as seen on the screen, or as applied directly to the sun or any heavenly body large enough to be measured by it." since the sun's diameter is about , miles, each graduation (in the case above specified) corresponds to one- th part of , miles--that is, to a length of , miles on the sun's surface. any other case can be treated in precisely the same manner. it will be found easy so to place the screen that the distance between successive graduations (as seen projected upon the screen) may correspond to any desired unit of linear measurement--say an inch. then if the observer use transparent tracing-paper ruled with faint lines forming squares half-an-inch in size, he can comfortably copy directly from the screen any solar phenomena he may be struck with. a variety of methods of drawing will suggest themselves. mr. howlett, in the paper i have quoted from above, describes a very satisfactory method, which those who are anxious to devote themselves seriously to solar observation will do well to study. it is necessary that the observer should be able to determine approximately where the sun's equator is situated at the time of any observation, in order that he may assign to any spot or set of spots its true position in relation to solar longitude and latitude. mr. howlett shows how this may be done by three observations of the sun made at any fixed hour on successive days. perhaps the following method will serve the purpose of the general observer sufficiently well:-- the hour at which the sun crosses the meridian must be taken for the special observation now to be described. this hour can always be learnt from 'dietrichsen's almanac'; but noon, civil time, is near enough for practical purposes. now it is necessary first to know the position of the ecliptic with reference to the celestial equator. of course, at noon a horizontal line across the sun's disc is parallel to the equator, but the position of that diameter of the sun which coincides with the ecliptic is not constant: at the summer and winter solstices this diameter coincides with the other, or is horizontal at noon; at the spring equinox the sun (which travels on the ecliptic) is passing towards the north of the equator, crossing that curve at an angle of - / °, so that the ecliptic coincides with that diameter of the sun which cuts the horizontal one at an angle of - / ° and has its _left_ end above the horizontal diameter; and at the autumn equinox the sun is descending and the same description applies, only that the diameter (inclined - / ° to the horizon) which has its _right_ end uppermost, now represents the ecliptic. for intermediate dates, use the following little table:-- -------------------------------------------------------------------------- date. |dec. |jan. |jan. |feb. |feb. |mar. |mar. (circiter.) | |june |may |may |apr. |apr. | -------------------+-------+------+-------+------+-------+-------+-------- inclination of |left |left |left |left |left |left |left ecliptical diameter| | | | | | | of sun to the | ° ' | ° ' | ° ' | ° ' | ° ' | ° ' | ° ' horizon.[ ] |right |right |right |right |right |right |right -------------------+-------+------+-------+------+-------+-------+-------- date. | |dec. |nov. |nov. |oct. |oct. | (circiter.) |jan. |july |july |aug. |aug. |sept. |sept. -------------------------------------------------------------------------- now if our observer describe a circle, and draw a diameter inclined according to above table, this diameter would represent the sun's equator if the axis of the sun were square to the ecliptic-plane. but this axis is slightly inclined, the effect of which is, that on or about june the sun is situated as shown in fig. with respect to the ecliptic _ab_; on or about september he is situated as shown in fig. ; on or about december as shown in fig. ; and on or about march as shown in fig. . the inclination of his equator to the ecliptic being so small, the student can find little difficulty in determining with sufficient approximation the relation of the sun's polar axis to the ecliptic on intermediate days, since the equator is never more _inclined_ than in figs. and , never more _opened out_ than in figs. and . having then drawn a line to represent the sun's ecliptical diameter inclined to the horizontal diameter as above described, and having (with this line to correspond to _ab_ in figs. - ) drawn in the sun's equator suitably inclined and opened out, he has the sun's actual presentation (at noon) as seen with an erecting eye-piece. holding his picture upside down, he has the sun's presentation as seen with an astronomical eye-piece--and, finally, looking at his picture from behind (without inverting it), he has the presentation seen when the sun is projected on the screen. hence, if he make a copy of this last view of his diagram upon the centre of his screen, and using a low power, bring the whole of the sun's image to coincide with the circle thus drawn (to a suitable scale) on the screen, he will at once see what is the true position of the different sun-spots. after a little practice the construction of a suitably sized and marked circle on the screen will not occupy more than a minute or two. [illustration: _fig. ._] [illustration: _fig. ._] [illustration: _fig. ._] [illustration: _fig. ._] it must be noticed that the sun's apparent diameter is not always the same. he is nearer to us in winter than in summer, and, of course, his apparent diameter is greater at the former than at the latter season. the variation of the apparent diameter corresponds (inversely) to the variation of distance. as the sun's greatest distance from the earth is , , miles (pretty nearly) and his least , , , his greatest, mean, and least apparent diameters are as , - / , and respectively; that is, as , , and respectively. mr. howlett considers that with a good -inch telescope, applied in the manner we have described, all the solar features may be seen, except the separate granules disclosed by first-class instruments in the hands of such observers as dawes, huggins, or secchi. faculæ may, of course, be well seen. they are to be looked for near spots which lie close to the sun's limb. when the sun's general surface is carefully scrutinised, it is found to present a mottled appearance. this is a somewhat delicate feature. it results, undoubtedly, from the combined effect of the granules separately seen in powerful instruments. sir john herschel has stated that he cannot recognise the marbled appearance of the sun with an achromatic. mr. webb, however, has seen this appearance with such a telescope, of moderate power, used with direct vision; and certainly i can corroborate mr. howlett in the statement that this appearance may be most distinctly seen when the image of the sun is received within a well-darkened room. my space will not permit me to enter here upon the discussion of any of those interesting speculations which have been broached concerning solar phenomena. we may hope that the great eclipse of august, , which promises to be the most favourable (for effective observation) that has ever taken place, will afford astronomers the opportunity of resolving some important questions. it seems as if we were on the verge of great discoveries,--and certainly, if persevering and well-directed labour would seem in any case to render such discoveries due as man's just reward, we may well say that he deserves shortly to reap a harvest of exact knowledge respecting solar phenomena. the end. footnotes: [footnote : such a telescope is most powerful with the shortest sight. it may be remarked that the use of a telescope often reveals a difference in the sight of the two eyes. in my own case, for instance, i have found that the left eye is very short-sighted, the sight of the right eye being of about the average range. accordingly with my left eye a - / -foot object-glass, alone, forms an effective telescope, with which i can see jupiter's moons quite distinctly, and under favourable circumstances even saturn's rings. i find that the full moon is too bright to be observed in this way without pain, except at low altitudes.] [footnote : betelgeuse--commonly interpreted the giant's shoulder--_ibt-al-jauza_. the words, however, really signify, "the armpit of the central one," orion being so named because he is divided centrally by the equator.] [footnote : i have never been able to see more than four with a - / -inch aperture. i give a view of the trapezium as seen with an -inch equatorial.] [footnote : sir w. herschel several times saw [epsilon] lyræ as a double. bessel also relates that when he was a lad of thirteen he could see this star double. i think persons having average eye-sight could see it double if they selected a suitable hour for observation. my own eye-sight is not good enough for this, but i can distinctly see this star wedged whenever the line joining the components is inclined about ° to the horizon, and also when lyra is near the zenith.] [footnote : they were so described by admiral smyth in . mr. main, in , describes them as straw-coloured and reddish, while mr. webb, in , saw them pale-yellow and _lilac_!] [footnote : or the observer may sweep from [omicron] towards [nu], looking for r about two-fifths of the way from [omicron] to [nu].] [footnote : here a single period only is taken, to get back to a convenient hour of the evening.] [footnote : here a single period only is taken, to get back to a convenient hour of the evening.] [footnote : i have constructed a zodiac-chart, which will enable the student to mark in the path of a planet, at any season of the year, from the recorded places in the almanacs.] [footnote : it is convenient to remember that through precession a star near the ecliptic shifts as respects the r.a. and dec. lines, through an arc of one degree--or nearly twice the moon's diameter--in about years, all other stars through a less arc.] [footnote : mercury is best seen when in quadrature to the sun, but _not_ (as i have seen stated) at those quadratures in which he attains his maximum elongation from the sun. this will appear singular, because the maximum elongation is about °, the minimum only about °. but it happens that in our northern latitudes mercury is always _south_ of the sun when he attains his maximum elongation, and this fact exercises a more important effect than the mere amount of elongation.] [footnote : it does not seem to me that the difficulty of detecting mercury is due to the difficulty "of identifying it amongst the surrounding stars, during the short time that it can be seen" (hind's 'introduction to astronomy'). there are few stars which are comparable with mercury in brilliancy, when seen under the same light.] [footnote : i may notice another error sometimes made. it is said that the shadow of a satellite _appears_ elliptical when near the edge of the disc. the shadow is _in reality_ elliptical when thus situated, but _appears_ circular. a moment's consideration will show that this should be so. the part of the disc concealed by a _satellite_ near the limb is also elliptical, but of course appears round.] [footnote : from a paper by mr. breen, in the 'popular science review,' october, .] [footnote : 'intellectual observer' for july, , to which magazine the reader is referred for full details of mr. howlett's method of observation, and for illustrations of the appliances he made use of, and of some of his results.] [footnote : as the sun does not attain such an altitude as ° during two months in the year, it is well to notice that the true length of the sun's apparent solar diameter is determinable even immediately after sun-rise, if the line of graduation is made to coincide with the _horizontal_ diameter of the picture on the screen--for refraction does not affect the length of this diameter.] [footnote : the words "left" and "right" indicate which end of the sun's ecliptical diameter is uppermost at the dates in upper or lower row respectively.] london: printed by w. clowes and sons, duke street, stamford street, and charing cross. generously made available by the internet archive/american libraries.) transcriber's note obvious typographical errors have been corrected. a list of corrections is found at the end of the text. [illustration: lilly's hieroglyphs (published in )] myths and marvels of astronomy by richard a. proctor author of "rough ways made smooth," "the expanse of heaven," "our place among infinities," "pleasant ways in science," etc., etc. _new edition_ longmans, green, and co. london, new york, and bombay _printed by_ ballantyne, hanson & co _at the ballantyne press_ preface. the chief charm of astronomy, with many, does not reside in the wonders revealed to us by the science, but in the lore and legends connected with its history, the strange fancies with which in old times it has been associated, the half-forgotten myths to which it has given birth. in our own times also, astronomy has had its myths and fancies, its wild inventions, and startling paradoxes. my object in the present series of papers has been to collect together the most interesting of these old and new astronomical myths, associating with them, in due proportion, some of the chief marvels which recent astronomy has revealed to us. to the former class belong the subjects of the first four and the last five essays of the present series, while the remaining essays belong to the latter category. throughout i have endeavoured to avoid technical expressions on the one hand, and ambiguous phraseology (sometimes resulting from the attempt to avoid technicality) on the other. i have, in fact, sought to present my subjects as i should wish to have matters outside the range of my special branch of study presented for my own reading. richard a. proctor. contents. page i. astrology ii. the religion of the great pyramid iii. the mystery of the pyramids iv. swedenborg's visions of other worlds v. other worlds and other universes vi. suns in flames vii. the rings of saturn viii. comets as portents ix. the lunar hoax x. on some astronomical paradoxes xi. on some astronomical myths xii. the origin of the constellation-figures myths and marvels of astronomy i. _astrology._ signs and planets, in aspects sextile, quartile, trine, conjoined, or opposite; houses of heaven, with their cusps, hours, and minutes; almuten, almochoden, anahibazon, catahibazon; a thousand terms of equal sound and significance.--_guy mannering._ ... come and see! trust thine own eyes. a fearful sign stands in the house of life, an enemy: a fiend lurks close behind the radiance of thy planet--oh! be warned!--coleridge. astrology possesses a real interest even in these days. it is true that no importance attaches now even to the discussion of the considerations which led to the rejection of judicial astrology. none but the most ignorant, and therefore superstitious, believe at present in divination of any sort or kind whatsoever. divination by the stars holds no higher position than palmistry, fortune-telling by cards, or the indications of the future which foolish persons find in dreams, tea-dregs, salt-spilling, and other absurdities. but there are two reasons which render the history of astrology interesting. in the first place, faith in stellar influences was once so widespread that astrological terminology came to form a part of ordinary language, insomuch that it is impossible rightly to understand many passages of ancient and mediæval literature, or rightly to apprehend the force of many allusions and expressions, unless the significance of astrological teachings to the men of those times be recognised. in the second place, it is interesting to examine how the erroneous teachings of astrology were gradually abandoned, to note the way in which various orders of mind rejected these false doctrines or struggled to retain them, and to perceive how, with a large proportion of even the most civilised races, the superstitions of judicial astrology were long retained, or are retained even to this very day. the world has still to see some superstitions destroyed which are as widely received as astrology ever was, and which will probably retain their influence over many minds long after the reasoning portion of the community have rejected them. even so far back as the time of eudoxus the pretensions of astrologers were rejected, as cicero informs us ('de div.' ii. ). and though the romans were strangely superstitious in such matters, cicero reasons with excellent judgment against the belief in astrology. gassendi quotes the argument drawn by cicero against astrology, from the predictions of the chaldæans that cæsar, crassus, and pompey would die 'in a full old age, in their own houses, in peace and honour,' whose deaths, nevertheless, were 'violent, immature, and tragical.' cicero also used an argument whose full force has only been recognised in modern times. 'what contagion,' he asked, 'can reach us from the planets, whose distance is almost infinite?' it is singular that seneca, who was well acquainted with the uniform character of the planetary motions, seems to have entertained no doubt respecting their influence. tacitus expresses some doubts, but was on the whole inclined to believe in astrology. 'certainly,' he says, 'the majority of mankind cannot be weaned from the opinion that at the birth of each man his future destiny is fixed; though some things may fall out differently from the predictions, by the ignorance of those who profess the art; and thus the art is unjustly blamed, confirmed as it is by noted examples in all ages.'[ ] probably, the doubt suggested by the different fortunes and characters of men born at the same time must have occurred to many before cicero dwelt upon it. pliny, who followed cicero in this, does not employ the argument quite correctly, for he says that, 'in every hour, in every part of the world, are born lords and slaves, kings and beggars.' but of course, according to astrological principles, it would be necessary that two persons, whose fortunes were to be alike, should be born, not only in the same hour, but in the same place. the fortunes and character of jacob and esau, however, should manifestly have been similar, which was certainly not the case, if their history has been correctly handed down to us. an astrologer of the time of julius cæsar, named publius nigidius figulus, used a singular argument against such reasoning. when an opponent urged the different fortunes of men born nearly at the same instant, nigidius asked him to make two contiguous marks on a potter's wheel which was revolving rapidly. when the wheel was stopped, the two marks were found to be far apart. nigidius is said to have received the name of figulus (the potter), in remembrance of the story; but more probably he was a potter by trade, and an astrologer only during those leisure hours which he could devote to charlatanry. st. augustine, who relates the story (which i borrow from whewell's 'history of the inductive sciences'), says, justly, that the argument of nigidius was as fragile as the ware made on the potter's wheel. the belief must have been all but universal in those days that at the birth of any person who was to hold an important place in the world's history the stars would either be ominously conjoined, or else some blazing comet or new star would make its appearance. for we know that some such object having appeared, or some unusual conjunction of planets having occurred, near enough to the time of christ's birth to be associated in men's minds with that event, it came eventually to be regarded as belonging to his horoscope, and as actually indicating to the wise men of the east (chaldæan astrologers, doubtless) the future greatness of the child then born. it is certain that that is what the story of the star in the east means as it stands. theologians differ as to its interpretation in points of detail. some think the phenomenon was meteoric, others that a comet then made its appearance, others that a new star shone out, and others that the account referred to a conjunction of jupiter, saturn, and mars, which occurred at about that time. as a matter of detail it may be mentioned, that none of these explanations in the slightest degree corresponds with the account, for neither meteor, nor comet, nor new star, nor conjoined planets, would go before travellers from the east, to show them their way to any place. yet the ancients sometimes regarded comets as guides. whichever view we accept, it is abundantly clear that an astrological significance was attached by the narrator to the event. and not so very long ago, when astrologers first began to see that their occupation was passing from them, the wise men of the east were appealed to against the enemies of astrology,[ ]--very much as moses was appealed to against copernicus and galileo, and more recently to protect us against certain relationships which darwin, wallace, and huxley unkindly indicate for the human race divine. although astronomers now reject altogether the doctrines of judicial astrology, it is impossible for the true lover of that science to regard astrology altogether with contempt. astronomy, indeed, owes much more to the notions of believers in astrology than is commonly supposed. astrology bears the same relation to modern astronomy that alchemy bears to modern chemistry. as it is probable that nothing but the hope of gain, literally in this case _auri sacra fames_, would have led to those laborious researches of the alchemists which first taught men how to analyse matter into its elementary constituents, and afterwards to combine these constituents afresh into new forms, so the belief that, by carefully studying the stars, men might acquire the power of predicting future events, first directed attention to the movements of the celestial bodies. kepler's saying, that astrology, though a fool, was the daughter of a wise mother,[ ] does not by any means present truly the relationship between astrology and astronomy. rather we may say that astrology and alchemy, though foolish mothers, gave birth to those wise daughters, astronomy and chemistry. even this way of speaking scarcely does justice to the astrologers and alchemists of old times. their views appear foolish in the light of modern scientific knowledge, but they were not foolish in relation to what was known when they were entertained. modern analysis goes far to demonstrate the immutability, and, consequently, the non-transmutability of the metals, though it is by no means so certain as many suppose that the present position of the metals in the list of _elements_ is really correct. certainly a chemist of our day would be thought very unwise who should undertake a series of researches with the object of discovering a mineral having such qualities as the alchemists attributed to the philosopher's stone. but when as yet the facts on which the science of chemistry is based were unknown, there was nothing unreasonable in supposing that such a mineral might exist, or the means of compounding it be discovered. nay, many arguments from analogy might be urged to show that the supposition was altogether probable. in like manner, though the known facts of astronomy oppose themselves irresistibly to any belief in planetary influences upon the fates of men and nations, yet before those facts were discovered it was not only not unreasonable, but was in fact, highly reasonable to believe in such influences, or at least that the sun, and moon, and stars moved in the heavens in such sort as to indicate what would happen. if the wise men of old times rejected the belief that 'the stars in their courses fought' for or against men, they yet could not very readily abandon the belief that the stars were for signs in the heavens of what was to befall mankind. if we consider the reasoning now commonly thought valid in favour of the doctrine that other orbs besides our earth are inhabited, and compare it with the reasoning on which judicial astrology was based, we shall not find much to choose between the two, so far as logical weight is concerned. because the only member of the solar system which we can examine closely is inhabited, astronomers infer a certain degree of probability for the belief that the other planets of the system are also inhabited. and because the only sun we know much about is the centre of a system of planets, astronomers infer that probably the stars, those other suns which people space, are also the centres of systems; although no telescope which man can make would show the members of a system like ours, attending on even the nearest of all the stars. the astrologer had a similar argument for his belief. the moon, as she circles around the earth, exerts a manifest influence upon terrestrial matter--the tidal wave rising and sinking synchronously with the movements of the moon, and other consequences depending directly or indirectly upon her revolution around the earth. the sun's influence is still more manifest; and, though it may have required the genius of a herschel or of a stephenson to perceive that almost every form of terrestrial energy is derived from the sun, yet it must have been manifest from the very earliest times that the greater light which rules the day rules the seasons also, and, in ruling them, provides the annual supplies of vegetable food, on which the very existence of men and animals depends. if these two bodies, the sun and moon, are thus potent, must it not be supposed, reasoned the astronomers of old, that the other celestial bodies exert corresponding influences? _we_ know, but they did not know, that the moon rules the tides effectually because she is near to us, and that the sun is second only to the moon in tidal influence because of his enormous mass and attractive energy. we know also that his position as fire, light, and life of the earth and its inhabitants, is due directly to the tremendous heat with which the whole of his mighty frame is instinct. not knowing this, the astronomers of old times had no sufficient reason for distinguishing the sun and moon from the other celestial bodies, so far at least as the general question of celestial influences was concerned. so far as particulars were concerned, it was not altogether so clear to them as it is to us, that the influence of the sun must be paramount in all respects save tidal action, and that of the moon second only to the sun's in other respects, and superior to his in tidal sway alone. many writers on the subject of life in other worlds are prepared to show (as brewster attempts to do, for example) that jupiter and saturn are far nobler worlds than the earth, because superior in this or that circumstance. so the ancient astronomers, in their ignorance of the actual conditions on which celestial influences depend, found abundant reasons for regarding the feeble influences exerted by saturn, jupiter, and mars, as really more potent than those exerted by the sun himself upon the earth. they reasoned, as milton afterwards made raphaël reason, that 'great or bright infers not excellence,' that saturn or jupiter, though 'in comparison so small, nor glist'ring' to like degree, may yet 'of solid good contain more plenty than the sun.' supposing the influence of a celestial body to depend on the magnitude of its sphere, in the sense of the old astronomy (according to which each planet had its proper sphere, around the earth as centre), then the influence of the sun would be judged to be inferior to that of either saturn, jupiter, or mars; while the influences of venus and mercury, though inferior to the influence of the sun, would still be held superior to that of the moon. for the ancients measured the spheres of the seven planets of their system by the periods of the apparent revolution of those bodies around the celestial dome, and so set the sphere of the moon innermost, enclosed by the sphere of mercury, around which in turn was the sphere of venus, next the sun's, then, in order, those of mars, jupiter, and saturn. we can readily understand how they might come to regard the slow motions of the sphere of saturn and jupiter, taking respectively some thirty and twelve years to complete a revolution, as indicating power superior to the sun's, whose sphere seemed to revolve once in a single year. many other considerations might have been urged, before the copernican theory was established, to show that, possibly, some of the planets exert influences more effective than those of the sun and moon. it is, indeed, clear that the first real shock sustained by astrology came from the arguments of copernicus. so long as the earth was regarded as the centre round which all the celestial bodies move, it was hopeless to attempt to shake men's faith in the influences of the stars. so far as i know, there is not a single instance of a believer in the old ptolemaic system who rejected astrology absolutely. the views of bacon--the last of any note who opposed the system of copernicus[ ]--indicate the extreme limits to which a ptolemaist could go in opposition to astrology. it may be worth while to quote bacon's opinion in this place, because it indicates at once very accurately the position held by believers in astrology in his day, and the influence which the belief in a central fixed earth could not fail to exert on the minds of even the most philosophical reasoners. 'astrology,' he begins, 'is so full of superstition that scarce anything sound can be discovered in it; though we judge it should rather be purged than absolutely rejected. yet if any one shall pretend that this science is founded not in reason and physical contemplations, but in the direct experience and observation of past ages, and therefore not to be examined by physical reasons, as the chaldæans boasted, he may at the same time bring back divination, auguries, soothsaying, and give in to all kinds of fables; for these also were said to descend from long experience. but we receive astrology as a part of physics, without attributing more to it than reason and the evidence of things allow, and strip it of its superstition and conceits. thus we banish that empty notion about the horary reign of the planets, as if each resumed the throne thrice in twenty-four hours, so as to leave three hours supernumerary; and yet this fiction produced the division of the week,[ ] a thing so ancient and so universally received. thus likewise we reject as an idle figment the doctrine of horoscopes, and the distribution of the houses, though these are the darling inventions of astrology, which have kept revel, as it were, in the heavens. and lastly, for the calculation of nativities, fortunes, good or bad hours of business, and the like fatalities, they are mere levities, that have little in them of certainty and solidity, and may be plainly confuted by physical reasons. but here we judge it proper to lay down some rules for the examination of astrological matters, in order to retain what is useful therein, and reject what is insignificant. thus, . let the greater revolutions be retained, but the lesser, of horoscopes and houses, be rejected--the former being like ordnance which shoot to a great distance, whilst the other are but like small bows, that do no execution. . the celestial operations affect not all kinds of bodies, but only the more sensible, as humours, air, and spirits. . all the celestial operations rather extend to masses of things than to individuals, though they may obliquely reach some individuals also which are more sensible than the rest, as a pestilent constitution of the air affects those bodies which are least able to resist it. . all the celestial operations produce not their effects instantaneously, and in a narrow compass, but exert them in large portions of time and space. thus predictions as to the temperature of a year may hold good, but not with regard to single days. . there is no fatal necessity in the stars; and this the more prudent astrologers have constantly allowed. . we will add one thing more, which, if amended and improved, might make for astrology--viz. that we are certain the celestial bodies have other influences besides heat and light, but these influences act not otherwise than by the foregoing rules, though they lie so deep in physics as to require a fuller explanation. so that, upon the whole, we must register as needed,[ ] an astrology written in conformity with these principles, under the name of _astrologia sana_.' he then proceeds to show what this just astrology should comprehend--as, , the doctrine of the commixture of rays; , the effect of nearest approaches and farthest removes of planets to and from the point overhead (the planets, like the sun, having their summer and winter); , the effects of distance, 'with a proper enquiry into what the vigour of the planets may perform of itself, and what through their nearness to us; for,' he adds, but unfortunately without assigning any reason for the statement, 'a planet is more brisk when most remote, but more communicative when nearest;' , the other accidents of the planet's motions as they pursue their wand'ring course, now high, now low, then hid, progressive, retrograde, or standing still; , all that can be discovered of the general nature of the planets and fixed stars, considered in their own essence and activity; , lastly, let this just astrology, he says, 'contain, from tradition, the particular natures and alterations of the planets and fixed stars; for' (here is a reason indeed) 'as these are delivered with general consent, they are not lightly to be rejected, unless they directly contradict physical considerations. of such observations let a just astrology be formed; and according to these alone should schemes of the heavens be made and interpreted.' the astrology thus regarded by bacon as sane and just did not differ, as to its primary object, from the false systems which now seem to us so absurd. 'let this astrology be used with greater confidence in prediction,' says bacon, 'but more cautiously in election, and in both cases with due moderation. thus predictions may be made of comets, and all kinds of meteors, inundations, droughts, heats, frosts, earthquakes, fiery eruptions, winds, great rains, the seasons of the year, plagues, epidemic diseases, plenty, famine, wars, seditions, sects, transmigrations of people, and all commotions, or great innovations of things, natural and civil. predictions may possibly be made more particular, though with less certainty, if, when the general tendencies of the times are found, a good philosophical or political judgment applies them to such things as are most liable to accidents of this kind. for example, from a foreknowledge of the seasons of any year, they might be apprehended more destructive to olives than grapes, more hurtful in distempers of the lungs than the liver, more pernicious to the inhabitants of hills than valleys, and, for want of provisions, to monks than courtiers, etc. or if any one, from a knowledge of the influence which the celestial bodies have upon the spirits of mankind, should find it would affect the people more than their rulers, learned and inquisitive men more than the military, etc. for there are innumerable things of this kind that require not only a general knowledge gained from the stars which are the agents, but also a particular one of the passive subjects. nor are elections to be wholly rejected, though not so much to be trusted as predictions; for we find in planting, sowing, and grafting, observations of the moon are not absolutely trifling, and there are many particulars of this kind. but elections are more to be curbed by our rules than predictions; and this must always be remembered, that election only holds in such cases where the virtue of the heavenly bodies, and the action of the inferior bodies also, is not transient, as in the examples just mentioned; for the increases of the moon and planets are not sudden things. but punctuality of time should here be absolutely rejected. and perhaps there are more of these instances to be found in civil matters than some would imagine.' the method of inquiry suggested by bacon as proper for determining the just rules of the astrology he advocated, was, as might be expected, chiefly inductive. there are, said he, 'but four ways of arriving at this science, viz.-- , by future experiments; , past experiments; , traditions; , physical reasons.' but he was not very hopeful as to the progress of the suggested researches. it is vain, he said, to think at present of future experiments, because many ages are required to procure a competent stock of them. as for the past, it is true that past experiments are within our reach, 'but it is a work of labour and much time to procure them. thus astrologers may, if they please, draw from real history all greater accidents, as inundations, plagues, wars, seditions, deaths of kings, etc., as also the positions of the celestial bodies, not according to fictitious horoscopes, but the above-mentioned rules of their revolutions, or such as they really were at the time, and, when the event conspires, erect a probable rule of prediction.' traditions would require to be carefully sifted, and those thrown out which manifestly clashed with physical considerations, leaving those in full force which complied with such considerations. lastly, the physical reasons worthiest of being enquired into are those, said bacon, 'which search into the universal appetites and passions of matter, and the simple genuine motions of the heavenly bodies.' it is evident there was much which, in our time at least, would be regarded as wild and fanciful in the 'sound and just astrology' advocated by bacon. yet, in passing, it may be noticed that even in our own time we have seen similar ideas promulgated, not by common astrologers and fortune-tellers (who, indeed, know nothing about such matters), but by persons supposed to be well-informed in matters scientific. in a roundabout way, a new astrology has been suggested, which is not at all unlike bacon's 'astrologia sana,' though not based, as he proposed that astrology should be, on experiment, or tradition, or physical reasons. it has been suggested, first, that the seasons of our earth are affected by the condition of the sun in the matter of spots, and very striking evidence has been collected to show that this must be the case. for instance, it has been found that years when the sun has been free from spots have been warmer than the average; and it has also been found that such years have been cooler than the average: a double-shotted argument wholly irresistible, especially when it is also found that when the sun has many spots the weather has sometimes been exceptionally warm and sometimes exceptionally cold. if this be not considered sufficient, then note that in one country or continent or hemisphere the weather, when the sun is most spotted (or least, as the case may be), may be singularly hot, while in another country, continent, or hemisphere, the weather may be as singularly cold. so with wind and calm, rain and drought, and so forth. always, whether the sun is very much spotted or quite free from spots, something unusual in the way of weather must be going on somewhere, demonstrating in the most significant way the influence of sun-spots or the want of sun-spots on the weather. it is true that captious minds might say that this method of reasoning proved too much in many ways, as, for example, thus--always, whether the sun is very much spotted or quite free from spots, some remarkable event, as a battle, massacre, domestic tragedy on a large scale, or the like, may be going on, demonstrating in the most significant way the influence of sun-spots or the want of sun-spots on the passions of men--which sounds absurd. but the answer is twofold. first, such reasoning is captious, and secondly, it is not certain that sun-spots, or the want of them, may not influence human passions; it may be worth while to enquire into this possible solar influence as well as the other, which can be done by crossing the hands of the new fortune-tellers with a sufficient amount of that precious metal which astrologers have in all ages dedicated to the sun. that the new system of divination is not solely solar, but partly planetary also, is seen when we remember that the sun-spots wax and wane in periods of time which are manifestly referable to the planetary motions. thus, the great solar spot-period lasts about eleven years, the successive spotless epochs being separated on the average by about that time; and so nearly does this period agree with the period of the planet jupiter's revolution around the sun, that during eight consecutive spot-periods the spots were most numerous when jupiter was farthest from the sun, and it is only by going back to the periods preceding these eight that we find a time when the reverse happened, the spots being most numerous when jupiter was nearest to the sun. so with various other periods which the ingenuity of messrs. de la rue and balfour stewart has detected, and which, under the closest scrutiny, exhibit almost exact agreement for many successive periods, preceded and followed by almost exact disagreement. here, again, the captious may argue that such alternate agreements and disagreements may be noted in every case where two periods are not very unequal, whether there be any connection between them or not; but much more frequently when there is no connection: and that the only evidence really proving a connection between planetary motions and the solar spots would be constant agreement between solar spot periods and particular planetary periods. but the progress of science, and especially the possible erection of a new observatory for finding out ('for a consideration') how sun-spots affect the weather, etc., ought not to be interfered with by captious reasoners in this objectionable manner. nor need any other answer be given them. seeing, then, that sun-spots manifestly affect the weather and the seasons, while the planets rule the sun-spots, it is clear that the planets really rule the seasons. and again, seeing that the planets rule the seasons, while the seasons largely affect the well-being of men and nations (to say nothing of animals), it follows that the planets influence the fates of men and nations (and animals). _quod erat demonstrandum._ let us return, however, to the more reasonable astrology of the ancients, and enquire into some of the traditions which bacon considered worthy of attention in framing the precepts of a sound and just astrology. it was natural that the astrologers of old should regard the planetary influences as depending in the main on the position of the celestial bodies on the sky above the person or place whose fortunes were in question. thus two men at the same moment in rome and in persia would by no means have the same horoscope cast for their nativities, so that their fortunes, according to the principles of judicial astrology, would be quite different. in fact it might happen that two men, born at the same instant of time, would have all the principal circumstances of their lives contrasted--planets riding high in the heavens of one being below the horizon of the other, and _vice versâ_. the celestial sphere placed as at the moment of the native's birth was divided into twelve parts by great circles supposed to pass through the point overhead, and its opposite, the point vertically beneath the feet. these twelve divisions were called 'houses.' their position is illustrated in the following figure, taken from raphaël's astrology. [illustration: particular significations of the _twelve celestial houses_, according to various astrological authors. sun-rise. cusp of the _ascendant_. life and health cusp of the _second house_. riches cusp of the _third house_. kindred and short journeys cusp of the _fourth house_. inheritances mid-night. cusp of the _fifth house_. children cusp of the _sixth house_. sickness cusp of the _seventh house_. marriage sun-set. cusp of the _eighth house_. death cusp of the _ninth house_. long journeys cusp of the _mid-heaven_. honor noon-day. cusp of the _eleventh house_. friends cusp of the _twelfth house_. enemies ] the first, called the ascendant house, was the portion rising above the horizon at the east. it was regarded as the house of life, the planets located therein at the moment of birth having most potent influence on the life and destiny of the native. such planets were said to rule the ascendant, being in the ascending house; and it is from this usage that our familiar expression that such and such an influence is 'in the ascendant' is derived. the next house was the house of riches, and was one-third of the way from the east below the horizon towards the place of the sun at midnight. the third was the house of kindred, short journeys, letters, messages, etc. it was two-thirds of the way towards the place of the midnight sun. the fourth was the house of parents, and was the house which the sun reached at midnight. the fifth was the house of children and women, also of all sorts of amusements, theatres, banquets, and merry-making. the sixth was the house of sickness. the seventh was the house of love and marriage. these three houses (the fifth, sixth, and seventh) followed in order from the fourth, so as to correspond to the part of the sun's path below the horizon, between his place at midnight and his place when descending in the west. the seventh, opposite to the first, was the descendant. the eighth house was the first house above the horizon, lying to the west, and was the house of death. the ninth house, next to the mid-heaven on the west, was the house of religion, science, learning, books, and long voyages. the tenth, which was in the mid-heaven, or region occupied by the sun at midday, was the house of honour, denoting credit, renown, profession or calling, trade, preferment, etc. the eleventh house, next to the mid-heaven on the east, was the house of friends. lastly, the twelfth house was the house of enemies. the houses were not all of equal potency. the _angular_ houses, which are the first, the fourth, the seventh, and the tenth--lying east, north, west, and south--were first in power, whether for good or evil. the second, fifth, eighth, and eleventh houses were called _succedents_, as following the angular houses, and next to them in power. the remaining four houses--viz. the third, sixth, ninth, and twelfth houses--were called _cadents_, and were regarded as weakest in influence. the houses were regarded as alternately masculine and feminine: the first, third, fifth, etc., being masculine; while the second, fourth, sixth, etc., were feminine. the more particular significations of the various houses are shown in the accompanying figure from the same book. [illustration: a celestial diagram representing at one view the various symbolical significations of the _twelve heavenly houses_; according to ancient manuscript writers of the twelfth century; _and not to be found in authors_. brethren of friends, fathers of kings, sickness of public enemies, wives of enemies, death of servants, long journeys of children, friends of brethren, thoughts of the asker. the end of youth, brethren of private enemies, fathers and grandsires of friends, king's sons, enemies of wives, magistery of children, private enemies of brethren. sects, dreams, churches, fathers of private enemies, sons of friends, sickness of kings, enemies of the religious, trade of servants, private enemies of fathers. dead men's goods, castles, treasure hid, the fate of the corpse in the grave, money of brethren, children of private enemies, sickness of friends, king's enemies, friends of servants. cards, dice, brethren's brethren, father's money, sickness of private enemies, enemies of friends, death of kings, friends of enemies, enemies of servants. vassals, children's money, brethren's fathers, father's brethren, enemies' enemies, death of friends, journeys and religion of kings, lay dignities, enemies of wives. fines, pleas, laws, nuptials, death of enemies, friends of brethren, sons of friends, sisters of brethren, death of enemies and of great beasts, religion of friends. labour, sorrow, inheritance of the dead, money of enemies, brethren of servants, sickness of brethren, dignity of friends, king's friends, enemies of religious persons. prophets, prayers, visions, omens, divine worship, wife's brethren, fathers of servants, children's children, sickness of fathers, enemies of brethren, friends of friends, enemies of kings. judges, brethren of enemies, servants, fathers of enemies, children of servants, sickness of sons, death of brethren, friends of enemies, enemies of friends. knights, esquires, children of enemies, sickness of servants, enemies and wives of offspring, death of fathers, journeys of brethren, enemies of enemies. envy, sorrow, guile, long hidden wrath, money of friends, brethren of kings, sickness of wives, servants' enemies, death of children, trade of brethren, a prison. ] it will be easily understood how these houses were dealt with in erecting a scheme of nativity. the position of the planets at the moment of the native's birth, in the several houses, determined his fortunes with regard to the various matters associated with these houses. thus planets of good influence in the native's ascendant, or first house, signified generally a prosperous life; but if at the same epoch a planet of malefic influence was in the seventh house, then the native, though on the whole prosperous, would be unfortunate in marriage. a good planet in the tenth house signified good fortune and honour in office or business, and generally a prosperous career as distinguished from a happy life; but evil planets in the ninth house would suggest to the native caution in undertaking long voyages, or entering upon religious or scientific controversies. similar considerations applied to questions relating to horary astronomy, in which the position of the planets in the various houses at some epoch guided the astrologer's opinion as to the fortune of that hour, either in the life of a man or the career of a state. in such inquiries, however, not only the position of the planets, etc., at the time had to be considered, but also the original horoscope of the person, or the special planets and signs associated with particular states. thus if jupiter, the most fortunate of all the planets, was in the ascendant, or in the house of honour, at the time of the native's birth, and at some epoch this planet was ill-aspected or afflicted by other planets potent for evil in the native's horoscope, then that epoch would be a threatening one in the native's career. the sign gemini was regarded by astrologers as especially associated with the fortunes of london, and accordingly they tell us that the great fire of london, the plague, the building of london bridge, and other events interesting to london, all occurred when this sign was in the ascendant, or when special planets were in this sign.[ ] the signs of the zodiac in the various houses were in the first place to be noted, because not only had these signs special powers in special houses, but the effects of the planets in particular houses varied according to the signs in which the planets were situated. if we were to follow the description given by the astrologers themselves, not much insight would be thrown upon the meaning of the zodiacal signs. for instance, astrologers say that aries is a vernal, dry, fiery, masculine, cardinal, equinoctial, diurnal, movable, commanding, eastern, choleric, violent, and quadrupedalian sign. we may, however, infer generally from their accounts the influences which they assigned to the zodiacal signs. aries is the house and joy of mars, signifies a dry constitution, long face and neck, thick shoulders, swarthy complexion, and a hasty, passionate temper. it governs the head and face, and all diseases relating thereto. it reigns over england, france, switzerland, germany, denmark, lesser poland, syria, naples, capua, verona, etc. it is a masculine sign, and is regarded as fortunate. taurus gives to the native born under his auspices a stout athletic frame, broad bull-like forehead, dark curly hair, short neck, and so forth, and a dull apathetic temper, exceedingly cruel and malicious if once aroused. it governs the neck and throat, and reigns over ireland, great poland, part of russia, holland, persia, asia minor, the archipelago, mantua, leipsic, etc. it is a feminine sign, and unfortunate. gemini is the house of mercury. the native of gemini will have a sanguine complexion and tall, straight figure, dark eyes quick and piercing, brown hair, active ways, and will be of exceedingly ingenious intellect. it governs the arms and shoulders, and rules over the south-west parts of england, america, flanders, lombardy, sardinia, armenia, lower egypt, london, versailles, brabant, etc. it is a masculine sign, and fortunate. cancer is the house of the moon and exaltation of jupiter, and its native will be of fair but pale complexion, round face, grey or mild blue eyes, weak voice, the upper part of the body large, slender arms, small feet, and an effeminate constitution. it governs the breast and the stomach, and reigns over scotland, holland, zealand, burgundy, africa, algiers, tunis, tripoli, constantinople, new york, etc. it is a feminine sign, and unfortunate. the native born under leo will be of large body, broad shoulders, austere countenance, with dark eyes and tawny hair, strong voice, and leonine character, resolute and ambitious, but generous, free, and courteous. leo governs the heart and back, and reigns over italy, bohemia, france, sicily, rome, bristol, bath, taunton, philadelphia, etc. it is a masculine sign, and fortunate. virgo is the joy of mercury. its natives are of moderate stature, seldom handsome, slender but compact, thrifty and ingenious. it governs the abdomen, and reigns over turkey both in europe and asia, greece, and mesopotamia, crete, jerusalem, paris, lyons, etc. it is a feminine sign, and generally unfortunate. libra is the house of venus. the natives of libra are tall and well made, elegant in person, round-faced and ruddy, but plain-featured and 'inclined to eruptions that disfigure the face when old; they' (the natives) 'are of sweet disposition, just and upright in dealing.' it governs the lumbar regions, and reigns over austria, alsace, savoy, portugal, livonia, india, ethiopia, lisbon, vienna, frankfort, antwerp, charleston, etc. it is a masculine sign, and fortunate. scorpio is, like aries, the house of mars, 'and also his joy.' its natives are strong, corpulent, and robust, with large bones, 'dark curly hair and eyes' (presumably the eyes dark only, not curly), middle stature, dusky complexion, active bodies; they are usually reserved in speech. it governs the region of the groin, and reigns over judæa, mauritania, catalonia, norway, west silesia, upper batavia, barbary, morocco, valentia, messina, etc. it is feminine, and unfortunate. (it would appear likely, by the way, that astrology was a purely masculine science.) sagittarius is the house and joy of jupiter. its natives are well formed and tall, ruddy, handsome, and jovial, with fine clear eyes, chestnut hair, and oval fleshy face. they are 'generally jolly fellows at either bin or board,' active, intrepid, generous, and obliging. it governs the legs and thighs,[ ] and reigns over arabia felix, spain, hungary, moravia, liguria, narbonne, cologne, avignon, etc. it is masculine, and of course fortunate. capricorn is the house of saturn and exaltation of mars. this sign gives to its natives a dry constitution and slender make, with a long thin visage, thin beard (a generally goaty aspect, in fact), dark hair, long neck, narrow chin, and weak knees. it governs, nevertheless, the knees and hams, and reigns over india, macedonia, thrace and greece, mexico, saxony, wilna, mecklenburgh, brandenburg, and oxford. it is feminine, and unfortunate. aquarius also is the house of saturn. its natives are robust, steady, strong, healthy, and of middle stature; delicate complexion, clear but not pale, sandy hair, hazel eyes, and generally an honest disposition. it governs the legs and ankles, and reigns over arabia, petræa, tartary, russia, denmark, lower sweden, westphalia, hamburg, and bremen. it is masculine, and fortunate. pisces is the house of jupiter and exaltation of venus. its natives are short, pale, thick-set, and round-shouldered (like fish), its character phlegmatic and effeminate. it governs the feet and toes, and reigns over portugal, spain, egypt, normandy, galicia, ratisbon, calabria, etc. it is feminine, and therefore, naturally, unfortunate. let us next consider the influences assigned to the various planets and constellations. though we can understand that in old times the planets and stars were regarded as exercising very potent influences upon the fates of men and nations,[ ] it is by no means easy to understand how astrologers came to assign to each planet its special influence. that is, it is not easy to understand how they could have been led to such a result by actual reasoning, still less by any process of observation.[ ] there was a certain scientific basis for the belief in the possibility of determining the special influences of the stars; and we should have expected to find some scientific process adopted for the purpose. yet, so far as can be judged, the influences assigned to the planets depended on entirely fanciful considerations. in some cases we seem almost to see the line along which the fancies of the old astrologers led them, just as in some cases we can perceive how mythological superstitions (which are closely related to astrological ideas) had their origin; though it is not quite clear whether the planets were first regarded as deities with special qualities, and these qualities afterwards assigned to the planetary influences, or whether the planetary influences were first assigned, and came eventually to be regarded as the qualities of the deities associated with the several planets. it is easy, for instance, to understand why astrologers should have regarded the sun as the emblem of kingly power and dignity, and equally easy to understand why, to the sun regarded as a deity, corresponding qualities should have been ascribed; but it is not easy to determine whether the astrological or the sabaistic superstitions were the earlier. and in like manner of the moon and planets. there seems to me no sufficient evidence in favour of whewell's opinion, that 'in whatever manner the sun, moon, and planets came to be identified with gods and goddesses, the characters ascribed to these gods and goddesses, regulated the virtues and powers of the stars which bear their names.' as he himself very justly remarks, 'we do not possess any of the speculations of the earlier astrologers; and we cannot, therefore, be certain that the notions which operated in men's minds when the art had its birth, agreed with the views on which it was afterwards defended.' he does not say why he infers that, though at later periods supported by physical analogies, it was originally suggested by mythological beliefs. quite as probably mythological beliefs were suggested by astrological notions. some of these beliefs, indeed, seem manifestly to have been so suggested; as the character of the deity mercury, from the rapid motions of the planet mercury, and the difficulty of detecting it; the character of mars from the blood-red hue of the planet when close to the horizon, and so forth. let us examine, however, the characteristics ascribed by astrologers to various planets. it is unfortunate for astrology that, despite the asserted careful comparison of events with the planetary positions preceding and indicating them, nothing was ever observed which seemed to suggest the possibility that there may be an unknown planet ruling very strongly the affairs of men. astrologers tell us now that uranus is a very potent planet; yet the old astrologers seem to have got on very well without him. by the way, one of the moderns, the grave raphaël, gives a very singular account of the discovery of uranus, in a book published sixteen years before neptune was discovered by just such a process as raphaël imagined in the case of uranus. he says that drs. halley, bradley, and others, having frequently observed that saturn was disturbed in his motion by some force exerted from beyond his orbit, and being unable to account for the disturbance on the known principles of gravitation, pursued their enquiry into the matter, 'till at length the discovery of this hitherto unknown planet covered their labours with success, and has enabled us to enlarge our present solar system to nearly double its bounds.' of course there is not a word of truth in this; uranus having been discovered by accident long after halley and bradley were in the grave. but the account suggests what might have been, and curiously anticipates the actual manner in which neptune was discovered. astrologers agree in attributing evil effects to uranus. but the evil he does is always peculiarly strange, unaccountable, and totally unexpected. he causes the native born under his influence to be of a very eccentric and original disposition, romantic, unsettled, addicted to change, a seeker after novelty; though, if the moon or mercury have a good aspect towards uranus, the native will be profound in the secret sciences, magnanimous, and lofty of mind. but let all beware of marriage when uranus is in the seventh house, or afflicting the moon. and in general, let the fair sex remember that uranus is peculiarly hostile to them, and very evil in love. saturn is the greater infortune of the old system of astrology, and is by universal experience acknowledged to be the most potent, evil, and malignant of all the planets. those born under him are of dark and pale complexion, with small, black, leering eyes, thick lips and nostrils, large ears, thin face, lowering looks, cloudy aspect, and seemingly melancholy and unhappy; and though they have broad shoulders, they have but short lips and a thin beard, they are in character austere and reserved, covetous, laborious, and revengeful; constant in friendship, and good haters. the most remarkable and certain characteristic of the saturnine man is that, as an old author observes 'he will never look thee in the face.' 'if they have to love any one, these saturnines,' says another old author, 'they love most constantly; and if they hate, they hate to the death.' the persons signified symbolically by saturn are grandparents, and other old persons, day labourers, paupers, beggars, clowns, husbandmen of the meaner sort, and especially undertakers, sextons, and gravediggers. chaucer thus presents the chief effects which saturn produces in the fortunes of men and nations--saturn himself being the speaker:-- ... quod saturne my cours, that hath so wide for to turne, hath more power than wot any man. min is the drenching in the sea so wan, min is the prison in the derke cote, min is the strangel and hanging by the throte, the murmure and the cherles rebelling, the groyning, and the prive empoysoning, i do vengaunce and pleine correction, while i dwell in the signe of the leon; min is the ruine of the high halles, the falling of the toures and of the walles upon the minour or the carpenter: i slew sampson in shaking the piler. min ben also the maladies colde, the derke tresons, and the castes olde: my loking is the fader of pestilence. jupiter, on the contrary, though saturn's next neighbour in the solar system, produces effects of an entirely contrary kind. he is, in fact, the most propitious of all the planets, and the native born under his influence has every reason to be jovial in fact as he is by nature. such a native will be tall and fair, handsome and erect, robust, ruddy, and altogether a good-looking person, whether male or female. the native will also be religious, or at least a good moral honest man, unless jupiter be afflicted by the aspects of saturn, mars, or uranus; in which case he may still be a jolly fellow, no man's enemy but his own--only he will probably be his own enemy to a very considerable extent, squandering his means and ruining his health by gluttony and intoxication. the persons represented by jupiter (when he is not afflicted) are judges, counsellors, church dignitaries, from cardinals to curates, scholars, chancellors, barristers, and the highest orders of lawyers, woollendrapers (possibly there may be some astral significance in the woolsack), and clothiers. when jupiter is afflicted, however, he denotes quacks and mountebanks, knaves, cheats, and drunkards. the influence of the planet on the fortunes is nearly always good. astrologers, who to a man reverence dignities, consider great britain fortunate in that the lady whom, with customary effusion, they term 'our most gracious queen,' was born when jupiter was riding high in the heavens near his culmination, this position promising a most fortunate and happy career. the time has passed when the fortunes of this country were likely to be affected by such things; but we may hope, for the lady's own sake, that this prediction has been fulfilled. astrologers assert the same about the duke of wellington, assigning midnight, may , , as the hour of his birth. there is some doubt both as to the date and place of the great soldier's birth; but the astrologer finds in the facts of his life the means of removing all such doubts.[ ] next in order comes mars, inferior only in malefic influence to saturn, and called by the old astrologers the lesser infortune. the native born under the influence of mars is usually of fierce countenance, his eyes sparkling, or sharp and darting, his complexion fiery or yellowish, and his countenance scarred or furrowed. his hair is reddish or sandy, unless mars chances to be in a watery sign, in which case the hair will be flaxen; or in an earthly sign, in which case the hair will be chestnut. the martialist is broad-shouldered, steady, and strong, but short,[ ] and often bony and lean. in character the martialist is fiery and choleric, naturally delighting in war and contention, but generous and magnanimous. this when mars is well aspected; should the planet be evil aspected, then will the native be treacherous, thievish, treasonable, cruel, and wicked. the persons signified by mars are generals, soldiers, sailors (if he is in a watery sign), surgeons, chemists, doctors, armourers, barbers, curriers, smiths, carpenters, bricklayers, sculptors, cooks, and tailors. when afflicted with mercury or the moon, he denotes thieves, hangmen, and 'all cut throat people.' in fact, except the ploughboy, who belongs to saturn, all the members of the old septet, 'tinker, tailor, soldier, sailor, apothecary, ploughboy, thief,' are favourites with mars. the planet's influence is not quite so evil as saturn's, nor are the effects produced by it so long-lasting. 'the influence of saturn,' says an astrologer, 'may be compared to a lingering but fatal consumption; that of mars to a burning fever.' he is the cause of anger, quarrels, violence, war, and slaughter. the sun comes next; for it must be remembered that, according to the old system of astronomy, the sun was a planet. persons born under the sun as the planet ruling their ascendant, would be more apt to be aware of the fact than saturnine, jovial, martial, or any other folk, because the hour of birth, if remembered, at once determines whether the native is a solar subject or not. the solar native has generally a round face (like pictures of the sun in old books of astronomy), with a short chin; his complexion somewhat sanguine; curling sandy hair, and a white tender skin. as to character, he is bold and resolute, desirous of praise, of slow speech and composed judgment; outwardly decorous, but privately not altogether virtuous. the sun, in fact, according to astrologers, is the natural significator of respectability; for which i can discover no reason, unless it be that the sun travelling always in the ecliptic has no latitude, and so solar folk are allowed none. when the sun is ill aspected, the native is both proud and mean, tyrannical and sycophantic, exceedingly unamiable, and generally disliked because of his arrogance and ignorant pomposity. the persons signified by the sun are emperors, kings, and titled folk generally, goldsmiths, jewellers, and coiners. when 'afflicted,' the sun signifies pretenders either to power or knowledge. the sun's influence is not in itself either good or evil, but is most powerful for good when he is favourably aspected, and for evil when he is afflicted by other planets. venus, the next in order, bore the same relation to the greater fortune jupiter which mars bore to saturn the greater ill-fortune. she was the lesser fortune, and her influence was in nearly all respects benevolent. the persons born under the influence of this planet are handsome, with beautiful sparkling hazel or black eyes (but another authority assigns the subject of venus, 'a full eye, usually we say goggle-eyed,' by which we do not usually imply beauty), ruddy lips, the upper lip short, soft smooth hair, dimples in the cheek and chin, an amorous look and a sweet voice. one old astrologer puts the matter thus pleasantly:--'the native of venus hath,' quoth he, 'a love-dimple in the chin, a lovely mouth, cherry lips, and a right merry countenance.' in character the native of venus is merry 'to a fault,' but of temper engaging, sweet and cheerful, unless she be ill aspected, when her native is apt to be too fond of pleasure and amusement. that her influence is good is shown (in the opinion of raphaël, writing in ) by the character of george iv., 'our present beloved monarch and most gracious majesty, who was born just as this benevolent star' was in the ascendant; 'for it is well known to all europe what a refined and polished genius, and what exquisite taste, the king of england possesses, which therefore may be cited as a most illustrious proof of the celestial science; a proof likewise which is palpably demonstrable, even to the most casual observer, since the time of his nativity is taken from the public journals of the period, and cannot be gainsaid.' 'this illustrious and regal horoscope is replete with wonderful verifications of planetary influence, and england cannot but prosper while she is blessed with the mild and beneficent sway of this potent monarch.' strengthened in faith by this convincing proof of the celestial science, we proceed to notice that venus is the protectrice of musicians, embroiderers, perfumers, classic modellers, and all who work in elegant attire or administer to the luxuries of the great; but when she is afflicted, she represents 'the lower orders of the votaries of voluptuousness.' mercury is considered by astrologers 'a cold, dry, melancholy star.' the mercurial is neither dark nor fair, but between both, long-faced, with high forehead and thin sharp nose, 'thin beard (many times none at all), slender of body, and with small weak eyes;' long slender hands and fingers are 'especial marks of mercury,' says raphaël. in character the mercurial is busy and prattling. but when well affected, mercury gives his subjects a strong, vigorous, active mind, searching and exhaustive, a retentive memory, a natural thirst for knowledge.[ ] the persons signified by mercury are astrologers, philosophers, mathematicians, politicians, merchants, travellers, teachers, poets, artificers, men of science, and all ingenious, clever men. when he is ill affected, however, he represents pettifoggers, cunning vile persons, thieves, messengers, footmen, and servants, etc. the moon comes last in planetary sequence, as nearest to the earth. she is regarded by astrologers as a cold, moist, watery, phlegmatic planet, variable to an extreme, and, like the sun, partaking of good or evil according as she is aspected favourably or the reverse. her natives are of good stature, fair, and pale, moon-faced, with grey eyes, short arms, thick hands and feet, smooth, corpulent and phlegmatic body. when she is in watery signs, the native has freckles on the face, or, says lilly, 'he or she is blub-cheeked, not a handsome body, but a muddling creature.' unless the moon is very well aspected, she ever signifies an ordinary vulgar person. she signifies sailors (not as mars does, the fighting-men of war-ships, but nautical folk generally) and all persons connected with water or any kind of fluid; also all who are engaged in inferior and common offices. we may note, in passing, that to each planet a special metal is assigned, as also particular colours. chaucer, in the chanones yemannes' tale, succinctly describes the distribution of the metals among the planets:-- sol gold is, and luna silver we threpe; mars iren, mercurie silver we clepe: saturnus led, and jupiter is tin, and venus coper, by my [the chanones yemannes'] faderkin. the colours are thus assigned:--to saturn, black; to jupiter, mixed red and green; to mars, red; to the sun, yellow or yellow-purple; to venus, white or purple; to mercury, azure blue; to the moon, a colour spotted with white and other mixed colours. again, the planets were supposed to have special influence on the seven ages of human life. the infant, 'mewling and puking in the nurse's arms,' was very appropriately dedicated to the moist moon; the whining schoolboy (did schoolboys whine in the days of good queen bess?) was less appropriately assigned to mercury, the patron of those who eagerly seek after knowledge: then very naturally, the lover sighing like furnace was regarded as the special favourite of venus. thus far the order has been that of the seven planets of the ancient astrology, in supposed distance. now, however, we have to pass over the sun, finding mars the patron of mid life, appropriately (in this respect) presiding over the soldier full of strange oaths, and so forth; the 'justice in fair round belly with good capon lined' is watched over by the respectable sun; maturer age by jupiter; and, lastly, old age by saturn. colours were also assigned to the twelve zodiacal signs--to aries, white and red; to taurus, white and lemon; to gemini, white and red (the same as aries); to cancer, green or russet; to leo, red or green; to virgo, black speckled with blue; to libra, black, or dark crimson, or tawny colour; to scorpio, brown; to sagittarius, yellow, or a green sanguine (this is as strange a colour as the _gris rouge_ of molière's _l'avare_); capricorn, black or russet, or a swarthy brown; to aquarius, a sky-coloured blue; to pisces, white glistening colour (like a fish just taken out of the water). the chief fixed stars had various influences assigned to them by astrologers. these influences were mostly associated with the imaginary figures of the constellations. thus the bright star in the head of aries, called by some the ram's horn, was regarded as dangerous and evil, denoting bodily hurts. the star menkar in the whale's jaw denoted sickness, disgrace, and ill-fortune, with danger from great beasts. betelgeux, the bright star on orion's right shoulder, denoted martial honours or wealth; bellatrix, the star on orion's left shoulder, denoted military or civic honours; rigel, on orion's left foot, denoted honours; sirius and procyon, the greater and lesser dog stars, both implied wealth and renown. star clusters seem to have portended loss of sight; at least we learn that the pleiades were 'eminent stars,' but denoting accidents to the sight or blindness, while the cluster præsepe or the beehive in like manner threatened blindness. the cluster in perseus does not seem to have been noticed by astrologers. the variable star algol or caput medusæ, which marks the head of gorgon, was accounted 'the most unfortunate, violent, and dangerous star in the heavens.' it is tolerably clear that the variable character of this star had been detected long before montanari (to whom the discovery is commonly attributed) noticed the phenomenon. the name algol is only a variation of al-ghúl, the monster or demon, and it cannot be doubted that the demoniac, gorgonian character assigned to this star was suggested by its ominous change, as though it were the eye of some fierce monster slowly winking amid the gloom of space. the two stars called the aselli, which lie on either side of the cluster præsepe, 'are said' (by astrologers) 'to be of a burning nature, and to give great indications of a violent death, or of violent and severe accidents by fire.' the star called cor hydræ, or the serpent's heart, denotes trouble through women (said i not rightly that astrology was a masculine science?); the lion's heart, regulus, implied glory and riches; deneb, the lion's tail, misfortune and disgrace. the southern scale of libra meant bad fortune, while the northern was eminently fortunate. astrology was divided into three distinct branches--the doctrine of nativities, horary astrology, and state astrology. the first assigned the rules for determining the general fortunes of the native, by drawing up his scheme of nativity or casting his horoscope. it took into account the positions of the various planets, signs, stars, etc., at the time of the native's birth; and as the astrologer could calculate the movements of the planets thereafter, he could find when those planets which were observed by the horoscope to be most closely associated with the native's fortunes would be well aspected or the reverse. thus the auspicious and unlucky epochs of the native's life could be predetermined. the astrologer also claimed some degree of power to rule the planets, not by modifying their movements in any way, but by indicating in what way the ill effects portended by their positions could be prevented. the arabian and persian astrologers, having less skill than the followers of ptolemy, made use of a different method of determining the fortunes of men, not calculating the positions of the planets for many years following the birth of the native, but assigning to every day after his birth a whole year of his life and for every two hours' motion of the moon one month. thus the positions of the stars and planets, twenty-one days after the birth of the native, would indicate the events corresponding to the time when he would have completed his twenty-first year. there was another system called the placidian, in which the effects of the positions of the planets were judged with sole reference to the motion of the earth upon her axis. it is satisfactory to find astrologers in harmony amongst each other as to these various methods, which one would have supposed likely to give entirely different results. 'each of them,' says a modern astrologer, 'is not only correct and approved by long-tried practice, but may be said to defy the least contradiction from those who will but take the pains to examine them (and no one else should deliver an opinion upon the subject). although each of the above methods are different, yet they by no means contradict each other, but each leads to _true results_, and in many instances they each lead to the foreknowledge of the same event; in which respect they may be compared to the ascent of a mountain by different paths, where, although some paths are longer and more difficult than others, they notwithstanding all lead to the same object.' all which, though plausible in tone labours under the disadvantage of being untrue. ptolemy is careful to point out, in his celebrated work the 'tetrabiblos,' that, of all events whatsoever which take place after birth, the most essential is the continuance of life. 'it is useless,' he says, 'to consider what events might happen to the native in later years if his life does not extend, for instance, beyond one year. so that the enquiry into the duration of life takes precedence of all others.' in order to deal properly with this question, it is necessary to determine what planet shall be regarded as the hyleg, apheta, or lord of life, for the native. next the anareta, or destroyer of life, must be ascertained. the anaretic planets are, by nature, saturn, mars, and uranus, though the sun, moon, and mercury may be endowed with the same fatal influence, if suitably afflicted. the various ways in which the hyleg, or giver of life, may be afflicted by the anareta, correspond to the various modes of death. but astrologers have always been singularly careful, in casting horoscopes, to avoid definite reference to the native's death. there are but few cases where the actual day of death is said to have been assigned. one is related in clarendon's 'history of the rebellion.' he tells us that william earl of pembroke died at the age of fifty, on the day upon which his tutor sandford had predicted his decease. burton, the author of the 'anatomy of melancholy,' having cast his own horoscope, and ascertained that he was to die on january , , is said to have committed suicide in order that the accuracy of his calculations might not be called in question. a similar story is related of cardan by dr. young (sidrophel vapulans), on the authority of gassendi, who, however, says only that either cardan starved himself, or, being confident in his art, took the predicted day for a fatal one, and by his fears made it so. gassendi adds that while cardan pretended to describe the fates of his children in his voluminous commentaries, he all the while never suspected, from the rules of his great art, that his dearest son would be condemned in the flower of his youth to be beheaded on a scaffold, by an executioner of justice, for destroying his own wife by poison. horary astrology relates to particular questions, and is a comparatively easy branch of the science. the art of casting nativities requires many years of study; but horary astrology 'may be well understood,' says lilly, 'in less than a quarter of a year.' 'if a proposition of any nature,' he adds, 'be made to any individual, about the result of which he is anxious, and therefore uncertain whether to accede to it or not, let him but note the hour and minute when it was _first_ made, and erect a figure of the heavens, and his doubts will be instantly resolved. he may thus in five minutes learn whether the affair will succeed or not: and consequently whether it is prudent to accept the offer made or not. if he examine the sign on the first house of the figure, the planet therein, or the planet ruling the sign, _will exactly describe the party making the offer_, both in person and character, and this may at once convince the enquirer for truth of the reality of the principles of the science. moreover, the descending sign, etc., _will describe his own person and character_--a farther proof of the truth of the science.' there is one feature of horary astrology which is probably almost as ancient as any portion of the science, yet which remains even to the present day, and will probably remain for many years to come. i refer to the influence which the planets were supposed to exert on the successive hours of every day--a belief from which the division of time into weeks of seven days unquestionably had its origin--though we may concede that the subdivision of the lunar month into four equal parts was also considered in selecting this convenient measure of time. every hour had its planet. now dividing twenty-four by seven, we get three and three over; whence, each day containing twenty-four hours, it follows that in each day the complete series of seven planets was run through three times, and three planets of the next series were used. the order of the planets was that of their distances, as indicated above. saturn came first, then jupiter, mars, the sun, venus, mercury, and the moon. beginning with saturn, as ruling the first hour of saturn's day (saturday), we get through the above series three times, and have for the last three hours of the day, saturn, jupiter, and mars. thus the next hour, the first hour of the next day, belongs to the sun--sunday follows saturday. we again run three times through the series, and the three remaining hours are governed by the sun, venus, and mercury,--giving the moon as the first planet for the next day. monday thus follows sunday. the last three hours of monday are ruled by the moon, saturn, and jupiter; leaving mars to govern the next day--martis dies, mardi, tuesday or tuisco's day. proceeding in the same way, we get mercury for the next day, mercurii dies, mercredi, wednesday or woden's day; jupiter for the next day, jovis dies, jeudi, thursday or thor's day; venus for the next day, veneris dies, vendredi, friday or freya's day; and so we come to saturday again.[ ] the period of seven days, which had its origin in, and derived its nomenclature from astrological ideas, shows by its wide prevalence how widely astrological superstitions were once spread among the nations. as whewell remarks (though, for reasons which will readily be understood he was by no means anxious to dwell upon the true origin of the sabbatical week), 'the usage is found over all the east; it existed among the arabians, assyrians, and egyptians. the same week is found in india, among the brahmins; it has there also its days marked by the names of the heavenly bodies; and it has been ascertained that the same day has, in that country, the name corresponding with its designation in other nations.... the period has gone on without interruption or irregularity from the earliest recorded times to our own days, traversing the extent of ages and the revolutions of empires; the names of ancient deities, which were associated with the stars, were replaced by those of the objects of the worship of our teutonic ancestors, according to their views of the correspondence of the two mythologies; and the quakers, in rejecting these names of days, have cast aside the most ancient existing relic of astrological as well as idolatrous superstition. not only do the names remain, but some of the observances connected with the old astrological systems remain even to this day. as ceremonies derived from pagan worship are still continued, though modified in form, and with a different interpretation, in christian and especially roman catholic observances, so among the jews and among christians the rites and ceremonies of the old egyptian and chaldæan astrology are still continued, though no longer interpreted as of yore. the great jewish lawgiver and those who follow him seem, for example, to have recognised the value of regular periods of rest (whether really required by man or become a necessity through long habit), but to have been somewhat in doubt how best to continue the practice without sanctioning the superstitions with which it had been connected. at any rate two different and inconsistent interpretations were given in the earlier and later codes of law. but whether the jews accepted the sabbath because they believed that an all-powerful being, having created the world in six days, required and took rest ('and was refreshed') on the seventh, as stated in exodus (xx. and xxxi. ), or whether they did so in remembrance of their departure from egypt, as stated in deuteronomy (v. ), there can be no question that among the egyptians the sabbath or saturn's day was a day of rest because of the malignant nature of the powerful planet-deity who presided over that day. nor can it be seriously doubted that the jews descended from the old chaldæans, among whom (as appears from stone inscriptions recently discovered) the very word sabbath was in use for a seventh day of rest connected with astrological observances, were familiar with the practice even before their sojourn in egypt. they had then probably regarded it as a superstitious practice to be eschewed like those idolatrous observances which had caused terah to remove with abraham and lot from ur of the chaldees. at any rate, we find no mention of the seventh day of rest as a religious observance until after the exodus.[ ] it was not their only religious observance having in reality an astrological origin. indeed, if we examine the jewish sacrificial system as described in numbers xxviii. and elsewhere, we shall find throughout a tacit reference to the motions or influences of the celestial bodies. there was the morning and evening sacrifice guided by the movements of the sun; the sabbath offering, determined by the predominance of saturn; the offering of the new moon, depending on the motions of the moon; and lastly, the paschal sacrifice, depending on the combined movements of the sun and moon--made, in fact, during the lunation following the sun's ascending passage of the equator at the sign of aries. let us return, however, after this somewhat long digression, to astrological matters. horary astrology is manifestly much better fitted than the casting of nativities for filling the pocket of the astrologer himself; because only one nativity can be cast, but any number of horary questions can be asked. it is on account of their skill in horary astrology that the zadkiels of our own time have occasionally found their way into the twelfth house, or house of enemies. even lilly himself, not devoting, it would seem, five minutes to inquire into the probable success of the affair, was indicted in by a half-witted young woman, because he had given judgment respecting stolen goods, receiving two shillings and sixpence, contrary to an act made under and provided by the wise and virtuous king james, first of england and sixth of scotland. state astrology relates to the destinies of kingdoms, thrones, empires, and may be regarded as a branch of horary science relating to subjects (and rulers) of more than ordinary importance. in former ages all persons likely to occupy an important position in the history of the world had their horoscopes erected; but in these degenerate days neither the casting of nativities nor the art of ruling the planets flourishes as it should do. our zadkiels and raphaëls publish, indeed, the horoscopes of kings and emperors, princes and princesses, and so forth; but their fate is as that of benedict (according to beatrice)--men 'wonder they will still be talking, for nobody marks them.' even those whose horoscopes have been erected show no proper respect for the predictions made in their behalf. thus the prince of wales being born when sagittarius was in the ascendant should have been, according to zadkiel, a tall man, with oval face, ruddy complexion, somewhat dusky, and so forth; but i understand he has by no means followed these directions as to his appearance. the sun, being well aspected, prognosticated honours--a most remarkable and unlooked-for circumstance, strangely fulfilled by the event; but then being in cancer, in sextile with mars, the prince of wales was to be partial to maritime affairs and attain naval glory, whereas as a field-marshal he can only win military glory. (i would not be understood to say that he is not quite as competent to lead our fleets as our battalions into action.) the house of wealth was occupied by jupiter, aspected by saturn, which betokened great wealth through inheritance--a prognostication, says professor miller, which is not unlikely to come true. the house of marriage was unsettled by the conflicting influences of venus, mars, and saturn; but the first predominating, the prince, after some trouble in his matrimonial speculations, was to marry a princess of high birth, and one not undeserving of his kindest and most affectionate attention, probably in . as to the date, an almanack informs me that the prince married a danish princess in march , which looks like a most culpable neglect of the predictions of our national astrologer. again, in may , when saturn was stationary in the ascending degree, the prince ought to have been injured by a horse, and also to have received a blow on the left side of the head, near the ear; but reprehensibly omitted both these ceremonies. a predisposition to fever and epileptic attacks was indicated by the condition of the house of sickness. the newspapers described, a few years since, a serious attack of fever; but as most persons have some experience of the kind, the fulfilment of the prediction can hardly be regarded as very wonderful. epileptic attacks, which, as less common, might have saved the credit of the astrologers, have not visited 'this royal native.' the position of saturn in capricorn betokened loss or disaster in one or other of the places ruled over by capricorn--which, as we have seen, are india, macedonia, thrace, greece, mexico, saxony, wilna, mecklenburgh, brandenburgh, and oxford. professor miller expresses the hope that oxford was the place indicated, and the disaster nothing more serious than some slight scrape with the authorities of christchurch. but princes never get into scrapes with college dons. probably some one or other of the 'hair-breadth 'scapes' chronicled by the reporters of his travels in india was the event indicated by the ominous position of saturn in capricorn. a remarkable list of characteristics were derived by zadkiel from the positions of the various planets and signs in the twelve houses of the 'royal native.' some, of course, were indicated in more ways than one, which will explain the parenthetical notes in the following alphabetical table which professor miller has been at the pains to draw up from zadkiel's predictions. the prince was to be 'acute, affectionate, amiable, amorous, austere, avaricious, beneficent, benevolent, brave, brilliant, calculated for government' (a quality which may be understood two ways), 'candid, careful of his person, careless, compassionate, courteous (twice over), delighting in eloquence, discreet, envious, fond of glory, fond of learning, fond of music, fond of poetry, fond of sports, fond of the arts and sciences, frank, full of expedients, generous (three times), gracious, honourable, hostile to crime, impervious, ingenious, inoffensive, joyous, just (twice), laborious, liberal, lofty, magnanimous, modest, noble, not easy to be understood (!), parsimonious, pious (twice), profound in opinion, prone to regret his acts, prudent, rash, religious, reverent, self-confident, sincere, singular in mode of thinking, strong, temperate, unreserved, unsteady, valuable in friendship, variable, versatile, violent, volatile, wily, and worthy.' zadkiel concludes thus:--'the square of saturn to the moon will add to the gloomy side of the picture, and give a tinge of melancholy at times to the native's character, and also a disposition to look at the dark side of things, and lead him to despondency; nor will he be at all of a sanguine character, but cool and calculating, though occasionally rash. yet, all things considered, though firm and sometimes positive in opinion, this royal native, if he live to mount the throne, will sway the sceptre of these realms in moderation and justice, and be a pious and benevolent man, and a merciful sovereign.' fortunately, the time has long since passed when swaying the sceptre of these realms had any but a figurative meaning, or when englishmen who obeyed their country's laws depended on the mercy of any man, or when even bad citizens were judged by princes. but we still prefer that princes should be well-mannered gentlemen, and therefore it is sincerely to be hoped that zadkiel's prediction, so far as it relates to piety and benevolence, may be fulfilled, should this 'royal native' live to mount the throne. as for mercy, it is a goodly quality even in these days and in this country; for if the law no longer tolerates cruelty to men, even on the part of princes, who once had prescribed rights in that direction, there are still some cruel, nay brutal sports in which 'royal natives' might sometimes be tempted to take part. wherefore let us hope that, even in regard to mercy, the predictions of astrologers respecting this 'royal native' may be fulfilled. passing however, from trivialities, let us consider the lessons which the history of astrology teaches us respecting the human mind, its powers and weaknesses. it has been well remarked by whewell that for many ages 'mysticism in its various forms was a leading character both of the common mind and the speculations of the most intelligent and profound reasoners.' thus mysticism was the opposite of that habit of thought which science requires, 'namely, clear ideas, distinctly employed to connect well-ascertained facts; inasmuch as the ideas in which it dealt were vague and unstable, and the temper in which they were contemplated was an urgent and aspiring enthusiasm, which could not submit to a calm conference with experience upon even terms.' we have seen what has been the history of one particular form of the mysticism of ancient and mediæval ages. if we had followed the history of alchemy, magic, and other forms of mysticism, we should have seen similar results. true science has gradually dispossessed science falsely so called, until now none but the weaker minds hold by the tenets formerly almost universally adopted. in mere numbers, believers in the ancient superstitions may be by no means insignificant; but they no longer have any influence. it has become a matter of shame to pay any attention to what those few say or do who not merely hold but proclaim the ancient faith in these matters. we can also see why this has been. in old times enthusiasm usurped the place of reason in these cases; but opinions so formed and so retained could not maintain their ground in the presence of reasoning and experience. so soon as intelligent and thoughtful men perceived that facts were against the supposed mysterious influences of the stars, the asserted powers of magicians, the pretended knowledge of alchemists, the false teachings of magic, alchemy, and astrology, were rejected. the lesson thus learned respecting erroneous doctrines which were once widely prevalent has its application in our time, when, though the influence of those teachings has passed away, other doctrines formerly associated with them still hold their ground. men in old times, influenced by erroneous teachings, wasted their time and energies in idle questionings of the stars, vain efforts to find arcana of mysterious power, and to acquire magical authority over the elements. is it altogether clear that in these our times men are not hampered, prevented to some degree from doing all the good they might do in the short life-time allotted to them, by doctrines of another kind? is there in our day no undue sacrifice of present good in idle questionings? is there no tendency to trust in a vain fetishism to prevent or remove evils which energy could avert or remedy? the time will come, in my belief, when the waste of those energies which in these days are devoted (not merely with the sanction, but the high approval, of some of the best among us) to idle aims, will be deplored as regretfully--but, alas, as idly--as the wasted speculations and labours of those whom whewell has justly called the most intelligent and profound reasoners of the 'stationary age' of science. the words with which whewell closes his chapter on the 'mysticism of the middle ages' have their application to the mysticism of the nineteenth century:--'experience collects her stores in vain, or ceases to collect them, when she can only pour them into the flimsy folds of the lap of mysticism, who is, in truth, so much absorbed in looking for the treasures which are to fall from the skies, that she heeds little how scantily she obtains, or how loosely she holds, such riches as she might find beside her.' ii. _the religion of the great pyramid._ during the last few years a new sect has appeared which, though as yet small in numbers, is full of zeal and fervour. the faith professed by this sect may be called the religion of the great pyramid, the chief article of their creed being the doctrine that that remarkable edifice was built for the purpose of revealing--in the fulness of time, now nearly accomplished--certain noteworthy truths to the human race. the founder of the pyramid religion is described by one of the present leaders of the sect as 'the late worthy john taylor, of gower street, london;' but hitherto the chief prophets of the new faith have been in this country professor smyth, astronomer royal for scotland, and in france the abbé moigno. i propose to examine here some of the facts most confidently urged by pyramidalists in support of their views. but it will be well first to indicate briefly the doctrines of the new faith. they may be thus presented: the great pyramid was erected, it would seem, under the instructions of a certain semitic king, probably no other than melchizedek. by supernatural means, the architects were instructed to place the pyramid in latitude ° north; to select for its figure that of a square pyramid, carefully oriented; to employ for their unit of length the sacred cubit corresponding to the , , th part of the earth's polar axis; and to make the side of the square base equal to just so many of these sacred cubits as there are days and parts of a day in a year. they were further, by supernatural help, enabled to square the circle, and symbolised their victory over this problem by making the pyramid's height bear to the perimeter of the base the ratio which the radius of a circle bears to the circumference. moreover, the great precessional period, in which the earth's axis gyrates like that of some mighty top around the perpendicular to the ecliptic, was communicated to the builders with a degree of accuracy far exceeding that of the best modern determinations, and they were instructed to symbolise that relation in the dimensions of the pyramid's base. a value of the sun's distance more accurate by far than modern astronomers have obtained (even since the recent transit) was imparted to them, and they embodied that dimension in the height of the pyramid. other results which modern science has achieved, but which by merely human means the architects of the pyramid could not have obtained, were also supernaturally communicated to them; so that the true mean density of the earth, her true shape, the configuration of land and water, the mean temperature of the earth's surface, and so forth, were either symbolised in the great pyramid's position, or in the shape and dimensions of its exterior and interior. in the pyramid also were preserved the true, because supernaturally communicated, standards of length, area, capacity, weight, density, heat, time, and money. the pyramid also indicated, by certain features of its interior structure, that when it was built the holy influences of the pleiades were exerted from a most effective position--the meridian, through the points where the ecliptic and equator intersect. and as the pyramid thus significantly refers to the past, so also it indicates the future history of the earth, especially in showing when and where the millennium is to begin. lastly, the apex or crowning stone of the pyramid was no other than the antitype of that stone of stumbling and rock of offence, rejected by builders who knew not its true use, until it was finally placed as the chief stone of the corner. whence naturally, 'whosoever shall fall upon it'--that is, upon the pyramid religion--'shall be broken; but on whomsoever it shall fall it will grind him to powder.' if we examine the relations actually presented by the great pyramid--its geographical position, dimensions, shape, and internal structure--without hampering ourselves with the tenets of the new faith on the one hand, or on the other with any serious anxiety to disprove them, we shall find much to suggest that the builders of the pyramid were ingenious mathematicians, who had made some progress in astronomy, though not so much as they had made in the mastery of mechanical and scientific difficulties. the first point to be noticed is the geographical position of the great pyramid, so far, at least, as this position affects the aspect of the heavens, viewed from the pyramid as from an observatory. little importance, i conceive, can be attached to purely geographical relations in considering the pyramid's position. professor smyth notes that the pyramid is peculiarly placed with respect to the mouth of the nile, standing 'at the southern apex of the delta-land of egypt.' this region being shaped like a fan, the pyramid, set at the part corresponding to the handle, was, he considers, 'that monument pure and undefiled in its religion through an idolatrous land, alluded to by isaiah; the monument which was both "an altar to the lord in the midst of the land of egypt, and a pillar at the border thereof," and destined withal to become a witness in the latter days, and before the consummation of all things, to the same lord, and to what he hath purposed upon man kind.' still more fanciful are some other notes upon the pyramid's geographical position: as (i.) that there is more land along the meridian of the pyramid than on any other all the world round; (ii.) that there is more land in the latitude of the pyramid than in any other; and (iii.) that the pyramid territory of lower egypt is at the centre of the dry land habitable by man all the world over. it does not seem to be noticed by those who call our attention to these points that such coincidences prove too much. it might be regarded as not a mere accident that the great pyramid stands at the centre of the arc of shore-line along which lie the outlets of the nile; or it might be regarded as not a mere coincidence that the great pyramid stands at the central point of all the habitable land-surface of the globe; or, again, any one of the other relations above mentioned might be regarded as something more than a mere coincidence. but if, instead of taking only one or other of these four relations, we take all four of them, or even any two of them, together, we must regard peculiarities of the earth's configuration as the result of special design which certainly have not hitherto been so regarded by geographers. for instance, if it was by a special design that the pyramid was placed at the centre of the nile delta, and also by special design that the pyramid was placed at the centre of the land-surface of the earth, if these two relations are each so exactly fulfilled as to render the idea of mere accidental coincidence inadmissible, then it follows, of necessity, that it is through no merely accidental coincidence that the centre of the nile delta lies at the centre of the land-surface of the earth; in other words, the shore-line along which lie the mouths of the nile has been designedly curved so as to have its centre so placed. and so of the other relations. the very fact that the four conditions _can_ be fulfilled simultaneously is evidence that a coincidence of the sort may result from mere accident.[ ] indeed, the peculiarity of geographical position which really seems to have been in the thoughts of the pyramid architects, introduces yet a fifth condition which by accident could be fulfilled along with the four others. it would seem that the builders of the pyramid were anxious to place it in latitude °, as closely as their means of observation permitted. let us consider what result they achieved, and the evidence thus afforded respecting their skill and scientific attainments. in our own time, of course, the astronomer has no difficulty in determining with great exactness the position of any given latitude-parallel. but at the time when the great pyramid was built it must have been a matter of very serious difficulty to determine the position of any required latitude-parallel with a great degree of exactitude. the most obvious way of dealing with the difficulty would have been by observing the length of shadows thrown by upright posts at noon in spring and autumn. in latitude ° north, the sun at noon in spring (or, to speak precisely, on the day of the vernal equinox) is just twice as far from the horizon as he is from the point vertically overhead; and if a pointed post were set exactly upright at true noon (supposed to occur at the moment of the vernal or autumnal equinox), the shadow of the post would be exactly half as long as a line drawn from the top of the pole to the end of the shadow. but observations based on this principle would have presented many difficulties to the architects of the pyramid. the sun not being a point of light, but a globe, the shadow of a pointed rod does not end in a well-defined point. the moment of true noon, which is not the same as ordinary or civil noon, never does agree exactly with the time of the vernal or autumnal equinox, and may be removed from it by any interval of time not exceeding twelve hours. and there are many other circumstances which would lead astronomers, like those who doubtless presided over the scientific preparations for building the great pyramid, to prefer a means of determining the latitude depending on another principle. the stellar heavens would afford practically unchanging indications for their purpose. the stars being all carried round the pole of the heavens, as if they were fixed points in the interior of a hollow revolving sphere, it becomes possible to determine the position of the pole of the star sphere, even though no bright conspicuous star actually occupies that point. any bright star close by the pole is seen to revolve in a very small circle, whose centre is the pole itself. such a star is our present so-called pole-star; and, though in the days when the great pyramid was built, that star was not near the pole, another, and probably a brighter star lay near enough to the pole[ ] to serve as a pole-star, and to indicate by its circling motion the position of the actual pole of the heavens. this was at that time, and for many subsequent centuries, the leading star of the great constellation called the dragon. the pole of the heavens, we know, varies in position according to the latitude of the observer. at the north pole it is exactly overhead; at the equator the poles of the heavens are both on the horizon; and, as the observer travels from the equator towards the north or south pole of the earth, the corresponding pole of the heavens rises higher and higher above the horizon. in latitude ° north, or one-third of the way from the equator to the pole, the pole of the heavens is raised one-third of the way from the horizon to the point vertically overhead; and when this is the case the observer knows that he is in latitude °. the builders of the great pyramid, with the almost constantly clear skies of egypt, may reasonably be supposed to have adopted this means of determining the true position of that thirtieth parallel on which they appear to have designed to place the great building they were about to erect. it so happens that we have the means of forming an opinion on the question whether they used one method or the other; whether they employed the sun or the stars to guide them to the geographical position they required. in fact, were it not for this circumstance, i should not have thought it worth while to discuss the qualities of either method. it will presently be seen that the discussion bears importantly on the opinion we are to form of the skill and attainments of the pyramid architects. every celestial object is apparently raised somewhat above its true position by the refractive power of our atmosphere, being most raised when nearest the horizon and least when nearest the point vertically overhead. this effect is, indeed, so marked on bodies close to the horizon that if the astronomers of the pyramid times had observed the sun, moon, and stars attentively when so placed, they could not have failed to discover the peculiarity. probably, however, though they noted the time of rising and setting of the celestial bodies, they only made instrumental observations upon them when these bodies were high in the heavens. thus they remained ignorant of the refractive powers of the air.[ ] now, if they had determined the position of the thirtieth parallel of latitude by observations of the noonday sun (in spring or autumn), then since, owing to refraction, they would have judged the sun to be higher than he really was, it follows that they would have supposed the latitude of any station from which they observed to be lower than it really was. for the lower the latitude the higher is the noonday sun at any given season. thus, when really in latitude ° they would have supposed themselves in a latitude lower than °, and would have travelled a little further north to find the proper place, as they would have supposed, for erecting the great pyramid. on the other hand, if they determined the place from observations of the movements of stars near the pole of the heavens, they would make an error of a precisely opposite nature. for the higher the latitude the higher is the pole of the heavens; and refraction, therefore, which apparently raises the pole of the heavens, gives to a station the appearance of being in a higher latitude than it really is, so that the observer would consider he was in latitude north when in reality somewhat south of that latitude. we have only then to inquire whether the great pyramid was set north or south of latitude °, to ascertain whether the pyramid architects observed the noonday sun or circumpolar stars to determine their latitude; always assuming (as we reasonably may) that those architects did propose to set the pyramid in that particular latitude, and that they were able to make very accurate observations of the apparent positions of the celestial bodies, but that they were not acquainted with the refractive effects of the atmosphere. the answer comes in no doubtful terms. the centre of the great pyramid's base lies about one mile and a third _south_ of the thirtieth parallel of latitude; and from this position the pole of the heavens, as raised by refraction, would appear to be very near indeed to the required position. in fact, if the pyramid had been set about half a mile still farther south the pole would have _seemed_ just right. of course, such an explanation as i have here suggested appears altogether heretical to the pyramidalists. according to them the pyramid architects knew perfectly well where the true thirtieth parallel lay, and knew also all that modern science has discovered about refraction; but set the pyramid south of the true parallel and north of the position where refraction would just have made the apparent elevation of the pole correct, simply in order that the pyramid might correspond as nearly as possible to each of two conditions, whereof both could not be fulfilled at once. the pyramid would indeed, they say, have been set even more closely midway between the true and the apparent parallels of ° north, but that the jeezeh hill on which it is set does not afford a rock foundation any farther north. 'so very close,' says professor smyth, 'was the great pyramid placed to the northern brink of its hill, that the edges of the cliff might have broken off under the terrible pressure had not the builders banked up there most firmly the immense mounds of rubbish which came from their work, and which strabo looked so particularly for years ago, but could not find. here they were, however, and still are, utilised in enabling the great pyramid to stand on the very utmost verge of its commanding hill, within the limits of the _two_ required latitudes, as well as over the centre of the land's physical and radial formation, and at the same time on the sure and proverbially wise foundation of rock.' the next circumstance to be noted in the position of the great pyramid (as of all the pyramids) is that the sides are carefully oriented. this, like the approximation to a particular latitude, must be regarded as an astronomical rather than a geographical relation. the accuracy with which the orientation has been effected will serve to show how far the builders had mastered the methods of astronomical observation by which orientation was to be secured. the problem was not so simple as might be supposed by those who are not acquainted with the way in which the cardinal points are correctly determined. by solar observations, or rather by the observations of shadows cast by vertical shafts before and after noon, the direction of the meridian, or north and south line, can theoretically be ascertained. but probably in this case, as in determining the latitude, the builders took the stars for their guide. the pole of the heavens would mark the true north; and equally the pole-star, when below or above the pole, would give the true north, but, of course, most conveniently when below the pole. nor is it difficult to see how the builders would make use of the pole-star for this purpose. from the middle of the northern side of the intended base they would bore a slant passage tending always from the position of the pole-star at its lower meridional passage, that star at each successive return to that position serving to direct their progress; while its small range, east and west of the pole, would enable them most accurately to determine the star's true mid-point below the pole; that is, the true north. when they had thus obtained a slant tunnel pointing truly to the meridian, and had carried it down to a point nearly below the middle of the proposed square base, they could, from the middle of the base, bore vertically downwards, until by rough calculation they were near the lower end of the slant tunnel; or both tunnels could be made at the same time. then a subterranean chamber would be opened out from the slant tunnel. the vertical boring, which need not be wider than necessary to allow a plumb-line to be suspended down it, would enable the architects to determine the point vertically below the point of suspension. the slant tunnel would give the direction of the true north, either from that point or from a point at some known small distance east or west of that point.[ ] thus, a line from some ascertained point near the mouth of the vertical boring to the mouth of the slant tunnel would lie due north and south, and serve as the required guide for the orientation of the pyramid's base. if this base extended beyond the opening of the slant tunnel, then, by continuing this tunnelling through the base tiers of the pyramid, the means would be obtained of correcting the orientation. this, i say, would be the course naturally suggested to astronomical architects who had determined the latitude in the manner described above. it may even be described as the only very accurate method available before the telescope had been invented. so that if the accuracy of the orientation appears to be greater than could be obtained by the shadow method, the natural inference, even in the absence of corroborative evidence, would be that the stellar method, and no other, had been employed. now, in , nouet, by refined observations, found the error of orientation measured by less than minutes of arc, corresponding roughly to a displacement of the corners by about - / inches from their true position, as supposed to be determined from the centre; or to a displacement of a southern corner by inches on an east and west line from a point due south of the corresponding northern corner. this error, for a base length of inches, would not be serious, being only one inch in about five yards (when estimated in the second way). yet the result is not quite worthy of the praise given to it by professor smyth. he himself, however, by much more exact observations, with an excellent altazimuth, reduced the alleged error from minutes to only - / , or to - ths of its formerly supposed value. this made the total displacement of a southern corner from the true meridian through the corresponding northern corner, almost exactly one foot, or one inch in about twenty-one yards--a degree of accuracy rendering it practically certain that some stellar method was used in orienting the base. now there _is_ a slanting tunnel occupying precisely the position of the tunnel which should, according to this view, have been formed in order accurately to orient the pyramid's base, assuming that the time of the building of the pyramid corresponded with one of the epochs when the star alpha draconis was distant ° ' from the pole of the heavens. in other words, there is a slant tunnel directed northwards and upwards from a point deep down below the middle of the pyramid's base, and inclined ° ' to the horizon, the elevation of alpha draconis at its lower culmination when ° ' from the pole. the last epoch when the star was thus placed was _circiter_ b.c.; the epoch next before that was b.c. between these two we should have to choose, on the hypothesis that the slant tunnel was really directed to that star when the foundations of the pyramid were laid. for the next epoch before the earlier of the two named was about , b.c., and the pyramid's date cannot have been more remote than b.c. the slant tunnel, while admirably fulfilling the requirements suggested, seems altogether unsuited for any other. its transverse height (that is, its width in a direction perpendicular to its upper and lower faces) did not amount to quite four feet; its breadth was not quite three feet and a half. it was, therefore, not well fitted for an entrance passage to the subterranean chamber immediately under the apex of the pyramid (with which chamber it communicates in the manner suggested by the above theory). it could not have been intended to be used for observing meridian transits of the stars in order to determine sidereal time; for close circumpolar stars, by reason of their slow motion, are the least suited of all for such a purpose. as professor smyth says, in arguing against this suggested use of the star, 'no observer in his senses, in any existing observatory, when seeking to obtain the time, would observe the transit of a circumpolar star for anything else than _to get the direction of the meridian to adjust his instrument by_.' (the italics are his.) it is precisely such a purpose (the adjustment, however, not of an instrument, but of the entire structure of the pyramid itself), that i have suggested for this remarkable passage--this 'cream-white, stone-lined, long tube,' where it traverses the masonry of the pyramid, and below that dug through the solid rock to a distance of more than feet. let us next consider the dimensions of the square base thus carefully placed in latitude ° north to the best of the builders' power, with sides carefully oriented. it seems highly probable that, whatever special purpose the pyramid was intended to fulfil, a subordinate idea of the builders would have been to represent symbolically in the proportions of the building such mathematical and astronomical relations as they were acquainted with. from what we know by tradition of the men of the remote time when the pyramid was built, and what we can infer from the ideas of those who inherited, however remotely, the modes of thought of the earliest astronomers and mathematicians, we can well believe that they would look with superstitious reverence on special figures, proportions, numbers, and so forth. apart from this, they may have had a quasi-scientific desire to make a lasting record of their discoveries, and of the collected knowledge of their time. it seems altogether probable, then, that the smaller unit of measurement used by the builders of the great pyramid was intended, as professor smyth thinks, to be equal to the , , th part of the earth's diameter, determined from their geodetical observations. it was perfectly within the power of mechanicians and mathematicians so experienced as they undoubtedly were--the pyramid attests so much--to measure with considerable accuracy the length of a degree of latitude. they could not possibly (always setting aside the theory of divine inspiration) have known anything about the compression of the earth's globe, and therefore could not have intended, as professor smyth supposes, to have had the , , th part of the earth's polar axis, as distinguished from any other, for their unit of length. but if they made observations in or near latitude ° north on the supposition that the earth is a globe, their probable error would exceed the difference even between the earth's polar and equatorial diameters. both differences are largely exceeded by the range of difference among the estimates of the actual length of the sacred cubit, supposed to have contained twenty-five of these smaller units. and, again, the length of the pyramid base-side, on which smyth bases his own estimate of the sacred cubit, has been variously estimated, the largest measure being inches, and the lowest inches. the fundamental theory of the pyramidalists, that the sacred cubit was exactly one , , th part of the earth's polar diameter, and that the side of the base contained as many cubits and parts of a cubit as there are days and parts of a day in the tropical year (or year of seasons), requires that the length of the side should be inches, lying between the limits indicated, but still so widely removed from either that it would appear very unsafe to base a theory on the supposition that the exact length is or was inches. if the measures inches and inches were inferior, and several excellent measures made by practised observers ranged around the length inches, the case would be different. but the best recent measures gave respectively and inches; and smyth exclaims against the unfairness of sir h. james in taking as 'therefore the [probable] true length of the side of the great pyramid when perfect,' calling this 'a dishonourable shelving of the honourable older observers with their larger results.' the only other measures, besides these two, are two by colonel howard vyse and by the french _savants_, giving respectively and · inches. the pyramidalists consider inches a fair mean value from these four. the natural inference, however, is, that the pyramid base is not now in a condition to be satisfactorily measured; and assuredly no such reliance can be placed on the mean value inches that, on the strength of it, we should believe what otherwise would be utterly incredible, viz. that the builders of the great pyramid knew 'both the size and shape of the earth exactly.' 'humanly, or by human science, finding it out in that age was, of course, utterly impossible,' says professor smyth. but he is so confident of the average value derived from widely conflicting base measures as to assume that this value, not being humanly discoverable, was of necessity 'attributable to god and to his divine inspiration.' we may agree, in fine, with smyth, that the builders of the pyramid knew the earth to be a globe; that they took for their measure of length the sacred cubit, which, by their earth measures, they made very fairly approximate to the , , th part of the earth's mean diameter; but there seems no reason whatever for supposing (even if the supposition were not antecedently of its very nature inadmissible) that they knew anything about the compression of the earth, or that they had measured a degree of latitude in their own place with very wonderful accuracy.[ ] but here a very singular coincidence may be noticed, or, rather, is forced upon our notice by the pyramidalists, who strangely enough recognise in it fresh evidence of design, while the unbeliever finds in it proof that coincidences are no sure evidence of design. the side of the pyramid containing - / times the sacred cubit of pyramid inches, it follows that the diagonal of the base contains , such inches, and the two diagonals together contain , pyramid inches, or almost exactly as many inches as there are years in the great precessional period. 'no one whatever amongst men,' says professor smyth after recording various estimates of the precessional period, 'from his own or school knowledge, knew anything about such a phenomenon, until hipparchus, some years after the great pyramid's foundation, had a glimpse of the fact; and yet it had been ruling the heavens for ages, and was recorded in jeezeh's ancient structure.' to minds not moved to most energetic forgetfulness by the spirit of faith, it would appear that when a square base had been decided upon, and its dimensions fixed, with reference to the earth's diameter and the year, the diagonals of the square base were determined also; and, if it so chanced that they corresponded with some other perfectly independent relation, the fact was not to be credited to the architects. moreover it is manifest that the closeness of such a coincidence suggests grave doubts how far other coincidences can be relied upon as evidence of design. it seems, for instance, altogether likely that the architects of the pyramid took the sacred cubit equal to one , , th part of the earth's diameter for their chief unit of length, and intentionally assigned to the side of the pyramid's square base a length of just so many cubits as there are days in the year; and the closeness of the coincidence between the measured length and that indicated by this theory strengthens the idea that this was the builder's purpose. but when we find that an even closer coincidence immediately presents itself, which manifestly is a coincidence _only_, the force of the evidence before derived from mere coincidence is _pro tanto_ shaken. for consider what this new coincidence really means. its nature may be thus indicated: take the number of days in the year, multiply that number by , and increase the result in the same degree that the diagonal of a square exceeds the side--then the resulting number represents very approximately the number of years in the great precessional period. the error, according to the best modern estimates, is about one th part of the true period. this is, of course, a merely accidental coincidence, for there is no connection whatever in nature between the earth's period of rotation, the shape of a square, and the earth's period of gyration. yet this merely accidental coincidence is very much closer than the other supposed to be designed could be proved to be. it is clear, then, that mere coincidence is a very unsafe evidence of design. of course the pyramidalists find a ready reply to such reasoning. they argue that, in the first place, it may have been by express design that the period of the earth's rotation was made to bear this particular relation to the period of gyration in the mighty precessional movement: which is much as though one should say that by express design the height of monte rosa contains as many feet as there are miles in the th part of the sun's distance.[ ] then, they urge, the architects were not bound to have a square base for the pyramid; they might have had an oblong or a triangular base, and so forth--all which accords very ill with the enthusiastic language in which the selection of a square base had on other accounts been applauded. next let us consider the height of the pyramid. according to the best modern measurements, it would seem that the height when (if ever) the pyramid terminated above in a pointed apex, must have been about feet. and from the comparison of the best estimates of the base side with the best estimates of the height, it seems very likely indeed that the intention of the builders was to make the height bear to the perimeter of the base the same ratio which the radius of a circle bears to the circumference. remembering the range of difference in the base measures it might be supposed that the exactness of the approximation to this ratio could not be determined very satisfactorily. but as certain casing stones have been discovered which indicate with considerable exactness the slope of the original plane-surfaces of the pyramid, the ratio of the height to the side of the base may be regarded as much more satisfactorily determined than the actual value of either dimension. of course the pyramidalists claim a degree of precision indicating a most accurate knowledge of the ratio between the diameter and the circumference of a circle; and the angle of the only casing stone measured being diversely estimated at ° ' and ° - / ', they consider ° ' · " the true value, and infer that the builders regarded the ratio as · to . the real fact is, that the modern estimates of the dimensions of the casing stones (which, by the way, ought to agree better if these stones are as well made as stated) indicate the values · and · for the ratio; and all we can say is, that the ratio really used lay _probably_ between these limits, though it may have been outside either. now the approximation of either is not remarkably close. it requires no mathematical knowledge at all to determine the circumference of a circle much more exactly. 'i thought it very strange,' wrote a circle-squarer once to de morgan (_budget of paradoxes_, p. ), 'that so many great scholars in all ages should have failed in finding the true ratio, and have been determined to try myself.' 'i have been informed,' proceeds de morgan, 'that this trial makes the diameter to the circumference as to , giving the ratio equal to · exactly. the result was obtained by the discoverer in three weeks after he first heard of the existence of the difficulty. this quadrator has since published a little slip and entered it at stationers' hall. he says he has done it by actual measurement; and i hear from a private source that he uses a disc of twelve inches diameter which he rolls upon a straight rail.' the 'rolling is a very creditable one; it is as much below the mark as archimedes was above it. its performer is a joiner who evidently knows well what he is about when he measures; he is not wrong by in .' such skilful mechanicians as the builders of the pyramid could have obtained a closer approximation still by mere measurement. besides, as they were manifestly mathematicians, such an approximation as was obtained by archimedes must have been well within their power; and that approximation lies well within the limits above indicated. professor smyth remarks that the ratio was 'a quantity which men in general, and all human science too, did not begin to trouble themselves about until long, long ages, languages, and nations had passed away after the building of the great pyramid; and after the sealing up, too, of that grand primeval and prehistoric monument of the patriarchal age of the earth according to scripture.' i do not know where the scripture records the sealing up of the great pyramid; but it is all but certain that during the very time when the pyramid was being built astronomical observations were in progress which, for their interpretation, involved of necessity a continual reference to the ratio in question. no one who considers the wonderful accuracy with which, nearly two thousand years before the christian era, the chaldæans had determined the famous cycle of the saros, can doubt that they must have observed the heavenly bodies for several centuries before they could have achieved such a success; and the study of the motions of the celestial bodies compels 'men to trouble themselves' about the famous ratio of the circumference to the diameter. we now come upon a new relation (contained in the dimensions of the pyramid as thus determined) which, by a strange coincidence, causes the height of the pyramid to appear to symbolise the distance of the sun. there were pyramid inches, or british inches, in the height of the pyramid according to the relations already indicated. now, in the sun's distance, according to an estimate recently adopted and freely used,[ ] there are , , miles or thousand millions of inches--that is, there are approximately as many thousand millions of inches in the sun's distance as there are inches in the height of the pyramid. if we take the relation as exact we should infer for the sun's distance thousand millions of inches, or , , miles--an immense improvement on the estimate which for so many years occupied a place of honour in our books of astronomy. besides, there is strong reason for believing that, when the results of recent observations are worked out, the estimated sun distance will be much nearer this pyramid value than even to the value , , recently adopted. this result, which one would have thought so damaging to faith in the evidence from coincidence--nay, quite fatal after the other case in which a close coincidence had appeared by merest accident--is regarded by the pyramidalist as a perfect triumph for their faith. they connect it with another coincidence, viz. that, assuming the height determined in the way already indicated, then it so happens that the height bears to half a diagonal of the base the ratio to . seeing that the perimeter of the base symbolises the annual motion of the earth round the sun, while the height represents the radius of a circle with that perimeter, it follows that the height should symbolise the sun's distance. 'that line, further,' says professor smyth (speaking on behalf of mr. w. petrie, the discoverer of this relation), 'must represent' this radius 'in the proportion of to , , , ' (or _ten_ raised to power _nine_), 'because amongst other reasons to is practically the shape of the great pyramid.' for this building 'has such an angle at the corners, that for every ten units its structure advances inwards on the diagonal of the base, it practically rises upwards, or points to sunshine' (_sic_) 'by _nine_. nine, too, out of the ten characteristic parts (viz. five angles and five sides) being the number of those parts which the sun shines on in such a shaped pyramid, in such a latitude near the equator, out of a high sky, or, as the peruvians say, when the sun sets on the pyramid with all its rays.' the coincidence itself on which this perverse reasoning rests is a singular one--singular, that is, as showing how close an accidental coincidence may run. it amounts to this, that if the number of days in the year be multiplied by , and a circle be drawn with a circumference containing times as many inches as there are days in the year, the radius of the circle will be very nearly one , , , th part of the sun's distance. remembering that the pyramid inch is assumed to be one , , th part of the earth's diameter, we shall not be far from the truth in saying that, as a matter of fact, the earth by her orbital motion traverses each day a distance equal to two hundred times her own diameter. but, of course, this relation is altogether accidental. it has no real cause in nature.[ ] such relations show that mere numerical coincidences, however close, have little weight as evidence, except where they occur in series. even then they require to be very cautiously regarded, seeing that the history of science records many instances where the apparent law of a series has been found to be falsified when the theory has been extended. of course this reason is not quoted in order to throw doubt on the supposition that the height of the pyramid was intended to symbolise the sun's distance. that supposition is simply inadmissible if the hypothesis, according to which the height was already independently determined in another way, is admitted. either hypothesis might be admitted were we not certain that the sun's distance could not possibly have been known to the builders of the pyramid; or both hypotheses may be rejected: but to admit both is out of the question. considering the multitude of dimensions of length, surface, capacity, and position, the great number of shapes, and the variety of material existing within the pyramid, and considering, further, the enormous number of relations (presented by modern science) from among which to choose, can it be wondered at if fresh coincidences are being continually recognised? if a dimension will not serve in one way, use can be found for it in another; for instance, if some measure of length does not correspond closely with any known dimension of the earth or of the solar system (an unlikely supposition), then it can be understood to typify an interval of time. if, even after trying all possible changes of that kind, no coincidence shows itself (which is all but impossible), then all that is needed to secure a coincidence is that the dimensions should be manipulated a little. let a single instance suffice to show how the pyramidalists (with perfect honesty of purpose) hunt down a coincidence. the slant tunnel already described has a transverse height, once no doubt uniform, now giving various measures from · pyramid inches to · inches, so that the vertical height from the known inclination of the tunnel would be estimated at somewhere between · inches and · . neither dimension corresponds very obviously with any measured distance in the earth or solar system. nor when we try periods, areas, etc., does any very satisfactory coincidence present itself. but the difficulty is easily turned into a new proof of design. putting all the observations together (says professor smyth), 'i deduced · pyramid inches to be the transverse height of the entrance passage; and computing from thence with the observed angle of inclination the vertical height, that came out · of the same inches. but the sum of those two heights, or the height taken up and down, equals inches, which length, as elsewhere shown, is the general pyramid linear representation of a day of twenty-four hours. and the mean of the two heights, or the height taken one way only, and impartially to the middle point between them, equals fifty inches; which quantity is, therefore, the general pyramid linear representation of only half a day. in which case, let us ask what the entrance passage has to do with half rather than a whole day?' on relations such as these, which, if really intended by the architect, would imply an utterly fatuous habit of concealing elaborately what he desired to symbolise, the pyramidalists base their belief that 'a mighty intelligence did both think out the plans for it, and compel unwilling and ignorant idolators, in a primal age of the world, to work mightily both for the future glory of the one true god of revelation, and to establish lasting prophetic testimony touching a further development, still to take place, of the absolutely divine christian dispensation.' iii. _the mystery of the pyramids._ few subjects of inquiry have proved more perplexing than the question of the purpose for which the pyramids of egypt were built. even in the remotest ages of which we have historical record, nothing seems to have been known certainly on this point. for some reason or other, the builders of the pyramids concealed the object of these structures, and this so successfully that not even a tradition has reached us which purports to have been handed down from the epoch of the pyramids' construction. we find, indeed, some explanations given by the earliest historians; but they were professedly only hypothetical, like those advanced in more recent times. including ancient and modern theories, we find a wide range of choice. some have thought that these buildings were associated with the religion of the early egyptians; others have suggested that they were tombs; others, that they combined the purposes of tombs and temples, that they were astronomical observatories, defences against the sands of the great desert, granaries like those made under joseph's direction, places of resort during excessive overflows of the nile; and many other uses have been suggested for them. but none of these ideas are found on close examination to be tenable as representing the sole purpose of the pyramids, and few of them have strong claims to be regarded as presenting even a chief object of these remarkable structures. the significant and perplexing history of the three oldest pyramids--the great pyramid of cheops, shofo, or suphis, the pyramid of chephren, and the pyramid of mycerinus; and the most remarkable of all the facts known respecting the pyramids generally, viz., the circumstance that one pyramid after another was built as though each had become useless soon after it was finished, are left entirely unexplained by all the theories above mentioned, save one only, the tomb theory, and that does not afford by any means a satisfactory explanation of the circumstances. i propose to give here a brief account of some of the most suggestive facts known respecting the pyramids, and, after considering the difficulties which beset the theories heretofore advanced, to indicate a theory (new so far as i know) which seems to me to correspond better with the facts than any heretofore advanced; i suggest it, however, rather for consideration than because i regard it as very convincingly supported by the evidence. in fact, to advance any theory at present with confident assurance of its correctness, would be simply to indicate a very limited acquaintance with the difficulties surrounding the subject. let us first consider a few of the more striking facts recorded by history or tradition, noting, as we proceed, whatever ideas they may suggest as to the intended character of these structures. it is hardly necessary to say, perhaps, that the history of the great pyramid is of paramount importance in this inquiry. whatever purpose pyramids were originally intended to subserve, must have been conceived by the builders of _that_ pyramid. new ideas may have been superadded by the builders of later pyramids, but it is unlikely that the original purpose can have been entirely abandoned. some great purpose there was, which the rulers of ancient egypt proposed to fulfil by building very massive pyramidal structures on a particular plan. it is by inquiring into the history of the first and most massive of these structures, and by examining its construction, that we shall have the best chance of finding out what that great purpose was. according to herodotus, the kings who built the pyramids reigned not more than twenty-eight centuries ago; but there can be little doubt that herodotus misunderstood the egyptian priests from whom he derived his information, and that the real antiquity of the pyramid-kings was far greater. he tells us that, according to the egyptian priests, cheops 'on ascending the throne plunged into all manner of wickedness. he closed the temples, and forbade the egyptians to offer sacrifice, compelling them instead to labour one and all in his service, viz., in building the great pyramid.' still following his interpretation of the egyptian account, we learn that one hundred thousand men were employed for twenty years in building the great pyramid, and that ten years were occupied in constructing a causeway by which to convey the stones to the place and in conveying them there. 'cheops reigned fifty years; and was succeeded by his brother chephren, who imitated the conduct of his predecessor, built a pyramid--but smaller than his brother's--and reigned fifty-six years. thus during one hundred and six years, the temples were shut and never opened.' moreover, herodotus tells us that 'the egyptians so detested the memory of these kings, that they do not much like even to mention their names. hence they commonly call the pyramids after philition, a shepherd who at that time fed his flocks about the place.' 'after chephren, mycerinus, son of cheops, ascended the throne, he reopened the temples, and allowed the people to resume the practice of sacrifice. he, too, left a pyramid, but much inferior in size to his father's. it is built, for half of its height, of the stone of ethiopia,' or, as professor smyth (whose extracts from rawlinson's translation i have here followed) adds 'expensive red granite.' 'after mycerinus, asychis ascended the throne. he built the eastern gateway of the temple of vulcan (phtha); and, being desirous of eclipsing all his predecessors on the throne, left as a monument of his reign a pyramid of brick.' this account is so suggestive, as will presently be shown, that it may be well to inquire whether it can be relied on. now, although there can be no doubt that herodotus misunderstood the egyptians in some matters, and in particular as to the chronological order of the dynasties, placing the pyramid kings far too late, yet in other respects he seems not only to have understood them correctly, but also to have received a correct account from them. the order of the kings above named corresponds with the sequence given by manetho, and also found in monumental and hieroglyphic records. manetho gives the names suphis i., suphis ii., and mencheres, instead of cheops, chephren, and mycerinus; while, according to the modern egyptologists, herodotus's cheops was shofo, shufu, or koufou; chephren was shafre, while he was also called nou-shofo or noum-shufu as the brother of shofo; and mycerinus was menhere or menkerre. but the identity of these kings is not questioned. as to the true dates there is much doubt, and it is probable that the question will long continue open; but the determination of the exact epochs when the several pyramids were built is not very important in connection with our present inquiry. we may, on the whole, fairly take the points quoted above from herodotus, and proceed to consider the significance of the narrative, with sufficient confidence that in all essential respects it is trustworthy. there are several very strange features in the account. in the first place, it is manifest that cheops (to call the first king by the name most familiar to the general reader) attached great importance to the building of his pyramid. it has been said, and perhaps justly, that it would be more interesting to know the plan of the architect who devised the pyramid than the purpose of the king who built it. but the two things are closely connected. the architect must have satisfied the king that some highly important purpose in which the king himself was interested, would be subserved by the structure. whether the king was persuaded to undertake the work as a matter of duty, or only to advance his own interests, may not be so clear. but that the king was most thoroughly in earnest about the work is certain. a monarch in those times would assuredly not have devoted an enormous amount of labour and material to such a scheme unless he was thoroughly convinced of its great importance. that the welfare of his people was not considered by cheops in building the great pyramid is almost equally certain. he might, indeed, have had a scheme for their good which either he did not care to explain to them or which they could not understand. but the most natural inference from the narrative is that his purpose had no reference whatever to their welfare. for though one could understand his own subjects hating him while he was all the time working for their good, it is obvious that his memory would not have been hated if some important good had eventually been gained from his scheme. many a far-seeing ruler has been hated while living on account of the very work for which his memory has been revered. but the memory of cheops and his successors was held in detestation. may we, however, suppose that, though cheops had not the welfare of his own people in his thoughts, his purpose was nevertheless not selfish, but intended in some way to promote the welfare of the human race? i say his purpose, because, whoever originated the scheme, cheops carried it out; it was by means of his wealth and through his power that the pyramid was built. this is the view adopted by professor piazzi smyth and others, in our own time, and first suggested by john taylor. 'whereas other writers,' says smyth, 'have generally esteemed that the mysterious persons who directed the building of the great pyramid (and to whom the egyptians, in their traditions, and for ages afterwards, gave an immoral and even abominable character) must therefore have been very bad indeed, so that the world at large has always been fond of standing on, kicking, and insulting that dead lion, whom they really knew not; he, mr. john taylor, seeing how religiously bad the egyptians themselves were, was led to conclude, on the contrary, that those _they_ hated (and could never sufficiently abuse) might, perhaps, have been pre-eminently good; or were, at all events, of _different religious faith_ from themselves.' 'combining this with certain unmistakable historical facts,' mr. taylor deduced reasons for believing that the directors of the building designed to record in its proportions, and in its interior features, certain important religious and scientific truths, not for the people then living, but for men who were to come years or so after. i have already considered at length (see the preceding essay) the evidence on which this strange theory rests. but there are certain matters connecting it with the above narrative which must here be noticed. the mention of the shepherd philition, who fed his flocks about the place where the great pyramid was built, is a singular feature of herodotus's narrative. it reads like some strange misinterpretation of the story related to him by the egyptian priests. it is obvious that if the word philition did not represent a people, but a person, this person must have been very eminent and distinguished--a shepherd-king, not a mere shepherd. rawlinson, in a note on this portion of the narrative of herodotus, suggests that philitis was probably a shepherd-prince from palestine, perhaps of philistine descent, 'but so powerful and domineering, that it may be traditions of his oppressions in that earlier age which, mixed up afterwards in the minds of later egyptians with the evils inflicted on their country by the subsequent shepherds of better known dynasties, lent so much fear to their religious hate of shepherd times and that name.' smyth, somewhat modifying this view, and considering certain remarks of manetho respecting an alleged invasion of egypt by shepherd-kings, 'men of an ignoble race (from the egyptian point of view) who had the confidence to invade our country, and easily subdued it to their power without a battle,' comes to the conclusion that some shemite prince, 'a contemporary of, but rather older than, the patriarch abraham,' visited egypt at this time, and obtained such influence over the mind of cheops as to persuade him to erect the pyramid. according to smyth, the prince was no other than melchizedek, king of salem, and the influence he exerted was supernatural. with such developments of the theory we need not trouble ourselves. it seems tolerably clear that certain shepherd-chiefs who came to egypt during cheops' reign were connected in some way with the designing of the great pyramid. it is clear also that they were men of a different religion from the egyptians, and persuaded cheops to abandon the religion of his people. taylor, smyth, and the pyramidalists generally, consider this sufficient to prove that the pyramid was erected for some purpose connected with religion. 'the pyramid,' in fine, says smyth, 'was charged by god's inspired shepherd-prince, in the beginning of human time, to keep a certain message secret and inviolable for years, and it has done so; and in the next thousand years it was to enunciate that message to all men, with more than traditional force, more than all the authenticity of copied manuscripts or reputed history; and that part of the pyramid's usefulness is now beginning.' there are many very obvious difficulties surrounding this theory; as, for example (i.) the absurd waste of power in setting supernatural machinery at work years ago with cumbrous devices to record its object, when the same machinery, much more simply employed now, would effect the alleged purpose far more thoroughly; (ii.) the enormous amount of human misery and its attendant hatreds brought about by this alleged divine scheme; and (iii.) the futility of an arrangement by which the pyramid was only to subserve its purpose when it had lost that perfection of shape on which its entire significance depended, according to the theory itself. but, apart from these, there is a difficulty, nowhere noticed by smyth or his followers, which is fatal, i conceive, to this theory of the pyramid's purpose. the second pyramid, though slightly inferior to the first in size, and probably far inferior in quality of masonry, is still a structure of enormous dimensions, which must have required many years of labour from tens of thousands of workmen. now, it seems impossible to explain why chephren built this second pyramid, if we adopt smyth's theory respecting the first pyramid. for either chephren knew the purpose for which the great pyramid was built, or he did not know it. if he knew that purpose, and it was that indicated by smyth, then he also knew that no second pyramid was wanted. on that hypothesis, all the labour bestowed on the second pyramid was wittingly and wilfully wasted. this, of course is incredible. but, on the other hand, if chephren did not know what was the purpose for which the great pyramid was built, what reason could chephren have had for building a pyramid at all? the only answer to this question seems to be that chephren built the second pyramid in hopes of finding out why his brother had built the first, and this answer is simply absurd. it is clear enough that whatever purpose cheops had in building the first pyramid, chephren must have had a similar purpose in building the second; and we require a theory which shall at least explain why the first pyramid did not subserve for chephren the purpose which it subserved or was meant to subserve for cheops. the same reasoning may be extended to the third pyramid, to the fourth, and in fine to all the pyramids, forty or so in number, included under the general designation of the pyramids of ghizeh or jeezeh. the extension of the principle to pyramids later than the second is especially important as showing that the difference of religion insisted on by smyth has no direct bearing on the question of the purpose for which the great pyramid itself was constructed. for mycerinus either never left or else returned to the religion of the egyptians. yet he also built a pyramid, which, though far inferior in size to the pyramids built by his father and uncle, was still a massive structure, and relatively more costly even than theirs, because built of expensive granite. the pyramid built by asychis, though smaller still, was remarkable as built of brick; in fact, we are expressly told that asychis desired to eclipse all his predecessors in such labours, and accordingly left this brick pyramid as a monument of his reign. we are forced, in fact, to believe that there was some special relation between the pyramid and its builder, seeing that each one of these kings wanted a pyramid of his own. this applies to the great pyramid quite as much as to the others, despite the superior excellence of that structure. or rather, the argument derives its chief force from the superiority of the great pyramid. if chephren, no longer perhaps having the assistance of the shepherd-architects in planning and superintending the work, was unable to construct a pyramid so perfect and so stately as his brother's, the very fact that he nevertheless built a pyramid shows that the great pyramid did not fulfil for chephren the purpose which it fulfilled for cheops. but, if smyth's theory were true, the great pyramid would have fulfilled finally and for all men the purpose for which it was built. since this was manifestly not the case, that theory is, i submit, demonstrably erroneous. it was probably the consideration of this point, viz. that each king had a pyramid constructed for himself, which led to the theory that the pyramids were intended to serve as tombs. this theory was once very generally entertained. thus we find humboldt, in his remarks on american pyramids, referring to the tomb theory of the egyptian pyramids as though it were open to no question. 'when we consider,' he says, 'the pyramidical monuments of egypt, of asia, and of the new continent, from the same point of view, we see that, though their form is alike, their destination was altogether different. the group of pyramids of ghizeh and at sakhara in egypt; the triangular pyramid of the queen of the scythians, zarina, which was a stadium high and three in circumference, and which was decorated with a colossal figure; the fourteen etruscan pyramids, which are said to have been enclosed in the labyrinth of the king porsenna, at clusium--were reared to serve as the sepulchres of the illustrious dead. nothing is more natural to men than to commemorate the spot where rest the ashes of those whose memory they cherish whether it be, as in the infancy of the race, by simple mounds of earth, or, in later periods, by the towering height of the tumulus. those of the chinese and of thibet have only a few metres of elevation. farther to the west the dimensions increase; the tumulus of the king alyattes, father of croesus, in lydia, was six stadia, and that of ninus was more than ten stadia in diameter. in the north of europe the sepulchre of the scandinavian king gormus and the queen daneboda, covered with mounds of earth, are three hundred metres broad, and more than thirty high.' but while we have abundant reason for believing that in egypt, even in the days of cheops and chephren, extreme importance was attached to the character of the place of burial for distinguished persons, there is nothing in what is known respecting earlier egyptian ideas to suggest the probability that any monarch would have devoted many years of his subjects' labour, and vast stores of material, to erect a mass of masonry like the great pyramid, solely to receive his own body after death. far less have we any reason for supposing that many monarchs in succession would do this, each having a separate tomb built for him. it might have been conceivable, had only the great pyramid been erected, that the structure had been raised as a mausoleum for all the kings and princes of the dynasty. but it seems utterly incredible that such a building as the great pyramid should have been erected for one king's body only--and that, not in the way described by humboldt, when he speaks of men commemorating the spot where rest the remains of those whose memory they cherish, but at the expense of the king himself whose body was to be there deposited. besides, the first pyramid, the one whose history must be regarded as most significant of the true purpose of these buildings, was not built by an egyptian holding in great favour the special religious ideas of his people, but by one who had adopted other views and those not belonging, so far as can be seen, to a people among whom sepulchral rites were held in exceptional regard. a still stronger objection against the exclusively tombic theory resides in the fact that this theory gives no account whatever of the characteristic features of the pyramids themselves. these buildings are all, without exception, built on special astronomical principles. their square bases are so placed as to have two sides lying east and west, and two lying north and south, or, in other words, so that their four faces front the four cardinal points. one can imagine no reason why a tomb should have such a position. it is not, indeed, easy to understand why any building at all, except an astronomical observatory, should have such a position. a temple perhaps devoted to sun-worship, and generally to the worship of the heavenly bodies, might be built in that way. for it is to be noticed that the peculiar figure and position of the pyramids would bring about the following relations:--when the sun rose and set south of the east and west points, or (speaking generally) between the autumn and the spring equinoxes, the rays of the rising and setting sun illuminated the southern face of the pyramid; whereas during the rest of the year, that is, during the six months between the spring and autumn equinoxes, the rays of the rising and setting sun illuminated the northern face. again, all the year round the sun's rays passed from the eastern to the western face at solar noon. and lastly, during seven months and a half of each year, namely, for three months and three quarters before and after midsummer, the noon rays of the sun fell on all four faces of the pyramid, or, according to a peruvian expression (so smyth avers), the sun shone on the pyramid 'with all his rays.' such conditions as these might have been regarded as very suitable for a temple devoted to sun-worship. yet the temple theory is as untenable as the tomb theory. for, in the first place, the pyramid form--as the pyramids were originally built, with perfectly smooth slant-faces, not terraced into steps as now through the loss of the casing-stones--was entirely unsuited for all the ordinary requirements of a temple of worship. and further, this theory gives no explanation of the fact that each king built a pyramid, and each king only one. similar difficulties oppose the theory that the pyramids were intended to serve as astronomical observatories. for while their original figure, however manifestly astronomical in its relations, was quite unsuited for observatory work, it is manifest that if such had been the purpose of pyramid-building, so soon as the great pyramid had once been built, no other would be needed. certainly none of the pyramids built afterwards could have subserved any astronomical purpose which the first did not subserve, or have subserved nearly so well as the great pyramid those purposes (and they are but few) which that building may be supposed to have fulfilled as an astronomical observatory. of the other theories mentioned at the beginning of this paper none seem to merit special notice, except perhaps the theory that the pyramids were made to receive the royal treasures, and this theory rather because of the attention it received from arabian literati, during the ninth and tenth centuries, than because of any strong reasons which can be suggested in its favour. 'emulating,' says professor smyth, 'the enchanted tales of bagdad,' the court poets of al mamoun (son of the far-famed haroun al raschid) 'drew gorgeous pictures of the contents of the pyramid's interior.... all the treasures of sheddad ben ad the great antediluvian king of the earth, with all his medicines and all his sciences, they declared were there, told over and over again. others, though, were positive that the founder-king was no other than saurid ibn salhouk, a far greater one than the other; and these last gave many more minute particulars, some of which are at least interesting to us in the present day, as proving that, amongst the egypto-arabians of more than a thousand years ago, the jeezeh pyramids, headed by the grand one, enjoyed a pre-eminence of fame vastly before all the other pyramids of egypt put together; and that if any other is alluded to after the great pyramid (which has always been the notable and favourite one, and chiefly was known then as the east pyramid), it is either the second one at jeezeh, under the name of the west pyramid; or the third one, distinguished as the coloured pyramid, in allusion to its red granite, compared with the white limestone casings of the other two (which, moreover, from their more near, but by no means exact, equality of size, went frequently under the affectionate designation of "the pair").' the report of ibn abd alkohm, as to what was to be found in each of these three pyramids, or rather of what, according to him, was put into them originally by king saurid, runs as follows: 'in the western pyramid, thirty treasuries filled with store of riches and utensils, and with signatures made of precious stones, and with instruments of iron and vessels of earth, and with arms which rust not, and with glass which might be bended and yet not broken, and with strange spells, and with several kinds of _alakakirs_ (magical precious stones) single and double, and with deadly poisons, and with other things besides. he made also in the east' (the great pyramid) 'divers celestial spheres and stars, and what they severally operate in their aspects, and the perfumes which are to be used to them, and the books which treat of these matters. he put also into the coloured pyramid the commentaries of the priests in chests of black marble, and with every priest a book, in which the wonders of his profession and of his actions and of his nature were written, and what was done in his time, and what is and what shall be from the beginning of time to the end of it.' the rest of this worthy's report relates to certain treasurers placed within these three pyramids to guard their contents, and (like all or most of what i have already quoted) was a work of imagination. ibn abd alkohm, in fact, was a romancist of the first water. perhaps the strongest argument against the theory that the pyramids were intended as strongholds for the concealment of treasure, resides in the fact that, search being made, no treasure has been discovered. when the workmen employed by caliph al mamoun, after encountering manifold difficulties, at length broke their way into the great ascending passage leading to the so-called king's chamber, they found 'a right noble apartment, thirty-four feet long, seventeen broad, and nineteen high, of polished red granite throughout, walls, floor, and ceiling, in blocks squared and true, and put together with such exquisite skill that the joints are barely discernible to the closest inspection. but where is the treasure--the silver and the gold, the jewels, medicines, and arms?--these fanatics look wildly around them, but can see nothing, not a single _dirhem_ anywhere. they trim their torches, and carry them again and again to every part of that red-walled, flinty hall, but without any better success. nought but pure polished red granite, in mighty slabs, looks upon them from every side. the room is clean, garnished too, as it were, and, according to the ideas of its founders, complete and perfectly ready for its visitors so long expected, so long delayed. but the gross minds who occupy it now, find it all barren, and declare that there is nothing whatever for them in the whole extent of the apartment from one end to another; nothing except an empty stone chest without a lid.' it is, however, to be noted that we have no means of learning what had happened between the time when the pyramid was built and when caliph al mamoun's workmen broke their way into the king's chamber. the place may, after all, have contained treasures of some kind; nor, indeed, is it incompatible with other theories of the pyramid to suppose that it was used as a safe receptacle for treasures. it is certain, however, that this cannot have been the special purpose for which the pyramids were designed. we should find in such a purpose no explanation whatever of any of the most stringent difficulties encountered in dealing with other theories. there could be no reason why strangers from the east should be at special pains to instruct an egyptian monarch how to hide and guard his treasures. nor, if the great pyramid had been intended to receive the treasures of cheops, would chephren have built another for his own treasures, which must have included those gathered by cheops. but, apart from this, how inconceivably vast must a treasure-hoard be supposed to be, the safe guarding of which would have repaid the enormous cost of the great pyramid in labour and material! and then, why should a mere treasure-house have the characteristics of an astronomical observatory? manifestly, if the pyramids were used at all to receive treasures, it can only have been as an entirely subordinate though perhaps convenient means of utilising these gigantic structures. having thus gone through all the suggested purposes of the pyramids save two or three which clearly do not possess any claim to serious consideration, and having found none which appear to give any sufficient account of the history and principal features of these buildings, we must either abandon the inquiry or seek for some explanation quite different from any yet suggested. let us consider what are the principal points of which the true theory of the pyramids should give an account. in the first place, the history of the pyramids shows that the erection of the first great pyramid was in all probability either suggested to cheops by wise men who visited egypt from the east, or else some important information conveyed to him by such visitors caused him to conceive the idea of building the pyramid. in either case we may suppose, as the history indeed suggests, that these learned men, whoever they may have been, remained in egypt to superintend the erection of the structure. it may be that the architectural work was not under their supervision; in fact, it seems altogether unlikely that shepherd-rulers would have much to teach the egyptians in the matter of architecture. but the astronomical peculiarities which form so significant a feature of the great pyramid were probably provided for entirely under the instructions of the shepherd chiefs who had exerted so strange an influence upon the mind of king cheops. next, it seems clear that self-interest must have been the predominant reason in the mind of the egyptian king for undertaking this stupendous work. it is true that his change of religion implies that some higher cause influenced him. but a ruler who could inflict such grievous burdens on his people in carrying out his purpose that for ages afterwards his name was held in utter detestation, cannot have been solely or even chiefly influenced by religious motives. it affords an ample explanation of the behaviour of cheops, in closing the temples and forsaking the religion of his country, to suppose that the advantages which he hoped to secure by building the pyramid depended in some way on his adopting this course. the visitors from the east may have refused to give their assistance on any other terms, or may have assured him that the expected benefit could not be obtained if the pyramid were erected by idolaters. it is certain, in any case, that they were opposed to idolatry; and we have thus some means of inferring who they were and whence they came. we know that one particular branch of one particular race in the east was characterised by a most marked hatred of idolatry in all its forms. terah and his family, or, probably, a sect or division of the chaldæan people, went forth from ur of the chaldees, to go into the land of canaan--and the reason why they went forth we learn from a book of considerable historical interest (the book of judith) to have been because 'they would not worship the gods of their fathers who were in the land of the chaldæans.' the bible record shows that members of this branch of the chaldæan people visited egypt from time to time. they were shepherds, too, which accords well with the account of herodotus above quoted. we can well understand that persons of this family would have resisted all endeavours to secure their acquiescence in any scheme associated with idolatrous rites. neither promises nor threats would have had much influence on them. it was a distinguished member of the family, the patriarch abraham, who said: 'i have lift up mine hand unto the lord, the most high god, the possessor of heaven and earth, that i will not take from a thread even to a shoe-latchet, and that i will not take anything that is thine, lest thou shouldest say, i have made abram rich.' vain would all the promises and all the threats of cheops have been to men of this spirit. such men might help him in his plans, suggested, as the history shows, by teachings of their own, but it must be on their own conditions, and those conditions would most certainly include the utter rejection of idolatrous worship by the king in whose behalf they worked, as well as by all who shared in their labours. it seems probable that they convinced both cheops and chephren, that unless these kings gave up idolatry, the purpose, whatever it was, which the pyramid was erected to promote, would not be fulfilled. the mere fact that the great pyramid was built either directly at the suggestion of these visitors, or because they had persuaded cheops of the truth of some important doctrine, shows that they must have gained great influence over his mind. rather we may say that he must have been so convinced of their knowledge and power as to have accepted with unquestioning confidence all that they told him respecting the particular subject over which they seemed to possess so perfect a mastery. but having formed the opinion, on grounds sufficiently assured, that the strangers who visited egypt and superintended the building of the great pyramid were kinsmen of the patriarch abraham, it is not very difficult to decide what was the subject respecting which they had such exact information. they or their parents had come from the land of the chaldæans, and they were doubtless learned in all the wisdom of their chaldæan kinsmen. they were masters, in fact, of the astronomy of their day, a science for which the chaldæans had shown from the earliest ages the most remarkable aptitude. what the actual extent of their astronomical knowledge may have been it would be difficult to say. but it is certain, from the exact knowledge which later chaldæans possessed respecting long astronomical cycles, that astronomical observations must have been carried on continuously by that people for many hundreds of years. it is highly probable that the astronomical knowledge of the chaldæans in the days of terah and abraham was much more accurate than that possessed by the greeks even after the time of hipparchus.[ ] we see indeed, in the accurate astronomical adjustment of the great pyramid, that the architects must have been skilful astronomers and mathematicians; and i may note here, in passing, how strongly this circumstance confirms the opinion that the visitors were kinsmen of terah and abraham. all we know from herodotus and manetho, all the evidence from the circumstances connected with the religion of the pyramid-kings, and the astronomical evidence given by the pyramids themselves, tends to assure us that members of that particular branch of the chaldæan family which went out from ur of the chaldees because they would not worship the gods of the chaldæans, extended their wanderings to egypt, and eventually superintended the erection of the great pyramid so far as astronomical and mathematical relations were concerned. but not only have we already decided that the pyramids were not intended solely or chiefly to sub serve the purpose of astronomical observatories, but it is certain that cheops would not have been personally much interested in any astronomical information which these visitors might be able to communicate. unless he saw clearly that something was to be gained from the lore of his visitors, he would not have undertaken to erect any astronomical buildings at their suggestion, even if he had cared enough for their knowledge to pay any attention to them whatever. most probably the reply cheops would have made to any communications respecting mere astronomy, would have run much in the style of the reply made by the turkish cadi, imaum ali zadè to a friend of layard's who had apparently bored him about double stars and comets: 'oh my soul! oh my lamb!' said ali zadè, 'seek not after the things which concern thee not. thou camest unto us, and we welcomed thee: go in peace. of a truth thou hast spoken many words; and there is no harm done, for the speaker is one and the listener is another. after the fashion of thy people thou hast wandered from one place to another until thou art happy and content in none. listen, oh my son! there is no wisdom equal unto the belief in god! he created the world, and shall we liken ourselves unto him in seeking to penetrate into the mysteries of his creation? shall we say, behold this star spinneth round that star, and this other star with a tail goeth and cometh in so many years! let it go! he from whose hand it came will guide and direct it. but thou wilt say unto me, stand aside, oh man, for i am more learned than thou art, and have seen more things. if thou thinkest that thou art in this respect better than i am, thou art welcome. i praise god that i seek not that which i require not. thou art learned in the things i care not for; and as for that which thou hast seen, i defile it. will much knowledge create thee a double belly, or wilt thou seek paradise with thine eyes?' such, omitting the references to the creator, would probably have been the reply of cheops to his visitors, had they only had astronomical facts to present him with. or, in the plenitude of his kingly power, he might have more decisively rejected their teaching by removing their heads. but the shepherd-astronomers had knowledge more attractive to offer than a mere series of astronomical discoveries. their ancestors had watched from the centres of their sleeping flocks those radiant mercuries, that seemed to move carrying through æther in perpetual round decrees and resolutions of the gods; and though the visitors of king cheops had themselves rejected the sabaistic polytheism of their kinsmen, they had not rejected the doctrine that the stars in their courses affect the fortunes of men. we know that among the jews, probably the direct descendants of the shepherd-chiefs who visited cheops, and certainly close kinsmen of theirs, and akin to them also in their monotheism, the belief in astrology was never regarded as a superstition. in fact, we can trace very clearly in the books relating to this people that they believed confidently in the influences of the heavenly bodies. doubtless the visitors of king cheops shared the belief of their chaldæan kinsmen that astrology is a true science, 'founded' indeed (as bacon expresses their views) 'not in reason and physical contemplations, but in the direct experience and observation of past ages.' josephus records the jewish tradition (though not as a tradition but as a fact) that 'our first father, adam, was instructed in astrology by divine inspiration,' and that seth so excelled in the science, that, 'foreseeing the flood and the destruction of the world thereby, he engraved the fundamental principles of his art (astrology) in hieroglyphical emblems, for the benefit of after ages, on two pillars of brick and stone.' he says farther on that the patriarch abraham, 'having learned the art in chaldæa, when he journeyed into egypt taught the egyptians the sciences of arithmetic and astrology.' indeed, the stranger called philitis by herodotus may, for aught that appears, have been abraham himself; for it is generally agreed that the word philitis indicated the race and country of the visitors, regarded by the egyptians as of philistine descent and arriving from palestine. however, i am in no way concerned to show that the shepherd-astronomers who induced cheops to build the great pyramid were even contemporaries of abraham and melchizedek. what seems sufficiently obvious is all that i care to maintain, namely, that these shepherd-astronomers were of chaldæan birth and training, and therefore astrologers, though, unlike their chaldæan kinsmen, they rejected sabaism or star-worship, and taught the belief in one only deity. now, if these visitors were astrologers, who persuaded cheops, and were honestly convinced themselves, that they could predict the events of any man's life by the chaldæan method of casting nativities, we can readily understand many circumstances connected with the pyramids which have hitherto seemed inexplicable. the pyramid built by a king would no longer be regarded as having reference to his death and burial, but to his birth and life, though after his death it might receive his body. each king would require to have his own nativity-pyramid, built with due symbolical reference to the special celestial influences affecting his fortunes. every portion of the work would have to be carried out under special conditions, determined according to the mysterious influences ascribed to the different planets and their varying positions-- now high, now low, then hid. progressive, retrograde, or standing still. if the work had been intended only to afford the means of predicting the king's future, the labour would have been regarded by the monarch as well bestowed. but astrology involved much more than the mere prediction of future events. astrologers claimed the power of ruling the planets--that is, of course, not of ruling the motions of those bodies, but of providing against evil influences or strengthening good influences which they supposed the celestial orbs to exert in particular aspects. thus we can understand that while the mere basement layers of the pyramid would have served for the process of casting the royal nativity, with due mystic observances, the further progress of building the pyramid would supply the necessary means and indications for ruling the planets most potent in their influence upon the royal career. remembering the mysterious influence which astrologers ascribed to special numbers, figures, positions, and so forth, the care with which the great pyramid was so proportioned as to indicate particular astronomical and mathematical relations is at once explained. the four sides of the square base were carefully placed with reference to the cardinal points, precisely like the four sides of the ordinary square scheme of nativity.[ ] the eastern side faced the ascendant, the southern faced the mid-heaven, the western faced the descendant, and the northern faced the imum coeli. again, we can understand that the architects would have made a circuit of the base correspond in length with the number of days in the year--a relation which, according to prof. p. smyth, is fulfilled in this manner, that the four sides contain one hundred times as many pyramid inches as there are days in the year. the pyramid inch, again, is itself mystically connected with astronomical relations, for its length is equal to the five hundred millionth part of the earth's diameter, to a degree of exactness corresponding well with what we might expect chaldæan astronomers to attain. prof. smyth, indeed, believes that it was exactly equal to that proportion of the earth's polar diameter--a view which would correspond with his theory that the architects of the great pyramid were assisted by divine inspiration; but what is certainly known about the sacred cubit, which contained twenty-five of these inches, corresponds better with the diameter which the chaldæan astronomers, if they worked very carefully, would have deduced from observations made in their own country, on the supposition which they would naturally have made that the earth is a perfect globe, not compressed at the poles. it is not indeed at all certain that the sacred cubit bore any reference to the earth's dimensions; but this seems tolerably well made out--that the sacred cubit was about inches in length, and that the circuit of the pyramid's base contained a hundred inches for every day of the year. relations such as these are precisely what we might expect to find in buildings having an astrological significance. similarly, it would correspond well with the mysticism of astrology that the pyramid should be so proportioned as to make the height be the radius of a circle whose circumference would equal the circuit of the pyramid's base. again, that long slant tunnel, leading downwards from the pyramid's northern face, would at once find a meaning in this astrological theory. the slant tunnel pointed to the pole-star of cheops' time, when due north below the true pole of the heavens. this circumstance had no observational utility. it could afford no indication of time, because a pole-star moves very slowly, and the pole-star of cheops' day must have been in view through that tunnel for more than an hour at a time. but, apart from the mystical significance which an astrologer would attribute to such a relation, it may be shown that this slant tunnel is precisely what the astrologer would require in order to get the horoscope correctly. another consideration remains to be mentioned which, while strengthening the astrological theory of the pyramids, may bring us even nearer to the true aim of those who planned and built these structures. it is known also that the chaldæans from the earliest times pursued the study of alchemy in connection with astrology, not hoping to discover the philosopher's stone by chemical investigations alone, but by carrying out such investigations under special celestial influence. the hope of achieving this discovery, by which he would at once have had the means of acquiring illimitable wealth, would of itself account for the fact that cheops expended so much labour and material in the erection of the great pyramid, seeing that, of necessity, success in the search for the philosopher's stone would be a main feature of his fortunes, and would therefore be astrologically indicated in his nativity-pyramid, or perhaps even be secured by following mystical observances proper for ruling his planets. the elixir of life may also have been among the objects which the builders of the pyramids hoped to discover. it may be noticed, as a somewhat significant circumstance, that, in the account given by ibn abd alkohm of the contents of the various pyramids, those assigned to the great pyramid relate entirely to astrology and associated mysteries. it is, of course, clear that abd alkohm drew largely on his imagination. yet it seems probable that there was also some basis of tradition for his ideas. and certainly one would suppose that, as he assigned a treasurer to the east pyramid ('a statue of black agate, his eyes open and shining, sitting on a throne with a lance'), he would have credited the building with treasure also, had not some tradition taught otherwise. but he says that king saurid placed in the east pyramid, not treasures, but 'divers celestial spheres and stars, and what they severally operate in their aspects, and the perfumes which are to be used to them, and the books which treat of these matters.'[ ] but, after all, it must be admitted that the strongest evidence in favour of the astrological (and alchemical) theory of the pyramids is to be found in the circumstance that all other theories seem untenable. the pyramids were undoubtedly erected for some purpose which was regarded by their builders as most important. this purpose certainly related to the personal fortunes of the kingly builders. it was worth an enormous outlay of money, labour, and material. this purpose was such, furthermore, that each king required to have his own pyramid. it was in some way associated with astronomy, for the pyramids are built with most accurate reference to celestial aspects. it also had its mathematical and mystical bearings, seeing that the pyramids exhibit mathematical and symbolical peculiarities not belonging to their essentially structural requirements. and lastly, the erection of the pyramids was in some way connected with the arrival of certain learned persons from palestine, and presumably of chaldæan origin. all these circumstances accord well with the theory i have advanced; while only some of them, and these not the most characteristic, accord with any of the other theories. moreover, no fact known respecting the pyramids or their builders is inconsistent with the astrological (and alchemical) theory. on the whole, then, if it cannot be regarded as demonstrated (in its general bearing, of course, for we cannot expect any theory about the pyramids to be established in minute details), the astrological theory may fairly be described as having a greater degree of probability in its favour than any hitherto advanced. iv. _swedenborg's visions of other worlds._ if it were permitted to men to select a sign whereby they should know that a message came from the supreme being, probably the man of science would select for the sign the communication of some scientific fact beyond the knowledge of the day, but admitting of being readily put to the test. the evidence thus obtained in favour of a revelation would correspond in some sense to that depending on prophecies; but it would be more satisfactory to men having that particular mental bent which is called the scientific. whether this turn of mind is inherent or the result of training, it certainly leads men of science to be more exacting in considering the value of evidence than any men, except perhaps lawyers. in the case of the student of science, st. paul's statement that 'prophecies' 'shall fail' has been fulfilled, whereas it may be doubted whether evidence from 'knowledge' would in like manner 'vanish away.' on the contrary, it would grow stronger and stronger, as knowledge from observation, from experiment, and from calculation continually increased. it can scarcely be said that this has happened with such quasi-scientific statements as have actually been associated with revelation. if we regard st. paul's reference to knowledge as relating to such statements as these, then nothing could be more complete than the fulfilment of his own prediction, 'whether there be prophecies, they shall fail; whether there be tongues, they shall cease; whether there be knowledge, it shall vanish away.' the evidence from prophecies fails for the exact inquirer, who perceives the doubts which exist (among the most earnest believers) as to the exact meaning of the prophetic words, and even in some cases as to whether prophecies have been long since fulfilled or relate to events still to come. the evidence from 'tongues' has ceased, and those are dust who are said to have spoken in strange tongues. the knowledge which was once thought supernatural has utterly vanished away. but if, in the ages of faith, some of the results of modern scientific research had been revealed, as the laws of the solar system, the great principle of the conservation of energy, or the wave theory of light, or if some of the questions which still remain for men of science to solve had been answered in those times, the evidence for the student of science would have been irresistible. of course he will be told that even then he would have hardened his heart; that the inquiry after truth tending naturally to depravity of mind, he would reject even evidence based on his beloved laws of probability; that his 'wicked and adulterous generation seeketh "in vain" after a sign,' and that if he will not accept moses and the prophets, neither would he believe though one rose from the dead. still the desire of the student of science to base his faith on convincing evidence (in a matter as important to him as to those who abuse him) does seem to have something reasonable in it after all. the mental qualities which cause him to be less easily satisfied than others, came to him in the same way as his bodily qualities; and even if the result to which his mental training leads him is as unfortunate as some suppose, that training is not strictly speaking so heinously sinful that nothing short of the eternal reprobation meted out to him by earthly judges can satisfy divine justice. so that it may be thought not a wholly unpardonable sin to speak of a sign which, had it been accorded, would have satisfied even the most exacting student of science. apart, too, from all question of faith, the mere scientific interest of divinely inspired communications respecting natural laws and processes would justify a student of science in regarding them as most desirable messages from a being of superior wisdom and benevolence. if prophecies and tongues, why not knowledge, as evidence of a divine mission? such thoughts are suggested by the claim of some religious teachers to the possession of knowledge other than that which they could have gained by natural means. the claim has usually been quite honest. the teacher of religion tests the reality of his mission in simple _à priori_ confidence that he has such a mission, and that therefore some one or other of the tests he applies will afford the required evidence. to one, says st. paul, is given the word of wisdom; to another, the word of knowledge; to another, faith; to another, the gift of healing; to another, the working of miracles; to another, prophecy; to another, the discerning of spirits; to another, divers kinds of tongues: and so forth. if a man like mahomet, who believes in his mission to teach, finds that he cannot satisfactorily work miracles--that mountains will not be removed at his bidding--then some other evidence satisfies him of the reality of his mission. swedenborg, than whom, perhaps, no more honest man ever lived, said and believed that to him had been granted the discerning of spirits. 'it is to be observed,' he said, 'that a man may be instructed by spirits and angels if his interiors be so open as to enable him to speak and be in company with them, for man in his essence is a spirit, and is with spirits as to his interiors; so that he whose interiors are opened by the lord may converse with them, as man with man. _this privilege i have enjoyed daily now for twelve years._' it indicates the fulness of swedenborg's belief in this privilege that he did not hesitate to describe what the spirits taught him respecting matters which belong rather to science than to faith; though it must be admitted that probably he supposed there was small reason for believing that his statements could ever be tested by the results of scientific research. the objects to which his spiritual communications related were conveniently remote. i do not say this as desiring for one moment to suggest that he purposely selected those objects, and not others which might be more readily examined. he certainly believed in the reality of the communications he described. but possibly there is some law in things visionary, corresponding to the law of mental operation with regard to scientific theories; and as the mind theorises freely about a subject little understood, but cautiously where many facts have been ascertained, so probably exact knowledge of a subject prevents the operation of those illusions which are regarded as supernatural communications. it is in a dim light only that the active imagination pictures objects which do not really exist; in the clear light of day they can no longer be imagined. so it is with mental processes. probably there is no subject more suitable in this sense for the visionary than that of life in other worlds. it has always had an attraction for imaginative minds, simply because it is enwrapped in so profound a mystery; and there has been little to restrain the fancy, because so little is certainly known of the physical condition of other worlds. recently, indeed, a somewhat sudden and severe check has been placed on the liveliness of imagination which had enabled men formerly to picture to themselves the inhabitants of other orbs in space. spectroscopic analysis and exact telescopic scrutiny will not permit some speculations to be entertained which formerly met with favour. yet even now there has been but a slight change of scene and time. if men can no longer imagine inhabitants of one planet because it is too hot, or of another because it is too cold, of one body because it is too deeply immersed in vaporous masses, or of another because it has neither atmosphere nor water, we have only to speculate about the unseen worlds which circle round those other suns, the stars; or, instead of changing the region of space where we imagine worlds, we can look backward to the time when planets now cold and dead were warm with life, or forward to the distant future when planets now glowing with fiery heat shall have cooled down to a habitable condition. swedenborg's imaginative mind seems to have fully felt the charm of this interesting subject. it was, indeed, because of the charm which he found in it, that he was readily persuaded into the belief that knowledge had been supernaturally communicated to him respecting it. 'because i had a desire,' he says, 'to know if there are other earths, and to learn their nature and the character of their inhabitants, it was granted me by the lord to converse and have intercourse with spirits and angels who had come from other earths, with some for a day, with some for a week, and with some for months. from them i have received information respecting the earths from and near which they are, the modes of life, customs and worship of their inhabitants, besides various other particulars of interest, all which, having come to my knowledge in this way, i can describe as things which i have seen and heard.' it is interesting (psychologically) to notice how the reasoning which had convinced swedenborg of the existence of other inhabited worlds is attributed by him to the spirits. 'it is well known in the other life,' he says, 'that there are many earths with men upon them; for there (that is, in the spiritual life) every one who, from a love of truth and consequent use, desires it, is allowed to converse with the spirits of other earths, so as to be assured that there is a plurality of worlds, and be informed that the human race is not confined to one earth only, but extends to numberless earths.... i have occasionally conversed on this subject with the spirits of our earth, and the result of our conversation was that a man of enlarged understanding may conclude from various considerations that there are many earths with human inhabitants upon them. for it is an inference of reason that masses so great as the planets are, some of which exceed this earth in magnitude, are not empty bodies, created only to be carried in their motion round the sun, and to shine with their scanty light for the benefit of one earth only; but that they must have a nobler use. he who believes, as every one ought to believe, that the deity created the universe for no other end than the existence of the human race, and of heaven from it (for the human race is the seminary of heaven), must also believe that wherever there is an earth there are human inhabitants. that the planets which are visible to us, being within the boundary of our solar system, are earths, may appear from various considerations. they are bodies of earthy matter, because they reflect the sun's light, and when seen through the telescope appear, not as stars shining with a flaming lustre, but as earths, variegated with obscure spots. like our earth, they are carried round the sun by a progressive motion, through the path of the zodiac, whence they have years and seasons of the year, which are spring, summer, autumn, and winter; and they rotate upon their axes, which makes days, and times of the day, as morning, midday, evening, and night. some of them also have satellites, which perform their revolutions about their globes, as the moon does about ours. the planet saturn, as being farthest from the sun, has besides an immense luminous ring, which supplies that earth with much, though reflected, light. how is it possible for anyone acquainted with these facts, and who thinks from reason, to assert that such bodies are uninhabited?' remembering that this reasoning was urged by the spirits, and that during twelve years swedenborg's interiors had been opened in such sort that he could converse with spirits from other worlds, it is surprising that he should have heard nothing about uranus or neptune, to say nothing of the zone of asteroids, or again, of planets as yet unknown which may exist outside the path of neptune. he definitely commits himself, it will be observed, to the statement that saturn is the planet farthest from the sun. and elsewhere, in stating where in these spiritual communications the 'idea' of each planet was conceived to be situated, he leaves no room whatever for uranus and neptune, and makes no mention of other bodies in the solar system than those known in his day. this cannot have been because the spirits from then unknown planets did not feel themselves called upon to communicate with the spirit of one who knew nothing of their home, for he received visitors from worlds in the starry heavens far beyond human ken. it would almost seem, though to the faithful swedenborgian the thought will doubtless appear very wicked, that the system of swedenborg gave no place to uranus and neptune, simply because he knew nothing about those planets. otherwise, what a noble opportunity there would have been for establishing the truth of swedenborgian doctrines by revealing to the world the existence of planets hitherto unknown. before the reader pronounces this a task beneath the dignity of the spirits and angels who taught swedenborg it will be well for him to examine the news which they actually imparted. i may as well premise, however, that it does not seem to me worth while to enter here at any length into swedenborg's descriptions of the inhabitants of other worlds, because what he has to say on this subject is entirely imaginative. there is a real interest for us in his ideas respecting the condition of the planets, because those ideas were based (though unconsciously) upon the science of his day, in which he was no mean proficient. and even where his mysticism went beyond what his scientific attainments suggested, a psychological interest attaches to the workings of his imagination. it is as curious a problem to trace his ideas to their origin as it sometimes is to account for the various phases of a fantastic dream, such a dream, for instance, as that which armadale, the doctor, and midwinter, in 'armadale,' endeavour to connect with preceding events. but swedenborg's visions of the behaviour and appearance of the inhabitants of other earths have little interest, because it is hopeless to attempt to account for even their leading features. for instance, what can we make of such a passage as the following, relating to the spirits who came from mercury?--'some of them are desirous to appear, not like the spirits of other earths as men, but as crystalline globes. their desire to appear so, although they do not, arises from the circumstance that the knowledges of things immaterial are in the other life represented by crystals.' yet some even of these more fanciful visions significantly indicate the nature of swedenborg's philosophy. one can recognise his disciples and his opponents among the inhabitants of various favoured and unhappy worlds, and one perceives how the wiser and more dignified of his spiritual visitors are made to advocate his own views, and to deride those of his adversaries. some of the teachings thus circuitously advanced are excellent. for instance, swedenborg's description of the inhabitants of mercury and their love of abstract knowledge contains an instructive lesson. 'the spirits of mercury imagine,' he says, 'that they know so much, that it is almost impossible to know more. but it has been told them by the spirits of our earth, that they do not know many things, but few, and that the things which they know not are comparatively infinite, and in relation to those they do know are as the waters of the largest ocean to those of the smallest fountain; and further, that the first advance to wisdom is to know, acknowledge, and perceive that what we do know, compared with what we do not know, is so little as hardly to amount to anything.'[ ] so far we may suppose that swedenborg presents his own ideas, seeing that he is describing what has been told the mercurial spirits by the spirits of our earth, of whom (during these spiritual conversations) he was one. but he proceeds to describe how angels were allowed to converse with the mercurial spirits in order to convince them of their error. 'i saw another angel,' says he, after describing one such conversation, 'conversing with them; he appeared at some altitude to the right; he was from our earth, and he enumerated very many things of which they were ignorant.... as they had been proud on account of their knowledges, on hearing this they began to humble themselves. their humiliation was represented by the sinking of the company which they formed, for that company then appeared as a volume or roll, ... as if hollowed in the middle and raised at the sides.... they were told what that signified, that is, what they thought in their humiliation, and that those who appeared elevated at the sides were not as yet in any humiliation. then i saw that the volume was separated, and that those who were not in humiliation were remanded back towards their earth, the rest remaining.' little being known to swedenborg, as indeed little is known to the astronomers of our own time, about mercury, we find little in the visions relating to that planet which possesses any scientific interest. he asked the inhabitants who were brought to him in visions about the sun of the system, and they replied that it looks larger from mercury than as seen from other worlds. this of course was no news to swedenborg. they explained further, that the inhabitants enjoy a moderate temperature, without extremes of heat or cold. 'it was given to me,' proceeds swedenborg, 'to tell them that it was so provided by the lord, that they might not be exposed to excessive heat from their greater proximity to the sun, since heat does not arise from the sun's nearness, but from the height and density of the atmosphere, as appears from the cold on high mountains even in hot climates; also that heat is varied according to the direct or oblique incidence of the sun's rays, as is plain from the seasons of winter and summer in every region.' it is curious to find thus advanced, in a sort of lecture addressed to visionary mercurials, a theory which crops up repeatedly in the present day, because the difficulty which suggests it is dealt with so unsatisfactorily for the most part in our text-books of science. continually we hear of some new paradoxist who propounds as a novel doctrine the teaching that the atmosphere, and not the sun, is the cause of heat. the mistake was excusable in swedenborg's time. in fact it so chanced that, apart from the obvious fact on which the mistake is usually based--the continued presence, namely, of snow on the summits of high mountains even in the torrid zone--it had been shown shortly before by newton, that the light fleecy clouds seen sometimes even in the hottest weather above the wool-pack or cumulus clouds are composed of minute crystals of ice. seeing that these tiny crystals can exist under the direct rays of the sun in hot summer weather, many find it difficult to understand how those rays can of themselves have any heating power. yet in reality the reasoning addressed by swedenborg to his mercurial friends was entirely erroneous. if he could have adventured as far forth into time as he did into space, and could have attended in the spirit the lectures of one john tyndall, a spirit of our earth, he would have had this matter rightly explained to him. in reality the sun's heat is as effective directly at the summit of the highest mountain as at the sea-level. a thermometer exposed to the sun in the former position indicates indeed a slightly higher temperature than one similarly exposed to the sun (when at the same altitude) at the sea-level. but the air does not get warmed to the same degree, simply because, owing to its rarity and relative dryness, it fails to retain any portion of the heat which passes through it. it is interesting to notice how swedenborg's scientific conceptions of the result of the (relatively) airless condition of our moon suggested peculiar fancies respecting the lunar inhabitants. interesting, i mean, psychologically: for it is curious to see scientific and fanciful conceptions thus unconsciously intermingled. of the conscious intermingling of such conceptions instances are common enough. the effects of the moon's airless condition have been often made the subject of fanciful speculations. the reader will remember how scheherazade, in 'the poet at the breakfast table,' runs on about the moon. 'her delight was unbounded, and her curiosity insatiable. if there were any living creatures there, what odd things they must be. they couldn't have any lungs nor any hearts. what a pity! did they ever die? how could they expire if they didn't breathe? burn up? no air to burn in. tumble into some of those horrid pits, perhaps, and break all to bits. she wondered how the young people there liked it, or whether there were any young people there. perhaps nobody was young and nobody was old, but they were like mummies all of them--what an idea!--two mummies making love to each other! so she went on in a rattling, giddy kind of way, for she was excited by the strange scene in which she found herself, and quite astonished the young astronomer with her vivacity.' but swedenborg's firm belief that the fancies engendered in his mind were scientific realities is very different from the conscious play of fancy in the passage just quoted. it must be remembered that swedenborg regarded his visions with as much confidence as though they were revelations made by means of scientific instruments; nay, with even more confidence, for he knew that scientific observations may be misunderstood, whereas he was fully persuaded that his visions were miraculously provided for his enlightenment, and that therefore he would not be allowed to misunderstand aught that was thus revealed to him. 'it is well known to spirits and angels,' he says, 'that there are inhabitants in the moon, and in the moons or satellites which revolve about jupiter and saturn. even those who have not seen and conversed with spirits who are from them entertain no doubt of their being inhabited, for they, too, are earths, and where there is an earth there is man; man being the end for which every earth exists, and without an end nothing was made by the great creator. every one who thinks from reason in any degree enlightened, must see that the human race is the final cause of creation.' the moon being inhabited then by human beings, but being very insufficiently supplied with air, it necessarily follows that these human beings must be provided in some way with the means of existing in that rare and tenuous atmosphere. tremendous powers of inspiration and expiration would be required to make that air support the life of the human body. although swedenborg could have had no knowledge of the exact way in which breathing supports life (for priestley was his junior by nearly half a century), yet he must clearly have perceived that the quantity of air inspired has much to do with the vitalising power of the indraught. no ordinary human lungs could draw in an adequate supply of air from such an atmosphere as the moon's; but by some great increase of breathing power it might be possible to live there: at least, in swedenborg's time there was no reason for supposing otherwise. reason, then, having convinced him that the lunar inhabitants must possess extraordinary breathing apparatus, and presumably most powerful voices, imagination presented them to him accordingly. 'some spirits appeared overhead,' he says, 'and thence were heard voices like thunder; for their voices sounded precisely like thunder from the clouds after lightning. i supposed it was a great multitude of spirits who had the art of giving voices with such a sound. the more simple spirits who were with me derided them, which greatly surprised me. but the cause of their derision was soon discovered, which was, that the spirits who thundered were not many, but few, and were as little as children, and that on former occasions they (the thunderers) had terrified them by such sounds, and yet were unable to do them the least harm. that i might know their character, some of them descended from on high, where they thundered; and, what surprised me, one carried another on his back, and the two thus approached me. their faces appeared not unhandsome, but longer than those of other spirits. in stature they were like children of seven years old, but the frame was more robust, so that they were like men. it was told me by the angels that they were from the moon. he who was carried by the other came to me, applying himself to my left side under the elbow, and thence spoke. he said, that when they utter their voices they thunder in this way,'--and it seems likely enough that if there are any living speaking beings in the moon, their voice, could they visit the earth, would be found to differ very markedly from the ordinary human voice. 'in the spiritual world their thunderous voices have their use. for by their thundering the spirits from the moon terrify spirits who are inclined to injure them, so that the lunar spirits go in safety where they will. to convince me the sound they make was of this kind, he (the spirit who was carried by the other) retired, but not out of sight, and thundered in like manner. they showed, moreover, that the voice was thundered by being uttered from the abdomen like an eructation. it was perceived that this arose from the circumstance that the inhabitants of the moon do not, like the inhabitants of other earths, speak from the lungs, but from the abdomen, and thus from air collected there, the reason of which is that the atmosphere with which the moon is surrounded is not like that of other earths.' in his intercourse with spirits from jupiter, swedenborg heard of animals larger than those that live on the earth. it has been a favourite idea of many believers in other worlds than ours, that though in each world the same races of animals exist, they would be differently proportioned; and there has been much speculation as to the probable size of men and other animals in worlds much larger or much smaller than the earth. when as yet ideas about other worlds were crude, the idea prevailed that giants exist in the larger orbs, and pygmies in the smaller. whether this idea had its origin in conceptions as to the eternal fitness of things or not, does not clearly appear. it seems certainly at first view natural enough to suppose that the larger beings would want more room and so inhabit the larger dwelling-places. it was a pleasing thought that, if we could visit jupiter or saturn, we should find the human inhabitants there in bigness to surpass earth's giant sons; but that if we could visit our moon or mercury, or whatever smaller worlds there are, we should find men now less than smallest dwarfs, in narrow room throng numberless, like that pygmæan race beyond the indian mount; or fairy elves, whose midnight revels, by a forest side or fountain, some belated peasant sees, or dreams he sees. later the theory was started that the size of beings in various worlds depends on the amount of light received from the central sun. thus wolfius asserted that the inhabitants of jupiter are nearly fourteen feet high, which he proved by comparing the quantity of sunlight which reaches the jovians with that which we terrenes receive. recently, however, it has been noted that the larger the planet, the smaller in all probability must be the inhabitants, if any. for if there are two planets of the same density but unequal size, gravity must be greater at the surface of the larger planet, and where gravity is great large animals are cumbered by their weight. it is easy to see this by comparing the muscular strength of two men similarly proportioned, but unequal in height. suppose one man five feet in height, the other six; then the cross section of any given muscle will be less for the former than for the latter in the proportion of twenty-five (five times five) to thirty-six (six times six). roughly, the muscular strength of the bigger man will be half as great again as that of the smaller. but the weights of the men will be proportioned as (five times five times five) to (six times six times six), so that the weight of the bigger man exceeds that of the smaller nearly as seven exceeds four, or by three-fourths. the taller man exceeds the smaller, then, much more in weight than he does in strength; he is accordingly less active in proportion to his size. within certain limits, of course, size increases a man's effective as well as his real strength. for instance, our tall man in the preceding illustration cannot lift his own weight as readily as the small man can lift his; but he can lift a weight of three hundred pounds as easily as the small man can lift a weight of two hundred pounds. when we get beyond certain limits of height, however, we get absolute weakness as the result of the increase of weight. swift's brobdingnags, for instance, would have been unable to stand upright; for they were six times as tall as men, and therefore each brobdingnag would have weighed times as much as a man, but would have possessed only thirty-six times the muscular power. their weight would have been greater, then, in a sixfold greater degree than their strength, and, so far as their mere weight was concerned, their condition would have resembled that of an ordinary man under a load five times exceeding his own weight. as no man could walk or stand upright under such a load, so the brobdingnags would have been powerless to move, despite, or rather because of, their enormous stature. applying the general considerations here enunciated to the question of the probable size of creatures like ourselves in other planets, we see that men in jupiter should be much smaller, men in mercury much larger, than men on the earth. so also with other animals. but swedenborg's spirit visitors from these planets taught differently. 'the horses of our earth,' he says, 'when seen by the spirits of jupiter, appeared to me smaller than usual, though rather robust; which arose from the idea those spirits had respecting them. they informed me that among them there are animals similar, though much larger; but that they are wild, and in the woods, and that when they come in sight they cause terror though they are harmless; they added that their terror of them is natural or innate.'[ ] on the other hand the inhabitants of mercury, who might be thirteen feet high yet as active as our men, appeared slenderer than terrene men. 'i was desirous to know,' says swedenborg, 'what kind of face and person the people in mercury have, compared with those of the people on our earth. there therefore stood before me a female exactly resembling the women on that earth. her face was beautiful, but it was smaller than that of a woman of our earth; she was more slender, but of equal height; she wore a linen head-dress, not artfully yet gracefully disposed. a man also was presented. he, too, was more slender than the men of our earth; he wore a garment of deep blue, closely fitted to his body without folds or flowing skirts. such, i learn, were the personal form and costume of the humans of that earth. afterwards there was shown me a species of the oxen and cows, which did not indeed differ much from those on our earth, except that they were smaller, and made some approach to the stag and hind species.' we have seen, too, that the lunar spirits were no larger than children seven years old. one passage of swedenborg's description of jupiter is curious. 'although on that earth,' he says, 'spirits speak with men' (_i.e._ with jovian men) 'man in his turn does not speak with spirits, except to say, when instructed, _that he will do so no more_,'--which we should regard as a bull if it were not news from the jovian spirit world. 'nor is man allowed to tell anyone that a spirit has spoken to him; if he does so, he is punished. those spirits of jupiter when they were with me, at first supposed they were with a man of their own earth; but when in my turn i spoke with them, and thought of publishing what passed between us and so relating it to others, then, because they were not allowed to chastise me, they discovered they were with a stranger.' it has been a favourite idea with those who delight in the argument from design, that the moons of the remoter planets have been provided for the express purpose of making up for the small amount of sunlight which reaches those planets. jupiter receives only about one twenty-seventh part of the light which we receive from the sun; but then, has he not four moons to make his nights glorious? saturn is yet farther away from the sun, and receives only the ninetieth part of the light we get from the sun; but then he has eight moons and his rings, and the nocturnal glory of his skies must go far to compensate the saturnians for the small quantity of sunlight they receive. the saturnian spirits who visited swedenborg were manifestly indoctrinated with these ideas. for they informed him that the nocturnal light of saturn is so great that some saturnians worship it, calling it the lord. these wicked spirits are separated from the rest, and are not tolerated by them. 'the nocturnal light,' say the spirits, 'comes from the immense ring which at a distance encircles that earth, and from the moons which are called the satellites of saturn.' and again, being questioned further 'concerning the great ring which appears from our earth to rise above the horizon of that planet, and to vary its situations, they said that it does not appear to them as a ring, but only as a snow-white substance in heaven in various directions.' unfortunately for our faith in the veracity of these spirits, it is certain that the moons of saturn cannot give nearly so much light as ours, while the rings are much more effective as darkeners than as illuminators. one can readily calculate the apparent size of each of the moons as seen from saturn, and thence show that the eight discs of the moons together are larger than our moon's disc in about the proportion of forty-five to eight. so that if they were all shining as brightly as our full moon and all full at the same time, their combined light would exceed hers in that degree. but they are not illuminated as our moon is. they are illuminated by the same remote sun which illuminates saturn, while our moon is illuminated by a sun giving her as much light as we ourselves receive. our moon then is illuminated ninety times more brightly than the moons of saturn, and as her disc is less than all theirs together, not as one to ninety, but as sixteen to ninety, it follows that all the saturnian moons, if full at the same time, would reflect to saturn one-sixteenth part of the light which we receive from the full moon.[ ] as regards the rings of saturn, nothing can be more certain than that they tend much more to deprive saturn of light then to make up by reflection for the small amount of light which saturn receives directly from the sun. the part of the ring which lies between the planet and the sun casts a black shadow upon saturn, this shadow sometimes covering an extent of surface many times exceeding the entire surface of our earth. the shadow thus thrown upon the planet creeps slowly, first one way, then another, northwards and southwards over the illuminated hemisphere of the planet (as pictured in the th plate of my treatise on saturn), requiring for its passage from the arctic to the antarctic regions and back again to the arctic regions of the planet, a period nearly equal to that of a generation of terrestrial men. nearly thirty of our years the process lasts, during half of which time the northern hemisphere suffers, and during the other half the southern. the shadow band, which be it remembered stretches right athwart the planet from the extreme eastern to the extreme western side of the illuminated hemisphere, is so broad during the greater part of the time that in some regions (those corresponding to our temperate zones) the shadow takes two years in passing, during which time the sun cannot be seen at all, unless for a few moments through some chinks in the rings, which are known to be not solid bodies, but made up of closely crowded small moons. and the slow passage of this fearful shadow, which advances at the average rate of some twenty miles a day, but yet hangs for years over the regions athwart which it sweeps, occurs in the very season when the sun's small direct supply of heat would require to be most freely compensated by nocturnal light--in the winter season, namely, of the planet. moreover, not only during the time of the shadow's passage, but during the entire winter half of the saturnian year, the ring reflects no light during the night time, the sun being on the other or summer side of the ring's plane.[ ] the only nocturnal effect which would be observable would be the obliteration of the stars covered by the ring system. it is strange that, this being so, the spirits from saturn should have made no mention of the circumstance; and even more strange that these spirits and others should have asserted that the moons and rings of saturn compensate for the small amount of light directly received from the sun. most certainly a swedenborg of our own time would find the spirits from saturn more veracious and more communicative about these matters, though even what _he_ would hear from the spirits would doubtless appear to sceptics of the twenty-first century to be no more than he could have inferred from the known facts of the science of his day. but swedenborg was not content merely to receive visits from the inhabitants of other planets in the solar system. he was visited also by the spirits of earths in the starry heaven; nay, he was enabled to visit those earths himself. for man, even while living in the world, 'is a spirit as to his interiors, the body which he carries about in the world only serving him for performing functions in this natural or terrestrial sphere, which is the lowest.' and to certain men it is granted not only to converse as a spirit with angels and spirits, but to traverse in a spiritual way the vast distances which separate world from world and system from system, all the while remaining in the body. swedenborg was one of these. 'the interiors of my spirit,' he says, 'are opened by the lord, so that while i am in the body i can at the same time be with angels in heaven, and not only converse with them, but behold the wonderful things which are there and describe them, that henceforth it may no more be said, "who ever came from heaven to assure us it exists and tell us what is there?" he who is unacquainted with the arcana of heaven cannot believe that man can see earths so remote, and give any account of them from sensible experience. but let him know that spaces and distances, and consequently progressions, existing in the natural world, in their origin and first causes are changes of the state of the interiors; that with angels and spirits progressions appear according to changes of state; and that by changes of state they may be apparently translated from one place to another, and from one earth to another, even to earths at the boundaries of the universe; so likewise may man as to his spirit, his body still remaining in its place. this has been the case with me.' before describing his visits to earths in the starry heavens, swedenborg is careful to indicate the probability that such earths exist. 'it is well known to the learned world,' he says, 'that every star is a sun in its place, remaining fixed like the sun of our earth.' the proper motions of the stars had, alas! not been discovered in swedenborg's day, nor does he seem to have been aware what a wild chase he was really entering upon in his spiritual progressions. conceive the pursuit of sirius or vega as either sun rushed through space with a velocity of thirty or forty miles in every second of time! to resume, however, the account which swedenborg gives of the ideas of the learned world of his day. 'it is the distance which makes a star appear in a small form; consequently' (the logical necessity is not manifest, however) 'each star, like the sun of our system, has around it planets which are earths; and the reason these are not visible to us is because of their immense distance and their having no light but from their own star, which light cannot be reflected so far as to reach us.' 'to what other end,' proceeds this most convincing reasoning, 'can be so immense a heaven with such a multitude of stars? for man is the end for which the universe was created. it has been ascertained by calculation that supposing there were in the universe a million earths, and on every earth three hundred millions of men and two hundred generations within six thousand years, and that to every man or spirit was allotted a space of three cubic ells, the collective number of men or spirits could not occupy a space equal to a thousandth part of this earth, thus not more than that occupied by one of the satellites of jupiter or saturn; a space on the universe almost undiscernible, for a satellite is hardly visible to the naked eye. what would this be for the creator of the universe, to whom the whole universe filled with earths could not be enough' (for what?), 'seeing that he is infinite.' however, it is not on this reasoning alone that swedenborg relies. he tells us, honestly beyond all doubt, that he knows the truth of what he relates. 'the information i am about to give,' he says, 'respecting the earths in the starry heaven is from experimental testimony; from which it will likewise appear how i was translated thither as to my spirit, the body remaining in its place.' his progress in his first star-hunt was to the right, and continued for about two hours. he found the boundary of our solar system marked first by a white but thick cloud, next by a fiery smoke ascending from a great chasm. here some guards appeared, who stopped some of the company, because these had not, like swedenborg and the rest, received permission to pass. they not only stopped those unfortunates, but tortured them, conduct for which terrestrial analogues might possibly be discovered. having reached another system, he asked the spirits of one of the earths there how large their sun was and how it appeared. they said it was less than the sun of our earth, and has a flaming appearance. our sun, in fact, is larger than other suns in space, for from that earth starry heavens are seen, and a star larger than the rest appears, which, say those spirits, 'was declared from heaven' to be the sun of swedenborg's earthly home. what swedenborg saw upon that earth has no special interest. the men there, though haughty, are loved by their respective wives because they, the men, are good. but their goodness does not appear very manifest from anything in the narrative. the only man seen by swedenborg took from his wife 'the garment which she wore, and threw it over his own shoulders; loosening the lower part, which flowed down to his feet like a robe (much as a man of our earth might be expected to loosen the tie-back of the period, if he borrowed it in like manner) he thus walked about clad.' he next visited an earth circling round a star, which he learned was one of the smaller sort, not far from the equator. its greater distance was plain from the circumstance that swedenborg was two days in reaching it. in this earth he very nearly fell into a quarrel with the spirits. for hearing that they possess remarkable keenness of vision, he 'compared them with eagles which fly aloft, and enjoy a clear and extensive view of objects beneath.' at this they were indignant, supposing, poor spirits, 'that he compared them to eagles as to their rapacity, and consequently thought them wicked.' he hastened to explain, however, that he 'did not liken them to eagles as to their rapacity, but as to sharpsightedness.' swedenborg's account of a third earth in the star-depths contains a very pretty idea for temples and churches. the temples in that earth 'are constructed,' he says, of trees, not cut down, but growing in the place where they were first planted. on that earth, it seems, there are trees of an extraordinary size and height; these they set in rows when young, and arrange in such an order that they may serve when they grow up to form porticoes and colonnades. in the meanwhile, by cutting and pruning, they fit and prepare the tender shoots to entwine one with another, and join together so as to form the groundwork and floor of the temple to be constructed, and to rise at the sides as walls, and above to bend into arches to form the roof. in this manner they construct the temple with admirable art, elevating it high above the ground. they prepare also an ascent into it, by continuous branches of the trees, extended from the trunk and firmly connected together. moreover, they adorn the temple without and within in various ways, by disposing the foliage into particular forms; thus they build entire groves. but it was not permitted me to see the nature of these temples, only i was informed that the light of their sun is let in by apertures amongst the branches, and is everywhere transmitted through crystals; whereby the light falling on the walls is refracted in colours like those of the rainbow, particularly blue and orange, of which they are fondest. such is their architecture, which they prefer to the most magnificent palaces of our earth.' other earths in the starry heavens were visited by swedenborg, but the above will serve sufficiently to illustrate the nature of his observations. one statement, by the way, was made to him which must have seemed unlikely ever to be contravened, but which has been shown in our time to be altogether erroneous. in the fourth star-world he visited, he was told that that earth, which travels round its sun in days of fifteen hours each, is one of the least in the universe, being scarcely german miles, say english miles, in circumference. this would make its diameter about english miles. but there is not one of the whole family of planetoids which has a diameter so great as this, and many of these earths must be less than fifty miles in diameter. now swedenborg remarks that he had his information from the angels, 'who made a comparison in all these particulars with things of a like nature on our earth, according to what they saw in me or in my memory. their conclusions were formed by angelic ideas, whereby are instantly known the measure of space and time in a just proportion with respect to space and time elsewhere. angelic ideas, which are spiritual, in such calculations infinitely excel human ideas, which are natural.' he must therefore have met, unfortunately, with untruthful angels. the real source of swedenborg's inspirations will be tolerably obvious--to all, at least, who are not swedenborgians. but our account of his visions would not be complete in a psychological sense without a brief reference to the personal allusions which the spirits and angels made during their visits or his wanderings. his distinguished rival, christian wolf, was encountered as a spirit by spirits from mercury, who 'perceived that what he said did not rise above the sensual things of the natural man, because in speaking he thought of honour, and was desirous, as in the world (for in the other world every one is like his former self), to connect various things into series, and from these again continually to deduce others, and so form several chains of such, which they did not see or acknowledge to be true, and which, therefore, they declared to be chains which neither cohered in themselves nor with the conclusions, calling them the obscurity of authority;' so they ceased to question him further, and presently left him. similarly, a spirit who in this world had been a 'prelate and a preacher,' and 'very pathetic, so that he could deeply move his hearers,' got no hearing among the spirits of a certain earth in the starry heavens; for they said they could tell 'from the tone of the voice whether a discourse came from the heart or not;' and as his discourse came not from the heart, 'he was unable to teach them, whereupon he was silent.' convenient thus to have spirits and angels to confirm our impressions of other men, living or dead. apart from the psychological interest attaching to swedenborg's strange vision, one cannot but be strongly impressed by the idea pervading them, that to beings suitably constituted all that takes place in other worlds might be known. modern science recognises a truth here; for in that mysterious ether which occupies all space, messages are at all times travelling by which the history of every orb is constantly recorded. no world, however remote or insignificant; no period, however distant--but has its history thus continually proclaimed in ever widening waves. nay, by these waves also (to beings who could read their teachings aright) the future is constantly indicated. for, as the waves which permeate the ether could only be situated as they actually are, at any moment, through past processes, each one of which is consequently indicated by those ethereal waves, so also there can be but one series of events in the future, as the sequel of the relations actually indicated by the ethereal undulations. these, therefore, speak as definitely and distinctly of the future as of the past. could we but rid us of the gross habiliments of flesh, and by some new senses be enabled to feel each order of ethereal undulations, even of those only which reach our earth, all knowledge of the past and future would be within our power. the consciousness of this underlies the fancies of swedenborg, just as it underlies the thought of him who sang-- there's not an orb which thou behold'st but in his motion like an angel sings, still quiring to the young-eyed cherubim. but while this muddy vesture of decay doth grossly close us in, we cannot hear it. v. _other worlds and other universes._ if any one shall gravely tell me that i have spent my time idly in a vain and fruitless inquiry after what i can never become sure of, the answer is that at this rate he would put down all natural philosophy, as far as it concerns itself in searching into the nature of such things. in such noble and sublime studies as these, 'tis a glory to arrive at probability, and the search itself rewards the pains. but there are many degrees of probable, some nearer to the truth than others, in the determining of which lies the chief exercise of our judgment. and besides the nobleness and pleasure of the studies, may we not be so bold as to say that they are no small help to the advancement of wisdom and morality?--huyghens, _conjectures concerning the planetary worlds_. the interest with which astronomy is studied by many who care little or nothing for other sciences is due chiefly to the thoughts which the celestial bodies suggest respecting life in other worlds than ours. there is no feeling more deeply seated in the human heart--not the belief in higher than human powers, not the hope of immortality, not even the fear of death--than the faith in realms of life where other conditions are experienced than those we are acquainted with here. it is not vulgar curiosity or idle fancy that suggests the possibilities of life in other worlds. it has been the conviction of the profoundest thinkers, of men of highest imagination. the mystery of the star-depths has had its charm for the mathematician as well as for the poet; for the exact observer as for the most fruitful theoriser; nay, for the man of business as for him whose life is passed in communing with nature. if we analyse the interest with which the generality of men inquire into astronomical matters apparently not connected with the question of life in other worlds, we find in every case that it has been out of this question alone or chiefly that that interest has sprung. the great discoveries made during the last few years respecting the sun for example, might seem remote from the subject of life in other worlds. it is true that sir william herschel thought the sun might be the abode of living creatures; and sir john herschel even suggested the possibility that the vast streaks of light called the solar willow-leaves, objects varying from two hundred to a thousand miles in length, might be living creatures whose intense lustre was the measure of their intense vitality. but modern discoveries had rendered all such theories untenable. the sun is presented to us as a mighty furnace, in whose fires the most stubborn elements are not merely melted but vaporised. the material of the sun has been analysed, the motions and changes taking place on his surface examined, the laws of his being determined. how, it might be asked, is the question of life in other worlds involved in these researches? the faith of sir david brewster in the sun as the abode of life being dispelled, how could discoveries respecting the sun interest those who care about the subject of the plurality of worlds? the answer to these questions is easily found. the real interest which solar researches have possessed for those who are not astronomers has resided in the evidence afforded respecting the sun's position as the fire, light, and life of the system of worlds whereof our world is one. the mere facts discovered respecting the sun would be regarded as so much dry detail were they not brought directly into relation with our earth and its wants, and therefore with the wants of the other earths which circle round the sun; but when thus dealt with they immediately excite attention and interest. i do not speak at random in asserting this, but describe the result of widely ranging observation. i have addressed hundreds of audiences in great britain and america on the subject of recent solar discoveries, and i have conversed with many hundreds of persons of various capacity and education, from men almost uncultured to men of the highest intellectual power; and my invariable experience has been that solar research derives its chief interest when viewed in relation to the sun's position as the mighty ruler, the steadfast sustainer, the beneficent almoner of the system of worlds to which our earth belongs. it is the same with other astronomical subjects. few care for the record of lunar observations, save in relation to the question whether the moon is or has been the abode of living creatures. the movements of comets and meteors, and the discoveries recently made respecting their condition, have no interest except in relation to the position of these bodies in the economy of solar systems, or to the possible part which they may at one time have performed in building up worlds and suns. none save astronomers, and few only of these, care for researches into the star-depths, except in connection with the thought that every star is a sun and therefore probably the light and fire of a system of worlds like those which circle around our own sun. it is singular how variously this question of life in other worlds has been viewed at various stages of astronomical progress. from the time of pythagoras, who first, so far as is known, propounded the general theory of the plurality of worlds, down to our own time, when brewster and chalmers on the one hand, and whewell on the other, have advocated rival theories probably to be both set aside for a theory at once intermediate to and more widely ranging in time and space than either, the aspect of the subject has constantly varied, as new lights have been thrown upon it from different directions. it may be interesting briefly to consider what has been thought in the past on this strangely attractive question, and then to indicate the view towards which modern discoveries seem manifestly to point--a view not likely to undergo other change than that resulting from clearer vision and closer approach. in other words, i shall endeavour to show that the theory to which we are now led by all the known facts is correct in general, though, as fresh knowledge is obtained, it may undergo modification in details. we now see the subject from the right point of view, though as science progresses we may come to see it more clearly and definedly. when men believed the earth to be a flat surface above which the heavens were arched as a tent or canopy, they were not likely to entertain the belief in other worlds than ours. during the earlier ages of mankind ideas such as these prevailed. the earth had been fashioned into its present form and condition, the heavens had been spread over it, the sun, and moon, and stars had been set in the heavens for its use and adornment, and there was no thought of any other world. but while this was the general belief, there was already a school of philosophy where another doctrine had been taught. pythagoras had adopted the belief of apollonius pergæus that the sun is the centre of the planetary paths, the earth one among the planets--a belief inseparable from the doctrine of the plurality of worlds. much argument has been advanced to show that this belief never was adopted before the time of copernicus, and unquestionably it must be admitted that the theory was not presented in the clear and simple form to which we have become accustomed. but it is not necessary to weigh the conflicting arguments for and against the opinion that pythagoras and others regarded the earth as not the fixed centre of the universe. the certain fact that the doctrine of the plurality of worlds was entertained (i do not say adopted) by them, proves sufficiently that they cannot have believed the earth to be fixed and central. the idea of other worlds like our earth is manifestly inconsistent with the belief that the earth is the central body around which the whole universe revolves. that this is so is well illustrated by the fate of the unfortunate giordano bruno. he was one of the first disciples of copernicus, and, having accepted the doctrine that the earth travels round the sun as one among his family of planets, was led very naturally to the belief that the other planets are inhabited. he went farther, and maintained that as the earth is not the only inhabited world in the solar system, so the sun is not the only centre of a system of inhabited worlds, but each star a sun like him, about which many planets revolve. this was one of the many heresies for which bruno was burned at the stake. it is easy, also, to recognise in the doctrine of many worlds as the natural sequel of the copernican theory, rather than in the features of this theory itself, the cause of the hostility with which theologians regarded it, until, finding it proved, they discovered that it is directly taught in the books which they interpret for us so variously. the copernican theory was not rejected--nay, it was even countenanced--until this particular consequence of the theory was recognised. but within a few years from the persecution of bruno, galileo was imprisoned, and the last years of his life made miserable, because it had become clear that in setting the earth adrift from its position as centre of the universe, he and his brother copernicans were sanctioning the belief in other worlds than ours. again and again, in the attacks made by clericals and theologians upon the copernican theory, this lamentable consequence was insisted upon. unconscious that they were advancing the most damaging argument which could be conceived for the cause they had at heart, they maintained, honestly but unfortunately, that with the new theory came the manifest inference that our earth is not the only and by no means the most important world in the universe--a doctrine manifestly inconsistent (so they said) with the teachings of the scriptures. it was naturally only by a slow progression that men were able to advance into the domain spread before them by the copernican theory, and to recognise the real minuteness of the earth both in space and time. they more quickly recognised the earth's insignificance in space, because the new theory absolutely forced this fact upon them. if the earth, whose globe they knew to be minute compared with her distance from the sun, is really circling around the sun in a mighty orbit many millions of miles in diameter, it follows of necessity that the fixed stars must lie so far away that even the span of the earth's orbit is reduced to nothing by comparison with the vast depths beyond which lie even the nearest of those suns. this was tycho brahe's famous and perfectly sound argument against the copernican theory. 'the stars remain fixed in apparent position all the time, yet the copernicans tell us that the earth from which we view the stars is circling once a year in an orbit many millions of miles in diameter; how is it that from so widely ranging a point of view we do not see widely different celestial scenery? who can believe that the stars are so remote that by comparison the span of the earth's path is a mere point?' tycho's argument was of course valid.[ ] of two things one. either the earth does not travel round the sun, or the stars are much farther away than men had conceived possible in tycho's time. his mistake lay in rejecting the correct conclusion because simply it made the visible universe seem many millions of times vaster than he had supposed. yet the universe, even as thus enlarged, was but a point to the universe visible in our day, which in turn will dwindle to a point compared with the universe as men will see it a few centuries hence; while that or the utmost range of space over which men can ever extend their survey is doubtless as nothing to the real universe of occupied space. such has been the progression of our ideas as to the position of the earth in space. forced by the discoveries of copernicus to regard our earth as a mere point compared with the distances of the nearest fixed stars, men gradually learned to recognise those distances which at first had seemed infinite as in their turn evanescent even by comparison with that mere point of space over which man is able by instrumental means to extend his survey. though there has been a similar progression in men's ideas as to the earth's position in time, that progression has not been carried to a corresponding extent. men have not been so bold in widening their conceptions of time as in widening their conceptions of space. it is here and thus that, in my judgment, the subject of life in other worlds has been hitherto incorrectly dealt with. men have given up as utterly idle the idea that the existence of worlds is to be limited to the special domain of space to which our earth belongs; but they are content to retain the conception that the domain of time to which our earth's history belongs, 'this bank and shoal of time' on which the life of the earth is cast, is the period to which the existence of other worlds than ours should be referred. this, which is to be noticed in nearly all our ordinary treatises on astronomy, appears as a characteristic peculiarity of works advocating the theory of the plurality of worlds. brewster and dick and chalmers, all in fact who have taken that doctrine under their special protection, reason respecting other worlds as though, if they failed to prove that other orbs are inhabited _now_, or are at least _now_ supporting life in some way or other, they failed of their purpose altogether. the idea does not seem to have occurred to them that there is room and verge enough in eternity of time not only for activity but for rest. they must have all the orbs of space busy at once in the one work which they seem able to conceive as the possible purpose of those bodies--the support of life. the argument from analogy, which they had found effective in establishing the general theory of the plurality of worlds, is forgotten when its application to details would suggest that not _all_ orbs are _at all times_ either the abode of life or in some way subserving the purposes of life. we find, in all the forms of life with which we are acquainted, three characteristic periods--first the time of preparation for the purposes of life; next, the time of fitness for those purposes; and thirdly, the time of decadence tending gradually to death. we see among all objects which exist in numbers, examples of all these stages existing at the same time. in every race of living creatures there are the young as yet unfit for work, the workers, and those past work; in every forest there are saplings, seed-bearing trees, and trees long past the seed-bearing period. we know that planets, or rather, speaking more generally, the orbs which people space, pass through various stages of development, during some only of which they can reasonably be regarded as the abode of life or supporting life; yet the eager champion of the theory of many worlds will have them all in these life-bearing or life-supporting stages, none in any of the stages of preparation, none in any of the stages of decrepitude or death. this has probably had its origin in no small degree from the disfavour with which in former years the theory of the growth and development of planets and systems of planets was regarded. until the evidence became too strong to be resisted, the doctrine that our earth was once a baby world, with many millions of years to pass through before it could be the abode of life, was one which only the professed atheist (so said too many divines) could for a moment entertain; while the doctrine that not the earth alone, but the whole of the solar system, had developed from a condition utterly unlike that through which it is now passing, could have had its origin only in the suggestions of the evil one. both doctrines were pronounced to be so manifestly opposed to the teachings of moses, and not only so, but so manifestly inconsistent with the belief in a supreme being, that--that further argument was unnecessary, and denunciation only was required. so confident were divines on these points, that it would not have been very wonderful if some few students of science had mistaken assertion for proof, and so concluded that the doctrines towards which science was unmistakably leading them really were inconsistent with what they had been taught to regard as the word of god. whether multiplied experiences taught men of science to wait before thus deciding, or however matters fell out, it certainly befell before very long that the terrible doctrine of cosmical development was supported by such powerful evidence, astronomical and terrestrial, as to appear wholly irresistible. then, not only was the doctrine accepted by divines, but shown to be manifestly implied in the sacred narrative of the formation of the earth and heavens, sun, and moon, and stars; while upon those unfortunate students of science who had not changed front in good time, and were found still arguing on the mistaken assumption that the development of our system was not accordant with that ancient narrative, freshly forged bolts were flung from the olympus of orthodoxy. so far as the other argument--from the inconsistency of the development theory with belief in a supreme being--was concerned, the student of science was independent of the interpretations which divines claim the sole right of assigning to the ancient books. science has done so much more than divinity (which in fact has done nothing) to widen our conceptions of space and time, that she may justly claim full right to deal with any difficulties arising from such enlargement of our ideas. with the theological difficulty science would not care to deal at all, were she not urged to do so by the denunciations of divines; and when, so urged, she touches that difficulty, she is quickly told that the difficulty is insuperable, and not long after that it has no existence, and (on both accounts) that it should have been left alone. but with the difficulty arising from the widening of our ideas respecting space and time, science may claim good, almost sole, right to deal. the path to a solution of the problem is not difficult to find. at a first view, it does seem to those whose vision had been limited to a contracted field, that the wide domain of time and space in which processes of development are found to take place is the universe itself, that to deny the formation of our earth by a special creative act is to deny the existence of a creator, that to regard the beginning of our earth as a process of development is to assert that development has been in operation from the beginning of all things. but when we recognise clearly that vastness and minuteness, prolonged and brief duration, are merely relative, we perceive that in considering our earth's history we have to deal only with small parts of space and brief periods of time, by comparison with all space and all time. our earth is very large compared with a tree or an animal, but very small compared with the solar system, a mere point compared with the system of stars to which the sun belongs, and absolutely as nothing compared with the universe of space; and in like manner, while the periods of her growth and development occupy periods very long-lasting compared with those required for the growth and development of a tree or an animal, they are doubtless but brief compared with the eras of the development of our solar system, a mere instant compared with the eras of the development of star-systems, and absolutely evanescent compared with eternity. we have no more reason for rejecting the belief in a creator because our earth or the solar system is found to have developed to its present condition from an embryonic primordial state, than we have had ever since men first found that animals and trees are developed from the germ. the region of development is larger, the period of development lasts longer, but neither the one nor the other is infinite; and being finite, both one and the other are simply nothing by comparison with infinity. it is a startling thought, doubtless, that periods of time compared with which the life of a man, the existence of a nation, nay, the duration of the human race itself, sink into insignificance, should themselves in turn be dwarfed into nothingness by comparison with periods of a still higher order. but the thought is not more startling than that other thought which we have been compelled to admit--the thought that the earth on which we live, and the solar system to which it belongs, though each so vast that all known material objects are as nothing by comparison, are in turn as nothing compared with the depths of space separating us from even the nearest among the fixed stars. one thought, as i have said, we have been compelled to admit, the other has not as yet been absolutely forced upon us. though men have long since given up the idea that the earth and heavens have endured but a few thousand years, it is still possible to believe that the birth of our solar system, whether by creative act or by the beginning of processes of development, belongs to the beginning of all time. but this view cannot be regarded as even probable. although it has never been proved that any definite relation must subsist between time (occupied by events) and space (occupied by matter), the mind naturally accepts the belief that such a relation exists. as we find the universe enlarging under the survey of science, our conceptions of the duration of the universe enlarge also. when the earth was supposed to be the most important object in creation, men might reasonably assign to time itself (regarded as the interval between the beginning of the earth and the consummation of all things when the earth should perish) a moderate duration; but it is equally reasonable that, as the insignificance of the earth's domain in space is recognised, men should recognise also the presumable insignificance of the earth's existence in time. in this respect, although we have nothing like the direct evidence afforded by the measurement of space, we yet have evidence which can scarcely be called in question. we find in the structure of our earth the signs of its former condition. we see clearly that it was once intensely hot! and we know from experimental researches on the cooling of various earths that many millions of years must have been required by the earth in cooling down from its former igneous condition. we may doubt whether bischoff's researches can be relied upon in details, and so be unwilling to assign with him a period of millions of years to a single stage of the process of cooling. but that the entire process lasted tens of millions and probably hundreds of millions of years cannot be doubted. recognising such enormous periods as these in the development of one of the smallest fruits of the great solar tree of life, we cannot but admit at least the reasonableness of believing that the larger fruits (jupiter, for instance, with times as much matter, and saturn with times) must require periods still vaster, probably many times larger. indeed, science shows not only that this view is reasonable, but that no other view is possible. for the mighty root of the tree of life, the great orb of the sun, containing _thousand_ times as much matter as the earth, yet mightier periods would be needed. the growth and development of these, the parts of the great system, must of necessity require much shorter time-intervals than the growth and development of the system regarded as a whole. the enormous period when the germs only of the sun and planets existed as yet, when the chaotic substance of the system had not yet blossomed into worlds, the mighty period which is to follow the death of the last surviving member of the system, when the whole scheme will remain as the dead trunk of a tree remains after the last leaf has fallen, after the last movement of sap within the trunk--these periods must be infinite compared with those which measure the duration of even the mightiest separate members of the system. but all this has been left unnoticed by those who have argued in support of the brewsterian doctrine of a plurality of worlds. they argue as if it had never been shown that every member of the solar system, as of all other such systems in space, has to pass through an enormously long period of preparation before becoming fit to be the abode of life, and that after being fit for life (for a period very long to our conceptions, but by comparison with the other exceedingly short) it must for countless ages remain as an extinct world. or else they reason as though it had been proved that the relatively short life-bearing periods in the existence of the several planets must of necessity synchronise, instead of all the probabilities lying overwhelmingly the other way. while this has been (in my judgment) a defect in what may be called the brewsterian theory of other worlds, a defect not altogether dissimilar has characterised the opposite or whewellite theory. very useful service was rendered to astronomy by whewell's treatise upon, or rather against, the plurality of worlds, calling attention as it did to the utter feebleness of the arguments on which men had been content to accept the belief that other planets and other systems are inhabited. but some among the most powerfully urged arguments against that belief tacitly relied on the assumption of a similarity of general condition among the members of the solar system. for instance, the small mean density of jupiter and saturn had, on the brewsterian theory, been explained as probably due to vast hollow spaces in those planets' interiors--an explanation which (if it could be admitted) would leave us free to believe that jupiter and saturn may be made of the same materials as our own earth. with this was pleasantly intermixed the conception that the inhabitant of these planets may have his 'home in subterranean cities warmed by central fires, or in crystal caves cooled by ocean tides, or may float with the nereids upon the deep, or mount upon wings as eagles, or rise upon the pinions of the dove, that he may flee away and be at rest,' with much more in the same fanciful vein. we now know that there can be no cavities more than a few miles below the crust of a planet, simply because, under the enormous pressures which would exist, the most solid matter would be perfectly plastic. but while whewell's general objection to the theory that jupiter or saturn is in the same condition as our earth thus acquires new force, the particular explanation which he gave of the planet's small density is open to precisely the same general objection. for he assumes that, because the planet's mean density is little greater than that of water, the planet is probably a world of water and ice with a cindery nucleus, or in fact just such a world as would be formed if a sufficient quantity of water in the same condition as the water of our seas were placed at jupiter's greater distance from the sun, around a nucleus of earthy or cindery matter large enough to make the density of the entire planet thus formed equal to that of jupiter, or about one-third greater than the density of water. in this argument there are in reality two assumptions, of precisely the same nature as those which whewell set himself to combat. it is first assumed that some material existing on a large scale in our earth, and nearly of the same density as jupiter, must constitute the chief bulk of that planet, and secondly that the temperature of jupiter's globe must be that which a globe of such material would have if placed where jupiter is. the possibility that jupiter may be in an entirely different stage of planetary life--or, in other words, that the youth, middle life, and old age of that planet may belong to quite different eras from the corresponding periods of our earth's life--is entirely overlooked. rather, indeed, it may be said that the extreme probability of this, on any hypothesis respecting the origin of the solar system, and its absolute certainty on the hypothesis of the development of that system, are entirely overlooked. a fair illustration of the erroneous nature of the arguments which have been used, not only in advocating rival theories respecting the plurality of worlds, but also in dealing with subordinate points, may be presented as follows: imagine a wide extent of country covered with scattered trees of various size, and with plants and shrubs, flowers and herbs, down to the minutest known. let us suppose a race of tiny creatures to subsist on one of the fruits of a tree of moderate size, their existence as a race depending entirely on the existence of the fruit on which they subsist, while the existence of the individuals of their race lasts but for a few minutes. furthermore, let there be no regular fruit season either on their tree or in their region of vegetable life, but fruits forming, growing, and decaying all the time. let us next conceive these creatures to be possessed of a power of reasoning respecting themselves, their fruit world, the tree on which it hangs, and to some degree even respecting such other trees, plants, flowers, and so forth, as the limited range of their vision might be supposed to include. it would be a natural thought with them, when first they began to exercise this power of reasoning, that their fruit home was the most important object in existence, and themselves the chief and noblest of living beings. it would also be very natural that they should suppose the formation of their world to correspond with the beginning of time, and the formation of their race to have followed the formation of their world by but a few seconds. they would conclude that a supreme being had fashioned their world and themselves by special creative acts, and that what they saw outside their fruit world had been also specially created, doubtless to subserve their wants. let us now imagine that gradually, by becoming more closely observant than they had been, by combining together to make more complete observations, and above all by preserving the records of observations made by successive generations, these creatures began to obtain clearer ideas respecting their world and the surrounding regions of space. they would find evidence that the fruit on which they lived had not been formed precisely as they knew it, but had undergone processes of development. the distressing discovery would be made that this development could not possibly have taken place in a few seconds, but must have required many hours, nay, even several of those enormous periods called by us days. this, however, would only be the beginning of their troubles. gradually the more advanced thinkers and the closest observers would perceive that not only had their world undergone processes of development, but that its entire mass had been formed by such processes--that in fact it had not been created at all, in the sense in which they had understood the word, but had _grown_. this would be very dreadful to these creatures, because they would not readily be able to dispossess their minds of the notion that they were the most important beings in the universe, their domain of space coextensive with the universe, the duration of their world coextensive with time. but passing over the difficulties thus arising, and the persecution and abuse to which those would be subjected who maintained the dangerous doctrine that their fruit home had been developed, not created, let us consider how these creatures would regard the question of other worlds than their own. at first they would naturally be unwilling to admit the possibility that other worlds as important as their own could exist. but if after a time they found reason to believe that their world was only one of several belonging to a certain tree system, the idea would occur to them, and would gradually come to be regarded as something more than probable, that those other fruit worlds, like their own, might be the abode of living creatures. and probably at first, while as yet the development of their own world was little understood, they would conceive the notion that all the fruits, large or small, upon their tree system were in the same condition as their own, and either inhabited by similar races or at least in the same full vigour of life-bearing existence. but so soon as they recognised the law of development of their own world, and the relation between such development and their own requirements, they would form a different opinion, if they found that only during certain stages of their world's existence life could exist upon it. if, for instance, they perceived that their fruit world must once have been so bitter and harsh in texture that no creatures in the least degree like themselves could have lived upon it, and that it was passing slowly but surely through processes by which it would become one day dry and shrivelled and unable to support living creatures, they would be apt, if their reasoning powers were fairly developed, to inquire whether other fruits which they saw around them on their tree system were either in the former or in the latter condition. if they found reason to believe certain fruits were in one or other of these stages, they would regard such fruits as not yet the abode of life or as past the life-supporting era. it seems probable even that another idea would suggest itself to some among their bolder thinkers. recognising in their own world in several instances what to their ideas resembled absolute waste of material or of force, it might appear to them quite possible that some, perhaps even a large proportion, of the fruits upon their tree were not only not supporting life at the particular epoch of observation, but never had supported life and never would--that, through some cause or other, life would never appear upon such fruits even when they were excellently fitted for the support of life. they might even conceive that some among the fruits of their tree had failed or would fail to come to the full perfection of fruit life. looking beyond their own tree--that is, the tree to which their own fruit world belonged--they would perceive other trees, though their visual powers might not enable them to know whether such trees bore fruit, whether they were in other respects like their own, whether those which seemed larger or smaller were really so, or owed their apparent largeness to nearness, or their apparent smallness to great distance. they would be apt perhaps to generalise a little too daringly respecting these remote tree systems, concluding too confidently that a shrub or a flower was a tree system like their own, or that a great tree, every branch of which was far larger than their entire tree system, belonged to the same order and bore similar fruit. they might mistake, also, in forgetting the probable fact that as every fruit in their own tree system had its own period of life, very brief compared with the entire existence of the fruit, so every tree might have its own fruit-bearing season. thus, contemplating a tree which they supposed to be like their own in its nature, they might say, 'yonder is a tree system crowded with fruits, each the abode of many myriads of creatures like ourselves:' whereas in reality the tree might be utterly unlike their own, might not yet have reached or might long since have passed the fruit-bearing stage, might when in that stage bear fruit utterly unlike any they could even imagine, and each such fruit during its brief life-bearing condition might be inhabited by living beings utterly unlike any creatures they could conceive. yet again, we can very well imagine that the inhabitants of our fruit world, though they might daringly overleap the narrow limits of space and time within which their actual life or the life of their race was cast, though they might learn to recognise the development of their own world and of others like it, even from the very blossom, would be utterly unable to conceive the possibility that the tree itself to which their world belonged had developed by slow processes of growth from a time when it was less even than their own relatively minute home. still less would it seem credible to them, or even conceivable, that the whole forest region to which they belonged, containing many orders of trees differing altogether from their own tree system, besides plants and shrubs, and flowers and herbs (forms of vegetation of whose use they could form no just conception whatever), had itself grown; that once the entire forest domain had been under vast masses of water--the substance which occasionally visited their world in the form of small drops; that such changes were but minute local phenomena of a world infinitely higher in order than their own; that that world in turn was but one of the least of the worlds forming a yet higher system; and so on _ad infinitum_. such ideas would seem to them not merely inconceivable, but many degrees beyond the widest conceptions of space and time which they could regard as admissible. our position differs only in degree, not in kind, from that of these imagined creatures, and the reasoning which we perceive (though they could not) to be just for such creatures is just for us also. it was perfectly natural that before men recognised the evidences of development in the structure of our earth they should regard the earth and all things upon the earth and visible from the earth as formed by special creative acts precisely as we see them now. but so soon as they perceived that the earth is undergoing processes of development and has undergone such processes in the past, it was reasonable, though at first painful, to conclude that on this point they had been mistaken. yet as we recognise the absurdity of the supposition that, because fruits and trees grow, and were not made in a single instant as we know them, therefore there is no supreme being, so may we justly reject as absurd the same argument, enlarged in scale, employed to induce the conclusion that because planets and solar systems have been developed to their present condition, and were not created in their present form, therefore there is no creator, no god. i do not know that the argument ever has been used in this form; but it has been used to show that those who believe in the development of worlds and systems must of necessity be atheists, an even more mischievous conclusion than the other; for none who had not examined the subject would be likely to adopt the former conclusion, but many might be willing to believe that a number of their fellow-men hold obnoxious tenets, without inquiring closely or at all into the reasoning on which the assertion had been based. but it is more important to notice how our views respecting other worlds should be affected by those circumstances in the evidence _we_ have, which correspond with the features of the evidence on which the imagined inhabitants of the fruit world would form their opinion. it was natural that when men first began to reason about themselves and their home they should reject the idea of other worlds like ours, and perhaps it was equally natural that when first the idea was entertained that the planets may be worlds like ours, men should conceive that all those worlds are in the same condition as ours. but it would be, or rather it _is_, as unreasonable for men to maintain such an opinion now, when the laws of planetary development are understood, when the various dimensions of the planets are known, and when the shortness of the life-supporting period of a planet's existence compared with the entire duration of the planet has been clearly recognised, as it would be for the imagined inhabitants of a small fruit on a tree to suppose that all the other fruits on the tree, though some manifestly far less advanced in development and others far more advanced than their own, were the abode of the same forms of life, though these forms were seen to require those conditions, and no other, corresponding to the stage of development through which their own world was passing. viewing the universe of suns and worlds in the manner here suggested, we should adopt a theory of other worlds which would hold a position intermediate between the brewsterian and the whewellite theories. (it is not on this account that i advocate it, let me remark in passing, but simply because it accords with the evidence, which is not the case with the others.) rejecting on the one hand the theory of the plurality of worlds in the sense implying that all existing worlds are inhabited, and on the other hand the theory of but one world, we should accept a theory which might be entitled the paucity of worlds, only that relative not absolute paucity must be understood. it is absolutely certain that this theory is the correct one, if we admit two postulates, neither of which can be reasonably questioned--viz., first, that the life-bearing era of any world is short compared with the entire duration of that world; and secondly, that there can have been no cause which set all the worlds in existence, not simultaneously, which would be amazing enough, but (which would be infinitely more surprising) in such a way that after passing each through its time of preparation, longer for the large worlds and shorter for the small worlds, they all reached at the same time the life-bearing era. but quite apart from this antecedent probability, amounting as it does to absolute certainty if these two highly probably postulates are admitted, we have the actual evidence of the planets we can examine--that evidence proving incontestably, as i have shown elsewhere, that such planets as jupiter and saturn are still in the state of preparation, still so intensely hot that no form of life could possibly exist upon them, and that such bodies as our moon have long since passed the life-bearing stage, and are to all intents and purposes defunct. but may we not go farther? recognising in our own world, in many instances, what to our ideas resembles waste--waste seeds, waste lives, waste races, waste regions, waste forces--recognising superfluity and superabundance in all the processes and in all the works of nature, should it not appear at least possible that some, perhaps even a large proportion, of the worlds in the multitudinous systems peopling space, are not only not now supporting life, but never have supported life and never will? does this idea differ in kind, however largely to our feeble conceptions it may seem to differ in degree, from the idea of the imagined creatures on a fruit, that some or even many fruits excellently fitted for the support of life might not subserve that purpose? and as those creatures might conceive (as we _know_) that some fruits, even many, fail to come to the full perfection of fruit life, may not we without irreverence conceive (as higher beings than ourselves may _know_) that a planet or a sun may fail in the making? we cannot say that in such a case there would be a waste or loss of material, though we may be unable to conceive how the lost sun or planet could be utilised. our imagined insect reasoners would be unable to imagine that fruits plucked from their tree system were otherwise than wasted, for they would conceive that their idea of the purpose of fruits was the only true one; yet they would be altogether mistaken, as we may be in supposing the main purpose of planetary existence is the support of life. in like manner, when we pass in imagination beyond the limits of our own system, we may learn a useful lesson from the imagined creatures' reasoning about other tree systems than that to which their world belonged. astronomers have been apt to generalise too daringly respecting remote stars and star systems, as though our solar system were a true picture of all solar systems, the system of stars to which our sun belongs a true picture of all star systems. they have been apt to forget that, as every world in our own system has its period of life, short by comparison with the entire duration of the world, so each solar system, each system of such systems, may have its own life-bearing season, infinitely long according to our conceptions, but very short indeed compared with the entire duration of which the life-bearing season would be only a single era. lastly, though men may daringly overleap the limits of time and space within which their lives are cast, though they may learn to recognise the development of their own world and of others like it even from the blossom of nebulosity, they seem unable to rise to the conception that the mighty tree which during remote æons bore those nebulous blossoms sprang itself from cosmical germs. we are unable to conceive the nature of such germs; the processes of development affecting them belong to other orders than any processes we know of, and required periods compared with which the inconceivable, nay, the inexpressible periods required for the development of the parts of our universe, are as mere instants. yet have we every reason which analogy can afford to believe that even the development of a whole universe such as ours should be regarded as but a minute local phenomenon of a universe infinitely higher in order, that universe in turn but a single member of a system of such universes, and so on, even _ad infinitum_. to reject the belief that this is possible is to share the folly of beings such as we have conceived regarding their tiny world as a fit centre whence to measure the universe, while yet, from such a stand-point, this little earth on which we live would be many degrees beyond the limits where for them the inconceivable would begin. to reject the belief that this is not only possible, but real, is to regard the few short steps by which man has advanced towards the unknown as a measurable approach towards limits of space, towards the beginning and the end of all things. until it can be shown that space is bounded by limits beyond which neither matter nor void exists, that time had a beginning before which it was not and tends to an end after which it will exist no more, we may confidently accept the belief that the history of our earth is as evanescent in time as the earth itself is evanescent in space, and that nothing we can possibly learn about our earth, or about the system it belongs to, or about systems of such systems, can either prove or disprove aught respecting the scheme and mode of government of the universe itself. it is true now as it was in days of yore, and it will remain true as long as the earth and those who dwell on it endure, that what men know is nothing, the unknown infinite. vi. _suns in flames._ in november news arrived of a catastrophe the effects of which must in all probability have been disastrous, not to a district, or a country, or a continent, or even a world, but to a whole system of worlds. the catastrophe happened many years ago--probably at least a hundred--yet the messenger who brought the news has not been idle on his way, but has sped along at a rate which would suffice to circle this earth eight times in the course of a second. that messenger has had, however, to traverse millions of millions of miles, and only reached our earth november . the news he brought was that a sun like our own was in conflagration; and on a closer study of his message something was learned as to the nature of the conflagration, and a few facts tending to throw light on the question (somewhat interesting to ourselves) whether our own sun is likely to undergo a similar mishap at any time. what would happen if he did, we know already. the sun which has just met with this disaster--that is, which so suffered a few generations ago--blazed out for a time with several hundred times its former lustre. if our sun were to increase as greatly in light and heat, the creatures on the side of our earth turned towards him at the time would be destroyed in an instant. those on the dark or night hemisphere would not have to wait for their turn till the earth, by rotating, carried them into view of the destroying sun. in much briefer space the effect of his new fires would be felt all over the earth's surface. the heavens would be dissolved and the elements would melt with fervent heat. in fact no description of such a catastrophe, as affecting the night half of the earth, could possibly be more effective and poetical than st. peter's account of the day of the lord, coming 'as a thief in the night; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also and the works that are therein being burned up;' though i imagine the apostle would have been scarce prepared to admit that the earth was in danger from a solar conflagration. indeed, according to another account, the sun was to be turned into darkness and the moon into blood, before that great and notable day of the lord came--a description corresponding well with solar and lunar eclipses, the most noteworthy 'signs in the heavens,' but agreeing very ill with the outburst of a great solar conflagration. before proceeding to inquire into the singular and significant circumstances of the recent outburst, it may be found interesting to examine briefly the records which astronomy has preserved of similar catastrophes in former years. these may be compared to the records of accidents on the various railway lines in a country or continent. those other suns which we can stars are engines working the mighty mechanism of planetary systems, as our sun maintains the energies of our own system; and it is a matter of some interest to us to inquire in how many cases, among the many suns within the range of vision, destructive explosions occur. we may take the opportunity, later, to inquire into the number of cases in which the machinery of solar systems appears to have broken down. the first case of a solar conflagration on record is that of the new star observed by hipparchus some years ago. in his time, and indeed until quite recently, an object of this kind was called a new star, or a temporary star. but we now know that when a star makes its appearance where none had before been visible, what has really happened has been that a star too remote to be seen has become visible through some rapid increase of splendour. when the new splendour dies out again, it is not that a star has ceased to exist; but simply that a faint star which had increased greatly in lustre has resumed its original condition. hipparchus's star must have been a remarkable object, for it was visible in full daylight, whence we may infer that it was many times brighter than the blazing dog-star. it is interesting in the history of science, as having led hipparchus to draw up a catalogue of stars, the first on record. some moderns, being sceptical, rejected this story as a fiction; but biot examining chinese chronicles[ ] relating to the times of hipparchus, finds that in b.c. (about nine years before the date of hipparchus's catalogue) a new star was recorded as having appeared in the constellation scorpio. the next new star (that is, stellar conflagration) on record is still more interesting, as there appears some reason for believing that before long we may see another outburst of the same star. in the years , , and , brilliant stars appeared in the region of the heavens between cepheus and cassiopeia. sir j. herschel remarks, that, 'from the imperfect account we have of the places of the two earlier, as compared with that of the last, which was well determined, as well as from the tolerably near coincidence of the intervals of their appearance, we may suspect them, with goodricke, to be one and the same star, with a period of or perhaps of years.' the latter period may very reasonably be rejected, as one can perceive no reason why the intermediate returns of the star to visibility should have been overlooked, the star having appeared in a region which never sets. it is to be noted that, the period from to being years, and that from to only years, the period of this star (if goodricke is correct in supposing the three outbursts to have occurred in the same star) would seem to be diminishing. at any time, then, this star might now blaze out in the region between cassiopeia and cepheus, for more than years have already passed since its last outburst. as the appearance of a new star led hipparchus to undertake the formation of his famous catalogue, so did the appearance of the star in cassiopeia, in , lead the danish astronomer tycho brahe to construct a new and enlarged catalogue. (this, be it remembered, was before the invention of the telescope.) returning one evening (november , , old style) from his laboratory to his dwelling-house, he found, says sir j. herschel, 'a group of country people gazing at a star, which he was sure did not exist an hour before. this was the star in question.' the description of the star and its various changes is more interesting at the present time, when the true nature of these phenomena is understood, than it was even in the time when the star was blazing in the firmament. it will be gathered from that description and from what i shall have to say farther on about the results of recent observations on less splendid new stars, that, if this star should reappear in the next few years, our observers will probably be able to obtain very important information from it. the message from it will be much fuller and more distinct than any we have yet received from such stars, though we have learned quite enough to remain in no sort of doubt as to their general nature. the star remained visible, we learn, about sixteen months, during which time it kept its place in the heavens without the least variation. 'it had all the radiance of the fixed stars, and twinkled like them; and was in all respects like sirius, except that it surpassed sirius in brightness and magnitude.' it appeared larger than jupiter, which was at that time at his brightest, and was scarcely inferior to venus. _it did not acquire this lustre gradually_, but shone forth at once of its full size and brightness, 'as if,' said the chroniclers of the time, 'it had been of instantaneous creation.' for three weeks it shone with full splendour, during which time it could be seen at noonday 'by those who had good eyes, and knew where to look for it.' but before it had been seen a month, it became visibly smaller, and from the middle of december till march , when it entirely disappeared, it continually diminished in magnitude. 'as it decreased in size, it varied in colour: at first its light was white and extremely bright; it then became yellowish; afterwards of a ruddy colour like mars; and finished with a pale livid white resembling the colour of saturn.' all the details of this account should be very carefully noted. it will presently be seen that they are highly characteristic. those who care to look occasionally at the heavens to know whether this star has returned to view may be interested to learn whereabouts it should be looked for. the place may be described as close to the back of the star-gemmed chair in which cassiopeia is supposed to sit--a little to the left of the seat of the chair, supposing the chair to be looked at in its normal position. but as cassiopeia's chair is always inverted when the constellation is most conveniently placed for observation, and indeed as nine-tenths of those who know the constellation suppose the chair's legs to be the back, and _vice versâ_, it may be useful to mention that the star was placed somewhat thus with respect to the straggling w formed by the five chief stars of cassiopeia. there is a star not very far from the place here indicated, but rather nearer to the middle angle of the w. this, however, is not a bright star; and cannot possibly be mistaken for the expected visitant. (the place of tycho's star is indicated in my school star-atlas and also in my larger library atlas. the same remark applies to both the new stars in the serpent-bearer, presently to be described.) [illustration] in august the astronomer fabricius observed a new star in the neck of the whale, which also after a time disappeared. it was not noticed again till the year , when an observer rejoicing in the name of phocyllides holwarda observed it, and, keeping a watch, after it had vanished, upon the place where it had appeared, saw it again come into view nine months after its disappearance. since then it has been known as a variable star with a period of about days hours. when brightest this star is of the second magnitude. it indicates a somewhat singular remissness on the part of the astronomers of former days, that a star shining so conspicuously for a fortnight, once in each period of - / days, should for so many years have remained undetected. it may, perhaps, be thought that, noting this, i should withdraw the objection raised above against sir j. herschel's idea that the star in cassiopeia may return to view once in years, instead of once in years. but there is a great difference between a star which at its brightest shines only as a second-magnitude star, so that it has twenty or thirty companions of equal or greater lustre above the horizon along with it, and a star which surpasses three-fold the splendid sirius. we have seen that even in tycho brahe's day, when probably the stars were not nearly so well known by the community at large, the new star in cassiopeia had not shone an hour before the country people were gazing at it with wonder. besides, cassiopeia and the whale are constellations very different in position. the familiar stars of cassiopeia are visible on every clear night, for they never set. the stars of the whale, at least of the part to which the wonderful variable star belongs, are below the horizon during rather more than half the twenty-four hours; and a new star there would only be noticed, probably (unless of exceeding splendour), if it chanced to appear during that part of the year when the whale is high above the horizon between eventide and midnight, or in the autumn and early winter. it is a noteworthy circumstance about the variable star in the whale, deservedly called mira, or the wonderful, that it does not always return to the same degree of brightness. sometimes it has been a very bright second-magnitude star when at its brightest, at others it has barely exceeded the third magnitude. hevelius relates that during the four years between october and december , mira did not show herself at all! as this star fades out, it changes in colour from white to red. towards the end of september , a new star made its appearance in the constellation ophiuchus, or the serpent-bearer. its place was near the heel of the right foot of 'ophiuchus huge.' kepler tells us that it had no hair or tail, and was certainly not a comet. moreover, like the other fixed stars, it kept its place unchanged, showing unmistakably that it belonged to the star-depths, not to nearer regions. 'it was exactly like one of the stars, except that in the vividness of its lustre, and the quickness of its sparkling, it exceeded anything that he had ever seen before. it was every moment changing into some of the colours of the rainbow, as yellow, orange, purple, and red; though it was generally white when it was at some distance from the vapours of the horizon.' in fact, these changes of colour must not be regarded as indicating aught but the star's superior brightness. every very bright star, when close to the horizon, shows these colours, and so much the more distinctly as the star is the brighter. sirius, which surpasses the brightest stars of the northern hemisphere full four times in lustre, shows these changes of colour so conspicuously that they were regarded as specially characteristic of this star, insomuch that homer speaks of sirius (not by name, but as the 'star of autumn') shining most beautifully 'when laved of ocean's wave'--that is, when close to the horizon. and our own poet, tennyson, following the older poet, sings how the fiery sirius alters hue, and bickers into red and emerald. the new star was brighter than sirius, and was about five degrees lower down, when at its highest above the horizon, than sirius when _he_ culminates. five degrees being equal to nearly ten times the apparent diameter of the moon, it will be seen how much more favourable the conditions were in the case of kepler's star for those coloured scintillations which characterised that orb. sirius never rises very high above the horizon. in fact, at his highest (near midnight in winter, and, of course, near midday in summer) he is about as high above the horizon as the sun at midday in the first week in february. kepler's star's greatest height above the horizon was little more than three-fourths of this, or equal to about the sun's elevation at midday on january or in any year. like tycho brahe's star, kepler's was brighter even than jupiter, and only fell short of venus in splendour. it preserved its lustre for about three weeks, after which time it gradually grew fainter and fainter until some time between october and february , when it disappeared. the exact day is unknown, as during that interval the constellation of the serpent-bearer is above the horizon in the day-time only. but in february , when it again became possible to look for the new star in the night-time, it had vanished. it probably continued to glow with sufficient lustre to have remained visible, but for the veil of light under which the sun concealed it, for about sixteen months altogether. in fact, it seems very closely to have resembled tycho's star, not only in appearance and in the degree of its greatest brightness, but in the duration of its visibility. in the year a new star appeared in the constellation cygnus, attaining the third magnitude. it remained visible, but not with this lustre, for nearly two years. after it had faded almost out of view, it flickered up again for awhile, but soon after it died out, so as to be entirely invisible. whether a powerful telescope would still have shown it is uncertain, but it seems extremely probable. it may be, indeed, that this new star in the swan is the same which has made its appearance within the last few weeks; but on this point the evidence is uncertain. on april , , mr. hind (superintendent of the nautical almanac, and discoverer of ten new members of the solar system) noticed a new star of the fifth magnitude in the serpent-bearer, but in quite another part of that large constellation than had been occupied by kepler's star. a few weeks later, it rose to the fourth magnitude. but afterwards its light diminished until it became invisible to ordinary eyesight. it did not vanish utterly, however. it is still visible with telescopic power, shining as a star of the eleventh magnitude, that is five magnitudes below the faintest star discernible with the unaided eye. this is the first new star which has been kept in view since its apparent creation. but we are now approaching the time when it was found that as so-called new stars continue in existence long after they have disappeared from view, so also they are not in reality new, but were in existence long before they became visible to the naked eye. on may , , shortly before midnight, mr. birmingham, of tuam, noticed a star of the second magnitude in the northern crown, where hitherto no star visible to the naked eye had been known. dr. schmidt, of athens, who had been observing that region of the heavens the same night, was certain that up to p.m., athens local time, there was no star above the fourth magnitude in the place occupied by the new star. so that, if this negative evidence can be implicitly relied on, the new star must have sprung at least from the fourth, and probably from a much lower magnitude, to the second, in less than three hours--eleven o'clock at athens corresponding to about nine o'clock by irish railway time. a mr. barker, of london, canada, put forward a claim to having seen the new star as early as may --a claim not in the least worth investigating, so far as the credit of first seeing the new star is concerned, but exceedingly important in its bearing on the nature of the outburst affecting the star in corona. it is unpleasant to have to throw discredit on any definite assertion of facts; unfortunately, however, mr. barker, when his claim was challenged, laid before mr. stone, of the greenwich observatory, such very definite records of observations made on may , , , and , that we have no choice but either to admit these observations, or to infer that he experienced the delusive effects of a very singular trick of memory. he mentions in his letter to mr. stone that he had sent full particulars of his observations on those early dates to professor watson, of ann arbor university, on may ; but (again unfortunately) instead of leaving that letter to tell its own story in professor watson's hands, he asked professor watson to return it to him: so that when mr. stone very naturally asked professor watson to furnish a copy of this important letter, professor watson had to reply, 'about a month ago, mr. barker applied to me for this letter, and i returned it to him, as requested, without preserving a copy. i can, however,' he proceeded, 'state positively that he did not mention any actual observation earlier than may . he said he thought he had noticed a strange star in the crown about two weeks before the date of his first observation--may --but not particularly, and that he did not recognise it until the th. he did not give any date, and did not even seem positive as to identity.... when i returned the letter of may , i made an endorsement across the first page, in regard to its genuineness, and attached my signature. i regret that i did not preserve a copy of the letter in question; but if the original is produced, it will appear that my recollection of its contents is correct.' i think no one can blame mr. stone, if, on the receipt of this letter, he stated that he had not the 'slightest hesitation' in regarding mr. barker's earlier observations as 'not entitled to the slightest credit.'[ ] it may be fairly taken for granted that the new star leapt very quickly, if not quite suddenly, to its full splendour. birmingham, as we have seen, was the first to notice it, on may . on the evening of may , schmidt of athens discovered it independently, and a few hours later it was noticed by a french engineer named courbebaisse. afterwards, baxendell of manchester, and others independently saw the star. schmidt, examining argelander's charts of , stars (charts which i have had the pleasure of mapping in a single sheet), found that the star was not a new one, but had been set down by argelander as between the ninth and tenth magnitudes. referring to argelander's list, we find that the star had been twice observed--viz., on may , , and on march , . birmingham wrote at once to mr. huggins, who, in conjunction with the late dr. miller, had been for some time engaged in observing stars and other celestial objects with the spectroscope. these two observers at once directed their telescope armed with spectroscopic adjuncts--the telespectroscope is the pleasing name of the compound instrument--to the new-comer. the result was rather startling. it may be well, however, before describing it, to indicate in a few words the meaning of various kinds of spectroscopic evidence. the light of the sun, sifted out by the spectroscope, shows all the colours but not all the tints of the rainbow. it is spread out into a large rainbow-tinted streak, but at various places (a few thousand) along the streak there are missing tints; so that in fact the streak is crossed by a multitude of dark lines. we know that these lines are due to the absorptive action of vapours existing in the atmosphere of the sun, and from the position of the lines we can tell what the vapours are. thus, hydrogen by its absorptive action produces four of the bright lines. the vapour of iron is there, the vapour of sodium, magnesium, and so on. again, we know that these same vapours, which, by their absorptive action, cut off rays of certain tints, emit light of just those tints. in fact, if the glowing mass of the sun could be suddenly extinguished, leaving his atmosphere in its present intensely heated condition, the light of the faint sun which would thus be left us would give (under spectroscopic scrutiny) those very rays which now seem wanting. there would be a spectrum of multitudinous bright lines, instead of a rainbow-tinted spectrum crossed by multitudinous dark lines. it is, indeed, only by contrast that the dark lines appear dark, just as it is only by contrast that the solar spots seem dark. not only the penumbra but the umbra of a sun-spot, not only the umbra but the nucleus, not only the nucleus but the deeper black which seems to lie at the core of the nucleus, shine really with a lustre far exceeding that of the electric light, though by contrast with the rest of the sun's surface the penumbra looks dark, the umbra darker still, the nucleus deep black, and the core of the nucleus jet black. so the dark lines across the solar spectrum mark where certain rays are relatively faint, though in reality intensely lustrous. conceive another change than that just imagined. conceive the sun's globe to remain as at present, but the atmosphere to be excited to many times its present degree of light and splendour: then would all these dark lines become bright, and the rainbow-tinted background would be dull or even quite dark by contrast. this is not a mere fancy. at times, local disturbances take place in the sun which produce just such a change in certain constituents of the sun's atmosphere, causing the hydrogen, for example, to glow with so intense a heat that, instead of its lines appearing dark, they stand out as bright lines. occasionally, too, the magnesium in the solar atmosphere (over certain limited regions only, be it remembered) has been known to behave in this manner. it was so during the intensely hot summer of , insomuch that the italian observer tacchini, who noticed the phenomenon, attributed to such local overheating of the sun's magnesium vapour the remarkable heat from which we then for a time suffered. now, the stars are suns, and the spectrum of a star is simply a miniature of the solar spectrum. of course, there are characteristic differences. one star has more hydrogen, at least more hydrogen at work absorbing its rays, and thus has the hydrogen lines more strongly marked than they are in the solar spectrum. another star shows the lines of various metals more conspicuously, indicating that the glowing vapours of such elements, iron, copper, mercury, tin, and so forth, either hang more densely in the star's atmosphere than in our sun's, or, being cooler, absorb their special tints more effectively. but speaking generally, a stellar spectrum is like the solar spectrum. there is the rainbow-tinted streak, which implies that the source of light is glowing solid, liquid, or highly compressed vaporous matter, and athwart the streak there are the multitudinous dark lines which imply that around the glowing heart of the star there are envelopes of relatively cool vapours. we can understand, then, the meaning of the evidence obtained from the new star in the northern crown. in the first place, the new star showed the rainbow-tinted streak crossed by dark lines, which indicated its sun-like nature. _but, standing out on that rainbow-tinted streak as on a dark background, were four exceedingly bright lines--lines so bright, though fine, that clearly most of the star's light came from the glowing vapours to which these lines belonged._ three of the lines belonged to hydrogen, the fourth was not identified with any known line. let us distinguish between what can certainly be concluded from this remarkable observation, and what can only be inferred with a greater or less degree of probability. it is absolutely certain that when messrs. huggins and miller made their observation (by which time the new star had faded from the second to the third magnitude), enormous masses of hydrogen around the star were glowing with a heat far more intense than that of the star itself within the hydrogen envelope. it is certain that the increase in the star's light, rendering the star visible which before had been far beyond the range of ordinary eyesight, was due to the abnormal heat of the hydrogen surrounding that remote sun. but it is not so clear whether the intense glow of the hydrogen was caused by combustion or by intense heat without combustion. the difference between the two causes of increased light is important; because on the opinion we form on this point must depend our opinion as to the probability that our sun may one day experience a similar catastrophe, and also our opinion as to the state of the sun in the northern crown after the outburst. to illustrate the distinction in question, let us take two familiar cases of the emission of light. a burning coal glows with red light, and so does a piece of iron placed in a coal fire. but the coal and the iron are undergoing very different processes. the coal is burning, and will presently be consumed; the iron is not burning (except in the sense that it is burning hot, which means only that it will make any combustible substance burn which is brought into contact with it), and it will not be consumed though the coal fire be maintained around it for days and weeks and months. so with the hydrogen flames which play at all times over the surface of our own sun. they are not burning like the hydrogen flames which are used for the oxy-hydrogen lantern. were the solar hydrogen so burning, the sun would quickly be extinguished. they are simply aglow with intensity of heat, as a mass of red-hot iron is aglow; and, so long as the sun's energies are maintained, the hydrogen around him will glow in this way without being consumed. as the new fires of the star in the crown died out rapidly, it is possible that in their case there was actual combustion. on the other hand, it is also possible, and perhaps on the whole more probable, that the hydrogen surrounding the star was simply set glowing with increased lustre owing to some cause not as yet ascertained. let us see how these two theories have been actually worded by the students of science themselves who have maintained them. 'the sudden blazing forth of this star,' says mr. huggins, 'and then the rapid fading away of its light, suggest the rather bold speculation that in consequence of some great internal convulsion, a large volume of hydrogen and other gases was evolved from it, the hydrogen, by its combination with some other element,' in other words, by _burning_, 'giving out the light represented by the bright lines, and at the same time heating to the point of vivid incandescence the solid matter of the star's surface.' 'as the liberated hydrogen gas became exhausted' (i now quote not huggins's own words, but words describing his theory in a book which he has edited) 'the flame gradually abated, and, with the consequent cooling, the star's surface became less vivid, and the star returned to its original condition.' on the other hand, the german physicists, meyer and klein, consider the sudden development of hydrogen, in quantities sufficient to explain such an outburst, exceedingly unlikely. they have therefore adopted the opinion, that the sudden blazing out of the star was occasioned by the violent precipitation of some mighty mass, perhaps a planet, upon the globe of that remote sun, 'by which the momentum of the falling mass would be changed into molecular motion, or in other words into heat and light.' it might even be supposed, they urge, that the star in the crown, by its swift motion, may have come in contact with one of the star clouds which exist in large numbers in the realms of space. 'such a collision would necessarily set the star in a blaze and occasion the most vehement ignition of its hydrogen.' fortunately, our sun is safe for many millions of years to come from contact from any one of its planets. the reader must not, however, run away with the idea that the danger consists only in the gradual contraction of planetary orbits sometimes spoken of. that contraction, if it is taking place at all, of which we have not a particle of evidence, would not draw mercury to the sun's surface for at least ten million millions of years. the real danger would be in the effects which the perturbing action of the larger planets might produce on the orbit of mercury. that orbit is even now very eccentric, and must at times become still more so. it might, but for the actual adjustment of the planetary system, become so eccentric that mercury could not keep clear of the sun; and a blow from even small mercury (only weighing, in fact, millions of millions of millions of tons), with a velocity of some miles per second, would warm our sun considerably. but there is no risk of this happening in mercury's case--though the unseen and much more shifty vulcan (in which planet i beg to express here my utter disbelief) might, perchance, work mischief if he really existed. as for star clouds lying in the sun's course, we may feel equally confident. the telescope assures us that there are none immediately on the track, and we know, also, that, swiftly though the sun is carrying us onwards through space,[ ] many millions of years must pass before he is among the star families towards which he is rushing. of the danger from combustion, or from other causes of ignition than those considered by meyer and klein, it still remains to speak. but first, let us consider what new evidence has been thrown upon the subject by the observations made on the star which flamed out last november. the new star was first seen by professor schmidt, who has had the good fortune of announcing to astronomers more than one remarkable phenomenon. it was he who discovered in november that a lunar crater had disappeared, an announcement quite in accordance with the facts of the case. we have seen that he was one of the independent discoverers of the outburst in the northern crown. on november , at the early hour of . in the evening (showing that schmidt takes time by the forelock at his observatory), he noticed a star of the third magnitude in the constellation of the swan, not far from the tail of that southward-flying celestial bird. he is quite sure that on november , the last preceding clear evening, the star was not there. at midnight its light was very yellow, and it was somewhat brighter than the neighbouring star eta pegasi, on the flying horse's southernmost knee (if anatomists will excuse my following the ordinary usage which calls the wrist of the horse's fore-arm the knee). he sent news of the discovery forthwith to leverrier, the chief of the paris observatory; and the observers there set to work to analyse the light of the stranger. unfortunately the star's suddenly acquired brilliancy rapidly faded. m. paul henry estimated the star's brightness on december as equal only to that of a fifth-magnitude star. moreover, the colour, which had been very yellow on november , was by this time 'greenish, almost blue.' on december , m. cornu, observing during a short time when the star was visible through a break between clouds, found that the star's spectrum consisted almost entirely of bright lines. on december , he was able to determine the position of these lines, though still much interrupted by clouds. he found three bright lines of hydrogen, the strong (really double) line of sodium, the (really triple) line of magnesium, and two other lines. one of these last seemed to agree exactly in position with a bright line belonging to the corona seen around the sun during total eclipse.[ ] the star has since faded gradually in lustre until, at present, it is quite invisible to the naked eye. we cannot doubt that the catastrophe which befell this star is of the same general nature as is that which befell the star in the northern crown. it is extremely significant that all the elements which manifested signs of intense heat in the case of the star in the swan, are characteristic of our sun's outer appendages. we know that the coloured flames seen around the sun during total solar eclipse consist of glowing hydrogen, and of glowing matter giving a line so near the sodium line that in the case of a stellar spectrum it would, probably, not be possible to distinguish one from the other. into the prominences there are thrown from time to time masses of glowing sodium, magnesium, and (in less degree) iron and other metallic vapours. lastly, in that glorious appendage, the solar corona, which extends for hundreds of thousands of miles from the sun's surface, there are enormous quantities of some element, whose nature is as yet unknown, showing under spectroscopic analysis the bright line which seems to have appeared in the spectrum of the flaming sun in the swan. this evidence seems to me to suggest that the intense heat which suddenly affected this star had its origin from without. at the same time, i cannot agree with meyer and klein in considering that the cause of the heat was either the downfall of a planetary mass on the star, or the collision of the star with a star-cloudlet, or nebula, traversing space in one direction while the star swept onwards in another. a planet could not very well come into final conflict with its sun at one fell swoop. it would gradually draw nearer and nearer, not by the narrowing of its path, but by the change of the path's shape. the path would, in fact, become more and more eccentric; until, at length, at its point of nearest approach, the planet would graze its primary, exciting an intense heat where it struck, but escaping actual destruction that time. the planet would make another circuit, and again graze its sun, at or near the same part of the planet's path. for several circuits this would continue, the grazes not becoming more effective each time, but rather less. the interval between them, however, would grow continually less and less. at last the time would come when the planet's path would be reduced to the circular form, its globe touching its sun's all the way round, and then the planet would very quickly be reduced to vapour, and partly burned up, its substance being absorbed by its sun. but all the successive grazes would be indicated to us by accessions in the star's lustre, the period between each seeming outburst being only a few months at first, and becoming gradually less and less (during a long course of years, perhaps even of centuries), until the planet was finally destroyed. nothing of this sort has happened in the case of any so-called new star. as for the rush of a star through a nebulous mass, that is a theory which would scarcely be entertained by any one acquainted with the enormous distances separating the gaseous star-clouds properly called nebulæ. there may be small clouds of the same sort scattered much more densely through space; but we have not a particle of evidence that this actually is the case. all we certainly _know_ about star-cloudlets suggest that the distances separating them from each other are comparable with those which separate star from star, in which case the idea of a star coming into collision with a star-cloudlet, and still more the idea of this occurring several times in a century, is wild in the extreme. on the whole, the theory seems more probable than any of these, that enormous flights of large meteoric masses travel around those stars which thus occasionally break forth in conflagration, such flights travelling on exceedingly eccentric paths, and requiring enormously long periods to complete each circuit of their vast orbits. in conceiving this, we are not imagining anything new. such a meteoric flight would differ only in degree not kind from meteoric flights which are known to circle around our own sun. i am not sure, indeed, that it can be definitely asserted that our sun has no meteoric appendages of the same nature as those which, if this theory be true, excite to intense periodic activity the sun round which they circle. we know that comets and meteors are closely connected, every comet being probably (many certainly) attended by flights of meteoric masses. the meteors which produce the celebrated november showers of falling stars follow in the track of a comet invisible to the naked eye. may we not reasonably suppose, then, that those glorious comets which have not only been visible but conspicuous, shining even in the day-time, and brandishing round tails which, like that of the 'wonder in heaven, the great dragon,' seemed to 'draw the third part of the stars of heaven,' are followed by much denser flights of much more massive meteors? now some among these giant comets have paths which carry them very close to our sun. newton's comet, with its tail a hundred millions of miles in length, all but grazed the sun's globe. the comet of , whose tail, says sir j. herschel, 'stretched half-way across the sky,' must actually have grazed the sun, though but lightly, for its nucleus was within , miles of his surface, and its head was more than , miles in diameter. and these are only two among the few comets whose paths are known. at any time we might be visited by a comet mightier than either, travelling on an orbit intersecting the sun's surface, followed by flights of meteoric masses enormous in size and many in number, which, falling on the sun's globe with the enormous velocity corresponding to their vast orbital range and their near approach to the sun--a velocity of some miles per second--would, beyond all doubt, excite his whole frame, and especially his surface regions, to a degree of heat far exceeding what he now emits. we have had evidence of the tremendous heat to which the sun's surface would be excited by the downfall of a shower of large meteoric masses. carrington and hodgson, on september , , observed (independently) the passage of two intensely bright bodies across a small part of the sun's surface--the bodies first increasing in brightness, then diminishing, then fading away. it is generally believed that these were meteoric masses raised to fierce heat by frictional resistance. now so much brighter did they appear, or rather did that part of the sun's surface appear through which they had rushed, that carrington supposed the dark glass screen used to protect the eye had broken, and hodgson described the brightness of this part of the sun as such that the part shone like a brilliant star on the background of the glowing solar surface. mark, also, the consequences of the downfall of those two bodies only. a magnetic disturbance affected the whole frame of the earth at the very time when the sun had been thus disturbed. vivid auroras were seen not only in both hemispheres, but in latitudes where auroras are very seldom witnessed. 'by degrees,' says sir j. herschel, 'accounts began to pour in of great auroras seen not only in these latitudes, but at rome, in the west indies, in the tropics within eighteen degrees of the equator (where they hardly ever appear); nay, what is still more striking, in south america and in australia--where, at melbourne, on the night of september , the greatest aurora ever seen there made its appearance. these auroras were accompanied with unusually great electro-magnetic disturbances in every part of the world. in many places the telegraph wires struck work. they had too many private messages of their own to convey. at washington and philadelphia, in america, the electric signal-men received severe electric shocks. at a station in norway the telegraphic apparatus was set fire to; and at boston, in north america, a flame of fire followed the pen of bain's electric telegraph, which writes down the message upon chemically prepared paper.' seeing that where the two meteors fell the sun's surface glowed thus intensely, and that the effect of this accession of energy upon our earth was thus well marked, can it be doubted that a comet, bearing in its train a flight of many millions of meteoric masses, and falling directly upon the sun, would produce an accession of light and heat whose consequences would be disastrous? when the earth has passed through the richer portions (not the actual nuclei, be it remembered) of meteor systems, the meteors visible from even a single station have been counted by tens of thousands, and it has been computed that millions must have fallen upon the whole earth. these were meteors following in the train of very small comets. if a very large comet followed by no denser a flight of meteors, but each meteoric mass much larger, fell directly upon the sun, it would not be the outskirts but the nucleus of the meteoric train which would impinge upon him. they would number thousands of millions. the velocity of downfall of each mass would be more than miles per second. and they would continue to pour in upon him for several days in succession, millions falling every hour. it seems not improbable that, under this tremendous and long-continued meteoric hail, his whole surface would be caused to glow as intensely as that small part whose brilliancy was so surprising in the observation made by carrington and hodgson. in that case, our sun, seen from some remote star whence ordinarily he is invisible, would shine out as a new sun, for a few days, while all things living on our earth, and whatever other members of the solar system are the abode of life, would inevitably be destroyed. the reader must not suppose that this idea has been suggested merely in the attempt to explain outbursts of stars. the following passage from a paper of considerable scientific interest by professor kirkwood, of bloomington, indiana, a well-known american astronomer, shows that the idea had occurred to him for a very different reason. he speaks here of a probable connection between the comet of and the great sun-spot which appeared in june . i am not sure, however, but that we may regard the very meteors which seem to have fallen on the sun on september , , as bodies travelling in the track of the comet of --just as the november meteors seen in - , , etc., until , were bodies certainly following in the track of the telescopic comet of . 'the opinion has been expressed by more than one astronomer,' he says, speaking of carrington's observation, 'that this phenomenon was produced by the fall of meteoric matter upon the sun's surface. now, the fact may be worthy of note that the comet of actually grazed the sun's atmosphere about three months before the appearance of the great sun-spot of the same year. had it approached but little nearer, the resistance of the atmosphere would probably have brought its entire mass to the solar surface. even at its actual distance it must have produced considerable atmospheric disturbance. but the recent discovery that a number of comets are associated with meteoric matter, travelling in nearly the same orbits, suggests the inquiry whether an enormous meteorite following in the comet's train, and having a somewhat less perihelion distance, may not have been precipitated upon the sun, thus producing the great disturbance observed so shortly after the comet's perihelion passage.' there are those, myself among the number, who consider the periodicity of the solar spots, that tide of spots which flows to its maximum and then ebbs to its minimum in a little more than eleven years, as only explicable on the theory that a small comet having this period, and followed by a meteor train, has a path intersecting the sun's surface. in an article entitled 'the sun a bubble,' which appeared in the 'cornhill magazine' for october , i remarked that from the observed phenomena of sun-spots we might be led to suspect the existence of some as yet undetected comet with a train of exceptionally large meteoric masses, travelling in a period of about eleven years round the sun, and having its place of nearest approach to that orb so close to the solar surface that, when the main flight is passing, the stragglers fall upon the sun's surface. in this case, we could readily understand that, as this small comet unquestionably causes our sun to be variable to some slight degree in brilliancy, in a period of about eleven years, so some much larger comet circling around mira, in a period of about days, may occasion those alternations of brightness which have been described above. it may be noticed in passing, that it is by no means certain that the time when the sun is most spotted is the time when he gives out least light. though at such times his surface is dark where the spots are, yet elsewhere it is probably brighter than usual; at any rate, all the evidence we have tends to show that when the sun is most spotted, his energies are most active. it is then that the coloured flames leap to their greatest height and show their greatest brilliancy, then also that they show the most rapid and remarkable changes of shape. supposing there really is, i will not say danger, but a possibility, that our sun may one day, through the arrival of some very large comet travelling directly towards him, share the fate of the suns whose outbursts i have described above, we might be destroyed unawares, or we might be aware for several weeks of the approach of the destroying comet. suppose, for example, the comet, which might arrive from any part of the heavens, came from out that part of the star-depths which is occupied by the constellation taurus--then, if the arrival were so timed that the comet, which might reach the sun at any time, fell upon him in may or june, we should know nothing of that comet's approach: for it would approach in that part of the heavens which was occupied by the sun, and his splendour would hide as with a veil the destroying enemy. on the other hand, if the comet, arriving from the same region of the heavens, so approached as to fall upon the sun in november or december, we should see it for several weeks. for it would then approach from the part of the heavens high above the southern horizon at midnight. astronomers would be able in a few days after it was discovered to determine its path and predict its downfall upon the sun, precisely as newton calculated the path of _his_ comet and predicted its near approach to the sun. it would be known for weeks then that the event which newton contemplated as likely to cause a tremendous outburst of solar heat, competent to destroy all life upon the surface of our earth, was about to take place; and, doubtless, the minds of many students of science would be exercised during that interval in determining whether newton was right or wrong. for my own part, i have very little doubt that, though the change in the sun's condition in consequence of the direct downfall upon his surface of a very large comet would be but temporary, and in that sense slight--for what are a few weeks in the history of an orb which has already existed during thousands of millions of years?--yet the effect upon the inhabitants of the earth would be by no means slight. i do not think, however, that any students of science would remain, after the catastrophe, to estimate or to record its effects. fortunately, all that we have learned hitherto from the stars favours the belief that, while a catastrophe of this sort may be possible, it is exceedingly unlikely. we may estimate the probabilities precisely in the same way that an insurance company estimates the chance of a railway accident. such a company considers the number of accidents which occur among a given number of railway journeys, and from the smallness of the number of accidents compared with the largeness of the number of journeys estimates the safety of railway travelling. our sun is one among many millions of suns, any one of which (though all but a few thousands are actually invisible) would become visible to the naked eye, if exposed to the same conditions as have affected the suns in flames described in the preceding pages. seeing, then, that during the last two thousand years or thereabouts, only a few instances of the kind, certainly not so many as twenty, have been recorded, while there is reason to believe that some of these relate to the same star which has blazed out more than once, we may fairly consider the chance exceedingly small that during the next two thousand, or even the next twenty thousand years, our sun will be exposed to a catastrophe of the kind. we might arrive at this conclusion independently of any considerations tending to show that our sun belongs to a safe class of system-rulers, and that all, or nearly all, the great solar catastrophes have occurred among suns of a particular class. there are, however, several considerations of the kind which are worth noting. in the first place, we may dismiss as altogether unlikely the visit of a comet from the star-depths to our sun, on a course carrying the comet directly upon the sun's surface. but if, among the comets travelling in regular attendance upon the sun, there be one whose orbit intersects the sun's globe, then that comet must several times ere this have struck the sun, raising him temporarily to a destructive degree of heat. now, such a comet must have a period of enormous length, for the races of animals now existing upon the earth must all have been formed since that comet's last visit--on the assumption, be it remembered, that the fall of a large comet upon the sun, or rather the direct passage of the sun through the meteoric nucleus of a large comet, would excite the sun to destructive heat. if all living creatures on the earth are to be destroyed when some comet belonging to the solar system makes its next return to the sun, that same comet at its last visit must have raised the sun to an equal, or even greater intensity of heat, so that either no such races as at present exist had then come into being, or, if any such existed, they must at that time have been utterly destroyed. we may fairly believe that all comets of the destructive sort have been eliminated. judging from the evidence we have on the subject, the process of the formation of the solar system was one which involved the utilisation of cometic and meteoric matter; and it fortunately so chanced that the comets likely otherwise to have been most mischievous--those, namely, which crossed the track of planets, and still more those whose paths intersected the globe of the sun--were precisely those which would be earliest and most thoroughly used up in this way. secondly, it is noteworthy that all the stars which have blazed out suddenly, except one, have appeared in a particular region of the heavens--the zone of the milky way (all, too, on one half of that zone). the single exception is the star in the northern crown, and that star appeared in a region which i have found to be connected with the milky way by a well-marked stream of stars, not a stream of a few stars scattered here and there, but a stream where thousands of stars are closely aggregated together, though not quite so closely as to form a visible extension of the milky way. in my map of , stars this stream can be quite clearly recognised; but, indeed, the brighter stars scattered along it form a stream recognisable with the naked eye, and have long since been regarded by astronomers as such, forming the stars of the serpent and the crown, or a serpentine streak followed by a loop of stars shaped like a coronet. now the milky way, and the outlying streams of stars connected with it, seem to form a region of the stellar universe where fashioning processes are still at work. as sir w. herschel long since pointed out, we can recognise in various parts of the heavens various stages of development, and chief among the regions where as yet nature's work seems incomplete, is the galactic zone--especially that half of it where the milky way consists of irregular streams and clouds of stellar light. as there is no reason for believing that our sun belongs to this part of the galaxy, but on the contrary good ground for considering that he belongs to the class of insulated stars, few of which have shown signs of irregular variation, while none have ever blazed suddenly out with many hundred times their former lustre, we may fairly infer a very high degree of probability in favour of the belief that, for many ages still to come, the sun will continue steadily to discharge his duties as fire, light, and life of the solar system. vii. _the rings of saturn._ the rings of saturn, always among the most interesting objects of astronomical research, have recently been subjected to close scrutiny under high telescopic powers by mr. trouvelot, of the harvard observatory, cambridge, u.s. the results which he has obtained afford very significant evidence respecting these strange appendages, and even throw some degree of light on the subject of cosmical evolution. the present time, when saturn is the ruling planet of the night, seems favourable for giving a brief account of recent speculations respecting the saturnian ring-system, especially as the observations of mr. trouvelot appear to remove all doubt as to the true nature of the rings, if indeed any doubt could reasonably be entertained after the investigations made by european and american astronomers when the dark inner ring had but recently been recognised. it may be well to give a brief account of the progress of observation from the time when the rings were first discovered. in passing, i may remark that the failure of galileo to ascertain the real shape of these appendages has always seemed to me to afford striking evidence of the importance of careful reasoning upon all observations whose actual significance is not at once apparent. if galileo had been thus careful to analyse his observations of saturn, he could not have failed to ascertain their real meaning. he had seen the planet apparently attended by two large satellites, one on either side, 'as though supporting the aged saturn upon his slow course around the sun.' night after night he had seen these attendants, always similarly placed, one on either side of the planet, and at equal distances from it. then in he had again examined the planet, and lo, the attendants had vanished, 'as though saturn had been at his old tricks, and had devoured his children.' but after a while the attendant orbs had reappeared in their former positions, had seemed slowly to grow larger, until at length they had presented the appearance of two pairs of mighty arms encompassing the planet. if galileo had reasoned upon these changes of appearance, he could not have failed, as it seems to me, to interpret their true meaning. the three forms under which the rings had been seen by him sufficed to indicate the true shape of the appendage. because saturn was seen with two attendants of apparently equal size and always equi-distant from him, it was certain that there must be some appendage surrounding him, and extending to that distance from his globe. because this appendage disappeared, it was certain that it must be thin and flat. because it appeared at another time with a dark space between the arms and the planet, it was certain that the appendage is separated by a wide gap from the body of the planet. so that galileo might have concluded--not doubtfully, but with assured confidence--that the appendage is a thin flat ring nowhere attached to the planet, or, as huyghens said some forty years later, saturn '_annulo cingitur tenui, plano, nusquam cohærente_.' whether such reasoning would have been accepted by the contemporaries of galileo may be doubtful. the generality of men are not content with reasoning which is logically sound, but require evidence which they can easily understand. very likely huyghens' proof from direct observation, though in reality not a whit more complete and far rougher, would have been regarded as the first true proof of the existence of saturn's ring, just as sir w. herschel's observation of one star actually moving round another was regarded as the first true proof of the physical association of certain stars, a fact which michell had proved as completely and far more neatly half a century earlier, by a method, however, which was 'caviare to the general.' however, as matters chanced, the scientific world was not called upon to decide between the merits of a discovery made by direct observation and one effected by means of abstract reasoning. it was not until saturn had been examined with much higher telescopic power than galileo could employ, that the appendage which had so perplexed the florentine astronomer was seen to be a thin flat ring, nowhere touching the planet, and considerably inclined to the plane in which saturn travels. we cannot wonder that the discovery was regarded as a most interesting one. astronomers had heretofore had to deal with solid masses, either known to be spheroidal, like the earth, the sun, the moon, jupiter, and venus, or presumed to be so, like the stars. the comets might be judged to be vaporous masses of various forms; but even these were supposed to surround or to attend upon globe-shaped nuclear masses. here, however, in the case of saturn's ring, was a quoit-shaped body travelling around the sun in continual attendance upon saturn, whose motions, no matter how they varied in velocity or direction, were so closely followed by this strange attendant that the planet remained always centrally poised within the span of its ring-girdle. to appreciate the interest with which this strange phenomenon was regarded, we must remember that as yet the law of gravity had not been recognised. huyghens discovered the ring (or rather perceived its nature) in , but it was not till that newton first entertained the idea that the moon is retained in its orbit about the earth by the attractive energy which causes unsupported bodies to fall earthwards; and he was unable to demonstrate the law of gravity before . now, in a general sense, we can readily understand in these days how a ring around a planet continues to travel along with the planet despite all changes of velocity or direction of motion. for the law of gravity teaches that the same causes which tend to change the direction and velocity of the planet's motion tend in precisely the same degree to change the direction and velocity of the ring's motion. but when huyghens made his discovery it must have appeared a most mysterious circumstance that a ring and planet should be thus constantly associated--that during thousands of years no collision should have occurred whereby the relatively delicate structure of the ring had been destroyed. only six years later a discovery was made by two english observers, william and thomas ball, which enhanced the mystery. observing the northern face of the ring, which was at that time turned earthwards, they perceived a black stripe of considerable breadth dividing the ring into two concentric portions. the discovery did not attract so much attention as it deserved, insomuch that when cassini, ten years later, announced the discovery of a corresponding dark division on the southern surface, none recalled the observation made by the brothers ball. cassini expressed the opinion that the ring is really divided into two, not merely marked by a dark stripe on its southern face. this conclusion would, of course, have been an assured one, had the previous observation of a dark division on the northern face been remembered. with the knowledge which we now possess, indeed, the darkness of the seeming stripe would be sufficient evidence that there must be a real division there between the rings; for we know that no mere darkness of the ring's substance could account for the apparent darkness of the stripe. it has been well remarked by professor tyndall, that if the moon's whole surface could be covered with black velvet, she would yet appear white when seen on the dark background of the sky. and it may be doubted whether a circular strip of black velvet miles wide, placed where we see the dark division between the rings, would appear nearly as dark as that division. since we could only admit the possibility of some substance resembling our darker rocks occupying this position (for we know of nothing to justify the supposition that a substance as dark as lampblack or black velvet could be there), we are manifestly precluded from supposing that the dark space is other than a division between two distinct rings. yet sir w. herschel, in examining the rings of saturn with his powerful telescopes, for a long time favoured the theory that there is no real division. he called it the 'broad black mark,' and argued that it can neither indicate the existence of a zone of hills upon the ring, nor of a vast cavernous groove, for in either case it would present changes of appearance (according to the ring's changes of position) such as he was unable to detect. it was not until the year , eleven years after his observations had commenced, that, perceiving a corresponding broad black mark upon the ring's southern face, herschel expressed a 'suspicion' that the ring is divided into two concentric portions by a circular gap nearly miles in width. he expressed at the same time, very strongly, his belief that this division was the only one in saturn's ring-system. a special interest attached at that time to the question whether the ring is divided or not, for laplace had then recently published the results of his mathematical inquiry into the movements of such a ring as saturn's, and, having _proved_ that a single solid ring of such enormous width could not continue to move around the planet, had expressed the _opinion_ that saturn's ring consists in reality of many concentric rings, each turning, with its own proper rotation rate, around the central planet. it is singular that herschel, who, though not versed in the methods of the higher mathematics, had considerable native power as a mathematician, was unable to perceive the force of laplace's reasoning. indeed, this is one of those cases where clearness of perception rather than profundity of mathematical insight was required. laplace's equations of motion did not express all the relations involved, nor was it possible to judge, from the results he deduced, how far the stability of the saturnian rings depended on the real structure of these appendages. one who was well acquainted with mechanical matters, and sufficiently versed in mathematics to understand how to estimate generally the forces acting upon the ring-system, could have perceived as readily the general conditions of the problem as the most profound mathematician. one may compare the case to the problem of determining whether the action of the moon in causing the tidal wave modifies in any manner the earth's motion of rotation. we know that as a mathematical question this is a very difficult one. the astronomer royal, for example, not long ago dealt with it analytically, and deduced the conclusion that there is no effect on the earth's rotation, presently however, discovering by a lucky chance a term in the result which indicates an effect of that kind. but if we look at the matter in its mechanical aspect, we perceive at once, without any profound mathematical research, that the retardation so hard to detect mathematically must necessarily take place. as sir e. beckett says in his masterly work, _astronomy without mathematics_, 'the conclusion is as evident without mathematics as with them, when once it has been suggested.' so when we consider the case of a wide flat ring surrounding a mighty planet like saturn, we perceive that nothing could possibly save such a ring from destruction if it were really one solid structure. to recognise this the more clearly, let us first notice the dimensions of the planet and rings. we have in saturn a globe about , miles in mean diameter, an equatorial diameter being about , miles, the polar diameter , miles. the attractive force of this mighty mass upon bodies placed on its surface is equal to about one-fifth more than terrestrial gravity if the body is near the pole of saturn, and is almost exactly the same as terrestrial gravity if the body is at the planet's equator. its action on the matter of the ring is, of course, very much less, because of the increased distance, but still a force is exerted on every part of the ring which is comparable with the familiar force of terrestrial gravity. the outer edge of the outer ring lies about , miles from the planet's centre, the inner edge of the inner ring (i speak throughout of the ring-system as known to sir w. herschel and laplace) about , miles from the centre, the breadth of the system of bright rings being about , miles. between the planet's equator and the inner edge of the innermost bright ring there intervenes a space of about , miles. roughly speaking, it may be said that the attraction of the planet on the substance of the ring's inner edge is less than gravity at saturn's equator (or, which is almost exactly the same thing, is less than terrestrial gravity) in about the proportion of to ; or, still more roughly, the inner edge of saturn's inner bright ring is drawn inwards by about half the force of gravity at the earth's surface. the outer edge is drawn towards saturn by a force less than terrestrial gravity in the proportion of about to --say roughly that the force thus exerted by saturn on the matter of the outer edge of the ring-system is equivalent to about one-fifth of the force of gravity at the earth's surface. it is clear, first, that if the ring-system did not rotate, the forces thus acting on the material of the rings would immediately break them into fragments, and, dragging these down to the planet's equator, would leave them scattered in heaps upon that portion of saturn's surface. the ring would in fact be in that case like a mighty arch, each portion of which would be drawn towards saturn's centre by its own weight. this weight would be enormous if bessel's estimate of the mass of the ring-system is correct. he made the mass of the ring rather greater than the mass of the earth--an estimate which i believe to be greatly in excess of the truth. probably the rings do not amount in mass to more than a fourth part of the earth's mass. but even that is enormous, and subjected as is the material of the rings to forces varying from one-half to a fifth of terrestrial gravity, the strains and pressures upon the various parts of the system would exceed thousands of times those which even the strongest material built up into their shape could resist. the system would no more be able to resist such strains and pressures than an arch of iron spanning the atlantic would be able to sustain its own weight against the earth's attraction. it would be necessary then that the ring-system should rotate around the planet. but it is clear that the proper rate of rotation for the outer portion would be very different from the rate suited for the inner portion. in order that the inner portion should travel around saturn entirely relieved of its weight, it should complete a revolution in about seven hours twenty-three minutes. the outer portion, however, should revolve in about thirteen hours fifty-eight minutes, or nearly fourteen hours. thus the inner part should rotate in little more than half the time required by the outer part. the result would necessarily be that the ring-system would be affected by tremendous strains, which it would be quite unable to resist. the existence of the great division would manifestly go far to diminish the strains. it is easily shown that the rate of turning where the division is, would be once in about eleven hours and twenty-five minutes, not differing greatly from the mean between the rotation-periods for the outside and for the inside edges of the system. even then, however, the strains would be hundreds of times greater than the material of the ring could resist. a mass comparable in weight to our earth, compelled to rotate in (say) nine hours when it ought to rotate in eleven or in seven, would be subjected to strains exceeding many times the resistances which the cohesive power of its substance could afford. that would be the condition of the inner ring. and in like manner the outer ring, if it rotated in about twelve hours and three-quarters, would have its outer portions rotating too fast and its inner portions too slowly, because their proper periods would be fourteen hours and eleven hours and a half respectively. nothing but the division of the ring into a number of narrow hoops could possibly save it from destruction through the internal strains and pressures to which its material would be subjected. even this complicated arrangement, however, would not save the ring-system. if we suppose a fine hoop to turn around a central attracting body as the rings of saturn rotate around the planet, it may be shown that unless the hoop is so weighted that its centre of gravity is far from the planet, there will be no stability in the resulting motions; the hoop will before long be made to rotate eccentrically, and eventually be brought into destructive collision with the central planet. it was here that laplace left the problem. nothing could have been more unsatisfactory than his result, though it was accepted for nearly half a century unquestioned. he had shown that a weighted fine hoop may possibly turn around a central attracting mass without destructive changes of position, but he had not proved more than the bare possibility of this, while nothing in the appearance of saturn's rings suggests that any such arrangement exists. again, manifestly a multitude of narrow hoops, so combined as to form a broad flat system of rings, would be constantly in collision _inter se_. besides, each one of them would be subjected to destructive strains. for though a fine uniform hoop set rotating at a proper rate around an attracting mass at its centre would be freed from all strains, the case is very different with a hoop so weighted as to have its centre of gravity greatly displaced. laplace had saved the theoretical stability of the motions of a fine ring at the expense of the ring's power of resisting the strains to which it would be exposed. it seems incredible that such a result (expressed, too, very doubtingly by the distinguished mathematician who had obtained it) should have been accepted so long almost without question. there is nothing in nature in the remotest degree resembling the arrangement imagined by laplace, which indeed appears on _à priori_ grounds impossible. it was not claimed for it that it removed the original difficulties of the problem; and it introduced others fully as serious. so strong, however, is authority in the scientific world that none ventured to express any doubts except sir w. herschel, who simply denied that the two rings were divided into many, as laplace's theory required. as time went on and the signs of many divisions were at times recognised, it was supposed that laplace's reasoning had been justified; and despite the utter impossibility of the arrangement he had suggested, that arrangement was ordinarily described as probably existing. at length, however, a discovery was made which caused the whole question to be reopened. on november , , w. bond, observing the planet with the telescope of the harvard observatory, perceived within the inner bright ring a feeble illumination which he was at a loss to understand. on the next night the faint light was better seen. on the th, tuttle, who was observing with bond, suggested the idea that the light within the inner bright ring was due to a dusky ring inside the system of bright rings. on november , mr. dawes in england perceived this dusky ring, and announced the discovery before the news had reached england that bond had already seen the dark ring. the credit of the discovery is usually shared between bond and dawes, though the usual rule in such matters would assign the discovery to bond alone. it was found that the dark ring had already been seen at rome so far back as , and again by galle at berlin in may . the roman observations were not satisfactory. those by galle, however, were sufficient to have established the fact of the ring's existence; indeed, in galle measured the dark ring. but very little attention was attracted to this interesting discovery, insomuch that when bond and dawes announced their observation of the dark ring in , the news was received by astronomers with all the interest attaching to the detection of before unnoted phenomena. it may be well to notice under what conditions the dark ring was detected in . in september the ring had been turned edgewise towards the sun, and as rather more than seven years are occupied in the apparent gradual opening out of the ring from that edge view to its most open appearance (when the outline of the ring-system is an eclipse whose lesser axis is nearly equal to half the greater), it will be seen that in november the rings were but slightly opened. thus the recognition of the dark ring within the bright system was made under unfavourable conditions. for four preceding years--that is, from the year --the rings had been as little or less opened; and again for several years preceding , though the rings had been more open, the planet had been unfavourably placed for observation in northern latitudes, crossing the meridian at low altitudes. still, in and , when the rings were most open, although the planet was never seen under favourable conditions, the opening of the rings, then nearly at its greatest, made the recognition of the dark ring possible; and we have seen that galle then made the discovery. when bond rediscovered the dark ring, everything promised that before long the appendage would be visible with telescopes far inferior in power to the great harvard refractor. year after year the planet was becoming more favourably placed for observation, while all the time the rings were opening out. accordingly it need not surprise us to learn that in the dark ring was seen with a telescope less than three inches and a half in aperture. even so early as , mr. hartnup, observing the planet with a telescope eight inches and a half in aperture, found that 'the dark ring could not be overlooked for an instant.' but while this increase in the distinctness of the dark ring was to be expected, from the mere fact that the ring was discovered under relatively unfavourable conditions, yet the fact that saturn was thus found to have an appendage of a remarkable character, perfectly obvious even with moderate telescopic power, was manifestly most surprising. the planet had been studied for nearly two centuries with telescopes exceeding in power those with which the dark ring was now perceived. some among these telescopes were not only of great power, but employed by observers of the utmost skill. the elder herschel had for a quarter of a century studied saturn with his great reflectors eighteen inches in aperture, and had at times turned on the planet his monstrous (though not mighty) four-feet mirror. schröter had examined the dark space within the inner bright ring for the special purpose of determining whether the ring-system is really disconnected from the globe. he had used a mirror nineteen inches in aperture, and he had observed that the dark space seen on either side of saturn inside the ring-system not only appeared dark, but actually darker than the surrounding sky. this was presumably (though not quite certainly) an effect of contrast only, the dark space being bounded all round by bright surfaces. if real, the phenomenon signified that whereas the space outside the ring, where the satellites of the planet travel, was occupied by some sort of cosmical dust, the space within the ring-system was, as it were, swept and garnished, as though all the scattered matter which might otherwise have occupied that region had been either attracted to the body of the planet or to the rings.[ ] but manifestly the observation was entirely inconsistent with the supposition that there existed in schröter's time a dark or dusky ring within the bright system. again, the elder struve made the most careful measurement of the whole of the ring-system in , when the system was as well placed for observation as in (or, in other words, as well placed as it can possibly be); but though he used a telescope nine inches and a half in aperture, and though his attention was specially attracted to the inner edge of the inner bright ring (_which seemed to him indistinct_), he did not detect the dark ring. yet we have seen that in , under much less favourable conditions, a less practised observer, using a telescope of less aperture, found that the dark ring could not be overlooked for an instant. it is manifest that all these considerations point to the conclusion that the dark ring is a new formation, or, at the least, that it has changed notably in condition during the present century. i have hitherto only considered the appearance of the dusky ring as seen on either side of the planet's globe within the bright rings. the most remarkable feature of the appendage remains still to be mentioned--the fact, namely, that the bright body of the planet can be seen through this dusky ring. where the dark ring crosses the planet, it appears as a rather dark belt, which might readily be mistaken for a belt upon the planet's surface; for the outline of the planet can be seen through the ring as through a film of smoke or a crape veil. now it is worthy of notice that whereas the dark ring was not detected outside the planet's body until , nor generally recognised by astronomers until , the dark belt across the planet, really caused by the dusky ring, was observed more than a century earlier. in the younger cassini saw it, and perceived that it was not curved enough for a belt really belonging to the planet. hadley again observed that the belt attended the ring as this opened out and closed, or, in other words, that the dark belt belonged to the ring, not to the body of the planet. and in many pictures of saturn's system a dark band is shown along the inner edge of the inner bright ring where it crosses the body of the planet. it seems to me that we have here a most important piece of evidence respecting the rings. it is clear that the inner part of the inner bright ring has for more than a century and a half (how much more we do not know) been partially transparent, and it is probable that within its inner edge there has been all the time a ring of matter; but this ring has only within the last half-century gathered consistency enough to be discernible. it is manifest that the existence of the dark belt shown in the older pictures would have led directly to the detection of the dark ring, had not this appendage been exceedingly faint. thus, while the observation of the dark belt across the planet's face proves the dusky ring to have existed in some form long before it was perceived, the same fact only helps to render us certain that the dark ring has changed notably in condition during the present century. the discovery of this singular appendage, an object unique in the solar system, naturally attracted fresh attention to the question of the stability of the rings. the idea was thrown out by the elder bond that the new ring may be fluid, or even that the whole ring-system may be fluid, and the dark ring simply thinner than the rest. it was thought possible that the ring-system is of the nature of a vast ocean, whose waves are steadily advancing upon the planet's globe. the mathematical investigation of the subject was also resumed by professor benjamin pierce, of harvard, and it was satisfactorily demonstrated that the stability of a system of actual rings of solid matter required so nice an adjustment of so many narrow rings as to render the system far more complex than even laplace had supposed. 'a stable formation can,' he said, 'be nothing other than a very great number of separate narrow rigid rings, each revolving with its proper relative velocity.' as was well remarked by the late professor nichol, 'if this arrangement or anything like it were real, how many new conditions of instability do we introduce. observation tells us that the division between such rings must be extremely narrow, so that the slightest disturbance by external or internal causes would cause one ring to impinge upon another; and we should thus have the seed of perpetual catastrophes.' nor would such a constitution protect the system against dissolution. 'there is no escape from the difficulties, therefore, but through the final rejection of the idea that saturn's rings are rigid or in any sense a solid formation.' the idea that the ring-system may be fluid came naturally next under mathematical scrutiny. strangely enough, the physical objections to the theory of fluidity appear to have been entirely overlooked. before we could accept such a theory, we must admit the existence of elements differing entirely from those with which we are familiar. no fluid known to us could retain the form of the rings of saturn under the conditions to which they are exposed. but the mathematical examination of the subject disposed so thoroughly of the theory that the rings can consist of continuous fluid masses, that we need not now discuss the physical objections to the theory. there remains only the theory that the saturnian ring-system consists of discrete masses analogous to the streams of meteors known to exist in great numbers within the solar system. the masses may be solid or fluid, may be strewn in relatively vacant space, or may be surrounded by vaporous envelopes; but that they are discrete, each free to travel on its own course, seemed as completely demonstrated by pierce's calculations as anything not actually admitting of direct observation could possibly be. the matter was placed beyond dispute by the independent analysis to which clerk maxwell subjected the mathematical problem. it had been selected in as the subject for the adams prize essay at cambridge, and clerk maxwell's essay, which obtained the prize, showed conclusively that only a system of many small bodies, each free to travel upon its course under the varying attractions to which it was subjected by saturn itself, and by the saturnian satellites, could possibly continue to girdle a planet as the rings of saturn girdle him. it is clear that all the peculiarities hitherto observed in the saturnian ring-system are explicable so soon as we regard that system as made up of multitudes of small bodies. varieties of brightness simply indicate various degrees of condensation of these small satellites. thus the outer ring had long been observed to be less bright than the inner. of course it did not seem impossible that the outer ring might be made of different materials; yet there was something bizarre in the supposition that two rings forming the same system were thus different in substance. it would not have been at all noteworthy if different parts of the same ring differed in luminosity--in fact, it was much more remarkable that each zone of the system seemed uniformly bright all round. but that one zone should be of one tint, another of an entirely different tint, was a strange circumstance so long as the only available interpretation seemed to be that one zone was made (throughout) of one substance, the other of another. if this was strange when the difference between the inner and outer bright rings was alone considered, how much stranger did it seem when the multitudinous divisions in the rings were taken into account! why should the ring-system, , miles in width, be thus divided into zones of different material? an arrangement so artificial is quite unlike all that is elsewhere seen among the subjects of the astronomer's researches. but when the rings are regarded as made up of multitudes of small bodies, we can quite readily understand how the nearly circular movements of all of these, at different rates, should result in the formation of rings of aggregation and rings of segregation, appearing at the earth's distance as bright rings and faint rings. the dark ring clearly corresponds in appearance with a ring of thinly scattered satellites. indeed, it seems impossible otherwise to account for the appearance of a dusky belt across the globe of the planet where the dark ring crosses the disc. if the material of the dark ring were some partly transparent solid or fluid substance, the light of the planet received through the dark ring added to the light reflected by the dark ring itself, would be so nearly equivalent to the light received from the rest of the planet's disc, that either no dark belt would be seen, or the darkening would be barely discernible. in some positions a bright belt would be seen, not a dark one. but a ring of scattered satellites would cast as its shadow a multitude of black spots, which would give to the belt in shadow a dark grey aspect. a considerable proportion of these spots would be hidden by the satellites forming the dark ring, and in every case where a spot was wholly or partially hidden by a satellite, the effect (at our distant station where the separate satellites of the dark ring are not discernible) would simply be to reduce _pro tanto_ the darkness of the grey belt of shadow. but certainly more than half the shadows of the satellites would remain in sight; for the darkness of the ring at the time of its discovery showed that the satellites were very sparsely strewn. and these shadows would be sufficient to give to the belt a dusky hue, such as it presented when first discovered.[ ] the observations which have recently been made by mr. trouvelot indicate changes in the ring-system, and especially in the dark ring, which place every other theory save that to which we have thus been led entirely out of the question. it should be noted that mr. trouvelot has employed telescopes of unquestionable excellence and varying in aperture from six inches to twenty-six inches, the latter aperture being that of the great telescope of the washington observatory (the largest refractor in the world). he has noted in the first place that the interior edge of the outer bright ring, which marks the outer limit of the great division, is irregular, but whether the irregularity is permanent or not he does not know. the great division itself is found not to be actually black, but, as was long since noted by captain jacob, of the madras observatory, a very dark brown, as though a few scattered satellites travelled along this relatively vacant zone of the system. mr. trouvelot has further noticed that the shadow of the planet upon the rings, and especially upon the outer ring, changes continually in shape, a circumstance which he attributes to irregularities in the surface of the rings. for my own part, i should be disposed to attribute these changes in the shape of the planet's shadow (noted by other observers also) to rapid changes in the deep cloud-laden atmosphere of the planet. passing on, however, to less doubtful observations, we find that the whole system of rings has presented a clouded and spotted aspect during the last four years. mr. trouvelot specially describes this appearance as observed on the parts of the ring outside the disc, called by astronomers the _ansæ_ (because of their resemblance to handles), and it would seem, therefore, that the spotted and cloudy portions are seen only where the background on which the rings are projected is black. this circumstance clearly suggests that the darkness of these parts is due to the background, or, in other words, that the sky is in reality seen through those parts of the ring-system, just as the darkness of the slate-coloured interior ring is attributed, on the satellite theory, to the background of sky visible through the scattered flight of satellites forming the dark ring. the matter composing the dark ring has been observed by mr. trouvelot to be gathered in places into compact masses, which prevent the light of the planet from being seen through those portions of the dark ring where the matter is thus massed together. it is clear that such peculiarities could not possibly present themselves in the case of a continuous solid or fluid ring-system, whereas they would naturally occur in a ring formed of multitudes of minute bodies travelling freely around the planet. the point next to be mentioned is still more decisive. when the dark ring was carefully examined with powerful telescopes during the ten years following its discovery by bond, at which time it was most favourably placed for observation, it was observed that the outline of the planet could be seen across the entire breadth of the dark ring. all the observations agreed in this respect. it was, indeed, noticed by dawes that outside the planet's disc the dark ring showed varieties of tint, its inner half being darker than its outer portion. lassell, observing the planet under most favourable conditions with his two-feet mirror at malta, could not perceive these varieties of tint, which therefore we may judge to have been either not permanent or very slightly marked. but, as i have said, all observers agreed that the outline of the planet could be seen athwart the entire width of the dark ring. mr. trouvelot, however, has found that during the last four years the planet has not been visible through the whole width of the dark ring, but only through the inner half of the ring's breadth. it appears, then, that either the inner portion is getting continually thinner and thinner--that is, the satellites composing it are becoming continually more sparsely strewn--or that the outer portion is becoming more compact, doubtless by receiving stray satellites from the interior of the inner bright ring. it is clear that in saturn's ring-system, if not in the planet itself, mighty changes are still taking place. it may be that the rings are being so fashioned under the forces to which they are subjected as to be on their way to becoming changed into separate satellites, inner members of that system which at present consists of eight secondary planets. but, whatever may be the end towards which these changes are tending, we see processes of evolution taking place which may be regarded as typifying the more extensive and probably more energetic processes whereby the solar system itself reached its present condition. i ventured more than ten years ago, in the preface to my treatise upon the planet saturn, to suggest the possibility 'that in the variations perceptibly proceeding in the saturnian ring-system a key may one day be found to the law of development under which the solar system has reached its present condition.' this suggestion seems to me strikingly confirmed by the recent discoveries. the planet saturn and its appendages, always interesting to astronomers, are found more than ever worthy of close investigation and scrutiny. we may here, as it were, seize nature in the act, and trace out the actual progress of developments which at present are matters rather of theory than of observation. viii. _comets as portents_ the blazing star, threat'ning the world with famine, plague, and war; to princes death; to kingdoms many curses; to all estates inevitable losses; to herdsmen rot; to ploughmen hapless seasons; to sailors storms; to cities civil treasons. although comets are no longer regarded with superstitious awe as in old times, mystery still clings to them. astronomers can tell what path a comet is travelling upon, and say whence it has come and whither it will go, can even in many cases predict the periodic returns of a comet, can analyse the substance of these strange wanderers, and have recently discovered a singular bond of relationship between comets and those other strange visitants from the celestial depths, the shooting stars. but astronomy has hitherto proved unable to determine the origin of comets, the part they perform in the economy of the universe, their real structure, the causes of the marvellous changes of shape which they undergo as they approach the sun, rush round him, and then retreat. as sir john herschel has remarked: 'no one, hitherto, has been able to assign any single point in which we should be a bit better or worse off, materially speaking, if there were no such thing as a comet. persons, even thinking persons, have busied themselves with conjectures; such as that they may serve for fuel for the sun (into which, however, they never fall), or that they may cause warm summers, which is a mere fancy, or that they may give rise to epidemics, or potato-blights, and so forth.' and though, as he justly says, 'this is all wild talking,' yet it will probably continue until astronomers have been able to master the problems respecting comets which hitherto have foiled their best efforts. the unexplained has ever been and will ever be marvellous to the general mind. just as unexplored regions of the earth have been tenanted in imagination by anthropophagi and men whose heads do grow beneath their shoulders, so do wondrous possibilities exist in the unknown and the ill-understood phenomena of nature. in old times, when the appearance and movements of comets were supposed to be altogether uncontrolled by physical laws, it was natural that comets should be regarded as signs from heaven, tokens of divine wrath towards some, and of the interposition of divine providence in favour of others. as seneca well remarked: 'there is no man so dull, so obtuse, so turned to earthly things, who does not direct all the powers of his mind towards things divine when some novel phenomenon appears in the heavens. while all follows its usual course up yonder, familiarity robs the spectacle of its grandeur. for so is man made. however wonderful may be what he sees day after day, he looks on it with indifference; while matters of very little importance attract and interest him if they depart from the accustomed order. the host of heavenly constellations beneath the vault of heaven, whose beauty they adorn, attract no attention; but if any unusual appearance be noticed among them, at once all eyes are turned heavenwards. the sun is only looked on with interest when he is undergoing eclipse. men observe the moon only under like conditions.... so thoroughly is it a part of our nature to admire the new rather than the great. the same is true of comets. when one of these fiery bodies of unusual form appears, every one is eager to know what it means; men forget other objects to inquire about the new arrival; they know not whether to wonder or to tremble; for many spread fear on all sides, drawing from the phenomenon most grave prognostics.' there is no direct reference to comets in the bible, either in the old testament or the new. it is possible that some of the signs from heaven recorded in the bible pages were either comets or meteors, and that even where in some places an angel or messenger from god is said to have appeared and delivered a message, what really happened was that some remarkable phenomenon in the heavens was interpreted in a particular manner by the priests, and the interpretation afterwards described as the message of an angel. the image of the 'flaming sword which turned every way' may have been derived from a comet; but we can form no safe conclusion about this, any more than we can upon the question whether the 'horror of great darkness' which fell upon abraham (genesis xv. ) when the sun was going down, was caused by an eclipse;[ ] or whether the going back of the shadow upon the dial of ahaz was caused by a mock sun. the star seen by the wise men from the east may have been a comet, since the word translated 'star' signifies any bright object seen in the heavens, and is in fact the same word which homer, in a passage frequently referred to, uses to signify either a comet or a meteor. the way in which it appeared to go before them, when (directed by herod, be it noticed) they went to bethlehem, almost due south of jerusalem, would correspond to a meridian culmination low down--for the star had manifestly not been visible in the earlier evening, since we are told that they rejoiced when they saw the star again. it was probably a comet travelling southwards; and, as the wise men had travelled from the east, it had very likely been first seen in the west as an evening star, wherefore its course was retrograde--that is, supposing it _was_ a comet.[ ] it may possibly have been an apparition of halley's comet, following a course somewhat similar to that which it followed in the year , when the perihelion passage was made on november , and the comet running southwards disappeared from northern astronomers, though in january it was '_received_' by sir j. herschel, to use his own expression, 'in the southern hemisphere.' there was an apparition of halley's comet in the year , or seventy years after the nativity; and the period of the comet varies, according to the perturbing influences affecting the comet's motion, from sixty-nine to eighty years. homer does not, to the best of my recollection, refer anywhere directly to comets. pope, indeed, who made very free with homer's references to the heavenly bodies,[ ] introduces a comet--and a red one, too!--into the simile of the heavenly portent in book iv.:-- as the red comet from saturnius sent to fright the nations with a dire portent (a fatal sign to armies in the plain, or trembling sailors on the wintry main), with sweeping glories glides along in air, and shakes the sparkles from its blazing hair: between two armies thus, in open sight, shot the bright goddess in a trail of light. but homer says nothing of this comet. if homer had introduced a comet, we may be sure it would not have shaken sparkles from its blazing tail. homer said simply that 'pallas rushed from the peaks of heaven, like the bright star sent by the son of crafty-counselled kronus (as a sign either to sailors, or the broad array of the nations), from which many sparks proceed.' strangely enough, pingré and lalande, the former noted for his researches into ancient comets, the latter a skilful astronomer, agree in considering that homer really referred to a comet, and they even regard this comet as an apparition of the comet of . they cite in support of this opinion the portent which followed the prayer of anchises, 'Æneid,' book ii. , etc.: 'scarce had the old man ceased from praying, when a peal of thunder was heard on the left, and a star, gliding from the heavens amid the darkness, rushed through space followed by a long train of light; we saw the star,' says Æneas, 'suspended for a moment above the roof, brighten our home with its fires, then, tracing out a brilliant course, disappear in the forests of ida; then a long train of flame illuminated us, and the place around reeked with the smell of sulphur. overcome by these startling portents, my father arose, invoked the gods, and worshipped the holy star.' it is impossible to recognise here the description of a comet. the noise, the trail of light, the visible motion, the smell of sulphur, all correspond with the fall of a meteorite close by; and doubtless virgil simply introduced into the narrative the circumstances of some such phenomenon which had been witnessed in his own time. to base on such a point the theory that the comet of was visible at the time of the fall of troy, the date of which is unknown, is venturesome in the extreme. true, the period calculated for the comet of , when pingré and lalande agreed in this unhappy guess, was years; and if we multiply this period by five we obtain years, taking from which leaves years b.c., near enough to the supposed date of the capture of troy. unfortunately, encke (the eminent astronomer to whom we owe that determination of the sun's distance which for nearly half a century held its place in our books, but has within the last twenty years been replaced by a distance three millions of miles less) went over afresh the calculations of the motions of that famous comet, and found that, instead of years, the most probable period is about years. the difference amounts only to years; but even this small difference rather impairs the theory of lalande and pingré.[ ] three hundred and seventy-one years before the christian era, a comet appeared which aristotle (who was a boy at the time) has described. diodorus siculus writes thus respecting it: 'in the first year of the d olympiad, alcisthenes being archon of athens, several prodigies announced the approaching humiliation of the lacedæmonians; a blazing torch of extraordinary size, which was compared to a flaming beam, was seen during several nights.' guillemin, from whose interesting work on comets i have translated the above passage, remarks that this same comet was regarded by the ancients as having not merely presaged but produced the earthquakes which caused the towns of helice and bura to be submerged. this was clearly in the thoughts of seneca when he said of this comet that as soon as it appeared it brought about the submergence of bura and helice. in those times, however, comets were not regarded solely as signs of disaster. as the misfortunes of one nation were commonly held to be of advantage to other nations, so the same comet might be regarded very differently by different nations or different rulers. thus the comet of the year b.c. was regarded by timoleon of corinth as presaging the success of his expedition against corinth. 'the gods announced,' said diodorus siculus, 'by a remarkable portent, his success and future greatness; a blazing torch appeared in the heavens at night, and went before the fleet of timoleon until he arrived in sicily.' the comets of the years b.c. and b.c. were not regarded as portents of death, but as signalising, the former the birth, the latter the accession, of mithridates. the comet of b.c. was held by some to be the soul of julius cæsar on its way to the abode of the gods. bodin, a french lawyer of the sixteenth century, regarded this as the usual significance of comets. he was, indeed, sufficiently modest to attribute the opinion to democritus, but the whole credit of the discovery belonged to himself. he maintained that comets only indicate approaching misfortunes because they are the spirits or souls of illustrious men, who for many years have acted the part of guardian angels, and, being at last ready to die, celebrate their last triumph by voyaging to the firmament as flaming stars. 'naturally,' he says, 'the appearance of a comet is followed by plague, pestilence, and civil war; for the nations are deprived of the guidance of their worthy rulers, who, while they were alive, gave all their efforts to prevent intestine disorders.' pingré comments justly on this, saying that 'it must be classed among base and shameful flatteries, not among philosophic opinions.' usually, however, it must be admitted that the ancients, like the men of the middle ages, regarded comets as harbingers of evil. 'a fearful star is the comet,' says pliny, 'and not easily appeased, as appeared in the late civil troubles when octavius was consul; a second time by the intestine war of pompey and cæsar; and, in our own time, when, claudius cæsar having been poisoned, the empire was left to domitian, in whose reign there appeared a blazing comet.' lucan tells us of the second event here referred to, that during the war 'the darkest nights were lit up by unknown stars' (a rather singular way of saying that there were no dark nights); 'the heavens appeared on fire, flaming torches traversed in all directions the depths of space; a comet, that fearful star which overthrows the powers of the earth, showed its horrid hair.' seneca also expressed the opinion that some comets portend mischief: 'some comets,' he said, 'are very cruel and portend the worst misfortunes; they bring with them and leave behind them the seeds of blood and slaughter.' it was held, indeed, by many in those times a subject for reproach that some were too hard of heart to believe when these signs were sent. it was a point of religious faith that 'god worketh' these 'signs and wonders in heaven.' when troubles were about to befall men, 'nation rising against nation, and kingdom against kingdom, with great earthquakes in divers places, and famines, and pestilences, and fearful sights,' then 'great signs shall there be from heaven.' says josephus, commenting on the obstinacy of the jews in such matters, 'when they were at any time premonished from the lips of truth itself, by prodigies and other premonitory signs of their approaching ruin, they had neither eyes nor ears nor understanding to make a right use of them, but passed them over without heeding or so much as thinking of them; as, for example, what shall we say of the comet in the form of a sword that hung over jerusalem for a whole year together?' this was probably the comet described by dion cassius (_hist. roman._ lxv. ) as having been visible between the months of april and december in the year a.d. this or the comet of a.d. might have been halley's comet. the account of josephus as to the time during which it was visible would not apply to halley's, or, indeed, to any known comet whatever; doubtless he exaggerated. he says: 'the comet was of the kind called _xiphias_, because their tail resembles the blade of a sword,' and this would apply fairly well to halley's comet as seen in , , and ; though it is to be remembered that comets vary very much even at successive apparitions, and it would be quite unsafe to judge from the appearance of a comet seen eighteen centuries ago that it either was or was not the same as some comet now known to be periodic. the comet of a.d. is interesting as having given rise to a happy retort from vespasian, whose death the comet was held to portend. seeing some of his courtiers whispering about the comet, 'that hairy star,' he said, 'does not portend evil to me. it menaces rather the king of the parthians. he is a hairy man, but i am bald.' anna comnena goes even beyond josephus. he only rebuked other men for not believing so strongly as he did himself in the significance of comets--a rebuke little needed, indeed, if we can judge from what history tells us of the terrors excited by comets. but the judicious daughter of alexius was good enough to approve of the wisdom which provided these portents. speaking of a remarkable comet which appeared before the irruption of the gauls into the roman empire, she says: 'this happened by the usual administration of providence in such cases; for it is not fit that so great and strange an alteration of things as was brought to pass by that irruption of theirs should be without some previous denunciation and admonishment from heaven.' socrates, the historian (b. , c. ), says that when gainas besieged constantinople, 'so great was the danger which hung over the city, that it was presignified and portended by a huge blazing comet which reached from heaven to the earth, the like whereof no man had ever seen before.' and cedrenus, in his 'compendium of history,' states that a comet appeared before the death of johannes tzimicas, the emperor of the east, which foreshadowed not alone his death, but the great calamities which were to befall the roman empire by reason of their civil wars. in like manner, the comet of announced the death of attila, that of the death of valentinian. the death of merovingius was announced by the comet of , of chilperic by that of , of the emperor maurice by that of , of mahomet by that of , of louis the debonair by that of , and of the emperor louis ii. by that of . nay, so confidently did men believe that comets indicated the approaching death of great men, that they did not believe a very great man _could_ die without a comet. so they inferred that the death of a very great man indicated the arrival of a comet; and if the comet chanced not to be visible, so much the worse--not for the theory, but--for the comet. 'a comet of this kind,' says pingré, 'was that of the year , presaging the death of charlemagne.' so guillemin quotes pingré; but he should rather have said, such was the comet whose arrival was announced by charlemagne's death--and in no other way, for it was not seen by mortal man. the reader who chances to be strong as to his dates may have observed that some of the dates above mentioned for comets do not accord exactly with the dates of the events associated with those comets. thus louis the debonair did not die in , but in . this, however, is a matter of very little importance. if some men, after their comet has called for them, are 'an unconscionable time in dying,' as charles ii. said of himself, it surely must not be considered the fault of the comet. louis himself regarded the comet of as his death-warrant; the astrologers admitted as much: what more could be desired? the account of the matter given in a chronicle of the time, by a writer who called himself 'the astronomer,' is curious enough: 'during the holy season of easter, a phenomenon, ever fatal and of gloomy foreboding, appeared in the heavens. as soon as the emperor, who paid attention to such phenomena, received the first announcement of it, he gave himself no rest until he had called a certain learned man and myself before him. as soon as i arrived, he anxiously asked me what i thought of such a sign. i asked time of him, in order to consider the aspect of the stars, and to discover the truth by their means, promising to acquaint him on the morrow; but the emperor, persuaded that i wished to gain time, which was true, in order not to be obliged to announce anything fatal to him, said to me: "go on the terrace of the palace, and return at once to tell me what you have seen, for i did not see this star last evening, and you did not point it out to me; but i know that it is a comet; tell me what you think it announces to me." then, scarcely allowing me time to say a word, he added: "there is still another thing you keep back: it is that a change of reign and the death of a prince are announced by this sign." and as i advanced the testimony of the prophet, who said: "fear not the signs of the heavens as the nations fear them," the prince, with his grand nature and the wisdom which never forsook him, said: "we must only fear him who has created both us and this star. but, as this phenomenon may refer to us, let us acknowledge it as a warning from heaven."' accordingly, louis himself and all his court fasted and prayed, and he built churches and monasteries. but all was of no avail. in little more than three years he died; showing, as the historian raoul glaber remarked, that 'these phenomena of the universe are never presented to man without surely announcing some wonderful and terrible event.' with a range of three years in advance, and so many kings and princes as there were about in those days, and are still, it would be rather difficult for a comet to appear without announcing some such wonderful and terrible event as a royal death. the year a.d. was by all but common consent regarded as the date assigned for the end of the world. for a thousand years satan had been chained, and now he was to be loosened for a while. so that when a comet made its appearance, and, terrible to relate, continued visible for nine days, the phenomenon was regarded as something more than a nine days' wonder. besides the comet, a very wonderful meteor was seen. 'the heavens opened, and a kind of flaming torch fell upon the earth, leaving behind a long track of light like the path of a flash of lightning. its brightness was so great that it frightened not only those who were in the fields, but even those who were in their houses. as this opening in the sky slowly closed men saw with horror the figure of a dragon, whose feet were blue, and whose head' [like that of dickens's dwarf] 'seemed to grow larger and larger.' a picture of this dreadful meteor accompanies the account given by the old chronicler. for fear the exact likeness of the dragon might not be recognised (and, indeed, to see it one must 'make believe a good deal'), there is placed beside it a picture of a dragon to correspond, which picture is in turn labelled 'serpens cum ceruleis pedibus.' it was considered very wicked in the year to doubt that the end of all things was at hand. but somehow the world escaped that time. in the year halley's comet appeared to announce to the saxons the approaching conquest of england by william the norman. a contemporary poet made a singular remark, which may have some profound poetical meaning, but certainly seems a little indistinct on the surface. he said that 'the comet had been more favourable to william than nature had been to cæsar; the latter had no hair, but william had received some from the comet.' this is the only instance, so far as i know, in which a comet has been regarded as a perruquier. a monk of malmesbury spoke more to the purpose, according to then received ideas, in thus apostrophising the comet: 'here art thou again, cause of tears to many mothers! it is long since i saw thee last, but i see thee now more terrible than ever; thou threatenest my country with complete ruin.' halley's comet, with its inconveniently short period of about seventy-seven years, has repeatedly troubled the nations and been regarded as a sign sent from heaven: ten million cubic miles of head, ten billion leagues of tail, all provided for the sole purpose of warning one petty race of earth-folks against the evils likely to be brought against them by another. this comet has appeared twenty-four times since the date of its first recorded appearance, which some consider to have been b.c., and others refer to a few years later. it may be interesting to quote here babinet's description of the effects ascribed in to this comet, often the terror of nations, but the triumph of mathematicians, as the first whose motions were brought into recognisable obedience to the laws of gravity.[ ] 'the mussulmans, with mahomet ii. at their head, were besieging belgrade, which was defended by huniade, surnamed the exterminator of the turks. halley's comet appeared and the two armies were seized with equal fear. pope calixtus iii., himself seized by the general terror, ordered public prayers and timidly anathematised the comet and the enemies of christianity. he established the prayer called the noon _angelus_, the use of which is continued in all catholic churches. the franciscans (_frères mineurs_) brought , defenders to belgrade, besieged by the conqueror of constantinople, the destroyer of the eastern empire. at last the battle began; it continued two days without ceasing. a contest of two days caused , combatants to bite the dust. the franciscans, unarmed, crucifix in hand, were in the front rank, invoking the papal exorcism against the comet, and turning upon the enemy that heavenly wrath of which none in those times dared doubt.' the great comet of has been regarded as the occasion of the emperor charles v.'s abdication of the imperial throne; a circumstance which seems rendered a little doubtful by the fact that he had already abdicated when the comet appeared--a mere detail, perhaps, but suggesting the possibility that cause and effect may have been interchanged by mistake, and that it was charles's abdication which occasioned the appearance of the comet. according to gemma's account the comet was conspicuous rather from its great light than from the length of its tail or the strangeness of its appearance. 'its head equalled jupiter in brightness, and was equal in diameter to nearly half the apparent diameter of the moon.' it appeared about the end of february, and in march presented a terrible appearance, according to ripamonte. 'terrific indeed,' says sir j. herschel, 'it might well have been to the mind of a prince prepared by the most abject superstition to receive its appearance as a warning of approaching death, and as specially sent, whether in anger or in mercy, to detach his thoughts from earthly things, and fix them on his eternal interests. such was its effect on the emperor charles v., whose abdication is distinctly ascribed by many historians to this cause, and whose words on the occasion of his first beholding it have even been recorded-- "his ergo indiciis me mea fata vocant"-- the language and the metrical form of which exclamation afford no ground for disputing its authenticity, when the habits and education of those times are fairly considered.' it is quite likely that, having already abdicated the throne, charles regarded the comet as signalling his retirement from power--an event which he doubtless considered a great deal too important to be left without some celestial record. but the words attributed to him are in all probability apocryphal. the comet of was remarkable for the strangeness of its aspect, which in some respects resembled that of the comet of , called donati's. it required only the terror with which such portentous objects were witnessed in the middle ages to transform the various streamers, curved and straight, extending from such an object, into swords and spears, and other signs of war and trouble. doubtless, we owe to the fears of the middle ages the strange pictures claiming to present the actual aspect of some of the larger comets. halley's comet did not escape. it was compared to a straight sword at one visit, to a curved scimitar in , and even at its last return in there were some who recognised in the comet a resemblance to a misty head. other comets have been compared to swords of fire, bloody crosses, flaming daggers, spears, serpents, fiery dragons, fish, and so forth. but in this respect no comet would seem to have been comparable with that of , of which andrew paré writes as follows: 'this comet was so horrible and dreadful, and engendered such terror in the minds of men, that they died, some from fear alone, others from illness engendered by fear. it was of immense length and blood-red colour; at its head was seen the figure of a curved arm, holding a large sword in the hand as if preparing to strike. at the point of this sword were three stars; and on either side a number of axes, knives, and swords covered with blood, amongst which were many hideous human faces with bristling beards and hair.' such peculiarities of shape, and also those affecting the position and movements of comets, were held to be full of meaning. as bayle pointed out in his 'thoughts about the comet of ,' these fancies are of great antiquity. pliny tells us that in his time astrologers claimed to interpret the meaning of a comet's position and appearance, and that also of the direction towards which its rays pointed. they could, moreover, explain the effects produced by the fixed stars whose rays were conjoined with the comet's. if a comet resembles a flute, then musicians are aimed at; when comets are in the less dignified parts of the constellations, they presage evil to immodest persons; if the head of a comet forms an equilateral triangle or a square with fixed stars, then it is time for mathematicians and men of science to tremble. when they are in the sign of the ram, they portend great wars and widespread mortality, the abasement of the great and the elevation of the small, besides fearful droughts in regions over which that sign predominates; in the virgin, they imply many grievous ills to the female portion of the population; in the scorpion, they portend a plague of reptiles, especially locusts; in the fishes, they indicate great troubles from religious differences, besides war and pestilence. when, like the one described by milton, they 'fire the length of ophiuchus huge,' they show that there will be much mortality caused by poisoning. the comet of , which led bayle to write the treatise to which reference has just been made, was one well calculated to inspire terror. indeed, if the truth were known, that comet probably brought greater danger to the inhabitants of the earth than any other except the comet of --the danger not, however, being that derived from possible collision between the earth and a comet, but that arising from the possible downfall of a large comet upon the sun, and the consequent enormous increase of the sun's heat. that, according to newton, is the great danger men have to fear from comets; and the comet of was one which in that sense was a very dangerous one. there is no reason why a comet from outer space should not fall straight towards the sun, as at one time the comet of was supposed to be doing. all the comfort that science can give the world on that point is that such a course for a comet is only one out of many millions of possible courses, all fully as likely; and that, therefore, the chance of a comet falling upon the sun is only as one in many millions. still, the comet of made a very fair shot at the sun, and a very slight modification of its course by jupiter or saturn might have brought about the catastrophe which newton feared. whether, if a comet actually fell upon the sun, anything very dreadful would happen, is not so clear. newton's ideas respecting comets were formed in ignorance of many physical facts and laws which in our day render reasoning upon the subject comparatively easy. yet, even in our time, it is not possible to assert confidently that such fears are idle. during the solar outburst witnessed by carrington and hodgson in september , it is supposed that the sun swallowed a large meteoric mass; and, as great cornets are probably followed by many such masses, it seems reasonable to infer that if such a comet fell upon the sun, his surface being pelted with such exceptionally large masses, stoned with these mighty meteoric balls, would glow all over (or nearly so) as brightly as a small spot of that surface glowed upon that occasion. now that portion was so bright that carrington thought 'that by some chance a ray of light had penetrated a hole in the screen attached to the object-glass by which the general image is thrown in shade, for the brilliancy was fully equal to that of direct sunlight.' manifestly, if the whole surface of the sun, or any large portion of the surface, were caused to glow with that exceeding brilliancy, surpassing ordinary sunlight in the same degree that ordinary sunlight surpassed the shaded solar image in carrington's observations, the result would be disastrous in the extreme for the inhabitants of that half of the earth which chanced to be in sunlight at the time; and if (as could scarcely fail to happen) the duration of that abnormal splendour were more than half a day, then the whole earth would probably be depopulated by the intense heat. the danger, as i have said, is slight--partly because there is small chance of a collision between the sun and a comet, partly because we have no certain reasons for assuming that a collision would be followed by the heating of the sun for a while to a very high temperature. looking around at the suns which people space, and considering their history, so far as it has been made known to us, for the last two thousand years, we find small occasion for fear. those suns seem to have been for the most part safe from any sudden or rapid accessions of heat; and if they travel thus safely in their mighty journeys through space, we may well believe that our sun also is safe. nevertheless, there _have_ been catastrophes here and there. now one sun and now another has blazed out with a hundred times its usual lustre, gradually losing its new fires and returning to its customary brightness; but after what destruction among those peopling its system of worlds who shall say? spectroscopic analysis, that powerful help to the modern astronomical inquirer, has shown in one of these cases that just such changes had taken place as we might fairly expect would follow if a mighty comet fell into the sun. if this interpretation be correct, then we are not wholly safe. any day might bring us news of a comet sailing full upon our sun from out the depths of space. then astronomers would perhaps have the opportunity of ascertaining the harmlessness of a collision between the ruler of our system and one of the long-tailed visitors from the celestial spaces. or possibly, astronomers and the earth's inhabitants generally might find out the reverse, though the knowledge would not avail them much, seeing that the messenger who would bring it would be the king of terrors himself. it was well, perhaps, that newton's discovery of the law of gravitation, and the application of this law to the comets of and (the latter our old friend halley's comet, then properly so called as studied by him), came in time to aid in removing to some slight degree the old superstitions respecting comets. for in england many remembered the comets of the great plague and of the great fire of london. these comets came so closely upon the time of the plague and the fire respectively, that it was not wonderful if even the wiser sort were struck by the coincidence and could scarcely regard it as accidental. it is not easy for the student of science in our own times, when the movements of comets are as well understood as those of the most orderly planets, to place himself in the position of men in the times when no one knew on what paths comets came, or whither they retreated after they had visited our sun. taught as men were, on the one hand, that it was wicked to question what seemed to be the teaching of the scriptures, that changes or new appearances in the heavens were sent to warn mankind of approaching troubles, and perplexed as they were, on the other, by the absence of any real knowledge respecting comets and meteors, it was not so easy as we might imagine from our own way of viewing these matters, to shake off a superstition which had ruled over men's minds for thousands of years. no sect had been free from this superstition. popes and priests had taught their followers to pray against the evil influences of comets and other celestial portents; luther and melanchthon had condemned in no measured terms the rashness and impiety of those who had striven to show that the heavenly bodies and the earth move in concordance with law--those 'fools who wish to reverse the entire science of astronomy.' a long interval had elapsed between the time when the copernican theory was struggling for existence--when, but that more serious heresies engaged men's attention and kept religious folk by the ears, that astronomical heresy would probably have been quenched in blood--and the forging by newton of the final link of the chain of reasoning on which modern astronomy is based; but in those times the minds of men moved more slowly than in ours. the masses still held to the old beliefs about the heavenly bodies. defoe, indeed, speaking of the terror of men at the time of the great plague, says that they 'were more addicted to prophecies and astrological conjurations, dreams, and old wives' tales, than ever they were before or since.' but in reality, it was only because of the great misery then prevailing that men seemed more superstitious than usual; for misery brings out the superstitions--the fetishisms, if we may so speak--which are inherent in many minds, but concealed from others in prosperous times, out of shame, or perhaps a worthier feeling. even in our own times great national calamities would show that many superstitions exist which had been thought extinct, and we should see excited among the ill-educated that particular form of persecution which arises, not from zeal for religion and not from intolerance, but from the belief that the troubles have been sent because of unbelief and the fear that unless some expiation be made the evil will not pass away from the midst of the people. it is at such times of general affliction that minds of the meaner sort have proved 'zealous even to slaying.' the influence of strange appearances in the heavens on even thoughtful and reasoning minds, at such times of universal calamity, is well shown by defoe's remarks on the comets of the years and . 'the old women,' he says, 'and the phlegmatic, hypochondriacal part of the other sex, whom i could almost call old women too, remarked that those two comets passed directly over the city' [though that appearance must have depended on the position whence these old women, male and female, observed the comet], 'and that so very near the houses, that it was plain they imported something peculiar to the city alone; and that the comet before the pestilence was of a faint, dull, languid colour, and its motion very heavy, solemn, and slow; but that the comet before the fire was bright and sparkling, or, as others said, flaming, and its motion swift and furious: and that accordingly one foretold a heavy judgment, slow but severe, terrible and frightful, as was the plague; but the other foretold a stroke, sudden, swift, and fiery, as was the conflagration. nay, so particular some people were, that, as they looked upon that comet preceding the fire, they fancied that they not only saw it pass swiftly and fiercely, and could perceive the motion with their eye, but even that they heard it; that it made a mighty rushing noise, fierce and terrible, though at a distance and but just perceivable. i saw both these stars, and must confess had i had so much the common notion of such things in my head, that i was apt to look upon them as the forerunners and warnings of god's judgments, and especially when, the plague having followed the first, i yet saw another of the same kind, i could not but say, god had not yet sufficiently scourged the city' [london]. the comets of and , though they did not bring plagues or conflagrations immediately, yet were not supposed to have been altogether without influence. the convenient fiction, indeed, that some comets operate quickly and others slowly, made it very difficult for a comet to appear to which some evil effects could not be ascribed. if any one can find a single date, since the records of history have been carefully kept, which was so fortunately placed that, during no time following it within five years, no prince, king, emperor, or pope died, no war was begun, or ended disastrously for one side or the other engaged in it, no revolution was effected, neither plague nor pestilence occurred, neither droughts nor floods afflicted any nation, no great hurricanes, earthquakes, volcanic outbursts, or other trouble was recorded, he will then have shown the bare possibility that a comet might have appeared which seemed to presage neither abrupt nor slow-moving calamities. but it is not possible to name such a date, nor even a date which was not followed within two years at the utmost by a calamity such as superstition might assign to a comet. and so closely have such calamities usually followed, that scarce a comet could appear which might not be regarded as the precursor of very quickly approaching calamity. even if a comet had come which seemed to bring no trouble, nay, if many such comets had come, men would still have overlooked the absence of any apparent fulfilment of the predicted troubles. henry iv. well remarked, when he was told that astrologers predicted his death because a certain comet had been observed: 'one of these days they will predict it truly, and people will remember better the single occasion when the prediction will be fulfilled than the many other occasions when it has been falsified by the event.' the troubles connected with the comets of and were removed farther from the dates of the events themselves than usual, at least so far as the english interpretation of the comets was concerned. 'the great comet in ,' says one, 'followed by a lesser comet in , was evidently the forerunner of all those remarkable and disastrous events that ended in the revolution of . it also evidently presaged the revocation of the edict of nantes, and the cruel persecution of the protestants, by the french king louis xiv., afterwards followed by those terrible wars which, with little intermission, continued to ravage the finest parts of europe for nearly twenty-four years.' if in some respects the fears inspired by comets have been reduced by modern scientific discoveries respecting these bodies, yet in other respects the very confidence engendered by the exactness of modern astronomical computations has proved a source of terror. there is nothing more remarkable, for instance, in the whole history of cometary superstition, than the panic which spread over france in the year , in consequence of a rumour that the mathematician lalande had predicted the occurrence of a collision between a comet and the earth, and that disastrous effects would inevitably follow. the foundation of the rumour was slight enough in all conscience. it had simply been announced that lalande would read before the academy of sciences a paper entitled 'reflections on those comets which can approach the earth.' that was absolutely all; yet, from that one fact, not only were vague rumours of approaching cometic troubles spread abroad, but the statement was definitely made that on may or , , 'a comet would encounter the earth.'[ ] so great was the fear thus excited, that, in order to calm it, lalande inserted in the 'gazette de france' of may , , the following advertisement:--'m. lalande had not time to read his memoir upon comets which may approach the earth and cause changes in her motions; but he would observe that it is impossible to assign the epochs of such events. the next comet whose return is expected is the one which should return in eighteen years; but it is not one of those which can hurt the earth.' this note had not the slightest effect in restoring peace to the minds of unscientific frenchmen. m. lalande's study was crowded with anxious persons who came to inquire about his memoir. certain devout folk, 'as ignorant as they were imbecile,' says a contemporary journal, begged the archbishop of paris to appoint forty hours' prayer to avert the danger and prevent the terrible deluge. for this was the particular form most men agreed that the danger would take. that prelate was on the point, indeed, of complying with their request, and would have done so, but that some members of the academy explained to him that by so doing he would excite ridicule. far more effective, and, to say truth, far better judged, was the irony of voltaire, in his deservedly celebrated 'letter on the pretended comet.' it ran as follows:-- 'grenoble, may , . 'certain parisians who are not philosophers, and who, if we are to believe them, will not have time to become such, have informed me that the end of the world approaches, and will occur without fail on the th of this present month of may. they expect, that day, a comet, which is to take our little globe from behind and reduce it to impalpable powder, according to a certain prediction of the academy of sciences which has not yet been made. 'nothing is more likely than this event; for james bernouilli, in his "treatise upon the comet" of , predicted expressly that the famous comet of would return with terrible uproar (_fracas_) on may , ; he assured us that in truth its perruque would signify nothing mischievous, but that its tail would be an infallible sign of the wrath of heaven. if james bernouilli mistook, it is, after all, but a matter of fifty-four years and three days. 'now, so small an error as this being regarded by all geometricians as of little moment in the immensity of ages, it is manifest that nothing can be more reasonable than to hope (_sic, espérer_) for the end of the world on the th of this present month of may , or in some other year. if the thing should not come to pass, "omittance is no quittance" (_ce qui est différé, n'est pas perdu_). 'there is certainly no reason for laughing at m. trissotin, triple idiot though he is (_tout trissotin qu'il est_), when he says to madame philaminte (molière's "femmes savantes," acte iv. scène ), 'nous l'avons en dormant, madame, échappé belle; un monde près de nous a passé tout du long, est chu tout au travers de notre tourbillon; et, s'il eût en chemin rencontré notre terre, elle eût été brisée en morceaux comme verre. 'a comet coursing along its parabolic orbit may come full tilt against our earth. but then, what will happen? either that comet will have a force equal to that of our earth, or greater, or less. if equal, we shall do the comet as much harm as it will do us, action and reaction being equal; if greater, the comet will bear us away with it; if less, we shall bear away the comet. 'this great event may occur in a thousand ways, and no one can affirm that our earth and the other planets have not experienced more than one revolution, through the mischance of encountering a comet on their path. 'the parisians will not desert their city on the th inst.; they will sing songs, and the play of "the comet and the world's end" will be performed at the opéra comique.' the last touch is as fine in its way as sydney smith's remark that, if london were destroyed by an earthquake, the surviving citizens would celebrate the event by a public dinner among the ruins. voltaire's prediction was not fulfilled exactly to the letter, but what actually happened was even funnier than what his lively imagination had suggested. it was stated by a parisian professor in (as a reason why the academy of sciences should refute an assertion then rife to the effect that biela's comet would encounter the earth that year) that during the cometic panic of 'there were not wanting people who knew too well the art of turning to their advantage the alarm inspired by the approaching comet, and _places in paradise were sold at a very high rate_.[ ] the announcement of the comet of may produce similar effects,' he said, 'unless the authority of the academy apply a prompt remedy; and this salutary intervention is at this moment implored by many benevolent persons.' in recent years the effects produced on the minds of men by comets have been less marked than of yore, and appear to have depended a good deal on circumstances. the comet of the year (called donati's), for example, occasioned no special fears, at least until napoleon iii. made his famous new-year's day speech, after which many began to think the comet had meant mischief. but the comet of , though less conspicuous, occasioned more serious fears. it was held by many in italy to presage a very great misfortune indeed, viz. the restoration of francis ii. to the throne of the two sicilies. others thought that the downfall of the temporal power of the papacy and the death of pope pius ix. were signified. i have not heard that any very serious consequences were expected to follow the appearance of coggia's comet in . the great heat which prevailed during parts of the summer of was held by many to be connected in some way with a comet which some very unskilful telescopist constructed in his imagination out of the glare of jupiter in the object-glass of his telescope. another benighted person, seeing the pleiades low down through a fog, turned them into a comet, about the same time. possibly the idea was, that since comets are supposed to cause great heats, great heats may be supposed to indicate a comet somewhere; and with minds thus prepared, it was not wonderful, perhaps, that telescopic glare, or an imperfect view of our old friends the pleiades, should have been mistaken for a vision of the heat-producing comet. it should be a noteworthy circumstance to those who still continue to look on comets as signs of great catastrophes, that a war more remarkable in many respects than any which has ever yet been waged between two great nations--a war swift in its operations and decisive in its effects--a war in which three armies, each larger than all the forces commanded by napoleon i. during the campaign of , were captured bodily--should have been begun and carried on to its termination without the appearance of any great comet. the civil war in america, a still more terrible calamity to that great nation than the success of moltke's operations to the french, may be regarded by believers as presignified by the great comet of . but it so chances that the war between france and germany occurred near the middle of one of the longest intervals recorded in astronomical annals as unmarked by a single conspicuous comet--the interval between the years and . if the progress of just ideas respecting comets has been slow, it must nevertheless be regarded as on the whole satisfactory. when we remember that it was not a mere idle fancy which had to be opposed, not mere terrors which had to be calmed, but that the idea of the significance of changes in the heavens had come to be regarded by mankind as a part of their religion, it cannot but be thought a hopeful sign that all reasoning men in our time have abandoned the idea that comets are sent to warn the inhabitants of this small earth. obeying in their movements the same law of gravitation which guides the planets in their courses, the comets are tracked by the skilful mathematician along those remote parts of their course where even the telescope fails to keep them in view. not only are they no longer regarded as presaging the fortunes of men on this earth, but men on this earth are able to predict the fortunes of comets. not only is it seen that they cannot influence the fates of the earth or other planets, but we perceive that the earth and planets by their attractive energies influence, and in no unimportant degree, the fates of these visitants from outer space. encouraging, truly, is the lesson taught us by the success of earnest study and careful inquiry in determining some at least among the laws which govern bodies once thought the wildest and most erratic creatures in the whole of god's universe. ix. _the lunar hoax._ then he gave them an account of the famous moon hoax, which came out in . it was full of the most barefaced absurdities, yet people swallowed it all; and even arago is said to have treated it seriously as a thing that could not well be true, for mr. herschel would have certainly notified him of these marvellous discoveries. the writer of it had not troubled himself to invent probabilities, but had borrowed his scenery from the 'arabian nights' and his lunar inhabitants from 'peter wilkins.'--oliver wendell holmes (in _the poet at the breakfast-table_). in one of the earliest numbers of 'macmillan's magazine, the late professor de morgan, in an article on scientific hoaxing, gave a brief account of the so-called 'lunar hoax'--an instance of scientific trickery frequently mentioned, though probably few are familiar with the real facts. de morgan himself possessed a copy of the second english edition of the pamphlet, published in london in . but the original pamphlet edition, published in america in september , is not easily to be obtained. the proprietors of the new york 'sun,' in which the fictitious narrative first appeared, published an edition of , copies, and every copy was sold in less than a month. lately a single copy of that edition was sold for three dollars seventy-five cents.[ ] the pamphlet is interesting in many respects, and i propose to give here a brief account of it. but first it may be well to describe briefly the origin of the hoax. it is said that after the french revolution of nicollet, a french astronomer of some repute, especially for certain lunar observations of a very delicate and difficult kind, left france in debt and also in bad odour with the republican party. according to this story, arago the astronomer was especially obnoxious to nicollet, and it was as much with the view of revenging himself on his foe as from a wish to raise a little money that nicollet wrote the moon-fable. it is said further that arago was entrapped, as nicollet desired, and circulated all over paris the wonders related in the pamphlet, until nicollet wrote to his friend bouvard explaining the trick. so runs the story, but the story cannot be altogether true. nicollet may have prepared the narrative and partly written it, but there are passages in the pamphlet as published in america which no astronomer could have written. possibly there is some truth in de morgan's supposition that the original work was french. this may have been nicollet's: and the american edition was probably enlarged by the translator, who, according to this account, was richard alton locke,[ ] to whom in america the whole credit, or discredit, of the hoax is commonly attributed. there can be no doubt that either the french version was much more carefully designed than the american, or there was no truth in the story that arago was deceived by the narrative; for in its present form the story, though clever, could not for an instant have deceived any one acquainted with the most elementary laws of optics. the whole story turns on optical rather than on astronomical considerations; but every astronomer of the least skill is acquainted with the principles on which the construction of optical instruments depends. though the success of the deception recently practised on m. chasles by the forger of the pascal papers has been regarded as showing how easily mathematicians may be entrapped, yet even m. chasles would not have been deceived by bad mathematics; and arago, a master of the science of optics, could not but have detected optical blunders which would be glaring to the average cambridge undergraduate. but let us turn to the story itself. the account opens with a passage unmistakably from an american hand, though purporting, be it remembered, to be quoted from the 'supplement to the edinburgh journal of science.' 'in this unusual addition to our journal, we have the happiness of making known to the british public, and thence to the whole civilised world, recent discoveries in astronomy which will build an imperishable monument to the age in which we live, and confer upon the present generation of the human race a proud distinction through all future time. it has been poetically said' [where and by whom?] 'that the stars of heaven are the hereditary regalia of man, as the intellectual sovereign of the animal creation. he may now fold the zodiac around him with a loftier consciousness of his mental supremacy.' to the american mind enwrapment in the star-jewelled zodiac may appear as natural as their ordinary oratorical references to the star-spangled banner; but the idea is essentially transatlantic, and not even the most poetical european astronomer could have risen to such a height of imagery. passing over several pages of introductory matter, we come to the description of the method by which a telescope of sufficient magnifying power to show living creatures in the moon was constructed by sir john herschel. it had occurred, it would seem, to the elder herschel to construct an improved series of parabolic and spherical reflectors 'uniting all the meritorious points in the gregorian and newtonian instruments, with the highly interesting achromatic discovery of dolland'(_sic_). [this is much as though one should say that a clever engineer had conceived the idea of constructing an improved series of railway engines, combining all the meritorious points in stationary and locomotive engines, with _isaac_ watts' highly ingenious discovery of screw propulsion. for the gregorian and newtonian instruments simply differ in sending the rays received from the great mirror in different directions, and dolland's discovery relates to the ordinary forms of telescopes with large lens, not with large mirror.] however, accumulating infirmities and eventually death prevented sir william herschel from applying his plan, which 'evinced the most profound research in optical science, and the most dexterous ingenuity in mechanical contrivance. but his son, sir john herschel, nursed and cradled in the observatory, and a practical astronomer from his boyhood, determined upon testing it at whatever cost. within two years of his father's death he completed his new apparatus, and adapted it to the old telescope with nearly perfect success.' a short account of the observations made with this instrument, now magnifying six thousand times, follows, in which most of the astronomical statements are very correctly and justly worded, being, in fact, borrowed from a paper by sir w. herschel on observation of the moon with precisely that power. but this great improvement upon all former telescopes still left the observer at a distance of forty miles from the moon; and at that distance no object less than about twenty yards in diameter could be distinguished, and even objects of that size 'would appear only as feeble, shapeless points.' sir john 'had the satisfaction to know that if he could leap astride a cannon-ball, and travel upon its wings of fury for the respectable period of several millions of years, he would not obtain a more enlarged view of the more distant stars than he could now possess in a few minutes of time; and that it would require an ultra-railroad speed of fifty miles an hour for nearly the livelong year, to secure him a more favourable inspection of the gentle luminary of the night;' but 'the exciting question whether this "observed" of all the sons of men, from the days of eden to those of edinburgh, be inhabited by beings, like ourselves, of consciousness and curiosity, was left to the benevolent index of natural analogy, or to the severe tradition that the moon is tenanted only by the hoary _solitaire_, whom the criminal code of the nursery had banished thither for collecting fuel on the sabbath-day.'[ ] but the time had arrived when the great discovery was to be made, by which at length the moon could be brought near enough, by telescopic power, for living creatures on her surface to be seen if any exist. the account of the sudden discovery of the new method, during a conversation between sir john herschel and sir david brewster, is one of the most cleverly conceived (though also one of the absurdest) passages in the pamphlet. 'about three years ago, in the course of a conversational discussion with sir david brewster upon the merits of some ingenious suggestions by the latter, in his article on optics in the "edinburgh encyclopædia," p. , for improvements in newtonian reflectors, sir john herschel adverted to the convenient simplicity of the old astronomical telescopes that were without tubes, and the object-glass of which, placed upon a high pole, threw the focal image to a distance of and even feet. dr. brewster readily admitted that a tube was not necessary, provided the focal image were conveyed into a dark apartment and there properly received by reflectors.... the conversation then became directed to that all-invincible enemy, the paucity of light in powerful magnifiers. after a few moments' silent thought, sir john diffidently enquired whether it would not be possible to effect _a transfusion of artificial light through the focal object of vision_! sir david, somewhat startled at the originality of the idea, paused awhile, and then hesitatingly referred to the refrangibility of rays, and the angle of incidence. sir john, grown more confident, adduced the example of the newtonian reflector, in which the refrangibility was corrected by the second speculum, and the angle of incidence restored by the third.' all this part of the narrative is simply splendid in absurdity. hesitating references to refrangibility and the angle of incidence would have been sheerly idiotic under the supposed circumstances; and in the newtonian reflector (which has only two specula or mirrors) there is no refrangibility to be corrected; apart from which, 'correcting refrangibility' has no more meaning than 'restoring the angle of incidence.' '"and," continued sir john, "why cannot the illuminating microscope, say the hydro-oxygen, be applied to render distinct, and, if necessary, even to magnify, the focal object?" sir david sprung from his chair' [and well he might, though not] 'in an ecstasy of conviction, and, leaping half-way to the ceiling, exclaimed, "thou art the man!" each philosopher anticipated the other in presenting the prompt illustration that if the rays of the hydro-oxygen microscope, passed through a drop of water containing the larvæ of a gnat and other objects invisible to the naked eye, rendered them not only keenly but firmly magnified to dimensions of many feet; so could the same artificial light, passed through the faintest focal object of a telescope, both distinctify (to coin a new word for an extraordinary occasion) and magnify its feeblest component members. the only apparent desideratum was a recipient for the focal image which should transfer it, without refranging it, to the surface on which it was to be viewed under the revivifying light of the microscopic reflectors.' singularly enough, the idea here mentioned does not appear to many so absurd as it is in reality. it is known that the image formed by the large lens of an ordinary telescope or the large mirror of a reflecting telescope is a real image; not a merely virtual image like that which is seen in a looking-glass. it can be received on a sheet of paper or other white surface just as the image of surrounding objects can be thrown upon the white table of the camera obscura. it is this real image, in fact, which we look at in using a telescope of any sort, the portion of such a telescope nearest to the eye being in reality a microscope for viewing the image formed by the great lens or mirror, as the case may be. and it does not seem to some altogether absurd to speak of illuminating this image by transfused light, or of casting by means of an illuminating microscope a vastly enlarged picture of this image upon a screen. but of course the image being simply formed by the passage of rays (which originally came from the object whose image they form) through a certain small space, to send _other_ rays (coming from some other luminous object) through the same small space, is not to improve, but, so far as any effect is produced at all, to impair, the distinctness of the image. in fact, if these illuminating rays reached the eye, they would seriously impair the distinctness of the image. their effect may be compared exactly with the effect of rays of light cast upon the image in a camera obscura; and, to see what the effect of such rays would be, we need only consider why it is that the camera _is_ made 'obscura,' or dark. the effect of the transfusion of light through a telescopic image may be easily tried by any one who cares to make the experiment. he has only to do away with the tube of his telescope (substituting two or three straight rods to hold the glass in its place), and then in the blaze of a strong sun to direct the telescope on some object lying nearly towards the sun. or if he prefer artificial light for the experiment, then at night let him direct the telescope so prepared upon the moon, while a strong electric light is directed upon the place where the focal image is formed (close in front of the eye). the experiment will not suggest very sanguine hopes of good result from the transfusion of artificial light. yet, to my own knowledge, not a few who were perfectly well aware that the lunar hoax was not based on facts, have gravely reasoned that the principle suggested might be sound, and, in fact, that they could see no reason why astronomers should not try it, even though it had been first suggested as a joke. to return, however, to the narrative. 'the co-operative philosophers, having hit upon their method, determined to test it practically. they decided that a medium of the purest plate-glass (which it is said they obtained, by consent, be it observed, from the shop-window of m. desanges, the jeweller to his ex-majesty charles x., in high street) was the most eligible they could discover. it answered perfectly with a telescope which magnified a hundred times, and a microscope of about thrice that power.' thus fortified by experiment, and 'fully sanctioned by the high optical authority of sir david brewster, sir john laid his plan before the royal society, and particularly directed to it the attention of his royal highness the duke of sussex, the ever munificent patron of science and the arts. it was immediately and enthusiastically approved by the committee chosen to investigate it, and the chairman, who was the royal president' (this continual reference to royalty is manifestly intended to give a british tone to the narrative), 'subscribed his name for a contribution of £ , , with a promise that he would zealously submit the proposed instrument as a fit object for the patronage of the privy purse. he did so without delay; and his majesty, on being informed that the estimated expense was £ , , naïvely enquired if the costly instrument would conduce to any improvement in _navigation_. on being informed that it undoubtly would, the sailor king promised a _carte blanche_ for any amount which might be required.' all this is very clever. the 'sailor king' comes in as effectively to give _vraisemblance_ to the narrative as 'crabtree's little bronze shakspeare that stood over the fireplace,' and the 'postman just come to the door with a double letter from northamptonshire.' then comes a description of the construction of the object-glass, twenty-four feet in diameter, 'just six times the size of the elder herschel's;' who, by the way, never made a telescope with an object-glass. the account of sir john herschel's journey from england, and even some details of the construction of the observatory, were based on facts, indeed, so many persons in america as well as in england were acquainted with some of these circumstances, that it was essential to follow the facts as closely as possible. of course, also, some explanation had to be given of the circumstance that nothing had before been heard respecting the gigantic instrument taken out by sir john herschel. 'whether,' says the story, 'the british government were sceptical concerning the promised splendour of the discoveries, or wished them to be scrupulously veiled until they had accumulated a full-orbed glory for the nation and reign in which they originated, is a question which we can only conjecturally solve. but certain it is that the astronomer's royal patrons enjoined a masonic taciturnity upon him and his friends until he should have officially communicated the results of his great experiment.' it was not till the night of january , , that the mighty telescope was at length directed towards our satellite. the part of the moon selected was on the eastern part of her disc. 'the whole immense power of the telescope was applied, and to its focal image about one half of the power of the microscope. on removing the screen of the latter, the field of view was covered throughout its entire area with a beautifully distinct and even vivid representation of _basaltic rock_. its colour was a greenish brown; and the width of the columns, as defined by their interstices on the canvas, was invariably twenty-eight inches. no fracture whatever appeared in the mass first presented; but in a few seconds a shelving pile appeared, of five or six columns' width, which showed their figure to be hexagonal, and their articulations similar to those of the basaltic formation at staffa. this precipitous cliff was profusely covered with a dark red flower, precisely similar, says dr. grant, to the papaver rhoeus, or rose poppy, of our sublunary cornfields; and this was the first organic production of nature in a foreign world ever revealed to the eyes of men.' it would be wearisome to go through the whole series of observations thus fabled, and only a few of the more striking features need be indicated. the discoveries are carefully graduated in interest. thus we have seen how, after recognising basaltic formations, the observers discovered flowers: they next see a lunar forest, whose 'trees were of one unvaried kind, and unlike any on earth except the largest kind of yews in the english churchyards.' (there is an american ring in this sentence, by the way, as there is in one, a few lines farther on, where the narrator having stated that by mistake the observers had the sea of clouds instead of a more easterly spot in the field of view, proceeds to say: 'however, the moon was a free country, and we not as yet attached to any particular province.') next a lunar ocean is described, 'the water nearly as blue as that of the deep sea, and breaking in large white billows upon the strand, while the action of very high tides was quite manifest upon the face of the cliffs for more than a hundred miles.' after a description of several valleys, hills, mountains and forests, we come to the discovery of animal life. an oval valley surrounded by hills, red as the purest vermilion, is selected as the scene. 'small collections of trees, of every imaginable kind, were scattered about the whole of this luxuriant area; and here our magnifiers blessed our panting hopes with specimens of conscious existence. in the shade of the woods we beheld brown quadrupeds having all the external characteristics of the bison, but more diminutive than any species of the bos genus in our natural history.' then herds of agile creatures like antelopes are described, 'abounding on the acclivitous glades of the woods.' in the contemplation of these sprightly animals the narrator becomes quite lively. 'this beautiful creature,' says he, 'afforded us the most exquisite amusement. the mimicry of its movements upon our white painted canvas was as faithful and luminous as that of animals within a few yards of the camera obscura. frequently, when attempting to put our fingers upon its beard, it would suddenly bound away as if conscious of our earthly impertinence; but then others would appear, whom we could not prevent nibbling the herbage, say or do to them what we would.' a strange amphibious creature, of a spherical form, rolling with great velocity along a pebbly beach, is the next object of interest, but is presently lost sight of in a strong current setting off from the angle of an island. after this there are three or four pages descriptive of various lunar scenes and animals, the latter showing a tendency, singular considering the circumstances, though very convenient for the narrator, to become higher and higher in type as the discoveries proceed, until an animal somewhat of the nature of the missing link is discovered. it is found in the endymion (a circular walled plain) in company with a small kind of reindeer, the elk, the moose, and the horned bear, and is described as the biped beaver. it 'resembles the beaver of the earth in every other respect than in its destitution of a tail, and its invariable habit of walking upon only two feet. it carries its young in its arms like a human being, and moves with an easy gliding motion. its huts are constructed better and higher than those of many tribes of human savages, and, from the appearance of smoke in nearly all of them, there is no doubt of its being acquainted with the use of fire. still, its head and body differ only in the points stated from that of the beaver; and it was never seen except on the borders of lakes and rivers, in which it has been observed to immerse for a period of several seconds.' the next step towards the climax brings us to domestic animals, 'good large sheep, which would not have disgraced the farms of leicestershire or the shambles of leadenhall market; we fairly laughed at the recognition of so familiar an acquaintance in so distant a land. presently they appeared in great numbers, and, on reducing the lenses, we found them in flocks over a great part of the valley. i need not say how desirous we were of finding shepherds to these flocks, and even a man with blue apron and rolled-up sleeves would have been a welcome sight to us, if not to the sheep; but they fed in peace, lords of their own pastures, without either protector or destroyer in human shape.' in the meantime, discussion had arisen as to the lunar locality where men, or creatures resembling them, would most likely be found. herschel had a theory on the subject--viz., that just where the balancing or libratory swing of the moon brings into view the greatest extent beyond the eastern or western parts of that hemisphere which is turned earthwards in the moon's mean or average position, lunar inhabitants would probably be found, and nowhere else. this, by the way (speaking seriously), is a rather curious anticipation of a view long subsequently advanced by hansen, and for a time adopted by sir j. herschel, that possibly the remote hemisphere of the moon may be a fit abode for living creatures, the oceans and atmosphere which are wanting on the nearer hemisphere having been (on this hypothesis) drawn over to the remoter because of a displacement of the moon's centre of gravity. i ventured in one of my first books on astronomy to indicate objections to this theory, the force of which sir j. herschel admitted in a letter addressed to me on the subject. taking, then, an opportunity when the moon had just swung to the extreme limit of her balancing, or, to use technical terms, when she had attained her maximum libration in longitude, the observers approached the level opening to lake langrenus, as the narrator calls this fine walled plain, which, by the way, is fully thirty degrees of lunar longitude within the average western limit of the moon's visible hemisphere. 'here the valley narrows to a mile in width, and displays scenery on both sides picturesque and romantic beyond the powers of a prose description. imagination, borne on the wings of poetry, could alone gather similes to portray the wild sublimity of this landscape, where dark behemoth crags stood over the brows of lofty precipices, as if a rampart in the sky; and forests seemed suspended in mid-air. on the eastern side there was one soaring crag, crested with trees, which hung over in a curve like three-fourths of a gothic arch, and being of a rich crimson colour, its effect was most strange upon minds unaccustomed to the association of such grandeur with such beauty. but, whilst gazing upon them in a perspective of about half a mile, we were thrilled with astonishment to perceive four successive flocks of large winged creatures, wholly unlike any kind of birds, descend with a slow even motion from the cliffs on the western side and alight upon the plain. they were first noticed by dr. herschel, who exclaimed: "now, gentlemen, my theories against your proofs, which you have often found a pretty even bet, we have here something worth looking at. i was confident that if ever we found beings in human shape it would be in this longitude, and that they would be provided by their creator with some extraordinary powers of locomotion." ... we counted three parties of these creatures, of twelve, nine, and fifteen in each, walking erect towards a small wood near the base of the eastern precipices. certainly they _were_ like human beings, for their wings had now disappeared, and their attitude in walking was both erect and dignified.... they averaged four feet in height, were covered, except on the face, with short and glossy copper-coloured hair, lying snugly upon their backs, from the top of the shoulders to the calves of the legs. the face, which was of a yellowish flesh colour, was a slight improvement upon that of the large orang outang, being more open and intelligent in its expression, and having a much greater expansion of forehead. the mouth, however, was very prominent, though somewhat relieved by a thick beard upon the lower jaw, and by lips far more human than those of any species of the simia genus. in general symmetry of body and limbs they were infinitely superior to the orang outang; so much so, that, but for their long wings, lieutenant drummond said they would look as well on a parade ground as some of the old cockney militia.... these creatures were evidently engaged in conversation; their gesticulation, more particularly the varied action of their hands and arms, appeared impassioned and emphatic. we hence inferred that they were rational beings, and, although not perhaps of so high an order as others which we discovered the next month on the shores of the bay of rainbows, that they were capable of producing works of art and contrivance.... they possessed wings of great expansion, similar in construction to those of the bat, being a semi-transparent membrane united in curvilinear divisions by means of straight radii, united at the back by the dorsal integuments. but what astonished us very much was the circumstance of this membrane being continued from the shoulders to the legs, united all the way down, though gradually decreasing in width' (very much as fuseli depicted the wings of his satanic majesty, though h.s.m. would seem to have the advantage of the lunar bat-men in not being influenced by gravity[ ]). 'the wings seemed completely under the command of volition, for those of the creatures whom we saw bathing in the water spread them instantly to their full width, waved them as ducks do theirs to shake off the water, and then as instantly closed them again in a compact form. our further observation of the habits of these creatures, who were of both sexes, led to results so very remarkable, that i prefer they should be first laid before the public in dr. herschel's own work, where i have reason to know they are fully and faithfully stated, however incredulously they may be received.... we scientifically denominated them the vespertilio-homo or bat-man; and they are doubtless innocent and happy creatures, notwithstanding that some of their amusements would but ill comport with our terrestrial notions of decorum.' the omitted passages were suppressed in obedience to dr. grant's private injunction. 'these, however, and other prohibited passages,' were to be presently 'published by dr. herschel, with the certificates of the civil and military authorities of the colony, and of several episcopal, wesleyan, and other ministers, who in the month of march last were permitted, under stipulation of temporary secrecy, to visit the observatory, and become eye-witnesses of the wonders which they were requested to attest. we are confident that his forthcoming volumes will be at once the most sublime in science, and the most intense in general interest, that ever issued from the press.' the actual climax of the narrative, however, is not yet reached. the inhabitants of langrenus, though rational, do not belong to the highest orders of intelligent lunarians. herschel, ever ready with theories, had pointed out that probably the most cultivated races would be found residing on the slopes of some active volcano, and, in particular, that the proximity of the flaming mountain bullialdus (about twenty degrees south and ten east of the vast crater tycho, the centre whence extend those great radiations which give to the moon something of the appearance of a peeled orange) 'must be so great a local convenience to dwellers in this valley during the long periodical absence of solar light, as to render it a place of popular resort for the inhabitants of all the adjacent regions, more especially as its bulwark of hills afforded an infallible security against any volcanic eruption that could occur.' our observers therefore applied their full power to explore it. 'rich, indeed, was our reward. the very first object in this valley that appeared upon our canvas was a magnificent work of art. it was a temple--a fane of devotion or of science, which, when consecrated to the creator, is devotion of the loftiest order, for it exhibits his attributes purely, free from the masquerade attire and blasphemous caricature of controversial creeds, and has the seal and signature of his own hand to sanction its aspirations. it was an equi-angular temple, built of polished sapphire, or of some resplendent blue stone, which, like it, displayed a myriad point of golden light twinkling and scintillating in the sunbeams.... the roof was composed of yellow metal, and divided into three compartments, which were not triangular planes inclining to the centre, but subdivided, curved, and separated so as to present a mass of violently agitated flames rising from a common source of conflagration, and terminating in wildly waving points. this design was too manifest and too skilfully executed to be mistaken for a single moment. through a few openings in these metallic flames we perceived a large sphere of a darker kind of metal nearly of a clouded copper colour, which they enclosed and seemingly raged around, as if hieroglyphically consuming it.... what did the ingenious builders mean by the globe surrounded by flames? did they, by this, record any past calamity of _their_ world, or predict any future one of _ours_?' (why, by the way, should the past theory be assigned to the moon and the future one to our earth?) 'i by no means despair of ultimately solving not only these but a thousand other questions which present themselves respecting the objects in this planet; for not the millionth part of her surface has yet been explored, and we have been more desirous of collecting the greatest possible number of new facts than of indulging in speculative theories, however seductive to the imagination.' after this we have an account of the behaviour of the vespertilio-homo at meals. 'they seemed eminently happy, and even polite; for individuals would select large and bright specimens of fruit, and throw them archwise across to some friend who had extracted the nutriment from those scattered around him.' however, the lunar men are not on the whole particularly interesting beings according to this account. 'so far as we could judge, they spent their happy hours in collecting various fruits in the woods, in eating, flying, bathing, and loitering about the summits of precipices.' one may say of them what huxley is reported to have said of the spirits as described by spiritualists, that no student of science would care to waste his time inquiring about such a stupid set of people. such are the more interesting and characteristic portions of a narrative, running in the original to forty or fifty large octavo pages. in its day the story attracted a good deal of notice, and, even when every one had learned the trick, many were still interested in a _brochure_ which was so cleverly conceived and had deceived so many. to this day the lunar hoax is talked of in america, where originally it had its chief--or, one may rather say, its only real--success as a hoax. it reached england too late to deceive any but those who were unacquainted with herschel's real doings, and no editors of public journals, i believe, gave countenance to it at all. in america, on the contrary, many editors gave the narrative a distinguished place in their columns. some indeed expressed doubts, and others followed the safe course of the 'philadelphia inquirer,' which informed its readers that 'after an attentive perusal of the whole story they could decide for themselves;' adding that, 'whether true or false, the narrative is written with consummate ability and possesses intense interest.' but others were more credulous. according to the 'mercantile advertiser' the story carried 'intrinsic evidence of being an authentic document.' the 'albany daily advertiser' had read the article 'with unspeakable emotions of pleasure and astonishment.' the 'new york times' announced that 'the writer (dr. andrew grant) displays the most extensive and accurate knowledge of astronomy; and the description of sir john's recently improved instruments, the principle on which the inestimable improvements were founded, the account of the wonderful discoveries in the moon, etc., all are probable and plausible, and have an air of intense verisimilitude.' the 'new yorker' considered the discoveries 'of astounding interest, creating a new era in astronomy and science generally.'[ ] in our time a trick of the kind could hardly be expected to succeed so well, even if as cleverly devised and as well executed. the facts of popular astronomy and of general popular science have been more widely disseminated. america, too, more than any other great nation, has advanced in the interval. it was about two years after this pamphlet had appeared, that j. quincy adams used the following significant language in advocating the erection of an astronomical observatory at washington: 'it is with no feeling of pride as an american that the remark may be made, that on the comparatively small territorial surface of europe there are existing more than of these lighthouses of the skies; while throughout the whole american hemisphere there is but one.' at present, some of the finest observatories in the world belong to american cities, or are attached to american colleges; and much of the most interesting astronomical work of this country has been achieved by american observers. yet we still hear from time to time of the attempted publication of hoaxes of greater or less ingenuity. it is singular (and i think significant) how often these relate to the moon. there would seem to be some charm about our satellite for the minds of paradoxists and hoaxers generally. nor are these tricks invariably detected at once by the general public, or even by persons of some culture. i remember being gravely asked (in january ) whether an account given in the 'new york world,' purporting to describe how the moon's frame was gradually cracking, threatening eventually to fall into several separate fragments, was in reality based on fact. in the far west, at lincoln, nebraska, a lawyer asked me, not long since, why i had not described the great discoveries recently made by means of a powerful reflector erected near paris. according to the 'chicago times,' this powerful instrument had shown buildings in the moon, and bands of workers could be seen with it who manifestly were undergoing some kind of penal servitude, for they were chained together. it was clear, from the presence of these and the absence of other inhabitants, that the side of the moon turned earthwards is a dreary and unpleasant place of abode, the real 'happy hunting grounds' of the moon lying on her remote and unseen hemisphere. as gauges of general knowledge, scientific hoaxes have their uses, just as paradoxical works have. no one, certainly no student of science, can thoroughly understand how little some persons know about science, until he has observed how much will be believed, if only published with the apparent authority of a few known names, and announced with a sufficient parade of technical verbiage; nor is it so easy as might be thought, even for those who are acquainted with the facts, to disprove either a hoax or a paradox. nothing, indeed, can much more thoroughly perplex and confound a student of science than to be asked to prove, for example, that the earth is not flat, or the moon not inhabited by creatures like ourselves; for the circumstance that such a question is asked implies ignorance so thorough of the very facts on which the proof must be based, as to render argument all but hopeless from the outset. i have had a somewhat wide experience of paradoxists, and have noted the experience of de morgan and others who, like him, have tried to convince them of their folly. the conclusion at which i have arrived is, that to make a rope of sand were an easy task compared with the attempt to instil the simpler facts of science into paradoxical heads. i would make some remarks, in conclusion, upon scientific or quasi-scientific papers not intended to deceive, but yet presenting imaginary scenes, events, and so forth, described more or less in accordance with scientific facts. imaginary journeys to the sun, moon, planets, and stars; travels over regions on the earth as yet unexplored; voyages under the sea, through the bowels of the earth, and other such narratives, may, perhaps, be sometimes usefully written and read, so long as certain conditions are fulfilled by the narrator. in the first place, while adopting, to preserve the unities, the tone of one relating facts which actually occurred, he should not suffer even the simplest among his readers to lie under the least misapprehension as to the true nature of the narrative. again, since of necessity established facts must in such a narrative appear in company with the results of more or less probable surmise, the reader should have some means of distinguishing where fact ends and surmise begins. for example, in a paper i once wrote, entitled 'a journey to saturn,' i was not sufficiently careful to note that while the appearances described in the approach towards the planet were in reality based on the observed appearances as higher and higher telescopic powers are applied to the planet, others supposed to have been seen by the visitors to saturn when actually within his system, were only such as might possibly or probably be seen, but for which we have no real evidence. in consequence of this omission, i received several inquiries about these matters. 'is it true,' some wrote, 'that the small satellite hyperion' (scarce discernible in powerful telescopes, while titan and japetus on either side are large) 'is only one of a ring of small satellites travelling between the orbits of the larger moons?'--as the same planets travel between the paths of mars and jupiter. others asked on what grounds it was said that the voyagers found small moons circling about titan, the giant moon of the saturnian system, as the moons of jupiter and saturn circle around those giant members of the solar system. in each case, i was reduced to the abject necessity of explaining that there was no evidence for the alleged state of things, which, however, might nevertheless exist. scientific fiction which has to be interpreted in that way is as bad as a joke that has to be explained. in my 'journey to the sun' i was more successful (it was the earlier essay, however); insomuch that professor young, of dartmouth college (hanover, n.h.), one of the most skilful solar observers living, assured me that, with scarcely a single exception, the various phenomena described corresponded exactly with the ideas he had formed respecting the probable condition of our luminary.[ ] but i must confess that my own experience has not been, on the whole, favourable to that kind of popular science writing. it appears to me that the more thoroughly the writer of such an essay has studied any particular scientific subject, the less able must he be to write a fictitious narrative respecting it. just as those ignorant of any subject are often the readiest to theorise about it, because least hampered by exact knowledge, so i think that the careful avoidance of any exact study of the details of a scientific subject must greatly facilitate the writing of a fictitious narrative respecting it. but unfortunately a narrative written under such conditions, however interesting to the general reader, can scarcely forward the propagation of scientific knowledge, one of the qualities claimed for fables of the kind. as an instance in point, i may cite jules verne's 'voyage to the moon,' where (apart, of course, from the inherent and intentional absurdity of the scheme itself), the circumstances which are described are calculated to give entirely erroneous ideas about the laws of motion. nothing could be more amusing, but at the same time nothing more scientifically absurd, than the story of the dead dog satellite, which, flung out of the travelling projectile, becomes a veritable satellite, moving always beside the voyagers; for, with whatever velocity the dog had been expelled by them, with that same velocity would he have retreated continually from their projectile abode, whose own attraction on the dog would have had no appreciable effect in checking his departure. again, the scene when the projectile reaches the neutral point between the earth and moon, so that there is no longer any gravity to keep the travellers on the floor of their travelling car, is well conceived (though, in part, somewhat profane); but in reality the state of things described as occurring there would have prevailed throughout the journey. the travellers would no more be drawn earthwards (as compared with the projectile itself) than we travellers on the earth are drawn sunwards with reference to the earth. the earth's attracting force on the projectile and on the travellers would be equal all through the journey, not solely when the projectile reached the neutral point; and being equal on both, would not draw them together. it may be argued that the attractions were equal before the projectile set out on its journey, and therefore, if the reasoning just given were correct, the travellers ought not to have had any weight keeping them on the floor of the projectile before it started, 'which is absurd.' but the pressure upon the floor of the projectile at rest is caused by the floor being kept from moving; let it be free to obey gravity, and there will no longer be any pressure: and throughout the journey to the moon, the projectile, like the travellers it contains, is obeying the action of gravity. unfortunately, those who are able to follow the correct reasoning in such matters are not those to whom jules verne's account would suggest wrong ideas about matters dynamical; the young learner who _is_ misled by such narratives is neither able to reason out the matter for himself, nor to understand the true reasoning respecting it. he is, therefore, apt to be set quite at sea by stories of the kind, and especially by the specious reasoning introduced to explain the events described. in fine, it would seem that such narratives must be valued for their intrinsic interest, just like other novels or romances, not for the quality sometimes claimed for them of combining instruction with amusement. x. _on some astronomical paradoxes._ for many years the late professor de morgan contributed to the columns of the 'athenæum' a series of papers in which he dealt with the strange treatises in which the earth is flattened, the circle squared, the angle divided into three, the cube doubled (the famous problem which the delphic oracle set astronomers), and the whole of modern astronomy shown to be a delusion and a snare. he treated these works in a quaint fashion: not unkindly, for his was a kindly nature; not even earnestly, though he was thoroughly in earnest; yet in such sort as to rouse the indignation of the unfortunate paradoxists. he was abused roundly for what he said, but much more roundly when he declined further controversy. paradoxists of the ignorant sort (for it must be remembered that not all are ignorant) are, indeed, well practised in abuse, and have long learned to call mathematicians and astronomers cheats and charlatans. they freely used their vocabulary for the benefit of de morgan, whom they denounced as a scurrilous scribbler, a defamatory, dishonest, abusive, ungentlemanly, and libellous trickster. he bore this shower of abuse with exceeding patience and good nature. he had not been wholly unprepared for it, in fact; and, as he had a purpose in dealing with the paradoxists, he was satisfied to continue that quiet analysis of their work which so roused their indignation. he found in them a curious subject of study; and he found an equally curious subject of study in their disciples. the simpler--not to say more foolish--paradoxists, whose wonderful discoveries are merely amazing misapprehensions, were even more interesting to de morgan than the craftier sort who make a living, or try to make a living, out of their pretended theories. indeed, these last he treated, as they deserved, with a scathing satire quite different from his humorous and not ungenial comments on the wonderful theories of the honest paradoxists. there is one special use to which the study of paradox-literature may be applied, which--so far as i know--has not hitherto been much attended to. it may be questioned whether half the strange notions into which paradoxists fall must not be ascribed to the vagueness of too many of our scientific treatises. a half-understood explanation, or a carelessly worded account of some natural phenomenon, leads the paradoxist, whose nature is compounded of conceit and simplicity, to originate a theory of his own on the subject. once such a theory has been devised, it takes complete possession of the paradoxist's mind. all the facts of which he thenceforward hears, which bear in the least on his favourite craze, appear to give evidence in its favour, even though in reality they are most obviously opposed to it. he learns to look upon himself as an unappreciated newton, and to see the bitterest malevolence in those who venture to question his preposterous notions. he is fortunate if he do not suffer his theories to withdraw him from his means of earning a livelihood, or if he do not waste his substance in propounding and defending them. one of the favourite subjects for paradox-forming is the accepted theory of the solar system. our books on astronomy too often present this theory in such sort that it seems only a _successor_ of ptolemy's; and the impression is conveyed that, like ptolemy's, it may be one day superseded by some other theory. this is quite enough for the paradoxist. if a new theory is to replace the one now accepted, why should not _he_ be the new copernicus? he starts upon the road without a tithe of the knowledge that old ptolemy possessed, unaware of the difficulties which ptolemy met and dealt with--free, therefore, because of his perfect ignorance, to form theories at which ptolemy would have smiled. he has probably heard of the centrics and eccentrics scribbled o'er cycle and epicycle, orb in orb, which disfigured the theories of the ancients; but he is quite unconscious that every one of those scribblings had a real meaning, each being intended to account for some observed peculiarity of planetary motion, which _must_ be accounted for by any theory which is to claim acceptance. in this happy unconsciousness that there are any peculiarities requiring explanation, knowing nothing of the strange paths which the planets are seen to follow on the heavenly vault, their wand'ring course now high, now low, then hid, progressive, retrograde, or standing still, he placidly puts forward--and presently very vehemently urges--a theory which accounts for none of these things. it has often seemed to me that a large part of the mischief--for let it be remembered that the published errors of the paradoxist are indicative of much unpublished misapprehension--arises from the undeserved contempt with which our books of astronomy too often treat the labours of ptolemy, tycho brahe, and others who advocated erroneous theories. if the simple truth were told, that the theory of ptolemy was a masterpiece of ingenuity and that it was worked out by his followers in a way which merits the highest possible praise, while the theory of tycho brahe was placed in reality on a sounder basis than that of copernicus, and accounted as well and as simply for observed appearances, the student would begin to realise the noble nature of the problem which those great astronomers dealt with. and again, if stress were laid upon the fact that tycho brahe devoted years upon years of his life to secure such observations of the planets as might settle the questions at issue, the student would learn something of the spirit in which the true lover of science proceeds. it seems to me, also, that far too little is said about the kind of work by which kepler and newton finally established the accepted theories. there is a strange charm in the history of those twenty years of kepler's life during which he was analysing the observations made by tycho brahe. surrounded with domestic trials and anxieties, which might well have claimed his whole attention, tried grievously by ill-health and bodily anguish, he laboured all those years upon erroneous theories. the very worst of these had infinitely more evidence in its favour than the best which the paradoxists have brought forth. there was not one of those theories which nine out of ten of his scientific contemporaries would not have accepted ungrudgingly. yet he wrought these theories one after another to their own disproof. _nineteen_ of them he tried and rejected--the twentieth was the true theory of the solar system. perhaps nothing in the whole history of astronomy affords a nobler lesson to the student of science--unless, indeed, it be the calm philosophy with which newton for eighteen years suffered the theory of the universe to remain in abeyance, because faulty measurements of the earth prevented his calculations from agreeing with observed facts. but, as professor tyndall has well remarked--and the paradoxist should lay the lesson well to heart--'newton's action in this matter was the normal action of the scientific mind. if it were otherwise--if scientific men were not accustomed to demand verification, if they were satisfied with the imperfect while the perfect is attainable--their science, instead of being, as it is, a fortress of adamant, would be a house of clay, ill fitted to bear the buffetings of the theologic storms to which it has been from time to time, and is at present, exposed.' the fame of newton has proved to many paradoxists an irresistible attraction; it has been to these unfortunates as the candle to the fluttering moth. circle-squaring, as we shall presently see, has had its attractions, nor have earth-fixing and earth-flattening been neglected; but attacking the law of gravitation has been the favourite work of paradoxists. newton has been praised as surpassing the whole human race in genius; mathematicians and astronomers have agreed to laud him as unequalled; why should not paradoxus displace him and be praised in like manner? it would be unfair, perhaps, to say that the paradoxist consciously argues thus. he doubtless in most instances convinces himself that he has really detected some flaw in the theory of gravitation. yet it is impossible not to recognise, as the real motive of every paradox-monger, the desire to have that said of him which has been said of newton: '_genus humanum ingenio superavit._' i remember a curious instance of this which occurred soon after the appearance of the comet of . it chanced that, while that object was under discussion, reference was made to the action of a repulsive force exerted by the sun upon the matter of the comet's tail. on this, some one addressed a long letter to a glasgow newspaper, announcing that he had long ago proved that the sun's attraction alone is insufficient to account for the planetary motions. his reasoning was amazingly simple. if the sun's attraction is powerful enough to keep the outer planets in their course, it must be too powerful for venus and mercury close by the sun; if it only just suffices to keep these in their course, it cannot possibly be powerful enough to restrain the outer planets. the writer of this letter said that he had been very badly treated by scientific bodies. he had announced his discovery to the royal astronomical society, the royal society, the imperial academy at paris, and other scientific bodies; but they had one and all refused to listen to him. he had forsaken or neglected his trade for several years in order to give attention to the new and (as he thought) the true theory of the universe. he complained in a specially bitter manner of the unfavourable comments which men of science had made upon his views in private letters addressed to him in reply to his communications. there is something melancholy even in what is most ridiculous in cases of this sort. the simplicity which supposes that considerations so obvious as those adduced could escape the scrutiny, not of newton only, but of all who have followed in the same track during two centuries, is certainly stupendous; nor can one fail to smile at seeing a difficulty, such as might naturally suggest itself to a beginner, and such as half-a-dozen words from an expert would clear up, regarded gravely as a discovery calculated to make its author famous for all time. yet, when one considers the probable consequences of the blunder to the unhappy enthusiast, and perchance to his family, it is difficult not to feel a sense of pity, quite apart from that pity allied to contempt which is excited by his mistake. a few words added to the account of newton's theory, which the paradoxist had probably read in some astronomical treatise, would have prevented all this mischief. indeed, this difficulty, which, as we have said, is a natural one, should be dealt with and removed in any account of the planetary system intended for beginners. the simple statement that the outer planets move more slowly than the inner, and so _require_ a smaller force to keep them in their course, would have sufficed, not, perhaps, altogether to remove the difficulty, but to show the beginner where the explanation was to be looked for. it was in connection with this subject of gravitation that one of the most well-meaning of the paradoxists--the late mr. james reddie--came under professor de morgan's criticism. mr. reddie was something more than well-meaning. he was earnestly desirous of advancing the interests of science, as well as of defending religion from what he mistakenly supposed to be the dangerous teachings of the newtonians. he founded for these purposes the victoria institute, of which society he was the secretary from the time of its institution until his decease, some years since; and, probably, many who declined to join that society because of the anti-newtonian proclivities of its secretary, were unaware that to that secretary the institute owed its existence. it so chanced that i had myself a good deal of correspondence with mr. reddie (who was, however, personally unknown to me). this correspondence served to throw quite a new light on the mental habitudes and ways of thinking of the honest paradoxist. i believe that professor de morgan hardly gave mr. reddie credit for the perfect honesty which he really possessed. it may have been that a clear reasoner like de morgan could hardly (despite his wide experience) appreciate the confusion of mind which is the normal characteristic of the paradoxist. but certainly the very candid way in which mr. reddie admitted, in the correspondence above named, that he had not known some facts and had misunderstood others, afforded to my mind the most satisfactory proofs of his straightforwardness. it may be instructive to consider a few of those paradoxes of mr. reddie's which professor de morgan found chief occasion to pulverise. in a letter to the astronomer-royal mr. reddie announced that he was about to write 'a paper intended to be hereafter published, elaborating more minutely and discussing more rigidly than before the glaring fallacies, dating from the time of newton, relating to the motion of the moon.' he proceeded to 'indicate the nature of the issues he intended to raise.' he had discovered that the moon does not, as a matter of fact, go round the earth at the rate of miles an hour, as astronomers say, but follows an undulatory path round the sun at a rate varying between , and , miles an hour; because, while the moon seems to go round the earth, the latter is travelling onwards at the rate of , miles an hour round the sun. of course he was quite right in his facts, and quite wrong in his inferences; as the astronomer-royal pointed out in a brief letter, closing with the remark that, 'as a very closely occupied man,' mr. airy could 'not enter further into the matter.' but further mr. reddie persisted in going, though he received no more letters from greenwich. his reply to sir g. airy contained, in fact, matter enough for a small pamphlet. now here was certainly an amazing fact. a well-known astronomical relation, which astronomers have over and over again described and explained, is treated as though it were something which had throughout all ages escaped attention. it is not here the failure to comprehend the _rationale_ of a simple explanation which is startling, but the notion that an obvious fact had been wholly overlooked. of like nature was the mistake which brought mr. reddie more especially under professor de morgan's notice. it is known that the sun, carrying with him his family of planets, is speeding swiftly through space--his velocity being estimated as probably not falling short of , miles per hour. it follows, of course, that the real paths of the planets in space are not closed curves, but spirals of different orders. how, then, can the theory of copernicus be right, according to which the planets circle in closed orbits round the sun? here was mr. reddie's difficulty; and like the other, it appeared to his mind as a great discovery. he was no whit concerned by the thought that astronomers ought surely to have noticed the difficulty before. it did not seem in the least wonderful that he, lightly reading a book or two of popular astronomy, should discover that which laplace, the herschels, leverrier, airy, adams, and a host of others, who have given their whole lives to astronomy, had failed to notice. accordingly, mr. reddie forwarded to the british association (in session at newcastle) a paper controverting the theory of the sun's motion. the paper was declined with thanks by that bigoted body 'as opposed to newtonian astronomy.' 'that paper i published,' says mr. reddie, 'in september , with an appendix, in both thoroughly exhibiting the illogical reasoning and absurdities involved in the theory; and with what result? the members of section a of the british association, and fellows of the royal society and of the royal astronomical society, to whom i sent copies of my paper, were, without exception, _dumb_.' professor de morgan, however, having occasion to examine mr. reddie's publications some time after, was in no sort dumb, but in very plain and definite terms exhibited their absurdity. after all, however, the real absurdity consisted, not in the statements which mr. reddie made, nor even in the conclusions which he drew from them, but in the astounding simplicity which could suppose that astronomers were unaware of the facts which their own labours had revealed. in my correspondence with mr. reddie i recognised the real source of the amazing self-complacency displayed by the true paradoxist. the very insufficiency of the knowledge which a paradoxist possesses of his subject, affords the measure of his estimate of the care with which other men have studied that subject. because the paradoxist is ready to pronounce an opinion about matters he has not studied, it does not seem strange to him that newton and his followers should be equally ready to discuss subjects they had not inquired into. another very remarkable instance was afforded by mr. reddie's treatment of the subject of comets. and here, by the way, i shall quote a remark made by sir john herschel soon after the appearance of the comet of . 'i have received letters,' he said, 'about the comets of the last few years, enough to make one's hair stand on end at the absurdity of the theories they propose, and at the ignorance of the commonest laws of optics, of motion, of heat, and of general physics, they betray in their writers.' in the present instance, the correspondence showed that the paradoxist supposed the parabolic paths of some comets to be regarded by astronomers as analogous to the parabolic paths traversed by projectiles. he expressed considerable astonishment when i informed him that, in the first place, projectiles do not travel on truly parabolic paths; and secondly, that in all respects their motion differs essentially from that which astronomers ascribe to comets. these last move more and more quickly until they reach what is called the vertex of the parabola (the point of such a path which lies nearest to the sun): projectiles, on the contrary, move more and more slowly as they approach the corresponding point of their path; and further, the comet first approaches and then recedes from the centre of attraction--the projectile first recedes from and then approaches the attracting centre. the earth-flatteners form a considerable section of the paradoxical family. they experienced a practical rebuff, a few years since, which should to some degree have shaken their faith in the present chief of their order. to do this chief justice, he is probably far less confident about the flatness of the earth than any of his disciples. under the assumed name of parallax he visited most of the chief towns of england, propounding what he calls his system of zetetic astronomy. why he should call himself parallax it would be hard to say; unless it be that the verb from which the word is derived signifies primarily to shift about or dodge, and secondarily to alter a little, especially for the worse. his employment of the word zetetic is less doubtful, as he claims for his system that it alone is founded on the true seeking out of nature's secrets. the experimental basis of the theory of parallax is mainly this: having betaken himself to a part of the bedford canal, where there is an uninterrupted water-line of about six miles, he tested the water surface for signs of curvature, and (as he said) found none. it chanced, unfortunately, that a disciple--mr. john hampden, of swindon--accepted the narrative of this observation in an unquestioning spirit; and was so confident that the bedford canal has a truly plane surface, that he wagered five hundred pounds on his opinion, challenging the believers in the earth's rotundity to repeat the experiment. the challenge was accepted by mr. wallace, the eminent naturalist; and the result may be anticipated. three boats were to be moored in a line, three miles or so between each. each carried a mast of given length. if, when the summits of the first and last masts were seen in a line through a telescope, the summit of the middle mast was not found to be above the line, then mr. hampden was to receive five hundred pounds from mr. wallace. if, on the contrary, the top of the middle mast was found, as the accepted theory said it should be, to be several feet above the line joining the tops of the two outer masts, then mr. hampden was to lose the five hundred pounds he had so rashly ventured. everything was conducted in accordance with the arrangements agreed upon. the editor of a well-known sporting paper acted as stakeholder, and unprejudiced umpires were to decide as to what actually was seen through the telescope. it need scarcely be said that the accepted theory held its own, and that mr. hampden lost his money. he scarcely bore the loss with so good a grace as was to have been expected from a philosopher merely desirous of ascertaining the truth. his wrath was not expended on parallax, whom he might have suspected of having led him astray; nor does he seem to have been angry with himself, as would have seemed natural. all his anger was reserved for those who still continued to believe in the earth's rotundity. whether he believed that the bedford water had risen under the middle boat to oblige mr. wallace, or how it came to pass that his own chosen experiment had failed him, does not appear. the subsequent history of this matter has been unpleasant. it illustrates, unfortunately but too well, the mischief which may ensue from the tricks of those who make a trade of paradox--tricks which would be scarce possible, however, if text-books of science were more carefully written, and by those only who are really acquainted with the subject of which they treat. the book which originally led to mr. hampden's misfortunes, and has misled not a few, ought to have deceived none. i have already mentioned the statement on which parallax (whose true name is rowbotham) rested his theory. of course, if that statement had been true--if he had, with his eye a few inches from the surface of the water of the bedford canal, seen an object close to the surface six miles from him--there manifestly would have been something wrong in the accepted theory about the earth's rotundity. so, also, if a writer were to announce a new theory of gravity, stating as the basis of his theory that a heavy missile which he had thrown into the air had gone upwards on a serpentine course to the moon, any one who accepted the statement would be logically bound to admit at least that the fact described was inconsistent with the accepted theory. but no one would accept such a statement; and no one should have accepted mr. rowbotham's statement. his statement was believed, however, and perhaps is still believed by many. twenty years ago de morgan wrote that 'the founder of the zetetic astronomy gained great praise from provincial newspapers for his ingenuity in proving that the earth is a flat, surrounded by ice,' with the north polar ice in the middle. 'some of the journals rather incline to this view; but the "leicester advertiser" thinks that the statement "would seem to invalidate some of the most important conclusions of modern astronomy;" while the "norfolk herald" is clear that "there must be great error on one side or the other." ... the fact is worth noting that from - arguments on the roundness or flatness of the earth did itinerate. i have no doubt they did much good, for very few persons have any distinct idea of the evidence for the rotundity of the earth. the "blackburn standard" and "preston guardian" (december and , ) unite in stating that the lecturer ran away from his second lecture at burnley, having been rather too hard pressed, at the end of his first lecture, to explain why the large hull of a ship disappeared before the masts. the persons present and waiting for the second lecture assuaged their disappointment by concluding that the lecturer had slipped off the ice edge of his flat disc, and that he would not be seen again till he peeped up on the opposite side.' ... 'the zetetic system,' proceeds de morgan, 'still lives in lectures and books; as it ought to do, for there is no way of teaching a truth comparable to opposition. the last i heard of it was in lectures at plymouth, in october . since this time a prospectus has been issued of a work entitled "the earth not a globe;" but whether it has been published i do not know.' the book was published soon after the above was written, and de morgan gives the following quaint account of it: 'august , . the zetetic astronomy has come into my hands. when in i went to see the great exhibition i heard an organ played by a performer who seemed very desirous of exhibiting one particular stop. "what do you think of that stop?" i was asked. "that depends on the name of it," said i "oh! what can the name of it have to do with the sound? 'that which we call a rose,' etc." "the name has everything to do with it: if it be a flute stop i think it very harsh; but if it be a railway-whistle stop, i think it very sweet." so as to this book: if it be childish, it is clever; if it be mannish, it is unusually foolish. the flat earth floating tremulously on the sea; the sun moving always over the flat, giving day when near enough, and night when too far off; the self-luminous moon, with a semi-transparent invisible moon created to give her an eclipse now and then; the new law of perspective, by which the vanishing of the hull before the masts, usually thought to prove the earth globular, really proves it flat;--all these and other things are well fitted to form exercises for a person who is learning the elements of astronomy. the manner in which the sun dips into the sea, especially in tropical climates, upsets the whole. mungo park, i think, gives an african hypothesis which explains phenomena better than this. the sun dips into the western ocean, and the people there cut him in pieces, fry him in a pan, and then join him together again; take him round the under way, and set him up in the east. i hope this book will be read, and that many will be puzzled by it; for there are many whose notions of astronomy deserve no better fate. there is no subject on which there is so little accurate conception as on that of the motions of the heavenly bodies.[ ] the author, though confident in the extreme, neither impeaches the honesty of those whose opinion he assails, nor allots them any future inconvenience: in these points he is worthy to live on a globe and to rotate in twenty-four hours.' i chanced to reside near plymouth when mr. rowbotham lectured there in october . it will readily be understood that, in a town where there are so many naval men, his lectures were not altogether so successful as they have sometimes been in small inland towns. numbers of naval officers, however, who were thoroughly well assured of the fact that the earth is a globe, were not able to demolish the crafty arguments of parallax publicly, during the discussions which he challenged at the close of each lecture. he was too skilled in that sort of evasion which his assumed name (as interpreted by liddell and scott) suggests, to be readily cornered. when an argument was used which he could not easily meet, or seem to meet, he would say simply: 'well, sir, you have now had your fair share of the discussion; let some one else have his turn.' it was stated in the newspapers that one of his audience was so wrathful with the lecturer on account of these evasions, that he endeavoured to strike parallax with a knobbed stick at the close of the second lecture; but probably there was no real foundation for the story. mr. rowbotham did a very bold thing, however, at plymouth. he undertook to prove, by observations made with a telescope upon the eddystone lighthouse from the hoe and from the beach, that the surface of the water is flat. from the beach usually only the lantern can be seen. from the hoe the whole of the lighthouse is visible under favourable conditions. duly on the morning appointed, mr. rowbotham appeared. from the hoe a telescope was directed towards the lighthouse, which was well seen, the morning being calm and still, and tolerably clear. on descending to the beach it was found that, instead of the whole lantern being visible as usual, only half could be seen--a circumstance doubtless due to the fact that the air's refractive power, which usually diminishes the dip due to the earth's curvature by about one-sixth part, was less efficient that morning than usual. the effect of the peculiarity was manifestly unfavourable to mr. rowbotham's theory. the curvature of the earth produced a greater difference than usual between the appearance of a distant object as seen from a certain high station and from a certain low station (though still the difference fell short of that which would be shown if there were no air). but parallax claimed the peculiarity observable that morning as an argument in favour of his flat earth. it is manifest, he said, that there is something wrong about the accepted theory; for it tells us that so much less of the lighthouse should be seen from the beach than from the hoe, whereas less still was seen. and many of the plymouth folk went away from the hoe that morning, and from the second lecture, in which parallax triumphantly quoted the results of the observation, with the feeling which had been expressed seven years before in the 'leicester advertiser,' that 'some of the most important conclusions of modern astronomy had been seriously invalidated.' if our books of astronomy, in referring to the effects of the earth's curvature, had only been careful to point out how surveyors and sailors and those who build lighthouses take into account the modifying effects of atmospheric refraction, and how these effects have long been known to vary with the temperature and pressure of the air, this mischief would have been avoided. it would not be fair to say of the persons misled on that occasion by parallax that they deserved no better; since the fault is not theirs as readers, but that of careless or ill-informed writers. another experiment conducted by parallax the same morning was creditable to his ingenuity. nothing better, perhaps, was ever devised to deceive people, apparently by ocular evidence, into the belief that the earth is flat--nor is there any clearer evidence of the largeness of the earth's globe compared with our ordinary measures. on the hoe, some ninety or a hundred feet above the sea-level, he had a mirror suspended in a vertical position facing the sea, and invited the bystanders to look in that mirror at the sea-horizon. to all appearance the line of the horizon corresponded exactly with the level of the eye-pupils of the observer. now, of course, when we look into a mirror whose surface is exactly vertical, the line of sight to the eye-pupils of our image in the mirror is exactly horizontal; whereas the line of sight from the eyes to the image of the sea-horizon is depressed exactly as much as the line from the eyes to the real sea-horizon. here, then, seemed to be proof positive that there is no depression of the sea-horizon; for the horizontal line to the image of the eye-pupil seemed to coincide exactly with the line to the image of the sea-horizon. it is not necessary to suppose here that the mirror was wrongly adjusted, though the slightest error of adjustment would affect the result either favourably or unfavourably for parallax's flat-earth theory. it is a matter of fact that, if the mirror were perfectly vertical, only very acute vision could detect the depression of the image of the sea-horizon below the image of the eye-pupil. the depression can easily be calculated for any given circumstances. parallax encouraged observers to note very closely the position of the eye-pupil in the image, so that most of them approached the image within about ten inches, or the glass within about five. now, in such a case, for a height of one hundred feet above the sea-level the image of the sea-horizon would be depressed below the image of the eye-pupil by less than three hundredths of an inch--an amount which could not be detected by one eye in a hundred. the average diameter of the pupil itself is one-fifth of an inch, or about seven times as great as the depression of the sea-horizon in the case supposed. it would require very close observation and a good eye to determine whether a horizontal line seen on either side of the head were on the level of the centres of the eye-pupils, or lower by about one-seventh of the breadth of either pupil. the experiment is a pretty one, however, and well worth trying by any one who lives near to the sea-shore and sea-cliffs. but there is a much more effective experiment which can be much more easily tried--only it is open to the disadvantage that it at once demolishes the argument of our friend parallax. it occurred to me while i was writing the above paragraph. let a very small mirror (it need not be larger than a sixpence) be so suspended to a small support and so weighted that when left to itself it hangs with its face perfectly vertical--an arrangement which any competent optician will easily secure--and let a fine horizontal line or several horizontal lines be marked on the mirror; which, by the way, should be a metallic one, as its indications will then be altogether more trustworthy. this mirror can be put into the waistcoat pocket and conveniently carried to much greater height than the mirror used by parallax. now, at some considerable height--say five or six hundred feet above the sea-level, but a hundred or even fifty will suffice--look into this small mirror while _facing_ the sea. the true horizon will then be seen to be visibly below the centre of the eye-pupil--visibly in this case because the horizontal line traced on the mirror can be made to coincide with the sea-horizon exactly, and will then be found _not_ to coincide with the centre of the eye-pupil. such an instrument could be readily made to show the distance of the sea-horizon, which at once determines the height of the observer above the sea-level. for this purpose all that would be necessary would be a means of placing the eye at some definite distance from the small mirror, and a fine vertical scale on the mirror to show the exact depression of the sea-horizon. for balloonists such an instrument would sometimes be useful, as showing the elevation independently of the barometer, whenever any portion of the sea-horizon was in view. the mention of balloon experiences leads me to another delusive argument of the earth-flatteners.[ ] it has been the experience of all aeronauts that, as the balloon rises, the appearance of the earth is by no means what would be expected from the familiar teachings in our books of astronomy. there is a picture in most of these books representing the effect of ascent above the sea-level in depressing the line of sight to the horizon, and bringing more and more into view the convexity of the earth's globe. one would suppose, from the picture, that when an observer is at a great height the earth would appear to rise under him, like some great round and well-curved shield whose convexity was towards him. instead of this, the aeronaut finds the earth presenting the appearance of a great hollow basin, or of the concave side of a well-curved shield. the horizon seems to rise as he rises, while the earth beneath him sinks lower and lower. a somewhat similar phenomenon may be noted when, after ascending the landward side of a high cliff, we come suddenly upon a view of the sea--invariably the sea-horizon is higher than we expected to find it. _only_, in this case, the surface of the sea seems to rise from the beach below towards the distant horizon convexly not concavely; the reason of which i take to be this, that the waves, and especially long rollers or uniform large ripples, teach the eye to form true conceptions of the shape of the sea-surface even when the eye is deceived as to the position of the sea-horizon. indeed, i should much like to know what would be the appearance of the sea from a balloon when no land was in sight (though i do not particularly wish to make the observation myself): the convexity discernible, for the reason just named, would contend strangely with the concavity imagined, for the reason now to be indicated. the deception arises from the circumstance that the scene displayed below and around the balloon is judged by the eye from the experience of more familiar scenes. the horizon is depressed, but so little that the eye cannot detect the depression, especially where the boundary of the horizon is irregular. it is here that the text-book pictures mislead; for they show the depression as far too great to be overlooked, setting the observer sometimes about two thousand miles above the sea-level. the eye, then, judges the horizon to be where it usually is--on the same level as the observer; but looking downwards, the eye perceives, and at once appreciates if it does not even exaggerate, the great depth at which the earth lies below the balloon. the appearance, then, as judged by the eye, is that of a mighty basin whose edge rises up all round to the level of the balloon, while its bottom lies two or three miles or more below the balloon. the zetetic faithful reason about this matter as though the impressions of the senses were trustworthy under all conditions, familiar or otherwise; whereas, in point of fact, we know that the senses often deceive, even under familiar conditions, and almost always deceive under conditions, which are not familiar. a person, for example, accustomed to the mist and haze of our british air, is told by the sense of sight, when he is travelling where a clearer atmosphere prevails, that a mountain forty miles from him is a hill a few miles away. on the other hand, an italian travelling through the highlands is impressed with the belief that all the features of the scenery are much larger (because he supposes them much more remote) than they really are. a hundred such instances of deception might easily be cited. the conditions under which the aeronaut observes the earth are certainly less familiar than those under which the briton views the alps and apennines, or the italian views ben lomond or ben lawers. it would be rash, therefore, even if no other evidence were available, to reject the faith that the earth is a globe because, as seen from a balloon, it looks like a basin. indeed, to be strictly logical, the followers of parallax ought on this account to adopt the faith that the earth is not flat, but basin-shaped, which hitherto they have not been ready to do. we have seen that parallax describes a certain experiment on the bedford level, which, if made as he states, would have shown certainly that something was wrong in the accepted system--for a six-mile straight-edge along water would be as severe a blow to the belief in a round earth, as a straight line on the sea-surface from queenstown to new york. another curious experiment adorns his little book, which, if it could be repeated successfully before a dozen trustworthy witnesses, would rather astonish men of science. having, he says, by certain reasoning--altogether erroneous, but that is a detail--convinced himself that, on the accepted theory, a bullet fired vertically upwards ought to fall far to the west of the place whence it was fired, he carefully fixed an air-gun in a vertical position, and fired forty bullets vertically upwards. all these fell close to the gun--which is not surprising, though it must have made such an experiment rather dangerous; but two fell back into the barrel itself--which certainly was very surprising indeed. one might fairly challenge the most experienced gunner in the world to achieve one such vertical shot in a thousand trials; two in forty bordered on the miraculous. the earth-flatteners i have been speaking of claim, as one of their objects, the defence of scripture. but some of the earth-flatteners of the last generation (or a little farther back) took quite another view of the matter. for instance, sir richard phillips, a more vehement earth-flattener than parallax, was so little interested in defending the scriptures, that in he was sentenced to a year's imprisonment for selling a book regarded as atheistic. in he attempted the conversion of professor de morgan, opening the correspondence with the remark that he had 'an inveterate abhorrence of all the pretended wisdom of philosophy derived from the monks and doctors of the middle ages, and not less those of higher name who merely sought to make the monkish philosophy more plausible, or so to disguise it as to mystify the mob of small thinkers.' he seems himself to have succeeded in mystifying many of those whom he intended to convert. admiral smyth gives the following account of an interview he had with phillips: 'this pseudo-mathematical knight once called upon me at bedford, without any previous acquaintance, to discuss "those errors of newton, which he almost blushed to name," and which were inserted in the "principia" to "puzzle the vulgar." he sneered with sovereign contempt at the "trinity of gravitating force, projectile force, and void space," and proved that all change of place is accounted for by motion.' [startling hypothesis!] 'he then exemplified the conditions by placing some pieces of paper on a table, and slapping his hand down close to them, thus making them fly off, which he termed applying the momentum. all motion, he said, is in the direction of the forces; and atoms seek the centre by "terrestrial centripetation"--a property which causes universal pressure; but in what these attributes of pushing and pulling differ from gravitation and attraction was not expounded. many of his "truths" were as mystified as the conundrums of rabelais; so nothing was made of the motion.' a favourite subject of paradoxical ideas has been the moon's motion of rotation. strangely enough, de morgan, who knew more about past paradoxists than any man of his time, seems not to have heard of the dispute between keill and bentley over this matter in . he says, 'there was a dispute on the subject, in , between james ferguson and an anonymous opponent; and i think there have been others;' but the older and more interesting dispute he does not mention. bentley, who was no mathematician, pointed out in a lecture certain reasons for believing that the moon does not turn on her axis, or has no axis on which she turns. keill, then only nineteen years old, pointed out that the arguments used by bentley proved that the moon does rotate instead of showing that she does not. (twenty years later keill was appointed savilian professor of astronomy at oxford. he was the first holder of that office to teach the newtonian astronomy.) in recent times, as most of my readers know, the paradox that the moon does not rotate has been revived more than once. in it was sustained by mr. jellinger symons, one of whose staunchest supporters, mr. h. perigal, had commenced the attack a few years earlier. of course, the gist of the argument against the moon's rotation lies in the fact that the moon always keeps the same face turned towards the earth, or very nearly so. if she did so exactly, and if her distance from the earth were constantly the same, then her motion would be exactly the same as though she were rigidly connected with the earth, and turned round an axis at the earth. the case may be thus illustrated: through the middle of a large orange thrust one short rod vertically, and another long rod horizontally; thrust the further end of the latter through a small apple, and now turn the whole affair round the short vertical rod as an axis. then the apple will move with respect to the orange as the moon would move with respect to the earth on the suppositions just made. no one in this case would say that the apple was turning round on its axis, since its motion would be one of rotation round the upright axis through the orange. therefore, say the opponents of the moon's rotation, no one should say that the moon turns round on her axis. of course, the answer would be obvious even if the moon's motions were as supposed. the moon is not connected with the earth as the apple is with the orange in the illustrative case. if the apple, without rigid connection with the orange, were carried round the orange so as to move precisely as if it were so connected, it would unquestionably have to rotate on its axis, as any one will find who may try the experiment. thus for the straight rod thrust through the apple substitute a straight horizontal bar carrying a small basin of water in which the apple floats. sway the bar steadily and slowly round, and it will be found (if a mark is placed on the apple) that the apple no longer keeps the same face towards the centre of motion; but that, to cause it to do so, a slow motion of rotation must be communicated to the apple in the same direction and at the same rate (neglecting the effects of the friction of the water against the sides of the basin) as the bar is rotating. in my 'treatise on the moon' i have described and pictured a simple apparatus by which this experiment may easily be made. but, of course, such experiments are not essential to the argument by which the paradox is overthrown. this argument simply is, that the moon as she travels on her orbit round the sun--the real centre of her motion--turns every part of her equator in succession towards him once in a lunar month. at the time of new moon the sun illuminates the face of the moon turned from us; at the time of full moon he illuminates the face which has been gradually brought round to him as the moon has passed through her first two quarters. as she passes onwards to new moon again, the face we see is gradually turned from him until he shines full upon the other face. and so on during successive lunations. this could not happen unless the moon rotated. again, if we lived on the moon we should find the heaven of the fixed stars turning round from east to west once in rather more than twenty-seven days; and unless we supposed, as we should probably do for a long time, that our small world was the centre of the universe, and that the stars turned round it, we should be compelled to admit that it was turning on its own axis from west to east once in the time just named. there would be no escape. the mere fact that all the time the stars thus seemed to be turning round the moon, the earth would not so seem to move, but would lie always in the same direction, would in no sort help to remove the difficulty. lunarian paradoxists would probably argue that she was in some way rigidly connected with the moon; but even they would never think of arguing that their world did not turn on its axis, _unless_ they maintained that it was the centre of the universe. this, i think, they would very probably do; but as yet terrestrial paradoxists have not, i believe, maintained this hypothesis. i once asked mr. perigal whether that was the true theory of the universe--the moon central, the earth, sun, and heavens carried round her. he admitted that his objections to accepted views were by no means limited to the moon's rotation; and, if i remember rightly, he said that the idea i had thrown out in jest was nearer the truth than i thought, or used words to that effect. but as yet the theory has not been definitely enunciated that the moon is the boss of the universe. comets, as already mentioned, have been the subjects of paradoxes innumerable; but as yet comets have been so little understood, even by astronomers, that paradoxes respecting them cannot be so readily dealt with as those relating to well-established facts. among thoroughly paradoxical ideas respecting comets, however, may be mentioned one whose author is a mathematician of well-deserved repute--professor tait's 'sea-bird theory' of comets' tails. according to this theory, the rapid formation of long tails and the rapid changes of their position may be explained on the same principle that we explain the rapid change of appearance of a flight of sea-birds, when, from having been in a position where the eye looks athwart it, the flight assumes a position where the eye looks at it edgewise. in the former position it is scarcely visible (when at a distance), in the latter it is seen as a well-defined streak; and as a very slight change of position of each bird may often suffice to render an extensive flight thus visible throughout its entire length, which but a few moments before had been invisible, so the entire length of a comet's tail may be brought into view, and apparently be formed in a few hours, through some comparatively slight displacement of the individual meteorites composing it. this paradox--for paradox it unquestionably is--affords a curious illustration of the influence which mathematical power has on the minds of men. every one knows that professor tait has potential mathematical energy competent to dispose, in a very short time, of all the difficulties involved in his theory; therefore few seem to inquire whether this potential energy has ever been called into action. it is singular, too, that other mathematicians of great eminence have been content to take the theory on trust. thus sir w. thomson, at the meeting of the british association at edinburgh, described the theory as disposing easily of the difficulties presented by newton's comet in . glashier, in his translation of guillemin's 'les comètes,' speaks of the theory as one not improbably correct, though only to be established by rigid investigation of the mathematical problems involved. in reality, not five minutes' inquiry is needed to show any one acquainted with the history of long-tailed comets that tait's theory is quite untenable. take newton's comet. it had a tail ninety millions of miles long, extending directly from the sun as the comet approached him, and seen, four days later, extending to the same distance, and still directly from the sun, as the comet receded from him in an entirely different direction. according to tait's sea-bird theory, the earth was at both these epochs in the plane of a sheet of meteorites forming the tail; but on each occasion the sun also was in the same plane, for the edge of the sheet of meteorites was seen to be directly in a line with the sun. the comet's head, of course, was in the same plane; but three points, not in a straight line, determine a plane. hence we have, as the definite result of the sea-bird theory, that the layer or stratum of meteorites, forming the tail of newton's comet, lay in the same plane which contained the sun, the earth, and the comet. but the comet crossed the ecliptic (the plane in which the earth travels round the sun) between the epochs named, crossing it at a great angle. when crossing it, then, the great layer of meteorites was in the plane of the ecliptic; before crossing it the layer was greatly inclined to that plane one way, and after crossing it the layer was greatly inclined to that plane another way. so that we have in no way escaped the difficulty which the sea-bird theory was intended to remove. if it was a startling and, indeed, incredible thing that the particles along a comet's tail should have got round in four days from the first to the second position of the tail considered above, it is as startling and incredible that a mighty layer of meteorites should have shifted bodily in the way required by the sea-bird theory. nay, there is an element in our result which is still more startling than any of the difficulties yet mentioned; and that is, the singular care which the great layer of meteorites would seem to have shown to keep its plane always passing through the earth, with which it was in no way connected. why should this preference have been shown by the meteor flock for our earth above all the other members of the solar system?--seeing that the sea-bird theory _requires_ that this comet, and not newton's comet alone but all others having tails, should not only be thus complaisant with respect to our little earth, but should behave in a totally different way with respect to every other member of the sun's family. we can understand that, while several have been found who have applauded the sea-bird paradox for what it _might_ do in explaining comets' tails, its advocates have as yet not done much to reconcile it with cometic observation. the latest astronomical paradox published is perhaps still more startling. it relates to the planet venus, and is intended to explain the appearance presented by this planet when crossing the sun's face, or, technically, when in transit. at this time she is surrounded by a ring of light, which appears somewhat brighter than the disc of the sun itself. before fully entering on the sun's face, also, the part of venus's globe as yet outside the sun's disc is seen to be girt round by a ring of exceedingly bright light--so bright, indeed, that it has left its record in photographs where the exposure was only for the small fraction of a second allowable in the case of so intensely brilliant a body as the sun. astronomers have not found it difficult to explain either peculiarity. it has been proved clearly in other ways that venus has an atmosphere like our own, but probably denser. as the sun is raised into view above the horizon (after he has really passed below the horizon plane) by the bending power of our air upon his rays, so the bending power of venus's air brings the sun into our view round the dark body of the planet. but the new paradox advances a much bolder theory. instead of an atmosphere such as ours, venus has a glass envelope; and instead of a surface of earth and water, in some cases covered with clouds, venus has a surface shining with metallic lustre.[ ] the author of this theory, mr. jos. brett, startled astronomers by announcing, a few years ago, that with an ordinary telescope he could see the light of the sun's corona without the aid of an eclipse, though astronomers had observed that the delicate light of the corona fades out of view with the first returning rays of the sun after total eclipse. the latest paradoxist, misled by the incorrect term 'centrifugal force,' proposes to 'modify, if not banish,' the old-fashioned astronomy. what is called centrifugal force is in truth only inertia. in the familiar instance of a body whirled round by a string, the breaking of the string no more implies that an active force has pulled away the body, than the breaking of a rope by which a weight is pulled implies that the weight has exerted an active resistance. of course, here again the text-books are chiefly in fault. such are a few among the paradoxes of various orders by which astronomers, like the students of other sciences, have been from time to time amused. it is not altogether, as it may seem at first sight, 'a sin against the twenty-four hours' to consider such matters; for much may be learned not only from the study of the right road in science, but from observing where and how men may go astray. i know, indeed, few more useful exercises for the learner than to examine a few paradoxes, when leisure serves, and to consider how, if left to his own guidance, he would confute them. xi. _on some astronomical myths._ the expression 'astronomical myth' has recently been used, on the title-page of a translation from the french, as synonymous with false systems of astronomy. it is not, however, in that sense that i here use it. the history of astronomy presents the records of some rather perplexing observations, not confirmed by later researches, but yet not easily to be explained away or accounted for. such observations humboldt described as belonging to the myths of an uncritical period; and it is in that sense that i employ the term 'astronomical myth' in this essay. i propose briefly to describe and comment on some of the more interesting of these observations, which, in whatever sense they are to be interpreted, will be found to afford a useful lesson. it is hardly necessary, perhaps, to point out that the cases which i include here i regard as really cases in which astronomers have been deceived by illusory observations. other students of astronomy may differ from me as respects some of these instances. i do not wish to dogmatise, but simply to describe the facts as i see them, and the impressions which i draw from them. those who view the facts differently will not, i think, have to complain that i have incorrectly described them. at the outset, let me point out that some observations which were for a long time regarded as mythical have proved to be exact. for instance, when as yet very few telescopes existed, and those very feeble, galileo's discovery of moons travelling round jupiter was rejected as an illusion for which satan received the chief share of credit. there is an amusing and yet in one aspect almost pathetic reference to this in his account of his earlier observations of saturn. he had seen the planet apparently attended on either side by two smaller planets, as if helping old saturn along. but on december , ,[ ] turning his telescope on the planet, he found to his infinite amazement not a trace of the companion planets could be seen; there in the field of view of his telescope was the golden-tinted disc of the planet as smoothly rounded as the disc of mars or jupiter. 'what,' he wrote, 'is to be said concerning so strange a metamorphosis? are the two lesser stars consumed after the manner of the solar spots? have they vanished or suddenly fled? has saturn, perhaps, devoured his children? or were the appearances, indeed, illusion or fraud with which the glasses have so long deceived me as well as many others to whom i have shown them? now, perhaps, is the time come to revive the well-nigh withered hopes of those who, guided by more profound contemplations, have discovered the fallacy of the new observations, and demonstrated the utter impossibility of the existence of those things which the telescope appears to show. i do not know what to say in a case so surprising, so unlooked for, and so novel. the shortness of the time, the unexpected nature of the event, the weakness of my understanding, and the fear of being mistaken, have greatly confounded me.' we now know that these observations, as well as those made soon after by hevelius, though wrongly interpreted, were correct enough. nay, we know that if either galileo or hevelius had been at the pains to reason out the meaning of the alternate visibility and disappearance of objects looking like attendant planets, they must have anticipated the discovery made in by huyghens, that saturn's globe is girdled about by a thin flat ring so vast that, if a score of globes like our earth were set side by side, the range of that row of worlds would be less than the span of the saturnian ring system. there is a reference in galileo's letter to the solar spots; 'are the two lesser stars,' he says, 'consumed after the manner of the solar spots?' when he thus wrote the spots were among the myths or fables of astronomy, and an explanation was offered, by those who did not reject them utterly, which has taken its place among forsaken doctrines, those broken toys of astronomers. it is said that when scheiner, himself a jesuit, communicated to the provincial of the jesuits his discovery of the spots on the sun, the latter, a staunch aristotelian, cautioned him not to see these things. 'i have read aristotle's writings from beginning to end many times,' he said, 'and i can assure you i have nowhere found in them anything similar to what you mention' [amazing circumstances!] 'go, therefore, my son, tranquillise yourself; be assured that what you take for spots on the sun are the faults of your glasses or your eyes.' as the idea was obviously inadmissible that a celestial body could be marked by spots, the theory was started that the dark objects apparently seen on the sun's body were in reality small planets revolving round the sun, and a contest arose for the possession of these mythical planets. tardé maintained that they should be called _astra borbonia_, in honour of the royal family of france; but c. malapert insisted that they should be called _sidera austriaca_. meantime the outside world laughed at the spots, and their names, and the astronomers who were thought to have invented both. 'fabritius puts only three spots,' wrote burton in his 'anatomy of melancholy,' 'and those in the sun; apelles , and those without the sun, floating like the cyanean isles in the euxine sea. tardé the frenchman hath observed , and those neither spots nor clouds as galileus supposed, but planets concentric with the sun, and not far from him, with regular motions. christopher schemer' [a significant way of spelling scheiner's name], 'a german suisser jesuit, divides them _in maculas et faculas_, and will have them to be fixed _in solis superficie_ and to absolve their periodical and regular motions in or dayes; holding withall the rotation of the sun upon his centre, and are all so confident that they have made schemes and tables of their motions. the hollander censures all; and thus they disagree among themselves, old and new, irreconcilable in their opinions; thus aristarchus, thus hipparchus, thus ptolomæus, thus albategnius, etc., with their followers, vary and determine of these celestial orbs and bodies; and so whilst these men contend about the sun and moon, like the philosophers in lucian, it is to be feared the sun and moon will hide themselves, and be as much offended as she was with those, and send another message to jupiter, by some new-fangled icaromenippus, to make an end of all these curious controversies, and scatter them abroad.' it is well to notice how in this, as in many other instances, the very circumstance which makes scientific research trustworthy caused the unscientific to entertain doubt. if men of science were to arrange beforehand with each other what observations they should publish, how their accounts should be ended, what theories they would endeavour to establish, their results would seem far more trustworthy, their theories far more probable, than according to the method actually adopted. science, which should be exact, seems altogether inexact, because one observer seems to obtain one result, another a different result. scientific theories seem unworthy of reliance because scientific men entertain for a long time rival doctrines. but in another and a worthier sense than as the words are used in the 'critic,' when men of science do agree their agreement is wonderful. it _is_ wonderful, worthy of all admiration, because before it has been attained errors long entertained have had to be honestly admitted; because the taunt of inconsistency is not more pleasant to the student of science than to others, and the man who having a long time held one doctrine adopts and enforces another (one perhaps which he had long resisted), is sure to be accused by the many of inconsistency, the truly scientific nature of his procedure being only recognised by the few. the agreement of men of science ought to be regarded also as most significant in another sense. so long as there is room for refusing to admit an important theory advanced by a student of science, it is natural that other students of science should refuse to do so; for in admitting the new theory they are awarding the palm to a rival. in strict principle, of course, this consideration ought to have no influence whatever; as a matter of fact, however, men of science, being always men and not necessarily strengthened by scientific labours against the faults of humanity, the consideration has and must always have influence. therefore, when the fellow-writers and rivals of newton or of his followers gave in their adhesion to the newtonian theory; when in our own time--but let us leave our own time alone, in this respect--when, speaking generally, a novel doctrine, or some new generalisation, or some great and startling discovery, is admitted by rival students of the branch of astronomy to which it belongs, the probability is great that the weight of evidence has been found altogether overwhelming. let us now, however, turn to cases in which, while many observations seem to point to some result, it has appeared that, after all, those observations must have been illusory. a striking instance in point is found in the perplexing history of the supposed satellite of venus. on january , , the celebrated astronomer, j.d. cassini saw a crescent shaped and posited like venus, but smaller, on the western side of the planet. more than fourteen years later, he saw a crescent east of the planet. the object continued visible in the latter case for half an hour, when the approach of daylight obliterated the planet and this phantom moon from view. the apparent distance of the moon from venus was in both cases small, viz., only one diameter of the planet in the former case, and only three-fifths of that diameter in the latter. next, on october , , old style, the optician short, who had had considerable experience in observation, saw a small star perfectly defined but less luminous than venus, at a distance from the planet equal to about one-third of the apparent diameter of our moon. this is a long distance, and would correspond to a distance from venus certainly not less than the moon's distance from the earth. short was aware of the risk of optical illusion in such matters, and therefore observed venus with a second telescope; he also used four eye-pieces of different magnifying power. he says that venus was very distinct, the air very pure, insomuch that he was able to use a power of . the seeming moon had a diameter less than a third of venus's, and showed the same phase as the planet. its disc was exceedingly well defined. he observed it several times during a period of about one hour. still more convincing, to all appearance, is the account of the observations made by m. montaigne, as presented to the academy of sciences at paris by m. baudouin in . the transit of venus which was to take place on june in that year led to some inquiry as to the satellite supposed to have been seen by cassini and short, for of course a transit would be a favourable occasion for observing the satellite. m. montaigne, who had no faith in the existence of such an attendant, was persuaded to look for it early in . on may he saw a little crescent moon about twenty minutes of arc (nearly two-thirds the apparent diameter of our moon) from the planet. he repeated his observation several times that night, always seeing the small body, but not quite certain, despite its crescent shape, whether it might not be a small star. on the next evening, and again on may and , he saw the small companion apparently somewhat farther from venus and in a different position. he found that it could be seen when venus was not in the field of view. the following remarks were made respecting these observations in a french work, 'dictionnaire de physique,' published in :--'the year will be celebrated in astronomy in consequence of the discovery that was made on may of a satellite circulating round venus. we owe it to m. montaigne, member of the society of limoges. m. baudouin read before the academy of sciences at paris a very interesting memoir, in which he gave a determination of the revolution and distance of the satellite. from the calculations of this expert astronomer we learn that the new star has a diameter about one-fourth that of venus, is distant from venus almost as far as the moon from our earth, has a period of nine days seven hours' [much too short, by the way, to be true, expert though m. baudouin is said to have been], 'and its ascending node'--but we need not trouble ourselves about its ascending node. three years later rödkier, at copenhagen, march and , , saw the satellite of venus with a refracting telescope feet long, which should have been effective if longitude has any virtue. he could not see the satellite with another telescope which he tried. but several of his friends saw it with the long telescope. amongst others, horrebow, professor of astronomy, saw the satellite on march and , after taking several precautions to prevent optical illusion. a few days later montbaron, at auxerre, who had heard nothing of these observations, saw a satellite, and again on march and it appeared, always in a different position. it should be added that scheuten asserted that during the transit of venus was accompanied by a small satellite in her motion across the sun's face. so confidently did many believe in this satellite of venus that frederick the great, who for some reason imagined that he was entitled to dispose as he pleased of the newly discovered body, proposed to assign it away to the mathematician d'alembert, who excused himself from accepting the questionable honour in the following terms:-- 'your majesty does me too much honour in wishing to baptize this new planet with my name. i am neither great enough to become the satellite of venus in the heavens, nor well enough (_assez bien portant_) to be so on the earth, and i am too well content with the small place i occupy in this lower world to be ambitious of a place in the firmament.' it is not at all easy to explain how this phantom satellite came to be seen. father hell, of vienna--the same astronomer whom sir g. airy suspects of falling asleep during the progress of the transit of venus in --made some experiments showing how a false image of the planet might be seen beside the true one, the false image being smaller and fainter, like the moons seen by schort (as hell called short), cassini, and the rest. and more recently sir david brewster stated that wargentin 'had in his possession a good achromatic telescope, which always showed venus with such a satellite.' but hell admitted that the falsehood of the unreal venus was easily detected, and brewster adds to his account of wargentin's phantom moon, that 'the deception was discovered by turning the telescope about its axis.' as admiral smyth well remarks, to endeavour to explain away in this manner the observations made by cassini and short 'must be a mere pleasantry, for it is impossible such accurate observers could have been deceived by so gross a neglect.' smyth, by the way, was a believer in the moon of venus. 'the contested satellite is perhaps extremely minute,' he says, 'while some parts of its body may be less capable of reflecting light than others; and when the splendour of its primary and our inconvenient station for watching it are considered, it must be conceded that, however slight the hope may be, search ought not to be relinquished.' setting aside scheuten's asserted recognition of a dark body near venus during the transit of , venus has always appeared without any attendant when in transit. as no one else claimed to have seen what scheuten saw in , though the transit was observed by hundreds, of whom many used far finer telescopes than he, we must consider that he allowed his imagination to deceive him. during the transit of , and again on december - , , venus certainly had no companion during her transit. what, then, was it that cassini, short, montaigne, and the rest supposed they saw? the idea has been thrown out by mr. webb that mirage caused the illusion. but he appears to have overlooked the fact that though an image of venus formed by mirage would be fainter than the planet, it would not be smaller. it might, according to the circumstances, be above venus or below, or even somewhat towards either side, and it might be either a direct or an inverted image, but it could not possibly be a diminished image. single observations like cassini's or short's might be explained as subjective phenomena, but this explanation will not avail in the case of the copenhagen observations. i reject, as every student of astronomy will reject, the idea of wilful deception. occasionally an observer may pretend to see what he has not seen, though i believe this very seldom happens. but even if cassini and the rest had been notoriously untrustworthy persons instead of being some of them distinguished for the care and accuracy with which their observations were made and recorded, these occasional views of a phantom satellite are by no means such observations as they would have invented. no distinction was to be gained by observations which could not be confirmed by astronomers possessing more powerful telescopes. cassini, for example, knew well that nothing but his well-earned reputation could have saved him from suspicion or ridicule when he announced that he had seen venus attended by a satellite. it seems to me probable that the false satellite was an optical illusion brought about in a different way from those referred to by hell and brewster, though among the various circumstances which in an imperfect instrument might cause such a result i do not undertake to make a selection. it is certain that venus's satellite has vanished with the improvement of telescopes, while it is equally certain that even with the best modern instruments illusions occasionally appear which deceive even the scientific elect. three years have passed since i heard the eminent observer otto struve, of pulkowa, give an elaborate account of a companion to the star procyon, describing the apparent brightness, distance, and motions of this companion body, for the edification of the astronomer-royal and many other observers. i had visited but a few months before the observatory at washington, where, with a much more powerful telescope, that companion to procyon had been systematically but fruitlessly sought for, and i entertained a very strong opinion, notwithstanding the circumstantial nature of struve's account and his confidence (shared in unquestioningly by the observers present), that he had been in some way deceived. but i could not then see, nor has any one yet explained, how this could be. the fact, however, that he had been deceived is now undoubted. subsequent research has shown that the pulkowa telescope, though a very fine instrument, possesses the undesirable quality of making a companion orb for all first-class stars in the position where o. struve and his assistant lindenau saw the supposed companion of procyon. i may as well point out, however, that theories so wild have recently been broached respecting venus, that far more interesting explanations of the enigma than this optical one may be looked for presently. it has been gravely suggested by mr. jos. brett, the artist, that venus has a surface of metallic brilliancy, with a vitreous atmosphere,--which can only be understood to signify a glass case. this stupendous theory has had its origin in an observation of considerable interest which astronomers (it is perhaps hardly necessary to say) explain somewhat differently. when venus has made her entry in part upon the sun's face at the beginning of transit, there is seen all round the portion of her disc which still remains outside the sun an arc of light so brilliant that it records its photographic trace during the instantaneous exposure required in solar photography. it is mathematically demonstrable that this arc of light is precisely what _should_ be seen if venus has an atmosphere like our earth's. but mathematical demonstration is not sufficient (or perhaps we may say it is too much) for some minds. therefore, to simplify matters, venus has been provided with a mirror surface and a glass case. (see preceding essay, on astronomical paradoxes, for further details.) the enigma next to be considered is of a more doubtful character than the myth relating to the satellite of venus. astronomers are pretty well agreed that venus has no moon, but many, including some deservedly eminent, retain full belief in the story of the planet vulcan. more than seventeen years ago the astronomical world was startled by the announcement that a new planet had been discovered, under circumstances unlike any which had heretofore attended the discovery of fresh members of the solar system. at that time astronomers had already become accustomed to the discovery, year after year, of several asteroids, which are in reality planets, though small ones. in fact, no less than fifty-six of these bodies were then known, whereof fifty-one had been discovered during the years - inclusive, not one of these years having passed without the detection of an asteroid. but all these planets belonged to one family, and as there was every reason to believe that thousands more travel in the same region of the solar system, the detection of a few more among the number had no longer any special interest for astronomers. the discovery of the first known member of the family had indeed been full of interest, and had worthily inaugurated the present century, on the first day of which it was made. for it had been effected in pursuance of a set scheme, and astronomers had almost given up all hopes of success in that scheme when piazzi announced his detection of little ceres. again the discovery of the next few members of the family had been interesting as revealing the existence of a new order of bodies in the solar system. no one had suspected the possibility that besides the large bodies which travel round the sun, either singly or attended by subordinate families of moons, there might be a ring of many planets. this was what the discovery of ceres, pallas, juno, and vesta seemed to suggest, unless--still stranger thought--these were but fragments of a mighty planet which had been shattered in long-past ages by some tremendous explosion. since then, however, this startling theory has been (itself) exploded. year after year new members of the ring of multitudinous planets are discovered, and that, not as was recently predicted, in numbers gradually decreasing, but so rapidly that more have been discovered during the last ten years than during the preceding twenty. the discovery of the giant planet uranus, an orb exceeding our earth twelve and a half times in mass and seventy-four times in volume, was a matter of much greater importance, so far as the dignity of the planetary system was concerned, for it is known that the whole ring of asteroids together does not equal one-tenth part of the earth in mass, while uranus exceeds many times in volume the entire family of terrestrial planets--mercury, venus, the earth, and mars. the detection of uranus, unlike that of ceres, was effected by accident. sir w. herschel was looking for double stars of a particular kind in the constellation gemini when by good fortune the stranger was observed. the interest with which astronomers received the announcement of the discovery of uranus, though great, was not to be compared with that with which they deservedly welcomed the discovery of neptune, a larger and more massive planet, revolving at a distance one-half greater even than the mighty space which separates uranus from the sun, a space so great that by comparison with it the range of , , of miles, which forms the diameter of our earth's orbit, seems quite insignificant. it was not, however, the vastness of neptune's mass or volume, or the awful remoteness of the path along which he pursues his gloomy course, which attracted the interest of astronomers, but the strangeness of the circumstances under which the planet had been detected. his influence had been felt for many years before astronomers thought of looking for him, and even when the idea had occurred to one or two, it was considered, and that, too, by an astronomer as deservedly eminent as sir g. airy, too chimerical to be reasonably entertained. all the world now knows how leverrier, the greatest living master of physical astronomy, and adams, then scarce known outside cambridge, both conceived the idea of finding the planet, not by the simple method of looking for it with a telescope, but by the mathematical analysis of the planet's disturbing influence upon known members of the solar system. all know, too, that these mathematicians succeeded in their calculations, and that the planet was found in the very region and close to the very point indicated first by adams, and later, but independently, and (fortunately for him more publicly) by leverrier. none of these instances of the discovery of members of the solar system resembled in method or details the discovery announced early in the year . it was not amid the star-depths and in the darkness of night that the new planet was looked for, but in broad day, and on the face of the sun himself. it was not on the outskirts of the solar system that the planet was supposed to be travelling, but within the orbit of mercury, hitherto regarded as of all planets the nearest to the sun. it was not hoped that any calculation of the perturbations of other planets would show the place of the stranger, though certain changes in the orbit of mercury seemed clearly enough to indicate the stranger's existence. early in leverrier had announced that the position of mercury's path was not precisely in agreement with calculations based on the adopted estimates of the masses of those planets which chiefly disturb the motions of mercury. the part of the path where mercury is nearest to the sun, and where, therefore, he travels fastest, had slightly shifted from its calculated place. this part of the path was expected to move, but it had moved more than was expected; and of course mercury having his region of swiftest motion somewhat differently placed than was anticipated, himself moved somewhat differently. leverrier found that to explain this feature of mercury's motion either the mass of venus must be regarded as one-tenth greater than had been supposed, or some unknown cause must be regarded as affecting the motion of mercury. a planet as large as mercury, about midway between mercury and the sun, would account for the observed disturbance; but leverrier rejected the belief that such a planet exists, simply because he could not 'believe that it would be invisible during total eclipses of the sun.' 'all difficulties disappear,' he added, 'if we admit, in place of a single planet, small bodies circulating between mercury and the sun.' considering their existence as not at all improbable, he advised astronomers to watch for them. it was on january , , that leverrier thus wrote. on december , , a letter had been addressed by a m. lescarbault of orgères to leverrier, through m. vallée, hon. inspector-general of roads and bridges, announcing that on march , , about four in the afternoon, lescarbault had seen a round black spot on the face of the sun, and had watched it as it passed across like a planet in transit--not with the slow motion of an ordinary sun-spot. the actual time during which the round spot was visible was one hour, seventeen minutes, nine seconds, the rate of motion being such that, had the spot crossed the middle of the sun's disc, at the same rate, the transit would have lasted more than four hours. the spot thus merely skirted the sun's disc, being at no time more than about one forty-sixth part of the sun's apparent diameter from the edge of the sun. lescarbault expressed his conviction that on a future day, a black spot, perfectly round and very small, will be seen passing over the sun, and 'this point will very probably be the planet whose path i observed on march , .' 'i am persuaded,' he added, 'that this body is the planet, or one of the planets, whose existence in the vicinity of the sun m. leverrier had made known a few months ago' (referring to the preliminary announcement of results which leverrier published afterwards more definitely). leverrier, when the news of lescarbault's observation first reached him, was surprised that the observation should not have been announced earlier. he did not consider the delay sufficiently justified by lescarbault's statement that he wished to see the spot again. he therefore set out for orgères, accompanied by m. vallée. 'the predominant feeling in leverrier's mind,' says abbé moigno, 'was the wish to unmask an attempt to impose upon him, as the person more likely than any other astronomer to listen to the allegation that his prophecy had been fulfilled.' 'one should have seen m. lescarbault,' says moigno, 'so small, so simple, so modest, and so timid, in order to understand the emotion with which he was seized, when leverrier, from his great height, and with that blunt intonation which he can command, thus addressed him: "it is then you, sir, who pretend to have observed the intra-mercurial planet, and who have committed the grave offence of keeping your observation secret for nine months. i warn you that i have come here with the intention of doing justice to your pretensions, and of demonstrating either that you have been dishonest or deceived. tell me, then, unequivocally, what you have seen."' this singular address did not bring the interview, as one might have expected, to an abrupt end. the lamb, as the abbé calls the doctor, trembling, stammered out an account of what he had seen. he explained how he had timed the passage of the black spot. 'where is your chronometer?' asked leverrier. 'it is this watch, the faithful companion of my professional journeys.' 'what! with that old watch, showing only minutes, dare you talk of estimating seconds. my suspicions are already too well confirmed.' 'pardon me, i have a pendulum which beats seconds.' 'show it me.' the doctor brings down a silk thread to which an ivory ball is attached. fixing the upper end to a nail, he draws the ball a little from the vertical, counts the number of oscillations, and shows that his pendulum beats seconds; he explains also how his profession, requiring him to feel pulses and count pulsations, he has no difficulty in mentally keeping record of successive seconds. having been shown the telescope with which the observation was made, the record of the observation (on a piece of paper covered with grease and laudanum, and doing service as a marker in the 'connaissance des temps,' or french nautical almanac), leverrier presently inquired if lescarbault had attempted to deduce the planet's distance from the sun from the period of its transit. the doctor admitted that he had attempted this, but, being no mathematician, had failed to achieve success with the problem. he showed the rough draughts of his futile attempts at calculation on a board in his workshop, 'for,' said he naïvely, 'i am a joiner as well as an astronomer.' the interview satisfied leverrier that a new planet, travelling within the orbit of mercury, had really been discovered. 'with a grace and dignity full of kindness,' says a contemporary narrative of these events,[ ] 'he congratulated lescarbault on the important discovery which he had made.' anxious to obtain some mark of respect for the discoverer of vulcan, leverrier made inquiry concerning his private character, and learned from the village curé, the juge de paix, and other functionaries, that he was a skilful physician and a worthy man. with such high recommendations, m. leverrier requested from m. rouland, the minister of public instruction, the decoration of the legion of honour for m. lescarbault. the minister, in a brief but interesting statement of his claim, communicated this request to the emperor, who, by a decree dated january , conferred upon the village astronomer the honours so justly due to him. his professional brethren in paris were equally solicitous to testify their regard; and mm. felix roubaud, legrande, and caffe, as delegates of the scientific press, proposed to the medical body, and to the scientific world in paris, to invite lescarbault to a banquet in the hôtel du louvre on january . the announcement of the supposed discovery caused astronomers to re-examine records of former observations of black spots moving across the sun. several such records existed, but they had gradually come to be regarded as of no real importance. wolff of zurich published a list of no fewer than twenty such observations made since . carrington added many other cases. comparing together three of these observations, wolff found that they would be satisfied by a planet having a period of revolution of days, agreeing fairly with the period of rather more than - / days inferred by leverrier for lescarbault's planet. but the entire set of observations of black spots require that there should be at least three new planets travelling between mercury and the sun. many observers also set themselves the task of searching for vulcan, as the supposed new planet was called. they have continued fruitlessly to observe the sun for this purpose until the present time. while the excitement over lescarbault's discovery was at its height, another observer impugned not only the discovery but the honesty of the discoverer. m. liais, a french astronomer of considerable skill, formerly of the paris observatory, but at the time of lescarbault's achievement in the service of the brazilian government, published a paper, 'sur la nouvelle planète annoncée par m. lescarbault,' in which he endeavoured to establish the four following points:-- first, the observation of lescarbault was never made. secondly, leverrier was mistaken in considering that a planet such as vulcan might have escaped detection when off the sun's face. thirdly, that vulcan would certainly have been seen during total solar eclipses, if the planet had a real objective existence. fourthly, m. leverrier's reasons for believing that the planet exists are based on the supposition that astronomical observations are more precise than they really are. probably, liais's objections would have had more weight with leverrier had the fourth point been omitted. it was rash in a former subordinate to impugn the verdict of the chief of the paris observatory on a matter belonging to that special department of astronomy which an observatory chief might be expected to understand thoroughly. it is thought daring in the extreme for one outside the circles of official astronomy (as newton in flamstead's time, sir w. herschel in maskelyne's, and sir j. herschel in the present century), to advance or maintain an opinion adverse to that of some official chief, but for a subordinate (even though no longer so), to be guilty of such rash procedure 'is most tolerable and not to be endured,' as a typical official has said. accordingly, very little attention was paid by leverrier to liais's objections. yet, in some respects, what m. liais had to say was very much to the point. at the very time when lescarbault was watching the black spot on the sun's face, liais was examining the sun with a telescope of much greater magnifying power, and saw no such spot. his attention was specially directed to the edge of the sun (where lescarbault saw the spot) because he was engaged in determining the decrease of the sun's brightness near the edge. moreover, he was examining the very part of the sun's edge where lescarbault saw the planet enter, at a time when it must have been twelve minutes in time upon the face of the sun, and well within the margin of the solar disc. the negative evidence here is strong; though it must always be remembered that negative evidence requires to be overwhelmingly strong before it can be admitted as effective against positive evidence. it seems at a first view utterly impossible that liais, examining with a more powerful telescope the region where lescarbault saw the spot, could have failed to see it had it been there; but experience shows that it is not impossible for an observer engaged in examining phenomena of one class to overlook a phenomenon of another class, even when glaringly obvious. all we can say is that liais was not likely to have overlooked lescarbault's planet had it been there; and we must combine this probability against vulcan's existence with arguments derived from other considerations. there is also the possibility of an error in time. as the writer in the 'north british review' remarks, 'twelve minutes is so short a time that it is just possible that the planet may not have entered upon the sun during the time that liais observed it.' the second and third arguments are stronger. in fact, i do not see how they can be resisted. it is, in the first place, clear from lescarbault's account that vulcan must have a considerable diameter--certainly if vulcan's diameter in miles were only half the diameter of mercury, it would have been all but impossible for lescarbault with his small telescope to see vulcan at all, whereas he saw the black spot very distinctly. say vulcan has half the diameter of mercury, and let us compare the brightness of these two planets when at their greatest apparent distances from the sun, that is, when each looks like a half-moon. the distance of mercury exceeds the estimated distance of vulcan from the sun as exceeds , so that vulcan is more strongly illuminated in the proportion of times to times , or to --say at least to . but having a diameter but half as large the disc of vulcan could be but about a fourth of mercury's at the same distance from us (and they would be at about the same distance from us when seen as half-moons). hence vulcan would be brighter than mercury in the proportion of to . of course being so near the sun he would not be so easily seen; and we could never expect to see him at all, perhaps, with the naked eye--though even this is not certain. but mercury, when at the same apparent distance from the sun, and giving less light than at his greatest seeming distance, is quite easily seen in the telescope. much more easily, then, should vulcan be seen, if a telescope were rightly directed at such a time, or when vulcan was anywhere near his greatest seeming distance from the sun. now it is true astronomers do not know precisely when or where to look for him. but he passes from his greatest distance on one side of the sun to his greatest distance on the other in less than ten days, according to the computed period, and certainly (that is, if the planet exists) in a very short time. the astronomer has then only to examine day after day a region of small extent on either side of the sun, for ten or twelve days in succession (an hour's observation each day would suffice), to be sure of seeing vulcan. yet many astronomers have made such search many times over, without seeing any trace of the planet. during total solar eclipses, again, the planet has been repeatedly looked for unsuccessfully--though it should at such a time be a very conspicuous object, when favourably placed, and could scarcely fail of being very distinctly seen wherever placed. the fourth argument of lescarbault's is not so effective, and in fact he gets beyond his depth in dealing with it. but it is to be noticed that a considerable portion of the discrepancy between mercury's observed and calculated motions has long since been accounted for by the changed estimate of the earth's mass as compared with the sun's, resulting from the new determination of the sun's distance. however, the arguments depending on this consideration would not be suited to these pages. there was one feature in liais's paper which was a little unfortunate. he questioned lescarbault's honesty. he said 'lescarbault contradicts himself in having first asserted that he saw the planet enter upon the sun's disc, and having afterwards admitted to leverrier that it had been on the disc some seconds before he saw it, and that he had merely inferred the time of its entry from the rate of its motion afterwards. if this one assertion be fabricated, the whole may be so.' 'he considers these arguments to be strengthened,' says the 'north british review,' 'by the assertion which, as we have seen, perplexed leverrier himself, that if m. lescarbault had actually seen a planet on the sun, he could not have kept it secret for nine months.' this charge of dishonesty, unfortunate in itself, had the unfortunate effect of preventing lescarbault or the abbé moigno from replying. the latter simply remarked that the accusation was of such a nature as to dispense him from any obligation to refute it. this was an error of judgment, i cannot but think, if an effective reply was really available. the remarks with which the north british reviewer closes his account may be repeated now, so far as they relate to the force of the negative evidence, with tenfold effect. 'since the first notice of the discovery in the beginning of january the sun has been anxiously observed by astronomers; and the limited area around him in which the planet _must be_, if he is not upon the sun, has doubtless been explored with equal care by telescopes of high power, and processes by which the sun's direct light has been excluded from the tube of the telescope as well as the eye of the observer, and yet no planet has been found. this fact would entitle us to conclude that no such planet exists if its existence had been merely conjectured, or if it had been deduced from any of the laws of planetary distance, or even if leverrier or adams had announced it as the probable result of planetary perturbations. if the finest telescopes cannot rediscover a planet which with the small power used by lescarbault has a visible disc, within so limited an area of which the sun is the centre, or rather within a narrow belt of that circle, we should unhesitatingly declare that no such planet exists. but the question assumes a very different aspect when it involves moral considerations. if,' proceeds the reviewer, writing in august , 'after the severe scrutiny which the sun and its vicinity will undergo before and after and during his total eclipse in july, no planet shall be seen; and if no round black spot distinctly separable from the usual solar spots shall be seen on the solar spots' (_sic_, presumably solar disc was intended), 'we will not dare to say that it does not exist. we cannot doubt the honesty of m. lescarbault, and we can hardly believe that he was mistaken. no solar spot, no floating scoria, could maintain in its passage over the sun a circular and uniform shape, and we are confident that no other hypothesis but that of an intra-mercurial planet can explain the phenomena seen and measured by m. lescarbault, a man of high character, possessing excellent instruments, and in every way competent to use them well, and to describe clearly and correctly the results of his observations. time, however, tries facts as well as speculations. the phenomena observed by the french astronomer may never be again seen, and the disturbance of mercury which rendered it probable may be otherwise explained. should this be the case, we must refer the round spot on the sun to some of those illusions of the eye or of the brain which have sometimes disturbed the tranquillity of science.' the evidence which has accumulated against vulcan in the interval since this was written is not negative only, but partly positive, as the following instance, which i take from my own narrative at the time in a weekly journal, serves to show:--after more than sixteen years of fruitless watching, astronomers learned last august ( ) that in the month of april vulcan had been seen on the sun's disc in china. on april , it appeared, herr weber, an observer of considerable skill, stationed at pecheli, had seen a small round spot on the sun, looking very much as a small planet might be expected to look. a few hours later he turned his telescope upon the sun, and lo! the spot had vanished, precisely as though the planet had passed away after the manner of planets in transit. he forwarded the news of his observation to europe. the astronomer wolff, well known for his sun-spot studies, carefully calculated the interval which had passed since lescarbault saw vulcan on march , , and to his intense satisfaction was enabled to announce that this interval contained the calculated period of the planet an exact number of times. leverrier at paris received the announcement still more joyfully; while the abbé moigno, who gave vulcan its name, and has always staunchly believed in the planet's existence, congratulated lescarbault warmly upon this new view of the shamefaced vulcan. not one of those who already believed in the planet had the least doubt as to the reality of weber's observations, and of these only lescarbault himself received the news without pleasure. he, it seems, has never forgiven the germans for destroying his observatory and library during the invasion of france in , and apparently would prefer that his planet should never be seen again rather than that a german astronomer should have seen it. but the joy of the rest and lescarbault's sorrow were alike premature. it was found that the spot seen by weber had not only been observed at the madrid observatory, where careful watch is kept upon the sun, but had been photographed at greenwich; and when the description of its appearance, as seen in a powerful telescope at one station, and its picture as photographed by a fine telescope at the other, came to be examined, it was proved unmistakably that the spot was an ordinary sun-spot (not even quite round), which had after a few hours disappeared, as even larger sun-spots have been known to do in even a shorter time. it is clear that had not weber's spot been fortunately seen at madrid and photographed at greenwich, his observation would have been added to the list of recorded apparitions of vulcan in transit, for it fitted in perfectly with the theory of vulcan's real existence. i think, indeed, for my own part, that the good fortune was weber's. had it so chanced that thick weather in madrid and at greenwich had destroyed the evidence actually obtained to show that what weber described he really saw, although it was not what he thought, some of the more suspicious would have questioned whether, in the euphonious language of the north british reviewer, 'the round spot on the sun' was not due 'to one of those illusions of the eye or of the brain which have sometimes disturbed the tranquillity of science.' of course no one acquainted with m. weber's antecedents would imagine for a moment that he had invented the observation, even though the objective reality of his spot had not been established. but if a person who is entirely unknown, states that he has seen vulcan, there is antecedently some degree of probability in favour of the belief that the observation is as much a myth as the planet itself. some observations of vulcan have certainly been invented. i have received several letters purporting to describe observations of bodies in transit over the sun's face, either the rate of transit, the size of the body, or the path along which it was said to move, being utterly inconsistent with the theory that it was an intra-mercurial planet, while yet (herein is the suspicious circumstance of such narratives) the epoch of transit accorded in the most remarkable manner with the period assigned to vulcan. a paradoxist in america (of louisville, kentucky) who had invented a theory of the weather, in which the planets, by their influence on the sun, were supposed to produce all weather-changes, the nearer planets being the most effective, found his theory wanted vulcan very much. accordingly, he saw vulcan crossing the sun's face in september, which, being half a year from march, is a month wherein, according to lescarbault's observation, vulcan may be seen in transit, and by a strange coincidence the interval between our paradoxist's observation and lescarbault's exactly contained a certain number of times the period calculated by leverrier for vulcan. this was a noble achievement on the part of our paradoxist. at one stroke it established his theory of the weather, and promised to ensure him text-book immortality as one of the observers of vulcan. but, unfortunately, a student of science residing in st. louis, after leaving the louisville paradoxist full time to parade his discovery, heartlessly pointed out that an exact number of revolutions of vulcan after lescarbault's march observation, must of necessity have brought the planet on that side of the sun on which the earth lies in march, so that to see vulcan so placed on the sun's face in september was to see vulcan through the sun, a very remarkable achievement indeed. the paradoxist was abashed, the reader perhaps imagines. not in the least. the planet's period must have been wrongly calculated by leverrier--that was all: the real period was less than half as long as leverrier had supposed; and instead of having gone a certain number of times round since lescarbault had seen it, vulcan had gone twice as many times round and half once round again. the circumstance that if vulcan's period had been thus short, the time of crossing the sun's face would have been much less than, according to lescarbault's account, it actually was, had not occurred to the louisville weather-prophet.[ ] leverrier's faith in vulcan, however, has remained unshaken. he has used all the observations of spots which, like weber's, have been seen only for a short time. at least he has used all which have not, like weber's, been proved to be only transient sun-spots. selecting those which fit in well with lescarbault's observation, he has pointed out how remarkable it is that they show this accord. the possibility that some of them might be explicable as weber's proved to be, and that some even may have been explicable as completely, but less satisfactorily, in another way, seems to have been thought scarcely worth considering. using the imperfect materials available, but with exquisite skill--as a phidias might model an exquisite figure of materials that would presently crumble into dust--leverrier came to the conclusion that vulcan would cross the sun's disc on or about march , . 'he, therefore,' said sir g. airy, addressing the astronomical society, 'circulated a despatch among his friends, asking them carefully to observe the sun on march .' sir g. airy, humouring his honoured friend, sent telegrams to india, australia, and new zealand, requesting that observations might be made every two hours or oftener. leverrier himself wrote to santiago de chili and other places, so that, including american and european observations, the sun could be watched all through the twenty-four hours on march , , and . 'without saying positively that he believed or disbelieved in the existence of the planet,' proceeds the report, 'sir g. airy thought, since m. leverrier was so confident, that the opportunity ought not to be neglected by anybody who professed to take an interest in the progress of planetary astronomy.' it is perhaps unnecessary to add that observations were made as requested. many photographs of the sun also were taken during the hours when vulcan, if he exists at all, might be expected to cross the sun's face. but the 'planet of romance,' as abbé moigno has called vulcan, failed to appear, and the opinion i had expressed last october ('english mechanic and world of science,' for october , , p. ), that vulcan might perhaps better be called the 'planet of fiction' was _pro tanto_ confirmed. nevertheless, i would not be understood to mean by the word 'fiction' aught savouring of fraud so far as lescarbault is concerned--i prefer the north briton's view of lescarbault's spot, that so to speak, it was ... the blot upon his brain, that _would_ show itself without. i have left small space to treat of other fancied discoveries among the orbs of heaven. yet there are some which are not only interesting but instructive, as showing how even the most careful observers may be led astray. in this respect the mistakes into which observers of great and well deserved eminence have fallen are specially worthy of attention. with the description of three such mistakes, made by no less an astronomer than sir w. herschel, i shall bring this paper to a close. when sir w. herschel examined the planet uranus with his most powerful telescope he saw the planet to all appearance girt about by two rings at right angles to one another. the illusion was so complete that herschel for several years remained in the belief that the rings were real. they were, however, mere optical illusions, due to the imperfect defining qualities of the telescope with which he observed the planet. later he wrote that 'the observations which tend to ascertain' (indicate?) 'the existence of rings not being satisfactorily supported, it will be proper that surmises of them should either be given up, as ill-founded, or at least reserved till superior instruments can be provided.' sir w. herschel was more completely misled by the false uranian satellites. he had seen, as he supposed, no less than six of these bodies. as only two of these had been seen again, while two more were discovered by lassell, the inference was that uranus has eight satellites in all. these for a long time flourished in our text-books of astronomy; and many writers, confident in the care and skill of sir w. herschel, were unable for a long time to believe that he had been deceived. thus admiral smyth, in his 'celestial cycle,' wrote of those who doubted the extra satellites:--'they must have but a meagre notion of sir w. herschel's powerful means, his skill in their application, and his method of deliberate procedure. so far from doubting there being six satellites' (this was before lassell had discovered the other two) 'it is highly probable that there are still more.' whewell, also, in his 'bridgewater treatise,' says, 'that though it no longer appears probable that uranus has a ring like saturn, he has at least five satellites which are visible to us, and we believe that the astronomer will hardly deny that he' (uranus, not the astronomer), 'may possibly have thousands of smaller ones circulating about him.' but in this case sir w. herschel, anxiously though he endeavoured to guard against the possibility of error, was certainly mistaken. uranus may, for anything that is known to the contrary, have many small satellites circulating about him, but he certainly has not four satellites (besides those known) which could have been seen by sir w. herschel with the telescope he employed. for the neighbourhood of the planet has been carefully examined with telescopes of much greater power by observers who with those telescopes have seen objects far fainter than the satellites supposed to have been seen by the elder herschel. the third of the herschelian myths was the lunar volcano in eruption, which he supposed he had seen in progress in that part of the moon which was not at the time illuminated by the sun's rays. he saw a bright star-like point of light, which corresponded in position with the crater of the lunar mountain aristarchus. he inferred that a volcano was in active eruption because the brightness of the point of light varied from time to time, and also because he did not remember to have seen it before under the same conditions. there is no doubt something very remarkable in the way in which this part of the moon's surface shines when not illumined by the sun. if it were always bright we should conclude at once that the earth-light shining upon it rendered it visible. for it must be remembered that the part of the moon which looks dark (or seems wanting to the full disc) is illuminated by our earth, shining in the sky of the moon as a disc thirteen times as large as that of the moon we see, and with the same proportion of its disc sunlit as is dark in the moon's disc. thus when the moon is nearly new our earth is shining in the lunar skies as a nearly full moon thirteen times as large as ours. the light of this noble moon must illumine the moon's surface much more brightly than a terrestrial landscape is illumined by the full moon, and if any parts of her surface are very white they will shine out from the surface around, just as the snow-covered peak of a mountain shines out upon a moonlit night from among the darker hills and dales and rocks and forests of the landscape. but herschel considered that the occasional brightness of the crater aristarchus could not be thus explained. the spot had been seen before the time of herschel's observations by cassini and others. it has been seen since by captain kater, francis baily, and many others. dr. maskelyne tells us that in march it was seen by the naked eye by two persons. baily thus describes the appearance presented by this lunar crater on december , : 'directed telescope to the moon, and pointing it to the dark part in the vicinity of aristarchus, soon saw the outline of that mountain very distinctly, formed like an irregular nebula. nearly in the centre was a light resembling that of a star of the ninth or tenth magnitude. it appeared by glimpses, but at times was brilliant, and visible for several seconds together.' there can be little doubt, however, that the apparent brightness of this lunar crater, or rather of its summit, is due to some peculiar quality in the surface, which may perhaps be covered by some crystalline or vitreous matter poured out in the far distant time when the crater was an active one. prof. shaler, who examined the crater when it was illuminated only by earthshine, with the fine -inch telescope of the harvard observatory (cambridge u.s.), says that he has been able to recognise nearly all the craters over miles in diameter in the dark part. 'there are several degrees of brightness,' he says, 'observable in the different objects which shine out by the earth-light. this fact probably explains the greater part of the perplexing statements concerning the illumination of certain craters. it certainly accounts for the volcanic activity which has so often been supposed to be manifested by aristarchus. under the illumination by the earth-light this is by far the brightest object on the dark part of the moon's face, and is visible much longer and with poorer glasses than any other object there.' here my record of astronomical myths must be brought to a close. it will be noticed that in every instance either the illusion has affected the actual observations of eminent and skilful astronomers, or has caused such astronomers to put faith for a while in illusory observations. had i cared to include the mistakes which have been made by or have misled observers of less experience, i could have filled many sheets for each page of the present article. but it has seemed to me more instructive to show how error may affect the observations even of the most careful and deservedly eminent astronomers, how even the most cautious may be for a time misled by the mistakes of inferior observers, especially when the fact supposed to have been observed accords with preconceived opinions. xii. _the origin of the constellation-figures._ although the strange figures which astronomers still allow to straggle over their star maps no longer have any real scientific interest, they still possess a certain charm, not only for the student of astronomy, but for many who care little or nothing about astronomy as a science. when i was giving a course of twelve lectures in boston, america, a person of considerable culture said to me, 'i wish you would lecture about the constellations; i care little about the sun and moon and the planets, and not much more about comets; but i have always felt great interest in the bears and lions, the chained and chaired ladies, king cepheus and the rescuer perseus, orion, ophiuchus, hercules, and the rest of the mythical and fanciful beings with which the old astronomers peopled the heavens. i say with carlyle, "why does not some one teach me the constellations, and make me at home in the starry heavens, which are always overhead, and which i don't half know to this day."' we may notice, too, that the poets by almost unanimous consent have recognised the poetical aspect of the constellations, while they have found little to say about subjects which belong especially to astronomy as a science. milton has indeed made an archangel reason (not unskilfully for milton's day) about the ptolemaic and copernican systems, while tennyson makes frequent reference to astronomical theories. 'there sinks the nebulous star we call the sun, if that hypothesis of theirs be sound,' said ida; but she said no more, save 'let us down and rest,' as though the subject were wearisome to her. again, in the palace of art the soul of the poet having built herself that 'great house so royal, rich, and wide,' thither-- ... when all the deep unsounded skies shuddered with silent stars, she clomb, and as with optic glasses her keen eyes pierced through the mystic dome, regions of lucid matter taking forms, brushes of fire, hazy gleams, clusters and beds of worlds and beelike swarms of suns, and starry streams: she saw the snowy poles of moonless mars, that marvellous round of milky light below orion, and those double stars whereof the one more bright is circled by the other. but the poet's soul so wearied of these astronomical researches that the beautiful lines i have quoted disappeared (more's the pity) from the second and all later editions. such exceptions, indeed, prove the rule. poets have been chary in referring to astronomical researches and results, full though these have been of unspeakable poetry; while from the days of homer to those of tennyson, the constellations which 'garland the heavens' have always been favourite subjects of poetic imagery. it is not my present purpose, however, to discuss the poetic aspect of the constellations. i propose to inquire how these singular figures first found their way to the heavens, and, so far as facts are available for the purpose, to determine the history and antiquity of some of the more celebrated constellations. long before astronomy had any existence as a science men watched the stars with wonder and reverence. those orbs, seemingly countless--which bespangle the dark robe of night--have a charm and beauty of their own apart from the significance with which the science of astronomy has invested them. the least fanciful mind is led to recognise on the celestial concave the emblems of terrestrial objects, pictured with more or less distinctness among the mysterious star-groupings. we can imagine that long before the importance of the study of the stars was recognised, men had begun to associate with certain star-groups the names of familiar objects animate or inanimate. the flocks and herds which the earliest observers of the heavens tended would suggest names for certain sets of stars, and thus the bull, the ram, the kids, would appear in the heavens. other groups would remind those early observers of the animals from whom they had to guard their flocks, or of the animals to whose vigilance they trusted for protection, and thus the bear, the lion, and the dogs would find their place among the stars. the figures of men and horses, and of birds and fishes, would naturally enough be recognised, nor would either the implements of husbandry, or the weapons by which the huntsman secured his prey, remain unrepresented among the star-groupings. and lastly, the altar on which the first-fruits of harvest and vintage were presented, or the flesh of lambs and goats consumed, would be figured among the innumerable combinations which a fanciful eye can recognise among the orbs of heaven. in thus suggesting that the first observers of the heavens were shepherds, huntsmen, and husbandmen, i am not advancing a theory on the difficult questions connected with the origin of exact astronomy. the first observations of the heavens were of necessity made by men who depended for their subsistence on a familiarity with the progress and vicissitudes of the seasons, and doubtless preceded by many ages the study of astronomy as a science. and yet the observations made by those early shepherds and hunters, unscientific though they must have been in themselves, are full of interest to the student of modern exact astronomy. the assertion may seem strange at first sight, but is nevertheless strictly true, that if we could but learn with certainty the names assigned to certain star-groups, before astronomy had any real existence, we could deduce lessons of extreme importance from the rough observations which suggested those old names. in these days, when observations of such marvellous exactness are daily and nightly made, when instruments capable of revealing the actual constitution of the stars are employed, and observers are so numerous, it may seem strange to attach any interest to the question whether half-savage races recognised in such and such a star-group the likeness of a bear, or in another group the semblance of a ship. but though we could learn more, of course, from exacter observations, yet even such rough and imperfect records would have their value. if we could be certain that in long-past ages a star-group really resembled some known object, we should have in the present resemblance of that group to the same object evidence of the general constancy of stellar lustre, or if no resemblance could be recognised we should have reason to doubt whether other suns (and therefore our own sun) may not be liable to great changes. the subject of the constellation-figures as first known is interesting in other ways. for instance, it is full of interest to the antiquarian (and most of us are to some degree antiquarians) as relating to the most ancient of all human sciences. the same mental quality which causes us to look with interest on the buildings raised in long-past ages, or on the implements and weapons of antiquity, renders the thought impressive that the stars which we see were gazed on perhaps not less wonderingly in the very infancy of the human race. it is, again, a subject full of interest to the chronologist to inquire in what era of the world's history exact astronomy began, the moon was assigned her twenty-eight zodiacal mansions, the sun his twelve zodiacal signs. it is well known, indeed, that newton himself did not disdain to study the questions thus suggested; and the speculations of the ingenious dupuis found favour with the great mathematician laplace. unfortunately, the evidence is not sufficiently exact to be very trustworthy. in considering, for instance, the chronological inquiries of newton, one cannot but feel that the reliance placed by him on the statements made by different writers is not justified by the nature of those statements, which were for the most part vague in the extreme. we owe many of them to poets who, knowing little of astronomy, mixed up the phenomena of their own time with those which they found recorded in the writings of astronomers. some of the statements left by ancient writers are indeed ludicrously incongruous; insomuch that grotius not unjustly said of the account of the constellations given by the poet aratus, that it could be assigned to no fixed epoch and to no fixed place. however, this would not be the place to discuss details such as are involved in exact inquiries. i have indicated some of these in an appendix to my treatise on 'saturn,' and others in the preface to my 'gnomonic star atlas'; but for the most part they do not admit very readily of familiar description. let us turn to less technical considerations, which fortunately are in this case fully as much to the point as exact inquiries, seeing that there is no real foundation for such inquiries in any of the available evidence. the first obvious feature of the old constellations is one which somehow has not received the attention it deserves. it is as instructive as any of those which have been made the subject of profound research. there is a great space in the heavens over which none of the old constellations extend, except the river eridanus as now pictured, but we do not know where this winding stream of stars was supposed by the old observers to come to an end. this great space surrounds the southern pole of the heavens, and thus shows that the first observers of the stars were not acquainted with the constellations which can be seen only from places far south of chaldæa, persia, egypt, india, china, and indeed of all the regions to which the invention of astronomy has been assigned. whatever the first astronomers were, however profound their knowledge of astronomy may have been (as some imagine), they had certainly not travelled far enough towards the south to know the constellations around the southern pole. if they had been as well acquainted with geography as some assert, if even any astronomer had travelled as far south as the equator, we should certainly have had pictured in the old star charts some constellations in that region of the heavens wherein modern astronomers have placed the octant, the bird of paradise, the sword-fish, the flying-fish, toucan, the net, and other uncelestial objects. in passing i may note that this fact disposes most completely of a theory lately advanced that the constellations were invented in the southern hemisphere, and that thus is to be explained the ancient tradition that the sun and stars have changed their courses. for though all the northern constellations would have been more or less visible from parts of the southern hemisphere near the equator, it is absurd to suppose that a southern observer would leave untenanted a full fourth of the heavens round the southern or visible pole, while carefully filling up the space around the northern or unseen pole with incomplete constellations whose northern unknown portions would include that pole. supposing it for a moment to be true, as a modern advocate of the southern theory remarks, that 'one of the race migrating from one side to the other of the equator would take his position from the sun, and fancy he was facing the same way when he looked at it at noon, and so would think the motion of the stars to have altered instead of his having turned round,' the theory that astronomy was brought to us from south of the equator cannot possibly be admitted in presence of that enormous vacant region around the southern pole. i think, however, that, apart from this, a race so profoundly ignorant as to suppose any such thing, to imagine they were looking north when in reality they were looking south, can hardly be regarded as the first founders of the science of astronomy. the great gap i have spoken of has long been recognised. but one remarkable feature in its position has not, to the best of my remembrance, been considered--the vacant space is eccentric with regard to the southern pole of the heavens. the old constellations, the altar, the centaur, and the ship argo, extend within twenty degrees of the pole, while the southern fish and the great sea-monster cetus, which are the southernmost constellations on the other side, do not reach within some sixty degrees of the pole. of course, in saying that this peculiarity has not been considered, i am not suggesting that it has not been noticed, or that its cause is in any way doubtful or unknown. we know that the earth, besides whirling once a day on its axis, and rushing on its mighty orbit around the sun (spanning some , , of miles) reels like a gigantic top, with a motion so slow that , years are required for a single circuit of the swaying axis around an imaginary line upright to the plane in which the earth travels. and we know that in consequence of this reeling motion the points of the heavens opposite the earth's poles necessarily change. so that the southern pole, now eccentrically placed amid the region where there were no constellations in old times, was once differently situated. but the circumstance which seems to have been overlooked is this, that by calculating backwards to the time when the southern pole was in the centre of that vacant region, we have a much better chance of finding the date (let us rather say the century) when the older constellations were formed, than by any other process. we may be sure not to be led very far astray; for we are not guided by one constellation but by several, whereas all the other indications which have been followed depend on the supposed ancient position of single constellations. and then most of the other indications are such as might very well have belonged to periods following long after the invention of the constellations themselves. an astronomer might have ascertained, for instance, that the sun in spring was in some particular part of the ram or of the fishes, and later a poet like aratus might describe that relation (erroneously for his own epoch) as characteristic of one or other constellation; but who is to assure us that the astronomer who noted the relation correctly may not have made his observation many hundreds of years after those constellations were invented? whereas, there was one period, and only one period, when the most southernmost of the old constellations could have marked the limits of the region of sky visible from some northern region. thus, too, may we form some idea of the latitude in which the first observers lived. for in high latitudes the southernmost of the old constellations would not have been visible at all, and in latitudes much lower than a certain latitude, presently to be noted, these constellations would have ridden high above the southern horizon, other star-groups showing below them which were not included among the old constellations. i have before me as i write a picture of the southern heavens, drawn by myself, in which this vacant space--eccentric in position but circular in shape--is shown. the centre lies close by the lesser magellanic cloud--between the stars kappa toucani and eta hydri of our modern maps, but much nearer to the last named. near this spot, then, we may be sure, lay the southern pole of the star-sphere when the old constellations, or at least the southern ones, were invented. (if there had been astronomers in the southern hemisphere eta hydri would certainly have been their pole-star.) now it is a matter of no difficulty whatever to determine the epoch when the southern pole of the heavens was thus placed.[ ] between and years before the christian era the southern constellations had the position described, the invisible southern pole lying at the centre of the vacant space of the star-sphere--or rather of the space free from constellations. it is noteworthy that for other reasons this period, or rather a definite epoch within it, is indicated as that to which must be referred the beginning of exact astronomy. amongst others must be mentioned this--that in the year b.c. _quam proximè_, the pleiades rose to their highest above the horizon at noon (or technically made their noon culmination), at the spring equinox. we can readily understand that to minds possessed with full faith in the influence of the stars on the earth, this fact would have great significance. the changes which are brought about at that season of the year, in reality, of course, because of the gradual increase in the effect of the sun's rays as he rises higher and higher above the celestial equator, would be attributed, in part at least, to the remarkable star-cluster coming then close by the sun on the heavens, though unseen. thus we can readily understand the reference in job to the 'sweet influences of the pleiades.' again at that same time, b.c. when the sun and the pleiades opened the year (with commencing spring) together, the star alpha of the dragon, which was the pole-star of the period, had that precise position with respect to the true pole of the heavens which is indicated by the slope of the long passage extending downwards aslant from the northern face of the great pyramid; that is to say, when due north below the pole (or at what is technically called its sub-polar meridional passage) the pole-star of the period shone directly down that long passage, and i doubt not could be seen not only when it came to that position during the night, but also when it came there during the day-time. but some other singular relations are to be noted in connection with the particular epoch i have indicated. it is tolerably clear that in imagining figures of certain objects in the heavens, the early observers would not be apt to picture these objects in unusual positions. a group of stars may form a figure so closely resembling that of a familiar object that even a wrong position would not prevent the resemblance from being noticed, as for instance the 'chair,' the 'plough,' and so forth. but such cases are not numerous; indeed, to say the truth, one must 'make believe a good deal' to see resemblance between the star-groups and _most_ of the constellation-figures, even under the most favourable conditions. when there is no very close resemblance, as is the case with all the large constellations, position must have counted for something in determining the association between a star-group and a known object. now the constellations north of the equator assume so many and such various positions that this special consideration does not apply very forcibly to them. but those south of the equator are only seen above the southern horizon, and change little in position during their progress from east to west of the south point. the lower down they are the less they change in position. and the very lowest--such as those were, for instance, which i have been considering in determining the position of the southern pole--are only fully visible when due south. they must, then, in all probability, have stood upright or in their natural position when so placed, for if they were not rightly placed then they only were so when below the horizon and consequently invisible. let us, then, inquire what was the position of the southernmost constellations when fully seen above the southern horizon at midnight. the centaur stood then as he does now, upright; only--whereas now in egypt, chaldæa, india, persia, and china, only the upper portions of his figure rise above the horizon, he then stood, the noblest save orion of all the constellations, with his feet (marked by the bright alpha and beta still belonging to the constellation, and by the stars of the southern cross which have been taken from it) upon the horizon itself. in latitude twenty degrees or so north he may still be seen thus placed when due south. the centaur was represented in old times as placing an offering upon the altar, which was pictured, says manilius, as bearing a fire of incense represented by stars. this to a student of our modern charts seems altogether perplexing. the centaur carries the wolf on the end of his spear; but instead of placing the wolf (not a very acceptable meat offering, one would suppose) upon the altar, he is directing this animal towards the base of the altar, whose top is downwards, the flames represented there tending (naturally) downwards also. it is quite certain the ancient observers did not imagine anything of this sort. as i have said, aratus tells us the celestial centaur was placing an offering _upon_ the altar, which was therefore upright, and manilius describes the altar as ferens thuris, stellis imitantibus, ignem, so that the fire was where it should be, on the top of an upright altar, where also on the sky itself were stars looking like the smoke from incense fires. now that was precisely the appearance presented by the stars forming the constellation at the time i have indicated, some years b.c. setting the altar upright above the southern horizon (that is, inverting the absurd picture at present given of it) we see it just where it should be placed to receive the centaur's offering. a most remarkable portion of the milky way is then seen to be directly above the altar in such a way as to form a very good imitation of smoke ascending from it. this part of the milky way is described by sir j. herschel, who studied it carefully during his stay at the cape of good hope, as forming a complicated system of interlaced streaks and masses which covers the tail of scorpio (extending from the altar which lies immediately south of the scorpion's tail). the milky way divides, in fact, just above the altar as the constellation was seen years ago above the southern horizon, one branch being that just described, the other (like another stream of smoke) 'passing,' says herschel, 'over the stars iota of the altar, theta and iota of the scorpion, etc., to gamma of the archer, where it suddenly collects into a vivid oval mass, so very rich in stars that a very moderate calculation makes their number exceed , .' nothing could accord better with the descriptions of aratus and manilius. but there is another constellation which shows in a more marked way than either the centaur or the altar that the date when the constellations were invented must have been near that which i have named. both ara and centaurus look now in suitable latitudes (about twenty degrees north) as they looked in higher latitudes (about forty degrees north) years ago. for, the reeling motion of our earth has changed the place of the celestial pole in such a way as only to depress these constellations southwards without much changing their _position_; they are nearly upright when due south now as they were years ago, only lower down. but the great ship argo has suffered a much more serious displacement. one cannot now see this ship _like_ a ship at any time or from any place on the earth's surface. if we travel south till the whole constellation comes into visibility above the southern horizon at the proper season (january and february for the midnight hours) the keel of the ship is aslant, the stern being high above the waist (the fore part is wanting). if we travel still further south, we can indeed reach places where the course of the ship is so widened, and the changes of position so increased, that she appears along part of her journey on an even keel, but then she is high above the horizon. now years ago she stood on the horizon itself at her southern culmination, with level keel and upright mast. in passing i may note that for my own part i imagine that this great ship represented the ark, its fore part being originally the portion of the centaur now forming the horse, so that the centaur was represented as a man (not as a man-horse) offering a gift on the altar. thus in this group of constellations i recognise the ark, and noah going up from the ark towards the altar 'which he builded unto the lord; and took of every clean beast, and of every clean fowl, and offered burnt offerings on the altar.' i consider further that the constellation-figures of the ship, the man with an offering, and the altar, painted or sculptured in some ancient astrological temple, came at a later time to be understood as picturing a certain series of events, interpreted and expanded by a poetical writer into a complete narrative. without venturing to insist on so heterodox a notion, i may remark as an odd coincidence that probably such a picture or sculpture would have shown the smoke ascending from the altar which i have already described, and in this smoke there would be shown the bow of sagittarius; which, interpreted and expanded in the way i have mentioned, might have accounted for the 'bow set in the clouds, for a token of a covenant.' it is noteworthy that all the remaining constellations forming the southern limit of the old star-domes or charts, were watery ones--the southern fish, over which aquarius is pouring a quite unnecessary stream of water, the great sea monster towards which in turn flow the streams of the river eridanus. the equator, too, was then occupied along a great part of its length by the great sea serpent hydra, which reared its head above the equator, very probably indicated then by a water horizon, for nearly all the signs below it were then watery. at any rate, as the length of hydra then lay horizontally above the ship, whose masts reached it, we may well believe that this part of the picture of the heavens showed a sea-horizon and a ship, the great sea serpent lying along the horizon. on the back of hydra is the raven, which again may be supposed by those who accept the theory mentioned above to have suggested the raven which went forth to and fro from the ark. he is close enough to the rigging of argo to make an easy journey of it. the dove, however, must not be confounded with the modern constellation columba, though this is placed (suitably enough) near the ark. we must suppose the idea of the dove was suggested by a bird pictured in the rigging of the celestial ship. the sequence in which the constellations came above the horizon as the year went round corresponded very satisfactorily with the theory, fanciful though this seem to some. first aquarius pouring streams of water, the three fishes (pisces and piscis australis), and the great sea monster cetus, showing how the waters prevailed over the highest hills, then the ark sailing on the waters, a little later the raven (corvus), the man descending from the ark and offering a gift on the altar, and last the bow set amid the clouds. the theory just described may not meet with much favour. but wilder theories of the story of the deluge have been adopted and advocated with considerable confidence. one of the wildest, i fear, is the astronomer-royal's, that the deluge was simply a great rising of the nile; and sir g. airy is so confident respecting this that he says, 'i cannot entertain the smallest doubt that the flood of noah was a flood of the nile;' precisely as he might say, 'i cannot entertain the smallest doubt that the earth moves round the sun.' on one point we can entertain very little doubt indeed. if it ever rained before the flood, which seems probable, and if the sun ever shone on falling rain, which again seems likely, nothing short of a miracle could have prevented the rainbow from making its appearance before the flood. the wildest theory that can be invented to explain the story of the deluge cannot be wilder than the supposition that the rays of sunlight shining on falling raindrops could have ever failed to show the prismatic colours. the theory i have suggested above, without going so far as strongly to advocate it, far less insist upon it, is free at any rate from objection on this particular score, which cannot be said of the ordinary theory. i am not yet able, however, to say that 'i cannot entertain the smallest doubt' about my theory. we may feel tolerably sure that the period when the old southern constellations were formed must have been between and years before the present era, a period, by the way, including the date usually assigned to the deluge,--which, however, must really occupy our attention no further. in fact, let us leave the watery constellations lying below the equator of those remote times and seek at once the highest heavens above them. here, at the northern pole of these days, we find the great dragon, which in any astrological temple of the time must have formed the highest or crowning constellation, surrounding the very key-stone of the dome. he has fallen away from that proud position since. in fact, even years ago he only held to the pole, so to speak, by his tail, and we have to travel back years or so to find the pole situate in a portion of the length of the dragon which can be regarded as central. one might almost, if fancifully disposed, recognise the gradual displacement of the dragon from his old place of honour, in certain traditions of the downfall of the great dragon whose 'tail drew the third part of the stars of heaven.' the central position of the dragon, for even when the pole-star had drawn near to the dragon's tail the constellation was still central, will remind the classical reader of homer's description of the shield of hercules-- the scaly horror of a dragon, coil'd full in the central field, unspeakable, with eyes oblique retorted, that ascant shot gleaming fire. (_elton's translation._) i say homer's description, for i cannot understand how any one who compares together the description of the shield of achilles in the iliad and that of the shield of hercules in the fragmentary form in which we have it, can doubt for a moment that both descriptions came from the same hand. (the theory that hesiod composed the latter poem can scarcely be entertained by any scholar.) as i long since pointed out in my essay 'a new theory of achilles' shield' ('light science,' first series), no poet so inferior as actually to borrow homer's words in part of the description of the shield of hercules could have written the other parts not found in the shield of achilles. 'i cannot for my own part entertain the slightest doubt'--that is to say, i think it altogether probable--that homer composed the lines supposed to describe the shield of hercules long before he introduced the description, pruned and strengthened, into that particular part of the iliad where it served his purpose best. and i have as little doubt that the original description, of which we only get fragments in either poem, related to something far more important than a shield. the constellations are not suitable adornments for the shield of fighting man, even though he was under the special care of a celestial mother and had armour made for him by a celestial smith. yet we learn that achilles' shield displayed-- the starry lights that heav'n's high convex crown'd the pleiads, hyads, and the northern beam, and great orion's more refulgent beam,-- to which, around the cycle of the sky, the bear revolving, points his golden eye,-- still shines exalted. and so forth. the shield of hercules displayed at its centre the polar constellation the dragon. we read also that-- there was the knight of fair-hair'd danae born, perseus. orion is not specially mentioned, but orion, lepus, and the dogs seem referred to:-- men of chase were taking the fleet hares; two keen-toothed dogs bounded beside. homer would find no difficulty in pluralising the mighty hunter and the hare into huntsmen and hares when utilising a description originally referring to the constellation. i conceive that the original description related to one of those zodiac temples whose remains are still found in egypt, though the egyptian temples of this kind were probably only copies of more ancient chaldæan temples. we know from assyrian sculptures that representations of the constellations (and especially the zodiacal constellations) were common among the babylonians; and, as i point out in the essay above referred to, 'it seems probable that in a country where sabæanism or star-worship was the prevailing form of religion, yet more imposing proportions would be given to zodiac temples than in egypt.' my theory, then, respecting the two famous 'shields' is that homer in his eastern travels visited imposing temples devoted to astronomical observation and star-worship, and that nearly every line in both descriptions is borrowed from a poem in which he described a temple of this sort, its domed zodiac, and those illustrations of the labours of different seasons and of military or judicial procedures which the astrological proclivities of star-worshippers led them to associate with the different constellations. for the arguments on which this theory is based i have not here space. they are dealt with in the essay from which i have quoted. one point only i need touch upon here, besides those i have mentioned already. it may be objected that the description of a zodiac temple has nothing to connect it with the subject of the iliad. this is certainly true; but no one who is familiar with homer's manner can doubt that he would work in, if he saw the opportunity, a poem on some subject outside that of the iliad, so modifying the language that the description would correspond with the subject in hand. there are many passages, though none of such length, in both the iliad and the odyssey, which seem thus to have been brought into the poem; and other passages not exactly of this kind yet show that homer was not insensible to the advantage of occasionally using memory instead of invention. any one who considers attentively the aspect of the constellation draco in the heavens, will perceive that the drawing of the head in the maps is not correct; the head is no longer pictured as it must have been conceived by those who first formed the constellation. the two bright stars beta and gamma are now placed on a head in profile. formerly they marked the two eyes. i would not lay stress on the description of the dragon in the shield of hercules, 'with eyes oblique retorted, that askant shot gleaming fire;' for all readers may not be prepared to accept my opinion that that description related to the constellation draco. but the description of the constellation itself by aratus suffices to show that the two bright stars i have named marked the eyes of the imagined monster--in fact, aratus's account singularly resembles that given in the shield of hercules. 'swol'n is his neck,' says aratus of the dragon-- ... eyes charg'd with sparkling fire his crested head illume. as if in ire, to helice he turns his foaming jaw, and darts his tongue, barb'd with a blazing star. and the dragon's head with sparkling eyes can be recognised to this day, so soon as this change is made in its configuration, whereas no one can recognise the remotest resemblance to a dragon's head in profile. the star barbing the dragon's tongue would be xi of the dragon according to aratus's account, for so only would the eyes be turned towards helice the bear. but when aratus wrote, the practice of separating the constellations from each other had been adopted; in fact, he derived his knowledge of them chiefly from eudoxus, the astronomer and mathematician, who certainly would not have allowed the constellations to be intermixed. in the beginning, there are reasons for believing it was different, and if a group of stars resembled any known object it would be called after that object, even though some of the stars necessary to make up the figure belonged already to some other figure. this being remembered, we can have no difficulty in retorting the dragon's head more naturally--not to the star xi of the dragon, but to the star iota of hercules. the four stars are situated thus, [illustration] the larger ones representing the eyes; and so far as the head is concerned it is a matter of indifference whether the lower or the upper small star be taken to represent the tongue. but, as any one will see who looks at these stars when the dragon is best placed for ordinary (non-telescopic) observation, the attitude of the animal is far more natural when the star iota of hercules marks the tongue, for then the creature is situated like a winged serpent hovering above the horizon and looking downwards, whereas when the star xi marks the tongue, the hovering dragon is looking upwards and is in an unnaturally constrained position. (i would not, indeed, claim to understand perfectly all the ways of dragons; still it may be assumed that a dragon hovering above the horizon would rather look downwards in a natural position than upwards in an awkward one.) the star iota of hercules marks the heel of this giant, called the kneeler (engonasin) from time immemorial. he must have been an important figure on the old zodiac temples, and not improbably his presence there as one of the largest and highest of the human figures may have caused a zodiac-dome to be named after hercules. the dome of hercules would come near enough to the title, 'the shield of hercules,' borne by the fragmentary poem dealt with above. the foot of the kneeling man was represented on the head of the dragon, the dragon having hold of the heel. and here, again, some imagine that a sculptured representation of these imagined figures in the heavens may have been interpreted and expanded into the narrative of a contest between the man and the old serpent the dragon, ophiuchus the serpent-bearer being supposed to typify the eventual defeat of the dragon. this fancy might be followed out like that relating to the deluge; but the present place would be unsuitable for further inquiries in that particular direction. some interest attaches to the constellation ophiuchus, to my mind, in the evidence it affords respecting the way in which the constellations were at first intermixed. i have mentioned one instance in which, as i think, the later astronomers separated two constellations which had once been conjoined. many others can be recognised when we compare the actual star-groups with the constellation-figures as at present depicted. no one can recognise the poop of a ship in the group of stars now assigned to the stern of argo, but if we include the stars of the greater dog, and others close by, a well-shaped poop can be clearly seen. the head of the lion of our maps is as the head of a dog, so far as stars are concerned; but if stars from the crab on one side and from virgo on the other be included in the figure, and especially berenice's hair to form the tuft of the lion's tail, a very fine lion with waving mane can be discerned, with a slight effort of the imagination. so with bootes the herdsman. he was of old 'a fine figure of a man,' waving aloft his arms, and, as his name implies, shouting lustily at the retreating bear. now, and from some time certainly preceding that of eudoxus, one arm has been lopped off to fashion the northern crown, and the herdsman holds his club as close to his side as a soldier holds his shouldered musket. the constellation of the great bear, once i conceive the only bear (though the lesser bear is a very old constellation), has suffered wofully. originally it must have been a much larger bear, the stars now forming the tail marking part of the outline of the back; but first some folks who were unacquainted with the nature of bears turned the three stars (the horses of the plough) into a long tail, abstracting from the animal all the corresponding portion of his body, and then modern astronomers finding a great vacant space where formerly the bear's large frame extended, incontinently formed the stars of this space into a new constellation, the hunting dogs. no one can recognise a bear in the constellation as at present shaped, but any one who looks attentively at the part of the skies occupied by the constellation will recognise (always 'making believe a good deal') a monstrous bear, with the proper small head of creatures of the bear family, and with exceedingly well-developed plantigrade feet. of course this figure cannot at all times be recognised with equal facility; but before midnight during the last four or five months in the year, the bear occupies positions favouring his recognition, being either upright on his feet, or as if descending a slope, or squatting on his great haunches. as a long-tailed animal the creature is more like one of those wooden toy-monkeys which used to be made for children, and may be now, in which the sliding motion of a ringed rod carried the monkey over the top of a stick. the little bear has i think been borrowed from the dragon, which was certainly a winged monster originally. now the astronomers who separated from each other, and in so doing spoiled the old constellation-figures, seem to have despaired of freeing ophiuchus from his entanglements. the serpent is twined around his body, the scorpion is clawing at one leg. the constellation makers have _per fas et nefas_ separated scorpio from the serpent holder, spoiling both figures. but the serpent has been too much for them, insomuch that they have been reduced to the abject necessity of leaving one part of the serpent on one side of the region they allow to ophiuchus, and the other part of the serpent to the other. a group of constellations whose origin and meaning are little understood remains to be mentioned. close by the dragon is king cepheus, beside him his wife cassiopeia (the seated lady), near whom is andromeda the chained lady. the sea monster cetus is not far away, though not near enough to threaten her safety, the ram and triangle being between the monster's head and her feet, the fishes intervening between the body of the monster and her fair form. close at hand is perseus, the rescuer, with a sword (looking very much like a reaping-hook in all the old pictures) in his right hand, and bearing in his left the head of medusa. the general way of accounting for the figures thus associated has been by supposing that, having a certain tradition about cepheus and his family, men imagined in the heavens the pictorial representation of the events of the tradition. i have long believed that the actual order in this and other cases was the reverse of this, that men imagined certain figures in the heavens, pictured these figures in their astronomical temples or observatories, and made stories to fit the pictures afterwards, probably many generations afterwards. be this as it may, we can at present give no satisfactory explanation of the group of constellations. wilford gives an account, in his 'asiatic researches,' of a conversation with a pundit or astronomer respecting the names of the indian constellations. 'asking him,' he says, 'to show me in the heavens the constellation antarmada, he immediately pointed to andromeda, though i had not given him any information about it beforehand. he afterwards brought me a very rare and curious work in sanscrit, which contained a chapter devoted to _upanachatras_, or extra-zodiacal constellations, with drawings of _capuja_ (cepheus) and of _casyapi_ (cassiopeia) seated and holding a lotus-flower in her hand, of antarmada charmed with the fish beside her, and last of _paraseia_ (perseus), who, according to the explanation of the book, held the head of a monster which he had slain in combat; blood was dropping from it, and for hair it had snakes.' some have inferred from the circumstance that the indian charts thus showed the cassiopeian set of constellations, that the origin of these figures is to be sought in india. but probably both the indian and the greek constellation-figures were derived from a much older source. the zodiacal twelve are in some respects the most important and interesting of all the ancient constellations. if we could determine the origin of these figures, their exact configuration as at first devised, and the precise influences assigned to them in the old astrological systems, we should have obtained important evidence as to the origin of astronomy itself. not indeed that the twelve signs of the zodiac were formed at the beginning or even in the early infancy of astronomy. it seems abundantly clear that the division of the zodiac (which includes the moon's track as well as the sun's) had reference originally to the moon's motions. she circuits the star-sphere in about twenty-seven days and a third, while the lunation or interval from new moon to new moon is, as we all know, about twenty-nine days and a half in length. it would appear that the earliest astronomers, who were of course astrologers also, of all nations--the indian, egyptian, chinese, persian, and chaldæan astronomers--adopted twenty-eight days (probably as a rough mean between the two periods just named) for their chief lunar period, and divided the moon's track round the ecliptic into twenty-eight portions or mansions. how they managed about the fractions of days outstanding--whether the common lunation was considered or the moon's motion round the star-sphere--is not known. the very circumstance, however, that they were for a long time content with their twenty-eight lunar mansions shows that they did not seek great precision at first. doubtless they employed some rough system of 'leap-months' by which, as occasion required, the progress of the month was reconciled with the progress of the moon, just as by our leap-years the progress of the year is reconciled with the progress of the sun or seasons. the use of the twenty-eight-day period naturally suggested the division of time into weeks of seven days each. the ordinary lunar month is divided in a very obvious manner into four equal parts by the lunar aspects. every one can recognise roughly the time of full moon and the times of half moon before and after full, while the time of new moon is recognised from these two last epochs. thus the four quarters of the month, or roughly the four weeks of the month, would be the first time-measure thought of;--after the day, which is the necessary foundation of all time measures. the nearest approach which can be made to a quarter-month in days is the week of seven days; and although some little awkwardness arose from the fact that four weeks differ appreciably from a lunar month, this would not long prevent the adoption of the week as a measure of time. in fact, just as our years begin on different days of the week without causing any inconvenience, so the ancient months might be made to begin with different week-days. all that would be necessary to make the week measure fairly well the quarters of the month, would be to start each month on the proper or nearest week-day. to inform people about this, some ceremony could be appointed for the day of the new moon, and some signal employed to indicate the time when this ceremony was to take place. this--the natural and obvious course--we find was the means actually adopted, the festival of the new moon and the blowing of trumpets in the new moon being an essential part of the arrangements adopted by nations who used the week as a chief measure of time. the seven days were not affected by the new moons so far as the nomenclature of these days, or special duties connected with any one of them, might be concerned. originally the idea may have been to have festivals and sacrifices at the time of new moon, first quarter, full moon, and third quarter; but this arrangement would naturally (and did, as we know, actually) give way before long to a new moon festival regulating the month and seventh-day festivals, each class of festival having its appropriate sacrifices and duties. this, i say, was the natural course. its adoption _may_ have been aided by the recognition of the fact that the seven planets of the old system of astronomy might conveniently be taken to rule the days and the hours in the way described in the essay on astrology. that that nomenclature and that system of association between the planets and the hours, days, and weeks of time-measurement was eventually adopted, is certain; but whether the convenience and apparent mystical fitness of this arrangement led to the use of weekly festivals in conjunction with monthly ones, or whether those weekly festivals were first adopted in the way described above, or whether (which seems altogether more likely) both sets of considerations led to the arrangement, we cannot certainly tell. the arrangement was in every way a natural one; and one may say, considering all the circumstances, that it was almost an inevitable one. there was, however, another possible arrangement, viz., the division of time into ten-day periods, three to each month, with corresponding new moon festivals. but as the arrival of the moon at the _thirds_ of her progress are not at all so well marked as her arrival at the quarters, and as there is no connection between the number ten and the planets, this arrangement was far less likely to be adopted than the other. accordingly we find that only one or two nations adopted it. six sets of five days would be practically the same arrangement; five sets of six for each month would scarcely be thought of, as with that division the use of simple direct observations of the moon for time measurement, which was the real aim of all such divisions, would not be convenient or indeed even possible for the generality of persons. few could tell easily when the moon is two-fifths or four-fifths full, whereas every one can tell when she is half-full or quite full (the requisite for weekly measurement); and it would be possible to guess pretty nearly when she is one-third or two-thirds full, the requisite for the tridecennial division. my object in the above discussion of the origin of the week (as distinguished from the origin of the sabbath, which i considered in the essay on astrology), has been to show that the use of the twelve zodiacal signs was in every case preceded by the use of the twenty-eight lunar mansions. it has been supposed that those nations in whose astronomy the twenty-eight mansions still appear, adopted one system, while the use of the twelve signs implies that another system had been adopted. thus the following passage occurs in mr. blake's version of flammarion's 'history of the heavens:'--'the chinese have twenty-eight constellations, though the word _sion_ does not mean a group of stars, but simply a mansion or hotel. in the coptic and ancient egyptian the word for constellations has the same meaning. they also have twenty-eight, and the same number is found among the arabians, persians, and indians. among the chaldæans or accadians we find no sign of the number twenty-eight. the ecliptic, or "yoke of the sky," with them, as we see in the newly-discovered tablet, was divided into twelve divisions, as now, and the only connection that can be imagined between this and the twenty-eight is the opinion of m. biot, who thinks that the chinese had originally only twenty-four mansions, four more being added by chenkung, b.c., and that they corresponded with the twenty-four stars, twelve to the north and twelve to the south, that marked the twelve signs of the zodiac amongst the chaldæans. but under this supposition the twenty-eight has no reference to the moon, whereas we have every reason to believe it has.' the last observation is undoubtedly correct--the twenty-eight mansions have been mansions of the moon from the beginning. but in this very circumstance, as also in the very tablets referred to in the preceding passage, we find all the evidence needed to show that originally the chaldæans divided the zodiac into twenty-eight parts. for we find from the tablets that, like the other nations who had twenty-eight zodiacal mansions, the chaldæans used a seven-day period, derived from the moon's motions, every seventh day being called _sabbatu_, and held as a day of rest. we may safely infer that the chaldæan astronomers, advancing beyond those of other nations, recognised the necessity of dividing the zodiac with reference to the sun's motions instead of the moon's. they therefore discarded the twenty-eight lunar mansions, and adopted instead twelve solar signs; this number twelve, like the number twenty-eight itself, being selected merely as the most convenient approximation to the number of parts into which the zodiac was naturally divided by another period. thus the twenty-eighth part of the zodiac corresponds roughly with the moon's daily motion, and the twelfth part of the zodiac corresponds roughly with the moon's monthly motion; and both the numbers twenty-eight and twelve admit of being subdivided, while twenty-nine (a nearer approach than twenty-eight to the number of days in a lunation) and thirteen (almost as near an approach as twelve to the number of months in a year) do not. it seems to me highly probable that the date to which all inquiries into the origin of the constellations and the zodiacal signs seems to point--viz. b.c.--was the date at which the chaldæan astronomers definitely adopted the new system, the lunisolar instead of lunar division of the zodiac and of time. one of the objects which the architects of the great pyramid (not the king who built it) may have had was not improbably this--the erection of a building indicating the epoch when the new system was entered upon, and defining in its proportions, its interior passages, and other features, fundamental elements of the new system. the great difficulty, an overwhelming difficulty it has always seemed to me, in accepting the belief that the year b.c. defined the beginning of exact astronomy, has been this, that several of the circumstances insisted upon as determining that date imply a considerable knowledge of astronomy. thus astronomers must have made great progress in their science before they could select as a day for counting from, the epoch when the slow reeling motion of the earth (the so-called precessional motion) brought the pleiades centrally south, at noon, at the time of the vernal equinox. the construction of the great pyramid, again, in all its astronomical features, implies considerable proficiency in astronomical observation. thus the year b.c. may very well be regarded as defining the introduction of a new system of astronomy, but certainly not the beginning of astronomy itself. of course we may cut the knot of this difficulty, as prof. smyth and abbé moigno do, by saying that astronomy began b.c., the first astronomers being instructed supernaturally, so that the astronomical minerva came into full-grown being. but i apprehend that argument against such a belief is as unnecessary as it would certainly be useless. and now let us consider how this theory accords with the result to which we were led by the position of the great vacant space around the southern pole. so far as the date is concerned, we have already seen that the epoch b.c. accords excellently with the evidence of the vacant space. but this evidence, as i mentioned at the outset, establishes more than the date; it indicates the latitude of the place where the most ancient of ptolemy's forty-eight constellations were first definitely adopted by astronomers. if we assume that at this place the southernmost constellations were just fully seen when due south, we find for the latitude about thirty-eight degrees north. (the student of astronomy who may care to test my results may be reminded here that it is not enough to show that every star of a constellation would when due south be above the horizon of the place--what is wanted is, that the whole constellation when towards the south should be visible at a single view. however, the whole constellation may not have included all the stars now belonging to it.) the station of the astronomers who founded the new system can scarcely have been more than a degree or two north of this latitude. on the other side, we may go a little further, for by so doing we only raise the constellations somewhat higher above the southern horizon, to which there is less objection than to a change thrusting part of the constellations below the horizon. still it may be doubted whether the place where the constellations were first formed was less than or degrees north of the equator. the great pyramid, as we know, is about degrees north of the equator; but we also know that its architects travelled southwards to find a suitable place for it. one of their objects may well have been to obtain a fuller view of the star-sphere south of their constellations. i think from to degrees north would be about the most probable limits, and from to degrees north the certain limits of the station of the first founders of solar zodiacal astronomy. what their actual station may have been is not so easily established. some think the region lay between the sources of the oxus (amoor) and indus, others that the station of these astronomers was not very far from mount ararat--a view to which i was led long ago by other considerations discussed in the first appendix to my treatise on 'saturn and its system.' at the epoch indicated, the first constellation of the zodiac was not, as now, the fishes, nor, as when a fresh departure was made by hipparchus, the ram, but the bull, a trace of which is found in virgil's words-- candidus auratis aperit cum cornibus annum taurus. the bull then was the spring sign, the pleiades and ruddy aldebaran joining their rays with the sun's at the time of the vernal equinox. the midsummer sign was the lion (the bright cor leonis nearly marking the sun's highest place). the autumn sign was the scorpion, the ruddy antares and the stars clustering in the head of the scorpion joining their rays with the sun's at the time of the autumnal equinox. and lastly the winter sign was the water bearer, the bright fomalhaut conjoining his rays with the sun's at midwinter. it is noteworthy that all these four constellations really present some resemblance to the objects after which they are named. the scorpion is in the best drawing, but the bull's head is well marked, and, as already mentioned, a leaping lion can be recognised. the streams of stars from the urn of aquarius and the urn itself are much better defined than the urn bearer. i have not left myself much space to speak of the finest of all the constellations, the glorious orion--the giant in his might, as he was called of old. in this noble asterism the figure of a giant ascending a slope can be readily discerned when the constellation is due south. at the time to which i have referred the constellation orion was considerably below the equator, and instead of standing nearly upright when due south high above the horizon, as now in our northern latitudes, he rose upright above the south-eastern horizon. the resemblance to a giant figure must then have been even more striking than it is at present (except in high northern latitudes, where orion, when due south, is just fully above the horizon). the giant orion has long been identified with nimrod; and those who recognise the antitypes of the ark in argo, of the old dragon in draco, and of the first and second adams in the kneeling hercules defeated by the serpent and the upright ophiuchus triumphant over the serpent, may, if they so please, find in the giant orion, the two dogs, the hare, and the bull (whom orion is more directly dealing with), the representations of nimrod, that mighty hunter before the lord, his hunting dogs, and the animals he hunted. pegasus, formerly called the horse, was regarded in very ancient times as the steed of nimrod. in modern astronomy the constellations no longer have the importance which once attached to them. they afford convenient means for naming the stars, though i think many observers would prefer the less attractive but more business-like methods adopted by piazzi and others, according to which a star rejoices in no more striking title than 'piazzi xiiih. ,' or 'struve, .' they still serve, however, to teach beginners the stars, and probably many years will pass before even exact astronomy dismisses them altogether to the limbo of discarded symbolisms. it is, indeed, somewhat singular that astronomers find it easier to introduce new absurdities among the constellations than to get rid of these old ones. the new and utterly absurd figures introduced by bode still remain in many charts despite such inconvenient names as _honores frederici_, _globum Ærostaticum_ and _machina pneumatica_; and i have very little doubt that a new constellation, if it only had a specially inconvenient title, would be accepted. but when francis baily tried to simplify the heavens by removing many of bode's absurd constellations, he was abused by many as violently as though he had proposed the rejection of the newtonian system. i myself tried a small measure of reform in the three first editions of my 'library atlas,' but have found it desirable to return to the old nomenclature in the fourth. the end. _printed by_ ballantyne, hanson & co. _edinburgh and london_ footnotes: [ ] these reflections were suggested to tacitus by the conduct of thrasyllus (chief astrologer of the emperor tiberius), when his skill was tested by his imperial employer after a manner characteristic of that agreeable monarch. the story runs thus (i follow whewell's version): 'those who were brought to tiberius on any important matter, were admitted to an interview in an apartment situated on a lofty cliff in the island of capreæ. they reached this place by a narrow path, accompanied by a single freedman of great bodily strength; and on their return, if the emperor had conceived any doubts of their trustworthiness, a single blow buried the secret and its victim in the ocean below. after thrasyllus had, in this retreat, stated the results of his art as they concerned the emperor, tiberius asked him whether he had calculated how long he himself had to live. the astrologer examined the aspect of the stars, and while he did this showed hesitation, alarm, increasing terror, and at last declared that "the present hour was for him critical, perhaps fatal." tiberius embraced him, and told him "he was right in supposing he had been in danger, but that he should escape it," and made him henceforward his confidential counsellor.' it is evident, assuming the story to be true (as seems sufficiently probable), that the emperor was no match for the charlatan in craft. it was a natural thought on the former's part to test the skill of his astrologer by laying for him a trap such as the story indicates--a thought so natural, indeed, that it probably occurred to thrasyllus himself long before tiberius put the plan into practice. even if thrasyllus had not been already on the watch for such a trick, he would have been but a poor trickster himself if he had not detected it the moment it was attempted, or failed to see the sole safe course which was left open to him. probably, with a man of the temper of tiberius, such a counter-trick as galeotti's in _quentin durward_ would have been unsafe. [ ] the belief in the influence of the stars and the planets on the fortunes of the new-born child was still rife when shakespeare made glendower boast: at my nativity the front of heaven was full of fiery shapes of burning cressets; know, that at my birth the frame and huge foundation of the earth shook like a coward. and shakespeare showed himself dangerously tainted with freethought in assigning (even to the fiery hotspur) the reply: so it would have done at the same season, if your mother's cat had kittened, though yourself had ne'er been born. in a similar vein butler, in _hudibras_ ridiculed the folly of those who believe in horoscopes and nativities: as if the planet's first aspect the tender infant did infect in soul and body, and instil all future good and future ill; which in their dark fatalities lurking, at destined periods fall a-working, and break out, like the hidden seeds of long diseases, into deeds, in friendships, enmities, and strife. and all th' emergencies of life. [ ] preface to the _rudolphine tables_. [ ] it is commonly stated that bacon opposed the copernican theory because he disliked gilbert, who had advocated it. 'bacon,' says one of his editors, 'was too jealous of gilbert to entertain one moment any doctrine that he advanced.' but, apart from the incredible littleness of mind which this explanation imputes to bacon, it would also have been an incredible piece of folly on bacon's part to advocate an inferior theory while a rival was left to support a better theory. bacon saw clearly enough that men were on their way to the discovery of the true theory, and, so far as in him lay, he indicated how they should proceed in order most readily to reach the truth. it must, then, have been from conviction, not out of mere contradiction, that bacon declared himself in favour of the ptolemaic system. in fact, he speaks of the diurnal motion of the earth as 'an opinion which we can demonstrate to be most false;' doubtless having in his thoughts some such arguments as misled tycho brahe. [ ] to bacon's theological contemporaries this must have seemed a dreadful heresy, and possibly in our own days the assertion would be judged scarcely less harshly, seeing that the observance of the (so-called) sabbath depends directly upon the belief in quite another origin of the week. yet there can be little question that the week really had its origin in astrological formulæ. [ ] in bohn's edition the word 'defective' is here used, entirely changing the meaning of the sentence. bacon registers an _astrologia sana_ amongst the things needed for the advancement of learning, whereas he is made to say that such an astrology must be registered as defective. [ ] the astrologers were exceedingly ingenious in showing that their art had given warning of the great plague and fire of london. thus, the star which marks the bull's northern horn--and which is described by ptolemy as like mars--was, they say, exactly in that part of the sign gemini which is the ascendant of london, in . lilly, however, for whom they claim the credit of predicting the year of this calamity, laid no claim himself to that achievement; nay, specially denied that he knew when the fire was to happen. the story is rather curious. in lilly had published his _monarchy or no monarchy_, which contained a number of curious hieroglyphics. amongst these were two (see frontispiece) which appeared to portend plague and fire respectively. the hieroglyphic of the plague represents three dead bodies wrapped in death-clothes, and for these bodies two coffins lie ready and two graves are being dug; whence it was to be inferred that the number of deaths would exceed the supply of coffins and graves. the hieroglyphic of the fire represents several persons, gentlefolk on one side and commonfolk on the other, emptying water vessels on a furious fire into which two children are falling headlong. the occurrence of the plague in attracted no special notice to lilly's supposed prediction of that event, though probably many talked of the coincidence as remarkable. but when in the great fire occurred, the house of commons summoned lilly to attend the committee appointed to enquire into the cause of the fire. 'at two of the clock on friday, the th of october ,' he attended in the speaker's chamber, 'to answer such questions as should then and there be asked him.' sir robert brooke spoke to this effect: 'mr. lilly, this committee thought fit to summon you to appear before them this day, to know if you can say anything as to the cause of the late fire, or whether there might be any design therein. you are called the rather hither, because in a book of yours long since printed, you hinted some such thing by one of your hieroglyphics.' unto which he replied: 'may it please your honours, after the beheading of the late king, considering that in the three subsequent years the parliament acted nothing which concerned the settlement of the nation's peace, and seeing the generality of the people dissatisfied, the citizens of london discontented, and the soldiery prone to mutiny, i was desirous, according to the best knowledge god had given me, to make enquiry by the art i studied, what might, from that time, happen unto the parliament and nation in general. at last, having satisfied myself as well as i could, and perfected my judgment therein, i thought it most convenient to signify my intentions and conceptions thereof in forms, shapes, types, hieroglyphics, etc., without any commentary, that so my judgment might be concealed from the vulgar, and made manifest only unto the wise; i herein imitating the examples of many wise philosophers who had done the like. having found, sir, that the great city of london should be sadly afflicted with a great plague, and not long after with an exorbitant fire, i framed these two hieroglyphics, as represented in the book, which in effect have proved very true.' 'did you foresee the year?' said one. 'i did not,' said lilly; 'nor was desirous; of that i made no scrutiny. now, sir, whether there was any design of burning the city, or any employed to that purpose, i must deal ingenuously with you, that since the fire i have taken much pains in the search thereof, but cannot or could not give myself the least satisfaction therein. i conclude that it was the finger of god only; but what instruments he used thereunto i am ignorant.' [ ] sir toby belch and sir andrew aguecheek were evidently not well taught in astrology. 'shall we set about some revels?' says the latter. 'what shall we do else?' says toby; 'were we not born under taurus?' 'taurus, that's sides and heart,' says sapient andrew. 'no, sir,' responds toby, 'it's legs and thighs. let me see thee caper.' [ ] 'this is the excellent foppery of the world, that, when we are sick in fortune (often the surfeit of our own behaviour), we make guilty of our disasters the sun, moon, and stars: as if we were villains on necessity; fools by heavenly compulsion; knaves, thieves, and treacherous by spherical predominance; drunkards, liars, and adulterers, by inforced obedience of planetary influence; and all that we are, evil, by a divine thrusting on.'--shakespeare (_king lear_). [ ] there are few things more remarkable, or to reasoning minds more inexplicable, than the readiness with which men undertook in old times, and even now undertake, to interpret omens and assign prophetic significance to casual events. one can understand that foolish persons should believe in omens, and act upon the ideas suggested by their superstitions. the difficulty is to comprehend how these superstitions came into existence. for instance, who first conceived the idea that a particular line in the palm of the hand is the line of life; and what can possibly have suggested so absurd a notion? to whom did the thought first present itself that the pips on playing-cards are significant of future events; and why did he think so? how did the 'grounds' of a teacup come to acquire that deep significance which they now possess for mrs. gamp and betsy prig? if the believers in these absurdities be asked _why_ they believe, they answer readily enough either that they themselves or their friends have known remarkable fulfilments of the ominous indications of cards or tea-dregs, which must of necessity be the case where millions of forecasts are daily made by these instructive methods. but the persons who first invented those means of divination can have had no such reasons. they must have possessed imaginations of singular liveliness and not wanting in ingenuity. it is a pity that we know so little of them. [ ] wellington lived too long for the astrologers, his death within the year having unfortunately been predicted by them many times during the last fifteen years of his life. some astrologers were more cautious, however. i have before me his horoscope, carefully calculated, _secundum artem_, by raphaël in , with results 'sufficiently evincing the surprising verity and singular accuracy of astrological calculations, when founded on the correct time of birth, and mathematically calculated. i have chosen,' he proceeds, 'the nativity of this illustrious native, in preference to others, as the subject is now living, and, consequently, all possibility of making up any fictitious horoscope is at once set aside; thus affording me a most powerful shield against the insidious representations of the envious and ignorant traducer of my sublime science.' by some strange oversight, however, raphaël omits to mention anything respecting the future fortunes of wellington, showing only how wonderfully wellington's past career had corresponded with his horoscope. [ ] 'i have still observed,' says an old author, 'that your right martialist doth seldom exceed in height, or be at the most above a yard or a yard and a half in height' (which is surely stint measure). 'it hath been always thus,' said that right martialist sir geoffrey hudson to julian peveril; 'and in the history of all ages, the clean tight dapper little fellow hath proved an overmatch for his burly antagonist. i need only instance, out of holy writ, the celebrated downfall of goliath and of another lubbard, who had more fingers in his hand, and more inches to his stature, than ought to belong to an honest man, and who was slain by a nephew of good king david; and of many others whom i do not remember; nevertheless, they were all philistines of gigantic stature. in the classics, also, you have tydeus, and other tight compact heroes, whose diminutive bodies were the abode of large minds.' [ ] it is likely that swedenborg in his youth studied astrology, for in his visions the mercurial folk have this desire of knowledge as their distinguishing characteristic. [ ] it is singular that, when there is this perfectly simple explanation of the origin of the nomenclature of the days of the week, an explanation given by ancient historians and generally received, whewell should have stated that 'various accounts are given, all the methods proceeding upon certain arbitrary arithmetical processes connected in some way with astrological views.' speaking of the arrangement of the planets in the order of their supposed distances, and of the order in which the planets appear in the days of the week, he says, 'it would be difficult to determine with certainty why the former order was adopted, and how and why the latter was derived from it.' but, in reality, there is no difficulty about either point. the former arrangement corresponded precisely with the periodic times of the seven planets of the old egyptian system (unquestionably far more ancient than the system adopted by the greeks), while the latter springs directly from the former. assign to the hours of the day, successively, the seven planets in the former order, continuing the sequence without interruption day after day, and in the course of seven days each one of the planets will have ruled the first hour of a day, in the order,--saturn, the sun, the moon, mars, mercury, jupiter, and venus. what arbitrary arithmetical process there is in this it would be difficult to conceive. arithmetic does not rule the method at all. nor has any other method ever been suggested; though this method has been presented in several ways, some arithmetical and some geometrical. we need then have no difficulty in understanding what seems so perplexing to whewell, the universality, namely, of the notions 'which have produced this result,' for the notions were not fantastic, but such as naturally sprang from the ideas on which astrology itself depends. [ ] the following remarks by the astronomer-royal on this subject seem to me just, in the main. they accord with what i had said earlier in my essay on saturn and the sabbath of the jews ('our place among infinities,' th essay). 'the importance which moses attached to it [the hebdomadal rest] is evident; and, with all reverence, i recognise to the utmost degree the justice of his views. no direction was given for religious ceremonial' (he seems to have overlooked numbers xxviii. , and cognate passages), 'but it was probably seen that the health given to the mind by a rest from ordinary cares, and by the opportunity of meditation, could not fail to have a most beneficial religious effect. but, to give sanction to this precept, the authority of at least a myth was requisite. i believe it was simply for this reason that the myth of the six days of creation was preserved. it is expressly cited in the first delivery of the commandments, as the solemn authority (exodus xxxi. ) for the command. it is remarkable that at the second mention of the commandment (deuteronomy v.) no reference is made to the creation; perhaps, after the complete establishment of jehovistic ideas in the minds of the israelites, they had nearly lost the recollection of the elohistic account, and it was not thought desirable to refer to it' (airy, 'on the early hebrew scriptures,' p. ). it must be regarded as a singular instance of the persistency of myths, if this view be correct, that a myth which had become obsolete for the jews between the time of moses and that of the writer (whoever he may have been) who produced the so-called mosaic book of deuteronomy, should thereafter have been revived, and have come to be regarded by the jews themselves and by christians as the word of god. [ ] of course it may be argued that nothing in the world is the result of _mere_ accident, and some may assert that even matters which are commonly regarded as entirely casual have been specially designed. it would not be easy to draw the precise line dividing events which all men would regard as to all intents and purposes accidental from those which some men would regard as results of special providence. but common sense draws a sufficient distinction, at least for our present purpose. [ ] this star, called _thuban_ from the arabian _al-thúban_, the dragon, is now not very bright, being rated at barely above the fourth magnitude, but it was formerly the brightest star of the constellation, as its name indicates. bayer also assigned to it the first letter of the greek alphabet; though this is not absolutely decisive evidence that so late as his day it retained its superiority over the second magnitude stars to which bayer assigned the second and third greek letters. in the year b.c., or thereabouts, the star was at its nearest to the true north pole of the heavens, the diameter of the little circle in which it then moved being considerably less than one-fourth the apparent diameter of the moon. at that time the star must have seemed to all ordinary observation an absolutely fixed centre, round which all the other stars revolved. at the time when the pyramid was built this star was about sixty times farther removed from the true pole, revolving in a circle whose apparent diameter was about seven times as great as the moon's. yet it would still be regarded as a very useful pole-star, especially as there are very few conspicuous stars in the neighbourhood. [ ] even that skilful astronomer hipparchus, who may be justly called the father of observational astronomy, overlooked this peculiarity, which ptolemy would seem to have been the first to recognise. [ ] it would only be by a lucky accident, of course, that the direction of the slant tunnel's axis and that of the vertical from the selected central point would lie in the same vertical plane. the object of the tunnelling would, in fact, be to determine how far apart the vertical planes through these points lay, and the odds would be great against the result proving to be zero. [ ] it may, perhaps, occur to the reader to inquire what diameter of the earth, supposed to be a perfect sphere, would be derived from a degree of latitude measured with absolute accuracy near latitude °. a degree of latitude measured in polar regions would indicate a diameter greater even than the equatorial; one measured in equatorial regions would indicate a diameter less even than the polar. near latitude ° the measurement of a degree of latitude would indicate a diameter very nearly equal to the true polar diameter of the earth. in fact, if it could be proved that the builders of the pyramid used for their unit of length an exact subdivision of the polar diameter, the inference would be that, while the coincidence itself was merely accidental, their measurement of a degree of latitude in their own country had been singularly accurate. by an approximate calculation i find that, taking the earth's compression at - , the diameter of the earth, estimated from the accurate measurement of a degree of latitude in the neighbourhood of the great pyramid, would have made the sacred cubit--taken at one , , th of the diameter--equal to · british inches; a closer approximation than professor smyth's to the estimated mean probable value of the sacred cubit. [ ] it is, however, almost impossible to mark any limits to what may be regarded as evidence of design by a coincidence-hunter. i quote the following from the late professor de morgan's _budget of paradoxes_. having mentioned that occurs less frequently than any other digit in the number expressing the ratio of circumference to diameter of a circle, he proceeds: 'a correspondent of my friend piazzi smyth notices that is the number of most frequency, and that - / is the nearest approximation to it in simple digits. professor smyth, whose work on egypt is paradox of a very high order, backed by a great quantity of useful labour, the results of which will be made available by those who do not receive the paradoxes, is inclined to see confirmation for some of his theory in these phenomena.' in passing, i may mention as the most singular of these accidental digit relations which i have yet noticed, that in the first digits of the square root of , the number occurs more than twice as often as either or , which each occur eight times, and occurring each nine times, and occurring no less than eighteen times. [ ] i have substituted this value in the article 'astronomy,' of the _british encyclopædia_, for the estimate formerly used, viz. , , miles. but there is good reason for believing that the actual distance is nearly , , miles. [ ] it may be matched by other coincidences as remarkable and as little the result of the operation of any natural law. for instance, the following strange relation, introducing the dimensions of the sun himself, nowhere, so far as i have yet seen, introduced among pyramid relations, even by pyramidalists: 'if the plane of the ecliptic were a true surface, and the sun were to commence rolling along that surface towards the part of the earth's orbit where she is at her mean distance, while the earth commenced rolling upon the sun (round one of his great circles), each globe turning round in the same time,--then, by the time the earth had rolled its way once round the sun, the sun would have almost exactly reached the earth's orbit. this is only another way of saying that the sun's diameter exceeds the earth's in almost exactly the same degree that the sun's distance exceeds the sun's diameter.' [ ] it has been remarked that, though hipparchus had the enormous advantage of being able to compare his own observations with those recorded by the chaldæans, he estimated the length of the year less correctly than the chaldæans. it has been thought by some that the chaldæans were acquainted with the true system of the universe, but i do not know that there are sufficient grounds for this supposition. diodorus siculus and apollonius myndius mention, however, that they were able to predict the return of comets, and this implies that their observations had been continued for many centuries with great care and exactness. [ ] the language of the modern zadkiels and raphaëls, though meaningless and absurd in itself, yet, as assuredly derived from the astrology of the oldest times, may here be quoted. (it certainly was not invented to give support to the theory i am at present advocating.) thus runs the jargon of the tribe: 'in order to illustrate plainly to the reader what astrologers mean by the "houses of heaven," it is proper for him to bear in mind the four cardinal points. the eastern, facing the rising sun, has at its centre the first grand angle or first house, termed the horoscope or ascendant. the northern, opposite the region where the sun is at midnight, or the _cusp_ of the lower heaven or nadir, is the imum coeli, and has at its centre the fourth house. the western, facing the setting sun, has at its centre the third grand angle or seventh house or descendant. and lastly, the southern, facing the noonday sun, has at its centre the astrologer's tenth house, or mid-heaven, the most powerful angle or house of honour.' 'and although,' proceeds the modern astrologer, 'we cannot in the ethereal blue discern these lines or terminating divisions, both reason and experience assure us that they certainly exist; therefore the astrologer has certain grounds for the choice of his four angular houses' (out of twelve in all) 'which, resembling the palpable demonstration they afford, are in the astral science esteemed the most powerful of the whole. '--raphaël's _manual of astrology_. [ ] arabian writers give the following account of egyptian progress in astrology and the mystical arts: nacrawasch, the progenitor of misraim, was the first egyptian prince, and the first of the magicians who excelled in astrology and enchantment. retiring into egypt with his family of eighty persons, he built essous, the most ancient city of egypt, and commenced the first dynasty of misraimitish princes, who excelled as cabalists, diviners, and in the mystic arts generally. the most celebrated of the race were naerasch, who first represented by images the twelve signs of the zodiac; gharnak, who openly described the arts before kept secret; hersall, who first worshipped idols; sehlouk, who worshipped the sun; saurid (king saurid of ibn abd alkohm's account), who erected the first pyramids and invented the magic mirror; and pharaoh, the last king of the dynasty, whose name was afterwards taken as a kingly title, as cæsar later became a general imperial title. [ ] it is noteworthy how swedenborg here anticipates a saying of laplace, the greatest mathematician the world has known, save newton alone. newton's remark that he seemed but as a child who had gathered a few shells on the shores of ocean, is well known. laplace's words, '_ce que nous connaissons est peu de chose; ce que nous ignorons est immense_,' were not, as is commonly stated, his last. de morgan gives the following account of laplace's last moments, on the authority of laplace's friend and pupil, the well-known mathematician poisson: 'after the publication (in ) of the fifth volume of the mécanique céleste, laplace became gradually weaker, and with it musing and abstracted. he thought much on the great problems of existence, and often muttered to himself, "_qu'est-ce que c'est que tout cela!_" after many alternations he appeared at last so permanently prostrated that his family applied to his favourite pupil, m. poisson, to try to get a word from him. poisson paid a visit, and after a few words of salutation, said, "j'ai une bonne nouvelle à vous annoncer: on a reçu au bureau des longitudes une lettre d'allemagne annonçant que m. bessel a vérifié par l'observation vos découvertes théoriques sur les satellites de jupiter." laplace opened his eyes and answered with deep gravity. "_l'homme ne poursuit que des chimères._" he never spoke again. his death took place march , .' [ ] the reason assigned by swedenborg is fanciful enough. 'in the spiritual sense,' he says, 'a horse signifies the intellectual principle formed from scientifics, and as they are afraid of cultivating the intellectual faculties by worldly sciences, from this comes an influx of fear. they care nothing for scientifics which are of human erudition.' [ ] similar reasoning applies to the moons of jupiter, and it so chances that the result in their case comes out exactly the same as in the case of saturn; all the jovian moons, if full together, would reflect only the sixteenth part of the light which we receive from the full moon. it is strange that scientific men of considerable mathematical power have used the argument from design apparently supplied by the satellites, without being at the pains to test its validity by the simple mathematical calculations necessary to determine the quantity of light which these bodies can reflect to the planets round which they travel. brewster and whewell, though they took opposite sides in the controversy about other inhabited worlds, agreed in this. brewster, of course, holding the theory that all the planets are inhabited, very naturally accepted the argument from design in this case. whewell, in opposing that theory, did not dwell at all upon the subjects of the satellites. but in his 'bridgewater treatise on astronomy and general physics,' he says, 'taking only the ascertained cases of venus, the earth, jupiter, and saturn, we conceive that a person of common understanding will be strongly impressed with the persuasion that the satellites are placed in the system with a view to compensate for the diminished light of the sun at greater distances. mars is an exception; some persons might conjecture from this case that the arrangement itself, like other useful arrangements, has been brought about by some wider law which we have not yet detected. but whether or not we entertain such a guess (it can be nothing more), we see in other parts of creation so many examples of apparent exceptions to rules, which are afterwards found to be capable of explanation, or to be provided for by particular contrivances, that no one familiar with such contemplations will, by one anomaly, be driven from the persuasion that the end which the arrangements of the satellites seem suited to answer is really one of the ends of their creation.' [ ] the reader who cares enough about such subjects to take the necessary trouble, can easily make a little model of saturn and his ring system, which will very prettily illustrate the effect of the rings both in reflecting light to the planet's darkened hemisphere and in cutting off light from the planet's illuminated hemisphere. take a ball, say an ordinary hand-ball, and pierce it through the centre with a fine knitting-needle. cut out a flat ring of card, proportioned to the ball as the ring system of saturn to his ball. (if the ball is two inches in diameter, strike out on a sheet of cardboard two concentric circles, one of them with a radius of a little more than an inch and a half, the other with a radius of about two inches and three-eights, and cut out the ring between these two circles.) thrust the knitting-needle through this ring in such a way that the ball shall lie in the middle of the ring, as the globe of saturn hangs (without knitting-needle connections) in the middle of his ring system. thrust another knitting-needle centrally through the ball square to the plane of the ring, and use this second needle, which we may call the polar one, as a handle. now take the ball and ring into sunlight, or the light of a lamp or candle, holding them so that the shadow of the ring is as thin as possible. this represents the position of the shadow at the time of saturnian spring or autumn. cause the shadow slowly to shift until it surrounds the part of the ball through which the polar needle passes on one side. this will represent the position of the shadow at the time of midwinter for the hemisphere corresponding to that side of the ball. notice that while the shadow is traversing this half of the ball, the side of the ring which lies towards that half is in shadow, so that a fly or other small insect on that half of the ball would see the darkened side of the ring. a saturnian correspondingly placed would get no reflected sunlight from the ring system. move the ball and ring so that the shadow slowly returns to its first position. you will then have illustrated the changes taking place during one half of a saturnian year. continue the motion so that the shadow passes to the other half of the ball, and finally surrounds the other point through which the polar needle passes. the polar point which the shadow before surrounded will now be seen to be in the light, and this half of the ball will illustrate the hemisphere of saturn where it is midsummer. it will also be seen that the side of the ring towards this half of the ball is now in the light, so that a small insect on this half of the ball would see the bright side of the ring. a saturnian correspondingly placed would get reflected sunlight from the ring system _both by day and by night_. moving the ball and ring so that the shadow returns to its first position, an entire saturnian year will have been illustrated. these changes can be still better shown with a saturnian orrery (see plate viii. of my saturn), which can be very easily constructed. [ ] not 'of course' because tycho used it, for, like other able students of science, he made mistakes from time to time. thus he argued that the earth cannot rotate on her axis, because if she did bodies raised above her surface would be left behind--an argument which even the mechanical knowledge of his own time should have sufficed to invalidate, though it is still used from time to time by paradoxers of our own day. [ ] chinese chronicles contain other references to new stars. the annals of ma-touan-lin, which contain the official records of remarkable appearances in the heavens, include some phenomena which manifestly belong to this class. thus they record that in the year a star appeared between the stars which mark the hind feet of the centaur. this star remained visible from december in that year until july in the next (about the same time as tycho brahe's and kepler's new stars, presently to be described). another star, assigned by these annals to the year , seems to be the same as a star referred to by hepidannus as appearing a.d. . it was of extraordinary brilliancy, and remained visible in the southern part of the heavens during three months. the annals of ma-touan-lin assign to it a position low down in sagittarius. [ ] still a circumstance must be mentioned which tends to show that the star may have been visible a few hours earlier than dr. schmidt supposed. mr. m. walter, surgeon of the th regiment, then stationed in north india, wrote (oddly enough, on may , , the first anniversary of mr. birmingham's discovery) as follows to mr. stone:--'i am certain that this same conflagration was distinctly perceptible here at least six hours earlier. my knowledge of the fact came about in this wise. the night of the th of may last year was exceedingly sultry, and about eight o'clock on that evening i got up from the tea-table and rushed into my garden to seek a cooler atmosphere. as my door opens towards the east, the first object that met my view was the northern crown. my attention was at once arrested by the sight of a strange star outside the crown' (that is, outside the circlet of stars forming the diadem, not outside the constellation itself). the new star 'was then certainly quite as bright--i rather thought more so--as its neighbour alphecca,' the chief gem of the crown. 'i was so much struck with its appearance, that i exclaimed to those indoors, "why, here is a new comet!'" he made a diagram of the constellation, showing the place of the new star correctly. unfortunately, mr. walter does not state why he is so confident, a year after the event, that it was on the th of may, and not on the th, that he noticed the new star. if he fixed the date only by the star's appearance as a second-magnitude star, his letter proves nothing; for we know that on the th it was still shining as brightly as alphecca, though on the th it was perceptibly fainter. [ ] the velocity of three or four miles per second inferred by the elder struve must now be regarded (as i long since pointed out would prove to be the case) as very far short of the real velocity of our system's motion through stellar space. [ ] m. cornu's observations are full of interest, and he deserves considerable credit for his energy in availing himself of the few favourable opportunities he had for making them. but he goes beyond his province in adding to his account of them some remarks, intended apparently as a reflection on mr. huggins's speculations respecting the star in the northern crown. '_i_,' says m. cornu, 'will not try to form any hypothesis about the cause of the outburst. to do so would be unscientific, and such speculations, though interesting, cumber science wofully.' this is sheer nonsense, and comes very ill from an observer whose successes in science have been due entirely to the employment of methods of observation which would have had no existence had others been as unready to think out the meaning of observed facts as he appears to be himself. [ ] the same peculiarity has been noticed since the discovery of the dark ring, the space within that ring being observed by coolidge and g. bond at harvard in to be apparently darker than the surrounding sky. [ ] i cannot understand why mr. webb, in his interesting little work, _celestial objects for common telescopes_, says that the satellite theory of the rings certainly seems insufficient to account for the phenomena of the dark ring. it seems, on the contrary, manifest that the dark ring can scarcely be explained in any other way. the observations recently made are altogether inexplicable on any other theory. [ ] a gentleman, whose acquaintance i made in returning from america last spring, assured me that he had found demonstrative evidence showing that a total eclipse of the moon then occurred; for he could prove that abraham's vision occurred at the time of full moon, so that it could not otherwise have been dark when the sun went down (v. ). but the horror of great darkness occurred when the sun was going down, and total eclipses of the moon do not behave that way--at least, in our time. [ ] it is not easy to understand what else it could have been. the notion that a conjunction of three planets, which took place shortly before the time of christ's birth, gave rise to the tradition of the star in the east, though propounded by a former president of the astronomical society, could hardly be entertained by an astronomer, unless he entirely rejected matthew's account, which the author of this theory, being a clergyman, can scarcely have done. [ ] as, for instance, when he makes homer say of the moon that around her throne the vivid planets roll, and stars unnumbered gild the glowing pole. it is difficult, indeed, to understand how so thorough an astronomer as the late admiral smyth could have called the passage in which these lines occur one of the finest bursts of poetry in our language, except on the principle cleverly cited by waller when charles ii. upbraided him for the warmth of his panegyric on cromwell, that 'poets succeed better with fiction than with truth.' macaulay, though not an astronomer, speaks more justly of the passage in saying that this single passage contains more inaccuracies than can be found in all wordsworth's 'excursion.' [ ] it may be necessary to throw in here a few words of explanation, lest the non-astronomical reader should run away with the idea that the so-called exact science is a very inexact science indeed, so far as comets are concerned. the comet of was one of those which travel on a very eccentric orbit. coming, indeed, from out depths many times more remote than the path even of the remotest planet, neptune, this comet approached nearer to the sun than any which astronomers have ever seen, except only the comet of . when at its nearest its nucleus was only a sixth part of the sun's diameter from his surface. thus the part of the comet's orbit along which astronomers traced its motion was only a small part at one end of an enormously long oval, and very slight errors of observation were sufficient to produce very large errors in the determination of the nature of the comet's orbit. encke admitted that the period might, so far as the comparatively imperfect observations made in were concerned, be any whatever, from years to many millions of years, or even to infinity--that is, the comet might have a path not re-entering into itself, but carrying the comet for ever away from the sun after its one visit to our system. [ ] for a portion of the passages which i have quoted in this essay i am indebted to guillemin's 'treatise on comets,' a useful contribution to the literature of the subject, though somewhat inadequate so far as exposition is concerned. [ ] something very similar happened only a few years ago, so that we cannot afford to laugh too freely at the terrors of france in . it was reported during the winter of - , that plantamour, the swiss astronomer, had predicted the earth's destruction by a comet on august , . yet there was no other foundation for this rumour than the fact that plantamour, in a lecture upon comets and meteors, had stated that the meteors seen on august , , and are bodies following in the track of a comet whose orbit passes very near to the earth's. it was very certainly known to astronomers that there could be no present danger of a collision with this comet, for the comet has a period of at least years, and had last passed close to the earth's orbit (not to the earth herself, be it understood) in . but it was useless to point this out. many people insisted on believing that on august , , the earth would come into collision, possibly disastrous, with a mighty comet, which plantamour was said to have detected and to have shown by a profound calculation to be rushing directly upon our unfortunate earth. [ ] a rather amusing mistake was made by the stenographers of a new york paper in reporting the above sentence, which i happened to quote in a lecture upon comets and meteors. instead of paradise they wrote paris. those acquainted with pitman's system of short-hand, the one most commonly employed by reporters, will easily understand how the mistake was made, the marks made to represent the consonants p, r, d, and s differing little from those made to represent the consonants p, r, and s (the 'd' or 't' sound is represented, or may be represented, by simply shortening the length of the sign for the preceding consonant). the mistake led naturally to my remarking in my next lecture that i had not before known how thoroughly synonymous the words are in america, though i had heard it said that 'good americans, when they die, go to paris.' [ ] on the occasion of my first visit to america, in , i for the first time succeeded in obtaining a copy of this curious pamphlet. it had been mentioned to me (by emerson, i think) as an amusing piece of trickery played off by a scientific man on his brethren; and dr. wendell holmes, who was present, remarked that he had a copy in his possession. this he was good enough to lend me. soon after, a valued friend in new york presented me with a copy. [ ] this locke must not be confounded with richard lock, the circle-squarer and general paradoxist, who flourished a century earlier. [ ] the nurses' tale is, that the man was sent to the moon by moses for gathering sticks on the sabbath, and they refer to the cheerful story in numbers xv. - . according to german nurses the day was not the sabbath, but sunday. their tale runs as follows: 'ages ago there went one sunday an old man into the woods to hew sticks. he cut a faggot and slung it on a stout staff, cast it over his shoulder, and began to trudge home with his burthen. on his way he met a handsome man in sunday suit, walking towards the church. the man stopped, and asked the faggot-bearer; "do you know that this is sunday on earth, when all must rest from their labours?" "sunday on earth or monday in heaven, it's all one to me?" laughed the wood-cutter. "then bear your bundle for ever!" answered the stranger. "and as you value not sunday on earth, yours shall be a perpetual moon-day in heaven; you shall stand for eternity in the moon, a warning to all sabbath-breakers." thereupon the stranger vanished; and the man was caught up with his staff and faggot into the moon, where he stands yet.' according to some narrators the stranger was christ; but whether from german laxity in such matters or for some other reason, no text is quoted in evidence, as by the more orthodox british nurses. luke vi. - might serve. [ ] milton's opinion may be quoted against me here; and as received ideas respecting angels, good and bad, the fall of man, and many other such matters, are due quite as much to milton as to any other authority, his opinion must not be lightly disregarded. but though, when milton's satan 'meets a vast vacuity' where his wings are of no further service to him, 'all unawares flutt'ring his pennons vain, plumb down he drops ten thousand fathoms deep, and to this hour down had been falling, had not by ill chance the strong rebuff of some tumultuous cloud, instinct with fire and nitre, hurried him as many miles aloft,' yet this was written nearly a quarter of a century before newton had established the law of gravity. moreover, there is no evidence to show in what direction satan fell; 'above is below and below above,' says richter, 'to one stripped of gravitating body;' and whether satan was under the influence of gravity or not, he would be practically exempt from its action when in the midst of that 'dark, illimitable ocean' of space, 'without bound, without dimensions, where length, breadth, and height, and time and place are lost.' his lighting 'on niphates' top,' and overleaping the gate of paradise, may be used as arguments either way. on the whole, i must (according to my present lights) claim for satan a freedom from all scientific restraints. this freedom is exemplified by his showing all the kingdoms of the world from an exceeding high mountain, thus affording the first practical demonstration of the flat-earth theory, the maintenance of which led to poor mr. hampden's incarceration. [ ] the _sun_ itself claimed to have established the veracity of the account in a manner strongly recalling a well-known argument used by orthodox believers in the bible account of the cosmogony. either, say these, moses discovered how the world was made, or the facts were revealed to him by some one who had made the discovery: but moses could not have made the discovery, knowing nothing of the higher departments of science; therefore, the account came from the only being who could rationally be supposed to know anything about the beginning of the world. 'either,' said the _new york sun_, speaking of a mathematical problem discussed in the article, 'that problem was predicated by us or some other person, who has thereby made the greatest of all modern discoveries in mathematical astronomy. we did not make it, for we know nothing of mathematics whatever; therefore, it was made by the only person to whom it can rationally be ascribed, namely herschel the astronomer, its only avowed and undeniable author.' in reality, notwithstanding this convincing argument, the problem was stolen by locke from a paper by olbers, shortly before published, and gave the method followed by beer and mädler throughout their selenographical researches in - . [ ] i had at the same time the good fortune to satisfy in equal degree, though quite unexpectedly, an english student of the sun, who at that time bore me no great good-will. something in the article chanced to suggest that it came from another, presumably a rival, hand; while an essay which appeared about the same time (the spring of ) was commonly but erroneously attributed to me. accordingly, a leading article in _nature_ was devoted to the annihilation of the writer supposed to be myself, and to the lavish and quite undeserved laudation of the article i had written, which was selected as typifying all the good qualities which an article of the kind should possess. those acquainted with the facts were not a little amused by the mistake. [ ] the astronomer-royal once told me that he had found that few persons have a clear conception of the fact that the stars rise and set. still fewer know how the stars move, which stars rise and set, which are always above the horizon, which move on large circles, which on small ones; though a few hours' observation on half-a-dozen nights in the year (such observations being continuous, but made only at hourly intervals) would show dearly how the stars move. it is odd to find even some who write about astronomy making mistakes on matters so elementary. for instance, in a primer of astronomy recently published, it is stated that the stars which pass overhead in london rise and set on a slant--the real fact being that _those_ stars never rise or set at all, never coming within some two dozen moon-breadths of the horizon. [ ] in passing let me note that, of course, i am not discussing the arguments of paradoxists with the remotest idea of disproving them. they are not, in reality, worth the trouble. but they show where the general reader of astronomical text-books, and other such works, is likely to go astray, and thus conveniently indicate matters whose explanation may be useful or interesting. [ ] sterne anticipated this paradoxist in (jestingly) attributing glassiness to an inferior planet. he made the inhabitants, however, not the air, glassy. 'the intense heat of the country,' he says, speaking of the planet mercury, 'must, i think, long ago have vitrified the bodies of the inhabitants to suit them for the climate; so that all the tenements of their souls may be nothing else, for aught the soundest philosophy can show to the contrary, but one fine transparent body of clear glass; so that till the inhabitant grows old and tolerably wrinkled, whereby the rays of light become monstrously refracted, or return reflected from the surface, etc., his soul might as well play the fool out o' doors as in her own house.' [ ] it will be seen from table x. of my treatise on saturn that the ring disappeared on december , remaining invisible (because turning its dark side earthwards) till the spring of . but on december , the ring must have been quite invisible in a telescope so feeble as galileo's. the ring then would have been little more than a fine line of light as seen with one of our powerful modern telescopes. [ ] _north british review_ for august . [ ] he had, indeed, at an earlier stage, shown a marvellous ignorance of astronomy by the remark, which doubtless appeared to him a safe one, that when he saw a planet on the sun in september he supposed it was mercury; a september transit of mercury being as impossible as an eclipse of the sun during the moon's third quarter. [ ] it is, by the way, somewhat amusing to find baron humboldt referring a question of this sort to the great mathematician gauss, and describing the problem as though it involved the most profound calculations. ten minutes should suffice to deal with any problem of the kind. transcriber's note the following typographical errors were corrected. page error correction julias julius genuis genius artficers artificers signfies signifies footnote preplexing perplexing chaldean chaldæan chaldeans chaldæans peruquier perruquier peruque perruque northfolk norfolk ascant askant harper's library of living thought are the planets inhabited? by e. walter maunder, f.r.a.s. superintendent of the solar department, royal observatory greenwich author of "astronomy without a telescope" "the royal observatory, greenwich, its history and work" "the astronomy of the bible," "the heavens and their story" etc. harper & brothers london and new york albemarle street, w. _published march, _ contents chapter page i. the question stated ii. the living organism iii. the sun iv. the distribution of the elements in space v. the moon vi. the canals of mars vii. the condition of mars viii. the illusions of mars ix. venus, mercury and the asteroids x. the major planets xi. when the major planets cool xii. the final question index are the planets inhabited? chapter i the question stated the first thought that men had concerning the heavenly bodies was an obvious one: they were lights. there was a greater light to rule the day; a lesser light to rule the night; and there were the stars also. in those days there seemed an immense difference between the earth upon which men stood, and the bright objects that shone down upon it from the heavens above. the earth seemed to be vast, dark, and motionless; the celestial lights seemed to be small, and moved, and shone. the earth was then regarded as the fixed centre of the universe, but the copernican theory has since deprived it of this pride of place. yet from another point of view the new conception of its position involves a promotion, since the earth itself is now regarded as a heavenly body of the same order as some of those which shine down upon us. it is amongst them, and it too moves and shines--shines, as some of them do, by reflecting the light of the sun. could we transport ourselves to a neighbouring world, the earth would seem a star, not distinguishable in kind from the rest. but as men realized this, they began to ask: "since this world from a distant standpoint must appear as a star, would not a star, if we could get near enough to it, show itself also as a world? this world teems with life; above all, it is the home of human life. men and women, gifted with feeling, intelligence, and character, look upward from its surface and watch the shining members of the heavenly host. are none of these the home of beings gifted with like powers, who watch in their turn the movements of that shining point which is our world?" this is the meaning of the controversy on the plurality of worlds which excited so much interest some sixty years ago, and has been with us more or less ever since. it is the desire to recognize the presence in the orbs around us of beings like ourselves, possessed of personality and intelligence, lodged in an organic body. this is what is meant when we speak of a world being "inhabited." it would not, for example, at all content us if we could ascertain that jupiter was covered by a shoreless ocean, rich in every variety of fish; or that the hard rocks of the moon were delicately veiled by lichens. just as no richness of vegetation and no fulness and complexity of animal life would justify an explorer in describing some land that he had discovered as being "inhabited" if no men were there, so we cannot rightly speak of any other world as being "inhabited" if it is not the home of intelligent life. if the life did not rise above the level of algæ or oysters, the globe on which they flourish would be uninhabited in our estimation, and its chief interest would lie in the possibility that in the course of ages life might change its forms and develop hereafter into manifestations with which we could claim a nearer kinship. on the other hand, of necessity we are precluded from extending our enquiry to the case of disembodied intelligences, if such be conceived possible. all created existences must be conditioned, but if we have no knowledge of what those conditions may be, or means for attaining such knowledge, we cannot discuss them. nothing can be affirmed, nothing denied, concerning the possibility of intelligences existing on the moon or even in the sun if we are unable to ascertain under what limitations those particular intelligences subsist. gnomes, sylphs, elves, and fairies, and all similar conceptions, escape the possibility of discussion by our ignorance of their properties. as nothing can be asserted of them they remain beyond investigation, as they are beyond sight and touch. the only beings, then, the presence of which would justify us in regarding another world as "inhabited" are such as would justify us in applying that term to a part of our own world. they must possess intelligence and consciousness on the one hand; on the other, they must likewise have corporeal form. true, the form might be imagined as different from that we possess; but, as with ourselves, the intelligent spirit must be lodged in and expressed by a living material body. our enquiry is thus rendered a physical one; it is the necessities of the living body that must guide us in it; a world unsuited for living organisms is not, in the sense of this enquiry, a "habitable" world. the discussion, as it was carried on sixty years ago by dr. whewell and sir david brewster, was essentially a metaphysical, almost a theological one, and it was chiefly considered in its supposed relationship to certain religious conceptions. it was urged that it was derogatory to the wisdom and goodness of the creator to suppose that he would have created so many great and glorious orbs without having a definite purpose in so doing, and that the only purpose for which a world could be made was that it might be inhabited. so, again, when dr. a. r. wallace revived the discussion in , he clearly had a theological purpose in his opening paper, though he was taking the opposite view from that held by brewster half a century earlier. for myself, if there be any theological significance attaching to the solving of this problem, i do not know what it is. if we decide that there are very many inhabited worlds, or that there are few, or that there is but one--our own--i fail to see how it should modify our religious beliefs. for example: explorers have made their way across the antarctic continent to the south pole but have found no "inhabitant" there. has this fact any theological bearing? or if, on the contrary, a race of men had been discovered there, what change would it have made in the theological position of anyone? and if this be so with regard to a new continent on this earth, why should it be different with regard to the continents of another planet? the problem therefore seems not to be theological or metaphysical, but purely physical. we have simply to ask with regard to each heavenly body which we pass in review: "are its physical conditions, so far as we can ascertain them, such as would render the maintenance of life possible upon it?" the question is not at all as to how life is generated on a world, but as to whether, if once in action on a particular world, its activities could be carried on. chapter ii the living organism a world for habitation, then, is a world whereon living organisms can exist that are comparable in intelligence with men. but "men" presuppose the existence of living organisms of inferior grades. therefore a world for habitation must first of all be one upon which it is possible for living organisms, as such, to exist. it does not concern us in the present connection how life first came into existence on this planet. it is sufficient that we know from experience that life does exist here; and in whatsoever way it was first generated here, in that same way we may consider that it could have been generated on another planet. nor need any question trouble us as to the precise line of demarkation to be drawn between inorganic and organic substances, or amongst the latter, between plants and animals. these are important subjects for discussion, but they do not affect us here, for we are essentially concerned with the highest form of organism, the one furthest from these two dividing lines. it suffices that living organisms do exist here, and exist under well-defined conditions. wanting these conditions, they perish. we can, to a varying degree, determine the physical conditions prevailing upon the heavenly bodies, and we can ascertain whether these physical conditions would be favourable, unfavourable, or fatal to the living organism. what is a living organism? a living organism is such that, though it is continually changing its substance, its identity, as a whole, remains essentially the same. this definition is incomplete, but it gives us a first essential approximation, it indicates the continuance of the whole, with the unceasing change of the details. were this definition complete, a river would furnish us with a perfect example of a living organism, because, while the river remains, the individual drops of water are continually changing. there is then something more in the living organism than the continuity of the whole, with the change of the details. an analogy, given by max verworn, carries us a step further. he likens life to a flame, and takes a gas flame with its butterfly shape as a particularly appropriate illustration. here the shape of the flame remains constant, even in its details. immediately above the burner, at the base of the flame, there is a completely dark space; surrounding this, a bluish zone that is faintly luminous; and beyond this again, the broad spread of the two wings that are brightly luminous. the flame, like the river, preserves its identity of form, while its constituent details--the gases that feed it--are in continual change. but there is not only a change of material in the flame; there is a change of condition. everywhere the gas from the burner is entering into energetic combination with the oxygen of the air, with evolution of light and heat. there is change in the constituent particles as well as change of the constituent particles; there is more than the mere flux of material through the form; there is change of the material, and in the process of that change energy is developed. a steam-engine may afford us a third illustration. here fresh material is continually being introduced into the engine there to suffer change. part is supplied as fuel to the fire there to maintain the temperature of the engine; so far the illustration is analogous to that of the gas flame. but the engine carries us a step further, for part of the material supplied to it is water, which is converted into steam by the heat of the fire, and from the expansion of the steam the energy sought from the machine is derived. here again we have change in the material with development of energy; but there is not only work done in the subject, there is work done by it. but the living organism differs from artificial machines in that, of itself and by itself, it is continuously drawing into itself non-living matter, converting it into an integral part of the organism, and so endowing it with the qualities of life. and from this non-living matter it derives fresh energy for the carrying on of the life of the organism. the engine and the butterfly gas flame do not give us, any more than the river, a complete picture of the living organism. the form of the river is imposed upon it from without; the river is defined by its bed, by the contour of the country through which it flows. the form and size of the flame are equally defined by exterior conditions; they are imposed upon it by the shape of the burner and the pressure of the gas passing through it. the form of the engine is as its designer has made it. but the form of the living organism is imposed upon it from within; and, as far as we can tell, is inherent in it. here is the wonder and mystery of life: the power of the living organism to assimilate dead matter, to give it life and bring it into the law and unity of the organism itself. but it cannot do this indiscriminately; it is not able thus to convert every dead material; it is restricted, narrowly restricted, in its action. "one of the chief characteristics of living matter is found in the continuous range of chemical reactions which take place between living cells and their inorganic surroundings. without cease certain substances are taken up and disappear in the endless round of chemical reactions in the cell. other substances which have been produced by the chemical reactions in living matter pass out of the cell and reappear in inorganic nature as waste products of the life process. the whole complex of these chemical transformations is generally called _metabolism_. inorganic matter contrasts strikingly with living substance. however long a crystal or a piece of metal is kept in observation, there is no change of the substance, and the molecules remain the same and in the same number. for living matter the continuous change of substances is an indispensable condition of existence. to stop the supply of food material for a certain time is sufficient to cause a serious lesion of the life process or even the death of the cell. but the same happens when we hinder the passing out of the products of chemical transformation from the cell. on the other hand, we may keep a crystal of lifeless matter in a glass tube carefully shut up from all exchange of substance with the external world for as many years as we like. the existence of this crystal will continue without end and without change of any of its properties. there is no known living organism which could remain in a dry resting state for an infinitely long period of time. the longest lived are perhaps the spores of mosses which can exist in a dry state more than a hundred years. as a rule the seeds of higher plants show their vital power already weakened after ten years; most of them do not germinate if kept more than twenty to thirty years. these experiences lead to the opinion that even dry seeds and spores of lower plants in their period of rest of vegetation continue the processes of metabolism to a certain degree. this supposition is confirmed by the fact that a very slight respiration and production of carbonic acid can be proved when the seeds contain a small percentage of water. it seems as if life were weakened in these plant organs to a quite imperceptible degree, but never, not even temporarily, really suspended. "life is, therefore, quite inseparable from chemical reactions, and on the whole what we call life is nothing else but a complex of innumerable chemical reactions in the living substance which we call protoplasm."[ ] the essential quality, therefore, of life is continual change, but not mere change in general. it is that special process of the circulation of matter which we call metabolism, and this circulation is always connected with a particular chemical substance--protoplasm. in this substance five elements are always present and predominant--carbon, oxygen, nitrogen, hydrogen, and sulphur. the compounds which these five elements form with each other are most complex and varied, and they also admit to combination--but in smaller proportions--some of the other elements, of which phosphorus, potassium, calcium, magnesium, and iron are the most important. for protoplasm--using the term in the most general sense--is a chemical substance, not a mere mixture of a number of chemical elements, nor a mere mechanical structure. "however differently the various plasma substances behave in detail, they always exhibit the same general composition as the other albuminoids out of the five 'organo-genetic elements'--namely in point of weight, - % carbon, - % oxygen, - % nitrogen, - % hydrogen, and - % sulphur."[ ] haeckel, the writer just quoted, describes the plasm, the universal basis of all the vital phenomena, in the following terms: "in every case where we have with great difficulty succeeded in examining the plasm as far as possible and separating it from the plasma-products, it has the appearance of a colourless, viscous substance, the chief physical property of which is its peculiar thickness and consistency. the physicist distinguishes three conditions of inorganic matter--solid, fluid, and gaseous. active living protoplasm cannot be strictly described as either fluid or solid in the physical sense. it presents an intermediate stage between the two which is best described as viscous; it is best compared to a cold jelly, or solution of glue. just as we find the latter substance in all stages between the solid and the fluid, so we find in the case of protoplasm. the cause of this softness is the quantity of water contained in the living matter, which generally amounts to a half of its volume and weight. the water is distributed between the plasma molecules or the ultimate particles of living matter in much the same way as it is in the crystals of salts, but with the important difference that it is very variable in quantity in the plasm. on this depends the capacity for the absorption or imbibition in the plasm, and the mobility of its molecules, which is very important for the performance of the vital actions. however, this capacity of absorption has definite limits in each variety of plasm; living plasm is not soluble in water, but absolutely resists the penetration of any water beyond this limit."[ ] and czapek further tells us that "the most striking feature of cell life is the fact that an enormous number of chemical reactions take place within the narrowest space. most plant cells do not exceed · to · millimetres in diameter. their greatest volume therefore can only be an eighth of a cubic millimetre. nevertheless, in this minute space we notice in every stage of cell life a considerable number of chemical reactions which are carried on contemporaneously, without one disturbing the other in the slightest degree."[ ] it is clear if organic bodies were built up of chemical compounds of small complexity and great stability that this continuous range of chemical reactions, this unceasing metabolism, could not take place. it is therefore a necessary condition for organic substances that they should be built up of chemical compounds that are most complex and unstable. "exactly those substances which are most important for life possess a very high molecular weight, and consequently very large molecules, in comparison with inorganic matter. for example: egg-albumin is said to have the molecular weight of at least , , starch more than , , whilst the molecular weight of hydrogen is , of sulphuric acid and of potassium nitrate about , and the molecular weight of the heaviest metal salts does not exceed about ."[ ] to sum up: the living organism, whether it be a simple cell, or the ordered community of cells making up the perfect plant or animal, is an entity, a living individual, wherein highly complex and unstable compounds are unceasingly undergoing chemical reactions, a metabolism essentially associated with protoplasm. but these complex compounds are, nevertheless, formed by the combinations of but a few of the elements now known to us. many writers on the subject of the habitability of other worlds, from contemplating the rich and apparently limitless variety of the forms of life, and the diversity of the conditions under which they exist, have been led to assume that the basis of life must itself also in like manner be infinitely broad and infinitely varied. in this they are mistaken. as we have seen, the elements entering into the composition of organic bodies are, in the main, few in number. the temperatures at which they can exist are likewise strictly limited. but, above all, that circulation of matter which we call life--the metabolism of vital processes--requires for its continuance the presence of one indispensable factor--water. protoplasm itself, as czapek puts it, is practically an _albumin sol_; that is to say, it is a chemical substance of which the chief constituents are albuminous matter and water, and the protoplasm can only take from without material dissolved in water; it can only eject matter in the same way. this _osmosis_ is an indispensable condition in the vital process. and the "streaming" of protoplasm, its continual movement in the cell, can only be carried on in water. water is the compound of oxygen and hydrogen in the proportion of two atoms of hydrogen to one of oxygen. it is familiar to us in three states: solid, liquid, and gaseous, or ice, water, and steam. but it is only in the liquid state that water is available for carrying on the processes of life. this fact limits the temperatures at which the organic functions can be carried on, for water under terrestrial conditions is only liquid for a hundred degrees; it freezes at ° centigrade, it boils at ° centigrade. necessarily, our experiences are mostly confined within this range, and therefore we are apt unconsciously to assume that this range is all the range that is possible, whereas it is but a very small fraction of the range conceivable, and indeed existing, in cosmical space. in its liquid state water is a general solvent, and yet pure water is neutral in its qualities, both characteristics being essential to its usefulness as a vehicle for the protoplasmic actions. naturally, this function of water as a solvent can only exist when water is in the liquid state; solid water, that is ice, neither dissolves nor flows; and water, when heated to boiling point, passes into vapour, and so leaves the organism moistureless, and therefore dead. it is possible to grind a living organism to a pulp so that the structure of the cells is practically destroyed, and yet for some reactions which are quite peculiar to life still to show themselves for some appreciable time. but when the cell-pulp is heated to the temperature of boiling water, these chemical processes cannot be longer observed. what is left may then be considered as definitely dead. water is, then, indispensable for the living organism; but there are two great divisions of such organisms--plants and animals. animals are generally, but not universally, free to move, and therefore to travel to seek their food. but their food is restricted; they cannot directly convert inorganic matter to their own use; they can only assimilate organic material. the plant, on the other hand, unlike the animal, can make use of inorganic material. plant life, therefore, requires an abundant supply of water in which the various substances necessary for its support can be dissolved; it must either be in water, or, if on land, there must be an active circulation of water both through the atmosphere and through the soil, so as to bring to it the food that it requires. animal life presupposes plant life, for it is always dependent upon it. many writers have assumed that life is very widely distributed in connection with this planet. the assumption is a mistaken one, as has been well pointed out by garrett p. serviss, a charming writer on astronomical subjects: "on the earth we find animated existence confined to the surface of the crust of the globe, to the lower and denser strata of the atmosphere, and to the film of water that constitutes the oceans. it does not exist in the heart of the rocks forming the body of the planet nor in the void of space surrounding it outside the atmosphere. as the earth condensed from the original nebula, and cooled and solidified, a certain quantity of matter remained at its surface in the form of free gases and unstable compounds, and, within the narrow precincts where these things were, lying like a thin shell between the huge inert globe of permanently combined elements below, and the equally unchanging realm of the ether above, life, a phenomenon depending upon ceaseless changes, combinations and re-combinations of chemical elements in unstable and temporary union, made its appearance, and there only we find it at the present time."[ ] "the huge inert globe of permanently combined elements below, and the equally unchanging realm of the ether above," offer no home for the living organism; least of all for the highest of such organisms--man. both must be tempered to a condition which will permit and favour continual change, the metabolism which is the essential feature of life. "when the earth had to be prepared for the habitation of man, a veil, as it were, of intermediate being was spread between him and its darkness, in which were joined, in a subdued measure, the stability and the insensibility of the earth, and the passion and perishing of mankind. "but the heavens, also, had to be prepared for his habitation. between their burning light,--their deep vacuity, and man, as between the earth's gloom of iron substance, and man, a veil had to be spread of intermediate being;--which should appease the unendurable glory to the level of human feebleness, and sign the changeless motion of the heavens with the semblance of human vicissitude. between the earth and man arose the leaf. between the heaven and man came the cloud. his life being partly as the falling leaf and partly as the flying vapour."[ ] the leaf and the cloud are the signs of a habitable world. the leaf--that is to say, plant life, vegetation--is necessary because animal life is not capable of building itself up from inorganic material. this step must have been previously taken by the plant. the cloud, that is to say water-vapour, is necessary because the plant in its turn cannot directly assimilate to itself the nitrogen from the atmosphere. the food for the plant is brought to it by water, and it assimilates it by the help of water. it is, therefore, upon the question of the presence of water that the question of the habitability of a given world chiefly turns. in the physical sense, man is "born of water," and any world fitted for his habitation must "stand out of the water and in the water." chapter iii the sun the sun is, of all the heavenly bodies, the most impressive, and has necessarily, at all times, attracted the chief attention of men. there are only two of the heavenly bodies that appear to be more than points of light, only two that show a surface to the naked eye, and the sun, being so much the brighter of the two, and the obvious source of all our light and heat, and the fosterer of vegetation, readily takes the premier place in interest. in the present day we know too much about the sun for anyone to suppose that it can be the home of organic life; but it is not many years since its habitability was seriously suggested even by so high an authority as sir william herschel. he conceived that it was possible that its stores of light and heat might be confined to a relatively thin shell in its upper atmosphere, and that below this shell a screen of clouds might so check radiation downward that it would be possible for an inner nucleus to exist which should be cool and solid. this fancied inner globe would then necessarily enjoy perpetual daylight, and a climate which knew no variation from pole to pole. to its inhabitants the entire heavens would be generally luminous, the light not being concentrated into any one part of the vault; and it was supposed that, ignorant of time, a happy race might flourish, cultivating the far-spread solar fields, in perpetual daylight, and in the serenity of a perpetual spring that was distracted by no storm. the picture thus conjured up is a pleasing one, though probably, to the restless sons of earth, it would seem to suffer somewhat from monotony. but we now know that it corresponds in not a single detail to the actual facts. the study of solar conditions carried on through the last hundred years has revealed to us, not serenity and peace, but storm, stress, and commotion on the most gigantic scale. but though we now can dismiss from our minds the possibility that the sun can be inhabited, yet it is of such importance to the maintenance of life on this planet, and by parity of reasoning to life on any other planet, that a review of its conditions forms a necessary introduction to our subject. further, those conditions themselves will bring out certain principles that are of necessary application when we come to consider the case of particular planets. the distance of the sun from the earth is often spoken of as the "astronomical unit"; it is the fundamental measure of astronomy, and all our information as to the sizes and distances of the various planets rests upon it. and, as we shall shortly see, the particular problem with which we are engaged--the habitability of worlds--is directly connected with these two factors: the size of the world in question, and its distance from the sun. the distance of the sun has been determined by several different methods the principles of which do not concern us here, but they agree in giving the mean distance of the sun as a little less than , , miles; that is to say, it would require , worlds as large as our own to be put side by side in order to bridge the chasm between the two. or a traveller going round the earth at its equator would have to repeat the journey times before he had traversed a space equal to the sun's distance. but knowing the sun's distance, we are able to deduce its actual diameter, its superficial extent, and its volume, for its apparent diameter can readily be measured. its actual diameter then comes out as , miles, or · times that of the earth. its surface exceeds that of the earth , times; its volume, , , times. but the weight of the sun is known as well as its size; this follows as a consequence of gravitation. for the planets move in orbits under the influence of the sun's attraction; the dimensions of their orbits are known, and the times taken in describing them; the amount of the attractive force therefore is also known, that is to say, the mass of the sun. this is , times the mass of the earth; and as the latter has been determined as equal to about , , , , , , , tons that of the sun would be equal to , , , , , , , , , tons. it will be seen that the proportion of the volume of the sun to that of the earth is greater than the proportion of its mass to the earth's mass--almost exactly four times greater; so that the mean density of the sun can be only one-fourth that of the earth. yet, if we calculate the force of gravity at the surfaces of both sun and earth, we find that the sun has a great preponderance. its mass is , times that of the earth, but to compare it with the attraction of the earth's surface we must divide by ( · ){ }, since the distance of the sun's centre from its surface is · times as great as the corresponding distance in the case of the earth, and the force of gravity diminishes as the square of the increased distance. this gives the force of gravity at the solar surface as · times its power at the surface of the earth, so that a body weighing one ton here would weigh tons cwt. if it were taken to the sun.[ ] this relation is one of great importance when we realize that the pressure of the earth's atmosphere is · lb. on the square inch at the sea level; that is to say, if we could take a column of air one square inch in section, extending from the surface of the earth upwards to the very limit of the atmosphere, we should find that it would have this weight. if we construct a water barometer, the column of water required to balance the atmosphere must be feet high, while the height of the column of mercury in a mercurial barometer is inches high, for the weight of cubic inches of mercury or of cubic inches of water ( × = ) is · lb. if, now, we ascend a mountain, carrying a mercurial barometer with us we should find that it would fall about one inch for the first feet of our ascent; that is to say, we should have left one-thirtieth of the atmosphere below us by ascending feet. as we went up higher we should find that we should have to climb more than feet further in order that the barometer might fall another inch; and each successive inch, as we went upward, would mean a longer climb. at the height of feet the barometer would have fallen three inches; we should have passed through one-tenth of the atmosphere. at the height of feet, we should have passed through one-fifth of the atmosphere, the barometer would have dropped six inches; and so on, until at about three and a third miles above sea level the barometer would read fifteen inches, showing that we had passed through half the atmosphere. mont blanc is not quite three miles high, so that in europe we cannot climb to the height where half the atmosphere is left below us, and there is no terrestrial mountain anywhere which would enable us to double the climb; that is to say, to ascend six and two-third miles. could we do so, however, we should find that the barometer had fallen to seven and a half inches; that the second ascent of three and a third miles had brought us through half the remaining atmosphere, so that only one-fourth still remained above us. in the celebrated balloon ascent made by mr. coxwell and mr. glaisher on september , , an even greater height was attained, and it was estimated that the barometer fell at its lowest reading to seven inches, which would correspond to a height of , feet. but on the sun, where the force of gravity is · times as great as at the surface of the earth, it would, if all the other conditions were similar, only be necessary to ascend one furlong, instead of three and a third miles, in order to reach the level of half the surface pressure, and an ascent of two furlongs would bring us to the level of quarter pressure, and so on. if then the solar atmosphere extends inwards, below the apparent surface, it should approximately double in density with each furlong of descent. these considerations, if taken alone, would point to a mean density of the sun not as we know it to be, less than that of the earth, but immeasurably greater; but the discordance is sufficiently explained when we come to another class of facts. these relate to the temperature of the sun, and to the enormous amount of light and heat which it radiates forth continually. this entirely transcends our power to understand or appreciate. nevertheless, the astonishing figures which the best authorities give us may, by their vastness, convey some rough general impression that may be of service. thus prof. c. a. young puts the total quantity of sunlight as equivalent to , , , , , , , , , standard candles. the intensity of sunlight at each point of the sun's surface is variously expressed as , times that of a standard candle, times that of the metal in a bessemer converter, times that of a calcium light, or, · times that of an electric arc. the same authority estimates at _calories_ the value of the _solar constant_; that is to say, the heat which, if our atmosphere were removed, would be received from the sun in a minute of time upon a square metre of the earth's surface that had the sun in its zenith, would be sufficient to raise the temperature of a kilogram of water degrees centigrade. this would involve that the heat radiation from each square metre of the sun's surface would equal , , calories; or sufficient to melt through in each minute of time a shell of ice surrounding the sun to the thickness of · feet. prof. abbot's most recent determination of the solar constant diminishes these estimates by one third; but he still gives the probable temperature of the solar surface as not far short of degrees centigrade, or about , degrees fahrenheit. the sun, then, presents us with temperatures and pressures which entirely surpass our experience on the earth. the temperatures, on the one hand, are sufficient to convert into a permanent gas every substance with which we are acquainted; the pressures, on the other hand, apart from the high temperatures, would probably solidify every element, and the sun, as a whole, would present itself to us as a comparatively small solid globe, with a density like that of platinum. with both factors in operation, we have the result already given: a huge globe, more than one hundred times the diameter of the earth, yet only one-fourth its density, and gaseous probably throughout the whole of its enormous bulk. what effect have these two factors, so stupendous in scale, upon its visible surface? what is the appearance of the sun? it appears to be a large glowing disc, sensibly circular in outline, with its edge fairly well-defined both as seen in the telescope and as registered on photographs. in the spectroscope, or when in an eclipse of the sun the moon covers the whole disc, a narrow serrated ring is seen surrounding the rim, like a velvet pile of a bright rose colour. this crimson rim, the sierra or _chromosphere_ as it is usually called, is always to be found edging the entire sun, and therefore must carpet the surface everywhere. but under ordinary conditions, we do not see the chromosphere itself, but look down through it on the _photosphere_, or general radiating surface. this, to the eye, certainly looks like a definite shell, but some theorists have been so impressed with the difficulty of conceiving that a gaseous body like the sun could, under the conditions of such stupendous temperatures as there exist, have any defined limit at all, that they deny that what we see on the sun is a real boundary, and argue that it only appears so to us through the effects of the anomalous refraction or dispersion of light. such theories introduce difficulties greater and more numerous than those that they clear away, and they are not generally accepted by practical observers of the sun. they seem incompatible with the apparent structure of the photosphere, which is everywhere made up of a complicated mottling: minute grains somewhat resembling those of rice in shape, of intense brightness, and irregularly scattered. this mottling is sometimes coarsely, sometimes finely textured; in some regions it is sharp and well defined, in others misty or blurred, and in both cases they are often arranged in large elaborate patterns, the figures of the pattern sometimes extending for a hundred thousand miles or more in any direction. the rice-like grains or granules of which these figures are built up, and the darker pores between them, are, on the other hand, comparatively small, and do not, on the average, exceed two to four hundred miles in diameter. but the sun shows us other objects of quite a different order in their dimensions. here and there the bright granules of the photosphere become disturbed and torn apart, and broad areas are exposed which are relatively dark. these are _sunspots_, and in the early stages of their development they are usually arranged in groups which tend to be stretched out parallel to the sun's equator. a group of spots in its later stages of development is more commonly reduced to a single round, well-defined, dark spot. these groups, when near the edge of the sun, are usually seen to be accompanied by very bright markings, arranged in long irregular lines, like the foam on an incoming tide. these markings are known as the _faculae_, from their brightness. in the spectroscope, when the serrated edges of the chromosphere are under observation, every now and then great _prominences_, or tongues and clouds of flame, are seen to rise up from them, sometimes changing their form and appearance so rapidly that the motion can almost be followed by the eye. an interval of fifteen or twenty minutes has frequently been sufficient to transform, quite beyond recognition, a mass of flame fifty thousand miles in height. sometimes a prominence of these, or even greater, dimensions has formed, developed, risen to a great distance from the sun, and completely disappeared within less than half an hour. the velocity of the gas streams in such eruptions often exceeds one hundred miles a second; sometimes, though only rarely, it reaches a speed twice as great. sunspots do not offer us examples of motions of this order of rapidity, but the areas which they affect are not less astonishing. many spot groups have been seen to extend over a length of one hundred thousand, or one hundred and fifty thousand miles, and to cover a total area of a thousand million square miles. indeed, the great group of february, , at its greatest extent, covered an area four times as great as this. again, in the normal course of the development of a spot group, the different members of the group frequently show a kind of repulsion for each other in the early stages of the group's history, and the usual speed with which they move away from each other is three hundred miles an hour. the spots, the faculae, the prominences, are all, in different ways, of the nature of storms in an atmosphere; that is to say, that, in the great gaseous bulk of the sun, certain local differences of constitution, temperature, and pressure are marked by these different phenomena. from this point of view it is most significant that many spots are known to last for more than a month; some have been known to endure for even half a year. the nearest analogy which the earth supplies to these disturbances may be found in tropical cyclones, but these are relatively of far smaller area, and only last a few days at the utmost, while a hundred miles an hour is the greatest velocity they ever exhibit, and this, fortunately, only under exceptional circumstances. for a wind of such violence mows down buildings and trees as a scythe the blades of grass; and were tornadoes moving at a rate of miles an hour as common upon the earth as spots are upon the sun, it would be stripped bare of plants and animals, as well as of men and of all their works. it is not an accident that the sun, when storm-swept, shows this violence of commotion, but a necessary consequence of its enormous temperature and pressures. as we have seen, the force of gravity at its surface is · times that at the surface of the earth, where a body falls · feet in the first second of time; on the sun, therefore, a body would fall feet in the first second; and the atmospheric motions generally would be accelerated in the same proportion. the high temperatures, the great pressures, the violent commotions which prevail on the sun are, therefore, the direct consequence of its enormous mass. the sun is, then, not merely the type and example of the chief source of light and heat in a given planetary system; it indicates to us that size and mass are the primary tokens by which we may judge the temperature of a world, and the activity to be expected in its changes. chapter iv the distribution of the elements in space it is now an old story, but still possessing its interest, how fraunhofer analysed the light of the sun by making it pass through a narrow slit and a prism, and found that the broad rainbow-tinted band of light so obtained was interrupted by hundreds of narrow dark lines, images in negative of the slit; and how kirchhoff succeeded in proving that two of these dark lines were caused by the white light of the solar photosphere having suffered absorption at the sun by passing through a stratum of glowing sodium vapour. from that time forward it has been known that the sun is surrounded by an atmosphere of intensely heated gases, among which figure many of those elements familiar to us in the solid form on the earth, such as iron, cobalt, nickel, copper, manganese, and the like. these metals, here the very types of solid bodies, are permanent gases on the sun. the sun, then, is in an essentially gaseous condition, enclosed by the luminous shell which we term the photosphere. this shell prof. c. a. young and the majority of astronomers regard as consisting of a relatively thin layer of glowing clouds, justifying the quaint conceit of r. a. proctor, who spoke of the sun as a "bubble"; that is, a globe of gas surrounded by an envelope so thin in comparison as to be a mere film. there has been much difference of opinion as to the substance forming these clouds, but the theory is still widely held which was first put forward by dr. johnstone stoney in , that they are due to the condensation of carbon, the most refractory of all known elements. prof. abbot, however, refuses to believe in a surface of this nature, holding that the temperature of the sun is too high even at the surface to permit any such condensation. the application of the spectroscope to astronomy is not confined to the sun, but reaches much further. the stars also yield their spectra, and we are compelled to recognize that they also are suns; intensely heated globes of glowing gas, rich in the same elements as those familiar to us on the earth and known by their spectral lines to be present on the sun. the stars, therefore, cannot themselves be inhabited worlds any more than the sun, and at a stroke the whole of the celestial luminaries within the furthest range of our most powerful telescopes are removed from our present search. only those members of our solar system that shine by reflecting the light of the sun can be cool enough for habitation; the true stars cannot be inhabited, for, whatever their quality and order, they are all suns, and must necessarily be in far too highly heated a condition to be the abode of life. many of them may, perhaps, be a source of light and heat to attendant planets, but there is no single instance in which such a planet has been directly observed; no dark, non-luminous body has ever been actually seen in attendance on a star. many double or multiple stars are known, but these are all instances in which one sun-like body is revolving round another of the same order.[ ] we see no body shining by reflected light outside the limits of the solar system. planets to the various stars may exist in countless numbers, but they are invisible to us, and we cannot discuss conditions where everything is unknown. enquiry in such a case is useless, and speculation vain. the stars, as revealed to us by the spectroscope are all of the same order as the sun, but they are not all of the same species. quite a large number of stars, of which arcturus is one of the best-known examples, show spectra that are essentially the same as that of the sun, but there are other stars of which the spectra bear little or no semblance to it. nevertheless, it remains true that, on the whole, stellar spectra bear witness to the presence of just the same elements as we recognize in the sun, though not always in the same proportions or in the same conditions--hydrogen, calcium, sodium, magnesium, iron, titanium, and many more are recognized in nearly all. it is true that not all the known terrestrial elements have yet been identified in either sun or stars; but, in general, those missing are either "negative" elements like the halogens, or elements of great atomic weight like mercury and platinum. that elements of one class should, as a rule, reveal their presence in sun and stars wherever these are placed, and, correspondingly, that other classes should as generally fail to show themselves, indicate that such absence is more likely to be due to the general structure of the stellar photospheres and reversing layers than to any irregularity in the distribution of matter in the universe. it is easy, for example, to conceive that the heavy metals may lie somewhat deeper down within the sun or star than those of low atomic weight. in the case of the sun, there seems a clear connection between atomic weight and the distinctness with which the element is recognized in the spectrum of the photosphere, the lower atomic weights showing themselves more conspicuously. it is clear that not all elements present in a sun or star show themselves in its spectrum. oxygen is very feebly represented by its elemental lines, but the flutings of titanium oxide are found in sunspots, and with great distinctness in a certain type of stars. nitrogen, too, though not directly recognized, proves its presence by the lines of cyanogen. the case of helium is one of particular interest; this element was recognized by a very bright yellow line in the solar prominences before it was known to exist on the earth; indeed, it received the name _helium_ because it then seemed to be a purely solar constituent. now it is seen as a strong absorption line in the spectrum of many stars; but for some reason it is not in general seen as an absorption line over the sun's disc, and if our sun were removed to such distance so as to appear to us only as a star, we should have no evidence that it contained any helium at all. so far, then, as the evidence of the spectroscope goes, the elements present in the earth are present throughout the whole extent of the universe within our view: the same elements and with the same qualities. for the lines of the spectrum of an element are the revelation of its innermost molecular structure, so that we can confidently affirm that hydrogen and oxygen on sirius, arcturus, or the sun, are essentially the same elements as hydrogen and oxygen on the earth. on a planet attached to any of these stars, the two gases would combine together to form water under just the same conditions as they do here on the earth; and at suitable temperatures that water would be a neutral liquid, capable of dissolving just the same chemical substances that it does here. it would freeze as it does here; it would evaporate as it does here; it would be water as completely in all its qualities and conditions as earthly water is. and what applies to one element or compound applies to all. throughout the whole extent of space, the same building materials have been employed, and throughout they retain the same qualities. hydrogen is seen in the spectra of nearly all stars, and also in those of nebulæ. the elemental lines of oxygen are not indeed seen in stellar spectra, but that the element is present is shown by the flutings of titanium oxide which distinguish stars like antares. nitrogen and carbon again are not recognized by their elemental lines, but the lines of cyanogen are seen in the spectra of comets and of sunspots, and hydrocarbon flutings in the spectra of comets and red stars; while in a few of the hottest stars even sulphur has recently been identified.[ ] all the five organo-genetic elements are therefore abundantly diffused through space; the materials for protoplasm, "the albuminous substance with water," are at hand everywhere. this being so, it is reasonable to infer that if organic life exists elsewhere than on this earth, its essential feature, there as here, is the metabolism of nitrogenous carbon compounds in association with protoplasm. but it is objected that "we are not yet able to identify all the lines in solar or stellar spectra; may not some of these lines be due to elements of which we know nothing here, and may not such new elements form complex and unstable compounds with each other, or with some of those familiar to us, that would take the place of the five organo-generators, and so give rise to a physical basis of life, different from that we know on this earth?" but the development of mendeléeff's periodic law has shown that the elements are not to be regarded as disconnected entities. the law as given in mendeléeff's own words, runs: "the properties of the elements as well as the forms and properties of their compounds are in periodic dependence on, or (expressing ourselves algebraically) form a periodic function of the atomic weights of the elements." in other words, they form a series, not only as it regards their atomic weights, but also as it regards their own properties and the forms and properties of their compounds. we are no longer at liberty, as we might have been many years ago, to call into fancied existence new elements having no relation in their properties and compounds to those with which we are acquainted. new elements, no doubt, will be discovered in the future, as in the past; and indeed we may be able to discover them and learn their atomic weights and properties without ever being able to handle them in a terrestrial laboratory. in a series of remarkable papers communicated to the royal astronomical society during the past year ( - ), dr. j. w. nicholson has given the result of his computation of the positions of the spectral lines of two elements of simple structure, and has found that the resulting lines correspond, for one dynamical system, to the chief unidentified lines observed in the spectra of nebulæ, and for the other, to the chief unidentified lines in the spectrum of the corona. the latter element is probably associated with the halogens, but of much lower atomic weight (namely, · ), than fluorine; he therefore gives it the name of _protofluorine_. the other element, to which he gives the name _nebulium_, will have an atomic weight of · . prof. max wolf, of heidelberg, has recently pointed out[ ] the evidence of the presence of two other unknown gases in the ring nebula in lyra, and there is no reason to suppose that the process of discovery has come to an end. but we cannot imagine that we shall discover any new elements that are more abundant and more universally diffused than the five which give us protoplasm--"the physical basis of life." to take an analogy from the solar system: many hundreds of planetoids have now been discovered between the orbits of mars and jupiter, and probably many hundreds more remain to be discovered; but of one thing we are certain, that none of the planetoids yet to be discovered will be of the same rank as either of those two guardians, mars and jupiter, who revolve on the confines of the planetoidal zone. indeed, ceres, the planetoid first discovered, has a greater mass than the aggregate of all discovered since, and probably of all that exist in the zone. water is essential for life here, but the quality in water which restricts the range of terrestrial life is that it freezes at ° centigrade, and boils at ° centigrade; it is only in the liquid state during the intermediate range of degrees. in order to extend the range for living organisms, we should have, therefore, to discover a new vehicle, that, possessing all the other qualities of water, is not restricted to the liquid state within the same limits. but we are at once met with the difficulty that the first essential for the vehicle is that it should be abundant, and there are no other elements more abundant than hydrogen and oxygen. this new vehicle must, like water, be both neutral and stable, or it would itself interfere with the highly unstable compounds that are a necessity for metabolism. and, if we could find this new vehicle, liquid at temperatures outside the ° to ° centigrade, have we any reason to suppose that protoplasm itself would be able to endure these outlying temperatures? looking through the range of substances available, we can only say that none other presents itself as approaching water in suitability for its essential office. if we, ourselves, were able to create a vehicle, could we imagine one more perfectly suited? chapter v the moon the sun and moon offer to our sight almost exactly the same apparent diameters; to the eye, they look the same size. but as we know the sun to be times as distant as the moon, it is necessarily times as large; its surface must exceed that of the moon by the square of , or , ; its volume by the cube of , or , , . as the sun is of low mean density, its mass does not exceed that of the moon in quite the same high ratio; but it is equal in mass to , , moons. compared with the sun, the moon is therefore an insignificant little ball--a mere particle; but as a world for habitation it possesses some advantages over the sun. the first glance at it in a telescope is sufficient to assure the observer that he is looking at a solid, substantial globe. it is not only substantial, it is rugged; its surface is broken up into mountains, hills, valleys, and plains; the mountains stand out in sensible relief; it looks like a ball of solid silver boldly embossed and chased. so far all is to the good for the purpose of habitation. wherever men are, they must have a solid platform on which to stand; they must have a stable terrene whereon their food may grow, and this the moon could supply. "the earth's gloom of iron substance" is necessary for man here, and the moon appears to offer a like stability. another favourable condition is that we know that the moon receives from the sun a sufficient supply of light and heat. each square yard of its surface receives, on the average, the same amount of light and heat that would fall upon a square yard on the earth that was presented towards the sun at the same inclination; and we know from our own experience that this is sufficient for the maintenance of life. and the moon is near enough for us to subject her to a searching scrutiny. every part of the hemisphere turned toward us has been repeatedly examined, measured, and photographed; to that extent our knowledge of its topography is more complete than of the world on which we live. there are no unexplored regions on our side of the moon. the great photographs taken in recent years at the observatories of paris and of the university of chicago have shown thousands of "crater-pits," not more than a mile across; and narrow lines on the moon's surface have been detected with a breadth less than one-tenth of this. an elevation on the moon, if it rose up abruptly from an open plain, would make its presence apparent by the shadow which it would cast soon after sunrise or near sunset; in this way an isolated building, if it were as large as the great pyramid of ghizeh, would also show itself, and all our great towns and cities would be apparent as areas of indistinct mottling, though the details of the cities would not be made out. but if vegetation took the same forms on the moon as on the earth, and passed through the same changes, we should have no difficulty in perceiving the evidence of its presence. if we were transported to the moon and turned our eyes earthward, we should not need the assistance of any telescope in order to detect terrestrial changes which would be plainly connected with the seasonal changes of vegetation. the earth would present to us a disc four times the apparent diameter of the moon, and on that disc canada would offer as great an area as the whole of the moon does to us. we could easily follow with the naked eye the change from the glittering whiteness of the aspect of canada when snow-covered in winter, to the brown, green and gold which would succeed each other during the brighter months of the year. and this type of change would alternate between the northern and southern hemispheres, for the winter of canada is the summer of the argentine, and conversely. we ought, therefore, to have no difficulty in observing seasonal changes on the moon, if such take place. but nothing of the kind has ever been remarked; no changes sufficiently pronounced for us to be sure of them are ever witnessed. here and there some slight mutations have been suspected, nearly all accomplishing their cycle in the course of a lunar day; so that it is difficult to separate them from changes purely apparent, brought about by the change in the incidence of the illumination. the difference in appearance of a given area on the moon when viewed under a low sun and when the sun is on the meridian is very striking. in the first case everything is in the boldest relief; the shadows are long and intensely black; the whole area under examination in the telescope seems as if it might be handled. under the high sun, the contrasts are gone; the scenery appears flat, many of the large conspicuous markings are only recognized with difficulty. thus the terse remark of mädler, "the full moon knows no maginus," has become a proverb amongst selenographers; yet maginus is a fine walled plain some eighty miles in diameter, and its rampart attains a height in parts of , feet. maginus lies near tycho, which has been well named "the lunar metropolis," for from it radiates the principal system of bright streaks conspicuous on the full moon. these white streaks appear when the shadows have vanished or are growing short; they are not seen under a low sun. the changes which appear to take place in the lunar formations owing to the change in their illumination are much more striking and varied than would be anticipated. but the question arises whether all the changes that are associated with the progress of the lunar day can be ascribed to this effect. thus, prof. w. h. pickering writes concerning a well-known pair of little craters of about nine miles in diameter, "known as messier and messier a, situated side by side not far from the centre of the mare fecunditatis. when the sun rises first on them, the eastern one, a, is triangular and larger than messier, which latter is somewhat pear-shaped. about three days after sunrise they both suddenly turn white, messier rapidly grows in size, soon surpasses a, and also becomes triangular in shape. six days after sunrise the craters are again nearly of the same size, owing to the diminution of messier. the shape of a has become irregular, and differs in different lunations. at nine days after sunrise the craters are exactly alike in size and shape, both now being elliptical, with their major axes lying in a nearly n. and s. direction. just before sunset a is again the larger, being almost twice the size of messier."[ ] some observers explain this cycle of changes as due merely to the peculiar contour of the two objects, the change in the lighting during the lunar day altering their apparent figures. prof. w. h. pickering, on the other hand, while recognizing that some portion of the change of shape is probably due to the contour of the ground, conceives that, in order to explain the whole phenomenon, it is necessary to suppose that a white layer of hoar frost is formed periodically round the two craters. it is also alleged that whereas mädler described the two craters as being exactly alike eighty years ago, messier a is now distinctly the larger; but it is very doubtful whether mädler's description can be trusted to this degree of nicety. if it could, this would establish a permanent change in the actual structure of the lunar surface at this point. there are several other cases of the same order of ambiguity. the most celebrated is linné, a white spot about six miles in diameter on the mare serentatis. this object appears to change in size during the progress of the lunar day, and, as with messier, some selenographers consider that it has also suffered an actual permanent change in shape within the last sixty or seventy years. here again the evidence is not decisive; neison is by no means convinced that a change has taken place, yet does not think it impossible that linné may once have been a crater with steep walls which have collapsed into its interior through the force of gravity. another type of suspected change is associated with the neighbourhood of aristarchus, the brightest formation on the moon, so bright indeed that sir william herschel, observing it when illuminated by earthshine in the dark portion of the moon, thought that he was watching a lunar volcano in eruption. in , on september , the late major molesworth noticed that the crater was at that time under the rays of the setting sun, and filled with shadow, and the inner terraces, which should have been invisible, were seen as faint, knotted, glimmering streaks under both the eastern and western walls, and the central peak was also dimly discernible. he thought this unusual lighting up of rocks on which the sun had already set might be due either to phosphorescence produced by long exposure to the sun's rays, or to inherent heat, or to reflected glare from the western rampart. still more important, both major molesworth and mr. walter goodacre, each on more than one occasion, observed what seemed to be a faint bluish mist on the inner slope of the east wall, soon after sunrise, but this was visible only for a short time. other selenographers too, on rare occasions, have made observations accordant with these, relating to various regions on the moon. these, and a few other similar instances, are all that selenography has to offer by way of evidence of actual lunar change. of seeming change there is abundance, but beyond that we have only cases for controversy, and one of the most industrious of the present-day observers of the moon, m. philip fauth, declares that "as a student of the moon for the last twenty years, and as probably one of the few living investigators who have kept in practical touch with the results of selenography, he is bound to express his conviction that no eye has ever seen a physical change in the plastic features of the moon's surface."[ ] in this matter of change, then, the earth and moon stand in the greatest contrast to each other. as we have seen, from the view-point of the moon, the appearance of the earth would change so manifestly with the progress of the seasons that no one could fail to remark the difference, even though observing with the naked eye. but from the view-point of the earth, the moon when examined by our most experienced observers, armed with our most powerful telescopes, offers us only a few doubtful enigmatical instances of possible change confined to small isolated localities; we see no evidence that the "gloom of iron substance" below is ever concealed by a veil of changing vegetation, or that "between the burning light and deep vacuity" of the heavens above, the veil of the flying vapour has ever been spread out. we see the moon so clearly that we are assured it holds no water to nourish plant life; we see it so clearly because there is no air to carry the vapour that might dim our view. life is change, and a planet where there is no change, or where that change is very small, can be no home for life. the "stability and insensibility" are indeed required in the platform upon which life is to appear, but there must be the presence of "the passion and the perishing," or life will be unable to find a home. we infer the absence of water and air from the moon not only from the unchanging character of its features and the distinctness with which we see them; we are able to make direct observations. galileo, the first man to observe the moon to better advantage than with the naked eye, was not long before he decided that the moon contained no water, for though milton, in a well-known passage, makes galileo discover "rivers or mountains on her spotty globe," galileo himself wrote: "i do not believe that the body of the moon is composed of earth and water." the name of _maria_ was given to the great grey plains of the moon by hevelius, but this was simply for convenience of nomenclature, not because he actually believed them to be seas. one observation is, in itself, sufficient to prove that the maria are not water surfaces. the moon's "terminator," that is to say, the line dividing the part in sunlight from that in darkness, is clearly irregular when it passes over the great plains; were they actually sea it would be a bright line and perfectly smooth. the grey plains are therefore not expanses of water now, nor were they in time past. it is obvious that in some remote antiquity their surface was in a fluid condition, but it was the fluidity of molten rock. this is seen by the way in which the maria have invaded, breached, broken down, and submerged many of the circular formations on their margins. thus the mare humorum has swept away half the wall of the rings, hippalus and doppelmayer, and far out in the open plain of the mare nubium, great circles like kies, and that immediately north of flamsteed, stand up in faint relief as of half-submerged rings. clearly there was a period after the age in which the great ring mountains and walled plains came into existence, when an invasive flood attacked and partially destroyed a large proportion of them. and the flood itself evidently became more viscous and less fluid the further it spread from its original centre of action, for the ridges and crumpling of the surface indicate that the material found more and more difficulty in its flow. we have evidence just as direct that there is no atmosphere. this is very strikingly shown when the moon, in its monthly progress among the stars, passes before one of them and occults it. such an occultation is instantaneous, and is particularly impressive when either a disappearance or a reappearance occurs at the defective limb; that is to say, at the limb which is not illuminated by the sun, and is therefore invisible. the observer may have a bright star in the field of view, showing steadily in a cloudless sky; there is not a hint of a weakening in its light; suddenly it is gone. the first experience of such an observation is most disconcerting; it is hardly less disconcerting to observe the reappearance at the dark limb. one moment the field of view of the telescope is empty; the next, without any sort of dawning, a bright star is shining steadily in the void, and it almost seems to the observer as if an explosion had taken place. if the moon had an atmosphere extending upwards from its surface in all directions and of any appreciable density, an occultation would not be so exceedingly abrupt; and, in particular, if the occultation were watched through a spectroscope, then, at the disappearance, the spectrum of the star would not vanish as a whole, but the red end would go first, and the rest of the spectrum would be swept out of sight successively, from orange to the violet. this does not happen; the whole spectrum goes out together, and it is clear that no appreciable atmosphere can exist on the moon. in actual observation so inappreciable is it that its density at the moon's surface is variously estimated as / th of that of the earth by neison, and as / th by w. h. pickering. if the moon possessed an atmosphere bearing the same proportion to her total mass as we find in the case of the earth, she would have a density of one-fortieth of our atmosphere at the sea level. the moon is at the same mean distance from the sun as the earth, and therefore, surface for surface, receives from it on the average the same amount of light and heat. but it makes a very different use of these supplies. bright as the moon appears when seen at the full on some winter night, it has really but a very low power of reflection, and is only bright by contrast with the darkness of the midnight sky. if the full moon is seen in broad daylight, it is pale and ghost-like. sir john herschel has put it on record that when in south africa he often had the opportunity of comparing the moon with the face of table mountain, the sun shining full upon both, and the moon appeared no brighter than the weathered rock. the best determinations of the _albedo_ of the moon, that is to say, of its reflective power, give it as · , so that only one-sixth of the incident light is reflected, the other five-sixths being absorbed. it is difficult to obtain a good determination of the earth's _albedo_, but the most probable estimate puts it as about · , or three times as great as that of the moon. this high reflective power is partly to be accounted for by the great extent of the terrestrial polar caps, but chiefly by the clouds and dust layer always present in its atmosphere. a larger proportion, therefore, of the solar rays are employed in heating the soil of the moon than in heating that of the earth, and in this connection the effect of an important difference between the two worlds must be noted. the earth rotates on its axis in hours minutes seconds, the mean length of its rotation as referred to the sun being hours. the rotation of the moon, on the other hand, takes days hours minutes to accomplish, giving a mean rotation, as referred to the sun, of days hours minutes. the lunar surface is therefore exposed uninterruptedly to the solar scorching for very nearly fifteen of our days at a time, and it is, in turn, exposed to the intense cold of outer space for an equal period. as the surface absorbs heat so readily, it must radiate it as quickly; hence radiation must go on with great rapidity during the long lunar night. lord rosse and prof. very have both obtained measures of the change in the lunar heat radiation during the progress of a total eclipse of the moon, with the result that the heat disappeared almost completely, though not quite at the same time as the light. prof. langley succeeded in obtaining from the moon, far down in the long wave lengths of the infra-red, a heat spectrum which was only partly due to reflection from the sun; part coming from the lunar soil itself, which, having absorbed heat from the sun, radiated it out again almost immediately. in , prof. very, following up langley's line of work, concluded that the temperature of the lunar soil must range through about ° centigrade, considerably exceeding ° at the height of the lunar day, and falling to about the temperature of liquid air during the lunar night. so wide a range of temperature must be fatal to living organisms, particularly when the range is repeated at short, regular intervals of time. but this range of temperature comes directly from the length of the moon's rotation period; for the longer the day of the moon, the higher the temperature which may be attained in it; the longer the night, the greater the cold which will in turn be experienced. we learn, therefore, that the time of rotation of a planet is an important factor in its habitability. chapter vi the canals of mars both of the two worlds best placed for our study are thus, for different reasons, ruled out of court as worlds for habitation. the sun by its vastness, its intolerable heat and the violence of its changes, has to be rejected on the one hand, while the moon, so small, and therefore so rigid, unchanging and bare, is rejected on the other. of the other heavenly bodies, the planet mars is the one that we see to best advantage. two other planets, eros and venus, at times come nearer to us, but neither offers us on such occasions equal facilities for their examination. but of mars it has been asserted not only that it is inhabited, but that we know it to be the case, since the evidence of the handiwork of intelligent beings is manifest to us, even across the tremendous gulf of forty or more million miles of space. a claim so remarkable almost captures the position by its audacity. there is a natural desire among men to believe the marvellous, and the very boldness of the assertion goes no small way to overcome incredulity. and when we consider how puny are men as we see them on this our planet, how minute their greatest works, how superhuman any undertaking would be which could demonstrate our existence to observers on another planet, we must admit that it is a marvel that there should be any evidence forthcoming that could bear one way or another on the solution of a problem so difficult. the first fact that we have to remember with regard to the planet mars is the smallness of its apparent size. to the eye it is nearly a star--a point of light without visible surface. it is almost twice the size of the moon in actual diameter, but as its mean distance from the earth is times that of the moon, its mean apparent diameter is times smaller. we cannot, however, watch mars in all parts of its orbit; it is best placed for observation, and, therefore, most observed, when in opposition, and oppositions may be favourable or unfavourable. at the most favourable opposition, mars is times as distant as the moon; at the least favourable, times; so that on such occasions its apparent size varies from / th of the diameter of the moon to / th. but a telescope with a magnifying power of could never, under the most perfect conditions, show mars, even in the closest opposition, as well as the moon is seen with the naked eye, for the practical magnifying power of a telescope is never as great as the theoretical. in practice, a child's spy-glass magnifying some six diameters will show the full moon to better advantage than mars has ever been seen, even in our most powerful telescopes. the small apparent size of the planet explains how it was that galileo does not seem to have been able to detect any markings upon it. in , huyghens laid the foundation stone of areography by observing some dark spots, and determining from their apparent movements that the planet had a rotation on its axis, which it accomplished in about the same time as the earth. small and rough as are the drawings that huyghens made, the identification of one or two of his spots is unmistakable. seven years later, in , both cassini and hooke made a number of sketches, and those by hooke have been repeatedly used in modern determinations of the rotation period of the planet. the next great advance was made by sir william herschel, who, during the oppositions of , , , and , determined the inclination of the axis of mars to the plane of its orbit, measured its polar and equatorial diameters, and ascertained the amount of the polar flattening. he paid also special attention to two bright white spots upon the planet, and he showed that these formed round the planet's poles and increased in size as the winter of each several hemisphere drew on and diminished again with the advance of summer, behaving therefore as do the snow caps of our own polar regions. the next stage in the development of our knowledge of mars must be ascribed to the two german astronomers, beer and mädler, who made a series of drawings in the years , and , by means of a telescope of inches aperture, from which they were able to construct a chart of the entire globe. this chart may be considered classic, for the features which it represents have been observed afresh at each succeeding opposition. mars, therefore, possesses a permanent topography, and some of the markings in question can be identified, not only in the rough sketches made by sir william herschel, but even in those made by hooke and cassini as far back as the year . in the forty years that followed, the planet was studied by many of the most skilled observers, particularly by mr. j. n. lockyer in , and the rev. w. r. dawes in . in , the late mr. n. e. green, drawing-master to queen victoria, and a distinguished painter in water colours, made a series of sketches of the planet from a station in the island of madeira feet above sea-level. when the opposition was over, mr. green collected together a large number of drawings, and formed a chart of the planet, much richer in detail than any that had preceded it, and from his skill, experience and training as an artist he reproduced the appearance of the planet with a fidelity that had never been equalled before and has never been surpassed since. at this time it was generally assumed that mars was a miniature of our own world. the brighter districts of its surface were supposed to be continents, the darker, seas. as sir william herschel had already pointed out long before, the little world evidently had its seasons, its axis being inclined to the plane of its orbit at much the same angle as is the case with the earth; it had its polar caps, presumably of ice and snow; its day was but very little longer than that of the earth; and the only important difference seemed to be that it had a longer year, and was a little further off the sun. but the general conclusion was that it was so like the earth in its conditions that we had practically found out all that there was to know; all that seemed to be reserved for future research was that a few minor details of the surface might be filled in as the power of our telescopes was increased. but fortunately for progress, this sense of satisfaction was to be rudely disturbed. as mars, in its progress round the sun, receded from the earth, or rather as the earth moved away from it, the astronomers who observed so diligently during the autumn of turned their attention to other objects. one of them, however, schiaparelli, the most distinguished astronomer on the continent of europe, still continued to watch the planet, and, as the result of his labours, he published some months later the first of a magnificent series of _memoirs_, bringing to light what appeared to be a new feature. his drawings not only showed the "lands" and "seas," that is to say the bright and dark areas, that green and his predecessors had drawn, but also a number of fine, narrow, dark lines crossing the "lands" in every direction. these narrow lines are the markings which have since been so celebrated as the "canals of mars," and the discussion as to the real nature of these canals has focussed attention upon mars in a way that, perhaps, nothing else could have done. before the study of planetary markings was left almost entirely to the desultory labours of amateurs, skilled though many of them were; since , the most powerful telescopes of the great public observatories of the world have been turned upon mars, and the most skilful and experienced of professional astronomers have not been ashamed to devote their time to it. there is no need to pass in review the whole of the immense mass of observations that have been accumulated since schiaparelli brought out the first of his great memoirs. that memoir gave rise to an immediate controversy, for many astronomers of skill and experience had observed the planet in without detecting the network of lines which schiaparelli had revealed, and it was natural that they should feel some reluctance in accepting results so strange and novel. but little by little this controversy has passed. we now know that the "canals" vary much in their visibility, and "curiously enough the canals are most conspicuous, not at the time the planet is nearest to the earth and its general features are in consequence best seen, but as the planet goes away the canals come out. the fact is that the orbital position and the seasonal epoch conspire to a masking of the phenomena." this was the chief reason why schiaparelli's discoveries seemed at first to stand so entirely without corroboration; the "canals" did not become conspicuous until after most observers had desisted from following the planet. another reason was that, in , mars was low down in the sky for northern observatories, and good definition is an essential for their recognition. but the careful examination of drawings made in earlier oppositions, especially those made by dawes and green, afforded confirmation of not a few of schiaparelli's "canals"; even in a few of the easiest and most conspicuous had been delineated by other astronomers before any rumour of schiaparelli's work had come abroad, and as mars came under observation again and again at successive oppositions, the number of those who were able to verify schiaparelli's discoveries increased. it has now long been known that the great italian astronomer was not the victim of a mere optical illusion; there were actual markings on the planet mars where he had represented them; markings which, when seen under like conditions and with equal instrumental equipment, did present the appearance of straight, narrow lines. the "canals of mars" are not mere figments of the imagination, but have a real objective basis. as this controversy has passed away, another and a very different one has arisen out of an unfortunate mistranslation of the term chosen by schiaparelli to indicate these linear streaks. in conformity with the type of nomenclature adopted by previous areographers who had divided mars into "seas," "continents," "islands," "isthmuses," "straits" and the like, schiaparelli had called the narrow lines he detected "_canali_", that is to say "channels," but without intending to convey the idea of artificial construction. indeed, he himself was careful to point out that these designations "were not intended to prejudge the nature of the spot, and were nothing but an artifice for helping the memory and for shortening descriptions." and he added, "we speak in the same way of the lunar seas, although we well know that there are no true seas on the moon." but "_canali_" was unhappily rendered in english as "canals," instead of "channels." "channel" would have left the nature of the marking an open question, but, in english, "canal" means an artificial waterway. here then the question as to whether or no mars is inhabited comes definitely before us. have we sufficient grounds for believing that the "canals" are artificial constructions, or may they be merely natural formations? in , mr. percival lowell founded at flagstaff, arizona, u.s.a., a well-equipped observatory for the special study of mars, and he has continued his scrutiny of the planet from that time to the present with the most unrelaxing perseverance. the chief results that he has obtained have been the detection of many new "canals"; the discovery of a number of dark, round dots, termed by him "oases," at the junctions of the "canals"; and the demonstration that the "canals" and certain of the dusky regions are subject to strictly seasonal change, as really as the polar caps themselves. in addition, he has formed the conclusion, which he has supported with much ingenuity and skill, that the regularity of the "canals" and "oases" quite precludes the possibility of their being natural formations. hence there has arisen the second controversy: that on the nature of the "canals"; for mr. lowell considers that their presence proves the existence of inhabitants on mars, who, by means of a titanic system of irrigation, are fighting a losing battle against the gradual desiccation of their planet. in a paper published in the _international scientific review_, "scientia," in january, , mr. lowell gave a summary of his argument. "organic life needs water for its existence. this water we see exists on mars, but in very scant amount, so that if life of any sort exists there, it must be chiefly dependent on the semi-annual unlocking of the polar snows for its supply, inasmuch as there are no surface bodies of it over the rest of the planet. now the last few years, beginning with schiaparelli in , and much extended since at flagstaff, have shown: "the surface of the planet to be very curiously meshed by a fine network of lines and spots. "now if one considers first the appearance of this network of lines and spots, and then its regular behaviour, he will note that its geometrism precludes its causation on such a scale by any natural process and, on the other hand, that such is precisely the aspect which an artificial irrigating system, dependent upon the melting of the polar snows, would assume. since water is only to be had at the time it is there unlocked, and since for any organic life it must be got, it would be by tapping the disintegrated cap, and only so, that it could be obtained. if mars be inhabited, therefore, it is precisely such a curious system we should expect to see, and only by such explanation does it seem possible to account for the facts. "these lines are the so-called canals of mars. it is not supposed that what we see is the conduit itself. on the contrary, the behaviour of these lines indicates that what we are looking at is vegetation. now, vegetation can only be induced by a water-supply. what we see resembles the yearly inundation of the nile, of which to a spectator in space the river itself might be too narrow to be seen, and only the verdured country on its banks be visible. this is what we suppose to be the case with mars. however the water be conducted, whether in covered conduits, which seems probable, or not, science is not able to state, but the effects of it are so palpable and so exactly in accord with what such a system of irrigation would show, that we are compelled to believe that such is indeed its _vera causa_." beside the bulky _memoirs_ in which prof. lowell has published the scientific results obtained at his observatory at flagstaff, and papers and articles appearing in various scientific journals, he has brought out three books of a more popular character: "_mars_"; "_mars and its canals_"; and "_mars as the abode of life_." in these he shows that to the assiduity of the astronomer he adds the missionary's zeal and eagerness for converts as he pleads most skilfully for the acceptance of his chosen doctrine of the presence of men on mars. in the last of the three books mentioned, he deals directly with "proofs of life on mars." the presence of vegetation may be inferred from seasonal changes of tint, just as an observer on the moon might with the naked eye watch effects on the earth. but though "vegetable life could thus reveal itself directly, animal life could not. not by its body but by its mind would it be known. across the gulf of space it could be recognized only by the imprint it had made on the face of mars." "confronting the observer are lines and spots that but impress him the more, as his study goes on, with their non-natural look. so uncommonly regular are they, and on such a scale as to raise suspicions whether they can be by nature regularly produced" (p. ). "... unnatural regularity, the observations showed, betrays itself in everything to do with the lines: in their surprising straightness, their amazing uniformity throughout, their exceeding tenuity, and their immense length" (p. ). "as a planet ages, its surface water grows scarce. its oceans in time dry up, its rivers cease to flow, its lakes evaporate (p. ).... now, in the struggle for existence, water must be got.... its procuring depends on the intelligence of the organisms that stand in need of it.... as a planet ages, any organisms upon it will share in its development. they must evolve with it, indeed, or perish. at first they change only, as environment offers opportunity, in a lowly, unconscious way. but, as brain develops, they rise superior to such occasioning.... the last stage in the expression of life upon a planet's surface must be that just antecedent to its dying of thirst.... with an intelligent population this inevitable end would be long foreseen.... both polar caps would be pressed into service in order to utilize the whole available supply and also to accommodate most easily the inhabitants of each hemisphere" (pp. - ). "that intelligence should thus mutely communicate its existence to us across the far reaches of space, itself remaining hid, appeals to all that is highest and most far-reaching in man himself. more satisfactory than strange this; for in no other way could the habitation of the planet have been revealed. it simply shows again the supremacy of mind.... thus, not only do the observations we have scanned lead us to the conclusion that mars at this moment is inhabited, but they land us at the further one that these denizens are of an order whose acquaintance was worth the making" (p. ). for the moment, let us leave prof. lowell's argument as he puts it. whether we accept it or not, it remains that it is a marvellous achievement of the optician's skill and the observer's devotion that from a planet so small and so distant as mars any evidence should be forthcoming at all that could bear upon the question of the existence of intelligent organisms upon its surface. but it is of the utmost significance to note that the whole question turns upon the presence of water--of water in the liquid state, of water in a sufficient quantity; and the final decision, for mr. lowell's contention, or against it, must turn on that one point. the search for life on mars is essentially a search for water; a search for water, not only in the present state of mars, but in its past as well. for, without water in sufficient quantities in the past, life on mars could not have passed through the evolutionary development necessary to its attaining its highest expression,--that where the material living organism has become the tabernacle and instrument of the conscious intelligent spirit. chapter vii the condition of mars the planet mars is the debatable ground between two opinions. here, the two opposing views join issue; the controversy comes to a focus. the point in debate is whether certain markings--some linear, some circular--are natural or artificial. if, it is argued, some are truly like a line, without curve or break, as if drawn with pen, ink, and ruler; or others, so truly circular, without deviation or break, as if drawn with pen, ink, and compass; if, moreover, when we obtain more powerful telescopes, erected in better climates for observing, these markings become more truly lines and circles the better we see them; then they are _artificial_, not natural structures. but artificial structures imply artificers. and if the structures are so designed as to meet the needs of a living organism, it implies that the living organism that designed them must have a reasonable mind lodged in a natural body. if, then, the "lines" and "circles" that prof. lowell and his disciples assert to be artificial canals and oases are really such, they premise the order of being that we call man. but these canals and oases also premise the liquid that we call water--water that flows and water utilized in cultivation. in this chapter we will leave out of count the first premiss--man--and only deal with what concerns the second premiss--water; with water that flows and is utilized in vegetation. planetary statistics +--------------------------------------------++--------++---------------- | || minor || inner | ||planets.|| +--------------------------------------------++--------++-------+-------+ | || ceres || moon |mercury| +--------------------------------------------++--------++-------+-------+ |proportions of the planets:-- || || | | | diameter in miles || || | | | " [symbol] = || · || · | · | | surface, [symbol] = || · || · | · | | volume, [symbol] = || · || · | · | | density, water = || · ? || · | · | | " [symbol] = || · ? || · | · | | mass, [symbol] = || · || · | · | | gravity at surface, [symbol] = || · || · | · | | rate of fall, feet in the first second || · || · | · | | albedo || · || · | · | | || || | | |details of orbit:-- || || | | | mean distance from sun in millions of miles|| · || · | · | | " " earth's distance = || · || · | · | | period of revolution, in years || · || · | · | | velocity, in miles per second || · || · | · | | eccentricity || · || · | · | | aphelion distance, perihelion = || · || · | · | | inclination of equator to orbit || (?) || °· ´| (?) | | || || d h m | d | | rotation period || (?) || · · | (?) | | || || | | |atmosphere, assuming the total mass of the || || | | | atmosphere to be proportional to the mass || || | | | of the planet:-- || || | | | pressure at the surface in lb. per sq. in. || · || · | · | | " " " in "atmospheres" || · || · | · | | level of half surface pressure in miles || · || · | · | | boiling point of water at the surface || || °c | °c | | || || | | |temperature:-- || || | | | light and heat received from sun, || || | | | [symbol] = || · || · | · | | reciprocal of square-root of distance, || || | | | [symbol] = || · || · | · | | equatorial temp. of ideal planet, absolute || || ° | ° | | " " " " centigrade|| - || + | + | | average temp. of ideal planet, absolute || || | | | " " " " centigrade || - || + | + | | upper limit under zenith sun, absolute || || | | | " " " " centigrade || - || + | + | | average temp. of equivalent disc, absolute || || | | | " " " " centigrade|| - || + | + | | || || | | +--------------------------------------------++--------++-------+-------+ ------------------++--------++--------------------------------------+ planets. || || outer planets. | || || | +--------+--------++--------++---------+---------+--------+---------+ | mars | venus || earth || uranus | neptune | saturn | jupiter | +--------+--------++--------++---------+---------+--------+---------+ | | || || | | | | | | || || | | | | | · | · || · || · | · | · | · | | · | · || · || · | · | · | · | | · | · || · || · | · | · | · | | · | · || · || · | · | · | · | | · | · || · || · | · | · | · | | · | · || · || · | · | · | · | | · | · || · || · | · | · | · | | · | · || · || · | · | · | · | | · | · || · ? || · | · | · | · | | | || || | | | | | | || || | | | | | · | · || · || · | · | · | · | | · | · || · || · | · | · | · | | · | · || · || · | · | · | · | | · | · || · || · | · | · | · | | · | · || · || · | · | · | · | | · | · || · || · | · | · | · | | °· ´ | (?) || °· ´|| (?) | (?) | °· ´| °· ´ | |h m s | || h m s|| h m | | h m | h m | | · · | (?) || · · || · (?) | (?) | · ±| · ± | | | || || | | | | | | || || | | | | | | || || | | | | | | || || | | | | | · | · || · || · | · | · | · | | · | · || · || · | · | · | · | | · | · || · || · | · | · | · | | °c | °c || °c || °c | °c | °c | °c | | | || || | | | | | | || || | | | | | · | · || · || · | · | · | · | | · | · || · || · | · | · | · | | ° | ° || ° || ° | ° | ° | ° | | - | + || + || - | - | - | - | | | || || | | | | | - | + || + || - | - | - | - | | | || || | | | | | + | + || + || - | - | - | - | | | || || | | | | | + | + || + || - | - | - | - | | | || || | | | | +--------+--------++--------++---------+---------+--------+---------+ for in regard to this particular premiss we can do away with hypothesis, and deal only with certain physical facts that are not controversial and are not in dispute. the first of this series of facts concerning mars about which there can be no controversy or dispute relates to its size and mass. as the foregoing table shows, it comes between the moon and the earth in these respects. the figures show at a glance that mars ranks in its dimensions between the moon and the earth, and that, on the whole, it is more like to the moon than it is to the earth. but in what way would this affect mars as a suitable home for life? in many ways; and amongst these the distribution of its atmosphere and the sluggishness of its atmospheric circulation are not the least important. it was mentioned in chapter iii that at a height of about three and a third miles the barometer will stand at inches, or half its mean height at sea level, showing that one half the atmosphere has been passed through. mont blanc, the highest mountain in europe, is under miles in height, so that it is not possible, in europe, to climb to the level of half-pressure; mt. everest, the highest mountain in the world, is not quite six miles high, so that no part of the solid substance of our planet reaches up to the level of the quarter pressure. on a very few occasions daring aeronauts have soared into the empyrean higher than the summits of even our loftiest mountains, but the excursion has been a dangerous one, and they have with difficulty brought their life back from so rare and cold, so inhospitable a region. when gay-lussac, in , attained a height of , feet above sea level, the thermometer, which on the ground read ° c., sank to ° below zero, and the rare atmosphere was so dry that paper crumpled up as if it had been placed near the fire, and his pulse rose to pulsations a minute instead of his normal . when mr. glaisher and mr. coxwell made their celebrated ascent between and o'clock on the afternoon of september , , they found that at a height of , feet the temperature sank to - · °; at , feet to - · °; and at , feet the temperature was down to - · ° c. at this height the rarefaction of the air was so great and the cold so intense that mr. glaisher fainted, and mr. coxwell's hands being rendered numb and useless by the cold, he was only able to bring about their descent in time by pulling the string of the safety valve with his teeth. yet when they attained this height they were far above all cloud or mist, and the sun's rays fell full upon them. the sun's rays had all the force that they had at the surface of the earth, but in the rare atmosphere of seven miles above the earth, the radiation from every particle not in direct sunlight was so great that while the right hand, exposed to the sun, might burn, the left hand, protected from his direct rays, might freeze. but gravity at the surface of mars is much feebler than at the surface of the earth, and in order to reach the level of half-pressure a martian mountaineer would have to climb, not three and a third miles, but eight and three-quarter miles; that is to say, the distance to be ascended is in the inverse proportion of the force of gravity at the surface of the planet. the atmosphere of mars, therefore, is much deeper than that of the earth, and one great cause of precipitation here is much weakened there. a current of air heavily laden with moisture, if it encounters a range of mountains, is forced upwards, and consequently expands, owing to the diminished pressure. the expansion brings about a cooling, and from both causes the atmosphere is unable to retain as much water-vapour as it carried before. on mars, the same relative expansion and cooling would only follow if the ascent were nearly three times as great, and the feeble force of gravity has its effect in another way; for just as a weight on mars will only fall six feet in the first second as against sixteen on the earth, so a dense and heavy column of air will fall with proportionate slowness and a light column ascend in the same languid manner. an ascending current on mars would therefore take / · × / · = / · , or seven times as long to attain the same relative expansion as on the earth. the winds of mars are therefore sluggish, and precipitation is slight. so far at least it resembles "the island valley of avilion; where falls not hail, or rain, or any snow, nor ever wind blows loudly;" and r. a. proctor, acute and accurate writer on planetary physics as he was, fell into a mistake when he referred to mars as being "hurricane-swept." there are no hurricanes on mars; its fiercest winds can never exceed in violence what a sailor would call a "capful." this holds good for mars, but it also holds good for every planet where the force of gravity at the surface is relatively feeble. the greater the force of gravity the more active the atmospheric circulation, and more violent its disturbances; the feebler the action of gravity the more languid the circulation, and the slighter the disturbances. the atmosphere of mars is relatively deeper than that of the earth, so that we, in observing the details of its surface, are looking down through an immense thickness of an obscuring medium. and yet the details of the surface are seen with remarkable distinctness; not as clearly indeed as we can see those of the moon, but nearly so. for instance, the "canals" appear to have a breadth of from to miles, corresponding to / th, and / th, of a second of arc, at an average opposition. the oases, as a rule, are about miles in diameter, that is to say about half a second of arc. these are extraordinarily fine details to be perceived and held, even if mars had no atmosphere at all; it would certainly be impossible to detect them unless the atmosphere were exceedingly thin and transparent. for we must remember that, though our own atmosphere is a hindrance to our observing, yet the atmosphere of the planet into which we are looking is a greater hindrance still. like the lace curtains of the window of a house, it is a much greater obstacle to looking inward than to looking outward, and as the perfect distinctness with which we see the moon is a proof that it is practically without an atmosphere, so the great detail visible on mars bears unmistakable testimony to the slightness of the atmospheric veil around that planet. and when we turn again to the statistics of mars, we see that this must inevitably be the case. of two planets, one heavier than the other, it is not possible to suppose that the lighter should secure the greater proportional amount of atmosphere. with planets, as with persons, it is the most powerful that gets the lion's share: "to him that hath it is given, and from him that hath not is taken away even that which he seemeth to have." but if we assume that mars has acquired an atmosphere proportional to its mass, then we see from the table that this must be a little less than / th of that of the earth; exactly · . it is distributed over a smaller surface, · . consequently the amount of air above each square inch of martian surface is · ÷ · = · . but since the force of gravity at the surface of mars is less than on the earth, this column of air will only weigh · × · = · ; or one-seventh of the column of air resting on a square inch of the earth's surface. the pressure at the surface of mars will therefore be · lb.; and the aneroid barometer would read · inches. (in order to express the diminished pressure of the martian atmosphere, it is necessary to refer it to the aneroid barometer. the mercury in a mercurial barometer, or the water in a water barometer would lose in weight in consequence of the diminished force of gravity in the same proportion as the air would, and the mercurial barometer would read · inches.) but a pressure of · lb. on the square inch is far less than that experienced by coxwell and glaisher in their great ascent; it is about one-half the pressure that is experienced on the top of the very highest terrestrial mountains. but the habitable regions of the earth do not extend even so far upward as to the level of a pressure of · lb. on the square inch; that is, of half the terrestrial surface pressure. plant life dies out before we reach that point, and though birds or men may occasionally attain greater heights, they cannot domicile there, and are, indeed, only able thus to ascend in virtue of nourishment which they have procured in more favoured regions. if we could suppose the conditions of the whole earth changed to correspond with those prevailing at the summit of mt. everest, or even at the summit of mont blanc, it is clear that the life now present on this planet would be extinguished, and that speedily. much more would this be the case if the atmosphere were diminished to one half the pressure on the summit of the highest earthly mountain. the tenuity of the atmosphere on mars has another consequence. here water freezes at ° c. and boils at ° c.; so that for one hundred degrees it remains in a liquid condition. on mars, under the assumed conditions, water would boil at ° c., and the range of temperature within which it would be liquid would be much curtailed. but it is only water in the liquid state that is useful for sustaining life. the above estimate of the density of the atmosphere of mars is an outside limit, for it assumes that mars has retained an atmosphere to the full proportion of its mass. but as the molecules of a gas are in continual motion, and in every direction, the lighter, most swiftly moving molecules must occasionally be moving directly outwards from the planet at the top of their speed, and in this case, if the speed of recession should exceed that which the gravity of the planet can control, the particle is lost to the planet for ever. a small planet therefore is subject to a continual drain upon its atmosphere, a drain of the lightest constituents. hence it is, no doubt, that free hydrogen is not a constituent of the atmosphere of the earth. to what extent, then, has the atmosphere of mars fallen below its full proportion? mr. lowell has adopted an ingenious method of obtaining some light on this question, by comparing the relative albedoes of the earth and mars; that is to say the relative power of reflection possessed by the two planets. of course the method is rough; we have first of all no satisfactory means of determining the albedo of the earth itself, and mr. lowell puts it higher than most astronomers would do; then there is the difficulty of determining what portion of the total albedo is to be referred to the atmosphere and what to the actual soil or surface of the planet. but, on the whole, mr. lowell concludes that the amount of atmosphere above the unit of surface of mars is · of that above the unit of surface of the earth. this would bring down the pressure on each square inch of mars to · lb., and the aneroid barometer would read · inches; and water would boil at ° c. the range of temperature from day to night, from summer to winter, at any place on the planet would be increased, while the range within which water could retain its liquid form would be diminished. these statistics may seem rather dull and tiresome, but if we are to deal with the problem before us at all, it is important to understand that one factor in the condition of a planet cannot be altered and all the other factors retained unchanged. it will be seen that in computing the density of the atmosphere of mars, we had to take into consideration not only the diameter of the planet, but the surface, which varies as the square of the diameter; the volume, which varies as the cube; the mass, which varies in a higher power still; and various combinations of these numbers. novelists who write tales of journeys to other worlds or of the inhabitants of other worlds visiting this one, usually assume that the atmosphere is of the same density on all planets, and the action of gravity unchanged. in their view it is only that men would have a little less ground to walk upon on mars, and a good deal more on jupiter. dean swift, in _gulliver's travels_, made the lilliputians take a truer view of the effect of the alteration of one dimension, for, finding that gulliver was twelve times as tall as the average lilliputian, they did not appoint him the rations of twelve lilliputians, which would have been rather poor feeding for that veracious mariner, but allotted him the cube of twelve, viz. seventeen hundred and twenty-eight rations. mr. j. holt schooling, in one of his ingenious and interesting statistical papers, tried to bring home the vast extent of the british empire by supposing that it seceded, and taking the portion of earth that has fallen to it, set up a world of its own--the planet "victoria." he allots to the british empire per cent of the land surface of the world. if the earth were divided so as to form two globes with surfaces in proportion of to , the smaller globe, which would correspond to mr. schooling's new planet "victoria," would be less than half the present earth in diameter; it would be considerably smaller than mars. but "the rest of the world" would be · of the present earth in diameter, or very nearly the size of venus, and it would contain just eight-ninths of the substance of the earth, leaving only one-ninth for "victoria." the statistics given above will suggest to the reader that, could such a secession be carried out, the inhabitants of the british empire would not be happier for the change during the very short continued existence that remained to them. the "rest of the world" could spare our fraction of the planet much better than we could spare theirs. this is a principle which applies to worlds anywhere; not merely within the limits of the solar system but wherever they exist. everywhere the surface must vary with the square of the diameter; the volume with the cube; everywhere the smaller planet must have the rarer atmosphere, and with a rare atmosphere the extreme range of temperature must be great, while the range of temperature within which water will flow will be restricted. our earth stands as the model of a world of the right size for the maintenance of life; much smaller than our earth would be too small; much larger, as we shall see later, would be too large. so far we have dealt with mars as if it received the same amount of light and heat from the sun that the earth does. but, as the table shows, from its greater distance from the sun, mars receives per unit of surface only about three-sevenths of the light and heat of that received by the earth. the inclination of the axis of mars is almost the same as that of the earth, so that the general character of the seasons is not very different on the two planets, and the torrid, temperate, and frigid zones have almost the same proportions. the length of the day is also nearly the same for both, the martian day being slightly longer; but the most serious factor is the greater distance of mars, and the consequent diminution in the light and heat received from the sun. the light and heat received by the earth are not so excessive that we could be content to see them diminished, even by per cent, but for mars they are diminished by per cent. how can we judge the effect of so important a difference? the mean temperature of our earth is supposed to be about °f., or °c. three-sevenths of this would give us °c. as the mean temperature of mars, which would signify a planet not impossible for life. but the zero of the centigrade scale is not the absolute zero; it only marks the freezing-point of water. the absolute zero is computed to be - ° on the centigrade scale; the temperature of the earth on the absolute scale therefore should be taken as °, and three-sevenths of this would give ° of absolute temperature. but this is ° below freezing-point, and no life could exist on a planet under such conditions. but the mean temperature of mars cannot be computed quite so easily. the hotter a body is the more rapidly it radiates heat; the cooler it is the slower its radiation. according to stefan's law, the radiation varies for a perfect radiator with the th power of the absolute temperature; so that if mars were at ° abs., while the earth were at ° abs., the earth would be radiating its heat nearly times faster than mars. the heat income of mars would therefore be in a much higher proportion than its expenditure; and necessarily its heat capital would increase until income and expenditure balanced. prof. poynting has made the temperature of the planets under the th power law of radiation the subject of an interesting enquiry, and the figures which he has obtained for mars and other planets are included in the table. the equatorial and average temperatures are given under the assumption that mars possesses an atmosphere as efficient as our own in equalizing the temperature of the whole planet. if, on the other hand, its atmosphere has no such regulating power, then under the zenith sun the upper limit of the temperature of a portion of its surface reflecting one-eighth would be, as shown in the table, °c. this would imply that the temperature on the dark side of the planet was very nearly at the absolute zero. "if we regard mars as resembling our moon, and take the moon's effective average temperature as ° abs., the corresponding temperature for mars is ° abs., and the highest temperature is four-fifths of ° = ° abs. but the surface of mars has probably a higher coefficient of absorption than the surface of the moon--it certainly has for light--so that we may put his effective average temperature, on this supposition, some few degrees above ° abs., and his equatorial temperature some degrees higher still. it appears as exceedingly probable, then, that whether we regard mars as like the earth or, going to the other extreme, as like the moon, the temperature of his surface is everywhere below the freezing-point of water."[ ] as the atmospheric circulation on mars must be languid, and the atmosphere itself is very rare, the general condition of the planet will approximate rather to the lunar type than to the terrestrial, and the extremes, both of heat and cold, will approach those which would prevail on a planet without a regulating atmosphere. there is another way of considering the effect on the climate of mars and its great distance from the sun, which, though only rough and crude, may be helpful to some readers. if we take the earth at noonday at the time of the equinox, then a square yard at the equator has the sun in its zenith, and is fully presented to its light and heat. but, as we move away from the equator, we find that each higher latitude is less fully presented to the sun, until, when we reach latitude - / °--in other words just outside the arctic circle-- square yards are presented to the sun so as to receive only as much of the solar radiation as square yards receive at the equator. we may take, then, latitude - / ° as representing mars, while the equator represents the earth. or, we may take it that we should compare the climate of archangel with the climate of singapore. now the mean temperature of latitude - / °, say the latitude of archangel, is just about freezing-point ( °c.), while that of the equator is about °c. we should therefore expect from this a difference between the mean temperatures of the earth and mars of °; that is to say, as the earth stands at °c, mars would be at - °c. but, on the earth, the evaporation and precipitation is great, and the atmospheric circulation vigorous. evaporation is always going on in equatorial regions, and the moisture-laden winds are continually moving polewards, carrying with them vast stores of heat to be liberated as the rain falls. the oceanic currents have the same effect, and how great the modification which they introduce may be seen by comparing the climates of labrador and scotland. there appear to be no great oceans on mars. the difference of ° which we find on the earth between the equator and the edge of the arctic circle is a difference which remains after the convection currents of air and sea have done much to reduce the temperature of the equator and to raise that of high latitudes. if we suppose that their effect has been to reduce this difference to one half of what it would have been were each latitude isolated from the rest, we shall not be far wrong, and we should get a range of ° as the true equivalent difference between the mean temperatures of singapore and archangel; i.e. of the earth and mars; and mars would stand at - °c. the closeness with which this figure agrees with that reached by prof. poynting suggests that it is a fair approximation to the correct figure. the size of mars taught us that we have in it a planet with an atmosphere of but one half the density of that prevailing on the top of our highest mountain; the distance of mars from the sun showed us that it must have a mean temperature close to that of freezing mercury. what chance would there be for life on a world the average condition of which would correspond to that of a terrestrial mountain top, ten miles high and in the heart of the polar regions? but mars in the telescope does not look like a cold planet. as we look at it, and note its bright colour, the small extent of the white caps presumed to be snow, and the high latitudes in which the dark markings--presumed to be water or vegetation--are seen, it seems difficult to suppose that the mean temperature of the planet is lower than that of the earth. thus on the wonderful photographs taken by prof. barnard in , the nilosyrtis with the protonilus is seen as a dark canal. now the protonilus is in north lat. °, and on the date of observation--september , --the winter solstice of the northern hemisphere of mars was just past. there would be nothing unusual for the ground to be covered with snow and the water to be frozen in a corresponding latitude if in a continental situation on the earth. then, again, in the summer, the white polar caps of mars diminish to a far greater extent than the snow and ice caps of the earth; indeed, one of the martian caps has been known to disappear completely. yet, as the accompanying diagram will show, something of this kind is precisely what we ought to expect to see. the diagram has been constructed in the following manner: a curve of mean temperatures has been laid down for every ° of latitude on the earth, derived as far as possible from accepted isothermals in continental countries in the northern hemisphere. from this curve ordinates have been drawn at each °, upward to show average deviation from the mean temperature for the hottest part of the day in summer, downward for the deviation for the coldest part of the night in winter. obviously, on the average, the range from maximum to minimum will increase from the equator to the poles. the mean temperature of the earth has been taken as °c, and as representing that prevailing in about ° lat. the diagram shows that the maximum temperature of no place upon the earth's surface approaches the boiling-point of water, and that it is only within the polar circle that the mean temperature is below freezing-point. water, therefore, on the earth must be normally in the liquid state. in constructing a similar diagram for mars, three modifications have to be made. first of all, the mean temperature of the planet must be considerably lower than that of the earth. next, since the atmospheric circulation is languid and there are no great oceans, the temperatures of different latitudes cannot be equalized to the same extent as on the earth. it follows, therefore, that the range in mean temperature from equator to pole must be considerably greater on mars than on the earth. thirdly, the range in temperature in any latitude, from the hottest part of the day in summer to the coldest part of the night in winter, must be much greater than with us; partly on account of the very slight density of the atmosphere, and partly on account of the length of the martian year. [illustration: thermographs of the earth and mars] we cannot know the exact figures to adopt, but the general type of the thermograph for mars as compared with that of the earth will remain. the mean temperature of mars will be lower, the range of temperature from equator to pole will be greater, and the extremes of temperature in any given latitude more pronounced than upon the earth. and the general lesson of the diagram may be summed up in a sentence. the maximum temperature on the planet is well above freezing-point, and the part of the planet at maximum temperature is precisely the part that we see the best. but while this is so, it is clear that water on mars must normally be in the state of ice; mars is essentially a frozen planet; and the extremes of cold experienced there, not only every year but every night, far transcend the bitterest extremes of our own polar regions. the above considerations do not appear to render it likely that there is any vegetation on mars. a planet ice-bound every night and with its mean temperature considerably below freezing-point does not seem promising for vegetation. if vegetation exists, it must be of a kind that can pass through all the stages of its life-history during the few bright hours of the martian day. every night will be for it a winter, a winter of undescribable frost, which it could only endure in the form of spores. so if there be vegetation it must be confined to some hardy forms of a low type. at a distance of forty millions of miles it is not easy to discriminate between the darkness of sheets of water and the darkness of stretches of vegetation. some of the so-called "seas" may possibly be really of the latter class, but that there must be expanses of water on the planet is clear, for if there were no water surfaces there would be no evaporation; and if there were no evaporation from whence could come the supply of moisture that builds up the winter pole cap? the great american astronomer, prof. newcomb, gave in _harper's weekly_ for july , , an admirable summary of the verdict of science as to the character of the meteorology of mars. "the most careful calculation shows that if there are any considerable bodies of water on our neighbouring planet they exist in the form of ice, and can never be liquid to a depth of more than one or two inches, and that only within the torrid zone and during a few hours each day.... there is no evidence that snow like ours ever forms around the poles of mars. it does not seem possible that any considerable fall of such snow could ever take place, nor is there any necessity of supposing actual snow or ice to account for the white caps. at a temperature vastly below any ever felt in siberia, the smallest particles of moisture will be condensed into what we call hoar frost, and will glisten with as much whiteness as actual snow.... thus we have a kind of martian meteorological changes, very slight indeed and seemingly very different from those of our earth, but yet following similar lines on their small scale. for snowfall substitute frostfall; instead of feet or inches say fractions of a millimetre, and instead of storms or wind substitute little motions of an air thinner than that on the top of the himalayas, and we shall have a general description of martian meteorology." what we know of mars, then, shows us a planet, icebound every night, but with a day temperature somewhat above freezing-point. as we see it, we look upon its warmest regions, and the rapidity with which it is cleared of ice, snow, and cloud shows the atmosphere to be rare and the moisture little in amount and readily evaporated. the seas are probably shallow depressions, filled with ice to the bottom, but melted as to their surfaces by day. from the variety of tints noted in the seas, and the recurrent changes in their outlines, they are composed of congeries of shallow pools, fed by small sluggish streams; great ocean basins into which great rivers discharge themselves are quite unknown. chapter viii the illusions of mars the two preceding chapters have led to two opposing, two incompatible conclusions. in chapter vi, a summary was given of prof. lowell's claim to have had ocular demonstration of the handiwork of intelligent organisms on mars. in chapter vii, it was shown that the indispensable condition for living organisms, water in the liquid state, is only occasionally present there, the general temperature being much below freezing-point, so that living organisms of high development and more than ephemeral existence are impossible. prof. lowell argues that the appearance of the network of lines and spots formed by the canals and oases, and its regular behaviour, "preclude its causation on such a scale by any natural process," his assumption being that he has obtained finality in his seeing of the planet, and that no improvement in telescopes, no increase in experience, no better eyesight will ever break up the perfect regularity of form and position, which he gives to the canals, into finer and more complex detail. but the history of our knowledge of the planet's surface teaches us a different lesson. two small objects appear repeatedly on the drawings made by beer and mädler in ; these are two similar dark spots, the one isolated, the other at the end of a gently curved line. both spots resemble in form and character the oases of prof. lowell, and the curved line, at the termination of which one of the spots appears, represents closely the appearance presented by several of the canals. in the year no better drawings of mars had appeared; and in representing these two spots as truly circular and the curved line as narrow, sharp, and uniform, beer and mädler undoubtedly portrayed the planet as actually they saw it. the one marking was named by schiaparelli the lacus solis, the other, the sinus sabæus, and they are two of the best known and most easily recognized of the planet's features; so that it is easy to trace the growth of our knowledge of both of them from up to the present time. they were drawn by dawes in , by schiaparelli in and the succeeding years, by lowell in and since, and by antoniadi in and . but whereas the drawings of beer and mädler, made by the aid of a telescope of inches aperture, show the two spots as exactly alike, in those of dawes, made with a telescope of inches, the resemblance between the two has entirely vanished, and neither is shown as a plain circular dot. since then, observers of greater experience and equipped with more powerful instruments have directed their attention to these two objects, and a mass of complicated structure has been brought out in the regions which were so simple in the sight of beer and mädler, so that not a trace of resemblance remains between the two objects that to them appeared indistinguishable. now the gradation in size, from the lacus solis down to the smallest oasis of lowell, is a complete one. if a future development in the power of telescopes should equal the advance made from the -inch of beer and mädler, to the -inch which antoniadi used in , is it reasonable to suppose that prof. lowell's oases will refuse to yield to such improvement, and will all still show themselves as uniform spots, precisely circular in outline? it is clear that beer and mädler would have been mistaken if they had argued that the apparently perfect circularity of the two oases which they observed proved them to be artificial, because the increase in telescopic power has since shown us that neither is circular. the obvious reason why they appeared so round to beer and mädler was that they were too small to be defined in their instruments; their minor irregularities were therefore invisible, and their apparent circularity covered detail of an altogether different form. beer and mädler only drew two such spots; lowell shows about two hundred. beer and mädler's two spots seemed to them exactly alike; these two spots as we see them to-day have no resemblance to each other. prof. lowell's two hundred oases, with few exceptions, seem all of the same character; is it possible to suppose, if telescopes develop in the future as they have done in the past, that the two hundred oases will preserve their uniformity of appearance any more than the lacus solis and the head of the sinus sabæus? if a novice begins to work upon mars with a small telescope, he will draw the lacus solis and the sinus sabæus as two round, uniform spots, and as he gains experience, and his instrumental power is increased, he will begin to detect detail in them, and draw them as dawes and schiaparelli and others have shown them later. it is no question of planetary change; it is a question of experience and of "seeing." there is a much simpler explanation of the regularity of the canals and oases than to suppose that an industrious population of geometers have dug them out or planted them; it is connected with the nature of vision. a telegraph wire seen against a background of a bright cloud can be discerned at an amazing distance--in fact, at , times the breadth of the wire; a distance at which the wire subtends a breadth of a second of arc. for average normal sight the perception of the wire will be quite unmistakable, but at the same time it would be quite untrue to say that the perception of the wire was of the nature of defined vision, as would be seen at once if small objects of irregular shape were threaded on the wire; these would have to be many times the breadth of the wire in order to be detected. again, if instead of a wire of very great length extending right across the field of view of both eyes, a short, black line be drawn on a white ground, it will be found that as the length of the line is diminished below a certain point so its breadth must be increased. if the observer is distant from the line times its length, then the breadth must be increased to be equal to the length, and the object, whatever its actual shape, can be just recognized as a small circular spot, which will subtend about seconds of arc. but though a black spot, seconds in diameter, can be perceived on a white ground, we have not yet attained to defined vision. for if we place two black spots each seconds of arc in diameter, near each other, they will not be seen as separate spots unless there is a clear space between them of six times that amount. nearer than that they will give the impression that they form one circular spot, or an oval one, or even a uniform straight line, according to the amount of separation. if two equal round spots be placed so that the distance between their centres is equal to two diameters, then the diameter of each spot must be, at least, seconds of arc for them to be distinctly defined; that is to say for the spots to be seen as two separate objects. it will be seen that there is a wide range between objects that are large enough to be quite unmistakably perceived, and objects which are large enough to have their true outline really defined. it is a question of seconds of arc in the one case and of minutes of arc in the other. within this range, between the limit at which objects can be just perceived and that where they can be just defined, objects must all appear as of one of two forms--the straight line and the circular dot. this depends upon the structure of the eye and of the retina; the eye being essentially a lens with its defining power necessarily limited by its aperture, and the retina a sensitive screen built up of an immense number of separate elements each of which can only transmit a single sensation. different eyes will have different limits, both for the smallest objects which can be discerned and for the smallest objects that can be defined, but for any sight the range between the two will be of the order just indicated. prof. lowell has drawn attention to the "strangely economic character of both the canals and oases in the matter of form." it is true that straight lines and circles are economic forms, but they are economic not only in the construction of irrigation works but also in vision. "the circle is the figure which encloses the maximum area for the minimum average distance from its centre to any point situated within it;" therefore, if a small spot be perceived by the sight but be too small to have its actual outline defined, it will be recognized by the eye as being truly circular, on the principle of economy of effort. so, again, a straight line is the shortest that can be drawn between two points; and a straight line can be perceived as such when of an angular breadth quite times less than that of the smallest spot. a straight line is that which gives the least total excitement in order to produce an appreciable impression, and therefore the smallest appreciable impression produces the effect of a straight line. it is sufficient, then, for us to suppose that the surface of mars is dotted over with minute irregular markings, with a tendency to aggregate in certain directions, such as would naturally arise in the process of the cooling of a planet when the outer crust was contracting above an unyielding nucleus. if these markings are fairly near each other it is not necessary, in order to produce the effect of "canals," that they should be individually large enough to be seen. they may be of any conceivable shape, provided that they are separately below the limit of defined vision, and are sufficiently sparsely scattered. in this case the eye inevitably sums up the details (which it recognizes but cannot resolve) into lines essentially "canal-like" in character. wherever there is a small aggregation of these minute markings, an impression will be given of a circular spot, or, to use prof. lowell's nomenclature, an "oasis." if the aggregation be greater still and more extended, we shall have a shaded area--a "sea." the above remarks apply to observation with the unaided eye, but the same principle applies yet more strongly to telescopic vision. no star is near enough or sufficiently large to give the least impression of a true disc; its diameter is indistinguishable; it is for us a mathematical point, "without parts or magnitude." but the image of a star formed by a telescope is not a point but a minute disc, surrounded by a series of diffraction rings. this disc is "spurious," for the greater the aperture of the telescope the smaller the apparent disc. that which holds good for a bright point like a star holds good for every individual point of a planetary surface when viewed through the telescope; that is to say, each point is represented by a minute disc; all lines and outlines therefore are slightly blurred, so that minute irregularities are inevitably smoothed out. when we come to photographs, the process is carried to a third stage. the image is formed by the telescope, subject to all the limitations of telescopic images, and is received on a plate essentially granular in structure, and is finally examined by the eye. the granular structure of the plate acts as the third factor in concealing irregularities and simplifying details; a third factor in producing the two simplest types of form--the straight line and the circular dot. prof. lowell describes the canals as like lines drawn with pen, ink and ruler, but not a few of our best observers have advanced much beyond this stage. even as far back as , some of the canals were losing their strict rectilinear appearance to schiaparelli, and the observers of the planet who have been best favoured by the power of the telescope at their disposal, by the atmospheric conditions under which they worked, and by their own skill and experience--such as antoniadi, barnard, cerulli, denning, millochau, molesworth, phillips, stanley williams and others--have found them to show evident signs of resolution. thus, in , antoniadi found that of canals, were resolved into disconnected knots of diffused shadings, were seen as irregular lines, as more or less dark bands; and he found that, in good seeing, there was no trace whatever of the geometrical network. the progress of observation, therefore, has left prof. lowell behind, and has dispelled the fable which he has defended with so much ingenuity. but, indeed, there never was any more reason for taking seriously his theory as to the presence of artificial waterways on mars than for believing in the actual existence of the weird creatures described by h. g. wells in the _war of the worlds_. there are too many oversights in the canal theory. thus no source is indicated for the moisture supposed to be locked up in the winter pole cap. prof. lowell holds that there are no large bodies of water on the planet; that the so-called seas are really cultivated land. in this case there could be little or no evaporation, and so no means by which the polar deposits could be recruited. yet it is certain that the supply of the winter pole cap must come from the evaporation of water in some region or other. and here is another oversight of the artificial canal theory. the canals are supposed to be necessary for the conveyance of water from the pole towards the equator; although, as this was "uphill," vast pumping stations at short intervals had to be predicated. but it is not supposed that the water needed to travel by way of the canals to the poles. if, however, the moisture is conveyed as vapour through the atmosphere to the pole as winter approaches, it cannot be impossible that it should be conveyed in the same manner from the pole as summer draws on, and in that case the artificial canals would not be needed. if the canals are necessary for conveying the water in one direction, they would be necessary for the opposite direction. but there would be something too farcical in the idea of the careful martians dispatching their water first to the pole to be frozen there, and then, after it had been duly frozen and melted again, fetching it back along thousands of miles and through numerous pumping stations for use in irrigating their fields. of all the many hundreds of canals only a few actually touch the polar caps. but on the theory that the entire canal system is fed by the polar cap in summer, the carrying capacity of the polar canals should be equal to, if not greater than, that of the entire system outside the polar circle. a glance at the charts of the planet shows that the polar canals could not supply a twentieth part of the water needed for those in the equatorial regions. another oversight is that of the significance of the alleged uniformity and breadth of the canals. prof. lowell repeatedly insists that the canals are of even breadth from end to end, and spring into existence at once throughout their whole length. this statement is in itself a proof that the canals cannot be what he supposes them to be. an irrigation system could not have these characteristics; the region fertilized would take time to develop; we should see the canal extending itself gradually across the continent, and its breadth would not be uniform from end to end, but the region fertilized would grow narrower with increase of distance from the fountain head of the canal. under what conditions can we see straight lines, perfectly uniform from end to end, spring into existence, in their entirety, without going through any stages of growth? when the lines are not actual images, but are suggested by markings perceived, but not perfectly defined. in and , in conjunction with mr. evans, the headmaster of greenwich hospital school, i tried a number of experiments on this point, with the aid of about two hundred of the boys of the school. they had several qualifications in respect of these experiments; they were keen-sighted, well drilled; accustomed to do what they were told without asking questions; and they knew nothing whatsoever of astronomy, certainly nothing about mars. a diagram was hung up, based upon some drawing or other of the planet made by schiaparelli, lowell or other martian observer, but the canals were not inserted; only a few dots or irregular markings were put in here and there. and the boys were arranged at different distances from the diagram and told to draw exactly what they saw. those nearest the diagram were able to detect the little irregular markings and represented them under their true forms. those at the back of the room could not see anything of them, and only represented the broadest features of the diagram, the continents and seas. those in the middle of the room were too far off to define the minute markings, but were near enough for those markings to produce some impression upon them; and that impression always was of a network of straight lines, sometimes with dots at the points of meeting. advancing from a distance toward the diagram the process of development became quite clear. at the back of the room no straight lines were seen; as the observer came slowly forward, first one straight line would appear completely, then another, and so on, until all the chief canals drawn by schiaparelli and lowell in the region represented had come into evidence in their proper places. advancing still further, the canals disappeared, and the little irregular markings which had given rise to them were perceived in their true forms. these experiments at the greenwich hospital school were merely the repetition of similar ones that i had myself made privately twelve years earlier, leading me to the conclusion, published in , that the canals of mars were simply the summation of a complexity of detail too minute to be separately discerned. a little later, in his work "_marte nel - _," dr. cerulli independently arrived at the same conclusion, and wrote: "these lines are formed by the eye ... which utilizes ... the dark elements which it finds along certain directions"; and "a large number of these elements forms a broad band"; and "a smaller number of them gives rise to a narrow line." also, "the marvellous appearance of the lines in question has its origin, not in the reality of the thing, but in the inability of the present telescope to show faithfully such a reality." in , prof. newcomb made some experiments in the same direction and reached the same general conclusion. more recently still, prof. w. h. pickering has worked on the same lines and with the same result. the venerable george pollock, formerly the senior master of the supreme court and king's remembrancer, sent to me, in his st year, the following note as affording an apt illustration of the true nature of the canaliform markings on mars: "on saturday last, journeying in a motor-car, i came into a broad road bounded by a dark wood. looking up i was amazed to see distinct, well-defined, vertical, parallel white lines, the wood forming the dark background. on getting nearer, these lines resolved themselves into spots, and they proved to be the white insulators supporting the telegraph wires." prof. lowell has objected that all experiments and illustrations of this kind are irrelevant; only observations upon the planet itself ought to be taken into account. but such observations have been made upon the planet itself with just the same result. observers have seen streaks upon mars--knotted, broken, irregular, full of detail--and when the planet has receded to a greater distance, the very same marking has shown itself as a narrow straight line, uniform from end to end, as if drawn with pen, ink and ruler. the greater distance has caused the irregularities, seen when nearer at hand, to disappear. in this, and not in any gigantic engineering works, is the explanation of the artificiality of the markings on mars as prof. lowell sees them. that artificiality has already disappeared under better seeing with more powerful telescopes. this chapter is entitled "the illusions of mars." yet the illusions of mars are not the straight lines and round dots of the canal system, but the forced and curious interpretation which has been put upon them. if the planet be within a certain range of distance and under examination with a certain telescopic power, the straight lines and round dots are inevitable. their artificiality is not a function of the actual martian details themselves, but of the mode in which, under given conditions, we are obliged to see them. chapter ix venus, mercury and the asteroids of all the planets, venus appears, to the unassisted eye, by far the loveliest. when seen in the early morning before sunrise--its "western elongation"--or after sundown in the evening--its "eastern elongation"--and still more as it attains its greatest brilliancy, it has attracted attention everywhere and in all ages. it then shines with brilliance ten times as great as jupiter in opposition, and the brightest members of the heavenly host look pale and dim beside it. it is emphatically the morning or the evening star, lucifer, or vesper, herald or follower of the sun; it can even assert itself in the presence of the lord of day, for it has often been seen at noonday by watchers who knew where to look; sometimes by the general crowd. but in the telescope venus appears less satisfying. it is a pretty spectacle indeed to watch the phases of the gleaming little globe of silver, for, like the moon under varying illumination from the sun, it undergoes change of apparent shape. but the surface of the planet yields little detail, and that little is illusive and ill-defined. the clear-cut outlines and black shadows of the moon have no place here, nor do the ruddy plains and blue-grey "seas" of mars find any analogues. all that can be observed beyond the changes of phase are a few faint, ill-defined patches, where the molten silver of the general surface is slightly dimmed and tarnished, and perhaps one or two spots, not less evasive and difficult to fix, that exceed the rest of the surface in brightness. this very difficulty in making out the markings on venus is hopeful for our search; it points to a veiling over the planet, a veiling by an atmosphere. and the statistics of the table show that venus closely resembles our earth in size and mass, and therefore probably in atmospheric equipment. if we assume that the atmosphere of any planet is in direct proportion to its mass--and as venus is so nearly the twin of the earth there is no reason to expect any great difference between the two in this respect--the atmosphere of venus would have a pressure of about · lb. on the square inch, and the level of half pressure would be nearly four miles above the surface. in other words the atmosphere would be both thinner and deeper than that of the earth, but the difference would not be important in amount. but venus is nearer to the sun than the earth, and receives nearly double the light and heat. its theoretical equatorial temperature is °abs., or °c, and its corresponding mean temperature is ° c. but water under a pressure of · lb. will boil at ° c, so that at the equator of venus the upper limit for water as a liquid is just passed, but, for the planet in general, a fairly safe margin is maintained. here then is sufficient explanation why the topography of venus is concealed. the atmosphere will always be abundantly charged with water-vapour, and an almost unbroken screen of clouds be spread throughout its upper regions. such a screen will greatly protect the planet from the full scorching of the sun, and tend to equalize the temperature of day and night, of summer and winter, of equator and poles. the temperature range will be slight, and there will be no wide expanses of polar ice. water that flows will be abundant everywhere. so far all the facts connected with venus are favourable for life, even though the picture called up to the mind may not seem inviting to us. for views of the heavens must be rare; the sun must seldom pierce through the cloud veil; there is no moon and the stars must be almost always hidden. the earth with its moon might form a beautiful ornament at times in the midnight sky if the cloud-shell should occasionally open, but on the whole, the planet is shut up to itself in a perpetual vapour-bath, and its condition will approach that of some of the most humid countries in the terrestrial tropics during the height of their rainy seasons. but it would seem that life both of plants and animals, under such conditions, might flourish and be abundant. the mean temperature would not, in general, be high enough to drive off the water as steam, nor low enough to congeal it into ice; it would remain water--water that flows. but there is still a possible hindrance to life on venus, a hindrance that actually exists in the case of mercury. mercury, the "twinkler," is not an easy object in our northern latitudes, but, in countries near the tropics, is often quite conspicuous, a little scintillating gem of light in the bright sky, before sunrise or after sunset. in the telescope it is not so attractive as venus, partly because it is smaller, partly because, though it receives more than three times as much light from the sun, it is duller in hue. yet it is not quite so secretive as its neighbour, and a certain number of markings have been detected upon its disc, markings which, like those of the moon, appear to be permanent. a glance at the table will show that this was to be expected. in size, mercury comes between the moon and mars, and the atmospheric veil ought therefore to be, as it evidently is, very slight and transparent; offering little or no hindrance to an observer scanning it from another world. the other necessary consequences of small size and mass will follow; the feeble force of gravitation, the languid atmospheric circulation, the extreme range of temperatures, the low temperature at which water will boil. but the heat to which mercury is exposed far transcends our terrestrial experience. in the mean it receives nearly seven times as much heat from the sun as the earth does, but this supply is not maintained uniformly, for mercury moves round the sun in a very eccentric orbit, so that when in aphelion it receives, surface for surface, only about four times as much heat as the earth, but some six weeks later when in perihelion it receives more than eleven times. the great range of temperature due to the thinness of the atmosphere must therefore be further increased by the varying distance of the planet from the sun. a reference to prof. poynting's figures shows that the mean temperature of mercury must approximate to ° c., while water will boil at ° c. or even lower. here, then, is a condition the exact reverse of mars. water as a liquid will be rare on mercury, not because it is congealed, but because it is evaporated; on the dark side of the planet it may, indeed, pass into ice, but on the side exposed to the sun it must exist normally as a constituent of the atmosphere. water in a liquid state, water that flows, must be almost unknown. but we have good reason to believe that that which is the dark side of mercury at one time is always dark; that which is exposed to the sun is always exposed to it. since mercury wears no concealing veil of atmosphere, and displays markings that can be identified and followed, a surprising circumstance has come to light. in , schiaparelli discovered that mercury, instead of rotating on its axis in about hours like the earth and mars, rotates in days; that is to say, it always turns the same face towards the sun, just as the moon turns the same face towards the earth. this fact, confirmed theoretically by prof. g. h. darwin in his development of the theory of tidal friction, puts the condition of mercury in quite a new light. no alternation of day or night refreshes and restores the little world; one hemisphere is for ever exposed to the blasting heat of the sun, seven times hotter for it than for the earth; the other hemisphere is for ever exposed to the darkness and cold of outer space, a range from something like ° c. above freezing-point, to ° c. below. it is true that between the two hemispheres there is a "debatable land," for, owing to the ellipticity of the orbit, the face turned to the sun is not exactly the same at all times, and a region about ° in width on each side of the planet, that is to say, rather more than a quarter of its entire surface, has one day and one night in each period of days, but these more favoured sections can scarcely be considered more habitable than the rest. the conditions of mercury are so unfavourable for life that, even if this remarkable relation of rotation period to revolution did not hold good, it would still be impossible to regard it as a world for habitation. but its case shows that a further condition of habitability has to be satisfied by a planet. size and distance from the sun afford the first two conditions; a suitable rotation period is now seen to be a third. and it is possible that in this very particular venus fails to qualify. schiaparelli, the first observer of his time, assisted by the clear italian sky, believed that he had demonstrated that venus, like mercury, rotates once in her year; her day being thus equal in length to of ours, and the face that she turns to the sun being always the same. and in her case this statement requires practically no qualification, for, her orbit being nearly circular, there is hardly any libration; a place that has the sun in its zenith has it so for ever; one on the night side of venus can never have a sunrise, or gladden in the daylight. the side exposed to the sun will wither in a temperature of about ° c., in which all moisture will be evaporated; the side remote from it will be bound in eternal ice. in neither hemisphere will water exist in the liquid state; in neither hemisphere will life be possible. but as yet the evidence is not conclusive that venus has this long rotation period. several observers of high rank believe that our neighbour rotates in nearly the same time as the earth, but its markings are so faint and elusive that the problem is a difficult one. the spectroscopic method of determining the speed of rotation has been equally indecisive. until, therefore, the rotation period has been decided, the habitability of venus must remain in question. if it always turns the same face to the sun, there can be no more life upon it than upon mercury; if on the contrary it rotates in much the same time as the earth, then, so far as we know, it may well be a habitable world. whether it is actually inhabited is a matter at present entirely beyond our knowledge. a page or two back we touched lightly on the eccentricity of the orbit of mercury--lightly, because it was not the chief factor in disabling the planet for habitation. but the condition introduced by this eccentricity is one which of itself would be sufficient to put it out of court. in the six weeks in which mercury moves from aphelion to perihelion, it approaches the sun by fourteen millions of miles, and the heat received by it is increased - / times. then, in the next six weeks, it recedes as far, and there is a like diminution. in other words, six weeks makes a greater proportional change in this one planet's condition than we should experience if our earth were transported from its own orbit to that of mars. but there are other members of the solar system whose orbits are so elongated that that of mercury seems in comparison almost circular. these are the comets, some of which all but graze the surface of the sun at perihelion, and then recede from him for periods that it takes even thousands of years to complete. but without dwelling on such extreme cases, two of the best known of the periodic comets may be taken as examples of the rest. encke's is the comet of shortest period, returning in about · years. at perihelion it is millions of miles from the sun; one-third the distance of the earth. it receives, therefore, at this part of its orbit, times as much light and heat as the earth. but at aphelion it retreats deep into the region of the asteroids, and is much more than four times the mean distance of the earth. at this part of its orbit it receives but / th as much heat as the earth. by far the most famous of all the comets is that known by the name of halley, and its mean period is years. at perihelion it comes within the orbit of venus; indeed, nearly halfway between that and the orbit of mercury. at aphelion it recedes to thirty-five times the distance of the earth, far beyond the orbit of neptune. the range in its light and heat from the sun is from times that of the earth to less than / th; or, in other words, the supply of heat at one time is nearly times that at another, and of the years of its period, only days are spent within the orbit of the earth. comets cannot be homes of life; they are not sufficiently condensed; indeed, they are probably but loose congeries of small stones. but even if comets were of planetary size it is clear that life could not be supported on them; water could not remain in the liquid state on a world that rushed from one such extreme of temperature to another. between the orbits of mars and jupiter there are scattered an untold number of little planets commonly known as asteroids or minor planets. minor planets indeed they are, for the one first discovered--ceres-- probably outweighs all the rest, known and unknown, put together, though something like have already been detected, and the list grows at the rate of about one a week. as the table shows, ceres is so small that the earth exceeds it in volume times; even the moon is times as large. the mass of ceres is not known; being so small, its density is probably less than that of the moon, so that the earth may easily outweigh it , times. the unfavourable conditions resulting from smallness of size that the moon presents are therefore exaggerated exceedingly in the case of ceres; its atmosphere must approach in tenuity what we should regard as a vacuum in a terrestrial laboratory, and water as a liquid be entirely unknown. its distance from the sun is another hostile factor; for in consequence it receives per unit of surface only per cent of the light and heat that falls on the earth; its maximum temperature under a zenith sun will fall far below freezing-point, the minimum on the dark side will approach the absolute zero. with ceres the whole of the asteroidal family can be dismissed as possible abodes of life. no astronomer can regard them as such. yet they have their lesson to teach. life can exist on the earth only on the upper face of its crust, and in a very thin film of air and water; but the enormous solid bulk within, inert though it be, that supports the stage on which the great drama of life is played, is as really essential as air and water themselves. if that bulk were much smaller and less massive life could find no place upon its surface. chapter x the major planets it is a striking change to pass from ceres, the giant of the minor planets, to jupiter, the giant of the major planets. instead of a world that the earth exceeds in volume times, we are confronted by one that exceeds the earth times. ceres, when viewed through a large telescope, is just able to present a perceptible disc; jupiter offers the largest shown by any heavenly body after the sun and moon. and that disc is one that never fails to charm the attentive student, for it abounds in colour, movement and change. the late prof. james keeler, an observer of the first rank, having the advantage of observing the planet from the summit of mt. hamilton and with the great -inch telescope of the lick observatory, thus describes the aspect of the planet in . "seen with this instrument on a fine night, the disc of jupiter was a most beautiful object, covered with a wealth of detail which could not possibly be accurately represented in a drawing.... scarcely any portion of jupiter, except the red spot and the extreme polar regions, was of a uniform tint, the surface being mottled with flocculent and more or less irregular cloud masses.... the equatorial zone, occupying the space between the red belts, was marked in the centre by a salmon-coloured stripe, which was occasionally interrupted by an extension of the white clouds on the sides of the zone. the edges were brilliant white, and were formed of rounded cloud-like masses, which at certain places extended into the red belts as long streamers.... near their junction with the equatorial zone, the streamers were white and definite in outline, but they became redder in tint toward their outer extremities, and more diffuse, until they were lost in the general red colour of the background. when the seeing was good they were seen to be formed of irregular rounded or feathery clouds, fading toward the outer ends, until the structure could no longer be distinguished.... the portions of the equatorial zone surrounding the roots of well-marked streamers were somewhat brighter than at other places, and it is a curious circumstance that they were almost invariably suffused with a pale olive-green colour, which seemed to be associated with great disturbance, and which was rarely seen elsewhere.... the red belts presented on all occasions the appearance of a passive medium, in which the phenomena of the streamers and other forms ... were manifested. the phenomena would be exactly reproduced by streamers of cloudy white matter floating in a semi-transparent reddish fluid, sometimes submerged and sometimes rising to the surface.... the dark spots frequently seen on the red belts usually occupied spaces left by sharp turns in the streamers, and they were of the same colour as the belts, but deeper in tint, as if the fluid medium could be seen to a greater depth."[ ] in other words, jupiter is a striped or banded planet, the bands lying along the direction of turning. these bands are coloured in varying tints, and the planet rotates very rapidly, for the details in the bands pass quickly from one limb to the other. and not only is the speed of rotation of the whole very rapid--jupiter turns about its axis in a little less than ten hours, so that a particle at its equator moves through miles in each minute--but the various items that form the bands rotate in different times. they may also alter their form and their colour. jupiter seems, then, to be a planet with a great and rapidly changing atmosphere that extends above a shoreless sea formed of some liquified substance or substances--the whole in a state of flux. but if we turn back to the table, we see that jupiter at its mean distance from the sun is · times that of the earth; that is to say, it receives only / th of the light and heat that we receive. but in chapter viii, we learnt from mars that as this receives only / ths of the earth's light and heat, its mean temperature would sink to - °c.; the earth's being °c. mars is therefore almost always a frozen planet; frozen except on its mere surface when this is exposed to the full rays of the sun. no sea there would ever be melted to a depth of more than a few inches, even at noonday in midsummer. and yet mars has at least ten times the advantages of jupiter. jupiter, then, must be a frozen planet through and through; no liquid of any sort can exist on its surface; no vapour of any substance can exist in its atmosphere. it must be icebound even at its summer noonday. yet, from the description given by prof. keeler, it is manifestly not so; and another item in the table emphasizes that it cannot be so. the density of the sun is · that of water, jupiter's is · , showing that but a very small proportion (if any) of its bulk can be solid; the rest must be vaporous, or at least fluid. how then can we reconcile these inconsistencies? it is in the dimensions of jupiter that we find the answer. the mass of the planet is times that of the earth; it is indeed nearly three times as great as that of all the other planets put together. but the aggregation of so vast an amount of material is of itself a source of heat; the chief source at the present time of the enormous output of heat from the sun is ascribed to its gradual contraction; the slow falling of its substance, if we may so express it, a little nearer to its centre. the great mass of jupiter points to its inherent store of heat being much greater than that of any other planet. and of two bodies equally hot, the larger must cool more slowly than the smaller. if, therefore, all the members of the solar system had at one and the same moment possessed the same surface temperature, that equality would have ceased directly they began to radiate their heat into space; the temperature of the smaller bodies falling more rapidly than those of the larger. this is another example of the principle that has already been noted, that the properties of a small world are not those of a large world divided by a constant factor. it is not possible to conceive a model of the solar system in which all the significant factors should be true to the same scale. if the diameters and distances were all made on a one-tenth scale, the surfaces would be one-hundredth of reality, the volumes one-thousandth. but a radiating body radiates from its surface, while the store of heat from which that radiation is kept up is supplied by its volume. it follows, therefore, that a large and heavy world must differ from a small light world, not merely in scale, but also in kind. the surface of a world is all that we see of it; it is, therefore, very commonly all that we consider. but unseen, and hence often unconsidered, beneath the surface lies its substance or mass, and it is this that determines the state and condition of the surface; it is the underlying power. two men may be contending in a financial struggle; to the eye they may look alike, equally prosperous; both may have the same amount of money actually in their pockets; but the one has nothing else, the other has a great banking account and vast investments, and is, in fact, a millionaire; and it is his unseen power and resources that will make themselves felt. jupiter therefore introduces us to a new factor in world-condition; not all its heat is derived from the sun; much is inherent to it. and though it is not possible at present to say that the mass of jupiter being so much its inherent heat must be this or that quantity as a function of that mass, yet in general, and neglecting other considerations, we can say that of two worlds the one with the greater mass will be that with the higher inherent temperature. this factor of inherent temperature was one that did not require to be noticed in dealing with the moon, or venus, or mars, for these and all the planets yet noticed are less in size, surface, volume, and mass than the earth, and hence possess less inherent heat. it is only now that the greater planets are being considered that the question of a source of heat, other than the sun, can arise. but the evidence of such heat on jupiter is not to be disputed. the albedo or reflective index of jupiter has been put by the late prof. g. bond, of harvard college observatory, as higher than unity; in other words, that it emits more light than it receives. this is now generally regarded as an excessive estimate, but the albedo of the disc as a whole cannot be put lower than · , or about that of white paper. but many of the "belts" or dark regions are of a dull copper tint, and the polar caps are dusky, so that bond's estimate must be realized for the most brilliant "zones," as the brighter regions are called; certainly for the whitest of the white spots. no direct evidence of inherent luminosity has been obtained, for the satellites disappear entirely in eclipse. but though their shadows in transit appear very dark, it is clear that they are not absolutely black, since sometimes such a shadow is not distinguishable in darkness from the satellite that casts it; a delicate proof that the background on which it falls has some intrinsic luminosity. unless there is the counteracting effect of a high temperature, the atmosphere of jupiter would have a pressure at the surface of lb. to the square inch, and the level of half pressure be attained at a mile and a quarter; the reverse condition to that on mars would obtain, and the atmosphere of jupiter would be much denser and much shallower than that of the earth. denser it probably is; shallower it cannot be, for the great white spots, each often five or six thousand miles in diameter, that range themselves at times along the equatorial regions till they look like the portholes of a ship, evidently rise from depths great even as compared with their size. but it is only by intense heat that the effect of the great mass of jupiter in constricting its atmosphere within shallow depths can be overcome. again, the extraordinary lightness of the planet, so little above the density of water, points in the same direction. so, not less unmistakably, do the magnitude and rapidity of the atmospheric movements. the clouds and storms of our own atmosphere are worked by solar heat; solar heat it is that draws up the vapours and provides the chief part of the energy manifested in the speed and strength of the air-current. but solar heat can only give / th the amount of that energy at the distance of jupiter, so that, if they were entirely dependent on solar radiation, the winds of jupiter should be very feeble. further, the difference of presentment due to the difference of latitude is a fruitful cause of inequalities of temperature and pressure in the terrestrial atmosphere. but as a degree of latitude on jupiter is eleven times as wide as on the earth, such inequalities connected with a given difference in latitude are spread over eleven times the distance that they would be on the earth, and are, therefore, so much the less pronounced. yet, across a gulf of millions of miles we can clearly discern the bright zones of jupiter now narrowing down and constricting the red belts, now thrust apart by them, and can detect changes taking place in an hour of time over areas equal to that of a terrestrial hemisphere. a notable peculiarity of jupiter is found in the proper motions of its spots. many of the white spots are exceedingly swift, giving a rotation period of h. m. while the equatorial belt in general gives a period m. longer; so that in rotations (nearly days) a white spot will have passed entirely round the belt, gaining upon it at a rate of nearly miles an hour. the most famous of all the markings in jupiter is the great red spot, which became conspicuous in , since when the spot itself, or at least the nest in which it lay, has always been visible. it has been identified with a great red spot observed by hooke and cassini in - , that appeared and vanished again eight times between and . it therefore has had a history practically as long as our telescopic knowledge of the planet, and may be looked upon as in some sort a permanent feature. yet that it is not in the nature of a portion of a solid crust is clear. it occupies on jupiter much the position and relative area of australia on the earth, but whereas australia of necessity rotates in one piece with all the other continents, the great red spot has a rotation period which is neither that of the equatorial belt, nor of the quickly moving white spots, and is not itself stable. an "australia on the loose" is impossible, even unthinkable here, but the great red spot, for all its long duration, is mobile and inconstant, and is therefore no portion of a solid permanent crust. the giant planet jupiter, therefore, offers us an example of what we may call a "semi-sun"; a world still bubbling with tremendous energies of its own, still pulsing with its own inherent heat, still without a solid crust; probably without a solid nucleus, liquid or vaporous throughout. whatever the future may hold for such an orb, it is clearly no world for habitation at present. full of colour, and movement, and change as it is, it lacks the earth's "gloom of iron substance," which is necessary, no less than its veiling by the plant, as a stage for "the passion and perishing of mankind." but if jupiter be a semi-sun, still a source of heat, perhaps even of light, can it yield the means of life to its satellites? for jupiter is sun-like, not merely in its own condition, but also in that it is the centre and ruler of a system of its own. we know already of eight satellites revolving round it. of these eight, only four--the four discovered by galileo, in the first days of his possession of a telescope--need be considered; the other four are of the same order of size as the asteroids, and are indeed much smaller than ceres. but the galilean satellites are of a higher rank. europa, the smallest, is in size a twin to the moon; callisto, the outermost, is almost exactly the size of mercury; io, the innermost, is midway between the two in its dimensions. but ganymede, the largest, is almost comparable with mars, its diameter being · that of the earth instead of the · of mars. but the moon, mercury, and mars have all been shown, on the ground of their small size, to be worlds unfit for habitation; the satellites of jupiter are, therefore, all rejected on the same score. nor can the greater nearness of their immediate primary compensate for their remoteness from the sun. it is true that jupiter presents to ganymede a disc with more than times the apparent area that the sun presents to the earth, but to make up for the falling-off of the solar radiation, each unit of this area should radiate about / th as much heat as each unit of the sun's surface. in other words, the absolute surface temperature of jupiter should be / th that of the sun, or about ° c., and this is higher than can be admitted. the sun and jupiter together cannot put ganymede in as favourable a position as mars, much less as favourable as the earth. the case of jupiter carries with it those of saturn, uranus, and neptune. all three, from their high albedoes and low densities, are still in a vaporous condition; still in some sort, semi-suns; sources of a certain amount of heat, and not recipients merely. the days are yet far distant when a solid crust can form on any one of them, and the water condense from the steamy atmosphere to form oceans, seas, and rivers. not till then, if at all, when water as a liquid, water that flows, is present, can life begin to appear and enter on its long course of change. chapter xi when the major planets cool the question has been asked: "it is evident that life cannot exist at the present time on the outer planets, since they are in a highly heated and quasi-solar condition; but when they cool down, as cool they must, and a solid crust is formed, may not a time come when they will be habitable? it seems impossible to think that worlds so beautiful to our eyes and so vast in scale are destined never to be peopled by intelligent beings." it is clearly difficult to answer satisfactorily a question that requires so deep a plunge into the recesses of the unknown future; yet, so far as our knowledge goes, there is no reason to think that jupiter will be more habitable then than it is now. the difficulty of the small supply of light and heat received from the sun would apparently still remain, if indeed, the cooling of the sun itself would not increase it. we do not know of any means by which our sun could so increase its radiation as to supply to jupiter from to times as much heat as it now receives, and this would be necessary to place it in the same favoured condition as the earth. if so great a change were to take place in the sun, life would be scorched out of existence on all planets nearer than jupiter, and, similarly, if the solar emission were increased to meet the necessities of uranus or neptune, even jupiter would fall a victim. but we may consider it as a conceivable case that a planet of the exact dimensions of jupiter may be revolving in an annual period of the same length as his, round some star that is capable of affording it adequate nourishment; and so with the three other giant planets. the actual jupiter and saturn of the solar system have, so far as we can tell, neither present nor future as habitable worlds, but we can consider what would be the case of imaginary bodies of similar dimensions in systems where the supply of heat would be sufficient. or we can neglect the question of temperature altogether, as we did at first in the case of mars. all the four planets must shrink much in volume before their solidification will take place. their average density at present but little exceeds that of water; indeed, saturn is not so dense as water; yet we must suppose that the same elements are in general common to the earth and to them all. if we assume, then, that the four planets all cool to the point of solidification, their densities must be much increased, and their volumes correspondingly diminished. since all four greatly exceed the earth in mass, it is but natural to expect that, when they have assumed the terrestrial condition, they will be more closely compacted than the earth, and their densities in consequence will be greater. it will, however, be simpler if we assume exactly the same density for them as for the earth. jupiter will then have shrunk to about one-fourth of its present volume, and the statistics for the four planets will run as in the following table: statistics of the four outer planets if with the same density as the earth proportions of the planets:-- uranus neptune saturn jupiter diameter in miles do [symbol] = · · · · surface, [symbol] = · · · · mass and volume, [symbol] = · · · · gravity at surface, [symbol] = · · · · rate of fall, feet in the first second · · · · atmosphere, assuming the total mass of the atmosphere to be proportional to the mass of the planet:-- pressure at the surface in lb. per square inch · · · · pressure at the surface in "atmospheres" · · · · level of half-pressure in miles · · · · boiling point of water at surface °c °c °c °c jupiter offers two peculiarities. in its shrunken condition, its diameter, instead of being eleven times that of the earth, will be not quite seven, and the force of gravity at the surface will be greater than that of the earth in the same proportion. a man who here weighs lb. will there weigh over lb.; and the muscular effort of movement will be increased in the same ratio. the athlete who here can clear a height ft. in. will there, with like pains, surmount inches; and other efforts will be in the same proportion. the atmosphere, supposing it to be in proportion to the mass of jupiter, will exercise a pressure of - / "atmospheres," or more than lb., to the square inch. following on this enormous pressure at the surface would be the rapidity with which the atmosphere would thin out in the upward direction. the level of half-pressure would be attained by ascending less than half a mile in height; that is to say, there would be a difference of pressure of lb. on the square inch from that experienced at the sea-level. we know from the fact that fishes live at enormous depths in the ocean, that living organisms can be constructed to endure great pressures, but they are not constructed to endure great alterations of pressure. the deep-sea fishes are as instantly killed by being brought up to the surface, as the surface fishes or the land animals would be if they were plunged into the depths. and it is clear that on jupiter a low range of hills that on the earth would be considered only an easy climb, would be an impassable barrier, not only from the immense exertion of mounting it, but chiefly from the unendurable change of pressure which the ascent would involve. the sevenfold gravity of jupiter, taken in connection with this enormous atmospheric pressure, would tend to make the meteorological disturbances of the planet violent far beyond anything of which the earth can furnish an example. the atmosphere would possess a high viscosity, and differences in condition, pressure and saturation would tend to accumulate, until at length the balance would be restored with explosive suddenness and force. here our most violent tornadoes may reach a speed of miles an hour; on jupiter, gales of five or six times that velocity would be common. we cannot conceive that living organisms would be able to grow, flourish and multiply where the conditions were so cataclysmic. this difficulty must always exist where the planet is great in mass, and the force of gravity high at the surface. the case of saturn is not so extreme as that of jupiter, though it is probably sufficiently severe to exclude it from the ranks of worlds that could ever be dwelt in. the atmospheric pressure would be about "atmospheres," or more than lb. on the square inch. the level of half-pressure would be reached at about three-quarters of a mile, and the force of gravity be nearly - / times that of the earth. but the serious condition for saturn would come from that feature which renders it by far the most attractive of all the planets seen in the telescope, the presence of the wonderful ring system. to us, viewing saturn from afar, and from practically the same direction as the sun, the rings are seen lit up; but to a dweller on saturn, the rings during the day are between his world and the sun, and hence turn their dark side toward him. more than that, the telescope shows us that the rings cast a shadow on the planet; in other words, they eclipse part of it; and this shadow changes its position with the progress of the saturnian year. proctor computed that if the rings were a hundred miles in thickness, the equator would suffer, in consequence, total eclipse for nearly ten days at each equinox, and partial eclipse for about forty days more. moving away from the equator, each higher latitude would have a longer and longer period of eclipse in the winter half of its year; the higher the latitude, the later after the autumnal equinox the eclipse would begin, and the longer it would last, until about latitude ° was reached. here the eclipses would begin nearly three terrestrial years after the time of the autumnal equinox. at first the sun would be eclipsed only in the morning and evening of each day, but the length of the daily eclipse would increase, until the sun was hidden the whole day long. this period of total eclipse would last for about years months, terrestrial reckoning, or with the periods of partial eclipse, years and nearly months. whatever the efficiency of the sun that afforded light and heat to such a planet, it is clear that such eclipses must be fatal to life in two ways: light and heat would be cut off from wide regions of the planet for long periods of time, and terrible meteorological convulsions must follow in the train. here on the earth, though a total eclipse generally lasts only two or three minutes, the atmospheric disturbance is perceptible, and the fall of temperature very marked, and it does not require much reflection to see that the analogous disturbance in an atmosphere twenty times as dense must be terrific indeed during an eclipse that lasts not a few minutes only, but for more than six of our years. the case of uranus introduces us to another class of conditions fatal to habitability. the equator of jupiter is inclined only ° to the plane of its orbit; the difference in its seasons is, therefore, almost imperceptible; there is hardly any alteration in the incidence of the solar rays; it is, as if on the earth, the height of the sun at noon in mid-winter were what it actually is on the th of march, and its height at midsummer the same as we observe on march . the inclination of the equator of saturn is considerably greater than that of mars or the earth, so that its seasons are more pronounced, but not to an extent that would introduce any radical difference. but for uranus, the inclination of the equator to the plane of the orbit is °. if this were the case for the earth, the noonday sun for london would be, at the spring equinox, - / ° high as at present, but its altitude day by day would increase with great rapidity, and before the end of april, the sun at noon would be right in the zenith, and ° above the horizon at midnight. at midsummer, indeed, it would be only ° high at noonday, but it would be north of the zenith instead of south, and at technical midnight, it would still be ° in altitude, thus moving round in a very small circle, only ° in diameter. from about april to august --that is to say, for days--the sun would never set, and unlike the summer day of our own polar regions now, wherein the sun, though always present, is always low down in the sky, for much of that period it would pass the meridian quite close to the zenith. as the year of uranus is times the length of our year, the london of uranus would have to endure not far short of years continuous scorching. and the winter would be as long; the perpetual day of summer would be replaced by a night as enduring. more than years of unbroken darkness, of unmitigated cold, cannot possibly ever consist with the conditions necessary for life upon a planet. whatever the brightness of the imagined sun of uranus, if for years at a time that sun were below the horizon, the water on the planet must be congealed, and during the years of unbroken day all the water would be as certainly evaporated. thus, though uranus is not burdened by the enormous mass of jupiter, nor overshadowed, like saturn, by a system of rings, the extraordinary inclination of its axis introduces a condition which is as fatal to it, as a world to dwell in, as any of the disabilities of the other planets. it is curious that these four outer planets, that resemble each other so strikingly in many of their conditions--in their vast size, high albedo, low density, and vaporous envelopes, that show, in their spectra, not merely the lines of reflected sunlight, but also special lines due to their own atmospheres (the chief of these being common to all the four planets)--should yet, in the inclination of their axes to the plane of their orbits, display every possible variety. the axis of jupiter is almost normal to its orbit, that of uranus lies almost in the plane of its orbit. the axes of saturn and neptune have a mean inclination, but it would appear that the rotation of neptune is in the reverse direction to that of planets in general, so that the true inclination is usually taken as being the complement of the observed angle, as if the axis were turned right over. it is uncertain whether this would have any important effect upon the habitability of the planet, but it supplies the fourth possible case for the position of the axis. chapter xii the final question in passing in review the various members of the solar system, it has been seen that there are many conditions that have to be fulfilled before a planet can be regarded as the possible abode of life, because there are many conditions necessary in order that water may exist on its surface in the liquid state. the size and mass of the planet are restricted within quite narrow limits; and a world much larger or much smaller than our own is necessarily excluded. the supply of light and heat received from the sun must not fall much below that received by the earth, nor greatly exceed it; in other words, the distance of the planet from its sun is somewhat precisely fixed, since the light and heat vary inversely not as the distance, but as its square. of course, in different systems, with suns of different power, the most favourable distance will not be the same in each; but in any system there will be one most advantageous distance, and no great departure from it will be possible. this condition further implies that the planetary orbits must be nearly circular; pronounced eccentricity, such as the orbits of even our short-period comets display, would be fatal to the persistence of water in the liquid state, and hence to the continuance of life. a wide discordance between the planes of the planet's equator and of its orbit, by rendering the seasons extravagantly diverse, would act as prejudicially as an eccentric orbit, and a rotation period equal to that of revolution would mean that one hemisphere was eternally frozen while the other was exposed to perpetual heat. it follows that in any given system there can be at most only one or two planets upon which life can find a home, and this only where the right conditions of size and mass, of rotation period, inclination of axis, and shape of orbit, all co-exist in a globe at the proper distance. but the type of system offered by our sun and his planets is not the only one that exists. a very large proportion of stars are binaries--two suns revolve round their common centre of gravity. in many cases the two suns are separable in the telescope, and their relative movements can be measured; in other cases, termed "spectroscopic binaries," we only learn that a star which appears absolutely single has two components from the evidence of its spectrum; the spectroscope revealing two sets of lines that vibrate to and fro with respect to each other. yet, again, a third class of double stars has made itself known in the "algol variables." the optical double stars are cases where the two components are far distant from each other, and hence can be distinguished in our telescopes as separate points of light. the "spectroscopic binaries" are cases where the two components are too close to be separately perceived, but where the two are not greatly unequal in brightness, so that the spectrum of the one does not overpower that of the other. the "algol variables" are cases where the two components are of very unequal brightness, and, being very close to each other, are so placed with respect to the earth that the fainter partly eclipses the brighter in its revolution round it, and so causes a temporary diminution in its light at regular intervals. all these three classes of binary systems are now known to be very numerous. prof. campbell estimates that fully one star in six is a spectroscopic binary. but there must be many binary systems that do not reveal themselves--double stars where the companion is too faint or too close to be detected, algol systems where the companion does not pass before its primary--and it seems almost certain that simple systems, like that of which our sun is the unchallenged autocrat, must be comparatively rare. but the problem of the movements of a planet attendant upon two or more suns is one of amazing complexity, and our greatest mathematicians have as yet only been able to deal with the approximate solution of a few very special cases. these are, however, sufficient to show that the orbit of a planet so placed would be most irregular; the variations in the supplies of light and heat received would be as great as even comets experience within the solar system, and, what would be more disastrous still, these variations would not be periodic but irregular. one year would be unlike that which preceded it, and would be followed by changed conditions in the next. plants and animals would never have the chance of acclimatizing themselves to these ever-changing vicissitudes. the stability of condition essential for the maintenance of water in a liquid state would be wanting; and, in consequence, life could neither come into existence, nor persist if it once appeared. so far, therefore, our line of thought has led us to recognize that life can exist in comparatively few of the innumerable stellar systems strewn through infinite space, and in any given system it can at best find only one or two homes. the conditions for a life-bearing planet are thus both numerous and stringent--there is no elasticity about them. it is not sufficient that a planet might fulfil many or even most of these conditions; failure in one is failure altogether; "one black ball excludes;" the candidate who fails in a single subject is "ploughed" without mercy. and in most cases the failure is final; no opportunity is given to the candidate to "sit" again. but space is not the only horizon along which our thought must be directed; there is also the horizon of time. every world must have its past and its future, as well as its present. for some worlds the conditions are so fixed that, like jupiter and saturn, they are not now worlds that can be dwelt in, they never were in that condition, and they never can be; their enormous mass forbids it. mercury and the moon at the other end of the planetary scale are also permanently disabled; their insignificant size excludes them. there was also a time when the earth was not a world of habitation; it was "without form and void"; hot and vaporous, even as the four outer planets are now. now it is inhabited, but there may come a time when this phase of its history has run its course, and either from a falling off in the tribute of light and heat rendered to it by the sun, or from the gradual desiccation of the surface, or, perchance, from the slow loss of its atmosphere, it may approach the condition of mars, and in its turn be no longer an abode of life. many planets are essentially debarred from ever entering on the vital stage; but of those to which such a stage is possible, it can only form an incident in the entire duration of the orb. and if our earth is any type or example of the vital stage in general, vast aeons must run their course from the first appearance of the humblest germs of life up to the bringing forth of life in conscious intelligence. one hundred million years are freely spoken of in this connection by those who study the crust of the earth and those who are occupied with the relations of the varied forms of life. man is the latest arrival on this planet, and however far back we try to push the time of his earliest appearance, it is beyond question that that time, relatively to the entire duration of the earth since a solid crust began to form, is but as yesterday. if, from some other globe in the depths of space, this world of ours could have been watched during the long aeons that elapsed from its first separation from the solar nebula down to the time when it first possessed a surface of land and water, and from that time, again, throughout the hypothetical one hundred million years that preceded the advent of man, then, during all those aeons, those imagined observers would have had under their scrutiny a world as yet without inhabitant. the earth now is in the inhabited condition, but science gives us no clue as to how long that condition will endure; rather such hints as are afforded us would seem to point to its lasting but for a brief season as compared with the indefinite duration which preceded it, and the indefinite duration which shall follow. if this thought be sound, it places before us an entirely new and most serious consideration. the world predestined for habitation must not only have its size within certain narrow limits, its distance from its central sun in a certain narrow zone, its rotation period, the inclination of its axis, the eccentricity of its orbit, all suitable alike, but even if in these and in all other necessaries it is perfectly adapted for habitation, yet it will be only during a relatively small fraction of its entire duration that intelligent life, clothed in material form, will find a place upon it. let us sum shortly what we know and what we conclude. we know that this, our earth, is a habitable globe, for we ourselves are living upon it. we know what constitutes the physical basis of our life, and under what conditions on this earth it flourishes, and under what conditions it is destroyed. if we turn our eyes from this, our earth, and look out upon the starry skies, we see the other planets of our system, and the suns which are the centres of other systems. from the consideration of the planets in our own system, we have seen how stringent and how many are the conditions imposed for life to be possible. round our sun there is but a narrow zone in which a habitable world may circle; in this zone there is room for but few worlds, and we actually know of three alone, the earth, the moon, and venus. we know that the earth can be and is inhabited; that the moon is not and cannot be inhabited; and that venus, though of habitable size, may yet be subject to the fatal disqualification of always turning the same face to the sun. of other planetary systems than our own, we actually know of none, but we assume that there are such, and as numerous as there are suns in the starry depths. but of these planetary systems we can rule out, as containing no habitable member, all such as circle round double or multiple suns or, indeed, round any single star that, from whatever cause, is largely variable and, therefore, much less stable than our own. mira ceti, which in months increases its brightness times, may stand as an example. probably these disqualifications rule out of court the great proportion of the stellar systems. of the few, comparatively speaking, single and stable suns that remain in the heavenly abyss, we must conclude, from what we know of our solar system, that they, too, have but a narrow zone, outside of which no world would be fit to dwell in; whilst in the zone the few worlds which might exist must violate no one of many strict conditions. if we assume that there are a hundred million stars within the ken of our telescopes, we may well believe that not more than one in a hundred of these would fulfil the condition of being a single and stable sun, such as ours. of the planets revolving round these million suns--stable and efficient suns--can we expect that in more cases than one in a hundred there will be a planet in the habitable zone fulfilling all the other conditions of habitability, of size, mass, inclination of axis, circular orbit, and rotation? of these ten thousand earths which may be made fit for the habitation of man, can we assume that even one in a hundred is now at that epoch in its history when it is no longer "without form and void," when a division has been made between the waters under the firmament and those that are above the firmament; when the waters under the heaven have been gathered into one place, and the dry land has appeared, and when the earth and the waters have brought forth life abundantly? out of a hundred million of planetary systems throughout the depths of space, can we suppose that there are even one hundred worlds that are actually inhabited at the present moment? these numbers and proportions certainly are not, and cannot be, based on knowledge; they are given as illustrations only; but, vague as they are, they suggest that our earth may be neither one of many inhabited earths, nor yet unique, but one of a few--indeed of a very few. and then the objection is raised: "if our own earth is but one of, perhaps, two inhabited worlds in the solar system; and of perhaps one or two hundred inhabited worlds throughout the furthest space that we can scan; why is all this waste?" of all the countless millions of stellar systems without living organisms as inhabitants, we cannot tell the purpose for the simple reason that we do not know it; but of "waste" in the solar system, there is no question. relatively speaking, this is quite insignificant, for we cannot consider that as "waste material" which is useful and, indeed, essential to existence. for, consider first the material in the earth itself. its total volume is , , , cubic miles, but man only lives _upon_ its surface of less than million square miles in extent, and he can not probe down as far as ten miles below it, through the depths of ocean or by his deepest mine. thus we are left with over thousand million of cubic miles that man, or plant, or beast can never make direct use of. but without this thousand million cubic miles that he can never sow nor reap, the overlying platform on which he dwells would be useless for retaining the air or the water by which he lives. no less essential is the sun; its vast bulk of , , , , , , , , , tons can, in no single unit, be counted "waste," for it is from this that the heat and light necessary for life on the earth is derived. but the tonnage of all the planets combined is but · per cent of the sun alone; and a wastage, if such it is, like this is insignificant from a material point of view. there is a type of politician at the present day who is convinced that the highest purpose to which land can be put is to build upon it; that being, in general, the use giving the highest money return per square foot, though the return does not always fall to the builder. it has taken not a little agitation and popular pressure to enforce the truth that cultivated land is also of use. but there are few who realize that land that is neither built upon nor cultivated is also essential. our barren moors and bleak hillsides, "wastelands" as we call them, are absolutely necessary as collectors of the water by which we live. from them our springs take their source; and they supply our cities with the first necessity of life. we find, then, in this universe so far as we can know it, that space is lavishly provided, matter is lavishly scattered, time is unsparingly drawn upon, but life in any form, and especially in its highest form, is, relatively speaking, very sparsely given. that very circumstance surely points to the overwhelming importance of conscious, intelligent life, and the insignificance of lifeless matter in comparison with it. we have to exhaust arithmetic in computing the size, the mass, the output of heat and light of our sun, yet it is but the hearth-fire and lamp of terrestrial life; and its amazing agglomeration of matter and energy is ungrudgingly devoted to this humble purpose. whatever view we hold as to the scheme of the universe; whether with the unthinking we fail to recognize thought and purpose behind its marvellous manifestations, or, with the thoughtful, realize that only infinite thought could provide so wonderfully for the bringing forth of thought in living material organisms, the conclusion still remains: living intelligences are, by the direct testimony of the universe itself, its noblest and most precious product. the plea is often made that as we find life adapting itself to a great variety of conditions on this earth, we must not set limits to its power of adaption to the conditions of other worlds. but this plea is an unthinking one. the range of conditions through which we find life on this earth is as nothing to the range given by the varied sizes and positions of the different planets; and even on our earth, life in the unfavoured regions--the tops of mountains, the polar snows, the waterless deserts, the ocean depths--is only possible because there are more favoured regions close at hand, and there are, as it were, "crumbs that fall from the rich man's table." a well-known littérateur in setting forth "a hundred ways of making money" gave great prominence to the method of living as caretaker in an empty house. but residing in an empty house does not, in itself, supply the means of sustenance; these have to be furnished by the wealthier man who employs the caretaker. another plea for vague sentiment in this matter is that we cannot expect that intelligent beings on other worlds would have the same form as man, and if not the same form, then, that the same conditions of existence would not hold good for them as for us. both contentions are unsound. protoplasm is the physical basis of all the life that we know, whatever its form; though these forms are to be counted by the million, and are as diverse as they are numerous. and everywhere and always, water is found essential to protoplasmic life. of life of any other kind we do not know any examples; we have no instance; if such exist, then they are beyond our ken. and neither anthropologist nor biologist would admit that the form of intelligent life was an unrelated accident. whether the form brought the intelligence, or the intelligence the form, or both were evolved together, the one reacting on the other, the human form and the human intelligence are associated, and we feel this to be so of necessity. in , dr. eugene dubois found in java a molar tooth and a portion of a skull, and later the thigh bone of the left leg, and two more teeth. such as they were, these relics appeared nearer in form to the corresponding fragments of an average australian than to those of an ape, and on this ground intelligence was claimed for the creature of which they were the remains, and it was given the name of pithecanthropus, or ape-man. the discovery aroused much discussion, but on all sides it was unhesitatingly assumed that the difference between the form of pithecanthropus and that of the most similar ape was an index of its superior intelligence over the ape, just in so far as that difference was in the direction of the modern human form. the same remark applies to the recent discovery of very ancient human remains in sussex. never at any time has it been supposed that the physical frame has followed any other path in the evolution of intelligence than that which brought forth man. the flesh-eating animals have attained efficiency in hunting and warfare by variation along many types of form; the herbivora have been not less varied in the forms by which as races they secured themselves from destruction; but thought has been associated with the development of one type or form only, and the entire future of thought on this planet rested neither with mammoth nor cave-bear, but with the possessor of the erect stature, the upward look, the differentiation of hand and foot, even in their crudest and earliest stages. swift, in _gulliver's travels_, conceived of a land where the intelligence and conscience of man dwelt in the form of the horse, and the human form tabernacled the instincts of the beast. h. g. wells, in his _war of the worlds_, attributed intelligence to monsters--half-cuttlefish and half-anemone,--and the human form to their helpless, unresisting prey. both conceptions are as scientifically absurd as they are gross and revolting; and if it were possible for the skeleton of creatures from other worlds to be brought to us here, then biologists would as confidently pronounce on their intelligence as they do on the extinct forms of bygone ages--the nearer to the human form, the nearer to the human mind. we have found the figures of reindeer, horse, and mammoth scratched in outline on a mammoth tusk; but though the artist has left no other trace, we need no further evidence of his bodily form. neither horse, nor reindeer, nor mammoth made those rough outlines; they were drawn by a man. more striking still, france yields us chipped flints by the million, flints so slightly shaped that it is in dispute whether they may not have been so broken by the action of torrents. but there are only two theories about them; either they were so chipped by natural action, or they were designedly so chipped by creatures resembling ourselves in head and hand. the question that has been dealt with in this volume is a scientific one, and the attempt has been made to treat it as such, and to argue from known physical facts as to the conditions of worlds which we cannot visit. but by many the question is generally discussed wholly apart from physical facts at all, and it becomes one of sentiment and of religious sympathy. yet, curiously enough, the division between those who think that all worlds must be inhabited and those who think that our own world stands alone is not coincident with any line of theological divisions, but rather cuts across all such. some believers in christianity argue that since god has filled this world with life, life has been his purpose in the world, and must therefore have been his purpose in all other worlds--they too must be filled with life in like manner. other believers argue that this world was the scene of the incarnation of our lord, and is therefore unique in that respect; and that this uniqueness sets its stamp upon this world in all respects. opponents to christianity are divided into the same two classes, the one arguing that wherever there is matter the inevitable course of evolution will produce life, and eventually intelligent life. the other class are equally clear that all forms of life are special, the result of the particular environment, and that it is unreasonable to expect that any other world has had the same history as our own, or that the same special conditions have prevailed elsewhere. in other words the belief that there are other inhabited worlds has depended chiefly neither on science nor on religious belief, but upon sentiment. there are some who like to think themselves, and the race to which they belong, altogether exceptional; others delight in finding themselves reflected wherever they look. so far as science has progressed and can return an answer to an enquiry that exceeds so far the bounds of our direct observation, it dissents from both orders of thought. the conditions of life are indeed narrow, special, restricted; intelligent, organic life must, relatively speaking, be a rarity in the universe, but we lack the information that would enable us to affirm with any confidence that such life is only to be found upon this world of ours. heavy as the odds are against any particular world being an inhabited one, yet when the limitless extent of space is considered, and the innumerable numbers of stars and systems of stars, it seems but reasonable to conclude that though inhabited worlds are relatively rare, the absolute number of them may be considerable; considerable, if not at one particular moment of time, yet when the whole duration of the universe is admitted. but there is a religious question connected with this enquiry; one that goes down to the very roots of man's deepest thoughts and aspirations. as individuals our days on the earth are as a shadow, and there is none abiding; as individuals we pass and disappear; and though the race remains, yet as far as science can guide us and enable us to penetrate the future, the same lot awaits the race as well. slowly but surely the water of a planet will combine with its substance or disappear into its crust. the cooling of the sun, though it may be long delayed, would seem to be inevitable in the sequel. "oh, life as futile then as frail. * * * * what hope of answer or redress? behind the veil, behind the veil." it is to this veil that we are now brought. it seems impossible to believe that life, so rare a fruit of the universe, intelligent life, conscious life, to which the long course of evolution has been so manifestly leading up all through the long ages, should have no better destiny than a final and hopeless extinction; that this earth and all the efforts and aspirations of the long generations of men should have no worthier end than to swing, throughout the eternal ages, an empty, frozen heap of dust, circling round the extinct cinder that was once its sun. if we look backward, we seem to discern clear signs of progress; if we look forward, we discern nothing but the veil. science is but organized experience, and experience of the future we have none. there was a time when on this world there was no life; a time when life began. how did it begin? under what conditions? of that great change--when non-living matter first became endowed with life, became so endowed not by the action and intervention of other living matter, but without it--we have no knowledge, no experience. and so long as this continues to be the case, that change, the greatest physical change that has yet taken place in the history of the universe, the first change of the non-living into the living, is outside the reach of science; it lies beyond its border. we may guess and speculate about it, but speculation is not science; we may spin words about it with the utmost skill of the dialectician, but metaphysics is not science; it can never come within the scope of science until it has first come within the scope of experience. there is, therefore, a veil behind us as well as the one that encloses us in front; and as hitherto science has failed to pierce the veil of the past, it is even less able to pierce the veil of the future; for of the future we have no experience. * * * * * here, then, our enquiry must end, for it is an enquiry of physical science; the search for living material organisms endowed with intelligence. how life first came upon this earth, or when, or where, is beyond the power of science to determine. yet it did come. there was a time when there was no life here; none, not even the humblest form of it; nor was there any hint or foreshadowing of it, still less of all its infinities of form, and possibilities of development. once life was not, yet life came, and now, life is abundant, but abundant only in worlds quite exceptional in their conditions, and therefore few in number; it is even conceivable that this earth of ours may be unique. but life as we know it, protoplasmic life, life dependent upon water, the life of intelligence united to the material organism, is under sentence of death. has it any future beyond that veil? is there any kind of life not subject to these narrow limitations; not under the inexorable decree? to questions such as these science has no reply to give; it is even more helpless to answer them than to determine how life first came; its experience does not reach so far. science can examine the present conditions of physical life, but whether or no that life can undergo a change greater than that which passed upon the old inorganic world, it cannot determine. it has no experience. but if science is dumb, if the utmost exertion of human energy and power of research can throw no light on a future of which we have no experience, we are not left without an answer. a voice has been heard, the voice of the son of god himself: "i am the resurrection and the life. he that believeth on me, though he were dead, yet shall he live." and accepting his word, the church in all ages, and among all nations, peoples, and tongues, has made reply: "i look for the resurrection of the dead and the life of the world to come." index abbot, c. g., , albedo of earth, , ---- jupiter, ---- mars, ---- moon, _albumin sol_, algol-type stars, , , antares, antoniadi, e. m., , archangel, climate of, , arcturus, , aristarchus, lunar crater, "astronomical unit," atmosphere of, mars, ----, moon, ----, sun, ----, venus, barnard, e. e., , beer, , , bond, g. p., , brewster, sir david, calcium, , callisto, satellite of jupiter, calory, campbell, w. w., carbon, , , , carbonic acid, cassini, , , ceres, minor planet, , , , , cerulli, v., , chromosphere, , cobalt, comet, encke's, ----, halley's, ----, spectra, copernican theory, copper, corona, coxwell, , , cyanogen, , czapek, f., , darwin, sir g. h., dawes, w. r., , , , denning, w. f., dispersion, anomalous, doppelmayer, lunar crater, dubois, eugene, eros, minor planet, europa, satellite of jupiter, evans, j. e., faculae, , fauth, p., flamsteed, lunar crater, fluorine, fraunhofer, galileo, , , ganymede, satellite of jupiter, , gay-lussac, glaisher, j., , , goodacre, w., green, n. e., , , greenwich hospital school, , "gulliver's travels," , haeckel, e., halogens, , "harper's weekly," helium, herschel, sir j., ---- sir w., , , , , hevelius, hippalus, lunar crater, hooke, r., , , huyghens, hydrocarbons, hydrogen, , , , , , "inhabitant," "inhabited" worlds, , , io, satellite of jupiter, iron, , , jupiter, - ----, belts, , ----, great red spot, ----, proper motion of spots, ----, satellites of, , ----, white spots, , keeler, j. e., , kies, lunar crater, kirchhoff, lacus solis, , , langley, s. p., lilliputians, , linné, lunar crater, lockyer, j. n., lowell, p., , , , , , , , , , , , , , , , , lucifer, mädler, , , , , maginus, lunar crater, magnesium, , manganese, mare fecunditatis, ---- humerum, ---- nubium, ---- serenitatis, mars, canals of, - , , , ----, conditions of, - ----, illusions of, - ----, meteorology of, - ----, oases of, , , , ----, thermograph of, , ----, winds of, mendeléeff, mercury, - messier, lunar crater, , metabolism, , , , , millechau, milton, mira ceti, molesworth, p. b., , moon, - ----, "terminator" of, mont blanc, , , mount everest, , nature of vision, nebulae, spectrum of, , nebulium, negative elements, neison, e., , neptune, , newcomb, s., , nicholson, j. w., nickel, nilosyrtis, "canal" on mars, nitrogen, , , , observatory, chicago, ----, harvard college, ----, lick, ----, paris, occultation, , organic life, definition of, organism, living, - organo-genetic elements, , , osmosis, oxygen, , , , , periodic law, mendeléeff's, phillips, t. e. r., phosphorus, photosphere, , , pickering, w. h., , , , pithecanthropus, planetary statistics, table of, , , platinum, "plurality of worlds," pollock, master, potassium, poynting, j. h., , , , proctor, r. a., , prominences, , , protofluorine, protonilus, "canal" on mars, protoplasm, , , , , , , pyramid, great, refraction, anomalous, reversing layer, "rice-grains," of sun's surface, , ring nebula in lyra, rosse, lord, ruskin, j., saturn, ----, rings of, schiaparelli, g. v., , , , , , , , , , , schooling, t. holt, "scientia," "semi-suns," , serviss, garrett p., singapore, climate of, , sinus sabaeus, marking on mars, , sirius, sodium, , "solar constant," spectroscopic binaries, , spectrum, ----, heat, "spurious" disc, stars, double, ----, multiple, ----, red, ----, spectra of, , , stefan's law, stoney, g. johnstone, "streaming," sulphur, , sun, - sunspots, , , , ----, spectra of, swift, dean, , table mountain, thermograph of mars, , titanium, , , tornadoes, , "twinkler," tycho, lunar crater, uranus, , venus, , - verworn, max, very, f. w., vesper, "victoria," hypothetical planet, wallace, a. r., "war of the worlds," , waste, , water, indispensable factor, , wells, h. g., , whewell, williams, a. stanley, wolf, max, young, c. a., , william brendon and son, ltd. printers, plymouth footnotes: [ ] _chemical phenomena in life_, pp. - , by dr. frederick czapek (harper's library of living thought). the reader is strongly recommended to study this work in the present connection. [ ] _wonders of life_, by ernst haeckel, professor at jena university, p. . [ ] _wonders of life_, pp. - . [ ] _chemical phenomena in life_, p. . [ ] _ibid._, p. . [ ] _other worlds_, by garrett p. serviss, pp. - . [ ] _modern painters_, by john ruskin. [ ] if this experiment could be carried out, it would be necessary to use a spring balance. if the object were weighed in a pair of scales or by a steelyard, the counterbalancing weights would be likewise affected in the same proportion, so that the equilibrium would be undisturbed. [ ] the movements of not a few double stars point to perturbations caused by the attraction of unseen bodies. there are also a number of instances known of "eclipse" or "algol-type" variable stars, in which the presence of a dark companion is indicated by the diminution of the light of the star at regular intervals. [ ] _proc. r. soc._, lxxx, , . [ ] _nature_, lxxx, (april th, ). [ ] "periodic changes upon the moon," _memoirs_, british astronomical association, vol. xiii, p. . [ ] _the moon_, by philip fauth, p. . [ ] _radiation in the solar system: its effects on temperature, and its pressure on small bodies_, by dr. j. h. poynting (_phil. trans. of the royal society_, vol. a). [ ] _publ. of the astron. soc. of the pacific_, vol. ii, pp. - . harper's library of living though arthur holmes the age of the earth and associated problems. _illustrated_ gives us the result of the latest research into this field of enquiry. the radioactive minerals are shown to be recording their own age with the exquisite accuracy of a chronometer--their records checking physical, astronomical, and geological methods of computation. prof. a. w. bickerton the birth of worlds and systems _illustrated_ _preface by prof. ernest rutherford, f.r.s._ a graphic account of the formation of new stars from the collision of dead suns or other celestial bodies. the theory throws light on many astronomical problems, and with its conception of an immortal cosmos, is of great philosophical importance. prof. svante arrhenius the life of the universe _ vols. illustrated_ "we can thoroughly recommend these volumes. the information is accurate, useful, and most suggestive. there are many for whom the first chapters of genesis are a subtle allegory covering the profoundest truths, and we are grateful to the author for having set out this mass of facts."--_the globe._ sir oliver lodge, f.r.s. the ether of space _illustrated_ "this work by the great physicist will be found to possess an abiding charm and an intellectual stimulation."-_observer._ "opens up new views into the nature of the universe. precise and lucid, it summarises our knowledge of the substance which fills all space and penetrates all matter--the substratum of matter itself."--_birmingham post._ _please write for announcements and descriptive list:_ harper & brothers, albemarle street, london, w. harper's library of living thought _foolscap vo, gilt tops, decorative covers, richly gilt backs per volume: cloth s. d. net. leather s. d. net._ prof. arthur keith, m.d. (hunterian professor royal college of surgeons) ancient types of man _illustrated_ "the kind of book that only a master of his subject could write. it must interest every thinking person."_--british medical journal._ prof. frederick czapek chemical phenomena in life discusses in clear, concise terms the great question--"can life be explained by physics and chemistry?" it deals with the life-processes of plants, the molecular structure of protoplasm, organic synthesis in the cell, the nature of ferments, and the subject of inheritance. sir a. tilden, f.r.s. the elements speculations as to their nature and origin _diagrams, &c._ points to the conclusion that the elements resulted from a change in some primal essence, and discusses "whether all may not be suffering a slow waste, which, in the long run, must lead back to the primal chaos." sir william ramsay, f.r.s. elements and electrons _diagrams_ the electron--"the atom of electricity"--is shown to be separable from matter, and to be capable under certain circumstances of independent existence. the book shows that the electron must be regarded as a kind of "element" itself, with much stronger claims to "elementary" or undecomposable characters than the bodies hitherto ranked as elements. transcriber's notes: passages in italics are indicated by _italics_. superscripted characters are indicated by {superscript}. the original text includes symbols which are represented by [symbol] in this text version. proofreading team. [illustration: maria mitchell] maria mitchell life, letters, and journals compiled by phebe mitchell kendall illustrated contents chapter i the parents--home life--education, teachers, books--astronomical instruments--solar eclipse of --teaching--appointment as librarian of nantucket atheneum--friendships for young people--extracts from diary, --music--the piano--society--story-telling--housework--extract from diary, chapter ii "sweeping" the heavens--discovery of the comet, --frederick vi. and the comet--letters from g. p. bond and hon. edward everett--admiral smyth--american academy--american association for the advancement of science--extract from diary, --dorothea dix--esther--divers extracts from diary, , --comet of --computations for comet--visit to cape cod--sandwich and plymouth--pilgrim hall--rev. james freeman clarke--accidents in observing chapter iii wires in the transit instrument--deacon greele--smithsonian fund--"doing"--rachel in "phèdre" and "adrienne"--emerson--the hard winter chapter iv southern tour--chicago--st. louis--scientific academy of st. louis--dr. pope--dr. seyffarth--mississippi river--sand-bars--cherry blossoms--eclipse of sun--natchez--new orleans--slave market--negro church--the "peculiar institution"--bible--judge smith--travelling without escort--savannah--rice plantations--negro children--miss murray--charleston--drive--condition of slaves--old buildings--miss rutledge--mr. capers--class meeting--hospitality--mrs. holbrook--miss pinckney--manners--portraits--miss pinckney's father--george washington--augusta--nashville--mrs. fogg--mrs. polk--charles sumner--mammoth cave--chattanooga chapter v first european tour--liverpool--london--rev. james martineau--mr. john taylor--mr. lassell--liverpool observatory--the hawthornes--shop-keepers and waiters--greenwich observatory--sir george airy--visits to greenwich--herr struvé's mission to england--dinner party--general sabine--westminster abbey--newton's monument--british museum--four great men--st. paul's--dr. johnson--opera--aylesbury--admiral smyth's family--amateur astronomers--hartwell house--dr. lee chapter vi cambridge--dr. whewell--table conversation--professor challis--professor adams--customs--professor sedgwick--caste--king's chapel--fellows-- ambleside--coniston waters--the lakes--miss southey--collingwood--letter to her father--herschels--london rout--professor stokes--dr. arnott--edinboro'--observatory--glasgow observatory--professor nichol--dungeon ghyll--english language--english and americans--boys and beggars chapter vii adams and leverrier--the discovery of the planet neptune--extract from papers--professor bond, of cambridge, mass.--paris--imperial observatory--mons. and mme. leverrier--reception at leverrier's--rooms in observatory--rome--impressions--apartments in rome and paris--customs--holy week--vespers at st. peter's--women--frederika bremer--paul akers--harriet hosmer--collegio romano--father secchi--galileo--visit to the roman observatory--permission from cardinal antonelli--spectroscope chapter viii mrs. somerville--berlin--humboldt--mrs. mitchell's illness and death--removal to lynn, mass.--telescope presented to miss mitchell by elizabeth peabody and others--letters from admiral smyth--colors of stars--extract from letter to a friend--san marino medal--other extracts chapter ix life at vassar college--anxious mammas--faculty meetings--president hill--professor peirce--burlington, ia., and solar eclipse--classes at vassar--professor mitchell and her pupils--extracts from diary--aids --scholarships--address to her students--imagination in science--"i am but a woman"--maria mitchell endowment fund--emperor of brazil--president raymond's death--dome parties--comet, --the apple-tree--"honor girls"--mr. matthew arnold chapter x second visit to europe--russia--extracts from diary and letters--custom-house peculiarities--russian railways--domes--russian thermometers and calendars--the drosky and drivers--observatory at pulkova--herr struvé--scientific position of russia--language-- religion--democracy of the church--government--a russian family--london, --frances power cobbe--bookstores in london--glasgow college for girls chapter xi papers--science--eclipse of , denver, colorado--colors of stars chapter xii religious matters--president taylor's remarks--sermons--george macdonald--rev. dr. peabody--dr. lyman abbott--professor henry--meeting of the american scientific association at saratoga--professor peirce-- concord school of philosophy--emerson--miss peabody--dr. harris--easter flowers--whittier--rich days--cooking schools--anecdotes chapter xiii letter-writing--woman suffrage--membership in various societies.--women's congress at syracuse, n.y.--picnic at medfield, mass.--degrees from different colleges--published papers.--failure in health--resigns her position at vassar college--letters from various persons--death--conclusion appendix introductory note by hon. edward everett correspondence relative to the danish medal chapter i - birth--parents--home surroundings and early life maria mitchell was born on the island of nantucket, mass., aug. , . she was the third child of william and lydia [coleman] mitchell. her ancestors, on both sides, were quakers for many generations; and it was in consequence of the intolerance of the early puritans that these ancestors had been obliged to flee from the state of massachusetts, and to settle upon this island, which, at that time, belonged to the state of new york. for many years the quakers, or friends, as they called themselves, formed much the larger part of the inhabitants of nantucket, and thus were enabled to crystallize, as it were, their own ideas of what family and social life should be; and although in course of time many "world's people" swooped down and helped to swell the number of islanders, they still continued to hold their own methods, and to bring up their children in accordance with their own conceptions of "divine light." mr. and mrs. mitchell were married during the war of ; the former lacking one week of being twenty-one years old, and the latter being a few months over twenty. the people of nantucket by their situation endured many hardships during this period; their ships were upon the sea a prey to privateers, and communication with the mainland was exposed to the same danger, so that it was difficult to obtain such necessaries of life as the island could not furnish. there were still to be seen, a few years ago, the marks left on the moors, where fields of corn and potatoes had been planted in that trying time. so the young couple began their housekeeping in a very simple way. mr. mitchell used to describe it as being very delightful; it was noticed that mrs. mitchell never expressed herself on the subject,--it was she, probably, who had the planning to do, to make a little money go a great way, and to have everything smooth and serene when her husband came home. mrs. mitchell was a woman of strong character, very dignified, honest almost to an extreme, and perfectly self-controlled where control was necessary. she possessed very strong affections, but her self-control was such that she was undemonstrative. she kept a close watch over her children, was clearheaded, knew their every fault and every merit, and was an indefatigable worker. it was she who looked out for the education of the children and saw what their capacities were. mr. mitchell was a man of great suavity and gentleness; if left to himself he would never have denied a single request made to him by one of his children. his first impulse was to gratify every desire of their hearts, and if it had not been for the clear head of the mother, who took care that the household should be managed wisely and economically, the results might have been disastrous. the father had wisdom enough to perceive this, and when a child came to him, and in a very pathetic and winning way proffered some request for an unusual indulgence, he generally replied, "yes, if mother thinks best." mr. mitchell was very fond of bright colors; as they were excluded from the dress of friends, he indulged himself wherever it was possible. if he were buying books, and there was a variety of binding, he always chose the copies with red covers. even the wooden framework of the reflecting telescope which he used was painted a brilliant red. he liked a gay carpet on the floor, and the walls of the family sitting-room in the house on vestal street were covered with paper resplendent with bunches of pink roses. suspended by a cord from the ceiling in the centre of this room was a glass ball, filled with water, used by mr. mitchell in his experiments on polarization of light, flashing its dancing rainbows about the room. at the back of this house was a little garden, full of gay flowers: so that if the garb of the young mitchells was rather sombre, the setting was bright and cheerful, and the life in the home was healthy and wide-awake. when the hilarity became excessive the mother would put in her little check, from time to time, and the father would try to look as he ought to, but he evidently enjoyed the whole. as mr. mitchell was kind and indulgent to his children, so he was the sympathetic friend and counsellor of many in trouble who came to him for help or advice. as he took his daily walk to the little farm about a mile out of town, where, for an hour or two he enjoyed being a farmer, the people would come to their doors to speak to him as he passed, and the little children would run up to him to be patted on the head. he treated animals in the same way. he generally kept a horse. his children complained that although the horse was good when it was bought, yet as mr. mitchell never allowed it to be struck with a whip, nor urged to go at other than a very gentle trot, the horse became thoroughly demoralized, and was no more fit to drive than an old cow! there was everything in the home which could amuse and instruct children. the eldest daughter was very handy at all sorts of entertaining occupations; she had a delicate sense of the artistic, and was quite skilful with her pencil. the present kindergarten system in its practice is almost identical with the home as it appeared in the first half of this century, among enlightened people. there is hardly any kind of handiwork done in the kindergarten that was not done in the mitchell family, and in other families of their acquaintance. the girls learned to sew and cook, just as they learned to read,--as a matter of habit rather than of instruction. they learned how to make their own clothes, by making their dolls' clothes,--and the dolls themselves were frequently home-made, the eldest sister painting the faces much more prettily than those obtained at the shops; and there was a great delight in gratifying the fancy, by dressing the dolls, not in quaker garb, but in all of the most brilliant colors and stylish shapes worn by the ultra-fashionable. there were always plenty of books, and besides those in the house there was the atheneum library, which, although not a free library, was very inexpensive to the shareholders. there was another very striking difference between that epoch and the present. the children of that day were taught to value a book and to take excellent care of it; as an instance it may be mentioned that one copy of colburn's "algebra" was used by eight children in the mitchell family, one after the other. the eldest daughter's name was written on the inside of the cover; seven more names followed in the order of their ages, as the book descended. with regard to their reading, the mother examined every book that came into the house. of course there were not so many books published then as now, and the same books were read over and over. miss edgeworth's stories became part of their very lives, and young's "night thoughts," and the poems of cowper and bloomfield were conspicuous objects on the bookshelves of most houses in those days. mr. mitchell was very apt, while observing the heavens in the evening, to quote from one or the other of these poets, or from the bible. "an undevout astronomer is mad" was one of his favorite quotations. among the poems which maria learned in her childhood, and which was repeatedly upon her lips all through her life, was, "the spacious firmament on high." in her latter years if she had a sudden fright which threatened to take away her senses she would test her mental condition by repeating that poem; it is needless to say that she always remembered it, and her nerves instantly relapsed into their natural condition. the lives of maria mitchell and her numerous brothers and sisters were passed in simplicity and with an entire absence of anything exciting or abnormal. the education of their children is enjoined upon the parents by the "discipline," and in those days at least the parents did not give up all the responsibility in that line to the teachers. in maria mitchell's childhood the children of a family sat around the table in the evenings and studied their lessons for the next day,--the parents or the older children assisting the younger if the lessons were too difficult. the children attended school five days in the week,--six hours in the day,--and their only vacation was four weeks in the summer, generally in august. the idea that children over-studied and injured their health was never promulgated in that family, nor indeed in that community; it seems to be a notion of the present half-century. maria's first teacher was a lady for whom she always felt the warmest affection, and in her diary, written in her later years, occurs this allusion to her: "i count in my life, outside of family relatives, three aids given me on my journey; they are prominent to me: the woman who first made the study-book charming; the man who sent me the first hundred dollars i ever saw, to buy books with; and another noble woman, through whose efforts i became the owner of a telescope; and of these, the first was the greatest." as a little girl, maria was not a brilliant scholar; she was shy and slow; but later, under her father's tuition, she developed very rapidly. after the close of the war of , when business was resumed and the town restored to its normal prosperity, mr. mitchell taught school,--at first as master of a public school, and afterwards in a private school of his own. maria attended both of these schools. mr. mitchell's pupils speak of him as a most inspiring teacher, and he always spoke of his experiences in that capacity as very happy. when her father gave up teaching, maria was put under the instruction of mr. cyrus peirce, afterwards principal of the first normal school started in the united states. mr. peirce took a great interest in maria, especially in developing her taste for mathematical study, for which she early showed a remarkable talent. the books which she studied at the age of seventeen, as we know by the date of the notes, were bridge's "conic sections," hutton's "mathematics," and bowditch's "navigator." at that time prof. benjamin peirce had not published his "explanations of the navigator and almanac," so that maria was obliged to consult many scientific books and reports before she could herself construct the astronomical tables. mr. mitchell, on relinquishing school-teaching, was appointed cashier of the pacific bank; but although he gave up teaching, he by no means gave up studying his favorite science, astronomy, and maria was his willing helper at all times. mr. mitchell from his early youth was an enthusiastic student of astronomy, at a time, too, when very little attention was given to that study in this country. his evenings, when pleasant, were spent in observing the heavens, and to the children, accustomed to seeing such observations going on, the important study in the world seemed to be astronomy. one by one, as they became old enough, they were drafted into the service of counting seconds by the chronometer, during the observations. some of them took an interest in the thing itself, and others considered it rather stupid work, but they all drank in so much of this atmosphere, that if any one had asked a little child in this family, "who was the greatest man that ever lived?" the answer would have come promptly, "herschel." maria very early learned the use of the sextant. the chronometers of all the whale ships were brought to mr. mitchell, on their return from a voyage, to be "rated," as it was called. for this purpose he used the sextant, and the observations were made in the little back yard of the vestal-street home. there was also a clumsy reflecting telescope made on the herschelian plan, but of very great simplicity, which was put up on fine nights in the same back yard, when the neighbors used to flock in to look at the moon. afterwards mr. mitchell bought a small dolland telescope, which thereafter, as long as she lived, his daughter used for "sweeping" purposes. after their removal to the bank building there were added to these an "altitude and azimuth circle," loaned to mr. mitchell by west point academy, and two transit instruments. a little observatory for the use of the first was placed on the roof of the bank building, and two small buildings were erected in the yard for the transits. there was also a much larger and finer telescope loaned by the coast survey, for which service mr. mitchell made observations. at the time when maria mitchell showed a decided taste for the study of astronomy there was no school in the world where she could be taught higher mathematics and astronomy. harvard college, at that time, had no telescope better than the one which her father was using, and no observatory except the little octagonal projection to the old mansion in cambridge occupied by the late dr. a.p. peabody. however, every one will admit that no school nor institution is better for a child than the home, with an enthusiastic parent for a teacher. at the time of the annular eclipse of the sun in the totality was central at nantucket. the window was taken out of the parlor on vestal street, the telescope, the little dolland, mounted in front of it, and with maria by his side counting the seconds the father observed the eclipse. maria was then twelve years old. at sixteen miss mitchell left mr. peirce's school as a pupil, but was retained as assistant teacher; she soon relinquished that position and opened a private school on traders' lane. this school too she gave up for the position of librarian of the nantucket atheneum, which office she held for nearly twenty years. this library was open only in the afternoon, and on saturday evening. the visitors were comparatively few in the afternoon, so that miss mitchell had ample leisure for study,--an opportunity of which she made the most. her visitors in the afternoon were elderly men of leisure, who enjoyed talking with so bright a girl on their favorite hobbies. when they talked miss mitchell closed her book and took up her knitting, for she was never idle. with some of these visitors the friendship was kept up for years. it was in this library that she found la place's "mécanique céleste," translated by her father's friend, dr. bowditch; she also read the "theoria motus," of gauss, in its original latin form. in her capacity as librarian miss mitchell to a large extent controlled the reading of the young people in the town. many of them on arriving at mature years have expressed their gratitude for the direction in which their reading was turned by her advice. miss mitchell always had a special friendship for young girls and boys. many of these intimacies grew out of the acquaintance made at the library,--the young girls made her their confidante and went to her for sympathy and advice. the boys, as they grew up, and went away to sea, perhaps, always remembered her, and made a point, when they returned in their vacations, of coming to tell their experiences to such a sympathetic listener. "april , . a young sailor boy came to see me to-day. it pleases me to have these lads seek me on their return from their first voyage, and tell me how much they have learned about navigation. they always say, with pride, 'i can take a lunar, miss mitchell, and work it up!' "this boy i had known only as a boy, but he has suddenly become a man and seems to be full of intelligence. he will go once more as a sailor, he says, and then try for the position of second mate. he looked as if he had been a good boy and would make a good man. "he said that he had been ill so much that he had been kept out of temptation; but that the forecastle of a ship was no place for improvement of mind or morals. he said the captain with whom he came home asked him if he knew me, because he had heard of me. i was glad to find that the captain was a man of intelligence and had been kind to the boy." miss mitchell was an inveterate reader. she devoured books on all subjects. if she saw that boys were eagerly reading a certain book she immediately read it; if it were harmless she encouraged them to read it; if otherwise, she had a convenient way of _losing_ the book. in november, when the trustees made their annual examination, the book appeared upon the shelf, but the next day after it was again lost. at this time nantucket was a thriving, busy town. the whale-fishery was a very profitable business, and the town was one of the wealthiest in the state. there was a good deal of social and literary life. in a friend's family neither music nor dancing was allowed. mr. and mrs. mitchell were by no means narrow sectarians, but they believed it to be best to conform to the rules of friends as laid down in the "discipline." george fox himself, the founder of the society, had blown a blast against music, and especially instrumental music in churches. it will be remembered that the methodists have but recently yielded to the popular demand in this respect, and have especially favored congregational singing. it is most likely that george fox had no ear for music himself, and thus entailed upon his followers an obligation from which they are but now freeing themselves. there was plenty of singing in the mitchell family, and the parents liked it, especially the father, who, when he sat down in the evening with the children, would say, "now sing something." but there could be no instruction in singing; the children sang the songs that they picked up from their playmates. however, one of the daughters bought a piano, and maria's purse opened to help that cause along. it would not have been proper for mr. mitchell to help pay for it, but he took a great interest in it, nevertheless. so indeed did the mother, but she took care not to express herself outwardly. the piano was kept in a neighboring building not too far off to be heard from the house. maria had no ear for music herself, but she was always to be depended upon to take the lead in an emergency, so the sisters put their heads together and decided that the piano must be brought into the house. when they had made all the preparations the father and mother were invited to take tea with their married daughter, who lived in another part of the town and had been let into the secret. the piano was duly removed and placed in an upper room called the "hall," where mr. mitchell kept the chronometers, where the family sewing was done, and where the larger part of the books were kept,--a beautiful room, overlooking "the square," and a great gathering-place for all their young friends. when the piano was put in place, the sisters awaited the coming of the parents. maria stationed herself at the foot of the stairs, ready to meet them as they entered the front door; another, half-way between, was to give the signal to a third, who was seated at the piano. the footsteps were heard at the door, the signal was given; a lively tune was started, and maria confronted the parents as they entered. "what's that?" was the exclamation. "well," said maria, soothingly, "we've had the piano brought over." "why, of all things!" exclaimed the mother. the father laid down his hat, walked immediately upstairs, entered the hall, and said, "come, daughter, play something lively!" so that was all. but that was not all for mr. mitchell; he had broken the rules accepted by the friends, and it was necessary for some notice to be taken of it, so a dear old friend and neighbor came to deal with him. now, to be "under dealings," as it is called, was a very serious matter,--to be spoken of only under the breath, in a half whisper. "i hear that thee has a piano in thy house," said the old friend. "yes, my daughters have," was the reply. "but it is in thy house," pursued the friend. "yes; but my home is my children's home as well as mine," said mr. mitchell, "and i propose that they shall not be obliged to go away from home for their pleasures. i don't play on the piano." it so happened that mr. mitchell held the property of the "monthly meeting" in his hands at the time, and it was a very improper thing for the accredited agent of the society to be "under dealings," as mr. mitchell gently suggested. this the friend had not thought of, and so he said, "well, william, perhaps we'd better say no more about it." when the father came home after this interview he could not keep it to himself. if it had been the mother who was interviewed she would have kept it a profound secret,--because she would not have liked to have her children get any fun out of the proceedings of the old friend. but mr. mitchell told the story in his quiet way, the daughters enjoyed it, and declared that the piano was placed upon a firm foothold by this proceeding. the news spread abroad, and several other young quaker girls eagerly seized the occasion to gratify their musical longings in the same direction. [footnote: it is pleasant to note that this objection to music among friends is a thing of the past, and that the friends' school at providence, r.i., which is under the control of the "new england yearly meeting of friends," has music in its regular curriculum.] few women with scientific tastes had the advantages which surrounded miss mitchell in her own home. her father was acquainted with the most prominent scientific men in the country, and in his hospitable home at nantucket she met many persons of distinction in literature and science. she cared but little for general society, and had always to be coaxed to go into company. later in life, however, she was much more socially inclined, and took pleasure in making and receiving visits. she could neither dance nor sing, but in all amusements which require quickness and a ready wit she was very happy. she was very fond of children, and knew how to amuse them and to take care of them. as she had half a dozen younger brothers and sisters, she had ample opportunity to make herself useful. she was a capital story-teller, and always had a story on hand to divert a wayward child, or to soothe the little sister who was lying awake, and afraid of the dark. she wrote a great many little stories, printed them with a pen, and bound them in pretty covers. most of them were destroyed long ago. maria took her part in all the household work. she knew how to do everything that has to be done in a large family where but one servant is kept, and she did everything thoroughly. if she swept a room it became clean. she might not rearrange the different articles of furniture in the most artistic manner, but everything would be clean, and there would be nothing left crooked. if a chair was to be placed, it would be parallel to something; she was exceedingly sensitive to a line out of the perpendicular, and could detect the slightest deviation from that rule. she had also a sensitive eye in the matter of color, and felt any lack of harmony in the colors worn by those about her. maria was always ready to "bear the brunt," and could at any time be coaxed by the younger children to do the things which they found difficult or disagreeable. the two youngest children in the family were delicate, and the special care of the youngest sister devolved upon maria, who knew how to be a good nurse as well as a good playfellow. she was especially careful of a timid child; she herself was timid, and, throughout her life, could never witness a thunder-storm with any calmness. on one of those occasions so common in an american household, when the one servant suddenly takes her leave, or is summarily dismissed, miss mitchell describes her part of the family duties: "oct. , . this morning i arose at six, having been half asleep only for some hours, fearing that i might not be up in time to get breakfast, a task which i had volunteered to do the preceding evening. it was but half light, and i made a hasty toilet. i made a fire very quickly, prepared the coffee, baked the graham bread, toasted white bread, trimmed the solar lamp, and made another fire in the dining-room before seven o'clock. "i always thought that servant-girls had an easy time of it, and i still think so. i really found an hour too long for all this, and when i rang the bell at seven for breakfast i had been waiting fifteen minutes for the clock to strike. "i went to the atheneum at . , and having decided that i would take the newark and cambridge places of the comet, and work them up, i did so, getting to the three equations before i went home to dinner at . . i omitted the corrections of parallax and aberrations, not intending to get more than a rough approximation. i find to my sorrow that they do not agree with those from my own observations. i shall look over them again next week. "at noon i ran around and did up several errands, dined, and was back again at my post by . . then i looked over my morning's work,--i can find no mistake. i have worn myself thin trying to find out about this comet, and i know very little now in the matter. "i saw, in looking over cooper, elements of a comet of which resemble what i get out for this, from my own observations, but i cannot rely upon my own. "i saw also, to-day, in the 'monthly notices,' a plan for measuring the light of stars by degrees of illumination,--an idea which had occurred to me long ago, but which i have not practised. "october . yesterday i was again reminded of the remark which mrs. stowe makes about the variety of occupations which an american woman pursues. "she says it is this, added to the cares and anxieties, which keeps them so much behind the daughters of england in personal beauty. "and to-day i was amused at reading that one of her party objected to the introduction of waxed floors into american housekeeping, because she could seem to see herself down on her knees doing the waxing. "but of yesterday. i was up before six, made the fire in the kitchen, and made coffee. then i set the table in the dining-room, and made the fire there. toasted bread and trimmed lamps. rang the breakfast bell at seven. after breakfast, made my bed, and 'put up' the room. then i came down to the atheneum and looked over my comet computations till noon. before dinner i did some tatting, and made seven button-holes for k. i dressed and then dined. came back again to the atheneum at . , and looked over another set of computations, which took me until four o'clock. i was pretty tired by that time, and rested by reading 'cosmos.' lizzie e. came in, and i gossiped for half an hour. i went home to tea, and that over, i made a loaf of bread. then i went up to my room and read through (partly writing) two exercises in german, which took me thirty-five minutes. "it was stormy, and i had no observing to do, so i sat down to my tatting. lizzie e. came in and i took a new lesson in tatting, so as to make the pearl-edged. i made about half a yard during the evening. at a little after nine i went home with lizzie, and carried a letter to the post-office. i had kept steadily at work for sixteen hours when i went to bed." chapter ii - miss mitchell's comet--extracts from diary--the comet miss mitchell spent every clear evening on the house-top "sweeping" the heavens. no matter how many guests there might be in the parlor, miss mitchell would slip out, don her regimentals as she called them, and, lantern in hand, mount to the roof. on the evening of oct. , , there was a party of invited guests at the mitchell home. as usual, maria slipped out, ran up to the telescope, and soon returned to the parlor and told her father that she thought she saw a comet. mr. mitchell hurried upstairs, stationed himself at the telescope, and as soon as he looked at the object pointed out by his daughter declared it to be a comet. miss mitchell, with her usual caution, advised him to say nothing about it until they had observed it long enough to be tolerably sure. but mr. mitchell immediately wrote to professor bond, at cambridge, announcing the discovery. on account of stormy weather, the mails did not leave nantucket until october . frederick vi., king of denmark, had offered, dec. , , a gold medal of the value of twenty ducats to the first discoverer of a telescopic comet. the regulations, as revised and amended, were republished, in april, , in the "astronomische nachrichten." when this comet was discovered, the king who had offered the medal was dead. the son, frederick vii., who had succeeded him, had not the interest in science which belonged to his father, but he was prevailed upon to carry out his father's designs in this particular case. the same comet had been seen by father de vico at rome, on october , at . p.m., and this fact was immediately communicated by him to professor schumacher, at altona. on the th of october, at . p.m., the comet was observed by mr. w.r. dawes, at kent, england, and on the th it was seen by madame rümker, the wife of the director of the observatory at hamburg. the following letter from the younger bond will show the cordial relations existing between the observatory at cambridge and the smaller station at nantucket: cambridge, oct. , . dear maria: there! i think that is a very amiable beginning, considering the way in which i have been treated by you! if you are going to find any more comets, can you not wait till they are announced by the proper authorities? at least, don't kidnap another such as this last was. if my object were to make you fear and tremble, i should tell you that on the evening of the th i was sweeping within a few degrees of your prize. i merely throw out the hint for what it is worth. it has been very interesting to watch the motion of this comet among the stars with the great refractor; we could almost see it move. an account of its passage over the star mentioned by your father when he was here, would make an interesting notice for one of the foreign journals, which we would readily forward.... [here follow mr. bond's observations.] respectfully, your obedient servant, g. p. bond. hon. edward everett, who at that time was president of harvard college, took a great interest in the matter, and immediately opened a correspondence with the proper authorities, and sent a notice of the discovery to the "astronomische nachrichten." the priority of miss mitchell's discovery was immediately admitted throughout europe. the king of denmark very promptly referred the matter to professor schumacher, who reported in favor of granting the medal to miss mitchell, and the medal was duly struck off and forwarded to mr. everett. among european astronomers who urged miss mitchell's claim was admiral smyth, whom she knew through his "celestial cycle," and who later, on her visit to england, became a warm personal friend. madame rümker, also, sent congratulations. mr. everett announced the receipt of the medal to miss mitchell in the following letter: cambridge, march , . my dear miss mitchell: i have the pleasure to inform you that your medal arrived by the last steamer; it reached me by mail, yesterday afternoon. i went to boston this morning, hoping to find you at the adams house, to put it into your own hand. as your return to nantucket prevented this, i, of course, retain it, subject to your orders, not liking to take the risk again of its transmission by mail. having it in this way in my hand, i have taken the liberty to show it to some friends, such as w.c. bond, professor peirce, the editors of the "transcript," and the members of my family,--which i hope you will pardon. i remain, my dear miss mitchell, with great regard, very faithfully yours, edward everett.[footnote: see appendix.] in miss mitchell was elected to membership by the "american academy of arts and sciences," unanimously; she was the first and only woman ever admitted. in the diploma the printed word "fellow" is erased, and the words "honorary member" inserted by dr. asa gray, who signed the document as secretary. some years later, however, her name is found in the list of fellows of this academy, also of the american institute and of the american association for the advancement of science. for many years she attended the annual conventions of this last-mentioned association, in which she took great interest. the extract below refers to one of these meetings, probably that of : "august . it is really amusing to find one's self lionized in a city where one has visited quietly for years; to see the doors of fashionable mansions open wide to receive you, which never opened before. i suspect that the whole corps of science laughs in its sleeves at the farce. "the leaders make it pay pretty well. my friend professor bache makes the occasions the opportunities for working sundry little wheels, pulleys, and levers; the result of all which is that he gets his enormous appropriations of $ , out of congress, every winter, for the maintenance of the united states coast survey. "for a few days science reigns supreme,--we are fêted and complimented to the top of our bent, and although complimenters and complimented must feel that it is only a sort of theatrical performance, for a few days and over, one does enjoy acting the part of greatness for a while! i was tired after three days of it, and glad to take the cars and run away. "the descent into a commoner was rather sudden. i went alone to boston, and when i reached out my free pass, the conductor read it through and handed it back, saying in a gruff voice, 'it's worth nothing; a dollar and a quarter to boston.' think what a downfall! the night before, and 'one blast upon my bugle horn were worth a hundred men!' now one man alone was my dependence, and that man looked very much inclined to put me out of the car for attempting to pass a ticket that in his eyes was valueless. of course i took it quietly, and paid the money, merely remarking, 'you will pass a hundred persons on this road in a few days on these same tickets.' "when i look back on the paper read at this meeting by mr. j---- in his uncouth manner, i think when a man is thoroughly in earnest, how careless he is of mere _words!_" in miss mitchell was asked by the late admiral davis, who had just taken charge of the american nautical almanac, to act as computer for that work,--a proposition to which she gladly assented, and for nineteen years she held that position in addition to her other duties. this, of course, made a very desirable increase to her income, but not necessarily to her expenses. the tables of the planet venus were assigned to her. in this year, too, she was employed by professor bache, of the united states coast survey, in the work of an astronomical party at mount independence, maine. " . i was told that miss dix wished to see me, and i called upon her. it was dusk, and i did not at once see her; her voice was low, not particularly sweet, but very gentle. she told me that she had heard professor henry speak of me, and that professor henry was one of her best friends, the truest man she knew. when the lights were brought in i looked at her. she must be past fifty, she is rather small, dresses indifferently, has good features in general, but indifferent eyes. she does not brighten up in countenance in conversing. she is so successful that i suppose there must be a hidden fire somewhere, for heat is a motive power, and her cold manners could never move legislatures. i saw some outburst of fire when mrs. hale's book was spoken of. it seems mrs. hale wrote to her for permission to publish a notice of her, and was decidedly refused; another letter met with the same answer, yet she wrote a 'life' which miss dix says is utterly false. "in her general sympathy for suffering humanity, miss dix seems neglectful of the individual interest. she has no family connection but a brother, has never had sisters, and she seemed to take little interest in the persons whom she met. i was surprised at her feeling any desire to see me. she is not strikingly interesting in conversation, because she is so grave, so cold, and so quiet. i asked her if she did not become at times weary and discouraged; and she said, wearied, but not discouraged, for she had met with nothing but success. there is evidently a strong will which carries all before it, not like the sweep of the hurricane, but like the slow, steady, and powerful march of the molten lava. "it is sad to see a woman sacrificing the ties of the affections even to do good. i have no doubt miss dix does much good, but a woman needs a home and the love of other women at least, if she lives without that of man." the following entry was made many years after:-- "august, . i have just seen miss dix again, having met her only once for a few minutes in all the eighteen years. she listened to a story of mine about some girls in need, and then astonished me by an offer she made me." "feb. , . i think dr. hall [in his 'life of mary ware'] does wrong when he attempts to encourage the use of the _needle_. it seems to me that the needle is the chain of woman, and has fettered her more than the laws of the country. "once emancipate her from the 'stitch, stitch, stitch," the industry of which would be commendable if it served any purpose except the gratification of her vanity, and she would have time for studies which would engross as the needle never can. i would as soon put a girl alone into a closet to meditate as give her only the society of her needle. the art of sewing, so far as men learn it, is well enough; that is, to enable a person to _take the stitches_, and, if necessary, to make her own garments in a strong manner; but the dressmaker should no more be a universal character than the carpenter. suppose every man should feel it is his duty to do his own mechanical work of _all_ kinds, would society be benefited? would the work be well done? yet a woman is expected to know how to do all kinds of sewing, all kinds of cooking, all kinds of any _woman's_ work, and the consequence is that life is passed in learning these only, while the universe of truth beyond remains unentered. "may , . i could not help thinking of esther [a much-loved cousin who had recently died] a few evenings since when i was observing. a meteor flashed upon me suddenly, very bright, very short-lived; it seemed to me that it was sent for me especially, for it greeted me almost the first instant i looked up, and was gone in a second,--it was as fleeting and as beautiful as the smile upon esther's face the last time i saw her. i thought when i talked with her about death that, though she could not come to me visibly, she might be able to influence my feelings; but it cannot be, for my faith has been weaker than ever since she died, and my fears have been greater." a few pages farther on in the diary appears this poem: "esther "living, the hearts of all around sought hers as slaves a throne; dying, the reason first we found-- the fulness of her own. "she gave unconsciously the while a wealth we all might share-- to me the memory of the smile that last i saw her wear. "earth lost from out its meagre store a bright and precious stone; heaven could not be so rich before, but it has richer grown." "sept. , . i am surprised to find the verse which i picked up somewhere and have always admired-- "'oh, reader, had you in your mind such stores as silent thought can bring, oh, gentle reader, you would find a tale in everything'-- belonging to wordsworth and to one of wordsworth's simple, i am almost ready to say _silly_, poems. i am in doubt what to think of wordsworth. i should be ashamed of some of his poems if i had written them myself, and yet there are points of great beauty, and lines which once in the mind will not leave it. "oct. , . people have to learn sometimes not only how much the heart, but how much the head, can bear. my letter came from cambridge [the harvard observatory], and i had some work to do over. it was a wearyful job, but by dint of shutting myself up all day i did manage to get through with it. the good of my travelling showed itself then, when i was too tired to read, to listen, or to talk; for the beautiful scenery of the west was with me in the evening, instead of the tedious columns of logarithms. it is a blessed thing that these pictures keep in the mind and come out at the needful hour. i did not call them, but they seemed to come forth as a regulator for my tired brain, as if they had been set sentinel-like to watch a proper time to appear. "november, . there is said to be no up or down in creation, but i think the _world_ must be _low_, for people who keep themselves constantly before it do a great deal of stooping! "dec. , . last night we had the first meeting of the class in elocution. it was very pleasant, but my deficiency of ear was never more apparent to myself. we had exercises in the ascending scale, and i practised after i came home, with the family as audience. h. says my ear is competent only to vulgar hearing, and i cannot appreciate nice distinctions.... i am sure that i shall never say that if i had been properly educated i should have made a singer, a dancer, or a painter--i should have failed less, perhaps, in the last. ... coloring i might have been good in, for i do think my eyes are better than those of any one i know. "feb. , . if i should make out a calendar by my feelings of fatigue, i should say there were six saturdays in the week and one sunday. "mr. ---- somewhat ridicules my plan of reading milton with a view to his astronomy, but i have found it very pleasant, and have certainly a juster idea of milton's variety of greatness than i had before. i have filled several sheets with my annotations on the 'paradise lost,' which i may find useful if i should ever be obliged to teach, either as a schoolma'am or a lecturer. [footnote: this paper has been printed since miss mitchell's death in "poet-lore," june-july, .] "march , . i 'swept' last night two hours, by three periods. it was a grand night--not a breath of air, not a fringe of a cloud, all clear, all beautiful. i really enjoy that kind of work, but my back soon becomes tired, long before the cold chills me. i saw two nebulae in leo with which i was not familiar, and that repaid me for the time. i am always the better for open-air breathing, and was certainly meant for the wandering life of the indian. "sept. , . i am just through with a summer, and a summer is to me always a trying ordeal. i have determined not to spend so much time at the atheneum another season, but to put some one in my place who shall see the strange faces and hear the strange talk. "how much talk there is about religion! giles [footnote: rev. henry giles.] i like the best, for he seems, like myself, to have no settled views, and to be religious only in feeling. he says he has no piety, but a great sense of infinity. "yesterday i had a shaker visitor, and to-day a catholic; and the more i see and hear, the less do i care about church doctrines. the catholic, a priest, i have known as an atheneum visitor for some time. he talked to-day, on my asking him some questions, and talked better than i expected. he is plainly full of intelligence, full of enthusiasm for his religion, and, i suspect, full of bigotry. i do not believe he will die a catholic priest. a young man of his temperament must find it hard to live without family ties, and i shall expect to hear, if i ever hear of him again, that some good little irish girl has made him forget his vows. "my visitors, in other respects, have been of the average sort. four women have been delighted to make my acquaintance--three men have thought themselves in the presence of a superior being; one offered me twenty-five cents because i reached him the key of the museum. one woman has opened a correspondence with me, and several have told me that they knew friends of mine; two have spoken of me in small letters to small newspapers; one said he didn't see me, and one said he did! i have become hardened to all; neither compliment nor quarter-dollar rouses any emotion. my fit of humility, which has troubled me all summer, is shaken, however, by the first cool breeze of autumn and the first walk taken without perspiration. "sept. , . on the evening of the th, while 'sweeping,' there came into the field the two nebulae in ursa major, which i have known for many a year, but which to my surprise now appeared to be three. the upper one, as seen from an inverting telescope, appeared double-headed, like one near the dolphin, but much more decided than that, the space between the two heads being very plainly discernible and subtending a decided angle. the bright part of this object was clearly the old nebula--but what was the appendage? had the nebula suddenly changed? was it a comet, or was it merely a very fine night? father decided at once for the comet; i hesitated, with my usual cowardice, and forbade his giving it a notice in the newspaper. "i watched it from . to . almost without cessation, and was quite sure at . that its position had changed with regard to the neighboring stars. i counted its distance from the known nebula several times, but the whole affair was difficult, for there were flying clouds, and sometimes the nebula and comet were too indistinct to be definitely seen. "the th was cloudy and the th the same, with the variety of occasional breaks, through which i saw the nebula, but not the comet. "on the st came a circular, and behold mr. van arsdale had seen it on the th, but had not been sure of it until the th, on account of the clouds. "i was too well pleased with having really made the discovery to care because i was not first. "let the dutchman have the reward of his sturdier frame and steadier nerves! "especially could i be a christian because the th was cloudy, and more especially because i dreaded the responsibility of making the computations, _nolens volens_, which i must have done to be able to call it mine.... "i made observations for three hours last night, and am almost ill to-day from fatigue; still i have worked all day, trying to reduce the places, and mean to work hard again to-night. "sept. , . i began to recompute for the comet, with observations of cambridge and washington, to-day. i have had a fit of despondency in consequence of being obliged to renounce my own observations as too rough for use. the best that can be said of my life so far is that it has been industrious, and the best that can be said of me is that i have not pretended to what i was not. "october . as soon as i had run through the computations roughly for the comet, so as to make up my mind that by my own observations (which were very wrong) the perihelion was passed, and nothing more to be hoped for from observations, i seized upon a pleasant day and went to the cape for an excursion. we went to yarmouth, sandwich, and plymouth, enjoying the novelty of the new car-route. it really seemed like railway travelling on our own island, so much sand and so flat a country. "the little towns, too, seemed quaint and odd, and the old gray cottages looked as if they belonged to the last century, and were waked from a long nap by the railway whistle. "i thought sandwich a beautiful, and plymouth an interesting, town. i would fain have gone off into some poetical quotation, such as 'the breaking waves dashed high' or 'the pilgrim fathers, where are they?' but k., who had been there before, desired me not to be absurd, but to step quietly on to the half-buried rock and quietly off. younger sisters know a deal, so i did as i was bidden to do, and it was just as well not to make myself hoarse without an appreciative audience. "i liked the picture by sargent in pilgrim hall, but seeing plymouth on a mild, sunny day, with everything looking bright and pleasant, it was difficult to conceive of the landing of the pilgrims as an event, or that the settling of such a charming spot required any heroism. "the picture, of course, represents the dreariness of winter, and my feelings were moved by the chilled appearance of the little children, and the pathetic countenance of little peregrine white, who, considering that he was born in the harbor, is wonderfully grown up before they are welcomed by samoset. according to history little peregrine was born about december and samoset met them about march ; so he was three months old, but he is plainly a forward child, for he looks up very knowingly. such a child had immortality thrust upon him from his birth. it must have had a deadening influence upon him to know that he was a marked man whether he did anything worthy of mark or not. he does not seem to have made any figure after his entrance into the world, though he must have created a great sensation when he came. "october . i have just gone over my comet computations again, and it is humiliating to perceive how very little more i know than i did seven years ago when i first did this kind of work. to be sure, i have only once in the time computed a parabolic orbit; but it seems to me that i know no more in general. i think i am a little better thinker, that i take things less upon trust, but at the same time i trust myself much less. the world of learning is so broad, and the human soul is so limited in power! we reach forth and strain every nerve, but we seize only a bit of the curtain that hides the infinite from us. "will it really unroll to us at some future time? aside from the gratification of the affections in another world, that of the intellect must be great if it is enlarged and its desires are the same. "nov. , . yesterday james freeman clarke, the biographer of margaret fuller, came into the atheneum. it was plain that he came to see me and not the institution.... he rushed into talk at once, mostly on people, and asked me about my astronomical labors. as it was a kind of flattery, i repaid it in kind by asking him about margaret fuller. he said she did not strike any one as a person of intellect or as a student, for all her faculties were kept so much abreast that none had prominence. i wanted to ask if she was a lovable person, but i did not think he would be an unbiassed judge, she was so much attached to him. "dec. , . the love of one's own sex is precious, for it is neither provoked by vanity nor retained by flattery; it is genuine and sincere. i am grateful that i have had much of this in my life. "the comet looked in upon us on the th. it made a twilight call, looking sunny and bright, as if it had just warmed itself in the equinoctial rays. a boy on the street called my attention to it, but i found on hurrying home that father had already seen it, and had ranged it behind buildings so as to get a rough position. "it was piping cold, but we went to work in good earnest that night, and the next night on which we could see it, which was not until april. "i was dreadfully busy, and a host of little annoyances crowded upon me. i had a good star near it in the field of my comet-seeker, but _what_ star? "on that rested everything, and i could not be sure even from the catalogue, for the comet and the star were so much in the twilight that i could get no good neighboring stars. we called it arietes, or . "then came a waxing moon, and we waxed weary in trying to trace the fainter and fainter comet in the mists of twilight and the glare of moonlight. "next i broke a screw of my instrument, and found that no screw of that description could be bought in the town. "i started off to find a man who could make one, and engaged him to do so the next day. the next day was fast day; all the world fasted, at least from labor. "however, the screw was made, and it fitted nicely. the clouds cleared, and we were likely to have a good night. i put up my instrument, but scarcely had the screw-driver touched the new screw than out it flew from its socket, rolled along the floor of the 'walk,' dropped quietly through a crack into the gutter of the house-roof. i heard it click, and felt very much like using language unbecoming to a woman's mouth. "i put my eye down to the crack, but could not see it. there was but one thing to be done,--the floor-boards must come up. i got a hatchet, but could do nothing. i called father; he brought a crowbar and pried up the board, then crawled under it and found the screw. i took good care not to lose it a second time. "the instrument was fairly mounted when the clouds mounted to keep it company, and the comet and i again parted. "in all observations, the blowing out of a light by a gust of wind is a very common and very annoying accident; but i once met with a much worse one, for i dropped a chronometer, and it rolled out of its box on to the ground. we picked it up in a great panic, but it had not even altered its rate, as we found by later observations. "the glaring eyes of the cat, who nightly visited me, were at one time very annoying, and a man who climbed up a fence and spoke to me, in the stillness of the small hours, fairly shook not only my equanimity, but the pencil which i held in my hand. he was quite innocent of any intention to do me harm, but he gave me a great fright. "the spiders and bugs which swarm in my observing-houses i have rather an attachment for, but they must not crawl over my recording-paper. rats are my abhorrence, and i learned with pleasure that some poison had been placed under the transit-house. "one gets attached (if the term may be used) to certain midnight apparitions. the aurora borealis is always a pleasant companion; a meteor seems to come like a messenger from departed spirits; and the blossoming of trees in the moonlight becomes a sight looked for with pleasure. "aside from the study of astronomy, there is the same enjoyment in a night upon the housetop, with the stars, as in the midst of other grand scenery; there is the same subdued quiet and grateful seriousness; a calm to the troubled spirit, and a hope to the desponding. "even astronomers who are as well cared for as are those of cambridge have their annoyances, and even men as skilled as they are make blunders. "i have known one of the bonds,[footnote: of the harvard college observatory.] with great effort, turn that huge telescope down to the horizon to make an observation upon a blazing comet seen there, and when he had found it in his glass, find also that it was not a comet, but the nebula of andromeda, a cluster of stars on which he had spent much time, and which he had made a special object of study. "dec. , . they were wonderful men, the early astronomers. that was a great conception, which now seems to us so simple, that the earth turns upon its axis, and a still greater one that it revolves about the sun (to show this last was worth a man's lifetime, and it really almost cost the life of galileo). somehow we are ready to think that they had a wider field than we for speculation, that truth being all unknown it was easier to take the first step in its paths. but is the region of truth limited? is it not infinite?... we know a few things which were once hidden, and being known they seem easy; but there are the flashings of the northern lights--'across the lift they start and shift;' there is the conical zodiacal beam seen so beautifully in the early evenings of spring and the early mornings of autumn; there are the startling comets, whose use is all unknown; there are the brightening and flickering variable stars, whose cause is all unknown; and the meteoric showers--and for all of these the reasons are as clear as for the succession of day and night; they lie just beyond the daily mist of our minds, but our eyes have not yet pierced through it." chapter iii - extracts from diary--rachel--emerson--a hard winter "jan. , . i put some wires into my little transit this morning. i dreaded it so much, when i found yesterday that it must be done, that it disturbed my sleep. it was much easier than i expected. i took out the little collimating screws first, then i drew out the tube, and in that i found a brass plate screwed on the diaphragm which contained the lines. i was at first a little puzzled to know which screws held this diaphragm in its place, and, as i was very anxious not to unscrew the wrong ones, i took time to consider and found i need turn only two. then out slipped the little plate with its three wires where five should have been, two having been broken. as i did not know how to manage a spider's web, i took the hairs from my own head, taking care to pick out white ones because i have no black ones to spare. i put in the two, after first stretching them over pasteboard, by sticking them with sealing-wax dissolved in alcohol into the little grooved lines which i found. when i had, with great labor, adjusted these, as i thought, firmly, i perceived that some of the wax was on the hairs and would make them yet coarser, and they were already too coarse; so i washed my little camel's-hair brush which i had been using, and began to wash them with clear alcohol. almost at once i washed out another wire and soon another and another. i went to work patiently and put in the five perpendicular ones besides the horizontal one, which, like the others, had frizzled up and appeared to melt away. with another hour's labor i got in the five, when a rude motion raised them all again and i began over. just at one o'clock i had got them all in again. i attempted then to put the diaphragm back into its place. the sealing-wax was not dry, and with a little jar i sent the wires all agog. this time they did not come out of the little grooved lines into which they were put, and i hastened to take out the brass plate and set them in parallel lines. i gave up then for the day, but, as they looked well and were certainly in firmly, i did not consider that i had made an entire failure. i thought it nice ladylike work to manage such slight threads and turn such delicate screws; but fine as are the hairs of one's head, i shall seek something finer, for i can see how clumsy they will appear when i get on the eyepiece and magnify their imperfections. they look parallel now to the eye, but with a magnifying power a very little crook will seem a billowy wave, and a faint star will hide itself in one of the yawning abysses. "january . finding the hairs which i had put into my instrument not only too coarse, but variable and disposed to curl themselves up at a change of weather, i wrote to george bond to ask him how i should procure spider lines. he replied that the web from cocoons should be used, and that i should find it difficult at this time of year to get at them. i remembered at once that i had seen two in the library room of the atheneum, which i had carefully refrained from disturbing. i found them perfect, and unrolled them.... fearing that i might not succeed in managing them, i procured some hairs from c.'s head. c. being not quite a year old, his hair is remarkably fine and sufficiently long.... i made the perpendicular wires of the spider's webs, breaking them and doing the work over again a great many times.... i at length got all in, crossing the five perpendicular ones with a horizontal one from c.'s spinning-wheel.... after twenty-four hours' exposure to the weather, i looked at them. the spider-webs had not changed, they were plainly used to a chill and made to endure changes of temperature; but c.'s hair, which had never felt a cold greater than that of the nursery, nor a change more decided than from his mother's arms to his father's, had knotted up into a decided curl!--n.b. c. may expect ringlets. "january . horace greeley, in an article in a recent number of the 'tribune,' says that the fund left by smithson is spent by the regents of that institution in publishing books which no publisher would undertake and which do no good to anybody. now in our little town of nantucket, with our little atheneum, these volumes are in constant demand.... "i do not suppose that such works as those issued by the smithsonian regents are appreciated by all who turn them over, but the ignorant learn that such things exist; they perceive that a higher cultivation than theirs is in the world, and they are stimulated to strive after greater excellence. so i steadily advocate, in purchasing books for the atheneum, the lifting of the people. 'let us buy, not such books as the people want, but books just above their wants, and they will reach up to take what is put out for them.' "sept. , . to know what one ought to do is certainly the hardest thing in life. 'doing' is comparatively easy; but there are no laws for your individual case--yours is one of a myriad. "there are laws of right and wrong in general, but they do not seem to bear upon any particular case. "in chess-playing you can refer to rules of movement, for the chess-men are few, and the positions in which they may be placed, numerous as they are, have a limit. "but is there any limit to the different positions of human beings around you? is there any limit to the peculiarities of circumstances? "here a man, however much of a copyist he may be by nature, comes down to simple originality, unless he blindly follows the advice of some friend; for there is no precedent in anything exactly like his case; he must decide for himself, and must take the step alone; and fearfully, cautiously, and distrustingly must we all take many of our steps, for we see but a little way at best, and we can foresee nothing at all. "september . i read this morning an article in 'putnam's magazine,' on rachel. i have been much interested in this woman as a genius, though i am pained by the accounts of her career in point of morals, and i am wearied with the glitter of her jewelry. night puts on a jewelled robe which few admire, compared with the admiration for marketable jewelry. the new york 'tribune' descends to the rating of the value of those worn by her, and it is the prominent point, or rather it makes the multitude of prominent points, when she is spoken of. "the writer in 'putnam' does not go into these small matters, but he attempts a criticism on acting, to which i am not entirely a convert. he maintains that if an actor should really show a character in such light that we could not tell the impersonation from the reality, the stage would lose its interest. i do not think so. we should draw back, of course, from physical suffering; but yet we should be charmed to suppose anything real, which we had desired to see. if we felt that we really met cardinal wolsey or henry viii. in his days of glory, would it not be a lifelong memory to us, very different from the effect of the stage, and if for a few moments we really _felt_ that we had met them, would it not lift us into a new kind of being? "what would we not give to see julius caesar and the soothsayer, just as they stood in rome as shakspere represents them? why, we travel hundreds of miles to see the places noted for the doings of these old romans; and if we could be made to believe that we met one of the smaller men, even, of that day, our ecstasy would be unbounded. 'a tin pan so painted as to deceive is atrocious,' says this writer. of course, for we are not interested in a tin pan; but give us a portrait of shakspere or milton so that we shall feel that we have met them, and i see no atrocity in the matter. we honor the homes of these men, and we joy in the hope of seeing them. what would be beyond seeing them in life? "october . i saw rachel in 'phèdre' and in 'adrienne.' i had previously asked a friend if i, in my ignorance of acting, and in my inability to tell good from poor, should really perceive a marked difference between rachel and her aids. she thought i should. i did indeed! in 'phèdre,' which i first saw, she was not aided at all by her troupe; they were evidently ill at ease in the greek dress and in greek manners; while she had assimilated herself to the whole. it is founded on the play of euripides, and even to rachel the passion which she represents as phèdre must have been too strange to be natural. hippolytus refuses the love which phèdre offers after a long struggle with herself, and this gives cause for the violent bursts in which rachel shows her power. it was an outburst of passion of which i have no conception, and i felt as if i saw a new order of being; not a woman, but a personified passion. the vehemence and strength were wonderful. it was in parts very touching. there was as fine an opportunity for aricia to show some power as for phèdre, but the automaton who represented aricia had no power to show. oenon, whom i took to be the sister sarah, was something of an actress, but her part was so hateful that no one could applaud her. i felt in reading 'phèdre,' and in hearing it, that it was a play of high order, and that i learned some little philosophy from some of its sentiments; but for 'adrienne' i have a contempt. the play was written by scribe specially for rachel, and the french acting was better done by the other performers than the greek. i have always disliked to see death represented on the stage. rachel's representation was awful! i could not take my eyes from the scene, and i held my breath in horror; the death was so much to the life. it is said that she changes color. i do not know that she does, but it looked like a ghastly hue that came over her pale face. "i was displeased at the constant standing. neither as greeks nor as frenchmen did they sit at all; only when dying did rachel need a chair. they made love standing, they told long stories standing, they took snuff in that position, hat in hand, and rachel fainted upon the breast of some friend from the same fatiguing attitude. "the audience to hear 'adrienne' was very fine. the unitarian clergymen and the divinity students seemed to have turned out. "most of the two thousand listeners followed with the book, and when the last word was uttered on the french page, over turned the two thousand leaves, sounding like a shower of rain. the applause was never very great; it is said that rachel feels this as a boston peculiarity, but she ought also to feel the compliment of so large an audience in a city where foreigners are so few and the population so small compared to that of new york. "nov. , . last night i heard emerson give a lecture. i pity the reporter who attempts to give it to the world. i began to listen with a determination to remember it in order, but it was without method, or order, or system. it was like a beam of light moving in the undulatory waves, meeting with occasional meteors in its path; it was exceedingly captivating. it surprised me that there was not only no commonplace thought, but there was no commonplace expression. if he quoted, he quoted from what we had not read; if he told an anecdote, it was one that had not reached us. at the outset he was very severe upon the science of the age. he said that inventors and discoverers helped themselves very much, but they did not help the rest of the world; that a great man was felt to the centre of the copernican system; that a botanist dried his plants, but the plants had their revenge and dried the botanist; that a naturalist bottled up reptiles, but in return the man was bottled up. "there was a pitiful truth in all this, but there are glorious exceptions. professor peirce is anything but a formula, though he deals in formulae. "the lecture turned at length upon beauty, and it was evident that personal beauty had made emerson its slave many a time, and i suppose every heart in the house admitted the truth of his words.... "it was evident that mr. emerson was not at ease, for he declared that good manners were more than beauty of face, and good expression better than good features. he mentioned that sir philip sydney was not handsome, though the boast of english society; and he spoke of the astonishing beauty of the duchess of hamilton, to see whom hundreds collected when she took a ride. i think in these cases there is something besides beauty; there was rank in that of the duchess, in the case of sydney there was no need of beauty at all. "dec. , . all along this year i have felt that it was a hard year--the hardest of my life. and i have kept enumerating to myself my many trials; to-day it suddenly occurred to me that my blessings were much more numerous. if mother's illness was a sore affliction, her recovery is a great blessing; and even the illness itself has its bright side, for we have joyed in showing her how much we prize her continued life. if i have lost some friends by death, i have not lost all. if i have worked harder than i felt that i could bear, how much better is that than not to have as much work as i wanted to do. i have earned more money than in any preceding year; i have studied less, but have observed more, than i did last year. i have saved more money than ever before, hoping for europe in ." ... miss mitchell from her earliest childhood had had a great desire to travel in europe. she received a very small salary for her services in the atheneum, but small as it was she laid by a little every year. she dressed very simply and spent as little as possible on herself--which was also true of her later years. she took a little journey every year, and could always have little presents ready for the birthdays and christmas days, and for the necessary books which could not be found in the atheneum library, and which she felt that she ought to own herself,--all this on a salary which an ordinary school-girl in these days would think too meagre to supply her with dress alone. in this family the children were not ashamed to say, "i can't afford it," and were taught that nothing was cheap that they could not pay for--a lesson that has been valuable to them all their lives. ".... . deacon greeley, of boston, urged my going to boston and giving some lectures to get money. i told him i could not think of it just now, as i wanted to go to europe. 'on what money?' said he. 'what i have earned,' i replied. 'bless me!' said he; 'am i talking to a capitalist? what a mistake i have made.'" during the time of the prosperity of the town, the winters were very sociable and lively; but when the inhabitants began to leave for more favorable opportunities for getting a livelihood, the change was felt very seriously, especially in the case of an exceptionally stormy winter. here is an extract showing how miss mitchell and her family lived during one of these winters: "jan. , . hard winters are becoming the order of things. winter before last was hard, last winter was harder, and this surpasses all winters known before. "we have been frozen into our island now since the th. no one cared much about it for the first two or three days; the sleighing was good, and all the world was out trying their horses on main street--the racecourse of the world. day after day passed, and the thermometer sank to a lower point, and the winds rose to a higher, and sleighing became uncomfortable; and even the dullest man longs for the cheer of a newspaper. the 'nantucket inquirer' came out for awhile, but at length it had nothing to tell and nothing to inquire about, and so kept its peace. "after about a week a vessel was seen off siasconset, and boarded by a pilot. her captain said he would go anywhere and take anybody, as all he wanted was a harbor. two men whose business would suffer if they remained at home took passage in her, and with the pilot, patterson, she left in good weather and was seen off chatham at night. it was hoped that patterson would return and bring at least a few newspapers, but no more is known of them. our postmaster thought he was not allowed to send the mails by such a conveyance. "yesterday we got up quite an excitement because a large steamship was seen near the haul-over. she set a flag for a pilot, and was boarded. it was found that she was out of course, twenty days from glasgow, bound to new york. what the european news is we do not yet know, but it is plain that we are nearer to europe than to hyannis. christians as we are, i am afraid we were all sorry that she did not come ashore. we women revelled in the idea of the rich silks she would probably throw upon the beach, and the men thought a good job would be made by steamboat companies and wreck agents. "last night the weather was so mild that a plan was made for cutting out the steamboat; all the irishmen in town were ordered to be on the harbor with axes, shovels, and saws at seven this morning. the poor fellows were exulting in the prospect of a job, but they are sadly balked, for this morning at seven a hard storm was raging--snow and a good north-west wind. what has become of the english steamer no one knows, but the wind blows off shore, so she will not come any nearer to us. "inside of the house we amuse ourselves in various ways. f.'s family and ours form a club meeting three times a week, and writing 'machine poetry' in great quantities. occasionally something very droll puts us in a roar of laughter. f., e., and k. are, i think, rather the smartest, though mr. m. has written rather the best of all. at the next meeting, each of us is to produce a sonnet on a subject which we draw by lot. i have written mine and tried to be droll. k. has written hers and is serious. "i am sadly tried by this state of things. i cannot hear from cambridge (the nautical almanac office), and am out of work; it is cloudy most of the time, and i cannot observe; and i had fixed upon just this time for taking a journey. my trunk has been half packed for a month. "january . foreseeing that the thermometer would show a very low point last night, we sat up until near midnight, when it stood one and one-half below zero. the stars shone brightly, and the wind blew freshly from west north-west. "this morning the wind is the same, and the mercury stood at six and one-half below zero at seven o'clock, and now at ten a.m. is not above zero. the coffin school dismissed its scholars. miss f. suffered much from the exposure on her way to school. "the 'inquirer' came out this morning, giving the news from europe brought by the steamer which lies off 'sconset. no coal has yet been carried to the steamer, the carts which started for 'sconset being obliged to return. "there are about seven hundred barrels of flour in town; it is admitted that fresh meat is getting scarce; the streets are almost impassable from the snow-drifts. "k. and i have hit upon a plan for killing time. we are learning poetry--she takes twenty lines of goldsmith's 'traveller,' and i twenty lines of the 'deserted village.' it will take us twenty days to learn the whole, and we hope to be stopped in our course by the opening of the harbor. considering that k. has a fiancé from whom she cannot hear a word, she carries herself very amicably towards mankind. she is making herself a pair of shoes, which look very well; i have made myself a morning-dress since we were closed in. "last night i took my first lesson in whist-playing. i learned in one evening to know the king, queen, and jack apart, and to understand what my partner meant when she winked at me. "the worst of this condition of things is that we shall bear the marks of it all our lives. we are now sixteen daily papers behind the rest of the world, and in those sixteen papers are items known to all the people in all the cities, which will never be known to us. how prices have fluctuated in that time we shall not know--what houses have burned down, what robberies have been committed. when the papers do come, each of us will rush for the latest dates; the news of two weeks ago is now history, and no one reads history, especially the history of one's own country. "i bought a copy of 'aurora leigh' just before the freezing up, and i have been careful, as it is the only copy on the island, to circulate it freely. it must have been a pleasant visitor in the four or five households which it has entered. we have had dr. kane's book and now have the 'japan expedition.' "the intellectual suffering will, i think, be all. i have no fear of scarcity of provisions or fuel. there are old houses enough to burn. fresh meat is rather scarce because the english steamer required so much victualling. we have a barrel of pork and a barrel of flour in the house, and father has chickens enough to keep us a good while. "there are said to be some families who are in a good deal of suffering, for whom the howard society is on the lookout. mother gives very freely to bridget, who has four children to support with only the labor of her hands. "the coffin school has been suspended one day on account of the heaviest storm, and the unitarian church has had but one service. no great damage has been done by the gales. my observing-seat came thundering down the roof one evening, about ten o'clock, but all the world understood its cry of 'stand from under,' and no one was hurt. several windows were blown in at midnight, and houses shook so that vases fell from the mantelpieces. "the last snow drifted so that the sleighing was difficult, and at present the storm is so smothering that few are out. a. has been out to school every day, and i have not failed to go out into the air once a day to take a short walk. "january . we left the mercury one below zero when we went to bed last night, and it was at zero when we rose this morning. but it rises rapidly, and now, at eleven a.m., it is as high as fifteen. the weather is still and beautiful; the english steamer is still safe at her moorings. "our little club met last night, each with a sonnet. i did the best i could with a very bad subject. k. and e. rather carried the honors away, but mr. j. m.'s was very taking. our 'crambo' playing was rather dull, all of us having exhausted ourselves on the sonnets. we seem to have settled ourselves quietly into a tone of resignation in regard to the weather; we know that we cannot 'get out,' any more than sterne's starling, and we know that it is best not to fret. "the subject which i have drawn for the next poem is 'sunrise,' about which i know very little. k. and i continue to learn twenty lines of poetry a day, and i do not find it unpleasant, though the 'deserted village' is rather monotonous. "we hear of no suffering in town for fuel or provisions, and i think we could stand a three months' siege without much inconvenience as far as the physicals are concerned. "january . the ice continues, and the cold. the weather is beautiful, and with the thermometer at fourteen i swept with the telescope an hour and a half last night, comfortably. the english steamer will get off to-morrow. it is said that they burned their cabin doors last night to keep their water hot. many people go out to see her; she lies off 'sconset, about half a mile from shore. we have sent letters by her which, i hope, may relieve anxiety. "k. bought a backgammon board to-day. clifford [the little nephew] came in and spent the morning. "january . we have had now two days of warm weather, but there is yet no hope of getting our steamboat off. day before yesterday we went to 'sconset to see the english steamer. she lay so near the shore that we could hear the orders given, and see the people on board. when we went down the bank the boats were just pushing from the shore, with bags of coal. they could not go directly to the ship, but rowed some distance along shore to the north, and then falling into the ice drifted with it back to the ship. when they reached her a rope was thrown to them, and they made fast and the coal was raised. we watched them through a glass, and saw a woman leaning over the side of the ship. the steamer left at five o'clock that day. "it was worth the trouble of a ride to 'sconset to see the masses of snow on the road. the road had been cleared for the coal-carts, and we drove through a narrow path, cut in deep snow-banks far above our heads, sometimes for the length of three or four sleighs. we could not, of course, turn out for other sleighs, and there was much waiting on this account. then, too, the road was much gullied, and we rocked in the sleigh as we would on shipboard, with the bounding over hillocks of snow and ice. "now, all is changed: the roads are slushy, and the water stands in deep pools all over the streets. there is a dense fog, very little wind, and that from the east. the thermometer above thirty-six. "[mails arrived february , and our steamboat left february .]" chapter iv southern tour in miss mitchell made a tour in the south, having under her charge the young daughter of a western banker. "march , . i left meadville this morning at six o'clock, in a stage-coach for erie. i had, early in life, a love for staging, but it is fast dying out. nine hours over a rough road are enough to root out the most passionate love of that kind. "our stage was well filled, but in spite of the solid base we occasionally found ourselves bumping up against the roof or falling forward upon our opposite neighbors. "stage-coaches are, i believe, always the arena for political debate. to-day we were all on one side, all buchanan men, and yet all anti-slavery. it seemed reasonable, as they said, that the south should cease to push the slave question in regard to kansas, now that it has elected its president. "when i took the stage out to meadville on the 'mud-road,' it was filled with fremont men, and they seemed to me more able men, though they were no younger and no more cultivated. "march . i believe any one might travel from maine to georgia and be perfectly ignorant of the route, and yet be well taken care of, mainly from the good-nature in every one. "i found from nantucket to chicago more attention than i desired. i had a short seat in one of the cars, through the night. i did not think it large enough for two, and so coiled myself up and went to sleep. there were men standing all around. once one of them came along and said something about there being room for him on my seat. another man said, 'she's asleep, don't disturb her.' i was too selfish to offer the half of a short seat, and too tired to reason about the man's being, possibly, more tired than i. "i was invariably offered the seat near the window that i might lean against the side of the car, and one gentleman threw his shawl across my knees to keep me warm (i was suffering with heat at the time!). another, seeing me going to chicago alone, warned me to beware of the impositions of hack-drivers; telling me that i must pay two dollars if i did not make a bargain beforehand. i found it true, for i paid one dollar for going a few steps only. "one peculiarity in travelling from east to west is, that you lose the old men. in the cars in new england you see white-headed men, and i kept one in the train up to new york, and one of grayish-tinted hair as far as erie; but after cleveland, no man was over forty years old. "for hundreds of miles the prairie land stretches on the illinois central railroad between chicago and st. louis. it may be pleasant in summer, but it is a dreary waste in winter. the space is too broad and too uniform to have beauty. the girdle of trees would be pretty, doubtless, if seen near, but in the distance and in winter it is only a black border to a brown plain. "the state of illinois must be capitally adapted to railroads on account of this level, and but little danger can threaten a train from running off of the track, as it might run on the soil nearly as well as on the rails. "our engine was uncoupled, and had gone on for nearly half a mile without the cars before the conductor perceived it. "the time from chicago to st. louis is called fifteen hours and a quarter; we made it twenty-three. "if the prairie land is good farming-land, illinois is destined to be a great state. if its people will think less of the dollar and more of the refinements of social life and the culture of the mind, it may become the great state of the union yet. "march . planter's hotel, st. louis. we visited mercantile hall and the library. the lecture-room is very spacious and very pretty. no gallery hides the frescoed walls, and no painful economy has been made of the space on the floor. " th. i begin to perceive the commerce of st. louis. we went upon the levee this morning, and for miles the edge was bordered with the pipes of steamboats, standing like a picket-fence. then we came to the wholesale streets, and saw the immense stores for dry-goods and crockery. "to-day i have heard of a scientific association called the 'scientific academy of st. louis,' which is about a year old, and which is about to publish a volume of transactions, containing an account of an artesian well, and of some inscriptions just sent home from nineveh, which mr. gust. seyffarth has deciphered. "mr. seyffarth must be a remarkable man; he has translated a great many inscriptions, and is said to surpass champollion. he has published a work on egyptian astronomy, but no copy is in this country. "dr. pope, who called on me, and with whom i was much pleased, told me of all these things. western men are so proud of their cities that they spare no pains to make a person from the eastern states understand the resources, and hopes, and plans of their part of the land. "rev. dr. eliot i have not seen. he is about to establish a university here, for which he has already $ , , and the academic part is already in a state of activity. "rev. mr. staples tells me that dr. eliot puts his hands into the pockets of his parishioners, who are rich, up to the elbows. "altogether, st. louis is a growing place, and the west has a large hand and a strong grasp. "doctor seyffarth is a man of more than sixty years, gray-haired, healthy-looking, and pleasant in manners. he has spent long years of labor in deciphering the inscriptions found upon ancient pillars, egyptian and arabic, dating five thousand years before christ. i asked him if he found the observations continuous, and he said that he did not, but that they seem to be astrological pictures of the configuration of the planets, and to have been made at the birth of princes. "he has just been reading the slabs sent from nineveh by mr. marsh; their date is only about five hundred years b.c. "mr. seyffarth's published works amount to seventy, and he was surprised to find a whole set of them in the astor library in new york. "march . we came on board of the steamer 'magnolia,' this morning, in great spirits. we were a little late, and miss s. rushed on board as if she had only new orleans in view. i followed a little more slowly, and the brigadier-general came after, in a sober and dignified manner. "we were scarcely on board when the plank was pulled in, and a few minutes passed and we were afloat on the mississippi river. miss s. and myself were the only lady passengers; we had, therefore, the whole range of staterooms from which to choose. each could have a stateroom to herself, and we talked in admiration of the pleasant times we should have, watching the scenery from the stateroom windows, or from the saloon, reading, etc. "we started off finely. i, who had been used only to the rough waters of the atlantic coast, was surprised at the steady gliding of the boat. i saw nothing of the mingling of the waters of the missouri and the mississippi of which i had been told. perhaps i needed somebody to point out the difference. "the two banks of the river were at first much alike, but after a few hours the left bank became more hilly, and at intervals presented bluffs and rocks, rude and irregular in shape, which we imagined to be ruins of some old castle. "at intervals, too, we passed steamers going up to st. louis, all laden with passengers. we exulted in our majestic march over the waters. i thought it the very perfection of travelling, and wished that all my family and all my friends were on board. "i wondered at the stupidity of the rest of the world, and thought that they ought all to leave the marts of business, to step from the desk, the counting-room, and the workshop on board the 'magnolia,' and go down the length of the 'father of waters.' "and so they would, i suppose, but for sand-bars. here we are five hours out, and fast aground! we were just at dinner, the captain making himself agreeable, the dinner showing itself to be good, when a peculiar motion of the boat made the captain heave a sigh--he had been heaving the lead all the morning. 'ah,' he said, 'just what i feared; we've got to one of those bad places, and we are rubbing the bottom.' "i asked very innocently if we must wait for the tide, and was informed that there was no tide felt on this part of the river. miss s. turned a little pale, and showed a loss of appetite. i was a little bit moved, but kept it to myself and ate on. "as soon as dinner was over, we went out to look at the prospect of affairs. we were close into the land, and could be put on shore any minute; the captain had sent round a little boat to sound the waters, and the report brought back was of shallow water just ahead of us, but more on the right and left. "while we stood on deck a small boat passed, and a sailor very gleefully called out the soundings as he threw the lead, 'eight and a half-nine.' "but we are still high and dry now at two o'clock p.m. they are shaking the steamer, and making efforts to move her. they say if she gets over this, there is no worse place for her to meet. "i asked the captain of what the bottom is composed, and he says, 'of mud, rocks, snags, and everything.' "he is now moving very cautiously, and the boat has an unpleasant tremulous motion. "march . latitude about thirty-eight degrees. we are just where we stopped at noon yesterday--there is no change, and of course no event. one of our crew killed a 'possum yesterday, and another boat stopped near us this morning, and seems likely to lie as long as we do on the sand-bar. "we read shakspere this morning after breakfast, and then betook ourselves to the wheel-house to look at the scenery again. while there a little colored boy came to us bearing a waiter of oranges, and telling us that the captain sent them with his compliments. we ate them greedily, because we had nothing else to do. " st. still the sand-bar. no hope of getting off. we heard the pilot hail a steamboat which was going up to st. louis, and tell them to send on a lighter, and i suppose we must wait for that.... it is my private opinion that this great boat will not get off at all, but will lie here until she petrifies.... "march . we left the 'magnolia' after four days and four hours upon the sand-bar near turkey island, upon seeing the 'woodruff' approach. we left in a little rowboat, and it seemed at first as if we could not overtake the steamer; but the captain saw us and slackened his speed. "miss s. and i clutched hands in a little terror as our small boat seemed likely to run under the great steamer, but our oarsmen knew their duty and we were safely put on board of the 'woodruff.' "march . we stopped at cairo at eight o'clock this morning. mr. s. went on shore and brought newspapers on board. the cairo paper i do not think of high order. i saw no mention in it of the detention of the 'magnolia'! "march . yesterday we count as a day of events. it began to look sunny on the banks, especially on the kentucky side, and miss s. and i saw cherry-blossoms. we remembered the eclipse, and mr. s. having brought with him a piece of broken glass from one of the windows of the 'magnolia,' i smoked it over a piece of candle which i had brought from room no. of the planter's house at st. louis, and we prepared to see the eclipse. "i expected to see the moon on at five o'clock and twenty minutes, but as i had no time i could not tell when to look for it. "it was not on at that time by my watch, but in ten minutes after was so far on that i think my time cannot be much wrong. "it was a little cloudy, so that we saw the sun only 'all flecked with bars,' and caught sight of the phenomenon at intervals. "we were at a coal-landing at the time, and not far from madrid. the boat stopped so long to take in an immense pile of corn-bags that our passengers went on shore--such of them as could climb the slippery bank. "when we saw them coming back laden with peach-blossoms, and saw the little children dressing their hats with them, we were seized with a longing for them, and mr. s. offered to go and get us some; we begged to go too, but he objected. "we were really envious of his good luck when we saw him jump into a country wagon, drawn by oxen which trotted off like horses, and, waving his handkerchief to us, ride off in great glee. he came back with an armful of peach-tree branches. whose orchard he robbed at our instigation i cannot say. a little girl, the daughter of the captain, pulled some blossoms open, and showed us that the fruit germs were not dead, but would have become peaches if we had not coveted them. "the th was also our first night steam-boating. after passing cairo the river is considered safe for night travel, and the boat started on her way at . p.m. we had been out about half an hour when a lady who was playing cards threw down her cards and rushed with a shriek to her stateroom. i perceived then that there had been a peculiar motion to the boat and that it suddenly stopped. we found that one of the paddle-wheels was caught in a snag, but there was no harm done. it made us a little nervous, but we slept well enough after it. "when i look out upon the river, i wonder that boats are not continually snagged. little trees are sticking up on all sides, and sometimes we seem to be going over a meadow and pushing among rushes. "a yawl, which was sent out yesterday to sound, was snagged by a stump which was high out of water; probably they were carried on to it by a current. the little boat whirled round and round, and the men were plainly frightened, for they dropped their oars and clutched the sides of the boat. they got control, however, in a few minutes, and had the jeers of the men left on the steamer for their pains. "march . we stopped at natchez before breakfast this morning, and, having half an hour, we took a carriage and drove through the city. it was like driving through a succession of gardens: roses were hanging over the fences in the richest profusion, and the arbor-vitae was ornamenting every little nook, and adorning every cottage. "natchez stands on a high bluff, very romantic in appearance; jagged and rugged, as if volcanoes had been at work in a time long past, for tall trees grew in the ravines. "most of our lady passengers are, like ourselves, on a tour of pleasure; six of them go with us to the st. charles hotel. some are from keokuk, ia., and i think i like these the best. one young lady goes ashore to spend some time on a plantation, as a governess. she looks feeble, and we all pity her. "to-day we pass among plantations on both sides of the river. we begin to see the live-oak--a noble tree. the foliage is so thick and dark that i have learned to know it by its color. the magnolia trees, too, are becoming fragrant. "march . we are at length in new orleans, and up three flights at the st. charles, in a dark room. "the peculiarities of the city dawn upon me very slowly. i first noticed the showy dress of the children, then the turbaned heads of the black women in the streets, and next the bouquet-selling boys with their french phrases. "april . this morning we went to a slave market. it looked on first entrance like an intelligence office. men, women, and children were seated on long benches parallel with each other. all rose at our entrance, and continued standing while we were there. we were told by the traders to walk up and down the passage between them, and talk with them as we liked. as mr. s. passed the men, several lifted their hands and said, 'here's the boy that will suit you; i can do any kind of work.' some advertised themselves with a good deal of tact. one woman pulled at my shawl and asked me to buy her. i told her that i was not a housekeeper. 'not married?' she asked.--'no.'--'well, then, get married and buy me and my husband.' "there was a girl among them whiter than i, who roused my sympathies very much. i could not speak to her, for the past and the future were too plainly told in her face. i spoke to another, a bright-looking girl of twelve. 'where were you raised?'--'in kentucky.'--'and why are you to be sold?'--'the trader came to kentucky, bought me, and brought me here.' i thought what right had i to be homesick, when that poor girl had left all her kindred for life without her consent. "i could hold my tongue and look around without much outward show of disgust, but to talk pleasantly to the trader i could not consent. he told me that he had been brought up in the business, but he thought it a pity. "no buyers were present, so there was no examination that was painful to look upon. "the slaves were intelligent-looking, and very healthy and neat in appearance. those who belonged to one owner were dressed alike--some in striped pink and white dresses, others in plaid, all a little showy. the men were in thick trousers and coarse dark-blue jackets. "april . we have been this morning to a negro church. we found it a miserable-looking house, mostly unpainted and unplastered, but well filled with the swarthy faces. they were singing when we entered; we were pointed to a good seat. "there may have been fifty persons present, all well dressed; the women in the fanciful checkered headdresses so much favored by the negro race, the men in clean collars, nankin trousers, and dark coats. all showed that they were well kept and well fed. "the audience was increased by new comers frequently, and these, whatever the exercise might be, shook hands with those around them as they seated themselves, and joined immediately in the services. the singing was by the whole congregation, the minister lining out the hymns as in the early times in new england. "several persons carried on the exercises from the pulpit, and in the prayers and sermon the audience took an active part, responding in groans, 'oh, yes,' or 'amen,' sometimes performing a kind of chant to accompany the words.... a negro minister said in his prayer, 'o god, we are not for much talking.' i was delighted at the prospect of a short discourse, but i found his 'not much talking' exactly corresponded to 'a good deal' in my use of words. he talked for a full hour. "there was something pleasing in the earnestness of the preacher and the sympathetic feeling of the audience, but their peculiar condition was not alluded to, and probably was not felt. "the discourse was almost ludicrous at times, and at times was pathetic. i saved up a few specimens: "'o god, you have said that where one or two are gathered together in your name, there will you be; if anything stands between us that you can't come, put it aside.' "'god wants a kingdom upon earth with which he can coin-cide, and that kingdom are your heart.' "'god is near you when you are at the wash-tub or the ironing-table.' "'brethren, i thought last sabbath i wouldn't live to this; a man gets such a notion sometimes.' "april , alabama river. some lessons we of the north might learn from the south, and one is a greater regard for human life. i asked the captain of our boat if they had any accidents in these waters. he said, 'we don't kill people at the south, we gave that up some years ago; we leave it to the north, and the north seems to be capable of doing it.' "the reason for this is, that they are in no hurry. the southern character is opposed to haste. safety is of more worth than speed, and there is no hurry. "every one at the south introduces its 'peculiar institution' into conversation. "they talk as i expected southern people of intelligence to talk; they lament the evil, and say, 'it is upon us, what can we do? to give them freedom would be cruel.' "southerners fall back upon the bible at once; there is more of the old-fashioned religion at the south than at the north; that is, they are not intellectual religionists. they are shocked by the irreligion of massachusetts, and by theodore parker. they read the bible, and can quote it; they are ready with it as an argument at every turn. i am of course not used to the warfare, and so withdraw from the fight. "one argument which three persons have brought up to me is the superior condition of the blacks now, to what it would have been had their parents remained in africa, and they been children of the soil. i make no answer to this, for if this is an argument, it would be our duty to enslave the heathen, instead of attempting to enlighten them. "we hear some anecdotes which are amusing. a judge smith, of south carolina, moved to alabama, and became a prominent man there. he was sent to the senate. he was violently opposed by a young man who said that but for his gray hair he would challenge him. judge smith said, 'you are not the first coward who has taken shelter beneath my gray hairs.' "the same judge smith, when a proposition came before the senate to build a state penitentiary, said, 'wall in the city of mobile; you will have your penitentiary and its inmates.' "so far i have found it easier to travel without an escort south and west than at the north; that is, i have more care taken of me. every one is courteous, too, in speech. i know that they cannot love massachusetts, but they are careful not to wound my feelings. they acknowledge it to be the great state in education; they point to a pretty village and say, 'almost as neat as a new england village.' "savannah, april .... to-day we left town at ten o'clock for a drive in any direction that we liked. mr. f. and i went in a buggy, and miss s. cantered behind us on her horse. "the road that we took led to some rice plantations ten miles out of the city. our path was ornamented by the live-oaks, cedar trees, the dogwood, and occasionally the mistletoe, and enlivened sometimes by the whistle of the mocking-bird. down low by the wheels grew the wild azalea and the jessamine. above our heads the spanish moss hung from the trees in beautiful drapery. "by mistake we drove into the plantation grounds of mr. gibbons, a man of wealth, who is seldom on his lands, and where the avenues are therefore a little wild, and the roads a little rough. "we came afterwards upon a road leading under the most magnificent oaks that i ever saw. i felt as if i were under the arched roof of some ancient cathedral. "the trees were irregularly grouped and of immense size, throwing their hundreds of arms far upon the background of heaven, and bearing the drapery of the spanish moss fold upon fold, as if they sought to keep their raiment from touching the earth. i was perfectly delighted, and think it the finest picture i have yet seen. "retracing our steps, we sought the plantation of mr. potter--a very different one from that of mr. gibbons, as all was finish and neatness; a fine mansion well stored with books, and some fine oaks, some of which mr. potter had planted himself. "mr. potter walked through the fields with us, and, stopping among the negro huts, he said to a little boy, 'call the children and give us some singing.' the little boy ran off, shouting, 'come and sing for massa;' and in a few minutes the little darkies might be seen running through the fields and tumbling over the fences in their anxiety to get to us, to the number of eighteen. "they sat upon the ground around us and began their song. the boy who led sang 'early in the morning,' and the other seventeen brought in a chorus of 'let us think of jesus.' then the leader set up something about 'god almicha,' to which the others brought in another chorus. "they were a dirty and shabby looking set, but as usual fat, even to the little babies, whom the larger boys were tending. one little girl as she passed mr. potter carelessly put her hand in his and said, 'good morning, massa.' "mrs. g. tells me an anecdote which shows the southern sentiment on the one subject. the ladies of charleston were much pleased with miss murray, and got up for her what they called a murray testimonial, a collection of divers pretty things made by their own hands. the large box was ready to be sent to england, but alas for miss murray! while they were debating in what way it should be sent to ensure its reaching her without cost to herself, in an unwise moment she sent twenty-five dollars to 'bleeding kansas,' and the fit of good feeling towards her ebbed; the 'testimonial' remains unsent. "april , charleston. this place is somewhat like boston in its narrow streets, but unlike boston in being quiet; as is all the south. quiet and moderation seem to be the attributes of southern cities. you need not hurry to a boat for fear it will leave at the hour appointed; it never does. "we took a carriage and drove along the battery. the snuff of salt air did me good. "then we went on to a garden of roses, owned and cultivated by a colored woman. she has some twenty acres devoted to flowers and vegetables, and she owns twenty 'niggers.' the universal term for slaves is 'niggers.' 'nigger, bring that horse,' 'nigger, get out of the way,' will be said by the finest gentleman, and 'my niggers' is said by every one. "i do not believe that the slaves are badly treated; there may be cases of it, but i have seen them only sleek, fat, and lazy. "the old buildings of charleston please me exceedingly. the houses are built of brick, standing end to the street, three stories in height, with piazza above piazza at the side; with flower gardens around, and magnolias at the gates; the winding steps to the mansions festooned with roses. "i have just called on miss rutledge, who lives in the second oldest house in the city; herself a fine specimen of antiquity, in her double-ruffled cap and plaided black dress; she chatted away like a young person, using the good old english. "april . to-day mr. capers called on me. i was pleased with the account he gave me of his college life, and of a meeting held by his class thirty years after they graduated. some thirty of them assembled at the revere house in boston; they spread a table with viands from all sections of the country. mr. capers sent watermelons, and another gentleman from kentucky sent the wines of his state. "they sat late at table; they renewed the old friendships and talked over college scenes, and when it was near midnight some one proposed that each should give a sketch of his life, so they went through in alphabetical order. "adams was the first. he said, 'you all remember how i waited upon table in commons. you know that i afterwards went through college, but you do not know that to this man [and he pointed to a classmate] i was indebted for the money that paid for my college course.' "anderson was the second, and he told of his two wives: of the first, much; of the second, little. bowditch came next, and he said he would tell of anderson's second wife, who was a miss lockworth, of lexington, ky. "anderson, a widower, and his brother went to lexington, carrying with them a letter of introduction to the father of the young lady. "while the brother was making an elaborate toilet, anderson strolled out, and came, in his walk, upon a beautiful residence, and saw, within the enclosure, some inviting grounds. he stopped and spoke to the porter, and found it was mr. lockworth's. he told the porter that he had letters to mr. lockworth, and was intending to call upon him. the porter was very communicative, and told him a good deal. anderson asked if there were not a pretty daughter. the porter asked him to walk around. as he entered the gate he reached a dollar to the man, and, being much pleased, when he came out he reached the porter another dollar. "anderson went back to the hotel, told his brother about it, and they set out together to deliver the letter. the brother knew mr. lockworth, and as they met him in the parlor, he walked up, shook hands with him, and asked to present his brother, lars anderson. 'no introduction is necessary,' said mr. lockworth; and putting his hand into his pocket, drawing out the two dollars, he added, 'i am already in your debt just this sum!' the 'pretty daughter' was sitting upon the sofa. "mr. capers told me that their autobiographies drew smiles and tears alternately; they continued till one o'clock; then one of the class said, 'brothers, do you know that not a wineglass has yet been turned up, not a drop of wine drunk? and all were at once so impressed with the conviction that they had all been lifted above the needs of the flesh that they refused to drink, and one of the clergymen of the class kneeling in prayer, they all knelt at once, even to some idle spectators who were looking on. "april . nothing can exceed the hospitality shown to us. we have several invitations for each day, and calls without limit. "i had heard mrs. holbrook described as a wonder, and i found her a very pleasing woman, all ready to talk, and talking with a richness of expression which shows a full mind. mrs. holbrook was a rutledge, and it was amusing, after seeing her, to open miss bremer's 'homes of the new world,' and read her extravagant comments. miss bremer was certainly made happy at belmont. "april . to-day i have been to see miss pinckney. she is the last representative of her name, is over eighty, and still retains the animation of youth, though somewhat shaken in her physical strength by age. i found her sitting in an armchair, her feet resting upon a cushion, surrounded by some half-dozen callers. "she rose at once when i entered, and insisted upon my occupying her seat, while she took a less comfortable one. "the walls of the room were ornamented with portraits of major-general pinckney by stuart, stuart's washington, one by morris of general thomas pinckney, and a portrait of miss pinckney's mother. "miss pinckney is a very plain woman, but much beloved for her benevolence. "it is said that on looking over her diary which she keeps, recording the reasons for her many gifts to her friends and to her slaves, such entries as these will be found: "'$---- to mary, because she is married.' "'$---- to julia, because she has no husband.' "miss pinckney showed me among her centre-table ornaments a miniature of washington; one of her grandmother, of exceeding beauty; one of each of the pinckneys whose portraits are on the walls. "charleston is full of ante-revolution houses, and they please me. they were built when there was no hurry; they were built to last, and they have lasted, and will yet last for the children of their present possessors. "nothing can be happier in expression than the faces of the colored children. they have what must be the ease of the lower classes in a despotic country. the slaves have no care, no ambition; their place is a fixed one--they know it, and take all the good they can get. the children are fat, sleek, and, inheriting no nervous longings from their parents, are on a constant grin--at play with loud laughs and high leaps. "may . it does not follow because the slaves are sleek and fat and really happy--for happy i believe they are--that slavery is not an evil; and the great evil is, as i always supposed, in the effect upon the whites. the few southern gentlemen that i know interest me from their courtesy, agreeable manners, and ready speech. they also strike me as childlike and fussy. i catch myself feeling that i am the man and they are women; and i see this even in the captain of a steamer. then they all like to talk sentiment--their religion is a feeling. "may . the negroes are remarkable for their courtesy of manner. those who belong to good families seem to pride themselves upon their dress and style. "a lady walking in charleston is never jostled by black or white man. the white man steps out of her way, the black man does this and touches his hat. the black woman bows--she is distinguished by her neat dress, her clean plaid head-dress, and her upright carriage. it would be well for some of our young ladies to carry burdens on their heads, even to the risk of flattening the instep, if by that means they could get the straight back of a slave. "mrs. w., who takes us out to drive, comes with her black coachman and a little boy. the coachman wears white gloves, and looks like a gentleman. the little boy rings door-bells when we stop. "when it rained the other day, mrs. w. dropped the window of the carriage, and desired the two to put on their shawls, for fear they would take cold. they are plainly a great care to their owners, for they are like children and cannot take care of themselves; and yet in another way the masters are like children, from the constant waiting upon that they receive. one would think, where one class does all the thinking and the other all the working, that masters would be active thinkers and slaves ready workers; but neither result seems to happen--both are listless and inactive. "may . i asked miss pinckney to-day if she remembered george washington. she and mrs. poinsett spoke at once. "'oh, yes, we were children,' said mrs. poinsett; 'but my father would have him come to see us, and he took each of us in his arms and kissed us; and at another time we went to mt. vernon and made him a visit.' "never were more intelligent old ladies than mrs. poinsett and miss pinckney. the latter stepped around like a young girl, and brought a heavy book to show me the sketch of her sister, marie henrietta pinckney, who, in the nullification time of , wrote a pamphlet in defence of the state. "miss pinckney's father was the originator of the celebrated maxim, 'millions for defence, but not one cent for tribute.' their house was the headquarters for the nullifiers, and they had serenades, she said, without number. "it was pleasant to hear the old ladies chatter away, and it was interesting to think of the distinguished men who had been under that roof, and of the cultivated and beautiful women who had adorned the mansion. "miss pinckney, when i left, followed me to the door, and put into my hands an elegant little volume of poems, called 'reliquiai.' "they seem to be simple effusions of some person who died early. "may . we left charleston, its old houses and its good people, on monday, and reached augusta the same day. "augusta is prettily laid out, but the place is of little interest; and for the hotel where we stayed, i can only give this advice to its inmates: 'don't examine a black spot upon your pillow-case; go to sleep at once, and keep asleep if you can.' "when we were on the road from augusta to atlanta, the conductor said, 'if you are going on to nashville, you will be on the road in the night; people don't love to go on that road in the night. i don't know why.' "when we came to the nashville road, i thought that i knew 'why.' the road runs around the base of a mountain, while directly beneath it, at a great depth, runs a river. a dash off the track on one side would be against the mountain, on the other side would be into the river, while the sharp turns seem to invite such a catastrophe. when we were somewhat wrought up to a nervous excitement, the cars would plunge into the darkness of a tunnel--darkness such as i almost felt. "it was a picturesque but weary ride, and we were tired and hungry when we reached nashville. "may . to-day we have been out for a two-hours' drive. it is warm, cloudy, and looks like a tempest; we are too tired for much effort. "mrs. fogg, of nashville, took us to call on the widow of president polk. we found her at home, though apparently just ready for a walk. she is still in mourning, and tells me that she has not travelled fifty miles from home in the last eight years. "she spoke to me of governor briggs (of massachusetts), an old friend; of professor hare; and said that among her cards, on her return from a journey some years ago, she found charles sumner's; and forgetting at the moment who he was, she asked the servant who he was. 'the abolitionist senator from massachusetts--i asked him in,' was the reply. "mrs. polk talks readily, is handsome, elegant in figure, and shows at once that she is well read. she told me that she reads all the newspaper reports of the progress of science. she lives simply, as any new england woman would, though her house is larger than most private residences. "mrs. fogg told me many anecdotes of dorothea dix. that lady was, at one time, travelling alone, and was obliged to stop at some little village tavern. as she lay half asleep upon the sofa, the driver of the stage in which she was to take passage came into the room, approached her, and held a light to her closed eyes. she did not dare to move nor utter a sound, but when he turned away she opened her eyes and watched him. he went to the mail-bags, opened them, took out the letters, hastily broke the seals, took out money enclosed, put it into his pocket, closed the bags, and again approached her with his lamp. she shut her eyes and pretended to sleep again; then at the proper time entered the stage and pursued her journey. at the end of the journey she reported his conduct to the proper authorities. "i was a little doubtful about the propriety of going to the mammoth cave without a gentleman escort, but if two ladies travel alone they must have the courage of men. so i called the landlord as soon as we arrived at the cave house, and asked if we could have mat, who i had been told was the best guide now that stephen is ill. the landlord promised mat to me for two days. after dinner we made our first attempt. "the ground descends for some two hundred feet towards the mouth of the cave; then you come to a low hill, and you descend through a small aperture not at all imposing, in front of which trickles a little stream. for some little while we needed no light, but soon the guide lighted and gave to each of us a little lamp. mat took the lead, i came next, miss s. followed, and an old slave brought up in the rear. "i confess that i shuddered as i came into the darkness. our lamps, of course, gave but feeble light; we barely saw at first where our feet must step. "i looked up, trying in vain to find the ceiling or the walls. all was darkness. in about an hour we saw more clearly. the chambers are, many of them, elliptical in shape; the ceiling is of mixed dark and white color, and looks much like the sky on a cloudy moonlight evening. "a friend of ours, who has been much in the cave, says, 'if the top were lifted off, and the whole were exposed to view, no woman would ever enter it again.' "we clambered over heaps of rocks, we descended ladders, wound through narrow passages, passed along chambers so low that we crouched for the whole length, entered upon lofty halls, ascended ladders, and crossed a bridge over a yawning abyss. "every nightmare scene that i had ever dreamed of seemed to be realized. i shuddered several times, and was obliged to reason with myself to assure me of safety. occasionally we sat down and rested upon some flat rock. "miss s., who has a great taste for costuming, wound her plaid shawl about her shoulders, turbaned her head with a green veil, swung her lamp upon a stick which she rested upon her shoulder, and then threw herself upon a rock in a most picturesque attitude. the guide took a lower seat, and his dirty tin cup, swung across his breast, looked like an ornament as the light struck it; his swarthy face was bright, and i wondered what our friends at home would give for a picture. "one of these elliptical halls has its ceiling immensely far off, and of the deepest black, until our feeble little lights strike upon innumerable points, when it shines forth like a dark starlight night. the stars are faint, but they look so exceedingly like the heavens that one easily forgets that it is not reality. "the guide asked us to be seated, while he went behind down a descent with the lights, to show us the creeping over of the shadows of the rocks, as if a dark cloud passed over the starlit vault. the black cloud crept on and on as the guide descended, until a fear came over us, and we cried out together to him to come back, not to leave us in total darkness. he begged that he might go still lower and show us entire darkness, but we would not permit it. "guin's dome. what the name means i can't say. the guide tells you to pause in your scrambling over loose stones and muddy soil,--which you are always willing to do,--and to put your head through a circular aperture, and to look up while he lights the bengal light; you obey, and look up upon columns of fluted, snowy whiteness; he tells you to look down, and you follow the same pillars down--up to heights which the light cannot climb, down to depths on which it cannot fall. "you shudder as you look up, and you shudder as you look down. indeed, the march of the cave is a series of shudders. geologists may enjoy it, a large party may be merry in it; but if the 'underground railroad' of the slaves is of that kind, i should rather remain a slave than undertake a runaway trip! "may . to-day we retraced our steps from nashville to chattanooga. it had been raining nearly all night, and we found, when not far from the latter place, that the streams were pouring down from the high lands upon the car-track, so that we came through rivers. when we dashed into the dark tunnel it was darker than ever from the darkness of the day, and it seemed to me that the darkness pressed upon me. i am sure i should keep my senses a very little while if i were confined in a dark place. "as we came out of the tunnel, the water from the hill above dashed upon the cars; and although it did not break the panes of glass, it forced its way through and sprinkled us. "the route, with all its terrors, is beautiful, and the trees are now much finer than they were ten days ago. "may . there is this great difference between niagara and other wonders of the world: that of it you get no idea from descriptions, or even from paintings. of the 'mammoth cave' you have a conception from what you are told; of the natural bridge you get a really truthful impression from a picture. but cave and bridge are in still life. niagara is all activity and change. no picture gives you the varying form of the water or the change of color; no description conveys to your mind the ceaseless roar. so, too, the ocean must be unrepresentable to those who have not looked upon it. "the natural bridge stands out bold and high, just as you expect to see it. you are agreeably disappointed, however, on finding that you can go under the arch and be completely in the coolness of its shade while you look up for two hundred feet to the rocky black and white ceiling above. "one of the prettiest peculiarities is the fringing above of the trees which hang over the edge, and looking out past the arch the wooded banks of the ravine are very pleasant. from above, one has the pain always attendant to me upon looking down into an abyss, but at the same time one obtains a better conception of the depth of the valley. it is well worth seeing, partly for itself, partly because it can be reached only by a ride among the hills of the blue ridge." chapter v first european tour--liverpool--the hawthornes--london--greenwich observatory--admiral smyth--dr. lee shortly after her return from the south, miss mitchell started again for a tour in europe with the same young girl. miss mitchell carried letters from eminent scientific people in this country to such persons as it would be desirable for her to know in europe; especially to astronomers and mathematicians. when miss mitchell went to europe she took her almanac work with her, and what time she was not sight-seeing she was continuing that work. her wisdom in this respect was very soon apparent. she had not been in england many weeks when a great financial crisis took place in the united states, and the father of her young charge succumbed to the general failure. the young lady was called home, but after considering the matter seriously miss mitchell decided to remain herself, putting the young lady into careful hands for the return passage from liverpool. miss mitchell enjoyed the society of the scientific people whom she met in england to her heart's content. she was very cordially received, and the astronomers not only opened their observatories to her, but welcomed her into their family life. on arriving at liverpool, miss mitchell delivered the letters to the astronomers living in or near that city, and visited their observatories. "aug. , . i brought a letter from professor silliman to mr. john taylor, cotton merchant and astronomer; and to-day i have taken tea with him. he is an old man, nearly eighty i should think, but full of life, and talks by the hour on heathen mythology. he was the principal agent in the establishment of the liverpool observatory, but disclaims the honor, because it was established on so small a scale, compared with his own gigantic plan. mr. taylor has invented a little machine, for showing the approximate position of a comet, having the elements. "he has also made additions to the globes made by de morgan, so that they can be used for any year and show the correct rising and setting of the stars. "he struck me as being a man of taste, but of no great profundity. he has a painting which he believes to be by guido; it seemed to me too fresh in its coloring for the sixteenth century. "august , p.m. i put down my pen, because old mr. taylor called, and while he was here rev. james martineau came. mr. martineau is one of the handsomest men i ever saw. he cannot be more than thirty, or if he is he has kept his dark hair remarkably. he has large, bluish-gray eyes, and is tall and elegant in manner. he says he is just packed to move to london. he gave me his london address and hoped he should see me there; but i doubt if he does, for i did not like to tell him my address unless he asked for it, for fear of seeming to be pushing. "august,... i have been to visit mr. lassell. he called yesterday and asked me to dine with him to-day. he has a charming place, about four miles out of liverpool; a pretty house and grounds. "mr. lassell has constructed two telescopes, both on the newtonian plan; one of ten, the other of twenty, feet in length. each has its separate building, and in the smaller building is a transit instrument. "mr. lassell must have been a most indefatigable worker as well as a most ingenious man; for, besides constructing his own instruments, he has found time to make discoveries. he is, besides, very genial and pleasant, and told me some good anecdotes connected with astronomical observations. "one story pleased me very much. our massachusetts astronomer, alvan clark, has long been a correspondent of mr. dawes, but has never seen him. wishing to have an idea of his person, and being a portrait painter, mr. clark sent to mr. dawes for his daguerreotype, and from that painted a likeness, which he has sent out to liverpool, and which is said to be excellent. "mr. lassell looks in at the side of his reflecting telescopes by means of a diagonal eye-piece; when the instrument is pointed at objects of high altitude he hangs a ladder upon the dome and mounts; the ladder moves around with the dome. mr. lassell works only for his own amusement, and has been to malta,--carrying his larger telescope with him,--for the sake of clearer skies. neither mr. lassell nor mr. hartnup [footnote: of the liverpool observatory.] makes regular observations. "the misses lassell, four in number, seem to be very accomplished. they take photographs of each other which are beautiful, make their own picture-frames, and work in the same workshop with their father. one of them told me that she made observations on my comet, supposing it to belong to mr. dawes, who was a friend of hers. "they keep an album of the autographs of their scientific visitors, and among them i saw those of professor young, of dartmouth, and of professor loomis. "august . i have just returned from a visit to the liverpool observatory, under the direction of mr. hartnup. it is situated on waterloo dock, and the pier of the observatory rests upon the sandstone of that region, the telescope is an equatorial; like many good instruments in our country, it is almost unused. "mr. hartnup's observatory is for nautical purposes. i found him a very gentlemanly person, and very willing to show me anything of interest about the observatory; but they make no regular series of astronomical observations, other than those required for the commerce of liverpool. "mr. hartnup has a clock which by the application of an electric current controls the action of other clocks, especially the town clock of liverpool--distant some miles. the current of electricity is not the motive power, but a corrector. "much attention is paid to meteorology. the pressure of the wind, the horizontal motion, and the course are recorded upon sheets of paper running upon cylinders and connected with the clock; the instrument which obeys the voice of the wind being outside. "aug. , . i did not send my letter to mr. hawthorne until yesterday, supposing that he was not in the city; but yesterday when rev. james martineau called on me, he said that he had not yet left. mr. martineau said that it would be a great loss to liverpool when mr. hawthorne went away. "i sent my letter at once; from all that i had heard of mr. hawthorne's shyness, i thought it doubtful if he would call, and i was therefore very much pleased when his card was sent in this morning. mr. hawthorne was more chatty than i had expected, but not any more diffident. he remained about five minutes, during which time he took his hat from the table and put it back once a minute, brushing it each time. the engravings in the books are much like him. he is not handsome, but looks as the author of his books should look; a little strange and odd, as if not of this earth. he has large, bluish-gray eyes; his hair stands out on each side, so much so that one's thoughts naturally turn to combs and hair-brushes and toilet ceremonies as one looks at him." later, when miss mitchell was in paris, alone, on her way to rome, she sent to the hawthornes, who were also in paris, asking for the privilege of joining them, as they too were journeying in the same direction. she says in her diary: "mrs. hawthorne was feeble, and she told me that she objected, but that mr. hawthorne assured her that i was a person who would give no trouble; therefore she consented. we were about ten days on the journey to rome, and three months in rome; living, however, some streets asunder. i saw them nearly every day. like everybody else, i found mr. hawthorne very taciturn. his few words were, however, very telling. when i talked french, he told me it was capital: 'it came down like a sledge-hammer.' his little satirical remarks were such as these: it was march and i took a bunch of violets to rosa; notched white paper was wound around them, and mr. hawthorne said, 'they have on a cambric ruffle." "generally he sat by an open fire, with his feet thrust into the coals, and an open volume of thackeray upon his knees. he said that thackeray was the greatest living novelist. i sometimes suspected that the volume of thackeray was kept as a foil, that he might not be talked to. he shrank from society, but rode and walked." extract from a letter. rome, feb. , . ... the hawthornes are invaluable to me, because the little ones come to my room every day and i go there when i like. mrs. hawthorne sometimes walks with us, mr. h. _never_. he has a horror of sight-seeing and of emotions in general, but i like him very much, and when i say i like _him_ it only means that i like _her_ a little more. julian, the boy, is in love with me. when i was last there mr. h. came home with me; as he put on his coat he turned to julian and said, "julian, i should think with your _tender interest_ in miss mitchell you wouldn't let me escort her home." "we arrived in rome in the evening. mrs. h. was somewhat of an invalid, and mr. hawthorne tried in vain to make the servant understand that she must have a fire in her room. he spoke no word of french, german, or italian, but he said emphatically, 'make a fire in mrs. hawthorne's room.' worn out with his efforts, he turned to me and said, 'do, miss mitchell, tell the servant what i want; your french is excellent! englishmen and frenchmen understand it equally well.' so i said in execrable french, 'make a fire,' and pointed to the grate; of course the gesture was understood. "mr. hawthorne was minutely and scrupulously honest; i should say that he was a rigid temperance man. once i heard mrs. hawthorne say to the clerk, 'send some brandy to mr. hawthorne at once.' we were six in the party. when i paid my bill i heard mr. hawthorne say to miss s., the teacher, who took all the business cares, 'don't let miss mitchell pay for one-sixth of my brandy.' "so if we ordered tea for five, and six partook of it, he called the waiter and said, 'six have partaken of the tea, although there was no tea added; to the amount.' "i told mr. hawthorne that a friend of mine, miss w., desired very much to see him, as she admired him very much. he said, 'don't let her see me, let her keep her little lamp burning.' "he was a sad man; i could never tell why. i never could get at anything of his religious views. "he was wonderfully blest in his family. mrs. hawthorne almost worshipped him. she was of a very serious and religious turn of mind. "i dined with them the day that una was sixteen years old. we drank her health in cold water. mr. hawthorne said, 'may you live happily, and be ready to go when you must.' "he joined in the family talk very pleasantly. one evening we made up a story. one said, 'a party was in rome;' another said, 'it was a pleasant day;' another said, 'they took a walk.' it came to hawthorne's turn, and he said, 'do put in an incident;' so rosa said, 'then a bear jumped from the top of st. peter's!' the story went no further. "i was with the family when they first went to st. peter's. hawthorne turned away saying, 'the st. peter's of my imagination was better.' "i think he could not have been well, he was so very inactive. if he walked out he took rosa, then a child of six, with him. he once came with her to my room, but he seemed tired from the ascent of the stairs. i was on the fifth floor. "i have been surprised to see that he made severe personal remarks in his journal, for in the three months that i knew him i never heard an unkind word; he was always courteous, gentle, and retiring. mrs. hawthorne said she took a wifely pride in his having no small vices. mr. hawthorne said to miss s., 'i have yet to find the first fault in mrs. hawthorne.' "one day mrs. hawthorne came to my room, held up an inkstand, and said, 'the new book will be begun to-night.' "this was 'the marble faun.' she said, 'mr. hawthorne writes after every one has gone to bed. i never see the manuscript until it is what he calls _clothed_'.... mrs. h. says he never knows when he is writing a story how the characters will turn out; he waits for _them_ to influence _him_. "i asked her if zenobia was intended for margaret fuller, and she said, 'no;' but mr. hawthorne admitted that margaret fuller seemed to be around him when he was writing it. "london, august. we went out for our first walk as soon as breakfast was over, and we walked on regent street for hours, looking in at the shop windows. the first view of the street was beautiful, for it was a misty morning, and we saw its length fade away as if it had no end. i like it that in our first walk we came upon a crowd standing around 'punch.' it is a ridiculous affair, but as it is as much a 'peculiar institution' as is southern slavery, i stopped and listened, and after we came into the house miss s. threw out some pence for them. we rested after the shop windows of regent street, took dinner, and went out again, this time to piccadilly. "the servility of the shopkeepers is really a little offensive. 'what shall i have the honor of showing you?' they say. "our chambermaid, at our lodgings, thanks us every time we speak to her. "i feel ashamed to reach a four-penny piece to a stout coachman who touches his hat and begs me to remember him. sometimes i am ready to say, 'how can i forget you, when you have hung around me so closely for half an hour?' "our waiter at the adelphi hotel, at liverpool, was a very respectable middle-aged man, with a white neck-cloth; he looked like a methodist parson. he waited upon us for five days with great gravity, and then another waiter told us that we could give our waiter what we pleased. we were charged £ for 'attendance' in the bill, but i very innocently gave half as much more, as fee to the 'parson,' "august . to-day we took a brougham and drove around for hours. of course we didn't _see_ london, and if we stay a month we shall still know nothing of it, it is so immense. i keep thinking, as i go through the streets, of 'the rats and the mice, they made such a strife, he had to go to london,' etc., and especially 'the streets were so wide, and the lanes were so narrow;' for i never saw such narrow streets, even in boston. "we have begun to send out letters, but as it is 'out of season' i am afraid everybody will be at the watering-places. the greenwich observatory. "the observatory was founded by charles ii. the king that 'never said a foolish thing and never did a wise one' was yet sagacious enough to start an institution which has grown to be a thing of might, and this, too, of his own will, and not from the influence of courtiers. one of the hospital buildings of greenwich, then called the 'house of delights,' was the residence of henrietta maria, and the young prince probably played on the little hill now the site of the observatory. "but charles, though he started an observatory, did not know very well what was needed. the first building consisted of a large, octagonal room, with windows all around; it was considered sufficiently firm without any foundation, and sufficiently open to the heavens with no opening higher than windows. this room is now used as a place of deposit for instruments, and busts and portraits of eminent men, and also as the dancing-hall for the director's family. "under mr. airy's [footnote: the late sir george airy.] direction, the walls of the observing-room have become pages of its history. the transit instruments used by halley, bradley, and pond hang side by side; the zenith sector with which bradley discovered the 'aberration of light,' now moving rustily on its arc, is the ornament of another room; while the shelves of the computing-room are filled with volumes of unpublished observations of flamstead and others. "the observatory stands in greenwich park, the prettiest park i have yet seen; being a group of small hills. they point out oaks said to belong to elizabeth's time--noble oaks of any time. the observatory is one hundred and fifty feet above the sea level. the view from it is, of course, beautiful. on the north the river, the little thames, big with its fleet, is winding around the isle of dogs; on the left london, always overhung with a cloud of smoke, through which st. paul's and the houses of parliament peep. "mr. airy was exceedingly kind to me, and seemed to take great interest in showing me around. he appeared to be much gratified by my interest in the history of the observatory. he is naturally a despot, and his position increases this tendency. sitting in his chair, the zero-point of longitude for the world, he commands not only the little knot of observers and computers around him, but when he says to london, 'it is one o'clock,' london adopts that time, and her ships start for their voyages around the globe, and continue to count their time from that moment, wherever the english flag is borne. "it is singular what a quiet motive-power science is, the breath of a nation's progress. "mr. airy is not favorable to the multiplication of observatories. he predicted the failure of that at albany. he says that he would gladly destroy one-half of the meridian instruments of the world, by way of reform. i told him that my reform movement would be to bring together the astronomers who had no instruments and the instruments which had no astronomers. "mr. airy is exceedingly systematic. in leading me by narrow passages and up steep staircases, from one room to another of the irregular collection of rooms, he was continually cautioning me about my footsteps, and in one place he seemed to have a kind of formula: 'three steps at this place, ten at this, eleven at this, and three again.' so, in descending a ladder to the birthplace of the galvanic currents, he said, 'turn your back to the stairs, step down with the right foot, take hold with the right hand; reverse the operation in ascending; do not, on coming out, turn around at once, but step backwards one step first.' "near the throne of the astronomical autocrat is another proof of his system, in a case of portfolios. these contain the daily bills, letters, and papers, as they come in and are answered in order. when a portfolio is full, the papers are removed and are sewed together. each year's accumulation is bound, and the bound volumes of mr. airy's time nearly cover one side of his private room. "mr. airy replies to all kinds of letters, with two exceptions: those which ask for autographs, and those which request him to calculate nativities. both of these are very frequent. "in the drawing-room mr. airy is cheery; he loves to recite ballads and knows by heart a mass of verses, from 'a, apple pie,' to the 'lady of the lake.' "a lover of nature and a close observer of her ways, as well in the forest walk as in the vault of heaven, mr. airy has roamed among the beautiful scenery of the lake region until he is as good a mountain guide as can be found. he has strolled beside grassmere and ascended helvellyn. he knows the height of the mountain peaks, the shingles that lie on their sides, the flowers that grow in the valleys, the mines beneath the surface. "at one time the government survey planted what is called a 'man' on the top of one of the hills of the lake region. in a dry season they built up a stone monument, right upon the bed of a little pond. the country people missed the little pond, which had seemed to them an eye of nature reflecting heaven's blue light. they begged for the removal of the surveyor's pile, and mr. airy at once changed the station. "the established observatories of england do not step out of their beaten path to make discoveries--these come from the amateurs. in this respect they differ from america and germany. the amateurs of england do a great deal of work, they learn to know of what they and their instruments are capable, and it is done. "the library of greenwich observatory is large. the transactions of learned societies alone fill a small room; the whole impression of the thirty volumes of printed observations fills a wall of another room, and the unpublished papers of the early directors make of themselves a small manuscript library. "october , . we have just returned from our fourth visit to greenwich, like the others twenty-four hours in length. we go again to-morrow to meet the sabines. "herr struve, the director of the pulkova observatory, is at greenwich, with his son karl. the old gentleman is a magnificent-looking fellow, very large and well proportioned; his great head is covered with white hair, his features are regular and handsome. when he is introduced to any one he thrusts both hands into the pockets of his pantaloons, and bows. i found that the son considered this position of the hands particularly _english_. however, the old gentleman did me the honor to shake hands with me, and when i told him that i brought a letter to him from a friend in america, he said, 'it is quite unnecessary, i know you without.' he speaks very good english. "herr struve's mission in england is to see if he can connect the trigonometrical surveys of the two countries. it is quite singular that he should visit england for this purpose, so soon after russia and england were at war. one of his sons was an army surgeon at the crimea. "five visitors remained all night at the observatory. i slept in a little round room and miss s. in another, at the top of a little jutting-out, curved building. mrs. airy says, 'mr. airy got permission of the board of visitors to fit up some of the rooms as lodging-rooms.' mr. airy said, 'my dear love, i did as i always do: i fitted them up first, and then i reported to the board that i had done it.' "october . another dinner-party at the observatory, consisting of the struves, general and mrs. sabine, professor and mrs. powell, mr. main, and ourselves; more guests coming to tea. "mrs. airy told me that she should arrange the order of the guests at table to please herself; that properly all of the married ladies should precede me, but that i was really to go first, with mr. airy. to effect this, however, she must explain it to mrs. sabine, the lady of highest rank. "so we went out, professor airy and myself, professor powell and mrs. sabine, general sabine and mrs. powell, mr. charles struve and miss s., mr. main, mrs. airy, and professor struve. "general sabine is a small man, gray haired and sharp featured, about seventy years old. he smiles very readily, and is chatty and sociable at once. he speaks with more quickness and ease than most of the englishmen i have met. mrs. sabine is very agreeable and not a bit of a blue-stocking. "the chat at table was general and very interesting. mr. airy says, 'the best of a good dinner is the amount of talk.' he talked of the great 'leviathan' which he and struve had just visited, then anecdotes were told by others, then they went on to comic poetry. mr. airy repeated 'the lost heir,' by hood. general sabine told droll anecdotes, and the point was often lost upon me, because of the local allusions. one of his anecdotes was this: 'archbishop whately did not like a professor named robert daly; he said the irish were a very contented people, they were satisfied with one _bob daily_.' i found that a 'bob' is a shilling. "when the dinner was over, the ladies left the room, and the gentlemen remained over their wine; but not for long, for mr. airy does not like it, and struve hates it. "then, before tea, others dropped in from the neighborhood, and the tea was served in the drawing-room, handed round informally. "august . westminster abbey interested me more than i had expected. we went into the chapels and admired the sculpture when the guide told us we ought, and stopped with interest sometimes over some tomb which he did not point out. "i stepped aside reverently when i found i was standing on the stone which covers the remains of dr. johnson. it is cracked across the middle. garrick lies by the side of johnson, and i thought at first that goldsmith lay near; but it is only a monument--the body is interred in temple churchyard. "you are continually misled in this way unless you refer at every minute to your guide-book, and to go through europe reading a guide-book which you can read at home seems to be a waste of time. on the stone beneath which addison lies is engraved the verse from tickell's ode: "'ne'er to these chambers where the mighty rest,' etc. "the base of newton's monument is of white marble, a solid mass large enough to support a coffin; upon that a sarcophagus rests. the remains are not enclosed within. as i stepped aside i found i had been standing upon a slab marked 'isaac newton,' beneath which the great man's remains lie. "on the side of the sarcophagus is a white marble slab, with figures in bas-relief. one of these imaginary beings appears to be weighing the planets on a steel-yard. they hang like peas! another has a pair of bellows and is blowing a fire. a third is tending a plant. "on this sarcophagus reclines a figure of newton, of full size. he leans his right arm upon four thick volumes, probably 'the principia,' and he points his left hand to a globe above his head on which the goddess urania sits; she leans upon another large book. "newton's head is very fine, and is probably a portrait. the left hand, which is raised, has lost two fingers. i thought at first that this had been the work of some 'undevout astronomer,' but when i came to 'read up' i found that at one time soldiers were quartered in the abbey, and probably one of them wanted a finger with which to crowd the tobacco into his pipe, and so broke off one. "august . to-day we have been to the far-famed british museum. i carried an 'open sesame' in the form of a letter given to me by professor henry, asking for me special attention from all societies with which the 'smithsonian' at washington is connected. "i gave the paper first to a police officer; a police officer is met at every turn in london. he handed it to another official, who said, 'you'd better go to the secretary.' "i walked in the direction towards which he pointed, a long way, until i found the secretary. he called another man, and asked him to show me whatever i wanted to see. "this man took me into another room, and consigned me to still another man--the fifth to whom i had been referred. no. was an intelligent and polite person, and he began to talk about america at once. "i asked to see anything which had belonged to newton, and he told me they had one letter only,--from newton to leibnitz,--which he showed me. it was written in latin, with diagrams and formulae interspersed. the reply of leibnitz, copied by newton, was also in their collection, and an order from newton written while he was director of the mint. "no. also showed me the illuminated manuscripts of the collection; they are kept locked in glass-topped cases, and a curtain protects them from the light. we saw also the oldest copy of the bible in the world. "the art of printing has brought incalculable blessings; but as i looked at a neat manuscript book by queen elizabeth, copied from another as a present to her father, i could not help thinking it was much better than worsted work! "a much-worn prayer-book was shown me, said to be the one used by lady jane grey when on the scaffold. nothing makes me more conscious that i am on foreign soil than the constant recurrence of associations connected with the executioner's block. we hung the quakers and we burned the witches, but we are careful not to remember the localities of our barbarisms; we show instead the plymouth rock or the washington elm. "among other things, we were shown the 'magna charta'--a few fragments of worn-out paper on which some words could be traced; now carefully preserved in a frame, beneath a glass. "thus far england has impressed me seriously; i cannot imagine how it has ever earned the name of 'merrie england.' "august . there are four great men whose haunts i mean to seek, and on whose footsteps i mean to stand: newton, shakspere, milton, and johnson. "to-day i told the driver to take me to st. martin's, where the guide-book says that newton lived. he put me down at the newton hotel, but i looked in vain to its top to see anything like an observatory. "i went into a wine-shop near, and asked a girl, who was pouring out a dram, in which house newton lived. she pointed, not to the hotel, but to a house next to a church, and said, 'that's it--don't you see a place on the top? that's where he used to study nights.' "it is a little, oblong-shaped observatory, built apparently of wood, and blackened by age. the house is a good-looking one--it seems to be of stone. the girl said the rooms were let for shops. "next i told the driver to take me to fleet street, to gough square, and to bolt court, where johnson lived and died. "bolt court lies on fleet street, and it is but few steps along a narrow passage to the house, which is now a hotel, where johnson died; but you must walk on farther through the narrow passage, a little fearful to a woman, to see the place where he wrote the dictionary. the house is so completely within a court, in which nothing but brick walls could be seen, that one wonders what the charm of london could be, to induce one to live in that place. but a great city always draws to itself the great minds, and there johnson probably found his enjoyment. "august . we took st. paul's church to-day. we took tickets for the vaults, the bell, the crypt, the whispering-gallery, the clock and all. we did not know what was before us. it was a little tiresome as far as the library and the room of nelson's trophies, but to my surprise, when the guide said, 'go that way for the clock,' he did not take the lead, but pointed up a staircase, and i found myself the pioneer in the narrowest and darkest staircase i ever ascended. it was really perfect darkness in some of the places, and we had to feel our way. we all took a long breath when a gleam of light came in at some narrow windows scattered along. at the top, in front of the clock works, stood a woman, who began at once to tell us the statistics of the pendulum, to which recital i did not choose to listen. she was not to go down with us, and, panting with fatigue and trembling with fright, we groped our way down again. "there was another long, but easy, ascent to the 'whispering-gallery,' which is a fine place from which to look down upon the interior of the church. the man in attendance looked like a respectable elderly gentleman. he told us to go to the opposite side of the gallery, and he would whisper to us. we went around, and, worn out with fatigue, dropped upon a bench. "the man began to whisper, putting his mouth to an opening in the wall; we heard noises, but could not tell what he said. "to my amazement, this very respectable-looking elderly gentleman, as we passed him in going out, whispered again, and as this time he put his mouth close to my ear, i understood! he said, 'if you will give anything for the whisper, it will be gratefully received.' there are notices all over the church forbidding fees, and i felt that the man was a beggar at best--more properly a pickpocket. "a figure of dr. johnson stands in one of the aisles of the church. it must be like him, for it is exceedingly ugly. "september . we have been three weeks in london 'out of season,' but with plenty of letters. at present we have as many acquaintances as we desire. last night we were at the opera, to-night we go out to dine, and to-morrow evening to a dance, the next day to admiral smyth's. "the opera fatigued me, as it always does. i tired my eyes and ears in the vain effort to appreciate it. mario was the great star of the evening, but i knew no difference. "one little circumstance showed me how an american, with the best intentions, may offend against good manners. american-like we had secured very good seats, were in good season, and as comfortable as the very narrow seats would permit us to be, before most of the audience arrived. the house filled, and we sat at our ease, feeling our importance, and quite unconscious that we were guilty of any impropriety. while the curtain was down, i heard a voice behind me say to the gentleman who was with us, 'is the lady on your left with you?'--'yes,' said mr. r.--'she wears a bonnet, which is not according to rule.'--'too late now,' said mr. r.--'it is my fault,' said the attendant; 'i ought not to have admitted her; i thought it was a hood.' "i was really in hopes that i should be ordered out, for i was exceedingly fatigued and should have been glad of some fresh air. on looking around, i saw that only the 'pit' wore bonnets. "september . we left london yesterday for aylesbury. it is two hours by railroad. like all railroads in england, it runs seemingly through a garden. in many cases flowers are cultivated by the roadside. "from aylesbury to stone, the residence of admiral smyth, it is two miles of stage-coach riding. stage-coaches are now very rare in england, and i was delighted with the chance for a ride. "we found the stage-coach crowded. the driver asked me if we were for st. john's lodge, and on my replying in the affirmative gave me a note which mrs. smyth had written to him, to ask for inside seats. the note had reached him too late, and he said we must go on the outside. he brought a ladder and we got up. for a minute i thought, 'what a height to fall from!' but the afternoon was so lovely that i soon forgot the danger and enjoyed the drive. there were six passengers on top. "aylesbury is a small town, and stone is a very small village. the driver stopped at what seemed to be a cultivated field, and told me that i was at my journey's end. on looking down i saw a wheelbarrow near the fence, and i remembered that mrs. smyth had said that one would be waiting for our luggage, and i soon saw mrs. smyth and her daughter coming towards us. it was a walk of about an eighth of a mile to the 'lodge'--a pleasant cottage surrounded by a beautiful garden. "admiral smyth's family go to a little church seven hundred years old, standing in the midst of tombstones and surrounded by thatched cottages. english scenery seems now (september) much like our southern scenery in april--rich and lovely, but wanting mountains and water. an english village could never be mistaken for an american one: the outline against the sky differs; a thatched cottage makes a very wavy line on the blue above. "we find enough in st. john's lodge, in the admiral's library, and in the society of the cultivated members of his family to interest us for a long time. "the admiral himself is upwards of sixty years of age, noble-looking, loving a good joke, an antiquarian, and a good astronomer. i picked up many an anecdote from him, and many curious bits of learning. "he tells a good story, illustrative of his enthusiasm when looking at a crater in the moon. he says the night was remarkably fine, and he applied higher and higher powers to his glass until he seemed to look down into the abyss, and imagining himself standing on its verge he felt himself falling in, and drew back with a shudder which lasted even after the illusion was over. "in speaking of stratford-upon-avon, the admiral told me that the lucy family, one of whose ancestors drove shakspere from his grounds, and who is caricatured in justice shallow, still resides on the same spot as in shakspere's time. he says no family ever retained their characteristics more decidedly. "some years ago one of this family was invited to a shakspere dinner. he resented the well-meant invitation, saying they must surely have forgotten how that _person_ treated his ancestor! "the amateur astronomers of england are numerous, but they are not like those of america. "in america a poor schoolmaster, who has some bright boys who ask questions, buys a glass and becomes a star-gazer, without time and almost without instruments; or a watchmaker must know the time, and therefore watches the stars as time-keepers. in almost all cases they are hard-working men. "in england it is quite otherwise. a wealthy gentleman buys a telescope as he would buy a library, as an ornament to his house. "admiral smyth says that no family is quite civilized unless it possesses a copy of some encyclopaedia and a telescope. the english gentleman uses both for amusement. if he is a man of philosophical mind he soon becomes an astronomer, or if a benevolent man he perceives that some friend in more limited circumstances might use it well, and he offers the telescope to him, or if an ostentatious man he hires some young astronomer of talent, who comes to his observatory and makes a name for him. then the queen confers the honor of knighthood, not upon the young man, but upon the owner of the telescope. sir james south was knighted for this reason. "we have been visiting hartwell house, an old baronial residence, now the property of dr. lee, a whimsical old man. "this house was for years the residence of louis xviii., and his queen died here. the drawing-room is still kept as in those days; the blue damask on the walls has been changed by time to a brown. the rooms are spacious and lofty, the chimney-pieces of richly carved marble. the ceiling of one room has fine bas-relief allegorical figures. "books of antiquarian value are all around--one whole floor is covered with them. they are almost never opened. in some of the rooms paintings are on the walls above the doors. "dr. lee's modern additions are mostly paintings of himself and a former wife, and are in very bad taste. he has, however, two busts of mrs. somerville, from which i received the impression that she is handsome, but mrs. smyth tells me she is not so; certainly she is sculpturesque. "the royal family, on their retreat from hartwell house, left their prayer-book, and it still remains on its stand. the room of the ladies of the bedchamber is papered, and the figure of a pheasant is the prevailing characteristic of the paper. the room is called 'the pheasant room.' one of the birds has been carefully cut out, and, it is said, was carried away as a memento by one of the damsels. "dr. lee is second cousin to sir george lee, who died childless. he inherits the estate, but not the title. the estate has belonged to the lees for four hundred years. as the doctor was a lee only through his mother, he was obliged to take her name on his accession to the property. he applied to parliament to be permitted to assume the title, and, being refused, from a strong tory he became a liberal, and delights in currying favor with the lowest classes; he has twice married below his rank. being remotely connected with the hampdens, he claims john hampden as one of his family, and keeps a portrait of him in a conspicuous place. "a summer-house on the grounds was erected by lady elizabeth lee, and some verses inscribed on its walls, written by her, show that the lees have not always been fools. "but dr. lee has his way of doing good. being fond of astronomy, he has bought an eight and a half feet equatorial telescope, and with a wisdom which one could scarcely expect, he employed admiral smyth to construct an observatory. he has also a fine transit instrument, and the admiral, being his near neighbor, has the privilege of using the observatory as his own. in the absence of the lees he has a private key, with which he admits himself and mrs. smyth. they make the observations (mrs. smyth is a very clever astronomer), sleep in a room called 'the admiral's room,' find breakfast prepared for them in the morning, and return to their own house when they choose. "i saw in the observatory a timepiece with a double second-hand; one of these could be stopped by a touch, and would, in that way, show an observer the instant when he thought a phenomenon, as an occultation for instance, had occurred, and yet permit him to go on with his count of the seconds, and, if necessary, correct his first impression. "admiral smyth is a hard worker, but i suspect that many of the amateur astronomers of england are dr. lees--rich men who, as a hobby, ride astronomy and employ a good astronomer. dr. lee gives the use of a good instrument to the curate; another to mr. payson, of cambridge, who has lately found a little planet. "i saw at admiral smyth's some excellent photographs of the moon, but in england they have not yet photographed the stars." chapter vi first european tour continued--cambridge university--ambleside--miss southey---the herschels--a london rout--edinboro' and glasgow observatories--"reflections and mutterings" "if any one wishes to know the customs of centuries ago in england, let him go to cambridge. "sitting at the window of the hotel, he will see the scholars, the fellows, the masters of arts, and the masters of colleges passing along the streets in their different gowns. very unbecoming gowns they are, in all cases; and much as the wearers must be accustomed to them, they seem to step awkwardly, and to have an ungraceful feminine touch in their motions. "everything that you see speaks of the olden time. even the images above the arched entrance to the courts around which the buildings stand are crumbling slowly, and the faces have an unearthly expression. "if the visitor is fortunate enough to have an introduction to one of the college professors, he will be taken around the buildings, to the libraries, the 'combination' room to which the fellows retire to chat over their wine, and perhaps even to the kitchen. "our first knowledge of cambridge was the entrance to trinity college and the master's lodge. "we arrived in cambridge just about at lunch time--one o'clock. "mrs. airy said to me, 'although we are invited to be guests of dr. whewell, he is quite too mighty a man to come to meet us." her sons, however, met us, and we walked with them to dr. whewell's. "the master's lodge, where dr. whewell lives, is one of the buildings composing the great pile of trinity college. one of the rooms in the lodge still remains nearly as in the time of henry viii. it is immense in size, and has two oriel windows hung with red velvet. in this room the queen holds her court when she is in cambridge; for the lodge then becomes a palace, and the 'master' retires to some other apartments, and comes to dinner only when asked. "it is said that the present master does not much like to submit to this position. "in this great room hang full-length portraits of henry and elizabeth. on another wall is a portrait of newton, and on a third the sweet face of a young girl, dr. whewell's niece, of whom i heard him speak as 'kate.' "dr. whewell received us in this room, standing on a rug before an open fireplace; a wood fire was burning cheerily. mrs. airy's daughter, a young girl, was with us. "dr. whewell shook hands with us, and we stood. i was very tired, but we continued to stand. in an american gentleman's house i should have asked if i might sit, and should have dropped upon a chair; here, of course, i continued to stand. after, perhaps, fifteen minutes, dr. whewell said, 'will you sit?' and the four of us dropped upon chairs as if shot! "the master is a man to be noted, even physically. he is much above ordinary size, and, though now gray-haired, would be extraordinarily handsome if it were not for an expression of ill-temper about the mouth. "an englishmen is proud; a cambridge man is the proudest of englishmen; and dr. whewell, the proudest of cambridge men. "in the opinion of a cambridge man, to be master of trinity is to be master of the world! "at lunch, to which we stayed, dr. whewell talked about american writers, and was very severe upon them; some of them were friends of mine, and it was not pleasant. but i was especially hurt by a remark which he made afterwards. americans are noted in england for their use of slang. the english suppose that the language of sam slick or of nasby is the language used in cultivated society. they do not seem to understand it, and i have no doubt to-day that lowell's comic poems are taken seriously. so at this table, dr. whewell, wishing to say that we would do something in the way of sight-seeing very thoroughly, turning to me, said, 'we'll go the whole hog, miss mitchell, as you say in america.' "i turned to the young american girl who sat next to me, and said, 'miss s., did you ever hear that expression except on the street?' 'never,' she replied. "afterwards he said to me, 'you in america think you know something about the english language, and you get out your webster's dictionary, and your worcester's dictionary, but we here in cambridge think we know rather more about english than you do.' "after lunch we went to the observatory. the cambridge observatory has the usual number of meridian instruments, but it has besides a good equatorial telescope of twenty feet in length, mounted in the english style; for mr. airy was in cambridge at the time of its establishment. in this pretty observatory, overlooking the peaceful plains, with some small hills in the distance, mr. and mrs. airy passed the first year of their married life. "professor challis, the director, is exceedingly short, thick-headed (in appearance), and, like many of the english, thick-tongued. while i was looking at the instruments, mrs. airy came into the equatorial house, bringing mr. adams, the rival of leverrier, [footnote: see chapter vii.]--another short man, but bright-looking, with dark hair and eyes, and again the thick voice, this time with a nasal twang. he is a fellow of pembroke college, and master of arts. if mr. adams had become a fellow of his own college, st. john, he must have gone into holy orders, as it is called; this he was not willing to do; he accepted a fellowship from pembroke. "mr. adams is a merry little man, loves games with children, and is a favorite with young ladies. "at . we went again to the lodge to dine. we were a little late, and the servant was in a great hurry to announce us; but i made him wait until my gloves were on, though not buttoned. he announced us with a loud voice, and dr. whewell came forward to receive us. being announced in this way, the other guests do not wait for an introduction. there was a group of guests in the drawing-room, and those nearest me spoke to me at once. "dinner was announced immediately, and dr. whewell escorted me downstairs, across an immense hall, to the dining-room, outside of which stood the waiters, six in number, arranged in a straight line, in livery, of course. one of them had a scarlet vest, short clothes, and drab coat. "as i sat next to the master, i had a good deal of talk with him. he was very severe upon americans; he said that emerson did not write good english, and copied carlyle! i thought his severity reached really to discourtesy, and i think he perceived it when he asked me if i knew emerson personally, and i replied that i did, and that i valued my acquaintance with him highly. "i got a little chance to retort, by telling him that we had outgrown mrs. hemans in america, and that we now read mrs. browning more. he laughed at it, and said that mrs. browning's poetry was so coarse that he could not tolerate it, and he was amused to hear that any people had got above mrs. hemans; and he asked me if we had outgrown homer! to which i replied that they were not similar cases. "altogether, there was a tone of satire in dr. whewell's remarks which i did not think amiable. "there were, as there are very commonly in english society, some dresses too low for my taste; and the wine-drinking was universal, so that i had to make a special point of getting a glass of water, and was afraid i might drink all there was on the table! "before the dessert came on, saucers were placed before each guest, and a little rose-water dipped into them from a silver basin; then each guest washed his face thoroughly, dipping his napkin into the saucer. professor willis, who sat next to me, told me that this was a custom peculiar to cambridge, and dating from its earliest times. "the finger bowls came on afterwards, as usual. "it is customary for the lady of the house or the 'first lady' to turn to her nearest neighbor at the close of dinner and say, 'shall we retire to the drawing-room?' now, there was no lady of the house, and i was in the position of first lady. they might have sat there for a thousand years before i should have thought of it. i drew on my gloves when the other ladies drew on theirs, and then we waited. mrs. airy saw the dilemma, made the little speech, and the gentlemen escorted us to the door, and then returned to their wine. "we went back to the drawing-room and had coffee; after coffee new guests began to come, and we went into the magnificent room with the oriel windows. "professor sedgwick came early--an old man of seventy-four, already a little shattered and subject to giddiness. he is said to be very fond of young ladies even now, and when younger made some heartaches; for he could not give up his fellowship and leave cambridge for a wife; which, to me, is very unmanly. he is considered the greatest geologist in england, and of course they would say 'in the world,' and is much loved by all who know him. he came to cambridge a young man, and the elms which he saw planted are now sturdy trees. it is pleasant to hear him talk of cambridge and its growth; he points to the stately trees and says, 'those trees don't look as old as i, and they are not.' "i did not see professor adams at that time, but i spent the whole of monday morning walking about the college with him. i asked him to show me the place where he made his computations for neptune, and he was evidently well pleased to do so. "we laughed over a roll, which we saw in the college library, containing a list of the ancestors of henry viii.; among them was jupiter. "professor adams tells me that in wales genealogical charts go so far back that about half-way between the beginning and the present day you find this record: 'about this time the world was created'! "november . at lunch to-day dr. whewell was more interesting than i had seen him before. he asked me about laura bridgman, and said that he knew a similar case. he contended, in opposition to mrs. airy and myself, that loss of vision was preferable to loss of hearing, because it shut one out less from human companionship. "dr. whewell's self-respect and immense self-esteem led him to imperiousness of manner which touches the border of discourtesy. he loves a good joke, but his jests are serious. he writes verses that are touchingly beautiful, but it is difficult to believe, in his presence, that he writes them. mrs. airy said that dr. whewell and i _riled_ each other! "i was at an evening party, and the airy boys, young men of eighteen and twenty, were present. they stood the whole time, occasionally leaning against a table or the piano, in their blue silk gowns. i urged them to sit. 'of course not,' they said; 'no undergraduate sits in the master's presence!' "i went to three services on 'scarlet sunday,' for the sake of seeing all the sights. "the costumes of cambridge and oxford are very amusing, and show, more than anything i have seen, the old-fogyism of english ways. dr. whewell wore, on this occasion, a long gown reaching nearly to his feet, of rich scarlet, and adorned with flowing ribands. the ribands did not match the robe, but were more of a crimson. "i wondered that a strong-minded man like dr. whewell could tolerate such trappings for a moment; but it is said that he is rather proud of them, and loves all the etiquette of the olden time, as also, it is said, does the queen. "in these robes dr. whewell escorted me to church--and of course we were a great sight! "before dinner, on this scarlet sunday, there was an interval when the master was evidently tried to know what to do with me. at length he hit upon an expedient. 'boys,' he said to the young airys, 'take miss mitchell on a walk!' "i was a little surprised to find myself on a walk, 'nolens volens;' so as soon as we were out of sight of the master of trinity, i said, 'now, young gentlemen, as i do not want to go to walk, we won't go!' "it was hard for me to become accustomed to english ideas of caste. i heard professor sedgwick say that miss herschel, the daughter of sir john and niece to caroline, married a gordon. 'such a great match for her!' he added; and when i asked what match could be great for a daughter of the herschels, i was told that she had married one of the queen's household, and was asked to _sit_ in the presence of the queen! "when i hear a missionary tell that the pariah caste sit on the ground, the peasant caste lift themselves by the thickness of a leaf, and the next rank by the thickness of a stalk, it seems to me that the heathen has reached a high state of civilization--precisely that which victoria has reached when she permits a herschel to sit in her presence! "the university of cambridge consists of sixteen colleges. i was told that, of these, trinity leads and st. john comes next. "trinity has always led in mathematics; it boasts of newton and byron among its graduates. milton belonged to christ church college; the mulberry tree which he planted still flourishes. "even to-day, a young scholar of trinity expressed his regret to me that milton did not belong to the college in which he himself studied. he pointed out the rooms occupied by newton, and showed us 'newton's bridge,' 'which will surely fall when a greater man than he walks over it'! "milton first planned the great poem, 'paradise lost,' as a drama, and this manuscript, kept within a glass case, is opened to the page on which the _dramatis personae_ are planned and replanned. on the opposite page is a part of 'lycidas,' neatly written and with few corrections. "the most beautiful of the college buildings is king's chapel. a cambridge man is sure to take you to one of the bridges spanning the wretched little stream called the 'silver cam,' that you may see the architectural beauties of this building. "it is well to attend service in one or the other of the chapels, to see assembled the young men, who are almost all the sons of the nobility or gentry. the propriety of their conduct struck me. "the fellows of the colleges are chosen from the 'scholars' who are most distinguished, as the 'scholars' are chosen from the undergraduates. they receive an income so long as they remain connected with the college and unmarried. "they have also the use of rooms in the college; they dine in the same hall with the undergraduates, but their tables are placed upon a raised dais; they have also little garden-places given them. "'what are their duties?' i asked mr. airy. 'none at all; _they_ are the college. it would not be a seat of learning without them.' "they say in cambridge that dr. whewell's book, 'plurality of worlds,' reasons to this end: the planets were created for this world; this world for man; man for england; england for cambridge; and cambridge for dr. whewell! "ambleside, september . we have spent the sunday in ascending a mountain, i have a minute route marked out for me by professor airy, who has rambled among the lakes and mountains of cumberland and westmoreland for months, and says that no man lives who knows them better than he. "in accordance with these directions, i took a one-horse carriage this morning for coniston waters, in order to ascend the 'old man.' the waiter at the 'salutation' at ambleside, which we made headquarters, told me that i could not make the ascent, as the day would not be fine; but i have not travelled six months for nothing, and i knew he was saying, 'you are fine american geese; you are not to leave my house until you have been well plucked!'--which threat he will of course keep, but i shall see all the 'old men' that i choose. so i borrowed the waiter's umbrella, when he said it would rain, and off we went in an open carriage, a drive of seven miles, up hill and down dale, among mountains and around ponds (lakes _they_ called them), in the midst of rich lands and pretty mansions, with occasionally a castle, and once a ruin, to diversify the scenery. "arrived at coniston hotel, the waiter said the same thing: 'it's too cloudy to ascend the "old man;"' but as soon as it was found that if it was too cloudy we did not intend to stay, it cleared off amazingly fast, and the ponies were ordered. i thought at first of walking up, but, having a value for my feet and not liking to misuse them, i mounted a pony and walked him. "he was beautifully stupid, but i could not help thinking of henry colman, the agriculturist, who, when in england, went on a fox-hunt. he said, 'think of my poor wife's old husband leaping a fence!' "but i soon forgot any fear, for the pony needed nothing from me or the guide, but scrambled about any way he chose; and the scenery was charming, for although the mountains are not very high, they are thrown together very beautifully and remind me of those of the hudson highlands. then the little lakes were lovely, and occasionally we came to a tarn or pond, and exceedingly small waterfalls were rushing about everywhere, without any apparent object in view, but evidently looking for something. and spite of the weatherwise head-waiter of the 'salutation' and of him of coniston inn, the day was beautiful. we had to give up the ponies when we were half a mile from the top, and clamber up ourselves. the guide was very intelligent, and pointed out the lakes, windermere, coniston; and the mountains, helvellyn, skiddaw, and saddleback; but at one time he spoke a name that i couldn't understand, and forgetting that i was in england and not in america, i asked him to _spell_ it. he replied, 'theys call it so always.' he did not fail, however, to ask questions like a yankee, if he couldn't spell like one. 'which way be ye coming?'--'from america.'--'ye'll be going to scotland like?'--'yes.'--'ye'll be spending much money before ye are home again.' "when we were quite on top of the mountain i asked what the white glimmering was in the distance, and he said it was, what i supposed, an arm of the sea. "the shadows of the flying clouds were very pretty falling on the hills around us, and the villages in the valleys beneath looked like white dots on the green. "sunday, sept. , . we have been to see miss southey to-day. i sent the letter which mrs. airy gave me yesterday, and with it a note saying that i would call to-day if convenient. "miss southey replied at once, saying that she should be happy to see me. she lives in a straggling, irregular cottage, like most of the cottages around keswick, but beautifully situated, though far from the lake. "southey himself lived at greta hall, a much finer place, for many years, but he never owned it, and the gentleman who bought it will permit no one to see it. "miss southey's house is overgrown with climbing plants, has windows opening to the ground, and is really a summer residence, not a good winter home. "when southey, in his decline, married a second wife, the family scattered, and this daughter, the only unmarried one, left him. "we were shown into a pleasant parlor comfortably furnished, especially with books and engravings, portraits of southey, wordsworth, and others. "miss southey soon came down; she is really pretty, having the fresh english complexion and fair hair. she seems to be a very simple, pleasant person; chatty, but not too much so. she is much engrossed by the care of three of her brother's children, an old aunt, and a servant, who, having been long in the family, has become a dependant. miss southey spoke at once of the americans whom she had known, ticknor being one. "the old aunt asked after a new york lady who had visited southey at greta hall, but her niece reminded her that it must have been before i was born! "miss southey said that her father felt that he knew as many americans as englishmen, and that she wanted very much to go to america. i told her that she would be in danger of being 'lionized;' she said, 'oh, i should like that, for of course it is gratifying to know how much my father was valued there." "i asked after the children, and miss southey said that the little boy had called out to her, 'oh! aunt katy, the ameriky ladies have come! "the three children were called in; the boy, about six years old, of course wouldn't speak to me. "the best portrait of southey in his daughter's collection is a profile in wax--a style that i have seen several times in england, and which i think very pretty. "we went down to lodore, the scene of the poem, 'how does the water come down,' etc., and found it about as large as the other waterfalls around here--a little dripping of water among the stones. collingwood, nov. , . my dear father: this is sir john herschel's place. i came last night just at dusk. according to english ways, i ought to have written a note, naming the hour at which i should reach etchingham, which is four miles from collingwood; but when i left liverpool i went directly on, and a letter would have arrived at the same time that i did. i stopped in london one night only, changed my lodging-house, that i might pay a pound a week only for letting my trunk live in a room, instead of two pounds, and started off again. i reached etchingham at ten minutes past four, took a cab, and set off for sir john's. it is a large brick house, no way handsome, but surrounded by fine grounds, with beautiful trees and a very large pond. the family were at dinner, and i was shown into the drawing-room. there was just the light of a coal fire, and as i stood before it sir john bustled in, an old man, much bent, with perfectly white hair standing out every way. he reached both hands to me, and said, "we had no letter and so did not expect you, but you are always welcome in this house." lady herschel followed--very noble looking; she does not look as old as i, but of course must be; but english women, especially of her station, do not wear out as we do, who are "jacks at all trades." i found a fire in my room, and a cup of tea and crackers were immediately sent up. the herschels have several children; i have not seen caroline, louise, william, and alexander, but belle, and amelie, and marie, and julie, and rosa, and francesca, and constance, and john are at home! the children are not handsome, but are good-looking, and well brought up of course, and highly educated. the children all come to table, which is not common in england. think what a table they must set when the whole twelve are at home! the first object that struck me in the house was borden's map of massachusetts, hanging in the hall opposite the entrance. over the mantelpiece in the dining-room is a portrait of sir william herschel. in the parlor is a portrait of caroline herschel, and busts of sir william, sir john, and the eldest daughter. i spent the evening in looking at engravings, sipping tea, and talking. sir john is like the elder mr. bond, except that he talks more readily; but he is womanly in his nature, not a tyrant like whewell. sir john is a better listener than any man i have met in england. he joins in all the chit-chat, is one of the domestic circle, and tells funny little anecdotes. (so do whewell and airy.) the herschels know abbot lawrence and edward everett--and everywhere these two have left a good impression. but i am certainly mortified by anecdotes that i hear of "pushing" americans. mrs. ---- sought an introduction to sir john herschel to tell him about an abridgment of his astronomy which she had made, and she intimated to him that in consequence of her abridgment his work was, or would be, much more widely known in america. lady herschel told me of it, and she remarked, "i believe sir john was not much pleased, for he does not like abridgments." i told her that i had never heard of the abridgment. there are other guests in the house: a lady whose sister was among those killed in india; and her husband, who is an officer in the army. we have all been playing at "spelling" this evening, with the letters, as we did at home last winter. sunday, th. i thought of going to london to-day, but was easily persuaded to stay and go with lady herschel to-morrow. all this afternoon i have spent listening to sir john, who has shown me his father's manuscript, his aunt's, beautifully neat, and he told me about his cape observations. the telescope used at the cape of good hope lies in the barn (the glass, of course, taken care of) unused; and sir john now occupies himself with writing only. he made many drawings at the cape, which he showed me, and very good ones they are. lady herschel offers me a letter to mrs. somerville, who is godmother to one of her children. i am afraid i shall have no letter to leverrier, for every one seems to dislike him. lady herschel says he is one of the few persons whom she ever asked for an autograph; he was her guest, and he refused! just as i was coming away, sir john bustled up to me with a sheet of paper, saying that he thought i would like some of his aunt's handwriting and he would give it to me. he had before given me one of his own calculations; he says if there were no "war, pestilence, or famine," and one pair of human beings had been put upon the globe at the time of cheops, they would not only now fill the earth, but if they stood upon each other's heads, they would reach a hundred times the distance to neptune! i turned over their scrap-books, and sir john's poetry is much better than many of the specimens they had carefully kept, by sir william hamilton. sir william hamilton's sister had some specimens in the book, and also lady herschel and her brother. lady herschel is the head of the house--so is mrs. airy--so, i suspect, is the wife in all well-ordered households! i perceived that sir john did not take a cup of tea until his wife said, "you can have some, my dear." mr. airy waits and waits, and then says, "my dear, i shall lose all my flesh if i don't have something to eat and drink." i am hoping to get to paris next week, about the d. i have had just what i wanted in england, as to society. "november . a few days ago i received a card, 'mrs. baden powell, at home november .' of course i did not know if it was a tea party or a wedding reception. so i appealed to mrs. airy. she said, 'it is a london rout. i never went to one, but you'll find a crowd and a good many interesting people.' "i took a cab, and went at nine o'clock. the servant who opened the door passed me to another who showed me the cloak-room. the girl who took my shawl numbered it and gave me a ticket, as they would at a public exhibition. then she pointed to the other end of the room, and there i saw a table with tea and coffee. i took a cup of coffee, and then the servant asked my name, _yelled_ it up the stairs to another, and he announced it at the drawing-room door just as i entered. "mrs. powell and the professor were of course standing near, and mrs. admiral smyth just behind. to my delight, i met four english persons whom i knew, and also prof. henry b. rogers, who is a great society man. "people kept coming until the room was quite full. i was very glad to be introduced to professor stokes, who is called the best mathematician in england, and is a friend of adams. he is very handsome--almost all englishmen are handsome, because they look healthy; but professor stokes has fine black eyes and dark hair and good features. he looks very young and innocent. stokes is connected with cambridge, but lives in london, just as professor powell is connected with oxford, but also lives in london. several gentlemen spoke to me without a special introduction--one told me his name was dr. townby [qy., toynbie], and he was a great admirer of emerson--the first case of the sort i have met. "dr. townby is a young man not over thirty, full of enthusiasm and progress, like an american. he really seemed to me all alive, and is either a genius or crazy--the shade between is so delicate that i can't always tell to which a person belongs! i asked him if babbage was in the room, and he said, 'not yet,' so i hoped he would come. "he told me that a fine-looking, white-headed, good-featured old man was roget, of the 'thesaurus;' and another old man in the corner was dr. arnott, of the 'elements of physics.' i had supposed he was dead long ago. afterwards i was introduced to him. he is an old man, but not much over sixty; his hair is white, but he is full of vigor, short and stout, like almost all englishmen and englishwomen. i have met only two women taller than myself, and most of them are very much shorter. dr. arnott told me he was only now finishing the 'elements,' which he first published in . he intends now to publish the more mathematical portions with the other volumes. he was very sociable, and i told him he had twenty years ago a great many readers in america. he said he supposed he had more there than in england, and that he believed he had made young men study science in many instances. "i asked him if babbage was in the room, and he too said, 'not yet.' dr. arnott asked me if i wore as many stockings when i was observing as the herschels--he said sir william put on twelve pairs and caroline fourteen! "i stayed until eleven o'clock, then i said 'good-by,' and just as i stepped upon the threshold of the drawing-room to go out, a broad old man stepped upon it, and the servant announced 'mr. babbage,' and of course that glimpse was all i shall ever have! "edinboro', september . the people of edinboro', having a passion for grecian architecture, and being very proud of the athenian character of their city, seek to increase the resemblance by imitations of ancient buildings. "grecian pillars are seen on calton hill in great numbers, and the observatory would delight an old greek; its four fronts are adorned by grecian pillars, and it is indeed beautiful as a structure; but the greeks did not build their temples for astronomical observations; they probably adapted their architecture to their needs. "this beautiful building was erected by an association of gentlemen, who raised a good deal of money, but, of course, not enough. they built the grecian temple, but they could not supply it with priests. "about a hundred years ago colin maclaurin had laid the foundation of an observatory, and the curious gothic building, which still stands, is the first germ. we laugh now at the narrow ideas of those days, which seemed to consider an observatory a lookout only; but the first step in a work is a great step--the others are easily taken. there was added to the building of maclaurin a very small transit room, and then the present edifice followed. "when the builders of the observatory found that they could not support it, they presented it to the british government; so that it is now a government child, but it is not petted, like the first-born of greenwich. "there are three instruments; an excellent transit instrument of six and a half inches' aperture, resting on its y's of solid granite. the corrections of the errors of the instrument by means of little screws are given up, and the errors which are known to exist are corrected in the computations. "professor smyth finds that although the two pillars upon which the instrument rests were cut from the same quarry, they are unequally affected by changes of temperature; so that the variation of the azimuth error, though slight, is irregular. "the collimation plate they correct with the micrometer, so that they consider some position-reading of the micrometer-head the zero point, and correct that for the error, which they determine by reflection in a trough of mercury. with this instrument they observe on certain stars of the british catalogue, whose places are not very well determined, and with a mural circle of smaller power they determine declinations. "the observatory possesses an equatorial telescope, but it is of mixed composition. the object glass was given by dr. lee, the eye-pieces by some one else, and the two are put together in a case, and used by professor smyth for looking at the craters in the moon; of these he has made fine drawings, and has published them in color prints. "the whole staff of the observatory consists of professor smyth, mr. wallace, an old man, and mr. williamson, a young man. "the city of edinboro' has no amateur astronomers, and there are two only, of note, in scotland: sir william bisbane and sir william keith murray. "from the observatory, the view of edinboro' is lovely. 'auld reekie,' as the scotch call it, always looks her best through a mist, and a scotch mist is not a rare event--so we saw the city under its most becoming veil. "october, . i stopped in glasgow a few hours, and went to the observatory, which is also the private residence of professor nichol. miss nichol received me, and was a very pleasant, blue-eyed young lady. "i found that the observatory boasts of two good instruments: a meridian circle, which must be good, from its appearance, and a newtonian telescope, differently mounted from any i had seen; cased in a composition tube which is painted bright blue--rather a striking object. the iron mounting seemed to me good. it was of the german kind, but modified. it seemed to me that it could be used for observations far from the meridian. the iron part was hollow, so that the clock was inside, as was the azimuth circle, and thus space was saved. "they have a wind and rain self-register, and a self-registering barometer, marking on a cylinder turned by a clock, the paper revolving once an hour. "when i was at dungeon ghyll, a little ravine among the english lakes, down which trickles an exceedingly small stream of water, but which is, nevertheless, very picturesque,--as i followed the old man who shows it for a sixpence, he asked if we had come a long way. 'from america,' i replied. 'we have many americans here,' said he; 'it is much easier to understand their language than that of other foreigners; they speak very good english, better than the french or germans.' "i felt myself a little annoyed and a good deal amused. i supposed that i spoke the language that addison wrote, and here was a westmoreland guide, speaking a dialect which i translated into english before i could understand it, complimenting me upon my ability to speak my own tongue. "i learned afterwards, as i journeyed on, to expect no appreciation of my country or its people. the english are strangely deficient in curiosity. i can scarcely imagine an englishwoman a gossip. "i found among all classes a knowledge of the extent of america; by the better classes its geography was understood, and its physical peculiarities. one astronomer had bound the scientific papers from america in green morocco, as typical of a country covered by forests. among the most intelligent men whom i met i found an appreciation of the different characters of the states. everywhere massachusetts was honored; everywhere i met the horror of the honest englishman at the slave system; but anything like a discriminating knowledge of our public men i could not meet. webster had been heard of everywhere. they assured me that our _really great_ men were known, our really great deeds appreciated; but this is not true. they make mistakes in their measure of our men; second-rate men who have travelled are of course known to the men whom they have met; these travellers have not perhaps thought it necessary to mention that they represent a secondary class of people, and they are considered our 'first men.' the english forget that all americans travel. "i was vexed when i saw some of our most miserable novels, bound in showy yellow and red, exposed for sale. a friend told me that they had copied from the cheap publications of america. it may be so, but they have outdone us in the cheapness of the material and the showy covers. i never saw yellow and red together on any american book. "the english are far beyond us in their highest scholarship, but why should they be ignorant of our scholars? the englishman is proud, and not without reason; but he may well be proud of the american offshoot. it is not strange that england produces fine scholars, when we consider that her colleges confer fellowships on the best undergraduates. "england differs from america in the fact that it has a past. well may the great men of the present be proud of those who have gone before them; it is scarcely to be hoped that the like can come after them; and yet i suppose we must admit that even now the strong minds are born across the water. "at the same time england has a class to which we have happily no parallel in our country--a class to which even english gentlemen liken the sepoys, and who would, they admit, under like circumstances be guilty of like enormities. but the true englishman shuts his eyes for a great part of the time to the steps in the social scale down which his race descends, and looks only at the upper walks. he has therefore a glance of patronizing kindness for the people of the united states, and regards us of new england as we regard our rich brethren of the west. "i wondered what was to become of the english people! their island is already crowded with people, the large towns are numerous and are very large. suppose for an instant that her commerce is cut off, will they starve? it is an illustration of moral power that, little island as that of great britain is, its power is the great power of the world. "crowded as the people are, they are healthy. i never saw, i thought, so many ruddy faces as met me at once in liverpool. dirty children in the street have red cheeks and good teeth. nowhere did i see little children whose minds had outgrown their bodies. they do not live in the school-room, but in the streets. one continually meets little children carrying smaller ones in their arms; little girls hand in hand walk the streets of london all day. there are no free schools, and they have nothing to do. beggars are everywhere, and as importunate as in italy. for a well-behaved common people i should go to paris; for clean working-women i should look in paris. "i saw a little boy in england tormenting a smaller one. he spat upon his cap, and then declared that the little one did it. the little one sobbed and said he didn't. i gave the little one a penny; he evidently did not know the value of the coin, and appealed to the bigger boy. 'is it a penny?' he asked, with a look of amazement. 'yes,' said the bigger. off ran the smaller one triumphant, and the bigger began to cry, which i permitted him to do." chapter vii - first european tour continued--leverrier and the paris observatory--rome--harriet hosmer--observatory of the collegio romano--secchi at this time, the feeling between astronomers of great britain and those of the united states was not very cordial. it was the time when adams and leverrier were contending to which of them belonged the honor of the discovery of the planet neptune, and each side had its strong partisans. among miss mitchell's papers we find the following with reference to this subject: "... adams, a graduate of cambridge, made the calculations which showed how an unseen body must exist whose influences were felt by uranus. it was a problem of great difficulty, for he had some half-dozen quantities touching uranus which were not accurately known, and as many wholly unknown concerning the unseen planet. we think it a difficult question which involves three or four unknown quantities with too few circumstances, but this problem involved twelve or thirteen, so that x, y, z reached pretty high up into the alphabet. but adams, having worked the problem, carried his work to airy, the astronomer royal of england, and awaited his comments. a little later leverrier, the french astronomer, completed the same problem, and waiting for no authority beyond his own, flung his discovery out to the world with the self-confidence of a frenchman.... "... when the news of the discovery of neptune reached this country, i happened to be visiting at the observatory in cambridge, mass. professor bond (the elder) had looked for the planet the night before i arrived at his house, and he looked again the evening that i came. "his observatory was then a small, round building, and in it was a small telescope; he had drawn a map of a group of stars, one of which he supposed was not a star, but the planet. he set the telescope to this group, and asking his son to count the seconds, he allowed the stars to pass by the motion of the earth across the field. if they kept the relative distance of the night before, they were all stars; if any one had approached or receded from the others, it was a planet; and when the father looked at his son's record he said, 'one of those has moved, and it is the one which i thought last night was the planet.' he looked again at the group, and the son said, 'father, do give me a look at the new planet--you are the only man in america that can do it!' and then we both looked; it looked precisely like a small star, and george and i both asked, 'what made you think last night that it was the new planet?' mr. bond could only say, 'i don't know, it looked different from the others.' "it is always so--you cannot get a man of genius to explain steps, he leaps. "after the discovery of this planet, professor peirce, in our own country, declared that it was not the planet of the theory, and therefore its discovery was a happy accident. but it seemed to me that it was the planet of the theory, just as much if it varied a good deal from its prescribed place as if it varied a little. so you might have said that uranus was not the uranus of the theory. "sir john herschel said, 'its movements have been felt trembling along the far-reaching line of our analysis, with a certainty hardly inferior to ocular demonstration.' i consider it was superior to ocular demonstration, as the action of the mind is above that of the senses. adams, in his study at cambridge, england, and leverrier in his closet at paris, poring over their logarithms, knew better the locus of that outside planet than all the practical astronomers of the world put together.... "of course in paris i went to the imperial observatory, to visit leverrier. i carried letters from professor airy, who also sent a letter in advance by post. leverrier called at my hotel, and left cards; then came a note, and i went to tea. "leverrier had succeeded arago. arago had been a member of the provisional government, and had died. leverrier took exactly opposite ground, politically, to that of arago; he stood high with the emperor. "he took me all over the observatory. he had a large room for a ballroom, because in the ballroom science and politics were discussed; for where a press is not free, salons must give the tone to public opinion. "both leverrier and madame leverrier said hard things about the english, and the english said hard things about leverrier. "the astronomical observatory of paris was founded on the establishment of the academy of sciences, in the reign of louis xiv. the building was begun in and finished in ; like other observatories of that time, it was quite unfit for use. "john dominie cassini came to it before it was finished, saw its defects, and made alterations; but the whole building was afterwards abandoned. m. leverrier showed me the transit instrument and the mural circle. he has, like mr. airy, made the transit instrument incapable of mechanical change for its corrections of error, so that it depends for accuracy upon its faults being known and corrected in the computations. "all the early observatories of europe seem to have been built as temples to urania, and not as working-chambers of science. the royal observatory at greenwich, the imperial observatory of paris, and the beautiful structure on calton hill, edinboro', were at first wholly useless as observatories. that of greenwich had no steadiness, while every pillar in the astronomical temple of edinboro', though it may tell of the enlightenment of greece, hides the light of the stars from the scottish observer. well might struve say that 'an observatory should be simply a box to hold instruments.' "the leverriers speak english about as well as i do french, and we had a very awkward time of it. m. leverrier talked with me a little, and then talked wholly to one of the gentlemen present. madame was very chatty. "leverrier is very fine-looking; he is fair-haired full-faced, altogether very healthy-looking. his wife is really handsome, the children beautiful. i was glad that i could understand when leverrier said to the children, 'if you make any more noise you go to bed.' "while i was there, a woman as old as i rushed in, in bonnet and shawl, and flew around the room, kissed madame, jumped the children about, and shook hands with monsieur; and there was a great amount of screaming and laughing, and all talked at once. as i could not understand a word, it seemed to me like a theatre. "i asked monsieur when i could see the observatory, and he answered, 'whenever it suits your convenience.' "december . i went to leverrier's again last evening by special invitation. four gentlemen and three ladies received me, all standing and bowing without speaking. monsieur was, however, more sociable than before, and shrieked out to me in french as though i were deaf. "the ladies were in blue dresses; a good deal of crinoline, deep flounces, high necks, very short, flowing sleeves, and short undersleeves; the dresses were brocade and the flounces much trimmed, madame's with white plush. "the room was cold, of course, having no carpet, and a wood fire in a very small fireplace. "the gentlemen continued standing or promenading, and taking snuff. "except leverrier, no one of them spoke to me. the ladies all did, and all spoke french. the two children were present again--the little girl five years old played on the piano, and the boy of nine played and sang like a public performer. he promenaded about the room with his hands in his pockets, like a man. i think his manners were about equal to -----'s, as occasionally he yelled and was told to be quiet. "about ten o'clock m. leverrier asked me to go into the observatory, which connects with the dwelling. they are building immense additional rooms, and are having a great telescope, twenty-seven feet in focal length, constructed. "with leverrier's bad english and my bad french we talked but little, but he showed me the transit instrument, the mural circle, the computing-room, and the private office. he put on his cloak and cap, and said, 'voila le directeur!' "one room, he told me, had been arago's, and arago had his bed on one side. m. leverrier said, 'i do not wish to have it for my room.' he is said to be much opposed to arago, and to be merciless towards his family. "he showed me another room, intended for a reception-room, and explained to me that in france one had to make science come into social life, for the government must be reached in order to get money. "there were huge globes in one room that belonged to cassini. if what he showed me is not surpassed in the other rooms, i don't think much of their instruments. "m. leverrier said he had asked m. chacornac to meet me, but he was not there. i felt that we got on a little better, but not much, and it was evident that he did not expect me to understand an observatory. we did not ascend to the domes. "leverrier has telegraphic communication with all europe except great britain. "it was quite singular that they made such different remarks to me. leverrier said that they had to make science popular. "airy said, 'in england there is no astronomical public, and we do not need to make science popular.' "jan. , . i am in rome! i have been here four days, and already i feel that i would rather have that four days in rome than all the other days of my travels! i have been uncomfortable, cold, tired, and subjected to all the evils of travelling; but for all that, i would not have missed the sort of realization that i have of the existence of the past of great glory, if i must have a thousand times the discomfort. i went alone yesterday to st. peter's and the vatican, and today, taking murray, i went alone to the roman forum, and stood beside the ruined porticos and the broken columns of the temple. then i pushed on to the coliseum, and walked around its whole circumference. i could scarcely believe that i really stood among the ruins, and was not dreaming! i really think i had more enjoyment for going alone and finding out for myself. afterwards the hawthornes called, and i took mrs. h. to the same spot.... "i really feel the impressiveness of rome. all europe has been serious to me; rome is even sad in its seriousness. you cannot help feeling, in the coliseum, some little of the influence of the scenes that have been enacted there, even if you know little about them; you must remember that the vast numbers of people who have been within its walls for ages have not been common minds, whether they were christian martyrs or travelling artists.... "i think if i had never heard before of the reputation of the pictures and statues of the vatican, i should have perceived their superiority. there is more idea of _action_ conveyed by the statuary than i ever received before--they do not seem to be _dead_. "january . i have finer rooms than i had in paris, but the letting of apartments is better managed in paris. there you always find a _concierge_, who tells you all you want to know, and who speaks several languages. in rome you enter a narrow, dark passage, and look in vain for a door. then you go up a flight of stairs, and see a door with a string; you pull the string, and a woman puts her mouth to a square hole, covered with tin punctured with holes, and asks what you want. you tell her, and she tells you to go up higher; you repeat the process, and at last reach the rooms. the higher up the better, because you get some sun, and one learns the value of sunlight. i saw no sun in paris in my room, and here i have it half of the day, and it seems very pleasant. "all the customs of the people differ from those of paris.... "a little of italian art enters into the ornaments of rooms and furniture, but anything like mechanical skill seems to be unheard of; and i dare say the pretty stamp used on the butter i have, which represents some antique picture, was cut by some northern hand. i could make a better cart than those that i see on the streets, and i could _almost_ make as good horses as those that draw them!... "it is holy week. i have spent seven hours at a time at st. peter's, in terrible crowds, for ten days, and now i go no more. the ladies are seated, but as the ceremonies are in different parts of the immense building, they rush wildly from one to the other; with their black veils they look like furies let loose! i stayed five hours to-day to see the pope wash feet, which was very silly; for i saw mother wash them much more effectually twenty years ago! "the crowd is better worth seeing than the ceremony, if one could only see it without being in it. i shall not try to hear the 'miserere'--i have given up the study of music! since i failed to appreciate mario, i sha'n't try any more! "i go to the storys' on sunday evening to look at st. peter's lighting up. "march . i have been to vespers at st. peter's. they begin an hour before sunset. when my work is done for the day, i walk to st. peter's. this is sunday, and the floor was full of kneeling worshippers, but that makes no difference. i walk about among them. "i was there an hour to-day before i saw a person that i knew; then i met the nicholses and went with them into a side chapel to hear vespers. then i saw next the waterstons, then miss lander; but i was unusually short of friends, i generally meet so many more. "there were kneeling women to-day with babies in their arms. the babies of the lower classes have their legs so wrapped up that they cannot move them; they look like small pillows even when they are six months old. i think it must dwarf them. we americans are a tall people. i am a very tall woman here. i think that p.'s height would cause a sensation in the streets. my servant admires my height very much. "march . i called on miss bremer to-day, having heard that she desired to see me. she is a 'little woman in black,' but not so plain; her face is a little red, but her complexion is fair and the expression very pleasing. she chatted away a good deal; asked me about astronomy, and how i came to study it. i told her that my father put me to it, and she said she was just writing a story on the affection of father and daughter. she told me i had good eyes. it is a long time now since any one has told me that! "miss bremer and mrs. w. met in my room and remained an hour. miss bremer is quiet and unpretending. mrs. w. is flashy and brilliant, and, as i usually say when i don't understand a person, a little insane; she had the floor all the time after she came in. she gave a sketch of her life from her birth up, mentioning incidentally that she had been a belle, surrounded with beaux, the pride of her parents, with a reputation for intellect, etc. "i had been urging miss bremer into an interesting talk before mrs. w. appeared, and i felt what a pity it was that she hadn't the same propensity to talk that the latter had. she talked very pleasantly, however, and i thought what a pity it was that i shall not see her again; for i leave rome in three days for florence. "i was in rome for a winter, an idler by necessity for six weeks. it is the very place of all the world for an idler. "on the pleasant days there are the ruins to visit, the campagna to stroll over, the villas and their grounds to gather flowers in, the forum to muse in, the pincian hill or the capitoline for a gossiping walk with some friend. "on rainy days it is all art. there are the cathedrals, the galleries, and the studios of the thousand artists; for every winter there are a thousand artists in rome. "a rainy day found me in the studio of paul akers. as i was looking at some of his models, the studio door opened and a pretty little girl, wearing a jaunty hat and a short jacket, into the pockets of which her hands were thrust, rushed into the room, seemingly unconscious of the presence of a stranger, began a rattling, all-alive talk with mr. akers, of which i caught enough to know that a ride over the campagna was planned, as i heard mr. akers say, 'oh, i won't ride with you--i'm afraid to!' after which he turned to me and introduced harriet hosmer. "i was just from old conservative england, and i had been among its most conservative people. i had caught something of its old musty-parchment ideas, and the cricket-like manners of harriet hosmer rather troubled me. it took some weeks for me to get over the impression of her madcap ways; they seemed childish. "i went to her studio and saw 'puck,' a statue all fun and frolic, and i imagined all was fun to the core of her heart. "as a general rule, people disappoint you as you know them. to know them better and better is to know more and more weaknesses. harriet hosmer parades her weaknesses with the conscious power of one who knows her strength, and who knows you will find her out if you are worthy of her acquaintance. she makes poor jokes--she's a little rude--a good deal eccentric; but she is always _true_. "in the town where she used to live in massachusetts they will tell you a thousand anecdotes of her vagaries--but they are proud of her. "she does not start on a false scent; she knows the royal character of the game before she hunts. "a lady who is a great rider said to me a few days since: 'of course i do not ride like harriet hosmer, but, if you will notice, there is method in harriet hosmer's madness. she does not mount a horse until she has examined him carefully.' "at the time when i saw her, she was thinking of her statue of zenobia. she was studying the history of palmyra, reading up on the manners and customs of its people, and examining eastern relics and costumes. "if she heard that in the sacristy of a certain cathedral, hundreds of miles away, were lying robes of eastern queens, she mounted her horse and rode to the spot, for the sake of learning the lesson they could teach. "day after day alone in her studio, she studied the subject. think what knowledge of the country, of the history of the people, must be gathered, must be moulded, to bring into the face and bearing of its queen the expression of the race! think what familiar acquaintance with the human form, to represent a lifelike figure at all! "for years after i came home i read the newspapers to see if i could find any notice of the statue of zenobia; and i did at length see this announcement: 'the statue of zenobia, by miss hosmer, is on exhibition at childs & jenks'.' "it was after five years. all through those five years, miss hosmer had kept her projects steadily turned in this direction. "whatever may be the criticism of art upon her work, no one can deny that she is above the average artist. "but she is herself, as a woman, very much above herself in art. if there came to any struggling artist in rome the need of a friend,--and of the thousand artists in rome very few are successful,--harriet hosmer was that friend. "i knew her to stretch out a helping hand to an unfortunate artist, a poor, uneducated, unattractive american, against whom the other americans in rome shut their houses and their hearts. when the other americans turned from the unsuccessful artist, harriet hosmer reached forth the helping hand. "when harriet hosmer knew herself to be a sculptor, she knew also that in all america was no school for her. she must leave home, she must live where art could live. she might model her busts in the clay of her own soil, but who should follow out in marble the delicate thought which the clay expressed? the workmen of massachusetts tended the looms, built the railroads, and read the newspapers. the hard-handed men of italy worked in marble from the designs put before them; one copied the leaves which the sculptor threw into the wreaths around the brows of his heroes; another turned with his tool the folds of the drapery; another wrought up the delicate tissues of the flesh; none of them dreamed of ideas: they were copyists,--the very hand-work that her head needed. "and to italy she went. for her school she sought the studio of gibson--the greatest sculptor of the time. "she resolved 'to scorn delights and live laborious days;' and there she has lived and worked for years. "she fashions the clay to her ideal--every little touch of her fingers in the clay is a thought; she thinks in clay. "the model finished and cast in the dull, hard, inexpressive plaster, she stands by the workmen while they put it into the marble. she must watch them, for a touch of the tool in the wrong place might alter the whole expression of the face, as a wrong accent in the reader will spoil a line of poetry. "collegio romano; secchi. there was another observatory which had a reputation and was known in america. it was the observatory of the collegio romano, and was in the monastery behind the church of st. ignasio. its director was the father secchi who had visited the united states, and was well known to the scientists of this country. "i said to myself, 'this is the land of galileo, and this is the city in which he was tried. i knew of no sadder picture in the history of science than that of the old man, galileo, worn by a long life of scientific research, weak and feeble, trembling before that tribunal whose frown was torture, and declaring that to be false which he knew to be true. and i know of no picture in the history of religion more weakly pitiable than that of the holy church trembling before galileo, and denouncing him because he found in the book of nature truths not stated in their own book of god--forgetting that the book of nature is also a book of god. "it seems to be difficult for any one to take in the idea that two truths cannot conflict. "galileo was the first to see the four moons of jupiter; and when he announced the fact that four such moons existed, of course he was met by various objections from established authority. one writer declared that as astrologers had got along very well without these planets, there could be no reason for their starting into existence. "but his greatest heresy was this: he was tried, condemned, and punished for declaring that the sun was the centre of the system, and that the earth moved around it; also, that the earth turned on its axis. "for teaching this, galileo was called before the assembled cardinals of rome, and, clad in black cloth, was compelled to kneel, and to promise never again to teach that the earth moved. it is said that when he arose he whispered, 'it does move!' "he was tried at the hall of sopre minerva. in fewer than two hundred years from that time the church of st. ignasio was built, and the monastery on whose walls the instruments of the modern observatory stand. "it is a very singular fact, but one which seems to show that even in science 'the blood of the martyrs is the seed of the church,' that the spot where galileo was tried is very near the site of the present observatory, to which the pope was very liberal. "from the hall of sopre minerva you make but two turns through short streets to the fontenelle de borghese, in the rear of which stands the present observatory. "indeed, if a cardinal should, at the hall of sopre minerva, call out to secchi, 'watchman, what of the night?' secchi could hear the question; and no bolder views emanate from any observatory than those which secchi sends out. "i sent a card to secchi, and awaited a call, well satisfied to have a little more time for listless strolling among ruins and into the studios. and so we spent many an hour: picking up land shells from the top of the coliseum, gathering violets in the upper chambers of the palace of the caesars,--for the overgrown walls made climbing very easy,--or, resting upon some broken statue on the forum, we admired the arches of the temple of peace, thrown upon the rich blue of the sunny skies. "returning one day from a drive, i met two priests descending one of the upper flights of stairs in the house where i lived. as my rooms had been blessed once, and holy water sprinkled upon them, i thought perhaps another process of that kind had just been gone through, and was about to pass them, when one of them, accosting me, asked if i were the signorine mitchell,--changing his italian to good english as he saw that i was, and introducing himself as father secchi. he told me that the younger man was a young _religieux_, and the two turned and went back with me. "i recalled, as i saw father secchi, an anecdote i had heard, no way to his credit,--except for ingenious trickery. it was said that coming to america he brought with him the object-glass of a telescope, at a time when scientific apparatus paid a high duty. being asked by some official what the article was, he replied, 'my looking-glass,' and in that way passed it off as personal wardrobe, so escaped the duty. (it may have been de vico.) "father secchi had brought with him, to show me, negatives of the planet saturn,--the rings showing beautifully, although the image was not more than half an inch in size. "i was ignorant enough of the ways of papal institutions, and, indeed, of all italy, to ask if i might visit the roman observatory. i remembered that the days of galileo were days of two centuries since. i did not know that my heretic feet must not enter the sanctuary,--that my woman's robe must not brush the seats of learning. "the father's refusal was seen in his face at once, and i felt that i had done something highly improper. the father said that he would have been most happy to have me visit him, but he had not the power--it was a religious institution--he had already applied to his superior, who was not willing to grant permission--the power lay with the holy father or one of his cardinals. i was told that mrs. somerville, the most learned woman in all europe, had been denied admission; that the daughter of sir john herschel, in spite of english rank, and the higher stamp of nature's nobility, was at that time in rome, and could not enter an observatory which was at the same time a monastery. "if i had before been mildly desirous of visiting the observatory, i was now intensely anxious to do so. father secchi suggested that i should see cardinal antonelli in person, with a written application in my hand. this was not to be thought of--to ask an interview with the wily cardinal! from a letter to her father. ... i am working to get admitted to see the observatory, but it cannot be done without special permission from the pope, and i don't like to be "presented." if i can get permission without the humbug of putting on a black veil and receiving a blessing from pius, i shall; but i shrink from the formality of presentation. i know thou'd say "be presented." "our minister at that time had the reputation of being very careless of the needs and wishes of his countrymen, and i was not surprised to find a long delay. "in the course of my waiting, i had told my story to a young italian gentleman, the nephew of a monseigneur; a monseigneur being next in rank to a cardinal. he assured me that permission would never be obtained by our minister. "after a fortnight's waiting i received a permit, written on parchment, and signed by cardinal antonelli. "when the young italian next called, i held the parchment up in triumph, and boasted that minister ---- had at length moved in the matter. the young man coolly replied, 'yes, i spoke to my uncle last evening, and asked him to urge the matter with cardinal antonelli; but for that it would never have come!' there had been 'red tape,' and i had not seen it. "at the same time that the formal missive was sent to me, a similar one was sent to father secchi, authorizing him to receive me. the father called at once to make the arrangements for my visit. i made the most natural mistake! i supposed that the doors which opened to one woman, opened to all, and i asked to take with me my italian servant, a quick-witted and bright-eyed woman, who had escorted me to and from social parties in the evening, and who had learned in these walks the names of the stars, receiving them from me in english, and giving back to me the sweet italian words; and who had come to think herself quite an astronomer. father secchi refused at once. he said i was to meet him at the church of st. ignasio at one and a half hours before ave marie, and he would conduct me through the church into the observatory. my servant might come into the church with me. the ave marie bell rings half an hour after sunset. "at the appointed time, the next fine day,--and all days seem to be fine,--we set out on our mission. "when we entered the church we saw, far in the distance, father secchi, standing just behind a pillar. he slipped out a little way, as much as to say, 'i await you,' but did not come forward to meet us; so the woman and i passed along through the rows of kneeling worshippers, by the strolling students, and past the lounging tourists--who, guide-book in hand, are seen in every foreign church--until we came to the standpoint from which the father had been watching us. "then the italian woman put up a petition, not one word of which i could understand, but the gestures and the pointing showed that she begged to go on and enter the monastery and see the observatory. father secchi said, 'no, the holy father gave permission to one only,' and alone i entered the monastery walls. "through long halls, up winding staircases, occasionally stopped by some priest who touched his broad hat and asked 'parlate italiano?' occasionally passed by students, often stopped by pictures on the walls,--once to be introduced to a professor; then through the library of the monastery, full of manuscripts on which monks had worked away their lives; then through the astronomical library, where young astronomers were working away theirs, we reached at length the dome and the telescope. "one observatory is so much like another that it does not seem worth while to describe father secchi's. this observatory has a telescope about the size of that at washington (about twelve inches). secchi had no staff, and no prescribed duties. the base of the observatory was the solid foundation of the old roman building. the church was built in , and the monastery in part at that time, certainly the dome of the room in which was the meridian instrument. "the staircase is cut out of the old roman walls, which no roll of carriage, except that of the earthquake chariot, can shake. "having no prescribed duties, secchi could follow his fancies--he could pick up comets as he picked up bits of mosaic upon the roman forum. he learns what himself and his instruments can do, and he keeps to that narrow path. "he was at that time much interested in celestial photography. "italy must be the very paradise of astronomers; certainly i never saw objects so well before; the purity of the air must be very superior to ours. we looked at venus with a power of , but it was not good. jupiter was beautiful, and in broad daylight the belts were plainly seen. with low powers the moon was charming, but the air would not bear high ones. "father secchi said he had used a power of , , but that was more common. i have rarely used . saturn was exquisite; the rings were separated all around; the dusky ring could be seen, and, of course, the shadow of the ball upon the ring. "the spectroscopic method of observing starlight was used by secchi as early as by any astronomer. by this method the starlight is analyzed, and the sunlight is analyzed, and the two compared. if it does not disclose absolutely what are the peculiarities of starlight and sunlight, relatively, it traces the relationship. "in order to be successful in this kind of observation, the telescope must keep very accurately the motion of the earth in its axis; and so the papal government furnishes nice machinery to keep up with this motion,--the same motion for declaring whose existence galileo suffered! the two hundred years had done their work. "i should have been glad to stay until dark to look at nebulae, but the father kindly informed me that my permission did not extend beyond the daylight, which was fast leaving us, and conducting me to the door he informed me that i must make my way home alone, adding, 'but we live in a civilized country.' "i did not express to him the doubt that rose to my thoughts! the ave marie bell rings half an hour after sunset, and before that time i must be out of the observatory and at my own house." chapter viii - first european tour concluded--mrs. somerville--humboldt--mrs. mitchell's death--removal to lynn, mass.--present of an equatorial telescope-extracts from letters "i had no hope, when i went to europe, of knowing mrs. somerville. american men of science did not know her, and there had been unpleasant passages between the savants of europe and those of the united states which made my friends a little reluctant about giving me letters. "professor henry offered to send me letters, and said that among them should be one to mrs. somerville; but when his package came, no such letter appeared, and i did not like to press the matter,--indeed, after i had been in england i was not surprised at any amount of reluctance. they rarely asked to know my friends, and yet, if they were made known to them, they did their utmost. "so i went to europe with no letter to mrs. somerville, and no letter to the herschels. "i was very soon domesticated with the airys, and really felt my importance when i came to sleep in one of the round rooms of the royal observatory. i dared give no hint to the airys that i wanted to know the herschels, although they were intimate friends. 'what was i that i should love them, save for feeling of the pain?' but one fine day a letter came to mrs. airy from lady herschel, and she asked, 'would not miss mitchell like to visit us?' of course miss mitchell jumped at the chance! mrs. airy replied, and probably hinted that miss mitchell 'could be induced,' etc. "if the airys were old friends of mrs. somerville, the herschels were older. the airys were just and kind to me; the herschels were lavish, and they offered me a letter to mrs. somerville. "so, provided with this open sesame to mrs. somerville's heart, i called at her residence in florence, in the spring of . "i sent in the letter and a card, and waited in the large florentine parlor. in the open fireplace blazed a wood fire very suggestive of american comfort--very deceitful in the suggestion, for there is little of home comfort in italy. "after some little delay i heard a footstep come shuffling along the outer room, and an exceedingly tall and very old man entered the room, in the singular head-dress of a red bandanna turban, approached me, and introduced himself as dr. somerville, the husband. "he was very proud of his wife, and very desirous of talking about her, a weakness quite pardonable in the judgment of one who is desirous to know. he began at once on the subject. mrs. somerville, he said, took great interest in the americans, for she claimed connection with the family of george washington. "washington's half-brother, lawrence, married anne fairfax, who was one of the scotch family. when lieutenant fairfax was ordered to america, washington wrote to him as a family relative, and asked him to make him a visit. lieutenant fairfax applied to his commanding officer for permission to accept, and it was refused. they never met, and much to the regret of the fairfax family the letter of washington was lost. the fairfaxes of virginia are of the same family, and occasionally some member of the american branch returns to see his scotch cousins. "while dr. somerville was eagerly talking of these things, mrs. somerville came tripping into the room, speaking at once with the vivacity of a young person. she was seventy-seven years old, but appeared twenty years younger. she was not handsome, but her face was pleasing; the forehead low and broad; the eyes blue; the features so regular, that in the marble bust by chantrey, which i had seen, i had considered her handsome. "neither bust nor picture, however, gives a correct idea of her, except in the outline of the head and shoulders. "she spoke with a strong scotch accent, and was slightly affected with deafness, an infirmity so common in england and scotland. "while mrs. somerville talked, the old gentleman, seated by the fire, busied himself in toasting a slice of bread on a fork, which he kept at a slow-toasting distance from the coals. an english lady was present, learned in art, who, with a volubility worthy of an american, rushed into every little opening of mrs. somerville's more measured sentences with her remarks upon recent discoveries in _her_ specialty. whenever this occurred, the old man grew fidgety, moved the slice of bread backwards and forwards as if the fire were at fault, and when, at length, the english lady had fairly conquered the ground, and was started on a long sentence, he could bear the eclipse of his idol no longer, but, coming to the sofa where we sat, he testily said, 'mrs. somerville would rather talk on science than on art.' "mrs. somerville's conversation was marked by great simplicity; it was rather of the familiar and chatty order, with no tendency to the essay style. she touched upon the recent discoveries in chemistry or the discovery of gold in california, of the nebulae, more and more of which she thought might be resolved, and yet that there might exist nebulous matters, such as compose the tails of comets, of the satellites, of the planets, the last of which she thought had other uses than as subordinates. she spoke with disapprobation of dr. whewell's attempt to prove that our planet was the only one inhabited by reasoning beings; she believed that a higher order of beings than ourselves might people them. "on subsequent visits there were many questions from mrs. somerville in regard to the progress of science in america. she regretted, she said, that she knew so little of what was done in our country. "from lieutenant maury, alone, she received scientific papers. she spoke of the late dr. (nathaniel) bowditch with great interest, and said she had corresponded with one of his sons. she asked after professor peirce, whom she considered a great mathematician, and of the bonds, of cambridge. she was much interested in their photography of the stars, and said it had never been done in europe. at that time photography was but just applied to the stars. i had carried to the royal astronomical society the first successful photograph of a star. it was that of mizar and alcor, in the great bear. (since that time all these things have improved.) "the last time i saw mrs. somerville, she took me into her garden to show me her rose-bushes, in which she took great pride. mrs. somerville was not a mathematician only, she spoke italian fluently, and was in early life a good musician. "i could but admire mrs. somerville as a woman. the ascent of the steep and rugged path of science had not unfitted her for the drawing-room circle; the hours of devotion to close study have not been incompatible with the duties of wife and mother; the mind that has turned to rigid demonstration has not thereby lost its faith in those truths which figures will not prove. 'i have no doubt,' said she, in speaking of the heavenly bodies, 'that in another state of existence we shall know more about these things.' "mrs. somerville, at the age of seventy-seven, was interested in every new improvement, hopeful, cheery, and happy. her society was sought by the most cultivated people in the world. [she died at ninety-two.] "berlin, may , . humboldt had replied to my letter of introduction by a note, saying that he should be happy to see me at p.m., may . of course i was punctual. humboldt is one of several residents in a very ordinary-looking house on oranienberge strasse. "all along up the flight of stairs to his room were printed notices telling persons where to leave packages and letters for alexander humboldt. "the servant showed me at first into a sort of anteroom, hung with deers' horns and carpeted with tigers' skins, then into the study, and asked me to take a seat on the sofa. the room was very warm; comfort was evidently carefully considered, for cushions were all around; the sofa was handsomely covered with worsted embroidery. a long study-table was full of books and papers. "i had waited but a few moments when humboldt came in; he was a smaller man than i had expected to see. he was neater, more 'trig,' than the pictures represent him; in looking at the pictures you feel that his head is too large,--out of proportion to the body,--but you do not perceive this when you see him. "he bowed in a most courtly manner, and told me he was much obliged to me for coming to see him, then shook hands, and asked me to sit, and took a chair near me. "there was a clock in sight, and i stayed but half an hour. he talked every minute, and on all kinds of subjects: of dr. bache, who was then at the head of the u.s. coast survey; of dr. gould, who had recently returned from long years in south america; of the washington observatory and its director, lieutenant maury; of the dudley observatory, at albany; of sir george airy, of the greenwich observatory; of professor enke's comet reputation; of argelander, who was there observing variable stars; of mrs. somerville and goldschmidt, and of his brother. "it was the period when the subject of admitting kansas as a slave state was discussed--he touched upon that; it was during the administration of president buchanan, and he talked about that. "having been nearly a year in europe, i had not kept up my reading of american newspapers, but humboldt could tell me the latest news, scientifically and politically. to my ludicrous mortification, he told me of the change of position of some scientific professor in new york state, and when i showed that i didn't know the location of the town, which was clinton, he told me if i would look at the map, which lay upon the table, i should find the town somewhere between albany and buffalo. "humboldt was always considered a good-tempered, kindly-natured man, but his talk was a little fault-finding. "he said: 'lieutenant maury has been useful, but for the director of an observatory he has put forth some strange statements in the 'geography of the sea.' "he asked me if mrs. somerville was now occupied with pure mathematics. he said: 'there she is strong. i never saw her but once. she must be over sixty years old.' in reality she was seventy-seven. he spoke with admiration of mrs. somerville's 'physical geography,'--said it was excellent because so concise. 'a german woman would have used more words.' "humboldt asked me if they could apply photography to the small stars--to the eighth or ninth magnitude. i had asked the same question of professor bond, of cambridge, and he had replied, 'give me $ , , and we can do it; but it is very expensive.' "humboldt spoke of the fifty-three small planets, and gave his opinion that they could not be grouped together; that there was no apparent connection. "having lost all his teeth, humboldt's articulation was indistinct--he talked very rapidly. his hair was thin and very white, his eyes very blue, his nose too broad and too flat; yet he was a handsome man. he wore a white necktie, a black dress-coat, buttoned up, but not so much so that it hid a figured dark-blue and white waistcoat. he was a little deaf. he told me that he was eighty-nine years old, and that he and bonpland, alone, were living of those who in early life were on expeditions together; that bonpland was eighty-five, and much the more vigorous of the two. "he said that we had gone backwards, morally, in america since he was there,--that then there were strong men there: jefferson, and hamilton, and madison; that the three months he spent in america were spent almost wholly with jefferson. "in the course of conversation he told me that the fifth volume of 'cosmos' was in preparation. he urged me to go to see argelander on my way to london; he followed me out, still urging me to do this, and at the same time assured me that kansas would go all right. "it was singular that humboldt should advise me to use the sextant; it was the first instrument that i ever used, and it is a very difficult one. no young aspirant in science ever left humboldt's presence uncheered, and no petty animosities come out in his record. you never heard of humboldt's complaining that any one had stolen his thunder,--he knew that no one could lift his bolts. "when i came away, he thanked me again for the visit, followed me into the anteroom, and made a low bow." in mrs. mitchell was taken suddenly ill, and although partial recovery followed, her illness lasted for six years, during which time maria was her constant nurse. for most of the six years her mother's condition was such that merely a general care was needed, but it used to be said that maria's eyes were always upon her. when the opportunity to go to europe came, an older sister came with her family to take maria's place in the home; and when miss mitchell returned she found her mother so nearly in the state in which she had left her, that she felt justified in having taken the journey. mrs. mitchell died in , and a few months after her death mr. mitchell and his daughter removed to lynn, mass.--miss mitchell having purchased a small house in that city, in the rear of which she erected the little observatory brought from nantucket. she was very much depressed by her mother's death, and absorbed herself as much as possible in her observations and in her work for the nautical almanac. soon after her return from europe she had been presented with an equatorial telescope, the gift of american women, through miss elizabeth peabody. the following letter refers to this instrument: letter from admiral smyth. st. john's lodge, near aylesbury, - -' . my dear miss mitchell: ... we are much pleased to hear of your acquisition of an equatorial instrument under a revolving roof, for it is a true scientific luxury as well as an efficient implement. the aperture of your object-glass is sufficient for doing much useful work, but, if i may hazard an opinion to you, do not attempt too much, for it is quality rather than quantity which is now desirable. i would therefore leave the multiplication of objects to the larger order of telescopes, and to those who are given to sweep and ransack the heavens, of whom there is a goodly corps. now, for your purpose, i would recommend a batch of neat, but not over-close, binary systems, selected so as to have always one or the other on hand. i, however, have been bestirring myself to put amateurs upon a more convenient and, i think, a better mode of examining double stars than by the wire micrometer, with its faults of illumination, fiddling, jumps, and dirty lamps. this is by the beautiful method of rock-crystal prisms, not the rochon method of double-image, but by thin wedges cut to given angles. i have told mr. alvan clark my "experiences." and i hope he will apply his excellent mind to the scheme. i am insisting upon this point in some astronomical twaddle which i am now printing, and of which i shall soon have to request your acceptance of a copy. there is a very important department which calls for a zealous amateur or two, namely, the colors of double stars, for these have usually been noted after the eye has been fatigued with observing in illuminated fields. the volume i hope to forward--_en hommage_--will contain all the pros and cons of this branch. there is, for ultimate utility, nothing like forming a plan and then steadily following it. those who profess they will attend to everything often fall short of the mark. the division of labor leads to beneficial conclusions as well in astronomy as in mechanics and arts. mrs. smyth and my daughter unite with me in wishing you all happiness and success; and believe me my dear miss mitchell, yours very faithfully, w. h. smyth. in regard to the colors of stars, miss mitchell had already begun their study, as these extracts from her diary show: "feb. , . i am just learning to notice the different colors of the stars, and already begin to have a new enjoyment. betelgeuse is strikingly red, while rigel is yellow. there is something of the same pleasure in noticing the hues that there is in looking at a collection of precious stones, or at a flower-garden in autumn. blue stars i do not yet see, and but little lilac except through the telescope. "feb. , .... i swept around for comets about an hour, and then i amused myself with noticing the varieties of color. i wonder that i have so long been insensible to this charm in the skies, the tints of the different stars are so delicate in their variety. ... what a pity that some of our manufacturers shouldn't be able to steal the secret of dyestuffs from the stars, and astonish the feminine taste by new brilliancy in fashion. [footnote: see chapter xi.] [nantucket], april [ ]. my dear: your father just gave me a great fright by "tapping at my window" (i believe poe's was a door, wasn't it?) and holding up your note. i was busy examining some star notices just received from russia or germany,--i never knew where dorpat is.--and just thinking that my work was as good as theirs. i always noticed that when school-teachers took a holiday in order to visit other institutions they came home and quietly said, "no school is better or as good as mine." and then i read your note, and perceive your reading is as good as mrs. kemble's. now, being _modest_, i always felt afraid the reason i thought you such a good reader was because i didn't know any better, but if all the world is equally ignorant, it makes it all right.... i've been intensely busy. i have been looking for the little inferior planet to cross the sun, which it hasn't done, and i got an article ready for the paper and then hadn't the courage to publish--not for fear of the readers, but for fear that i should change my own ideas by the time 'twas in print. i am hoping, however, to have something by the meeting of the scientific association in august,--some paper,--not to get reputation for myself,--my reputation is so much beyond me that as policy i should keep quiet,--but in order that my telescope may show that it is at work. i am embarrassed by the amount of work it might do--as you do not know which of mrs. browning's poems to read, there are so many beauties. the little republic of san marino presented miss mitchell, in , with a bronze medal of merit, together with the _ribbon_ and _letters patent_ signed by the two captains regent. this medal she prized as highly as the gold one from denmark. "nantucket, may , [ ].... i send you a notice of an occultation; the last sentence and the last figures are mine. you and i can never occult, for have we not always helped one another to shine? do you have worcester's dictionary? i read it continually. did you feast on 'the marble faun'? i have a charming letter from una hawthorne, herself a poet by nature, all about 'papa's book.' ought not mr. hawthorne to be the happiest man alive? he isn't, though! do save all the anecdotes you possibly can, piquant or not; starved people are not over-nice. lynn, jan. [ ]. ... i very rarely see the b----s; they go to a different church, and you know with that class of people "not to be with us is to be against us." indeed, i know very little of lynn people. if i can get at mr. j., when you come to see me i'll ask him to tea. he has called several times, but he's in such demand that he must be engaged some weeks in advance! would you, if you lived in lynn, want to fall into such a mass of idolaters? i was wretchedly busy up to december , but have got into quiet seas again. i have had a great deal of company--not a person that i did not want to see, but i can't make the days more than twenty-four hours long, with all my economy of time. this week professor crosby, of salem, comes up with his graduating class and his corps of teachers for an evening. they remained in lynn until miss mitchell was called to vassar college, in , as professor of astronomy and director of the observatory. chapter ix - life at vassar college in her life at vassar college there was a great deal for miss mitchell to get accustomed to; if her duties had been merely as director of the observatory, it would have been simply a continuation of her previous work. but she was expected, of course, to teach astronomy; she was by no means sure that she could succeed as a teacher, and with this new work on hand she could not confine herself to original investigation--that which had been her great aim in life. but she was so much interested in the movement for the higher education of women, an interest which deepened as her work went on, that she gave up, in a great measure, her scientific life, and threw herself heart and soul into this work. for some years after she went to vassar, she still continued the work for the nautical almanac; but after a while she relinquished that, and confined herself wholly to the work in the college. " . vassar college brought together a mass of heterogeneous material, out of which it was expected that a harmonious whole would evolve--pupils from all parts of the country, of different habits, different training, different views; teachers, mostly from new england, differing also; professors, largely from massachusetts, yet differing much. and yet, after a year, we can say that there has been no very noisy jarring of the discordant elements; small jostling has been felt, but the president has oiled the rough places, and we have slid over them. "... miss ---- is a bigot, but a very sincere one. she is the most conservative person i ever met. i think her a very good woman, a woman of great energy.... she is very kind to me, but had we lived in the colonial days of massachusetts, and had she been a power, she would have burned me at the stake for heresy! "yesterday the rush began. miss lyman [the lady principal] had set the twenty teachers all around in different places, and i was put into the parlor to talk to 'anxious mothers.' "miss lyman had a hoarse cold, but she received about two hundred students, and had all their rooms assigned to them. "while she had one anxious mamma, i took two or three, and kept them waiting until she could attend to them. several teachers were with me. i made a rush at the visitors as they entered, and sometimes i was asked if i were lady principal, and sometimes if i were the matron. this morning miss lyman's voice was gone. she must have seen five hundred people yesterday. "among others there was one miss mitchell, and, of course, that anxious mother put that girl under my special care, and she is very bright. then there were two who were sent with letters to me, and several others whose mothers took to me because they were frightened by miss lyman's _style_. "one lady, who seemed to be a bright woman, got me by the button and held me a long time--she wanted this, that, and the other impracticable thing for the girl, and told me how honest her daughter was; then with a flood of tears she said, 'but she is not a christian. i know i put her into good hands when i put her here.' (then i was strongly tempted to avow my unitarianism.) miss w., who was standing by, said, 'miss lyman will be an excellent spiritual adviser,' and we both looked very serious; when the mother wiped her weeping eyes and said, 'and, miss mitchell, will you ask miss lyman to insist that my daughter shall curl her hair? she looks very graceful when her hair is curled, and i want it insisted upon,' i made a note of it with my pencil, and as i happened to glance at miss w. the corners of her mouth were twitching, upon which i broke down and laughed. the mother bore it very good-naturedly, but went on. she wanted to know who would work some buttonholes in her daughter's dress that was not quite finished, etc., and it all ended in her inviting me to make her a visit. "oct. , . our faculty meetings always try me in this respect: we do things that other colleges have done before. we wait and ask for precedent. if the earth had waited for a precedent, it never would have turned on its axis! "sept. , . i have written to-day to give up the nautical almanac work. i do not feel sure that it will be for the best, but i am sure that i could not hold the almanac and the college, and father is happy here. "i tell miss lyman that my father is so much pleased with everything here that i am afraid he will be immersed!" [footnote: vassar college, though professedly unsectarian, was mainly under baptist control.] only those who knew vassar college in its earlier days can tell of the life that the father and daughter led there for four years. mr. mitchell died in . [illustration: the father and daughter] "jan. , . meeting dr. hill at a private party, i asked him if harvard college would admit girls in fifty years. he said one of the most conservative members of the faculty had said, within sixteen days, that it would come about in twenty years. i asked him if i could go into one of professor peirce's recitations. he said there was nothing to keep me out, and that he would let me know when they came. "at eleven a.m., the next friday, i stood at professor peirce's door. as the professor came in i went towards him, and asked him if i might attend his lecture. he said 'yes.' i said 'can you not say "i shall be happy to have you"?' and he said 'i shall be happy to have you,' but he didn't look happy! "it was with some little embarrassment that mrs. k. and i seated ourselves. sixteen young men came into the room; after the first glance at us there was not another look, and the lecture went on. professor peirce had filled the blackboard with formulae, and went on developing them. he walked backwards and forwards all the time, thinking it out as he went. the students at first all took notes, but gradually they dropped off until perhaps only half continued. when he made simple mistakes they received it in silence; only one, that one his son (a tutor in college), remarked that he was wrong. the steps of his lesson were all easy, but of course it was impossible to tell whence he came or whither he was going.... "the recitation-room was very common-looking--we could not tolerate such at vassar. the forms and benches of the recitation-room were better for taking notes than ours are. "the professor was polite enough to ask us into the senior class, but i had an engagement. i asked him if a young lady presented herself at the door he _could_ keep her out, and he said 'no, and i shouldn't.' i told him i would send some of my girls. "oct. , . resolved, in case of my outliving father and being in good health, to give my efforts to the intellectual culture of women, without regard to salary; if possible, connect myself with liberal christian institutions, believing, as i do, that happiness and growth in this life are best promoted by them, and that what is good in this life is good in any life." in august, , miss mitchell, with several of her vassar students, went to burlington, ia., to observe the total eclipse of the sun. she wrote a popular account of her observations, which was printed in "hours at home" for september, . her records were published in professor coffin's report, as she was a member of his party. "sept. , . my classes came in to-day for the first time; twenty-five students--more than ever before; fine, splendid-looking girls. i felt almost frightened at the responsibility which came into my hands--of the possible _twist_ which i might give them. " . i never look upon the mass of girls going into our dining-room or chapel without feeling their nobility, the sovereignty of their pure spirit." the following letter from miss mitchell, though written at a later date, gives an idea of the practical observing done by her classes: my dear miss ----: i reply to your questions concerning the observatory which you propose to establish. and, first, let me congratulate you that you begin _small_. a large telescope is a great luxury, but it is an enormous expense, and not at all necessary for teaching.... my beginning class uses only a small portable equatorial. it stands out-doors from a.m. to p.m. the girls are encouraged to use it: they are expected to determine the rotation of the sun on its axis by watching the spots--the same for the planet jupiter; they determine the revolution of titan by watching its motions, the retrograde and direct motion of the planets among the stars, the position of the sun with reference to its setting in winter and summer, the phases of venus. all their book learning in astronomy should be mathematical. the astronomy which is not mathematical is what is so ludicrously called "geography of the heavens"--is not astronomy at all. my senior class, generally small, say six, is received as a class, but in practical astronomy each girl is taught separately. i believe in _small_ classes. i instruct them separately, first in the use of the meridian instrument, and next in that of the equatorial. they obtain the time for the college by meridian passage of stars; they use the equatorial just as far as they can do with very insufficient mechanism. we work wholly on planets, and they are taught to find a planet at any hour of the day, to make drawings of what they see, and to determine positions of planets and satellites. with the clock and chronograph they determine difference of right ascension of objects by the electric mode of recording. they make, sometimes, very accurate drawings, and they learn to know the satellites of saturn (titan, rhea, etc.) by their different physiognomy, as they would persons. they have sometimes measured diameters. if you add to your observatory a meridian instrument, i should advise a small one. _size_ is not so important as people generally suppose. nicety and accuracy are what is needed in all scientific work; startling effects by large telescopes and high powers are too suggestive of sensational advertisement. the relation between herself and her pupils was quite remarkable--it was very cordial and intimate; she spoke of them always as her "girls," but at the same time she required their very best work, and was intolerant of shirking, or of an ambition to do what nature never intended the girl in question to do. one of her pupils writes thus: "if it were only possible to tell you of what professor mitchell did for one of her girls! 'her girls!' it meant so much to come into daily contact with such a woman! there is no need of speaking of her ability; the world knows what that was. but as her class-room was unique, having something of home in its belongings, so its atmosphere differed from that of all others. anxiety and nervous strain were left outside of the door. perhaps one clue to her influence may be found in her remark to the senior class in astronomy when ' entered upon its last year: 'we are women studying together.' "occasionally it happened that work requiring two hours or more to prepare called for little time in the class. then would come one of those treats which she bestowed so freely upon her girls, and which seemed to put them in touch with the great outside world. letters from astronomers in europe or america, or from members of their families, giving delightful glimpses of home life; stories of her travels and of visits to famous people; accounts of scientific conventions and of large gatherings of women,--not so common then as now,--gave her listeners a wider outlook and new interests. "professor mitchell was chairman of a standing committee of the american association for the advancement of women,--that on women's work in science,--and some of her students did their first work for women's organizations in gathering statistics and filling out blanks which she distributed among them. "the benefits derived from my college course were manifold, but time and money would have been well spent had there been no return but that of two years' intercourse with maria mitchell." another pupil, and later her successor at vassar college, miss mary w. whitney, has said of her method of teaching: "as a teacher, miss mitchell's gift was that of stimulus, not that of drill. she could not drill; she would not drive. but no honest student could escape the pressure of her strong will and earnest intent. the marking system she held in contempt, and wished to have nothing to do with it. 'you cannot mark a human mind,' she said, 'because there is no intellectual unit;' and upon taking up her duties as professor she stipulated that she should not be held responsible for a strict application of the system." "july, . my students used to say that my way of teaching was like that of the man who said to his son, 'there are the letters of the english alphabet--go into that corner and learn them.' "it is not exactly my way, but i do think, as a general rule, that teachers talk too much! a book is a very good institution! to read a book, to think it over, and to write out notes is a useful exercise; a book which will not repay some hard thought is not worth publishing. the fashion of lecturing is becoming a rage; the teacher shows herself off, and she does not try enough to develop her pupils. "the greatest object in educating is to give a right habit of study.... * * * * * "... not too much mechanical apparatus--let the imagination have some play; a cube may be shown by a model, but let the drawing upon the blackboard represent the cube; and if possible let nature be the blackboard; spread your triangles upon land and sky. "one of my pupils always threw her triangles on the celestial vault above her head.... "a small apparatus well used will do wonders. a celebrated chemist ordered his servant to bring in the laboratory--on a tray! newton rolled up the cover of a book; he put a small glass at one end, and a large brain at the other--it was enough. * * * * * "when a student asks me, 'what specialty shall i follow?' i answer, 'adopt some one, if none draws you, and wait.' i am confident that she will find the specialty engrossing. "feb. , . when i came to vassar, i regretted that mr. vassar did not give full scholarships. by degrees, i learned to think his plan of giving half scholarships better; and to-day i am ready to say, 'give no scholarships at all.' "i find a helping-hand lifts the girl as crutches do; she learns to like the help which is not self-help. "if a girl has the public school, and wants enough to learn, she will learn. it is hard, but she was born to hardness--she cannot dodge it. labor is her inheritance. "i was born, for instance, incapable of appreciating music. i mourn it. should i go to a music-school, therefore? no, avoid the music-school; it is a very expensive branch of study. when the public school has taught reading, writing, and arithmetic, the boy or girl has his or her tools; let them use these tools, and get a few hours for study every day. "... do not give educational aid to sickly young people. the old idea that the feeble young man must be fitted for the ministry, because the more sickly the more saintly, has gone out. health of body is not only an accompaniment of health of mind, but is the cause; the converse may be true,--that health of mind causes health of body; but we all know that intellectual cheer and vivacity act upon the mind. if the gymnastic exercise helps the mind, the concert or the theatre improves the health of the body. "let the unfortunate young woman whose health is delicate take to the culture of the woods and fields, or raise strawberries, and avoid teaching. "better give a young girl who is poor a common-school education, a little lift, and tell her to work out her own career. if she have a distaste to the homely routine of life, leave her the opportunity to try any other career, but let her understand that she stands or falls by herself. "... not every girl should go to college. the over-burdened mother of a large family has a right to be aided by her daughter's hands. i would aid the mother and not the daughter. "i would not put the exceptionally smart girl from a _very_ poor family into college, unless she is a genius; and a genius should wait some years to _prove_ her genius. "endow the already established institution with money. endow the woman who shows genius with _time_. "a case at johns hopkins university is an excellent one. a young woman goes into the institution who is already a scholar; she shows what she can do, and she takes a scholarship; she is not placed in a happy valley of do nothing,--she is put into a workshop, where she can work. "... we are all apt to say, 'could we have had the opportunity in life that our neighbor had,'--and we leave the unfinished sentence to imply that we should have been geniuses. "no one ever says, 'if i had not had such golden opportunities thrust upon me, i might have developed by a struggle'! but why look back at all? why turn your eyes to your shadow, when, by looking upward, you see your rainbow in the same direction? "but our want of opportunity was our opportunity--our privations were our privileges--our needs were stimulants; we are what we are because we had little and wanted much; and it is hard to tell which was the more powerful factor.... * * * * * "small aids to individuals, large aid to masses. * * * * * "the russian czar determined to found an observatory, and the first thing he did was to take a million dollars from the government treasury. he sends to america to order a thirty-five inch telescope from alvan clark,--not to promote science, but to surpass other nations in the size of his glass. 'to him that hath shall be given.' read it, 'to him that hath _should_ be given.' * * * * * "to give wisely is hard. i do not wonder that the millionaire founds a new college--why should he not? millionaires are few, and he is a man by himself--he must have views, or he could not have earned a million. but let the man or woman of ordinary wealth seek out the best institution already started,--the best girl already in college,--and give the endowment. "i knew a rich woman who wished to give aid to some girls' school, and she travelled in order to find that institution which gave the most solid learning with the least show. she found it where few would expect it,--in tennessee. it was worth while to travel. "the aid that comes need not be money; let it be a careful consideration of the object, and an evident interest in the cause. "when you aid a teacher, you improve the education of your children. it is a wonder that teachers work as well as they do. i never look at a group of them without using, mentally, the expression, 'the noble army of martyrs'! "the chemist should have had a laboratory, and the observatory should have had an astronomer; but we are too apt to bestow money where there is no man, and to find a man where there is no money. * * * * * "if every girl who is aided were a very high order of scholar, scholarship would undoubtedly conquer poverty; but a large part of the aided students are ordinary. they lack, at least, executive power, as their ancestors probably did. poverty is a misfortune; misfortunes are often the result of blamable indiscretion, extravagance, etc. "it is one of the many blessings of poverty that one is not obliged to 'give wisely.'" . _to her students:_ "i cannot expect to make astronomers, but i do expect that you will invigorate your minds by the effort at healthy modes of thinking.... when we are chafed and fretted by small cares, a look at the stars will show us the littleness of our own interests. "... but star-gazing is not science. the entrance to astronomy is through mathematics. you must make up your mind to steady and earnest work. you must be content to get on slowly if you only get on thoroughly.... "the phrase 'popular science' has in itself a touch of absurdity. that knowledge which is popular is not scientific. "the laws which govern the motions of the sun, the earth, planets, and other bodies in the universe, cannot be understood and demonstrated without a solid basis of mathematical learning. * * * * * "every formula which expresses a law of nature is a hymn of praise to god. * * * * * "you cannot study anything persistently for years without becoming learned, and although i would not hold reputation up to you as a very high object of ambition, it is a wayside flower which you are sure to have catch at your skirts. "whatever apology other women may have for loose, ill-finished work, or work not finished at all, you will have none. "when you leave vassar college, you leave it the _best educated women in the world_. living a little outside of the college, beyond the reach of the little currents that go up and down the corridors, i think i am a fairer judge of your advantages than you can be yourselves; and when i say you will be the best educated women in the world, i do not mean the education of text-books, and class-rooms, and apparatus, only, but that broader education which you receive unconsciously, that higher teaching which comes to you, all unknown to the givers, from daily association with the noble-souled women who are around you." " . when astronomers compare observations made by different persons, they cannot neglect the constitutional peculiarities of the individuals, and there enters into these computations a quantity called 'personal equation.' in common terms, it is that difference between two individuals from which results a difference in the _time_ which they require to receive and note an occurrence. if one sees a star at one instant, and records it, the record of another, of the same thing, is not the same. "it is true, also, that the same individual is not the same at all times; so that between two individuals there is a mean or middle individual, and each individual has a mean or middle self, which is not the man of to-day, nor the man of yesterday, nor the man of to-morrow; but a middle man among these different selves.... * * * * * "we especially need imagination in science. it is not all mathematics, nor all logic, but it is somewhat beauty and poetry. "there will come with the greater love of science greater love to one another. living more nearly to nature is living farther from the world and from its follies, but nearer to the world's people; it is to be of them, with them, and for them, and especially for their improvement. we cannot see how impartially nature gives of her riches to all, without loving all, and helping all; and if we cannot learn through nature's laws the certainty of spiritual truths, we can at least learn to promote spiritual growth while we are together, and live in a trusting hope of a greater growth in the future. "... the great gain would be freedom of thought. women, more than men, are bound by tradition and authority. what the father, the brother, the doctor, and the minister have said has been received undoubtingly. until women throw off this reverence for authority they will not develop. when they do this, when they come to truth through their investigations, when doubt leads them to discovery, the truth which they get will be theirs, and their minds will work on and on unfettered. [ .] "i am but a woman! "for women there are, undoubtedly, great difficulties in the path, but so much the more to overcome. first, no woman should say, 'i am but a woman!' but a woman! what more can you ask to be? "born a woman--born with the average brain of humanity--born with more than the average heart--if you are mortal, what higher destiny could you have? no matter where you are nor what you are, you are a power--your influence is incalculable; personal influence is always underrated by the person. we are all centres of spheres--we see the portions of the sphere above us, and we see how little we affect it. we forget the part of the sphere around and before us--it extends just as far every way. "another common saying, 'it isn't the way,' etc. who settles the way? is there any one so forgetful of the sovereignty bestowed on her by god that she accepts a leader--one who shall capture her mind? "there is this great danger in student life. now, we rest all upon what socrates said, or what copernicus taught; how can we dispute authority which has come down to us, all established, for ages? "we must at least question it; we cannot accept anything as granted, beyond the first mathematical formulae. question everything else. "'the world is round, and like a ball seems swinging in the air.'[ ] [footnote : from peter parley's primary geography.] "no such thing! the world is not round, it does not swing, and it doesn't _seem_ to swing! "i know i shall be called heterodox, and that unseen lightning flashes and unheard thunderbolts will be playing around my head, when i say that women will never be profound students in any other department except music while they give four hours a day to the _practice_ of music. i should by all means encourage every woman who is born with musical gifts to study music; but study it as a science and an art, and not as an accomplishment; and to every woman who is not musical, i should say, 'don't study it at all;' you cannot afford four hours a day, out of some years of your life, just to be agreeable in company upon _possible_ occasions. "if for four hours a day you studied, year after year, the science of language, for instance, do you suppose you would not be a linguist? do you put the mere pleasing of some social party, and the reception of a few compliments, against the mental development of four hours a day of study of something for which you were born? "when i see that girls who are required by their parents to go through with the irksome practising really become respectable performers, i wonder what four hours a day at something which they loved, and for which god designed them, would do for them. "i should think that to a real scientist in music there would be something mortifying in this rush of all women into music; as there would be to me if i saw every girl learning the constellations, and then thinking she was an astronomer! "jan. , . at the meeting of graduates at the deacon house, the speeches that were made were mainly those of dr. r. and professor b. i am sorry now that i did not at least say that the college is what it is mainly because the early students pushed up the course to a collegiate standard. "jan. , . it has become a serious question with me whether it is not my duty to beg money for the observatory, while what i really long for is a quiet life of scientific speculation. i want to sit down and study on the observations made by myself and others." during her later years at vassar, miss mitchell interested herself personally in raising a fund to endow the chair of astronomy. in march, , she wrote: "i have been in new york quite lately, and am quite hopeful that miss ---- will do something for vassar. mrs. c., of newburyport, is to ask whittier, who is said to be rich, and ---- told me to get anything i could out of her father. but after all i am a poor beggar; my ideas are small!" since miss mitchell's death, the fund has been completed by the alumnae, and is known as the maria mitchell endowment fund. with $ , appropriated by the trustees it amounts to $ , . "june , . i had imagined the emperor of brazil to be a dark, swarthy, tall man, of forty-five years; that he would not really have a crown upon his head, but that i should feel it was somewhere around, handy-like, and that i should know i was in royal presence. but he turns out to be a large, old man,--say, sixty-five,--broad-headed and broad-shouldered, with a big white beard, and a very pleasant, even chatty, manner. "once inside of the dome, he seemed to feel at home; to my astonishment he asked if alvan clark made the glass of the equatorial. as he stepped into the meridian-room, and saw the instruments, he said, 'collimators?' i said, 'you have been in observatories before.' 'oh, yes, cambridge and washington,' he replied. he seemed much more interested in the observatory than i could possibly expect. i asked him to go on top of the roof, and he said he had not time; yet he stayed long enough to go up several times. i am told that he follows out, remarkably, his own ideas as to his movements." in , miss mitchell went to denver, colorado, to observe the total eclipse of the sun. she was accompanied by several of her former pupils. she prepared an account of this eclipse, which will be found in chapter xi. "aug. , . dr. raymond [president of vassar college] is dead. i cannot quite take it in. i have never known the college without him, and it will make all things different. "personally, i have always been fond of him; he was very enjoyable socially and intellectually. officially he was, in his relations to the students, perfect. he was cautious to a fault, and has probably been very wise in his administration of college affairs. he was broad in his religious views. he was not broad in his ideas of women, and was made to broaden the education of women by the women around him. "june , . the dome party to-day was sixty-two in number. it was breakfast, and we opened the dome; we seated forty in the dome and twenty in the meridian-room." this "dome party" requires a few words of explanation, because it was unique among all the vassar festivities. the week before commencement, miss mitchell's pupils would be informed of the approaching gathering by a notice like the following: circular. the annual dome party will be held at the observatory on saturday, the th, at p.m. you are cordially invited to be present. m. m. [as this gathering is highly intellectual, you are invited to bring poems.] it was, at first, held in the evening, but during the last years was a breakfast party, its character in other respects remaining the same. little tables were spread under the dome, around the big telescope; the flowers were roses from miss mitchell's own garden. the "poems" were nonsense rhymes, in the writing of which miss mitchell was an adept. each student would have a few verses of a more or less personal character, written by miss mitchell, and there were others written by the girls themselves; some were impromptu; others were set to music, and sung by a selected glee-club. "june , . we have written what we call our dome poetry. some nice poems have come in to us. i think the vassar girls, in the main, are magnificent, they are so all-alive.... "may , . vassar is getting pretty. i gathered lilies of the valley this morning. the young robins are out in a tree close by us, and the phoebe has built, as usual, under the front steps. "i am rushing dome poetry, but so far show no alarming symptoms of brilliancy." a former student writes as follows about the dome poetry: "at the time it was read, though it seemed mere merry nonsense, it really served a more serious purpose in the work of one who did nothing aimlessly. this apparent nonsense served as the vehicle to convey an expression of approbation, affection, criticism, or disapproval in such a merry mode that even the bitterest draught seemed sweet." " , july . we left vassar, june , on the steamer 'galatea,' from new york to providence. i looked out of my state-room window, and saw a strange-looking body in the northern sky. my heart sank; i knew instantly that it was a comet, and that i must return to the observatory. calling the young people around me, and pointing it out to them, i had their assurance that it was a comet, and nothing but a comet. "we went to bed at nine, and i arose at six in the morning. as soon as i could get my nieces started for providence, i started for stonington,--the most easy of the ways of getting to new york, as i should avoid point judith. "i went to the boat at the stonington wharf about noon, and remained on board until morning--there were few passengers, it was very quiet, and i slept well. "arriving in new york, i took cars at a.m. for poughkeepsie, and reached the college at dinner-time. i went to work the same evening. "as i could not tell at what time the comet would pass the meridian, i stationed myself at the telescope in the meridian-room by p.m., and watched for the comet to cross. as it approached the meridian, i saw that it would go behind a scraggy apple-tree. i sent for the watchman, mr. crumb, to come with a saw, and cut off the upper limbs. he came back with an axe, and chopped away vigorously; but as one limb after another fell, and i said, 'i need more, cut away,' he said, 'i think i must cut down the whole tree.' i said, 'cut it down.' i felt the barbarism of it, but i felt more that a bird might have a nest in it. "i found, when i went to breakfast the next morning, that the story had preceded me, and i was called 'george washington.' "but for all this, i got almost no observation; the fog came up, and i had scarcely anything better than an estimation. i saw the comet blaze out, just on the edge of the field, and i could read its declination only. "on the th, th, and july st, i obtained good meridian passages, and the r.a. must be very good. "jan. , . there is a strange sentence in the last paragraph of dr. jacobi's article on the study of medicine by women, to the effect that it would be better for the husband always to be superior to the wife. why? and if so, does not it condemn the ablest women to a single life? "march , , p.m. i start for faculty, and we probably shall elect what are called the 'honor girls.' i dread the struggle that is pretty certain to come. each of us has some favorite whom she wishes to put into the highest class, and whom she honestly believes to be of the highest order of merit. i never have the whole ten to suit me, but i can truly say that at this minute i do not care. i should be sorry not to see s., and w., and p., and e., and g., and k. on the list of the ten, but probably that is more than i ought to expect. the whole system is demoralizing and foolish. girls study for prizes, and not for learning, when 'honors' are at the end. the unscholarly motive is wearing. if they studied for sound learning, the cheer which would come with every day's gain would be health-preserving. "... i have seven advanced students, and to-day, when i looked around to see who should be called to help look out for meteors, i could consider only _one_ of them not already overworked, and she was the post-graduate, who took no honors, and never hurried, and has always been an excellent student. "... we are sending home some girls already [november ], and ---- is among them. i am somewhat alarmed at the dropping down, but ---- does an enormous amount of work, belongs to every club, and writes for every club and for the 'vassar miscellany,' etc.; of course she has the headache most of the time. "sometimes i am distressed for fear dr. clarke [footnote: author of "sex in education."] is not so far wrong; but i do not think it is the study--it is the morbid conscientiousness of the girls, who think they must work every minute. "april , . miss herschel came to the college on the th, and stayed three days. she is one of the little girls whom i saw, twenty-three years since, playing on the lawn at sir john herschel's place, collingwood. "... miss herschel was just perfect as a guest; she fitted in beautifully. the teachers gave a reception for her, ---- gave her his poem, and henry, the gardener, found out that the man in whose employ he lost a finger was her brother-in-law, in leeds! "jan. , . mr. [matthew] arnold has been to the college, and has given his lecture on emerson. the audience was made up of three hundred students, and three hundred guests from town. never was a man listened to with so much attention. whether he is right in his judgment or not, he held his audience by his manly way, his kindly dissection, and his graceful english. socially, he charmed us all. he chatted with every one, he smiled on all. he said he was sorry to leave the college, and that he felt he must come to america again. we have not had such an awakening for years. it was like a new volume of old english poetry. "march , . in february, , i counted seconds for father, who observed the annular eclipse at nantucket. i was twelve and a half years old. in , fifty-four years later, i counted seconds for a class of students at vassar; it was the same eclipse, but the sun was only about half-covered. both days were perfectly clear and cold." chapter x second european tour--russia--frances power cobbe--"the glasgow college for girls" in , miss mitchell spent the summer in europe, and availed herself of this opportunity to visit the government observatory at pulkova, in russia. "eydkuhnen, wednesday, july , . certainly, i never in my life expected to spend twenty-four hours in this small town, the frontier town of prussia. here i remembered that our little bags would be examined, and i asked the guard about it, but he said we need not trouble ourselves; we should not be examined until we reached the first russian town of wiersbelow. so, after a mile more of travel, we came to wiersbelow. knowing that we should keep our little compartment until we got to st. petersburg, we had scattered our luggage about; gloves were in one place, veil in another, shawl in another, parasol in another, and books all around. "the train stopped. imagine our consternation! two officials entered the carriage, tall russians in full uniform, and seized everything--shawls, books, gloves, bags; and then, looking around very carefully, espied w's poor little ragged handkerchief, and seized that, too, as a contraband article! we looked at one another, and said nothing. the tall russian said something to us; we looked at each other and sat still. the tall russians looked at one another, and there was almost an official smile between them. "then one turned to me, and said, very distinctly, 'passy-port.' 'oh,' i said, 'the passports are all right; where are they?' and we produced from our pockets the passports prepared at washington, with the official seal, and we delivered them with a sort of air as if we had said, 'you'll find that they do things all right at washington.' "the tall russians got out, and i was about to breathe freely, when they returned, and said something else--not a word did i understand; they exchanged a look of amusement, and w. and i, one of amazement; then one of them made signs to us to get out. the sign was unmistakable, and we got out, and followed them into an immense room, where were tables all around covered with luggage, and about a hundred travellers standing by; and our books, shawls, gloves, etc., were thrown in a heap upon one of these tables, and we awoke to the disagreeable consciousness that we were in a custom-house, and only two out of a hundred travellers, and that we did not understand one word of russian. "but, of course, it could be only a few minutes of delay, and if german and french failed, there is always left the language of signs, and all would be right. "after, perhaps, half an hour, two or three officials approached us, and, holding the passports, began to talk to us. how did they know that those two passports belonged to us? out of two hundred persons, how could they at once see that the woman whose age was given at more than half a century, and the lad whose age was given at less than a score of years, were the two fatigued and weary travellers who stood guarding a small heap of gloves, books, handkerchiefs, and shawls? two of the officials held up the passports to us, pointed to the blank page, shook their heads ominously; the third took the passports, put them into his vest pocket, buttoned up his coat, and motioned to us to follow him. "we followed; he opened the door of an ordinary carriage, waved his hand for us to get in, jumped in himself, and we found we were started back. we could not cross the line between germany and russia. "we meekly asked where we were to go, and were relieved when we found that we went back only to the nearest town, but that the passports must be sent to konigsberg, sixty miles away, to be endorsed by the russian ambassador--it might take some days. w. was very much inclined to refuse to go back and to attempt a war of words, but it did not seem wise to me to undertake a war against the russian government; i know our country does not lightly go into an 'unpleasantness' of that kind.... "so we went back to eydkuhnen,--a little miserable german village. we took rooms at the only hotel, and there we stayed twenty-four hours. before the end of that time, we had visited every shop in the village, and aired our german to most of our fellow-travellers whom we met at the hotel. "the landlord took our part, and declared it was hard enough on simple travellers like ourselves to be stopped in such a way, and that russia was the only country in europe which was rigid in that respect. happily, our passports were back in twenty-four hours, and we started again; our trunks had been registered for st. petersburg, and to st. petersburg they had gone, ahead of us; and of the small heap of things thrown down promiscuously at the custom-house, the whole had not come back to us--it was not very important. i learned how to wear one glove instead of two, or to go without. "we had the ordeal of the custom-house to pass again; but once passed, and told that we were free to go on, it was like going into a clear atmosphere from a fog. we crossed the custom-house threshold into another room, and we found ourselves in russia, and in an excellent, well-furnished, and cheery restaurant. we lost the german smoke and the german beer; we found hot coffee and clean table-cloths. "we did not return to our dusty, red-velvet palace, but we entered a clean, comfortable compartment, with easy sofas, for the night. we started again for st. petersburg; we were now four days from london. i will omit the details of a break-down that night, and another change of cars. we had some sleep, and awoke in the morning to enjoy russia. "and, first, of russian railroads. when the railroads of russia were planned, the emperor nicholas allowed a large sum of money for the building. the engineer showed him his plan. the road wound by slight curves from one town to another. this did not suit the emperor at all. he took his ruler, put it down upon the table, and said: 'i choose to have my roads run so.' of course the engineer assented--he had his large fund granted; a straight road was much cheaper to build than a curved one. as a consequence, he built and furnished an excellent road. "at every 'verst,' which is not quite a mile, a small house is placed at the roadside, on which, in very large figures, the number of versts from st. petersburg is told. the train runs very smoothly and very slowly; twenty miles an hour is about the rate. of course the journey seemed long. for a large part of the way it was an uninhabited, level plain; so green, however, that it seemed like travelling on prairies. occasionally we passed a dreary little village of small huts, and as we neared st. petersburg we passed larger and better built towns, which the dome of some cathedral lighted up for miles. "the road was enlivened, too, by another peculiarity. the restaurants were all adorned by flags of all colors, and festooned by vines. at one place the green arches ran across the road, and we passed under a bower of evergreens. i accepted this, at first, as a russian peculiarity, and was surprised that so much attention was paid to travellers; but i learned that it was not for us at all. the duke of edinboro' had passed over the road a few days before, on his way to st. petersburg, for his betrothal to the only daughter of the czar, and the decorations were for him; and so we felt that we were of the party, although we had not been asked. "we approached st. petersburg just at night, and caught the play of the sunlight on the domes. it is a city of domes--blue domes, green domes, white domes, and, above all, the golden dome of the cathedral of st. isaac's. "it is almost never a single dome. st. isaac's central, gilded dome looms up above its fellow domes, but four smaller ones surround it. "it was summer; the temperature was delightful, about like our october. the showers were frequent, there was no dust and no sultry air. "there must be a great deal of nice mechanical work required in st. petersburg, for on the nevsky perspective, the principal street, there were a great many shops in which graduating and measuring instruments of very nice workmanship were for sale. especially i noticed the excellence of the thermometers, and i naturally stopped to read them. figures are a common language, but it was clear that i was in another planet; i could not read the thermometers! i judged that the weather was warm enough for the thermometer to be at . i read, say, . and then i remembered that the russians do not put their freezing point at , as we do, and i was obliged to go through a troublesome calculation before i could tell how warm it was. "but i came to a still stranger experience. i dated my letters august , and went to my banker's, before i sealed them, to see if there were letters for me. the banker's little calendar was hanging by his desk, and the day of the month was on exhibition, in large figures. i read, july ! this was distressing! was i like alice in wonderland? did time go backward? surely, i had dated august . could i be in error twelve days? and then i perceived that twelve days was just the difference of old and new calendars. "how many times i had taught students that the russians still counted their time by the 'old style,' but had never learned it myself! and so i was obliged to teach myself new lessons in science. the earth turns on its axis just the same in russia as in boston, but you don't get out of the sunlight at the boston sunset hour. "when the thermometer stands at in st. petersburg, it does not freeze as it does in boston. on the contrary, it is very warm in st. petersburg, for it means what does in boston. and if you leave london on the d of july, and are five days on the way to st. petersburg, a week after you get there it is still the d of july! and we complain that the day is too short! "another peculiarity. we strolled over the city all day; we came back to our hotel tired; we took our tea; we talked over the day; we wrote to our friends; we planned for the next day; we were ready to retire. we walked to the window--the sun was striking on all the chimney tops. it doesn't seem to be right even for the lark to go to sleep while the sun shines. we looked at our watches; but the watches said nine o'clock, and we went off to our beds in daytime; and we awoke after the first nap to perceive that the sun still shone into the room. "like all careful aunts, i was unwilling that my nephew should be out alone at night. he was desirous of doing the right thing, but urged that at home, as a little boy, he was always allowed to be out until dark, and he asked if he could stay out until dark! alas for the poor lad! there was no dark at all! i could not consent for him to be out all night, and the twilight was not over. you may read and read that the summer day at st. petersburg is twenty hours long, but until you see that the sun scarcely sets, you cannot take it in. "i wondered whether the laboring man worked eight or ten hours under my window; it seemed to me that he was sawing wood the whole twenty-four! "w. came in one night after a stroll, and described a beautiful square which he had come upon accidentally. i listened with great interest, and said, 'i must go there in the morning; what is the name of it?'--'i don't know,' he replied.--'why didn't you read the sign?' i asked.--'i can't read,' was the reply.--'oh, no; but why didn't you ask some one?'--'i can't speak,' he answered. neither reading nor speaking, we had to learn st. petersburg by our observation, and it is the best way. most travellers read too much. "there are learned institutions in st. petersburg: universities, libraries, picture-galleries, and museums; but the first institution with which i became acquainted was the drosky. the drosky is a very, very small phaeton. it has the driver's seat in front, and a very narrow seat behind him. one person can have room enough on this second seat, but it usually carries two. invariably the drosky is lined with dark-blue cloth, and the drosky-driver wears a dark-blue wrapper, coming to the feet, girded around the waist by a crimson sash. he also wears a bell-shaped hat, turned up at the side. you are a little in doubt, if you see him at first separated from his drosky, whether he is a market-woman or a serving-man, the dress being very much like a morning wrapper. but he is rarely six feet away from his carriage, and usually he is upon it, sound asleep! "the trunks having gone to st. petersburg in advance of ourselves, our first duty was to get possession of them. they were at the custom-house, across the city. my nephew and i jumped upon a drosky--we could not say that we were really _in_ the drosky, for the seat was too short. the drosky-driver started off his horse over the cobble-stones at a terrible rate. i could not keep my seat, and i clung to w. he shouted, 'don't hold by me; i shall be out the next minute!' what could be done? i was sure i shouldn't stay on half a minute. blessings on the red sash of the drosky-man--i caught at that! he drove faster and faster, and i clung tighter and tighter, but alarmed at two immense dangers: first, that i should stop his breath by dragging the girdle so tightly; and, next, that when it became unendurable to him, he would loosen it in front. "i could not perceive that he was aware of my existence at all! he had only one object in life,--to carry us across the city to our place of destination, and to get his copecks in return. "in a few days i learned to like the jolly vehicles very much. they are so numerous that you may pick one up on any street, whenever you are tired of walking. "my principal object in visiting st. petersburg was the astronomical observatory at pulkova, some twelve miles distant. "i had letters to the director, otto von struve, but our consul declared that i must also have one from him, for struve was a very great man. i, of course, accepted it. "we made the journey by rail and coach, but it would be better to drive the whole way. "most observatories are temples of silence, and quiet reigns. as we drove into the grounds at pulkova, a small crowd of children of all ages, and servants of all degrees, came out to meet us. they did not come out to do us honor, but to gaze at us. i could not understand it until i learned that the director of the observatory has a large number of aids, and they, with all their families, live in large houses, connected with the central building by covered ways. "all about the grounds, too, were small observatories,--little temples,--in which young men were practising for observations on the transit of venus. these little buildings, i afterwards learned, were to be taken down and transported, instruments and all, to the coast of asia. "the director of the observatory is otto struve--his father, wilhelm struve, preceded him in this office. properly, the director is herr von struve; but the old russian custom is still in use, and the servants call him wilhelm-vitch; that is, 'the son of william.' "when i bought a photograph of the present emperor, alexander, i saw that he was called nicholas-vitch. "herr struve received us courteously, and an assistant was called to show us the instruments. all observatories are much alike; therefore i will not describe this, except in its peculiarities. one of these was the presence of small, light, portable rooms, i.e., baseless boxes, which rolled over the instruments to protect them; two sides were of wood, and two sides of green silk curtains, which could, of course, be turned aside when the boxes, or little rooms, were rolled over the apparatus. being covered in this way, the heavy shutters can be left open for weeks at a time. "everything was on a large scale--the rooms were immense. "the director has three assistants who are called 'elder astronomers,' and two who are called 'adjunct astronomers.' each of these has a servant devoted to him. i asked one of the elder astronomers if he had rooms in the observatory, and he answered, 'yes, my rooms are ft. by .' "they seem to be amused at the size of their lodgings, for mr. struve, when he told me of his apartments, gave me at once the dimensions,-- ft. by ft. "the room in which we dined with the family of herr struve was immense. i spoke of it, and he said, 'we cannot open our windows in the winter,--the winters are so severe,--and so we must have good air without it.' their drawing-room was also very large; the chairs (innumerable, it seemed to me) stood stiffly around the walls of the room. the floor was painted and highly varnished, and flower-pots were at the numerous windows on little stands. it was scrupulously neat everywhere. "there was very little ceremony at dinner; we had the delicious wild strawberries of the country in great profusion; and the talk, the best part of the dinner, was in german, russian, and english. "madame struve spoke german, russian, and french, and complained that she could not speak english. she said that she had spent three weeks with an english lady, and that she must be very stupid not to speak english. "i noticed that in one of the rooms, which was not so very immense, there was a circular table, a small centre-carpet, and chairs around the table; i have been told that 'in society' in russia, the ladies sit in a circle, and the gentlemen walk around and talk consecutively with the ladies,--kindly giving to each a share of their attention. "they assured me that the winters were charming, the sleighing constant, and the social gatherings cheery; but think of four hours, only, of daylight in the depth of the winter. their dread was the spring and the autumn, when the mud is deep. "everything in the observatory which could be was built of wood. they have the fir, which is very indestructible; it is supposed to show no mark of change in two hundred years. "wood is so susceptible of ornamentation that the pretty villages of russia--and there are some that look like new england villages--struck us very pleasantly, after the stone and brick villages of england. "i try, when i am abroad, to see in what they are superior to us,--not in what they are inferior. "our great idea is, of course, freedom and self-government; probably in that we are ahead of the rest of the world, although we are certainly not so much in advance as we suppose; but we are sufficiently inflated with our own greatness to let that subject take care of itself when we travel. we travel to learn; and i have never been in any country where they did not do something better than we do it, think some thoughts better than we think, catch some inspiration from heights above our own--as in the art of italy, the learning of england, and the philosophy of germany. "let us take the scientific position of russia. when, half a century ago, john quincy adams proposed the establishment of an astronomical observatory, at a cost of $ , , it was ridiculed by the newspapers, considered utopian, and dismissed from the public mind. when our government, a few years since, voted an appropriation of $ , for a telescope for the national observatory, it was considered magnificent. yet, a quarter of a century since ( ), russia founded an astronomical observatory. the government spent $ , on instruments, $ , , on buildings, and annually appropriated $ , for salaries of observers. i naturally thought that a million and a half dollars, and oriental ideas, combined, would make the observatory a showy place; i expected that the observatory would be surmounted by a gilded dome, and that 'pearly gates' would open as i approached. there is not even a dome! "the central observation-room is a cylinder, and its doors swing back on hinges. wherever it is possible, wood is used, instead of stone or brick. i could not detect, in the whole structure, anything like carving, gilding, or painting, for mere show. it was all for science; and its ornamentations were adapted to its uses, and came at their demand. "in our country, the man of science leads an isolated life. if he has capabilities of administration, our government does not yet believe in them. "the director of the observatory at pulkova has the military rank of general, and he is privy councillor to the czar. every subordinate has also his military position--he is a soldier. "what would you think of it, if the director of any observatory were one of the president's cabinet at washington, in virtue of his position? struve's position is that of a member of the president's cabinet. "here is another difference: ours is a democratic country. we recognize no caste; we are born 'free and equal.' we honor labor; work is ennobling. these expressions we are all accustomed to use. do we live up to them? many a rich man, many a man in fine social position, has married a school-teacher; but i never heard it spoken of as a source of pride in the alliance until i went to despotic russia. struve told me, as he would have told of any other honor which had been his, that his wife, as a girl, had taught school in st. petersburg. and then madame struve joined in the conversation, and told me how much the subject of woman's education still held her interest. "st. petersburg is about the size of philadelphia. struve said, 'there are thousands of women studying science in st. petersburg.' how many thousand women do you suppose are studying science in the whole state of new york? i doubt if there are five hundred. "then again, as to language. it is rare, even among the common people, to meet one who speaks one language only. if you can speak no russian, try your poor french, your poor german, or your good english. you may be sure that the shopkeeper will answer in one or another, and even the drosky-driver picks up a little of some one of them. "of late, the russian government has founded a medical school for women, giving them advantages which are given to men, and the same rank when they graduate; the czar himself contributed largely to the fund. "one wonders, in a country so rich as ours, that so few men and women gratify their tastes by founding scholarships and aids for the tuition of girls--it must be such a pleasant way of spending money. "then as regards religion. i am never in a country where the catholic or greek church is dominant, but i see with admiration the zeal of its followers. i may pity their delusions, but i must admire their devotion. if you look around in one of our churches upon the congregation, five-sixths are women, and in some towns nineteen-twentieths; and if you form a judgment from that fact, you would suppose that religion was entirely a 'woman's right.' in a catholic church or greek church, the men are not only as numerous as the women, but they are as intense in their worship. well-dressed men, with good heads, will prostrate themselves before the image of the holy virgin as many times, and as devoutly, as the beggar-woman. "i think i saw a russian gentleman at st. isaac's touch his forehead to the floor, rise and stand erect, touch the floor again, and rise again, ten times in as many minutes; and we were one day forbidden entrance to a church because the czar was about to say his prayers; we found he was making the pilgrimage of some seventy churches, and praying in each one. "christians who believe in public prayer, and who claim that we should be instant in prayer, would consider it a severe tax upon their energies to pray seventy times a day--they don't care to do it! "then there is the _democracy_ of the church. there are no pews to be sold to the highest bidder--no 'reserved seats;' the oneness and equality before god are always recognized. a russian gentleman, as he prays, does not look around, and move away from the poor beggar next to him. at st. peter's the crowd stands or kneels--at st. isaac's they stand; and they stand literally on the same plane. "i noticed in the crowd at st. isaac's, one festival day, young girls who were having a friendly chat; but their religion was ever in their thoughts, and they crossed themselves certainly once a minute. their religion is not an affair of sunday, but of every day in the week. "the drosky-driver, certainly the most stupid class of my acquaintance in russia, never forgets his prayers; if his passenger is never so much in a hurry, and the bribe never so high, the drosky-driver will check his horse, and make the sign of the cross as he passes the little image of the virgin,--so small, perhaps, that you have not noticed it until you wonder why he slackens his pace. "then as to government. we boast of our national freedom, and we talk about universal suffrage, the 'home of the free,' etc. yet the serfs in russia were freed in march, , just before our civil war began. they freed their serfs without any war, and each serf received some acres of land. they freed twenty-three millions, and we freed four or five millions of blacks; and all of us, who are old enough, remember that one of the fears in freeing the slaves was the number of lawless and ignorant blacks who, it was supposed, would come to the north. "we talk about _universal_ suffrage; a larger part of the antiquated russians vote than of americans. just as i came away from st. petersburg i met a moscow family, travelling. we occupied the same compartment car. it was a family consisting of a lady and her three daughters. when they found where i had been, they asked me, in excellent english, what had carried me to st. petersburg, and then, why i was interested in pulkova; and so i must tell them about american girls, and so, of course, of vassar college. "they plied me with questions: 'do you have women in your faculty? do men and women hold the same rank?' i returned the questions: 'is there a girl's college in moscow?' 'no,' said the youngest sister, with a sigh, 'we are always _going_ to have one.' the eldest sister asked: 'do women vote in america?' 'no,' i said. 'do women vote in russia?' she said 'no;' but her mother interrupted her, and there was a spicy conversation between them, in russian, and then the mother, who had rarely spoken, turned to me, and said: 'i vote, but i do not go to the polls myself. i send somebody to represent me; my vote rests upon my property.' "have you not read a story, of late, in the newspapers, about some excellent women in a little town in connecticut whose pet heifers were taken by force and sold because they refused to pay the large taxes levied upon them by their townsmen, they being the largest holders of property in the town? that circumstance could not have happened in barbarous russia; there, the owner of property has a right to say how it shall be used. "'why do you ask me about our government?' i said to the russian girls. 'are you interested in questions of government?' they replied, 'all russian women are interested in questions of that sort.' how many american women are interested in questions concerning government? "these young girls knew exactly what questions to ask about vassar college,--the course of study, the diploma, the number of graduates, etc. the eldest said: 'we are at once excited when we hear of women studying; we have longed for opportunities to study all our lives. our father was the engineer of the first russian railroad, and he spent two years in america." "i confess to a feeling of mortification when one of these girls asked me, 'did you ever read the translation of a russian book?' and i was obliged to answer 'no.' this girl had read american books in the original. they were talking russian, french, german, and english, and yet mourning over their need of education; and in general education, especially in that of women, i think we must be in advance of them. "one of these sisters, forgetting my ignorance, said something to me in russian. the other laughed. 'what did she say?' i asked. the eldest replied, 'she asked you to take her back with you, and educate her.' 'but,' i said, 'you read and speak your languages--the learning of the world is open to you--found your own college!' and the young girl leaned back on the cushions, drew her mantle around her, and said, 'we have not the energy of the american girl!' "the energy of the american girl! the rich inheritance which has come down to her from men and women who sought, in the new world, a better and higher life. "when the american girl carries her energy into the great questions of humanity, into the practical problems of life; when she takes home to her heart the interests of education, of government, and of religion, what may we not hope for our country! london, . "it was the th of august, and i had no hope that miss cobbe could be at her town residence, but i felt bound to deliver mrs. howe's letter, and i wished to give her a vassar pamphlet; so i took a cab and drove; it was at an enormous distance from my lodging--she told me it was six miles. i was as much surprised as delighted when the girl said she was at home, for the house had painters in it, the carpets were up, and everything looked uninhabitable. the girl came back, after taking my card, and asked me if i would go into the studio, and so took me through a pretty garden into a small building of two rooms, the outer one filled with pictures and books. i had never heard that miss cobbe was an artist, and so i looked around, and was afraid that i had got the wrong miss cobbe. but as i glanced at the table i saw the 'contemporary review,' and i took up the first article and read it--by herbert spencer. i had become somewhat interested in a pretty severe criticism of the modes of reasoning of mathematical men, and had perceived that he said the problems of concrete sciences were harder than any of the physical sciences (which i admitted was all true), when a very white dog came bounding in upon me, and i dropped the book, knowing that the dog's mistress must be coming,--and miss cobbe entered. she looked just as i expected, but even larger; but then her head is magnificent because so large. she was very cordial at once, and told me that miss davies had told her i was in london. she said the studio was that of her friend. i could not refrain from thanking her for her books, and telling her how much we valued them in america, and how much good i believed they had done. she colored a very little, and said, 'nothing could be more gratifying to me.' "i had heard that she was not a women's rights woman, and she said, 'who could have told you that? i am remarkably so. i write suffrage articles continually--i sign petitions.' "i was delighted to find that she had been an intimate friend of mrs. somerville; had corresponded with her for years, and had a letter from her after she was ninety-two years of age, when she was reading quaternions for amusement. she said that mrs. somerville would probably have called herself a unitarian, but that really she was a theist, and that it came out more in her later life. she said she was correcting proof of the life by the daughters; that the life was intensely interesting; that mrs. somerville mourned all her life that she had not had the advantages of education. "i asked her how i could get a photograph of mrs. somerville, and she said they could not be bought. she told me, without any hint from me, that she would give vassar college a plaster cast of the bust of mrs. somerville. [footnote: this bust always stood in miss mitchell's parlor at the observatory.] she said, as women grew older, if they lived independent lives, they were pretty sure to be 'women's rights women.' she said the clergy--the broadest, who were in harmony with her--were very courteous, and that since she had grown old (she's about forty-five) all men were more tolerant of her and forgot the difference of sex. "i felt drawn to her when she was most serious. i told her i had suffered much from doubt, and asked her if she had; and she said yes, when she was young; but that she had had, in her life, rare intervals when she believed she held communion with god, and on those rare periods she had rested in the long intermissions. she laughed, and the tears came to her eyes, all together; she was _quick_, and all-alive, and so courteous. when she gave me a book she said, 'may i write your whole name? and may i say "from your friend"?' "then she hurried on her bonnet, and walked to the station with me; and her round face, with the blond hair and the light-blue eyes, seemed to me to become beautiful as she talked. "in edinburgh i asked for a photograph of mary somerville, and the young man behind the counter replied, 'i don't know who it is.' "in london i asked at a bookstore, which the murrays recommended, for a photograph of mrs. somerville and of sir george airy, and the man said if they could be had in london he would get them; and then he asked, 'are they english?' and i informed him that sir george airy was the astronomer royal! * * * * * "'the glasgow college for girls.' seeing a sign of this sort, i rang the door-bell of the house to which it was attached, entered, and was told the lady was at home. as i waited for her, i took up the 'prospectus,' and it was enough,--'music, dancing, drawing, needlework, and english' were the prominent features, and the pupils were children. all well enough,--but why call it a college? "when the lady superintendent came in, i told her that i had supposed it was for more advanced students, and she said, 'oh, it is for girls up to twenty; one supposes a girl is finished by twenty.' "i asked, as modestly as i could, 'have you any pupils in latin and mathematics?' and she said, 'no, it's for girls, you know. dr. m. hopes we shall have some mathematics next year.' 'and,' i asked, 'some latin?' 'yes, dr. m. hopes we shall have some latin; but i confess i believe latin and mathematics all bosh; give them modern languages and accomplishments. i suppose your school is for professional women.' "i told her no; that the daughters of our wealthiest people demand learning; that it would scarcely be considered 'good society' when the women had neither latin nor mathematics. "'oh, well,' she said, 'they get married here so soon.' "when i asked her if they had lady teachers, she said 'oh, no [as if that would ruin the institution]; nothing but first-class masters.' "it was clear that the women taught the needlework." chapter xi papers--science [ ]--the denver eclipse [ ]--colors of stars "the dissemination of information in regard to science and to scientific investigations relieves the scientist from the small annoyances of extreme ignorance. "no one to-day will expect to receive a letter such as reached sir john herschel some years ago, asking for the writer's horoscope to be cast; or such as he received at another time, which asked, shall i marry? and have i seen _her_? "nor can it be long, if the whole population is somewhat educated, that i shall be likely to receive, as i have done, applications for information as to the recovery of stolen goods, or to tell fortunes. "when crossing the atlantic, an irish woman came to me and asked me if i told fortunes; and when i replied in the negative, she asked me if i were not an astronomer. i admitted that i made efforts in that direction. she then asked me what i could tell, if not fortunes. i told her that i could tell when the moon would rise, when the sun would rise, etc. she said, 'oh,' in a tone which plainly said, 'is _that_ all?' "only a few winters since, during a very mild winter, a young lad who was driving a team called out to me on the street, and said he had a question to ask me. "i stopped; and he asked, 'shall we lose our ice-crop this winter?' "it was january, and it was new england. it took very little learning and no alchemy to foretell that the month of february and the neighborhood of boston would give ice enough; and i told him that the ice-crop would be abundant; but i was honest enough to explain to him that my outlook into the future was no better than his. "one of the unfavorable results of the attempt to popularize science is this: the reader of popular scientific books is very likely to think that he understands the science itself, when he merely understands what some writer says about science. "take, for example, the method of determining the distance of the moon from the earth--one of the easiest problems in physical astronomy. the method can be told in a few sentences; yet it took a hundred years to determine it with any degree of accuracy--and a hundred years, not of the average work of mankind in science, but a hundred years during which able minds were bent to the problem. "still, with all the school-masters, and all the teaching, and all the books, the ignorance of the unscientific world is enormous; they are ignorant both ways--they underrate the scientific people and they overrate them. there is, on the one hand, the irish woman who is disappointed because you cannot tell fortunes, and, on the other hand, the cultivated woman who supposes that you must know _all_ science. "i have a friend who wonders that i do not take my astronomical clock to pieces. she supposes that because i am an astronomer, i must be able to be a clock-maker, while i do not handle a tool if i can help it! she did not expect to take her piano to pieces because she was musical! she was as careful not to tinker it as i was not to tinker the clock, which only an expert in clock-making was prepared to handle. "... only a few weeks since i received a letter from a lady who wished to come to make me a visit, and to 'scan the heavens,' as she termed it. now, just as she wrote, the clock, which i was careful not to meddle with, had been rapidly gaining time, and i was standing before it, watching it from hour to hour, and slightly changing its rate by dropping small weights upon its pendulum. time is so important an element with the astronomer, that all else is subordinate to it. "then, too, the uneducated assume the unvarying exactness of mathematical results; while, in reality, mathematical results are often only approximations. we say the sun is , , miles from the earth, plus or minus a probable error; that is, we are right, probably, within, say, , miles; or, the sun is , , minus , miles, or it is , , plus , miles off; and this probable error is only a probability. "if we make one more observation it cannot agree with any one of our determinations, and it changes our probable error. [illustration: bust of maria mitchell. _from original made by miss emma f. brigham in _] "this ignorance of the masses leads to a misconception in two ways; the little that a scientist can do, they do not understand,--they suppose him to be godlike in his capacity, and they do not see results; they overrate him and they underrate him--they underrate his work. "there is no observatory in this land, nor in any land, probably, of which the question is not asked, 'are they doing anything? why don't we hear from them? they should make discoveries, they should publish.' "the one observation made at greenwich on the planet neptune was not published until after a century or more--it was recorded as a star. the observation had to wait a hundred years, about, before the time had come when that evening's work should bear fruit; but it was good, faithful work, and its time came. "kepler was years in passing from one of his laws to another, while the school-boy, to-day, rattles off the three as if they were born of one breath. "the scientist should be free to pursue his investigations. he cannot be a scientist and a school-master. if he pursues his science in all his intervals from his class-work, his classes suffer on account of his engrossments; if he devotes himself to his students, science suffers; and yet we all go on, year after year, trying to work the two fields together, and they need different culture and different implements. " . in the eclipse of this year, the dark shadow fell first on the united states thirty-eight degrees west of washington, and moved towards the south-east, a circle of darkness one hundred and sixteen miles in diameter; circle overlapping circle of darkness until it could be mapped down like a belt. "the mapping of the dark shadow, with its limitations of one hundred and sixteen miles, lay across the country from montana, through colorado, northern and eastern texas, and entered the gulf of mexico between galveston and new orleans. this was the region of total eclipse. looking along this dark strip on the map, each astronomer selected his bit of darkness on which to locate the light of science. "but for the distance from the large cities of the country, colorado seemed to be a most favorable part of the shadow; it was little subject to storms, and reputed to be enjoyable in climate and abundant in hospitality. "my party chose denver, col. i had a friend who lived in denver, and she was visiting me. i sought her at once, and with fear and trembling asked, 'have you a bit of land behind your house in denver where i could put up a small telescope?' 'six hundred miles,' was the laconic reply! "i felt that the hospitality of the rocky mountains was at my feet. space and time are so unconnected! for an observation which would last two minutes forty seconds, i was offered six hundred miles, after a journey of thousands. "a journey from boston to denver makes one hopeful for the future of our country. we had hour after hour and day after day of railroad travel, over level, unbroken land on which cattle fed unprotected, summer and winter, and which seemed to implore the traveller to stay and to accept its richness. it must be centuries before the now unpeopled land of western kansas and colorado can be crowded. "we started from boston a party of two; at cincinnati a third joined us; at kansas city we came upon a fourth who was ready to fall into our ranks, and at denver two more awaited us; so we were a party of six--'all good women and true.' "all along the road it had been evident that the country was roused to a knowledge of the coming eclipse; we overheard remarks about it; small telescopes travelled with us, and our landlord at kansas city, when i asked him to take care of a chronometer, said he had taken care of fifty of them in the previous fortnight. our party had three telescopes and one chronometer. "we had travelled so comfortably all along the santa fé road, from kansas city to pueblo, that we had forgotten the possibility of other railroad annoyances than those of heat and dust until we reached pueblo. at pueblo all seemed to change. we left the santa fé road and entered upon that of the rio grande. "which road was to blame, it is not for me to say, but there was trouble at once about our 'round-trip ticket.' that settled, we supposed all was right. "in sending out telescopes so far as from boston to denver, i had carefully taken out the glasses, and packed them in my trunks. i carried the chronometer in my hand. "it was only five hours' travel from pueblo to denver, and we went on to that city. the trunks, for some unexplained reason, or for no reason at all, chose to remain at pueblo. "one telescope-tube reached denver when we did; but a telescope-tube is of no value without glasses. we learned that there was a war between the two railroads which unite at pueblo, and war, no matter where or when it occurs, means ignorance and stupidity. "the unit of measure of value which the railroad man believes in is entirely different from that in which the scientist rests his faith. "a war between two railroads seemed very small compared with two minutes forty seconds of observation of a total eclipse. one was terrestrial, the other cosmic. "it was wednesday when we reached denver. the eclipse was to occur the following monday. "we haunted the telegraph-rooms, and sent imploring messages. we placed ourselves at the station, and watched the trains as they tossed out their freight; we listened to every express-wagon which passed our door without stopping, and just as we were trying to find if a telescope could be hired or bought in denver, the glasses arrived. "it was now friday; we must put up tents and telescopes, and test the glasses. "it rained hard on friday--nothing could be done. it rained harder on saturday. it rained hardest of all on sunday, and hail mingled with the rain. but monday morning was clear and bright. it was strange enough to find that we might camp anywhere around denver. our hostess suggested to us to place ourselves on 'mccullough's addition.' in new york or boston, if i were about to camp on private grounds i should certainly ask permission. in the far west you choose your spot of ground, you dig post-holes and you pitch tents, and you set up telescopes and inhabit the land; and then the owner of the land comes to you, and asks if he may not put up a fence for you, to keep off intruders, and the nearest residents come to you and offer aid of any kind. "our camping-place was near the house occupied by sisters of charity, and the black-robed, sweet-faced women came out to offer us the refreshing cup of tea and the new-made bread. "all that we needed was 'space,' and of that there was plenty. "our tents being up and the telescopes mounted, we had time to look around at the view. the space had the unlimitedness that we usually connect with sea and sky. our tents were on the slope of a hill, at the foot of which we were about six thousand feet above the sea. the plain was three times as high as the hills of the hudson-river region, and there arose on the south, almost from west to east, the peaks upon peaks of the rocky mountains. one needs to live upon such a plateau for weeks, to take in the grandeur of the panorama. "it is always difficult to teach the man of the people that natural phenomena belong as much to him as to scientific people. camping parties who put up telescopes are always supposed to be corporations with particular privileges, and curious lookers-on gather around, and try to enter what they consider a charmed circle. we were remarkably free from specialists of this kind. camping on the south-west slope of the hill, we were hidden on the north and east, and another party which chose the brow of the hill was much more attractive to the crowd. our good serving-man was told to send away the few strollers who approached; even our friends from the city were asked to remove beyond the reach of voice. "there is always some one to be found in every gathering who will not submit to law. at the time of the total eclipse in iowa, in , there passed in and out among our telescopes and observers an unknown, closely veiled woman. the remembrance of that occasion never comes to my mind without the accompaniment of a fluttering green veil. "this time it was a man. how he came among us and why he remained, no one can say. each one supposed that the others knew, and that there was good reason for his presence. if i was under the tent, wiping glasses, he stood beside me; if the photographer wished to make a picture of the party, this man came to the front; and when i asked the servant to send off the half-vagrant boys and girls who stood gazing at us, this man came up and said to me in a confidential tone, 'they do not understand the sacredness of the occasion, and the fineness of the conditions.' there was something regal in his audacity, but he was none the less a tramp. "persons who observe an eclipse of the sun always try to do the impossible. they seem to consider it a solemn duty to see the first contact of sun and moon. the moon, when seen in the daytime, looks like a small faint cloud; as it approaches the sun it becomes wholly unseen; and an observer tries to see when this unseen object touches the glowing disc of the sun. "when we look at any other object than the sun, we stimulate our vision. a good observer will remain in the dark for a short time before he makes a delicate observation on a faint star, and will then throw a cap over his head to keep out strong lights. "when we look at the sun, we at once try to deaden its light. we protect our eyes by dark glasses--the less of sunlight we can get the better. we calculate exactly at what point the moon will touch the sun, and we watch that point only. the exact second by the chronometer when the figure of the moon touches that of the sun, is always noted. it is not only valuable for the determination of longitude, but it is a check on our knowledge of the moon's motions. therefore, we try for the impossible. "one of our party, a young lady from california, was placed at the chronometer. she was to count aloud the seconds, to which the three others were to listen. two others, one a young woman from missouri, who brought with her a fine telescope, and another from ohio, besides myself, stood at the three telescopes. a fourth, from illinois, was stationed to watch general effects, and one special artist, pencil in hand, to sketch views. "absolute silence was imposed upon the whole party a few minutes before each phenomenon. "of course we began full a minute too soon, and the constrained position was irksome enough, for even time is relative, and the minute of suspense is longer than the hour of satisfaction. [footnote: as the computed time for the first contact drew near, the breath of the counter grew short, and the seconds were almost gasped and threatened to become inaudible, when miss mitchell, without moving her eye from the tube of the telescope, took up the counting, and continued until the young lady recovered herself, which she did immediately.] "the moon, so white in the sky, becomes densely black when it is closely ranging with the sun, and it shows itself as a black notch on the burning disc when the eclipse begins. "each observer made her record in silence, and then we turned and faced one another, with record in hand--we differed more than a second; it was a large difference. "between first contact and totality there was more than an hour, and we had little to do but look at the beautiful scenery and watch the slow motion of a few clouds, on a height which was cloud-land to dwellers by the sea. "our photographer begged us to keep our positions while he made a picture of us. the only value to the picture is the record that it preserves of the parallelism of the three telescopes. you would say it was stiff and unnatural, did you not know that it was the ordering of nature herself--they all point to the centre of the solar system. "as totality approached, all again took their positions. the corona, which is the 'glory' seen around the sun, was visible at least thirteen minutes before totality; each of the party took a look at this, and then all was silent, only the count, on and on, of the young woman at the chronometer. when totality came, even that ceased. "how still it was! "as the last rays of sunlight disappeared, the corona burst out all around the sun, so intensely bright near the sun that the eye could scarcely bear it; extending less dazzlingly bright around the sun for the space of about half the sun's diameter, and in some directions sending off streamers for millions of miles. "it was now quick work. each observer at the telescopes gave a furtive glance at the un-sunlike sun, moved the dark eye-piece from the instrument, replaced it by a more powerful white glass, and prepared to see all that could be seen in two minutes forty seconds. they must note the shape of the corona, its color, its seeming substance, and they must look all around the sun for the 'interior planet.' "there was certainly not the beauty of the eclipse of . then immense radiations shot out in all directions, and threw themselves over half the sky. in , the rosy prominences were so many, so brilliant, so fantastic, so weirdly changing, that the eye must follow them; now, scarcely a protuberance of color, only a roseate light around the sun as the totality ended. but if streamers and prominences were absent, the corona itself was a great glory. our special artist, who made the sketch for my party, could not bear the light. "when the two minutes forty seconds were over, each observer left her instrument, turned in silence from the sun, and wrote down brief notes. happily, some one broke through all rules of order, and shouted out, 'the shadow! the shadow!' and looking toward the southeast we saw the black band of shadow moving from us, a hundred and sixty miles over the plain, and toward the indian territory. it was not the flitting of the closer shadow over the hill and dale: it was a picture which the sun threw at our feet of the dignified march of the moon in its orbit. "and now we looked around. what a strange orange light there was in the north-east! what a spectral hue to the whole landscape! was it really the same old earth, and not another planet? "great is the self-denial of those who follow science. they who look through telescopes at the time of a total eclipse are martyrs; they severely deny themselves. the persons who can say that they have seen a total eclipse of the sun are those who rely upon their eyes. my aids, who touched no glasses, had a season of rare enjoyment. they saw mercury, with its gleam of white light, and mars, with its ruddy glow; they saw regulus come out of the darkening blue on one side of the sun, venus shimmer and procyon twinkle near the horizon, and arcturus shine down from the zenith. "_we_ saw the giant shadow as it _left_ us and passed over the lands of the untutored indian; _they_ saw it as it approached from the distant west, as it fell upon the peaks of the mountain-tops, and, in the impressive stillness, moved directly for our camping-ground. "the savage, to whom it is the frowning of the great spirit, is awe-struck and alarmed; the scholar, to whom it is a token of the inviolability of law, is serious and reverent. "there is a dialogue in some of the old school-readers, and perhaps in some of the new, between a tutor and his two pupils who had been out for a walk. one pupil complained that the way was long, the road was dusty, and the scenery uninteresting; the other was full of delight at the beauties he had found in the same walk. one had walked with his eyes intellectually closed; the other had opened his eyes wide to all the charms of nature. in some respects we are all, at different times, like each of these boys: we shut our eyes to the enjoyments of nature, or we open them. but we are capable of improving ourselves, even in the use of our eyes--we see most when we are most determined to see. the _will_ has a wonderful effect upon the perceptive faculties. when we first look up at the myriads of stars seen in a moonless evening, all is confusion to us; we admire their brilliancy, but we scarcely recognize their grouping. we do not feel the need of knowing much about them. "a traveller, lost on a desert plain, feels that the recognition of one star, the pole star, is of itself a great acquisition; and all persons who, like mariners and soldiers, are left much with the companionship of the stars, only learn to know the prominent clusters, even if they do not know the names given to them in books. "the daily wants of the body do not require that we should say "'give me the ways of wandering stars to know the depths of heaven above and earth below.' but we have a hunger of the mind which asks for knowledge of all around us, and the more we gain, the more is our desire; the more we see, the more are we capable of seeing. "besides learning to see, there is another art to be learned,--_not to see_ what is not. "if we read in to-day's paper that a brilliant comet was seen last night in new york, we are very likely to see it to-night in boston; for we take every long, fleecy cloud for a splendid comet. "when the comet of was expected, a few years ago, to reappear, some young men in cambridge told professor bond that they had seen it; but professor bond did not see it. continually are amateurs in astronomy sending notes of new discoveries to bond, or some other astronomers, which are no discoveries at all! "astronomers have long supposed the existence of a planet inferior to mercury; and m. leverrier has, by mathematical calculation, demonstrated that such a planet exists. he founded his calculations upon the supposed discovery of m. lesbarcault, who declares that it crossed the sun's disc, and that he saw it and made drawings. the internal evidence, from the man's account, is that he was an honest enthusiast. i have no doubt that he followed the path of a solar spot, and as the sun turned on its axis he mistook the motion for that of the dark spot; or perhaps the spot changed and became extinct, and another spot closely resembling it broke out and he was deceived; his wishes all the time being 'father to the thought.' "the eye is as teachable as the hand. every one knows the most prominent constellations,--the pleiades, the great bear, and orion. many persons can draw the figures made by the most brilliant stars in these constellations, and very many young people look for the 'lost pleiad.' but common observers know these stars only as bright objects; they do not perceive that one star differs from another in glory; much less do they perceive that they shine with differently colored rays. "those who know sirius and betel do not at once perceive that one shines with a brilliant white light and the other burns with a glowing red, as different in their brilliancy as the precious stones on a lapidary's table, perhaps for the same reason. and so there is an endless variety of tints of paler colors. "we may turn our gaze as we turn a kaleidoscope, and the changes are infinitely more startling, the combinations infinitely more beautiful; no flower garden presents such a variety and such delicacy of shades. "but beautiful as this variety is, it is difficult to measure it; it has a phantom-like intangibility--we seem not to be able to bring it under the laws of science. "we call the stars garnet and sapphire; but these are, at best, vague terms. our language has not terms enough to signify the different delicate shades; our factories have not the stuff whose hues might make a chromatic scale for them. "in this dilemma, we might make a scale of colors from the stars themselves. we might put at the head of the scale of crimson stars the one known as hind's, which is four degrees west of rigel; we might make a scale of orange stars, beginning with betel as orange red; then we should have betelgeuze, aldebaran, ß ursae minoris, altair and _a_ canis, _a_ lyrae, the list gradually growing paler and paler, until we come to a lyrae, which might be the leader of a host of pale yellow stars, gradually fading off into white. "most of the stars seen with the naked eye are varieties of red, orange, and yellow. the reds, when seen with a glass, reach to violet or dark purple. with a glass, there come out other colors: very decided greens, very delicate blues, browns, grays, and white. if these colors are almost intangible at best, they are rendered more so by the variations of the atmosphere, of the eye, and of the glass. but after these are all accounted for, there is still a real difference. two stars of the class known as double stars, that is, so little separated that considerable optical power is necessary to divide them, show these different tints very nicely in the same field of the telescope. "then there comes in the chance that the colors are complementary; that the eye, fatigued by a brilliant red in the principal star, gives to the companion the color which would make up white light. this happens sometimes; but beyond this the reare innumerable cases of finely contrasted colors which are not complementary, but which show a real difference of light in the stars; resulting, perhaps, from distance,--for some colors travel farther than others, and all colors differ in their order of march,--perhaps from chemical differences. "single blue or green stars are never seen; they are always given as the smaller companion of a pair. "out of several hundred observed by mr. bishop, forty-five have small companions of a bluish, or greenish, or purplish color. almost all of these are stars of the eighth to tenth magnitude; only once are both seen blue, and only in one case is the large one blue. in almost every case the large star is yellow. the color most prevailing is yellow; but the varieties of yellow are very great. "we may assume, then, that the blue stars are faint ones, and probably distant ones. but as not all faint stars or distant ones are blue, it shows that there is a real difference. in the star called piscium, the small star shows a peculiar snuffy-brown tinge. "of two stars in the constellation ursa minoris, not double stars, one is orange and the other is green, both very vivid in color. "from age to age the colors of some prominent stars have certainly changed. this would seem more likely to be from change of place than of physical constitution. "nothing comes out more clearly in astronomical observations than the immense activity of the universe. 'all change, no loss, 'tis revolution all.' "observations of this kind are peculiarly adapted to women. indeed, all astronomical observing seems to be so fitted. the training of a girl fits her for delicate work. the touch of her fingers upon the delicate screws of an astronomical instrument might become wonderfully accurate in results; a woman's eyes are trained to nicety of color. the eye that directs a needle in the delicate meshes of embroidery will equally well bisect a star with the spider web of the micrometer. routine observations, too, dull as they are, are less dull than the endless repetition of the same pattern in crochet-work. "professor chauvenet enumerates among 'accidental errors in observing,' those arising from imperfections in the senses, as 'the imperfection of the eye in measuring small spaces; of the ear, in estimating small intervals of time; of the touch, in the delicate handling of an instrument.' "a girl's eye is trained from early childhood to be keen. the first stitches of the sewing-work of a little child are about as good as those of the mature man. the taking of small stitches, involving minute and equable measurements of space, is a part of every girl's training; she becomes skilled, before she is aware of it, in one of the nicest peculiarities of astronomical observation. "the ear of a child is less trained, except in the case of a musical education; but the touch is a delicate sense given in exquisite degree to a girl, and her training comes in to its aid. she threads a needle almost as soon as she speaks; she touches threads as delicate as the spider-web of a micrometer. "then comes in the girl's habit of patient and quiet work, peculiarly fitted to routine observations. the girl who can stitch from morning to night would find two or three hours in the observatory a relief." chapter xii religious beliefs--comments on sermons--concord school--whittier--cooking schools--anecdotes partly in consequence of her quaker training, and partly from her own indifference towards creeds and sects, miss mitchell was entirely ignorant of the peculiar phrases and customs used by rigid sectarians; so that she was apt to open her eyes in astonishment at some of the remarks and sectarian prejudices which she met after her settlement at vassar college. she was a good learner, however, and after a while knew how to receive in silence that which she did not understand. "miss mitchell," asked one good missionary, "what is your favorite position in prayer?" "flat upon my back!" the answer came, swift as lightning. in she wrote in her diary: "there is a god, and he is good, i say to myself. i try to increase my trust in this, my only article of creed." miss mitchell never joined any church, but for years before she left nantucket she attended the unitarian church, and her sympathies, as long as she lived, were with that denomination, especially with the more liberally inclined portion. there were always a few of the teachers and' some of the students who sympathized with her in her views; but she usually attended the college services on sunday. president taylor, of vassar college, in his remarks at her funeral, stated that all her life professor mitchell had been seeking the truth,--that she was not willing to accept any statement without studying into the matter herself,--"and," he added, "i think she has found the truth she was seeking." miss mitchell never obtruded her views upon others, nor did she oppose their views. she bore in silence what she could not believe, but always insisted upon the right of private judgment. miss w., a teacher at vassar, was fretting at being obliged to attend chapel exercises twice a day when she needed the time for rest and recreation, and applied to miss mitchell for help in getting away from it. after some talk miss mitchell said: "oh, well, do as _i_ do--sit back folding your arms, and think of something pleasant!" "sunday, dec. , . we heard two sermons: the first in the afternoon, by rev. mr. a., baptist, the second in the evening, by rev. mr. b., congregationalist. "rev. mr. a. took a text from deuteronomy, about 'moses;' rev. mr. b. took a text from exodus, about 'moses;' and i am told that the sermon on the preceding sunday was about moses. "it seems to me strange that since we have the history of christ in the new testament, people continue to preach about moses. "rev. mr. a. was a man of about forty years of age. he chanted rather than read a hymn. he chanted a sermon. his description of the journey of moses towards canaan had some interesting points, but his manner was affected; he cried, or pretended to cry, at the pathetic points. i hope he really cried, for a weakness is better than an affectation of weakness. he said, 'the unbeliever is already condemned.' it seems to me that if anything would make me an infidel, it would be the threats lavished against unbelief. "mr. b. is a self-made man, the son of a blacksmith. he brought the anvil, the hammer, and bellows into the pulpit, and he pounded and blew, for he was in earnest. i felt the more respect for him because he was in earnest. but when he snapped his fingers and said, 'i don't care that for the religion of a man which does not begin with prayer,' i was provoked at his forgetfulness of the character of his audience. " . i am more and more disgusted with the preaching that i hear!... why cannot a man act himself, be himself, and think for himself? it seems to me that naturalness alone is power; that a borrowed word is weaker than our own weakness, however small we may be. if i reach a girl's heart or head, i know i must reach it through my own, and not from bigger hearts and heads than mine. "march, . there was something so genuine and so sincere in george macdonald that he took those of us who were _emotional_ completely--not by storm so much as by gentle breezes.... what he said wasn't profound except as it reached the depths of the heart.... he gave us such broad theological lessons! in his sermon he said, 'don't trouble yourself about what you _believe_, but _do_ the will of god.' his consciousness of the existence of god and of his immediate supervision was felt every minute by those who listened.... "he stayed several days at the college, and the girls will never get over the good effects of those three days--the cheerier views of life and death. "... rev. dr. peabody preached for us yesterday, and was lovely. everyone was charmed in spite of his old-fashioned ways. his voice is very bad, but it was such a simple, common-sense discourse! mr. vassar said if that was unitarianism, it was just the right thing. "aug. , . went to a baptist church, and heard rev. mr. f. 'christ the way, the only way.' the sermon was wholly without logic, and yet he said, near its close, that those who had followed him must be convinced that this was true. he said a traveller whom he met on the cars admitted that we all desired heaven, but believed that there were as many ways to it as to boston. mr. f. said that god had prepared but one way, just as the government in those countries of the old world whose cities were upon almost inaccessible pinnacles had prepared one way of approach. (it occurred to me that if those governments possessed godlike powers, they would have made a great many ways.) "mr. f. was very severe upon those who expect to be saved by their own deserts. he said, 'you tender a farthing, when you owe a million.' i could not see what they owed at all! at this point he might well have given some attention to 'good works;' and if he must mention 'debt,' he might well remind them that they sat in an unpaid-for church! "it was plain that he relied upon his anecdotes for the hold upon his audience, and the anecdotes were attached to the main discourse by a very slender thread of connection. i felt really sad to know that not a listener would lead a better life for that sermon--no man or woman went out cheered, or comforted, or stimulated. "on the whole, it is strange that people who go to church are no worse than they are! "sept. , . a clergyman said, in his sermon, 'i do not say with the frenchman, if there were no god it would be well to invent one, but i say, if there were no future state of rewards and punishments, it would be better to believe in one.' did he mean to say, 'better to believe a lie'? "march , . dr. lyman abbott preached. i was surprised to find how liberal congregational preaching had become, for he said he hoped and expected to see women at the bar and in the pulpit, although he believed they would always be exceptional cases. he preached mainly on the motherhood of god, and his whole sermon was a tribute to womanhood.... i rejoice at the ideal womanhood of purity which he put before the girls. i wish some one would preach purity to young men. "july , . i went to hear rev. mr. ---- at the universalist church. he enumerated some of the dangers that threaten us: one was 'the doctrines of scientists,' and he named tyndale, huxley, and spencer. i was most surprised at his fear of these men. can the study of truth do harm? does not every true scientist seek only to know the truth? and in our deep ignorance of what is truth, shall we dread the search for it? "i hold the simple student of nature in holy reverence; and while there live sensualists, despots, and men who are wholly self-seeking, i cannot bear to have these sincere workers held up in the least degree to reproach. and let us have truth, even if the truth be the awful denial of the good god. we must face the light and not bury our heads in the earth. i am hopeful that scientific investigation, pushed on and on, will reveal new ways in which god works, and bring to us deeper revelations of the wholly unknown. "the physical and the spiritual seem to be, at present, separated by an impassable gulf; but at any moment that gulf may be overleaped--possibly a new revelation may come.... "april, . i called on professor henry at the smithsonian institute. he must be in his eightieth year; he has been ill and seems feeble, but he is still the majestic old man, unbent in figure and undimmed in eye. "i always remember, when i see him, the remark of dorothy dix, 'he is the truest man that ever lived.' "we were left alone for a little while, and he introduced the subject of his nearness to death. he said, 'the national academy has raised $ , , the interest of which is for myself and family as long as any of us live [he has daughters only], and in view of my death it is a great comfort to me.' i ventured to ask him if he feared death at all. he said, 'not in the least; i have thought of it a great deal, and have come to feel it a friend. i _cherish_ the belief in immortality; i have suffered much, at times, in regard to that matter.' scientifically considered, only, he thought the probability was on the side of continued existence, as we must believe that spirit existed independent of matter. "he went to a desk and pulled out from a drawer an old copy of 'gregory's astronomy,' and said, 'that book changed my whole life--i read it when i was sixteen years old; i had read, previously, works of the imagination only, and at sixteen, being ill in bed, that book was near me; i read it, and determined to study science.' i asked him if a life of science was a good life, and he said that he felt that it was so. "... when i was travelling with miss s., who was near-sighted and kept her eyes constantly half-shut, it seemed to me that every other young lady i met had wide, staring eyes. now, after two years sitting by a person who never reasons, it strikes me that every other person whom i meet has been thinking hard, and his logic stands out a prominent characteristic. "aug. , . scientific association met at saratoga. ... professor peirce, now over seventy years old, was much the same as ever. he went on in the cars with us, and was reading mallock's 'is life worth living?' and i asked, 'is it?' to which professor peirce replied, 'yes, i think it is.' then i asked, 'if there is no future state, is life worth living?' he replied, 'indeed it is not; life is a cruel tragedy if there is no immortality.' i asked him if he conceived of the future life as one of embodiment, and he said 'yes; i believe with st paul that there is a spiritual body....' "professor peirce's paper was on the 'heat of the sun;' he considers the sun fed not by impact of meteors, but by the compression of meteors. i did not think it very sound. he said some good things: 'where the truth demands, accept; what the truth denies, reject.' "concord, mass., . to establish a school of philosophy had been the dream of alcott's life; and there he sat as i entered the vestry of a church on one of the hottest days in august. he looked full as young as he did twenty years ago, when he gave us a 'conversation' in lynn. elizabeth peabody came into the room, and walked up to the seat of the rulers; her white hair streamed over her shoulders in wild carelessness, and she was as careless as ever about her whole attire, but it was beautiful to see the attention shown to her by mr. alcott and mr. sanborn. "emerson entered,--pale, thin, almost ethereal in countenance,--followed by his daughter, who sat beside him and watched every word that he uttered. on the whole, it was the same emerson--he stumbled at a quotation as he always did; but his thoughts were such as only emerson could have thought, and the sentences had the emersonian pithiness. he made his frequent sentences very emphatic. it was impossible to see any thread of connection; but it always was so--the oracular sentences made the charm. the subject was memory.' he said, 'we remember the selfishness or the wrong act that we have committed for years. it is as it should be--memory is the police-officer of the universe.' 'architects say that the arch never rests, and so the past never rests.' (was it, never sleeps?) 'when i talk with my friend who is a genealogist, i feel that i am talking with a ghost.' "the little vestry, fitted perhaps for a hundred people, was packed with two hundred,--all people of an intellectual cast of face,--and the attention was intense. the thermometer was ninety in the shade! "i did not speak to mr. emerson; i felt that i must not give him a bit of extra fatigue. "july , . the school of philosophy has built a shanty for its meetings, but it is a shanty to be proud of, for it is exactly adapted to its needs. it is a long but not low building, entirely without finish, but water-tight. a porch for entrance, and a recess similar at the opposite end, which makes the place for the speakers. there was a small table upon the platform on which were pond lilies, some shelves around, and a few busts--one of socrates, i think. "i went in the evening to hear dr. harris on 'philosophy.' the rain began to come down soon after i entered, and my philosophy was not sufficient to keep me from the knowledge that i had neither overshoes nor umbrella; i remembered, too, that it was but a narrow foot-path through the wet grass to the omnibus. but i listened to dr. harris, and enjoyed it. he lauded fichte as the most accurate philosopher following kant--he said not of the greatest _breadth_, but the most acute. "after dr. harris' address, mr. alcott made a few remarks that were excellent, and said that when we had studied philosophy for fifteen years, as the lecturer had done, we might know something; but as it was, he had pulled us to pieces and then put us together again. "the audience numbered sixty persons. "may, . i have just finished miss peabody's account of channing. i have been more interested in miss peabody than in channing, and have felt how valuable she must have been to him. how many of channing's sermons were instigated by her questions! ... miss peabody must have been very remarkable as a young woman to ask the questions which she asked at twenty. "april, . the waste of flowers on easter sunday distressed me. something is due to the flowers themselves. they are massed together like a bushel of corn, and look like red and white sugar-plums as seen in a confectioner's window. "a pillow of flowers is a monstrosity. a calla lily in a vase is a beautiful creation; so is a single rose. but when the rose is crushed by a pink on each side of it, and daisies crush the pinks, and azaleas surround the daisies, there is no beauty and no fitness. "the cathedral had no flowers. "aug. , . we visited whittier; we found him at lunch, but he soon came into the parlor. he was very chatty, and seemed glad to see us. mrs. l. was with me, and whittier was very ready to write in the album which she brought with her, belonging to her adopted son. we drifted upon theological subjects, and i asked mr. whittier if he thought that we fell from a state of innocence; he replied that he thought we were better than adam and eve, and if they fell, they 'fell up.' "his faith seems to be unbounded in the goodness of god, and his belief in moral accountability. he said, 'i am a good deal of a quaker in my conviction that a light comes to me to dictate to me what is right.' we stayed about an hour, and we were afraid it would be too much for him; but miss johnson, his cousin, who lives with him, assured us that it was good for him; and he himself said that he was sorry to have us go. "one thing that he said, i noted: that his fancy was for farm-work, but he was not strong enough; he had as a young man some literary ambition, but never thought of attaining the reputation which had come to him. "july , . i have had two or three rich days! on friday last i went to holderness, n.h., to the asquam house; i had been asked by mrs. t. to join her party. there were at this house mr. whittier, mr. and mrs. cartland, professor and mrs. johnson, of yale, mr. williams, the chinese scholar, his brother, an episcopal clergyman, and several others. the house seemed full of fine, cultivated people. we stayed two days and a half. "and first of the scenery. the road up to the house is a steep hill, and at the foot of the hill it winds and turns around two lakes. the panorama is complete one hundred and eighty degrees. beyond the lakes lie the mountains. we do not see mt. washington. the house has a piazza nearly all around it. we had a room on the first floor--large, and with two windows opening to the floor. "the programme of the day's work was delightfully monotonous. for an hour or so after breakfast we sat in the ladies' parlor, we sewed, and we told anecdotes. whittier talked beautifully, almost always on the future state and his confidence in it. occasionally he touched upon persons. he seems to have loved lydia maria child greatly. "when the cool of the morning was over, we went out upon the piazza, and later on we went under the trees, where, it is said, whittier spends most of the time. "there was little of the old-time theology in his views; his faith has been always very firm. mr. cartland asked me one day if i really felt there was any doubt of the immortality of the soul. i told him that on the whole i believed it more than i doubted it, but i could not say that i felt no doubt. whittier asked me if there were no immortality if i should be distressed by it, and i told him that i should be exceedingly distressed; that it was the only thing that i craved. he said that 'annihilation was better for the wicked than everlasting punishment,' and to that i assented. he said that he thought there might be persons so depraved as not to be worth saving. i asked him if god made such. nobody seemed ready to reply. besides myself there was another of the party to whom a dying friend had promised to return, if possible, but had not come. "whittier believed that they did sometimes come. he said that of all whom he had lost, no one would be so welcome to him as lydia maria child. "we held a little service in the parlor of the hotel, and mrs. c. read the fourteenth chapter of john. rev. mr. w. read a sermon from 'the pure in heart shall see god," written by parkhurst, of new york. he thought the child should be told that in heaven he should have his hobby-horse. after the service, when we talked it over, i objected to telling the child this. whittier did not object; he said that luther told his little boy that he should have a little dog with a golden tail in heaven. "aug. , . i have been to see an exhibition of a cooking school. i found sixteen girls in the basement of a school-house. they had long tables, across which stretched a line of gas-stoves and jets of gas. some of the girls were using saucepans; they set them upon the stove, and then sat down where they could see a clock while the boiling process went on. "at one table a girl was cutting out doughnuts; at another a girl was making a pudding--a layer of bits of bread followed by a layer of fruit. each girl had her rolling-pin, and moulding-board or saucepan. "the chief peculiarity of these processes was the cleanliness. the rolling-pins were clean, the knives were clean, the aprons were clean, the hands were clean. not a drop was spilled, not a crumb was dropped. "if into the kitchen of the crowded mother there could come the utensils, the commodities, the clean towels, the ample _time_, there would come, without the lessons, a touch of the millennium. "i am always afraid of manual-labor schools. i am not afraid that these girls could not read, for every american girl reads, and to read is much more important than to cook; but i _am_ afraid that not all can _write_--some of them were not more than twelve years old. "and what of the boys? must a common cook always be a girl? and must a boy not cook unless on the top of the ladder, with the pay of the president of harvard college? "i am jealous for the schools; i have heard a gentleman who stands high in science declare that the cooking schools would eventually kill out every literary college in the land--for women. but why not for men? if the food for the body is more important than the food for the mind, let us destroy the latter and accept the former, but let us not continue to do what has been tried for fifteen hundred years,--to keep one half of the world to the starvation of the mind, in order to feed better the physical condition of the other half. "let us have cooks; but let us leave it a matter of choice, as we leave the dressmaking and the shoe-making, the millinery and the carpentry,--free to be chosen! "there are cultivated and educated women who enjoy cooking; so there are cultivated men who enjoy kensington embroidery. who objects? but take care that some rousing of the intellect comes first,--that it may be an enlightened choice,--and do not so fill the day with bread and butter and stitches that no time is left for the appreciation of whittier, letting at least the simple songs of daily life and the influence of rhythm beautify the dreary round of the three meals a day." miss mitchell had a stock of conundrums on hand, and was a good guesser. she told her stories at all times when they happened to come into her mind. she would arrive at her sister's house, just from poughkeepsie on a vacation, and after the threshold was crossed and she had said "good morning," in a clear voice to be heard by all within her sight, she would, perhaps, say, "well, i have a capital story which i must tell before i take my bonnet off, or i shall forget it!" and there went with her telling an action, voice, and manner which added greater point to the story, but which cannot be described. one of her associates at vassar, in recalling some of her anecdotes, writes: "professor mitchell was quite likely to stand and deliver herself of a bright little speech before taking her seat at breakfast. it was as though the short walk from the observatory had been an inspiration to thought." she was quick at repartee. on one occasion charlotte cushman and her friend miss stebbins were visiting miss mitchell at vassar. miss mitchell took them out for a drive, and pointed out the different objects of interest as they drove along the banks of the hudson. "what is that fine building on the hill?" asked miss cushman.--"that," said miss mitchell, "was a boys' school, originally, but it is now used as a hotel, where they charge five dollars a day!"--"five dollars a day?" exclaimed miss cushman; "jupiter ammon!"--"no," said miss stebbins, "jupiter mammon!"--"not at all," said miss mitchell, "jupiter _gammon!_" "farewell, maria," said an old friend, "i hope the lord will be with thee." "good-by," she replied, "i _know_ he will be with you." a characteristic trait in miss mitchell was her aversion to receiving unsolicited advice in regard to her private affairs. "a suggestion is an impertinence," she would often say. the following anecdote shows how she received such counsel: a literary man of more than national reputation said to one of her admirers, "i, for one, cannot endure your maria mitchell." at her solicitation he explained why; and his reason was, as she had anticipated, founded on personal pique. it seems he had gone up from new york to poughkeepsie especially to call upon professor mitchell. during the course of conversation, with that patronizing condescension which some self-important men extend to all women indiscriminately, he proceeded to inform her that her manner of living was not in accordance with his ideas of expediency. "now," he said, "instead of going for each one of your meals all the way from your living-rooms in the observatory over to the dining-hall in the college building, i should think it would be far more convenient and sensible for you to get your breakfast, at least, right in your own apartments. in the morning you could make a cup of coffee and boil an egg with almost no trouble." at which professor mitchell drew herself up with the air of a tragic queen, saying, "and is my time worth no more than to boil eggs?" chapter xiii miss mitchell's letters--woman suffrage--membership in various societies--published articles--death--conclusion miss mitchell was a voluminous letter writer and an excellent correspondent, but her letters are not essays, and not at all in the approved style of the "complete letter writer." if she had any particular thing to communicate, she rushed into the subject in the first line. in writing to her own family and intimate friends, she rarely signed her full name; sometimes she left it out altogether, but ordinarily "m.m." was appended abruptly when she had expressed all that she had to say. she wrote as she talked, with directness and promptness. no one, in watching her while she was writing a letter, ever saw her pause to think what she should say next or how she should express the thought. when she came to that point, the "m.m." was instantly added. she had no secretiveness, and in looking over her letters it has been almost impossible to find one which did not contain too much that was personal, either about herself or others, to make it proper; especially as she herself would be very unwilling to make the affairs of others public. "oct. , . i have spent $ on dress this year. i have a very pretty new felt bonnet of the fashionable shape, trimmed with velvet; it cost only $ , which, of course, was pitifully cheap for broadway. if thou thinks after $ it wouldn't be extravagant for me to have a waterproof cloak and a linsey-woolsey morning dress, please to send me patterns of the latter material and a description of waterproofs of various prices. they are so ugly, and i am so ditto, that i feel if a few dollars, more or less, would make me look better, even in a storm, i must not mind it." "my orthodoxy is settled beyond dispute, i trust, by the following circumstance: the editor of a new york magazine has written to me to furnish an article for the christmas number on 'the star in the east.' i have ventured, in my note of declination, to mention that if i investigated that subject i might decide that there was no star in the case, and then what would become of me, and _where should i go_? since that he has not written, so i may have hung myself! " . april . i have 'done' new york very much as we did it thirty years ago. on saturday i went to miss booth's reception, and it was like miss lynch's, only larger than miss lynch's was when i was there.... miss booth and a friend live on fifty-ninth street, and have lived together for years. miss booth is a nice-looking woman. she says she has often been told that she looked like me; she has gray hair and black eyes, but is fair and well-cut in feature. i had a very nice time. "on sunday i went to hear frothingham, and he was at his very best. the subject was 'aspirations of man,' and the sermon was rich in thought and in word. ... frothingham's discourse was more cheery than usual; he talked about the wonderful idea of personal immortality, and he said if it be a dream of the imagination let us worship the imagination. he spoke of mrs. child's book on 'aspirations,' and i shall order it at once. the only satire was such a sentence as this: on speaking of a piece of egyptian sculpture he said, 'the gates of heaven opened to the good, not to the orthodox.' "to-day, monday, i have been to a public school (a primary) and to stewart's mansion. i asked the majordomo to take us through the rooms on the lower floor, which he did. i know of no palace which comes up to it. the palaces always have a look as if at some point they needed refurbishing up. i suppose that mrs. stewart uses that dining-room, but it did not look as if it was made to eat in. i still like gérôme's 'chariot race' better than anything else of his. the 'horse fair' was too high up for me to enjoy it, and a little too mixed up. " . st. petersburg is another planet, and, strange to say, is an agreeable planet. some of these europeans are far ahead of us in many things. i think we are in advance only in one universal democracy of freedom. but then, that is everything. "nov. , . i think you are right to decide to make your home pleasant at any sacrifice which involves _only_ silence. and you are so all over a radical, that it won't hurt you to be toned down a little, and in a few years, as the world moves, your family will have moved one way and you the other a little, and you will suddenly find yourself on the same plane. it is much the way that has been between miss ---- and myself. to-day she is more of a women's rights woman than i was when i first knew her, while i begin to think that the girls would better dress at tea-time, though i think on that subject we thought alike at first, so i'll take another example. "i have learned to think that a _young_ girl would better not walk to town alone, even in the daytime. when i came to vassar i should have allowed a child to do it. but i never knew _much_ of the world--never shall--nor will you. and as we were both born a little deficient in worldly caution and worldly policy, let us receive from others those, lessons,--_do as well as we can_, and keep our _heart_ unworldly if our manners take on something of those ways. "oct. , .... i have scarcely got over the _tire_ of the congress [footnote: the annual meeting of the association for the advancement of women, of which miss mitchell was president. it was held at syracuse, n.y., in .] yet, although it is a week since i returned. i feel as if a great burden was lifted from my soul. you will see my 'speech' in the 'woman's journal,' but in the last sentence it should be 'eastward' and not '_earth_ward.' it was a grand affair, and babies came in arms. school-boys stood close to the platform, and school-girls came, books in hand. the hall was a beautiful opera-house, and could hold at least one thousand seven hundred. it was packed and jammed, and rough men stood in the aisles. when i had to speak to announce a paper i stood _very still_ until they became quiet. once, as i stood in that way, a man at the extreme rear, before i had spoken a word, shouted out, 'louder!' we all burst into a laugh. then, of course, i had to make them quiet again. i lifted the little mallet, but i did not strike it, and they all became still. i was surprised at the good breeding of such a crowd. in the evening about half was made up of men. i could not have believed that such a crowd would keep still when i asked them to. "they say i did well. think of my developing as a president of a social science society in my old age!" miss mitchell took no prominent part in the woman suffrage movement, but she believed in it firmly, and its leaders were some of her most highly valued friends. "sept. , . went to a picnic for woman suffrage at a beautiful grove at medfield, mass. it was a gathering of about seventy-five persons (mostly from needham), whose president seemed to be vigorous and good-spirited. "the main purpose of the meeting was to try to affect public sentiment to such an extent as to lead to the defeat of a man who, when the subject of woman suffrage was before the legislature, said that the women had all they wanted now--that they could get anything with 'their eyes as bright as the buttons on an angel's coat.' lucy stone, mr. blackwell, rev. mr. bush, miss eastman, and william lloyd garrison spoke. "garrison did not look a day older than when i first saw him, forty years ago; he spoke well--they said with less fire than he used in his younger days. garrison said what every one says--that the struggle for women was the old anti-slavery struggle over again; that as he looked around at the audience beneath the trees, it seemed to be the same scene that he had known before. "... we had a very good bit of missionary work done at our table (at vassar) to-day. a man whom we all despise began to talk against voting by women. i felt almost inclined to pay him something for his remarks. "a group from the washington women suffrage association stopped here to-day.... i liked susan b. anthony very much. she seemed much worn, but was all alive. she is eighteen months younger than i, but seems much more alert. i suppose brickbats are livelier than logarithms!" miss mitchell was a member of several learned societies. she was the first woman elected to membership of the american academy of arts and sciences, whose headquarters are at boston. in she was chosen a member of the american philosophical society, a society founded by benjamin franklin, in philadelphia. the american association for the advancement of science made her a member in the early part of its existence. miss mitchell was one of the earliest members of the american association for the advancement of women. at one period she was president of the association, and for many years served as chairman of the committee on science. in this latter capacity she reached, through circulars and letters, women studying science in all parts of the country; and the reports, as shown from year to year, show a wonderful increase in the number of such women. she was a member, also, of the new england women's club, of boston, and after her annual visit at christmas she entertained her students at vassar with descriptions of the receptions and meeting of that body. she was also a member of the new york sorosis. she received the degree of ph.d. from rutgers female college in , her first degree of ll.d. from hanover college in , and her last ll.d. from columbia college in . miss mitchell had no ambition to appear in print, and most of her published articles were in response to applications from publishers. a paper entitled "mary somerville" appeared in the "atlantic monthly" for may, . there were several articles in "silliman's journal,"--mostly results of observations on jupiter and saturn,--a few popular science papers in "hours at home," and one on the "herschels," printed in "the century" just after her death. miss mitchell also read a few lectures to small societies, and to one or two girls' schools; but she never allowed such outside work to interfere with her duties at vassar college, to which she devoted herself heart and soul. when the failure of her health became apparent to the members of her family, it was with the utmost difficulty that miss mitchell could be prevailed upon to resign her position. she had fondly hoped to remain at vassar until she should be seventy years old, of which she lacked about six months. it was hoped that complete rest might lead to several years more of happy life for her; but it was not to be so--she died in lynn, june , . it was one of miss mitchell's boasts that she had earned a salary for over fifty years, without any intermission. she also boasted that in july, , when she slipped and fell, spraining herself so that she was obliged to remain in the house a day or two, it was the first time in her memory when she had remained in the house a day. in fact, she made a point of walking out every day, no matter what the weather might be. a serious fall, during her illness in lynn, stopped forever her daily walks. she had resigned her position in january, . the resignation was laid on the table until the following june, at which time the trustees made her professor emeritus, and offered her a home for life at the observatory. this offer she did not accept, preferring to live with her family in lynn. the following extracts from letters which she received at this time show with what reverence and love she was regarded by faculty and students. "jan. , .... you may be sure that we shall be glad to do all we can to honor one whose faithful service and honesty of heart and life have been among the chief inspirations of vassar college throughout its history. of public reputation you have doubtless had enough, but i am sure you cannot have too much of the affection and esteem which we feel toward you, who have had the privilege of working, with you." "jan. , . you will consent, you _must_ consent, to having your home here, and letting the work go. it is not astronomy that is wanted and needed, it is maria mitchell.... the richest part of my life here is connected with you.... i cannot picture vassar without you. there's nothing to point to!" "may , . in all the great wonder of life, you have given me more of what i have wanted than any other creature ever gave me. i hoped i should amount to something for your sake." dr. eliza m. mosher, at one time resident physician at the college, said of her: "she was quick to withdraw objections when she was convinced of error in her judgment. i well remember her opposition to the ground i took in my 'maiden speech' in faculty meeting, and how, at supper, she stood, before sitting down, to say, 'you were right this afternoon. i have thought the matter over, and, while i do not like to believe it, i think it is true.'" of her rooms at the observatory, miss grace anna lewis, who had been a guest, wrote thus: "her furniture was plain and simple, and there was a frank simplicity corresponding therewith which made me believe she chose to have it so. it looked natural for her. i think i should have been disappointed had i found her rooms fitted up with undue elegance." "professor mitchell's position at vassar gave astronomy a prominence there that it has never had in any other college for women, and in but few for men. i suppose it would have made no difference what she had taught. doubtless she never suspected how many students endured the mathematical work of junior astronomy in order to be within range of her magnetic personality." (from "wide awake," september, .) a graduate writes: "her personality was so strong that it was felt all over the college, even by those who were not in her department, and who only admired her from a distance." extract from a letter written after her death by a former pupil: "i count maria mitchell's services to vassar and her pupils infinitely valuable, and her character and attainments great beyond anything that has yet been told.... i was one of the pupils upon whom her freedom from all the shams and self-deceptions made an impression that elevated my whole standard, mental and moral.... the influence of her own personal character sustains its supreme test in the evidence constantly accumulating, that it strengthens rather than weakens with the lapse of time. her influence upon her pupils who were her daily companions has been permanent, character-moulding, and unceasingly progressive." president taylor, in his address at her funeral, said: "if i were to select for comment the one most striking trait of her character, i should name her _genuineness_. there was no false note in maria mitchell's thinking or utterance.... "one who has known her kindness to little children, who has watched her little evidences of thoughtful care for her associates and friends, who has seen her put aside her own long-cherished rights that she might make the way of a new and untried officer easier, cannot forget the tenderer side of her character.... "but if would be vain for me to try to tell just what it was in miss mitchell that attracted us who loved her. it was this combination of great strength and independence, of deep affection and tenderness, breathed through and through with the sentiment of a perfectly genuine life, which has made for us one of the pilgrim-shrines of life the study in the observatory of vassar college where we have known her _at home_, surrounded by the evidences of her honorable professional career. she has been an impressive figure in our time, and one whose influence lives." introductory note on the th of december, , a gold medal of the value of twenty ducats was founded, at the suggestion of professor schumacher, of altona, by his majesty frederic vi., at that time king of denmark, to be awarded to any person who should first discover a telescopic comet. this foundation and the conditions on which the medal would be awarded were announced to the public in the "astronomische nachrichten" for the th of march, . the regulations underwent a revision after a few years, and in april, ("astronomische nachrichten," no. ), were republished as follows: " . the medal will be given to the first discoverer of any comet, which, at the time of its discovery, is invisible to the naked eye, and whose periodic time is unknown. " . the discoverer, if a resident of any part of europe except great britain, is to make known his discovery to mr. schumacher at altona. if a resident in great britain, or any other quarter of the globe except the continent of europe, he is to make his discovery known directly to mr. francis baily, london. [since mr. baily's decease, g.b. airy, esq., astronomer royal, has been substituted in this and in the th and th articles of the regulations.] " . this communication must be made by the _first post_ after the discovery. if there is no regular mail at the place of discovery, the first opportunity of any other kind must be made use of, without waiting for other observations. exact compliance with this condition is indispensable. if this condition is not complied with, and only one person discovers the comet, no medal will be given for the discovery. otherwise, the medal will be assigned to the discoverer who earliest complies with the condition. " . the communication must not only state as exactly as possible the time of the discovery, in order to settle the question between rival claims, but also as near as may be the place of the comet, and the direction in which it is moving, as far as these points can be determined from the observations of one night. " . if the observations of one night are not sufficient to settle these points, the enunciation of the discovery must still be made, in compliance with the third article. as soon as a second observation is made, it must be communicated in like manner with the first, and with it the longitude of the place where the discovery is made, unless it take place at some known observatory. the expectation of obtaining a second observation will never be received as a satisfactory reason for postponing the communication of the first. " . the medal will be assigned twelve months after the discovery of the comet, and no claim will be admitted after that period. " . messrs. baily and schumacher are to decide if a discovery has been made. if they differ, mr. gauss, of göttingen, is to decide. " . messrs. baily and schumacher have agreed to communicate mutually to each other every announcement of a discovery. "altona, april, ." on the st of october, , at half-past ten o'clock, p.m., a telescopic comet was discovered by miss maria mitchell, of nantucket, nearly vertical above polaris about five degrees. the further progress and history of the discovery will sufficiently appear from the following correspondence. on the d of october the same comet was seen at half-past seven, p.m., at rome, by father de vico, and information of the fact was immediately communicated by him to professor schumacher at altona. on the th of october, at twenty minutes past nine, p.m., it was observed by mr. w.r. dawes, at camden lodge, cranbrook, kent, in england, and on the th it was seen by madame rümker, the wife of the director of the observatory at hamburg. mr. schumacher, in announcing this last discovery, observes: [footnote: "astronomische nachrichten," no. .] "madame rümker has for several years been on the lookout for comets, and her persevering industry seemed at last about to be rewarded, when a letter was received from father de vico, addressed to the editor of this journal, from which it appeared that the same comet had been observed by him on the d instant at rome." not deeming it probable that his daughter had anticipated the observers of this country and europe in the discovery of this comet, no steps were taken by mr. mitchell with a view to obtaining the king of denmark's medal. prompt information, however, of the discovery was transmitted by mr. mitchell to his friend, william c. bond, esq., director of the observatory at cambridge. the observations of the messrs. bond upon the comet commenced on the th of october; and on the th were transmitted by me to mr. schumacher, for publication in the "astronomische nachrichten." it was stated in the memorandum of the messrs. bond that the comet was seen by miss mitchell on the st instant. this notice appeared in the "nachrichten" of dec. , , and the priority of miss mitchell's discovery was immediately admitted throughout europe. my attention had been drawn to the subject of the king of denmark's comet medal by some allusion to it in my correspondence with professor schumacher, in reference to the discovery of telescopic comets by mr. george p. bond, of the observatory at cambridge. having learned some weeks after miss mitchell's discovery that no communication had been made on her behalf to the trustees of the medal, and aware that the regulations in this respect were enforced with strictness, i was apprehensive that it might be too late to supply the omission. still, however, as the spirit of the regulations had been complied with by mr. mitchell's letter to mr. bond of the d of october, it seemed worth while at least to make the attempt to procure the medal for his daughter. although the attempt might be unsuccessful, it would at any rate cause the priority of her discovery to be more authentically established than it might otherwise have been. i accordingly wrote to mr. mitchell for information on the subject, and applied for, and obtained from mr. bond, mr. mitchell's original letter to him of the d of october, with the nantucket postmark. these papers were transmitted to professor schumacher, with a letter dated th and th january. on the th of february i wrote a letter to my much esteemed friend, captain w.h. smyth, r.n., formerly president of the astronomical society at london, requesting him to interest himself with professor schumacher to obtain the medal for miss mitchell. captain smyth entered with great readiness into the matter, and addressed a note on the subject to mr. airy, the astronomer royal, at greenwich. mr. airy kindly wrote to professor schumacher without loss of time; but it was their united opinion that a compliance with the condition relative to immediate notice of a discovery was indispensable, and that it was consequently out of their power to award the medal to miss mitchell. mr. schumacher suggested, as the only means by which this difficulty could be overcome, an application to the danish government, through the american legation at copenhagen. conceiving that the correspondence could be carried on more promptly through the danish legation at washington, i addressed a letter on the th of april to mr. steene-billé, chargé d'affaires of the king of denmark in this country, and sent with it copies of the documents which had been forwarded to professor schumacher. mr. steene-billé, however, was of opinion that the application, if made at all, should be made through the american legation at copenhagen; but he expressed at the same time a confident opinion that, owing to the condition and political relations of denmark, the application would necessarily prove unavailing. it was at this time that the difficulties in schleswig-holstein were at their height, and it seemed hopeless at such a moment, and in face of the opinion of the official representative of the danish government in this country, to engage its attention to an affair of this kind. no further attempt was accordingly made by me, for some weeks, to pursue the matter. in fact, a report reached the united states that the medal had actually been awarded to father de vico. although this was believed by me to be an unfounded rumor, the regulations allowing one year for the presentation of claims, there was reason to apprehend that it proceeded from some quarter well informed as to what would probably take place at the expiration of the twelvemonth. on the th of august, father de vico, who had left rome in the spring in consequence of the troubles there, made a visit to cambridge, in company with the right rev. bishop fitzpatrick, of boston, and on this occasion informed me that he had received an intimation from professor schumacher that the comet-medal would be awarded to miss mitchell. i was disposed to think that father de vico labored under some misapprehension as to the purport of professor schumacher's communications, as afterwards appeared to be the case. i felt encouraged, however, by his statement not only to renew my correspondence on the subject with professor schumacher, but i determined, on the th of august, to address a letter to r.p. fleniken, esq., chargé d'affaires of the united states at copenhagen. this letter was accompanied with copies of the original papers. mr. fleniken entered with great zeal and interest into the subject. he lost no time in bringing it before the danish government by means of a letter to the count de knuth, the minister at that time for foreign affairs, and of another to the king of denmark himself. his majesty, with the most obliging promptness, ordered a reference of the case to professor schumacher, with directions to report thereon without delay. mr. schumacher had been for a long time in possession of the documents establishing miss mitchell's priority, which was, indeed, admitted throughout scientific europe. professor schumacher immediately made his report in favor of granting the medal to miss mitchell, and this report was accepted by the king. the result was forthwith communicated by the count de knuth to mr. fleniken, with the gratifying intelligence that the king had ordered the medal to be awarded to miss mitchell, and that it would be delivered to him for transmission as soon as it could be struck off. this has since been done. it must be regarded as a striking proof of an enlightened interest for the promotion of science, not less than of a kind regard for the rights and feelings of the individual most concerned in this decision, that the king of denmark should have bestowed his attention upon this subject, at a period of so much difficulty and alarm for europe in general and his own kingdom in particular. it would not have been possible to act more promptly in a season of the profoundest tranquillity. his majesty has on this occasion shown that he is animated by the same generous zeal for the encouragement of astronomical research which led his predecessor to found the medal; while he has performed an act of gracious courtesy toward a stranger in a distant land which must ever be warmly appreciated by her friends and countrymen. nor ought the obliging agency of the count de knuth, the minister of foreign affairs, to be passed without notice. the slightest indifference on his part, even the usual delays of office, would have prevented the application from reaching the king before the expiration of the twelvemonth within which all claims must, by the regulations, be presented. no one can reflect upon the pressure of business which must have existed in the foreign office at copenhagen during the past year, without feeling that the count de knuth must largely share his sovereign's zeal for science, as well as his love of justice. nothing else will account for the attention bestowed at such a political crisis on an affair of this kind. the same attention appears to have been given to the subject by his successor, count moltka. it was quite fortunate for the success of the application that the office of chargé d'affaires of the united states at copenhagen happened to be filled by a gentleman disposed to give it his prompt and persevering support. a matter of this kind, of course, lay without the province of his official duties. but no subject officially committed to him by the instructions of his government could have been more zealously pursued. on the very day on which my communication of the th of august reached him, mr. fleniken addressed his letters to the minister of foreign affairs and to the king, and he continued to give his attention to the subject till the object was happily effected, and the medal placed in his hands. the event itself, however insignificant in the great world of politics and business, is one of pleasing interest to the friends of american science, and it has been thought proper that the following record of it should be preserved in a permanent form. i have regretted the frequent recurrence of my own name in the correspondence, and have suppressed several letters of my own which could be spared, without rendering less intelligible the communications of the other parties, to whom the interest and merit of the transaction belong. edward everett. cambridge, st february, . correspondence hon. william mitchell to william c. bond, esq., cambridge. "nantucket, mo. d, . "my dear friend: i write now merely to say that maria discovered a telescopic comet at half-past ten on the evening of the first instant, at that hour nearly vertical above polaris five degrees. last evening it had advanced westwardly; this evening still further, and nearing the pole. it does not bear illumination, but maria has obtained its right ascension and declination, and will not suffer me to announce it. pray tell me whether it is one of george's; if not, whether it has been seen by anybody. maria supposes it may be an old story. if quite convenient, just drop a line to her; it will oblige me much. i expect to leave home in a day or two, and shall be in boston next week, and i would like to have her hear from you before i can meet you. i hope it will not give thee much trouble amidst thy close engagements. "our regards are to all of you, most truly, "william mitchell." * * * * * hon. edward everett to hon. william mitchell. "cambridge, th january, . "dear sir: i take the liberty to inquire of you whether any steps have been taken by you, on behalf of your daughter, by way of claiming the medal of the king of denmark for the first discovery of a telescopic comet. the regulations require that information of the discovery should be transmitted by the next mail to mr. airy, the astronomer royal, if the discovery is made elsewhere than on the continent of europe. if made in the united states, i understand from mr. schumacher that information may be sent to the danish minister at washington, who will forward it to mr. airy,--but it must be sent by next mail. "in consequence of non-compliance with these regulations, mr. george bond has on one occasion lost the medal. i trust this may not be the case with miss mitchell. "i am, dear sir, with much respect, faithfully yours, "edward everett." * * * * * extract from a letter of the hon. william mitchell to hon. edward everett. "nantucket, st mo. th, . "esteemed friend: thy kind letter of the th instant reached me duly. no steps were taken by my daughter in claim of the medal of the danish king. on the night of the discovery, i was fully satisfied that it was a comet from its location, though its real motion at this time was so nearly opposite to that of the earth (the two bodies approaching each other) that its apparent motion was scarcely appreciable. i urged very strongly that it should be published immediately, but she resisted it as strongly, though she could but acknowledge her conviction that it was a comet. she remarked to me, 'if it is a new comet, our friends, the bonds, have seen it. it may be an old one, so far as relates to the discovery, and one which we have not followed.' she consented, however, that i should write to william c. bond, which i did by the first mail that left the island after the discovery. this letter did not reach my friend till the th or th, having been somewhat delayed here and also in the post-office at cambridge. "referring to my journal i find these words: 'maria will not consent to have me announce it as an original discovery.' "the stipulations of his majesty have, therefore, not been complied with, and the peculiar circumstances of the case, her sex, and isolated position, may not be sufficient to justify a suspension of the rules. nevertheless, it would gratify me that the generous monarch should know that there is a love of science even in this to him remote corner of the earth. "i am thine, my dear friend, most truly, "william mitchell." * * * * * hon. edward everett to professor schumacher, at altona. "cambridge, th january, . "dear sir: your letter of the th october, accompanying the 'planeten-circulär,' reached me but a few days since. if you would be so good as to forward to the care of john miller, esq., henrietta street, covent garden, london, any letter you may do me the favor to write to me, it would reach me promptly. "the regulations relative to the king of denmark's medal have not hitherto been understood in this country. i shall take care to give publicity to them. not only has mr. bond lost the medal to which you think he would have been entitled, [footnote: mr. schumacher had remarked to me, in his letter of the th of october, that mr. george p. bond would have received the medal for the comet first seen by him as a nebulous object on the th of february, , if his observation made at that time had been communicated, according to the regulations, to the trustees of the medal.] but i fear the same has happened to miss mitchell, of nantucket, who discovered the comet of last october on the first day of that month. i think it was not seen in europe till the third. "i remain, dear sir, with great respect, faithfully yours, "edward everett." * * * * * hon. edward everett to hon. william mitchell. "cambridge, th january, . "dear sir: i have your esteemed favor of the th, which reached me this day. i am fearful that the rigor deemed necessary in enforcing the regulations relative to the king of denmark's prize may prevent your daughter from receiving it. i learn from mr. schumacher's letter, that, besides mr. george bond, dr. bremeker lost the medal because he allowed a single post-day to pass before he announced his discovery. there could, in his case, be no difficulty in establishing the fact of his priority, nor any doubt of the good faith with which it was asserted. but inasmuch as miss mitchell's discovery was actually made known to mr. bond by the next mail which left your island, it is possible--barely possible--that this may be considered as a substantial compliance with the regulation. at any rate, it is worth trying; and if we can do no more we can establish the lady's claim to all the credit of the prior discovery. i shall therefore apply to mr. bond for the letter which you wrote, and if it contains nothing improper to be seen by others we will forward it to the danish minister at washington with a certified extract from your journal. i will have a certified copy of all these papers prepared and sent to mr. schumacher; and if any departure from the letter of the regulations is admissible, this would seem to be a case for it. i trust miss mitchell's retiring disposition will not lead her to oppose the taking of these steps. "i am, dear sir, with great respect, faithfully yours, [signed] "edward everett." * * * * * postscript to mr. everett's letter to professor schumacher of the th january, . "p.s.--the foregoing was written to go by the steamer of the th, but was a few hours too late. i have since received some information in reference to the comet of october which leads me to hope that you may feel it in your power to award the medal to miss maria mitchell. miss mitchell saw the comet at half-past ten o'clock on the evening of october st. her father, a skilful astronomer, made an entry in his journal to that effect. on the third day of october he wrote a letter to mr. bond, the director of our observatory, announcing the discovery. this letter was despatched the following day, being the first post-day after the discovery of the comet. this letter i transmit to you, together with letters from mr. mitchell and mr. bond to myself. nantucket, as you are probably aware, is a small, secluded island, lying off the extreme point of the coast of massachusetts. mr. mitchell is a member of the executive council of massachusetts and a most respectable person. "as the claimant is a young lady of great diffidence, the place a retired island, remote from all the high-roads of communication; as the conditions have not been well understood in this country; and especially as there was a substantial compliance with them--i hope his majesty may think miss maria mitchell entitled to the medal. "cambridge, th january, . * * * * * extract from a letter from mr. everett to captain w.h. smyth, r.n., late president of the royal astronomical society, london, dated cambridge, th february, . "i have lately been making interest with mr. schumacher to cause the king of denmark's medal to be given to miss mitchell for the discovery of the comet to which her name has been given, if i mistake not, in the journal of your society as well as in the 'nachrichten.' she unquestionably discovered it at half-past ten on the evening of the st of october; it was not, i think, seen in europe till the d. her father, on the d, wrote a letter to mr. bond, the director of our observatory, informing him of this discovery; and this letter was sent by the first mail that left the little out-of-the-way island (nantucket) after the discovery. the _spirit_ of the regulations was therefore complied with. but as the _letter_ requires that the notice should be given either to the danish minister resident in the country or to mr. airy, if the discovery is made elsewhere than on the continent of europe, it is possible that some demur may be made. the precise terms of the regulations have not been sufficiently made known in this country. as the claim in this case is really a just one, the claimant a lady, industrious, vigilant, a good astronomer and mathematician, i cannot but hope she will succeed; and if you have the influence with schumacher which you ought to have, i would take it kindly if you would use it in her favor." * * * * * captain smyth to mr. everett. " cheyne walk, chelsea, th march, . "my dear sir: on the receipt of your last letter, i forthwith wrote to the astronomer royal, urging the claims of miss mitchell, of nantucket, and he immediately replied, saying that he would lose no time in consulting his official colleague, mr. schumacher, on the subject. i have just received the accompanying letter from greenwich, by which you will perceive how the matter stands at present; i say at present, because, however the claim may be considered as to the technical form of application, there is no doubt whatever of her fully meriting the award. "i am, my dear sir, very faithfully yours, [signed] "w.h. smyth." * * * * * g.b. airy, esq., to captain smyth. "royal observatory, greenwich, th march, . "my dear sir: i have received mr. schumacher's answer in regard to miss mitchell's supposed claims for the king of denmark's medal. we agree, without the smallest hesitation, that we cannot award the medal. we have in all cases acted strictly in conformity with the published rules; and i am convinced, and i believe that mr. schumacher is convinced, that it is absolutely necessary that we do not depart from them. "mr. schumacher suggests, as the only way in which miss mitchell's claim in equity could be urged, that application might be made on her part, through the american legation, to the king of denmark; and the king can, if he pleases, make exception to the usual rules. "i am, my dear sir, yours most truly, [signed] "g.b. airy." * * * * * hon. edward everett to r.p. fleniken. "cambridge, mass., th august, . "dear sir: without the honor of your personal acquaintance, i take the liberty of addressing you on a subject which i am confident will interest you as a friend of american science. you are doubtless aware that by the liberality of one of the kings of denmark, the father, i believe, of his late majesty, a foundation was made for a gold medal to be given to the first discoverer of a telescopic comet. mr. schumacher, of altona, and mr. baily, of london (and since his decease mr. airy, astronomer royal at greenwich), were made the trustees of this foundation. among the regulations established for awarding the medal was this: that the discoverer should, by the first mail which leaves the place of his residence after the discovery, give notice thereof to mr. schumacher if the discovery is made on the continent of europe, and to mr. airy if made in any other part of the world; provided that, if the discovery be made in america, the notice may be given to the danish minister at washington. it has been deemed necessary to adhere with great strictness to this regulation, in order to prevent fraudulent claims. "on the first day of october last, at about half-past ten o'clock in the evening, a telescopic comet was discovered, in the island of nantucket, by miss maria mitchell, daughter of hon. w. mitchell, one of the executive council of this state. mr. mitchell made an entry of the discovery at the time in his journal. in consequence of miss mitchell's diffidence, she would not allow any publicity to be given to her discovery till its reality was ascertained. her father, however, by the first mail that left nantucket for the mainland, addressed a letter to mr. w.c. bond, director of the observatory in this place, acquainting him with his daughter's discovery. a copy of this letter i herewith transmit to you. the comet was not discovered in europe till the d of october, when it was seen by father de vico, the celebrated astronomer at rome. "you perceive from this statement that, if mr. mitchell had addressed his letter to the danish minister at washington instead of mr. bond, his daughter would have been entitled to the medal, under the strict terms of the regulations. but these regulations have not been generally understood in this country; and as the fact of miss mitchell's prior discovery is undoubted, and recognized throughout europe, it would be a pity that she should lose the medal on a mere technical punctilio. the comet is constantly called 'miss mitchell's comet' in the monthly journal of the royal astronomical society at london, and in the 'astronomische nachrichten,' the well-known astronomical journal, edited by mr. schumacher himself, at altona. father de vico (who, with his brothers of the society of jesuits, has left rome since the revolution there) was at this place (cambridge) three days ago, and spoke of miss mitchell's priority as an undoubted fact. "last winter i addressed a letter to mr. schumacher, acquainting him with the foregoing facts relative to the discovery, and transmitting to him the _original_ letter of mr. mitchell to mr. bond, dated d october, bearing the original nantucket postmark of the th. i also wrote to capt. w. h. smyth, late president of the royal astronomical society of england, desiring him to speak to mr. airy on the subject. he did so, and mr. airy wrote immediately to mr. schumacher. mr. schumacher in his reply expressed the opinion, in which mr. airy concurs, that _under the regulations_ it is not in their power to award the medal to miss mitchell. they suggest, however, that an application should be made, through the american legation at the danish court, to his majesty the king of denmark, for authority, under the present circumstances, to dispense with the literal fulfilment of the conditions. "it is on this subject that i take the liberty to ask your good offices. i accompany my letter with copies of a portion of the correspondence which has been had on the subject, and i venture to request you to address a note to the proper department of the danish government, to the end that authority should be given to messrs. schumacher and airy to award the medal to miss mitchell, _provided they are satisfied that she first discovered the comet_. "i will only add that, should you succeed in effecting this object, you will render a very acceptable service to all the friends of science in america. "i remain, dear sir, with high consideration, your obedient, faithful servant, [signed] "edward everett. "to r. p. fleniken, esq., chargé d'affaires of the united states of america at copenhagen." * * * * * r.p. fleniken, esq., to the count de knuth. "légation des etats unis d'amérique,} à copenhague, le septembre, . } "monsieur le ministre: j'ai l'honneur de remettre sous ce pli à votre excellence une lettre que j'ai reçue d'un de mes concitoyens les plus distingués, avec une correspondance touchant une matière à laquelle il me semble que le danemark ne soit guère moins intéressé que ne le sont les etats unis; le premier y ayant contribué le digne motif, l'autre en ayant heureusement accompli l'objet. "je recommande ces documents à l'examination attentive de votre excellence, sachant bien l'intérêt profond qu'elle ne manque jamais de prendre à de tels sujets, et la réputation éminente de cultivateur des sciences et de la littérature, dont elle jouit avec tant de justice. j'y ai joint une lettre de moi-même, adressée à sa majesté le roi de danemark. "la matière dont il est question, monsieur, sera d'autant plus intéressante à votre excellence, qu'on peut la regarder comme une voix de réponse adressée à l'ancienne scandinavie, proclaimant les prodiges merveilleux de la science moderne, des bords mêmes du vinland des vikinger hardis et entreprenants du dixième et de l'onzième siècles. "je prie votre excellence de vouloir bien soumettre tous les documents ci-joints à l'oeil de sa majesté, et dans le cas heureux ou vous seriez d'avis que ma compatriote, mlle. mitchell, puisse avec justice revendiquer la récompense génereuse instituée par le roi frédéric vi., alors, monsieur, je prie votre excellence de vouloir bien appuyer de ses propres estimables et puissantes recommandations l'application des amis de la jeune demoiselle. "je m'empresse à cette occasion, monsieur, de renouveler à votre excellence l'assurance de ma considération très distinguée. "r.p. fleniken. "a son excellence m. le comte de knuth, ministre d'etat, et chef du département des affaires etrangères. translation. [footnote: this and the other translations of the french letters are printed as received in this country.] "legation of the united states of america,} city of copenhagen, september th, . } "sir: i have the honor to communicate to you a letter from a distinguished citizen of my own country, together with a correspondence relating to a subject in which denmark and the united states appear somewhat equally interested, the former in furnishing a laudable motive, and the latter as happily achieving the object. "i commend these papers to your careful examination, being well aware of the deep interest you take in all such subjects, and of the eminent reputation you so justly enjoy as a gentleman of science and of literature. they are accompanied by a letter from myself addressed to his majesty the king of denmark. "this subject will not be the less interesting to you, sir, as it would appear to be a returning voice addressed to ancient scandinavia, speaking of the wonderful achievements of modern science, from the 'vinland' of the hardy and enterprising 'northmen' of the tenth and the eleventh centuries. "i beg, therefore, that you will obligingly lay them all before his majesty, and should they happily impress you that my countrywoman, miss mitchell, is fairly entitled to the generous offering of king frederic vi., be pleased, sir, to accompany the application of her friends in her behalf by your own very valuable and potent recommendation. "i avail myself of this occasion to renew to your excellency the assurance of my most distinguished consideration. [signed]. "r.p. fleniken. "to his excellency the count de knuth, minister of state and chief of the department of foreign affairs. * * * * * r. p. fleniken, esq., to the king of denmark. "légation des etats unis d'amérique,} à copenhague, le septembre, . } "sire: le soussigné a l'honneur, par l'intermédiaire de m. votre ministre d'état et chef du département des affaires étrangères, de soumettre à votre majesté une lettre d'un citoyen très distingué des etats unis, accompagnée de la copie d'une correspondance concernant une matière a laquelle votre majesté, souverain également distingué par la libéralité généreuse qu'elle fait voir dans ses rapports sociaux et politiques, et par l'admiration ardente qu'elle manifeste envers la science et la littérature, ne peut manquer de prendre un vif intérêt. "le soussigné se félicite beaucoup d'être l'intermédiaire par les mains duquel ces documents arrivent sous l'oeil de votre majesté, étant persuadé que la lecture en fournira à votre majesté l'occasion de recourir avec une grande satisfaction patriotique, comme protecteur éminent des sciences, à l'institution d'un de ses illustres prédécesseurs; et ce souvenir de la haute position à laquelle le danemark s'est élevé dans les arts et les sciences, ne lui sera peut-être pas moins doux quand elle songe que c'est justement sur cette même côte, où déjà au dixième siècle l'intrépidité et l'esprit hardi de ses ancêtres scandinaves les avaient amenés à la découverte du grand continent occidental et à la fondation d'une colonie, que vient de s'accomplir cette conquête de la science, dont parlent les dits papiers. "le soussigné ose donc espérer, qu'à la suite d'une examination attentive des lettres ci-jointes, et desquelles il paraîtrait être généralement reconnu qu'à mlle. mitchell des etats unis est dû l'honneur d'avoir la première découvert la comète télescopique qui aujourd'hui porte son nom, que votre majesté ne trouvera point dans la réserve louable qui empêcha cette jeune demoiselle de se précipiter à la poursuite d'une renommée publique, une cause suffisante de lui refuser le prix de sa brilliante découverte; mais qu'au contraire elle donnera l'ordre de lui expédier la médaille, autant comme une récompense due à ses éminents talents scientifiques, que pour témoigner combien votre majesté sait apprécier cette modestie charmante qui s'opposa à ce que mlle. mitchell recherchât une célébrité publique et scientifique, avec le seul but de remplir une forme tout-à-fait technique. "le soussigné, chargé d'affaires des etats unis de l'amérique, saisit avec empressement cette occasion d'offrir à votre majesté l'expression de sa considération la plus haute et la plus distinguée. "r.p. fleniken. "À sa majesté frederic vii., roi de danemark, duc de slesvig et de holstein." * * * * * translation. "legation of the united states of america,} city of copenhagen, september th, . } "sire: the undersigned has the honor, through your majesty's minister of state and chief of the department of foreign affairs, to communicate to you a letter from a very distinguished citizen of the united states, together with copies of a correspondence relating to a subject in which your majesty, alike distinguished for generous liberality in social and political affairs as a sovereign, as well as an ardent admirer of science and of literature, will doubtless feel a lively interest. "the undersigned is happy to be the medium through which those papers reach the eye of your majesty, feeling sensible that their perusal will furnish occasion to your majesty to recur with much national pleasure to the act of one of your illustrious predecessors as a distinguished patron of science; and this recurrence to the eminent position that denmark has attained in the arts and the sciences may perhaps not be the less pleasurable from the fact that the trophy of science to which the papers allude was achieved on the very coast where, as far back as the tenth century, the intrepidity and enterprise of your majesty's scandinavian ancestors first discovered and planted a colony upon the great western continent. "the undersigned therefore hopes that, after a careful examination of the accompanying papers, from which it would seem to be admitted that miss mitchell, of the united states, is entitled to the honor of first discovering the telescopic comet bearing her name, your majesty will not be able to perceive in that commendable delicacy which forbade her hastily seeking public notoriety a sufficient motive for withholding from her the reward of her eminent discovery; but, on the contrary, will direct the medal to be awarded to her, not only as a suitable encouragement to her distinguished scientific attainments, but also as evincing your majesty's appreciation of that beautiful virtue which withheld her from rushing into public and scientific renown merely to comply with a purely technical condition. "the undersigned, american chargé d'affaires, gladly improves this very pleasant occasion to tender to your majesty the expression of his high and most distinguished consideration. [signed] "r. p. fleniken. "to his majesty frederic vii., king of denmark, duke of schleswig and holstein." * * * * * the count de knuth to mr. fleniken. "copenhague, ce octobre, . "monsieur: j'ai eu l'honneur de recevoir votre office du du passé, par lequel vous avez exprimé le désir que la médaille instituée par feu le roi frédéric vi., en récompense de la découverte de comètes télescopiques, fût accordée à mlle. maria mitchell, de nantucket dans les etats unis d'amérique. "après avoir examiné les pièces justificatives que vous avez bien voulu me communiquer relativement à cette réclamation, je ne saurais que partager votre avis, monsieur, qu'il paraît hors de doute que la découverte de la comète en question est effectivement dûe aux savantes recherches de mlle. mitchell; et que ce n'est que faute de n'avoir pas observé les formalités prescrites, qu'elle n'a point jusqu'ici reçu une marque de distinction à laquelle elle paraît avoir de si justes titres. "le savant astronome, le professeur schumacher, ayant également recommandé mlle. mitchell à la faveur qu'elle sollicite maintenant, je me suis empressé de référer cette question au roi, mon auguste maître, en mettant en même temps sous les yeux de sa majesté la lettre que vous lui avez adressée à ce sujet; et c'est avec bien du plaisir que je me vois aujourd'hui à même de vous faire part, monsieur, que sa majesté n'a point hésité à satisfaire à votre demande, en accordant à mlle. mitchell la médaille qu'elle ambitionne. "aussitôt que cette médaille sera frappée, je m'empresserai de vous la faire parvenir. "en attendant je saisis avec bien du plaisir cette occasion pour vous renouveler, monsieur, les assurances de ma considération très distinguée. "f.w. knuth. "À monsieur fleniken, chargé d'affaires des etats unis d'amérique." * * * * * translation. "copenhagen, th october, . "sir: i have had the honor to receive your communication of the th ultimo, in which you express the desire that the medal instituted by his late majesty, frederic vi., as a reward for the discovery of telescopic comets, should be granted to miss maria mitchell, of nantucket, in the united states of america. "on examination of the justificatory pieces which you have been good enough to forward me, relating to her claim, i cannot do otherwise than participate in your opinion, sir, that it would appear to admit of no doubt that the discovery of the comet in question was really due to miss mitchell's learned researches; and that her not having as yet received a mark of distinction to which she seems to have such a just claim was entirely owing to her not having observed the prescribed forms. "the learned astronomer, professor schumacher, having likewise recommended miss mitchell to the favor which she now solicits, i hasten to refer this question to the king, my august master, at the same time laying before his majesty the letter which you have addressed to him on this subject; and i have much pleasure in being now enabled to inform you, sir, that his majesty has not hesitated to grant your request by awarding to miss mitchell the medal which she desires. "as soon as this medal is struck, i will have it forwarded to you, and meanwhile have much pleasure in availing myself of this occasion to renew to you, sir, the assurances of my most distinguished consideration. [signed] "f.w. knuth. "to mr. fleniken, chargé d'affaires of the united states of america." * * * * * mr. fleniken to the count de knuth. "légation des etats unis d'amérique, à copenhague, le octobre, . "monsieur: le soussigné a eu l'honneur de recevoir l'office que votre excellence lui a addressé en date d'hier pour lui faire part de la nouvelle heureuse que sa majesté, après avoir examiné les documents que vous avez bien voulu lui soumettre, ayant pour objet d'établir le fait que mlle. mitchell ait la première découvert la comète télescopique d'octobre de l'an dernier, a bien voulu trouver ces preuves suffisantes, et a ordonné qu'on frappe une médaille, afin de la lui faire présenter comme une marque de distinction que sa majesté croit qu'elle mérite en effet, quoiqu'elle n'ait pas rigoureusement observé les formalités prescrites par le roi frédéric vi., fondateur de ce don. "le soussigné s'empresse donc d'assurer votre excellence et en même temps de vous prier, monsieur, de vouloir bien faire parvenir cette assurance à sa majesté, que cet acte signalé de libéralité ne peut manquer d'être dignement et hautement apprécié par les institutions scientifiques des etats unis, par mlle. mitchell qui est l'objet de cette distinction généreuse, et par les nombreux amis scientifiques de cette dame; enfin, par tous ceux qui prennent de l'intérêt à la réussite heureuse des recherches astronomiques. "le soussigné ne peut terminer cette communication sans exprimer à votre excellence (en la priant de porter aussi ses sentiments à la connaissance de sa majesté) sa vive appréciation de ce noble et éclatant acte de justice, si promptement et si généreusement rendu à sa jeune compatriote par le roi de danemark, et il saisit avec empressement cette occasion de renouveler à votre excellence les assurances de sa considération très distinguée. "r.p. fleniken. "À son excellence m. le comte de knuth, ministre d'etat et chef du département des affaires etrangères." * * * * * translation. "legation of the united states,} copenhagen, october th, . } "sir: the undersigned has the honor to acknowledge the receipt of your excellency's communication of yesterday's date, conveying to him the gratifying intelligence that his majesty, from an examination of the evidence which you obligingly laid before him, tending to establish the fact of miss mitchell's having discovered the telescopic comet of october, last, has been pleased to consider it quite satisfactory, and has ordered a medal to be struck for her as a mark of distinction to which his majesty deems her entitled, notwithstanding her omission to comply with the prescribed conditions of frederic vi., who instituted the donation. "the undersigned, therefore, begs to express to you, sir, and through you to his majesty, the assurance that this eminent act of liberality cannot fail to be duly and highly appreciated by the scientific institutions of his own country, by miss mitchell herself, who is the object of this generous distinction, and by her numerous scientific friends, as well as by all who feel an interest in successful astronomical achievements. "the undersigned cannot close this communication without expressing to you and to the king his own unaffected appreciation of this noble and distinguished act of justice, so promptly and so generously bestowed upon his unobtrusive countrywoman by the king of denmark, and avails himself of the occasion to renew to your excellency the assurance of his most distinguished consideration. [signed] "r.p. fleniken. "to his excellency the count de knuth, minister of state, etc., etc., etc." astronomy with an opera-glass a popular introduction to the study of the starry heavens with the simplest of optical instruments with maps and directions to facilitate the recognition of the constellations and the principal stars visible to the naked eye by garrett p. serviss "known are their laws; in harmony unroll the nineteen-orbed cycles of the moon. and all the signs through which night whirls her car from belted orion back to orion and his dauntless hound, and all poseidon's, all high zeus' stars bear on their beams true messages to man." poste's aratus. _third edition_ new york d. appleton and company london: caxton house, paternoster square copyright, , by d appleton and company. to the reader in the pages that follow, the author has endeavored to encourage the study of the heavenly bodies by pointing out some of the interesting and marvelous phenomena of the universe that are visible with little or no assistance from optical instruments, and indicating means of becoming acquainted with the constellations and the planets. knowing that an opera-glass is capable of revealing some of the most beautiful sights in the starry dome, and believing that many persons would be glad to learn the fact, he set to work with such an instrument and surveyed all the constellations visible in the latitude of new york, carefully noting everything that it seemed might interest amateur star-gazers. all the objects thus observed have not been included in this book, lest the multiplicity of details should deter or discourage the very readers for whom it was specially written. on the other hand, there is nothing described as visible with an opera-glass or a field-glass which the author has not seen with an instrument of that description, and which any person possessing eye-sight of average quality and a competent glass should not be able to discern. but, in order to lend due interest to the subject, and place it before the reader in a proper light and true perspective, many facts have been stated concerning the objects described, the ascertainment of which has required the aid of powerful telescopes, and to observers with such instruments is reserved the noble pleasure of confirming with their own eyes those wonderful discoveries which the looker with an opera-glass can not hope to behold unless, happily, he should be spurred on to the possession of a telescope. yet even to glimpse dimly these distant wonders, knowing what a closer view would reveal, is a source of no mean satisfaction, while the celestial phenomena that lie easily within reach of an opera-glass are sufficient to furnish delight and instruction for many an evening. it should be said that the division of the stars used in this book into the "stars of spring," "stars of summer," "stars of autumn," and "stars of winter," is purely arbitrary, and intended only to indicate the seasons when certain constellations are best situated for observation or most conspicuous. the greater part of the matter composing this volume appeared originally in a series of articles contributed by the author to "the popular science monthly" in -' . the reception that those articles met with encouraged him to revise and enlarge them for publication in the more permanent form of a book. g. p. s. brooklyn, n. y., _september, ._ contents. page introduction popular interest in the phenomena of the heavens. the opera-glass as an instrument of observation for beginners in star-study. testing an opera-glass. chapter i. the stars of spring _description of the constellations_--auriga, the charioteer; berenice's hair; cancer, the crab [the manger]; canis minor, the lesser dog; corvus, the crow; crateris, the cup; gemini, the twins; hydra, the water-serpent; leo, the lion; ursa major, the greater bear [the great dipper]; ursa minor, the lesser bear [the pole-star]. a circular index-map, maps on a larger scale, of the constellations described, and pictures of remarkable objects. chapter ii. the stars of summer _description of the constellations_--aquila, the eagle; boötes, the herdsman, or bear-diver; canes venatici, the hunting-dogs; cygnus, the swan [the northern cross]; delphinus, the dolphin; draco, the dragon; hercules [the great sun-swarm, m]; libra, the balance; lyra, the harp; the northern crown; ophiuchus et serpens, the serpent-bearer and the serpent; sagitta, the arrow; sagittarius, the archer; scorpio, the scorpion; sobieski's shield; taurus poniatowskii, poniatowsky's bull; virgo, the virgin [the field of the nebulæ]; vulpecula, the little fox. a circular index-map, maps, on a larger scale, of the constellations described, and pictures of remarkable objects. chapter iii. the stars of autumn _description of the constellations_--andromeda [the great nebula]; aquarius, the water-bearer; aries, the ram; capricornus, the goat; cassiopeia; cepheus; cetus, the whale [mira, the wonderful variable star]; pegasus, the winged horse. perseus [algol, the demon-star]; pisces, the fishes; piscis australis, the southern fish; the triangles. a circular index-map, maps on a larger scale, of the constellations described, and pictures of remarkable objects. chapter iv. the stars of winter _description of the constellations_--argo, jason's ship; canis major, the great dog [sirius]; eridanus, the river po; lepus, the hare; monoceros, the unicorn; orion [the great nebula]; taurus, the bull [the pleiades and hyades]. a circular index-map, maps on a larger scale, of the constellations described, and pictures of remarkable objects. chapter v. the moon, the planets, and the sun description of lunar "seas," mountains, and "craters," with a map of the moon, and cuts showing its appearance with a field-glass. _opera-glass observation of_--the sun (one cut), mercury, venus, mars, jupiter and his satellites (one cut), saturn, uranus (three cuts). astronomy with an opera-glass. introduction. star-gazing was never more popular than it is now. in every civilized country many excellent telescopes are owned and used, often to very good purpose, by persons who are not practical astronomers, but who wish to see for themselves the marvels of the sky, and who occasionally stumble upon something that is new even to professional star-gazers. yet, notwithstanding this activity in the cultivation of astronomical studies, it is probably safe to assert that hardly one person in a hundred knows the chief stars by name, or can even recognize the principal constellations, much less distinguish the planets from the fixed stars. and of course they know nothing of the intellectual pleasure that accompanies a knowledge of the stars. modern astronomy is so rapidly and wonderfully linking the earth and the sun together, with all the orbs of space, in the bonds of close physical relationship, that a person of education and general intelligence can offer no valid excuse for not knowing where to look for sirius or aldebaran, or the orion nebula, or the planet jupiter. as australia and new zealand and the islands of the sea are made a part of the civilized world through the expanding influence of commerce and cultivation, so the suns and planets around us are, in a certain sense, falling under the dominion of the restless and resistless mind of man. we have come to possess vested intellectual interests in mars and saturn, and in the sun and all his multitude of fellows, which nobody can afford to ignore. a singular proof of popular ignorance of the starry heavens, as well as of popular curiosity concerning any uncommon celestial phenomenon, is furnished by the curious notions prevailing about the planet venus. when venus began to attract general attention in the western sky in the early evenings of the spring of , speculation quickly became rife about it, particularly on the great brooklyn bridge. as the planet hung dazzlingly bright over the new jersey horizon, some people appeared to think it was the light of liberty's torch, mistaking the bronze goddess's real flambeau for a part of the electric-light system of the metropolis. finally (to judge from the letters written to the newspapers, and the questions asked of individuals supposed to know something about the secrets of the sky), the conviction seems to have become pretty widely distributed that the strange light in the west was no less than an electrically illuminated balloon, nightly sent skyward by mr. edison, for no other conceivable reason than a wizardly desire to mystify his fellow-men. i have positive information that this ridiculous notion has been actually entertained by more than one person of intelligence. and as venus glowed with increasing splendor in the serene evenings of june, she continued to be mistaken for some petty artificial light instead of the magnificent world that she was, sparkling out there in the sunshine like a globe of burnished silver. yet venus as an evening star is not so rare a phenomenon that people of intelligence should be surprised at it. once in every days she reappears at the same place in the sunset sky-- "gem of the crimson-colored even, companion of retiring day." no eye can fail to note her, and as the nearest and most beautiful of the earth's sisters it would seem that everybody should be as familiar with her appearance as with the face of a friend. but the popular ignorance of venus, and the other members of the planetary family to which our mother, the earth, belongs, is only an index of the denser ignorance concerning the stars--the brothers of our great father, the sun. i believe this ignorance is largely due to mere indifference, which, in its turn, arises from a false and pedantic method of presenting astronomy as a creature of mathematical formulæ, and a humble handmaiden of the art of navigation. i do not, of course, mean to cast doubt upon the scientific value of technical work in astronomy. the science could not exist without it. those who have made the spectroscope reveal the composition of the sun and stars, and who are now making photography picture the heavens as they are, and even reveal phenomena which lie beyond the range of human vision, are the men who have taken astronomy out of its swaddling-clothes, and set it on its feet as a progressive science. but when one sees the depressing and repellent effect that has evidently been produced upon the popular mind by the ordinary methods of presenting astronomy, one can not resist the temptation to utter a vigorous protest, and to declare that this glorious science is not the grinning mathematical skeleton that it has been represented to be. perhaps one reason why the average educated man or woman knows so little of the starry heavens is because it is popularly supposed that only the most powerful telescopes and costly instruments of the observatory are capable of dealing with them. no greater mistake could be made. it does not require an optical instrument of any kind, nor much labor, as compared with that expended in the acquirement of some polished accomplishments regarded as indispensable, to give one an acquaintance with the stars and planets which will be not only pleasurable but useful. and with the aid of an opera-glass most interesting, gratifying, and, in some instances, scientifically valuable observations may be made in the heavens. i have more than once heard persons who knew nothing about the stars, and probably cared less, utter exclamations of surprise and delight when persuaded to look at certain parts of the sky with a good glass, and thereafter manifest an interest in astronomy of which they would formerly have believed themselves incapable. being convinced that whoever will survey the heavens with a good opera-glass will feel repaid many fold for his time and labor, i have undertaken to point out some of the objects most worthy of attention, and some of the means of making acquaintance with the stars. first, a word about the instrument to be used. galileo made his famous discoveries with what was, in principle of construction, simply an opera-glass. this form of telescope was afterward abandoned because very high magnifying powers could not be employed with it, and the field of view was restricted. but, on account of its brilliant illumination of objects looked at, and its convenience of form, the opera-glass is still a valuable and, in some respects, unrivaled instrument of observation. in choosing an opera-glass, see first that the object-glasses are achromatic, although this caution is hardly necessary, for all modern opera-glasses, worthy of the name, are made with achromatic objectives. but there are great differences in the quality of the work. if a glass shows a colored fringe around a bright object, reject it. let the diameter of the object-glasses, which are the large lenses in the end farthest from the eye, be not less than an inch and a half. the magnifying power should be at least three or four diameters. a familiar way of estimating the magnifying power is by looking at a brick wall through one barrel of the opera-glass with one eye, while the other eye sees the wall without the intervention of the glass. then notice how many bricks seen by the naked eye are required to equal in thickness one brick seen through the glass. that number represents the magnifying power. the instrument used by the writer in making most of the observations for this book has object-glasses . inch in diameter, and a magnifying power of about . times. see that the fields of view given by the two barrels of the opera-glass coincide, or blend perfectly together. if one appears to partially overlap the other when looking at a distant object, the effect is very annoying. this fault arises from the barrels of the opera-glass being placed too far apart, so that their optical centers do not coincide with the centers of the observer's eyes. [illustration: a very bad field.] occasionally, on account of faulty centering of the lenses, a double image is given of objects looked at, as illustrated in the accompanying cut. in such a case the glass is worthless; but if the effect is simply the addition of a small, crescent-shaped extension on one side of the field of view without any reduplication, the fault may be overlooked, though it is far better to select a glass that gives a perfectly round field. some glasses have an arrangement for adjusting the distance between the barrels to suit the eyes of different persons, and it would be well if all were made adjustable in the same way. don't buy a cheap glass, but don't waste your money on fancy mountings. what the rev. t. w. webb says of telescopes is equally true of opera-glasses: "inferior articles may be showily got up, and the outside must go for nothing." there are a few makers whose names, stamped upon the instrument, may generally be regarded as a guarantee of excellence. but the best test is that of actual performance. i have a field-glass which i found in a pawn-shop, that has no maker's name upon it, but in some respects is quite capable of bearing comparison with the work of the best advertised opticians. and this leads me to say that, by the exercise of good judgment, one may occasionally purchase superior glasses at very reasonable prices in the pawn-shops. ask to be shown the old and well-tried articles; you may find among them a second-hand glass of fine optical properties. if the lenses are not injured, one need not trouble one's self about the worn appearance of the outside of the instrument; so much the more evidence that somebody has found it well worth using. a good field or marine glass is in some respects better than an opera-glass for celestial observations. it possesses a much higher magnifying power, and this gives sometimes a decided advantage. but, on the other hand, its field of view is smaller, rendering it more difficult to find and hold objects. besides, it does not present as brilliant views of scattered star-clusters as an opera-glass does. for the benefit of those who possess field-glasses, however, i have included in this brief survey certain objects that lie just beyond the reach of opera-glasses, but can be seen with the larger instruments. i have thought it advisable in the descriptions of the constellations which follow to give some account of their mythological origin, both because of the historical interest which attaches to it, and because, while astronomers have long since banished the constellation figures from their maps, the names which the constellations continue to bear require some explanation, and they possess a literary and romantic interest which can not be altogether disregarded in a work that is not intended for purely scientific readers. chapter i. the stars of spring. having selected your glass, the next thing is to find the stars. of course, one could sweep over the heavens at random on a starry night and see many interesting things, but he would soon tire of such aimless occupation. the observer must know what he is looking at in order to derive any real pleasure or satisfaction from the sight. it really makes no difference at what time of the year such observations are begun, but for convenience i will suppose that they are begun in the spring. we can then follow the revolution of the heavens through a year, at the end of which the diligent observer will have acquired a competent knowledge of the constellations. the circular map, no. , represents the appearance of the heavens at midnight on the st of march, at eleven o'clock on the th of march, at ten o'clock on the st of april, at nine o'clock on the th of april, and at eight o'clock on the st of may. the reason why a single map can thus be made to show the places of the stars at different hours in different months will be plain upon a little reflection. in consequence of the earth's annual journey around the sun, the whole heavens make one apparent revolution in a year. this revolution, it is clear, must be at the rate of ° in a month, since the complete circuit comprises °. but, in addition to the annual revolution, there is a diurnal revolution of the heavens which is caused by the earth's daily rotation upon its axis, and this revolution must, for a similar reason, be performed at the rate of ° for each of the twenty-four hours. it follows that in two hours of the daily revolution the stars will change their places to the same extent as in one month of the annual revolution. it follows also that, if one could watch the heavens throughout the whole twenty-four hours, and not be interrupted by daylight, he would behold the complete circuit of the stars just as he would do if, for a year, he should look at the heavens at a particular hour every night. suppose that at nine o'clock on the st of june we see the star spica on the meridian; in consequence of the rotation of the earth, two hours later, or at eleven o'clock, spica will be ° west of the meridian. but that is just the position which spica would occupy at nine o'clock on the st of july, for in one month (supposing a month to be accurately the twelfth part of a year) the stars shift their places ° toward the west. if, then, we should make a map of the stars for nine o'clock on the st of july, it would answer just as well for eleven o'clock on the st of june, or for seven o'clock on the st of august. [illustration: map .] the center of the map is the zenith, or point overhead. the reader must now exercise his imagination a little, for it is impossible to represent the true appearance of the concave of the heavens on flat paper. holding the map over your head, with the points marked east, west, north, and south in their proper places, conceive of it as shaped like the inside of an open umbrella, the edge all around extending clear down to the horizon. suppose you are facing the south, then you will see, up near the zenith, the constellation of leo, which can be readily recognized on the map by six stars that mark out the figure of a sickle standing upright on its handle. the large star in the bottom of the handle is regulus. having fixed the appearance and situation of this constellation in your mind, go out-of-doors, face the south, and try to find the constellation in the sky. with a little application you will be sure to succeed. using leo as a basis of operations, your conquest of the sky will now proceed more rapidly. by reference to the map you will be able to recognize the twin stars of gemini, southwest of the zenith and high up; the brilliant lone star, procyon, south of gemini; the dazzling sirius, flashing low down in the southwest; orion, with all his brilliants, blazing in the west; red aldebaran and the pleiades off to his right; and capella, bright as a diamond, high up above orion, toward the north. in the southeast you will recognize the quadrilateral of corvus, with the remarkably white star spica glittering east of it. next face the north. if you are not just sure where north is, try a pocket-compass. this advice is by no means unnecessary, for there are many intelligent persons who are unable to indicate true north within many degrees, though standing on their own doorstep. having found the north point as near as you can, look upward about forty degrees from the horizon, and you will see the lone twinkler called the north or pole star. forty degrees is a little less than half-way from the horizon to the zenith. by the aid of the map, again, you will be able to find, high up in the northeast, near the zenith, the large dipper-shaped figure in ursa major, and, when you have once noticed that the two stars in the outer edge of the bowl of the dipper point almost directly to the pole-star, you will have an unfailing means of picking out the latter star hereafter, when in doubt.[a] continuing the curve of the dipper-handle, in the northeast, your eye will be led to a bright reddish star, which is arcturus, in the constellation boötes. [a] let the reader remember that the distance between the two stars in the brim of the bowl of the dipper is about ten degrees, and he will have a measuring-stick that he can apply in estimating other distances in the heavens. in the same way you will be able to find the constellations cassiopeia, cepheus, draco, and perseus. don't expect to accomplish it all in an hour. you may have to devote two or three evenings to such observation, and make many trips indoors to consult the map, before you have mastered the subject; but when you have done it you will feel amply repaid for your exertions, and you will have made for yourself silent friends in the heavens that will beam kindly upon you, like old neighbors, on whatever side of the world you may wander. having fixed the general outlines and location of the constellations in your mind, and learned to recognize the chief stars, take your opera-glass and begin with the constellation leo and the star regulus. contrive to have some convenient rest for your arms in holding the glass, and thus obtain not only comfort but steadiness of vision. a lazy-back chair makes a capital observing-seat. be very particular, too, to get a sharp focus. remember that no two persons' eyes are alike, and that even the eyes of the same observer occasionally require a change. in looking for a difficult object, i have sometimes suddenly brought the sought-for phenomenon into view by a slight turn of the focusing-screw. you will at once be gratified by the increased brilliancy of the star as seen by the glass. if the night is clear, it will glow like a diamond. yet regulus, although ranked as a first-magnitude star, and of great repute among the ancient astrologers, is far inferior in brilliancy to such stars as capella and arcturus, to say nothing of sirius. by consulting map no. you will next be able to find the celebrated star bearing the name of the greek letter gamma ([gamma]). if you had a telescope, you would see this star as a close and beautiful double, of contrasted colors. but it is optically double, even with an opera-glass. you can not fail to see a small star near it, looking quite close if the magnifying power of your glass is less than three times. you will be struck by the surprising change of color in turning from regulus to gamma--the former is white and the latter deep yellow. it will be well to look first at one and then at the other, several times, for this is a good instance of what you will meet with many times in your future surveys of the heavens--a striking contrast of color in neighboring stars. one can thus comprehend that there is more than one sense in which to understand the scriptural declaration that "one star differeth from another in glory." the radiant point of the famous november meteors, which, in and , filled the sky with fiery showers, is near gamma. turn next to the star in leo marked zeta ([zeta]). if your glass is a pretty large and good one, and your eye keen, you will easily see three minute companion stars keeping company with zeta, two on the southeast, and one, much closer, toward the north. the nearest of the two on the south is faint, being only between the eighth and ninth magnitude, and will probably severely test your powers of vision. next look at epsilon ([epsilon]), and you will find near it two seventh-magnitude companions, making a beautiful little triangle. [illustration: map .] away at the eastern end of the constellation, in the tail of the imaginary lion, upon whose breast shines regulus, is the star beta ([beta]) leonis, also called denebola. it is almost as bright as its leader, regulus, and you will probably be able to catch a tinge of blue in its rays. south of denebola, at a distance of nineteen minutes of arc, or somewhat more than half the apparent diameter of the moon, you will see a little star of the sixth magnitude, which is one of the several "companions" for which denebola is celebrated. there is another star of the eighth magnitude in the same direction from denebola, but at a distance of less than five minutes, and this you may be able to glimpse with a powerful field-glass, under favorable conditions. i have seen it well with a field-glass of . -inch aperture, and a magnifying power of seven times. but it requires an experienced eye and steady vision to catch this shy twinkler. when looking for a faint and difficult object, the plan pursued by telescopists is to avert the eye from the precise point upon which the attention is fixed, in order to bring a more sensitive part of the retina into play than that usually employed. look toward the edge of the field of view, while the object you are seeking is in the center, and then, if it can be seen at all with your glass, you will catch sight of it, as it were, out of the corner of your eye. the effect of seeing a faint star in this way, in the neighborhood of a large one, whose rays hide it from direct vision, is sometimes very amusing. the little star seems to dart out into view as through a curtain, perfectly distinct, though as immeasurably minute as the point of a needle. but the instant you direct your eyes straight at it, presto! it is gone. and so it will dodge in and out of sight as often as you turn your eyes. if you will sweep carefully over the whole extent of leo, whose chief stars are marked with their greek-letter names on our little map, you will be impressed with the power of your glass to bring into sight many faint stars in regions that seem barren to the naked eye. an opera-glass of . aperture will show ten times as many stars as the naked eye can see. a word about the "lion" which this constellation is supposed to represent. it requires a vivid imagination to perceive the outlines of the celestial king of beasts among the stars, and yet somebody taught the people of ancient india and the old egyptians to see him there, and there he has remained since the dawn of history. modern astronomers strike him out of their charts, together with all the picturesque multitude of beasts and birds and men and women that bear him company, but they can not altogether banish him, or any of his congeners, for the old names, and, practically, the old outlines of the constellations are retained, and always will be retained. the lion is the most conspicuous figure in the celebrated zodiac of dendera; and, indeed, there is evidence that before the story of hercules and his labors was told this lion was already imagined shining among the stars. it was characteristic of the greeks that they seized him for their own, and tried to rob him of his real antiquity by pretending that jupiter had placed him among the stars in commemoration of hercules's victory over the nemæan lion. in the hebrew zodiac leo represented the lion of judah. it was thus always a lion that the ancients thought they saw in this constellation. in the old star-maps the lion is represented as in the act of springing upon his prey. his face is to the west, and the star regulus is in his heart. the sickle-shaped figure covers his breast and head, gamma being in the shoulder, zeta in the mane of the neck, mu and epsilon in the cheek, and lambda in the jaws. the fore-paws are drawn up to the breast and represented by the stars zi and omicron. denebola is in the tuft of the tail. the hind-legs are extended downward at full length, in the act of springing. starting from the star delta in the hip, the row consisting of theta, iota, tau, and upsilon, shows the line of the hind-legs. leo had an unsavory reputation among the ancients because of his supposed influence upon the weather. the greatest heat of summer was felt when the sun was in this constellation: "most scorching is the chariot of the sun, and waving spikes no longer hide the furrows when he begins to travel with the lion." looking now westwardly from the sickle of leo, at a distance about equal to twice the length of the sickle, your eye will be caught by a small silvery spot in the sky lying nearly between two rather faint stars. this is the famous præsepe, or manger, in the center of the constellation cancer. the two stars on either side of it are called the aselli, or the ass's colts, and the imagination of the ancients pictured them feeding from their silver manger. turn your glass upon the manger and you will see that it consists of a crowd of little stars, so small and numerous that you will probably not undertake to count them, unless you are using a large field-glass. galileo has left a delightful description of his surprise and gratification when he aimed his telescope at this curious cluster and other similar aggregations of stars and discovered what they really were. using his best instrument, he was able to count thirty-six stars in the manger. the manger was a famous weather-sign in olden times, and aratus, in his "diosemia," advises his readers to-- "... watch the manger: like a little mist far north in cancer's territory it floats. its confines are two faintly glimmering stars; these are two asses that a manger parts, which suddenly, when all the sky is clear, sometimes quite vanishes, and the two stars seem to have closer moved their sundered orbs. no feeble tempest then will soak the leas; a murky manger with both stars shining unaltered is a sign of rain." like other old weather-saws, this probably possesses a gleam of sense, for it is only when the atmosphere is perfectly transparent that the manger can be clearly seen; when the air is thick with mist, the harbinger of coming storm, it fades from sight. the constellation cancer, or the crab, was represented by the egyptians under the figure of a scarabæus. the observer will probably think that it is as easy to see a beetle as a crab there. cancer, like leo, is one of the twelve constellations of the zodiac, the name applied to the imaginary zone ° degrees wide and extending completely around the heavens, the center of which is the ecliptic or annual path of the sun. the names of these zodiacal constellations, in their order, beginning at the west and counting round the circle, are: aries, taurus, gemini, cancer, leo, virgo, libra, scorpio, sagittarius, capricornus, aquarius, and pisces. cancer has given its name to the circle called the tropic of cancer, which indicates the greatest northerly declination of the sun in summer, and which he attains on the st or d of june. but, in consequence of the precession of the equinoxes, all of the zodiacal constellations are continually shifting toward the east, and cancer has passed away from the place of the summer solstice, which is now to be found in gemini. below the manger, a little way toward the south, your eye will be caught by a group of four or five stars of about the same brightness as the aselli. this marks the head of hydra, and the glass will show a striking and beautiful geometrical arrangement of the stars composing it. hydra is a very long constellation, and trending southward and eastward from the head it passes underneath leo, and, sweeping pretty close down to the horizon, winds away under corvus, the tail reaching to the eastern horizon. the length of this skyey serpent is about °. its stars are all faint, except alphard, or the hydra's heart, a second-magnitude star, remarkable for its lonely situation, southwest of regulus. a line from gamma leonis through regulus points it out. it is worth looking at with the glass on account of its rich orange-tint. hydra is fabled to be the hundred-headed monster that was slain by hercules. it must be confessed that there is nothing very monstrous about it now except its length. the most timid can look upon it without suspecting its grisly origin. coming back to the manger as a starting-point, look well up to the north and west, and at a distance somewhat less than that between regulus and the manger you will see a pair of first-magnitude stars, which you will hardly need to be informed are the celebrated twins, from which the constellation gemini takes its name. the star marked [alpha] in the map is castor, and the star marked [beta] is pollux. no classical reader needs to be reminded of the romantic origin of these names. a sharp contrast in the color of castor and pollux comes out as soon as the glass is turned upon them. castor is white, with occasionally, perhaps, a suspicion of a green ray in its light. pollux is deep yellow. castor is a celebrated double star, but its components are far too close to be separated with an opera-glass, or even the most powerful field-glass. you will be at once interested by the singular _cortége_ of small stars by which both castor and pollux are surrounded. these little attendant stars, for such they seem, are arrayed in symmetrical groups--pairs, triangles, and other figures--which, it seems difficult to believe, could be unintentional, although it would be still more difficult to suggest any reason why they should be arranged in that way. [illustration: map .] our map will show you the position of the principal stars of the constellation. castor and pollux are in the heads of the twins, while the row of stars shown in the map xi ([xi]), gamma ([gamma]), nu ([nu]), mu ([mu]), and eta ([eta]), marks their feet, which are dipped in the edge of the milky-way. one can spend a profitable and pleasurable half-hour in exploring the wonders of gemini. the whole constellation, from head to foot, is gemmed with stars which escape the naked eye, but it sparkles like a bead-spangled garment when viewed with the glass. owing to the presence of the milky-way, the spectacle around the feet of the twins is particularly magnificent. and here the possessor of a good opera-glass can get a fine view of a celebrated star-cluster known in the catalogues as m. it is situated a little distance northwest of the star eta, and is visible to the naked eye, on a clear, moonless night, as a nebulous speck. with a good glass you will see two wonderful streams of little stars starting, one from eta and the other from mu, and running parallel toward the northwest; m is situated between these star-streams. the stars in the cluster are so closely aggregated that you will be able to clearly separate only the outlying ones. the general aspect is like that of a piece of frosted silver over which a twinkling light is playing. a field-glass brings out more of the component stars. the splendor of this starry congregation, viewed with a powerful telescope, may be guessed at from admiral smyth's picturesque description: "it presents a gorgeous field of stars, from the ninth to the sixteenth magnitude, but with the center of the mass less rich than the rest. from the small stars being inclined to form curves of three or four, and often with a large one at the root of the curve, it somewhat reminds one of the bursting of a sky-rocket." and webb adds that there is an "elegant festoon near the center, starting with a reddish star." no one can gaze upon this marvelous phenomenon, even with the comparatively low powers of an opera-glass, and reflect that all these swarming dots of light are really suns, without a stunning sense of the immensity of the material universe. it is an interesting fact that the summer solstice, or the point which the sun occupies when it attains its greatest northerly declination, on the longest day of the year, is close by this great cluster in gemini. in the glare of the sunshine those swarming stars are then concealed from our sight, but with the mind's eye we can look past and beyond our sun, across the incomprehensible chasm of space, and behold them still shining, their commingled rays making our great god of day seem but a lonely wanderer in the expanse of the universe. it was only a short distance southwest of this cluster that one of the most celebrated discoveries in astronomy was made. there, on the evening of march , , william herschel observed a star whose singular aspect led him to put a higher magnifying power on his telescope. the higher power showed that the object was not a star but a planet, or a comet, as herschel at first supposed. it was the planet uranus, whose discovery "at one stroke doubled the breadth of the sun's dominions." the constellation of gemini, as the names of its two chief stars indicate, had its origin in the classic story of the twin sons of jupiter and leda: "fair leda's twins, in time to stars decreed, one fought on foot, one curbed the fiery steed." castor and pollux were regarded by both the greeks and the romans as the patrons of navigation, and this fact crops out very curiously in the adventures of st. paul. after his disastrous shipwreck on the island of melita he embarked again on a more prosperous voyage in a ship bearing the name of these very brothers. "and after three months," writes the celebrated apostle (acts xxviii, ) "we departed in a ship of alexandria, which had wintered in the isle, whose sign was castor and pollux." we may be certain that paul was acquainted with the constellation of gemini, not only because he was skilled in the learning of his times, but because, in his speech on mars hill, he quoted a line from the opening stanzas of aratus's "phenomena," a poem in which the constellations are described. the map will enable you next to find procyon, or the little dog-star, more than twenty degrees south of castor and pollux, and almost directly below the manger. this star will interest you by its golden-yellow color and its brightness, although it is far inferior in the latter respect to sirius, or the great dog-star, which you will see flashing splendidly far down beneath procyon in the southwest. about four degrees northwest of procyon is a third-magnitude star, called gomelza, and the glass will show you two small stars which make a right-angled triangle with it, the nearer one being remarkable for its ruddy color. procyon is especially interesting because it is attended by an invisible star, which, while it has escaped all efforts to detect it with powerful telescopes, nevertheless reveals its presence by the effect of its attraction upon procyon. it is a curious fact that both of the so-called dog-stars are thus attended by obscure or dusky companion-stars, which, notwithstanding their lack of luminosity, are of great magnitude. in the case of sirius, the improvement in telescopes has brought the mysterious attendant into view, but procyon's mate remains hidden from our eyes. but it can not escape the ken of the mathematician, whose penetrating mental vision has, in more than one instance, outstripped the discoveries of the telescope. almost half a century ago the famous bessel announced his conclusion--in the light of later developments it may well be called discovery--that both sirius and procyon were binary systems, consisting each of a visible and an invisible star. he calculated the probable period of revolution, and found it to be, in each case, approximately fifty years. sixteen years after bessel's death, one of alvan clark's unrivaled telescopes at last revealed the strange companion of sirius, a huge body, half as massive as the giant dog-star itself, but ten thousand times less brilliant, and more recent observations have shown that its period of revolution is within six or seven months of the fifty years assigned by bessel. if some of the enormous telescopes that have been constructed in the past few years should succeed in rendering procyon's companion visible also, it is highly probable that bessel's prediction would receive another substantial fulfillment. the mythological history of canis minor is somewhat obscure. according to various accounts it represents one of diana's hunting-dogs, one of orion's hounds, the egyptian dog-headed god anubis, and one of the dogs that devoured their master actæon after diana had turned him into a stag. the mystical dr. seiss leaves all the ancient myth-makers far in the rear, and advances a very curious theory of his own about this constellation, in his "gospel in the stars," which is worth quoting as an example of the grotesque fancies that even in our day sometimes possess the minds of men when they venture beyond the safe confines of this terraqueous globe. after summarizing the various myths we have mentioned, he proceeds to identify procyon, putting the name of the chief star for the constellation, "as the starry symbol of those heavenly armies which came forth along with the king of kings and lord of lords to the battle of the great day of god almighty, to make an end of misrule and usurpation on earth, and clear it of all the wild beasts which have been devastating it for these many ages." the reader will wonder all the more at this rhapsody after he has succeeded in picking out the modest little dog in the sky. sirius, orion, aldebaran, and the pleiades, all of which you will perceive in the west and southwest, are generally too much involved in the mists of the horizon to be seen to the best advantage at this season, although it will pay you to take a look through the glass at sirius. but the splendid star capella, in the constellation auriga, may claim a moment's attention. you will find it high up in the northwest, half-way between orion and the pole-star, and to the right of the twins. it has no rival near, and its creamy-white light makes it one of the most beautiful as well as one of the most brilliant stars in the heavens. its constitution, as revealed by the spectroscope, resembles that of our sun, but the sun would make but a sorry figure if removed to the side of this giant star. about seven and a half degrees above capella, and a little to the left, you will see a second-magnitude star called menkalina. two and a half times as far to the left, or south, in the direction of orion, is another star of equal brightness to menkalina. this is el nath, and marks the place where the foot of auriga, or the charioteer, rests upon the point of the horn of taurus. capella, menkalina, and el nath make a long triangle which covers the central part of auriga. the naked eye shows two or three misty-looking spots within this triangle, one to the right of el nath, one in the upper or eastern part of the constellation, near the third-magnitude star theta ([theta]), and another on a line drawn from capella to el nath, but much nearer to capella. turn your glass upon these spots, and you will be delighted by the beauty of the little stars to whose united rays they are due. el nath has around it some very remarkable rows of small stars, and the whole constellation of auriga, like that of gemini, glitters with star-dust, for the milky-way runs directly through it. with a powerful field-glass you may try a glimpse at the rich star-clusters marked m, m, and ^ . [illustration: map .] the mythology of auriga is not clear, but the ancients seem to have been of one mind in regarding the constellation as representing the figure of a man carrying a goat and her two kids in his arms. auriga was also looked upon as a beneficent constellation, and the goat and kids were believed to be on the watch to rescue shipwrecked sailors. as capella, which represents the fabled goat, shines nearly overhead in winter, and would ordinarily be the first bright star to beam down through the breaking clouds of a storm at that season, it is not difficult to imagine how it got its reputation as the seaman's friend. dr. seiss has so spirited a description of the imaginary figure contained in this constellation that i can not refrain from quoting it: "the figure itself is that of a mighty man seated on the milky-way, holding a band or ribbon in his right hand, and with his left arm holding up on his shoulder a she-goat which clings to his neck and looks out in astonishment upon the terrible bull; while in his lap are two frightened little kids which he supports with his great hand." it is scarcely necessary to add that dr. seiss insists that auriga, as a constellation, was invented long before the time of the greeks, and was intended prophetically to represent that good shepherd who was to come and rescue the sinful world. if any reader wishes to exercise his fancy by trying to trace the outlines of this figure, he will find the head of auriga marked by the star delta ([delta]) and the little group near it. capella, in the heart of the goat, is just below his left shoulder, and menkalina marks his right shoulder. el nath is in his right foot, and iota ([iota]) in his left foot. the stars epsilon ([epsilon]), zeta ([zeta]), eta ([eta]), and lambda ([lambda]) shine in the kids which lie in auriga's lap. the faint stars scattered over the eastern part of the constellation are sometimes represented as forming a whip with many lashes, which the giant flourishes with his right hand. let us turn back to denebola in the lion's tail. now glance from it down into the southeast, and you will see a brilliant star flashing well above the horizon. this is spica, the chief twinkler of virgo, and it is marked on our circular map. then look into the northwest, and at about the same distance from denebola, but higher above the horizon than spica, you will catch the sparkling of a large, reddish star. it is arcturus in boötes. the three, denebola, spica, and arcturus, mark the corners of a great equilateral triangle. nearly on a line between denebola and arcturus, and somewhat nearer to the former, you will perceive a curious twinkling, as if gossamers spangled with dew-drops were entangled there. one might think the old woman of the nursery rhyme who went to sweep the cobwebs out of the sky had skipped this corner, or else that its delicate beauty had preserved it even from her housewifely instincts. this is the little constellation called berenice's hair. your opera-glass will enable you to count twenty or thirty of the largest stars composing this cluster, which are arranged, as so often happens, with a striking appearance of geometrical design. the constellation has a very romantic history. it is related that the young queen berenice, when her husband was called away to the wars, vowed to sacrifice her beautiful tresses to venus if he returned victorious over his enemies. he did return home in triumph, and berenice, true to her vow, cut off her hair and bore it to the temple of venus. but the same night it disappeared. the king was furious, and the queen wept bitterly over the loss. there is no telling what might have happened to the guardians of the temple, had not a celebrated astronomer named conon led the young king and queen aside in the evening and showed them the missing locks shining transfigured in the sky. he assured them that venus had placed berenice's lustrous ringlets among the stars, and, as they were not skilled in celestial lore, they were quite ready to believe that the silvery swarm they saw near arcturus had never been there before. and so for centuries the world has recognized the constellation of berenice's hair. look next at corvus and crater, the crow and the cup, two little constellations which you will discover on the circular map, and of which we give a separate representation in map . you will find that the stars delta ([delta]) and eta ([eta]), in the upper left-hand corner of the quadrilateral figure of corvus, make a striking appearance. the little star zeta ([zeta]) is a very pretty double for an opera-glass. there is a very faint pair of stars close below and to the right of beta ([beta]). this forms a severe test. only a good opera-glass will show these two stars as a single faint point of light. a field-glass, however, will show both, one being considerably fainter than the other. crater is worth sweeping over for the pretty combinations of stars to be found in it. you will observe that the interminable hydra extends his lengthening coils along under both of the constellations. in fact, both the cup and the crow are represented as standing upon the huge serpent. the outlines of a cup are tolerably well indicated by the stars included under the name crater, but the constellation of the crow might as well have borne any other name so far as any traceable likeness is concerned. one of the legends concerning corvus avers that it is the daughter of the king of phocis, who was transformed into a crow to escape the pursuit of neptune. she is certainly safe in her present guise. arcturus and spica, and their companions, may be left for observation to a more convenient season, when, having risen higher, they can be studied to better advantage. it will be well, however, to merely glance at them with the glass in order to note the great difference of color--spica being brilliantly white and arcturus almost red. [illustration: map .] we will now turn to the north. you have already been told how to find the pole-star. look at it with your glass. the pole-star is a famous double, but its minute companion can only be seen with a telescope. as so often happens, however, it has another companion for the opera-glass, and this latter is sufficiently close and small to make an interesting test for an inexperienced observer armed with a glass of small power. it must be looked for pretty close to the rays of the large star, with such a glass. it is of the seventh magnitude. with a large field-glass several smaller companions may be seen, and a very excellent glass may show an . -magnitude star almost hidden in the rays of the seventh-magnitude companion. with the aid of map no. find in ursa minor, which is the constellation to which the pole-star belongs, the star beta ([beta]), which is also called kochab (the star marked [alpha] in the map is the pole-star). kochab has a pair of faint stars nearly north of it, about one degree distant. with a small glass these may appear as a single star, but a stronger glass will show them separately. [illustration: map .] and now for ursa major and the great dipper--draco, cepheus, cassiopeia, and the other constellations represented on the circular map, being rather too near the horizon for effective observation at this time of the year. first, as the easiest object, look at the star in the middle of the handle of the dipper (this handle forms the tail of ursa major), and a little attention will show you, without the aid of a glass, if your eye-sight is good, that the star is double. a smaller star seems to be almost in contact with it. the larger of these two stars is called mizar and the smaller alcor--the horse and his rider the arabs said. your glass will, of course, greatly increase the distance between alcor and mizar, and will also bring out a clear difference of color distinguishing them. now, if you have a very powerful glass, you may be able to see the sidus ludovicianum, a minute star which a german astronomer discovered more than a hundred and fifty years ago, and, strangely enough, taking it for a planet, named it after a german prince. the position of the sidus ludovicianum, with reference to mizar and alcor, is represented in the accompanying sketch. you must look very sharply if you expect to see it, and your opera-glass will have to be a large and strong one. a field-glass, however, can not fail to show it. sweep along the whole length of the dipper's handle, and you will discover many fine fields of stars. then look at the star alpha ([alpha]) in the outer edge of the bowl nearest to the pole-star. there is a faint star, of about the eighth magnitude, near it, in the direction of beta ([beta]). this will prove a very difficult test. you will have to try it with averted vision. if you have a field-glass, catch it first with that, and, having thus fixed its position in your mind, try to find it with the opera-glass. its distance is a little over half that between mizar and alcor. it is of a reddish color. you will notice nearly overhead three pairs of pretty bright stars in a long, bending row, about half-way between leo and the dipper. these mark three of ursa major's feet, and each of the pairs is well worth looking at with a glass, as they are beautifully grouped with stars invisible to the naked eye. the letters used to designate the stars forming these pairs will be found upon our map of ursa major. the scattered group of faint stars beyond the bowl of the dipper forms the bear's head, and you will find that also a field worth a few minutes' exploration. [illustration: mizar, alcor, and the sidus ludovicianum.] the two bears, ursa major and ursa minor, swinging around the pole of the heavens, have been conspicuous in the star-lore of all ages. according to fable, they represent the nymph calisto, with whom jupiter was in love, and her son arcas, who were both turned into bears by juno, whereupon jupiter, being unable to restore their form, did the next best thing he could by placing them among the stars. ursa major is calisto, or helica, as the greeks called the constellation. the greek name of ursa minor was cynosura. the use of the pole-star in navigation dates back at least to the time of the phoenicians. the observer will note the uncomfortable position of ursa minor, attached to the pole by the end of its long tail. but, after all, no one can expect to derive from such studies as these any genuine pleasure or satisfaction unless he is mindful of the real meaning of what he sees. the actual truth seems almost too stupendous for belief. the mind must be brought into an attitude of profound contemplation in order to appreciate it. from this globe we can look out in every direction into the open and boundless universe. blinded and dazzled during the day by the blaze of that star, of which the earth is a near and humble dependent, we are shut in as by a curtain. but at night, when our own star is hidden, our vision ranges into the depths of creation, and we behold them sparkling with a multitude of other suns. with so simple an aid as that of an opera-glass we penetrate still deeper into the profundities of space, and thousands more of these strange, far-away suns come into sight. they are arranged in pairs, sets, rows, streams, clusters--here they gleam alone in distant splendor, there they glow and flash in mighty swarms. this is a look into heaven more splendid than the imagination of bunyan pictured; here is a celestial city whose temples are suns, and whose streets are the pathways of light. chapter ii. the stars of summer. let us now suppose that the earth has advanced for three months in its orbit since we studied the stars of spring, and that, in consequence, the heavens have made one quarter of an apparent revolution. then we shall find that the stars which in spring shone above the western horizon have been carried down out of sight, while the constellations that were then in the east have now climbed to the zenith, or passed over to the west, and a fresh set of stars has taken their place in the east. in the present chapter we shall deal with what may be called the stars of summer; and, in order to furnish occupation for the observer with an opera-glass throughout the summer months, i have endeavored to so choose the constellations in which our explorations will be made, that some of them shall be favorably situated in each of the months of june, july, and august. the circular map represents the heavens at midnight on the st of june; at eleven o'clock, on the th of june; at ten o'clock, on the st of july; at nine o'clock, on the th of july; and at eight o'clock, on the st of august. remembering that the center of the map is the point over his head, and that the edge of it represents the circle of the horizon, the reader, by a little attention and comparison with the sky, will be able to fix in his mind the relative situation of the various constellations. the maps that follow will show him these constellations on a larger scale, and give him the names of their chief stars. [illustration: map .] the observer need not wait until midnight on the st of june in order to find some of the constellations included in our map. earlier in the evening, at about that date, say at nine o'clock, he will be able to see many of these constellations, but he must look for them farther toward the east than they are represented in the map. the bright stars in boötes and virgo, for instance, instead of being over in the southwest, as in the map, will be near the meridian; while lyra, instead of shining high overhead, will be found climbing up out of the northeast. it would be well to begin at nine o'clock, about the st of june, and watch the motions of the heavens for two or three hours. at the commencement of the observations you will find the stars in boötes, virgo, and lyra in the positions i have just mentioned, while half-way down the western sky will be seen the sickle of leo. the brilliant procyon and capella will be found almost ready to set in the west and northwest, respectively. between procyon and capella, and higher above the horizon, shine the twin stars in gemini. in an hour procyon, capella, and the twins will be setting, and spica will be well past the meridian. in another hour the observer will perceive that the constellations are approaching the places given to them in our map, and at midnight he will find them all in their assigned positions. a single evening spent in observations of this sort will teach him more about the places of the stars than he could learn from a dozen books. taking, now, the largest opera-glass you can get (i have before said that the diameter of the object-glasses should not be less than . inch, and, i may add, the larger they are the better), find the constellation scorpio, and its chief star antares. the map shows you where to look for it at midnight on the st of june. if you prefer to begin at nine o'clock at that date, then, instead of looking directly in the south for scorpio, you must expect to see it just rising in the southeast. you will recognize antares by its fiery color, as well as by the striking arrangement of its surrounding stars. there are few constellations which bear so close a resemblance to the objects they are named after as scorpio. it does not require a very violent exercise of the imagination to see in this long, winding trail of stars a gigantic scorpion, with its head to the west, and flourishing its upraised sting that glitters with a pair of twin stars, as if ready to strike. readers of the old story of phaeton's disastrous attempt to drive the chariot of the sun for a day will remember it was the sight of this threatening monster that so terrified the ambitious youth as he dashed along the zodiac, that he lost control of apollo's horses, and came near burning the earth up by running the sun into it. antares rather gains in redness when viewed with a glass. its color is very remarkable, and it is a curious circumstance that with powerful telescopes a small, bright-green star is seen apparently almost touching it. antares belongs to secchi's third type of suns, that in which the spectroscopic appearances suggest the existence of a powerfully absorptive atmosphere, and which are believed on various grounds to be, as lockyer has said, "in the last visible stage of cooling"; in other words, almost extinct. this great, red star probably in actual size exceeds our sun, and no one can help feeling the sublime nature of those studies which give us reason to think that here we can actually behold almost the expiring throes of a giant brother of our giant sun. only, the lifetime of a sun is many millions of years, and its gradual extinction, even after it has reached a stage as advanced as that of antares is supposed to be, may occupy a longer time than the whole duration of the human race. a little close inspection with the naked eye will show three fifth- or sixth-magnitude stars above antares and sigma ([sigma]), which form, with those stars, the figure of an irregular pentagon. an opera-glass shows this figure very plainly. the nearest of these stars to antares, the one directly above it, is known by the number , and belongs to scorpio, while the farthest away, which marks the northernmost corner of the pentagon, is rho in ophiuchus. try a powerful field-glass upon the two stars just named. take first. you will without much difficulty perceive that it has a little star under its wing, below and to the right, and more than twice as far away above it there is another faint star. then turn to rho. look sharp and you will catch sight of two companion stars, one close to rho on the right and a little below, and the other still closer and directly above rho. the latter is quite difficult to be seen distinctly, but the sight is a very pretty one. the opera-glass will show a number of faint stars scattered around antares. turn now to beta ([beta]) in scorpio, with the glass. a very pretty pair of stars will be seen hanging below [beta]. sweeping downward from this point to the horizon you will find many beautiful star-fields. the star marked nu ([nu]) is a double which you will be able to separate with a powerful field-glass, the distance between its components being ". [illustration: map .] and next let us look at a star-cluster. you will see on map no. an object marked m, near antares. its designation means that it is no. in messier's catalogue of nebulæ. it is not a true nebula, but a closely compacted cluster of stars. with the opera-glass, if you are looking in a clear and moonless night, you will see it as a curious nebulous speck. with a field-glass its real nature is more apparent, and it is seen to blaze brighter toward the center. it is, in fact, one of those universes within the universe where thousands of suns are associated together by some unknown law of aggregation into assemblages of whose splendor the slight view that we can get gives us but the faintest conception. the object above and to the right of antares, marked in the map m., is a nebula, and although the nebula itself is too small to be seen with an opera-glass (a field-glass shows it as a mere wisp of light), yet there is a pretty array of small stars in its neighborhood worth looking at. besides, this nebula is of special interest, because in a star suddenly took its place. at least, that is what seemed to have happened. what really did occur, probably, was that a variable or temporary star, situated between us and the nebula, and ordinarily too faint to be perceived, received a sudden and enormous accession of light, and blazed up so brightly as to blot out of sight the faint nebula behind it. if this star should make its appearance again, it could easily be seen with an opera-glass, and so it will not be useless for the reader to know where to look for it. the quarter of the heavens with which we are now dealing is famous for these celestial conflagrations, if so they may be called. the first temporary star of which there is any record appeared in the constellation of the scorpion, near the head, years before christ. it must have been a most extraordinary phenomenon, for it attracted attention all over the world, and both greek and chinese annals contain descriptions of it. in a. d. a temporary star shone out in the tail of scorpio. in a. d. arabian astronomers, under the caliph al-mamoun, the son of haroun-al-raschid, who broke into the great pyramid, observed a temporary star, that shone for four months in the constellation of the scorpion. in there was a temporary star, of a bluish color, in the tail of scorpio, and in another in the head of the constellation. besides these there are records of the appearance of four temporary stars in the neighboring constellation of ophiuchus, one of which, that of , is very famous, and will be described later on. it is conceivable that these strange outbursts in and near scorpio may have had some effect in causing this constellation to be regarded by the ancients as malign in its influence. we shall presently see some examples of star-clusters and nebulæ with which the instruments we are using are better capable of dealing than with the one described above. in the mean time, let us follow the bending row of stars from antares toward the south and east. when you reach the star mu ([mu]), you are not unlikely to stop with an exclamation of admiration, for the glass will separate it into two stars that, shining side by side, seem trying to rival each other in brightness. but the next star below [mu], marked zeta ([zeta]), is even more beautiful. it also separates into two stars, one being reddish and the other bluish in color. the contrast in a clear night is very pleasing. but this is not all. above the two stars you will notice a curious nebulous speck. now, if you have a powerful field-glass, here is an opportunity to view one of the prettiest sights in the heavens. the field-glass not only makes the two stars appear brighter, and their colors more pronounced, but it shows a third, fainter star below them, making a small triangle, and brings other still fainter stars into sight, while the nebulous speck above turns into a charmingly beautiful little star-cluster, whose components are so close that their rays are inextricably mingled in a maze of light. this little cut is an attempt to represent the scene, but no engraving can reproduce the life and sparkle of it. [illustration: zeta scorpionis.] following the bend of the scorpion's tail upward, we come to the pair of stars in the sting. these, of course, are thrown wide apart by the opera-glass. then let us sweep off to the eastward a little way and find the cluster known as m. you will see it marked on the map. above it, and near enough to be included in the same field of view, is m., a smaller cluster. both of these have a sparkling appearance with an opera-glass, and by close attention some of the separate stars in m. may be detected. with a field-glass these clusters become much more striking and starry looking, and the curious radiated structure of m. comes out. in looking at such objects we can not too often recall to our minds the significance of what we see--that these glimmering specks are the lights in the windows of the universe which carry to us, across inconceivable tracts of space, the assurance that we and our little system are not alone in the heavens; that all around us, and even on the very confines of immensity, nature is busy, as she is here, and the laws of light, heat, gravitation (and why not of life?), are in full activity. the clusters we have just been looking at lie on the borders of scorpio and sagittarius. let us cross over into the latter constellation, which commemorates the centaur chiron. we are now in another, and even a richer, region of wonders. the milky-way, streaming down out of the northeast, pours, in a luminous flood, through sagittarius, inundating that whole region of the heavens with seeming deeps and shallows, and finally bursting the barriers of the horizon disappears, only to glow with redoubled splendor in the southern hemisphere. the stars zeta ([zeta]), tau ([tau]), sigma ([sigma]), phi ([phi]), lambda ([lambda]), and mu ([mu]) indicate the outlines of a figure sometimes called the milk-dipper, which is very evident when the eye has once recognized it. on either side of the upturned handle of this dipper-like figure lie some of the most interesting objects in the sky. let us take the star [mu] for a starting-point. sweep downward and to the right a little way, and you will be startled by a most singular phenomenon that has suddenly made its appearance in the field of view of your glass. you may, perhaps, be tempted to congratulate yourself on having got ahead of all the astronomers, and discovered a comet. it is really a combination of a star-cluster with a nebula, and is known as m. sir john herschel has described the "nebulous folds and masses" and dark oval gaps which he saw in this nebula with his large telescope at the cape of good hope. but no telescope is needed to make it appear a wonderful object; an opera-glass suffices for that, and a field-glass reveals still more of its marvelous structure. the reader will recollect that we found the summer solstice close to a wonderful star-swarm in the feet of gemini. singularly enough the winter solstice is also near a star-cluster. it is to be found near a line drawn from m. to the star [mu] sagittarii, and about one third of the way from the cluster to the star. there is another less conspicuous star-cluster still closer to the solstitial point here, for this part of the heavens teems with such aggregations. on the opposite side of the star [mu]--that is to say, above and a little to the left--is an entirely different but almost equally attractive spectacle, the swarm of stars called m. here, again, the field-glass easily shows its superiority over the opera-glass, for magnifying power is needed to bring out the innumerable little twinklers of which the cluster is composed. but, whether you use an opera-glass or a field-glass, do not fail to gaze long and steadily at this island of stars, for much of its beauty becomes evident only after the eye has accustomed itself to disentangle the glimmering rays with which the whole field of view is filled. try the method of averted vision, and hundreds of the finest conceivable points of light will seem to spring into view out of the depths of the sky. the necessity of a perfectly clear night, and the absence of moonlight, can not be too much insisted upon for observations such as these. everybody knows how the moonlight blots out the smaller stars. a slight haziness, or smoke, in the air produces a similar effect. it is as important to the observer with an opera-glass to have a transparent atmosphere as it is to one who would use a telescope; but, fortunately, the work of the former is not so much interfered with by currents of air. always avoid the neighborhood of any bright light. electric lights in particular are an abomination to star-gazers. the cloud of stars we have just been looking at is in a very rich region of the milky-way, in the little modern constellation called "sobieski's shield," which we have not named upon our map. sweeping slowly upward from m. a little way with the field-glass, we will pass in succession over three nebulous-looking spots. the second of these, counting upward, is the famous horseshoe nebula. its wonders are beyond the reach of our instrument, but its place may be recognized. look carefully all around this region, and you will perceive that the old gods, who traveled this road (the milky-way was sometimes called the pathway of the gods), trod upon golden sands. off a little way to the east you will find the rich cluster called m. but do not imagine the thousands of stars that your opera-glass or field-glass reveals comprise all the riches of this golconda of the heavens. you might ply the powers of the greatest telescope in a vain attempt to exhaust its wealth. as a hint of the wonders that lie hidden here, let me quote father secchi's description of a starry spot in this same neighborhood, viewed with the great telescope at rome. after telling of "beds of stars superposed upon one another," and of the wonderful geometrical arrangement of the larger stars visible in the field, he adds: "the greater number are arranged in spiral arcs, in which one can count as many as ten or twelve stars of the ninth to the tenth magnitude following one another in a curve, like beads upon a string. sometimes they form rays which seem to diverge from a common focus, and, what is very singular, one usually finds, either at the center of the rays, or at the beginning of the curve, a more brilliant star of a red color, which seems to lead the march. it is impossible to believe that such an arrangement can be accidental." the reader will recall the somewhat similar description that admiral smyth and mr. webb have given of a star-cluster in gemini (see chapter i). the milky look of the background of the galaxy is, of course, caused by the intermingled radiations of inconceivably minute and inconceivably numerous stars, thousands of which become separately visible, the number thus distinguishable varying with the size of the instrument. but the most powerful telescope yet placed in human hands can not sound these starry deeps to the bottom. the evidence given by prof. holden, the director of the lick observatory, on this point is very interesting. speaking of the performance of the gigantic telescope on mount hamilton, thirty-six inches in aperture, he says: "the milky-way is a wonderful sight, and i have been much interested to see that there is, even with our superlative power, no final resolution of its finer parts into stars. there is always the background of unresolved nebulosity on which hundreds and thousands of stars are studded--each a bright, sharp, separate point." the groups of stars forming the eastern half of the constellation of sagittarius are worth sweeping over with the glass, as a number of pretty pairs may be found there. sagittarius stands in the old star-maps as a centaur, half-horse-half-man, facing the west, with drawn bow, and arrow pointed at the scorpion. [illustration: map .] next let us pass to the double constellation adjoining scorpio and sagittarius on the north--ophiuchus and the serpent. these constellations, as our map shows, are curiously intermixed. the imagination of the old star-gazers, who named them, saw here the figure of a giant grasping a writhing serpent with his hands. the head of the serpent is under the northern crown, and its tail ends over the star-gemmed region that we have just described, called "sobieski's shield." ophiuchus stands, as figured in flamsteed's "atlas," upon the back of the scorpion, holding the serpent with one hand below the neck, this hand being indicated by the pair of stars marked epsilon ([epsilon]) and delta ([delta]), and with the other near the tail. the stars tau ([tau]) and nu ([nu]) indicate the second hand. the giant's face is toward the observer, and the star alpha ([alpha]), also called ras alhague, shines in his forehead, while beta ([beta]) and gamma ([gamma]) mark his right shoulder. ophiuchus has been held to represent the famous physician Æsculapius. one may well repress the tendency to smile at these fanciful legends when he reflects upon their antiquity. there is no doubt that this double constellation is at least three thousand years old--that is to say, for thirty centuries the imagination of men has continued to shape these stars into the figures of a gigantic man struggling with a huge serpent. if it possesses no other interest, then it at least has that which attaches to all things ancient. like many other of the constellations it has proved longer-lived than the mightiest nations. while greece flourished and decayed, while rome rose and fell, while the scepter of civilization has passed from race to race, these starry creations of fancy have shone on unchanged. the mind that would ignore them now deserves compassion. the reader will observe a little circle in the map, and near it the figures . this indicates the spot where one of the most famous temporary stars on record appeared in the year . at first it was far brighter than any other star in the heavens; but it quickly faded, and in a little over a year disappeared. it is particularly interesting, because kepler--the quaintest, and not far from the greatest, figure in astronomical history--wrote a curious book about it. some of the philosophers of the day argued that the sudden outburst of the wonderful star was caused by the chance meeting of atoms. kepler's reply was characteristic, as well as amusing: "i will tell those disputants, my opponents, not my own opinion, but my wife's. yesterday, when i was weary with writing, my mind being quite dusty with considering these atoms, i was called to supper, and a salad i had asked for was set before me. 'it seems, then,' said i, aloud, 'that if pewter dishes, leaves of lettuce, grains of salt, drops of water, vinegar and oil, and slices of egg, had been flying about in the air from all eternity, it might at last happen by chance that there would come a salad.' 'yes,' says my wife, 'but not so nice and well-dressed as this of mine is.'" while there are no objects of special interest for the observer with an opera-glass in ophiuchus, he will find it worth while to sweep over it for what he may pick up, and, in particular, he should look at the group of stars southeast of [beta] and [gamma]. these stars have been shaped into a little modern asterism called taurus poniatowskii, and it will be noticed that five of them mark the outlines of a letter v, resembling the well-known figure of the hyades. also look at the stars in the head of serpens, several of which form a figure like a letter [x]. a little west of theta ([theta]) in the tail of serpens, is a beautiful swarm of little stars, upon which a field-glass may be used with advantage. the star [theta] is itself a charming double, just within the separating power of a very powerful field-glass under favorable circumstances, the component stars being only about one third of a minute apart. do not fail to notice the remarkable subdivisions of the milky-way in this neighborhood. its current seems divided into numerous channels and bays, interspersed with gaps that might be likened to islands, and the star [theta] appears to be situated upon one of these islands of the galaxy. this complicated structure of the milky-way extends downward to the horizon, and upward through the constellation cygnus, and of its phenomenal appearance in that region we shall have more to say further on. directly north of ophiuchus is the constellation hercules, interesting as occupying that part of the heavens toward which the proper motion of the sun is bearing the earth and its fellow-planets, at the rate, probably, of not less than , , miles in a year--a stupendous voyage through space, of whose destination we are as ignorant as the crew of a ship sailing under sealed orders, and, like whom, we must depend upon such inferences as we can draw from courses and distances, for no other information comes to us from the flagship of our squadron. [illustration: map .] in the accompanying map we have represented the beautiful constellations lyra and the northern crown, lying on either side of hercules. the reader should note that the point overhead in this map is not far from the star eta ([eta]) in hercules. the bottom of the map is toward the south, the right-hand side is west, and the left-hand side east. it is important to keep these directions in mind, in comparing the map with the sky. for instance, the observer must not expect to look into the south and see hercules half-way up the sky, with lyra a little east of it; he must look for hercules nearly overhead, and lyra a little east of the zenith. the same precautions are not necessary in using the maps of scorpio, sagittarius, and ophiuchus, because those constellations are nearer the horizon, and so the observer does not have to imagine the map as being suspended over his head. the name hercules sufficiently indicates the mythological origin of the constellation, and yet the greeks did not know it by that name, for aratus calls it "the phantom whose name none can tell." the northern crown, according to fable, was the celebrated crown of ariadne, and lyra was the harp of orpheus himself, with whose sweet music he charmed the hosts of hades, and persuaded pluto to yield up to him his lost eurydice. with the aid of the map you will be able to recognize the principal stars and star-groups in hercules, and will find many interesting combinations of stars for yourself. an object of special interest is the celebrated star-cluster m. you will find it on the map between the stars eta ([eta]) and zeta ([zeta]). while an opera-glass will only show it as a faint and minute speck, lying nearly between two little stars, it is nevertheless well worth looking for, on account of the great renown of this wonderful congregation of stars. sir william herschel computed the number of stars contained in it as about fourteen thousand. it is roughly spherical in shape, though there are many straggling stars around it evidently connected with the cluster. in short, it is _a ball of suns_. the reader should not mistake what that implies, however. these suns, though truly solar bodies, are probably very much smaller than our sun. mr. gore has computed their average diameter to be forty-five thousand miles, and the distance separating each from the next to be , , , miles. it may not be uninteresting to inquire what would be the appearance of the sky to dwellers within such a system of suns. adopting mr. gore's estimates, and supposing , , , miles to be very nearly the uniform distance apart of the stars in the cluster, and forty-five thousand miles their uniform diameter, then, starting with a single star in the center, their arrangement might be approximately in concentric spherical shells, situated about , , , miles apart. the first shell, counting outward from the center, would contain a dozen stars, each of which, as seen by an observer stationed upon a planet at the center of the cluster, would shine eleven hundred times as bright as sirius appears to us. the number of the stars in each shell would increase as they receded from the center in proportion to the squares of the radii of the successive shells, while their luminosity, as seen from the center, would vary inversely as those squares. still, the outermost stars--the total number being limited to fourteen or fifteen thousand--would appear to our observer at the center of the system about five times as brilliant as sirius. it is clear, then, that he would be dwelling in a sort of perpetual daylight. his planet might receive from the particular sun around which it revolved as brilliant a daylight as our sun gives to us, but let us see what would be the illumination of its night side. adopting zöllner's estimate of the light of the sun as , times as great as that of the full moon, and choosing among the various estimates of the light of sirius as compared with the sun / as probably the nearest the truth, we find that the moon sends us about sixty-five hundred times as much light as sirius does. now, since the dozen stars nearest the center of the cluster would each appear to our observer eleven hundred times as bright as sirius, all of them together would give a little more than twice as much light as the full moon sheds upon the earth. but as only half the stars in the cluster would be above the horizon at once we must diminish this estimate by one half, in order to obtain the amount of light that our supposititious planet would receive on its night side from the nearest stars in the cluster. and since the number of these stars increases with their distance from the center in the same ratio as their light diminishes, it follows that the total light received from the cluster would exceed that received from the dozen nearest stars as many times as there were spherical shells in the cluster. this would be about fifteen times, and accordingly all the stars together would shed, at the center, some thirty times as much light as that of the moon. dividing this again by two, because only half of the stars could be seen at once, we find that the night side of our observer's planet would be illuminated with fifteen times as much light as the full moon sheds upon the earth. it is evident, too, that our observer would enjoy the spectacle of a starry firmament incomparably more splendid than that which we behold. only about three thousand stars are visible to our unassisted eyes at once on any clear night, and of those only a few are conspicuous, and two thirds are so faint that they require some attention in order to be distinguished. but the spectator at the center of the hercules cluster would behold some seven thousand stars at once, the faintest of which would be five times as brilliant as the brightest star in our sky, while the brighter ones would blaze like nearing suns. one effect of this flood of starlight would be to shut out from our observer's eyes all the stars of the outside universe. they would be effaced in the blaze of his sky, and he would be, in a manner, shut up within his own little star-system, knowing nothing of the greater universe beyond, in which we behold his multitude of luminaries, diminished and blended by distance into a faintly shining speck, floating like a silvery mote in a sunbeam. if our observer's planet, instead of being situated in the center of the cluster, circled around one of the stars at the outer edge of it, the appearance of his sky would be, in some respects, still more wonderful, the precise phenomena depending upon the position of the planet's orbit and the station of the observer. less than half of his sky would be filled, at any time, by the stars of the cluster, the other half opening upon outer space and appearing by comparison almost starless--a vast, cavernous expanse, with a few faint glimmerings out of its gloomy depths. the plane of the orbit of his planet being supposed to pass through the center of the spherical system, our observer would, during his year, behold the night at one season blazing with the splendors of the clustered suns, and at another emptied of brilliant orbs and faintly lighted with the soft glow of the milky-way and the feeble flickering of distant stars, scattered over the dark vault. the position of the orbit, and the inclination of the planet's axis might be such that the glories of the cluster would not be visible from one of its hemispheres, necessitating a journey to the other side of the globe to behold them.[b] [b] a similar calculation of the internal appearances of the hercules cluster, which i made, was published in in the "new york sun." of course, it is not to be assumed that the arrangement of the stars in the cluster actually is exactly that which we have imagined. still, whatever the arrangement, so long as the cluster is practically spherical, and the stars composing it are of nearly uniform size and situated at nearly uniform distances, the phenomena we have described would fairly represent the appearances presented to inhabitants of worlds situated in such a system. as to the possibility of the existence of such worlds and inhabitants, everybody must draw his own conclusions. astronomy, as a science, is silent upon that question. but there shine the congregated stars, mingling their rays in a message of light, that comes to us across the gulf, proclaiming their brotherhood with our own glorious sun. mathematicians can not unravel the interlocking intricacies of their orbits, and some would, perhaps _a priori_, have said that such a system was impossible, but the telescope has revealed them, and there they are! what purposes they subserve in the economy of the universe, who shall declare? if you have a field-glass, by all means try it upon m. it will give you a more satisfactory view than an opera-glass is capable of doing, and will magnify the cluster so that there can be no possibility of mistaking it for a star. compare this compact cluster, which only a powerful telescope can partially resolve into its component stars, with m. and m., described before, in order to comprehend the wide variety in the structure of these aggregations of stars. the northern crown, although a strikingly beautiful constellation to the naked eye, offers few attractions to the opera-glass. let us turn, then, to lyra. i have never been able to make up my mind which of three great stars is entitled to precedence--vega, the leading brilliant of lyra, arcturus in boötes, or capella in auriga. they are the three leaders of the northern firmament, but which of them should be called the chief, is very hard to say. at any rate, vega would probably be generally regarded as the most beautiful, on account of the delicate bluish tinge in its light, especially when viewed with a glass. there is no possibility of mistaking this star because of its surpassing brilliancy. two faint stars close to vega on the east make a beautiful little triangle with it, and thus form a further means of recognition, if any were needed. your opera-glass will show that the floor of heaven is powdered with stars, fine as the dust of a diamond, all around the neighborhood of vega, and the longer you gaze the more of these diminutive twinklers you will discover. [illustration: map .] now direct your glass to the northernmost of the two little stars near vega, the one marked epsilon ([epsilon]) in the map. you will perceive that it is composed of two stars of almost equal magnitude. if you had a telescope of considerable power, you would find that each of these stars is in turn double. in other words, this wonderful star which appears single to the unassisted eye, is in reality quadruple, and there is reason to think that the four stars composing it are connected in pairs, the members of each pair revolving around their common center while the two pairs in turn circle around a center common to all. with a field-glass you will be able to see that the other star near vega, zeta ([zeta]), is also double, the distance between its components being three quarters of a minute, while the two stars in [epsilon] are a little less than ½' apart. the star beta ([beta]) is remarkably variable in brightness. you may watch these variations, which run through a regular period of about days, ¾ hours, for yourself. between beta and gamma ([gamma]) lies the beautiful ring nebula, but it is hopelessly beyond the reach of the optical means we are employing. let us turn next to the stars in the west. in consulting the accompanying map of virgo and boötes (map no. ), the observer is supposed to face the southwest, at the hours and dates mentioned above as those to which the circular map corresponds. he will then see the bright star spica in virgo not far above the horizon, while arcturus will be half-way up the sky, and the northern crown will be near the zenith. the constellation virgo is an interesting one in mythological story. aratus tells us that the virgin's home was once on earth, where she bore the name of justice, and in the golden age all men obeyed her. in the silver age her visits to men became less frequent, "no longer finding the spirits of former days"; and, finally, when the brazen age came with the clangor of war: "justice, loathing that race of men, winged her flight to heaven; and fixed her station in that region where still by night is seen the virgin goddess near to bright boötes." the chief star of virgo, spica, is remarkable for its pure white light. to my eye there is no conspicuous star in the sky equal to it in this respect, and it gains in beauty when viewed with a glass. with the aid of the map the reader will find the celebrated binary star gamma ([gamma]) virginis, although he will not be able to separate its components without a telescope. it is a curious fact that the star epsilon ([epsilon]) in virgo has for many ages been known as the grape-gatherer. it has borne this name in greek, in latin, in persian, and in arabic, the origin of the appellation undoubtedly being that it was observed to rise just before the sun in the season of the vintage. it will be observed that the stars [epsilon], [delta], [gamma], [eta], and [beta], mark two sides of a quadrilateral figure of which the opposite corner is indicated by denebola in the tail of leo. within this quadrilateral lies the marvelous field of the nebulæ, a region where with adequate optical power one may find hundreds of these strange objects thronging together, a very storehouse of the germs of suns and worlds. unfortunately, these nebulæ are far beyond the reach of an opera-glass, but it is worth while to know where this curious region is, even if we can not behold the wonders it contains. the stars omicron ([omicron]), pi ([pi]), etc., forming a little group, mark the head of virgo. the autumnal equinox, or the place where the sun crosses the equator of the heavens on his southerly journey about the st of september, is situated nearly between the stars [eta] and [beta] virginis, a little below the line joining them, and somewhat nearer to [eta]. both [eta] and [zeta] virginis are almost exactly upon the equator of the heavens. the constellation libra, lying between virgo and scorpio, does not contain much to attract our attention. its two chief stars, [alpha] and [beta], may be readily recognized west of and above the head of scorpio. the upper one of the two, [beta], has a singular greenish tint, and the lower one, [alpha], is a very pretty double for an opera-glass. the constellation of libra appears to have been of later date than the other eleven members of the zodiacal circle. its two chief stars at one time marked the extended claws of scorpio, which were afterward cut off (perhaps the monster proved too horrible even for its inventors) to form libra. as its name signifies, libra represents a balance, and this fact seems to refer the invention of the constellation back to at least three hundred years before christ, when the autumnal equinox occurred at the moment when the sun was just crossing the western border of the constellation. the equality of the days and nights at that season readily suggests the idea of a balance. milton, in "paradise lost," suggests another origin for the constellation of the balance in the account of gabriel's discovery of satan in paradise: "... now dreadful deeds might have ensued, nor only paradise in this commotion, but the starry cope of heaven, perhaps, or all the elements at least had gone to wrack, disturbed and torn with violence of this conflict, had not soon the eternal, to prevent such horrid fray, hung forth in heaven his golden scales, yet seen betwixt astrea and the scorpion sign." just north of virgo's head will be seen the glimmering of berenice's hair. this little constellation was included among those described in the chapter on "the stars of spring," but it is worth looking at again in the early summer, on moonless nights, when the singular arrangement of the brighter members of the cluster at once strikes the eye. [illustration: berenice's hair.] boötes, whose leading brilliant, arcturus, occupies the center of our map, also possesses a curious mythical history. it is called by the greeks the bear-driver, because it seems continually to chase ursa major, the great bear, in his path around the pole. the story is that boötes was the son of the nymph calisto, whom juno, in one of her customary fits of jealousy, turned into a bear. boötes, who had become a famous hunter, one day roused a bear from her lair, and, not knowing that it was his mother, was about to kill her, when jupiter came to the rescue and snatched them both up into the sky, where they have shone ever since. lucan refers to this story when, describing brutus's visit to cato at night, he fixes the time by the position of these constellations in the heavens: "'twas when the solemn dead of night came on, when bright calisto, with her shining son, now half the circle round the pole had run." boötes is not specially interesting for our purposes, except for the splendor of arcturus. this star has possessed a peculiar charm for me ever since boyhood, when, having read a description of it in an old treatise on uranography, i felt an eager desire to see it. as my search for it chanced to begin at a season when arcturus did not rise till after a boy's bed-time, i was for a long time disappointed, and i shall never forget the start of surprise and almost of awe with which i finally caught sight of it, one spring evening, shooting its flaming rays through the boughs of an apple-orchard, like a star on fire. when near the horizon, arcturus has a remarkably reddish color; but, after it has attained a high elevation in the sky, it appears rather a deep yellow than red. there is a scattered cluster of small stars surrounding arcturus, forming an admirable spectacle with an opera-glass on a clear night. to see these stars well, the glass should be slowly moved about. many of them are hidden by the glare of arcturus. the little group of stars near the end of the handle of the great dipper, or, what is the same thing, the tail of the great bear, marks the upraised hand of boötes. between berenice's hair and the tail of the bear you will see a small constellation called canes venatici, the hunting-dogs. on the old star-maps boötes is represented as holding these dogs with a leash, while they are straining in chase of the bear. you will find some pretty groupings of stars in this constellation. and now we will turn to the east. our next map shows cygnus, a constellation especially remarkable for the large and striking figure that it contains, called the northern cross, aquila the eagle, the dolphin, and the little asterisms sagitta and vulpecula. in consulting the map, the observer is supposed to face toward the east. in aquila the curious arrangement of two stars on either side of the chief star of the constellation, called altair, at once attracts the eye. within a circle including the two attendants of altair you will probably be able to see with the naked eye only two or three stars in addition to the three large ones. now turn your glass upon the same spot, and you will see eight or ten times as many stars, and with a field-glass still more can be seen. watch the star marked eta ([eta]), and you will find that its light is variable, being sometimes more than twice as bright as at other times. its changes are periodical, and occupy a little over a week. the eagle is fabled to have been the bird that jupiter kept beside his throne. a constellation called antinous, invented by tycho brahe, is represented on some maps as occupying the lower portion of the space given to aquila. the dolphin is an interesting little constellation, and the ancients said it represented the very animal on whose back the famous musician arion rode through the sea after his escape from the sailors who tried to murder him. but some modern has dubbed it with the less romantic name of job's coffin, by which it is sometimes called. it presents a very pretty sight to the opera-glass. cygnus, the swan, is a constellation whose mythological history is not specially interesting, although, as remarked above, it contains one of the most clearly marked figures to be found among the stars, the famous northern cross. the outlines of this cross are marked with great distinctness by the stars alpha ([alpha]), epsilon ([epsilon]), gamma ([gamma]), delta ([delta]), and beta ([beta]), together with some fainter stars lying along the main beam of the cross between [beta] and [gamma]. the star [beta], also called albireo, is one of the most beautiful double stars in the heavens. the components are sharply contrasted in color, the larger star being golden-yellow, while the smaller one is a deep, rich blue. with a field-glass of . -inch aperture and magnifying seven times i have sometimes been able to divide this pair, and to recognize the blue color of the smaller star. it will be found a severe test for such a glass. [illustration: map .] about half-way from albireo to the two stars [zeta] and [epsilon] in aquila is a very curious little group, consisting of six or seven stars in a straight row, with a garland of other stars hanging from the center. to see it best, take a field-glass, although an opera-glass shows it. i have indicated the place of the celebrated star cygni in the map, because of the interest attaching to it as the nearest to us, so far as we know, of all the stars in the northern hemisphere, and with one exception the nearest star in all the heavens. yet it is very faint, and the fact that so inconspicuous a star should be nearer than such brilliants as vega and arcturus shows how wide is the range of magnitude among the suns that light the universe. the actual distance of cygni is something like , times as great as the distance from the earth to the sun. the star omicron ([omicron]) is very interesting with an opera-glass. the naked eye sees a little star near it. the glass throws them wide apart, and divides [omicron] itself into two stars. now, a field-glass, if of sufficient power, will divide the larger of these stars again into two--a fine test. sweep around [alpha] and [gamma] for the splendid star-fields that abound in this neighborhood; also around the upper part of the figure of the cross. we are here in one of the richest parts of the milky-way. between the stars [alpha], [gamma], [epsilon], is the strange dark gap in the galaxy called the coal-sack, a sort of hole in the starry heavens. although it is not entirely empty of stars, its blackness is striking in contrast with the brilliancy of the milky-way in this neighborhood. the divergent streams of the great river of light in this region present a very remarkable appearance. [illustration: map .] finally, we come to the great dragon of the sky. in using the map of draco and the neighboring constellations, the reader is supposed to face the north. the center of the upper edge of the map is directly over the observer's head. one of the stories told of this large constellation is that it represents a dragon that had the temerity to war against minerva. the goddess "seized it in her hand, and hurled it, twisted as it was, into the heavens round the axis of the world, before it had time to unwind its contortions." others say it is the dragon that guarded the golden apples in the garden of the hesperides, and that was slain by the redoubtable hercules. at any rate, it is plainly a monster of the first magnitude. the stars [beta], [gamma], [xi], [nu], and [mu] represent its head, while its body runs trailing along, first sweeping in a long curve toward cepheus, and then bending around and passing between the two bears. try [nu] with your opera-glass, and if you succeed in seeing it double you may congratulate yourself on your keen sight. the distance between the stars is about '. notice the contrasted colors of [gamma] and [beta], the former being a rich orange and the latter white. as you sweep along the winding way that draco follows, you will run across many striking fields of stars, although the heavens are not as rich here as in the splendid regions that we have just left. you will also find that cepheus, although not an attractive constellation to the naked eye, is worth some attention with an opera-glass. the head and upper part of the body of cepheus are plunged in the stream of the milky way, while his feet are directed toward the pole of the heavens, upon which he is pictured as standing. cepheus, however, sinks into insignificance in comparison with its neighbor cassiopeia, but that constellation belongs rather to the autumn sky, and we shall pass it by here. chapter iii. the stars of autumn. in the "fifth evening" of that delightful, old, out-of-date book of fontenelle's, on the "plurality of worlds," the astronomer and the marchioness, who have been making a wonderful pilgrimage through the heavens during their evening strolls in the park, come at last to the starry systems beyond the "solar vortex," and the marchioness experiences a lively impatience to know what the fixed stars will turn out to be, for the astronomer has sharpened her appetite for marvels. "tell me," says she, eagerly, "are they, too, inhabited like the planets, or are they not peopled? in short, what can we make of them?" the astronomer answers his charming questioner, as we should do to-day, that the fixed stars are so many suns. and he adds to this information a great deal of entertaining talk about the planets that may be supposed to circle around these distant suns, interspersing his conversation with explanations of "vortexes," and many quaint conceits, in which he is helped out by the ready wit of the marchioness. finally, the impressionable mind of the lady is overwhelmed by the grandeur of the scenes that the astronomer opens to her view, her head swims, infinity oppresses her, and she cries for mercy. "you show me," she exclaims, "a perspective so interminably long that the eye can not see the end of it. i see plainly the inhabitants of the earth; then you cause me to perceive those of the moon and of the other planets belonging to our vortex (system), quite clearly, yet not so distinctly as those of the earth. after them come the inhabitants of planets in the other vortexes. i confess, they seem to me hidden deep in the background, and, however hard i try, i can barely glimpse them at all. in truth, are they not almost annihilated by the very expression which you are obliged to use in speaking of them? you have to call them inhabitants of one of the planets contained in one out of the infinity of vortexes. surely we ourselves, to whom the same expression applies, are almost lost among so many millions of worlds. for my part, the earth begins to appear so frightfully little to me that henceforth i shall hardly consider any object worthy of eager pursuit. assuredly, people who seek so earnestly their own aggrandizement, who lay schemes upon schemes, and give themselves so much trouble, know nothing of the vortexes! i am sure my increase of knowledge will redound to the credit of my idleness, and when people reproach me with indolence i shall reply: 'ah! if you but knew the history of the fixed stars!'" it is certainly true that a contemplation of the unthinkable vastness of the universe, in the midst of which we dwell upon a speck illuminated by a spark, is calculated to make all terrestrial affairs appear contemptibly insignificant. we can not wonder that men for ages regarded the earth as the center, and the heavens with their lights as tributary to it, for to have thought otherwise, in those times, would have been to see things from the point of view of a superior intelligence. it has taken a vast amount of experience and knowledge to convince men of the parvitude of themselves and their belongings. so, in all ages they have applied a terrestrial measure to the universe, and imagined they could behold human affairs reflected in the heavens and human interests setting the gods together by the ears. [illustration: map. .] this is clearly shown in the story of the constellations. the tremendous truth that on a starry night we look, in every direction, into an almost endless vista of suns beyond suns and systems upon systems, was too overwhelming for comprehension by the inventors of the constellations. so they amused themselves, like imaginative children, as they were, by tracing the outlines of men and beasts formed by those pretty lights, the stars. they turned the starry heavens into a scroll filled with pictured stories of mythology. four of the constellations with which we are going to deal in this chapter are particularly interesting on this account. they preserve in the stars, more lasting than parchment or stone, one of the oldest and most pleasing of all the romantic stories that have amused and inspired the minds of men--the story of perseus and andromeda--a better story than any that modern novelists have invented. the four constellations to which i refer bear the names of andromeda, perseus, cassiopeia, and cepheus, and are sometimes called, collectively, the royal family. in the autumn they occupy a conspicuous position in the sky, forming a group that remains unrivaled until the rising of orion with his imperial _cortége_. the reader will find them in map no. , occupying the northeastern quarter of the heavens. this map represents the visible heavens at about midnight on september st, ten o'clock p. m. on october st, and eight o'clock p. m. on november st. at this time the constellations that were near the meridian in summer will be found sinking in the west, hercules being low in the northwest, with the brilliant lyra and the head of draco suspended above it; aquila, "the eagle of the winds," soars high in the southwest; while the cross of cygnus is just west of the zenith; and sagittarius, with its wealth of star-dust, is disappearing under the horizon in the southwest. far down in the south the observer catches the gleam of a bright lone star of the first magnitude, though not one of the largest of that class. it is fomalhaut, in the mouth of the southern fish, piscis australis. a slight reddish tint will be perceived in the light of this beautiful star, whose brilliance is enhanced by the fact that it shines without a rival in that region of the sky. fomalhaut is one of the important "nautical stars," and its position was long ago carefully computed for the benefit of mariners. the constellation of piscis australis, which will be found in our second map, does not possess much to interest us except its splendid leading star. in consulting map , the observer is supposed to be facing south, or slightly west of south, and he must remember that the upper part of the map reaches nearly to the zenith, while at the bottom it extends down to the horizon. [illustration: map .] to the right, or west, of fomalhaut, and higher up, is the constellation of capricornus, very interesting on many accounts, though by no means a striking constellation to the unassisted eye. the stars alpha ([alpha]), called giedi, and beta ([beta]), called dabih, will be readily recognized, and a keen eye will perceive that alpha really consists of two stars. they are about six minutes of arc apart, and are of the third and the fourth magnitude respectively. these stars, which to the naked eye appear almost blended into one, really have no physical connection with each other, and are slowly drifting apart. the ancient astronomers make no mention of giedi being composed of two stars, and the reason is plain, when it is known that in the time of hipparchus, as flammarion has pointed out, their distance apart was not more than two thirds as great as it is at present, so that the naked eye could not have detected the fact that there were two of them; and it was not until the seventeenth century that they got far enough asunder to begin to be separated by eyes of unusual power. with an ordinary opera-glass they are thrown well apart, and present a very pretty sight. considering the manner in which these stars are separating, the fact that both of them have several faint companions, which our powerful telescopes reveal, becomes all the more interesting. a suggestion of sir john herschel, concerning one of these faint companions, that it shines by reflected light, adds to the interest, for if the suggestion is well founded the little star must, of course, be actually a planet, and granting that, then some of the other faint points of light seen there are probably planets too. it must be said that the probabilities are against herschel's suggestion. the faint stars more likely shine with their own light. even so, however, these two systems, which apparently have met and are passing one another, at a distance small as compared with the space that separates them from us, possess a peculiar interest, like two celestial fleets that have spoken one another in the midst of the ocean of space. the star beta, or dabih, is also a double star. the companion is of a beautiful blue color, generally described as "sky-blue." it is of the seventh magnitude, while the larger star is of magnitude three and a half. the latter is golden-yellow. the blue of the small star can be seen with either an opera- or a field-glass, but it requires careful looking and a clear and steady atmosphere. i recollect discovering the color of this star with a field-glass, and exclaiming to myself, "why, the little one is as blue as a bluebell!" before i knew that that was its hue as seen with a telescope. trying my opera-glass upon it i found that the color was even more distinct, although the small star was then more or less enveloped in the yellow rays of the large one. the distance between the two stars in dabih is nearly the same as that between the components of [epsilon] lyræ, and the comparative difficulty of separating them is an instructive example of the effect of a large star in concealing a small one close beside it. the two stars in [epsilon] lyræ are of nearly equal brightness, and are very easily separated and distinguished, but in [beta] capricorni, or dabih, one star is about twenty times as bright as the other, and consequently the fainter star is almost concealed in the glare of its more brilliant neighbor. with the most powerful glass at your disposal, sweep from the star zeta ([zeta]) eastward a distance somewhat greater than that separating alpha and beta, and you will find a fifth-magnitude star beside a little nebulous spot. this is the cluster known as m, one of those sun-swarms that overwhelm the mind of the contemplative observer with astonishment, and especially remarkable in this case for the apparent vacancy of the heavens immediately surrounding the cluster, as if all the stars in that neighborhood had been drawn into the great assemblage, leaving a void around it. of course, with the instrument that our observer is supposed to be using, merely the _existence_ of this solar throng can be detected; but, if he sees that it is there, he may be led to provide himself with a telescope capable of revealing its glories. admiral smyth remarks that, "although capricorn is not a striking object, it has been the very pet of all constellations with astrologers," and he quotes from an old almanac of the year , that "whoso is borne in capcorn schal be ryche and wel lufyd." the mythological account of the constellation is that it represents the goat into which pan was turned in order to escape from the giant typhon, who once on a time scared all the gods out of their wits, and caused them to change themselves into animals, even jupiter assuming the form of a ram. according to some authorities, piscis australis represents the fish into which venus changed herself on that interesting occasion. directly above piscis australis, and to the east or left of capricorn, the map shows the constellation of aquarius, or the water-bearer. some say this commemorates ganymede, the cup-bearer of the gods. it is represented in old star-maps by the figure of a young man pouring water from an urn. the star alpha ([alpha]) marks his right shoulder, and beta ([beta]) his left, and gamma ([gamma]), zeta ([zeta]), eta ([eta]), and pi ([pi]) indicate his right hand and the urn. from this group a current of small stars will be recognized, sweeping downward with a curve toward the east, and ending at fomalhaut; this represents the water poured from the urn, which the southern fish appears to be drinking. in fact, according to the pictures in the old maps, the fish succeeds in swallowing the stream completely, and it vanishes from the sky in the act of entering his distended mouth! it is worthy of remark that in greek, latin, and arabic this constellation bears names all of which signify "a man pouring water." the ancient egyptians imagined that the setting of aquarius caused the rising of the nile, as he sank his huge urn in the river to fill it. alpha aquarii was called by the arabs sadalmelik, which is interpreted to mean the "king's lucky star," but whether it proved itself a lucky star in war or in love, and what particular king enjoyed its benign influence and recorded his gratitude in its name, we are not informed. thus, at every step, we find how shreds of history and bits of superstition are entangled among the stars. surely, humanity has been reflected in the heavens as lastingly as it has impressed itself upon the earth. starting from the group of stars just described as forming the water-bearer's urn, follow with a glass the winding stream of small stars that represent the water. several very pretty and striking assemblages of stars will be encountered in its course. the star tau ([tau]) is double and presents a beautiful contrast of color, one star being white and the other reddish-orange--two solar systems, it may be, apparently neighbors as seen from the earth, in one of which daylight is white and in the other red! point a good glass upon the star marked nu ([nu]), and you will see, somewhat less than a degree and a half to the west of it, what appears to be a faint star of between the seventh and eighth magnitudes. you will have to look sharp to see it. it is with your mind's eye that you must gaze, in order to perceive the wonder here hidden in the depths of space. that faint speck is a nebula, unrivaled for interest by many of the larger and more conspicuous objects of that kind. lord rosse's great telescope has shown that in form it resembles the planet saturn; in other words, that it consists apparently of a ball surrounded by a ring. but the spectroscope proves that it is a gaseous mass, and the micrometer--supposing its distance to be equal to that of the stars, and we have no reason to think it less--that it must be large enough to fill the whole space included within the orbit of neptune! here, then, as has been said, we seem to behold a genesis in the heavens. if laplace's nebular hypothesis, or any of the modifications of that hypothesis, represents the process of formation of a solar system, then we may fairly conclude that such a process is now actually in operation in this nebula in aquarius, where a vast ring of nebulous matter appears to have separated off from the spherical mass within it. this may not be the true explanation of what we see there, but, whatever the explanation is, there can be no question of the high significance of this nebula, whose shape proclaims unmistakably the operation of great metamorphic forces there. of course, with his insignificant optical means, our observer can see nothing of the strange form of this object, the detection of which requires the aid of the most powerful telescopes, but it is much to know where that unfinished creation lies, and to see it, even though diminished by distance to a mere speck of light. turn your glass upon the star shown in the map just above mu ([mu]) and epsilon ([epsilon]). you will find an attractive arrangement of small stars in its neighborhood. the star marked is double to the naked eye, and the row of stars below it is well worth looking at. the star delta ([delta]) indicates the place where, in , tobias mayer narrowly escaped making a discovery that would have anticipated that which a quarter of a century later made the name of sir william herschel world-renowned. the planet uranus passed near delta in , and tobias mayer saw it, but it moved so slowly that he took it for a fixed star, never suspecting that his eyes had rested upon a member of the solar system whose existence was, up to that time, unknown to the inhabitants of adam's planet. above aquarius you will find the constellation pegasus. it is conspicuously marked by four stars of about the second magnitude, which shine at the corners of a large square, called the great square of pegasus. this figure is some fifteen degrees square, and at once attracts the eye, there being few stars visible within the quadrilateral, and no large ones in the immediate neighborhood to distract attention from it. one of the four stars, however, as will be seen by consulting map , does not belong to pegasus, but to the constellation andromeda. mythologically, this constellation represents the celebrated winged horse of antiquity: "now heaven his further wandering flight confines, where, splendid with his numerous stars, he shines." the star alpha ([alpha]) is called markab; beta ([beta]) is scheat, and gamma ([gamma]) is algenib; the fourth star in the square, belonging to andromeda, is called alpheratz. although pegasus presents a striking appearance to the unassisted eye, on account of its great square, it contains little to attract the observer with an opera-glass. it will prove interesting, however, to sweep with the glass carefully over the space within the square, which is comparatively barren to the naked eye, but in which many small stars will be revealed, of whose existence the naked-eye observer would be unaware. the star marked pi ([pi]) is an interesting double, which can be separated by a good eye without artificial aid, and which, with an opera-glass, presents a fine appearance. and now we come to map no. , representing the constellations cetus, pisces, aries, and the triangles. in consulting it the observer is supposed to face the southeast. cetus is a very large constellation, and from the peculiar conformation of its principal stars it can be readily recognized. the head is to the east, the star alpha ([alpha]), called menkar, being in the nose of this imaginary inhabitant of the sky-depths. the constellation is supposed to represent the monster that, according to fable, was sent by neptune to devour the fair andromeda, but whose bloodthirsty design was happily and gallantly frustrated by perseus, as we shall learn from starry mythology further on. although bearing the name cetus, the whale, the pictures of the constellation in the old maps do not present us with the form of a whale, but that of a most extraordinary scaly creature with enormous jaws filled with large teeth, a forked tongue, fore-paws armed with gigantic claws, and a long, crooked, and dangerous-looking tail. indeed, aratus does not call it a "whale," but a "sea-monster," and dr. seiss would have us believe that it was intended to represent the leviathan, whose terrible prowess is celebrated in the book of job. [illustration: map .] by far the most interesting object in cetus is the star mira. this is a famous variable--a sun that sometimes shines a thousand-fold more brilliantly than at others! it changes from the second magnitude to the ninth or tenth, its period from maximum to maximum being about eleven months. during about five months of that time it is completely invisible to the naked eye; then it begins to appear again, slowly increasing in brightness for some three months, until it shines as a star of the second magnitude, being then as bright as, if not brighter than, the most brilliant stars in the constellation. it retains this brilliance for about two weeks, and then begins to fade again, and, within three months, once more disappears. there are various irregularities in its changes, which render its exact period somewhat uncertain, and it does not always attain the same degree of brightness at its maximum. for instance, in , mira was almost equal in brilliance to a first-magnitude star, but frequently at its greatest brightness it is hardly equal to an ordinary star of the second magnitude. by the aid of our little map you will readily be able to find it. you will perceive that it has a slightly reddish tint. watch it from one of its maxima, and you will see it gradually fade from sight until, at last, only the blackness of the empty sky appears where, a few months before, a conspicuous star was visible. keep watch of that spot, and in due course you will perceive mira shining there again--a mere speck, but slowly brightening--and in three months more the wonderful star will blaze again with renewed splendor. knowing that our own sun is a variable star--though variable only to a slight degree, its variability being due to the spots that appear upon its surface in a period of about eleven years--we possess some light that may be cast upon the mystery of mira's variations. it seems not improbable that, in the case of mira, the surface of the star at the maximum of spottedness is covered to an enormously greater extent than occurs during our own sun-spot maxima, so that the light of the star, instead of being merely dimmed to an almost imperceptible extent, as with our sun, is almost blotted out. when the star blazes with unwonted splendor, as in , we may fairly assume that the pent-up forces of this perishing sun have burst forth, as in a desperate struggle against extinction. but nothing can prevail against the slow, remorseless, unswerving progress of that obscuration, which comes from the leaking away of the solar heat, and which constitutes what we may call the death of a sun. and that word seems peculiarly appropriate to describe the end of a body which, during its period of visible existence, not only presents the highest type of physical activity, but is the parent and supporter of all forms of life upon the planets that surround it. we might even go so far as to say that possibly mira presents to us an example of what our sun will be in the course of time, as the dead and barren moon shows us, as in a magician's glass, the approaching fate of the earth. fortunately, human life is a mere span in comparison with the æons of cosmic existence, and so we need have no fear that either we or our descendants for thousands of generations shall have to play the tragic _rôle_ of campbell's "last man," and endeavor to keep up a stout heart amid the crash of time by meanly boasting to the perishing sun, whose rays have nurtured us, that, though his proud race is ended, we have confident anticipations of immortality. i trust that, when man makes his exit from this terrestrial stage, it will not be in the contemptible act of kicking a fallen benefactor. there are several other variable stars in cetus, but none possessing much interest for us. the observer should look at the group of stars in the head, where he will find some interesting combinations, and also at chi, which is the little star shown in the map near zeta ([zeta]). this is a double that will serve as a very good test of eye and instrument, the smaller companion-star being of only seven and a half magnitude. directly above cetus is the long, straggling constellation of pisces, the fishes. the northern fish is represented by the group of stars near andromeda and the triangles. a long band or ribbon, supposed to bind the fish together, trends thence first southeast and then west until it joins a group of stars under pegasus, which represents the western fish, not to be confounded with the southern fish described near the beginning of this chapter, which is a separate constellation. fable has, however, somewhat confounded these fishes; for while, as i have remarked above, the southern fish is said to represent venus after she had turned herself into a fish to escape from the giant typhon, the two fishes of the constellation we are now dealing with are also fabled to represent venus and her interesting son cupid under the same disguise assumed on precisely the same occasion. if typhon, however, was so great a brute that even cupid's arrows were of no avail against him, we should, perhaps, excuse mythology for duplicating the record of so wondrous an event. you will find it very interesting to take your glass and, beginning with the attractive little group in the northern fish, follow the windings of the ribbon, with its wealth of tiny stars, to the western fish. when you have arrived at that point, sweep well over the sky in that neighborhood, and particularly around and under the stars iota ([iota]), theta ([theta]), lambda ([lambda]), and kappa ([kappa]). if you are using a powerful glass, you will be surprised and delighted by what you see. below the star omega ([omega]), and to the left of lambda, is the place which the sun occupies at the time of the spring equinox--in other words, one of the two crossing-places of the equinoctial or the equator of the heavens, and the ecliptic, or the sun's path. the prime meridian of the heavens passes through this point. you can trace out this great circle, from which astronomical longitudes are reckoned, by drawing an imaginary line from the equinoctial point just indicated through [alpha] in andromeda and [beta] in cassiopeia to the pole-star. to the left of pisces, and above the head of cetus, is the constellation aries, or the ram. two pretty bright stars, four degrees apart, one of which has a fainter star near it, mark it out plainly to the eye. these stars are in the head of the ram. the brightest one, alpha ([alpha]), is called hamal; beta ([beta]) is named sheratan; and its fainter neighbor is mesarthim. according to fable, this constellation represents the ram that wore the golden fleece, which was the object of the celebrated expedition of the argonauts. there is not much in the constellation to interest us, except its historical importance, as it was more than two thousand years ago the leading constellation of the zodiac, and still stands first in the list of the zodiacal signs. owing to the precession of the equinoxes, however, the vernal equinoctial point, which was formerly in this constellation, has now advanced into the constellation pisces, as we saw above. gamma ([gamma]), arietis, is interesting as the first telescopic double star ever discovered. its duplicity was detected by dr. hooke while watching the passage of a comet near the star in . singularly enough, the brightest star in the constellation, now bearing the letter [alpha], originally did not belong to the constellation. tycho brahe finally placed it in the head of aries. the little constellation of the triangles, just above aries, is worth only a passing notice. insignificant as it appears, this little group is a very ancient constellation. it received its name, deltoton, from the greek letter [delta]. [illustration: map .] the reader must now be introduced to the "royal family." although the story of perseus and andromeda is, of course, well known to nearly all readers, yet, on account of the great beauty and brilliancy of the group of constellations that perpetuate the memory of it among the stars, it is worth recalling here. it will be remembered that, as perseus was returning through the air from his conquest of the gorgon medusa, he saw the beautiful andromeda chained to a rock on the sea-coast, waiting to be devoured by a sea-monster. the poor girl's only offense was that her mother, cassiopeia, had boasted for her that she was fairer than the sea-beauty, atergatis, and for this neptune had decreed that all the land of the ethiopians should be drowned and destroyed unless andromeda was delivered up as a sacrifice to the dreadful sea-monster. when perseus, dropping down to learn why this maiden was chained to the rocks, heard from andromeda's lips the story of her woes, he laughed with joy. here was an adventure just to his liking, and besides, unlike his previous adventures, it involved the fate of a beautiful woman with whom he was already in love. could he save her? well, wouldn't he! the sea-monster might frighten a kingdom full of ethiops, but it could not shake the nerves of a hero from greece. he whispered words of encouragement to andromeda, who could scarce believe the good news that a champion had come to defend her after all her friends and royal relations had deserted her. neither could she feel much confidence in her young champion's powers when suddenly her horrified gaze met the awful leviathan of the deep advancing to his feast! but perseus, with a warning to andromeda not to look at what he was about to do, sprang with his winged sandals up into the air. and then, as charles kingsley has so beautifully told the story-- "on came the great sea-monster, coasting along like a huge black galley, lazily breasting the ripple, and stopping at times by creek or headland to watch for the laughter of girls at their bleaching, or cattle pawing on the sand-hills, or boys bathing on the beach. his great sides were fringed with clustering shells and sea-weeds, and the water gurgled in and out of his wide jaws as he rolled along, dripping and glistening in the beams of the morning sun. at last he saw andromeda, and shot forward to take his prey, while the waves foamed white behind him, and before him the fish fled leaping. "then down from the height of the air fell perseus like a shooting-star--down to the crest of the waves, while andromeda hid her face as he shouted. and then there was silence for a while. "at last she looked up trembling, and saw perseus springing toward her; and, instead of the monster, a long, black rock, with the sea rippling quietly round it." perseus had turned the monster into stone by holding the blood-freezing head of medusa before his eyes; and it was fear lest andromeda herself might see the gorgon's head, and suffer the fate of all who looked upon it, that had led him to forbid her watching him when he attacked her enemy. afterward he married her, and cassiopeia, andromeda's mother, and cepheus, her father, gave their daughter's rescuer a royal welcome, and all the ethiops rose up and blessed him for ridding the land of the monster. and now, if we choose, we can, any fair night, see the principal characters of this old romance shining in starry garb in the sky. aratus saw them there in his day, more than two hundred years before christ, and has left this description in his "skies," as translated by poste: "nor shall blank silence whelm the harassed house of cepheus; the high heavens know their name, for zeus is in their line at few removes. cepheus himself by she-bear cynosure, iasid king stands with uplifted arms. from his belt thou castest not a glance to see the first spire of the mighty dragon. "eastward from him, heaven-troubled queen, with scanty stars but lustrous in the full-mooned night, sits cassiopeia. not numerous nor double-rowed the gems that deck her form, but like a key which through an inward-fastened folding-door men thrust to knock aside the bolts, they shine in single zigzag row. she, too, o'er narrow shoulders stretching uplifted hands, seems wailing for her child. "for there, a woful statue-form, is seen andromeda, parted from her mother's side. long i trow thou wilt not seek her in the nightly sky, so bright her head, so bright her shoulders, feet, and girdle. yet even there she has her arms extended, and shackled even in heaven; uplifted, outspread eternally are those fair hands. "her feet point to her bridegroom perseus, on whose shoulder they rest. he in the north-wind stands gigantic, his right hand stretched toward the throne where sits the mother of his bride. as one bent on some high deed, dust-stained he strides over the floor of heaven." the makers of old star-maps seem to have vied in the effort to represent with effect the figures of andromeda, perseus, and cassiopeia among the stars, and it must be admitted that some of them succeeded in giving no small degree of life and spirit to their sketches. the starry riches of these constellations are well matched with their high mythological repute. lying in and near the milky-way, they are particularly interesting to the observer with an opera-glass. besides, they include several of the most celebrated wonders of the firmament. in consulting map no. , the observer is supposed to face the east and northeast. we will begin our survey with andromeda. the three chief stars of this constellation are of the second magnitude, and lie in a long, bending row, beginning with alpha ([alpha]), or alpheratz, in the head, which, as we have seen, marks one corner of the great square of pegasus. beta ([beta]), or mirach, with the smaller stars mu ([mu]) and nu ([nu]), form the girdle. the third of the chief stars is gamma ([gamma]), or almaach, situated in the left foot. the little group of stars designated lambda ([lambda]), kappa ([kappa]), and iota ([iota]), mark the extended right hand chained to the rock, and zeta ([zeta]) and some smaller stars southwest of it show the left arm and hand, also stretched forth and shackled. in searching for picturesque objects in andromeda, begin with alpheratz and the groups forming the hands. below the girdle will be seen a rather remarkable arrangement of small stars in the mouth of the northern fish. now follow up the line of the girdle to the star nu ([nu]). if your glass has a pretty wide field, your eye will immediately catch the glimmer of the great nebula of andromeda in the same field with the star. this is the oldest or earliest discovered of the nebulæ, and, with the exception of that in orion, is the grandest visible in this hemisphere. of course, not much can be expected of an opera-glass in viewing such an object; and yet a good glass, in clear weather and the absence of the moon, makes a very attractive spectacle of it. [illustration: the great andromeda nebula.] by turning the eyes aside, the nebula can be seen, extended as a faint, wispy light, much elongated on either side of the brighter nucleus. the cut here given shows, approximately, the appearance of the nebula, together with some of the small stars in its neighborhood, as seen with a field-glass. with large telescopes it appears both larger and broader, expanding to a truly enormous extent, and in bond's celebrated picture of it we behold gigantic rifts running lengthwise, while the whole field of sky in which it is contained appears sprinkled over with minute stars apparently between us and the nebula. it was in, or, probably more properly speaking, in line with, this nebula that a new star suddenly shone out in , and, after flickering and fading for a few months, disappeared. that the outburst of light in this star had any real connection with the nebula is exceedingly improbable. although it appeared to be close beside the bright nucleus of the nebula, it is likely that it was really hundreds or thousands of millions of miles either this side or the other side of it. why it should suddenly have blazed into visibility, and then in so short a time have disappeared, is a question as difficult as it is interesting. the easiest way to account for it, if not the most satisfactory, is to assume that it is a variable star of long period, and possessing a very wide range of variability. one significant fact that would seem to point to some connection between star and the nebula, after all, is that a similar occurrence was noticed in the constellation scorpio in , and to which i have previously referred (see chapter ii). in that case a faint star projected against the background of a nebula, suddenly flamed into comparatively great brilliance, and then faded again. the chances against the accidental superposition of a variable star of such extreme variability upon a known nebula occurring twice are so great that, for that reason alone, we might be justified in thinking some mysterious causal relation must in each case exist between the nebula and the star. the temptation to indulge in speculation is very great here, but it is better to wait for more light, and confess that for the present these things are inexplicable. it will be found very interesting to sweep with the glass slowly from side to side over andromeda, gradually approaching toward cassiopeia or perseus. the increase in the richness of the stratum of faint stars that apparently forms the background of the sky will be clearly discernible as you approach the milky-way, which passes directly through cassiopeia and perseus. it may be remarked that the milky-way itself, in that splendidly rich region about sagittarius (described in the "stars of summer"), is not nearly so effective an object with an opera-glass as it is above cygnus and in the region with which we are now dealing. this seems to be owing to the smaller magnitude of its component stars in the southern part of the stream. there the background appears more truly "milky," while in the northern region the little stars shine distinct, like diamond-specks, on a black background. the star nu, which serves as a pointer to the great nebula, is itself worth some attention with a pretty strong glass on account of a pair of small stars near it. the star gamma ([gamma]) is interesting, not only as one of the most beautiful triples in the heavens (an opera-glass is far too feeble an instrument to reveal its companions), but because it serves to indicate the radiant point of the biela meteors. there was once a comet well known to astronomers by the name of its discoverer, biela. it repeated its visits to the neighborhood of the sun once in every six or seven years. in this comet astonished all observers by splitting into two comets, which continued to run side by side, like two equal racers, in their course around the sun. each developed a tail of its own. in , when the twin comets were due again, the astronomical world was on the _qui vive_, and they did not disappoint expectation, for back they came out of the depths of space, still racing, but much farther apart than they had been before, alternating in brightness as if the long struggle had nearly exhausted them, and finally, like spent runners, growing faint and disappearing. they have never been seen since. in , when the comets should have been visible, if they still existed, a very startling thing happened. out of the northern heavens, along the track of the missing comets, where the earth crossed it, on the night of the th of november came glistening and dashing the fiery spray of a storm of meteors. it was the dust and fragments of the lost comet of biela, which, after being split in two in , had evidently continued the process of disintegration until its cometary character was completely lost. it seems to have made a truly ghostly exit, for right after the meteor swarm of a mysterious cometary body was seen, which was supposed at the time to be the missing comet itself, and which, it is not altogether improbable, may have been a fragment of it. three days after the meteors burst over europe, it occurred to professor klinkerfues, of berlin, that if they came from biela's comet the comet itself ought to be seen in the southern hemisphere retreating from its encounter with the earth. on november th he sent his now historical telegram to mr. pogson, an astronomer at madras; "biela touched earth november th. search near theta centauri." for thirty-six hours after the receipt of this extraordinary request mr. pogson was prevented by clouds from scanning the heavens with his telescope. when the sky cleared at last, behold there was a comet in the place indicated in the telegram! it was glimpsed again the next night, and then clouds intervened, and not a trace of it was ever seen afterward. but every year, on the th of november, when the earth crosses the orbit of the lost comet, meteoric fragments come plunging into our atmosphere, burning as they fly. ordinarily their number is small, but when, as in , a swarm of the meteors is in that part of their orbit which the earth crosses, there is a brilliant spectacle. in this occurred, and the world was treated to one of the most splendid meteoric displays on record. [illustration: the attendants of alpha persei.] next let us turn to perseus. the bending row of stars marking the center of this constellation is very striking and brilliant. the brightest star in the constellation is alpha, or algenib, in the center of the row. the head of perseus is toward cassiopeia, and in his left hand he grasps the head of medusa, which hangs down in such a way that its principal star beta, or algol, forms a right angle with algenib and almaach in andromeda. this star algol, or the demon, as the arabs call it, is in some respects the most wonderful and interesting in all the heavens. it is as famous for the variability of its light as mira, but it differs widely from that star both in its period, which is very short, and in the extent of the changes it undergoes. during about two days and a half, algol is equal in brilliance to algenib, which is a second-magnitude star; then it begins to fade, and in the course of about four and a half hours it sinks to the fourth magnitude, being then about equal to the faint stars near it. it remains thus obscured for only a few minutes, and then begins to brighten again, and in about four and a half hours more resumes its former brilliance. this phenomenon is very easily observed, for, as will be seen by consulting our little map, algol can be readily found, and its changes are so rapid that under favorable circumstances it can be seen in the course of a single night to run through the whole gamut. of course, no optical instrument whatever is needed to enable one to see these changes of algol, for it is plainly visible to the naked eye throughout, but it will be found interesting to watch the star with an opera-glass. its periodic time from minimum to minimum is two days, twenty hours, and forty-nine minutes, lacking a few seconds. any one can calculate future minima for himself by adding the periodic time above given to the time of any observed minimum. while spots upon its surface may be the cause of the variations in the light of mira, it is believed that the more rapid changes of algol may be due to another cause; namely, the existence of a huge, dark body revolving swiftly around it at close quarters in an orbit whose plane is directed edgewise toward the earth, so that at regular intervals this dark body causes a partial eclipse of algol. notwithstanding the attacks that have been made upon this theory, it seems to hold its ground, and it will probably continue to find favor as a working hypothesis until some fresh light is cast upon the problem. it hardly needs to be said that the dark body in question, if it exists, must be of enormous size, bearing no such insignificant proportion to the size of algol as the earth does to the sun, but being rather the rival in bulk of its shining brother--a blind companion, an extinguished sun. there was certainly great fitness in the selection of the little group of stars of which this mysterious algol forms the most conspicuous member, to represent the awful head of the gorgon carried by the victorious perseus for the confusion of his enemies. in a darker age than ours the winking of this demon-star must have seemed a prodigy of sinister import. turn now to the bright star algenib, or alpha persei. you will find with the glass an exceedingly attractive spectacle there. in my note-book i find this entry, made while sweeping over perseus for materials for this chapter: "the field about alpha is one of the finest in the sky for an opera-glass. stars conspicuously ranged in curving lines and streams. a host follows alpha from the east and south." the picture on page will give the reader some notion of the exceeding beauty of this field of stars, and of the singular manner in which they are grouped, as it were, behind their leader. a field-glass increases the beauty of the scene. the reader will find a starry cluster marked on map as the "great cluster." this object can be easily detected by the naked eye, resembling a wisp of luminous cloud. it marks the hand in which perseus clasps his diamond sword, and, with a telescope of medium power, it is one of the most marvelously beautiful objects in the sky--a double swarm of stars, bright enough to be clearly distinguished from one another, and yet so numerous as to dazzle the eye with their lively beams. an opera-glass does not possess sufficient power to "resolve" this cluster, but it gives a startling suggestion of its half-hidden magnificence, and the observer will be likely to turn to it again and again with increasing admiration. sweep from this to alpha persei and beyond to get an idea of the procession of suns in the milky-way. the nebulous-looking cluster marked m appears with an opera-glass like a faint comet. about a thousand years ago the theologians undertook to reconstruct the constellation figures, and to give them a religious significance. they divided the zodiac up among the twelve apostles, st. peter taking the place of aries, with the triangles for his mitre. in this reconstruction perseus was transmogrified into st. paul, armed with a sword in one hand and a book in the other; cassiopeia became mary magdalene; while poor andromeda, stripped of all her beauty and romance, was turned into a sepulchre! next look at cassiopeia, which is distinctly marked out by the zigzag row of stars so well described by aratus. here the milky-way is so rich that the observer hardly needs any guidance; he is sure to stumble upon interesting sights for himself. the five brightest stars are generally represented as indicating the outlines of the chair or throne in which the queen sits, the star zeta ([zeta]) being in her head. look at zeta with a good field-glass, and you will see a singular and brilliant array of stars near it in a broken half-circle, which may suggest the notion of a crown. near the little star kappa ([kappa]) in the map will be seen a small circle and the figures . this shows the spot where the famous temporary star, which has of late been frequently referred to as the "star of bethlehem," appeared. it was seen in , and carefully observed by the famous astronomer tycho brahe. it seems to have suddenly burst forth with a brilliance that outshone every other star in the heavens, not excepting sirius itself. but its supremacy was short-lived. in a few months it had sunk to the second magnitude. it continued to grow fainter, exhibiting some remarkable changes of color in the mean time, and in less than a year and a half it disappeared. it has never been seen since. but in , and again in , a star is said to have suddenly blazed out near that point in the heavens. there is no certainty about these earlier apparitions, but, assuming that they are not apocryphal, they might possibly indicate that the star seen by tycho was a periodical one, its period considerably exceeding three hundred years. carrying this supposed period back, it was found that an apparition of this star might have occurred about the time of the birth of christ. it did not require a very prolific imagination to suggest its identity with the so-called star of the magi, and hence the legend of the star of bethlehem and its impending reappearance, of which we have heard so much of late. it will be observed, from the dates given above, that, even supposing them to be correct, no definite period is indicated for the reappearance of the star. in one case the interval is three hundred and eight years, and in the other three hundred and nineteen years. in short, there are too many suppositions and assumptions involved to allow of any credence being given to the theory of the periodicity of tycho's wonderful star. at the same time, nobody can say it is impossible that the star should appear again, and so it may be interesting for the reader to know where to look for it. many of the most beautiful sights of this splendid constellation are beyond the reach of an opera-glass, and reserved for the grander powers of the telescope. we will pause but briefly with cepheus, for the old king's constellation is comparatively dim in the heavens, as his part in the dramatic story of andromeda was contemptible, and he seems to have got among the stars only by virtue of his relationship to more interesting persons. he does possess one gem of singular beauty--the star mu, which may be found about two and a half degrees south of the star nu ([nu]). it is the so-called "garnet star," thus named by william herschel, who advises the observer, in order to appreciate its color, to glance from it to alpha cephei, which is a white star. mu is variable, changing from the fourth to the sixth magnitude in a long period of five or six years. its color is changeable, like its light. sometimes it is of a deep garnet hue, and at other times it is orange-colored. upon the whole, it appears of a deeper red than any other star visible to the naked eye. if you have a good field-glass, try its powers upon the star delta ([delta]) cephei. this is a double star, the components being about forty-one seconds of arc apart, the larger of four and one half magnitude, and the smaller of the seventh magnitude. the latter is of a beautiful blue color, while the larger star is yellow or orange. with a good eye, a steady hand, and a clear glass, magnifying not less than six diameters, you can separate them, and catch the contrasted tints of their light. besides being a double star, delta is variable. chapter iv. the stars of winter. i have never beheld the first indications of the rising of orion without a peculiar feeling of awakened expectation, like that of one who sees the curtain rise upon a drama of absorbing interest. and certainly the magnificent company of the winter constellations, of which orion is the chief, make their entrance upon the scene in a manner that may be described as almost dramatic. first in the east come the world-renowned pleiades. at about the same time capella, one of the most beautiful of stars, is seen flashing above the northeastern horizon. these are the sparkling ushers to the coming spectacle. in an hour the fiery gleam of aldebaran appears at the edge of the dome below the pleiades, a star noticeable among a thousand for its color alone, besides being one of the brightest of the heavenly host. the observer familiar with the constellations knows, when he sees this red star which marks the eye of the angry bull, taurus, that just behind the horizon stands orion with starry shield and upraised club to meet the charge of his gigantic enemy. with aldebaran rises the beautiful v-shaped group of the hyades. presently the star-streams of eridanus begin to appear in the east and southeast, the immediate precursors of the rising of orion: "and now the river-flood's first winding reach the becalmed mariner may see in heaven, as he watches for orion to espy if he hath aught to say of the night's measure or the slumbering winds." the first glimpse we get of the hero of the sky is the long bending row of little stars that glitter in the lion's skin which, according to mythology, serves him for a shield. the great constellation then advances majestically into sight. first of its principal stars appears bellatrix in the left shoulder; then the little group forming the head, followed closely by the splendid betelgeuse, "the martial star," flashing like a decoration upon the hero's right shoulder. then come into view the equally beautiful rigel in the left foot, and the striking row of three bright stars forming the belt. below these hangs another starry pendant marking the famous sword of orion, and last of all appears saiph in the right knee. there is no other constellation containing so many bright stars. it has two of the first magnitude, betelgeuse and rigel; the three stars in the belt, and bellatrix in the left shoulder, are all of the second magnitude; and besides these there are three stars of the third magnitude, more than a dozen of the fourth, and innumerable twinklers of smaller magnitudes, whose commingled scintillations form a celestial illumination of singular splendor. "thus graced and armed he leads the starry host." by the time orion has chased the bull half-way up the eastern slope of the firmament, the peerless dog-star, sirius, is flaming at the edge of the horizon, while farther north glitters procyon, the little dog-star, and still higher are seen the twin stars in gemini. when these constellations have advanced well toward the meridian, as shown in our circular map, their united radiance forms a scene never to be forgotten. counting one of the stars in gemini as of the first rank, there are no less than seven first-magnitude stars ranged around one another in a way that can not fail to attract the attention and the admiration of the most careless observer. aldebaran, capella, the twins, procyon, sirius, and rigel mark the angles of a huge hexagon, while betelgeuse shines with ruddy beauty not far from the center of the figure. the heavens contain no other naked-eye view comparable with this great array, not even the glorious celestial region where the southern cross shines supreme, being equal to it in splendor. as an offset to the discomforts of winter observations of the stars, the observer finds that the softer skies of summer have no such marvelous brilliants to dazzle his eyes as those that illumine the hyemal heavens. to comprehend the real glories of the celestial sphere in the depth of winter one should spend a few clear nights in the rural districts of new york or new england, when the hills, clad with sparkling blankets of crusted snow, reflect the glitter of the living sky. in the pure frosty air the stars seem splintered and multiplied indefinitely, and the brighter ones shine with a splendor of light and color unknown to the denizen of the smoky city, whose eyes are dulled and blinded by the glare of streetlights. there one may detect the delicate shade of green that lurks in the imperial blaze of sirius, the beautiful rose-red light of aldebaran, the rich orange hue of betelgeuse, the blue-white radiance of rigel, and the pearly luster of capella. if you have never seen the starry heavens except as they appear from city streets and squares, then, i had almost said, you have never seen them at all, and especially in the winter is this true. i wish i could describe to you the impression that they can make upon the opening mind of a country boy, who, knowing as yet nothing of the little great world around him, stands in the yawning silence of night and beholds the illimitably great world above him, looking deeper than thought can go into the shining vistas of the universe, and overwhelmed with the wonder of those marshaled suns. [illustration: map. .] looking now at map , we see the heavens as they appear at midnight on the st of december, at o'clock p. m. on the st of january, and at o'clock p. m. on the st of february. in the western half of the sky we recognize andromeda, pegasus, pisces, cetus, aries, cassiopeia, and other constellations that we studied in the "stars of autumn." far over in the east we see rising leo, cancer, and hydra, which we included among the "stars of spring." occupying most of the southern and eastern heavens are the constellations which we are now to describe under the name of the "stars of winter," because in that season they are seen under the most favorable circumstances. i have already referred to the admirable way in which the principal stars of some of these constellations are ranged round one another. by the aid of the map the observer can perceive the relative position of the different constellations, and, having fixed this in his mind, he will be prepared to study them in detail. [illustration: map .] let us now begin with map no. , which shows us the constellations of eridanus, lepus, orion, and taurus. eridanus is a large though not very conspicuous constellation, which is generally supposed to represent the celebrated river now known as the po. it has had different names among different peoples, but the idea of a river, suggested by its long, winding streams of stars, has always been preserved. according to fable, it is the river into which phaeton fell after his disastrous attempt to drive the chariot of the sun for his father phoebus, and in which hare-brained adventure he narrowly missed burning the world up. the imaginary river starts from the brilliant star rigel, in the left foot of orion, and flows in a broad upward bend toward the west; then it turns in a southerly direction until it reaches the bright star gamma ([gamma]), where it bends sharply to the north, and then quickly sweeps off to the west once more, until it meets the group of stars marking the head of cetus. thence it runs south, gradually turning eastward, until it flows back more than half-way to orion. finally it curves south again and disappears beneath the horizon. throughout the whole distance of more than ° the course of the stream is marked by rows of stars, and can be recognized without difficulty by the amateur observer. the first thing to do with your opera-glass, after you have fixed the general outlines of the constellation in your mind by naked-eye observations, is to sweep slowly over the whole course of the stream, beginning at rigel, and following its various wanderings. eridanus ends in the southern hemisphere near a first-magnitude star called achernar, which is situated in the stream, but can not be seen from our latitudes. along the stream you will find many interesting groupings of the stars. in the map see the pair of stars below and to the right of nu ([nu]). these are the two omicrons, the upper one being [omicron]¹ and the lower one [omicron]². the latter is of an orange hue, and is remarkable for the speed with which it is flying through space. there are only one or two stars whose proper motion, as it is called, is more rapid than that of [omicron]² in eridanus. it changes its place nearly seven minutes of arc in a century. the records of the earliest observations we possess show that near the beginning of the christian era it was about half-way between [omicron]¹ and [nu]. its companion [omicron]¹, on the contrary, seems to be almost stationary, so that [omicron]² will gradually draw away from it, passing on toward the southwest until, in the course of centuries, it will become invisible from our latitudes. this flying star is accompanied by two minute companions, which in themselves form a close and very delicate double star. these two little stars, of only . and . magnitude, respectively, are, of course beyond the ken of the observer with an opera-glass. the system of which they form a part, however, is intensely interesting, since the appearances indicate that they belong, in the manner of satellites, to [omicron]², and are fellow-voyagers of that wonderful star. [illustration: the "golden horns" of taurus.] having admired the star-groups of eridanus, one of the prettiest of which is to be seen around beta ([beta]), let us turn next to taurus, just above or north of eridanus. two remarkable clusters at once attract the eye, the hyades, which are shaped somewhat like the letter [v], with aldebaran in the upper end of the left-hand branch, and the pleiades, whose silvery glittering has made them celebrated in all ages. the pleiades are in the shoulder and the hyades in the face of taurus, aldebaran most appropriately representing one of his blazing eyes as he hurls himself against orion. the constellation-makers did not trouble themselves to make a complete bull, and only the head and fore-quarters of the animal are represented. if taurus had been completed on the scale on which he was begun, there would have been no room in the sky for aries; one of the fishes would have had to abandon his celestial swimming-place, and even the fair andromeda would have found herself uncomfortably situated. but, as if to make amends for neglecting to furnish their heavenly bull with hind-quarters, the ancients gave him a most prodigious and beautiful pair of horns, which make the beholder feel alarm for the safety of orion. starting out of the head above the hyades, as illustrated in our cut, the horns curve upward and to the east, each being tipped by a bright star. along and between the horns runs a scattered and broken stream of minute stars which seem to be gathered into knots just beyond the end of the horns, where they dip into the edge of the milky-way. many of these stars can be seen, on a dark night, with an ordinary opera-glass, but, to see them well, one should use as large a field-glass as he can obtain. with such a glass their appearance almost makes one suspect that virgil had a poetic prevision of the wonders yet to be revealed by the telescope when he wrote, as rendered by dryden, of the season-- "when with his _golden horns_ in full career the bull beats down the barriers of the year." below the tips of the horns, and over orion's head, there are also rich clusters of stars, as if the bull were flaunting shreds of sparkling raiment torn from some celestial victim of his fury. with an ordinary glass, however, the observer will not find this star-sprinkled region around the horns of taurus as brilliant a spectacle as that presented by the hyades and the group of stars just above them in the bull's ear. the two stars in the tips of the horns are both interesting, each in a different way. the upper and brighter one of the two, marked beta ([beta]) in map no. , is called el nath. it is common to the left horn of taurus and the right foot of auriga, who is represented standing just above. it is a singularly white star. this quality of its light becomes conspicuous when it is looked at with a glass. the most inexperienced observer will hardly fail to be impressed by the pure whiteness of el nath, in comparison with which he will find that many of the stars he had supposed to be white show a decided tinge of color. the star in the tip of the right or southern horn, zeta ([zeta]), is remarkable, not on its own account, but because it serves as a pointer to a famous nebula, the discovery of which led messier to form his catalogue of nebulæ. this is sometimes called the "crab nebula," from the long sprays of nebulous matter which were seen surrounding it with lord rosse's great telescope. our little sketch is simply intended to enable the observer to locate this strange object. if he wishes to study its appearance, he must use a powerful telescope. but with a first-rate field-glass he can see it as a speck of light in the position shown in the cut, where the large star is zeta and the smaller ones are faint stars, the relative position of which will enable the observer to find the nebula, if he keeps in mind that the top of the cut is toward the north. it is noteworthy that this nebula for a time deceived several of the watchers who were on the lookout for the predicted return of halley's comet in . [illustration: the crab nebula.] and now let us look at the hyades, an assemblage of stars not less beautiful than their more celebrated sisters the pleiades. the leader of the hyades is aldebaran, or alpha tauri, and his followers are worthy of their leader. the inexperienced observer is certain to be surprised by the display of stars which an opera-glass brings to view in the hyades. our illustration will give some notion of their appearance with a large field-glass. the "brackish poet," of whose rhymes admiral smyth was so fond, thus describes the hyades: "in lustrous dignity aloft see alpha tauri shine, the splendid zone he decorates attests the power divine: for mark around what glitt'ring orbs attract the wandering eye, you'll soon confess no other star has such attendants nigh." the redness of the light of aldebaran is a very interesting phenomenon. careful observation detects a decided difference between its color and that of betelgeuse, or alpha orionis, which is also a red star. it differs, too, from the brilliant red star of summer, antares. aldebaran has a trace of rose-color in its light, while betelgeuse is of a very deep orange, and antares may be described as fire-red. these shades of color can easily be detected by the naked eye after a little practice. first compare aldebaran and betelgeuse, and glance from each to the brilliant white, or bluish-white, star rigel in orion's foot. upon turning the eye back from rigel to aldebaran the peculiar color of the latter is readily perceived. spectroscopic analysis has revealed the presence in aldebaran of hydrogen, sodium, magnesium, calcium, iron, bismuth, tellurium, antimony, and mercury. and so modern discoveries, while they have pushed back the stars to distances of which the ancients could not conceive, have, at the same time, and equally, widened the recognized boundaries of the physical universe and abolished forever the ancient distinction between the heavens and the earth. it is a plain road from the earth to the stars, though mortal feet can not tread it. [illustration: the hyades.] keeping in mind that in our little picture of the hyades the top is north, the right hand west, and the left hand east, the reader will be able to identify the principal stars in the group. aldebaran is readily recognized, because it is the largest of all. the bright star near the upper edge of the picture is epsilon tauri, and its sister star, forming the point of the [v], is gamma tauri. the three brightest stars between epsilon and gamma, forming a little group, are the deltas, while the pair of stars surrounded by many smaller ones, half-way between aldebaran and gamma, are the thetas. these stars present a very pretty appearance, viewed with a good glass, the effect being heightened by a contrast of color in the two thetas. the little pair southeast of aldebaran, called the sigmas, is also a beautiful object. the distance apart of these stars is about seven minutes of arc, while the distance between the two thetas is about five and a half minutes of arc. these measures may be useful to the reader in estimating the distances between other stars that he may observe. it will also be found an interesting test of the eye-sight to endeavor to see these stars as doubles without the aid of a glass. persons having keen eyes will be able to accomplish this. north of the star epsilon will be seen a little group in the ear of the bull (see cut, "the golden horns of taurus"), which presents a brilliant appearance with a small glass. the southernmost pair in the group are the kappas, whose distance apart is very nearly the same as that of the thetas, described above; but i think it improbable that anybody could separate them with the naked eye, as there is a full magnitude between them in brightness, and the smaller star is only of magnitude . , while sixth-magnitude stars are generally reckoned as the smallest that can be seen by the naked eye. above the kappas, and in the same group in the ear, are the two upsilons, forming a wider pair. next we come to the pleiades: "though small their size and pale their light, wide is their fame." in every age and in every country the pleiades have been watched, admired, and wondered at, for they are visible from every inhabited land on the globe. to many they are popularly known as the seven stars, although few persons can see more than six stars in the group with the unaided eye. it is a singular fact that many of the earliest writers declare that only six pleiades can be seen, although they all assert that they are seven in number. these seven were the fabled daughters of atlas, or the atlantides, whose names were merope, alcyone, celæno, electra, taygeta, asterope, and maia. one of the stories connected with them is that merope married a mortal, whereupon her star grew dim among her sisters. another fable assures us that electra, unable to endure the sight of the burning of troy, hid her face in her hands, and so blotted her star from the sky. while we may smile at these stories, we can not entirely disregard them, for they are intermingled with some of the richest literary treasures of the world, and they come to us, like some old keepsake, perfumed with the memory of a past age. the mythological history of the pleiades is intensely interesting, too, because it is world-wide. they have impressed their mark, in one way or another, upon the habits, customs, traditions, language, and history of probably every nation. this is true of savage tribes as well as of great empires. the pleiades furnish one of the principal links that appear to connect the beginnings of human history with that wonderful prehistoric past, where, as through a gulf of mist, we seem to perceive faintly the glow of a golden age beyond. the connection of the pleiades with traditions of the flood is most remarkable. in almost every part of the world, and in various ages, the celebration of a feast or festival of the dead, dimly connected by traditions with some great calamity to the human race in the past, has been found to be directly related to the pleiades. this festival or rite, which has been discovered in various forms among the ancient hindoos, egyptians, persians, peruvians, mexicans, druids, etc., occurs always in the month of november, and is regulated by the culmination of the pleiades. the egyptians directly connected this celebration with a deluge, and the mexicans, at the time of the spanish conquest, had a tradition that the world had once been destroyed at the time of the midnight culmination of the pleiades. among the savages inhabiting australia and the pacific island groups a similar rite has been discovered. it has also been suggested that the japanese feast of lanterns is not improbably related to this world-wide observance of the pleiades, as commemorating some calamitous event in the far past which involved the whole race of man in its effects. the pleiades also have a supposed connection with that mystery of mysteries, the great pyramid of cheops. it has been found that about the year b. c., when the beginning of spring coincided with the culmination of the pleiades at midnight, that wonderful group of stars was visible, just at midnight, through the mysterious southward-pointing passage of the pyramid. at the same date the then pole-star, alpha draconis, was visible through the northward-pointing passage of the pyramid. another curious myth involving the pleiades as a part of the constellation taurus is that which represents this constellation as the bull into which jupiter changed himself when he carried the fair europa away from phoenicia to the continent that now bears her name. in this story the fact that only the head and fore-quarters of the bull are visible in the sky is accounted for on the ground that the remainder of his body is beneath the water through which he is swimming. here, then, is another apparent link with the legends of the flood, with which the pleiades have been so strangely connected, as by common consent among many nations, and in the most widely separated parts of the earth. with the most powerful field-glass you may be able to see all of the stars represented in our picture of the pleiades. with an ordinary opera-glass the fainter ones will not be visible; yet even with such a glass the scene is a remarkable one. not only all of the "seven sisters," but many other stars, can be seen twinkling among them. the superiority of alcyone to the others, which is not so clear to the naked eye, becomes very apparent. alcyone is the large star below the middle of the picture with a triangle of little stars beside it. to the left or east of alcyone the two most conspicuous stars are atlas and pleione. the latter--which is the uppermost one--is represented too large in the picture. it requires a sharp eye to see pleione without a glass, while atlas is plainly visible to the unaided vision, and is always counted among the naked-eye pleiades, although it does not bear the name of one of the mythological sisters, but that of their father. the bright star below and to the right of alcyone is merope; the one near the right-hand edge of the picture, about on a level with alcyone, is electra. above, or to the north of electra, are two bright stars lying in a line pointing toward alcyone; the upper one of these, or the one farthest from alcyone, is taygeta, and the other is maia. above taygeta and maia, and forming a little triangle with them, is a pair of stars which bears the name of asterope. about half-way between taygeta and electra, and directly above the latter, is celæno. [illustration: the pleiades.] the naked-eye observer will probably find it difficult to decide which he can detect the more easily, celæno or pleione, while he will discover that asterope, although composed of two stars, as seen with a glass, is so faint as to be much more difficult than either celæno or pleione. unless, as is not improbable, the names have become interchanged in the course of centuries, the brightness of these stars would seem to have undergone remarkable changes. the star of merope, it will be remembered, was said to have become indistinct, or disappeared, because she married a mortal. at present merope is one of those that can be plainly seen with the naked-eye, while the star of asterope, who was said to have had the god mars for her spouse, has faded away until only a glass can show it. it would appear, then, that notwithstanding an occasional temporary eclipse, it is, in the long run, better to marry a plain mortal than a god. electra, too, who hid her eyes at the sight of burning troy, seems to have recovered from her fright, and is at present, next to alcyone, the brightest star in the cluster. but, however we may regard those changes in the brightness of the pleiades which are based upon tradition, there is no doubt that well-attested changes have taken place in the comparative brilliancy of stars in this cluster since astronomy became an exact science. observations of the proper motions of the pleiades have shown that there is an actual physical connection between them; that they are, literally speaking, a flight of suns. their common motion is toward the southwest, under the impulse of forces that remain as yet beyond the grasp of human knowledge. alcyone was selected by mädler as the central sun around which the whole starry system revolved, but later investigations have shown that his speculation was not well founded, and that, so far as we can determine, the proper motions of the stars are not such as to indicate the existence of any common center. they appear to be flying with different velocities in every direction, although--as in the case of the pleiades--we often find groups of them associated together in a common direction of flight. still another curious fact about the pleiades is the existence of some rather mysterious nebulous masses in the cluster. in temple discovered an extensive nebula, of a broad oval form, with the star merope immersed in one end of it. subsequent observations showed that this strange phenomenon was variable. sometimes it could not be seen; at other times it was very plain and large. in jeaurat's chart of the pleiades, made in , a vast nebulous mass is represented near the stars atlas and pleione. this has since been identified by goldschmidt as part of a huge, ill-defined nebula, which he thought he could perceive enveloping the whole group of the pleiades. many observers, however, could never see these nebulous masses, and were inclined to doubt their actual existence. within the past few years astronomical photography, having made astonishing progress, has thrown new light upon this mysterious subject. the sensitized plate of the camera, when applied at the focus of a properly constructed telescope, has proved more effective than the human retina, and has, so to speak, enabled us to see beyond the reach of vision by means of the pictures it makes of objects which escape the eye. in november, , paul and prosper henry turned their great photographing telescope upon the pleiades, and with it discovered a nebula apparently attached to the star maia. the most powerful telescopes in the world had never revealed this to the eye. yet of its actual existence there can be no question. their photograph also showed the merope nebula, although much smaller, and of a different form from that represented by its discoverer and others. there evidently yet remains much to be discovered in this singular group, and the mingling of nebulous matter with its stars makes tennyson's picturesque description of the pleiades appear all the more life-like: "many a night i saw the pleiads, rising through the mellow shade, glitter like _a swarm of fire-flies tangled in a silver braid_." the reader should not expect to be able to see the nebulæ in the pleiades with an opera-glass. i have thought it proper to mention these singular objects only in order that he might be in possession of the principal and most curious facts about those interesting stars.[c] [footnote c: the henry brothers have continued the photographic work described above, and their later achievements are even more interesting and wonderful. they have found that there are many nebulous masses involved in the group of the pleiades, and have photographed them. one of the most amazing phenomena in their great photograph of the pleiades is a long wisp or streak of nebulous matter, along which eight or nine stars are strung in a manner which irresistibly suggests an intimate connection between the stars and the nebula. this recalls the recent (august, ) discovery made by prof. holden, with the great lick telescope, concerning the structure of the celebrated ring nebula in lyra, which, it appears, is composed of concentric ovals of stars and nebulous stuff, so arranged that we must believe they are intimately associated in a most wonderful community.] orion will next command our attention. you will find the constellation in map no. : "eastward beyond the region of the bull stands great orion; whoso kens not him in cloudless night gleaming aloft, shall cast his eyes in vain to find a brighter sign in all the heaven." to the naked eye, to the opera-glass, and to the telescope, orion is alike a mine of wonders. this great constellation embraces almost every variety of interesting phenomena that the heavens contain. here we have the grandest of the nebulæ, some of the largest and most beautifully colored stars, star-streams, star-clusters, nebulous stars, variable stars. i have already mentioned the positions of the principal stars in the imaginary figure of the great hunter. i may add that his upraised arm and club are represented by the stars seen in the map above alpha ([alpha]) or betelgeuse, one of which is marked nu ([nu]), and another, in the knob of the club, chi ([chi]). i have also, in speaking of aldebaran, described the contrast in the colors of betelgeuse and beta ([beta]) or rigel. betelgeuse, it may be remarked, is slightly variable. sometimes it appears brighter than rigel, and sometimes less brilliant. it is interesting to note that, according to secchi's division of the stars into types, based upon their spectra, betelgeuse falls into the third order, which seems to represent a type of suns in which the process of cooling, and the formation of an absorptive envelope or shell, have gone on so far that we may regard them as approaching the point of extinction. rigel, on the other hand, belongs to the first order or type which represents suns that are probably both hotter and younger in the order of development. so, then, we may look upon the two chief stars of this great constellation as representing two stages of cosmical existence. betelgeuse shows us a sun that has almost run its course, that has passed into its decline, and that already begins to faint and flicker and grow dim before the on-coming and inevitable fate of extinction; but in rigel we see a sun blazing with the fires of youth, splendid in the first glow of its solar energies, and holding the promise of the future yet before it. rigel belongs to a new generation of the universe; betelgeuse to the universe that is passing. we may pursue this comparison one step farther back and see in the great nebula, which glows dimly in the middle of the constellation, between rigel triumphant and betelgeuse languishing, a still earlier cosmical condition--the germ of suns whose infant rays may illuminate space when rigel itself is growing dim. [illustration: the sword of orion and the great nebula.] turn your glass upon the three stars forming the belt. you will not be likely to undertake to count all the twinkling lights that you will see, especially as many of them appear and disappear as you turn your attention to different parts of the field. sweep all around the belt and also between the belt and gamma ([gamma]) or bellatrix. according to the old astrologers, women born under the influence of the star bellatrix were lucky, and provided with good tongues. of course, this was fortunate for their husbands too! below the belt will be seen a short row of stars hanging downward and representing the sword. in the middle of this row is the great orion nebula. the star theta ([theta]) involved in the nebula is multiple, and the position of this little cluster of suns is such that, as has been said, they seem to be feeding upon the substance of the nebula surrounding them. other stars are seen scattered in different parts of the nebula. this phenomenon can be plainly seen with an opera-glass. our picture of the sword of orion shows its appearance with a good field-glass. with such a glass several fine test-objects will be found in the sword. one of the best of these is formed by the two five-pointed stars seen in the picture close together above the nebula. no difficulty will be encountered in separating these stars with a field-glass, but it will require a little sharp watching to detect the small star between the two and just above the line joining them. so, the bending row of faint stars above and to the right of the group just described will be found rather elusive as individuals, though easily glimpsed as a whole. of the great nebula itself not much detail can be seen. yet by averting the eyes the extension of the nebulous light in every direction from the center can be detected and traced, under favorable circumstances, to a considerable distance. the changes that this nebula certainly has undergone in the brilliancy, if not in the form, of different parts of it, are perhaps indications of the operation of forces, which we know must prevail there, and whose tendency can only be in the direction of condensation, and the ultimate formation of future suns and worlds. yet, as the appearance of the nebula in great telescopes shows, we can not expect that the processes of creation will here produce a homologue of our solar system. the curdled appearance of the nebula indicates the formation of various centers of condensation, the final result of which will doubtless be a group of stars like some of those which we see in the heavens, and whose common motion shows that they are bound together in the chains of reciprocal gravitation. the pleiades are an example of such a group. do not fail to look for a little star just west of rigel, which, with a good opera-glass, appears to be almost hidden in the flashing rays of its brilliant companion. if you have also a field-glass, after you have detected this shy little twinkler with your opera-glass, try the larger glass upon it. you will find then that the little star originally seen is not the only one there. a still smaller star, which had before been completely hidden, will now be perceived. i may add that, with telescopes, rigel is one of the most beautiful double stars in the sky, having a little blue companion close under its wing. run your glass along the line of little stars forming the lion's skin or shield that orion opposes to the onset of taurus. here you will find some interesting combinations, and the star marked on the map [pi]^ will especially attract your eye, because it is accompanied, about fifteen minutes to the northwest, by a seventh-magnitude star of a rich orange hue. look next at the little group of three stars forming the head of orion. although there is no nebula here, yet these stars, as seen with the naked eye, have a remarkably nebulous look, and ptolemy regarded the group as a nebulous star. the largest star is called lambda ([lambda]); the others are phi ([phi]) one and two. an opera-glass will show another star above ([lambda]), and a fifth star below [phi]^ which is the farthest of the two phis from lambda. it will also reveal a faint twinkling between [lambda] and [phi]^ . a field-glass shows that this twinkling is produced by a pretty little row of three stars of the eighth and ninth magnitudes. in fact, orion is such a striking object in the sky that more than one attempt has been made to steal away its name and substitute that of some modern hero. the university of leipsic, in , formally resolved that the stars forming the belt and sword of orion should henceforth be known as the constellation of napoleon. as if to offset this, an englishman proposed to rename orion for the british naval bull-dog nelson. but "orion armed" has successfully maintained his name and place against all comers. as becomes the splendor of his constellation, orion is a tremendous hero of antiquity, although it must be confessed that his history is somewhat shadowy and uncertain, even for a mythological story. all accounts agree, however, that he was the mightiest hunter ever known, and the hebrews claimed that he was no less a person than nimrod himself. [illustration: map .] the little constellations of lepus and columba, below orion, need not detain us long. you will find in them some pretty combinations of stars. in lepus is the celebrated "crimson star," which has been described as resembling a drop of blood in color--a truly marvelous hue for a sun--but, as it is never brighter than the sixth magnitude, and from that varies down to the ninth, we could hardly hope to see its color well with an opera-glass. besides, the observer would have difficulty in finding it. we will now turn to the constellation of canis major, represented in map no. . although, as a constellation, it is not to be compared with the brilliant orion, yet, on account of the unrivaled magnificence of its chief star, canis major presents almost as attractive a scene as its more extensive rival. everybody has heard of sirius, or the dog-star, and everybody must have seen it flashing and scintillating so splendidly in the winter heavens, that to call it a first-magnitude star does it injustice, since no other star of that magnitude is at all comparable with it. sirius, in fact, stands in a class by itself as the brightest star in the sky. its light is white, with a shade of green, which requires close watching to be detected. when it is near the horizon, or when the atmosphere is very unsteady, sirius flashes prismatic colors like a great diamond. the question has been much discussed, as to whether sirius was formerly a red star. it is described as red by several ancient authors, but it seems to be pretty well established that these descriptions are most of them due to a blunder made by cicero in his translation of the astronomical poem of aratus. it is not impossible, though it is highly improbable, that sirius has changed color. so intimately was sirius connected in the minds of the ancient egyptians with the annual rising of the nile, that it was called the nile-star. when it appeared in the morning sky, just before sunrise, the season of the overflowing of the great river was about to begin, and so the appearance of this star was regarded as foretelling the coming of the floods. the dog-days got their name from sirius, as they occur at the time when that star rises with the sun. your eyes will be fairly dazzled when you turn your glass upon this splendid star. by close attention you will be able to perceive a number of faint stars, mere points by comparison, in the immediate neighborhood of sirius. there are many interesting objects in the constellation. the star marked nu ([nu]) in the map is really triple, as the smallest glass will show. look next at the star-group m. the cloud of minute stars of which it is composed can be very well seen with a field-glass or a powerful opera-glass. the star is of a very ruddy color that contrasts beautifully with the light of epsilon ([epsilon]), which can be seen in the same field of view with an opera-glass. between the stars delta ([delta]) and [omicron]¹ and [omicron]² there is a remarkable array of minute stars, as shown in the accompanying cut. one never sees stars arranged in streams or rows, like these, without an irresistible impression that the arrangement can not be accidental; that some law must have been in operation which associated them together in the forms which we see. yet, when we reflect that these are all suns, how far do we seem to be from understanding the meaning of the universe! [illustration: delta canis majoris and its neighbors.] the extraordinary size and brilliancy of sirius might naturally enough lead one to suppose that it is the nearest of the stars, and such it was once believed to be. observations of stellar parallax, however, show that this was a mistake. the distance of sirius is so great that no satisfactory determination of it has yet been made. we may safely say, though, that that distance is, at the least calculation, , , , , miles. in other words, sirius is about , times as far from the earth as the sun is. then, since light diminishes as the square of the distance increases, the sun, if placed as far from us as sirius is, would send us, in round numbers, , , , times less light than we now receive from it. but sirius actually sends us only about , , , times less light than the sun does; consequently sirius must shine , , , / , , , = times as brilliantly as the sun. if we adopt wollaston's estimate of the light of sirius, as compared with that of the sun, viz., / , , , , we shall still find that the actual brilliancy of that grand star is more than fourteen times as great as that of our sun. but as observations on the companion of sirius show that sirius's mass is fully twenty times the sun's, and since the character of sirius's spectrum indicates that its intrinsic brightness, surface for surface, is much superior to the sun's, it is probable that our estimate of the star's actual brilliancy, as compared with what the sun would possess at the same distance, viz., seventy-two times, is much nearer the truth. it is evident that life would be insupportable upon the earth if it were placed as near to sirius as it is to the sun. if the earth were a planet belonging to the system of sirius, in order to enjoy the same amount of heat and light it now receives, it would have to be removed to a distance of nearly , , miles, or eight and a half times its distance from the sun. its time of revolution around sirius would then be nearly five and a half years, or, in other words, the year would be lengthened five and a half times. but, as i have said, the estimate of sirius's distance used in these calculations is the smallest that can be accepted. good authorities regard the distance as being not less than , , , , miles; in which case the star's brilliancy must be as much as times greater than that of the sun! and yet even sirius is probably not the greatest sun belonging to the visible universe. there can be little doubt that canopus, in the southern hemisphere, is a grander sun than sirius. to our eyes, canopus is only about half as bright as sirius, and it ranks as the second star in the heavens in the order of brightness. but while sirius's distance is measurable, that of canopus is so unthinkably immense that astronomers can get no grip upon it. if it were only twice as remote as sirius, it would be equal to two of the latter, but in all probability its distance is much greater than that. and possibly even canopus is not the greatest gem in the coronet of creation. sirius, as we saw when talking of procyon (see chapter i), is a double star. for many years after bessel had declared his belief that the dog-star was subjected to the attraction of an invisible companion, telescopes failed to reveal the accompanying star.[d] finally, in , a new telescope that alvan clark had just finished and was testing, brought the hidden star into view. the suggestion that it may shine by reflected light from sirius has been made. in that case it must, of course, be a planet, but a planet of such stupendous magnitude that the imagination can scarcely grasp it; a planet probably as large as our sun, perhaps larger; a planet equal in size to more than a million earths! but, as was remarked of the faint stars in alpha capricornis, it is probable that the hypothesis of reflected light is not the true one. more probably the companion of sirius shines with light of its own, though its excessive faintness in comparison with its bulk indicates that its condition must be very different from that of an ordinary star. [d] the following extract from a letter by bessel to humboldt, written in (see "cosmos," vol. iii, p. ), is interesting, in view of the discoveries made since then: "at all events i continue in the belief that procyon and sirius are true double stars, consisting of a visible and an invisible star. no reason exists for considering luminosity an essential property of these bodies. the fact that numberless stars are visible is evidently no proof against the existence of an equally incalculable number of invisible ones. the physical difficulty of a change in the proper motion is satisfactorily set aside by the hypothesis of dark stars." readers of voltaire will remember that the hero of his extraordinary story of "micromegas" came from an imaginary planet circling around sirius. inasmuch as voltaire, together with dean swift, ascribed two moons to mars many years before they were discovered (probably suggested by a curiously mistaken interpretation by kepler of an anagram in which galileo had concealed his discovery of the ring of saturn), it is all the more interesting that the great infidel should have imagined an enormous planet circling around the dog-star. but voltaire went far astray when he ascribed a gigantic stature to his "sirian." he makes micromegas, whose world was , , times larger in circumference than the earth, more than twenty miles tall, so that when he visited our little planet he was able to wade through the oceans and step over the mountains without inconvenience, and, when he had scooped up some of the inhabitants on his thumb-nail, was obliged to use a powerful microscope in order to see them. voltaire should rather have gone to some of the most minute of the asteroids for his giant, for under the tremendous gravitation of such a world as he has described micromegas himself would have been a fit subject for microscopic examination. but, however much we may doubt the stature of voltaire's visitor from sirius, we can not doubt the soundness of the conclusion at which he arrived, after having, by an ingenious arrangement, succeeded in holding a conversation with some earthly philosophers under his microscope, namely, that these infinitely little creatures possessed a pride that was almost infinitely great. east and south of canis major, which, by-the-way, is said to represent one of orion's hounds, is part of the constellation argo, which stands for the ship in which jason sailed in search of the golden fleece. the observer will find many objects of interest here, although some of them are so close to the horizon in our latitudes that much of their brilliancy is lost. note the two stars [zeta] and [pi] near the lower edge of the map, then sweep slowly over the space lying between them. about half-way your attention will be arrested by a remarkable stellar arrangement, in which a beautiful half-circle of small stars curving above a larger star, which is reddish in color, is conspicuous. this neighborhood will be found rich in stars that the naked eye can not see. just below the star [eta], in canis major, is another fine group. the star [pi], which is deep yellow or orange, has three little stars above it, two of which form a pretty pair. the star [xi] has a companion, which forms a fine test for an opera-glass, and is well worth looking for. look also at the cluster m, just above and to the west of [xi]. the stars [mu] and [kappa] are seen double with an opera-glass. the two neighboring clusters, m and ^ , are very interesting objects. to see them well, use a powerful field-glass. a "fiery fifth-magnitude star," as webb calls it, can be seen in the field at the same time. the presence of the milky-way is manifest by the sprinkling of stars all about this region. in fact, the attentive observer will before this have noticed that the majority of the most brilliant constellations lie either in the milky-way or along its borders. cassiopeia, as we saw, sits athwart the galaxy whose silvery current winds in and out among the stars of her "chair"; perseus is aglow with its sheen as it wraps him about like a mantle of stars; taurus has the tips of his horns dipped in the great stream; it flows between the shining feet of gemini and the head and shoulders of orion as between starry banks; the peerless sirius hangs like a gem pendent from the celestial girdle. in the southern hemisphere we should find the beautiful constellation of the ship argo, containing canopus, sailing along the milky-way, blown by the breath of old romance on an endless voyage; the southern cross glitters in the very center of the galaxy; and the bright stars of the centaur might be likened to the heads of golden nails pinning this wondrous scarf, woven of the beams of millions of tiny stars, against the dome of the sky. passing back into the northern hemisphere we find scorpio, sagittarius, aquila, the dolphin, cygnus, and resplendent lyra, all strung along the course of the milky-way. turning now to the constellation monoceros, we shall find a few objects worthy of attention. this constellation is of comparatively modern origin, having been formed by bartschius, whose chief title to distinction is that he married the daughter of john kepler. the region around the stars , , and will be found particularly rich, and the cluster ^ shows well with a strong glass. look also at the cluster m, and compare its appearance with that of the clusters in argo. with these constellations we finish our review of the stellar wonders that lie within the reach of so humble an instrument as an opera-or field-glass. we have made the circuit of the sky, and the hosts that illumine the vernal heavens are now seen advancing from the east, and pressing close upon the brighter squadrons of winter. their familiar figures resemble the faces of old friends whom we are glad to welcome. these starry acquaintances never grow wearisome. their interest for us is as fathomless as the deeps of space in which they shine. the man never yet lived whose mind could comprehend the full meaning of the wondrous messages that they flash to us upon the wings of light. as we watch them in their courses, the true music of the spheres comes to our listening ears, the chorus of creation--faint with distance, for it is by slow approaches that man draws near to it--chanting the grandest of epics, the poem of the universe; and the theme that runs through it all is the reign of law. do not be afraid to become a star-gazer. the human mind can find no higher exercise. he who studies the stars will discover-- "an endless fountain of immortal drink pouring unto us from heaven's brink." chapter v. the moon, the planets, and the sun. "it is a most beautiful and delightful sight," exclaims galileo, in describing the discoveries he had made with his telescope, "to behold the body of the moon, which is distant from us nearly sixty semi-diameters of the earth, as near as if it was at a distance of only two of the same measures.... and, consequently, any one may know with the certainty that is due to the use of our senses that the moon assuredly does not possess a smooth and polished surface, but one rough and uneven, and, just like the face of the earth itself, is everywhere full of vast protuberances, deep chasms, and sinuosities." there was, perhaps, nothing in the long series of discoveries with which galileo astonished the world after he had constructed his telescope, which, as he expresses it, "was devised by me through god's grace first enlightening my mind," that had a greater charm for him than his lunar observations. certainly there was nothing which he has described with greater enthusiasm and eloquence. and this could hardly have been otherwise, for the moon was the first celestial object to which galileo turned his telescope, and then for the first time human eyes may be said to have actually looked into another world than the earth, though his discoveries and those of his successors have not realized all the poetic fancies about the moon contained in the verses that are ascribed to orpheus: "and he another wandering world has made which gods selene name, and men the moon. it mountains, cities has, and temples grand." yet galileo's observations at once upset the theory, for which apollonius was responsible, and which seems to have been widely prevalent up to his time, that the moon was a smooth body, polished like a mirror, and presenting in its light and dark spots reflections of the continents and oceans of the earth. he also demonstrated that its surface was covered with plains and mountains, but the "cities and temples" of the moon have remained to our time only within the ken of romance. galileo's telescope, as i have before remarked, was, in the principle of its construction, simply an opera-glass of one tube. he succeeded in making a glass of this kind that magnified thirty diameters, a very much higher power than is given to the opera-and field-glasses of to-day. yet he had to contend with the disadvantages of single lenses, achromatic combinations of glass for optical purposes not being contrived until nearly a hundred years after his death, and so his telescope did not possess quite as decided a superiority over a modern field-glass as the difference in magnifying power would imply. in fact, if the reader will view the moon with a first-rate field-glass, he will perceive that the true nature of the surface of the lunar globe can be readily discerned with such an instrument. even a small opera-glass will reveal much to the attentive observer of the moon; but for these observations the reader should, if possible, make use of a field-glass, and the higher its power the better. the illustrations accompanying this chapter were made by the author with the aid of a glass magnifying seven diameters. of course, the first thing the observer will wish to see will be the mountains of the moon, for everybody has heard of them, and the most sluggish imagination is stirred by the thought that one can look off into the sky and behold "the eternal hills" of another planet as solid and substantial as our own. but the chances are that, if left to their own guidance, ninety-nine persons out of a hundred would choose exactly the wrong time to see these mountains. at any rate, that is my experience with people who have come to look at the moon through my telescope. unless warned beforehand, they invariably wait until full moon, when the flood of sunshine poured perpendicularly upon the face of our satellite conceals its rugged features as effectually as if a veil had been drawn over them. begin your observations with the appearance of the narrowest crescent of the new moon, and follow it as it gradually fills, and then you will see how beautifully the advancing line of lunar sunrise reveals the mountains, over whose slopes and peaks it is climbing, by its ragged and sinuous outline. the observer must keep in mind the fact that he is looking straight down upon the tops of the lunar mountains. it is like a view from a balloon, only at a vastly greater height than any balloon has ever attained. even with a powerful telescope the observer sees the moon at an apparent distance of several hundred miles, while with a field-glass, magnifying seven diameters, the moon appears as if thirty-five thousand miles off. the apparent distance with galileo's telescope was eight thousand miles. recollect how when seen from a great height the rugosities of the earth's surface flatten out and disappear, and then try to imagine how the highest mountains on the earth would look if you were suspended thirty-five thousand miles above them, and you will, perhaps, rather wonder at the fact that the moon's mountains can be seen at all. it is the contrast of lights and shadows that not only reveals them to us, but enables us to measure their height. on the moon shadows are very much darker than upon the earth, because of the extreme rarity of the moon's atmosphere, if indeed it has any atmosphere at all. by stepping around the corner of a rock there, one might pass abruptly from dazzling noonday into the blackness of midnight. the surface of the moon is extraordinarily rough and uneven. it possesses broad plains, which are probably the bottoms of ancient seas that have now dried up, but these cover only about two fifths of the surface visible to us, and most of the remaining three fifths are exceedingly rugged and mountainous. many of the mountains of the moon are, foot for foot, as lofty as the highest mountains on the earth, while all of them, in proportion to the size of the moon's globe, are much larger than the earth's mountains. it is obvious, then, that the sunshine, as it creeps over these alpine landscapes in the moon, casting the black shadows of the peaks and craters many miles across the plains, and capping the summits of lofty mountains with light, while the lower regions far around them are yet buried in night, must clearly reveal the character of the lunar surface. mountains that can not be seen at all when the light falls perpendicularly upon them, or, at the most, appear then merely as shining points, picture themselves by their shadows in startling silhouettes when illuminated laterally by the rising sun. but at full moon, while the mountains hide themselves in light, the old sea-beds are seen spread out among the shining table-lands with great distinctness. even the naked eye readily detects these as ill-defined, dark patches upon the face of the moon, and to their presence are due the popular notions that have prevailed in all quarters of the world about the "man in the moon," the "woman in the moon," "jacob in the moon," the "hare in the moon," the "toad in the moon," and so on. but, however clearly one may imagine that he discerns a man in the moon while recalling the nursery-rhymes about him, an opera-glass instantly puts the specter to flight, and shows the round lunar disk diversified and shaded like a map.[e] [e] i should, perhaps, qualify the statement in the text slightly in favor of a lunar lady to whom mr. henry m. parkhurst first called my attention. about nine days after new moon a rather pretty and decidedly feminine face appears on the western half of the disk. it is formed by the mountains and table-lands embraced by the sea of serenity, the sea of tranquillity, the sea of vapors, etc., and is best seen with the aid of an opera-glass of low power. the face is readily distinguishable on rutherfurd's celebrated photograph of the full moon. it is necessary for this purpose to turn the photograph upside down, since it is a telescopic picture, and consequently reversed. the crater tycho forms a breastpin for the lady, and menelaus glitters like a diamond ornament in her hair, while the range of the apennines resembles a sort of coronet resting on her forehead. this same woman in the moon, it appears, was described by dr. james thompson years ago, and, for aught i know, she may be the diana to whom herrick sang: "queen and huntress chaste and fair, seated in thy silver chair, now the sun is laid to sleep, state in wonted manner keep. hesperus entreats thy light, goddess excellently bright." a feature of the full moon's surface that instantly attracts attention is the remarkable brightness of the southern part of the disk, and the brilliant streaks radiating from a bright point near the lower edge. the same simile almost invariably comes to the lips of every person who sees this phenomenon for the first time--"it looks like a peeled orange." the bright point, which is the great crater-mountain tycho, looks exactly like the pip of the orange, and the light-streaks radiating from it in all directions bear an equally striking resemblance to the streaks that one sees upon an orange after the outer rind has been removed. i shall have something more to say about these curious streaks further on; in the mean time, let us glance at our little sketch-map of the moon. [illustration: map of the moon.] the so-called seas are marked on the map, for the purpose of reference, by the letters which they ordinarily bear in lunar maps. the numerals indicate craters, or ring-plains, and mountain-ranges. the following key-list will enable the reader to identify all the objects that are lettered or numbered upon the map. i have given english translations of the latin names which the old astronomers bestowed upon the seas: _seas, gulfs, and marshes._ a. the crisian sea. b. humboldt sea. c. the sea of cold. d. the lake of death. e. the lake of dreams. f. the marsh of sleep. g. the sea of tranquillity. h. the sea of serenity. i. the marsh of mists. k. the marsh of putrefaction. l. the sea of vapors. m. the central gulf. n. the gulf of heats. o. the sea of showers. p. the bay of rainbows. q. the ocean of storms. r. the bay of dew. s. the sea of clouds. t. the sea of humors. v. the sea of nectar. x. the sea of fertility. z. the south sea. _mountains and crater rings._ . grimaldi. . letronne. . gassendi. . euclides. . bullialdus. . pitatus. . schickhard. . longomontanus. . tycho. . maginus. . clavius. . newton. . maurolycus. . stöfler. . walter. . regiomontanus. . purbach. . arzachel. . alphonsus. . ptolemaus. . hipparchus. . albategnius. . theophilus. . cyrillus. . catharina. . the altai mts. . piccolomini. . petavius. . langrenus. . proclus. . cleomedes. . atlas. . hercules. . posidonius. . plinius. . menelaus. . manilius. . the caucasus mts. . eudoxus. . aristotle. . the alps. . plato. . archimedes. . the apennines. . eratosthenes. . copernicus. . the carpathian mts. . timocharis. . lambert. . euler. . aristarchus. . kepler. . flamsteed. the early selenographers certainly must have been men of vivid imagination, and the romantic names they gave to the lunar landscapes, and particularly to the "seas," add a charm of their own to the study of the moon. who would not wish to see the "bay of rainbows," or the "lake of dreams," or the "sea of tranquillity," if for no other reason than a curiosity to know what could have induced men to give to these regions in the moon such captivating titles? or who would not desire to visit them if he could? though no doubt we should find them, like the "delectable mountains" in the "pilgrim's progress," most charming when seen from afar. the limited scale of our map, of course, renders it impossible to represent upon it more than a comparatively small number of the lunar mountains that have received names. in selecting those to be put in the map i have endeavored to choose such as, on account of their size, their situation, or some striking peculiarity, would be most likely to attract the attention of a novice. the observer must not expect to see them all at once, however. the lunar features change their appearance to a surprising extent, in accordance with the direction of their illumination. some great mountain-masses and ring-plains, or craters, which present scenes of magnificence when the sun is rising or setting upon them, disappear under a perpendicular light, such as they receive at full moon. the great crater-plain, known as maginus, numbered in our map, is one of these. the broken mountain-wall surrounding this vast depressed plain rises in some places to a height of over fourteen thousand feet above the valley within, and the spectacle of sunrise upon maginus, seen with a powerful telescope, is a most impressive sight, and even with a field-glass is very interesting. yet, a few days later, maginus vanishes, as if it had been swallowed up, and as beer and mädler have expressed it, "the full moon knows no maginus." the still grander formation of mountain, plain, and crater, called clavius ( in the map), disappears almost as completely as maginus at full moon, yet, under the proper illumination, it presents a splendid pageant of light and shadow. on the other hand, some of the lunar mountains shine vividly at full moon, and can be well seen then, though, of course, only as light spots, since at that time they cast no shadows. menelaus ( in the map), aristarchus ( ), proclus ( ), copernicus ( ), and kepler ( ), are among these shining mountains. aristarchus is the most celebrated of them all, being the brightest point on the moon. it can even be seen glimmering on the dark side of the moon--that is to say, when no light reaches it except that which is reflected from the earth. with a large telescope, aristarchus is so dazzlingly bright under a high sun, that the eye is partly blinded in gazing at it. it consists of a mountain-ring surrounding a circular valley, about twenty-eight miles in diameter. the flanks of these mountains, especially on their inner slopes, and the floor of the valley within, are very bright, while a peak in the center of the valley, about as high as storm-king mountain on the hudson, shines with piercing brilliancy. sir william herschel mistook it for a volcano in action. it certainly is not an active volcano, but just what makes it so dazzling no one knows. the material of which this mountain is formed would seem to possess a higher reflective power than that of any other portion of the moon's surface. one is irresistibly reminded of the crystallized mountains described in the celebrated "moon hoax" of richard adams locke. with an opera-glass you can readily recognize aristarchus as a bright point at full moon. with a field-glass it is better seen, and some of the short, light rays surrounding it are perceived, while, when the sun is rising upon it, about four days after first quarter, its crateriform shape can be detected with such a glass. the visibility of aristarchus on the dark side of the moon leads us to a brief consideration of the illumination by the earth of that portion of the moon's surface which is not touched directly by sunlight at new and old moon. this phenomenon is shown in the accompanying illustration. not only can the outlines of the dark part of the moon be seen under such circumstances, but even the distinction in color between the dusky "seas" and the more brilliant table-lands and mountain-regions can be perceived, and with powerful telescopes many minor features come into sight. a little consideration must convince any one, as it convinced galileo more than two hundred and seventy-five years ago, that the light reflected from the earth upon the moon is sufficient to produce this faint illumination of the lunar landscapes. we have only to recall the splendors of a night that is lighted by a full moon, and then to recollect that at new or old moon the earth is "full" as seen from our satellite, and that a full earth must give some fourteen times as much light as a full moon, in order to realize the brilliancy of an earth-lit night upon the moon. as the moon waxes to us, the earth wanes to the moon, and _vice versa_, and so the phenomenon of earth-shine on the lunar surface must be looked for before the first quarter and after the last quarter of the moon. [illustration: sunrise on the sea of serenity, and theophilus and other craters.] the reader will find it an attractive occupation to identify, by means of the map, the various "seas," "lakes," and "marshes," for not only are they interesting on account of the singularity of their names, but they present many remarkable differences of appearance, which may be perceived with the instrument he is supposed to be using. the oval form of the crisian sea (a), which is the first of the "seas" to come into sight at new moon, makes it a very striking object. with good telescopes, and under favorable illumination, a decidedly green tint is perceived in the crisian sea. it measures about two hundred and eighty by three hundred and fifty-five miles in extent, and is, perhaps, the deepest of all the old sea-beds visible on the moon. it is surrounded by mountains, which can be readily seen when the sun strikes athwart them a few days after new or full moon. on the southwestern border a stupendous mountain-promontory, called cape agarum, projects into the crisian sea fifty or sixty miles, the highest part rising precipitously eleven thousand feet above the floor of the sea. i have seen cape agarum very clearly defined with a field-glass. near the eastern border is the crater-mountain proclus, which i have already mentioned as possessing great brilliancy under a high sun, being in this respect second only to aristarchus. from the foot of proclus spreads away the somewhat triangular region called the marsh of sleep (f). the term "golden-brown," which has been applied to it, perhaps describes its hue well enough. with a telescope it is a most interesting region, but with less powerful instruments one must be content with recognizing its outline and color. the broad, dark-gray expanse of the sea of tranquillity (g) will be readily recognized by the observer, and he will be interested in the mottled aspect which it presents in certain regions, caused by ridges and elevations, which, when this sea-bottom was covered with water, may have formed shoals and islands. the sea of fertility (x) is remarkable for its irregular surface, and the long, crooked bays into which its southern extremity is divided. the sea of nectar (v) is connected with the sea of tranquillity by a broad strait (one would naturally anticipate from their names that there must be some connection between them), while between it and the sea of fertility runs the range of the pyrenees mountains, twelve thousand feet high, flanked by many huge volcanic mountain-rings. the sea of serenity (h), lying northeast of the sea of tranquillity, is about four hundred and twenty miles broad by four hundred and thirty miles long, being very nearly of the same area as our caspian sea. it is deeper than the sea of tranquillity, and a greenish hue is sometimes detected in its central parts. it deepens toward the middle. three quarters of its shore-line are bordered by high mountains, and many isolated elevations and peaks are scattered over its surface. in looking at these dried-up seas of the moon, one is forcibly reminded of the undulating and in some places mountainous character of terrestrial sea-bottoms, as shown by soundings and the existence of small islands in the deep sea, like the bermudas, the azores and st. helena. the sea of serenity is divided nearly through the center by a narrow, bright streak, apparently starting from the crater-mountain menelaus ( in the map), but really taking its rise at tycho far in the south. this curious streak can be readily detected even with a small opera-glass. just what it is no one is prepared to say, and so the author of the "moon hoax" was fairly entitled to take advantage of the romancer's license, and declare that "its edge throughout its whole length of three hundred and forty miles is an acute angle of solid quartz-crystal, brilliant as a piece of derbyshire spar just brought from the mine, and containing scarcely a fracture or a chasm from end to end!" along the southern shore, on either side of menelaus, extends the high range of the hæmus mountains. south and southeast of the sea of serenity are the sea of vapors (l), the central gulf (m), and the gulf of heats (n). the observer will notice at full moon three or four curious dark spots in the region occupied by these flat expanses. on the north and northwest of the sea of serenity are the lake of death (d), and the lake of dreams (e), chiefly remarkable for their names. the sea of showers (o) is a very interesting region, not only in itself, but on account of its surroundings. its level is very much broken by low, winding ridges, and it is variegated by numerous light-streaks. at its western end it blends into the marsh of mists (i) and the marsh of putrefaction (k). on its northeast border is the celebrated sinus iridum, or bay of rainbows (p), upon which selenographers have exhausted the adjectives of admiration. the bay is semicircular in form, one hundred and thirty-five miles long and eighty-four miles broad. its surface is dark and level. at either end a splendid cape extends into the sea of showers, the eastern one being called cape heraclides, and the western cape laplace. they are both crowned by high peaks. along the whole shore of the bay runs a chain of gigantic mountains, forming the southern border of a wild and lofty plateau, called the sinus iridum highlands. of course, a telescope is required to see the details of this "most magnificent of all lunar landscapes," and yet much can be done with a good field-glass. with such an instrument i have seen the capes at the ends of the bay projecting boldly into the dark, level expanse surrounding them, and the high lights of the bordering mountains sharply contrasted with the dusky semicircle at their feet, and have been able to detect the presence of the low ridges that cross the front of the bay like shoals, separating it from the "sea" outside. two or three days after first quarter, the shadows of the peaks about the bay of rainbows may be seen. the bay of dew (r) above the bay of rainbows, and the sea of cold (c), are the northernmost of the dark levels visible. it was in keeping with the supposed character of this region of the moon that riccioli named two portions of it the land of hoar frost and the land of drought. extending along the eastern side of the disk is the great ocean of storms (q), while between the ocean of storms and the middle of the moon lies the sea of clouds (s). both of these are very irregular in outline, and much broken by ridges and mountains. the sea of humors (t), although comparatively small, is one of the most easily seen of all the lunar plains. to the naked eye it looks like a dark, oval patch on the moon. with a telescope it is seen, under favorable conditions, to possess a decided green tint. humboldt sea (b) and the south sea (z) belong principally to that part of the moon which is always turned away from the earth, and only their edges project into the visible hemisphere, although, under favorable librations, their farther borders, lined as usual with mountain-peaks, may be detected. for our purposes they possess little interest. let us now glance at some of the mountains and "craters." the dark oval called grimaldi ( ) can be detected by the naked eye, or at least it has been thus seen, although it requires a sharp eye; and perhaps a shade or a pair of eye-glasses of london smoke-glass, to take off the glare of the moon, should be used in looking for it.[f] it is simply a plain, containing some fourteen thousand square miles, remarkable for its dark color, and surrounded by mountains. schickhard ( ) is another similar plain, nearly as large, but not possessing the same dark tint in the interior. the huge mountains around schickhard make a fine spectacle when the sun is rising upon them shortly before full moon. [f] there are other uses to which such eye-glasses may be put by sky-gazers. i habitually carry a pair for studying clouds. it is wonderful how much the effect of great cloud-masses is heightened by them, especially when seen in a bright light. delicate curls and striæ of cirrus, which escape the uncovered eye in the glare of sunlight, can be readily detected and studied by the use of neutral-tinted eye-glasses or spectacles. tycho ( ) is the most famous of the crater-mountains, though not the largest. it is about fifty-four miles across and three miles deep. in its center is a peak five or six thousand feet high. tycho is the radial point of the great light-streaks that, as i have already remarked, cause the southern half of the moon to be likened to a peeled orange. it is a tough problem in selenography to account for these streaks. they are best seen at full moon. they can not be seen at all until the sun has risen to a certain elevation above them, ° according to neison; but, when they once become visible, they dominate everything. they turn aside for neither mountains nor plains, but pass straight on their courses over the ruggedest regions of the moon, retaining their brilliancy undiminished, and pouring back such a flood of reflected light that they completely conceal some of the most stupendous mountain-masses across which they lie. they clearly consist of different material from that of which the most of the moon's surface is composed--a material possessing a higher reflective power. in this respect they resemble aristarchus and other lunar craters that are remarkable for their brilliancy under a high illumination. tycho itself, the center or hub, from which these streaks radiate like spokes, is very brilliant in the full moon. but immediately around tycho there is a dark rim some twenty-five miles broad. beyond this rim the surface becomes bright, and the bright region extends about ninety miles farther. out of it spring the great rays or streaks, which vary from ten to twenty miles in width, and many of which are several hundred miles long--one, which we have already mentioned as extending across the sea of serenity, being upward of two thousand miles in length. it has been truly said that we have nothing like these streaks upon the earth, and so there is no analogy to go by in trying to determine their nature. it has been suggested that if the moon had been split or shattered from within by some tremendous force, and molten matter from the interior had been thrust up into the cracks thus formed, and had cooled there into broad seams of rock, possessing a higher reflective power than the surrounding surface of the moon, then the appearances presented would not be unlike what we actually see. but there are serious objections to such a view, which we have not space to discuss here. it is enough to say that the nature of these streaks is still a question awaiting solution, and here is an opportunity for an important discovery, but not one to be achieved with an opera-glass. i may add an interesting suggestion as to the nature of these streaks made by the rev. mr. grensted. he holds that the air and water of the moon were chemically, and not mechanically, absorbed in the process of oxidation which went on at the time when her surface temperature was above a red heat. having a much larger surface in proportion to her bulk than the earth, the oxidation of the moon has, he thinks, extended much deeper than that of the earth, and her atmosphere and oceans have been exhausted in the process. both the earth and the moon, he maintains, have metallic nuclei, and the streaks about tycho and copernicus, and some other lunar craters, may be dikes of pure and shining metal, which have escaped oxidation owing to the comparatively small supply of lunar oxygen. upon this theory aristarchus must be a metallic mountain. [illustration: sunrise on clavius, tycho, plato, etc.] clavius ( ) is one of the most impressive of all the lunar formations. there probably does not exist anywhere upon the earth so wild a scene upon a corresponding scale of grandeur. of course, its details are far beyond the reach of the instrument we are supposed to be using, and yet, even with a field-glass, or a powerful opera-glass, some of its main features are visible. it is represented in our picture of the half-moon, being the lowest and largest of the ring-like forms seen at the inner edge of the illuminated half of the disk; the rays of the rising sun touching the summits of some of the peaks in its interior have brought them into sight as a point of light, and at the same time, reaching across the gulf within, have lighted up the higher slopes of the great mountain-wall on the farther or eastern side of the crater-valley, making it resemble a semicircle of light projecting into the blackness of the still unilluminated plains around it. i should advise every reader to take advantage of any opportunity that may be presented to him to see clavius with a powerful telescope when the sun is either rising or setting upon it. neison has given a spirited description of the scene, as follows: the sunrise on clavius commences with the illumination of a few peaks on the western wall, but soon rapidly extends along the whole wall of clavius, which then presents the appearance of a great double bay of the dark night-side of the moon penetrating so deep into the illuminated portion as to perceptibly blunt the southern horn to the naked eye. within the dark bay some small, bright points soon appear--the summits of the great ring-plains within--followed shortly by similar light-points near the center, due to peaks on the walls of the smaller ring-plains, these light-islands gradually widening and forming delicate rings of light in the dark mass of shadow still enveloping the floor of clavius. far in the east then dimly appear a few scarcely perceptible points, rapidly widening into a thin bright line, the crest of the great southeastern wall of clavius, the end being still lost far within the night-side of the moon. by the period the extreme summit of the lofty wall of clavius on the east becomes distinct, fine streaks of light begin to extend across the dark mass of shadow on the interior of clavius, from the light breaking through some of the passes on the west wall and illuminating the interior; and these streaks widen near the center and form illuminated spots on the floor, when both east and west it still lies deeply immersed in shadow, strongly contrasting with the now brightly illuminated crest of the lofty east wall and the great circular broad rings of light formed by the small ring-plains within clavius. the illumination of the interior of clavius now proceeds rapidly, and forms a magnificent spectacle: the great, brightly illuminated ring-plains on the interior, with their floors still totally immersed in shadow; the immense steep line of cliffs on the east and southeast are now brilliantly illuminated, though the entire surface at their base is still immersed in the shades of night; and the great peaks on the west towering above the floor are thrown strongly into relief against the dark shadow beyond them. newton ( ) is the deepest of the great crateriform chasms on the moon. some of the peaks on its walls rise twenty-four thousand feet above the interior gulf. its shadow, and those of its gigantic neighbors--for the moon is here crowded with colossal walls, peaks, and craters--may be seen breaking the line of sunlight below clavius, in our illustration. i have just spoken of these great lunar formations as chasms. the word describes very well the appearance which some of them present when the line separating day and night on the moon falls across them, but the reader should not be led by it into an erroneous idea of their real character. such formations as newton, which is one hundred and forty miles long by seventy broad, may more accurately be described as vast depressed plains, generally containing peaks and craters, which are surrounded by a ring of steep mountains, or mountain-walls, that rise by successive ridges and terraces to a stupendous height. the double chain of great crater-plains reaching half across the center of the moon contains some of the grandest of these strange configurations of conjoined mountain, plain, and crater. the names of the principal ones can be learned from the map, and the reader will find it very interesting to watch them coming into sight about first quarter, and passing out of sight about third quarter. at such times, with a field-glass, some of them look like enormous round holes in the inner edge of the illuminated half of the moon. theophilus ( ), cyrillus ( ), and catharina ( ), are three of the finest walled plains on the moon--theophilus, in particular, being a splendid specimen of such formations. this chain of craters may be seen rapidly coming into sunlight at the edge of the sea of nectar, in our picture of "sunrise on the sea of serenity," etc. the altai mountains ( ) are a line of lofty cliffs, two hundred and eighty miles in length, surmounting a high table-land. the caucasus mountains ( ) are a mass of highlands and peaks, which introduce us to a series of formations resembling those of the mountainous regions of the earth. the highest peak in this range is about nineteen thousand feet. between the caucasus and the apennines ( ) lies a level pass, or strait, connecting the sea of serenity with the sea of showers. the apennines are the greatest of the lunar mountain-chains, extending some four hundred and sixty miles in length, and containing one peak twenty-one thousand feet high, and many varying from twelve thousand to nearly twenty thousand. it will thus be seen that the apennines of the earth sink into insignificance in comparison with their gigantic namesakes on the moon. as this range runs at a considerable angle to the line of sunrise, its high peaks are seen tipped with sunlight for a long distance beyond the generally illuminated edge about the time of first quarter. even with the naked eye the sun-touched summits of the lunar apennines may at that time be detected as a tongue of light projecting into the dark side of the moon. the alps ( ) are another mountain-mass of great elevation, whose highest peak is a good match for the mont blanc of the earth, after which it has been named. plato ( ) is a very celebrated dark and level plain, surrounded by a mountain-ring, and presenting in its interior many puzzling and apparently changeable phenomena which have given rise to much speculation, but which, of course, lie far beyond the reach of opera-glasses. plato is seen in the picture of "sunrise on clavius," etc., on page , being the second ring from the top. if ariosto had had a telescope, we might have suspected that it was this curious plain that he had in mind when he described that strange valley in the moon, in which was to be found everything that was lost from the earth, including lost wits; and where the redoubtable knight astolpho, having been sent in search of the missing wit of the great orlando, was astonished to find what he sought carefully preserved in a vial along with other similar vials belonging to many supposedly wise people of the earth, whom nobody suspected of keeping a good part of their sapience in the moon. copernicus ( ) is the last of the lunar formations that we shall describe. it bears a general resemblance to tycho, and is slightly greater in diameter; it is, however, not quite so deep. it has a cluster of peaks in the center, whose tops may be detected with a field-glass, as a speck of light when the rays of the morning sun, slanting across the valley, illuminate them while their environs are yet buried in night. copernicus is the center of a system of light-streaks somewhat resembling those of tycho, but very much shorter. we must not dismiss the moon without a few words as to its probable condition. it was but natural, after men had seen the surface of the moon diversified with hills and valleys like another earth, that the opinion should find ready acceptance that beings not unlike ourselves might dwell upon it. nothing could possibly have been more interesting than the realization of such a fancy by the actual discovery of the lunar inhabitants, or at least of unmistakable evidence of their existence. the moon is so near to the earth, as astronomical distances go, and the earth and the moon are so intimately connected in the companionship of their yearly journey around the sun, and their greater journey together with the sun and all his family, through the realms of space, that we should have looked upon the lunar inhabitants, if any had existed, as our neighbors over the way--dwelling, to be sure, upon a somewhat more restricted domain than ours, vassals of the earth in one sense, yet upon the whole very respectable and interesting people, with whom one would be glad to have a closer acquaintance. but, alas! as the powers of the telescope increased, the vision of a moon crowded with life faded, until at last the cold fact struck home that the moon is, in all probability, a frozen and dried-up globe, a mere planetary skeleton, which could no more support life than the humboldt glacier could grow roses. and yet this opinion may go too far. there is reason for thinking that the moon is not absolutely airless, and, while it has no visible bodies of water, its soil may, after all, not be entirely arid and desiccated. there are observations which hint at visible changes in certain spots that could possibly be caused by vegetation, and there are other observations which suggest the display of electric luminosity in a rarefied atmosphere covering the moon. to declare that no possible form of life can exist under the conditions prevailing upon the lunar surface would be saying too much, for human intelligence can not set bounds to creative power. yet, within the limits of life, such as we know them, it is probably safe to assert that the moon is a dead and deserted world. in other words, if a race of beings resembling ourselves, or resembling any of our contemporaries in terrestrial life, ever existed upon the moon, they must long since have perished. that such beings may have existed, is possible, particularly if it be true, as generally believed, that the moon once had a comparatively dense atmosphere and water upon its surface, which have now, in the process of cooling of the lunar globe, been withdrawn into its interior. it certainly does not detract from the interest with which we study the rugged and beautiful scenery of the moon to reflect that if we could visit those ancient sea-bottoms, or explore those glittering mountains, we might, perchance, find there some remains or mementos of a race that flourished, and perhaps was all gathered again to its fathers, before man appeared upon the earth. that slight physical changes, such as the downfall of mountain-walls or crater-cones, still occasionally occur upon the moon, is an opinion entertained by some selenographers, and apparently justified by observation. the enormous changes of temperature, from burning heat under a cloudless sun to the freezing cold of space at night with no atmospheric blanket to retain heat (which has generally been assumed to be the condition of things on the moon), would naturally exert a disintegrating effect upon the lunar rocks. but the question is now in dispute whether the surface of the moon ever rises above the freezing-point of water, even under a midday sun. mankind has always been a little piqued by the impossibility of seeing the other side of the moon, and all sorts of odd fancies have been indulged in regard to it. among the most curious is the ancient belief that the souls of the good who die on earth are transported to that side of the moon which is turned away from the earth; while the souls of the wicked sojourn on this side, in full view of the scene of their evil deeds. the visible side of the moon--with its tremendous craters, its yawning chasms, its frightful contrasts of burning sunshine and cimmerian darkness, its airless and arid plains and dried-up sea-bottoms exposed to the pitiless cold of open space, and heated, if heated at all, by scorching sunbeams as fierce as naked flame--would certainly appear to be in a proper condition to serve as a purgatory. but we have no reason to think that the other side is any better off in these respects. in fact, the glimpses that we get of it around the corners, so to speak, indicate that the whole round globe of the moon is as ragged, barren, and terrible as that portion of it which is turned to our view. the planets.--in attempting to view the planets with an opera-glass, too much must not be expected; and yet interesting views can sometimes be obtained. the features of their surfaces, of course, can not be detected even with a powerful field-glass, but the difference between the appearance of a large planet and that of the stars will at once strike the observer. mercury, which, on account of its nearness to the sun and its rapid changes of place, comparatively few persons ever see, can perhaps hardly be called an interesting object for an opera-glass, and yet the beauty of the planet is greatly increased when viewed with such aid. mercury is brilliant enough to be readily distinguishable, even while the twilight is still pretty bright; and i have had most charming views of the shy planet, glittering like a globule of shining metal through the fading curtain of a winter sunset. venus is, under favorable circumstances, a very interesting planet for opera-glass observations. the crescent phase can be seen with a powerful glass near inferior conjunction, and, even when the form of the planet can not be discerned, its exceeding brilliancy makes it an attractive object. the flood of light which venus pours forth, and which is so dazzling that it baffles the best telescopes, to a greater or less extent, in any effort to descry the features of that resplendent disk, is evidently reflected from a cloud-burdened atmosphere. while these clouds render the planet surprisingly lustrous to our eyes, they must, of course, keep the globe beneath them most of the time in shadow. it is a source of keen regret that the surface of venus can not be seen as clearly as that of mars, for, _a priori_, there is rather more reason to regard venus as possibly an inhabited world than any other of the earth's sister planets, not excepting mars. still, even if we could plainly make out the presence of oceans and continents on venus, that fact would hardly be any better indication of the possibility of life there than is furnished by the phenomena of its atmosphere. it is an interesting reflection that in admiring the brilliancy of this splendid planet the light that produces so striking an effect upon our eyes has but a few minutes before traversed the atmosphere of a distant world, which, like our own air, may furnish the breath of life to millions of intelligent creatures, and vibrate with the music of tongues speaking languages as expressive as those of the earth. mars, being both more distant and smaller than venus, does not present so splendid a scene, and yet when it is at or near opposition it is a superb object even for an opera-glass, its deep reddish-yellow color presenting a fine contrast to that of most of the stars. it can often be seen in conjunction with, or near to, the moon and stars, and the beauty of these phenomena is in some cases greatly enhanced by the use of a glass. to find mars (and the same remark applies to the other planets), take its right ascension and declination for the required date from the nautical almanac, and then mark its place upon a planisphere or any good star-map. this planet is at the present time ( ) slowly drawing nearer to the earth at each opposition, and in it will be closer to us than at any time since , when its two minute satellites were discovered. it will consequently grow brighter every year until then. how splendidly it shines when at its nearest approach to the earth may be inferred from the fact that in it was so brilliant as actually to cause a panic. this was doubtless owing to its peculiar redness. i well remember the almost startling appearance which the planet presented in the autumn of . mars is especially interesting because of the apparently growing belief that it may be an inhabited world, and because of certain curious markings on its surface that can only be seen under favorable conditions. the recent completion of the great lick telescope and other large glasses, and the approach of the planet to a favorable opposition, give reason to hope that within the next few years a great deal of light will be cast upon some of the enigmatical features of mars's surface. [illustration: jupiter and his moons. (seen with a field-glass; seven diameters.)] jupiter, although much more distant than mars, is ordinarily a far more conspicuous phenomenon in the sky on account of his vast bulk. his interest to observers with an opera-glass depends mainly upon his four moons, which, as they circle about him, present a miniature of the solar system. with a strong opera-glass one or two of jupiter's little family of moons may occasionally be caught sight of as excessively minute dots of light half-hidden in the glare of the planet. if you succeed under favorable circumstances in seeing one of these moons with your glass, you will be all the more astonished to learn that there are several apparently well-authenticated instances of one of the moons of jupiter having been seen with the naked eye. with a field-glass, however, you will have no difficulty in seeing all of the moons when they are properly situated. if you miss one or more of them, you may know that it is either between you and the planet, or behind the planet, or buried in the planet's shadow, or else so close to the planet as to be concealed by its radiance. it will be best for the observer to take out of the nautical almanac the "configurations of jupiter's satellites" for the evenings on which he intends to make his observations, recollecting that the position of the whole system, as there given, is reversed, or presented as seen with an astronomical telescope, which inverts objects looked at, as an opera-glass does not. in order to bring the satellites into the positions in which he will see them, our observer has only to turn the page in the nautical almanac showing their configurations upside down. of course, since the motions of the satellites, particularly of the inner ones, are very rapid, their positions are continually changing, and their configurations are different every night. if the observer has any doubt about his identification of them, or thinks they may be little stars, he has only to carefully note their position and then look at them again the next evening. he may even notice their motion in the course of a single evening, if he begins early and follows them for three or four hours. it is impossible to describe the peculiar attractions of the scene presented by the great planet and his four little moons on a serene evening to an observer armed with a powerful glass. probably much of the impressiveness of the spectacle is owing to the knowledge that those little points of light, shining now in a row and now in a cluster, are actually, at every instant, under the government of their giant neighbor and master, and that as we look upon them, obediently making their circuits about him, never venturing beyond a certain distance away, we behold a type of that gravitational mastery to which our own little planet is subject as it revolves around its still greater ruler, the sun, to whose control even jupiter in his turn must submit. the beautiful planet saturn requires for the observation of its rings magnifying powers far beyond those of the instruments with which our readers are supposed to be armed. it would be well, however, for the observer to trace its slow motion among the stars with the aid of the nautical almanac, and he should be able with a good field-glass to see, under favorable circumstances, the largest of its eight moons, titan. this is equal in brilliancy to an . magnitude star. its position with respect to saturn on any given date can be learned from the ephemeris. it may appear somewhat presumptuous to place uranus, a planet which it required the telescope and the eye of a herschel to discover, in a list of objects for the opera-glass. but it must not be forgotten that uranus was seen certainly several, and probably many, times before herschel's discovery, being simply mistaken, on account of the slowness of its motion, for a fixed star. when near opposition, uranus looks as bright as a sixth-magnitude star, and can be easily detected with the naked eye when its position is known. with an opera-glass (and still more readily with a field-glass) this distant planet can be watched as it moves deliberately onward in its gigantic orbit. its passage by neighboring stars is an exceedingly interesting phenomenon, and it is in this way that you may recognize the planet. on the evening of may , , i knew, from the co-ordinates given in the nautical almanac, that uranus was to be found a short distance east of mars, which was then only a few degrees from the well-known star gamma virginis. accordingly, i turned my opera-glass upon mars, and at once saw a star in the expected position, which i knew was uranus. but there were other small stars in the field, and, supposing i had not been certain which was uranus, how could i have recognized it? the answer is plain: simply by watching for a night or two to see which star moved. that star would, of course, be uranus. the accompanying cuts will show the motions of mars and uranus with respect to neighboring stars at that time, and will serve as an example of the method of distinguishing a planet from the fixed stars by its change of place. in the first cut we have the two planets and three neighboring stars as they appeared on may th. these stars were best seen with a field-glass, although an opera-glass readily showed them. [illustration: mars and uranus, may , .] [illustration: mars and uranus, june , .] [illustration: mars and uranus, june , .] on june st the relative positions of the planets and stars were as shown in the second cut. a glance suffices to show that not only mars but uranus also has shifted its position with respect to the three immovable stars. this change of place alone would have sufficed to indicate the identity of uranus. to make sure, the inexperienced observer had only to continue his observations a few nights longer. on june th mars and uranus were in conjunction, and their position, as well as that of the same set of three stars, is shown in the third cut. it will be seen that while mars had changed its place very much more than uranus, yet that the latter planet had now moved so far from its original position on may th, that there could be no possibility that the merest tyro in star-gazing would fail to notice the change. whenever the observer sees an object which he suspects to be a planet, he can satisfy himself of its identity by making a series of little sketches like the above, showing the position of the suspected object on successive evenings, with respect to neighboring stars. the same plan suffices to identify the larger planets, in the case of which no glass is necessary. the observer can simply make a careful estimate by the naked eye of the supposed planet's distance and bearing from large stars near it, and compare them with similar observations made on subsequent evenings. the sun.--that spots upon the sun may be seen with no greater optical aid than that of an opera-glass is perhaps well known to many of my readers, for during the past ten years public attention has been drawn to sun-spots in an especial manner, on account of their supposed connection with meteorology, and in that time there have been many spots upon the solar disk which could not only be seen with an opera-glass, but even with the unassisted eye. at present ( ) we are near a minimum period of sun-spots, and the number to be seen even with a telescope is comparatively very small, yet only a few days before this page was written there was a spot on the sun large enough to be conspicuous with the aid of a field-glass. during the time of a spot-maximum the sun is occasionally a wonderful object, no matter how small the power of the instrument used in viewing it may be. strings of spots of every variety of shape sometimes extend completely across the disk. our illustration shows the appearance of the sun, as drawn by the author on the st of september, . every one of the spots and spot-groups there represented could be seen with a good field-glass, and nearly all of them with an opera-glass. [illustration: the sun, september , .] as in all such cases, our interest in the phenomena increases in proportion to our understanding of their significance and their true scale of magnitude. in glancing from side to side of the sun's disk, the eye ranges over a distance of more than , miles--not a mere ideal distance, or an expanse of empty space, but a distance filled by an actual and, so to speak, tangible body, whose diameter is of that stupendous magnitude. one sees at a glance, then, the enormous scale on which these spots are formed. the earth placed beside them would be but a speck, and yet they are mere pits in the surface of the sun, filled perhaps with partially cooled metallic vapors, which have been cast up from the interior, and are settling back again. it is worth anybody's while to get a glimpse at a sun-spot if he can, for, although he may see it merely as a black dot on the shining disk, yet it represents the play of physical forces whose might and power are there exercised on a scale really beyond human comprehension. the imagination of milton or dante would have beheld the mouth of hell yawning in a sun-spot. in order to view the sun it is, of course, necessary to contrive some protection for the eyes. this may be constructed by taking two strips of glass four or five inches long and an inch wide, and smoking one of them until you can without discomfort look at the sun through it. then place the two strips together, with the smoked surface inside--taking care to separate them slightly by pieces of cardboard placed between the ends--and fasten the edges together with strips of paper gummed on. then, by means of a rubber band, fasten the dark glass thus prepared over the eye-end of your opera-glass in such a way that both of the lenses are completely covered by it. it will require a little practice to enable you to get the sun into the field of view and keep it there, and for this purpose you should assume a posture--sitting, if possible--which will enable you to hold the glass very steady. then point the glass nearly in the direction of the sun, and move it slowly about until the disk comes in sight. it is best to carefully focus your instrument on some distant object before trying to look at the sun with it. as there is some danger of the shade-glass being cracked by the heat, especially if the object-glasses of the instrument are pretty large, it would be well to get the strips of glass for the shade large enough to cover the object-end of the instrument instead of the eye-end. at a little expense an optician will furnish you with strips of glass of complementary tints, which, when fastened together, give a very pleasing view of the sun without discoloring the disk. dark red with dark blue or green answer very well; but the color must be very deep. the same arrangement, of course, will serve for viewing an eclipse of the sun. a word, finally, about the messenger which brings to us all the knowledge we possess of the contents and marvels of space--light. without the all-pervading luminiferous ether, narrow indeed would be our acquaintance with the physical creation. this is a sympathetic bond by which we may conceive that intelligent creatures throughout the universe are united. light tells us of the existence of suns and systems so remote that the mind shrinks from the attempt to conceive their distance; and light bears back again to them a similar message in the feeble glimmering of our own sun. and can any one believe that there are no eyes out yonder to receive, and no intelligence to interpret that message? sir humphry davy has beautifully expressed a similar thought in one of his philosophical romances: in jupiter you would see creatures similar to those in saturn, but with different powers of locomotion; in mars and venus you would find races of created forms more analogous to those belonging to the earth; but in every part of the planetary system you would find one character peculiar to all intelligent natures, a sense of receiving impressions from light by various organs of vision, and toward this result you can not but perceive that all the arrangements and motions of the planetary bodies, their satellites and atmospheres, are subservient. the spiritual natures, therefore, that pass from system to system in progression toward power and knowledge preserve at least this one invariable character, and their intellectual life may be said to depend more or less upon the influence of light.[g] [g] see "consolations in travel, or, the last days of a philosopher"; dialogue i. light is a result, and an expression, of the energy of cosmical life. the universe lives while light exists. but when the throbbing energies of all the suns are exhausted, and space is filled with universal gloom, the light of intelligence must vanish too. one can not read the wonderful messages of light--one can not study the sun, the moon, and the stars in any manner--without perceiving that the physical universe is enormously greater than he had thought, and that the creation, of which the earth is an infinitesimal part, is almost infinitely more magnificent in actual magnitude than the imaginary domain which men of old times pictured as the dwelling-place of the all-controlling gods; without feeling that he has risen to a higher plane, and that his intellectual life has taken a nobler aim and a broader scope. index. achernar, . albireo ([beta] cygni), . alcor, . alcyone, . mädler's "central sun," . aldebaran, , , , , , . algenib ([alpha] persei), , . algol, the demon-star, . probable cause of variation of, . al-mamoun, the caliph, observation of a temporary star, . almaach ([gamma] andromedæ), , . alphard, . alpha andromedæ, . agnarii (sadalmelik), . arietis (hamal), . capricorni (giedi), . ceti (menkar), . draconis, formerly the pole-star, . libræ, . ophiuchi (ras alhague), . orionis (betelgeuse), , , . pegasi (markab), . ursæ majoris, . alpheratz ([alpha] andromedæ), . alps, the lunar, . altai mountains, . altair, . andromedæ, map of, . mythology of, . antares, , , . antinous, . apennines, the lunar, . apollonius, regarded the moon as a mirror, . aquarius, map of, . mythology of, . aquila, map of, . mythology of, . aratus, description of the manger, . the "diosemia" of, . the phenomena of, . story of virgo, . description of the "royal family," . description of cetus, . arcturus, , , , , . argo, map of, . mythology of, . aries, map of, . mythology of, . ariosto, story of a trip to the moon, . aristarchus, the shining mountain, . aselli, . asterope, . atlas, . auriga, map of, . mythology of, . star swarms in, . autumn, map of the stars of, . bartschius invents monoceros, . bay of dew, . bay of rainbows, . bear's head, stars forming the, . bellatrix, , . belt, orion's, , . berenice's hair, the constellation of, . picture of, . bessel, studies of sirius and procyon, . letter about "dark stars," . beta andromedæ (mirach), . arietis (sheratan), . capricorni (dabih), . cassiopeia, . beta corvi, . cygni (albireo), . libræ, . leonis (denebola), . lyræ, . pegasi, . scorpionis, . ursæ minoris (kochab), . betelgeuse ([alpha] orionis), , , . bethlehem, the so-called star of, . biela's comet, it breaks up, . biela meteors, radiant point of the, . boötes, map of, . mythology of, . calisto, another name of ursa major, . cancer, map of, . mythology of, . canes venatici, . canis major, map of, . mythology of, . canis minor, map of, . mythology of, . canopus, . capella, , , , , . cape heraclides, . laplace, . capricornus, map of, . mythology of, . cassiopeia, map of, . mythology of, . castor, . catharina, . caucasus mountains, . celæno, . central gulf, . "central sun," mädler's ideas about a, . cepheus, map of, , . cetus, map of, . mythology of, . chi ceti, . clavius, , , . coal-sack, . comet, biela's, . comet, halley's, the crab nebula mistaken for, . constellations, origin of, , , . along the milky-way, . the zodiacal, . constellations, st. paul's knowledge of, . copernicus, . corvus, map of, . mythology of, . "crimson star," . crisian sea, . cynosura, a name of ursa minor, . cygnus, map of, . cyrillus, . dabih ([beta] capricorni), . dark stars, bessel's suggestion about, . davy, humphry, on life in other worlds, . delta canis majoris, . cephei, . tauri, . deltoton, . denebola ([beta] leonis), , , . dipper, the great, , . dog-days, origin of the, . dog-star, . dolphin, map of the, . mythology of the, . draco, map of, . mythology of, . el nath, , . epsilon leonis, . lyræ, . tauri, . virginis, . equinox, autumnal, . vernal, . eridanus, map of, . eta aquilæ, . field-glass, . field of the nebulæ, . flammarion, on [alpha] capricorni, . flood traditions connected with the pleiades, , . focus, importance of a sharp, . fomalhaut, . fontenelle, "plurality of worlds," . galileo, his telescope an opera-glass, . his description of præsepe, . his description of the moon, . power of his telescope, . gamma andromedæ, , . leonis, . pegasi, . tauri, . virginis, . "garnet star" (mu cephei), . gemini, map of, . mythology of, . genesis, a celestial, . giedi ([alpha] capricorni), . glass, use of smoked or colored, , . goldschmidt sees a nebula in the pleiades, . gomelza, . gore, estimate of the stars in m, . "grape-gatherer" ([epsilon] virginis), . grensted, rev. mr., suggestion about lunar rays, . grimaldi, . halley's comet and crab nebula, . hamal ([alpha] arietis), . hæmus mountains, . henry, paul and prosper, photographs of the pleiades, . hercules, map of, . mythology of, . motion of solar system toward, . herschel, william, discovers uranus, . computation of stars in m, . advice about seeing star-colors, . thinks he sees lunar volcano, . john, description of m, . suggestion about [alpha] capricorni, . holden, prof., on the milky-way, . structure of ring nebula, . hooke, discovers first telescopic double star, . hyades, , , , . hydra, map of part of, . mythology of, . hydra's heart (alphard), . humboldt sea, . jeaurat, chart of the pleiades, . job's coffin, . jupiter, . satellites of, . kappa argus, . tauri, . kepler observes the star of , . kingsley, story of andromeda, . "king's lucky star," . kochab (beta ursæ minoris), . lake of death, . of dreams, . land of drought, . of hoar frost, . leo, map of, . mythology of, . sickle-shaped figure in, , . lepus, map of, . lick telescope, views of milky-way, . views of ring nebula, . light, the messenger of the universe, . in a star-cluster, . libra, description and mythology of, . life, does it exist beyond the earth? , , , , , . locke, richard adams, author of the "moon hoax," . lyra, map of, . mythology of, . mädler, on the "central sun," . maginus, . maia, , . man in the moon, . manger (præsepe), . marine glass, . markab ([alpha] pegasi), . marsh of mists, . of putrefaction, . of sleep, . mars, . medusa, the head of, . menelaus, . menkalina, . menkar ([alpha] ceti), . mercury, . merope, , . mesarthim, . meteors, radiant point of november, . radiant point of biela, . micromegas, the story of, . milk-dipper, . milky-way, , , , , , , , , . mira ([omicron] ceti), . probable cause of its variations, . milton, account of libra, . mirach ([beta] andromedæ), . mizar, . moon, mountains of the, . shadows on the, . map of the, . list of mountains, "seas," etc., . inhabitableness of the, . the other side of the, . "moon hoax," , . monoceros, map of, . mu argus, . scorpionis, . nebulæ (and star-clusters): m, . m, . m, . m, . m, . m, . m, . m, . m, . m, . m, . m, . m, . m, . m, . m, . m, . ^ , . ^ , . ^ , . andromeda, great nebula in, , . aquarius, nebula in, . crab nebula, , . field of the nebulæ, . horseshoe nebula, . orion, great nebula in, . perseus, great cluster in, . pleiades, nebulæ in the, . ring nebula in lyra, . nebular hypothesis, . neison, description of sunrise on clavius, . newton, . "nile-star," . northern cross, , . northern crown, map of the, . northern fish, , . nu andromedæ, , . aquarii, a pointer to a nebula, . canis majoris, . draconis, . scorpionis, . ocean of storms, . omicron ceti (mira), , . cygni, . omicron two eridani, a flying-star, . opera-glass, views of the stars with, . how to choose a good, . magnifying power of, . defects of, . ophiuchus and serpens, map of, . mythology of, . orion, map of, . mythology of, . great array of stars around, . riches of, . spectacle of the rising of, . orpheus, fancies about the moon, . pegasus, map of, . mythology of, . perseus, map of, . mythology of, . great cluster in, . phantom, another name of hercules, . photography, astronomical, , . pi argus, . five orionis, . pegasi, . pisces, map of, . mythology of, . piscis australis, . plato, . pleiades, , , , . names of the, . mythology of, . and the flood, , . and the great pyramid, . picture of the, . common motion of the, . pleione, , . pole-star, , . pollux, . præsepe (the manger), . prime meridian, . proclus, . procyon, , . pyramid of cheops and the pleiades, . pyrenees mountains, . ras alhague ([alpha] ophiuchi), . rays of the moon, . regulus, , . revolution of the heavens, , . rho ophiuchi, . rigel, , , , . ring nebula, . "royal family," , . rutherford, photograph of the moon, . sadalmelik ([alpha] aquarii), . sagitta, map of, . sagittarius, map of, . mythology of, . saiph, . saturn, . scorpio, map of, . mythology of, . pair of stars in sting of, . schickhard, . sea of clouds, . sea of cold, . sea of fertility, . sea of humors, . sea of nectar, . sea of serenity, . sea of showers, . sea of tranquillity, . sea of vapors, . secchi, father, types of the stars, . description of a star-swarm, . seiss, rev. dr., on canis minor, . description of auriga, . sheratan ([beta] arietis), . sidus ludovicianum, . sirius, , , . color of, . size and distance of, . the companion of, , . its light compared with the sun's, . sigma tauri, . sixty-one cygni, . smyth, admiral, on capricorn, . description of aldebaran, . description of m, . solstice, summer, , . winter, . sobieski's shield, . solar system, voyaging of, in space, . southern cross, , . south sea, . spectroscopic analysis, , . spica, , , , . spring, map of the stars of, . square of pegasus, . st. paul, acquainted with the constellations, . star-clusters (see nebulæ, etc.). star-cluster, light in a, . summer, map of the stars of, . sun, opera-glass observations of the, . the, a variable star, . sword of orion, . taurus, map of, . mythology of, . the "golden horns" of, . poniatowskii, . tau aquarii, . taygeta, . temporary stars: b. c. the first on record, . a. d., . , . , . , tycho's star, . , . , , . , , . , . temple, discovers a nebula in the pleiades, . tennyson, describes the pleiades, . theophilus, . theta orionis, . serpentis, . tauri, . tobias mayer, sees the planet neptune, . triangles, map of the, . mythology of, . twenty-two canis majoris, . scorpii, . tycho brahe, invents antinous, . places hamal in aries, . studies the star of , . tycho, , . upsilon tauri, . uranus, discovery of, . how to find, . ursa major, map of, . mythology of, . stars in the feet of, . ursa minor, map of, . mythology of, . vega, . venus, mistaken for artificial light, . opera-glass observation of, . virgil, description of taurus, . virgo, map of, . mythology of, . vision, seeing with averted, . voltaire, story of "micromegas," . vulpecula, map of, . webb, rev. t. w., on telescopes, . on m, . western fish, . winter, brilliancy of the heavens in, . map of the stars of, . woman in the moon, . zeta corvi, . cassiopeia, . leonis, . lyræ, . scorpionis, . tauri, a pointer to the crab nebula, . zi argus, . zodiac, . zodiac, divided among the twelve apostles, . of dendera, . zöllner, estimate of sirius's light, . the end. note: project gutenberg also has an html version of this file which includes the original illustrations. see -h.htm or -h.zip: (http://www.gutenberg.net/dirs/ / / / / / -h/ -h.htm) or (http://www.gutenberg.net/dirs/ / / / / / -h.zip) other worlds by garrett p. serviss. * * * * * * other worlds. their nature and possibilities in the light of the latest discoveries. illustrated. mo. cloth, $ . net; postage additional. no science has ever equaled astronomy in its appeal to the imagination, and recently popular interest in the wonders of the starry heavens has been stimulated by surprising discoveries and imaginary discoveries, as well as by a marked tendency of writers of fiction to include other worlds and their possible inhabitants within the field of romance. mr. serviss's new book on "other worlds, their nature and possibilities in the light of the latest discoveries," summarizes what is known. with helpful illustrations, the most interesting facts about the planets venus, mars, jupiter, saturn, etc., as well as about the nearest of all other worlds, the moon, are presented in a popular manner, and always from the point of view of human interest--a point that is too seldom taken by writers on science. astronomy with an opera-glass. a popular introduction to the study of the starry heavens with the simplest of optical instruments. illustrated. vo. cloth, $ . . "by its aid thousands of people who have resigned themselves to the ignorance in which they were left at school, by our wretched system of teaching by the book only, will thank mr. serviss for the suggestions he has so well carried out."--_new york times._ pleasures of the telescope. a descriptive guide to amateur astronomers and all lovers of the stars. illustrated. vo. cloth, $ . . "the volume will be found interesting by those for whom it is written, and will inspire many with a love for the study of astronomy, one of the most far-reaching of the sciences."--_milwaukee journal._ d. appleton and company, new york. * * * * * * [illustration: chart of mars. after schiaparelli.] other worlds their nature, possibilities and habitability in the light of the latest discoveries. by garrett p. serviss author of "astronomy with an opera-glass" and "pleasures of the telescope" with charts and illustrations "shall we measure the councils of heaven by the narrow impotence of human faculties, or conceive that silence and solitude reign throughout the mighty empire of nature?" --dr. thomas chalmers. new york d. appleton and company copyright, , by d. appleton and company. to the memory of william jay youmans. preface the point of view of this book is human interest in the other worlds around us. it presents the latest discoveries among the planets of the solar system, and shows their bearing upon the question of life in those planets. it points out the resemblances and the differences between the earth and the other worlds that share with it in the light of the sun. it shows what we should see and experience if we could visit those worlds. while basing itself upon facts, it does not exclude the discussion of interesting probabilities and theories that have commanded wide popular attention. it points out, for instance, what is to be thought of the idea of interplanetary communication. it indicates what must be the outlook of the possible inhabitants of some of the other planets toward the earth. as far as may be, it traces the origin and development of the other worlds of our system, and presents a graphic picture of their present condition as individuals, and of their wonderful contrasts as members of a common family. in short, the aim of the author has been to show how wide, and how rich, is the field of interest opened to the human mind by man's discoveries concerning worlds, which, though inaccessible to him in a physical sense, offer intellectual conquests of the noblest description. and, finally, in order to assist those who may wish to recognize for themselves these other worlds in the sky, this book presents a special series of charts to illustrate a method of finding the planets which requires no observatory and no instruments, and only such knowledge of the starry heavens as anybody can easily acquire. g.p.s. borough of brooklyn, new york city, _september, ._ contents chapter i _introductory_ remarkable popular interest in questions concerning other worlds and their inhabitants--theories of interplanetary communication--the plurality of worlds in literature--romances of foreign planets--scientific interest in the subject--opposing views based on telescopic and spectroscopic revelations--changes of opinion--desirability of a popular presentation of the latest facts--the natural tendency to regard other planets as habitable--some of the conditions and limitations of the problem--the solar system viewed from outer space--the resemblances and contrasts of its various planets--three planetary groups recognized--the family character of the solar system chapter ii _mercury, a world of two faces and many contrasts_ grotesqueness of mercury considered as a world--its dimensions, mass, and movements--the question of an atmosphere--mercury's visibility from the earth--its eccentric orbit, and rapid changes of distance from the sun--momentous consequences of these peculiarities--a virtual fall of fourteen million miles toward the sun in six weeks--the tremendous heat poured upon mercury and its great variations--the little planet's singular manner of rotation on its axis--schiaparelli's astonishing discovery--a day side and a night side--interesting effects of libration--the heavens as viewed from mercury--can it support life? chapter iii _venus, the twin of the earth_ a planet that matches ours in size--its beauty in the sky--remarkable circularity of its orbit--probable absence of seasons and stable conditions of temperature and weather on venus--its dense and abundant atmosphere--seeing the atmosphere of venus from the earth--is the real face of the planet hidden under an atmospheric veil?--conditions of habitability--all planetary life need not be of the terrestrial type--the limit fixed by destructive temperature--importance of air and water in the problem--reasons why venus may be a more agreeable abode than the earth--splendor of our globe as seen from venus--what astronomers on venus might learn about the earth--a serious question raised--does venus, like mercury, rotate but once in the course of a revolution about the sun?--reasons for and against that view chapter iv _mars, a world more advanced than ours_ resemblances between mars and the earth--its seasons and its white polar caps--peculiar surface markings--schiaparelli's discovery of the canals--his description of their appearance and of their duplication--influence of the seasons on the aspect of the canals--what are the canals?--mr. lowell's observations--the theory of irrigation--how the inhabitants of mars are supposed to have taken advantage of the annual accession of water supplied by the melting of the polar caps--wonderful details shown in charts of mars--curious effects that may follow from the small force of gravity on mars--imaginary giants--reasons for thinking that mars may be, in an evolutionary sense, older than the earth--speculations about interplanetary signals from mars, and their origin--mars's atmosphere--the question of water--the problem of temperature--eccentricities of mars's moons chapter v _the asteroids, a family of dwarf worlds_ only four asteroids large enough to be measured--remarkable differences in their brightness irrespective of size--their widely scattered and intermixed orbits--eccentric orbit of eros--the nearest celestial body to the earth except the moon--its existence recorded by photography before it was discovered--its great and rapid fluctuations in light, and the curious hypotheses based upon them--is it a fragment of an exploded planet?--the startling theory of olbers as to the origin of the asteroids revived--curious results of the slight force of gravity on an asteroid--an imaginary visit to a world only twelve miles in diameter chapter vi _jupiter, the greatest of known worlds_ jupiter compared with our globe--his swift rotation on his axis--remarkable lack of density--the force of gravity on jupiter--wonderful clouds--strange phenomena of the great belts--brilliant display of colors--the great red spot and the many theories it has given rise to--curious facts about the varying rates of rotation of the huge planet's surface--the theory of a hidden world in jupiter--when jupiter was a companion star to the sun--the miracle of world-making before our eyes--are jupiter's satellites habitable?--magnificent spectacles in the jovian system chapter vii _saturn, a prodigy among planets_ the wonder of the great rings--saturn's great distance and long year--the least dense of all the planets--it would float in water--what kind of a world is it?--sir humphry davy's imaginary inhabitants of saturn--facts about the rings, which are a phenomenon unparalleled in the visible universe--the surprising nature of the rings, as revealed by mathematics and the spectroscope--the question of their origin and ultimate fate--dr. dick's idea of their habitability--swedenborg's curious description of the appearance of the rings from saturn--is saturn a globe of vapor, or of dust?--the nine satellites and "roche's limit"--the play of spectacular shadows in the saturnian system--uranus and neptune--is there a yet undiscovered planet greater than jupiter? chapter viii _the moon, child of the earth and the sun_ the moon a favorite subject for intellectual speculation--its nearness to the earth graphically illustrated--ideas of the ancients--galileo's discoveries--what first raised a serious question as to its habitability--singularity of the moon's motions--appearance of its surface to the naked eye and with the telescope--the "seas" and the wonderful mountains and craters--a terrible abyss described--tycho's mysterious rays--difference between lunar and terrestrial volcanoes--mountain-ringed valleys--gigantic cracks in the lunar globe--slight force of gravity of the moon and some interesting deductions--the moon a world of giantism--what kind of atmospheric gases can the moon contain--the question of water and of former oceans--the great volcanic cataclysm in the moon's history--evidence of volcanic and other changes now occurring--is there vegetation on the moon?--lunar day and night--the earth as seen from the moon--discoveries yet to be made chapter ix _how to find the planets_ it is easy to make acquaintance with the planets and to follow them among the stars--the first step a knowledge of the constellations--how this is to be acquired--how to use the nautical almanac in connection with the charts in this book--the visibility of mercury and venus--the oppositions of mars, jupiter, and saturn index list of illustrations page chart of mars _frontispiece_ diagram showing causes of day and night on portions of mercury regions of day and night on mercury venus's atmosphere seen as a ring of light view of jupiter _facing_ three views of saturn _facing_ diagram showing the moon's path through space the lunar alps, apennines, and caucasus _facing_ the moon at first and last quarter _facing_ phases and rotation of the moon charts showing the zodiacal constellations: . from right ascension hours to hours . " " " " " . " " " " " . " " " " " . " " " " " . " " " " " other worlds chapter i introductory other worlds and their inhabitants are remarkably popular subjects of speculation at the present time. every day we hear people asking one another if it is true that we shall soon be able to communicate with some of the far-off globes, such as mars, that circle in company with our earth about the sun. one of the masters of practical electrical science in our time has suggested that the principle of wireless telegraphy may be extended to the transmission of messages across space from planet to planet. the existence of intelligent inhabitants in some of the other planets has become, with many, a matter of conviction, and for everybody it presents a question of fascinating interest, which has deeply stirred the popular imagination. the importance of this subject as an intellectual phenomenon of the opening century is clearly indicated by the extent to which it has entered into recent literature. poets feel its inspiration, and novelists and romancers freely select other planets as the scenes of their stories. one tells us of a visit paid by men to the moon, and of the wonderful things seen, and adventures had, there. lucian, it is true, did the same thing eighteen hundred years ago, but he had not the aid of hints from modern science to guide his speculations and lend verisimilitude to his narrative. another startles us from our sense of planetary security with a realistic account of the invasion of the earth by the terrible sons of warlike mars, seeking to extend their empire by the conquest of foreign globes. sometimes it is a trip from world to world, a kind of celestial pleasure yachting, with depictions of creatures more wonderful than-- "the anthropophagi and men whose heads do grow beneath their shoulders"-- that is presented to our imagination; and sometimes we are informed of the visions beheld by the temporarily disembodied spirits of trance mediums, or other modern thaumaturgists, flitting about among the planets. then, to vary the theme, we find charming inhabitants of other worlds represented as coming down to the earth and sojourning for a time on our dull planet, to the delight of susceptible successors of father adam, who become, henceforth, ready to follow their captivating visitors to the ends of the universe. in short, writers of fiction have already established interplanetary communication to their entire satisfaction, thus vastly and indefinitely enlarging the bounds of romance, and making us so familiar with the peculiarities of our remarkable brothers and sisters of mars, venus, and the moon, that we can not help feeling, notwithstanding the many divergences in the descriptions, that we should certainly recognize them on sight wherever we might meet them. but the subject is by no means abandoned to the tellers of tales and the dreamers of dreams. men of science, also, eagerly enter into the discussion of the possibilities of other worlds, and become warm over it. around mars, in particular, a lively war of opinions rages. not all astronomers have joined in the dispute--some have not imagination enough, and some are waiting for more light before choosing sides--but those who have entered the arena are divided between two opposed camps. one side holds that mars is not only a world capable of having inhabitants, but that it actually has them, and that they have given visual proof of their existence and their intelligence through the changes they have produced upon its surface. the other side maintains that mars is neither inhabited nor habitable, and that what are taken for vast public works and engineering marvels wrought by its industrious inhabitants, are nothing but illusions of the telescope, or delusions of the observer's mind. both adduce numerous observations, telescopic and spectroscopic, and many arguments, scientific and theoretic, to support their respective contentions, but neither side has yet been able to convince or silence the other, although both have made themselves and their views intensely interesting to the world at large, which would very much like to know what the truth really is. and not only mars, but venus--the beauteous twin sister of the earth, who, when she glows in the evening sky, makes everybody a lover of the stars--and even mercury, the moor among the planets, wearing "the shadowed livery of the burnished sun," to whom he is "a neighbor and near bred," and jupiter, saturn, and the moon itself--all these have their advocates, who refuse to believe that they are lifeless globes, mere reflectors of useless sunshine. the case of the moon is, in this respect, especially interesting, on account of the change that has occurred in the opinions held concerning its physical condition. for a very long time our satellite was confidently, and almost universally, regarded as an airless, waterless, lifeless desert, a completely "dead world," a bare, desiccated skull of rock, circling about the living earth. but within a few years there has been a reaction from this extreme view of the lifelessness of the moon. observers tell us of clouds suddenly appearing and then melting to invisibility over volcanic craters; of evidences of an atmosphere, rare as compared with ours, yet manifest in its effects; of variations of color witnessed in certain places as the sunlight drifts over them at changing angles of incidence; of what seem to be immense fields of vegetation covering level ground, and of appearances indicating the existence of clouds of ice crystals and deposits of snow among the mountainous lunar landscapes. thus, in a manner, the moon is rehabilitated, and we are invited to regard its silvery beams not as the reflections of the surface of a desert, but as sent back to our eyes from the face of a world that yet has some slight remnants of life to brighten it. the suggestion that there is an atmosphere lying close upon the shell of the lunar globe, filling the deep cavities that pit its face and penetrating to an unknown depth in its interior, recalls a speculation of the ingenious and entertaining fontenelle, in the seventeenth century--recently revived and enlarged upon by the author of one of our modern romances of adventure in the moon--to the effect that the lunar inhabitants dwell beneath the surface of their globe instead of on the top of it. now, because of this widespread and continually increasing interest in the subject of other worlds, and on account of the many curious revelations that we owe to modern telescopes and other improved means of investigation, it is certainly to be desired that the most important and interesting discoveries that have lately been made concerning the various globes which together with the earth constitute the sun's family, should be assembled in a convenient and popular form--and that is the object of this book. fact is admittedly often stranger and more wonderful than fiction, and there are no facts that appeal more powerfully to the imagination than do those of astronomy. technical books on astronomy usually either ignore the subject of the habitability of the planets, or dismiss it with scarcely any recognition of the overpowering human interest that it possesses. hence, a book written specially from the point of view of that subject would appear calculated to meet a popular want; and this the more, because, since mr. proctor wrote his other worlds than ours and m. flammarion his pluralité des mondes habités, many most important and significant discoveries have been made that, in several notable instances, have completely altered the aspect in which the planets present themselves for our judgment as to their conditions of habitability. no doubt the natural tendency of the mind is to regard all the planets as habitable worlds, for there seems to be deeply implanted in human nature a consciousness of the universality of life, giving rise to a conviction that one world, even in the material sense, is not enough for it, but that every planet must belong to its kingdom. we are apt to say to ourselves: "the earth is one of a number of planets, all similarly circumstanced; the earth is inhabited, why should not the others also be inhabited?" what has been learned of the unity in chemical constitution and mechanical operation prevailing throughout the solar system, together with the continually accumulating evidence of the common origin of its various members, and the identity of the evolutionary processes that have brought them into being, all tends to strengthen the _a priori_ hypothesis that life is a phenomenon general to the entire system, and only absent where its essential and fundamental conditions, for special and local, and perhaps temporary, reasons, do not exist. if we look for life in the sun, for instance, while accepting the prevalent conception of the sun as a center of intense thermal action, we must abandon all our ideas of the physical organization of life formed upon what we know of it from experimental evidence. we can not imagine any form of life that has ever been presented to our senses as existing in the sun. but this is not generally true of the planets. life, in our sense of it, is a planetary, not a solar, phenomenon, and while we may find reasons for believing that on some of the planets the conditions are such that creatures organized like ourselves could not survive, yet we can not positively say that every form of living organism must necessarily be excluded from a world whose environment would be unsuited for us and our contemporaries in terrestrial life. although our sole knowledge of animated nature is confined to what we learn by experience on the earth, yet it is a most entertaining, and by no means unedifying, occupation, to seek to apply to the exceedingly diversified conditions prevailing in the other planets, as astronomical observations reveal them to us, the principles, types, and limitations that govern the living creatures of our world, and to judge, as best we can, how far those types and limits may be modified or extended so that those other planets may reasonably be included among the probable abodes of life. in order to form such judgments each planet must be examined by itself, but first it is desirable to glance at the planetary system as a whole. to do this we may throw off, in imagination, the dominance of the sun, and suppose ourselves to be in the midst of open space, far removed both from the sun and the other stars. in this situation it is only by chance, or through foreknowledge, that we can distinguish our sun at all, for it is lost among the stars; and when we discover it we find that it is only one of the smaller and less conspicuous members of the sparkling host. we rapidly approach, and when we have arrived within a distance comparable with that of its planets, we see that the sun has increased in apparent magnitude, until now it enormously outshines all the other stars, and its rays begin to produce the effect of daylight upon the orbs that they reach. but we are in no danger of mistaking its apparent superiority to its fellow stars for a real one, because we clearly perceive that our nearness alone makes it seem so great and overpowering. and now we observe that this star that we have drawn near to has attending it a number of minute satellites, faintly shining specks, that circle about it as if charmed, like night-wandering insects, by its splendor. it is manifest to us at the first glance that without the sun these obedient little planets would not exist; it is his attraction that binds them together in a system, and his rays that make them visible to one another in the abyss of space. although they vary in relative size, yet we observe a striking similarity among them. they are all globular bodies, they all turn upon their axes, they all travel about the sun in the same direction, and their paths all lie very nearly in one plane. some of them have one or more moons, or satellites, circling about them in imitation of their own revolution about the sun. their family relationship to one another and to the sun is so evident that it colors our judgment about them as individuals; and when we happen to find, upon closer approach, that one of them, the earth, is covered with vegetation and water and filled with thousands of species of animated creatures, we are disposed to believe, without further examination, that they are all alike in this respect, just as they are all alike in receiving light and heat from the sun. this preliminary judgment, arising from the evident unity of the planetary system, can only be varied by an examination of its members in detail. one striking fact that commands our attention as soon as we have entered the narrow precincts of the solar system is the isolation of the sun and its attendants in space. the solar system occupies a disk-shaped, or flat circular, expanse, about , , , miles across and relatively very thin, the sun being in the center. from the sun to the nearest star, or other sun, the distance is approximately five thousand times the entire diameter of the solar system. but the vast majority of the stars are probably a hundred times yet more remote. in other words, if the solar system be represented by a circular flower-bed ten feet across, the nearest star must be placed at a distance of nine and a half miles, and the great multitude of the stars at a distance of nine hundred miles! or, to put it in another way, let us suppose the sun and his planets to be represented by a fleet of ships at sea, all included within a space about half a mile across; then, in order that there might be no shore relatively nearer than the nearest fixed star is to the sun, we should have to place our fleet in the middle of the pacific ocean, while the distance of the main shore of the starry universe would be so immense that the whole surface of the earth would be far too small to hold the expanse of ocean needed to represent it! from these general considerations we next proceed to recall some of the details of the system of worlds amid which we dwell. besides the earth, the sun has seven other principal planets in attendance. these eight planets fall into two classes--the terrestrial planets and the major, or jovian, planets. the former class comprises mercury, venus, the earth, and mars, and the latter jupiter, saturn, uranus, and neptune. i have named them all in the order of their distance from the sun, beginning with the nearest. the terrestrial planets, taking their class name from _terra_, the earth, are relatively close to the sun and comparatively small. the major planets--or the jovian planets, if we give them a common title based upon the name of their chief, jupiter or jove--are relatively distant from the sun and are characterized both by great comparative size and slight mean density. the terrestrial planets are all included within a circle, having the sun for a center, about , , miles in radius. the space, or gap, between the outermost of them, mars, and the innermost of the jovian planets, jupiter, is nearly two and a half times as broad as the entire radius of the circle within which they are included. and not only is the jovian group of planets widely separated from the terrestrial group, but the distances between the orbits of its four members are likewise very great and progressively increasing. between jupiter and saturn is a gap , , miles across, and this becomes , , miles between saturn and uranus, and more than , , , miles between uranus and neptune. all of these distances are given in round numbers. finally, we come to some very extraordinary worlds--if we can call them worlds at all--the asteroids. they form a third group, characterized by the extreme smallness of its individual members, their astonishing number, and the unusual eccentricities and inclinations of their orbits. they are situated in the gap between the terrestrial and the jovian planets, and about of them have been discovered, while there is reason to think that their real number may be many thousands. the largest of them is less than miles in diameter, and many of those recently discovered may be not more than ten or twenty miles in diameter. what marvelous places of abode such little planets would be if it were possible to believe them inhabited, we shall see more clearly when we come to consider them in their turn. but without regard to the question of habitability, the asteroids will be found extremely interesting. in the next chapter we proceed to take up the planets for study as individuals, beginning with mercury, the one nearest the sun. chapter ii mercury, a world of two faces and many contrasts mercury, the first of the other worlds that we are going to consider, fascinates by its grotesqueness, like a piece of chinese ivory carving, so small is it for its kind and so finished in its eccentric details. in a little while we shall see how singular mercury is in many of the particulars of planetary existence, but first of all let us endeavor to obtain a clear idea of the actual size and mass of this strange little planet. compared with the earth it is so diminutive that it looks as if it had been cut out on the pattern of a satellite rather than that of an independent planet. its diameter, , miles, only exceeds the moon's by less than one half, while both jupiter and saturn, among their remarkable collections of moons, have each at least one that is considerably larger than the planet mercury. but, insignificant though it be in size, it holds the place of honor, nearest to the sun. it was formerly thought that mercury possessed a mass greatly in excess of that which its size would seem to imply, and some estimates, based upon the apparent effect of its attraction on comets, made it equal in mean density to lead, or even to the metal mercury. this led to curious speculations concerning its probable metallic composition, and the possible existence of vast quantities of such heavy elements as gold in the frame of the planet. but more recent, and probably more correct, computations place mercury third in the order of density among the members of the solar system, the earth ranking as first and venus as second. mercury's density is now believed to be less than the earth's in the ratio of to . accepting this estimate, we find that the force of gravity upon the surface of mercury is only one third as great as upon the surface of the earth--i.e., a body weighing pounds on the earth would weigh only pounds on mercury. this is an important matter, because not only the weight of bodies, but the density of the atmosphere and even the nature of its gaseous constituents, are affected by the force of gravity, and if we could journey from world to world, in our bodily form, it would make a great difference to us to find gravity considerably greater or less upon other planets than it is upon our own. this alone might suffice to render some of the planets impossible places of abode for us, unless a decided change were effected in our present physical organization. one of the first questions that we should ask about a foreign world to which we proposed to pay a visit, would relate to its atmosphere. we should wish to know in advance if it had air and water, and in what proportions and quantities. however its own peculiar inhabitants might be supposed able to dispense with these things, to _us_ their presence would be essential, and if we did not find them, even a planet that blazed with gold and diamonds only waiting to be seized would remain perfectly safe from our invasion. now, in the case of mercury, some doubt on this point exists. messrs. huggins, vogel, and others have believed that they found spectroscopic proof of the existence of both air and the vapor of water on mercury. but the necessary observations are of a very delicate nature, and difficult to make, and some astronomers doubt whether we possess sufficient proof that mercury has an atmosphere. at any rate, its atmosphere is very rare as compared with the earth's, but we need not, on that account, conclude that mercury is lifeless. possibly, in view of certain other peculiarities soon to be explained, a rare atmosphere would be decidedly advantageous. being much nearer the sun than the earth is, mercury can be seen by us only in the same quarter of the sky where the sun itself appears. as it revolves in its orbit about the sun it is visible, alternately, in the evening for a short time after sunset and in the morning for a short time before sunrise, but it can never be seen, as the outer planets are seen, in the mid-heaven or late at night. when seen low in the twilight, at evening or morning, it glows with the brilliance of a bright first-magnitude star, and is a beautiful object, though few casual watchers of the stars ever catch sight of it. when it is nearest the earth and is about to pass between the earth and the sun, it temporarily disappears in the glare of the sunlight; and likewise, when it it is farthest from the earth and passing around in its orbit on the opposite side of the sun, it is concealed by the blinding solar rays. consequently, except with the instruments of an observatory, which are able to show it in broad day, mercury is never visible save during the comparatively brief periods of time when it is near its greatest apparent distance east or west from the sun. the nearer a planet is to the sun the more rapidly it is compelled to move in its orbit, and mercury, being the nearest to the sun of all the planets, is by far the swiftest footed among them. but its velocity is subject to remarkable variation, owing to the peculiar form of the orbit in which the planet travels. this is more eccentric than the orbit of any other planet, except some of the asteroids. the sun being situated in one focus of the elliptical orbit, when mercury is at perihelion, or nearest to the sun, its distance from that body is , , miles, but when it is at aphelion, or farthest from the sun, its distance is , , miles. the difference is no less than , , miles! when nearest the sun mercury darts forward in its orbit at the rate of twenty-nine miles in a second, while when farthest from the sun the speed is reduced to twenty-three miles. now, let us return for a moment to the consideration of the wonderful variations in mercury's distance from the sun, for we shall find that their effects are absolutely startling, and that they alone suffice to mark a wide difference between mercury and the earth, considered as the abodes of sentient creatures. the total change of distance amounts, as already remarked, to , , miles, which is almost half the entire distance separating the planet from the sun at perihelion. this immense variation of distance is emphasized by the rapidity with which it takes place. mercury's periodic time, i.e., the period required for it to make a single revolution about the sun--or, in other words, the length of its year--is eighty-eight of our days. in just one half of that time, or in about six weeks, it passes from aphelion to perihelion; that is to say, in six weeks the whole change in its distance from the sun takes place. in six weeks mercury falls , , miles--for it _is_ a fall, though in a curve instead of a straight line--falls , , miles toward the sun! and, as it falls, like any other falling body it gains in speed, until, having reached the perihelion point, its terrific velocity counteracts its approach and it begins to recede. at the end of the next six weeks it once more attains its greatest distance, and turns again to plunge sunward. of course it may be said of every planet having an elliptical orbit that between aphelion and perihelion it is falling toward the sun, but no other planet than mercury travels in an orbit sufficiently eccentric, and approaches sufficiently near to the sun, to give to the mind so vivid an impression of an actual, stupendous fall! next let us consider the effects of this rapid fall, or approach, toward the sun, which is so foreign to our terrestrial experience, and so appalling to the imagination. first, we must remember that the nearer a planet is to the sun the greater is the amount of heat and light that it receives, the variation being proportional to the inverse square of the distance. the earth's distance from the sun being , , miles, while mercury's is only , , , it follows, to begin with, that mercury gets, on the average, more than six and a half times as much heat from the sun as the earth does. that alone is enough to make it seem impossible that mercury can be the home of living forms resembling those of the earth, for imagine the heat of the sun in the middle of a summer's day increased six or seven fold! if there were no mitigating influences, the face of the earth would shrivel as in the blast of a furnace, the very stones would become incandescent, and the oceans would turn into steam. still, notwithstanding the tremendous heat poured upon mercury as compared with that which our planet receives, we can possibly, and for the sake of a clearer understanding of the effects of the varying distance, which is the object of our present inquiry, find a loophole to admit the chance that yet there may be living beings there. we might, for instance, suppose that, owing to the rarity of its atmosphere, the excessive heat was quickly radiated away, or that there was something in the constitution of the atmosphere that greatly modified the effective temperature of the sun's rays. but, having satisfied our imagination on this point, and placed our supposititious inhabitants in the hot world of mercury, how are we going to meet the conditions imposed by the rapid changes of distance--the swift fall of the planet toward the sun, followed by the equally swift rush away from it? for change of distance implies change of heat and temperature. it is true that we have a slight effect of this kind on the earth. between midsummer (of the northern hemisphere) and midwinter our planet draws , , miles nearer the sun, but the change occupies six months, and, at the earth's great average distance, the effect of this change is too slight to be ordinarily observable, and only the astronomer is aware of the consequent increase in the apparent size of the sun. it is not to this variation of the sun's distance, but rather to the changes of the seasons, depending on the inclination of the earth's axis, that we owe the differences of temperature that we experience. in other words, the total supply of heat from the sun is not far from uniform at all times of the year, and the variations of temperature depend upon the distribution of that supply between the northern and southern hemispheres, which are alternately inclined sunward. but on mercury the supply of solar heat is itself variable to an enormous extent. in six weeks, as we have seen, mercury diminishes its distance from the sun about one third, which is proportionally ten times as great a change of distance as the earth experiences in six months. the inhabitants of mercury in those six pregnant weeks see the sun expand in the sky to more than two and a half times its former magnitude, while the solar heat poured upon them swiftly augments from something more than four and a half times to above eleven times the amount received upon the earth! then, immediately, the retreat of the planet begins, the sun visibly shrinks, as a receding balloon becomes smaller in the eyes of its watchers, the heat falls off as rapidly as it had previously increased, until, the aphelion point being reached, the process is again reversed. and thus it goes on unceasingly, the sun growing and diminishing in the sky, and the heat increasing and decreasing by enormous amounts with astonishing rapidity. it is difficult to imagine any way in which atmospheric influences could equalize the effects of such violent changes, or any adjustments in the physical organization of living beings that could make such changes endurable. but we have only just begun the story of mercury's peculiarities. we come next to an even more remarkable contrast between that planet and our own. during the paris exposition of a little company of astronomers was assembled at the juvisy observatory of m. flammarion, near the french capital, listening to one of the most surprising disclosures of a secret of nature that any _savant_ ever confided to a few trustworthy friends while awaiting a suitable time to make it public. it was a secret as full of significance as that which galileo concealed for a time in his celebrated anagram, which, when at length he furnished the key, still remained a riddle, for then it read: "the mother of the loves imitates the shapes of cynthia," meaning that the planet venus, when viewed with a telescope, shows phases like those of the moon. the secret imparted in confidence to the knot of astronomers at juvisy came from a countryman of galileo's, signor g. v. schiaparelli, the director of the observatory of milan, and its purport was that the planet mercury always keeps the same face directed toward the sun. schiaparelli had satisfied himself, by a careful series of observations, of the truth of his strange announcement, but before giving it to the world he determined to make doubly sure. early in he withdrew the pledge of secrecy from his friends and published his discovery. no one can wonder that the statement was generally received with incredulity, for it was in direct contradiction to the conclusions of other astronomers, who had long believed that mercury rotated on its axis in a period closely corresponding with that of the earth's rotation--that is to say, once every twenty-four hours. schiaparelli's discovery, if it were received as correct, would put mercury, as a planet, in a class by itself, and would distinguish it by a peculiarity which had always been recognized as a special feature of the moon, viz., that of rotating on its axis in the same period of time required to perform a revolution in its orbit, and, while this seemed natural enough for a satellite, almost nobody was prepared for the ascription of such eccentric conduct to a planet. the italian astronomer based his discovery upon the observation that certain markings visible on the disk of mercury remained in such a position with reference to the direction of the sun as to prove that the planet's rotation was extremely slow, and he finally satisfied himself that there was but one rotation in the course of a revolution about the sun. that, of course, means that one side of mercury always faces toward the sun while the opposite side always faces away from it, and neither side experiences the alternation of day and night, one having perpetual day and the other perpetual night. the older observations, from which had been deduced the long accepted opinion that mercury rotated, like the earth, once in about twenty-four hours, had also been made upon the markings on the planet's disk, but these are not easily seen, and their appearances had evidently been misinterpreted. the very fact of the difficulty of seeing any details on mercury tended to prevent or delay corroboration of schiaparelli's discovery. but there were two circumstances that contributed to the final acceptance of his results. one of these was his well-known experience as an observer and the high reputation that he enjoyed among astronomers, and the other was the development by prof. george darwin of the theory of tidal friction in its application to planetary evolution, for this furnished a satisfactory explanation of the manner in which a body, situated as near the sun as mercury is, could have its axial rotation gradually reduced by the tidal attraction of the sun until it coincided in period with its orbital revolution. accepting the accuracy of schiaparelli's discovery, which was corroborated in every particular in by percival lowell in a special series of observations on mercury made with his -inch telescope at flagstaff, arizona, and which has also been corroborated by others, we see at once how important is its bearing on the habitability of the planet. it adds another difficulty to that offered by the remarkable changes of distance from the sun, and consequent variations of heat, which we have already discussed. in order to bring the situation home to our experience, let us, for a moment, imagine the earth fallen into mercury's dilemma. there would then be no succession of day and night, such as we at present enjoy, and upon which not alone our comfort but perhaps our very existence depends, but, instead, one side of our globe--it might be the asiatic or the american half--would be continually in the sunlight, and the other side would lie buried in endless night. and this condition, so suggestive of the play of pure imagination, this plight of being a two-faced world, like the god janus, one face light and the other face dark, must be the actual state of things on mercury. there is one interesting qualification. in the case just imagined for the earth, supposing it to retain the present inclination of its axis while parting with its differential rotation, there would be an interchange of day and night once a year in the polar regions. on mercury, whose axis appears to be perpendicular, a similar phenomenon, affecting not the polar regions but the eastern and western sides of the planet, is produced by the extraordinary eccentricity of its orbit. as the planet alternately approaches and recedes from the sun its orbital velocity, as we have already remarked, varies between the limits of twenty-three and thirty-five miles per second, being most rapid at the point nearest the sun. but this variation in the speed of its revolution about the sun does not, in any manner, affect the rate of rotation on its axis. the latter is perfectly uniform and just fast enough to complete one axial turn in the course of a single revolution about the sun. the accompanying figure may assist the explanation. [illustration: diagram showing that, owing to the eccentricity of its orbit, and its varying velocity, mercury, although making but one turn on its axis in the course of a revolution about the sun, nevertheless experiences on parts of its surface the alternation of day and night.] let us start with mercury in perihelion at the point _a_. the little cross on the planet stands exactly under the sun and in the center of the illuminated hemisphere. the large arrows show the direction in which the planet travels in its revolution about the sun, and the small curved arrows the direction in which it rotates on its axis. now, in moving along its orbit from _a_ to _b_ the planet, partly because of its swifter motion when near the sun, and partly because of the elliptical nature of the orbit, traverses a greater angular interval with reference to the sun than the cross, moving with the uniform rotation of the planet on its axis, is able to traverse in the same time. as drawn in the diagram, the cross has moved through exactly ninety degrees, or one right angle, while the planet in its orbit has moved through considerably more than a right angle. in consequence of this gain of the angle of revolution upon the angle of rotation, the cross at _b_ is no longer exactly under the sun, nor in the center of the illuminated hemisphere. it appears to have shifted its position toward the west, while the hemispherical cap of sunshine has slipped eastward over the globe of the planet. in the next following section of the orbit the planet rotates through another right angle, but, owing to increased distance from the sun, the motion in the orbit now becomes slower until, when the planet arrives at aphelion, _c_, the angular difference disappears and the cross is once more just under the sun. on returning from aphelion to perihelion the same phenomena recur in reverse order and the line between day and night on the planet first shifts westward, attaining its limit in that respect at _d_, and then, at perihelion, returns to its original position. now, if we could stand on the sunward hemisphere of mercury what, to our eyes, would be the effect of this shifting of the sun's position with regard to a fixed point on the planet's surface? manifestly it would cause the sun to describe a great arc in the sky, swinging to and fro, in an east and west line, like a pendulum bob, the angular extent of the swing being a little more than forty-seven degrees, and the time required for the sun to pass from its extreme eastern to its extreme western position and back again being eighty-eight days. but, owing to the eccentricity of the orbit, the sun swings much faster toward the east than toward the west, the eastward motion occupying about thirty-seven days and the westward motion about fifty-one days. [illustration: the regions of perpetual day, perpetual night, and alternate day and night on mercury. in the left-hand view the observer looks at the planet in the plane of its equator; in the right-hand view he looks down on its north pole.] another effect of the libratory motion of the sun as seen from mercury is represented in the next figure, where we have a view of the planet showing both the day and the night hemisphere, and where we see that between the two there is a region upon which the sun rises and sets once every eighty-eight days. there are, in reality, two of these lune-shaped regions, one at the east and the other at the west, each between , and , miles broad at the equator. at the sunward edge of these regions, once in eighty-eight days, or once in a mercurial year, the sun rises to an elevation of forty-seven degrees, and then descends again straight to the horizon from which it rose; at the nightward edge, once in eighty-eight days, the sun peeps above the horizon and quickly sinks from sight again. the result is that, neglecting the effects of atmospheric refraction, which would tend to expand the borders of the domain of sunlight, about one quarter of the entire surface of mercury is, with regard to day and night, in a condition resembling that of our polar regions, where there is but one day and one night in the course of a year--and on mercury a year is eighty-eight days. one half of the remaining three quarters of the planet's surface is bathed in perpetual sunshine and the other half is a region of eternal night. and now again, what of life in such a world as that? on the night side, where no sunshine ever penetrates, the temperature must be extremely low, hardly greater than the fearful cold of open space, unless modifying influences beyond our ken exist. it is certain that if life flourishes there, it must be in such forms as can endure continual darkness and excessive cold. some heat would be carried around by atmospheric circulation from the sunward side, but not enough, it would seem, to keep water from being perpetually frozen, or the ground from being baked with unrelaxing frost. it is for the imagination to picture underground dwellings, artificial sources of heat, and living forms suited to unearthlike environment. what would be the mental effects of perpetual night upon a race of intelligent creatures doomed to that condition? perhaps not quite so grievous as we are apt to think. the constellations in all their splendor would circle before their eyes with the revolution of their planet about the sun, and with the exception of the sun itself--which they could see by making a journey to the opposite hemisphere--all the members of the solar system would pass in succession through their mid-heaven, and two of them would present themselves with a magnificence of planetary display unknown on the earth. venus, when in opposition under the most favorable circumstances, is scarcely more than , , miles from mercury, and, showing herself at such times with a fully illuminated disk--as, owing to her position within the orbit of the earth, she never can do when at her least distance from us--she must be a phenomenon of unparalleled beauty, at least four times brighter than we ever see her, and capable, of course, of casting a strong shadow. the earth, also, is a splendid star in the midnight sky of mercury, and the moon may be visible to the naked eye as a little attendant circling about its brilliant master. the outer planets are slightly less conspicuous than they are to us, owing to increase of distance. the revolution of the heavens as seen from the night side of mercury is quite different in period from that which we are accustomed to, although the apparent motion is in the same direction, viz., from east to west. the same constellations remain above the horizon for weeks at a time, slowly moving westward, with the planets drifting yet more slowly, but at different rates, among them; the nearer planets, venus and the earth, showing the most decided tendency to loiter behind the stars. on the side where eternal sunlight shines the sky of mercury contains no stars. forever the pitiless blaze of day; forever, "all in a hot and copper sky the bloody sun at noon." as it is difficult to understand how water can exist on the night hemisphere, except in the shape of perpetual snow and ice, so it is hard to imagine that on the day hemisphere water can ever be precipitated from the vaporous form. in truth, there can be very little water on mercury even in the form of vapor, else the spectroscope would have given unquestionable evidence of its presence. those who think that mercury is entirely waterless and almost, if not quite, airless may be right. in these respects it would then resemble the moon, and, according to some observers, it possesses another characteristic lunar feature in the roughening of its surface by what seem to be innumerable volcanic craters. but if we suppose mercury to possess an atmosphere much rarer than that of the earth, we may perceive therein a possible provision against the excessive solar heat to which it is subjected, since, as we see on high mountains, a light air permits a ready radiation of heat, which does not become stored up as in a denser atmosphere. as the sun pours its heat without cessation upon the day hemisphere the warmed air must rise and flow off on all sides into the night hemisphere, while cold air rushes in below, to take its place, from the region of frost and darkness. the intermediate areas, which see the sun part of the time, as explained above, are perhaps the scene of contending winds and tempests, where the moisture, if there be any, is precipitated, through the rapid cooling of the air, in whelming floods and wild snow-storms driven by hurrying blasts from the realm of endless night. enough seems now to have been said to indicate clearly the hopelessness of looking for any analogies between mercury and the earth which would warrant the conclusion that the former planet is capable of supporting inhabitants or forms of life resembling those that swarm upon the latter. if we would still believe that mercury is a habitable globe we must depend entirely upon the imagination for pictures of creatures able to endure its extremes of heat and cold, of light and darkness, of instability, swift vicissitude, and violent contrast. in the next chapter we shall study a more peaceful and even-going world, yet one of great brilliancy, which possesses some remarkable resemblances to the earth, as well as some surprising divergences from it. chapter iii venus, the twin of the earth we come now to a planet which seems, at the first glance, to afford a far more promising outlook than mercury does for the presence of organic life forms bearing some resemblance to those of the earth. one of the strongest arguments for regarding venus as a world much like ours is based upon its remarkable similarity to the earth in size and mass, because thus we are assured that the force of gravity is practically the same upon the two planets, and the force of gravity governs numberless physical phenomena of essential importance to both animal and vegetable life. the mean diameter of the earth is , miles; that of venus is , miles. the difference is so slight that if the two planets were suspended side by side in the sky, at such a distance that their disks resembled that of the full moon, the eye would notice no inequality between them. the mean density of venus is about nine tenths of that of the earth, and the force of gravity upon its surface is in the ratio of about to as compared to its force on the surface of the earth. a man removed to venus would, consequently, find himself perceptibly lighter than he was at home, and would be able to exert his strength with considerably greater effect than on his own planet. but the difference would amount only to an agreeable variation from accustomed conditions, and would not be productive of fundamental changes in the order of nature. being, like mercury, nearer to the sun than the earth is, venus also is visible to us only in the morning or the evening sky. but her distance from the sun, slightly exceeding , , miles, is nearly double that of mercury, so that, when favorably situated, she becomes a very conspicuous object, and, instead of being known almost exclusively by astronomers, she is, perhaps, the most popular and most admired of all the members of the planetary system, especially when she appears in the charming rôle of the "evening star." as she emerges periodically from the blinding glare of the sun's immediate neighborhood and begins to soar, bright as an electric balloon, in the twilight, she commands all eyes and calls forth exclamations of astonishment and admiration by her singular beauty. the intervals between her successive reappearances in the evening sky, measured by her synodic period of days, are sufficiently long to give an element of surprise and novelty to every return of so dazzling a phenomenon. even the light of the full moon silvering the tree tops does not exercise greater enchantment over the mind of the contemplative observer. in either of her rôles, as morning or as evening star, venus has no rival. no fixed star can for an instant bear comparison with her. what she lacks in vivacity of light--none of the planets twinkles, as do all of the true stars--is more than compensated by the imposing size of her gleaming disk and the striking beauty of her clear lamplike rays. her color is silvery or golden, according to the state of the atmosphere, while the distinction of her appearance in a dark sky is so great that no eye can resist its attraction, and i have known an unexpected glimpse of venus to put an end to an animated conversation and distract, for a long time, the attention of a party of ladies and gentlemen from the social occupation that had brought them together. as a telescopic object venus is exceedingly attractive, even when considered merely from the point of view of simple beauty. both mercury and venus, as they travel about the sun, exhibit phases like those of the moon, but venus, being much larger and much nearer to the earth than mercury, shows her successive phases more effectively, and when she shines as a thin crescent in the morning or evening twilight, only a very slight magnifying power is required to show the sickle form of her disk. a remarkable difference between venus and mercury comes out as soon as we examine the shape of the former's orbit. venus's mean distance from the sun is , , miles, and her orbit is so nearly a circle, much more nearly than that of any other planet, that in the course of a revolution her distance from the sun varies less than a million miles. the distance of the earth varies , , miles, and that of mercury , , . her period of revolution, or the length of her year, is of our days. when she comes between the sun and the earth she approaches us nearer than any other planet ever gets, except the asteroid eros, her distance at such times being , , miles, or about one hundred and ten times the distance of the moon. being nearer to the sun in the ratio of to , venus receives almost twice as much solar light and heat as we get, but less than one third as much as mercury gets. there is reason to believe that her axis, instead of being considerably inclined, like that of the earth, is perpendicular to the plane of her orbit. thus venus introduces to us another novelty in the economy of worlds, for with a perpendicular axis of rotation she can have no succession of seasons, no winter and summer flitting, one upon the other's heels, to and fro between the northern and southern hemispheres; but, on the contrary, her climatic conditions must be unchangeable, and, on any particular part of her surface, except for local causes of variation, the weather remains the same the year around. so, as far as temperature is concerned, venus may have two regions of perpetual winter, one around each pole; two belts of perpetual spring in the upper middle latitudes, one on each side of the equator; and one zone of perpetual summer occupying the equatorial portion of the planet. but, of course, these seasonal terms do not strictly apply to venus, in the sense in which we employ them on the earth, for with us spring is characterized rather by the change in the quantity of heat and other atmospheric conditions that it witnesses than by a certain fixed and invariable temperature. to some minds it may appear very undesirable, from the point of view of animate existences, that there should be no alternation of seasons on the surface of a planet, but, instead, fixed conditions of climate; yet it is not clear that such a state of affairs might not be preferable to that with which we are familiar. even on the earth, we find that tropical regions, where the seasonal changes are comparatively moderate, present many attractions and advantages in contrast with the violent and often destructive vicissitudes of the temperate zones, and nature has shown us, within the pale of our own planet, that she is capable of bringing forth harvests of fruit and grain without the stimulus of alternate frost and sunshine. even under the reign of perpetual summer the fields and trees find time and opportunity to rest and restore their productive forces. the circularity of venus's orbit, and the consequently insignificant change in the sun's distance and heating effect, are other elements to be considered in estimating the singular constancy in the operation of natural agencies upon that interesting planet, which, twin of the earth though it be in stature, is evidently not its twin in temperament. and next as to the all-important question of atmosphere. in what precedes, the presence of an atmosphere has been assumed, and, fortunately, there is very convincing evidence, both visual and spectroscopic, that venus is well and abundantly supplied with air, by which it is not meant that venus's air is precisely like the mixture of oxygen and nitrogen, with a few other gases, which we breathe and call by that name. in fact, there are excellent reasons for thinking that the atmosphere of venus differs from the earth's quite as much as some of her other characteristics differ from those of our planet. but, however it may vary from ours in constitution, the atmosphere of venus contains water vapor, and is exceedingly abundant. listen to professor young: "its [venus's] atmosphere is probably from one and a half to two times as extensive and as dense as our own, and the spectroscope shows evidence of the presence of water vapor in it." and prof. william c. pickering, basing his statement on the result of observations at the mountain observatory of arequipa, says: "we may feel reasonably certain that at the planet's [venus's] surface the density of its atmosphere is many times that of our own." we do not have to depend upon the spectroscope for evidence that venus has a dense atmosphere, for we can, in a manner, _see_ her atmosphere, in consequence of its refractive action upon the sunlight that strikes into it near the edge of the planet's globe. this illumination of venus's atmosphere is witnessed both when she is nearly between the sun and the earth, and when, being exactly between them, she appears in silhouette against the solar disk. during a transit of this kind, in , many observers, and the present writer was one, saw a bright atmospheric bow edging a part of the circumference of venus when the planet was moving upon the face of the sun--a most beautiful and impressive spectacle. even more curious is an observation made in by prof. c.s. lyman, of yale college, who, when venus was very near the sun, saw her atmosphere _in the form of a luminous ring_. a little fuller explanation of this appearance may be of interest. when approaching inferior conjunction--i.e., passing between the earth and sun--venus appears, with a telescope, in the shape of a very thin crescent. professor lyman watched this crescent, becoming narrower day after day as it approached the sun, and noticed that its extremities gradually extended themselves beyond the limits of a semicircle, bending to meet one another on the opposite side of the invisible disk of the planet, until, at length, they did meet, and he beheld a complete ring of silvery light, all that remained visible of the planet venus! the ring was, of course, the illuminated atmosphere of the planet refracting the sunlight on all sides around the opaque globe. in m. flammarion witnessed the same phenomenon in similar circumstances. one may well envy those who have had the good fortune to behold this spectacle--to actually see, as it were, the air that the inhabitants of another world are breathing and making resonant with all the multitudinous sounds and voices that accompany intelligent life. but perhaps some readers will prefer to think that even though an atmosphere is there, there is no one to breathe it. [illustration: venus's atmosphere seen as a ring of light.] as the visibility of venus's atmosphere is unparalleled elsewhere in the solar system, it may be worth while to give a graphic illustration of it. in the accompanying figure the planet is represented at three successive points in its advance toward inferior conjunction. as it approaches conjunction it slowly draws nearer the earth, and its apparent diameter consequently increases. at _a_ a large part of the luminous crescent is composed of the planet's surface reflecting the sunshine; at _b_ the ratio of the reflecting surface to the illuminated atmosphere has diminished, and the latter has extended, like the curved arms of a pair of calipers, far around the unilluminated side of the disk; at _c_ the atmosphere is illuminated all around by the sunlight coming through it from behind, while the surface of the planet has passed entirely out of the light--that is to say, venus has become an invisible globe embraced by a circle of refracted sunshine. we return to the question of life. with almost twice as much solar heat and light as we have, and with a deeper and denser atmosphere than ours, it is evident, without seeking other causes of variation, that the conditions of life upon venus are notably different from those with which we are acquainted. at first sight it would seem that a dense atmosphere, together with a more copious supply of heat, might render the surface temperature of venus unsuitable for organic life as we understand it. but so much depends upon the precise composition of the atmosphere and upon the relative quantities of its constituents, that it will not do to pronounce a positive judgment in such a case, because we lack information on too many essential points. experiment has shown that the temperature of the air varies with changes in the amount of carbonic acid and of water vapor that it contains. it has been suggested that in past geologic ages the earth's atmosphere was denser and more heavily charged with vapors than it is at present; yet even then forms of life suited to their environment existed, and from those forms the present inhabitants of our globe have been developed. there are several lines of reasoning which may be followed to the conclusion that venus, as a life-bearing world, is younger than the earth, and, according to that view, we are at liberty to imagine our beautiful sister planet as now passing through some such period in its history as that at which the earth had arrived in the age of the carboniferous forests, or the age of the gigantic reptiles who ruled both land and sea. but, without making any assumptions as to the phase of evolution which life may have attained on venus, it is also possible to think that the planet's thick shell of air, with its abundant vapors, may serve as a shield against the excessive solar radiation. venus is extraordinarily brilliant, its reflective power being greatly in excess of mercury's, and it has often been suggested that this may be due to the fact that a large share of the sunlight falling upon it is turned back before reaching the planet's surface, being reflected both from the atmosphere itself and from vast layers of clouds. even when viewed with the most powerful telescopes and in the most favoring circumstances, the features of venus's surface are difficult to see, and generally extremely difficult. they consist of faint shadowy markings, indefinite in outline, and so close to the limit of visibility that great uncertainty exists not only as to their shape and their precise location upon the planet, but even as to their actual existence. no two observers have represented them exactly alike in drawings of the planet, and, unfortunately, photography is as yet utterly unable to deal with them. mr. percival lowell, in his special studies of venus in , using a -inch telescope of great excellence, in the clear and steady air of arizona, found delicate spokelike streaks radiating from a rounded spot like a hub, and all of which, in his opinion, were genuine and definite markings on the planet's surface. but others, using larger telescopes, have failed to perceive the shapes and details depicted by mr. lowell, and some are disposed to ascribe their appearances to venus's atmosphere. mr. lowell himself noticed that the markings seemed to have a kind of obscuring veil over them. in short, all observers of venus agree in thinking that her atmosphere, to a greater or less extent, serves as a mask to conceal her real features, and the possibilities of so extensive an atmosphere with reference to an adjustment of the peculiar conditions of the planet to the requirements of life upon it, are almost unlimited. if we could accurately analyze that atmosphere we would have a basis for more exact conclusions concerning venus's habitability. but the mere existence of the atmosphere is, in itself, a strong argument for the habitability of the planet, and as to the temperature, we are really not compelled to imagine special adaptations by means of which it may be brought into accord with that prevailing upon the earth. as long as the temperature does not rise to the _destructive_ point, beyond which our experience teaches that no organic life can exist, it may very well attain an elevation that would mean extreme discomfort from our point of view, without precluding the existence of life even in its terrestrial sense. and would it not be unreasonable to assume that vital phenomena on other planets must be subject to exactly the same limitations that we find circumscribing them in our world? that kind of assumption has more than once led us far astray even in dealing with terrestrial conditions. it is not so long ago, for instance, since life in the depths of the sea was deemed to be demonstrably impossible. the bottom of the ocean, we were assured, was a region of eternal darkness and of frightful pressure, wherein no living creatures could exist. yet the first dip of the deep-sea trawl brought up animals of marvelous delicacy of organization, which, although curiously and wonderfully adapted to live in a compressed liquid, collapsed when lifted into a lighter medium, and which, despite the assumed perpetual darkness of their profound abode, were adorned with variegated colors and furnished with organs of phosphorescence whereby they could create for themselves all the light they needed. even the fixed animals of the sea, growing, like plants, fast to the rocks, are frequently vivid with living light, and there is a splendid suggestion of nature's powers of adaptation, which may not be entirely inapplicable to the problems of life on strange planets, in alexander agassiz's statement that species of sea animals, living below the depths to which sunlight penetrates, "may dwell in total darkness and be illuminated at times merely by the movements of abyssal fishes through the forests of phosphorescent alcyonarians." in attempting to judge the habitability of a planet such as venus we must first, as far as possible, generalize the conditions that govern life and restrict its boundaries. on the earth we find animated existence confined to the surface of the crust of the globe, to the lower and denser strata of the atmosphere, and to the film of water that constitutes the oceans. it does not exist in the heart of the rocks forming the body of the planet nor in the void of space surrounding it outside the atmosphere. as the earth condensed from the original nebula, and cooled and solidified, a certain quantity of matter remained at its surface in the form of free gases and unstable compounds, and, within the narrow precincts where these things were, lying like a thin shell between the huge inert globe of permanently combined elements below, and the equally unchanging realm of the ether above, life, a phenomenon depending upon ceaseless changes, combinations and recombinations of chemical elements in unstable and temporary union, made its appearance, and there only we find it at the present time. it is because air and water furnish the means for the continual transformations by which the bodies of animals and plants are built up and afterward disintegrated and dispersed, that we are compelled to regard their presence as prerequisites to the existence, on any planet, of life in any of the forms in which we are acquainted with it. but if we perceive that another world has an atmosphere, and that there is water vapor in its atmosphere--both of which conditions are fulfilled by venus--and if we find that that world is bathed in the same sunshine that stimulates the living forces of our planet, even though its quantity or intensity may be different, then it would seem that we are justified in averring that the burden of proof rests upon those who would deny the capability of such a world to support inhabitants. the generally accepted hypothesis of the origin of the solar system leads us to believe that venus has experienced the same process of evolution as that which brought the earth into its present condition, and we may fairly argue that upon the rocky shell of venus exists a region where chemical combinations and recombinations like those on the surface of the earth are taking place. it is surely not essential that the life-forming elements should exist in exactly the same states and proportions as upon the earth; it is enough if some of them are manifestly present. even on the earth these things have undergone much variation in the course of geological history, coincidently with the development of various species of life. just at present the earth appears to have reached a stage where everything contributes to the maintenance of a very high organization in both the animal and vegetable kingdoms. so each planet that has attained the habitable stage may have a typical adjustment of temperature and atmospheric constitution, rendering life possible within certain limits peculiar to that planet, and to the special conditions prevailing there. admitting, as there is reason for doing, that different planets may be at different stages of development in the geological and biological sense, we should, of course, not expect to find them inhabited by the same living species. and, since there is also reason to believe that no two planets upon arriving at the same stage of evolution as globes would possess identical gaseous surroundings, there would naturally be differences between their organic life forms notwithstanding the similarity of their common phase of development in other respects. thus a departure from the terrestrial type in the envelope of gases covering a planet, instead of precluding life, would only tend to vary its manifestations. after all, why should the intensity of the solar radiation upon venus be regarded as inimical to life? the sunbeams awaken life. it is not impossible that relative nearness to the sun may be an advantage to venus from the biologic point of view. she gets less than one third as much heat as mercury receives on the average, and she gets it with almost absolute uniformity. at aphelion mercury is about two and four tenths times hotter than venus; then it rushes sunward, and within forty-four days becomes six times hotter than venus. in the meantime the temperature of the latter, while high as compared with the earth's, remains practically unchanged. not only may mercury's temperature reach the destructive point, and thus be too high for organic life, but mercury gets nothing with either moderation or constancy. it is a world both of excessive heat and of violent contrasts of temperature. venus, on the other hand, presents an unparalleled instance of invariableness and uniformity. she may well be called the favorite of the sun, and, through the advantages of her situation, may be stimulated by him to more intense vitality than falls to the lot of the earth. it is open, at least to the writers of the interplanetary romances now so popular, to imagine that on venus, life, while encompassed with the serenity that results from the circular form of her orbit, and the unchangeableness of her climates, is richer, warmer, more passionate, more exquisite in its forms and more fascinating in its experiences, keener of sense, capable of more delicious joys, than is possible to it amid the manifold inclemencies of the colder earth. we have seen that there is excellent authority for saying that venus's atmosphere is from one and a half to two times as dense and as extensive as ours. here is an interesting suggestion of aerial possibilities for her inhabitants. if man could but fly, how would he take to himself wings and widen his horizons along with the birds! give him an atmosphere the double in density of that which now envelopes him, take off a little of his weight, thereby increasing the ratio of his strength and activity, put into his nervous system a more puissant stimulus from the life-giving sun, and perchance he _would_ fly. well, on venus, apparently, these very conditions actually exist. how, then, do intellectual creatures in the world of venus take wing when they choose? upon what spectacle of fluttering pinions afloat in iridescent air, like a raphael dream of heaven and its angels, might we not look down if we could get near enough to our brilliant evening star to behold the intimate splendors of its life? as venus herself would be the most brilliant member of the celestial host to an observer stationed on the night side of mercury, so the earth takes precedence in the midnight sky of venus. for the inhabitants of venus mercury is a splendid evening and morning star only, while the earth, being an outer planet, is visible at times in that part of the sky which is directly opposite to the place of the sun. the light reflected from our planet is probably less dazzling than that which venus sends to us, both because, at our greater distance, the sunlight is less intense, and because our rarer atmosphere reflects a smaller proportion of the rays incident upon it. but the earth is, after all, a more brilliant phenomenon seen from venus than the latter is seen from the earth, for the reason that the entire illuminated disk of the earth is presented toward our sister planet when the two are at their nearest point of approach, whereas, at that time, the larger part of the surface of venus that is turned earthward has no illumination, while the illuminated portion is a mere crescent. owing, again, to the comparative rarity of the terrestrial atmosphere, it is probable that the inhabitants of venus--assuming their existence--enjoy a superb view of the continents, oceans, polar snows, and passing clouds that color and variegate the face of the earth. our astronomers can study the full disk of venus only when she is at her greatest distance, and on the opposite side of the sun from us, where she is half concealed in the glare. the astronomers of venus, on the other hand, can study the earth under the most favorable conditions of observation--that is to say, when it is nearest to them and when, being in opposition to the sun, its whole disk is fully illuminated. in fact, there is no planet in the entire system which enjoys an outlook toward a sister world comparable with that which venus enjoys with regard to the earth. if there be astronomers upon venus, armed with telescopes, it is safe to guess that they possess a knowledge of the surface of the earth far exceeding in minuteness and accuracy the knowledge that we possess of the features of any heavenly body except the moon. they must long ago have been able to form definite conclusions concerning the meteorology and the probable habitability of our planet. it certainly tends to increase our interest in venus when, granting that she is inhabited, we reflect upon the penetrating scrutiny of which the earth may be the object whenever venus--as happens once every days--passes between us and the sun. the spectacle of our great planet, glowing in its fullest splendor in the midnight sky, pied and streaked with water, land, cloud, and snow, is one that might well excite among the astronomers of another world, so fortunately placed to observe it, an interest even greater than that which the recurrence of total solar eclipses occasions upon the earth. for the inhabitants of venus the study of the earth must be the most absorbing branch of observational astronomy, and the subject, we may imagine, of numberless volumes of learned memoirs, far exceeding in the definiteness of their conclusions the books that we have written about the physical characteristics of other members of the solar system. and, if we are to look for attempts on the part of the inhabitants of other worlds to communicate with us by signals across the ether, it would certainly seem that venus is the most likely source of such efforts, for from no other planet can those features of the earth that give evidence of its habitability be so clearly discerned. of one thing it would seem we may be certain: if venus has intellectual inhabitants they possess far more convincing evidence of our existence than we are likely ever to have of theirs. in referring to the view of the earth from mercury it was remarked that the moon is probably visible to the naked eye. from venus the moon is not only visible, but conspicuous, to the naked eye, circling about the earth, and appearing at times to recede from it to a distance of about half a degree--equal to the diameter of the full moon as we see it. the disk of the earth is not quite four times greater in diameter than that of the moon, and nowhere else in the solar system is there an instance in which two bodies, no more widely different in size than are the moon and the earth, are closely linked together. the moons of the other planets that possess satellites are relatively so small that they appear in the telescope as mere specks beside their primaries, but the moon is so large as compared with the earth that the two must appear, as viewed from venus, like a double planet. to the naked eye they may look like a very wide and brilliant double star, probably of contrasted colors, the moon being silvery white and the earth, perhaps, now of a golden or reddish tinge and now green or blue, according to the part of its surface turned toward venus, and according, also, to the season that chances to be reigning over that part. such a spectacle could not fail to be of absorbing interest, and we can not admit the possibility of intelligent inhabitants on venus without supposing them to watch the motions of the moon and the earth with the utmost intentness. the passage of the moon behind and in front of the earth, and its eclipses when it goes into the earth's shadow, could be seen without the aid of telescopes, while, with such instruments, these phenomena would possess the highest scientific interest and importance. because the earth has a satellite so easily observable, the astronomers of venus could not remain ignorant of the exact mass of our planet, and in that respect they would outstrip us in the race for knowledge, since, on account of the lack of a satellite attending venus, we have been able to do no more than make an approximate estimate of her mass. with telescopes, too, in the case of a solar eclipse occurring at the time of the earth's opposition, they could see the black spot formed by the shadow of the moon, where the end of its cone moved across the earth like the point of an invisible pencil, and could watch it traversing continents and oceans, or thrown out in bold contrast upon the white background of a great area of clouds. indeed, the phenomena which our globe and its satellite present to venus must be so varied and wonderful that one might well wish to visit that planet merely for the sake of beholding them. thus far we have found so much of brilliant promise in the earth's twin sister that i almost hesitate to approach another phase of the subject which may tend to weaken the faith of some readers in the habitability of venus. it may have been observed that heretofore nothing has been said as to the planet's rotation period, but, without specifically mentioning it, i have tacitly assumed the correctness of the generally accepted period of about twenty-four hours, determined by de vico and other observers. this period, closely accordant with the earth's, is, as far as it goes, another argument for the habitability of venus. but now it must be stated that no less eminent an authority than schiaparelli holds that venus, as well as mercury, makes but a single turn on its axis in the course of a revolution about the sun, and, consequently, is a two-faced world, one side staring eternally at the sun and the other side wearing the black mask of endless night. schiaparelli made this announcement concerning venus but a few weeks after publishing his discovery of mercury's peculiar rotation. he himself appears to be equally confident in both cases of the correctness of his conclusions and the certainty of his observation. as with mercury, several other observers have corroborated him, and particularly percival lowell in this country. mr. lowell, indeed, seems unwilling to admit that any doubt can be entertained. nevertheless, very grave doubt is entertained, and that by many, and probably by the majority, of the leading professional astronomers and observers. in fact, some observers of great ability, equipped with powerful instruments, have directly contradicted the results of schiaparelli and his supporters. the reader may ask: "why so readily accept schiaparelli's conclusions with regard to mercury while rejecting them in the case of venus?" the reply is twofold. in the first place the markings on venus, although mr. lowell sketched them with perfect confidence in , are, by the almost unanimous testimony of those who have searched for them with telescopes, both large and small, extremely difficult to see, indistinct in outline, and perhaps evanescent in character. the sketches of no two observers agree, and often they are remarkably unlike. the fact has already been mentioned that mr. lowell noticed a kind of veil partially obscuring the markings, and which he ascribed, no doubt correctly, to the planet's atmosphere. but he thinks that, notwithstanding the atmospheric veil, the markings noted by him were unquestionably permanent features of the planet's real surface. inasmuch, however, as his drawings represent things entirely different from what others have seen, there seems to be weight in the suggestion that the radiating bands and shadings noticed by him were in some manner illusory, and perhaps of atmospheric origin. if the markings were evidently of a permanent nature and attached to the solid shell of the planet, and if they were of sufficient distinctness to be seen in substantially the same form by all observers armed with competent instruments, then whatever conclusion was drawn from their apparent motion as to the period of the planet's rotation would have to be accepted. in the case of mercury the markings, while not easily seen, appear to be sufficiently distinct to afford confidence in the result of observations based upon them, but venus's markings have been represented in so many different ways that it seems advisable to await more light before accepting any extraordinary, and in itself improbable, conclusion based upon them. it should also be added that in spectroscopic observations by belopolski at pulkova gave evidence that venus really rotates rapidly on her axis, in a period probably approximating to the twenty-four hours of the earth's rotation, thus corroborating the older conclusions. belopolski's observation, it may be remarked, was based upon what is known as the doppler principle, which is employed in measuring the motion of stars in the line of sight, and in other cases of rapidly moving sources of light. according to this principle, when a source of light, either original or reflected, is approaching the observer, the characteristic lines in its spectrum are shifted toward the blue end, and when it is retreating from the observer the lines are shifted toward the red end. now, in the case of a planet rotating rapidly on its axis, it is clear that if the observer is situated in, or nearly in, the plane of the planet's equator, one edge of its disk will be approaching his eye while the opposite edge is retreating, and the lines in the spectrum of a beam of light from the advancing edge will be shifted toward the blue, while those in the spectrum of the light coming from the retreating edge will be shifted toward the red. and, by carefully noting the amount of the shifting, the velocity of the planet's rotation can be computed. this is what was done by belopolski in the case of venus, with the result above noted. secondly, the theory that venus rotates but once in the course of a revolution finds but slight support from the doctrine of tidal friction, as compared with that which it receives when applied to mercury. the effectiveness of the sun's attraction in slowing down the rotation of a planet through the braking action of the tides raised in the body of the planet while it is yet molten or plastic, varies inversely as the sixth power of the planet's distance. for mercury this effectiveness is nearly three hundred times as great as it is for the earth, while for venus it is only seven times as great. while we may admit, then, that mercury, being relatively close to the sun and subject to an enormous braking action, lost rotation until--as occurred for a similar reason to the moon under the tidal attraction of the earth--it ended by keeping one face always toward its master, we are not prepared to make the same admission in the case of venus, where the effective force concerned is comparatively so slight. it should be added, however, that no certain evidence of polar compression in the outline of venus's disk has ever been obtained, and this fact would favor the theory of a very slow rotation because a plastic globe in swift rotation has its equatorial diameter increased and its polar diameter diminished. if venus were as much flattened at the poles as the earth is, it would seem that the fact could not escape detection, yet the necessary observations are very difficult, and venus is so brilliant that her light increases the difficulty, while her transits across the sun, when she can be seen as a round black disk, are very rare phenomena, the latest having occurred in and , and the next not being due until . upon the whole, probably the best method of settling the question of venus's rotation is the spectroscopic method, and that, as we saw, has already given evidence for the short period. even if it were established that venus keeps always the same face to the sun, it might not be necessary to abandon altogether the belief that she is habitable, although, of course, the obstacles to that belief would be increased. venus's orbit being so nearly circular, and her orbital motion so nearly invariable, she has but a very slight libration with reference to the sun, and the east and west lunes on her surface, where day and night would alternate once in her year of days, would be so narrow as to be practically negligible. but, owing to her extensive atmosphere, there would be a very broad band of twilight on venus, running entirely around the planet at the inner edge of the light hemisphere. what the meteorological conditions within this zone would be is purely a matter of conjecture. as in the case of mercury, we should expect an interchange of atmospheric currents between the light and dark sides of the planet, the heated air rising under the influence of the unsetting sun in one hemisphere, and being replaced by an indraught of cold air from the other. the twilight band would probably be the scene of atmospheric conflicts and storms, and of immense precipitation, if there were oceans on the light hemisphere to charge the air with moisture. it has been suggested that ice and snow might be piled in a vast circle of glaciers, belting the planet along the line between perpetual day and night, and that where the sunbeams touched these icy deposits near the edge of the light hemisphere a marvelous spectacle of prismatic hills of crystal would be presented! it may be remarked that it would be the inhabitants of the dark hemisphere who would enjoy the beautiful scene of the earth and the moon in opposition. chapter iv mars, a world more advanced than ours mars is the fourth planet in the order of distance from the sun, and the outermost member of the terrestrial group. its mean distance is , , miles, variable, through the eccentricity of its orbit, to the extent of about , , miles. it will be observed that this is only a million miles less than the variation in mercury's distance from the sun, from which, in a previous chapter, were deduced most momentous consequences; but, in the case of mars, the ratio of the variation to the mean distance is far smaller than with mercury, so that the effect upon the temperature of the planet is relatively insignificant. mars gets a little less than half as much solar light and heat as the earth receives, its situation in this respect being just the opposite to that of venus. its period of orbital revolution, or the length of its year, is of our days. the diameter of mars is , miles, and its density is per cent of the earth's density. gravity on its surface is only per cent of terrestrial gravity--i.e., a one hundred-pound weight removed from the earth to mars would there weigh but thirty-eight pounds. mars evidently has an atmosphere, the details of which we shall discuss later. the poles of the planet are inclined from a perpendicular to the plane of its orbit at very nearly the same angle as that of the earth's poles, viz., ° ´. its rotation on its axis is also effected in almost the same period as the earth's, viz., hours, minutes. when in opposition to the sun, mars may be only about , , miles from the earth, but its average distance when in that position is more than , , miles, and may be more than , , . these differences arise from the eccentricities of the orbits of the two planets. when on the farther side of the sun--i.e., in conjunction with the sun as seen from the earth--mars's average distance from us is about , , miles. in consequence of these great changes in its distance, mars is sometimes a very conspicuous object in the sky, and at other times inconspicuous. the similarity in the inclination of the axis of the two planets results in a close resemblance between the seasons on mars and on the earth, although, owing to the greater length of its year, mars's seasons are much longer than ours. winter and summer visit in succession its northern and southern hemispheres just as occurs on the planet that we inhabit, and the torrid, temperate, and frigid zones on its surface have nearly the same angular width as on the earth. in this respect mars is the first of the foreign planets we have studied to resemble the earth. around each of its poles appears a circular white patch, which visibly expands when winter prevails upon it, and rapidly contracts, sometimes almost completely disappearing, under a summer sun. from the time of sir william herschel the almost universal belief among astronomers has been that these gleaming polar patches on mars are composed of snow and ice, like the similar glacial caps of the earth, and no one can look at them with a telescope and not feel the liveliest interest in the planet to which they belong, for they impart to it an appearance of likeness to our globe which at first glance is all but irresistible. to watch one of them apparently melting, becoming perceptibly smaller week after week, while the general surface of the corresponding hemisphere of the planet deepens in color, and displays a constantly increasing wealth of details as summer advances across it, is an experience of the most memorable kind, whose effect upon the mind of the observer is indescribable. early in the history of the telescope it became known that, in addition to the polar caps, mars presented a number of distinct surface features, and gradually, as instruments increased in power and observers in skill, charts of the planet were produced showing a surface diversified somewhat in the manner that characterizes the face of the earth, although the permanent forms do not closely resemble those of our planet. two principal colors exist on the disk of mars--dark, bluish gray or greenish gray, characterizing areas which have generally been regarded as seas, and light yellowish red, overspreading broad regions looked upon as continents. it was early observed that if the dark regions really are seas, the proportion of water to land upon mars is much smaller than upon the earth. for two especial reasons mars has generally been regarded as an older or more advanced planet than the earth. the first reason is that, accepting laplace's theory of the origin of the planetary system from a series of rings left off at the periphery of the contracting solar nebula, mars must have come into existence earlier than the earth, because, being more distant from the center of the system, the ring from which it was formed would have been separated sooner than the terrestrial ring. the second reason is that mars being smaller and less massive than the earth has run through its developments a cooling globe more rapidly. the bearing of these things upon the problems of life on mars will be considered hereafter. and now, once more, schiaparelli appears as the discoverer of surprising facts about one of the most interesting worlds of the solar system. during the exceptionally favorable opposition of mars in , when an american astronomer, asaph hall, discovered the planet's two minute satellites, and again during the opposition of , the italian observer caught sight of an astonishing network of narrow dark lines intersecting the so-called continental regions of the planet and crossing one another in every direction. schiaparelli did not see the little moons that hall discovered, and hall did not perceive the enigmatical lines that schiaparelli detected. hall had by far the larger and more powerful telescope; schiaparelli had much the more steady and favorable atmosphere for astronomical observation. yet these differences in equipment and circumstances do not clearly explain why each observer should have seen what the other did not. there may be a partial explanation in the fact that an observer having made a remarkable discovery is naturally inclined to confine his attention to it, to the neglect of other things. but it was soon found that schiaparelli's lines--to which he gave the name "canals," merely on account of their shape and appearance, and without any intention to define their real nature--were excessively difficult telescopic objects. eight or nine years elapsed before any other observer corroborated schiaparelli's observations, and notwithstanding the "sensation" which the discovery of the canals produced they were for many years regarded by the majority of astronomers as an illusion. but they were no illusion, and in schiaparelli added to the astonishment created by his original discovery, and furnished additional grounds for skepticism, by announcing that, at certain times, many of the canals geminated, or became double! he continued his observations at each subsequent opposition, adding to the number of the canals observed, and charting them with classical names upon a detailed map of the planet's surface. at length in perrotin, at nice, detected many of schiaparelli's canals, and later they were seen by others. in schiaparelli greatly extended his observations, and in and some of the canals were studied with the -inch telescope of the lick observatory, and in the last-named year a very elaborate series of observations upon them was made by percival lowell and his associates, prof. william c. pickering and mr. a.e. douglass, at flagstaff, arizona. mr. lowell's charts of the planet are the most complete yet produced, containing canals to which separate names have been given, besides more than a hundred other markings also designated by individual appellations. it should not be inferred from the fact that schiaparelli's discovery in excited so much surprise and incredulity that no glimpse of the peculiar canal-like markings on mars had been obtained earlier than that. at least as long ago as mr. dawes, in england, had seen and sketched half a dozen of the larger canals, or at least the broader parts of them, especially where they connect with the dark regions known as seas, but dawes did not see them in their full extent, did not recognize their peculiar character, and entirely failed to catch sight of the narrower and more numerous ones which constitute the wonderful network discovered by the italian astronomer. schiaparelli found no less than sixty canals during his first series of observations in . let us note some of the more striking facts about the canals which schiaparelli has described. we can not do better than quote his own words: "there are on this planet, traversing the continents, long dark lines which may be designated as _canals_, although we do not yet know what they are. these lines run from one to another of the somber spots that are regarded as seas, and form, over the lighter, or continental, regions a well-defined network. their arrangement appears to be invariable and permanent; at least, as far as i can judge from four and a half years of observation. nevertheless, their aspect and their degree of visibility are not always the same, and depend upon circumstances which the present state of our knowledge does not yet permit us to explain with certainty. in a great number were seen which were not visible in , and in all those which had been seen at former oppositions were found again, together with new ones. sometimes these canals present themselves in the form of shadowy and vague lines, while on other occasions they are clear and precise, like a trace drawn with a pen. in general they are traced upon the sphere like the lines of great circles; a few show a sensible lateral curvature. they cross one another obliquely, or at right angles. they have a breadth of two degrees, or kilometres [ miles], and several extend over a length of eighty degrees, or , kilometres [nearly , miles]. their tint is very nearly the same as that of the seas, usually a little lighter. every canal terminates at both its extremities in a sea, or in another canal; there is not a single example of one coming to an end in the midst of dry land. "this is not all. in certain seasons these canals become double. this phenomenon seems to appear at a determinate epoch, and to be produced simultaneously over the entire surface of the planet's continents. there was no indication of it in , during the weeks that preceded and followed the summer solstice of that world. a single isolated case presented itself in . on the th of december, this year--a little before the spring equinox, which occurred on mars on the st of january, --i noticed the doubling of the nile [a canal thus named] between the lakes of the moon and the ceraunic gulf. these two regular, equal, and parallel lines caused me, i confess, a profound surprise, the more so because a few days earlier, on the d and the th of december, i had carefully observed that very region without discovering anything of the kind. "i awaited with curiosity the return of the planet in , to see if an analogous phenomenon would present itself in the same place, and i saw the same thing reappear on the th of january, , one month after the spring equinox--which occurred on the th of december, . the duplication was still more evident at the end of february. on this same date, the th of january, another duplication had already taken place, that of the middle portion of the canal of the cyclops, adjoining elysium. [elysium is a part of one of the continental areas.] "yet greater was my astonishment when, on the th of january, i saw the canal jamuna, which was then in the center of the disk, formed very rigidly of two parallel straight lines, crossing the space which separates the niliac lake from the gulf of aurora. at first sight i believed it was an illusion, caused by fatigue of the eye and some new kind of strabismus, but i had to yield to the evidence. after the th of january i simply passed from wonder to wonder; successively the orontes, the euphrates, the phison, the ganges, and the larger part of the other canals, displayed themselves very clearly and indisputably duplicated. there were not less than twenty examples of duplication, of which seventeen were observed in the space of a month, from the th of january to the th of february. "in certain cases it was possible to observe precursory symptoms which are not lacking in interest. thus, on the th of january, a light, ill-defined shade extended alongside the ganges; on the th and the th one could only distinguish a series of white spots; on the th the shadow was still indecisive, but on the st the duplication was perfectly clear, such as i observed it until the d of february. the duplication of the euphrates, of the canal of the titans, and of the pyriphlegethon also began in an uncertain and nebulous form. "these duplications are not an optical effect depending on increase of visual power, as happens in the observation of double stars, and it is not the canal itself splitting in two longitudinally. here is what is seen: to the right or left of a pre-existing line, without any change in the course and position of that line, one sees another line produce itself, equal and parallel to the first, at a distance generally varying from six to twelve degrees--i.e., from to kilometres ( to miles); even closer ones seem to be produced, but the telescope is not powerful enough to distinguish them with certainty. their tint appears to be a quite deep reddish brown. the parallelism is sometimes rigorously exact. there is nothing analogous in terrestrial geography. everything indicates that here there is an organization special to the planet mars, probably connected with the course of its seasons."[ ] [footnote : l'astronomie, vol. i, , pp. _et seq._] schiaparelli adds that he took every precaution to avoid the least suspicion of illusion. "i am absolutely sure," he says, "of what i have observed." i have quoted his statement, especially about the duplication of the canals, at so much length, both on account of its intrinsic interest and because it has many times been argued that this particular phenomenon must be illusory even though the canals are real. one of the most significant facts that came out in the early observations was the evident connection between the appearance of the canals and the seasonal changes on mars. it was about the time of the spring equinox, when the white polar caps had begun to melt, that schiaparelli first noticed the phenomenon of duplication. as the season advanced the doubling of the canals increased in frequency and the lines became more distinct. in the meantime the polar caps were becoming smaller. broadly speaking, schiaparelli's observation showed that the doubling of the canals occurred principally a little after the spring equinox and a little before the autumn equinox; that the phenomenon disappeared in large part at the epoch of the winter solstice, and disappeared altogether at the epoch of the summer solstice. moreover, he observed that many of the canals, without regard to duplication, were invisible at times, and reappeared gradually; faint, scarcely visible lines and shadows, deepened and became more distinct until they were clearly and sharply defined, and these changes, likewise, were evidently seasonal. the invariable connection of the canals at their terminations with the regions called seas, the fact that as the polar caps disappeared the sealike expanses surrounding the polar regions deepened in color, and other similar considerations soon led to the suggestion that there existed on mars a wonderful system of water circulation, whereby the melting of the polar snows, as summer passed alternately from one hemisphere to the other, served to reenforce the supply of water in the seas, and, through the seas, in the canals traversing the broad expanses of dry land that occupy the equatorial regions of the planet. the thought naturally occurred that the canals might be of artificial origin, and might indicate the existence of a gigantic system of irrigation serving to maintain life upon the globe of mars. the geometrical perfection of the lines, their straightness, their absolute parallelism when doubled, their remarkable tendency to radiate from definite centers, lent strength to the hypothesis of an artificial origin. but their enormous size, length, and number tended to stagger belief in the ability of the inhabitants of any world to achieve a work so stupendous. after a time a change of view occurred concerning the nature of the expanses called seas, and mr. lowell, following his observations of , developed the theory of the water circulation and irrigation of mars in a new form. he and others observed that occasionally canals were visible cutting straight across some of the greenish, or bluish-gray, areas that had been regarded as seas. this fact suggested that, instead of seas, these dark expanses may rather be areas of marshy ground covered with vegetation which flourishes and dies away according as the supply of water alternately increases and diminishes, while the reddish areas known as continents are barren deserts, intersected by canals; and as the water released by the melting of the polar snows begins to fill the canals, vegetation springs up along their sides and becomes visible in the form of long narrow bands. according to this theory, the phenomena called canals are simply lines of vegetation, the real canals being individually too small to be detected. it may be supposed that from a central supply canal irrigation ditches are extended for a distance of twenty or thirty miles on each side, thus producing a strip of fertile soil from forty to sixty miles wide, and hundreds, or in some cases two or three thousands, of miles in length. the water supply being limited, the inhabitants can not undertake to irrigate the entire surface of the thirsty land, and convenience of circulation induces them to extend the irrigated areas in the form of long lines. the surface of mars, according to lowell's observation, is remarkably flat and level, so that no serious obstacle exists to the extension of the canal system in straight bands as undeviating as arcs of great circles. wherever two or more canals meet, or cross, a rounded dark spot from a hundred miles, or less, to three hundred miles in diameter, is seen. an astonishing number of these appear on mr. lowell's charts. occasionally, as occurs at the singular spot named lacus solis, several canals converging from all points of the compass meet at a central point like the spokes of a wheel; in other cases, as, for instance, that of the long canal named eumenides, with its continuation orcus, a single conspicuous line is seen threading a large number of round dark spots, which present the appearance of a row of beads upon a string. these circular spots, which some have regarded as lakes, mr. lowell believes are rather oases in the great deserts, and granting the correctness of his theory of the canals the aptness of this designation is apparent.[ ] [footnote : the reader can find many of these "canals" and "oases," as well as some of the other regions on mars that have received names, in the frontispiece.] wherever several canals, that is to say, several bands of vegetation or bands of life, meet, it is reasonable to assume that an irrigated and habitable area of considerable extent will be developed, and in such places the imagination may picture the location of the chief centers of population, perhaps in the form of large cities, or perhaps in groups of smaller towns and villages. the so-called lacus solis is one of these localities. so, likewise, it seems but natural that along the course of a broad, well-irrigated band a number of expansions should occur, driving back the bounds of the desert, forming rounded areas of vegetation, and thus affording a footing for population. wherever two bands cross such areas would be sure to exist, and in almost every instance of crossing the telescope actually shows them. as to the gemination or duplication of many of the lines which, at the beginning of the season, appear single, it may be suggested that, in the course of the development of the vast irrigation system of the planet parallel bands of cultivation have been established, one receiving its water supply from the canals of the other, and consequently lagging a little behind in visibility as the water slowly percolates through the soil and awakens the vegetation. or else, the character of the vegetation itself may differ as between two such parallel bands, one being supplied with plants that spring up and mature quickly when the soil about their roots is moistened, while the plants in the twin band respond more slowly to stimulation. objection has been made to the theory of the artificial origin of the canals of mars on the ground, already mentioned, that the work required to construct them would be beyond the capacity of any race of creatures resembling man. the reply that has been made to this is twofold. in the first place, it should be remembered that the theory, as mr. lowell presents it, does not assert that the visible lines are the actual canals, but only that they are strips of territory intersected, like holland or the center of the plain of lombardy, by innumerable irrigation canals and ditches. to construct such works is clearly not an impossible undertaking, although it does imply great industry and concentration of effort. in the second place, since the force of gravity on mars is in the ratio of only to compared with the earth's, it is evident that the diminished weight of all bodies to be handled would give the inhabitants of mars an advantage over those of the earth in the performance of manual labor, provided that they possess physical strength and activity as great as ours. but, in consequence of this very fact of the slighter force of gravity, a man upon mars could attain a much greater size, and consequently much greater muscular strength, than his fellows upon the earth possess without being oppressed by his own weight. in other words, as far as the force of gravity may be considered as the decisive factor, mars could be inhabited by giants fifteen feet tall, who would be relatively just as active, and just as little impeded in their movements by the weight of their bodies, as a six-footer is upon the earth. but they would possess far more physical strength than we do, while, in doing work, they would have much lighter materials to deal with. whether the theory that the canals of mars really are canals is true or not, at any rate there can now be no doubt as to the existence of the strange lines which bear that designation. the suggestion has been offered that their builders may no longer be in existence, mars having already passed the point in its history where life must cease upon its surface. this brings us to consider again the statement, made near the beginning of this chapter, that mars is, perhaps, at a more advanced stage of development than the earth. if we accept this view, then, provided there was originally some resemblance between mars's life forms and those of the earth, the inhabitants of that planet would, at every step, probably be in front of their terrestrial rivals, so that at the present time they should stand well in advance. mr. lowell has, perhaps, put this view of the relative advancement in evolution of mars and its inhabitants as picturesquely as anybody. "in mars," he says, "we have before us the spectacle of a world relatively well on in years, a world much older than the earth. to so much about his age mars bears witness on his face. he shows unmistakable signs of being old. advancing planetary years have left their mark legible there. his continents are all smoothed down; his oceans have all dried up.... mars being thus old himself, we know that evolution on his surface must be similarly advanced. this only informs us of its condition relative to the planet's capabilities. of its actual state our data are not definite enough to furnish much deduction. but from the fact that our own development has been comparatively a recent thing, and that a long time would be needed to bring even mars to his present geological condition, we may judge any life he may support to be not only relatively, but really older than our own. from the little we can see such appears to be the case. the evidence of handicraft, if such it be, points to a highly intelligent mind behind it. irrigation, unscientifically conducted, would not give us such truly wonderful mathematical fitness in the several parts to the whole as we there behold.... quite possibly such martian folk are possessed of inventions of which we have not dreamed, and with them electrophones and kinetoscopes are things of a bygone past, preserved with veneration in museums as relics of the clumsy contrivances of the simple childhood of the race. certainly what we see hints at the existence of beings who are in advance of, not behind us, in the journey of life."[ ] [footnote : mars, by percival lowell, p. _et seq._] granted the existence of such a race as is thus described, and to them it might not seem a too appalling enterprise, when their planet had become decrepit, with its atmosphere thinned out and its supply of water depleted, to grapple with the destroying hand of nature and to prolong the career of their world by feats of chemistry and engineering as yet beyond the compass of human knowledge. it is confidence, bred from considerations like these, in the superhuman powers of the supposed inhabitants of mars that has led to the popular idea that they are trying to communicate by signals with the earth. certain enigmatical spots of light, seen at the edge of the illuminated disk of mars, and projecting into the unilluminated part--for mars, although an outer planet, shows at particular times a gibbous phase resembling that of the moon just before or just after the period of full moon--have been interpreted by some, but without any scientific evidence, as of artificial origin. upon the assumption that these bright points, and others occasionally seen elsewhere on the planet's disk, are intended by the martians for signals to the earth, entertaining calculations have been made as to the quantity of light that would be required in the form of a "flash signal" to be visible across the distance separating the two planets. the results of the calculations have hardly been encouraging to possible investors in interplanetary telegraphy, since it appears that heliographic mirrors with reflecting surfaces measured by square miles, instead of square inches, would be required to send a visible beam from the earth to mars or _vice versa_. the projections of light on mars can be explained much more simply and reasonably. various suggestions have been made about them; among others, that they are masses of cloud reflecting the sunshine; that they are areas of snow; and that they are the summits of mountains crowned with ice and encircled with clouds. in fact, a huge mountain mass lying on the terminator, or the line between day and night, would produce the effect of a tongue of light projecting into the darkness without assuming that it was snow-covered or capped with clouds, as any one may convince himself by studying the moon with a telescope when the terminator lies across some of its most mountainous regions. to be sure, there is reason to think that the surface of mars is remarkably flat; yet even so the planet may have some mountains, and on a globe the greater part of whose shell is smooth any projections would be conspicuous, particularly where the sunlight fell at a low angle across them. another form in which the suggestion of interplanetary communication has been urged is plainly an outgrowth of the invention and surprising developments of wireless telegraphy. the human mind is so constituted that whenever it obtains any new glimpse into the arcana of nature it immediately imagines an indefinite and all but unlimited extension of its view in that direction. so to many it has not appeared unreasonable to assume that, since it is possible to transmit electric impulses for considerable distances over the earth's surface by the simple propagation of a series of waves, or undulations, without connecting wires, it may also be possible to send such impulses through the ether from planet to planet. the fact that the electric undulations employed in wireless telegraphy pass between stations connected by the crust of the earth itself, and immersed in a common atmospheric envelope, is not deemed by the supporters of the theory in question as a very serious objection, for, they contend, electric waves are a phenomenon of the ether, which extends throughout space, and, given sufficient energy, such waves could cross the gap between world and world. but nobody has shown how much energy would be needed for such a purpose, and much less has anybody indicated a way in which the required energy could be artificially developed, or cunningly filched from the stores of nature. it is, then, purely an assumption, an interesting figment of the mind, that certain curious disturbances in the electrical state of the air and the earth, affecting delicate electric instruments, possessing a marked periodicity in brief intervals of time, and not yet otherwise accounted for, are due to the throbbing, in the all-enveloping ether, of impulses transmitted from instruments controlled by the _savants_ of mars, whose insatiable thirst for knowledge, and presumably burning desire to learn whether there is not within reach some more fortunate world than their half-dried-up globe, has led them into a desperate attempt to "call up" the earth on their interplanetary telephone, with the hope that we are wise and skilful enough to understand and answer them. in what language they intend to converse no one has yet undertaken to tell, but the suggestion has sapiently been made that, mathematical facts being invariable, the eternal equality of two plus two with four might serve as a basis of understanding, and that a statement of that truth sent by electric taps across the ocean of ether would be a convincing assurance that the inhabitants of the planet from which the message came at least enjoyed the advantages of a common-school education. but, while speculation upon this subject rests on unverified, and at present unverifiable, assumptions, of course everybody would rejoice if such a thing were possible, for consider what zest and charm would be added to human life if messages, even of the simplest description, could be sent to and received from intelligent beings inhabiting other planets! it is because of this hold that it possesses upon the imagination, and the pleasing pictures that it conjures up, that the idea of interplanetary communication, once broached, has become so popular a topic, even though everybody sees that it should not be taken too seriously. the subject of the atmosphere of mars can not be dismissed without further consideration than we have yet given it, because those who think the planet uninhabitable base their opinion largely upon the assumed absence of sufficient air to support life. it was long ago recognized that, other things being equal, a planet of small mass must possess a less dense atmosphere than one of large mass. assuming that each planet originally drew from a common stock, and that the amount and density of its atmosphere is measured by its force of gravity, it can be shown that mars should have an atmosphere less than one fifth as dense as the earth's. dr. johnstone stoney has attacked the problem of planetary atmospheres in another way. knowing the force of gravity on a planet, it is easy to calculate the velocity with which a body, or a particle, would have to start radially from the planet in order to escape from its gravitational control. for the earth this critical velocity is about seven miles per second; for mars about three miles per second. estimating the velocity of the molecules of the various atmospheric gases, according to the kinetic theory, dr. stoney finds that some of the smaller planets, and the moon, are gravitationally incapable of retaining all of these gases in the form of an atmosphere. among the atmospheric constituents that, according to this view, mars would be unable permanently to retain is water vapor. indeed, he supposes that even the earth is slowly losing its water by evaporation into space, and on mars, owing to the slight force of gravity there, this process would go on much more rapidly, so that, in this way, we have a means of accounting for the apparent drying up of that planet, while we may be led to anticipate that at some time in the remote future the earth also will begin to suffer from lack of water, and that eventually the chasms of the sea will yawn empty and desolate under a cloudless sky. but it is not certain that the original supply of atmospheric elements was in every case proportional to the respective force of gravity of a planet. the fact that venus appears to have an atmosphere more extensive and denser than the earth's, although its force of gravity is a little less than that of our globe, indicates at once a variation as between these two planets in the amount of atmospheric material at their disposal. this may be a detail depending upon differences in the mode, or in the stage, of their evolution. thus, after all, dr. stoney's theory may be substantially correct and yet mars may retain sufficient water to form clouds, to be precipitated in snow, and to fill its canals after each annual melting of the polar caps, because the original supply was abundant, and its escape is a gradual process, only to be completed by age-long steps. even though the evidence of the spectroscope, as far as it goes, seems to lend support to the theory that there is no water vapor in the atmosphere of mars, we can not disregard the visual evidence that, nevertheless, water vapor exists there. what are the polar caps if they are not snow? frozen carbon dioxide, it has been suggested; but this is hardly satisfactory, for it offers no explanation of the fact that when the polar caps diminish, and in proportion as they diminish, the "seas" and the canals darken and expand, whereas a reasonable explanation of the correlation of these phenomena is offered if we accept the view that the polar caps consist of snow. then there are many observations on record indicating the existence of clouds in mars's atmosphere. sometimes a considerable area of its surface has been observed to be temporarily obscured, not by dense masses of cloud such as accompany the progress of great cyclonic storms across the continents and oceans of the earth, but by comparatively thin veils of vapor such as would be expected to form in an atmosphere so comparatively rare as that of mars. and these clouds, in some instances at least, appear, like the cirrus streaks and dapples in our own air, to float at a great elevation. mr. douglass, one of mr. lowell's associates in the observations of at flagstaff, arizona, observed what he believed to be a cloud over the unilluminated part of mars's disk, which, by micrometric measurement and estimate, was drifting at an elevation of about fifteen miles above the surface of the planet. this was seen on two successive days, november th and november th, and it underwent curious fluctuations in visibility, besides moving in a northerly direction at the rate of some thirteen miles an hour. but, upon the whole, as mr. lowell remarks, the atmosphere of mars is remarkably free of clouds. the reader will remember that mars gets a little less than half as much heat from the sun as the earth gets. this fact also has been used as an argument against the habitability of the planet. in truth, those who think that life in the solar system is confined to the earth alone insist upon an almost exact reproduction of terrestrial conditions as a _sine qua non_ to the habitability of any other planet. venus, they think, is too hot, and mars too cold, as if life were rather a happy accident than the result of the operation of general laws applicable under a wide variety of conditions. all that we are really justified in asserting is that venus may be too hot and mars too cold for _us_. of course, if we adopt the opinion held by some that the temperature on mars is constantly so low that water would remain perpetually frozen, it does throw the question of the kind of life that could be maintained there into the realm of pure conjecture. the argument in favor of an extremely low temperature on mars is based on the law of the diminution of radiant energy inversely as the square of the distance, together with the assumption that no qualifying circumstances, or no modification of that law, can enter into the problem. according to this view, it could be shown that the temperature on mars never rises above - ° f. but it is a view that seems to be directly opposed to the evidence of the telescope, for all who have studied mars under favorable conditions of observation have been impressed by the rapid and extensive changes that the appearance of its surface undergoes coincidently with the variation of the planet's seasons. it has its winter aspect and its summer aspect, perfectly distinct and recognizable, in each hemisphere by turns, and whether the polar caps be snow or carbon dioxide, at any rate they melt and disappear under a high sun, thus proving that an accumulation of heat takes place. professor young says: "as to the temperature of mars we have no certain knowledge. on the one hand, we know that on account of the planet's distance from the sun the intensity of solar radiation upon its surface must be less than here in the ratio of to ( . )^ --i.e., only about per cent as great as with us; its 'solar constant' must be less than calories against our . then, too, the low density of its atmosphere, probably less at the planet's surface than on the tops of our highest mountains, would naturally assist to keep down the temperature to a point far below the freezing-point of water. but, on the other hand, things certainly _look_ as if the polar caps were really masses of _snow_ and _ice_ deposited from vapor in the planet's atmosphere, and as if these actually melted during the martian summer, sending floods of water through the channels provided for them, and causing the growth of vegetation along their banks. we are driven, therefore, to suppose either that the planet has sources of heat internal or external which are not yet explained, or else, as long ago suggested, that the polar 'snow' may possibly be composed of something else than frozen _water_."[ ] [footnote : general astronomy, by charles a. young. revised edition, , p. .] even while granting the worst that can be said for the low temperature of mars, the persistent believer in its habitability could take refuge in the results of recent experiments which have proved that bacterial life is able to resist the utmost degree of cold that can be applied, microscopic organisms perfectly retaining their vitality--or at least their power to resume it--when subjected to the fearfully low temperature of liquid air. but then he would be open to the reply that the organisms thus treated are in a torpid condition and deprived of all activity until revived by the application of heat; and the picture of a world in a state of perpetual sleep is not particularly attractive, unless the fortunate prince who is destined to awake the slumbering beauty can also be introduced into the romance.[ ] [footnote : many of the present difficulties about temperatures on the various planets would be beautifully disposed of if we could accept the theory urged by mr. cope whitehouse, to the effect that the sun is not really a hot body at all, and that what we call solar light and heat are only local manifestations produced in our atmosphere by the transformation of some other form of energy transmitted from the sun; very much as the electric impulses carried by a wire from the transmitting to the receiving station on a telephone line are translated by the receiver into waves of sound. according to this theory, which is here mentioned only as an ingenuity and because something of the kind so frequently turns up in one form or another in popular semi-scientific literature, the amount of heat and light on a planet would depend mainly upon local causes.] to an extent which most of us, perhaps, do not fully appreciate, we are indebted for many of the pleasures and conveniences and some of the necessities of life on our planet to its faithful attendant, the moon. neither mercury nor venus has a moon, but mars has two moons. this statement, standing alone, might lead to the conclusion that, as far as the advantages a satellite can afford to the inhabitants of its master planet are concerned, the people of mars are doubly fortunate. so they would be, perhaps, if mars's moons were bodies comparable in size with our moon, but in fact they are hardly more than a pair of very entertaining astronomical toys. the larger of the two, phobos, is believed to be about seven miles in diameter; the smaller, deimos, only five or six miles. their dimensions thus resemble those of the more minute of the asteroids, and the suggestion has even been made that they may be captured asteroids which have fallen under the gravitational control of mars. the diameters just mentioned are professor pickering's estimates, based on the amount of light the little satellites reflect, for they are much too small to present measurable disks. deimos is , miles from the center of mars and , miles from its surface. phobos is , miles from the center of the planet and only , from the surface. deimos completes a revolution about the planet in thirty hours and eighteen minutes, and phobos in the astonishingly short period--although, of course, it is in strict accord with the law of gravitation and in that sense not astonishing--of seven hours and thirty-nine minutes. since mars takes twenty-four hours and thirty-seven minutes for one rotation on its axis, it is evident that phobos goes round the planet three times in the course of a single martian day and night, rising, contrary to the general motion of the heavens, in the west, running in a few hours through all the phases that our moon exhibits in the course of a month, and setting, where the sun and all the stars rise, in the east. deimos, on the other hand, has a period of revolution five or six hours longer than that of the planet's axial rotation, so that it rises, like the other heavenly bodies, in the east; but, because its motion is so nearly equal, in angular velocity, to that of mars's rotation, it shifts very slowly through the sky toward the west, and for two or three successive days and nights it remains above the horizon, the sun overtaking and passing it again and again, while, in the meantime, its protean face swiftly changes from full circle to half-moon, from half-moon to crescent, from crescent back to half, and from half to full, and so on without ceasing. and during this time phobos is rushing through the sky in the opposite direction, as if in defiance of the fundamental law of celestial revolution, making a complete circuit three times every twenty-four hours, and changing the shape of its disk four times as rapidly as deimos does! truly, if we were suddenly transported to mars, we might well believe that we had arrived in the mother world of lunatics, and that its two moons were bewitched. yet it must not be supposed that all the peculiarities just mentioned would be clearly seen from the surface of mars by eyes like ours. the phases of phobos would probably be discernible to the naked eye, but those of deimos would require a telescope in order to be seen, for, notwithstanding their nearness to the planet, mars's moons are inconspicuous phenomena even to the martians themselves. professor young's estimate is that phobos may shed upon mars one-sixtieth and deimos one-twelve-hundredth as much reflected moonlight as our moon sends to the earth. accordingly, a "moonlit night" on mars can have no such charm as we associate with the phrase. but it is surely a tribute to the power and perfection of our telescopes that we have been able to discover the existence of objects so minute and inconspicuous, situated at a distance of many millions of miles, and half concealed by the glaring light of the planet close around which they revolve. if mars's moons were as massive as our moon is they would raise tremendous tides upon mars, and would affect the circulation of water in the canals, but, in fact, their tidal effects are even more insignificant than their light-giving powers. but for astronomers on mars they would be objects of absorbing interest. upon quitting mars we pass to the second distinctive planetary group of the solar system, that of the asteroids. chapter v the asteroids, a family of dwarf worlds beyond mars, in the broad gap separating the terrestrial from the jovian planets, are the asteroids, of which nearly five hundred have been discovered and designated by individual names or numbers. but any statement concerning the known number of asteroids can remain valid for but a short time, because new ones are continually found, especially by the aid of photography. very few of the asteroids are of measurable size. among these are the four that were the first to be discovered--ceres, pallas, juno, and vesta. their diameters, according to the measurements of prof. e.e. barnard, of the yerkes observatory, are as follows: ceres, miles; pallas, miles; juno, miles; vesta, miles. it is only necessary to mention these diameters in order to indicate how wide is the difference between the asteroids and such planets as the earth, venus, or mars. the entire surface of the largest asteroid, ceres, does not equal the republic of mexico in area. but ceres itself is gigantic in comparison with the vast majority of the asteroids, many of which, it is believed, do not exceed twenty miles in diameter, while there may be hundreds or thousands of others still smaller--ten miles, five miles, or perhaps only a few rods, in diameter! curiously enough, the asteroid which appears brightest, and which it would naturally be inferred is the largest, really stands third in the order of measured size. this is vesta, whose diameter, according to barnard, is only miles. it is estimated that the surface of vesta possesses about four times greater light-reflecting power than the surface of ceres. some observations have also shown a variation in the intensity of the light from vesta, a most interesting fact, which becomes still more significant when considered in connection with the great variability of another most extraordinary member of the asteroidal family, eros, which is to be described presently. the orbits of the asteroids are scattered over a zone about , , miles broad. the mean distance from the sun of the nearest asteroid, eros, is , , miles, and that of the most distant, thule, , , miles. wide gaps exist in the asteroidal zone where few or no members of the group are to be found, and prof. daniel kirkwood long ago demonstrated the influence of jupiter in producing these gaps. almost no asteroids, as he showed, revolve at such a distance from the sun that their periods of revolution are exactly commensurable with that of jupiter. originally there may have been many thus situated, but the attraction of the great planet has, in the course of time, swept those zones clean. many of the asteroids have very eccentric orbits, and their orbits are curiously intermixed, varying widely among themselves, both in ellipticity and in inclination to the common plane of the solar system. considered with reference to the shape and position of its orbit, the most unique of these little worlds is eros, which was discovered in by de witt, at berlin, and which, on account of its occasional near approach to the earth, has lately been utilized in a fresh attempt to obtain a closer approximation to the true distance of the sun from the earth. the mean distance of eros from the sun is , , miles, its greatest distance is , , miles, and its least distance , , miles. it will thus be seen that, although all the other asteroids are situated beyond mars, eros, at its mean distance, is nearer to the sun than mars is. when in aphelion, or at its greatest distance, eros is outside of the orbit of mars, but when in perihelion it is so much inside of mars's orbit that it comes surprisingly near the earth. indeed, there are times when eros is nearer to the earth than any other celestial body ever gets except the moon--and, it might be added, except meteors and, by chance, a comet, or a comet's tail. its least possible distance from the earth is less than , , miles, and it was nearly as close as that, without anybody knowing or suspecting the fact, in , four years in advance of its discovery. yet the fact, strange as the statement may seem, had been recorded without being recognized. after de witt's discovery of eros in , at a time when it was by no means as near the earth as it had been some years before, prof. e.c. pickering ascertained that it had several times imprinted its image on the photographic plates of the harvard observatory, with which pictures of the sky are systematically taken, but had remained unnoticed, or had been taken for an ordinary star among the thousands of star images surrounding it. from these telltale plates it was ascertained that in it had been in perihelion very near the earth, and had shone with the brilliance of a seventh-magnitude star. it will, unfortunately, be a long time before eros comes quite as near us as it did on that occasion, when we failed to see it, for its close approaches to the earth are not frequent. prof. solon i. bailey selects the oppositions of eros in and as probably the most favorable that will occur during the first half of the twentieth century. we turn to the extraordinary fluctuations in the light of eros, and the equally extraordinary conclusions drawn from them. while the little asteroid, whose diameter is estimated to be in the neighborhood of twenty or twenty-five miles, was being assiduously watched and photographed during its opposition in the winter of - , several observers discovered that its light was variable to the extent of more than a whole magnitude; some said as much as two magnitudes. when it is remembered that an increase of one stellar magnitude means an accession of light in the ratio of . to , and an increase of two magnitudes an accession of . to , the significance of such variations as eros exhibited becomes immediately apparent. the shortness of the period within which the cycle of changes occurred, about two hours and a half, made the variation more noticeable, and at the same time suggested a ready explanation, viz., that the asteroid was rapidly turning on its axis, a thing, in itself, quite in accordance with the behavior of other celestial bodies and naturally to be expected. but careful observation showed that there were marked irregularities in the light fluctuations, indicating that eros either had a very strange distribution of light and dark areas covering its surface, or that instead of being a globular body it was of some extremely irregular shape, so that as it rotated it presented successively larger and smaller reflecting surfaces toward the sun and the earth. one interesting suggestion was that the little planet is in reality double, the two components revolving around their common center of gravity, like a close binary star, and mutually eclipsing one another. but this theory seems hardly competent to explain the very great fluctuation in light, and a better one, probably, is that suggested by prof. e.c. pickering, that eros is shaped something like a dumb-bell. we can picture such a mass, in imagination, tumbling end over end in its orbit so as to present at one moment the broad sides of both bells, together with their connecting neck, toward the sun, and, at the same time, toward the observer on the earth, and, at another moment, only the end of one of the bells, the other bell and the neck being concealed in shadow. in this way the successive gain and loss of sixfold in the amount of light might be accounted for. owing to the great distance the real form of the asteroid is imperceptible even with powerful telescopes, but the effect of a change in the amount of reflecting surface presented produces, necessarily, an alternate waxing and waning of the light. as far as the fluctuations are concerned, they might also be explained by supposing that the shape of the asteroid is that of a flat disk, rotating about one of its larger diameters so as to present, alternately, its edge and its broadside to the sun. and, perhaps, in order completely to account for all the observed eccentricities of the light of eros, the irregularity of form may have to be supplemented by certain assumptions as to the varying reflective capacity of different parts of the misshapen mass. the invaluable harvard photographs show that long before eros was recognized as an asteroid its light variations had been automatically registered on the plates. some of the plates, prof. e.c. pickering says, had had an exposure of an hour or more, and, owing to its motion, eros had formed a trail on each of these plates, which in some cases showed distinct variations in brightness. differences in the amount of variation at different times will largely depend upon the position of the earth with respect to the axis of rotation. another interesting deduction may be made from the changes that the light of eros undergoes. we have already remarked that one of the larger asteroids, and the one which appears to the eye as the most brilliant of all, vesta, has been suspected of variability, but not so extensive as that of eros. olbers, at the beginning of the last century, was of the opinion that vesta's variations were due to its being not a globe but an angular mass. so he was led by a similar phenomenon to precisely the same opinion about vesta that has lately been put forth concerning eros. the importance of this coincidence is that it tends to revive a remarkable theory of the origin of the asteroids which has long been in abeyance, and, in the minds of many, perhaps discredited. this theory, which is due to olbers, begins with the startling assumption that a planet, perhaps as large as mars, formerly revolving in an orbit situated between the orbits of mars and jupiter, was destroyed by an explosion! although, at first glance, such a catastrophe may appear too wildly improbable for belief, yet it was not the improbability of a world's blowing up that led to a temporary abandonment of olbers's bold theory. the great french mathematician lagrange investigated the explosive force "which would be necessary to detach a fragment of matter from a planet revolving at a given distance from the sun," and published the results in the connaissance des temps for . "applying his results to the earth, lagrange found that if the velocity of the detached fragment exceeded that of a cannon ball in the proportion of to the fragment would become a comet with a direct motion; but if the velocity rose in the proportion of to the motion of the comet would be retrograde. if the velocity was less than in either of these cases the fragment would revolve as a planet in an elliptic orbit. for any other planet besides the earth the velocity of explosion corresponding to the different cases would vary in the inverse ratio of the square root of the mean distance. it would therefore manifestly be less as the planet was more distant from the sun. in the case of each of the four smaller planets (only the four asteroids, ceres, pallas, juno, and vesta, were known at that time), the velocity of explosion indicated by their observed motion would be less than twenty times the velocity of a cannon ball."[ ] [footnote : grant's history of physical astronomy, p. .] instead, then, of being discredited by its assumption of so strange a catastrophe, olbers's theory fell into desuetude because of its apparent failure to account for the position of the orbits of many of the asteroids after a large number of those bodies had been discovered. he calculated that the orbits of all the fragments of his exploded planet would have nearly equal mean distances, and a common point of intersection in the heavens, through which every fragment of the original mass would necessarily pass in each revolution. at first the orbits of the asteroids discovered seemed to answer to these conditions, and olbers was even able to use his theory as a means of predicting the position of yet undetected asteroids. only ceres and pallas had been discovered when he put forth his theory, but when juno and vesta were found they fell in with his predictions so well that the theory was generally regarded as being virtually established; while the fluctuations in the light of vesta, as we have before remarked, led olbers to assert that that body was of a fragmental shape, thus strongly supporting his explosion hypothesis. afterward, when the orbits of many asteroids had been investigated, the soundness of olbers's theory began to be questioned. the fact that the orbits did not all intersect at a common point could easily be disposed of, as professor newcomb has pointed out, by simply placing the date of the explosion sufficiently far back, say millions of years ago, for the secular changes produced by the attraction of the larger planets would effectively mix up the orbits. but when the actual effects of these secular changes were calculated for particular asteroids the result seemed to show that "the orbits could never have intersected unless some of them have in the meantime been altered by the attraction of the small planets on each other. such an action is not impossible, but it is impossible to determine it, owing to the great number of these bodies and our ignorance of their masses."[ ] [footnote : popular astronomy, by simon newcomb, p. .] yet the theory has never been entirely thrown out, and now that the discovery of the light fluctuations of eros lends support to olbers's assertion of the irregular shape of some of the asteroids, it is very interesting to recall what so high an authority as professor young said on the subject before the discovery of eros: "it is true, as has often been urged, that this theory in its original form, as presented by olbers, can not be correct. no _single_ explosion of a planet could give rise to the present assemblage of orbits, nor is it possible that even the perturbations of jupiter could have converted a set of orbits originally all crossing at one point (the point of explosion) into the present tangle. the smaller orbits are so small that, however turned about, they lie wholly inside the larger and can not be made to intersect them. if, however, we admit a _series_ of explosions, this difficulty is removed; and if we grant an explosion at all, there seems to be nothing improbable in the hypothesis that the fragments formed by the bursting of the parent mass would carry away within themselves the same forces and reactions which caused the original bursting, so that they themselves would be likely enough to explode at some time in their later history."[ ] [footnote : general astronomy, by charles a. young. revised edition, , p. .] the rival theory of the origin of the asteroids is that which assumes that the planetary ring originally left off from the contracting solar nebula between the orbits of mars and jupiter was so violently perturbed by the attraction of the latter planet that, instead of being shaped into a single globe, it was broken up into many fragments. either hypothesis presents an attractive picture; but that which presupposes the bursting asunder of a large planet, which might at least have borne the germs of life, and the subsequent shattering of its parts into smaller fragments, like the secondary explosions of the pieces of a pyrotechnic bomb, certainly is by far the more impressive in its appeal to the imagination, and would seem to offer excellent material for some of the extra-terrestrial romances now so popular. it is a startling thought that a world can possibly carry within itself, like a dynamite cartridge, the means of its own disruption; but the idea does not appear so extremely improbable when we recall the evidence of collisions or explosions, happening on a tremendous scale, in the case of new or temporary stars.[ ] [footnote : "since the discovery of eros, the extraordinary position of its orbit has led to the suggestion that possibly mars itself, instead of being regarded as primarily a major planet, belonging to the terrestrial group, ought rather to be considered as the greatest of the asteroids, and a part of the original body from which the asteroidal system was formed."--j. bauschinger, astronomische nachrichten, no. .] coming to the question of life upon the asteroids, it seems clear that they must be excluded from the list of habitable worlds, whatever we may choose to think of the possible habitability of the original planet through whose destruction they may have come into existence. the largest of them possesses a force of gravity far too slight to enable it to retain any of the gases or vapors that are recognized as constituting an atmosphere. but they afford a captivating field for speculation, which need not be altogether avoided, for it offers some graphic illustrations of the law of gravitation. a few years ago i wrote, for the entertainment of an audience which preferred to meet science attired in a garb woven largely from the strands of fancy, an account of some of the peculiarities of such minute globes as the asteroids, which i reproduce here because it gives, perhaps, a livelier picture of those little bodies, from the point of view of ordinary human interest, than could be presented in any other way. a waif of space one night as i was waiting, watch in hand, for an occultation, and striving hard to keep awake, for it had been a hot and exhausting summer's day, while my wife--we were then in our honeymoon--sat sympathetically by my side, i suddenly found myself withdrawn from the telescope, and standing in a place that appeared entirely strange. it was a very smooth bit of ground, and, to my surprise, there was no horizon in sight; that is to say, the surface of the ground disappeared on all sides at a short distance off, and beyond nothing but sky was visible. i thought i must be on the top of a stupendous mountain, and yet i was puzzled to understand how the face of the earth could be so far withdrawn. presently i became aware that there was some one by me whom i could not see. "you are not on a mountain," my companion said, and as he spoke a cold shiver ran along my back-bone; "you are on an asteroid, one of those miniature planets, as you astronomers call them, and of which you have discovered several hundred revolving between the orbits of mars and jupiter. this is the little globe that you have glimpsed occasionally with your telescope, and that you, or some of your fellows, have been kind enough to name menippe." then i perceived that my companion, whose address had hardly been reassuring, was a gigantic inhabitant of the little planet, towering up to a height of three quarters of a mile. for a moment i was highly amused, standing by his foot, which swelled up like a hill, and straining my neck backward to get a look up along the precipice of his leg, which, curiously enough, i observed was clothed in rough homespun, the woolly knots of the cloth appearing of tremendous size, while it bagged at the knee like any terrestrial trousers' leg. his great head and face i could see far above me, as it were, in the clouds. yet i was not at all astonished. "this is all right," i said to myself. "of course on menippe the people must be as large as this, for the little planet is only a dozen miles in diameter, and the force of gravity is consequently so small that a man without loss of activity, or inconvenience, can grow three quarters of a mile tall." suddenly an idea occurred to me. "just to think what a jump i can make! why, only the other day i was figuring it out that a man could easily jump a thousand feet high from the surface of menippe, and now here i actually am on menippe. i'll jump." the sensation of that glorious rise skyward was delightful beyond expression. my legs seemed to have become as powerful as the engines of a transatlantic liner, and with one spring i rose smoothly and swiftly, and as straight as an arrow, surmounting the giant's foot, passing his knee and attaining nearly to the level of his hip. then i felt that the momentum of my leap was exhausted, and despite my efforts i slowly turned head downward, glancing in affright at the ground a quarter of a mile below me, on which i expected to be dashed to pieces. but a moment's thought convinced me that i should get no hurt, for with so slight a force of gravity it would be more like floating than falling. just then the menippean caught me with his monstrous hand and lifted me to the level of his face. "i should like to know," i said, "how you manage to live up here; you are so large and your planet is so little." "now, you are altogether too inquisitive," replied the giant. "you go!" he stooped down, placed me on the toe of his boot, and drew back his foot to kick me off. it flashed into my mind that my situation had now become very serious. i knew well what the effects of the small attractive force of these diminutive planets must be, for i had often amused myself with calculations about them. in this moment of peril i did not forget my mathematics. it was clear that if the giant propelled me with sufficient velocity i should be shot into space, never to return. how great would that velocity have to be? my mind worked like lightning on this problem. the diameter of menippe i knew did not exceed twelve miles. its mean density, as near as i could judge, was about the same as that of the earth. its attraction must therefore be as its radius, or nearly times less than that of the earth. a well-known formula enables us to compute the velocity a body would acquire in falling from an infinite distance to the earth or any other planet whose size and force of gravity are known. the same formula, taken in the opposite sense, of course, shows how fast a body must start from a planet in order that it may be freed from its control. the formula is v = square root of ( gr.), in which "g" is the acceleration of gravity, equal for the earth to feet in a second, and "r" is the radius of the attracting body. on menippe i knew "g" must equal about one twentieth of a foot, and "r" , feet. like a flash i applied the formula while the giant's muscles were yet tightening for the kick: , Ã� / Ã� = , , the square root of which is a fraction more than . fifty-six feet in a second, then, was the critical velocity with which i must be kicked off in order that i might never return. i perceived at once that the giant would be able to accomplish it. i turned and shouted up at him: "hold on, i have something to say to you!" i dimly saw his mountainous face puckered into mighty wrinkles, out of which his eyes glared fiercely, and the next moment i was sailing into space. i could no more have kept a balance than the earth can stand still upon its axis. i had become a small planet myself, and, like all planets, i rotated. yet the motion did not dizzy me, and soon i became intensely interested in the panorama of creation that was spread around me. for some time, whenever my face was turned toward the little globe of menippe, i saw the giant, partly in profile against the sky, with his back bent and his hands upon his knees, watching me with an occasional approving nod of his big head. he looked so funny standing there on his little seven-by-nine world, like a clown on a performing ball, that, despite my terrible situation, i shook my sides with laughter. there was no echo in the profundity of empty space. soon menippe dwindled to a point, and i saw her inhospitable inhabitant no more. then i watched the sun and the blazing firmament around, for there was at the same time broad day and midnight for me. the sunlight, being no longer diffused by an atmosphere, did not conceal the face of the sky, and i could see the stars shining close to the orb of day. i recognized the various planets much more easily than i had been accustomed to do, and, with a twinge at my heart, saw the earth traveling along in its distant orbit, splendid in the sunshine. i thought of my wife sitting alone by the telescope in the darkness and silence, wondering what had become of me. i asked myself, "how in the world can i ever get back there again?" then i smiled to think of the ridiculous figure i cut, out here in space, exposed to the eyes of the universe, a rotating, gyrating, circumambulating astronomer, an animated teetotum lost in the sky. i saw no reason to hope that i should not go on thus forever, revolving around the sun until my bones, whitening among the stars, might be revealed to the superlative powers of some future telescope, and become a subject of absorbing interest, the topic of many a learned paper for the astronomers of a future age. afterward i was comforted by the reflection that in airless space, although i might die and my body become desiccated, yet there could be no real decay; even my garments would probably last forever. the _savants_, after all, should never speculate on my bones. i saw the ruddy disk of mars, and the glinting of his icy poles, as the beautiful planet rolled far below me. "if i could only get there," i thought, "i should know what those canals of schiaparelli are, and even if i could never return to the earth, i should doubtless meet with a warm welcome among the martians. what a lion i should be!" i looked longingly at the distant planet, the outlines of whose continents and seas appeared most enticing, but when i tried to propel myself in that direction i only kicked against nothingness. i groaned in desperation. suddenly something darted by me flying sunward; then another and another. in a minute i was surrounded by strange projectiles. every instant i expected to be dashed in pieces by them. they sped with the velocity of lightning. hundreds, thousands of them were all about me. my chance of not being hit was not one in a million, and yet i escaped. the sweat of terror was upon me, but i did not lose my head. "a comet has met me," i said. "these missiles are the meteoric stones of which it is composed." and now i noticed that as they rushed along collisions took place, and flashes of electricity darted from one to another. a pale luminosity dimmed the stars. i did not doubt that, as seen from the earth, the comet was already flinging the splendors of its train upon the bosom of the night. while i was wondering at my immunity amid such a rain of death-threatening bolts, i became aware that their velocity was sensibly diminishing. this fact i explained by supposing that i was drawn along with them. notwithstanding the absence of any collision with my body, the overpowering attraction of the whole mass of meteors was overcoming my tangential force and bearing me in their direction. at first i rejoiced at this circumstance, for at any rate the comet would save me from the dreadful fate of becoming an asteroid. a little further reflection, however, showed me that i had gone from the frying-pan into the fire. the direction of my expulsion from menippe had been such that i had fallen into an orbit that would have carried me around the sun without passing very close to the solar body. now, being swept along by the comet, whose perihelion probably lay in the immediate neighborhood of the sun, i saw no way of escape from the frightful fate of being broiled alive. even where i was, the untempered rays of the sun scorched me, and i knew that within two or three hundred thousand miles of the solar surface the heat must be sufficient to melt the hardest rocks. i was aware that experiments with burning-glasses had sufficiently demonstrated that fact. but perforce i resigned myself to my fate. at any rate it would the sooner be all over. in fact, i almost forgot my awful situation in the interest awakened by the phenomena of the comet. i was in the midst of its very head. i was one of its component particles. i was a meteor among a million millions of others. if i could only get back to the earth, what news could i not carry to signor schiaparelli and mr. lockyer and dr. bredichin about the composition of comets! but, alas! the world could never know what i now saw. nobody on yonder gleaming earth, watching the magnificent advance of this "specter of the skies," would ever dream that there was a lost astronomer in its blazing head. i should be burned and rent to pieces amid the terrors of its perihelion passage, and my fragments would be strewn along the comet's orbit, to become, in course of time, particles in a swarm of aerolites. perchance, through the effects of some unforeseen perturbation, the earth might encounter that swarm. thus only could i ever return to the bosom of my mother planet. i took a positive pleasure in imagining that one of my calcined bones might eventually flash for a moment, a falling star, in the atmosphere of the earth, leaving its atoms to slowly settle through the air, until finally they rested in the soil from which they had sprung. from such reflections i was aroused by the approach of the crisis. the head of the comet had become an exceedingly uncomfortable place. the collisions among the meteors were constantly increasing in number and violence. how i escaped destruction i could not comprehend, but in fact i was unconscious of danger from that source. i had become in spirit an actual component of the clashing, roaring mass. tremendous sparks of electricity, veritable lightning strokes, darted about me in every direction, but i bore a charmed life. as the comet drew in nearer to the sun, under the terrible stress of the solar attraction, the meteors seemed to crowd closer, crashing and grinding together, while the whole mass swayed and shrieked with the uproar of a million tormented devils. the heat had become terrific. i saw stone and iron melted like snow and dissipated in steam. stupendous jets of white-hot vapor shot upward, and, driven off by the electrical repulsion of the sun, streamed backward into the tail. suddenly i myself became sensible of the awful heat. it seemed without warning to have penetrated my vitals. with a yell i jerked my feet from a boiling rock and flung my arms despairingly over my head. "you had better be careful," said my wife, "or you'll knock over the telescope." i rubbed my eyes, shook myself, and rose. "i must have been dreaming," i said. "i should think it was a very lively dream," she replied. i responded after the manner of a young man newly wed. at this moment the occultation began. chapter vi jupiter, the greatest of known worlds when we are thinking of worlds, and trying to exalt the imagination with them, it is well to turn to jupiter, for there is a planet worth pondering upon! a world thirteen hundred times as voluminous as the earth is a phenomenon calculated to make us feel somewhat as the inhabitant of a rural village does when his amazed vision ranges across the million roofs of a metropolis. jupiter is the first of the outer and greater planets, the major, or jovian, group. his mean diameter is , miles, and his average girth more than , miles. an inhabitant of jupiter, in making a trip around his planet, along any great circle of the sphere, would have to travel more than , miles farther than the distance between the earth and the moon. the polar compression of jupiter, owing to his rapid rotation, amounts in the aggregate to more than , miles, the equatorial diameter being , miles and the polar diameter , miles. jupiter's mean distance from the sun is , , miles, and the eccentricity of his orbit is sufficient to make this distance variable to the extent of , , miles; but, in view of his great average distance, the consequent variation in the amount of solar light and heat received by the planet is not of serious importance. when he is in opposition to the sun as seen from the earth jupiter's mean distance from us is about , , miles. his year, or period of revolution about the sun, is somewhat less than twelve of our years ( . years). his axis is very nearly upright to the plane of his orbit, so that, as upon venus, there is practically no variation of seasons. gigantic though he is in dimensions, jupiter is the swiftest of all the planets in axial rotation. while the earth requires twenty-four hours to make a complete turn, jupiter takes less than ten hours (nine hours fifty-five minutes), and a point on his equator moves, in consequence of axial rotation, between , and , miles in an hour. the density of the mighty planet is slight, only about one quarter of the mean density of the earth and virtually the same as that of the sun. this fact at once calls attention to a contrast between jupiter and our globe that is even more significant than their immense difference in size. the force of gravity upon jupiter's surface is more than two and a half times greater than upon the earth's surface (more accurately . times), so that a hundred-pound weight removed from the planet on which we live to jupiter would there weigh pounds, and an average man, similarly transported, would be oppressed with a weight of at least pounds. but, as a result of the rapid rotation of the great planet, and the ellipticity of its figure, the unfortunate visitor could find a perceptible relief from his troublesome weight by seeking the planet's equator, where the centrifugal tendency would remove about twenty pounds from every one hundred as compared with his weight at the poles. if we could go to the moon, or to mercury, venus, or mars, we may be certain that upon reaching any of those globes we should find ourselves upon a solid surface, probably composed of rock not unlike the rocky crust of the earth; but with jupiter the case would evidently be very different. as already remarked, the mean density of that planet is only one quarter of the earth's density, or only one third greater than the density of water. consequently the visitor, in attempting to set foot upon jupiter, might find no solid supporting surface, but would be in a situation as embarrassing as that of milton's satan when he undertook to cross the domain of chaos: "fluttering his pinions vain, plumb down he drops, ten thousand fathom deep, and to this hour down had been falling had not, by ill chance, the strong rebuff of some tumultuous cloud. instinct with fire and niter, hurried him as many miles aloft; that fury stayed, quenched in a boggy syrtis, neither sea nor good dry land, nigh foundered, as he fares, treading the crude consistence, half on foot, half flying." the probability that nothing resembling a solid crust, nor, perhaps, even a liquid shell, would be found at the visible surface of jupiter, is increased by considering that the surface density must be much less than the mean density of the planet taken as a whole, and since the latter but little exceeds the density of water, it is likely that at the surface everything is in a state resembling that of cloud or smoke. our imaginary visitor upon reaching jupiter would, under the influence of the planet's strong force of gravity, drop out of sight, with the speed of a shot, swallowed up in the vast atmosphere of probably hot, and perhaps partially incandescent, gases. when he had sunk--supposing his identity could be preserved--to a depth of thousands of miles he might not yet have found any solid part of the planet; and, perchance, there is no solid nucleus even at the very center. the cloudy aspect of jupiter immediately strikes the telescopic observer. the huge planet is filled with color, and with the animation of constant movement, but there is no appearance of markings, like those on mars, recalling the look of the earth. there are no white polar caps, and no shadings that suggest the outlines of continents and oceans. what every observer, even with the smallest telescope, perceives at once is a pair of strongly defined dark belts, one on either side of, and both parallel to, the planet's equator. these belts are dark compared with the equatorial band between them and with the general surface of the planet toward the north and the south, but they are not of a gray or neutral shade. on the contrary, they show decided, and, at times, brilliant colors, usually of a reddish tone. more delicate tints, sometimes a fine pink, salmon, or even light green, are occasionally to be seen about the equatorial zone, and the borders of the belts, while near the poles the surface is shadowed with bluish gray, imperceptibly deepening from the lighter hues of the equator. all this variety of tone and color makes of a telescopic view of jupiter a picture that will not quickly fade from the memory; while if an instrument of considerable power is used, so that the wonderful details of the belts, with their scalloped edges, their diagonal filaments, their many divisions, and their curious light and dark spots, are made plain, the observer is deeply impressed with the strangeness of the spectacle, and the more so as he reflects upon the enormous real magnitude of that which is spread before his eye. the whole earth flattened out would be but a small blotch on that gigantic disk! then, the visible rotation of the great jovian globe, whose effects become evident to a practised eye after but a few minutes' watching, heightens the impression. and the presence of the four satellites, whose motions in their orbits are also evident, through the change in their positions, during the course of a single not prolonged observation, adds its influence to the effectiveness of the scene. indeed, color and motion are so conspicuous in the immense spectacle presented by jupiter that they impart to it a powerful suggestion of life, which the mind does not readily divest itself of when compelled to face the evidence that jupiter is as widely different from the earth, and as diametrically opposed to lifelike conditions, as we comprehend them, as a planet possibly could be. the great belts lie in latitudes about corresponding to those in which the trade-winds blow upon the earth, and it has often been suggested that their existence indicates a similarity between the atmospheric circulation of jupiter and that of the world in which we live. no doubt there are times when the earth, seen with a telescope from a distant planet, would present a belted appearance somewhat resembling that of jupiter, but there would almost certainly be no similar display of colors in the clouds, and the latter would exhibit no such persistence in general form and position as characterizes those of jupiter. our clouds are formed by the action of the sun, producing evaporation of water; on jupiter, whose mean distance from the sun is more than five times as great as ours, the intensity of the solar rays is reduced to less than one twenty-fifth part of their intensity on the earth, so that the evaporation can not be equally active there, and the tendency to form aerial currents and great systems of winds must be proportionally slight. in brief, the clouds of jupiter are probably of an entirely different origin from that of terrestrial clouds, and rather resemble the chaotic masses of vapor that enveloped the earth when it was still in a seminebulous condition, and before its crust had formed. although the strongest features of the disk of jupiter are the great cloud belts, and the white or colored spots in the equatorial zone, yet the telescope shows many markings north and south of the belts, including a number of narrower and fainter belts, and small light or dark spots. none of them is absolutely fixed in position with reference to others. in other words, all of the spots, belts, and markings shift their places to a perceptible extent, the changes being generally very slow and regular, but occasionally quite rapid. the main belts never entirely disappear, and never depart very far from their mean positions with respect to the equator, but the smaller belts toward the north and south are more or less evanescent. round or oblong spots, as distinguished from belts, are still more variable and transient. the main belts themselves show great internal commotion, frequently splitting up, through a considerable part of their length, and sometimes apparently throwing out projections into the lighter equatorial zone, which occasionally resemble bridges, diagonally spanning the broad space between the belts. [illustration: jupiter as seen at the lick observatory in . the great red spot is visible, together with the indentation in the south belt.] perhaps the most puzzling phenomenon that has ever made its appearance on jupiter is the celebrated "great red spot," which was first noticed in , although it has since been shown to be probably identical with a similar spot seen in , and possibly with one noticed in . this spot, soon after its discovery in , became a clearly defined red oval, lying near the southern edge of the south belt in latitude about °. its length was nearly one third of the diameter of the disk and its width almost one quarter as great as its length. translated into terrestrial measure, it was about , miles long and , miles broad. in it seemed to deepen in color until it became a truly wonderful object, its redness of hue irresistibly suggesting the idea that it was something hot and glowing. during the following years it underwent various changes of appearance, now fading almost to invisibility and now brightening again, but without ever completely vanishing, and it is still ( ) faintly visible. nobody has yet suggested an altogether probable and acceptable theory as to its nature. some have said that it might be a part of the red-hot crust of the planet elevated above the level of the clouds; others that its appearance might be due to the clearing off of the clouds above a heated region of the globe beneath, rendering the latter visible through the opening; others that it was perhaps a mass of smoke and vapor ejected from a gigantic volcano, or from the vents covering a broad area of volcanic action; others that it might be a vast incandescent slag floating upon the molten globe of the planet and visible through, or above, the enveloping clouds; and others have thought that it could be nothing but a cloud among clouds, differing, for unknown reasons, in composition and cohesion from its surroundings. all of these hypotheses except the last imply the existence, just beneath the visible cloud shell, of a more or less stable and continuous surface, either solid or liquid. when the red spot began to lose distinctness a kind of veil seemed to be drawn over it, as if light clouds, floating at a superior elevation, had drifted across it. at times it has been reduced in this manner to a faint oval ring, the rim remaining visible after the central part has faded from sight. one of the most remarkable phenomena connected with the mysterious spot is a great bend, or scallop, in the southern edge of the south belt adjacent to the spot. this looks as if it were produced by the spot, or by the same cause to which the spot owes its existence. if the spot were an immense mountainous elevation, and the belt a current of liquid, or of clouds, flowing past its base, one would expect to see some such bend in the stream. the visual evidence that the belt is driven, or forced, away from the neighborhood of the spot seems complete. the appearance of repulsion between them is very striking, and even when the spot fades nearly to invisibility the curve remains equally distinct, so that in using a telescope too small to reveal the spot itself one may discover its location by observing the bow in the south belt. the suggestion of a resemblance to the flowing of a stream past the foot of an elevated promontory, or mountain, is strengthened by the fact, which was observed early in the history of the spot, that markings involved in the south belt have a quicker rate of rotation about the planet's axis than that of the red spot, so that such markings, first seen in the rear of the red spot, gradually overtake and pass it, and eventually leave it behind, as boats in a river drift past a rock lying in the midst of the current. this leads us to another significant fact concerning the peculiar condition of jupiter's surface. not only does the south belt move perceptibly faster than the red spot, but, generally speaking, the various markings on the surface of the planet move at different rates according as they are nearer to or farther from the equator. between the equator and latitude ° or ° there is a difference of six minutes in the rotation period--i.e., the equatorial parts turn round the axis so much faster than the parts north and south of them, that in one rotation they gain six minutes of time. in other words, the clouds over jupiter's equator flow past those in the middle latitudes with a relative velocity of miles per hour. but there are no sharp lines of separation between the different velocities; on the contrary, the swiftness of rotation gradually diminishes from the equator toward the poles, as it manifestly could not do if the visible surface of jupiter were solid. in this respect jupiter resembles the sun, whose surface also has different rates of rotation diminishing from the equator. measured by the motion of spots on or near the equator, jupiter's rotation period is about nine hours fifty minutes; measured by the motion of spots in the middle latitudes, it is about nine hours fifty-six minutes. the red spot completes a rotation in a little less than nine hours and fifty-six minutes, but its period can not be positively given for the singular reason that it is variable. the variation amounts to only a few seconds in the course of several years, but it is nevertheless certain. the phenomenon of variable motion is not, however, peculiar to the red spot. mr. w.f. denning, who has studied jupiter for a quarter of a century, says: "it is well known that in different latitudes of jupiter there are currents, forming the belts and zones, moving at various rates of speed. in many instances the velocity changes from year to year. and it is a singular circumstance that in the same current a uniform motion is not maintained in all parts of the circumference. certain spots move faster than others, so that if we would obtain a fair value for the rotation period of any current it is not sufficient to derive it from one marking alone; we must follow a number of objects distributed in different longitudes along the current and deduce a mean from the whole."[ ] [footnote : the observatory, no. , december, .] nor is this all. observation indicates that if we could look at a vertical section of jupiter's atmosphere we should behold an equally remarkable contrast and conflict of motions. there is evidence that some of the visible spots, or clouds, lie at a greater elevation than others, and it has been observed that the deeper ones move more rapidly. this fact has led some observers to conclude that the deep-lying spots may be a part of the actual surface of the planet. but if we could think that there is any solid nucleus, or core, in the body of jupiter, it would seem, on account of the slight mean density of the planet, that it can not lie so near the visible surface, but must be at a depth of thousands, perhaps tens of thousands, of miles. since the telescope is unable to penetrate the cloudy envelope we can only guess at the actual constitution of the interior of jupiter's globe. in a spirit of mere speculative curiosity it has been suggested that deep under the clouds of the great planet there may be a comparatively small solid globe, even a habitable world, closed round by a firmament all its own, whose vault, raised , or , miles above the surface of the imprisoned planet, appears only an unbroken dome, too distant to reveal its real nature to watchers below, except, perhaps, under telescopic scrutiny; enclosing, as in a shell, a transparent atmosphere, and deriving its illumination partly from the sunlight that may filter through, but mainly from some luminous source within. but is not jupiter almost equally fascinating to the imagination, if we dismiss all attempts to picture a humanly impossible world shut up within it, and turn rather to consider what its future may be, guided by the not unreasonable hypothesis that, because of its immense size and mass, it is still in a chaotic condition? mention has been made of the resemblance of jupiter to the sun by virtue of their similar manner of rotation. this is not the only reason for looking upon jupiter as being, in some respects, almost as much a solar as a planetary body. its exceptional brightness rather favors the view that a small part of the light by which it shines comes from its own incandescence. in size and mass it is half-way between the earth and the sun. jupiter is eleven times greater than the earth in diameter and thirteen hundred times greater in volume; the sun is ten times greater than jupiter in diameter and a thousand times greater in volume. the mean density of jupiter, as we have seen, is almost exactly the same as the sun's. now, the history of the solar system, according to the nebular hypothesis, is a history of cooling and condensation. the sun, a thousand times larger than jupiter, has not yet sufficiently cooled and contracted to become incrusted, except with a shell of incandescent metallic clouds; jupiter, a thousand times smaller than the sun, has cooled and contracted until it is but slightly, if at all, incandescent at its surface, while its thickening shell, although still composed of vapor and smoke, and still probably hot, has grown so dense that it entirely cuts off the luminous radiation from within; the earth, to carry the comparison one step further, being more than a thousand times smaller than jupiter, has progressed so far in the process of cooling that its original shell of vapor has given place to one of solid rock. a sudden outburst of light from jupiter, such as occurs occasionally in a star that is losing its radiance through the condensation of absorbing vapors around it, would furnish strong corroboration of the theory that jupiter is really an extinguished sun which is now on the way to become a planet in the terrestrial sense. not very long ago, as time is reckoned in astronomy, our sun, viewed from the distance of the nearer fixed stars, may have appeared as a binary star, the brighter component of the pair being the sun itself and the fainter one the body now called the planet jupiter. supposing the latter to have had the same intrinsic brilliance, surface for surface, as the sun, it would have radiated one hundred times less light than the sun. a difference of one hundredfold between the light of two stars means that they are six magnitudes apart; or, in other words, from a point in space where the sun appeared as bright as what we call a first-magnitude star, its companion, jupiter, would have shone as a sixth-magnitude star. many stars have companions proportionally much fainter than that. the companion of sirius, for instance, is at least ten thousand times less bright than its great comrade. looking at jupiter in this way, it interests us not as the probable abode of intelligent life, but as a world in the making, a world, moreover, which, when it is completed--if it ever shall be after the terrestrial pattern--will dwarf our globe into insignificance. that stupendous miracle of world-making which is dimly painted in the grand figures employed by the writers of genesis, and the composers of other cosmogonic legends, is here actually going on before our eyes. the telescope shows us in the cloudy face of jupiter the moving of the spirit upon the face of the great deep. what the final result will be we can not tell, but clearly the end of the grand processes there in operation has not yet been reached. the interesting suggestion was made and urged by mr. proctor that if jupiter itself is in no condition at present to bear life, its satellites may be, in that respect, more happily circumstanced. it can not be said that very much has been learned about the satellites of jupiter since proctor's day, and his suggestion is no less and no more probable now than it was when first offered. there has been cumulative evidence that jupiter's satellites obey the same law that governs the rotation of our moon, viz., that which compels them always to keep the same face turned toward their primary, and this would clearly affect, although it might not preclude, their habitability. with the exception of the minute fifth satellite discovered by barnard in , they are all of sufficient size to retain at least some traces of an atmosphere. in fact, one of them is larger than the planet mars, and another is of nearly the same size as that planet, while the smallest of the four principal ones is about equal to our moon. under the powerful attraction of jupiter they travel rapidly, and viewed from the surface of that planet they would offer a wonderful spectacle. they are continually causing solar eclipses and themselves undergoing eclipse in jupiter's shadow, and their swiftly changing aspects and groupings would be watched by an astronomer on jupiter with undying interest. but far more wonderful would be the spectacle presented by jupiter to inhabitants dwelling on his moons. from the nearer moon, in particular, which is situated less than , miles from jupiter's surface, the great planet would be an overwhelming phenomenon in the sky. its immense disk, hanging overhead, would cover a circle of the firmament twenty degrees in diameter, or, in round numbers, forty times the diameter of the full moon as seen from the earth! it would shed a great amount of light and heat, and thus would more or less effectively supply the deficit of solar radiation, for we must remember that jupiter and his satellites receive from the sun less than one twenty-fifth as much light and heat as the earth receives. the maze of contending motions, the rapid flow and eddying of cloud belts, the outburst of strange fiery spots, the display of rich, varied, and constantly changing colors, which astonish and delight the telescopic observer on the earth, would be exhibited to the naked eye of an inhabitant of jupiter's nearest moon far more clearly than the greatest telescope is able to reveal them to us. here, again, the mind is carried back to long past ages in the history of the planet on which we dwell. it is believed by some that our moon may have contained inhabitants when the earth was still hot and glowing, as jupiter appears to be now, and that, as the earth cooled and became habitable, the moon gradually parted with its atmosphere and water so that its living races perished almost coincidently with the beginning of life on the earth. if we accept this view and apply it to the case of jupiter we may conclude that when that enormous globe has cooled and settled down to a possibly habitable condition, its four attendant moons will suffer the fate that overtook the earth's satellite, and in their turn become barren and death-stricken, while the great orb that once nurtured them with its light and heat receives the promethean fire and begins to bloom with life. chapter vii saturn, a prodigy among planets one of the first things that persons unaccustomed to astronomical observations ask to see when they have an opportunity to look through a telescope is the planet saturn. many telescopic views in the heavens disappoint the beginner, but that of saturn does not. even though the planet may not look as large as he expects to see it from what he has been told of the magnifying power employed, the untrained observer is sure to be greatly impressed by the wonderful rings, suspended around it as if by a miracle. no previous inspection of pictures of these rings can rob them of their effect upon the eye and the mind. they are overwhelming in their inimitable singularity, and they leave every spectator truly amazed. sir john herschel has remarked that they have the appearance of an "elaborately artificial mechanism." they have even been regarded as habitable bodies! what we are to think of that proposition we shall see when we come to consider their composition and probable origin. in the meantime let us recall the main facts of saturn's dimensions and situation in the solar system. saturn is the second of the major, or jovian, group of planets, and is situated at a mean distance from the sun of , , miles. we need not consider the eccentricity of its orbit, which, although relatively not very great, produces a variation of , , miles in its distance from the sun, because, at its immense mean distance, this change would not be of much importance with regard to the planet's habitability or non-habitability. under the most favorable conditions saturn can never be nearer than , , miles to the earth, or eight times the sun's distance from us. it receives from the sun about one ninetieth of the light and heat that we get. [illustration: saturn in its three principal phases as seen from the earth. from a drawing by bond.] saturn takes twenty-nine and a half years to complete a journey about the sun. like jupiter, it rotates very rapidly on its axis, the period being ten hours and fourteen minutes. its axis of rotation is inclined not far from the same angle as that of the earth's axis ( ° ´), so that its seasons should resemble ours, although their alternations are extremely slow in consequence of the enormous length of saturn's year. not including the rings in the calculation, saturn exceeds the earth in size times. the addition of the rings would not, however, greatly alter the result of the comparison, because, although the total surface of the rings, counting both faces, exceeds the earth's surface about times, their volume, owing to their surprising thinness, is only about six times the volume of the earth, and their mass, in consequence of their slight density, is very much less than the earth's, perhaps, indeed, inappreciable in comparison. saturn's mean diameter is , miles, and its polar compression is even greater than that of jupiter, a difference of , miles--almost comparable with the entire diameter of the earth--existing between its equatorial and its polar diameter, the former being , and the latter , miles. we found the density of jupiter astonishingly slight, but that of saturn is slighter still. jupiter would sink if thrown into water, but saturn would actually float, if not "like a cork," yet quite as buoyantly as many kinds of wood, for its mean density is only three quarters that of water, or one eighth of the earth's. in fact, there is no known planet whose density is so slight as saturn's. thus it happens that, notwithstanding its vast size and mass, the force of gravity upon saturn is nearly the same as upon our globe. upon visiting venus we should find ourselves weighing a little less than at home, and upon visiting saturn a little more, but in neither case would the difference be very important. if the relative weight of bodies on the surfaces of planets formed the sole test of their habitability, venus and saturn would both rank with the earth as suitable abodes for men. but the exceedingly slight density of saturn seems to be most reasonably accounted for on the supposition that, like jupiter, it is in a vaporous condition, still very hot within--although but slightly, if at all, incandescent at the surface--and, therefore, unsuited to contain life. it is hardly worth while to speculate about any solid nucleus within, because, even if such a thing were possible, or probable, it must lie forever hidden from our eyes. but if we accept the theory that saturn is in an early formative stage, and that, millions of years hence, it may become an incrusted and habitable globe, we shall, at least, follow the analogy of what we believe to have been the history of the earth, except that saturn's immense distance from the sun will always prevent it from receiving an amount of solar radiation consistent with our ideas of what is required by a living world. of course, since one can imagine what he chooses, it is possible to suppose inhabitants suited to existence in a world composed only of whirling clouds, and a poet with the imagination of a milton might give us very imposing and stirring images of such creatures and their chaotic surroundings, but fancies like these can have no basis in human experience, and consequently can make no claim upon scientific recognition. or, as an alternative, it might be assumed that saturn is composed of lighter elements and materials than those which constitute the earth and the other solid planets in the more immediate neighborhood of the sun. but such an assumption would put us entirely at sea as regards the forms of organic life that could exist upon a planet of that description, and, like sir humphry davy in the vision, that occupies the first chapter of his quaintly charming consolations in travel, or, the last days of a philosopher, we should be thrown entirely upon the resources of the imagination in representing to ourselves the nature and appearance of its inhabitants. yet minds of unquestioned power and sincerity have in all ages found pleasure and even profit in such exercises, and with every fresh discovery arises a new flight of fancies like butterflies from a roadside pool. as affording a glimpse into the mind of a remarkable man, as well as a proof of the fascination of such subjects, it will be interesting to quote from the book just mentioned davy's description of his imaginary inhabitants of saturn: "i saw below me a surface infinitely diversified, something like that of an immense glacier covered with large columnar masses, which appeared as if formed of glass, and from which were suspended rounded forms of various sizes which, if they had not been transparent, i might have supposed to be fruit. from what appeared to me to be analogous to bright-blue ice, streams of the richest tint of rose color or purple burst forth and flowed into basins, forming lakes or seas of the same color. looking through the atmosphere toward the heavens, i saw brilliant opaque clouds, of an azure color, that reflected the light of the sun, which had to my eyes an entirely new aspect and appeared smaller, as if seen through a dense blue mist. "i saw moving on the surface below me immense masses, the forms of which i find it impossible to describe. they had systems for locomotion similar to those of the morse, or sea-horse, but i saw, with great surprise, that they moved from place to place by six extremely thin membranes, which they used as wings. their colors were varied and beautiful, but principally azure and rose color. i saw numerous convolutions of tubes, more analogous to the trunk of the elephant than to anything else i can imagine, occupying what i supposed to be the upper parts of the body. it was with a species of terror that i saw one of them mounting upward, apparently flying toward those opaque clouds which i have before mentioned. "'i know what your feelings are,' said the genius; 'you want analogies, and all the elements of knowledge to comprehend the scene before you. you are in the same state in which a fly would be whose microscopic eye was changed for one similar to that of man, and you are wholly unable to associate what you now see with your former knowledge. but those beings who are before you, and who appear to you almost as imperfect in their functions as the zoophytes of the polar sea, to which they are not unlike in their apparent organization to your eyes, have a sphere of sensibility and intellectual enjoyment far superior to that of the inhabitants of your earth. each of those tubes, which appears like the trunk of an elephant, is an organ of peculiar motion or sensation. they have many modes of perception of which you are wholly ignorant, at the same time that their sphere of vision is infinitely more extended than yours, and their organs of touch far more perfect and exquisite.'" after descanting upon the advantages of saturn's position for surveying some of the phenomena of the solar system and of outer space, and the consequent immense advances that the saturnians have made in astronomical knowledge, the genius continues: "'if i were to show you the different parts of the surface of this planet you would see the marvelous results of the powers possessed by these highly intellectual beings, and of the wonderful manner in which they have applied and modified matter. those columnar masses, which seem to you as if rising out of a mass of ice below, are results of art, and processes are going on within them connected with the formation and perfection of their food. the brilliant-colored fluids are the results of such operations as on the earth would be performed in your laboratories, or more properly in your refined culinary apparatus, for they are connected with their system of nourishment. those opaque azure clouds, to which you saw a few minutes ago one of those beings directing his course, are works of art, and places in which they move through different regions of their atmosphere, and command the temperature and the quantity of light most fitted for their philosophical researches, or most convenient for the purposes of life.'"[ ] [footnote : davy, of course, was aware that, owing to increase of distance, the sun would appear to an inhabitant of saturn with a disk only one ninetieth as great in area as that which it presents to our eyes.] but, while saturn does not appear, with our present knowledge, to hold out any encouragement to those who would regard it as the abode of living creatures capable of being described in any terms except those of pure imagination, yet it is so unique a curiosity among the heavenly bodies that one returns again and again to the contemplation of its strange details. saturn has nine moons, but some of them are relatively small bodies--the ninth, discovered photographically by professor pickering in , being especially minute--and others are situated at great distances from the planet, and for these reasons, together with the fact that the sunlight is so feeble upon them that, surface for surface, they have only one ninetieth as much illumination as our moon receives, they can not make a very brilliant display in the saturnian sky. to astronomers on saturn they would, of course, be intensely interesting because of their perturbations and particularly the effect of their attraction on the rings. this brings us again to the consideration of those marvelous appendages, and to the statement of facts about them which we have not yet recalled. if the reader will take a ball three inches in diameter to represent the globe of saturn, and, out of the center of a circular piece of writing-paper seven inches in diameter, will cut a round hole three and three quarter inches across, and will then place the ball in the middle of the hole in the paper, he will have a very fair representation of the relative proportions of saturn and its rings. to represent the main gap or division in the rings he might draw, a little more than three eighths of an inch from the outer edge of the paper disk, a pencil line about a sixteenth of an inch broad. perhaps the most striking fact that becomes conspicuous in making such a model of the saturnian system is the exceeding thinness of the rings as compared with their enormous extent. they are about , miles across from outer edge to outer edge, and about , miles broad from outer edge to inner edge--including the gauze ring presently to be mentioned--yet their thickness probably does not surpass one hundred miles! in fact, the sheet of paper in our imaginary model is several times too thick to represent the true relative thickness of saturn's rings. several narrow gaps in the rings have been detected from time to time, but there is only one such gap that is always clearly to be seen, the one already mentioned, situated about , miles from the outer edge and about , miles in width. inside of this gap the broadest and brightest ring appears, having a width of about , miles. for some reason this great ring is most brilliant near the gap, and its brightness gradually falls off toward its inner side. at a distance of something less than , miles from the planet--or perhaps it would be more correct to say above the planet, for the rings hang directly over saturn's equator--the broad, bright ring merges into a mysterious gauzelike object, also in the form of a ring, which extends to within , or , miles of the planet's surface, and therefore itself has a width of say , miles. in consequence of the thinness of the rings they completely disappear from the range of vision of small telescopes when, as occurs once in every fifteen years, they are seen exactly edgewise from the earth. in a telescope powerful enough to reveal them when in that situation they resemble a thin, glowing needle run through the ball of the planet. the rings will be in this position in , and again in . the opacity of the rings is proved by the shadow which they cast upon the ball of the planet. this is particularly manifest at the time when they are edgewise to the earth, for the sun being situated slightly above or below the plane of the rings then throws their shadow across saturn close to its equator. when they are canted at a considerable angle to our line of sight their shadow is seen on the planet, bordering their outer edge where they cross the ball. the gauze ring, the detection of which as a faintly luminous phenomenon requires a powerful telescope, can be seen with slighter telescopic power in the form of a light shade projected against the planet at the inner edge of the broad bright ring. the explanation of the existence of this peculiar object depends upon the nature of the entire system, which, instead of being, as the earliest observers thought it, a solid ring or series of concentric rings, is composed of innumerable small bodies, like meteorites, perhaps, in size, circulating independently but in comparatively close juxtaposition to one another about saturn, and presenting to our eyes, because of their great number and of our enormous distance, the appearance of solid, uniform rings. so a flock of ducks may look from afar like a continuous black line or band, although if we were near them we should perceive that a considerable space separates each individual from his neighbors. the fact that this is the constitution of saturn's rings can be confidently stated because it has been mathematically proved that they could not exist if they were either solid or liquid bodies in a continuous form, and because the late prof. james e. keeler demonstrated with the spectroscope, by means of the doppler principle, already explained in the chapter on venus, that the rings circulate about the planet with varying velocities according to their distance from saturn's center, exactly as independent satellites would do. it might be said, then, that saturn, instead of having nine satellites only, has untold millions of them, traveling in orbits so closely contiguous that they form the appearance of a vast ring. as to their origin, it may be supposed that they are a relic of a ring of matter left in suspension during the contraction of the globe of saturn from a nebulous mass, just as the rings from which the various planets are supposed to have been formed were left off during the contraction of the main body of the original solar nebula. other similar rings originally surrounding saturn may have become satellites, but the matter composing the existing rings is so close to the planet that it falls within the critical distance known as "roche's limit," within which, owing to the tidal effect of the planet's attraction, no body so large as a true satellite could exist, and accordingly in the process of formation of the saturnian system this matter, instead of being aggregated into a single satellite, has remained spread out in the form of a ring, although its substance long ago passed from the vaporous and liquid to the solid form. we have spoken of the rings as being composed of meteorites, but perhaps their component particles may be so small as to answer more closely to the definition of dust. in these rings of dust, or meteorites, disturbances are produced by the attraction of the planet and that of the outer satellites, and it is yet a question whether they are a stable and permanent feature of saturn, or will, in the course of time, be destroyed.[ ] [footnote : for further details about saturn's rings, see the tides, by g.h. darwin, chap. xx.] it has been thought that the gauze ring is variable in brightness. this would tend to show that it is composed of bodies which have been drawn in toward the planet from the principal mass of the rings, and these bodies may end their career by falling upon the planet. this process, indefinitely continued, would result in the total disappearance of the rings--saturn would finally swallow them, as the old god from whom the planet gets its name is fabled to have swallowed his children. near the beginning of this chapter reference was made to the fact that saturn's rings have been regarded as habitable bodies. that, of course, was before the discovery that they were not solid. knowing what we now know about them, even dr. thomas dick, the great scotch popularizer of astronomy in the first half of the nineteenth century, would have been compelled to abandon his theory that saturn's rings were crowded with inhabitants. at the rate of to the square mile he reckoned that they could easily contain , , , , people. he even seems to have regarded their edges--in his time their actual thinness was already well known--as useful ground for the support of living creatures, for he carefully calculated the aggregate area of these edges and found that it considerably exceeded the area of the entire surface of the earth. indeed, dr. dick found room for more inhabitants on saturn's rings than on saturn itself, for, excluding the gauze ring, undiscovered in his day, the two surfaces of the rings are greater in area than the surface of the globe of the planet. he did not attack the problem of the weight of bodies on worlds in the form of broad, flat, thin, surfaces like saturn's rings, or indulge in any reflections on the interrelations of the inhabitants of the opposite sides, although he described the wonderful appearance of saturn and other celestial objects as viewed from the rings. but all these speculations fall to the ground in face of the simple fact that if we could reach saturn's rings we should find nothing to stand upon, except a cloud of swiftly flying dust or a swarm of meteors, swayed by contending attractions. and, indeed, it is likely that upon arriving in the immediate neighborhood of the rings they would virtually disappear! seen close at hand their component particles might be so widely separated that all appearance of connection between them would vanish, and it has been estimated that from saturn's surface the rings, instead of presenting a gorgeous arch spanning the heavens, may be visible only as a faintly gleaming band, like the milky way or the zodiacal light. in this respect the mystic swedenborg appears to have had a clearer conception of the true nature of saturn's rings than did dr. dick, for in his book on the earths in the universe he says--using the word "belt" to describe the phenomenon of the rings: "being questioned concerning that great belt which appears from our earth to rise above the horizon of that planet, and to vary its situations, they [the inhabitants of saturn] said that it does not appear to them as a belt, but only as somewhat whitish, like snow in the heaven, in various directions." in view of such observations as that of prof. e.e. barnard, in , showing that a satellite passing through the shadow of saturn's rings does not entirely disappear--a fact which proves that the rings are partially transparent to the sunlight--one might be tempted to ask whether saturn itself, considering its astonishing lack of density, is not composed, at least in its outer parts, of separate particles of matter revolving independently about their center of attraction, and presenting the appearance of a smooth, uniform shell reflecting the light of the sun. in other words, may not saturn be, exteriorly, a globe of dust instead of a globe of vapor? certainly the rings, incoherent and translucent though they be, reflect the sunlight to our eyes, at least from the brighter part of their surface, with a brilliance comparable with that of the globe of the planet itself. as bearing on the question of the interior condition of saturn and jupiter, it should, perhaps, be said that mathematical considerations, based on the figures of equilibrium of rotating liquid masses, lead to the conclusion that those planets are comparatively very dense within. professor darwin puts the statement very strongly, as follows: "in this way it is known with certainty that the central portions of the planets jupiter and saturn are much denser, compared to their superficial portions, than is the case with the earth."[ ] [footnote : the tides, by g.h. darwin, p. .] the globe and rings of saturn witness an imposing spectacle of gigantic moving shadows. the great ball stretches its vast shade across the full width of the rings at times, and the rings, as we have seen, throw their shadow in a belt, whose position slowly changes, across the ball, sweeping from the equator, now toward one pole and now toward the other. the sun shines alternately on each side of the rings for a space of nearly fifteen years--a day fifteen years long! and then, when that face of the ring is turned away from the sun, there ensues a night of fifteen years' duration also. whatever appearance the rings may present from the equator and the middle latitudes on saturn, from the polar regions they would be totally invisible. as one passed toward the north, or the south, pole he would see the upper part of the arch of the rings gradually sink toward the horizon until at length, somewhere in the neighborhood of the polar circle, it would finally disappear, hidden by the round shoulder of the great globe. uranus, neptune, and the suspected ultraneptunian planet what has been said of jupiter and saturn applies also to the remaining members of the jovian group of planets, uranus and neptune, viz., that their density is so small that it seems probable that they can not, at the present time, be in a habitable planetary condition. all four of these outer, larger planets have, in comparatively recent times, been solar orbs, small companions of the sun. the density of uranus is about one fifth greater than that of water, and slightly greater than that of neptune. uranus is , miles in diameter, and neptune , miles. curiously enough, the force of gravity upon each of these two large planets is a little less than upon the earth. this arises from the fact that in reckoning gravity on the surface of a planet not only the mass of the planet, but its diameter or radius, must be considered. gravity varies directly as the mass, but inversely as the square of the radius, and for this reason a large planet of small density may exercise a less force of gravity at its surface than does a small planet of great density. the mean distance of uranus from the sun is about , , , miles, and its period of revolution is eighty-four years; neptune's mean distance is about , , , miles, and its period of revolution is about years. uranus has four satellites, and neptune one. the remarkable thing about these satellites is that they revolve _backward_, or contrary to the direction in which all the other satellites belonging to the solar system revolve, and in which all the other planets rotate on their axis. in the case of uranus, the plane in which the satellites revolve is not far from a position at right angles to the plane of the ecliptic; but in the case of neptune, the plane of revolution of the satellites is tipped much farther backward. since in every other case the satellites of a planet are situated nearly in the plane of the planet's equator, it may be assumed that the same rule holds with uranus and neptune; and, that being so, we must conclude that those planets rotate backward on their axes. this has an important bearing on the nebular hypothesis of the origin of the solar system, and at one time was thought to furnish a convincing argument against that hypothesis; but it has been shown that by a modification of laplace's theory the peculiar behavior of uranus and neptune can be reconciled with it. very little is known of the surfaces of uranus and neptune. indications of the existence of belts resembling those of jupiter have been found in the case of both planets. there are similar belts on saturn, and as they seem to be characteristic of large, rapidly rotating bodies of small density, it was to be expected that they would be found on uranus and neptune. the very interesting opinion is entertained by some astronomers that there is at least one other great planet beyond neptune. the orbits of certain comets are relied upon as furnishing evidence of the existence of such a body. prof. george forbes has estimated that this, as yet undiscovered, planet may be even greater than jupiter in mass, and may be situated at a distance from the sun one hundred times as great as the earth's, where it revolves in an orbit a single circuit of which requires a thousand years. whether this planet, with a year a thousand of our years in length, will ever be seen with a telescope, or whether its existence will ever, in some other manner, be fully demonstrated, can not yet be told. it will be remembered that neptune was discovered by means of computations based upon its disturbing attraction on uranus before it had ever been recognized with the telescope. but when the astronomers in the observatories were told by their mathematical brethren where to look they found the planet within half an hour after the search began. so it is possible the suspected great planet beyond neptune may be within the range of telescopic vision, but may not be detected until elaborate calculations have deduced its place in the heavens. as a populous city is said to furnish the best hiding-place for a man who would escape the attention of his fellow beings, so the star-sprinkled sky is able to conceal among its multitudes worlds both great and small until the most painstaking detective methods bring them to recognition. chapter viii the moon, child of the earth and the sun very naturally the moon has always been a great favorite with those who, either in a scientific or in a literary spirit, have speculated about the plurality of inhabited worlds. the reasons for the preference accorded to the moon in this regard are evident. unless a comet should brush us--as a comet is suspected of having done already--no celestial body, of any pretensions to size, can ever approach as near to the earth as the moon is, at least while the solar system continues to obey the organic laws that now control it. it is only a step from the earth to the moon. what are , miles in comparison with the distances of the stars, or even with the distances of the planets? jupiter, driving between the earth and the moon, would occupy more than one third of the intervening space with the chariot of his mighty globe; saturn, with broad wings outspread, would span more than two thirds of the distance; and the sun, so far from being able to get through at all, would overlap the way more than , miles on each side. in consequence, of course, of its nearness, the moon is the only member of the planetary system whose principal features are visible to the naked eye. in truth, the naked eye perceives the larger configurations of the lunar surface more clearly than the most powerful telescope shows the details on the disk of mars. long before the time of galileo and the invention of the telescope, men had noticed that the face of the moon bears a resemblance to the appearance that the earth would present if viewed from afar off. in remote antiquity there were philosophers who thought that the moon was an inhabited world, and very early the romancers took up the theme. lucian, the voltaire of the second century of our era, mercilessly scourged the pretenders of the earth from an imaginary point of vantage on the moon, which enabled him to peer down into their secrets. lucian's description of the appearance of the earth from the moon shows how clearly defined in his day had become the conception of our globe as only an atom in space. "especially did it occur to me to laugh at the men who were quarreling about the boundaries of their land, and at those who were proud because they cultivated the sikyonian plain, or owned that part of marathon around oenoe, or held possession of a thousand acres at acharnæ. of the whole of greece, as it then appeared to me from above, being about the size of four fingers, i think attica was in proportion a mere speck. so that i wondered on what condition it was left to these rich men to be proud."[ ] [footnote : ikaromenippus; or, above the clouds. prof. d.c. brown's translation.] such scenes as lucian beheld, in imagination, upon the earth while looking from the moon, many would fain behold, with telescopic aid, upon the moon while looking from the earth. galileo believed that the details of the lunar surface revealed by his telescope closely resembled in their nature the features of the earth's surface, and for a long time, as the telescope continued to be improved, observers were impressed with the belief that the moon possessed not only mountains and plains, but seas and oceans also. it was the discovery that the moon has no perceptible atmosphere that first seriously undermined the theory of its habitability. yet, as was remarked in the introductory chapter, there has of late been some change of view concerning a lunar atmosphere; but the change has been not so much in the ascertained facts as in the way of looking at those facts. but before we discuss this matter, it will be well to state what is known beyond peradventure about the moon. its mean distance from the earth is usually called, for the sake of a round number, , miles, but more accurately stated it is , miles. this is variable to the extent of more than , miles, on account of the eccentricity of its orbit, and the eccentricity itself is variable, in consequence of the perturbing attractions of the earth and the sun, so that the distance of the moon from the earth is continually changing. it may be as far away as , miles and as near as , miles. although the orbit of the moon is generally represented, for convenience, as an ellipse about the earth, it is, in reality, a varying curve, having the sun for its real focus, and always concave toward the latter. this is a fact that can be more readily explained with the aid of a diagram. [illustration: the moon's path with respect to the sun and the earth.] in the accompanying cut, when the earth is at _a_ the moon is between it and the sun, in the phase called new moon. at this point the earth's orbit about the sun is more curved than the moon's, and the earth is moving relatively faster than the moon, so that when it arrives at _b_ it is ahead of the moon, and we see the latter to the right of the earth, in the phase called first quarter. the earth being at this time ahead of the moon, the effect of its attraction, combined with that of the sun, tends to hasten the moon onward in its orbit about the sun, and the moon begins to travel more swiftly, until it overtakes the earth at _c_, and appears on the side opposite the sun, in the phase called full moon. at this point the moon's orbit about the sun has a shorter radius of curvature than the earth's. in traveling from _c_ to _d_ the moon still moves more rapidly than the earth, and, having passed it, appears at _d_ to the left of the earth, in the phase called third quarter. now, the earth being behind the moon, the effect of its attraction combined with the sun's tends to retard the moon in its orbit about the sun, with the result that the moon moves again less rapidly than the earth, and the latter overtakes it, so that, upon reaching _e_, the two are once more in the same relative positions that they occupied at _a_, and it is again new moon. thus it will be seen that, although the real orbit of the moon has the sun for its center of revolution, nevertheless, in consequence of the attraction of the earth, combined in varying directions with that of the sun, the moon, once every month, makes a complete circuit of our globe. the above explanation should not be taken for a mathematical demonstration of the moon's motion, but simply for a graphical illustration of how the moon appears to revolve about the earth while really obeying the sun's attraction as completely as the earth does. there is no other planet that has a moon relatively as large as ours. the moon's diameter is , miles. its volume, compared with the earth's, is in the ratio of to , and its density is about six tenths of the earth's. this makes its mass to that of our globe about as to . in other words, it would take eighty-one moons to counterbalance the earth. before speaking of the force of gravity on the moon we will examine the character of the lunar surface. to the naked eye the moon's face appears variegated with dusky patches, while a few points of superior brilliance shine amid the brighter portions, especially in the southern and eastern quarters, where immense craters like tycho and copernicus are visible to a keen eye, gleaming like polished buttons. with a telescope, even of moderate power, the surface of the moon presents a scene of astonishing complexity, in which strangeness, beauty, and grandeur are all combined. the half of the moon turned earthward contains an area of , , square miles, a little greater than the area of south america and a little less than that of north america. of these , , square miles, about , , square miles are occupied by the gray, or dusky, expanses, called in lunar geography, or selenography, _maria_--i.e., "seas." whatever they may once have been, they are not now seas, but dry plains, bordered in many places by precipitous cliffs and mountains, varied in level by low ridges and regions of depression, intersected occasionally by immense cracks, having the width and depth of our mightiest river cañons, and sprinkled with bright points and crater pits. the remaining , , square miles are mainly occupied by mountains of the most extraordinary character. owing partly to roughness of the surface and partly to more brilliant reflective power, the mountainous regions of the moon appear bright in comparison with the dull-colored plains. some of the lunar mountains lie in long, massive chains, with towering peaks, profound gorges, narrow valleys, vast amphitheaters, and beetling precipices. looking at them with a powerful telescope, the observer might well fancy himself to be gazing down from an immense height into the heart of the untraveled himalayas. but these, imposing though they are, do not constitute the most wonderful feature of the mountain scenery of the moon. appearing sometimes on the shores of the "seas," sometimes in the midst of broad plains, sometimes along the course of mountain chains, and sometimes in magnificent rows, following for hundreds of miles the meridians of the lunar globe, are tremendous, mountain-walled, circular chasms, called craters. frequently they have in the middle of their depressed interior floors a peak, or a cluster of peaks. their inner and outer walls are seamed with ridges, and what look like gigantic streams of frozen lava surround them. the resemblance that they bear to the craters of volcanoes is, at first sight, so striking that probably nobody would ever have thought of questioning the truth of the statement that they are such craters but for their incredible magnitude. many of them exceed fifty miles in diameter, and some of them sink two, three, four, and more miles below the loftiest points upon their walls! there is a chasm, miles long and broad, named newton, situated about miles from the south pole of the moon, whose floor lies , feet below the summit of a peak that towers just above it on the east! this abyss is so profound that the shadows of its enclosing precipices never entirely quit it, and the larger part of its bottom is buried in endless night. one can not but shudder at the thought of standing on the broken walls of newton, and gazing down into a cavity of such stupendous depth that if chimborazo were thrown into it, the head of the mighty andean peak would be thousands of feet beneath the observer. a different example of the crater mountains of the moon is the celebrated tycho, situated in latitude about ° south, corresponding with the latitude of southern new zealand on the earth. tycho is nearly circular and a little more than miles across. the highest point on its wall is about , feet above the interior. in the middle of its floor is a mountain , or , feet high. tycho is especially remarkable for the vast system of whitish streaks, or rays, which starting from its outer walls, spread in all directions over the face of the moon, many of them, running, without deviation, hundreds of miles across mountains, craters, and plains. these rays are among the greatest of lunar mysteries, and we shall have more to say of them. [illustration: the lunar alps, apennines, and caucasus. photographed with the lick telescope.] copernicus, a crater mountain situated about ° north of the equator, in the eastern hemisphere of the moon, is another wonderful object, miles in diameter, a polygon appearing, when not intently studied, as a circle, , or , feet deep, and having a group of relatively low peaks in the center of its floor. around copernicus an extensive area of the moon's surface is whitened with something resembling the rays of tycho, but more irregular in appearance. copernicus lies within the edge of the great plain named the _oceanus procellarum_, or "ocean of storms," and farther east, in the midst of the "ocean," is a smaller crater mountain, named kepler, which is also enveloped by a whitish area, covering the lunar surface as if it were the result of extensive outflows of light-colored lava. in one important particular the crater mountains of the moon differ from terrestrial volcanoes. this difference is clearly described by nasmyth and carpenter in their book on the moon: "while the terrestrial crater is generally a hollow on a mountain top, with its flat bottom high above the level of the surrounding country, those upon the moon have their lowest points depressed more or less deeply below the general surface of the moon, the external height being frequently only a half or one third of the internal depth." it has been suggested that these gigantic rings are only "basal wrecks" of volcanic mountains, whose conical summits have been blown away, leaving vast crateriform hollows where the mighty peaks once stood; but the better opinion seems to be that which assumes that the rings were formed by volcanic action very much as we now see them. if such a crater as copernicus or the still larger one named theophilus, which is situated in the western hemisphere of the moon, on the shore of the "sea of nectar," ever had a conical mountain rising from its rim, the height attained by the peak, if the average slope were about °, would have been truly stupendous--fifteen or eighteen miles! there is a kind of ring mountains, found in many places on the moon, whose forms and surroundings do not, as the craters heretofore described do, suggest at first sight a volcanic origin. these are rather level plains of an oval or circular outline, enclosed by a wall of mountains. the finest example is, perhaps, the dark-gray plato, situated in ° of north latitude, near an immense mountain uplift named the lunar alps, and on the northern shore of the _mare imbrium_, or "sea of showers." plato appears as an oval plain, very smooth and level, about miles in length, and completely surrounded by mountains, quite precipitous on the inner side, and rising in their highest peaks to an elevation of , to , feet. enclosed plains, bearing more or less resemblance to plato--sometimes smooth within, and sometimes broken with small peaks and craters or hilly ridges--are to be found scattered over almost all parts of the moon. if our satellite was ever an inhabited world like the earth, while its surface was in its present condition, these valleys must have presented an extraordinary spectacle. it has been thought that they may once have been filled with water, forming lakes that recall the curious crater lake of oregon. [illustration: the moon at first and last quarter (western and eastern hemispheres). photographed with the lick telescope.] it is not my intention to give a complete description of the various lunar features, and i mention but one other--the "clefts" or "rills," which are to be seen running across the surface like cracks. one of the most remarkable of these is found in the _oceanus procellarum_, near the crater-mountain aristarchus, which is famed for the intense brilliance of its central peak, whose reflective power is so great that it was once supposed to be aflame with volcanic fire. the cleft, or crack, in question is very erratic in its course, and many miles in length, and it terminates in a ringed plain named herodotus not far east of aristarchus, breaking through the wall of the plain and entering the interior. many other similar chasms or cañons exist on the moon, some crossing plains, some cleaving mountain walls, and some forming a network of intersecting clefts. mr. thomas gwyn elger has this to say on the subject of the lunar clefts: "if, as seems most probable, these gigantic cracks are due to contractions of the moon's surface, it is not impossible, in spite of the assertions of the text-books to the effect that our satellite is now a 'changeless world,' that emanations may proceed from these fissures, even if, under the monthly alternations of extreme temperatures, surface changes do not now occasionally take place from this cause also. should this be so, the appearance of new rills and the extension and modification of those already existing may reasonably be looked for." mr. elger then proceeds to describe his discovery in , in the ring-plain mersenius, of a cleft never noticed before, and which seems to have been of recent formation.[ ] [footnote : the moon, a full description and map of its principal features, by thomas gwyn elger, . those who desire to read detailed descriptions of lunar scenery may consult, in addition to mr. elger's book, the following: the moon, considered as a planet, a world, and a satellite, by james nasmyth and james carpenter, ; the moon, and the condition and configurations of its surface, by edmund neison, . see also annals of harvard college observatory, vol. xxxii, part ii, , for observations made by prof. william h. pickering at the arequipa observatory.] we now return to the question of the force of lunar gravity. this we find to be only one sixth as great as gravity on the surface of the earth. it is by far the smallest force of gravity that we have found anywhere except on the asteroids. employing the same method of comparison that was made in the case of mars, we compute that a man on the moon could attain a height of thirty-six feet without being relatively more unwieldy than a six-foot descendant of adam is on the earth. whether this furnishes a sound reason for assuming that the lunar inhabitants, if any exist or have ever existed, should be preposterous giants is questionable; yet such an assumption receives a certain degree of support from the observed fact that the natural features of the moon are framed on an exaggerated scale as compared with the earth's. we have just observed that the moon is characterized by vast mountain rings, attaining in many cases a diameter exceeding fifty miles. if these are volcanic craters, it is evident, at a glance, that the mightiest volcanoes of the earth fall into insignificance beside them. now, the slight force of gravity on the moon has been appealed to as a reason why volcanic explosions on the lunar globe should produce incomparably greater effects than upon the earth, where the ejected materials are so much heavier. the same force that would throw a volcanic bomb a mile high on the earth could throw it six miles high on the moon. the giant cannon that we have placed in one of our coast forts, which is said to be able to hurl a projectile to a distance of fifteen miles, could send the same projectile ninety miles on the moon. an athlete who can clear a horizontal bar at a height of six feet on the earth could clear the same bar at a height of thirty-six feet on the moon. in other words, he could jump over a house, unless, indeed, the lunarians really are giants, and live in houses proportioned to their own dimensions and to the size of their mountains. in that case, our athlete would have to content himself with jumping over a lunarian, whose head he could just clear--with the hat off. these things are not only amusing, but important. there can be no question that the force of gravity on the moon actually is as slight as it has just been described. so, even without calling in imaginary inhabitants to lend it interest, the comparative inability of the moon to arrest bodies in motion becomes a fact of much significance. it has led to the theory that meteorites may have originally been shot out of the moon's great volcanoes, when those volcanoes were active, and may have circulated about the sun until various perturbations have brought them down upon the earth. a body shot radially from the surface of the moon would need to have a velocity of only about a mile and a half in a second in order to escape from the moon's control, and we can believe that a lunar volcano when in action could have imparted such a velocity, all the more readily because with modern gunpowders we have been able to give to projectiles a speed one half as great as that needed for liberation from lunar gravity. another consequence of the small gravitative power of the moon bears upon the all-important question of atmosphere. according to the theory of dr. johnstone stoney, heretofore referred to, oxygen, nitrogen, and water vapor would all gradually escape from the moon, if originally placed upon it, because, by the kinetic theory, the maximum velocities of their molecules are greater than a mile and a half per second. the escape would not occur instantly, nor all at once, for it would be only the molecules at the upper surface of the atmosphere which were moving with their greatest velocity, and in a direction radial to the center of the moon, that would get away; but in the course of time this gradual leakage would result in the escape of all of those gases.[ ] [footnote : the discovery of free hydrogen in the earth's atmosphere, by professor dewar, , bears upon the theory of the escape of gases from a planet, and may modify the view above expressed. since hydrogen is theoretically incapable of being permanently retained in the free state by the earth, its presence in the atmosphere indicates either that there is an influx from space or that it emanates from the earth's crust. in a similar way it may be assumed that atmospheric gases can be given off from the crust of the moon, thus, to a greater or less extent, supplying the place of the molecules that escape.] after it had been found that, to ordinary tests, the moon offered no evidence of the possession of an atmosphere, and before dr. stoney's theory was broached, it was supposed by many that the moon had lost its original supply of air by absorption into its interior. the oxygen was supposed to have entered into combination with the cooling rocks and minerals, thus being withdrawn from the atmosphere, and the nitrogen was imagined to have disappeared also within the lunar crust. for it seems to have always been tacitly assumed that the phenomenon to be accounted for was not so much the _absence_ of a lunar atmosphere as its _disappearance_. but disappearance, of course, implies previous existence. in like manner it has always been a commonly accepted view that the moon probably once had enough water to form lakes and seas. these, it has been calculated, could have been absorbed into the lunar globe as it cooled off. but johnstone stoney's theory offers another method by which they could have escaped, through evaporation and the gradual flight of the molecules into open space. possibly both methods have been in operation, a portion of the constituents of the former atmosphere and oceans having entered into chemical combinations in the lunar crust, and the remainder having vanished in consequence of the lack of sufficient gravitative force to retain them. but why, it may be asked, should it be assumed that the moon ever had things which it does not now possess? perhaps no entirely satisfactory reply can be made. some observers have believed that they detected unmistakable indications of alluvial deposits on lunar plains, and of the existence of beaches on the shores of the "seas." messrs. loewy and puiseux, of the paris observatory, whose photographs of the moon are perhaps the finest yet made, say on this subject: "there exists, from the point of view of relief, a general similarity between the 'seas' of the moon and the plateaux which are covered to-day by terrestrial oceans. in these convex surfaces are more frequent than concave basins, thrown back usually toward the verge of the depressed space. in the same way the 'seas' of the moon present, generally at the edges, rather pronounced depressions. in one case, as in the other, we observe normal deformations of a shrinking globe shielded from the erosive action of rain, which tends, on the contrary, in all the abundantly watered parts of the earth to make the concave surfaces predominate. the explanation of this structure, such as is admitted at present by geologists, seems to us equally valid for the moon."[ ] [footnote : comptes rendus, june , july , .] it might be urged that there is evidence of former volcanic activity on the moon of such a nature that explosions of steam must have played a part in the phenomena, and if there was steam, of course there was water. but perhaps the most convincing argument tending to show that the moon once had a supply of water, of which some remnant may yet remain below the surface of the lunar globe, is based upon the probable similarity in composition of the earth and the moon. this similarity results almost equally whether we regard the moon as having originated in a ring of matter left off from the contracting mass that became the earth, or whether we accept the suggestion of prof. g.h. darwin, that the moon is the veritable offspring of the earth, brought into being by the assistance of the tidal influence of the sun. the latter hypothesis is the more picturesque of the two, and, at present, is probably the more generally favored. it depends upon the theory of tidal friction, which was referred to in chapter iii, as offering an explanation of the manner in which the rotation of the planet mercury has been slowed down until its rotary period coincides with that of its revolution. the gist of the hypothesis in question is that at a very early period in its history, when the earth was probably yet in a fluid condition, it rotated with extreme rapidity on its axis, and was, at the same time, greatly agitated by the tidal attraction of the sun, and finally huge masses were detached from the earth which, ultimately uniting, became the moon.[ ] [footnote : the tides, by g.h. darwin, chapter xvi.] born in this manner from the very substance of the earth, the moon would necessarily be composed, in the main, of the same elements as the globe on which we dwell, and is it conceivable that it should not have carried with it both air and water, or the gases from which they were to be formed? if the moon ever had enough of these prime requisites to enable it to support forms of life comparable with those of the earth, the disappearance of that life must have been a direct consequence of the gradual vanishing of the lunar air and water. the secular drying up of the oceans and wasting away of the atmosphere on our little neighbor world involved a vast, all-embracing tragedy, some of the earlier scenes of which, if theories be correct, are now reenacted on the half-desiccated planet mars--a planet, by the way, which in size, mass, and ability to retain vital gases stands about half-way between the earth and the moon. one of the most interesting facts about the moon is that its surface affords evidence of a cataclysm which has wiped out many, and perhaps nearly all, of the records of its earlier history, that were once written upon its face. even on the earth there have been geological catastrophes destroying or burying the accumulated results of ages of undisturbed progress, but on the moon these effects have been transcendent. the story of the tremendous disaster that overtook the moon is partly written in its giant volcanoes. although it may be true, as some maintain, that there is yet volcanic action going on upon the lunar surface, it is evident that such action must be insignificant in comparison with that which took place ages ago. there is a spot in the western hemisphere of the moon, on the border of a placid bay or "sea," that i can never look at without a feeling of awe and almost of shrinking. there, within a space about miles in length by in width, is an exhibition of the most terrifying effects of volcanic energy that the eye of man can anywhere behold. three immense craters--theophilus, miles across and - / miles deep; cyrillus, miles across and , feet deep; and catharina, miles across and from , to , feet deep--form an interlinked chain of mountain rings, ridges, precipices, chasms, and bottomless pits that take away one's breath. but when the first impression of astonishment and dismay produced by this overwhelming spectacle has somewhat abated, the thoughtful observer will note that here the moon is telling him a part of her wonderful story, depicted in characters so plain that he needs no instruction in order to decipher their meaning. he will observe that this ruin was not all wrought at once or simultaneously. theophilus, the crater-mountain at the northwestern end of the chain, whose bottom lies deepest of all, is the youngest of these giants, though the most imposing. for a distance of forty miles the lofty wall of theophilus has piled itself upon the ruins of the wall of cyrillus, and the circumference of the circle of its tremendous crater has been forcibly thrust within the original rim of the more ancient crater, which was thus rudely compelled to make room for its more vigorous rival and successor. the observer will also notice that catharina, the huge pit at the southeastern end of the chain, bears evidence of yet greater age. its original walls, fragments of which still stand in broken grandeur, towering to a height of , feet, have, throughout the greater part of their circuit, been riddled by the outbreak of smaller craters, and torn asunder and thrown down on all sides. in the vast enclosure that was originally the floor of the crater-mountain catharina, several crater rings, only a third, a quarter, or a fifth as great in diameter, have broken forth, and these in turn have been partially destroyed, while in the interior of the oldest of them yet smaller craters, a nest of them, mere etnas, cotopaxis, and kilaueas in magnitude, simple pinheads on the moon, have opened their tiny jaws in weak and ineffective expression of the waning energies of a still later epoch, which followed the truly heroic age of lunar vulcanicity. this is only one example among hundreds, scattered all over the moon, which show how the surface of our satellite has suffered upheaval after upheaval. it is possible that some of the small craters, not included within the walls of the greater ones, may represent an early stage in the era of volcanic activity that wrecked the moon, but where larger and smaller are grouped together a certain progression can be seen, tending finally to extinction. the internal energies reached a maximum and then fell off in strength until they died out completely. it can hardly be supposed that the life-bearing phase of lunar history--if there ever was one--could survive the outbreak of the volcanic cataclysm. north america, or europe, if subjected to such an experience as the continental areas of the moon have passed through, would be, in proportion, worse wrecked than the most fearfully battered steel victim of a modern sea fight, and one can readily understand that, in such circumstances, those now beautiful and populous continents would exhibit, from a distance, scarcely any token of their present topographical features, to say nothing of any relics of their occupation by living creatures. there are other interesting glimpses to be had of an older world in the moon than that whose scarred face is now beautified for us by distance. not far from theophilus and the other great crater-mountains just described, at the upper, or southern, end of the level expanse called the "sea of nectar," is a broad, semicircular bay whose shores are formed by the walls of a partially destroyed crater named fracastorius. it is evident that this bay, and the larger part of the "sea of nectar," have been created by an outwelling of liquid lavas, which formed a smooth floor over a portion of the pre-existing surface of the moon, and broke down and submerged a large part of the mountain ring of fracastorius, leaving the more ancient walls standing at the southern end, while, outlined by depressions and corrugations in the rocky blanket, are certain half-defined forms belonging to the buried world beneath. near copernicus, some years ago, as dr. edward s. holden pointed out, photographs made with the great lick telescope, then under his direction, showed, in skeleton outline, a huge ring buried beneath some vast outflow of molten matter and undiscerned by telescopic observers. and mr. elger, who was a most industrious observer and careful interpreter of lunar scenery, speaks of "the undoubted existence of the relics of an earlier lunar world beneath the smooth superficies of the _maria_." although, as already remarked, it seems necessary to assume that any life existing in the moon prior to its great volcanic outburst must have ceased at that time, yet the possibility may be admitted that life could reappear upon the moon after its surface had again become quiet and comparatively undisturbed. germs of the earlier life might have survived, despite the terrible nature of the catastrophe. but the conditions on the moon at present are such that even the most confident advocates of the view that the lunar world is not entirely dead do not venture to assume that anything beyond the lowest and simplest organic forms--mainly, if not wholly, in the shape of vegetation--can exist there. the impression that even such life is possible rests upon the accumulating evidence of the existence of a lunar atmosphere, and of visible changes, some apparently of a volcanic character and some not, on the moon's surface. prof. william h. pickering, who is, perhaps, more familiar with the telescopic and photographic aspects of the moon than any other american astronomer, has recorded numberless instances of change in minute details of the lunar landscapes. he regards some of his observations made at arequipa as "pointing very strongly to the existence of vegetation upon the surface of the moon in large quantities at the present time." the mountain-ringed valley of plato is one of the places in the lunar world where the visible changes have been most frequently observed, and more than one student of the moon has reached the conclusion that something very like the appearances that vegetation would produce is to be seen in that valley. professor pickering has thoroughly discussed the observations relating to a celebrated crater named linné in the _mare serenitatis_, and after reading his description of its changes of appearance one can hardly reject his conclusion that linné is an active volcanic vent, but variable in its manifestations. this is only one of a number of similar instances among the smaller craters of the moon. the giant ones are evidently entirely extinct, but some of the minor vents give occasional signs of activity. nor should it be assumed that these relatively slight manifestations of volcanic action are really insignificant. as professor pickering shows, they may be regarded as comparable with the greatest volcanic phenomena now witnessed on the earth, and, speaking again of plato, he says of its evidences of volcanic action: "it is, i believe, more active than any area of similar size upon the earth. there seems to be no evidences of lava, but the white streaks indicate apparently something analogous to snow or clouds. there must be a certain escape of gases, presumably steam and carbonic acid, the former of which, probably, aids in the production of the white markings."[ ] [footnote : annals of harvard college observatory, vol. xxxii, part ii, .] to professor pickering we owe the suggestion that the wonderful rays emanating from tycho consist of some whitish substance blown by the wind, not from tycho itself, but from lines of little volcanic vents or craters lying along the course of the rays. this substance may be volcanic powder or snow, in the form of minute ice crystals. mr. elger remarks of this theory that the "confused network of streaks" around copernicus seems to respond to it more happily than the rays of tycho do, because of the lack of definiteness of direction so manifest in the case of the rays. as an encouragement to amateur observers who may be disposed to find out for themselves whether or not changes now take place in the moon, the following sentence from the introduction to professor pickering's chapter on plato in the harvard observatory annals, volume xxxii, will prove useful and interesting: "in reviewing the history of selenography, one must be impressed by the singular fact that, while most of the astronomers who have made a special study of the moon, such as schroeter, maedler, schmidt, webb, neison, and elger, have all believed that its surface was still subject to changes readily visible from the earth, the great majority of astronomers who have paid little attention to the subject have quite as strenuously denied the existence of such changes." in regard to the lunar atmosphere, it may be said, in a word, that even those who advocate the existence of vegetation and of clouds of dust or ice crystals on the moon do not predicate any greater amount, or greater density, of atmosphere than do those who consider the moon to be wholly dead and inert. professor pickering himself showed, from his observations, that the horizontal refraction of the lunar atmosphere, instead of being less than ´´, as formerly stated, was less than . ´´. yet he found visual evidence that on the sunlit side of the moon this rare atmosphere was filled to a height of four miles with some absorbing medium which was absent on the dark side, and which was apparently an emanation from the lunar crust, occurring after sunrise. and messrs. loewy and puiseux, of the paris observatory, say, after showing reasons for thinking that the great volcanic eruptions belong to a recent period in the history of the moon, that "the diffusion of cinders to great distances infers a gaseous envelope of a certain density.... the resistance of the atmosphere must have been sufficient to retard the fall of this dust [the reference is to the white trails, like those from tycho], during its transport over a distance of more than , kilometers [ miles]."[ ] [footnote : comptes rendus, june , july , .] we come now to a brief consideration of certain peculiarities in the motions of the moon, and in the phenomena of day and night on its surface. the moon keeps the same side forever turned toward the earth, behaving, in this respect, as mercury does with regard to the sun. the consequence is that the lunar globe makes but one rotation on its axis in the course of a month, or in the course of one revolution about the earth. some of the results of this practical identity of the periods of rotation and revolution are illustrated in the diagram on page . the moon really undergoes considerable libration, recalling the libration of mercury, which was explained in the chapter on that planet, and in consequence we are able to see a little way round into the opposite lunar hemisphere, now on this side and now on the other, but in the diagram this libration has been neglected. if it had been represented we should have found that, instead of only one half, about three fifths of the total superficies of the moon are visible from the earth at one time or another. [illustration: phases and rotation of the moon.] perhaps it should be remarked that in drawing the moon's orbit about the earth as a center we offer no contradiction to what was shown earlier in this chapter. the moon does travel around the earth, and its orbit about our globe may, for our present purpose, be treated independently of its motion about the sun. let the central globe, then, represent the earth, and let the sun be supposed to shine from the left-hand side of the diagram. a little cross is erected at a fixed spot on the globe of the moon. at _a_ the moon is between the earth and the sun, or in the phase of new moon. the lunar hemisphere facing the earth is now buried in night, except so far as the light reflected from the earth illuminates it, and this illumination, it is interesting to remember, is about fourteen times as great--reckoned by the relative areas of the reflecting surfaces--as that which the full moon sends to the earth. an inhabitant of the moon, standing beside the cross, sees the earth in the form of a huge full moon directly above his head, but, as far as the sun is concerned, it is midnight for him. in the course of about seven days the moon travels to _b_. in the meantime it has turned one quarter of the way around its axis, and the spot marked by the cross is still directly under the earth. for the lunar inhabitant standing on that spot the sun is now on the point of rising, and he sees the earth no longer in the shape of a full moon, but in that of a half-moon. the lunar globe itself appears, at the same time from the earth, as a half-moon, being in the position or phase that we call first quarter. seven more days elapse, and the moon arrives at _c_, opposite to the position of the sun, and with the earth between it and the solar orb. it is now high noon for our lunarian standing beside the cross, while the earth over his head appears, if he sees it at all, only as a black disk close to the sun, or--as would sometimes be the case--covering the sun, and encircled with a beautiful ring of light produced by the refraction of its atmosphere. (recall the similar phenomenon in the case of venus.) the moon seen from the earth is now in the phase called full moon. another lapse of seven days, and the moon is at _d_, in the phase called third quarter, while the earth, viewed from the cross on the moon, which is still pointed directly at it, appears again in the shape of a huge half-moon. during the next seven days the moon returns to its original position at _a_, and becomes once more new moon, with "full earth" shining upon it. now it is evident that in consequence of the peculiar law of the moon's rotation its days and nights are each about two of our weeks, or fourteen days, in length. that hemisphere of the moon which is in the full sunlight at _a_, for instance, is buried in the middle of night at _c_. the result is different than in the case of mercury, because the body toward which the moon always keeps the same face directed is not the luminous sun, but the non-luminous earth. it is believed that the moon acquired this manner of rotation in consequence of the tidal friction exercised upon it by the earth. the tidal attraction of the earth exceeds that of the sun upon the moon because the earth is so much nearer than the sun is, and tidal attraction varies inversely as the cube of the distance. in fact, the braking effect of tidal friction varies inversely as the sixth power of the distance, so that the ability of the earth to stop the rotation of the moon on its axis is immensely greater than that of the sun. this power was effectively applied while the moon was yet a molten mass, so that it is probable that the moon has rotated just as it does now for millions of years. as was remarked a little while ago, the moon traveling in an elliptical orbit about the earth has a libratory movement which, if represented in our picture, would cause the cross to swing now a little one way and now a little the other, and thus produce an apparent pendulum motion of the earth in the sky, similar to that of the sun as seen from mercury. but it is not necessary to go into the details of this phenomenon. the reader, if he chooses, can deduce them for himself. but we may inquire a little into the effects of the long days and nights of the moon. in consequence of the extreme rarity of the lunar atmosphere, it is believed that the heat of the sun falling upon it during a day two weeks in length, is radiated away so rapidly that the surface of the lunar rocks never rises above the freezing temperature of water. on the night side, with no warm atmospheric blanket such as the earth enjoys, the temperature may fall far toward absolute zero, the most merciful figure that has been suggested for it being ° below the zero of our ordinary thermometers! but there is much uncertainty about the actual temperature on the moon, and different experiments, in the attempt to make a direct measurement of it, have yielded discordant results. at one time, for instance, lord rosse believed he had demonstrated that at lunar noon the temperature of the rocks rose above the boiling-point of water. but afterward he changed his mind and favored the theory of a low temperature. in this and in other respects much remains to be discovered concerning our interesting satellite, and there is plenty of room, and an abundance of original occupation, for new observers of the lunar world. chapter ix how to find the planets there is no reason why everybody should not know the principal planets at sight nearly as well as everybody knows the moon. it only requires a little intelligent application to become acquainted with the other worlds that have been discussed in the foregoing chapters, and to be able to follow their courses through the sky and recognize them wherever they appear. no telescope, or any other instrument whatever, is required for the purpose. there is but one preliminary requirement, just as every branch of human knowledge presupposes its a b c. this is an acquaintance with the constellations and the principal stars--not a difficult thing to obtain. almost everybody knows the "great dipper" from childhood's days, except, perhaps, those who have had the misfortune to spend their youth under the glare of city lights. some know orion when he shines gloriously in the winter heavens. many are able to point out the north star, or pole star, as everybody should be able to do. all this forms a good beginning, and may serve as the basis for the rapid acquirement of a general knowledge of the geography of the heavens. if you are fortunate enough to number an astronomer among your acquaintance--an amateur will do as well as a professor--you may, with his aid, make a short cut to a knowledge of the stars. otherwise you must depend upon books and charts. my astronomy with an opera-glass was prepared for this very purpose. for simply learning the constellations and the chief stars you need no opera-glass or other instrument. with the aid of the charts, familiarize yourself with the appearance of the constellations by noticing the characteristic arrangements of their chief stars. you need pay no attention to any except the bright stars, and those that are conspicuous enough to thrust themselves upon your attention. learn by observation at what seasons particular constellations are on, or near, the meridian--i.e., the north and south line through the middle of the heavens. make yourself especially familiar with the so-called zodiacal constellations, which are, in their order, running around the heavens from west to east: aries, taurus, gemini, cancer, leo, virgo, libra, scorpio, sagittarius, capricornus, aquarius, and pisces. the importance of these particular constellations arises from the fact that it is across them that the tracks of the planets lie, and when you are familiar with the fixed stars belonging to them you will be able immediately to recognize a stranger appearing among them, and will correctly conclude that it is one of the planets.[ ] how to tell which planet it may be, it is the object of this chapter to show you. as an indispensable aid--unless you happen already to possess a complete star atlas on a larger scale--i have drawn the six charts of the zodiacal constellations and their neighbors that are included in this chapter. [footnote : in our latitudes, planets are never seen in the northern quarter of the sky. when on the meridian, they are always somewhere between the zenith and the southern horizon.] [illustration: chart no. .--from right ascension hours to hours; declination ° north to ° south.] having learned to recognize the constellations and their chief stars on sight, one other step, an extremely easy one, remains to be taken before beginning your search for the planets--buy the american ephemeris and nautical almanac for the current year. it is published under the direction of the united states naval observatory at washington, and can be purchased for one dollar. this book, which may appear to you rather bulky and formidable for an almanac, contains hundreds of pages and scores of tables to which you need pay no attention. they are for navigators and astronomers, and are much more innocent than they look. the plain citizen, seeking only an introduction to the planets, can return their stare and pass by, without feeling in the least humiliated. [illustration: chart no. .--from right ascension hours to hours; declination ° north to ° south.] in the front part of the book, after the long calendar, and the tables relating to the sun and the moon, will be found about thirty pages of tables headed, in large black letters, with the names of the planets--mercury, venus, mars, jupiter, saturn, etc. two months are represented on each page, and opposite the number of each successive day of the month the position of the planet is given in hours, minutes, and seconds of right ascension, and degrees, minutes, and seconds of north and south declination, the sign + meaning north, and the sign - south. do not trouble yourself with the seconds in either column, and take the minutes only when the number is large. the hours of right ascension and the degrees of declination are the main things to be noticed. right ascension, by the way, expresses the distance of a celestial body, such as a star or a planet, east of the vernal equinox, or the first point of aries, which is an arbitrary point on the equator of the heavens, which serves, like the meridian of greenwich on the earth, as a starting-place for reckoning longitude. the entire circuit of the heavens along the equator is divided into twenty-four hours of right ascension, each hour covering ° of space. if a planet then is in right ascension (usually printed for short r.a.) h. m. s., it is on the meridian of the vernal equinox, or the celestial greenwich; if it is in r.a. h., it will be found ° east of the vernal equinox, and so on. [illustration: chart no. .--from right ascension hours to hours; declination ° north to ° south.] declination (printed d. or dec.) expresses the distance of a celestial body north or south of the equator of the heavens. with these explanations we may proceed to find a planet by the aid of the nautical almanac and our charts. i take, for example, the ephemeris for the year , and i look under the heading "jupiter" on page , for the month of july. opposite the th day of the month i find the right ascension to be h. m., neglecting the seconds. now minutes are so near to half an hour that, for our purposes, we may say jupiter is in r.a. h. m. i set this down on a slip of paper, and then examine the declination column, where i find that on july jupiter is in south declination (the sign - meaning south, as before explained) ° ´ ´´, which is almost ° ´, and, for our purposes, we may call this ° ´, which is what i set down on my slip. [illustration: chart no. .--from right ascension hours to hours; declination ° north to ° south.] next, i turn to chart no. , in this chapter, where i find the meridian line of r.a. h. running through the center of the chart. i know that jupiter is to be looked for about m. east, or to the left, of that line. at the bottom and top of the chart, every twenty minutes of r.a. is indicated, so that it is easy, with the eye, or with the aid of a ruler, to place the vertical line at some point of which jupiter is to be found. [illustration: chart no. .--from right ascension hours to hours; declination ° north to ° south.] then i consult my note of the declination of the planet. it is south ° ´. on the vertical borders of the chart i find the figures of the declination, and i observe that ° dec., which represents the equator of the heavens, is near the top of the chart, while each parallel horizontal line across the chart indicates ° north or south of its next neighbor. next to the bottom of the chart i find the parallel of °, and i see that every five degrees is indicated by the figures at the sides. by the eye, or with the aid of a ruler, i easily estimate where the horizontal line of ° would fall, and since ´ is the third of a degree i perceive that it is, for the rough purpose of merely finding a conspicuous planet, negligible, although it, too, can be included in the estimate, if thought desirable. having already found the vertical line on which jupiter is placed and having now found the horizontal line also, i have simply to regard their crossing point, which will be the situation of the planet among the stars. i note that it is in the constellation sagittarius in a certain position with reference to a familiar group of stars in that constellation, and when i look at the heavens, there, in the place thus indicated, jupiter stands revealed. [illustration: chart no. .--from right ascension hours to hours ( ii.); declination ° north to ° south.] the reader will readily perceive that, in a precisely similar manner, any planet can be located, at any time of the year, and at any point in its course about the heavens. but it may turn out that the place occupied by the planet is too near the sun to render it easily, or at all, visible. such a case can be recognized, either from a general knowledge of the location of the constellations at various seasons, or with the aid of the nautical almanac, where at the beginning of each set of monthly tables in the calendar the sun's right ascension and declination will be found. in locating the sun, if you find that its right ascension differs by less than an hour, one way or the other, from that of the planet sought, it is useless to look for the latter. if the planet is situated west of the sun--to the right on the chart--then it is to be looked for in the east before sunrise. but if it is east of the sun--to the left on the chart--then you must seek it in the west after sunset. for instance, i look for the planet mercury on october , . i find its r.a. to be h. m. and its dec. ° ´. looking at the sun's place for october th, i find it to be r.a. h. m. and dec. ° ´. placing them both on chart no. , i discover that mercury is well to the east, or left hand of the sun, and will consequently be visible in the western sky after sundown. additional guidance will be found by noting the following facts about the charts: the meridian (the north and south line) runs through the middle of chart no. between and o'clock p.m. on november st, between and o'clock p.m. on december st, and between and o'clock p.m. on january st. the meridian runs through the middle of chart no. between and o'clock p.m. on january st, between and o'clock p.m. on february st, and between and o'clock p.m. on march st. the meridian runs through the middle of chart no. between and o'clock p.m. on march st, between and o'clock p.m. on april st, and between and o'clock p.m. on may st. the meridian runs through the middle of chart no. between and o'clock p.m. on may st, between and o'clock p.m. on june st, and between and o'clock p.m. on july st. the meridian runs through the middle of chart no. between and o'clock p.m. on july st, between and o'clock p.m. on august st, and between and o'clock p.m. on september st. the meridian runs through the middle of chart no. between and o'clock p.m. on september st, between and o'clock p.m. on october st, and between and o'clock p.m. on november st. note well, also, these particulars about the charts: chart no. includes the first four hours of right ascension, from h. to h. inclusive; chart no. includes h. to h.; chart no. , h. to h.; chart no. , h. to h.; chart no. , h. to h.; and chart no. , h. to h., which completes the circuit. in the first three charts the line of °, or the equator, is found near the bottom, and in the last three near the top. this is a matter of convenience in arrangement, based upon the fact that the ecliptic, which, and not the equator, marks the center of the zodiac, indicates the position of the tracks of the planets among the stars; and the ecliptic, being inclined ° to the plane of the equator, lies half to the north and half to the south of the latter. those who, after all, may not care to consult the ephemeris in order to find the planets, may be able to locate them, simply from a knowledge of their situation among the constellations. some ordinary almanacs tell in what constellations the principal planets are to be found at various times of the year. having once found them in this way, it is comparatively easy to keep track of them thereafter through a general knowledge of their movements. jupiter, for instance, requiring a period of nearly twelve years to make a single journey around the sun, moves about ° eastward among the stars every year. the zodiacal constellations are roughly about ° in length, and as jupiter was in sagittarius in , he will be in capricornus in . saturn, requiring nearly thirty years for a revolution around the sun, moves only between ° and ° eastward every year, and, being in conjunction with jupiter in sagittarius in , does not get beyond the border of that constellation in . jupiter having been in opposition to the sun june , , will be similarly placed early in august, , the time from one opposition of jupiter to the next being days. saturn passes from one opposition to the next in days, so that having been in that position july , , it reaches it again about july , . mars requires about days to complete a revolution, and comes into conjunction with the earth, or opposition to the sun--the best position for observation--on the average once every days. mars was in opposition near the end of february, , and some of its future oppositions will be in march, ; may, ; july, ; and september, . the oppositions of and will be unusually favorable ones, for they will occur when the planet is comparatively near the earth. when a planet is in opposition to the sun it is on the meridian, the north and south line, at midnight. mercury and venus being nearer the sun than the earth is, can never be seen very far from the place of the sun itself. venus recedes much farther from the solar orb than mercury does, but both are visible only in the sunset or the sunrise sky. all almanacs tell at what times these planets play their respective rôles as morning or as evening stars. in the case of mercury about days on the average elapse between its reappearances; in the case of venus, about days. the latter, for instance, having become an evening star at the end of april, , will become an evening star again in december, . with the aid of the nautical almanac and the charts the amateur will find no difficulty, after a little practise, in keeping track of any of the planets. in the back part of the nautical almanac will be found two pages headed "phenomena: planetary configurations." with the aid of these the student can determine the position of the planets with respect to the sun and the moon, and with respect to one another. the meaning of the various symbols used in the tables will be found explained on a page facing the calendar at the beginning of the book. from these tables, among other things, the times of greatest elongation from the sun of the planets mercury and venus can be found. it may be added that only bright stars, and stars easily seen, are included in the charts, and there will be no danger of mistaking any of these stars for a planet, if the observer first carefully learns to recognize their configurations. neither mars, jupiter, nor saturn ever appears as faint as any of the stars, except those of the first magnitude, included in the charts. uranus and neptune being invisible to the naked eye--uranus can occasionally be just glimpsed by a keen eye--are too faint to be found without the aid of more effective appliances. index agassiz, alexander, on deep-sea animals, . asteroids, the, , . brightness of, . imaginary adventures on, . life on, . number of, known, . orbits of, . origin of, , . size of, . aristarchus, lunar crater, . atmosphere, importance of, . bailey, solon i., on oppositions of eros, . barnard, e.e., discovers fifth satellite of jupiter, . measures asteroids, . on saturn's rings, . belopolski, on rotation of venus, . ceres, an asteroid, , . clefts in the moon, . copernicus, lunar crater, , . darwin, george h., on jupiter and saturn, . on origin of moon, . theory of tidal friction, . davy, sir humphry, on saturn, . dawes sees canals on mars, . deimos, satellite of mars, . denning, w.f., description of jupiter, . de vico on rotation of venus, . dewar, james, discovers free hydrogen in air, . de witt discovers eros, . dick, thomas, on saturn, . douglass, a.e., sees mars's canals, . sees clouds in mars, . doppler's principle, , . earth and moon's orbit, . birth of moon from, . change of distance from sun, . less advanced than mars, . older than venus, . seen from mercury, . seen from venus, - , . seen from moon, . earth, similarity to venus, . supposed signals to and from mars, . elger, t.g., on cracks in moon, . on tycho's rays, . ephemeris, how to use, , . eros, an asteroid, - , , . flammarion, c., observes venus's atmosphere, . on plurality of worlds, . forbes, prof. george, on ultra-neptunian planet, . galileo on lunar world, . gravity, as affecting life on planets, , . hall, asaph, discovers mars's moons, . herodotus, lunar crater, . herschel, sir john, on saturn, . holden, e.s., on photograph of lunar crater, . huggins on mercury's atmosphere, . inhabitants of foreign planets, , , . interplanetary communication, , , , , . juno, an asteroid, . jupiter, cloudy aspect of, . density of, . distance of, . equatorial belts on, . future of, . gravity on, . great red spot on, . markings outside the belts, . and the nebular theory, . once a companion star, . polar compression of, . possibly yet incandescent, . question of a denser core, . resemblance of, to sun, . rotation of, , . satellites of, , . seen from satellites, . size of, . solar light and heat on, . south belt of, . surface conditions of, . theories about the red spot, . trade-winds and the belts of, . various rates of rotation of, . visibility of rotation of, . keeler, j.e., on saturn's rings, . kepler, lunar crater, . kinetic theory of gases, . kirkwood, daniel, on asteroids, . lagrange on olbers's theory, . lick observatory and mars's canals, . life, a planetary phenomenon, . in sea depths, . on planets, , . prime requisites of, . resisting extreme cold, . universality of, . loewy and puiseux, on lunar atmosphere, . on lunar "seas," . lowell, percival, description of mars, . on markings of venus, . on mercury's rotation, . on rotation of venus, . sees mars's canals, . theory of martian canals, . lucian, on appearance of earth from moon, . lyman, c.s., observes venus's atmosphere, . mars, age of, . atmosphere of, , , . bands of life on, . canals on, . described by schiaparelli, . gemination of, , . have builders of, disappeared? . and irrigation, . and lines of vegetation, . and seasonal changes, . and water circulation, . carbon dioxide on, . circular spots or "oases" on, . colors of, . dimensions of, . distance of, , . enigmatical lights on, . gravity on, . inclination of axis, . length of year, . lowell's theory of, . light and heat on, . moonlight on, . orbit of, . polar caps of, , . possible size of inhabitants, . satellites of, , , . seasons on, . supposed signals from, , . temperature of, , . water vapor on, . mercury, atmosphere of, , , , . day and night on, , , . dimensions, . earth seen from, . habitability of, , , . heavens seen from, , . heat and light on, , . holds place of honor, . length of year, . mass of, . moon visible from, . resemblances to moon, . rotation of, . shape of orbit, . sun as seen from, . velocity in orbit, . venus seen from, . virtual fall toward sun, . visibility of, . water on, . moon, the area of surface, . atmosphere of, , , , , . clouds on, , . constitution of, . craters, . day and night on, . distance of, , . density of, . former cataclysm on, . former life on, , . giantism on, , . gravity on, , , . libration of, . meteorites and, . mountains on, . the older world in, . origin of, . phases and motions of, . rotation of, . seas of, . size of, . snow on, . speculation about, . temperature of, . vegetation on, , , . visibility of features of, . nasmyth and carpenter on lunar craters, . neptune, description of, - . newcomb, simon, on olbers's theory, . newton, lunar crater, . olbers's theory of planetary explosion, . on vesta's light, . pallas, an asteroid, . perrotin sees canals on mars, . phobos, satellite of mars, . pickering, e.c., discovers ninth moon of saturn, . finds eros on harvard plates, . on shape of eros, . on light of eros, . pickering, w.h., on lunar atmosphere, . observes changes in moon, . sees mars's canals, . theory of tycho's rays, . on venus's atmosphere, . planets, classification of, . how to find, , . resemblances among, . plato, lunar ring plain, . plurality of worlds in literature, . subject ignored, . proctor, r.a., on jupiter's moons, . on other worlds, . roche's limit, . rosse, lord, on temperature of moon, . saturn, age of, . composition of, . density of, . distance of, . the gauze ring, - . gravity on, . inclination of axis, . interior of, . length of year, . popular telescopic object, . rings of, , . gaps in, . origin of, . periodic disappearance of, . seen from planet, . shadow of, . rotation of, . satellites of, . size of, . schiaparelli discovers canals on mars, . describes martian canals, . discovers mercury's rotation, , . on rotation of venus, . solar system, shape and size of, . unity of, . viewed from space, . stoney, johnstone, on atmospheres of planets, . on escape of gases from moon, . sun, the, isolation in space, . no life on, . resemblances with jupiter, . swedenborg, on saturn's rings, . tidal friction, , , , . tycho, lunar crater, . ultra-neptunian planet, . uranus, description of, - . venus, age of, . atmosphere of, , , , , . absence of seasons on, . density of, . distance of, , . gravity on, , . inclination of axis, . life on, , , , , , , , . light and heat on, - . orbit of, . phases of, . resemblances of, to earth, . rotation of, , , . size of, . twilight on, . visibility of, . vesta, an asteroid, , , . vogel on mercury's atmosphere, . wireless telegraphy, , . young, c.a., on olbers's theory of asteroids, . on temperature of mars, . on venus's atmosphere, . zodiac, the, . the end a new book by prof. groos. the play of man. by karl groos, professor of philosophy in the university of basel, and author of "the play of animals." translated, with the author's cooperation, by elizabeth l. baldwin, and edited, with a preface and appendix, by prof. j. mark baldwin, of princeton university. mo. cloth, $ . 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"one can here get a clear conception of the relative condition of the stars and constellations, and of the existent universe so far as it is disclosed to view. the author presents his wonderful and at times bewildering facts in a bright and cheery spirit that makes the book doubly attractive."--_boston home journal._ d. appleton and company, new york. curiosities of the sky by garrett serviss curiosities of the sky was first published in and the text is in the public domain. the transcription was done by william mcclain (info@sattre-press.com), . a printed version of this book is available from sattre press (http://csky.sattre-press.com). it includes extensive annotations, a new introduction and all the original photographs and diagrams. _________________________________________________________________ preface what froude says of history is true also of astronomy: it is the most impressive where it transcends explanation. it is not the mathematics of astronomy, but the wonder and the mystery that seize upon the imagination. the calculation of an eclipse owes all its prestige to the sublimity of its data; the operation, in itself, requires no more mental effort than the preparation of a railway time-table. the dominion which astronomy has always held over the minds of men is akin to that of poetry; when the former becomes merely instructive and the latter purely didactic, both lose their power over the imagination. astronomy is known as the oldest of the sciences, and it will be the longest-lived because it will always have arcana that have not been penetrated. some of the things described in this book are little known to the average reader, while others are well known; but all possess the fascination of whatever is strange, marvelous, obscure, or mysterious -- magnified, in this case, by the portentous scale of the phenomena. the idea of the author is to tell about these things in plain language, but with as much scientific accuracy as plain language will permit, showing the wonder that is in them without getting away from the facts. most of them have hitherto been discussed only in technical form, and in treatises that the general public seldom sees and never reads. among the topics touched upon are: * the strange unfixedness of the ``fixed stars,'' the vast migrations of the suns and worlds constituting the universe. * the slow passing out of existence of those collocations of stars which for thousands of years have formed famous ``constellations,'' preserving the memory of mythological heroes and heroines, and perhaps of otherwise unrecorded history. * the tendency of stars to assemble in immense clouds, swarms, and clusters. * the existence in some of the richest regions of the universe of absolutely black, starless gaps, deeps, or holes, as if one were looking out of a window into the murkiest night. * the marvelous phenomena of new, or temporary, stars, which appear as suddenly as conflagrations, and often turn into something else as eccentric as themselves. * the amazing forms of the ``whirlpool,'' ``spiral,'' ``pinwheel,'' and ``lace,'' or ``tress,'' nebulæ. * the strange surroundings of the sun, only seen in particular circumstances, but evidently playing a constant part in the daily phenomena of the solar system. * the mystery of the zodiacal light and the gegenschein. * the extraordinary transformations undergone by comets and their tails. * the prodigies of meteorites and masses of stone and metal fallen from the sky. * the cataclysms that have wrecked the moon. * the problem of life and intelligence on the planet mars. * the problematical origin and fate of the asteroids. * the strange phenomena of the auroral lights. an attempt has been made to develop these topics in an orderly way, showing their connection, so that the reader may obtain a broad general view of the chief mysteries and problems of astronomy, and an idea of the immense field of discovery which still lies, almost unexplored, before it. the windows of absolute night to most minds mystery is more fascinating than science. but when science itself leads straight up to the borders of mystery and there comes to a dead stop, saying, ``at present i can no longer see my way,'' the force of the charm is redoubled. on the other hand, the illimitable is no less potent in mystery than the invisible, whence the dramatic effect of keats' ``stout cortez'' staring at the boundless pacific while all his men look at each other with a wild surmise, ``silent upon a peak in darien.'' it is with similar feelings that the astronomer regards certain places where from the peaks of the universe his vision seems to range out into endless empty space. he sees there the shore of his little isthmus, and, beyond, unexplored immensity. the name, ``coal-sacks,'' given to these strange voids is hardly descriptive. rather they produce upon the mind the effect of blank windows in a lonely house on a pitch-dark night, which, when looked at from the brilliant interior, become appalling in their rayless murk. infinity seems to acquire a new meaning in the presence of these black openings in the sky, for as one continues to gaze it loses its purely metaphysical quality and becomes a kind of entity, like the ocean. the observer is conscious that he can actually see the beginning of its ebon depths, in which the visible universe appears to float like an enchanted island, resplendent within with lights and life and gorgeous spectacles, and encircled with screens of crowded stars, but with its dazzling vistas ending at the fathomless sea of pure darkness which encloses all. the galaxy, or milky way, surrounds the borders of our island in space like a stellar garland, and when openings appear in it they are, by contrast, far more impressive than the general darkness of the interstellar expanse seen in other directions. yet even that expanse is not everywhere equally dark, for it contains gloomy deeps discernable with careful watching. here, too, contrast plays an important part, though less striking than within the galactic region. some of sir william herschel's observations appear to indicate an association between these tenebrious spots and neighboring star clouds and nebulæ. it is an illuminating bit of astronomical history that when he was sweeping the then virgin heavens with his great telescopes he was accustomed to say to his sister who, note-book in hand, waited at his side to take down his words, fresh with the inspiration of discovery: ``prepare to write; the nebulæ are coming; here space is vacant.'' the most famous of the ``coal-sacks,'' and the first to be brought to general attention before astronomers had awakened to the significance of such things, lies adjacent to the ``southern cross,'' and is truly an amazing phenomenon. it is not alone the conspicuousness of this celestial vacancy, opening suddenly in the midst of one of the richest parts of the galaxy, that has given it its fame, but quite as much the superstitious awe with which it was regarded by the early explorers of the south seas. to them, as well as to those who listened in rapt wonder to their tales, the ``coal-sack'' seemed to possess some occult connection with the mystic ``cross.'' in the eyes of the sailors it was not a vacancy so much as a sable reality in the sky, and as, shuddering, they stared at it, they piously crossed themselves. it was another of the magical wonders of the unknown south, and as such it formed the basis of many a ``wild surmise'' and many a sea-dog's yarn. scientific investigation has not diminished its prestige, and today no traveler in the southern hemisphere is indifferent to its fascinating strangeness, while some find it the most impressive spectacle of the antarctic heavens. all around, up to the very edge of the yawning gap, the sheen of the milky way is surpassingly glorious; but there, as if in obedience to an almighty edict, everything vanishes. a single faint star is visible within the opening, producing a curious effect upon the sensitive spectator, like the sight of a tiny islet in the midst of a black, motionless, waveless tarn. the dimensions of the lagoon of darkness, which is oval or pear-shaped, are eight degrees by five, so that it occupies a space in the sky about one hundred and thirty times greater than the area of the full moon. it attracts attention as soon as the eye is directed toward the quarter where it exists, and by virtue of the rarity of such phenomena it appears a far greater wonder than the drifts of stars that are heaped around it. now that observatories are multiplying in the southern hemisphere, the great austral ``coal-sack'' will, no doubt, receive attention proportioned to its importance as one of the most significant features of the sky. already at the sydney observatory photographs have shown that the southern portion of this dead sea of space is not quite ``bottomless,'' although its northern part defies the longest sounding lines of the astronomer. there is a similar, but less perfect, ``coal-sack'' in the northern hemisphere, in the constellation of ``the swan,'' which, strange to say, also contains a well-marked figure of a cross outlined by stars. this gap lies near the top of the cross-shaped figure. it is best seen by averted vision, which brings out the contrast with the milky way, which is quite brilliant around it. it does not, however, exercise the same weird attraction upon the eye as the southern ``coal-sack,'' for instead of looking like an absolute void in the sky, it rather appears as if a canopy of dark gauze had been drawn over the stars. we shall see the possible significance of this appearance later. just above the southern horizon of our northern middle latitudes, in summer, where the milky way breaks up into vast sheets of nebulous luminosity, lying over and between the constellations scorpio and sagittarius, there is a remarkable assemblage of ``coal-sacks,'' though none is of great size. one of them, near a conspicuous star-cluster in scorpio, m , is interesting for having been the first of these strange objects noted by herschel. probably it was its nearness to m which suggested to his mind the apparent connection of such vacancies with star-clusters which we have already mentioned. but the most marvelous of the ``coal-sacks'' are those that have been found by photography in sagittarius. one of barnard's earliest and most excellent photographs includes two of them, both in the star-cluster m . the larger, which is roughly rectangular in outline, contains one little star, and its smaller neighbor is lune-shaped -- surely a most singular form for such an object. both are associated with curious dark lanes running through the clustered stars like trails in the woods. along the borders of these lanes the stars are ranked in parallel rows, and what may be called the bottoms of the lanes are not entirely dark, but pebbled with faint stellar points. one of them which skirts the two dark gaps and traverses the cluster along its greatest diameter is edged with lines of stars, recalling the alignment of the trees bordering a french highway. this road of stars cannot be less than many billions of miles in length! all about the cluster the bed of the galaxy is strangely disturbed, and in places nearly denuded, as if its contents had been raked away to form the immense stack and the smaller accumulations of stars around it. the well-known ``trifid nebula'' is also included in the field of the photograph, which covers a truly marvelous region, so intricate in its mingling of nebulæ, star-clusters, star-swarms, star-streams, and dark vacancies that no description can do it justice. yet, chaotic as it appears, there is an unmistakable suggestion of unity about it, impressing the beholder with the idea that all the different parts are in some way connected, and have not been fortuitously thrown together. miss agnes m. clerke made the striking remark that the dusky lanes in m are exemplified on the largest scale in the great rift dividing the milky way, from cygnus in the northern hemisphere all the way to the ``cross'' in the southern. similar lanes are found in many other clusters, and they are generally associated with flanking rows of stars, resembling in their arrangement the thick-set houses and villas along the roadways that traverse the approaches to a great city. but to return to the black gaps. are they really windows in the star-walls of the universe? some of them look rather as if they had been made by a shell fired through a luminous target, allowing the eye to range through the hole into the void space beyond. if science is discretely silent about these things, what can the more venturesome and less responsible imagination suggest? would a huge ``runaway sun,'' like arcturus, for instance, make such an opening if it should pass like a projectile through the milky way? it is at least a stimulating inquiry. being probably many thousands of times more massive than the galactic stars, such a stellar missile would not be stopped by them, though its direction of flight might be altered. it would drag the small stars lying close to its course out of their spheres, but the ultimate tendency of its attraction would be to sweep them round in its wake, thus producing rather a star-swarm than a vacancy. those that were very close to it might be swept away in its rush and become its satellites, careering away with it in its flight into outer space; but those that were farther off, and they would, of course, greatly outnumber the nearer ones, would tend inward from all sides toward the line of flight, as dust and leaves collect behind a speeding motor (though the forces operating would be different), and would fill up the hole, if hole it were. a swarm thus collected should be rounded in outline and bordered with a relatively barren ring from which the stars had been ``sucked'' away. in a general sense the m cluster answers to this description, but even if we undertook to account for its existence by a supposition like the above, the black gaps would remain unexplained, unless one could make a further draft on the imagination and suggest that the stars had been thrown into a vast eddy, or system of eddies, whose vortices appear as dark holes. only a maelstrom-like motion could keep such a funnel open, for without regard to the impulse derived from the projectile, the proper motions of the stars themselves would tend to fill it. perhaps some other cause of the whirling motion may be found. as we shall see when we come to the spiral nebulæ, gyratory movements are exceedingly prevalent throughout the universe, and the structure of the milky way is everywhere suggestive of them. but this is hazardous sport even for the imagination -- to play with suns as if they were but thistle-down in the wind or corks in a mill-race. another question arises: what is the thickness of the hedge of stars through which the holes penetrate? is the depth of the openings proportionate to their width? in other words, is the milky way round in section like a rope, or flat and thin like a ribbon? the answer is not obvious, for we have little or no information concerning the relative distances of the faint galactic stars. it would be easier, certainly, to conceive of openings in a thin belt than in a massive ring, for in the first case they would resemble mere rifts and breaks, while in the second they would be like wells or bore-holes. then, too, the fact that the milky way is not a continuous body but is made up of stars whose actual distances apart is great, offers another quandary; persistent and sharply bordered apertures in such an assemblage are a priori as improbable, if not impossible, as straight, narrow holes running through a swarm of bees. the difficulty of these questions indicates one of the reasons why it has been suggested that the seeming gaps, or many of them, are not openings at all, but opaque screens cutting off the light from stars behind them. that this is quite possible in some cases is shown by barnard's later photographs, particularly those of the singular region around the star rho ophiuchi. here are to be seen somber lanes and patches, apparently forming a connected system which covers an immense space, and which their discoverer thinks may constitute a ``dark nebula.'' this seems at first a startling suggestion; but, after all, why should their not be dark nebulæ as well as visible ones? in truth, it has troubled some astronomers to explain the luminosity of the bright nebulæ, since it is not to be supposed that matter in so diffuse a state can be incandescent through heat, and phosphorescent light is in itself a mystery. the supposition is also in accord with what we know of the existence of dark solid bodies in space. many bright stars are accompanied by obscure companions, sometimes as massive as themselves; the planets are non-luminous; the same is true of meteors before they plunge into the atmosphere and become heated by friction; and many plausible reasons have been found for believing that space contains as many obscure as shining bodies of great size. it is not so difficult, after all, then, to believe that there are immense collections of shadowy gases and meteoric dust whose presence is only manifested when they intercept the light coming from shining bodies behind them. this would account for the apparent extinguishment of light in open space, which is indicated by the falling off in relative number of telescopic stars below the tenth magnitude. even as things are, the amount of light coming to us from stars too faint to be seen with the naked eye is so great that the statement of it generally surprises persons who are unfamiliar with the inner facts of astronomy. it has been calculated that on a clear night the total starlight from the entire celestial sphere amounts to one-sixtieth of the light of the full moon; but of this less than one-twenty-fifth is due to stars separately distinguished by the eye. if there were no obscuring medium in space, it is probable that the amount of starlight would be noticeably and perhaps enormously increased. but while it seems certain that some of the obscure spots in the milky way are due to the presence of ``dark nebulæ,'' or concealing veils of one kind or another, it is equally certain that there are many which are true apertures, however they may have been formed, and by whatever forces they may be maintained. these, then, are veritable windows of the galaxy, and when looking out of them one is face to face with the great mystery of infinite space. there the known universe visibly ends, but manifestly space itself does not end there. it is not within the power of thought to conceive an end to space, for the instant we think of a terminal point or line the mind leaps forward to the beyond. there must be space outside as well as inside. eternity of time and infinity of space are ideas that the intellect cannot fully grasp, but neither can it grasp the idea of a limitation to either space or time. the metaphysical conceptions of hypergeometry, or fourth-dimensional space, do not aid us. having, then, discovered that the universe is a thing contained in something indefinitely greater than itself; having looked out of its windows and found only the gloom of starless night outside -- what conclusions are we to draw concerning the beyond? it seems as empty as a vacuum, but is it really so? if it be, then our universe is a single atom astray in the infinite; it is the only island in an ocean without shores; it is the one oasis in an illimitable desert. then the milky way, with its wide-flung garland of stars, is afloat like a tiny smoke-wreath amid a horror of immeasurable vacancy, or it is an evanescent and solitary ring of sparkling froth cast up for a moment on the viewless billows of immensity. from such conclusions the mind instinctively shrinks. it prefers to think that there is something beyond, though we cannot see it. even the universe could not bear to be alone -- a crusoe lost in the cosmos! as the inhabitants of the most elegant château, with its gardens, parks, and crowds of attendants, would die of loneliness if they did not know that they have neighbors, though not seen, and that a living world of indefinite extent surrounds them, so we, when we perceive that the universe has limits, wish to feel that it is not solitary; that beyond the hedges and the hills there are other centers of life and activity. could anything be more terrible than the thought of an isolated universe? the greater the being, the greater the aversion to seclusion. only the infinite satisfies; in that alone the mind finds rest. we are driven, then, to believe that the universal night which envelopes us is not tenantless; that as we stare out of the star-framed windows of the galaxy and see nothing but uniform blackness, the fault is with our eyes or is due to an obscuring medium. since our universe is limited in extent, there must be other universes beyond it on all sides. perhaps if we could carry our telescopes to the verge of the great ``coal-sack'' near the ``cross,'' being then on the frontier of our starry system, we could discern, sparkling afar off in the vast night, some of the outer galaxies. they may be grander than ours, just as many of the suns surrounding us are immensely greater than ours. if we could take our stand somewhere in the midst of immensity and, with vision of infinite reach, look about us, we should perhaps see a countless number of stellar systems, amid which ours would be unnoticeable, like a single star among the multitude glittering in the terrestial sky on a clear night. some might be in the form of a wreath, like our own; some might be globular, like the great star-clusters in hercules and centaurus; some might be glittering circles, or disks, or rings within rings. if we could enter them we should probably find a vast variety of composition, including elements unknown to terrestrial chemistry; for while the visible universe appears to contain few if any substances not existing on the earth or in the sun, we have no warrant to assume that others may not exist in infinite space. and how as to gravitation? we do not know that gravitation acts beyond the visible universe, but it is reasonable to suppose that it does. at any rate, if we let go its sustaining hand we are lost, and can only wander hopelessly in our speculations, like children astray. if the empire of gravitation is infinite, then the various outer systems must have some, though measuring by our standards an imperceptible, attractive influence upon each other, for gravitation never lets go its hold, however great the space over which it is required to act. just as the stars about us are all in motion, so the starry systems beyond our sight may be in motion, and our system as a whole may be moving in concert with them. if this be so, then after interminable ages the aspect of the entire system of systems must change, its various members assuming new positions with respect to one another. in the course of time we may even suppose that our universe will approach relatively close to one of the others; and then, if men are yet living on the earth, they may glimpse through the openings which reveal nothing to us now, the lights of another nearing star system, like the signals of a strange squadron, bringing them the assurance (which can be but an inference at present) that the ocean of space has other argosies venturing on its limitless expanse. there remains the question of the luminiferous ether by whose agency the waves of light are borne through space. the ether is as mysterious as gravitation. with regard to ether we only infer its existence from the effects which we ascribe to it. evidently the ether must extend as far as the most distant visible stars. but does it continue on indefinitely in outer space? if it does, then the invisibility of the other systems must be due to their distance diminishing the quantity of light that comes from them below the limit of perceptibility, or to the interposition of absorbing media; if it does not, then the reason why we cannot see them is owing to the absence of a means of conveyance for the light waves, as the lack of an interplanetary atmosphere prevents us from hearing the thunder of sun-spots. (it is interesting to recall that mr edison was once credited with the intention to construct a gigantic microphone which should render the roar of sun-spots audible by transforming the electric vibrations into sound-waves). on this supposition each starry system would be enveloped in its own globule of ether, and no light could cross from one to another. but the probability is that both the ether and gravitation are ubiquitous, and that all the stellar systems are immersed in the former like clouds of phosphorescent organisms in the sea. so astronomy carries the mind from height to greater height. men were long in accepting the proofs of the relative insignificance of the earth; they were more quickly convinced of the comparative littleness of the solar system; and now the evidence assails their reason that what they had regarded as the universe is only one mote gleaming in the sunbeams of infinity. star-clouds, star-clusters, and star-streams in the preceding chapter we have seen something of the strangely complicated structure of the galaxy, or milky way. we now proceed to study more comprehensively that garlanded ``pathway of the gods.'' judged by the eye alone, the milky way is one of the most delicately beautiful phenomena in the entire realm of nature -- a shimmer of silvery gauze stretched across the sky; but studied in the light of its revelations, it is the most stupendous object presented to human ken. let us consider, first, its appearance to ordinary vision. its apparent position in the sky shifts according to the season. on a serene, cloudless summer evening, in the absence of the moon, whose light obscures it, one sees the galaxy spanning the heavens from north to southeast of the zenith like a phosphorescent arch. in early spring it forms a similar but, upon the whole, less brilliant arch west of the zenith. between spring and summer it lies like a long, faint, twilight band along the northern horizon. at the beginning of winter it again forms an arch, this time spanning the sky from east to west, a little north of the zenith. these are its positions as viewed from the mean latitude of the united states. even the beginner in star-gazing does not have to watch it throughout the year in order to be convinced that it is, in reality, a great circle, extending entirely around the celestial sphere. we appear to be situated near its center, but its periphery is evidently far away in the depths of space. although to the casual observer it seems but a delicate scarf of light, brighter in some places than in others, but hazy and indefinite at the best, such is not its appearance to those who study it with care. they perceive that it is an organic whole, though marvelously complex in detail. the telescope shows that it consists of stars too faint and small through excess of distance to be separately visible. of the hundred million suns which some estimates have fixed as the probable population of the starry universe, the vast majority (at least thirty to one) are included in this strange belt of misty light. but they are not uniformly distributed in it; on the contrary, they are arrayed in clusters, knots, bunches, clouds, and streams. the appearance is somewhat as if the galaxy consisted of innumerable swarms of silver-winged bees, more or less intermixed, some massed together, some crossing the paths of others, but all governed by a single purpose which leads them to encircle the region of space in which we are situated. from the beginning of the systematic study of the heavens, the fact has been recognized that the form of the milky way denotes the scheme of the sidereal system. at first it was thought that the shape of the system was that of a vast round disk, flat like a cheese, and filled with stars, our sun and his relatively few neighbors being placed near the center. according to this view, the galactic belt was an effect of perspective; for when looking in the direction of the plane of the disk, the eye ranged through an immense extension of stars which blended into a glimmering blur, surrounding us like a ring; while when looking out from the sides of the disk we saw but few stars, and in those directions the heavens appeared relatively blank. finally it was recognized that this theory did not correspond with the observed appearances, and it became evident that the milky way was not a mere effect of perspective, but an actual band of enormously distant stars, forming a circle about the sphere, the central opening of the ring (containing many scattered stars) being many times broader than the width of the ring itself. our sun is one of the scattered stars in the central opening. as already remarked, the ring of the galaxy is very irregular, and in places it is partly broken. with its sinuous outline, its pendant sprays, its graceful and accordant curves, its bunching of masses, its occasional interstices, and the manifest order of a general plan governing the jumble of its details, it bears a remarkable resemblance to a garland -- a fact which appears the more wonderful when we recall its composition. that an elm-tree should trace the lines of beauty with its leafy and pendulous branches does not surprise us; but we can only gaze with growing amazement when we behold a hundred million suns imitating the form of a chaplet! and then we have to remember that this form furnishes the ground-plan of the universe. as an indication of the extraordinary speculations to which the mystery of the milky way has given rise, a theory recently ( ) proposed by prof. george c. comstock may be mentioned. starting with the data (first) that the number of stars increases as the milky way is approached, and reaches a maximum in its plane, while on the other hand the number of nebulæ is greatest outside the milky way and increases with distance from it, and (second) that the milky way, although a complete ring, is broad and diffuse on one side through one-half its course -- that half alone containing nebulæ -- and relatively narrow and well defined on the opposite side, the author of this singular speculation avers that these facts can best be explained by supposing that the invisible universe consists of two interpenetrating parts, one of which is a chaos of indefinite extent, strewn with stars and nebulous dust, and the other a long, broad but comparatively thin cluster of stars, including the sun as one of its central members. this flat star-cluster is conceived to be moving edgewise through the chaos, and, according to professor comstock, it acts after the manner of a snow-plough sweeping away the cosmic dust and piling it on either hand above and below the plane of the moving cluster. it thus forms a transparent rift, through which we see farther and command a view of more stars than through the intensified dust-clouds on either hand. this rift is the milky way. the dust thrown aside toward the poles of the milky way is the substance of the nebulæ which abound there. ahead, where the front of the star-plough is clearing the way, the chaos is nearer at hand, and consequently there the rift subtends a broader angle, and is filled with primordial dust, which, having been annexed by the vanguard of the star-swarm, forms the nebulæ seen only in that part of the milky way. but behind, the rift appears narrow because there we look farther away between dust-clouds produced ages ago by the front of the plough, and no scattered dust remains in that part of the rift. in quoting an outline of this strikingly original theory the present writer should not be understood as assenting to it. that it appears bizarre is not, in itself, a reason for rejecting it, when we are dealing with so problematical and enigmatical a subject as the milky way; but the serious objection is that the theory does not sufficiently accord with the observed phenomena. there is too much evidence that the milky way is an organic system, however fantastic its form, to permit the belief that it can only be a rift in chaotic clouds. as with every organism, we find that its parts are more or less clearly repeated in its ensemble. among all the strange things that the milky way contains there is nothing so extraordinary as itself. every astronomer must many times have found himself marveling at it in those comparatively rare nights when it shows all its beauty and all its strangeness. in its great broken rifts, divisions, and spirals are found the gigantic prototypes of similar forms in its star-clouds and clusters. as we have said, it determines the general shape of the whole sidereal system. some of the brightest stars in the sky appear to hang like jewels suspended at the ends of tassels dropped from the galaxy. among these pendants are the pleiades and the hyades. orion, too, the ``mighty hunter,'' is caught in ``a loop of light'' thrown out from it. the majority of the great first-magnitude stars seem related to it, as if they formed an inner ring inclined at an angle of some twenty degrees to its plane. many of the long curves that set off from it on both sides are accompanied by corresponding curves of lucid stars. in a word, it offers every appearance of structural connection with the entire starry system. that the universe should have assumed the form of a wreath is certainly a matter for astonishment; but it would have been still more astonishing if it had been a cube, a rhomboid, or a dodecahedron, for then we should have had to suppose that something resembling the forces that shape crystals had acted upon the stars, and the difficulty of explaining the universe by the laws of gravitation would have been increased. from the milky way as a whole we pass to the vast clouds, swarms, and clusters of stars of which it is made up. it may be, as some astronomers hold, that most of the galactic stars are much smaller than the sun, so that their faintness is not due entirely to the effect of distance. still, their intrinsic brilliance attests their solar character, and considering their remoteness, which has been estimated at not less than ten thousand to twenty thousand light-years (a light-year is equal to nearly six thousand thousand million miles) their actual masses cannot be extremely small. the minutest of them are entitled to be regarded as real suns, and they vary enormously in magnitude. the effects of their attractions upon one another can only be inferred from their clustering, because their relative movements are not apparent on account of the brevity of the observations that we can make. but imagine a being for whom a million years would be but as a flitting moment; to him the milky way would appear in a state of ceaseless agitation -- swirling with ``a fury of whirlpool motion.'' the cloud-like aspect of large parts of the galaxy must always have attracted attention, even from naked-eye observers, but the true star-clouds were first satisfactorily represented in barnard's photographs. the resemblance to actual clouds is often startling. some are close-packed and dense, like cumuli; some are wispy or mottled, like cirri. the rifts and modulations, as well as the general outlines, are the same as those of clouds of vapor or dust, and one notices also the characteristic thinning out at the edges. but we must beware of supposing that the component suns are thickly crowded as the particles forming an ordinary cloud. they look, indeed, as if they were matted together, because of the irradiation of light, but in reality millions and billions of miles separate each star from its neighbors. nevertheless they form real assemblages, whose members are far more closely related to one another than is our sun to the stars around him, and if we were in the milky way the aspect of the nocturnal sky would be marvelously different from its present appearance. stellar clouds are characteristic of the galaxy and are not found beyond its borders, except in the ``magellanic clouds'' of the southern hemisphere, which resemble detached portions of the milky way. these singular objects form as striking a peculiarity of the austral heavens as does the great ``coal-sack'' described in chapter . but it is their isolation that makes them so remarkable, for their composition is essentially galactic, and if they were included within its boundaries they would not appear more wonderful than many other parts of the milky way. placed where they are, they look like masses fallen from the great stellar arch. they are full of nebulæ and star-clusters, and show striking evidences of spiral movement. star-swarms, which are also characteristic features of the galaxy, differ from star-clouds very much in the way that their name would imply -- i.e., their component stars are so arranged, even when they are countless in number, that the idea of an exceedingly numerous assemblage rather than that of a cloud is impressed on the observer's mind. in a star-swarm the separate members are distinguishable because they are either larger or nearer than the stars composing a ``cloud.'' a splendid example of a true star-swarm is furnished by chi persei, in that part of the milky way which runs between the constellations perseus and cassiopeia. this swarm is much coarser than many others, and can be seen by the naked eye. in a small telescope it appears double, as if the suns composing it had divided into two parties which keep on their way side by side, with some commingling of their members where the skirts of the two companies come in contact. smaller than either star-clouds or star-swarms, and differing from both in their organization, are star-clusters. these, unlike the others, are found outside as well as inside the milky way, although they are more numerous inside its boundaries than elsewhere. the term star-cluster is sometimes applied, though improperly, to assemblages which are rather groups, such, for instance, as the pleiades. in their most characteristic aspect star-clusters are of a globular shape -- globes of suns! a famous example of a globular star-cluster, but one not included in the milky way, is the ``great cluster in hercules.'' this is barely visible to the naked eye, but a small telescope shows its character, and in a large one it presents a marvelous spectacle. photographs of such clusters are, perhaps, less effective than those of star-clouds, because the central condensation of stars in them is so great that their light becomes blended in an indistinguishable blur. the beautiful effect of the incessant play of infinitesimal rays over the apparently compact surface of the cluster, as if it were a globe of the finest frosted silver shining in an electric beam, is also lost in a photograph. still, even to the eye looking directly at the cluster through a powerful telescope, the central part of the wonderful congregation seems almost a solid mass in which the stars are packed like the ice crystals in a snowball. the same question rises to the lips of every observer: how can they possibly have been brought into such a situation? the marvel does not grow less when we know that, instead of being closely compacted, the stars of the cluster are probably separated by millions of miles; for we know that their distances apart are slight as compared with their remoteness from the earth. sir william herschel estimated their number to be about fourteen thousand, but in fact they are uncountable. if we could view them from a point just within the edge of the assemblage, they would offer the appearance of a hollow hemisphere emblazoned with stars of astonishing brilliancy; the near-by ones unparalleled in splendor by any celestial object known to us, while the more distant ones would resemble ordinary stars. an inhabitant of the cluster would not know, except by a process of ratiocination, that he was dwelling in a globular assemblage of suns; only from a point far outside would their spherical arrangement become evident to the eye. imagine fourteen-thousand fire-balloons with an approach to regularity in a spherical space -- say, ten miles in diameter; there would be an average of less than thirty in every cubic mile, and it would be necessary to go to a considerable distance in order to see them as a globular aggregation; yet from a point sufficiently far away they would blend into a glowing ball. photographs show even better than the best telescopic views that the great cluster is surrounded with a multitude of dispersed stars, suggestively arrayed in more or less curving lines, which radiate from the principle mass, with which their connection is manifest. these stars, situated outside the central sphere, look somewhat like vagrant bees buzzing round a dense swarm where the queen bee is sitting. yet while there is so much to suggest the operation of central forces, bringing and keeping the members of the cluster together, the attentive observer is also impressed with the idea that the whole wonderful phenomenon may be the result of explosion. as soon as this thought seizes the mind, confirmation of it seems to be found in the appearance of the outlying stars, which could be as readily explained by the supposition that they have been blown apart as that they have flocked together toward a center. the probable fact that the stars constituting the cluster are very much smaller than our sun might be regarded as favoring the hypothesis of an explosion. of their real size we know nothing, but, on the basis of an uncertain estimate of their parallax, it has been calculated that they may average forty-five thousand miles in diameter -- something more than half the diameter of the planet jupiter. assuming the same mean density, fourteen thousand such stars might have been formed by the explosion of a body about twice the size of the sun. this recalls the theory of olbers, which has never been altogether abandoned or disproved, that the asteroids were formed by the explosion of a planet circulating between the orbits of mars and jupiter. the asteroids, whatever their manner of origin, form a ring around the sun; but, of course, the explosion of a great independent body, not originally revolving about a superior center of gravitational force, would not result in the formation of a ring of small bodies, but rather of a dispersed mass of them. but back of any speculation of this kind lies the problem, at present insoluble: how could the explosion be produced? (see the question of explosions in chapters and ). then, on the other hand, we have the observation of herschel, since abundantly confirmed, that space is unusually vacant in the immediate neighborhood of condensed star-clusters and nebulæ, which, as far as it goes, might be taken as an indication that the assembled stars had been drawn together by their mutual attractions, and that the tendency to aggregation is still bringing new members toward the cluster. but in that case there must have been an original condensation of stars at that point in space. this could probably have been produced by the coagulation of a great nebula into stellar nuclei, a process which seems now to be taking place in the orion nebula. a yet more remarkable globular star-cluster exists in the southern hemisphere, omega centauri. in this case the central condensation of stars presents an almost uniform blaze of light. like the hercules cluster, that in centaurus is surrounded with stars scattered over a broad field and showing an appearance of radial arrangement. in fact, except for its greater richness, omega centauri is an exact duplicate of its northern rival. each appears to an imaginative spectator as a veritable ``city of suns.'' mathematics shrinks from the task of disentangling the maze of motions in such an assemblage. it would seem that the chance of collisions is not to be neglected, and this idea finds a certain degree of confirmation in the appearance of ``temporary stars'' which have more than once blazed out in, or close by, globular star-clusters. this leads up to the notable fact, first established by professor bailey a few years ago, that such clusters are populous with variable stars. omega centauri and the hercules cluster are especially remarkable in this respect. the variables found in them are all of short period and the changes of light show a noteworthy tendency to uniformity. the first thought is that these phenomena must be due to collisions among the crowded stars, but, if so, the encounters cannot be between the stars themselves, but probably between stars and meteor swarms revolving around them. such periodic collisions might go on for ages without the meteors being exhausted by incorporation with the stars. this explanation appears all the more probable because one would naturally expect that flocks of meteors would abound in a close aggregation of stars. it is also consistent with perrine's discovery -- that the globular star clusters are powdered with minute stars strewn thickly among the brighter ones. in speaking of professor comstock's extraordinary theory of the milky way, the fact was mentioned that, broadly speaking, the nebulæ are less numerous in the galactic belt than in the comparatively open spaces on either side of it, but that they are, nevertheless, abundant in the broader half of the milky way which he designates as the front of the gigantic ``plough'' supposed to be forcing its way through the enveloping chaos. in and around the sagittarius region the intermingling of nebulæ and galactic star clouds and clusters is particularly remarkable. that there is a causal connection no thoughtful person can doubt. we are unable to get away from the evidence that a nebula is like a seed-ground from which stars spring forth; or we may say that nebulæ resemble clouds in whose bosom raindrops are forming. the wonderful aspect of the admixtures of nebulæ and star-clusters in sagittarius has been described in chapter . we now come to a still more extraordinary phenomenon of this kind -- the pleiades nebulæ. the group of the pleiades, although lying outside the main course of the galaxy, is connected with it by a faint loop, and is the scene of the most remarkable association of stars and nebulous matter known in the visible universe. the naked eye is unaware of the existence of nebulæ in the pleiades, or, at the best, merely suspects that there is something of the kind there; and even the most powerful telescopes are far from revealing the full wonder of the spectacle; but in photographs which have been exposed for many hours consecutively, in order to accumulate the impression of the actinic rays, the revelation is stunning. the principle stars are seen surrounded by, and, as it were, drowned in, dense nebulous clouds of an unparalleled kind. the forms assumed by these clouds seem at first sight inexplicable. they look like fleeces, or perhaps more like splashes and daubs of luminous paint dashed carelessly from a brush. but closer inspection shows that they are, to a large extent, woven out of innumerable threads of filmy texture, and there are many indications of spiral tendencies. each of the bright stars of the group -- alcyone, merope, maia, electra, taygeta, atlas -- is the focus of a dense fog (totally invisible, remember, alike to the naked eye and to the telescope), and these particular stars are veiled from sight behind the strange mists. running in all directions across the relatively open spaces are nebulous wisps and streaks of the most curious forms. on some of the nebular lines, which are either straight throughout, or if they change direction do so at an angle, little stars are strung like beads. in one case seven or eight stars are thus aligned, and, as if to emphasize their dependence upon the chain which connects them, when it makes a slight bend the file of stars turns the same way. many other star rows in the group suggest by their arrangement that they, too, were once strung upon similar threads which have now disappeared, leaving the stars spaced along their ancient tracks. we seem forced to the conclusion that there was a time when the pleiades were embedded in a vast nebula resembling that of orion, and that the cloud has now become so rare by gradual condensation into stars that the merest trace of it remains, and this would probably have escaped detection but for the remarkable actinic power of the radiant matter of which it consists. the richness of many of these faint nebulous masses in ultra-violet radiations, which are those that specifically affect the photographic plate, is the cause of the marvelous revelatory power of celestial photography. so the veritable unseen universe, as distinguished from the ``unseen universe'' of metaphysical speculation, is shown to us. a different kind of association between stars and nebulæ is shown in some surprising photographic objects in the constellation cygnus, where long, wispy nebulæ, billions of miles in length, some of them looking like tresses streaming in a breeze, lie amid fields of stars which seem related to them. but the relation is of a most singular kind, for notwithstanding the delicate structure of the long nebulæ they appear to act as barriers, causing the stars to heap themselves on one side. the stars are two, three, or four times as numerous on one side of the nebulæ as on the other. these nebulæ, as far as appearance goes, might be likened to rail fences, or thin hedges, against which the wind is driving drifts of powdery snow, which, while scattered plentifully all around, tends to bank itself on the leeward side of the obstruction. the imagination is at a loss to account for these extraordinary phenomena; yet there they are, faithfully giving us their images whenever the photographic plate is exposed to their radiations. thus the more we see of the universe with improved methods of observation, and the more we invent aids to human senses, each enabling us to penetrate a little deeper into the unseen, the greater becomes the mystery. the telescope carried us far, photography is carrying us still farther; but what as yet unimagined instrument will take us to the bottom, the top, and the end? and then, what hitherto untried power of thought will enable us to comprehend the meaning of it all? stellar migrations to the untrained eye the stars and the planets are not distinguishable. it is customary to call them all alike ``stars.'' but since the planets more or less rapidly change their places in the sky, in consequence of their revolution about the sun, while the stars proper seem to remain always in the same relative positions, the latter are spoken of as ``fixed stars.'' in the beginnings of astronomy it was not known that the ``fixed stars'' had any motion independent of their apparent annual revolution with the whole sky about the earth as a seeming center. now, however, we know that the term ``fixed stars'' is paradoxical, for there is not a single really fixed object in the whole celestial sphere. the apparent fixity in the positions of the stars is due to their immense distance, combined with the shortness of the time during which we are able to observe them. it is like viewing the plume of smoke issuing from a steamer, hull down, at sea: if one does not continue to watch it for a long time it appears to be motionless, although in reality it may be traveling at great speed across the line of sight. even the planets seem fixed in position if one watches them for a single night only, and the more distant ones do not sensibly change their places, except after many nights of observation. neptune, for instance, moves but little more than two degrees in the course of an entire year, and in a month its change of place is only about one-third of the diameter of the full moon. yet, fixed as they seem, the stars are actually moving with a speed in comparison with which, in some cases, the planets might almost be said to stand fast in their tracks. jupiter's speed in his orbit is about eight miles per second, neptune's is less than three and one-half miles, and the earth's is about eighteen and one-half miles; while there are ``fixed stars'' which move two hundred or three hundred miles per second. they do not all, however, move with so great a velocity, for some appear to travel no faster than the planets. but in all cases, notwithstanding their real speed, long-continued and exceedingly careful observations are required to demonstrate that they are moving at all. no more overwhelming impression of the frightful depths of space in which the stars are buried can be obtained than by reflecting upon the fact that a star whose actual motion across the line of sight amounts to two hundred miles per second does not change its apparent place in the sky, in the course of a thousand years, sufficiently to be noticed by the casual observer of the heavens! there is one vast difference between the motions of the stars and those of the planets to which attention should be at once called: the planets, being under the control of a central force emanating from their immediate master, the sun, all move in the same direction and in orbits concentric about the sun; the stars, on the other hand, move in every conceivable direction and have no apparent center of motion, for all efforts to discover such a center have failed. at one time, when theology had finally to accept the facts of science, a grandiose conception arose in some pious minds, according to which the throne of god was situated at the exact center of his creation, and, seated there, he watched the magnificent spectacle of the starry systems obediently revolving around him. astronomical discoveries and speculations seemed for a time to afford some warrant for this view, which was, moreover, an acceptable substitute for the abandoned geocentric theory in minds that could only conceive of god as a superhuman artificer, constantly admiring his own work. no longer ago than the middle of the nineteenth century a german astronomer, maedler, believed that he had actually found the location of the center about which the stellar universe revolved. he placed it in the group of the pleiades, and upon his authority an extraordinary imaginative picture was sometimes drawn of the star alcyone, the brightest of the pleiades, as the very seat of the almighty. this idea even seemed to gain a kind of traditional support from the mystic significance, without known historical origin, which has for many ages, and among widely separated peoples, been attached to the remarkable group of which alcyone is the chief. but since maedler's time it has been demonstrated that the pleiades cannot be the center of revolution of the universe, and, as already remarked, all attempts to find or fix such a center have proved abortive. yet so powerful was the hold that the theory took upon the popular imagination, that even today astronomers are often asked if alcyone is not the probable site of ``jerusalem the golden.'' if there were a discoverable center of predominant gravitative power, to which the motions of all the stars could be referred, those motions would appear less mysterious, and we should then be able to conclude that the universe was, as a whole, a prototype of the subsidiary systems of which it is composed. we should look simply to the law of gravitation for an explanation, and, naturally, the center would be placed within the opening enclosed by the milky way. if it were there the milky way itself should exhibit signs of revolution about it, like a wheel turning upon its hub. no theory of the star motions as a whole could stand which failed to take account of the milky way as the basis of all. but the very form of that divided wreath of stars forbids the assumption of its revolution about a center. even if it could be conceived as a wheel having no material center it would not have the form which it actually presents. as was shown in chapter , there is abundant evidence of motion in the milky way; but it is not motion of the system as a whole, but motion affecting its separate parts. instead of all moving one way, the galactic stars, as far as their movements can be inferred, are governed by local influences and conditions. they appear to travel crosswise and in contrary directions, and perhaps they eddy around foci where great numbers have assembled; but of a universal revolution involving the entire mass we have no evidence. most of our knowledge of star motions, called ``proper motions,'' relates to individual stars and to a few groups which happen to be so near that the effects of their movements are measurable. in some cases the motion is so rapid (not in appearance, but in reality) that the chief difficulty is to imagine how it can have been imparted, and what will eventually become of the ``runaways.'' without a collision, or a series of very close approaches to great gravitational centers, a star traveling through space at the rate of two hundred or three hundred miles per second could not be arrested or turned into an orbit which would keep it forever flying within the limits of the visible universe. a famous example of these speeding stars is `` groombridge,'' a star of only the sixth magnitude, and consequently just visible to the naked eye, whose motion across the line of sight is so rapid that it moves upon the face of the sky a distance equal to the apparent diameter of the moon every years. the distance of this star is at least , , , , miles, and may be two or three times greater, so that its actual speed cannot be less than two hundred, and may be as much as four hundred, miles per second. it could be turned into a new course by a close approach to a great sun, but it could only be stopped by collision, head-on, with a body of enormous mass. barring such accidents it must, as far as we can see, keep on until it has traversed our stellar system, whence in may escape and pass out into space beyond, to join, perhaps, one of those other universes of which we have spoken. arcturus, one of the greatest suns in the universe, is also a runaway, whose speed of flight has been estimated all the way from fifty to two hundred miles per second. arcturus, we have every reason to believe, possesses hundreds of times the mass of our sun -- think, then, of the prodigious momentum that its motion implies! sirius moves more moderately, its motion across the line of sight amounting to only ten miles per second, but it is at the same time approaching the sun at about the same speed, its actual velocity in space being the resultant of the two displacements. what has been said about the motion of sirius brings us to another aspect of this subject. the fact is, that in every case of stellar motion the displacement that we observe represents only a part of the actual movement of the star concerned. there are stars whose motion carries them straight toward or straight away from the earth, and such stars, of course, show no cross motion. but the vast majority are traveling in paths inclined from a perpendicular to our line of sight. taken as a whole, the stars may be said to be flying about like the molecules in a mass of gas. the discovery of the radial component in the movements of the stars is due to the spectroscope. if a star is approaching, its spectral lines are shifted toward the violet end of the spectrum by an amount depending upon the velocity of approach; if it is receding, the lines are correspondingly shifted toward the red end. spectroscopic observation, then, combined with micrometric measurements of the cross motion, enables us to detect the real movement of the star in space. sometimes it happens that a star's radial movement is periodically reversed; first it approaches, and then it recedes. this indicates that it is revolving around a near-by companion, which is often invisible, and superposed upon this motion is that of the two stars concerned, which together may be approaching or receding or traveling across the line of sight. thus the complications involved in the stellar motions are often exceedingly great and puzzling. yet another source of complication exists in the movement of our own star, the sun. there is no more difficult problem in astronomy than that of disentangling the effects of the solar motion from those of the motions of the other stars. but the problem, difficult as it is, has been solved, and upon its solution depends our knowledge of the speed and direction of the movement of the solar system through space, for of course the sun carries its planets with it. one element of the solution is found in the fact that, as a result of perspective, the stars toward which we are going appear to move apart toward all points of the compass, while those behind appear to close up together. then the spectroscopic principle already mentioned is invoked for studying the shift of the lines, which is toward the violet in the stars ahead of us and toward the red in those that we are leaving behind. of course the effects of the independent motions of the stars must be carefully excluded. the result of the studies devoted to this subject is to show that we are traveling at a speed of twelve to fifteen miles per second in a northerly direction, toward the border of the constellations hercules and lyra. a curious fact is that the more recent estimates show that the direction is not very much out of a straight line drawn from the sun to the star vega, one of the most magnificent suns in the heavens. but it should not be inferred from this that vega is drawing us on; it is too distant for its gravitation to have such an effect. many unaccustomed thoughts are suggested by this mighty voyage of the solar system. whence have we come, and whither do we go? every year of our lives we advance at least , , miles. since the traditional time of adam the sun has led his planets through the wastes of space no less than , , , miles, or more than times the distance that separates him from the earth. go back in imagination to the geologic ages, and try to comprehend the distance over which the earth has flown. where was our little planet when it emerged out of the clouds of chaos? where was the sun when his ``thunder march'' began? what strange constellations shone down upon our globe when its masters of life were the monstrous beasts of the ``age of reptiles''? a million years is not much of a span of time in geologic reckoning, yet a million years ago the earth was farther from its present place in space than any of the stars with a measurable parallax are now. it was more than seven times as far as sirius, nearly fourteen times as far as alpha centauri, three times as far as vega, and twice as far as arcturus. but some geologists demand two hundred, three hundred, even one thousand million years to enable them to account for the evolutionary development of the earth and its inhabitants. in a thousand million years the earth would have traveled farther than from the remotest conceivable depths of the milky way! other curious reflections arise when we think of the form of the earth's track as it follows the lead of the sun, in a journey which has neither known beginning nor conceivable end. there are probably many minds which have found a kind of consolation in the thought that every year the globe returns to the same place, on the same side of the sun. this idea may have an occult connection with our traditional regard for anniversaries. when that period of the year returns at which any great event in our lives has occurred we have the feeling that the earth, in its annual round, has, in a manner, brought us back to the scene of that event. we think of the earth's orbit as a well-worn path which we traverse many times in the course of a lifetime. it seems familiar to us, and we grow to have a sort of attachment to it. the sun we are accustomed to regard as a fixed center in space, like the mill or pump around which the harnessed patient mule makes his endless circuits. but the real fact is that the earth never returns to the place in space where it has once quitted. in consequence of the motion of the sun carrying the earth and the other planets along, the track pursued by our globe is a vast spiral in space continually developing and never returning upon its course. it is probable that the tracks of the sun and the others stars are also irregular, and possibly spiral, although, as far as can be at present determined, they appear to be practically straight. every star, wherever it may be situated, is attracted by its fellow-stars from many sides at once, and although the force is minimized by distance, yet in the course of many ages its effects must become manifest. looked at from another side, is there not something immensely stimulating and pleasing to the imagination in the idea of so stupendous a journey, which makes all of us the greatest of travelers? in the course of a long life a man is transported through space thirty thousand million miles; halley's comet does not travel one-quarter as far in making one of its immense circuits. and there are adventures on this voyage of which we are just beginning to learn to take account. space is full of strange things, and the earth must encounter some of them as it advances through the unknown. many singular speculations have been indulged in by astronomers concerning the possible effects upon the earth of the varying state of the space that it traverses. even the alternation of hot and glacial periods has sometimes been ascribed to this source. when tropical life flourished around the poles, as the remains in the rocks assure us, the needed high temperature may, it has been thought, have been derived from the presence of the earth in a warm region of space. then, too, there is a certain interest for us in the thought of what our familiar planet has passed through. we cannot but admire it for its long journeying as we admire the traveler who comes to us from remote and unexplored lands, or as we gaze with a glow of interest upon the first locomotive that has crossed a continent, or a ship that has visited the arctic or antarctic regions. if we may trust the indications of the present course, the earth, piloted by the sun, has come from the milky way in the far south and may eventually rejoin that mighty band of stars in the far north. while the stars in general appear to travel independently of one another, except when they are combined in binary or trinary systems, there are notable exceptions to this rule. in some quarters of the sky we behold veritable migrations of entire groups of stars whose members are too widely separated to show any indications of revolution about a common center of gravity. this leads us back again to the wonderful group of the pleiades. all of the principle stars composing that group are traveling in virtually parallel lines. whatever force set them going evidently acted upon all alike. this might be explained by the assumption that when the original projective force acted upon them they were more closely united than they are at present, and that in drifting apart they have not lost the impulse of the primal motion. or it may be supposed that they are carried along by some current in space, although it would be exceedingly difficult, in the present state of our knowledge, to explain the nature of such a current. yet the theory of a current has been proposed. as to an attractive center around which they might revolve, none has been found. another instance of similar ``star-drift'' is furnished by five of the seven stars constituting the figure of the ``great dipper.'' in this case the stars concerned are separated very widely, the two extreme ones by not less than fifteen degrees, so that the idea of a common motion would never have been suggested by their aspect in the sky; and the case becomes the more remarkable from the fact that among and between them there are other stars, some of the same magnitude, which do not share their motion, but are traveling in other directions. still other examples of the same phenomenon are found in other parts of the sky. of course, in the case of compact star-clusters, it is assumed that all the members share a like motion of translation through space, and the same is probably true of dense star-swarms and star-clouds. the whole question of star-drift has lately assumed a new phase, in consequence of the investigations of kapteyn, dyson, and eddington on the ``systematic motions of the stars.'' this research will, it is hoped, lead to an understanding of the general law governing the movements of the whole body of stars constituting the visible universe. taking about eleven hundred stars whose proper motions have been ascertained with an approach to certainty, and which are distributed in all parts of the sky, it has been shown that there exists an apparent double drift, in two independent streams, moving in different and nearly opposed directions. the apex of the motion of what is called ``stream i'' is situated, according to professor kapteyn, in right ascension °, declination south °, which places it just south of the constellation orion; while the apex of ``stream ii'' is in right ascension °, declination south °, placing it in the constellation ara, south of scorpio. the two apices differ very nearly ° in right ascension and about ° in declination. the discovery of these vast star-streams, if they really exist, is one of the most extraordinary in modern astronomy. it offers the correlation of stellar movements needed as the basis of a theory of those movements, but it seems far from revealing a physical cause for them. as projected against the celestial sphere the stars forming the two opposite streams appear intermingled, some obeying one tendency and some the other. as professor dyson has said, the hypothesis of this double movement is of a revolutionary character, and calls for further investigation. indeed, it seems at first glance not less surprising than would be the observation that in a snow-storm the flakes over our heads were divided into two parties and driving across each other's course in nearly opposite directions, as if urged by interpenetrating winds. but whatever explanation may eventually be found for the motions of the stars, the knowledge of the existence of those motions must always afford a new charm to the contemplative observer of the heavens, for they impart a sense of life to the starry system that would otherwise be lacking. a stagnant universe, with every star fixed immovably in its place, would not content the imagination or satisfy our longing for ceaseless activity. the majestic grandeur of the evolutions of the celestial hosts, the inconceivable vastness of the fields of space in which they are executed, the countless numbers, the immeasurable distances, the involved convolutions, the flocking and the scattering, the interpenetrating marches and countermarches, the strange community of impulsion affecting stars that are wide apart in space and causing them to traverse the general movement about them like aides and despatch-bearers on a battle-field -- all these arouse an intensity of interest which is heightened by the mystery behind them. the passing of the constellations from a historical and picturesque point of view, one of the most striking results of the motions of the stars described in the last chapter is their effect upon the forms of the constellations, which have been watched and admired by mankind from a period so early that the date of their invention is now unknown. the constellations are formed by chance combinations of conspicuous stars, like figures in a kaleidoscope, and if our lives were commensurate with the æons of cosmic existence we should perceive that the kaleidoscope of the heavens was ceaselessly turning and throwing the stars into new symmetries. even if the stars stood fast, the motion of the solar system would gradually alter the configurations, as the elements of a landscape dissolve and recombine in fresh groupings with the traveler's progress amid them. but with the stars themselves all in motion at various speeds and in many directions, the changes occur more rapidly. of course, ``rapid'' is here understood in a relative sense; the wheel of human history to an eye accustomed to the majestic progression of the universe would appear to revolve with the velocity of a whirling dynamo. only the deliberation of geological movements can be contrasted with the evolution and devolution of the constellations. and yet this secular fluctuation of the constellation figures is not without keen interest for the meditative observer. it is another reminder of the swift mutability of terrestial affairs. to the passing glance, which is all that we can bestow upon these figures, they appear so immutable that they have been called into service to form the most lasting records of ancient thought and imagination that we possess. in the forms of the constellations, the most beautiful, and, in imaginative quality, the finest, mythology that the world has ever known has been perpetuated. yet, in a broad sense, this scroll of human thought imprinted on the heavens is as evanescent as the summer clouds. although more enduring than parchment, tombs, pyramids, and temples, it is as far as they from truly eternizing the memory of what man has fancied and done. before studying the effects that the motions of the stars have had and will have upon the constellations, it is worth while to consider a little further the importance of the stellar pictures as archives of history. to emphasize the importance of these effects it is only necessary to recall that the constellations register the oldest traditions of our race. in the history of primeval religions they are the most valuable of documents. leaving out of account for the moment the more familiar mythology of the greeks, based on something older yet, we may refer for illustration to that of the mysterious maya race of america. at izamal, in yucatan, says mr stansbury hagar, is a group of ruins perched, after the mexican and central-american plan, on the summits of pyramidal mounds which mark the site of an ancient theogonic center of the mayas. here the temples all evidently refer to a cult based upon the constellations as symbols. the figures and the names, of course, were not the same as those that we have derived from our aryan ancestors, but the star groups were the same or nearly so. for instance, the loftiest of the temples at izamal was connected with the sign of the constellation known to us as cancer, marking the place of the sun at the summer solstice, at which period the sun was supposed to descend at noon like a great bird of fire and consume the offerings left upon the altar. our scorpio was known to the mayas as a sign of the ``death god.'' our libra, the ``balance,'' with which the idea of a divine weighing out of justice has always been connected, seems to be identical with the mayan constellation teoyaotlatohua, with which was associated a temple where dwelt the priests whose special business it was to administer justice and to foretell the future by means of information obtained from the spirits of the dead. orion, the ``hunter'' of our celestial mythology, was among the mayas a ``warrior,'' while sagittarius and others of our constellations were known to them (under different names, of course), and all were endowed with a religious symbolism. and the same star figures, having the same significance, were familiar to the peruvians, as shown by the temples at cuzco. thus the imagination of ancient america sought in the constellations symbols of the unchanging gods. but, in fact, there is no nation and no people that has not recognized the constellations, and at one period or another in its history employed them in some symbolic or representative capacity. as handled by the greeks from prehistoric times, the constellation myths became the very soul of poetry. the imagination of that wonderful race idealized the principal star groups so effectively that the figures and traditions thus attached to them have, for civilized mankind, displaced all others, just as greek art in its highest forms stands without parallel and eclipses every rival. the romans translated no heroes and heroines of the mythical period of their history to the sky, and the deified cæsars never entered that lofty company, but the heavens are filled with the early myths of the greeks. herakles nightly resumes his mighty labors in the stars; zeus, in the form of the white ``bull,'' taurus, bears the fair europa on his back through the celestial waves; andromeda stretches forth her shackled arms in the star-gemmed ether, beseeching aid; and perseus, in a blaze of diamond armor, revives his heroic deeds amid sparkling clouds of stellar dust. there, too, sits queen cassiopeia in her dazzling chair, while the great king, cepheus, towers gigantic over the pole. professor young has significantly remarked that a great number of the constellations are connected in some way or other with the argonautic expedition -- that strangely fascinating legend of earliest greek story which has never lost its charm for mankind. in view of all this, we may well congratulate ourselves that the constellations will outlast our time and the time of countless generations to follow us; and yet they are very far from being eternal. let us now study some of the effects of the stellar motions upon them. we begin with the familiar figure of the ``great dipper.'' he who has not drunk inspiration from its celestial bowl is not yet admitted to the circle of olympus. this figure is made up of seven conspicuous stars in the constellation ursa major, the ``greater bear.'' the handle of the ``dipper'' corresponds to the tail of the imaginary ``bear,'' and the bowl lies upon his flank. in fact, the figure of a dipper is so evident and that of a bear so unevident, that to most persons the ``great dipper'' is the only part of the constellation that is recognizable. of the seven stars mentioned, six are of nearly equal brightness, ranking as of the second magnitude, while the seventh is of only the third magnitude. the difference is very striking, since every increase of one magnitude involves an increase of two-and-a-half times in brightness. there appears to be little doubt that the faint star, which is situated at the junction of the bowl and the handle, is a variable of long period, since three hundred years ago it was as bright as its companions. but however that may be, its relative faintness at the present time interferes but little with the perfection of the ``dipper's'' figure. in order the more readily to understand the changes which are taking place, it will be well to mention both the names and the greek letters which are attached to the seven stars. beginning at the star in the upper outer edge of the rim of the bowl and running in regular order round the bottom and then out to the end of the handle, the names and letters are as follows: dubhe ({\alpha}), merak ({\beta}), phaed ({\gamma}), megrez ({\delta}), alioth ({\epsilon}), mizar ({\zeta}), and benetnasch ({\eta}). megrez is the faint star already mentioned at the junction of the bowl and handle, and mizar, in the middle of the handle, has a close, naked-eye companion which is named alcor. the arabs called this singular pair of stars ``the horse and rider.'' merak and duhbe are called ``the pointers,'' because an imaginary line drawn northward through them indicates the pole star. now it has been found that five of these stars -- viz., merak, phaed, megrez, alioth, and mizar (with its comrade) -- are moving with practically the same speed in an easterly direction, while the other two, dubhe and benetnasch, are simultaneously moving westward, the motions of benetnasch being apparently more rapid. the consequence of these opposed motions is, of course, that the figure of the ``dipper'' cannot always have existed and will not continue to exist. in the accompanying diagrams it has been thought interesting to show the relative positions of these seven stars, as seen from the point which the earth now occupies, both in the past and in the future. arrows attached to the stars in the figure representing the present appearance of the ``dipper'' indicate the directions of the motions and the distances over which they will carry the stars in a period of about five hundred centuries. the time, no doubt, seems long, but remember the vast stretch of ages through which the earth has passed, and then reflect that no reason is apparent why our globe should not continue to be a scene of animation for ten thousand centuries yet to come. the fact that the little star alcor placed so close to mizar should accompany the latter in its flight is not surprising, but that two of the principal stars of the group should be found moving in a direction directly opposed to that pursued by the other five is surprising in the highest degree; and it recalls the strange theory of a double drift affecting all the stars, to which attention was called in the preceding chapter. it would appear that benetnasch and dubhe belong to one ``current,'' and merak, phaed, megrez, alioth, and mizar to the other. as far as is known, the motion of the seven stars are not shared by the smaller stars scattered about them, but on the theory of currents there should be such a community of motion, and further investigation may reveal it. from the ``great dipper'' we turn to a constellation hardly less conspicuous and situated at an equal distance from the pole on the other side -- cassiopeia. this famous star-group commemorating the romantic queen of ethiopia whose vain boasting of her beauty was punished by the exposure of her daughter andromeda to the ``sea monster,'' is well-marked by five stars which form an irregular letter ``w'' with its open side toward the pole. three of these stars are usually ranked as of the second magnitude, and two of the third; but to ordinary observation they appear of nearly equal brightness, and present a very striking picture. they mark out the chair and a part of the figure of the beautiful queen. beginning at the right-hand, or western, end of the ``w,'' their greek letter designations are: beta ({\beta}), alpha ({\alpha}), gamma ({\gamma}), delta ({\delta}), and epsilon ({\epsilon}). four of them, beta, alpha, delta, and epsilon are traveling eastwardly at various speeds, while the fifth, gamma, moves in a westerly direction. the motion of beta is more rapid than that of any of the others. it should be said, however, that no little uncertainty attaches to the estimates of the rate of motion of stars which are not going very rapidly, and different observers often vary considerably in their results. in the beautiful ``northern crown,'' one of the most perfect and charming of all the figures to be found in the stars, the alternate combining and scattering effects of the stellar motions are shown by comparing the appearance which the constellation must have had five hundred centuries ago with that which it has at present and that which it will have in the future. the seven principle stars of the asterism, forming a surprisingly perfect coronet, have movements in three directions at right angles to one another. that in these circumstances they should ever have arrived at positions giving them so striking an appearance of definite association is certainly surprising; from its aspect one would have expected to find a community of movement governing the brilliants of the ``crown,'' but instead of that we find evidence that they will inevitably drift apart and the beautiful figure will dissolve. a similar fate awaits such asterisms as the ``northern cross'' in cygnus; the ``crow'' (corvus), which stands on the back of the great ``sea serpent,'' hydra, and pecks at his scales; ``job's coffin'' (delphinus); the ``great square of pegasus''; the ``twins'' (gemini); the beautiful ``sickle'' in leo; and the exquisite group of the hyades in taurus. in the case of the hyades, two controlling movements are manifest: one, affecting five of the stars which form the well-known figure of a letter ``v,'' is directed northerly; the other, which controls the direction of two stars, has an easterly trend. the chief star of the group, aldebaran, one of the finest of all stars both for its brilliance and its color, is the most affected by the easterly motion. in time it will drift entirely out of connection with its present neighbors. although the hyades do not form so compact a group as the pleiades in the same constellation, yet their appearance of relationship is sufficient to awaken a feeling of surprise over the fact that, as with the stars of the ``dipper,'' their association is only temporary or apparent. the great figure of orion appears to be more lasting, not because its stars are physically connected, but because of their great distance, which renders their movements too deliberate to be exactly ascertained. two of the greatest of its stars, betelgeuse and rigel, possess, as far as has been ascertained, no perceptible motion across the line of sight, but there is a little movement perceptible in the ``belt.'' at the present time this consists of an almost perfect straight line, a row of second-magnitude stars about equally spaced and of the most striking beauty. in the course of time, however, the two right-hand stars, mintaka and alnilam (how fine are these arabic star names!) will approach each other and form a naked-eye double, but the third, alnita, will drift away eastward, so that the ``belt'' will no longer exist. for one more example, let us go to the southern hemisphere, whose most celebrated constellation, the ``southern cross,'' has found a place in all modern literatures, although it has no claim to consideration on account of association with ancient legends. this most attractive asterism, which has never ceased to fascinate the imagination of christendom since it was first devoutly described by the early explorers of the south, is but a passing collocation of brilliant stars. yet even in its transfigurations it has been for hundreds of centuries, and will continue to be for hundreds of centuries to come, a most striking object in the sky. our figures show its appearance in three successive phases: first, as it was fifty thousand years ago (viewed from the earth's present location); second, as it is in our day; and, third, as it will be an equal time in the future. the nearness of these bright stars to one another -- the length of the longer beam of the ``cross'' is only six degrees -- makes this group very noticeable, whatever the arrangement of its components may be. the largest star, at the base of the ``cross,'' is of the first magnitude, two of the others are of the second magnitude, and the fourth is of the third. other stars, not represented in the figures, increase the effect of a celestial blazonry, although they do not help the resemblance to a cross. but since the motion of the solar system itself will, in the course of so long a period as fifty thousand years, produce a great change in the perspective of the heavens as seen from the earth, by carrying us nearly nineteen trillion miles from our present place, why, it may be asked, seek to represent future appearances of the constellations which we could not hope to see, even if we could survive so long? the answer is: because these things aid the mind to form a picture of the effects of the mobility of the starry universe. only by showing the changes from some definite point of view can we arrive at a due comprehension of them. the constellations are more or less familiar to everybody, so that impending changes of their forms must at once strike the eye and the imagination, and make clearer the significance of the movements of the stars. if the future history of mankind is to resemble its past and if our race is destined to survive yet a million years, then our remote descendents will see a ``new heavens'' if not a ``new earth,'' and will have to invent novel constellations to perpetuate their legends and mythologies. if our knowledge of the relative distances of the stars were more complete, it would be an interesting exercise in celestial geometry to project the constellations probably visible to the inhabitants of worlds revolving around some of the other suns of space. our sun is too insignificant for us to think that he can make a conspicuous appearance among them, except, perhaps, in a few cases. as seen, for instance, from the nearest known star, alpha centauri, the sun would appear of the average first magnitude, and consequently from that standpoint he might be the gem of some little constellation which had no sirius, or arcturus, or vega to eclipse him with its superior splendor. but from the distance of the vast majority of the stars the sun would probably be invisible to the naked eye, and as seen from nearer systems could only rank as a fifth or sixth magnitude star, unnoticed and unknown except by the star-charting astronomer. conflagrations in the heavens suppose it were possible for the world to take fire and burn up -- as some pessimists think that it will do when the divine wrath shall have sufficiently accumulated against it -- nobody out of our own little corner of space would ever be aware of the catastrophe! with all their telescopes, the astronomers living in the golden light of arcturus or the diamond blaze of canopus would be unable to detect the least glimmer of the conflagration that had destroyed the seat of adam and his descendents, just as now they are totally ignorant of its existence. but at least fifteen times in the course of recorded history men looking out from the earth have beheld in the remote depths of space great outbursts of fiery light, some of them more splendidly luminous than anything else in the firmament except the sun! if they were conflagrations, how many million worlds like ours were required to feed their blaze? it is probable that ``temporary'' or ``new'' stars, as these wonderful apparitions are called, really are conflagrations; not in the sense of a bonfire or a burning house or city, but in that of a sudden eruption of inconceivable heat and light, such as would result from the stripping off the shell of an encrusted sun or the crashing together of two mighty orbs flying through space with a hundred times the velocity of the swiftest cannon-shot. temporary stars are the rarest and most erratic of astronomical phenomena. the earliest records relating to them are not very clear, and we cannot in every instance be certain that it was one of these appearances that the ignorant and superstitious old chroniclers are trying to describe. the first temporary star that we are absolutely sure of appeared in , and is known as ``tycho's star,'' because the celebrated danish astronomer (whose remains, with his gold-and-silver artificial nose -- made necessary by a duel -- still intact, were disinterred and reburied in ) was the first to perceive it in the sky, and the most assiduous and successful in his studies of it. as the first fully accredited representative of its class, this new star made its entry upon the scene with becoming éclat. it is characteristic of these phenomena that they burst into view with amazing suddenness, and, of course, entirely unexpectedly. tycho's star appeared in the constellation cassiopeia, near a now well-known and much-watched little star named kappa, on the evening of november , . the story has often been repeated, but it never loses interest, how tycho, going home that evening, saw people in the street pointing and staring at the sky directly over their heads, and following the direction of their hands and eyes he was astonished to see, near the zenith, an unknown star of surpassing brilliance. it outshone the planet jupiter, and was therefore far brighter than the first magnitude. there was not another star in the heavens that could be compared with it in splendor. tycho was not in all respects free from the superstitions of his time -- and who is? -- but he had the true scientific instinct, and immediately he began to study the stranger, and to record with the greatest care every change in its aspect. first he determined as well as he could with the imperfect instruments of his day, many of which he himself had invented, the precise location of the phenomena in the sky. then he followed the changes that it underwent. at first it brightened until its light equaled or exceeded that of the planet venus at her brightest, a statement which will be appreciated at its full value by anyone who has ever watched venus when she plays her dazzling rôle of ``evening star,'' flaring like an arc light in the sunset sky. it even became so brilliant as to be visible in full daylight, since, its position being circumpolar, it never set in the latitude of northern europe. finally it began to fade, turning red as it did so, and in march, , it disappeared from tycho's searching gaze, and has never been seen again from that day to this. none of the astronomers of the time could make anything of it. they had not yet as many bases of speculation as we possess today. tycho's star has achieved a romantic reputation by being fancifully identified with the ``star of bethlehem,'' said to have led the wondering magi from their eastern deserts to the cradle-manger of the savior in palestine. many attempts have been made to connect this traditional ``star'' with some known phenomenon of the heavens, and none seems more idle than this. yet it persistently survives, and no astronomer is free from eager questions about it addressed by people whose imagination has been excited by the legend. it is only necessary to say that the supposition of a connection between the phenomenon of the magi and tycho's star is without any scientific foundation. it was originally based on an unwarranted assumption that the star of tycho was a variable of long period, appearing once every three hundred and fifteen years, or thereabout. if that were true there would have been an apparition somewhere near the traditional date of the birth of christ, a date which is itself uncertain. but even the data on which the assumption was based are inconsistent with the theory. certain monkish records speak of something wonderful appearing in the sky in the years and , and these were taken to have been outbursts of tycho's star. investigation shows that the records more probably refer to comets, but even if the objects seen were temporary stars, their dates do not suit the hypothesis; from to there is a gap of years, and from to one of only years; moreover years have now ( ) elapsed since tycho saw the last glimmer of his star. upon a variability so irregular and uncertain as that, even if we felt sure that it existed, no conclusion could be found concerning an apparition occurring years ago. in the year (the year in which giordano bruno was burned at the stake for teaching that there is more than one physical world), a temporary star of the third magnitude broke out in the constellation cygnus, and curiously enough, considering the rarity of such phenomena, only four years later another surprisingly brilliant one appeared in the constellation ophiuchus. this is often called ``kepler's star,'' because the great german astronomer devoted to it the same attention that tycho had given to the earlier phenomenon. it, too, like tycho's, was at first the brightest object in the stellar heavens, although it seems never to have quite equaled its famous predecessor in splendor. it disappeared after a year, also turning of a red color as it became more faint. we shall see the significance of this as we go on. some of kepler's contemporaries suggested that the outburst of this star was due to a meeting of atoms in space, and idea bearing a striking resemblance to the modern theory of ``astronomical collisions.'' in , , and temporary stars made their appearance, but none of them was of great brilliance. in one of the second magnitude broke forth in the ``northern crown'' and awoke much interest, because by that time the spectroscope had begun to be employed in studying the composition of the stars, and huggins demonstrated that the new star consisted largely of incandescent hydrogen. but this star, apparently unlike the others mentioned, was not absolutely new. before its outburst it had shown as a star of the ninth magnitude (entirely invisible, of course, to the naked eye), and after about six weeks it faded to its original condition in which it has ever since remained. in a temporary star appeared in the constellation cygnus, and attained at one time the brightness of the second magnitude. its spectrum and its behavior resembled those of its immediate predecessor. in , astronomers were surprised to see a sixth-magnitude star glimmering in the midst of the hazy cloud of the great andromeda nebula. it soon absolutely disappeared. its spectrum was remarkable for being ``continuous,'' like that of the nebula itself. a continuous spectrum is supposed to represent a body, or a mass, which is either solid or liquid, or composed of gas under great pressure. in january, , a new star was suddenly seen in the constellation auriga. it never rose much above the fourth magnitude, but it showed a peculiar spectrum containing both bright and dark lines of hydrogen. but a bewildering surprise was now in store; the world was to behold at the opening of the twentieth century such a celestial spectacle as had not been on view since the times of tycho and kepler. before daylight on the morning of february , , the rev. doctor anderson, of edinburgh, an amateur astronomer, who had also been the first to see the new star in auriga, beheld a strange object in the constellation perseus not far from the celebrated variable star algol. he recognized its character at once, and immediately telegraphed the news, which awoke the startled attention of astronomers all over the world. when first seen the new star was no brighter than algol (less than the second magnitude), but within twenty-four hours it was ablaze, outshining even the brilliant capella, and far surpassing the first magnitude. at the spot in the sky where it appeared nothing whatever was visible on the night before its coming. this is known with certainty because a photograph had been made of that very region on february , and this photograph showed everything down to the twelfth magnitude, but not a trace of the stranger which burst into view between the st and the nd like the explosion of a rocket. upon one who knew the stars the apparition of this intruder in a well-known constellation had the effect of a sudden invasion. the new star was not far west of the zenith in the early evening, and in that position showed to the best advantage. to see capella, the hitherto unchallenged ruler of that quarter of the sky, abased by comparison with this stranger of alien aspect, for there was always an unfamiliar look about the ``nova,'' was decidedly disconcerting. it seemed to portend the beginning of a revolution in the heavens. one could understand what the effect of such an apparition must have been in the superstitious times of tycho. the star of tycho had burst forth on the northern border of the milky way; this one was on its southern border, some forty-five degrees farther east. astronomers were well-prepared this time for the scientific study of the new star, both astronomical photography and spectroscopy having been perfected, and the results of their investigations were calculated to increase the wonder with which the phenomenon was regarded. the star remained at its brightest only a few days; then, like a veritable conflagration, it began to languish; and, like the reflection of a dying fire, as it sank it began to glow with the red color of embers. but its changes were spasmodic; once about every three days it flared up only to die away again. during these fluctuations its light varied alternately in the ratio of one to six. finally it took a permanent downward course, and after a few months the naked eye could no longer perceive it; but it remained visible with telescopes, gradually fading until it had sunk to the ninth magnitude. then another astonishing change happened: in august photographs taken at the yerkes observatory and at heidelberg showed that the ``nova'' was surrounded by a spiral nebula! the nebula had not been there before, and no one could doubt that it represented a phase of the same catastrophe that had produced the outburst of the new star. at one time the star seemed virtually to have disappeared, as if all its substance had been expanded into the nebulous cloud, but always there remained a stellar nucleus about which the misty spiral spread wider and ever wider, like a wave expanding around a center of disturbance. the nebula too showed a variability of brightness, and four condensations which formed in it seemed to have a motion of revolution about the star. as time went on the nebula continued to expand at a rate which was computed to be not less than twenty thousand miles per second! and now the star itself, showing indications of having turned into a nebula, behaved in a most erratic manner, giving rise to the suspicion that it was about to burst out again. but this did not occur, and at length it sunk into a state of lethargy from which it has to the present time not recovered. but the nebulous spiral has disappeared, and the entire phenomena as it now ( ) exists consists of a faint nebulous star of less than the ninth magnitude. the wonderful transformations just described had been forecast in advance of the discovery of the nebulous spiral encircling the star by the spectroscopic study of the latter. at first there was no suggestion of a nebular constitution, but within a month or two characteristic nebular lines began to appear, and in less than six months the whole spectrum had been transformed to the nebular type. in the mean time the shifting of the spectral lines indicated a complication of rapid motions in several directions simultaneously. these motions were estimated to amount to from one hundred to five hundred miles per second. the human mind is so constituted that it feels forced to seek an explanation of so marvelous a phenomenon as this, even in the absence of the data needed for a sound conclusion. the most natural hypothesis, perhaps, is that of a collision. such a catastrophe could certainly happen. it has been shown, for instance, that in infinity of time the earth is sure to be hit by a comet; in the same way it may be asserted that, if no time limit is fixed, the sun is certain to run against some obstacle in space, either another star, or a dense meteor swarm, or one of the dark bodies which there is every reason to believe abound around us. the consequences of such a collision are easy to foretell, provided that we know the masses and the velocities of the colliding bodies. in a preceding chapter we have discussed the motions of the sun and stars, and have seen that they are so swift that an encounter between any two of them could not but be disastrous. but this is not all; for as soon as two stars approached within a few million miles their speed would be enormously increased by their reciprocal attractions and, if their motion was directed radially with respect to their centers, they would come together with a crash that would reduce them both to nebulous clouds. it is true that the chances of such a ``head-on'' collision are relatively very small; two stars approaching each other would most probably fall into closed orbits around their common center of gravity. if there were a collision it would most likely be a grazing one instead of a direct front-to-front encounter. but even a close approach, without any actual collision, would probably prove disastrous, owing to the tidal influence of each of the bodies on the other. suns, in consequence of their enormous masses and dimensions and the peculiarities of their constitution, are exceedingly dangerous to one another at close quarters. propinquity awakes in them a mutually destructive tendency. consisting of matter in the gaseous, or perhaps, in some cases, liquid, state, their tidal pull upon each other if brought close together might burst them asunder, and the photospheric envelope being destroyed the internal incandescent mass would gush out, bringing fiery death to any planets that were revolving near. without regard to the resulting disturbance of the earth's orbit, the close approach of a great star to the sun would be in the highest degree perilous to us. but this is a danger which may properly be regarded as indefinitely remote, since, at our present location in space, we are certainly far from every star except the sun, and we may feel confident that no great invisible body is near, for if there were one we should be aware of its presence from the effects of its attraction. as to dark nebulæ which may possibly lie in the track that the solar system is pursuing at the rate of , , miles per year, that is another question -- and they, too, could be dangerous! this brings us directly back to ``nova persei,'' for among the many suggestions offered to explain its outburst, as well as those of other temporary stars, one of the most fruitful is that of a collision between a star and a vast invisible nebula. professor seeliger, of munich, first proposed this theory, but it afterward underwent some modifications from others. stated in a general form, the idea is that a huge dark body, perhaps an extinguished sun, encountered in its progress through space a widespread flock of small meteors forming a dark nebula. as it plunged into the swarm the friction of the innumerable collisions with the meteors heated its surface to incandescence, and being of vast size it then became visible to us as a new star. meanwhile the motion of the body through the nebula, and its rotation upon itself, set up a gyration in the blazing atmosphere formed around it by the vaporized meteors; and as this atmosphere spread wider, under the laws of gyratory motion a rotation in the opposite direction began in the inflamed meteoric cloud outside the central part of the vortex. thus the spectral lines were caused to show motion in opposite directions, a part of the incandescent mass approaching the earth simultaneously with the retreat of another part. so the curious spectroscopic observations before mentioned were explained. this theory might also account for the appearance of the nebulous spiral first seen some six months after the original outburst. the sequent changes in the spectrum of the ``nova'' are accounted for by this theory on the assumption, reasonable enough in itself, that at first the invading body would be enveloped in a vaporized atmosphere of relatively slight depth, producing by its absorption the fine dark lines first observed; but that as time went on and the incessant collisions continued, the blazing atmosphere would become very deep and extensive, whereupon the appearance of the spectral lines would change, and bright lines due to the light of the incandescent meteors surrounding the nucleus at a great distance would take the place of the original dark ones. the vortex of meteors once formed would protect the flying body within from further immediate collisions, the latter now occurring mainly among the meteors themselves, and then the central blaze would die down, and the original splendor of the phenomenon would fade. but the theories about nova persei have been almost as numerous as the astronomers who have speculated about it. one of the most startling of them assumed that the outburst was caused by the running amuck of a dark star which had encountered another star surrounded with planets, the renewed outbreaks of light after the principal one had faded being due to the successive running down of the unfortunate planets! yet another hypothesis is based on what we have already said of the tidal influence that two close approaching suns would have upon each other. supposing two such bodies which had become encrusted, but remained incandescent and fluid within, to approach within almost striking distance; they would whirl each other about their common center of gravity, and at the same time their shells would burst under the tidal strain, and their glowing nuclei being disclosed would produce a great outburst of light. applying this theory to a ``nova,'' like that of in the ``northern crown,'' which had been visible as a small star before the outbreak, and which afterward resumed its former aspect, we should have to assume that a yet shining sun had been approached by a dark body whose attraction temporarily burst open its photosphere. it might be supposed that in this case the dark body was too far advanced in cooling to suffer the same fate from the tidal pull of its victim. but a close approach of that kind would be expected to result in the formation of a binary system, with orbits of great eccentricity, perhaps, and after the lapse of a certain time the outburst should be renewed by another approximation of the two bodies. a temporary star of that kind would rather be ranked as a variable. the celebrated french astronomer, janssen, had a different theory of nova persei, and of temporary stars in general. according to his idea, such phenomena might be the result of chemical changes taking place in a sun without interference by, or collision with, another body. janssen was engaged for many years in trying to discover evidence of the existence of oxygen in the sun, and he constructed his observatory on the summit of mount blanc specially to pursue that research. he believed that oxygen must surely exist in the sun since we find so many other familiar elements included in the constitution of the solar globe, and as he was unable to discover satisfactory evidence of its presence he assumed that it existed in a form unknown on the earth. if it were normally in the sun's chromosphere, or coronal atmosphere, he said, it would combine with the hydrogen which we know is there and form an obscuring envelope of water vapor. it exists, then, in a special state, uncombined with hydrogen; but let the temperature of the sun sink to a critical point and the oxygen will assume its normal properties and combine with the hydrogen, producing a mighty outburst of light and heat. this, janssen thought, might explain the phenomena of the temporary stars. it would also, he suggested, account for their brief career, because the combination of the elements would be quickly accomplished, and then the resulting water vapor would form an atmosphere cutting off the radiation from the star within. this theory may be said to have a livelier human interest than some of the others, since, according to it, the sun may carry in its very constitution a menace to mankind; one does not like to think of it being suddenly transformed into a gigantic laboratory for the explosive combination of oxygen and hydrogen! but while janssen's theory might do for some temporary stars, it is inadequate to explain all the phenomena of nova persei, and particularly the appearance of the great spiral nebula that seemed to exhale from the heart of the star. upon the whole, the theory of an encounter between a star and a dark nebula seems best to fit the observations. by that hypothesis the expanding billow of light surrounding the core of the conflagration is very well accounted for, and the spectroscopic peculiarities are also explained. dr gustov le bon offers a yet more alarming theory, suggesting that temporary stars are the result of atomic explosion; but we shall touch upon this more fully in chapter . twice in the course of this discussion we have called attention to the change of color invariably undergone by temporary stars in the later stages of their career. this was conspicuous with nova persei which glowed more and more redly as it faded, until the nebulous light began to overpower that of the stellar nucleus. nothing could be more suggestive of the dying out of a great fire. moreover, change of color from white to red is characteristic of all variable stars of long period, such as ``mira'' in cetus. it is also characteristic of stars believed to be in the later stages of evolution, and consequently approaching extinction, like antares and betelgeuse, and still more notably certain small stars which ``gleam like rubies in the field of the telescope.'' these last appear to be suns in the closing period of existence as self-luminous bodies. between the white stars, such as sirius and rigel, and the red stars, such as aldebaran and alpha herculis, there is a progressive series of colors from golden yellow through orange to deep red. the change is believed to be due to the increase of absorbing vapors in the stellar atmosphere as the body cools down. in the case of ordinary stars these changes no doubt occupy many millions of years, which represent the average duration of solar life; but the temporary stars run through similar changes in a few months: they resemble ephemeral insects -- born in the morning and doomed to perish with the going down of the sun. explosive and whirling nebulæ one of the most surprising triumphs of celestial photography was professor keeler's discovery, in , that the great majority of the nebulæ have a distinctly spiral form. this form, previously known in lord rosse's great ``whirlpool nebula,'' had been supposed to be exceptional; now the photographs, far excelling telescopic views in the revelation of nebular forms, showed the spiral to be the typical shape. indeed, it is a question whether all nebulæ are not to some extent spiral. the extreme importance of this discovery is shown in the effect that it has had upon hitherto prevailing views of solar and planetary evolution. for more than three-quarters of a century laplace's celebrated hypothesis of the manner of origin of the solar system from a rotating and contracting nebula surrounding the sun had guided speculation on that subject, and had been tentatively extended to cover the evolution of systems in general. the apparent forms of some of the nebulæ which the telescope had revealed were regarded, and by some are still regarded, as giving visual evidence in favor of this theory. there is a ``ring nebula'' in lyra with a central star, and a ``planetary nebula'' in gemini bearing no little resemblance to the planet saturn with its rings, both of which appear to be practical realizations of laplace's idea, and the elliptical rings surrounding the central condensation of the andromeda nebula may be cited for the same kind of proof. but since keeler's discovery there has been a decided turning away of speculation another way. the form of the spiral nebulæ seems to be entirely inconsistent with the theory of an originally globular or disk-shaped nebula condensing around a sun and throwing or leaving off rings, to be subsequently shaped into planets. some astronomers, indeed, now reject laplace's hypothesis in toto, preferring to think that even our solar system originated from a spiral nebula. since the spiral type prevails among the existing nebulæ, we must make any mechanical theory of the development of stars and planetary systems from them accord with the requirements which that form imposes. a glance at the extraordinary variations upon the spiral which professor keeler's photographs reveal is sufficient to convince one of the difficulty of the task of basing a general theory upon them. in truth, it is much easier to criticize laplace's hypothesis than to invent a satisfactory substitute for it. if the spiral nebulæ seem to oppose it there are other nebulæ which appear to support it, and it may be that no one fixed theory can account for all the forms of stellar evolution in the universe. our particular planetary system may have originated very much as the great french mathematician supposed, while others have undergone, or are now undergoing, a different process of development. there is always a too strong tendency to regard an important new discovery and the theories and speculations based upon it as revolutionizing knowledge, and displacing or overthrowing everything that went before. upon the plea that ``laplace only made a guess'' more recent guesses have been driven to extremes and treated by injudicious exponents as ``the solid facts at last.'' before considering more recent theories than laplace's, let us see what the nature of the photographic revelations is. the vast celestial maelstrom discovered by lord rosse in the ``hunting dogs'' may be taken as the leading type of the spiral nebulæ, although there are less conspicuous objects of the kind which, perhaps, better illustrate some of their peculiarities. lord rosse's nebula appears far more wonderful in the photographs than in his drawings made with the aid of his giant reflecting telescope at parsonstown, for the photographic plate records details that no telescope is capable of showing. suppose we look at the photograph of this object as any person of common sense would look at any great and strange natural phenomenon. what is the first thing that strikes the mind? it is certainly the appearance of violent whirling motion. one would say that the whole glowing mass had been spun about with tremendous velocity, or that it had been set rotating so rapidly that it had become the victim of ``centrifugal force,'' one huge fragment having broken loose and started to gyrate off into space. closer inspection shows that in addition to the principal focus there are various smaller condensations scattered through the mass. these are conspicuous in the spirals. some of them are stellar points, and but for the significance of their location we might suppose them to be stars which happen to lie in a line between us and the nebula. but when we observe how many of them follow most faithfully the curves of the spirals we cannot but conclude that they form an essential part of the phenomenon; it is not possible to believe that their presence in such situations is merely fortuitous. one of the outer spirals has at least a dozen of these star-like points strung upon it; some of them sharp, small, and distinct, others more blurred and nebulous, suggesting different stages of condensation. even the part which seems to have been flung loose from the main mass has, in addition to its central condensation, at least one stellar point gleaming in the half-vanished spire attached to it. some of the more distant stars scattered around the ``whirlpool'' look as if they too had been shot out of the mighty vortex, afterward condensing into unmistakable solar bodies. there are at least two curved rows of minute stars a little beyond the periphery of the luminous whirl which clearly follow lines concentric with those of the nebulous spirals. such facts are simply dumbfounding for anyone who will bestow sufficient thought upon them, for these are suns, though they may be small ones; and what a birth is that for a sun! look now again at the glowing spirals. we observe that hardly have they left the central mass before they begin to coagulate. in some places they have a ``ropy'' aspect; or they are like peascods filled with growing seeds, which eventually will become stars. the great focus itself shows a similar tendency, especially around its circumference. the sense that it imparts of a tremendous shattering force at work is overwhelming. there is probably more matter in that whirling and bursting nebula than would suffice to make a hundred solar systems! it must be confessed at once that there is no confirmation of the laplacean hypothesis here; but what hypothesis will fit the facts? there is one which it has been claimed does so, but we shall come to that later. in the meanwhile, as a preparation, fix in the memory the appearance of that second spiral mass spinning beside its master which seems to have spurned it away. for a second example of the spiral nebulæ look at the one in the constellation triangulum. god, how hath the imagination of puny man failed to comprehend thee! here is creation through destruction with a vengeance! the spiral form of the nebula is unmistakable, but it is half obliterated amid the turmoil of flying masses hurled away on all sides with tornadic fury. the focus itself is splitting asunder under the intolerable strain, and in a little while, as time is reckoned in the cosmos, it will be gyrating into stars. and then look at the cyclonic rain of already finished stars whirling round the outskirts of the storm. observe how scores of them are yet involved in the fading streams of the nebulous spirals; see how they have been thrown into vast loops and curves, of a beauty that half redeems the terror of the spectacle enclosed within their lines -- like iridescent cirri hovering about the edges of a hurricane. and so again are suns born! let us turn to the exquisite spiral in ursa major; how different its aspect from that of the other! one would say that if the terrific coil in triangulum has all but destroyed itself in its fury, this one on the contrary has just begun its self-demolition. as one gazes one seems to see in it the smooth, swift, accelerating motion that precedes catastrophe. the central part is still intact, dense, and uniform in texture. how graceful are the spirals that smoothly rise from its oval rim and, gemmed with little stars, wind off into the darkness until they have become as delicate as threads of gossamer! but at bottom the story told here is the same -- creation by gyration! compare with the above the curious mass in cetus. here the plane of the whirling nebula nearly coincides with our line of sight and we see the object at a low angle. it is far advanced and torn to shreds, and if we could look at it perpendicularly to its plane it is evident that it would closely resemble the spectacle in triangulum. then take the famous andromeda nebula (see frontispiece), which is so vast that notwithstanding its immense distance even the naked eye perceives it as an enigmatical wisp in the sky. its image on the sensitive plate is the masterpiece of astronomical photography; for wild, incomprehensible beauty there is nothing that can be compared with it. here, if anywhere, we look upon the spectacle of creation in one of its earliest stages. the andromeda nebula is apparently less advanced toward transformation into stellar bodies than is that in triangulum. the immense crowd of stars sprinkled over it and its neighborhood seem in the main to lie this side of the nebula, and consequently to have no connection with it. but incipient stars (in some places clusters of them) are seen in the nebulous rings, while one or two huge masses seem to give promise of transformation into stellar bodies of unusual magnitude. i say ``rings'' because although the loops encompassing the andromeda nebula have been called spirals by those who wish utterly to demolish laplace's hypothesis, yet they are not manifestly such, as can be seen on comparing them with the undoubted spirals of the lord rosse nebula. they look quite as much like circles or ellipses seen at an angle of, say, fifteen or twenty degrees to their plane. if they are truly elliptical they accord fairly well with laplace's idea, except that the scale of magnitude is stupendous, and if the andromeda nebula is to become a solar system it will surpass ours in grandeur beyond all possibility of comparison. there is one circumstance connected with the spiral nebulæ, and conspicuous in the andromeda nebula on account of its brightness, which makes the question of their origin still more puzzling; they all show continuous spectra, which, as we have before remarked, indicate that the mass from which the light comes is either solid or liquid, or a gas under heavy pressure. thus nebulæ fall into two classes: the ``white'' nebulæ, giving a continuous spectrum; and the ``green'' nebulæ whose spectra are distinctly gaseous. the andromeda nebula is the great representative of the former class and the orion nebula of the latter. the spectrum of the andromeda nebula has been interpreted to mean that it consists not of luminous gas, but of a flock of stars so distant that they are separately indistinguishable even with powerful telescopes, just as the component stars of the milky way are indistinguishable with the naked eye; and upon this has been based the suggestion that what we see in andromeda is an outer universe whose stars form a series of elliptical garlands surrounding a central mass of amazing richness. but this idea is unacceptable if for no other reason than that, as just said, all the spiral nebulæ possess the same kind of spectrum, and probably no one would be disposed to regard them all as outer universes. as we shall see later, the peculiarity of the spectra of the spiral nebulæ is appealed to in support of a modern substitute for laplace's hypothesis. finally, without having by any means exhausted the variety exhibited by the spiral nebulæ, let us turn to the great representative of the other species, the orion nebula. in some ways this is even more marvelous than the others. the early drawings with the telescope failed to convey an adequate conception either of its sublimity or of its complication of structure. it exists in a nebulous region of space, since photographs show that nearly the whole constellation is interwoven with faintly luminous coils. to behold the entry of the great nebula into the field even of a small telescope is a startling experience which never loses its novelty. as shown by the photographs, it is an inscrutable chaos of perfectly amazing extent, where spiral bands, radiating streaks, dense masses, and dark yawning gaps are strangely intermingled without apparent order. in one place four conspicuous little stars, better seen in a telescope than in the photograph on account of the blurring produced by over-exposure, are suggestively situated in the midst of a dark opening, and no observer has ever felt any doubt that these stars have been formed from the substance of the surrounding nebula. there are many other stars scattered over its expanse which manifestly owe their origin to the same source. but compare the general appearance of this nebula with the others that we have studied, and remark the difference. if the unmistakably spiral nebulæ resemble bursting fly-wheels or grindstones from whose perimeters torrents of sparks are flying, the orion nebula rather recalls the aspect of a cloud of smoke and fragments produced by the explosion of a shell. this idea is enforced by the look of the outer portion farthest from the bright half of the nebula, where sharply edged clouds with dark spaces behind seem to be billowing away as if driven by a wind blowing from the center. next let us consider what scientific speculation has done in the effort to explain these mysteries. laplace's hypothesis can certainly find no standing ground either in the orion nebula or in those of a spiral configuration, whatever may be its situation with respect to the grand nebula of andromeda, or the ``ring'' and ``planetary'' nebulæ. some other hypothesis more consonant with the appearances must be found. among the many that have been proposed the most elaborate is the ``planetesimal hypothesis'' of professors chamberlin and moulton. it is to be remarked that it applies to the spiral nebulæ distinctively, and not to an apparently chaotic mass of gas like the vast luminous cloud in orion. the gist of the theory is that these curious objects are probably the result of close approaches to each other of two independent suns, reminding us of what was said on this subject when we were dealing with temporary stars. of the previous history of these appulsing suns the theory gives us no account; they are simply supposed to arrive within what may be called an effective tide-producing distance, and then the drama begins. some of the probable consequences of such an approach have been noticed in chapter ; let us now consider them a little more in detail. tides always go in couples; if there is a tide on one side of a globe there will be a corresponding tide on the other side. the cause is to be found in the law that the force of gravitation varies inversely as the square of the distance; the attraction on the nearest surface of the body exercised by another body is greater than on its center, and greater yet than on its opposite surface. if two great globes attract each other, each tends to draw the other out into an ellipsoidal figure; they must be more rigid than steel to resist this -- and even then they cannot altogether resist. if they are liquid or gaseous they will yield readily to the force of distortion, the amount of which will depend upon their distance apart, for the nearer they are the greater becomes the tidal strain. if they are encrusted without and liquid or gaseous in the interior, the internal mass will strive to assume the figure demanded by the tidal force, and will, if it can, burst the restraining envelope. now this is virtually the predicament of the body we call a sun when in the immediate presence of another body of similarly great mass. such a body is presumably gaseous throughout, the component gases being held in a state of rigidity by the compression produced by the tremendous gravitational force of their own aggregate mass. at the surface such a body is enveloped in a shell of relatively cool matter. now suppose a great attracting body, such as another sun, to approach near enough for the difference in its attraction on the two opposite sides of the body and on its center to become very great; the consequence will be a tidal deformation of the whole body, and it will lengthen out along the line of the gravitational pull and draw in at the sides, and if its shell offers considerable resistance, but not enough to exercise a complete restraint, it will be violently burst apart, or blown to atoms, and the internal mass will leap out on the two opposite sides in great fiery spouts. in the case of a sun further advanced in cooling than ours the interior might be composed of molten matter while the exterior crust had become rigid like the shell of an egg; then the force of the ``tidal explosion'' produced by the appulse of another sun would be more violent in consequence of the greater resistance overcome. such, then, is the mechanism of the first phase in the history of a spiral nebula according to the planetesimal hypothesis. two suns, perhaps extinguished ones, have drawn near together, and an explosive outburst has occured in one or both. the second phase calls for a more agile exercise of the imagination. to simplify the case, let us suppose that only one of the tugging suns is seriously affected by the strain. its vast wings produced by the outburst are twisted into spirals by their rotation and the contending attractions exercised upon them, as the two suns, like battleships in desperate conflict, curve round each other, concentrating their destructive energies. then immense quantities of débris are scattered about in which eddies are created, and finally, as the sun that caused the damage goes on its way, leaving its victim to repair its injuries as it may, the dispersed matter cools, condenses, and turns into streams of solid particles circling in elliptical paths about their parent sun. these particles, or fragments, are the ``planetesimals'' of the theory. in consequence of the inevitable intersection of the orbits of the planetesimals, nodes are formed where the flying particles meet, and at these nodes large masses are gradually accumulated. the larger the mass the greater its attraction, and at last the nodal points become the nuclei of great aggregations from which planets are shaped. this, in very brief form, is the planetesimal hypothesis which we are asked to substitute for that based on laplace's suggestion as an explanation of the mode of origin of the solar system; and the phenomena of the spiral nebulæ are appealed to as offering evident support to the new hypothesis. we are reminded that they are elliptical in outline, which accords with the hypothesis; that their spectra are not gaseous, which shows that they may be composed of solid particles like the planetesimals; and that their central masses present an oval form, which is what would result from the tidal effects, as just described. we also remember that some of them, like the lord rosse and the andromeda nebulæ, are visually double, and in these cases we might suppose that the two masses represent the tide-burst suns that ventured into too close proximity. it may be added that the authors of the theory do not insist upon the appulse of two suns as the only way in which the planetesimals may have originated, but it is the only supposition that has been worked out. but serious questions remain. it needs, for instance, but a glance at the triangulum monster to convince the observer that it cannot be a solar system which is being evolved there, but rather a swarm of stars. many of the detached masses are too vast to admit of the supposition that they are to be transformed into planets, in our sense of planets, and the distances of the stars which appear to have been originally ejected from the focal masses are too great to allow us to liken the assemblage that they form to a solar system. then, too, no nodes such as the hypothesis calls for are visible. moreover, in most of the spiral nebulæ the appearances favor the view that the supposititious encountering suns have not separated and gone each rejoicing on its way, after having inflicted the maximum possible damage on its opponent, but that, on the contrary, they remain in close association like two wrestlers who cannot escape from each other's grasp. and this is exactly what the law of gravitation demands; stars cannot approach one another with impunity, with regard either to their physical make-up or their future independence of movement. the theory undertakes to avoid this difficulty by assuming that in the case of our system the approach of the foreign body to the sun was not a close one -- just close enough to produce the tidal extrusion of the relatively insignificant quantity of matter needed to form the planets. but even then the effect of the appulse would be to change the direction of flight, both of the sun and of its visitor, and there is no known star in the sky which can be selected as the sun's probable partner in their ancient pas deux. that there are unconquered difficulties in laplace's hypothesis no one would deny, but in simplicity of conception it is incomparably more satisfactory, and with proper modifications could probably be made more consonant with existing facts in our solar system than that which is offered to replace it. even as an explanation of the spiral nebulæ, not as solar systems in process of formation, but as the birthplaces of stellar clusters, the planetesimal hypothesis would be open to many objections. granting its assumptions, it has undoubtedly a strong mathematical framework, but the trouble is not with the mathematics but with the assumptions. laplace was one of the ablest mathematicians that ever lived, but he had never seen a spiral nebula; if he had, he might have invented a hypothesis to suit its phenomena. his actual hypothesis was intended only for our solar system, and he left it in the form of a ``note'' for the consideration of his successors, with the hope that they might be able to discover the full truth, which he confessed was hidden from him. it cannot be said that that truth has yet been found, and when it is found the chances are that intuition and not logic will have led to it. the spiral nebulæ, then, remain among the greatest riddles of the universe, while the gaseous nebulæ, like that of orion, are no less mysterious, although it seems impossible to doubt that both forms give birth to stars. it is but natural to look to them for light on the question of the origin of our planetary system; but we should not forget that the scale of the phenomena in the two cases is vastly different, and the forces in operation may be equally different. a hill may have been built up by a glacier, while a mountain may be the product of volcanic forces or of the upheaval of the strata of the planet. the banners of the sun as all the world knows, the sun, a blinding globe pouring forth an inconceivable quantity of light and heat, whose daily passage through the sky is caused by the earth's rotation on its axis, constitutes the most important phenomenon of terrestial existence. viewed with a dark glass to take off the glare, or with a telescope, its rim is seen to be a sharp and smooth circle, and nothing but dark sky is visible around it. except for the interference of the moon, we should probably never have known that there is any more of the sun than our eyes ordinarily see. but when an eclipse of the sun occurs, caused by the interposition of the opaque globe of the moon, we see its immediate surroundings, which in some respects are more wonderful than the glowing central orb. these surroundings, although not in the sense in which we apply the term to the gaseous envelope of the earth, may be called the sun's atmosphere. they consist of two very different parts -- first, the red ``prominences,'' which resemble tongues of flame ascending thousands of miles above the sun's surface; and, second, the ``corona,'' which extends to distances of millions of miles from the sun, and shines with a soft, glowing light. the two combined, when well seen, make a spectacle without parallel among the marvels of the sky. although many attempts have been made to render the corona visible when there is no eclipse, all have failed, and it is to the moon alone that we owe its revelation. to cover the sun's disk with a circular screen will not answer the purpose because of the illumination of the air all about the observer. when the moon hides the sun, on the other hand, the sunlight is withdrawn from a great cylinder of air extending to the top of the atmosphere and spreading many miles around the observer. there is then no glare to interfere with the spectacle, and the corona appears in all its surprising beauty. the prominences, however, although they were discovered during an eclipse, can now, with the aid of the spectroscope, be seen at any time. but the prominences are rarely large enough to be noticed by the naked eye, while the streamers of the corona, stretching far away in space, like ghostly banners blown out from the black circle of the obscuring moon, attract every eye, and to this weird apparition much of the fear inspired by eclipses has been due. but if the corona has been a cause of terror in the past it has become a source of growing knowledge in our time. the story of the first scientific observation of the corona and the prominences is thrillingly interesting, and in fact dramatic. the observation was made during the eclipse of , which fortunately was visible all over central and southern europe so that scores of astronomers saw it. the interest centers in what happened at pavia in northern italy, where the english astronomer francis baily had set up his telescope. the eclipse had begun and bailey was busy at his telescope when, to quote his own words in the account which he wrote for the memoirs of the royal astronomical society: i was astounded by a tremendous burst of applause from the streets below, and at the same moment was electrified by the sight of one of the most brilliant and splendid phenomena that can well be imagined; for at that instant the dark body of the moon was suddenly surrounded with a corona, or kind of bright glory, similar in shape and magnitude to that which painters draw round the heads of saints... pavia contains many thousand inhabitants, the major part of whom were at this early hour walking about the streets and squares or looking out of windows in order to witness this long-talked-of phenomenon; and when the total obscuration took place, which was instantaneous, there was a universal shout from every observer which ``made the welkin ring,'' and for the moment withdrew my attention from the object with which i was immediately occupied. i had, indeed, expected the appearance of a luminous circle round the moon during the time of total obscurity; but i did not expect, from any of the accounts of preceding eclipses that i had read, to witness so magnificent an exhibition as that which took place... splendid and astonishing, however, as this remarkable phenomenon really was, and although it could not fail to call forth the admiration and applause of every beholder, yet i must confess that there was at the same time something in its singular and wonderful appearance that was appalling... but the most remarkable circumstance attending the phenomenon was the appearance of three large protuberances apparently emanating from the circumference of the moon, but evidently forming a portion of the corona. they had the appearance of mountains of a prodigious elevation; their color was red tinged with lilac or purple; perhaps the color of the peach-blossom would more nearly represent it. they somewhat resembled the tops of the snowy alpine mountains when colored by the rising or the setting sun. they resembled the alpine mountains in another respect, inasmuch as their light was perfectly steady, and had none of that flickering or sparkling motion so visible in other parts of the corona... the whole of these protuberances were visible even to the last moment of total obscuration, and when the first ray of light was admitted from the sun they vanished, with the corona, altogether, and daylight was instantly restored. i have quoted nearly all of this remarkable description not alone for its intrinsic interest, but because it is the best depiction that can be found of the general phenomena of a total solar eclipse. still, not every such eclipse offers an equally magnificent spectacle. the eclipses of and , for instance, which were seen by the writer, the first in south carolina and the second in spain, fell far short of that described by bailey in splendor and impressiveness. of course, something must be allowed for the effect of surprise; bailey had not expected to see what was so suddenly disclosed to him. but both in and the amount of scattered light in the sky was sufficient in itself to make the corona appear faint, and there were no very conspicuous prominences visible. yet on both occasions there was manifest among the spectators that mingling of admiration and awe of which bailey speaks. the south carolinians gave a cheer and the ladies waved their handkerchiefs when the corona, ineffably delicate of form and texture, melted into sight and then in two minutes melted away again. the spaniards, crowded on the citadel hill of burgos, with their king and his royal retinue in their midst, broke out with a great clapping of hands as the awaited spectacle unfolded itself in the sky; and on both occasions, before the applause began, after an awed silence a low murmur ran through the crowds. at burgos it is said many made the sign of the cross. it was not long before bailey's idea that the prominences were a part of the corona was abandoned, and it was perceived that the two phenomena were to a great extent independent. at the eclipse of , which the astronomers, aroused by the wonderful scene of , and eager to test the powers of the newly invented spectroscope, flocked to india to witness, janssen conceived the idea of employing the spectroscope to render the prominences visible when there was no eclipse. he succeeded the very next day, and these phenomena have been studied in that way ever since. there are recognized two kinds of prominences -- the ``erruptive'' and the ``quiescent.'' the latter, which are cloud-like in form, may be seen almost anywhere along the edge of the sun; but the former, which often shoot up as if hurled from mighty volcanoes, appear to be associated with sun-spots, and appear only above the zones where spots abound. either of them, when seen in projection against the brilliant solar disk, appears white, not red, as against a background of sky. the quiescent prominences, whose elevation is often from forty thousand to sixty thousand miles, consist, as the spectroscope shows, mainly of hydrogen and helium. the latter, it will be remembered, is an element which was known to be in the sun many years before the discovery that it also exists in small quantities on the earth. a fact which may have a significance which we cannot at present see is that the emanation from radium gradually and spontaneously changes into helium, an alchemistical feat of nature that has opened many curious vistas to speculative thinkers. the eruptive prominences, which do not spread horizontally like the others, but ascend with marvelous velocity to elevations of half a million miles or more, are apparently composed largely of metallic vapors -- i.e. metals which are usually solid on the earth, but which at solar temperatures are kept in a volatilized state. the velocity of their ascent occasionally amounts to three hundred or four hundred miles per second. it is known from mathematical considerations that the gravitation of the sun would not be able to bring back any body that started from its surface with a velocity exceeding three hundred and eighty-three miles per second; so it is evident that some of the matter hurled forth in eruptive prominences may escape from solar control and go speeding out into space, cooling and condensing into solid masses. there seems to be no reason why some of the projectiles from the sun might not reach the planets. here, then, we have on a relatively small scale, explosions recalling those which it has been imagined may be the originating cause of some of the sudden phenomena of the stellar heavens. of the sun-spots it is not our intention here specifically to speak, but they evidently have an intimate connection with eruptive prominences, as well as some relation, not yet fully understood, with the corona. of the real cause of sun-spots we know virtually nothing, but recent studies by professor hale and others have revealed a strange state of things in the clouds of metallic vapors floating above them and their surroundings. evidences of a cyclonic tendency have been found, and professor hale has proved that sun-spots are strong magnetic fields, and consist of columns of ionized vapors rotating in opposite directions in the two hemispheres. a fact which may have the greatest significance is that titanium and vanadium have been found both in sun-spots and in the remarkable variable mira ceti, a star which every eleven months, or thereabout, flames up with great brilliancy and then sinks back to invisibility with the naked eye. it has been suggested that sun-spots are indications of the beginning of a process in the sun which will be intensified until it falls into the state of such a star as mira. stars very far advanced in evolution, without showing variability, also exhibit similar spectra; so that there is much reason for regarding sunspots as emblems of advancing age. the association of the corona with sun-spots is less evident than that of the eruptive prominences; still such an association exists, for the form and extent of the corona vary with the sun-spot period of which we shall presently speak. the constitution of the corona remains to be discovered. it is evidently in part gaseous, but it also probably contains matter in the form of dust and small meteors. it includes one substance altogether mysterious -- ``coronium.'' there are reasons for thinking that this may be the lightest of all the elements, and professor young, its discoverer, said that it was ``absolutely unique in nature; utterly distinct from any other known form of matter, terrestial, solar, or cosmical.'' the enormous extent of the corona is one of its riddles. since the development of the curious subject of the ``pressure of light'' it has been proposed to account for the sustentation of the corona by supposing that it is borne upon the billows of light continually poured out from the sun. experiment has proved, what mathematical considerations had previously pointed out as probable, that the waves of light exert a pressure or driving force, which becomes evident in its effects if the body acted upon is sufficiently small. in that case the light pressure will prevail over the attraction of gravitation, and propel the attenuated matter away from the sun in the teeth of its attraction. the earth itself would be driven away if, instead of consisting of a solid globe of immense aggregate mass, it were a cloud of microscopic particles. the reason is that the pressure varies in proportion to the surface of the body acted upon, while the gravitational attraction is proportional to the volume, or the total amount of matter in the body. but the surface of any body depends upon the square of its diameter, while the volume depends upon the cube of the diameter. if, for instance, the diameter is represented by , the surface will be proportional to × , or , and the volume to × × , or ; but if the diameter is taken as , the surface will be × , or , and the volume × × , or . now, the ratio of to is twice as great as that of to . if the diameter is still further decreased, the ratio of the surface to the volume will proportionally grow larger; in other words, the pressure will gain upon the attraction, and whatever their original ratio may have been, a time will come, if the diminution of size continues, when the pressure will become more effective than the attraction, and the body will be driven away. supposing the particles of the corona to be below the critical size for the attraction of a mass like that of the sun to control them, they would be driven off into the surrounding space and appear around the sun like the clouds of dust around a mill. we shall return to this subject in connection with the zodiacal light, the aurora, and comets. on the other hand, there are parts of the corona which suggest by their forms the play of electric or magnetic forces. this is beautifully shown in some of the photographs that have been made of the corona during recent eclipses. take, for instance, that of the eclipse of . the sheaves of light emanating from the poles look precisely like the ``lines of force'' surrounding the poles of a magnet. it will be noticed in this photograph that the corona appears to consist of two portions: one comprising the polar rays just spoken of, and the other consisting of the broader, longer, and less-defined masses of light extending out from the equatorial and middle-latitude zones. yet even in this more diffuse part of the phenomenon one can detect the presence of submerged curves bearing more or less resemblance to those about the poles. just what part electricity or electro-magnetism plays in the mechanism of the solar radiation it is impossible to say, but on the assumption that it is a very important part is based the hypothesis that there exists a direct solar influence not only upon the magnetism, but upon the weather of the earth. this hypothesis has been under discussion for half a century, and still we do not know just how much truth it represents. it is certain that the outbreak of great disturbances on the sun, accompanied by the formation of sun-spots and the upshooting of eruptive prominences (phenomena which we should naturally expect to be attended by action), have been instantly followed by corresponding ``magnetic storms'' on the earth and brilliant displays of the auroral lights. there have been occasions when the influence has manifested itself in the most startling ways, a great solar outburst being followed by a mysterious gripping of the cable and telegraph systems of the world, as if an invisible and irresistible hand had seized them. messages are abruptly cut off, sparks leap from the telegraph instruments, and the entire earth seems to have been thrown into a magnetic flurry. these occurrences affect the mind with a deep impression of the dependence of our planet on the sun, such as we do not derive from the more familiar action of the sunlight on the growth of plants and other phenomena of life depending on solar influences. perhaps the theory of solar magnetic influence upon the weather is best known in connection with the ``sun-spot cycle.'' this, at any rate, is, as already remarked, closely associated with the corona. its existence was discovered in by the german astronomer schwabe. it is a period of variable length, averaging about eleven years, during which the number of spots visible on the sun first increases to a maximum, then diminishes to a minimum, and finally increases again to a maximum. for unknown reasons the period is sometimes two or three years longer than the average and sometimes as much shorter. nevertheless, the phenomena always recur in the same order. starting, for instance, with a time when the observer can find few or no spots, they gradually increase in number and size until a maximum, in both senses, is reached, during which the spots are often of enormous size and exceedingly active. after two or three years they begin to diminish in number, magnitude, and activity until they almost or quite disappear. a strange fact is that when a new period opens, the spots appear first in high northern and southern latitudes, far from the solar equator, and as the period advances they not only increase in number and size, but break out nearer and nearer to the equator, the last spots of a vanishing period sometimes lingering in the equatorial region after the advance-guard of its successor has made its appearance in the high latitudes. spots are never seen on the equator nor near the poles. it was not very long after the discovery of the sun-spot cycle that the curious observation was made that a striking coincidence existed between the period of the sun-spots and another period affecting the general magnetic condition of the earth. when a curved line representing the varying number of sun-spots was compared with another curve showing the variations in the magnetic state of the earth the two were seen to be in almost exact accord, a rise in one curve corresponding to a rise in the other, and a fall to a fall. continued observation has proved that this is a real coincidence and not an accidental one, so that the connection, although as yet unexplained, is accepted as established. but does the influence extend further, and directly affect the weather and the seasons as well as the magnetic elements of the earth? a final answer to this question cannot yet be given, for the evidence is contradictory, and the interpretations put upon it depend largely on the predilections of the judges. but, in a broad sense, the sun-spots and the phenomena connected with them must have a relation to terrestial meteorology, for they prove the sun to be a variable star. reference was made, a few lines above, to the resemblance of the spectra of sun-spots to those of certain stars which seem to be failing through age. this in itself is extremely suggestive; but if this resemblance had never been discovered, we should have been justified in regarding the sun as variable in its output of energy; and not only variable, but probably increasingly so. the very inequalities in the sun-spot cycle are suspicious. when the sun is most spotted its total light may be reduced by one-thousandth part, although it is by no means certain that its outgiving of thermal radiations is then reduced. a loss of one-thousandth of its luminosity would correspond to a decrease of . of a stellar magnitude, considering the sun as a star viewed from distant space. so slight a change would not be perceptible; but it is not alone sun-spots which obscure the solar surface, its entire globe is enveloped with an obscuring veil. when studied with a powerful telescope the sun's surface is seen to be thickly mottled with relatively obscure specks, so numerous that it has been estimated that they cut off from one-tenth to one-twentieth of the light that we should receive from it if the whole surface were as brilliant as its brightest parts. the condition of other stars warrants the conclusion that this obscuring envelope is the product of a process of refrigeration which will gradually make the sun more and more variable until its history ends in extinction. looking backward, we see a time when the sun must have been more brilliant than it is now. at that time it probably shone with the blinding white splendor of such stars as sirius, spica, and vega; now it resembles the relatively dull procyon; in time it will turn ruddy and fall into the closing cycle represented by antares. considering that once it must have been more radiantly powerful than at present, one is tempted to wonder if that could have been the time when tropical life flourished within the earth's polar circles, sustained by a vivific energy in the sun which it has now lost. the corona, as we have said, varies with the sun-spot cycle. when the spots are abundant and active the corona rises strong above the spotted zones, forming immense beams or streamers, which on one occasion, at least, had an observed length of ten million miles. at the time of a spot minimum the corona is less brilliant and has a different outline. it is then that the curved polar rays are most conspicuous. thus the vast banners of the sun, shaken out in the eclipse, are signals to tell of its varying state, but it will probably be long before we can read correctly their messages. the zodiacal light mystery there is a singular phenomenon in the sky -- one of the most puzzling of all -- which has long arrested the attention of astronomers, defying their efforts at explanation, but which probably not one in a hundred, and possibly not one in a thousand, of the readers of this book has ever seen. yet its name is often spoken, and it is a conspicuous object if one knows when and where to look for it, and when well seen it exhibits a mystical beauty which at the same time charms and awes the beholder. it is called ``the zodiacal light,'' because it lies within the broad circle of the zodiac, marking the sun's apparent annual path through the stars. what it is nobody has yet been able to find out with certainty, and books on astronomy usually speak of it with singular reserve. but it has given rise to many remarkable theories, and a true explanation of it would probably throw light on a great many other celestial mysteries. the milky way is a more wonderful object to look upon, but its nature can be comprehended, while there is a sort of uncanniness about the zodiacal light which immediately impresses one upon seeing it, for its part in the great scheme of extra-terrestrial affairs is not evident. if you are out-of-doors soon after sunset -- say, on an evening late in the month of february -- you may perceive, just after the angry flush of the dying winter's day has faded from the sky, a pale ghostly presence rising above the place where the sun went down. the writer remembers from boyhood the first time it was pointed out to him and the unearthly impression that it made, so that he afterward avoided being out alone at night, fearful of seeing the spectral thing again. the phenomenon brightens slowly with the fading of the twilight, and soon distinctly assumes the shape of an elongated pyramid of pearly light, leaning toward the south if the place of observation is in the northern hemisphere. it does not impress the observer at all in the same manner as the milky way; that looks far off and is clearly among the stars, but the zodiacal light seems closer at hand, as if it were something more intimately concerning the earth. to all it immediately suggests a connection, also, with the sunken sun. if the night is clear and the moon absent (and if you are in the country, for city lights ruin the spectacles of the sky), you will be able to watch the apparition for a long time. you will observe that the light is brightest near the horizon, gradually fading as the pyramidal beam mounts higher, but in favorable circumstances it may be traced nearly to the meridian south of the zenith, where its apex at last vanishes in the starlight. it continues visible during the evenings of march and part of april, after which, ordinarily, it is seen no more, or if seen is relatively faint and unimpressive. but when autumn comes it appears again, this time not like a wraith hovering above the westward tomb of the day-god, but rather like a spirit of the morning announcing his reincarnation in the east. the reason why the zodiacal light is best seen in our latitudes at the periods just mentioned is because at those times the zodiac is more nearly perpendicular to the horizon, first in the west and then in the east; and, since the phenomenon is confined within the borders of the zodiac, it cannot be favorably placed for observation when the zodiacal plane is but slightly inclined to the horizon. its faint light requires the contrast of a background of dark sky in order to be readily perceptible. but within the tropics, where the zodiac is always at a favorable angle, the mysterious light is more constantly visible. nearly all observant travelers in the equatorial regions have taken particular note of this phenomenon, for being so much more conspicuous there than in the temperate zones it at once catches the eye and holds the attention as a novelty. humboldt mentions it many times in his works, for his genius was always attracted by things out of the ordinary and difficult of explanation, and he made many careful observations on its shape, its brilliancy, and its variations; for there can be no doubt that it does vary, and sometimes to an astonishing degree. it is said that it once remained practically invisible in europe for several years in succession. during a trip to south africa in an english astronomer, mr e. w. maunder, found a remarkable difference between the appearance of the zodiacal light on his going and coming voyages. in fact, when crossing the equator going south he did not see it at all; but on returning he had, on march th, when one degree south of the equator, a memorable view of it. it was a bright, clear night, and the zodiacal light was extraordinarily brilliant -- brighter than he had ever seen it before. the milky way was not to be compared with it. the brightest part extended ° from the sun. there was a faint and much narrower extension which they could just make out beyond the pleiades along the ecliptic, but the greater part of the zodiacal light showed as a broad truncated column, and it did not appear nearly as conical as he had before seen it. when out of the brief twilight of intertropical lands, where the sun drops vertically to the horizon and night rushes on like a wave of darkness, the zodiacal light shoots to the very zenith, its color is described as a golden tint, entirely different from the silvery sheen of the milky way. if i may venture again to refer to personal experiences and impressions, i will recall a view of the zodiacal light from the summit of the cone of mt etna in the autumn of the year (more briefly described in astronomy with the naked eye). there are few lofty mountains so favorably placed as etna for observations of this kind. it was once resorted to by prof. george e. hale, in an attempt to see the solar corona without an eclipse. rising directly from sea-level to an elevation of nearly eleven thousand feet, the observer on its summit at night finds himself, as it were, lost in the midst of the sky. but for the black flanks of the great cone on which he stands he might fancy himself to be in a balloon. on the occasion to which i refer the world beneath was virtually invisible in the moonless night. the blaze of the constellations overhead was astonishingly brilliant, yet amid all their magnificence my attention was immediately drawn to a great tapering light that sprang from the place on the horizon where the sun would rise later, and that seemed to be blown out over the stars like a long, luminous veil. it was the finest view of the zodiacal light that i had ever enjoyed -- thrilling in its strangeness -- but i was almost disheartened by the indifference of my guide, to whom it was only a light and nothing more. if he had no science, he had less poetry -- rather a remarkable thing, i thought, for a child of his clime. the light appeared to me to be distinctly brighter than the visible part of the milky way which included the brilliant stretches in auriga and perseus, and its color, if one may speak of color in connection with such an object, seemed richer than that of the galactic band; but i did not think of it as yellow, although humboldt has described it as resembling a golden curtain drawn over the stars, and du chaillu in equatorial africa found it of a bright yellow color. it may vary in color as in conspicuousness. the fascination of that extraordinary sight has never faded from my memory. i turned to regard it again and again, although i had never seen the stellar heavens so brilliant, and it was one of the last things i looked for when the morning glow began softly to mount in the east, and sicily and the mediterranean slowly emerged from the profound shadow beneath us. the zodiacal light seems never to have attracted from astronomers in general the amount of careful attention that it deserves; perhaps because so little can really be made of it as far as explanation is concerned. i have referred to the restraint that scientific writers apparently feel in speaking of it. the grounds for speculation that it affords may be too scanty to lead to long discussions, yet it piques curiosity, and as we shall see in a moment has finally led to a most interesting theory. once it was the subject of an elaborate series of studies which carried the observer all round the world. that was in -- , during the united states exploring expedition that visited the then little known japan. the chaplain of the fleet, the rev. mr jones, went out prepared to study the mysterious light in all its phases. he saw it from many latitudes on both sides of the equator, and the imagination cannot but follow him with keen interest in his world-circling tour, keeping his eyes every night fixed upon the phantasm overhead, whose position shifted with that of the hidden sun. he demonstrated that the flow extends at times completely across the celestial dome, although it is relatively faint directly behind the earth. on his return the government published a large volume of his observations, in which he undertook to show that the phenomenon was due to the reflection of sunlight from a ring of meteoric bodies encircling the earth. but, after all, this elaborate investigation settled nothing. prof. e. e. barnard has more recently devoted much attention to the zodiacal light, as well as to a strange attendant phenomenon called the ``gegenschein,'' or counterglow, because it always appears at that point in the sky which is exactly opposite the sun. the gegenschein is an extremely elusive phenomenon, suitable only for eyes that have been specially trained to see it. professor newcomb has cautiously remarked that it is said that in that point of the heavens directly opposite the sun there is an elliptical patch of light... this phenomenon is so difficult to account for that its existence is sometimes doubted; yet the testimony in its favor is difficult to set aside. it certainly cannot be set aside at all since the observations of barnard. i recall an attempt to see it under his guidance during a visit to mount hamilton, when he was occupied there with the lick telescope. of course, both the gegenschein and the zodiacal light are too diffuse to be studied with telescopes, which, so to speak, magnify them out of existence. they can only be successfully studied with the naked eye, since every faintest glimmer that they afford must be utilized. this is especially true of the gegenschein. at mount hamilton, mr barnard pointed out to me its location with reference to certain stars, but with all my gazing i could not be sure that i saw it. to him, on the contrary, it was obvious; he had studied it for months, and was able to indicate its shape, its boundaries, its diameter, and the declination of its center with regard to the ecliptic. there is not, of course, the shadow of a doubt of the existence of the gegenschein, and yet i question if one person in a million has ever seen or ever will see it. the zodiacal light, on the other hand, is plain enough, provided that the time and the circumstances of the observation are properly chosen. in the attempts to explain the zodiacal light, the favorite hypothesis has been that it is an appendage of the sun -- perhaps simply an extension of the corona in the plane of the ecliptic, which is not very far from coinciding with that of the sun's equator. this idea is quite a natural one, because of the evident relation of the light to the position of the sun. the vast extension of the equatorial wings of the corona in gave apparent support to this hypothesis; if the substance of the corona could extend ten million miles from the sun, why might it not extend even one hundred million, gradually fading out beyond the orbit of the earth? a variation of this hypothesis assumes that the reflection is due to swarms of meteors circling about the sun, in the plane of its equator, all the way from its immediate neighborhood to a distance exceeding that of the earth. but in neither form is the hypothesis satisfactory; there is nothing in the appearance of the corona to indicate that it extends even as far as the planet mercury, while as to meteors, the orbits of the known swarms do not accord with the hypothesis, and we have no reason to believe that clouds of others exist traveling in the part of space where they would have to be in order to answer the requirements of the theory. the extension of the corona in did not resemble in its texture the zodiacal light. now, it has so often happened in the history of science that an important discovery in one branch has thrown unexpected but most welcome light upon some pending problem in some other branch, that a strong argument might be based upon that fact alone against the too exclusive devotion of many investigators to the narrow lines of their own particular specialty; and the zodiacal light affords a case in point, when it is considered in connection with recent discoveries in chemistry and physics. from the fact that atoms are compound bodies made up of corpuscles at least a thousand times smaller than the smallest known atom -- a fact which astounded most men of science when it was announced a few years ago -- a new hypothesis has been developed concerning the nature of the zodiacal light (as well as other astronomical riddles), and this hypothesis comes not from an astronomer, but from a chemist and physicist, the swede, svante arrhenius. in considering an outline of this new hypothesis we need neither accept nor reject it; it is a case rather for suspension of judgment. to begin with, it carries us back to the ``pressure of light'' mentioned in the preceding chapter. the manner in which this pressure is believed generally to act was there sufficiently explained, and it only remains to see how it is theoretically extended to the particles of matter supposed to constitute the zodiacal light. we know that corpuscles, or ``fragments of atoms'' negatively electrified, are discharged from hot bodies. streams of these ``ions'' pour from many flames and from molten metals; and the impact of the cathode and ultra-violet rays causes them to gush even from cold bodies. in the vast laboratory of the sun it is but reasonable to suppose that similar processes are taking place. ``as a very hot metal emits these corpuscles,'' says prof. j. j. thomson, ``it does not seem an improbable hypothesis that they are emitted by that very hot body, the sun.'' let it be assumed, then, that the sun does emit them; what happens next? negatively charged corpuscles, it is known, serve as nuclei to which particles of matter in the ordinary state are attracted, and it is probable that those emitted from the sun immediately pick up loads in this manner and so grow in bulk. if they grow large enough the gravitation of the sun draws them back, and they produce a negative charge in the solar atmosphere. but it is probable that many of the particles do not attain the critical size which, according to the principles before explained, would enable the gravitation of the sun to retain them in opposition to the pressure of the waves of light, and with these particles the light pressure is dominant. clouds of them may be supposed to be continually swept away from the sun into surrounding space, moving mostly in or near the plane of the solar equator, where the greatest activity, as indicated by sunspots and related phenomena, is taking place. as they pass outward into space many of them encounter the earth. if the earth, like the moon, had no atmosphere the particles would impinge directly on its surface, giving it a negative electric charge. but the presence of the atmosphere changes all that, for the first of the flying particles that encounter it impart to it their negative electricity, and then, since like electric charges repel like, the storm of particles following will be sheered off from the earth, and will stream around it in a maze of hyperbolic paths. those that continue on into space beyond the earth may be expected to continue picking up wandering particles of matter until their bulk has become so great that the solar attraction prevails again over the light pressure acting upon them, and they turn again sunward. passing the earth on their return they will increase the amount of dust-clouds careering round it; and these will be further increased by the action of the ultra-violet rays of the sunlight causing particles to shoot radially away from the earth when the negative charge of the upper atmosphere has reached a certain amount, which particles, although starting sunward, will be swept back to the earth with the oncoming streams. as the final result of all this accumulation of flying and gyrating particles in the earth's neighborhood, we are told that the latter must be transformed into the semblance of a gigantic solid-headed comet provided with streaming tails, the longest of them stretching away from the direction of the sun, while another shorter one extends toward the sun. this shorter tail is due to the particles that we have just spoken of as being driven sunward from the earth by the action of ultra-violet light. no doubt this whole subject is too technical for popular statement; but at any rate the general reader can understand the picturesque side of the theory, for its advocates assure us that if we were on the moon we would doubtless be able to see the comet-like tails of the earth, and then we could appreciate the part that they play in producing the phenomenon of the zodiacal light. that the light as we see it could be produced by the reflection of sunlight from swarms of particles careering round the earth in the manner supposed by arrhenius' hypothesis is evident enough; and it will be observed that the new theory, after all, is only another variant of the older one which attributes the zodiacal light to an extension of the solar corona. but it differs from the older theory in offering an explanation of the manner in which the extension is effected, and it differentiates between the corona proper and the streams of negative particles shot away from the sun. in its details the hypothesis of arrhenius also affords an explanation of many peculiarities of the zodiacal light, such as that it is confined to the neighborhood of the ecliptic, and that it is stronger on the side of the earth which is just turning away from a position under the sun than on the other side; but it would carry us beyond our limits to go into these particulars. the gegenschein, according to this theory, is a part of the same phenomenon as the zodiacal light, for by the laws of perspective it is evident that the reflection from the streams of particles situated at a point directly opposite to the sun would be at a maximum, and this is the place which the gegenschein occupies. apart from its geometrical relations to the position of the sun, the variability of the zodiacal light appears to affirm its solar dependence, and this too would be accounted for by arrhenius' hypothesis better than by the old theory of coronal extension. the amount of corpuscular discharge from the sun must naturally be governed by the state of relative activity or inactivity of the latter, and this could not but be reflected in the varying splendor of the zodiacal light. but much more extended study than has yet been given to the subject will be required before we can feel that we know with reasonable certainty what this mysterious phenomenon really is. by the hypothesis of arrhenius every planet that has an atmosphere must have a zodiacal light attending it, but the phenomenon is too faint for us to be able to see it in the case, for instance, of venus, whose atmosphere is very abundant. the moon has no corresponding ``comet's tail'' because, as already explained, of the lack of a lunar atmosphere to repel the streams by becoming itself electrified; but if there were a lunar zodiacal light, no doubt we could see it because of the relative nearness of our satellite. marvels of the aurora one of the most vivid recollections of my early boyhood is that of seeing my father return hastily into the house one evening and call out to the family: ``come outside and look at the sky!'' ours was a country house situated on a commanding site, and as we all emerged from the doorway we were dumbfounded to see the heavens filled with pale flames which ran licking and quivering over the stars. instantly there sprang into my terrified mind the recollection of an awful description of ``the day of judgment'' (the dies iræ), which i had heard with much perturbation of spirit in the dutch reformed church from the lips of a tall, dark-browed, dreadfully-in-earnest preacher of the old-fashioned type. my heart literally sank at sight of the spectacle, for it recalled the preacher's very words; it was just as he had said it would be, and it needed the assured bearing of my elders finally to convince me that that day of wrath, o dreadful day, when heaven and earth shall pass away, as david and the sibyl say had not actually come upon us. and even the older members of the household were not untouched with misgivings when menacing spots of crimson appeared, breaking out now here, now there, in the shuddering sky. toward the north the spectacle was appalling. a huge arch spanned an unnaturally dark segment resting on the horizon, and above this arch sprang up beams and streamers in a state of incessant agitation, sometimes shooting up to the zenith with a velocity that took one's breath, and sometimes suddenly falling into long ranks, and marching, marching, marching, like an endless phalanx of fiery specters, and moving, as i remember, always from east to west. the absolute silence with which these mysterious evolutions were performed and the quavering reflections which were thrown upon the ground increased the awfulness of the exhibition. occasionally enormous curtains of lambent flame rolled and unrolled with a majestic motion, or were shaken to and fro as if by a mighty, noiseless wind. at times, too, a sudden billowing rush would be made toward the zenith, and for a minute the sky overhead would glow so brightly that the stars seemed to have been consumed. the spectacle continued with varying intensity for hours. this exhibition occurred in central new york, a latitude in which the aurora borealis is seldom seen with so much splendor. i remember another similar one seen from the city of new york in november, . on this last occasion some observers saw a great upright beam of light which majestically moved across the heavens, stalking like an apparition in the midst of the auroral pageant, of whose general movements it seemed to be independent, maintaining always its upright posture, and following a magnetic parallel from east to west. this mysterious beam was seen by no less than twenty-six observers in different parts of the country, and a comparison of their observations led to a curious calculation indicating that the apparition was about one hundred and thirty-three miles tall and moved at the speed of ten miles per second! but, as everybody knows, it is in the arctic regions that the aurora, or the ``northern lights,'' can best be seen. there, in the long polar night, when for months together the sun does not rise, the strange coruscations in the sky often afford a kind of spectral daylight in unison with the weird scenery of the world of ice. the pages in the narratives of arctic exploration that are devoted to descriptions of the wonderful effects of the northern lights are second to none that man has ever penned in their fascination. the lights, as i have already intimated, display astonishing colors, particularly shades of red and green, as they flit from place to place in the sky. the discovery that the magnetic needle is affected by the aurora, quivering and darting about in a state of extraordinary excitement when the lights are playing in the sky, only added to the mystery of the phenomenon until its electro-magnetic nature had been established. this became evident as soon as it was known that the focus of the displays was the magnetic pole; and when the far south was visited the aurora australis was found, having its center at the south magnetic pole. then, if not before, it was clear that the earth was a great globular magnet, having its poles of opposite magnetism, and that the auroral lights, whatever their precise cause might be, were manifestations of the magnetic activity of our planet. after the invention of magnetic telegraphy it was found that whenever a great aurora occurred the telegraph lines were interrupted in their operation, and the ocean cables ceased to work. such a phenomenon is called a ``magnetic storm.'' the interest excited by the aurora in scientific circles was greatly stimulated when, in the last half of the nineteenth century, it was discovered that it is a phenomenon intimately associated with disturbances on the sun. the ancient ``zurich chronicles,'' extending from the year to the year , in which both sun-spots visible to the naked eye and great displays of the auroral lights were recorded, first set rudolf wolf on the track of this discovery. the first notable proof of the suspected connection was furnished with dramatic emphasis by an occurrence which happened on september , . near noon on that day two intensely brilliant points suddenly broke out in a group of sun-spots which were under observation by mr r. c. carrington at his observatory at redhill, england. the points remained visible for not more than five minutes, during which interval they moved thirty-five thousand miles across the solar disk. mr r. hodgson happened to see the same phenomenon at his observatory at highgate, and thus all possibility of deception was removed. but neither of the startled observers could have anticipated what was to follow, and, indeed, it was an occurrence which has never been precisely duplicated. i quote the eloquent account given by miss clerke in her history of astronomy during the nineteenth century. this unique phenomenon seemed as if specially designed to accentuate the inference of a sympathetic relation between the earth and the sun. from august to september , , a magnetic storm of unparalleled intensity, extent, and duration was in progress over the entire globe. telegraphic communication was everywhere interrupted -- except, indeed, that it was in some cases found practicable to work the lines without batteries by the agency of the earth-currents alone; sparks issued from the wires; gorgeous auroras draped the skies in solemn crimson over both hemispheres, and even in the tropics; the magnetic needle lost all trace of continuity in its movements and darted to and fro as if stricken with inexplicable panic. the coincidence was even closer. at the very instant of the solar outburst witnessed by carrington and hodgson the photographic apparatus at kew registered a marked disturbance of all the three magnetic elements; while shortly after the ensuing midnight the electric agitation culminated, thrilling the whole earth with subtle vibrations, and lighting up the atmosphere from pole to pole with coruscating splendors which perhaps dimly recall the times when our ancient planet itself shone as a star. if this amazing occurrence stood alone, and as i have already said it has never been exactly duplicated, doubt might be felt concerning some of the inferences drawn from it; but in varying forms it has been repeated many times, so that now hardly anyone questions the reality of the assumed connection between solar outbursts and magnetic storms accompanied by auroral displays on the earth. it is true that the late lord kelvin raised difficulties in the way of the hypothesis of a direct magnetic action of the sun upon the earth, because it seemed to him that an inadmissible quantity of energy was demanded to account for such action. but no calculation like that which he made is final, since all calculations depend upon the validity of the data; and no authority is unshakable in science, because no man can possess omniscience. it was lord kelvin who, but a few years before the thing was actually accomplished, declared that aerial navigation was an impracticable dream, and demonstrated its impracticability by calculation. however the connection may be brought about, it is as certain as evidence can make it that solar outbursts are coincident with terrestial magnetic disturbances, and coincident in such a way as to make the inference of a causal connection irresistible. the sun is only a little more than a hundred times its own diameter away from the earth. why, then, with the subtle connection between them afforded by the ether which conveys to us the blinding solar light and the life-sustaining solar heat, should it be so difficult to believe that the sun's enormous electric energies find a way to us also? no doubt the impulse coming from the sun acts upon the earth after the manner of a touch upon a trigger, releasing energies which are already stored up in our planet. but besides the evidence afforded by such occurrences as have been related of an intimate connection between solar outbreaks and terrestial magnetic flurries, attended by magnificent auroral displays, there is another line of proof pointing in the same direction. thus, it is known that the sun-spot period, as remarked in a preceding chapter, coincides in a most remarkable manner with the periodic fluctuations in the magnetic state of the earth. this coincidence runs into the most astonishing details. for instance, when the sun-spot period shortens, the auroral period shortens to precisely the same extent; as the short sun-spot periods usually bring the most intense outbreaks of solar activity, so the corresponding short auroral periods are attended by the most violent magnetic storms; a secular period of about two hundred and twenty-two years affecting sun-spots is said to have its auroral duplicate; a shorter period of fifty-five and a half years, which some observers believe that they have discovered appears also to be common to the two phenomena; and yet another ``superposed'' period of about thirty-five years, which some investigators aver exists, affects sun-spots and aurora alike. in short, the coincidences are so numerous and significant that one would have to throw the doctrine of probability to the winds in order to be able to reject the conclusion to which they so plainly lead. but still the question recurs: how is the influence transmitted? here arrhenius comes once more with his hypothesis of negative corpuscles, or ions, driven away from the sun by light-pressure -- a hypothesis which seems to explain so many things -- and offers it also as an explanation of the way in which the sun creates the aurora. he would give the aurora the same lineage with the zodiacal light. to understand the application of this theory we must first recall the fact that the earth is a great magnet having its two opposite poles of magnetism, one near the arctic and the other near the antarctic circle. like all magnets, the earth is surrounded with ``lines of force,'' which, after the manner of the curved rays we saw in the photograph of a solar eclipse, start from a pole, rising at first nearly vertically, then bend gradually over, passing high above the equator, and finally descending in converging sheaves to the opposite pole. now the axis of the earth is so placed in space that it lies at nearly a right angle to the direction of the sun, and as the streams of negatively charged particles come pouring on from the sun (see the last preceding chapter), they arrive in the greatest numbers over the earth's equatorial regions. there they encounter the lines of magnetic force at the place where the latter have their greatest elevation above the earth, and where their direction is horizontal to the earth's surface. obeying a law which has been demonstrated in the laboratory, the particles then follow the lines of force toward the poles. while they are above the equatorial regions they do not become luminescent, because at the great elevation that they there occupy there is virtually no atmosphere; but as they pass on toward the north and the south they begin to descend with the lines of force, curving down to meet at the poles; and, encountering a part of the atmosphere comparable in density with what remains in an exhausted crookes tube, they produce a glow of cathode rays. this glow is conceived to represent the aurora, which may consequently be likened to a gigantic exhibition of vacuum-tube lights. anybody who recalls his student days in the college laboratory and who has witnessed a display of northern lights will at once recognize the resemblance between them in colors, forms, and behavior. this resemblance had often been noted before arrhenius elaborated his hypothesis. without intending to treat his interesting theory as more than a possibly correct explanation of the phenomena of the aurora, we may call attention to some apparently confirmatory facts. one of the most striking of these relates to a seasonal variation in the average number of auroræ. it has been observed that there are more in march and september than at any other time of the year, and fewer in june and december; moreover (and this is a delicate test as applied to the theory), they are slightly rarer in june than in december. now all these facts seem to find a ready explanation in the hypothesis of arrhenius, thus: ( ) the particles issuing from the sun are supposed to come principally from the regions whose excitement is indicated by the presence of sun-spots (which accords with hale's observation that sun-spots are columns of ionized vapors), and these regions have a definite location on either side of the solar equator, seldom approaching it nearer than within ° or ° north or south, and never extending much beyond ° toward either pole; ( ) the equator of the sun is inclined about ° to the plane of the earth's orbit, from which it results that twice in a year -- viz., in june and december -- the earth is directly over the solar equator, and twice a year -- viz., in march and september -- when it is farthest north or south of the solar equator, it is over the inner edge of the sun-spot belts. since the corpuscles must be supposed to be propelled radially from the sun, few will reach the earth when the latter is over the solar equator in june and december, but when it is over, or nearly over, the spot belts, in march and september, it will be in the line of fire of the more active parts of the solar surface, and relatively rich streams of particles will reach it. this, as will be seen from what has been said above, is in strict accord with the observed variations in the frequency of auroræ. even the fact that somewhat fewer auroræ are seen in june than in december also finds its explanation in the known fact that the earth is about three million miles nearer the sun in the winter than in the summer, and the number of particles reaching it will vary, like the intensity of light, inversely as the square of the distance. these coincidences are certainly very striking, and they have a cumulative force. if we accept the theory, it would appear that we ought to congratulate ourselves that the inclination of the sun's equator is so slight, for as things stand the earth is never directly over the most active regions of the sun-spots, and consequently never suffers from the maximum bombardment of charged particles of which the sun is capable. incessant auroral displays, with their undulating draperies, flitting colors, and marching columns might not be objectionable from the point of view of picturesqueness, but one magnetic storm of extreme intensity following closely upon the heels of another, for months on end, crazing the magnetic needle and continually putting the telegraph and cable lines out of commission, to say nothing of their effect upon ``wireless telegraphy'', would hardly add to the charms of terrestrial existence. one or two other curious points in connection with arrhenius' hypothesis may be mentioned. first, the number of auroræ, according to his explanation, ought to be greatest in the daytime, when the face of the earth on the sunward side is directly exposed to the atomic bombardment. of course visual observation can give us no information about this, since the light of the aurora is never sufficiently intense to be visible in the presence of daylight, but the records of the magnetic observatories can be, and have been, appealed to for information, and they indicate that the facts actually accord with the theory. behind the veil of sunlight in the middle of the afternoon, there is good reason to believe, auroral exhibitions often take place which would eclipse in magnificence those seen at night if we could behold them. observation shows, too, that auroræ are more frequent before than after midnight, which is just what we should expect if they originate in the way that arrhenius supposes. second, the theory offers an explanation of the alleged fact that the formation of clouds in the upper air is more frequent in years when auroræ are most abundant, because clouds are the result of the condensation of moisture upon floating particles in the atmosphere (in an absolutely dustless atmosphere there would be no clouds), and it has been proved that negative ions like those supposed to come from the sun play a master part in the phenomena of cloud formation. yet another singular fact, almost mystical in its suggestions, may be mentioned. it seems that the dance of the auroral lights occurs most frequently during the absence of the moon from the hemisphere in which they appear, and that they flee, in greater part, to the opposite hemisphere when the moon's revolution in an orbit considerably inclined to the earth's equator brings her into that where they have been performing. arrhenius himself discovered this curious relation of auroral frequency to the position of the moon north or south of the equator, and he explains it in this way. the moon, like the earth, is exposed to the influx of the ions from the sun; but having no atmosphere, or almost none, to interfere with them, they descend directly upon her surface and charge her with an electric negative potential to a very high degree. in consequence of this she affects the electric state of the upper parts of the earth's atmosphere where they lie most directly beneath her, and thus prevents, to a large extent, the negative discharges to which the appearance of the aurora is due. and so ``the extravagant and erring spirit'' of the aurora avoids the moon as hamlet's ghost fled at the voice of the cock announcing the awakening of the god of day. there are even other apparent confirmations of the hypothesis, but we need not go into them. we shall, however, find one more application of it in the next chapter, for it appears to be a kind of cure-all for astronomical troubles; at any rate it offers a conceivable solution of the question, how does the sun manage to transmit its electric influence to the earth? and this solution is so grandiose in conception, and so novel in the mental pictures that it offers, that its acceptance would not in the least detract from the impression that the aurora makes upon the imagination. strange adventures of comets the fears and legends of ancient times before science was born, and the superstitions of the dark ages, sedulously cultivated for theological purposes by monks and priests, have so colored our ideas of the influence that comets have had upon the human mind that many readers may be surprised to learn that it was the apparition of a wonderful comet, that of , which led to the foundation of our greatest astronomical institution, the harvard college observatory. no doubt the comet superstition existed half a century ago, as, indeed, it exists yet today, but in this case the marvelous spectacle in the sky proved less effective in inspiring terror than in awakening a desire for knowledge. even in the sixteenth century the views that enlightened minds took of comets tended powerfully to inspire popular confidence in science, and halley's prediction, after seeing and studying the motion of the comet which appeared in , that it would prove to be a regular member of the sun's family and would be seen returning after a period of about seventy-six years, together with the fulfillment of that prediction, produced a revulsion from the superstitious notions which had so long prevailed. then the facts were made plain that comets are subject to the law of gravitation equally with the planets; that there are many which regularly return to the neighborhood of the sun (perihelion); and that these travel in orbits differing from those of the planets only in their greater eccentricity, although they have the peculiarity that they do not, like the planets, all go round the sun in the same direction, and do not keep within the general plane of the planetary system, but traverse it sometimes from above and sometimes from below. other comets, including most of the ``great'' ones, appear to travel in parabolic or, in a few cases, hyperbolic orbits, which, not being closed curves, never bring them back again. but it is not certain that these orbits may not be extremely eccentric ellipses, and that after the lapse of hundreds, or thousands, of years the comets that follow them may not reappear. the question is an interesting one, because if all orbits are really ellipses, then all comets must be permanent members of the solar system, while in the contrary case many of them are simply visitors, seen once and never to be seen again. the hypothesis that comets are originally interlopers might seem to derive some support from the fact that the certainly periodic ones are associated, in groups, with the great outer planets, whose attraction appears to have served as a trap for them by turning them into elliptical orbits and thus making them prisoners in the solar system. jupiter, owing to his great mass and his commanding situation in the system, is the chief ``comet-catcher;'' but he catches them not for himself, but for the sun. yet if comets do come originally from without the borders of the planetary system, it does not, by any means, follow that they were wanderers at large in space before they yielded to the overmastering attraction of the sun. investigation of the known cometary orbits, combined with theoretical considerations, has led some astronomers to the conclusion that as the sun travels onward through space he ``picks up en route'' cometary masses which, without belonging strictly to his empire, are borne along in the same vast ``cosmical current'' that carries the solar system. but while no intelligent person any longer thinks that the appearance of a great comet is a token from the heavenly powers of the approaching death of a mighty ruler, or the outbreak of a devastating war, or the infliction of a terrible plague upon wicked mankind, science itself has discovered mysteries about comets which are not less fascinating because they are more intellectual than the irrational fancies that they have displaced. to bring the subject properly before the mind, let us see what the principal phenomena connected with a comet are. at the present day comets are ordinarily ``picked up'' with the telescope or the photographic plate before any one except their discoverer is aware of their existence, and usually they remain so insignificant in appearance that only astronomers ever see them. yet so great is the prestige of the word ``comet'' that the discovery of one of these inconspicuous wanderers, and its subsequent movements, become items of the day's news which everybody reads with the feeling, perhaps, that at least he knows what is going on in the universe even if he doesn't understand it. but a truly great comet presents quite a different proposition. it, too, is apt to be detected coming out of the depths of space before the world at large can get a glimpse of it, but as it approaches the sun its aspect undergoes a marvelous change. agitated apparently by solar influence, it throws out a long streaming tail of nebulous light, directed away from the sun and looking as if blown out like a pennon by a powerful wind. whatever may be the position of the comet with regard to the sun, as it circles round him it continually keeps its tail on the off side. this, as we shall soon see, is a fact of capital importance in relation to the probable nature of comets' tails. almost at the same time that the formation of the tail is observed a remarkable change takes place in the comet's head, which, by the way, is invariably and not merely occasionally its most important part. on approaching the sun the head usually contracts. coincidently with this contraction a nucleus generally makes its appearance. this is a bright, star-like point in the head, and it probably represents the totality of solid matter that the comet possesses. but it is regarded as extremely unlikely that even the nucleus consists of a uniformly solid mass. if it were such, comets would be far more formidable visitors when they pass near the planets than they have been found to be. the diameter of the nucleus may vary from a few hundred up to several thousand miles; the heads, on the average, are from twenty-five thousand to one hundred thousand miles in diameter, although a few have greatly exceeded these dimensions; that of the comet of , one of the most stupendous ever seen, was a million and a quarter miles in diameter! as to the tails, not withstanding their enormous length -- some have been more than a hundred million miles long -- there is reason to believe that they are of extreme tenuity, ``as rare as vacuum.'' the smallest stars have been seen shining through their most brilliant portions with undiminished luster. after the nucleus has been formed it begins to throw out bright jets directed toward the sun. a stream, and sometimes several streams, of light also project sunward from the nucleus, occasionally appearing like a stunted tail directed oppositely to the real tail. symmetrical envelopes which, seen in section, appear as half circles or parabolas, rise sunward from the nucleus, forming a concentric series. the ends of these stream backward into the tail, to which they seem to supply material. ordinarily the formation of these ejections and envelopes is attended by intense agitation of the nucleus, which twists and turns, swinging and gyrating with an appearance of the greatest violence. sometimes the nucleus is seen to break up into several parts. the entire heads of some comets have been split asunder in passing close around the sun; the comet of retreated into space after its perihelion passage with five heads instead of the one that it had originally, and each of these heads had its own tail! the possession of the spectroscope has enabled astronomers during later years to study the chemical composition of comets by analyzing their light. at first the only substances thus discovered in them were hydro-carbon compounds, due evidently to the gaseous envelopes in which some combination of hydrogen with carbon existed. behind this gaseous spectrum was found a faint continuous spectrum ascribed to the nucleus, which apparently both reflects the sunlight and gives forth the light of a glowing solid or liquid. subsequently sodium and iron lines were found in cometary spectra. the presence of iron would seem to indicate that some of these bodies may be much more massive than observations on their attractive effects have indicated. in some recent comets, such as morehouse's, in , several lines have been found, the origin of which is unknown. without going back of the nineteenth century we may find records of some of the most extraordinary comets that man has ever looked upon. in , still spoken of as ``the year of the comet,'' because of the wonderful vintage ascribed to the skyey visitor, a comet shaped like a gigantic sword amazed the whole world, and, as it remained visible for seventeen months, was regarded by superstitious persons as a symbol of the fearful happenings of napoleon's russian campaign. this comet, the extraordinary size of whose head, greatly exceeding that of the sun itself, has already been mentioned, was also remarkable for exhibiting so great a brilliancy without approaching even to the earth's distance from the sun. but there was once a comet (and only once -- in the year ) which never got nearer to the sun than four times the distance of the earth and yet appeared as a formidable object in the sky. as professor young has remarked, ``it must have been an enormous comet to be visible from such a distance.'' and we are to remember that there were no great telescopes in the year . that comet affects the imagination like a phantom of space peering into the solar system, displaying its enormous train afar off (which, if it had approached as near as other comets, would probably have become the celestial wonder of all human memory), and then turning away and vanishing in the depths of immensity. in a comet appeared which was so brilliant that it could be seen in broad day close beside the sun! this was the first authenticated instance of that kind, but the occurrence was to be repeated, as we shall see in a moment, less than forty years later. the splendid comet of , usually called donati's, is remembered by many persons yet living. it was, perhaps, both as seen by the naked eye and with the telescope, the most beautiful comet of which we have any record. it too marked a rich vintage year, still remembered in the vineyards of france, where there is a popular belief that a great comet ripens the grape and imparts to the wine a flavor not attainable by the mere skill of the cultivator. there are ``comet wines,'' carefully treasured in certain cellars, and brought forth only when their owner wishes to treat his guests to a sip from paradise. the year saw another very remarkable comet, of an aspect strangely vast and diffuse, which is believed to have swept the earth with its immense tail when it passed between us and the sun on the night of june th, an event which produced no other known effect than the appearance of an unwonted amount of scattered light in the sky. the next very notable comet was the ``great southern comet'' of , which was not seen from the northern hemisphere. it mimicked the aspect of the famous comet of , and to the great surprise of astronomers appeared to be traveling in the same path. this proved to be the rising of the curtain for an astronomical sensation unparalleled in its kind; for two years later another brilliant comet appeared, first in the southern hemisphere, and it too followed the same track. the startling suggestion was now made that this comet was identical with those of and , its return having been hastened by the resistance experienced in passing twice through the coronal envelope, and there were some who thought that it would now swing swiftly round and then plunge straight into the sun, with consequences that might be disastrous to us on account of the ``flash of heat'' that would be produced by the impact. nervous people were frightened, but observation soon proved that the danger was imaginary, for although the comet almost grazed the sun, and must have rushed through two or three million miles of the coronal region, no retardation of its immense velocity was perceptible, and it finally passed away in a damaged condition, as before remarked, and has never since appeared. then the probable truth was perceived -- viz., that the three comets ( , , and ) were not one identical body, but three separate ones all traveling in the same orbit. it was found, too, that a comet seen in bore similar insignia of relationship. the natural inference was that these four bodies had once formed a single mass which had been split apart by the disruptive action of the sun. strength was lent to this hypothesis by the fact that the comet of was apparently torn asunder during its perihelion passage, retreating into space in a dissevered state. but prof. george forbes has a theory that the splitting of the original cometary mass was effected by an unknown planet, probably greater than jupiter, situated at a hundred times the earth's distance from the sun, and revolving in a period of a thousand years. he supposes that the original comet was not that of , but one seen in , which has since been ``missing,'' and that its disruption occurred from an encounter with the supposititious planet about the year . truly from every point of view comets are the most extraordinary of adventurers! the comet of was likewise remarkable for being visible, like its predecessor of , in full daylight in close proximity to the sun. the story of its detection when almost in contact with the solar disk is dramatic. it had been discovered in the southern hemisphere only a couple of weeks before its perihelion, which occurred on september th, and on the forenoon of that day it was seen by doctor common in england, and by doctor elkin and mr finlay at the cape of good hope, almost touching the sun. it looked like a dazzling white bird with outspread wings. the southern observers watched it go right into the sun, when it instantly disappeared. what had happened was that the comet in passing its perihelion point had swung exactly between the earth and the sun. on the following morning it was seen from all parts of the world close by the sun on the opposite side, and it remained thus visible for three days, gradually receding from the solar disk. it then became visible for northern observers in the morning sky before sunrise, brandishing a portentous sword-shaped tail which, if it had been in the evening sky, would have excited the wonder of hundreds of millions, but situated where it was, comparatively few ever saw it. the application of photography to the study of comets has revealed many curious details which might otherwise have escaped detection, or at best have remained subject to doubt. it has in particular shown not only the precise form of the tails, but the remarkable vicissitudes that they undergo. professor barnard's photographs of brooks' comet in suggested, by the extraordinary changes in the form of the tail which they revealed, that the comet was encountering a series of obstructions in space which bent and twisted its tail into fantastic shapes. the reader will observe the strange form into which the tail was thrown on the night of october st. a cloud of meteors through which the comet was passing might have produced such deformations of its tail. in the photograph of daniels' comet of , a curious striping of the tail will be noticed. the short bright streaks seen in the photograph, it may be explained, are the images of stars which are drawn out into lines in consequence of the fact that the photographic telescope was adjusted to follow the motion of the comet while the stars remained at rest. but the adventures of comets are not confined to possible encounters with unknown obstacles. we have referred to the fact that the great planets, and especially jupiter, frequently interfere with the motions of comets. this interference is not limited to the original alteration of their orbits from possible parabolas to ellipses, but is sometimes exercised again and again, turning the bewildered comets into elliptical paths of all degrees of eccentricity. a famous example of this kind of planetary horse-play is furnished by the story of lexell's missing comet. this comet was first seen in . investigation showed that it was moving in an orbit which should bring it back to perihelion every five and a half years; yet it had never been seen before and, although often searched for, has never been seen since. laplace and leverrier proved mathematically that in it had approached so close to jupiter as to be involved among the orbits of his satellites. what its track had been before is not known, but on that occasion the giant planet seized the interloper, threw it into a short elliptic orbit and sent it, like an arrested vagrant, to receive sentence at the bar of the sun. on this journey it passed within less than , , miles of the earth. the form of orbit which jupiter had impressed required, as we have said, its return in about five and a half years; but soon after it had the misfortune a second time to encounter jupiter at close range, and he, as if dissatisfied with the leniency of the sun, or indignant at the stranger's familiarity, seized the comet and hurled it out of the system, or at any rate so far away that it has never since been able to rejoin the family circle that basks in the immediate rays of the solar hearth. nor is this the only instance in which jupiter has dealt summarily with small comets that have approached him with too little deference. the function which jupiter so conspicuously fulfills as master of the hounds to the sun is worth considering a little more in detail. to change the figure, imagine the sun in its voyage through space to be like a majestic battleship surrounded by its scouts. small vessels (the comets, as they are overhauled by the squadron, are taken in charge by the scouts, with jupiter for their chief, and are forced to accompany the fleet, but not all are impressed. if a strange comet undertakes to run across jupiter's bows the latter brings it to, and makes prize of it by throwing it into a relatively small ellipse with the sun for its focus. thenceforth, unless, as happened to the unhappy comet of lexell, it encounters jupiter again in such a way as to be diverted by him into a more distant orbit, it can never get away. about thirty comets are now known to have thus been captured by the great planet, and they are called ``jupiter's comet family.'' but, on the other hand, if a wandering comet crosses the wake of the chief planetary scout the latter simply drives it away by accelerating its motion and compels it to steer off into open space. the transformation of comets into meteors will be considered in the next chapter, but here, in passing, mention may be made of the strange fate of one member of jupiter's family, biela's comet, which, having become over bold in its advances to its captor, was, after a few revolutions in is impressed orbit, torn to pieces and turned into a flock of meteors. and now let us return to the mystery of comets' tails. that we are fully justified in speaking of the tails of comets as mysterious is proved by the declaration of sir john herschel, who averred, in so many words, that ``there is some profound secret and mystery of nature concerned in this phenomenon,'' and this profound secret and mystery has not yet been altogether cleared up. nevertheless, the all-explaining hypothesis of arrhenius offers us once more a certain amount of aid. comets' tails, arrhenius assures us, are but another result of the pressure of light. the reader will recall the applications of this theory to the zodiacal light and the aurora. in the form in which we now have to deal with it, the supposition is made that as a comet approaches the sun eruptions of vapor, due to the solar heat, occur in its nucleus. these are naturally most active on the side which is directly exposed to the sun, whence the appearance of the immense glowing envelopes that surround the nucleus on the sunward side. among the particles of hydro-carbon, and perhaps solid carbon in the state of fine dust, which are thus set free there will be many whose size is within the critical limit which enables the light-waves from the sun to drive them away. clouds of such particles, then, will stream off behind the advancing comet, producing the appearance of a tail. this accounts for the fact that the tails of comets are always directed away from the sun, and it also explains the varying forms of the tails and the extraordinary changes that they undergo. the speed of the particles driven before the light-waves must depend upon their size and weight, the lightest of a given size traveling the most swiftly. by accretion certain particles might grow, thus losing velocity and producing the appearance of bunches in the tail, such as have been observed. the hypothesis also falls in with the researches of bredichin, who has divided the tails of comets into three principal classes -- viz.: ( ) those which appear as long, straight rays; ( ) those which have the form of curved plumes or scimitars; ( ) those which are short, brushy, and curved sharply backward along the comet's path. in the first type he calculates the repulsive force at from twelve to fifteen times the force of gravity; in the second at from two to four times; and in the third at about one and a half times. the straight tails he ascribes to hydrogen because the hydrogen atom is the lightest known; the sword-shaped tails to hydro-carbons; and the stumpy tails to vaporized iron. it will be seen that, if the force driving off the tails is that which arrhenius assumes it to be, the forms of those appendages would accord with those that bredichin's theory calls for. at the same time we have an explanation of the multiple tails with which some comets have adorned themselves. the comet of , for instance, had at one time no less than seven tails spread in a wide curved brush behind it. donati's comet of also had at least two tails, the principal one sword-shaped and the other long, narrow, and as straight as a rule. according to bredichin, the straight tail must have been composed of hydrogen, and the other of some form of hydro-carbon whose atoms are heavier than those of hydrogen, and, consequently, when swept away by the storm of light-waves, followed a curvature depending upon the resultant of the forces operating upon them. the seven tails of the comet of presented a kind of diagram graphically exhibiting its complex composition, and, if we knew a little more about the constituents of a comet, we might be able to say from the amount of curvature of the different tails just what were the seven substances of which that comet consisted. if these theories seem to the reader fantastic, at any rate they are no more fantastic than the phenomena that they seek to explain. meteors, fire-balls, and meteorites one of the most terrorizing spectacles with which the heavens have ever caused the hearts of men to quake occurred on the night of november , . on that night north america, which faced the storm, was under a continual rain of fire from about ten o'clock in the evening until daybreak. the fragments of a comet had struck the earth. but the meaning of what had happened was not discovered until long afterward. to the astronomers who, with astonishment not less than that of other people, watched the wonderful scene, it was an unparalleled ``shower of meteors.'' they did not then suspect that those meteors had once formed the head of a comet. light dawned when, a year later, prof. denison olmsted, of yale college, demonstrated that the meteors had all moved in parallel orbits around the sun, and that these orbits intersected that of the earth at the point where our planet happened to be on the memorable night of november th. professor olmsted even went so far as to suggest that the cloud of meteors that had encountered the earth might form a diffuse comet; but full recognition of the fact that they were cometary débris came later, as the result of further investigation. the key to the secret was plainly displayed in the spectacle itself, and was noticed without being understood by thousands of the terror-stricken beholders. it was an umbrella of fire that had opened overhead and covered the heavens; in other words, the meteors all radiated from a particular point in the constellation leo, and, being countless as the snowflakes in a winter tempest, they ribbed the sky with fiery streaks. professor olmsted showed that the radiation of the meteors from a fixed point was an effect of perspective, and in itself a proof that they were moving in parallel paths when they encountered the earth. the fact was noted that there had been a similar, but incomparably less brilliant, display of meteors on the same day of november, , and it was rightly concluded that these had belonged to the same stream, although the true relationship of the phenomena was not immediately apprehended. olmsted ascribed to the meteors a revolution about the sun once in every six months, bringing them to the intersection of their orbit with that of the earth every november th; but later investigators found that the real period was about thirty-three and one-quarter years, so that the great displays were due three times in a century, and their return was confidently predicted for the year . the appearance of the meteors in , a year before the great display, was ascribed to the great length of the stream which they formed in space -- so great that they required more than two years to cross the earth's orbit. in the earth had encountered a relatively rare part of the stream, but in , on returning to the crossing-place, it found there the richest part of the stream pouring across its orbit. this explanation also proved to be correct, and the predicted return in was duly witnessed, although the display was much less brilliant than in . it was followed by another in . in the mean time olmsted's idea of a cometary relationship of the meteors was demonstrated to be correct by the researches of schiaparelli and others, who showed that not only the november meteors, but those of august, which are seen more or less abundantly every year, traveled in the tracks of well-known comets, and had undoubtedly an identical origin with those comets. in other words the comets and the meteor-swarms were both remnants of original masses which had probably been split up by the action of the sun, or of some planet to which they had made close approaches. the annual periodicity of the august meteors was ascribed to the fact that the separation had taken place so long ago that the meteors had become distributed all around the orbit, in consequence of which the earth encountered some of them every year when it arrived at the crossing-point. then leverrier showed that the original comet associated with the november meteors was probably brought into the system by the influence of the planet uranus in the year of the christian era. afterward alexander herschel identified the tracks of no less than seventy-six meteor-swarms (most of them inconspicuous) with those of comets. the still more recent researches of mr w. f. denning make it probable that there are no meteors which do not belong to a flock or system probably formed by the disintegration of a cometary mass; even the apparently sporadic ones which shoot across the sky, ``lost souls in the night,'' being members of flocks which have become so widely scattered that the earth sometimes takes weeks to pass through the region of space where their paths lie. the november meteors should have exhibited another pair of spectacles in and , and their failure to do so caused at first much disappointment, until it was made plain that a good reason existed for their absence. it was found that after their last appearance, in , they had been disturbed in their movements by the planets jupiter and saturn, whose attractions had so shifted the position of their orbit that it no longer intersected that of the earth, as it did before. whether another planetary interference will sometime bring the principal mass of the november meteors back to the former point of intersection with the earth's orbit is a question for the future to decide. it would seem that there may be several parallel streams of the november meteors, and that some of them, like those of august, are distributed entirely around the orbit, so that every mid-november we see a few of them. we come now to a very remarkable example of the disintegration of a comet and the formation of a meteor-stream. in biela, of josephstadt, austria, discovered a comet to which his name was given. calculation showed that it had an orbital period of about six and a half years, belonging to jupiter's ``family.'' on one of its returns, in , it astonished its watchers by suddenly splitting in two. the two comets thus formed out of one separated to a distance of about one hundred and sixty thousand miles, and then raced side by side, sometimes with a curious ligature connecting them, like siamese twins, until they disappeared together in interplanetary space. in they came back, still nearly side by side, but now the distance between them had increased to a million and a quarter of miles. after that, at every recurrence of their period, astronomers looked for them in vain, until , when an amazing thing happened. on the night of november th, when the earth was crossing the plane of the orbit of the missing comet, a brilliant shower of meteors burst from the northern sky, traveling nearly in the track which the comet should have pursued. the astronomers were electrified. klinkerfues, of göttingen, telegraphed to pogson, of madras: ``biela touched earth; search near theta centauri.'' pogson searched in the place indicated and saw a cometary mass retreating into the southern heavens, where it was soon swallowed from sight! since then the biela meteors have been among the recognized periodic spectacles of the sky, and few if any doubt that they represent a portion of the missing comet whose disintegration began with the separation into two parts in . the comet itself has never since been seen. the first display of these meteors, sometimes called the ``andromedes,'' because they radiate from the constellation andromeda, was remarkable for the great brilliancy of many of the fire-balls that shot among the shower of smaller sparks, some of which were described as equaling the full moon in size. none of them is known to have reached the earth, but during the display of the same meteors in a meteoric mass fell at mazapil in northern mexico (it is now in the museum at vienna), which many have thought may actually be a piece of the original comet of biela. this brings us to the second branch of our subject. more rare than meteors or falling stars, and more startling, except that they never appear in showers, are the huge balls of fire which occasionally dart through the sky, lighting up the landscapes beneath with their glare, leaving trains of sparks behind them, often producing peals of thunder when they explode, and in many cases falling upon the earth and burying themselves from a few inches to several feet in the soil, from which, more than once, they have been picked up while yet hot and fuming. these balls are sometimes called bolides. they are not really round in shape, although they often look so while traversing the sky, but their forms are fragmentary, and occasionally fantastic. it has been supposed that their origin is different from that of the true meteors; it has even been conjectured that they may have originated from the giant volcanoes of the moon or have been shot out from the sun during some of the tremendous explosions that accompany the formation of eruptive prominences. by the same reasoning some of them might be supposed to have come from some distant star. others have conjectured that they are wanderers in space, of unknown origin, which the earth encounters as it journeys on, and lord kelvin made a suggestion which has become classic because of its imaginative reach -- viz., that the first germs of life may have been brought to the earth by one of these bodies, ``a fragment of an exploded world.'' it is a singular fact that astronomers and scientific men in general were among the last to admit the possibility of solid masses falling from the sky. the people had believed in the reality of such phenomena from the earliest times, but the savants shook their heads and talked of superstition. this was the less surprising because no scientifically authenticated instance of such an occurrence was known, and the stones popularly believed to have fallen from the sky had become the objects of worship or superstitious reverence, a fact not calculated to recommend them to scientific credence. the celebrated ``black stone'' suspended in the kaaba at mecca is one of these reputed gifts from heaven; the ``palladium'' of ancient troy was another; and a stone which fell near ensisheim, in germany, was placed in a church as an object to be religiously venerated. many legends of falling stones existed in antiquity, some of them curiously transfigured by the imagination, like the ``lion of the peloponnesus,'' which was said to have sprung down from the sky upon the isthmus of corinth. but near the beginning of the nineteenth century, in , a veritable shower of falling stones occurred at l'aigle, in northern france, and this time astronomers took note of the phenomenon and scientifically investigated it. thousands of the strange projectiles came from the sky on this occasion, and were scattered over a wide area of country, and some buildings were hit. four years later another shower of stones occurred at weston, conn., numbering thousands of individuals. the local alarm created in both cases was great, as well it might be, for what could be more intimidating than to find the blue vault of heaven suddenly hurling solid missiles at the homes of men? after these occurrences it was impossible for the most skeptical to doubt any longer, and the regular study of ``aerolites,'' or ``meteorites,'' began. one of the first things recognized was the fact that fire-balls are solid meteorites in flight, and not gaseous exhalations in the air, as some had assumed. they burn in the air during their flight, and sometimes, perhaps, are entirely consumed before reaching the ground. their velocity before entering the earth's atmosphere is equal to that of the planets in their orbits -- viz., from twenty to thirty miles per second -- a fact which proves that the sun is the seat of the central force governing them. their burning in the air is not difficult to explain; it is the heat of friction which so quickly brings them to incandescence. calculation shows that a body moving through the air at a velocity of about a mile per second will be brought, superficially, to the temperature of ``red heat'' by friction with the atmosphere. if its velocity is twenty miles per second the temperature will become thousands of degrees. this is the state of affairs with a meteorite rushing into the earth's atmosphere; its surface is liquefied within a few seconds after the friction begins to act, and the melted and vaporized portion of its mass is swept backward, forming the train of sparks that follows every great fire-ball. however, there is one phenomenon connected with the trains of meteorites which has never been satisfactorily explained: they often persist for long periods of time, drifting and turning with the wind, but not ceasing to glow with a phosphorescent luminosity. the question is, whence comes this light? it must be light without heat, since the fine dust or vapor of which the train can only consist would not retain sufficient heat to render it luminous for so long a time. an extremely remarkable incident of this kind occurred on february , , when an immense fire-ball that passed over southern england left a train that remained visible during two hours, assuming many curious shapes as it was drifted about by currents in the air. but notwithstanding the enormous velocity with which meteorites enter the air they are soon slowed down to comparatively moderate speed, so that when they disappear they are usually traveling not faster than a mile a second. the courses of many have been traced by observers situated along their track at various points, and thus a knowledge has been obtained of their height above the ground during their flight and of the length of their visible courses. they generally appear at an elevation of eighty or a hundred miles, and are seldom visible after having descended to within five miles of the ground, unless the observer happens to be near the striking-point, when he may actually witness the fall. frequently they burst while high in the air and their fragments are scattered like shrapnel over the surface of the ground, sometimes covering an area of several square miles, but of course not thickly; different fragments of the same meteorite may reach the ground at points several miles apart. the observed length of their courses in the atmosphere varies from fifty to five hundred miles. if they continued a long time in flight after entering the air, even the largest of them would probably be consumed to the last scrap, but their fiery career is so short on account of their great speed that the heat does not have time to penetrate very deeply, and some that have been picked up immediately after their fall have been found cold as ice within. their size after reaching the ground is variable within wide limits; some are known which weigh several tons, but the great majority weigh only a few pounds and many only a few ounces. meteorites are of two kinds: stony meteorites and iron meteorites. the former outnumber the latter twenty to one; but many stone meteorites contain grains of iron. nickel is commonly found in iron meteorites, so that it might be said that that redoubtable alloy nickel-steel is of cosmical invention. some twenty-five chemical elements have been found in meteorites, including carbon and the ``sun-metal,'' helium. the presence of the latter is certainly highly suggestive in connection with the question of the origin of meteorites. the iron meteorites, besides metallic iron and nickel, of which they are almost entirely composed, contain hydrogen, helium, and carbonic oxide, and about the only imaginable way in which these gases could have become absorbed in the iron would be through the immersion of the latter while in a molten or vaporized state in a hot and dense atmosphere composed of them, a condition which we know to exist only in the envelopes of the sun and the stars. the existence of carbon in the canyon diablo iron meteorites is attended by a circumstance of the most singular character -- a very ``fairy tale of science.'' in some cases the carbon has become diamond! these meteoric diamonds are very small; nevertheless, they are true diamonds, resembling in many ways the little black gems produced by moissan's method with the aid of the electric furnace. the fact that they are found embedded in these iron meteorites is another argument in favor of the hypothesis of the solar or stellar origin of the latter. to appreciate this it is necessary to recall the way in which moissan made his diamonds. it was by a combination of the effects of great heat, great pressure, and sudden or rapid superficial cooling on a mass of iron containing carbon. when he finally broke open his iron he found it a pudding stuffed with miniature black diamonds. when a fragment of the canyon diablo meteoric iron was polished in philadelphia over fifteen years ago it cut the emery-wheel to pieces, and examination showed that the damage had been effected by microscopic diamonds peppered through the mass. how were those diamonds formed? if the sun or sirius was the laboratory that prepared them, we can get a glimpse at the process of their formation. there is plenty of heat, plenty of pressure, and an abundance of vaporized iron in the sun and the stars. when a great solar eruption takes place, masses of iron which have absorbed carbon may be shot out with a velocity which forbids their return. plunged into the frightful cold of space, their surfaces are quickly cooled, as moissan cooled his prepared iron by throwing it into water, and thus the requisite stress is set up within, and, as the iron solidifies, the included carbon crystallizes into diamonds. whether this explanation has a germ of truth in it or not, at any rate it is evident that iron meteorites were not created in the form in which they come to us; they must once have been parts of immeasurably more massive bodies than themselves. the fall of meteorites offers an appreciable, though numerically insignificant, peril to the inhabitants of the earth. historical records show perhaps three or four instances of people being killed by these bodies. but for the protection afforded by the atmosphere, which acts as a very effective shield, the danger would doubtless be very much greater. in the absence of an atmosphere not only would more meteorites reach the ground, but their striking force would be incomparably greater, since, as we have seen, the larger part of their original velocity is destroyed by the resistance of the air. a meteorite weighing many tons and striking the earth with a velocity of twenty or thirty miles per second, would probably cause frightful havoc. it is a singular fact that recent investigations seem to have proved that an event of this kind actually happened in north america -- perhaps not longer than a thousand or two thousand years ago. the scene of the supposed catastrophe is in northern central arizona, at coon butte, where there is a nearly circular crater in the middle of a circular elevation or small mountain. the crater is somewhat over four thousand feet in diameter, and the surrounding rim, formed of upturned strata and ejected rock fragments, rises at its highest point one hundred and sixty feet above the plain. the crater is about six hundred feet in depth -- that is, from the rim to the visible floor or bottom of the crater. there is no evidence that volcanic action has ever taken place in the immediate neighborhood of coon butte. the rock in which the crater has been made is composed of horizontal sandstone and limestone strata. between three hundred and four hundred million tons of rock fragments have been detached, and a large portion hurled by some cause out of the crater. these fragments lie concentrically distributed around the crater, and in large measure form the elevation known as coon butte. the region has been famous for nearly twenty years on account of the masses of meteoric iron found scattered about and known as the ``canyon diablo'' meteorites. it was one of these masses, which consist of nickel-iron containing a small quantity of platinum, and of which in all some ten tons have been recovered for sale to the various collectors throughout the world, that as before mentioned destroyed the grinding-tool at philadelphia through the cutting power of its embedded diamonds. these meteoric irons are scattered about the crater-hill, in concentric distribution, to a maximum distance of about five miles. when the suggestion was first made in that a monster meteorite might have created by its fall this singular lone crater in stratified rocks, it was greeted with incredulous smiles; but since then the matter has assumed a different aspect. the standard iron company, formed by messrs. d. m. barringer, b. c. tilghman, e. j. bennitt, and s. j. holsinger, having become, in , the owner of this freak of nature, sunk shafts and bored holes to a great depth in the interior of the crater, and also trenched the slopes of the mountain, and the result of their investigations has proved that the meteoric hypothesis of origin is correct. (see the papers published in the proceedings of the academy of natural sciences of philadelphia, december, , wherein it is proved that the united states geological survey was wrong in believing this crater to have been due to a steam explosion. since that date there has been discovered a great amount of additional confirmatory proof). material of unmistakably meteoric origin was found by means of the drills, mixed with crushed rock, to a depth of six hundred to seven hundred feet below the floor of the crater, and a great deal of it has been found admixed with the ejected rock fragments on the outer slopes of the mountain, absolutely proving synchronism between the two events, the formation of this great crater and the falling of the meteoric iron out of the sky. the drill located in the bottom of the crater was sent, in a number of cases, much deeper (over one thousand feet) into unaltered horizontal red sandstone strata, but no meteoric material was found below this depth (seven hundred feet, or between eleven and twelve hundred feet below the level of the surrounding plain), which has been assumed as being about the limit of penetration. it is not possible to sink a shaft at present, owing to the water which has drained into the crater, and which forms, with the finely pulverized sandstone, a very troublesome quicksand encountered at about two hundred feet below the visible floor of the crater. as soon as this water is removed by pumping it will be easy to explore the depths of the crater by means of shafts and drifts. the rock strata (sandstone and limestone) of which the walls consist present every appearance of having been violently upturned by a huge body penetrating the earth like a cannon-ball. the general aspect of the crater strikingly resembles the impression made by a steel projectile shot into an armor-plate. mr tilghman has estimated that a meteorite about five hundred feet in diameter and moving with a velocity of about five miles per second would have made just such a perforation upon striking rocks of the character of those found at this place. there was some fusion of the colliding masses, and the heat produced some steam from the small amount of water in the rocks. as a result there has been found at depth a considerable amount of fused quartz (original sandstone), and with it innumerable particles or sparks of fused nickel-iron (original meteorite). a projectile of that size penetrating eleven to twelve hundred feet into the rocky shell of the globe must have produced a shock which was perceptible several hundred miles away. the great velocity ascribed to the supposed meteorite at the moment of striking could be accounted for by the fact that it probably plunged nearly vertically downward, for it formed a circular crater in the rocky crust of the earth. in that case it would have been less retarded by the resistance of the atmosphere than are meteorites which enter the air at a lower angle and shoot ahead hundreds of miles until friction has nearly destroyed their original motion when they drop upon the earth. some meteoric masses of great size, such as peary's iron meteorite found at cape york, greenland, and the almost equally large mass discovered at bacubirito, mexico, appear to have penetrated but slightly on striking the earth. this may be explained by supposing that they pursued a long, horizontal course through the air before falling. the result would be that, their original velocity having been practically destroyed, they would drop to the ground with a velocity nearly corresponding to that which gravity would impart within the perpendicular distance of their final fall. a six-hundred-and-sixty-pound meteorite, which fell at knyahinya, hungary, striking at an angle of ° from the vertical, penetrated the ground to a depth of eleven feet. it has been remarked that the coon butte meteorite may have fallen not longer ago than a few thousand years. this is based upon the fact that the geological indications favor the supposition that the event did not occur more than five thousand years ago, while on the other hand the rings of growth in the cedar-trees growing on the slopes of the crater show that they have existed there about seven hundred years. prof. william h. pickering has recently correlated this with an ancient chronicle which states that at cairo, egypt, in the year , ``many stars passed with a great noise.'' he remarks that cairo is about °, by great circle, from coon butte, so that if the meteorite that made the crater was a member of a flock of similar bodies which encountered the earth moving in parallel lines, some of them might have traversed the sky tangent to the earth's surface at cairo. that the spectacle spoken of in the chronicle was caused by meteorites he deems exceedingly probable because of what is said about ``a great noise;'' meteorites are the only celestial phenomena attended with perceptible sounds. professor pickering conjectures that this supposed flock of great meteorites may have formed the nucleus of a comet which struck the earth, and he finds confirmation of the idea in the fact that out of the ten largest meteorites known, no less than seven were found within nine hundred miles of coon butte. it would be interesting if we could trace back the history of that comet, and find out what malicious planet caught it up in its innocent wanderings and hurled it with so true an aim at the earth! this remarkable crater is one of the most interesting places in the world, for there is absolutely no record of such a mass, possibly an iron-headed comet, from outer space having come into collision with our earth. the results of the future exploration of the depths of the crater will be awaited with much interest. the wrecking of the moon there are sympathetic moods under whose influence one gazes with a certain poignant tenderness at the worn face of the moon; that little ``fossil world'' (the child of our mother earth, too) bears such terrible scars of its brief convulsive life that a sense of pity is awakened by the sight. the moon is the wonder-land of the telescope. those towering mountains, whose ``proud aspiring peaks'' cast silhouettes of shadow that seem drawn with india-ink; those vast plains, enchained with gentle winding hills and bordered with giant ranges; those oval ``oceans,'' where one looks expectant for the flash of wind-whipped waves; those enchanting ``bays'' and recesses at the seaward feet of the alps; those broad straits passing between guardian heights incomparably mightier than gibraltar; those locket-like valleys as secluded among their mountains as the vale of cashmere; those colossal craters that make us smile at the pretensions of vesuvius, etna, and cotopaxi; those strange white ways which pass with the unconcern of roman roads across mountain, gorge, and valley -- all these give the beholder an irresistible impression that it is truly a world into which he is looking, a world akin to ours, and yet no more like our world than pompeii is like naples. its air, its waters, its clouds, its life are gone, and only a skeleton remains -- a mute but eloquent witness to a cosmical tragedy without parallel in the range of human knowledge. one cannot but regret that the moon, if it ever was the seat of intelligent life, has not remained so until our time. think what the consequences would have been if this other world at our very door had been found to be both habitable and inhabited! we talk rather airily of communicating with mars by signals; but mars never approaches nearer than , , miles, while the moon when nearest is only a little more than , miles away. given an effective magnifying power of five thousand diameters, which will perhaps be possible at the mountain observatories as telescopes improve, and we should be able to bring the moon within an apparent distance of about forty miles, while the corresponding distance for mars would be more than seven thousand miles. but even with existing telescopic powers we can see details on the moon no larger than some artificial constructions on the earth. st peter's at rome, with the vatican palace and the great piazza, if existing on the moon, would unquestionably be recognizable as something else than a freak of nature. large cities, with their radiating lines of communication, would at once betray their real character. cultivated tracts, and the changes produced by the interference of intelligent beings, would be clearly recognizable. the electric illumination of a large town at night would probably be markedly visible. gleams of reflected sunlight would come to us from the surfaces of the lakes and oceans, and a huge ``liner'' traversing a lunar sea could probably be followed by its trail of smoke. as to communications by ``wireless'' signals, which certain enthusiasts have thought of in connection with mars, in the case of the moon they should be a relatively simple matter, and the feat might actually be accomplished. think what a literature would grow up about the moon if it were a living world! its very differences from the earth would only accentuate its interest for us. night and day on the moon are each two weeks in length; how interesting it would be to watch the manner in which the lunarians dealt with such a situation as that. lunar and terrestrial history would keep step with each other, and we should record them both. truly one might well wish to have a neighbor world to study; one would feel so much the less alone in space. it is not impossible that the moon did at one time have inhabitants of some kind. but, if so, they vanished with the disappearance of its atmosphere and seas, or with the advent of its cataclysmic age. at the best, its career as a living world must have been brief. if the water and air were gradually absorbed, as some have conjectured, by its cooling interior rocks, its surface might, nevertheless, have retained them for long ages; but if, as others think, their disappearance was due to the escape of their gaseous molecules in consequence of the inability of the relatively small lunar gravitation to retain them, then the final catastrophe must have been as swift as it was inevitable. accepting darwin's hypothesis, that the moon was separated from the earth by tidal action while both were yet plastic or nebulous, we may reasonably conclude that it began its career with a good supply of both water and air, but did not possess sufficient mass to hold them permanently. yet it may have retained them long enough for life to develop in many forms upon its surface; in fact, there are so many indications that air and water have not always been lacking to the lunar world that we are driven to invent theories to explain both their former presence and their present absence. but whatever the former condition of the moon may have been, its existing appearance gives it a resistless fascination, and it bears so clearly the story of a vast catastrophe sculptured on its rocky face that the thoughtful observer cannot look upon it without a feeling of awe. the gigantic character of the lunar features impresses the beholder not less than the universality of the play of destructive forces which they attest. let us make a few comparisons. take the lunar crater called ``tycho'', which is a typical example of its kind. in the telescope tycho appears as a perfect ring surrounding a circular depression, in the center of which rises a group of mountains. its superficial resemblance to some terrestrial volcanic craters is very striking. vesuvius, seen from a point vertically above, would no doubt look something like that (the resemblance would have been greater when the monte del cavallo formed a more complete circuit about the crater cone). but compare the dimensions. the remains of the outer crater ring of vesuvius are perhaps half a mile in diameter, while the active crater itself is only two or three hundred feet across at the most; tycho has a diameter of fifty-four miles! the group of relatively insignificant peaks in the center of the crater floor of tycho is far more massive than the entire mountain that we call vesuvius. the largest known volcanic crater on the earth, aso san, in japan, has a diameter of seven miles; it would take sixty craters like aso san to equal tycho in area! and tycho, though one of the most perfect, is by no means the largest crater on the moon. another, called ``theophilus,'' has a diameter of sixty-four miles, and is eighteen thousand feet deep. there are hundreds from ten to forty miles in diameter, and thousands from one to ten miles. they are so numerous in many places that they break into one another, like the cells of a crushed honeycomb. the lunar craters differ from those of the earth more fundamentally than in the matter of mere size; they are not situated on the tops of mountains. if they were, and if all the proportions were the same, a crater like tycho might crown a conical peak fifty or one hundred miles high! instead of being cavities in the summits of mountains, the lunar craters are rather gigantic sink-holes whose bottoms in many cases lie two or three miles below the general surface of the lunar world. around their rims the rocks are piled up to a height of from a few hundred to two or three thousand feet, with a comparatively gentle inclination, but on the inner side they fall away in gigantic broken precipices which make the dizzy cliffs of the matterhorn seem but ``lover's leaps.'' down they drop, ridge below ridge, crag under crag, tottering wall beneath wall, until, in a crater named ``newton,'' near the south lunar pole, they attain a depth where the rays of the sun never reach. nothing more frightful than the spectacle which many of these terrible chasms present can be pictured by the imagination. as the lazy lunar day slowly advances, the sunshine, unmitigated by clouds or atmospheric veil of any kind, creeps across their rims and begins to descend the opposite walls. presently it strikes the ragged crest of a ridge which had lain hidden in such darkness as we never know on the earth, and runs along it like a line of kindling fire. rocky pinnacles and needles shoot up into the sunlight out of the black depths. down sinks the line of light, mile after mile, and continually new precipices and cliffs are brought into view, until at last the vast floor is attained and begins to be illuminated. in the meanwhile the sun's rays, darting across the gulf, have touched the summits of the central peaks, twenty or thirty miles from the crater's inmost edge, and they immediately kindle and blaze like huge stars amid the darkness. so profound are some of these awful craters that days pass before the sun has risen high enough above them to chase the last shadows from their depths. although several long ranges of mountains resembling those of the earth exist on the moon, the great majority of its elevations assume the crateriform aspect. sometimes, instead of a crater, we find an immense mountain ring whose form and aspect hardly suggest volcanic action. but everywhere the true craters are in evidence, even on the sea-beds, although they attain their greatest number and size on those parts of the moon -- covering sixty per cent of its visible surface -- which are distinctly mountainous in character and which constitute its most brilliant portions. broadly speaking, the southwestern half of the moon is the most mountainous and broken, and the northeastern half the least so. right down through the center, from pole to pole, runs a wonderful line of craters and crateriform valleys of a magnitude stupendous even for the moon. another similar line follows the western edge. three or four ``seas'' are thrust between these mountainous belts. by the effects of ``libration'' parts of the opposite hemisphere of the moon which is turned away from the earth are from time to time brought into view, and their aspect indicates that that hemisphere resembles in its surface features the one which faces the earth. there are many things about the craters which seem to give some warrant for the hypothesis which has been particularly urged by mr g. k. gilbert, that they were formed by the impact of meteors; but there are also many things which militate against that idea, and, upon the whole, the volcanic theory of their origin is to be preferred. the enormous size of the lunar volcanoes is not so difficult to account for when we remember how slight is the force of lunar gravity as compared with that of the earth. with equal size and density, bodies on the moon weigh only one-sixth as much as on the earth. impelled by the same force, a projectile that would go ten miles on the earth would go sixty miles on the moon. a lunar giant thirty-five feet tall would weigh no more than an ordinary son of adam weighs on his greater planet. to shoot a body from the earth so that it would not drop back again, we should have to start it with a velocity of seven miles per second; a mile and a half per second would serve on the moon. it is by no means difficult to believe, then, that a lunar volcano might form a crater ring eight or ten times broader than the greatest to be found on the earth, especially when we reflect that in addition to the relatively slight force of gravity, the materials of the lunar crust are probably lighter than those of our terrestrial rocks. for similar reasons it seems not impossible that the theory mentioned in a former chapter -- that some of the meteorites that have fallen upon the earth originated from the lunar volcanoes -- is well founded. this would apply especially to the stony meteorites, for it is hardly to be supposed that the moon, at least in its superficial parts, contains much iron. it is surely a scene most strange that is thus presented to the mind's eye -- that little attendant of the earth's (the moon has only one-fiftieth of the volume, and only one-eightieth of the mass of the earth) firing great stones back at its parent planet! and what can have been the cause of this furious outbreak of volcanic forces on the moon? evidently it was but a passing stage in its history; it had enjoyed more quiet times before. as it cooled down from the plastic state in which it parted from the earth, it became incrusted after the normal manner of a planet, and then oceans were formed, its atmosphere being sufficiently dense to prevent the water from evaporating and the would-be oceans from disappearing continually in mist. this, if any, must have been the period of life in the lunar world. as we look upon the vestiges of that ancient world buried in the wreck that now covers so much of its surface, it is difficult to restrain the imagination from picturing the scenes which were once presented there; and, in such a case, should the imagination be fettered? we give it free rein in terrestrial life, and it rewards us with some of our greatest intellectual pleasures. the wonderful landscapes of the moon offer it an ideal field with just enough half-hidden suggestions of facts to stimulate its powers. the great plains of the mare imbrium and the mare serenitatis (the ``sea of showers'' and the ``sea of serenity''), bordered in part by lofty mountain ranges precisely like terrestrial mountains, scalloped along their shores with beautiful bays curving back into the adjoining highlands, and united by a great strait passing between the nearly abutting ends of the ``lunar apennines'' and the ``lunar caucasus,'' offer the elements of a scene of world beauty such as it would be difficult to match upon our planet. look at the finely modulated bottom of the ancient sea in mr ritchey's exquisite photograph of the western part of the mare serenitatis, where one seems to see the play of the watery currents heaping the ocean sands in waving lines, making shallows, bars, and deeps for the mariner to avoid or seek, and affording a playground for the creatures of the main. what geologist would not wish to try his hammer on those rocks with their stony pages of fossilized history? there is in us an instinct which forbids us to think that there was never any life there. if we could visit the moon, there is not among us a person so prosaic and unimaginative that he would not, the very first thing, begin to search for traces of its inhabitants. we would look for them in the deposits on the sea bottoms; we would examine the shores wherever the configuration seemed favorable for harbors and the sites of maritime cities -- forgetting that it may be a little ridiculous to ascribe to the ancient lunarians the same ideas that have governed the development of our race; we would search through the valleys and along the seeming courses of vanished streams; we would explore the mountains, not the terrible craters, but the pinnacled chains that recall our own alps and rockies; seeking everywhere some vestige of the transforming presence of intelligent life. perhaps we should find such traces, and perhaps, with all our searching, we should find nothing to suggest that life had ever existed amid that universal ruin. look again at the border of the ``sea of serenity'' -- what a name for such a scene! -- and observe how it has been rent with almost inconceivable violence, the wall of the colossal crater posidonius dropping vertically upon the ancient shore and obliterating it, while its giant neighbor, le monnier, opens a yawning mouth as if to swallow the sea itself. a scene like this makes one question whether, after all, those may not be right who have imagined that the so-called sea bottoms are really vast plains of frozen lava which gushed up in floods so extensive that even the mighty volcanoes were half drowned in the fiery sea. this suggestion becomes even stronger when we turn to another of the photographs of mr ritchey's wonderful series, showing a part of the mare tranquilitatis (``sea of tranquility''!). notice how near the center of the picture the outline of a huge ring with radiating ridges shows through the sea bottom; a fossil volcano submerged in a petrified ocean! this is by no means the only instance in which a buried world shows itself under the great lunar plains. yet, as the newer craters in the sea itself prove, the volcanic activity survived this other catastrophe, or broke out again subsequently, bringing more ruin to pile upon ruin. yet notwithstanding the evidence which we have just been considering in support of the hypothesis that the ``seas'' are lava floods, messrs. loewy and puiseux, the selenographers of the paris observatory, are convinced that these great plains bear characteristic marks of the former presence of immense bodies of water. in that case we should be forced to conclude that the later oceans of the moon lay upon vast sheets of solidified lava; and thus the catastrophe of the lunar world assumes a double aspect, the earliest oceans being swallowed up in molten floods issuing from the interior, while the lands were reduced to chaos by a universal eruption of tremendous volcanoes; and then a period of comparative quiet followed, during which new seas were formed, and new life perhaps began to flourish in the lunar world, only to end in another cataclysm, which finally put a term to the existence of the moon as a life-supporting world. suppose we examine two more of mr ritchey's illuminating photographs, and, first, the one showing the crater theophilus and its surroundings. we have spoken of theophilus before, citing the facts that it is sixty-four miles in diameter and eighteen thousand feet deep. it will be noticed that it has two brother giants -- cyrillus the nearer, and catharina the more distant; but theophilus is plainly the youngest of the trio. centuries, and perhaps thousands of years, must have elapsed between the periods of their upheaval, for the two older craters are partly filled with débris, while it is manifest at a glance that when the south eastern wall of theophilus was formed, it broke away and destroyed a part of the more ancient ring of cyrillus. there is no more tremendous scene on the moon than this; viewed with a powerful telescope, it is absolutely appalling. the next photograph shows, if possible, a still wilder region. it is the part of the moon lying between tycho and the south pole. tycho is seen in the lower left-hand part of the picture. to the right, at the edge of the illuminated portion of the moon, are the crater-rings, longomontanus and wilhelm i, the former being the larger. between them are to be seen the ruins of two or three more ancient craters which, together with portions of the walls of wilhelm i and longomontanus, have been honeycombed with smaller craters. the vast crateriform depression above the center of the picture is clavius, an unrivaled wonder of lunar scenery, a hundred and forty-two miles in its greatest length, while its whole immense floor has sunk two miles below the general surface of the moon outside the ring. the monstrous shadow-filled cavity above clavius toward the right is blancanus, whose aspect here gives a good idea of the appearance of these chasms when only their rims are in the sunlight. but observe the indescribable savagery of the entire scene. it looks as though the spirit of destruction had gone mad in this spot. the mighty craters have broken forth one after another, each rending its predecessor; and when their work was finished, a minor but yet tremendous outbreak occurred, and the face of the moon was gored and punctured with thousands of smaller craters. these relatively small craters (small, however, only in a lunar sense, for many of them would appear gigantic on the earth) recall once more the theory of meteoric impact. it does not seem impossible that some of them may have been formed by such an agency. one would not wish for our planet such a fate as that which has overtaken the moon, but we cannot be absolutely sure that something of the kind may not be in store for it. we really know nothing of the ultimate causes of volcanic activity, and some have suggested that the internal energies of the earth may be accumulating instead of dying out, and may never yet have exhibited their utmost destructive power. perhaps the best assurance that we can find that the earth will escape the catastrophe that has overtaken its satellite is to be found in the relatively great force of its gravitation. the moon has been the victim of its weakness; given equal forces, and the earth would be the better able to withstand them. it is significant, in connection with these considerations, that the little planet mercury, which seems also to have parted with its air and water, shows to the telescope some indications that it is pitted with craters resembling those that have torn to pieces the face of the moon. upon the whole, after studying the dreadful lunar landscapes, one cannot feel a very enthusiastic sympathy with those who are seeking indications of the continued existence of some kind of life on the moon; such a world is better without inhabitants. it has met its fate; let it go! fortunately, it is not so near that it cannot hide its scars and appear beautiful -- except when curiosity impels us to look with the penetrating eyes of the astronomer. the great mars problem let any thoughtful person who is acquainted with the general facts of astronomy look up at the heavens some night when they appear in their greatest splendor, and ask himself what is the strongest impression that they make upon his mind. he may not find it easy to frame an answer, but when he has succeeded it will probably be to the effect that the stars give him an impression of the universality of intelligence; they make him feel, as the sun and the moon cannot do, that his world is not alone; that all this was not made simply to form a gorgeous canopy over the tents of men. if he is of a devout turn of mind, he thinks, as he gazes into those fathomless deeps and among those bewildering hosts, of the infinite multitude of created beings that the almighty has taken under his care. the narrow ideas of the old geocentric theology, which made the earth god's especial footstool, and man his only rational creature, fall away from him like a veil that had obscured his vision; they are impossible in the presence of what he sees above. thus the natural tendency, in the light of modern progress, is to regard the universe as everywhere filled with life. but science, which is responsible for this broadening of men's thoughts concerning the universality of life, itself proceeds to set limits. of spiritual existences it pretends to know nothing, but as to physical beings, it declares that it can only entertain the supposition of their existence where it finds evidence of an environment suited to their needs, and such environment may not everywhere exist. science, though repelled by the antiquated theological conception of the supreme isolation of man among created beings, regards with complacency the probability that there are regions in the universe where no organic life exists, stars which shine upon no inhabited worlds, and planets which nourish no animate creatures. the astronomical view of the universe is that it consists of matter in every stage of evolution: some nebulous and chaotic; some just condensing into stars (suns) of every magnitude and order; some shaped into finished solar bodies surrounded by dependent planets; some forming stars that perhaps have no planets, and will have none; some constituting suns that are already aging, and will soon lose their radiant energy and disappear; and some aggregated into masses that long ago became inert, cold, and rayless, and that can only be revivified by means about which we can form conjectures, but of which we actually know nothing. as with the stars, so with the planets, which are the satellites of stars. all investigations unite to tell us that the planets are not all in the same state of development. as some are large and some small, so some are, in an evolutionary sense, young, and some old. as they depend upon the suns around which they revolve for their light, heat, and other forms of radiant energy, so their condition varies with their distance from those suns. many may never arrive at a state suitable for the maintenance of life upon their surfaces; some which are not at present in such a state may attain it later; and the forms of life themselves may vary with the peculiar environment that different planets afford. thus we see that we are not scientifically justified in affirming that life is ubiquitous, although we are thus justified in saying that it must be, in a general sense, universal. we might liken the universe to a garden known to contain every variety of plant. if on entering it we see no flowers, we examine the species before us and find that they are not of those which bloom at this particular season, or perhaps they are such as never bear flowers. yet we feel no doubt that we shall find flowers somewhere in the garden, because there are species which bloom at this season, and the garden contains all varieties. while it is tacitly assumed that there are planets revolving around other stars than the sun, it would be impossible for us to see them with any telescope yet invented, and no instrument now in the possession of astronomers could assure us of their existence; so the only planetary system of which we have visual knowledge is our own. excluding the asteroids, which could not from any point of view be considered as habitable, we have in the solar system eight planets of various sizes and situated at various distances from the sun. of these eight we know that one, the earth, is inhabited. the question, then, arises: are there any of the others which are inhabited or habitable? since it is our intention to discuss the habitability of only one of the seven to which the question applies, the rest may be dismissed in a few words. the smallest of them, and the nearest to the sun, is mercury, which is regarded as uninhabitable because it has no perceptible supply of water and air, and because, owing to the extraordinary eccentricity of its orbit, it is subjected to excessive and very rapid alterations in the amount of solar heat and light poured upon its surface, such alterations being inconsistent with the supposition that it can support living beings. even its average temperature is more than six and a half times that prevailing on the earth! another circumstance which militates against its habitability is that, according to the results of the best telescopic studies, it always keeps the same face toward the sun, so that one half of the planet is perpetually exposed to the fierce solar rays, and the other half faces the unmitigated cold of open space. venus, the next in distance from the sun, is almost the exact twin of the earth in size, and many arguments may be urged in favor of its habitability, although it is suspected of possessing the same peculiarity as mercury, in always keeping the same side sunward. unfortunately its atmosphere appears to be so dense that no permanent markings on its surface are certainly visible, and the question of its actual condition must, for the present, be left in abeyance. mars, the first planet more distant from the sun than the earth, is the special subject of this chapter, and will be described and discussed a few lines further on. jupiter, saturn, uranus, and neptune, the four giant planets, all more distant than mars, and each more distant than the other in the order named, are all regarded as uninhabitable because none of them appears to possess any degree of solidity. they may have solid or liquid nuclei, but exteriorly they seem to be mere balls of cloud. of course, one can imagine what he pleases about the existence of creatures suited to the physical constitution of such planets as these, but they must be excluded from the category of habitable worlds in the ordinary sense of the term. we go back, then, to mars. it will be best to begin with a description of the planet. mars is miles in diameter; its surface is not much more than one-quarter as extensive as that of the earth (. ). its mean distance from the sun is , , miles, , , miles greater than that of the earth. since radiant energy varies inversely as the square of distance, mars receives less than half as much solar light and heat as the earth gets. mars' year (period of revolution round the sun) is days. its mean density is per cent of the earth's, and the force of gravity on its surface is per cent of that on the surface of the earth; i.e., a body weighing one hundred pounds on the earth would, if transported to mars, weigh but thirty-eight pounds. the inclination of its equator to the plane of its orbit differs very little from that of the earth's equator, and its axial rotation occupies hours minutes. so that the length of day and night, and the extent of the seasonal changes on mars, are almost precisely the same as on the earth. but owing to the greater length of its year, the seasons of mars, while occurring in the same order, are almost twice as long as ours. the surface of the planet is manifestly solid, like that of our globe, and the telescope reveals many permanent markings on it, recalling the appearance of a globe on which geographical features have been represented in reddish and dusky tints. around the poles are plainly to be seen rounded white areas, which vary in extent with the martian seasons, nearly vanishing in summer and extending widely in winter. the most recent spectroscopic determinations indicate that mars has an atmosphere perhaps as dense as that to be found on our loftiest mountain peaks, and there is a perceptible amount of watery vapor in this atmosphere. the surface of the planet appears to be remarkably level, and it has no mountain ranges. no evidences of volcanic action have been discovered on mars. the dusky and reddish areas were regarded by the early observers as respectively seas and lands, but at present it is not believed that there are any bodies of water on the planet. there has never been much doubt expressed that the white areas about the poles represent snow. it will be seen from this brief description that many remarkable resemblances exist between mars and the earth, and there is nothing wonderful in the fact that the question of the habitability of the former has become one of extreme and wide-spread interest, giving rise to the most diverse views, to many extraordinary speculations, and sometimes to regrettably heated controversy. the first champion of the habitability of mars was sir william herschel, although even before his time the idea had been suggested. he was convinced by the revelations of his telescopes, continually increasing in power, that mars was more like the earth than any other planet. he could not resist the testimony of the polar snows, whose suggestive conduct was in such striking accord with what occurs upon the earth. gradually, as telescopes improved and observers increased in number, the principal features of the planet were disclosed and charted, and ``areography,'' as the geography of mars was called, took its place among the recognized branches of astronomical study. but it was not before that a fundamentally new discovery in areography gave a truly sensational turn to speculation about life on ``the red planet.'' in that year mars made one of its nearest approaches to the earth, and was so situated in its orbit that it could be observed to great advantage from the northern hemisphere of the earth. the celebrated italian astronomer, schiaparelli, took advantage of this opportunity to make a trigonometrical survey of the surface of mars -- as coolly and confidently as if he were not taking his sights across a thirty-five-million-mile gulf of empty space -- and in the course of this survey he was astonished to perceive that the reddish areas, then called continents, were crossed in many directions by narrow, dusky lines, to which he gave the suggestive name of ``canals.'' thus a kind of firebrand was cast into the field of astronomical speculation, which has ever since produced disputes that have sometimes approached the violence of political faction. at first the accuracy of schiaparelli's observations was contested; it required a powerful telescope, and the most excellent ``seeing,'' to render the enigmatical lines visible at all, and many searchers were unable to detect them. but schiaparelli continued his studies in the serene sky of italy, and produced charts of the gridironed face of mars containing so much astonishing detail that one had either to reject them in toto or to confess that schiaparelli was right. as subsequent favorable oppositions of mars occurred, other observers began to see the ``canals'' and to confirm the substantial accuracy of the italian astronomer's work, and finally few were found who would venture to affirm that the ``canals'' did not exist, whatever their meaning might be. when schiaparelli began his observations it was generally believed, as we have said, that the dusky areas on mars were seas, and since schiaparelli thought that the ``canals'' invariably began and ended at the shores of the ``seas,'' the appropriateness of the title given to the lines seemed apparent. their artificial character was immediately assumed by many, because they were too straight and too suggestively geometrical in their arrangement to permit the conclusion that they were natural watercourses. a most surprising circumstance noted by schiaparelli was that the ``canals'' made their appearance after the melting of the polar snow in the corresponding hemisphere had begun, and that they grew darker, longer, and more numerous in proportion as the polar liquidation proceeded; another very puzzling observation was that many of them became double as the season advanced; close beside an already existing ``canal,'' and in perfect parallelism with it, another would gradually make its appearance. that these phenomena actually existed and were not illusions was proved by later observations, and today they are seen whenever mars is favorably situated for observation. in the closing decade of the nineteenth century, mr percival lowell took up the work where schiaparelli had virtually dropped it, and soon added a great number of ``canals'' to those previously known, so that in his charts the surface of the wonderful little planet appears covered as with a spider's web, the dusky lines criss-crossing in every direction, with conspicuous knots wherever a number of them come together. mr lowell has demonstrated that the areas originally called seas, and thus named on the earlier charts, are not bodies of water, whatever else they may be. he has also found that the mysterious lines do not, as schiaparelli supposed, begin and end at the edges of the dusky regions, but often continue on across them, reaching in some cases far up into the polar regions. but schiaparelli was right in his observation that the appearance of the ``canals'' is synchronous with the gradual disappearance of the polar snows, and this fact has become the basis of the most extraordinary theory that the subject of life in other worlds has ever given birth to. now, the effect of such discoveries, as we have related, depends upon the type of mind to whose attention they are called. many are content to accept them as strange and inexplicable at present, and to wait for further light upon them; others insist upon an immediate inquiry concerning their probable nature and meaning. such an inquiry can only be based upon inference proceeding from analogy. mars, say mr lowell and those who are of his opinion, is manifestly a solidly incrusted planet like the earth; it has an atmosphere, though one of great rarity; it has water vapor, as the snows in themselves prove; it has the alternation of day and night, and a succession of seasons closely resembling those of the earth; its surface is suggestively divided into regions of contrasting colors and appearance, and upon that surface we see an immense number of lines geometrically arranged, with a system of symmetrical intersections where the lines expand into circular and oval areas -- and all connected with the annual melting of the polar snows in a way which irresistibly suggests the interference of intelligence directed to a definite end. why, with so many concurrent circumstances to support the hypothesis, should we not regard mars as an inhabited globe? but the differences between mars and the earth are in many ways as striking as their resemblances. mars is relatively small; it gets less than half as much light and heat as we receive; its atmosphere is so rare that it would be distressing to us, even if we could survive in it at all; it has no lakes, rivers, or seas; its surface is an endless prairie. and its ``canals'' are phenomena utterly unlike anything on the earth. yet it is precisely upon these divergences between the earth and mars, this repudiation of terrestrial standards, that the theory of ``life on mars,'' for which mr lowell is mainly responsible, is based. because mars is smaller than the earth, we are told it must necessarily be more advanced in planetary evolution, the underlying cause of which is the gradual cooling and contraction of the planet's mass. mars has parted with its internal heat more rapidly than the earth; consequently its waters and its atmosphere have been mostly withdrawn by chemical combinations, but enough of both yet remain to render life still possible on its surface. as the globe of mars is evolutionally older than that of the earth, so its forms of organic life may be proportionally further advanced, and its inhabitants may have attained a degree of cultivated intelligence much superior to what at present exists upon the earth. understanding the nature and the causes of the desiccation of their planet, and possessing engineering science and capabilities far in advance of ours, they may be conceived to have grappled with the stupendous problem of keeping their world in a habitable condition as long as possible. supposing them to have become accustomed to live in their rarefied atmosphere (a thing not inconceivable, since men can live for a time at least in air hardly less rare), the most pressing problem for them is that of a water-supply, without which plant life cannot exist, while animal life in turn depends for its existence upon vegetation. the only direction in which they can seek water is that of the polar regions, where it is alternately condensed into snow and released in the liquid form by the effect of the seasonal changes. it is, then, to the annual melting of the polar snow-fields that the martian engineers are supposed to have recourse in supplying the needs of their planet, and thus providing the means of prolonging their own existence. it is imagined that they have for this purpose constructed a stupendous system of irrigation extending over the temperate and equatorial regions of the planet. the ``canals'' represent the lines of irrigation, but the narrow streaks that we see are not the canals themselves, but the irrigated bands covered by them. their dark hue, and their gradual appearance after the polar melting has begun, are due to the growth of vegetation stimulated by the water. the rounded areas visible where several ``canals'' meet and cross are called by mr lowell ``oases.'' these are supposed to be the principal centers of population and industry. it must be confessed that some of them, with their complicated systems of radiating lines, appear to answer very well to such a theory. no attempt to explain them by analogy with natural phenomena on the earth has proved successful. but a great difficulty yet remains: how to explain the seemingly miraculous powers of the supposed engineers? here recourse is had once more to the relative smallness of the planet. we have remarked that the force of gravity on mars is only thirty-eight per cent of that on the earth. a steam-shovel driven by a certain horse-power would be nearly three times as effective there as here. a man of our stature on mars would find his effective strength increased in the same proportion. but just because of the slight force of gravity there, a martian might attain to the traditional stature of goliath without finding his own weight an encumbrance to his activity, while at the same time his huge muscles would come into unimpeded play, enabling him single-handed to perform labors that would be impossible to a whole gang of terrestrial workmen. the effective powers of huge machines would be increased in the same way; and to all this must be added the fact that the mean density of the materials of which mars is composed is much less than that of the constituents of the earth. combining all these considerations, it becomes much less difficult to conceive that public works might be successfully undertaken on mars which would be hopelessly beyond the limits of human accomplishment. certain other difficulties have also to be met; as, for instance, the relative coldness of the climate of mars. at its distance it gets considerably less than half as much light and heat as we receive. in addition to this, the rarity of its atmosphere would naturally be expected to decrease the effective temperature at the planet's surface, since an atmosphere acts somewhat like the glass cover of a hot-house in retaining the solar heat which has penetrated it. it has been calculated that, unless there are mitigating circumstances of which we know nothing, the average temperature at the surface of mars must be far below the freezing-point of water. to this it is replied that the possible mitigating circumstances spoken of evidently exist in fact, because we can see that the watery vapor condenses into snow around the poles in winter, but melts again when summer comes. the mitigating agent may be supposed to exist in the atmosphere where the presence of certain gases would completely alter the temperature gradients. it might also be objected that it is inconceivable that the martian engineers, however great may be their physical powers, and however gigantic the mechanical energies under their control, could force water in large quantities from the poles to the equator. this is an achievement that measures up to the cosmical standard. it is admitted by the champions of the theory that the difficulty is a formidable one; but they call attention to the singular fact that on mars there can be found no chains of mountains, and it is even doubtful if ranges of hills exist there. the entire surface of the planet appears to be almost ``as smooth as a billiard ball,'' and even the broad regions which were once supposed to be seas apparently lie at practically the same level as the other parts, since the ``canals'' in many cases run uninterruptedly across them. lowell's idea is that these sombre areas may be expanses of vegetation covering ground of a more or less marshy character, for while the largest of them appear to be permanent, there are some which vary coincidently with the variations of the canals. as to the kind of machinery employed to force the water from the poles, it has been conjectured that it may have taken the form of a gigantic system of pumps and conduits; and since the martians are assumed to be so far in advance of us in their mastery of scientific principles, the hypothesis will at least not be harmed by supposing that they have learned to harness forces of nature whose very existence in a manageable form is yet unrecognized on the earth. if we wish to let the imagination loose, we may conjecture that they have conquered the secret of those intra-atomic forces whose resistless energy is beginning to become evident to us, but the possibility of whose utilization remains a dream, the fulfillment of which nobody dares to predict. such, in very brief form, is the celebrated theory of mars as an inhabited world. it certainly captivates the imagination, and if we believe it to represent the facts, we cannot but watch with the deepest sympathy this gallant struggle of an intellectual race to preserve its planet from the effects of advancing age and death. we may, indeed, wonder whether our own humanity, confronted by such a calamity, could be counted on to meet the emergency with equal stoutness of heart and inexhaustibleness of resource. up to the present time we certainly have shown no capacity to confront nature toe to toe, and to seize her by the shoulders and turn her round when she refuses to go our way. if we could get into wireless telephonic communication with the martians we might learn from their own lips the secret of their more than ``roman recovery.'' the riddle of the asteroids between the orbits of mars and jupiter revolves the most remarkable system of little bodies with which we are acquainted -- the asteroids, or minor planets. some six hundred are now known, and they may actually number thousands. they form virtually a ring about the sun. the most striking general fact about them is that they occupy the place in the sky which should be occupied, according to bode's law, by a single large planet. this fact, as we shall see, has led to the invention of one of the most extraordinary theories in astronomy -- viz., that of the explosion of a world! bode's law, so-called, is only an empiric formula, but until the discovery of neptune it accorded so well with the distances of the planets that astronomers were disposed to look upon it as really representing some underlying principle of planetary distribution. they were puzzled by the absence of a planet in the space between mars and jupiter, where the ``law'' demanded that there should be one, and an association of astronomers was formed to search for it. there was a decided sensation when, in , piazzi, of palermo, announced that he had found a little planet which apparently occupied the place in the system which belonged to the missing body. he named it ceres, and it was the first of the asteroids. the next year olbers, of bremen, while looking for ceres with his telescope, stumbled upon another small planet which he named pallas. immediately he was inspired with the idea that these two planets were fragments of a larger one which had formerly occupied the vacant place in the planetary ranks, and he predicted that others would be found by searching in the neighborhood of the intersection of the orbits of the two already discovered. this bold prediction was brilliantly fulfilled by the finding of two more -- juno in , and vesta in . olbers would seem to have been led to the invention of his hypothesis of a planetary explosion by the faith which astronomers at that time had in bode's law. they appear to have thought that several planets revolving in the gap where the ``law'' called for but one could only be accounted for upon the theory that the original one had been broken up to form the several. gravitation demanded that the remnants of a planet blown to pieces, no matter how their orbits might otherwise differ, should all return at stated periods to the point where the explosion had occurred; hence olbers' prediction that any asteroids that might subsequently be discovered would be found to have a common point of orbital intersection. and curiously enough all of the first asteroids found practically answered to this requirement. olbers' theory seemed to be established. after the first four, no more asteroids were found until , when one was discovered; then, in , three more were added to the list; and after that searchers began to pick them up with such rapidity that by the close of the century hundreds were known, and it had become almost impossible to keep track of them. the first four are by far the largest members of the group, but their actual sizes remained unknown until less than twenty years ago. it was long supposed that vesta was the largest, because it shines more brightly than any of the others; but finally, in , barnard, with the lick telescope, definitely measured their diameters, and proved to everybody's surprise that ceres is really the chief, and vesta only the third in rank. his measures are as follows: ceres, miles; pallas, miles; vesta, miles; and juno, miles. they differ greatly in the reflective power of their surfaces, a fact of much significance in connection with the question of their origin. vesta is, surface for surface, rather more than three times as brilliant as ceres, whence the original mistake about its magnitude. nowadays new asteroids are found frequently by photography, but physically they are most insignificant bodies, their average diameter probably not exceeding twenty miles, and some are believed not to exceed ten. on a planet only ten miles in diameter, assuming the same mean density as the earth's, which is undoubtedly too much, the force of gravity would be so slight that an average man would not weigh more than three ounces, and could jump off into space whenever he liked. although the asteroids all revolve around the sun in the same direction as that pursued by the major planets, their orbits are inclined at a great variety of angles to the general plane of the planetary system, and some of them are very eccentric -- almost as much so as the orbits of many of the periodic comets. it has even been conjectured that the two tiny moons of mars and the four smaller satellites of jupiter may be asteroids gone astray and captured by those planets. two of the asteroids are exceedingly remarkable for the shapes and positions of their orbits; these are eros, discovered in , and t. g., , found eight years later. the latter has a mean distance from the sun slightly greater than that of jupiter, while the mean distance of eros is less than that of mars. the orbit of eros is so eccentric that at times it approaches within , , miles of the earth, nearer than any other regular member of the solar system except the moon, thus affording an unrivaled means of measuring the solar parallax. but for our present purpose the chief interest of eros lies in its extraordinary changes of light. these changes, although irregular, have been observed and photographed many times, and there seems to be no doubt of their reality. their significance consists in their possible connection with the form of the little planet, whose diameter is generally estimated at not more than twenty miles. von oppolzer found, in , that eros lost three-fourths of its brilliancy once in every two hours and thirty-eight minutes. other observers have found slightly different periods of variability, but none as long as three hours. the most interesting interpretation that has been offered of this phenomenon is that it is due to a great irregularity of figure, recalling at once olbers' hypothesis. according to some, eros may be double, the two bodies composing it revolving around each other at very close quarters; but a more striking, and it may be said probable, suggestion is that eros has a form not unlike that of a dumb-bell, or hour-glass, turning rapidly end over end so that the area of illuminated surface presented to our eyes continually changes, reaching at certain times a minimum when the amount of light that it reflects toward the earth is reduced to a quarter of its maximum value. various other bizarre shapes have been ascribed to eros, such, for instance, as that of a flat stone revolving about one of its longer axes, so that sometimes we see its face and sometimes its edge. all of these explanations proceed upon the assumption that eros cannot have a simple globular figure like that of a typical planet, a figure which is prescribed by the law of gravitation, but that its shape is what may be called accidental; in a word, it is a fragment, for it seems impossible to believe that a body formed in interplanetary space, either through nebular condensation or through the aggregation of particles drawn together by their mutual attractions, should not be practically spherical in shape. nor is eros the only asteroid that gives evidence by variations of brilliancy that there is something abnormal in its constitution; several others present the same phenomenon in varying degrees. even vesta was regarded by olbers as sufficiently variable in its light to warrant the conclusion that it was an angular mass instead of a globe. some of the smaller ones show very notable variations, and all in short periods, of three or four hours, suggesting that in turning about one of their axes they present a surface of variable extent toward the sun and the earth. the theory which some have preferred -- that the variability of light is due to the differences of reflective power on different parts of the surface -- would, if accepted, be hardly less suggestive of the origin of these little bodies by the breaking up of a larger one, because the most natural explanation of such differences would seem to be that they arose from variations in the roughness or smoothness of the reflecting surface, which would be characteristic of fragmentary bodies. in the case of a large planet alternating expanses of land and water, or of vegetation and desert, would produce a notable variation in the amount of reflection, but on bodies of the size of the asteroids neither water nor vegetation could exist, and an atmosphere would be equally impossible. one of the strongest objections to olbers' hypothesis is that only a few of the first asteroids discovered travel in orbits which measurably satisfy the requirement that they should all intersect at the point where the explosion occurred. to this it was at first replied that the perturbations of the asteroidal orbits, by the attractions of the major planets, would soon displace them in such a manner that they would cease to intersect. one of the first investigations undertaken by the late prof. simon newcomb was directed to the solution of this question, and he arrived at the conclusion that the planetary perturbations could not explain the actual situation of the asteroidal orbits. but afterward it was pointed out that the difficulty could be avoided by supposing that not one but a series of explosions had produced the asteroids as they now are. after the primary disruption the fragments themselves, according to this suggestion, may have exploded, and then the resulting orbits would be as ``tangled'' as the heart could wish. this has so far rehabilitated the explosion theory that it has never been entirely abandoned, and the evidence which we have just cited of the probably abnormal shapes of eros and other asteroids has lately given it renewed life. it is a subject that needs a thorough rediscussion. we must not fail to mention, however, that there is a rival hypothesis which commends itself to many astronomers -- viz., that the asteroids were formed out of a relatively scant ring of matter, situated between mars and jupiter and resembling in composition the immensely more massive rings from which, according to laplace's hypothesis, the planets were born. it is held by the supporters of this theory that the attraction of the giant jupiter was sufficient to prevent the small, nebulous ring that gave birth to the asteroids from condensing like the others into a single planet. but if we accept the explosion theory, with its corollary that minor explosions followed the principal one, we have still an unanswered question before us: what caused the explosions? the idea of a world blowing up is too titanic to be shocking; it rather amuses the imagination than seriously impresses it; in a word, it seems essentially chimerical. we can by no appeal to experience form a mental picture of such an occurrence. even the moon did not blow up when it was wrecked by volcanoes. the explosive nebulæ and new stars are far away in space, and suggest no connection with such a catastrophe as the bursting of a planet into hundreds of pieces. we cannot conceive of a great globe thousands of miles in diameter resembling a pellet of gunpowder only awaiting the touch of a match to cause its sudden disruption. somehow the thought of human agency obtrudes itself in connection with the word ``explosion,'' and we smile at the idea that giant powder or nitro-glycerine could blow up a planet. yet it would only need enough of them to do it. after all, we may deceive ourselves in thinking, as we are apt to do, that explosive energies lock themselves up only in small masses of matter. there are many causes producing explosions in nature, every volcanic eruption manifests the activity of some of them. think of the giant power of confined steam; if enough steam could be suddenly generated in the center of the earth by a downpour of all the waters of the oceans, what might not the consequences be for our globe? in a smaller globe, and it has never been estimated that the original asteroid was even as large as the moon, such a catastrophe would, perhaps, be more easily conceivable; but since we are compelled in this case to assume that there was a series of successive explosions, steam would hardly answer the purpose; it would be more reasonable to suppose that the cause of the explosion was some kind of chemical reaction, or something affecting the atoms composing the exploding body. here dr gustav le bon comes to our aid with a most startling suggestion, based on his theory of the dissipation of intra-atomic energy. it will be best to quote him at some length from his book on the evolution of forces. ``it does not seem at first sight,'' says doctor le bon, very comprehensible that worlds which appear more and more stable as they cool could become so unstable as to afterward dissociate entirely. to explain this phenomenon, we will inquire whether astronomical observations do not allow us to witness this dissociation. we know that the stability of a body in motion, such as a top or a bicycle, ceases to be possible when its velocity of rotation descends below a certain limit. once this limit is reached it loses its stability and falls to the ground. prof. j. j. thomson even interprets radio-activity in this manner, and points out that when the speed of the elements composing the atoms descends below a certain limit they become unstable and tend to lose their equilibria. there would result from this a commencement of dissociation, with diminution of their potential energy and a corresponding increase of their kinetic energy sufficient to launch into space the products of intra-atomic disintegration. it must not be forgotten that the atom being an enormous reservoir of energy is by this very fact comparable with explosive bodies. these last remain inert so long as their internal equilibria are undisturbed. so soon as some cause or other modifies these, they explode and smash everything around them after being themselves broken to pieces. atoms, therefore, which grow old in consequence of the diminution of a part of their intra-atomic energy gradually lose their stability. a moment, then, arrives when this stability is so weak that the matter disappears by a sort of explosion more or less rapid. the bodies of the radium group offer an image of this phenomenon -- a rather faint image, however, because the atoms of this body have only reached a period of instability when the dissociation is rather slow. it probably precedes another and more rapid period of dissociation capable of producing their final explosion. bodies such as radium, thorium, etc., represent, no doubt, a state of old age at which all bodies must some day arrive, and which they already begin to manifest in our universe, since all matter is slightly radio-active. it would suffice for the dissociation to be fairly general and fairly rapid for an explosion to occur in a world where it was manifested. these theoretical considerations find a solid support in the sudden appearances and disappearances of stars. the explosions of a world which produce them reveal to us, perhaps, how the universes perish when they become old. as astronomical observations show the relative frequency of these rapid destructions, we may ask ourselves whether the end of a universe by a sudden explosion after a long period of old age does not represent its most general ending. here, perhaps, it will be well to stop, since, entrancing as the subject may be, we know very little about it, and doctor le bon's theory affords a limitless field for the reader's imagination. _________________________________________________________________ a printed version of this book is available from sattre press (http://csky.sattre-press.com). it includes extensive annotations, a new introduction and all the original photographs and diagrams. the source and mode of solar energy throughout the universe. by i. w. heysinger, m.a., m.d. illustrated. philadelphia: j. b. lippincott company. . contents. page introduction chapter i. statement of the problem of solar energy chapter ii. the constitution and phenomena of the sun chapter iii. the mode of solar energy chapter iv. the source of solar energy chapter v. the distribution and conservation of solar energy chapter vi. the phenomena of the stars chapter vii. temporary stars, meteors, and comets chapter viii. the phenomena of comets chapter ix. interpretation of cometic phenomena chapter x. the resolvable nebulæ, star-clusters and galaxies chapter xi. the gaseous nebulæ chapter xii. the nebular hypothesis: its basis and its difficulties chapter xiii. the genesis of solar systems and galaxies chapter xiv. the mosaic cosmogony chapter xv. conclusion. the harmony of nature's laws and operations reference index of authorities cited classified index of subject-matter list of illustrations. page figs. to . types from nature, illustrating development of a solar system from the attenuated matter of space frontispiece. fig. . a typical sun-spot fig. . structure of the sun, analytical illustration of fig. . electrical polarities of sun and planets fig. . ideal view of the generation and transmission of planetary electricity fig. . the aurora borealis, view of fig. . diffused brush discharge of an electrical machine fig. . planetary generation and transmission of electrical energy to the sun, analytical illustration of fig. . gradual discharge of electricity from one conductor to another in a partial vacuum fig. . sudden electrical discharge through the atmosphere fig. . position of planets with reference to the generation of sun-spots; maximum and minimum of electrical action fig. . analysis of a typical sun-spot fig. . retardation of sun-spots in their travel across the solar face; development to the rear and recession in front figs. and . complex lines of planetary electrical action upon the sun produced by the inclination of the solar axis to the plane of the ecliptic figs. to . examples of electrical repulsion: fig. , similarly electrified pith-balls; fig. , the electrical windmill; fig. , repulsion of a flame; fig. , self-repulsion around a conductor; fig. , attraction between opposite and repulsion between similar electricities; fig. , mutual repulsion between similar + electrospheres of the earth and the moon; fig. , mutual repulsion between the similar--electrospheres of sun and comet figs. to . spectra of solar light, incandescent sodium and calcium, and the absorption and bright-line spectra of hydrogen gas figs. to . reversal and neutralization of spectroscopic lines of hydrogen in the light of a variable star like betelgeuse fig. . a double-sun nebula in process of development into a solar system fig. . double stars with complementary colors, interpretation of the phenomena of fig. . a solar system which would explain the regular variability of the star mira fig. . lineal nebula in sobieski's crown which has been affected by currents in the ocean of space figs. to . four stages in the phenomena of a new or temporary star, a "star in flames;" reversal of the hydrogen lines in its spectrum figs. and . illustration of repulsion of the tail of a comet by the similarly electrified solar electrosphere; comparison with similar repulsion in a vacuum-chamber experiment figs. and . the electroscope, and mutual electrical repulsion in a bundle of dry straws fig. . experiment with a candle and currents of air from between two disks, illustrating the radial semi-rotation of a comet's tail during perihelion figs. to . four non-systemic gaseous nebulæ: fig. , crab nebula; fig. , dumb-bell nebula; fig. , lineal nebula in sobieski's crown; fig. , catherine-wheel nebula. the latter illustrates the formation of a planetary nebula with a hollow center, or else dispersion into the elements of space again fig. . great spiral nebula in canes venatici and a small adjacent nebula affected thereby figs. to . four gaseous nebulæ in process of development into solar systems: fig. , divergent spiral; fig. , later stage of a similar spiral; fig. , subsequent stage of rupture of the nearly circular convolutions of a similar nebula; fig. , the same stage in the development of a solar system with a double sun fig. . nucleated planetary nebula, showing its external ring split and held apart, in part of its circumference, by electrical repulsion fig. . divergent spiral nebula on cover of book. introduction. this work is not presented to the reader as a treatise on astronomy, although the different phenomena pertaining to that splendid science are reviewed with some detail, and the established facts bearing upon the subjects discussed are briefly cited in the very words of the great writers upon whose authority they rest. a considerable experience in chemistry, electricity, and the other allied physical sciences long since convinced the author of this work that some simple and uniform principle must control the production of the physical phenomena of astronomy,--some general law capable of being extended in its application to the widest, as well as applied to the narrowest, limits of that science. knowing the absolute certainty of a magnetic and electrical connection between the sun and the earth, as evidenced by the reflected energy of sun-spots, auroras, etc., and that no known cause except electricity could account for some, at least, of the cometic phenomena, it seemed that any comprehensive law must at all events include this mode of energy as an effective cause, and that if the law be uniform in its application, it must equally exclude all others which may be either antagonistic or not necessary. a careful investigation was therefore made of those less generally known principles concerned in the generation and transformations of electrical energy, in order to determine the sufficiency or insufficiency of this agency in the grander operations of nature (for, of course, mere currents of electricity could play no part in these phenomena), with the result that every line of research led irresistibly to the conclusions presented in this work. these investigations, specifically directed, at first, to the source and mode of the solar energy of our own system alone, were found to be equally applicable to others, and were successively extended to the whole sidereal, nebular, and cometic field, and finally to space itself, for all the phenomena of which it seemed to furnish an adequate and harmonious interpretation. the fact, when once demonstrated, that the true source of solar energy is not to be found in the sun itself, but in the potential energy of space, served as a guiding principle, and, by its continuously extended application, was found to cover perfectly the source and mode of all solar energy. every step of the investigation has been based on the established facts of science and the observations of eminent astronomers as laid down by the best authorities; and the quotations herein made from their works are full and fair, and are properly credited in every case, and taken from books easily accessible to the general reader. it is hoped that further attention may be directed to this field of research by far more capable investigators than the author of this work, so that systematic astronomy may no longer bear the reproach that it is largely an empirical science, but that it may henceforth be based upon rational and comprehensive principles, capable of universal extension and of general scientific application. the authorities cited in this work include many illustrious names: proctor, tyndall, helmholtz, langley, huggins, newcomb, young, flammarion, balfour stewart, r. kalley miller, herschel, nichol, lord rosse, urbanitsky, crookes, fraunhofer, ball, and many others, all of whom are known throughout the world as among the master minds of science. from them we have drawn the rich stores of knowledge of the phenomena with which this work deals, and which we have so fully and freely cited, as the basis of the splendid superstructure which astronomy to-day reveals. no one will venture to controvert the statements of fact made by these eminent men, and, where conflict of opinion has arisen among them, we have quoted all parties, so that the reader can form his own conclusion, in each case, for himself. so diverse, apparently, are the phenomena reviewed that they present the aspect of a great picture-gallery, in which the paintings totally differ from each other in subject, in treatment, and in origin, their only common qualities being those of grandeur and fidelity to truth and to the principles of art. but they are not merely paintings, they are the moving panorama of creation, and, diverse as they may appear, they will be found to show the same "handling," which reveals the same universal artist; they have, in truth, a common mode of development and a common principle of construction, obscure as these may seem to be. for thousands of years "natural history," so called, was studied and taught; zoölogy was a well-known science far back in old historic times. but it was left for modern biological research to turn from these fixed and fully-developed forms of life, and go back to trace their primal development through what is now the science of embryology, and thus we have learned that nature traverses the same paths in forming a man as in producing a frog or a bird. the process is carried further along in one case than in another, but the lines of development are almost identical; and the tracing out of these common lines and their subsequent divergencies has shed a flood of new light upon these dark and hitherto unknown places, so that we are now fairly on the true highway of physical life at last. when adult forms were alone compared, animal with animal, no common ground of origin or development could be discerned; nature was believed to work by "special creations," and vast cataclysms were devised to utterly destroy the organic life of one terrestrial epoch after another, leaving a few hardy accidental survivors, or "types," perchance, to trace back their lines of descent beyond such periods of cyclical destruction. all this is now changed, and these views, so recently held and taught, have been abandoned forever, and continuously operative natural processes of development, modified by environment and heredity, have taken their place, and biology now has a future as well as a past. and so it must be with the less complex, but far more extended, creations and transformations in the vast fields of astronomical science with which this book is concerned. hitherto we have here, too, dealt with "special creations" and cataclysms; henceforth we must follow the uniform and eternal laws of progressive development. among the multitude of hitherto unsolved problems of astronomy we may enumerate the following: why sun-spots travel faster around the sun when near his equator than when more distant from it. the physical causes of sun-spots, faculæ, and solar prominences. why the number and size of sun-spots seem to affect terrestrial magnetism. the rational interpretation of the eleven-year and the long sun-spot cycles. the origin of the aurora borealis. the causes of the periodicity of regularly variable stars. how to explain, in accordance with the nebular hypothesis, why algol and its companion, which are not greatly different in mass and volume, and both obviously gaseous, should so differ in character, one being a bright sun and the other a dark planet. whether there are great, compact, but dark bodies, comparable to suns and planets in magnitude, and unconnected with any solar system, floating about in space. why double and multiple stars are so frequently of contrasted or complementary colors. why regularly variable stars are longer in decline than in growth of brilliancy, since such decline is no criterion of loss of heat, but rather the reverse. why the sun and fixed stars have atmospheres largely composed of free hydrogen, and the planets have atmospheres of free oxygen and nitrogen. why a small and sometimes even scarcely visible star occasionally is seen to suddenly blaze up, in a few hours, to hundreds of times its normal brilliancy, and then far more gradually fade, through months and years, back to its former state, in which thenceforth it continues to maintain its original lustre. why comets, when they have tails, always project these appendages radially from the direction of the sun. how to account for the presence of cyanogen, and how for the absence of oxygen and the constant presence of hydrocarbon vapors around the nuclei of comets. why some comets split up into separate comets and others sometimes show multiple tails. why comets, when they pass around and behind the sun, in some cases reappear shorn of their splendor and in other cases with their splendor greatly enhanced. whence comets are derived, where is their permanent abiding-place, and how did they originally reach those distant regions which they occupy before entering our system, if merely the débris left behind from contraction of the mass of plasma out of which our solar system is supposed to have been formed. why so many of the irresolvable nebulæ present the appearance of divergent spirals of many different forms. how to account for the annular nebulæ with hollow centers and for those partially-completed planetary nebulæ, so called, which afterwards appear to retrograde into diffused gaseous nebulæ again or gradually disappear. what is the ultimate constitution of interstellar space? have the fixed stars planetary systems like our own, or not? must they have such, or merely may they have? what principle of conservation of energy is it possible to apply to the vast quantities of light and heat which constantly disappear in the interstellar realms of space? how to account for this enormous emission of solar energy during the long period of time requisite for the development of the earth during its past geological ages. how to explain why the moon always presents the same face to the earth. why, if the law of gravity prevails there, there are no visible traces of atmosphere or moisture in the moon. what is the basic principle on which depends the ratio of mean planetary distances, , , , , , etc., always plus ? what is the origin of the planetary satellites and the cause of their irregular distribution, and what the origin of saturn's rings? how was the belt of asteroids formed between mars and jupiter? why is the orbit of neptune relatively compressed against that of uranus? why is the mass of neptune out of its proper proportion compared with those of jupiter, saturn, uranus, and neptune in a diminishing series? what is the rational interpretation and what the origin of the sun's corona and the cause of the coronal streamers? there are many other problems equally difficult which are encountered in the study of this noble science, but the above are surely sufficiently striking. any complete interpretation of these various phenomena, even singly, would seem to be an important step in advance; then how much more so if the explanation of one and all of these is to be found in a single, all-embracing cause, a few simple and uniformly operative principles, as unquestionably operative here as in the other fields of science to which they pertain, and which, once thoroughly comprehended and rigidly applied, will be found to elucidate all the multifarious phenomena of sidereal space so clearly and precisely that any intelligent observer and reasoner can determine each question finally for himself, and solve not only these, but all the other astronomical problems and paradoxes which have from time to time arisen? it is not to be understood that this sublime science and these illimitable realms are to be laid off with the metes and bounds of a farmer's meadow, for all the lines of the different sciences are linked together at a thousand points, but that the operative principles which nature constantly employs once firmly grasped, the intricacy of each series of phenomena encountered will become gradually lessened, link by link, as observations and deductions are more closely and rationally made along these well-established lines of research, instead of here and there, empirically, and at hap-hazard, as has been the only method hitherto possible to pursue. when the relatively few fixed principles which control the operations of nature in the field of astronomy are thoroughly comprehended, for on this vast panorama she lays her colors with a heavy brush, we can study her phenomena and interpret her processes even more readily than the kindred sciences have enabled us to do in the adjacent fields of biology, wherein the splendid achievements of less than a quarter of a century past have not only aroused the interest and enthusiasm of the world, but already point the way to still grander triumphs yet to come. the source and mode of solar energy. chapter i. statement of the problem of solar energy. in endeavoring to present a new and rational interpretation of the source and mode of solar energy, based upon the established principles of recent science, it becomes necessary to briefly cite the facts bearing upon the problem to be solved and the authorities for their support, as well as to describe concisely the different hypotheses at present in vogue, and to point out the well-established insufficiency of these theories, one and all, to account for or explain the difficulties encountered, and which so far have remained as an unsolved enigma. and this problem of solar energy is the grandest and most important question of all physics, for upon the light and heat of the sun depend all physical life and its consequences, animal and vegetable, past, present, and future. if within finite time, and relatively, compared with the enormous vistas of the past, a very brief time, this source of energy is to cease, and our whole system be involved in darkness and death, such darkness and death must be eternal; for the dead sun in his final stage of condensation will be as fixed and unchangeable as the operation of eternal laws can make it, and henceforth there can be no revival or reversals, no turning back of the hand upon the dial, while the laws of nature continue; and outside the uniform operation of the laws of nature there is no source, or mode, or continuance of solar energy conceivable. it is true that when our system shall have ran down to its culmination in death, other present systems may continue for a time to exist and new ones spring into being; but these, too, must inevitably follow the same course, and likewise end in eternal darkness, until finally the great experiment of creation shall have ended in eternal failure. the changes we see in progress around us, however, are not of this nature. the individual dies, but the forces which gave life and strength to the race persist, and others will take his place, and the same forces will continue to operate with constant renewals, since we draw our light and heat and life from without; but in the death of suns and their attendant planets there is no analogous process, for such suns are constantly expending their enormous energies in the support of life external to themselves, and only the smallest part of this energy, even, can ever be utilized by themselves or by other suns or planets under any mode of interpretation now in vogue, the boundless realms of so-called inert and empty space receiving the same proportionate quota of light and heat as the almost microscopic points in the sky which constitute the suns and systems we see, and practically all, or nearly all, of this enormous energy is an absolute dead waste; so that whether receiving new supplies from a constant rain of adjacent meteor streams, or from the gradual contraction of the solar volume, the vast realms of space are the useless recipients of what can never return to the sun again, and, of course, in such case the inevitable end can be predicted; for contraction of volume, with a given mass, must have an effective limit, and meteoric aggregation must also find an effective limit, if the planets are not to be thrown out of place as they continue to revolve around the sun. all accepted theories begin with a primordial impulse, the energies of which are of necessity constantly frittered away and wasted, until finally all light and heat and life must cease to exist, and that at a stage in which no further impulse can ever be given, since the whole universe will have passed through every possible stage of degradation down to the final one of universal and eternal death. and yet this is the best that science has to suggest; the only comfort offered us is that it will not happen in our time, and so, "after us the deluge." the nebular hypothesis, so called, of laplace, has required much modification, in the light of more recent science, but the essential principles of this theory are still generally accepted, for they fairly well account for the primal connection of the sun and planets, and the position of the central sun within, with the orbital and rotational planetary movements, as no other theory has yet done. by this theory the limits of our solar system were once occupied by an attenuated gaseous nebula containing within itself all the matter which now forms our solar system. this great nebular mass, primordially assumed, was given by gravity a slow but gradually increasing rotation upon its center; the force of gravity acted more strongly upon this rotating body as it contracted, so that rings of nebulous matter were successively thrown off, which coalesced into single masses and these finally into planets. these planetary globes themselves, as they coalesced and contracted, left behind or threw off rings of their outer matter, which, in turn, became moons, and finally our solar system with its central sun was evolved as we now see it; development continued, the planets cooled and condensed, life appeared when the conditions became suitable, and the original progressive condensation of the central mass--the sun--still continuing, the evolution of light and heat continues, and will continue in a correlative degree. as our moon has passed, apparently, beyond the stage of life, and is cold, airless, waterless, and dead, so will the earth pass; and the larger planets, such as jupiter and saturn, which have not yet reached the life stage of condensation, are still hot, but they, too, will pass through the present stage of the earth, then through that in which the moon now is; and the central sun, still glowing, but more and more dimly, will itself pass through the stages in which jupiter and saturn now are, then through that of our present earth, and finally into that of the moon, long before which time the emission of all light and heat will have ceased from the sun to its encircling planets, and finally the sun itself will sink into eternal frigidity, and all its store of light and heat will have been dissipated into boundless space, and the possibility of anything resembling what we know as life will have been forever extinguished. in considering the question of the sun's energy, the author of the article "sun," in appleton's cyclopædia, says, "how to account for the supply of the prodigious amount of heat constantly radiated from the solar surface has offered a boundless field of hypothesis. one conjecture is that the sun is now giving off the heat imparted to it at its creation, and that it is gradually cooling down ( ). another ascribed it to combustion ( ), and a third to currents of electricity ( ). newton and buffon conjectured that comets might be the aliment of the sun ( ); and of late years a somewhat similar theory (first broached by mr. waterston in ) has been in vogue,--viz., that a stream of meteoric matter constantly pouring into the sun from the regions of space supplies its heat, by the conversion into it of the arrested motion ( ). as the sun may, indeed, derive a small amount of heat from this cause, it deserves more attention than previous conjectures. but conjecture and hypothesis may be said to have given place to views which claim a higher title, as it is now becoming generally recognized, in accordance with modern physical theories of heat, that in the gravitation of the sun's mass toward its center, and in its consequent condensation, sufficient heat must be evolved to supply the present radiation, enormous as this undoubtedly is. it appears to be susceptible of full demonstration that a contraction of the sun's volume of a given definite amount, which is yet so slight as to be invisible to the most powerful telescope, is competent to furnish a heat-supply equal to all that can have been emitted during historical periods. according to this theory, then (which is largely due to the development by helmholtz of mayer's great generalization), the sun's mass remains unaltered, and its temperature nearly constant, while its size is slowly diminishing as it contracts; so slowly, however, that the supply may be reckoned on through periods almost infinite as measured by the known past of our race, and which are in any case to be counted by millions of years ( )." to these must be added the hypothesis of dr. siemens, fully described in professor proctor's "mysteries of time and space." this ingenious theory, in brief, is that the rotation of the sun on its axis causes a suction in the manner of a fan, at the poles, and a tangential projection, at the equator, of a disk-like stream of gaseous matter into space. the light and heat of the sun, dispersed through space, slowly but continuously act upon the compound gases with which space is universally pervaded to disassociate them into their elements. the disassociated gases thus sucked in at the solar poles at an extremely low temperature are brought into a state of combustion by friction and condensation, thus generating new supplies of light and heat, and the gases thus reunited by combustion are again projected into space, to be again slowly disassociated by the operation of the sun's light and heat. the result of this combustion is to form aqueous vapor and carbonic acid and carbonic oxide, and these gases, when disassociated in space, are resolved into carbon, oxygen, and hydrogen, which again and again are thus recombined and again and again decomposed as they pass over the sun's surface ( ). the seven hypotheses above described are the only ones now in vogue, and a brief analysis will show that no single one of them, nor all combined, will give sufficient results to account for the essential difficulties or known conditions of the problem. the first and second hypotheses are answered by the fact set forth by helmholtz (popular scientific lectures, article "on the origin of the planetary system"), that, if the mass of the sun were composed of the two elements capable by combination of producing the greatest possible light and heat,--to wit, hydrogen and oxygen in the proportions in which they unite to form water,--"calculation shows that under the above supposition the heat resulting from their combustion would be sufficient to keep up the radiation of heat from the sun three thousand and twenty-one years. that, it is true, is a long time, but even profane history teaches that the sun has lighted and warmed us for three thousand years, and geology puts it beyond doubt that this period must be extended to millions of years." the third hypothesis relates to currents of electricity. we have no knowledge of currents of electricity which could produce, however multiplied or intensified, such light and heat as are constantly poured forth from the sun into all space. that electricity is the intermediate cause of our sun's energy, and of all solar energy, it is the purpose of this work to demonstrate, but not electric currents, which find their attractiveness to theorists in the vague suggestion of which professor proctor speaks, referring to comets, in his article on "cometic mysteries," "that perhaps this is an electrical phenomenon; perhaps that other feature is electrical, too; perhaps all or most of the phenomena of comets depend on electricity." but he adds, "it is so easy to make such suggestions, so difficult to obtain evidence in their favor having the slightest scientific value. still, i hold the electrical idea to be well worth careful study. whatever credit may hereafter be given to any electrical theory of comets will be solely and entirely due to those who may help to establish it upon a basis of sound evidence,--none whatever to the mere suggestion, which has been made time and again since it was first advanced by fontanelle." it will be seen that the present work, in demonstrating the true source and mode of solar energy, in itself presents a full and sufficient explanation of all the cometic mysteries referred to, as well as all those pertaining to other solar systems in space, and the multifarious phenomena which they present. indeed, the philosophic mind will not be satisfied with the sufficiency of any hypothesis which will not unlock the mysteries and clearly explain the phenomena of other systems,--of comets, variable and temporary stars, double stars, and all the complicated celestial economy which to the eye of the mere observer presents a bewildering scene of the operation of independent and inscrutable forces. the fifth hypothesis cited, that of meteoric impact, doubtless plays a part, as we know from the generation of light and heat by the constant passage of similar bodies through our own atmosphere. and we know, of course, that the sun, by its vastly-increased attraction, must be subjected to the constant impact of such meteoric bodies in enormous numbers. but the fatal defect in the theory is that such impacts, to produce the radiant energy of the sun, must constantly add to its mass in like proportion, and as the motions and distances of the planets in their orbits are regulated and preserved by virtue of the substantially constant mass of the sun, any progressive and considerable increase in its mass must constantly bring the planets nearer and nearer, and thus increase their orbital velocity. helmholtz quotes from sir william thomson's investigation, that, "assuming it to hold, the mass of the sun should increase so rapidly that the consequences would have shown themselves in the accelerated motion of the planets. the entire loss of heat from the sun cannot, at all events, be produced in this way; at the most a portion, which, however, may not be inconsiderable." r. kalley miller, in "the romance of astronomy," says, "but more recent observations have led sir william thomson to a modification of his theory. he has calculated that if the meteoric shower were sufficiently heavy to make up for the sun's whole expenditure of heat, the matter of the corona must be so dense as seriously to perturb the orbits of certain comets which pass very close to his surface,--a result which is found not to be the case. but the meteoric theory is only thrown back a step. if the sun's mass were originally formed, as is not at all improbable, by the agglomeration of these particles, sir william thomson has calculated that the heat generated by their thus falling together would be sufficient to account for a supply of twenty million years of solar heat at the present rate of emission. and thus, though the meteors are not sufficient to maintain the energy of our system unimpaired, they may yet have been the original storehouse from which all that energy was derived.... but if the economy of our system be spared long enough, the day must come when the sun with age has become wan; when the matter of the corona has all been drawn in and used up without avail; when the lavish luxuriance with which he has showered abroad his light and heat has finally exhausted all his stores. he has still power, aided by the resisting medium, to drag his satellites one by one down upon his surface; and the shock of each successive impact will, for a brief period, give him a fresh tenure of life. when the earth crashes into the sun it will supply him with a store of heat for nearly a century, while jupiter's large mass will extend the period by nearly thirty thousand years. but when the last of the planets is swallowed up, the sun's energies will rapidly die out and a deep and deathly gloom gather about nature's grave. looking into the ages of a future eternity, we can see nothing but a cold and burnt-out mass remaining of that glorious orb which went forth in the morning of time, joyful as a bridegroom from his chamber, and rejoicing as a strong man to run a race." the sixth hypothesis is that to which most credence is now given. it is that of evolution of energy by condensation of volume. professor proctor ("the sun as a perpetual machine") says, "in company with this great mystery of seeming waste comes the yet more difficult problem, how to explain the apparent continuance of solar light and heat during millions of years. we know from the results of geological research that the earth has been exposed to the action of the solar rays with their present activity during at least a hundred million years. yet it is difficult to see how, on any hypothesis of the generation of solar heat, or by combining together all possible modes of heat generation, a supply for more than twenty millions of years in the past and a possible supply for as long a period in the future can be accounted for." of these vast periods of terrestrial existence in the past we quote the following from a recent publication: "professor c. d. wolcott expresses the opinion that geologic time is not to be measured by hundreds of years, but simply by tens of millions. this is widely different from the conclusion arrived at by sir charles lyell, who, basing his estimate on modifications of certain specimens of marine life, assigned , , years as the required geological period; darwin claimed , , years; crowell, about , , ; geike, from , , upward; mcgee, upham, and other recent authorities claim from , , up to , , ." helmholtz ("on the origin of the planetary system") says, "it is probable rather that a great part of this heat, which was produced by condensation, began to radiate into space before this condensation was complete. but the heat which the sun could have previously developed by its condensation would have been sufficient to cover its present expenditure for not less than , , of years of the past.... we may therefore assume with great probability that the sun will still continue in its condensation, even if it only attained the density of the earth, though it will probably become far denser in its interior, owing to its far greater pressure; this would develop fresh quantities of heat, which would be sufficient to maintain for an additional , , of years the same intensity of sunshine as that which is now the source of all terrestrial life." of this process of condensation professor ball, in his recent work, "in the high heavens," says, "it goes without saying that the welfare of the human race is necessarily connected with the continuance of the sun's beneficent action. we have indeed shown that the few other direct or indirect sources of heat which might conceivably be relied upon are in the very nature of things devoid of necessary permanence. it becomes, therefore, of the utmost interest to inquire whether the sun's heat can be calculated on indefinitely. here is indeed a subject which is literally of the most vital importance, so far as organic life is concerned. if the sun shall ever cease to shine, then it must be certain that there is a term beyond which human existence, or indeed organic existence of any type whatever, cannot any longer endure on the earth. we may say once for all that the sun contains just a certain number of units of heat, actual or potential, and that he is at the present moment shedding that heat around with the most appalling extravagance." quoting from professor langley, he says, "we feel certain that the incessant radiation from the sun must be producing a profound effect on its stores of energy. the only way of reconciling this with the total absence of evidence of the expected changes is to be found in the supposition that such is the mighty mass of the sun, such the prodigious supply of heat or what is the equivalent of heat which it contains, that the grand transformation through which it is passing proceeds at a rate so slow that, during the ages accessible to our observations, the results achieved have been imperceptible.... we cannot, however, attribute to the sun any miraculous power of generating heat. that great body cannot disobey those laws which we have learned from experiments in our laboratories. of course no one now doubts that the great law of the conservation of energy holds good. we do not in the least believe that because the sun's heat is radiated away in such profusion it is therefore entirely lost. it travels off, no doubt, to the depths of space, and as to what may become of it there we have no information. everything we know points to the law that energy is as indestructible as matter itself. the heat scattered from the sun exists at least as ethereal vibration, if in no other form. but it is most assuredly true that this energy, so copiously dispensed, is lost to our solar system. there is no form in which it is returned, or in which it can be returned. the energy of the system is as surely declining as the store of energy of the clock declines according as the weight runs down. in the clock, however, the energy is restored by winding up the weight, but there is no analogous process known in our system." the purpose of the present work, however, is to clearly demonstrate that just such a process is actually being carried on, and has been so carried on from the beginning, and will be forever. this writer continues reviewing the suppositions formerly entertained, that the sun was a heated body gradually cooling down, or that it was undergoing absolute combustion, and shows that they were utterly insufficient. he then refers to the theory of meteoric supply, of which he says, "it can, however, be shown that there are not enough meteors in existence to supply a sufficient quantity of heat to the sun to compensate the loss by radiation. the indraught of meteoric matter may, indeed, certainly tend in some small degree to retard the ultimate cooling of the great luminary, but its effect is so small that we can quite afford to overlook it from the point of view that we are taking in these pages. it is to helmholtz we are indebted for the true solution of the long-vexed problem. he has demonstrated in the clearest manner where the source of the sun's heat lies.... a gaseous globe like the sun, when it parts with its heat, observes laws of a very different type from those which a cooling solid follows. as the heat disappears by radiation the body contracts; the gaseous object, however, decreases in general much more than a solid body would do for the same loss of heat.... the globe of gas unquestionably radiates heat and loses it, and the globe, in consequence of that loss, shrinks to a smaller size.... in the facts just mentioned we have an explanation of the sustained heat of the sun. of course we cannot assume that in our calculations the sun is to be treated as if it were gaseous throughout its entire mass, but it approximates so largely to the gaseous state in the greater part of its bulk that we can feel no hesitation in adopting the belief that the true cause has been found." regarding the constitution of the sun, it may be stated, however, that we only see its photosphere, which is the visible sun, and the whole volume has a density about that of water; but no man has ever seen the body of the sun itself. in this respect it is like the planet jupiter: we only know that its density cannot be less than one-fourth the density of the earth's solid globe. if the photosphere extend to a depth of one thousand, ten thousand, or a hundred thousand miles, the density of the sun's body or core will be correspondingly increased. even computing the whole visible volume, the density is far greater than that of any gas we know, even with the solar pressure of gravity; with the sun's metallic vapors, if the whole core were already vaporized, we would not, to say the least, be likely to observe the sun-spots and other solar phenomena as we find them actually to occur; this, however, will be more fully considered later on. the author continues, "but there is a boundary to the prospect of the continuance of the sun's radiation. of course, as the loss of heat goes on the gaseous parts will turn into liquids, and as the process is still further protracted the liquids will transform into solids. thus, we look forward to a time when the radiation of the sun can be no longer carried on in conformity with the laws which dictate the loss of heat from a gaseous body. when this state is reached the sun may, no doubt, be an incandescent solid with a brilliance as great as is compatible with that condition, but the further loss of heat will then involve loss of temperature.... there seems no escape from the conclusion that the continuous loss of solar heat must still go on, so that the sun will pass through the various stages of brilliant incandescence, of glowing redness, of dull redness, until it ultimately becomes a dark and non-luminous star.... there is thus a distinct limit to man's existence on the earth, dictated by the ultimate exhaustion of the sun.... the utmost amount of heat that it would ever have been possible for the sun to contain would, according to this authority (professor langley), supply its radiation for eighteen million years at the present rate.... it seems that the sun has already dissipated about four-fifths of the energy with which it may have originally been endowed. at all events, it seems that, radiating energy at its present rate, the sun may hold out for four million years or for five million years, but not for ten million years.... we have seen that it does not seem possible for any other source of heat to be available for replenishing the waning stores of the luminary." he concludes by saying that the original heat may have been imparted as the result of some great collision, the solar body having itself been dark before the collision occurred, and that it may be reinvigorated by a repetition of a similar startling process, but indicates in general terms that such an operation would be bad for the round world and all contained therein. it would, in fact, be rough treatment for even a hopeless case. condensation of the solar volume is unquestionably a source of heat, for we know that the solid or liquid interior of the earth increases in temperature at a definite ratio as we descend through its crust; but long before the sun shall have become contracted to the density of the earth all its heat will have become substantially internal heat, and it can then supply no more by radiation to its surrounding planets. it will be seen that the radiant energy of the sun on any of the above hypotheses is not sufficient to account even for the life period of the earth in the past, and that its future period of energy must be still more brief. professor ball ("in the high heavens"), basing his views on laplace's "nebular hypothesis," says, "looking back into the remote ages, we thus see that the sun was larger and larger the further back we project our view. if we go sufficiently far back, we seem to come to a time when the sun, in a more or less completely gaseous state, filled up the surrounding space out to the orbit of mercury, or, earlier still, out to the orbit of the remotest planet." according to this hypothesis, all these brilliant suns, the author says, will "settle down into dark bodies like the earth," and that "every analogy would teach us that the dark and non-luminous bodies in the universe are far more numerous than the brilliant suns. we can never see the dark objects; we can discern their presence only indirectly. all the stars that we can see are merely those bodies which at this epoch of their career happen for the time to be so highly heated as to be luminous.... it may happen that there are dark bodies in the vicinity of some of the bright stars to which these stars act as illuminants, just in the same way as the sun disperses light to the planets." one would naturally suppose, however, that there must be some sort of laws to govern such stupendous operations, and that nature is not merely engaged in blowing bubbles. to quote professor newcomb: "at the present time we can only say that the nebular hypothesis is indicated by the general tendencies of the laws of nature; that it has not been proved to be inconsistent with any fact; that it is almost a necessary consequence of the only theory by which we can account for the origin and conservation of the sun's heat; but that it rests on the assumption that this conservation is to be explained by the laws of nature as we now see them in operation. should any one be sceptical as to the sufficiency of these laws to account for the present state of things, science can furnish no evidence strong enough to overthrow his doubts until the sun shall be found growing smaller by actual measurement, or the nebulæ be actually seen to condense into stars and systems." while the validity of the views set forth in the present volume does not depend on the sufficiency or insufficiency of the nebular hypothesis, and in fact requires the condensation as well as the expansion of the solar volume under the influence of heat to be recognized and its extreme importance pointed out, yet it must not be supposed that this great generalization of kant and laplace, based on the views presented originally by sir william herschel, is established, or that the difficulties in its way are not so enormous as to be almost insuperable. professor ball points out that thousands of bodies occupy our solar system, and together compose it as a whole; that these have orbits of every sort of eccentricity and direction, and occupying all possible planes which can pass through the sun; that the bodies circle around the sun, some backward and others forward, and that only the planets seem to conform to some common order; and without this order, which may be accidental, so far as our knowledge goes, the system would have been disrupted long since, if it ever could have begun its operations; and that in this view the heavens may be strewn with wrecks of systems which failed to survive from inherent want of harmony,--that is to say, as based on observation only. whether the nebular hypothesis be a universal or a partial law of development, or whether the real processes be quite different, cannot, however, depend on the continued maintenance and evolution of the sun's energy, as this source must in truth be sought for in quite a different direction. the remaining hypothesis (the seventh) is considered in detail in professor proctor's work, "mysteries of time and space." the fatal defect in dr. siemens's theory is, that his gases will not be projected from the sun's equator. professor proctor says, "thus the centripetal tendency of matter at the sun's equator is very much greater (many hundreds of times greater) than its centrifugal tendency, and there is not the slightest possibility of matter being projected into space from the sun's surface by centrifugal tendency. nor is there any part of the sun's mass where the centrifugal tendency is greater than at the surface near the equator. so that, whatever else the sun may be doing to utilize his mighty energies, he is certainly not throwing off matter constantly from his equatorial regions, as dr. siemens's theory requires." there are other difficulties which professor proctor considers, such as the doubt as to the power of the sun's rays to disassociate combined gases in space, and also that, since both light and heat must be utilized in this work, if the sun's energies are to be perpetually renewed, these forces would sensibly disappear in work, and the result would be that the fixed stars would be invisible beyond their domains, and their light, when not totally cut off, would be greatly diminished, in any event, as distances increased, which is not the case. besides, these gases thus disassociated could never be entirely used by the sun, and the remainder would be wasted, and the part wasted would vastly exceed that utilized, probably in as great proportion of waste as that of the sun's light not utilized by the planets, which gather but one two-hundred-and-thirty-two-millionths of the whole. it may be further added that these gases would be mechanically mixed, the combined and the disassociated, and this would be mostly the case in those parts nearest the sun, so that large volumes of spent and useless gases would have to be carried in to no purpose whatever. in fact, these gases would gradually form a closed circuit of supply and discharge, and surrounding space would be but slightly affected. professor proctor concludes, "we have, in fact, the fallacy of perpetual motion in a modified form." it will be apparent that under any single one, or all, of these hypotheses, the future prospect for created forms and continued existence is hopeless, and that the inevitable result must do violence to every conception of either an intelligent creative power or the operations of universal law. the mind revolts from the continued degradation and destruction of all organic creation, while the malevolent and iconoclastic forces of nature hold high revel over final ruin and eternal destruction, brought about by their own incessant efforts, striking out blindly to make or mar, and they alone the deathless survivors, the half-blind fates and furies of the eternal future. it betokens, not the processes of orderly government, but the reign of anarchy. note.--since this work has been in press, at the annual meeting of the british association, august , , lord salisbury, the president, delivered a powerful and lucid address on the present status of scientific knowledge and its limitations. with reference to the antiquity of the earth we quote the following: "it is evident, from the increase of heat as we descend into the earth, that the earth is cooling, and we know, by experiment within certain wide limits, the rate at which its substances--the matters of which it is constituted--are found to cool. it follows that we can approximately calculate how hot it was so many million years ago; but if at any time it was hotter at the surface by fifty degrees fahrenheit than it is now, life would then have been impossible upon the planet, and, therefore, we can without much difficulty fix a date before which organic life on earth cannot have existed. basing himself on these considerations, lord kelvin limited the period of organic life upon the earth to a hundred million years, and professor tait, in a still more penurious spirit, cut that hundred down to ten." if a period of anything like ten million years, even, has been requisite to cool the earth's surface only fifty degrees in temperature, what time must have elapsed since the terrestrial globe had a temperature high enough to effect the difficult chemical combinations of many of the elements which compose its structure? and even this must have been far less than the vast cycles of time during which original consolidation was effected. through all these ages the sun must have been pouring out his radiant energy at at least his present rate. radiation of heat from the earth may have been relatively less rapid from a denser carbon-laden atmosphere in times past than at present, but it never could have been more so. the whole address cited is, indeed, strongly corroborative of the facts upon which the present work is based. chapter ii. the constitution and phenomena of the sun. the various theories thus reviewed, while not sufficient in themselves to account for the facts of our own solar system, are fatally defective in another respect. while they aim to account for the sun's light and heat, they all fail to consider the active medium of the solar light and heat in the sun itself. it is not simply a highly-heated central mass glowing in space. it is a vast orb surrounded by different envelopes of incandescent vapors or gases, and by far the most vast in volume, as well as in light and heat-radiating power, are the photosphere and its superincumbent chromosphere, composed almost entirely of free hydrogen gas in a state of intense incandescence. whence comes this enormous mass of hydrogen? and how explain the entire absence of free hydrogen gas from our own atmosphere and its replacement by oxygen? there is a recent theory propounded by mr. a. mott, which is set forth in detail in professor ball's "in the high heavens," and which endeavors to account for the remarkable absence of free hydrogen gas from the earth's atmosphere, for, as the author states, "it is a singular fact that hydrogen in the free state is absent from our atmosphere." the theory, in brief, is that the molecules of hydrogen gas have an average speed of about a mile a second,--which, however, is only one-seventh that required to shoot them off into space,--but that these molecules are continually changing their velocity, and may sometimes attain a speed of seven miles a second; the result is that "every now and then a molecule of hydrogen succeeds in bolting away from the earth altogether and escaping into open space." during past ages the molecules of hydrogen would thus have gradually wiggled up through the air, and finally disappeared into outer darkness for good and all; and thus "the fact that there is at present no free hydrogen in the air over our heads may be accounted for." since the molecules of oxygen have only a velocity of a quarter mile a second, that unfortunate gas remains behind and is consumed. the first difficulty with this theory is to explain how, if the hydrogen wiggled off in this unceremonious manner, it ever wiggled on. there is no objection to a gait of this rapidity, however; it is highly creditable, in fact; but we have a right to expect some degree of consistency in even so light-headed a body as hydrogen gas. the article quoted thus continues: "if the mass of the earth were very much larger than it is, then the velocities with which the molecules of hydrogen wend their way would never be sufficiently high to enable them to quit the earth altogether, and consequently we might in such a case expect to find our atmosphere largely charged with hydrogen." it will be seen that, according to this theory, hydrogen is able to achieve a speed of seven miles per second under exceptional excitement, and that this molecular velocity is just enough, and no more than enough, to give it egress. we know that jupiter's mass is three hundred times as great as that of the earth, and the attraction of gravity is so powerful on the surface of that planet that, as the writer just quoted says, "walking, or even standing, would involve the most fearful exertion, while rising from bed in the morning would be a difficult, indeed, probably, an impossible, process." we also know that the atmosphere of this planet is laden with enormous clouds floating at various altitudes and with incessant movements. we are told that "the molecular speed of aqueous vapor averages only one-third of that attained by the molecules of hydrogen." of course, on the planet jupiter, hydrogen would have no chance of escape at all: it would just have to stay and take it, like the rest of us. jupiter must thus have an atmosphere like our own, except that it is "largely charged with hydrogen." of the clouds upon this planet, professor ball says, "in fact, the longer we look at jupiter the more we become convinced that the surface of the planet is swathed with a mighty volume of clouds so dense and so impenetrable that our most powerful telescopes have never yet been able to pierce through them down to the solid surface of the planet." with the densities, molecular velocities, and specific gravity of the oxygen, nitrogen, and the hydrogen, with which latter the atmosphere of jupiter must be "largely charged," as it is said, it is difficult to understand how such enormous clouds of aqueous vapors, themselves composed of oxygen, which is a very slow-footed gas, and hydrogen, could travel about with such facility; we ought to find them packed down like london fog, to say the least, upon the surface of that planet, with the supernatant gases all adrift overhead. jupiter is a hot body; it has not yet cooled down; and if it is provided with volcanoes, such as its great red spot and the analogies of the earth and moon would suggest, we can tell pretty nearly what would have happened long ago with a jovian atmosphere like ours; but "largely charged with hydrogen," if we compare it with, say, an equal mass of dynamite touched off by a volcanic explosion; there would not have been enough of old jupiter left to swear by, and what was left would not have had any atmosphere at all. on mars, the same writer thinks the oxygen would still cling, like the fragrance of the rose, but that all the molecules of the fleet-footed and excitable hydrogen would long since have taken french leave, as it did from the earth; but at the moon, on account of its small size and mass, both gases would have gone off incontinently together. "it is now easy," the author says, "to account for the absence of atmosphere from the moon.... neither of the gases, oxygen or nitrogen, to say nothing of hydrogen, could possibly exist in the free state on a globe of the mass and dimensions of our satellite.... indeed, the weight of every object on the moon would be reduced to the sixth part of that which the same object has on earth." nevertheless, it may be said that the moon has considerable weight, as weights go, but with a comet it is quite a different matter. "these bodies," the author says, "demonstrate conclusively that the quantity of matter even in a comet is extremely small when compared with its bulk. the conclusion thus arrived at is confirmed by the fact that our efforts to obtain the weight of a comet have hitherto proved unsuccessful.... it has thus been demonstrated that, notwithstanding the stupendous bulk of a great comet, its mass must have been so inconsiderable as to have been insufficient to disturb even such unimportant members of the solar system as the satellites of jupiter." now, here is a state of things; for the spectroscope shows that comets are fully provided with a large supply of hydrogen, enough and to spare for ornament, even, and of nitrogen also, while it is the abnormally fugacious oxygen which has, apparently, taken its departure. of course, such facts demonstrate the untenability of the theory, which is, besides, in direct contradiction with the laws governing gaseous diffusion. gases pass into each other with the same velocity as into a vacuum, and it is not to be imagined that the molecules of hydrogen could thus move individually off, unless forced upward by the pressure of some other gas, which the law of gaseous diffusion makes impossible. we should as readily expect to see a tumbler full of iron balls, into the interstices of which loose sand has been poured, manifest a similar phenomenon by the wiggling out of the less dense sand at the top of the glass. one might also ask whence, if this theory had any substantial basis, could come the enormous volumes of hydrogen gas in the atmosphere of a new or temporary star, in a few hours, or the changes manifested in the atmospheres of the variable stars. so, also, the nebular or any other hypothesis of creation would be impossible under this theory, as the heavier and less mobile gaseous elements would remain behind, or be condensed nearest the center of gravity of the aggregating nebula, while the more rapid gases would disappear outwardly, and in consequence the sun would be found to be composed of the heavier elements exclusively, and each of the planets, in turn, would consist of only one or two elements, in accordance with the more and more mobile character of their molecular movements, and the uniformity of chemical constitution between the sun and planets, as well as the fixed stars, would not be found to exist. the theory, in fact, is an example of the endeavor to explain an easily understood difficulty by a less easily understood impossibility. none of the different theories even attempt to account for the prodigious volumes of hydrogen in the solar atmosphere, and without its presence the sun, so far as we know, would be almost an inert mass, considered as a source of energy for the supply of our planetary system. we know, of course, that meteors contain sometimes as much as six volumes of gases, largely composed of hydrogen, at our own atmospheric pressure. but the pressure at the sun's surface is more than twenty-seven times that at the surface of the earth, and yet the volume of hydrogen there existing visibly is vaster beyond computation than any possible mass of meteoric material could supply. so, also, while it may be granted that condensation of volume must vastly raise the solar temperature, how could it produce the enormous masses of hydrogen, the lightest of all the elements, unless they have been temporarily occluded and finally thrown out from within, which is impossible? these vast volumes of hydrogen are to be considered first of all in any attempt whatever to solve the problem of the source and mode of solar energy. considering the phenomena presented within the limits of our own solar system alone, we find that the earth is one of a single family of planets, each of which very closely resembles it, and all of which circle, in slightly elliptical orbits, at various distances around the sun, their orbits occupying substantially the same plane, thus making our solar system a flat disk of space occupied by the sun as a center, with the planets and their satellites moving harmoniously around it. the planets differ from each other in size, mass, and temperature, but each is surrounded by an envelope of aqueous vapor, suspended in an atmosphere substantially like our own. professor proctor, in his "light science for leisure hours," says of the planet jupiter, "his real surface is always veiled by his dense and vapor-laden atmosphere. saturn, venus, and mercury are similarly circumstanced." of mars he says that it is "distinctly marked (in telescopes of sufficient power) with continents and oceans which are rarely concealed by vapors." now, whence comes this aqueous vapor surrounding all the planets? whether received originally from the diffused nebular mass from which our solar system is supposed to have been condensed, or attracted by the force of gravity from interplanetary space, like the meteors which fall upon the earth's surface, it is evident that interplanetary space must once have been pervaded with aqueous vapor, since the nebular mass from which our solar system was constituted must have occupied at least the space embraced within its largest planetary orbit, and doubtless much more; and if so, such aqueous vapor, and other vapors also, must still persist in space, just as the meteoric particles which so constantly manifest themselves in our atmosphere. if the planets had no common origin, the evidence is equally conclusive, since then this identical substance could only have been derived from a common source, which can only be interplanetary space. this also is in accordance with the laws of attraction, which would operate to gather and condense the rarefied aqueous vapor of space around the planetary masses in definite proportions. in his "familiar essays on scientific subjects," professor proctor says, "in fact, we do thus recognize in the spectra of mars, venus, and other planets the presence of aqueous vapor in their atmosphere;" and in his "mysteries of time and space" he says, "we may admit the possibility that the aqueous vapor and carbon compounds are present in stellar or interplanetary space." but in addition to this aqueous vapor which surrounds the planetary bodies, we find free oxygen in vast quantities, and, with this, free nitrogen in mechanical admixture, and these together constitute the atmosphere we breathe, and which sustains organic life by a process of slow combustion. but we find no free hydrogen either in our own atmosphere or in that of other planets. turning now to the sun, we find that it is surrounded by an atmosphere as well as the planets, but that this atmosphere is composed not of free oxygen, but of free hydrogen. in his article, "oxygen in the sun," professor proctor says, "fourteen only of the elements known to us, or less than a quarter of the total number, were thus found to be present in the sun's constitution; and of these all were metals, if we regard hydrogen as metallic.... but most remarkable of all, and most perplexing, was the absence of all trace of oxygen and nitrogen, two gases which could not be supposed wanting in the substance of the great ruling center of the planetary system." the researches of dr. draper indicated, however, that oxygen could be found in the sun; not in his external atmosphere but far down within his surface. professor proctor says, "dr. draper mentions that he has found no traces of oxygen above the photosphere." such free oxygen cannot be associated with the hydrogen, however, even if its presence be finally determined, but it may be due to the deoxidation of solid compounds precipitated upon the sun from space, and held at a temperature above that of disassociation, as hydrogen is sometimes generated at the surface of the earth. the vast mass of the solar atmosphere is composed of hydrogen gas, with which are found commingled vapors of the various elements which enter into the sun's constitution, and this solar atmosphere corresponds in proportion, speaking generally, with our own atmosphere, except that the volume of solar hydrogen is vastly greater than that of terrestrial oxygen, for the reason, as will be explained, that water contains two volumes of the former to one of the latter. in appleton's cyclopædia the sun is thus described, (article by professors langley and proctor): "to sum up briefly the received hypotheses of the physical constitution of the sun: of its internal structure we know nothing, but we can infer, from the low density of the solar globe as a whole, that no considerable portion is solid or liquid. the regions we examine appear to consist of cloud layers at several levels floating in a complex atmosphere, in which probably most of the elements are known to us, and certainly many of them exist in the form of vapor. outside this complex atmosphere extend envelopes of simpler constitution, though into them occasionally arise the vapors which ordinarily lie lower down. the sierra, for instance, consists in the main of glowing hydrogen gas and that gas, whatever it may be, which produces the line near the orange-yellow sodium lines. the prominence region may be regarded as simply the extension of the sierra." of these prominences, professor ball says, "the memorable discovery made by janssen and lockyer, independently, in , showed that the prominences could be observed without the help of an eclipse, by the happy employment of the peculiar refrangibility of the rosy light which these prominences emit.... we can now obtain, not, as heretofore, merely isolated views of special prominences through the widely opened slit of the spectroscope, but we are furnished, after a couple of minutes' exposure, with a complete photograph of the prominences surrounding the sun.... the incandescent region of the chromosphere from which these prominences arise is also recorded with accuracy." resuming our quotation from appleton's cyclopædia: "the inner corona is still simpler than the sierra, so far as its gaseous constitution is concerned; but here meteoric and cometic matter appears, extending to the outer corona and to great distances beyond even the visible limits of the zodiacal. returning to the photosphere, we find it subject to continual fluctuations, both from local causes of agitation and from the subjacent vapor acting by its elasticity to burst through it; the faculæ, which are found to be above the general level of the photosphere, are taken to be heapings up of the luminous matter like the crested surges of the sea. all the strata are subject to great movements, which sometimes have the character of uniform progression analogous to our trade-winds, and sometimes are violent, and resemble in their effects our tornadoes and whirlwinds. eruptive action appears to operate from time to time with exceeding violence, but whether the enormous velocities of outrush are due to true explosive action (which would compel us to believe that the sun is enclosed by a liquid shell, so as to resemble a gigantic bubble) or to the uprising of lighter vapors from enormous depths, as heated currents rise in our own atmosphere, is not as yet certainly known." the sierra, or chromosphere, is thus described in the same article: "the sierra presents four aspects: , smooth with defined outline; , smooth but with no defined outline; , fringed with filaments; and, , irregularly fringed with small flames. the prominences may be divided into three orders,--heaps, jets, and plumes. the heaped prominences need no special description. the jets ... originate generally in rectilinear jets either vertical or oblique, very bright and very well defined. they rise to a great height, often to a height of at least eighty thousand miles, and occasionally to more than twice that; then bending back, fall again upon the sun like the jets of our fountains. then they spread into figures resembling gigantic trees more or less rich in branches. their luminosity is intense, insomuch that they can be seen through the light clouds into which the sierra breaks up. their spectrum indicates the presence of many elements besides hydrogen. when they have reached a certain height they cease to grow, and become transformed into exceedingly bright masses, which eventually separate into fleecy clouds. the jet prominences last but a short time--rarely an hour, frequently but a few minutes,--and they are only to be seen in the neighborhood of the spots. wherever there are jet prominences there also are faculæ. the plume prominences are distinguished from the jets in not being characterized by any signs of an eruptive origin. they often extend to an enormous height; they last longer than the jets, though subject to rapid changes of figure; and, lastly, they are distributed indifferently over the sun's surface. it would seem that in the jets a part of the photosphere is lifted up, whereas in the case of plumes only the sierra is disturbed." of these eruptions professor ball says, "vast masses of vapors are frequently expelled from the interior of the sun by convulsive throes with a speed of three hundred, four hundred, and sometimes nearly a thousand miles a second.... the spectroscope enables the observer actually to witness the ascent of these solar prominences." the corona, which extends beyond the chromosphere, has been determined by its continuous spectrum to be a vast envelope extending at least a million miles from the sun's surface. "it cannot be a solar atmosphere," professor proctor observes in his article on this subject, in his "mysteries of time and space."... "it will be seen, then, how inconceivably great the pressure exerted by a solar atmosphere some eight thousand times as deep as ours would necessarily be, let the nature of the gases composing it be what it may."... "if a man could be placed on the solar surface, his own weight would crush him as effectually as though while on earth a weight of a couple of tons were heaped upon him.... now, it happens that we know quite well that the pressure exerted by the real solar atmosphere, even close by the bright surface which forms the visible globe of the sun, is nothing like so great as it would be if the corona formed part of that atmosphere." in the article "sun," in appleton's cyclopædia, it is stated that "mr. arthur w. wright, of yale college, has succeeded in showing that this light (the zodiacal) is not emitted from incandescent gas, but reflected from particles or small bodies, and hence derived from the sun."... "there is reason to believe that the true solar corona extends much farther (than a million miles), and that, in reality, the zodiacal light forms the outer part of the solar corona." proctor, again, in his article on the corona, says, "it would seem to follow that the corona is due to bodies of some sort travelling around the sun, and by their motion preserved either from falling towards him (in which case the corona would quickly disappear) or from producing any pressure upon his surface, as an atmosphere would." in his article on "the sun as a perpetual machine," he says, "there is every reason for regarding the zodiacal as consisting in the main of meteorolithic masses, a sort of cosmical dust, rushing through interplanetary space with planetary velocities. to such matter, assuming, as we well may, that space really is occupied by attenuated vapors, ... the luminosity of the zodiacal would be attributable to particles of dust emitting light reflected by the sun or by phosphorescence (this last may be seriously questioned). but there is another cause for luminosity of these particles which may deserve a passing consideration. each particle would be electrified by gaseous friction in its acceleration, and its electric tension would be vastly increased in its forcible removal, in the same way as the fine dust of the desert has been observed by werner siemens to be in a state of high electrification on the apex of the cheops pyramid. would not the zodiacal light also find explanation by slow electric discharges backward from the dust towards the sun?" it may be observed in passing that such electrical glow is much more prominently, and more likely to be, the result of induction than of friction. in the article "sun," previously quoted, professor young says, "there is surrounding the sun, beyond any further reasonable doubt, a mass of self-luminous gaseous matter, whose spectrum is characterized by the green line kirchhoff. the precise extent of this it is hardly possible to consider as determined, but it must be many times the thickness of the red hydrogen portion of the sierra, perhaps, on an average, ' or ', with occasional horns of twice that height. it is not at all unlikely that it may even turn out to have no upper limit, but to extend from the sun indefinitely into space." in the same article the sun's apparent diameter is placed at about ', so that the thickness of the above gaseous envelope would be not less than one-fourth the sun's diameter, or more than two hundred thousand miles. this coronal envelope, extending out from the solar body until gradually merged into the attenuated matter of space, has a light so feeble that it can only be clearly observed during total eclipse. professor ball ("in the high heavens") says, "the sunlight is so intense that if it be reduced sufficiently by any artifice, the coronal light also suffers so much abatement that, owing to its initial feebleness, it ceases altogether to be visible." during the great eclipse of it was photographed, and of these photographs the same author says, "one of the most remarkable features in the structure of the corona is the presence of streamers or luminous rays extending from the north and south poles of the sun. these rays are generally more or less curved, and it is doubtful whether the phenomena they exhibit are not in some way a consequence of the rotation of the sun. this consideration is connected with the question as to how far the corona itself shares in that rotation of the sun with which astronomers are familiar. i should perhaps rather have said that rotation of the sun's photosphere which, as the sun-spots prove, is accomplished once every twenty-five days. even this shell of luminous matter does not revolve as a rigid mass would do. by some mysterious law the equatorial portions accomplish their revolution in a shorter period than is required by those zones of the photosphere which lie nearer the north and south poles of the luminary. as to how the parts of the sun which are interior to the photosphere may revolve, we are quite ignorant.... we have no means of knowing to what extent the corona shares in the rotation. it would seem certain that the lower parts which lie comparatively near the surface must be affected by the rapid rotation of the photosphere; but it is very far from certain that this rotation can be shared to any great extent by those parts of the corona which lie at a distance from the sun's surface as great as the solar radius or diameter.... the corona presents a curious green line that seems to denote some invariable constituent of the sun's outer atmosphere, but the element to which this green line owes its origin is wholly unknown." the same author quotes from dr. huggins as follows: "it is interesting to read what dr. huggins has to tell us about the solar corona. the nature of this marvellous appendage to the sun is still a matter of uncertainty. there can, however, be no doubt that the corona consists of highly-attenuated matter driven outward from the sun by some repulsive force, and it is also clear that if this force be not electric, it must at least be something of a very kindred character.... so far as the spectrum of the corona is concerned, we may summarize what is known in the words of dr. huggins: 'the green coronal line has no known representative in terrestrial substances, nor has schuster been able to recognize any of our elements in the other lines of the corona.'" the account given by general myer--quoted in professor proctor's article, "the sun's corona"--of the great eclipse of , as viewed from an altitude of five thousand five hundred feet above sea-level, is as follows: "as a centre stood the full and intensely black disk of the moon, surrounded by an aureola of soft bright light, through which shot out, as if from the circumference of the moon, straight, massive silvery rays, seeming distinct and separate from each other, to a distance of two or three diameters of the lunar disk; the whole spectacle showing as upon a background of diffused rose-colored light. the silvery rays were longest and most prominent at four points of the circumference, ... apparently equidistant from each other. there was no motion of the rays: they seemed concentric." three diameters would make these rays extend two and a half million miles at least from the sun's photosphere, or even its chromosphere. the coincidence between these rays and those observed (see above) in the eclipse of must be noted, since these latter were conceived at one time to be meteor streams. as those seen in radiated from the poles, and were curved in form, while those last noted radiated at four equidistant points, none polar, and were straight, it will be seen that, if both phenomena were of the same class, they could not have been due to meteor streams. the sun's spots, which we will next refer to, are deep, relatively dark, but in fact extremely bright depressions in the photosphere. "many spots are of enormous size" (see article, "sun"); "one had a diameter exceeding fifty thousand miles, and many far larger than this have been seen. the spots are not scattered over the whole surface of the sun, but are for the most part confined to two belts between latitude five degrees and thirty degrees, on either side of the solar equator. an equatorial zone six degrees wide is almost entirely free from spots.... the inclination of the solar equator is about seven degrees.... the spots on the sun usually have a dark central region called the umbra, within which is a still darker part called the nucleus, while around this there is a fringe of fainter shade than the umbra, called the penumbra. although the umbra and nucleus appear dark, however, it is not to be supposed that they are really dark; ... though the nucleus looks perfectly black by contrast with the general surface, it shines in reality with a light unbearably brilliant when viewed alone, while his thermal measurements show that the heat from the nucleus is even greater proportionately than the light, and not very greatly below the heat of the surrounding surface.... the recognition of a nucleus within the umbra would seem to indicate that a third cloud layer (besides the outer or photosphere and a darker cloud layer beneath) exists within the second or internal layer of herschel's theory. but the observations of professor langley show that most probably all the features of the solar photosphere yet observed are phenomena of cloud envelopes, since he has been able to recognize cloud forms at one level floating over cloud forms at a lower level, while even in the (relatively) darkest depths of the nucleus clouds are still to be perceived, though so deep down that their outlines can be barely discerned." professor ball says of the heat-wave of , "as to the activity of the sun during the past summer, a very striking communication has recently been made by one of the most rising american astronomers, mr. george e. hale, of chicago. he has invented an ingenious apparatus for photographing on the same plate at one exposure both the bright spots and the protuberances of the sun.... on the th of july a photograph of the sun showed a large spot. another photograph taken in a few minutes exhibited a bright band; twenty-seven minutes later a further exposure displayed an outburst of brilliant faculæ all over the spot. at the end of an hour the faculæ had all vanished and the spot was restored to its original condition. it was not a mere coincidence that our magnetic observatories exhibited considerable disturbances the next day, and that brilliant auroras were noted." carrington's observations have shown that spots in different solar latitudes travel at different rates. "taking two parts of the visible solar surface in the same longitude, but one in latitude forty-five degrees (say), the other on the equator, the latter will advance farther and farther in longitude from the former, gaining daily about two degrees, so that in the course of about one hundred and eighty days it will have gained a complete revolution. that is to say, the sun's equator makes about two revolutions more per annum than regions in forty-five degrees north and south solar latitude." the sun is about , miles in diameter; its density is one-fourth that of the earth; its mass is , times greater, and its volume , , . gravity at its surface is . times that of the earth; its distance is approximately , , miles; it rotates upon its axis, which is inclined to the planetary plane at an angle of seven degrees, once in twenty-five and one-third days, apparently increased to thirty days by the earth's orbital advance in the same direction around the sun; and it has a motion around its center,--a true orbital motion,--due to displacement by gravity of the planetary masses, which, however, is always within its own mass. the above, in brief, is, so far as we know, the constitution of the sun and its appendages. its internal globe is surrounded by a glowing gaseous envelope, the photosphere, which is the visible orb, composed of cloud masses of glowing hydrogen gas intermingled with vapors of many of our terrestrial elements, all in a state of apparent disassociation. of the constitution of the sun's mass, professor ball says, "professor rowland has shown that thirty-six terrestrial elements are certainly indicated in the solar spectrum, while eight others are doubtful. fifteen elements have not been found, though sought for, and ten elements have not yet been compared with the sun's spectrum. reasons are also given for showing that, though fifteen elements had no lines corresponding to those shown in the solar spectrum, yet there is but little evidence to show that they are really absent from the sun. dr. huggins epitomizes these very interesting results in the striking remark, 'it follows that if the whole earth were heated to the temperature of the sun, its spectrum would resemble very closely the solar spectrum.'" outside the photosphere is the simpler chromosphere, composed largely of hydrogen, and merging into the corona at a distance of hundreds of thousands of miles from the sun's apparent surface, and this corona extends outward to a vast distance, and is itself largely composed of self-luminous matter, the action of gravity being counterbalanced by the centrifugal force of orbital rotation, or more probably by electrical repulsion. the metallic vapors in the sun's photosphere are suspended in glowing hydrogen, which vastly preponderates over all the others in mass and volume, the incandescence of which is the principal source of solar light and heat. the planets revolve in elliptical orbits around this central sun, and crossing these orbits at various angles rush streams of cometic matter and comets and meteoric bodies, in streams and clouds, which, swiftly sweeping around at various distances, are again thrown off into space. meteors constantly fall into the sun's mass, as they do upon the earth; but the grand key-note of all his life and energy, so far as we can perceive, is the vast envelope of glowing hydrogen gas. conversely, the planetary envelopes are of relatively cool oxygen mixed with nitrogen gas, which hold in suspension diffused aqueous vapors. if our own aqueous vapors are derived by the attraction of gravity from the interplanetary space, as they must have been, we can be sure that, were the sun at a sufficiently low temperature, he, too, would gather to himself a surrounding envelope of aqueous vapor, larger than our own in proportion to his mass, and larger than that of all the planets together, the combined mass of which he exceeds by seven hundred and fifty times. we should also expect similar aggregations of aqueous vapors to surround all the fixed stars in proportion to their various masses, yet we do not find aqueous vapor there, but hydrogen instead. and in the distant telescopic nebulæ we still find hydrogen and nitrogen; even in the comets we find free hydrogen in vast predominance, but not free oxygen; so that we may roughly divide the bodies of stellar space into two grand categories,--those with atmospheres of hydrogen and those with atmospheres of oxygen. it is true that the latter are limited to the planets of our own system, so far as direct observation goes, for we cannot see such dark planets as exist beyond our own solar system; but if such planets exist, as they must, for reasons stated later on, and revolve around their own central suns, we may infer, with the strength of demonstration almost, that if their suns correspond to our sun in this respect, their planets will correspond to our planets in a similar respect. but the bodies with atmospheres of oxygen are those which rotate around the sun substantially as a center, while with reference to themselves the sun is more or less a fixed body in space. it is true that our whole system is drifting through space, at present in the direction of the constellation lyra, and directly away from that portion of space occupied by sirius and canopus, with an annual motion of probably hundreds of millions of miles. professor ball ("in the high heavens") says, "in conclusion, it would seem that the sun and the whole solar system are bound on a voyage to that part of the sky which is marked by the star delta lyræ. it also appears that the speed with which this motion is urged is such as to bring us every day about , miles nearer to this part of the sky. in one year the solar system accomplishes a journey of no less than , , miles." a speed of eight miles per second gives an annual rate of , , miles. this speed, however, is greatly exceeded by many stars (as determined by displacement of the lines of the spectrum); the star no. , of groombridge's catalogue (see "in the high heavens"), has a rate of two hundred miles per second. the author says, "indeed, in some cases stellar velocities are attained which appear to be even greater than that just mentioned. we do not, therefore, make any extravagant supposition in adopting a speed of twenty miles per second," which he takes as the average. "i have adopted this particular velocity as fairly typical of sidereal motions generally. it is rather larger than the speed with which the earth moves in its orbit." the distances, of course, are equally enormous. this author says, "the nearest star, as far as we yet know, in the northern hemisphere is cygni.... i think we cannot be far wrong in adopting a value of fifty millions of millions of miles.... in the course of a million years a star with the average speed of twenty miles a second would move over a distance which was about a dozen times as great as the distance between cygni and the solar system." this assuming that the solar system is at rest, which is not the case, as the author says, "unless binary, stars do not remain in proximity, so far as we know; the general rule appears to be that of universal movement through space." this drift through space, however, no more affects the terms of the problem than the rotation of the earth upon its axis or its orbital motion affects the operations of an electric machine as the handle may be rotated to or from the direction of these motions. both machine and reservoir of energy occupying a fixed relation with reference to each other, the positions of each are the same as though absolutely fixed. this is true of gravitation, likewise, as well as of all other natural and universal forces. the fact established, then, that attenuated aqueous vapor is diffused throughout the interplanetary space occupied by our own solar system, and that it tends to surround our sun and planetary bodies with aqueous envelopes of increased density, proportionate to the action of gravity, the question arises, is there any known force which will act through such interplanetary space to decompose such aqueous vapor into its constituent elements and deposit hydrogen gas around the sun and oxygen gas around the planets, and which, while maintaining a planetary temperature such as we find on the planets, will at the same time raise the hydrogen envelope of the sun to such a temperature of incandescence that it will become a glowing sphere of heated hydrogen, in which other constituents of the sun's mass will be raised to incandescence and partially volatilized in the intense heat of that incandescent gas; in which, in fact, the phenomena of the sun will become manifest? if so, two vastly important corollaries are inevitable: first, that the fixed stars, which also shine with the light of their own glowing hydrogen, are themselves surrounded by a similar aqueous vapor, diffused through their own adjacent space, and that, in consequence, not only our own planetary distances, but all interstellar space, as far as the utmost distance of the faintest fixed stars, is likewise pervaded by the same attenuated aqueous vapor, and that this is the grand source from which is derived all solar energy, not only of our own sun, but of all the other flaming orbs of space; and, second, which is still more important to us as citizens of the universe, that each flaming hydrogen sun must have surrounding it a correlative dark planetary system of its own, and that the complement of glowing hydrogen, as an incandescent envelope of the central orb, necessitates the corresponding supplement of cool oxygen as an envelope for each of such planetary bodies; in other words, that without such planets as our system possesses, there can be no suns such as our own and the other suns we see. vast orbs might be conceived of as rotating in eternal darkness without associated satellites, but the incandescent atmosphere of hydrogen must have--not may have, but must have--subordinate planets substantially similar to ours, surrounded by atmospheres substantially similar to our own (for we find free nitrogen in comets, in meteorites, and in the faintest nebulæ), and these planets are thus fitted, so far as we can know, for the support of organic life and for the same orderly courses of nature as we see manifest around us. they must be cool, for at the planetary poles there must be a moderate temperature in contrast with the solar pole, which becomes, of necessity, highly heated; they must have an atmosphere of oxygen in order that the solar center may have an atmosphere of hydrogen; these planetary atmospheres must be supplied with nitrogen, because nitrogen is universally available, and similar causes operating under similar circumstances will produce like effects; these atmospheres must be charged with condensed aqueous vapors, and, if cool enough, must have deposited water in liquid form, for aqueous vapors when condensed by gravity are the correlated sources of supply of their respective gaseous components at both solar and planetary poles; and these planets must rotate in orderly periods around their central suns, or the aqueous vapors cannot be regularly and continuously disassociated into their elemental gases. these planets may be few or many--perhaps even a single one sometimes--for each sun, but they must be large enough or numerous enough to operate by their aggregate mass, so as to disassociate around the planets as much oxygen as their central sun disassociates of hydrogen in their combining proportions,--that is, two volumes of hydrogen for each one of oxygen. we will therefore find in such planets all the potentialities of life--we can see and study these planets, though physically invisible, as easily and as thoroughly as we do our own, for having the relationship of constitution between our own planets and our sun, we may thereby learn the essential relationship between any fixed star and its planets by directly studying the constitution of such star alone. among the planets of our own system neptune and mercury, and those which exist adjacent to their boundaries, can be studied with difficulty and uncertainty; but what astronomer doubts that they are constituted much like the other planets, and have passed, or will pass, through such stages of progress as we find apparent among those more directly under our observation? while we shall thus find universality and harmony among all the starry systems, we shall not find identity; but with the guiding light of demonstrated scientific principles, we may apply our knowledge as a key to unlock the mysteries of the most distant stars. the milky way will gleam with new meaning, sirius, aldebaran, the pleiades, will send us messages of fellowship, and the established sphere of creative energy will have expanded, with all its wondrous mechanism, to fill the universe. when we see at night a vast factory building with every window lighted, one who understands the operation and mechanism essential to the work of a mill sees not alone the illuminated windows, but the looms in motion, the flying shuttles, the spindles humming, the wheels turning, and all the complicated machinery in active operation. and he can even picture operatives at work in their various avocations, and the flashing windows, though themselves silent, are the visible index of the light within which illuminates and makes possible the work there performed. and so, when thus comprehended, the flaming stars, but points of light in the archways of the sky, themselves will reveal to us the wondrous workings within the realm which they illuminate and warm and vivify. we may also reasonably infer, as will be more fully explained further on, that there can be no actual basis for the opinion sometimes expressed, that great, dark, solid orbs--independent worlds, in fact--are drifting about through space at random, as it were, like homeless vagabonds. in these sparsely-occupied domains the head of each household, as in every well-regulated family, has all its different members gathered around in strict subordination, to aid in the support of the establishment. no sun no planets; no planets no sun, is the general statement of the sidereal formula. like a sexual duality, the mutually correlated parts constitute a single, composite, and interdependent whole: one generates, concentrates, and transmits; the other receives, transforms, and delivers. note.--regarding the absence of oxygen from the sun's atmosphere we quote the following from lord salisbury's very recent address (see note at end of chapter i.): "it is a great aggravation of the mystery which surrounds the question of the elements, that, among the lines which are absent from the spectrum of the sun, those of nitrogen and oxygen stand first. oxygen constitutes the largest portion of the solid and liquid substances of our planet, so far as we know it; and nitrogen is very far the predominant constituent of our atmosphere. if the earth is a detached bit whirled off the mass of the sun, as cosmogonists love to tell us, how comes it that in leaving the sun we cleaned him out so completely of his nitrogen and oxygen that not a trace of these gases remains behind to be discovered even by the sensitive vision of the spectroscope?" we shall find that the absence of oxygen in the solar envelope is a necessary corollary of its presence in those of the planets. the same is true, possibly, of nitrogen. ammoniacal vapors are decomposable into hydrogen and nitrogen, and hydrocarbon gases into hydrogen and carbon, just as aqueous vapors are resolvable into hydrogen and oxygen. in the earlier stages of the earth's development we have abundant evidence of an atmosphere heavily laden with carbonic vapors, which have disappeared, to remain stored as fixed carbon, and the oxygen has also largely disappeared, to constitute the enormous mass of oxides in the earth's mass, while the nitrogen remains to dilute the remaining oxygen and constitute the air we breathe. their common correlative, hydrogen, intermingled with metallic vapors, composes the vast atmosphere of the sun. chapter iii. the mode of solar energy. but is there such an available force? there is one, and only one,--electricity, when properly generated and suitably applied. it is an axiom of electrical science that any fluid which will at all conduct a current of electricity can be decomposed by a current of electricity. (see urbanitsky's work, "electricity in the service of man," cassell's edition, page .) it is there stated (page ), "we have frequently had occasion to mention certain chemical effects of electricity,--namely, the decomposition of gaseous compounds into simple gases." page , "whatever the substances we expose to the action of the galvanic current, decomposition takes place proportional to the strength of the current." page , "hydrogen is always evolved at the negative pole of the battery and oxygen at the positive pole. the gases can then be collected in different tubes, the hydrogen tube receiving twice as much gas as the oxygen tube; since water consists of two volumes of hydrogen and one volume of oxygen, it follows that the galvanic current decomposes water into its constituents. as chemically pure water has so great a resistance as almost to force us to consider it a non-conductor, it is generally acidulated with sulphuric acid. the smallest amount of acid diminishes the resistance considerably. the silent discharge is far more effective in bringing about this transformation than the spark discharge." page , "gases are bad conductors of electricity; if it had been otherwise, we should never have become acquainted with electricity, as it would have been conducted away by the air as fast as it was generated. the vacuum also does not conduct electricity, but moist air becomes a partial conductor. moist air also will spoil the insulation of non-conducting supports. all bodies are more or less hygroscopic, and the moisture condensed on their surfaces thus turns the best insulators into conductors. change of temperature also influences conductivity." page , "when using induction machines, the moisture of the air often causes experiments to fail, especially before large audiences. the atmosphere becomes saturated with moisture, and it is often impossible to get the machine in working order." several desiccating devices are mentioned by the authors of this work, as used with such machines, to prevent such dissipation or conduction of electricity from the machine into space by the aqueous vapor of the atmosphere. in describing the aurora borealis (page ), these authors say, "the rarefied air is nearer the earth at the poles than the equator, in consequence of the earth's centrifugal motion, and, the earth being negatively electrified, negative electricity will flow from this point, directed against the positively electrified upper layers of rarefied air." same work, pages , , "the resistance (in liquids) diminishes as the temperature increases, a result which is exactly opposite to what occurs with metals. conductivity for carbon increases with the temperature, thus agreeing with the action of liquids." page , "to determine the resistance in liquids, the above methods cannot be employed, liquids being decomposed by the electrical current." referring to the voltaic arc and the spark of the induction apparatus (page ), it is said, "dry air under great pressure offers a high resistance, but a perfect vacuum is a perfect insulator, and between these extremes there are degrees of rarification which admit of a flow of electricity." in general, it is said that electrical decomposition requires that the electrolyte be in liquid form, but this is not universally true, and throughout interplanetary space may not be true at all. in ferguson's work on electricity, it is stated that, "the passage of electricity through compound gases in a state of great rarity, as in the so-called vacuum tubes, frequently separates them up into their constituents." so, also, the opinion that electricity cannot be readily conducted through dry gases is refuted by the play of the auroral streamers. the distance from the surface of the earth of these electrical waves and the auroral arch is variously estimated at from seventy to two hundred and sixty-five miles, and in one instance "at a height of from four thousand to six thousand miles;" see article in appleton's cyclopædia. certainly there could be no sensible moisture at the temperatures there prevalent, and especially at night and during the fall and winter months when these displays are very frequent. whether the currents be due to induction, as between neighboring bodies one of which is electrified, or from direct emission, as in brush discharges, there must obviously be some medium of contact and continuity for the free transference of electrical energy through space. regarding the rationale of electrolysis ("electricity in the service of man"), after discussing certain other theories, the authors say, "clausius, too, assumes an electrified condition of the molecules of each electrode, but he neither attributes to the galvanic current the force of direction nor power of decomposing. he points out that both the molecules of fluids and also their atoms are in continual motion. the atoms in molecules of fluids are held together but by a moderate force, and the molecules themselves constantly undergo changes both of synthesis and analysis. the galvanic current merely effects a regulated motion of the atoms; the positive ions are attracted by the negative electrode, and the negative ions by the positive electrode, and by this means are separated out from the liquid." page , "the upper layers of air are more or less electrified, so as to have a potential differing from that of the earth, but how their electrical condition has been produced is not at present known. condensation of water-vapor is supposed to produce electricity. close to the earth the air has little or no electricity; the farther from the earth the greater the amount of electricity in the air." referring to the sparking discharge, it is said, page , "the density of the air, however, has to be taken into account; the sparking distance is lessened in denser air, and becomes greater when the atmospheric pressure is diminished. not only the density, but also the chemical composition of the medium influences the sparking distance. faraday found the distances considerably less in chlorine gas, but twice as long in hydrogen gas as in air." page , "the sparking distance increases at a somewhat greater rate than the difference of potential of the discharging bodies.... when the sparking distance becomes very great ... it is proportional to the difference of potential." page , "there is a difference of potential between the earth and points in the air above. in fine weather the potential is higher the higher we go, increasing usually at the rate of twenty to forty volts for each foot." it will be seen that, continued upward at this rate, the increased electrical pressure for each mile of elevation would be between , and , volts, or for each one hundred miles more than , , volts; and at an altitude of one thousand miles, if carried so far, the potential would be between one and two hundred million volts, an electrical pressure quite inconceivable to us. such a potential in currents of enormous quantity continually flowing from the earth to the sun would certainly decompose any aqueous vapors condensed around these bodies. but the question at once arises, what reason is there to suppose that such currents could possibly flow between the earth and the sun, across that vast intervening region of space, a distance of more than , , miles? and would not the resistance to such currents in transit be so enormous that the entire potential, however great, would have been practically lost long before reaching the sun? to this there is a complete and irrefutable answer, not based upon any abstract theory, but upon established fact. it is an absolute certainty that electrical currents of enormous quantity and high potential are constantly passing between the earth and the sun, and that these currents have so free a passage--far more free than through any metallic circuits that we know of--that they pass over this enormous distance absolutely without appreciable resistance. we may note in this connection the well-known facts, now being largely utilized, though the art is still in its infancy, of telegraphing and transmitting all sorts of electrical currents over large distances without wires or any conductors, except those furnished by nature. of the currents between the earth and the sun, professor proctor, in his "light science for leisure hours," says, "remembering the influence which the sun has been found to exercise upon the magnetic needle, the question will naturally arise, has the sun anything to do with magnetic storms? we have clear evidence that he has. on the st of september, , messrs. carrington and hodgson were observing the sun, one at oxford and the other in london. their scrutiny was directed to certain large spots which at that time marked the sun's face. suddenly a bright light was seen by each observer to break out on the sun's surface and to travel, slowly in appearance, but in reality at the rate of about seven thousand miles in a minute, across a part of the solar disk. now, it was found afterwards that the self-registering magnetic instruments at kew had made at that very instant a strongly-marked jerk. it was learned that at that moment a magnetic storm prevailed in the west indies, in south america, and in australia. the signal men in the telegraph stations at washington and philadelphia received strong electric shocks; the pen of bain's telegraph was followed by a flame of fire; and in norway the telegraphic machinery was set on fire. at night great auroras were seen in both hemispheres. it is impossible not to connect these startling magnetic indications with the remarkable appearance observed upon the sun's disk. but there is other evidence. magnetic storms prevail more commonly in some years than in others. in those years in which they occur most frequently it is found that the ordinary oscillations of the magnetic needle are more extensive than usual. now, when these peculiarities had been noticed for many years, it was found that there was an alternate and systematic increase and diminution in intensity of magnetic action, and that the period of the variation was about eleven years. but at the same time a diligent observer had been recording the appearance of the sun's face from day to day and from year to year. he had found that the solar spots are in some years more freely displayed than in others, and he had determined the period in which the spots had successively presented with maximum frequency to be about eleven years. on a comparison of the two sets of observations it was found (and has now been placed beyond a doubt by many years of continual observation) that magnetic perturbations are most energetic when the sun is most spotted, and vice versa. for so remarkable a phenomenon as this none but a cosmical cause can suffice. we can neither say that the spots cause the magnetic storms nor that the magnetic storms cause the spots. we must seek for a cause producing at once both sets of phenomena." it will be observed that the phenomena seen in the sun were marked at the same instant by violent electric perturbations on earth. hence something must have passed with the velocity of light, which we know to be at the rate of , miles per second, or in about eight minutes from the sun to the earth. but it is stated in "electricity in the service of man," page , that, "according to the theoretical calculations of kirchhoff, as well as of ayrton and perry, the velocity of electricity in a wire without resistance would be equal to the velocity of light." hence we perceive that the apparent difficulty has vanished in the light of observed fact, and that currents of electricity do pass and are constantly passing between the earth and the sun without the slightest loss of speed,--that is to say, without resistance. we shall find in the sequel that the above phenomena were caused most probably by a partial interruption of a constant direct current from the earth to the sun, instead of by an opposite return current from the sun to the earth. in further illustration of the above facts we quote the following, page , "electricity in the service of man:" "many attempts have been made to find a connection between the spots and prominences in the sun and the electrical phenomena on the earth. professor forster says that by numerous magnetic observations of the last thirty or forty years it has been proved that the formation of black spots on the surface of the sun, and the generation of pillars and clouds of glowing gases in the immediate neighborhood of the sun, stand in close connection with certain deviations in direction and intensity of the earth's magnetic forces." professor proctor, in his "light science for leisure hours," says, "from all this it appears, incontestably, that there is an intimate connection between the causes of auroras and those of terrestrial magnetism.... the magnetic needle not only swayed responsively to auroras observable in the immediate neighborhood, but to auroras in progress hundreds and thousands of miles away. nay, as inquiry progressed, it was discovered that the needles in our northern observatories are swayed by influences associated even with the occurrence of auroras around the southern polar regions.... could we only associate auroras with terrestrial magnetism, we should still have done much to enhance the interest which the beautiful phenomenon is calculated to excite. but when once this association has been established, others of even greater interest are brought into recognition; for terrestrial magnetism has been clearly shown to be influenced directly by the action of the sun.... we already begin to see, then, that auroras are associated in some mysterious way with the action of the solar rays. the phenomenon which had been looked on for so many ages as a mere spectacle, caused perhaps by some process in the upper regions of the air of a simple local character, has been brought into the range of planetary phenomena. as surely as the brilliant planets which deck the nocturnal skies are illuminated by the same orb which gives us our days and seasons, so are they subject to the same mysterious influence which causes the northern banners to wave respondently over the starlit depths of heaven. nay, it is even probable that every flicker and coruscation of our auroral displays correspond with similar manifestations upon every planet which travels round the sun." in professor ball's late work, "in the high heavens," the author says, "dr. schuster suggests that there may be an electric connection between the sun and the planets. in fact, with some limitations, we might even assert that there must be such a connection. it is well known that great outbreaks on the sun have been immediately followed, i might almost say accompanied, by remarkable magnetic disturbances on the earth. the instances that are recorded of this connection are altogether too remarkable to be set aside as mere coincidences. dr. huggins has not referred in this connection to hertz's astonishing discoveries; but it seems quite possible that research along this line may throw light on the subject, at present so obscure, of the electric relation between the sun and the earth." of this common electrical relationship between our sun and the different planets, and of these with each other, professor proctor says, in his article, "terrestrial magnetism," "interesting as are the bonds of union which copernicus and kepler and newton have traced in the relations of our system, it would seem as though we were approaching the traces of a yet more wonderful law of association. we see the earth's magnetism responding to the solar influences, not merely in those rhythmic motions which belong to the periodic variations, but in sudden thrills affecting the whole framework of our globe. the magnetic storms which are called into action by such solar disturbances as the one of september, , are, we may feel sure, not peculiar to our own earth. the other planets feel the same influence,--not, perhaps, in exactly the same way, but according to the constitution and physical habitudes which respectively belong to them. so that one can scarce conceive a subject of study at once more promising and more interesting." of these prophetic shadows which science often seems to cast before, professor nichol, in his "architecture of the heavens" (referring to sir william herschel), says, "without difficulty or pretence he there casts aside an idea which had not been questioned before, unless in a few of those obscure, indefinite speculations which, strangely enough, often prelude important discoveries." these facts are thus incontestably established: that electric currents of enormous energy and vast quantity are constantly passing without appreciable resistance and with the speed of light between the earth and the sun; that such currents cannot be conducted through vacua, or through dry gases, or through a dense medium; and that, whatever other matter may exist in the intervening space, such space is pervaded throughout by an attenuated vapor of such constitution and density that it will transmit such electrical currents with the highest conceivable efficiency. we know that such passage of these currents cannot depend upon the ether of space which is acted upon by the sun to produce the ethereal undulatory vibrations of light and heat, for, after we have produced the most perfect vacuum possible, we find that the rays of light continue to pass through it as freely as they pass through space, while currents of electricity cannot be made to pass at all. hence we know to a certainty that the medium which transmits these enormous currents of electricity must be a vapor capable of conducting electricity, that it must hence be decomposable by the electric current, and that when decomposed one of its elements must consist of hydrogen gas and the other of oxygen; in other words, that this conducting medium must consist of attenuated aqueous vapor, commingled doubtless with other vapors which themselves, like the acid of the acidulated water used in electrolysis, aid in the conduction of these enormous currents. we also know that such vapors in space will be necessarily attracted, by gravitation, around the solar and planetary bodies immersed therein, and must form condensed vaporous atmospheres or cloud masses, and if these are decomposed by the passage of such currents of electricity, that hydrogen gas will be liberated at the solar galvanic pole and oxygen at the terrestrial or other planetary pole, precisely as we find to be the case in nature. will such gaseous envelopes, then, have the same temperature for each gas when thus liberated, or will the hydrogen envelope of the sun be heated to incandescence, due to the passage of the electrical current? the temperature of interplanetary space is probably very low. of this professor ball says, "what this may be is a matter of some uncertainty, but from all the evidence available it seems plain that we may put it at not less than three hundred degrees below zero;" and the same author adds, "the temperature is taken to be sixty-four degrees below zero, being presumably that at the confines of the atmosphere." whatever the temperature of space, or its variations, may be, the passage of the planetary electricity through the condensed hydrogen envelope of the sun will produce great changes in the heat of that body and of the solar core within. while with a small electrolytic apparatus we find no special differences of temperature in the gases, with large quantities of electricity, driven at a high potential, we find that a new and startling result ensues. something of this sort is seen in the operation of electric arc-light lamps, now in common use, in which two slightly separated carbon points are traversed by a current of considerable potential. the current is driven across the intervening space between the points, carrying with it an atmosphere of disintegrated carbon, through which the electricity is carried at its highest speed, and a most brilliant light is produced. in "electricity in the service of man," page , it is said, "we may conclude from this that the current does not cease when the arc of light is formed. the resistance of the arc seems to be only very slight; in fact, the current must be conducted by it." of the structure and constitution of the luminous electrosphere, or arc, produced in these lamps, "professor j. a. fleming," says the scientific american, "has shown that the well-known color of the light of the electric arc from carbon points is due to the incandescence of the carbon filling the space between the positive and the negative rods. the true arc is here, and exists in a space filled with the vapor of carbon, which has a brilliant violet color. examined by the spectroscope, the central axis of the carbon arc gives a spectrum marked by two bright violet bands. outside this is an aureole of carbon vapor of yellow or golden color. the electrical strain of the arc occurs chiefly at the surface of the crater which forms at the end of the positive rod, where, in fact, the principal work of generating light is done; for eighty per cent. of the total light of the arc comes from the incandescent carbon at this place. thus, in a sense, the arc light is mainly an incandescent light, the effect being produced by the layer of carbon which is being constantly evaporated at an extremely elevated temperature. hence the light of the carbon arc is not, and can never be, white, as it is sometimes described as being, but must always be tinted violet by the carbon vapor normally present between the rods." the significance of the above-quoted extract will be readily perceived when we come to consider the action of the direct planetary electrical currents upon the solar envelope, the effects in both cases being substantially identical. the quantity and intensity of the electric current, as it passes through the incandescent arc to the negative pole, and thence back to the dynamo, are diminished exactly in proportion to the energy expended in the generation of the light and heat of the arc. it is precisely the same as in the operation of a turbine water-wheel; if working at its highest efficiency, the discharged water is almost deprived of force: its gravity has been converted into work. in the electric light this conversion is only partial, owing to atmospheric and other conditions; but in the case of the solar envelope and its core, it is nearly, if not altogether, perfect, so that the currents of electricity are almost entirely converted into light and heat, or expended in the electrolytic decomposition of the surrounding aqueous vapors, and do not reappear as electricity, but as converted solar energy. brilliant, however, as the light rays are in a powerful arc lamp,--perhaps the nearest to solar light we can produce,--the obscure heat rays are far more numerous and powerful. on page of the work just cited a table is given, showing the proportion of visible and invisible rays emitted by different illuminants, and with the electric lamp, even, ninety per cent. of all the rays emitted by the voltaic arc are heat rays, which are obscure and invisible. but the startling effects of electricity of large quantity and high potential, in the decomposition of water, are far more strikingly exhibited by an apparatus shown in at the chicago exhibition by a firm from brussels, and which is described in the electrical review as follows: "an ordinary wooden pail is three-quarters filled with water slightly acidulated; a lead plate about nine inches broad by sixteen inches long dips to the bottom of the pail and is connected to an incandescent dynamo machine capable of giving over one hundred and fifty ampères. the iron rod, or article to be heated, is connected to the pole of the dynamo and simply dipped into the water; it immediately becomes heated and rapidly rises to a melting temperature; only that portion of the metal completely immersed becomes heated, and the heating is so rapid that neither the water nor that portion of the metal out of the water becomes very warm. wrought iron and steel actually melt if long enough held under water. a carbon rod subjected to this process becomes amorphous carbon, proving that a temperature of at least four thousand degrees centigrade has been reached, and it is stated that with two hundred and twenty volts' pressure a temperature of eight thousand degrees centigrade has been reached. there are various theories to account for this phenomenon, but from close observation it appears to be a case of arc heating. the moment the metal is plunged into the water it is enveloped in hydrogen gas decomposed from the water. this envelope of gas parts the water and metal, forming an arc, which raises the surrounding gaseous envelope to an enormous temperature; the metal surrounded by this arc is almost immediately raised to the same temperature. a flame of burning hydrogen appears around the metal on the surface of the water. the principle of the method is the same as that on which the burning of an arc light between two carbon points under water depends. an arc lamp will burn quite steadily under water if the connections are made water-proof; the arc itself requires no protection." it will be seen that the process above described is precisely analogous to that involved in the problem of the sun's energy. the planets correspond with the leaden plates, upon which oxygen is disengaged from the water, while at the same moment the liberated hydrogen necessarily appears at the opposite pole. the generation of hydrogen gas forms an envelope or atmosphere of hydrogen around the sun which forces back the aqueous vapor. the current, in passing through this gaseous envelope to the metal core within, intensely heats the hydrogen, which rapidly communicates its rising heat to the central core. if this core is composed of metals, and the temperature be raised sufficiently high, which only depends upon the quantity and working pressure of the electricity employed, the metal core will be volatilized in whole or in part, and, if of mixed metals, we will find the presence of these elements revealed in the spectroscopic lines corresponding thereto, and the flames and flashes of hydrogen at the surfaces beyond the envelope, at the surface of contact with the matter of space, will be also seen. in fact, such an experiment, properly prepared, could be made to show roughly most of the phenomena of solar light and heat as they actually appear, such as sun-spots, prominences, jets, plumes, faculæ, the photosphere, chromosphere, absorption bands, vortical disturbances, metallic vapors, and the complete solar spectrum, with the different fraunhofer lines. in the case of the sun, these currents must be measured by millions of ampères, and possibly by hundreds of millions of volts, instead of by mere hundreds, while the hydrogen envelope extends outward from the sun's surface hundreds of thousands of miles until, perhaps, finally merged into the corona. as the currents pass from the planets and planetoids (for not only the larger planets, but all the planetary bodies of our system must contribute, if any of them contribute) to the sun, or rather to the sphere of its electrical action, without resistance, so long as these planets generate constant currents of the same, or nearly the same, potential, so long will the sun maintain his constant light and heat; if these are increased or diminished, the sun's light and heat will be temporarily, but only temporarily, increased or diminished; and this process must continue, without further loss or change, indefinitely into the future. whatever the sun may gain by increment of meteoric masses may pass for what it is worth, but the gradual contraction of his volume cannot proceed while his present temperature is maintained by the passage of such currents,--that is to say, his light and heat will remain constant, and also his mass and volume, so long as the electric currents which pass from the planets to the sun and the constitution of space which surrounds the sun and planets themselves remain constant. it now remains to consider how such enormous currents of electricity can be generated and maintained. we know, of course, that chemical changes cannot operate to produce them. they must be derived from something contained in or diffused through interplanetary space, and the planets themselves must be the means by which such currents of electricity are brought into effective operation. on our own earth we have many kinds of mechanically-constructed electrical apparatus which generate electricity, to use a popular expression, or which, more properly, separate the opposite potentials from an unstable electrical tension or equilibrium of the matter of space. these machines practically take positive electricity from the mutually-balanced electric potentials of which the earth and its surrounding gaseous envelope are the vast common storehouse, in such manner that the positive electricity thus drawn out from and again passing into the common storehouse shall, during such transit, be compelled to pass through channels which will cause it to do work, at the expense of its potential or pressure, during its passage, or in which electricity is raised in its electro-motive force from a lower to a higher potential or pressure, just as the pressure of water is increased when delivered from a greater or a still greater height, or steam, when confined in space under higher and still higher temperatures. but none of these machines actually generate electricity ab initio; they merely put into effective operation the pre-existing force. the mass of the earth is of irregularly negative polarity, the air above is positive, and as we ascend, the potential, or voltage, or pressure increases at a nearly uniform rate of from twenty to forty volts for each foot. the earth is thus surrounded by an electrosphere as well as an atmosphere, and the two are not coincident, for while the pressure of the atmosphere diminishes as we ascend, that of the electrosphere increases. the moon, too, and each planet must have its electrosphere, and around the sun's core we can see the solar electrosphere in its visible glory. thus, all our planets rotate upon their axes and revolve around the sun, each surrounded by an enormous electrosphere, just as an electrical induction machine is surrounded, when in operation, with an electrosphere of its own, and which, by breaking connection with the conductor which carries away its current, becomes, when shown in a darkened room, clearly visible. in "electricity in the service of man" it is said, page , "the inductive action of the machine is quite as rapid and as powerful when both collectors are removed and nothing is left but the two rotating disks and their respective contact or neutralizing brushes. the whole apparatus then bristles with electricity, and if viewed in the dark presents a most beautiful appearance, being literally bathed with luminous brush discharges." this is a true aurora. let us now examine some of these more recent electric machines,--the later induction, not the older frictional machines, for it is obvious that the rotation of the planets, if they operate as electric generators, or separators, must act by induction and not by friction. the frictional machines are of the old type and are well known from the books; in these a glass disk or cylinder is rubbed upon in its rotation by an amalgamated (so called) friction pad fixed securely to the bed of the machine. but more recently these have been replaced by far more powerful and simple machines which operate entirely by induction, like approaching thunderclouds, for instance, and in which one or more glass disks are merely rotated rapidly and freely in the air, these disks having a number of light metallic sectors, such as bits of tin-foil, pasted on their outer sides at equal radial intervals, and with metallic collecting brushes which, however, barely graze the surfaces of the rotating disk. there is no pressure and no friction, except that of the disks as they freely revolve in the atmosphere. in the above-quoted work, page , is a description of wimshurst's influence machine, one of the most recent and most powerful, which we condense as follows: this machine was produced about . it consists of two circular disks of thin glass fourteen and one-half inches in diameter in the sample described, attached at their centers to loose bosses, so as to be rotated by cords and pulleys operated by a handle, in opposite directions. the disks rotate parallel with each other and are not more than one-eighth of an inch apart, and have their surfaces well varnished; and attached by cement to their outer surfaces are twelve or more radial, sector-shaped plates of thin brass- or tin-foil, disposed around the disks at equal distances apart. these sectors take the place of the "inductors" of holtz's instrument, and appear to act also as carriers, though the exact nature of their action is somewhat mysterious. it appears, however, probable that those acting for the time as carriers on the one disk act at the same time as inductors on the other. the two sectors on the same diameter of each disk, at opposite sides of the center, are twice in each revolution momentarily placed in metallic connection with one another by means of a pair of fine wire brushes attached to the ends of a bent metal rod loosely pivoted at the center of each disk, the metal sectors just grazing the tips of the wire brushes as they pass. there is one of these bent rods on the outside of each disk, and their position as pivoted on their center can be varied at will, both with reference to the one on the opposite side and to the position of the fixed collecting combs. the efficiency of the machine varies with their position, and the maximum appears to be generally when the brushes touch the disks on diameters crossing the position of the collecting combs at about forty-five degrees, and with the bent rods on opposite sides at right angles to each other. the collecting combs are simple forks with collecting points turned inward, which forks embrace the opposite sides of the disks outside, which freely rotate between them, and they are supported on insulated posts. these supports may be small leyden jars or condensers, with discharging knobs, or may be connected with similar condensers at a distance, or arranged in batteries or otherwise. the presence of the collecting combs is not necessary to the operation of the machine, their sole function being to carry away the positive electricity as generated. the machine is self-exciting, and it is believed that the initial action must be due to friction in the layer of air contained between the plates, which, as above stated, are only about one-eighth of an inch apart. it is nearly independent of atmospheric conditions, and not liable to reverse its polarity, as are the voss machines. the voss machine uses a larger glass disk which does not rotate, but is fixed, and which has a central opening three inches wide, with a different arrangement of tin-foil disks or sectors, and a smaller glass disk rotates parallel with it. the holtz machine is somewhat similar, using a single rotating, well-varnished glass disk revolving opposite a well-varnished larger disk, the latter provided with three sector-shaped openings or windows, with varnished paper inductors or flaps passing through these windows so as to touch the revolving disk. there are also two series of fine metal points held by brass bars provided with insulated handles and discharging knobs. it is only necessary to give a general idea of the construction and operation of such machines, as their specific construction can be readily learned from the books. of the mode of operation, however, it is said, "what takes place when the machine is in action is of a very complicated nature, and can hardly be said to be perfectly understood." with a wimshurst machine having disks of a diameter of fourteen and one-half inches "there is produced under ordinary atmospheric conditions a powerful spark discharge between the knobs when they are separated by a distance of four and one-half inches, a pint size leyden jar being in connection with each knob (one on each opposite diameter of the two disks), and these four-and-one-half-inch discharges take place in regular succession at every two and a half turns of the handle. it is usual to construct the machine with small leyden jars or condensers attached to conductors, by which the spark is materially increased. a machine has been constructed with plates seven feet in diameter, which, it was believed, would give sparks thirty inches long; but no leyden jars have been found to withstand its discharge, all being pierced by the enormous tension." three of toepler's induction machines (see page , "electricity in the service of man"), connected together, gave a current which maintained a platinum wire one-fifth of a millimeter thick continually at a red heat, and was also capable of decomposing water. chapter iv. the source of solar energy. the remarkable resemblance between the mode of operation and effects of these electrical induction machines and the vast rotating electrosphere of the earth must be at once apparent. the operation is precisely the same, and the results must, pari passu, be substantially similar. we need not seek for precise parallelism of structure, because these machines themselves, it has been shown, widely differ in structure among themselves. but the almost infinitely more vast terrestrial electrosphere, which cannot be less than ten thousand miles in diameter, and perhaps much more (if we may form an opinion from the relative magnitude of the field of action of the hydrogen envelope which constitutes the solar electrosphere), rotating in the attenuated vapors of space, among which vapors that of water plays a most important part, and which vapors constantly impinge with various disturbances of contact against the more and more attenuated layers of the terrestrial atmosphere, and which gradually, from within outward, less and less partakes of the earth's rotation until, finally, its rotatory movement is lost in the vast ocean of space, establishes the certainty that enormous quantities of electricity must there be disengaged, precisely as in the machines which we have described, and to learn the potential or active pressure of this electricity we have only to consider the fact that we find a rise so rapid, as we ascend through our atmosphere, that the potential increases by from twenty to forty volts for each foot. that these currents are transmitted to the sun without appreciable resistance we already know, and that they are there transformed into light and heat we can, from the previously cited experiments, see. but it may be urged that the resistance of such attenuated vapors in space, and the generation of electricity in such quantities, would inevitably retard and finally destroy planetary motion. the sufficient answer to this is found in the consideration that the same facts must exist under any possible mode of organization of our solar system, and that such interference, besides, must have absolutely prevented its formation at all, if such were the case. all the matter of our planetary system together is only one seven-hundred-and-fiftieth that of the sun; if this were added to the sun's bulk it would but slightly enlarge it. but all this solar and planetary matter together, if distributed over the space occupied by our planetary system,--and, by the nebular hypothesis of the organization of our solar system, this is requisite,--and having an axial diameter one-half that of its equatorial (see proctor's "familiar essays on scientific subjects,"--"oxygen in the sun"), would have had a density of only about one four-hundred-thousandth that of hydrogen gas at atmospheric pressure. this nebular mass must have had a diameter at least sixty times that of the distance of the earth from the sun and a depth of thirty times its distance. that this enormous mass of attenuated matter should ever have been made to rotate as a whole by any force of attraction, repulsion, or rotation, with a tenuity so great that, if measured by an equal volume of hydrogen gas,--the lightest substance known to us,--it would have furnished material for four hundred thousand such systems as ours, presupposes a resistance so slight that the planets themselves, when coagulated out of such a mass, could never in any conceivable time exhibit retardation from such a source; and we know to a certainty that such attenuated vapors do exist in space, for electricity cannot be transmitted through a vacuum, and it is transmitted with perfect freedom between the earth and the sun. but it may be said that the laws were then different. if they were different then, they are doubtless different now. if, on the other hand, we assume that the bodies of which our solar system is composed were simply aggregated into concrete masses from meteoric dust, the difficulty is not lessened; for if the resistances to their operation now are such as to perceptibly retard their motions, they must have operated still more powerfully to originally prevent them; while, if hurled forth by an almighty fiat, complete from the hand of creative energy, the same force which impelled them forward must have also established the laws under which they now move. it is calculated that our earth must be losing time, by tidal retardation, at the rate of one-half the moon's diameter in each twelve hundred years (see proctor, "light science for leisure hours,"--"our chief timepiece losing time"), and that "the length of a day is now more by about one eighty-fourth part of a second than it was two thousand years ago." perhaps, however, we may discover that these changes are themselves periodic and increase in cycles to a maximum, and then diminish, as is the case with magnetic, planetary, and stellar variations, and other similar changes, when sufficiently long observed; for while such changes may very well accompany a theory under which our system and all other systems are slowly running down to decay and death, it is entirely incompatible with the primal forces under which they must have been originally formed. in other words, if the tides are dragging back our earth without compensation, this dragging back can only come from the oceanic deposit of water on the earth from the aqueous vapors of space which do not partake of the planetary rotation and orbital movement of the earth. but if these can now retard the earth's motion, they must have originally prevented it in the beginning. this loss of time is, moreover, merely inferential from mathematical computations, and its basis is found in the belief that all the operations of nature are in a slow process of degradation, and the calculated loss itself may be merely theoretical, and not true in fact. professor proctor himself concedes the uncertainty of this alleged retardation when he says in the same article, "at this rate of change our day would merge into a lunar month in the course of thirty-six thousand millions of years. but after a while the change will take place more slowly, and some trillion or so of years will elapse before the full change is effected." while the processes of nature are generally believed to be running down, everything is bent to that belief; but the forces of nature must, nevertheless, be uniform and supreme, for it is by these forces that the expected results are to be achieved. that changes occur constantly is inevitable, but the source of these must be looked for in the interaction of original forces, and not in the degradation of systems. there is reason to believe, in fact, that the repulsion of the terrestrial electrosphere by that of the moon may itself be sufficient to counteract such retarding force of lunar gravity, for the tides upon earth are not merely oceanic, but atmospheric, and on the latter the electrical repulsion of the moon must act very powerfully and with directly counteractive effect. let us now apply the preceding principles to the problem under review. all planetary space is pervaded with attenuated vapors or gases, among which aqueous vapor occupies a leading place. the planets and all planetary bodies, having opposite electrical polarity from the central and relatively fixed sun, by their orbital motions around and constant subjection thereto act as enormous induction machines, which generate electricity from the ocean of attenuated aqueous vapor, each planet being surrounded by an enormous electrosphere, carried with the planet in its axial and orbital movements, the successive atmospheric envelopes gradually diminishing in rotational velocity until merged into the outer ocean of space. as the planets advance in their orbits they plunge into new and fresh fields, and, as the whole solar system gradually moves onward through space, these fields are never re-occupied. these electrospheres, by their rotation, generate enormous quantities of electricity at an extremely high potential,--so high that we can scarcely even conceive it,--and this electricity flows in a constant current to the sun, where it disappears as electricity, to reappear in the form of solar light and heat. these planetary currents also flow towards such other negatively electrified bodies as may exist in space--the comets and fixed stars, for example--in proportion to their distance; for, since resistance is not appreciable between ourselves and the sun, as is also the case with light, so, like light, our electricity must pass outward as well as inward to take part in the harmonious operations of the whole universe. but it should be noted that the distribution of electric energy in the form of currents is quite different from that of light or other radiant energy; for while light is diffused from a center outward through space, electric currents, on the contrary, are concentrated and directed along lines of force to concrete centers of opposite polarity. as a consequence, the intensity of light decreases according to the squares of the distances traversed plus the resistance to the passage of the light itself, while the electric current is only diminished by the resistance of the medium through which it passes. as the light of the sun has a velocity of one hundred and eighty-eight thousand miles per second, and the electric current between the earth and the sun the same, it will be seen that the resistance is practically alike for these two forms of energy. indeed, the striking resemblance between the ethereal vibrations which constitute light and heat and exceedingly rapid alternating currents of electricity through molecular media may suggest that the transformation of one force into the other is some sort of a "step-up" or "step-down" process, much higher in degree, but of the same character as the well-known analogous electrical transformations used in the arts. it should also be borne in mind that, while the intensity of light diminishes according to the above law, the quantity remains the same, less resistance, as the area covered increases precisely in the same proportion as the intensity diminishes,--that is, in the ratio of squares. around the earth and other planets gravity attracts the aqueous vapors in increased density, the same as around the sun; but the electric currents passing between the planets and the sun decompose this aqueous vapor into its constituent gases, hydrogen and oxygen. the oxygen is deposited within the positive electrospheres of the planetary bodies, where it mingles with nitrogen to form our atmosphere and those of the other planets. in this float the aqueous vapors condensed from space, which are lighter than air. (see tyndall, "the forms of water:" "it also sends up a quantity of aqueous vapor which, being far lighter than air, helps the latter to rise.") these aqueous vapors, condensed into clouds and precipitated upon the earth, form our oceans and their affluents. the hydrogen gas disengaged upon the sun's surface forms a similar envelope, which is penetrated by the planetary electric currents, and is thus highly heated and rendered incandescent; the glowing hydrogen transmits its heat to the sun's mass within, which is thus raised to, and permanently maintained in, a liquid or densely gaseous state, its metallic constituents being volatilized in part, and these metallic vapors mingle with the lower strata of hydrogen to form the sun's photosphere, while, above, the glowing hydrogen grows more pure, and finally, at a distance of hundreds of thousands of miles, is merged into the corona, which is composed, in part at least, of cosmical dust rotating around and repelled by the sun, and which shines partly by reflected light, partly by that of the relatively cooler hydrogen, and partly, perhaps, by electrification of its constituents by the powerful currents passing through it. each of the planetary bodies, large or small, takes its proportionate part in the generation and transmission of electricity, according to its volume, mass, and motion. as an adjunct to this electrical sequence we have learned that any interruption of such currents between the generator and the receiver will cause the generating apparatus to glow with diffused electrical light, as is the case with the wimshurst machine already described. when such connection is removed, it is said, "the whole apparatus bristles with electricity, and if viewed in the dark presents a most beautiful appearance, being literally bathed with luminous brush discharges." such a phenomenon recalls at once the aurora borealis; and when we find this as a sequence of the electrical storm of the first of september, , before described ("at night great auroras were seen in both hemispheres"), and connect with this the persistence of electricity upon insulated surfaces (see "electricity in the service of man," page : "glass being a bad conductor, the electricity does not spread all over the plate, but remains where it is produced"), we shall inevitably conclude that there was some partial interruption in the current flowing from the earth to the sun at that moment; and if we recall that at that very instant "suddenly a bright light was seen by each observer to break out on the sun's surface and to travel across a part of the solar disk," we shall learn that the processes connected with the production of such a bright light will interrupt in part the terrestrial current. we can readily understand that if this bright light exceeded in electrical intensity that due to the earth's current, it might temporarily reverse the polarity of the afferent current or retard its flow, like the so-called "backwater" of a mill. it would be like attempting to discharge steam at sixty pounds' pressure into a vessel filled with other steam at sixty-one pounds. whence, then, came this bright light? perhaps from the conjoint action of some other planet, perhaps from sudden chemical disassociation beneath the surface, perhaps by the abnormal piling up of depths of transparent glowing hydrogen or other local disturbance. and this leads to the consideration of the uniformity of solar action. the planetary electrospheres will be constant in their operation if the constitution of surrounding space remains uniform; but we shall find reason to believe that there are currents in the ocean of space, as there are currents in our own seas, and electrical generation will necessarily vary when such currents are encountered. the sun itself in such case, however, will become an automatic regulator, for his density being but one-fourth that of the earth, and the spectroscope having shown his chemical composition to a large extent, we know that his mass must be either liquid or vaporous, and perhaps in part both. such masses readily respond to variations of temperature, expanding as it rises and contracting as it falls. hence, if a portion of space were reached where the action of the planetary electrospheres was increased by relative increase of temperature in some interstellar "gulf stream," the sun's volume would expand and compensation be at once established, while, conversely, with diminution of such planetary action, the solar volume would contract and an increased supply from his reserve store be given out thereby. in this way the condensation relied upon to give us heat for seven or seventeen million years becomes a compensating mechanism, self-operative through the most distant cycles of time. we shall also find in such electric currents an explanation of sun-spots. it is not meant that a full knowledge can be obtained of their minute constitution, nor is it necessary; but the equatorial belt of six degrees, nearly free from sun-spots, we can readily understand to be caused--since sun-spots are depressions in the photosphere down to the deeper and denser cloud strata beneath--by the equatorial piling up of the sun's atmosphere by its rotation. any point on the sun's equator travels at four times the rotational velocity of one on the earth's equator, but the sun's attraction of gravity is twenty-seven and one-tenth times that of the earth, so that the piling up of an atmosphere of hydrogen would be considerable, and such depressions would not ordinarily exist there. similarly, near the sun's poles we should find a gradual darkening, as is the case; but from five degrees to thirty degrees latitude, the sun, in its rotation, by reason of the inclination of its axis, passes at every point directly beneath the planets, or within their area of control, and here we find the solar spots in their greatest number, size, and intensity. these sun-spots cross the face of the sun in about fifteen days, and vary in development from year to year, having a cycle of . years from maximum to maximum. they also have a long cycle of about fifty-six years. (see article "the sun," in appleton's cyclopædia.) "wolf, in , presented a formula by which the frequency of spots is connected with the motions of the four bodies, venus, the earth, jupiter, and saturn. professor loomis, of yale college, has since advocated a theory (suggested by the present writer [proctor] in , in 'saturn and his system,' page , note) that the long cycle of fifty-six years is related to the successive conjunctions of saturn and jupiter. but the association is as yet very far from being demonstrated, to say the least." should such fact be established, an explanation for it will be found in the direct impact of the condensed electric currents from several planets approaching conjunction, and raising a portion of the sun's atmosphere suddenly to a higher temperature and volatilizing an abnormal proportion of the semi-vaporous metallic core beneath. this would form an upburst piling the intensely heated faculæ up on the sides and revealing the relatively darker masses of cloud beneath, the cooler supernatant hydrogen pouring in from the upper layers to fill the returning void. this is precisely what is seen in such spots and their surrounding disturbances. in the article "the sun," above quoted, we read, "mr. huggins has found that several of the absorption bands belonging to the solar spectrum are wider in the spectrum of a spot, a circumstance indicative of increased absorption so far as the vapors corresponding to such lines are concerned.... near the great spots or groups of spots there are often seen streaks more luminous than the neighboring surface, called faculæ. they are oftenest seen towards the borders of the disk." this writer also describes "luminous bridges across spots which sink into the vortex and are replaced by others of the numberless cloud-like forms from one hundred to one thousand miles in diameter, the brilliancy of which so greatly exceeds that of the intervening spaces that they must be recognized as the principal radiators of the solar light and heat." the apparent retardation of the spots most distant from the sun's equator may also be partially, at least, explained by planetary currents of electricity, as the equatorial atmosphere is deeper and more likely to carry forward such vortices when formed, while the planets act more directly on the sun's mass beneath their direct influence. let us consider this retardation of sun-spots somewhat more in detail. take, for example, the case of a large planet at such orbital position that its direct line of electrical impact will penetrate the photosphere at (say) seven degrees north solar latitude, which is about fifty-two thousand miles from his equator. during its annual revolution this planet will traverse, with its line of energy, every point of the sun's surface down to seven degrees south latitude and back again to its initial point, thus tracing a close spiral around the sun for fourteen degrees, or about one hundred and four thousand miles in width. the centrifugal force of the solar rotation piles up the photosphere and the chromosphere around the sun's equator, precisely as our atmosphere is piled up around our own equator. if the planet be a large one (for distance has but little to do with these electrical currents at planetary distances, in which they differ entirely from light, heat, and gravity), or if there be two planets nearly in conjunction, the body of the chromosphere and the surface of the photosphere will gradually become highly heated, for currents of electricity, of themselves, do not directly heat the solar core any more than a like current heats the under carbon of an arc lamp, the high temperature in both cases being altogether due to the incandescent heat of the interposed arc or envelope. faculæ of intense brightness will then appear upon the photosphere, and these will be driven forward and also outward in the direction of the higher latitudes, producing an oblique forward movement from difference of rotational speed at different portions of the sun's surface. similar phenomena are constantly observed on the surface of the earth in the generation and behavior of cyclones and other atmospheric disturbances. they may be compared to the wake of a vessel anchored in a strong tide-way. these faculæ will slowly raise the temperature of the surface of the sun's core beneath to the point of eruptive volatilization, and particularly so if the planet is receding from, instead of advancing towards, the solar equator. at some point in advance of the line of planetary energy an eruption of volatilized metals will suddenly occur, first thrusting up a vast area of the photosphere and then bursting it asunder, which will drive these ruptured masses with enormous speed forward and obliquely outward from the equator. such faculæ (see proctor's "light science") sometimes reach a velocity of seven thousand miles per minute, while the sun's rotational movement at the equator is less than seventy miles per minute. this sudden eruption will be almost immediately succeeded by great expansion and consequent fall of temperature, so that within a few hours the heavy volatile metals begin to condense and rapidly recede into their crater, and the faculæ in front and at the sides will now stream inward to occupy this vacuum with constantly accelerated velocity, pouring over the edges like the rush of waters at the falls of niagara. as they sweep downward over the inner rim of the funnel, these streams of faculæ will glow with increased whiteness, and appear to be sharply cut off at their inner ends; but this is only apparently so, and is due to the position of the observer, who looks almost directly downward upon these descending streams. it is for the same reason that the faculæ appear more brilliant when near the borders of the solar disk (see page ). any good view of a sun-spot when analyzed will show the streams of faculæ thus pouring inward, and they are among the most peculiar and conspicuous phenomena to be observed. the drawings of professor langley, reproduced in the popular science monthly for september, , and july, , are particularly striking in their illustration of these effects, though their significance and interpretation were not then at hand. but while these heavy metallic vapors so rapidly condense and subside in the forward or initial portion of the sun-spot under observation, new depths of intensely-heated faculæ are generated behind, and these operate with renewed energy upon the fresh surface of the solar core in rear of the original seat of eruption; so that each sun-spot, while in an active state, will exhibit two entirely distinct aspects, the forward portion of the crater in a state of rapid condensation and subsidence of the recently erupted metallic vapors, and with inflowing streams of incandescent hydrogen from the front and sides, and the rear portion of the crater up to its rearward wall, and even streaming forth from beneath it, in a state of violent eruption. the large volcanic craters of the hawaiian islands exhibit similar partial eruptions and subsidences progressing simultaneously in the same depths. the sudden formation of the great incandescent loops and plumes to which professor langley calls especial attention, and which have hitherto been so perplexing, can now be readily understood and explained. if one of these inflowing streams be carried partially down into and across the crater, and then caught, in its advance, by the uprush in the central or rear portions of the cavity, it will be at once swept upward alongside the ascending eruption, and either scattered at its forward extremity into sprays and plumes, or else thrown forward bodily in the form of a more or less complete loop. in a sun-spot fifty thousand miles in diameter, such a loop, having a long diameter of twenty thousand miles, if we give a speed to the faculæ of seven thousand miles per minute, would be formed in about seven minutes, during which the sun-spot would itself have advanced less than five hundred miles across the face of the sun. the luminous bridges which form so suddenly across portions of the crater may be explained in a similar manner: they are streams of faculæ floated on the nearly balanced uprush of metallic vapors from beneath. it will thus be seen that a sun-spot is not merely a fixed eruption, like a volcano, but rather a continuous series of eruptions, like a line of activity following, for example, the great terrestrial volcanic curve which extends up the western coast of america, across the pacific ocean and asia, and into central and southern europe, for during its progression its scene of action is constantly being shifted to the rear; it is like a furrow cut by a plough, in which the upturned sod is constantly falling in at one end of the furrow while the plough is cutting a new furrow at the other, except that in this case the plough is relatively fixed overhead, and the field itself passes along beneath it. consequently, the center of activity of a sun-spot is only in its rear portions, generally considered, and the whole sun-spot is gradually retreating, by successive filling up in front and opening out behind, farther and farther to the rear,--that is to say, to the east,--so that retardation relatively to the rotational advance of the photosphere necessarily ensues. but when the sun-spot is developed upon or near the equatorial line this retardation is not so considerable, for the deeper layers of the photosphere in those regions are slower to act and require greater energy to affect them, so that all except deep and violent eruptions fail to show themselves at the surface at all, and the heated faculæ are carried directly forward along the surface of the equatorial swell, so that the center of activity is driven forward more rapidly than in the higher latitudes, and the rate of progression is more nearly coincident with that of the photosphere. but if these facts are correctly stated and explained, we may have to revise our calculations of the sun's rotational period, for retardation to some extent must occur in all cases, if in any. a sun-spot, we thus perceive, is an elongated wave or ridge of eruption along the rotational direction of the sun's body. why, then, it may be asked, is not this line of eruption continuous entirely around the sun? for the same reason, it may be answered, that our own cyclones are not continuous, though caused substantially in the same manner, and that volcanic eruptions only occur at long intervals, though the forces at work are continuous. lowering of temperature follows swiftly after eruption, and as the deeper structures of the solar nucleus become gradually affected, instead of volatilization of the outer layers of the surface, we will have diffused gaseous expansion of large portions, and finally of the entire solar mass, which cannot as a whole be volatilized by any conceivable planetary energy. we see these operations exemplified in heating a bar of copper in a bunsen flame; the latter first turns green from surface volatilization of the copper, but as the heat is communicated to the deeper structures the green flame disappears, and the whole additional heat goes to raise the temperature of the mass. these processes in the sun are thus seen to be self-compensatory in their nature. they are the means provided to distribute the restricted areas of abnormally heated photosphere over the solar surface, and finally to cause the absorption of the whole excess of heat in the sun's central mass. the balance is so evenly maintained, however, that, were all the planets equally distributed with reference to the sun's surface, such sun-spots would be the exception and not the rule, and their distribution would be equal and constant; but, as the planets continually change their positions with reference to the sun and to each other, only by some such provision of nature could the internal structure of the sun be maintained without serious derangement, or, indeed, final disruption. so nature distributes her stores of heat upon the earth. these beautiful self-compensations we shall find suddenly appearing, as we advance, in all parts of the field of astronomical research. it may seem like temerity to advance statements so positive and specific as to the cause, constitution, and progression of sun-spots, in the absence of any considerable accumulation of observations to sustain them, but the few examples which we have noted are in accordance with these views, and when attention is once called to the basic principles on which they depend, observations will doubtless be made in abundance to prove or disprove what has been here stated. the mere fact of a differential rate of advance among sun-spots, as they pass across the solar face, of itself demonstrates that the active causes of these phenomena must be extra-solar, and if so, their only possible dynamic source must be looked for in the planets, and the remaining conclusions will of necessity follow as a corollary. we may even, by merely examining an accurate drawing of a sun-spot, determine its position and direction upon the solar sphere from which it was delineated by its lines of active eruption and influx of faculæ, and also whether it be a new spot or one which has passed entirely beyond its active stage and is about to finally disappear. as for the faculæ which striate the photosphere, the mottlings and so-called "willow-leaves," any one who will quietly gaze downward upon the turbid surface of the mississippi or other similar river, in mid-channel, will see plenty of such faculæ: the river is full of them. the heavier, intermingled clay, slowly subsiding, is caught up in the turmoil beneath the surface and swept upward in elongated ovals and eddies, the larger swells nearly colorless, and others of all shades of ochre and yellow, and the whole as richly mottled, sometimes, as the variegated pattern of a persian carpet. if we substitute for the subsiding clay the rapidly sinking heavy metallic vapors, and enlarge the scale from the dimensions of the river to those of the sun, we will have the mottled solar surface with its kaleidoscopic changes, the so-called "willow-leaves," and the faculæ in all their glory. a careful study of the sun will show most clearly that only in some such explanation as the present view affords can a rational basis for its varied phenomena be found. if the sun's equator were coincident with the plane of the planetary orbits, it is obvious that all the planetary energies would be directed, whatever the position of the planets around the sun, immediately upon this equatorial great circle, and that, at each revolution upon his axis, corresponding nearly to our calendar month, the same part of his sphere would be exposed to these direct currents, so that the intensity would be, in its aggregate, nearly a constant quantity. but, by reason of the sun's axial inclination of seven degrees to the plane of the planetary orbits, a far more complex and important condition of affairs ensues. it will be seen at once that the plane of the planetary orbits intersects the sun's equator at opposite sides, and that, from a minimum of nothing, this line reaches a maximum, twice in each circumference, of seven degrees, one north and the other south of the equator, and that this arc of fourteen degrees, thus traversed by every planet in its orbital rotation around the sun, measures more than one hundred thousand miles from north to south upon the solar surface, nearly one-half the distance which separates the earth from the moon. if all the planets were in conjunction or nearly so, on one side of the sun, for example, and in the vertical plane of the sun's axis, they would continue to deliver their electrical currents with their greatest intensity upon a single point of his surface fifty-two thousand miles north of his equator, while the opposite point, one hundred and four thousand miles distant, would be unaffected by any direct currents at all. conversely, if in conjunction on the opposite side of the sun, they would continue to deliver these currents upon a corresponding point fifty-two thousand miles south of the equator; but if in conjunction in the vertical plane transverse to the sun's axial inclination, these currents on either side of the sun would be delivered directly upon the solar equator. the importance of this will be understood when it is considered that for many of our years such planets as jupiter and saturn must continue to direct their currents upon a very slowly changing point of the sun's surface, by reason of their vast annual rotational period, while with the earth and the interior planets these various points are struck with ever-increasing rapidity as the year decreases in length with the different planets, the earth, venus, and mercury. there is a solar equinoctial, so to speak, just as there is a terrestrial equinoctial in which the sun crosses the line twice each year, and the meteorological disturbances faintly shown on the earth at such times are vastly increased on the sun, and rendered far more complex by the interaction of many planets upon the sun, instead of a single sun upon each planet. while our equinoctial has to do with gravity and light and heat, and probably magnetism, the solar equinoctial deals with the vast electrical streams which feed its fires and set it boiling with furious energy, first at one point, then at another, until the increment has been absorbed and adjusted, and thus equalized throughout his mass. what a new interest this must arouse in our study of sun-spots, faculæ, prominences, sun-storms, and the vast panorama of solar action hung up before our astonished eyes! a new world here awaits its columbus. but not only the planets thus gather, so to speak, electricity for the sun's support from space; the moon also must do its part, as it rotates in the same manner, subject to the sun, and has its own motion through space. but an examination of the moon shows no atmosphere and no aqueous matter visible to us, and also the singular fact that it constantly presents one side only to the earth. r. kalley miller, in his "romance of astronomy," article "the moon," says, "after an elaborate analysis, professor hausen, of gotha, found that it could be accounted for only by supposing that the side of the moon nearest us was lighter than the other, and hence that its center of gravity was not at its center of figure, but considerably nearer the side of it which is always turned away from us. he calculates the distance between these centers to be nearly thirty-five miles, evidently a most important eccentricity, when we remember that the radius of the moon is little over a thousand miles. it must have been produced by some great internal convulsion after the moon assumed its solid state; but the forces required to produce this disruption are less than might at first sight appear necessary, owing to the fact that the force of gravitation and the weight of matter are six times less at the moon than with us." those who are fond of the so-called "argument of design" will be gratified to learn that, if the moon had a rotation upon its own axis similar to that of the earth, all life--past, present or future--would have been impossible on that satellite or planet; and that, on the contrary,--provided she always turns the same side of her surface to the earth,--it is quite possible that air, water, and life may exist, or may have existed, on the opposite side of the moon, but not otherwise. in fact, air and water must now exist on the opposite side; and, since her whole supply will thus be condensed upon half her surface or less, even with her small force of gravity, it may be quite sufficient in quantity and density for the support of animal, vegetable, or even human life. by reason of this difference in the lunar center of gravity, the side presented to the earth in physical position is similar to the summit of a mountain upon the earth's surface two hundred miles high, and surely we would not expect to find much air or water or life at that altitude. but the opposite side would resemble a champagne country at the foot of this enormous mountain, and might be well fitted for human existence. now, we know that similar electricities repel each other, and air or gases charged with similar electricities are equally self-repellent. professor tyndall, in his "lessons in electricity," says, "the electricity escaping from a point or flame into the air renders the air self-repulsive. the consequence is, that when the hand is placed over a point mounted on the prime conductor of a good machine, a cold blast is distinctly felt.... the blast is called the 'electric wind.' wilson moved bodies by its action; faraday caused it to depress the surface of a liquid; hamilton employed the reaction of the electric wind to make pointed wires rotate. the wind was also found to promote evaporation." while electrical repulsion is doubtless analogous to, and correlative with, the attraction of gravitation, this force, and even gravity itself, has been sometimes interpreted as derived from the mutually interacting molecules of space itself. we may even learn somewhat of how such repulsions of similar and attractions of opposite electrospheres might occur. we constantly speak of positive and negative electricity as though these were different fluids, but such expressions are employed only in the same manner as the analogous terms, heat and cold. we know, of course, that cold is the relative absence of heat, the dividing line being not a fixed, but a constantly changing one, so that one body is cold to another by reason of relative, and not absolute, deprivation of heat. it is well known, however, that cold, which is purely a negative state, manifests the same apparent radiant energy as heat. a vessel near an iceberg is exposed to a wave of cold, precisely as of heat from a heated body at the same distance. this, of course, is due to abstraction and not to increment. all space being occupied by attenuated matter in a state of unstable electrical equilibrium, as we say, which simply means a condition ready to be raised or lowered in tension by absorption from or into outside media, all concrete bodies floating in that space must have an electrical potential either equal to, or higher, or else lower than that of their surrounding space. a solitary body in space, if we can conceive of such, in either a higher or lower state of electrical tension, would be drawn upon from all sides to equalize the distribution and restore the general average. but if two bodies occupy the same field, and are widely different from each other in electrical potential, one higher and the other lower than that of space, this distribution will be towards each other, and must be manifested by mutual attraction. but if, on the contrary, these two bodies are both equally higher or lower than the spatial average, they have nothing to give to each other, but have this difference to give to or receive only from outer space, and hence they will be drawn apart or, as we say, mutually repelled. the case is similar to what we see in the case of bodies of water at various levels. suppose there be a lake of a fixed level, and communicating with it and with each other, by open channels, two ponds of water occupying an island in the middle of the lake. if one of these ponds be higher in level and the other lower than the lake, their waters will rapidly converge, the higher flowing into the lower; but if both are at the same level, and higher than the lake, they will flow apart into the lake. or, if both are at the same level, and lower than the lake, the water of the latter will equally flow from outside into both ponds, and their waters will still be held separate from each other. the analogies of these various levels may be pursued to any desired extent, as electrical tensions find their most exact analogies in the pressures of bodies of water at different levels and of different quantities, and these analogies are those most constantly used in the interpretation of such electrical phenomena. the great electrical activity of the electrospheres of the earth and moon, while they discharge their tremendous currents directly into the sun, at the same time must cause their similarly electrified atmospheres to mutually repel each other, while gravity continues to operate to maintain the earth and moon at their fixed distances from each other, and to retain their gaseous envelopes around their own bodies. the result must be that these similarly electrified atmospheres repel each other with a force proportioned to their masses of atmosphere and the intensity of the electricities of each. the moon's axial rotation being completed but once in twenty-eight days, and that of the earth once in each day, and the moon's mass and volume being so much less than those of the earth, whatever of electrified air or moisture she may have (and she must have both, proportionate to her attributes) would have been driven as by a cyclone to the opposite side of the moon and there retained. now, with an atmosphere and water only on one side of the moon, and that the side opposite the earth, it is obvious that a rotation on her axis at all resembling that of the earth would carry every part of her surface, at each complete rotation, from a region of air and moisture into one deprived of both, and in such a condition she would of necessity be deprived of both life and its possibility; hence, as the laws of nature compel the lunar atmosphere and moisture to reside permanently on the side always opposite the earth, a co-ordinate arrest of the moon's axial motion with reference to the earth could alone compensate for such a state of things, and, curiously enough, we find as a solitary exception, compared with the planets, that such is the case. the moon unquestionably has both atmosphere and water on its opposite side. in his recent work, "in the high heavens," professor ball reviews the physical conditions of the other planets as possible abodes of life. he pronounces against the moon because night and day would each be a fortnight in length; but this is surely no objection, for even in norway and greenland such nights and days are not uncommon at different seasons, and thousands of human beings, even as at present constituted on earth, spend their lives there in content and happiness. that the moon also would be terribly scorched by the long day and frozen by the long night does not necessarily follow, for the atmosphere of mars, that author says, "to a large extent mitigates the fierceness with which the sun's rays would beat down on the globe if it were devoid of such protection." as the moon's opposite face must have a double quota both of atmosphere and clouds, the difficulty will be correspondingly less than on mars; and as for the "lightness" of bodies on the moon, they would probably get along quite as well as mosquitoes and like "birds of prey" in the marshes along our coasts. the author refers constantly to our bodies; for example, "could we live on a planet like neptune?" no, we could not; we would be dead before we got there. nor could we live in the bark of a tree, or at the bottom of the ocean, or in a globule of serum; but living beings are found there nevertheless. the principle is that wherever life is possible there we may expect to find life; and surely life is, or has been, or will be possible, not only on the moon, so far as our knowledge of physical conditions can go, but also on some of the other planets. of course each planet has its life stage, but this applies not only to the earth, but to all the other planets as well, and not only to the planets of our own system, but to those of all other solar systems. each has had, or will have, its stage in which life is possible, and these planets may be like human habitations, in which whole races at times migrate from one home to another. there is no conceivable reason why this may not be the general law of creation, and every analogy leads us to believe that it is so. it has been recently announced that, from telescopic observations, the atmosphere of mars must be at least as attenuated as that among the highest mountainous regions of the earth, if this planet has any atmosphere at all. that it must be far less dense than that of the earth at sea-level is obvious, for the mass and volume of mars are very much less than those of our own planet; but that mars is devoid of a gaseous envelope or atmosphere is contrary to what we know of all sidereal physics. the sun, the fixed stars, the comets, the nebulæ, and even the meteorolithic fragments which fall upon the earth, all show the same elementary chemical constitution as the earth itself, and we cannot believe that mars alone is differently constituted from every other body we have been able to examine. we have direct evidence, on this planet, of polar snows and their melting away under the sun's heat; we see the apparent areas of sea and land; it has its moons as the earth has hers, and exhibits all the characteristic phenomena of the earth and other planets. all sidereal bodies that we know of, except, perhaps, our moon, which exception we have fully accounted for, are found to be surrounded by gaseous envelopes or atmospheres of some sort. the sun, the fixed stars, the nuclei of comets, the condensing nebulæ, the planets jupiter and the earth, which are those under our most direct observation, and even the meteorites, when examined, reveal the presence of many times their own volumes of independent atmospheric gases; and whatever may be the theory of the origin or development of mars, it must have been subjected to the same influences, the same environment, and the same processes of creation as those of our solar system generally; and that this body alone should possess no gaseous envelope--for the denial of atmosphere denies, at the same time, the presence of any or all surrounding gases--is quite incredible. only the most positive, direct, and long-continued proofs of such fact could be accepted, and even then the history of all scientific progress shows that what are believed to be facts themselves fluctuate like fancies till, by their accumulated force, they solidify into universally accepted demonstration. the fact, moreover, that the atmospheres of the smaller planets are more attenuated than our own and those of the larger ones denser has no bearing, in itself, on the probability of the existence of life on these other planets, for in our own atmosphere oxygen, which is the efficient element, is diluted with four times its quantity of inert nitrogen. these proportions doubtless vary largely in other atmospheres, so that the oxygen may be much richer in some and far poorer, relatively, in others. the mere fact that the presence of nitrogen, probably, and aqueous vapor, certainly, depends on the gravity of the mass of each planet, while the oxygen is due to electrolytic decomposition induced by the combined volume, mass, and rotation, and other causes,--such as the axial inclination of such planets, for example,--renders these variations in the constitution of planetary atmospheres a certainty. as mars has a diameter much more than one-half that of the earth, and a diurnal rotational period nearly the same, while his mass, which controls the action of gravity, is only about one-ninth that of the earth (see appleton's cyclopædia), it is obvious that his oxygen-gathering power, compared with that for accumulating nitrogen and aqueous vapor, is much higher than that of the earth, and we should expect to find there an attenuated atmosphere very rich in oxygen, and with a relatively smaller proportion of aqueous vapor, or even water, on his surface. such seem to be the facts as far as observed. in operating an electric machine the strength of the current is directly proportionate to the speed of rotation,--that is to say, to the velocity of the generating surface; for example, of the wimshurst induction machine it is stated (page , "electricity in the service of man"), "these four-and-one-half inch discharges take place in regular succession at every two and a half turns of the handle." it is also a well-established law of electrolysis that "the amount of decomposition effected by the current is in proportion to the current strength." professor ferguson ("electricity," page ) says of the voltameter, an instrument devised by faraday, and used for testing the strength of currents by the proportionate decomposition of acidulated water, "mixed gases rise into the tube, and the quantity of gas given off in a given time measures the strength of the current." roughly estimating the diameter of mars at five-eighths, the surface velocity at three-fifths, and the mass at one-ninth those of the earth, this planet should have an atmosphere containing about sixty per cent. of oxygen and forty of nitrogen, with a barometric pressure at sea-level of about six and one-half inches of mercury. this would be an excellent atmosphere,--about equal in its quota of oxygen for each respiration to that of the higher areas of persia, a great country for roses. the aqueous vapors lying low and near the surface would serve as a vaporous screen to concentrate and retain the sun's heat and retard radiation from that planet. nothing in particular seems to be the matter with mars. on the contrary, the mass of jupiter is so great, and his attraction of gravity so powerful, that it is only by his exceedingly rapid diurnal rotation (once in less than ten hours) that it is possible for him to accumulate any effective percentage of oxygen at all. but there is certainly plenty of water there. we may approximately compute, in general terms, the proportion of oxygen in the atmospheres of the other planets in the same way. neptune, it is true, is so far distant from the sun that the solar orb only "appears about the same magnitude as venus when at its greatest brilliancy, as viewed from the earth," but we must not forget that "the intensity of the sun's light would be more than ten thousand times greater than that of venus" (professor dunkin, in "the midnight sky"). unless the moon gathers a portion of the earth's oxygen (the planetary satellites, like saturn's rings, thus constituting in their rotations a constituent part of the planets themselves), the percentage of this gas in her atmosphere must be exceedingly small, for her axial rotation has a period of a whole lunar month, being the same as that of her revolution around the earth as a center. the absence of apparent atmosphere and moisture from the visible lunar surface has already been mentioned and explained. the means by which this fact has been approximately determined are described by professor dunkin, in "the midnight sky," as follows: "among the many proofs of the non-existence of a lunar atmosphere, it may be mentioned that no water can be seen; at least there is not a sufficient quantity in any one spot so as to be visible from the earth. again, there are no clouds; for if there were, we should immediately discover them by the variable light and shade which they would produce. but one great proof of the absence of any large amount of vapor being suspended over the lunar surface is the sudden extinction of a star when occulted by the moon. the author has been a constant observer of these phenomena, and, though his experience is of long standing, he has never observed an occultation of a star or planet, especially at the unilluminated edge of a young moon, without having his conviction confirmed that there is no appreciable lunar atmosphere.... professor challis has subjected the results of a large number of these observations to a severe mathematical test, but he has not been able to discover the slightest trace of any effect produced by a lunar atmosphere." in appleton's cyclopædia, article "the moon," it is stated that "schröter (about ) claimed to have discovered indications of vegetation on the surface of the moon. these consist of certain traces of a greenish tint which appear and reappear periodically; much as the white spots covering the polar regions of mars.... as we are able, under the most favorable conditions, to use upon the moon telescopic powers which have the effect of bringing the satellite to within one hundred and fifty to one hundred and twenty miles of us, we should doubtless notice any such marked changes on her surface as the passage of the seasons produces, for example, on our own globe." very recently (august , ), it has been stated, professor gathmann has observed a peculiar green spot about forty by seventy miles in area near the crater of tycho brahe, "on the northwestern edge of the satellite's upper limb," which had disappeared twenty-two hours afterwards. we understand, of course, that the moon's librations, by the variation of position of the lunar body, enable us to see, at times, around the edge of this satellite somewhat, so that, instead of observing only one-half, we can in this way see nearly six-tenths of her surface, but not at the same time, of course. when the moon is dark it occupies a position between the earth and the sun, and only its opposite face is illuminated. in this position the attraction of solar gravity and the attraction of the electrically opposite solar electrosphere both accumulate their forces upon the moon's atmosphere in the same line as the repulsion of the earth's similar electricity, so that the lunar moisture and atmosphere are, at this part of her subordinate orbit, most powerfully forced away from the direction of the earth. as the moon now proceeds towards her first quarter, the terrestrial repulsion drives her atmosphere radially outward, while solar gravity and electrical attraction tend to hold it in the direction of the sun. the result will be an electrospheric libration, so to speak, and the moon's atmosphere and moisture will be carried around towards its illuminated face and, to some extent, will overlap the area of terrestrial repulsion. but as the moon advances this will gradually diminish, soon cease, and finally be reversed as it again approaches darkness. we can now understand why the green surface, if it really was due to vegetation, appeared along the lunar margin at the time described above, and also that the observation of planetary occultations "at the unilluminated edge of the young moon" was the very worst part of the moon and its orbit in which to look for air or moisture; as the sun's influence is then directly away from the unilluminated surface of the moon, and his "pull" would have, in fact, still further denuded the very portion most persistently examined, and where this absence of atmosphere was especially noted. when considering the transference of energy from the peripheral regions of the solar system to the center, its conversion there into a new form of molecular force, and its subsequent distribution, we find a curious and instructive parallel in the action of the reflex nervous system of animal life. this system is one in which the brain or other conscious center of nerve-energy takes no part. tickle the foot of a child, for example, and its whole muscular system is thrown into uncontrollable convulsions of laughter. here an exciting contact with the terminal filaments of the afferent or sensory nerves is rapidly carried into the local nerve-center of this part of the system,--that is, the sensory column of the spinal cord. this center of ganglionic nerve-matter lies directly against the corresponding motor mass, both freely communicating with each other. the sensory current passing into its central ganglion undergoes some peculiar change of character, probably one of intensification, such as is observed in the action of the condenser of an electrical machine, through which sensory ganglion, thus raised in potential, it passes to the motor ganglion adjacent, where it is instantly transformed into an entirely different form of energy. the sensory character has now entirely disappeared, and it has been converted into and is flashed forth as motor energy to the different muscles of the body, which are immediately contracted, the violent molecular motion of the fibres being at once converted into muscular motion in mass. the changes are entirely analogous to those we see in the different conversions of energy in our solar system. considering the surface of the body as a planetary electrosphere, it is acted upon by excitation from without; currents of energy are engendered, which are at once transmitted to the sensory ganglion, corresponding to the hydrogen atmosphere or electrosphere of the sun; intensification of action here ensues, the current passing through this ganglion or atmosphere into the solar body itself, which corresponds to the motor ganglion; both ganglia are now highly excited; the electrical force is converted into the radiant molecular motor energy of heat and light in the sun and muscular excitement in the body, and these are flashed forth and find scope for their action within the body of the subject or upon the surface of the planets, which lie, like the muscular structure of the body, within the genetic electrosphere where, acted upon from without and by agencies entirely external, moving contact has induced the primary molecular action, which was then instantaneously transferred to the center, there converted into another form, that of motor energy, and thence sent forth to produce action in the muscles of the body in the one case, and in the other upon the planetary bodies and their satellites and other structures which occupy surrounding space. chapter v. the distribution and conservation of solar energy. what, then, becomes of the light and heat flashed forth with eternal energy from the fiery waves of the sun's incandescent atmosphere? professor ball ("in the high heavens") says, "much of what has been said with regard to light may be repeated with regard to heat. we know that radiant heat consists of ethereal undulations of the same character as the waves of light. hence we see that the heat or the light radiated from a glowing gas is mainly provided at the expense of the energy possessed by the molecules in virtue of their internal oscillations." conversely, of course, the ethereal undulations thus induced by high molecular motion in the heated gas or vapor must disappear in so-called absorption or transference by contact with other molecules, themselves devoid of such specific internal oscillations. the heat motion then disappears as heat by its conversion into work, just as the motion of a belt in a mill disappears in the work of the machine which it drives. one two-hundred-and-thirty-two-millionth part of the radiant solar energy, we know, is caught by the flying planets of our system in the forms of heat and light, adapted to sustain life and its continued potentiality, and we know that this solar energy is the sole source of all the development and maintenance of the planets as the possible abodes of organic life, past, present or future. but what of the vast total, of which we consume so minute a fraction? it is true that, in addition to the planets, space is occupied by many small meteoric bodies, which manifest themselves to us as shooting stars and meteorites, but the mass of these is too trifling to be estimated. professor helmholtz, in his "popular scientific lectures," says, "according to alexander herschel's estimates, each stone is, on an average, at a distance of four hundred and fifty miles from its neighbors." when these bodies enter our atmosphere by force of the earth's attraction they are heated by its atmospheric friction to incandescence, and in most cases are even volatilized before reaching the earth's surface. the vast volumes of solar heat and light, however, are poured forth from the sun indiscriminately in all directions into illimitable space, wherein all the masses of concrete matter, including the stars, are relatively far less in volume than the flying motes of the purest morning air which sparkle in the flood of light sent forth by the rising sun. is all the rest wasted? professor balfour stewart, in his work "the conservation of energy," says, "if this be the fate of the high-temperature energy of the universe, let us think for a moment what will happen to its visible energy. we have spoken already about a medium pervading space, the office of which appears to be to degrade and ultimately extinguish all differential motion, just as it tends to reduce and ultimately equalize all difference in temperature. thus, the universe would ultimately become an equally heated mass, utterly worthless as far as the production of work is concerned, since such production depends upon difference of temperature." it is obvious that the starting-point taken by the author last quoted, but which, nevertheless, is in accordance with the views now generally prevalent, is diametrically opposed to that sought to be established in this work. professor stewart takes the sun's inherent energy as the initial point of departure, and reasons from that as to the final consequence when all its light and heat shall have been distributed or dissipated into the attenuated medium which occupies space, and which will be thus slowly heated until all space has been raised in temperature to that of the last dying sun, when all will thenceforth remain unchanged and unchangeable, silent, dark, and dead, to all eternity. on the contrary, the purpose of the present work is to establish a directly opposite principle, based, however, on demonstrated scientific facts and not on theory, that the medium which pervades all space was originally in the same equally and universally potential state (with its molecules raised to a tension constituting an unstable equilibrium) in which, practically, professor stewart's argument leaves it finally, and that this universal molecular energy of position was permanently maintained by the employment of the forces which afterwards, transformed into light and heat, were shed abroad by the sun in the work of again overcoming the intermolecular tension of cohesion, and that the light and heat of the sun are merely caught up again by these same or other molecules and successively employed in the same manner, while the planetary electrospheres utilize these same forces of internal tension in the generation of electricity, which, sent to the sun, is converted into light and heat, and these are again transferred to their original source. the rotation of the planets is the grand exciting cause, and the process, in its complete cycle of development, has live stages: first, planetary generation; second, transference by currents of electricity to the sun; third, conversion into light and heat; fourth, emission; and, fifth, reabsorption and conversion again into molecular energy of position. all space is thus found to be pervaded by extremely attenuated vapors, which contain the elemental constituents out of which suns and planets are evolved under favorable circumstances of development, and, among other vapors, aqueous vapor, and that these are the agency upon which the planetary electrospheres operate in their generation of electrical currents, and which vapors, in turn, by absorption of the solar energy of radiation, again transform this energy into mutually balanced electric potential, until it is once more disengaged as electricity by the rotating planetary electrospheres, and so on in a constant circuit forever repeated. it differs from perpetual motion, however, in that the planetary rotation is the external and not the internal generative cause, since the electrical forces neither cause nor control these motions; they belong to the realm of gravity. the disassociation, moreover, is electrical and not chemical disassociation. the tensions are against cohesion and not against chemical affinity; are, in fact, similar to those which constitute our atmosphere a vast electrical reservoir; and the aqueous vapors, through all their changes, permanently remain as aqueous vapors, except those condensed portions disassociated by electrolytic action at the electrospheric poles, and which have no relation to the attenuated vapors of space, except in that the latter are their sources of supply. the process is analogous to what we see around us at all times in the atmosphere. while the process described by professor stewart resembles the emptying of the inherent water of a cloud, in the form of rain, into an ocean which never yields up its water again, so that, when the cloud has rained itself out, it is gone forever, the processes here sketched are like the vapors which are caught up by the heated air, carried over the thirsty lands, distributed in rain to fertilize and vivify them, then gathered in a thousand tiny rills from countless fountains, again descending to the sea and again carried up in vapor, and so on over and over in unceasing round. it is the difference between an old-fashioned flintlock musket and a modern magazine rifle, except that the magazine is always full. this great ocean of space was primordially charged with these potential vapors; it is the constitution of space itself. we are so accustomed to consider space as empty, and that it is nothingness, the antithesis of something or anything, that it is a negation or a blank, that it requires an effort to even think of it as a fully stocked establishment with all the goods necessary for use or ornament, in the latest styles and of prime quality, only not made up, and that all our suns and worlds are merely tailoring establishments where the operatives cut and fit and make them up to order. when more goods are wanted they have to go to the store. is space, then, eternal, and is this constant round of energies to be eternal? if one is eternal, so is the other, and surely nothing can be more eternal than space, and we cannot conceive of any other space than this space. out of it came all created things, and so long as the orbs rotate without retardation, so long will these interchanges go on without impairment, and that they do so rotate is the necessary corollary of the fact that they ever began to rotate. if rotation, on the contrary, was imparted by special creative power, then the same power established the laws by which they rotate, and took cognizance of resistance as well. whatever the impulse was, it still remains; whatever caused the rotation to begin maintains it; if the cause is eternal the rotation may be eternal; and, in any case, its period must be measured by cycles of æons, to which the allotted lifetime of a dying sun--a few million years, perhaps--is but as the sunburst of a morning-glory flower to the hoary age of a mighty planet. compared with the popular view of the sun's life-period, we may formulate the terms of an equation in which the sun's mass, compared with the realms of infinite space, is as the sun's lifetime--on a basis of contraction of his volume--to the lifetime which actually is to be. as one of the terms is practically infinite, so must be the answer to the problem. professor stewart says, "we cannot help believing that there is a material medium of some kind between the sun and the earth; indeed, the undulatory theory of light requires this belief." it has already been shown that the transmission of electricity also requires it, but that there must be a medium quite different from the undulatory ether. professor proctor ("mysteries of time and space") says, "we may admit the possibility that the aqueous vapor and carbon compounds are present in stellar or interplanetary space." again he says, "assuming, as we well may, that space is really occupied by attenuated vapors." the same writer says further, "to this end all thoughtful study of the mechanism seems to tend (associating, perhaps, our visible universe with others, permeating it as the ether of space permeates the densest solids, and in turn with others so permeated by it); there may be that constant interchange, that perpetual harmony, of which goethe sung: 'balanced worlds from change defending, while everywhere diffused is harmony unending.'" the light and heat poured forth from the sun are, as stated, in the form of radiated energy. they penetrate the attenuated vapors as far as vision extends, and doubtless farther, but they cannot reach the boundaries of space, for even the mind of man cannot reach those limits. aqueous vapor absorbs heat; we know this without any demonstration, for the radiated heat of the earth is arrested by a veil of clouds, so that on cloudy nights frost will not form. so also the sun shining into water will raise its temperature, as in a glass globe, and such absorption of heat by aqueous vapors or water would be much more manifest were not a large part employed in loosening the tension of the constituent molecules, since, when thus employed, it is not manifest as sensible heat. professor tyndall, in "the forms of water," states that "the quantity of heat which would raise the temperature of a pound of water one degree would raise the temperature of a pound of iron ten degrees." professor stewart, in "the conservation of energy," says, "that peculiar motion which is imparted by heat when absorbed into a body is, therefore, one variety of molecular energy.... part of the energy of absorbed heat is spent in pulling asunder the molecules of the body under the attractive force which binds them together, and thus a store of energy of position is laid up, which disappears again after the body is cooled. "heat will only be changed into work while it passes from a body of high temperature to one of low.... at very high temperatures it is possible that most compounds are decomposed, and the temperature at which this takes place, for any compound, has been termed its temperature of disassociation. heat energy is changed into electrical separation when tourmalines and certain other crystals are heated." it may be added that it is also changed into electrical energy by the operation of all electrical machines, as molecular motions are all mutually interconvertible, and heat itself is only a mode of such motion. of radiant energy, the same writer says, "this form of energy [radiant heat] is converted into absorbed heat whenever it falls upon an opaque substance ... and heats it. it is a curious question to ask what becomes of the radiant light from the sun that is not absorbed either by the planets of our system or by any of the stars. we can only reply to such a question that, as far as we can judge from our present knowledge, the radiant energy that is not absorbed must be conceived to be traversing space at the rate of one hundred and eighty-eight thousand miles a second." we know, of course, that aqueous vapors are partially opaque to heat rays, as the radiated heat of the earth is partially arrested by such vapors in the atmosphere, but they are apparently transparent to the rays of light. but we know that this cannot be entirely true in fact, for light rays only differ from heat rays in the comparative length of their waves or impulses, while rays of light are always accompanied--when emitted by a thermally incandescent body--by a much larger number of those of heat. as a body is raised in temperature radiant dark rays first appear; these being raised higher, become visible as light, and new dark rays are radiated behind them, and this continues till after the state of highest incandescence is reached and the invisible chemical rays beyond the spectrum appear. it is like a crowd surging forth in flight from the doors of a building; as the speed of those in front increases to a run, others follow more slowly in the mass, and as these gain speed others continue to follow, while the great mass of laggards still trails along in a lengthening line to the rear. the perception of light is itself merely due to the constitution of the optic apparatus of the observer, which only takes cognizance of vibrations radiated from the middle portion of the scale, just as the ear does with sounds, and not to any actual difference in their mode of production. that heat rays and light rays are identical in constitution can be readily shown by the experiment described by professor tyndall in his "forms of water," in which an opaque screen of iodine solution in bisulphide of carbon was employed to arrest, in a beam of light, all the light waves (to which it is entirely opaque), while transmitting the dark rays. these non-luminous rays are then converged by a lens: "let us, then, by means of our opaque solution, isolate our dark waves and converge them on the cotton. it explodes as before.... at the same dark focus sheets of platinum are raised to vivid redness; ... a diamond is caused to glow like a star, being afterwards gradually dissipated." sir william herschel (see article "spectrum," appleton's cyclopædia) says, "if we call light those rays which illuminate objects, and radiant heat those which heat bodies, it may be inquired whether light be essentially different from radiant heat. in answer to which i would suggest that we are not allowed by the rules of philosophizing to admit of two different causes to explain certain effects, if they may be accounted for by one."... "tyndall, by similar experiments, found that the thermal energy of the invisible radiation of a very powerful electric light is eight times that of the visible.... seebeck showed that the position of maximum heat in the spectrum changes with the nature of the prism and sometimes occurs in the red." melconi, with prisms of alcohol and water, found it in the yellow. athermic bands are also found in the heat-spectrum, corresponding to the fraunhofer lines seen in the visible spectrum. we may illustrate this successive development of more and more rapid light-waves by conceiving of a harp having musical strings of various length and thickness, but not strung up, so that, when swept by the hand, the vibrations are felt, but no musical tones are produced. if, now, all the strings are simultaneously and gradually stretched while under continuous vibration, we will first hear the hum of the lighter strings, but deep down in the scale; and as the tension gradually increases the pitch of these will rise higher and higher and be succeeded by other new tones below, until the whole register is simultaneously sounded. and if the tension be further increased, the vibrations of the upper strings will gradually grow so rapid that the ear can take no cognizance of them, corresponding to the invisible chemical rays of the spectrum, while the middle strings will be sounding loudly, and others will be slowly vibrating below the musical scale, but without sound, corresponding to the invisible heat rays. in addition to this gradual ascent of pitch along the scale, however, there is reason to believe that sympathetic vibrations are induced in the spectrum of thermal and chemical light corresponding to the over-tones in music and to those hidden rhythms which differentiate the "timbre" of one kind of musical instrument from that of another, so that a definite wave-length will not only repeat itself among adjacent molecules, but will give rise to harmonious vibrations quite different in amplitude and velocity. an example of this is found in some of the phenomena of phosphorescence and fluorescence, in which chemical rays totally invisible are able, under suitable conditions, to excite molecular movements corresponding to parts of the visible spectrum, and quite different in wave-lengths and in rapidity. this process is precisely the converse of what we perceive in thermal light; in the latter case the colors ascend, loaded with invisible heat rays; in the former they descend, loaded with invisible chemical rays, only noted, perhaps, by their actinic action on the photographic plate. others, as the sulphide of calcium paints and the like, repeat their own vibrations for many hours, and we find in certain chemical salts of some rare metals, as lanthanum and cerium, the curious property of suddenly raising the whole scale, as in a recently introduced gas-lamp, in which a skeleton mantle of these oxides glows with a wondrously beautiful white light under the relatively low temperature of a small bunsen burner; similar phenomena are manifested in the behavior of electric discharges in attenuated gases, as well as in what is known to children as "fox-fire," wood undergoing slow decomposition in damp places, or in the self-luminous secretions (corresponding, perhaps, to ptomaines or like products) of glow-worms and other animals. if we ever--as we probably soon shall--reach that point where we can illuminate our dwellings with "cold candles," as the inhabitants of tropical countries carry about a few fire-flies in a paper box for a lantern on dark nights, it must be by the study of these phenomena. but meantime "old sol" will continue to discharge his accumulating stores of both heat and light, for both these are essential, not only for use upon the planets, but throughout all the realms of space. in the transformation into and emission of his radiant energy the sun is not a chemical engine, but a mill,--one of those which "grind slowly, but they grind exceeding small." the difference between radiated thermal light and heat is obviously one of degree only and not of kind. the undulations of light may be compared to the thrust of a rapier, and the more massive waves of radiant heat to the blow of a bludgeon, but the same resistance which arrests the advance of the one must retard and finally arrest that of the other, if sufficiently extended. within the limits of a space in which professor stewart conceives that the first rays of light which ever flashed forth at the dawn of creation, in the primal æons of the universe, are still to this day, along their original lines of radiation, "traversing space at the rate of one hundred and eighty-eight thousand miles per second," there must certainly be room enough and absorption enough (which even a few yards of mist will supply) to curb these runaway steeds somewhere along their lines of flaming passage. at that very point they are at work acting upon the molecules of the attenuated vapors of space, and assisting to re-establish the potential energy which has there been converted, into another form of force by the planetary rotations of the solar systems of those distant regions. by the law of the diffusion of gases, and that of the diffusion or transference of heat-energy from molecule to molecule, the vast realms of interstellar space must tend to be all brought into approximate uniformity of tensions, and the force abstracted at those points of space occupied by the relatively few and insignificant solar systems will be returned, not directly at the identical places where such solar systems may exist, but at every part of space to which their radiant energy extends. as we give from our own supplies to other systems for their support, so they, in turn, give back again to us. it is said that in the earliest days of creation the stars sang together; they still sing together, planets and suns, as "jura answers from her misty shroud back to the joyous alps, who call to her aloud." when old earth lifts his brimming beaker from the great crystal sea and drains it to the good health of all the stars of heaven, they each respond with fiery energy, and by their merry twinkle we may know how highly they appreciate the toast. we are all one family,--but what a family! comets, planets, double stars, variable stars, stars of complementary colors, blue, yellow, orange, and red stars, stars which blaze up in sudden conflagration, apparently new stars, nebulæ half star and half vapor, nebulæ all vapor and others all stars, the vast milky-way like a wondrous river of hundreds of millions of solar systems, the insulated stars scattered through space like watchmen on the distant hills beyond the city walls, streams of stars, stars which are parting from each other in space like scattering families, and those which travel together in groups like pioneers in a strange country,--all these and doubtless other unknown types and forms compose this sidereal family. will they fall into their categories as lawful subjects, so as to be properly classified in a single scheme of the visible order of creation, or shall we fail to interpret their apparent mysteries when we apply the same principles which have been successfully applied to the phenomena of our own solar system? let us see. in examining the sun, we find that a beam of its light passed through a prism is thrown upon the wall in a wedge-shaped streak of rainbow-tinted colors. fraunhofer, many years ago, found that this spectrum was crossed at irregular intervals by a series of dark lines, of variable width and distance apart, of which he catalogued more than five hundred. these lines were subsequently found to correspond in the aggregate, in their position in the spectrum, with a series of bright lines of different colors which formed the separate spectra of various metals when burned, in vapor or powder, in the flame of an alcohol lamp. each of these transverse lines was found to have a fixed and invariable position in the extended scale of the spectrum, and scarcely any lines of the different elements are alike; so that, when the spectrum is properly magnified under telescopic observation and the lines identified, we have the means of determining the presence or absence of such elements in the vaporous constitution of any incandescent body by examination of its spectrum. in this way many of our terrestrial elements are found to exist in the sun,--so many, in fact, that we know that the sun's nucleus, or core, must be composed substantially of the same elements, the same sort of matter, as exists on earth,--that we are, in fact, "a chip of the old block." but it was found--and this is the real basis of spectrum analysis--that if a certain metal or other element be burned in the flame of an alcohol lamp, and a more brilliant flame of the same metal or element burned in another lamp be observed through the first flame, it will be seen that, "while the general illumination of the spectrum is increased, the previous bright lines characterizing the element are now replaced by dark lines or lines relatively very faint; in a word, the spectrum characteristic of the given element is exactly reversed" (appleton's cyclopædia, article "spectrum analysis"). we have referred to this fact above in considering the origin of sun-spots, showing that they are due to increased heat acting upon the core of the sun so as to volatilize an abnormally large proportion of the elements usually in a more condensed state upon the surface of the solar body beneath its hydrogen envelope. these vapors, thus raised in temperature, are driven upward by their volatilization into the incandescent atmosphere of hydrogen, and the vaporous matters in the higher strata thus produce the characteristic absorption bands of these elements, while the overheated vapors, by a vast uprush from beneath, hurl aside the more highly heated hydrogen above to appear as faculæ around the sun-spot, the cooler upper layers of hydrogen following downward the subsiding vaporous metallic uprush as it sinks back beneath the photospheric level. it is obvious that by similar spectrum analysis we may determine to a large extent the constitution of the fixed stars and other self-luminous bodies of space and interpret the phenomena which they exhibit. we quote the following from the previously cited article in appleton's cyclopædia, by professor proctor: "spectroscopic analysis applied to the stars has shown that they resemble the sun in general constitution and condition. but characteristic differences exist, insomuch that the stars have been divided into four orders distinguished by their spectra. these are thus presented by secchi, who examined more than five hundred star spectra: the first type is represented by alpha lyræ, sirius, etc., and includes most of the stars shining with a white light, as altair, regulus, rigel, the stars beta, gamma, epsilon, zeta, and eta of ursa major, etc. these give a spectrum showing all the seven colors, and crossed usually by many lines, but always by the four lines of hydrogen, very dark and strong. the breadth of these four lines indicates a very deep, absorptive stratum at a high temperature and at great pressure. nearly half the stars observed by secchi [more than two hundred out of five hundred] showed this spectrum. the second type includes most of the yellow stars, as capella, pollux, arcturus, aldebaran, alpha of ursa major, procyon, etc. the fraunhofer lines are well seen in the red and blue, but not so well in the yellow. the resemblance of this spectrum to the sun suggests that stars of this type resemble the sun closely in physical constitution and condition. about one-third of the stars observed by secchi [more than one hundred and fifty out of five hundred] showed this spectrum. the third type includes antares, alpha of orion, and alpha of hercules, beta of pegasus, mira, and most of the stars shining with a red light. the spectra show bands of lines; according to secchi, there are shaded bands, but a more powerful spectroscope shows multitudes of fine lines. the spectra resemble somewhat the spectrum of a sun-spot, and secchi has advanced the theory that these stars are covered in great part by spots like those of the sun. about one hundred [out of five hundred] of the observed stars belong to this type." (it should be noted that the presence of sun-spots is no evidence of diminished heat in a sun; see professor proctor in his "myths and marvels of astronomy," article "suns in flames:" "it may be noticed, in passing, that it is by no means certain that the time when the sun is most spotted is the time when he gives out least light.... all the evidence we have tends to show that when the sun is most spotted his energies are most active. it is then that the colored flames leap to their greatest height and show their greatest brilliancy, then also that they show the most rapid and remarkable changes of shape.") ... "the fourth type differs from the preceding in the arrangement and appearance of the bands. it includes only faint stars. a few stars, as gamma of cassiopeia, eta of argus, beta of lyra, etc., show the lines of hydrogen bright instead of dark, as though surrounded by hydrogen glowing with a heat more intense than that of the central orb itself around which the hydrogen exists." all the above five hundred stars reveal the presence of hydrogen under precisely such conditions as conform to the general principle involved in the source and mode of solar energy as herein stated. but a single star (betelgeuse) was observed by huggins and miller in england which showed the lines of sodium, magnesium, iron, bismuth, and calcium, "but found those of hydrogen wanting." of the spectrum of this gas, professor ball says, "the hydrogen spectrum appears to present a simplicity not found in the spectrum of any other gas, and therefore it is with great interest that we examine the spectra of the white stars, in which the dark lines of hydrogen are unusually strong and broad." referring to the new star in the northern crown, which burst forth in , the same writer says, "the feature which made the spectrum of the new star essentially distinct from that of any other star that had been previously observed was the presence of certain bright lines superposed on a spectrum with dark lines of one of the ordinary types. the position of certain of these lines showed that one of the luminous gases must be hydrogen." of this particular star (betelgeuse) it is said (proctor's "familiar essays"), "red stars and variable stars affect the neighborhood of the milky way or of well-marked star-streams. the constellation orion is singularly rich in objects of this class. it is here that the strange 'variable' betelgeuse lies. at present this star shows no sign of variation, but a few years ago it exhibited remarkable changes." we thus see that betelgeuse is a variable star, and it must have passed in its different variations between the limits of extreme brilliancy, in which the lines of hydrogen appear bright, and that of a less brilliant stage, in which they appear dark,--that is, as absorption bands. it has thus, in fact, run the gamut, so to speak, of color changes, and now occupies an intermediate position in the scale. in his article "star unto star," the same writer says, "on this view we may fairly assume that the darkness of the hydrogen lines is a characteristic of stars at a much higher temperature than our sun and suns of the same class." we have already seen that the spectra of stars of the fourth type--appleton's cyclopædia, "spectrum analysis"--"show the lines of hydrogen bright instead of dark, as though surrounded by hydrogen glowing with a heat more intense than that of the central orb itself." professor dunkin says, in his work "the midnight sky," "one of the conclusions drawn by kirchhoff from these experiments is that each incandescent gas weakens, by absorption, rays of the same degree of refrangibility as those it emits; or, in other words, that the spectrum of each incandescent gas is reversed when this gas is traversed by rays of the same refrangibility emanating from an intensely luminous source which gives of itself a continuous spectrum like that of the sun." ... "the third division, including betelgeuse, antares, alpha herculis, and others of like color, seems to be affected by something peculiar in their physical composition, as if their photospheres contained a quantity of gas at a lower temperature than usual. the stars in this class have generally a ruddy tint, probably owing to their light having undergone some modification while passing through an absorbing atmosphere.... a great number of the stars in the third division are variable in their lustre." we may therefore readily conclude that midway between the inverted lines which constitute the dark absorption bands and the faint spectra which show the bright lines of hydrogen direct there must be an atmosphere of glowing hydrogen superposed upon a deeper one in such proportion that it will not reveal its presence in the spectroscope at all; for when the dark and light bands, which occupy precisely the same position in the spectrum, are of approximately equal intensity the result will obviously be the neutralization of both. that among a myriad suns, some with dark hydrogen lines and some with bright, there should occur occasionally an example corresponding to this point of divergence, and especially among variable stars, is not only to be expected, but is, in fact, confirmatory of the general hypothesis itself. it is an exception which emphatically proves the rule, when we can trace the operative cause which has produced it. chapter vi. the phenomena of the stars. let us now consider the phenomena of the double stars. these were formerly believed to be single orbs, but the more powerful telescopes of recent years have shown them to consist of two suns, each substantially similar to our own sun, revolving around each other at a relatively small distance apart. in appleton's cyclopædia, article "star," we read, "it is noteworthy that few simple stars show such colors as blue, green, violet, or indigo; but among double and multiple star systems not only are these colors recognized, but such colors as lilac, olive, gray, russet, and so on. a beautiful feature in many double stars remains to be noticed: it is often found that the components exhibit complementary colors. this is oftener seen among unequal doubles, and then the larger component shows a color from the red end of the spectrum, as red, orange, or yellow, while the smaller shows the corresponding color from the blue end, as green, blue, or purple. the colors are real, not merely the result of contrast, for when the larger star is concealed the color of the smaller remains (in most cases) unchanged. spectrum analysis shows that the colors of many double stars are due to the absorptive vapors cutting off certain portions of the light.... the components are circling around each other, or rather around their common center of gravity." professor ball, in his work "in the high heavens," says, "there is no more pleasing phenomenon in sidereal astronomy than that presented by the contrasted hues often exhibited by double stars.... it seemed not at all impossible that there might be some optical explanation of colors so vividly contrasted emanating from points so contiguous. it was also remembered that blue stars were generally only present as one member of an associated pair.... when, however, dr. huggins showed that the actual spectrum of the object demonstrated that the cause of the color in each star arose from absorption by its peculiar atmosphere, it became impossible to doubt the reality of the phenomena. since then it has been for physicists to explain why two closely neighboring stars should differ so widely in their atmospheric constituents, for it can be no longer contended that their beautiful hues arise from an optical illusion." of these double stars with complementary colors we quote the following from professor dunkin (who, in turn, quotes from admiral smyth, the author of "sidereal chromatics"): "in eta cassiopeiæ the large star is a dull white and the smaller one lilac; in gamma andromedæ, a deep yellow and sea-green; in iota cancri, a dusky orange and a sapphire blue; in delta corvi, a bright yellow and purple; and in albiero, or beta cygni, yellow and blue. in most of the remaining stars of the list the contrasting colors are equally marked, and also in many others which are not included in it." some of these double stars are variable in their colors, as are the ordinary single variables, and, of course, for a similar reason,--to wit, the varying intensity of more or less cumulative planetary impacts. the interpretation, of course, as explained below, is that these suns, each one of different mass and consequently of different electrical resistance, are arranged in parallel circuit along a single line of electric current; a pair of different-sized arc or incandescent lamps, similarly arranged, would exhibit precisely the same phenomena. a compound solar system of this sort, apparently, with double sun and single planetary system in process of formation, nearly completed from a spiral nebula, is shown in a gaseous nebula within the constellation ursa minor, illustrated in lord rosse's drawing (see nichols "architecture of the heavens," plate x., lower figure). more than three thousand of these binary stars have been catalogued, and some of them make a complete revolution about their common centers of gravity--so distant are they from each other--in periods of not less than sixty, or even eighty, years. of the double star mizar,--the middle one of the three which form the tail of the great bear,--professor ball states that, by new methods of spectroscopic analysis, the component stars which form this double have been found to be one hundred and fifty millions of miles apart, while alcor, a smaller star, visible to the naked eye, and enormously farther from mizar than are the components of the latter from each other, moves through space in a parallel direction and with the same velocity as its double companion. what the connection may be, if any, we do not know, but their identical course is obviously related to some common circumstance of origin, as is the probable case with those other groups of stars which drift through space together. they show that solar systems are not necessarily individual creations, but may be formed in groups at the same period of time, and by the operation of natural laws simultaneously directed upon or into the creative matter from which solar systems are built up and sent along their way. it has been already shown that our sun has a motion around the center of gravity of our own solar system, as a whole, similar to that of the binary stars around each other, but that, by reason of his vast relative mass (seven hundred and fifty to one for all the planets), this center is always within the confines of his own volume. if, however, our sun were divided into two suns one, two, or five million miles apart, each revolving around a common center of gravity situated between the two, and the planets revolving around the same center of gravity, but relatively more distant, the planets would thus rotate around both suns as a common center, and with the electric polarity of both suns the same, as must necessarily be the case, they would present phenomena precisely similar to those exhibited by the double stars. and such might very easily be the case in even a system so small as our own, for the planet mercury has so elliptical an orbit that its distance from the sun varies in different parts of its annual movement from twenty-eight to forty-five millions of miles. there would then be mutual electric repulsion of the two solar electrospheres, such as we see in the case of comets and in the sun's corona and long streamers. professor proctor, article "the sun's long streamers," says, "these singular appendages, like the streamers seen by professor abbe, extend directly from the sun, as if he exerted some repellent action.... i cannot but think that the true explanation of these streamers, whatever it may be (i am not in the least prepared to say what it is), will be found whensoever astronomers have found an explanation of comets' tails.... whether the repulsive force is electrical, magnetic, or otherwise, does not at present concern us, or rather does concern us, but at present we are quite unable to answer the question." a similar example is to be found in the self-repellent positive electrospheres of the earth and moon, illustrated on a previous page, which, in fact, are types among planets of precisely what we find in double stars. now, if these double central suns, with a common system of planets revolving around them both, differ one from the other in size, they will differ also in the depth and density of their hydrogen atmospheres, and the electric forces directed against them will produce different results in each. in one we will have high temperature, great volatilization, and wide absorption bands; in the other, a shallow atmosphere, a temperature below that of an extensive volatilization of its metallic components, and a spectrum rich in light at the blue end, while the former one will be correspondingly richer in the yellow and red rays at the opposite and lower end of the spectrum. one, in fact, will manifest the phenomena of blue-white stars, the other, those of orange-red, but variously modified in a chromatic series. the case may be extended to multiple stars, and complementary colors, more or less perfect, may be almost predicated as the law of compound solar bodies having cores like that of our sun, but each of different mass, and surrounded by hydrogen atmospheres of different depths and densities, both acted upon by the same exterior planetary electrical currents. it is certainly true of double stars, and probably so of all the others. of course such enormously massive double suns presuppose enormous planets, rotating around them at enormous distances; but when we compare the distance of our own satellite, the moon, from the earth with the distance of neptune from the sun, and consider that the light of the sun will reach neptune in about four hours, and then compare this distance with the inconceivable distances of space requisite to retard and merge all radiant energy into the diffused molecular energy of position, our wonder will cease. we have also to consider those single stars which (see appleton's cyclopædia, article "star") are variable in their brilliancy. "these stars may be divided into periodic variables, irregular variables, and temporary stars. periodic variable stars are those which undergo increase and diminution of light at regular intervals. thus, the star mira, or omicron of cetus, varies in lustre, in a period of three hundred and thirty-one and one-third days, from the second magnitude to a faintness such that the star can only be seen with a powerful telescope, and thence to the second magnitude again. it shines for about a fortnight as a star of the second magnitude, and then remains invisible for five months, the decrease of lustre occupying about three months, the increase about seven weeks. such is the general course of its phases. it does not always, however, return to the same degree of brightness, nor increase and diminish by the same gradations; neither are the successive intervals of its maxima equal. from recent observations and inquiries into its history, the mean period would appear to be subject to a cyclical fluctuation embracing eighty-eight such periods, and having the effect of gradually lengthening and shortening alternately those intervals to the extent of twenty-five days one way and the other. the irregularities in the degree of brightness attained at the maximum are probably also periodical.... it suggests a probable explanation of these changes of brightness, that when the star is near its minimum, its color changes from white to a full red, which, from what we know of the spectra of colored stars, seems to indicate that the loss of brightness is due to the formation of many spots over the surface of this distant sun. "algol is another remarkable variable, passing, however, much more rapidly through all its changes. it is ordinarily a second-magnitude star, but during about seven hours in each period of sixty-nine hours its lustre first diminishes until the star is reduced to a fourth magnitude, and after it has remained twenty minutes at its minimum its lustre is gradually restored. it remains a second-magnitude star for about sixty-two hours in each period of sixty-nine hours. these changes seem to correspond to what might be expected if a large opaque orb is circling around this distant sun in a period of sixty-nine hours, transiting its disk at regular intervals." of this star, professor ball says, "applying the improved spectroscopic process to algol, he [vogel] determined on one night that algol was retreating from the earth at a speed of twenty-six miles per second.... when vogel came to repeat his observations, he found that algol was again moving with the same velocity, but this time towards the earth instead of from it.... it appeared that the movements were strictly periodic; that is to say, for one day and ten hours the star is moving towards us, and then for a like time it moves from us, the maximum speed being ... twenty-six miles a second.... it is invariably found that every time the movement of retreat is concluded the star loses its brilliance, and regains it again at the commencement of the return movement.... the spectroscopic evidence admits of no other interpretation save that there must be another mighty body in the immediate vicinity of algol.... algol must be attended by a companion star which, if not absolutely as devoid of intrinsic light as the earth or the moon, is nevertheless dark relatively to algol. once in each period of revolution this obscure body intrudes itself between the earth and algol, cutting off a portion of the direct light from the star and thus producing the well-known effect." this is, in fact, a periodic transit or eclipse of algol by a planet, such as we see in eclipses of our own sun by the moon and the inner planets, except that algol's planet is apparently single like our moon with reference to the earth, and that it is relatively much larger than any of our own planets, as we would necessarily suppose it to be, if solitary. its mass has been computed by the effects which it produces, and we learn that it is not a dark sun with a brilliant planet, but a brilliant sun with a dark planet, just as our solar system presents. "algol, at the moment of its greatest eclipse, has lost about three-fifths of its light; it therefore follows that the dark satellite must have covered three-fifths of the bright surface.... the period of maximum obscuration is about twenty minutes, and we know the velocity of the bright star, which, along with the period of revolution, gives the magnitude of the orbit." from these data it has been computed that the globe of algol itself is about one-fourth larger than that of our visible sun, but its mass is so much less that its weight is only one-half that of our sun, so that its body is probably gaseous. the author concludes, "no one, however, will be likely to doubt that it is the law of gravitation, pure and simple, which prevails in the celestial spaces, and consequently we are able to make use of it to explain the circumstances attending the movements of algol's dark companion. this body is the smaller of the two, and the speed with which it moves is double as great as that of algol, so that it travels over as many miles in a second as an express train can get over in an hour. the companion of algol is about the same size as our sun, but has a mass only one-fourth as great. this indicates a globe of matter which must be largely in the gaseous state, but which, nevertheless, seems to be devoid of intrinsic luminosity. their distance [apart] is always some three million miles. this is, however, an unusually short distance when compared with the dimensions of the two globes themselves." with this exception, the author says, "the movements of algol and its companion are not very dissimilar to movements in the solar system with which we are already familiar." it will be seen that the want of luminosity in the dark companion of algol finds a ready explanation in the fact that it is a planet, acting precisely as our own planets do, and that the luminosity of algol itself is directly attributable to the electricity developed by the presence of this planet rotating axially and orbitally around it, and the darkness of the planet itself is the necessary correlative of the heat and light of its sun. the planet has about one-half the density of saturn, while algol has one-half the density of the sun, and hence we should expect to find on algol an atmosphere largely composed of glowing hydrogen, and on its planet an atmosphere largely composed of oxygen, in which, doubtless, float enormous clouds of aqueous vapor. the interpretation is direct and conclusive, and upon no other hypothesis can the facts be explained, for their close connection with each other demonstrates their common origin, and their masses are not so different one from the other as to permit, on any theory of their coequal origin as suns, one to glow with the fires of youth and energy and the other to have grown dark and dead from old age and exhaustion, and especially so if still in its gaseous stage, which is that which must characterize its highest state of incandescent energy from the most active condensation of its volume, if the nebular hypothesis has any validity whatever. in fact, this example alone, if the constitution of algol's dark satellite is really gaseous, must go very far to throw the gravest doubt, in itself, on the validity of this hypothesis. the star beta, of the constellation lyra, has a full period of twelve days and twenty-two hours, divided into two periods of six days and eleven hours, in each of which the star has a maximum brightness of about the three and one-half magnitude, but in one period the minimum is about the four and one-third magnitude, while in the other it is about the four and one-half magnitude. this peculiarity points, it is said, to an opaque orb with a satellite, the satellite being occulted by the primary in the alternative transits, and therefore the loss of light is less. the star delta of cepheus is quite different, however, for, while it takes only one, day and fourteen hours in passing from its minimum to maximum of brightness, it occupies three days and nineteen hours, or somewhat more than double this time, in passing from maximum to minimum. two or three hundred of these variable stars are already known. the above examples are cited in detail because they furnish the strongest possible proof of the truth of the hypothesis which we are endeavoring to present. while the movements of the stars algol and beta lyræ may find an adequate interpretation in the one case in a large occulting planet, and in the other in an occulting planet with a satellite, it is obvious that mira and delta cephei cannot be explained except by the presence of planetary bodies or satellites which do not mechanically occult the light of their suns. in these regularly variable stars it is the light which varies, but of course the solar heat must vary also,--that is to say, the solar energy varies regularly, but with unequal periods of growth and decline and with larger periods of cyclical variation in addition. such variations can only be produced by the action of permanently connected and orbitally rotating planetary bodies, acting dynamically through space, to regularly increase and diminish the solar energy, and such bodies can only do this by their orbital positions with reference to each other and to the central sun itself. in this case, since the activity of solar energy is most unquestionably varied by the planetary energies, by their position and movements, at least a portion of solar energy must be due to planetary action, and if this be so, it may be affirmed with certainty that substantially all solar energy may be produced in the same way; for, otherwise, we seek for two diverse causes to produce a single effect, which may be produced by one. we have no knowledge, however, of any planetary energy which could operate to increase or diminish the energy of the central sun in its emission of light, except that which we have already presented, and no theory of our own sun's energy hitherto advanced has ever taken cognizance of the planetary energies of our system as an effective cause for those of the sun. but while the sun's energy is--as it must be in this case--the outcome of that of the planets, it is equally obvious that the planets themselves can have no permanent, inherent energy of their own to generate or modify such energy of the sun, since they are in fact supplied by the solar energy, and their motions are controlled and regulated by the sun itself. hence the inference is irresistible that the planets must derive their primary force from an external source not solar, and this they can only do by means of their rotation in space, and the only force derivable from space of which we have any knowledge is electricity, so that the circle thus becomes complete. how now shall we explain these periodical aberrations of energy? the color of a star, as we know, is no criterion of its age or size. the color is due to atmospheric absorption of the radiant light. the double stars, for example, revolve around each other at regular periods, and they are necessarily of nearly the same age, as sidereal ages are computed, but they frequently differ one from the other in color, and multiple stars may be all different each from the others; and the color, as before stated, is no criterion of size, for a small sun, with its glowing hydrogen in a state of high incandescence, and with few absorption bands in its spectrum, will appear bluish-white, or of that specific type of stars, without reference to size, while a much larger sun, with its light darkened by broad absorption bands and sun-spots, will appear orange or red; and, consequently, difference of color can be no criterion of distance, since a blue-white star of small size will outshine a red orb of much greater magnitude, whether it be more or less distant. the variable stars, for these reasons, belong to the order of red stars mostly, if not altogether. we must also bear in mind that sun-spots do not diminish the solar heat, as they are the result of increased and not of diminished energy. electric currents of high potential pass directly, as we know, along the lines of least resistance to their opposite center of polarity, so that two planets nearly in conjunction with each other transmit their currents almost directly towards the sun's center, and upon the same point of solar latitude, while, if at right angles with the sun, they must deliver their electricity along converging lines and thus strike the solar surface at different points. currents of electricity of high potential also (see "electricity in the service of man," page ), by their own passage, facilitate the passage of succeeding currents, so that generators discharging along the same lines find less and less resistance. it is true that we find no appreciable resistance in the passage of these currents between the earth and the sun, as their velocity is that of light, but both light and electricity may be equally retarded by resistance in a small degree. we know also that in the condensed hydrogen atmosphere of the sun there must be resistance, and also that the resistance in fluids diminishes as the temperature rises. considering now the variable star mira, as above described, we observe, as is the case with delta cephei, also cited, that the period between its greatest light, in a descending scale, and its least is about twice as long as its rise from minimum to maximum. during a period of four years ( to ) it is said that it was not visible at all. if mira be considered a relatively small sun, with its axis strongly inclined to the planetary plane, and having three planets only, two of them constituting a double planet, like the earth and moon, but nearly equal in size, and having a rotation about the sun in nearly eleven months and a rotation about each other in the same period, and, besides these, a much more distant large planet, something like our jupiter, with an orbital period of many years, so that the cycle of relative positions is complete in about eighty-eight of the shorter periods of variation, we would have such results as we see in mira. twice in each revolution of the double planet its two members and their sun would be in conjunction, and we would have great brilliancy and whiteness until the metallic elements began to volatilize in increased proportions; then an era of wide absorption bands and redness, gradually increasing to a maximum after its periods of greatest light, and then slowly diminishing as the double planet advanced in its rotation; and, finally, as it again approached conjunction, the brilliant hydrogen illumination, subsequently followed by the gradually darkened spectrum, and so on, while the large outer planet by its various positions would first relatively retard and then accelerate the variation until its grand cycle was complete. the permanent disappearance for years, if true, may be due to other causes, which will be referred to in considering the phenomena of new and temporary stars. many of the irregular variables may doubtless be similarly explained,--our own sun, in fact, being a variable with a period of about eleven years,--and doubtless the apparent irregularity in most cases is due to lack of sufficient time for observation. those stars which are in fact really irregular in their variation owe their changes, doubtless, to the same causes which produce new stars, so called, and "suns in flames," which will be next considered. among the countless stars of heaven a great catastrophe seems occasionally to occur. a star bursts out into sudden flame, to all appearance, or a great fixed star appears where no star had ever been seen before. in professor proctor's article, "suns in flames" ("myths and marvels of astronomy"), we will find an extended discussion of these wonderful phenomena. the astronomer tycho brahe described the one which appeared in as follows: "it suddenly shone forth in the constellation cassiopeia with a splendor exceeding that of stars of the first magnitude, or even jupiter or venus at their brightest, and could be seen by the naked eye on the meridian at full day. its brilliancy gradually diminished from the time of its first appearance, and at the end of sixteen months it entirely disappeared, and has never been seen since. during the whole time of its apparition its place in the heavens remained unaltered, and it had no annual parallax, so that its distance was of the same order as that of the fixed stars." tycho described its changes of color as follows: first, as having been of a bright white; afterwards of a reddish-yellow, like mars or aldebaran; and, lastly, of a leaden white, like saturn. in a first-magnitude star suddenly appeared in the right foot of ophiucus. "it presented appearances resembling those shown by the former, and disappeared after a few months." many other cases are cited by astronomers, and in "a star appeared in the northern crown, the observations of which threw great light on the subject of so-called new stars. in the first place, it was found that where this new star appeared there had been a tenth-magnitude star; the new star, then, was in reality a star long known, which had acquired new brilliancy. "when first observed with this abnormal lustre, it was shining as a star of the second magnitude. examined with the spectroscope, its light revealed a startling state of things in those remote depths of space. the usual stellar spectrum, rainbow-tinted and crossed by dark lines, was seen to be crossed also by four exceedingly bright lines, the spectrum of glowing hydrogen.... the greater part of the star's light manifestly came from this glowing hydrogen, though it can scarcely be doubted that the rest of the spectrum was brighter than before the outburst, the materials of the star being raised to an intense heat. the maximum brightness exceeded that of a tenth-magnitude star nearly eight hundred times. after shining for a short time as a second-magnitude star, it diminished rapidly in lustre, and it is now between the ninth and tenth magnitudes" (appleton's cyclopædia). of this new star, professor ball says, "another memorable achievement in the early part of dr. huggins's career is connected with the celebrated new star that burst forth in the crown in . it seemed a fortunate coincidence that just at the moment when the spectroscope was beginning to be applied to the sidereal heavens a star of such marvellous character should have presented itself.... the feature which made the spectrum of the new star essentially distinct from that of any other star that had been previously observed was the presence of certain bright lines superposed on a spectrum with dark lines of one of the ordinary types. the position of certain of these lines showed that one of the luminous gases must be hydrogen.... the spectroscope showed that there must have been something which we may describe as a conflagration of hydrogen on a stupendous scale, and this outburst would account for the sudden increase in luminosity of the star, and also to some extent explain how so stupendous an illumination, once kindled, could dwindle away in so short a time as a few days." it will be seen that these new stars leap suddenly into great brilliancy: it is a matter of a few hours only. after remaining a very short time in this stage of abnormal incandescence, they gradually die out again in lustre and revert to their original condition; they are not consumed either in body or atmosphere. several theories have been advanced to account for these remarkable phenomena; see "suns in flames," by professor proctor. one is, in effect, that by some sudden "internal convulsion a large volume of hydrogen and other gases was evolved from it, the hydrogen by its combination with some other element giving out the lines represented by the bright lines, and at the same time heating to a point of vivid incandescence the solid matter of the star's surface.... as the liberated hydrogen gas became exhausted the flame gradually abated, and with the consequent cooling the star's surface became less vivid and the star returned to its original condition;" which, by the way, it never could have done if its atmosphere had been exposed to such a disintegration, without the construction of an entirely new atmosphere precisely similar to the one just destroyed. the process would be one of simple combustion. it requires the evolution of enormous volumes of hydrogen from within the planet, and of other enormous volumes of something else, by which to burn it up and yet not burn up the original hydrogen envelope. this other element could not have previously existed outside the solar body and contiguous thereto, or it would have burned up the ordinary hydrogen envelope of the sun long before, as well as the metallic vapors floating therein. both these mutually hostile gases must have come from within, and this is manifestly impossible, as we should thus have explosion and solar destruction, but not combustion. there is no reason to believe that hydrogen, the lightest of elements, could have remained occluded within the solar mass, to the exclusion of the heavier metals, if disassociated, and if held combined no such sudden liberation could occur. besides, such convulsion would be impossible in any sun at all resembling ours, as any further liberation of gases from internal condensation must be due to solar contraction, hence gradual, and not sudden. moreover, such liberation of hydrogen gas from within would show its spectrum loaded, at its earliest eruption, with absorption bands; and, finally, the convulsion presupposes as great an activity, and consequently as great a difficulty, before the phenomenon as the phenomenon itself presents; for such vast disturbance of mass would be more difficult to account for, and require more energy to produce, than the results themselves. moreover, the whole mass of the star appeared to increase equally in temperature, as shown by the spectrum, and, if produced by an internal convulsion, this must have extended to, if not proceeded from, its core; so that while the combustion of hydrogen might have ceased in a very brief time, the intense heat of the solar mass could not have been dissipated for thousands of years. it would, in fact, have disrupted the whole orb. another theory is that this vast incandescence was caused by the "violent precipitation of some mighty mass--perhaps a planet--upon the globe of that remote sun, by which the momentum of the falling mass would be changed into molecular motion; in other words, into heat and light." this theory is no more plausible than the other, since it fails to account for the enormous volume of hydrogen, with bright lines, as a result of such contact; while professor proctor very clearly shows that such contact would have been preceded, necessarily, by repeated partial grazings, as the outside body repeatedly passed in swifter and closer passage by the sun in its gradually approaching orbital revolutions, and that the increase of light and heat must have been measured by years instead of by hours. the same difficulties exist in the supposed passage of the star through nebulæ or star clouds, of which professor proctor says, "as for the rush of a star through a nebulous mass, that is a theory which would scarcely be entertained by any one acquainted with the enormous distances separating them.... all we certainly know suggests that the distances separating them from each other are comparable with those which separate star from star." in fact, no tenable theory has been advanced which will cover the phenomena. professor proctor describes a star which flamed out in . at midnight, november , a star of the third magnitude was noticed in the constellation of the swan; its light was very yellow; its brilliancy rapidly faded. on december it was equal to a star of the fifth magnitude only, and the color, which had been yellow, was now greenish-blue. "the star's spectrum at this time consisted almost entirely of bright lines. december he found three bright lines of hydrogen, the strong double line of sodium, the triple line of magnesium, and two other lines. one of these last seemed to agree exactly in position with a bright line belonging to the corona seen around the sun during total eclipse." the star afterwards faded away gradually until quite invisible to the naked eye. it will be noticed that none of the above elements--sodium, potassium, or magnesium--are such as would combine with hydrogen to produce the phenomena in question. professor proctor concludes, "this evidence seems to me to suggest that the intense heat which suddenly affected this star had its origin from without." he suggests possible meteoric flights; but meteoric stones themselves are separated in space by enormous distances, and these, if converged in orbital flight, would present the same phenomena of successive grazings as a small planet approaching under like circumstances, and by their gradually increasing incandescence we should certainly have other elements visible in the spectroscope besides those observed. and these meteoric bodies, if projected into the sun, would pass in a very brief time through the hydrogen envelope, producing only local phenomena, so that their first blow would be manifested in volatilization of the outer portions of the mass and broad absorption bands, and consequent redness of the planet, exhibiting great heat, but not great light. in such case the bright lines of hydrogen, if they appeared at all, would only be visible as an after-consequence, and not at the earliest moment of conflagration,--that is, the star might grow from red to white, but by no possibility the reverse. it is, however, characteristic of these new stars that their first flash, as it were, is into the incandescence of directly glowing hydrogen, with its bright lines, then through a series of gradually increasing sun-spots, and finally a slow return to their original condition and apparent magnitude. it is obviously a surface phenomenon of the solar atmosphere, primarily, then followed by consequences involving only the outer surface of the solar core, but with no observable permanent change in the character or constitution of the mass of the sun itself. these characteristics are invariable, and the sequence of phenomena is the same in all the cases observed. chapter vii. temporary stars, meteors, and comets. what, then, is the probable cause of these terrific conflagrations, as they appear to us? take an ordinary electric induction machine,--a holtz or a wimshurst,--and, if the surrounding air is moist, as we operate it we will find that the results are poor, the sparks short and relatively few; but let us take the machine into another room in which the atmosphere is dry and crisp. a wondrous change will occur, and instead of a current which could scarcely flash across a few inches of space, we will now have so great an increase of energy that its tension will even cause the spark to perforate and destroy the glass walls of the heavy leyden jars in which it is condensed. the vast realms of space, with their attenuated vapors, are the field in which the planetary electric generators operate, and into which, likewise, myriads of suns constantly pour their light and heat. we may consider this space, according to the popular view, to be uniform in constitution and density throughout all its parts,--that it is, in fact, like a vast, silent, and motionless dead sea. but this cannot possibly be true, any more than throughout the vast compass of our own atmosphere; for while some parts of space are peopled by millions of solar systems, others, as we can plainly see, so far as telescopic vision extends, are comparatively vacant. far more electricity is being abstracted (so to speak) in some parts of space than in others, and far more heat and light are being poured back to restore the equilibrium in some than in others. we have already seen that the temperature at the exterior surface of the terrestrial atmosphere is estimated to be more than two hundred degrees higher than in the realms of open interplanetary space; hence there must be currents,--currents of rotation like cyclones, vortical currents like whirlwinds, currents of transmission like our land- and sea-breezes and the trade-winds,--and, in fact, all space must be in a state of constant displacement and replacement, and, if visible, we should see it like a vast room filled with smoke, in which currents of every shape and direction and of all velocities would be manifest. such currents could throw nebulæ during their condensation into rotation which could never rotate of their own motion, or gather to centers of aggregation vast whirling clouds of spatial matter, and in the spiral nebulæ we may see many such movements of rotation in apparent active progress. of these we read in appleton's cyclopædia, "they have the appearance of a maelstrom of stellar matter, and are among the most interesting objects in the heavens." in professor nichol's splendid work ("the architecture of the heavens," ) we may see magnificent engravings of these wonderful phenomena, from the drawings by lord rosse, and no one can study these figures without realizing the presence of vast currents in space. in the great spiral nebula in the constellation canes venatici (see illustration in chapter xii.) we perceive that the tail of the smaller nebula has been drawn into the outer convolution of the great spiral, against the radial repulsion of the latter nebula, as we can see by its curvature. this can only be due to a tremendous inflowing current in space. were the deflection due to gravity the trend would be to the center and not to the outer convolution of the larger nebula. professor nichol says, "the spiral figure is characteristic of an extensive class of galaxies." not only in the spiral, but in other forms of nebulæ we may observe these currents of space, so that we cannot fail to perceive that they exist, and we should even conclude, a priori, that these must exist. in the elongated linear nebula in sobieski's crown, illustrated above, its length is deflected into irregular curves apparently due to counter-currents of space. these gaseous nebulæ, flammarion says, "appear like immense vaporous clouds tossed about by some rough winds, pierced with deep rents, and broken in jagged portions." it may be said generally that every sun, as it drifts through space, must leave a wake of increased electric potential among the molecules which line its pathway. beyond the limits of every vortex extend radial or tangential, polar or equatorial, streams of space, and these must extend without limit until deflected or neutralized by other conditions. throughout all space, just as in our own atmosphere, but vastly more slowly, there must be an infinitude of movements in every direction,--movements in lines, circles, vortices, ellipses and irregular curvatures, and of all possible varieties of mass and volume. suppose, now, a sailing vessel lighted with incandescent lamps, the electrical currents for the support of which are derived from the chemical action of sea-water on multiple pairs of suitable metallic plates arranged to extend downward as a galvanic battery into the ocean as the ship sails along, and that these plates, by the chemical action of the sea-water at ordinary, temperatures, should furnish a sufficient current to properly light the vessel. if the constancy of such current depended on the average temperature of the sea-water, at, say, sixty degrees fahrenheit, we should find that, on suddenly crossing into the gulf stream, with a temperature twenty degrees higher, the energy of the battery would be rapidly increased and the lights would glow with increased brilliancy until, on emerging from the gulf stream at its opposite side, the original status would be gradually restored. if these distant solar systems, in their drift through space, should encounter a corresponding stream under an increased molecular tension, more highly heated, for example, or charged with electrical potential by the surrounding solar systems, or otherwise, we should expect a similar result to ensue,--that the currents would be increased suddenly, both in quantity and intensity, and all the phenomena of "blazing" stars be revealed in the precise order in which we see them. professor proctor seems to have had some such idea of space vaguely in his mind when he says, in his "familiar essays," "one is invited to believe that the star may have been carried by its proper motions into regions where there is a more uniform distribution of the material whence this orb recruits its fires. it may be that, in the consideration of such causes of variation affecting our sun in long-past ages, a more satisfactory explanation than any yet obtained may be found of the problem geologists found so perplexing,--the former existence of a tropical climate in places within the temperate zone, or even near the arctic regions. sir john herschel long since pointed to the variation of the sun as a possible cause of such changes of climate." in confirmation of the view that such changes may be due to the passage of a solar system into or through such a "gulf stream" of space, we quote the following from professor proctor's "suns in flames:" "it is noteworthy that all the stars which have blazed out suddenly, except one, have appeared in a particular region of the heavens,--the zone of the milky way (all, too, in one-half of that zone). the single exception is the star in the northern crown, and that star appeared in a region which i have found to be connected with the milky way by a well-marked stream of stars; not a stream of a few stars scattered here and there, but a stream where thousands of stars are closely aggregated together, though not quite so closely as to form a visible extension of the milky way.... now, the milky way and the outlying streams of stars connected with it seem to form a region of the stellar universe where fashioning processes are still at work." in just such regions of potential energy should we look for such currents in space, as, on our own earth, the gulf stream and the trade-winds, as well as cyclones and other atmospheric movements, find their origin under precisely parallel circumstances,--to wit, the outpour upon and direct precipitation of increased quantities of heat at the tropics or other local centers of such development. the effects of such an increase of quantity and potential in an electrical current are clearly illustrated in the device previously referred to, in which electrolytic decomposition was effected in a pail of water; we find it also in the burning out of the brushes and commutators in dynamo-electric machines and in telegraphic apparatus during thunder-storms and the like. allowing a solar system a drift through space only equal to that of our own, which has a relatively slow movement, it would traverse such a "gulf stream" of space seven hundred thousand miles wide in a single day. but it may not even have passed through; it may merely have grazed the margin of such a current; for the motions of solar systems are not controlled by the same forces as those upon which their electrical energies depend. professor ball, in his chapter on the great heat-wave of , says, "towards the end of july an extraordinarily high temperature, even for that period of the year, prevailed over a very large part of the north american continent. the so-called heat-wave then seems to have travelled eastward and crossed the atlantic ocean; ... a fortnight after the occurrence of unusually great heat in the new world there was a similar experience in the old world.... this discussion will at all events enable us to make some reply to the question which has often been asked, as to what was the cause of the great heat-wave.... it is, however, quite possible that certain changes in progress on the sun may act in a specific manner on our climate.... it cannot be denied that local, if not general, changes in the sun's temperature must be the accompaniment of the violent disturbances by which our luminary is now and then agitated. it is, indeed, well known that there are occasional outbreaks of solar activity, and that these recur in a periodic manner; it is accordingly not without interest to notice that the present year has been one of the periods of this activity. we are certainly not going so far as to say that any connection has been definitely established between a season of exuberant sun-spots and a season remarkable for excessive warmth; but, as we know that there is a connection between the magnetic condition of the earth and the state of solar activity, it is by no means impossible that climate and sun-spots may also stand in some relationship to each other." these local deviations are doubtless due to planetary positions with reference to the sun, but more general variations must depend upon the constitution of such parts of space as the solar system may occupy; but even then they will be but temporary, since the sun's volume will rapidly expand or contract so as finally to restore the normal emission of solar heat, as will be further explained later on in this work. there are other causes also, readily conceivable, for such increased electrical action; for instance, in that thickly-peopled region of space, two solar systems adjacent might easily have their exterior planets so related to each other as suddenly, at their points of nearest approach, to cause one or more to direct an abnormally large electrical current into the sun of the adjacent system; this would correspond in electric energy, in fact, to a violent "perturbation" in its orbit by the action of gravity produced by a neighboring planet or system. no reversal of polarity could take place between these planets under these circumstances, any more than between the earth and the moon. in some portions of the milky way, doubtless, suns blaze by dozens across the sky at night, and by day as well, to which, in our more solitary skies, we are strangers. revolving in perfect harmony, perturbations must nevertheless be frequent, and to what limits they may there be confined we shall never know until we realize the extent of these galaxies and the relative contiguity of their solar systems to each other. it is enough to show how such variations may occur; in what particular way they do occur does not affect the question of their origin. even if such increased energy were to continue by permanently increased planetary action, it is not necessary to suppose that a corresponding permanent increase of light and heat would result on the part of the sun, for its density is such (only one-fourth that of the earth) that, under the tremendous force of its gravity (twenty-seven and one-tenth times that of the earth), its constituents cannot be maintained in solid form, but must be, as before stated, either liquid or gaseous, and perhaps in part both. now, as it has been computed that the sun, by contraction to its present density, would have evolved its present light and heat for a period of millions of years, it is obvious that any increase in its present volume, without increase of mass, would produce precisely opposite and compensated results, so that the sun could receive from outside sources as much heat as would expand its present volume to that at the initial point of such assumed condensation without increased emission of light and heat. the sun is thus, in effect, a self-compensating machine, and its passage through a region of increased electrical generation would first manifest itself in a vast increase of brilliancy, due to higher incandescence of its hydrogen envelope; this, in turn, would be communicated to the deeper structures of the sun, producing increased volatilization and dark absorption bands, and finally to the whole solar mass, expanding its volume in proportion to the heat absorbed. hence we should see precisely the phenomena that we do see in flaming stars or so-called new stars. we find such compensations all through nature, and it is simply in accordance with her universal laws that they occur. it is a singular circumstance that the catastrophe which is foretold in the biblical record as the termination of all human life on earth, for the present cycle, at least, should be almost literally in accordance with the phenomena characteristic of such an increase of solar energy, and one produced in some such manner. if the temperature of the solar atmosphere were rapidly raised by increased planetary action to a point which would reverse the lines of hydrogen from dark to bright, say to a brightness eight hundred times that of the normal, as in the case of the temporary star cited, though the heat would not, of course, be increased in any such proportion, yet the heavens would be indeed rolled up as a scroll, and all life would be extinguished in a very brief period. but the planets would continue to roll along their orbits, the integrity of the earth's mass would still be intact, and after a few days or weeks the sun would begin to decline in brightness, the volatilized vapors would slowly recede within the solar atmosphere, and the temperature would gradually fall again to its normal, leaving, however, a lifeless world to roll on its way henceforth, but as bright and cheerful in all its possibilities, when the former conditions had gradually become restored, as before. perhaps some distant astronomer in the neighborhood of sirius--if we shall have travelled so far away by that time--might send a note to the morning papers to announce that the temporary star near alpha centauri had again receded to the tenth magnitude. in due time--perhaps a thousand years--all would be ready for a new development of life, and the cycle would continue as before. perchance, too, in some deep abyss, or buried far beneath the surface, some germs of life might still continue to exist; and from these, like the seeds resurrected from buried mummies, a new life might again begin, guided along once more through vast ages in a progressive ascent from development to development until, in some new and strange forms, the higher types of life might again appear. to these there would indeed be revealed a new heaven and a new earth. who knows how many such cycles of life may have come and gone on earth, in which, like the dwellers of jerusalem, new peoples have built new cities, one above another, upon the unknown graves of the past? in the words of tennyson,-- "a wondrous eft was of old the lord and master of earth, for him did his high sun flame, and his river billowing ran, and he felt himself in his force to be nature's crowning race. as nine months go to the shaping an infant ripe for his birth, so many a million of ages have gone to the making man: he now is first, but is he the last?" whatever the coming, the progress, or the going of life on earth, the course of our solar system will go on the same, the processes of creation unchanged and her mechanism unimpaired. it is obvious that no such conditions could prevail in the return to unorganizable chaos which must be the consequence of any possible planetary collisions in space. no conceivable process of creation could return a system disrupted into meteorites to an operative solar system again. even the nebular hypothesis contemplates nothing of that sort as, by the wildest conjecture, ever possible. but with us the danger is far distant. professor proctor says, in his article "suns in flames," "as sir william herschel long since pointed out, we can recognize in various parts of the heavens various stages of development, and chief among the regions where as yet nature's work seems incomplete is the galactic zone,--especially that half of it where the milky way consists of irregular streams and clouds of stellar light. as there is no reason for believing that our sun belongs to this part of the galaxy, but, on the contrary, good ground for considering that he belongs to the class of insulated stars, few of which have shown signs of irregular variation, while none have ever blazed suddenly out with many hundred times their former lustre, we may fairly infer a very high degree of probability in favor of the belief that, for many ages still to come, the sun will continue steadily to discharge his duties as fire, light, and life of the solar system." the passage of our system through gradually changing regions of space, as contrasted with streams or vortices, could not affect our sun's light even temporarily, as the contraction and expansion of its volume would fully compensate for any such gradual or partial variation, and, by position, he is far from likely to pass into any of those whirlpools or torrents of space which seem to mark at irregular intervals the region of the irregularly variable stars. allied in appearance to such stars which suddenly flame out in space, but totally different in reality, are comets. these strangers to our own system have excited the wonder and astonishment of mankind from the earliest ages. they seem to defy all rules and all explanation; but, when properly examined, they will fall inevitably into the general scheme of the source and mode of solar energy which we have endeavored to present. these bodies enter our solar system from without. appleton's cyclopædia says, "schiaparelli, to whom the discovery is in part due, considers the meteors to be dispersed portions of the comet's original substance,--that is, of the substance with which the comet entered the solar domain." professor proctor, "meteoric astronomy," says, "a word or two may be permitted on the question of the condition of comets freshly arriving on the scene of the solar system. it is assumed sometimes that the train of meteors already exists when the comet first comes within the solar domain." in the "romance of astronomy" (r. kalley miller, m.a.) it is said, "in a sort of debatable territory between our own solar system and the infinite stellar universe around we come upon these erratic and anomalous bodies--the comets; some of which have accidentally become permanent attendants upon our sun; others have only paid it a single casual visit in the course of their wanderings through space, and are not likely again to come within the range of its attracting influence; while countless millions are doubtless scattered throughout the realms of the infinite, whose existence will never be revealed to human ken at all." professor helmholtz, in fact (see addendum to his lecture on the origin of the planetary system), advanced the idea in a speculative way, that our terrestrial life might have had its origin in one of these meteoric bodies by the "transmission of organisms through space." in professor proctor's article on comets ("mysteries of time and space") he says, "the paths followed by comets show no resemblance either to the planetary orbits or to each other. here we see a comet travelling in a path of moderate extent and not very eccentric; then another which rushes from a distance of two or three thousand millions of miles, approaches the sun with ever-increasing velocity until nearer to him than parts of his own corona (as seen in eclipses), sweeps around him with inconceivable rapidity, and makes off again to where the aphelion of its orbit lies far out in space beyond the most distant known planet,--neptune. some comets travel in a direct, some in a retrograde path; a few near the plane of the earth's orbit, many in planes showing every variety of inclination. some comets regularly return after intervals of a few years; some after hundreds of years; others are only seen once or twice, and then unaccountably vanish; and not a few show by the paths they follow that they have come from interstellar space to pay our system but a single visit, passing out again to traverse we know not what other systems or regions.... when we have said that these objects obey the law of gravity, we have mentioned the only circumstance--as it would appear--in which they conform to the relations observed in terrestrial and planetary arrangements. and even this law--the widest yet revealed to man--they seem to obey half unwillingly. we see the head of a comet tracing out systematically enough its proper orbit, while the comet's tail is all unruly and disobedient.... the fact, then, is demonstrated that two of the meteor streams encountered by the earth are so far associated with two comets as to travel on the same orbits. we may not unsafely infer that all the meteor systems are in like manner associated with other comets. nor is it very rash to assume that all comets are in like manner associated with meteor systems." concerning the influence of gravitation of the planets, the same author says ("meteoric astronomy"), "now, the circumstances under which a comet approaching the sun on a parabolic or hyperbolic orbit can be thus affected must be regarded as exceptional. the planet's influence must, in the first place, be very energetically exercised; in other words, the arriving comet must pass very close to the planet, for under any other circumstances the sun's influence so enormously outvies the planet's that the figure of the cometic orbit would be very little affected. moreover, the planet's attraction must produce an important balance of retardation. the planet will inevitably accelerate the comet up to a certain point, and afterwards will retard it; the latter influence must greatly exceed the former. to show how greatly the comet must be retarded, it is only necessary to mention that the actual velocity of the november meteors when they cross the orbit of uranus is less than one-third of the velocity with which uranus himself travels, but their velocity at the same distance from the sun, when they were approaching him from some distant stellar domain, exceeded the velocity of uranus in his orbit in the proportion of about seven to five.... it follows, not merely as a probable inference, but, i think, as a demonstrated conclusion, that if the november meteors came originally into our system as a comet travelling sunward from infinity, then either that comet was very compact or else uranus captured only a small portion of the comet, the remaining portions moving thenceforth on orbits wholly different from the path of the november meteors.... no other planet than uranus can have brought about the subjection of this comet to solar rule." in his article on comets he says, "it may be well here to consider a case in which some active force (other than gravity) exerted by the sun seems to have brought the destruction of a comet, or at least to have broken up the comet into unrecognizable fragments." he refers to biela's comet, with an orbital period of six and two-thirds years, and a path which was found to approach very near to the path of the earth. in the comet crossed the earth's track several weeks before the arrival of the earth at the same point without appreciable interference. on its second return, in - , it was found to be divided into two comets travelling side by side; in they reappeared, still divided, and gradually diverging from each other. since then they have never reappeared, though diligently sought for at every period. professor proctor adds, "it has been seen again, though not as a comet; nay, the occasion on which it was seen in the way referred to was predicted, and the prediction fulfilled, even in details. for a full account of its reappearance--as a meteor stream--i refer the reader to my essay on biela's comet in 'familiar science studies.'" in miller's "romance of astronomy" we read, "encke's comet, which possesses the smallest orbit of any connected with our system, is sensibly drawing nearer and nearer to the sun at every revolution." in professor proctor's "cometic mysteries," the author says, "we hear it stated that the nucleus of a comet is made up of meteoric stones (professor p. g. tait says--for unknown reasons--that they resemble 'paving stones or even bricks') as confidently as though the earth had at some time passed through the nucleus of a comet, and some of our streets were now paved with stones which had fallen to the earth on such an occasion. as a matter of fact, all that has yet been proved is that meteoric bodies follow in the track (which is very different from the tail) of some known comets, and that probably all comets are followed by trains of meteors. these may have come out of the head or nucleus in some way as yet unexplained; but it is by no means certain that they have done so, and it is by many astronomers regarded as more than doubtful. the most important point to be noticed in the behavior of large comets as they approach the sun is, that usually the side of the coma which lies towards the sun is the scene of intense disturbance. streams of luminous matter seem to rise continually towards the sun, attaining a certain distance from the head, when, assuming a cloud-like appearance, they seem to form an envelope around the nucleus. this envelope gradually increases its distance from the sun, growing fainter and larger, while within it the process is repeated and a new envelope is formed. this, in turn, ascends from the nucleus, expanding as it does so, while within it a new envelope is formed. meanwhile the first one formed has grown fainter, perhaps has disappeared. but sometimes the process goes on so rapidly (a day or two sufficing for the formation of a complete new envelope) that several envelopes will be seen at the same time,--the outermost faintest, the innermost most irregular in shape and most varied in brightness, while the envelope or envelopes between are the best developed and most regular. the matter raised up in these envelopes seems to have undergone a certain change of character, causing it no longer to obey the sun's attractive influence, but to experience a strong repulsive action from him, whereby it is apparently swept away with great rapidity to form the tail. 'it flows past the nucleus,' says dr. huggins, 'on all sides, still ever expanding and shooting backward until a tail is formed in the direction opposite to the sun. this tail is usually curved, though sometimes rays or extra tails sensibly straight are also seen.'" in "the sun as a perpetual machine," professor proctor says, "take, again, the phenomena of comets, which still remain among the greatest of nature's mysteries. we have reason to believe ... that the nucleus of a comet consists of an aggregation of stones similar to meteorites. adopting this view, and assuming that these stones have absorbed somewhere gases to the amount of six times their volume (taken at atmospheric pressure), we may ask, what will be the effect of such a mass of stones advancing towards the sun at a velocity reaching in perihelion the prodigious rate of three hundred and sixty-six miles per second (as observed in the comet of ), being twenty-three times our orbital rate of motion?" professor ball says, "one of the most important results of the great shower of was the demonstration that the swarm of little bodies to which that shower owed its origin was connected with a comet. the swarm was found, in fact, to follow the exact track which the comet pursued around the sun.... of this connection between the cometary orbits and revolving swarms of meteors many other instances could be cited. i may refer to the remarkable lists published by the british association, in which, beside the name of the comet or the designation which astronomers had affixed to it, the meteoric swarm with which the comet is associated is also given.... on these grounds it appears to be perfectly certain that the origin of the shooting stars which appear in swarms cannot be disassociated from the origin of the comets by which those swarms are accompanied." the author makes a distinction between such ordinary shooting stars and meteorites, and attributes the appearance of the latter on earth to masses thrown forth from some volcano somewhere, but this has nothing to do with the special phenomena to be interpreted. it may be said, however, that the presence of olefiant gas as one of the occluded gases in a meteorite (four and fifty-five-hundredths per cent., as stated by professor proctor, in his article "the sun as a perpetual machine"), and the remarkable fact, stated in the article "spectrum analysis" in appleton's cyclopædia, that, in winnecke's comet of , "the bands agree in position with those obtained as the spectrum of carbon, by passing the electric spark through olefiant gas, "would lead one to consider a cometic origin, for this particular meteorite at least, to be highly probable. professor ball further says, "there have been several instances in which a comet has approached so close to a planet that the attraction between the two bodies must have had significant influence on the planet, if the cometary mass had been at all comparable with that of the more robust body. the most celebrated instance is presented in the case of lexell's comet, which happened to cross the track of jupiter. the effect upon this body was so overwhelming that it was wrenched from its original path and started afresh along a wholly different track." the same writer, speaking of the tails of comets, says, "i have no intention to discuss here the vexed question of the tails of comets. i do not now inquire whether the repulsion by which the tail is produced be due to the intense radiation from the sun, or to electricity, or to some other agent. it is sufficient for our present purpose to note that, even if the tails of comets do gravitate towards the sun, the attraction is obscured by a more powerful repulsive force.... nor do the directions in which the comets move exhibit any conformity; some move round the sun in one direction, some move in the opposite direction. even the planes which contain the orbits of the comets are totally different from each other. instead of being inclined at only a very few degrees to their mean position, the planes of the comets hardly follow any common law; they are inclined at all sorts of directions. in no respect do the comets obey those principles which are necessary to prevent constitutional disorder in the planetary system.... now, all we have hitherto seen with regard to comets tends to show that the masses of comets are extremely small. attempts have been made to measure them, but have always failed, because the scales in which we have attempted to weigh them have been too coarse to weigh anything of the almost spiritual texture of a comet. it is unnecessary to go as far as some have done, and to say that the weight of a large comet may be only a few pounds or a few ounces. it might be more reasonable to suppose that the weight of a large comet was thousands of tons, though even thousands of tons would be far too small a weight to admit of being measured by the very coarse balance which is at our disposal." in the chapter "visitors from the sky," the same author says, "as such a comet in its progress across the heavens passes between us and the stars, those stars are often seen twinkling brilliantly right through the many thousand miles of cometary matter which their rays have to traverse. the lightest haze in our atmosphere would suffice to extinguish the faint gleam of these small stars; indeed, a few feet of mist would have more power of obstructing the stellar light than cometary material scores of thousands of miles thick. it is true that the central portions of many of these comets often exhibit much greater density than is found in the exterior regions; still, in the great majority of such objects there is no opacity, even in the densest part, sufficient to put out a star. in the case of the more splendid bodies of this description, it may be supposed that the matter is somewhat more densely aggregated as well as more voluminous; still, however, it will be remembered that the great comet of passed over arcturus, and that the star was seen shining brilliantly, notwithstanding the interposition of a cometary curtain millions of miles in thickness. so far as i know, no case is known in which the nucleus of a really bright and great comet has been witnessed in the act of passage over a considerable star. it would indeed be extremely interesting to ascertain whether in such case the star experienced any considerable diminution in its lustre." chapter viii. the phenomena of comets. from the extracts thus cited we may form a fairly clear idea of the phenomena which comets present, and these facts represent about all that we know of these mysterious objects. they approach the sun in a nearly radial direction, thus cutting the planetary orbits transversely. they approach the sun from all directions and at all angles, without reference to the common plane in which all the planetary orbits lie. they have no rotation on their own axes, as the planets have, but, like an aggregated mass of meteorites or cosmical dust, rush inward from the exterior realms of space, so that their course is diametrically opposite that of the planets and the other cosmical bodies which constitute our solar system. such a body as a comet, in fact, would present in its approach to our solar system very much the phenomena of an approaching exterior sun, corresponding far more closely in appearance and behavior to our own sun than to any of the planets. such a body could not generate positive electricity, as the planets do, but, on the contrary, must have an electrosphere of negative, or at least neutral, polarity. on its approach to our planetary system the batteries of all the planets would be at once turned upon the intruder, and it would be rapidly thrown into the same state of active electrical polarity as the sun. the aqueous vapor condensed around its nucleus by gravity in its approach through space, or buried among the meteoric particles constituting the comet, would be necessarily decomposed into its constituent gases, just as in the case of the sun, by the positive electrical currents from the planetary electrospheres, and the disassociated hydrogen would form the negative electrosphere of the comet, glowing with its own luminosity, by gaseous incandescence. "we should then observe, during its continued approach to the sun, phenomena similar to those which we might expect to manifest themselves during the approach of a minute solar body towards the sun, characterized by a rapid increase of velocity, due to attraction of gravity, and tremendous mutual repulsion between the solar and cometic electrospheres. we should see the luminous hydrogen and associated gases boiling upward, and thence drawn forward from the nucleus by the combined gravity of the sun's mass, that of the planetary masses, and the opposite polarity of the planetary electrospheres, while they would be, at the same time, repelled backward by the enormous repulsive force of the negative electrosphere of the sun. as a result, we should find these gases in a state of ebullition, forced forward under great excitement and disturbance, boiling, eddying about, driven to and fro in all directions until the sun's repulsive force had overcome the different attractions, when these luminous clouds or envelopes would be swept swiftly off to the rear, as by a powerful current of wind, around the margins of the nucleus, and they would be seen to stream backward from the sun as an elongated envelope or tail. new volumes of gas would pour to the front, attracted from deeper depths, and these, on reaching the cometary electrosphere, would be again repelled by the solar activity and driven to the rear, while the gases thus driven backward, themselves similarly electrified, would mutually repel each other as they streamed backward around the margins of the nucleus. let us now see what these gases are: if they are such as appear in the sun's electrosphere, we will know that such must be their action; if, on the contrary, they are such as appear in planetary electrospheres, we will find any such attempted explanation to be a failure. quoting largely from dr. huggins, professor proctor, in his "cometic mysteries," says, "the spectrum of the brightest comet of that year was partly continuous, and on this continuous spectrum many of the well-known fraunhofer lines could be traced. this made it certain that part of the comet's light was reflected sunlight, though dr. huggins considers also that a part of the continuous spectrum of every comet is due to inherent light. on this point some doubt may be permitted. it is one thing for special bands to show themselves, for some substances may become self-luminous under special conditions at very moderate temperatures; it is quite another thing that the solid parts of a comet's substance should become incandescent. i venture to express my opinion that this can scarcely happen, except in the case of comets which approach very near to the sun. besides the continuous spectrum with dark lines, the photograph showed also a spectrum of bright lines. 'these lines,' says dr. huggins, 'possessed extreme interest, for there was certainly contained within this hieroglyphic writing some new information. a discussion of the position of these new lines showed them to be undoubtedly the same lines which appear in certain compounds of carbon. not long before professors liveing and dewar had found from their laboratory experiments that these lines are only present when nitrogen is also present, and that they indicate a nitrogen compound of carbon,--namely, cyanogen. two other bright groups were also seen in the photograph, confirming the presence of hydrogen,--carbon and nitrogen.' it is worthy of notice that only a few days later dr. h. draper succeeded in obtaining a photograph of the same comet's spectrum. it appeared to him to confirm dr. huggins's statements, except only that the dark fraunhofer lines were not visible, the photograph having probably been taken under less favorable conditions.... but the latest comet has brought with it fresh news. its spectrum is not like that given by the comets we are considering. the bright lines of sodium are seen in it, and also other bright lines and groups of lines which have not yet been shown to be identical with any belonging to the hydrocarbon groups, but probably are so.... the cyanogen groups are not seen.... but it is manifest that this comet underwent important changes.... in april was found simply a faint continuous spectrum; in may the three bands associated with carbon were present, though faint, while there was no trace whatever of the sodium band. on the contrary, in june the nucleus of the comet gave a very strong and extended continuous spectrum with an excessively strong bright line in the orange-yellow identical with the well-known double sodium line of the solar spectrum. on this ... it is necessary to conclude that during the last fortnight of may the spectrum of wells's comet had changed in a manner of which the history of science furnishes no precedent." it should be observed that the elements carbon and hydrogen closely resemble each other, not only in their multifarious chemical affinities and reactions, but in their electric polarities, and the hydrocarbon compounds, like their constituents, carbon and hydrogen, are electrically similar to each other, an example of this similarity of the elements being found in the identical action of the carbon arc and hydrogen envelope in the heating and lighting experiments with electrical currents hereinbefore described. we have already seen that carbon follows quite a different law from the other concrete elements, in the fact that its electrical resistance diminishes as the temperature rises; it also differs widely from the other solid elements in its atomic heat, which has a value much less than one-half the mean constant, which is . . of this matter of specific heat, professor fownes, in his work on chemistry (bridges' edition), says, "dulong and petit observed in the course of their investigation a most remarkable circumstance. if the specific heats of bodies be computed upon equal weights, numbers are obtained all different and exhibiting no simple relations among themselves; but if, instead of equal weights, quantities be taken in the proportion of the atomic weights, an almost perfect coincidence in the numbers will be observed, showing that some exceedingly intimate connection must exist between the relations of bodies to heat and their chemical nature; and when the circumstance is taken into view that relations of even a still closer kind link together chemical and electrical phenomena, it is not too much to expect that ere long some law may be discovered far more general than any with which we are yet acquainted.... nevertheless, this law must not be understood as perfectly general, for there are three elements--namely, carbon, boron, and silicon" [these form a single group of elements in chemical classification]--"which exhibit decided exceptions to it." organic chemistry is substantially based upon the almost infinitely interchanging relations among carbon-hydrogen radicals, supplemented by a few other elements. according to professor fownes, "organic chemistry is in fact the chemistry of carbon compounds." the position of carbon among the elements is something like that of camphor among the oils, the latter being a volatile oil, but concrete in form. with a concrete element having the peculiar character of carbon we can well understand its universal chemical and electrical relationship with gaseous hydrogen in the grandest operations of nature. cyanogen is an electrically similar compound of carbon with the addition of nitrogen. of these elements it will be seen that nitrogen and hydrogen are found to exist also in the gaseous nebulæ, and with the probable addition there of oxygen; but in comets the quota of active oxygen must be sought for in the correlated planetary, and not in the cometic, atmospheres, as is the case with the sun. of the presence of the vapor of carbon in comets professor ball says, "this is a very singular fact, when it is remembered that carbon is one of the substances essentially associated with life in the forms in which we know it." professor huggins says, "since that time the light from some twenty comets has been examined by different observers. the general close agreement in all cases, notwithstanding some small divergencies, of the bright bands in the cometary light with those seen in the spectrum of hydrocarbons justifies us fully in ascribing the original light of these comets to matter which contains carbon in combination with hydrogen." we may learn something further of the constitution of comets, perhaps, by considering the chemical reactions which their spectra seem to indicate. the following extract is from a recent article on the manufacture of illuminating gas: "ammonia contains . parts of nitrogen and . of hydrogen. it is not produced by a direct combination, for nitrogen can be caught and wedded only by a hot and skilful wooing. in the gas retort, at a temperature of degrees and in the presence of lime, soda, or potash, it will combine with carbon and form cyanogen, and then further combine with the alkali to form a cyanide. there is steam in the retort, and, as nearly as the gas chemists can make out, the nitrogen promptly divorces itself, gives up the carbon to the oxygen of the steam, and, taking the hydrogen to itself, becomes, for the time at least, a fixed, if volatile, substance, but ever ready to enter into new alliances." it will be remembered that in the comets examined by professors huggins and draper the spectroscope revealed both cyanogen and the double line of sodium. the function of the sodium is readily understood, as by its presence it enables the nitrogen in the cometic atmosphere to combine with a part of the carbon of the gaseous hydrocarbons which constitute this atmosphere, and thus produce the cyanogen. but to effect this combination requires in the retort a temperature of degrees. if the combining temperature around the nucleus of a comet is the same, it will show that the temperature of this comet's nucleus must be very high, and, while many times less than that of the sun's photosphere, it still clearly illustrates the powerful character of the impact of the planetary electrical currents upon the comet, and its tremendous repulsion by the similarly electrified solar electrosphere. the second one of the above reactions, that from cyanogen to ammonia, is due to the steam or aqueous vapor in the retort. but in the case of the comet all the aqueous vapor and its constituent oxygen have disappeared by electrolytic decomposition long before the combining temperature of cyanogen has been reached; so that the sodium, the hydrocarbons, and the cyanogen alone appear, and the oxygen compounds are missing. but on the reversal of polarity of this comet by contact with a planetary electrosphere, should such ever occur, and its consequent assumption of positive electricity, the oxygen would again appear, and, if the temperature had not yet receded below that of the reaction which produces ammoniacal vapors, we might expect, should a fragment of this comet enter our atmosphere as a meteorite, to find ammonia as well as sodium as a constituent thereof; otherwise the ammonia would be replaced by carbonic oxide and carbonic acid, by the action of oxygen upon the hydrocarbons, and water by the action of oxygen upon the hydrogen of the same, at much lower temperatures than would suffice for the generation of ammonia. the cyanogen would then perhaps remain as cyanide of sodium, unless decomposed by contact with the meteoric metallic iron at a high temperature, as occurs in the operation known in the arts as "case-hardening." the presence of microscopic diamonds in meteorites may be accounted for by a somewhat similar reducing reaction under heat and the active force of the planetary and solar voltaic arc. in the popular view comets are always associated with tails, but, in fact, comets without tails are far more numerous than those to which these appendages pertain; the tails, when such exist, are the direct result of the repulsive energy of the solar electrosphere, and are only manifested when their proximity to the sun has aroused sufficient activity to swiftly sweep backward from the sun with inconceivable velocity the gaseous matter concentrated in and around the nucleus. as these tails owe their formation to the sun's repulsive energy, they must always extend radially outward from the sun, and by the self-repulsive energy of the diverse constituents of the tails themselves these will be broken occasionally into two, four, or six lateral strands, and (possibly by the attraction of the different planetary electrospheres) curvatures may be apparent along the sweep of the comets' tails corresponding, in effect, with perturbations produced by gravity in the orbit of the nucleus. of these various phenomena, professor proctor, in his article on comets, says, "a very large number of comets have no visible tails. when first seen in the telescope a comet usually presents a small, round disk of hazy light, somewhat brighter near the center. as the comet approaches the sun the disk lengthens, and, if the comet is to be a tailed one, traces begin to be observed of a streakiness in the comet's light. gradually a tail is formed, which is turned always from the sun. the tail grows brighter and larger, and the head becomes developed into a coma surrounding a distinctly marked nucleus. presently the comet is lost to view through its near approach to the sun; but after a while it is again seen, sometimes wonderfully changed in aspect through the effects of solar heat. some comets are brighter and more striking after passing their point of nearest approach to the sun than before; others are quite shorn of their splendor when they reappear." this change of aspect is not due to solar heat, but to the energetic repulsion of the solar electrosphere. the force of gravity irresistibly impels the comet forward to the sun's electrical vortex, and the change of aspect is due to the repulsion of its entire stock of free gaseous matter into space in case its supply is small, or to its increased development and pouring forth in case the supply is large. it is like the volatilization by a heated atmosphere of ammoniacal gas, for instance, absorbed in water. the ebullition is vastly increased by the heat, but if the entire stock of ammonia has been driven off in its passage through the heated medium, it will emerge with the residual water quiescent; otherwise, in a state of increased agitation. the same author, in "cometic mysteries," says, "repulsion of the cometary matter could only take place if this matter, after it has been driven off from the nucleus, and the sun have both high electric potentials of the same kind." his further guess, however, that it is analogous to the aurora, is wide of the mark; it is due, in fact, to the mutual repulsion of their similar negative electrospheres, the cometic electrosphere, however, being so much smaller than that of the sun that the latter shows no appreciable disturbance, as is the case, under similar circumstances, with the electrospheres of the earth and moon. in the article last quoted it is said, "there is a dark space immediately behind the nucleus,--that is, where the nucleus, if solid, would throw its shadow if there were matter to receive the light all round so that the shadow could be seen." this presents, it is stated, a great difficulty. the author, by a happy guess,--almost an inspiration, in fact, of which this splendid writer and observer was so full,--suggests in a foot-note a possible explanation, which, while not in itself correct, suggests an analogous process very like what we actually see. "if the particles forming the envelopes are minute flat bodies, and if anything in the circumstances under which these particles are driven off into the tail causes them to always so arrange themselves that the planes in which they severally lie pass through the axis of the tail (which, if the tail is an electrical phenomenon, might very well happen), then we should find the region behind the nucleus very dark or almost black, for the particles in the direction of the line of sight there would be turned edgewise towards us, whereas those on either side or in the prolongation of the envelopes would turn their faces towards the observer." as a matter of fact, the envelope streaming backward from the nucleus forms a hollow tube, the opposite sides of which exhibit the same mutual repulsion as both exhibit towards the sun; hence the phenomenon would be similar to that exhibited by blowing into a closed bag of porous material covered with wisps of cotton, for example, and the gases, in addition to their rush backward from the sun, would also exhibit a radial rush outward from the longitudinal axis of the tail. this is what we actually observe, and sufficiently accounts for the phenomenon, be it altogether or only partially real, and not merely, as that author thinks it may be, apparent. it is said, in the same article, that "bredichen has shown that where there are three tails to a comet their forms correspond with the theory that the envelopes raised from the head are principally formed of hydrogen, carbon, and iron; but this ... seems open at present to considerable doubt." at all events, these separate tails are self-repulsive, or they would be merged into each other by the sun's repulsive energy; in fact, they occupy the resultant of the direction produced by the line of the sun's repulsion and those of their own mutually repellent force,--that is to say, radial or divergent. it must not be supposed that these tails are of insignificant proportions. "when we see the tail of a comet occupying a volume thousands of times greater than that of the sun itself, the question naturally suggests itself, 'how does it happen that so vast a body can sweep through the solar system without deranging the motion of every planet?' conceding even an extreme tenuity to the substance composing so vast a volume, one would still expect its mass to be tremendous. for instance, if we supposed the whole mass of the tail of the comet of to consist of hydrogen gas (the lightest substance known to us), yet even then the mass of the tail would have largely exceeded that of the sun. every planet would have been dragged from its orbit by so vast a mass passing so near. we know, on the contrary, that no such effects were produced. the length of our year did not change by a single second.... thus we are forced to admit that the actual substance of the comet was inconceivably rare.... from what we have already seen, it will be manifest that the formation of comets' tails is a process of a very marvellous nature, apparently involving forces other than those with which we are acquainted. the tail, ninety million miles in length, which was seen stretching from the head of newton's comet nearly along the path which the retreating comet had to traverse, must, it would seem, have been formed by some force far more active than the force of gravity. the distance traversed by the comet in the last four weeks of its approach to the sun under gravity was no greater than that over which the matter of the tail, seen after the comet had circled around the sun, had been carried in a few hours. yet we have no other evidence of any repulsive force at all being exerted by the sun,--at least no evidence which can be regarded as demonstrative,--and still less have we any evidence of a repulsive force exceeding in energy the sun's attracting power." (proctor.) chapter ix. interpretation of cometic phenomena. now, curiously enough, we have in constant use in our laboratories a little instrument called the electroscope, in which we have manifested very clearly a repulsive force exceeding in energy the earth's attracting power, and very greatly exceeding it. it is described in "electricity in the service of man" as follows: "if we rub a large glass rod with a silk pad, we observe that it will attract light bodies, then, after contact, repel them. during the process we may notice a peculiar noise, and if the experiment be carried out in the dark we may further notice sparks passing between the rod and the rubber, and also that the rod becomes luminous. if we suspend a pith-ball by means of a silk thread, on bringing the rubbed rod near the pith-ball it will move towards the rod, touch it, and then be repelled. if the glass rod be again brought near the pith-ball, it will move away from the glass rod, and continue to be repelled until it has been touched by some other body.... in order to ascertain whether electricity is communicated by electrified bodies to non-electrified bodies when brought into contact, let us suspend two pith-balls from the same point of support by threads of uniform silk, and touch the pith-balls with the rubbed glass rod. the balls fly from the rod and also from one another. on bringing near them a third pith-ball or any other light body, we find that, though they repel one another, they are attracted by the light body, showing that they have become electrified by contact with the rubbed glass rod. from this we conclude that an unelectrified body may be electrified by contact with an electrified body, and also that there is repulsion after contact. there is mutual repulsion between two electrified bodies, but there is attraction between a single electrified body and one that is unelectrified." the mutual repulsion of these pith-balls is the exact measure of the strength of electrification. hung side by side to the knob of a prime conductor of an electrical machine, the mutual repulsion of the similar electrospheres of these pith-balls drives them apart against the earth's gravity and holds them extended, if the electrical tension be sufficient, to their widest limit of divergence. it is, in effect, precisely similar to the action of the solar and cometic electrospheres (see illustration in a previous chapter, page ), each being similarly electrified and communicating with the other across a space which, as before stated, is freely traversable by electric currents without appreciable resistance. that such electrospheres are flaming with heat does not interfere with such self-repellent action; in fact, it intensifies it. in professor tyndall's "lessons in electricity" we read, "flames and glowing embers act like points; they also rapidly discharge electricity. the electricity escaping from a point or flame renders the air self-repulsive. the consequence is that when the hand is placed over a point mounted on the prime conductor of a machine in good action a cold blast is distinctly felt.... wilson moved bodies by its action, faraday caused it to depress the surface of a liquid, hamilton employed the reaction of the electric wind to make pointed wires rotate. the 'wind' was also found to promote evaporation." let us now apply these principles to the tails of comets. if we conceive the sun and comet to be analogous to our pith-balls, one enormously larger than the other, however, and hung by vaporous conducting cords from the combined generating planetary electrospheres, both sun and cometic nucleus surrounded each by a vaporous envelope, and suspended so that they will hang from parallel cords, say a dozen million miles apart, and with no currents of electricity as yet in operation, we will find that the sun and comet will be simply attracted towards each other by the force of gravity, so that their suspending cords will converge. if the planetary electrical machines now commence their rotations, and currents of electricity begin to pass in quantity and intensity like those which pass between the earth and the sun, both the solar and cometic pith-balls will become similarly electrified, and their gaseous atmospheres, instead of drawing towards each other, will become luminous and self-repulsive. the atmosphere which surrounds the cometic pith-ball, by reason of its great tenuity, will be driven backward with extreme velocity, while the solar pith-ball electrosphere will be so little affected that its repulsion will be imperceptible. all the gaseous matter, however, of the smaller pith-ball will be forced off in a direction opposite that of the larger one, and this repulsive energy will even carry the pith-balls apart, causing the suspending cords to widely diverge from each other, while the force of gravity of the earth tends to bring them nearer together. if the gravity of the larger pith-ball, however, was equal, relatively, to that of the sun, the result would be that the solid pith-balls would be mutually attracted by gravitation and only the electrified atmospheres, would be mutually repelled. this experiment would present phenomena similar to those we are now considering. (see illustration, page .) in describing newton's comet, with a tail ninety million miles long projected backward both from the sun and the comet, when it disappeared in the light of the sun, and exhibiting a similar tail, also ninety million miles long, when, less than four days afterwards, it reappeared from behind the sun, but with the tail now directed forward from the comet, but in both cases extended radially outward from the sun, it is obvious that this whole tail must have made a sweeping change of direction of nearly one hundred and eighty degrees upon the nucleus as its center. professor proctor says, "as sir john herschel remarks, we cannot look on the tail of a comet as something whirled round like a stick as the comet circles around its perihelion sweep. the tail with which the comet reappeared must have been an entirely new formation." it is true that a comet's tail cannot be conceived of as being whirled round like a stick, but we can very readily conceive of it as something like a flame composed of incandescent gases, and it may very easily be blown round a stick; and this is precisely what must happen in the case of a comet. construct, for experiment, a little apparatus consisting of a blow-pipe adapted to deliver a current of air between two horizontal metal disks, say an eighth of an inch apart, one perforated at the center to admit the nozzle of the blow-pipe. by directing a constant current of air through the latter, it will be deflected so as to blow radially outward in all directions and in the same plane. now take a stick with a flame on the end of it, or a lighted candle, and with it approach this center of repellent energy in the plane of the space between the disks and along an ellipse representing the orbit of a comet. as the flame approaches the improvised solar center it will be driven backward from the wick of the candle almost along the line of its approach, and as it passes around the center it will be constantly blown outward in a radial direction until, when it recedes after perihelion, the flame will be seen pointed almost directly ahead. at all times the direction of the flame will lie along the radial lines prolonged outward from the center through the wick of the candle, and it will not be a new flame generated at every change of its direction, but the same flame constantly forced outward by the repulsive force of the central atmosphere in this case or the solar electrosphere in the case of the sun. this experiment is an accurate and conclusive exhibit of the phenomena of solar repulsion in its action upon the tail of a comet. it is analogous in principle to the repulsion of the pith-balls and the electric wind and (in application) to the phenomena presented by comets in their movements to, around, and from the sun. this repulsion is not operative in effect against the wick of the candle,--that is to say, it is not the repulsion of the nucleus which determines the direction of the tail, but the repulsion by direct outblow of the sun, so to speak, upon the incandescent gases of the tail itself. this fact clearly demonstrates that the repulsion of like electrospheres is the cause of the phenomenon, and, when once understood, the process is quite as simple as that of the original formation of the tail itself, which no one disputes. there is to be further considered the theoretical resistance of space to the projection and deflection of such enormous volumes of attenuated matter as appear in comets' tails. while it may not be absolutely necessary to offer an explanation of this apparent difficulty, in view of the fact that such projection and deflection do actually occur, still, the well-known laws of the diffusion of gases, in accordance with which any gaseous matter will traverse any other gaseous matter with the same velocity as, and with no more resistance than, in a vacuum, will show that this difficulty has been much overrated, while for the twin difficulty, how to account for the persistence of luminosity at such vast distances from its source, we may quote from professor proctor, "cometic mysteries," who, in turn, quotes as follows: "comets travel in what must be regarded as to all intents and purposes a vacuum. from dr. crookes' experiments on very high vacua we may infer that there is very little loss of heat, except by radiation." by "intents and purposes" we understand, of course, as a cause of resistance, and certainly there is no reason to believe that the attenuated vapors of space are sufficient in density to cause any rapid diffusion of heat by convection, as contrasted with that of radiation. we have seen that comets of short period sometimes disappear, and that their disappearance is frequently followed by the appearance of trains of meteors. in other words, they have apparently lost their cometic properties and become permanent adjuncts to our solar system. a curious confirmation of this fact is to be found in the character of the occluded gases which are contained in such meteorites as sometimes fall upon the earth's surface. of this professor proctor says, "we have reason to believe that the nucleus of a comet consists of an aggregation of stones similar to meteorites." speaking of the condition in which meteorites reach the earth, he says, "they are known to contain as much as six times their own volume of gases (taken at atmospheric pressure). in one of these meteorites recently examined by dr. flight, the following percentages of various gases were noted: of carbonic oxide, . ; of carbonic acid gas, . ; of hydrogen, . ; of olefiant gas, . ; and of nitrogen, . ." the presence of olefiant gas at once suggests the hydrocarbons of the cometic nucleus. the presence of this gas cannot be accounted for by the passage of the meteorite through our atmosphere, nor can that of hydrogen, and these are two characteristic gases, together with the vapor of carbon, constantly found to exist in comets. as before explained, the advent of a comet into our solar system is that of a stranger, with electric polarity the opposite of that of the planetary electrospheres and identical with that of the sun. under the combined influence of the solar gravity and perturbation by the gravity of the planets these foreign bodies tend to shorten their periods, and finally fall into the ordinary array of the bodies which compose our own solar system. but when this occurs they will, in turn, become contributors to, instead of antagonists of, the energy of the sun; in other words, they must then conform electrically to the condition of the family into which they have married,--that is to say, the planets,--and a reversal of their electrical polarity will take place. this reversal of polarity is no novelty in the operation of electrical apparatus. in "electricity in the service of man" we read as follows of the voss induction machine: "this machine is exceedingly powerful in favorable weather, but has an important defect in a tendency to self-reversal, which is apt to occur at a stoppage. this defect can be produced in a voss machine, when desired, by holding a metal point to the positive brush k. the two derived inductive circuits are beautifully manifested when this machine is worked in the dark. a luminous stream is seen pouring towards the collecting comb l on whichever side of the machine the comb is positive." it will thus be seen that simple contact of a neutral (or negatively opposite) body will reverse the electrical polarity of this machine, or even the interruption of its motion will do so at times. possibly a similar reversal may be produced in a comet by the contact in whole or in part of its nucleus with a planetary electrosphere, since the action of gravity is entirely independent of that of the attraction or repulsion of the electrospheres of both planetary and cometic bodies. such reversal of polarity in a comet would at once extinguish its luminosity, and the generation of oxygen would at once replace the prior generation of hydrogen, and herein we may find explained the presence of carbonic oxide in large volume and carbonic acid in small volume in the meteorite above referred to, and of which gases professor proctor says, "it is quite certain these gases were not taken up by the meteorolite during its flight through the air." these aggregations of discrete meteoric bodies, loosely adherent by mutual gravity alone, would be gradually torn apart by planetary interference and dragged into streams of small bodies, thenceforth traversing space in elliptical orbits around the sun, just as do the planets and planetoids. cyanogen, also, the deadly gas so frequently found to exist in enormous quantities in the nuclei of comets, would at once disappear, by double conversion into carbonic acid, or oxide, and ammonia, or nitrogen, so that this danger, as the result of a comet's possible approach to the earth's atmosphere, may be dismissed from apprehension. it will be seen that all the enormous difficulties in the phenomena of comets find an explanation in the operation of the same universal laws which we have endeavored to apply to the other sidereal bodies. in conclusion, we may cite the following from dr. huggins: "broadly, the different applications of principles of electricity which have been suggested group themselves about the common idea that great electrical disturbances are set up by the sun's action in connection with the vaporization of some of the matter of the nucleus, and that the tail is probably matter carried away, possibly in connection with electric discharges, under an electrical influence of repulsion exerted by the sun. this view necessitates the supposition that the sun is strongly electrified, either negatively or positively, and, further, that in the processes taking place in the comet, either of vaporization or of some other kind, the matter thrown out by the nucleus has become strongly electrified in the same way as the sun,--that is, negatively if the sun's electricity is negative, or positively if the sun's is positive. the enormous disturbances which the spectroscope shows to be always at work in the sun must be accompanied by electrical changes of equal magnitude, but we know nothing as to how far these are all, or the great majority of them, in one direction, so as to cause the sun to maintain permanently a high electrical state, whether positive or negative." the above speculations will have thus become demonstrated facts (though not in the mode suggested by the above writer) as soon as we clearly understand that, instead of the sun's "enormous disturbances" producing "electrical changes of equal magnitude," it is the electrical changes of equal magnitude which themselves cause the sun's disturbances, and that the sun's negative electrical polarity is permanently fixed by the opposite and positive polarity of the planetary electrospheres, and that all these various phenomena are but the normal expression of a single universal law, and are all due to the constant interaction of planetary, solar, and cometic electrospheres, in accordance with the well-established principles of electrical science. if, however, we consider, as is generally believed to be the case, the sun itself to be the sole prime source of its visible energy, nothing but difficulty and vague speculation can be looked for on every hand; but by relegating the solar orb to its proper place, and taking as the starting-point the true source of all energy,--to wit, the hidden forces embodied in the vapors or gases of interstellar space,--the whole process and mode of action will logically follow, and obscurity and difficulty together disappear. this principle, properly understood, is a master-key which will unlock every problem and interpret every enigma which the realms of interstellar space can present. chapter x. the resolvable nebulÆ, star-clusters and galaxies. when we come to consider the nebulæ, and endeavor to learn what part electricity has to play in the phenomena presented by these singular objects, we must recollect, in order to give them their due importance, that they are neither few in number nor uniform in constitution. of the nebulæ, professor proctor ("star-clouds and star-mist") says, "when the depths of the heavens are explored with a powerful telescope a number of strange cloud-like objects are brought into view. it is startling to consider that if the eye of man suddenly acquired the light-gathering power of a large telescope, and if at the same time all the single stars disappeared, we should see on the celestial vault a display of the mysterious objects called nebulæ or star-clouds exceeding in number all the stars which can now be seen on the darkest night in winter. the whole sky would seem mottled with these singular objects." as telescopes, with the advances of constructive art, increased in power, these luminous clouds became more and more clearly defined, and many of them became resolved into clusters of stars, galaxies of suns like the milky way, of which latter our solar system is a constituent part, but more distant from us than the separately visible stars of that galaxy, and each separated from the relatively adjacent clusters by intervals of space comparable only with those which separate them from our own system. of these glorious star-clusters, says flammarion, in "the wonders of the heavens," "in the bosom of infinite space, the unfathomable depth of which we have tried to comprehend, float rich clusters of stars, each separated by immense intervals. we shall soon show that all the stars are suns like ours, shining with their own light, and foci of as many systems of worlds. now, the stars are not scattered in all parts of space at hazard; they are grouped as the members of many families. if we compared the ocean of the heavens with the ocean of the earth, we should say that the isles which sprinkle this ocean do not rise separately in all parts of the sea, but that they are united here and there in archipelagoes more or less rich.... they are all collected in tribes, most of which count their members by millions." says professor nichol, "system on system of majesty unspeakable float through the fathomless ocean of space. our galaxy, with splendors that seem illimitable, is only a unit among unnumbered throngs; we can think of it, in comparison with creation, but as we were wont to think of one of its own stars. "of these glorious star-clusters the same writer says, "that no one has ever seen them in a telescope of adequate power without uttering a shout of wonder." these mist-like star-clouds were successively resolved, nebula by nebula, until science settled into the belief that with telescopes of adequate power all nebulæ might be so resolved, and the capacity of telescopes to thus resolve nebulæ became a test of their power. but spectrum analysis finally entered the lists with new methods of investigation, and the comparatively tiny spectroscope at a single leap passed far beyond the utmost limits of the highest telescopic vision, and at one blow struck the whole category of nebulæ into two widely different classes,--those composed of discrete stars grouped like the suns of our own milky way, and exhibiting the characteristic spectra of such bodies, and those composed of diffused gaseous matter not yet condensed into suns, and showing the disconnected spectral lines of simple elemental gases. the line of division was clear, direct, positive, and beyond all dispute. yet beyond these two classes further research has disclosed certain vast nebulæ in which some portions exhibit true solar spectra more or less modified and others true gaseous spectra, each apparently merging into the other by gradations so faint and delicate that the inference is irresistible that in these nebulæ we see the processes of galactic and solar creation at various stages of their development. of these nebulæ, professor ball says, "in one of his most remarkable papers, sir w. herschel presents us with a summary of his observations on the nebulæ, arranged in such a manner as to suggest his theory of the gradual transmutation of nebulæ into stars. he first shows us that there are regions in the heavens where a faint diffused nebulosity is all that can be detected by the telescope. there are other nebulæ in which a nucleus can be just discerned, others again in which the nucleus is easily seen, and still others where the nucleus is a bright star-like point. the transition from an object of this kind to a nebulous star is very natural, while the nebulous stars pass into the ordinary stars by a few graduated stages. it is thus possible to enumerate a series of objects, beginning at one end with the most diffused nebulosity and ending at the other with an ordinary fixed star or group of stars. each object in the series differs but slightly from the object just before it and just after it." and of these composite nebulæ, he adds, "the great nebula in orion is known to be the most glorious body of its class that the heavens display. seen through a powerful telescope, ... the appearance of this grand 'light stain' is of indescribable glory. it is a vast volume of bluish gaseous material with hues of infinite softness and delicacy. here it presents luminous tracts which glow with an exquisite blue light; there it graduates off until it is impossible to say where the nebula ceases and the black sky begins." with reference to these distant galaxies of apparently complete solar systems like our own, the same principles must regulate the conversion of this energy of planetary electricity into the energy of solar light and heat as we see manifested in our own sun. the light of the individual stars is sufficient evidence of this; but the question may be asked, is the electrical interaction between separate galaxies and between different solar systems in the same galaxy universal, or are these operations merely local? in other words, is the source and the mode of solar energy in accordance with a single universal law of and between all created universes, or is it limited in effective energy to the members of each individual solar system alone? the answer is, that it is not less universal than the law of gravitation and no more so. there is a prevalent popular fallacy that the force of gravity is such that the movements, not only of solar systems, but of whole galaxies, and of all the illimitable systems of galaxies, are under its effective control, and that the whole universe of boundless space acknowledges its overwhelming sway. but nothing can be further from the truth. we know, of course, that the law is universal, as expressed in the statement of its terms by newton, but the mere statement of the law itself, as applied to interstellar distances, refutes the idea that solar systems and galaxies can rotate around any common center by virtue of the attraction of gravitation as a controlling force. the universality of the law itself has even been doubted. professor ball says, "in the first book about astronomy which i read in my boyhood there was a glowing description.... i allude to the discovery, or the alleged discovery, of a certain 'central sun' about which it was believed or stated that all the bodies in the universe revolved.... it was too good to be true. no one ever hears anything about the central sun hypothesis nowadays.... it must be, then, admitted that when the law of gravitation is spoken of as being universal, we are using language infinitely more general than the facts absolutely warrant. at the present moment we only know that gravitation exists to a very small extent in a certain indefinite small portion of space. our knowledge would have to be enormously increased before we could assert that gravitation was in operation throughout this very limited region; and even when we have proved this, we should only have made an infinitesimal advance to a proof that gravitation is absolutely universal." anyone who chooses may prove for himself that the force exercised by gravitation between the multitudinous suns of our own galaxy, the milky way, and our earth must be quite infinitesimal, and totally unable to control the motions of our own solar system in a definite orbit through universal space. we know that the law which regulates the intensity of light at various distances is the same as the law of gravity,--that is to say, the proportion is directly as the mass and inversely as the square of the distance. we know also that the stars which compose the milky way are similarly constituted, generally considered, to our own sun, and that under similar circumstances the emission of light, roughly speaking, will vary according to the magnitude of these distant suns. now, if any one will stand, at the darkest hour of the night, when the moon is absent and the sky perfectly cloudless, when the "stars that oversprinkle all the heavens seem to twinkle with a crystalline delight," and sweep with his gaze all the concave hemisphere of the sky, and then compare the light which is radiated around him with the gorgeous effulgence of the noonday summer sun, he can pretty closely compare the relative attraction of gravity which all those distant suns together can exercise upon our earth with that of our own sun. under control of the latter, the earth sweeps around in her orbit at the rate of about twenty miles per second; all these suns could not give our solar system even a minute fraction of that. of this starlight professor ball says, "the sun certainly must receive some heat by the radiation from the stars; but this is quite infinitesimal in comparison with his own stupendous radiation." any such attraction, of course, could not control the motions of our solar system, and much less that of many of the others. "the night has a thousand eyes, and the day but one, but the light of the whole world dies when the day is done." we can also demonstrate the fact mathematically by an exceedingly rough calculation, which, however, will be sufficient for our purpose. of the milky way, which comprises only the stars of our own sidereal system, professor ball says, "one hundred million stars are presumed to be disposed in a flat circular layer of such dimensions that a ray of light would require thirty thousand years to traverse one diameter." (the most recent estimates make the number of the stars which compose the milky way several times one hundred million, occupying a correspondingly greater amplitude of space. the number in any case is sufficiently stupendous.) our solar system is located in space at the apex of a vast transverse cleft, and nearly at the center of this disk. let us leave out of consideration the lower half of the milky way, as we look upward on a starlit night, and conceive this galaxy to extend only across the midnight sky above us like an archway, with fifty million suns, visible and invisible, exposed in the field of our vision. the nearest of all the fixed stars to us is that known as alpha centauri,--not visible, however, in our northern skies. this star is about two hundred and thirty thousand times as far from our sun as is the earth. if of the same mass as our sun, it must exert upon us an attractive force of gravity one fifty-three-billionth that of our own sun. next in distance is the star no. of the constellation cygnus. this may be three times as distant, and is certainly not less than twice. the light of the former will reach the earth in three and one-quarter years; that of the latter in not less than six and one-half years, perhaps much more. these are our nearest stellar neighbors. while the former will attract us with only one fifty-three-thousand-millionth that of the sun, the latter will attract us with less than one two-hundred-thousand-millionth that of our sun. conceive, then, a square pyramid extending radially upward for three thousand times the mean of these distances to the upper probable limits of the milky way, a light-distance of fifteen thousand years, and that this pyramid expands according to the squares of its distances, so that it will contain within it, equally distributed, all the stars (fifty million) of the upper half of the disk of the milky way; the sum total of all these attractions could not reach one twenty-millionth part of that of our sun upon the earth. if we continue to pile galaxies, in the same perpetual recession, behind each other to all infinity, we still could not engender sufficient attractive force to control the observed movements of the multitudinous stars of space. the very statement of the law of gravitation itself disproves it; for if we multiply orbs and systems according to any principle of aggregation that we know of in the way of distribution of such systems, or anything possible, with due regard to their own mutually interacting movements in space, we could never reach the inside limits of such a sphere of control, because the piling up of orb behind orb adds but an infinitesimal fraction to the force of gravity, for as the orbs themselves multiply in distance progressively by hundreds, their relative attractions inversely diminish by ten thousands. no possible increase of suns directly in mass could compensate for such an inverse ratio of squares, even if all intergalactic space were peopled with suns, instead of being, in fact, like a vast ocean, with a few small clusters of islands scattered here and there throughout its illimitable extent. of these vast realms of space, professor ball asks, "is our sidereal system to be regarded as an oceanic island in space, or is it in such connection with the systems in other parts of space as might lead us to infer that the various systems had a common character? the evidence seems to show that the stars in our system are probably not permanently associated together, but that in the course of time some stars enter our system and other stars leave it, in such manner as to suggest that the bodies visible to us are fairly typical of the general contents of the universe. the strongest evidence that can be presented on this subject is met with in the peculiar circumstances of one particular star. the star in question is known as no. of groombridge's catalogue. it is a small star, not to be seen without the aid of a telescope.... we shall probably be quite correct in assuming that the distance is not less than two hundred billions of miles.... the velocity is no less than two hundred miles per second.... the star sweeps along through our system with this stupendous velocity.... the velocity being over twenty-five miles a second, the attraction can never overcome the velocity, so that the star seems destined to escape." of the star alcyone he says, "doubtless that star is thousands of billions of miles from the earth; doubtless the light from it requires thousands of years--and some astronomers have said millions of years--to span the abyss which intervenes between our globe and those distant regions." and yet these stars, these galaxies, and even all the nebulæ we see or ever shall see, are merely in the vestibule of space; we have scarcely even yet lifted the outer curtain at the entrance of those vast realms. that the popular, but pseudo-scientific, idea of a series of ever-widening concentric orbits, increasing at every new expansion by an inconceivable ratio, is incredible we can well understand, and it is a satisfaction to know that such a wild hypothesis finds no warrant in the dicta or the demonstrations of science. and it is in the failure of gravity to control over the intervening space which lies between those vastly distant centers that we may hope to find the inklings of a more far-reaching law, by which nebulæ like that of orion crystallize out into separate star systems, just as in the rocks, whether igneous, metamorphic, or sedimentary, we find the attraction of cohesion yield to that of crystallization, until the whole cleft rock blazes with countless garnets in the schist and quartz crystals in the gneiss, or reveals the yellow specks of olivine in volcanic ejections. we shall find in the processes concerned with the development of living things the workings of a similar great law, perhaps the same. wherever there is the possibility of life, there we find life. there seems to be an all-pervading vital tension, so to speak, an energizing force, which drives the evolution and ascent of life forward and upward by successive leaps, as it were, from type to type, from race to race, and even from nation to nation. in this universal forward movement we may dimly discern the primordial creative and developing impulse, constantly acting, but manifesting visible change only at intervals as gathering forces accumulate and equilibrium is disturbed. it manifests itself in all the fields of nature,--vital, chemical, molecular, molar, systemic. it is the ever-acting, eternal past, present, and future, the macrocosm and the microcosm, the panurgus, the brahma, the ancient of days, and cannot be silenced or evaded: "they reckon ill who leave me out, when me they fly i am the wings." r. kalley miller, in his "romance of astronomy," says, "it would be hopeless to attempt expressing in ordinary language the vast distance at which these clusters of stars are situated from us. if we were to reckon it in miles, or even in millions of miles, figures would pile upon figures till in their number all definite idea of their value was lost. we must choose another unit to measure these infinitudes of space,--a unit compared with which the dimensions of our own solar system shrink into absolute nothingness. the velocity of light is such that it would flash fifteen times from pole to pole of our earth between two beats of the pendulum. it bridges the huge chasm that separates us from the sun in little more than eight minutes. but the light that shows us these faint star-clusters has been travelling with this frightful velocity for more than two million years since it left its distant source. we see them to-day in the fields of our telescopes, not as they are now, but as they were countless ages before the creation of man upon the earth. what they are now who can tell?" the movements of solar systems through space are unquestionably controlled by some wider law than that of gravitation, and it still remains for science to seek its hidden principles and discover its mode of operation. we know that some stars travel alone, like the star already noted, no. of groombridge's catalogue; that others travel in pairs, like the double star mizar and its companion alcor; and others in groups, like the stars beta, gamma, delta, epsilon and zeta, of the constellation ursa major; that we are driving towards the constellation lyra and leaving behind us sirius and its fellows, and that many, if not all, of the stars whose motions we can measure have a rapid movement through space, but under what control, in accord with what hidden harmony, and under what general plan they move, we do not know; but the laws of electrical action of the circling planets upon their central suns, and of these upon space, we can readily account for by the similar operation of the same laws within our own solar domain; and we know by the similar terms of the ratio of distribution of light that this is commensurate in extent with the law of gravity, and operates in a like proportion of energy over all intervening distances; so that wherever our sun presents a visible point of light, there it is pouring its energy into space, and every sun we can see, every galaxy, every star-cluster, nay, every nebula, is likewise pouring into the interplanetary space of our own solar system its proportionate quota of energy. the very fact that we can see the star shine is itself the fullest evidence that this is so, and evidence also that the law of gravitation there, too, is still in force, operating over these same distances, and with the same proportionate energy. knowing all this, we can read with a new light the grand vistas of the skies, with their starry denizens, and claim them all as parts of our own family; and the mutual interchange of attractive energy and of light and heat will not fail between us until those inconceivable distances shall have been reached which human knowledge can never span and where speculation fails; and even there, from out those dark abysses,--dark to our human eyes,--the call will still faintly reach us, and our response will reach them also, though we shall never have tangible evidence that such mutual ties continue to exist. industriously our planets gather their mighty energies from the surrounding springs of space, as one dips water from a crystal stream; we hand it over to our sun, and he, the royal high-priest, sprinkles it in glittering diamond-sprays over all those countless suns and their subject worlds, and they are baptized with an eternal baptism into our common brotherhood and we into theirs. our familiar planets, mars, jupiter, neptune, the earth, and even our little moon, seem to raise their voices and take actual part in the councils of almighty power, to move about as perpetual benefactors, gathering and spreading beneficence abroad, instead of cowering, a hapless few, like storm-stayed travellers, around the dying embers of our poor old sun, passive recipients of the light and heat and life which we have been taught to believe are slowly sinking into ashes and fading away in eternal darkness and death. one swift glance into these boundless truths is better for the human soul than the slow passage of whole hopeless centuries, which leave as their inevitable legacy on earth a vast and final catastrophe, in which everything that gave us light and heat and being must perish forever. has it, indeed, come to this, that the last word which science has to offer is, "after us the deluge"? by no means. we have merely been endeavoring to measure the right hand of god by weighing and measuring a single isolated one of his countless multitude of suns. it is as though one standing beside a great water-wheel should estimate its power and rotation by measuring the width and depth of the buckets and calculating the weight of water which its thirty-two receptacles contain, saying, "at its present rate in so many seconds it will cease to move." but we take him to the water-gate, and show it wide open; to the great dam above it which contains cubic miles of water; and still beyond that to the mighty fountains bursting forth with their rush and roar from the rock-ribbed fastnesses of the eternal hills, and pouring their unfailing flood-tide down forever and ever. and we do not pause even here: we show him the vapors rising from the spent water again, condensing into clouds, pouring down in torrents of rain among the hills, and that these continuously feed the sources of the fountains, which in turn supply the wheel almost to bursting. and so it is with the glorious mechanism of the heavens. the source of solar energy is not to be found in the sun itself, but in his environment; and he himself, in all his glory, is but the king, crowned with gold, blazing with rich apparel, and scattering benefits among his satellites, not from his own private treasury, but who himself is enriched by the mighty tribute with which his willing subjects continually endow him, and to whom alone he owes all his pride and power and wealth and magnificence, and which he, in turn, so freely expends, transmuted in form alone, in the perpetual improvement and welfare of his domain. he is the faithful ruler, but not the creator; the beneficent monarch, but not the god. chapter xi. the gaseous nebulÆ. when we reach the irresolvable nebulæ, we unquestionably have approached the creative period of solar systems and in many cases of whole galaxies. these are multifarious in form, but all can be reduced to a few comprehensive types. in determining the question as to whether these irresolvable nebulæ were composed of distinct stars like the milky way, but too distant to be resolved from their mist-like light into discrete stars by the most powerful telescopes, or whether they were gaseous in constitution,--that is, composed of diffused gaseous elements not condensed into solar bodies,--the spectroscope became the final and infallible test. of this instrument, thus used, professor proctor, in his "star-clouds and star-mist," says, "a very few words will explain the whole matter to readers who remember the three fundamental laws of this new mode of investigation,--viz., that, first, light from a burning solid or liquid source gives the rainbow-colored streak of light commonly known as the prismatic spectrum; secondly, when vapors surround such a source of light, the rainbow-colored streak is crossed by dark lines; and, thirdly, when the source of light is gas, there is no longer a rainbow-colored streak, but merely a finite number of bright lines." dr. huggins selected for investigation the small planetary nebula in the dragon. he says, "when i had directed the telescope armed with the spectrum apparatus to this nebula, i at first suspected that some derangement of the instrument had taken place, for no spectrum was seen, but only a short line of light. i then found that the light of this nebula, unlike any other extra-terrestrial light which had yet been subjected by me to prismatic analysis, was of definite colors, and therefore could not form a spectrum. a great part of the light is monochromatic, and so remains concentrated in a bright line occupying a position in the spectrum corresponding to its color. careful examination showed a narrower and much fainter line near the one first discovered. beyond this point, about three times as far from the first line, was a third exceedingly faint line. from the position of one of the bright lines it is inferred the gas nitrogen is one of the constituents of the nebula; another line indicates the existence of the gas hydrogen in that far-off system; the third line has not yet been associated with any known terrestrial element, though it is near one belonging to the metal barium, and still nearer one belonging to oxygen; a fourth line occasionally seen belongs to hydrogen." professor proctor says, "dr. huggins examined a large number of the planetary nebulæ (so called), obtaining in each case a spectrum which indicates gaseity. in some cases only one line could be seen, in others two, more commonly three, and in a few instances four. when these lines were seen they invariably corresponded in position with those already described. the single line sometimes seen corresponded with the brightest line of the three; and when a second line was visible, this also was no new line, but agreed with the second brightest line in the three-line spectrum. the fourth line was seen only in the spectrum of a very bright, small, blue planetary nebula, but was later observed in other cases, and especially in the great orion nebula." at this time the latter was not visible, but when dr. huggins had opportunity to examine it, he says, "the telescopic observations of this nebula seem to show that it is suitable to a crucial test of the usually received opinion that the resolution of a nebula into bright stellar points is a certain indication that the nebula consists of discrete stars." professor proctor says, "a simple glance resolved the difficulty. the light from the brightest part of the nebula--the very part which under lord rosse's great reflector blazed with innumerable points of light--gave a spectrum identical in all respects with that which huggins had obtained from the planetary nebulæ. thus, what had been deemed boldness in herschel--namely, that he should have associated the wildest and most fantastic nebula in the heavens with the circular and (in ordinary telescopes) almost uniformly luminous planetary nebulæ--was unexpectedly confirmed." the spectrum of this nebula has more recently been photographed by a long exposure in the camera of the prepared plate. of the result, professor proctor thus speaks, "the nebula is seen to be in great part gaseous, and, where gaseous, to shine in the main with the tints described above; but parts of the nebula are not gaseous, and those portions which are so are not all constituted in the same manner.... that portion which is called the fish's mouth gives a continuous spectrum; in other words, the same spectrum which we obtain from a star or a star-cluster. this is the spectrum arising from a glowing solid or liquid mass, or if from a gaseous body, then the gaseous body must be in a state of great compression.... but the stars thus forming must be immersed in the glowing gas forming the general substance of the nebula.... it would be absurd to suppose that the nebula is a flat surface; ... nebulous matter lies also, in all probability (certainly one might fairly say), between us and the stellar aggregration as well as on the farther side." further, the same author says, "if, as is probable, the luminosity of the gaseous portion of the orion nebula is accompanied by but a relatively small proportion of heat, then the rays from the violet and ultra-violet part of the spectrum are likely to give us much more complete information respecting the constitution of these nebulous masses than can be derived from the visible part of the spectrum." in the recent work of professor ball, "in the high heavens," that author says, "there are, however, good grounds for believing that nebulæ really do undergo some changes, at least as regards brightness; but whether these changes are such as herschel's theory would seem to require is quite another question. perhaps the best-authenticated instance is that of the variable nebula in the constellation of taurus, discovered by mr. hind in . at the time of its discovery this object was a small nebula about one minute in diameter, with a central condensation of light. d'arrest, the distinguished astronomer of copenhagen, found in that this nebula had vanished. on the th of december, , the nebula was again seen in the powerful refractor at pulkova, but on december , , mr. hind failed to detect it with the telescope by which it had been originally discovered.... in , o. struve, observing at pulkova, detected another nebulous spot in the vicinity of the place of the missing object, but this also has now vanished. struve, however, does not consider that the nebula of is distinct from hind's nebula, but he says, 'what i see is certainly the variable nebula itself, only in altered brightness and spread over a larger space. some traces of nebulosity are still to be seen exactly on the spot where hind and d'arrest placed the variable nebula. it is a remarkable circumstance that this nebula is in the vicinity of a variable star which changes somewhat irregularly from the ninth to the twelfth magnitude. at the time of the discovery in both the star and the nebula were brighter than they have since become.'... it must be admitted that the changes are such as would not be expected if herschel's theory were universally true. another remarkable occurrence in modern astronomy may be cited as having some bearing on the question as to the actual evidence for or against herschel's theory. on november , , dr. schmidt noticed a new star of the third magnitude in the constellation cygnus.... the brilliancy gradually declined until, on the th of december, mr. hind found it to be of the sixth magnitude. the spectrum ... exhibited several bright lines which indicated that the star differed from other stars by the possession of vast masses of glowing gaseous material.... september , , it was then below the tenth magnitude and of a decidedly bluish tint. viewed through the spectroscope, its light was almost completely monochromatic, and appeared to be indistinguishable from that which is often found to come from nebulæ.... it would seem certain that we have an instance before us in which a star has changed into a planetary nebula of small angular diameter.... professor pickering, however, has since found slight traces of a continuous spectrum, but the object has now become so extremely faint that such observations are very difficult.... for the nebular theory we require evidence of the conversion of nebulæ into stars." and not, it may be added, of stars into nebulæ. of the irregular nebulæ, professor proctor says, "it may well chance, as long since suggested by professor clark, of cincinnati, and as more cautiously hinted by dr. huggins, that in the varieties of constitution observed in the irregular nebulæ, and the evidence such varieties afford of progressive changes, we may find not merely direct evidence of the development of suns and sun-systems from the great masses of nebulous matter, but even what would be a far more important and impressive result,--actual evidence of the development of so-called elements from substances really elementary, or, at any rate, one stage nearer the elementary condition than are our hydrogen, nitrogen, oxygen, carbon, and so forth. the peculiarity of the spectral indications of the presence of nitrogen and hydrogen in the nebula, that only one line of nitrogen and two or three lines of hydrogen are discernible, instead of a complete spectrum of either element as seen under any known conditions, seems suggestive of what may be called a more elemental condition of hydrogen and nitrogen." whether this be so, or whether these peculiarities are due to self-obscuration, or mutual reversal of the familiar lines due to the enormous disturbances of the nebular mass which must exist, it is certain that there is one terrestrial substance, at least, which acts invariably, in combination and chemical affinity, as a simple element in inorganic chemistry, but which is, in fact, compound,--to wit, the hypothetical radical ammonium, which is closely allied with the simple alkaline metals potassium and sodium, forming with them a single group; and yet, while the others have always remained as fixed, primitive elements, the hypothetical element ammonium alone is a composite substance consisting of hydrogen and nitrogen, two of the invariable gaseous constituents of all these nebulæ. in comets we find, vaguely expressed, an occasional strongly marked sodium line, and also the spectrum of carbon; in these gaseous nebulæ we find, as yet, no trace of carbon, and this element is so closely allied to hydrogen in its chemical affinities and reactions as to suggest that it may be the same element or some alloy of it, or in some allotropic form, as we find to be the case with other simple elements under special conditions. in organic chemistry--the chemistry of organic life--we find almost innumerable compound radicals which act as simple elements in combination, but which we can combine and separate into their constituents at will; to all intents and purposes, in their various reactions, they behave as elemental substances, and were it not that our analyses are able to resolve them, as the spectroscope resolves the nebulæ, we might well believe that here also we were dealing with simple primary elements. it is almost certain that great discoveries in this field of chemistry are not far distant, which will recall with wondering surprise the now universally exploded fallacies of the "philosopher's stone" and the "universal solvent." indeed, we may find in the electrical energies of the planets and the self-repulsive force of the electrospheres of the earth and moon possible grounds for investigating anew some of the abandoned tenets of astrology, in the hope that the light of science may disclose some basis, at least, for what, at one time,--and for nearly all time, in fact,--was the universally accepted belief, not only of the ignorant, but of those the wisest and most learned of their day and generation. if the planets by their position can cloud the sun, nearly a million miles in diameter, with spots, or shed the brilliance of the aurora borealis over all our skies, may they not also cloud the embryonic intellect, or charge it with energies for a career of prosperity or of disaster? may not the unseen currents, or the electric storms around us, or the vast electrical phenomena of the sun as well affect the sprouting germs of the husbandman or some abnormally rapid development of an insect pest as the light, the warmth, the moisture, or the cold, which, to our coarser vision, are alone apparent? fancy and fallacy revel luxuriantly where science fails, but truth existed long before science was systematized, and the supercilious condemnation of once generally accepted views without examination is merely pseudo-science, and scarcely a single grade higher in the scale than ignorant superstition itself. and every new advance in knowledge requires a new overhauling of abandoned material, just as a new advance in metallurgical knowledge enables us sometimes to work over again our once-rejected mining dumps with decided profit. indeed, science itself is but a collection of observed facts reduced to system, and among the shrewd and practical miners there is a well-known saying, "the ore is where you find it," which has frequently put scientific assertion to the blush. a study of the beautiful mezzotint plates, from the drawings of the earl of rosse, contained in professor nichol's splendid work, "the architecture of the heavens," will clearly disclose the forms, as revealed by a powerful telescope, of many of these gaseous nebulæ. of such nebulæ, appleton's cyclopædia says, "nebulæ proper, or those which have not been definitely resolved, are found in nearly every quarter of the firmament, though abounding especially near those regions which have fewest stars. scarcely any are found near the milky way, and the great mass of them lie in the two opposite spaces farthest removed from this circle. their forms are very various, and often undergo strange and unexpected changes as the power of the telescope with which they are viewed is increased, so as not to be recognizable in some cases as the same objects." an example of this is shown in plate x. (figs. and ) of professor nichol's work, which gives a greatly enlarged view of those shown in figs. and of plate ix. (for fig. of nichol's plate x., see illustration of nebula with double sun, in previous chapter.) professor nichol says, "in every instance examined, save one, the planetary nebulæ are nebulæ with hollow centers." the inference which this writer makes, that such a planetary nebula consists of "a grand annular cluster of stars," has been since disproved by the discoveries of the spectroscope, but the telescopic form remains true, and still awaits further interpretation. while the irresolvable nebulæ seem to seek some retired spot in space for their processes, like certain animals when incubating, this rule is not universal. of this, appleton's cyclopædia says, "the density of nebular distribution increased with the distance from the galactic zone for the irresolvable nebulæ, but diminished with that distance for the clusters.... there is not a gradual condensation of nebulæ towards two opposite regions, near the poles of the galactic zone, but the nebulæ are gathered into streams, nodules, and irregular aggregations such as we find in the grouping of stars.... between stars and nebulæ their arrangement follows the law of contrast. there are two remarkable exceptions to this law,--the magellanic clouds. in these, where stars of all orders, from the ninth magnitude to irresolvable stellar aggregations, are as richly gathered as in the galactic zone, nebulæ of all orders are also gathered richly, even more so than anywhere else over the whole heavens." in the same work, article "nebula," it is stated of the planetary nebulæ, "there are several which have perfectly the appearance of a ring, and are called annular nebulæ.... some appear to be physically connected in pairs like double stars. most of the small nebulæ have the general appearance of a bright central nucleus enveloped in a nebulous veil. this nucleus is sometimes concentrated as a star and sometimes diffused. the enveloping veil is sometimes circular and sometimes elliptical, with every degree of eccentricity between a circle and a straight line. there are some which, with a general disposition to symmetry of form, have great branching arms or filaments with more or less precision of outline. an example of this is lord rosse's crab nebula. another remarkable object is the nebula in andromeda, which is visible with the naked eye, and is the only one which was discovered before the invention of the telescope. simon marius ( ) describes its appearance as that of a candle shining through horn. besides the above, which have comparatively regular forms, there are others more diffused and devoid of symmetry of shape. a remarkable example is the great nebula in orion, discovered by huygens in .... the great nebula in argo is another example of this class." the number of nebulæ recognized in all the heavens is upward of five thousand, and new ones are being constantly discovered. of these objects, professor nichol says, "the spiral figure is characteristic of an extensive class of galaxies. majestic associations of orbs, arranged in this winding form, with branches issuing like a divergent geometric curve from a globular cluster." these nebulæ, however, are not associations of orbs; they are gaseous nebulæ apparently in process of evolution. this author (professor nichol) presents views of such spiral nebulæ either foreshortened to the view, so as to form a long ellipse, or with the convolutions of the spiral apparently raised from the horizontal plane into a conical form, and showing the black streaks of space which lie between the convolutions, others seen in side view, others in front, and, in fact, presented to the eye in every position for observation. the author wrote before the days of the spectroscope, and that he should conceive these vast objects to be spirals made up of blazing suns like our milky way--vast galaxies, in fact--was an inevitable conclusion at that time; but we now know that these spiral nebulæ are gaseous, are apparently in process of manufacture, and we can see them in their different stages of evolution, and may perhaps learn something about the processes by which solar systems and galaxies of suns are formed. of one of these strange but exceedingly instructive objects, professor ball, in his work "in the high heavens," says, "fig. represents one of the famous spiral nebulæ (that of canes venatici) discovered many years ago by the late earl of rosse. the object is invisible to the naked eye. it seems like a haze surrounding the stars, which the telescope discloses in considerable numbers, as shown in the picture. when viewed through an instrument of sufficient power, a marvellous spectacle is revealed. there are wisps and patches of glowing cloud-like material which shine not as our clouds do, by reflecting to us the sunlight. this celestial cloud is no doubt self-luminous; it is, in fact, composed of vapors so intensely heated that they glow with fervor. as i write, i have lord rosse's elaborate drawing of this nebula before me, and on the margin of this stupendous object the nebula fades away so tenderly that it is almost impossible to say where the luminosity terminates. probably this nebula will in some remote age condense down into more solid substances. it contains, no doubt, enough material to make many globes as big as our earth. before, however, it settles down into dark bodies like the earth, it will have to pass through stages in which its condensing materials will form bright sun-like bodies. it seems as if this process of condensation might almost be witnessed at the present time in some parts of the great object. there are also some very striking nebulæ which are often spoken of as planetary. they are literally balls of bluish-colored gas or vapor, apparently more dense than that which forms the nebula now under consideration. such globes are doubtless undergoing condensation, and may be regarded as incipient worlds." of these spiral nebulæ it is said, in appleton's cyclopædia, "many of them had been long known as nebulæ, but their characteristic spiral form had never been suspected. they have the appearance of a maelstrom of stellar matter, and are among the most interesting objects in the heavens." of their spectra it is said, "the bright-line spectrum is given by all the irregular nebulæ hitherto examined and by the planetary nebulæ." that is to say, these nebulæ are gaseous in constitution, and have not yet reached the stage of solar condensation which marks the existence of individual suns. chapter xii. the nebular hypothesis: its basis and its difficulties. "there sinks the nebulous star we call the sun, if that hypothesis of theirs be sound."--tennyson. while the nebular theory of laplace is now the generally accepted scientific hypothesis of the formation of our solar system and of all solar systems, it finds its strongest support in the mode in which it seeks to account for the heat and light of the sun,--that is, that the central orb, gradually condensing down from an original volume as large as the orbit of neptune, at least, after disengaging the planetary rings, continued to condense to its present volume, and still so continues, the molecular motions arrested by condensation under gravity reappearing in the form of the energy of light and heat, and that this process of degradation will continue until, finally, the sun becomes a solid inert mass, incapable by further condensation of exciting the ethereal undulations in space which constitute heat and light, when the whole process will finally cease, the sun will die out, the planets continue to rotate in darkness, and the whole machinery be left running through an eternal night, like a vast mill in the hands of a negligent watchman (or rather no watchman at all), left to run itself alone, dark, empty, lifeless, and deserted, through the long and silent watches of the night. while the source and mode of solar energy set forth in this work are to be as readily accounted for if we accept as valid laplace's nebular hypothesis as by any other theory, yet such basis is not essential for its support; for while the planetary rotations and the central sun are the necessary consequence, according to laplace's hypothesis, of their mode of formation,--are, in fact, just what we actually find them to be under any hypothesis,--electrical generation and transformation will proceed just the same whether the planets and sun were formed originally in one mode or in another. but, since this generally accepted hypothesis accounts for the light and heat of the sun, to a certain extent at least, and for a certain relatively brief period, while no other hypothesis has been able to sufficiently account for it at all, and while this hypothesis also finds both support and contradiction in many observed phenomena of our solar system, it may well occur that this hypothesis itself, based upon the necessity of accounting for the sun's light and heat, and which latter afford it its strongest basis of support, may, if the basis upon which the theory rests be found to be otherwise explicable, still remain as an end, while originally presented only as a means, and thus be held as an obstacle to the acceptance of the widely different interpretation of known facts herein presented, in the absence of any other hypothesis capable of explaining the same facts in accordance with this presentation of planetary electrical generation and the solar transformation of this energy into light and heat. herbert spencer mentions an instance of such perversion of means into an end as occurring during the agitation for the repeal of the corn laws in england, which extended over many years, during which organized efforts were made to influence parliament. a permanent commission was established, with official head-quarters permanently located in london, with clerks, secretaries, higher officers, and all the paraphernalia of a first-class establishment. the purpose of this institution was to act in behalf of the popular interests upon parliament by every available means to secure this great reform. after years of effort, he says, a clerk one day rushed, breathless, into the office from the house of commons and shouted, in accents of despair, "we are ruined; the bill has passed!" the nebular hypothesis, while generally accepted in lieu of a better one, has no actual primary basis beyond that of mere assumption. of it professor ball says, "the nebular theory ... seems, from the nature of the case, to be almost incapable of receiving any direct testimony." we have already quoted from professor newcomb that it must be accepted, with all its difficulties, until a different and sufficient explanation of solar energy shall be presented. as set forth in appleton's cyclopædia, the theory is as follows: "assuming, for the sake of the argument, a rare, homogeneous, nebulous matter, widely diffused through space, the following successive changes will, on physical principles, take place in it: , mutual gravitation of its atoms; , atomic repulsion; , evolution of heat by overcoming this repulsion; , molecular combination at a certain stage of condensation; followed by, , sudden and great disengagement of heat; , lowering of temperature by radiation and consequent precipitation of binary atoms, aggregating into irregular flocculi and floating in the rarer medium, just as water when precipitated from air collects into clouds; , each flocculus will move towards the common center of gravity of all; but, being an irregular mass in a resisting medium, this motion will be out of the rectilinear,--that is to say, not directly towards the common center of gravity, but towards one or the other side of it,--and thus, , a spiral movement will ensue, which will be communicated to the rarer medium through which the flocculus is moving; and, , a preponderating momentum and rotation of the whole mass in some one direction, converging in spirals towards the common center of gravity. certain subordinate actions are to be noticed also. mutual attraction will tend to produce groups of flocculi concentrating around local centers of gravity and acquiring a subordinate vortical movement. these conclusions are shown to be in entire harmony with the observed phenomena. in this genetic process, when the precipitated matter is aggregating into flocculi, there will be found here and there detached portions, like shreds of cloud in a summer sky, which will not coalesce with the larger internal masses, but will slowly follow without overtaking them. these fragments will assume characteristics of motion strikingly correspondent to those of the comets, whose physical constitution and distribution are seen to be completely accordant with the hypothesis." during this process, it is further stated, successive rings of nebulous matter will be thrown off and left behind, which are supposed to have coalesced into planets and their satellites, and the motion of rotation will become more and more rapid as condensation proceeds, until, finally, the last planet, mercury, will be left behind in annular form, and the sun will then become the central orb of all the planets, and condensation afterwards will proceed without further delivery of planetary rings. professor ball says, "if we go sufficiently far back, we seem to come to a time when the sun, in a more or less completely gaseous state, filled up the surrounding space out to the orbit of mercury, or, earlier still, out to the orbit of the remotest planet." there is nothing in the actively developing nebula illustrated on the following page which shows the slightest analogy, either in structure or the forces at work, to what is demanded by the nebular hypothesis. on the contrary, these radiating, spiral convolutions, springing from a center and extended, with interstratified dark spaces, out to the periphery, are entirely incompatible with that theory. there have not, so far, been observed in all the heavens any gaseous nebulæ which lend the slightest support to the nebular hypothesis. we should expect to find, if it were true, that many of the nucleated planetary nebulæ show exterior concentric rings of luminous matter, clearly defined, two, three, or a dozen in number, left behind by the contracting volume of the nebula, and coalescing into planets, and, within, the glowing disk from which new external rings are about to be left as a residuum. on the contrary, these nebulæ gradually fade away towards their margins, and imperceptibly disappear in the blackness of space. if they terminated abruptly, we might suppose that here, at least, was the orbit of a newly forming planet, but the regular and delicate gradation of luminosity from maximum to zero shows that no such sudden breaking off has occurred. in all these nebulæ we find every definitely marked structure to exhibit the operation of combined forces of gravity and internal repulsion nearly equally balanced, but each acting independently of the other. these phenomena are as universal as the forces of cohesion and repellent polarity in the "attraction particles" of cell-life which determine the segmentation, growth, and development of the living organism. we find here the primal modification and differentiation of material structure under the stress of directly opposite and interacting primitive forces, and it is doubtless the same whether in a cell or a system. it is not a residuum, but the vis a tergo. it is well known that there are many and great difficulties involved in the nebular hypothesis. as for the genesis of comets, it will be at once seen that the theory will only account for such comets as never venture much beyond the orbit of neptune, as well as those which have an orbital plane nearly coincident with that of the planets. but most comets come from illimitable space, far, far beyond neptune's circle and at all angles to the plane of the planetary orbits; and we have already seen that a disk of space of the diameter of neptune's orbit and half as thick (see proctor's "familiar essays") would, to contain all the matter of our solar system equally distributed, have a density of only one four-hundred-thousandth that of hydrogen gas at atmospheric pressure,--that is to say, such a volume of the lightest substance we know of would make four hundred thousand solar systems like our own. this author inquires if such a mass could, under any circumstances, rotate as a whole, and adds, "has it ever occurred, i often wonder, to those who glibly quote the nebular theory as originally propounded, to inquire how far some of the processes suggested by laplace are in accordance with the now well-known laws of physics?" but the great primal difficulty is in the first assumption of the theory, which is not only entirely gratuitous, but physically impossible. it is that this great plasma of nebulous material--in the case of our own solar system not less than six thousand million miles in diameter--should have in someway become aggregated into a homogeneous mass of the requisite tenuity, complete and perfect, and ready for the succeeding stages of the process, in which, however, the law of gravity has hitherto had no active operation whatever; for, if gravitation existed and operated therein, such homogeneous mass could never have been formed, nor ever existed even if formed. the very forces which alone could have brought this vast mass together must have been the identical forces which afterwards broke it up into the sun and planets, and the operation of the same force must have prevented its original formation at all. according to the theory, it was like a horse-race, in which all the participants stood silent and motionless until the judge cried, "go!" but the judge was the great creative force itself, and if the fiat reached to this extent, the same power could just as readily--nay, far more readily--have shot the sun and planets forth into rotation, as children scatter dough-balls, instead of holding in abeyance the control of universal law so as to (as a humorous writer speaks of the operations of a child in his investigation of a watch) "see the wheels go round." this is not nature's plan, so far as human knowledge goes. of course these masses gathering to this great nebulous center, if acted upon by gravitation, would have at once condensed around the center as a nucleus, and if rotation ever commenced, it must have commenced then, millions of years, doubtless, before the outlying masses had even got within hailing distance. when masses of people assemble at a camp-meeting, the first comers take the best places, and the late arrivals have to circulate around in the woods; they do not all gather in a circle and then make a grand rush. that would be fair, perhaps, but it is not nature. and this, unquestionably, is how, if ever formed at all, these nebulæ must have formed into systems. the fact that the orbital planes of very many of these asteroids are greatly inclined to the common planetary plane, and still more greatly inclined to one another, points almost unerringly to the existence during their stage of formation of some powerful force either of internal repulsion or external attraction. that no sufficiently large body could have been present to exercise such attraction so far outside the general planetary plane is self-evident, and if there had been such source of attraction, while the orbital planes of the asteroids might have been deflected from the common plane, they could not have been forced apart so as to differ largely among themselves. certainly nothing pertaining to the nebular hypothesis could have produced any such effects under any conceivable circumstances, and especially at so late a period of its progress, after all the principal planets had been completed. the only alternative is self-repulsion, and this could only have been due to the causes and their mode of operation already described in this work. in a modified degree these planes exhibit the same irregular orbital deflections as are so conspicuously visible in the orbits of comets, and they must have been unquestionably produced in the same manner. the barren bands or stripes in the area occupied by these asteroids, like the dark or vacant rings of the planet saturn, may have been largely affected by the perturbing attraction of the neighboring planet jupiter; but certainly no influence of that great planet (himself in the common planetary plane) could have operated to cast these forming planetoids into planes of diverse inclinations among themselves or to that of his own. on the contrary, his whole force must have been exerted to bring them into the closest harmony with his own orbital movements. omitting discussion of the technical difficulties in the application of the nebular theory to demonstrated facts, which may be found in the books, we may again repeat that this theory is not essential to account for the heat of the sun, which finds its real source elsewhere, while, nevertheless, the theory in itself is not incompatible with the views which we have endeavored to present and demonstrate. certain phenomena, however, have been considered in prior quotations in this work which may aid us to roughly indicate the successive processes by which the evolution of solar systems and galaxies may be explained on another basis which requires no violent assumptions to be made and no suspension of any of nature's universal laws. the same operations which we see around us at the present time in our own system, if extended to the dimensions of a nebular aggregation, would probably present the same phenomena as those we find partially disclosed in the gaseous nebulæ, particularly the spiral, and these would naturally determine the final production of solar systems such as our own. the gaseous nebulæ, not spiral, and the mixed nebulæ also, would fall into their appropriate categories in the same general plan, and a consistent mode of formation would be presented from the beginning to the end of the different processes. it should be observed that the spiral required by laplace's nebular theory is essentially a centripetal spiral. the spiral nebulæ we see in the heavens, however, are centrifugal spirals. this is clearly shown in plates xv., xii., and the frontispiece of nichol's "architecture of the heavens," as well as in plates xiii. and xiv. plate xv.--the open spiral--is directly contradictory of any phenomena which could occur in accordance with the nebular theory of laplace. the frontispiece shows the only form which such a nebula could assume at any stage of its career,--that is, a close spiral with nearly circular convolutions. but while this particular form is not only in entire accordance with the hypothesis which we are about to suggest, being in fact one of the later and necessary stages in its progress, any such spiral as that shown in plate xv. is utterly out of the question in the application of the nebular theory of laplace or in any of the more recent modifications thereof. the only hypothesis by which the various phenomena can be adequately explained must almost certainly be based upon the combined action of gravitation and electrospheric repulsion. we find in the corona of our own sun such phenomena manifested in the most striking degree, even in a completed system, and we can well understand that during the early stages of systemic development such phenomena would vastly transcend anything which we could now hope to observe around our own sun. we see this repulsion still more highly developed in the formation of the tails of comets. while these coronal rays are not visible to a distance of more, perhaps, than five million miles from the sun's disk, we have seen that the tail of newton's comet was shot forth to a distance of ninety million miles in a few days, as it were in a moment, by the tremendous electrical repulsion of the solar electrosphere, and that this enormous tail, which, if composed of hydrogen gas alone (it was, of course, enormously more attenuated), would have contained a mass much more than equal to the weight of the sun, was swung around over an arc of one hundred and eighty degrees, giving a radial sweep of the tail over a distance of two hundred and eighty millions of miles in less than four days. and the tails of many other comets have largely transcended in dimensions that of newton, above cited. we have learned much of the laws which regulate the development of storms, cyclones, whirlwinds, water-spouts, and other vortical phenomena in the atmosphere of our own earth, and can readily apply these principles to phenomena of vastly greater magnitude. we know that the matter of comets' tails is self-repulsive, as shown in multiple tails, as well as that it is repelled by an adjacent similarly electrified electrosphere,--that of the sun, for example,--as with pith-balls in the familiar class-room experiments; so that we can gather a very fair and complete idea of the processes of nature when dealing with such phenomena on a vastly more extended scale, in which our moments are measured by millions of years and our miles by the almost infinite distances of sidereal and nebular space. chapter xiii. the genesis of solar systems and galaxies. the processes of development of a solar system from the diffused elemental matter of space may then be roughly sketched as follows, premising that each stage may have possibly extended over vast periods of time, and the whole, perhaps, not been completed for millions of years. with the processes of creation time is as nothing. the area of space in which a solar system is about to be developed has hitherto maintained its molecular constituents in a state of gradually increased unstable equilibrium, whether such augmented instability may have been induced by a gradual rise of temperature from emission of the solar energy of other galaxies, by gradual diffusion from constantly operative centers, from currents or vortices of space, or by some primal inherent constitution of space itself, with constantly increasing tensions relieved by successive discharges, of which analogous instances are found in various other processes of nature, as, for example, ovulation, fission, and gemmation in the reproduction of life, regularly recurring epileptiform convulsions, regularly repeated spark discharges from electrical machines, or the ebullition of viscous fluids with their slowly recurring bursting bubbles. at some focal point of this area a rupture of tension will finally occur, induced by some sudden current or vortical movement, as we see sometimes in a pool of water gradually reduced in temperature below the freezing-point, when its whole surface, by the passage of a breath of wind even, will be suddenly flashed into crystals of ice. at this point of space there will be instituted a rapid expansion among the molecules and a consequent fall of temperature, followed by an inrush of the vaporous material surrounding this center of agitation, and a vortical movement will be established, with currents of spatial matter attracted to this vortex in constantly increasing streams. the molecular tensions will be successively unlocked as the circles of agitation continue to widen, and a condensed nucleus will form, rotating upon its axis and exhibiting the combined phenomena of gravity and centrifugal force. as the nucleus continues to increase in mass and density its temperature will constantly rise, while its speed of rotation will gradually diminish as its volume increases, and the aqueous vapors of space, as they gather around this rotating center of attraction, will be forced outward by centrifugal action and the heat of the nucleus, and form vast attenuated clouds,--not necessarily visible, however, to human sight,--and these clouds, in their various stratifications and disturbances, will gradually come to partake of the rotatory movement of the center, such movements, however, gradually fading away as they recede in space and in density. the cyclonic movements of these clouds of aqueous vapor upon themselves, but principally against the surrounding gases of space still under tension, will generate enormous quantities of electricity, which flash like thunder-clouds as they approach each other, with incessant streams of lightning and rolls of thunder. the growing and heating central nucleus is thus thrown into a state of high electrical opposite polarity, and its own constituent elements become self-repellent, just as we see in the sun's corona and in the phenomena of comets. the electrical tension of the central mass will gradually grow higher and higher, until a vast stream or streams of incandescent nebulous matter (for with double suns they may be multiple, or the internal repulsion may even cause division of the nucleus itself) will be suddenly driven outward in a radial direction along the lines of least resistance,--that is to say, in the plane of equatorial rotation, where centrifugal force is most effective. we can readily understand the self-repellent force of such an enormous mass of cosmical matter by considering that, in our own completed system, the repulsion of the solar electrosphere drove forth the tail of newton's comet, as before stated, to a distance of ninety million miles, and whirled it around a semicircle of this radius in less than four days. our most distant planet, neptune, is only thirty times this distance from the sun, and we see during every solar eclipse the coronal structure glowing to a distance of more than a million miles from the sun's disk, and the radial streamers driven forth five million miles, and even farther. (see illustrations of solar corona in guillemin's "the heavens.") the vast stream of radiating nebulous matter thus forced out by solar repulsion will likewise be acted upon with equal energy by its own internal self-repellent force. if we conceive a stream of water thrown vertically upward by a powerful force-pump, in which every drop of the fluid is endowed with tremendous self-repulsive energy, we should find an analogy to the phenomenon in question. we can see an example of this in the "crab nebula," illustrated in a previous chapter. the stream, acted upon by gravity downward, by the force of ejection upward, and by the internal force of repulsion both transversely and upward, would assume a pyriform shape, narrower beneath, largely swollen about its middle, and thence gradually decreasing in diameter to its termination in a rounded tuft, in advance of which would be driven forth detached sprays and wisps, while filaments and outlying parallel strands would mark its entire ascent, except towards its point of ejection, where the primal force which drove it out is greatly in excess of those of gravity and self-repulsion. it will be seen at a glance that these phenomena are precisely those which we observe in a comet's tail. (see illustrations of many comets having these characteristics in guillemin's "the heavens," lockyer's edition.) suppose, now, that this stream of water or the tail of a large comet were gradually wrapped around its point of emission by the rotation of this nucleus upon its axis. a spiral would form, very open or flaring at first, but gradually growing closer and more circular as the force of gravity drew its convolutions downward upon the interstratified clouds of aqueous vapor occupying, in compressed layers, the spaces between the adjacent coils of the spiral. there would be a composite action of forces observed: gravity would attract the convolutions and their interstratified layers of cloud equally, according to their densities, while the central repulsive force would repel the convolutions of the spiral along the same lines of force, but would not act at all upon the strata of clouds, and the force of internal self-repulsion would also tend to disrupt the convolutions of the spiral by expanding them outwardly. the outer convolution, however, would have no backward thrust from any internal repulsion beyond, while, within, gravity and solar repulsion would be more equally balanced, so that the outer coil would be relatively compressed in its rotation against the next inner convolution, and its ratio of distance would not be maintained. we find this exemplified in the case of neptune's, orbit in our own system. the inner convolution would also be abnormal, since the primal force of ejection must have been sufficient to carry the outward thrust of the whole spiral, and in consequence its flare would offer much greater resistance to the deflection of rotation, and it would have a more radial direction than those beyond. we shall find that the planet mercury, and the inner convolution which was eventually reabsorbed into the solar mass, exhibit these phenomena. between the outer and these inner convolutions the curve of the spiral would be approximately regular, with a fixed ratio of increase. in the planets of our solar system this ratio is that produced by constantly doubling the preceding number, the series being , , , , , etc. in other solar systems, however, the ratio may be quite different. in this abnormal flare of the inner convolution is doubtless to be found the rational basis of bode's empirical law of planetary distances, in which the arbitrary number must be added to each term of the above progression, making the series , , , , , etc. the inner coil between mercury and the sun was drawn into the solar mass on the disruption of the spiral, leaving, from the abnormally radial curvature of the inner portions of the spiral and its absence from the series, a vacant place which must be represented by the relatively fixed increment to be added to each term of the series. as the convolutions of the spiral become more and more compressed towards each other and more and more flattened against the interstratified cloud-layers, the force of internal repulsion becomes more and more active in its tendency to disrupt the spiral, since its forces are more direct and concentrated along lines nearly at right angles to the force of gravity. during the formation of the spiral we can easily conceive that--like a stream of water shooting over a cascade, or the multiple tails of some comets, or even a whole comet, as, for example, biela's, which was split up into two separate bodies by this force--some convolution, perhaps a single one of the series, will be laterally divided into a large number of nearly parallel strands, mutually held apart by their internal self-repulsion, and with cloud-layers interposed between these lateral strands. such a series of small planets as these would finally produce we find in the belt of our asteroids, the bulk of the convolution, probably, for the most part, however, scattered in space, since their aggregate mass is so small, and possibly, in part, coalesced into the mass of jupiter, to which mars, by his position, may also have contributed. not only may a whole convolution be thus split up, but along the spiral at many points the outer margins may be thrust outward, forming partially detached parallel strands, which may thus coalesce to form the satellites of the completed planets; while at the outer extremity of all, where the backward thrust of self-repulsion is wanting, enormous wisps, sprays, and tufts of nebulous matter would be driven entirely forth into the illimitable realms of outer space, but not necessarily, or even probably, into the space of other systems, which are so enormously distant; and there, in those unoccupied realms, they will remain to gyrate "in the solitude of their own originality," in the form of comets, until, at long intervals, they may chance to revisit the scenes of their earliest youth, to warm their frozen limbs for a brief period at the old and well-remembered parental fire, or finally, worn out with toil and travel, "come home at last to die." driven forth from the society of their fellows by their own unbalanced energies, these anarchists of the sky may form loose aggregations, granulated about multitudes of self-constituted minor centers; but, cut loose from all effective solar control during their period of coalescence, they must forever lack the consolidated form and complex organization of their prosperous and rotund brethren, the planets and their satellites, or even the tiny asteroids, who stayed home and, like the little pig, had bread and butter for breakfast. the disruptive energy of internal repulsion, as above stated, increases in force as the convolutions of the spiral become more and more compressed and the spiral becomes more and more circular in form. suddenly the coils of the spiral will be burst asunder, and this will occur along that particular radial line of gravitation where the central nucleus acts with its most effective force. the disruption will be simultaneous, as a general rule, in accordance with the principles which control ruptures of tension of bodies in a state of unstable equilibrium, and which we see exemplified in multiplied centers of crystallization, the simultaneous formation of mud-cracks, the giant's causeway, and other like phenomena. each convolution will now become a detached open ring, one of its broken extremities, however, being millions of miles farther from the central nucleus than the other. what occurs when a cometic body, negatively electrified, impinges upon the positive electrosphere of a planet, or when an electrical induction machine like voss's is touched by an oppositely electrified body, will now necessarily occur with these disrupted convolutions. their connection with the negatively electrified nucleus being broken, a reversal of electrical polarity will ensue from contact with the adjacent positively electrified clouds of aqueous vapor, and, instead of self-repulsion, mutual attraction will now prevail along the length of each of the open rings. held apart from the central nucleus by the interstratified cloud-layers, and acted upon by the double force of gravity and internal attraction, the component elements of these open rings will rapidly lose their luminosity and heat, and coalesce by a retrograde movement down the lines of their direction, thus approaching the sun along the segment of an ellipse, the nucleus, or sun, occupying one of the foci, the eccentricity of the ellipse being measured by the differential between the nearest point of the open ring and the part of the convolution which lies directly opposite and beyond the sun. in other words, the form of the spiral will determine the eccentricity of the ellipse, subject to perturbations, however, of various sorts. during this stage of coalescence from an open ring into a sphere, these bodies will take on, by cooling and condensation, their planetary forms; and as the forming spheres, by the retreat of their masses down the lines of approach to the sun, advance, their forward and nearer extremities will be more powerfully acted upon by gravity than those parts in the rear, and a forward plunge or axial movement of rotation will be set up. viscous matter,--pitch, for example,--molten by the sun's heat and flowing down a steep roof, exhibits a similar forward movement, the outer layers tending to roll over the inner ones in convoluted folds, the adhesion to the roof of the under surface corresponding to the retarding pull of the sun's attraction. in like manner are produced rotating eddies in streams of water having crooked channels, eddies of air under water-falls, and other analogous atmospheric disturbances. during the stage of coalescence of the planetary spheres the adjacent clouds of aqueous vapor will condense around them, and their hitherto diffused electrical energies will be concentrated by rotation in currents of enormous quantity and potential directly upon the sun, and a disassociation of the elements which compose these watery vapors will ensue, the result of which will be the deposit of hydrogen gas as an atmospheric envelope around the sun's body, and of oxygen around and through the bodies which constitute the planets. these gases will be disassociated in their combining proportions, two volumes of hydrogen at the sun for one volume of oxygen, distributed according to their relative electrical energies among the planets. this nascent oxygen will rapidly combine with the consolidating elements of the planets and, interpenetrating their solidifying bodies, form the vast mass of oxides which we find to constitute the bulk of our terrestrial mass, the residue, mechanically commingled with the condensed ever-present nitrogen, forming the planetary atmospheres. the condensation of volume of the planets will give rise to great elevation of temperature, while their currents of electricity, poured into the sun, will, by their passage through its enormously compressed hydrogen atmosphere, produce intense heat, and this, rapidly communicated to the solar core within, will raise its temperature to that of the sun as we now see it, and permanently maintain it in that state of incandescence. during the stage of coalescence of the planetary bodies, outlying strands of the spiral will follow the course of their adjacent masses in a nearly parallel movement, and will gradually coalesce into smaller bodies more directly under the influence of the gravity of their own adjacent planets, by their proximity, than of that of the sun. these bodies will thus rotate as satellites around their planets, and the forward shift of their centers of gravity, by their advance along their lines of coalescence, may result in a permanent displacement, of which we see an example in the moon, which constantly presents the same face to the earth, while having an axial rotation of its own with reference to the sun. (in this case the action of gravity may have been assisted, however, by the mutual repulsion of the lunar and terrestrial electrospheres forcing the atmosphere and moisture of the lunar mass to its opposite side and maintaining it there, where it would remain as a buffer against rotation.) in some cases we might find certain outlying strands of a convolution which, perturbed by external influences, may have been delayed in its conversion into spherical form, and this subordinate strand, pyriform itself, as it must have been, in shape, would thus form a spiral of minute discrete bodies, probably like the nucleus of a comet, finally assuming the shape of a series of rings, and rotating like a satellite around the neighboring planet, the inner and outer strands more attenuated and the middle ones more condensed, as we find to be the case with the rings of saturn. in the original spiral we have seen that, as a whole, it was of necessity pyriform in shape. the planets formed therefrom would thus be found to increase in size from within outward to a maximum, after which they would again decrease, but not to the original minimum, while the extreme outer planet would also be unduly enlarged by increment from partially dissipated terminal filaments, gradually attracted thereto from surrounding space. there is such an undue enlargement of the planet neptune, and this, with its relatively compressed orbit, before alluded to, renders it almost certain that neptune is in reality the outermost member of our planetary system. we find this gradation of size to be the case in our solar system, except where the series has been broken by the multitudinous separation, from violent internal repulsion, of one of the convolutions into parallel strands showing all sorts of perturbations, this being the convolution which occupied the region between the orbits of mars and jupiter, and which, by the coalescence of these numerous parallel strands into small planetary bodies, has filled the space with a belt of asteroids hundreds and perhaps thousands or even tens of thousands in number. it is probable that a law regulating the ellipticity of planetary orbits can be deduced from a consideration of the principles which have governed their inception, and with these are doubtless closely related those laws of laplace which have demonstrated that "in any system of bodies travelling in one direction around a central attracting orb, the eccentricities and inclinations, if small at any one time, would always continue inconsiderable." (appleton's cyclopædia, article "planet.") we have thus traced the genesis of a solar system from its earliest stages forward through its various changes until, complete and in working order, it is ready to be sent on its eternal course, either alone or as one of a vast congeries of similar systems, like the milky way. (see frontispiece for illustration of a series of types of development from a straight-tailed comet, through different curvatures, and spiral nebulæ of less and less divergence, until nearly circular, and finally terminating in a complete solar system.) these processes of creation may be isolated, or they may flash a hundred million solar systems into being together, as crystals flash forth in the rock; but, when once formed, they go forth each as eternal as space itself. but can we not go back one step farther still in the progressive stages of creative energy? whence came these powerful agencies by means of which all those distant regions became peopled with suns and worlds? the great source of all is to be found alone in space,--the so-called "empty space." but it is far from empty; all through it are diffused the attenuated vapors which, condensed, constitute our suns and planets, and all that is, or ever shall be, gaseous vapors, which are held poised, with their opposite tensions of cohesion and expansion, like the prince rupert drops which glass-blowers make for toys,--a little bulb of glass, chilled as it falls, molten, in a vessel of water. from one extremity projects a long, crooked stem, scarcely thicker at the end than a horse-hair, spun out from the molten glass as it hung from the glass-blower's rod. the bulbous body is as large, perhaps, as a nut; you can beat it with a hammer and it will not break; it is the hardest in structure of all glass. now, wrap this bulb up in a thick handkerchief, or you may be injured; hold it firmly, and break off the very tiniest tip of the long stem three, four, or even six inches from the bulb. there is a sudden shock; open your handkerchief, and lo! instead of the solid bulb, there is only a loose mass of white powder. if you put the bulb in a heavy glass vessel full of water and break off the tip of the tail, it will shatter the vessel into fragments. what is the explanation?--it is, of course, well known--simply that the molecules of glass were instantly arrested in their motion of adjustment as the glass was suddenly chilled by the water, and the molecular motion of shrinkage was arrested, leaving the individual molecules under a tremendous strain of position in their endeavor to reach their true places. they are rigidly fixed in this position of unstable equilibrium, one balancing the other; but let a single molecule be displaced,--a fragment so tiny that the eye can scarcely see it,--and the molecules, thus thrown out of mutual support against each other, must now rearrange themselves from the ruptured rigid mass, and, like a row of stood-up bricks, each of which thrusts the other forward, with a sudden explosive force the molecules assume their true position of stable equilibrium, but it is at the cost of the whole structure. to this same cause we owe the explosive force of our gunpowder, nitroglycerin, and all explosives; the molecules are held in unstable equilibrium, and the tension once relieved at a single point, be it ever so infinitesimal, the molecules of the whole mass rearrange themselves with explosive energy. strange that so harmless a substance as glycerin, by the mere replacement of an atom of nitrogen gas, should develop the energy of dynamite under a trifling molecular shock. so, also, the aqueous and perhaps other vapors of all space, attenuated though they be, and perhaps by reason of this very tenuity itself, as shown by the experiments of professor crookes with attenuated gases when acted upon by electricity, are held in the same state of unstable equilibrium. we know the potency of this instability from the terrific explosive combination of the gases which combine to form aqueous vapor. we may again refer to one of the well-known experiments of professor crookes with simple atmospheric air. enclosed in a cylindrical glass vessel, the electric spark passed freely; as it became more rarefied under an air-pump, new phenomena appeared, until, at a stage of high rarefaction, the molecules of these gases were driven forward by the electric current with such energy as first to raise the temperature of the opposite side of the cylinder to a red heat, then to melt, and finally to perforate the glass. the explanation is that the movements of closely aggregated molecules mutually interfere with each other; as they gain elbow-room by being reduced in number, they act with more directness, and consequently with more force: it is the difference between men fighting in a crowded room and out in an open field. it is possible that these molecular tensions of space, by the ready unlocking of the forces with which they are charged, may even aid in the rotation of the planets by acting upon their electrospheres in their drift through space, as charged thunder-clouds react upon each other, or the molecules of atmospheric air, in moderately high vacua, under electrical excitement, act upon the walls of the containing vessel, as in the experiments of professor crookes and others. the riddles of nature are like those of the sphinx,--they have more than one meaning. the tensions of the aggregated molecules of space are thus counterbalanced only so long as all space is equally occupied and a state of perfect quiescence exists in its every part. a molecular disturbance in one part is immediately communicated to adjacent parts, and finally to all. with the first movement, gravity asserts itself, for gravity exists and must exist in all parts, and must actively manifest itself whenever the perfect mutual balance of space is disturbed and a center of energy developed, and co-ordinately with the action of gravity begins that of electricity. movements among the molecules are converted into movement of mass; centripetal motion begets condensation, this begets sensible heat and vortical movement; then come the phenomena of electrical generation by moving contact with the gases of space, then repulsion and disassociation of the elements of the aqueous vapors, combination of simple into compound elements; and, the balance once disturbed, the state of unstable equilibrium is forever destroyed, and all space henceforth must exhibit constant change. there are whole segments of space absolutely blank, so far as visible systems are concerned, which seem to have been exhausted, for the present æons at least, to supply material for the vast adjacent galaxies which extend along their borders; see illustrations in proctor's "essays on astronomy," article "distribution of the nebulæ." it need not be supposed that such stage of perfect and universal quiescence ever existed in fact; it is like the nirvana of the buddhist philosophers,--a subjective and not an objective condition. we can have no knowledge of the existence, even, of material things, save from their phenomena, the manifestation of interchanging forces, upon which rests our threefold basis of knowledge, perception, cognition, and comparison. we know nothing of matter, except as affected by internal or external force, nor of force itself, except as it acts in one mode or another upon matter. all beyond this is, for us, without form and void. progressive change has always, doubtless, been the universal law of creation, and the great ocean of space is, and ever has been, and ever will be the highway through which perpetually plough the great caravels which bear the fortunes of creative energy, laden with life and light and heat, in their eternal progression. the creative impulse once given, if it, too, was not primeval in the eternal past, must have gone on from development to development, like the transmission of life, from age to age and from realm to realm. "the mills of the gods grind slowly;" in these vast areas time is absolutely nothing; the processes we see are but as the dip of a swallow's wing compared with an inconceivable futurity; but all our energies, and all the energies of planets and suns and systems and galaxies, and of whatever other and wider created forms may stretch onward to infinity, came forth from the ocean of space, and to this ocean all these energies continue to return again in ceaseless circuit. can we indicate any relationship of periodicity for the genesis of solar systems from space? there is a remarkable example of a somewhat similar periodicity in organic life for the rupture of tensions, so common that its analogous character and perfect regularity are scarcely even thought of. among the highest species of mammalia we find that, in a state of health, whether resident of the heights of the andes, the deserts of africa, the jungles of india, or the most densely populated centers of london; among rich or poor, high or low, idle or industrious, virtuous or vicious, ancient or modern, civilized or barbarous, black, white, red, or yellow, the ovum of the mature female rises to the surface of the ovary, and at intervals, almost uniform, of twenty-eight days, organic excitement ensues, the enclosing vesicle is ruptured, and the ovum escapes. the remarkable feature is not that these processes continuously succeed each other; but that under such diverse conditions and opposite circumstances, and with two separate ovaries operating at the same time, simultaneously or successively, this almost miraculous interval of no more and no less than twenty-eight days between the successive ruptures of tension and their attendant phenomena, should constantly persist. for its ultimate cause we must look back to the vis a tergo to which we have already alluded; and there may be, and doubtless is, a similarly acting remote cause which regulates the periodical development of solar systems or of galaxies, periods of intense activity, followed by intervals of exhaustion and recuperation, and again succeeded by another period of activity, and so on perpetually, for space is perpetual, infinite, and inexhaustible. it will be observed that the processes above roughly sketched are somewhat similar to those observed in the formation of so-called water-spouts, which usually terminate in dissipation in the atmosphere, or else in terrific thunder-storms, but which occasionally reach a sufficient energy of rotation to spin their central nuclei down towards, or even to, the surface of the sea, or, in desert regions, to that of the ground. there is no analogy with the theoretical and "assumed" primal mass of attenuated plasma of the nebular theory, or with its slow initial rotation, with the successive casting off of rings of nebulous matter. it may sometimes happen, however, that the repulsive electrical energy of the central nucleus may throw off its external envelopes with sufficient force to drive them entirely beyond the effective limit of its attractive forces, as occurs in the formation of embryonic comets as above described; in such case the nebula will be a variable one, with successively repeated aggregations and successive outbursts, periodical like the active stages of volcanoes; and, even when the nucleus has already presented a continuous solar spectrum, its energies may be thus expended, or more gradually, and finally dissipated like the electricity of a highly charged leyden jar exposed to a moist atmosphere. as a bottle of strongly effervescing liquid may blow itself empty, when suddenly opened, by the mutually repellent energy of its contained molecules, so if such a phenomenon were manifested in a radial direction from a central point, the repelled spray would show itself as a nebulous ring with a hollow center. an example of this sort is shown in the multiple-tailed "catherine-wheel" nebula (fig. of a previous illustration). if such an annular nebula should become ruptured into two portions by internal repulsion, the electrical polarity of the smaller fragment would be reversed, and the two arcs would separately coalesce and consolidate into a sun and a single planet, forming a solar system like that of algol, which has been already described. otherwise, the nebula would probably retrograde and disappear, by diffusion, into space again. we may expect to find abortive efforts of nature here, as we so constantly find them elsewhere, not merely in inorganic matter, but even among the processes of life. in professor proctor's article ("essays on astronomy") on the square-shouldered aspect of saturn, he mentions a hitherto unexplained circumstance of the earth's atmosphere--the curious fact that the barometrical pressure of the earth's atmosphere is somewhat higher between the poles and the equator than immediately over the latter, as might be supposed to be the case. this is a phenomenon of mutual repulsion similar to those manifested in the operations above described. the rotation of the earth on its axis forces the terrestrial atmosphere, by its centrifugal motion, in undue proportion, around the equatorial belt, causing the same sort of atmospheric thinning at the poles which we see in the solar photosphere at its corresponding parts. at the same time the highly electrified atmosphere, by its mutually repellent action, tends to force this swollen equatorial ring backward toward the poles. the resultant of these two repulsions is an area of maximum density part way between the poles and the equator. it is probable that this self-repellent equatorial swell may play some part in the sun's atmosphere, in extending, and also in limiting, the areas of eruptive sun-spots outward from his equator. while the nebulæ are more distant than many of the discrete stars revealed to us by the telescope, there is no reason to suppose that they are more distant than the star-clouds into which are merged the separate stars of the milky way, or the star-clusters seen in other portions of the sky. we know, in fact, that this is not so, for our telescopes show brilliant stars in very many cases which are components of the nebulæ themselves; and the fact that the nebulæ can be seen as having visible form, and not as mere points of light, is itself conclusive as to their relative distances. hence we need not be surprised to learn that these forming spirals will result each in the production of a single solar system, and not a galaxy of suns, as was once supposed. were such the case it would be impossible for us to observe the structure of the nebulæ at all, as their distances would be far too vast. of the forms of the gaseous nebulæ guillemin asks, "is the spiral the original form of those gaseous matters, the condensation of which may give, or has given, birth to each individual of this gigantic association?" the same author says of these apparently regularly formed nebulæ, "it is impossible not to recognize in them so many systems." many of the spiral nebulæ were formerly supposed to be globular aggregations of nebulous matter only, and their spiral character came as a great surprise with the use of more powerful telescopes; and many--nay, most--of these apparently globular nebulæ have totally changed their appearance when viewed with instruments of higher power, while the spirals have become more and more pronounced in character with every increase of telescopic vision. of one of such apparently globular nebulæ guillemin says, "the center is like a large globular nebula with a very marked condensation, whence radiate branches arranged in the form of spirals. in several points of these branches other centers of condensation are noticed. sir john herschel had classed this among the nebulæ of rounded, globular form, doubtless because the central nebulosity was the only one revealed by his telescope." the formation of the sub-centers in this nebula (which is between the great bear and boötes) should be particularly noted in connection with the coalescence of planets as above described. in a note to guillemin's work, professor lockyer says, "the proper motion of nebulæ has not yet been inquired into, because everybody, looking upon them as irresolvable star-clusters, thought them infinitely remote. now, however, that we know they are not clusters of stars, properly so called, it is possible that they may be much nearer to us than we imagine." in connection with the double-sun spiral nebula shown in the preceding illustration, guillemin says, "we have noticed nebulæ accompanied by systems of double or multiple stars, placed in a manner so symmetrical in the midst of the nebulosity that it is impossible to doubt the existence of a real connection between the stars and the nebulæ." and flammarion says of these apparently globular nebulæ, when under the observation of more powerful telescopes, "in the place where pale and whitish clouds gave out a calm and uniform light, the giant eye of the telescope has discerned alternately dark and luminous regions,"--that is to say, they reveal the operation of the opposite forces of attraction and repulsion, and are spiral. while gaseous nebulæ may be of any conceivable form, the direction and operation of the forces which will determine their character as solar systems must be similar, just as with the forms of organic life, and the only nebulæ which reveal a distinct systematic development in harmony with a working solar system are the spiral. there is no difficulty whatever in tracing such a nebula through all its formative stages, as we have done, and we can, in fact, see painted on the background of the sky every step of the shifting tableau through which such forms must pass. by the nebular hypothesis the whole course of development, of necessity, is rigidly forward to its culmination; but by employing the analogies presented to us in other operations of nature, we can readily account for variations, haltings, ineffectual efforts, uncompleted processes, and even reversals and redistributions into other secondary sources of energy. they equally comprise the agencies for the production of a single solar system or of a myriad, just as we see the vortical water-spouts or sand-storms either single, double, or multiple; they are flexible, as are all the processes of nature, and require no violent assumption of a prior physical basis known to us "ne'er before on sea or shore." they also account for the deviation from the normal of the orbits of neptune and mercury, for the formation of the asteroids and saturn's rings, for the different eccentricities and inclinations of the orbits, for the forward axial rotation of the planets and their satellites, and even for their perturbations and abnormalities; they furnish a basis for bode's empirical law, for the distribution of the planets in size, for the origin of comets and meteor streams, for kepler's laws, for the equal and permanent relation of eccentricities and inclinations, and for the fixed axial position of the moon with reference to the earth; they account for the free oxygen in the planetary and free hydrogen in the solar atmosphere, they employ the variation of volume of the sun as a regulator instead of an independent generator of light and heat, and they are in entire conformity with the established principles which govern the electrical generation of active forces, their transmission to the sun, their transformation into light and heat, and their return to the regions of space, where they continue to act with potential energy to all eternity, as they must do if space itself is eternal; and we surely know that, if anything whatever is eternal, space must be so. this great ocean--the home, the domain, the workshop of creative energy--is the last retreat of the human intellect; here it may find rest, and here alone. while solar systems may afford in their circling planets a possible dominion for finite life, and in their suns their daily bread; in the infinite and all-embracing realms of space, filled with the potentialities of all created forms, thrilled with the impulses of all creative force, is to be found the unfailing source of all, the dominion of the eternal architect, before whom nature bends the obedient knee, waits to hear his mighty voice, or swiftly runs to do his royal bidding. chapter xiv. the mosaic cosmogony. "one generation passeth away, and another generation cometh: but the earth abideth for ever."--bible. thus, as we have seen, through countless future ages will the sun, with his incandescent envelope of hydrogen, and the planets, with their life-sustaining atmospheres of oxygen, fulfil their appointed times and courses. but if we could conceive that all atmospheres, solar and planetary, were suddenly blotted out and forever annihilated, so that these great orbs thenceforth rolled along as they do now, but only as black globes in an ocean of space of stygian darkness, new atmospheres would at once begin to be formed, and these would soon again surround the sun and planets, precisely like those which now exist. sweeping along in darkness, the force of gravity would gather around each of these bodies vast accumulations of aqueous vapor and other gases condensed from the attenuated matter of surrounding space. the planets, by their axial rotations, would again generate from these regions, newly occupied as the system drifted along through space, electrical energy of enormous quantity and potential. earth would again hear the mighty mandate, "let there be light," and from her poles to her equator the skies would blaze with brush-light auroras. suddenly, with a mighty leap, the pent-up currents would flash across to their opposite electric pole, the auroras would gradually die away, and instantly the molecules of hydrogen would begin to sift out at the solar and those of oxygen at the planetary terminals. the electrical currents driving their furious pathway through the rapidly gathering hydrogen envelope, the sun would first begin to faintly flicker with hazy, nebulous light; the light would gather intensity, and soon flash and glow with energy; the solar nucleus within would become intensely heated and liquefied or partially volatilized, and again the solar streams of incandescent heat and light would radiate forth on every side; the commingled gases, oxygen and nitrogen, would once more surround each planetary globe, and we should have a new solar envelope just as we now see it, and new planetary atmospheres like our own; and then, and not till then, would the opposing generative forces permanently counterbalance each other and electrolytic decomposition become practically stationary, except to compensate for the slight variations constantly liable to occur in the complicated running of the mechanism. so the mutilated crustacean re-grows his lost claws, and so our own gaping wounds are healed by the great vis medicatrix naturæ. the most stable of all things is mutually balanced instability; perhaps there is no other form of stability. the "nebular hypothesis" of laplace concerns itself only with the aggregate matter of which our solar system is composed, and the force of gravity, including cohesion, ignoring the action of the equally powerful force of repulsion. but there is another nebular hypothesis much older than that of laplace and far more scientific, for it utilizes both the force of gravity and cohesion and the radiant force of repulsion in the generation of our solar system. we refer to what is known as the mosaic cosmogony. whatever the origin of this magnificent narrative may have been, whether written down by moses originally, or by him derived from the sacred learning of egypt, with which he was fully acquainted, or by the egyptian scribes drawn from ethiopia, and still further back from the sacred traditions of india, it bears internal evidence, when properly rendered from the hebrew record, of a knowledge of these stupendous phenomena (which no human eye could ever have beheld) which is most remarkable. the commonly accepted versions do not clearly bring out the full meaning of the original,--indeed, it would have been impossible for the earlier translators to have done so,--but when critically and etymologically rendered, very surprising coincidences with the succession of events as they must actually have occurred, and the principles involved in the successive stages of creation, will be found in nearly every part of the record. this record is embodied in the first chapter and first three verses of the second chapter of genesis. the hebrew was long believed to be an original, if not an inspired, language, but it is now well known to have been a derivative or root language, made up much like the english, and, like it, having the meanings of its words primarily determined by those of the root-stems from which they have been formed. the roots of these hebrew words are to be found among the languages of many older peoples, and nearly all of them have now been traced to their immediate origin. another source of error is in the so-called masoretic pointing, which was not introduced for a thousand years after the time of moses, and which has often changed the signification of the older words, and even the form of the words themselves; but by critical researches the roots and their combinations have been isolated, so that we are now able to possess a much mere accurate knowledge of the mosaic record than was possible in former times, for, of course, no original copies have come down to us. it is not a reconstruction of the record which has been made, but a careful editing by means of the derivation and true signification of the words used, and by careful comparison among the most ancient versions accessible to modern research. the english version, while imperfect in its rendering of this ancient narrative, is not to be considered by any means a false translation, but it largely errs in failing to give the full radical meaning of the words employed in the original. as an illustration of this indefiniteness of rendering in the ordinary english version let us consider the opening sentences of the narrative: "in the beginning god created the heaven and the earth. and the earth was without form, and void; and darkness was upon the face of the deep." in the "beginning" of what? does it mean the beginning of our own solar system? or of all systems? or of all space? or of jehovah (for he has not yet been mentioned or described)? or of the aleim themselves,--that is, did the work begin as soon as the forces began? and did the latter originate spontaneously, or otherwise? what "god" is meant? is it jehovah, or aleim, or some other god not yet mentioned or described? if we will take every name in the bible which is translated god (and it may be any of these according to the english rendering), we will have legion. we shall even find that the same word which is translated "god" was applied by jehovah on one occasion to moses. "created"? what is meant by this word? was the creating a creation out of nothing? out of something pre-existing? or something coexisting elsewhere? was the creation a direct or an indirect one? by the use of the forces of nature, or by overriding the forces of nature? was it a physical creation by an inconceivable action of mere thought, or will? and if so, was this thought, or will, god himself, or one of his attributes or powers only? "the heaven"? what heaven? was it that to which the virtuous are supposed to go after death? or was it some more physical heaven? was the heaven the atmospheric heaven, the interplanetary heaven, the heaven of interstellar space, or that more extended heaven which lies beyond our knowledge? was the heaven one of these which he created, or did he create all the different heavens of all the solar systems and nebulæ at the same time? "without form"? was the earth without any form at all? or merely without its present form? or without some particular form not mentioned? if the earth was a physical structure it must have had some form; what was it? "and void"? was the earth void like a soap-bubble? or void like a ray of light? or a vacuum? if it was empty, what was it that was empty? how could the heaven and earth be void after they had been brought into existence? "darkness was upon the face of the deep"? what deep? was it the sea not yet created? or the earth, which is anything but a "deep"? was it the atmosphere? or all space? if the latter, did all other systems of space wait for their light on ours? or did we wait on theirs? are there no new systems now forming, and none to be formed hereafter? if all space is meant, where was its outside, or its face? and what occupied the intervening regions? was it a physical face or the face of a vacuum? were these statements to be accepted by faith or reason? if the former, was it a faith which could only have come from the experience of after-ages? or was it based on the ipse dixit of moses? what was the basis of faith when the record was first written? was it from generally accepted tradition or by revelation? is the record anonymous or does it reveal the name of its author? if to be endorsed by knowledge and reason, why should not the narrative be strictly and accurately translated, even at the expense of conciseness and elegance of diction, in order that the exact force of every word shall be fully felt and recognized? if the record is from divine revelation, it is still more essential to know precisely what was revealed; otherwise we are no better than idolaters; we are worse, in fact, for we have changed and falsified the landmarks of religion, and bear false witness against god himself. we must not interpret genesis by records made long subsequently; it must speak for itself or not at all. when construed in accordance with the exact definition of the words themselves quite a new and strange light is thrown upon the history of the events thus recorded. the great importance of a strict construction of the translation and fidelity to the original is emphasized by the fact that the same word was never used in this record to express a different sense in different parts, nor were two different words ever used in different places to express the same meaning. it is, therefore, necessary to give every word of the original its exact fulness and force. the basis of the following critical translation is to be found in "mankind: their origin and destiny" (longmans & co., london, ), but a careful comparison has been made with other accepted authorities, and the root-meanings of the separate words have been carefully traced out, so that many necessary changes will be found to have been made in order to bring out the precise sense of the original. there is no actual literal, critical, etymological, and scientific rendering embraced in a single translation known to us, and which is complete in itself; but that which follows will be found, it is believed, to give every word its particular etymological shade of meaning, and to employ the same word in the same place, for the same purpose, and with the same signification as it was understood to have, in its original form, when first recorded. the specific root-meanings of the most important words used are further explained in detail in a separate section below. the use of aleim, "the powerful forces," in the plural, followed by the verb in the singular, is a hebraism, and indicates the collective character of the forces as specially energized, sent forth, and directed by jeove (jeova or jehovah is the chaldaic form of the word, the original hebrew being jeove), who does not appear by name in this narrative, though, as we shall see, specially delegated power from some higher source is that characteristic which is most emphasized throughout the record. these forces are personified, as is usual in ancient records (and, indeed, in modern thought), but they are in reality the "powers of god." the author of the work above referred to says, "the idea of moses was that there was a supreme god ... and that he only acts by means of his agents called aleim, the gods, in the plural and indefinite number, or embassadors, or voices." the ancient belief in the unity of all forces in one creative individuality is also most clearly shown in some of the oldest vedaic hymns of india (see max müller, "the veda"). "self (atman) is the lord of all things, self is the king of all things. as all the spokes of a wheel are contained in the nave and the circumference, all things are contained in this self; all selves are contained in this self. brahman (force) itself is but self." of the religion of the ancient egyptians (see "evolution and christianity," by j. f. york) it is said, "the chief theological characteristic of this first of all known civilized religions is the doctrine of the divine unity. as m. de rougé says, 'one idea predominates, that of a single and primeval god; everywhere and always it is one substance, self-existent, and an unapproachable god.'" the egyptian cosmogony, as the fragments have come down to us (see professor arnold guyot, "creation"), is as follows: . the original gaseous form, and the darkness of matter. . the successive transformations. . light, as the first step in this development. . the separation of the waters below from the waters above the expanse. . periods of development of indefinite length. . the sun, moon, and earth organized last. the word mlactou, which occurs several times repeated in the summing up of this narrative, explains the character of aleim most fully, as specially energized and directed agencies or forces. this word never has any other meaning. even when applied to a king it was not a king as a monarch, but as the specially directed agent of god. i. samuel xxviii. , "the lord hath sent the kingdom out of thine hand; ... because thou obeydst not the voice of the lord." when, in exodus xiii. it is said that "jeove went before them by day in a pillar of a cloud," this is explained, in chapter xiv. verse , to mean that this pillar of cloud by day and of fire by night was mlac, a messenger, or agent. it is translated "angel" in the english version, but it was not a personal angel; it was a specially energized and directed force. in the earliest times it was not the god of fire, or of force, or of justice which men feared, but fire, or force, or justice; the anthropomorphic conception came later with the generalization of all fire, all force, or all justice. we say now that a malefactor fears the law; what he really fears, however, is punishment. in this record we are dealing with the primordial forces of god,--gravity, electricity, attraction, repulsion, cohesion, vital force, etc., etc., but acting with special energy for a predetermined result. of these forces dr. mccosh says, in his work on christianity and positivism, "one god, with his infinitely varied perfections,--his power, his knowledge, his wisdom, his love, his mercy; we should see that one power blowing in the breeze, smiling in the sunshine, sparkling in the stars, quickening us as we bound along in the felt enjoyment of health, efflorescing in every form and hue of beauty, and showering down daily gifts upon us. the profoundest minds in our day, and in every day, have been fond of regarding this force, not as something independent of god, but as the very power of god acting in all action; so that in him we live, and move, and have our being." in more rugged and virile form this was precisely the old mosaic philosophy, the philosophy of the arcana of the egyptian temples, and of the vedaic age of the aryans of india. where was the radiant center of this unfailing search-light which has poured its broad belt of dazzling brightness down to our day from those old, prehistoric ages? so de jouvencel, in his "genesis according to science," says, "we should not place the works of nature on one side and nature on the other. nature is a work and not a person." the word which in the english version is translated "rested," in the concluding verses of the narrative, does not mean rested from fatigue, but rested as a pendulum rests when it ceases to vibrate. had the word been rendered "came to a state of rest," it would have been far more accurate and true to the sense of the original. what is meant is that these pent-up forces had operated, under the guidance of jeove, to rupture a state of unstable equilibrium in the attenuated matter of space, just as similar forces are now said to gather energy to produce a volcanic eruption of the earth's crust, preceded by earthquakes and other vast disturbances radiating from the center of rupture of these tensions between the molecules of matter, accompanied by explosive expansion and all the phenomena of disorganization and repulsion, and succeeded by condensation, development, harmony, and final quiescence of these specially energized and self-opposing forces in a newly formed state of molecular equilibrium. to quote from professor guyot, "god rests as the creator of the visible universe. the forces of nature are now in that admirable equilibrium which we now behold, and which is necessary to our existence." in "the unity of nature" the duke of argyle says, "we strain our imaginations to conceive the processes of creation, whilst in reality they are around us daily." the words which conclude the third verse of chapter ii. are also imperfectly rendered in our english version, and this defect has led to a popular misconception almost universal. they are construed to mean "created--and made," as though marking a broad class distinction between the different processes before described. from this the inference has been drawn that while, for the more subordinate features, the word rendered "made" indicated that these were stages in the process of creation merely involving the use of coexisting materials, in the grander features of the work it was supposed that there had been a creation ab initio,--that is, out of nothing. whole libraries have been written on this theme; but the words used bear no such meaning; on the contrary, they signify the exact opposite. there is, however, a broad distinction between the interpretation of the two words; but it is that the word which is to be rendered "fashioned like the work of a sculptor" is narrower and not broader in significance than the simple word "made;" so that the former is included in, but is not generically distinct from, the latter. the word bra means that these portions of creation were fashioned with the care and artistic skill of a sculptor, as contradistinguished from turning out the productions in mass; this distinction does not relate to the origin, but to the workmanship. however interstellar or primordial space was formed, or when, if it ever was formed, there is nothing in this record which excludes a pre-existent space substantially like that which now is. what we see in the sky, among the nebulæ, are later developments of like solar systems, in like manner, from the midst of the substance of the same illimitable and eternal space. but biology has an interest in this account of creation equally as great as has cosmology. the word bra is first applied to the formation of the individualized substance of the heavens and the earth. they were fashioned or carved out like a sculpture from something on which the forces could operate. there was, of course, creation involved, but it was a mental, not a physical process. when a sculptor has completed his clay figure he has brought forth a great creation, perhaps, and the "creation" is still his own, though the figure be cast in bronze by hired workmen in the foundry, who execute the sculptor's will at two dollars a day, it may be, each. beyond this mental element there is no more creation, in its widest sense, than when a boy "creates" a new point on his pencil by guiding his hand and knife to sharpen it. when the "diffused light" came, it is not said that it was "fashioned like the work of a sculptor," or that it was even "made;" but that it "came into existence." "let there be light, and there was light," as the english version has it. but when the radiant energy of the sun came to be formed, on the fourth day, it did not "come into existence," nor was it "fashioned like the work of a sculptor;" it was "made." the reason is that it was not a development from the preceding "diffused light," but a new kind of light, made mechanically by the electrolysis of aqueous vapor around the sun's body, forming a hydrogen envelope, and by driving the furious torrents of electricity from the planets through this atmosphere, while the auroral, "diffused light" of the earth was gradually dying away during the process. hence there was no room for the word bra, or for the word iei (came into existence) here; the word to be used was osh. and when life was first introduced,--vegetable life, the primal life,--the word used is not bra; this life was not "fashioned" or developed from other life. but when animal life was afterwards introduced, the word used is bra; it was a refashioning. what was this life fashioned out of? it was not "made;" it did not "begin to exist;" it was developed. in this manner the earth was finally filled with animal life. then came the introduction of the human race. here we again have the word bra, thrice repeated; but when this introduction of mankind was first projected, and before it was executed, it was in these words, "we will make [the root osh] mankind;" or, in the english version, "let us make man." there seems here to have been a gradual ascent of living organisms by development, almost precisely in accordance with the most recent teachings of science. two essentially different kinds of light were successively produced, independently of each other; the earlier kind "came into being," and the later "was made." the substance or entity of the heavens and of the earth, generically, "was fashioned." three successive introductions of organic life not essentially different from each other occurred; the first is described thus: "let the earth bring forth; ... and the earth brought forth," in the english version; or "there shall be made to grow; ... and there was caused to arise suddenly out of the ground ... vegetation," as more accurately rendered. the second form of organic life, in order of time, the animal, was "fashioned." the third form, mankind, was also "fashioned," and this was done long subsequently to the introduction of the second. if the word bra had any signification of original creation it would have been applied to the first creation of life, for it was far more wonderful and original that there should be vegetable life which grew and developed, which brought forth flowers and then fruit, which formed germinative seeds, and from these successively and continuously reproduced its multifarious species, than that animal life should have been introduced long afterwards to repeat these same things which vegetation had been, in all its forms, from the lowest to the highest, already doing for untold ages,--from the third period of the earth's long history to the fifth; and more especially still when we consider that vegetable life and animal life, in their lowest forms, have no positive line of division between them. and if osh, which is applied to the genesis of solar light, be capable of the signification of original creation, then this word should have been applied to the generation of the "diffused light" of the second day, for the genesis of light is far more wonderful and original than the subsequent production of sunlight, after the forming earth had existed for two whole formative periods, from the second to the fourth, under the constant illumination of this universally diffused auroral light. if, on the other hand, the words applied to the first generation of light and the first generation of life be held to mark an original creation, then these words are never applied in this whole narrative to the genesis of the entity of the heavens, or the earth, or the sun and moon, or to animal life, or the life of man. the radiant light and heat of the sun were not made until the fourth day, while the introduction of vegetable life dates from the long antecedent third day of creation. prior to the development of the sun's thermal light there could have been, as we have already shown, no free oxygen in the terrestrial atmosphere; and it is a remarkable circumstance that vegetation, which is the only form of organic life which could have existed and propagated its species in an atmosphere composed of carbonic, nitrogenous, and aqueous vapors, devoid of oxygen, is that particular form of life which has been selected for this purpose, and its advent placed prior to the making of the sun. it would have been far more reasonable (previous to our present knowledge of these things) to have placed the formation of the sun in advance of the introduction of life; it is surprising that this was not done, unless we give to these "ancients" a knowledge of the principles of natural science far beyond anything hitherto attributed to them. in the same connection there is described a stage preparatory to and leading up to the simultaneous development of the sun's light and heat, and the sifting out of hydrogen around the solar core, and of oxygen in the terrestrial atmosphere, which is equally remarkable. the "separation of the waters" described in verses and has never been fully rendered into english, or even understood in the original, as the words seemed meaningless in their literal sense until correctly interpreted by the facts set forth in the present work. we must first note that the separation of the waters of space to two opposite foci, with an intervening space of attenuated matter, and their condensation there into two entirely different bodies, was the work of the second day, while the formation of the terrestrial rain-clouds and seas, as connected together, was a work of the third day, and was not accomplished until then, which was long afterwards. these entirely different operations--different in time, place, character, and circumstance--have always been confounded with each other; but one is in reality systemic and the other merely local. in verse there was decreed an expanse or thinning (an attenuated region) in the center of the waters, and a separation was made by the formation of two "spots" (verse ), one under the expanse and the other above the expanse; the expanse was space, interplanetary space. professor arnold guyot, in his book on creation, says, "it is to be regretted that the english version has translated the hebrew word expanse by the word firmament.... the difficulties they [the commentators] have created for themselves arose ... from depriving it of its cosmogonic character and belittling it by reducing the great phenomena there described to a simple modification of the terrestrial atmosphere.... they forget that this thin covering of clouds is but a temporary and ever-changing one, and that the clouds are in that heaven rather than above it.... they forget that this is not the true heavens in which are spread the sun and moon and stars.... this grand day, so dwarfed and misunderstood, is the one in which are described the generations of the heavens, announced by moses, which otherwise find no place in the narrative of the creative week." the two foci of waters were the solar and terrestrial; around these bodies were gathered by the attraction of gravity, and there condensed, the aqueous vapors from the attenuated intervening matter of space; the earth by its rotation generated the enormous electrical currents which still continue; when these made their mighty leap across to the sun, the diffused auroral light around the earth gradually disappeared, hydrogen and oxygen began to be evolved at the opposite poles--the sun and the earth--from the condensed envelopes of aqueous vapor which surrounded them, the sun's hydrogen atmosphere was pierced, as in the pail-of-water experiment described in an earlier chapter of the present work, by the planetary electric currents, the sun became incandescent, and pari passu the earth became fitted, by the development of oxygen, for the abode of animal life. as taking part in this great mechanical transformation, the sun was said to have been "made;" it did not "come into being." just prior to the introduction of vegetable life--during the same creative epoch, in fact, and for the support of which life it was necessary--the waters under the expanse were condensed into rain-clouds and seas, and there is a curious reference (verse ) to the appearance of the earth's dryness "as produced by the action of an internal fire;" the gradual cooling of the earth by the radiation of its internal heat of condensation into space would account for this appearance, and, in connection with the diffused auroral light throughout the whole sky, would doubtless have sufficed for the support of vegetable life. in verse the fixed stars (the suns of other systems) are referred to, but in a parenthetical statement--almost deprecatory, in fact--that "the dim and almost extinct lights" the same forces created also, but when they were created is not stated in the record. the occasion for this incidental remark is to be found in the preceding statement that the two new luminaries, the sun and moon, were the two "superior bodies in size of the starry lights." having mentioned the stars in this comparison, the author feels called upon to add that the latter also had been similarly created,--that is, that they were not original existences, and of course they are not, but they were not created at that epoch, and are not said to have been. in chapter ii. verse , which opens the second narrative (quite a different history, by the way), jeove appears himself, joined with the aleim, and henceforth this personal connection is maintained; the english version translates this composite word "the lord god," which means the master god; the correct reading is, however, the "god of gods," or what we call the "god of the forces of nature," or the "god omnipotent." in the whole mosaic cosmogony there is nothing which can even suggest a gradually closing nebulous mass; the element of rotation is absent (and it would not have been understood by the people even if presented); but, with this exception, the processes of development are substantially in accord with what must really have taken place, and in the order described. but it is, as before stated, absolutely essential to understand the root-meanings of all the more important words used in the original. a superficial translation is not only meaningless, but misleading; whereas, when accurately understood, the record is one of the most remarkable ever presented to human intelligence. the words used were selected deliberately for their specific shades of meaning, and, unless these are properly rendered, to the uninformed the narrative will present a simple succession of startling phenomena, while to the educated student each of these changes carries within its verbal index its origin, its mode, and the knowledge of the forces at work. to the one it is a dramatic spectacle performed on the stage in front; to the other it is the same work as seen behind the curtain, with all the intermoving mechanism (the author's manuscript the sole guide), the interplay of complicated forces, the triumphant successes, the rapt attention, and even the sudden applause extorted at each wondrous climax from the skilled actors themselves, who are at the same time unceasingly engaged in working out the mighty drama of creation. one might readily believe that the original author of this record was thoroughly acquainted with the processes involved in the development of a solar system like our own from the diffused primordial matter of space, substantially as we have endeavored, in the present work, to deduce them from the most recent investigations and discoveries of science. of the watery vapors condensed above the expanse of space many of the ancient writers had a far more correct knowledge than had those who translated these chapters from the original into the various modern languages. in the psalms we read, "praise him, ... ye waters that be above the heavens;" in the song of the three holy children, "o all ye waters that be above the heavens." theophilus speaks of the "visible sky as having drawn to itself a portion of the waters of chaos at the time of the creation." saint augustine says that the firmament has been formed "between the upper and the lower waters," and quotes genesis i. and , as his authority. thousands of years ago, as far back as the days of the pythagoreans, and even long before, mankind was acquainted with the mariner's compass, telescopic tubes, and glass lenses; they knew that the moon receives her light by reflection from the sun, of the presence of mountains and valleys on the lunar surface, that her day and night are each a fortnight in length, that there were other planets known to the egyptians besides the seven known to the greeks (the brahmans reckoned fifteen of them), that the sun is the center of our planetary system, that the earth and the other planets revolve around it, that the earth is round and rotates on its own axis daily, that weight is a principal element in the maintenance of these rotations, that the fixed stars are suns, and that the milky way appears white from the number of stars which it contains. kircher quotes from an ancient syrian author the philosophy of the sidereal system, dividing it into many layers or spheres attached to orbits, each presided over by a spirit. in the eighth sphere are placed the fixed stars, "still higher two other layers of stars not less luminous, and of different sizes, the nebulæ and the small stars of the milky way, and the whole is surrounded by the celestial waters, which spread over the whole firmament, and which compose the great sea of light and the boundless ocean." the sources of all this wondrous knowledge can be traced back through chaldea, arabia, egypt, ethiopia, and, through the colony of meroë, to india. root-meanings of the principal words used in the mosaic narrative of creation. aleim ("corruptly called elohim by the modern jews, but always aleim in the synagogue copies") means the strong forces (or, by subsequent impersonation, subaltern gods), operating to carry out the purposes and execute the plans of jeove. al, the root, signifies strong, strength, a ram; al-e means strong in a personal sense; aleim (plural) means the forces, the strong-ones, the powers, and in egyptian mythology, the subordinate, or executive, gods, the demi-urgi. exodus vii. , "and the lord [jeove] said unto moses, see i have made thee a god [aleim] to pharaoh; thou shalt speak all that i command thee." bra, carved, cut, fashioned like the work of a sculptor, gave a new shape to, formed from unformed material. from br, a knife; br-i, to carve, to cut. brashit, in the commencement or beginning of individualized existence (with the initial preposition b-). b signifies in; it (which is related to at) signifies individualized existence; rash, a principle or beginning, or a commencement. at, connected with the chaldaic, signifies substance, essence, or individuality, "the thing itself" (latin, ens); it is correctly translated "individualized substance." eshmim, the combination of the preposition e with the substantive shmim, the word signifying of the visible heavens, or the planisphere. artz, the earth in a state of aridity, or as a generalized expression for the earth; ar signifies the earth, and the termination tz intensifies the signification of drought, whiteness, aridity; in contrast with this is adme, red earth, or productive earth or soil. u- is a conjunction, signifying and or then, in the sense of succession of time, something like our phrase "and then." teou does not mean "without form," nor does ubeou mean "and void," as rendered in our english version, at least not in the ordinary sense of these words. "teou refers to extinct life, or to existence shut up as in a tomb and in darkness, while u-beou refers to life which is about reappearing, but still hidden in the egg or the ovary, and waiting for the word which shall cause the dawn of creation to shine upon it." these words are more properly rendered "tomb-like darkness and undeveloped." eshc means darkness; not merely an intense darkness, but what may be denominated a "thick darkness;" it is an enshrouding darkness which compresses and hinders. it is precisely such a darkness as would be produced by the interstratified cloud-layers between the convolutions of a forming spiral nebula, or the cloud-strata surrounding the earth before electrolytic decomposition of the aqueous vapors had ensued. with the advent of the sun, in the narrative, this darkness and the term which expresses it disappear. teou-m is the word above explained, with the termination -m, expressing the idea of arrested, doubtful, indefinite, as applied to all existence; the word "undifferentiated nature" properly interprets its vagueness and general character of an abyss of being, in the etymological sense of "nature" as the totality of things at that time born or produced. rove means breath, in the sense of an expanding, liberating, or developing spirit; its literal meaning is "the breath, the spirit which dilates and frees." mrepht, brooded with incubating love; reph is composed of re, "to be full of good-will, to be agreeable," and eph, "to cover, to protect, to incubate, to brood." mim, the seeds of all beings, the waters. it is said, "the choice of this letter m, to signify water [the alphabetical egyptian letter m is represented by the two undulatory lines which in the hieroglyphics represent water], is connected with the egyptian ideas of the cause of the generation of living beings." numbers xxiv. , "he shall pour the waters out of his buckets, and the seed [zro] in the waters [b-mim]." the latter word is plural in form, but both singular and plural in sense. aour, diffused light; a light resembling the dawn, but quite distinct from the light of the sun. the latter was not established until the fourth day, and its advent is characterized by a new word, leair, "to cause light to move above the earth." joum is day, generically, and lile night. rqiô, the expanse; atrqiô, the individualized substance of the expanse. space, in the opinion of the egyptians, "not being a vacuum, but a material substance, moses could say, and was even compelled to say, 'the substance of space, that which constitutes it.'" osh, made. this word first occurs in verse , and is there applied to the making a separation between the waters or aqueous vapors condensed around the earth and those condensed around some similar spot "above, as regards the individuality of the expanse,"--to wit, the solar core or nucleus,--to which, attracted by gravity from the attenuated vapors of the space between, is due the subsequent establishment of the solar light and heat, as in an electrical arc light, and the presence of oxygen in the terrestrial atmosphere. these processes, involving the constitution of our atmosphere and of the sun's photosphere and chromosphere, were not completed until two subsequent cosmical periods had elapsed, from the third to the fifth. the word osh, in its different combinations and inflections, is also used in verse , where it signifies "making," as applied to fruit; "yielding" fruit, in verse ; "they made," as applied to the sun and moon, in verse ; "made," as applied to the entity of quadrupeds and higher animals generally, in verse ; "we will make," as applied to man, verse ; "had made," as applied to "every entity of creation," verse ; "had made," as applied to the specially directed work as mlactou, chapter ii. verse ; and finally, in the general summing up in verse of the second chapter, as an element in a compound substantive phrase "according to the making-act," or "in accordance with the making of creation." "oshout," it is said, "signifies a manual operation, carried on according to a previously conceived idea, or model." we find a similar use of the substantive infinitive with a preceding preposition in verse , chapter iii. "ctnout is derived from tne, a consoling word. tnout, the infinitive of the conjugation piel, adds to the word the act of causing to be done, and of doing with care." a similar construction, lraout, is employed in chapter ii. verse , translated in the english version, "and brought them unto adam to see what ..."; more literally, "as regards the act of seeing," or according to a vision, or show. that is, they were brought and presented to his sight. the object in writing these two words, bra and l-osh-out, together at the very end of the narrative was to conclusively establish the fact, beyond all possible doubt, that the whole work of creation was an orderly and harmonious progression. mlactou, which word is used twice in verse and once in verse of the second chapter, and not previously, is also introduced for specific emphasis. it means that the whole preceding work of creation was, in its nature, "the work of mlac," a messenger, or a specially energized and directed agency, sent to fulfil the appointed work of jeove. its purpose was to forever prevent the belief that the work of creation was due to mere natural forces, on the one hand, operating by chance; and, on the other, that these forces were independent gods carrying out their own purposes, and of their own will. it was set up as a double barrier against rationalism on the one side and polytheism on the other. it may be incidentally added that the popular belief that "adam was created out of the dust of the earth" is not in accordance with the original record. in the second narrative, chapter ii. verse , the word ophr is rendered "dust" in our english version, but it does not signify ordinary terrestrial dust at all; "its radical meaning is to volatilize a substance, to sublimate it." the true signification of the word used is analogous to a "material essence." the same word is used in numbers xxiii. as a synonym for "seed;" it is said that "the septuagint version translates ophr by sperma." the formation, described in the third chapter, of the female human being out of one of the ribs of adam, excised for that purpose (which is a matter of almost universal popular belief), is not, in reality, what is stated in the original. in verse of chapter ii. the words are rendered in our version, "and he took one of his ribs." what is really said, however, is "and he brought out another one from his sides." so the similar expression in verse in reality signifies, "caused to be made according to womankind the individualized substance of his side." the word translated "of his ribs" is precisely the same as is subsequently used by the same writer (exodus xxxvii. ) to designate the location of the supporting rings upon an altar of incense, and is there rendered, "by the two corners of it, upon the two sides." the defective translation is due to imperfect knowledge, at that time, of the processes of organic development. the true signification is that given in the "institutes of manu": "having divided his own sub-sistence, the mighty power became half male and half female." the words rendered "help meet" in verses and have a far higher meaning; "i will make him a help meet" should be translated, "i will cause to be made for him an overseeing help as a guide, an instructor, a revealer." and in verse of chapter iii., "and adam called his wife's name eve," the latter word is not translated; the correct rendering is, "and adam called the symbolic name of his wife the female serpent-wise revealer, she who explains, points out things, who instructs," for that is what the true root-meaning of eve signifies. the concluding words of this verse, "because she was the mother of all living," are obviously mistranslated, for not only was she not a mother at all, but she did not even conceive, as stated in the next chapter, until she had left the garden finally. the true signification is, "because she was the mother of all [spiritual, see verse , as contradistinguished from animal and vegetable] life." the female human being, the word translated woman, has the generic root-signification of "flame," while, prior to eve, that of the adamic man is the "red earth." as the male was formed from a material earthly essence, the female was created one remove further from the gross and material in the direction of the spiritual; and her powers were distinctively subjective, those of intuition, while those of the male were objective, those derived from instruction. even in the final curse (so called) the man turns back to the earth to earn his subsistence, while the woman turns forward to the instruction of the future men and women, the children; for the words, "in sorrow shalt thou bring forth children," have left one word of the original untranslated, and by supplying this the sense is entirely changed, "and conceiving, and bringing forth, in sorrow shalt thou bring up, care for, and train children." in those countries childbirth was never attended with much pain or sorrow. the obvious effect of the whole inspired or traditionary second narrative is to clearly differentiate the contrasted faculties of the two sexes, and the root-meanings of the words employed, whether moses himself perceived it or not, are a testimonial of the highest possible character for woman, instead of being, as rendered in the ordinary versions, a mark of inferiority, or even of degradation. in the garden scene, when she partook of the fruit of the tree of knowledge, she did not do it hastily or from mere temptation; it is said that "she considered it attentively;" the same word being used as was employed in the first narrative to mark the intense interest and almost superhuman character of the consideration by the aleim of the work, as its successive stages appeared, which they were delegated to perform, and which jeove himself directed. the prize, to her, far outweighed the penalty, and the aspiring sibyl dared to lift the innermost veil in the adytum of the temple, and grasp the lofty truths which made her as one of the aleim. so fell prometheus. and then, no sooner had the flame-crowned seer won her precious prize, than, woman-like, she turned and laid it before her husband, and he, the innocent one, "did eat." the serpent was not a mere snake, be it understood; it was the egyptian typhon, the dark spirit of doubt, the questioner, the tempter, the eternal if, the why, whence, what, and whither? it was her insatiable aspiration to reach the highest possible limits of human knowledge which gave strength to her daring, and not a childish fancy for an apple. all this, of course, is lost in the translation. it is as though the national standard of a mighty people had been disinterred from the remains of past ages, which had been borne aloft at the head of mighty armies for centuries, and for which thousands had gloriously died in battle in defence of a sacred cause, and which now, its past history untraced, has been catalogued as a brass bird of some sort mounted on a stick. it is to be regretted that there is no plain, popular work by a thoroughly capable scholar, without theological or anti-theological bias, which treats of the origin, form, root-derivation, usage, accurate signification, and construction of the comparatively few words employed in the ancient narratives which compose the first half-dozen chapters of genesis, and, we may add, the book of job; something like those inestimable works which deal with the ancient cosmogonic literature of egypt, babylonia, persia, india, china, phoenicia, and central america. nothing of this sort is to be found, at all events in a form accessible to the general reader, and such a work, in small compass, would be of the highest importance to popular instructors, to students, and to the public as well, for it would throw a flood of light on these extremely valuable but, hitherto, so illy-comprehended records. the mosaic narrative of creation. . aleim, the forces, fashioned like the work of a sculptor, in the commencement of individualized existence, the individualized substance of the heavens and the individualized substance of the earth. . and the earth was in tomb-like darkness and undeveloped, and there was compressive hindering darkness on the surface of undifferentiated nature. and the dilating and liberating spirit of the forces hovered with incubating love on the surface of the seeds of all beings, the waters. . then aleim said, there shall be a diffused light; and a diffused light was. . and aleim regarded with attention the individualized substance of the diffused light, because good. and aleim caused a separation to be made between the diffused light and between the compressive hindering darkness. . then aleim exclaimed for the diffused light, day! and for the compressive hindering darkness exclaimed, night! and there was a transition from light to darkness, and then there was a renewal of light; first day. . then aleim said, there shall be an expansion obtained by a thinning in the center of the waters, and there was that which caused a separation to be made by occupying a spot, the waters according to the waters. . and aleim made the individualized substance of the expanse, and caused a separation to exist by the occupation of the spot, of the waters which are under as regards the expanse of space, and by the occupation of the spot, of the waters which are above as regards the expanse of space; and it was so. . then aleim exclaimed for the expanse of space, the heavens! and there was a transition from light to darkness, and then there was a renewal of light; second day. . and aleim said, the waters which are underneath the heavens will tend directly, in order to meet in it, towards a single spot fixed upon for their meeting; and of dryness produced by the action of an internal fire the appearance shall be made; and it was so. . then aleim exclaimed for the dryness, earth! and for the spot fixed upon for the meeting of the waters exclaimed, seas! then aleim looked attentively at it, because good. . and aleim said, there shall be made to grow from the earth a dwarf vegetation which can be trodden under foot, a maturing plant causing to be sowed around it a seed, the strong and woody substance of fruit making fruit after his kind whose seed is in itself above the earth; and it was so. . and there was caused to arise suddenly and full of strength a dwarf vegetation, a maturing plant sowing around it seed after his kind; and the woody substance yielding fruit whose seed is in itself after his kind. then aleim considered it, because good. . and there was a transition from light to darkness, and then there was a renewal of light; third day. . then aleim said, there shall be starry-lights in the expanse of space of the heavens to separate between the duration of the day and between the duration of the night; and they shall be for signs, and for seasons, and for the days which make the year, and for the repetitions of years. . and they shall be for luminous bodies in the expanse of space of the heavens to cause light to move above the earth; and it was so. . and aleim made a double individualized substance, the superior in size and excellence of the starry-lights, the individualized substance which was the greater of the luminous bodies to represent the rule of the day, and the lesser luminous body to represent the rule of the night. of the dim and almost extinct lights [the stars] they made the individualized substance also. . and aleim established these individualized substances in the expanse of space of the heavens to make light move above the earth. . and to be representatives of dominion during the day and during the night, and to separate between the continuance of diffused light and between the continuance of compressive hindering darkness; then aleim looked attentively at it, because good. . and there was a transition from light to darkness, and then there was a renewal of light; fourth day. . then aleim said, the waters shall bring forth a swarm of swarming creatures having living breath; and that which flies, the birds, shall be made to fly with strength and fleetness above the earth in the space extended of the heavens. . and aleim fashioned like the work of a sculptor the individualized substance of those which are superior in size of the gigantic reptiles and every individualized substance having living breath, that moveth, which they had produced, swarming from the waters, according to their kind; and every individualized substance of flying thing with wings, after his kind. then aleim looked attentively at it, because good. . and aleim blessed these individualities by saying, propagate your species and multiply yourselves, and fill the individualized substance of the waters in the seas; and as for the flying thing, it shall multiply itself on the earth. . and there was a transition from light to darkness, and then there was a renewal of light; fifth day. . then aleim said, from the earth shall be brought forth the living breath according to its kind, the quadruped, and the being which moveth about, and the terrestrial animal according to its kind; and it was so. . and aleim made the individualized substance of the animal of the earth according to his kind, and the individualized substance of the quadruped according to his kind, and every individualized substance that moveth about of red earth according to his kind. then aleim regarded it, because good. . then aleim said, we will make mankind of a like order of intellect with ourselves, and they shall extend their dominion over the fish of the sea, and over the bird of the heavens, and over the quadruped, and over all of the earth, and over all the moving beings that move about over the earth. . and aleim fashioned like the work of a sculptor the individualized substance of mankind in the exactness of a shadow cast upon a wall; on this shadow aleim carved the individuality; male and female they fashioned the individualized substance. . then aleim blessed the individualized substance. and aleim said unto them, be fruitful and multiply and replenish the individualized substance of the earth, and subdue it, and extend your dominion over the fish of the sea, and over the birds of the heavens, and over all life of the being which moveth about over the earth. . and aleim said, behold i have given for you every useful plant-substance yielding seed, yielding seed which there is over the surface of all the earth, and every individualized substance of tree which has in it fruit pertaining to a tree yielding seed, yielding seed for you, it shall be for food. . and for all animal life of the earth, and for everything that flies in the heavens, and for every being that moveth over the surface of the earth which has in it living breath, every individualized substance which is a green maturing plant shall be for food. and it was so. . then aleim looked at every individualized substance which they had made, and behold it was as good as possible. and there was a transition from light to darkness, and then there was a renewal of light; sixth day. (chapter ii.) . then the finishing was made of the heavens, and of the earth, and of all the orderly arrangement. . and aleim [the forces] finished on the seventh day the divinely appointed and directed work which they had performed; and they came again to a state of rest on the seventh day from all the appointed work which they had done. . then aleim blessed the individualized substance of the seventh day and sanctified it, because in it they returned to their primitive condition from all the divinely appointed and directed work which the forces had fashioned like the work of a sculptor, in accordance with the making of creation. chapter xv. conclusion. the harmony of nature's laws and operations. we have passed before us the different orders of celestial phenomena; we have called down the denizens of the starry skies and placed them on the witness stand, and we have interrogated them in the light of the evidence which they have given before; we have compared their different statements, and have found that in their testimony they all finally agree. instead of confusion, we find order; instead of complexity, simplicity; instead of discord, harmony; and through all we see the orderly progress of nature with uniform step, from stage to stage, higher and higher, until at last she stands triumphant, the handmaid of creative power, in the very center of the arch of the universe. we have taken the simplest operations which we find in progress around us, and have extended them to larger operations, constantly keeping in view their relevancy and the facts which form their sole support. mere speculation has been excluded, and theory has found its every step based on an established fact. in this way we may hope to make place for further investigation in this field by abler minds, and that the conclusions of science may then become so well understood and so firmly established that to go back to the "dead-and-dying" theories of solar energies will be like going back to ptolemy and tycho for our astronomy. we have considered the hypothesis which bases the energy of our sun upon his inherent heat, upon combustion, upon the accretion of meteoric streams, and upon his slow and gradual condensation of volume; and have found that all these hypotheses, singly or combined, fail to account for his energy through the vistas of the past, during which we know he must have shone as he now shines, and fail to account for more than a slow but inevitable decline, in the relatively near future, into eternal darkness and death. we have found that all these theories are alike, in that they recognize the sun itself as the only source of his energy, that his enormous emission of light and heat is almost entirely wasted in empty space, and that this will go on with the same frightful waste until he has squandered his whole patrimony and ends his melancholy career in the poor-house or the dungeon. we have, however, seen that even this will not save the wretched client, for he has already spent far more than he ever could have received originally by inheritance, and far more than he could have gained by gifts pitched in in bulk--like the poor colored brother's potatoes--through the window. we have therefore gone over the case anew, and have learned that enormous electrical currents are constantly passing between the earth and the sun, with practically no resistance, and this irrespective of any hypothesis, actual or possible; and these facts have solved at the outset one of the greatest conceivable difficulties,--to wit, that of the transmission through space of such essential currents. turning our attention to the more recent advances in electricity and the arts of electrical construction, we have found that induction machines, as contradistinguished from the older friction machines, operate in a manner strongly suggestive of the rotation of a planet through space, and we learn that the electrical potential of the air overhead increases constantly by an enormous multiplying number as we ascend, proving great electrical action in the regions immediately surrounding the earth, and which we have called the terrestrial electrosphere. we have also found that sun-spots and solar storms and other disturbances are at once reflected in our earth-currents, and are followed immediately by great electrical disturbances here and by extensive auroral displays at night. experiment shows that similar auroral displays may be produced with an electrical machine by interruption of the current leading to its principal condenser, thus demonstrating that the currents are from the earth to the sun, and not the converse. we have also found that while the solar atmosphere is largely composed of hydrogen gas, that of the earth and other planets is largely composed of oxygen, and that these gases, the constituents of water, are separately disengaged at the opposite electrical poles by the electrolytic action of a powerful current of electricity applied to the decomposition of aqueous vapors, in accordance with the established electrical law that any fluid which will transmit a current may be decomposed by it; hence we learn that our interplanetary space contains attenuated aqueous vapors, which we have also learned to be true from other sources. as our other planets, as well as the earth, are found to be surrounded with an atmosphere of dilute oxygen, and with aqueous vapors suspended in it, we know that their action upon the sun must be similar to that of the earth, and that the congeries of planets thus unite in their supply of electricity to the sun in constant and enormous currents. examining now the effects of passing powerful electrical currents through a compressed envelope of hydrogen gas surrounding a conductor, we find that great heat ensues, that the hydrogen becomes highly incandescent, and that the metallic nucleus within is raised to an extremely high temperature, and we also observe the same effects when the current is transmitted through the separated carbons of an electrical arc light. we have thus accounted for the constant supply of the energy which, transformed into light and heat, as in the last-mentioned experiments, the sun pours forth perpetually into space. we have also learned that electrical induction machines derive their electrical currents from the surrounding air, and also that no electricity can be generated in, or transmitted through, a vacuum, and hence we learn that the planets, by the rotation of their electrospheres in contact with the attenuated vapors of space, generate these powerful electrical currents with which the sun is supplied, and that the sun merely restores to the ocean from which, in another form, it was abstracted the light and heat which he emits, and that, instead of all being wasted except that which falls upon the planets, in fact that is the only part which actually, in one sense at least, is wasted: all the rest is deposited in bank, but that is "spent." the important generalization is thus arrived at, that the true source of solar energy is to be found in the attenuated vapors of space, and that the mode is that of the generation of electricity by the rotating planetary electrospheres, its transference through the aqueous vapors of interplanetary space to the sun, its passage under resistance through the compressed hydrogen envelope, its transformation there into light and heat, and its final emission or backpouring into space again. the molecular motions which give rise to light and heat in their passage through the vast distances of space are finally retarded by and disappear as radiated energy in the restoration or increase of the intermolecular tension of the vapors of space, and these processes continue, and must continue, to all eternity, if the sun exists and his planets continue to revolve in orderly circuit around him. if there be any permanent degradation of energy, it must be with reference to the total volume of infinite, or at least indefinite, space, and not with reference to the relatively minute spark of fire which we call the sun. we have also learned that the moon's electrosphere is repelled by that of its neighbor, the earth, and that whatever vapor and atmosphere it may have can exist only on its opposite side; and we have also learned that, by reason of the moon's peculiar axial rotation with reference to the earth, any other arrangement of the lunar moisture and air, even if such were possible, would have absolutely prohibited all life on that subordinate planet at any stage of its existence whatever. we have applied the above principles to the fixed stars, and have learned that, by the same law, the resplendent star itself is proof conclusive that it, too, must have planets rotating around it, and that these planets must have an oxygen atmosphere and clouds of aqueous vapor like our own. we have interpreted the double and multiple stars, and, by an extension of the same law, explained their frequently contrasted or complementary colors. the new stars which blaze up in sudden conflagration and then die out have no secrets when this new light is turned upon them; they, too, are but the faithful followers of the law; and the temporary and variable stars likewise fall into their appropriate categories and obediently move on with the procession. the comets,--the banner-bearers of the sidereal hosts,--which from the earliest ages have defied science to read their cabalistic legend, find it now "writ large" and in plain english. even the meteorites, the cosmical dust, the unorganized débris of space, are found to be amenable to the same law. when we turn in wider gaze to spy out the fantastic nebulæ on the very outer fringe of visible things, after we have separated out the star-clusters and organized galaxies of suns, we apply our touchstone to the irresolvable gaseous nebulæ, and lo! their mystery dissolves at a touch. we have even been able to picture the processes of the creation of solar systems and whole galaxies of suns in which the same law finds scope, and by its infinite and harmonious extension we learn that nature moves with a comprehensive plan, and is uniform in her infinite variety and eternal in her ceaseless activity. we have been told that-- "the poem of the universe no rhythm has nor rhyme; some god recites the wondrous song, a stanza at a time." but it is all a mistake; the loftiest strains which ever inspired the soul of mozart or of beethoven had not the ineffable harmony, nor the sweetest songs of the greatest poets the perfect rhyme, ever repeated and ever varied, of the universe. its orderly progress is like the onward movement of a mighty army, and there is but one grand commander, "but one god," and nature, that showeth forth his handiwork, "is his prophet." we have found that the "course of nature," the eternally youthful mother, is the same, whether in spinning a tendril in the garden, in weaving a whirlwind in the atmosphere, or in elaborating from the universal vapors of primordial space a solar system or a galaxy. and it is not a convulsive, spasmodic nature that we find; we do not love to associate great explosions, cataclysms, the destruction of worlds, or the extinction of suns with our ideas of nature. these seem not to be of nature. the nature we love is the gentle mother, uniform in her operations, kindly in her ways, beneficent in her results; the nature of the rain, the sunshine, seed-time and harvest and the sprouting seed again; ever patient, ever responsive, but in all as firm and steadfast as the foundations of eternity itself. so we have found her. we have assumed nothing; we have observed and endeavored to deduce from observation her systematic plan, for this is the voice of her law, "the same yesterday, to-day, and forever." to quote the words of matthew arnold, from out the darkness of the past we seem to hear her say,-- "will ye claim for your great ones the gift to have rendered the gleam of my skies? race after race, man after man, have thought that my secret was theirs, --they are dust, they are changed, they are gone! i remain." canadian eclipse party . the proceedings of the canadian eclipse party . by commander ashe. director observatory, quebec. quebec: printed by middleton & dawson, at the "gazette" general printing establishment. . the canadian eclipse party . __________ before giving an account of my proceedings in reference to the eclipse, i think it only right, in justice to our party, to state that the arrangements were made very hastily, as it was not until the last moment that would admit of my reaching the station allotted to me by the american astronomers, viz., jefferson city, that i was informed that $ had been appropriated for the purpose of taking my telescope to iowa. the party consisted of mr. douglas, mr. falconer, and myself. as we had only three days to get ready, there was much to be done, dismounting the telescope and making cases for the several parts, and carefully packing photographic materials. instead of the stone support for telescope (eight inches aperture and feet focus) i had one made of wood, but as the centre of gravity was raised so high by using wood, i had to take great care in the formation of the base; however, the stability was excellent. our arrangements were all complete by the th of july, and we started that evening by the montreal boat. for the benefit of those who may undertake an expedition of a similar kind, it may be well to mention a few incidents that occurred during our journey, which, although trifling in themselves, may prove useful to future eclipse parties. i may mention that two of the cases, containing parts of the telescope, were directed "eclipse expidition," with three i's in expedition. this was pointed out to me at montreal, but the mistake is excusable, for evidently the more eyes we have in an astronomical expedition the better. with regard to original spelling, i will relate the following anecdote, which would have suited "artemus ward." the boatswain of a man-of-war has to keep a rough expense book of the different stores that he uses, and this is checked by the master, who on one occasion sent for mr. parks, and when lie came, he said: "oh, mr. parks, you have expended too much rope for those 'jib guys;' it will surely be found fault with; you had better reduce the quantity;" and on handing him the book, he said: "by the bye, b-l-o-x is not the way to spell blocks." the boatswain took the book very sulkily; and after he had taken two steps towards the door, he turned round, and said "well, sir, if b-l-o-x don't spell blocks, what do it spell?" we started on our journey by the evening train. when we arrived at port huron our first difficulty occurred; the custom-house officers would not pass our baggage, although we pointed out the great importance of our party, and also, that the moon would not wait an instant for us. they did not see it; so our baggage was locked up for the night. we took rooms at a small inn, and then mr. douglas and i went by rail to huron, to see the head of the customs. after going up two flights of stairs, we were shewn into a room which two gentlemen occupied. the chief was smoking, with the chair resting on its two hind legs and his resting on the table. we told our story, and shewed him a certificate from the american consul at quebec. he looked very hard at me, took the cigar out of his mouth, wrote a pass which he handed to me, and then resumed his cigar and former position. we began to thank him, but as he hid himself in smoke, we retreated down stairs. i never was more struck with the kindness of our american cousins than i was during this trip. on all occasions, they did all in their power to promote our convenience. in the morning we had time to see mr. muir, the director of the railway, who kindly gave us a free passage over his line, a kindness that was shewn to us by all the directors of the different lines that we travelled on. i may remark that the cases with the heavier parts of the telescope were broken, and i much feared that the instruments would be seriously damaged. mr. muir very kindly had outside cases put on, and i carried the most valuable part (the object glass) in my hand. after we left chicago, and before going to bed, we left word to be called before crossing the mississippi. it is not fair to judge of scenery from a view taken through the window of a railway car, but i must say that i was disappointed,--shallow, sluggish, and muddy; but then i ought to remember that i live on the banks of one of the finest and most beautiful rivers in the world. in the morning we were on the prairie, which is not so flat as i had expected to see it, but it is a beautiful undulating country, and if there were trees upon it nothing more could be desired. it was explained to me by a gentleman who was travelling with us, the reason why trees do not grow on this beautiful land. it appears that on the eastern bank of all rivers and streams only do trees grow; now without entering into the cause of the prairies catching fire, i will only say that in september, when the long grass is quite dry, they do catch fire, and then burn until it is stopped by a river, and as it always burns to windward, and as the wind generally blows in one direction, we have a solution why the trees only grow on one side of a river; and once the primeval forest is removed, it never has a chance of growing again, as the young trees are sure to be burnt, and the beautiful black soil of the prairie is enriched by the deposit of burnt grass. at one station where we stopped to water our engine, i saw two children of the soil; they have good reason to complain at their lot. the buffalo and antelope driven away, and if they are hungry they are told to go and dig; dig, how can they dig? let us reverse the picture. suppose that our cities and towns were by the indians turned into a prairie, and when we were hungry they told us to go away and catch a buffalo, a pretty hand i should make of catching a buffalo. the sooner the poor fellows are shot down or killed by small-pox, the sooner they will go to their happy hunting grounds. as the norway rat kills all other rats that it meets, so the savage must disappear, and the northern races of europe will exterminate them. there is one exception, the african negro, and no matter what you do to him he thrives under the treatment; whether free or in slavery he multiplies and is happy. strange that rum which kills the indian, only makes him fat. but the king of savages--the new zealander--has the fairest island, in the most favored clime, taken from him, and civilization forced upon him. there is no getting away from this civilization now. but i am thankful to say that i was at san francisco before it arrived there. when out shooting i saw the fresh foot-prints of a grizzly bear, and did not know how far the gentleman might have been from me at that moment. now, i should like to know how far you would have to travel, and how much you would have to spend, before you could experience the same delightful sensation. i have seen real indians with real bows and arrows, in vancouver's island; and the place where i then saw them, now has become the head-quarters of the pacific squadron; and the indians, instead of flattening their heads, no doubt have put on the grecian bend. where is all this to stop? it was pointed out to me that most of the telegraph-posts were struck by lightning; no wonder; for that king of natural forces, that for so many thousands of years has reigned supreme-splitting the granite rock, and shivering the mighty oak at his will--now to be brought into existence at the will of an apothecary boy, placed in two cups and locked up in a cupboard, and then made travel day and night, over hill and dale, and under the vast ocean, to carry messages at the bidding of man,--no wonder, i say, that he should try and knock the whole concern into a cocked hat! "boonsboro! twenty minutes for dinner!!" now, then, we shall have something in keeping with the prairie,--i suppose a deer roasted on a stake. nothing of the sort. i went into a nice dining-room; saw a quantity of pretty girls, or rather young ladies, with short sleeves and low dresses. "soup, sir! chicken, sir! peas, sir!" the station at rugby is nothing to it. after twenty minutes of capital feeding, we heard, "all aboard! all aboard!" and as we left, the father of these young ladies was standing at the door, and obliged us by taking half-a-dollar, a great improvement on the english system, where, on asking the waiter for your bill, he asks: "what 'ave you 'ad?" and begins to add accordingly. the next station was jefferson, , miles from quebec. here the boxes were again thrown out, and the train left for san francisco. the boxes were left at the station, and we drove up to the hotel, about half-a-mile from the station. as this was saturday, july st, we had exactly a week to select a site and to build an observatory-mount the telescope and take preliminary observations. the american parties were several weeks at their station before the day of the eclipse, and found it not too long to prepare. jefferson city is three years old, has about eight thousand inhabitants, and looks a thriving place. the next day, after church, mr. douglas and i rode across the prairie to a station situated about eight miles on the railway from jefferson. as it was nearer to the central line of eclipse, we wanted to see if it would do for the site of our observatory. i forgot to mention that the day before i left quebec, in pulling off my boot i broke the tendon of the plantaris muscle, which made me quite lame. however, the six days' comparative rest made it much better, but still it was far from well. "we started for our ride across the prairie about two o'clock, and reached the station in about an hour and a-half. we crossed several streams and some marshy ground, and started several prairie chickens. after examining the place, and finding that it would be very inconvenient to get the material there, we thought that it would be better to remain at jefferson, and we mounted to return. after we had left some time, and as i was suffering from my leg, and could not ride fast, i persuaded mr. douglas to ride on, and get back before sunset to keep an appointment with a carpenter, and not to mind me, as i could ride slowly back. he very reluctantly did so, and when i was left alone, i felt quite at home, steering my horse across the boundless prairie by the setting sun. now, my horse had crossed many streams, and soft wet places in going out, so i took it for granted that he knew more about the prairie than i did, and would not allow me to get into difficulties, and consequently steered a straight course for that point of the compass in the direction of jefferson. the sun had just touched the horizon. i was crossing some marshy ground with reeds up to my shoulders, when i saw my horse's nostrils distended, and his ears forward. i immediately put my helm down and brought him round, and just as i had done so, down he sank; i found myself up to my ankles in mud, and up to the calf of the leg in water; the horse was fixed immovable, no struggling, but snorting and dreadfully frightened. i have been in various situations of difficulty; but when i looked up and saw the tall reeds far above my head, and the sun setting, i must confess that i thought my case a serious one. i remembered the fate of a young french officer of the combined fleet that was at anchor at the entrance to the "dardanelles," who went on shore to shoot, and as he did not return that night, we landed in the morning to look for him, and not far from the ship, we found him in a bog up to his waist, his gun a few feet in front of him, and he quite dead. i knew that if a man once gets up to his waist, it would be impossible to extricate himself; however, when i dismounted i sank up to my knees, and although that was not the place to philosophize, still i did so, and i began to think what is the reason that a man in struggling works himself down, and i immediately discovered that on raising the heel i produced a vacuum, as the mud prevents either water or air getting underneath the foot, and so with lbs. to the square inch, in addition to your weight you soon disappear. that being the case, i did not attempt to raise the foot, but moved it backwards and forwards in a horizontal position until i made the hole so big, that water got under the foot, when i could lift it up with the greatest ease. after extricating myself i tore down some reeds and made a platform round my horse, then i patted his neck, and spoke good-naturedly to him, and then went astern, and by means of his tail worked him backwards and forwards with a rolling kind of motion to let the water well round his feet, and lastly went ahead, passed the bridle over his neck, and sat down with it in my hands right ahead. now, then, old boy, "up she rises," the horse began to struggle, i kept the head-rope taut, and he was freeing himself bravely. if i let go the bridle too soon, he would go back; if i held on too long, he would be upon me, and not only kill me but bury me, so at the critical moment i let go, and rolled over and over amongst the reeds, and the horse floundered past me. when i got on my feet no horse was to be seen, but only the tops of the reeds moving as he was making his way out. i thought i had not improved my situation much, for with my leg i could not walk a mile, and, of course, the horse had shaped his course for the stable. however, when i emerged from the reeds, i saw the dear old fellow standing as still as if he were in his stable. but now came another difficulty with my lame leg, i could not put a foot into the stirrup, perhaps he might have been in a circus and taught to lay down, so i began kicking his forelegs and lifting up one and then the other--but no--he had no idea of it: then i thought i would lash his feet together with the bridle and throw him down, but there might be some difficulty in my remaining on his back when he floundered to get up, well, if the worst comes to the worst, i will lash myself to his tail and make him tow me home; but an idea struck me, i lengthened the near stirrup to about a foot and a-half of the ground, and then lengthened the other and brought it over on the same side, and here i had a nice little ladder to walk up which i did, and then knelt on the saddle and dropped into my seat. i could not help shaking hands with myself, and patting my steed on the neck, i then commenced my journey home, which i reached just before dark. [photograph: view of jefferson city, iowa, from observatory.] we had agreed to erect the observatory about half a mile from the station, on a rising part of the prairie; carpenters were engaged, and an arrangement made with a lumber merchant, who would supply what i wanted and take it back when i had done with it, only charging us for the damage done to the stuff. early on monday morning, the instruments were carted out and unpacked; and at sunset the four walls of the observatory were up. now, as we thought it not advisable to leave all these things open on the prairie, it was agreed that some one should sleep there--and, of course, it was my duty to remain. they sent down a mattrass, pillow, and blanket; there was no wood to build a large fire outside, but i collected some chips, and lit a small fire inside, and placed my mattrass alongside. a little after sunset a musquito looked over the wall, and then sounded the assembly; on they came, and i with my head in the smoke kept blowing the fire, putting on wet grass to make a smoke; but, after half an hour at this work, i found out the fact that man was not intended for a pair of bellows, and although i assisted the action by compressing my sides with my hands, still at the end of the half hour that i blew i found that i was blown. when once my head was out of the smoke, the musquitoes flew at me; i stood up to fight them, but in so doing i had to fight myself also. now an army was drawn up in contiguous columns on my cheeks, the skirmishers advancing through my eye-brows; at their first volley i felt as if i was struck with a hackle. i really think that they work their stings like the needle of a sewing machine. maddened, i struck myself a fearful blow with both hands in the face, and had the satisfaction of making them "leave that," and so i fought myself and the musquitoes for some time: still they attacked me with an impetuosity truly marvellous, and where one fell two took his place. i was getting weak; a storming party had now taken possession of my right ear; i clenched my fist, and with a swinging blow, cleared the ear, but knocked myself down. exhausted and worn out, i put my hands into my pockets, and gave them my head. in that half-dreamy state, the long, long hours were passed; and after they had breakfasted, dined and supped, they began to discuss me. "ah," said one, "if you want a good drink, strike between the corner of the eye and the nose." "no, no," said a large party; "if you want a draught of good sparkling astronomer, sink your pump in his temple." "you are wrong," said a dissipated old fellow with frayed wings; "just creep up his cuff, and harpoon his wrist, and there you will drink until you lift yourself off your legs." then they sung the following song. "the blood of the indian is dark and flat, and that of the buffalo hard to come at but the blood of the astronomer is clear and bright: we will dance and we'll drink the live-long night. chorus:-"how jolly we are with flights so airy; happy is the mosquito that dwells on the prairie." and then they quarrelled and fought with each other, and made speeches,--and so the dreary hours dragged along; but when the eastern horizon was tinted with beams of light, they staggered off to their respective marshes-some to die of apoplexy, others of _delirium tremens_. verdict--served them right. from dawn until six, i had a refreshing sleep, and when my relief came, i awoke up, and began to think whether i had heard all this, or only dreamt it. i suppose i dreamt it. the work now made rapid progress: doors with locks, dark room settled, platform for telescope support firmly laid. the next day, began to mount the telescope, but when we came to screw in the object-glass, we found out that the brass seat in the tube had been pressed into an oval. what was to be done? no one in jefferson that knew anything about it; too late to send it anywhere; here was a great break-down. however, a mr. kelly said he would try; and after some hours' hard work, he got the object-glass screwed home, but could not be unscrewed; so the flats that hold the bolts that secure the object-glass to the telescope could not be put on, but we secured it as well as we could. it is important to mention that before arriving at jefferson, we made the acquaintance of a mr. vail, from philadelphia, who was going to des moines to observe the eclipse, and as i had a -inch telescope by dolland, without an observer, i asked him to join our party and observe the eclipse with it, which he kindly consented to do; and his report is of the very greatest consequence, as it confirms, in a most striking manner, the details that are seen in the negatives. by friday night, all preparations were made, and we retired to rest with great doubts about having a fine day. [photograph: clear for action.] however, saturday came at last, and the morning was hazy and overcast; but about eight, the clouds began to break and mr. vail and i took some observations for "time." the afternoon was cloudless; but still a haze near the horizon. at half-past three, we "beat to quarters." mr. douglas shut himself up in the dark room; i took charge of the telescope; mr. stanton, with a light cloth, covered and uncovered the "object glass;" mr. vail had his telescope nicely adjusted; and mr. falconer was seated in a very good position to observe the dark shadow crossing the country, and to note any other phenomena. at h. m. s., local mean time, the first contact took place, and the first photogram taken, shewing a slight indentation on the sun's limb. we took the partial eclipse with an eye-piece, giving a -inch picture but as it was hazy, i removed it before totality, and took the photograms in the principal focus. i may remark that no one could have had a better view of the eclipse than i had. as i stood in rear of the telescope, i had only to count the double beats of the pendulum of the "driving clock," which i did without taking my eyes off the moon. i exposed the plates of totality for ten seconds, then withdrew the holder, and handed it to mr. douglas. we took several photograms of the partial eclipse before totality, four during totality, and two after; but the weather had become so hazy, immediately after the sun made its appearance, that we could hardly get a picture. as all the reports are published, it only remains for the jefferson party to give theirs, and the eclipse of can be fully discussed. there are one or two points that the negatives of our party will throw a light upon. with regard to the bright band on the sun, bordering the moon, in the pictures of the partial eclipse, it is well known that, there is nothing surrounding the moon that could produce that effect; and also, that the photograms taken at burlington, shew, beyond a doubt, that it is no optical illusion. dr. curtis has suggested that it is caused by diffraction; still, i very much doubt if diffraction could produce such a uniform dark broad band, so well defined, as is seen in those photograms. one of the photograms of the partial eclipse that we took before totality, shews the cusps and edge of the moon to be double, giving the appearance of a band surrounding the moon. this is caused by the reflection of the moon from the second or underside of the glass, which happens when the sun is not in the centre of the field; and by holding the negative of a partial eclipse so that the light will fall obliquely on it, you will see a dark band surrounding the moon's limb, from the same cause. "bailey's beads." in the eclipse of , i had the honor of being attached to the american expedition that went to the coast of labrador. professor alexander, dr. f. a. barnard and myself, who were observing with telescopes, all exclaimed at the same time, "bailey's beads!" it is very true, that at otumwa a picture at the last instant, just before totality, was taken, "shewing the sun's edge cut by the peaks of the lunar mountains into irregular spots;" but these were not the bailey beads that i saw in labrador, and i am confident that neither professor alexander nor dr. barnard will accept that solution. in the report of mr. w. s. gilman, junr., who observed the eclipse at sioux city, mr. farrel gives a description and drawing of bailey's beads; and what he saw in , i saw in , the film of light broken into rectangular pieces, which appeared to swim along the edge of the moon like drops of water. a crowd had followed us from the town, and took a position near the observatory, as, no doubt, they thought that we would select the best place for observing the eclipse. on the last glimpse of day-light vanishing, the crowd never fail to give expression to their feelings with a noise that is unlike anything else that i have ever heard. it is not like the noise that a crowd makes on seeing a lovely rocket burst, or that which they make on seeing some acrobat perform a wonderful feat. no; there is an expression of terror in it. it is not a shout; it is a moan. before giving a description of the photograms of the total eclipse, it will be necessary to refute some opinions that have gratuitously been given respecting them. after i had carefully examined the negatives, and made drawings, i had the drawings and the negatives compared by mr. langton, who expressed his opinion that they were faithful copies; and when i found that it would be many months before i could get funds to print my report, it was agreed upon, after consulting some friends, that the negatives of totality should be sent to england. unfortunately, i selected mr. de la rue as the fittest person to examine them. he never acknowledged the receipt of them, and, after many months, mr. falconer, who had returned to england, sent me a copy of a letter to him, from mr. de la rue: "the observatory, cranford, middlesex, "dec. th, . "my dear sir,-i am very sorry to have caused any uneasiness to commander ashe; but one circumstance and another have delayed my writing to him. i have received his papers, which i sent to the astronomical, and later on, the original negatives, which arrived safely, although commander ashe had neglected the precaution of protecting them with a covering of glass. there is evidence in these negatives of the telescope having moved, or, perhaps, followed irregularly, during the exposure of the plates, and this renders the dealing with the negatives very difficult; moreover, it contradicts the theory set forth by commander ashe in respect to a certain terrace-like formation in the prominences, and also the rapid shooting out of a certain prominence. the american photographs are very much more perfect than those sent by commander ashe; in fact, they leave nothing to be desired. to correct the defects of duplication in commander ashe's photographs, would entail some expense, [i understand that mr. de la rue has spent pounds, in patching up major tennant's photograms.] and much trouble; and it would be necessary for hint to re-write his paper. "i have only returned to my house (after an absence of a year) a few months ago, and have had major tennant's paper to see through the press; so that my correspondence has fallen greatly into arrears. wishing you the compliments of the season, i am, with best regards, "yours sincerely, "warren de la rue. "alexander pytts falconer, esq., "bath." here is a very serious charge. i am accused of foisting on the public a marvellous account of the eclipse, which my own negatives contradict; but i shall have no difficulty in shewing conclusively that mr. de la rue has made a blunder, when he says that "there is evidence of the telescope having moved, or, perhaps, followed irregularly." it would have been better had mr. de la rue produced his evidence before he takes upon himself to assert that the negatives contradict my statements. but the crimes i am charged with are, that on the th of august last, some person or persons did, accidentally or maliciously, disturb the telescope, during the exposure of plates nos. iii. and iv., and that the said plates mislead, and are not faithful representations of the phenomena seen and also, that they contradict the statements of commander ashe, with regard to the "rapid shooting out of a certain prominence." in clearing myself of these heavy charges, i shall divide my evidence into two parts-negative and positive. in the first place, the telescope was firmly placed upon a platform made by the heavy sleepers borrowed from the railway station, and surrounded by boards, as may be seen in the photograms; and commander ashe has been too long at sea to travel miles with a heavy telescope, and then not to be able to give it stability. there were four persons inside the building--mr. falconer, seated some distance from the telescope, observing the general appearance of the eclipse with the naked eye; mr. stanton upon a platform, ready to uncover and cover the object-glass with a light cloth; mr. douglas in the dark room, and myself at the telescope, which was firmly clamped in hour-angle, and declination. the people outside were at a distance upon an elevation, and were quite still. the telescope, if it moved, must have moved in hour-angle, or declination, or in both; if it moved in hour-angle, the endless screw must have tripped upon the driving-wheel, which it could not do without making a noise, which would have been heard by me. if it moved in declination, mr. stanton must have moved it in uncovering the object-glass; but in so doing, he must have given the telescope a pretty hard blow, of which he must have been aware. but neither mr. stanton nor myself are aware of any disturbance of the telescope. there was no wind, which would only have caused a vibration, and given a blurred image. in examining nos. i. and ii. photograms, the limb of the moon may be clearly traced, and there is not a shadow of suspicion of any relative motion in the telescope. here we have proof that the driving clock was performing its duty well for the first half of totality; and no one will have the hardihood to say that it altered its rate in the next minute and a-half. in looking at no. iv. photogram, we see that a point of light is double. now, we will suppose this duplication was caused by the telescope receiving a smart blow; then, by drawing a line through the two positions of the same object, we get the direction of the motion. now, look to the right and we see a protuberance with a triplicate form. here, then, the telescope must have received two blows; and by drawing a line along the top of the three figures, we get the direction of the motion, or disturbance; and on looking at the different directions of the two motions, we see that the telescope moved two ways at once, and also, that one part of the plate was disturbed once, whilst another part of the same plate was disturbed twice--which is absurd; and lastly, mr. vail who had not seen the photograms when he wrote his report, gives a description of certain lines and cracks that are to be seen in the negatives when they are examined by a lens. how is it possible to get over this? here, an american gentleman sees with a telescope exactly what is photographed. but this is negative testimony; i will now prove, conclusively, giving geometrical evidence, that mr. de la rue has made an egregious misstatement. the reader will have it in his power to corroborate this testimony. place a piece of paper behind the photograms iii. and iv. (taken in the principal focus), and with a needle make holes in four or five different places, taking care not to mark the bottom of a protuberance, which is a notch, but where you can see distinctly the limb of the moon; then remove the paper and find the centre of three holes, and draw a circle through them; and if it passes over the other holes, you have positive proof that the centre did not move during the exposure. now, look at the lithograph, and you will see a circle drawn through five marks made upon the limb of the moon of no. iii., and through four marks made upon the limb of the moon of no. iv.--_q. e. d._ [photograph: iv.] having proved that the very remarkable photograms taken at jefferson are correct representations of the phenomena seen at that place, i will proceed to describe the details of the four negatives that are to be seen when examined with a lens. the moment the sun disappeared, out flashed the corona, which resembled an aurora, and no doubt belongs to the sun, and not to the moon. no. i. shews the continuous mass of red matter with the flame-like appearance of the so-called "ear of corn;" a little to the left are seen two detached red lumps, like glowing coals; and underneath is seen the slightest trace of a prominence that is to play a conspicuous part in the eclipse. no. ii., the limb of the moon, is seen completely round, and a little more is seen of the prominence underneath. now, it is time to remark that the flame-like mass in no. i., and the detached prominences in nos. i. and ii., appear to cut in upon the limb of the moon. dr. curtis, after trying several experiments, is firmly convinced that this appearance is entirely due to a photographic effect, by excessive overexposure of the plates. i have to remark, that nothing was more conspicuous than the indentations of the glowing masses upon the limb of the moon. remember that these protuberances were not dazzling lights, but could be contemplated with the greatest comfort; and the eye is so fastidious, that in running round the limb of the moon, it immediately detects the sudden break in the circumference. but i have a theory, and it is dangerous to trust the eye of a man with a theory, without good support. directly after the eclipse, some of those outside joined us, and the conversation was upon the extraordinary shooting-out of the prominence, which they were all describing. in the midst of the conversation, a carpenter touched me on the arm, and said: "but what were the notches on the moon?" now, this is conclusive evidence, and would be taken in any court of law. remember, the word "notches," (the language of a carpenter) is his own, and no other word do i think so applicable. i answered that i did not know, and that nothing puzzled me more. on examining the negatives with a lens, i saw the limb of the moon distinctly through the prominence; and further, that the part on the moon was a similar and inverted figure to the upper part, and i was convinced that the "notch" was caused by reflection of the protuberance on the surface of the moon. let b f be the height of the protuberance, and l b the line of sight, tangent to the point b, and let the lines of sight, both direct and reflected, be considered parallel to each other; now, through the point d draw a tangent, and let the incident ray, f d, and the reflected ray, o d, make equal angles with it; then, the exterior angle, o d c, is equal to the angles d ac and a c d; take away the right angles, d and a, and we have the remaining angles, o d e and c, equal; and b a (the depth of the notch) is equal to the versine of the angle of reflection. [illustration: fig , fig , fig ] in measuring the enlarged photogram, b c was . inches, and b f, . inches; and as b c, the moon's semi-diameter on the th august, subtended an angle of '. " = _a_. let c f subtend an angle = _y_. . cot. _a_ ' " then cotan. _y_ = ------------- = . = _y_. . . = _a_. ' " angle subtended by protuberance = . . as mr. douglas had no one to help him in the dark room, there was some delay in getting no. iii. plate; but whilst i was waiting for it, out shot an enormous flame from the bright point before mentioned. it shot out in about three seconds, not unlike a jet of gas from a coal in the grate and when it reached its greatest height (about one-third higher than that seen in photogram), it was blown off to the left, just like a flame acted on by a "blow-pipe," and came to a point. the part blown off was a bright white flame. (see lithograph.) now, as my veracity, after mr. de la rue's letter, is doubtful, and as this phenomenon was not seen any where else besides jefferson, i must substantiate the fact, mr. falconer, in his report to me, gives a drawing which is very similar to fig. no. ; he says: "it assumed the shape of a red-hot crooked bar of iron; this, resting on the dazzling silvery coronal light, gave a strange and wonderous addition to the glorious scene we now beheld." but it was seen by all, and can be attested to by hundreds. when no. iii. plate was ready, it had lost about one-third of its height, and its flame-like appearance. when no. iii, plate is examined with a lens, all the lines that are shewn in fig. are seen; and here i must make an extract from the report of mr. vail, who was observing the eclipse with an excellent -inch telescope, by dolland, and who made his report long before i had examined the negatives with a lens. in speaking of this protuberance, he says: "its outlines were perfectly well defined, and were not curves, but rather irregularly broken straight lines, and throughout it seemed marked by similar lines. it reminded me of the appearance one sometimes sees on the face of a cliff, where the rock is broken by horizontal and vertical lines." now, it is most evident that mr. vail saw with a telescope what i photographed; and further, it would be impossible to have these delicate lines in a photogram, if there was any relative motion. without entering into any discussion about what the protuberances are, or are not, i will only say that when the flame burnt out, the residium was a cinder, and which is shewn in photogram no. iii.; this quickly tumbled down into a great heap, as seen in no. iv. but the fault of the canadian party consists in not having photograms similar to those of the american astronomers, which all more or less agree with each other. this is extremely hard, and although i congratulate those gentlemen on their well earned reputation, still i trust that our photograms, instead of contradicting one another, will be found consistent. i believe that jefferson city was the most westerly place where photograms of the eclipse were taken, and directly totality finished with us, it commenced at des moines, so that the photograms taken there must be compared with ours. there is a general belief that the protuberances do not change their form, at least but slowly, so it is of great consequence to substantiate my statement, which is, that whilst waiting for no. iii. plate this protuberance shot out, and when no. iii. photogram was taken it had lost its flamelike appearance, and about one-third its height. no iv. photogram shews the great prominence much reduced in height and increased in breadth, as if it had tumbled into a heap of burning matter. i cannot say whether all prominences are formed by the shooting-out of a flame, and then tumbling into a heap, but i do say that the great protuberance was formed in that manner. in looking at the des moines photogram, taken near the end of the eclipse, (i don't mean the engraving,) you see a great heap, not very unlike that seen in no. iv.; and dr. curtis remarks "that there is the same appearance of vast volumes of matter tossed up into an irregular heap by the ejecting force, and sinking back again. on all sides in long vertical rolls." this is a very good description of what actually took place. unfortunately, the long exposure of sixty-six seconds gives a softened appearance, and what should have appeared as a heap of cinders, now looks like a fluid. [photographs: ii, i, iv, iii] i now come to the most remarkable photogram that has ever been taken of an eclipse. no. iv. was taken as near the limb of the sun as it is possible to take one, for on shutting down the slide, out burst the sun. in this photogram you can see two luminous concentric bands running from a to e, separated by a dark space, or rather a dark band, which takes its origin on a part of the protuberance a. (see fig. .) these bands are crossed by numerous bright rays, all parallel to themselves and to the protuberances a and e. there are two bright beams, and both, together with the bright rays, are divided by this dark band. at e is seen the protuberance with a triplicate form, and appears to be three parallel planes of light; upon the upper one there appears a dark line, similar to those seen upon fig. . now, on looking at the des moines photogram, you actually see the stumps of these three parallel planes; could anything be more satisfactory? i will leave it to others to discuss these various phenomena, which will throw much light on the physical constitution of the sun, but will recapitulate some of the facts deduced from our observations. the corona belongs to the sun, and not to the moon. some of the protuberances are formed by the shooting of a flame, which burns out, leaving something that looks like a cinder, which crumbles into a heap, and then retains that form for some time; that there are luminous gases that surround the sun in concentric strata divided by a non-luminous layer; that the notches on the limb of the moon are the reflections of the upper part of the protuberances from the surface of the moon; that at a great distance from the sun there is a violent current of gas in an opposite direction to the motion of the sun upon its axis; that the light band surrounding the moon's limb in photograms of the partial eclipse, may be caused by the reflection from the second or under side of the plate. in conclusion, i congratulate those gentlemen who so kindly assisted me on our complete success, especially my dear friend and old ship-mate, professor stephen alexander, without whose assistance no canadian party would have been formed; and also, mr. vail, of philadelphia, who kindly joined our party, and whose annexed report gives such ample proof of the value of our negatives. e. d. ashe, commander, royal navy, director observatory, quebec. june nd, . report of mr. vail. "boston, august , . "commander ashe, _quebec observatory_. "dear sir,--i owe you an apology for not writing earlier, and communicating my observations on the eclipse; but since i parted from you at detroit, i have been so constantly on the move, as to seem to have no opportunity. i will now state briefly a few phenomena that i noticed at the time of the eclipse, most of which i think were communicated to you verbally before. "after the clouds that partially obscured the sari on the morning of the th had passed away, i observed that though the atmosphere was hazy, and the sky by no means blue, there was an unusual stillness and freedom from agitation in the air, so that the outlines of the spots on the sun were clearly defined in the small dolland telescope that i had under my charge, and this satisfactory condition of the air for telescopic observation continued until after the end of totality. the first contact was at h. m. s local time. it was probably about s. after this, before you were notified that the eclipse had begun, two or three seconds being lost in determining whether it was the limb of the moon, indenting the edge of the sun, or not. your first photograph was therefore probably five or six seconds after the beginning. the passage of the edge of the moon over the larger spot on the sun, i noted as follows:-- h. m. s. contact with the penumbra................. " " " umbra.................... complete obscuration of umbra............. "the time both of the beginning and end of totality, for reasons verbally stated to you, i failed to note. of the phenomena during totality, those which i most noted were, first, the disappearance of the last rays of the sun in an irregular broken line of light, succeeded at or near this point by a band or corona of a silvery white light almost as bright as the face of full moon. this though much wider at this point than elsewhere, was soon observed to extend in an entire ring around the dark body of the moon; from this luminous ring, rays of light seemed to shoot out at right angles on every side, diverging as it were from the centre of it. in some places they seemed to extend out nearly half the diameter of the moon from the bright ring; in others, not one fourth so far. but the most remarkable appearance of all, and that which attracted the attention of every one who witnessed the eclipse, whether seen with the naked eye or with the telescope, were the red protuberances that shot up immediately on the disappearance of the sun, from various places, on the edge of the moon; their position your photograph will fix better than i describe. the largest was on the lower edge of the moon, and was by my estimate, when highest, not less than two minutes in altitude from the edge of the moon, or about , miles. its colour was a bright _pinkish red_, its outlines were well defined, and were not curves, but rather irregularly-broken straight lines, and throughout it seemed marked by similar lines. it reminded me of the appearance one sometimes sees on the face of a cliff where the rock is broken by horizontal and vertical lines. the same or nearly the same appearance would be presented if one were to view columnal basaltic rocks, from a point where the rocks in the rear would rise above those in front. i would therefore suggest whether these lines may not have a similar origin, and each be the outline of a vast column of luminous matter thrown up above the atmosphere of the sun. there was a constant fluctuation in the height of these coloured protuberances during the total eclipse; the large one was the only one that was seen throughout the whole time, and that remained visible for some time after the edge of the sun appeared. the general phenomena, such as the darkness, the shining of the stars, &c, i had less opportunity of noticing than yourself and others, who were without a telescope, and will therefore say nothing about them. i have made no attempt to put my observations into any regular form, but have hastily written such as i thought might be of use to you, leaving it entirely to you to make any use of them. "very truly yours, "hugh d. vail." _________ mr. falconer's observations. "_to captain_ ashe, _r.n., &c., observatory, quebec:_ "dear sir,--as requested by you, i now give you the results of such observations as were made by me on the th of august last, during the progress of the eclipse. the limbs of the moon could be clearly defined beyond the s. and s.e. limbs of the sun. shortly before totality, there appeared on the sun's northern limb several watery-looking globules, which merged into each other as they passed from west to east, and then disappeared. at this instant, also, appeared distinct long, brilliant, yellow, rays of light, running east and west, and far away, and as straight as if ruled; others again ran north and south, and reminded me of the glory ancient painters depict around the heads of saints. on the southern limb appeared, just at totality, a large circular opening, or ring of bright silvery light, which assumed the shape of a red-hot crooked bar of iron. this, resting on the dazzling silvery coronal light, gave a strange and wondrous addition to the glorious scene we now beheld. several constellations shone brightly fourth, and a star or two low down on the western horizon. i must not omit the strange protuberances seen at this moment: on the eastern side was one like a tongue bent upwards, with streaks of a reddish hue; the others the shape of knobs, dark and colorless, and rugged in outline. "i now come to the general appearance of the land and sky, and the changes that took place over the vast prairie, stretching far and wide, upon which you had erected your observatory. it was long before any appearance of gloom or darkness occurred, not till h. m., when a hazy gloom gradually spread over the broad expanse which surrounded us. at h. m. was seen a dense cloud approaching from the n.w. and s.w., rolling along in its course and obscuring everything around. indeed, it had the appearance of a coming storm, and seemed in part to issue from the prairie. it did not reach or envelope the observatory. in front of this was a lurid, unearthly glare, clear and bright, of a greenish tinge; the dense prairie grass around might have contributed to this effect. presently, when totality took place, all became comparatively dark; every tongue was hushed amongst the groups of persons who had come out on foot, or were seated in their waggons, from jefferson and the country around. and what did they behold? a wondrous sight! at the moment of totality, burst forth the beautiful coronal light of the brightness of burnished silver! upon the southern portion of this ring of light, rested that curved, elongated protuberance, of a fiery redness, rendered more ruddy in contrast with the dazzling silvery light of the corona. "several constellations shone bright and clear; several stars also were observed above the western horizon. all these gave the scene a magnificence and grandeur. wonder and admiration sat upon every face uplifted to the sky. every voice was hushed. sublime, indeed, was the scene presented. in reverential awe the groups stood mute. each one seemed to ponder within himself over the glorious scene in front of him. "presently, the light of the sun suddenly bursts forth; the clouds which covered the vast prairie lift, and gradually roll away. then along the western horizon are displayed long bright streaks of light, as seen at the approach of coming day. the purple hue upon the distant prairie vanishes. the stars also disappear, and the momentary night is turned into day! "a murmur is now heard, and voices arise, proclaiming the sublimity of the scene they had just witnessed, one of the most wondrous and imposing sights presented to the human eye, in the firmament of heaven! the words of the psalmist involuntarily fell from the lips: 'the heavens declare the glory of god, and the firmament sheweth his handiwork.' "at the approach of totality, the station-master informed me his poultry quietly went to roost. in jefferson city, the swallows flew down upon the ground, amidst the granite boulders, and remained till the light returned, when they arose and flew wildly about. "it remains only for me, in conclusion, to thank you and mr. douglas for inviting me to join this highly-interesting expedition, and to congratulate you and mr. douglas upon the great success which attended your photographic operations. "i have to thank you for beholding the wondrous and vast prairies west of the mississippi. 'haec olim meminisse juvabit.' "i remain, dear capt. ashe, yours very faithfully, "alex. pytts falconer. "glenalla, quebec, _august_ _th_, ." mars and its mystery [illustration: lowell's globe of mars, . _frontispiece_] mars and its mystery by edward s. morse member national academy of sciences author of "japanese homes and their surroundings," "glimpses of china and chinese homes," etc. _illustrated_ boston little, brown, and company copyright, , by little, brown, and company. _all rights reserved_ published october, the university press, cambridge, u. s. a. to percival lowell who has by his energy and scientific spirit established a new standard for the study of mars this book is affectionately inscribed preface the following pages have been written for the general reader. the controversies over the interpretation of the curious markings of mars and the wide divergence of opinion as to their nature first turned my attention to the matter. the question of intelligence in other worlds is of perennial interest to everyone, and that question may possibly be settled by an unprejudiced study of our neighboring planet mars. knowing the many analogies between mars and the earth, we are justified in asking what conditions really exist in mars. instead of flouting at every attempt to interpret the various and complicated markings of its surface, we should soberly consider any rational explanation of these enigmas from the postulate that the two spheres, so near together in space, cannot be so far apart physically, and from the fact that as intelligence is broadly modifying the appearance of the surface of the earth, a similar intelligence may also be marking the face of mars. a student familiar with a general knowledge of the heavens, a fair acquaintance with the surface features of the earth, with an appreciation of the doctrine of probabilities, and capable of estimating the value of evidence, is quite as well equipped to examine and discuss the nature of the markings of mars as the astronomer. if, furthermore, he is gifted with imagination and is free from all prejudice in the matter, he may have a slight advantage. astronomers are probably the most exact of all students as to their facts, and in this discussion there is no attempt to introduce evidence they do not supply, as the frequent quotations from their writings will show. having studied mars through nearly one presentation of the planet with the great refractor at the lowell observatory, what i saw with my own eyes, uninfluenced by what others saw, will be presented in a short chapter at the end of this book. i wish to express my obligations to professor percival lowell for the privileges of his observatory, for many of the illustrations in this book, and for his unbounded hospitality during my visit to flagstaff. i am also deeply indebted to mr. russell robb for valuable assistance during the preparation of the manuscript. e. s. m. salem, massachusetts, october, . contents page i. introduction ii. immeasurable distances of space iii. other worlds inhabited iv. lowell's book on mars v. testimony of astronomers vi. the study of planetary markings vii. difficulties of seeing viii. variation in drawing ix. theories regarding the canals x. comments and criticism xi. atmosphere and moisture xii. notes on irrigation xiii. variety of conditions under which life exists xiv. my own work xv. what the martians might say of us xvi. schiaparelli, lowell, perrotin, thollon xvii. last words index list of illustrations lowell's globe of mars _frontispiece_ fig. . planisphere of earth _page_ plates i. tobacco cultivation under cloth, porto rico _page_ ii. drawings of solar corona " iii. chinese bowl, showing crackle " iv. mud cracks on shore of roger's lake, arizona " v. natural lines, cracks, fissures, etc. " vi. artificial lines, railways, streets, canals, etc. " vii. dome of lowell observatory, flagstaff, arizona " viii. twenty-four inch telescope, lowell observatory " ix. drawings of canals of mars by the author " portraits giovanni virginio schiaparelli _page_ percival lowell " henri perrotin " m. thollon " _life not wholly unlike that on the earth may therefore exist upon mars for anything we know to the contrary._ simon newcomb. mars and its mystery i introduction had some one asked, fifty years ago, is the sun composed of chemical elements with which we are familiar? shall we ever know? the question would not have been deemed worthy of a second thought. realizing what has been accomplished, not only regarding the constitution of the sun, but of the most remote stars, we are encouraged to ask: is mars inhabited? shall we ever know? to what groups of students are we to appeal for an answer? if we want to know the diameter of mars, its weight, the form of its orbit, the inclination of its axis, the period of its revolution around the sun, and its rotation period, its ephemeris and its albedo, we ask the astronomer, for he has the instruments with which to observe and measure, and the mathematical knowledge necessary to reduce the measurements. if mars were incandescent, we should appeal to the astrophysicist for information regarding its chemical composition. if, however, we want to know the probability of mars being the abode of life, we should appeal to one who is familiar with the conditions of life upon our own globe. if the question is asked as to the existence of intelligence on the planet, we endeavor to trace evidences of its surface markings, and their character, whether natural or artificial. knowing how profoundly man has changed the appearance of the surface features of our own globe in the removal of vast forests, in the irrigation of enormous tracts of sterile plain, the filling up of certain areas, like peking, tokio, london, with material having a different reflecting surface, we are to scan the surface of mars for similar modifications, and for an answer ask those who are familiar with physical geography, with meteorology, with geology, including the character of natural cracks or crannies, deep cañon, or range of mountains, or any of the great cataclysms which have scarred the face of the earth. taking the great mass of facts as they are presented to us by astronomers, to what class are we to appeal as to the probability of life in other worlds? what class will form the most rational conclusions? will it be the circle-squarers, perpetual-motion cranks, spiritualists, survivals of a past who believe the world is flat, those who have "anthropomorphic conceptions of the supreme" and hebraic conceptions of the origin of things, or will it be those who value observation and experiment, who appreciate the importance of large numbers, and who are endowed with a tithe of imagination? most certainly the latter class. in approaching the interpretation of the markings of mars we should first glance at a brief historical summary of what has already been done. we should examine the testimony of those who have seen and drawn the canals; we are then better prepared to examine the records of the latest observations and the explanation of their nature. in the meantime an inquiry must be made as to whether the mathematical astronomer, after all, is best fitted to judge of the surface features of a planet. next we should take up in the following order the evidences, which are overwhelming, that a network of lines, geodetic in their character, mark the surface of mars. it has been claimed that these lines show the result of irrigation, and, therefore, the irrigation features of our own planet should be examined. it has been objected that many astronomers have not been able to see the markings, and consequently their existence has been doubted. it will then be proper to point out that the difficulties of seeing are very great, and that the acutest eyesight, coupled with long practice, is necessary to recognize the markings. it has been objected that the drawings of the minuter details of mars vary with different observers. it will be necessary to show that every kind of research employing graphic representation labors under the same difficulty, and none more so than astronomy. it has been objected that there is not sufficient moisture and atmosphere in mars to sustain life, and this must be answered by those only who are familiar with conditions affecting life on our own planet. various theories have been advanced, some of them physical, to explain the markings of mars, and these must be considered, and, if possible, answered. comments and criticism are difficult to repress, as the discoveries of schiaparelli and the additional discoveries and deductions of lowell have evoked discussions, which, in some instances, have been harsh and unreasonable, and, in one case, positively ridiculous. schiaparelli has been called an impostor, and lowell has come in for his full share of vituperation and innuendo. if this portion of the discussion is considered unparliamentary, the attitude and language of certain astronomers have provoked it. a brief account is presented of what the author was enabled to draw of the martian details, with a transcript of his notes made at the time of observation, and finally a little imaginary sketch is given as to how the world would look from mars; and if similar kinds of astronomers existed there, what comments and objections they might offer as to the inhabitability of the earth. such flights of the imagination are justified in that it gives one a chance to appreciate the weakness of some of the arguments urged against the idea of intelligence in mars. it will be objected that some of the names herein quoted are not recognized as astronomers. i can only say that in every instance i have found references to the writings and essays of those that might be objected to in the pages of the "observatory," and other reputable astronomical journals, and in no instances accompanied by adverse comment or criticism. if astronomers--even the distinguished schiaparelli--quote these names in scientific memoirs, i may venture to do the same in a book written for the general reader. the objection, however, has always presented itself with every controversy; it was conspicuously marked in the passionate discussions over darwin's "origin of species." the intelligent laity recognized the truth of darwin's proposition long before the zoölogist began to waver. essays by the unprofessional supporting darwin's contention were discredited because the writers were not trained naturalists. the history of invention is crowded with instances where devices and processes have been invented by men whose trades or professions were the least likely to enable them to originate such ideas. ii immeasurable distances of space _it is therefore perfectly reasonable to suppose that beings not only animated but endowed with reason inhabit countless worlds in space._ simon newcomb. until within recent centuries, man has not only believed that he and his kind were the only intelligent creatures in the universe, but that the little round ball on which he lived was the dominant part thereof. so rooted for ages was this conviction that it became fixed in man's mental structure, and hence the survival of the idea that still lingers in the minds of a few to-day. the conclusion was natural, however, for the behavior of the starry heavens and the sun and the moon seemed sufficient evidence that man, and the surface upon which he lived, was the centre of the universe. the stars were bright points of light, the moon a silver disk, and the sun a heat and light giving ball of fire, equally diminutive and not far away. let one realize for a moment the experience of these early people. everything aerial, with the exception of feathery birds, fluffy bats and flying insects, was composed of the lightest particles--cottony seeds, reluctantly falling snow-flakes, motes in the air, smoke and vaporous cloud, and, in contrast, the rock-foundationed and irregular surface upon which the people dwelt, and flat as far as man had reached. what wonder, then, that man viewed these brilliant points and dazzling disks as objects of no great size and not far away, hauled across the heavens by unseen spirits of some kind. the marvel of it all is, not that they believed as they did, but that any other views of cosmography could have been established. and yet the successive increments of astronomical knowledge, founded apparently on the soundest mathematics, were adopted in their turn. what more convincing than the epicyclic theory of ptolemy, buttressed by figures so ingenious and convincing, that the theory might have lasted till now except for the truer understanding of planetary movements in relation to that of the earth? all through this history are found traces of the barriers erected by prejudiced conservatives, of which the attitude of tycho brahe is a good example, though in this case it was probably his belief in the hebraic conception of the universe which excited his opposition to kepler's views, a conception which, unfortunately for the progress of astronomical research, still lingers among certain observers to-day and places them in precisely the same category with tycho brahe. with the gradual accumulation of knowledge it was found that of all the innumerable illuminated bodies in the heavens, only one,--just one,--the moon, revolved around the earth, and that the earth instead of being all dominant in the affairs of the universe, played a very minor part, and, instead of being master, was a very humble midget revolving around the sun; that, indeed, with the exception of the moon, there were visible to the naked eye only three bright points of light in the whole range of the heavens more insignificant in size,--mercury, venus, and mars,--while the other planets were vastly larger, and had many more satellites revolving around them. then it was found that, with the exception of the few planets, the myriad stars had no connection with the sun whatsoever, that the sun was no longer the centre of a great universe. later it was discovered through spectroscopic analysis that all the myriad of stars were composed of chemical elements similar to our sun. here, then, was the startling revelation that our sun was simply a star, and that the stars represented a "universe of suns," and, if we could get near any one star of the millions that sparkle in the heavens telescopically, we should see it as a round ball emitting light and heat. it was perhaps humiliating to find that our sun was so insignificant in size that from sirius, for example, it could not be seen with the naked eye, so small indeed that in the close companionship of other stars it would be swallowed up by their greater size and brilliancy. to assume, then, that our sun, so identical to the stars in heat and light emitting properties, was the only sun that had revolving around it a few minute balls, would be as absurd as if one should go on a pebbly beach, extending from labrador to florida for example, and picking up a single pebble, should have the hardihood to assert that this pebble was the only one, among the millions of pebbles, upon which would be found the bits of seaweed and little snails which it might support. the overwhelming vastness of the universe is entirely beyond the grasp of the human mind. the mere statement that it requires so many years for the light to reach us from a certain star, the parallax of which has been rudely established, affords one only a faint glimmer of the truth. the swing of our earth about the sun gives us a base line of , , of miles, and yet, with this enormous base from which to subtend an angle, only a very few of the myriad of stars show the slightest displacement; the others exhibit no more signs of divergence than if while looking at them we had simply moved our heads from one side to the other! fixed stars they appear to be, and are so called, though we are told they are all drifting in various directions, as our star-sun is. only by reducing all these vast distances and dimensions to a minute scale can the mind realize the futility of ever comprehending the illimitable distances of space. in order to consider the attitude of the earth in relation to the sun and the nearest fixed star, we will reduce the sun's diameter of , miles to the dimensions of a ball one inch in diameter; the earth reduced to the same scale would be a minute speck less than one one-hundredth of an inch in diameter; a perforation in paper made by the finest cambric needle would represent the size of this minute speck, the earth. following this scale we should place this speck nine feet from the inch ball, this distance representing , , of miles, the earth's distance from the sun; mars would be a still smaller speck a step farther off. let us now proceed to boston common, for example, and on the smooth playground place our inch ball representing the sun; taking three good steps we should place our minute speck, representing the earth, upon the ground where it would be immediately lost in the fine gravel; another step and we would place a still smaller particle, representing mars. how big a circle on the earth's surface, using the inch ball as a centre, should we have to describe in order to include the nearest fixed star? such a circle would reach to detroit, michigan, and columbus, ohio, or wilmington, north carolina! to find a circle which would include eight other fixed stars next in distance, and only eight of the thousands which render the heavens so beautiful on a clear winter's night--we should run such a circle through the centre of hudson bay, the waters of southern greenland, lake winnipeg, and new orleans! in this broad way only can we form a dim conception of the overwhelming distances of space, and, in this absolutely unthinkable space, our little sun, with its constant rain of meteoric dust, an occasional comet, and its microscopic planets are literally bunched together. to admit, as we must then, that one of these motes has had irrigating canals on various parts of its surface since prehistoric times, and the other mote has nothing of the sort despite the geodetic lines that are seen marking its surface, is simply preposterous. their disposition, their visibility coincident with the martian summer, becoming apparent only when the snow caps melt, their convergence towards centres of distribution, all go to prove by the simplest analogy an identity of structure. certainly the overwhelming force of lowell's observations and arguments baffles any other reasonable explanation of the character and purpose of these markings. here are the lines, some following the arcs of great circles, all appearing precisely when they should appear, and in progressive strength from the north when the vivifying water from the melting snow cap first starts the vegetation. why certain parallels or doublings are observed in some of the canals is about as puzzling to us as the checkerboard townships of the west would appear to a martian, where some would be yellow with the ripening grain while others, uncultivated, would appear of a different color. iii other worlds inhabited _whether the other fixed stars have similar planetary companions or not is to us a matter of pure conjecture, which may or may not enter into our conception of the universe. but probably every thoughtful person believes with regard to those distant suns that there is in space something besides our system on which they shine._ tyndall. it would be a waste of time to attempt an interpretation of the markings of mars as a result of intelligent effort, if it could be proved beyond a reasonable doubt that our globe was not only unique among the bodies which probably accompany the innumerable suns, but was the only body, among them all, sustaining creatures of intelligence. if life exists in other planets of a nature with which we are familiar, then the physical conditions must be similar to those of our own planet. later we shall point out the infinite variety of conditions under which life--even man--exists on this globe, and it will be shown that the question of higher or lower temperature, more or less humidity, higher or lower atmospheric pressure, greater or less force of gravity, can have but little weight in discussing the probability of life in other worlds. in a planet devoid of atmosphere, or a sphere glowing with its own heat, we may decide without question that life does not exist. even in a globe in many respects like our own it would be hazardous to conjecture the kinds of organic forms in which it is manifested. reasoning from analogy, if life exists in mars, or other spheres in infinite space, it must have originated under much the same conditions as it originated here; at the outset the most primitive bits of protoplasm. but has life appeared in mars? tyndall, in graphic words, pictures the rounding of worlds from nebulous haze, and then says, "for eons, the immensity of which overwhelms man's conception, the earth was unfit to maintain what we call life. it is now covered with visible living things. they are not formed of matter different from that around them. they are, on the contrary, bone of its bone and flesh of its flesh." mars must come in the same category. it is a part of the original nidus from which our world was condensed, and however life originated in the past, the conditions for its origin, at least, must have been as favorable on the surface of mars, as on the surface of the earth, and, so far as we know to the contrary, even more favorable. in the beginning, mars cooled and hardened with all those behaviors of contraction, condensation of vapor on its surface, erosion, etc., and it is impossible to avoid the conviction that life, as on our earth, arose under the same physical conditions. recalling the resemblance which mars bears to the earth, and the data which have already been established, we behold a world in many respects like ours, with its sunsets and sunrises, winds that sweep over its surface, the dust storms from the deserts, its snow-storms and snow-drifts, its dazzling fields of white in the north, with an occasional snow-storm that whitens the planet far down in latitude; the seasonal changes, and, most important of all, the melting ice caps, with rivulets and torrents, temporary arctic seas and frozen pools, its great expanses of vegetation and sterile plains. we have in mars the variety of conditions under which life has assumed its infinite variety of aspects on the earth, and which, by analogy, should have passed through similar stages in mars. life at the outset must have been protoplasmic; then came contractile tissue, muscular bundles, hardened structures within and without for their support, nerves to animate the muscles, and protection for nerve-trunk, either rigid or flexible. hard parts might vary under a different force of gravity, though there might appear types of structure that could be classified with our own. all such conditions, however, are mere surmises, for about such matters we can reason only from analogy. the first proposition to establish is that the conception of the plurality of worlds is not unreasonable, and second, that many of the most eminent astronomers have believed in the inhabitability of other worlds, and this justifies a reasonable man to follow the inquiry. the belief is based upon legitimate analogies which have thus far guided man in every generalization, in the establishment of principles, and are continually appealed to in the details of every day's experience. from remote times it has been taken for granted by the best minds that other worlds besides ours sustain life. the early belief in the plurality of worlds was based on the idea that since spheres like ours had been fashioned by the almighty they must have been made for the same purpose for which our globe seemed intended, to sustain life, and scripture was freely quoted in support of the idea. sir david brewster, in his book "more worlds than one," says that the doctrine of the plurality of worlds was maintained by almost all the distinguished astronomers and writers who have flourished since the true figure of the earth was determined: "giordano bruno of nola, kepler, and tycho believed in it; and cardinal cusa and bruno, before the discovery of binary systems among the stars, believed also that the stars were inhabited. sir isaac newton likewise adopted it, and dr. bentley, master of trinity college, in his eighth sermon on the confutation of atheism from the origin and frame of the world, has ably maintained the same doctrine. in our own day we may number among its supporters the distinguished names of laplace, sir william and sir john herschel, dr. chalmers, isaac taylor, and m. arago." the attitude of the intelligent world to-day is well shown in a recent number of london "nature," where in a review of a book by wallace, endeavoring to show that this world alone sustains life, the reviewer ends by saying: "to consider this earth as the only inhabited body in the stellar universe, a reversion to prehistoric ideas, may or may not be an advance, but it will require very strong arguments before we can be brought to consider that its isolation in the cosmos is indeed a fact." until the discovery by schiaparelli of the network of lines in mars, laid out with seemingly intelligent precision, the arguments for the inhabitability of other worlds were based entirely upon analogy. sir richard owen, the great comparative anatomist, in supporting the contention that life existed in other planets, said: "the grounds of belief vary with the probability of a proposition; if nothing better than analogy can be had--on analogy will belief be based." professor o. m. mitchell, the first director of the cincinnati observatory, in his work on "popular astronomy," says, in regard to the doctrine of the plurality of worlds: "it would be most incredible to assert, as some have done, that our planet, so small and insignificant in its proportions when compared with other planets with which it is allied, is the only world in the whole universe filled with sentient, rational and intelligent beings capable of comprehending the grand mysteries of the physical universe." the eminent french astronomer, m. flammarion, has, in an eloquent passage in his "plurality of worlds," portrayed the vastness of the universe and the utter insignificance of our earth in the immensity of space: "if advancing with the velocity of light[ ] we could traverse from century to century this unlimited number of suns and spheres without ever meeting any limit to this prodigious immensity where god brings forth worlds and beings; looking behind, but no longer knowing in what part of the infinite to find this grain of dust called the earth, we should stop fascinated and confounded by such a spectacle, and uniting our voice to the concert of universal nature we should say from the depths of our soul, almighty god! how senseless we were to believe that there was nothing beyond the earth, and that our abode alone possessed the privilege of reflecting thy greatness and honor." compare these elevating thoughts with the shrunken attitude of one who has the conceit to imagine that he and his kind are not only alone in the universe but superadds to this monstrous conception the idea that the millions of great suns are designedly waltzing around solely for his edification and amusement, unmindful of the heedless way in which the millions of his race regard the overpowering majesty of the heavens. to the thousand millions that live to-day, and the thousand, thousand millions that have perished in the past, the starry heavens have never excited an emotion grateful, reverent, or curious, unless a flaming comet, or an eclipse of the sun or moon occurred, and then with superstitious fear have they gone grovelling in the dust. an astronomer imbued with hebraic conceptions of the universe is poorly equipped to appreciate the arguments in favor of life in other worlds. he may be keen in perceiving lines in the spectrum, and the significance of their lateral displacement, but possessed with a belief--the result of early training--that a little two-legged human molecule could command the sun and moon to stand still, a realization of his own insignificance, or the possibility of intelligence in other worlds, must forever remain beyond his grasp. emerson said "the dogmas shrivel as dry leaves at the door of the observatory." they never shrivel for such minds, but grow and flourish with a density that obscures by, its rankness every rational conception of the heavens above. as an illustration of the attitude of such mentalities we have to go back fifty years, for few survive to-day. edward hitchcock, professor of geology and theology at amherst, wrote a book just fifty years ago entitled "plurality of worlds," in which he denounces the idea; but observe the precise way in which he lays down the law: "the planets had no vital tendencies, they could have had such given only by an additional act or series of acts of creative power. as mere inert globes, they had no settled destiny to be the seats of life; they could have had such a destiny only by the appointment of him who creates living things and puts them in the places which he chooses for them" (page ). it may be objected that it is useless to bring up these old theological conceptions, as the world has happily gone beyond them, and only in an atavistic manner do we find a few still holding them; nevertheless it may be safely asserted that fifty years hence we shall look back upon the attitude of certain astronomers to-day with much the same pity and amusement which excites us when we regard the attitude of a similar class in the middle of the last century. tyndall expresses the universal belief of thinkers in whatever line of work, that life is by no means confined to this earth. he says: "whether the other fixed stars have similar planetary companions or not is to us a matter of pure conjecture, which may or may not enter into our conception of the universe. but probably every thoughtful man believes, with regard to these distant suns, that there is, in space, something besides our system on which they shine." one class of objectors to the idea that other worlds are inhabited endeavors to show that our position in the universe is unique, that the solar system itself is quite unlike anything existing elsewhere, and, to cap the climax, that our own little world has just the right amount of water, air, and gravitational force to enable it to be the abode of intelligent life, and nowhere else in the broad expanse of heaven can such physical habitudes be found as will enable life to originate or to exist! in a memoir on the "evolution of the solar system," by professor t. j. j. see, the author, while not denying the possibility of other systems like our own, still considers our system unique. here are his words: "therefore, while observation gives us no grounds for denying the existence of other systems like our own, it does not enable us to affirm, or even to render probable, that such systems do exist." because a number of binary stars have been discovered in which the two stars are nearly equal in mass, and their orbits highly eccentric, he therefore concludes that the millions of stars that stud the heavens are probably without satellites. the unreasonableness of this attitude is emphasized by realizing that these innumerable suns are similar to our own sun, as revealed by the spectroscope, and have a similar eruptive energy. professor newcomb, however, says: "evidence is continually increasing that dark and opaque worlds like ours exist and revolve around their primaries." had mr. see discovered that every star of the many million was accompanied by another star nearly equal in mass, with its marked eccentric behavior, then only would he be justified in his inference that our solar system was indeed unique. when one realizes that the stars are at such unimaginable distances that the highest powers of the telescope reveal even the nearest of them only as points of light--not as disks--and when one further realizes that the satellites of our sun, even the largest of them, are diminutive globes compared to the vastness of the sun, it seems unreasonable if not impossible to entertain the idea that none of these remote stars are accompanied by satellites, and that, therefore, this little sun of ours stands without parallel in the universe. tyndall, in his famous reply to the critics of his belfast address, in speaking of the origin of life, referred to the nebular theory as follows: "according to it our sun and planets were once diffused through space as an impalpable haze out of which by condensation came the solar system. what caused it to condense? loss of heat. what rounded the sun and planets? that which rounds a tear, molecular force." in these terse and graphic expressions we are made to understand the universality of law. so far as we have sounded the depths of the stellar universe we see the same obedience to gravitational laws, the same flashing lines in the spectrum. we encounter no phenomena that cannot be explained, or at least inferred, by the knowledge we have obtained from our little mote of the cosmos. mr. see thinks it remarkable that "previous investigators have almost invariably approached the problem of cosmogony from the point of view of the planets and satellites, and that no considerable attempt has been made to inquire into the development of the great number of systems observed among the fixed stars." it is true our planetary system has been used as a standard of measurement for the universe, and a very comprehensive standard it has proved to be. the law of universal gravitation was based on terrestrial and lunar observations, spectroscopic analysis was determined in a terrestrial laboratory. as george iles says, a coal of fire may be raked from a grate and broken up to illustrate the rapid cooling of smaller masses. even a child's spinning top may be used in an astronomical lecture. the study of our sun led to the study of the fixed stars, and so our little system has thus far furnished us with examples and illustrations by which we interpret the universe. in our solar system we have a fair sample of the cosmos in miniature, though our sun is so modest in size, compared with the great orbs that appeal to us by their number and brilliancy. so far as our telescopes have sounded the heavens we find nebulous clouds in their structure showing inchoate masses, orbital and spiral arrangements, condensations in their centres. we have the binaries with their extraordinary properties, we have variables with their dark bodies revolving around their primaries. in our little system we also have dark bodies revolving around a luminous primary, from one of which we endeavor to interpret the mysteries of the universe; we have loose masses, as in comets with enormously elongated orbits; we have spheres of insignificant size, with small bodies revolving around them, and these epitomes revolving around a central sun; we have one of these bodies with meteoric rings; and, in the case of our own globe, a satellite of such size that except in the form of its orbit it might well represent a binary in embryo;--and, finally, a host of bodies big enough to reflect the rays of the sun, pursuing their various orbital paths. we are told that the stars are as distant from each other as we are from them. we may regard these systems of nebulæ, variables, doubles, etc., as different kinds or species of heavenly bodies; and to assert that our system is the only individual of the species in the universe seems contrary to all celestial analogy, for do we not have hundreds of binaries, thousands of variables, millions of suns, revealing the same fiery energy and consuming the same elemental fuel? professor newcomb in his "reminiscences" describes his first sweeping the heavens, at random, with the then new twenty-six inch refractor at the naval observatory and discovering a little cluster of stars so small and faint that the individual stars eluded even the great power of this instrument. he says: "i could not help the vain longing which one must sometimes feel under such circumstances, to know what beings might live on planets belonging to what, from an earthly point of view, seemed to be on the border of creation itself." one would suppose that this expression of a longing to ascertain the character of the beings inhabiting planets circling these distant suns would induce one to study a planet analogous to our earth, and so near in comparison to these unimaginable distances as to be within a hand's grasp, so to speak. the little interest professor newcomb has taken in the subject is well expressed in his late book "astronomy for everybody." in his chapter on mars, in which _everybody_ is certainly interested, he says: "the reader will excuse me for saying anything in this chapter about the possible inhabitants of mars. he knows just as much of the subject as i do, and that is nothing at all." he might at least have given the various pronouncements of schiaparelli, lowell, and others as to the probable character of these remarkable markings on mars, and their supposed significance. while professor newcomb's attitude on the question of the plurality of worlds has been somewhat conservative in the past he has lately, however, expressed himself on the question in no uncertain terms. in a recent article in "harper's magazine," entitled "probability of life in other worlds," he has lent his sanction to the rational idea that other worlds may be the abode of intelligent creatures. his recognition of the principle will do much to offset the influence if it ever had any, of a recent book published in england by alfred russel wallace, in which the distinguished author attempts to show that this world stands alone as the abode of intelligent life. despite his epoch-making work with darwin, nearly fifty years ago, which must forever merit our gratitude, and the charm of his various essays on protective coloring, mimicry, theory of birds' nests, etc., he has since those lucid days expressed convictions of such a nature that if a future demorgan should write on human paradoxes he would classify mr. wallace as chief among them. a profound believer in evolution, he exempts man from the inexorable logic of the principle with about as much reason as if, confessing his belief in the nebular hypothesis, he should insist that the earth was an exception. but to return to professor newcomb's recent utterances. in the above-mentioned article he says: "not only does life, but intelligence, flourish on this globe under great variety of conditions as regards temperature and surroundings, and no sound reason can be shown why, under certain conditions which are frequent in the universe, intelligent beings should not acquire the highest development." again he says: "life, not wholly unlike that on the earth, may therefore exist upon mars, for anything we know to the contrary. more than this we cannot say." in his final summing up professor newcomb says: "it is therefore perfectly reasonable to suppose that beings not only animated but endowed with reason inhabit countless worlds in space." it would seem as if a mind capable of entertaining an idea of our uniqueness in the universe betrays the survival of a mental condition which, centuries ago, regarded the stars as bits of luminous material expressly designed to illuminate this little earth, around which they all pursued their daily paths. iv lowell's book on mars _this whole arrangement presents an indescribable simplicity and symmetry which cannot be the work of chance._ schiaparelli, in writing of the canals. in a discussion of the surface markings of mars a broad sketch of what has already been accomplished in the study of that planet should be given for the general reader. i know of no better way of doing this than by giving a brief abstract of percival lowell's epoch-making work entitled "mars." in this book he presents in a clear and striking manner the results of his own work covering continuous observations of the planet for many years. the preface is dated from flagstaff, arizona, . since that time he has issued three volumes of memoirs, in quarto, of the lowell observatory, and a number of bulletins in which he presents many additional facts confirming previous observations, besides new observations; and finally, in a late bulletin, he has presented photographs of mars made by his assistant, mr. lampland, in which a number of canals plainly show, thus setting forever at rest the question of the subjective character of the markings. the student must, however, follow the advice of an english reviewer and by all means read the book. "to determine," says mr. lowell, "whether a planet be the abode of life in the least resembling that with which we are acquainted, two questions about it must be answered in turn: first, are its physical conditions such as render it, in our general sense, habitable; and secondly, are there any signs of its actual habitation? these problems must be attacked in their order, for unless we can answer the first satisfactorily, it were largely futile to seek for evidence of the second." the reason why mars in certain years becomes so conspicuous is that its orbit is highly eccentric. every two years--the period of its revolution about the sun--brings it nearest to the sun, and once in fifteen years we find ourselves between it and the sun at its nearest approach. huyghens, in , made a drawing of the dark region on mars now known as the syrtis major, and, through its disappearance and reappearance, he discovered that the planet rotated on its axis, and roughly determined a daily period of twenty-four hours. for the first time it was known that mars had a day and a night. as some doubts existed as to the correctness of huyghens's figures, cassini in determined anew the rotation period of mars and found it to be twenty-four hours and forty minutes. from the white polar caps, the study of which we first owe to maraldi, it was found that the tilt of its axis to the plane of its orbit was very nearly the same as that of the earth. as this inclination determines the seasons, it was seen that mars, like the earth, had its spring, summer, autumn, and winter. a polar flattening was also observed which was slightly in excess of ours. "to all forms of life of which we have any conception, two things in nature are vital, air and water." has it an atmosphere? without air no change could take place. the moon without air remains unchanged, except what gravitation accomplishes in pulling down crater walls. "with mars it is otherwise. over the surface of that planet changes do occur, changes upon a scale vast enough to be visible from the earth." the first sign of change occurs in the polar snow cap. it dwindles in size every two years (the time of a single revolution of mars around the sun). for nearly two hundred years these white polar caps have been observed to wax and wane. as the martian winter comes on in the northern hemisphere, for example, the polar cap extends its borders to the temperate zone. as summer comes on the snow cap is seen to dwindle gradually away, till by early autumn it presents but a tiny patch a few hundred miles across. schiaparelli observed changes in tint which he noticed were correlated with the seasons. in observations were made continuously from early june till late in november. these dates, in mars, represent the last of april till the last of august. during this time marked changes took place in the bluish-green areas of the planet. a wave of seasonal change swept down from the pole to the equator. the fact of this occurrence constitutes positive proof of the presence of an atmosphere. in another way the evidence was shown. a series of measurements of the polar and equatorial diameters of mars were made, and these indicated that a visible layer of twilight atmosphere had been measured. this, lowell explains by a diagram and other data. it is found, according to lowell's observations, that the atmosphere is much freer from clouds than had been supposed. he shows conclusively that it is much rarer than that of the earth. appearances have been seen, however, which are best explained by assuming them to be clouds. during the opposition of , mr. douglass, at that time an assistant astronomer at the lowell observatory, made a special study of the terminator of mars.[ ] a careful study of the terminator for almost every degree of latitude was made, and irregularities were detected. of this large number, were not only recorded, but measured; and of these, were depressions, and were elevations of the surface. many of these irregularities were supposed to be clouds, but the arguments to support this attribution are too technical to be presented here. unmistakable clouds have also been seen moving at a definite rate of speed, as if carried along by the wind. "to sum up, now, what we know about the atmosphere of mars: we have proof positive that mars has an atmosphere; we have reason to believe this atmosphere to be very thin,--thinner at least by half than the air upon the summit of the himalayas,--and in constitution, not to differ greatly from our own." as to the existence of water on the planet, one has only to consider the polar snow caps. in the height of the southern winter, the polar cap of snow measures over two thousand miles across, covering fifty-five degrees of latitude, with one unbroken waste of white. as spring advances the snow begins to melt, disappearing rapidly as summer comes on, and, as it melts, a dark band is seen bordering this edge. as the snow recedes the dark band recedes. this band is, therefore, not a permanent marking on the planet, but obviously water, the result of the melting snow--an arctic sea, in fact. this band is irregular, varying in width in different longitudes, as if the water filled up large areas of depression. when finally the snow cap disappears, as it did for the first time on record on the notable occasion of october , , the dark band, which had become thinner, disappeared also, leaving only a yellow stretch of surface. an additional proof that this dark band is water, was established by professor w. h. pickering, for he discovered that the light reflected from its surface was polarized. the absurdity of the suggestion that these white polar caps are not snow, but congealed carbonic acid gas, is fully shown by lowell. the asymmetry of the outline of these snow caps is paralleled by the irregularity of the earth's polar caps. glints of brilliant light are seen to flash out from this region, as if produced by sunlight reflected from a sloping surface. on comparing these flashes of light with observations made by green, in , they were found to be in the same place. detached fields of snow were also observed below the receding line, an evidence that these regions were at a higher elevation. as before stated, on october , , for the first time in the record of polar observations, the southern polar cap disappeared entirely. in this connection it may be of interest to observe that in the united states, in the summer of , the temperature ranged a few degrees above the normal. (for this fact i am indebted to professor cleveland abbe, e. s. m.) the large, irregular, dark regions on the planet have been supposed to be bodies of water, or seas, and have been described and named as such by astronomers. lowell shows, however, that there is every reason to doubt this conclusion. "to begin with, they are of every grade of tint,--a very curious feature for seas to exhibit, unless they were everywhere but a few feet deep; which, again, is a most singular characteristic for seas that cover hundreds of thousands of square miles in extent,--seas, that is, as large as the bay of bengal. the martian surface would have to be amazingly flat for this to be possible. we know it to be relatively flat, but to be as flat as all this would seem to pass the bounds of credible simplicity. here, also, professor w. h. pickering's polariscope investigations come in with effect, for he found the light from the supposed seas to show no trace of polarization. hence, these were probably not water." lowell also shows that if these regions were seas, or water surfaces of the shallowest kind, sunlight would certainly be reflected from some portion of the surface so as to be visible from the earth. a calculation of the region from which such a beam of light might be reflected has been carefully made, but no light of this nature has ever been seen. these regions change in color, and schiaparelli suggested that in some way these changes were dependent on the martian seasons. lowell, by continuous observations covering many presentations of the planet, has demonstrated that the changes in color are synchronous with the seasons, and they further show that these regions change in expanse as well. the reader must refer to lowell's book to understand the very minute way in which the author traces out the behavior of these so-called seas as the martian summer advances and autumn comes on. his evidence is overwhelming that the regions heretofore regarded as seas are vast tracts of vegetation, doubtless on lower levels, or depressions of the surface, old sea bottoms, in fact, where springs and the natural settlings of stray waters might keep the ground sufficiently moist to support a scanty growth. the regions not marked by the dark shading, from their reddish and yellowish tinge, have always been regarded as land, probably desert land, as they remain fixed from year to year, dead and unchangeable as deserts are. the question naturally arises, if the water of mars is piled up at the poles as snow, how does it find its way back on its melting? a discovery made by schiaparelli in revealed the existence of various lines marking the surface which he called _canali_, or channels.[ ] these lines cover the face of the planet like a net, they are laid out with geodetic precision. "the lines start from points on the coast of the blue-green regions, commonly well-marked bays, and proceed directly to what seem centres in the middle of the continent, since, most surprisingly, they meet there other lines that have come to the same spot with apparently a like determinate intent." in other words these lines--fine, straight, dark, as if cut by an engraver, some of them running for hundreds of miles--converge at certain centres. they all start, as schiaparelli first observed, from definite regions and terminate at definite points. many of them follow the arcs of great circles. these lines may be thirty or more miles in width, apparently preserving the same width throughout, though slightly wider where they leave the dark bands. they run in every direction, a number often converging at a common centre, and, when they do so, a round, dark area appears which lowell has called an oasis. in the clear and steady atmosphere of flagstaff, mr. lowell, by the aid of his superb telescope, has added about four times as many canals as are shown on schiaparelli's chart. these canals form an intricate network of lines, and no one can contemplate these curious features without being impressed by their artificial character. schiaparelli, who first discovered them in , continued his observations from year to year despite the fact that no one else could see them. in the course of a few years he discovered a still more remarkable condition, and this was that a number of the canals appeared double. this, indeed, seemed an optical illusion, and by no means strengthened his position, as the single canals proclaimed by him were supposed to be figments of the imagination. undeterred by the general scepticism, schiaparelli established, at each fresh opposition, his previous announcements. for nine years no one was able to confirm his marvellous discoveries. in the year , however, perrotin, at nice, with his assistant, thollon, managed to make out a number of the canals, single and double, which were carefully drawn. reference to perrotin's work will be made further on. the reason why so few have seen them is the lack of observers with acute eyesight and patient devotion to the work, coupled with unsteady air. size of aperture seems to be of little importance. that schiaparelli, with an - / inch glass, discovered the canals, while with the twenty-six inch glass of the naval observatory at washington they have never been seen, is emphatic evidence of what a clear and steady atmosphere means in the study of delicate planetary markings. the artificiality of the canals is shown by the "supernaturally regular appearance of the system, upon three distinct counts: first, the straightness of the lines; second, their individually uniform width; and, third, their systematic radiation from special points." it was the mathematical shape of the ohio mounds that first suggested their artificial character. that these lines are artificial and not natural is seen in the fact that at times they are not visible. the lines while temporary in appearance are permanently in place. "not only do they not change in position during one opposition; they seem not to do so from one opposition to another." "unchangeable, apparently, in position, the canals are otherwise among the most changeable features of the martian disk." the order of their appearance synchronizes with the changes of the season, as the snow caps begin to melt the canals begin to appear; in appearance strengthened first at the borders of the polar seas and gradually stretching down towards the equator. in minute detail lowell presents the successive visibility of the different canals. to account for all these phenomena we have to look at our own earth for a parallel, and we see it in the great irrigation tracks of the west, and in the vast irrigated regions in india depending upon the melting of the himalaya snow cap. the accumulative evidence is overwhelming that here is a dry planet, and an intelligence of some kind that can only survive by utilizing the few remaining sources of water supply. it is to the merit of professor w. h. pickering, to whom professor lowell gives the credit of having first suggested the idea of irrigation to account for the great width of the canals. what we see, then, is not the canal, which may be a slender stream of water, but a broad band of vegetation irrigated from these narrow channels. these lines penetrate and cross the dark regions in various directions, which again offer additional proof that the so-called seas are not seas but areas of vegetation sparsely scattered, against which the irrigated portions are of sufficient strength and color to show.[ ] among the most interesting features of the planet's surface are the round, or oval spots which lowell calls oases; these invariably occur at the junction of the canals. "in spite of the great number of the spots, not one of them stands isolate. there is not a single instance of a spot that is not connected by a canal to the rest of the dark areas." there appears to be no spot that has not two or more canals running to it, and apparently no canal junction is without its spot. the majority of the spots are to miles in diameter. there are many smaller ones. these spots, like the canals, appear and disappear coincidently with seasonal changes. the canals and the oases follow the same method and order in their growth. "both are affected by one progressive change that sweeps over the face of the planet from the pole to the equator." the reader cannot dwell too strongly on the fact that the visibility of these various markings appears first in northern latitudes, and gradually darkens toward the equator, precisely the reverse of the unfolding of plant life on the earth. from mars our earth would show its tropical vegetation the year round, while in mars no tropical vegetable coloration would appear until water from the melting polar snow caps animates its growth. lowell shows conclusively that the seas are not seas, nor the canals waterways, nor the spots lakes. apparently, the spots appear not so much by an increase in size as by a deepening in tint. they start, it would seem, as big as they are to be, but faint in tone; they then proceed to darken throughout. if these spots are areas of vegetation, the explanation of their appearance is at once evident. even more markedly unnatural is another phenomenon of this phenomenal system, of which almost every one has heard and almost nobody has seen,--the double canals. upon a part of the disk where, up to that time, a single canal has been visible, of a sudden, some night, in place of the single canal, twin canals are perceived, similar in character and inclination, absolutely parallel, reminding one of the twin rails of a railroad track. the regularity of the thing is startling. in details the doubles vary, chiefly, it would seem, in the distance the twin lines lie apart. lowell says the widest he has seen is the ganges, in which six degrees separate the two lines,--in the narrowest, the phison, four degrees and a quarter. from to miles of clear country is found between the paralleling lines. "one element of mystery may be eliminated at the outset.... it is perceived of a sudden, by the observer, because of some specially favorable night. but it has been for some time developing. so much is apparent from my observations. suggestions of duality occurred weeks before the thing stood definitely revealed. furthermore, the gemination may lie concealed from the observer some time after it is quite complete, owing to lack of favorable atmospheric conditions. for it takes emphatically steady air to see it unmistakably." each canal has its individual behavior of doubling, and the varying widths, and their evident seasonal relations utterly forbid the conception that their appearance is due to optical illusion. mr. lowell feels tolerably sure that the doubling, or gemination of the canals, show that the phenomenon is not only seasonal but vegetal. why it should take this form is one of the most pregnant problems about the planet. for it is the most artificial-looking phenomenon of an artificial-looking disk. we quote a paragraph from the concluding chapter in his book: "to review, now, the chain of reasoning by which we have been led to regard it probable that upon the surface of mars we see the effects of local intelligence. we find, in the first place, that the broad physical conditions of the planet are not antagonistic to some form of life; secondly, that there is an apparent dearth of water upon the planet's surface, and, therefore, if beings of sufficient intelligence inhabited it, they would have to resort to irrigation to support life; thirdly, that there turns out to be a network of markings covering the disk, precisely counterparting what a system of irrigation would look like; and, lastly, that there is a set of spots placed where we should expect to find the lands thus artificially fertilized, and behaving as such constructed oases should. all this, of course, may be a set of coincidences, signifying nothing; but the probability points the other way. as to details of explanation, any we may adopt will undoubtedly be found, on closer acquaintance, to vary from the actual martian state of things; for any martian life must differ markedly from our own." * * * * * in this brief résumé of lowell's work on mars but scant justice has been done to the many novel and convincing suggestions in explanation of the varied features marking the surface of mars. there are many enigmas, however, awaiting solution, if we endeavor to explain them by comparison with the methods pursued by man on this earth, and mr. lowell frankly admits the many difficulties in the way of a clear solution. i have already mentioned how puzzling the checker-board appearance of our western townships would seem to a martian, but this comparison does not help us to understand the so-called gemination of the canals, though we might have parallel sets of canals, as we have parallel lines of railways. the enormous distance which the water travels in the martian canals must presuppose an artificial method of urging it on. precisely how this operation might be accomplished is a question to be solved by the mechanical and hydraulic engineer. beside the doubling, or so-called gemination, of the canals, there are other enigmas in the markings. at certain times there has been observed in the equatorial region of mars a number of white spots, which have greatly puzzled the student of mars and for which no explanation has yet been offered. that they are not clouds is seen in the fact that they do not move or drift. furthermore these white spots are fixed features of the region, as they appear in the same places. it might be suggested that they represent snow-capped elevations or mountain peaks, but this is difficult to believe, as an examination of the terminator of mars reveals no evidences of high elevations. these white spots appear only in mid-summer, which would argue against the idea of their being snow caps, as in mid-summer they would certainly melt and disappear. the time of their appearance coincides with the time of greatest equatorial heat. for a reasonable suggestion it might be offered that these white spots are due to vegetation of some kind. the cotton belt of the south, if one could imagine the cotton bolls a little larger and more crowded together, would make white areas. masses of white flowers, such as the whiteweed or daisy, may be seen covering hundreds of acres of meadow land in new england. i have noticed from the tops of mountains in new hampshire, in july, extensive meadow lands resembling fields of snow from the profusion of white daisies. the blossoming of fruit trees in the santa clara valley, california, whitens the surface for miles. since the appearance of these white spots in mars corresponds with the period of greatest evaporation, it is conceivable that an intelligence in mars might utilize the same method which has been recently adopted in connecticut and porto rico in the raising of tobacco; namely, to protect the fields with white cotton cloth; or, as in florida, where extensive orange groves are covered with white cloth to guard against sudden frost. that this supposition has something to commend it may be seen in the accompanying reproduction of a photograph (plate i), made in porto rico, of tobacco plantations when the fields are covered with white cloth supported on suitable frames. this picture appeared in an article by eugene p. lyle, jr., on porto rico, in the january number of "world's work," to the publishers of which we are indebted for the privilege of using it. these various guesses may all be wrong, as, after all, we are judging mars from conditions belonging to our own planet. this, however, we are compelled to do, as we have no other standards of comparison. [illustration: plate i tobacco cultivation under cloth, porto rico] v testimony of astronomers _that there may be types of life of some kind on mars is, i should think, quite likely._ sir robert ball. in the following chapter are presented abstracts from memoirs, communications, etc., of a few among the many astronomers and observers who have recognized the markings on the planet, and, in many cases, have made drawings of them. before presenting these few brief records, i have compiled, from camille flammarion's great work on mars, the names of those astronomers whose drawings he reproduces in this monograph, for such it is. a brief examination of flammarion's volume will give one an idea of the extent and variety of work which has already been accomplished in interpreting the surface features of mars, and the number of astronomers who have made contributions to the subject. flammarion divides these observations into three periods; the first, beginning with the rude drawing of fontana, in , followed by huyghens, in , cassini, in , and many others up to harding, in . in this period the drawings were rude, though a number of the more conspicuous features were established, and above all, the existence of what was interpreted as snow in the white polar caps. astronomically many points were determined, such as an approximation of the period of revolution, the distance of mars from the sun, the diameter of the planet, its mass, the inclination of its axis, the eccentricity of its orbit, its period of rotation, etc. the second period begins with the remarkable work of beer and mäedler, in and subsequent years. to them belongs the honor of being the first astronomers to make a chart of the planet. an advance standard was set for future studies, and the work which followed revealed details in the surface markings never before suspected. the second period, from to , includes the observations and drawings of beer and mäedler, ; sir john herschel, ; galle, ; warren de la rue, ; webb, ; secchi, ; liais, ; schmidt, ; lockyer, ; phillips, ; lassell, ; knott, ; kaiser, ; dawes, ; franzenne, ; williams, ; proctor, ; lahardeley, ; burton, ; wilson, ; gledhill, ; flammarion, ; terby, ; green, ; trouvelot, ; lohse, ; holden, . the third period extends from to , when flammarion published his book. the following drawings are given: flammarion, - ; paul and prosper henry, ; neisten, - - - ; terby, - - ; van ertborn, ; cruls, ; dreyer, - ; lohse, - - - ; green, ; schiaparelli, - ; maunder, ; konkoly, ; boeddicker, - ; burton, ; trouvelot, ; knoble, ; denning, ; perrotin and thollon, ; proctor, ; perrotin, ; holden and keeler, ; wislicenus, - ; w. h. pickering, ; williams, ; giovannozzi, ; guillaume, . it is impossible to follow these various drawings of mars from the earliest ones of the first period, many of little value, to the slow yet certain advance as seen in the more detailed drawings of the second period, without realizing the gradual improvement of the telescope, coupled with a greater number of observers endowed with better eyesight and impelled by deeper interest in the work. in the third period, culminating with the great work of schiaparelli, and confirmed by the remarkable observations of perrotin and thollon, we see the results of still more arduous devotion to the work; a great advance in telescopes, with better definition, and, in the case of the observations at nice and milan, a steadier atmosphere through which to observe. flammarion brought his work up to . lowell's work on mars, though of a kind with schiaparelli, is, in every circumstance accompanying it, so remarkable that we may well consider the standard now set by him as the beginning of another period; and this period will fix a standard which will consist in securing observers who, in the language of sir david gill, have a special faculty, an inborn capacity, a delight in the exercise of exceptional acuteness of eyesight and natural dexterity, coupled with the gift of imagination as to the true meaning of what they observe. with this standard established, there must also go a perfect telescope for definition, mounted on an elevation a mile and a half or more above the level of the sea, in a region of the clearest and steadiest atmosphere in the world. one cannot help reflecting on these various drawings presented in flammarion's work, and wondering what the results would have been if all these astronomers could have had telescopes as incomparable as that at flagstaff, perched on some high mountain peak with a clear and steady atmosphere continuous for weeks, and, superadded to all these advantages, independent fortunes to enable them to transport their telescopes thousands of miles south when a favorable opposition of mars occurred at a low altitude. the astronomers who have advanced certain theories to explain the markings may be counted as admitting their existence, whatever they may be. among the other astronomers to be referred to are, first, those who admit the markings, and have in all likelihood seen them; second, those who have observed and made drawings of the markings; and, third, those who have drawn them and admit, or at least do not deny, their artificiality. miss agnes m. clerke, an astronomical writer of great merit, who has written a most lucid and comprehensive "history of astronomy in the nineteenth century," says: "the canals of mars are an existent and permanent phenomenon." mr. thomas lindsay, of toronto, read some notes before the astronomical society of that city in regard to the phenomenon of the so-called doubling of the canals and the explanation advanced that it was due to errors in focusing. "it had been stated by several english observers that, by racking the eyepiece within or without the focus, all the phenomena might be produced." in the case of mars, however, he asks: "how is it possible that all the observers had their telescopes unadjusted, and, if any one had, would he not be immediately aware of it?" mr. lindsay thought that the theory was too obviously opposed to the simplest kind of common sense to merit a moment's consideration. mr. john a. patterson, in his presidential address before the astronomical society of toronto, in speaking of mars, said the discoveries rest on the bed rock of scientific evidence; and, after speaking of the supposed spectroscopic evidence that there was no atmosphere in mars, refers to the polar snow caps, their melting, and the lines of vegetation that are supposed to mark the margin of the canals, and he asks: "is it possible that all these may be consistent with no vapor floating above the surface? is it sound philosophy to conclude that the condition of things on our own little world gauges the possibilities and relations that exist in our sister world? dame nature does not turn out all her products in one pattern." mr. denning, in the "astronomische nachrichten," no. , gives the result of his observations on mars in . he says the canals, without doubt, are objective features; changes in the appearance of these markings he attributes to vaporous condensations. one rotation period of the planet satisfies the observation of all the markings, thus proving them to be definite features of the planet's surface rather than drifting vapors such as are seen when observing jupiter and saturn. in spite of these admissions mr. denning, in , while repeating his convictions as to the objectivity of the canals, denied their sharp outline. of the ten canals he drew, eight were discovered by schiaparelli, and two were discovered by lowell. denning observed these lines with a ten inch reflector. schiaparelli compared them in sharpness to lines of a steel engraving. it rests with the reader to judge who is most likely to be correct in his description of the character of the lines--mr. denning with a ten inch reflector, in a poor atmosphere, or schiaparelli and lowell, with a twenty-six and a twenty-four inch refractor, respectively, in a far superior atmosphere. among the many who have seen and drawn the canals comes first, of course, professor schiaparelli, the discoverer of them. it is only necessary to state here that he first detected these enigmatical markings, which he named _canali_, in . in the opposition of , he not only confirmed the discoveries of , but added new _canali_, and for the first time saw the curious process of doubling, or gemination. astronomers in various parts of the world searched in vain for these markings, and despite the exalted character and remarkable work of the distinguished italian in other lines of astronomic research, it was feared that, in this instance, schiaparelli had been the victim of an hallucination. it is true that from the time of huyghens, in , a few astronomers, such as secchi, schroeter, kaiser, and dawes, have detected and drawn a few faint lines which seemed to be identical with the _canali_ of schiaparelli. it was not until , however, that perrotin and thollon with a twenty-nine inch refractor of the nice observatory, first began to confirm the discoveries of schiaparelli, and since that time observers in various parts of the world have detected and drawn these remarkable lines. the cumulative testimony of these men as to the veritable existence of these markings cannot be set aside. it seems strange that nine years should elapse before an astronomer with an interest in the subject, coupled with an acute vision and the patience to observe assiduously, should arise to confirm the existence of these markings, but in another chapter i have called attention to the little interest astronomers have manifested in planetary markings of any kind. it has been shown elsewhere that acute vision, with a clear and, above all, a steady atmosphere, are the chief essentials in making out the markings. it is curious to note the attitude of some astronomers, who, having seen the canals and even drawn them, denied their veritability. their explanations cover "illusions due to the property of light itself, the inability of the eye to maintain its mechanism of accommodation, the behavior of air waves, temporary alteration of the focus of the eye, undetected astigmatism," etc., etc. but, to return to the astronomers who have drawn them. on the unfavorable opposition of , schiaparelli declares that "the _canali_ had all the distinctness of an engraving on steel, with the magical beauty of a colored engraving." he furthermore says: "as far as we have been able to observe them hitherto, they are certainly fixed configurations upon the planet, the nilosyrtis has been seen in that place for nearly one hundred years and some of the others for at least thirty years." in this connection it is interesting to quote from schiaparelli who, until many years after he discovered the canals of mars, had no doubt of their natural origin. as late as , he still considered them natural. in speaking of the canals, he says: "it is not necessary to suppose here the work of intelligent beings; and in spite of the almost geometric appearance of their whole system, for the present we incline to believe that they are product of the evolution of a planet, much as on the earth is the english channel, or the channel of mozambique." this extract may be found in a memoir in "natura ed arte," , page . on page of the same memoir schiaparelli illustrates the elasticity of his mind and a thoroughly unprejudiced attitude by saying: "their singular aspect, and the fact that they are drawn with absolute geometric precision, as if they were the product of rule and compass, have induced some people to see in them the work of intelligent beings, inhabitants of the planet. _i should be very careful not to combat this supposition, which involves no impossibility._" (the italics are ours.) his comparison of the martian lines with the english channel and the channel of mozambique, if he means any resemblance in form and not in the manner of formation, is most unfortunate, for on the whole face of the earth he could not have mentioned surface features more totally unlike any feature of the martian surface, as drawn by him, than these two channels: the english channel, miles wide at its mouth and miles long, tapering to the straits of dover; the mozambique channel, hour-glass shaped, , miles long, and, at its narrowest part, miles wide, and at either end nearly miles wide. had he suggested the red sea, , miles long, or the straits of malacca, miles long, a nearer resemblance to the canals of mars might have been seen, though even here it would be impossible to find their counterparts in mars. these channels are merging with the ocean, are nearly half the width of their length, and enlarge at both ends, while the _canali_ of mars run for hundreds of miles as straight as ruled lines. how slight the resemblance is may be appreciated by comparing the following figure of the earth (fig. ), upon which the red sea, the english and the mozambique channels and the straits of malacca are indicated. [illustration: fig. .] in schiaparelli becomes still more convinced of their artificiality. in his memoir xxv, in the reale academia del lincei, in speaking of the canals, he says: "this whole arrangement presents an indescribable simplicity and symmetry which cannot possibly be the work of chance." in a letter to mr. lowell, dated december , , he writes: "your theory of vegetation becomes more and more probable." mr. a. stanley williams, in the "observatory" for june, , in a paper entitled "notes on mars," described the appearance of certain canals, regions, etc., in great detail. he notices that at the crossing of the canals a little dark spot occurs, a feature, he says, which was first elucidated by professor lowell in . mr. williams also noticed the black streak bordering the northern snow cap, which mr. lowell in his book on mars has interpreted as a body of water resulting from the melting snow. in the quarterly journal of the astronomical society of wales, the rev. theo. e. r. phillips publishes an excellent drawing of mars in color. in this drawing he shows a large number of regions, a number of canals, and other features which, he says, "came out with the clearness and sharpness of an engraving, and bore no resemblance to the 'diffused streaks' or amorphous smudges one sees for the canals in imperfect seeing." in this drawing the polar snow caps show with remarkable vividness. professor w. h. pickering, in a continuous record of observations on mars, published in the "annals of the lowell observatory," records under august : "the dark north canals are also noticeable, and, had they looked as they now do, could not possibly have been missed on the th." dr. phil. fauth has, with a seven inch objective, drawn and published sixty-three drawings of mars in which a great many canals are shown, a list of which he presents in his memoir on the subject. the lamented perrotin, for some time director of the nice observatory, in company with m. janssen, at meudon, observed mars through the great equatorial ( - / inch), and published the results in the "comptes rendues" (vol. cxxiv, no. ). he describes the several zones, the northern equatorial zone "being more particularly the zone of the extraordinary canals, the discovery of which we owe to schiaparelli, and to which we ourselves, by our publication, in , called the attention of the astronomical world." the london "nature," march , , in noting the death of m. henry perrotin, speaks of him as one of the ablest advocates of astronomical science. he devoted much time to mars. "aware that he was working at the extreme limit of visibility, and knowing the tendency for self-deception to creep in and impair the value of such delicate observations, he sought opportunities of making similar measures and records with different instruments, and under varied conditions, in order to remove, so far as possible, the evils of bias and partiality from the results of his researches." dr. terby of louvain, in a memoir entitled "physical observations of mars," a translation of which appeared in the "astronomical and astrophysical journal," no. , identifies many of schiaparelli's _canali_ and other details depicted in schiaparelli's map of mars. in conclusion dr. terby says: "after what we have seen we dare affirm that henceforth the progress of areography will be in the hands of those alone who, freeing themselves from the shackles of doubt, will resolutely engage in the way traced by the celebrated astronomer of milan. a new era has begun in the study of mars by the discovery of canals and their doubling, and by the micrometric determination of one hundred and fourteen fundamental points on the map, an era succeeding to that which was inaugurated a half century ago by the construction of the first two hemispheres and by the approximate fixing of fourteen points by mäedler." dr. terby further says: "but these results have an incontestable value in the presence of the incredulity with which certain astronomers still consider the beautiful discoveries of milan. who would believe it? in spite of the beautiful drawings of m. perrotin one reads still that the discoveries of m. schiaparelli have not been confirmed by the largest instruments." in "astronomy and astrophysics," no. , is published a series of contributions on mars by professors edward c. holden, william h. pickering, c. a. young, lewis swift, george c. comstock, e. e. barnard, and h. c. wilson. all of these men are astronomers and all are connected as directors or observers with various observatories in the united states. many sent sketches, most of them saw the canals, all saw the polar snow caps and darker regions. to say that these astronomers were sketching details which existed only in their imagination is simply preposterous. professor herbert a. howe, director of the chamberlin observatory, at denver, in his "elements of descriptive astronomy" says: "if we have simply to answer the question, 'would a man, as constituted at present, if transported to mars find it possible to exist there?' the most probable answer is, 'no.' while one must not be dogmatic, it may be said, with some assurance, that the man would gasp a few times and die. however, it is conceivable that manlike beings might find a home there." mr. howe could have said without being dogmatic that a man thus transported would die of what is known as caisson disease. among those who assert that the canals are artificial we have professor percival lowell as pre-eminent. he has erected an observatory in the region of one of the clearest atmospheres in the world, has furnished it with the finest telescope that clark ever made, and for the chief purpose of studying the surface features of mars. in his interesting book on mars he has presented the results of his observations in so lucid and convincing a manner that a reviewer of the english edition of the work, in an english astronomical journal, is led to write: "we may say at once that we feel bound to accept these observations as sufficient evidence of the real existence of the markings without expressing an opinion as to what they may be." the reviewer ends by saying: "indeed, there is a subtle deftness in the way mr. lowell deals with his observations which gives the impression that he has been there and seen it all, and it is really hard to say why we cannot accept his conclusions. it is probable, because we are shy to receive new facts at a first statement. in time, no doubt, we shall be willing to accept his deductions (or facts) as to the markings. we were about to advance objections, but they seem poor, and really it is a case where each person must read and form his own ideas--but by all means read." we have already presented a summary of his observations. we may add here, however, an extract from his book on the solar system. in this mr. lowell says of mars: "what we see hints of the existence of beings who are in advance of, not behind us in the journey of life," and again: "life on mars must take on a very different guise from what it wears on the earth. it is certain there can be no man there--that is as certain as anything can be. but this does not preclude a local intelligence equal to, and perhaps easily superior to, our own. we seem to have evidence that something of the sort does exist there at the present moment and has made imprint of its existence far exceeding anything we have left on mother earth." george w. morehouse, in his "wilderness of worlds," says: "taken all together we must regard mars as probably an inhabited world and very similar to the earth." mr. hector macpherson, jr., member of the astronomical society of france, in his interesting book "astronomers of to-day," says, in regard to mr. lowell's book on mars: "he does not ask us to believe anything fantastical or extravagant. his hypothesis has been framed to account for all the various martian features. at present we can only say that it is the most comprehensive and probable theory yet advanced to explain the phenomena of the red planet." professor todd, director of the astronomical observatory at amherst college, in his book on stars and telescopes, in referring to drawings of a region in the southern portion of mars, known as the solis lacus, and a complicated drawing of another region, says: "whether one views this marvellous and intricate system as a whole, or in some portion of high detail, it is difficult to escape the conviction that the _canali_ have, at least in part, been designed and executed with a definite end in view." there are many who do not deny the existence of some forms of life on the planet, but are not prepared to admit the existence of intelligent creatures. sir robert ball expresses himself as follows: "that there may be types of life of some kind on mars is, i should think, quite likely." the number of astronomers above quoted, who have seen and drawn the canals, might be augmented, but a sufficient number have been cited to show that the evidence of the presence of these markings does not rest with a few, furthermore, some of these observers can only interpret the markings as the result of intelligent action. it may be urged that among those quoted are some whose opinion may not have great weight since they are not professional astronomers. one must insist that the study of planetary markings as well as the interpretation of their meanings comes not only within the province of planetary astronomers, but that any broad-minded man, with an acute eye and familiar with the sciences connected with the surface features of the earth, is quite competent to make observations of his own and to judge of the merits of the question. vi the study of planetary markings _their singular aspect, and the fact that they are drawn with absolute geometric precision as if they were the product of rule and compass, have induced some people to see in them the work of intelligent beings, inhabitants of the planet. i should be careful not to combat this supposition which involves no impossibility._ schiaparelli. it is a question whether, after all, the study of planetary markings comes within the province of astronomers. not more, perhaps, than the study of physical geography and subjects connected with the surface features of the earth, comes under the cognizance of those whose profession it is to determine the oscillation of the pole, the earth's movements due to the moon, etc. indeed, these lines of research are strictly astronomical. with the study of the surface markings of the moon, or mars, features of an entirely different kind are to be interpreted, and quite a different equipment is necessary. it is no wonder, then, that astronomers, the most conservative of all classes of investigators, should view with suspicion the results of the work of schiaparelli, lowell and others. immersed in mathematics, trusting in nothing that cannot be measured and reckoned, as a class holding their imagination in abeyance, is it any surprise that they should present an attitude of indifference and even hostility to the work of those who, differently equipped mentally, have attempted a definition and solution of the riddle of the martian markings? to appreciate how foreign to the studies of an astronomer is the interpretation of the canals of mars, one has simply to scan the index of any astronomical publication, or the titles of papers in the transactions of astronomical societies. for example, take volumes xx and xxi of the "astronomical journal" and tabulate the papers, memoirs, etc., therein published, numbering two hundred and thirty-eight, and we find of these, seventy-four on the stars; sixty-two on the comets; nineteen on planets and satellites, mostly mathematical; eighteen on the sun; eighteen on the asteroids; fifteen on eros; ten on polar motion and latitude; four on nova persei; and seventeen miscellaneous, consisting of logarithms, instruments, gegenschein, etc.; and only one on mars, and this on the polar snow caps! as to the question whether it is more important to add another to the thousands of variable stars and binaries, and hundreds of asteroids, already determined, or to consider whether we are alone in the universe and, if so, the significance of it, i think with the intelligent public there can be no doubt. a fair sample of the subjects which occupy the astronomers' mind, and which are so remote from the study of planetary markings, and have so little interest for the public, may be gathered from the following list selected at random from an astronomical publication. notes on variable stars; maxima and minima of long period variables; micrometrical measurements of the companion of procyon; the problem of three bodies; ephemeris of comet a, ; on the eruptive energy of the stars; eclipse cycles; determinations of the aberration-constant from right ascension; theory of a resisting medium upon bodies moving in parabolic orbits; weights and systematic corrections of meridian observation in right ascension and declination; and other titles equally profound. many of these memoirs consist of hundreds of pages of figures, and, as a friend of mine observed, not a column footed up! take for example a title like the following: "method of developing the perturbative functions, also precepts for executing their development." this memoir is accompanied by pages of algebraic formulæ which the layman turns over in despair, the only illumination consisting of a few words in english which render the gloom still more apparent,--such words as "hence," "or," "we therefore have," "if we put." of what we "have," and why we "put," we are left in profound ignorance. now i venture to believe that the great world of humanity takes but little interest in such pages, or in the kinds of titles above given, though fully realizing that they mean something and represent important steps in astronomic research. it would add greatly to the value of these contributions if a brief summary in plain english could be given at the end of these papers, but it is the rarest event that these collectors of data ever make any generalizations, or form any deductions. my faith in the appalling character of algebraic formulæ[ ] received a rude shock when i learned of an experience of louise michel, the anarchist, who was transported for life to new caledonia (afterwards pardoned). on arriving at the savage island, true to her humanitarian instincts, "she immediately established a school for native children, who by a curious freak of their minds, she noted with rejoicing, took naturally to algebra before they learned arithmetic!" hovenden quotes huxley as saying that mathematics "is that study that knows nothing of observation, nothing of induction, nothing of experiment, nothing of causation." he also quotes the words of clerk maxwell, who said, in regard to mathematicians, that it was "doubtful whether the ideas as expressed in symbols had ever quite found their way out of the equations into their minds." they never seem to appeal to the doctrine of probabilities nor do they in any way permit imagination to act as a stimulus to suggestive thought. least of all would a layman ridicule or question the painstaking labor involved in astronomic work, though he cannot see a glimmer of light or intelligence in the enigmatical pages. a certain class of astronomers might take a lesson from an intelligent public in ceasing to scoff and ridicule what they are unable to see themselves in the martian markings. the chief work of these men indicates the cold precise measuring of points of light in the heavens, the determination of orbits, elements and ephemeris of heavenly bodies, the determination of solar parallax, etc., most of the subjects strictly mathematical, a question of careful measurements for which the necessary instruments are at hand, or simply sweeping the heavens for a new variable, binary or asteroid. parallaxes and orbits are matters of measurement to be reckoned by the figures of anybody else. it is obvious from all this that little or no interest is manifested by astronomers in planetary markings, least of all in those of mars. the exasperating feature of the matter is that they persistently repudiate the observation of others equally well equipped, and endowed with the same enthusiasm and devotion to their work. the way in which the gatherers of the raw material arrogate to themselves the science of astronomy, relegating the thinkers and generalizers to the limbo of speculation, is as if the book-keepers of a corporation should assume themselves to be the master-minds of the concern and the banker, or financier, at the head of it, a dreamer not worth regarding. an illustration of the conservativeness of astronomers in regard to planetary markings is shown in their cautious attitude concerning the polar snow caps of mars. here are white polar caps on mars, precisely where they ought to be if they _are_ snow, they wax and wane at the time they should and at no other time, a dark band appears at their borders as the caps in turn diminish in size, which has been interpreted as water due to the melting snow, and no other substance known could possibly reproduce these varying conditions. professor c. a. young, in describing these white areas, says: "the one which happens to be turned toward the sun continually diminishes in size, while the other increases, the process being reversed with the seasons of the planet." after these admissions professor young cautiously says: "these are believed to be ice caps." sir john herschel says: "the variety in the spots may arise from the planet not being destitute of atmosphere and clouds, and what adds greatly to the probability of this is the appearance of brilliant white spots at the poles--one of which appears in our figure--which have been conjectured with some probability to be snow, as they disappear when they have been long exposed to the sun, and are greatest when just emerging from the long night of the polar winter." had michael faraday been an astronomer, how long would it have taken him to pronounce these white polar caps snow and ice? de la rive, in his memoir of faraday, in speaking of his marvellous accomplishments, says: "one may easily understand what must be produced under such circumstances by a life thus wholly consecrated to science, when to a strong and vigorous intellect is joined a most brilliant imagination." tyndall, in his discourse "on the scientific use of the imagination," says: "bounded and conditioned by co-operant reason, imagination becomes the mightiest instrument of the physical discoverer. newton's passage from a falling apple to a falling moon was a leap of the imagination." that herbert hall turner, professor of astronomy in the university of oxford, does not regard the various contributions on the surface features of mars as belonging to astronomical science may be inferred from his interesting book lately published, entitled "astronomical discovery." this book presents to us the history of the discovery of uranus and eros, of neptune, bradley's aberration of light, schwabe and sun-spot period, the variation of latitude, etc., but not a word about the marvellous discoveries of the _canali_ of mars by schiaparelli, so fully confirmed by the observation and drawings of many others, and the great advances made by lowell in the discovery of new features with his lucid and rational interpretation of the seeming enigmas. astronomy, the oldest and most conservative of all the sciences, has been the last to subdivide. already one group of men has justified by its work a division of the science known as astrophysics. the lamented keeler, in explaining the difference between astronomy and astrophysics, said: "astrophysics seeks to ascertain the nature of the heavenly bodies, rather than their positions and motions in space, _what_ they are, rather than _where_ they are." this natural division suggests the propriety of making another division equally distinct, which should comprise the study and interpretation of the surface markings of the planets and satellites, under the name of planetology. the study would be the application to these bodies of the science of geology, in its broadest sense, meteorology, physical geography, geodesy, and related sciences. with the science of planetology established, the student of this science will no longer call to his aid the astronomer, and, least of all, the astrophysicist, nor will he be mindful of their criticism or neglect. he will appeal to the sciences which are involved in the study of the surface features of his own globe, in the interpretation of planetary detail. vii difficulties of seeing _it is contrary to all the analogies of nature to suppose that life began only on a single world._ simon newcomb. for years i had been familiar with different representations of mars in which the surface features had been strongly depicted in black and white; in other words, photo-reliefs, or engravings incorporated with the printed page. i had unwittingly come to believe that these features were equally distinct when one observed mars through the telescope. i had not then seen schiaparelli's original memoir in which his wonderful map presents the canals in light and tenuous lines, which are, however, as clear cut as the lines of a steel engraving, to use his words. for a long time i had hoped for a chance to observe mars through a large telescope in a clear and steady atmosphere. it seemed reasonable to me--knowing nothing about it--that one who had traced out under the microscope delicate lines and structural features in diaphanous membranes, who had, in fact, used a microscope with high powers for forty years, would find it child's play to make out the canals, oases, regions, etc., of mars, as represented in the various publications on the subject. professor percival lowell, of flagstaff, arizona, finally gave me the opportunity i so much desired, and, through his courtesy and kindness, i was enabled to observe mars every night for nearly six weeks through his twenty-four inch refractor, the last and probably the best telescope ever made by clark, mounted in one of the steadiest atmospheres in the world and at an altitude above sea-level of over , feet. imagine my surprise and chagrin when i first saw the beautiful disk of mars through this superb telescope. not a line! not a marking! the object i saw could only be compared in appearance to the open mouth of a crucible filled with molten gold. slight discolorations here and there and evanescent areas outlined for the tenth of a second, but not a determinate line or spot to be seen. had i stopped that night, or even a week later, i might have joined the ranks of certain observers and said "illusion" or something worse. and right here it was that my experience in microscopic work helped me, for, remembering the hours--nay, days--i had worked, in making out structural features in delicate organisms which my unprofessional friends could not see at all, i realized that patient observation would be required if i was to be successful in my efforts. my despair, however, was overwhelming when professor lowell and his assistants, looking for a few moments at the same object, would draw on paper the features which had been plainly revealed to them, consisting of definite shaded regions, a number of canals and other markings, of which, with the utmost scrutiny, i could hardly detect a trace. for the first time i realized that observing fixed diaphanous membranes under a microscope with rigid stand, and within four inches of one's nose, was quite a different matter from observing a brilliant disk , miles in diameter, , , miles away, with an oscillating atmosphere of unknown depth between. night after night i examined this golden, opalescent disk, drawing each time such features as i could convey by memory from the ocular to the drawing table, and, little by little, new features were detected, and to my delight the drawings agreed with those made by the others. since the drawings made by the four observers coincided, it was evident that we had not been victims of subjective phenomena. furthermore, as i discovered afterwards, by comparison, the drawings i made not only agreed with theirs but with those made by other observers, at different times, in other parts of the world. so slow were my acquisitions, however, that it soon became evident that at least months of continuous observation would be necessary before the more delicate markings would be revealed to me. it is interesting to learn that others have had a similar experience. mr. a. stanley williams, of england, in an article entitled "notes on mars" ("observatory," june, ), in stating the difficulties of observation, says: "my eye invariably requires at least two months of continuous observation of a planet before it acquires its full sensitiveness to the most minute details." in this connection it is well to state that mr. lowell began the observation of mars when he was a mere boy. his first telescope, which he still has, was a two and a quarter inch refractor. his observations were made from the roof of his house in boston, and with this small glass he defined the general shaded regions that huyghens had detected and drawn in . since then mr. lowell has observed in turn through a six inch, an eighteen inch of brashear, and, for the last few years, through a twenty-four inch refractor made by clark especially for this work. to refute the accumulated observations of mr. lowell one must have the same acute eye, and a record of the same continuous and devoted study. nothing short of that experience will avail. the jealous derision that has gone up from some observers endowed with less acuteness of vision is neither dignified nor just. were these martian details based upon the observations of lowell alone, one might be inclined to say that some vagary of the mind had led him to imagine these markings which were first detected by the great italian astronomer schiaparelli. up to the present time--to mention only a few--observations and drawings have been made by perrotin, thollon, and flammarion, of france; dr. phil. fauth, of germany; williams, of england; lowell, w. h. pickering, douglass, lampland, and schaeberle, of america, while many others have made drawings of the more conspicuous details. with this record it is impossible to deny the existence of these markings essentially as they are drawn. the difficulty of seeing the more delicate markings of the planet is unquestionable, and an examination of astronomical literature, from which we shall make numerous quotations, indicates only too plainly the acuteness of vision, and the time and care necessary to make competent observations. sir robert ball says, in one of his recent works: "the detection of the martian features indicates one of the utmost refinements of astronomical observations." macpherson, in his "astronomers of to-day," thus writes of schiaparelli, "professor schiaparelli's observations have been distinguished by his keen-sightedness and care. he has taken every precaution to avoid all disturbances resulting from personal equation, and has found it well to adopt the rule (which he here quotes) 'to abstain from everything which could affect the nervous system, from narcotics and alcohol, and especially from the abuse of coffee, which i found to be exceedingly prejudicial to the accuracy of observation.'" what i might have accomplished in the way of seeing had i followed the wise example of schiaparelli i do not know. a not too strict abstemiousness in any of these matters, coupled with long daily walks on the mesa, with its fascinating flora and fauna, found me in the observer's chair every night, somewhat fatigued mentally and physically. sir robert ball, in his "popular guide to the heavens," in describing the difficulty in making out the more delicate markings of mars, says: "it should be understood that in the unsteady air of england it is almost hopeless to expect many of the finer details; not even in the most favorable climates are they to be seen always, or all at once, and much training of the eye is required before it is fit to decide for or against the existence of these details on the verge of invisibility." as another illustration, perhaps, of the difficulties of seeing, sir robert, in the same book, says: "observers of mars are divided into two camps, those who see the canals, and those who do not. the former are in the strong position that they are perfectly sure that they see what they represent in their drawings." from the foregoing it must be evident that not only are the finer markings on mars most difficult to see even under the best conditions but that exceptional acuteness of vision, which few possess, united with long practice, is necessary to make out the tenuous lines which enclose the field of mars like a net. that mr. lowell has had a long and continuous practice, covering years, in observing mars through the steadiest of atmospheres and with a superb glass, is simply a statement of fact. it may be said without fear of contradiction that he has devoted more time to the observation of mars than all the other observers combined. has he then an exceptional acuteness of sight, coupled with indefatigable industry, in the pursuit of this quest to which he is devoting his life and fortune? the following instance will illustrate his marvellous eyesight. we were walking along the shores of a lake some miles from flagstaff, the expanse of shore left by the rapidly evaporating waters abounding with thousands of very small black spiders running hither and thither at our approach. i told him of one i had just seen in which the abdomen was covered with minute young spiders which the mother was carrying about with her--a well-known habit of certain species. this curious fact i had detected only while stooping close to the ground in search of minute shells. mr. lowell, while walking along, immediately began scanning the ground for the trace of a spider with minutely granulated abdomen, and finally exclaimed: "there is one of them!" on stooping down to examine the object it proved to my astonishment to be a female carrying its young in the way already described. this incident revealed a remarkable acuteness of vision to detect, while standing erect and walking, this tiny spider among hundreds of others of its species that were scampering away at our approach. not only is acuteness of vision necessary to one who is to study planetary markings, but of importance also is a clear, and above all a steady atmosphere; and, strange as it may appear, telescopes of moderate size seem to be the instruments with which the best work has been done. it is also true in astronomy, as in warfare, that it is not the biggest gun but the man behind the gun that does the most efficient work. as an evidence of the importance of steady atmosphere professor w. h. pickering, in his observations on the satellites of jupiter, says his work had two important bearings: "first, as showing the relative importance of atmosphere _versus_ aperture for delicate visual observations of this sort. in the same category would be included studies of planetary detail as distinguished from the examination of very faint objects. in other words, if an observer wishes to study very faint stars he must have a large telescope. if he wishes to study the neighboring planets and brighter satellites he may use a small telescope, but he must have a very good atmosphere." the importance of a clear and steady atmosphere, for delicate observation, is known to all astronomers. the rarity of such days, even in our clear atmosphere so superior to that of england, is not generally known. forty years ago dr. henry draper, in an address entitled "are other worlds inhabited?" in speaking of mars and the difficulties of seeing, said: "one of the greatest obstacles to distinct vision is our own atmosphere. its currents and motions tend to confuse the outlines of objects, and, according to my experience, a whole year may pass without the occurrence of more than one good night. the only remedy is to carry the telescope as high up on a mountain as possible, so as to leave below the more injurious portions of the atmosphere. it might be possible to work , feet above the sea in the neighborhood of the equator." i quote these words that the general reader may appreciate the advantages lowell has with his fine telescope south of all european observatories, in the latitude, say of algiers, at a high altitude, and in the dry and steady atmosphere of arizona, with uninterrupted seeing for weeks together, and each night far superior to any night which greenwich could ever be blessed with. professor w. h. pickering attests to the importance of a steady atmosphere in studying the moon from a station in jamaica, when he says that, with a five inch refractor, he was able to detect minute details which were not revealed by the far larger telescopes at harvard university. mr. w. d. barbour, president of the leeds astronomical society, using his four inch achromatic, says: "in one of those brief intervals of atmospheric steadiness i saw distinctly a number of well-known markings," the names of which he gives. dr. phil. fauth, using a seven inch refractor, made sixty-three drawings of mars, showing in wonderful detail the canals, oases, etc. mr. w. j. lockyer, in london "nature," testifies that "a keen and patient observer, sitting at the eyepiece of a comparatively small equatorially mounted telescope, if he makes his observations carefully, and with due regard to atmospheric conditions for good seeing, can do more useful and valuable work than one who has a large aperture at his command and employs it indifferently." mr. e. ledger, in speaking of dawes, who made a remarkable map of mars, says he was justly famed for the remarkable distinctness of his vision; he had detected and drawn a few lines which seemed to be identical to those of schiaparelli. in the authorities above quoted we have endeavored to show that a steady atmosphere, a persistent devotion to the work, accompanied by acute vision, and also a talent for observation, are all the factors needed, not only to confirm the remarkable discoveries of schiaparelli and lowell, but possibly to detect, at favorable moments, new features which have escaped the eyes of these keen observers. at this point we cannot resist giving the words of sir david gill, director of the royal observatory at the cape of good hope. professor s. w. burnham, of the lick observatory, in reviewing a memoir entitled "double star observations at the cape of good hope," quotes as follows from the preface: "sir david gill, in speaking of the routine character of the work involved in the investigation, says: 'there is no instance, as far as i know, of a long and valuable series of double star discovery and observation made by a mere assistant acting under orders. _it is a special faculty, an inborn capacity, a delight in the exercise of exceptional acuteness of eyesight and natural dexterity, coupled with the gift of imagination as to the true meaning of what he observes, that imparts to the observer the requisite enthusiasm for double star observing._ no amount of training or direction could have created the struves, a dawes, or a dembowski. _the great double star observer is born, not made_, and i believe that no extensive series of double star measurement will ever emanate from a regular observatory, through successive directorates, unless men are specially selected who have previously distinguished themselves in that field of work, and who were originally driven to it from sheer compulsion of inborn taste.'" if the reader will substitute the words _planetary markings_ for _double star_ in the above quotation from sir david gill's report, he will understand why we have ventured to italicise certain lines, and will appreciate their significance. in no stronger or truer words could one have emphasized the conditions involved in a critical study of the surface features of mars. in the experience of an astronomer, it is not an unusual occurrence that an object in the heavens, fairly conspicuous, remains unseen until by some lucky chance an observer sweeping the sky picks it up, and, having determined its position, it is promptly found by others. professor h. h. turner, in his "astronomical discovery of the nineteenth century," says: "it is a common experience in astronomy that an observer may fail to notice in a general scrutiny, some phenomenon which he can see perfectly well when his attention is called to it; when a man has made a discovery, and others are told what to look for, they often see it so easily that they are filled with amazement and chagrin that they never saw it before." in the rev. t. w. webb's interesting book on "celestial objects for common telescopes," a reminiscence of the author is given by a friend in which the following is related as illustrating the varying ability of observers in seeing. "a curious instance of difference of vision was well illustrated one superb evening when mr. webb and the writer were observing saturn with the nine and a half inch refractor at hardwick. mr. webb saw distinctly the division in the outer ring which the writer could not see a trace of, while the writer picked up a faint point of light which afterwards turned out to be enceladus (a satellite) which mr. webb could not see." in my brief observation of mars i probably might have made out many more details if i had permitted mr. lowell to tell me what to see, and where to look for them on the disk. this i would not allow him to do, nor did i study any of the numerous drawings in his own work, or the original memoirs of schiaparelli, or other works containing drawings of mars in his library. i would not learn the names of any of the regions, or canals, nor with a single exception do i know them now. only when i had finished my last night's observations, did mr. lowell take my drawings and write out a list of the various canals, oases, etc., which i had made out. thus, unaided, i drew simply what was plainly evident, though many other details flashed out for a second, which were not recorded, simply because i did not see them often enough to be sure of their precise position on the disk. mr. lowell points out one of the reasons why so many observers and astronomers have not seen the canals. in the third volume of the "annals of the lowell observatory" he refers to a certain series of observations of mars, made in , and says: "not only was there no sign of a canal, but even the main markings showed disheartingly indefinite." "this vacancy of expression was due to the martian date." "it was the very nick of time to see nothing, for the part of the planet most presented to the earth was then at the height of the dead season, and in this fact lies the key to much past undetection and present unbelief in the phenomenon of the canals." viii variation in drawing _let us not cheat ourselves with words. conservatism sounds finely and covers any amount of ignorance and fear._ percival lowell. much doubt has been expressed as to the existence of the so-called canals in mars and other surface markings of that planet in consequence of the discrepancy seen in the drawings of the more delicate features by various observers. while in the main a certain general resemblance is seen in the topographical character of the network of lines, and a more close resemblance in the darker markings, notably the syrtis major, the disagreement in the minor details has led certain astronomers to deny their existence altogether, or to insist that most of the markings were subjective, or due to poor focusing, or the result of aberration of the eye or lens. professor simon newcomb, in his "new astronomy for everybody," in speaking of the work of the observers at the lick observatory and the great telescope at their command coupled with favorable situation, says: "it is therefore noteworthy that the markings on the face of mars as presented by barnard do not quite correspond to the channels of schiaparelli and lowell." newcomb also reproduces in his book the drawings of a region in mars known as solis lacus, made by campbell and hussey, and finds they do not show an exact agreement between them. now such objections might have some weight if drawings made by different observers of the solar corona, for example, or the nebula of orion, or the milky way had any close resemblance. as a matter of fact, these various drawings depart far more widely from the originals, as shown by photographic reproduction, than do the various drawings of mars. mr. fison, in his "recent advances in astronomy," in speaking of the divergence in the drawings made by different observers, says: "in inspecting sketches of the delicate details of the corona of the sun made at the same place by different observers, it is difficult to believe that the same object has been represented." to appreciate how widely divergent such drawings are one has only to refer to the united states naval observatory publication on the total eclipse of the sun, july , . [illustration: plate ii drawings of the solar corona by various observers] as an indication of the dissimilarity of the drawings of the corona made at the same instant by different observers, many of whom are well-known astronomers, i may say that the various plates resemble in turn the following objects: a skate's egg-case; a circular battery discharging fire from one side while the smoke drifts away in the opposite direction; an ascidian, known as molgula, with an extra aperture, however; a snowshoe; a radiolarian; a fighting shield of an igorrote savage; an egg of a hair worm; a crushed spider, and other equally dissimilar objects. i have reproduced a few of these drawings (plate ii), that the reader may realize that my similes are not exaggerated. the many drawings which have been made of the nebula of orion, by astronomers of distinction, depart quite as widely from each other as do those of the solar corona. in volume xxv of the "naval observatory observations" is published a monograph of the central parts of the nebula of orion, by professor e. s. holden. he starts with a drawing made by huyghens in and ends with a drawing made by professor langley in . in a summary of the work the author says: "i am acquainted with but one drawing of the nebula which is entirely above criticism, that of the late g. p. bond. he was a skilled artist," etc. an examination of the drawings in this memoir are equally distracting. in looking at them casually they suggest respectively a japanese stocking pattern; an amoeba; an embryo cuttlefish; a plan of boston, and other forms equally divergent. mr. fison, in his book above quoted, writes as follows of other astronomical subjects: "drawings of the milky way as seen by the naked eye have been recently executed by two independent observers, mr. boeddicker and mr. eaton, each drawing the result of long and arduous observation, but in comparing them it is the exception rather than the rule to find any approximation in agreement in respect of the more delicate details." the drawings of the surface features of mars by different observers do vary in respect of the more delicate details, but in every case they represent a map of some kind and do not remind one of a wheelbarrow, baptismal font, or other incongruous objects. these divergent drawings of the same object are not confined to celestial bodies. one has only to examine works on ancient mexican and egyptian monuments, or those of classical archæology, to see the astounding caricatures and perversions. the various drawings of the famous dighton rock inscription, covering a period of two hundred years, are striking examples of the vagaries of an artist. moreover, the text accompanying the drawings often states that they were drawn with scrupulous care. the hieroglyphics are pecked out on the face of a rock in rough lines, half an inch wide and a third of an inch in depth. these marks are in enduring rock; it is the observer and his imperfect drawing which is at fault. the nebula of orion, the milky way, and, for the time being, the solar corona are permanent objective realities and have all been photographed, yet behold the drawings! it is unnecessary to state that the ability to draw varies quite as much with man as the ability to sing. a man may be an excellent observer and yet utterly unable to use a pencil, and any attempt on the part of one to draw who has no ability in that direction results in a fiasco. it is noteworthy that an artist with no knowledge of astronomy, or the art of telescopic observation, will make a more accurate drawing than one made by the best astronomer who has no ability as a draughtsman. concerning the drawings of mars, if one will turn to the "annals of the lowell observatory," volume i, plate xiv, he will there see drawings made on successive nights by mr. lowell and his assistants, mr. douglass and mr. drew, showing a remarkable agreement. after finishing my observations of mars, which covered nearly a complete presentation of the planet, i made a comparison between my drawings and those made by professor lowell and his secretary, miss leonard, and a few made by the assistant astronomers, mr. lampland and mr. slipher, and the agreement was almost absolute, the only difference being that their drawings portrayed additional features which in some cases i had caught a glimpse of but could not fix. i found it exceedingly difficult to draw in the correct positions details within a circle, and particularly when the axis of that circle was inclined some degrees from the vertical, indicated by a spider's thread in the ocular. i think any reasonable man will admit that the divergence seen in the various drawings of mars by different observers cannot be held as an argument against their existence. ix theories regarding the canals _in knowledge, that man only is to be condemned and despised who is not in a state of transition._ faraday. having shown to the satisfaction of any reasonable mind that the delicate lines, known as canals, do exist, it will be interesting to examine some of the theories which have been advanced to explain these markings, as well as some of the absurd deductions drawn from their existence. the late dr. j. joly, professor of geology in the university of dublin, in a paper on the origin of the canals of mars ("trans." royal soc., dublin) came to the conclusion that meteoric bodies, revolving on or near the surface of mars, produced these lines. in brief, he supposed that mars at various times in the early stages of his history, when his rotation period was much shorter, attracted small bodies, which, after whirling about the planet, finally came down on the crust and caused these lines. he conceived of satellites twice the diameter of phobos, or say, seventy-two miles in diameter, flying about mars at a distance of sixty-three miles, which would at this distance, by its attractive force, exert a stress on the supposed thin crust of mars of from fifteen to thirty tons per square foot, and thus rend the surface of the planet in a zone two hundred and twenty miles wide, thus forming two parallel ridges which might be visible to us as double canals. this preposterous idea takes no account of the greater attractive force of the earth, and that it too should have had precisely the same experience, more often repeated. no trace of such behaviors, however, has ever been detected. the moon, too, should have caught some of these heavy bodies, but while conspicuous cracks are seen on her surface, and delicate ridges are seen radiating from the larger volcanoes, not a trace of these great meteoric furrows has ever been observed. it takes no account of the chances--one in a million--that these cavorting meteors should meet at common centres, and if they did, the impossibility that they should stop abruptly and then start off in opposite directions. it takes no account of many of the lines following the arc of a great circle, or what finally became of three or four hundred of these meteors to tally with the number of the canals, unless it is supposed that some of them went whirling around the planet three or four times, changing their courses instantly and repeatedly. indeed, the advancement of such absurd ideas shows the desperate despair of a man who tries to escape the admission that the lines in question may be artificial--and hence the result of intelligence working to a definite end--by a conception as crazy as one might possibly get in a disordered dream. to heighten the absurdity of this theory, if that were possible, mr. j. l. e. dryer, who signs a notice of this paper, while calling attention to the fact that this hypothesis takes no account of the correlation of changes in the canals with seasonal changes on the planet, otherwise soberly says: "it must be conceded that there is nothing in the new hypothesis contrary to observed facts." mr. j. orr, in the pages of the "british astronomical journal," assuming that schiaparelli believed that the canals were excavated (despite the fact that schiaparelli called them _canali_, or channels), and compared them to the english channel and the channel of mozambique--for at the outset he had no doubt of their being natural configurations--proceeds to show the impossibility of an idea that was never entertained. his attempt is as childish and ridiculous as the theory he conjures up. mr. orr, taking it for granted that the only explanation offered for these lines is that they are excavated, concludes that a martian canal, like tartarus, "should be seventy feet in depth (one might ask, why not five hundred or five thousand?) and that the canals of mars would contain , , of our suez canals, and would require an army of two hundred million men, working for one thousand of our years, for their construction," and similar idiocies regarding the population of mars, which he concludes "must be , , , thus showing that all the adult males, and a large number of women, must have been engaged in the great work." in connection with this absurd travesty, let us pause for a moment to consider the extraordinary character of the president of this society before which this paper was read. a man who is the senior assistant of the royal observatory at greenwich, instead of rebuking this balderdash as entirely beside the question, stated as the result of an experiment with a lot of charity-school children, that the canals are merely illusions of the brain, and this in the face of the testimony of a number of astronomers, many of whom are highly distinguished, that the markings do exist. this man seriously commented on the paper by saying: "he hoped that mr. orr's statistical, but nevertheless amusing and instructive, paper might prove one more nail in the coffin of a very absurd idea which had certainly got most undue currency, namely, that the canals of mars could possibly be the work of human agents." equally astounding, too, is it that this nonsense the "astronomical journal of the pacific" republishes without a word of comment. but what could we expect of the mentality of the senior assistant of the royal observatory at greenwich, who, with the great vault of heaven crowded with enigmas awaiting an answer, should waste a particle of gray matter in trying to ascertain precisely where joshua stood when he commanded the sun to stand still so that he could have a little more time for his bloody work. even the day of the month is ascertained; he finds that the date of this murderous affair was about july , and that the sun must have risen exactly at a. m. and set at p. m. the moon, he concludes, must have been about its third quarter and was within half an hour of setting. he could not fix the year, however! fancy all this detail without a word of exegetical criticism, or comment on the precise words of joshua. "and he said in the sight of israel, sun, stand thou still upon gibeon; and thou, moon, in the valley of ajalon. and the sun stood still, and the moon stayed, until the people had avenged themselves upon their enemies." not even a pious query as to why the lord did not shower down a few more meteorites, rather than disarrange the whole solar system. such an attitude of the mind renders one incapable of appreciating anything in astronomic research beyond that which can be measured and photographed. the above is a fair illustration of the intolerable attitude of many of those who deny the existence of the canals, or, if admitting them as existent, resort to every expedient to disprove their artificial character. among the interesting suggestions as to the cause of the lines on mars is that proposed by professor w. h. pickering, who, while admitting that they represent bands of vegetation, believes that they have their counterpart on the moon, and that both are produced by volcanic forces, the cracking of the surface being the result of internal strain and stress. the fissures thus produced permit the escape of water vapor and carbon dioxide, and thus the natural irrigation of these cracks is effected and growth of vegetation follows. this opinion should have great weight, as professor pickering has made a profound study of lunar details, and is one of the foremost authorities on the subject. he has also drawn many of the surface features of mars, and was at one time connected with the lowell observatory. he it was who suggested irrigation to account for the great apparent width of the martian lines. in the "annals of the harvard college observatory," vol. liii, no. , professor pickering presents a study of a crater on the moon's surface, known as eratosthenes, accompanied by drawings and photographs of an area within the crater revealing a few irregular cracks which he thinks correspond to the well-known canals of mars; indeed, he calls these lines canals though he believes them to be cracks. a few spots, probably craterlets, he compares to the oases of lowell. that there is no atmosphere on the moon is admitted by all. professor pickering's keen eye has, however, detected a change in the appearance of these cracks which he attributes to vegetation, animated in its growth by water vapor and carbonic acid gas, as before remarked. in this supposition he may be right, though it seems difficult to believe that so deliquescent an organism as a plant could withstand a variation of temperature from two to three hundred degrees below zero, to one above that of boiling water. one might naturally ask why the greater cracks so conspicuous on the moon's surface, typical examples of which are found in the mare serenitis, mare triangulatis, and surroundings, do not emit aqueous vapor and carbon dioxide, and thus show similar features of widening and change of shade. admitting the correctness of pickering's views, it seems impossible to see any resemblance between this diminutive agglomeration of lines within a lunar crater, and the great geodetic lines sweeping for hundreds of miles across the face of mars. [illustration: plate iii chinese bowl, showing crackle] in the lunar crater, known as flammarion's circle, a most typical branching crack is seen. an examination of these lunar cracks, of which i made drawings through the great telescope at the lowell observatory, showed them to be cracks of the most unmistakable character, paralleled on the earth's surface, by sunbaked fissures. if volcanic forces have caused these cracks in the moon the same kind of energy should have produced the same general results in mars, and circular craters should equally be in evidence, for many of the lunar craters are sufficiently large to be detected were they on mars. they would certainly be indicated on the terminator, and yet not a trace of such markings has been found. it is rather extraordinary, too, that such earthquake fissures on any great scale should not have been filled with trap, silicate, or other injected material. indeed it is strange that such a triangulating arrangement of cracks has not been found on the earth's surface. [illustration: plate iv mud cracks on shore of roger's lake, arizona] in order to pronounce the lines on mars as simply cracks one should study the various kinds of cracks in similar surfaces on the earth. in such a study he would be amazed at the similarity of cracks. when there is a grain in the substance, as in wood, the cracks follow the grain, though even in this material they are discontinuous. in amorphous material they have essentially the same character; whether in the almost microscopic crackle of old satsuma pottery, or huge cracks in sun-dried mud, the areas enclosed are generally polygonal. if the material be of impalpable fineness the edges of the cracks are smooth and clean-cut, as in plate iii, from a chinese bowl; whereas if the material is coarse and pebbly the edges of the cracks are rough and irregular, as in plate iv, from the muddy shores of a lake. cracks arising from contraction never converge to a common centre, and when not connected with another crack they taper to a point. they begin at indefinite places and end in an equally indefinite manner. that there should be a common resemblance in cracks due to contraction is evident as they arise from a shrinking of the surface. the most ancient deposits, millions of ages ago, reveal mud cracks differing in no respect from those found to-day. we subjoin a few forms of cracks from various surfaces, to show their essential resemblance. it will be seen that the cracks in the moon are identical in character to those found on the mesa at flagstaff. they start from some indefinite point, are irregular in outline and end as indefinitely. a poor asphalt pavement offers one of the best opportunities for the study of the formation of various kinds of cracks and fissures. on the edge of a sloping sidewalk one may see the cracks due to a sliding, or lateral displacement of the surface; the effects of subsidence show a number of cracks around the area of depression; the growth of a tree crowding the asphalt shows the effect of lateral thrust, and an enlargement of a root below, or the effects of frost show cracks due to elevation. all these various cracks reveal the same features: they are discontinuous, they begin and end without definition. schiaparelli says in regard to the _canali_ of mars: "none of them have yet been seen cut off in the middle of the continent, remaining without beginning or without end." these lines on the surface of mars, as a writer in "nature" says, are almost without exception geodetically straight, supernaturally so, and this in spite of their leading in every possible direction. it is inconceivable that cracks should be laid out with such geodetic precision. we have seen that cracks have no definite beginning or termination; we have seen that the lines of mars begin and end at definite places. cracks are irregular, vary in width and differ entirely from the straight lines depicted by schiaparelli, lowell, and others. but if we admit them to be natural cracks in the crust we are compelled to admit that the forces implicated in such cracks must have been active many millions of years ago, as mars, being a much older planet than the earth, must have long since ceased to show those activities which the earth, even to-day, exhibits in such phenomena as earthquakes, subsidences, elevations, and the like. now cracks made at that early time in the history of the planet must have long since become filled with detritus and obliterated in other ways, and no evidence would show, even on close inspection, of their former existence, much less at a distance of , , of miles, more or less. [illustration: plate v . pottery crackle inches . mud cracks feet . asphalt cracks inches . earth cracks feet . crater cracks, moon miles . _a._ moon _b._ africa miles miles natural lines cracks, fissures, etc.] in plate v, page , are given six figures representing various cracks and fissures. no. represents the cracks in the glaze of japanese pottery, magnified. no. shows the mud cracks on the edge of a lake, to the extent of two feet. no. is a series of cracks in an asphalt pavement, covering about two feet. no. shows the form of cracks in the surface of a mesa in arizona, the result of the summer heat, the length being about ten feet. no. is a tracing from a drawing by professor w. h. pickering showing cracks in the lunar crater eratosthenes, with an extent of fifty-five miles. the original drawing represented a much greater widening of the lines which professor pickering believes to be due to vegetation. i endeavored to trace the centre of each line and professor pickering said in regard to my tracing: "in one or two instances you have assumed that a crack went through the middle of a broad space, whereas, for aught we know, it may have gone along either edge, but otherwise the tracing obviously follows the outlines of my drawing." it evidently gives a _cachet_ of what appears to be veritable cracks on the surface, and it is interesting to compare this drawing with the cracks in the asphalt. in no. are two drawings; one marked a represents cracks in a region of the moon known as flammarion's circle, the other b represents the great rift in southern africa, probably the most stupendous phenomenon in geological history. this rift has been traced from the valley of the jordan through the dead sea, into the gulf of akaba, thence into the red sea, which it follows the entire length, then turning southwesterly into africa and branching, one branch takes in lake tanganyika, and the other branch lake nyassa. a portion north of nyassa is still problematical. here is a crack , miles long, most of it filled with detritus, water, or forest. it would be an interesting question whether such a fracture would be visible even from the moon. a glance at these various figures will give one a conception of the similarity of cracks, their irregular contour, their indeterminate origin, and ending. cracks arising from shrinkage vary only in the material in which the crack takes place; the conditions resulting from shrinkage or pulling apart are precisely the same. [illustration: plate vi . railroads, illinois miles . streets, montreal / mile . irrigation canals, arizona - / miles . canals, groningen, holland miles . mars, schiaparelli's map . mars, lowell's globe artificial lines railways, streets, canals, etc.] let us now glance at a series of figures on plate vi, page ; their artificial character may be recognized at once. they are all designed for channels or thoroughfares for the transportation of men, merchandise, or water. no. represents a tracing from a railroad map of a county in illinois. the convergence of lines to common centres, and, in one case, parallel lines may be seen. the length of the region represented is thirty-seven miles. no. is a tracing of streets in a district of montreal, covering an extent of half a mile. no. is a tracing of a small region near phoenix, arizona, showing irrigating canals. the larger ones follow contour lines of the surface; the smaller ones are usually laid out in rectangular form to correspond with the original land sections and sub-sections, the boundary lines of which run north and south, east and west. no. represents the canals converging on groningen, holland. no. is a tracing from a hemispherical map of mars made by schiaparelli, and no is traced from a photograph of a globe on which lowell has carefully drawn the canals, oases, etc., of mars covering a land extent of , miles. the remarkable artificiality of all these figures must be admitted. the lines on the first four figures are laid out by an intelligence for similar purposes. no. for the conveyance of passengers and freight; no. for the traffic of a city; no. for the conveyance of water; no. for purposes of navigation, and nos. and , according to lowell's view, for the conveyance of water from melting polar snow caps for irrigation purposes. a simple, rational explanation, as their great width and geodetic precision forbid any other. let one contemplate these lines of mars and compare them with the natural cracks on plate v and he will appreciate the emphatic words of lowell when he says: "the mere aspect is enough to cause all theories about glaciation, fissures, or surface cracks to die an instant and natural death." consider any other possible tracing of lines on the face of the earth as the result of nature's forces, such as river beds, cañons, chasms, fissures, faults, rifts, precipitous valleys, fiords, the results of sharp folds in the strata, parallel chains of mountains, and none of these lines would be straight, none of them would be of uniform width, and few of them would have the enormous breadth of the martian lines, they would begin nowhere and, with the exception of the rivers, end nowhere. this definition holds good as the result of natural forces from the microscopic crackle on a dinner plate, to a crack in the earth's crust fifteen hundred miles long. having briefly alluded to some of the theories advanced to explain the geodetic network of lines encircling mars--theories in one case so puerile, and in another case an interpretation so monstrous, though endorsed by astronomers of standing--we turn to the suggestion that these various lines are artificial, that they were designed for a definite purpose, namely, to conduct water from those regions alone where water is found for the purposes of irrigation. we shall call attention to a parallel case where the great ice caps and glaciers of the himalaya mountains supply water, by their melting, for thousands of miles of irrigating canals. let us ask ourselves whether if the snows of the himalayas gradually failed, the crowded millions of india would not if necessary reach out to the farthest north for this precious fluid? our great centres of population at the present time are reaching out in every direction for water supply. how long would it take new york city to decide in case of water famine to tap the great lakes to the north, or to establish pipe lines to the north pole, if it were necessary to go that distance for water? from the foregoing it is seen that the question of water supply has engaged the energies of man from pre-historic times. these great irrigating works are found, however, in regions of sterility, or light rainfall, from the rude irrigating canals of ancient peru and arizona to the marvellous accomplishments of the hydraulic engineer in india and egypt. this demand for more water is not, however, confined to regions of sterility, the reaching out of cities for supplies of water for potable purposes and for the wasteful disposal of sewage was inevitable. what shall we say, however, of the notes of warning in regions of rain? england is considered a land of humidity and copious rains, and yet the alarm is already sounded that in the no distant future an appalling catastrophe may threaten her in the failure of her water supply. in a special despatch to the "new york herald," mr. bently, president of the royal meteorological society, is quoted as saying at its annual meeting, "so enormous now is the drain upon the country's available supplies, so much have the growth of cities, the disappearance of forested areas, the extent of street surface impervious to moisture, and the diversion of the rivers, lakes, and other natural fresh water reservoirs from their natural function of irrigators and distributors of the all essential moisture to the land interfered in england with nature's arrangements, that english engineers and meteorologists at no distant date may find a task of almost insuperable difficulty awaiting their endeavors." dr. mill, a rainfall expert, on being consulted by a "daily mail" correspondent regarding this alarming statement, was of the opinion that the question would require early consideration. we quote his words as follows: "legislation is needed in the immediate future for the regulation of the rivers. the great question is how to store the water which at present runs to waste on the coasts." "the planting of trees on the high water-sheds is one of the first solutions of the problem. the chief difficulty lies in the scarcity of suitable land available for building large reservoirs, and at some future date the services of engineers will be required to reform the present arrangement of reservoirs." "in austria the government issues an annual report on the condition of the danube and detailed statistics of the rainfall, with a view to storing all the available water supplies. the work done by the austrian government i am doing in regard to the british isles on my own responsibility, but the rainfall and the river conditions are only a portion of a much larger problem." the above quotations indicate that even now an alarm is felt in countries of fair rainfall regarding the possible failure of the water supply in the near future and is perhaps a premonition as to what may be absorbing our energies in centuries to come. such possibilities as here suggested may offer an additional clew to an interpretation of the martian markings. the unnatural straightness of these interlacing lines on mars, many of them following the arcs of great circles, their uniform width throughout, their always starting from definite areas, their convergence to common centres, and their varying visibility synchronizing with the martian seasons finds no parallel in natural phenomena. if in the mind's eye we were to survey the earth from mars the only feature we should find at all paralleling the lines in mars would be found in the level regions of the west, where, for thousands of miles, the land extends in vast level stretches. in these regions would be found lines of railroads running in straight courses, starting from definite places, converging to common centres, their sides, in certain seasons, conspicuous with ripening grain fields, or again the work of the united states reclamation bureau running its irrigating canals in various directions through that great region. both these kinds of lines would be artificial and both designed for purposes of conveyance--in the one case, merchandise and passengers, in the other case, water. if the martian lines are not artificial some other theories must be offered than those thus far advanced to explain their origin and purpose. the phenomenon of the extraordinary doubling of the canals when first announced was immediately disbelieved; when, however, other observers confirmed schiaparelli's discoveries, and it became evident that these double lines had a veritable existence, the phenomenon was regarded as an evidence that profound physical changes were going on in the planet. thus in mr. stanislaus maunier, in "la nature,"[ ] in alluding to the remarkable discovery of the doubling of the canals, says: "mars at this moment is the theatre of phenomena of stupendous grandeur which will be adequate in a few years to impress profound changes in its aspect." this was written in , and continuous observations of the planet since that time have shown no profound changes, or changes of any kind beyond those which periodically occur with the seasons. since mars is a much older planet than the earth, it seems reasonable to believe that it is more stable, that volcanoes and earthquakes have long ceased to manifest their activities, that erosive action by water is no longer in evidence, subsidence and elevation of continental areas no longer occur. from this condition of the planet it is impossible to believe that the curious phenomenon of the doubling or gemination of the canals can be due to any physical changes now taking place. schiaparelli said that many of the ingenious suppositions advanced to account for this doubling of the canals would not have been proposed had their authors been able to examine the gemination with their own eyes; he further says: "it is far easier to explain the gemination if we are willing to introduce the forces pertaining to organic nature; here the field of plausible supposition is immense," and in this field of suppositions he suggests "changes of vegetation over vast areas." let any intelligent mind soberly consider this rational suggestion of schiaparelli's and compare it with other theories that have been advanced, and he will be compelled to admit that vegetation alone gives us at least a clew to the extraordinary behavior of these parallel lines. to understand the symmetry, the suddenness, and the vast extent of this phenomenon, the further explanation of vegetation superinduced by artificial methods will alone complete the answer. sir robert ball cannot conceive how mars, a much older planet, should develop synchronously with the earth creatures of intelligence, an event which he insists should have occurred ages earlier in its history. in this supposition he is quite right, for if there are creatures of intelligence in mars these should have appeared much earlier, and that is probably what has happened. the problem is one parallel to that urged by sir boyd dawkins in regard to the evidences of man in the tertiary rocks. dawkins argued that since the mammals in the tertiary had changed so profoundly, many types becoming extinct, if man had lived at that time he also should have been affected by the same influences, and should have changed accordingly. it has been clearly pointed out by cope and others that the moment intelligence became a factor in natural selection it was seized upon to the relative exclusion of physical characteristics, hence but little change, otherwise than an intellectual one, has taken place in man since his progenitors took to the trees and made up by agility, cunning, and alertness what they lacked in physical strength. in the same way, if, in the past history of mars, an intelligent creature appeared he must have survived under precisely similar conditions, and long after favorable environments had passed that were implicated in making him what he was. admitting that there is an intelligent creature of some kind in mars, is it reasonably conceivable that he should have caused such changes in the surface features of that planet as to be visible from the earth? professor newcomb concludes, in a recent article in "harper's magazine," that "we cannot expect to see any signs of the works of inhabitants in mars, if such exist." let us, however, reverse the proposition and ask ourselves if man has been implicated in any changes in the surface appearance of the earth that would be visible from mars? and i think the question can be answered in only one way. the vast cities such as pekin, tokio, london, and new york, with their great expanse of tiled and slated roofs, and sterile streets, would certainly have a different albedo from the grass and trees in the immediate outskirts of such places. the tracts of land reclaimed from the sea, and still more the enormous areas which have been rendered green by irrigation, must, of all contrasts, be markedly conspicuous. to realize the extent of this work, it is only necessary to state that in egypt , , acres depend upon irrigation, and this area to be vastly increased in a short time; the western states of america with , , acres, and this area being rapidly augmented by the work of the united states reclamation bureau; in india , , acres under irrigation, and this being continually added to; above all, however, the vast extent of territory from which the dark forests have been removed in this country, and more particularly in china, must make a visible landmark. if one can recall the appearance of forests in the southern and middle part of maine, say from bethel or bangor, fifty years ago, he will remember that from the top of any hill a stretch of dark blue forest was to be seen as far as the eye could reach, and now from the same elevations one can see only an occasional clump of blue forest, while the remaining surface is, according to the season, either bright green, yellow with ripening vegetation, or white with snow, out of which the dark clumps of forest growth are most conspicuous. considering the contrasting colors in one year covering hundreds of thousands of square miles in various portions of the country, the question naturally arises which of these contrasts would be most conspicuous,--the colors just mentioned of solid land surfaces of vegetation, snow, and desert, or diaphanous clouds with their gray shadows. we are told that jupiter, with the mean distance at opposition of nearly , , miles, shows its clouds, its red spot, and the shadow transits of its satellites. surely if these conditions are seen from the earth, the changes in the earth's appearance above described might be seen from mars, which at its nearest opposition is only , , miles away, and, conversely, any change of similar character in mars would certainly be visible from the earth. x comments and criticism _nothing is more difficult and requires more caution than philosophical deduction, nor is there anything more adverse to its accuracy than fixity of opinion._ faraday. it will be of interest to examine the writings of certain astronomers, and writers on astronomy, to appreciate the unreasonable conservatism, not to say narrow-mindedness, which color their opinions. it ill becomes students of science to ridicule the honest and persistent labors of such men as schiaparelli, lowell, perrotin, and others, unless they can show an equal devotion to the work. they do not recall the deluge of essays, reviews, and sober treatises which followed darwin's great work, viewing the evidences of darwin not thoughtfully, nor based upon any knowledge of the subject, but with contempt, and, in many instances, with vituperation. so rapid, however, was the recognition of darwin's interpretation of nature's facts that most of these writers lived long enough to see their protests entirely discredited, or to become enthusiastic advocates of the theory. in their own domain of astronomy these writers are equally forgetful of the earnest and even bitter controversies regarding the demonstration by chandler of the oscillation of the poles, and consequent variation of latitude, and the final establishment of chandler's views, in the teeth of opposition, by the greatest astronomers. the character of this irrelevant and adverse criticism may be appreciated by subjoining a few examples. the most amazing of all these expressions is to be found in the report of the british astronomical association, for . it seems that a committee had been appointed by the association to report on the surface features of mars. e. walter maunder was made director of the committee. twenty-six observers, of whom twenty-one were inhabitants of great britain, sent in the result of their work accompanied by drawings. a summary of this work was published in the form of memoranda accompanied by a mercator projection map of mars, individual planisphere drawings, as well as colored plates; these together represented twenty-eight single canals, five double canals, nine oases, as well as the dark regions so long familiar to astronomers. this was a somewhat remarkable contribution considering the complaints from the different observers in regard to the weather, and the prejudiced, and negligent part played by the man at the helm. that i am not unjust in these statements may be understood by quoting from the report showing the conditions under which the english observers labored, the delinquent part which mr. maunder, the director, played in the matter, and the conclusions which mr. maunder arrived at after this unsatisfactory performance. he says: "the opposition of proved on the whole a very disappointing one. although mars at opposition was almost at its nearest approach to the earth, it was far from being well placed for observation by european astronomers owing to its great southerly declination, and consequent low altitude.[ ] the weather during the autumn of was for the most part very unfavorable for observation of so difficult an object, and several members who joined the section at the beginning were unable to contribute either drawings or report." now i beg the reader to carefully note the part the director played in this important work. here are his words; there is no need of italicizing them. "none of the few evenings which the director was able to give to the examination of the planet was really suitable for the purpose, and as the pressure of other duties rendered it impossible for him to supply any detailed help to the members, the section was at a very serious disadvantage." he certainly is frank enough to state the disadvantages the section was under with such a man at the head. realizing the conditions of seeing in the fog and soot-begrimed atmosphere of england, the low altitude of mars, and the loss to the committee of the assistance which a director might have given to the work had he been able to approach the subject in a broad and unprejudiced manner, one is naturally led to ask what this committee would have accomplished if each member in turn had had an opportunity of observing mars at a high altitude with a twenty-four inch refractor of remarkable definition, at an elevation of , feet above the sea-level, in an atmosphere so clear and steady that stars of the third and fourth magnitude may be seen to set at the horizon line. mr. maunder in speaking of the nomenclature used in his report says, "the term 'canal' has also been retained, though 'canals' in the sense of being artificial productions, the markings of mars which bear that name, are certainly not. it is difficult, indeed, to understand how so preposterous an idea obtained currency for a moment even by the most ignorant." it is impossible to repress one's amazement at these expressions after the confessions he makes as to his official functions on the committee, and i appeal to any honest and unprejudiced mind if a more incompetent person of the class to which he belongs could have been found in england for the directorship of such a body. in this connection we cannot refrain from giving a few paragraphs from a paper entitled "can organic life exist in the planetary system?" by c. a. stetefeldt. the author says: "we must, however, acknowledge that if other suns in the universe have planets--and there is no reason why they should not--many of them may present physical conditions identical with, or similar to, those existing on the earth, and that therefore their organic life may be similar to our own. further, i am far from denying that, under favorable circumstances, creatures may be evolved upon planets which revolve around other suns, whose mental capacity is as much superior to man's as that of the latter is to the lowest form of vertebrates." having made these liberal admissions in regard to the universe at large he attempts to show that none of the planets outside the earth could sustain life, and finally closes in this extraordinary manner: "in concluding this investigation we cannot help admiring the inductive acumen of the theologians who considered the earth the most important of the planets, and the centre of creation. although their opinions were not based upon scientific facts, they _arrived at the truth nevertheless_." (italics ours.) familiar as every one is with the attitude of theologians for the last several centuries concerning astronomical discovery i think it may be safely said that this is the first instance on record where they have been credited with an induction not based on observed facts worth quoting in an astronomical paper. and this contribution also appeared in the publications of the "astronomical society of the pacific," volume vi, no. , without a word of comment! how different was the behavior of the "journal" when a report of percival lowell's lecture on mars, written by dr. edward everett hale, was reproduced in its pages. the following comments were made by edward s. holden, then director of the lick observatory: "something is seen, no doubt, but i may add that nothing has been observed at the lick observatory during the years - , so far as i know, which goes to confirm the very striking conclusions here described." it may be added that during the years - nothing was seen of the fifth moon of jupiter. the discovery of this satellite with the lick telescope was not due to any special efforts on the part of the director. the rev. e. ledger, "nineteenth century magazine," volume liii, , p. , in an article entitled "the canals of mars--are they real?" presents an excellent account of the successive observers of mars, and the results of their work, and the objections of those who could not see the canals, or saw them imperfectly. he recalls maunder's childish experiments, and is greatly impressed by them. he then says: "astronomers are no doubt very well acquainted with the laws of optics as applied to the eye. they have made, and may yet make, many experiments connected with their action. they are accustomed to allow for individual peculiarities in observation, as, for instance, when what is termed personal equation affects the rapidity with which different observers touch a key to record what they see. they may therefore skilfully judge of the effect produced in observations of mars by such processes of the eye, or of the brain, or nervous system as i have referred to." he strongly thinks it would be well "if some skilful nerve specialist and oculist could work in conjunction with some of these practised observers who have seen the canals. they might both assist in observing, and at the same time carry out careful researches into the optical delusions which brain or eye may experience in connection with telescopic observation." this is certainly a happy thought of the reverend author, only it would seem in this case that a larger and more diversified corps of specialists, including alienists, is needed to attend to that class of astronomers who are suffering with mental strabismus. it might be advisable to call in the services of a bacteriologist to make cultures of new forms of microbes which may be involved in rendering a man incapable of estimating the value of evidence. it is the exception rather than the rule in astronomical science that one finds such unfounded and prejudiced utterances as those above commented upon. the glamour of astrology still lingers, in the public eye in its respect and awe for the astronomer's work. every eclipse seems in the nature of a prophecy. the public contributes liberally for the support of eclipse expeditions, observatories, and the like, and these contributions would be still more liberal if the public could realize the profound significance of the researches now being carried on by director pickering at harvard, director campbell at lick, director hale at the solar observatory, mount wilson, and many others. their observations are received without question. the thoughtful man would only ask that like credence should be given to the work of every earnest student unless disproved, even though the field of investigation covers regions hitherto but little explored, and yet of the very greatest interest to the human race. xi atmosphere and moisture _if in any planet we could detect the traces of vegetable life it would at once be a strong argument for the existence of animals there and vice versa._ henry draper. schiaparelli points out that "the polar snows of mars prove in an incontrovertible manner that the planet, like the earth, is surrounded by an atmosphere capable of transporting vapor from one place to another." mr. e. e. barnard, in the "astrophysical journal," volume xvii, no. , in speaking of the polar caps, says: "there seems no definite proof that they are not as much ice and snow as that which we have to deal with in our own terrestrial winters. so much is at least suggested by the great seasonal changes they undergo from winter to summer. there seems to be a general belief now that mars certainly has an atmosphere. this atmosphere seems to be very much less than our own, and yet it is of sufficient density to produce the phenomena of the polar caps by condensation and evaporation and also to produce, though rarely, some form of clouds." among those who have claimed to have established the existence of water vapor in mars by the spectroscope are rutherford, secchi, huggins, janssen, and vogel; and these declare the existence of a martian atmosphere similar to our own in composition. mr. campbell can find no spectroscopic indication of an atmosphere charged with water vapor. lewis e. jewell says: "the spectroscopic proof of the presence of a fair amount of water in the atmosphere of mars must be regarded as unattainable." professor lowell, despite the aid the admission of water vapor in mars would give to his position, also doubts whether the spectroscope is able to detect the evidence through our own moisture-laden atmosphere. after a minute and exhaustive study of the polar snow caps by the combined observations of lowell, douglass and w. h. pickering, mr. lowell says: "it is interesting that the cap should so simply tell us of these three important things: the presence of air, the presence of water, and the presence of a temperature, not incomparable with that of the earth." seasonal changes on mars have long been recognized and admitted by astronomers, and these changes are on so vast a scale as to be distinctly visible from the earth. without an atmosphere the surface of mars would be inert. schiaparelli was the first to notice that at successive oppositions the same regions showed different degrees of darkness and accounted for these variations by seasonal change. mr. denning believes that certain changes in the appearance of the markings to be due to vaporous condensations. sir norman lockyer believed he saw the obscuration of a large region by clouds, this obscuration continuing for some hours. a bright spot on the terminator of mars, discovered by douglass at the lowell observatory, and which led to the newspaper excitement that signals were being made, was seen to move and finally disappear and its appearance, drift and disappearance is interpreted by lowell as a cloud illuminated by the sun and carried along by the wind. the presence of clouds, judging from my own brief experience, was certainly suggested at times by the peculiar way in which a large region known as syrtis major disappeared and flashed out again. this behavior might be expected of the tenuous lines as a result of refraction and other disturbances in our own atmosphere; when, however, a large, dark region at one time stands out firm, clear and sharp-cut as the stroke of a japanese brush, then gradually fades out and remains obscure for some time we are inclined to believe that sir norman lockyer's interpretation is true and that in such a case drifting clouds or sudden vaporous condensation produced the obscuration. from an article on mars by sir robert ball, republished in the "annual report of the smithsonian institution" for , we quote the following: "the discussion we have just given will prepare us to believe that a planet with the size and mass of mars may be expected to be encompassed with an atmosphere. our telescopic observations completely bear this out. it is perfectly certain that there is a certain shell of gaseous material investing mars. this is shown in various ways. we note the gradual obscuration of objects on the planet as they approach the edge of the disk, where they are necessarily viewed through a greatly increased thickness of martian atmosphere. we also observe the clearness with which objects are exhibited at the centre of the disk of mars, and though this may be in some measure due to the absence of distortion from the effects of foreshortening, it undoubtedly arises to some extent from the fact that objects in this position are viewed through a comparatively small thickness of the atmosphere enveloping the planet. clouds are also sometimes seen apparently floating in the upper region of mars. this, of course, is possible only on the supposition that there must be an atmosphere which formed the vehicle by which clouds were borne along. it is, however, quite obvious that the extent of the martian atmosphere must be quite insignificant when compared with that by which our earth is enveloped. it is a rare circumstance for any of the main topographical features, such as the outlines of its so-called continents, or the coasts of its so-called seas, to be obscured by clouds to an extent which is appreciable except by very refined observations." professor w. h. pickering made seven photographs of mars on april , and within twenty-four hours made seven additional photographs of the same region. the second series of photographs showed an area of white extending from the polar snow cap far down toward the equator, covering a surface which he estimated to be as large as the united states. it afterwards slowly disappeared. how shall we account for this sudden apparition of a vast area of white which the photographs of twenty-four hours before did not reveal. a boy of ten, as well as the philosopher would simply say a snow-storm had taken place in mars. is it, then, unreasonable to picture whirling snowflakes, snow-drifts, and dazzling whiteness from the sun's rays, and in the rapid melting of the snow, broad rivers and turbulent brooks with water areas frozen at night? but why should we be compelled to imagine as naked the surface through which these waters find their way? soil there must be from the continual erosion of running water. the character of the rock exposures we cannot guess at, but a picture of bare rock and lifeless ground is unthinkable. such wide-spread storms without an atmosphere could not occur. the seasonal appearance of these snows and their slow disappearance not only indicates an atmosphere, but an atmosphere disturbed by established currents which convey the moisture-laden air to regions of congelation. a number of observers who have detected clouds in mars described them as being yellowish in color. what more probable than that these yellowish masses are simply dust-storms such as one may often see whirling along over our american deserts? when the gusts of wind are fitful like squalls at sea, the obscuration would be fitful, to clear up again. the vast areas of desert land in mars renders this supposition very probable. since the above was written, my attention has been called to an early "bulletin of the lowell observatory," in which mr. lowell, in discussing the appearance of a certain large projection on the terminator of mars, says: "finally, its color leads me to believe it not a cloud of water-vapor, but a cloud of dust. other phenomena of the planet bear out this supposition." xii notes on irrigation _your theory of vegetation becomes more and more probable._ schiaparelli in a letter to lowell. let one stand on some peak of the verd mountains, northeast from phoenix, arizona, overlooking the gila river as it follows its course across the desert, and after the river is lost to view he will notice that the foliage along its banks marks its course. if one takes this view in winter time, the uniform gray of the plains, unbroken by a single shade of color blends with the light blue of the distant plomas and castle dome mountains on the southwest horizon. in the early spring when the water is first let into the irrigating channels with their innumerable divergent ditches, a shade of green may be seen emerging from the monotonous yellow-gray of the hot and sterile plain, first conspicuous near the source of the water supply, and then following along to phoenix, tempe, and other regions till in full efflorescence these cities stand out like great green carpets spread upon the earth. from this mountain top not a trace of an irrigating ditch, large or small, would be discerned, except here and there a glint of reflected sunlight, but the effects of the life-giving waters can be traced in broad bands to the remotest limits of the water channels, when they would end as abruptly as they had begun. if we examine railroad maps, the lines of which represent the road-beds utilized to convey passengers and freight to various places, we shall observe that in mountainous regions the lines run very irregularly, often paralleling mountain chains, or following rivers. on level areas such as iowa, texas, and other states, the railroads run for hundreds of miles in straight lines, at times converging towards large centres of population. their occasional parallelism and radiation from centres, all present a certain _cachet_ in angles of approach and alignment that reminds one strongly of similar features in the markings of mars. if each railroad were bordered by a wide growth of trees with sterile desert between, these broad bands as seen from mars would be identical with the appearance of similar lines in mars as seen from the earth. in mars, however, there are no high elevations since the terminator of mars stands out clear cut and not jagged as in the moon. the planet being devoid of hill ranges, and large oceans, the canals can run in straight lines for hundreds of miles. if it were possible to conceive by analogy a creature on mars furnished with a telescope, he would undoubtedly correlate the irrigating regions of arizona as similar in nature to his own canals. the irregularity of the rivers running through such regions would puzzle him quite as much as we are puzzled by the absolute straightness of the martian canals. he would, of course, observe that in our winter the irrigating areas became invisible, to appear again as our summer advanced. his own experience of vegetation arising from irrigation alone and starting from the north when the first water from the melting snow cap animated the growth of plant life, and proceeding slowly towards the equator would prevent him from understanding the reverse condition on our planet, with the shade of green being perennial at the equator and spreading slowly north with the advance of summer. the marvels of irrigation are impossible to conceive of without first seeing a parched land before the water channels are dug and the exuberant vegetation springing with the water's advent. the illimitable stretches of arid plain, no green, rarely an evidence of life, and then usually in hideous shapes like the hissing and purple-mouthed gila monster; hot pale dust; blinding sunlight; ragged clumps of gray sagebrush, rebuking by their hopeless color and dishevelled appearance, the intolerable condition of their existence; angular cacti, surviving because of their vicious needles, and then literally a step only from this sterile waste, and one finds himself wading through rich, soft alfalfa, under the deep shade of cottonwood trees, glistening threads of water when the overhanging vegetation does not hide the channels, brilliant flowers, singing birds, fat cattle and vociferous children. in this apparently irreclaimable desert of arizona, have sprung up prosperous cities, great farms and fruit orchards. about phoenix, more than one hundred and twenty-five thousand acres are under the richest and most profitable cultivation, and all due to a little narrow canal which conveys the water from salado river, and distributes it by narrow ditches, so narrow, indeed, as to be invisible except on the nearest approach. there have already been constructed in the gila valley alone, two hundred and fifty miles of ditches, and four hundred miles of parallels. mr. ray stannard baker, in the "century" for july, , presents in a graphic way, the marvels of irrigation. major j. w. powell, during the later years of his life devoted his whole time and energy to urging the reclamation of desert lands in the west by irrigation. in his reports on the subject he estimated that a region equal in size to new england, new york, pennsylvania, and west virginia could be recovered from the desert sands of arizona and other regions in the west. in india, millions of pounds have been spent for irrigating canals and ditches. a single canal with its tributaries drawing water from the ganges measures , miles in length, bringing into cultivation one million acres of land at an expense of fifteen millions of dollars. the idea of irrigation is not due to the advanced intellect of man; it has been the result of dire necessity and is of great antiquity. mr. frank hamilton cushing discovered evidences of the most extensive irrigating canals among the ancient pueblo indians of arizona. sir c. scott moncrief, in his address as president of the engineering section of the british association for the advancement of science, describes the various forms of irrigation. the primitive method consists in raising water by human labor. early egyptian sculpture depicts laborers raising water by means of buckets, and along the banks of the nile the same method may be seen to-day. other methods of raising water are by pumps driven by windmills. in certain regions artesian wells furnish water for irrigation. the importance of irrigation is best shown in the fact, that, while the rainfall in cairo is, on an average, one and four tenths inches a year, yet in the immediate neighborhood land brings $ per acre; this value being due to irrigation alone. in speaking of water storage for supplying the irrigating canals the author says: "when there is no moderating lake, a river fed by a glacier has a precious source of supply. the hotter the weather the more rapidly will the ice melt, and this is just when irrigation is most wanted." (judging from this dictum, the condition in mars is ideal.) in speaking of the great assouan reservoir in egypt, he says: "the sale value of land irrigated by its waters will be increased by about $ , , . the increase in irrigation areas in our western states may be appreciated by the following figures. in it amounted to , , acres; in , to , , acres. now it is at least , , acres. without irrigation this land sold for four or five dollars per acre; with irrigation it brings forty dollars per acre. xiii variety of conditions under which life exists _not only does life but intelligence flourish on this globe under a great variety of conditions, as regards temperature and surroundings, and no sound reason can be shown why under certain conditions which are frequent in the universe, intelligent beings should not acquire the highest development._ simon newcomb. the argument most often urged against the idea that life exists in mars is that there is no atmosphere in that planet, or if there is one it is so rarefied that it could not sustain life as we know it. according to proctor, we have heretofore been led to consider the planet's physical condition as adapted to the wants of creatures which exist upon our own earth rather than to ascertain the conditions which might obtain to enable life to exist on the surface of other planets. it is highly probable that if an air-breathing animal of our earth were instantly immersed in an atmosphere as rare as that of mars, it would perish in a short time. precisely what a species through thousands of generations of selection and survival might adapt itself to, is an open question. leaving this contention for a moment, let us consider the almost infinite variety of conditions under which life exists on our globe, and we shall find that any and all conditions which the surface of mars may offer, if experienced gradually through successive generations, would not be inimical to terrestrial life from the lowest to the highest, including even man. mr. garrett p. serviss, in discussing the question of life, in his book "other worlds," said: "would it not be unreasonable to assume that vital phenomena on other planets must be subject to exactly the same limitations that we find circumscribing them in our world? that kind of assumption has more than once led us far astray even in dealing with terrestrial conditions. it is not so long ago, for instance, since life in the depths of the sea was deemed to be demonstrably impossible. the bottom of the ocean, we were assured, was a region of eternal darkness and of frightful pressure, wherein no living creatures could exist. yet the first dip of the deep-sea trawl brought up animals of marvellous delicacy of organization, which, although curiously and wonderfully adapted to live in a compressed liquid, collapsed when lifted into a lighter medium." one has only to make himself familiar with the wide range of conditions under which life in various forms exists on the earth, to realize that the introduction of martian conditions here would not be such an overwhelming calamity, and if these conditions could be introduced by minute increments covering thousands of centuries, it is not unreasonable to believe that myriads of forms would survive the change, and among those that survive would be precisely the kinds that thrive under the most diverse conditions here--namely, man and the higher hymenoptera, the ants. to enumerate, in the broadest way, the variety of conditions under which life exists here, one has only to enumerate creatures living in the deepest abysses of the ocean; high up on the slopes of the himalayas; swarming in arctic seas; withstanding the hot glare of a tropical sun; living deep in the ground; breeding in the darkest caves; flourishing in desert regions; thriving in water below freezing, and again in water nearly at the boiling point. professor jeffries wyman, in a memoir on "living organisms in heated water," has collected data showing that fishes are found living in water ranging from ° to ° fahrenheit. he also found that low forms of plant life exist in water of various temperatures as high as: ° f. observed by dr. hooker in sorujkund; ° " " " capt. strachey in thibet; ° " " " humboldt in latrinchera; ° " " " dr. brewer in california; ° " " " descloizeaux in iceland. if we consider man alone, we find him at aden, on the red sea, at a temperature of ° in the shade, and in siberia at ° below zero; grovelling in mines deep in the earth, and living in great communities ten thousand feet above sea-level; fighting battles on the slopes of the himalayas, at an altitude of , feet; nomadic on sterile tracts; sweltering under the glaring sun of the equator, and existing in regions of perpetual snow and ice, and without sunlight for six months of the year. such are a few of the varied conditions to which man has become accustomed since he emerged from his tropical and arboreal relatives. the question finally comes down to the effect of the rarefaction of air on life. an inquiry as to how far man can stand changes of atmospheric pressure is of interest in this connection, for we know that sudden changes are accompanied by mountain sickness, at great elevations, and caisson disease under great pressure. large birds soar among the high peaks of the andes and drop at once to sea-level. i have dredged delicate mollusks at a depth of one hundred and fifty fathoms of water and kept them alive for weeks in an aquarium. man, while showing a sensitiveness to changes in barometric pressure when experienced suddenly, can nevertheless get accustomed to great ranges of pressure. the cities of bogota and quito are , feet above the level of the sea and yet in quito when de saussure, the naturalist, became so ill from the rarefaction that he could hardly find energy enough to read his instruments, and his servants, digging holes in the snow, fainted from the exertion, the natives were pursuing their various activities, and bull-fights were going on! one has only to read the accounts of the english expedition to thibet to learn that troops fought in skirmishes at the height of , feet. mr. douglas w. freshfield (in "scot. geo. mag.," april, ) gives an account of mountain sickness in the sikkim himalaya. he says the effect of high altitude was different in different individuals; some men were entirely free from it, and among them a goorkha, who ran back in a pass at an altitude of , feet to hurry up some loiterers. another member of the party, an englishman, actually gained in weight, and had an increased appetite. here, then, are a few men among a small number, without previous experience in rarefied air, feeling no disturbance, and, in one case, actually benefited by it! the question arises as to what natural selection would do among a hundred million say, who, through many centuries, might be subject to a gradual attenuation of the air. the result of rarefaction of the atmosphere and the absence of moisture is associated with marked hygienic influences. the hadley climatological laboratory of the university of new mexico has made special investigations as to the increased lung capacity of those living at high altitudes, the relation of dry soil to health, etc. important work has been done by drs. john weinzirl, c. edw. magnusson, f. s. maltby, and mrs. w. c. hadley, and their investigations go to prove that high altitudes and absence of moisture are favorable to the health of man on this world, and by analogy would not be inimical to the survival of certain forms of life in mars. dr. s. e. solby (in "medical climatology," p. , ), in describing the effects of rarefaction of the air says: "the amount of air taken in at each breath becomes greater, and the air-cells, many of which are at lower altitudes often unused, are dilated." if we consider the atmospheric pressure under which a man can work and live, we find equal adaptability. mr. gardner d. hiscox, in his work on "compressed air, its production, uses, and applications," says: "experience has taught that the ill effects are in proportion to the rapidity with which the transmission is made from compressed air to the normal atmosphere. that while the pressure remains stationary all subjective phenomena disappear." he speaks of pressure of forty or fifty pounds to the square inch, and says that, at these pressures, taste, smell, and the sense of touch lose their acuteness. in the "engineering record" for january , , there is an interesting article on "caisson disease." it says that twenty pounds pressure per square inch is common on foundation work in new york, and that bridge piers have been built when pressures of nearly fifty pounds were required. the deepest pneumatic work in new york was done in the east river gas tunnel, when the maximum pressure was about forty-seven to fifty pounds per square inch above atmospheric. in the gas tunnel four men died from the effects of heavy pressure, while none died from that reason under bridge work. the article further says that ordinarily "strong young men in proper condition do not suffer from working two four-hour shifts daily, under pressure up to twenty-five or thirty pounds; above that limit injurious effects may be felt," etc. let any reasonable man consider the meaning of these data. without any selective action on the race, without even a graded increase of pressure from boyhood up, these workmen perform hard labor of stone excavation at these pressures, and in the same way, without previous experience, men are fighting battles at , feet altitude, and in one instance growing fat at , feet. eminent german and french scientists have studied the effects of pneumatic pressure by numerous experiments on men and animals. one experimenter subjected a great number of dogs, cats, rabbits, guinea-pigs, and other animals to repeated pressures up to one hundred pounds, and carefully observed the effects of the varying conditions, some of which were fatal, while others were apparently harmless. the experiments showed that sudden release from heavy pressures was fatal, but that if three or four hours were occupied in reducing a pressure of one hundred pounds, it was harmless. with these facts one cannot help wondering whether even man himself could not exist on mars if allowed time to get accustomed to the rare atmosphere through thousands of generations of minute increments of adaptation. as a matter of fact we use but a small portion of our lung capacity. let any one experiment with himself and observe that after he has inspired the accustomed quantity of air he can continue for some time to inspire more air, and also when he has expired the accustomed quantity of air in normal breathing, he can continue to expire a great deal more air. professor jeffries wyman, the famous lecturer on comparative anatomy at harvard, used to tell us that we ordinarily inspired about twenty cubic inches of air but we could inspire one hundred cubic inches more by an effort; also that having expired the ordinary quantity we could expire a hundred cubic inches more and when the lungs were removed from the body, an extra hundred cubic inches could be forced from them. a surgeon friend tells me that many men live and work with the greater portion of both lungs diseased, and unable to perform their functions. it would be an interesting inquiry to ascertain what other species of the animal kingdom has so wide a range as man. the dog evidently follows him in all altitudes and at all temperatures. the group of insects to which the bees, wasps, and ants belong, have always been recognized as standing highest in intelligence among the invertebrates. in the great work of dr. and mrs. peckham on wasps are shown manifestations of intelligence among the wasps that are simply startling, and the remarkable work of miss adele m. fielde on the ants adds greatly to the evidences of their unique intelligence. the ant stands among the invertebrates much as man does among the vertebrates. one has only to state concretely that ants practise a division of labor; distinguish certain colors; estimate numbers; recognize friends and enemies; harvest seeds, and, it is said, raise them, hence are called agricultural ants; have insect cows and milk them; collect leaves which they chop up for the purpose of raising a kind of fungus upon which they live; organize raids and fight battles in masses; enslave other species; build covered ways and tunnels; and perform other acts of a similar nature. bearing these statements in mind it is an interesting fact that at altitudes in arizona, where man finds it impossible to live except by fetching water from regions below, the ant, equally dependent on water, has survived on these high tablelands, and manages to raise huge colonies. in wandering over the mesa at flagstaff, at an elevation of over , feet, the extreme dryness of the ground is indicated by long cracks which appear on the surface. here, where hardly any insect is found except an occasional roaming butterfly, the ant has survived and is met with in great numbers. even a rare solitary insect known as the velvet ant, and consequently without communal aid, is found chirping merrily amidst these arid surroundings. in this connection, it is interesting to observe that creatures endowed with the highest intelligence, both vertebrate and invertebrate, manage to survive in considerable numbers in regions devoid of water. one conveys it to his habitations from lower levels, the other digs wells or manages to utilize the moisture from the roots of trees. xiv my own work _snow caps of solid carbonic acid gas, a planet cracked in a positively monomaniacal manner meteors ploughing tracks across its surface with such mathematical precision that they must have been educated to the performance, and so forth and so on, in hypotheses each more astounding than its predecessor, commend themselves to man, if only by such means he may escape the admission of anything approaching his kind._ percival lowell. i am led to present these few brief memoranda of my own work in order to meet questions which would naturally be asked as to whether i had ever seen mars through a telescope, and if so did i make out any markings or canals. [illustration: plate vii dome of the lowell observatory, flagstaff, arizona] it was my good fortune to have the privilege of observing mars every night at the lowell observatory (see plate vii) for thirty-four days, covering an almost complete presentation of the planet. a few nights were cloudy and no observations were made. with these exceptions i was in the observer's chair several times each evening. the twenty-four inch refractor of which i had the use was the last telescope clark ever made, and he pronounced it his best one. this instrument (plate viii) is mounted on a mesa near the town of flagstaff, arizona, at a height of over , feet above sea-level, in an atmosphere of remarkable clarity and steadiness. i have already stated on page my first experiences in observing and will only present the brief notes i made at the time of observation. better results would have accompanied these efforts had i followed the custom of michael faraday and asked what was i to look at, what was i expected to see? i had been somewhat prejudiced as to the existence of the canals by the comments of sporadic observers, many of whom, by the way, had never been able to see them, and denying that any one else ever had, straightway proceeded to suggest a theory to explain their presence! careful to avoid any bias in the matter i rigidly refused to allow either professor lowell or his assistants to suggest where i might find a canal or a marking on the disk. the night before i left the observatory for home i asked mr. lowell for the first time, to indicate the position of some conspicuous canal which i had not seen. this he did and examining the region which i supposed he had indicated on the disk i searched in vain for the line. in doing so another line was detected and drawn, and on confessing my failure to see the line he had described, showed him my drawing, when he exclaimed, "why, you have got it," and sure enough when he showed me his drawing and repeated the directions he had given me, i found that i had been looking at the wrong pole of the planet. [illustration: plate viii twenty-four inch telescope of the lowell observatory, flagstaff, arizona] in one stage of great discouragement i came across a statement made by mr. a. stanley williams which has already been quoted, namely, that he had to observe continually for two months before sufficient sensitiveness enabled him to make out the more delicate markings. that i might have seen more had i been acclimated, and had been accustomed to telescopic observation there is no doubt. the record is poor enough and yet under the conditions mentioned the results may be of interest to the reader. may . midnight. saw planet for the first time. a beautiful luminous disk with shades of tone dimly visible. southern pole cap white and seen. may . certain details sufficiently distinct to make out dark areas, and at times a line or two. may . occasional flashes of a few lines, while broad darkened area and cuniform area on right visible, and, in one flash, a line supporting the wedge as well as basal line. with no better seeing conditions than last night, more details came out, and for the first time i am encouraged to believe that each day an improvement will take place. i saw enough to make my first drawing. may . bad seeing. i made out only the broad southern band, the line at the northern pole and the wedge-shaped area to the right below, also a slight discoloration in the middle. may . not very good seeing. could make out but little more than i did last night. may . seeing about the same, perhaps slightly less. saw rift in southern dark band and north pole appeared luminous. may . mr. lowell informed me this morning that the luminous appearance around the north pole that i saw last night was the result of a snowstorm. seeing fair. considerable vibration of planet. saw new snow field of the northern pole distinctly outlined and much confused markings. looked in vain for spots but could not discern them. may . seeing clearer, and for the first time i made out distinctly two spots, or oases. mr. lowell informed me that schiaparelli had never seen them. the snow which fell on may was still conspicuous. may . with a headache and a seedy condition from not being acclimated, i yet found an improvement in my seeing capacities. i made out a promontory in the southern dark belt, also a canal running down from the trivium. may . bad seeing. could not define snow cap though dark southern band showed. made no drawing. may . am in despair of seeing anything when the others see so much. i must have an old and worn-out retina. in looking, lines flash out at times but it is impossible to locate them. i can certainly see more than huyghens did, but not much more. may . heavens very cloudy and mars obscured. may . poor seeing--saw but a few markings. may . snow and hail storm in the afternoon. temperature ° at night. seeing zero, and consequently no observation. may . to-night markings and more particularly shades seemed abundant yet so evanescent that only an intimate knowledge by long study could define them. i gave up in despair. may . saw a little more than i saw last night but did not see a trace of things that mr. lowell and his assistants apparently saw without effort. i realize that it requires a special training to observe the flickering evanescent markings on mars. june . though the best night yet for steady atmosphere i saw but little more and have come to the conclusion that it will take months of continuous observation before i can see anything. june . i went to the observatory to-night in despair of ever seeing anything more. got into the observing chair and immediately saw a number of markings i had not seen before, as my drawings show. i have purposely refrained from studying the maps, and so do not know the names of the lines detected. june . atmosphere so unsteady that it was impossible to make anything out of mars, so after struggling awhile gave it up in disgust. june . seeing about , yet manage to see a few planetary details. june . i find a slow advance in my ability to see the markings though it is exasperating that the janitor of the observatory talks about plainly seeing certain details which he indicates to me by a sketch, and looking at the region i can see no trace of a canal or anything else. june . seeing very good and in my observations tonight added another canal. it is a most difficult matter to catch the fleeting lines as they appear with startling distinctness to instantly vanish again. june . seeing fairly good. could make out but little more. color of regions very strong and vivid. june . seeing a little better than last night. added three new canals, and these canals flashed out three or four times before i was willing to record them, and then i did not believe them till mr. lowell showed me a drawing he had made just before, and the two drawings corresponded. june . looked at eight o'clock and the markings of larger features came out strong and dark and yet the seeing was not estimated high. june . rather poor seeing though some of the dark regions came out with remarkable distinctness. every day i notice a very slight improvement in detecting lines. markings formerly made out with great difficulty are now instantly recognized. june . in my observations to-night added one new canal and completed another, and was able to detect one that mr. lowell had not seen during the evening--a well-known one he says. it simply shows that one must continually observe as the lines flash out for a single instant. june . made out still another canal to-night. the markings show very clear, in fact some parts were vivid in distinctness and the lower part of syrtis major dark blue. june . poor seeing, yet i was able to see a few of the prominent features and defined the wedge-shaped region below. on plate ix i give a few of my drawings of mars in which are indicated the lines i saw many times and was able to fix. other lines flashed out for an instant but these were not recorded, simply because i could not definitely locate them. [illustration: plate ix may may snow fell may june june - june june - drawings of canals of mars by the author] the expression "poor seeing" in the above notes must be taken in a comparative sense with relation to the usual conditions of the atmosphere of flagstaff. poor seeing, therefore, at flagstaff would be equal, if not superior, to the best seeing at much lower levels. an astronomer who resigned his position in a western observatory for duties at mount wilson, california, told me that for thirty consecutive nights the seeing was superior to the best nights he had observed in at his former post. xv what the martians might say of us _o wad some power the giftie gie us, to see oursels as others see us!_ robert burns. for every single perplexity of interpretation we encounter in our study of the surface markings of mars, the martian would encounter a dozen perplexities in interpreting the various features on the surface of the earth. admitting the conclusions of lowell of the existence of intelligence in mars, and that that intelligence has been associated for ages with a planet having only slight elevations of land, a tenuous atmosphere, a scarcity of water which has been utilized for ages through artificial channels, as we have done in various parts of the world since prehistoric times, having vast tracts of sterile plains, and, within these sterile tracts large oases fed by irrigating canals, regions of sparse vegetation, and no large bodies of water; with these conditions going beyond the history of these intelligences, what must be the martian interpretation of the surface features of this world? it is a perfectly fair inquiry, for by such means we may appreciate the attitude of some of our interpreters of mars. in examining the earth, then, as we have examined mars, the martian would find large yellow and reddish areas, extensive greenish areas, and, besides, large regions of varying shades of blue, possibly, occupying three-fourths of the earth's surface. the yellow areas he would interpret as desert land, the greenish areas he might consider vegetation, but what would he make out of the larger regions of blue? this would certainly puzzle him, because, unfamiliar with oceans, he could not believe that such vast tracts could really be water. he would easily interpret the polar snow caps, and the waters at their edges, but the oceans would be impossible to solve. the suggestion, by some audacious interpreter, that this vast blue area was water, would be answered by showing that these so-called bodies of water bordered vast tracts of sandy deserts with no canals running into them for irrigation or navigation purposes. even the polar snow caps would be doubted, because they seemed to extend far down into temperate latitudes; and on their recedence in summer, there would be seen no dark, bordering seas as the result of their melting. the vegetation, instead of unfolding at the north and gradually extending southward, would unfold in a contrary direction, appearing first in south temperate latitudes and developing northward. the perennial character of the vegetation in the tropics would puzzle him. even if he recognized oases in the deserts of america and africa, the results of artesian wells or springs, he could not believe them to be vegetation; for he would detect no irrigating canals running into them. he would come to the conclusion that no creature could possibly exist on the earth, as the tremendous force of gravitation with great atmospheric pressure would forbid the existence of any organic forms. the immense clouds veiling the surface must at times suffer condensation, and the impact of raindrops would, from their velocity and weight, smash everything in the way of life. life, if it existed in forms supported by appendages, must have legs of iron to sustain its weight, and a crust like a turtle to be impervious to raindrops, and this would be contrary to all martian analogy. the courses of rivers, if detected, would puzzle him from their irregularity, unless he dared to suggest that these long sinuous channels extending for thousands of miles were identical to the little rivulets he had studied near his own poles. in fact, about the only feature outside the polar snow caps that he would instantly recognize, would be the great ice cap of the himalayas. india, that vast region extending from latitude ° nearly to the equator, with its great plains and sterile regions, with its overpowering heat, and a dense population, depends for the sustenance of many of its millions upon the thousands of miles of irrigating canals, fed from the melting snow caps of the himalayas. india has no great lakes, but in the northern plains great rivers course their way to the sea. the ganges and the indus and their tributaries derive their waters from the melting glaciers, and from these, a most extensive irrigating system of canals and reservoirs draw their waters. as the heat increases the ice melts more rapidly, and so more water is supplied at just the time when it is most needed. the whole scheme is on so vast a scale that a martian would recognize its meaning, though he would wonder at the tortuous outlines of the larger canals. flammarion has, in a similar manner, presented the arguments of martian astronomers as to whether life exists anywhere but upon the planet mars. he says, among other fancies, that the sapient martian argues that houses could not be built on the earth, on account of the violence with which building materials, such as bricks, blocks, etc., would drop, and thus endanger life. believing that mars is rightly balanced as to temperature, the earth being so much nearer the sun, would be too hot for life to exist. the martian conceives himself to be supremely complete "even to the point that artists wishing to represent god in our sanctuaries have figured him in the image of a martian man." the martian considers our year too short. in his reflections he says: "during the period in which one of us attains the middle age of fifty years those on earth have become decrepit old men of ninety-four, if, indeed, they are not already dead." seriously, if there is an intelligence in mars, it must have evolved along the same general lines as intelligence has developed on the earth. being an older planet, it must have outgrown many of the vagaries and illusions which still hamper man in his progress here. in the dim past, however, we can imagine some martian astronomer with the enigma of our earth before him, and the great vault of heaven with its thousands of riddles unanswered, consulting records and covering pages with mathematical formulæ to ascertain the precise spot upon which grew the bean stalk by which a martian jack ascended to encounter the giant. indeed, the imagination can conjure up an infinite number of parallels. if mars is an older sphere, we trust it has long outgrown the superstitions which still hamper man in his interpretation of the inexorable phenomena of nature on this little planet. we may hope that they have finally reached that stage when a dictum similar to that of huxley forms an engraved tablet in their temples of worship. these are his words: "science is teaching the world that the ultimate court of appeal is observation and experiment, and not authority. she is teaching it to estimate the value of evidence; she is creating a firm and living faith in the existence of immutable moral and physical laws, perfect obedience to which is the highest possible aim of an intelligent being." xvi schiaparelli, lowell, perrotin, thollon _every age has its problem, by solving which humanity is helped forward._ heinrich heine. in previous pages allusion has been made to the distinguished character of the astronomers who have contributed to a knowledge of the surface markings of mars. testimony from astronomical sources has been quoted as to their keen-sightedness in this work which, as sir robert ball has said, "indicates one of the utmost refinements of astronomical observation." that the reader may better understand the eminence of some of those whose names will forever be associated with the investigation of the surface features of mars the following brief records are given. [illustration: giovanni virginio schiaparelli] the two astronomers most widely known in connection with the study of mars are professor giovanni schiaparelli and professor percival lowell. lowell had just graduated from harvard, at the age of twenty-one, when schiaparelli, at the age of forty-two, made his first great discovery of the _canali_ of mars. macpherson, in his valuable history of the "astronomers of to-day," says of schiaparelli: "his studies of meteoric astronomy, of mars, venus, and mercury, of double stars and of stellar distribution, have given him a place second to none among living students of the heavens." from the same interesting book we gather the following facts: schiaparelli was born in sabigliano, in piedmont, in . he attended the usual schools in his native town and then entered the university of turin as a student of mathematics and architecture. before he was twenty years old he decided to devote himself to the study of astronomy. at the age of twenty-four he was an assistant in the celebrated observatory of pulkova. when the kingdom of italy was organized he became an assistant in the brera observatory, milan. he became suddenly famous at the age of twenty-seven by the discovery of a new asteroid. in he became director of the observatory. schiaparelli's first great discovery was the relationship between comets and meteoric showers. in he was accorded the gold medal of the royal astronomical society for his various astronomical discoveries. professor simon newcomb gives him high praise when he says: "among the individual observers schiaparelli may be assigned the first place in view of his long continued study of the planets under a fine italian sky, the conscientious minuteness of his examinations, and his eminence as an investigator." schiaparelli's researches into the relation of comets and meteors "were developed in , in his remarkable work 'le stelle cardenti,' which is, according to sir norman lockyer, one of the greatest contributions to astronomical literature which the nineteenth century has produced." macpherson closes his interesting memoir of schiaparelli by saying: "his devotion to astronomy, his singularly accurate observations and his wonderful discoveries have secured for him an exalted position among the greatest astronomers of modern times." for a further appreciation of the work of schiaparelli the reader is referred to macpherson's "astronomers of to-day." in this brief sketch the reader may judge of the eminent character of one who insists that the lines in mars are a persistent feature of its surface, whatever one's interpretation of them may be. [illustration: percival lowell] percival lowell was born in boston in . he was graduated from harvard in , and prepared for his graduating thesis an essay on the nebular hypothesis. lowell is a many-sided man. early interested in mathematics, he became one of the founders of the mathematical and physical society of boston. a visit to japan, where he lived a number of years, resulted in the writing of three interesting books: "the soul of the far east," ; "noto," ; and "occult japan," . during his residence in japan he was chosen foreign secretary and adviser to the korean special commission, then about to visit the united states, which he accompanied. on his return to korea he was the guest of the korean government, and this experience prompted him to write "a korean coup d' État," , and his well-known volume, "choson, the land of the morning calm," . on his return to america he undertook an eclipse expedition to tripoli with professor todd. his early interest in astronomical subjects was now fully awakened, and the red planet, which he had observed in boyhood with a small telescope from the roof of his father's house, aroused his interest on account of the heated discussions over schiaparelli's discoveries. with an impetuosity and enthusiasm which characterizes all his work, he set about to secure a proper region and a sufficient elevation for an observatory site. this was found in northern arizona at an elevation of over , feet. here, then, was established the lowell observatory with a twenty-four inch refractor made by clark especially for this observatory, the last, and, according to the maker's words, the best telescope he had ever made. lowell insisted that the location of an observatory was a much more important factor than the size of the instrument, and says: "when this is recognized, as it eventually will be, it will become the fashion to put up observatories where they may see rather than be seen." it may be said with truth that, for the first time in the history of astronomy, an observatory has been erected and fitted for the special purpose of studying the surface features of mars. during unfavorable oppositions lowell has turned his attention to the other planets, notably mercury and venus, with the result of adding many new and interesting details concerning these bodies. three volumes of quarto memoirs and many bulletins from the lowell observatory attest to his industry. he has been fortunate in securing talented assistants, and their contributions may be found in the various publications of the observatory. the character and importance of lowell's work may be understood by stating that the "british nautical almanac" is to adopt for the future the value of the position of the axis of mars, and the tilt of the planet's equator to its ecliptic, which was furnished by professor lowell in compliance with a request. mr. lowell is a fellow of the american academy of arts and sciences; member of the royal asiatic society of great britain; american philosophical society; société astronomique de france; american astronomical and astrophysical society; astronomische gesellschaft; société belge d'astronomie; fellow of the american geographical society; honorary member sociedad astronomica de mexico; and others. in he was awarded the janssen medal of the astronomical society of france for his researches on mars. mr. macpherson, in his memoir on lowell, says that "mr. lowell, by his unwearied devotion to astronomy, has already gained for himself an enduring reputation." [illustration: henri perrotin] m. henry perrotin and his assistant, m. thollon, have been quoted in previous pages as having markedly confirmed the discoveries of schiaparelli. through the courtesy of professor lowell i am enabled to present the likenesses of these two astronomers. i am indebted to the exhaustive work of miss agnes m. clerke, entitled the "history of astronomy during the nineteenth century," for the following memoranda of some of the work accomplished by these men. perrotin made a series of observations on venus fully confirming schiaparelli's inference of synchronous rotation and revolution: "a remarkable collection of drawings made by mr. lowell in appeared decisive in favor of the views of schiaparelli." in other words, venus, like the moon, presents the same face to the sun in its revolution about that luminary. perrotin has made important observations on the rings of saturn; his double-star measurements are also considered work of the highest character. [illustration: m. thollon] thollon has made many spectroscopic studies, among which were delicate experiments showing the lateral displacement of lines in the solar spectrum arising from the sun's rotation. in the annals of the nice observatory he published a great atlas consisting of thirty-three maps, exhibiting in quadruplicate a subdivision of the solar spectrum under varied conditions of weather and zenith distance. he also studied the spectrum of the great comet of , and by the displacement of its lines estimated that the comet was receding from the earth at the rate of from sixty-one to seventy-six kilometers per second. the leland prize was awarded to thollon for a hand drawing he made of the prismatic spectrum obtained with bisulphide of carbon prisms of high dispersive power. the character and reputation of these men, as well as others who have been quoted in these pages, must be weighed against the few who, not content with denying the existence of the _canali_ in mars, have in strong language abused those who accept them as veritable markings on the planet's surface. xvii last words _the uniformity of the course of nature will appear as the ultimate major premise of all inductions._ john stuart mill. the final question is, do the lines as depicted and described by various observers exist on the surface of mars? those who have made the greatest addition to our knowledge of the character of these lines, and have constructed maps based on martian latitude and longitude are accredited on other grounds as being endowed with remarkable acuteness of vision coupled with persistence and painstaking care in observation. the most successful work has been accomplished with instruments of fine definition in regions of steady atmosphere and high altitude, or at intervals of clarity and steadiness in regions otherwise unfavorable. finally, and most convincing of all, mr. lowell's assistant, mr. lampland, after many attempts has succeeded in photographing the more conspicuous linear markings. _the lines do exist essentially as figured by schiaparelli and lowell._ it now rests with the objectors to suggest any better interpretation of the markings of mars than that they are the results of intelligent effort. the mediæval attitude of some astronomers regarding this question recalls the story of scheiner, a jesuit brother, who, independently of galileo and fabricius, discovered spots on the sun. eager with enthusiasm he informed his superior of his remarkable discovery and begged to be allowed to publish it to the world. the superior replied, "go, my son; tranquilize yourself and rest assured that what you take for spots on the sun are the faults of your glasses or of your eyes." this happened three hundred years ago, and yet to-day a few astronomers of this class still survive. if one will calmly reason about the matter, let him consider a parallel case of interpretation. he digs out from the ground a fragment of stone; its somewhat symmetrical shape suggests to him the idea that it may be a rude stone implement. if he wishes to know what kind of rock it is and its geological age, he refers it to a geologist; if he wishes to know its composition, he asks a mineralogist, who, if necessary, will analyze it for him. if, however, he is curious to know whether its peculiar, fractured surface is due to frost or other natural agency, or whether it is the work of some rude savage, he inquires of an archæologist, who alone will be able to tell him whether it is a worked stone or natural fragment. he will probably tell him whether it was shaped by paleolithic man, and whether it is a rough stone implement or a core, _reject_ or chip. so with the study of mars, as we have already pointed out, there are certain matters of information about the planet which the astronomer alone can impart, while the superficial markings are just as certainly to be interpreted by another class of students who may or not be familiar with astronomical methods. * * * * * it was quite natural that astronomers, the most conservative of all classes of observers, should have doubted the first announcement of schiaparelli of the startling discovery of the _canali_ marking the face of the planet, the more so as year after year went by and yet with the utmost efforts of astronomers nothing of the nature of schiaparelli's lines could be seen. what added greatly to the doubt about the lines, and at the same time strengthened the idea that the lines were illusory, was the subsequent announcement by schiaparelli--undeterred by the universal skepticism--that at times the lines appeared double. what more convincing evidence could be offered than that the phenomenon was purely subjective? a few astronomers expressed their doubts in a courteous though hesitating manner. professor young, in his valuable text-book, "elements of astronomy" ( ), in correctly reporting schiaparelli's discovery says: "he is so careful and experienced an observer that his results cannot be lightly rejected; and yet it is not easy to banish a vague suspicion of some error or illusion, partly because his observations have thus far received so little confirmation from others, and partly because his 'canals' are so difficult to explain. they can hardly be _rivers_, because they are quite straight; nor can they be _artificial_ water-ways since the narrowest of them are forty or fifty miles wide. to add to the mystery, he finds that at certain times many of them become _doubled_,--the two which replace the former single one running parallel to each other for hundreds, and sometimes thousands, of miles, with a space of or miles between them. he thinks that this _gemination_ of the canals follows the course of the planet's seasons." the overpowering belief that this world alone sustained creatures of intelligence formed an obstructive barrier to any and all attempts made to uphold--at least by analogy--the idea of intelligence in other worlds. one cannot but regret that some philosopher had not, years before schiaparelli's time, expressed the conviction that mars might perhaps be more favorable to the existence of intelligent life than our own world, and with this conviction proceed to formulate the conditions which must of necessity exist: namely, that the planet being a much older world than ours, its waters had mostly vanished by chemical combination with the rocks and otherwise. following this assumption, the philosopher might have insisted that in the last extremity the melting snow caps would be utilized by the supposed intelligences to furnish water for potable and irrigating purposes. the philosopher might have superadded to this idea the prediction that, when telescopes were strong enough and eyes were keen enough, evidence of the truth of this supposition would be found in canals of some sort and that such lines should be carefully sought for. fancy the exultation of schiaparelli when at last he found the lines precisely as indicated. such an announcement from so distinguished an astronomer would have been hailed with acclaim. alas! for the conservatism of astronomers, such powers of prevision are sadly wanting. le verrier's prediction of an outer planet was a matter of dead certainty. the perturbations of uranus could not be accounted for except by the assumption of an outside body, and had it not been for the characteristic reserve of english astronomers, adams might have had the full credit. so rare are predictions of this nature in the history of astronomy that this instance will probably be quoted to the end of time. the masses, still ignorant of the certainty of mathematical astronomy, regard the prediction of an eclipse as in the nature of a prophecy. the liberal attitude of naturalists stands in marked contrast, and the history of their work is filled with examples of prediction and repeated confirmations. until the middle of the last century--grounded in the belief of special creation--how wonderfully rapid was the conversion of naturalists to the theory of evolution after darwin had offered his rational views on the subject. the existence of forms was predicted, based on the idea of evolution, and these have been found again and again. our museums display in their cases remains of fossil animals which complete many series undreamed of in pre-darwinian days. this wonderful work has been accomplished without resort to algebraic formulæ, and yet when mathematics can be applied, as it is in the law of variation, quantitative studies in heredity, and statistical methods generally, it is promptly seized upon by the biologist. * * * * * to one unconvinced of the existence of some signs of intelligent activity in mars the suggestions that have been made to account for certain appearances in the planet will seem absurd. if, on the other hand, he finds himself in agreement with those who believe the markings are the result of intelligent effort, then he is justified in using the various artificial markings of the surface of the earth as standards of comparison in explaining the many curious markings of mars. indeed, he is compelled to do so, just as would be demanded of him if he should stand on some high mountain peak in some hitherto unexplored region of africa and should minutely scan the hazy stretch of plains below. large white spots in equatorial regions which could not possibly be snow-covered hills, might be masses of white flowers or cloth-covered areas for the better cultivation of certain plants. lines that dimly stretched across the surface might be rivers, cañons, rifts, or bands of irrigation, according to their character. as we compare the circular markings on the moon with our terrestrial craters and fissures, and cracks on its surface with similar fissures on the earth, so we are forced to compare the markings on the surface of mars with what seems analogous to them on the surface of our own earth. once proved that the markings of mars are due to erosion, cracks, encircling meteors big enough to raise ridges by their attractive force, then all that has been written in demonstration of their artificial character goes for naught. the intelligent reader unprejudiced in the matter will, however, judge for himself the merits of our contention and will determine the reasonableness of the comparisons that have been made by lowell in solving the mystery of mars. index algebraic formulæ, . american astronomers, holden, pickering, young, swift, comstock, barnard, wilson, drew the more conspicuous canals, . ancient irrigation, . ants surviving at high altitudes, ; unique intelligence, . astronomer's chief work, ; conservatism, . astronomers who have seen the canals, . astronomical subjects remote from martian studies, . atmosphere and moisture, barnard and others, , ; sir robert ball, . austria's care of water, . ball, sir robert, difficulties of observation, ; life on mars quite likely, , ; objection to mars being inhabited, . barbour, w. d., with a four inch achromatic, . barnard's, dr., description of dark regions, . bees, wasps, and ants, . _canali_ supposed to mean canals, . canals appear double, ; artificiality of, ; as distinct as engraved lines, ; chain of reasoning in regard to, ; double, ; of mars, ; unchangeable in position, . cassini, . chandler's oscillation of pole, . checkerboard appearance of west, . clerke's, agnes m., expressions, . clouds in mars, ; in mars, sir norman lockyer, . comments and criticism, . committee of british astronomical association, . conception of life in other worlds, . conservatism of astronomers, . cracks all of the same nature, ; discontinuous, ; in asphalt pavement, . cultivation under cloth, porto rico, . dark regions not seas, . dawes, remarkable distinctness of vision, . de la rive, memoir of faraday, . denning's, mr., testimony, , . difficulties of seeing, . dighton rock, . draper, dr. henry, "are other worlds inhabited?" ; difficulties of seeing, ; high altitudes for telescopes, . drawings of mars by different observers, . dust storms in mars, . earth, a standard, , , ; early ideas regarding the, ; improbability of its being unique, . earth's distance from the sun, ; temperature above normal, . emerson's expressions, . england's unsteady atmosphere, . epicyclic theory of ptolemy, . "evolution of the solar system," t. j. j. see, . failure of water in england, . faraday's, michael, attitude, . fauth, dr. phil., ; drawings of mars, . first look at mars, . fison's, mr., comments, . flammarion's picture of the earth from mars, ; work on mars, . fruit trees, santa clara valley, . gill's, sir david, testimony, . hebraic conceptions, astronomers imbued with, . hebraic conceptions of the universe, . herschel, sir john, on snow caps, . high altitudes favorable to health, . holden, e. s., on nebula of orion, . howe's, herbert a., remarks, , . huxley's estimate of mathematicians, . huyghens, . ice caps of himalaya, . iles, george, illustration of cooling bodies, . illusions, supposes, . irrelevant criticism, . irrigation, ancient in arizona, in egypt, in india, ; marvels of, ; notes on, . joly's, dr. j., theory, . keeler's definition of astrophysics, . lampland, photographs of mars, . ledger's, rev. e., canals of mars, . liberal attitude of naturalists, . life at high altitudes, ; in other worlds, garrett p. serviss, ; under atmospheric pressure, . lindsay's, thomas, expressions, . lines of artificial character, . lockyer, sir norman, saw clouds in mars, . lockyer's, w. j., testimony, . lowell, percival, brief sketch of, ; different telescopes used by, ; gives reason why canals cannot always be seen, ; his acute eyesight, , ; his book on mars, ; his various publications, ; long practice in observing, ; snow caps prove atmosphere, ; on life on mars, , ; on twilight atmosphere in mars, . lung capacity, ; at high altitudes, . macpherson, hector, jr., agrees with lowell, . mars, appearance of earth from, ; beginning of life in, ; canals, ; canals continuous, ; dark regions change with the season, ; dark regions not seas, ; desert lands, ; detached fields of snow, ; disappearance of southern snow cap, ; distance from sun, ; double canals, , ; drawings of, coincided, ; glints of brilliant light, ; has it water? ; has life appeared in? ; life in, from analogy, ; much like the world, ; nearest approach to earth, ; oases, ; seasonal changes in, ; seasons, ; rarefaction of atmosphere in, ; rotation of, cassini, ; temperature of, ; terminator of, douglass, ; those who see and those who do not see, ; tilt of axis, ; white polar caps, . maunder, director of committee, . maunders's, e. w., comments, . maunier, stanislaus, on canal doubling, . maxwell, clerk, on mathematicians, . mediæval attitude of some astronomers, . michel, louise, teaching children, . morehouse, george w., believes mars is inhabited, , . my own work, . newcomb's, professor, opinion, ; other worlds inhabited, ; "reminiscences," . number of acres under irrigation, . observations of mars, st period, ; d period, ; d period, ; th period, lowell's work, . orr's, j., theory, . parallel case of interpretation, . patterson's, john a., expressions, . perrotin, brief sketch of, . perrotin and janssen describes the canals, ; and thollon, . perrotin's painstaking care, , . phillips', rev. theo. e. r., drawing, . pickering, w. h., canals seen by, ; shows importance of steady atmosphere, ; observations in jamaica by, ; polariscope observations by, - ; theory of, . planetology, . plurality of worlds, astronomer's belief in, ; edward hitchcock's views of the, ; flammarion's views of the, ; newcomb's attitude in regard to the, ; newcomb's belief in the, ; o. m. mitchell's views in regard to the, ; sir david brewster's views of the, ; sir richard owen's views in regard to the, ; tyndall's views of the, . polar snow cap, proof deduced from lowell, douglass, and pickering, . profound changes by man, . railroads in iowa and texas, . review of lowell's book, . rift in southern africa, . schiaparelli, abstemiousness when observing, ; brief sketch of, ; canals artificial, ; _canali_ natural, ; discovery, ; discovery of canals, ; does not deny intelligence in mars, ; suggestion as to doubling, . sea, so-called, land areas, . seasonal changes, . snow storms in mars, w. h. pickering, . solar system a standard for universe, . stars, bright points of light, ; similar to our sun, . stetefeldt's, c. a., views, . study of planetary markings, . sun and planets reduced to minute scale, . temperature under which man exists, . terby, dr., identifies many canals, . theories regarding canals, . thollon, brief sketch of, . titles of papers in astronomical journals, . todd, professor, says canals result of design, . turner, h. h., "astronomical discovery," ; on the difficulties of seeing, . tycho brahe, . tyndall on imagination, . tyndall's expressions on the nebular theory, ; reference to nebular theory, . unfolding of plant life on the earth, . variation in drawings by different observers, , ; of milky way, ; of nebula of orion, ; of solar corona, , . variety of conditions under which life exists, . vastness of the universe, . wallace, alfred russel, human paradox, ; review of, in london "nature," . water vapor, no spectroscopic proof of, campbell, . webb's, rev. t. w., difficulties of seeing, , . what the martians might say of us, . white spots in equatorial regions of mars, . white weed in new england, . williams, a. stanley, difficulty in observation, . would the work of man show in mars? . young, c. a., on snow caps, , ; on schiaparelli's discovery, . footnotes: [ ] some of our readers may not know that light travels, in round numbers, at the rate of , miles a second. [ ] the terminator represents the limit of light on that side of the planet in the shade, in other words, where the light terminates. in viewing the moon, when at quarter or half, the terminator is seen very ragged on account of the illumination of higher points on the surface. if the moon was as smooth as a billiard ball the terminator would be clear cut. [ ] the world in its ignorance of italian assumed that the word meant exclusively canals, and, if canals, then dug by shovels. what! a canal thirty miles wide and two thousand miles long dug in the snap of the finger? impossible conception, you say. we shall see later the sober utterances of a member of the british astronomical society on this gratuitous assumption, and an equally serious comment by the chief assistant of the royal observatory at greenwich (e. s. m.). [ ] the views so long held that the dark shaded regions were bodies of water, or seas, was disproved by the observations of pickering and douglass, who distinctly traced the course of the canals across these dark areas. the observations of dr. e. barnard certainly sustain the contention that they are land areas and probably depressions, representing ancient ocean beds. dr. barnard, using the telescope at the lick observatory, says: "under the best conditions these dark regions which are always shown, with smaller telescopes, of nearly uniform shade, broke up into a vast amount of very fine details. i hardly know how to describe the appearance of these 'seas' under these conditions. to those, however, who have looked down upon a mountainous country from a considerable elevation, perhaps some conception of the appearance presented by these dark regions may be had. from what i know of the appearance of the country about mt. hamilton, as seen from the observatory, i can imagine that, as viewed from a very great elevation, this region, broken by cañon, and slope and ridge, would look like the surface of these martian seas." [ ] sterling heiley, in "pearson's magazine," june, . [ ] a translation of which may be found in the "popular science monthly," vol. xxxv, p. . [ ] i may add that in a similar case an american student of mars moved his telescope to mexico and remounted it at a cost of some thousands of dollars. transcriber's notes: punctuation and spelling were made consistent when a predominant preference was found in this book; otherwise they were not changed. simple typographical errors were corrected; occasional unbalanced quotation marks retained; inconsistent hyphenation retained. ambiguous hyphens at the ends of lines were retained. page : quotation mark preceding 'the sale value' has no matching closing mark. page : "stetefelt's" is spelled "stetefeldt" on page . the latter is correct. page : "tycho brahe" probably should be indexed as "brahe, tycho". pleasures of the telescope an illustrated guide for amateur astronomers and a popular description of the chief wonders of the heavens for general readers by garrett p. serviss author of astronomy with an opera-glass "this being made, he yearned for worlds to make from other chaos out beyond our night-- for to create is still god's prime delight. the large moon, all alone, sailed her dark lake, and the first tides were moving to her might; then darkness trembled, and began to quake big with the birth of stars, and when he spake a million worlds leapt into radiant light." lloyd mifflin. _with many illustrations_ new york d. appleton and company copyright, , by d. appleton and company. preface by the introduction of a complete series of star maps, drawn specially for the use of the amateur and distributed through the body of the work, thus facilitating consultation, it is believed that this book makes a step in advance of its predecessors. the maps show all of the stars visible to the naked eye in the regions of sky represented, and, in addition, some stars that can only be seen with optical aid. the latter have been placed in the maps as guide posts in the telescopic field to assist those who are searching for faint and inconspicuous objects referred to in the text. as the book was not written for those who possess the equipment of an observatory, with telescopes driven by clockwork and provided with graduated circles, right ascensions and declinations are not given. all of the telescopic phenomena described are, however, represented in the maps. star clusters are indicated by a conventional symbol, and nebulæ by a little white circle; while a small cross serves to mark the places where notable new stars have appeared. the relative magnitudes of the stars are approximately shown by the dimensions of their symbols in the maps, the smaller stars being represented by white dots and the larger by star-shaped figures. in regard to binary stars, it should be remembered that, in many cases, their distances and angles of position change so rapidly that any statement concerning them remains valid only for a few years at the most. there is also much confusion among the measurements announced by different authorities. in general, the most recent measurements obtainable in are given in the text, but the observer who wishes to study close and rapid binaries will do well to revise his information about them as frequently as possible. an excellent list of double stars kept up to date, will be found in the annual companion to the observatory, published in london. in the lunar charts the plan of inserting the names of the principal formations has been preferred to that usually followed, of indicating them only by numbers, accompanied by a key list. even in the most detailed charts of the moon only a part of what is visible with telescopes can be shown, and the representation, at best, must be merely approximate. it is simply a question of what to include and what to omit; and in the present case the probable needs of the amateur observer have governed the selection--readiness and convenience of reference being the chief aim. it should, perhaps, be said here that the various chapters composing this book--like those of "astronomy with an opera-glass"--were, in their original form, with the single exception of chapter ix, published in appletons' popular science monthly. the author, it is needless to say, was much gratified by the expressed wish of many readers that these scattered papers should be revised and collected in a more permanent form. as bearing upon the general subject of the book, a chapter has been added, at the end, treating on the question of the existence of planets among the stars. this also first appeared in the periodical above mentioned. in conclusion, the author wishes for his readers as great a pleasure in the use of the telescope as he himself has enjoyed. g. p. s. borough of brooklyn, new york, _january, _. contents chapter i page the selection and testing of a glass how to get a good telescope--difference between reflectors and refractors--how a telescope is made achromatic--the way to test a telescope on stars. chapter ii in the starry heavens orion and its wonders, lepus, canis major, argo, monoceros, canis minor, and the head of hydra. chapter iii from gemini to leo and round about the zodiacal constellations gemini, cancer, and leo, and their neighbors auriga, the lynx, hydra, sextans, and coma berenices. chapter iv virgo and her neighbors crater and corvus, hydra, virgo, the "field of the nebulæ," libra, boötes, and the great arcturus, canes venatici, and corona borealis. chapter v in summer star-lands scorpio and its red-green gem, ophiuchus, sagittarius, scutum sobieskii, capricornus, serpens, hercules, draco, aquila, and delphinus. chapter vi from lyra to eridanus lyra and its brilliant vega, cygnus, vulpecula, aquarius, equuleus, pegasus, cetus, and eridanus. chapter vii pisces, aries, taurus, and the northern mars the first double star ever discovered, the pleiades and their photographic wonders, the royal family of the sky, andromeda, cassiopeia, perseus and cepheus, ursa major, camelopardalus, ursa minor, and the pole star. chapter viii scenes on the planets jupiter, its belts and its moons--saturn, the ringed planet--saturn's moons and roche's limit--mars and its white polar caps and so-called seas and continents--venus and her atmosphere--the peculiar rotations of venus and mercury. chapter ix the mountains and plains of the moon and the spectacles of the sun peculiarities of the lunar landscapes--the so-called seas, the craters, the ring mountains, the inclosed plains, the mountain ranges, tycho's mysterious streaks, and other lunar features described--how to view the sun and its spots. chapter x are there planets among the stars? significance of dr. see's observations--why our telescopes do not show planets circling around distant suns--reasons for thinking that such planets may exist--the bearing of stellar evolution on the question. pleasures of the telescope chapter i the selection and testing of a glass "o telescope, instrument of much knowledge, more precious than any scepter! is not he who holds thee in his hand made king and lord of the works of god?"--john kepler. if the pure and elevated pleasure to be derived from the possession and use of a good telescope of three, four, five, or six inches aperture were generally known, i am certain that no instrument of science would be more commonly found in the homes of intelligent people. the writer, when a boy, discovered unexpected powers in a pocket telescope not more than fourteen inches long when extended, and magnifying ten or twelve times. it became his dream, which was afterward realized, to possess a more powerful telescope, a real astronomical glass, with which he could see the beauties of the double stars, the craters of the moon, the spots on the sun, the belts and satellites of jupiter, the rings of saturn, the extraordinary shapes of the nebulæ, the crowds of stars in the milky way, and the great stellar clusters. and now he would do what he can to persuade others, who perhaps are not aware how near at hand it lies, to look for themselves into the wonder-world of the astronomers. there is only one way in which you can be sure of getting a good telescope. first, decide how large a glass you are to have, then go to a maker of established reputation, fix upon the price you are willing to pay--remembering that good work is never cheap--and finally see that the instrument furnished to you answers the proper tests for a telescope of its size. there are telescopes and telescopes. occasionally a rare combination of perfect homogeneity in the material, complete harmony between the two kinds of glass of which the objective is composed, and lens surfaces whose curves are absolutely right, produces a telescope whose owner would part with his last dollar sooner than with it. such treasures of the lens-maker's art can not, perhaps, be commanded at will, yet, they are turned out with increasing frequency, and the best artists are generally able, at all times, to approximate so closely to perfection that any shortcoming may be disregarded. in what is said above i refer, of course, to the refracting telescope, which is the form of instrument that i should recommend to all amateurs in preference to the reflector. but, before proceeding further, it may be well to recall briefly the principal points of difference between these two kinds of telescopes. the purpose of a telescope of either description is, first, to form an image of the object looked at by concentrating at a focus the rays of light proceeding from that object. the refractor achieves this by means of a carefully shaped lens, called the object glass, or objective. the reflector, on the other hand, forms the image at the focus of a concave mirror. [illustration: image at the focus of a lens.] a very pretty little experiment, which illustrates these two methods of forming an optical image, and, by way of corollary, exemplifies the essential difference between refracting and reflecting telescopes, may be performed by any one who possesses a reading glass and a magnifying hand mirror. in a room that is not too brightly illuminated pin a sheet of white paper on the wall opposite to a window that, by preference, should face the north, or away from the position of the sun. taking first the reading glass, hold it between the window and the wall parallel to the sheet of paper, and a foot or more distant from the latter. by moving it to and fro a little you will be able to find a distance, corresponding to the focal length of the lens, at which a picture of the window is formed on the paper. this picture, or image, will be upside down, because the rays of light cross at the focus. by moving the glass a little closer to the wall you will cause the picture of the window to become indistinct, while a beautiful image of the houses, trees, or other objects of the outdoor world beyond, will be formed upon the paper. we thus learn that the distance of the image from the lens varies with the distance of the object whose image is formed. in precisely a similar manner an image is formed at the focus of the object glass of a refracting telescope. [illustration: image at the focus of a concave mirror.] take next your magnifying or concave mirror, and detaching the sheet of paper from the wall, hold it nearly in front of the mirror between the latter and the window. when you have adjusted the distance to the focal length of the mirror, you will see an image of the window projected upon the paper, and by varying the distance, as before, you will be able to produce, at will, pictures of nearer or more remote objects. it is in this way that images are formed at the focus of the mirror of a reflecting telescope. now, you will have observed that the chief apparent difference between these two methods of forming an image of distant objects is that in the first case the rays of light, passing through the transparent lens, are brought to a focus on the side opposite to that where the real object is, while in the second case the rays, being reflected from the brilliant surface of the opaque mirror, come to a focus on the same side as that on which the object itself is. from this follows the most striking difference in the method of using refracting and reflecting telescopes. in the refractor the observer looks toward the object; in the reflector he looks away from it. sir william herschel made his great discoveries with his back to the sky. he used reflecting telescopes. this principle, again, can be readily illustrated by means of our simple experiment with a reading glass and a magnifying mirror. hold the reading glass between the eye and a distant object with one hand, and with the other hand place a smaller lens such as a pocket magnifier, near the eye, and in line with the reading glass. move the two carefully until they are at a distance apart equal to the sum of the focal lengths of the lenses, and you will see a magnified image of the distant object. in other words, you have constructed a simple refracting telescope. then take the magnifying mirror, and, turning your back to the object to be looked at, use the small lens as before--that is to say, hold it between your eye and the mirror, so that its distance from the latter is equal to the sum of the focal lengths of the mirror and the lens, and you will see again a magnified image of the distant object. this time it is a reflecting telescope that you hold in your hands. the magnification of the image reminds us of the second purpose which is subserved by a telescope. a telescope, whether refracting or reflecting, consists of two essential parts, the first being a lens, or a mirror, to form an image, and the second a microscope, called an eyepiece, to magnify the image. the same eyepieces will serve for either the reflector or the refractor. but in order that the magnification may be effective, and serve to reveal what could not be seen without it, the image itself must be as nearly perfect as possible; this requires that every ray of light that forms the image shall be brought to a point in the image precisely corresponding to that from which it emanates in the real object. in reflectors this is effected by giving a parabolic form to the concave surface of the mirror. in refractors there is a twofold difficulty to be overcome. in the first place, a lens with spherical surfaces does not bend all the rays that pass through it to a focus at precisely the same distance. the rays that pass near the outer edge of the lens have a shorter focus than that of the rays which pass near the center of the lens; this is called spherical aberration. a similar phenomenon occurs with a concave mirror whose surface is spherical. in that case, as we have seen, the difficulty is overcome by giving the mirror a parabolic instead of a spherical form. in an analogous way the spherical aberration of a lens can be corrected by altering its curves, but the second difficulty that arises with a lens is not so easily disposed of: this is what is called chromatic aberration. it is due to the fact that the rays belonging to different parts of the spectrum have different degrees of refrangibility, or, in other words, that they come to a focus at different distances from the lens; and this is independent of the form of the lens. the blue rays come to a focus first, then the yellow, and finally the red. it results from this scattering of the spectral rays along the axis of the lens that there is no single and exact focus where all meet, and that the image of a star, for instance, formed by an ordinary lens, even if the spherical aberration has been corrected, appears blurred and discolored. there is no such difficulty with a mirror, because there is in that case no refraction of the light, and consequently no splitting up of the elements of the spectrum. in order to get around the obstacle formed by chromatic aberration it is necessary to make the object glass of a refractor consist of two lenses, each composed of a different kind of glass. one of the most interesting facts in the history of the telescope is that sir isaac newton could see no hope that chromatic aberration would be overcome, and accordingly turned his attention to the improvement of the reflecting telescope and devised a form of that instrument which still goes under his name. and even after chester more hall in , and john dollond in , had shown that chromatic aberration could be nearly eliminated by the combination of a flint-glass lens with one of crown glass, william herschel, who began his observations in , devoted his skill entirely to the making of reflectors, seeing no prospect of much advance in the power of refractors. a refracting telescope which has been freed from the effects of chromatic aberration is called achromatic. the principle upon which its construction depends is that by combining lenses of different dispersive power the separation of the spectral colors in the image can be corrected while the convergence of the rays of light toward a focus is not destroyed. flint glass effects a greater dispersion than crown glass nearly in the ratio of three to two. the chromatic combination consists of a convex lens of crown backed by a concave, or plano-concave, lens of flint. when these two lenses are made of focal lengths which are directly proportional to their dispersions, they give a practically colorless image at their common focus. the skill of the telescope-maker and the excellence of his work depend upon the selection of the glasses to be combined and his manipulation of the curves of the lenses. [illustration: achromatic object glass. _a_, crown glass; _b_, flint glass.] now, the reader may ask, "since reflectors require no correction for color dispersion, while that correction is only approximately effected by the combination of two kinds of lenses and two kinds of glass in a refractor, why is not the reflector preferable to the refractor?" the answer is, that the refractor gives more light and better definition. it is superior in the first respect because a lens transmits more light than a mirror reflects. professor young has remarked that about eighty-two per cent of the light reaches the eye in a good refractor, while "in a newtonian reflector, in average condition, the percentage seldom exceeds fifty per cent, and more frequently is lower than higher." the superiority of the refractor in regard to definition arises from the fact that any distortion at the surface of a mirror affects the direction of a ray of light three times as much as the same distortion would do at the surface of a lens. and this applies equally both to permanent errors of curvature and to temporary distortions produced by strains and by inequality of temperature. the perfect achromatism of a reflector is, of course, a great advantage, but the chromatic aberration of refractors is now so well corrected that their inferiority in that respect may be disregarded. it must be admitted that reflectors are cheaper and easier to make, but, on the other hand, they require more care, and their mirrors frequently need resilvering, while an object glass with reasonable care never gets seriously out of order, and will last for many a lifetime. enough has now, perhaps, been said about the respective properties of object glasses and mirrors, but a word should be added concerning eyepieces. without a good eyepiece the best telescope will not perform well. the simplest of all eyepieces is a single double-convex lens. with such a lens the magnifying power of the telescope is measured by the ratio of the focal length of the objective to that of the eye lens. suppose the first is sixty inches and the latter half an inch; then the magnifying power will be a hundred and twenty diameters--i. e., the disk of a planet, for instance, will be enlarged a hundred and twenty times along each diameter, and its area will be enlarged the square of a hundred and twenty, or fourteen thousand four hundred times. but in reckoning magnifying power, diameter, not area, is always considered. for practical use an eyepiece composed of an ordinary single lens is seldom advantageous, because good definition can only be obtained in the center of the field. lenses made according to special formulæ, however, and called solid eyepieces, give excellent results, and for high powers are often to be preferred to any other. the eyepieces usually furnished with telescopes are, in their essential principles, compound microscopes, and they are of two descriptions, "positive" and "negative." the former generally goes under the name of its inventor, ramsden, and the latter is named after great dutch astronomer, huygens. the huygens eyepiece consists of two plano-convex lenses whose focal lengths are in the ratio of three to one. the smaller lens is placed next to the eye. both lenses have their convex surfaces toward the object glass, and their distance apart is equal to half the sum of their focal lengths. in this kind of eyepiece the image is formed between the two lenses, and if the work is properly done such an eyepiece is achromatic. it is therefore generally preferred for mere seeing purposes. in the ramsden eyepiece two plano-convex lenses are also used, but they are of equal focal length, are placed at a distance apart equal to two thirds of the focal length of either, and have their convex sides facing one another. with such an eyepiece the image viewed is beyond the farther or field lens instead of between the two lenses, and as this fact renders it easier to adjust wires or lines for measuring purposes in the focus of the eyepiece, the ramsden construction is used when a micrometer is to be employed. in order to ascertain the magnifying power which an eyepiece gives when applied to a telescope it is necessary to know the equivalent, or combined, focal length of the two lenses. two simple rules, easily remembered, supply the means of ascertaining this. the equivalent focal length of a negative or huygens eyepiece is equal to half the focal length of the larger or field lens. the equivalent focal length of a positive or ramsden eyepiece is equal to three fourths of the focal length of either of the lenses. having ascertained the equivalent focal length of the eyepiece, it is only necessary to divide it into the focal length of the object glass (or mirror) in order to know the magnifying power of your telescope when that particular eyepiece is in use. [illustration: negative eyepiece.] [illustration: positive eyepiece.] a first-class object glass (or mirror) will bear a magnifying power of one hundred to the inch of aperture when the air is in good condition--that is, if you are looking at stars. if you are viewing the moon, or a planet, better results will always be obtained with lower powers--say fifty to the inch at the most. and under ordinary atmospheric conditions a power of from fifty to seventy-five to the inch is far better for stars than a higher power. with a five-inch telescope that would mean from two hundred and fifty to three hundred and seventy-five diameters, and such powers should only be applied for the sake of separating very close double stars. as a general rule, the lowest power that will distinctly show what you desire to see gives the best results. the experienced observer never uses as high powers as the beginner does. the number of eyepieces purchased with a telescope should never be less than three--a very low power--say ten to the inch; a very high power, seventy-five or one hundred to the inch, for occasional use; and a medium power--say forty to the inch--for general use. if you can afford it, get a full battery of eyepieces--six or eight, or a dozen--for experience shows that different objects require different powers in order to be best seen, and, moreover, a slight change of power is frequently a great relief to the eye. there is one other thing of great importance to be considered in purchasing a telescope--the mounting. if your glass is not well mounted on a steady and easily managed stand, you might better have spent your money for something more useful. i have endured hours of torment while trying to see stars through a telescope that was shivering in the wind and dancing to every motion of the bystanders, to say nothing of the wriggling contortions caused by the application of my own fingers to the focusing screw. the best of all stands is a solid iron pillar firmly fastened into a brick or stone pier, sunk at least four feet in the ground, and surmounted by a well-made equatorial bearing whose polar axis has been carefully placed in the meridian. it can be readily protected from the weather by means of a wooden hood or a rubber sheet, while the tube of the telescope may be kept indoors, being carried out and placed on its bearing only when observations are to be made. with such a mounting you can laugh at the observatories with their cumbersome domes, for the best of all observatories is the open air. but if you dislike the labor of carrying and adjusting the tube every time it is used, and are both fond of and able to procure luxuries, then, after all, perhaps, you had better have the observatory, dome, draughts and all. the next best thing in the way of a mounting is a portable tripod stand. this may be furnished either with an equatorial bearing for the telescope, or an altazimuth arrangement which permits both up-and-down and horizontal motions. the latter is cheaper than the equatorial and proportionately inferior in usefulness and convenience. the essential principle of the equatorial bearing is motion about two axes placed at right angles to one another. when the polar axis is in the meridian, and inclined at an angle equal to the latitude of the place, the telescope can be moved about the two axes in such a way as to point to any quarter of the sky, and the motion of a star, arising from the earthy rotation, can be followed hour after hour without disturbing the instrument. when thus mounted, the telescope may be driven by clockwork, or by hand with the aid of a screw geared to a handle carrying a universal joint. and now for testing the telescope. it has already been remarked that the excellence of a telescope depends upon the perfection of the image formed at the focus of the objective. in what follows i have only a refractor in mind, although the same principles would apply to a reflector. with a little practice anybody who has a correct eye can form a fair judgment of the excellence of a telescopic image. suppose we have our telescope steadily mounted out of doors (if you value your peace of mind you will not try to use a telescope pointed out of a window, especially in winter), and suppose we begin our observations with the pole star, employing a magnifying power of sixty or seventy to the inch. our first object is to see if the optician has given us a good glass. if the air is not reasonably steady we had better postpone our experiment to another night, because we shall find that the star as seen in the telescope flickers and "boils," and behaves in so extraordinary a fashion that there is no more definition in the image than there is steadiness in a bluebottle buzzing on a window pane. but if the night is a fine one the star image will be quiescent, and then we may note the following particulars: the real image is a minute bright disk, about one second of arc in diameter if we are using a four-and-a-half or five-inch telescope, and surrounded by one very thin ring of light, and the fragments, so to speak, of one or possibly two similar rings a little farther from the disk, and visible, perhaps, only by glimpses. these "diffraction rings" arise from the undulatory nature of light, and their distance apart as well as the diameter of the central disk depend upon the length of the waves of light. if the telescope is a really good one, and both object glass and eyepiece are properly adjusted, the disk will be perfectly round, slightly softer at the edge, but otherwise equally bright throughout; and the ring or rings surrounding it will be exactly concentric, and not brighter on one side than on another. even if our telescope were only two inches or two inches and a half in aperture we should at once notice a little bluish star, the mere ghost of a star in a small telescope, hovering near the polar star. it is the celebrated "companion," but we shall see it again when we have more time to study it. now let us put the star out of focus by turning the focusing screw. suppose we turn it in such a way that the eyepiece moves slightly outside the focus, or away from the object glass. very beautiful phenomena immediately begin to make their appearance. a slight motion outward causes the little disk to expand perceptibly, and just as this expansion commences, a bright-red point appears at the precise center of the disk. but, the outward motion continuing, this red center disappears, and is replaced by a blue center, which gradually expands into a sort of flare over the middle of the disk. the disk itself has in the mean time enlarged into a series of concentric bright rings, graduated in luminosity with beautiful precision from center toward circumference. the outermost ring is considerably brighter, however, than it would be if the same gradation applied to it as applies to the inner rings, and it is surrounded, moreover, on its outer edge by a slight flare which tends to increase its apparent width. next let us return to the focus and then move the eyepiece gradually inside the focal point or plane. once more the star disk expands into a series of circles, and, if we except the color phenomena noticed outside the focus, these circles are precisely like those seen before in arrangement, in size, and in brightness. if they were not the same, we should pronounce the telescope to be imperfect. there is one other difference, however, besides the absence of the blue central flare, and that is a faint reddish edging around the outer ring when the expansion inside the focus is not carried very far. upon continuing to move the eyepiece inside or outside the focus we observe that the system of rings becomes larger, while the rings themselves rapidly increase in number, becoming at the same time individually thinner and fainter. [illustration: the star image.] by studying the appearance of the star disk when in focus and of the rings when out of focus on either side, an experienced eye can readily detect any fault that a telescope may have. the amateur, of course, can only learn to do this by considerable practice. any glaring and serious fault, however, will easily make itself manifest. suppose, for example, we observe that the image of a star instead of being perfectly round is oblong, and that a similar defect appears in the form of the rings when the eyepiece is put out of focus. we know at once that something is wrong; but the trouble may lie either in the object glass, in the eyepiece, in the eye of the observer himself, or in the adjustment of the lenses in the tube. a careful examination of the image and the out-of-focus circles will enable us to determine with which of these sources of error we have to deal. if the star image when in focus has a sort of wing on one side, and if the rings out of focus expand eccentrically, appearing wider and larger on one side than on the other, being at the same time brightest on the least expanded side, then the object glass is probably not at right angles to the axis of the tube and requires readjustment. that part of the object glass on the side where the rings appear most expanded and faintest needs to be pushed slightly inward. this can be effected by means of counterscrews placed for that purpose in or around the cell. but it, after we have got the object glass properly squared to the axis of the tube or the line of sight, the image and the ring system in and out of focus still appear oblong, the fault of astigmatism must exist either in the objective, the eyepiece, or the eye. the chances are very great that it is the eye itself that is at fault. we may be certain of this if we find, on turning the head so as to look into the telescope with the eye in different positions, that the oblong image turns with the head of the observer, keeping its major axis continually in the same relative position with respect to the eye. the remedy then is to consult an oculist and get a pair of cylindrical eyeglasses. if the oblong image does not turn round with the eye, but does turn when the eyepiece is twisted round, then the astigmatism is in the latter. if, finally, it does not follow either the eye or the eyepiece, it is the objective that is at fault. but instead of being oblong, the image and the rings may be misshapen in some other way. if they are three-cornered, it is probable that the object glass is subjected to undue pressure in its cell. this, if the telescope has been brought out on a cool night from a warm room, may arise from the unequal contraction of the metal work and the glass as they cool off. in fact, no good star image can be got while a telescope is assuming the temperature of the surrounding atmosphere. even the air inclosed in the tube is capable of making much trouble until its temperature has sunk to the level of that outside. half an hour at least is required for a telescope to adjust itself to out-of-door temperature, except in the summer time, and it is better to allow an hour or two for such adjustment in cold weather. any irregularity in the shape of the rings which persists after the lenses have been accurately adjusted and the telescope has properly cooled may be ascribed to imperfections, such as veins or spots of unequal density in the glass forming the objective. [illustration: the out-of-focus rings. , correct figure; and , spherical aberration.] the spherical aberration of an object glass may be undercorrected or overcorrected. in the former case the central rings inside the focus will appear faint and the outer ones unduly strong, while outside the focus the central rings will be too bright and the outer ones too feeble. but if the aberration is overcorrected the central rings will be overbright inside the focus and abnormally faint outside the focus. [illustration: two views of mars in . the smaller with a three-and-three-eighths-inch telescope; the larger with a nine-inch.] assuming that we have a telescope in which no obvious fault is discernible, the next thing is to test its powers in actual work. in what is to follow i shall endeavor to describe some of the principal objects in the heavens from which the amateur observer may expect to derive pleasure and instruction, and which may at the same time serve as tests of the excellence of his telescope. no one should be deterred or discouraged in the study of celestial objects by the apparent insignificance of his means of observation. the accompanying pictures of the planet mars may serve as an indication of the fact that a small telescope is frequently capable of doing work that appears by no means contemptible when placed side by side with that of the greater instruments of the observatories. chapter ii in the starry heavens "now constellations, muse, and signs rehearse; in order let them sparkle in thy verse."--manilius. let us imagine ourselves the happy possessors of three properly mounted telescopes of five, four, and three inches aperture, respectively. a fine midwinter evening has come along, the air is clear, cool, and steady, and the heavens, of that almost invisible violet which is reserved for the lovers of celestial scenery, are spangled with stars that hardly twinkle. we need not disturb our minds about a few thin clouds here and there floating lazily in the high air; they announce a change of weather, but they will not trouble us to-night. which way shall we look? our eyes will answer the question for us. however we may direct them, they instinctively return to the south, and are lifted to behold orion in his glory, now near the meridian and midway to the zenith, with taurus shaking the glittering pleiades before him, and canis major with the flaming dog star following at his heels. not only is orion the most brilliant of all constellations to the casual star-gazer, but it contains the richest mines that the delver for telescopic treasures can anywhere discover. we could not have made a better beginning, for here within a space of a few square degrees we have a wonderful variety of double stars and multiple stars, so close and delicate as to test the powers of the best telescopes, besides a profusion of star-clusters and nebulæ, including one of the supreme marvels of space, the great nebula in the sword. [illustration: map no. .] our star map no. will serve as a guide to the objects which we are about to inspect. let us begin operations with our smallest telescope, the three-inch. i may remark here that, just as the lowest magnifying power that will clearly reveal the object looked for gives ordinarily better results than a higher power, so the smallest telescope that is competent to show what one wishes to see is likely to yield more satisfaction, as far as that particular object is concerned, than a larger glass. the larger the object glass and the higher the power, the greater are the atmospheric difficulties. a small telescope will perform very well on a night when a large one is helpless. turn the glass upon beta (rigel), the white first-magnitude star in orion's left foot. observe whether the image with a high power is clear, sharp, and free from irregular wisps of stray light. look at the rings in and out of focus, and if you are satisfied with the performance, try for the companion. a good three-inch is certain to show it, except in a bad state of the atmosphere, and even then an expert can see it, at least by glimpses. the companion is of the ninth magnitude, some say the eighth, and the distance is about . ", angle of position (hereafter designated by p.) °.[ ] its color is blue, in decided contrast with the white light of its great primary. sir john herschel, however, saw the companion red, as others have done. these differences are doubtless due to imperfections of the eye or the telescope. in burnham believed he had discovered that the companion was an exceedingly close double star. no one except burnham himself succeeded in dividing it, and he could only do so at times. afterward, when he was at mount hamilton, he tried in vain to split it with the great thirty-six-inch telescope, in , , and . his want of success induced him to suggest that the component stars were in rapid motion, and so, although he admitted that it might not be double after all, he advised that it should be watched for a few years longer. his confidence was justified, for in aitken, with the lick telescope, saw and measured the distance of the extremely minute companion--distance . ", p. °. [ ] the angle of position measures the inclination to the meridian of a line drawn between the principal star and its companion; in other words, it shows in what direction from the primary we must look for the companion. it is reckoned from ° up to °, beginning at the north point and passing around by east through south and west to north again. thus, if the angle of position is ° or °, the companion is on the north side of the primary; if the angle is °, the companion is to the east; if °, to the south; if °, to the west, and so for intermediate angles. it must be remembered, however, that in the field of the telescope the top is south and the bottom north, unless a prism is used, when directions become complicated. east and west can be readily identified by noticing the motion of a star through the field; it moves toward the west and from the east. rigel has been suspected of a slight degree of variability. it is evidently a star of enormous actual magnitude, for its parallax escapes trustworthy measurement. it can only be ranked among the very first of the light-givers of the visible universe. spectroscopically it belongs to a peculiar type which has very few representatives among the bright stars, and which has been thus described: "spectra in which the hydrogen lines and the few metallic lines all appear to be of equal breadth and sharp definition." rigel shows a line which some believe to represent magnesium; but while it has iron lines in its spectrum, it exhibits no evidence of the existence of any such cloud of volatilized iron as that which helps to envelop the sun. for another test of what the three-inch will do turn to zeta, the lower, or left-hand, star in the belt. this is a triple, the magnitudes being second, sixth, and tenth. the sixth-magnitude star is about . " from the primary, p. °, and has a very peculiar color, hard to describe. it requires careful focusing to get a satisfactory view of this star with a three-inch telescope. use magnifying powers up to two hundred and fifty diameters. with our four-inch the star is much easier, and the five-inch shows it readily with a power of one hundred. the tenth-magnitude companion is distant ", p. °, and may be glimpsed with the three-inch. upon the whole, we shall find that we get more pleasing views of zeta orionis with the four-inch glass. just to the left of zeta, and in the same field of view with a very low power, is a remarkable nebula bearing the catalogue number . we must use our five-inch on this with a low power, but with zeta out of the field in order to avoid its glare. the nebula is exceedingly faint, and we can be satisfied if we see it simply as a hazy spot, although with much larger telescopes it has appeared at least half a degree broad. tempel saw several centers of condensation in it, and traced three or four broad nebulous streams, one of which decidedly suggested spiral motion. the upper star in the belt, delta, is double; distance, ", p. °; magnitudes, second and seventh very nearly; colors, white and green or blue. this, of course, is an easy object for the three-inch with a low magnifying power. it would be useless to look for the two fainter companions of delta, discovered by burnham, even with our five-inch glass. but we shall probably need the five-inch for our next attempt, and it will be well to put on a high power, say three hundred diameters. the star to be examined is the little brilliant dangling below the right-hand end of the belt, toward rigel. it appears on the map as eta. spare no pains in getting an accurate focus, for here is something worth looking at, and unless you have a trained eye you will not easily see it. the star is double, magnitudes third and sixth, and the distance from center to center barely exceeds ", p. °. a little tremulousness of the atmosphere for a moment conceals the smaller star, although its presence is manifest from the peculiar jutting of light on one side of the image of the primary. but in an instant the disturbing undulations pass, the air steadies, the image shrinks and sharpens, and two points of piercing brightness, almost touching one another, dart into sight, the more brilliant one being surrounded by an evanescent circle, a tiny ripple of light, which, as it runs round the star and then recedes, alternately embraces and releases the smaller companion. the wash of the light-waves in the atmosphere provokes many expressions of impatience from the astronomer, but it is often a beautiful phenomenon nevertheless. between eta and delta is a fifth-magnitude double star, sigma , which is worth a moment's attention. the primary, of a reddish color, has a very faint star, eleventh magnitude, at a distance of . ", p. °. still retaining the five-inch in use, we may next turn to the other end of the belt, where, just under zeta, we perceive the fourth-magnitude star sigma. he must be a person of indifferent mind who, after looking with unassisted eyes at the modest glimmering of this little star, can see it as the telescope reveals it without a thrill of wonder and a cry of pleasure. the glass, as by a touch of magic, changes it from one into eight or ten stars. there are two quadruple sets three and a half minutes of arc apart. the first set exhibits a variety of beautiful colors. the largest star, of fourth magnitude, is pale gray; the second in rank, seventh magnitude, distance ", p. °, presents a singular red, "grape-red" webb calls it; the third, eighth magnitude, distance ", p. °, is blue; and the fourth, eleventh magnitude, distance ", p. °, is apparently white. burnham has doubled the fourth-magnitude star, distance . ". the second group of four stars consists of three of the eighth to ninth magnitude, arranged in a minute triangle with a much fainter star near them. between the two quadruple sets careful gazing reveals two other very faint stars. while the five-inch gives a more satisfactory view of this wonderful multiple star than any smaller telescope can do, the four-inch and even the three-inch would have shown it to us as a very beautiful object. however we look at them, there is an appearance of association among these stars, shining with their contrasted colors and their various degrees of brilliance, which is significant of the diversity of conditions and circumstances under which the suns and worlds beyond the solar walk exist. from sigma let us drop down to see the wonders of orion's sword displayed just beneath. we can use with advantage any one of our three telescopes; but since we are going to look at a nebula, it is fortunate that we have a glass so large as five inches aperture. it will reveal interesting things that escape the smaller instruments, because it grasps more than one and a half times as much light as the four-inch, and nearly three times as much as the three-inch; and in dealing with nebulæ a plenty of light is the chief thing to be desired. the middle star in the sword is theta, and is surrounded by the celebrated nebula of orion. the telescope shows theta separated into four stars arranged at the corners of an irregular square, and shining in a black gap in the nebula. these four stars are collectively named the trapezium. the brightest is of the sixth magnitude, the others are of the seventh, seven and a half, and eighth magnitudes respectively. the radiant mist about them has a faint greenish tinge, while the four stars, together with three others at no great distance, which follow a fold of the nebula like a row of buttons on a coat, always appear to me to show an extraordinary liveliness of radiance, as if the strange haze served to set them off. [illustration: the trapezium with the fifth and sixth stars.] our three-inch would have shown the four stars of the trapezium perfectly well, and the four-inch would have revealed a fifth star, very faint, outside a line joining the smallest of the four and its nearest neighbor. but the five-inch goes a step farther and enables us, with steady gazing to see even a sixth star, of only the twelfth magnitude, just outside the trapezium, near the brightest member of the quartet. the lick telescope has disclosed one or two other minute points of light associated with the trapezium. but more interesting than the trapezium is the vast cloud, full of strange shapes, surrounding it. nowhere else in the heavens is the architecture of a nebula so clearly displayed. it is an unfinished temple whose gigantic dimensions, while exalting the imagination, proclaim the omnipotence of its builder. but though unfinished it is not abandoned. the work of creation is proceeding within its precincts. there are stars apparently completed, shining like gems just dropped from the hand of the polisher, and around them are masses, eddies, currents, and swirls of nebulous matter yet to be condensed, compacted, and constructed into suns. it is an education in the nebular theory of the universe merely to look at this spot with a good telescope. if we do not gaze at it long and wistfully, and return to it many times with unflagging interest, we may be certain that there is not the making of an astronomer in us. before quitting the orion nebula do not fail to notice an eighth-magnitude star, a short distance northeast of the great nebula, and nearly opposite the broad opening in the latter that leads in toward the gap occupied by the trapezium. this star is plainly enveloped in nebulosity, that is unquestionably connected with the larger mass of which it appears to form a satellite. at the lower end of the sword is the star iota, somewhat under the third magnitude. our three-inch will show that it has a bluish companion of seventh or eighth magnitude, at a little more than " distance, p. °, and the larger apertures will reveal a third star, of tenth magnitude, and reddish in color, distance ", p. °. close by iota we find the little double star sigma , whose components are of five and a half and six and a half magnitudes respectively, and separated ", p. °. above the uppermost star in the sword is a small star cluster, no. , which derives a special interest from the fact that it incloses a delicate double star, sigma , whose larger component is of the sixth magnitude, while the smaller is of the ninth, and the distance is only . ", p. °. we may try the four-inch on this object. having looked at alpha (betelgeuse), the great topaz star on orion's right shoulder, and admired the splendor of its color, we may turn the four-inch upon the star sigma , frequently referred to by its number as " orionis." it consists of one star of the sixth and another of sixth and a half magnitude, only . " apart, p. °. having separated them with a power of two hundred and fifty diameters on the four-inch, we may try them with a high power on the three-inch. we shall only succeed this time if our glass is of first-rate quality and the air is perfectly steady. the star lambda in orion's head presents an easy conquest for the three-inch, as it consists of a light-yellow star of magnitude three and a half and a reddish companion of the sixth magnitude; distance ", p. °. there is also a twelfth-magnitude star at ", p. °, and a tenth or eleventh magnitude one at ", p. °. these are tests for the five-inch, and we must not be disappointed if we do not succeed in seeing the smaller one even with that aperture. other objects in orion, to be found with the aid of our map, are: sigma , a double star, magnitude six and a half and seven, distance ", p. °; omicron sigma , otherwise named iota orionis, double, magnitude six and seven, distance ", p. °, requires five-inch glass; sigma , double, magnitudes six and a half and eight, distance . ", p. °; rho, double, magnitudes five and eight and a half, the latter blue, distance ", p. °, may be tried with a three-inch; tau, triple star, magnitudes four, ten and a half, and eleven, distances ", p. °, and ", p. °. burnham discovered that the ten-and-a-half magnitude star is again double, distance ", p. °. there is not much satisfaction in attempting tau orionis with telescopes of ordinary apertures; sigma otherwise _m_ orionis, double, magnitudes five and a half (greenish) and seven, distance . ", p. °, a pretty object; sigma , otherwise a , double, magnitudes five and seven, distance, . " or less, p. °, a rapid binary,[ ] which is at present too close for ordinary telescopes, although it was once within their reach; sigma , double, magnitudes six and eight, distance ", p. °, the smaller star pale blue--try it with a four-inch, but five-inch is better; sigma , double, magnitudes six and half and eight and a half, distance ", p. °; psi , double, magnitudes five and a half and eleven, distance ", or a little less, p. °; , star cluster, contains about twenty stars from the eighth to the eleventh magnitude; , nebula, faint, containing a triple star of the eighth magnitude, two of whose components are " apart, while the third is only . " from its companion, p. °; , star cluster, small and crowded; , star cluster, triangular shape, containing thirty stars, seventh to tenth magnitudes, one of which is a double, distance . ". [ ] the term "binary" is used to describe double stars which are in motion about their common center of gravity. let us now leave the inviting star-fields of orion and take a glance at the little constellation of lepus, crouching at the feet of the mythical giant. we may begin with a new kind of object, the celebrated red variable r leporis (map no. ). this star varies from the sixth or seventh magnitude to magnitude eight and a half in a period of four hundred and twenty-four days. hind's picturesque description of its color has frequently been quoted. he said it is "of the most intense crimson, resembling a blood-drop on the black ground of the sky." it is important to remember that this star is reddest when faintest, so that if we chance to see it near its maximum of brightness it will not impress us as being crimson at all, but rather a dull, coppery red. its spectrum indicates that it is smothered with absorbing vapors, a sun near extinction which, at intervals, experiences an accession of energy and bursts through its stifling envelope with explosive radiance, only to faint and sink once more. it is well to use our largest aperture in examining this star. we may also employ the five-inch for an inspection of the double star iota, whose chief component of the fifth magnitude is beautifully tinged with green. the smaller companion is very faint, eleventh magnitude, and the distance is about ", p. °. another fine double in lepus is kappa, to be found just below iota; the components are of the fifth and eighth magnitudes, pale yellow and blue respectively, distance . ", p. °; the third-magnitude star alpha has a tenth-magnitude companion at a distance of ", p. °, and its neighbor beta (map no. ), according to burnham, is attended by three eleventh-magnitude stars, two of which are at distances of ", p. °, and ", p. °, respectively, while the third is less than " from beta, p. °; the star gamma (map no. ) is a wide double, the distance being ", and the magnitudes four and eight. the star numbered is a remarkable multiple, but the components are too faint to possess much interest for those who are not armed with very powerful telescopes. [illustration: map no. .] from lepus we pass to canis major (map no. ). there is no hope of our being able to see the companion of alpha (sirius), at present ( ), even with our five-inch. discovered by alvan clark with an eighteen-inch telescope in , when its distance was " from the center of sirius, this ninth-magnitude star has since been swallowed up in the blaze of its great primary. at first, it slightly increased its distance, and from until most of the measures made by different observers considerably exceeded ". then it began to close up, and in the distance scarcely exceeded ". burnham was the last to catch sight of it with the lick telescope in that year. after that no human eye saw it until , when it was rediscovered at the lick observatory. since then the distance has gradually increased to nearly ". according to burnham, its periodic time is about fifty-three years, and its nearest approach to sirius should have taken place in the middle of . later calculations reduce the periodic time to forty-eight or forty-nine years. if we can not see the companion of the dog star with our instruments, we can at least, while admiring the splendor of that dazzling orb, reflect with profit upon the fact that although the companion is ten thousand times less bright than sirius, it is half as massive as its brilliant neighbor. imagine a subluminous body half as ponderous as the sun to be set revolving round it somewhere between uranus and neptune. remember that that body would possess one hundred and sixty-five thousand times the gravitating energy of the earth, and that five hundred and twenty jupiters would be required to equal its power of attraction, and then consider the consequences to our easy-going planets! plainly the solar system is not cut according to the sirian fashion. we shall hardly find a more remarkable coupling of celestial bodies until we come, on another evening, to a star that began, ages ago, to amaze the thoughtful and inspire the superstitious with dread--the wonderful algol in perseus. we may remark in passing that sirius is the brightest representative of the great spectroscopic type i, which includes more than half of all the stars yet studied, and which is characterized by a white or bluish-white color, and a spectrum possessing few or at best faint metallic lines, but remarkably broad, black, and intense lines of hydrogen. the inference is that sirius is surrounded by an enormous atmosphere of hydrogen, and that the intensity of its radiation is greater, surface for surface, than that of the sun. there is historical evidence to support the assertion, improbable in itself, that sirius, within eighteen hundred years, has changed color from red to white. with either of our telescopes we shall have a feast for the eye when we turn the glass upon the star cluster no. , some four degrees south of sirius. look for a red star near the center. observe the curving rows so suggestive of design, or rather of the process by which this cluster was evolved out of a pre-existing nebula. you will recall the winding streams in the great nebula of orion. another star cluster worth a moment's attention is no. , above and to the left of sirius. we had better use the five-inch for this, as many of the stars are very faint. not far away we find the double star , whose components are of the fifth and eighth magnitudes, distance . ", p. °. the small star is pale blue. cluster no. is a pleasing object with our largest aperture. in no. we have a faint nebula remarkable for the rows of minute stars in and near it. the star gamma is an irregular variable. in it is said to have almost disappeared, while at the beginning of the eighteenth century it was more than twice as bright as it is to-day. the reddish star delta is also probably variable. in my "astronomy with an opera glass" will be found a cut showing a singular array of small stars partly encircling delta. these are widely scattered by a telescope, even with the lowest power. eastward from canis major we find some of the stars of argo navis. sigma , of the sixth magnitude, has two minute companions at " distance, p. ° and °. the large star is itself double, but the distance, . ", p. °, places it beyond our reach. according to burnham, there is yet a fourth faint star at ", p. °. some three degrees and a half below and to the left of the star just examined is a beautiful star cluster, no. . nos. , , and are other star clusters well worth examination. a planetary nebula is included in . with very powerful telescopes this nebula has been seen ring-shaped. sigma , otherwise known as navis, is a pretty double, colors pale yellow and blue, magnitudes five and seven, distance . ", p. °. our three-inch will suffice for this. [illustration: map no. .] north of canis major and argo we find monoceros and canis minor (map no. ). the stars forming the western end of monoceros are depicted on map no. . we shall begin with these. the most interesting and beautiful is , a fine triple star, magnitudes five, six, and seven, distances . ", p. °, and . ", p. °. sir william herschel regarded this as one of the most beautiful sights in the heavens. it is a good object to try our three-inch on, although it should not be difficult for such an aperture. the star is also a triple, magnitudes six, ten, and eleven, distances . ", p. °, and ", p. °. we should glance at the star to admire its fine orange color. in we find a golden fifth-magnitude star, combined with a blue or lilac star of the seventh magnitude, distance ", p. °. sigma is a difficult double, magnitudes six and a half and twelve, distance ", p. °. sigma is double, magnitudes six and a half and eight, distance ", p. °. at the spot marked on the map we find an interesting cluster containing one star of the sixth magnitude. the remaining stars of monoceros will be found on map no. . the double and triple stars to be noted are s, or sigma (which is also a variable and involved in a faint nebula), magnitudes six and nine, distance . ", p. °; sigma , double, magnitudes five and a half and eight, distance ", p. °; sigma , triple, magnitudes five and a half, ten, and nine, distances ", p. °, and ", p. °. the clusters are , which has a minute triple star near the center; , one member of whose swarm is red; , very small but rich; and , interesting for the great number of ninth-magnitude stars that it contains. we should use the five-inch for all of these. canis minor and the head of hydra are also contained on map no. . procyon, alpha of canis minor, has several minute stars in the same field of view. there is, besides, a companion which, although it was known to exist, no telescope was able to detect until november, . it must be of immense mass, since its attraction causes perceptible perturbations in the motion of procyon. its magnitude is eight and a half, distance . ", p. °. one of the small stars just referred to, the second one east of procyon, distant one third of the moon's diameter, is an interesting double. our four-inch may separate it, and the five-inch is certain to do so. the magnitudes are seven and seven and a half or eight, distance . ", p. °. this star is variously named sigma and can. min. bode. star no. is a wide triple, magnitudes six, seven, and eight, distances , p. °, and ", p. °. procyon and its neighbors. in the head of hydra we find sigma , a double of the sixth and seventh magnitudes, distance . ", p. °. the larger star shows a fine yellow. in epsilon we have a beautiful combination of a yellow with a blue star, magnitudes four and eight, distance . ", p. °. finally, let us look at theta for a light test with the five-inch. the two stars composing it are of the fourth and twelfth magnitudes, distance ", p. °. the brilliant constellations of gemini and taurus tempt us next, but warning clouds are gathering, and we shall do well to house our telescopes and warm our fingers by the winter fire. there will be other bright nights, and the stars are lasting. chapter iii from gemini to leo and round about "if thou wouldst gaze on starry charioteer, and hast heard legends of the wondrous goat, vast looming shalt thou find on the twins' left, his form bowed forward."--poste's aratus. [illustration: map no. .] the zodiacal constellations of gemini, cancer, and leo, together with their neighbors auriga, the lynx, hydra, sextans, and coma berenices, will furnish an abundance of occupation for our second night at the telescope. we shall begin, using our three-inch glass, with alpha, the chief star of gemini (map no. ). this is ordinarily known as castor. even an inexperienced eye perceives at once that it is not as bright as its neighbor pollux, beta. whether this fact is to be regarded as indicating that castor was brighter than pollux in , when bayer attached their greek letters, is still an unsettled question. castor may or may not be a variable, but it is, at any rate, one of the most beautiful double stars in the heavens. a power of one hundred is amply sufficient to separate its components, whose magnitudes are about two and three, the distance between them being ", p. °. a slight yet distinct tinge of green, recalling that of the orion nebula, gives a peculiar appearance to this couple. green is one of the rarest colors among the stars. castor belongs to the same general spectroscopic type in which sirius is found, but its lines of hydrogen are broader than those seen in the spectrum of the dog star. there is reason for thinking that it may be surrounded with a more extensive atmosphere of that gaseous metal called hydrogen than any other bright star possesses. there seems to be no doubt that the components of castor are in revolution around their common center of gravity, although the period is uncertain, varying in different estimates all the way from two hundred and fifty to one thousand years; the longer estimate is probably not far from the truth. there is a tenth-magnitude star, distance ", p. °, which may belong to the same system. from castor let us turn to pollux, at the same time exchanging our three-inch telescope for the four-inch, or, still better, the five-inch. pollux has five faint companions, of which we may expect to see three, as follows: tenth magnitude, distance ", p. °; nine and a half magnitude, distance ", p. °, and ninth magnitude, distance ", p. °. burnham has seen a star of thirteen and a half magnitude, distance ", p. °, and has divided the tenth-magnitude star into two components, only . " apart, the smaller being of the thirteenth magnitude, and situated at the angle °. a calculation based on dr. elkin's parallax of . " for pollux shows that that star may be a hundredfold more luminous than the sun, while its nearest companion may be a body smaller than our planet jupiter, but shining, of course, by its own light. its distance from pollux, however, exceeds that of jupiter from the sun in the ratio of about one hundred and thirty to one. in the double star pi we shall find a good light test for our three-inch aperture, the magnitudes being six and eleven, distance ", p. °. the four-inch will show that kappa is a double, magnitudes four and ten, distance ", p. °. the smaller star is of a delicate blue color, and it has been suspected of variability. that it may be variable is rendered the more probable by the fact that in the immediate neighborhood of kappa there are three undoubted variables, s, t, and u, and there appears to be some mysterious law of association which causes such stars to group themselves in certain regions. none of the variables just named ever become visible to the naked eye, although they all undergo great changes of brightness, sinking from the eighth or ninth magnitude down to the thirteenth or even lower. the variable r, which lies considerably farther west, is well worth attention because of the remarkable change of color which it sometimes exhibits. it has been seen blue, red, and yellow in succession. it varies from between the sixth and seventh magnitudes to less than the thirteenth in a period of about two hundred and forty-two days. not far away we find a still more curious variable zeta; this is also an interesting triple star, its principal component being a little under the third magnitude, while one of the companions is of the seventh magnitude, distance ", p. °, and the other is of the eleventh magnitude or less, distance ", p. °. we should hardly expect to see the fainter companion with the three-inch. the principal star varies from magnitude three and seven tenths down to magnitude four and a half in a period of a little more than ten days. [illustration: wonderful nebula in gemini ( ).] with the four-or five-inch we get a very pretty sight in delta, which appears split into a yellow and a purple star, magnitudes three and eight, distance ", p. °. near delta, toward the east, lies one of the strangest of all the nebulæ. (see the figures on the map.) our telescopes will show it to us only as a minute star surrounded with a nebulous atmosphere, but its appearance with instruments of the first magnitude is so astonishing and at the same time so beautiful that i can not refrain from giving a brief description of it as i saw it in with the great lick telescope. in the center glittered the star, and spread evenly around it was a circular nebulous disk, pale yet sparkling and conspicuous. this disk was sharply bordered by a narrow _black_ ring, and outside the ring the luminous haze of the nebula again appeared, gradually fading toward the edge to invisibility. the accompanying cut, which exaggerates the brightness of the nebula as compared with the star, gives but a faint idea of this most singular object. if its peculiarities were within the reach of ordinary telescopes, there are few scenes in the heavens that would be deemed equally admirable. in the star eta we have another long-period variable, which is also a double star; unfortunately the companion, being of only the tenth magnitude and distant less than " from its third-magnitude primary, is beyond the reach of our telescopes. but eta points the way to one of the finest star clusters in the sky, marked on the map. the naked eye perceives that there is something remarkable in that place, and the opera glass faintly reveals its distant splendors, but the telescope fairly carries us into its presence. its stars are innumerable, varying from the ninth magnitude downward to the last limit of visibility, and presenting a wonderful array of curves which are highly interesting from the point of view of the nebular origin of such clusters. looking backward in time, with that theory to guide us, we can see spiral lines of nebulous mist occupying the space that now glitters with interlacing rows of stars. it is certainly difficult to understand how such lines of nebula could become knotted with the nuclei of future stars, and then gradually be absorbed into those stars; and yet, if such a process does not occur, what is the meaning of that narrow nebulous streak in the pleiades along which five or six stars are strung like beads on a string? the surroundings of this cluster, , as one sweeps over them with the telescope gradually drawing toward the nucleus, have often reminded me of the approaches to such a city as london. thicker and closer the twinkling points become, until at last, as the observers eye follows the gorgeous lines of stars trending inward, he seems to be entering the streets of a brilliantly lighted metropolis. other objects in gemini that we can ill miss are: , double, magnitudes three and eleven, distance ", p. °, colors yellow and blue; , double, magnitudes six and eight, distance ", p. °; gamma, remarkable for array of small stars near it; , double, magnitudes six and eight, distance . ", p. °, colors yellow and blue (very pretty); lambda, double, magnitudes four and eleven, distance ", p. °, color of larger star blue--try with the five-inch; epsilon, double, magnitudes three and nine, distance ", p. °. from gemini we pass to cancer. this constellation has no large stars, but its great cluster præsepe ( on map no. ) is easily seen as a starry cloud with the naked eye. with the telescope it presents the most brilliant appearance with a very low power. it was one of the first objects that galileo turned to when he had completed his telescope, and he wonderingly counted its stars, of which he enumerated thirty-six, and made a diagram showing their positions. the most interesting star in cancer is zeta, a celebrated triple. the magnitudes of its components are six, seven, and seven and a half; distances . ", p. °, and . ", p. °. we must use our five-inch glass in order satisfactorily to separate the two nearest stars. the gravitational relationship of the three stars is very peculiar. the nearest pair revolve around their common center in about fifty-eight years, while the third star revolves with the other two, around a center common to all three, in a period of six or seven hundred years. but the movements of the third star are erratic, and inexplicable except upon the hypothesis advanced by seeliger, that there is an invisible, or dark, star near it by whose attraction its motion is perturbed. in endeavoring to picture the condition of things in zeta cancri we might imagine our sun to have a companion sun, a half or a third as large as itself, and situated within what may be called planetary distance, circling with it around their center of gravity; while a third sun, smaller than the second and several times as far away, and accompanied by a _black_ or non-luminous orb, swings with the first two around another center of motion. there you would have an entertaining complication for the inhabitants of a system of planets! other objects in cancer are: sigma , double star, magnitudes six and six and a half, distance ", p. °; sigma , double, magnitudes both six, distance . ", p. °--four-inch should split it; iota, double, magnitudes four and a half and six and a half, distance ", p. °; , double magnitudes six and nine, distance . ", p. °; sigma , double, magnitudes both about the seventh, distance ", p. °; , star cluster, very beautiful with the five-inch glass. [illustration: map no. .] the constellation of auriga may next command our attention (map no. ). the calm beauty of its leading star capella awakens an admiration that is not diminished by the rivalry of orion's brilliants glittering to the south of it. although capella must be an enormously greater sun than ours, its spectrum bears so much resemblance to the solar spectrum that a further likeness of condition is suggested. no close telescopic companion to capella has been discovered. a ninth-magnitude companion, distant ", p. °, and two others, one of twelfth magnitude at ", p. °, the other of thirteenth magnitude at ", p. °, may be distant satellites of the great star, but not planets in the ordinary sense, since it is evident that they are self-luminous. it is a significant fact that most of the first-magnitude stars have faint companions which are not so distant as altogether to preclude the idea of physical relationship. but while capella has no visible companion, campbell, of the lick observatory, has lately discovered that it is a conspicuous example of a peculiar class of binary stars only detected within the closing decade of the nineteenth century. the nature of these stars, called spectroscopic binaries, may perhaps best be described while we turn our attention from capella to the second star in auriga beta (menkalina), which not only belongs to the same class, but was the first to be discovered. neither our telescopes, nor any telescope in existence, can directly reveal the duplicity of beta aurigæ to the eye--i. e., we can not see the two stars composing it, because they are so close that their light remains inextricably mingled after the highest practicable magnifying power has been applied in the effort to separate them. but the spectroscope shows that the star is double and that its components are in rapid revolution around one another, completing their orbital swing in the astonishingly short period of _four days_! the combined mass of the two stars is estimated to be two and a half times the mass of the sun, and the distance between them, from center to center, is about eight million miles. the manner in which the spectroscope revealed the existence of two stars in beta aurigæ is a beautiful illustration of the unexpected and, so to speak, automatic application of an old principle in the discovery of new facts not looked for. it was noticed at the harvard observatory that the lines in the photographed spectrum of beta aurigæ (and of a few other stars to be mentioned later) appeared single in some of the photographs and double in others. investigation proved that the lines were doubled at regular intervals of about two days, and that they appeared single in the interim. the explanation was not far to seek. it is known that all stars which are approaching us have their spectral lines shifted, by virtue of their motion of approach, toward the violet end of the spectrum, and that, for a similar reason, all stars which are receding have their lines shifted toward the red end of the spectrum. now, suppose two stars to be revolving around one another in a plane horizontal, or nearly so, to the line of sight. when they are at their greatest angular distance apart as seen from the earth one of them will evidently be approaching at the same moment that the other is receding. the spectral lines of the first will therefore be shifted toward the violet, and those of the second will be shifted toward the red. then if the stars, when at their greatest distance apart, are still so close that the telescope can not separate them, their light will be combined in the spectrum; but the spectral lines, being simultaneously shifted in opposite directions, will necessarily appear to be doubled. as the revolution of the stars continues, however, it is clear that their motion will soon cease to be performed in the line of sight, and will become more and more athwart that line, and as this occurs the spectral lines will gradually assume their normal position and appear single. this is the sequence of phenomena in beta aurigæ. and the same sequence is found in capella and in several other more or less conspicuous stars in various parts of the heavens. such facts, like those connecting rows and groups of stars with masses and spiral lines of nebula are obscure signboards, indicating the opening of a way which, starting in an unexpected direction, leads deep into the mysteries of the universe. southward from beta we find the star theta, which is a beautiful quadruple. we shall do best with our five-inch here, although in a fine condition of the atmosphere the four-inch might suffice. the primary is of the third magnitude; the first companion is of magnitude seven and a half, distance ", p. °; the second, of the tenth magnitude, distance ", p. °; and the third, of the tenth magnitude, distance ", p. °. we should look at the double sigma with one of our larger apertures in order to determine for ourselves what the colors of the components are. there is considerable diversity of opinion on this point. some say the larger star is pale red and the smaller light blue; others consider the color of the larger star to be greenish, and some have even called it white. the magnitudes are five and nine, distance ", p. °. auriga contains several noteworthy clusters which will be found on the map. the most beautiful of these is , in which about five hundred stars have been counted. the position of the new star of , known as nova aurigæ, is also indicated on the map. while this never made a brilliant appearance, it gave rise to a greater variety of speculative theories than any previous phenomenon of the kind. although not recognized until january , , this star, as photographic records prove, was in existence on december , . at its brightest it barely exceeded magnitude four and a half, and its maximum occurred within ten days after its first recognition. when discovered it was of the fifth magnitude. it was last seen in its original form with the lick telescope on april th, when it had sunk to the lowest limit of visibility. to everybody's astonishment it reappeared in the following august, and on the th of that month was seen shining with the light of a tenth-magnitude star, _but presenting the spectrum of a nebula!_ its visual appearance in the great telescope was now also that of a planetary nebula. its spectrum during the first period of its visibility had been carefully studied, so that the means existed for making a spectroscopic comparison of the phenomenon in its two phases. during the first period, when only a stellar spectrum was noticed, remarkable shiftings of the spectral lines occurred, indicating that two and perhaps three bodies were concerned in the production of the light of the new star, one of which was approaching the earth, while the other or the others receded with velocities of several hundred miles per second! on the revival in the form of a planetary nebula, while the character of the spectrum had entirely changed, evidences of rapid motion in the line of sight remained. but what was the meaning of all this? evidently a catastrophe of some kind had occurred out there in space. the idea of a collision involving the transformation of the energy of motion into that of light and heat suggests itself at once. but what were the circumstances of the collision? did an extinguished sun, flying blindly through space, plunge into a vast cloud of meteoric particles, and, under the lashing impact of so many myriads of missiles, break into superficial incandescence, while the cosmical wrack through which it had driven remained glowing with nebulous luminosity? such an explanation has been offered by seeliger. or was vogel right when he suggested that nova aurigæ could be accounted for by supposing that a wandering dark body had run into collision with a system of planets surrounding a decrepit sun (and therefore it is to be hoped uninhabited), and that those planets had been reduced to vapor and sent spinning by the encounter, the second outburst of light being caused by an outlying planet of the system falling a prey to the vagabond destroyer? or some may prefer the explanation, based on a theory of wilsing's, that _two_ great bodies, partially or wholly opaque and non-luminous at their surfaces, but liquid hot within, approached one another so closely that the tremendous strain of their tidal attraction burst their shells asunder so that their bowels of fire gushed briefly visible, amid a blaze of spouting vapors. and yet lockyer thinks that there was no solid or semisolid mass concerned in the phenomenon at all, but that what occurred was simply the clash of two immense swarms of meteors that had crossed one another's track. well, where nobody positively knows, everybody has free choice. in the meantime, look at the spot in the sky where that little star made its appearance and underwent its marvelous transformation, for, even if you can see no remains of it there, you will feel your interest in the problem it has presented, and in the whole subject of astronomy, greatly heightened and vivified, as the visitor to the field of waterloo becomes a lover of history on the spot. the remaining objects of special interest in auriga may be briefly mentioned: , triple star, magnitudes five, eight, and eleven, distances ", p. °, and ", p. °; , triple star, magnitudes five, seven and a half, and eleven, distances ", p. °, and . ", p. °, the last difficult for moderate apertures; lambda, double, magnitudes five and nine, distance ", p. °; epsilon, variable, generally of third magnitude, but has been seen of only four and a half magnitude; , double, magnitudes five and six, distance ", p. °; , , , and , clusters all well worth inspection, being especially beautiful. the inconspicuous lynx furnishes some fine telescopic objects, all grouped near the northwestern corner of the constellation. without a six-inch telescope it would be a waste of time to attack the double star , whose components are of sixth and eighth magnitudes, distance . ", p. °; but its neighbor, , a fine triple, is within our reach, the magnitudes being six, ten, and eight, distances ", p. °, and ", p. °. in lyncis we find one of the most attractive of triple stars, which in good seeing weather is not beyond the powers of a three-inch glass, although we shall have a far more satisfactory view of it with the four-inch. the components are of the sixth, seventh, and eighth magnitudes, distances . ", p. °, and . ", p. °. a magnifying power which just suffices clearly to separate the disks of the two nearer stars makes this a fine sight. a beautiful contrast of colors belongs to the double star , but unfortunately the star is at present very close, the distance between its sixth and seventh magnitude components not exceeding . ", position angle °. sigma is a pretty double, both stars being of the sixth magnitude, distance ", p. °. still finer is sigma , a double, whose stars are both a little above the seventh magnitude and nearly equal, distance ", p. °. a low power suffices to show the three stars in , their magnitudes being six and a half, seven and a half, and eight, distances ", p. °, and ", p. °. webb describes the two smaller stars as plum-colored. plum-colored suns! at the opposite end of the constellation are two fine doubles, sigma , magnitudes six and a half and seven, distance . ", p. °; and , magnitudes four and seven, distance . ", p. °. under the guidance of map no. we turn to leo, which contains one of the leading gems among the double stars, gamma, whose components, of the second and fourth magnitudes, are respectively yellow and green, the green star, according to some observers, having a peculiar tinge of red. their distance apart is . ", p. °, and they are undoubtedly in revolution about a common center, the probable period being about four hundred years. the three-inch glass should separate them easily when the air is steady, and a pleasing sight they are. the star iota is a closer double, and also very pretty, magnitudes four and eight, colors lemon and light blue, distance . ", p. °. other doubles are tau, magnitudes five and seven, distance ", p. °; , magnitudes seven and nine, distance ", p. °; , triple, magnitudes six, seven and a half, and ten, distance, . ", p. °, and ", p. °; , magnitudes four and a half and seven, distance . ", p. °; and , magnitudes six and nine, distance . ", p. °. leo contains a remarkable variable star, r, deep red in color, and varying in a space of a hundred and forty-four days from the fifth to the tenth magnitude. it has also several nebulæ, of which only one needs special mention, no. . this is spindle-shaped, and large telescopes show that it consists of three nebulæ. the observer with ordinary instruments finds little to see and little to interest him in these small, faint nebulæ. we may just glance at two double stars in the small constellation of sextans, situated under leo. these are: , magnitudes seven and eight, distance ", p. °; and , magnitudes six and seven, distance . ", p. °. [illustration: map no. .] coma berenices (map no. ) includes several interesting objects. let us begin with the star , a double, of magnitudes six and seven and a half, distance . ", p. °. the color of the smaller star is lilac. this hue, although not extremely uncommon among double stars elsewhere, recurs again and again, with singular persistence, in this little constellation. for instance, in the very next star that we look at, , we find a double whose smaller component is _lilac_. the magnitudes in are five and eight, distance ", p. °. so also the wide double , magnitudes five and a half and six, distance ", exhibits a tinge of _lilac_ in the smaller component; the triple , magnitudes five, eight, and nine, distances ", p. °, and . ", p. °, has four colors yellow, _lilac_, and blue, and the double , magnitudes five and six, distance ", p. °, combines an orange with a _lilac_ star, a very striking and beautiful contrast. we should make a mistake if we regarded this wonderful distribution of color among the double stars as accidental. it is manifestly expressive of their physical condition, although we can not yet decipher its exact meaning. the binary comæ berenicis is too close for ordinary telescopes, but it is highly interesting as an intermediate between those pairs which the telescope is able to separate and those--like beta aurigæ--which no magnifying power can divide, but which reveal the fact that they are double by the periodical splitting of their spectral lines. the orbit in comæ berenicis is a very small one, so that even when the components are at their greatest distance apart they can not be separated by a five-or six-inch glass. burnham, using the lick telescope, in made the distance . "; hall, using the washington telescope, in made it a trifle more than . ". he had measured it in as only . ". the period of revolution is believed to be about twenty-five years. in coma berenices there is an outlying field of the marvelous nebulous region of virgo, which we may explore on some future evening. but the nebulæ in coma are very faint, and, for an amateur, hardly worth the trouble required to pick them up. the two clusters included in the map, and , are bright enough to repay inspection with our largest aperture. [illustration: map no. .] although hydra is the largest constellation in the heavens, extending about seven hours, or °, in right ascension, it contains comparatively few objects of interest, and most of these are in the head or western end of the constellation, which we examined during our first night at the telescope. in the central portion of hydra, represented on map no. , we find its leading star alpha, sometimes called alphard, or cor hydræ, a bright second-magnitude star that has been suspected of variability. it has a decided orange tint, and is accompanied, at a distance of ", p. °, by a greenish tenth-magnitude star. bu. is a fine double, magnitudes eight and nine and a half, distance . ", p. °. the planetary nebula is about ' in diameter, pale blue in color, and worth looking at, because it is brighter than most objects of its class. tempel and secchi have given wonderful descriptions of it, both finding multitudes of stars intermingled with nebulous matter. for a last glimpse at celestial splendors for the night, let us turn to the rich cluster , in argo, just above the place where the stream of the milky way--here bright in mid-channel and shallowing toward the shores--separates into two or three currents before disappearing behind the horizon. it is by no means as brilliant as some of the star clusters we have seen, but it gains in beauty and impressiveness from the presence of one bright star that seems to captain a host of inferior luminaries. chapter iv virgo and her neighbors ... "that region where still by night is seen the virgin goddess near to bright boötes."--poste's aratus. [illustration: map no. .] following the order of right ascension, we come next to the little constellations crater and corvus, which may be described as standing on the curves of hydra (map no. ). beginning with crater, let us look first at alpha, a yellow fourth-magnitude star, near which is a celebrated red variable r. with a low power we can see both alpha and r in the same field of view, like a very wide double. there is a third star of ninth magnitude, and bluish in color, near r on the side toward alpha. r is variable both in color and light. when reddest, it has been described as "scarlet," "crimson," and "blood-colored"; when palest, it is a deep orange-red. its light variation has a period the precise length of which is not yet known. the cycle of change is included between the eighth and ninth magnitudes. while our three-inch telescope suffices to show r, it is better to use the five-inch, because of the faintness of the star. when the color is well seen, the contrast with alpha is very pleasing. there is hardly anything else in crater to interest us, and we pass over the border into corvus, and go at once to its chief attraction, the star delta. the components of this beautiful double are of magnitudes three and eight; distance ", p. °; colors yellow and purple. the night being dark and clear, we take the five-inch and turn it on the nebula , which the map shows just under the border of corvus in the edge of hydra. herschel believed he had resolved this into stars. it is a faint object and small, not exceeding one eighth of the moon's diameter. farther east in hydra, as indicated near the left-hand edge of map no. , is a somewhat remarkable variable, r hydræ. this star occasionally reaches magnitude three and a half, while at minimum it is not much above the tenth magnitude. its period is about four hundred and twenty-five days. [illustration: map no. .] while we have been examining these comparatively barren regions, glad to find one or two colored doubles to relieve the monotony of the search, a glittering white star has frequently drawn our eyes eastward and upward. it is spica, the great gem of virgo, and, yielding to its attraction, we now enter the richer constellation over which it presides (map no. ). except for its beauty, which every one must admire, spica, or alpha virginis, has no special claim upon our attention. some evidence has been obtained that, like beta aurigæ and capella, it revolves with an invisible companion of great mass in an orbit only six million miles in diameter. spica's spectrum resembles that of sirius. the faint star which our larger apertures show about ' northeast of spica is of the tenth magnitude. sweeping westward, we come upon sigma , a pretty little double with nearly equal components of about the sixth magnitude, distance . ", p. °. but our interest is not fully aroused until we reach gamma, a star with a history. the components of this celebrated binary are both nearly of the third magnitude, distance about . ", p. °. they revolve around their common center in something less than two hundred years. according to some authorities, the period is one hundred and seventy years, but it is not yet certainly ascertained. it was noticed about the beginning of the seventeenth century that gamma virginis was double. in the stars were so close together that no telescope then in existence was able to separate them, although it is said that the disk into which they had merged was elongated at pulkowa. in a few years they became easily separable once more. if the one-hundred-and-seventy-year period is correct, they should continue to get farther apart until about . according to asaph hall, their greatest apparent distance is . ", and their least apparent distance . "; consequently, they will never again close up beyond the separating power of existing telescopes. there is a great charm in watching this pair of stars even with a three-inch telescope--not so much on account of what is seen, although they are very beautiful, as on account of what we know they are doing. it is no slight thing to behold two distant stars obeying the law that makes a stone fall to the ground and compels the earth to swing round the sun. in theta we discover a fine triple, magnitudes four and a half, nine, and ten; distances ", p. °, and ", p. °. the ninth-magnitude star has been described as "violet," but such designations of color are often misleading when the star is very faint. on the other hand it should not be assumed that a certain color does not exist because the observer can not perceive it, for experience shows that there is a wide difference among observers in the power of the eye to distinguish color. i have known persons who could not perceive the difference of hue in some of the most beautifully contrasted colored doubles to be found in the sky. i am acquainted with an astronomer of long experience in the use of telescopes, whose eye is so deficient in color sense that he denies that there are any decided colors among the stars. such persons miss one of the finest pleasures of the telescope. in examining theta virginis we shall do best to use our largest aperture, viz., the five-inch. yet webb records that all three of the stars in this triple have been seen with a telescope of only three inches aperture. the amateur must remember in such cases how much depends upon practice as well as upon the condition of the atmosphere. there are lamentably few nights in a year when even the best telescope is ideally perfect in performance, but every night's observation increases the capacity of the eye, begetting a kind of critical judgment which renders it to some extent independent of atmospheric vagaries. it will also be found that the idiosyncrasies of the observer are reflected in his instrument, which seems to have its fits of excellence, its inspirations so to speak, while at other times it behaves as if all its wonderful powers had departed. another double that perhaps we had better not try with less than four inches aperture is virginis. the magnitudes are six and nine; distance, . ", p. °. colors yellow and blue. sigma is a fifth-magnitude star with a tenth-magnitude companion, distance only ", p. °. use the five-inch. and now we approach something that is truly marvelous, the "field of the nebulæ." this strange region, lying mostly in the constellation virgo, is roughly outlined by the stars beta, eta, gamma, delta, and epsilon, which form two sides of a square some ° across. it extends, however, for some distance into coma berenices, while outlying nebulæ belonging to it are also to be found in the eastern part of leo. unfortunately for those who expect only brilliant revelations when they look through a telescope, this throng of nebulæ consists of small and inconspicuous wisps as ill defined as bits of thistle-down floating high in the air. there are more than three hundred of them all told, but even the brightest are faint objects when seen with the largest of our telescopes. why do they congregate thus? that is the question which lends an interest to the assemblage that no individual member of it could alone command. it is a mystery, but beyond question it is explicable. the explanation, however, is yet to be discovered. the places of only three of the nebulæ are indicated on the map. no. has been described as resembling in shape a shuttle. its length is nearly one third of the moon's diameter. it is brightest near the center, and has several faint companions. no. is round, ' in diameter, and is accompanied by another round nebula in the same field of view toward the south. no. is double, and powerful telescopes show two more ghostly companions. there is an opportunity for good and useful work in a careful study of the little nebulæ that swim into view all over this part of virgo. celestial photography has triumphs in store for itself here. scattered over and around the region where the nebulæ are thickest we find eight or nine variable stars, three of the most remarkable of which, r, s, and u, may be found on the map. r is very irregular, sometimes attaining magnitude six and a half, while at other times its maximum brightness does not exceed that of an eighth-magnitude star. at minimum it sinks to the tenth or eleventh magnitude. its period is one hundred and forty-five days. u varies from magnitude seven or eight down to magnitude twelve or under and then regains its light, in a period of about two hundred and seven days. s is interesting for its brilliant red color. when brightest, it exceeds the sixth magnitude, but at some of its maxima the magnitude is hardly greater than the eighth. at minimum it goes below the twelfth magnitude. period, three hundred and seventy-six days. [illustration: map no. .] next east of virgo is libra, which contains a few notable objects (map no. ). the star alpha has a fifth-magnitude companion, distant about ", which can be easily seen with an opera glass. at the point marked a on the map is a curious multiple star, sometimes referred to by its number in piazzi's catalogues as follows: p. xiv. the two principal stars are easily seen, their magnitudes being six and seven and a half; distance ", p. °. burnham found four other faint companions, for which it would be useless for us to look. the remarkable thing is that these faint stars, the nearest of which is distant about " from the largest member of the group and the farthest about ", do not share, according to their discoverer, in the rapid proper motion of the two main stars. in iota we find a double a little difficult for our three-inch. the components are of magnitudes four and a half and nine, distance ", p. °. burnham discovered that the ninth-magnitude star consists of two of the tenth less than " apart, p. °. no astronomer who happens to be engaged in this part of the sky ever fails, unless his attention is absorbed by something of special interest, to glance at beta libræ, which is famous as the only naked-eye star having a decided green color. the hue is pale, but manifest.[ ] [ ] is the slight green tint perceptible in sirius variable? i am sometimes disposed to think it is. the star is a remarkable variable, belonging to what is called the algol type. its period, according to chandler, is days hours, minutes, . seconds. the time occupied by the actual changes is about twelve hours. at maximum the star is of magnitude five and at minimum of magnitude . . [illustration: map no. .] we may now conveniently turn northward from virgo in order to explore boötes, one of the most interesting of the constellations (map no. ). its leading star alpha, arcturus, is the brightest in the northern hemisphere. its precedence over its rivals vega and capella, long in dispute, has been settled by the harvard photometry. you notice that the color of arcturus, when it has not risen far above the horizon, is a yellowish red, but when the star is near mid-heaven the color fades to light yellow. the hue is possibly variable, for it is recorded that in arcturus appeared to have nearly lost its color. if it should eventually turn white, the fact would have an important bearing upon the question whether sirius was, as alleged, once a red or flame-colored star. but let us sit here in the starlight, for the night is balmy, and talk about arcturus, which is perhaps actually the greatest sun within the range of terrestrial vision. its parallax is so minute that the consideration of the tremendous size of this star is a thing that the imagination can not placidly approach. calculations, based on its assumed distance, which show that it _outshines the sun several thousand times_, may be no exaggeration of the truth! it is easy to make such a calculation. one of dr. elkin's parallaxes for arcturus is . ". that is to say, the displacement of arcturus due to the change in the observer's point of view when he looks at the star first from one side and then from the other side of the earth's orbit, , , miles across, amounts to only eighteen one-thousandths of a second of arc. we can appreciate how small that is when we reflect that it is about equal to the apparent distance between the heads of two pins placed an inch apart and viewed from a distance of a hundred and eighty miles! assuming this estimate of the parallax of arcturus, let us see how it will enable us to calculate the probable size or light-giving power of the star as compared with the sun. the first thing to do is to multiply the earth's distance from the sun, which may be taken at , , miles, by , , the number of seconds of arc in a radian, the base of circular measure, and then divide the product by the parallax of the star. performing the multiplication and division, we get the following: , , , , / . = , , , , , . the quotient represents miles! call it, in round numbers, a thousand millions of millions of miles. this is about , , times the distance from the earth to the sun. now for the second part of the calculation: the amount of light received on the earth from some of the brighter stars has been experimentally compared with the amount received from the sun. the results differ rather widely, but in the case of arcturus the ratio of the star's light to sunlight may be taken as about one twenty-five-thousand-millionth--i. e., , , , stars, each equal to arcturus, would together shed upon the earth as much light as the sun does. but we know that light varies inversely as the square of the distance; for instance, if the sun were twice as far away as it is, its light would be diminished for us to a quarter of its present amount. suppose, then, that we could remove the earth to a point midway between the sun and arcturus, we should then be , , times as far from the sun as we now are. in order to estimate how much light the sun would send us from that distance we must square the number , , and then take the result inversely, or as a fraction. we thus get / , , , , , representing the ratio of the sun's light at half the distance of arcturus to that at its real distance. but while receding from the sun we should be approaching arcturus. we should get, in fact, twice as near to that star as we were before, and therefore its light would be increased for us fourfold. now, if the amount of sunlight had not changed, it would exceed the light of arcturus only a quarter as much as it did before, or in the ratio of , , , / = , , , to . but, as we have seen, the sunlight would diminish through increase of distance to one , , , , th part of its original amount. hence its altered ratio to the light of arcturus would become , , , to , , , , , or to , . this means that if the earth were situated midway between the sun and arcturus, it would receive , times as much light from that star as it would from the sun! it is quite probable, moreover, that the heat of arcturus exceeds the solar heat in the same ratio, for the spectroscope shows that although arcturus is surrounded with a cloak of metallic vapors proportionately far more extensive than the sun's, yet, smothered as the great star seems in some respects to be, it rivals sirius itself in the intensity of its radiant energy. if we suppose the radiation of arcturus to be the same per unit of surface as the sun's, it follows that arcturus exceeds the sun about , times in volume, and that its diameter is no less than , , miles! imagine the earth and the other planets constituting the solar system removed to arcturus and set revolving around it in orbits of the same forms and sizes as those in which they circle about the sun. poor mercury! for that little planet it would indeed be a jump from the frying pan into the fire, because, as it rushed to perihelion, mercury would plunge more than , , miles beneath the surface of the giant star. venus and the earth would melt like snowflakes at the mouth of a furnace. even far-away neptune, the remotest member of the system, would swelter in torrid heat. but stop! look at the sky. observe how small and motionless the disks of the stars have become. back to the telescopes at once, for this is a token that the atmosphere is steady, and that "good seeing" may be expected. it is fortunate, for we have some delicate work before us. the very first double star we try in boötes, sigma , requires the use of the four-inch, and the five-inch shows it more satisfactorily. the magnitudes are sixth and ninth, distance ", p. °. on the other side of arcturus we find zeta, a star that we should have had no great difficulty in separating thirty years ago, but which has now closed up beyond the reach even of our five-inch. the magnitudes are both fourth, and the distance less than a quarter of a second; position angle changing. it is apparently a binary, and if so will some time widen again, but its period is unknown. the star , also known as sigma , near the southeastern edge of the constellation, is a pretty double, each component being of the seventh magnitude, distance ", p. °. just above zeta we come upon pi, an easy double for the three-inch, magnitudes four and six, distance " p. °. next is xi, a yellow and purple pair, whose magnitudes are respectively five and seven, distance less than ", p. °. this is undoubtedly a binary with a period of revolution of about a hundred and thirty years. its distance decreased about " between and . it was still decreasing in , when it had become . ". the orbital swing is also very apparent in the change of the position angle. the telescopic gem of boötes, and one of "the flowers of the sky," is epsilon, also known as mirac. when well seen, as we shall see it to-night, epsilon boötis is superb. the magnitudes of its two component stars are two and a half (according to hall, three) and six. the distance is about . ", p. °. the contrast of colors--bright orange yellow, set against brilliant emerald green--is magnificent. there are very few doubles that can be compared with it in this respect. the three-inch will separate it, but the five-inch enables us best to enjoy its beauty. it appears to be a binary, but the motion is very slow, and nothing certain is yet known of its period. in delta we have a very wide and easy double; magnitudes three and a half and eight and a half, distance ", p. °. the smaller star has a lilac hue. we can not hope with any of our instruments to see all of the three stars contained in , but two of them are easily seen; magnitudes four and seven, distance ", p. °. the smaller star is again double; magnitudes seven and eight, distance . ", p. °. it is clearly a binary, with a long period. a six-inch telescope that could separate this star at present would be indeed a treasure. sigma is another object rather beyond our powers, on account of the contrast of magnitudes. these are six and eight and a half; distance . ", p. °. other doubles are: (sigma ), magnitudes five and six, distance . ", p. °; (sigma ), magnitudes both nearly six, distance . ", p. °. smaller star light red; iota, magnitudes four and a half and seven and a half, distance ", p. °; kappa, magnitudes five and a half and eight, distance . ", p. °. some observers see a greenish tinge in the light of the larger star, the smaller one being blue. there are one or two interesting things to be seen in that part of canes venatici which is represented on map no. . the first of these is the star cluster . this will reward a good look with the five-inch. with large telescopes as many as one thousand stars have been discerned packed within its globular outlines. the star (sigma ) is a close binary with a period estimated at one hundred and twenty-five years. the magnitudes are six and seven or eight, distance about ", p. °. we may try for this with the five-inch, and if we do not succeed in separating the stars we may hope to do so some time, for the distance between them is increasing. although the nebula is a very wonderful object, we shall leave it for another evening. eastward from boötes shines the circlet of corona borealis, whose form is so strikingly marked out by the stars that the most careless eye perceives it at once. although a very small constellation, it abounds with interesting objects. we begin our attack with the five-inch on sigma , but not too confident that we shall come off victors, for this binary has been slowly closing for many years. the magnitudes are six and a half and seven, distance . ", p. °. not far distant is another binary, at present beyond our powers, eta. here the magnitudes are both six, distance . ", p. °. hall assigns a period of forty years to this star. the assemblage of close binaries in this neighborhood is very curious. only a few degrees away we find one that is still more remarkable, the star gamma. what has previously been said about comæ berenicis applies in a measure to this star also. it, too, has a comparatively small orbit, and its components are never seen widely separated. in their distance was . "; in they could not be split; in the distance had increased to . ", and in it had become . ", p. °. but in lewis made the distance only . ". the period has been estimated at one hundred years. while the group of double stars in the southern part of corona borealis consists, as we have seen, of remarkably close binaries, another group in the northern part of the same constellation comprises stars that are easily separated. let us first try zeta. the powers of the three-inch are amply sufficient in this case. the magnitudes are four and five, distance . ", p. °. colors, white or bluish-white and blue or green. next take sigma, whose magnitudes are five and six, distance ", p. °. with the five-inch we may look for a second companion of the tenth magnitude, distance ", p. °. it is thought highly probable that sigma is a binary, but its period has simply been guessed at. finally, we come to nu, which consists of two very widely separated stars, nu^ and nu^ , each of which has a faint companion. with the five-inch we may be able to see the companion of nu^ , the more southerly of the pair. the magnitude of the companion is variously given as tenth and twelfth, distance ", p. °. with the aid of the map we find the position of the new star of , which is famous as the first so-called temporary star to which spectroscopic analysis was applied. when first noticed, on may , , this star was of the second magnitude, fully equaling in brilliancy alpha, the brightest star of the constellation; but in about two weeks it fell to the ninth magnitude. huggins and miller eagerly studied the star with the spectroscope, and their results were received with deepest interest. they concluded that the light of the new star had two different sources, each giving a spectrum peculiar to itself. one of the spectra had dark lines and the other bright lines. it will be remembered that a similar peculiarity was exhibited by the new star in auriga in . but the star in corona did not disappear. it diminished to magnitude nine and a half or ten, and stopped there; and it is still visible. in fact, subsequent examination proved that it had been catalogued at bonn as a star of magnitude nine and a half in . consequently this "blaze star" of will bear watching in its decrepitude. nobody knows but that it may blaze again. perhaps it is a sun-like body; perhaps it bears little resemblance to a sun as we understand such a thing. but whatever it may be, it has proved itself capable of doing very extraordinary things. we have no reason to suspect the sun of any latent eccentricities, like those that have been displayed by "temporary" stars; yet, acting on the principle which led the old emperor-astrologer rudolph ii to torment his mind with self-made horoscopes of evil import, let us unscientifically imagine that the sun _could_ suddenly burst out with several hundred times its ordinary amount of heat and light, thereby putting us into a proper condition for spectroscopic examination by curious astronomers in distant worlds. but no, after all, it is far pleasanter to keep within the strict boundaries of science, and not imagine anything of the kind. chapter v in summer star-lands "i heard the trailing garments of the night sweep through her marble halls, i saw her sable skirts all fringed with light from the celestial walls."--h. w. longfellow. in the soft air of a summer night, when fireflies are flashing their lanterns over the fields, the stars do not sparkle and blaze like those that pierce the frosty skies of winter. the light of sirius, aldebaran, rigel, and other midwinter brilliants possesses a certain gemlike hardness and cutting quality, but antares and vega, the great summer stars, and arcturus, when he hangs westering in a july night, exhibit a milder radiance, harmonizing with the character of the season. this difference is, of course, atmospheric in origin, although it may be partly subjective, depending upon the mental influences of the mutations of nature. [illustration: map no. .] the constellation scorpio is nearly as striking in outline as orion, and its brightest star, the red antares (alpha in map no. ), carries concealed in its rays a green jewel which, to the eye of the enthusiast in telescopic recreation, appears more beautiful and inviting each time that he penetrates to its hiding place. we shall begin our night's work with this object, and the four-inch glass will serve our purpose, although the untrained observer would be more certain of success with the five-inch. a friend of mine has seen the companion of antares with a three-inch, but i have never tried the star with so small an aperture. when the air is steady and the companion can be well viewed, there is no finer sight among the double stars. the contrast of colors is beautifully distinct--fire-red and bright green. the little green star has been seen emerging from behind the moon, ahead of its ruddy companion. the magnitudes are one and seven and a half or eight, distance ", p. °. antares is probably a binary, although its binary character has not yet been established. a slight turn of the telescope tube brings us to the star sigma, a wide double, the smaller component of which is blue or plum-colored; magnitudes four and nine, distance ", p. °. from sigma we pass to beta, a very beautiful object, of which the three-inch gives us a splendid view. its two components are of magnitudes two and six, distance ", p. °; colors, white and bluish. it is interesting to know that the larger star is itself double, although none of the telescopes we are using can split it. burnham discovered that it has a tenth-magnitude companion; distance less than ", p. °. and now for a triple, which will probably require the use of our largest glass. up near the end of the northern prolongation of the constellation we perceive the star xi. the three-inch shows us that it is double; the five-inch divides the larger star again. the magnitudes are respectively five, five and a half, and seven and a half, distances . ", p. °, and ", p. °. a still more remarkable star, although one of its components is beyond our reach, is nu. with the slightest magnifying this object splits up into two stars, of magnitudes four and seven, situated rather more than " apart. a high power divides the seventh-magnitude companion into two, each of magnitude six and a half, distance . ", p. °. but (and this was another of burnham's discoveries) the fourth-magnitude star itself is double, distance . ", p. about °. the companion in this case is of magnitude five and a half. next we shall need a rather low-power eyepiece and our largest aperture in order to examine a star cluster, no. , which was especially admired by sir william herschel, who discovered that it was not, as messier had supposed, a circular nebula. herschel regarded it as the richest mass of stars in the firmament, but with a small telescope it appears merely as a filmy speck that has sometimes been mistaken for a comet. in a new star, between the sixth and seventh magnitude in brilliance, suddenly appeared directly in or upon the cluster, and the feeble radiance of the latter was almost extinguished by the superior light of the stranger. the latter disappeared in less than a month, and has not been seen again, although it is suspected to be a variable, and, as such, has been designated with the letter t. two other known variables, both very faint, exist in the immediate neighborhood. according to the opinion that was formerly looked upon with favor, the variable t, if it is a variable, simply lies in the line of sight between the earth and the star cluster, and has no actual connection with the latter. but this opinion may not, after all, be correct, for mr. bailey's observations show that variable stars sometimes exist in large numbers in clusters, although the variables thus observed are of short period. the cluster , just west of antares, is also worth a glance with the five-inch glass. it is dense, but its stars are very small, so that to enjoy its beauty we should have to employ a large telescope. yet there is a certain attraction in these far-away glimpses of starry swarms, for they give us some perception of the awful profundity of space. when the mind is rightly attuned for these revelations of the telescope, there are no words that can express its impressions of the overwhelming perspective of the universe. the southern part of the constellation ophiuchus is almost inextricably mingled with scorpio. we shall, therefore, look next at its attractions, beginning with the remarkable array of star clusters , , , and . all of these are small, ' or ' in diameter, and globular in shape. no. is the largest, and we can see some of the stars composing it. but these clusters, like those just described in scorpio, are more interesting for what they signify than for what they show; and the interest is not diminished by the fact that their meaning is more or less of a mystery. whether they are composed of pygmy suns or of great solar globes like that one which makes daylight for the earth, their association in spherical groups is equally suggestive. there are two other star clusters in ophiuchus, and within the limits of map no. , both of which are more extensive than those we have just been looking at. no. is ' or ' in diameter, also globular, brighter at the center, and surrounded by several comparatively conspicuous stars. no. is still larger, about half as broad as the moon, and many of its scattered stars are of not less than the ninth magnitude. with a low magnifying power the field of view surrounding the cluster appears powdered with stars. there are only two noteworthy doubles in that part of ophiuchus with which we are at present concerned: , whose magnitudes are five and seven, distance . ", p. °, colors yellow and red; and , magnitudes six and seven and a half, distance ", p. °, colors yellow or orange and blue. the first named is a binary whose period has not been definitely ascertained. the variable r has a period a little less than three hundred and three days. at its brightest it is of magnitude seven or eight, and at minimum it diminishes to about the twelfth magnitude. the spot where the new star of appeared is indicated on the map. this was, with the exception of tycho's star in , the brightest temporary star of which we possess a trustworthy account. it is frequently referred to as kepler's star, because kepler watched it with considerable attention, but unfortunately he was not as good an observer as tycho was. the star was first seen on october , , and was then brighter than jupiter. it did not, however, equal venus. it gradually faded and in march, , disappeared. about twelve degrees northwest of the place of the star of , and in that part of the constellation serpens which is included in map no. , we find the location of another temporary star, that of . it was first noticed by mr. hind on april th of that year, when its magnitude was not much above the seventh, and its color was red. it brightened rapidly, until on may d it was of magnitude three and a half. then it began to fade, but very slowly, and it has never entirely disappeared. it is now of the twelfth or thirteenth magnitude. in passing we may glance with a low power at nu serpentis, a wide double, magnitudes four and nine, distance ", p. °, colors contrasted but uncertain. sagittarius and its neighbor, the small but rich constellation scutum sobieskii, attract us next. we shall first deal with the western portions of these constellations which are represented on map no. . the star in sagittarius is a wide triple, magnitudes three and a half, nine and a half, and ten, distances ", p. °, and ", p. °. but the chief glory of sagittarius (and the same statement applies to scutum sobieskii) lies in its assemblage of star clusters. one of these, no. , also known as m , is plainly visible to the naked eye as a bright spot in the milky way. we turn our five-inch telescope, armed with a low magnifying power, upon this subject and enjoy a rare spectacle. as we allow it to drift through the field we see a group of three comparatively brilliant stars advancing at the front of a wonderful train of mingled star clusters and nebulous clouds. a little northwest of it appears the celebrated trifid nebula, no. on the map. there is some evidence that changes have occurred in this nebula since its discovery in the last century. barnard has made a beautiful photograph showing m and the trifid nebula on the same plate, and he remarks that the former is a far more remarkable object than its more famous neighbor. near the eastern border of the principal nebulous cloud there is a small and very black hole with a star poised on its eastern edge. this hole and the star are clearly shown in the photograph. cluster no. (m ) is usually described as resembling, to the naked eye, a protuberance on the edge of the milky way. it is nearly three times as broad as the moon, and is very rich in minute stars, which are at just such a degree of visibility that crowds of them continually appear and disappear while the eye wanders over the field, just as faces are seen and lost in a vast assemblage of people. this kind of luminous agitation is not peculiar to m , although that cluster exhibits it better than most others do on account of both the multitude and the minuteness of its stars. a slight sweep eastward brings us to yet another meeting place of stars, the cluster m , situated between the variables u and v. this is brilliant and easily resolved into its components, which include a number of double stars. the two neighboring variables just referred to are interesting. u has a period of about six days and three quarters, and its range of magnitude runs from the seventh down to below the eighth. v is a somewhat mysterious star. chandler removed it from his catalogue of variables because no change had been observed in its light by either himself, sawyer, or yendell. quirling, the discoverer of its variability, gave the range as between magnitudes . and . . it must, therefore, be exceedingly erratic in its changes, resembling rather the temporary stars than the true variables. in that part of scutum sobieskii contained in map no. we find an interesting double, sigma , whose magnitudes are six and nine, distance . ", p. °, colors white and orange. sigma is a triple, magnitudes seven, eight, and nine, distances ", p. °, and . ", p. °. the third star is, however, beyond our reach. the colors of the two larger are respectively yellow and violet. the star cluster is about one quarter as broad as the moon, and easily seen with our smallest aperture. [illustration: map no. .] passing near to the region covered by map no. , we find the remaining portions of the constellations sagittarius and scutum sobieskii. it will be advisable to finish with the latter first. glance at the clusters and . neither is large, but both are rich in stars. the nebula is a fine object of its kind. it brightens toward the center, and herschel thought he had resolved it into stars. the variable r is remarkable for its eccentricities. sometimes it attains nearly the fourth magnitude, although usually at maximum it is below the fifth, while at minimum it is occasionally of the sixth and at other times of the seventh or eighth magnitude. its period is irregular. turning back to sagittarius, we resume our search for interesting objects there, and the first that we discover is another star cluster, for the stars are wonderfully gregarious in this quarter of the heavens. the number our cluster bears on the map is , corresponding with m in messier's catalogue. it is very bright, containing many stars of the tenth and eleventh magnitudes, as well as a swarm of smaller ones. sir john herschel regarded the larger stars in this cluster as possessing a reddish tint. possibly there was some peculiarity in his eye that gave him this impression, for he has described a cluster in the constellation toucan in the southern hemisphere as containing a globular mass of rose-colored stars inclosed in a spherical shell of white stars. later observers have confirmed his description of the shape and richness of this cluster in toucan, but have been unable to perceive the red hue of the interior stars. the eastern expanse of sagittarius is a poor region compared with the western end of the constellation, where the wide stream of the milky way like a great river enriches its surroundings. the variables t and r are of little interest to us, for they never become bright enough to be seen without the aid of a telescope. in we find, however, an interesting double, which with larger telescopes than any of ours appears as a triple. the two stars that we see are of magnitudes six and seven and a half, distance ", p. °, colors yellow and blue. the third star, perhaps of thirteenth magnitude, is distant ", p. °. retaining map no. as our guide, we examine the western part of the constellation capricornus. its leader alpha is a naked-eye double, the two stars being a little more than ' apart. their magnitudes are three and four, and both have a yellowish hue. the western star is alpha^ , and is the fainter of the two. the other is designated as alpha^ . both are double. the components of alpha^ are of magnitudes four and eight and a half, distance ", p. °. with the washington twenty-six-inch telescope a third star of magnitude fourteen has been found at a distance of ", p. °. in alpha^ the magnitudes of the components are three and ten and a half, distance . ", p. °. the smaller star has a companion of the twelfth or thirteenth magnitude, distance . ", p. °. this, of course, is hopelessly beyond our reach. yet another star of magnitude nine, distance ", p. , we may see easily. dropping down to beta, we find it to be a most beautiful and easy double, possessing finely contrasted colors, gold and blue. the larger star is of magnitude three, and the smaller, the blue one, of magnitude six, distance ", p. °. between them there is a very faint star which larger telescopes than ours divide into two, each of magnitude eleven and a half; separated ", p. °. still farther south and nearly in a line drawn from alpha through beta we find a remarkable group of double stars, sigma, pi, rho, and omicron. the last three form a beautiful little triangle. we begin with sigma, the faintest of the four. the magnitudes of its components are six and nine, distance ", p. °. in pi the magnitudes are five and nine, distance . ", p. °; in rho, magnitudes five and eight, distance . ", p. ° (a third star of magnitude seven and a half is seen at a distance of ', p. °); in omicron, magnitudes six and seven, distance ", p. °. the star cluster is small, yet, on a moonless night, worth a glance with the five-inch. [illustration: map no. .] we now pass northward to the region covered by map no. , including the remainder of ophiuchus and serpens. beginning with the head of serpens, in the upper right-hand corner of the map, we find that beta, of magnitude three and a half, has a ninth-magnitude companion, distance ", p. °. the larger star is light blue and the smaller one yellowish. the little star nu is double, magnitudes five and nine, distance ", p. °, colors contrasted but uncertain. in delta we find a closer double, magnitudes three and four, distance . ", p. °. it is a beautiful object for the three-inch. the leader of the constellation, alpha, of magnitude two and a half, has a faint companion of only the twelfth magnitude, distance ", p. °. the small star is bluish. the variable r has a period about a week short of one year, and at maximum exceeds the sixth magnitude, although sinking at minimum to less than the eleventh. its color is ruddy. passing eastward, we turn again into ophiuchus, and find immediately the very interesting double, lambda, whose components are of magnitudes four and six, distance ", p. °. this is a long-period binary, and notwithstanding the closeness of its stars, our four-inch should separate them when the seeing is fine. we shall do better, however, to try with the five-inch. sigma consists of two stars of magnitudes six and seven and a half, distance ", p. °. sigma is a double of quite a different order. the magnitudes of its components are both six, the distance in . ", p. °. it is evidently a binary in rapid motion, as the distance changed from about a quarter of a second in to more than a second in . the star tau is a fine triple, magnitudes five, six, and nine, distances . ", p. °, and ", p. °. the close pair is a binary system with a long period of revolution, estimated at about two hundred years. we discover another group of remarkable doubles in , , and . in the first-named star the magnitudes are four and eight, distance ", p. °, colors finely contrasted, pale yellow and red. much more interesting, however, is , a binary whose components have completed a revolution since their discovery by sir william herschel, the period being ninety-five years. the magnitudes are four and six, or, according to hall, five and six, distance in . "; in , . ", according to maw. hall says the apparent distance when the stars are closest is about . ", and when they are widest . ". this star is one of those whose parallax has been calculated with a reasonable degree of accuracy. its distance from us is about , , times the distance of the sun, the average distance apart of the two stars is about , , , miles (equal to the distance of neptune from the sun), and their combined mass is three times that of the sun. hall has seen in the system of ophiuchi three stars of the thirteenth magnitude or less, at distances of about ", ", and " respectively. the star is also a close double, and beyond our reach. its magnitudes are six and seven, distance . ", p. °. it is, no doubt, a binary. three star clusters in ophiuchus remain to be examined. the first of these, no. , is partially resolved into stars by the five-inch. no. is globular, and has a striking environment of bystanding stars. it is about one quarter as broad as the full moon, and our largest aperture reveals the faint coruscation of its crowded components. no. is a coarser and more scattered star swarm--a fine sight! farther toward the east we encounter a part of serpens again, which contains just one object worth glancing at, the double theta, whose stars are of magnitudes four and four and a half, distance ", p. °. color, both yellow, the smaller star having the deeper hue. [illustration: map no. .] let us next, with the guidance of map no. , enter the rich star fields of hercules, and of the head and first coils of draco. according to argelander, hercules contains more stars visible to the naked eye than any other constellation, and he makes the number of them one hundred and fifty-five, nearly two thirds of which are only of the sixth magnitude. but heis, who saw more naked-eye stars than argelander, makes ursa major precisely equal to hercules in the number of stars, his enumeration showing two hundred and twenty-seven in each constellation, while, according to him, draco follows very closely after, with two hundred and twenty stars. yet, on account of the minuteness of the majority of their stars, neither of these constellations makes by any means as brilliant a display as does orion, to which argelander assigns only one hundred and fifteen naked-eye stars, and heis one hundred and thirty-six. we begin in hercules with the star kappa, a pretty little double of magnitudes five and a half and seven, distance ", p. °, colors yellow and red. not far away we find, in gamma, a larger star with a fainter companion, the magnitudes in this case being three and a half and nine, distance ", p. °, colors white and faint blue or lilac. one of the most beautiful of double stars is alpha herculis. the magnitudes are three and six, distance . ", p. °, colors orange and green, very distinct. variability has been ascribed to each of the stars in turn. it is not known that they constitute a binary system, because no certain evidence of motion has been obtained. another very beautiful and easily separated double is delta, magnitudes three and eight, distance ", p. °, colors pale green and purple. sweeping northwestward to zeta, we encounter a celebrated binary, to separate which at present requires the higher powers of a six-inch glass. the magnitudes are three and six and a half, distance in , . ", p. °; in , . ", p. °. the period of revolution is thirty-five years, and two complete revolutions have been observed. the apparent distance changes from . " to . ". they were at their extreme distance in . two pleasing little doubles are sigma , magnitudes six and nine, distance ", p. °, and sigma , magnitudes six and eight, distance ", p. °. at the northern end of the constellation is , a double that requires the light-grasping power of our largest glass. its magnitudes are six and twelve, distance ", p. °. in rho we discover another distinctly colored double, both stars being greenish or bluish, with a difference of tone. the magnitudes are four and five and a half, distance . ", p. °. but the double is yet more remarkable for the colors of its stars. their magnitudes are five and five and a half, distance ", p. °, colors, according to webb, "light apple-green and cherry-red." but other observers have noted different hues, one calling them both golden yellow. i think webb's description is more nearly correct. sigma is a very close double, requiring larger telescopes than those we are working with. its magnitudes are six and a half and eight, distance . ", p. °. it is probably a binary. sigma is also close, but our five-inch will separate it: magnitudes six and seven, distance . ", p. °. turning to , we have to deal with a triple, one of whose stars is at present beyond the reach of our instruments. the magnitudes of the two that we see are four and ten, distance ", p. °. the tenth-magnitude star is a binary of short period (probably less than fifty years), the distance of whose components was " in , " in , . " in , and . " in , when the position angle was °, and rapidly increasing. the distance is still much less than ". for a glance at a planetary nebula we may turn with the five-inch to no. . it is very small and faint, only " in diameter, and equal in brightness to an eighth-magnitude star. only close gazing shows that it is not sharply defined like a star, and that it possesses a bluish tint. its spectrum is gaseous. the chief attraction of hercules we have left for the last, the famous star cluster between eta and zeta, no. , more commonly known as m . on a still evening in the early summer, when the moon is absent and the quiet that the earth enjoys seems an influence descending from the brooding stars, the spectacle of this sun cluster in hercules, viewed with a telescope of not less than five-inches aperture, captivates the mind of the most uncontemplative observer. with the lick telescope i have watched it resolve into separate stars to its very center--a scene of marvelous beauty and impressiveness. but smaller instruments reveal only the in-running star streams and the sprinkling of stellar points over the main aggregation, which cause it to sparkle like a cloud of diamond dust transfused with sunbeams. the appearance of flocking together that those uncountable thousands of stars present calls up at once a picture of our lone sun separated from its nearest stellar neighbor by a distance probably a hundred times as great as the entire diameter of the spherical space within which that multitude is congregated. it is true that unless we assume what would seem an unreasonable remoteness for the hercules cluster, its component stars must be much smaller bodies than the sun; yet even that fact does not diminish the wonder of their swarming. here the imagination must bear science on its wings, else science can make no progress whatever. it is an easy step from hercules to draco. in the conspicuous diamond-shaped figure that serves as a guide-board to the head of the latter, the southernmost star belongs not to draco but to hercules. the brightest star in this figure is gamma, of magnitude two and a half, with an eleventh-magnitude companion, distant ", p. °. two stars of magnitude five compose nu, their distance apart being ", p. °. a more interesting double is , magnitudes five and five, distance . ", p. °. both stars are white, and they present a pretty appearance when the air is steady. they form a binary system of unknown period. sigma (also called draconis) is a triple, magnitudes six, six and a half, and six, distances . ", p. °, and ", p. °. sigma is an easy double, magnitudes six and a half and eight and a half, distance . ", p. °. the star eta is a very difficult double for even our largest aperture, on account of the faintness of one of its components. the magnitudes are two and a half and ten, distance . ", p. °. its near neighbor, sigma , may be a binary. its magnitudes are six and seven, distance ", p. °. in sigma we have another triple, magnitudes five, eight and a half, and seven, distances . ", p. °, and ", p. °, colors white, blue, and reddish. a fine double is epsilon, magnitudes five and eight, distance ", p. °. the nebula no. is of a planetary character, and interesting as occupying the pole of the ecliptic. a few years ago dr. holden, with the lick telescope, discovered that it is unique in its form. it consists of a double spiral, drawn out nearly in the line of sight, like the thread of a screw whose axis lies approximately endwise with respect to the observer. there is a central star, and another fainter star is involved in the outer spiral. the form of this object suggests strange ideas as to its origin. but the details mentioned are far beyond the reach of our instruments. we shall only see it as a hazy speck. no. is another nebula worth glancing at. it is tuttle's so-called variable nebula. [illustration: map no. .] there are three constellations represented on map no. to which we shall pay brief visits. first aquila demands attention. its doubles may be summarized as follows: , magnitudes five and nine, distance . ", p. °; pi, magnitudes six and seven, distance . ", p. °; , magnitudes six and ten, distance . ", p. °--requires the five-inch and good seeing; , magnitudes five and six, distance ", p. °; sigma , magnitudes six and eight, distance ", p. °; sigma , magnitudes six and seven, distance . ", p. °. the star eta is an interesting variable between magnitudes three and a half and . ; period, seven days, four hours, fourteen minutes. the small red variable r changes from magnitude six to magnitude seven and a half and back again in a period of three hundred and fifty-one days. star cluster no. is a striking object, its stars ranging from the ninth down to the twelfth magnitude. just north of aquila is the little constellation sagitta, containing several interesting doubles and many fine star fields, which may be discovered by sweeping over it with a low-power eyepiece. the star zeta is double, magnitudes five and nine, distance . ", p. °. the larger star is itself double, but far too close to be split, except with very large telescopes. in theta we find three components of magnitudes seven, nine, and eight respectively, distances . ", p. °, and ", p. °. a wide double is epsilon, magnitudes six and eight, distance ", p. °. nebula no. is planetary. turning to delphinus, we find a very beautiful double in gamma, magnitudes four and five, distance ", p. °, colors golden and emerald. the leader alpha, which is not as bright as its neighbor beta, and which is believed to be irregularly variable, is of magnitude four, and has a companion of nine and a half magnitude at the distance ", p. °. at a similar distance, ", p. °, beta has an eleventh-magnitude companion, and the main star is also double, but excessively close, and much beyond our reach. it is believed to be a swiftly moving binary, whose stars are never separated widely enough to be distinguished with common telescopes. chapter vi from lyra to eridanus "this orpheus struck when with his wondrous song he charmed the woods and drew the rocks along."--manilius. [illustration: map no. .] we resume our celestial explorations with the little constellation lyra, whose chief star, vega (alpha), has a very good claim to be regarded as the most beautiful in the sky. the position of this remarkable star is indicated in map no. . every eye not insensitive to delicate shades of color perceives at once that vega is not white, but blue-white. when the telescope is turned upon the star the color brightens splendidly. indeed, some glasses decidedly exaggerate the blueness of vega, but the effect is so beautiful that one can easily forgive the optical imperfection which produces it. with our four-inch we look for the well-known companion of vega, a tenth-magnitude star, also of a blue color deeper than the hue of its great neighbor. the distance is ", p. °. under the most favorable circumstances it might be glimpsed with the three-inch, but, upon the whole, i should regard it as too severe a test for so small an aperture. vega is one of those stars which evidently are not only enormously larger than the sun (one estimate makes the ratio in this case nine hundred to one), but whose physical condition, as far as the spectroscope reveals it, is very different from that of our ruling orb. like sirius, vega displays the lines of hydrogen most conspicuously, and it is probably a much hotter as well as a much more voluminous body than the sun. close by, toward the east, two fourth-magnitude stars form a little triangle with vega. both are interesting objects for the telescope, and the northern one, epsilon, has few rivals in this respect. let us first look at it with an opera glass. the slight magnifying power of such an instrument divides the star into two twinkling points. they are about two and a quarter minutes of arc apart, and exceptionally sharp-sighted persons are able to see them divided with the naked eye. now take the three-inch telescope and look at them, with a moderate power. each of the two stars revealed by the opera glass appears double, and a fifth star of the ninth magnitude is seen on one side of an imaginary line joining the two pairs. the northern-most pair is named epsilon_ , the magnitudes being fifth and sixth, distance ", p. °. the other pair is epsilon_ , magnitudes fifth and sixth, distance . ", p. °. each pair is apparently a binary; but the period of revolution is unknown. some have guessed a thousand years for one pair, and two thousand for the other. another guess gives epsilon_ a period of one thousand years, and epsilon_ a period of eight hundred years. hall, in his double-star observations, simply says of each, "a slow motion." purely by guesswork a period has also been assigned to the two pairs in a supposed revolution around their common center, the time named being about a million years. it is not known, however, that such a motion exists. manifestly it could not be ascertained within the brief period during which scientific observations of these stars have been made. the importance of the element of time in the study of stellar motions is frequently overlooked, though not, of course, by those who are engaged in such work. the sun, for instance, and many of the stars are known to be moving in what appear to be straight lines in space, but observations extending over thousands of years would probably show that these motions are in curved paths, and perhaps in closed orbits. if now in turn we take our four-inch glass, we shall see something else in this strange family group of epsilon lyræ. between epsilon_ and epsilon_ , and placed one on each side of the joining line, appear two exceedingly faint specks of light, which sir john herschel made famous under the name of the _debillissima_. they are of the twelfth or thirteenth magnitude, and possibly variable to a slight degree. if you can not see them at first, turn your eye toward one side of the field of view, and thus, by bringing their images upon a more sensitive part of the retina, you may glimpse them. the sight is not much, yet it will repay you, as every glance into the depths of the universe does. the other fourth-magnitude star near vega is zeta, a wide double, magnitudes fourth and sixth, distance ", p. °. below we find beta, another very interesting star, since it is both a multiple and an eccentric variable. it has four companions, three of which we can easily see with our three-inch; the fourth calls for the five-inch; the magnitudes are respectively four, seven or under, eight, eight and a half, and eleven; distances ", p. °; ", p. °; ", p. °; and ", p. °. the primary, beta, varies from about magnitude three and a half to magnitude four and a half, the period being twelve days, twenty-one hours, forty-six minutes, and fifty-eight seconds. two unequal maxima and minima occur within this period. in the spectrum of this star some of the hydrogen lines and the d_ line (the latter representing helium, a constituent of the sun and of some of the stars, which, until its recent discovery in a few rare minerals was not known to exist on the earth) are bright, but they vary in visibility. moreover, dark lines due to hydrogen also appear in its spectrum simultaneously with the bright lines of that element. then, too, the bright lines are sometimes seen double. professor pickering's explanation is that beta lyræ probably consists of two stars, which, like the two composing beta aurigæ, are too close to be separated with any telescope now existing, and that the body which gives the bright lines is revolving in a circle in a period of about twelve days and twenty-two hours around the body which gives the dark lines. he has also suggested that the appearances could be accounted for by supposing a body like our sun to be rotating in twelve days and twenty-two hours, and having attached to it an enormous protuberance extending over more than one hundred and eighty degrees of longitude, so that when one end of it was approaching us with the rotation of the star the other end would be receding, and a splitting of the spectral lines at certain periods would be the consequence. "the variation in light," he adds, "may be caused by the visibility of a larger or smaller portion of this protuberance." unfortunate star, doomed to carry its parasitical burden of hydrogen and helium, like sindbad in the clasp of the old man of the sea! surely, the human imagination is never so wonderful as when it bears an astronomer on its wings. yet it must be admitted that the facts in this case are well calculated to summon the genius of hypothesis. and the puzzle is hardly simplified by bélopolsky's observation that the body in beta lyræ giving dark hydrogen lines shows those lines also split at certain times. it has been calculated, from a study of the phenomena noted above, that the bright-line star in beta lyræ is situated at a distance of about fifteen million miles from the center of gravity of the curiously complicated system of which it forms a part. we have not yet exhausted the wonders of lyra. on a line from beta to gamma, and about one third of the distance from the former to the latter, is the celebrated ring nebula, indicated on the map by the number . we need all the light we can get to see this object well, and so, although the three-inch will show it, we shall use the five-inch. beginning with a power of one hundred diameters, which exhibits it as a minute elliptical ring, rather misty, very soft and delicate, and yet distinct, we increase the magnification first to two hundred and finally to three hundred, in order to distinguish a little better some of the details of its shape. upon the whole, however, we find that the lowest power that clearly brings out the ring gives the most satisfactory view. the circumference of the ring is greater than that of the planet jupiter. its ellipticity is conspicuous, the length of the longer axis being " and that of the shorter ". closely following the nebula as it moves through the field of view, our five-inch telescope reveals a faint star of the eleventh or twelfth magnitude, which is suspected of variability. the largest instruments, like the washington and the lick glasses, have shown perhaps a dozen other stars apparently connected with the nebula. a beautiful sparkling effect which the nebula presents was once thought to be an indication that it was really composed of a circle of stars, but the spectroscope shows that its constitution is gaseous. just in the middle of the open ring is a feeble star, a mere spark in the most powerful telescope. but when the ring nebula is photographed--and this is seen beautifully in the photographs made with the crossley reflector on mount hamilton by the late prof. j. e. keeler--this excessively faint star imprints its image boldly as a large bright blur, encircled by the nebulous ring, which itself appears to consist of a series of intertwisted spirals. not far away we find a difficult double star, , whose components are of magnitudes six and ten or eleven, distance . ", p. °. from lyra we pass to cygnus, which, lying in one of the richest parts of the milky way, is a very interesting constellation for the possessor of a telescope. its general outlines are plainly marked for the naked eye by the figure of a cross more than twenty degrees in length lying along the axis of the milky way. the foot of the cross is indicated by the star beta, also known as albireo, one of the most charming of all the double stars. the three-inch amply suffices to reveal the beauty of this object, whose components present as sharp a contrast of light yellow and deep blue as it would be possible to produce artificially with the purest pigments. the magnitudes are three and seven, distance . ", p. °. no motion has been detected indicating that these stars are connected in orbital revolution, yet no one can look at them without feeling that they are intimately related to one another. it is a sight to which one returns again and again, always with undiminished pleasure. the most inexperienced observer admires its beauty, and after an hour spent with doubtful results in trying to interest a tyro in double stars it is always with a sense of assured success that one turns the telescope to beta cygni. following up the beam of the imaginary cross along the current of the milky way, every square degree of which is here worth long gazing into, we come to a pair of stars which contend for the name-letter chi. on our map the letter is attached to the southernmost of the two, a variable of long period--four hundred and six days--whose changes of brilliance lie between magnitudes four and thirteen, but which exhibits much irregularity in its maxima. the other star, not named but easily recognized in the map, is sometimes called . it is an attractive double whose colors faintly reproduce those of beta. the magnitudes are five and eight, distance ", p. °. where the two arms of the cross meet is gamma, whose remarkable _cortége_ of small stars running in curved streams should not be missed. use the lowest magnifying power. at the extremity of the western arm of the cross is delta, a close double, difficult for telescopes of moderate aperture on account of the difference in the magnitudes of the components. we may succeed in dividing it with the five-inch. the magnitudes are three and eight, distance . ", p. °. it is regarded as a binary of long and as yet unascertained period. in omicron^ we find a star of magnitude four and orange in color, having two blue companions, the first of magnitude seven and a half, distance ", p. °, and the second of magnitude five and a half, distance ", p. °. farther north is psi, which presents to us the combination of a white five-and-a-half-magnitude star with a lilac star of magnitude seven and a half. the distance is ", p. °. a very pretty sight. we now pass to the extremity of the other arm of the cross, near which lies the beautiful little double , whose components are of magnitudes six and eight, distance . ", p. °. the colors are yellow and blue, conspicuous and finely contrasted. a neighboring double of similar hues is , in which the magnitudes are four and nine, distance ", p. °. sweeping a little way northward we come upon an interesting binary, lambda, which is unfortunately beyond the dividing power of our largest glass. a good seven-inch or seven-and-a-half-inch should split it under favorable circumstances. its magnitudes are six and seven, distance . ", p. °. the next step carries us to a very famous object, cygni, long known as the nearest star in the northern hemisphere of the heavens. it is a double which our three-inch will readily divide, the magnitudes being both six, distance ", p. °. the distance of cygni, according to hall's parallax of . ", is about , , , , miles. there is some question whether or not it is a binary, for, while the twin stars are both moving in the same direction in space with comparative rapidity, yet conclusive evidence of orbital motion is lacking. when one has noticed the contrast in apparent size between this comparatively near-by star, which the naked eye only detects with considerable difficulty, and some of its brilliant neighbors whose distance is so great as to be immeasurable with our present means, no better proof will be needed of the fact that the faintness of a star is not necessarily an indication of remoteness. we may prepare our eyes for a beautiful exhibition of contrasted colors once more in the star . this is really a quadruple, although only two of its components are close and conspicuous. the magnitudes are five, six, seven and a half, and twelve; distances . ", p. °; ", p. °; and ", p. °. the color of the largest star is white and that of its nearest companion blue; the star of magnitude seven and a half is also blue. the star cluster is a fine sight with our largest glass. in the map we find the place marked where the new star of made its appearance. this was first noticed on november , , when it shone with the brilliance of a star of magnitude three and a half. its spectrum was carefully studied, especially by vogel, and the very interesting changes that it underwent were noted. within a year the star had faded to less than the tenth magnitude, and its spectrum had completely changed in appearance, and had come to bear a close resemblance to that of a planetary nebula. this has been quoted as a possible instance of a celestial collision through whose effects the solid colliding masses were vaporized and expanded into a nebula. at present the star is very faint and can only be seen with the most powerful telescopes. compare with the case of nova aurigæ, previously discussed. underneath cygnus we notice the small constellation vulpecula. it contains a few objects worthy of attention, the first being the nebula , the "dumb-bell nebula" of lord rosse. with the four-inch, and better with the five-inch, we are able to perceive that it consists of two close-lying tufts of misty light. many stars surround it, and large telescopes show them scattered between the two main masses of the nebula. the lick photographs show that its structure is spiral. the star points out the place where a new star of the third magnitude appeared in . sigma is a close double, magnitudes six and eight, distance . ", p. °. [illustration: map no. .] we turn to map no. , and, beginning at the western end of the constellation aquarius, we find the variable t, which ranges between magnitudes seven and thirteen in a period of about two hundred and three days. its near neighbor sigma is a very close double, beyond the separating power of our five-inch, the magnitudes being six and seven, distance . ", p. °. sigma , also known as aquarii, is a good double for the three-inch. its magnitudes are six and eight, distance . ", p. °. in zeta we discover a beauty. it is a slow binary of magnitudes four and four, distance . ", p. °. according to some observers both stars have a greenish tinge. the star is a wider double, magnitudes six and eight, distance ", p. °, colors yellow and blue. the uncommon stellar contrast of white with light garnet is exhibited by tau, magnitudes six and nine, distance ", p. °. yellow and blue occur again conspicuously in psi, magnitudes four and a half and eight and a half, distance ", p. °. rose and emerald have been recorded as the colors exhibited in sigma , whose magnitudes are five and seven, distance . ", p. °. the variables s and r are both red. the former ranges between magnitudes eight and twelve, period two hundred and eighty days, and the latter between magnitudes six and eleven, period about three hundred and ninety days. the nebula is rosse's "saturn nebula," so called because with his great telescope it presented the appearance of a nebulous model of the planet saturn. with our five-inch we see it simply as a planetary nebula. we may also glance at another nebula, , which appears circular and is pinned with a little star at the edge. the small constellation equuleus contains a surprisingly large number of interesting objects. sigma is a rather close double, magnitudes six and eight, distance . ", p. °. sigma (the first star to the left of sigma , the name having accidentally been omitted from the map) is a beautiful triple, although the two closest stars, of magnitudes six and seven, can not be separated by our instruments. their distance in was . ", p. °, and they had then been closing rapidly since , when the distance was . ". the third star, of magnitude eight, is distant ", p. °. sigma consists of two stars, magnitudes six and seven, distance . ", p. . °. it is probably a binary. sigma is wider double, magnitudes both six, distance . ", p. °. another triple, one of whose components is beyond our reach, is gamma. here the magnitudes are fifth, twelfth, and sixth, distances ", p. ° and ". it would also be useless for us to try to separate delta, but it is interesting to remember that this is one of the closest of known double stars, the magnitudes being fourth and fifth, distance . ", p. °. these data are from hall's measurements in . the star is, no doubt, a binary. with the five-inch we may detect one and perhaps two of the companion stars in the quadruple beta. the magnitudes are five, ten, and two eleven, distances ", p. °; ", p. °; and . ", p. °. the close pair is comprised in the tenth-magnitude star. [illustration: map no. .] map no. introduces us to the constellation pegasus, which is comparatively barren to the naked eye, and by no means rich in telescopic phenomena. the star epsilon, of magnitude two and a half, has a blue companion of the eighth magnitude, distance ", p. °; colors yellow and violet. a curious experiment that may be tried with this star is described by webb, who ascribes the discovery of the phenomenon to sir john herschel. when near the meridian the small star in epsilon appears, in the telescope, underneath the large one. if now the tube of the telescope be slightly swung from side to side the small star will appear to describe a pendulumlike movement with respect to the large one. the explanation suggested is that the comparative faintness of the small star causes its light to affect the retina of the eye less quickly than does that of its brighter companion, and, in consequence, the reversal of its apparent motion with the swinging of the telescope is not perceived so soon. the third-magnitude star eta has a companion of magnitude ten and a half, distance ", p. °. the star beta, of the second magnitude, and reddish, is variable to the extent of half a magnitude in an irregular period, and gamma, of magnitude two and a half, has an eleventh-magnitude companion, distance ", p. °. [illustration: map no. .] our interest is revived on turning, with the guidance of map no. , from the comparative poverty of pegasus to the spacious constellation cetus. the first double star that we meet in this constellation is , whose components are of magnitudes six and nine, distance . ", p. °; colors, topaz and lilac. not far away is the closer double , composed of a sixth and a seventh magnitude star, distance . ", p. °. the four-inch is capable of splitting this star, but we shall do better to use the five-inch. in passing we may glance at the tenth-magnitude companion to eta, distance ", p. °. another wide pair is found in zeta, magnitudes three and nine, distance ", p. °. the next step brings us to the wonderful variable omicron, or mira, whose changes have been watched for three centuries, the first observer of the variability of the star having been david fabricius in . not only is the range of variability very great, but the period is remarkably irregular. in the time of hevelius, mira was once invisible for four years. when brightest, the star is of about the second magnitude, and when faintest, of the ninth magnitude, but at maximum it seldom exhibits the greatest brilliance that it has on a few occasions shown itself capable of attaining. ordinarily it begins to fade after reaching the fourth or fifth magnitude. the period averages about three hundred and thirty-one days, but is irregularly variable to the extent of twenty-five days. its color is red, and its spectrum shows bright lines, which it is believed disappear when the star sinks to a minimum. among the various theories proposed to account for such changes as these the most probable appears to be that which ascribes them to some cause analogous to that operating in the production of sun spots. the outburst of light, however, as pointed out by scheiner, should be regarded as corresponding to the maximum and not the minimum stage of sun-spot activity. according to this view, the star is to be regarded as possessing an extensive atmosphere of hydrogen, which, during the maximum, is upheaved into enormous prominences, and the brilliance of the light from these prominences suffices to swamp the photospheric light, so that in the spectrum the hydrogen lines appear bright instead of dark. it is not possible to suppose that mira can be the center of a system of habitable planets, no matter what we may think of the more constant stars in that regard, because its radiation manifestly increases more than six hundred fold, and then falls off again to an equal extent once in every ten or eleven months. i have met people who can not believe that the almighty would make a sun and then allow its energies "to go to waste," by not supplying it with a family of worlds. but i imagine that if they had to live within the precincts of mira ceti they would cry out for exemption from their own law of stellar utility. the most beautiful double star in cetus is gamma, magnitudes three and seven, distance ", p. °; hues, straw-color and blue. the leading star alpha, of magnitude two and a half, has a distant blue companion three magnitudes fainter, and between them are two minute stars, the southernmost of which is a double, magnitudes both eleven, distance ", p. °. the variable s ranges between magnitudes seven and twelve in a somewhat irregular period of about eleven months, while r ranges between the seventh and the thirteenth magnitudes in a period of one hundred and sixty-seven days. [illustration: map no. .] the constellation eridanus, represented in map no. , contains a few fine double stars, one of the most interesting of which is , a rather close binary. the magnitudes are four and eight, distance ", p. °. we shall take the five-inch for this, and a steady atmosphere and sharp seeing will be necessary on account of the wide difference in the brightness of the component stars. amateurs frequently fail to make due allowance for the effect of such difference. when the limit of separating power for a telescope of a particular aperture is set at " or ", as the case may be, it is assumed that the stars composing the doubles on which the test is made shall be of nearly the same magnitude, or at least that they shall not differ by more than one or two magnitudes at the most. the stray light surrounding a comparatively bright star tends to conceal a faint companion, although the telescope may perfectly separate them so far as the stellar disks are concerned. then, too, i have observed in my own experience that a very faint and close double is more difficult than a brighter pair not more widely separated, usually on account of the defect of light, and this is true even when the components of the faint double are of equal magnitude. sigma , otherwise known as eridani, is a superb object on account of the colors of its components, the larger star being a rich topaz and the smaller an ultramarine; while the difference in magnitude is not as great as in many of the colored doubles. the magnitudes are five and seven, distance . ", p. °. the star gamma, of magnitude two and a half, has a tenth-magnitude companion, distant ", p. °. sigma , also called eridani, consists of two stars of magnitudes six and nine, distance . ", p. °; colors, yellow and blue. the supposed binary character of this star has not yet been established. in omicron^ we come upon an interesting triple star, two of whose components at any rate we can easily see. the largest component is of the fourth magnitude. at a distance of ", p. °, we find a tenth-magnitude companion. this companion is itself double, the magnitudes of its components being ten and eleven, distance . ", p. °. hall says of these stars that they "form a remarkable system." he has also observed a fourth star of the twelfth magnitude, distant " from the largest star, p. °. this is apparently unconnected with the others, although it is only half as distant as the tenth-magnitude component is from the primary. sigma is interesting because of the similarity of its two components in size, both being of about the seventh magnitude, distance ", p. °. finally, we turn to the nebula . this is planetary in form and inconspicuous, but lassell has described it as presenting a most extraordinary appearance with his great reflector--a circular nebula lying upon another fainter and larger nebula of a similar shape, and having a star in its center. yet it may possibly be an immensely distant star cluster instead of a nebula, since its spectrum does not appear to be gaseous. chapter vii pisces, aries, taurus, and the northern stars "now sing we stormy skies when autumn weighs the year, and adds to nights and shortens days, and suns declining shine with feeble rays."--dryden's virgil. [illustration: map no. .] the eastern end of pisces, represented in map no. , includes most of the interesting telescopic objects that the constellation contains. we begin our exploration at the star numbered , a double that is very beautiful when viewed with the three-inch glass. the components are of magnitudes five and eight, distance . ", p. °. the larger star is yellow and the smaller deep blue. the star , while lacking the peculiar charm of contrasted colors so finely displayed in , possesses an attraction in the equality of its components which are both of the sixth magnitude and milk-white. the distance is . ", p. °. in we find a swift binary whose components are at present far too close for any except the largest telescopes. the distance in was only . ", p. °. the magnitudes are six and seven. in contrast with this excessively close double is psi, whose components are both of magnitude five and a half, distance ", p. °. dropping down to we come upon another very wide and pleasing double, magnitudes six and seven, distance ", p. °, colors white and lilac or pale blue. hardly less beautiful is zeta magnitudes five and six, distance ", p. °. finest of all is alpha, which exhibits a remarkable color contrast, the larger star being greenish and the smaller blue. the magnitudes are four and five, distance ", p. °. this star is a binary, but the motion is slow. the variable r ranges between magnitudes seven and thirteen, period three hundred and forty-four days. the constellation aries contains several beautiful doubles, all but one of which are easy for our smallest aperture. the most striking of these is gamma, which is historically interesting as the first double star discovered. the discovery was made by robert hooke in by accident, while he was following the comet of that year with his telescope. he expressed great surprise on noticing that the glass divided the star, and remarked that he had not met with a like instance in all the heavens. his observations could not have been very extensive or very carefully conducted, for there are many double stars much wider than gamma arietis which hooke could certainly have separated if he had examined them. the magnitudes of the components of gamma are four and four and a half, or, according to hall, both four; distance . ", p. °. a few degrees above gamma, passing by beta, is a wide double lambda, magnitudes five and eight, distance ", p. °, colors white and lilac or violet. three stars are to be seen in : magnitudes five and a half, ten, and nine, distances ", p. °, and ", p. °, colors white, blue, and lilac. the star is a very pretty double, magnitudes six and seven, distance . ", p. °. sigma consists of a topaz star combined with a sapphire, magnitudes six and nine, distance . ", p. °. the fourth-magnitude star has several faint companions. the magnitudes of two of these are eleven and nine, distances ", p. °, and ", p. °. we discover another triple in pi, magnitudes five, eight, and eleven, distances . ", p. °, and ", p. °. the double mentioned above as being too close for our three-inch glass is epsilon, which, however, can be divided with the four-inch, although the five-inch will serve us better. the magnitudes are five and a half and six, distance . ", p. °. the star has two companions, one of which is so close that our instruments can not separate it, while the other is too faint to be visible in the light of its brilliant neighbor without the aid of a very powerful telescope. [illustration: map no. .] we are now about to enter one of the most magnificent regions in the sky, which is hardly less attractive to the naked eye than orion, and which men must have admired from the beginning of their history on the earth, the constellation taurus (map no. ). two groups of stars especially distinguish taurus, the hyades and the pleiades, and both are exceedingly interesting when viewed with the lowest magnifying powers of our telescopes. we shall begin with a little star just west of the pleiades, sigma , also called tauri. this is a triple, but we can see it only as a double, the third star being exceedingly close to the primary. the magnitudes are six and a half, seven, and ten, distances . ", p. °, and ", p. °. in the pleiades we naturally turn to the brightest star eta, or alcyone, famous for having once been regarded as the central sun around which our sun and a multitude of other luminaries were supposed to revolve, and picturesque on account of the little triangle of small stars near it which the least telescopic assistance enables us to see. one may derive much pleasure from a study of the various groupings of stars in the pleiades. photography has demonstrated, what had long been suspected from occasional glimpses revealed by the telescope, that this celebrated cluster of stars is intermingled with curious forms of nebulæ. the nebulous matter appears in festoons, apparently attached to some of the larger stars, such as alcyone, merope, and maia, and in long, narrow, straight lines, the most remarkable of which, a faintly luminous thread starting midway between maia and alcyone and running eastward some ', is beaded with seven or eight stars. the width of this strange nebulous streak is, on an average, " or ", and there is, perhaps, no more wonderful phenomenon anywhere in celestial space. unfortunately, no telescope is able to show it, and all our knowledge about it is based upon photographs. it might be supposed that it was a nebulous disk seen edgewise, but for the fact that at the largest star involved in its course it bends sharply about ° out of its former direction, and for the additional fact that it seems to take its origin from a curved offshoot of the intricate nebulous mass surrounding maia. exactly at the point where this curve is transformed into a straight line shines a small star! in view of all the facts the idea does not seem to be very far-fetched that in the pleiades we behold an assemblage of suns, large and small, formed by the gradual condensation of a nebula, and in which evolution has gone on far beyond the stage represented by the orion nebula, where also a group of stars may be in process of formation out of nebulous matter. if we look a little farther along this line of development, we may perceive in such a stellar assemblage as the cluster in hercules, a still later phase wherein all the originally scattered material has, perhaps, been absorbed into the starry nuclei. [illustration: the chief stars in the pleiades.] the yellow star sigma has two companions: magnitudes six, nine, and nine and a half, distances ", p. °, and ", p. °. the star of the fifth magnitude has a companion of the ninth magnitude, distance ", p. °, colors emerald and purple, faint. an interesting variable, of the type of algol, is lambda, which at maximum is of magnitude three and four tenths and at minimum of magnitude four and two tenths. its period from one maximum to the next is about three days and twenty-three hours, but the actual changes occupy only about ten hours, and it loses light more swiftly than it regains it. a combination of red and blue is presented by phi (mistakenly marked on map no. as psi). the magnitudes are six and eight, distance ", p. °. a double of similar magnitudes is chi, distance ", p. °. between the two stars which the naked eye sees in kappa is a minute pair, each of less than the eleventh magnitude, distance ", p. °. another naked-eye double is formed by theta^ and theta^ , in the hyades. the magnitudes are five and five and a half, distance about ' ". the leading star of taurus, aldebaran (alpha), is celebrated for its reddish color. the precise hue is rather uncertain, but aldebaran is not orange as betelgeuse in orion is, and no correct eye can for an instant confuse the colors of these two stars, although many persons seem to be unable to detect the very plain difference between them in this respect. aldebaran has been called "rose-red," and it would be an interesting occupation for an amateur to determine, with the aid of some proper color scale, the precise hue of this star, and of the many other stars which exhibit chromatic idiosyncrasy. aldebaran is further interesting as being a standard first-magnitude star. with the four-inch glass we see without difficulty the tenth-magnitude companion following aldebaran at a distance of ", p. °. there is an almost inexplicable charm about these faint attendants of bright stars, which is quite different from the interest attaching to a close and nearly equal pair. the impression of physical relationship is never lacking though it may be deceptive, and this awakens a lively appreciation of the vast differences of magnitude that exist among the different suns of space. the actual size and might of this great red sun form an attractive subject for contemplation. as it appears to our eyes aldebaran gives one twenty-five-thousand-millionth as much light as the sun, but if we were placed midway between them the star would outshine the sun in the ratio of not less than to . and yet, gigantic as it is, aldebaran is possibly a pygmy in comparison with arcturus, whose possible dimensions were discussed in the chapter relating to boötes. although aldebaran is known to possess several of the metallic elements that exist in the sun, its spectrum differs widely from the solar spectrum in some respects, and more closely resembles that of arcturus. other interesting objects in taurus are sigma, divisible with the naked eye, magnitudes five and five and a half, distance '; sigma , double, magnitudes six and nine, distance . ", p. °; sigma , double, magnitudes six and seven, distance ", p. °--a pleasing sight; tau, triple, magnitudes four, ten and a half, and eleven, distances ", p. °, and ", p. °--the ten-and-a-half-magnitude star is itself double, as discovered by burnham; star cluster no. , not quite as broad as the moon, and containing some stars as large as the eleventh magnitude; and nebula no. , the so-called "crab nebula" of lord rosse, which our glasses will show only as a misty patch of faint light, although large telescopes reveal in it a very curious structure. [illustration: map no. .] we now turn to the cluster of circumpolar constellations sometimes called the royal family, in allusion to the well-known story of the ethiopian king cepheus and his queen cassiopeia, whose daughter andromeda was exposed on the seashore to be devoured by a monster, but who was saved by the hero perseus. all these mythologic personages are represented in the constellations that we are about to study.[ ] we begin with andromeda (map no. ). the leading star alpha marks one corner of the great square of pegasus. the first star of telescopic interest that we find in andromeda is , a double difficult on account of the faintness of the smaller component. the magnitudes are four and eleven, distance ", p. °. a few degrees north of the naked eye detects a glimmering point where lies the great nebula in andromeda. this is indicated on the map by the number . with either of our three telescopes it is an interesting object, but of course it is advisable to use our largest glass in order to get as much light as possible. all that we can see is a long, shuttle-shaped nebulous object, having a brighter point near the center. many stars are scattered over the field in its neighborhood, but the nebula itself, although its spectrum is peculiar in resembling that of a faint star, is evidently a gaseous or at any rate a meteoritic mass, since photographs show it to be composed of a series of imperfectly separated spirals surrounding a vast central condensation. this peculiarity of the andromeda nebula, which is invisible with telescopes although conspicuous in the photographs, has, since its discovery a few years ago, given a great impetus to speculation concerning the transformation of nebulæ into stars and star clusters. no one can look at a good photograph of this wonderful phenomenon without noticing its resemblance to the ideal state of things which, according to the nebular hypothesis, must once have existed in the solar system. it is to be remembered, however, that there is probably sufficient material in the andromeda nebula to make a system many times, perhaps hundreds or thousands of times, as extensive as that of which our sun is the center. if one contemplates this nebula only long enough to get a clear perception of the fact that creation was not ended when, according to the mosaic history, god, having in six days finished "the heavens and the earth and all the host of them," rested from all his work, a good blow will have been dealt for the cause of truth. systems far vaster than ours are now in the bud, and long before they have bloomed, ambitious man, who once dreamed that all these things were created to serve him, will probably have vanished with the extinguishment of the little star whose radiant energy made his life and his achievements briefly possible. [ ] for further details on this subject see astronomy with an opera-glass. in august, , a new star of magnitude six and a half made its appearance suddenly near the center of the andromeda nebula. within one year it had disappeared, having gradually dwindled until the great washington telescope, then the largest in use, no longer showed it. that this was a phenomenon connected with the nebula is most probable, but just what occurred to produce it nobody knows. the observed appearances might have been produced by a collision, and no better hypothesis has yet been suggested to account for them. near the opposite end of the constellation from alpha we find the most interesting of triple stars in gamma. the two larger components of this beautiful star are of magnitudes three and six, distance ", colors golden yellow and deep blue. the three-inch shows them finely. the smaller star is itself double, its companion being of magnitude eight, distance when discovered in . ", color bluish green. a few years ago this third star got so close to its primary that it was invisible even with the highest powers of the great lick telescope, but at present it is widening again. in october, , i had the pleasure of looking at gamma andromedæ with the lick telescope, and at that time it was possible just to separate the third star. the angle seemed too small for certain measurement, but a single setting of the micrometer by mr. barnard, to whose kindness i was indebted for my view of the star, gave . " as the approximate distance. in the distance had increased to . ", p. °. the brilliance of color contrast between the two larger stars of gamma andromedæ is hardly inferior to that exhibited in beta cygni, so that this star may be regarded as one of the most picturesque of stellar objects for small telescopes. other pleasing objects in this constellation are the binary star , magnitudes six and six and a half, distance ", p. °--the two stars are slowly closing and the five-inch glass is required to separate them: the richly colored variable r, which fades from magnitude five and a half to invisibility, and then recovers its light in a period of about four hundred and five days; and the bright star cluster , which covers a space about equal to the area of the full moon. just south of the eastern end of andromeda is the small constellation triangulum, or the triangles, containing two interesting objects. one of these is the beautiful little double , magnitudes five and six, distance . ", p. °, colors yellow and blue; and the other, the nebula , which equals in extent the star cluster in andromeda described above, but nevertheless appears very faint with our largest glass. its faintness, however, is not an indication of insignificance, for to very powerful telescopes it exhibits a wonderful system of nuclei and spirals--another bit of chaos that is yielding by age-long steps to the influence of demiurgic forces. a richer constellation than andromeda, both for naked-eye and telescopic observation, is perseus, which is especially remarkable for its star clusters. two of these, and , constitute the celebrated double cluster, sometimes called the sword-hand of perseus, and also chi persei. to the smallest telescope this aggregation of stars, ranging in magnitude from six and a half to fourteen, and grouped about two neighboring centers, presents a marvelous appearance. as an educative object for those unaccustomed to celestial observations it may be compared among star clusters to beta cygni among double stars, for the most indifferent spectator is struck with wonder in viewing it. all the other clusters in perseus represented on the map are worth examining, although none of them calls for special mention, except perhaps , where we may distinguish at least a hundred separate stars within an area less than one quarter as expansive as the face of the moon. among the double stars of perseus we note first eta, whose components are of magnitudes four and eight, distance ", colors white and pale blue. the double epsilon is especially interesting on account of an alleged change of color from blue to red which the smaller star undergoes coincidently with a variation of brightness. the magnitudes are three and eight, distance ", p. °. an interesting multiple is zeta, two of whose stars at least we can see. the magnitudes are three, nine, ten, and ten, distances ", p. °, ", and ". the chief attraction in perseus is the changeful and wonderful beta, or algol, the great typical star among the short-period variables. during the greater part of its period this star is of magnitude two and two tenths, but for a very short time, following a rapid loss of light, it remains at magnitude three and seven tenths. the difference, one magnitude and a half, corresponds to an actual difference in brightness in the ratio of . to . the entire loss of light during the declension occupies only four hours and a half. the star remains at its faintest for a few minutes only before a perceptible gain of light occurs, and the return to maximum is as rapid as was the preceding decline. the period from one minimum to the next is two days twenty hours forty-eight minutes fifty-three seconds, with an irregularity amounting to a few seconds in a year. the arabs named the star algol, or the demon, on account of its eccentricity which did not escape their attention; and when goodricke, in , applied a scientific method of observation to it, the real cause of its variations was suggested by him, but his explanation failed of general acceptance until its truth was established by prof. e. c. pickering in . this explanation gives us a wonderful insight into stellar constitution. according to it, algol possesses a companion as large as the sun, but invisible, both because of its proximity to that star and because it yields no light, and revolving in a plane horizontal to our line of sight. the period of revolution is identical with the period of algol's cycle of variation, and the diminution of light is caused by the interposition of the dark body as it sweeps along that part of its orbit lying between our point of view and the disk of algol. in other words, once in every two days twenty hours and forty-nine minutes algol, as seen from the earth, undergoes a partial eclipse. in consequence of the great comparative mass of its dark companion, algol itself moves in an orbit around their common center with a velocity quite sufficient to be detected by the shifting of the lines in its spectrum. by means of data thus obtained the mass, size, and distance apart of algol and its singular comrade have been inferred. the diameter of algol is believed to be about , , miles, that of the dark body about , miles, and the mean distance from center to center , , miles. the density of both the light and the dark star is slight compared with that of the sun, so that their combined mass is only two thirds as great as the sun's. mention has been made of a slight irregularity in algol's period of variation. basing his calculations upon this inequality, dr. chandler has put forward the hypothesis that there is another invisible body connected with algol, and situated at a distance from it of about , , , miles, and that around this body, which is far more massive than the others, algol and its companions revolve in a period of one hundred and thirty years! dr. chandler has earned the right to have his hypotheses regarded with respect, even when they are as extraordinary as that which has just been described. it needs no indulgence of the imagination to lend interest to algol; the simple facts are sufficient. how did that bright star fall in with its black neighbors? or were they created together? [illustration: map no. .] passing to the region covered by map no. , our eyes are caught by the curious figure, formed by the five brightest stars of the constellation cassiopeia, somewhat resembling the letter w. like perseus, this is a rich constellation, both in star clusters and double stars. among the latter we select as our first example sigma, in which we find a combination of color that is at once very unusual and very striking--green and blue. the magnitudes are five and seven, distance ", p. °. another beautiful colored double is eta, whose magnitudes are four and seven and a half, distance ", p. °, colors white and purple. this is one of the comparatively small number of stars the measure of whose distance has been attempted, and a keen sense of the uncertainty of such measures is conveyed by the fact that authorities of apparently equal weight place eta cassiopeiæ at such discordant distances as , , , , miles, , , , , miles, and , , , , miles. it will be observed that the difference between the greatest and the least of these estimates is about double the entire distance given by the latter. the same thing is practically true of the various attempts to ascertain the distance of the other stars which have a perceptible parallax, even those which are evidently the nearest. in some cases the later measures increase the distance, in other cases they diminish it; in no case is there anything like a complete accord. yet of course we are not to infer that it is hopeless to learn anything about the distances of the stars. with all their uncertainties and disagreements the few parallaxes we possess have laid a good foundation for a knowledge of the dimensions of at least the nearer parts of the universe. we find an interesting triple in psi, the magnitudes of the larger components being four and a half and eight and a half, distance ". the smaller star has a nine-and-a-half-magnitude companion, distance ". a more beautiful triple is iota, magnitudes four, seven, and eight, distances ", p. °, and . ", p. °. cassiopeia contains many star clusters, three of which are indicated in the map. of these is perhaps the most interesting, as it includes stars of many magnitudes, among which are a red one of the eighth magnitude, and a ninth-magnitude double whose components are " apart. not far from the star kappa we find the spot where the most brilliant temporary star on record made its appearance on november , . tycho brahe studied this phenomenon during the entire period of its visibility, which lasted until march, . it burst out suddenly with overpowering splendor, far outshining every fixed star, and even equaling venus at her brightest. in a very short time it began to fade, regularly diminishing in brightness, and at the same time undergoing changes of color, ending in red, until it disappeared. it has never been seen since, and the suspicion once entertained that it was a variable with a period considerably exceeding three hundred years has not been confirmed. there is a tenth-magnitude star near the place given by tycho as that occupied by the stranger. many other faint stars are scattered about, however, and tycho's measures were not sufficiently exact to enable us to identify the precise position of his star. if the phenomenon was due to a collision, no reappearance of the star is to be expected. camelopardalus is a very inconspicuous constellation, yet it furnishes considerable occupation for the telescope. sigma , of magnitude five, has a companion of magnitude nine and a half, distance ", °. sigma , also of the fifth magnitude, has a ninth-magnitude companion, distance only . ", p. °. according to some observers, the larger star is yellow and the smaller white. the star is a very pretty double, magnitudes both six, distance . ". its neighbor of magnitude six has an eighth-magnitude companion, distance . ", p. °. the star of magnitude five is also double, the companion of magnitude eight being distant only . ". a glance at star cluster , which shows a slight central condensation, completes our work in camelopardalus, and we turn to ursa major, represented in map no. . here there are many interesting doubles and triples. beginning with iota we find at once occupation for our largest glass. the magnitudes are three and ten, distance ", p. °. in the double star the magnitudes are four and nine, distance ", p. °. a more pleasing object is sigma^ , a greenish fifth-magnitude star which has an eighth-magnitude companion, distance . ", p. °. a good double for our four-inch glass is xi, whose magnitudes are four and five, distance . ", p. °. this is a binary with a period of revolution of about sixty years, and is interesting as the first binary star whose orbit was determined. savary calculated it in . near by is nu, a difficult double, magnitudes four and ten and a half, distance ", p. °. in we find again an easy double magnitudes six and eight, distance . ", p. °. another similar double is , magnitudes six and eight, distance . ", p. °. a third star, magnitude seven, is seen at a distance of " from the primary. we come now to ursa major's principal attraction zeta, frequently called mizar. the naked eye perceives near it a smaller star, named alcor. with the three-inch glass and a medium power we divide mizar into two bright stars brilliantly contrasted in color, the larger being white and the smaller blue-green. beside alcor, several fainter stars are seen scattered over the field of view, and, taken all in all, there are very few equally beautiful sights in the starry heavens. the magnitudes of the double are three and four, distance . ", p. °. the large star is again double, although no telescope has been able to show it so, its duplicity being revealed, like that of beta aurigæ, by the periodical splitting of the lines in its spectrum. ursa major contains several nebulæ which may be glimpsed with telescopes of moderate dimensions. an interesting pair of these objects, both of which are included in one field of view, is formed by and . the first named is the brighter of the two, its nucleus resembling a faint star. the nebula presents itself to us in the form of a faint, hazy star, but with large telescopes its appearance is very singular. according to a picture made by lord rosse, it bears no little resemblance to a skull, there being two symmetrically placed holes in it, each of which contains a star. [illustration: map no. .] the portion of canes venatici, represented in map no. , contains two or three remarkable objects. sigma is a close double, magnitudes six and seven, distance ", p. °. it is a pretty sight with the five-inch. the double star is singular in that its larger component is red and its smaller blue; magnitudes six and eight, distance . ", p. °. still more beautiful is , commonly called cor caroli. this double is wide, and requires but a slight magnifying power. the magnitudes are three and six, distance ", colors white or light yellow and blue. the nebula , although we can see it only as a pair of misty specks, is in reality a very wonderful object. lord rosse's telescope has revealed in it a complicated spiral structure, recalling the photographs of the andromeda nebula, and indicating that stupendous changes must be in process within it, although our records of observation are necessarily too brief to bring out any perceptible alteration of figure. it would seem that the astronomer has, of all men, the best reasons for complaining of the brevity of human life. lastly, we turn to ursa minor and the pole star. the latter is a celebrated double, not difficult, except with a telescope of less than three inches aperture in the hands of an inexperienced observer. the magnitudes are two and nine, distance . ". the small star has a dull blue color. in it was discovered by spectroscopic evidence that the pole star is triple. in pi' we see a wide double, magnitudes six and seven, distance ", p. °. this completes our survey of the starry heavens. chapter viii scenes on the planets "these starry globes far surpassed the earth in grandeur, and the latter looked so diminutive that our empire, which appeared only as a point on its surface, awoke my pity."--cicero, the dream of scipio. although amateurs have played a conspicuous part in telescopic discovery among the heavenly bodies, yet every owner of a small telescope should not expect to attach his name to a star. but he certainly can do something perhaps more useful to himself and his friends; he can follow the discoveries that others, with better appliances and opportunities, have made, and can thus impart to those discoveries that sense of reality which only comes from seeing things with one's own eyes. there are hundreds of things continually referred to in books and writings on astronomy which have but a misty and uncertain significance for the mere reader, but which he can easily verify for himself with the aid of a telescope of four or five inches aperture, and which, when actually confronted by the senses, assume a meaning, a beauty, and an importance that would otherwise entirely have escaped him. henceforth every allusion to the objects he has seen is eloquent with intelligence and suggestion. take, for instance, the planets that have been the subject of so many observations and speculations of late years--mars, jupiter, saturn, venus. for the ordinary reader much that is said about them makes very little impression upon his mind, and is almost unintelligible. he reads of the "snow patches" on mars, but unless he has actually seen the whitened poles of that planet he can form no clear image in his mind of what is meant. so the "belts of jupiter" is a confusing and misleading phrase for almost everybody except the astronomer, and the rings of saturn are beyond comprehension unless they have actually been seen. it is true that pictures and photographs partially supply the place of observation, but by no means so successfully as many imagine. the most realistic drawings and the sharpest photographs in astronomy are those of the moon, yet i think nobody would maintain that any picture in existence is capable of imparting a really satisfactory visual impression of the appearance of the lunar globe. nobody who has not seen the moon with a telescope--it need not be a large one--can form a correct and definite idea of what the moon is like. the satisfaction of viewing with one's own eyes some of the things the astronomers write and talk about is very great, and the illumination that comes from such viewing is equally great. just as in foreign travel the actual seeing of a famous city, a great gallery filled with masterpieces, or a battlefield where decisive issues have been fought out illuminates, for the traveler's mind, the events of history, the criticisms of artists, and the occurrences of contemporary life in foreign lands, so an acquaintance with the sights of the heavens gives a grasp on astronomical problems that can not be acquired in any other way. the person who has been in rome, though he may be no archæologist, gets a far more vivid conception of a new discovery in the forum than does the reader who has never seen the city of the seven hills; and the amateur who has looked at jupiter with a telescope, though he may be no astronomer, finds that the announcement of some change among the wonderful belts of that cloudy planet has for him a meaning and an interest in which the ordinary reader can not share. [illustration: jupiter seen with a five-inch telescope. shadow of a satellite visible.] jupiter is perhaps the easiest of all the planets for the amateur observer. a three-inch telescope gives beautiful views of the great planet, although a four-inch or a five-inch is of course better. but there is no necessity for going beyond six inches' aperture in any case. for myself, i should care for nothing better than my byrne five-inch of fifty-two inches' focal distance. with such a glass more details are visible in the dark belts and along the bright equatorial girdle than can be correctly represented in a sketch before the rotation of the planet has altered their aspect, while the shadows of the satellites thrown upon the broad disk, and the satellites themselves when in transit, can be seen sometimes with exquisite clearness. the contrasting colors of various parts of the disk are also easily studied with a glass of four or five inches' aperture. there is a charm about the great planet when he rides high in a clear evening sky, lording it over the fixed stars with his serene, unflickering luminousness, which no possessor of a telescope can resist. you turn the glass upon him and he floats into the field of view, with his _cortége_ of satellites, like a yellow-and-red moon, attended by four miniatures of itself. you instantly comprehend jupiter's mastery over his satellites--their allegiance is evident. no one would for an instant mistake them for stars accidentally seen in the same field of view. although it requires a very large telescope to magnify their disks to measurable dimensions, yet the smallest glass differentiates them at once from the fixed stars. there is something almost startling in their appearance of companionship with the huge planet--this sudden verification to your eyes of the laws of gravitation and of central forces. it is easy, while looking at jupiter amid his family, to understand the consternation of the churchmen when galileo's telescope revealed that miniature of the solar system, and it is gratifying to gaze upon one of the first battle grounds whereon science gained a decisive victory for truth. the swift changing of place among the satellites, as well as the rapidity of jupiter's axial rotation, give the attraction of visible movement to the jovian spectacle. the planet rotates in four or five minutes less than ten hours--in other words, it makes two turns and four tenths of a third turn while the earth is rolling once upon its axis. a point on jupiter's equator moves about twenty-seven thousand miles, or considerably more than the entire circumference of the earth, in a single hour. the effect of this motion is clearly perceptible to the observer with a telescope on account of the diversified markings and colors of the moving disk, and to watch it is one of the greatest pleasures that the telescope affords. it would be possible, when the planet is favorably situated, to witness an entire rotation of jupiter in the course of one night, but the beginning and end of the observation would be more or less interfered with by the effects of low altitude, to say nothing of the tedium of so long a vigil. but by looking at the planet for an hour at a time in the course of a few nights every side of it will have been presented to view. suppose the first observation is made between nine and ten o'clock on any night which may have been selected. then on the following night between ten and eleven o'clock jupiter will have made two and a half turns upon his axis, and the side diametrically opposite to that seen on the first night will be visible. on the third night between eleven and twelve o'clock jupiter will have performed five complete rotations, and the side originally viewed will be visible again. owing to the rotundity of the planet, only the central part of the disk is sharply defined, and markings which can be easily seen when centrally located become indistinct or disappear altogether when near the limb. approach to the edge of the disk also causes a foreshortening which sometimes entirely alters the aspect of a marking. it is advisable, therefore, to confine the attention mainly to the middle of the disk. as time passes, clearly defined markings on or between the cloudy belts will be seen to approach the western edge of the disk, gradually losing their distinctness and altering their appearance, while from the region of indistinct definition near the eastern edge other markings slowly emerge and advance toward the center, becoming sharper in outline and more clearly defined in color as they swing into view. watching these changes, the observer is carried away by the reflection that he actually sees the turning of another distant world upon its axis of rotation, just as he might view the revolving earth from a standpoint on the moon. belts of reddish clouds, many thousands of miles across, are stretched along on each side of the equator of the great planet he is watching; the equatorial belt itself, brilliantly lemon-hued, or sometimes ruddy, is diversified with white globular and balloon-shaped masses, which almost recall the appearance of summer cloud domes hanging over a terrestrial landscape, while toward the poles shadowy expanses of gradually deepening blue or blue-gray suggest the comparative coolness of those regions which lie always under a low sun. [illustration: eclipses and transits of jupiter's satellites. satellite i and the shadow of iii are seen in transit. iv is about to be eclipsed.] after a few nights' observation even the veriest amateur finds himself recognizing certain shapes or appearances--a narrow dark belt running slopingly across the equator from one of the main cloud zones to the other, or a rift in one of the colored bands, or a rotund white mass apparently floating above the equator, or a broad scallop in the edge of a belt like that near the site of the celebrated "red spot," whose changes of color and aspect since its first appearance in , together with the light it has thrown on the constitution of jupiter's disk, have all but created a new jovian literature, so thoroughly and so frequently have they been discussed. and, having noticed these recurring features, the observer will begin to note their relations to one another, and will thus be led to observe that some of them gradually drift apart, while others drift nearer; and after a time, without any aid from books or hints from observatories, he will discover for himself that there is a law governing the movements on jupiter's disk. upon the whole he will find that the swiftest motions are near the equator, and the slowest near the poles, although, if he is persistent and has a good eye and a good instrument, he will note exceptions to this rule, probably arising, as professor hough suggests, from differences of altitude in jupiter's atmosphere. finally, he will conclude that the colossal globe before him is, exteriorly at least, a vast ball of clouds and vapors, subject to tremendous vicissitudes, possibly intensely heated, and altogether different in its physical constitution, although made up of similar elements, from the earth. then, if he chooses, he can sail off into the delightful cloud-land of astronomical speculation, and make of the striped and spotted sphere of jove just such a world as may please his fancy--for a world of some kind it certainly is. for many observers the satellites of jupiter possess even greater attractions than the gigantic ball itself. as i have already remarked, their movements are very noticeable and lend a wonderful animation to the scene. although they bear classical names, they are almost universally referred to by their roman numbers, beginning with the innermost, whose symbol is i, and running outward in regular order ii, iii, and iv.[ ] the minute satellite much nearer to the planet than any of the others, which mr. barnard discovered with the lick telescope in , is called the fifth, although in the order of distance it would be the first. in size and importance, however, it can not rank with its comparatively gigantic brothers. of course, no amateur's telescope can afford the faintest glimpse of it. [ ] their names, in the same order as their numbers, are io, europa, ganymede, and callisto. satellite i, situated at a mean distance of , miles from jupiter's center--about , miles farther than the moon is from the earth--is urged by its master's overpowering attraction to a speed of miles per minute, so that it performs a complete revolution in about forty-two hours and a half. the others, of course, move more slowly, but even the most distant performs its revolution in several hours less than sixteen days. the plane of their orbits is presented edgewise toward the earth, from which it follows that they appear to move back and forth nearly in straight lines, some apparently approaching the planet, while others are receding from it. the changes in their relative positions, which can be detected from hour to hour, are very striking night after night, and lead to a great variety of arrangements always pleasing to the eye. the most interesting phenomena that they present are their transits and those of their round, black shadows across the face of the planet; their eclipses by the planet's shadow, when they disappear and afterward reappear with astonishing suddenness; and their occultations by the globe of jupiter. upon the whole, the most interesting thing for the amateur to watch is the passage of the shadows across jupiter. the distinctness with which they can be seen when the air is steady is likely to surprise, as it is certain to delight, the observer. when it falls upon a light part of the disk the shadow of a satellite is as black and sharply outlined as a drop of ink; on a dark-colored belt it can not so easily be seen. it is more difficult to see the satellites themselves in transit. there appears to be some difference among them as to visibility in such circumstances. owing to their luminosity they are best seen when they have a dark belt for a background, and are least easily visible when they appear against a bright portion of the planet. every observer should provide himself with a copy of the american ephemeris for the current year, wherein he will find all the information needed to enable him to identify the various satellites and to predict, by turning washington mean time into his own local time, the various phenomena of the transits and eclipses. while a faithful study of the phenomena of jupiter is likely to lead the student to the conclusion that the greatest planet in our system is not a suitable abode for life, yet the problem of its future, always fascinating to the imagination, is open; and whosoever may be disposed to record his observations in a systematic manner may at least hope to render aid in the solution of that problem. saturn ranks next to jupiter in attractiveness for the observer with a telescope. the rings are almost as mystifying to-day as they were in the time of herschel. there is probably no single telescopic view that can compare in the power to excite wonder with that of saturn when the ring system is not so widely opened but that both poles of the planet project beyond it. one returns to it again and again with unflagging interest, and the beauty of the spectacle quite matches its singularity. when saturn is in view the owner of a telescope may become a recruiting officer for astronomy by simply inviting his friends to gaze at the wonderful planet. the silvery color of the ball, delicately chased with half-visible shadings, merging one into another from the bright equatorial band to the bluish polar caps; the grand arch of the rings, sweeping across the planet with a perceptible edging of shadow; their sudden disappearance close to the margin of the ball, where they go behind it and fall straightway into night; the manifest contrast of brightness, if not of color, between the two principal rings; the fine curve of the black line marking the , -mile gap between their edges--these are some of the elements of a picture that can never fade from the memory of any one who has once beheld it in its full glory. [illustration: saturn seen with a five-inch telescope.] saturn's moons are by no means so interesting to watch as are those of jupiter. even the effect of their surprising number (raised to nine by professor pickering's discovery in of a new one which is almost at the limit of visibility, and was found only with the aid of photography) is lost, because most of them are too faint to be seen with ordinary telescopes, or, if seen, to make any notable impression upon the eye. the two largest--titan and japetus--are easily found, and titan is conspicuous, but they give none of that sense of companionship and obedience to a central authority which strikes even the careless observer of jupiter's system. this is owing partly to their more deliberate movements and partly to the inclination of the plane of their orbits, which seldom lies edgewise toward the earth. [illustration: polar view of saturn's system. the orbits of the five nearest satellites are shown. the dotted line outside the rings shows roche's limit.] but the charm of the peerless rings is abiding, and the interest of the spectator is heightened by recalling what science has recently established as to their composition. it is marvelous to think, while looking upon their broad, level surfaces--as smooth, apparently, as polished steel, though thirty thousand miles across--that they are in reality vast circling currents of meteoritic particles or dust, through which run immense waves, condensation and rarefaction succeeding one another as in the undulations of sound. yet, with all their inferential tumult, they may actually be as soundless as the depths of interstellar space, for struve has shown that those spectacular rings possess no appreciable mass, and, viewed from saturn itself, their (to us) gorgeous seeming bow may appear only as a wreath of shimmering vapor spanning the sky and paled by the rivalry of the brighter stars. in view of the theory of tidal action disrupting a satellite within a critical distance from the center of its primary, the thoughtful observer of saturn will find himself wondering what may have been the origin of the rings. the critical distance referred to, and which is known as roche's limit, lies, according to the most trustworthy estimates, just outside the outermost edge of the rings. it follows that if the matter composing the rings were collected into a single body that body would inevitably be torn to pieces and scattered into rings; and so, too, if instead of one there were several or many bodies of considerable size occupying the place of the rings, all of these bodies would be disrupted and scattered. if one of the present moons of saturn--for instance, mimas, the innermost hitherto discovered--should wander within the magic circle of roche's limit it would suffer a similar fate, and its particles would be disseminated among the rings. one can hardly help wondering whether the rings have originated from the demolition of satellites--saturn devouring his children, as the ancient myths represent, and encircling himself, amid the fury of destruction, with the dust of his disintegrated victims. at any rate, the amateur student of saturn will find in the revelations of his telescope the inspirations of poetry as well as those of science, and the bent of his mind will determine which he shall follow. professor pickering's discovery of a ninth satellite of saturn, situated at the great distance of nearly eight million miles from the planet, serves to call attention to the vastness of the "sphere of activity" over which the ringed planet reigns. surprising as the distance of the new satellite appears when compared with that of our moon, it is yet far from the limit where saturn's control ceases and that of the sun becomes predominant. that limit, according to prof. asaph hall's calculation, is nearly , , miles from saturn's center, while if our moon were removed to a distance a little exceeding , miles the earth would be in danger of losing its satellite through the elopement of artemis with apollo. although, as already remarked, the satellites of saturn are not especially interesting to the amateur telescopist, yet it may be well to mention that, in addition to titan and japetus, the satellite named rhea, the fifth in order of distance from the planet, is not a difficult object for a three-or four-inch telescope, and two others considerably fainter than rhea--dione (the fourth) and tethys (the third)--may be seen in favorable circumstances. the others--mimas (the first), enceladus (the second), and hyperion (the seventh)--are beyond the reach of all but large telescopes. the ninth satellite, which has received the name of ph[oe]be, is much fainter than any of the others, its stellar magnitude being reckoned by its discoverer at about . . mars, the best advertised of all the planets, is nearly the least satisfactory to look at except during a favorable opposition, like those of and , when its comparative nearness to the earth renders some of its characteristic features visible in a small telescope. the next favorable opposition will occur in . when well seen with an ordinary telescope, say a four-or five-inch glass, mars shows three peculiarities that may be called fairly conspicuous--viz., its white polar cap, its general reddish, or orange-yellow, hue, and its dark markings, one of the clearest of which is the so-called syrtis major, or, as it was once named on account of its shape, "hourglass sea." other dark expanses in the southern hemisphere are not difficult to be seen, although their outlines are more or less misty and indistinct. the gradual diminution of the polar cap, which certainly behaves in this respect as a mass of snow and ice would do, is a most interesting spectacle. as summer advances in the southern hemisphere of mars, the white circular patch surrounding the pole becomes smaller, night after night, until it sometimes disappears entirely even from the ken of the largest telescopes. at the same time the dark expanses become more distinct, as if the melting of the polar snows had supplied them with a greater depth of water, or the advance of the season had darkened them with a heavier growth of vegetation. [illustration: mars seen with a five-inch telescope.] the phenomena mentioned above are about all that a small telescope will reveal. occasionally a dark streak, which large instruments show is connected with the mysterious system of "canals," can be detected, but the "canals" themselves are far beyond the reach of any telescope except a few of the giants handled by experienced observers. the conviction which seems to have forced its way into the minds even of some conservative astronomers, that on mars the conditions, to use the expression of professor young, "are more nearly earthlike than on any other of the heavenly bodies which we can see with our present telescopes," is sufficient to make the planet a center of undying interest notwithstanding the difficulties with which the amateur is confronted in his endeavors to see the details of its markings. the illumination of venus's atmosphere at the beginning of her transit across the sun. in venus "the fatal gift of beauty" may be said, as far as our observations are concerned, to be matched by the equally fatal gift of brilliance. whether it be due to atmospheric reflection alone or to the prevalence of clouds, venus is so bright that considerable doubt exists as to the actual visibility of any permanent markings on her surface. the detailed representations of the disk of venus by mr. percival lowell, showing in some respects a resemblance to the stripings of mars, can not yet be accepted as decisive. more experienced astronomers than mr. lowell have been unable to see at all things which he draws with a fearless and unhesitating pencil. that there are some shadowy features of the planet's surface to be seen in favourable circumstances is probable, but the time for drawing a "map of venus" has not yet come. the previous work of schiaparelli lends a certain degree of probability to mr. lowell's observations on the rotation of venus. this rotation, according to the original announcement of schiaparelli, is probably performed in the same period as the revolution around the sun. in other words, venus, if schiaparelli and lowell are right, always presents the same side to the sun, possessing, in consequence, a day hemisphere and a night hemisphere which never interchange places. this condition is so antagonistic to all our ideas of what constitutes habitability for a planet that one hesitates to accept it as proved, and almost hopes that it may turn out to have no real existence. venus, as the twin of the earth in size, is a planet which the imagination, warmed by its sunny aspect, would fain people with intelligent beings a little fairer than ourselves; but how can such ideas be reconciled with the picture of a world one half of which is subjected to the merciless rays of a never-setting sun, while the other half is buried in the fearful gloom and icy chill of unending night? any amateur observer who wishes to test his eyesight and his telescope in the search of shades or markings on the disk of venus by the aid of which the question of its rotation may finally be settled should do his work while the sun is still above the horizon. schiaparelli adopted that plan years ago, and others have followed him with advantage. the diffused light of day serves to take off the glare which is so serious an obstacle to the successful observation of venus when seen against a dark sky. knowing the location of venus in the sky, which can be ascertained from the ephemeris, the observer can find it by day. if his telescope is not permanently mounted and provided with "circles" this may not prove an easy thing to do, yet a little perseverance and ingenuity will effect it. one way is to find, with a star chart, some star whose declination is the same, or very nearly the same, as that of venus, and which crosses the meridian say twelve hours ahead of her. then set the telescope upon that star, when it is on the meridian at night, and leave it there, and the next day, twelve hours after the star crossed the meridian, look into your telescope and you will see venus, or, if not, a slight motion of the tube will bring her into view. for many amateurs the phases of venus will alone supply sufficient interest for telescopic observation. the changes in her form, from that of a round full moon when she is near superior conjunction to the gibbous, and finally the half-moon phase as she approaches her eastern elongation, followed by the gradually narrowing and lengthening crescent, until she is a mere silver sickle between the sun and the earth, form a succession of delightful pictures. not very much can be said for mercury as a telescopic object. the little planet presents phases like those of venus, and, according to schiaparelli and lowell, it resembles venus in its rotation, keeping always the same side to the sun. in fact, schiaparelli's discovery of this peculiarity in the case of mercury preceded the similar discovery in the case of venus. there are markings on mercury which have reminded some astronomers of the moon, and there are reasons for thinking that the planet can not be a suitable abode for living beings, at least for beings resembling the inhabitants of the earth. uranus and neptune are too far away to present any attraction for amateur observers. chapter ix the mountains and plains of the moon, and the spectacles of the sun "... the moon, whose orb the tuscan artist views through optic glass at evening from the top of fesolé, or in valdarno, to descry new lands, rivers or mountains in her spotty globe."--paradise lost. the moon is probably the most interesting of all telescopic objects. this arises from its comparative nearness to the earth. a telescope magnifying , diameters brings the moon within an apparent distance of less than miles. if telescopes are ever made with a magnifying power of , diameters, then, provided that atmospheric difficulties can be overcome, we shall see the moon as if it were only about twenty miles off, and a sensitive astronomer might be imagined to feel a little hesitation about gazing so closely at the moon--as if he were peering into a neighbor world's window. but a great telescope and a high magnifying power are not required to interest the amateur astronomer in the study of the moon. our three-inch telescope is amply sufficient to furnish us with entertainment for many an evening while the moon is running through its phases, and we shall find delight in frequently changing the magnifying power as we watch the lunar landscapes, because every change will present them in a different aspect. it should be remembered that a telescope, unless a terrestrial eyepiece or prism is employed, reverses such an object as the moon top for bottom. accordingly, if the moon is on or near the meridian when the observations are made, we shall see the north polar region at the bottom and the south polar region at the top. in other words, the face of the moon as presented in the telescope will be upside down, north and south interchanging places as compared with their positions in a geographical map. but east and west remain unaltered in position, as compared with such a map--i. e., the eastern hemisphere of the moon is seen on the right and the western hemisphere on the left. it is the moon's western edge that catches the first sunlight when "new moon" begins, and, as the phase increases, passing into "first quarter" and from that to "full moon," the illumination sweeps across the disk from west to east. [illustration: lunar chart no. , northwest quarter.] the narrow sickle of the new moon, hanging above the sunset, is a charming telescopic sight. use a low power, and observe the contrast between the bright, smooth round of the sunward edge, which has almost the polish of a golden rim, and the irregular and delicately shaded inner curve, where the adjacent mountains and plains picturesquely reflect or subdue the sunshine. while the crescent grows broader new objects are continually coming into view as the sun rises upon them, until at length one of the most conspicuous and remarkable of the lunar "seas," the _mare crisium_, or sea of crises, lies fully displayed amid its encircling peaks, precipices, and craters. the _mare crisium_ is all in the sunlight between the third and fourth day after "new moon." it is about by miles in extent, and if ever filled with water must have been a very deep sea, since its arid bed lies at a great but not precisely ascertained depth below the general level of the moon. there are a few small craters on the floor of the _mare crisium_, the largest bearing the name of picard, and its borders are rugged with mountains. on the southwestern side is a lofty promontory, , feet in height, called cape agarum. at the middle of the eastern side a kind of bay opens deep in the mountains, whose range here becomes very narrow. southeast of this bay lies a conspicuous bright point, the crater mountain proclus, on which the sun has fully risen in the fourth day of the moon, and which reflects the light with extraordinary liveliness. adjoining proclus on the east and south is a curious, lozenge-shaped flat, broken with short, low ridges, and possessing a most peculiar light-brown tint, easily distinguished from the general color tone of the lunar landscapes. it would be interesting to know what was passing in the mind of the old astronomer who named this singular region _palus somnii_. it is not the only spot on the moon which has been called a "marsh," and to which an unexplained connection with dreams has been ascribed. nearly on the same meridian with proclus, at a distance of about a hundred miles northward, lies a fine example of a ring mountain, rather more than forty miles in diameter, and with peak-tipped walls which in some places are , feet in height, as measured from the floor within. this is macrobius. there is an inconspicuous central mountain in the ring. north of the _mare crisium_, and northwest of macrobius, we find a much larger mountain ring, oblong in shape and nearly eighty miles in its greatest diameter. it is named cleomenes. the highest point on its wall is about , feet above the interior. near the northeast corner of the wall yawns a huge and very deep crater, tralles, while at the northern end is another oblong crater mountain called burckhardt. from cleomenes northward to the pole, or to the northern extremity of the crescent, if our observations are made during new moon, the ground appears broken with an immense number of ridges, craters, and mountain rings, among which we may telescopically wander at will. one of the more remarkable of these objects, which may be identified with the aid of lunar chart no. , is the vast ringed plain near the edge of the disk, named gauss. it is more than a hundred and ten miles in diameter. owing to its situation, so far down the side of the lunar globe, it is foreshortened into a long ellipse, although in reality it is nearly a circle. a chain of mountains runs north and south across the interior plain. geminus, berzelius, and messala are other rings well worth looking at. the remarkable pair called atlas and hercules demand more than passing attention. the former is fifty-five and the latter forty-six miles in diameter. each sinks , feet below the summit of the loftiest peak on its encircling wall. both are full of interesting detail sufficient to occupy the careful observer for many nights. the broad ring bearing the name of endymion is nearly eighty miles in diameter, and has one peak , feet high. the interior plain is flat and dark. beyond endymion on the edge of the disk is part of a gloomy plain called the _mare humboltianum_. after glancing at the crater-shaped mountains on the western and southern border of the _mare crisium_, alhazen, hansen, condorcet, firmicus, etc., we pass southward into the area covered in lunar chart no. . the long dark plain south of the _mare crisium_ is the _mare fecunditatis_, though why it should have been supposed to be particularly fecund, or fertile, is by no means clear. on the western border of this plain, about three hundred miles from the southern end of the _mare crisium_, is the mountain ring, or circumvallation, called langrenus, about ninety miles across and in places , feet high. there is a fine central mountain with a number of peaks. nearly a hundred miles farther south, on the same meridian, lies an equally extensive mountain ring named vendelinus. the broken and complicated appearance of its northern walls will command the observer's attention. another similar step southward, and still on the same meridian brings us to a yet finer mountain ring, slightly larger than the others, and still more complicated in its walls, peaks, and terraces, and in its surroundings of craters, gorges, and broken ridges. this is petavius. west of petavius, on the very edge of the disk, is a wonderful formation, a walled plain named humboldt, which is looked down upon at one point near its eastern edge by a peak , feet in height. about a hundred and forty miles south of petavius is the fourth great mountain ring lying on the same meridian. its name is furnerius. look particularly at the brilliantly shining crater on the northeast slope of the outer wall of furnerius. [illustration: lunar chart no. , southwest quarter.] suppose that our observations are now interrupted, to be resumed when the moon, about "seven days old," is in its first quarter. if we had time, it would be a most interesting thing to watch the advance of the lunar sunrise every night, for new beauties are displayed almost from hour to hour; but, for the purposes of our description it is necessary to curtail the observations. at first quarter one half of the lunar hemisphere which faces the earth is illuminated by the sun, and the line of sunrise runs across some of the most wonderful regions of the moon. we begin, referring once more to lunar chart no. , in the neighborhood of the north pole of the moon. here the line along which day and night meet is twisted and broken, owing to the roughness of the lunar surface. about fifteen degrees southwest of the pole lies a remarkable square-cornered, mountain-bordered plain, about forty miles in length, called barrow. very close to the pole is a ring mountain, about twenty-five miles in diameter, whose two loftiest peaks, , to , feet high, according to neison, must, from their situation, enjoy perpetual day. the long, narrow, dark plain, whose nearest edge is about thirty degrees south of the pole, is the _mare frigoris_, bordered on both sides by uplands and mountains. at its southern edge we find the magnificent aristoteles, a mountain ring, sixty miles across, whose immense wall is composed of terraces and ridges running up to lofty peaks, which rise nearly , feet above the floor of the valley. about a hundred miles south of aristoteles is eudoxus, another fine mountain ring, forty miles in diameter, and quite as deep as its northern neighbor. these two make a most striking spectacle. we are now in the neighborhood of the greatest mountain chains on the moon, the lunar alps lying to the east and the lunar caucasus to the south of aristoteles and eudoxus, while still farther south, separated from the caucasus by a strait not more than a hundred miles broad, begins the mighty range of the lunar apennines. we first turn the telescope on the alps. as the line of sunrise runs directly across their highest peaks, the effect is startling. the greatest elevations are about , feet. the observer's eye is instantly caught by a great valley, running like a furrow through the center of the mountain mass, and about eighty or ninety miles in length. the sealike expanse south and southeast of the alps is the _mare imbrium_, and it is along the coast of this so-called sea that the alps attain their greatest height. the valley, or gorge, above mentioned, appears to cut through the loftiest mountains and to reach the "coast," although it is so narrowed and broken among the greater peaks that its southern portion is almost lost before it actually reaches the _mare imbrium_. opening wider again as it enters the _mare_, it forms a deep bay among precipitous mountains. the caucasus mountains are not so lofty nor so precipitous as the alps, and consequently have less attraction for the observer. they border the dark, oval plain of the _mare serenitatis_ on its northeastern side. the great bay running out from the _mare_ toward the northwest, between the caucasus and the huge mountain ring of posidonius, bears the fanciful name of _lacus somniorum_. in the old days when the moon was supposed to be inhabited, those terrestrial godfathers, led by the astronomer riccioli, who were busy bestowing names upon the "seas" and mountains of our patient satellite, may have pleased their imagination by picturing this arm of the "serene sea" as a peculiarly romantic sheet of water, amid whose magical influences the lunar gentlefolk, drifting softly in their silver galleons and barges, and enjoying the splendors of "full earth" poured upon their delightful little world, were accustomed to fall into charming reveries, as even we hard-headed sons of adam occasionally do when the waters under the keel are calm and smooth and the balmy air of a moonlit night invokes the twin spirits of poetry and music. posidonius, the dominating feature of the shore line here, is an extraordinary example of the many formations on the moon which are so different from everything on the earth that astronomers do not find it easy to bestow upon them names that truly describe them. it may be called a ring mountain or a ringed plain, for it is both. its diameter exceeds sixty miles, and the interior plain lies about , feet below the outer surface of the lunar ground. the mountain wall surrounding the ring is by no means remarkable for elevation, its greatest height not exceeding , feet, but, owing to the broad sweep of the curved walls, the brightness of the plain they inclose, and the picturesque irregularity of the silhouette of shadow thrown upon the valley floor by the peaks encircling it, the effect produced upon the observer is very striking and attractive. having finished with posidonius and glanced across the broken region of the taurus mountains toward the west, we turn next to consider the _mare serenitatis_. this broad gray plain, which, with a slight magnifying power, certainly looks enough like a sea to justify the first telescopists in thinking that it might contain water, is about by miles in extent, its area being , square miles. running directly through its middle, nearly in a north and south line, is a light streak, which even a good opera glass shows. this streak is the largest and most wonderful of the many similar rays which extend on all sides from the great crater, or ring, of tycho in the southern hemisphere. the ray in question is more than , miles long, and, like its shorter congeners, it turns aside for nothing; neither "sea," nor peak, nor mountain range, nor crater ring, nor gorge, nor cañon, is able to divert it from its course. it ascends all heights and drops into all depths with perfect indifference, but its continuity is not broken. when the sun does not illuminate it at a proper angle, however, the mysterious ray vanishes. is it a metallic vein, or is it volcanic lava or ash? was the globe of the moon once split open along this line? the _mare serenitatis_ is encircled by mountain ranges to a greater extent than any of the other lunar "seas." on its eastern side the caucasus and the apennines shut it in, except for a strait a hundred miles broad, by means of which it is connected with the _mare imbrium_. on the south the range of the hæmus mountains borders it, on the north and northwest the caucasus and the taurus mountains confine it, while on the west, where again it connects itself by a narrow strait with another "sea," the _mare tranquilitatis_, it encounters the massive uplift of mount argæus. not far from the eastern strait is found the remarkable little crater named linné, not conspicuous on the gray floor of the _mare_, yet easily enough found, and very interesting because a considerable change of form seems to have come over this crater some time near the middle of the nineteenth century. in referring to it as a crater it must not be forgotten that it does not form an opening in the top of a mountain. in fact, the so-called craters on the moon, generally speaking, are simply cavities in the lunar surface, whose bottoms lie deep below the general level, instead of being elevated on the summit of mountains, and inclosed in a conical peak. in regard to the alleged change in linné, it has been suggested, not that a volcanic eruption brought it about, but that a downfall of steep walls, or of an unsupported rocky floor, was the cause. the possibility of such an occurrence, it must be admitted, adds to the interest of the observer who regularly studies the moon with a telescope. just on the southern border of the _mare_, the beautiful ring menelaus lies in the center of the chain of the hæmus mountains. the ring is about twenty miles across, and its central peak is composed of some highly reflecting material, so that it shines very bright. the streak or ray from tycho which crosses the _mare serenitatis_ passes through the walls of menelaus, and perhaps the central peak is composed of the same substance that forms the ray. something more than a hundred miles east-southeast from menelaus, in the midst of the dark _mare vaporum_, is another brilliant ring mountain which catches the eye, manilius. it exceeds menelaus in brightness as well as in size, its diameter being about twenty-five miles. there is something singular underlying the dark lunar surface here, for not only is manilius extraordinarily brilliant in contrast with the surrounding plain, but out of that plain, about forty miles toward the east, projects a small mountain which is also remarkable for its reflecting properties, as if the gray ground were underlain by a stratum of some material that flashes back the sunlight wherever it is exposed. the crater mountain, sulpicius gallus, on the border of the _mare_, north of manilius and east of menelaus, is another example of the strange shining quality of certain formations on the moon. follow next the hæmus range westward until the attention falls upon the great ring mountain plinius, more than thirty miles across, and bearing an unusual resemblance to a fortification. mr. t. g. elger, the celebrated english selenographer, says of plinius that, at sunrise, "it reminds one of a great fortress or redoubt erected to command the passage between the _mare tranquilitatis_ and the _mare serenitatis_." but, of course, the resemblance is purely fanciful. men, even though they dwelt in the moon, would not build a rampart , feet high! mount argæus, at the southwest corner of the _mare serenitatis_, is a very wonderful object when the sun has just risen upon it. this occurs five days after the new moon. returning to the eastern extremity of the _mare_, we glance, in passing, at the precipitous mount hadley, which rises more than , feet above the level of the _mare_ and forms the northern point of the apennine range. passing into the region of the _mare imbrium_, whose western end is divided into the _palus putredinis_ on the south and the _palus nebularum_ on the north, we notice three conspicuous ring mountains, cassini near the alps, and aristillus and autolycus, a beautiful pair, nearly opposite the strait connecting the two _maria_. cassini is thirty-six miles in diameter, aristillus thirty-four, and autolycus twenty-three. the first named is shallow, only , feet in depth from the highest point of its wall, while aristillus carries some peaks on its girdle , feet high. autolycus, like cassini, is of no very great depth. westward from the middle of an imaginary line joining aristillus and cassini is the much smaller crater theætetus. outside the walls of this are a number of craterlets, and a french astronomer, charbonneaux, of the meudon observatory, reported in december, , that he had repeatedly observed white clouds appearing and disappearing over one of these small craters. south of the _mare vaporum_ are found some of the most notable of those strange lunar features that are called "clefts" or "rills." two crater mountains, in particular, are connected with them, ariadæus at the eastern edge of the _mare tranquilitatis_ and hyginus on the southern border of the _mare vaporum_. these clefts appear to be broad and deep chasms, like the cañons cut by terrestrial rivers, but it can not be believed that the lunar cañons are the work of rivers. they are rather cracks in the lunar crust, although their bottoms are frequently visible. the principal cleft from ariadæus runs eastward and passes between two neighboring craters, the southern of which is named silberschlag, and is noteworthy for its brightness. the hyginus cleft is broader and runs directly through the crater ring of that name. the observer will find much to interest him in the great, irregular, and much-broken mountain ring called julius cæsar, as well as in the ring mountains, godin, agrippa, and triesnecker. the last named, besides presenting magnificent shadows when the sunlight falls aslant upon it, is the center of a complicated system of rills, some of which can be traced with our five-inch glass. we next take up lunar chart no. , and pay a telescopic visit to the southwestern quarter of the lunar world. the _mare tranquilitatis_ merges through straits into two southern extensions, the _mare fecunditatis_ and the _mare nectaris_. the great ring mountains or ringed plains, langrenus, vendelinus, petavius, and furnerius, all lying significantly along the same lunar meridian, have already been noticed. their linear arrangement and isolated position recall the row of huge volcanic peaks that runs parallel with the shore of the pacific ocean in oregon and washington--mount jefferson, mount hood, mount st. helen's, mount tacoma--but these terrestrial volcanoes, except in elevation, are mere pins' heads in the comparison. in the eastern part of the _mare fecunditatis_ lies a pair of relatively small craters named messier, which possess particular interest because it has been suspected, though not proved, that a change of form has occurred in one or other of the pair. mädler, in the first half of the nineteenth century, represented the two craters as exactly alike in all respects. in webb discovered that they are not alike in shape, and that the easternmost one is the larger, and every observer easily sees that webb's description is correct. messier is also remarkable for the light streak, often said to resemble a comet's tail, which extends from the larger crater eastward to the shore of the _mare fecunditatis_. goclenius and guttemberg, on the highland between the _mare fecunditatis_ and the _mare nectaris_, are intersected and surrounded by clefts, besides being remarkable for their broken and irregular though lofty walls. guttemberg is forty-five miles and goclenius twenty-eight miles in diameter. the short mountain range just east of guttemberg, and bordering a part of the _mare nectaris_ on the west, is called the pyrenees. the _mare nectaris_, though offering in its appearance no explanation of its toothsome name--perhaps it was regarded as the drinking cup of the olympian gods--is one of the most singular of the dark lunar plains in its outlines. at the south it ends in a vast semicircular bay, sixty miles across, which is evidently a half-submerged mountain ring. but submerged by what? not water, but perhaps a sea of lava which has now solidified and forms the floor of the _mare nectaris_. the name of this singular formation is fracastorius. elger has an interesting remark about it. "on the higher portion of the interior, near the center," he says, "is a curious object consisting apparently of four light spots, arranged in a square, with a craterlet in the middle, all of which undergo notable changes of aspect under different phases." other writers also call attention to the fine markings, minute craterlets, and apparently changeable spots on the floor of fracastorius. we go now to the eastern side of the _mare nectaris_, where we find one of the most stupendous formations in the lunar world, the great mountain ring of theophilus, noticeably regular in outline and perfect in the completeness of its lofty wall. the circular interior, which contains in the center a group of mountains, one of whose peaks is , feet high, sinks , feet below the general level of the moon outside the wall! one of the peaks on the western edge towers more than , feet above the floor within, while several other peaks attain elevations of , to , feet. the diameter of the immense ring, from crest to crest of the wall, is sixty-four miles. theophilus is especially wonderful on the fifth and sixth days of the moon, when the sun climbs its shining pinnacles and slowly discloses the tremendous chasm that lies within its circles of terrible precipices. on the southeast theophilus is connected by extensions of its walls with a shattered ring of vast extent called cyrillus; and south from cyrillus, and connected with the same system of broken walls, lies the still larger ring named catharina, whose half-ruined walls and numerous crater pits present a fascinating spectacle as the shadows retreat before the sunrise advancing across them. these three--theophilus, cyrillus, and catharina--constitute a scene of surpassing magnificence, a glimpse of wonders in another world sufficient to satisfy the most riotous imagination. south of the _mare nectaris_ the huge ring mountain of piccolomini attracts attention, its massive walls surrounding a floor nearly sixty miles across, and rising in some places to an altitude of nearly , feet. it should be understood that wherever the height of the mountain wall of such a ring is mentioned, the reference level is that of the interior plain or floor. the elevation, reckoned from the outer side, is always very much less. the entire region south and east of theophilus and its great neighbors is marvelously rough and broken. approaching the center of the moon, we find a system of ringed plains even greater in area than any of those we have yet seen. hipparchus is nearly a hundred miles long from north to south, and nearly ninety miles broad from east to west. but its walls have been destroyed to such an extent that, after all, it yields in grandeur to a formation like theophilus. albategnius is sixty-five miles across, with peaks from , to , feet in height. sacrobosco is a confused mass of broken and distorted walls. aliacensis is remarkable for having a peak on the eastern side of its wall which is more than , feet high. werner, forty-five miles in diameter, is interesting because under its northeastern wall mädler, some seventy years ago, saw a light spot of astonishing brightness, unmatched in that respect by anything on the moon except the peak of aristarchus, which we shall see later. this spot seems afterward to have lost brilliance, and the startling suggestion has been made that its original brightness might have been due to its then recent deposit from a little crater that lies in the midst of it. walter is of gigantic dimensions, about one hundred miles in diameter. unlike the majority of the ringed plains, it departs widely from a circle. stöfler is yet larger than walter; but most interesting of all these gigantic formations is maurolycus, whose diameter exceeds one hundred and fifty miles, and which has walls , or , feet high. yet, astonishing though it may seem, this vast and complicated mass of mountain walls, craters, and peaks, is virtually unseen at full moon, owing to the perpendicularity of the sunlight, which prevents the casting of shadows. we shall next suppose that another period of about seven days has elapsed, the moon in the meantime reaching its full phase. we refer for guidance to lunar chart no. . the peculiarity of the northeastern quadrant which immediately strikes the eye is the prevalence of the broad plains called _maria_, or "seas." the northern and central parts are occupied by the _mare imbrium_, the "sea of showers" or of "rains," with its dark bay the _sinus Æstuum_, while the eastern half is covered by the vast _oceanus procellarum_, the "ocean of storms" or of "tempests." toward the north a conspicuous oval, remarkably dark in hue, immediately attracts our attention. it is the celebrated ringed plain of plato, about sixty miles in diameter and surrounded by a saw-edged rampart, some of whose pinnacles are , or , feet high. plato is a favorite subject for study by selenographers because of the changes of color which its broad, flat floor undergoes as the sun rises upon it, and also because of the existence of enigmatical spots and streaks whose visibility changes. south of plato, in the _mare imbrium_, rises a precipitous, isolated peak called pico, , feet in height. its resemblance in situation to the conical mountain pico in the azores strikes the observer. [illustration: lunar chart no. , northeast quarter.] eastward of plato a line of highlands, separating the _mare imbrium_ from the _mare frigoris,_ carries the eye to the beautiful semicircular _sinus iridum_, or "bay of rainbows." the northwestern extremity of this remarkable bay is guarded by a steep and lofty promontory called cape laplace, while the southeastern extremity also has its towering guardian, cape heraclides. the latter is interesting for showing, between nine and ten days after full moon, a singularly perfect profile of a woman's face looking out across the _mare imbrium_. the winding lines, like submerged ridges, delicately marking the floor of the _sinus iridum_ and that of the _mare_ beyond, are beautiful telescopic objects. the "bay" is about one hundred and thirty-five miles long by eighty-four broad. the _mare imbrium_, covering , square miles, is sparingly dotted over with craters. all of the more conspicuous of them are indicated in the chart. the smaller ones, like caroline herschel, helicon, leverrier, délisle, etc., vary from eight to twelve miles in diameter. lambert is seventeen miles in diameter, and euler nineteen, while timocharis is twenty-three miles broad and , feet deep below its walls, which rise only , feet above the surface of the _mare_. toward the eastern border of the sea, south of the harbinger mountains, we find a most remarkable object, the mountain ring, or crater plain, called aristarchus. this ring is not quite thirty miles in diameter, but there is nothing on the moon that can compare with it in dazzling brilliance. the central peak, , or , feet high, gleams like a mountain of crusted snow, or as if it were composed of a mass of fresh-broken white metal, or of compacted crystals. part of the inner slope of the east wall is equally brilliant. in fact, so much light is poured out of the circumvallation that the eye is partially blinded, and unable distinctly to see the details of the interior. no satisfactory explanation of the extraordinary reflecting power of aristarchus has ever been offered. its neighbor toward the east, herodotus, is somewhat smaller and not remarkably bright, but it derives great interest from the fact that out of a breach in its northern wall issues a vast cleft, or chasm, which winds away for nearly a hundred miles across the floor of the _mare_, making an abrupt turn when it reaches the foot of the harbinger mountains. the comparatively small crater, lichtenberg, near the northeastern limb of the moon, is interesting because mädler used to see in its neighborhood a pale-red tint which has not been noticed since his day. returning to the western side of the quadrant represented in lunar chart no. , we see the broad and beautifully regular ringed plain of archimedes, fifty miles in diameter and , feet deep. a number of clefts extend between the mountainous neighborhood of archimedes and the feet of the gigantic apennine mountains on the southwest. the little double crater, beer, between archimedes and timocharis, is very bright. the apennines extend about four hundred and eighty miles in a northwesterly and southeasterly direction. one of their peaks near the southern end of the range, mount huygens, is at least , feet high, and the black silhouettes of their sharp-pointed shadows thrown upon the smooth floor of the _mare imbrium_ about the time of first quarter present a spectacle as beautiful as it is unique. the apennines end at the southeast in the ring mountain, eratosthenes, thirty-eight miles across and very deep, one of its encircling chain of peaks rising , feet above the floor, and about half that height above the level of the _mare imbrium_. the shadows cast by eratosthenes at sunrise are magnificent. and now we come to one of the supreme spectacles of the moon, the immense ring or crater mountain copernicus. this is generally regarded as the grandest object that the telescope reveals on the earth's satellite. it is about fifty-six miles across, and its interior falls to a depth of , feet below the _mare imbrium_. its broad wall, composed of circle within circle of ridges, terraces, and precipices, rises on the east about , feet above the floor. on the inner side the slopes are very steep, cliff falling below cliff, until the bottom of the fearful abyss is attained. to descend those precipices and reach the depressed floor of copernicus would be a memorable feat for a mountaineer. in the center of the floor rises a complicated mountain mass about , feet high. all around copernicus the surface of the moon is dotted with countless little crater pits, and splashed with whitish streaks. northward lie the carpathian mountains, terminating on the east in tobias mayer, a ring mountain more than twenty miles across. the mountain ring kepler, which is also the center of a great system of whitish streaks and splashes, is twenty-two miles in diameter, and notably brilliant. finally, we turn to the southeastern quadrant of the moon, represented in lunar chart no. . the broad, dark expanse extending from the north is the _mare nubium_ on the west and the _oceanus procellarum_ on the east. toward the southeast appears the notably dark, rounded area of the _mare humorum_ inclosed by highlands and rings. we begin with the range of vast inclosures running southward near the central meridian, and starting with ptolemæus, a walled plain one hundred and fifteen miles in its greatest diameter and covering an area considerably exceeding that of the state of massachusetts. its neighbor toward the south, alphonsus, is eighty-three miles across. next comes arzachel, more than sixty-five miles in diameter. thebit, more than thirty miles across, is very deep. east of thebit lies the celebrated "lunar railroad," a straight, isolated wall about five hundred feet high and sixty-five miles long, dividing at its southern end into a number of curious branches, forming the buttresses of a low mountain. purbach is sixty miles broad, and south of that comes a wonderful region where the ring mountains hell, ball, lexell, and others, more or less connected with walls, inclose an area even larger than ptolemæus, but which, not being so distinctly bordered as some of the other inclosed plains, bears no distinctive name. [illustration: lunar chart no. , southeast quarter.] the next conspicuous object toward the south ranks with copernicus among the grandest of all lunar phenomena--the ring, or crater, tycho. it is about fifty-four miles in diameter and some points on its wall rise , feet above the interior. in the center is a bright mountain peak , feet high. but wonderful as are the details of its mountain ring, the chief attraction of tycho is its manifest relation to the mysterious bright rays heretofore referred to, which extend far across the surface of the moon in all directions, and of which it is the center. the streaks about copernicus are short and confused, constituting rather a splash than a regular system of rays; but those emanating from tycho are very long, regular, comparatively narrow, and form arcs of great circles which stretch away for hundreds of miles, allowing no obstacle to interrupt their course. southwest of tycho lies the vast ringed plain of maginus, a hundred miles broad and very wonderful to look upon, with its labyrinth of formations, when the sun slopes across it, and yet, like maurolycus, invisible under a vertical illumination. "the full moon," to use mädler's picturesque expression, "knows no maginus." still larger and yet more splendid is clavius, which exceeds one hundred and forty miles in diameter and covers , square miles of ground within its fringing walls, which carry some of the loftiest peaks on the moon, several attaining , feet. the floor is deeply depressed, so that an inhabitant of this singular inclosure, larger than massachusetts, connecticut, and rhode island combined, would dwell in land sunk two miles below the general level of the world about him. in the neighborhood of the south pole lies the large walled plain of newton, whose interior is the deepest known depression on the moon. it is so deep that the sunshine never touches the larger part of the floor of the inner abyss, and a peak on its eastern wall rises , feet sheer above the tremendous pit. other enormous walled plains are longomontanus, wilhelm i, schiller, bailly, and schickard. the latter is one hundred and thirty-four miles long and bordered by a ring varying from , to , feet in height. wargentin, the oval close to the moon's southeast limb, beyond schickard, is a unique formation in that, instead of its interior being sunk below the general level, it is elevated above it. it has been suggested that this peculiarity is due to the fact that the floor of wargentin was formed by inflation from below, and that it has cooled and solidified in the shape of a gigantic dome arched over an immense cavity beneath. a dome of such dimensions, however, could not retain its form unless partly supported from beneath. hainzel is interesting from its curious outline; cichus for the huge yawning crater on its eastern wall; capuanus for a brilliant shining crater also on its eastern wall; and mercator for possessing bright craters on both its east and its west walls. vitello has a bright central mountain and gains conspicuousness from its position at the edge of the dark _mare humorum_. agatharchides is the broken remnant of a great ring mountain. gassendi, an extremely beautiful object, is about fifty-five miles across. it is encircled with broken walls, craters and bright points, and altogether presents a very splendid appearance about the eleventh day of the moon's age. letronne is a half-submerged ring, at the southern end of the _oceanus procellarum_, which recalls fracastorius in the western lunar hemisphere. it lies, however, ten degrees nearer the equator than fracastorius. billy is a mountain ring whose interior seems to have been submerged by the dark substance of the _oceanus procellarum_, although its walls have remained intact. mersenius is a very conspicuous ring, forty miles in diameter, east of the _mare humorum_. vieta, fifty miles across, is also a fine object. grimaldi, a huge dusky oval, is nearly one hundred and fifty miles in its greatest length. the ring mountain landsberg, on the equator, and near the center of the visible eastern hemisphere, is worth watching because elger noticed changes of color in its interior in . bullialdus, in the midst of the _mare nubium_, is a very conspicuous and beautiful ring mountain about thirty-eight miles in diameter, with walls , feet high above the interior. those who wish to see the lunar mountains in all their varying aspects will not content themselves with views obtained during the advance of the sunlight from west to east, between "new moon" and "full moon," but will continue their observations during the retreat of the sunlight from east to west, after the full phase is passed. it is evident that the hemisphere of the moon which is forever turned away from the earth is quite as marvelous in its features as the part that we see. it will be noticed that the entire circle of the moon's limb, with insignificant interruptions, is mountainous. possibly the invisible side of our satellite contains yet grander peaks and crater mountains than any that our telescopes can reach. this probability is increased by the fact that the loftiest known mountain on the moon is never seen except in silhouette. it is a member of a great chain that breaks the lunar limb west of the south pole, and that is called the leibnitz mountains. the particular peak referred to is said by some authorities to exceed , feet in height. other great ranges seen only in profile are the dörfel mountains on the limb behind the ring plain bailly, the cordilleras, east of eichstadt, and the d'alembert mountains beyond grimaldi. the profile of these great mountains is particularly fine when they are seen during an eclipse of the sun. then, with the disk of the sun for a background, they stand out with startling distinctness. the sun when the sun is covered with spots it becomes a most interesting object for telescopic study. every amateur's telescope should be provided with apparatus for viewing the sun. a dark shade glass is not sufficient and not safe. what is known as a solar prism, consisting of two solid prisms of glass, cemented together in a brass box which carries a short tube for the eyepiece, and reflecting an image of the sun from their plane of junction--while the major remnant of light and heat passes directly through them and escapes from an opening provided for the purpose--serves very well. better and more costly is an apparatus called a helioscope, constructed on the principle of polarization and provided with prisms and reflectors which enable the observer, by proper adjustment, to govern very exactly and delicately the amount of light that passes into the eyepiece. furnished with an apparatus of this description we can employ either a three-, four-, or five-inch glass upon the sun with much satisfaction. for the amateur's purposes the sun is only specially interesting when it is spotted. the first years of the twentieth century will behold a gradual growth in the number and size of the solar spots as those years happen to coincide with the increasing phase of the sun-spot period. large sun spots and groups of spots often present an immense amount of detail which tasks the skill of the draughtsman to represent it. but a little practice will enable one to produce very good representations of sun spots, as well as of the whitish patches called faculæ by which they are frequently surrounded. for the simple purpose of exhibiting the spotted face of the sun without much magnifying power, a telescope may be used to project the solar image on a white sheet or screen. if the experiment is tried in a room, a little ingenuity will enable the observer to arrange a curtain covering the window used, in such a way as to exclude all the light except that which comes through the telescope. then, by placing a sheet of paper or a drawing board before the eyepiece and focusing the image of the sun upon it, very good results may be obtained. if one has a permanent mounting and a driving clock, a small spectroscope may be attached, for solar observations, even to a telescope of only four or five inches aperture, and with its aid most interesting views may be obtained of the wonderful red hydrogen flames that frequently appear at the edge of the solar disk. chapter x are there planets among the stars? "... and if there should be worlds greater than thine own, inhabited by greater things, and they themselves far more in number than the dust of thy dull earth, what wouldst thou think?"--byron's cain. this always interesting question has lately been revived in a startling manner by discoveries that have seemed to reach almost deep enough to touch its solution. the following sentences, from the pen of dr. t. j. j. see, of the lowell observatory, are very significant from this point of view: "our observations during -' have certainly disclosed stars more difficult than any which astronomers had seen before. among these obscure objects about half a dozen are truly wonderful, in that they seem to be dark, almost black in color, and apparently are shining by a dull reflected light. it is unlikely that they will prove to be self-luminous. if they should turn out dark bodies in fact, shining only by the reflected light of the stars around which they revolve, we should have the first case of planets--dark bodies--noticed among the fixed stars." of course, dr. see has no reference in this statement to the immense dark bodies which, in recent years, have been discovered by spectroscopic methods revolving around some of the visible stars, although invisible themselves. the obscure objects that he describes belong to a different class, and might be likened, except perhaps in magnitude, to the companion of sirius, which, though a light-giving body, exhibits nevertheless a singular defect of luminosity in relation to its mass. sirius has only twice the mass, but ten thousand times the luminosity, of its strange companion! yet the latter is evidently rather a faint, or partially extinguished, sun than an opaque body shining only with light borrowed from its dazzling neighbor. the objects seen by dr. see, on the contrary, are "apparently shining by a dull reflected light." if, however (as he evidently thinks is probable), these objects should prove to be really non-luminous, it would not follow that they are to be regarded as more like the planets of the solar system than like the dark companions of certain other stars. a planet, in the sense which we attach to the word, can not be comparable in mass and size with the sun around which it revolves. the sun is a thousand times larger than the greatest of its attendant planets, jupiter, and more than a million times larger than the earth. it is extremely doubtful whether the relation of sun and planet could exist between two bodies of anything like equal size, or even if one exceeded the other many times in magnitude. it is only when the difference is so great that the smaller of the two bodies is insignificant in comparison with the larger, that the former could become a cool, life-bearing globe, nourished by the beneficent rays of its organic comrade and master. judged by our terrestrial experience, which is all we have to go by, the magnitude of a planet, if it is to bear life resembling that of the earth, is limited by other considerations. even jupiter, which, as far as our knowledge extends, represents the extreme limit of great planetary size, may be too large ever to become the abode of living beings of a high organization. the force of gravitation on the surface of jupiter exceeds that on the earth's surface as . to . considering the effects of this on the weight and motion of bodies, the density of the atmosphere, etc., it is evident that jupiter would, to say the very least, be an exceedingly uncomfortable place of abode for beings resembling ourselves. but jupiter, if it is ever to become a solid, rocky globe like ours, must shrink enormously in volume, since its density is only . as compared with the earth. now, the surface gravity of a planet depends on its mass and its radius, being directly as the former and inversely as the square of the latter. but in shrinking jupiter will lose none of its mass, although its radius will become much smaller. the force of gravity will consequently increase on its surface as the planet gets smaller and more dense. the present mean diameter of jupiter is , miles, while its mass exceeds that of the earth in the ratio of to . suppose jupiter shrunk to three quarters of its present diameter, or , miles, then its surface gravity would exceed the earth's nearly five times. with one half its present diameter the surface gravity would become more than ten times that of the earth. on such a planet a man's bones would snap beneath his weight, even granting that he could remain upright at all! it would seem, then, that, unless we are to abandon terrestrial analogies altogether and "go it blind," we must set an upper limit to the magnitude of a habitable planet, and that jupiter represents such upper limit, if, indeed, he does not transcend it. the question then becomes, can the faint objects seen by dr. see and his fellow-observers, in the near neighborhood of certain stars, be planets in the sense just described, or are they necessarily far greater in magnitude than the largest planet, in the accepted sense of that word, which can be admitted into the category--viz., the planet jupiter? this resolves itself into another question: at what distance would jupiter be visible with a powerful telescope, supposing it to receive from a neighboring star an amount of illumination not less than that which it gets from the sun? to be sure, we do not know how far away the faint objects described by dr. see are; but, at any rate, we can safely assume that they are at the distance of the nearest stars, say somewhere about three hundred thousand times the earth's distance from the sun. the sun itself removed to that distance would appear to our eyes only as a star of the first magnitude. but zöllner has shown that the sun exceeds jupiter in brilliancy , , , times. seen from equal distances, however, the ratio would be about , , to . this would be the ratio of their light if both sun and jupiter could be removed to about the distance of the nearest stars. since the sun would then be only as bright as one of the stars of the first magnitude, and since jupiter would be , , times less brilliant, it is evident that the latter would not be visible at all. the faintest stars that the most powerful telescopes are able to show probably do not fall below the sixteenth or, at the most, the seventeenth magnitude. but a seventeenth-magnitude star is only between two and three million times fainter than the sun would appear at the distance above supposed, while, as we have seen, jupiter would be more than two hundred million times fainter than the sun. to put it in another way: jupiter, at the distance of the nearest stars, would be not far from one hundred times less bright than the faintest star which the largest telescope is just able, under the most exquisite conditions, to glimpse. to see a star so faint as that would require an object-glass of a diameter half as great as the length of the tube of the lick telescope, or say thirty feet! of course, jupiter might be more brilliantly illuminated by a brighter star than the sun; but, granting that, it still would not be visible at such a distance, even if we neglect the well-known concealing or blinding effect of the rays of a bright star when the observer is trying to view a faint one close to it. clearly, then, the obscure objects seen by dr. see near some of the stars, if they really are bodies visible only by light reflected from their surfaces, must be enormously larger than the planet jupiter, and can not, accordingly, be admitted into the category of planets proper, whatever else they may be. perhaps they are extreme cases of what we see in the system of sirius--i.e., a brilliant star with a companion which has ceased to shine as a star while retaining its bulk. such bodies may be called planets in that they only shine by reflected light, and that they are attached to a brilliant sun; but the part that they play in their systems is not strictly planetary. owing to their great mass they bear such sway over their shining companions as none of our planets, nor all of them combined, can exercise; and for the same reason they can not, except in a dream, be imagined to possess that which, in our eyes, must always be the capital feature of a planet, rendering it in the highest degree interesting wherever it may be found--sentient life. it does not follow, however, that there are no real planetary bodies revolving around the stars. as dr. see himself remarks, such insignificant bodies as our planets could not be seen at the distance of the fixed stars, "even if the power of our telescopes were increased a hundredfold, and consequently no such systems are _known_." this brings me to another branch of the subject. in the same article from which i have already quoted (recent discoveries respecting the origin of the universe, atlantic monthly, vol. lxxx, pages - ), dr. see sets forth the main results of his well-known studies on the origin of the double and multiple star systems. he finds that the stellar systems differ from the solar system markedly in two respects, which he thus describes: " . the orbits are highly eccentric; on the average twelve times more elongated than those of the planets and satellites. " . the components of the stellar systems are frequently equal and always comparable in mass, whereas our satellites are insignificant compared to their planets, and the planets are equally small compared to the sun." these peculiarities of the star systems dr. see ascribes to the effect of "tidal friction," the double stars having had their birth through fission of original fluid masses (just as the moon, according to george darwin's theory, was born from the earth), and the reaction of tidal friction having not only driven them gradually farther apart but rendered their orbits more and more eccentric. this manner of evolution of a stellar system dr. see contrasts with laplace's hypothesis of the origin of the planetary system through the successive separation of rings from the periphery of the contracting solar nebula, and the gradual breaking up of those rings and their aggregation into spherical masses or planets. while not denying that the process imagined by laplace may have taken place in our system, he discovers no evidence of its occurrence among the double stars, and this leads him to the following statement, in which believers in the old theological doctrine that the earth is the sole center of mortal life and of divine care would have found much comfort: "it is very singular that no visible system yet discerned has any resemblance to the orderly and beautiful system in which we live; and one is thus led to think that probably our system is unique in its character. at least it is unique among all _known_ systems." if we grant that the solar system is the only one in which small planets exist revolving around their sun in nearly circular orbits, then indeed we seem to have closed all the outer universe against such beings as the inhabitants of the earth. beyond the sun's domain only whirling stars, coupled in eccentric orbits, dark stars, some of them, but no planets--in short a wilderness, full of all energies except those of sentient life! this is not a pleasing picture, and i do not think we are driven to contemplate it. beyond doubt, dr. see is right in concluding that double and multiple star systems, with their components all of magnitudes comparable among themselves, revolving in exceedingly eccentric orbits under the stress of mutual gravitation, bear no resemblance to the orderly system of our sun with its attendant worlds. and it is not easy to imagine that the respective members of such systems could themselves be the centers of minor systems of planets, on account of the perturbing influences to which the orbits of such minor systems would be subjected. but the double and multiple stars, numerous though they be, are outnumbered a hundred to one by the single stars which shine alone as our sun does. what reason can we have, then, for excluding these single stars, constituting as they do the vast majority of the celestial host, from a similarity to the sun in respect to the manner of their evolution from the original nebulous condition? these stars exhibit no companions, such planetary attendants as they may have lying, on account of their minuteness, far beyond the reach of our most powerful instruments. but since they vastly outnumber the binary and multiple systems, and since they resemble the sun in having no large attendants, should we be justified, after all, in regarding our system as "unique"? it is true we do not know, by visual evidence, that the single stars have planets, but we find planets attending the only representative of that class of stars that we are able to approach closely--the sun--and we know that the existence of those planets is no mere accident, but the result of the operation of physical laws which must hold good in every instance of nebular condensation. two different methods are presented in which a rotating and contracting nebula may shape itself into a stellar or planetary system. the first is that described by laplace, and generally accepted as the probable manner of origin of the solar system--viz., the separation of rings from the condensing mass, and the subsequent transformation of the rings into planets. the planet saturn is frequently referred to as an instance of the operation of this law, in which the evolution has been arrested after the separation of the rings, the latter having retained the ring form instead of breaking and collecting into globes, forming in this case rings of meteorites, and reminding us of the comparatively scattered rings of asteroids surrounding the sun between the orbits of mars and jupiter. this laplacean process dr. see regards as theoretically possible, but apparently he thinks that if it took place it was confined to our system. the other method is that of the separation of the original rotating mass into two nearly equal parts. the mechanical possibility of such a process has been proved, mathematically, by poincaré and darwin. this, dr. see thinks, is the method which has prevailed among the stars, and prevailed to such a degree as to make the solar system, formed by the ring method, probably a unique phenomenon in the universe. is it not more probable that both methods have been in operation, and that, in fact, the ring method has operated more frequently than the other? if not, why do the single stars so enormously outnumber the double ones? it is of the essence of the fission process that the resulting masses should be comparable in size. if, then, that process has prevailed in the stellar universe to the practical exclusion of the other, there should be very few single stars; whereas, as a matter of fact, the immense majority of the stars are single. and, remembering that the sun viewed from stellar distances would appear unattended by subsidiary bodies, are we not justified in concluding that its origin is a type of the origin of the other single stars? while it is, as i have remarked, of the essence of the fission process that the resulting parts of the divided mass should be comparable in magnitude, it is equally of the essence of the ring, or laplacean process, that the bodies separated from the original mass should be comparatively insignificant in magnitude. as to the coexistence of the two processes, we have, perhaps, an example in the solar system itself. darwin's demonstration of the possible birth of the moon from the earth, through fission and tidal friction, does not apply to the satellites attending the other planets. the moon is relatively a large body, comparable in that respect with the earth, while the satellites of jupiter and saturn, for instance, are relatively small. but in the case of saturn there is visible evidence that the ring process of satellite formation has prevailed. the existing rings have not broken up, but their very existence is a testimony of the origin of the satellites exterior to them from other rings which did break up. thus we need not go as far away as the stars in order to find instances illustrating both the methods of nebular evolution that we have been dealing with. the conclusion, then, seems to be that we are not justified in assuming that the solar system is unique simply because it differs widely from the double and multiple star systems; and that we should rather regard it as probable that the vast multitude of stars which do not appear, when viewed with the telescope, or studied by spectroscopic methods, to have any attendants comparable with themselves in magnitude, have originated in a manner resembling that of the sun's origin, and may be the centers of true planetary systems like ours. the argument, i think, goes further than to show the mere possibility of the existence of such planetary systems surrounding the single stars. if those stars did not originate in a manner quite unlike the origin of the sun, then the existence of planets in their neighborhood is almost a foregone conclusion, for the sun could hardly have passed through the process of formation out of a rotating nebula without evolving planets during its contraction. and so, notwithstanding the eccentricities of the double stars, we may still cherish the belief that there are eyes to see and minds to think out in celestial space. index note.--double, triple, multiple, and colored stars, star clusters, nebulæ, and temporary stars will be found arranged under the heads of their respective constellations. andromeda, map no. , . stars: alpha, . gamma, . , . , . temporary star: , . cluster: , . variable: r, . nebula: , . aquarius, map no. , . stars: zeta, . tau, . psi, . , . sigma , . sigma ( ), . sigma , . variables: r, . s, . t, . nebulæ: (rosse's "saturn"), . , . aquila, map no. , . stars: pi, . , . , . , . sigma , . sigma , . cluster: , . variables: eta, . r, . argo: map no. , ; map no. , . stars: sigma , . sigma ( ), . clusters: , . , . , . , . nebula: , . aries, map no. , . stars: gamma, . epsilon, . lambda, . pi, . , . , . , . , . sigma , . auriga, map no. , . stars: alpha (capella), . beta (menkalina), . epsilon, . theta, . lambda, . , . , . , . sigma , . temporary star: , . clusters: , . , . , . , . , . boÖtes, map no. , . stars: alpha (arcturus), . delta, . epsilon (mirac), . zeta, . iota, . kappa, . , . xi, . pi, . sigma , . sigma ( ), . sigma ( ), . sigma ( ), . sigma , . camelopardalus, map no. , . stars: , . , . , . sigma , . sigma , . cluster: , . canes venatici, map no. , ; map no. , . stars: , . (cor caroli), . sigma , . sigma ( ), . cluster: , . nebula: , . canis major, map no. , . stars: alpha (sirius), . delta, . , . clusters: , . , . , . variable: gamma, . nebula: , . canis minor, map no. , . stars: alpha (procyon), . , . sigma ( can. min. bode), . cancer, map no. , . stars: zeta, . iota, . , . sigma , . sigma , . sigma , . clusters: præsepe, . , . capricornus, map no. , ; map no. , . stars: alpha, . beta, . omicron, . pi, . rho, . sigma, . cluster: , . cassiopeia, map no. , . stars: eta, . iota, . sigma, . psi, . temporary star: (tycho's), . cluster: , . cepheus, map no. , . cetus, map no. , . stars: alpha, . gamma, . zeta, . eta, . , . , . variables: omicron (mira), . r, . s, . columba, map no. , . coma berenices, map no. , . stars: , . , . , . , . , . , . clusters: , . , . corona borealis, map no. , . stars: gamma, . zeta, . eta, . nu, . sigma, . sigma , . temporary star: , . corvus, map no. , . star: delta, . crater, map no. , . variable: r, . cygnus, map no. , . stars: beta (albireo), . delta, . lambda, . , . omicron^ , . chi ( ), . psi, . , . , . , . temporary star: , . cluster: , . variable: chi, . delphinus, map no. , . stars: alpha, . beta, . gamma, . draco, map no. , ; map no. , . stars: gamma, . epsilon, . eta, . , . nu, . sigma , . sigma , . sigma ( ), . sigma , . nebulæ: , . , . equuleus, map no. , . stars: beta, . gamma, . sigma , . sigma , . sigma , . sigma , . eridanus, map no. , . stars: gamma, . omicron^ , . , . sigma ( ), . sigma ( ), . sigma , . nebula: , . gemini, map no. , . stars: alpha (castor), . beta (pollux), . gamma, . delta, . epsilon, . zeta, . eta, . kappa, . lambda, . , . pi, . , . , . cluster: , . variables: zeta, . eta, . r, . s, . t, . u, . nebula: , . hercules, map no. , ; map no. , . stars: alpha, . gamma, . delta, . zeta, . kappa, . , . rho, . , . , . sigma , . sigma , . sigma , . sigma , . nebulæ: (m ), . , . hydra, map no. , ; map no. , ; map no. , . stars: alpha, . epsilon, . theta, . bu. , . sigma , . variable: r, . nebulæ: , . , . lacerta, map no. , . leo, map no. , . stars: gamma, . iota, . tau, . , . , . , . , . variable: r, . nebula: , . leo minor, map no. , . lepus, map no. , ; map no. , . stars: alpha, . beta, . gamma, . iota, . , . variable: r, . libra, map no. , . stars: a, . alpha, . beta, . iota, . variable: delta, . lynx, map no. , . stars: , . , . , . , . , . , . sigma , . sigma , . sigma , . lyra, map no. , . stars: alpha (vega), . beta, . epsilon, . zeta, . , . variable: beta, . nebula: (ring), . monoceros, map no. , ; map no. , . stars: , . , . , . sigma , . sigma , . sigma , . sigma , . sigma , . clusters: , . , . , . , . , . variable: s, . ophiuchus, map no. , ; map no. , . stars: lambda, . tau, . , . , . , . , . , . sigma , . sigma , . temporary star: , . clusters: , . , . , . , . , . , . , . , . , . variable: r, . orion, map no. , . stars: alpha (betelgeuse), . beta (rigel), . delta, . zeta, . eta, . theta (trapezium), . iota, . lambda, . rho, . sigma, . tau, . psi^ , . sigma , . sigma , . sigma , . sigma , . sigma (a ), . sigma , . sigma , . sigma , . sigma ( ), . sigma , . omicron sigma (i), . clusters: , . , . , . , . nebulæ: great orion nebula, . , . , . pegasus, map no. , . stars: beta, . gamma, . epsilon, . eta, . perseus, map no. , . stars: epsilon, . zeta, . eta, . clusters: , . , . variable: beta (algol), . pisces, map no. , ; map no. , ; map no. , . stars: alpha, . zeta, . psi, . , . , . , . , . variable: r, . sagitta, map no. , . stars: epsilon, . zeta, . theta, . nebula: , . sagittarius, map no. , ; map no. , . stars: , . , . clusters: m , . , . (m ), . (m ), . , . variables: r, . t, . u, . v, . scorpio, map no. , . stars: alpha (antares), . beta, . nu, . xi, . sigma, . temporary star: , . clusters: , . , . scutum sobieskii, map no. , ; map no. , . stars: sigma , . sigma , . clusters: , . , . , . variable: r, . nebula: , . serpens, map no. , ; map no. , . stars: alpha, . beta, . delta, . theta, . nu, . variable: r, . taurus, map no. , . stars: alpha (aldebaran), . eta (alcyone), . theta, . kappa, . sigma, . tau, . phi, . chi, . , . sigma ( ), . sigma , . sigma , . sigma , . clusters: hyades, . pleiades, . , . variable: lambda, . nebulæ: in pleiades, . (crab net), . triangulum, map no. , . star: , . nebula: , . ursa major, map no. , . stars: zeta (mizar), . iota, . nu, . xi, . sigma^ , . , . , . , . nebulæ: , . , . , . ursa minor, map no. , . stars: alpha (pole star), . pi, . virgo, map no. , . stars: alpha (spica), . gamma, . theta, . , . sigma , . sigma , . variables: r, . s, . u, . nebulæ: field of the, . , . , . , . vulpecula, map no. , . star: sigma , . temporary star: , . nebula: (dumb bell), . the moon, most interesting of telescopic objects, ; telescopic views of moon reversed, . craters, ring mountains, and ringed plains: agatharchides, . agrippa, . albategnius, . alhazen, . aliacensis, . alphonsus, . archimedes, . ariadæus, . aristarchus, . aristillus, . aristoteles, . arzachel, . atlas, . autolycus, . bailly, . ball, . barrow, . beer, . berzelius, . billy, . bullialdus, . burckhardt, . capuanus, . cassini, . catharina, , cichus, . clavius, . cleomenes, . condorcet, . copernicus, . cyrillus, . délisle, endymion, . eratosthenes, . eudoxus, . euler, . firmicus, . fracastorius, , . furnerius, . gassendi, . gauss, . geminus, . goclenius, . godin, . grimaldi, . guttemberg, . hainzel, . hansen, . helicon, . hell, . hercules, . herodotus, . herschel, caroline, . hipparchus, . humboldt, . hyginus, . julius cæsar, . kepler, . lambert, . landsberg, . langrenus, , . letronne, . leverrier, . lexell, . lichtenberg, . linné, . longomontanus, . macrobius, . maginus, . manilius, . maurolycus, . menelaus, . mercator, . mersenius, . messala, . messier, . newton, . petavius, , . picard, . piccolomini, . pico, . plato, . plinius, . posidonius, , . proclus, . ptolemæus, . purbach, . sacrobosco, . schickard, . schiller, . silberschlag, . stöfler, . sulpicius gallus, . theætetus, . thebit, . theophilus, . timocharis, . tobias mayer, . tralles, . triesnecker, . tycho, , . vendelinus, , . vieta, . vitello, . walter, . wargentin, . werner, . wilhelm i, . _maria_, or "seas": _lacus somniorum_, . _mare crisium_, , , . _mare fecunditatis_, , . _mare frigoris_, , . _mare humboldtianum_, . _mare humorum_, , . _mare imbrium_, , , . _mare nectaris_, . _mare nubium_, . _mare serenitatis_, , , . _mare tranquilitatis_, . _mare vaporum_, , . _oceanus procellarum_, , , . _palus nebularum_, . _palus putredinis_, . _palus somnii_, . _sinus Æstuum_, . _sinus iridum_, , . other formations: alps mountains, . apennine mountains, , , . cape agarum, . cape heraclides, . cape laplace, . carpathian mountains, . caucasus mountains, . cordilleras mountains, . d'alembert mountains, . dörfel mountains, . hæmus mountains, . harbinger mountains, . leibnitz mountains, . "lunar railroad," . mt. argæus, , . mt. hadley, . mt. huygens, . pyrenees mountains, . taurus mountains, . the planets: are there planets among the stars? . mars, two views of, . best advertised of planets, . favorable oppositions of, . seen with -inch telescope, . polar caps of, . color of, . dark markings on, . "canals," . earthlike condition of, . mercury, phases of, . peculiar rotation of, . markings on, . probably not habitable, . jupiter, easiest planet for amateurs, . seen with -inch glass, . satellites, swift motions of, . velocity of planet's equator, . how to see all sides of, , . watching rotation of, . eclipses and transits of satellites, , . belts and clouds of, . different rates of rotation, . names and numbers of satellites, . saturn, next to jupiter in attractiveness, . seen with -inch glass, . its moons and their orbits, , . polar view of system, . roche's limit, , . origin of the rings, . pickering's ninth satellite, . the satellites as telescopic objects, . venus, her wonderful brilliance, . her atmosphere seen, . lowell's observations, . schiaparelli's observations, . her peculiar rotation, . how to see, in daytime, . neptune and uranus, . the sun, . shade glasses for telescopes in viewing, . solar prism, . helioscope, . periodicity of spots, . to see, by projection, . spectroscope for solar observation, . the telescope: refractors and reflectors, , . eyepieces, , , . aberration (chromatic), ; (spherical), , . achromatic telescopes, how made, . object glass, . magnifying power, . mountings, . rules for testing, . image of star in, . image in and out of focus, , , . astigmatism, . the end [illustration] s. _pleasures of the telescope_ _garrett p. serviss_ this book says to the amateur, in effect:--"what if you have not all advantages of clockwork and observatory equipment. you may know something of the witchery of the heavens even with a little telescope of three to five inches aperture!" "pleasures of the telescope" is popular in style rather than technical. for setting forth "the chief attractions of the starry heavens," a complete set of star-maps is included, showing "all the stars visible to the naked eye in the regions of sky represented, and in addition some stars that can only be seen with optical aid." in six chapters these twenty-six maps are described so plainly that the amateur can readily find all the interesting star-groups, clusters, and nebulæ, and also the colored or double stars. in the three concluding chapters the moon and planets receive special consideration. in the opening chapter the amateur is told how to select and test a glass. _booklovers bulletin._ transcriber's note minor errors and inconsistencies in punctuation and hyphenation have been silently corrected. some illustrations have been relocated a short distance within the text. original page numbers have been retained in the index. greek letters, used to identify stars, are replaced with the full name of the greek letter, e.g. alpha. upper case greek letters are shown by capitalising the initial letter, e.g. sigma a caret (^) is used to represent superscripts, e.g. nu^ and nu^ the following minor corrections have also been made: p : "wil" has been corrected to "will". p : sigma is not shown on map no. . the location of _m_ orionis is marked as sigma . this inconsistency has not been corrected. p : "for colors" has been corrected to "four colors". p : " , , , , , " has been corrected to " , , , , , ". p - : "magnical" has been corrected to "magical". p : a repeated "and" has been removed. [illustration: _fig. ._ the solar system.] meteoric astronomy: a treatise on shooting-stars, fire-balls, and aerolites. by daniel kirkwood, ll.d. professor of mathematics in washington and jefferson college. [illustration] philadelphia: j. b. lippincott & co. . entered, according to act of congress, in the year , by daniel kirkwood, ll.d., in the clerk's office of the district court of the united states for the western district of pennsylvania. preface. aristotle and other ancient writers regarded comets as meteors generated in the atmosphere. this opinion was generally accepted, even by the learned, until the observations of tycho, near the close of the sixteenth century, showed those mysterious objects to be more distant than the moon, thus raising them to the dignity of _celestial_ bodies. an achievement somewhat similar, and certainly no less interesting, was reserved for the astronomers of the _nineteenth_ century. this was the great discovery that _shooting-stars, fire-balls, and meteoric stones, are, like comets, cosmical bodies moving in conic sections about the sun_. dr. halley was the first to foretell the return of a comet, and the year will ever be known in history as that which witnessed the fulfillment of his prophecy. but in the department of _meteoric_ astronomy, a similar honor must now be awarded to the late dr. olbers. soon after the great star-shower of he inferred from a comparison of recorded facts that the november display attains a maximum at intervals of thirty-three or thirty-four years. he accordingly designated or as the time of its probable return; and the night of november th of the former year must always be memorable as affording the first verification of _his_ prediction. on that night several thousand meteors were observed in one hour from a single station. this remarkable display, together with the fact that another still more brilliant is looked for in november, , has given meteoric astronomy a more than ordinary degree of interest in the public mind. to gratify, in some measure, the curiosity which has been awakened, by presenting in a popular form the principal results of observation and study in this new field of research, is the main design of the following work. the first two chapters contain a popular view of what is known in regard to the star-showers of august and november, and also of some other epochs. the third is a description, in chronological order, of the most important falls of meteoric stones, together with the phenomena attending their descent. the fourth and following chapters to the eleventh inclusive, discuss various questions in the theory of meteors: such, for instance, as the relative number of aerolitic falls during different parts of the day, and also of the year; the coexistence of the different forms of meteoric matter in the same rings; meteoric dust; the stability of the solar system; the doctrine of a resisting medium; the extent of the atmosphere as indicated by meteors; the meteoric theory of solar heat; and the phenomena of variable and temporary stars. the twelfth chapter regards the rings of saturn as dense meteoric swarms, and accounts for the principal interval between them. the thirteenth presents various facts, not previously noticed, respecting the asteroid zone between mars and jupiter, with suggestions concerning their cause or explanation. as the nebular hypothesis furnishes a plausible account of the origin of meteoric streams, it seemed desirable to present an intelligible view of that celebrated theory. this accordingly forms the subject of the closing chapter. the greater part of the following treatise, it is proper to remark, was written before the publication (in england) of dr. phipson's volume on "meteors, aerolites, and falling-stars." the author has had that work before him, however, while completing his manuscript, and has availed himself of some of the accounts there given of recent phenomena. canonsburg, pa, _may, _. contents. page introduction chapter i. the meteors of november th- th chapter ii. other meteoric rings chapter iii. aerolites chapter iv. conjectures in regard to meteoric epochs chapter v. geographical distribution of meteoric stones--do aerolitic falls occur more frequently by day than by night?--do meteorites, bolides, and the matter of ordinary shooting-stars, coexist in the same rings? chapter vi. phenomena supposed to be meteoric--meteoric dust--dark days chapter vii. researches of reichenbach--theory of meteors--stability of the solar system--doctrine of a resisting medium chapter viii. does the number of aerolitic falls vary with the earth's distance from the sun?--relative numbers observed in the forenoon and afternoon--extent of the atmosphere as indicated by meteors chapter ix. the meteoric theory of solar heat chapter x. will the meteoric theory account for the phenomena of variable and temporary stars? chapter xi. the lunar and solar theories of the origin of aerolites chapter xii. the rings of saturn chapter xiii. the asteroid ring between mars and jupiter chapter xiv. origin of meteors--the nebular hypothesis appendix introduction. a general view of the solar system. the solar system consists of the sun, together with the planets and comets which revolve around him as the center of their motions. the sun is the great controlling orb of this system, and the source of light and heat to its various members. its magnitude is one million four hundred thousand times greater than that of the earth, and it contains more than seven hundred times as much matter as all the planets put together. mercury is the nearest planet to the sun; its mean distance being about thirty-seven millions of miles. its diameter is about three thousand miles, and it completes its orbital revolution in days. venus, the next member of the system, is sometimes our morning and sometimes our evening star. its magnitude is almost exactly the same as that of the earth. it revolves round the sun in days. the earth is the third planet from the sun in the order of distance; the radius of its orbit being about ninety-five millions of miles. it is attended by one satellite--the moon--the diameter of which is miles. mars is the first planet exterior to the earth's orbit. it is considerably smaller than the earth, and has no satellite. it revolves round the sun in days. the asteroids.--since the commencement of the present century a remarkable zone of telescopic planets has been discovered immediately exterior to the orbit of mars. these bodies are extremely small; some of them probably containing less matter than the largest mountains on the earth's surface. more than ninety members of the group are known at present, and the number is annually increasing. jupiter, the first planet exterior to the asteroids, is nearly five hundred millions of miles from the sun, and revolves round him in a little less than twelve years. this planet is ninety thousand miles in diameter and contains more than twice as much matter as all the other planets, primary and secondary, put together. jupiter is attended by four moons or satellites. saturn is the seventh planet in the order of distance--counting the asteroids as one. its orbit is about four hundred millions of miles beyond that of jupiter. this planet is attended by eight satellites, and is surrounded by three broad, flat rings. saturn is seventy-six thousand miles in diameter, and its mass or quantity of matter is more than twice that of all the other planets except jupiter. uranus is at double the distance of saturn, or nineteen times that of the earth. its diameter is about thirty-five thousand miles, and its period of revolution, eighty-four years. it is attended by four satellites. neptune is the most remote known member of the system; its distance being nearly three thousand millions of miles. it is somewhat larger than uranus; has certainly one satellite, and probably several more. its period is about one hundred and sixty-five years. a cannon-ball flying at the rate of five hundred miles per hour would not reach the orbit of neptune from the sun in less than six hundred and eighty years. these planets all move round the sun in the same direction--from west to east. their motions are nearly circular, and also nearly in the same plane. their orbits, except that of neptune, are represented in the frontispiece. it is proper to remark, however, that all representations of the solar system by maps and planetariums must give an exceedingly erroneous view either of the magnitudes or distances of its various members. if the earth, for instance, be denoted by a ball half an inch in diameter, the diameter of the sun, according to the same scale (sixteen thousand miles to the inch), will be between four and five feet; that of the earth's orbit, about one thousand feet; while that of neptune's orbit will be nearly six miles. to give an accurate representation of the solar system at a single view is therefore plainly impracticable. comets.--the number of comets belonging to our system is unknown. the appearance of more than seven hundred has been recorded, and of this number, the elements of about two hundred have been computed. they move in very eccentric orbits--some, perhaps, in parabolas or hyperbolas. the zodiacal light is a term first applied by dominic cassini, in , to a faint nebulous aurora, somewhat resembling the milky-way, apparently of a conical or lenticular form, having its base toward the sun, and its axis nearly in the direction of the ecliptic. the most favorable time for observing it is when its axis is most nearly perpendicular to the horizon. this, in our latitudes, occurs in march for the evening, and in october for the morning. the angular distance of its vertex from the sun is frequently seventy or eighty degrees, while sometimes, though rarely (except within the tropics), it exceeds even one hundred degrees. the zodiacal light is probably identical with the meteor called _trabes_ by _pliny_ and _seneca_. it was noticed in the latter part of the sixteenth century by tycho brahé, who "considered it to be an abnormal spring-evening twilight." it was described by descartes about the year , and again by childrey in . the first accurate description of the phenomenon was given, however, by cassini. this astronomer supposed the appearance to be produced by the blended light of an innumerable multitude of extremely small planetary bodies revolving in a ring about the sun. the appearance of the phenomenon as seen in this country is represented in fig. . [illustration: fig. .] for general readers it may not be improper to premise the following explanations: meteors are of two kinds, _cosmical_ and _terrestrial_: the former traverse the interplanetary spaces; the latter originate in the earth's atmosphere. _bolides_ is a general name for meteoric fire-balls of greater magnitude than shooting-stars. the _period_ of a planet, comet, or meteor is the time which it occupies in completing one orbital revolution. the motion of a heavenly body is said to be _direct_ when it is from west to east; and _retrograde_ when it is from east to west. _encke's hypothesis of a resisting medium._--the time occupied by encke's comet in completing its revolution about the sun is becoming less and less at each successive return. professor encke explains this fact by supposing the interplanetary spaces to be filled with an extremely rare fluid, the resistance of which to the cometary motion produces the observed contraction of the orbit. meteoric astronomy. chapter i. shooting-stars. i. the meteors of november th- th. although shooting-stars have doubtless been observed in all ages of the world, they have never, until recently, attracted the special attention of scientific men. the first exact observations of the phenomena were undertaken, about the close of the last century, by messrs. brandes and benzenberg. the importance, however, of this new department of research was not generally recognized till after the brilliant meteoric display of november th, . this shower of fire can never be forgotten by those who witnessed it.[ ] the display was observed from the west indies to british america, and from ° to ° west longitude from greenwich. captain hammond, of the ship restitution, had just arrived at salem, massachusetts, where he observed the phenomenon from midnight till daylight. he noticed with astonishment that precisely one year before, viz., on the th of november, , he had observed a similar appearance (although the meteors were less numerous) at mocha, in arabia. it was soon found, moreover, as a further and most remarkable coincidence, that an extraordinary fall of meteors had been witnessed on the th of november, . this was seen and described by andrew ellicott, esq., who was then at sea near cape florida. it was also observed in cumana, south america, by humboldt, who states that it was "simultaneously seen in the new continent, from the equator to new herrnhut, in greenland (lat. ° ´), and between ° and ° longitude." this wonderful correspondence of dates excited a very lively interest throughout the scientific world. it was inferred that a recurrence of the phenomenon might be expected, and accordingly arrangements were made for systematic observations on the th, th, and th of november. the periodicity of the shower was thus, in a very short time, placed wholly beyond question. the examination of old historical records led to the discovery of at least appearances of the november shower previous to the great fall of . the descriptions of these phenomena will be found collected in an interesting article by prof. h. a. newton, in the _american journal of science and arts_, for may, . they occurred in the years , , , , , , , , , , , and . besides these enumerated by professor newton as "the predecessors of the great exhibition on the morning of november th, ," we find others, less distinctly marked, in the catalogue of m. quetelet.[ ] these were in the years , , , , , and . from to , inclusive, quetelet's catalogue indicates partial returns of the november shower; making in all, up to the latter date, . in , november th, a straw roof was set on fire by a meteoric fire-ball, in the department de l'aine, france. on the th of november, , "at o'clock in the evening, the attention of observers in various parts of great britain was directed to a bright luminous body, apparently proceeding from the north, which, after making a rapid descent, in the manner of a rocket, suddenly burst, and scattering its particles into various beautiful forms, vanished in the atmosphere. this was succeeded by others all similar to the first, both in shape and the manner of its ultimate disappearance. the whole display terminated at ten o'clock, when dark clouds, which continued up till a late hour, overspread the earth, preventing any further observations."--_milner's gallery of nature_, p. . in , november th- th, meteors were observed in unusual numbers at vienna. one of extraordinary brilliancy, having an apparent magnitude equal to that of the full moon, was seen near cherburg. on several other returns of the november epoch the number of meteors observed has been greater than on ordinary nights; the distinctly marked exhibitions, however, up to , have all been enumerated. the shower of november , . the fact that all great displays of the november meteors have taken place at intervals of thirty-three or thirty-four years, or some multiple of that period, had led to a general expectation of a brilliant shower in . in this country, however, the public curiosity was much disappointed. the numbers seen were greater than on ordinary nights, but not such as would have attracted any special attention. the greatest number recorded at any one station was seen at new haven, by prof. newton. on the night of the th, were counted in five hours and twenty minutes, and on the following night, in five hours. this was about six times the ordinary number. a more brilliant display was, however, witnessed in europe. meteors began to appear in unusual frequency about eleven o'clock on the night of the th, and continued to increase with great rapidity for more than two hours; the maximum being reached a little after one o'clock. the edinburgh _scotsman_, of november th, contains a highly interesting description of the phenomenon as observed at that city. "standing on the calton hill, and looking westward," the editor remarks,--"with the observatory shutting out the lights of prince's street--it was easy for the eye to delude the imagination into fancying some distant enemy bombarding edinburgh castle from long range; and the occasional cessation of the shower for a few seconds, only to break out again with more numerous and more brilliant drops of fire, served to countenance this fancy. again, turning eastward, it was possible now and then to catch broken glimpses of the train of one of the meteors through the grim dark pillars of that ruin of most successful manufacture, the national monument; and in fact from no point in or out of the city was it possible to watch the strange rain of stars, pervading as it did all points of the heavens, without pleased interest, and a kindling of the imagination, and often a touch of deeper feeling that bordered on awe. the spectacle, of which the loftiest and most elaborate description could but be at the best imperfect--which truly should have been seen to be imagined--will not soon pass from the memories of those to whose minds were last night presented the mysterious activities and boundless fecundities of that universe of the heavens, the very unchangeableness of whose beauty has to many made it monotonous and of no interest." the appearance of the phenomenon, as witnessed at london, is minutely described in the _times_ of november th. the shower occurred chiefly between the hours of twelve and two. about one o'clock a single observer counted in two minutes. the whole number seen at greenwich was . the shower was also observed in different countries on the continent. _the meteors of compared with those of former displays._ the star shower of was much inferior to those of and .[ ] with these exceptions, however, it has, perhaps, been scarcely surpassed during the last years. historians represent the meteors of as innumerable, and as moving like rain in all possible directions.[ ] the exhibition of was no less magnificent. the stars, it is said, were seen to dash against each other like swarms of locusts; the phenomenon lasting till daybreak.[ ] the shower of is thus described in a portuguese chronicle, quoted by humboldt: "in the year , twenty-two days of the month of october being past, three months before the death of the king, dom pedro (of portugal), there was in the heavens a movement of stars, such as men never before saw or heard of. at midnight, and for some time after, all the stars moved from the east to the west; and after being collected together, they began to move, some in one direction, and others in another. and afterward they fell from the sky in such numbers, and so thickly together, that as they descended low in the air, they seemed large and fiery, and the sky and the air seemed to be in flames, and even the earth appeared as if ready to take fire. that portion of the sky where there were no stars, seemed to be divided into many parts, and this lasted for a long time." the following is humboldt's description of the shower of , as witnessed by himself and bonpland, in cumana, south america: "from half after two, the most extraordinary luminous meteors were seen toward the east.... thousands of bolides and falling stars succeeded each other during four hours. they filled a space in the sky extending from the true east ° toward the north and south. in an amplitude of ° the meteors were seen to rise above the horizon at e. n. e. and at e., describe arcs more or less extended, and fall toward the south, after having followed the direction of the meridian. some of them attained a height of °, and all exceeded ° or °.... mr. bonpland relates, that from the beginning of the phenomenon there was not a space in the firmament equal in extent to three diameters of the moon, that was not filled at every instant with bolides and falling-stars.... the guaiqueries in the indian suburb came out and asserted that the firework had begun at one o'clock.... the phenomenon ceased by degrees after four o'clock, and the bolides and falling-stars became less frequent; but we still distinguished some toward the northeast a quarter of an hour after sunrise." discussion of the phenomena. since the memorable display of november th, , the phenomena of shooting-stars have been observed and discussed by brandes, benzenberg, olbers, saigey, heis, olmsted, herrick, twining, newton, greg, and many others. in the elaborate paper of professor olmsted, it was shown that the meteors had their origin at a distance of more than miles from the earth's surface; that their paths diverged from a common point near the star _gamma leonis_; that in a number of instances they became visible about miles from the earth's surface; that their velocity was comparable to that of the earth in its orbit; and that in some cases their extinction occurred at an elevation of miles. it was inferred, moreover, that they consisted of combustible matter which took fire and was consumed in passing through the atmosphere; that this matter was derived from a nebulous body revolving round the sun in an elliptical orbit, but little inclined to the plane of the ecliptic; that its aphelion was near that point of the earth's orbit through which we annually pass about the th of november--the perihelion being a little within the orbit of mercury; and finally that its period was about one-half that of the earth. dr. olmsted subsequently modified his theory, having been led by further observations to regard the zodiacal light as the nebulous body from which the shooting-stars are derived. the latter hypothesis was also adopted by the celebrated biot. the fact that the position of the radiant point does not change with the earth's rotation, places the cosmical origin of the meteors wholly beyond question. the theory of a closed ring of nebulous matter revolving round the sun in an elliptical orbit which intersects that of the earth, affords a simple and satisfactory explanation of the phenomena. this theory was adopted by humboldt, arago, and others, shortly after the occurrence of the meteoric shower of . that the body which furnishes the material of these meteors moves in a closed or elliptical orbit is evident from the periodicity of the shower. it is also manifest from the partial recurrence of the phenomenon from year to year, that the matter is diffused around the orbit; while the extraordinary falls of , , , and , prove the diffusion to be far from uniform. elements of the orbit. future observations, it may be hoped, will ultimately lead to an accurate determination of the elements of this ring: many years, however, will probably elapse before all the circumstances of its motion can be satisfactorily known. professor newton, of yale college, has led the way in an able discussion of the observations.[ ] he has shown that the different parts of the ring are, in all probability, of very unequal density; that the motion is retrograde; and that the time, during which the meteors complete a revolution about the sun, must be limited to one of five accurately determined periods, viz.: · days, · days, · days, · days, or · years. he makes the inclination of the ring to the ecliptic about °. the five periods specified, he remarks, "are not all equally probable. some of the members of the group which visited us last november [ ] gave us the means of locating approximately the central point of the region from which the paths diverge. mr. g. a. nolen has, by graphical processes specially devised for the purpose, found its longitude to be °, and its latitude ° ´. this longitude is very nearly that of the point in the ecliptic toward which the earth is moving. hence the point from which the absolute motion of the bodies is directed (being in a great circle through the other two points) has the same longitude. the absolute motion of each meteor, then, is directed very nearly at right angles to a line from it to the sun, the deviation being probably not more than two or three degrees. "now, if in one year the group make ± / · revolutions, there is only a small portion of the orbit near the aphelion which fulfills the above condition. in like manner, if the periodic time is · years, only a small portion of the orbit near the perihelion fulfills it. on the other hand, if the annual motion is ± / · revolutions, the required condition is answered through a large part of the orbit. inasmuch as no reason appears why the earth should meet a group near its apsides rather than elsewhere, we must regard it as more probable that the group makes in one year either + / · , or - / · revolutions." professor newton concludes that the third of the above-mentioned periods, viz., · days, combines the greatest amount of probability of being the true one. we grant the force of the reasons assigned for its adoption. at least one consideration, however, in favor of the long period of · years is by no means destitute of weight: of nearly known bodies which revolve about the sun in orbits of small eccentricity, not one has a retrograde motion. now if this striking fact has resulted from a general cause, how shall we account for the backward motion of a meteoric ring, in an orbit almost circular, and but little inclined to the plane of the ecliptic? in such a case, is not the preponderance of probability in favor of the longer period? a revolution in · years corresponds to an ellipse whose major axis is · . consequently the aphelion distance would be somewhat greater than the mean distance of uranus. it may also be worthy of note, that five periods of the ring would be very nearly equal to two of uranus. the _monthly notices of the royal astronomical society_ for december, , and january, , contain numerous articles on the star shower of november th- th, . sir john herschel carefully observed the phenomena, and his conclusions in regard to the orbit are confirmatory of those of professor newton. "we are constrained to conclude," he remarks, "that the true line of direction, in space of each meteor's flight, lay in a plane at right angles to the earth's radius vector at the moment; and that therefore, except in the improbable assumption that the meteor was at that moment _in perihelio_ or _in aphelio_, its orbit would not deviate greatly from the circular form." the question is one to be decided by observation, and the only meteor whose track and time of flight seem to have been well observed, is that described by professor newton in _silliman's journal_ for january, , p. . the velocity in this case, if the estimated time of flight was nearly correct, was _inconsistent with the theory of a circular orbit_. it is also worthy of notice that dr. oppolzer's elements of the first comet of resemble, in a remarkable manner, those of the meteoric ring, supposing the latter to have a period of about - / years. schiaparelli's elements of the november ring, and oppolzer's elements of the comet of , are as follows: november comet of meteors. . longitude of perihelion ° ´ ° ´ longitude of ascending node. inclination perihelion distance · · eccentricity · · semi-axis major · · period, in years · · motion retrograde. retrograde. it seems very improbable that these coincidences should be accidental. leverrier and other astronomers have found elements of the meteoric orbit agreeing closely with those given by schiaparelli. should the identity of the orbits be fully confirmed, it will follow that the comet of _is a very large meteor_ of the november stream. the researches of professor c. bruhns, of leipzig, in regard to this group of meteors afford a probable explanation of the division of biela's comet--a phenomenon which has greatly perplexed astronomers for the last twenty years. adopting the period of - / years, professor bruhns finds that the comet passed extremely near, and probably _through_ the meteoric ring near the last of december, . it is easy to perceive that such a collision might produce the separation soon afterward observed. as the comet of biela makes three revolutions in twenty years, it was again at this intersection, or approximate intersection of orbits about the end of . but although the comet's position, with respect to the earth, was the same as in - , and although astronomers watched eagerly for its appearance, their search was unsuccessful. in short, _the comet is lost_. the denser portion of the meteoric stream was then approaching its perihelion. a portion of the arc had even passed that point, as a meteoric shower was observed at greenwich on the th of november, .[ ] the motion of the meteoric stream is retrograde; that of the comet, direct. did the latter plunge into the former, and was its non appearance the result of such collision and entanglement? [illustration: fig. . _probable orbit of the november meteors._] chapter ii. other meteoric rings. ii. the meteors of august th- th. muschenbroek, in his _introduction to natural philosophy_, published in , called attention to the fact that shooting-stars are more abundant in august than in any other part of the year. the annual periodicity of the maximum on the th or th of the month was first shown, however, by quetelet, shortly after the discovery of the yearly return of the november phenomenon. since that time an extraordinary number of meteors has been regularly observed, both in europe and america, from the th to the th of the month; the greatest number being generally seen on the th. in , edward heis, of aix-la-chapelle, saw meteors in one hour on the night of the th. in , he saw in ten minutes at the time of the maximum. in , on the night of the th, four observers, watching together at new haven, saw in three hours--from ten to one o'clock-- meteors. on the same night, at natick, massachusetts, two observers saw in about seven hours. at london, mercer county, pennsylvania, on the night of august th, , samuel s. gilson, esq., watching alone, saw meteors in forty minutes, and, with an assistant, in one hour and fifteen minutes. generally, the number observed per hour, at the time of the august maximum, is about nine times as great as on ordinary nights. like the november meteors, they have a common "radiant;" that is, their tracks, when produced backward, meet, or nearly meet, in a particular point in the constellation perseus. of the meteoric displays given in quetelet's "catalogue des principales apparitions d'étoiles filantes," seem to have been derived from the august ring. the first of these, with one exception, were observed in china during the last days of july, as follows: a.d. , july th. , " th- th. , " th- th. , " th. , " th. , " th. , " th- th. , " th- th. , " th- th. , " th- th. , " th- th. the next dates are , august d, and , august th. a comparison of these dates indicates a forward motion of the node of the ring along the ecliptic. this was pointed out several years since by boguslawski. a similar motion of the node has also been found in the case of the november ring. that these points should be stationary is, indeed, altogether improbable. the nodes of all the planetary orbits, it is well known, have a secular variation. on the evening of august th, , at about h. m., a meteor was seen by mr. e. c. herrick and prof. a. c. twining, at new haven, connecticut, which "was much more splendid than venus, and left a train of sparks which remained luminous for twenty seconds after the meteor disappeared." the same meteor was also accurately observed at burlington, new jersey, by mr. benjamin v. marsh. it was "conformable,"--that is, its track produced backward passed through the common radiant--and it was undoubtedly a member of the august group. the observations were discussed by professor h. a. newton, of yale college, who deduced from them the following approximate elements of the ring:[ ] semi-axis major · eccentricity · perihelion distance · inclination ° period days. motion, retrograde. the earth moving at the rate of , miles per hour, is at least five days in passing entirely through the ring. this gives a thickness of more than , , miles. the result of professor newton's researches on the orbit of this ring, though undertaken with inadequate data, and hence, in some respects, probably far from correct, is nevertheless highly interesting as being the first attempt to determine the orbit of shooting-stars. more recent investigations have shown a remarkable resemblance between the elements of these meteors and those of the third comet of . the former, by schiaparelli, and the latter, by oppolzer, are as follows: meteors of august th. comet iii., . longitude of perihelion ° ´ ° ascending node inclination perihelion distance · · period years(?). years(?). motion retrograde. retrograde. this similarity is too great to be accidental. _the august meteors and the third comet of probably belong to the same ring._ iii. the meteors of april th- th. the following dates of the april meteoric showers are extracted from quetelet's table previously referred to: a.d. , april th. , " th. , " th. , " th. , " th. , " th. , " th. , " th. , " th. , " th. , " th. , " th. , " th. , " th- th. the display of was witnessed in china, and is described as "very remarkable." that of was best observed in virginia, and was at its maximum between one and three o'clock. the alarm of fire had called many of the inhabitants of richmond from their houses, so that the phenomenon was generally witnessed. the meteors "seemed to fall from every point in the heavens, in such numbers as to resemble a shower of sky-rockets." some were of extraordinary magnitude. "one in particular, appeared to fall from the zenith, of the apparent size of a ball inches in diameter, that lighted the whole hemisphere for several seconds." the probability that the meteoric falls about the th of april are derived from a ring which intersects the earth's orbit, was first suggested by arago, in . the preceding list indicates a forward motion of the node. the radiant, according to mr. greg, is about _corona_. the number of meteors observed in , , and , was not very extraordinary. recent observations indicate april th- th as another epoch. the radiant is in virgo. iv. the meteors of december th- th. on the th of december, , a large meteoric stone fell in england. on the night, between the th and th of december, , professor brandes, then a student in göttingen, saw shooting-stars. on the th of the month, , a fall of meteoric stones, described by humboldt as "enormous," occurred near the village of macao, in brazil. during the last few years unusual numbers of shooting-stars have been noticed by different observers from the th to the th; the maximum occurring about the th. from a.d. , december d, to , december th- th, we find star showers in quetelet's catalogue, derived, probably, from this meteoric stream. as in other cases, the dates seem to show a progressive motion of the node. the position of the radiant, as determined by benjamin v. marsh, esq., of philadelphia, from observations in and , and also by r. p. greg, esq., of manchester, england, is at a point midway between castor and pollux. v. the meteors of january d- d. about the middle of the present century, mr. julius schmidt, of bonn, a distinguished and accurate observer, designated the d of january as a meteoric epoch; characterizing it, however, as "probably somewhat doubtful." recent observations, especially those of r. p. greg, esq., have fully confirmed it. the meteors for several hours are said to be as numerous as at the august maximum. the radiant is near the star _beta_ of the constellation böotes. quetelet's list contains at least five exhibitions which belong to this epoch. two or three others may also be referred to it with more or less probability. * * * * * several other meteoric epochs have been indicated; some of which, however, must yet be regarded as doubtful. in thirty years, from to , falls of bolides and meteoric stones occurred from the th to the th of november. such coincidences can hardly be accidental. unusual numbers of shooting-stars have also been seen about the th of july; from the th to the th of october, and about the middle of february. the radiant, for the last-mentioned epoch, is in _leo minor_. the numbers observed in october are said to be at present increasing. at least seven of the exhibitions in quetelet's catalogue are referable to this epoch. it is worthy of remark, moreover, that three of the dates specified by mr. greg as _aerolite_ epochs are coincident with those of shooting-stars; viz., february th- th, july th, and december th. the whole number of exhibitions enumerated in quetelet's catalogue is . in eighty-two instances the day of the month on which the phenomenon occurred is not specified. nearly two-thirds of the remainder, as we have seen, belong to established epochs, and the periodicity of others will perhaps yet be discovered. but reasons are not wanting for believing that our system is traversed by numerous meteoric streams besides those which actually intersect the earth's orbit. the asteroid region between mars and jupiter is probably occupied by such an annulus. the number of these asteroids increases as their magnitudes diminish; and this doubtless continues to be the case far below the limit of telescopic discovery. the zodiacal light is probably a dense meteoric ring, or rather, perhaps, a number of rings. we speak of it as _dense_ in comparison with others, which are invisible except by the ignition of their particles in passing through the atmosphere. from a discussion of the motions of the perihelia of mercury and mars, leverrier has inferred the existence of two rings of minute asteroids; one within the orbit of mercury, whose mass is nearly equal to that of mercury himself; the other at the mean distance of the earth, whose mass cannot _exceed_ the tenth part of the mass of the earth. within the last few years a distinguished european savant, buys-ballot, of utrecht, has discovered a short period of variation in the amount of solar heat received by the earth: the time from one maximum to another exceeding the period of the sun's apparent rotation by about twelve hours. the variation cannot therefore be due to any inequality in the heating power of the different portions of the sun's surface. the discoverer has suggested that it may be produced by a meteoric ring, whose period slightly exceeds that of the sun's rotation. such a zone might influence our temperature by partially intercepting the solar heat. general remarks. . the average number of shooting-stars seen in a clear, moonless night by a single observer, is about per hour. _one_ observer, however, sees only about one-fourth of those visible from his point of observation. about per hour might therefore be seen by watching the entire hemisphere. in other words, shooting-stars per day could be seen by the naked eye at any one point of the earth's surface, did the sun, moon, and clouds permit. . the mean altitude of shooting-stars above the earth's surface is about miles. . the number visible over the whole earth is about , times the number to be seen at any one point. hence the average number of those daily entering the atmosphere and having sufficient magnitude to be seen by the naked eye, is about , , . . the observations of pape and winnecke indicate that the number of meteors visible through the telescope, employed by the latter, is about times the number visible to the naked eye, or about , , per day.[ ] this is two per day, or , per century, for every square mile of the earth's surface. by increasing the optical power, this number would probably be indefinitely increased. at special times, moreover, such as the epochs of the great meteoric showers, the addition of foreign matter to our atmosphere is much greater than ordinary. it becomes, therefore, an interesting question whether sensible changes may not thus be produced in the atmosphere of our planet. . in august, , shooting-stars were doubly observed in england; that is, they were seen at two different stations. the average weight of these meteors, estimated--in accordance with the mechanical theory of heat--from the quantity of light emitted, was a little more than two ounces. . a meteoric mass exterior to the atmosphere, and consequently non-luminous, was observed on the evening of october th, , by edward heis, a distinguished european astronomer. it entered the field of view as he was observing the milky way, and he was enabled to follow it over or degrees of its path. it eclipsed, while in view, a number of the fixed stars. chapter iii. aerolites. it is now well known that much greater variety obtains in the structure of the solar system than was formerly supposed. this is true, not only in regard to the magnitudes and densities of the bodies composing it, but also in respect to the forms of their orbits. the whole number of planets, primary and secondary, known to the immortal author of the _mecanique celeste_, was only . this number has been more than quadrupled in the last quarter of a century. in laplace's view, moreover, all comets were strangers within the solar domain, having entered it from without. it is now believed that a large proportion originated in the system and belong properly to it. the gradation of planetary magnitudes, omitting such bodies as differ but little from those given, is presented at one view in the following table: name. diameter in miles. jupiter , uranus , the earth , mercury , the moon , rhea, saturn's th satellite , dione " th " vesta[ ] juno melpomene polyhymnia isis atalanta hestia the diminution doubtless continues indefinitely below the present limit of optical power. if, however, the orbits have small eccentricity, such asteroids could not become known to us unless their mean distances were nearly the same with that of the earth. but from the following table it will be seen that the variety is no less distinctly marked in the forms of the orbits: name. eccentricity. venus · the earth · jupiter · metis · mercury · pallas · polyhymnia · faye's comet · d'arrest's " · biela's " · encke's " · halley's " · fourth comet of · fifth comet of (donati's) · third comet of · were the eccentricities of the nearest asteroids equal to that of faye's comet, they would in perihelion intersect the earth's orbit. now, in the case of both asteroids and comets, the smallest are the most numerous; and as this doubtless continues below the limit of telescopic discovery, the earth ought to encounter such bodies in its annual motion. _it actually does so._ the number of _cometoids_ thus encountered in the form of _meteoric stones_, _fire-balls_, and _shooting-stars_ in the course of a single year amounts to many millions. the extremely minute, and such as consist of matter in the gaseous form, are consumed or dissipated in the upper regions of the atmosphere. no deposit from ordinary shooting-stars has ever been known to reach the earth's surface. but there is probably great variety in the physical constitution of the bodies encountered; and though comparatively few contain a sufficient amount of matter in the solid form to reach the surface of our planet, scarcely a year passes without the fall of meteoric stones in some part of the earth, either singly or in clusters. now, when we consider how small a proportion of the whole number are probably observed, it is obvious that the actual occurrence of the phenomenon can be by no means rare.[ ] although numerous instances of the fall of aerolites had been recorded, some of them apparently well authenticated, the occurrence long appeared too marvelous and improbable to gain credence with scientific men. such a shower of rocky fragments occurred, however, on the th of april, , at l'aigle, in france, as forever to dissipate all doubt on the subject. at one o'clock p.m., the heavens being almost cloudless, a tremendous noise, like that of thunder, was heard, and at the same time an immense fire-ball was seen moving with great rapidity through the atmosphere. this was followed by a violent explosion which lasted several minutes, and which was heard not only at l'aigle, but in every direction around it to the distance of seventy miles. immediately after a great number of meteoric stones fell to the earth, generally penetrating to some distance beneath the surface. the largest of these fragments weighed - / pounds. this occurrence very naturally excited great attention. m. biot, under the authority of the government, repaired to l'aigle, collected the various facts in regard to the phenomenon, took the depositions of witnesses, etc., and finally embraced the results of his investigations in an elaborate memoir. it would not comport with the design of the present treatise to give an extended list of these phenomena. the following account, however, includes the most important instances of the fall of aerolites, and also of the displays of meteoric fire-balls. . according to livy a number of meteoric stones fell on the alban hill, near rome, about the year b.c. this is the most ancient fall of aerolites on record. . b.c., about the year in which socrates was born. a mass of rock, described as "of the size of two millstones," fell at Ægos potamos, in thrace. an attempt to rediscover this meteoric mass, so celebrated in antiquity, was recently made, but without success. notwithstanding this failure, humboldt expressed the hope that, as such a body would be difficult to destroy, it may yet be found, "since the region in which it fell is now become so easy of access to european travelers." . a.d. an immense aerolite fell into the river (a branch of the tiber) at narni, in italy. it projected three or four feet above the surface of the water. . , november th. an aerolite, weighing two hundred and seventy-six pounds, fell at ensisheim, in alsace, penetrating the earth to the depth of three feet. this stone, or the greater portion of it, may still be seen at ensisheim. . , september th. at noon an almost total darkening of the heavens occurred at crema. "during this midnight gloom," says a writer of that period, "unheard-of thunders, mingled with awful lightnings, resounded through the heavens. * * * on the plain of crema, where never before was seen a stone the size of an egg, there fell pieces of rock of enormous dimensions and of immense weight. it is said that ten of these were found weighing a hundred pounds each." a monk was struck dead at crema by one of these rocky fragments. this terrific meteoric display is said to have lasted two hours, and aerolites were subsequently found. . , november th. a stone, weighing fifty-four pounds, fell on mount vaison, in provence. . , march th. a franciscan monk was killed at milan by the fall of a meteoric stone. . . two swedish sailors were killed on ship-board by the fall of an aerolite. . , july th. an extraordinary fire-ball was seen in england; its motion being opposite to that of the earth in its orbit. halley pronounced this meteor a cosmical body. (see philos. transact., vol. xxix.) . , june th. a stone weighing seventy-two pounds fell at larissa, in macedonia. . , march th. another great meteor was seen in england. its explosion occurred at an elevation of miles. notwithstanding its height, however, the report was like that of a broadside, and so great was the concussion that windows and doors were violently shaken. . , may th. two meteoric masses, consisting almost wholly of iron, fell near agram, the capital of croatia. the larger fragment, which weighs seventy-two pounds, is now in vienna. . . the concussion produced by a meteoric explosion threw down chimneys at aix, in provence, and was mistaken for an earthquake. . , july th. a large meteor exploded near paris, at an elevation of miles. . , august th. a fire-ball of extraordinary magnitude was seen in scotland, england, and france. it produced a rumbling sound like distant thunder, although its elevation above the earth's surface was miles at the time of its explosion. the velocity of its motion was equal to that of the earth in its orbit, and its diameter, according to sir charles blagden, was about half a mile. . , july th. between nine and ten o'clock at night a very large igneous meteor was seen near bourdeaux, france. over barbotan a loud explosion was heard, which was followed by a shower of meteoric stones of various magnitudes. . , july. a fall of about a dozen aerolites occurred at sienna, tuscany. . , december th. a large meteoric stone fell near wold cottage, in yorkshire, england. the following account of the phenomenon is taken from milner's _gallery of nature_, p. : "several persons heard the report of an explosion in the air, followed by a hissing sound; and afterward felt a shock, as if a heavy body had fallen to the ground at a little distance from them. one of these, a plowman, saw a huge stone falling toward the earth, eight or nine yards from the place where he stood. it threw up the mould on every side; and after penetrating through the soil, lodged some inches deep in solid chalk rock. upon being raised, the stone was found to weigh fifty-six pounds. it fell in the afternoon of a mild but hazy day, during which there was no thunder or lightning; and the noise of the explosion was heard through a considerable district." . , february th. a stone of ten pounds' weight fell in portugal. . , march th. a stone weighing twenty pounds fell at sules, near ville franche. . , march th. an aerolite weighing about twenty pounds fell at sale, department of the rhone. . , december th. a shower of meteoric stones fell at benares, in the east indies. an interesting account of the phenomenon was given by j. lloyd williams, f.r.s., then a resident in bengal. the sky had been perfectly clear for several days. at eight o'clock in the evening a large meteor appeared, which was attended with a loud rumbling noise. immediately after the explosion a sound was heard like that of heavy bodies falling in the neighborhood. next morning the fresh earth was found turned up in many places, and aerolites of various sizes were discovered beneath the surface. . , april th. the shower at l'aigle, previously described. . , december th. a large meteor exploded over weston, connecticut. the height, direction, velocity, and magnitude of this body were ably discussed by dr. bowditch in a memoir communicated to the american academy of arts and sciences in . the following condensed statement of the principal facts, embodied in dr. bowditch's paper, is extracted from the _people's magazine_ for january th, : "the meteor of was observed about a quarter-past six on monday morning. the day had just dawned, and there was little light except from the moon, which was just setting. it seemed to be half the diameter of the full moon; and passed, like a globe of fire, across the northern margin of the sky. it passed behind some clouds, and when it came out it flashed like heat lightning. it had a train of light, and appeared like a burning fire-brand carried against the wind. it continued in sight about half a minute, and, in about an equal space after it faded, three loud and distinct reports, like those of a four-pounder near at hand, were heard. then followed a quick succession of smaller reports, seeming like what soldiers call a running fire. the appearance of the meteor was as if it took three successive throes, or leaps, and at each explosion a rushing of stones was heard through the air, some of which struck the ground with a heavy fall. "the first fall was in the town of huntington, near the house of mr. merwin burr. he was standing in the road, in front of his house, when the stone fell, and struck a rock of granite about fifty feet from him, with a loud noise. the rock was stained a dark-red color, and the stone was principally shivered into very small fragments, which were thrown around to a distance of twenty feet. the largest piece was about the size of a goose egg, and was still warm. "the stones of the second explosion fell about five miles distant, near mr. william prince's residence, in weston. he and his family were in bed when they heard the explosion, and also heard a heavy body fall to the earth. they afterward found a hole in the earth, about twenty-five feet from the house, like a newly dug post-hole, about one foot in diameter, and two feet deep, in which they found a meteoric stone buried, which weighed thirty-five pounds. another mass fell half a mile distant, upon a rock, which it split in two, and was itself shivered to pieces. another piece, weighing thirteen pounds, fell a half a mile to the northeast, into a plowed field. "at the last explosion, a mass of stone fell in a field belonging to mr. elijah seely, about thirty rods from the house. this stone falling on a ledge, was shivered to pieces. it plowed up a large portion of the ground, and scattered the earth and stones to the distance of fifty or a hundred feet. some cattle that were near were very much frightened, and jumped into an inclosure. it was concluded that this last stone, before being broken, must have weighed about two hundred pounds. these stones were all of a similar nature, and different from any commonly found on this globe. when first found, they were easily reduced to powder by the fingers, but by exposure to the air they gradually hardened." . , november th. between nine and ten o'clock in the morning, an extraordinary meteor was seen in several of the new england states, new york, new jersey, the district of columbia, and virginia. the apparent diameter of the head was nearly equal to that of the sun, and it had a train, notwithstanding the bright sunshine, several degrees in length. its disappearance on the coast of the atlantic was followed by a series of the most terrific explosions. it is believed to have descended into the water, probably into delaware bay. a highly interesting account of this meteor, by prof. loomis, may be found in the _american journal of science and arts_ for january, . . , may st. about twenty minutes before one o'clock p.m., a shower of meteoric stones--one of the most extraordinary on record--fell in the s. w. corner of guernsey county, ohio. full accounts of the phenomena are given in _silliman's journal_ for july, , and january and july, , by professors e. b. andrews, e. w. evans, j. l. smith, and d. w. johnson. from these interesting papers we learn that the course of the meteor was about ° west of north. its visible track was over washington and noble counties, and the prolongation of its projection, on the earth's surface, passes directly through new concord, in the s. e. corner of muskingum county. the height of the meteor, when seen, was about miles, and its path was nearly parallel with the earth's surface. the sky, at the time, was, for the most part, covered with clouds over northwestern ohio, so that if any portion of the meteoric mass continued on its course, it was invisible. the velocity of the meteor, in relation to the earth's surface, was from to miles per second; and hence its absolute velocity in the solar system was from to miles per second. this would indicate an orbit of considerable eccentricity. "at new concord,[ ] muskingum county, where the meteoric stones fell, and in the immediate neighborhood, there were many distinct and loud reports heard. at new concord there were first heard in the sky, a little southeast of the zenith, a loud detonation, which was compared to that of a cannon fired at the distance of half a mile. after an interval of ten seconds another similar report. after two or three seconds another, and so on with diminishing intervals. twenty-three distinct detonations were heard, after which the sounds became blended together and were compared to the rattling fire of an awkward squad of soldiers, and by others to the roar of a railway train. these sounds, with their reverberations, are thought to have continued for two minutes. the last sounds seemed to come from a point in the southeast ° below the zenith. the result of this cannonading was the falling of a large number of stony meteorites upon an area of about ten miles long by three wide. the sky was cloudy, but some of the stones were seen first as 'black specks,' then as 'black birds,' and finally falling to the ground. a few were picked up within twenty or thirty minutes. the warmest was no warmer than if it had lain on the ground exposed to the sun's rays. they penetrated the earth from two to three feet. the largest stone, which weighed one hundred and three pounds, struck the earth at the foot of a large oak tree, and, after cutting off two roots, one five inches in diameter, and grazing a third root, it descended two feet ten inches into hard clay. this stone was found resting under a root that was not cut off. this would seemingly imply that it entered the earth obliquely." over thirty of the stones which fell were discovered, while doubtless many, especially of the smaller, being deeply buried beneath the soil, entirely escaped observation. the weight of the largest ten was four hundred and eighteen pounds. . , may th. early in the evening a very large and brilliant meteor was seen in france, from paris to the spanish border. at montauban, and in the vicinity, loud explosions were heard, and showers of meteoric stones fell near the villages of orgueil and nohic. the principal facts in regard to this meteor are the following: elevation when first seen, over miles. " at the time of its explosion " inclination of its path to the horizon ° or ° velocity per second, about miles, or equal to that of the earth's orbital motion. "this example," says prof. newton, "affords the strongest proof that the detonating and stone-producing meteors are phenomena not essentially unlike." the foregoing list contains but a small proportion even of those meteoric stones the date of whose fall is known. but besides these, other masses have been found so closely similar in structure to aerolites whose descent has been observed, as to leave no doubt in regard to their origin. one of these is a mass of iron and nickel, weighing sixteen hundred and eighty pounds, found by the traveler pallas, in , at abakansk, in siberia. this immense aerolite may be seen in the imperial museum at st. petersburg. on the plain of otumpa, in buenos ayres, is a meteoric mass - / feet in length, partly buried in the ground. its estimated weight is thirty-three thousand six hundred pounds. a specimen of this stone, weighing fourteen hundred pounds, has been removed and deposited in one of the rooms of the british museum. a similar block, of meteoric origin, weighing twelve or thirteen thousand pounds, was discovered some years since in the province of bahia, in brazil. some of the inferences derived from the examination of meteoric stones, and the consideration of the phenomena attending their fall, are the following: . r. p. greg, esq., of manchester, england, who has made luminous meteors a special study, has found that meteoric stone-falls occur with greater frequency than usual on or about particular days. he calls attention especially to five aerolite epochs, viz.: february th- th; may th; july th; november th, and december th. . it is worthy of remark that no new elements have been found in meteoric stones. humboldt, in his _cosmos_, called attention to this interesting fact. "i would ask," he remarks, "why the elementary substances that compose one group of cosmical bodies, or one planetary system, may not in a great measure be identical? why should we not adopt this view, since we may conjecture that these planetary bodies, like all the larger or smaller agglomerated masses revolving round the sun, have been thrown off from the once far more expanded solar atmosphere, and have been formed from vaporous rings describing their orbits round the central body?"[ ] . but while aerolites contain no elements but such as are found in the earth's crust, the manner in which these elements are combined and arranged is so peculiar that a skillful mineralogist will readily distinguish them from terrestrial substances. . of the eighteen or nineteen elements hitherto observed in meteoric stones, iron is found in the greatest abundance. the specific gravities vary from · to · : the former being that of the stone of alais, the latter, that of the meteorite of wayne county, ohio, described by professor j. l. smith in _silliman's journal_ for november, , p. . in most cases, however, the specific gravity is about or . . the contemplation of the heavenly bodies has often produced in thoughtful minds an intense desire to know something of their nature and physical constitution. this curiosity is gratified in the examination of aerolites. to handle, weigh, inspect, and analyze bodies that have wandered unnumbered ages through the planetary spaces--perhaps approaching in their perihelia within a comparatively short distance of the solar surface, and again receding in their aphelia to the limits of the planetary system--must naturally excite a train of pleasurable emotions. . it is highly probable that in pre-historic times, before the solar system had reached its present stage of maturity, those chaotic wanderers were more numerous in the vicinity of the earth's orbit than in recent epochs. even now the interior planets, mercury and venus, appear to be moving through the masses of matter which constitute the zodiacal light. it would seem probable, therefore, that they are receiving from this source much greater accretions of matter than the earth. . as mercury's orbit is very eccentric, he is beyond his mean distance during much more than half his period. hence, probably, the greater increments of meteoric matter are derived from such portions of the zodiacal light as have a longer period than mercury himself. if so, the tendency would be to diminish slowly the planet's mean motion. such a lengthening of the period has been actually discovered.[ ] chapter iv. conjectures in regard to meteoric epochs. it is highly probable that aerolites and shooting-stars are derived either from rings thrown off in the planes of the solar or planetary equators, or from streams of nebulous matter drawn into the solar system by the sun's attraction. such annuli or streams would probably each furnish an immense number of meteor-asteroids. if any rings intersect the earth's orbit, our planet must encounter such masses as happen at the same time to be passing the point of intersection. this must be repeated _at the same epoch_ in different years; the frequency of the encounter of course depending on the closeness and regularity with which the masses are distributed around the ring. accordingly it has been found that not only the meteors of november th and of the epochs named in chapter ii. have their respective radiants, but also those of many other nights. mr. alexander s. herschel, of collingwood, england, states that fifty-six such points of divergence are now well established. we have mentioned in a previous chapter that mr. greg, of manchester, has specified several epochs at which fire-balls appear, and meteoric stone-falls occur, with unusual frequency. the number of these periods will probably be increased by future observations. perhaps the following facts may justify the designation of july th- th as such an epoch: . on the th of july, , a large fire-ball was seen in göttingen. . on the th of july, , a fire-ball was seen in montgaillard. . on the th of july, , a brilliant meteor was seen in london. . on the th of july, , at about h. and m. p.m., a brilliant fire-ball passed over maryland and pennsylvania, and was seen also in virginia, delaware, new jersey, new york, and connecticut. its course was north, about thirty degrees east, and the projection of its path on the earth's surface passed about four miles west of lancaster, pennsylvania, and nearly through mauch chunk, in carbon county. when west of philadelphia its angle of elevation, as seen from that city, was forty-two degrees. consequently its altitude, when near lancaster, was about fifty-nine miles. the projection of its visible path, on the earth's surface, was at least two hundred and fifty miles in length. its height, when nearest gettysburg, was about seventy miles, and it disappeared at an elevation of about eighteen miles, near the south corner of wayne county, pennsylvania. its apparent diameter, as seen from york and lancaster, was about half that of the moon, and its estimated heliocentric velocity was between twenty and twenty-five miles. the author was assured by persons in harford county, maryland, and also in york, pennsylvania, that shortly after the disappearance of the meteor a distinct report, like that of a distant cannon, was heard. as might be expected, their estimates of the interval which elapsed were different; but daniel m. ettinger, esq., of york, who was paying particular attention, in expectation of a report, stated that it was a little over six minutes. this would indicate a distance of about seventy-five miles. the sound could not therefore have resulted from an explosion at or near the termination of the meteor's observed path. the inclination of the meteoric track to the surface of the earth was such that the body could not have passed out of the atmosphere. as no aerolites, however, were found beneath any part of its path, perhaps the entire mass may have been dissipated before reaching the earth.--_silliman's journal_ for may, . . on the th of july, , a remarkable fall of aerolites was witnessed at braunau, in bohemia. humboldt states that "the fallen masses of stone were so hot, that, after six hours, they could not be touched without causing a burn." an analysis of some of the fragments, by fischer and duflos, gave the following result: iron · nickel · cobalt · copper, manganese, arsenic, calcium, magnesium, silicium, carbon, chlorine and sulphur. · ------- · . on the th of july, , a brilliant fire-ball was seen at stone-easton, somerset, england. . on the th of july, , a large bolide was seen in london. . on the th of july, , a fire-ball was seen at senftenberg. . on the th of july, , a meteor, three times as large as jupiter, was seen at nottingham, england. . "one of the most celebrated falls that have occurred of late years is that which happened on the th of july, , between two and half-past two in the afternoon, at dhurmsala, in india. the aerolite in question fell with a most fearful noise, and terrified the inhabitants of the district not a little. several fragments were picked up by the natives, and carried religiously away, with the impression that they had been thrown from the summit of the himalayas by an invisible divinity. lord canning forwarded some of these stones to the british museum and to the vienna museum. mr. j. r. saunders also sent some of the stones to europe. it appears that, soon after their fall, the stones were _intensely cold_.[ ] they are ordinary earthy aerolites, having a specific gravity of · , containing fragments of iron and iron pyrites; they have an uneven texture, and a pale-gray color." . at a quarter-past ten o'clock on the evening of july th, , a large fire-ball was seen in new england.[ ] the hour of its appearance, it will be observed, was nearly the same with that of the bolide of july th, ; and it is also worthy of remark that their _directions_ were nearly the same. the meteor of had a tail three or four degrees in length, and the body, like that of , exploded with a loud report. . on the th of july, , an aerolite fell at mons, in belgium (quetelet's _physique du globe_, p. ). a forward motion of the node, somewhat less than that observed in the rings of november and august, would give a correspondence of dates between the falls of , , and . with the exception of the last, which is doubtful, these phenomena all occurred within a period of years. the epoch of november . it has been stated that in different years meteoric stones have fallen about the th of november. one of the most recent aerolites which can be assigned to this epoch is that which fell on the th of november, , at shalka, in bengal. it may be mentioned, as at least a coincidence, that the earth passes the approximate intersection of her orbit with that of biela's comet at the date of this epoch. do other bodies besides the two biela comets move in the same ellipse? it is worthy of remark that two star showers have been observed at this date: one in china, a.d. , the other in europe, (see quetelet's catalogue). it is certainly important that the meteors of this epoch should be carefully studied. chapter v. geographical distribution of meteoric stones--do aerolitic falls occur more frequently by day than by night?--do meteorites, bolides, and the matter of ordinary shooting-stars, coexist in the same rings? professor charles upham shepard, of amherst college, who has devoted special attention to the study of meteoric stones, has designated two districts of country, one in each continent, but both in the northern hemisphere, in which more than nine-tenths of all known aerolites have fallen. he remarks: "the fall of aerolites is confined principally to two zones; the one belonging to america is between ° and ° north latitude, and is about ° in length. its direction is more or less from northeast to southwest, following the general line of the atlantic coast. of all known occurrences of this phenomenon during the last fifty years, · per cent. have taken place within these limits, and mostly in the neighborhood of the sea. the zone of the eastern continent--with the exception that it extends ten degrees more to the north--lies between the same degrees of latitude, and follows a similar northeast direction, but is more than twice the length of the american zone. of all the observed falls of aerolites, · per cent. have taken place within this area, and were also concentrated in that half of the zone which extends along the atlantic." the facts as stated by professor shepard are, of course, unquestionable. it seems, however, extremely improbable that the districts specified should receive a much larger proportion of aerolites than others of equal extent. how, then, are the facts to be accounted for? we answer, the number of aerolites _seen_ to fall in a country depends upon the number of its inhabitants. the ocean, deserts, and uninhabited portions of the earth's surface afford no instances of such phenomena, simply for the want of observers. in sparsely settled countries the fall of aerolites would not unfrequently escape observation; and as such bodies generally penetrate the earth to some depth, the chances of discovery, when the fall is not observed, must be exceedingly rare. now the part of the american continent designated by professor shepard, it will be noticed, is the oldest and most thickly settled part of the united states; while that of the eastern continent stretches in like manner across the most densely populated countries of europe. this fact alone, in all probability, affords a sufficient explanation of prof. shepard's statement.[ ] _do aerolites fall more frequently by day than by night?_--mr. alexander s. herschel, of collingwood, england, has with much care and industry collected and collated the known facts in regard to bolides and aerolites. one result of his investigations is that a much greater number of meteoric stones are observed to fall by day than by night. from this he infers that, for the most part, the orbits in which they move are _interior_ to that of the earth. the fact, however, is obviously susceptible of a very different explanation--an explanation quite similar to that of the frequent falls in particular districts. _at night the number of observers is incomparably less; and hence many aerolites escape detection._ there would seem to be no cause, reason, or antecedent probability of these falls being more frequent at one hour than another in the whole twenty-four. _the coexistence of meteorites, bolides, and the matter of shooting-stars in the same rings?_--it has been stated on a previous page that several aerolite epochs are coincident with those of shooting-stars. is the number of such cases sufficient to justify the conclusion that the correspondence of dates is not accidental? we will consider, i. the epoch of november th- th. . , november th. a very large detonating meteor was seen at mansfield, thuringia, at two o'clock in the morning. the known rate of movement of the node brings this meteor within the november epoch. . , november th. a large fire-ball was seen at tubingen. the motion of the node brings this also within the epoch. . , november th. a bright meteoric light was observed at frankfort. . , november th. a large meteor was seen at göttingen and lilienthal. . , november th. a fire-ball, twenty-three miles high, was seen at london and edinburgh. . , november th. a splendid meteor was seen at dover and harts. . , november th. a fire-ball was seen in england. . , november th. a fire-ball was seen at gosport. . , november th. a fire-ball was seen at st. domingo. . , november th. a large detonating meteor was seen at cholimschk, russia. . , november th. a fire-ball appeared at potsdam. . , november th. a meteor was seen in full sunshine at sury, france. . , november th. a fire-ball was seen at bruneck. . , november th. a brilliant meteor was seen in the north of spain. . , november th. a fire-ball was seen in germany. . , november th. a meteor, two-thirds the size of the moon, was seen during the great meteoric shower in the united states. . , november th. a large fire-ball was seen in north america. . , november th. several aerolites fell near belmont, department de l'ain, france. . , november th. an aerolitic fall occurred at macao, brazil. . , november th. a remarkable fire-ball was seen in england. . , november th. a large fire-ball was seen at cherbourg. . , november th. an extraordinary meteor appeared in italy. "seen in the southern sky. varied in color; a bright cloud visible one and a half hour after; according to some a detonation heard fifteen minutes after bursting. seen also like a stream of fire between tunis and tripolis, where a shower of stones fell; some of them into the town of tripolis itself." . , november th. a large meteor was seen at mecklenburg and breslau. . , november th. a meteoric stone fell at trenzano, italy. . , november th. at athens, greece, a large number of bolides was seen by mr. j. f. julius schmidt, during the shower of shooting-stars. one of these fire-balls was of the first class, and left a train which was visible one hour to the naked eye. ii. the epoch of august th- th. . , august th. a meteoric stone fell in suffolk county, england. . , august th. an aerolite fell in holland. the observed motion of the node brings both these stone-falls within the epoch. . , august th. a large bolide was seen at greenwich. . , august th. a fire-ball was seen at northallerton. . , august th. a large meteor was seen in different parts of north america. . , august th. a fire-ball appeared at quedlinburg. . , august th. a bolide was seen at nurenberg. . , august th. a stone weighing seven and three-quarter pounds fell at tipperary, ireland. . , august th. in hungary a large fire-ball was seen to burst, with detonations. . , august th. a brilliant fire-ball was seen at augsburg. . , august th. a meteoric stone, weighing seven pounds, fell at slobodka, russia. . , august th. a meteorite fell at kadonah, agra. . , august th. a large meteor was seen in moravia. . , august th. "a large mass of fire fell down with a great explosion" near coblentz. . , august th. two meteoric stones fell in nobleboro', maine. . , august th. a fire-ball was seen at odensee. . , august th. a bright meteor appeared at halle. . , august th. a fire-ball was seen at worcestershire, england. . , august th. a bolide appeared at brussels. . , august th. a fine meteor was seen in germany. . , august th. a splendid fire-ball was seen at sea. . , august th. a bolide appeared at naples. . , august th. an aerolite fell at iwan, hungary. . , august th. a greenish fire-ball was seen at hamburg. . , august th. a large meteor was seen in brittany. . , august th. a fire-ball was seen at hamburg. . , august th. a brilliant meteor was seen at london and oxford. . , august th. a large irregular meteor, "like a bright cloud of smoke," was seen at brussels. . , august th. a meteor as large as the moon was seen in ireland. . , august th. a very large bolide was observed in paris. . , august th. a fire-ball was seen in paris. . , august th. a bolide was observed at glasgow. . , august th. a meteor twice as large as venus was seen at paris. . , august th. a large meteor was seen to separate into two parts. . , august th. a bluish meteor, five times as large as jupiter, was seen at nottingham. . , august th. a bolide was seen in paris. . , august th. a detonating meteor appeared in germany. . , august th. a meteoric stone fell near albany, new york. . , august th. a fine meteor was seen at athens. . , august th. a meteoric stone-fall occurred at pillistfer, russia. . , august th. an aerolite fell at shytal, india. iii. the epoch of december th- th. the following falls of meteoric stones have occurred at this epoch: . , december th. at wold cottage, england. . , december th. at benares, india. . , december th. at mässing, bavaria. . , december th. at luotolaks, finland. . , december th. at ausson, france. . , december th. at tirlemont, belgium. . , december th. at inly, near trebizond.[ ] iv. the epoch of april th- th. for this epoch we have the following aerolites: . , april th. at l'aigle, france. . , april th. at casignano, parma, italy. . , april th. at abkurpore, india. . , april th. at milena, croatia. v. the epoch of april th- th. . , april th. at doroninsk, russia. . , april th. at toulouse, france. . , april th. at zaborzika, russia. . , april th. at nerft, russia. the foregoing lists, which might be extended, are sufficient to establish the fact that meteoric stones are but the largest masses in the nebulous rings from which showers of shooting-stars are derived; a fact worthy of consideration whatever theory may be adopted in regard to the origin of such annuli. chapter vi. phenomena supposed to be meteoric--meteoric dust--dark days. it is well known that great variety has been found in the composition of aerolites. while some are extremely hard, others are of such a nature as to be easily reducible to powder. it is not impossible that when some of the latter class explode in the atmosphere they are completely pulverized, so that, reaching the earth in extremely minute particles, they are never discovered. it is very unlikely, moreover, that of the millions of shooting-stars that daily penetrate the atmosphere nothing whatever in the solid form should ever reach the earth's surface. indeed, the celebrated reichenbach, who devoted great attention to this subject, believed that he had actually discovered such deposits of meteoric matter. chladni and others have detailed instances of the fall of _dust_, supposed to be meteoric, from the upper regions of the atmosphere. the following may be regarded, with more or less probability, as instances of such phenomena: . a.d. , november th or th. a shower of black dust fell in the vicinity of constantinople. immediately before or about the time of the fall, according to old accounts, "the heavens appeared to be on fire," which seems to indicate a meteoric display of an extraordinary character. . on the d of december, , a considerable quantity of dark-colored matter fell from the atmosphere, at verde, in hanover. the fall was attended by intense light, as well as by a loud report resembling thunder. the substance which fell was hot when it reached the earth, as the planks on which a portion of it was found were slightly burnt, or charred. the date of this occurrence, allowance being made for the movement of the node, is included within the limits of the meteoric epoch of december th- th. . about a century later, viz., on the st of january, , a very extensive deposit of blackish matter, in appearance somewhat resembling charred paper, took place in norway and other countries in the north of europe. a portion of this substance, which had been carefully preserved, was analyzed by grotthus, and found to contain iron, silica, and other elements frequently met with in aerolites. . on the th of november, , red rain fell in sweden and russia, and on the same day in switzerland. it gave a reddish color to the waters of lake constance, to which it also imparted an acid taste. the rain which fell on this occasion deposited a sediment whose particles were attracted by the magnet. . in a luminous meteor exploded over the atlantic ocean, and at the same time a quantity of matter resembling sand descended to the surface. . according to chladni the explosion of a large bolide over peru, on the th of august, , was followed by a shower of cindery matter, the fall of which continued during three consecutive days. . on the th and th of march, , a shower of red dust fell in calabria, tuscany, and friuli. the deposit was sufficient to impart its color to the snow which was then upon the ground. that this dust was meteoric can scarcely be doubted, since at the same time a shower of aerolites fell at cutro, in calabria, attended by two loud reports resembling thunder. the shower of dust continued several hours, and was accompanied by a noise which was compared to the distant dashing of the waves of the ocean.[ ] . in november, , black rain and snow fell in canada. . on the d of may, , red rain fell near giessen. it deposited a dark-colored sediment which dr. zimmermann found to contain silica, oxide of iron, and various other substances observed in aerolites. it is well known that quantities of sand are often conveyed, by the trade-winds, from the continent of africa and deposited in the ocean. such sand-showers have sometimes occurred several hundred miles from the coast. volcanic matter also has been occasionally carried a considerable distance. the phenomena above described cannot, however, be referred to such causes; and there can be little doubt that most, if not all of them, were of meteoric origin. there is, in all probability, a regular gradation from the smallest visible shooting-stars to bolides and aerolites. no doubt a great number of very small meteoric stones penetrate beneath the earth's surface and escape observation. an interesting account of the accidental discovery of such _celestial pebbles_ has recently been given by professor haidinger, of vienna. the meteor from which they were derived _was but little larger than an ordinary shooting-star_. its track was visible, however, until it terminated at the earth's surface. professor haidinger's account is as follows: on the st of july, , about half-past nine o'clock in the evening, three inhabitants of the bourg of montpreis, in styria, saw a small luminous globe, very similar to a shooting-star, and followed by a luminous streak in the heavens, fall directly to the earth, which it attained close to the château that exists in the locality. the fall was accompanied by a whistling or hissing noise in the air, and terminated by a _slight_ detonation. the three observers, rushing to the spot where the meteor fell, immediately found a small cavity in the hard, sandy soil, from which they extracted three small meteoric stones about the size of nuts, and a quantity of black powder. for five to eight seconds these stones continued in a _state of incandescence_, and it was necessary to allow upwards of a quarter of an hour to elapse before they could be touched without inflicting a burn. they appear to have been ordinary meteoric stones, covered with the usual black rind. the possessors would not give them up to be analyzed. the details of this remarkable occurrence of the fall of an extremely small meteor, we owe to herr deschann, conservator of the museum of laibach, in carniola, and member of the austrian chamber of deputies. the following is perhaps the only instance on record in which a shooting-star _lower than the clouds_ has been undoubtedly observed. the date is one at which meteors are said to be more than usually numerous; and the radiant point for the epoch has been recently determined, by british observers, to be about _gamma cygni_. the meteor was seen by mr. david trowbridge, of hector, schuyler county, new york, who says: "on the evening of july th, , about h. m. p.m., a very bright meteor flashed out in cygnus, and moved from east to west with great rapidity. its path was about ° after i saw it. height above the northern horizon about °. duration of flight from one-half to one second. it left a beautiful train. the head was red and train blue. it was certainly _below_ the clouds. it passed between me and some cirro-stratus clouds, so dense as to hide ordinary stars completely. several others that saw it said it was _below_ the clouds."--_silliman's journal_ for sept. . it seems altogether probable that when a meteor thus descends, before its explosion or dissipation, into the lower atmospheric strata, at least portions of its mass must reach the earth's surface. meteoric transits--dark days. if shooting-stars and aerolites are derived from meteoric rings revolving round the sun in orbits nearly intersecting that of the earth, then ( ) these masses must sometimes transit the solar disk; ( ) if any of the rings contain either individual masses of considerable magnitude, or sufficiently dense swarms of meteoric asteroids, such transits may sometimes be observed; ( ) the passage of a dense meteoric cluster over the solar disk must partially intercept the sun's light and heat; and ( ) should both nodes of the ring very nearly intersect the earth's orbit, meteoric falls might occur when the earth is at either; in which case the epochs would be separated by an interval of about six months. have any such phenomena as those indicated been actually observed? the passage of dark spots across the sun, having a much more rapid motion than the solar maculæ, has been frequently noticed. the following instances are well authenticated: , june th. about mid-day the eminent french astronomer, messier, saw a great number of black points crossing the sun. rapidly moving spots were also seen by pastorff on the following dates: , october d, , july th and th, , october th, and on several subsequent occasions the same astronomer witnessed similar phenomena. another transit of this kind has been seen quite recently. on the th of may, , a small black spot was seen by coumbary to cross the solar disk. it seems difficult to account for these appearances (so frequently seen by experienced observers) unless we regard them as meteoric masses. partial interception of the sun's light and heat. numerous instances are on record of partial obscurations of the sun which could not be accounted for by any known cause. cases of such phenomena took place, according to humboldt, in the years , , and . another so-called _dark day_ occurred on the th of may, , and several more (some of still later date) might be specified. chladni and other physicists have regarded the transit of meteoric masses as the most probable cause of these obscurations. it is proper to remark, however, that the eminent french astronomer, faye, who has given the subject much attention, finds little or no evidence in support of this conjecture. an examination of meteorological records is said to have established two epochs of abnormal cold, viz., about the th of february and the th of may. the former was pointed out by brandes about the beginning of the present century; the latter by mädler, in . the may epoch occurs when the earth is in conjunction with one of the nodes of the november meteoric ring; and that of february has a similar relation to the august meteors. m. erman, a distinguished german scientist, soon after the discovery of the august and november meteoric epochs, suggested that those depressions of temperature might be explained by the intervention of the meteoric zones between the earth and the sun. the period, however, of the november meteors being still somewhat doubtful, their position with respect to the earth about the th of may is also uncertain. but however this may be, the following dates of aerolitic falls seem to indicate may th- th, or especially may th- th, as a meteoric epoch: (_a_) may th, , forsyth, georgia, u. s. a. (_b_) may th, , macerata, italy. (_c_) may th, , nashville, tennessee, u. s. a. (_d_) may th, , goruckpore, india. (_e_) may th, , vouillé, france. (_f_) may th, , oesel, baltic sea. (_g_) may th, , bremevörde, hanover. (_h_) may th, , near villanova, in catalonia, spain. (_i_) may th, , orgueil, france. all the foregoing, except that of may th, , may be found in shepard's list, _silliman's journal_ for january, . it has been shown in a former chapter that more than seven millions of shooting-stars of sufficient magnitude to be seen by the naked eye daily enter the earth's atmosphere. as the small ones are the most numerous, it is not improbable that an indefinitely greater number of meteoric particles, too minute to be visible, are being constantly, in this manner, arrested in their orbital motion. now, it would certainly be a very unwarranted conclusion that these atmospheric increments are all of a permanently gaseous form. in view of this strong probability that meteoric dust is daily reaching the earth's surface, baron von reichenbach, of vienna, conceived the idea of attempting its discovery. ascending to the tops of some of the german mountains, he carefully collected small quantities of the soil from positions in which it had not been disturbed by man. this matter, on being analyzed, was found to contain small portions of nickel and cobalt--elements rarely found in the mineral masses scattered over the earth's surface, but very frequently met with in aerolites. in short, reichenbach believed, and certainly not without some probability, that he had detected minute portions of meteoric matter. chapter vii. further researches of reichenbach--theory of meteors--stability of the solar system--doctrine of a resisting medium. the able and original researches of the celebrated reichenbach, who has made meteoric phenomena the subject of long-continued and enthusiastic investigation, have attracted the general attention of scientific men. it is proposed to present, in the following chapter, a brief _resumé_ of his views and conclusions. . _the constitution of comets._--it is a remarkable fact that cometary matter has no refractive power, as is manifest from the observations of stars seen through their substance.[ ] these bodies, therefore, are not gaseous; and the most probable theory in regard to their nature is that they consist of an infinite number of discrete, solid molecules, at great distances from each other, with very little attraction among themselves, or toward the nucleus, and having, therefore, great mobility. now baron reichenbach, having carefully examined a great number of meteoric stones, has found them for the most part composed of extremely minute globules, apparently cemented together. he hence infers that they have been comets--perhaps very small ones--whose component molecules have by degrees collected into single masses. . _the number of aerolites._--the average number of aerolitic falls in a year was estimated by schreibers, as previously stated, at . reichenbach, however, after a thorough discussion of the data at hand, makes the number much larger. he regards the probable annual average, for the entire surface of the earth, as not less than . this would give about twelve daily falls. they are of every variety as to magnitude, from a weight of less than a single ounce to over , pounds. the baron even suspects the meteoric origin of large masses of dolerite which all former geologists had considered native to our planet. in view of the fact that from the largest members of our planetary system down to the particles of meteoric dust there is an approximately regular gradation, and that the larger, at least in some instances, appear to have been formed by the aggregation of the smaller, he asks may not the earth itself have been formed by an agglomeration of meteorites? the learned author, from the general scope of his speculations, would thus seem to have adopted a form of the nebular hypothesis somewhat different from that proposed by laplace. . _composition and mean density of aerolites._--a large proportion of meteoric stones are similar in structure to the volcanic or plutonic rocks of the earth; and _all_ consist of elements identical with those in our planet's crust. their mean density, moreover, is very nearly the same with that of the earth. these facts are regarded by reichenbach as indicating that those meteoric masses which are daily becoming incorporated with our planet, have had a common origin with the earth itself. baron reichenbach's views, as presented by himself, will be found at length in _poggendorf's annalen_ for december, . _stability of the solar system._--the well-known demonstrations of the stability of the solar system, given by lagrange and laplace, are not to be accepted in an unlimited sense. they make no provision against the destructive agency of a resisting medium, or the entrance of matter into the solar domain from the interstellar spaces. in short, the conservative influence ascribed to these celebrated theorems extends only to the major planets; and even in their case it is to be understood as applying only to their mutual perturbations. the phenomena of shooting-stars and aerolites have demonstrated the existence of considerable quantities of matter moving in _unstable_ orbits. the amount of such matter within the solar system cannot now be determined; but the term probably includes the zodiacal light, many, if not all, of the meteoric rings, and a large number of comets. these unstable parts of the system are being gradually incorporated with the sun, the earth, and doubtless also with the other large planets. it is highly probable that at former epochs the quantity of such matter was much greater than at present, and that, unless new supplies be received _ab extra_, it must, by slow degrees, disappear from the system. the fact, now well established, of the extensive diffusion of meteoric matter through the interplanetary spaces has an obvious bearing on encke's theory of a resisting medium. if we grant the existence of such an ether, it would seem unphilosophical to ascribe to it one of the properties of a material fluid--the power of resisting the motion of all bodies moving through it--and to deny it such properties in other respects. its condensation, therefore, about the sun and other large bodies must be a necessary consequence. this condensation existed in the primitive solar spheroid, before the formation of the planets: the rotation of the spheroid would be communicated to the coexisting ether; and hence, _during the entire history of the planetary system, the ether has revolved around the sun in the same direction with the planets_. this condensed ether, it is also obvious, must participate in the progressive motion of the solar system. but again; even if we reject the doctrine of the development of the planetary bodies from a rotating nebula, we must still regard the density of the ether as increasing to the center of the system. the sun's rotation, therefore, would communicate motion to the first and denser portions; this motion would be transmitted outward through successive strata, with a constantly diminishing angular velocity. the motion of the planets themselves through the medium in nearly circular orbits would concur in imparting to it a revolution in the same direction. whether, therefore, we receive or reject the nebular hypothesis, the resistance of the ethereal medium to bodies moving in orbits of small eccentricity and in the direction of the sun's rotation, becomes an infinitesimal quantity. the hypothesis of encke, it is well known, was based solely on the observed acceleration of the comet which bears his name. more recently, however, a still greater acceleration has been found in the case of faye's comet. now as the meteoric matter of the solar system is a _known_ cause for such phenomena, sufficient, in all probability, both in mode and measure, the doctrine of a resisting ethereal medium would seem to be a wholly unnecessary assumption. chapter viii. does the number of aerolitic falls vary with the earth's distance from the sun?--relative numbers observed in the forenoon and afternoon--extent of the atmosphere as indicated by meteors. an analysis of any extensive table of meteorites and fire-balls proves that a greater number of aerolitic falls have been observed during the months of june and july, when the earth is near its aphelion, than in december and january, when near its perihelion. it is found, however, that the reverse is true in regard to bolides, or fire-balls. now the theory has been held by more than one physicist, that aerolites are the outriders of the asteroid ring between mars and jupiter; their orbits having become so eccentric that in perihelion they approach very near that of the earth. if this theory be the true one, the earth would probably encounter the greatest number of those meteor-asteroids when near its aphelion. the hypothesis therefore, it has been claimed, appears to be supported by well-known facts. the variation, however, in the observed number of aerolites may be readily accounted for independently of any theory as to their origin. the fall of meteoric stones would evidently be more likely to escape observation by night than by day, by reason of the relatively small number of observers. but the days are shortest when the earth is in perihelion, and longest when in aphelion; the ratio of their lengths being nearly equal to that of the corresponding numbers of aerolitic falls. on the other hand, it is obvious that fire-balls, unless of very extraordinary magnitude, would not be visible during the day. the _observed_ number will therefore be greatest when the nights are longest; that is, when the earth is near its perihelion. this, it will be found, is precisely in accordance with observation. it has been found, moreover, that a greater number of meteoric stones fall during the first half of the day, that is, from midnight to noon, than in the latter half, from noon to midnight. this would seem to indicate that a large proportion of the aerolites encountered by the earth have direct motion. _height of the atmosphere._--the weight of a given volume of mercury is , times that of an equal volume of air at the earth's surface; and since the mean height of the mercurial column in the barometer is about thirty inches, if the atmosphere were of uniform density its altitude would be about , feet, or nearly five miles. the density rapidly diminishes, however, as we ascend above the earth's surface. calling it unity at the sea level, the rate of variation is approximately expressed as follows: altitude in miles. density. / / / / / / / / etc. etc. from this table it will be seen that at the height of miles the air is one thousand times rarer than at the surface of the earth; and that, supposing the same rate of decrease to continue, at the height of miles the rarity would be one trillion times greater. the atmosphere, however, is not unlimited. when it becomes so rare that the force of repulsion between its particles is counterbalanced by the earth's attraction, no further expansion is possible. to determine the altitude of its superior surface is a problem at once difficult and interesting. not many years since about or miles were generally regarded as a probable limit. considerable light, however, has been thrown upon the question by recent observations in meteoric astronomy. several hundred detonating meteors have been observed, and their average height at the instant of their first appearance has been found to exceed miles. the great meteor of february d, , seen at brussels, geneva, paris, and elsewhere, was miles high when first seen, and a few apparently well-authenticated instances are known of a still greater elevation. we conclude, therefore, from the evidence afforded by meteoric phenomena, that the height of the atmosphere is certainly not _less_ than miles. it might be supposed, however, that the resistance of the air at such altitudes would not develop a sufficient amount of heat to give meteorites their brilliant appearance. this question has been discussed by joule, thomson, haidinger, and reichenbach, and may now be regarded as definitively settled. when the velocity of a meteorite is known the quantity of heat produced by its motion through air of a given density is readily determined. the temperature acquired is the equivalent of the force with which the atmospheric molecules are met by the moving body. this is about one degree (fahrenheit) for a velocity of feet per second, and it varies directly as the square of the velocity. a velocity, therefore, of miles in a second would produce a temperature of , , °. the weight of cubic feet of air at the earth's surface is about , , grains. this, consequently, is the weight of a column mile in length, and whose base or cross section is one square foot. the weight of a column of the same dimensions at a height of miles would be about / th of a grain. hence the heat acquired by a meteoric mass whose cross section is one square foot, in moving mile would be one grain raised - / degrees, or one-fifth of a grain ° in miles. this temperature would undoubtedly be sufficient to render meteoric bodies brilliantly luminous. but there have been indications of an atmosphere at an elevation of more than miles. a discussion of the best observations of the great aurora seen throughout the united states on the th of august, , gave miles as the height of the upper limit above the earth's surface. the aurora of september d, of the same year, had an elevation but little inferior, viz., miles. now, according to the observed rate of variation of density, at the height of miles, the atmosphere would be so rare that a sphere of it filling the orbit of neptune would contain less matter than / th of a cubic inch of air at the earth's surface. in other words, it would weigh less than / th of a grain. we are thus forced to the conclusion either that the law of variation is not the same at great heights as near the surface; or, that beyond the limits of the atmosphere of air, there is another of electricity, or of some other fluid. chapter ix. the meteoric theory of solar heat. of the various theories proposed by astronomers to account for the origin of the sun's light and heat, only two have at present any considerable number of advocates. these are-- . _the chemical theory_; according to which the light and heat of the sun are produced by the chemical combination of its elements; in other words, by an intense combustion. . _the meteoric theory_, which ascribes the heat of our central luminary to the fall of meteors upon its surface. the former is advocated with great ingenuity by professor ennis in a recent work on "_the origin of the stars, and the causes of their motions and their light_." it has, on the other hand, been ably opposed by dr. mayer, professor william thomson, and other eminent physicists. a brief examination of its claims may not be destitute of interest. if the sun's heat is produced by chemical action, whence comes the necessary supply of fuel to support the combustion? the quantity of solar heat radiated into space has been determined with at least an approximation to mathematical precision. we know also the amount produced by the combustion of a given quantity of coal. now it has been found by calculation that if the sun were a solid globe of coal, and a sufficient supply of oxygen were furnished to support its combustion, the amount of heat resulting from its consumption would be less than that actually emitted during the last years. in short, _no known_ elements would meet the demands of the case. but it is highly probable that the different bodies of the solar system are composed of the same elements. this view is sustained by the well-known fact that meteoric stones, which have reached us from different and distant regions of space, have brought us no new elementary substances. the _chemical_ theory of solar heat seems thus encumbered with difficulties well-nigh insuperable. professor ennis' mode of obviating this objection, though highly ingenious, is by no means conclusive. the latest analyses of the solar spectrum indicate, he affirms, the presence of numerous elements besides those with which we are acquainted. some of these may yield by their combustion a much greater amount of heat than the same quantity of any known elements in the earth's crust. "every star," he remarks, "as far as yet known, has a different set of fixed lines, although there are certain resemblances between them. they lead to the conclusion that each star has, in part at least, its peculiar modifications of matter, called simple elements; but the number of stars is infinite, and therefore the number of elements must be infinite."[ ] he argues, moreover, that in a globe so vast as the sun there may be forces in operation with whose nature we are wholly unacquainted. this leaving of the _known_ elements as well as the _known_ laws of nature for _unknown possibilities_ will hardly be satisfactory to unbiased minds. again: that the different bodies of the universe are composed of different elements is inferred by our author from the following among other considerations: "in our solar system mercury is sixty or eighty times more dense than one of the satellites of jupiter, and probably in a much greater proportion denser than the satellites of saturn. this indicates a wide difference between the nature of their elements." this statement is again repeated in a subsequent page.[ ] "the densities of the planets and their satellites prove that they are composed of very different elements. mercury is more than sixty times, and our earth about fifty times, more dense than the inner moon of jupiter. saturn is only about one-ninth as dense as the earth; it would float buoyantly on water. there is a high probability that the satellites of saturn and uranus are far lighter than those of jupiter. between the two extremes of the attendants of the sun, there is probably a greater difference in density than one hundred to one; and from one extreme to the other there are regular gradations of small amount. "the difference in constitution between the earth and the moon is seen in their densities: that of the moon being about half that of the earth. the nitrogen of our globe is found only in the atmosphere, and such substances as derive it from the atmosphere. the moon has no appreciable atmosphere, and therefore, in a high probability, no nitrogen." the statements here quoted were designed to show that the physical constitution of the sun and planets is widely different from that of the earth, and that the combustion of _some_ of the elements in this indefinite variety may account for the origin of solar heat. let us examine the facts. according to laplace the mass of jupiter's first satellite is · , that of jupiter being . the diameter is miles. hence the corresponding density is a little more than _one-fifth_ of the mean density of the earth. in other words, it is somewhat greater than the density of water, and very nearly equal to that of jupiter himself. professor ennis' value is therefore erroneous.[ ] in regard to the densities of the saturnian and uranian satellites nothing is known, and conjecture is useless. in short, saturn has the least mean density of all the planets, primary or secondary, so far as known. this may be owing to the great extent of his atmospheric envelope. the density of the moon is but three-fifths that of the earth: it is to be borne in mind, however, that the _mass_ and _pressure_ are also much less. with respect to meteorites the same author remarks that "like the moon, they are probably satellites of the earth; but being very small, they are liable to extraordinary perturbations, and hence strike the earth in many directions." here, again, his _facts_ are at fault; for ( ) the observed velocities of these bodies are inconsistent with the supposition of their being satellites of the earth; and ( ) the amount of perturbation of such bodies does not vary with their masses: a _small_ meteorite would fall toward the earth or any other planet with no greater velocity than a _large_ one. the meteoric theory. it has been shown in a previous chapter that immense numbers of meteoric asteroids are constantly traversing the planetary spaces--that many millions, in fact, daily enter the earth's atmosphere. reasons are not wanting for supposing the numbers of these bodies to increase with great rapidity as we approach the center of the system. moreover, on account of the greater force of gravity at the sun's surface the heat produced by their fall must be much greater than at the surface of the earth. it has been calculated that if one of these asteroids be arrested in perihelion by the solar atmosphere, the quantity of heat thus developed will be times greater than that produced by the combustion of an equal mass of coal. there can, therefore, be no reasonable doubt that a _portion_ of the sun's heat is produced by the impact of meteoric matter. in considering the probability that it is _chiefly_ so generated, the following questions naturally present themselves: . _what amount of matter precipitated upon the sun would develop the quantity of heat actually emitted?_--this question has been satisfactorily discussed by eminent physicists, and it will be sufficient for our purpose to give the result. according to professor william thomson, of glasgow, the present rate of emission would be kept up by a meteoric deposit which would form an annual stratum feet in thickness over the sun's surface. . _could such an increase of the sun's magnitude be detected by micrometrical measurement?_--this inquiry is readily answered in the negative. the apparent diameter would be augmented only one second in , years. . _is there any known or visible source from which this amount of meteoric matter may be supplied?_--thomson, mayer, and other distinguished writers regard the zodiacal light as the source of such meteorites. the inner portions of this immense "tornado" must be resisted in their motions by the solar atmosphere, and hence precipitated upon the sun's surface. . _would this increase of the sun's mass derange the motions of the solar system?_--to this question prof. ennis gives an affirmative answer; his first objection to the theory under consideration being stated as follows: "the constant accumulation of such materials, during hundreds of millions of years, would increase the body of the sun and its consequent gravity so greatly as to derange the entire solar system, by destroying the balance between the centripetal and centrifugal forces now acting on the planets."[ ] this, it must be confessed, would be a valid objection, if the meteoric matter were supposed to be derived from the extra-planetary spaces. as their source, however,--the zodiacal light--is interior to the earth's orbit, it can have no application to any planet exterior to venus. most probably the greater portion of the meteoric mass is even within the orbit of mercury, so that the effect of its convergence could scarcely be noticed even in the motion of the interior planets. in pre-historic time the zodiacal light may have extended far beyond the earth's orbit. if so, its convergence to its present dimensions was undoubtedly attended by an acceleration of the earth's mean motion. we can of course have no evidence that such a shortening of the year has never occurred. the second objection urged against the meteoric theory by the author of "the origin of the stars" is thus expressed: "as we must believe that all stars were lighted up by the same means, so we must believe, according to this theory, that the present interior heat of the earth and its former melted condition in both exterior and interior, was caused by the fall of meteorites. but if so, they must have gradually ceased to fall, as space became cleared of their presence, and we would now find a thick covering of meteorites on the earth's cooled surface. instead of this, we find them very rarely, and in accordance with their present very rare falls." to this it may be replied that the primitive igneous fluidity of the earth and planets was a necessary consequence of their condensation--a fact which has no inconsistency with the theory in question. a different _mechanical_ theory of the origin of solar heat is advocated by professor helmholtz in his interesting work _on the interaction of natural forces_. in regard to the sun he says: "if we adopt the very probable view, that the remarkably small density of so large a body is caused by its high temperature, and may become greater in time, it may be calculated that if the diameter of the sun were diminished only the ten-thousandth part of its present length, by this act a sufficient quantity of heat would be generated to cover the total emission for years. such a small change besides it would be difficult to detect by the finest astronomical observations."[ ] the same view is adopted by dr. joel e. hendricks, of des moines, iowa.[ ] chapter x. will the meteoric theory account for the phenomena of variable and temporary stars? having shown that meteor-asteroids are diffused in vast quantities throughout the universe; that according to eminent physicists the solar heat is produced by the precipitation of such matter on the sun's surface; and that leverrier has found it necessary to introduce the disturbing effect of meteoric rings in order fully to account for the motion of mercury's perihelion; we now propose extending the meteoric theory to a number of phenomena that have hitherto received no satisfactory explanation. variable and temporary stars. no theory as to the origin of the sun's light and heat would seem to be admissible unless applicable also to the sidereal systems. will the meteoric theory explain the phenomena of variable and temporary stars? "it has been remarked respecting variable stars, that in passing through their successive phases, they are subject to sensible irregularities, which have not hitherto been reduced to fixed laws. in general they do not always attain the same maximum brightness, their fluctuations being in some cases very considerable. thus, according to argelander, the variable star in _corona borealis_, which pigott discovered in , exhibits on some occasions such feeble changes of brightness, that it is almost impossible to distinguish the maxima from the minima by the naked eye; but after it has completed several of its cycles in this manner, its fluctuations all at once become so considerable, that in some instances it totally disappears. it has been found, moreover, that the light of variable stars does not increase and diminish symmetrically on each side of the maximum, nor are the successive intervals between the maxima exactly equal to each other."--_grant's history of physical astronomy_, p. . of the numerous hypotheses hitherto proposed to account for these phenomena we believe none can be found to include and harmonize all the facts of observation. the theories of herschel and maupertius fail to explain the irregularity in some of the periods; while those of newton and dunn afford no explanation of the periodicity itself.[ ] but let us suppose that among the fixed stars some have atmospheres of great extent, as was probably the case with the sun at a remote epoch in its history. let us also suppose the existence of nebulous rings, like those of our own system, moving in orbits so elliptical that in their perihelia they pass through the atmospheric envelopes of the central stars. such meteoric rings of varying density, like those revolving about the sun, would evidently produce the phenomena of variable stars. the resisting medium through which they pass in perihelion must gradually contract their orbits, or, in other words, diminish the intervals between consecutive maxima. such a shortening of the period is now well established in the case of _algol_. again, if a ring be influenced by perturbation the period will be variable, like that of _mira ceti_. a change, moreover, in the perihelion distance will account for the occasional increase or diminution of the apparent magnitude at the different maxima of the same star. but how are we to account for the variations of brightness observed in a number of stars where no order or periodicity in the variation has as yet been discovered? it is easy to perceive that either a single nebulous ring with more than one _hiatus_, or several rings about the same star, may produce phenomena of the character described. finally, if the matter of an elliptic ring should accumulate in a single mass, so as to occupy a comparatively small arc, its passage through perihelion might produce the phenomenon of a so-called temporary star. recent researches relating to nebulæ seem in some measure confirmatory of the view here presented. these observations have shown ( ) a change of position in some of these objects, rendering it probable that in certain cases they are not more distant than fixed stars visible to the naked eye; and ( ) a variation in the brilliancy of many small stars situated in the great nebula of orion, and also the existence of numerous masses of nebulous matter in the form of tufts apparently attached to stars,--facts regarded as indicative of a physical connection between the stars and nebulæ.[ ] chapter xi. the lunar and solar theories of the origin of aerolites. besides the _cosmical_ theory of aerolites which has been adopted in this work, and which is now accepted by a great majority of scientific men, at least four others have been proposed: ( ) the _atmospheric_, according to which they are formed, like hail, in the earth's atmosphere; ( ) the _volcanic_, which regards them as matter ejected with great force from terrestrial volcanoes; ( ) the _lunar_, which supposes them to have been thrown from craters in the moon; and ( ) the _solar_ hypothesis, according to which they are projected by some tremendous explosive force from the great central orb of our system. the first and second have been universally abandoned as untenable. the third and fourth, however, are entitled to consideration. the lunar theory. the theory which regards meteoric stones as products of eruption in lunar volcanoes was received with favor by the celebrated laplace: "as the gravity at the surface of the moon," he remarks, "is much less than at the surface of the earth, and as this body has no atmosphere which can oppose a sensible resistance to the motion of projectiles, we may conceive that a body projected with a great force, by the explosion of a lunar volcano, may attain and pass the limit, where the attraction of the earth commences to predominate over that of the moon. for this purpose it is sufficient that its initial velocity in the direction of the vertical may be meters in a second; then in place of falling back on the moon, it becomes a satellite of the earth, and describes about it an orbit more or less elongated. the direction of its primitive impulsion may be such as to make it move directly toward the atmosphere of the earth; or it may not attain it, till after several and even a great number of revolutions; for it is evident that the action of the sun, which changes in a sensible manner the distances of the moon from the earth, ought to produce in the radius vector of a satellite which moves in a very eccentric orbit, much more considerable variations, and thus at length so diminish the perigean distance of the satellite, as to make it penetrate our atmosphere. this body traversing it with a very great velocity, and experiencing a very sensible resistance, might at length precipitate itself on the earth; the friction of the air against its surface would be sufficient to inflame it, and make it detonate, provided that it contained ingredients proper to produce these effects, and then it would present to us all those phenomena which meteoric stones exhibit. if it was satisfactorily proved that they are not produced by volcanoes, or generated in our atmosphere, and that their cause must be sought beyond it, in the regions of the heavens, the preceding hypothesis, which likewise explains the identity of composition observed in meteoric stones, by an identity of origin, will not be devoid of probability."--_système du monde_, t. ii. cap. v. knowing the masses and volumes of the earth and moon, it is easy to estimate the force of gravity at their surfaces, the distance from each to the point of equal attraction, and the force with which a projectile must be thrown from the lunar surface to pass within the sphere of the earth's influence. it has been calculated that an initial velocity of about a mile and a half in a second would be sufficient for this purpose--a force not greater than that known to have been exerted by terrestrial volcanoes. the _possibility_, therefore, that volcanic matter from our satellite may reach the earth's surface seems fairly admissible. since the time of laplace, several distinguished european astronomers have regarded the lunar hypothesis as more or less probable. it was advocated as recently as by the late prof. j. p. nichol, of glasgow. this popular and interesting writer, after describing tycho, a large and well-known lunar crater, from which luminous rays or stripes radiate over a considerable part of the moon's surface, expresses the opinion that that immense cavity was formed by a single tremendous explosion. "reflecting," he remarks, "on the probable suddenness and magnitude of that force, or rather of that _explosive_ energy one of whose acts we have traced, as well as on the immense mass of matter which seems to have been thus violently dispersed, is not the inquiry a natural one, _where is that matter now_? it is a mass indeed which cannot well have wholly disappeared. it filled a cavern miles in breadth, and , feet deep--a cavern into which even now one might cast chimborazo and mont blanc, and room be left for teneriffe behind! like rocks flung aloft by our volcanoes, did this immense mass fall back in fragments to the surface of the moon, or was the expulsive force strong enough to give it an outward velocity sufficient to resist the attractive power of its parent globe? the moon, be it recollected, is very small in _mass_ compared with the earth, and her attractive energy greatly inferior accordingly. laplace has even calculated that the force urging a cannon-ball, increased to a degree quite within the limits of what is conceivable, could effect a final separation between our satellite and any of its component parts. it is _possible_ then, and, although not demonstrable, very far from a chimera, that the disrupted and expelled masses were, in the case of which we are speaking, driven conclusively into space; but if so, where are they now? where their new residence, and what their functions? in the emergency to which i refer, such fragments would necessarily wander among the interplanetary spaces in most irregular orbits, and chiefly in the neighborhood of the moon and the earth. now, while the planetary orbits are so nicely adjusted that neither confusion nor interference can ever occur, it is not at all likely that the same order could be established here; nay, it is next to certain, that in the course of its orbital revolution our globe would ever and anon come in contact with these lunar fragments; in other words, stones _would fall occasionally to its surface, and apparently from its atmosphere_."--_planetary system_, pp. , . we have preferred to give the views of these eminent scientists in their own language. olbers, biot, and poisson, who adopted the same theory, estimated the _initial_ velocity which would be necessary in order that lunar fragments might pass the point of equal attraction, and also the _final_, or acquired velocity on reaching the earth's surface. the several determinations of the former were as follows: according to olbers · miles a second. " biot · " " " laplace · " " " poisson · " " the mean being almost exactly a mile and a half. the velocity on reaching our planet, according to olbers, would be about six and a half miles. at the date of these calculations, however, the true velocity of aerolites had not been in any case satisfactorily determined. since that time it has been found in numerous instances to exceed _twenty miles a second_--a velocity greater than that of the earth's orbital motion. this fact of itself would seem fatal to the theory of a lunar origin. at the meeting of the american association for the advancement of science, in , dr. b. a. gould read a paper on the supposed lunar origin of aerolites, in which the hypothesis was subjected to the test of a rigid mathematical analysis. we will not attempt even an abstract of this interesting memoir. it amounts, however, to a virtual disproof of the lunar hypothesis. the solar theory. the theory which ascribes a solar origin to meteorites is not of recent date, having been held by diogenes laertius and other ancient greeks. among the moderns its advocates have been much less numerous than those of the lunar hypothesis. the late professor charles w. hackley, of new york, regarded shooting-stars, aerolites, and even comets, as matter projected with enormous force from the solar surface. the corona seen during total eclipses of the sun he supposed to be the emanations of this matter through the intervals of the luculi.--(see the proceedings of the american association for the advancement of science, fourteenth meeting, .) an ingenious theory, differing in its details from that of professor hackley, though somewhat similar in its general features, has lately been advocated by alexander wilcocks, m.d., of philadelphia, in a memoir read before the american philosophical society, may th, , and published in their proceedings. in regard to this hypothesis it seems sufficient to remark that it fails to give a satisfactory account of the annual periodicity of meteoric phenomena. chapter xii. the rings of saturn. until about the middle of the present century the rings of saturn were universally regarded as solid and continuous. the labors, however, of professors bond and pierce, of cambridge, massachusetts, as well as the more recent investigations of prof. maxwell, of england, have shown this hypothesis to be wholly untenable. the most probable opinion, based on the researches of these astronomers, is, that they consist of streams or clouds of meteoric asteroids. the zodiacal light and the zone of small planets between mars and jupiter appear to constitute analogous _primary_ rings. in the latter, however, a large proportion of the primitive matter seems to have collected in distinct, segregated masses. these meteoric zones have probably presented--what are not elsewhere found in the solar system--cases of commensurability in the planetary periods. the interior satellites of saturn are so near the ring as doubtless to exert great perturbative influence. unfortunately, the elements of the saturnian system as determined by different astronomers are somewhat discordant. this, however, is by no means surprising when we consider the great distance of the planet and the small magnitude of some of the satellites. for convenience of reference the mean apparent distances of the satellites, together with their periodic times, are given in the following table. the former are taken from hind's _solar system_; the latter from herschel's _outlines of astronomy_. table i.--the satellites of saturn. +-----------+------------------------+---------------+ | | | mean apparent | | name. | sidereal revolution. | distance. | +-----------+------------------------+---------------+ | | _d._ _h._ _m._ _s._ | ´´ | | mimas | · | · | | enceladus | · | · | | tethys | · | · | | dione | · | · | | rhea | · | · | | titan | · | · | | hyperion | ? | · ? | | japetus | · | · | +-----------+------------------------+---------------+ the late professor bessel devoted much attention to the theory of titan, whose mean distance he found to be · equatorial radii of the primary. struve's measurements of the ring are given in the second column of the following table. sir john herschel, however, regards the russian astronomer's interval between the rings as "somewhat too small."[ ] this remark is confirmed by the measurements of encke, whose results are given in column third. the fourth contains the _mean_ of struve's and encke's measurements; and the fifth, the same, expressed in equatorial radii of saturn. table ii.--the rings of saturn. +---------------------+---------+---------+----------+------------+ | | | | | in | | | struve. | encke. | mean. | semi-diam. | | | | | | of saturn. | +---------------------+---------+---------+----------+------------+ | equatorial radius | ´´ | ´´ | ´´ | | | of the planet | · | | | | | ext. semi-diameter | | | | | | of exterior ring | · | · | · | · | | int. semi-diameter | | | | | | of exterior ring | · | · | · | · | | ext. semi-diameter | | | | | | of interior ring | · | · | · | · | | int. semi diameter | | | | | | of interior ring | · | · | · | · | | breadth of interval | · | · | · | · | +---------------------+---------+---------+----------+------------+ the period of a satellite revolving at the distance, · , the interior limit of the interval = h. m. s. one-sixth of the period of dione = one-third " enceladus = one-half " mimas = one-fourth " tethys = and the period of a satellite at the distance, · , the exterior limit of the interval = the interval, therefore, occupies precisely the space in which the periods would be commensurable with those of the four members of the system immediately exterior. particles occupying this portion of the _primitive_ ring would always come into conjunction with one of these satellites in the same parts of their orbits. such orbits would become more and more eccentric until the matter moving in them would unite near one of the apsides with other portions of the ring. _we have thus a physical cause for the existence of this remarkable interval._ chapter xiii. the asteroid ring between mars and jupiter. the mean distances of the minor planets between mars and jupiter vary from · to · . the breadth of the zone is therefore , , miles greater than the distance of the earth from the sun; greater even than the entire interval between the orbits of mercury and mars. moreover, the _perihelion_ distance of some members of the group exceeds the _aphelion_ distance of others by a quantity equal to the whole interval between the orbits of mars and the earth. the _olbersian_ hypothesis of the origin of these bodies seems thus to have lost all claim to probability.[ ] professor alexander's theory of the disruption of a primitive discoidal planet of great equatorial diameter, is less objectionable; still, however, it requires confirmation. but whatever may have been the original constitution of the ring,[ ] its existence in its present form for an indefinite period is unquestioned. let us then consider some of the effects of its secular perturbation by the powerful mass of jupiter. _portions of the ring in which the periods of asteroids would be commensurable with that of jupiter._--the breadth of this zone is such as to contain several portions in which the periods of asteroids would be commensurable with that of jupiter. as in the case of the perturbation of saturn's ring by the interior satellites, the tendency of jupiter's influence would be to form gaps or chasms in the primitive ring. the mean distance of an asteroid whose period is / that of jupiter = · that of one whose period is / of jupiter's = · " " / " = · " " / " = · " " / " = · " " / " = · for the purpose of facilitating the comparison of these numbers with the mean distances of the asteroids and of observing whether any order obtains in the distribution of these mean distances in space, we have arranged the minor planets, in the following table, in the consecutive order of their periods: periods and distances of the asteroids. +------------+-------------+-----------+---------+ | order of | name. | distance. | period. | | discovery. | | | | +------------+-------------+-----------+---------+ | | flora | · | d | | | ariadne | · | · | | | feronia | · | · | | | harmonia | · | · | | | melpomene | · | · | | | sappho | · | · | | | victoria | · | · | | | euterpe | · | · | | | vesta | · | · | | | clio | · | · | | | urania | · | · | | | nemausa | · | · | | | metis | · | · | | | iris | · | · | | | echo | · | · | | | ausonia | · | · | | | phocea | · | · | | | massilia | · | · | | | asia | · | · | | | nysa | · | · | | | hebe | · | · | | | beatrice | · | · | | | isis | · | · | | | lutetia | · | · | | | fortuna | · | · | | | eurynome | · | · | | | parthenope | · | · | | | thetis | · | · | | | hestia | · | · | | | | · | · | | | amphitrite | · | · | | | astræa | · | · | | | egeria | · | · | | | irene | · | · | | | pomona | · | · | | | | · | · | | | melete | · | · | | | panopea | · | · | | | calypso | · | · | | | diana | · | · | | | thalia | · | · | | | fides | · | · | | | eunomia | · | · | | | io | · | · | | | virginia | · | · | | | thisbe | · | · | | | proserpina | · | · | | | maia | · | · | | | clytie | · | · | | | juno | · | · | | | eurydice | · | · | | | frigga | · | · | | | angelina | · | · | | | circe | · | · | | | concordia | · | · | | | alexandra | · | · | | | elpis | · | · | | | eugenia | · | · | | | leda | · | · | | | atalanta | · | · | | | niobe | · | · | | | alcmene | · | · | | | pandora | · | · | | | daphne | · | · | | | ceres | · | · | | | pallas | · | · | | | lætitia | · | · | | | galatea | · | · | | | bellona | · | · | | | leto | · | · | | | terpsichore | · | · | | | polyhymnia | · | · | | | aglaia | · | · | | | calliope | · | · | | | psyche | · | · | | | hesperia | · | · | | | danaë | · | · | | | leucothea | · | · | | | pales | · | · | | | semele | · | · | | | europa | · | · | | | doris | · | · | | | erato | · | · | | | themis | · | · | | | hygeia | · | · | | | euphrosyne | · | · | | | mnemosyne | · | · | | | antiope | · | · | | | freia | · | · | | | cybele | · | · | | | sylvia | · | · | +------------+-------------+-----------+---------+ remarks on the foregoing table. . the first two members of the group, flora and ariadne, have very nearly the same mean distance. immediately exterior to these, however, occurs a wide interval, including the distance at which seven periods of an asteroid would be equal to two of jupiter. . on the _outer_ limit of the ring freia, cybele, and sylvia have also nearly equal distances, and are separated from the next interior member by a wide space including the distance at which two periods would be equal to one of jupiter, and also that at which five would be equal to one of saturn. . besides these extreme members of the group, our table contains eighty-six minor planets, all of which are included between the distances · and · ; the mean interval between them being · . the distances are distributed as follows: · to · minimum. · to · maximum. · to · minimum. · to · } · to · } maximum. · to · · to · } minimum. · to · } · to · maximum. the clustering tendency is here quite apparent. . the three widest intervals between these bodies are-- (_a_) between leucothea and pales · , (_b_) " leto and terpsichore · , (_c_) " thetis and hestia · ; and these, it will be observed, are the three remaining distances, indicated on a previous page, at which the periods of the primitive meteoric asteroids would be commensurable with that of jupiter. now, if the original ring consisted of an indefinite number of separate particles moving with different velocities, according to their respective distances, those revolving at the distance · --in the interval between thetis and hestia--would make precisely three revolutions while jupiter completes one. a planetary particle at this distance would therefore always come in conjunction with jupiter in the same parts of its path: consequently its orbit would become more and more eccentric until the particle itself would unite with others, either exterior or interior, thus forming an asteroidal nucleus, while the primitive orbit of the particle would be left destitute of matter, like the interval in saturn's ring. . if the distribution of matter in the zone was originally nearly continuous, as in the case of saturn's rings, it would probably break up into a number of concentric annuli. on account, however, of the great perturbations to which they were subject, these narrow rings would frequently come in collision. after their rupture, and while the fragments were collecting in the form of asteroids, numerous intersections of orbits and new combinations of matter would occur, so as to leave, in the present orbits, but few traces of the rings from which the existing asteroids were derived. a comparison, however, of the elements of clytie and frigga shows a striking similarity; and professor lespiault has pointed out a corresponding likeness between the orbits of fides and maia. for these four asteroids the nodal lines and also the inclinations are nearly the same; while the periods differ by only a few days. it is probable, therefore, that they are all fragments of the same narrow ring. finally, as they all move nearly in the same plane, they must at some future time approach extremely near each other, and perhaps become united in one large asteroid. chapter xiv. origin of meteors--the nebular hypothesis. in regard to the physical history of those meteoric masses which, in such infinite numbers, traverse the interplanetary spaces, our knowledge is exceedingly limited. such as have reached the earth's surface consist of various elements in a state of combination. it has been remarked, however, by a distinguished scientist[ ] that "the character of the constituent particles of meteorites, and their general microscopical structure, differ so much from what is seen in terrestrial volcanic rocks, that it appears extremely improbable that they were ever portions of the moon, or of a planet, which differed from a large meteorite in having been the seat of a more or less modified volcanic action." as the celebrated nebular hypothesis seems to afford a very probable explanation of the origin of those bodies, whether in the form of rings or sporadic masses, its brief consideration may not be destitute of interest. we will merely premise that the existence of true nebulæ in the heavens--that is, of matter consisting of luminous gas--has been placed beyond doubt by the revelations of the spectroscope. as a group, our solar system is comparatively isolated in space; the distance of the nearest fixed star being at least seven thousand times that of neptune, the most remote known planet. besides the central or controlling orb, it contains, so far as known at present, ninety-nine primary planets, eighteen satellites, three planetary rings, and nearly eight hundred comets. in taking the most cursory view of this system we cannot fail to notice the following interesting facts in regard to the motions of its various members: . the sun rotates on his axis from west to east. . the primary planets all move nearly in the plane of the sun's equator. . the orbital motions of all the planets, primary and secondary, except the satellites of uranus and neptune, are in the same _direction_ with the sun's rotation. . the direction of the rotary motions of all the planets, primary and secondary, in so far as has been observed, is identical with that of their orbital revolutions; viz., from west to east. . the rings of saturn revolve about the planet in the same direction. . the planetary orbits are all nearly circular. . the _cometary_ is distinguished from the _planetary_ portion of the system by several striking characteristics: the orbits of comets are very eccentric and inclined to each other, and to the ecliptic at all possible angles. the motions of a large proportion of comets are _from east to west_. the physical constitution of the latter class of bodies is also very different from that of the former; the matter of which comets are composed being so exceedingly attenuated, at least in some instances, that fixed stars have been distinctly visible through what appeared to be the densest portion of their substance. none of these facts are accounted for by the law of gravitation. the sun's attraction can have no influence whatever in determining either the _direction_ of a planet's motion, or the eccentricity of its orbit. in other words, this power would sustain a planetary body moving from east to west, as well as from west to east; in an orbit having any possible degree of inclination to the plane of the sun's equator, no less than in one coincident with it; or, in a very eccentric ellipse, as well as in one differing but little from a circle. the consideration of the coincidences which we have enumerated led laplace to conclude that their explanation must be referred to the _mode_ of our system's formation--a conclusion which he regarded as strongly confirmed by the contemporary researches of sir william herschel. of the numerous nebulæ discovered and described by that eminent observer, a large proportion could not, even by his powerful telescope, be resolved into stars. in regard to many of these, it was not doubted that glasses of superior power would show them to be extremely remote sidereal clusters. on the other hand, a considerable number were examined which gave no indications of resolvability. these were supposed to consist of self-luminous, nebulous matter--the chaotic elements of future stars. the great number of these irresolvable nebulæ scattered over the heavens and apparently indicating the various stages of central condensation, very naturally suggested the idea that the solar system, and perhaps every other system in the universe, originally existed in a similar state. the sun was supposed by laplace to have been an exceedingly diffused, rotating nebula, of spherical or spheroidal form, extending beyond the orbit of the most distant planet; the planets as yet having no separate existence. this immense sphere of vapor, in consequence of the radiation of heat and the continual action of gravity, became gradually more dense, which condensation was necessarily attended by an increased angular velocity of rotation. at length a point was thus reached where the centrifugal force of the equatorial parts was equal to the central attraction. the condensation of the interior meanwhile continuing, the equatorial zone was detached, but necessarily continued to revolve around the central mass with the same velocity that it had at the epoch of its separation. if perfectly uniform throughout its entire circumference, which would be highly improbable, it would continue its motion in an unbroken ring, like that of saturn; if not, it would probably collect into several masses, having orbits nearly identical. "these masses should assume a spheroidal form, with a rotary motion in the direction of that of their revolution, because their inferior articles have a less real velocity than the superior; they have therefore constituted so many planets in a state of vapor. but if one of them was sufficiently powerful to unite successively by its attraction all the others about its center, the ring of vapors would be changed into one spheroidal mass, circulating about the sun, with a motion of rotation in the same direction with that of revolution."[ ] such, according to the theory of laplace, is the history of the formation of the most remote planet of our system. that of every other, both primary and secondary, would be precisely similar. in support of the nebular hypothesis, of which the foregoing is a brief general statement, we remark that _it furnishes a very simple explanation of the motions and arrangements of the planetary system_. in the first place, it is evident that the separation of a ring would take place at the equator of the revolving mass, where of course the centrifugal force would be greatest. these concentric rings--and consequently the resulting planets--would all revolve _in nearly the same plane_. it is evident also that the central body must have a revolution on its axis _in the same direction with the progressive motion of the planets_. again: at the breaking up of a ring, the particles of nebulous matter more distant from the sun would have a greater absolute velocity than those nearer to it, which would produce the observed _unity of direction in the rotary and orbital revolutions_. the motions of the satellites are explained in like manner. the hypothesis, moreover, accounts satisfactorily for the fact that the orbits of the planets are all nearly circular. and finally, it presents an obvious explanation of the rings of saturn. it would almost seem, indeed, as if these wonderful annuli had been left by the architect of nature, as an index to the creative process. the argument derived from the motions of the various members of the solar system is not new, having been forcibly stated by laplace, pontécoulant, nichol, and other astronomers. its full weight and importance, however, have not, we think, been duly appreciated. that a common physical cause has determined these motions, must be admitted by every philosophic mind. but apart from the nebular hypothesis, no such cause, adequate both in mode and measure, has ever been suggested;--indeed none, it seems to us, is conceivable. the phenomena which we have enumerated _demand_ an explanation, and this demand is met by the nebular hypothesis. it will be found, therefore, when closely examined, that the evidence afforded by the celestial motions is sufficient to give the theory of laplace a very high degree of probability. a comparison of the facts known in regard to comets, falling-stars, and meteoric stones, seems to warrant the inference that they are bodies of the same nature, and perhaps of similar origin; differing from each other mainly in the accidents of magnitude and density. the hypothesis of laplace very obviously accounts for the formation of planets and satellites, moving in the same direction, and in orbits nearly circular; but how, it may be asked, can the same theory explain the extremely eccentric, and in some cases retrograde, motions of comets and aerolites? this is the question to which we now direct our attention. after the nuclei of the solar and sidereal systems had been established in the primitive nebula, and when, in consequence, immense gaseous spheroids had collected around such nuclei, we may suppose that about the points of equal attraction between the sun and neighboring systems, portions of nebulous matter would be left in equilibrio. such outstanding nebulosities would gradually contract through the operation of gravity; and if, as would sometimes be the case, the solar attraction should preponderate, they would commence falling toward our system. unless disturbed by the planets they would probably move round the sun in parabolas. should they pass, however, near any of the large bodies of the system, their orbits might be changed into ellipses by planetary perturbation. such was the view of laplace in regard to the origin of comets. it seems probable, however, that many of these bodies originated _within_ the solar system, and belong properly to it. the outer rings thrown off by the planets may have been at too great distances from the primaries to form stable satellites. such masses would be separated by perturbation from their respective primaries, and would revolve round the sun in independent orbits. again: small portions of nebulous matter may have been abandoned as primary rings, at various intervals between the planetary orbits. at particular distances such rings would be liable to extraordinary perturbations, in consequence of which their orbits would ultimately assume an extremely elliptical form, like those of comets, and perhaps also those of meteors. it was shown in chapter xiii. that several such regions occur in the asteroid zone between mars and jupiter. we may add, in confirmation of this view, that there are twelve known comets whose periods are included between those of flora and jupiter. their motions are all direct; their orbits are less eccentric than those of other comets; and the mean of their inclinations is about the same as that of the asteroids. these facts certainly appear to indicate some original connection between these bodies and the zone of minor planets. the nebular hypothesis, it is thus seen, accounts satisfactorily for the origin of comets, aerolites, fire-balls, shooting-stars, and meteoric rings; regarding them all as bodies of the same nature, moving in cometary orbits about the sun. in this theory, the zodiacal light is an immense swarm of meteor-asteroids; so that the meteoric theory of solar heat, explained in a previous chapter, finds its place as a part of the same hypothesis. conclusion. some of the prominent results of observation and research in meteoric astronomy may be summed up as follows: . the shooting-stars of november, august, and other less noted epochs, are derived from elliptic rings of meteoric matter which intersect the earth's orbit. . meteoric stones and the matter of shooting-stars coexist in the same rings; the former being merely collections or aggregations of the latter. . the most probable period of the november meteors is thirty-three years and three months. leverrier's elements of this ring agree so closely with oppolzer's elements of the comet of as to render it probable that the latter is merely _a large meteor_ belonging to the same annulus. . the spectroscopic examination of this comet (of ) by william huggins, f.r.s., indicated that the nucleus was self-luminous, that the coma was rendered visible by reflecting solar light, and that "the material of the comet was similar to the matter of which the gaseous nebulæ consist." . the time of revolution of the august meteors is believed to be about years. m. schiaparelli has found a striking similarity between the elements of this ring and those of the third comet of . the same distinguished astronomer has shown, moreover, that a nebulous mass of considerable extent, drawn into the solar system _ab extra_, would form a _ring_ or _stream_. . the aerolitic epochs, established with more or less certainty, are the following: . february th- th. . march th- th. . april th- th. . april th- th. . may th- th; or especially, th- th. . may th. . july th- th. . july th. . august th- th. . october th- th. . november th- th. . november th- th. . december th- th. about one-half of this number are also known as shooting-star epochs. . the epoch of november th- th corresponds with that of the earth's crossing the orbit of biela's two comets. the aerolites of this epoch may therefore have been moving in nearly the same path. . a greater number of aerolitic falls are observed-- . by day than by night. . in the afternoon than in the forenoon. . when the earth is in aphelion than when in perihelion. the first fact is accounted for by the difference in the number of observers; the second indicates that a majority of aerolites have direct motion; and the third is dependent on the relative lengths of the day and night in the aphelic and perihelic portions of the orbit. . the observed velocities of meteorites are incompatible with the theory of their lunar origin. . if the meteoric swarm of november th has a period of thirty-three years, biela's comet passed very _near_, if not actually _through_ it toward the close of , about the time of the comet's separation. was the division of the cometary mass produced by the encounter? . the rings of saturn may be regarded as dense meteoric masses, and the principal or permanent division accounted for by the disturbing influence of the interior satellites. . the asteroidal space between mars and jupiter is probably a wide meteoric ring in which the largest aggregations are visible as minor planets. in the distribution of the mean distances of the known members of the group a clustering tendency is quite obvious. . the meteoric masses encountered by encke's comet may account for the shortening of the period of the latter without the hypothesis of an ethereal medium. appendix. a. the meteors of november th. the _american journal of science and arts_ for may, (received by the author after the first chapters of this work had gone to press), contains an interesting article by professor newton "on certain recent contributions to astro-meteorology." of the five possible periods of the november ring, first designated by professor n, it is now granted that the longest, viz., - / years, is most probably the true one. the results of leverrier's researches in regard to the epoch at which this meteoric mass was introduced into the solar system, are given in the same article. this distinguished astronomer supposes the group of meteors to have been thrown into an elliptic orbit by the disturbing influence of uranus. the meteoric stream, according to the most trustworthy elements of its orbit, passed extremely near that planet about the year of our era; which date is therefore assigned by leverrier as the probable time of its entrance into the planetary system. this result, however, requires confirmation. although the earliest display of the november meteors, so far as certainly known, was that of the year , several more ancient exhibitions may, with some probability, be referred to the same epoch. these are the phenomena of , , and , a.d., and , b.c. (see quetelet's catalogue.) the time of the year at which these showers occurred is not given. the _years_, however, correspond very well with the epochs of the maximum display of the november meteors. the intervals arranged in consecutive order, are as follows: from b.c. to a.d. , periods of · years each. " a.d. to " · , " · " " " · to " , " · " " " to " , " · " " " to " , " · " " " to " , " · " " " to " , " · " " " to " , " · " " " to " , " · " " " to " , " · " " " to " , " · " " " to " , " · " " " to " , " · " the first three dates are alone doubtful. the whole number of intervals from b.c. to a.d. is , and the mean length is · years. the perturbations of the ring by jupiter, saturn, and uranus, are doubtless considerable. it is worthy of note that-- periods of jupiter are nearly equal to of the ring. " saturn " " " " uranus " " " this group or stream has its perihelion at the orbit of the earth; its aphelion, at that of uranus. (see diagram, p. .) it must therefore produce star-showers at the latter as well as at the former. our planet, moreover, at each encounter appropriates a portion of the meteoric matter; while at the remote apsis of the stream uranus in all probability does the same. the matter of the ring will thus by slow degrees be gathered up by the two planets. b. comets and meteors. the recent researches and speculations of european astronomers in regard to the origin of comets and of meteoric streams, have suggested to the author the propriety of reproducing the following extracts from an article written by himself, in july, , and published in the _danville quarterly review_ for december of that year: "different views are entertained by astronomers in regard to the _origin_ of comets; some believing them to enter the solar system _ab extra_; others supposing them to have originated within its limits. the former is the hypothesis of laplace, and is regarded with favor by many eminent astronomers. it seems to afford a plausible explanation of the paucity of large comets during certain long intervals of time. in one hundred and fifty years, from to , sixteen comets were visible to the naked eye; of which eight appeared in the twenty-five years from to . again, during sixty years from to , only five comets were visible to the naked eye, while in the next fifty years there were double that number. now, according to laplace's hypothesis, patches of nebulous matter have been left nearly in equilibrium in the interstellar spaces. as the sun, in his progressive motion, approaches such clusters, they must, by virtue of his attraction, move toward the center of our system; the nearer portions with greater velocity than the more remote. the nebulous fragments thus introduced into our system would constitute comets; those of the same cluster would enter the solar domain at periods not very distant from each other; the forms of their orbits depending upon their original relative positions with reference to the sun's course, and also on planetary perturbations. on the other hand, the passage of the system through a region of space destitute of this chaotic vapor would be followed by a corresponding paucity of comets. "before the invention of the telescope, the appearance of a comet was a comparatively rare occurrence. the whole number visible to the naked eye during the last three hundred and sixty years has been fifty-five; or a mean of fifteen per century. the recent rate of telescopic discovery, however, has been about four or five annually. as many of these are extremely faint, it seems probable that an indefinite number, too small for detection, may be constantly traversing the solar domain. if we adopt laplace's hypothesis of the origin of comets, we may suppose an almost continuous fall of primitive nebular matter toward the center of the system--the _drops_ of which, penetrating the earth's atmosphere, produce _sporadic_ meteors; the larger aggregations forming comets. the disturbing influence of the planets may have transformed the original orbits of many of the former, as well as of the latter, into ellipses. it is an interesting fact that the motions of some luminous meteors--or _cometoids_, as perhaps they might be called--have been decidedly indicative of an origin beyond the limits of the planetary system. "but how are the phenomena of _periodic_ meteors to be accounted for, in accordance with this theory? "the division of biela's comet into two distinct parts suggests several interesting questions in cometary physics. the nature of the separating force remains to be discovered; 'but it is impossible to doubt that it arose from the divellent action of the sun, whatever may have been the mode of operation.' "'a signal manifestation of the influence of the sun,' says a distinguished writer, 'is sometimes afforded by the breaking up of a comet into two or more separate parts, on the occasion of its approach to the perihelion. seneca relates that ephoras, an ancient greek author, makes mention of a comet which before vanishing was seen to divide itself into two distinct bodies. the roman philosopher appears to doubt the possibility of such a fact; but keppler, with characteristic sagacity, has remarked that its actual occurrence was exceedingly probable. the latter astronomer further remarked that there were some grounds for supposing that two comets, which appeared in the same region of the heavens in the year , were the fragments of a comet that had experienced a similar dissolution. hevelius states that cysatus perceived in the head of the great comet of unequivocal symptoms of a breaking up of the body into distinct fragments. the comet when first seen in the month of november, appeared like a round mass of concentrated light. on the th of december it seemed to be divided into several parts. on the th of the same month it resembled a multitude of small stars. hevelius states that he himself witnessed a similar appearance in the head of the comet of .'[ ] edward biot, moreover, in his researches among the chinese records, found an account of 'three dome-formed comets' that were visible simultaneously in , and pursued very nearly the same apparent path. "another instance of a similar phenomenon is recorded by dion cassius, who states that a comet which appeared eleven years before our era, separated itself into several small comets. "these various examples are presented at one view, as follows: "i. ancient bipartition of a comet.--_seneca, quæst. nat._, _lib. vii. cap. xvi._ "ii. separation of a comet into a number of fragments, b.c.--_dion cassius._ "iii. three comets seen simultaneously pursuing the same orbit, a.d. --_chinese records--comptes rendus_, tom. xx. , p. . "iv. probable separation of a comet into parts, a.d. .--_hevelius_, _cometographia_, p. .--_keppler_, _de cometis_, p. . "v. indications of separation, .--_hevelius_, _cometographia_, p. . "vi. bipartition of biela's comet, - . "in view of these facts it seems highly probable, if not absolutely certain, that the process of division has taken place in several instances besides that of biela's comet. may not the force, whatever it is, that has produced _one_ separation, again divide the parts? and may not this action continue until the fragments become invisible? according to the theory now generally received, the periodic phenomena of shooting-stars are produced by the intersections of the orbits of such nebulous bodies with the earth's annual path. now there is reason to believe that these meteoric rings are very elliptical, and in this respect wholly dissimilar to the rings of primitive vapor which, according to the nebular hypothesis, were successively abandoned at the solar equator; in other words, that the matter of which they are composed moves in _cometary_ rather than _planetary_ orbits. may not our periodic meteors be the _debris_ of ancient but now disintegrated comets, whose matter has become distributed around their orbits?" c. biela's comet and the meteors of november th- th. at the close of chapter iv. it was suggested that the meteors of november th- th might possibly be derived from a ring of meteoric matter moving in the orbit of biela's comet. since that chapter was written similar conjectures have been started in the _astronomische nachrichten_[ ] by dr. edmund weiss and prof. d'arrest. the latter attempts to show that the december meteors may be derived from the same ring. the question will doubtless be decided at no distant day. d. the first comet of and the meteors of april th. recent investigations render it probable that the orbit of the first comet of is identical with that of the meteors of april th. the orbit is nearly perpendicular to the ecliptic. footnotes: [ ] for a full description, see silliman's journal for january and april, (prof. olmsted's article). also a valuable paper, in the july no. of the same year, by prof. twining. [ ] physique du globe, chap. iv. [ ] professor olmsted estimated the number of meteors, visible at new haven, during the night of november th- th, , at , . [ ] conde says, "there were seen, as it were lances, an infinite number of stars, which scattered themselves like rain to the right and left, and that year was called 'the year of stars.'" [ ] in , "on the last day of muharrem, stars shot hither and thither in the heavens, eastward and westward, and flew against one another like a scattering swarm of locusts, to the right and left; this phenomenon lasted until daybreak; people were thrown into consternation, and cried to god the most high with confused clamor."--quoted by prof. newton, in silliman's journal, may, . [ ] am. journ. of sci. and arts, may and july, . [ ] the stream or arc of meteors is several years in passing its node. the first indication of the approach of the display of was the appearance of meteors in unusual numbers at malta, on the th of november, . the great length of the arc is indicated, moreover, by the showers of and . [ ] silliman's journ. for sept. and nov., . [ ] the numerical results here given are those found by professor newton. see silliman's journ. for march, . [ ] the diameters of the asteroids are derived from a table by prof. lespiault, in the rep. of the smithsonian inst. for , p. . [ ] "it appears probable, from the researches of schreibers, that fall annually."--cosmos, vol. i. p. (bohn's ed.). reichenbach makes the number much greater. [ ] new concord is close to the guernsey county line. nearly all the stones fell in guernsey. [ ] cosmos, vol. i. p. . [ ] leverrier's annals of the observatory of paris, vol. i. p. . [ ] "this is a remarkable example of a stone arriving on the earth with a temperature approaching that of the interplanetary spaces. aerolites containing much iron, a substance which conducts heat well, get thoroughly heated by their passage through the atmosphere. but the stony aerolites, containing less iron, conducting heat badly, preserve in their interior the temperature of the locality from which they fall; their surface only is heated, and generally fused. when the stones are large, the _excessive cold_ of their interior portion, which must be nearly that of interplanetary space, is remarked; but when small, they remain hot for some time."--_dr. phipson._ [ ] silliman's journal, september, . [ ] the same explanation is given by t. m. hall, f.g.s., in the popular science review for oct. . [ ] this list contains nothing but _aerolites_. in the edinburgh review for january, , we find the following statements: "out of the large number of authentic aerolites preserved in mineralogical collections, two only--one on the th of august, and one on the th of november--are recorded to have fallen on star-shower dates. on the other hand, five or six meteorites, on the epoch of the th- th of october, belong to a date when star-showers, so far as is at present known, do not make their appearance." the inaccuracy of the former statement is sufficiently apparent. in regard to the latter we remark that quetelet's catalogue gives one star-shower on the th of october, and another on the th. [ ] the date of this remarkable occurrence is worthy of note as a probable aerolite epoch. from the th to the th of march we have the following falls of meteoric stones: . , march th. at halstead, essex, england. . , march th. at salés, france. . , march th. at alais, france. . , march th. at timochin, russia. . , march th. at kuleschofka, russia. . , march th- th. the phenomena above described. . , march th. at grüneberg, silesia. numerous fire-balls have appeared at the same epoch. [ ] the innermost or semi-transparent ring of saturn appears to be similarly constituted, as the body of the planet is seen through it without any distortion whatever. [ ] origin of the stars, p. . [ ] origin of the stars, p. . [ ] since the above was written prof. ennis has informed the author that, without making any estimate of his own, he adopted the density of jupiter's first satellite as given in lardner's _handbook of astronomy_. [ ] origin of the stars, p. . [ ] youman's correlation and conservation of forces, p. . [ ] iowa instructor and school journal for november, , p. . [ ] a recent hypothesis in regard to the temporary star of has been proposed by alexander wilcocks, m.d., of philadelphia. see journ. acad. nat. sci. of phila. for . [ ] gautier's notice of recent researches relating to nebulæ.--silliman's journal for jan. , and march, . [ ] outlines of astronomy, art. . [ ] a learned and highly interesting examination of this hypothesis will be found in a memoir "on the secular variations and mutual relations of the orbits of the asteroids," communicated to the am. acad. of arts and sciences, april th, , by simon newcomb, esq. [ ] for an explanation of the origin of the asteroids according to the nebular hypothesis, see an article by david trowbridge, a.m., in silliman's journal for nov. , and jan. . [ ] h. c. sorby, f.r.s. [ ] harte's trans. of laplace's syst. of the world, vol. ii., note vii. [ ] grant's hist. of phys. astr., p. . [ ] nos. and . publications of j. b. lippincott & co., phila. _will be sent by mail, post-paid, on receipt of price._ new america. by william hepworth dixon, editor of "the athenæum," and author of "the holy land," "william penn," etc. with illustrations from original photographs. third edition. complete in one volume, crown octavo. printed on tinted paper. extra cloth. price $ . . in these graphic volumes mr. dixon sketches american men and women, sharply, vigorously, and truthfully, under every aspect. the smart yankee, the grave politician, the senate and the stage, the pulpit and the prairie, loafers and philanthropists, crowded streets and the howling wilderness, the saloon and the boudoir, with women everywhere at full length--all passes on before us in some of the most vivid and brilliant pages ever written.--_dublin university magazine._ elements of art criticism. a text-book for schools and colleges, and a hand-book for amateurs and artists. by g. w. samson, d.d., president of columbian college, washington, d. c. second edition. crown vo. cloth. price $ . . this work comprises a treatise on the principles of man's nature as addressed by art, together with a historic survey of the methods of art execution in the departments of drawing, sculpture, architecture, painting, landscape gardening, and the decorative arts. the _round table_ says: "the work is incontestably one of great as well as unique value." history of the u. s. sanitary commission. being the general report of its work on the war of the rebellion. by charles j. stillÉ, professor in the university of pennsylvania. one vol. vo. cloth, beveled boards. price $ . . terra mariÆ; or, threads of maryland colonial history. by edward d. neill, one of the secretaries of the president of the united states. mo. extra cloth. price $ . . coming wonders, expected between and . by the rev. m. baxter, author of "the coming battle." one vol. mo. cloth. price $ . . _lippincott's pronouncing gazetteer of the world_, or geographical dictionary. revised edition, with an appendix containing nearly ten thousand new notices, and the most recent statistical information, according to the latest census returns, of the united states and foreign countries. lippincott's pronouncing gazetteer gives-- i.--a descriptive notice of the countries, islands, rivers, mountains, cities, towns, etc., in every part of the globe, with the most recent and authentic information. ii.--the names of all important places, etc., both in their native and foreign languages, with the pronunciation of the same--a feature never attempted in any other work. iii.--the classical names of all ancient places, so far as they can be accurately ascertained from the best authorities. iv.--a complete etymological vocabulary of geographical names. v.--an elaborate introduction, explanatory of the principles of pronunciation of names in the danish, dutch, french, german, greek, hungarian, italian, norwegian, polish, portuguese, russian, spanish, swedish, and welsh languages. comprised in a volume of over two thousand three hundred imperial octavo pages. price, $ . . from the hon. horace mann, ll.d., _late president of antioch college_. i have had your pronouncing gazetteer of the world before me for some weeks. having long felt the necessity of a work of this kind, i have spent no small amount of time in examining yours. it seems to me so important to have a comprehensive and authentic gazetteer in all our colleges, academies, and schools, that i am induced in this instance to depart from my general rule in regard to giving recommendations. your work has evidently been prepared with immense labor; and it exhibits proofs from beginning to end that knowledge has presided over its execution. the rising generation will be greatly benefited, both in the accuracy and extent of their information, should your work be kept as a book of reference on the table of every professor and teacher in the country. transcriber's notes: punctuation and spelling were made consistent when a predominant preference was found in this book; otherwise they were not changed. simple typographical errors were corrected; occasional unbalanced quotation marks retained. ambiguous hyphens at the ends of lines were retained. text uses both "star shower" and "star-shower"; not changed here. "keppler" is spelled that way in this text. a challenge to the johns hopkins university one hundred proofs that the earth is not a globe. dedicated to richard a. proctor, esq. "the greatest astronomer of the age." by wm. carpenter, referee for john hampden, esq., in the celebrated scientific wager, in ; author of 'common sense' on astronomy, (london, ;) proctor's planet earth; wallace's wonderful water; the delusion of the day, &c., &c. "upright, downright, straightforward." baltimore: printed and published by the author, no. chew street . twenty-five cents. five copies, postage paid, for one dollar. th edition: th thousand. index. the aeronaut sees for himself. standing water level. surveyors' "allowance." flow of rivers--the nile. lighthouses--cape hatteras. the sea-shore.--"coming up." a trip down chesapeake bay. the model globe useless. the sailor's level charts. the mariners' compass. the southern circumference. circumnavigation of the earth. meridians are straight lines. parallels of latitude--circles. sailing down and underneath. distance round the south. levelness required by man. the "level" of the astronomers. half the globe is cut off, now! no "up" or "down" in nature? the "spherical lodestone." no falsehoods wanted! no proof of "rotundity." a "most complete" failure. the first atlantic cable. earth's "curvature." which end goes down? a "hill of water." characteristics of a globe. horizon--level with the eye. much too small a globe. vanishing point of objects. we are not "fastened on." our "antipodes."--a delusion. horizon a level line. chesapeake bay by night. six months day and night. the "midnight sun." sun moves round the earth. suez canal-- miles--level. the "true level."--a curve. projectiles--firing east or west. bodies thrown upwards. firing in opposite direction. astronomer royal of england. an utterly meaningless theory. professor proctor's cylinder. proctor's false perspective. motion of the clouds. scriptural proof--a plane. the "standing order." more ice in the south. sun's accelerated pace, south. balloons not left behind. the moon's beams are cold. the sun and moon. not earth's shadow at all. rotating and revolving. proctor's big mistake. sun's distance from earth. no true "measuring-rod." sailing "round" a thing. telescopes--"hill of water." the laws of optics--glaisher. "dwelling" upon error. ptolemy's predictions. canal in china-- miles. mr. lockyer's false logic. beggarly alternatives. mr. lockyer's suppositions. north star seen from s. lat. "walls not parallel!" pendulum experiments. "delightful uncertainty." outrageous calculations. j. r. young's navigation. "tumbling over." circumnavigation--south. a disc--not a sphere. earth's "motion" unproven. moon's motion east to west. all on the wrong track. no meridianal "degrees." depression of north star. rivers flowing up-hill? miles in five seconds. miserable makeshifts. what holds the people on. luminous objects. practice against theory. unscientific classification. g. b. airy's "suppositions." astronomers give up theory. school-room "proofs" false. pictorial proof--earth a plane. laws of perspective ignored. "rational suppositions." it is the star that moves. hair-splitting calculation. how "time" is lost or gained. introduction. "parallax," the founder of the zetetic philosophy, is dead; and it now becomes the duty of those, especially, who knew him personally and who labored with him in the cause of truth against error, to begin, anew, the work which is left in their hands. dr. samuel b. rowbotham finished his earthly labours, in england, the country of his birth, december , , at the age of . he was, certainly, one of the most gifted of men: and though his labours as a public lecturer were confined within the limits of the british islands his published work is known all over the world and is destined to live and be republished when books on the now popular system of philosophy will be considered in no other light than as bundles of waste paper. for several years did "parallax" spread a knowledge of the facts which form the basis of his system without the slightest recognition from the newspaper press until, in january, , the people were informed by the "wilts independent" that lectures had been delivered by "a gentleman adopting the name of 'parallax,' to prove modern astronomy unreasonable and contradictory," that "great skill" was shown by the lecturer, and that he proved himself to be "thoroughly acquainted with the subject in all its bearings." such was the beginning--the end will not be so easily described. the truth will always find advocates--men who care not a snap of their fingers for the mere opinion of the world, whatever form it may take, whilst they know that they are the masters of the situation and that reason is king! in , "parallax" was described as "a paragon of courtesy, good temper, and masterly skill in debate." the author of the following hastily-gotten-up pages is proud of having spent many a pleasant hour in the company of samuel birley rowbotham. a complete sketch of the "zetetic philosophy" is impossible in a small pamphlet; and many things necessarily remain unsaid which, perhaps, should have been touched upon, but which would to some extent have interfered with the plan laid down--the bringing together, in a concise form, "one hundred proofs that the earth is not a globe." much may be gathered, indirectly, from the arguments in these pages, as to the real nature of the earth on which we live and of the heavenly bodies which were created for us. the reader is requested to be patient in this matter and not expect a whole flood of light to burst in upon him at once, through the dense clouds of opposition and prejudice which hang all around. old ideas have to be gotten rid of, by some people, before they can entertain the new; and this will especially be the case in the matter of the sun, about which we are taught, by mr. proctor, as follows: "the globe of the sun is so much larger than that of the earth that no less than , , globes as large as the earth would be wanted to make up together a globe as large as the sun." whereas, we know that, as it is demonstrated that the sun moves round over the earth, its size is proportionately less. we can then easily understand that day and night, and the seasons are brought about by his daily circuits round in a course concentric with the north, diminishing in their extent to the end of june, and increasing until the end of december, the equatorial region being the area covered by the sun's mean motion. if, then, these pages serve but to arouse the spirit of enquiry, the author will be satisfied. the right hand of fellowship in this good work is extended, in turn, to mr. j. lindgren, south first street, brooklyn, e. d., n. y., mr. m. c. flanders, lecturer, kendall, orleans county, n. y., and to mr. john hampden, editor of "parallax" (a new journal), cosmos house, balham, surrey, england. one hundred proofs that earth is not a globe. if man uses the senses which god has given him, he gains knowledge; if he uses them not, he remains ignorant. mr. r. a. proctor, who has been called "the greatest astronomer of the age," says: "the earth on which we live and move seems to be flat." now, he does not mean that it seems to be flat to the man who shuts his eyes in the face of nature, or, who is not in the full possession of his senses: no, but to the average, common sense, wide-awake, thinking man. he continues: "that is, though there are hills and valleys on its surface, yet it seems to extend on all sides in one and the same general level." again, he says: "there seems nothing to prevent us from travelling as far as we please in any direction towards the circle all round us, called the horizon, where the sky seems to meet the level of the earth." "the level of the earth!" mr. proctor knows right well what he is talking about, for the book from which we take his words, "lessons in elementary astronomy," was written, he tells us, "to guard the beginner against the captious objections which have from time to time been urged against accepted astronomical theories." the things which are to be defended, then, are these "accepted astronomical theories!" it is not truth that is to be defended against the assaults of error--oh, no: simply "theories," right or wrong, because they have been "accepted!" accepted! why, they have been accepted because it was not thought to be worth while to look at them. sir john herschel says: "we shall take for granted, from the outset, the copernican system of the world." he did not care whether it was the right system or a wrong one, or he would not have done that: he would have looked into it. but, forsooth, the theories are accepted, and, of course, the men who have accepted them are the men who will naturally defend them if they can. so, richard a. proctor tries his hand; and we shall see how it fails him. his book was published without any date to it at all. but there is internal evidence which will fix that matter closely enough. we read of the carrying out of the experiments of the celebrated scientist, alfred r. wallace, to prove the "convexity" of the surface of standing water, which experiments were conducted in march, , for the purpose of winning five hundred pounds from john hampden, esq., of swindon, england, who had wagered that sum upon the conviction that the said surface is always a level one. mr. proctor says: "the experiment was lately tried in a very amusing way." in or about the year , then, mr. proctor wrote his book; and, instead of being ignorant of the details of the experiment, he knew all about them. and whether the "amusing" part of the business was the fact that mr. wallace wrongfully claimed the five-hundred pounds and got it, or that mr. hampden was the victim of the false claim, it is hard to say. the "way" in which the experiment was carried out is, to all intents and purposes, just the way in which mr. proctor states that it "can be tried." he says, however, that the distance involved in the experiment "should be three or four miles." now, mr. wallace took up six miles in his experiment, and was unable to prove that there is any "curvature," though he claimed the money and got it; surely it would be "amusing" for anyone to expect to be able to show the "curvature of the earth" in three or four miles, as mr. proctor suggests! nay, it is ridiculous. but "the greatest astronomer of the age" says the thing can be done! and he gives a diagram: "showing how the roundness of the earth can be proved by means of three boats on a large sheet of water." (three or four miles.) but, though the accepted astronomical theories be scattered to the winds, we charge mr. proctor either that he has never made the experiment with the three boats, or, that, if he has, the experiment did not prove what he says it will. accepted theories, indeed! are they to be bolstered up with absurdity and falsehood? why, if it were possible to show the two ends of a four-mile stretch of water to be on a level, with the centre portion of that water bulged up, the surface of the earth would be a series of four-mile curves! but mr. proctor says: "we can set three boats in a line on the water, as at a, b, and c, (fig. ). then, if equal masts are placed in these boats, and we place a telescope, as shown, so that when we look through it we see the tops of the masts of a and c, we find the top of the mast b is above the line of sight." now, here is the point: mr. proctor either knows or he ought to know that we shall not find anything of the sort! if he has ever tried the experiment, he knows that the three masts will range in a straight line, just as common sense tells us they will. if he has not tried the experiment, he should have tried it, or have paid attention to the details of experiments by those who have tried similar ones a score of times and again. mr. proctor may take either horn of the dilemma he pleases: he is just as wrong as a man can be, either way. he mentions no names, but he says: "a person had written a book, in which he said that he had tried such an experiment as the above, and had found that the surface of the water was not curved." that person was "parallax," the founder of the zetetic philosophy. he continues: "another person seems to have believed the first, and became so certain that the earth is flat as to wager a large sum of money that if three boats were placed as in fig. , the middle one would not be above the line joining the two others." that person was john hampden. and, says mr. proctor, "unfortunately for him, some one who had more sense agreed to take his wager, and, of course, won his money." now, the "some one who had more sense" was mr. wallace. and, says proctor, in continuation: "he [hampden?] was rather angry; and it is a strange thing that he was not angry with himself for being so foolish, or with the person who said he had tried the experiment (and so led him astray), but with the person who had won his money!" here, then, we see that mr. proctor knows better than to say that the experiments conducted by "parallax" were things of the imagination only, or that a wrong account had been given of them; and it would be well if he knew better than to try to make his readers believe that either one or the other of these things is the fact: but, there is the old bedford canal now; and there are ten thousand places where the experiment may be tried! who, then, are the "foolish" people: those who "believe" the record of experiments made by searchers after truth, or those who shut their eyes to them, throw a doubt upon the record, charge the conductors of the experiments with dishonesty, never conduct similar experiments themselves, and declare the result of such experiments to be so and so, when the declaration can be proved to be false by any man, with a telescope, in twenty-four hours? mr. proctor:--the sphericity of the earth cannot be proved in the way in which you tell us it "can" be! we tell you to take back your words and remodel them on the basis of truth. such careless misrepresentations of facts are a disgrace to science--they are the disgrace of theoretical science to-day! mr. blackie, in his work on "self culture," says: "all flimsy, shallow, and superficial work, in fact, is a lie, of which a man ought to be ashamed." that the earth is an extended plane, stretched out in all directions away from the central north, over which hangs, for ever, the north star, is a fact which all the falsehoods that can be brought to bear upon it with their dead weight will never overthrow: it is god's truth the face of which, however, man has the power to smirch all over with his unclean hands. mr. proctor says: "we learn from astronomy that all these ideas, natural though they seem, are mistaken." man's natural ideas and conclusions and experimental results are, then, to be overthrown by--what! by "astronomy?" by a thing without a soul--a mere theoretical abstraction, the outcome of the dreamer? never! the greatest astronomer of the age is not the man, even, who can so much as attempt to manage the business. "we find," says mr. proctor, "that the earth is not flat, but a globe; not fixed, but in very rapid motion; not much larger than the moon, and far smaller than the sun and the greater number of the stars." first, then, mr. proctor, tell us how you find that the earth is not flat, but a globe! it does not matter that "we find" it so put down in that conglomeration of suppositions which you seek to defend: the question is, what is the evidence of it?--where can it be obtained? "the earth on which we live and move seems to be flat," you tell us: where, then, is the mistake? if the earth seem to be what it is not, how are we to trust our senses? and if it is said that we cannot do so, are we to believe it, and consent to be put down lower than the brutes? no, sir: we challenge you, as we have done many times before, to produce the slightest evidence of the earth's rotundity, from the world of facts around you. you have given to us the statement we have quoted, and we have the right to demand a proof; and if this is not forthcoming, we have before us the duty of denouncing the absurd dogma as worse than an absurdity--as a fraud--and as a fraud that flies in the face of divine revelation! well, then, mr. proctor, in demanding a proof of the earth's rotundity (or the frank admission of your errors), we are tempted to taunt you as we tell you that it is utterly out of your power to produce one; and we tell you that you do not dare even to lift up your finger to point us to the so-called proofs in the school-books of the day, for you know the measure of absurdity of which they are composed, and how disgraceful it is to allow them to remain as false guides of the youthful mind! mr. proctor: we charge you that, whilst you teach the theory of the earth's rotundity and mobility, you know that it is a plane; and here is the ground of the charge. in page , in your book, you give a diagram of the "surface on which we live," and the "supposed globe"--the supposed "hollow globe"--of the heavens, arched over the said surface. now, mr. proctor, you picture the surface on which we live in exact accordance with your verbal description. and what is that description? we shall scarcely be believed when we say that we give it just as it stands: "the level of the surface on which we live." and, that there may be no mistake about the meaning of the word "level," we remind you that your diagram proves that the level that you mean is the level of the mechanic, a plane surface, and not the "level" of the astronomer, which is a convex surface! in short, your description of the earth is exactly what you say it "seems to be," and, yet, what you say it is not: the very aim of your book being to say so! and we call this the prostitution of the printing press. and it is all the evidence that is necessary to bring the charge home to you, since the words and the diagram are in page of your own book. you know, then, that earth is a plane--and so do we. now for the evidence of this grand fact, that other people may know it as well as you: remembering, from first to last, that you have not dared to bring forward a single item from the mass of evidence which is to be found in the "zetetic philosophy," by "parallax," a work the influence of which it was the avowed object of your own book to crush!--except that of the three boats, an experiment which you have never tried, and the result of which has never been known, by anyone who has tried it, to be as you say it is! . the aeronaut can see for himself that earth is a plane. the appearance presented to him, even at the highest elevation he has ever attained, is that of a concave surface--this being exactly what is to be expected of a surface that is truly level, since it is the nature of level surfaces to appear to rise to a level with the eye of the observer. this is ocular demonstration and proof that earth is not a globe. . whenever experiments have been tried on the surface of standing water, this surface has always been found to be level. if the earth were a globe, the surface of all standing water would be convex. this is an experimental proof that earth is not a globe, . surveyors' operations in the construction of railroads, tunnels, or canals are conducted without the slightest "allowance" being made for "curvature," although it is taught that this so-called allowance is absolutely necessary! this is a cutting proof that earth is not a globe. . there are rivers that flow for hundreds of miles towards the level of the sea without falling more than a few feet--notably, the nile, which, in a thousand miles, falls but a foot. a level expanse of this extent is quite incompatible with the idea of the earth's "convexity." it is, therefore, a reasonable proof that earth is not a globe. . the lights which are exhibited in lighthouses are seen by navigators at distances at which, according to the scale of the supposed "curvature" given by astronomers, they ought to be many hundreds of feet, in some cases, down below the line of sight! for instance: the light at cape hatteras is seen at such a distance ( miles) that, according to theory, it ought to be nine-hundred feet higher above the level of the sea than it absolutely is, in order to be visible! this is a conclusive proof that there is no "curvature," on the surface of the sea--"the level of the sea,"--ridiculous though it is to be under the necessity of proving it at all: but it is, nevertheless, a conclusive proof that the earth is not a globe. . if we stand on the sands of the sea-shore and watch a ship approach us, we shall find that she will apparently "rise"--to the extent of her own height, nothing more. if we stand upon an eminence, the same law operates still; and it is but the law of perspective, which causes objects, as they approach us, to appear to increase in size until we see them, close to us, the size they are in fact. that there is no other "rise" than the one spoken of is plain from the fact that, no matter how high we ascend above the level of the sea, the horizon rises on and still on as we rise, so that it is always on a level with the eye, though it be two-hundred miles away, as seen by mr. j. glaisher, of england, from mr. coxwell's balloon. so that a ship five miles away may be imagined to be "coming up" the imaginary downward curve of the earth's surface, but if we merely ascend a hill such as federal hill, baltimore, we may see twenty-five miles away, on a level with the eye--that is, twenty miles level distance beyond the ship that we vainly imagined to be "rounding the curve," and "coming up!" this is a plain proof that the earth is not a globe. . if we take a trip down the chesapeake bay, in the day-time, we may see for ourselves the utter fallacy of the idea that when a vessel appears "hull down," as it is called, it is because the hull is "behind the water:" for, vessels have been seen, and may often be seen again, presenting the appearance spoken of, and away--far away--beyond those vessels, and, at the same moment, the level shore line, with its accompanying complement of tall trees, towering up, in perspective, over the heads of the "hull-down" ships! since, then, the idea will not stand its ground when the facts rise up against it, and it is a piece of the popular theory, the theory is a contemptible piece of business, and we may easily wring from it a proof that earth is not a globe. . if the earth were a globe, a small model globe would be the very best--because the truest--thing for the navigator to take to sea with him. but such a thing as that is not known: with such a toy as a guide, the mariner would wreck his ship, of a certainty! this is a proof that earth is not a globe. . as mariners take to sea with them charts constructed as though the sea were a level surface, however these charts may err as to the true form of this level surface taken as a whole, it is clear, as they find them answer their purpose tolerably well--and only tolerably well, for many ships are wrecked owing to the error of which we speak--that the surface of the sea is as it is taken to be, whether the captain of the ship "supposes" the earth to be a globe or anything else. thus, then, we draw, from the common system of "plane sailing," a practical proof that earth is not a globe. . that the mariners' compass points north and south at the same time is a fact as indisputable as that two and two makes four; but that this would be impossible if the thing were placed on a globe with "north" and "south" at the centre of opposite hemispheres is a fact that does not figure in the school-books, though very easily seen: and it requires no lengthy train of reasoning to bring out of it a pointed proof that the earth is not a globe. . as the mariners' compass points north and south at one time, and as the north, to which it is attracted, is that part of the earth situate where the north star is in the zenith, it follows that there is no south "point" or "pole" but that, while the centre is north, a vast circumference must be south in its whole extent. this is a proof that the earth is not a globe. . as we have seen that there is, really, no south point (or pole) but an infinity of points forming, together, a vast circumference--the boundary of the known world, with its battlements of icebergs which bid defiance to man's onward course in a southerly direction--so there can be no east or west "points," just as there is no "yesterday," and no "to-morrow." in fact, as there is one point that is fixed (the north), it is impossible for any other point to be fixed likewise. east and west are, therefore, merely directions at right angles with a north and south line: and as the south point of the compass shifts round to all parts of the circular boundary, (as it may be carried round the central north), so the directions east and west, crossing this line, continued, form a circle, at any latitude. a westerly circumnavigation, therefore, is a going round with the north star continually on the right hand, and an easterly circumnavigation is performed only when the reverse condition of things is maintained, the north star being on the left hand as the journey is made. these facts, taken together, form a beautiful proof that the earth is not a globe. . as the mariners' compass points north and south at one and the same time, and a meridian is a north and south line, it follows that meridians can be no other than straight lines. but, since all meridians on a globe are semicircles, it is an incontrovertible proof that the earth is not a globe. . "parallels of latitude" only--of all imaginary lines on the surface of the earth--are circles, which increase, progressively, from the northern centre to the southern circumference. the mariner's course in the direction of any one of these concentric circles is his longitude, the degrees of which increase to such an extent beyond the equator (going southwards) that hundreds of vessels have been wrecked because of the false idea created by the untruthfulness of the charts and the globular theory together, causing the sailor to be continually getting out of his reckoning. with a map of the earth in its true form all difficulty is done away with, and ships may be conducted anywhere with perfect safety. this, then, is a very important practical proof that the earth is not a globe. . the idea that, instead of sailing horizontally round the earth, ships are taken down one side of a globe, then underneath, and are brought up on the other side to get home again, is, except as a mere dream, impossible and absurd! and, since there are neither impossibilities nor absurdities in the simple matter of circumnavigation, it stands, without argument, a proof that the earth is not a globe. . if the earth were a globe, the distance round its surface at, say, "degrees" south latitude, could not possibly be any greater than it is at the same latitude north; but, since it is found by navigators to be twice the distance--to say the least of it--or, double the distance it ought to be according to the globular theory, it is a proof that the earth is not a globe. . human beings require a surface on which to live that, in its general character, shall be level; and since the omniscient creator must have been perfectly acquainted with the requirements of his creatures, it follows that, being an all-wise creator, he has met them thoroughly. this is a theological proof that the earth is not a globe. . the best possessions of man are his senses; and, when he uses them all, he will not be deceived in his survey of nature. it is only when some one faculty or other is neglected or abused that he is deluded. every man in full command of his senses knows that a level surface is a flat or horizontal one; but astronomers tell us that the true level is the curved surface of a globe! they know that man requires a level surface on which to live, so they give him one in name which is not one in fact! since this is the best that astronomers, with their theoretical science, can do for their fellow creatures--deceive them--it is clear that things are not as they say they are; and, in short, it is a proof that earth is not a globe. . every man in his senses goes the most reasonable way to work to do a thing. now, astronomers (one after another--following a leader), while they are telling us that earth is a globe, are cutting off the upper half of this supposititious globe in their books, and, in this way, forming the level surface on which they describe man as living and moving! now, if the earth were really a globe, this would be just the most unreasonable and suicidal mode of endeavoring to show it. so that, unless theoretical astronomers are all out of their senses together, it is, clearly, a proof that the earth is not a globe. . the common sense of man tells him--if nothing else told him--that there is an "up" and a "down" in nature, even as regards the heavens and the earth; but the theory of modern astronomers necessitates the conclusion that there is not: therefore, the theory of the astronomers is opposed to common sense--yes, and to inspiration--and this is a common sense proof that the earth is not a globe. . man's experience tells him that he is not constructed like the flies that can live and move upon the ceiling of a room with as much safety as on the floor: and since the modern theory of a planetary earth necessitates a crowd of theories to keep company with it, and one of them is that men are really bound to the earth by a force which fastens them to it "like needles round a spherical lodestone," a theory perfectly outrageous and opposed to all human experience, it follows that, unless we can trample upon common sense and ignore the teachings of experience, we have an evident proof that the earth is not a globe. . god's truth never--no, never--requires a falsehood to help it along. mr. proctor, in his "lessons," says: men "have been able to go round and round the earth in several directions." now, in this case, the word "several" will imply more than two, unquestionably: whereas, it is utterly impossible to circumnavigate the earth in any other than an easterly or a westerly direction; and the fact is perfectly consistent and clear in its relation to earth as a plane. now, since astronomers would not be so foolish as to damage a good cause by misrepresentation, it is presumptive evidence that their cause is a bad one, and--a proof that earth is not a globe. . if astronomical works be searched through and through, there will not be found a single instance of a bold, unhesitating, or manly statement respecting a proof of the earth's "rotundity." proctor speaks of "proofs which serve to show ... that the earth is not flat," and says that man "finds reason to think that the earth is not flat," and speaks of certain matters being "explained by supposing" that the earth is a globe; and says that people have "assured themselves that it is a globe;" but he says, also, that there is a "most complete proof that the earth is a globe:" just as though anything in the world could possibly be wanted but a proof--a proof that proves and settles the whole question. this, however, all the money in the united states treasury would not buy; and, unless the astronomers are all so rich that they don't want the cash, it is a sterling proof that the earth is not a globe. . when a man speaks of a "most complete" thing amongst several other things which claim to be what that thing is, it is evident that they must fall short of something which the "most complete" thing possesses. and when it is known that the "most complete" thing is an entire failure, it is plain that the others, all and sundry, are worthless. proctor's "most complete proof that the earth is a globe" lies in what he calls "the fact" that distances from place to place agree with calculation. but, since the distance round the earth at "degrees" south of the equator is twice the distance it would be on a globe, it follows that what the greatest astronomer of the age calls "a fact" is not a fact; that his "most complete proof" is a most complete failure; and that he might as well have told us, at once, that he has no proof to give us at all. now, since, if the earth be a globe, there would, necessarily, be piles of proofs of it all round us, it follows that when astronomers, with all their ingenuity, are utterly unable to point one out--to say nothing about picking one up--that they give us a proof that earth is not a globe. . the surveyor's plans in relation to the laying of the first atlantic telegraph cable, show that in miles--from valentia, ireland, to st. john's, newfoundland--the surface of the atlantic ocean is a level surface--not the astronomers' "level," either! the authoritative drawings, published at the time, are a standing evidence of the fact, and form a practical proof that earth is not a globe. . if the earth were a globe, it would, if we take valentia to be the place of departure, curvate downwards, in the miles across the atlantic to newfoundland, according to the astronomers' own tables, more than three-hundred miles; but, as the surface of the atlantic does not do so--the fact of its levelness having been clearly demonstrated by telegraph cable surveyors,--it follows that we have a grand proof that earth is not a globe. . astronomers, in their consideration of the supposed "curvature" of the earth, have carefully avoided the taking of that view of the question which--if anything were needed to do so--would show its utter absurdity. it is this:--if, instead of taking our ideal point of departure to be at valentia, we consider ourselves at st. john's, the miles of water between us and valentia would just as well "curvate" downwards as it did in the other case! now, since the direction in which the earth is said to "curvate" is interchangeable--depending, indeed, upon the position occupied by a man upon its surface--the thing is utterly absurd; and it follows that the theory is an outrage, and that the earth does not "curvate" at all:--an evident proof that the earth is not a globe. . astronomers are in the habit of considering two points on the earth's surface, without, it seems, any limit as to the distance that lies between them, as being on a level, and the intervening section, even though it be an ocean, as a vast "hill"--of water! the atlantic ocean, in taking this view of the matter, would form a "hill of water" more than a hundred miles high! the idea is simply monstrous, and could only be entertained by scientists whose whole business is made up of materials of the same description: and it certainly requires no argument to deduce, from such "science" as this, a satisfactory proof that the earth is not a globe. . if the earth were a globe, it would, unquestionably, have the same general characteristics--no matter its size--as a small globe that may be stood upon the table. as the small globe has top, bottom, and sides, so must also the large one--no matter how large it be. but, as the earth, which is "supposed" to be a large globe, has no sides or bottom as the small globe has, the conclusion is irresistible that it is a proof that earth is not a globe. . if the earth were a globe, an observer who should ascend above its surface would have to look downwards at the horizon (if it be possible to conceive of a horizon at all under such circumstances) even as astronomical diagrams indicate--at angles varying from ten to nearly fifty degrees below the "horizontal" line of sight! (it is just as absurd as it would be to be taught that when we look at a man full in the face we are looking down at his feet!) but, as no observer in the clouds, or upon any eminence on the earth, has ever had to do so, it follows that the diagrams spoken of are imaginary and false; that the theory which requires such things to prop it up is equally airy and untrue; and that we have a substantial proof that earth is not a globe. . if the earth were a globe, it would certainly have to be as large as it is said to be--twenty-five thousand miles in circumference. now, the thing which is called a "proof" of the earth's roundness, and which is presented to children at school, is, that if we stand on the sea-shore we may see the ships, as they approach us, absolutely "coming up," and that, as we are able to see the highest parts of these ships first, it is because the lower parts are "behind the earth's curve." now, since, if this were the case--that is, if the lower parts of these ships were behind a "hill of water" at all--the size of the earth, indicated by such a curve as this, would be so small that it would only be big enough to hold the people of a parish, if they could get all round it, instead of the nations of the world, it follows that the idea is preposterous; that the appearance is due to another and to some reasonable cause; and that, instead of being a proof of the globular form of the earth, it is a proof that earth is not a globe. . it is often said that, if the earth were flat, we could see all over it! this is the result of ignorance. if we stand on the level surface of a plain or a prairie, and take notice, we shall find that the horizon is formed at about three miles all around us: that is, the ground appears to rise up until, at that distance, it seems on a level with the eye-line or line of sight. consequently, objects no higher than we stand--say, six feet--and which are at that distance (three miles), have reached the "vanishing point," and are beyond the sphere of our unaided vision. this is the reason why the hull of a ship disappears (in going away from us) before the sails; and, instead of there being about it the faintest shadow of evidence of the earth's rotundity, it is a clear proof that earth is not a globe. . if the earth were a globe, people--except those on the top--would, certainly, have to be "fastened" to its surface by some means or other, whether by the "attraction" of astronomers or by some other undiscovered and undiscoverable process! but, as we know that we simply walk on its surface without any other aid than that which is necessary for locomotion on a plane, it follows that we have, herein, a conclusive proof that earth is not a globe. . if the earth were a globe, there certainly would be--if we could imagine the thing to be peopled all round--"antipodes:" "people who," says the dictionary, "living exactly on the opposite side of the globe to ourselves, have their feet opposite to ours:"--people who are hanging heads downwards whilst we are standing heads up! but, since the theory allows us to travel to those parts of the earth where the people are said to be heads downwards, and still to fancy ourselves to be heads upwards and our friends whom we have left behind us to be heads downwards, it follows that the whole thing is a myth--a dream--a delusion--and a snare; and, instead of there being any evidence at all in this direction to substantiate the popular theory, it is a plain proof that the earth is not a globe. . if we examine a true picture of the distant horizon, or the thing itself, we shall find that it coincides exactly with a perfectly straight and level line. now, since there could be nothing of the kind on a globe, and we find it to be the case all over the earth, it is a proof that the earth is not a globe. . if we take a journey down the chesapeake bay, by night, we shall see the "light" exhibited at sharpe's island for an hour before the steamer gets to it. we may take up a position on the deck so that the rail of the vessel's side will be in a line with the "light" and in the line of sight; and we shall find that in the whole journey the light will not vary in the slightest degree in its apparent elevation. but, say that a distance of thirteen miles has been traversed, the astronomers' theory of "curvature" demands a difference (one way or the other!) in the apparent elevation of the light, of feet inches! since, however, there is not a difference of hair's breadths, we have a plain proof that the water of the chesapeake bay is not curved, which is a proof that the earth is not a globe. . if the earth were a globe, there would, very likely, be (for nobody knows) six months day and six months night at the arctic and antarctic regions, as astronomers dare to assert there is:--for their theory demands it! but, as this fact--the six months day and six months night--is nowhere found but in the arctic regions, it agrees perfectly with everything else that we know about the earth as a plane, and, whilst it overthrows the "accepted theory," it furnishes a striking proof that earth is not a globe. . when the sun crosses the equator, in march, and begins to circle round the heavens in north latitude, the inhabitants of high northern latitudes see him skimming round their horizon and forming the break of their long day, in a horizontal course, not disappearing again for six months, as he rises higher and higher in the heavens whilst he makes his twenty-four hour circle until june, when he begins to descend and goes on until he disappears beyond the horizon in september. thus, in the northern regions, they have that which the traveller calls the "midnight sun," as he sees that luminary at a time when, in his more southern latitude, it is always midnight. if, then, for one-half the year, we may see for ourselves the sun making horizontal circles round the heavens, it is presumptive evidence that, for the other half-year, he is doing the same, although beyond the boundary of our vision. this, being a proof that earth is a plane, is, therefore, a proof that the earth is not a globe. . we have abundance of evidence that the sun moves daily round and over the earth in circles concentric with the northern region over which hangs the north star; but, since the theory of the earth being a globe is necessarily connected with the theory of its motion round the sun in a yearly orbit, it falls to the ground when we bring forward the evidence of which we speak, and, in so doing, forms a proof that the earth is not a globe. . the suez canal, which joins the red sea with the mediterranean, is about one hundred miles long; it forms a straight and level surface of water from one end to the other; and no "allowance" for any supposed "curvature" was made in its construction. it is a clear proof that the earth is not a globe. . when astronomers assert that it is "necessary" to make "allowance for curvature" in canal construction, it is, of course, in order that, in their idea, a level cutting may be had for the water. how flagrantly, then, do they contradict themselves when they say that the curved surface of the earth is a "true level!" what more can they want for a canal than a true level? since they contradict themselves in such an elementary point as this, it is an evidence that the whole thing is a delusion, and we have a proof that the earth is not a globe. . it is certain that the theory of the earth's rotundity and that of its mobility must stand or fall together. a proof, then, of its immobility is virtually a proof of its non-rotundity. now, that the earth does not move, either on an axis, or in an orbit round the sun or anything else, is easily proven. if the earth went through space at the rate of eleven-hundred miles in a minute of time, as astronomers teach us, in a particular direction, there would unquestionably be a difference in the result of firing off a projectile in that direction and in a direction the opposite of that one. but as, in fact, there is not the slightest difference in any such case, it is clear that any alleged motion of the earth is disproved, and that, therefore, we have a proof that the earth is not a globe. . the circumstances which attend bodies which are caused merely to fall from a great height prove nothing as to the motion or stability of the earth, since the object, if it be on a thing that is in motion, will participate in that motion; but, if an object be thrown upwards from a body at rest, and, again, from a body in motion, the circumstances attending its descent will be very different. in the former case, it will fall, if thrown vertically upwards, at the place from whence it was projected; in the latter case, it will fall behind--the moving body from which it is thrown will leave it in the rear. now, fix a gun, muzzle upwards, accurately, in the ground; fire off a projectile; and it will fall by the gun. if the earth travelled eleven-hundred miles a minute, the projectile would fall behind the gun, in the opposite direction to that of the supposed motion. since, then, this is not the case, in fact, the earth's fancied motion is negatived, and we have a proof that the earth is not a globe. . it is in evidence that, if a projectile be fired from a rapidly moving body in an opposite direction to that in which the body is going, it will fall short of the distance at which it would reach the ground if fired in the direction of motion. now, since the earth is said to move at the rate of nineteen miles in a second of time, "from west to east," it would make all the difference imaginable if the gun were fired in an opposite direction. but, as, in practice, there is not the slightest difference, whichever way the thing may be done, we have a forcible overthrow of all fancies relative to the motion of the earth, and a striking proof that the earth is not a globe. . the astronomer royal, of england, george b. airy, in his celebrated work on astronomy, the "ipswich lectures," says: "jupiter is a large planet that turns on his axis, and why do not we turn?" of course, the common sense reply is: because the earth is not a planet! when, therefore, an astronomer royal puts words into our mouth wherewith we may overthrow the supposed planetary nature of the earth, we have not far to go to pick up a proof that earth is not a globe. . it has been shown that an easterly or a westerly motion is necessarily a circular course round the central north. the only north point or centre of motion of the heavenly bodies known to man is that formed by the north star, which is over the central portion of the outstretched earth. when, therefore, astronomers tell us of a planet taking a westerly course round the sun, the thing is as meaningless to them as it is to us, unless they make the sun the northern centre of the motion, which they cannot do! since, then, the motion which they tell us the planets have is, on the face of it, absurd; and since, as a matter of fact, the earth can have no absurd motion at all, it is clear that it cannot be what astronomers say it is--a planet; and, if not a planet, it is a proof that earth is not a globe. . in consequence of the fact being so plainly seen, by everyone who visits the sea-shore, that the line of the horizon is a perfectly straight line, it becomes impossible for astronomers, when they attempt to convey, pictorially, an idea of the earth's "convexity," to do so with even a shadow of consistency: for they dare not represent this horizon as a curved line, so well known is it that it is a straight one! the greatest astronomer of the age, in page of his "lessons," gives an illustration of a ship sailing away, "as though she were rounding the top of a great hill of water;" and there--of a truth--is the straight and level line of the horizon clear along the top of the "hill" from one side of the picture to the other! now, if this picture were true in all its parts--and it is outrageously false in several--it would show that earth is a cylinder; for the "hill" shown is simply up one side of the level, horizontal line, and, we are led to suppose, down the other! since, then, we have such high authority as professor richard a. proctor that the earth is a cylinder, it is, certainly, a proof that the earth is not a globe. . in mr. proctor's "lessons in astronomy," page , a ship is represented as sailing away from the observer, and it is given in five positions or distances away on its journey. now, in its first position, its mast appears above the horizon, and, consequently, higher than the observer's line of vision. but, in its second and third positions, representing the ship as further and further away, it is drawn higher and still higher up above the line of the horizon! now, it is utterly impossible for a ship to sail away from an observer, under the conditions indicated, and to appear as given in the picture. consequently, the picture is a misrepresentation, a fraud, and a disgrace. a ship starting to sail away from an observer with her masts above his line of sight would appear, indisputably, to go down and still lower down towards the horizon line, and could not possibly appear--to anyone with his vision undistorted--as going in any other direction, curved or straight. since, then, the design of the astronomer-artist is to show the earth to be a globe, and the points in the picture, which would only prove the earth to be cylindrical if true, are not true, it follows that the astronomer-artist fails to prove, pictorially, either that the earth is a globe or a cylinder, and that we have, therefore, a reasonable proof that the earth is not a globe. . it is a well-known fact that clouds are continually seen moving in all manner of directions--yes, and frequently, in different directions at the same time--from west to east being as frequent a direction as any other. now, if the earth were a globe, revolving through space from west to east at the rate of nineteen miles in a second, the clouds appearing to us to move towards the east would have to move quicker than nineteen miles in a second to be thus seen; whilst those which appear to be moving in the opposite direction would have no necessity to be moving at all, since the motion of the earth would be more than sufficient to cause the appearance. but it only takes a little common sense to show us that it is the clouds that move just as they appear to do, and that, therefore, the earth is motionless. we have, then, a proof that the earth is not a globe. . we read in the inspired book, or collection of books, called the bible, nothing at all about the earth being a globe or a planet, from beginning to end, but hundreds of allusions there are in its pages which could not be made if the earth were a globe, and which are, therefore, said by the astronomer to be absurd and contrary to what he knows to be true! this is the groundwork of modern infidelity. but, since every one of many, many allusions to the earth and the heavenly bodies in the scriptures can be demonstrated to be absolutely true to nature, and we read of the earth being "stretched out" "above the waters," as "standing in the water and out of the water," of its being "established that it cannot be moved," we have a store from which to take all the proofs we need, but we will just put down one proof--the scriptural proof--that earth is not a globe. . a "standing order" exists in the english houses of parliament that, in the cutting of canals, &c., the datum line employed shall be a "horizontal line, which shall be the same throughout the whole length of the work." now, if the earth were a globe, this "order" could not be carried out: but, it is carried out: therefore, it is a proof that the earth is not a globe. . it is a well-known and indisputable fact that there is a far greater accumulation of ice south of the equator than is to be found at an equal latitude north: and it is said that at kerguelen, degrees south, kinds of plants exist, whilst, in iceland, degrees nearer the northern centre, there are species; and, indeed, all the facts in the case show that the sun's power is less intense at places in the southern region than it is in corresponding latitudes north. now, on the newtonian hypothesis, all this is inexplicable, whilst it is strictly in accordance with the facts brought to light by the carrying out of the principles involved in the zetetic philosophy of "parallax." this is a proof that the earth is not a globe. . every year the sun is as long south of the equator as he is north; and if the earth were not "stretched out" as it is, in fact, but turned under, as the newtonian theory suggests, it would certainly get as intensive a share of the sun's rays south as north; but the southern region being, in consequence of the fact stated, far more extensive than the region north, the sun, having to complete his journey round every twenty-four hours, travels quicker as he goes further south, from september to december, and his influence has less time in which to accumulate at any given point. since, then, the facts could not be as they are if the earth were a globe, it is a proof that the earth is not a globe. . the aeronaut is able to start in his balloon and remain for hours in the air, at an elevation of several miles, and come down again in the same county or parish from which he ascended. now, unless the earth drag the balloon along with it in its nineteen-miles-a-second motion, it must be left far behind, in space: but, since balloons have never been known thus to be left, it is a proof that the earth, does not move, and, therefore, a proof that the earth is not a globe. . the newtonian theory of astronomy requires that the moon "borrow" her light from the sun. now, since the sun's rays are hot and the moon's light sends with it no heat at all, it follows that the sun and moon are "two great lights," as we somewhere read; that the newtonian theory is a mistake; and that, therefore, we have a proof that the earth is not a globe. . the sun and moon may often be seen high in the heavens at the same time--the sun rising in the east and the moon setting in the west--the sun's light positively putting the moon's light out by sheer contrast! if the accepted newtonian theory were correct, and the moon had her light from the sun, she ought to be getting more of it when face to face with that luminary--if it were possible for a sphere to act as a reflector all over its face! but as the moon's light pales before the rising sun, it is a proof that the theory fails; and this gives us a proof that the earth is not a globe. . the newtonian hypothesis involves the necessity of the sun, in the case of a lunar eclipse, being on the opposite side of a globular earth, to cast its shadow on the moon: but, since eclipses of the moon have taken place with both the sun and the moon above the horizon, it follows that it cannot be the shadow of the earth that eclipses the moon; that the theory is a blunder; and that it is nothing less than a proof that the earth is not a globe. . astronomers have never agreed amongst themselves about a rotating moon revolving round a rotating and revolving earth--this earth, moon, planets and their satellites all, at the same time dashing through space, around the rotating and revolving sun, towards the constellation hercules, at the rate of four millions of miles a day! and they never will: agreement is impossible! with the earth a plane and without motion, the whole thing is clear. and if a straw will show which way the wind blows, this may be taken as a pretty strong proof that the earth is not a globe. . mr. proctor says: "the sun is so far off that even moving from one side of the earth to the other does not cause him to be seen in a different direction--at least the difference is too small to be measured." now, since we know that north of the equator, say degrees, we see the sun at mid-day to the south, and that at the same distance south of the equator we see the sun at mid-day to the north, our very shadows on the ground cry aloud against the delusion of the day and give us a proof that earth is not a globe. . there is no problem more important to the astronomer than that of the sun's distance from the earth. every change in the estimate changes everything. now, since modern astronomers, in their estimates of this distance, have gone all the way along the line of figures from three millions of miles to a hundred and four millions--to-day, the distance being something over , , ; it matters not how much: for, not many years ago, mr. hind gave the distance, "accurately," as , , !--it follows that they don't know, and that it is foolish for anyone to expect that they ever will know, the sun's distance! and since all this speculation and absurdity is caused by the primary assumption that earth is a wandering, heavenly body, and is all swept away by a knowledge of the fact that earth is a plane, it is a clear proof that earth is not a globe. . it is plain that a theory of measurements without a measuring-rod is like a ship without a rudder; that a measure that is not fixed, not likely to be fixed, and never has been fixed, forms no measuring-rod at all; and that as modern theoretical astronomy depends upon the sun's distance from the earth as its measuring-rod, and the distance is not known, it is a system of measurements without a measuring-rod--a ship without a rudder. now, since it is not difficult to foresee the dashing of this thing upon the rock on which zetetic astronomy is founded, it is a proof that earth is not a globe. . it is commonly asserted that "the earth must be a globe because people have sailed round it." now, since this implies that we can sail round nothing unless it be a globe, and the fact is well known that we can sail round the earth as a plane, the assertion is ridiculous, and we have another proof that earth is not a globe. . it is a fact not so well known as it ought to be that when a ship, in sailing away from us, has reached the point at which her hull is lost to our unaided vision, a good telescope will restore to our view this portion of the vessel. now, since telescopes are not made to enable people to see through a "hill of water," it is clear that the hulls of ships are not behind a hill of water when they can be seen through a telescope though lost to our unaided vision. this is a proof that earth is not a globe. . mr. glaisher, in speaking of his balloon ascends, says: "the horizon always appeared on a level with the car." now, since we may search amongst the laws of optics in vain for any principle that would cause the surface of a globe to turn its face upwards instead of downwards, it is a clear proof that the earth is not a globe. . the rev. d. olmsted, in describing a diagram which is supposed to represent the earth as a globe, with a figure of a man sticking out at each side and one hanging head downwards, says: "we should dwell on this point until it appears to us as truly up,"--in the direction given to these figures as it does with regard to a figure which he has placed on the top! now, a system of philosophy which requires us to do something which is, really, the going out of our minds, by dwelling on an absurdity until we think it is a fact, cannot be a system based on god's truth, which never requires anything of the kind. since, then, the popular theoretical astronomy of the day requires this, it is evident that it is the wrong thing, and that this conclusion furnishes us with a proof that the earth is not a globe. . it is often said that the predictions of eclipses prove astronomers to be right in their theories. but it is not seen that this proves too much. it is well known that ptolemy predicted eclipses for six-hundred years, on the basis of a plane earth, with as much accuracy as they are predicted by modern observers. if, then, the predictions prove the truth of the particular theories current at the time, they just as well prove one side of the question as the other, and enable us to lay claim to a proof that the earth is not a globe. . seven-hundred miles is said to be the length of the great canal, in china. certain it is that, when this canal was formed, no "allowance" was made for "curvature." yet the canal is a fact without it. this is a chinese proof that the earth is not a globe. . mr. j. n. lockyer says: "because the sun seems to rise in the east and set in the west, the earth really spins in the opposite direction; that is, from west to east." now, this is no better than though we were to say--because a man seems to be coming up the street, the street really goes down to the man! and since true science would contain no such nonsense as this, it follows that the so-called science of theoretical astronomy is not true, and, therefore, we have a proof that the earth is not a globe. . mr. lockyer says: "the appearances connected with the rising and setting of the sun and stars may be due either to our earth being at rest and the sun and stars travelling round it, or the earth itself turning round, while the sun and stars are at rest." now, since true science does not allow of any such beggarly alternatives as these, it is plain that modern theoretical astronomy is not true science, and that its leading dogma is a fallacy. we have, then, a plain proof that the earth is not a globe. . mr. lockyer, in describing his picture of the supposed proof of the earth's rotundity by means of ships rounding a "hill of water," uses these words:--"diagram showing how, when we suppose the earth is round, we explain how it is that ships at sea appear as they do." this is utterly unworthy of the name of science! a science that begins by supposing, and ends by explaining the supposition, is, from beginning to end, a mere farce. the men who can do nothing better than amuse themselves in this way must be denounced as dreamers only, and their leading dogma a delusion. this is a proof that earth is not a globe. . the astronomers' theory of a globular earth necessitates the conclusion that, if we travel south of the equator, to see the north star is an impossibility. yet it is well known this star has been seen by navigators when they have been more than degrees south of the equator. this fact, like hundreds of other facts, puts the theory to shame, and gives us a proof that the earth is not a globe. . astronomers tell us that, in consequence of the earth's "rotundity," the perpendicular walls of buildings are, nowhere, parallel, and that even the walls of houses on opposite sides of a street are not strictly so! but, since all observation fails to find any evidence of this want of parallelism which theory demands, the idea must be renounced as being absurd and in opposition to all well-known facts. this is a proof that the earth is not a globe. . astronomers have made experiments with pendulums which have been suspended from the interior of high buildings, and have exulted over the idea of being able to prove the rotation of the earth on its "axis," by the varying direction taken by the pendulum over a prepared table underneath--asserting that the table moved round under the pendulum, instead of the pendulum shifting and oscillating in different directions over the table! but, since it has been found that, as often as not, the pendulum went round the wrong way for the "rotation" theory, chagrin has taken the place of exultation, and we have a proof of the failure of astronomers in their efforts to substantiate their theory, and, therefore, a proof that earth is not a globe. . as to the supposed "motion of the whole solar system in space," the astronomer royal of england once said: "the matter is left in a most delightful state of uncertainty, and i shall be very glad if anyone can help us out of it." but, since the whole newtonian scheme is, to-day, in a most deplorable state of uncertainty--for, whether the moon goes round the earth or the earth round the moon has, for years, been a matter of "raging" controversy--it follows that, root and branch, the whole thing, is wrong; and, all hot from the raging furnace of philosophical phrensy, we find a glowing proof that earth is not a globe. . considerably more than a million earths would be required to make up a body like the sun--the astronomers tell us: and more than , suns would be wanted to equal the cubic contents of the star vega. and vega is a "small star!" and there are countless millions of these stars! and it takes , , years for the light of some of these stars to reach us at , , miles in a minute! and, says mr. proctor, "i think a moderate estimate of the age of the earth would be , , years!" "its weight," says the same individual, "is , , , , , , , tons!" now, since no human being is able to comprehend these things, the giving of them to the world is an insult--an outrage. and though they have all arisen from the one assumption that earth is a planet, instead of upholding the assumption, they drag it down by the weight of their own absurdity, and leave it lying in the dust--a proof that earth is not a globe. . mr. j. r. young, in his work on navigation, says: "although the path of the ship is on a spherical surface, yet we may represent the length of the path by a straight line on a plane surface." (and plane sailing is the rule.) now, since it is altogether impossible to "represent" a curved line by a straight one, and absurd to make the attempt, it follows that a straight line represents a straight line and not a curved one. and, since it is the surface of the waters of the ocean that is being considered by mr. young, it follows that this surface is a straight surface, and we are indebted to mr. young, a professor of navigation, for a proof that the earth is not a globe. . "oh, but if the earth is a plane, we could go to the edge and tumble over!" is a very common assertion. this is a conclusion that is formed too hastily, and facts overthrow it. the earth certainly is, just what man by his observation finds it to be, and what mr. proctor himself says it "seems" to be--flat; and we cannot cross the icy barrier which surrounds it. this is a complete answer to the objection, and, of course, a proof that earth is not a globe. . "yes, but we can circumnavigate the south easily enough," is often said--by those who don't know. the british ship challenger recently completed the circuit of the southern region--indirectly, to be sure--but she was three years about it, and traversed nearly , miles--a stretch long enough to have taken her six times round on the globular hypothesis. this is a proof that earth is not a globe. . the remark is common enough that we can see the circle of the earth if we cross the ocean, and that this proves it to be round. now, if we tie a donkey to a stake on a level common, and he eats the grass all around him, it is only a circular disc that he has to do with, not a spherical mass. since, then, circular discs may be seen anywhere--as well from a balloon in the air as from the deck of a ship, or from the standpoint of the donkey, it is a proof that the surface of the earth is a plane surface, and, therefore, a proof that the earth is not a globe. . it is "supposed," in the regular course of the newtonian theory, that the earth is, in june, about millions of miles ( , , ) away from its position in december. now, since we can, (in middle north latitudes), see the north star, on looking out of a window that faces it--and out of the very same corner of the very same pane of glass in the very same window--all the year round, it is proof enough for any man in his senses that we have made no motion at all. it is a proof that the earth is not a globe. . newtonian philosophers teach us that the moon goes round the earth from west to east. but observation--man's most certain mode of gaining knowledge--shows us that the moon never ceases to move in the opposite direction--from east to west. since, then, we know that nothing can possibly move in two, opposite directions at the same time, it is a proof that the thing is a big blunder; and, in short, it is a proof that the earth is not a globe. . astronomers tell us that the moon goes round the earth in about days. well, we may see her making her journey round, every day, if we make use of our eyes--and these are about the best things we have to use. the moon falls behind in her daily motion as compared with that of the sun to the extent of one revolution in the time specified; but that is not making a revolution. failing to go as fast as other bodies go in one direction does not constitute a going round in the opposite one--as the astronomers would have us believe! and, since all this absurdity has been rendered necessary for no other purpose than to help other absurdities along, it is clear that the astronomers are on the wrong track; and it needs no long train of reasoning to show that we have a proof that the earth is not a globe. . it has been shown that meridians are, necessarily, straight lines; and that it is impossible to travel round the earth in a north or south direction: from which it follows that, in the general acceptation of the word "degree,"--the th part of a circle--meridians have no degrees: for no one knows anything of a meridian circle or semicircle, to be thus divided. but astronomers speak of degrees of latitude in the same sense as those of longitude. this, then, is done by assuming that to be true which is not true. zetetic philosophy does not involve this necessity. this proves that the basis of this philosophy is a sound one, and, in short, is a proof that the earth is not a globe. . if we move away from an elevated object on or over a plain or a prairie, the height of the object will apparently diminish as we do so. now, that which is sufficient to produce this effect on a small scale is sufficient on a large one; and travelling away from an elevated object, no matter how high, over a level surface, no matter how far, will cause the appearance in question--the lowering of the object. our modern theoretical astronomers, however, in the case of the apparent lowering of the north star as we travel southward, assert that it is evidence that the earth is globular! but, as it is clear that an appearance which is fully accounted for on the basis of known facts cannot be permitted to figure as evidence in favor of that which is only a supposition, it follows that we rightfully order it to stand down, and make way for a proof that the earth is not a globe. . there are rivers which flow east, west, north, and south--that is, rivers are flowing in all directions over the earth's surface, and at the same time. now, if the earth were a globe, some of these rivers would be flowing up-hill and others down, taking it for a fact that there really is an "up" and a "down" in nature, whatever form she assumes. but, since rivers do not flow up-hill, and the globular theory requires that they should, it is a proof that the earth is not a globe. . if the earth were a globe, rolling and dashing through "space" at the rate of "a hundred miles in five seconds of time," the waters of seas and oceans could not, by any known law, be kept on its surface--the assertion that they could be retained under these circumstances being an outrage upon human understanding and credulity! but as the earth--that is, the habitable world of dry land--is found to be "standing out of the water and in the water" of the "mighty deep," whose circumferential boundary is ice, we may throw the statement back into the teeth of those who make it and flaunt before their faces the flag of reason and common sense, inscribed with--a proof that the earth is not a globe. . the theory of a rotating and revolving earth demands a theory to keep the water on its surface; but, as the theory which is given for this purpose is as much opposed to all human experience as the one which it is intended to uphold, it is an illustration of the miserable makeshifts to which astronomers are compelled to resort, and affords a proof that the earth is not a globe. . if we could--after our minds had once been opened to the light of truth--conceive of a globular body on the surface of which human beings could exist, the power--no matter by what name it be called--that would hold them on would, then, necessarily, have to be so constraining and cogent that they could not live; the waters of the oceans would have to be as a solid mass, for motion would be impossible. but we not only exist, but live and move; and the water of the ocean skips and dances like a thing of life and beauty! this is a proof that the earth is not a globe. . it is well known that the law regulating the apparent decrease in the size of objects as we leave them in the distance (or as they leave us) is very different with luminous bodies from what it is in the case of those which are non-luminous. sail past the light of a small lamp in a row-boat on a dark night, and it will seem to be no smaller when a mile off than it was when close to it. proctor says, in speaking of the sun: "his apparent size does not change,"--far off or near. and then he forgets the fact! mr. proctor tells us, subsequently, that, if the traveller goes so far south that the north star appears on the horizon, "the sun should therefore look much larger"--if the earth were a plane! therefore, he argues, "the path followed cannot have been the straight course,"--but a curved one. now, since it is nothing but common scientific trickery to bring forward, as an objection to stand in the way of a plane earth, the non-appearance of a thing which has never been known to appear at all, it follows that, unless that which appears to be trickery were an accident, it was the only course open to the objector--to trick. (mr. proctor, in a letter to the "english mechanic" for oct. , , boasts of having turned a recent convert to the zetetic philosophy by telling him that his arguments were all very good, but that "it seems as though [mark the language!] the sun ought to look nine times larger in summer." and mr. proctor concludes thus: "he saw, indeed, that, in his faith in 'parallax,' he had 'written himself down an ass.'") well, then: trickery or no trickery on the part of the objector, the objection is a counterfeit--a fraud--no valid objection at all; and it follows that the system which does not purge itself of these things is a rotten system, and the system which its advocates, with mr. proctor at their head, would crush if they could find a weapon to use--the zetetic philosophy of "parallax"--is destined to live! this is a proof that the earth is not a globe. . "is water level, or is it not?" was a question once asked of an astronomer. "practically, yes; theoretically, no," was the reply. now, when theory does not harmonize with practice, the best thing to do is to drop the theory. (it is getting too late, now, to say "so much the worse for the facts!") to drop the theory which supposes a curved surface to standing water is to acknowledge the facts which form the basis of zetetic philosophy. and since this will have to be done--sooner or later,--it is a proof that the earth is not a globe. . "by actual observation," says schoedler, in his "book of nature," "we know that the other heavenly bodies are spherical, hence we unhesitatingly assert that the earth is so also." this is a fair sample of all astronomical reasoning. when a thing is classed amongst "other" things, the likeness between them must first be proven. it does not take a schoedler to tell us that "heavenly bodies" are spherical, but "the greatest astronomer of the age" will not, now, dare to tell us that the earth is--and attempt to prove it. now, since no likeness has ever been proven to exist between the earth and the heavenly bodies, the classification of the earth with the heavenly bodies is premature--unscientific--false! this is a proof that earth is not a globe. . "there is no inconsistency in supposing that the earth does move round the sun," says the astronomer royal of england. certainly not, when theoretical astronomy is all supposition together! the inconsistency is in teaching the world that the thing supposed is a fact. since, then, the "motion" of the earth is supposition only--since, indeed, it is necessary to suppose it at all--it is plain that it is a fiction and not a fact; and, since "mobility" and "sphericity" stand or fall together, we have before us a proof that earth is not a globe. . we have seen that astronomers--to give us a level surface on which to live--have cut off one-half of the "globe" in a certain picture in their books. [see page .] now, astronomers having done this, one-half of the substance of their "spherical theory" is given up! since, then, the theory must stand or fall in its entirety, it has really fallen when the half is gone. nothing remains, then, but a plane earth, which is, of course, a proof that the earth is not a globe. . in "cornell's geography" there is an "illustrated proof of the form of the earth." a curved line on which is represented a ship in four positions, as she sails away from an observer, is an arc of degrees, or one-fifth of the supposed circumference of the "globe"--about , miles. ten such ships as those which are given in the picture would reach the full length of the "arc," making miles as the length of the ship. the man, in the picture, who is watching the ship as she sails away, is about miles high; and the tower, from which he takes an elevated view, at least miles high. these are the proportions, then, of men, towers, and ships which are necessary in order to see a ship, in her different positions, as she "rounds the curve" of the "great hill of water" over which she is supposed to be sailing: for, it must be remembered that this supposed "proof" depends upon lines and angles of vision which, if enlarged, would still retain their characteristics. now, since ships are not built miles long, with masts in proportion, and men are not quite miles high, it is not what it is said to be--a proof of rotundity--but, either an ignorant farce or a cruel piece of deception. in short, it is a proof that the earth is not a globe. . in "cornell's intermediate geography," ( ) page , is an "illustration of the natural divisions of land and water." this illustration is so nicely drawn that it affords, at once, a striking proof that earth is a plane. it is true to nature, and bears the stamp of no astronomer-artist. it is a pictorial proof that earth is not a globe. . if we refer to the diagram in "cornell's geography," page , and notice the ship in its position the most remote from the observer, we shall find that, though it is about , miles away, it is the same size as the ship that is nearest to him, distant about miles! this is an illustration of the way in which astronomers ignore the laws of perspective. this course is necessary, or they would be compelled to lay bare the fallacy of their dogmas. in short, there is, in this matter, a proof that the earth is not a globe. . mr. hind, the english astronomer, says: "the simplicity with which the seasons are explained by the revolution of the earth in her orbit and the obliquity of the ecliptic, may certainly be adduced as a strong presumptive proof of the correctness"--of the newtonian theory; "for on no other rational suppositions with respect to the relations of the earth and sun, can these and other as well-known phenomena, be accounted for." but, as true philosophy has no "suppositions" at all--and has nothing to do with "suppositions"--and the phenomena spoken of are thoroughly explained by facts, the "presumptive proof" falls to the ground, covered with the ridicule it so richly deserves; and out of the dust of mr. hind's "rational suppositions" we see standing before us a proof that earth is not a globe. . mr. hind speaks of the astronomer watching a star as it is "carried across the telescope by the diurnal revolution of the earth." now, this is nothing but downright absurdity. no motion of the earth could possibly carry a star across a telescope or anything else. if the star is carried across anything at all, it is the star that moves, not the thing across which it is carried! besides, the idea that the earth, if it were a globe, could possibly move in an orbit of nearly , , of miles with such exactitude that the cross-hairs in a telescope fixed on its surface would appear to glide gently over a star "millions of millions" of miles away is simply monstrous; whereas, with a fixed telescope, it matters not the distance of the stars, though we suppose them to be as far off as the astronomer supposes them to be; for, as mr. proctor himself says, "the further away they are, the less they will seem to shift." why, in the name of common sense, should observers have to fix their telescopes on solid stone bases so that they should not move a hair's-breadth, if the earth on which they fix them move at the rate of nineteen miles in a second? indeed, to believe that mr. proctor's mass of "six thousand million million million tons" is "rolling, surging, flying, darting on through space for ever" with a velocity compared with which a shot from a cannon is a "very slow coach," with such unerring accuracy that a telescope fixed on granite pillars in an observatory will not enable a lynx-eyed astronomer to detect a variation in its onward motion of the thousandth part of a hair's-breadth is to conceive a miracle compared with which all the miracles on record put together would sink into utter insignificance. captain r. j. morrison, the late compiler of "zadkeil's almanac," says: "we declare that this 'motion' is all mere 'bosh'; and that the arguments which uphold it are, when examined with an eye that seeks for truth only, mere nonsense, and childish absurdity." since, then, these absurd theories are of no use to men in their senses, and since there is no necessity for anything of the kind in zetetic philosophy, it is a "strong presumptive proof"--as mr. hind would say--that the zetetic philosophy is true, and, therefore, a proof that earth is not a globe. . mr. hind speaks of two great mathematicians differing only fifty-five yards in their estimate of the earth's diameter. why, sir john herschel, in his celebrated work, cuts off miles of the same thing to get "round numbers!" this is like splitting a hair on one side of the head and shaving all the hair off on the other! oh, "science!" can there be any truth in a science like this? all the exactitude in astronomy is in practical astronomy--not theoretical. centuries of observation have made practical astronomy a noble art and science, based--as we have a thousand times proved it to be--on a fixed earth; and we denounce this pretended exactitude on one side and the reckless indifference to figures on the other as the basest trash, and take from it a proof that the "science" which tolerates it is a false--instead of being an "exact"--science, and we have a proof that the earth is not a globe. . the sun, as he travels round over the surface of the earth, brings "noon" to all places on the successive meridians which he crosses: his journey being made in a westerly direction, places east of the sun's position have had their noon, whilst places to the west of the sun's position have still to get it. therefore, if we travel easterly, we arrive at those parts of the earth where "time" is more advanced, the watch in our pocket has to be "put on," or we may be said to "gain time." if, on the other hand, we travel westerly, we arrive at places where it is still "morning," the watch has to be "put back," and it may be said that we "lose time." but, if we travel easterly so as to cross the th meridian, there is a loss, there, of a day, which will neutralize the gain of a whole circumnavigation; and, if we travel westerly, and cross the same meridian, we experience the gain of a day, which will compensate for the loss during a complete circumnavigation in that direction. the fact of losing or gaining time in sailing round the world, then, instead of being evidence of the earth's "rotundity," as it is imagined to be, is, in its practical exemplification, an everlasting proof that the earth is not a globe. "and what then?" what then! no intelligent man will ask the question; and he who may be called an intellectual man will know that the demonstration of the fact that the earth is not a globe is the grandest snapping of the chains of slavery that ever took place in the world of literature or science. the floodgates of human knowledge are opened afresh and an impetus is given to investigation and discovery where all was stagnation, bewilderment and dreams! is it nothing to know that infidelity cannot stand against the mighty rush of the living water of truth that must flow on and on until the world shall look "up" once more "to him that stretched out the earth above the waters"--"to him that made great lights:--the sun to rule by day--the moon and stars to rule by night?" is it nothing to know and to feel that the heavenly bodies were made for man, and that the monstrous dogma of an infinity of worlds is overthrown for ever? the old-time english "family herald," for july , , says, in its editorial, that "the earth's revolution on its own axis was denied, against galileo and copernicus, by the whole weight of the church of rome." and, in an article on "the pride of ignorance," too!--the editor not knowing that if the earth had an axis to call its "own"--which the church well knew it had not, and, therefore, could not admit--it would not "revolve" on it; and that the theoretical motion on an axis is that of rotation, and not revolution! is it nothing to know that "the whole weight of the church of rome" was thrown in the right direction, although it has swayed back again like a gigantic pendulum that will regain its old position before long? is it nothing to know that the "pride of ignorance" is on the other side? is it nothing to know that, with all the bradlaughs and ingersolls of the world telling us to the contrary--biblical science is true? is it nothing to know that we are living on a body at rest, and not upon a heavenly body whirling and dashing through space in every conceivable way and with a velocity utterly inconceivable? is it nothing to know that we can look stedfastly up to heaven instead of having no heaven to look up to at all? is it nothing, indeed, to be in the broad daylight of truth and to be able to go on towards a possible perfection, instead of being wrapped in the darkness of error on the rough ocean of life, and finding ourselves stranded at last--god alone knows where? baltimore, maryland, u. s. a., august, . appendix to the second edition. the following letters remain unanswered, at the time of going to press, december , :-- " chew street, baltimore, nov. , . r. a. proctor, esq., st. joe, mo. sir: i have sent you two copies of my 'one hundred proofs that the earth is not a globe,' and, as several weeks have since elapsed and i have not heard from you, i write to inform you that if you have any remarks to make concerning that publication, and will let me have them in the course of a week or ten days, i will print them--if you say what you may wish to say in about five or six hundred words--in the second edition of the pamphlet, which will very soon be called for. allow me to say that, as this work is not only 'dedicated' to you but attacks your teachings, the public will be looking for something from your pen very shortly. i hope they may not be disappointed. yours in the cause of truth, w. carpenter." " chew street, baltimore, nov. , . spencer f. baird, esq., secretary of the smithsonian institution, washington, d. c. sir:--i had the pleasure, several weeks ago, of sending you my 'one hundred proofs that the earth is not a globe.' i hope you received them. a second edition is now called for, and i should esteem it a favor if you would write me a few words concerning them that i may print with this forthcoming edition as an appendix to them. if you think any of the 'hundred proofs' are unsound, i will print all you may have to say about them, if not over words, as above stated. i have made richard a. proctor, esq., a similar offer, giving him, of course, a little more space. i feel sure that the very great importance of this matter will prompt you to give it your immediate attention. i have the honor to be, sir, yours sincerely, wm. carpenter." copies of the first edition of this pamphlet have been sent to the leading newspapers of this country and of england, and to very many of the most renowned scientific men of the two countries--from the astronomer royal, of england, to dr. gilman, of johns hopkins university, baltimore. several copies have been sent to graduates of different universities, on application, in consequence of the subjoined advertisement, which has appeared in several newspapers:-- "wanted.--a scholar of ripe attainments to review carpenter's 'one hundred proofs that the earth is not a globe.' liberal remuneration offered. apply to wm. carpenter, chew street, baltimore. n. b.--no one need apply who has not courage enough to append his name to the review for publication." we should be pleased to hear from some of the gentlemen in time for the insertion of their courageous attacks in the third edition! opinions of the press. "this can only be described as an extraordinary book.... his arguments are certainly plausible and ingenious, and even the reader who does not agree with him will find a singular interest and fascination in analyzing the 'one hundred proofs.'... the proofs are set forth in brief, forcible, compact, very clear paragraphs, the meaning of which can be comprehended at a glance."--daily news, sept. . "throughout the entire work there are discernible traces of a strong and reliant mind, and such reliance as can only have been acquired by unbiassed observation, laborious investigation, and final conviction; and the masterly handling of so profound a theme displays evidence of grave and active researches. there is no groping wildly about in the vagueness of theoretical speculations, no empty hypotheses inflated with baseless assertions and false illustrations, but the practical and perspicuous conclusions of a mind emancipated from the prevailing influences of fashionable credence and popular prejudice, and subordinate only to those principles emanating from reason and common sense."--h. d. t., woodberry news, sept. , . "we do not profess to be able to overthrow any of his 'proofs.' and we must admit, and our readers will be inclined to do the same, that it is certainly a strange thing that mr. wm. carpenter, or anyone else, should be able to bring together 'one hundred proofs' of anything in the world if that thing is not right, while we keep on asking for one proof, that is really a satisfactory one, on the other side. if these 'hundred proofs' are nonsense, we cannot prove them to be so, and some of our scientific men had better try their hands, and we think they will try their heads pretty badly into the bargain."--the woodberry news, baltimore, sept. , . "this is a remarkable pamphlet. the author has the courage of his convictions, and presents them with no little ingenuity, however musty they may appear to nineteenth century readers. he takes for his text a statement of prof. proctor's that 'the earth on which we live and move seems to be flat,' and proceeds with great alacrity to marshal his hundred arguments in proof that it not only seems but is flat, 'an extended plane, stretched out in all directions away from the central north.' he enumerates all the reasons offered by scientists for a belief in the rotundity of the earth and evidently to his own complete satisfaction refutes them. he argues that the heavenly bodies were made solely to light this world, that the belief in an infinity of worlds is a monstrous dogma, contrary to bible teaching, and the great stronghold of the infidel; and that the church of rome was right when it threw the whole weight of its influence against galileo and copernicus when they taught the revolution of the earth on its axis."--michigan christian herald, oct. , . "so many proofs."--every saturday, sept. , . "a highly instructive and very entertaining work.... the book is well worth reading."--protector, baltimore, oct. , . "the book will be sought after and read with peculiar interest."--baltimore labor free press, oct. , . "some of them [the proofs] are of sufficient force to demand an answer from the advocates of the popular theory."--baltimore episcopal methodist, october , . "showing considerable smartness both in conception and argument."--western christian advocate, cincinnati, o., oct. , . "forcible and striking in the extreme."--brooklyn market journal. baltimore, maryland, u. s. a., december , . [appendix to third edition.] copy of letter from richard a. proctor, esq. montague street, russell square, london, w.c., dec., . w. carpenter, esq., baltimore. dear sir,--i am obliged to you for the copy of your "one hundred proofs that the earth is not a globe," and for the evident kindness of your intention in dedicating the work to me. the only further remark it occurs to me to offer is that i call myself rather a student of astronomy than an astronomer. yours faithfully, richard a. proctor. p.s. perhaps the pamphlet might more precisely be called "one hundred difficulties for young students of astronomy." [appendix to fourth edition.] copy of letter from spencer f. baird, esq. smithsonian institution, washington, d. c., jan. , . dear sir,--a copy of your "one hundred proofs that the earth is not a globe" was duly received, and was deposited in library of congress october , . [ ] a pressure of much more important work has prevented any attempt at reviewing these hundred proofs:--which however have doubtless been thoroughly investigated by the inquisitive astronomers and geodesists of the last four centuries. yours very respectfully, spencer f. baird, secretary s. i. mr. william carpenter, , chew street, baltimore, md. copy of a letter from one of the several applicants for the "one hundred proofs" for the purpose of reviewing them. the writer is professor of mathematics at the high school, auburn, n. y., and, in his application for the pamphlet, says: "am a yale graduate and a yale law school man: took the john a. porter prize (literary) ($ ) at yale college." auburn, dec. th, . my dear sir: your treatise was received. i have looked it over and noted it somewhat. a review of it to do it justice would be a somewhat long and laborious task. before i undertook so much thought i would write and ask what and how much you expect: how elaborately you wished it discussed: and what remuneration might be expected. it sets forth many new and strange doctrines which would have to be thoroughly discussed and mastered before reviewed. i am hard at work at present but would like to tackle this if it would be for my interest as well as yours. hope you will let me know very soon. very respectfully, to mr. w. carpenter, baltimore, md. frank strong. note.--unless a man be willing to sell his soul for his supposed worldly "interest," he will not dare to "tackle" the "one hundred proofs that the earth is not a globe." no man with well-balanced faculties will thus condemn himself. we charge the mathematicians of the world that, if they cannot say what they think of this pamphlet in a dozen words, they are entitled to no other name than--cowards! baltimore, maryland, may , . appendix to the fifth edition. editorial from the "new york world," of august , :-- the earth is flat. the iconoclastic tendencies of the age have received new impetus from mr. william carpenter, who comes forward with one hundred proofs that the earth is not a globe. it will be a sad shock to many conservatives who have since their childhood fondly held to the conviction that "the earth is round like an orange, a little flattened at the poles." to find that, after all, we have been living all these years on a prosaic and unromantic plane is far from satisfactory. we have rather gloried in the belief that the semi-barbarous nations on the other side of the earth did not carry their heads in the same direction in which ours point. it is hard to accept the assertion that the cannibals on savage islands are walking about on the same level with the civilized nations of our little world. but mr. carpenter has one hundred proofs that such is the unsatisfactory truth. not only that, but the iconoclast claims that we are not whirling through space at a terrible rate, but are absolutely stationary. some probability is given to this proposition by the present hot weather. the earth seems to be becalmed. if it were moving at the rate of nineteen miles a second wouldn't there be a breeze? this question is thrown out as perhaps offering the one hundred and first proof that the earth is not a globe. mr. carpenter may obtain the proof in detail at the office at our usual rates. a revolution will, of course, take place in the school geographies as soon as mr. carpenter's theories have been closely studied. no longer will the little boy answer the question as to the shape of the earth by the answer which has come ringing down the ages, "it's round like a ball, sir." no. he'll have to use the unpoetic formula, "it's flat like a pancake, sir." but, perhaps, after we have become used to the new idea it will not be unpleasant. the ancients flourished in the belief that the earth was a great plane. why shouldn't we be equally fortunate? it may be romantic but it is not especially comforting to think that the earth is rushing through space twisting and curving like a gigantic ball delivered from the hand of an enormous pitcher. something in the universe might make a base hit if we kept on and we would be knocked over an aerial fence and never found. perhaps, after all, it is safer to live on mr. carpenter's stationary plane. the "record," of philadelphia, june , , has the following, in the literary notes:--"under the title one hundred proofs that the earth is not a globe, mr. william carpenter, of baltimore, publishes a pamphlet which is interesting on account of the originality of the views advanced, and, from his standpoint, the very logical manner in which he seeks to establish their truth. mr. carpenter is a disciple of what is called the zetetic school of philosophy, and was referee for mr. john hampden when that gentleman, in , made a wager with mr. alfred r. wallace, of england, that the surface of standing water is always level, and therefore that the earth is flat. since then he has combated his views with much earnestness, both in writing and on the platform, and, whatever opinions we may have on the subject, a perusal of his little book will prove interesting and afford room for careful study." "the motto which he puts on the cover--'upright, downright, straightforward'--is well chosen, for it is an upright lie, a downright invention, and a straightforward butt of a bull at a locomotive."--the florida times union, dec. , . editor, charles h. jones. [pray, mr. jones, tell us what you mean by "an upright lie."!!] "we have received a pamphlet from a gentleman who thinks to prove that the earth is flat, but who succeeds only in showing that he is himself one."--new york herald, dec. , . [the reviewer, in this case, is, no doubt, a very "sharp" man, but his honesty--if he have any at all--is jagged and worn out. the "quotations" which he gives are fraudulent, there being nothing like them in the pamphlet.] "the author of the pamphlet is no 'flat,' though he may perhaps be called a 'crank.'"--st. catharines (can.) evening jour., dec. . "to say that the contents of the book are erudite and entertaining does not do mr. carpenter's astronomical ability half credit."--the sunday truth, buffalo, dec. , . "the entire work is very ingeniously gotten up.... the matter of perspective is treated in a very clever manner, and the coming up of 'hull-down' vessels on the horizon is illustrated by several well-worded examples."--buffalo times, dec. , . "the erudite author, who travels armed with plans and specifications to fire at the skeptical at a moment's notice, feels that he is doing a good work, and that his hundred anti-globular conclusions must certainly knock the general belief in territorial rotundity out of time."... "we trust that the distinguished author who has failed to coax richard proctor into a public discussion may find as many citizens willing to invest two shillings in his peculiar literature as he deserves."--buffalo courier, dec. , , and jan. , . "it is a pleasure now to see a man of mr. carpenter's attainments fall into line and take up the cudgels against the theories of the scientists who have taught this pernicious doctrine [the sphericity of the earth]."--rochester morning herald, jan. , . "as the game stands now, there is 'one horse' for prof. carpenter."--buffalo world, jan. , . "it is interesting to show how much can be said in favor of the flat world theory.... it is fairly well written, although, we believe filled with misstatements of facts."--rochester democrat and chronicle, jan. , . [we "believe" the editor cannot point one out.] "it is certainly worth twice the price, and will be read by all with peculiar interest."--scranton truth, march , . "mr. william carpenter has come to washington with a "hundred proofs that the earth is not a globe." he has a pamphlet on the subject which is ingenious, to say the least, and he is ominously eager to discuss the matter with any one who still clings to the absurd prejudices of the astronomers."--the hatchet, may , . "it contains some curious problems for solution, and the author boldly asserts that until they are solved the globular theory of the earth remains unproven, and is fallacious, &c."--the presbyterian, philadelphia, june , . "his reasoning is, to say the least, plausible, and the book interesting."--the item, philadelphia, june , . "mr. carpenter seems to have made a thorough investigation of the subject, and his arguments are practical and to the point."--sunday mercury, philadelphia, june , . "a gentleman has just called at the editorial rooms with a pamphlet which is designed to demonstrate that the earth is not a globe, but a flat disk; he also laid before us a chart from which it plainly appeared that the earth is a circular expanse of land, with the north pole in the exact center, and the antarctic sea flowing all around the land.... we went on to state that we lodged the care of all astronomical questions in the hands of rev. r. m. luther, to whom these perplexing matters are but as child's play.... our readers may, therefore, expect at an early date a judicial view of the astronomical and cosmological situation."--national baptist, philadelphia, july , . editor, dr. wayland. [we hope that the rev. r. m. luther will give us the means of publishing his decision before many more editions of the "hundred proofs" be issued. we are afraid that he finds the business much more than "child's play."] "'one hundred proofs that the earth is not a globe,' by william carpenter, is published by the author, whose novel and rather startling position is certainly fortified by a number of argumentative points, which, if they do not shake the reader's preconceived notions on the subject, will, at least, be found entertaining for the style in which they are put."--evening star, philadelphia, july , . "his 'proofs' go a long way towards convincing many that his ideas on the subject are practical and sensible."--fashion journal, philadelphia, july, . editor, mrs. f. e. benedict. "'one hundred proofs that the earth is not a globe' is a curious little pamphlet that we can commend to all interested in astronomy and related sciences. it may not upset received notions on the subject, but will give cause for much serious reflection. published by the author, wm. carpenter, baltimore, md. price cents."--the saturday evening post, philadelphia, july , . "here now is an able thinker of baltimore, professor william carpenter, who presents the claims of the zetetic philosophy to be considered the leading issue of our times.... one of the great proofs of the truth of the philosophy is that the regular astronomers do not dare to gainsay it.... they are well aware there is no south pole.... prof. carpenter, in a treatise that has reached us, furnishes proofs that the earth is flat, and while we cannot say that we understand all of them we appreciate the earnestness of his appeals to the moral people of the community to rise up and overthrow the miserable system of error that is being forced upon our children in the public schools, vitiating the very foundations of knowledge. what issue can be more noble or inspiring than truth vs. error? here is an issue on which there can be no trifling or compromise. in the great contest between those who hold the earth is flat and they who contend that it is round, let the flats assert themselves."--milwaukee sentinel, aug., . [from a long article, "the great zetetic issue."] letters to professor gilman, of the johns hopkins university. chew street, baltimore, september , . prof. gilman, johns hopkins university--sir: on the st ultimo i wrote to ask you if you received the pamphlet, which i left for you at the university twelve months ago, entitled "one hundred proofs that the earth is not a globe," and, if so, that you would kindly give me your opinion concerning it. i write, now, to ask you if you received my letter. i am quite sure that you will consider that the importance of the subject fully warrants the endeavor on my part to gain the views which may be entertained by you respecting it. the fifth edition will soon be called for, and anything you may urge--for or against--i shall be happy to insert in the "appendix." i send, herewith, a copy of the fourth edition of the pamphlet. yours sincerely, william carpenter. chew street, baltimore, october , . professor gilman--dear sir: i am now preparing the appendix for the fifth edition of my "one hundred proofs that the earth is not a globe," and i should be glad to receive your opinion of this work to insert in the said appendix. i can offer you from a few lines to a page, or two if necessary. of course, if this work as a whole be a fraud, it must be fraudulent in all its parts; and each one of the "hundred proofs" must contain a fallacy of some kind or other, and the thing would justify your disapprobation--expressed in few words or many. if, on the other hand, the work is what it professes to be, it will certainly claim your approval. yours sincerely, w. carpenter. chew street, baltimore, october , . prof. gilman--dear sir: a week ago i wrote you a letter to tell you that i should be glad to receive your opinion of the "hundred proofs that the earth is not a globe," of which work , copies are now in circulation. i wrote this work ( pages) in one week, without neglecting my daily business: surely, you can reply to it in a week from this time. i will give you from one to four pages, if you wish that amount of space, and send you fifty copies, if you desire to have them, without putting you to the slightest expense. i will even take any suggestion you please to make as to the title which shall be given to this extra edition of my work containing your reply or opinions. i should be sorry to be under the necessity of printing this letter, with others, in my next edition, in the place of any such reply or expression of opinion; for i feel sure there is no one in baltimore who is more capable of giving an opinion on this great subject. trusting to hear from you in a few days, i am, dear sir, yours truly, william carpenter. chew street, baltimore, october , . prof. gilman--sir: this is the fifth letter--and the last--to you, asking you for an expression of your opinion concerning the "one hundred proofs that the earth is not a globe." which would you prefer--to see my words, or yours, in print? i give you a week in which to decide. truly, william carpenter. the johns hopkins university, of baltimore. we are indebted to "scribner's monthly" for the following remarks concerning this institution:--"by the will of johns hopkins, a merchant of baltimore, the sum of $ , , was devoted to the endowment of a university and a hospital, $ , , being devoted to each. this is the largest single endowment ever made to an institution of learning in this country. to the bequest no burdensome conditions were attached."... "the physiological laboratory of the johns hopkins has no peer in this country, and the other laboratories few equals and no superiors." in the first annual report of the university ( ) we read:--"early in the month of february, , the trustees of the university having been apprised by the executors of johns hopkins, of the endowment provided by his will, took proper steps for organization and entering upon the practical duties of the trust, and addressed themselves to the selection of a president of the university. with this view the trustees sought the counsel and advice of the heads of several of the leading seats of learning in the country, and, upon unanimous recommendation and endorsement from these sources, the choice fell upon mr. daniel c. gilman, who, at the time, occupied the position of president of the university of california. "mr. gilman is a graduate of yale college, and for several years before his call to california, was a professor in that institution, taking an active part in the organization and development of 'the sheffield scientific school of yale college,' at new haven. upon receiving an invitation to baltimore, he resigned the office which he had held in california since , and entered upon the service of the johns hopkins university, may , ."--galloway cheston. "in the hunt for truth, we are not first hunters, and then men; we are first and always men, then hunters."--d. c. gilman, oct., . the "one hundred proofs that the earth is not a globe" have been running around within the observation of the master huntsman and his men for a year or more: now let the hunters prove themselves to be men; and the men, hunters. it is impossible to be successful hunters for truth, if error be allowed to go scot-free. nay, it is utterly impossible for the johns hopkins university to answer the purpose of its founder if its hunters for truth do not first hunt error with their hounds and hold it up to ridicule, and then, and always, keep a watchful eye for the truth lest they should injure it by their hot haste or wound it with their weapons. prof. daniel c. gilman, we charge you that the duties of your office render it imperative that, sooner or later, you lead your men into the field against the hundred proofs, to show the world that they are hunters worthy of the name--if, in your superior judgment, you decide that there is error to be slain--or, show that your hunters are worthy of the better name of men, by inducing them to follow and sustain you, out of the beaten track, in your endeavors to uphold god's truth, if, in your superior judgment, you tell them, "there is a truth to be upheld!" [end of the appendix to the fifth edition. nov. , .] professor proctor's proofs. "a proof, a proof!" cries student brown; says proctor, "very well, if that is all you want, indeed, i've plenty i can tell: but really i have scarcely time, or patience, now, to do it; you ought to know the earth's a globe, then, as a globe you'd view it. i knew it long ago: in truth, 'twas taught me in my cot, and, then, too old was i to doubt--too young to say 'twas not!" "and you have never questioned it?" "why should i, now, friend brown? i took it all for granted, just as daddy laid it down. and as my duty clearly was,--no other way i saw it-- and that's the reason why, of course, a globe i always draw it. and so you want a proof! ah ha: just cross the broad atlantic, and then a proof so strong you'll have, with joy 'twill send you [frantic!" "you mean, that i shall see the ships come round the old earth's side-- and up--and o'er the 'watery hill'--as into view they glide! no, proctor, no: you say, yourself, the earth so vast in size is, the surface seems a level one--indeed, to sight, it rises. and ships, when coming into view, seem 'bearing down upon us.' no, proctor, let us have a proof--no, no, come--mercy on us!" "well, brown, i've proofs that serve to show that earth, indeed, [a ball 'tis; but if you won't believe them--well, not mine but yours the fault is. why, everybody, surely, knows a planet must be round, and, since the earth a planet is, its shape at once is found. we know it travels round the sun, a thousand miles a minute, and, therefore, it must be a globe: a flat earth couldn't spin it. we know it on its axis turns with motion unperceived; and therefore, surely, plain it is, its shape must be believed. we know its weight put down in tons exactly as we weigh'd it; and, therefore, what could clearer be, if we ourselves had made it? we know its age--can figures lie?--its size--its weight--its motion; and then to say, ''tis all my eye,' shows madness in the notion. besides, the other worlds and suns--some cooling down--some hot!-- how can you say, you want a proof, with all these in the pot? no, brown: just let us go ahead; don't interfere at all; some other day i'll come and bring proof that earth's a ball!" "no, proctor, no:" said mr. brown; "'tis now too late to try it:-- a hundred proofs are now put down (and you cannot deny it) that earth is not a globe at all, and does not move through space: and your philosophy i call a shame and a disgrace. we have to interfere, and do the best that we are able to crush your theories and to lay the facts upon the table. god's truth is what the people need, and men will strive to preach it; and all your efforts are in vain, though you should dare impeach it. you've given half your theory up; the people have to know it:-- you smile, but, then, your book's enough: for that will plainly [show it. one-half your theory's gone, and, soon, the other half goes, too: so, better turn about, at once, and show what you can do. own up (as people have to do, when they have been deceived), and help the searcher after truth of doubt to be relieved. 'the only amaranthine flower is virtue;'--don't forget it-- 'the only lasting treasure, truth:'--and never strive to let it." odds and ends. "we do not possess a single evident proof in favor of the rotation"--of the earth--"around its axis."--dr. shoepfer. "to prove the impossibility of the revolution of the earth around the sun, will present no difficulty. we can bring self-evident proof to the contrary."--dr. shoepfer. "to reform and not to chastise, i am afraid is impossible.... to attack views in the abstract without touching persons may be safe fighting, indeed, but it is fighting with shadows."--pope. "both revelation and science agree as to the shape of the earth. the psalmist calls it the 'round world,' even when it was universally supposed to be a flat extended plain."--rev. dr. brewer. [what a mistake!?] "if the earth were a perfect sphere of equal density throughout, the waters of the ocean would be absolutely level--that is to say, would have a spherical surface everywhere equidistant from the earth's centre."--english "family herald," february , . "the more i consider them the more i doubt of all systems of astronomy. i doubt whether we can with certainty know either the distance or magnitude of any star in the firmament; else why do astronomers so immensely differ, even with regard to the distance of the sun from the earth? some affirming it to be only three, and others ninety millions of miles."--rev. john wesley, in his "journal." "i don't know that i ever hinted heretofore that the aeronaut may well be the most sceptical man about the rotundity of the earth. philosophy imposes the truth upon us; but the view of the earth from the elevation of a balloon is that of an immense terrestrial basin, the deeper part of which is that directly under one's feet. as we ascend, the earth beneath us seems to recede--actually to sink away--while the horizon gradually and gracefully lifts a diversified slope, stretching away farther and farther to a line that, at the highest elevation, seems to close with the sky. thus, upon a clear day, the aeronaut feels as if suspended at about an equal distance between the vast blue oceanic concave above and the equally expanded terrestrial basin below."--mr. elliott, baltimore. in the "scientific american," for april , , is a full report of a lecture delivered at berlin, by dr. shoepfer, headed "our earth motionless," which concludes thus:--"the poet goethe, whose prophetic views remained during his life wholly unnoticed, said the following: 'in whatever way or manner may have occurred this business, i must still say that i curse this modern theory of cosmogony, and hope that perchance there may appear in due time some young scientist of genius who will pick up courage enough to upset this universally disseminated delirium of lunatics. the most terrible thing in all this is that one is obliged to repeatedly hear the assurance that all the physicists adhere to the same opinion on this question. but one who is acquainted with men knows how it is done: good, intellectual, and courageous heads adorn their mind with such an idea for the sake of its probability; they gather followers and pupils, and thus form a literary power; their idea is finally worked out, exaggerated, and with a passionate impulse is forced upon society; hundreds and hundreds of noble-minded, reasonable people who work in other spheres, desiring to see their circle esteemed and dear to the interests of daily life, can do nothing better or more reasonable than to leave to other investigators their free scope of action, and add their voice in the benefit of that business which does not concern them at all. this is termed the universal corroboration of the truthfulness of an idea!'" the life of roger langdon told by himself with additions by his daughter ellen london elliot stock , paternoster row, e.c. "progress of astronomy" [_from "whitaker's almanack" for , under the heading "progress of astronomy."_] mr. langdon, station-master at silverton, on the great western railway, a self-taught astronomer, died on july , . mr. langdon made in his spare hours an -inch silver-on-glass mirror, grinding it on a machine of his own construction. in he contributed a paper to the _monthly notices_ of the royal astronomical society on "the markings of venus." preface the writing of this foreword to the biography of the late mr. roger langdon should have devolved upon one of the notable personages who had an admiration for him and his work, but unhappily they have all, or nearly all, passed away. unquestionably the person best fitted for the task would have been the late rev. h. fox strangways, rector of silverton during the period when mr. langdon acted as station-master there. they had a very cordial liking and respect for each other, and mr. strangways could doubtless have imparted a personal and intimate touch to this preface which would have been very valuable. when miss ellen langdon desired me to undertake this portion of the work i felt honoured, though diffident. a feeling that it was my clear duty to pay any mark of respect i could to the memory of this worthy man decided me to accept her invitation. my acquaintance with mr. langdon dates back to a few years before his death when my father was general manager of the great western railway and mr. langdon was still at work at silverton. my father's attention had been called to the personality and attainments of the silverton station-master, and as i was at that time doing a little journalism in odd moments it was suggested that i should run down and write something for the _great western magazine_, which i was very pleased to do. at that little wayside station just on the london side of exeter i therefore found myself one summer afternoon. the village of silverton, distant two miles from the station, was not visible, and the principal features in the immediate vicinity were the station-master's house, with the front garden between it and the station, and in the front garden a circular iron building with a cone-shaped revolving roof, which, i found, was an observatory sheltering a telescope for celestial observation. the tall, slightly stooping, white-bearded old station-master at once arrested attention. a dignified, patriarchal type of man, with a kindly, pleasant and simple manner, he was evidently much averse to all forms of affectation and cant. i was quickly made welcome and introduced to his wife and well-ordered home. we were immediately on excellent terms. i remember the eager pride with which he showed me his beloved telescope and its mounting and accessories, including the sidereal clock, and how i gazed under his direction at the heavenly objects which the night disclosed. the evening we spent together was a very memorable one. mr. langdon recounted the hardships and adventures of his career, and gave me an insight into the manifold difficulties and obstacles he had overcome in attaining the means of observing the celestial bodies in which he took so absorbing an interest. he also displayed for my amusement the ingenious church with chimes and other works of his hands. it is distinctly to be regretted that his autobiography ceases before the period when he made his four telescopes. his own account of his trials and difficulties and of the indefatigable inventive genius he showed in grappling with them would have been most instructive. his achievements become very impressive when his environment and paucity of means are remembered. long hours of duty at a little country station, the support and clothing of himself, his wife, and eight children who required to be educated and placed out in the world--all accomplished on a weekly wage, which from his marriage to old age averaged only _s._, and was in the earlier years much less--would have been enough to exhaust the energy and resources of any ordinary man. nevertheless mr. langdon found time and means to learn french, greek, and shorthand, to amuse his family and neighbours with lantern lectures, and to make and use effectively four telescopes, so that eventually his reputation spread to the royal astronomical society, before which he read a paper on his discoveries and observations. bear in mind that money was so scarce that he was practically reduced to make everything, even his tools, with his own hands from the crude materials, groping his way through the mists of uncertainty and disappointment to the haven of ultimate success. he was fortunate in his marriage, or he would probably never have succeeded as he did. he always referred to his wife as an inestimable blessing, and was, by her help, as free from home cares as a man with so small an income and eight children could be. the widow of the late rector of silverton bears testimony to the virtues and many good works of this estimable couple. their children rise up and call them blessed. their character and example even in this small locality and limited sphere must have been of very marked value. the career of roger langdon provides for all of us a striking illustration of what force of character will accomplish even in the humblest surroundings and in the face of the most serious obstacles. such men working persistently onwards and upwards with such slight recognition and encouragement are the real heroes of life, and their memory should be kept green for the benefit of those who come after them. h. clifton lambert. contents chapter page preface i. "why was i born?" ii. childhood's days iii. starting in life iv. my secret departure v. life in jersey vi. return and marriage vii. scientific achievements viii. closing years appendices chapter i "why was i born?" as earth's pageant passes by, let reflection turn thine eye inward, and observe thy breast; there alone dwells solid rest. that's a close immured tower which can mock all hostile power; to thyself a tenant be and inhabit safe and free. say not that the house is small girt up in a narrow wall the infinite creator can dwell there--and may not man? there content make thine abode with thyself and with thy god. i have no distinct recollection of my birth, although i believe i was a prominent actor in the performance. the very first thing, or rather, circumstance that i remember, was the birth of my sister, when i was two years and five months old. old nanny holland, who did duty as midwife, nurse and housekeeper, used to allow me to go out and play with the water and dabble in the mud; then she would call me in and smack me well and call me bad names, and shut me under the stairs until my pinafore was dry. i can quite well remember crying and asking myself, "why was i born?" especially as old nanny paid greater attention to me in this respect, than to any of my older brothers. then, as i grew older, there was my father who thoroughly believed that the stick was a cure for all complaints, and acting upon king solomon's advice, never spared the rod. on these occasions, i always asked myself the question, "why was i born?" as soon as old nanny had gone out of the house, i asked my mother if it was likely that old nanny would bring another baby next week; and when my kind and loving mother stroked my hair, and smiled and said "no," i was very soon out in the lane making bricks and building houses with mud. my mother did not smack me for this as old nanny had done, but she would call me and speak to me about making myself dirty, and somehow, whenever she spoke she was always obeyed. she used to have me by her knee and teach me dr. watts's hymns. i have lived to hear those hymns scoffed at, but i still think they might do good to some young people. now at the age of fifty i take great delight in the study of science and astronomy. who shall say that my dear good mother did not lay the foundation stone, and set my young mind thinking of the wonderful works of god, by teaching me-- i sing the almighty power of god that made the mountains rise, that spread the flowing seas abroad and built the lofty skies. i sing the wisdom that ordained the sun to rule the day. the moon shines full at his command and all the stars obey. this hymn, and other precepts taught by my gentle mother, sank deep into my mind, and set me thinking and pondering over the works of god, and led me to ask all sorts of questions, and i might say that i received all sorts of answers, which made me still more inquisitive, until my father would tell me to hold my tongue. i do not wish it to be understood that my father was a wrong-headed man, far from it; for i am sure that he possessed some of the finest qualities that adorn human nature. he possessed, in the very highest degree, the qualities of truth, justice, honour, and honesty of purpose; he considered it an exceedingly bad practice to owe anything to anybody, so he rose very early in the morning and took rest late that he might maintain his children, in what he termed "poor independence." moreover, he being the parish clerk and sunday school master--there was no week-day school--he had a very high veneration for the church. he was also choir master and organist. therefore he was a power in the village, and used his stick accordingly. woe to any bellringer who thoughtlessly entered the door of the church, without removing his hat from his head. "how dare you," he would say, "enter the sanctuary of the lord in that heathenish manner?" and the men i know very highly respected him, and obeyed his orders without a murmur. he would never allow cider, which was the drink of the country, to be brought inside the church gate; it was consecrated ground and was not to be defiled. he was like job in one thing, he was the father of seven sons and three daughters. the state of england at that time was very bad indeed, and the poor were really oppressed, especially in our remote part of the country. well, my father had enough to do to make both ends meet, and how he and my mother slaved and toiled to keep out of debt! my brothers and myself were sent to work at a very early age, at whatever we could get, and at this period, when the oppression was so great, i was always asking myself, "why was i born?" in the year , when i was four years of age, my father and mother had not heard of dr. jenner, and his plan of vaccination. if they had they would have surely fallen in with the idea, and would have acted upon it. it was the custom in those days that whenever small-pox made its appearance in the village, the mother of a family would take one of her children to the infected house, and place her healthy child in the bed of the person who had the malady. this was done so that the infection should not come upon her family unawares, but that she might be somewhat in a position to receive it, and with a little judicious management, generally to keep the disease under subjection; that is to say, she could generally manage so that only one of her children should be down with the small-pox at one time. whereas, if she allowed the infection to come upon her in its natural course, probably all her children would be down at once with the disease. now there was a boy who was said to be dying of small-pox, and whether it was ignorance, or superstition, or a combination of both, i do not know, but it was considered best, to let your children catch the small-pox from those who were suffering most violently. accordingly i was taken to the house where the boy lay dying, and there i was partly undressed and placed in the cradle by the side of the boy, and i was to stay there until i got warm and comfortable. as far as my own thoughts went in the matter i thought it very good fun, especially as when i was ill i should be out of the way of the stick at any rate. but while i was thinking over these matters, who should stalk into the room but old nanny holland. nanny was a sort of oracle in the village, besides being a kind of quack doctor, and what with her superior cunning, and evil temper, always excited more or less with gin, she held most of the poor women under her thumb, and when she approached the cradle where we were lying, i thought she looked more evil than usual. she looked at the cradle, then at the boy's mother, and said, "why don't you let the cheil (_child_) die? he can't die shut up in an infernal crile like this." and thereupon she dragged me out, and put me down, by no means lightly, upon the floor; she then tore away the foot of the cradle, so that the boy's feet could extend further down, and he was a corpse directly. it appears from nanny's theory, that although the child was in the agony of death, and with the last pang upon him, yet the vital spark could not part from him, until his crib was lengthened sufficiently to allow his feet to stretch downward without hindrance. i have sometimes thought that perhaps old nanny was more than half right in her theory. now, i cannot tell whether the virus of the boy's small-pox was too far spent, or whether i was an extraordinarily healthy subject, or whether perhaps old nanny frightened me, but certain it is, i did not catch the small-pox. therefore there was but one alternative, and that was, that i must be inoculated, or, as the villagers expressed, it "knockle-headed." as soon as i discovered this i really began to quake with fear, and to wonder why i was born. not that i feared the operation itself, as i had seen it performed on others, but i dreadfully feared the doctor who would perform upon me. i had not long to wait before my suspicions and fears were brought to a climax, for my mother took me off to nanny holland. nanny soon began to see about "knockle-heading" the children, and when she turned to me first, and i saw her coming towards me, with her surgical knife, my hair stood on end with fright. where she obtained the virus from i do not know, but she clawed hold of my arm, and stabbed a stocking needle through the skin, and lifting the skin upwards at the same time with a razor in her hand, cut a piece, about the size of a threepenny bit, three parts off, a bit of the skin being left in the way of a hinge; then with the point of an old knife, she plastered some matter into the wound, just as you might see a painter stopping a hole in a board with putty; then she replaced the slice of skin with the following caution, "now, youngster, if you scratch that off, i'll kill thee." my little sister was put through the same process, and louisa gard, a little chubby happy cherub of about four years of age, and a constant playmate of ours, was also operated upon. in due course old nanny's "matter" began to work. my sister was very ill with small-pox, and so also was little louisa. as for myself, i had it very slightly, in fact no one but my mother knew that i had the malady upon me. my sister got well in time, but of course the small-pox left its marks severely upon her. poor little louisa never rallied; or if she got over the small-pox, she had croup, which was too much for her, and she crossed over into the land of beulah. louisa and my sister and myself had attended the sunday school, for there was no week-day school. i asked mother if louisa would come back, and she said "no, but if you are a good boy, you will someday go where she is gone." then i would go out and look up at the stars, and wonder if i should see louisa flitting about from star to star, but my mother said, "no, you will not see her there, but you will meet her again at the last day; and if you grow up to be a good man, you will hear the great judge say, 'come, ye blessed of my father, and inherit the kingdom prepared for you, from the foundation of the world.'" this and other passages of scripture my mother taught me before i was really able to pronounce the words after her. all this was my religious instruction, besides what i learnt in the sunday school. chapter ii childhood's days in the curate-in-charge and his sister left our parish, and moved into berkshire. before the curate left he came to say good-bye to us. he also brought us some very useful things, which were most acceptable, for i know my mother had to struggle hard against wind and tide, as one might say, to keep us six great rollicking boys tidy, and how she did it as well as she did, with the scanty materials at her command, i really cannot conceive; but i do know that she many times went without food, so that we might have our fill. the curate looked at my sister's seamed face, then patted the baby, and said, "surely, mrs. langdon, you do not want so lovely a child to be disfigured with small-pox, do you?" "what can i do to avoid it?" asked my mother. "we have always been taught by our clergy that all these evil things are the 'lord's' will, so who can hinder it?" "god's will!" answered the curate. "have you not heard what everybody is talking about, i mean vaccination and cow-pox? vaccination is a process by which matter from a cow is inserted into your child's arm, and in the course of a few days the child will have what is called cow-pox; it is exceedingly mild, and the child will not suffer much, and if properly carried out, it is a sure preventive of small-pox." "dear, me," said my mother, "i wish i had known this sooner. i would gladly have had all my children vaccinated." "i am very glad to hear you say so," replied the curate, "but it was only accidentally that i mentioned it to you. i have spoken to several people about it, and i have found them so thoroughly prejudiced on the subject that i have found it prudent to hold my tongue. go to martock to dr. stuckey, and i know he will vaccinate the baby free of charge, and as i am leaving the village to-morrow i am very sorry that i shall not be able to know the result of the operation." my father had in the meantime come home to dinner, and had heard the latter part of the conversation, and he said, "i will take care you know the result, sir. when it is all over i will write and send you a letter, and let you know all about it, and my wife and myself are truly grateful to you for mentioning it." i have only to add that my father did write the letter to tell the good parson all about the baby having been vaccinated, and he had to pay one shilling and tenpence for that letter to be posted, besides having to walk several miles to the office. here i must say a few words in reference to nanny holland, and how it was that such an old shrew should be able to hold such power over nearly all the housewives of the village. in her younger days nanny had the reputation of being exceedingly skilful in midwifery. moreover nanny had been known more than once to set a broken leg, or arm, when the doctor was too busy, or, which was often the case, too drunk to attend. nanny was always ready to assist her neighbours in cases of sickness. she would go when called upon, whether by night or by day; and if any one hesitated to call her, she would not be any the better pleased, and would give them what she called a bit of the rough side of her tongue. but there was still another reason, and not an unimportant one, why the women in the village did not always consider it prudent to offend old nanny. at that time almost, if not quite, every wife in the village made her own bread, and nanny had the only oven in the parish; and there the women would go, carrying their dough with them, to be made into loaves and baked. in those days there was no electric telegraph, but somehow or other news would fly; and my baby sister had not been vaccinated many hours before the news reached old nanny's ears, and she took the first opportunity to call and find out particulars, so she came in with her teeth clenched, and her dark eyes sparkling with rage, and said, "i have just heard some news, and what dost thee think 'tis? why, i heard that thee's been down to thicky doctor and had thy chiel knockle-headed with vaccination." "well, so i have," replied my mother, "and i am only sorry that i did not know of vaccination sooner, so that i could have had all my children vaccinated." "whew," said nanny, "of all natural fools i ever knowed, thee art the cussedest fool of all. mind'ee, if thee brings thy dough to my bakehouse vriday next, i'll kick thee and thy dough out vaster than thee brought it in." and without another word old nanny went away, and from that day forward she always gave my mother a wide berth. whenever they met she would cross the road and pass along on the other side. i have stated that our curate-in-charge had left the village and gone into berkshire. the rector was a gentleman whom i had never seen. it was reported that he was squandering his time and his health and wealth on the turf, amongst thieves, black-legs, thimble-riggers, and other rogues and vagabonds. i know that it is not always prudent to believe all that is stated by the tongues of the villagers, but in this case i fear the accusation was only too true; in fact, the probabilities were that in this case the village gossip did not know all the truth. one thing was certain, he had to go about incognito as the bailiffs of the county court were constantly looking after him to serve him with a writ, or to arrest his person. only one good thing do i know of him; he used to send four pounds a year to my father for the maintenance of the sunday school. after our curate left, it was several months before we had another. the parsons of the neighbouring villages used to come, and sometimes we had morning service, and sometimes afternoon, and sometimes evening service, and more often no service at all. i remember on one occasion the bells were chimed at half-past ten, and the people came to church, but no parson came; the project again was tried at three, and again the people came, and again no parson, and as a sort of forlorn hope the bells were chimed again at six, and still no parson came. old george pant and a few others set up services about this time in the blacksmith's shop. now old george pant wore a wig, and other boys and myself used to go and peep through a large crack in the door of the blacksmith's shop, and watch him while he was praying. he used to get dreadfully excited and shake himself about, till by and by his wig would drop down behind him. i had seen george pant shake off his wig more than once, and the wicked thought entered my mind to try and steal that wig, which piece of theft i actually did accomplish on the very next sunday evening; and this is the way i did it. the door of the blacksmith's shop in which the meetings were held had in it several large cracks which i could easily put my hand through; and i noticed that when george was praying, he, and all his congregation, knelt with their backs towards the door; and so intent were they upon their devotions, that one could open the door and go in and out again, without attracting their attention. but i was too prudent to risk myself in far enough to pick up the coveted wig, when it should chance to fall; so i provided myself with a long stick, and tied a couple of eel-hooks to one end, and watched my opportunity through the crack in the door. i had not very long to wait. george began to pray, and presently down came the wig. directly it touched the ground, my fish-hook caught it up, and in another instant i was out of sight of the door with the wig under my arm. but i was no sooner at a safe distance, than i began to reflect, and i would have given the world to restore the wig to its place, but i knew i dared not do it. i knew that if i gave it up to its owner, he would freely forgive me, but my father would literally skin me. so i dug a hole in our orchard at the foot of the quince tree, dropped the wig in, and simply held my tongue. its sudden disappearance was a nine days' mystery in the village. meanwhile my father and mother were doing their level best to keep the sunday school going with no help from any one; and there we children were taught the catechism, and lessons from the old and new testaments; and the stick was frequently used. at length we received news that a new parson was coming, and all sorts of speculations were rife as to what sort of parson he would be. was he young or old? married or single? rich or poor? at last the bells were set ringing, and every boy blew his penny whistle and fired his pop-gun, because the reverend peter manonni scrope cornwall, m.a., with his two sons, and two daughters, and sister-in-law, miss brown, who was his housekeeper, had actually arrived. mr. cornwall took the curacy at £ per year, and an old tumbledown, damp, dismal den of a house to live in. now the rev. p. m. s. cornwall had to preach the gospel and educate his four children, keep up the dignity of his profession, visit and succour the sick, give to missionaries, buy books for the sunday school, and subscribe to the thirty-nine articles, besides giving an annual treat to one hundred children, all out of £ per annum. all of which he did to perfection. mr. cornwall was what may be called a good-natured, good-tempered sort of man; somewhat inclined to be stout; and i know that if any one was troubled in mind, body, or estate, they had only to go and open their heart to mr. cornwall and they would be sure to find a friend. moreover he had such a pleasant, benevolent-looking face that those who saw him were bound to love him. his sister-in-law, miss brown, was a lady of independent means, and when people went to their pastor to complain miss brown would almost surely be present, and she would put in a word here and there, as the case might require. she would blow people up if she discovered that their grief was brought about by their own naughtiness, as she would term it. she would tell them that they must be born again, and that they must go regularly to church, and after she had told them of their faults and how to mend them, she would dip her hand down into the recesses of her great wallet and bring up half-a-crown, and hand it over to the grieved one and say, "bless your heart, you must give god the glory, you must pray all day long, bless your heart, and say, 'create in me a clean heart, o god, and renew a right spirit within me,' and then you won't fall into trouble again, bless your heart." the very first sunday she was in the village she went and took full possession of the sunday school, and asked my father to give her the names of all the people who had children but did not send them to school, saying she would go round and ask the fathers and mothers to send their children. so during the week miss brown trudged from house to house, and asked the parents why the children were not sent to school. most of them began to make excuses. some had no clothes fit to go in, some had no shoes, some were sick with influenza, some were getting well, and others getting ill with small-pox; in fact, some had real excuses, and some made paltry excuses. but miss brown was equal to the occasion. those who were ill were to come to the parsonage for medicine, others were to come for clothes, shoes, or hats. anything and everything could be had for the fetching of it, and it was really astonishing how miss brown came by garments to suit nearly every child in the village; if she had been a marine store dealer, she could not have been possessed of more odds and ends, so that fathers and mothers as well as their children had not a shadow of an excuse for not coming to church or school. but the funniest thing was that miss brown did not let old george pant escape her notice. she called upon him, and his wife began to make excuse for him, that he had no hair on his head, and that he used to wear a wig, but some mischievous person had stolen it, and that george could not go to church and sit in a draught or he would catch such a cold that he would not get well again for months. miss brown listened as patiently as she could, and then said, "bless your heart, i have a wig that was my uncle's; and if george pant will come or send to the parsonage he shall be most welcome to it. it will just suit his complexion, bless your heart, and if people will only pray to the lord, he will always give them what is good for them, bless your heart. you must 'seek the lord while he may be found, and call upon him while he is near.'" the sunday school was held in the church, there being no school-house in the parish, and every sunday, in all weathers, at nine in the morning and half-past two in the afternoon, miss brown would be at the church door waiting to go in and open school; and i do believe that miss brown's great gold watch was always half an hour too fast, for my father, who was the very cream of punctuality, could not keep time with her. father kept the keys, and he was not always ready to open the church doors when the time was up by miss brown's great gold watch, and when he did arrive she would give him a gentle reminder that he was not in time by pulling out of her great wallet that great gold watch, and saying as she did so, "come, come, children, in to school; we are already two minutes late, and we have no time to lose; come and read." all had to read or learn a text, and were taught the catechism, before the afternoon service, which began at three o'clock. but miss brown was very tender-hearted towards her brother-in-law, the curate, and if that gentleman happened to have a cold, or a touch of the gout, which happened very often, then somehow there were fifty or fifty-five minutes between half-past two and three o'clock by miss brown's great gold watch, because "bless your heart, mr. cornwall has a bad cold and cannot walk very fast." also mr. cornwall came amongst us most sunday evenings and gave us some wholesome admonition, and he would tell us of all the most interesting things that were going on in the outer world, and of which we should never have heard without him. and when the dear man stood there in our midst telling us all these stories, his face beaming with goodness and kindness, and his hair as white as snow, i think i almost worshipped him. then about every sixth or seventh sunday he would preach a sermon specially to the young. thus did the rev. p. m. cornwall and miss brown take possession of the hearts of the people, both old and young, and in a very few years boys and girls grew up, and, as young men and maidens, still attended the sunday school--a school that could not be matched for miles around. on easter monday mr. cornwall would invite us to meet him in the churchyard, and we would join hands and encircle the church. then he would feed us with hot cross buns, and do all in his power, with the help of miss brown, to make us happy. it seems needless to record how much these two good people were beloved. chapter iii starting in life at the tender age of eight i was sent to work on a farm belonging to joseph greenham. for the princely sum of one shilling a week i had to mind sheep and pull up turnips in all winds and weathers, starting at six o'clock in the morning. very often i was out in the pouring rain all day and would go home very wet, and then my good mother had something to do to dry, not only my wet clothes, but also those of my four brothers. and i know it took her half the night to mend and tidy all our clothes. as soon as i was able i had to go driving plough, for in those days a man would not think of ploughing without a boy to drive the horses. now it was my sad fate to be placed under the hands of the most complete vagabond that it was possible for the spirit of all evil to beget. i cannot here tell--and if i could, nobody would credit--the dreadful usage which i received from his hands. although mr. greenham was my employer, yet to all intents and purposes jim the ploughman was my master. i was completely in his hands and under his control, and it was in his power to do what he thought fit. there was a public-house in our village kept by a widow, whose name, curiously enough, was temperance patch. jim was one of the best customers that temperance patch had. he spent all the money he could earn, beg or steal, in her house, and when he had no cash, he did not scruple to steal his employer's hay, corn, straw, eggs, fowls and potatoes; in fact everything portable was carried away to the _new inn_. i once thought it my duty to report to mr. greenham that jim had carried away a large bundle of hay, and when mr. greenham taxed him with theft, he cursed and swore, and said that i was a wicked young liar. after this, until i was thirteen years of age, my life was not worth the living; for i was thrashed and kicked and beaten most unmercifully by this brute. so i learned that a still tongue makes a wise head, and never once again did i say anything to any one, not even my mother, about the cruel treatment which it was my lot to receive. jim used to make me harness the horses long before i was tall enough to reach their heads, and beat and kick me if i could not do it quickly enough for his liking; and i used to wonder every day and all day, and ask myself, "why was i born?" sometimes jim would lie down under the hedge and go to sleep, making me plough the ground the while; and although i was but a child and scarcely tall enough to reach the plough handles, yet if he woke up and found any bad ploughing he would beat me to his heart's content. but with it all, he never could get me to tell the abominable lies that he would put in my mouth to tell mr. greenham so as to save him a scolding when he had been neglecting his work. i had learnt from mr. cornwall and also from my father that lying lips are an abomination to the lord, and this feeling was so strong within me that i could never corrupt my conscience and degrade myself to repeat jim's falsehoods, and i came in for many castigations accordingly. on one occasion mr. greenham took jim and me to the cellar, to lime some wheat before it was sown. while we were there mr. greenham was called away, and directly his back was turned jim caught up a dipper, as if he had not another moment to live, drew some cider and drank it greedily down; then he drew some more and offered it to me, but i refused. with an oath he pressed the edge of the dipper against my lips until they bled with the pressure; at the same time he held me by my hair, in order as he thought to pour the stolen liquid down my throat; but jim did not succeed in his purpose, so he drank it himself and threatened, using fearful imprecations, that if i ever said a word about it he would kill me on the spot. i don't think i should ever have said anything about it, but thieves are generally great fools. jim in his greedy haste did not turn the tap back as it was before, so that there were a few drops on the pavements. the dipper, also, was wet and smelled of cider. so mr. greenham accused him, but jim began to call god to witness that he was as innocent as a dove, and he had the impudence to refer to me to prove his honesty. the master asked me and i told him the simple truth, knowing full well that i should catch it soon. as soon as jim's guilt was discovered beyond dispute, he began to shed crocodile tears, and to lament and beg pardon in such a humble and seemingly contrite manner that the master's eyes were blinded, and he forgave him there and then. the next day we went into a field to plough, and now my punishment began. jim belaboured me with the horsewhip as long as he felt disposed. he knocked me down and tried to jerk the breath out of my body. then he wrenched my mouth open with a large nail and filled it with dirt. he allowed me to get on my legs again and resume ploughing for a time, but he soon began on me again. he struck me down and kicked me, and danced upon me, till i felt very faint and ill with loss of blood. i really thought my end had come, and i felt very glad. it may seem rather paradoxical, but that moment was the happiest moment of my life. i thought of dear miss brown, and her teachings: "blessed are they that mourn, for they shall be comforted. blessed are the pure in heart, for they shall see god. blessed are they which are persecuted for righteousness' sake, for theirs is the kingdom of heaven." all these and other precepts flashed into my mind, for i knew it was out of envy that i was so cruelly used. but somehow i refused to die at his bidding, so jim waited for another time to try and send me out of the world as if by accident. one of the horses was exceedingly ticklish when touched in a certain way upon its backbone, and could not bear to be touched on this particular spot with a curry comb, and sometimes when so irritated would let fly with both heels at once. so on the morning following the last punishment jim set me to clean some portion of the harness, and made me stand in a certain position directly behind the ticklish horse. there i worked away without any idea that mischief was brewing. jim, however, had laid all his plans, and if they had succeeded and i had been killed, he would have been found blameless. there was an open window to the stable exactly opposite and close to the ticklish horse, so that a man outside, by standing on a ledge of the wall, could put his hand through and touch the horse's back. i heard the horse make a noise, and on looking up saw jim's head outside the window, and his hand upon the horse's back. at the same moment the horse let fly, and one of his heels came against my left side and sent me dashing against the wall. i knew no more until i found myself in bed with my mother crying and washing the blood from my hair and face, and felt a great pain in my hip, where the horse's hoof struck. there was also a big scar on my head where i was knocked against the wall. i can only account for not having been finished off that time by the fact that the horse did not kick when it was first touched, but began to prance about, which arrested my attention and i moved close to his heels. if i had been a little further off his heels would have struck my head or the upper part of my body and i should not have been here to write. after lying in bed about a week, where i cogitated and wondered for what earthly purpose i was born, i had to go back under this fiend again. every other place in the parish was filled and my parents could not afford to keep me in idleness, so there was nothing for it, but to go back to work again as soon as possible. a few days after this the very same horse got restive in a field where we were and turned over a cartload of manure upon poor jim. i thought he was killed, in fact for a moment i hoped he was killed. but immediately i would have given worlds to have called back the thought. miss brown's words came upon me, quick as a lightning flash, "create in me a clean heart, o god, and renew a right spirit within me." other of her precepts came strongly into my mind, and i shook with fear, for i had learned that to wish a man dead amounted to the same thing as killing him. therefore, i felt that i had committed a most grievous sin, and i cannot express the joy i felt when i saw jim crawl out from under the cart unhurt. he began to curse and swear at the horse and me, saying it was all my fault, whereas it was his own fault, as in harnessing the horse he had negligently left the buckle of a strap under the cartsaddle, so that the buckle rested exactly upon the backbone of the horse and caused him to be restive. i was under jim's control for five years--years of my childhood, which i ought to be able to say were the happiest of my life. but they were just the reverse, and if i stated all that i suffered at his hands, no sane person would believe that such things could have been done with impunity. not many years ago mrs. beecher stowe shocked the refined feelings of the civilized world with her graphic account of the sufferings of the negro slaves in the united states of america. i cannot write my history in the shape and manner of a novel, with its parts and counter-parts, but what i have written are some of the main facts and features of my boyhood life. some people, those who have passed smoothly through their childhood, and have scarcely known sorrow, may ask whether it is possible that such things could have been done in england? my answer to this is, yes. it was not the parents, but the age that was to blame, as may be learnt from some of the works of charles dickens, and other writers who have given pictures of the period. i know that my brothers could write a parallel history, and they were not under the hands of so complete a blackguard as it fell to my lot to be under. when the season for ploughing was over i used to get a few weeks' relief from the hands of my tormentor. during such times i was sent into the fields minding sheep. these were days of pleasure and happiness. i had to work hard, but toil was a pleasure as long as i had no one to abuse and ill-use me. i was the happy possessor of a tattered testament, and i used to read from its torn pages. it began at the words, "let not your heart be troubled," and ended with the twenty-seventh chapter of the acts. i read the first and last chapters more than all the rest, and really knew them all, every word. now dear old mr. cornwall used to come out in the fields and find me out and ask me questions about scripture history, and i believe i used to answer him to his satisfaction for he called me a good boy. as far as i know it was the first time i had been called a good boy except by my mother, and i fancy i grew an inch taller all at once and that his calling me a good boy had a very strong influence in making me try to be good; but whenever he talked to me my conscience pricked me relative to old george pant's wig. i never could forgive myself for stealing it, and would have confessed to mr. cornwall concerning it, but i thought he would tell my father, and i did not want an extra thrashing. i used to leave work at six o'clock, and mr. cornwall told me that if i would come to the parsonage and pull up the weeds in his garden path he would give me a shilling. the idea of having a whole shilling, all in a lump, frightened me. i had never possessed a coin of the realm above the value of a halfpenny, and such halfpennies were, like angels' visits, few and far between, for the wages which i earned had to go for my maintainance. so i went every evening to accomplish the work, and was very particular to do it well, so that nothing should prevent the free and unconditional receipt of the shilling. i had been to crewkerne a few weeks previously and had seen a book in the printseller's window; it was _pinnock's catechism on astronomy_. my heart had been aching to obtain that book, but the price was ninepence, and i knew that if i saved up those very scarce halfpennies it would be years before i got ninepence, and so i thought i should never get the book. but now a new light had unexpectedly fallen upon the subject. my dream of possessing "pinnock" would now be realized, and that much sooner than i ever had imagined. i should now be able to run over to crewkerne and buy the book and have threepence change. therefore i finished my task, and swept and cleaned up all the weeds, and with a joyous heart i presented myself at the parsonage door for my promised shilling. "put not thy trust in princes" is a trite saying, but oh, how deeply and grievously i realized its truth, for i never received that shilling. mr. cornwall was laid up with a fit of gout, and what with the twinges of the malady and the business of his curacy, i suppose he had forgotten me. when mr. cornwall was upon his feet again i was too shy to ask him for the shilling and so it was passed by, and i was compelled to go without the pleasure of reading "pinnock" for several years. about two years after this mr. cornwall came out to see me in the fields. i had gone to another field a mile away, but had left my jacket and some tools and my fragment of a testament, all rolled up together in a corner of the hedge which i had been in the habit of using as a dining-room. so the parson thought he would be inquisitive. he opened my jacket and found an assortment of things that i had cut out of sticks and turnips. there were ships, soldiers, sailors, anything and everything, and i afterwards heard him tell father that he had fairly roared with laughter on finding them. a few days after this i saw the dear old face coming up the side of the hill where i was with the sheep. he was approaching very slowly; he never could walk very fast across the fields, because miss brown always would insist on his wearing a pair of her old clogs that he shouldn't catch cold, "bless your heart." when he came up to me he began to ask me questions and whether i found time to read the bible. so by degrees he got me to show him my fragment of a testament. he turned over the leaves and returned it to me. then he pulled out from his pocket a brand new bible. it was a reference bible, such a book as could not be bought at that time for less than seven or eight shillings. mr. cornwall gave it to me and told me to read, mark, learn, etc. in the fly leaf he had written:--"presented to roger langdon, for his good conduct at the sunday school, by the rev. p. m. s. cornwall." i cannot describe my feelings on that occasion. i believe i laughed and cried. i kept that bible, and carried it about with me wherever i went; until, a few days before i was married, it was stolen from my lodgings in bristol. chapter iv my secret departure since jim had compelled me to plough the ground while he slept, or otherwise idled his time, by the time i was twelve years of age i could plough a straight furrow. it was considered a crime of the deepest dye to plough a crooked one. there was a ploughing match to come off at haselbury, and mr. greenham entered me on the list as a first-class boy, and jim was entered as a first-class man; we then had to practise side by side in a field of clover. everybody said i should win the head prize, which for the boys was £ , while the head prize for men was £ with a society's coat and buttons. at length the important day arrived, and everybody went to the field where a piece of land or ridge must be ploughed in fourteen rounds; that is to say, the plough must go across the field and back again fourteen times, which makes twenty-eight furrows for each piece of land. neither more nor less must be ploughed in these twenty-eight furrows. all the furrows must be straight, no grass or weeds must be seen sticking up between them, and when finished the ridge must be level and even. now there was one ridge which had a hollow across it in a diagonal direction, and as all the pieces of land to be ploughed were drawn for by lot, this piece came to my share. i asked the umpires if they expected me to get that hollow up level, and they said yes, most decidedly; but they would give me an extra half hour to do it in. so at it we went, whistling the tune of "god speed the plough and the devil take the farmer." now, notwithstanding the hollows, i had the pleasure of hearing my work praised exceedingly, which gave me much courage. i finished my piece as soon as the rest had finished theirs. we then had the pleasure, the inestimable pleasure, of seeing the farmers and gentry eating roast reef and plum pudding in a decorated barn. the smell of these good things ought to have done us good. for myself i was very hungry, having had nothing to eat since before six o'clock in the morning. when they had dined we were called in one by one to receive our rewards. i heard my name called, and went into the barn to receive my prize. the squire, who was the spokesman, praised my work and said that i should make an excellent ploughman, and had it not been for the hollow, which i had not fetched up to their satisfaction, i should have been entitled to the first prize. but as it was i should be awarded the third prize of £ . now jim got nothing. i do not know why, for he was undoubtedly a good ploughman when he chose to put himself into his work. so he sent me home with the horses, as it was getting late, and said he would go and get my £ . i had no choice but to obey his orders, and i never saw my money, for he went straightway to temperance patch's, and spent it there. my poor mother went to him about it, but she received nothing but curses. so now i began seriously to think how i could get out of his power. i used to measure myself once a week to see how soon i should be tall enough for the army. i was thirteen years of age, and big and strong. so when her majesty queen victoria was crowned i went to yeovil, and for the first time in my life saw and heard a military band. i asked them to take me, as i could play a flute or cornet, but they replied, "you must grow a bit, and then call on us again." i grew a good deal during the next year, when i was fourteen. i was determined to bear jim's cruelty no longer, and i knew i was now tall enough for the army. so i made preparations for a start. i was receiving four shillings and sixpence a week, which was good wages at that time; but of course this all went for my maintenance. i went to my master, however, and asked him to keep my wages for three or four weeks, so that i might take it all in a lump, but i shrewdly held my tongue as to why. it was the only occasion on which i did not act quite openly with my mother, but in my mind i knew i should make it up to her later. so when my wages amounted to nearly £ , i asked mr. greenham for it and then made a start. i put together what few clothes i had and got up at three o'clock in the morning, with mixed feelings of joy and sorrow secretly bidding adieu to mother, father, brothers and sisters. on arriving at the end of the village i glanced back to take a last look at the church and steeple, which were just discernible in the grey dawn. i thought of mr. cornwall, and miss brown, some of whose teaching again came into my mind. i remembered that i had so often repeated the words, "the lord is my shepherd, i shall not want," and "i will lay me down in peace and take my rest, for it is thou, o lord, that makest me to dwell in safety." i now began to make tracks for weymouth, thirty-three miles distant, and beyond the first seven miles i did not know an inch of the way. i trudged on, however, like a snail, carrying all i possessed on my back. after about nine or ten hours' sharp walking i arrived at the quaint old town of dorchester, where i walked leisurely through the streets, looking at the old-fashioned houses, and such things as took my attention. just as i was going to turn into a shop to get some bread and cheese, a smart recruiting sergeant came striding up the street towards me. he clapped his hand upon my shoulder and said, "here, young man, will you enlist?" i do not know whether the perversity i showed is a characteristic of human nature, or whether it belonged to me individually, but if he had asked me to take a dose of poison, i could not have felt more vexed and annoyed; and when he showed me the shilling, the disgust i felt was beyond description. perhaps the fact of my being very hungry had something to do with it, but at any rate the idea of being a soldier went entirely out of my head for ever. after an hour's rest in dorchester, i travelled forward i still had about eight miles to go, and my feet were already blistered. i scarcely felt it, however, as i had often had them blistered when ploughing, with jim's ill-usage into the bargain; but now i had freed myself for ever from his cruelty, and i went along with a light heart. i reached weymouth about five o'clock in the afternoon. i had done the thirty-three miles, including stoppages, in fourteen hours. i walked round the harbour and asked every skipper i could find if he would take me on board his vessel in any capacity, but from all came the same answer, "no." so i began to think that the world and its inhabitants were not exactly what i had always thought or fancied they were. all the sea-songs i had heard, "the poor sailor boy," "the cabin boy," and others, had led me to believe that the skippers of vessels were only too glad to get hold of a boy when the chance offered; but now i found out my mistake. but one man told me that a shipowner in jersey was in want of hands. i made inquiries and soon found out that a steamer would start for jersey at nine o'clock the same evening. i went on board, paid ten shillings for my passage, and was soon off towards jersey. i cannot describe how much i enjoyed the view of the sea, especially when we began to lose sight of portland. the moon was shining and i could look around and see a great expanse of water. the sea was not rough, although there was a swell which was sufficient to toss the vessel up and down in what i soon found to be a very disagreeable manner. i soon began in right earnest to feel what the french call "mal de mer." after a twelve-hours' passage, which was considered good work at that time, we landed in jersey. i at once found my way to the shipowner, and he set me to work there and then unloading salt from a ship at one shilling and sixpence a day. the salt was not in lumps, but in the form of small grain. it was at that time used in england for manure; but in jersey and france it was used for salting butter, meat and fish. to any whose shoes have been worn off their feet, and their feet blistered and made sore with walking, i need hardly say that the salt soon found its way into my boots and made me nearly faint with pain. however, it was soon over, and in a couple of days i believe my feet were hard enough to walk upon flints without any inconvenience. sunday soon came round, and i brushed myself up and went to st. james' church. as i glanced over my person i found that my only suit was very much the worse for wear after unloading, but i had no idea of staying away from church on this account. in the afternoon i went to the town church and saw the soldiers walk down from fort regent, with their fife and drum band playing "the girl i left behind me" right down to the church door. as soon as they were out again the band struck up "the irish washerwoman." i thought it was very wicked, and wondered what my father would have said about it. on the monday morning, the judge of the island, who was my employer, came into the stores, where i was at work, racking brandy, and adding water and logwood and a drug, the name of which i do not remember, by which means we made three hogsheads out of two. now the judge was one of the richest merchants in the island, and therefore thought it his duty to set a good example to the other merchants, and to all seafaring men. he was a large importer of wine, rum, brandy, and gin, and was always very particular to see that we did not put too much water and logwood into the brandy, and water and sulphuric acid into the gin. so that his spirits were always considered the best in the island. the judge went regularly to church, and as he was walking round the stores on this particular monday morning he came up to me and said, "young man, i was much pleased to see you at church yesterday. are those the only clothes you have?" i confess that these remarks about my clothes did not please me, because on the sunday i had gone to church feeling unhappy at not being able to change my garments, and hoped that no one would notice me; and when i found that the judge had noticed me, i must say i felt seriously annoyed. i answered him civilly, however, that it was my only suit, but that i should get a new one at the first opportunity. the judge raised my wages there and then to two shillings a day, and in the afternoon called me into his private office and told me to go to his house as his wife would like to see me. during the rest of the day i was nearly beside myself, wondering what on earth such a great lady could want with a poor boy like me; therefore i was twisting and turning the thing over in my mind, wondering whether she could speak english, and what she would say to me and how i should answer her. at length evening came, and i went to the house and was presented to her. she was exceedingly plain in her dress, as all jersey ladies were, and i could scarcely believe it was the wife of the judge. however, she soon told me what she wanted me for. she said, "j'ai some clothes for de garçon; de coat was too much big, go to de tailor and have one gros cossack too make little from fit for yourself. reste vous one little minute, je donne vous some-sing for to mange." the drift of all this was that she gave me a meal and a left-off suit of the judge's. he was one of the largest men in the island. i was a mere stripling, and i believe i could have stood in one of the legs of the trousers. i could not get a tailor to do anything with such a suit unless i paid him more than a new suit of clothes, and i did not go to church for several sundays, until i had saved enough to buy myself a decent suit. now the judge was known to be a very pious man. he would take hold of little dirty urchins in the street when he heard them using bad language and reprimand them. his own people had to pull a long face and assume a virtue if they had it not. and when the postman came round with a petition to get off his sunday duty, everybody in the district signed it except the judge. he could not forego the pleasure of receiving and reading a few letters and allow the poor postman to go free on a sunday. i have been greatly amused more than once on going into an assize hall and seeing the judge sitting in his chair, looking as grave and solemn as only a judge can look. the clerk of the court read the queen's proclamation against vice and immorality; solemnly called upon the magistrates and sheriffs of counties to use all their power to suppress all kinds of vice and lewdness, especially sabbath breaking; and yet the judge could not allow the double rap at his door to cease on a sunday. i got on capitally with this lady; we seemed to understand each other at first sight. but do what i would, i could not bring myself to feel the respect due to her, simply because she wore a pair of old and dirty wooden shoes, a short, rough woollen skirt, a great red-patterned kerchief over her shoulders, and a large, stiff, white muslin cap on her head. altogether she cut such a figure, that i could not fancy she was the wife of a rich merchant and judge. but i found after a while that the ladies of jersey were exceedingly plain and unassuming. they assisted in house and dairy work; they milked and fed the cows. it was a very common thing to see the farmers' wives and daughters milk cows into one can, and goats into another; then, tying the cans together and slinging them across an old horse's back, they would perch themselves on the top, and set off to town at five o'clock in the morning, to sell the milk from door to door. they returned to breakfast and spent the remainder of the day working in the fields. i saw them, both in jersey and in france, actually ploughing, sowing, reaping and mowing; and yet these people were rich and had their thousands in the bank. after witnessing how hard the women had to work in jersey and france, i was not surprised that napoleon i. said that england was a paradise for women. i continued in the employ of the judge all the summer. my usual work was to adulterate the wine, brandy, gin, rum, and whisky; and though constantly amongst this firewater, i am thankful to say i did not acquire the taste for any of it. yet all who worked there could have what they liked. the judge gave carte blanche. i often thought what a paradise this would have been for jim, how he would have made himself a perfect walking swill-tub; but it would have soon killed him. i watched many strong sturdy fellows from devon and cornwall actually kill themselves with the accursed stuff. not that they were drunkards; nothing of the sort. but because brandy could be purchased at sixpence a bottle, so they would constantly be sipping it. they did not get drunk, but would take a little in winter to keep the cold out, and a little more in summer to keep out the heat; they would soon get "brain fever," or as some people would say, sunstroke, and die ramping mad. i have seen and known this in many cases both of men and women. chapter v life in jersey the judge had a fleet of ships of his own trading to nearly every corner of the globe, and in the months of september and october several vessels returned from the newfoundland fishery, laden with codfish, whalebone, sperm oil, and seal, beaver, fox, and other skins. he made me a sort of deputy-clerk, and i had to note down every article with its number and weight. this i did so much to his satisfaction that at christmas he actually gave me a sovereign as a present over and above my wages; and a few kind and complimentary words that he spoke made me feel as if i had suddenly grown an inch taller. now i began to feel very pleased and glad i was born. i began to think myself surpassingly rich, for i had three good suits of clothes, and six golden sovereigns in my pocket, and i thought of poor jim the ploughman, who used to go to bed early on saturday night to give his mother the opportunity to wash his one and only ragged shirt, so that he might have it clean on sunday morning. all this time i had thought greatly about my poor mother, for she had not the slightest idea of what had become of me. i had been away from april until christmas. i had never up to that time any idea of writing a letter as i had never been to a week-day school, but somehow or other i had acquired the art of writing. of course i could not realize how very deep her grief must have been, being only a male. we as men love our children, but our love at best cannot be measured or weighed against the deep and constant love of a mother for her child. now i thought how pleased she would be if i could only just put these six sovereigns into her hands. i thought whether i could manage to go for a week to see her, and return again; but on second thoughts i decided that would not do at all. so at last i wrote a letter, and told my mother that i was well and flourishing, but could not come to see her. if she would come and see me, however, i would send her the funds to pay her passage. she answered my letter by return of post to say that she would come, and so in the course of a few weeks i had the pleasure of meeting her on the pier, as she landed from the _atlantic_. the judge gave me as many holidays as i wanted, and so i was able to show my mother about the island. the first thing that surprised her was to see the french women wheeling heavy barrows of luggage about, while their lords and masters were swelling about with their gold-laced caps, gingham blouses, patent leather boots, and the everlasting pipe in their mouths. mother would gaze at the women in the street, and say, "well, i have worked hard in my time, but i am very glad i am not a frenchwoman." she was not surprised, she said, that the duke of wellington was able with a handful of englishmen to go over and thrash them frenchmen on their own ground. i thoroughly enjoyed myself during the five weeks mother stayed with me. father had given her four weeks, but i must confess to playing a trick on her so that she could stay with me another week. on the day that she was to leave i went down to the steamship office to ascertain at what time the packet started, and found it was timed to start at nine o'clock in the evening. but my mother did not trust quite to that, for she went herself to learn the hour of departure, so it was no fault of her own that she was left behind. well, we had high tea at six o'clock, and got a few friends together just to talk away the hours until nine o'clock. so while the ladies were thus employed, and helping my mother put her things together, i put the clock back three-quarters of an hour. no one noticed it, as the clock was in another room, and it turned out exactly as i desired. we went off, my mother and i, she shaking hands and bidding her friends good-bye and i laughing up my sleeve, knowing full well that the steam-packet would have left. when we arrived at the pier and she found it had gone without her she could scarcely believe her own senses. i knew that she had never been away from her home for twenty-four hours before, and i also knew that my father would be dreadfully vexed, especially as he would have to walk a long way to the foxwell turnpike to meet the weymouth coach. i ought to have been put in the stocks for practising such a joke, but somehow i felt it might be years before i should see her again. i took her over to elizabeth castle, and she stood close by the great gun when it was fired off, bearing the deafening noise and the vibration under our feet like a real old soldier; and then we went to gorée castle and saw the oyster fishing; and she was very much amused to see the oysters as they lay in heaps upon their shells, and to watch the mice, that swarmed around trying to get a nibble from the open oysters, which would close up their shells upon them. then i took her down to st. clement's bay, where her grandfather, a sergeant-major in one of the english regiments, landed when he was sent to the island to clear out the french. at the death of major peirson in the royal square the command fell upon my great-grandfather, who drove the french out of the town at the point of the bayonet literally into the sea, and at the time of our visit the guns which were used in that fight were stuck into the ground along the beach as a memorial of the occasion. at the time of my sojourn in jersey from to it contained twelve parishes. the capital town of st. heliers contained six or seven churches besides two roman catholic chapels, and several dissenting places of worship. there was a fine theatre, and a court-house in the royal square. this house did duty as the house of commons, guildhall, assize hall, and i know not what besides. i have seen great doings there when a new judge was being elected. i have also seen prisoners tried for various offences, but whether the prisoners were french or english or of any other nation, the whole of the business was carried on in the french language. if the prisoner at the bar did not happen to understand that language so much the worse for him. there was no such person as an interpreter, and i often heard sentence passed upon a prisoner who was quite ignorant of the nature of the trial or sentence until some kind friend who could speak both languages would tell him what he was to expect. mr. charles carus wilson, a man over seven feet in height and a member of the english bar, on one occasion stood up and told the judge that the prisoner had not had a fair trial, that he protested against it, and that he would report the circumstances to lord denman, the lord chief justice. the judge thereupon told mr. wilson that he had insulted the court and must pay a penalty of £ , and apologise to the court for such an insult. "indeed, i shall do neither one nor the other," replied mr. wilson. "then," said the judge, "you must go to prison during her majesty's pleasure." "very well," replied mr. wilson, "here's off to jail." so he walked through the streets in charge of a constable, his head and shoulders towering above the heads of the crowd which had gathered round. in prison they had to put two bedsteads and beds together to make it long enough for him to lie down. mr. wilson, however, took it very quietly and courteously and reported the whole matter to lord denman, who sent over a writ of habeas corpus. of course i wondered whatever that could be, but the steam packet arrived on a sunday morning, covered with flags and banners, and thousands of people went down to see the sight and wondered what was going to happen next. i do not know if the judges knew the meaning of it, but they were nearly frightened out of their wits. messengers were sent all over the island to call all the judges together. on monday morning they met and consulted, and the result of their deliberations was that they went themselves and opened the prison doors and asked mr. wilson if he would please to walk out. charles carus wilson, however, did not please to walk out. he merely replied, "you have sent me here for i know not what, and i do not feel disposed to be sent to prison and taken out again just as it suits your whims." so the upshot of it all was that they had to pay mr. wilson's fare and their own to london, and all had to appear before a judge of the queen's bench, and the jersey judges were fined £ each, and the poor woman, whose trial and sentence of seven years' transportation for stealing a hen and chicken had caused all the trouble, was freed. my employer had often told me that if i had been a few years older, he would have sent me to newfoundland to superintend his business there. as i was too young to fill such a responsible position, he proposed that i should join the _anchor_, a fine bark of tons. the captain, he said, was a "very nice gentleman," and on that vessel i should have an opportunity of learning the art of navigation, so that eventually i should be able to take charge of any ship belonging to the merchant service. i thought this was exceedingly kind, especially as he said he would provide me with an outfit, and i then and there closed with the bargain. the _anchor_ was in the harbour and i went on board and assisted in putting in a stock of provisions, ready for the voyage to the brazils. she was to sail in a fortnight, and i was rather glad that i was born, to fall in luck's way in the manner i had. there now appeared a prospect of my being placed in a position worth struggling for, which i knew was not usually the lot of one such as i. so i looked forward daily and hourly for the kind-hearted judge to supply me with the outfit. when, lo and behold, one morning, a day or two before the _anchor_ was ready to start, the judge told me that he intended to place jim drake in my place, because his father was dead, and he was a poor, friendless lad. "just as if i were not a poor, friendless lad," thought i. i don't think i wished jim drake dead, but i did wish that he had never been born. that he should step in and just open his mouth and catch the blessing that was intended for me was almost more than i could bear. so i had the mortification of seeing jim drake go off in the _anchor_, and i felt more disgusted than i can tell, especially as the skipper was a "very nice gentleman." but i afterwards found out that after all this happened for the best. the judge offered me more than one good situation. he had several other vessels, besides the _anchor_, but to all his offers i turned a deaf ear. if he had ill-used me, or kicked me and boxed my ears, i should have forgiven him; but he had deceived me, and for that offence i could not respect him. so i left him and sought other employment. i found work in a large blacksmith's shop, and here i had to work very hard indeed. i stayed for a time, and then engaged myself to another merchant. i went on board his vessel, which was a small sloop trading between france and the channel islands and occasionally visiting some english port, plymouth, poole or southampton. now this old merchant used to go over to havre or granville, proceed a few miles into the country, buy cows at from £ to £ each, and send them on board the _medora_, ordering them to be taken to st. aubin, jersey, while he himself went on before in the steam packet. the _medora_ in due course ran into harbour, and there would be the old sinner waiting to order the cows to poole, and at the next tide we would set sail for poole, where he again would be waiting to meet us, and the cattle would be unloaded and he would take them to the market or drive them round to the farmers and sell them for pure jersey cows, thereby gaining an enormous profit. one saturday afternoon when the wind was blowing a hurricane and the sea rolling and seething in all its majestic fury, as we were scudding along somewhere between guernsey and the isle of wight, we saw a fine bark in the distance, and on nearer approach we found it was the _anchor_, returning from the west indies laden with sugar. we got near enough to speak to each other, and i looked to see if i could make out jim drake, but i failed to do so. the vessel had encountered some very bad weather for nearly all the bulwarks were gone, and almost everything that is usually carried on deck had been washed away. the men were, like ourselves, tied fast with a rope's end to prevent their being washed overboard. after two or three days and nights in the channel, we at last found ourselves in weymouth harbour. we had been soaking wet and had had no sleep or food, nor did we seem to require any, but i went to an inn and got to bed and slept for ten hours. on arriving back in jersey the first thing i heard was that jim drake, who had just returned in the _anchor_, had summoned his skipper for cruelly ill-using him while at sea. it appeared that the _anchor_ was no sooner out of jersey than poor jim drake began to be sea-sick. so this skipper, the "very nice gentleman," took a rope, and the chief mate took another, and between them they belaboured poor jim, one hitting on one side and one on the other. so this nice pair seem to have taken a delight in ill-using the poor lad during the whole of the voyage. one day jim committed the awful crime of whistling. no one was allowed to whistle on board except the commander, and he only under extraordinary circumstances, such as when there was a dead calm he might whistle for a breeze. superstition ran so high on board that if a man whistled he was considered an evil genius; storm and tempest, fire and famine, aye, the devil himself would visit that ship, and from the moment jim drake whistled every man on board became his enemy. the skipper cursed, and the mate swore, and poor jim cried for mercy and said he did not know he was committing an offence. but they tied him to the mast and both took a rope and hammered away as if the subject they were operating upon had been a piece of cast-iron or a block of granite; and at last one took a handspike and gave jim a blow which broke his arm, and as there was no surgeon the limb was never set. this was the complaint brought before the court, and when i saw jim there i cried for pity as the poor lad stood there with his hand twisted round, the back being towards his thigh, the palm outwards, and the whole dangling useless by his side. now jim was an english lad, and the skipper a jerseyman, and all the business of the court was carried on in french. jim stated his evidence, but there was not a man amongst that crew who would corroborate it, and the skipper and the mate were two very respectable men. moreover the skipper was such a kind-hearted gentleman that he had actually given his men a double allowance of grog on their homeward voyage; so of course, as in duty bound, they all to a man spoke of his kind and generous conduct. so poor jim's complaint fell to the ground. and not only that, but the skipper took proceedings against him for trying to defame his fair character, and poor jim was sent to prison for three weeks. i cried with anguish for jim, yet thanked my lucky stars that i did not go on board the _anchor_. chapter vi return and marriage i had now been in jersey eight years and things began to get rather dead there. work was scarce and wages very low, and if any one wanted a small job done, there were always about a dozen men ready to do it for almost nothing; and what made it so was the number of irish pensioners. they had their pensions, and of course they would do any kind of work for less wages than any other man who had to live entirely by his labour. in fact, things were in a state of stagnation everywhere. i went across to france, and the people there were quarrelling with king louis philippe and casting all the blame of their poverty upon his shoulders, and saying how much better they should get on if they could only have a napoleon to rule over them. they were chalking _vive napoléon_ upon the pavements and walls. i went back to england and found things were not much better there. thousands of poor people were half-starved and half-clothed, and when they asked for work and wages to buy bread for themselves and their little ones, the commander-in-chief was ready to fire a volley of grape shot down the street. england and some of the continental nations were at very low water in - , and i think at their very lowest ebb in . some monarchs were obliged to abdicate, and in london in april of that year riot and rebellion were rampant. several thousand people paraded the streets starving. in the house of commons some one declared that it would be a wholesome proceeding to hang a few rebels. up jumped fergus o'connor, and cried, "whenever you hang a rebel you should make a point of hanging a tyrant too, and rebellion would soon die a natural death." soldiers were placed in every town in england, lest the owners of hungry stomachs should show too much anxiety to fill them with bread. at the same time it was said that millions of quarters of foreign grain were brought within sight of the shores of england, but were thrown into the sea by order of rich merchants rather than that corn should be brought in to reduce the price. such is the greed of man. the only work that i could get after i left jersey was in a canvas manufactory in somersetshire at eight shillings a week, and this was considered very good wages then. here i found poverty and wretchedness and oppression supreme. there were about four hundred people employed in the various departments of this business; old men and women, nearing four score, and little boys and girls from five years of age and upwards. some worked in the factory, and some who had hand-looms took their work to their homes. several of them had to walk four miles carrying their woof with them, where they had handlooms; and so these poor toilers worked from early dawn until ten o'clock at night weaving their woof into sail-cloth or canvas. when these poor slaves came to the factory for their work, the foreman would weigh out to each person so many pounds of chain and as many of woof to each person. the weaver had to take it home on his back and produce exactly forty-six yards length of canvas two feet wide. now if he happened to put a little too much energy or muscular strength into the work, all the woof would be used up before forty-six yards were made. consequently he would have to send to the factory for a few ounces more woof to finish the pieces, and the cloth therefore would be a trifle too stout, and consequently the weaver would be fined to the tune of any amount, according to the greed and temper of the master, from sixpence up to five shillings and sixpence, which was the wage due for weaving the whole piece. on the other hand if the weaver happened to be weakly and unable to use the strength required, he would come to his forty-six yards' length before he had shot in all his woof. the cloth therefore would not be quite stout enough, and the poor weaver would be fined. any frivolous pretext was resorted to to fine the workers. many a time have i seen poor men or women after toiling hard all the week coming to the pay office for their wages, but instead of receiving any being cursed at and told that it was a very great favour on their employers' part to give them work at all. and so these poor slaves would have to do without the "weaver's ox" (red herring) for their sunday dinner. a red herring was the greatest luxury these poor people could indulge in, and thrice blest was he who could afford three red herrings a week. there were many other pretexts for fining the weaver besides those mentioned. another was what was called in weaving a "gout"; that is, in the course of weaving there were some thick and gouty parts in the woof, where the thread was twice or thrice as thick as it should be. if the weaver was not careful, and allowed one of these thick gouty threads into the texture of the cloth, he was heavily fined. the cloth was also examined under a magnifying glass, and if found pin-holey or spotted, the weaver was fined. all these were certainly very superficial excuses for fining the people, for the masters themselves would throw the canvas down on the floor and walk over it. the cottages where these poor people had to live and do this weaving were shocking hovels. this firm was carried on by a man and his three sons. each of the sons took up a certain department. one superintended the spinning department, another the bleaching, and the third the weaving. on a certain whit monday at the village club festival an old farmer, in an after-dinner speech, took the liberty to tell the three young men that it was very shabby and mean of them to fine these poor weavers in the manner they did, and he thought it amounted to little less than downright robbery. then the young man from the spinning department jumped up, and with an oath cried, "they," meaning his brothers, "do not half fine them here. you should come over to my place and see my books." in less than a week this man was seized with a fit and fell dead; and the poor people, in their simplicity, said it was a judgment sent from heaven. the owners of the firm evidently did not think so, for they continued to fine the people as usual. on the following whit monday more after-dinner speeches were being made when the head of the bleaching department boasted that he had six hundred pounds which he had mulcted from his workers in fines. he forthwith built a new house and furnished it in grand style, and sent me to taunton to buy him a grand piano. the day for opening the new house was appointed, friends were invited and every preparation made for a grand feast. the day arrived, and the young boaster was riding up and down on his high-mettled white horse inspecting the arrangements when the horse, treading upon a half-rotten turnip, plunged and fell, and rolled completely over its rider, who was taken up and carried into the new house where he had never lived, a corpse. the simple-minded people also called this a judgment from above. very soon after this a large fire occurred at this factory which threw a great number of men, women, boys and girls out of work. as for myself i soon engaged myself to a solicitor, and my duties varied between serving writs and gardening. every time i delivered a writ i had to be put on my oath the following morning that i had so delivered, and then i used to receive three shillings and sixpence for my trouble. sometimes i had to make three or four journeys before i could accomplish the delivery, as so many well-known gentlemen would cleverly keep out of my way. on one occasion i had to serve a writ on a gentleman who always managed to be away when i called; but my master told me that if i had reasons for believing that he was at home i should walk in. the next evening when i called and the servant told me her master had just gone out, i pushed past her, and got into the dining-room, where the gentleman was just cutting away at a piece of roast beef. i handed the document to him and, bowing politely, retired as hastily as possible. as i turned to the door i heard something whirr past my shoulder and strike the wall beyond. it was the carving knife. another difficult case was that of a master carpenter who lived several miles from my master's office. it was summer time, and he went away to his works as soon as it was daylight and was not at home until nine in the evening. i could never find this gentleman at home although i called several evenings in succession, and at last i grew tired of walking so many miles several times a week. as there was a porch at the door i decided to sit down there and wait until he did come home. i sat and dozed until about a.m., when i heard some one moving inside the house. presently the door opened, and out came the gentleman i wanted. he was thunderstruck to see me there. i delivered the writ into his hands, and we both said "thank you," and went our ways. after i had been in this situation two years it was a very disagreeable blow to me to hear that my master was retiring from business and would have no further need of my services. he and i had got on together very comfortably for two years, and he was a kind employer, so i was really very sorry. i was now twenty-five years of age, and, like other young men, i thought it time to begin to see about "committing matrimony." i had become acquainted with miss anne warner, who lived at henley-on-thames, and she decided that she would marry me, if i got a permanent situation either on the railway or in the post office. accordingly, i applied for the railway, and was appointed as porter at bristol in may, . the bristol and exeter railway at that time was in its infancy. amongst the articles served out to me was a wooden staff or truncheon, to be used, if necessary, for clearing the station. in october of the same year i was married to miss warner at st. mary redcliffe's church, bristol. i knew that i was not to remain long at bristol, so my wife and i took rooms for a while. i was very soon sent as signalman to stoke canon, a village in devonshire, where at that time there was no station, but only a crossing. i had to leave my wife in bristol until i could obtain a suitable home, which i did as soon as possible, as keeping two homes going was a very expensive thing to do, and my wages at that time were only _s._ a week. my only accommodation when on duty, which was twelve hours a day, or an alternate week's night duty, was a sort of sentry box--a wretched affair, especially in cold weather, and for night duty. while at stoke canon i made a model of the village church. the only tools i had for this work were a pocket knife and a hammer. some time after i added a peal of bells which were set ringing by putting a penny in a slot in the roof of the church. in our eldest daughter was born, and in the following year i was removed to martock in somersetshire, where a son was born. in i was removed to durston, and in may another daughter was born. my wages were then £ a week, a very small income on which to keep ourselves and three children and pay rent, but my wife kept a school and had several neighbours' children to teach. we were very happy together, and i was glad that i was born! god in his works dost them love nature? dost thou love amid her wonders oft to rove, marking earth, sea, the heavens above, with curious eye? read, then, that open book; see where the name of god, inscribed there, urges thee on till thou declare, "my god, i see!" yet venture not, my soul, to come within fair earth's material dome without thy god: thou hast no home to compass thee. nature's fair works must e'er be read as penned by nature's sovereign head; else were its loveliest pages dead-- without his key. but by the polar star of grace, nature assumes her proper place, and thou mayst safely lead and trace her harmony. m. m. c. chapter vii scientific achievements [by miss ellen langdon] the above chapters were written by my father roger langdon, and i, his daughter, ellen, am continuing the story of his life. so i will begin by saying that the school kept by my mother was conducted in the same manner as were the church schools at that time. everything was very orderly and we just had to mind our p's and q's. our parish church and school were five miles away, so it was only possible for us to go there occasionally. we usually made the journey sitting on a trolley which father pushed part of the way, and then we would clamber up the railway bank and walk on to church. but there was another church nearer, to which as soon as we were big enough father would take us on alternate sunday mornings, when he was off duty. we had a large front room which was used as a schoolroom, and here we, with our neighbours' children, were taught reading, writing, arithmetic, spelling, geography, sewing, and the catechism. when father was on night duty (the duty in those days was twelve hours at a time) he would be at home in the morning, and sometimes he would take us for scripture. in this his teaching was unorthodox and advanced, and he always gave us plenty to think about. when later on we went to a school at taunton we found ourselves in most subjects in advance of children who had attended schools in the town. during the eight years my parents spent at durston three more sons were born to them; so there were now six of us, and i have often wondered since how mother managed to keep the school going with such a large family of her own. my grandmother, my mother's mother, was a frequent visitor, and would also be at work all day long. my father's father too would often come, and he used to make us stand by the harmonium and sing. i would like to say that my grandfather was one of father matthew's earliest converts to teetotalism, and he tried his best to get others to believe in the same thing. at this time father made a harmonium, which proved such a success that he was able to sell it, and with the proceeds he bought material to make a second. he also made a magic-lantern, and made slides of the stars and sun and moon and comets, and at christmas time he would invite the neighbours to see the lantern, and he would give them all the information he could on the subject. he also told us about the electricity which was some day to light up our houses and town and drive our railways and carriages. he told us about photography, and how we should live to see "living pictures." then for us he made all sorts of mechanical toys--walking dolls, wooden horses and boats. the forms which were used in the schoolroom he also made, as well as tables, chairs, our boots, and his own, a little carriage for the smaller children, and later on a perambulator. he was able to make anything he wished, but of course it all meant labour, and never a moment's idleness. and he had to put up with a great deal of enmity on account of his being so set against the drink traffic, and never going to the _railway hotel_ to spend his earnings. while we were at durston we had the pleasure of seeing garibaldi. his train was crossed on to another line. we children were playing as usual on the bank above the station, and when garibaldi's carriage stopped right in front of us of course we all screamed with delight, and our noise brought everybody out, and the men got so excited they crawled all over the top of the carriage and shouted for all they were worth. in my father was removed to taunton. his duty was at norton fitzwarren crossing, where there was no station, so he had only a small office to sit in, which, as he still had the night duty, he began to find very trying. house rent at taunton was heavy, so mother applied for a schoolmistress's situation, and she kept this post for nearly two years. meanwhile another son was born. but we were not very happy at taunton, for father was often ill during the time we were there. the school in which mother taught was next to our house, and was also used on sundays as a mission chapel. when we first went there they used to chant the _magnificat_ and _nunc dimittis_, and a hymn, to the accompaniment of an accordion. father did not like the sound, especially when he had to sleep on sunday before going on night duty; so he offered to lend his new harmonium if they would take care of it and could find some one to play it. the curate, the rev. j. jackson, was much pleased with the idea, and he persuaded a crippled lady, miss emma mockeridge, to undertake the duties of organist. she used to be wheeled down on sundays to play, and twice in the week besides she would gather the children round her in the schoolroom and teach them the hymn. so she came in all weathers from quite a long distance to do this work, and after a time she began to visit the people who attended the service, and told them that since mr. langdon would not accept any payment for the use of his harmonium it was their duty to provide themselves with a new one of their own. before very long she succeeded in collecting enough money to buy one. at this time mr. jackson held evening classes for men and charged them a halfpenny a week. father with some others went to him to learn greek, and got on very well with it. he greatly enjoyed these classes, and in later life would say how grateful he was for them. while we were at taunton there was an election, and father had a vote. a great deal of bribery went on; indeed the conservative member was afterwards unseated for bribery. mr. henry james (now lord james) was the liberal candidate, and he won the election. my father told us all about it, and said if any one meeting us in the town, or calling at the house, should offer to give us children anything we must on no account take anything. i do not know who the people were, but they mostly called in the evening, and would offer us groceries and other things. one evening i had been to the station to meet father, when a gentlemanly looking person came up to him, saying, "mr. langdon, i believe." father raised his hat and said, "yes, that is my name." "you have got a vote?" "yes, i have." "well, are you going to give it to us?" "that is my business, not yours." "come now, don't be a fool," said the gentleman, "you have got a little family; what will you do it for?" at the same time holding up one finger. "i am not to be bought," father replied. the gentleman then went on holding up fingers till he had got all his ten fingers up, and father at last cried, "no, i tell you no," to which the other replied, "you thundering fool." father raised his hat with "thank you, and you are another," and off we went. my father was deeply interested in astronomy, which he had studied a great deal with the help of books, and he had bought celestial and terrestrial globes. now he wanted a telescope. mr. nicholetts, a dear old gentleman who lived at petherton, had a telescope; and he often invited father to his house to look through it, and this gave father great pleasure and increased his ambition to possess one of his own. so for a few shillings he bought some second-hand lenses, and soon succeeded in making a small telescope, with a - / -inch reflector mounted on a wooden stand and swivel. this small instrument only whetted his desire for something better; so he sold it for _s._ _d._ and with the money obtained materials for another. after many difficulties and disappointments, which by sheer luck and hard work he surmounted, this second telescope was at last completed. this one had a four-inch reflector, and with its aid the ring and some of the satellites of the planet saturn could be seen. the crescent form of venus and some of the nebulæ were also plainly visible. and when father first saw the moon through it he said he was fairly astonished, for up to that time he had no idea how much of the physical features of the moon could be seen. in father was appointed station-master at silverton in devonshire. it was at the end of that we left taunton and took up our abode in our new home; and thus began what father always described as the happiest time of his life. for one thing he had from this time forward no more night duty, and his health improved considerably in consequence, so that he became stronger than he had been for many years. he greatly rejoiced, too, that there was no drinking bar at this station. another great advantage was that we were now within reach of exeter where there was a good school to which the younger children could be sent daily by train. in march my youngest sister was born; so now there were eight of us. but the following year my eldest brother was killed by an accident at the station. this was a terrible blow to both my parents, and the trouble turned father's auburn hair as white as snow. at silverton father made many friends, amongst them sir thomas acland and our good rector, rev. h. fox strangways and his lady. in my father became acquainted through the _english mechanic_ with dr. blacklock, and this gentleman gave him advice regarding the building of his telescopes; but it was all done by letter, for they never met, and it was wonderful how dr. blacklock found time with all his work to write so many letters as he did. father also received letters from mr. nasmyth, the inventor of the steam-hammer, on the same subject. mr. nasmyth took great interest in him and would write two or three sheets at a time, pointing out the difficulties and explaining how they might be overcome, and drawing diagrams of the tools he would need. after father had made the speculum, which he found exceedingly difficult, it had to be silvered on the front surface; and on this point dr. blacklock gave him valuable information which enabled father to do it successfully at the very first attempt. in father made a model in plaster of paris of the visible hemisphere of the moon, showing five hundred principal objects, hollows, craters, and mountains. this model he afterwards presented to the devon and exeter institute. mr. c. r. collins of teignmouth once wrote an article describing the discovery of a new crater on the moon by dr. hermann j. klein of cologne. going to his observatory father was able to show on this model of the moon, as the result of his own observations, this very crater. father had now made two telescopes, but he hoped to make another and still better one; so he set to work, and it was in the making of this that he received so much valuable advice from mr. nasmyth and dr. blacklock. this third telescope was a beautiful instrument. it had a six-inch speculum with a five foot focal length. with this he was enabled to detect certain markings upon the planet venus. in he read a paper before the royal astronomical society in london upon this subject. he said afterwards that he never was so nervous in his life as on this occasion, and he wished the earth would open and swallow him up. but his paper was very well received, and commended.[ ] he also made over a thousand drawings and photographs of the moon's surface. [ ] webb quotes from this paper in his book _celestial objects for common telescopes_. my father's observations are also mentioned in clarke's _history of astronomy during the nineteenth century_. much as father had accomplished, he was still bitten with the idea that his telescope was not so good as he would like, although it was a splendid one, and had cost him many weary hours' hard work to make. so he sold it for £ , and in the year set about making his fourth telescope--a very noble instrument, for which he had to build an observatory. he describes it thus: "an - / -inch silver-on-glass reflector mounted in a stout zinc tube, which turns in a cast-iron cradle on its own axis. the focal length is seven feet. there is a diagonal place for viewing the stars and a specially prepared glass wedge for observing the sun. the whole is mounted as an equatorial upon a strong cast-iron stand. it had two stout brass right ascension circles divided to seconds, and declination circles divided to minutes of arc. the telescope is furnished with a driving clock which keeps the celestial object in the field of view. the observatory is a circular iron building with conical-shaped revolving roof, two swing flaps of which give the required opening to the sky." this telescope took a long time to make, and night after night through many a weary month, when station duty was done, father would work at it for hours together in his home-made work-shop. but, as usual, the want of funds hampered him a good deal, and he found many difficulties to overcome; but he worked away with intense enthusiasm, and with the advice of dr. blacklock and mr. nasmyth, he at last completed this newtonian equatorial reflecting telescope fitted with a finder with ramsden eye-piece. he added to it a trap for taking photographs, the invention of his own brain, and in visiting greenwich observatory some years later he was pleased to find that the apparatus in use there for the same purpose was almost identical with his own. with this telescope my father photographed the transit of venus and took also several pictures of the sun and of the moon. to make his first telescope in he bought some second-hand lenses for a few shillings, and by means of a turning lathe he turned a stick upon which to roll the tin case, according to the size of the lens. the second telescope was a much more difficult undertaking for one whose acquaintance with mechanical processes was entirely self-acquired. he was as a man groping in the darkness. to obtain the special glass necessary for the speculum; to grind it to the most delicately accurate shape and density; to polish and silver the speculum; to make the metal tube to the requisite size and scale; to mount it with the necessary adjustments and accuracy; all these were so many enigmas which only his intense enthusiasm and perseverance enabled him to solve. to grind the speculum of the third telescope a special and very curious tool was necessary, and here mr. nasmyth gave father valuable information and sent a drawing of the tool. after making this tool according to mr. nasmyth's pattern, father found it applicable to metal specula only, and unsuitable to glass inasmuch as it would not parabolize, or work in the figure of a parabola. this was a great blow. however he eventually surmounted it by using ross's machine, a description of which he came across; after considerable inquiry. this grinding completed, he succeeded in polishing his speculum with a disc of pitch squares, an apparatus which gave him much thought and trouble. then came the silvering, and then the rolling and soldering of the tube, which was accomplished by means of a circular block of wood turned in the lathe to the required size. here another difficulty presented itself, for when it was done, the wood was found immovably fixed inside the tube, and it became necessary to procure a steel augur to bore it out. however, this taught father something, for in making the next telescope he used a number of laths fixed together and turned in the lathe. one of them was cut through diagonally so that the two parts when separated formed wedge-shaped sections, which could be readily knocked out when the case had been fixed; and thus the whole circular bundle could be easily removed. i know that the building of these telescopes was real hard work, and the difficulties and disappointments they involved were numerous, and were only overcome by sheer hard work and indomitable perseverance. the fourth telescope he had reason to be proud of. he was assisted with the adjustment of this as well as in making the pitch plate with which to polish the speculum by mr. newton, a gentleman from taunton. this was the telescope for which father built the observatory, and it has been described as a real triumph of skill. many a time after his day's work was done he would take his magic-lantern and give a lantern lecture on astronomy. he also wrote a paper, "a letter from the man in the moon," which was published in the _exe valley magazine_. another paper, "a journey with coggia's comet," appeared in _home words_. some of his mechanical toys, stoke canon church with its peal of bells, ships rocking on the ocean, and others, have often been shown at church sales of work, and so helped the funds. chapter viii closing years on several occasions during the early years at silverton, my father had trouble with drunken passengers. on one occasion a certain book salesman came to the station and called for a ticket to exeter, for which he tendered _d._ the parliamentary fare being _d._, my father asked him for the other _d._ the man began to abuse him and got on to the line, and would have been killed by an express, but father jumped down and dragged him back just in time to save both their lives. the man then struck father in the face with his umbrella and swore tremendously. after some trouble father succeeded in placing him outside the station gate and locked him out. the man finally paid _d._ for his ticket, and then threatened to kill father. of course he was summoned and had to pay heavy fines. father wrote regarding this case: "if i had caused the death of this man, i should have had to do at least twelve months' hard labour in one of her majesty's country mansions, and there would have been two and a half columns in _the times_, _the standard_, and _the daily telegraph_, expatiating on the carelessness of railway officials; but having saved his life at great risk of my own, i received as complete and satisfactory a blackguarding as it is possible to conceive." on another occasion father was not so fortunate in averting disaster. in a lady came to the station in her carriage to meet some friends who were going to spend easter with her. it was the day before good friday, and the trains were very late. the friends were coming in the down train from london, and she also wanted to see her son-in-law who was passing in the up train. while waiting she constantly crossed the line, first to the down platform, then to the up. a down train was signalled and off she went to the down platform. she was a very genial person and had been chatting pleasantly to every waiting passenger. this train was an express, and as soon as it passed by she saw an up train approaching. she immediately attempted to cross the line, probably thinking it was the stopping train, instead of which it was an express. father rushed out to warn her, but it was too late, for the engine was upon her, and she was instantly killed. the shock was very great to both of my parents and they could not sleep for weeks. up to father was single-handed, and used often to be on duty from a.m. till o'clock the following morning waiting for a coal train which used to come at any time in those days. afterwards a signalman was appointed, and then father's duty hours were a.m. till p.m. there are several people still living, both rich and poor, who could record the great courtesy they received at silverton station. many a time he would help some poor person along the lanes with her babies or her bundles, or show the way with his lamp to some benighted place perhaps two or three miles distant. in the autumn of father was very ill, and when he got better he was ordered away for a change of air, and i had the pleasure of going with him. we visited some relations and went on to london. if we had gone to the dullest place in the world i should have been quite happy so long as father was with me, for on all occasions he was just the same age as his children. but as it was we went to all the interesting places, and i don't know which of us enjoyed things the most. about this time the great western railway company took over what had been known as the bristol and exeter railway, and began to lay down narrow gauge-lines. the line near silverton runs through a valley, picturesque but wet. the station itself is about thirty feet lower than the floor of exeter cathedral. for several weeks during this time there had been a great deal of rain and the valley contained more water than usual. torrents ran down from the hills and flooded the valley of the culm to a depth of over five feet. a culvert which drains the main part of these hills passes under the railway close to silverton station, and this became blocked by two hurdles which had been carried down the stream and become fixed in an upright position right across the mouth of the culvert. consequently, leaves, brushwood, thousands of apples and other rubbish got fixed on one side of the hurdles, completely staying the torrent. the railway was quickly flooded, and at . p.m., after the station was closed for the night, down came the express known as "madame neilson's train," because it conveyed her regularly from london to her devonshire home. owing to the work of laying down the narrow-gauge rails there were a great many timbers collected on that part of the line, and these were lifted by the sudden flood, and floated about on the water. mother went to look at the flood just in time to see the express coming at a speed of sixty or seventy miles an hour, and she wondered if it would get safely through. the thought had scarcely entered her head when she saw the great engine rear itself up, as if it were a real live thing, then as suddenly drop down again, and she knew that it was off the line. the passengers got a shaking, but were otherwise none the worse, not even wetting their feet as they passed over planks laid across from their carriages to the platform. madame neilson and madame patti were both among the passengers, so here was a lively night for my mother and brothers. there is no railway hotel or other house near, so mother did her best to accommodate all these people, who were dreadfully hungry. they soon ate up all that was in our house, and there they had to wait for a relief train from exeter. my two young brothers were called out of their beds, to escort two gentlemen to the village of silverton two miles away. they started off full of excitement, and when they were about a quarter of a mile away the water was nearly up to their necks; but they all four went on, and my brothers had to try and get some bread for the hungry people. so they arrived in due course wet through and tired out, but they were none the worse the next day. altogether it was a most exciting night. father traced the origin of the flood and drew up plans, and received the thanks of the railway company. now there are some people, especially those who live in large towns, who may think that a small country station is a very dull place to live in, but that is because they have never tried it. apart from such occasional and exciting events as that just described, the country has interests and amusements of its own. when country people are waiting for a local train, particularly if it is a market train, and all the passengers, both rich and poor, are more or less acquainted with each other, every topic is discussed, and if the station-master has a few minutes to spare his opinion is sure to be asked. for example, when doctor temple was appointed bishop of exeter, it made such a stir that it was the talk of every one, and father's opinion was asked by every passenger. father had read about dr. temple, though he had not seen him. his reply to their question was always the same: "i rejoice to know that dr. temple is appointed; such men are needed in the church very much indeed. he will be the right man in the right place, and he will thoroughly do his duty, and he will be a hard worker. moreover, he will make the clergy work, and it is a thousand pities that so many churchmen have not yet realized what a strong man they will have amongst them." then up spake a countryman, "then, du yu 'old way un bein' a tay-totler, mr. langdon?" "why, of course i do," replied father. "that is the essence of the whole matter, and that is just why the exeter people are against him; but i for one am thankful, and i think it a great gain to the church of england to have at last a bishop who holds such opinions." "well now to be sure, mr. langdon, i knowed thee was long-headed, and i knowed thee was an ole liberal, now i knows thee beest an ole tay-totler." father was a broad churchman, and wished for church reform. he liked to hear good music and used to go regularly on sunday evenings to thorverton church, as the services there were very much to his liking. he used to say that the church was behind the times, and did not reach the people generally, chiefly because the clergy were all taken from one class, and in many cases they did not understand the poor. they were also educated over the heads of the people. in politics he was a supporter of mr. gladstone. my father had now taken up photography and had made a collapsible dark room that he could carry on his back. he succeeded, after a few failures, in taking some very good pictures of the moon in ; and in december he took a good one of the transit of venus. he also made an instantaneous shutter to his camera, by the help of which he was able to get some very good pictures, notably one of the old broad-gauge train known as the "flying dutchman." this was before dry plates were invented. the next thing he made was an excellent camera. a gentleman named mr. wellington gave him information both in regard to this and to photography in general, for which father was very grateful. in the winter of there was very deep snow, and our house was snowed up to a height of several feet, so that before he could open the station father had to dig his way out. no trains ran that day, and only one up and one down on the two succeeding days. after that the line was pretty clear again. in father and mother received another fearful blow by the death of my youngest brother, after a very short illness, at his lodgings in exeter. after this time father never appeared to be very well, and before long his health entirely gave way. during these years several visitors came to father's observatory, among them mr. clifton lambert, son of the general manager of the great western railway. this gentleman wrote a sketch of father's life, which was published in three different papers, _wit and wisdom_, june , the _great western railway magazine_, september , and in the summer number _western weekly news_, , just one month before he died. visitors would call at all hours, in the day to see the spots on the sun, and in the evening and at night to see the moon and stars. for many a year father had been a wonder to the simple country folk. they could not understand a man devoting his spare time to the study of the heavens, for the mere love of science. they had an idea that he could "rule the planets." father used to say that he was astonished at the amount of superstition prevailing in the minds of all sorts of people, not only the uneducated. even well-educated people would ask him if he could "rule their planets." he would say that he was ashamed to hear them ask such a question. people would come from long distances in the dead of night to have a look through the telescope. when asked how he had achieved so much, and brought up a large family in respectability, his answer was: "it is through the woman that the almighty gave me; she has done the most." it was in march that i went home to find father suffering from a complaint from which he could not hope to recover. he got gradually worse, and in july i was sent for again to come at once if i wished to see him alive. mother had done all she could do for him, and i was very much shocked to find him suffering dreadfully. but he was very cheerful, only longing for his sufferings to end. it was a painful time, but as we all gathered around his bed, he often made us laugh by his jokes. then there were quieter moments when he prayed the almighty to take him. on the evening before he died, he repeated part of psalm li. to me and some hymns, and that was the last. he passed peacefully away in the morning. through the kindness of sir thomas acland, he was buried in the private burial ground of the acland family, near his two sons. so live that thy summons comes to join the innumerable caravan which moves to that mysterious realm where each shall take his chamber in the silent halls of death. thou go not, like the quarry slave at night, scourged to his dungeon, but sustained and soothed. by an unfaltering trust, approach thy grave like one who wraps the drapery of his couch about him, and lies down to pleasant dreams. * * * * * speak of me as i am, nothing extenuate, nor set down aught in malice. appendices appendix i a list of parish clerks of the parish of chisleborough, copied from the register roger langdon, doctor of music, and clerk } of the parish of chisleborough, in the } county of somerset, from the year } years. to . } his son, james langdon, held the clerkship } . from until . } years. edward langdon, grandson of roger, held } the office from until , also } . organist. (father of the subject of } years. memoir.) } edward langdon, great grandson of roger, } . was also parish clerk and organist from } years. until . } peter langdon next held this same position } and died recently. great-great-grandson } . of roger langdon. } appendix ii observations of the planet venus, with a -inch silvered glass reflector. by r. langdon _communicated by j. norman lockyer, f.r.s._ [from the "monthly notices" of the royal astronomical society. vols. - , june, ] on may , , i had a good view of the planet venus, but i could not at first see her to my satisfaction as her light was so bright. she had more the appearance of a miniature sun than a star; but i put a diaphragm of blackened card in the eye-piece, and made a small hole through its centre with a piece of hot wire. i found this arrangement keep out to a great extent the glaring rays. i also sometimes used a slip of slightly tinted glass in front of the eye lens; this enabled me to bring the planet entirely under subjection. her shape was that of the moon when a little more than half full. i distinctly saw a dull, cloudy-looking mark along her bright limb, curving round parallel to it, and extending nearly across the disc, each end terminating in a point; joining this at the eastern extremity was another and darker mark of a club shape, its small end joining the point of the mark previously described. i watched these marks for half an hour. i saw some marks again the next evening, but before i could examine them the planet was hidden behind some clouds. on may , at . p.m., there was a cloud-like mark extending straight across the disc, and a club-shaped mark nearly in the centre, with its small end nearly touching the straight cloud. on the western limb another dark mark had made its appearance; it was not quite so large as the other, and it was not club-shaped; but its sides were parallel to each other till they approached the straight cloud, when they appeared to divide, each side curving round away from the other. i took much interest in watching these spots, as i had read that it was very doubtful whether any marks had ever been seen on this planet. i called several men to look at them, and they were able to describe them, although they had no previous knowledge or idea of what they were likely to see. one man was very confident it was the moon he was looking at, but when i pointed out to him that the moon was not in the neighbourhood, he said he thought it was the moon, because he could plainly see the dark patches on its surface. on may , at . p.m., there was a dark mark of a pear shape, extending from near the western edge to two-thirds the distance across the bright disc. this mark was not so dark as those seen on the st and th, but it was much larger. on july , at p.m., there were visible five dusky marks along the planet's terminator, and one nearly in the centre of the crescent, but they were not so well defined as those before described; but what seemed to me more remarkable was that the southern horn was rounded off considerably, whilst the northern horn was quite sharp, and ran out to a very fine thread-like point. on october , at . a.m., i saw venus as a beautiful little crescent. she was well defined, and both horns were as sharp as the finest pointed needles. i think i detected a dusky cloud-like mark about half way from the centre to the northern horn; but i am not quite sure about this as i had to leave my telescope before i could complete my sketch. on october , at . a.m., i was gratified with a sight which i had waited for and longed to see for many years; that was to have a good view of venus by daylight. i now had the longed-for opportunity, and it turned out as i expected. the superior light of the sun overcame that of the planet to such an extent that i was able to see her better than i had ever seen her before. i could now plainly perceive the jagged nature of the terminator, the unevenness of which could not be mistaken; but what was very remarkable, the northern horn was bent in towards the centre of the planet; it appeared as if a notch had been cut in the inside, and a slice cut off from the outside. i have no idea what was the cause of this appearance; i had never seen it so before, neither do i recollect ever having read of such a phenomenon. i did not perceive any markings on this occasion, but there was a kind of haziness along the whole length of the terminator; but i considered this at the time to have belonged to the terminator rather than to any markings on the disc. the terminator on this occasion was inky black. on november i saw venus every half hour during the day up to one o'clock. i made a sketch at . p.m. i could now distinctly see the jagged terminator; the nature of which was so much like that of the moon as it was possible to conceive; except that if we compare the moon's terminator to a piece of network, that of venus would be represented by a piece of fine lace. i could also see some thin, cloudy marks on her disc. the southern horn was very sharp; the northern one was a trifle rounded. i saw venus on february , (a few days before her inferior conjunction with the sun), and the bright part was an exceedingly beautiful fine crescent; but i and several other people could see the whole body of the planet in the same manner as we see the dark limb of the moon when _earth-shine_ is falling upon it; but i did not make any sketch at the time. i have observed venus a great many times besides those mentioned above, having made it my special work to do so, and have on several occasions strongly suspected markings to be visible; but i have not mentioned them, and have only described those times upon which i have no doubt of what i had seen. silverton station, near cullompton, devon. (sketches illustrating this and the following paper can be seen at the royal astronomical society's rooms.) appendix iii observations of the planet venus, with a -inch silvered glass reflector. by r. langdon [from the "monthly notices" of the royal astronomical society, june . vol. , page .] on january , , there was a cloudy mark, of a semicircular shape, extending nearly across the disc, and a dark spot in the centre; the illuminated disc itself was singularly egg-shaped. bad weather prevented me from constantly observing this planet, as i should like to have done, but on april , at p.m., i was viewing the planet with one of mr. browning's excellent achromatic eye-pieces, when i saw two exceedingly bright spots on the crescent--one close to the terminator towards the eastern horn, and the other in the centre of the crescent. these spots appeared like two drops of dew; they were glistening in such a manner as to cause the surrounding parts of the bright crescent to appear dull by contrast. cloudy weather prevented me seeing the planet again until the th, when the spots had disappeared, but the planet on this occasion was seen through the aurora, and the irregular and uneven appearance of the terminator was most beautifully depicted. the whole body of the planet also was distinctly visible. appendix iv a letter from the man in the moon (_left for the editor at the railway station_) [reprinted from the _exe valley magazine_] dear cousin,-- knowing how exceedingly anxious you must be to find out all you can respecting this little planet on which i live, i take this opportunity to send you a few lines to give you some little account of it. the moon, in many particulars, is like the earth on which you dwell; and perhaps there is no better way to give you a little more information about this planet than by instituting a comparison between it and the earth. i must presume you are aware that the earth is a globe, nearly round, like an orange; its circumference is about , miles, and its diameter , . the moon in this respect is like the earth, being also a globe, but it is only , miles in diameter, and about , miles in circumference. it would therefore take forty-nine moons to compose a globe the size of the earth. if you will take two threads and suspend an orange and a small cherry at six feet apart, you will then have a fair representation of the relative size and distance from each other of the earth and the moon. but the earth and the moon are not suspended by any visible or tangible object, but were launched forth in the beginning, and are kept in their places by the balance of attraction, constantly revolving, and travelling onward by the direction of him who also created the insignificant worm, and whose tender care is over all his works. you know, dear cousin, that the surface of the earth is diversified with large continents, which are dotted with chains of mountains and high hills, some of which are in a state of volcanic eruption. you have also great oceans of water lying in the hollows of your world. in the moon, too, we have mountains and hills, some of them very high and steep, thrown up ages ago by volcanic agency, though at present there is not a trace of existing fire or volcanic action, and you may safely consider the whole mass of the moon to be a huge, exhausted, burnt-out cinder. your mountains and hills are denuded--worn down--their sharp points and angles are worn away by frost, rain, and snow, and other atmospheric influences which have been constantly acting upon them for ages; but here in the moon we have no such thing as an atmosphere: we therefore have neither clouds nor rain, nor frost nor snow; and in the words of the poet-- here are no storms, no noise, but silence and eternal sleep. all here is as quiet and silent as the grave. sometimes, from the great heat of the sun, great masses of rock will split and crack, and come tumbling down from the sides of the cliffs; yet if you were close on the spot you would not hear the slightest sound, because there is no atmosphere by which sound can be propagated and conveyed. your fields are clad with verdure, and your pastures with flocks, so that as one of your inspired poets has sung-- the valley stands so thick with corn that they do laugh and sing, but neither verdure nor corn can exist upon the moon, as no plant-life can grow in a vacuum where there is no moisture. the crater mountains of the moon are its grand peculiarities. we see here that its whole surface has been upturned, convulsed and dislocated with forces of the greatest activity, the results of which remain to this day; so that our rocks are not levelled down by the fury of tempests, nor smoothed by the constant flow of water, as your earthly mountains are, but stand up in all their primitive sharpness. these volcanic craters are of all sizes, from fifty yards to as many miles across, and in the centre of some of them there stand up lofty hills. now if you could take up your position upon the highest peak of one of these central hills at the time the sun was rising, you would see the tops of the distant mountains forming a circle round you all illuminated by the sun's light; but as there is no atmosphere, there is no twilight, and consequently the great valley immediately beneath your feet would be in the very blackest of darkness. i know that you and others have often wondered what those dark grey patches are which you can see upon the moon, even with the unassisted eye. some people call them "the man in the moon, and his bundle of sticks," and the story goes that i went stealing sticks on a sunday, and for my wickedness was banished (sticks and all) into the moon! now i most strongly protest against this cruel libel; i never stole any sticks, even on a week-day, much less on a sunday, and i must say the people must have dreadful weak eyesight, and a dreadfully strong imagination, to see anything in these dark patches that can possibly be stretched into the shape of a man with a bundle of sticks at his back. so i hope you will kindly contradict this calumnious story whenever you can; indeed, in writing, it was partly my object to ask you to do so. one of the smallest dark markings that you can see on the moon with the naked eye is known to selenographers by the name of mare crisium, or the crisian sea; its width across from north to south is miles, and its length is miles from east to west, and it contains about , square miles, more than half as much again as the area of england and wales--rather a large size for a bundle of sticks, i opine. there are several other dark or grey patches on the moon, some smaller and some larger than the mare crisium, but they are all the beds or bottoms of what were once oceans, seas, and lakes, the waters of which have been dried up or evaporated many years ago. some think they have all gone over to that side of the moon which never turns round towards you, but i can tell you that is not the case; for if any water did exist on the moon's surface, the attraction of the earth would certainly draw it round to that side nearest to you, and so you would be able to see some signs of it, as well as clouds and vapours which would rise from it during the time of full moon. there are many other objects of interest, which i could mention to you, but i must draw my letter to a close; i will therefore only just give you the names of a few of those dark hollows which you can see with the unaided eye when the full moon is shining brightly. there is the "sea of tranquillity": its width from north to south is miles, and from east to west miles. there are also the "sea of serenity," the "sea of fogs," the "frozen sea," the "sea of vapours" and the "gulf of rainbows." this last named will appear to you of a greenish tint, and it is surrounded on nearly all sides with very lofty and steep mountains, some of them more than , feet high. then there are the "ocean of storms," the "gulf of dew," and the "sea of humours." this last will also appear of a green tint; it is very level, and is miles across. next come the "sea of nectar" and the "sea of fertility." all these were named "seas," because the ancient astronomers thought they contained water, and that they really were seas; but you are aware now that they contain no trace of water, so i need not inform you of that fact. and now dear cousin, i sincerely hope that what i have written will interest you, and if it does, and you will kindly let me know, i will write you another letter at some future time; but for the present i will say--farewell! your faithful servant and attached cousin, "the man in the moon." r. langdon. appendix v a journey with coggia's comet [reprinted from _home words_] this comet, which last year excited so much interest, is supposed by some to be the same which appeared in the year . if so, it is beyond the power of the human intellect to calculate the number of miles (millions upon millions) which it has travelled since that date; we may, however, in imagination, travel with it on one of its journeys. starting off then, as soon as it has made its perihelion passage, we are carried in the course of about six months to such a distance that this comparatively insignificant world (of which nevertheless we are all anxious to get a good slice) disappears entirely from our view, and the larger planets of this system are reduced to mere specks of light. the sun itself, which here scorches us at noonday, only appears there as a very minute star, just a small yellow speck. but meantime other suns, some of them of far greater magnitude and superior brilliancy than the sun we have left behind, gradually come into sight, and some of the "nebulæ," which appear to us here as so many bits of faint hazy light, some of them no larger than a crown piece, now appear to our unassisted vision in all the glorious majesty of suns and worlds and systems of worlds, all revolving round each other in the most regular and systematic order; for, as milton says in _paradise lost_, "order is heaven's first law." after our steed had carried us for the space of about seventy years in a direct onward course through systems of worlds by us from this world unseen, we should begin to return homewards, but by a different route from that by which we went out; and we should consequently have a constantly varying scene presented to our view. how awfully grand, for instance, would be the change, as we gradually lost sight of our yellow sun, to find ourselves arriving in sight and under the influence of a sun of a rich crimson red colour, and again after a few years to find ourselves in the presence of a green or a blue sun! yet it is more than probable that such would be the case, for the sky is spangled with suns of all colours. in the course of about years from the time when we set out we should be returned sufficiently near to this world to enable its inhabitants to catch sight of our steed's tail. and then, after all this long journey among the stars of years, we should have seen but a mere atom, just one grain of the works of him who knows the number of the hairs of our heads, and without whom a sparrow doth not fall to the ground! suppose we now inquire, what is the comet's probable business in coming amongst us once in years? are its duties those of a messenger or a scavenger, or both? it is well known that the sun is continually giving off light and heat, and consequently it must of necessity be gradually exhausting itself. it has been computed that were the mass of the sun composed of newcastle coal, with exhaustion going on at the present rate, the whole mass would be burnt out in , years. if the sun, then, is gradually being exhausted by giving off light and heat to his family of planets, and if the planets cannot give any portion of it back to him, seeing that they are entirely dependent upon the sun for their own physical and material existence--how is the sun's strength to be kept up so as to be equal to the demands made upon him? it is but reasonable to suppose that the comets (of which the firmament is said to be as full as the sea is of fishes) should bring some subtle fluid of which this system is being exhausted, and at the same time collect and carry away to other systems some noxious gas or other essence of which we have a superfluity, but which might be quite essential to the well-being of some other system; and that so a sort of healthy circulation in the universe around us might be kept up. ought we not, therefore, to look upon the appearance of a comet with some such feelings (only in a wider sense) as we would hail the arrival of a ship from a long voyage to a foreign clime, feeling sure it must come laden with some good store for the benefit of those whose business it is to stay at home performing faithfully their own several duties in their own several spheres? r. langdon. appendix vi the planet venus my dear nelly-bly,-- according to promise i send a sketch showing the different positions of the planet venus with regard to the earth during the past few months. i am astonished, not to say grieved, at the very great amount of ignorance and superstition which exists respecting the apparition of this planet recently as a "morning star." if you will refer to the sketch i will try and point out the various positions. june , , venus began to appear low down towards the western horizon as an "evening star," but as the evenings were then light i suppose it did not attract public attention. daily, however, the planet for a time was seen--after sunset--higher and higher in the western sky, until august , when it arrived in such a position with respect to the earth that it sent towards us the greatest amount of reflected light that it is possible it can send at any given time. the planet travelling through space in her orbit at the rate of sixty-nine thousand miles an hour overtook the earth (which is travelling in the same direction at fifty-eight thousand miles an hour) on september , when she was exactly in a straight line between us and the sun--called astronomically, "inferior conjunction." the moment she passes this point she becomes a morning star. she still moves on and leaves the earth behind, and when she arrives at the position shown on october she is at her greatest brilliancy as a morning star. from this time the planet's distance from us is rapidly increasing, and consequently her apparent size and brilliancy are as rapidly decreasing, and she is soon altogether lost in the rays of the sun and can only be seen by the aid of a telescope. venus makes a complete revolution round the sun in days and hours, but as the earth moves in the same direction but at a slower rate the planet overtakes the earth in about nineteen months, when we have her again as an evening and morning star respectively as before, and so on continually. and this is the star of bethlehem which has caused such a stir within the past two months. all sorts of ridiculous speculation and superstitious nonsense have been said concerning it. verily in this our day of rapid advancement we are almost, if not quite, as ignorant of astronomical matters as were the "wise men" of the east nearly two thousand years ago, or the natives of zululand of the present day. i hope i have made this plain to you; or if there is anything you do not understand, just ask the question and i will endeavour to supply the information. yours affectionately, r. langdon. ps.--it is rather singular that venus rotates upon her axis in such time as to produce a leap year once in four years as with us.--r. l. appendix vii silverton, _jan. , ._ dear bly,-- i thought i would send a sketch of the eclipse annie and i stayed up to see. we had a very beautiful, clear night, not a cloud to be seen. the moon entered its eastern edge into the penumbra at . and into the dark shadow at . ; and at p.m. the moon was in the centre of the shadow or totally eclipsed, but we could still see it appearing like a large orange and we could see all the principal craters and mountains through the shadow, and i was very interested to watch the stars disappear one after another behind the edge of the moon. of course i have not shown the sun in the sketch because there is not room, but you must imagine the sun to be about the size of a round table a good distance away to the left of the earth. now i will try and explain to you what the penumbra is and how it is produced. it is produced simply because the sun is so much larger than the earth, and you can make a little experiment and show freddy and william how it is done. place two lighted candles or lamps about the width of this paper apart which will represent the sun, then a little way off on a white cloth on the table place a teacup upside down which will do very well to represent the earth, then you will have the black shadow of the penumbra or half shadow on each side of it. a sixpence will represent the moon, which you can slide gradually across the shadows, and you will have the eclipse to a "t." it is necessary to have two candles or lamps because the sun is so much greater than the earth and the two candles' light combined represents the sun's light. a small orange perhaps would represent the earth as well or better than a teacup, because the shadow of a cup would not run out to a sharp point, but that of an orange would. that would not matter much either way, only freddy would most likely be asking the question and you would not be able to answer him. your affectionate dad. [printer's logo] * * * * * [transcriber's notes: page , "shilings" changed to "shillings" page , the word "to" may have been omitted before the word "each". it has been added in this text. "we got near enough to speak to each other ..."] generously made available by internet archive (https://archive.org) note: project gutenberg also has an html version of this file which includes the original illustrations. see -h.htm or -h.zip: (http://www.gutenberg.org/files/ / -h/ -h.htm) or (http://www.gutenberg.org/files/ / -h.zip) images of the original pages are available through internet archive. see https://archive.org/details/heavensabovepopu gillrich transcriber's note: text enclosed by underscores is in italics (_italics_). superscripts, such as p to the second power, are shown by the caret character "^" before the superscript, such as p^ . subscripts are similarly shown by an underscore before the subscript which is wrapped in curly braces, such as m_{ }. [illustration: spectra of various sources of light.] the heavens above: a popular handbook of astronomy the heavens above: a popular handbook of astronomy. by j. a. gillet, professor of physics in the normal college of the city of new york, and w. j. rolfe, formerly head master of the high school, cambridge, mass. with six lithographic plates and four hundred and sixty wood engravings. potter, ainsworth, & co., new york and chicago. . copyright by j. a. gillet and w. j. rolfe, . franklin press: rand, avery, and company, boston. preface. it has been the aim of the authors to give in this little book a brief, simple, and accurate account of the heavens as they are known to astronomers of the present day. it is believed that there is nothing in the book beyond the comprehension of readers of ordinary intelligence, and that it contains all the information on the subject of astronomy that is needful to a person of ordinary culture. the authors have carefully avoided dry and abstruse mathematical calculations, yet they have sought to make clear the methods by which astronomers have gained their knowledge of the heavens. the various kinds of telescopes and spectroscopes have been described, and their use in the study of the heavens has been fully explained. the cuts with which the book is illustrated have been drawn from all available sources; and it is believed that they excel in number, freshness, beauty, and accuracy those to be found in any similar work. the lithographic plates are, with a single exception, reductions of the plates prepared at the observatory at cambridge, mass. the remaining lithographic plate is a reduced copy of professor langley's celebrated sun-spot engraving. many of the views of the moon are from drawings made from the photographs in carpenter and nasmyth's work on the moon. the majority of the cuts illustrating the solar system are copied from the french edition of guillemin's "heavens." most of the remainder are from lockyer's "solar physics," young's "sun," and other recent authorities. the cuts illustrating comets, meteors, and nebulæ, are nearly all taken from the french editions of guillemin's "comets" and guillemin's "heavens." contents. i. the celestial sphere ii. the solar system i. theory of the solar system the ptolemaic system the copernican system tycho brahe's system kepler's system the newtonian system ii. the sun and planets i. the earth form and size day and night the seasons tides the day and time the year weight of the earth and precession ii. the moon distance, size, and motions the atmosphere of the moon the surface of the moon iii. inferior and superior planets inferior planets superior planets iv. the sun i. magnitude and distance of the sun ii. physical and chemical condition of the sun physical condition of the sun the spectroscope spectra chemical constitution of the sun motion at the surface of the sun iii. the photosphere and sun-spots the photosphere sun-spots iv. the chromosphere and prominences v. the corona v. eclipses vi. the three groups of planets i. general characteristics of the groups ii. the inner group of planets mercury venus mars iii. the asteroids iv. outer group of planets jupiter the satellites of jupiter saturn the planet and his moons the rings of saturn uranus neptune vii. comets and meteors i. comets general phenomena of comets motion and origin of comets remarkable comets connection between meteors and comets, physical and chemical constitution of comets ii. the zodiacal light iii. the stellar universe i. general aspect of the heavens ii. the stars the constellations clusters double and multiple stars new and variable stars distance of the stars proper motion of the stars chemical and physical constitution of the stars iii. nebulæ classification of nebulæ irregular nebulæ spiral nebulæ the nebular hypothesis iv. the structure of the stellar universe i. the celestial sphere. i. _the sphere._--a _sphere_ is a solid figure bounded by a surface which curves equally in all directions at every point. the rate at which the surface curves is called the _curvature_ of the sphere. the smaller the sphere, the greater is its curvature. every point on the surface of a sphere is equally distant from a point within, called the _centre_ of the sphere. the _circumference_ of a sphere is the distance around its centre. the _diameter_ of a sphere is the distance through its centre. the _radius_ of a sphere is the distance from the surface to the centre. the surfaces of two spheres are to each other as the squares of their radii or diameters; and the volumes of two spheres are to each other as the cubes of their radii or diameters. distances on the surface of a sphere are usually denoted in _degrees_. a degree is / of the circumference of the sphere. the larger a sphere, the longer are the degrees on it. a curve described about any point on the surface of a sphere, with a radius of uniform length, will be a circle. as the radius of a circle described on a sphere is a curved line, its length is usually denoted in degrees. the circle described on the surface of a sphere increases with the length of the radius, until the radius becomes °, in which case the circle is the largest that can possibly be described on the sphere. the largest circles that can be described on the surface of a sphere are called _great circles_, and all other circles _small circles_. any number of great circles may be described on the surface of a sphere, since any point on the sphere may be used for the centre of the circle. the plane of every great circle passes through the centre of the sphere, while the planes of all the small circles pass through the sphere away from the centre. all great circles on the same sphere are of the same size, while the small circles differ in size according to the distance of their planes from the centre of the sphere. the farther the plane of a circle is from the centre of the sphere, the smaller is the circle. by a _section_ of a sphere we usually mean the figure of the surface formed by the cutting; by a _plane section_ we mean one whose surface is plane. every plane section of a sphere is a circle. when the section passes through the centre of the sphere, it is a great circle; in every other case the section is a small circle. thus, _an_ and _sb_ (fig. ) are small circles, and _mm'_ and _sn_ are large circles. [illustration: fig. .] in a diagram representing a sphere in section, all the circles whose planes cut the section are represented by straight lines. thus, in fig. , we have a diagram representing in section the sphere of fig. . the straight lines _an_, _sb_, _mm'_, and _sn_, represent the corresponding circles of fig. . the _axis_ of a sphere is the diameter on which it rotates. the _poles_ of a sphere are the ends of its axis. thus, supposing the spheres of figs. and to rotate on the diameter _pp'_, this line would be called the axis of the sphere, and the points _p_ and _p'_ the poles of the sphere. a great circle, mm', situated half way between the poles of a sphere, is called the _equator_ of the sphere. every great circle of a sphere has two poles. these are the two points on the surface of the sphere which lie ° away from the circle. the poles of a sphere are the poles of its equator. [illustration: fig. .] . _the celestial sphere._--the heavens appear to have the form of a sphere, whose centre is at the eye of the observer; and all the stars seem to lie on the surface of this sphere. this form of the heavens is a mere matter of perspective. the stars are really at very unequal distances from us; but they are all seen projected upon the celestial sphere in the direction in which they happen to lie. thus, suppose an observer situated at _c_ (fig. ), stars situated at _a_, _b_, _d_, _e_, _f_, and _g_, would be projected upon the sphere at _a_, _b_, _d_, _e_, _f_, and _g_, and would appear to lie on the surface of the heavens. [illustration: fig. .] . _the horizon._--only half of the celestial sphere is visible at a time. the plane that separates the visible from the invisible portion is called the _horizon_. this plane is tangent to the earth at the point of observation, and extends indefinitely into space in every direction. in fig. , _e_ represents the earth, _o_ the point of observation, and _sn_ the horizon. the points on the celestial sphere directly above and below the observer are the poles of the horizon. they are called respectively the _zenith_ and the _nadir_. no two observers in different parts of the earth have the same horizon; and as a person moves over the earth he carries his horizon with him. [illustration: fig. .] the dome of the heavens appears to rest on the earth, as shown in fig. . this is because distant objects on the earth appear projected against the heavens in the direction of the horizon. [illustration: fig. .] the _sensible_ horizon is a plane tangent to the earth at the point of observation. the _rational_ horizon is a plane parallel with the sensible horizon, and passing through the centre of the earth. as it cuts the celestial sphere through the centre, it forms a great circle. _sn_ (fig. ) represents the sensible horizon, and _s'n'_ the rational horizon. although these two horizons are really four thousand miles apart, they appear to meet at the distance of the celestial sphere; a line four thousand miles long at the distance of the celestial sphere becoming a mere point, far too small to be detected with the most powerful telescope. [illustration: fig. .] [illustration: fig. .] . _rotation of the celestial sphere._--it is well known that the sun and the majority of the stars rise in the east, and set in the west. in our latitude there are certain stars in the north which never disappear below the horizon. these stars are called the _circumpolar_ stars. a close watch, however, reveals the fact that these all appear to revolve around one of their number called the _pole star_, in the direction indicated by the arrows in fig. . in a word, the whole heavens appear to rotate once a day, from east to west, about an axis, which is the prolongation of the axis of the earth. the ends of this axis are called the _poles_ of the heavens; and the great circle of the heavens, midway between these poles, is called the _celestial equator_, or the _equinoctial_. this rotation of the heavens is apparent only, being due to the rotation of the earth from west to east. . _diurnal circles._--in this rotation of the heavens, the stars appear to describe circles which are perpendicular to the celestial axis, and parallel with the celestial equator. these circles are called _diurnal circles_. the position of the poles in the heavens and the direction of the diurnal circles with reference to the horizon, change with the position of the observer on the earth. this is owing to the fact that the horizon changes with the position of the observer. [illustration: fig. .] when the observer is north of the equator, the north pole of the heavens is _elevated_ above the horizon, and the south pole is _depressed_ below it, and the diurnal circles are _oblique_ to the horizon, leaning to the south. this case is represented in fig. , in which _pp'_ represents the celestial axis, _eq_ the celestial equator, _sn_ the horizon, and _ab_, _cn_, _de_, _fg_, _sh_, _kl_, diurnal circles. _o_ is the point of observation, _z_ the zenith, and _z'_ the nadir. [illustration: fig. .] when the observer is south of the equator, as at _o_ in fig. , the south pole is _elevated_, the north pole _depressed_, and the diurnal circles are _oblique_ to the horizon, leaning to the north. when the diurnal circles are oblique to the horizon, as in figs. and , the celestial sphere is called an _oblique sphere_. when the observer is at the equator, as in fig. , the poles of the heavens are on the horizon, and the diurnal circles are _perpendicular_ to the horizon. when the observer is at one of the poles, as in fig. , the poles of the heavens are in the zenith and the nadir, and the diurnal circles are _parallel_ with the horizon. [illustration: fig. .] [illustration: fig. .] . _elevation of the pole and of the equinoctial._--at the equator the poles of the heavens lie on the horizon, and the celestial equator passes through the zenith. as a person moves north from the equator, his zenith moves north from the celestial equator, and his horizon moves down from the north pole, and up from the south pole. the distance of the zenith from the equinoctial, and of the horizon from the celestial poles, will always be equal to the distance of the observer from the equator. in other words, the elevation of the pole is equal to the latitude of the place. in fig. , _o_ is the point of observation, _z_ the zenith, and _sn_ the horizon. _np_, the elevation of the pole, is equal to _ze_, the distance of the zenith from the equinoctial, and to the distance of _o_ from the equator, or the latitude of the place. two angles, or two arcs, which together equal °, are said to be _complements_ of each other. _ze_ and _es_ in fig. are together equal to °: hence they are complements of each other. _ze_ is equal to the latitude of the place, and _es_ is the _elevation_ of the equinoctial above the horizon: hence the elevation of the equinoctial is equal to the complement of the latitude of the place. [illustration: fig. .] were the observer south of the equator, the zenith would be south of the equinoctial, and the south pole of the heavens would be the elevated pole. [illustration: fig. .] _ . four sets of stars._--at most points of observation there are four sets of stars. these four sets are shown in fig. . ( ) the stars in the neighborhood of the elevated pole _never set_. it will be seen from fig. , that if the distance of a star from the elevated pole does not exceed the elevation of the pole, or the latitude of the place, its diurnal circle will be wholly above the horizon. as the observer approaches the equator, the elevation of the pole becomes less and less, and the belt of circumpolar stars becomes narrower and narrower: at the equator it disappears entirely. as the observer approaches the pole, the elevation of the pole increases, and the belt of circumpolar stars becomes broader and broader, until at the pole it includes half of the heavens. at the poles, no stars rise or set, and only half of the stars are ever seen at all. ( ) the stars in the neighborhood of the depressed pole _never rise_. the breadth of this belt also increases as the observer approaches the pole, and decreases as he approaches the equator, to vanish entirely when he reaches the equator. the distance from the depressed pole to the margin of this belt is always equal to the latitude of the place. ( ) the stars in the neighborhood of the equinoctial, on the side of the elevated pole, _set, but are above the horizon longer than they are below it_. this belt of stars extends from the equinoctial to a point whose distance from the elevated pole is equal to the latitude of the place: in other words, the breadth of this third belt of stars is equal to the complement of the latitude of the place. hence this belt of stars becomes broader and broader as the observer approaches the equator, and narrower and narrower as he approaches the pole. however, as the observer approaches the equator, the horizon comes nearer and nearer the celestial axis, and the time a star is below the horizon becomes more nearly equal to the time it is above it. as the observer approaches the pole, the horizon moves farther and farther from the axis, and the time any star of this belt is below the horizon becomes more and more unequal to the time it is above it. the farther any star of this belt is from the equinoctial, the longer the time it is above the horizon, and the shorter the time it is below it. ( ) the stars which are in the neighborhood of the equinoctial, on the side of the depressed pole, _rise, but are below the horizon longer than they are above it_. the width of this belt is also equal to the complement of the latitude of the place. the farther any star of this belt is from the equinoctial, the longer time it is below the horizon, and the shorter time it is above it; and, the farther the place from the equator, the longer every star of this belt is below the horizon, and the shorter the time it is above it. at the equator every star is above the horizon just half of the time; and any star on the equinoctial is above the horizon just half of the time in every part of the earth, since the equinoctial and horizon, being great circles, bisect each other. . _vertical circles._--great circles perpendicular to the horizon are called _vertical circles_. all vertical circles pass through the zenith and nadir. a number of these circles are shown in fig. , in which _senw_ represents the horizon, and _z_ the zenith. [illustration: fig. .] the vertical circle which passes through the north and south points of the horizon is called the _meridian_; and the one which passes through the east and west points, the _prime vertical_. these two circles are shown in fig. ; _szn_ being the meridian, and _ezw_ the prime vertical. these two circles are at right angles to each other, or ° apart; and consequently they divide the horizon into four quadrants. [illustration: fig. .] . _altitude and zenith distance._--the _altitude_ of a heavenly body is its distance above the horizon, and its _zenith distance_ is its distance from the zenith. both the altitude and the zenith distance of a body are measured on the vertical circle which passes through the body. the altitude and zenith distance of a heavenly body are complements of each other. . _azimuth and amplitude.--azimuth_ is distance measured east or west from the meridian. when a heavenly body lies north of the prime vertical, its azimuth is measured from the meridian on the north; and, when it lies south of the prime vertical, its azimuth is measured from the meridian on the south. the azimuth of a body can, therefore, never exceed °. the azimuth of a body is the angle which the plane of the vertical circle passing through it makes with that of the meridian. the _amplitude_ of a body is its distance measured north or south from the prime vertical. the amplitude and azimuth of a body are complements of each other. . _alt-azimuth instrument._--an instrument for measuring the altitude and azimuth of a heavenly body is called an _alt-azimuth_ instrument. one form of this instrument is shown in fig. . it consists essentially of a telescope mounted on a vertical circle, and capable of turning on a horizontal axis, which, in turn, is mounted on the vertical axis of a horizontal circle. both the horizontal and the vertical circles are graduated, and the horizontal circle is placed exactly parallel with the plane of the horizon. when the instrument is properly adjusted, the axis of the telescope will describe a vertical circle when the telescope is turned on the horizontal axis, no matter to what part of the heavens it has been pointed. the horizontal and vertical axes carry each a pointer. these pointers move over the graduated circles, and mark how far each axis turns. to find the _azimuth_ of a star, the instrument is turned on its vertical axis till its vertical circle is brought into the plane of the meridian, and the reading of the horizontal circle noted. the telescope is then directed to the star by turning it on both its vertical and horizontal axes. the reading of the horizontal circle is again noted. the difference between these two readings of the horizontal circle will be the azimuth of the star. [illustration: fig. .] to find the _altitude_ of a star, the reading of the vertical circle is first ascertained when the telescope is pointed horizontally, and again when the telescope is pointed at the star. the difference between these two readings of the vertical circle will be the altitude of the star. . _the vernier._--to enable the observer to read the fractions of the divisions on the circles, a device called a _vernier_ is often employed. it consists of a short, graduated arc, attached to the end of an arm _c_ (fig. ), which is carried by the axis, and turns with the telescope. this arc is of the length of _nine_ divisions on the circle, and it is divided into _ten_ equal parts. if of the vernier coincides with any division, say , of the circle, of the vernier will be / of a division to the left of , will be / of a division to the left of , will be / , of a division to the left of , etc. hence, when coincides with , will be at - / ; when coincides with , will be at - / ; when coincides with , will be at - / , etc. [illustration: fig. .] to ascertain the reading of the circle by means of the vernier, we first notice the zero line. if it exactly coincides with any division of the circle, the number of that division will be the reading of the circle. if there is not an exact coincidence of the zero line with any division of the circle, we run the eye along the vernier, and note which of its divisions does coincide with a division of the circle. the reading of the circle will then be the number of the first division on the circle behind the of the vernier, and a number of tenths equal to the number of the division of the vernier, which coincides with a division of the circle. for instance, suppose of the vernier beyond of the circle, and of the vernier to coincide with of the circle. the reading of the circle will then be - / . . _hour circles._--great circles perpendicular to the celestial equator are called _hour circles_. these circles all pass through the poles of the heavens, as shown in fig. . _eq_ is the celestial equator, and _p_ and _p'_ are the poles of the heavens. the point _a_ on the equinoctial (fig. ) is called the _vernal equinox_, or the _first point of aries_. the hour circle, _app'_, which passes through it, is called the _equinoctial colure_. [illustration: fig. .] . _declination and right ascension._--the _declination_ of a heavenly body is its distance north or south of the celestial equator. the _polar distance_ of a heavenly body is its distance from the nearer pole. declination and polar distance are measured on hour circles, and for the same heavenly body they are complements of each other. [illustration: fig. .] the _right ascension_ of a heavenly body is its distance eastward from the first point of aries, measured from the equinoctial colure. it is equal to the arc of the celestial equator included between the first point of aries and the hour circle which passes through the heavenly body. as right ascension is measured eastward entirely around the celestial sphere, it may have any value from ° up to °. right ascension corresponds to longitude on the earth, and declination to latitude. . _the meridian circle._--the right ascension and declination of a heavenly body are ascertained by means of an instrument called the _meridian circle_, or _transit instrument_. a side-view of this instrument is shown in fig. . [illustration: fig. .] it consists essentially of a telescope mounted between two piers, so as to turn in the plane of the meridian, and carrying a graduated circle. the readings of this circle are ascertained by means of fixed microscopes, under which it turns. a heavenly body can be observed with this instrument, only when it is crossing the meridian. for this reason it is often called the _transit circle_. to find the declination of a star with this instrument, we first ascertain the reading of the circle when the telescope is pointed to the pole, and then the reading of the circle when pointed to the star on its passage across the meridian. the difference between these two readings will be the polar distance of the star, and the complement of them the declination of the star. to ascertain the reading of the circle when the telescope is pointed to the pole, we must select one of the circumpolar stars near the pole, and then point the telescope to it when it crosses the meridian, both above and below the pole, and note the reading of the circle in each case. the mean of these two readings will be the reading of the circle when the telescope is pointed to the pole. . _astronomical clock._--an _astronomical clock_, or _sidereal clock_ as it is often called, is a clock arranged so as to mark hours from to , instead of from to , as in the case of an ordinary clock, and so adjusted as to mark when the vernal equinox, or first point of aries, is on the meridian. as the first point of aries makes a complete circuit of the heavens in twenty-four hours, it must move at the rate of ° an hour, or of ° in four minutes: hence, when the astronomical clock marks , the first point of aries must be ° west of the meridian, and when it marks , ° west of the meridian, etc. that is to say, by observing an accurate astronomical clock, one can always tell how far the meridian at any time is from the first point of aries. . _how to find right ascension with the meridian circle._--to find the right ascension of a heavenly body, we have merely to ascertain the exact time, by the astronomical clock, at which the body crosses the meridian. if a star crosses the meridian at hour minutes by the astronomical clock, its right ascension must be °; if at hours, its right ascension must be °. to enable the observer to ascertain with great exactness the time at which a star crosses the meridian, a number of equidistant and parallel spider-lines are stretched across the focus of the telescope, as shown in fig. . the observer notes the time when the star crosses each spider-line; and the mean of all of these times will be the time when the star crosses the meridian. the mean of several observations is likely to be more nearly exact than any single observation. [illustration: fig. .] [illustration: fig. .] . _the equatorial telescope._--the _equatorial_ telescope is mounted on two axes,--one parallel with the axis of the earth, and the other at right angles to this, and therefore parallel with the plane of the earth's equator. the former is called the _polar axis_, and the latter the _declination axis_. each axis carries a graduated circle. these circles are called respectively the _hour circle_ and the _declination circle_. the telescope is attached directly to the declination axis. when the telescope is fixed in any declination, and then turned on its polar axis, the line of sight will describe a diurnal circle; so that, when the tube is once directed to a star, it can be made to follow the star by simply turning the telescope on its polar axis. in the case of large instruments of this class, the polar axis is usually turned by clock-work at the rate at which the heavens rotate; so that, when the telescope has once been pointed to a planet or other heavenly body, it will continue to follow the body and keep it steadily in the field of view without further trouble on the part of the observer. the great washington equatorial is shown in fig. . its object-glass is inches in diameter, and its focal length is - / feet. it was constructed by alvan clark & sons of cambridge, mass. it is one of the three largest refracting telescopes at present in use. the newall refractor at gateshead, eng., has an objective inches in diameter, and a focal length of feet. the great refractor at vienna has an objective inches in diameter. there are several large refractors now in process of construction. [illustration: fig. .] . _the wire micrometer._--large arcs in the heavens are measured by means of the graduated circles attached to the axes of the telescopes; but small arcs within the field of view of the telescope are measured by means of instruments called _micrometers_, mounted in the focus of the telescope. one of the most convenient of these micrometers is that known as the _wire micrometer_, and shown in fig. . the frame _aa_ covers two slides, _c_ and _d_. these slides are moved by the screws _f_ and _g_. the wires _e_ and _b_ are stretched across the ends of the slides so as to be parallel to each other. on turning the screws _f_ and _g_ one way, these wires are carried apart; and on turning them the other way they are brought together again. sometimes two parallel wires, _x_ and _y_, shown in the diagram at the right, are stretched across the frame at right angles to the wires _e_, _b_. we may call the wires _x_ and _y_ the _longitudinal_ wires of the micrometer, and _e_ and _b_ the _transverse_ wires. many instruments have only one longitudinal wire, which is stretched across the middle of the focus. the longitudinal wires are just in front of the transverse wires, but do not touch them. to find the distance between any two points in the field of view with a micrometer, with a single longitudinal wire, turn the frame till the longitudinal wire passes through the two points; then set the wires _e_ and _b_ one on each point, turn one of the screws, known as the _micrometer screw_, till the two wires are brought together, and note the number of times the screw is turned. having previously ascertained over what arc one turn of the screw will move the wire, the number of turns will enable us to find the length of the arc between the two points. the threads of the micrometer screw are cut with great accuracy; and the screw is provided with a large head, which is divided into a hundred or more equal parts. these divisions, by means of a fixed pointer, enable us to ascertain what fraction of a turn the screw has made over and above its complete revolutions. . _reflecting telescopes._--it is possible to construct mirrors of much larger size than lenses: hence reflecting telescopes have an advantage over refracting telescopes as regards size of aperture and of light-gathering power. they are, however, inferior as regards definition; and, in order to prevent flexure, it is necessary to give the speculum, or mirror, a massiveness which makes the telescope unwieldy. it is also necessary frequently to repolish the speculum; and this is an operation of great delicacy, as the slightest change in the form of the surface impairs the definition of the image. these defects have been remedied, to a certain extent, by the introduction of silver-on-glass mirrors; that is, glass mirrors covered in front with a thin coating of silver. glass is only one-third as heavy as speculum-metal, and silver is much superior to that metal in reflecting power; and when the silver becomes tarnished, it can be removed and renewed without danger of changing the form of the glass. _the herschelian reflector._--in this form of telescope the mirror is slightly tipped, so that the image, instead of being formed in the centre of the tube, is formed near one side of it, as in fig. . the observer can then view it without putting his head inside the tube, and therefore without cutting off any material portion of the light. in observation, he must stand at the upper or outer end of the tube, and look into it, his back being turned towards the object. from his looking directly into the mirror, it is also sometimes called the _front-view_ telescope. the great disadvantage of this arrangement is, that the rays cannot be brought to an exact focus when they are thrown so far to one side of the axis, and the injury to the definition is so great that the front-view plan is now entirely abandoned. [illustration: fig. .] _the newtonian reflector._--the plan proposed by sir isaac newton was to place a small plane mirror just inside the focus, inclined to the telescope at an angle of °, so as to throw the rays to the side of the tube, where they come to a focus, and form the image. an opening is made in the side of the tube, just below where the image is formed; and in this opening the eye-piece is inserted. the small mirror cuts off some of the light, but not enough to be a serious defect. an improvement which lessens this defect has been made by professor henry draper. the inclined mirror is replaced by a small rectangular prism (fig. ), by reflection from which the image is formed very near the prism. a pair of lenses are then inserted in the course of the rays, by which a second image is formed at the opening in the side of the tube; and this second image is viewed by an ordinary eye-piece. [illustration: fig. .] _the gregorian reflector._--this is a form proposed by james gregory, who probably preceded newton as an inventor of the reflecting telescope. behind the focus, _f_ (fig. ), a small concave mirror, _r_, is placed, by which the light is reflected back again down the tube. the larger mirror, _m_, has an opening through its centre; and the small mirror, _r_, is so adjusted as to form a second image of the object in this opening. this image is then viewed by an eye-piece which is screwed into the opening. [illustration: fig. .] _the cassegrainian reflector._--in principle this is the same with the gregorian; but the small mirror, _r_, is convex, and is placed inside the focus, _f_, so that the rays are reflected from it before reaching the focus, and no image is formed until they reach the opening in the large mirror. this form has an advantage over the gregorian, in that the telescope may be made shorter, and the small mirror can be more easily shaped to the required figure. it has, therefore, entirely superseded the original gregorian form. [illustration: fig. .] optically these forms of telescope are inferior to the newtonian; but the latter is subject to the inconvenience, that the observer must be stationed at the upper end of the telescope, where he looks into an eye-piece screwed into the side of the tube. on the other hand, the cassegrainian telescope is pointed directly at the object to be viewed, like a refractor; and the observer stands at the lower end, and looks in at the opening through the large mirror. this is, therefore, the most convenient form of all in management. [illustration: fig. .] the largest reflecting telescope yet constructed is that of lord rosse, at parsonstown, ireland. its speculum is feet in diameter, and its focal length feet. it is commonly used as a newtonian. this telescope is shown in fig. . the great telescope of the melbourne observatory, australia, is a cassegranian reflector. its speculum is feet in diameter, and its focal length is feet. it is shown in fig. . [illustration: fig. .] the great reflector of the paris observatory is a newtonian reflector. its mirror of silvered glass is feet in diameter, and its focal length is feet. this telescope is shown in fig. . . _the sun's motion among the stars._--if we notice the stars at the same hour night after night, we shall find that the constellations are steadily advancing towards the west. new constellations are continually appearing in the east, and old ones disappearing in the west. this continual advancing of the heavens towards the west is due to the fact that the sun's place among the stars is _continually moving towards the east_. the sun completes the circuit of the heavens in a year, and is therefore moving eastward at the rate of about a degree a day. [illustration: fig. .] this motion of the sun's place among the stars is due to the revolution of the earth around the sun, and not to any real motion of the sun. in fig. suppose the inner circle to represent the orbit of the earth around the sun, and the outer circle to represent the celestial sphere. when the earth is at _e_, the sun's place on the celestial sphere is at _s'_. as the earth moves in the direction _ef_, the sun's place on the celestial sphere must move in the direction _s't_: hence the revolution of the earth around the sun would cause the sun's place among the stars to move around the heavens in the same direction that the earth is moving around the sun. . _the ecliptic._--the circle described by the sun in its apparent motion around the heavens is called the _ecliptic_. the plane of this circle passes through the centre of the earth, and therefore through the centre of the celestial sphere; the earth being so small, compared with the celestial sphere, that it practically makes no difference whether we consider a point on its surface, or one at its centre, as the centre of the celestial sphere. the ecliptic is, therefore, a great circle. the earth's orbit lies in the plane of the ecliptic; but it extends only an inappreciable distance from the sun towards the celestial sphere. [illustration: fig. .] . _the obliquity of the ecliptic._--the ecliptic is inclined to the celestial equator by an angle of about - / °. this inclination is called the _obliquity of the ecliptic_. the obliquity of the ecliptic is due to the deviation of the earth's axis from a perpendicular to the plane of its orbit. the axis of a rotating body tends to maintain the same direction; and, as the earth revolves around the sun, its axis points all the time in nearly the same direction. the earth's axis deviates about - / ° from the perpendicular to its orbit; and, as the earth's equator is at right angles to its axis, it will deviate about - / ° from the plane of the ecliptic. the celestial equator has the same direction as the terrestrial equator, since the axis of the heavens has the same direction as the axis of the earth. [illustration: fig. .] suppose the globe at the centre of the tub (fig. ) to represent the sun, and the smaller globes to represent the earth in various positions in its orbit. the surface of the water will then represent the plane of the ecliptic, and the rod projecting from the top of the earth will represent the earth's axis, which is seen to point all the time in the same direction, or to lean the same way. the leaning of the axis from the perpendicular to the surface of the water would cause the earth's equator to be inclined the same amount to the surface of the water, half of the equator being above, and half of it below, the surface. were the axis of the earth perpendicular to the surface of the water, the earth's equator would coincide with the surface, as is evident from fig. . [illustration: fig. .] . _the equinoxes and solstices._--the ecliptic and celestial equator, being great circles, bisect each other. half of the ecliptic is north, and half of it is south, of the equator. the points at which the two circles cross are called the _equinoxes_. the one at which the sun crosses the equator from south to north is called the _vernal_ equinox, and the one at which it crosses from north to south the _autumnal_ equinox. the points on the ecliptic midway between the equinoxes are called the _solstices_. the one north of the equator is called the _summer_ solstice, and the one south of the equator the _winter_ solstice. in fig. , _eq_ is the celestial equator, _ece'c'_ the ecliptic, _v_ the vernal equinox, a the autumnal equinox, ec the winter solstice, and _e'c'_ the summer solstice. [illustration: fig. .] . _the inclination of the ecliptic to the horizon._--since the celestial equator is perpendicular to the axis of the heavens, it makes the same angle with it on every side: hence, at any place, the equator makes always the same angle with the horizon, whatever part of it is above the horizon. but, as the ecliptic is oblique to the equator, it makes different angles with the celestial axis on different sides; and hence, at any place, the angle which the ecliptic makes with the horizon varies according to the part which is above the horizon. the two extreme angles for a place more than - / ° north of the equator are shown in figs. and . the least angle is formed when the vernal equinox is on the eastern horizon, the autumnal on the western horizon, and the winter solstice on the meridian, as in fig. . the angle which the ecliptic then makes with the horizon is equal to the elevation of the equinoctial _minus_ - / °. in the latitude of new york this angle = ° - - / ° = - / °. [illustration: fig. .] the greatest angle is formed when the autumnal equinox is on the eastern horizon, the vernal on the western horizon, and the summer solstice is on the meridian (fig. ). the angle between the ecliptic and the horizon is then equal to the elevation of the equinoctial _plus_ - / °. in the latitude of new york this angle = ° + - / ° = - / °. of course the equinoxes, the solstices, and all other points on the ecliptic, describe diurnal circles, like every other point in the heavens: hence, in our latitude, these points rise and set every day. . _celestial latitude and longitude._--_celestial latitude_ is distance measured north or south from the ecliptic; and _celestial longitude_ is distance measured on the ecliptic eastward from the vernal equinox, or the first point of aries. great circles perpendicular to the ecliptic are called _celestial meridians_. these circles all pass through the poles of the ecliptic, which are some - / ° from the poles of the equinoctial. the latitude of a heavenly body is measured by the arc of a celestial meridian included between the body and the ecliptic. the longitude of a heavenly body is measured by the arc of the ecliptic included between the first point of aries and the meridian which passes through the body. there are, of course, always two arcs included between the first point of aries and the meridian,--one on the east, and the other on the west, of the first point of aries. the one on the _east_ is always taken as the measure of the longitude. . _the precession of the equinoxes._--the equinoctial points have a slow westward motion along the ecliptic. this motion is at the rate of about '' a year, and would cause the equinoxes to make a complete circuit of the heavens in a period of about twenty-six thousand years. it is called the _precession of the equinoxes_. this westward motion of the equinoxes is due to the fact that the axis of the earth has a slow gyratory motion, like the handle of a spinning-top which has begun to wabble a little. this gyratory motion causes the axis of the heavens to describe a cone in about twenty-six thousand years, and the pole of the heavens to describe a circle about the pole of the ecliptic in the same time. the radius of this circle is - / °. [illustration: fig. .] . _illustration of precession._--the precession of the equinoxes may be illustrated by means of the apparatus shown in fig. . the horizontal and stationary ring _ec_ represents the ecliptic; the oblique ring _e'q_ represents the equator; _v_ and _a_ represent the equinoctial point, and _e_ and _c_ the solstitial points; _b_ represents the pole of the ecliptic, _p_ the pole of the equator, and _po_ the celestial axis. the ring _e'q_ is supported on a pivot at _o_; and the rod _bp_, which connects _b_ and _p_, is jointed at each end so as to admit of the movement of _p_ and _b_. on carrying _p_ around _b_, we shall see that _e'q_ will always preserve the same obliquity to _ec_, and that the points _v_ and _a_ will move around the circle _ec_. the same will also be true of the points _e_ and _c_. . _effects of precession._--one effect of precession, as has already been stated, is the revolution of the pole of the heavens around the pole of the ecliptic in a period of about twenty-six thousand years. the circle described by the pole of the heavens, and the position of the pole at various dates, are shown in fig. , where o indicates the position of the pole at the birth of christ. the numbers round the circle to the left of o are dates a.d., and those to the right of o are dates b.c. it will be seen that the star at the end of the little bear's tail, which is now near the north pole, will be exactly at the pole about the year . it will then recede farther and farther from the pole till the year a.d., when it will be about forty-seven degrees away from the pole. it will be noticed that one of the stars of the dragon was the pole star about years b.c. there are reasons to suppose that this was about the time of the building of the great pyramid. a second effect of precession is the shifting of the signs along the zodiac. the _zodiac_ is a belt of the heavens along the ecliptic, extending eight degrees from it on each side. this belt is occupied by twelve constellations, known as the _zodiacal constellations_. they are _aries_, _taurus_, _gemini_, _cancer_, _leo_, _virgo_, _libra_, _scorpio_, _sagittarius_, _capricornus_, _aquarius_, and _pisces_. the zodiac is also divided into twelve equal parts of thirty degrees each, called _signs_. these signs have the same names as the twelve zodiacal constellations, and when they were first named, each sign occupied the same part of the zodiac as the corresponding constellation; that is to say, the sign aries was in the constellation aries, and the sign taurus in the constellation taurus, etc. now the signs are always reckoned as beginning at the vernal equinox, which is continually shifting along the ecliptic; so that the signs are continually moving along the zodiac, while the constellations remain stationary: hence it has come about that the _first point of aries_ (the _sign_) is no longer in the _constellation_ aries, but in pisces. [illustration: fig. .] fig. shows the position of the vernal equinox b.c. it was then in taurus, just south of the pleiades. it has since moved from taurus, through aries, and into pisces, as shown in fig. . [illustration: fig. .] [illustration: fig. .] since celestial longitude and right ascension are both measured from the first point of aries, the longitude and right ascension of the stars are slowly changing from year to year. it will be seen, from figs. and , that the declination is also slowly changing. . _nutation._--the gyratory motion of the earth's axis is not perfectly regular and uniform. the earth's axis has a slight tremulous motion, oscillating to and fro through a short distance once in about nineteen years. this tremulous motion of the axis causes the pole of the heavens to describe an undulating curve, as shown in fig. , and gives a slight unevenness to the motion of the equinoxes along the ecliptic. this nodding motion of the axis is called _nutation_. [illustration: fig. .] . _refraction._--when a ray of light from one of the heavenly bodies enters the earth's atmosphere obliquely, it will be bent towards a perpendicular to the surface of the atmosphere, since it will be entering a denser medium. as the ray traverses the atmosphere, it will be continually passing into denser and denser layers, and will therefore be bent more and more towards the perpendicular. this bending of the ray is shown in fig. . a ray which started from _a_ would enter the eye at _c_, as if it came from _i_: hence a star at _a_ would appear to be at _i_. [illustration: fig. .] atmospheric refraction displaces all the heavenly bodies from the horizon towards the zenith. this is evident from fig. . _od_ is the horizon, and _z_ the zenith, of an observer at _o_. refraction would make a star at _q_ appear at _p_: in other words, it would displace it towards the zenith. a star in the zenith is not displaced by refraction, since the rays which reach the eye from it traverse the atmosphere vertically. the farther a star is from the zenith, the more it is displaced by refraction, since the greater is the obliquity with which the rays from it enter the atmosphere. [illustration: fig. .] at the horizon the displacement by refraction is about half a degree; but it varies considerably with the state of the atmosphere. refraction causes a heavenly body to appear above the horizon longer than it really is above it, since it makes it appear to be on the horizon when it is really half a degree below it. the increase of refraction towards the horizon often makes the sun, when near the horizon, appear distorted, the lower limb of the sun being raised more than the upper limb. this distortion is shown in fig. . the vertical diameter of the sun appears to be considerably less than the horizontal diameter. [illustration: fig. .] . _parallax._--_parallax_ is the displacement of an object caused by a change in the point of view from which it is seen. thus in fig. , the top of the tower _s_ would be seen projected against the sky at _a_ by an observer at _a_, and at _b_ by an observer at _b_. in passing from _a_ to _b_, the top of the tower is displaced from _a_ to _b_, or by the angle _asb_. this angle is called the parallax of _s_, as seen from _b_ instead of _a_. [illustration: fig. .] the _geocentric parallax_ of a heavenly body is its displacement caused by its being seen from the surface of the earth, instead of from the centre of the earth. in fig. , _r_ is the centre of the earth, and _o_ the point of observation on the surface of the earth. stars at _s_, _s'_, and _s''_, would, from the centre of the earth, appear at _q_, _q'_, and _q''_; while from the point _o_ on the surface of the earth, these same stars would appear at _p_, _p'_ and _p''_, being displaced from their position, as seen from the centre of the earth, by the angles _qsp_, _q's'p'_, and _q''s''p''_. it will be seen that parallax displaces a body from the zenith towards the horizon, and that the parallax of a body is greatest when it is on the horizon. the parallax of a heavenly body when on the horizon is called its _horizontal parallax_. a body in the zenith is not displaced by parallax, since it would be seen in the same direction from both the centre and the surface of the earth. [illustration: fig. .] the parallax of a body at _s'''_ is _q'''s'''p_, which is seen to be greater than _qsp_; that is to say, the parallax of a heavenly body increases with its nearness to the earth. the distance and parallax of a body are so related, that, either being known, the other may be computed. . _aberration._--_aberration_ is a slight displacement of a star, owing to an apparent change in the direction of the rays of light which proceed from it, caused by the motion of the earth in its orbit. if we walk rapidly in any direction in the rain, when the drops are falling vertically, they will appear to come into our faces from the direction in which we are walking. our own motion has apparently changed the direction in which the drops are falling. [illustration: fig. .] in fig. let _a_ be a gun of a battery, from which a shot is fired at a ship, _de_, that is passing. let _abc_ be the course of the shot. the shot enters the ship's side at _b_, and passes out at the other side at _c_; but in the mean time the ship has moved from _e_ to _e_, and the part _b_, where the shot entered, has been carried to _b_. if a person on board the ship could see the ball as it crossed the ship, he would see it cross in the diagonal line _bc_; and he would at once say that the cannon was in the direction of _cb_. if the ship were moving in the opposite direction, he would say that the cannon was just as far the other side of its true position. now, we see a star in the direction in which the light coming from the star appears to be moving. when we examine a star with a telescope, we are in the same condition as the person who on shipboard saw the cannon-ball cross the ship. the telescope is carried along by the earth at the rate of eighteen miles a second: hence the light will appear to pass through the tube in a slightly different direction from that in which it is really moving: just as the cannon-ball appears to pass through the ship in a different direction from that in which it is really moving. thus in fig. , a ray of light coming in the direction _sot_ would appear to traverse the tube _ot_ of a telescope, moving in the direction of the arrow, as if it were coming in the direction _s'o_. [illustration: fig. .] as light moves with enormous velocity, it passes through the tube so quickly, that it is apparently changed from its true direction only by a very slight angle: but it is sufficient to displace the star. this apparent change in the direction of light caused by the motion of the earth is called _aberration of light_. . _the planets._--on watching the stars attentively night after night, it will be found, that while the majority of them appear _fixed_ on the surface of the celestial sphere, so as to maintain their relative positions, there are a few that _wander_ about among the stars, alternately advancing towards the east, halting, and retrograding towards the west. these wandering stars are called _planets_. their motions appear quite irregular; but, on the whole, their eastward motion is in excess of their westward, and in a longer or shorter time they all complete the circuit of the heavens. in almost every instance, their paths are found to lie wholly in the belt of the zodiac. [illustration: fig. .] fig. shows a portion of the apparent path of one of the planets. ii. the solar system. i. theory of the solar system. . _members of the solar system._--the solar system is composed of the _sun_, _planets_, _moons_, _comets_, and _meteors_. five planets, besides the earth, are readily distinguished by the naked eye, and were known to the ancients: these are _mercury_, _venus_, _mars_, _jupiter_, and _saturn_. these, with the _sun_ and _moon_, made up the _seven planets_ of the ancients, from which the seven days of the week were named. the ptolemaic system. . _the crystalline spheres._--we have seen that all the heavenly bodies appear to be situated on the surface of the celestial sphere. the ancients assumed that the stars were really fixed on the surface of a crystalline sphere, and that they were carried around the earth daily by the rotation of this sphere. they had, however, learned to distinguish the planets from the stars, and they had come to the conclusion that some of the planets were nearer the earth than others, and that all of them were nearer the earth than the stars are. this led them to imagine that the heavens were composed of a number of crystalline spheres, one above another, each carrying one of the planets, and all revolving around the earth from east to west, but at different rates. this structure of the heavens is shown in section in fig. . [illustration: fig. .] . _cycles and epicycles._--the ancients had also noticed that, while all the planets move around the heavens from west to east, their motion is not one of uniform advancement. mercury and venus appear to oscillate to and fro across the sun, while jupiter and saturn appear to oscillate to and fro across a centre which is moving around the earth, so as to describe a series of loops, as shown in fig. . [illustration: fig. .] the ancients assumed that the planets moved in exact circles, and, in fact, that all motion in the heavens was circular, the circle being the simplest and most perfect curve. to account for the loops described by the planets, they imagined that each planet revolved in a circle around a centre, which, in turn, revolved in a circle around the earth. the circle described by this centre around the earth they called the _cycle_, and the circle described by the planet around this centre they called the _epicycle_. . _the eccentric._--the ancients assumed that the planets moved at a uniform rate in describing the epicycle, and also the centre in describing the cycle. they had, however, discovered that the planets advance eastward more rapidly in some parts of their orbits than in others. to account for this they assumed that the cycles described by the centre, around which the planets revolved, were _eccentric_; that is to say, that the earth was not at the centre of the cycle, but some distance away from it, as shown in fig. . _e_ is the position of the earth, and _c_ is the centre of the cycle. the lines from _e_ are drawn so as to intercept equal arcs of the cycle. it will be seen at once that the angle between any pair of lines is greatest at _p_, and least at _a_; so that, were a planet moving at the same rate at _p_ and _a_, it would seem to be moving much faster at _p_. the point _p_ of the planet's cycle was called its _perigee_, and the point a its _apogee_. [illustration: fig. .] as the apparent motion of the planets became more accurately known, it was found necessary to make the system of cycles, epicycles, and eccentrics exceedingly complicated to represent that motion. the copernican system. . _copernicus._--copernicus simplified the ptolemaic system greatly by assuming that the earth and all the planets revolved about the sun as a centre. he, however, still maintained that all motion in the heavens was circular, and hence he could not rid his system entirely of cycles and epicycles. tycho brahe's system. . _tycho brahe._--tycho brahe was the greatest of the early astronomical observers. he, however, rejected the system of copernicus, and adopted one of his own, which was much more complicated. he held that all the planets but the earth revolved around the sun, while the sun and moon revolved around the earth. this system is shown in fig. . [illustration: fig. .] kepler's system. . _kepler._--while tycho brahe devoted his life to the observation of the planets. kepler gave his to the study of tycho's observations, for the purpose of discovering the true laws of planetary motion. he banished the complicated system of cycles, epicycles, and eccentrics forever from the heavens, and discovered the three laws of planetary motion which have rendered his name immortal. . _the ellipse._--an _ellipse_ is a closed curve which has two points within it, the sum of whose distances from every point on the curve is the same. these two points are called the _foci_ of the ellipse. [illustration: fig. .] one method of describing an ellipse is shown in fig. . two tacks, _f_ and _f'_, are stuck into a piece of paper, and to these are fastened the two ends of a string which is longer than the distance between the tacks. a pencil is then placed against the string, and carried around, as shown in the figure. the curve described by the pencil is an ellipse. the two points _f_ and _f'_ are the foci of the ellipse: the sum of the distances of these two points from every point on the curve is equal to the length of the string. when half of the ellipse has been described, the pencil must be held against the other side of the string in the same way, and carried around as before. the point _o_, half way between _f_ and _f'_, is called the _centre_ of the ellipse; _aa'_ is the _major axis_ of the ellipse, and _cd_ is the _minor axis_. . _the eccentricity of the ellipse._--the ratio of the distance between the two foci to the major axis of the ellipse is called the _eccentricity_ of the ellipse. the greater the distance between the two foci, compared with the major axis of the ellipse, the greater is the eccentricity of the ellipse; and the less the distance between the foci, compared with the length of the major axis, the less the eccentricity of the ellipse. the ellipse of fig. has an eccentricity of / . this ellipse scarcely differs in appearance from a circle. the ellipse of fig. has an eccentricity of / , and that of fig. an eccentricity of / . [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] . _kepler's first law._--kepler first discovered that _all the planets move from west to east in ellipses which have the sun as a common focus_. this law of planetary motion is known as _kepler's first law_. the planets appear to describe loops, because we view them from a moving point. the ellipses described by the planets differ in eccentricity; and, though they all have one focus at the sun, their major axes have different directions. the eccentricity of the planetary orbits is comparatively small. the ellipse of fig. has seven times the eccentricity of the earth's orbit, and twice that of the orbit of any of the larger planets except mercury; and its eccentricity is more than half of that of the orbit of mercury. owing to their small eccentricity, the orbits of the planets are usually represented by circles in astronomical diagrams. [illustration: fig. .] . _kepler's second law._--kepler next discovered that a planet's rate of motion in the various parts of its orbit is such that _a line drawn from the planet to the sun would always sweep over equal areas in equal times_. thus, in fig. , suppose the planet would move from _p_ to _p^ _ in the same time that it would move from _p^ _ to _p^ _, or from _p^ _ to _p^ _; then the dark spaces, which would be swept over by a line joining the sun and the planet, in these equal times, would all be equal. a line drawn from the sun to a planet is called the _radius vector_ of the planet. the radius vector of a planet is shortest when the planet is nearest the sun, or at _perihelion_, and longest when the planet is farthest from the sun, or at _aphelion_: hence, in order to have the areas equal, it follows that a planet must move fastest when at perihelion, and slowest at aphelion. _kepler's second law_ of planetary motion is usually stated as follows: _the radius vector of a planet describes equal areas in equal times in every part of the planet's orbit_. . _kepler's third law._--kepler finally discovered that the periodic times of the planets bear the following relation to the distances of the planets from the sun: _the squares of the periodic times of the planets are to each other as the cubes of their mean distances from the sun_. this is known as _kepler's third law_ of planetary motion. by _periodic time_ is meant the time it takes a planet to revolve around the sun. these three laws of kepler's are the foundation of modern physical astronomy. the newtonian system. . _newton's discovery._--newton followed kepler, and by means of his three laws of planetary motion made his own immortal discovery of the _law of gravitation_. this law is as follows: _every portion of matter in the universe attracts every other portion with a force varying directly as the product of the masses acted upon, and inversely as the square of the distances between them._ . _the conic sections._--the _conic sections_ are the figures formed by the various plane sections of a right cone. there are four classes of figures formed by these sections, according to the angle which the plane of the section makes with the axis of the cone. _opq_, fig. , is a right cone, and _on_ is its axis. any section, _ab_, of this cone, whose plane is perpendicular to the axis of the cone, is a _circle_. [illustration: fig. .] any section, _cd_, of this cone, whose plane is oblique to the axis, but forms with it an angle greater than _nop_, is an _ellipse_. the less the angle which the plane of the section makes with the axis, the more elongated is the ellipse. any section, _ef_, of this cone, whose plane makes with the axis an angle equal to _nop_, is a _parabola_. it will be seen, that, by changing the obliquity of the plane _cd_ to the axis _no_, we may pass uninterruptedly from the circle through ellipses of greater and greater elongation to the parabola. any section, _gh_, of this cone, whose plane makes with the axis _on_ an angle less than _nop_, is a _hyperbola_. [illustration: fig. .] it will be seen from fig. , in which comparative views of the four conic sections are given, that the circle and the ellipse are _closed_ curves, or curves which return into themselves. the parabola and the hyperbola are, on the contrary, _open_ curves, or curves which do not return into themselves. . _a revolving body is continually falling towards its centre of revolution._--in fig. let _m_ represent the moon, and _e_ the earth around which the moon is revolving in the direction _mn_. it will be seen that the moon, in moving from m to n, falls towards the earth a distance equal to _mn_. it is kept from falling into the earth by its orbital motion. [illustration: fig. .] the fact that a body might be projected forward fast enough to keep it from falling into the earth is evident from fig. . _ab_ represents the level surface of the ocean, _c_ a mountain from the summit of which a cannon-ball is supposed to be fired in the direction _ce_. _ad_ is a line parallel with _ce_; _db_ is a line equal to the distance between the two parallel lines _ad_ and _ce_. this distance is equal to that over which gravity would pull a ball towards the centre of the earth in a minute. no matter, then, with what velocity the ball _c_ is fired, at the end of a minute it will be somewhere on the line _ad_. suppose it were fired fast enough to reach the point _d_ in a minute: it would be on the line _ad_ at the end of the minute, but still just as far from the surface of the water as when it started. it will be seen, that, although it has all the while been falling towards the earth, it has all the while kept at exactly the same distance from the surface. the same thing would of course be true during each succeeding minute, till the ball came round to _c_ again, and the ball would continue to revolve in a circle around the earth. [illustration: fig. .] . _the form of a body's orbit depends upon the rate of its forward motion._--if the ball _c_ were fired fast enough to reach the line _ad_ beyond the point _d_, it would be farther from the surface at the end of the second than when it started. its orbit would no longer be circular, but _elliptical_. if the speed of projection were gradually augmented, the orbit would become a more and more elongated ellipse. at a certain rate of projection, the orbit would become a _parabola_; at a still greater rate, a _hyperbola_. . _the moon held in her orbit by gravity._--newton compared the distance _mn_ that the moon is drawn to the earth in a given time, with the distance a body near the surface of the earth would be pulled toward the earth in the same time; and he found that these distances are to each other inversely as the squares of the distances of the two bodies from the centre of the earth. he therefore concluded that _the moon is drawn to the earth by gravity_, and that the _intensity of gravity decreases as the square of the distance increases_. [illustration: fig. .] . _any body whose orbit is a conic section, and which moves according to kepler's second law, is acted upon by a force varying inversely as the square of the distance._--newton compared the distance which any body, moving in an ellipse, according to kepler's second law, is drawn towards the sun in the same time in different parts of its orbit. he found these distances in all cases to vary inversely as the square of the distance of the planet from the sun. thus, in fig. , suppose a planet would move from _k_ to _b_ in the same time that it would move from _k_ to _b_ in another part of its orbit. in the first instance the planet would be drawn towards the sun the distance _ab_, and in the second instance the distance _ab_. newton found that _ab : ab = (sk)^ : (sk)^ _. he also found that the same would be true when the body moved in a parabola or a hyperbola: hence he concluded that _every body that moves around the sun in an ellipse, a parabola, or a hyperbola, is moving under the influence of gravity_. [transcriber's note: in newton's equation above, (sk)^ means to group s and k together and square their product. in the original book, instead of using parentheses, there was a vinculum, a horizontal bar, drawn over the s and the k to express the same grouping.] [illustration: fig. .] . _the force that draws the different planets to the sun varies inversely as the squares of the distances of the planets from the sun._--newton compared the distances _jk_ and _ef_, over which two planets are drawn towards the sun in the same time, and found these distances to vary inversely as the squares of the distances of the planets from the sun: hence he concluded that _all the planets are held in their orbits by gravity_. he also showed that this would be true of any two bodies that were revolving around the sun's centre, according to kepler's third law. . _the copernican system._--the theory of the solar system which originated with copernicus, and which was developed and completed by kepler and newton, is commonly known as the _copernican system_. this system is shown in fig. . [illustration: fig. .] ii. the sun and planets. i. the earth. form and size. . _form of the earth._--in ordinary language the term _horizon_ denotes the line that bounds the portion of the earth's surface that is visible at any point. ( ) it is well known that the horizon of a plain presents the form of a circle surrounding the observer. if the latter moves, the circle moves also; but its form remains the same, and is modified only when mountains or other obstacles limit the view. out at sea, the circular form of the horizon is still more decided, and changes only near the coasts, the outline of which breaks the regularity. here, then, we obtain a first notion of the rotundity of the earth, since a sphere is the only body which is presented always to us under the form of a circle, from whatever point on its surface it is viewed. ( ) moreover, it cannot be maintained that the horizon is the vanishing point of distinct vision, and that it is this which causes the appearance of a circular boundary, because the horizon is enlarged when we mount above the surface of the plain. this will be evident from fig. , in which a mountain is depicted in the middle of a plain, whose uniform curvature is that of a sphere. from the foot of the mountain the spectator will have but a very limited horizon. let him ascend half way, his visual radius extends, is inclined below the first horizon, and reveals a more extended circular area. at the summit of the mountain the horizon still increases; and, if the atmosphere is pure, the spectator will see numerous objects where from the lower stations the sky alone was visible. [illustration: fig. .] this extension of the horizon would be inexplicable if the earth had the form of an extended plane. ( ) the curvature of the surface of the sea manifests itself in a still more striking manner. if we are on the coast at the summit of a hill, and a vessel appears on the horizon (fig. ), we see only the tops of the masts and the highest sails; the lower sails and the hull are invisible. as the vessel approaches, its lower part comes into view above the horizon, and soon it appears entire. [illustration: fig. .] in the same manner the sailors from the ship see the different parts of objects on the land appear successively, beginning with the highest. the reason of this will be evident from fig. , where the course of a vessel, seen in profile, is figured on the convex surface of the sea. [illustration: fig. .] as the curvature of the ocean is the same in every direction, it follows that the surface of the ocean is _spherical_. the same is true of the surface of the land, allowance being made for the various inequalities of the surface. from these and various other indications, we conclude that _the earth is a sphere_. . _size of the earth._--the size of the earth is ascertained by measuring the length of a degree of a meridian, and multiplying this by three hundred and sixty. this gives the circumference of the earth as about twenty-five thousand miles, and its diameter as about eight thousand miles. we know that the two stations between which we measure are one degree apart when the elevation of the pole at one station is one degree greater than at the other. . _the earth flattened at the poles._--degrees on the meridian have been measured in various parts of the earth, and it has been found that they invariably increase in length as we proceed from the equator towards the pole: hence the earth must curve less and less rapidly as we approach the poles; for the less the curvature of a circle, the larger the degrees on it. [illustration: fig. .] . _the earth in space._--in fig. we have a view of the earth suspended in space. the side of the earth turned towards the sun is illumined, and the other side is in darkness. as the planet rotates on its axis, successive portions of it will be turned towards the sun. as viewed from a point in space between it and the sun, it will present light and dark portions, which will assume different forms according to the portion which is illumined. these different appearances are shown in fig. . [illustration: fig. .] day and night. . _day and night._--the succession of day and night is due to _the rotation of the earth on its axis_, by which a place on the surface of the earth is carried alternately into the sunshine and out of it. as the sun moves around the heavens on the ecliptic, it will be on the celestial equator when at the equinoxes, and - / ° north of the equator when at the summer solstice, and - / ° south of the equator when at the winter solstice. . _day and night when the sun is at the equinoxes._--when the sun is at either equinox, the diurnal circle described by the sun will coincide with the celestial equator; and therefore half of this diurnal circle will be above the horizon at every point on the surface of the globe. at these times _day and night will be equal in every part of the earth_. [illustration: fig. .] [illustration: fig. .] the equality of days and nights when the sun is on the celestial equator is also evident from the following considerations: one-half of the earth is in sunshine all of the time; when the sun is on the celestial equator, it is directly over the equator of the earth, and the illumination extends from pole to pole, as is evident from figs. and , in the former of which the sun is represented as on the eastern horizon at a place along the central line of the figure, and in the latter as on the meridian along the same line. in each diagram it is seen that the illumination extends from pole to pole: hence, as the earth rotates on its axis, every place on the surface will be in the sunshine and out of it just half of the time. . _day and night when the sun is at the summer solstice._--when the sun is at the summer solstice, it will be - / ° north of the celestial equator. the diurnal circle described by the sun will then be - / ° north of the celestial equator; and more than half of this diurnal circle will be above the horizon at all places north of the equator, and less than half of it at places south of the equator: hence _the days will be longer than the nights at places north of the equator, and shorter than the nights at places south of the equator_. at places within - / ° of the north pole, the entire diurnal circle described by the sun will be above the horizon, so that the sun will not set. at places within - / ° of the south pole of the earth, the entire diurnal circle will be below the horizon, so that the sun will not rise. [illustration: fig. .] [illustration: fig. .] the illumination of the earth at this time is shown in figs. and . in fig. the sun is represented as on the western horizon along the middle line of the figure, and in fig. as on the meridian. it is seen at once that the illumination extends - / ° beyond the north pole, and falls - / ° short of the south pole. as the earth rotates on its axis, places near the north pole will be in the sunshine all the time, while places near the south pole will be out of the sunshine all the time. all places north of the equator will be in the sunshine longer than they are out of it, while all places south of the equator will be out of the sunshine longer than they are in it. . _day and night when the sun is at the winter solstice._--when the sun is at the winter solstice, it is - / ° south of the celestial equator. the diurnal circle described by the sun is then - / ° south of the celestial equator. more than half of this diurnal circle will therefore be above the horizon at all places south of the equator, and less than half of it at all places north of the equator: hence _the days will be longer than the nights south of the equator, and shorter than the nights at places north of the equator_. at places within - / ° of the south pole, the diurnal circle described by the sun will be entirely above the horizon, and the sun will therefore not set. at places within - / ° of the north pole, the diurnal circle described by the sun will be wholly below the horizon, and therefore the sun will not rise. the illumination of the earth at this time is shown in figs. and , and is seen to be the reverse of that shown in figs. and . [illustration: fig. .] [illustration: fig. .] . _variation in the length of day and night._--as long as the sun is north of the equinoctial, the nights will be longer than the days south of the equator, and shorter than the days north of the equator. it is just the reverse when the sun is south of the equator. the farther the sun is from the equator, the greater is the inequality of the days and nights. the farther the place is from the equator, the greater the inequality of its days and nights. when the distance of a place from the _north_ pole is less than the distance of the sun north of the equinoctial, it will have _continuous day without night_, since the whole of the sun's diurnal circle will be above the horizon. a place within the same distance of the _south_ pole will have _continuous night_. when the distance of a place from the _north_ pole is less than the distance of the sun south of the equinoctial, it will have _continuous night_, since the whole of the sun's diurnal circle will then be below the horizon. a place within the same distance of the _south_ pole will then have _continuous day_. at the _equator_ the _days and nights are always equal_; since, no matter where the sun is in the heavens, half of all the diurnal circles described by it will be above the horizon, and half of them below it. . _the zones._--it will be seen, from what has been stated above, that the sun will at some time during the year be directly overhead at every place within - / ° of the equator on either side. this belt of the earth is called the _torrid zone_. the torrid zone is bounded by circles called the _tropics_; that of _cancer_ on the north, and that of _capricorn_ on the south. it will also be seen, that, at every place within - / ° of either pole, there will be, some time during the year, a day during which the sun will not rise, or on which it will not set. these two belts of the earth's surface are called the _frigid zones_. these zones are bounded by the _arctic_ circles. the nearer a place is to the poles, the greater the number of days on which the sun does not rise or set. between the frigid zones and the torrid zones, there are two belts on the earth which are called the _temperate zones_. the sun is never overhead at any place in these two zones, but it rises and sets every day at every place within their limits. . _the width of the zones._--the distance the frigid zones extend from the poles, and the torrid zones from the equator, is exactly equal to _the obliquity of the ecliptic_, or the deviation of the axis of the earth from the perpendicular to the plane of its orbit. were this deviation forty-five degrees, the obliquity of the ecliptic would be forty-five degrees, the torrid zone would extend forty-five degrees from the equator, and the frigid zones forty-five degrees from the poles. in this case there would be no temperate zones. were this deviation fifty degrees, the torrid and frigid zones would overlap ten degrees, and there would be two belts of ten degrees on the earth, which would experience alternately during the year a torrid and a frigid climate. were the axis of the earth perpendicular to the plane of the earth's orbit, there would be no zones on the earth, and no variation in the length of day and night. . _twilight._--were it not for the atmosphere, the darkness of midnight would begin the moment the sun sank below the horizon, and would continue till he rose again above the horizon in the east, when the darkness of the night would be suddenly succeeded by the full light of day. the gradual transition from the light of day to the darkness of the night, and from the darkness of the night to the light of day, is called _twilight_, and is due to the _diffusion of light from the upper layers of the atmosphere_ after the sun has ceased to shine on the lower layers at night, or before it has begun to shine on them in the morning. [illustration: fig. .] let _abcd_ (fig. ) represent a portion of the earth, _a_ a point on its surface where the sun _s_ is setting; and let _sah_ be a ray of light just grazing the earth at _a_, and leaving the atmosphere at the point _h_. the point _a_ is illuminated by the whole reflective atmosphere _hgfe_. the point _b_, to which the sun has set, receives no direct solar light, nor any reflected from that part of the atmosphere which is below _alh_; but it receives a twilight from the portion _hlf_, which lies above the visible horizon _bf_. the point _c_ receives a twilight only from the small portion of the atmosphere; while at _d_ the twilight has ceased altogether. . _duration of twilight._--the astronomical limit of twilight is generally understood to be the instant when stars of the sixth magnitude begin to be visible in the zenith at evening, or disappear in the morning. twilight is usually reckoned to last until the sun's depression below the horizon amounts to eighteen degrees: this, however, varies; in the tropics a depression of sixteen or seventeen degrees being sufficient to put an end to the phenomenon, while in england a depression of seventeen to twenty-one degrees is required. the duration of twilight differs in different latitudes; it varies also in the same latitude at different seasons of the year, and depends, in some measure, on the meteorological condition of the atmosphere. when the sky is of a pale color, indicating the presence of an unusual amount of condensed vapor, twilight is of longer duration. this happens habitually in the polar regions. on the contrary, within the tropics, where the air is pure and dry, twilight sometimes lasts only fifteen minutes. strictly speaking, in the latitude of greenwich there is no true night from may to july , but constant twilight from sunset to sunrise. twilight reaches its minimum three weeks before the vernal equinox, and three weeks after the autumnal equinox, when its duration is an hour and fifty minutes. at midwinter it is longer by about seventeen minutes; but the augmentation is frequently not perceptible, owing to the greater prevalence of clouds and haze at that season of the year, which intercept the light, and hinder it from reaching the earth. the duration is least at the equator (an hour and twelve minutes), and increases as we approach the poles; for at the former there are two twilights every twenty-four hours, but at the latter only two in a year, each lasting about fifty days. at the north pole the sun is below the horizon for six months, but from jan. to the vernal equinox, and from the autumnal equinox to nov. , the sun is less than eighteen degrees below the horizon; so that there is twilight during the whole of these intervals, and thus the length of the actual night is reduced to two months and a half. the length of the day in these regions is about six months, during the whole of which time the sun is constantly above the horizon. the general rule is, _that to the inhabitants of an oblique sphere the twilight is longer in proportion as the place is nearer the elevated pole, and the sun is farther from the equator on the side of the elevated pole_. the seasons. . _the seasons._--while the sun is north of the celestial equator, places north of the equator are receiving heat from the sun by day longer than they are losing it by radiation at night, while places south of the equator are losing heat by radiation at night longer than they are receiving it from the sun by day. when, therefore, the sun passes north of the equator, the temperature begins to rise at places north of the equator, and to fall at places south of it. the rise of temperature is most rapid north of the equator when the sun is at the summer solstice; but, for some time after this, the earth continues to receive more heat by day than it loses by night, and therefore the temperature continues to rise. for this reason, the heat is more excessive after the sun passes the summer solstice than before it reaches it. . _the duration of the seasons._--summer is counted as beginning in june, when the sun is at the summer solstice, and as continuing until the sun reaches the autumnal equinox, in september. autumn then begins, and continues until the sun is at the winter solstice, in december. winter follows, continuing until the sun comes to the vernal equinox, in march, when spring begins, and continues to the summer solstice. in popular reckoning the seasons begin with the first day of june, september, december, and march. the reason why winter is counted as occurring after the winter solstice is similar to the reason why the summer is placed after the summer solstice. the earth north of the equator is losing heat most rapidly at the time of the winter solstice; but for some time after this it loses more heat by night than it receives by day: hence for some time the temperature continues to fall, and the cold is more intense after the winter solstice than before it. [illustration: fig. .] of course, when it is summer in the northern hemisphere, it is winter in the southern hemisphere, and the reverse. fig. shows the portion of the earth's orbit included in each season. it will be seen that the earth is at perihelion in the winter season for places north of the equator, and at aphelion in the summer season. this tends to mitigate somewhat the extreme temperatures of our winters and summers. [illustration: fig. .] . _the illumination of the earth at the different seasons._--fig. shows the earth as it would appear to an observer at the sun during each of the four seasons; that is to say, the portion of the earth that is receiving the sun's rays. figs. , , , and are enlarged views of the earth, as seen from the sun at the time of the summer solstice, of the autumnal equinox, of the winter solstice, and of the vernal equinox. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] fig. is, so to speak, a side view of the earth, showing the limit of sunshine on the earth when the sun is at the summer solstice; and fig. , showing the limit of sunshine when the sun is at the autumnal equinox. [illustration: fig. .] . _cause of the change of seasons._--variety in the length of day and night, and diversity in the seasons, depend upon _the obliquity of the ecliptic_. were there no obliquity of the ecliptic, there would be no inequality in the length of day and night, and but slight diversity of seasons. the greater the obliquity of the ecliptic, the greater would be the variation in the length of the days and nights, and the more extreme the changes of the seasons. tides. . _tides._--the alternate rise and fall of the surface of the sea twice in the course of a lunar day, or of twenty-four hours and fifty-one minutes, is known as the _tides_. when the water is rising, it is said to be _flood_ tide; and when it is falling, _ebb_ tide. when the water is at its greatest height, it is said to be _high_ water; and when at its least height, _low_ water. . _cause of the tides._--it has been known to seafaring nations from a remote antiquity that there is a singular connection between the ebb and flow of the tides and the diurnal motion of the moon. [illustration: fig. .] this tidal movement in seeming obedience to the moon was a mystery until the study of the law of gravitation showed it to be due to _the attraction of the moon on the waters of the ocean_. the reason why there are two tides a day will appear from fig. . let _m_ be the moon, _e_ the earth, and _em_ the line joining their centres. now, strictly speaking, the moon does not revolve around the earth any more than the earth around the moon; but the centre of each body moves around the common centre of gravity of the two bodies. the earth being eighty times as heavy as the moon, this centre is situated within the former, about three-quarters of the way from its centre to its surface, at the point _g_. the body of the earth itself being solid, every part of it, in consequence of the moon's attraction, may be considered as describing a circle once in a month, with a radius equal to _eg_. the centrifugal force caused by this rotation is just balanced by the mean attraction of the moon upon the earth. if this attraction were the same on every part of the earth, there would be everywhere an exact balance between it and the centrifugal force. but as we pass from _e_ to _d_ the attraction of the moon diminishes, owing to the increased distance: hence at _d_ the centrifugal force predominates, and the water therefore tends to move away from the centre _e_. as we pass from _e_ towards _c_, the attraction of the moon increases, and therefore exceeds the centrifugal force: consequently at _c_ there is a tendency to draw the water towards the moon, but still away from the centre _e_. at _a_ and _b_ the attraction of the moon increases the gravity of the water, owing to the convergence of the lines _bm_ and _am_, along which it acts: hence the action of the moon tends to make the waters rise at _d_ and _c_, and to fall at _a_ and _b_, causing two tides to each apparent diurnal revolution of the moon. . _the lagging of the tides._--if the waters everywhere yielded immediately to the attractive force of the moon, it would always be high water when the moon was on the meridian, low water when she was rising or setting, and high water again when she was on the meridian below the horizon. but, owing to the inertia of the water, some time is necessary for so slight a force to set it in motion; and, once in motion, it continues so after the force has ceased, and until it has acted some time in the opposite direction. therefore, if the motion of the water were unimpeded, it would not be high water until some hours after the moon had passed the meridian. the free motion of the water is also impeded by the islands and continents. these deflect the tidal wave from its course in such a way that it may, in some cases, be many hours, or even a whole day, behind its time. sometimes two waves meet each other, and raise a very high tide. in some places the tides run up a long bay, where the motion of a large mass of water will cause an enormous tide to be raised. in the bay of fundy both of these causes are combined. a tidal wave coming up the atlantic coast meets the ocean wave from the east, and, entering the bay with their combined force, they raise the water at the head of it to the height of sixty or seventy feet. . _spring-tides and neap-tides._--the sun produces a tide as well as the moon; but the tide-producing force of the sun is only about four-tenths of that of the moon. at new and full moon the two bodies unite their forces, the ebb and flow become greater than the average, and we have the _spring-tides_. when the moon is in her first or third quarter, the two forces act against each other; the tide-producing force is the difference of the two; the ebb and flow are less than the average; and we have the _neap-tides_. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] fig. shows the tide that would be produced by the moon alone; fig. , the tide produced by the combined action of the sun and moon; and fig. , by the sun and moon acting at right angles to each other. the tide is affected by the distance of the moon from the earth, being highest near the time when the moon is in perigee, and lowest near the time when she is in apogee. when the moon is in perigee, at or near the time of a new or full moon, unusually high tides occur. . _diurnal inequality of tides._--the height of the tide at a given place is influenced by the declination of the moon. when the moon has no declination, the highest tides should occur along the equator, and the heights should diminish from thence toward the north and south; but the two daily tides at any place should have the same height. when the moon has north declination, as shown in fig. , the highest tides on the side of the earth next the moon will be at places having a corresponding north latitude, as at _b_, and on the opposite side at those which have an equal south latitude. of the two daily tides at any place, that which occurs when the moon is nearest the zenith should be the greatest: hence, when the moon's declination is north, the height of the tide at a place in north latitude should be greater when the moon is above the horizon than when she is below it. at the same time, places south of the equator have the highest tides when the moon is below the horizon, and the least when she is above it. this is called the _diurnal inequality_, because its cycle is one day; but it varies greatly in amount at different places. [illustration: fig. .] . _height of tides._--at small islands in mid-ocean the tides never rise to a great height, sometimes even less than one foot; and the average height of the tides for the islands of the atlantic and pacific oceans is only three feet and a half. upon approaching an extensive coast where the water is shallow, the height of the tide is increased; so that, while in mid-ocean the average height does not exceed three feet and a half, the average in the neighborhood of continents is not less than four or five feet. the day and time. . _the day._--by the term _day_ we sometimes denote the period of sunshine as contrasted with that of the absence of sunshine, which we call _night_, and sometimes the period of the earth's rotation on its axis. it is with the latter signification that the term is used in this section. as the earth rotates on its axis, it carries the meridian of a place with it; so that, during each complete rotation of the earth, the portion of the meridian which passes overhead from pole to pole sweeps past every star in the heavens from west to east. the _interval between two successive passages of this portion of the meridian across the same star_ is the exact period of the complete rotation of the earth. this period is called a _sidereal day_. the sidereal day may also be defined as _the interval between two successive passages of the same star across the meridian_; the passage of the meridian across the star, and the passage or _transit_ of the star across the meridian, being the same thing looked at from a different point of view. the interval _between two successive passages of the meridian across the sun_, or _of the sun across the meridian_, is called a _solar day_. . _length of the solar day._--the solar day is a little longer than the sidereal day. this is owing to the sun's eastward motion among the stars. we have already seen that the sun's apparent position among the stars is continually shifting towards the east at a rate which causes it to make a complete circuit of the heavens in a year, or three hundred and sixty-five days. this is at the rate of about one degree a day: hence, were the sun and a star on the meridian together to-day, when the meridian again came around to the star, the sun would appear about one degree to the eastward: hence the meridian must be carried about one degree farther in order to come up to the sun. the solar day must therefore be _about four minutes longer_ than the sidereal day. [illustration: fig. .] [illustration: fig. .] the fact that the earth must make more than a complete rotation is also evident from figs. and . in fig. , _ba_ represents the plane of the meridian, and the small arrows indicate the direction the earth is rotating on its axis, and revolving in its orbit. when the earth is at , the sun is on the meridian at _a_. when the earth has moved to , it has made a complete rotation, as is shown by the fact that the plane of the meridian is parallel with its position at ; but it is evident that the meridian has not yet come up with the sun. in fig. , _oa_ represents the plane of the meridian, and _os_ the direction of the sun. the small arrows indicate the direction of the rotation and revolution of the earth. in passing from the first position to the second the earth makes a complete rotation, but the meridian is not brought up to the sun. . _inequality in the length of solar days._--the sidereal days are all of the same length; but the solar days differ somewhat in length. this difference is due to the fact that the sun's apparent position moves eastward, or _away from the meridian_, at a variable rate. there are three reasons why this rate is variable:-- ( ) the sun's eastward motion is due to the revolution of the earth in its orbit. now, the earth's orbital motion is _not uniform_, being fastest when the earth is at perihelion, and slowest when the earth is at aphelion: hence, other things being equal, solar days will be longest when the earth is at perihelion, and shortest when the earth is at aphelion. [illustration: fig. .] [illustration: fig. .] ( ) the sun's eastward motion is along the ecliptic. now, from figs. and , it will be seen, that, when the sun is at one of the equinoxes, it will be moving away from the meridian _obliquely_; and, from figs. and , that, when the sun is at one of the solstices, it will be moving away from the meridian _perpendicularly_: hence, other things being equal, the sun would move away from the meridian _fastest_, and the days be _longest_, when the sun is at the _solstices_; while it would move away from the meridian _slowest_, and the days be _shortest_, when the sun is at the _equinoxes_. that a body moving along the ecliptic must be moving at a variable angle to the meridian becomes very evident on turning a celestial globe so as to bring each portion of the ecliptic under the meridian in turn. [illustration: fig. .] [illustration: fig. .] ( ) the sun, moving along the ecliptic, always moves _in a great circle_, while the point of the meridian which is to overtake the sun moves in a diurnal circle, which is _sometimes a great circle_ and _sometimes a small circle_. when the sun is at the equinoxes, the point of the meridian which is to overtake it moves in a great circle. as the sun passes from the equinoxes to the solstices, the point of the meridian which is to overtake it moves on a smaller and smaller circle: hence, as we pass away from the celestial equator, the points of the meridian move slower and slower. therefore, other things being equal, the meridian will gain upon the sun _most rapidly_, and the days be _shortest_, when the sun is at the _equinoxes_; while it will gain on the sun _least rapidly_, and the days will be _longest_, when the sun is at the _solstices_. the ordinary or _civil day_ is the mean of all the solar days in a year. . _sun time and clock time._--it is noon by the sun when the sun is on the meridian, and by the clock at the middle of the civil day. now, as the civil days are all of the same length, while solar days are of variable length, it seldom happens that the middles of these two days coincide, or that sun time and clock time agree. the difference between sun time and clock time, or what is often called _apparent solar time_ and _mean solar time_, is called the _equation of time_. the sun is said to be _slow_ when it crosses the meridian after noon by the clock, and _fast_ when it crosses the meridian before noon by the clock. sun time and clock time coincide four times a year; during two intermediate seasons the clock time is ahead, and during two it is behind. * * * * * the following are the dates of coincidence and of maximum deviation, which vary but slightly from year to year:-- february true sun fifteen minutes slow. april true sun correct. may true sun four minutes fast. june true sun correct. july true sun six minutes slow. august true sun correct. november true sun sixteen minutes fast. december true sun correct. one of the effects of the equation of time which is frequently misunderstood is, that the interval from sunrise until noon, as given in the almanacs, is not the same as that between noon and sunset. the forenoon could not be longer or shorter than the afternoon, if by "noon" we meant the passage of the sun across the meridian; but the noon of our clocks being sometimes fifteen minutes before or after noon by the sun, the former may be half an hour nearer to sunrise than to sunset, or _vice versa_. the year. . _the year._--the _year_ is the time it takes the earth to revolve around the sun, or, what amounts to the same thing, _the time it takes the sun to pass around the ecliptic_. ( ) the time it takes the sun to pass from a star around to the same star again is called a _sidereal year_. this is, of course, the exact time it takes the earth to make a complete revolution around the sun. [illustration: fig. .] ( ) the time it takes the sun to pass around from the vernal equinox, or the _first point of aries_, to the vernal equinox again, is called the _tropical_ year. this is a little shorter than the sidereal year, owing to the precession of the equinoxes. this will be evident from fig. . the circle represents the ecliptic, _s_ the sun, and _e_ the vernal equinox. the sun moves around the ecliptic _eastward_, as indicated by the long arrow, while the equinox moves slowly _westward_, as indicated by the short arrow. the sun will therefore meet the equinox before it has quite completed the circuit of the heavens. the exact lengths of these respective years are:-- sidereal year . = days hours min sec tropical year . = days hours min sec since the recurrence of the seasons depends on the tropical year, the latter is the one to be used in forming the calendar and for the purposes of civil life generally. its true length is eleven minutes and fourteen seconds less than three hundred and sixty-five days and a fourth. it will be seen that the tropical year is about twenty minutes shorter than the sidereal year. ( ) the time it takes the earth to pass from its perihelion point around to the perihelion point again is called the _anomalistic year_. this year is about four minutes longer than the sidereal year. this is owing to the fact that the major axis of the earth's orbit is slowly moving around to the east at the rate of about ten seconds a year. this causes the perihelion point _p_ (fig. ) to move _eastward_ at that rate, as indicated by the short arrow. the earth _e_ is also moving eastward, as indicated by the long arrow. hence the earth, on starting at the perihelion, has to make a little more than a complete circuit to reach the perihelion point again. [illustration: fig. .] . _the calendar._--the _solar year_, or the interval between two successive passages of the same equinox by the sun, is days, hours, minutes, seconds. if, then, we reckon only days to a common or _civil year_, the sun will come to the equinox hours, minutes, seconds, or nearly a quarter of a day, later each year; so that, if the sun entered aries on the th of march one year, he would enter it on the st four years after, on the d eight years after, and so on. thus in a comparatively short time the spring months would come in the winter, and the summer months in the spring. among different ancient nations different methods of computing the year were in use. some reckoned it by the revolution of the moon, some by that of the sun; but none, so far as we know, made proper allowances for deficiencies and excesses. twelve moons fell short of the true year, thirteen exceeded it; days were not enough, were too many. to prevent the confusion resulting from these errors, julius cæsar reformed the calendar by making the year consist of days, hours (which is hence called a _julian_ year), and made every fourth year consist of days. this method of reckoning is called _old style_. but as this made the year somewhat too long, and the error in amounted to ten days, pope gregory xiii., in order to bring the vernal equinox back to the st of march again, ordered ten days to be struck out of that year, calling the next day after the th of october the th; and, to prevent similar confusion in the future, he decreed that three leap-years should be omitted in the course of every four hundred years. this way of reckoning time is called _new style_. it was immediately adopted by most of the european nations, but was not accepted by the english until the year . the error then amounted to eleven days, which were taken from the month of september by calling the d of that month the th. the old style is still retained by russia. according to the gregorian calendar, _every year whose number is divisible by four_ is a _leap-year_, except, that, _in the case of the years whose numbers are exact hundreds, those only are leap-years which are divisible by four after cutting off the last two figures_. thus the years , , , etc., are leap-years; , , , , , etc., are not. the error will not amount to a day in over three thousand years. . _the dominical letter._--the _dominical letter_ for any year is that which we often see placed against sunday in the almanacs, and is always one of the first seven in the alphabet. since a common year consists of days, if this number is divided by seven (the number of days in a week), there will be a remainder of one: hence a year commonly begins one day later in the week than the preceding one did. if a year of days begins on sunday, the next will begin on monday; if it begins on thursday, the next will begin on friday; and so on. if sunday falls on the st of january, the _first_ letter of the alphabet, or _a_, is the _dominical letter_. if sunday falls on the th of january (as it will the next year, unless the first is leap-year), the _seventh_ letter, _g_, is the dominical letter. if sunday falls on the th of january (as it will the third year, unless the first or second is leap-year), the _sixth_ letter, _f_, will be the dominical letter. thus, if there were no leap-years, the dominical letters would regularly follow a retrograde order, _g_, _f_, _e_, _d_, _c_, _b_, _a_. but _leap_-years have days, which, divided by seven, leaves two remainder: hence the years following leap-years will begin two days later in the week than the leap-years did. to prevent the interruption which would hence occur in the order of the dominical letters, leap-years have _two_ dominical letters, one indicating sunday till the th of february, and the other for the rest of the year. by _table i._ below, the dominical letter for any year (new style) for four thousand years from the beginning of the christian era may be found; and it will be readily seen how the table could be extended indefinitely by continuing the centuries at the top in the same order. to find the dominical letter by this table, _look for the hundreds of years at the top, and for the years below a hundred, at the left hand_. thus the letter for will be opposite the number , and in the column having at the top; that is, it will be _a_. in the same way, the letters for , which is a leap-year, will be found to be _fe_. having the dominical letter of any year, _table ii._ shows what days of every month of the year will be _sundays_. to find the sundays of any month in the year by this table, _look in the column, under the dominical letter, opposite the name of the month given at the left_. from the sundays the date of any other day of the week can be readily found. thus, if we wish to know on what day of the week christmas falls in , we look opposite december, under the letter _f_ (which we have found to be the dominical letter for the year), and find that the d of the month is a sunday; the th, or christmas, will then be wednesday. in the same way we may find the day of the week corresponding to any date (new style) in history. for instance, the th of june, , the day of the fight at bunker hill, is found to have been a _saturday_. these two tables then serve as a _perpetual almanac_. table i. --- --- --- ---- c e g ba b d f g a c e f g b d e fe ag cb dc d f a b c e g a b d f g ag cb ed fe f a c d e g b c d f a b cb ed gf ag a c e f g b d e f a c d .. ed gf ba cb .. c e g a .. b d f g .. a c e f .. gf ba dc ed .. e g b c .. d f a b .. c e g a .. ba dc fe gf .. g b d e .. f a c d .. e g b c .. dc fe ag ba table ii. a b c d e f g jan. . oct. . .. .. .. .. feb. - . .. .. .. march . nov. . .. .. .. .. .. .. .. april . july .. .. .. .. .. .. .. aug. . .. .. .. .. .. .. .. sept. . dec. . .. .. .. .. .. .. .. may. . .. .. .. .. .. .. .. june . .. weight of the earth and precession. . _the weight of the earth._--there are several methods of ascertaining the weight and mass of the earth. the simplest, and perhaps the most trustworthy method is to compare the pull of the earth upon a ball of lead with that of a known mass of lead upon it. the pull of a known mass of lead upon the ball may be measured by means of a torsion balance. one form of the balance employed for this purpose is shown in figs. and . two small balls of lead, _b_ and _b_, are fastened to the ends of a light rod _e_, which is suspended from the point _f_ by means of the thread _fe_. two large balls of lead, _w_ and _w_, are placed on a turn-table, so that one of them shall be just in front of one of the small balls, and the other just behind the other small ball. the pull of the large balls turns the rod around a little so as to bring the small balls nearer the large ones. the small balls move towards the large ones till they are stopped by the torsion of the thread, which is then equal to the pull of the large balls. the deflection of the rod is carefully measured. the table is then turned into the position indicated by the dotted lines in fig. , so as to reverse the position of the large balls with reference to the small ones. the rod is now deflected in the opposite direction, and the amount of deflection is again carefully measured. the second measurement is made as a check upon the accuracy of the first. the force required to twist the thread as much as it was twisted by the deflection of the rod is ascertained by measurement. this gives the pull of the two large balls upon the two small ones. we next calculate what this pull would be were the balls as far apart as the small balls are from the centre of the earth. we can then form the following proportion: the pull of the large balls upon the small ones is to the pull of the earth upon the small ones as the mass of the large balls is to the mass of the earth, or as the weight of the large balls is to the weight of the earth. of course, the pull of the earth upon the small balls is the weight of the small balls. in this way it has been ascertained that the mass of the earth is about . times that of a globe of water of the same size. in other words, the _mean density_ of the earth is about . . [illustration: fig. .] [illustration: fig. .] the weight of the earth in pounds may be found by multiplying the number of cubic feet in it by - / (the weight, in pounds, of one cubic foot of water), and this product by . . [illustration: fig. .] . _cause of precession._--we have seen that the earth is flattened at the poles: in other words, the earth has the form of a sphere, with a protuberant ring around its equator. this equatorial ring is inclined to the plane of the ecliptic at an angle of about - / °. in fig. this ring is represented as detached from the enclosed sphere. _s_ represents the sun, and _sc_ the ecliptic. as the point _a_ of the ring is nearer the sun than the point _b_ is, the sun's pull upon _a_ is greater than upon _b_: hence the sun tends to pull the ring over into the plane of the ecliptic; but the rotation of the earth tends to keep the ring in the same plane. the struggle between these two tendencies causes the earth, to which the ring is attached, to wabble like a spinning-top, whose rotation tends to keep it erect, while gravity tends to pull it over. the handle of the top has a gyratory motion, which causes it to describe a curve. the axis of the heavens corresponds to the handle of the top. ii. the moon. distance, size, and motions. . _the distance of the moon._--the moon is the nearest of the heavenly bodies. its distance from the centre of the earth is only about sixty times the radius of the earth, or, in round numbers, two hundred and forty thousand miles. the ordinary method of finding the distance of one of the nearer heavenly bodies is first to ascertain its horizontal parallax. this enables us to form a right-angled triangle, the lengths of whose sides are easily computed, and the length of whose hypothenuse is the distance of the body from the centre of the earth. [illustration: fig. .] horizontal parallax has already been defined ( ) as the displacement of a heavenly body when on the horizon, caused by its being seen from the surface, instead of the centre, of the earth. this displacement is due to the fact that the body is seen in a different direction from the surface of the earth from that in which it would be seen from the centre. horizontal parallax might be defined as the difference in the directions in which a body on the horizon would be seen from the surface and from the centre of the earth. thus, in fig. , _c_ is the centre of the earth, _a_ a point on the surface, and _b_ a body on the horizon of _a_. _ab_ is the direction in which the body would be seen from _a_, and _cb_ the direction in which it would be seen from _c_. the difference of these directions, or the angle _abc_, is the parallax of the body. the triangle _bac_ is right-angled at _a_; the side _ac_ is the radius of the earth, and the hypothenuse is the distance of the body from the centre of the earth. when the parallax _abc_ is known, the length of _cb_ can easily by found by trigonometrical computation. we have seen ( ) that the parallax of a heavenly body grows less and less as the body passes from the horizon towards the zenith. the parallax of a body and its altitude are, however, so related, that, when we know the parallax at any altitude, we can readily compute the horizontal parallax. the usual method of finding the parallax of one of the nearer heavenly bodies is first to find its parallax when on the meridian, as seen from two places on the earth which differ considerably in latitude: then to calculate what would be the parallax of the body as seen from one of these places and the centre of the earth: and then finally to calculate what would be the parallax were the body on the horizon. [illustration: fig. .] thus, we should ascertain the parallax of the body _b_ (fig. ) as seen from _a_ and _d_, or the angle _abd_. we should then calculate its parallax as seen from _a_ and _c_, or the angle _abc_. finally we should calculate what its parallax would be were the body on the horizon, or the angle _ab'c_. the simplest method of finding the parallax of a body _b_ (fig. ) as seen from the two points _a_ and _d_ is to compare its direction at each point with that of the same fixed star near the body. the star is so distant, that it will be seen in the same direction from both points: hence, if the direction of the body differs from that of the star ° as seen from one point, and ° ' as seen from the other point, the two lines _ab_ and _db_ must differ in direction by '; in other words, the angle _abd_ would be '. the method just described is the usual method of finding the parallax of the moon. . _the apparent size of the moon._--the _apparent size_ of a body is the visual angle subtended by it; that is, the angle formed by two lines drawn from the eye to two opposite points on the outline of the body. the apparent size of a body depends upon both its _magnitude_ and its _distance_. the apparent size, or _angular diameter_, of the moon is about thirty-one minutes. this is ascertained by means of the wire micrometer already described ( ). the instrument is adjusted so that its longitudinal wire shall pass through the centre of the moon, and its transverse wires shall be tangent to the limbs of the moon on each side, at the point where they are cut by the longitudinal wire. the micrometer screw is then turned till the wires are brought together. the number of turns of the screw needed to accomplish this will indicate the arc between the wires, or the angular diameter of the moon. [illustration: fig. .] in order to be certain that the longitudinal wire shall pass through the centre of the moon, it is best to take the moon when its disc is in the form of a crescent, and to place the longitudinal wire against the points, or _cusps_, of the crescent, as shown in fig. . [illustration: fig. .] . _the real size of the moon._--the real diameter of the moon is a little over one-fourth of that of the earth, or a little more than two thousand miles. the comparative sizes of the earth and moon are shown in fig. . [illustration: fig. .] the distance and apparent size of the moon being known, her real diameter is found by means of a triangle formed as shown in fig. . _c_ represents the centre of the moon, _cb_ the distance of the moon from the earth, and _ca_ the radius of the moon. _bac_ is a triangle, right-angled at _a_. the angle _abc_ is half the apparent diameter of the moon. with the angles _a_ and _b_, and the side _cb_ known, it is easy to find the length of _ac_ by trigonometrical computation. twice _ac_ will be the diameter of the moon. the volume of the moon is about one-fiftieth of that of the earth. . _apparent size of the moon on the horizon and in the zenith._.--the moon is nearly four thousand miles farther from the observer when she is on the horizon than when she is in the zenith. this is evident from fig. . _c_ is the centre of the earth, _m_ the moon on the horizon, _m'_ the moon in the zenith, and _o_ the point of observation. _om_ is the distance of the moon when she is on the horizon, and _om'_ the distance of the moon from the observer when she is in the zenith. _cm_ is equal to _cm'_, and _om_ is about the length of _cm_; but _om'_ is about four thousand miles shorter than _cm'_: hence _om'_ is about four thousand miles shorter than _om_. [illustration: fig. .] notwithstanding the moon is much nearer when at the zenith than at the horizon, it seems to us much larger at the horizon. this is a pure illusion, as we become convinced when we measure the disc with accurate instruments, so as to make the result independent of our ordinary way of judging. when the moon is near the horizon, it seems placed beyond all the objects on the surface of the earth in that direction, and therefore farther off than at the zenith, where no intervening objects enable us to judge of its distance. in any case, an object which keeps the same apparent magnitude seems to us, through the instinctive habits of the eye, the larger in proportion as we judge it to be more distant. . _the apparent size of the moon increased by irradiation._--in the case of the moon, the word _apparent_ means much more than it does in the case of other celestial bodies. indeed, its brightness causes our eyes to play us false. as is well known, the crescent of the new moon seems part of a much larger sphere than that which it has been said, time out of mind, to "hold in its arms." the bright portion of the moon as seen with our measuring instruments, as well as when seen with the naked eye, covers a larger space in the field of the telescope than it would if it were not so bright. this effect of _irradiation_, as it is called, must be allowed for in exact measurements of the diameter of the moon. [illustration: fig. .] . _apparent size of the moon in different parts of her orbit._--owing to the eccentricity of the moon's orbit, her distance from the earth varies somewhat from time to time. this variation causes a corresponding variation in her apparent size, which is illustrated in fig. . . _the mass of the moon._--the moon is considerably less dense than the earth, its mass being only about one-eightieth of that of the earth; that is, while it would take only about fifty moons to make the bulk of the earth, it would take about eighty to make the mass of the earth. one method of finding the mass of the moon is to compare her effect in producing the _tides_ with that of the sun. we first calculate what would be the moon's effect in producing the tides, were she as far off as the sun. we then form the following proportion: as the sun's effect in producing the tides is to the moon's effect at the same distance, so is the mass of the sun to the mass of the moon. the method of finding the mass of the sun will be given farther on. . _the orbital motion of the moon._--if we watch the moon from night to night, we see that she moves eastward quite rapidly among the stars. when the new moon is first visible, it appears near the horizon in the west, just after sunset. a week later the moon will be on the meridian at the same hour, and about a week later still on the eastern horizon. the moon completes the circuit of the heavens in a period of about thirty days, moving eastward at the rate of about twelve degrees a day. this eastward motion of the moon is due to the fact that she is revolving around the earth from west to east. [illustration: fig. .] . _the aspects of the moon._--as the moon revolves around the earth, she comes into different positions with reference to the earth and sun. these different positions of the moon are called the _aspects_ of the moon. the four chief aspects of the moon are shown in fig. . when the moon is at _m_, she appears in the opposite part of the heavens to the sun, and is said to be in _opposition_; when at _m'_ and at _m'''_, she appears ninety degrees away from the sun, and is said to be in _quadrature_; when at _m''_, she appears in the same part of the heavens as the sun, and is said to be in _conjunction_. . _the sidereal and synodical periods of the moon._--the _sidereal period_ of the moon is the time it takes her to pass around from a star to that star again, or the time it takes her to _make a complete revolution around the earth_. this is a period of about twenty-seven days and a third. it is sometimes called the _sidereal month_. the _synodical period_ of the moon is the time that it takes the moon to _pass from one aspect around to the same aspect again_. this is a period of about twenty-nine days and a half, and it is sometimes called the _synodical month_. [illustration: fig. .] the reason why the synodical period is longer than the sidereal period will appear from fig. . _s_ represents the position of the sun, _e_ that of the earth, and the small circle the orbit of the moon around the earth. the arrow in the small circle represents the direction the moon is revolving around the earth, and the arrow in the arc between _e_ and _e'_ indicates the direction of the earth's motion in its orbit. when the moon is at _m_{ }_, she is in conjunction. as the moon revolves around the earth, the earth moves forward in its orbit. when the moon has come round to _m_{ }_, so that _m_{ }m_{ }_ is parallel with _m_{ }m_{ }_, she will have made a complete or _sidereal_ revolution around the earth; but she will not be in conjunction again till she has come round to _m_, so as again to be between the earth and sun. that is to say, the moon must make more than a complete revolution in a synodical period. [illustration: fig. .] the greater length of the synodical period is also evident from fig. . _t_ represents the earth, and _l_ the moon. the arrows indicate the direction in which each is moving. when the earth is at _t_, and the moon at _l_, the latter is in conjunction. when the earth has reached _t'_, and the moon _l'_, the latter has made a sidereal revolution; but she will not be in conjunction again till the earth has reached _t''_, and the moon _l''_. . _the phases of the moon._--when the new moon appears in the west, it has the form of a _crescent_, with its convex side towards the sun, and its horns towards the east. as the moon advances towards quadrature, the crescent grows thicker and thicker, till it becomes a _half-circle_ at first quarter. when it passes quadrature, it begins to become convex also on the side away from the sun, or _gibbous_ in form. as it approaches opposition, it becomes more and more nearly circular, until at opposition it is a _full_ circle. from full moon to last quarter it is again gibbous, and at last quarter a half-circle. from last quarter to new moon it is again crescent; but the horns of the crescent are now turned towards the west. the successive phases of the moon are shown in fig. . [illustration: fig. .] . _cause of the phases of the moon._--take a globe, half of which is colored white and the other half black in such a way that the line which separates the white and black portions shall be a great circle which passes through the poles of the globe, and rotate the globe slowly, so as to bring the white half gradually into view. when the white part first comes into view, the line of separation between it and the black part, which we may call the _terminator_, appears concave, and its projection on a plane perpendicular to the line of vision is a concave line. as more and more of the white portion comes into view, the projection of the terminator becomes less and less concave. when half of the white portion comes into view, the terminator is projected as a straight line. when more than half of the white portion comes into view, the terminator begins to appear as a convex line, and this line becomes more and more convex till the whole of the white half comes into view, when the terminator becomes circular. [illustration: fig. .] the moon is of itself a dark, opaque globe; but the half that is towards the sun is always bright, as shown in fig. . this bright half of the moon corresponds to the white half of the globe in the preceding illustration. as the moon revolves around the earth, different portions of this illumined half are turned towards the earth. at new moon, when the moon is in conjunction, the bright half is turned entirely away from the earth, and the disc of the moon is black and invisible. between new moon and first quarter, less than half of the illumined side is turned towards the earth, and we see this illumined portion projected as a crescent. at first quarter, just half of the illumined side is turned towards the earth, and we see this half projected as a half-circle. between first quarter and full, more than half of the illumined side is turned towards the earth, and we see it as gibbous. at full, the whole of the illumined side is turned towards us, and we see it as a full circle. from full to new moon again, the phases occur in the reverse order. . _the form of the moon's orbit._--the orbit of the moon around the earth is an ellipse of slight eccentricity. the form of this ellipse is shown in fig. . _c_ is the centre of the ellipse, and _e_ the position of the earth at one of its foci. the eccentricity of the ellipse is only about one-eighteenth. it is impossible for the eye to distinguish such an ellipse from a circle. [illustration: fig. .] . _the inclination of the moon's orbit._--the plane of the moon's orbit is inclined to the ecliptic by an angle of about five degrees. the two points where the moon's orbit cuts the ecliptic are called her _nodes_. the moon's nodes have a westward motion corresponding to that of the equinoxes, but much more rapid. they complete the circuit of the ecliptic in about nineteen years. the moon's latitude ranges from ° north to ° south; and since, owing to the motion of her nodes, the moon is, during a period of nineteen years, ° north and ° south of every part of the ecliptic, her declination will range from - / ° + ° = - / ° north to - / ° + ° = - / ° south. . _the meridian altitude of the moon._--the _meridian altitude_ of any body is its altitude when on the meridian. in our latitude, the meridian altitude of any point on the equinoctial is forty-nine degrees. the meridian altitude of the summer solstice is ° + - / ° = - / °, and that of the winter solstice is ° - - / ° = - / °. the greatest meridian altitude of the moon is - / ° + ° = - / °, and its least meridian altitude, - / ° - ° = - / °. when the moon's meridian altitude is greater than the elevation of the equinoctial, it is said to run _high_, and when less, to run _low_. the full moon runs high when the sun is south of the equinoctial, and low when the sun is north of the equinoctial. this is because the full moon is always in the opposite part of the heavens to the sun. . _wet and dry moon._--at the time of new moon, the cusps of the crescent sometimes lie in a line which is nearly perpendicular with the horizon, and sometimes in a line which is nearly parallel with the horizon. in the former case the moon is popularly described as a _wet_ moon, and in the latter case as a _dry_ moon. [illustration: fig. .] the great circle which passes through the centre of the sun and moon will pass through the centre of the crescent, and be perpendicular to the line joining the cusps. now the ecliptic makes the least angle with the horizon when the vernal equinox is on the eastern horizon and the autumnal equinox is on the western. in our latitude, as we have seen, this angle is - / °: hence in our latitude, if the moon were at new on the ecliptic when the sun is at the autumnal equinox, as shown at _m_{ }_ (fig. ), the great circle passing through the centre of the sun and moon would be the ecliptic, and at new york would be inclined to the horizon at an angle of - / °. if the moon happened to be ° south of the ecliptic at this time, as at _m_{ }_, the great circle passing through the centre of the sun and moon would make an angle of only - / ° with the horizon. in either of these cases the line joining the cusps would be nearly perpendicular to the horizon. [illustration: fig. .] if the moon were at new on the ecliptic when the sun is near the vernal equinox, as shown at _m_{ }_ (fig. ), the great circle passing through the centres of the sun and moon would make an angle of - / ° with the horizon at new york; and were the moon ° north of the ecliptic at that time, as shown at _m_{ }_, this great circle would make an angle of - / ° with the horizon. in either of these cases, the line joining the cusps would be nearly parallel with the horizon. at different times, the line joining the cusps may have every possible inclination to the horizon between the extreme cases shown in figs. and . . _daily retardation of the moon's rising._--the moon rises, on the average, about fifty minutes later each day. this is owing to her eastward motion. as the moon makes a complete revolution around the earth in about twenty-seven days, she moves eastward at the rate of about thirteen degrees a day, or about twelve degrees a day faster than the sun. were the moon, therefore, on the horizon at any hour to-day, she would be some twelve degrees below the horizon at the same hour to-morrow. now, as the horizon moves at the rate of one degree in four minutes, it would take it some fifty minutes to come up to the moon so as to bring her upon the horizon. hence the daily retardation of the moon's rising is about fifty minutes; but it varies considerably in different parts of her orbit. there are two reasons for this variation in the daily retardation:-- ( ) the moon moves at a _varying rate in her orbit_; her speed being greatest at perigee, and least at apogee: hence, other things being equal, the retardation is greatest when the moon is at perigee, and least when she is at apogee. [illustration: fig. .] [illustration: fig. .] ( ) the moon moves at a _varying angle to the horizon_. the moon moves nearly in the plane of the ecliptic, and of course she passes both equinoxes every lunation. when she is near the autumnal equinox, her path makes the greatest angle with the eastern horizon, and when she is near the vernal equinox, the least angle: hence the moon moves away from the horizon fastest when she is near the autumnal equinox, and slowest when she is near the vernal equinox. this will be evident from figs. and . in each figure, _sn_ represents a portion of the eastern horizon, and _ec_, _e'c'_, a portion of the ecliptic. _ae_, in fig. , represents the autumnal equinox, and _aem_ the daily motion of the moon. _ve_, in fig. , represents the vernal equinox, and _vem'_ the motion of the moon for one day. in the first case this motion would carry the moon away from the horizon the distance _am_, and in the second case the distance _a'm'_. now, it is evident that _am_ is greater than _a'm'_: hence, other things being equal, the greatest retardation of the moon's rising will be when the moon is near the autumnal equinox, and the least retardation when the moon is near the vernal equinox. the least retardation at new york is twenty-three minutes, and the greatest an hour and seventeen minutes. the greatest and least retardations vary somewhat from month to month; since they depend not only upon the position of the moon in her orbit with reference to the equinoxes, but also upon the latitude of the moon, and upon her nearness to the earth. [illustration: fig. .] the direction of the moon's motion with reference to the ecliptic is shown in fig. , which shows the moon's motion for one day in july, . . _the harvest moon_--the long and short retardations in the rising of the moon, though they occur every month, are not likely to attract attention unless they occur at the time of full moon. the long retardations for full moon occur when the moon is near the autumnal equinox at full. as the full moon is always opposite to the sun, the sun must in this case be near the vernal equinox: hence the long retardations for full moon occur in the spring, the greatest retardation being in march. the least retardations for full moon occur when the moon is near the vernal equinox at full: the sun must then be near the autumnal equinox. hence the least retardations for full moon occur in the months of august, september, and october. the retardation is, of course, least for september; and the full moon of this month rises night after night less than half an hour later than the previous night. the full moon of september is called the "harvest moon," and that of october the "hunter's moon." . _the rotation of the moon._--a careful examination of the spots on the disc of the moon reveals the fact that she always presents the same side to the earth. in order to do this, she must rotate on her axis while making a revolution around the earth, or in about twenty-seven days. . _librations of the moon._--the moon appears to rock slowly to and fro, so as to allow us to see alternately a little farther around to the right and the left, or above and below, than we otherwise could. this apparent rocking of the moon is called _libration_. the moon has three librations:-- ( ) _libration in latitude._--this libration enables us to see alternately a little way around on the northern and southern limbs of the moon. this libration is due to the fact that the axis of the moon is not quite perpendicular to the plane of her orbit. the deviation from the perpendicular is six degrees and a half. as the axis of the moon, like that of the earth, maintains the same direction, the poles of the moon will be turned alternately six degrees and a half toward and from the earth. ( ) _libration in longitude._--this libration enables us to see alternately a little farther around on the eastern and western limbs of the moon. [illustration: fig. .] it is due to the fact that the moon's axial motion is uniform, while her orbital motion is not. at perigee her orbital motion will be in advance of her axial motion, while at apogee the axial motion will be in advance of the orbital. in fig. , _e_ represents the earth, _m_ the moon, the large arrow the direction of the moon's motion in her orbit, and the small arrow the direction of her motion of rotation. when the moon is at _m_, the line _ab_, drawn perpendicular to _em_, represents the circle which divides the visible from the invisible portion of the moon. while the moon is passing from _m_ to _m'_, the moon performs less than a quarter of a rotation, so that _ab_ is no longer perpendicular to _em'_. an observer on the earth can now see somewhat beyond _a_ on the western limb of the moon, and not quite up to _b_ on the eastern limb. while the moon is passing from _m'_ to _m''_, her axial motion again overtakes her orbital motion, so that the line _ab_ again becomes perpendicular to the line joining the centre of the moon to the centre of the earth. exactly the same side is now turned towards the earth as when the moon was at _m_. while the moon passes from _m''_ to _m'''_, her axial motion gets in advance of her orbital motion, so that _ab_ is again inclined to the line joining the centres of the earth and moon. a portion of the eastern limb of the moon beyond _b_ is now brought into view to the earth, and a portion of the western limb at _a_ is carried out of view. while the moon is passing from _m'''_ to _m_, the orbital motion again overtakes the axial motion, and _ab_ is again perpendicular to _me_. ( ) _parallactic libration._--while an observer at the centre of the earth would get the same view of the moon, whether she were on the eastern horizon, in the zenith, or on the western horizon, an observer on the surface of the earth does not get exactly the same view in these three cases. when the moon is on the eastern horizon, an observer on the surface of the earth would see a little farther around on the western limb of the moon than when she is in the zenith, and not quite so far around on the eastern limb. on the contrary, when the moon is on the western horizon, an observer on the surface of the earth sees a little farther around on the eastern limb of the moon than when she is in the zenith, and not quite so far around on her western limb. [illustration: fig. .] this will be evident from fig. . _e_ is the centre of the earth, and _o_ a point on its surface. _ab_ is a line drawn through the centre of the moon, perpendicular to a line joining the centres of the moon and the earth. this line marks off the part of the moon turned towards the centre of the earth, and remains essentially the same during the day. _cd_ is a line drawn through the centre of the moon perpendicular to a line joining the centre of the moon and the point of observation. this line marks off the part of the moon turned towards _o_. when the moon is in the zenith, _cd_ coincides with _ab_; but, when the moon is on the horizon, _cd_ is inclined to _ab_. when the moon is on the eastern horizon, an observer at _o_ sees a little beyond _b_, and not quite to _a_; and, when she is on the western horizon, he sees a little beyond _a_, and not quite to _b_. _b_ is on the western limb of the moon, and _a_ on her eastern limb. since this libration is due to the point from which the moon is viewed, it is called _parallactic_ libration; and, since it occurs daily, it is called _diurnal_ libration. [illustration: fig. .] . _portion of the lunar surface brought into view by libration._--the area brought into view by the first two librations is between one-twelfth and one-thirteenth of the whole lunar surface, or nearly one-sixth of the hemisphere of the moon which is turned away from the earth when the moon is at her state of mean libration. of course a precisely equal portion of the hemisphere turned towards us during mean libration is carried out of view by the lunar librations. if we add to each of these areas a fringe about one degree wide, due to the diurnal libration, and which we may call the _parallactic_ fringe, we shall find that the total area brought into view is almost exactly one-eleventh part of the whole surface of the moon. a similar area is carried out of view; so that the whole region thus swayed out of and into view amounts to two-elevenths of the moon's surface. this area is shown in fig. , which is a side view of the moon. [illustration: fig. .] . _the moon's path through space._--were the earth stationary, the moon would describe an ellipse around it similar to that of fig. ; but, as the earth moves forward in her orbit at the same time that the moon revolves around it, the moon is made to describe a sinuous path, as shown by the continuous line in fig. . this feature of the moon's path is greatly exaggerated in the upper portion of the diagram. the form of her path is given with a greater degree of accuracy in the lower part of the figure (the broken line represents the path of the earth); but even here there is considerable exaggeration. the complete serpentine path of the moon around the sun is shown, greatly exaggerated, in fig. , the broken line being the path of the earth. [illustration: fig. .] the path described by the moon through space is much the same as that described by a point on the circumference of a wheel which is rolled over another wheel. if we place a circular disk against the wall, and carefully roll along its edge another circular disk (to which a piece of lead pencil has been fastened so as to mark upon the wall), the curve described will somewhat resemble that described by the moon. this curve is called an _epicycloid_, and it will be seen that at every point it is concave towards the centre of the larger disk. in the same way the moon's orbit is _at every point concave towards the sun_. [illustration: fig. .] the exaggeration of the sinuosity in fig. will be more evident when it is stated, that, on the scale of fig. , the whole of the serpentine curve would lie _within the breadth_ of the fine circular line _mm'_. . _the lunar day._--the lunar day is twenty-nine times and a half as long as the terrestrial day. near the moon's equator the sun shines without intermission nearly fifteen of our days, and is absent for the same length of time. consequently, the vicissitudes of temperature to which the surface is exposed must be very great. during the long lunar night the temperature of a body on the moon's surface would probably fall lower than is ever known on the earth, while during the day it must rise higher than anywhere on our planet. [illustration: fig. .] it might seem, that, since the moon rotates on her axis in about twenty-seven days, the lunar day ought to be twenty-seven days long, instead of twenty-nine. there is, however, a solar, as well as a sidereal, day at the moon, as on the earth; and the solar day at the moon is longer than the sidereal day, for the same reason as on the earth. during the solar day the moon must make both a _synodical rotation_ and a _synodical revolution_. this will be evident from fig. , in which is shown the path of the moon during one complete lunation. _e_, _e'_, _e''_, etc., are the successive positions of the earth; and , , , , , the successive positions of the moon. the small arrows indicate the direction of the moon's rotation. the moon is full at and . at , _a_, at the centre of the moon's disk, will have the sun, which lies in the direction _as_, upon the meridian. before _a_ will again have the sun on the meridian, the moon must have made a synodical revolution; and, as will be seen by the dotted lines, she must have made more than a complete rotation. the rotation which brings the point _a_ into the same relation to the earth and sun is called a _synodical_ rotation. it will also be evident from this diagram that the moon must make a synodical rotation during a synodical revolution, in order always to present the same side to the earth. . _the earth as seen from the moon._--to an observer on the moon, the earth would be an immense moon, going through the same phases that the moon does to us; but, instead of rising and setting, it would only oscillate to and fro through a few degrees. on the other side of the moon it would never be seen at all. the peculiarities of the moon's motions which cause the librations, and make a spot on the moon's disk seem to an observer on the earth to oscillate to and fro, would cause the earth as a whole to appear to a lunar observer to oscillate to and fro in the heavens in a similar manner. it is a well-known fact, that, at the time of new moon, the dark part of the moon's surface is partially illumined, so that it becomes visible to the naked eye. this must be due to the light reflected to the moon from the earth. since at new moon the moon is between the earth and sun, it follows, that, when it is new moon at the earth, it must be _full earth_ at the moon: hence, while the bright crescent is enjoying full sunlight, the dark part of its surface is enjoying the light of the full _earth_. fig. represents the full earth as seen from the moon. [illustration: fig. .] the atmosphere of the moon. . _the moon has no appreciable atmosphere._--there are several reasons for believing that the moon has little or no atmosphere. ( ) had the moon an atmosphere, it would be indicated at the time of a solar eclipse, when the moon passes over the disk of the sun. if the atmosphere were of any considerable density, it would absorb a part of the sun's rays, so as to produce a dusky border in front of the moon's disk, as shown in fig. . in reality no such dusky border is ever seen; but the limb of the moon appears sharp, and clearly defined, as in fig. . [illustration: fig. .] [illustration: fig. .] if the atmosphere were not dense enough to produce this dusky border, its refraction would be sufficient to distort the delicate cusps of the sun's crescent in the manner shown at the top of fig. ; but no such distortion is ever observed. the cusps always appear clear and sharp, as shown at the bottom of the figure: hence it would seem that there can be no atmosphere of appreciable density at the moon. ( ) the absence of an atmosphere from the moon is also shown by the absence of twilight and of diffused daylight. upon the earth, twilight continues until the sun is eighteen degrees below the horizon; that is, day and night are separated by a belt twelve hundred miles in breadth, in which the transition from light to darkness is gradual. we have seen ( ) that this twilight results from the refraction and reflection of light by our atmosphere; and, if the moon had an atmosphere, we should notice a similar gradual transition from the bright to the dark portions of her surface. such, however, is not the case. the boundary between the light and darkness, though irregular, is sharply defined. close to this boundary the unillumined portion of the moon appears just as dark as at any distance from it. the shadows on the moon are also pitchy black, without a trace of diffused daylight. [illustration: fig. .] ( ) the absence of an atmosphere is also proved by the absence of refraction when the moon passes between us and the stars. let _ab_ (fig. ) represent the disk of the moon, and _cd_ an atmosphere supposed to surround it. let _sae_ represent a straight line from the earth, touching the moon at _a_, and let _s_ be a star situated in the direction of this line. if the moon had no atmosphere, this star would appear to touch the edge of the moon at _a_; but, if the moon had an atmosphere, a star behind the edge of the moon, at _s'_, would be visible at the earth; for the ray _s'a_ would be bent by the atmosphere into the direction _ae'_. so, also, on the opposite side of the moon, a star might be seen at the earth, although really behind the edge of the moon: hence, if the moon had an atmosphere, the time during which a star would be concealed by the moon would be less than if it had no atmosphere, and the amount of this effect must be proportional to the density of the atmosphere. the moon, in her orbital course across the heavens, is continually passing before, or _occulting_, some of the stars that so thickly stud her apparent path; and when we see a star thus pass behind the lunar disk on one side, and come out again on the other side, we are virtually observing the setting and rising of that star upon the moon. the moon's apparent diameter has been measured over and over again, and is known with great accuracy; the rate of her motion across the sky is also known with perfect accuracy: hence it is easy to calculate how long the moon will take to travel across a part of the sky exactly equal in length to her own diameter. supposing, then, that we observe a star pass behind the moon, and out again, it is clear, that, if there is no atmosphere, the interval of time during which it remains occulted ought to be exactly equal to the computed time which the moon would take to pass over the star. if, however, from the existence of a lunar atmosphere, the star disappears too late, and re-appears too soon, as we have seen it would, these two intervals will not agree; the computed time will be greater than the observed time, and the difference will represent the amount of refraction the star's light has sustained or suffered, and hence the extent of atmosphere it has had to pass through. comparisons of these two intervals of time have been repeatedly made, the most extensive being executed under the direction of the astronomer royal of england, several years ago, and based upon no less than two hundred and ninety-six occultation observations. in this determination the measured or telescopic diameter of the moon was compared with the diameter deduced from the occultations; and it was found that the telescopic diameter was greater than the occultation diameter by two seconds of angular measurement, or by about a thousandth part of the whole diameter of the moon. this discrepancy is probably due, in part at least, to _irradiation_ ( ), which augments the apparent size of the moon, as seen in the telescope as well as with the naked eye; but, if the whole two seconds were caused by atmospheric refraction, this would imply a horizontal refraction of one second, which is only one two-thousandth of the earth's horizontal refraction. it is possible that an atmosphere competent to produce this refraction would not make itself visible in any other way. but an atmosphere two thousand times rarer than our air can scarcely be regarded as an atmosphere at all. the contents of an air-pump receiver can seldom be rarefied to a greater extent than to about a thousandth of the density of air at the earth's surface; and the lunar atmosphere, if it exists at all, is thus proved to be twice as attenuated as what we commonly call a vacuum. the surface of the moon. [illustration: fig. .] . _dusky patches on the disk of the moon._--with the naked eye, large dusky patches are seen on the moon, in which popular fancy has detected a resemblance to a human face. with a telescope of low power, these dark patches appear as smooth as water, and they were once supposed to be seas. this theory was the origin of the name _mare_ (latin for _sea_), which is still applied to the larger of these plains; but, if there were water on the surface of the moon, it could not fail to manifest its presence by its vapor, which would form an appreciable atmosphere. moreover, with a high telescopic power, these plains present a more or less uneven surface; and, as the elevations and depressions are found to be permanent, they cannot, of course, belong to the surface of water. the chief of these plains are shown in fig. . they are _mare crisium_, _mare foecunditatis_, _mare nectaris_, _mare tranquillitatis_, _mare serenitatis_, _mare imbrium_, _mare frigoris_, and _oceanus procellarum_. all these plains can easily be recognized on the surface of the full moon with the unaided eye. . _the terminator of the moon._--the terminator of the moon is the line which separates the bright and dark portions of its disk. when viewed with a telescope of even moderate power, the terminator is seen to be very irregular and uneven. many bright points are seen just outside of the terminator in the dark portion of the disk, while all along in the neighborhood of the terminator are bright patches and dense shadows. these appearances are shown in figs. and , which represent the moon near the first and last quarters. they indicate that the surface of the moon is very rough and uneven. [illustration: fig. .] [illustration: fig. .] as it is always either sunrise or sunset along the terminator, the bright spots outside of it are clearly the tops of mountains, which catch the rays of the sun while their bases are in the shade. the bright patches in the neighborhood of the terminator are the sides of hills and mountains which are receiving the full light of the sun, while the dense shadows near by are cast by these elevations. . _height of the lunar mountains._--there are two methods of finding the height of lunar mountains:-- ( ) we may measure the length of the shadows, and then calculate the height of the mountains that would cast such shadows with the sun at the required height above the horizon. the length of a shadow may be obtained by the following method: the longitudinal wire of the micrometer ( ) is adjusted so as to pass through the shadow whose length is to be measured, and the transverse wires are placed one at each end of the shadow, as shown in fig. . the micrometer screw is then turned till the wires are brought together, so as to ascertain the length of the arc between them. we may then form the proportion: the number of seconds in the semi-diameter of the moon is to the number of seconds in the length of the shadow, as the length of the moon's radius in miles to the length of the shadow in miles. [illustration: fig. .] the height of the sun above the horizon is ascertained by measuring the angular distance of the mountain from the terminator. ( ) we may measure the distance of a bright point from the terminator, and then construct a right-angled triangle, as shown in fig. . a solution of this triangle will enable us to ascertain the height of the mountain whose top is just catching the level rays of the sun. [illustration: fig. .] _b_ is the centre of the moon, _m_ the top of the mountain, and _sam_ a ray of sunlight which just grazes the terminator at _a_, and then strikes the top of the mountain at _m_. the triangle _bam_ is right-angled at _a_. _ba_ is the radius of the moon, and _am_ is known by measurement; _bm_, the hypothenuse, may then be found by computation. _bm_ is evidently equal to the radius of the moon _plus_ the height of the mountain. by one or the other of these methods, the heights of the lunar mountains have been found with a great degree of accuracy. it is claimed that the heights of the lunar mountains are more accurately known than those of the mountains on the earth. compared with the size of the moon, lunar mountains attain a greater height than those on the earth. . _general aspect of the lunar surface._--a cursory examination of the moon with a low power is sufficient to show the prevalence of crater-like inequalities and the general tendency to _circular_ shape which is apparent in nearly all the surface markings; for even the large "seas" and the smaller patches of the same character repeat in their outlines the round form of the craters. it is along the terminator that we see these crater-like spots to the best advantage; as it is there that the rising or setting sun casts long shadows over the lunar landscape, and brings elevations into bold relief. they vary greatly in size; some being so large as to bear a sensible proportion to the moon's diameter, while the smallest are so minute as to need the most powerful telescopes and the finest conditions of atmosphere to perceive them. [illustration: fig. .] the prevalence of ring-shaped mountains and plains willbe evident from fig. , which is from a photograph of a model of the moon constructed by nasmyth. this same feature is nearly as marked in figs. and , which are copies of rutherfurd's photographs of the moon. . _lunar craters._--the smaller saucer-shaped formations on the surface of the moon are called _craters_. they are of all sizes, from a mile to a hundred and fifty miles in diameter; and they are supposed to be of volcanic origin. a high telescopic power shows that these craters vary remarkably, not only in size, but also in structure and arrangement. some are considerably elevated above the surrounding surface, others are basins hollowed out of that surface, and with low surrounding ramparts; some are like walled plains, while the majority have their lowest depression considerably below the surrounding surface; some are isolated upon the plains, others are thickly crowded together, overlapping and intruding upon each other; some have elevated peaks or cones in their centres, and some are without these central cones, while others, again, contain several minute craters instead; some have their ramparts whole and perfect, others have them broken or deformed, and many have them divided into terraces, especially on their inner sides. a typical lunar crater is shown in fig. . [illustration: fig. .] it is not generally believed that any active volcanoes exist on the moon at the present time, though some observers have thought they discerned indications of such volcanoes. [illustration: fig. .] . _copernicus._--this is one of the grandest of lunar craters (fig. ). although its diameter (forty-six miles) is exceeded by others, yet, taken as a whole, it forms one of the most impressive and interesting objects of its class. its situation, near the centre of the lunar disk, renders all its wonderful details conspicuous, as well as those of objects immediately surrounding it. its vast rampart rises to upwards of twelve thousand feet above the level of the plateau, nearly in the centre of which stands a magnificent group of cones, three of which attain a height of more than twenty-four hundred feet. many ridges, or spurs, may be observed leading away from the outer banks of the great rampart. around the crater, extending to a distance of more than a hundred miles on every side, there is a complex network of bright streaks, which diverge in all directions. these streaks do not appear in the figure, nor are they seen upon the moon, except at and near the full phase. they show conspicuously, however, by their united lustre on the full moon. this crater is seen just to the south-west of the large dusky plain in the upper part of fig. . this plain is _mare imbrium_, and the mountain-chain seen a little to the right of copernicus is named the _apennines_. copernicus is also seen in fig. , a little to the left of the same range. under circumstances specially favorable, myriads of comparatively minute but perfectly formed craters may be observed for more than seventy miles on all sides around copernicus. the district on the south-east side is specially rich in these thickly scattered craters, which we have reason to suppose stand over or upon the bright streaks. . _dark chasms._--dark cracks, or chasms, have been observed on various parts of the moon's surface. they sometimes occur singly, and sometimes in groups. they are often seen to radiate from some central cone, and they appear to be of volcanic origin. they have been called _canals_ and _rills_. [illustration: fig. .] one of the most remarkable groups of these chasms is that to the west of the crater named _triesneker_. the crater and the chasms are shown in fig. . several of these great cracks obviously diverge from a small crater near the west bank of the great one, and they subdivide as they extend from the apparent point of divergence, while they are crossed by others. these cracks, or chasms, are nearly a mile broad at the widest part, and, after extending full a hundred miles, taper away till they become invisible. [illustration: fig. .] . _mountain-ranges._--there are comparatively few mountain-ranges on the moon. the three most conspicuous are those which partially enclose mare imbrium; namely, the _apennines_ on the south, and the _caucasus_ and the _alps_ on the east and north-east. the apennines are the most extended of these, having a length of about four hundred and fifty miles. they rise gradually, from a comparatively level surface towards the south-west, in the form of innumerable small elevations, which increase in number and height towards the north-east, where they culminate in a range of peaks whose altitude and rugged aspect must form one of the most terribly grand and romantic scenes which imagination can conceive. the north-east face of the range terminates abruptly in an almost vertical precipice; while over the plain beneath, intensely black spire-like shadows are cast, some of which at sunrise extend full ninety miles, till they lose themselves in the general shading due to the curvature of the lunar surface. many of the peaks rise to heights of from eighteen thousand to twenty thousand feet above the plain at their north-east base (fig. ). [illustration: fig. .] fig. represents an ideal lunar landscape near the base of such a lunar range. owing to the absence of an atmosphere, the stars will be visible in full daylight. [illustration: fig. .] . _the valley of the alps._--the range of the _alps_ is shown in fig. . the great crater at the north end of this range is named _plato_. it is seventy miles in diameter. the most remarkable feature of the alps is the valley near the centre of the range. it is more than seventy-five miles long, and about six miles wide at the broadest part. when examined under favorable circumstances, with a high magnifying power, it is seen to be a vast flat-bottomed valley, bordered by gigantic mountains, some of which attain heights of ten thousand feet or more. [illustration: fig. .] . _isolated peaks._--there are comparatively few isolated peaks to be found on the surface of the moon. one of the most remarkable of these is that known as _pico_, and shown in fig. . its height exceeds eight thousand feet, and it is about three times as long at the base as it is broad. the summit is cleft into three peaks, as is shown by the three-peaked shadow it casts on the plain. . _bright rays._--about the time of full moon, with a telescope of moderate power, a number of bright lines may be seen radiating from several of the lunar craters, extending often to the distance of hundreds of miles. these streaks do not arise from any perceptible difference of level of the surface, they have no very definite outline, and they do not present any sloping sides to catch more sunlight, and thus shine brighter, than the general surface. indeed, one great peculiarity of them is, that they come out most forcibly when the sun is shining perpendicularly upon them: hence they are best seen when the moon is at full, and they are not visible at all at those regions upon which the sun is rising or setting. they are not diverted by elevations in their path, but traverse in their course craters, mountains, and plains alike, giving a slight additional brightness to all objects over which they pass, but producing no other effect upon them. "they look as if, after the whole surface of the moon had assumed its final configuration, a vast brush charged with a whitish pigment had been drawn over the globe in straight lines, radiating from a central point, leaving its trail upon every thing it touched, but obscuring nothing." [illustration: fig. .] the three most conspicuous craters from which these lines radiate are _tycho_, _copernicus_, and _kepler_. tycho is seen at the bottom of figs. and . kepler is a little to the left of copernicus in the same figures. it has been thought that these bright streaks are chasms which have been filled with molten lava, which, on cooling, would afford a smooth reflecting surface on the top. . _tycho._--this crater is fifty-four miles in diameter, and about sixteen thousand feet deep, from the highest ridge of the rampart to the surface of the plateau, whence rises a central cone five thousand feet high. it is one of the most conspicuous of all the lunar craters; not so much on account of its dimensions as from its being the centre from whence diverge those remarkable bright streaks, many of which may be traced over a thousand miles of the moon's surface (fig. ). tycho appears to be an instance of a vast disruptive action which rent the solid crust of the moon into radiating fissures, which were subsequently filled with molten matter, whose superior luminosity marks the course of the cracks in all directions from the crater as their common centre. so numerous are these bright streaks when examined by the aid of the telescope, and they give to this region of the moon's surface such increased luminosity, that, when viewed as a whole, the locality can be distinctly seen at full moon by the unassisted eye, as a bright patch of light on the southern portion of the disk. iii. inferior and superior planets. inferior planets. . _the inferior planets._--the _inferior planets_ are those which lie between the earth and the sun, and whose orbits are included by that of the earth. they are _mercury_ and _venus_. [illustration: fig. .] . _aspects of an inferior planet._--the four chief _aspects_ of an inferior planet as seen from the earth are shown in fig. , in which _s_ represents the sun, _p_ the planet, and _e_ the earth. when the planet is between the earth and the sun, as at _p_, it is said to be in _inferior conjunction_. when it is in the same direction as the sun, but beyond it, as at _p''_, it is said to be in _superior conjunction_. when the planet is at such a point in its orbit that a line drawn from the earth to it would be tangent to the orbit, as at _p'_ and _p'''_, it is said to be at its _greatest elongation_. [illustration: fig. .] . _apparent motion of an inferior planet._--when the planet is at _p_, if it could be seen at all, it would appear in the heavens at _a_. as it moves from _p_ to _p'_, it will appear to move in the heavens from _a_ to _b_. then, as it moves from _p'_ to _p''_, it will appear to move back again from _b_ to _a_. while it moves from _p''_ to _p'''_, it will appear to move from _a_ to _c_; and, while moving from _p'''_ to _p_, it will appear to move back again from _c_ to _a_. thus the planet will appear to oscillate to and fro across the sun from _b_ to _c_, never getting farther from the sun than _b_ on the west, or _c_ on the east: hence, when at these points, it is said to be at its _greatest western_ and _eastern elongations_. this oscillating motion of an inferior planet across the sun, combined with the sun's motion among the stars, causes the planet to describe a path among the stars similar to that shown in fig. . [illustration: fig. .] . _phases of an inferior planet._--an inferior planet, when viewed with a telescope, is found to present a succession of phases similar to those of the moon. the reason of this is evident from fig. . as an inferior planet passes around the sun, it presents sometimes more and sometimes less of its bright hemisphere to the earth. when the earth is at _t_, and venus at superior conjunction, the planet turns the whole of its bright hemisphere towards the earth, and appears _full_; it then becomes _gibbous_, _half_, and _crescent_. when it comes into _inferior conjunction_, it turns its dark hemisphere towards the earth: it then becomes _crescent_, _half_, _gibbous_, and _full_ again. . _the sidereal and synodical periods of an inferior planet._--the time it takes a planet to make a complete revolution around the sun is called the _sidereal period_ of the planet; and the time it takes it to pass from one aspect around to the same aspect again, its _synodical period_. [illustration: fig. .] the synodical period of an inferior planet is longer than its sidereal period. this will be evident from an examination of fig. . _s_ is the position of the sun, _e_ that of the earth, and _p_ that of the planet at inferior conjunction. before the planet can be in inferior conjunction again, it must pass entirely around its orbit, and overtake the earth, which has in the mean time passed on in its orbit to _e'_. while the earth is passing from _e_ to _e'_, the planet passes entirely around its orbit, and from _p_ to _p'_ in addition. now the arc _pp'_ is just equal to the arc _ee'_: hence the planet has to pass over the same arc that the earth does, and ° more. in other words, the planet has to gain ° on the earth. the synodical period of the planet is found by direct observation. . _the length of the sidereal period._--the length of the sidereal period of an inferior planet may be found by the following computation:-- let _a_ denote the synodical period of the planet, let _b_ denote the sidereal period of the earth, let _x_ denote the sidereal period of the planet. then _ °/b_ = the daily motion of the earth, and _ °/x_ = the daily motion of the planet, and _ °/x - °/b_ = the daily gain of the planet: also _ °/a_ = the daily gain of the planet: hence _ °/x - °/b = °/a_. dividing by °, we have _ /x - /b = /a_; clearing of fractions, we have _ab - ax = bx_: transposing and collecting, we have _(a + b)x = ab_: therefore _x = ab/a+b_. . _the relative distance of an inferior planet._--by the _relative distance_ of a planet, we mean its distance from the sun compared with the earth's distance from the sun. the relative distance of an inferior planet may be found by the following method:-- [illustration: fig. .] let _v_, in fig. , represent the position of venus at its greatest elongation from the sun, _s_ the position of the sun, and _e_ that of the earth. the line _ev_ will evidently be tangent to a circle described about the sun with a radius equal to the distance of venus from the sun at the time of this greatest elongation. draw the radius _sv_ and the line _se_. since _sv_ is a radius, the angle at _v_ is a right angle. the angle at _e_ is known by measurement, and the angle at _s_ is equal to °- the angle _e_. in the right-angled triangle _evs_, we then know the three angles, and we wish to find the ratio of the side _sv_ to the side _se_. the ratio of these lines may be found by trigonometrical computation as follows:-- _vs : es = sin sev : ._ substitute the value of the sine of sev, and we have _vs : es = . : ._ hence the relative distances of venus and of the earth from the sun are . and . superior planets. . _the superior planets._--the _superior planets_ are those which lie beyond the earth. they are _mars_, the _asteroids_, _jupiter_, _saturn_, _uranus_, and _neptune_. [illustration: fig. .] . _apparent motion of a superior planet._--in order to deduce the apparent motion of a superior planet from the real motions of the earth and planet, let _s_ (fig. ) be the place of the sun; , , , etc., the orbit of the earth; _a_, _b_, _c_, etc., the orbit of mars; and _cgl_ a part of the starry firmament. let the orbit of the earth be divided into twelve equal parts, each described in one month; and let _ab_, _bc_, _cd_, etc., be the spaces described by mars in the same time. suppose the earth to be at the point when mars is at the point _a_, mars will then appear in the heavens in the direction of _a_. when the earth is at , and mars at _c_, he will appear in the heavens at _c_. when the earth arrives at , mars will arrive at _d_, and will appear in the heavens at _d_. while the earth moves from to and from to , mars will appear to have advanced among the stars from _d_ to _e_ and from _e_ to _f_, in the direction from west to east. during the motion of the earth from to and from to , mars will appear to go backward from _f_ to _g_ and from _g_ to _h_, in the direction from east to west. during the motion of the earth from to and from to , mars will appear to advance from _h_ to _i_ and from _i_ to _k_, in the direction from west to east, and the motion will continue in the same direction until near the succeeding opposition. the apparent motion of a superior planet projected on the heavens is thus seen to be similar to that of an inferior planet, except that, in the latter case, the retrogression takes place near inferior conjunction, and in the former it takes place near opposition. [illustration: fig. .] . _aspects of a superior planet._--the four aspects of a superior planet are shown in fig. , in which _s_ is the position of the sun, _e_ that of the earth, and _p_ that of the planet. when the planet is on the opposite side of the earth to the sun, as at _p_, it is said to be in _opposition_. the sun and the planet will then appear in opposite parts of the heavens, the sun appearing at _c_, and the planet at _a_. when the planet is on the opposite side of the sun to the earth, as at _p''_, it is said to be in _superior conjunction_. it will then appear in the same part of the heavens as the sun, both appearing at _c_. when the planet is at _p'_ and _p'''_, so that a line drawn from the earth through the planet will make a right angle with a line drawn from the earth to the sun, it is said to be in _quadrature_. at _p'_ it is in its western quadrature, and at _p'''_ in its eastern quadrature. [illustration: fig. .] . _phases of a superior planet._--mars is the only one of the superior planets that has appreciable phases. at quadrature, as will appear from fig. , mars does not present quite the same side to the earth as to the sun: hence, near these parts of its orbit, the planet appears slightly gibbous. elsewhere in its orbit, the planet appears full. all the other superior planets are so far away from the sun and earth, that the sides which they turn towards the sun and the earth in every part of their orbit are so nearly the same, that no change in the form of their disks can be detected. . _the synodical period of a superior planet._--during a synodical period of a superior planet the earth must gain one revolution, or °, on the planet, as will be evident from an examination of fig. , in which _s_ represents the sun, _e_ the earth, and _p_ the planet at opposition. before the planet can be in opposition again, the earth must make a complete revolution, and overtake the planet, which has in the mean time passed on from _p_ to _p'_. [illustration: fig. .] in the case of most of the superior planets the synodical period is shorter than the sidereal period; but in the case of mars it is longer, since mars makes more than a complete revolution before the earth overtakes it. the synodical period of a superior planet is found by direct observation. . _the sidereal period of a superior planet._--the sidereal period of a superior planet is found by a method of computation similar to that for finding the sidereal period of an inferior planet:-- let _a_ denote the synodical period of the planet, let _b_ denote the sidereal period of the earth, let _x_ denote the sidereal period of the planet. then will _ °/b_ = daily motion of the earth, and _ °/x_ = daily motion of the planet; also _ °/b - °/x_ = daily gain of the earth. but _ °/a_ = daily gain of the earth: hence _ °/b - °/x = °/a_ _ /b - /x = /a_ _ax - ab = bx_ _(a-b)x = ab_ _x = ab/(a-b)_. [illustration: fig. .] . _the relative distance of a superior planet._--let _s_, _e_, and _m_, in fig. , represent the relative positions of the sun, the earth, and mars, when the latter planet is in opposition. let _e_ and _m_ represent the relative positions of the earth and mars the day after opposition. at the first observation mars will be seen in the direction _ema_, and at the second observation in the direction _ema_. but the fixed stars are so distant, that if a line, _ea_, were drawn to a fixed star at the first observation, and a line, _eb_, drawn from the earth to the same fixed star at the second observation, these two lines would be sensibly parallel; that is, the fixed star would be seen in the direction of the line _ea_ at the first observation, and in the direction of the line _eb_, parallel to _ea_, at the second observation. but if mars were seen in the direction of the fixed star at the first observation, it would appear back, or west, of that star at the second observation by the angular distance _bea_; that is, the planet would have retrograded that angular distance. now, this retrogression of mars during one day, at the time of opposition, can be measured directly by observation. this measurement gives us the value of the angle _bea_; but we know the rate at which both the earth and mars are moving in their orbits, and from this we can easily find the angular distance passed over by each in one day. this gives us the angles _esa_ and _msa_. we can now find the relative length of the lines _ms_ and _es_ (which represent the distances of mars and of the earth from the sun), both by construction and by trigonometrical computation. since _eb_ and _ea_ are parallel, the angle _eas_ is equal to _bea_. _sea = ° - (esa + eas)_ _esm = esa - msa_ _ems = ° - (sea + esm)_. we have then _ms : es = sin sea : sin ems._ substituting the values of the sines, and reducing the ratio to its lowest terms, we have _ms : es = . : ._ thus we find that the relative distances of mars and the earth from the sun are . and . by the simple observation of its greatest elongation, we are able to determine the relative distances of an inferior planet and the earth from the sun; and, by the equally simple observation of the daily retrogression of a superior planet, we can find the relative distances of such a planet and the earth from the sun. iv. the sun. i. magnitude and distance of the sun. [illustration: fig. .] . _the volume of the sun._--the apparent diameter of the sun is about ', being a little greater than that of the moon. the real diameter of the sun is , miles, or about a hundred and nine times that of the earth. as the diameter of the moon's orbit is only about , miles, or some sixty times the diameter of the earth, it follows that the diameter of the sun is nearly double that of the moon's orbit: hence, were the centre of the sun placed at the centre of the earth, the sun would completely fill the moon's orbit, and reach nearly as far beyond it in every direction as it is from the earth to the moon. the circumference of the sun as compared with the moon's orbit is shown in fig. . the volume of the sun is , , times that of the earth. . _the mass of the sun._--the sun is much less dense than the earth. the mass of the sun is only , times that of the earth, and its density only about a fourth that of the earth. to find the mass of the sun, we first ascertain the distance the earth would draw the moon towards itself in a given time, were the moon at the distance of the sun, and then form the proportion: as the distance the earth would draw the moon towards itself is to the distance that the sun draws the earth towards itself in the same time, so is the mass of the earth to the mass of the sun. although the mass of the sun is over three hundred thousand times that of the earth, the pull of gravity at the surface of the sun is only about twenty-eight times as great as at the surface of the earth. this is because the distance from the surface of the sun to its centre is much greater than from the surface to the centre of the earth. [illustration: fig. .] . _size of the sun compared with that of the planets._--the size of the sun compared with that of the larger planets is shown in fig. . the mass of the sun is more than seven hundred and fifty times that of all of the planets and moons in the solar system. in fig. is shown the apparent size of the sun as seen from the different planets. the apparent diameter of the sun decreases as the distance from it increases, and the disk of the sun decreases as the square of the distance from it increases. [illustration: fig. .] . _the distance of the sun._--the mean distance of the sun from the earth is about , , miles. owing to the eccentricity of the earth's orbit, the distance of the sun varies somewhat; being about , , miles less in january, when the earth is at perihelion, than in june, when the earth is at aphelion. "but, though the distance of the sun can easily be stated in figures, it is not possible to give any real idea of a space so enormous: it is quite beyond our power of conception. if one were to try to walk such a distance, supposing that he could walk four miles an hour, and keep it up for ten hours every day, it would take sixty-eight years and a half to make a single million of miles, and more than sixty-three hundred years to traverse the whole. "if some celestial railway could be imagined, the journey to the sun, even if our trains ran sixty miles an hour day and night and without a stop, would require over a hundred and seventy-five years. sensation, even, would not travel so far in a human lifetime. to borrow the curious illustration of professor mendenhall, if we could imagine an infant with an arm long enough to enable him to touch the sun and burn himself, he would die of old age before the pain could reach him; since, according to the experiments of helmholtz and others, a nervous shock is communicated only at the rate of about a hundred feet per second, or , miles a day, and would need more than a hundred and fifty years to make the journey. sound would do it in about fourteen years, if it could be transmitted through celestial space; and a cannon-ball in about nine, if it were to move uniformly with the same speed as when it left the muzzle of the gun. if the earth could be suddenly stopped in her orbit, and allowed to fall unobstructed toward the sun, under the accelerating influence of his attraction, she would reach the centre in about four months. i have said if she could be stopped; but such is the compass of her orbit, that, to make its circuit in a year, she has to move nearly nineteen miles a second, or more than fifty times faster than the swiftest rifle-ball; and, in moving twenty miles, her path deviates from perfect straightness by less than an eighth of an inch. and yet, over all the circumference of this tremendous orbit, the sun exercises his dominion, and every pulsation of his surface receives its response from the subject earth." (professor c. a. young: the sun.) . _method of finding the sun's distance._--there are several methods of finding the sun's distance. the simplest method is that of finding the actual distance of one of the nearer planets by observing its displacement in the sky as seen from widely separated points on the earth. as the _relative_ distances of the planets from each other and from the sun are well known, we can easily deduce the actual distance of the sun if we can find that of any of the planets. the two planets usually chosen for this method are mars and venus. ( ) the displacement of mars in the sky, as seen from two observatories which differ considerably in latitude, is, of course, greatest when mars is nearest the earth. now, it is evident than mars will be nearer the earth when in opposition than when in any other part of its orbit; and the planet will be least distant from the earth when it is at its perihelion point, and the earth is at its aphelion point, at the time of opposition. this method, then, can be used to the best advantage, when, at the time of opposition, mars is near its perihelion, and the earth near its aphelion. these favorable oppositions occur about once in fifteen years, and the last one was in . [illustration: fig. .] suppose two observers situated at _n'_ and _s'_ (fig. ), near the poles of the earth. the one at _n'_ would see mars in the sky at _n_, and the one at _s'_ would see it at _s_. the displacement would be the angle _nms_. each observer measures carefully the distance of mars from the same fixed star near it. the difference of these distances gives the displacement of the planet, or the angle _nms_. these observations were made with the greatest care in . ( ) venus is nearest the earth at the time of inferior conjunction; but it can then be seen only in the daytime. it is, therefore, impossible to ascertain the displacement of venus, as seen from different stations, by comparing her distances from a fixed star. occasionally, at the time of inferior conjunction, venus passes directly across the sun's disk. the last of these _transits_ of venus occurred in , and the next will occur in . it will then be over a hundred years before another will occur. [illustration: fig. .] suppose two observers, _a_ and _b_ (fig. ), near the poles of the earth at the time of a transit of venus. the observer at _a_ would see venus crossing the sun at _v_{ }_, and the one at _b_ would see it crossing at _v_{ }_. any observation made upon venus, which would give the distance and direction of venus from the centre of the sun, as seen from each station, would enable us to calculate the angular distance between the two chords described across the sun. this, of course, would give the displacement of venus on the sun's disk. this method was first employed at the last transits of venus which occurred before ; namely, those of and . there are three methods of observation employed to ascertain the apparent direction and distance of venus from the centre of the sun, called respectively the _contact method_, the _micrometric method_, and the _photographic method_. (_a_) in the _contact_ method, the observation consists in noting the exact time when venus crosses the sun's limb. to ascertain this it is necessary to observe the exact time of external and internal contact. this observation, though apparently simple, is really very difficult. with reference to this method professor young says,-- "the difficulties depend in part upon the imperfections of optical instruments and the human eye, partly upon the essential nature of light leading to what is known as diffraction, and partly upon the action of the planet's atmosphere. the two first-named causes produce what is called irradiation, and operate to make the apparent diameter of the planet, as seen on the solar disk, smaller than it really is; smaller, too, by an amount which varies with the size of the telescope, the perfection of its lenses, and the tint and brightness of the sun's image. the edge of the planet's image is also rendered slightly hazy and indistinct. [illustration: fig. .] "the planet's atmosphere also causes its disk to be surrounded by a narrow ring of light, which becomes visible long before the planet touches the sun, and, at the moment of internal contact, produces an appearance, of which the accompanying figure is intended to give an idea, though on an exaggerated scale. the planet moves so slowly as to occupy more than twenty minutes in crossing the sun's limb; so that even if the planet's edge were perfectly sharp and definite, and the sun's limb undistorted, it would be very difficult to determine the precise second at which contact occurs. but, as things are, observers with precisely similar telescopes, and side by side, often differ from each other five or six seconds; and, where the telescopes are not similar, the differences and uncertainties are much greater.... astronomers, therefore, at present are pretty much agreed that such observations can be of little value in removing the remaining uncertainty of the parallax, and are disposed to put more reliance upon the micrometric and photographic methods, which are free from these peculiar difficulties, though, of course, beset with others, which, however, it is hoped will prove less formidable." (_b_) of the _micrometric_ method, as employed at the last transit, professor young speaks as follows:-- "the micrometric method requires the use of a heliometer,--an instrument common only in germany, and requiring much skill and practice in its use in order to obtain with it accurate measures. at the late transit, a single english party, two or three of the russian parties, and all five of the german, were equipped with these instruments; and at some of the stations extensive series of measures were made. none of the results, however, have appeared as yet; so that it is impossible to say how greatly, if at all, this method will have the advantage in precision over the contact observations." (_c_) the following observations, with reference to the _photographic_ method, are also taken from professor young:-- "the americans and french placed their main reliance upon the photographic method, while the english and germans also provided for its use to a certain extent. the great advantage of this method is, that it makes it possible to perform the necessary measurements (upon whose accuracy every thing depends) at leisure after the transit, without hurry, and with all possible precautions. the field-work consists merely in obtaining as many and as good pictures as possible. a principal objection to the method lies in the difficulty of obtaining good pictures, i.e., pictures free from distortion, and so distinct and sharp as to bear high magnifying power in the microscopic apparatus used for their measurement. the most serious difficulty, however, is involved in the accurate determination of the scale of the picture; that is, of the number of seconds of arc corresponding to a linear inch upon the plate. besides this, we must know the exact greenwich time at which each picture is taken, and it is also extremely desirable that the _orientation_ of the picture should be accurately determined; that is, the north and south, the east and west points of the solar image on the finished plate. there has been a good deal of anxiety lest the image, however accurate and sharp when first produced, should alter, in course of time, through the contraction of the collodion film on the glass plate; but the experiments of rutherfurd, huggins, and paschen, seem to show that this danger is imaginary.... the americans placed the photographic telescope exactly in line with a meridian instrument, and so determined, with the extremest precision, the direction in which it was pointed. knowing this and the time at which any picture was taken, it becomes possible, with the help of the plumb-line image, to determine precisely the orientation of the picture,--an advantage possessed by the american pictures alone, and making their value nearly twice as great as otherwise it would have been. "the figure below is a representation of one of the american photographs reduced about one-half. _v_ is the image of venus, which, on the actual plate, is about a seventh of an inch in diameter; _aa'_ is the image of the plumb-line. the centre of the reticle is marked with a cross." [illustration: fig. .] the english photographs proved to be of little value, and the results of the measurements and calculations upon the american pictures have not yet been published. there is a growing apprehension that no photographic method can be relied upon. the most recent determinations by various methods indicate that the sun's distance is such that his parallax is about eighty-eight seconds. this would make the linear value of a second at the surface of the sun about four hundred and fifty miles. [illustration: plate .] ii. physical and chemical condition of the sun. physical condition of the sun. . _the sun composed mainly of gas._--it is now generally believed that the sun is mainly a ball of gas, or vapor, powerfully condensed at the centre by the weight of the superincumbent mass, but kept from liquefying by its exceedingly high temperature. the gaseous interior of the sun is surrounded by a layer of luminous clouds, which constitutes its visible surface, and which is called its _photosphere_. here and there in the photosphere are seen dark _spots_, which often attain an immense magnitude. these clouds float in the _solar atmosphere_, which extends some distance beyond them. the luminous surface of the sun is surrounded by a _rose-colored_ stratum of gaseous matter, called the _chromosphere_. here and there great masses of this chromospheric matter rise high above the general level. these masses are called _prominences_. outside of the chromosphere is the _corona_, an irregular halo of faint, pearly light, mainly composed of filaments and streamers, which radiate from the sun to enormous distances, often more than a million of miles. in fig. is shown a section of the sun, according to professor young. the accompanying lithographic plate gives a general view of the photosphere with its spots, and of the chromosphere and its prominences. . _the temperature of the sun._--those who have investigated the subject of the temperature of the sun have come to very different conclusions; some placing it as high as four million degrees fahrenheit, and others as low as ten thousand degrees. professor young thinks that rosetti's estimate of eighteen thousand degrees as the _effective temperature_ of the sun's surface is probably not far from correct. by this is meant the temperature that a uniform surface of lampblack of the size of the sun must have in order to radiate as much heat as the sun does. the most intense artificial heat does not exceed four thousand degrees fahrenheit. [illustration: fig. .] . _the amount of heat radiated by the sun._--a unit of heat is the amount of heat required to raise a pound of water one degree in temperature. it takes about a hundred and forty-three units of heat to melt a pound of ice without changing its temperature. a cubic foot of ice weighs about fifty-seven pounds. according to sir william herschel, were all the heat radiated by the sun concentrated on a cylinder of ice forty-five miles in diameter, it would melt it off at the rate of about a hundred and ninety thousand miles a second. professor young gives the following illustration of the energy of solar radiation: "if we could build up a solid column of ice from the earth to the sun, two miles and a quarter in diameter, spanning the inconceivable abyss of ninety-three million miles, and if then the sun should concentrate his power upon it, it would dissolve and melt, not in an hour, nor a minute, but in a single second. one swing of the pendulum, and it would be water; seven more, and it would be dissipated in vapor." [illustration: fig. .] this heat would be sufficient to melt a layer of ice nearly fifty feet thick all around the sun in a minute. to develop this heat would require the hourly consumption of a layer of anthracite coal, more than sixteen feet thick, over the entire surface of the sun; and the _mechanical equivalent_ of this heat is about ten thousand horse-power on every square foot of the sun's surface. . _the brightness of the sun's surface._--the sun's surface is a hundred and ninety thousand times as bright as a candle-flame, a hundred and forty-six times as bright as the calcium-light, and about three times and a half as bright as the voltaic arc. the sun's disk is much less bright near the margin than near the centre, a point on the limb of the sun being only about a fourth as bright as one near the centre of the disk. this diminution of brightness towards the margin of the disk is due to the increase in the absorption of the solar atmosphere as we pass from the centre towards the margin of the sun's disk; and this increased absorption is due to the fact, that the rays which reach us from near the margin have to traverse a much greater thickness of the solar atmosphere than those which reach us from the centre of the disk. this will be evident from fig. , in which the arrows mark the paths of rays from different parts of the solar disk. the spectroscope. [illustration: fig. .] . _the spectroscope as an astronomical instrument._--the _spectroscope_ is now continually employed in the study of the physical condition and chemical constitution of the sun and of the other heavenly bodies. it has become almost as indispensable to the astronomer as the telescope. . _the dispersion spectroscope._--the essential parts of the _dispersion_ spectroscope are shown in fig. . these are the _collimator tube_, the _prism_, and the _telescope_. the collimator tube has a narrow slit at one end, through which the light to be examined is admitted, and somewhere within the tube a lens for condensing the light. the light is dispersed on passing through the prism: it then passes through the objective of the telescope, and forms within the tube an image of the spectrum, which is examined by means of the eye-piece. the power of the spectroscope is increased by increasing the number of prisms, which are arranged so that the light shall pass through one after another in succession. such an arrangement of prisms is shown in fig. . one end of the collimator tube is seen at the left, and one end of the telescope at the right. sometimes the prisms are made long, and the light is sent twice through the same train of prisms, once through the lower, and once through the upper, half of the prisms. this is accomplished by placing a rectangular prism against the last prism of the train, as shown in fig. . [illustration: fig. .] [illustration: fig. .] . _the micrometer scale._--various devices are employed to obtain an image of a micrometer scale in the tube of the telescope beside that of the spectrum. [illustration: fig. .] one of the simplest of these methods is shown in fig. . _a_ is the telescope, _b_ the collimator, and _c_ the micrometer tube. the opening at the outer end of _c_ contains a piece of glass which has a micrometer scale marked upon it. the light from the candle shines through this glass, falls upon the surface of the prism _p_, and is thence reflected into the telescope, where it forms an enlarged image of the micrometer scale alongside the image of the spectrum. [illustration: fig. .] . _the comparison of spectra._--in order to compare two spectra, it is desirable to be able to see them side by side in the telescope. the images of two spectra may be obtained side by side in the telescope tube by the use of a little rectangular prism, which covers one-half of the slit of the collimator tube, as shown in fig. . the light from one source is admitted directly through the uncovered half of the slit, while the light from the other source is sent through the covered portion of the slit by reflection from the surface of the rectangular prism. this arrangement and its action will be readily understood from fig. . [illustration: fig. .] . _direct-vision spectroscope._--a beam of light may be dispersed, without any ultimate deflection from its course, by combining prisms of crown and flint glass with equal refractive, but unequal dispersive powers. such a combination of prisms is called a _direct-vision_ combination. one of three prisms is shown in fig. , and one of five prisms in fig. . [illustration: fig. .] [illustration: fig. .] a _direct-vision spectroscope_ (fig. ) is one in which a direct-vision combination of prisms is employed. _c_ is the collimator tube, _p_ the train of prisms, _f_ the telescope, and _r_ the comparison prism. [illustration: fig. .] . _the telespectroscope._--the spectroscope, when used for astronomical work, is usually combined with a telescope. the compound instrument is called a _telespectroscope_. the spectroscope is mounted at the end of the telescope in such a way that the image formed by the object-glass of the telescope falls upon the slit at the end of the collimator tube. a telespectroscope of small dispersive power is shown in fig. ; _a_ being the object-glass of the telescope, _cc_ the tube of the telescope, and _e_ the comparison prism at the end of the collimator tube. a more powerful instrument is shown in fig. . _a_ is the telescope, _c_ the collimator tube of the spectroscope, _p_ the train of prisms, and _e_ the telescope tube. fig. shows a still more powerful spectroscope attached to the great newall refractor ( ). [illustration: fig. .] [illustration: fig. .] . _the diffraction spectroscope._--a _diffraction_ spectroscope is one in which the spectrum is produced by reflection of the light from a finely ruled surface, or _grating_, as it is called, instead of by dispersion in passing through a prism. the essential parts of this instrument are shown in fig . this spectroscope may be attached to the telescope in the same manner as the dispersion spectroscope. when the spectroscope is thus used, the eye-piece of the telescope is removed. [illustration: fig. .] spectra. . _continuous spectra._--light from an incandescent solid or liquid which has suffered no absorption in the medium which it has traversed gives a spectrum consisting of a continuous colored band, in which the colors, from the red to the violet, pass gradually and imperceptibly into one another. the spectrum is entirely free from either light or dark lines, and is called a _continuous spectrum_. . _bright-lined spectra._--light from a luminous gas or vapor gives a spectrum composed of bright lines separated by dark spaces, and known as a _bright-lined spectrum_. it has been found that the lines in the spectrum of a substance in the state of a gas or vapor are the most characteristic thing about the substance, since no two vapors give exactly the same lines: hence, when we have once become acquainted with the bright-lined spectrum of any substance, we can ever after recognize that substance by the spectrum of its luminous vapor. even when several substances are mixed, they may all be recognized by the bright-lined spectrum of the mixture, since the lines of all the substances will be present in the spectrum of the mixture. this method of identifying substances by their spectra is called _spectrum analysis_. the bright-lined spectra of several substances are given in the frontispiece. the number of lines in the spectra of the elements varies greatly. the spectrum of sodium is one of the simplest, while that of iron is one of the most complex. the latter contains over six hundred lines. though no two vapors give identical spectra, there are many cases in which one or more of the spectral lines of one element coincide in position with lines of other elements. . _methods of rendering gases and vapors luminous._--in order to study the spectra of vapors and gases it is necessary to have some means of converting solids and liquids into vapor, and also of rendering the vapors and gases luminous. there are four methods of obtaining luminous vapors and gases in common use. [illustration: fig. .] ( ) _by means of the bunsen flame._--this is a very hot but an almost non-luminous flame. if any readily volatilized substance, such as the compounds of sodium, calcium, strontium, etc., is introduced into this flame on a fine platinum wire, it is volatilized in the flame, and its vapor is rendered luminous, giving the flame its own peculiar color. the flame thus colored may be examined by the spectroscope. the arrangement of the flame is shown in fig. . [illustration: fig. .] ( ) _by means of the voltaic arc._--an electric lamp is shown in fig. . when this lamp is to be used for obtaining luminous vapors, the lower carbon is made larger than the upper one, and hollowed out at the top into a little cup. the substance to be volatilized is placed in this cup, and the current is allowed to pass. the heat of the voltaic arc is much more intense than that of the bunsen flame: hence substances that cannot be volatilized in the flame are readily volatilized in the arc, and the vapor formed is raised to a very high temperature. ( ) _by means of the spark from an induction coil._--the arrangement of the coil for obtaining luminous vapors is shown in fig. . [illustration: fig. .] the terminals of the coil between which the spark is to pass are brought quite close together. when we wish to vaporize any metal, as iron, the terminals are made of iron. on the passage of the spark, a little of the iron at the ends of the terminals is evaporated; and the vapor is rendered luminous in the space traversed by the spark. a condenser is usually placed in the circuit. with the coil, the temperature may be varied at pleasure; and the vapor may be raised even to a higher temperature than with the electric lamp. to obtain a low temperature, the coil is used without the condenser. by using a larger and larger condenser, the temperature may be raised higher and higher. by means of the induction coil we may also heat gases to incandescence. it is only necessary to allow the spark to pass through a space filled with the gas. [illustration: fig. .] ( ) _by means of a vacuum tube._--the form of the vacuum tube commonly used for this purpose is shown in fig. . the gas to be examined, and which is contained in the tube, has very slight density: but upon the passage of the discharge from an induction coil or a holtz machine, through the tube, the gas in the capillary part of the tube becomes heated to a high temperature, and is then quite brilliant. . _reversed spectra._--if the light from an incandescent cylinder of lime, or from the incandescent point of an electric lamp, is allowed to pass through luminous sodium vapor, and is then examined with a spectroscope, the spectrum will be found to be a bright spectrum crossed by a single _dark_ line in the position of the yellow line of the sodium vapor. the spectrum of sodium vapor is _reversed_, its bright lines becoming dark and its dark spaces bright. with a spectroscope of any considerable power, the yellow line of sodium vapor is resolved into a double line. with a spectroscope of the same power, the dark sodium line of the reversed spectrum is seen to be a double line. it is found to be generally true, that the spectrum of the light from an incandescent solid or liquid which has passed _through a luminous vapor_ on its way to the spectroscope is made up of a bright ground crossed by dark lines; there being a dark line for every bright line that the vapor alone would give. . _explanation of reversed spectra._--it has been found that gases absorb and quench rays of the same degree of refrangibility as those which they themselves emit, and no others. when a solid is shining through a luminous vapor, this absorbs and quenches those rays from the solid which have the same degrees of refrangibility as those which it is itself emitting: hence the lines of the spectrum receive light from the vapor alone, while the spaces between the lines receive light from the solid. now, solids and liquids, when heated to incandescence, give a very much brighter light than vapors and gases at the same temperature: hence the lines of a reversed spectrum, though receiving light from the vapor or gas, appear dark by contrast. . _effect of increasing the power of the spectroscope upon the brilliancy of a spectrum._--an increase in the power of a spectroscope diminishes the brilliancy of a _continuous_ spectrum, since it makes the colored band longer, and therefore spreads the light out over a greater extent of surface; but, in the case of a _bright-lined_ spectrum, an increase of power in the spectroscope produces scarcely any alteration in the brilliancy of the lines, since it merely separates the lines farther without making the lines themselves any wider. in the case of a _reversed_ spectrum, an increase of power in the spectroscope dilutes the light in the spaces between the lines without diluting that of the lines: hence lines which appear dark in a spectroscope of slight dispersive power may appear bright in an instrument of great dispersive power. . _change of the spectrum with the density of the luminous vapor._--it has been found, that, as the density of a luminous vapor is diminished, the lines in its spectrum become fewer and fewer, till they are finally reduced to one. on the other hand, an increase of density causes new lines to appear in the spectrum, and the old lines to become thicker. . _change of the spectrum with the temperature of the luminous vapor._--it has also been found that the appearance of a bright-lined spectrum changes considerably with the temperature of the luminous vapor. in some cases, an increase of temperature changes the relative intensities of the lines; in other cases, it causes new lines to appear, and old lines to disappear. in the case of a compound vapor, an increase of temperature causes the colored bands (which are peculiar to the spectrum of the compound) to disappear, and to be replaced by the spectral lines of the elements of which the compound is made up. the heat appears to _dissociate_ the compound; that is, to resolve it into its constituent elements. in this case, each elementary vapor would give its own spectral lines. as the compound is not completely dissociated at once, it is possible, of course, for one or more of the spectral lines of the elementary vapors to co-exist in the spectrum with the bands of the compound. it has been found, that, in some cases, the spectra of the elementary gases change with the temperature of the gas; and lockyer thinks he has discovered conclusive evidence, in the spectra of the sun and stars, that many of the substances regarded as elementary are really resolved into simpler substances by the intense heat of the sun; in other words, that our so-called elements are really compounds. chemical constitution of the sun. . _the solar spectrum._--the solar spectrum is crossed transversely by a great number of fine dark lines, and hence it belongs to the class of _reversed_ spectra. these lines were first studied and mapped by fraunhofer, and from him they have been called _fraunhofer's lines_. [illustration: fig. .] a reduced copy of fraunhofer's map is shown in fig. . a few of the most prominent of the dark solar lines are designated by the letters of the alphabet. the other lines are usually designated by the numbers at which they are found on the scale which accompanies the map. this scale is usually drawn at the top of the map, as will be seen in some of the following diagrams. the two most elaborate maps of the solar spectrum are those of kirchhoff and angström. the scale on kirchhoff's map is an arbitrary one, while that of angström is based upon the wave-lengths of the rays of light which would fall upon the lines in the spectrum. [illustration: fig. .] the appearance of the spectrum varies greatly with the power of the spectroscope employed. fig. shows a portion of the spectrum as it appears in a spectroscope of a single prism: while fig. shows the _b_ group of lines alone, as they appear in a powerful diffraction spectroscope. [illustration: fig. .] . _the telluric lines._--there are many lines of the solar spectrum which vary considerably in intensity as the sun passes from the horizon to the meridian, being most intense when the sun is nearest the horizon, and when his rays are obliged to pass through the greatest depth of the earth's atmosphere. these lines are of atmospheric origin, and are due to the absorption of the aqueous vapor in our atmosphere. they are the same lines that are obtained when a candle or other artificial light is examined with a spectroscope through a long tube filled with steam. since these lines are due to the absorption of our own atmosphere, they are called _telluric lines_. a map of these lines is shown in fig. . [illustration: fig. .] . _the solar lines._--after deducting the telluric lines, the remaining lines of the solar spectrum are of solar origin. they must be due to absorption which takes place in the sun's atmosphere. they are, in fact, the reversed spectra of the elements which exist in the solar atmosphere in the state of vapor: hence we conclude that the luminous surface of the sun is surrounded with an atmosphere of luminous vapors. the temperature of this atmosphere, at least near the surface of the sun, must be sufficient to enable all the elements known on the earth to exist in it as vapors. [illustration: fig. .] . _chemical constitution of the sun's atmosphere._--to find whether any element which exists on the earth is present in the solar atmosphere, we have merely to ascertain whether the bright lines of its gaseous spectrum are matched by dark lines in the solar spectrum when the two spectra are placed side by side. in fig. , we have in no. a portion of the red end of the solar spectra, and in no. the spectrum of sodium vapor, both as obtained in the same spectroscope by means of the comparison prism. it will be seen that the double sodium line is exactly matched by a double dark line of the solar spectrum: hence we conclude that sodium vapor is present in the sun's atmosphere. fig. shows the matching of a great number of the bright lines of iron vapor by dark lines in the solar spectrum. this matching of the iron lines establishes the fact that iron vapor is present in the solar atmosphere. [illustration: fig. .] the following table (given by professor young) contains a list of all the elements which have, up to the present time, been detected with certainty in the sun's atmosphere. it also gives the number of bright lines in the spectrum of each element, and the number of those lines which have been matched by dark lines in the solar spectrum:-- elements. bright lines reversed. observer. lines. . iron kirchhoff. . titanium thalen. . calcium kirchhoff. . manganese angström. . nickel kirchhoff. . cobalt thalen. . chromium kirchhoff. . barium kirchhoff. . sodium kirchhoff. . magnesium kirchhoff. . copper? ? kirchhoff. . hydrogen angström. . palladium lockyer. . vanadium lockyer. . molybdenum lockyer. . strontium lockyer. . lead lockyer. . uranium lockyer. . aluminium angström. . cerium lockyer. . cadmium lockyer. . oxygen a ± bright h. draper. oxygen b ? schuster. in addition to the above elements, it is probable that several other elements are present in the sun's atmosphere; since at least one of their bright lines has been found to coincide with dark lines of the solar spectrum. there are, however, a large number of elements, no traces of which have yet been detected; and, in the cases of the elements whose presence in the solar atmosphere has been established, the matching of the lines is far from complete in the majority of the cases, as will be seen from the above table. this want of complete coincidence of the lines is undoubtedly due to the very high temperature of the solar atmosphere. we have already seen that the lines of the spectrum change with the temperature; and, as the temperature of the sun is far higher than any that we can produce by artificial means, we might reasonably expect that it would cause the disappearance from the spectrum of many lines which we find to be present at our highest temperature. lockyer maintains that the reason why no trace of the spectral lines of certain of our so-called elements is found in the solar atmosphere is, that these substances are not really elementary, and that the intense heat of the sun resolves them into simpler constituents. motion at the surface of the sun. . _change of pitch caused by motion of sounding body._--when a sounding body is moving rapidly towards us, the pitch of its note becomes somewhat higher than when the body is stationary; and, when such a body is moving rapidly from us, the pitch of its note is lowered somewhat. we have a good illustration of this change of pitch at a country railway station on the passage of an express-train. the pitch of the locomotive whistle is considerably higher when the train is approaching the station than when it is leaving it. . _explanation of the change of pitch produced by motion._--the pitch of sound depends upon the rapidity with which the pulsations of sound beat upon the drum of the ear. the more rapidly the pulsations follow each other, the higher is the pitch: hence the shorter the sound-waves (provided the sound is all the while travelling at the same rate), the higher the pitch of the sound. any thing, then, which tends to shorten the waves of sound tends also to raise its pitch, and any thing which tends to lengthen these waves tends to lower its pitch. when a sounding body is moving rapidly forward, the sound-waves are crowded together a little, and therefore shortened; when it is moving backward, the sound-waves are drawn out, or lengthened a little. the effect of the motion of a sounding body upon the length of its sonorous waves will be readily seen from the following illustration: suppose a number of persons stationed at equal intervals in a line on a long platform capable of moving backward and forward. suppose the men are four feet apart, and all walking forward at the same rate, and that the platform is stationary, and that, as the men leave the platform, they keep on walking at the same rate: the men will evidently be four feet apart in the line in front of the platform, as well as on it. suppose next, that the platform is moving forward at the rate of one foot in the interval between two men's leaving the platform, and that the men continue to walk as before: it is evident that the men will then be three feet apart in the line after they have left the platform. the forward motion of the platform has the effect of crowding the men together a little. were the platform moving backward at the same rate, the men would be five feet apart after they had left the platform. the backward motion of the platform has the effect of separating the men from one another. the distance between the men in this illustration corresponds to the length of the sound-wave, or the distance between its two ends. were a person to stand beside the line, and count the men that passed him in the three cases given above, he would find that more persons would pass him in the same time when the platform is moving forward than when it is stationary, and fewer persons would pass him in the same time when the platform is moving backward than when it is stationary. in the same way, when a sounding body is moving rapidly forward, the sound-waves beat more rapidly upon the ear of a person who is standing still than when the body is at rest, and less rapidly when the sounding body is moving rapidly backward. were the platform stationary, and were the person who is counting the men to be walking along the line, either towards or away from the platform, the effect upon the number of men passing him in a given time would be precisely the same as it would be were the person stationary, and the platform moving either towards or away from him at the same rate. so the change in the rapidity with which pulsations of sound beat upon the ear is precisely the same whether the ear is stationary and the sounding body moving, or the sounding body is stationary and the ear moving. . _change of refrangibility due to the motion of a luminous body._--refrangibility in light corresponds to pitch in sound, and depends upon the length of the luminous waves. the shorter the luminous waves, the greater the refrangibility of the waves. very rapid motion of a luminous body has the same effect upon the length of the luminous waves that motion of a sounding body has upon the length of the sonorous waves. when a luminous body is moving very rapidly towards us, its luminous waves are shortened a little, and its light becomes a little more refrangible; when the luminous body is moving rapidly from us, its luminous waves are lengthened a little, and its light becomes a little less refrangible. [illustration: fig. .] . _displacement of spectral lines._--in examining the spectra of the stars, we often find that certain of the dark lines are _displaced_ somewhat, either towards the red or the violet end of the spectrum. as the dark lines are in the same position as the bright lines of the absorbing vapor would be, a displacement of the lines towards the red end of the spectrum indicates a lowering of the refrangibility of the rays, due to a motion of the luminous vapor away from us; and a displacement of the lines towards the violet end of the spectrum indicates an increase of refrangibility, due to a motion of the luminous vapor towards us. from the amount of the displacement of the lines, it is possible to calculate the velocity at which the luminous gas is moving. in fig. is shown the displacement of the _f_ line in the spectrum of sirius. this is one of the hydrogen lines. _rv_ is the spectrum, _r_ being the red, and _v_ the violet end. the long vertical line is the bright _f_ line of hydrogen, and the short dark line to the left of it is the position of the _f_ line in the spectrum of sirius. it is seen that this line is displaced somewhat towards the red end of the spectrum. this indicates that sirius must be moving from us; and the amount of the displacement indicates that the star must be moving at the rate of some twenty-five or thirty miles a second. [illustration: fig. .] . _contortion of lines on the disk of the sun._--certain of the dark lines seen on the centre of the sun's disk often appear more or less distorted, as shown in fig. , which represents the contortion of the hydrogen line as seen at various times. and indicate a rapid motion of hydrogen away from us, or a _down-rush_ at the sun; and (in which the line at the centre is dark on one side, and bent towards the red end of the spectrum, and bright on the other side with a distortion towards the violet end of the spectrum) indicate a _down-rush_ of _cool_ hydrogen side by side with an _up-rush_ of _hot and bright_ hydrogen; indicates local _down-rushes_ associated with _quiescent_ hydrogen. the contorted lines, which indicate a violently agitated state of the sun's atmosphere, appear in the midst of other lines which indicate a quiescent state. this is owing to the fact that the absorption which produces the dark lines takes place at various depths in the solar atmosphere. there may be violent commotion in the lower layers of the sun's atmosphere, and comparative quiet in the upper layers. in this case, the lines which are due to absorption in the lower layers would indicate this disturbance by their contortions; while the lines produced by absorption in the upper layers would be free from contortion. it often happens, too, that the contortions are confined to one set of lines of an element, while other lines of the same element are entirely free from contortions. this is undoubtedly due to the fact that different layers of the solar atmosphere differ greatly in temperature; so that the same element would give one set of lines at one depth, and another set at another depth: hence commotion in the solar atmosphere at any particular depth would be indicated by the contortion of those lines of the element only which are produced by the temperature at that particular depth. a remarkable case of contortion witnessed by professor young is shown in fig. . three successive appearances of the _c_ line are shown. the second view was taken three minutes after the first, and the third five minutes after the second. the contortion in this case indicated a velocity ranging from two hundred to three hundred miles a second. [illustration: fig. .] . _contortion of lines on the sun's limb._--when the spectroscope is directed to the centre of the sun's disk, the distortion of the lines indicates only vertical motion in the sun's atmosphere; but, when the spectroscope is directed to the limb of the sun, displacements of the lines indicate horizontal motions in the sun's atmosphere. when a powerful spectroscope is directed to the margin of the sun's disk, so that the slit of the collimator tube shall be perpendicular to the sun's limb, one or more of the dark lines on the disk are seen to be prolonged by a bright line, as shown in fig. . but this prolongation, instead of being straight and narrow, as shown in the figure, is often widened and distorted in various ways, as shown in fig. . in the left-hand portion of the diagram, the line is deflected towards the red end of the spectrum; this indicates a violent wind on the sun's surface blowing away from us. in the right-hand portion of the diagram, the line is deflected towards the violet end of the spectrum; this indicates a violent wind blowing towards us. in the middle portion of the figure, the line is seen to be bent both ways; this indicates a cyclone, on one side of which the wind would be blowing from us, and on the other side towards us. [illustration: fig. .] [illustration: fig. .] the distortions of the solar lines indicate that the wind at the surface of the sun often blows with a velocity of _from one hundred to three hundred miles a second_. the most violent wind known on the earth has velocity of a hundred miles an hour. iii. the photosphere and sun spots. the photosphere. [illustration: fig. .] . _the granulation of the photosphere._--when the surface of the sun is examined with a good telescope under favorable atmospheric conditions, it is seen to be composed of minute grains of intense brilliancy and of irregular form, floating in a darker medium, and arranged in streaks and groups, as shown in fig. . with a rather low power, the general effect of the surface is much like that of rough drawing-paper, or of curdled milk seen from a little distance. with a high power and excellent atmospheric conditions, the _grains_ are seen to be irregular, rounded masses, some hundreds of miles in diameter, sprinkled upon a less brilliant background, and appearing somewhat like snow-flakes sparsely scattered over a grayish cloth. fig. is a representation of these grains according to secchi. [illustration: fig. .] with a very powerful telescope and the very best atmospheric conditions, the grains themselves are resolved into _granules_, or little luminous dots, not more than a hundred miles or so in diameter, which, by their aggregation, make up the grains, just as they, in their turn, make up the coarser masses of the solar surface. professor langley estimates that these granules constitute about one-fifth of the sun's surface, while they emit at least three-fourths of its light. . _shape of the grains._--the grains differ considerably in shape at different times and on different parts of the sun's surface. nasmyth, in , described them as _willow-leaves_ in shape, several thousand miles in length, but narrow and with pointed ends. he figured the surface of the sun as a sort of basket-work formed by the interweaving of such filaments. to others they have appeared to have the form of _rice-grains_. on portions of the sun's disk the elementary structure is often composed of long, narrow, blunt-ended filaments, not so much like willow-leaves as like bits of straw lying roughly parallel to each other,--a _thatch-straw_ formation, as it has been called. this is specially common in the immediate neighborhood of the spots. . _nature of the grains._--the grains are, undoubtedly, incandescent _clouds_ floating in the sun's atmosphere, and composed of partially condensed metallic vapors, just as the clouds of our atmosphere are composed of partially condensed aqueous vapor. rain on the sun is composed of white-hot drops of molten iron and other metals; and these drops are often driven with the wind with a velocity of over a hundred miles a second. as to the forms of the grains, professor young says, "if one were to speculate as to the explanation of the grains and thatch-straws, it might be that the grains are the upper ends of long filaments of luminous cloud, which, over most of the sun's surface, stand approximately vertical, but in the neighborhood of a spot are inclined so as to lie nearly horizontal. this is not certain, though: it may be that the cloud-masses over the more quiet portions of the solar surface are really, as they seem, nearly globular, while near the spots they are drawn out into filamentary forms by atmospheric currents." . _faculæ._--the _faculæ_ are irregular streaks of greater brightness than the general surface, looking much like the flecks of foam on the surface of a stream below a waterfall. they are sometimes from five to twenty thousand miles in length, covering areas immensely larger than a terrestrial continent. these faculæ are _elevated regions_ of the solar surface, ridges and crests of luminous matter, which rise above the general level of the sun's surface, and protrude through the denser portions of the solar atmosphere. when one of these passes over the edge of the sun's disk, it can be seen to project, like a little tooth. any elevation on the sun to be perceptible at all must measure at least half a second of an arc, or two hundred and twenty-five miles. the faculæ are most numerous in the neighborhood of the spots, and much more conspicuous near the limb of the sun than near the centre of the disk. fig. gives the general appearance of the faculæ, and the darkening of the limb of the sun. near the spots, the faculæ often undergo very rapid change of form, while elsewhere on the disk they change rather slowly, sometimes undergoing little apparent alteration for several days. [illustration: fig. .] . _why the faculæ are most conspicuous near the limb of the sun._--the reason why the faculæ are most conspicuous near the limb of the sun is this: the luminous surface of the sun is covered with an atmosphere, which, though not very thick compared with the diameter of the sun, is still sufficient to absorb a good deal of light. light coming from the centre of the sun's disk penetrates this atmosphere under the most favorable conditions, and is but slightly reduced in amount. the edges of the disk, on the other hand, are seen through a much greater thickness of atmosphere; and the light is reduced by absorption some seventy-five per cent. suppose, now, a facula were sufficiently elevated to penetrate quite through this atmosphere. its light would be undimmed by absorption on any part of the sun's disk; but at the centre of the disk it would be seen against a background nearly as bright as itself, while at the margin it would be seen against one only a quarter as bright. it is evident that the light of any facula, owing to the elevation, would be reduced less rapidly as we approach the edge of the disk than that of the general surface of the sun, which lies at a lower level. sun-spots. . _general appearance of sun-spots._--the general appearance of a well-formed sun-spot is shown in fig. . the spot consists of a very dark central portion of irregular shape, called the _umbra_, which is surrounded by a less dark fringe, called the _penumbra_. the penumbra is made up, for the most part, of filaments directed radially inward. [illustration: fig. .] there is great variety in the details of form in different sun-spots; but they are generally nearly circular during the middle period of their existence. during the period of their development and of their disappearance they are much more irregular in form. there is nothing like a gradual shading-off of the penumbra, either towards the umbra on the one side, or towards the photosphere on the other. the penumbra is separated from both the umbra and the photosphere by a sharp line of demarcation. the umbra is much brighter on the inner than on the outer edge, and frequently the photosphere is excessively bright at the margin of the penumbra. the brightness of the inner penumbra seems to be due to the crowding together of the penumbral filaments where they overhang the edge of the umbra. there is a general antithesis between the irregularities of the outer and inner edges of the penumbra. where an angle of the penumbral matter crowds in upon the umbra, it is generally matched by a corresponding outward extension into the photosphere, and _vice versa_. the umbra of the spot is far from being uniformly dark. many of the penumbral filaments terminate in little detached grains of luminous matter; and there are also fainter veils of a substance less brilliant, but sometimes rose-colored, which seem to float above the umbra. the umbra itself is made up of masses of clouds which are really intensely brilliant, and which appear dark only by contrast with the intenser brightness of the solar surface. among these clouds are often seen one or more minute circular spots much darker than the rest of the umbra. these darker portions are called _nuclei_. they seem to be the mouths of tubular orifices penetrating to unknown depths. the faint veils mentioned above continually melt away, and are replaced by others in some different position. the bright granules at the tips of the penumbral filaments seem to sink and dissolve, while fresh portions break off to replace them. there is a continual indraught of luminous matter over the whole extent of the penumbra. at times, though very rarely, patches of intense brightness suddenly break out, remain visible for a few minutes, and move over the spot with velocities as great as a hundred miles _a second_. the spots change their form and size quite perceptibly from day to day, and sometimes even from hour to hour. . _duration of sun-spots._--the average life of a sun-spot is two or three months: the longest on record is that of a spot observed in and , which lasted eighteen months. there are cases, however, where the disappearance of a spot is very soon followed by the appearance of another at the same point; and sometimes this alternate disappearance and re-appearance is several times repeated. while some spots are thus long-lived, others endure only a day or two, and sometimes only a few hours. . _groups of spots._--the spots usually appear not singly, but in groups. a large spot is often followed by a train of smaller ones to the east of it, many of which are apt to be irregular in form and very imperfect in structure, sometimes with no umbra at all, often with a penumbra only on one side. in such cases, when any considerable change of form or structure shows itself in the principal spot, it seems to rush westward over the solar surface, leaving its attendants trailing behind. when a large spot divides into two or more, as often happens, the parts usually seem to repel each other, and fly apart with great velocity. . _size of the spots._--the spots are sometimes of enormous size. groups have often been observed covering areas of more than a hundred thousand miles square, and single spots occasionally measure from forty to fifty thousand miles in diameter, the umbra being twenty-five or thirty thousand miles across. a spot, however, measuring thirty thousand miles over all, may be considered a large one. such a spot can easily be seen without a telescope when the brightness of the sun's surface is reduced by clouds or nearness to the horizon, or by the use of colored glass. during the years and spots were visible to the naked eye for a considerable portion of the time. the largest spot yet recorded was observed in . it had a breadth of more than a hundred and forty-three thousand miles, or nearly eighteen times the diameter of the earth, and covered about a thirty-sixth of the whole surface of the sun. [illustration: fig. .] fig. represents a group of sun-spots observed by professor langley, and drawn on the same scale as the small circle in the upper left-hand corner, which represents the surface of half of our globe. [illustration: fig. .] . _the penumbral filaments._--not unfrequently the penumbral filaments are curved spirally, indicating a cyclonic action, as shown in fig. . in such cases the whole spot usually turns slowly around, sometimes completing an entire revolution in a few days. more frequently, however, the spiral motion lasts but a short time; and occasionally, after continuing for a while in one direction, the motion is reversed. very often in large spots we observe opposite spiral movements in different portions of the umbra, as shown in figs. and . [illustration: fig. .] neighboring spots show no tendency to rotate in the same direction. the number of spots in which a decided cyclonic motion (like that shown in fig. ) appears is comparatively small, not exceeding two or three per cent of the whole. [illustration: fig. .] [illustration: fig. .] [illustration: plate .] plate ii. represents a typical sun-spot as delineated by professor langley. at the left-hand and upper portions of this great spot the filaments present the ordinary appearance, while at the lower edge, and upon the great overhanging branch, they are arranged very differently. the feathery brush below the branch, closely resembling a frost-crystal on a window-pane, is as rare as it is curious, and has not been satisfactorily explained. [illustration: fig. .] . _birth and decay of sun-spots._--the formation of a spot is sometimes gradual, requiring days or even weeks for its full development; and sometimes a single day suffices. generally, for some time before its appearance, there is an evident disturbance of the solar surface, indicated especially by the presence of many brilliant faculæ, among which _pores_, or minute black dots, are scattered. these enlarge, and between them appear grayish patches, in which the photospheric structure is unusually evident, as if they were caused by a dark mass lying below a thin veil of luminous filaments. this veil seems to grow gradually thinner, and finally breaks open, giving us at last the complete spot with its penumbra. some of the pores coalesce with the principal spot, some disappear, and others form the attendant train before described ( ). the spot when once formed usually assumes a circular form, and remains without striking change until it disappears. as its end approaches, the surrounding photosphere seems to crowd in, and overwhelm the penumbra. bridges of light (fig. ), often much brighter than the average of the solar surface, push across the umbra; the arrangement of the penumbra filaments becomes confused; and, as secchi expresses it, the luminous matter of the photosphere seems to tumble pell-mell into the chasm, which disappears, and leaves a disturbed surface marked with faculæ, which, in their turn, gradually subside. [illustration: fig. .] . _motion of sun-spots._--the spots have a regular motion across the disk of the sun from east to west, occupying about twelve days in the transit. a spot generally appears first on or near the east limb, and, after twelve or fourteen days, disappears at the west limb. at the end of another fourteen days, or more, it re-appears at the east limb, unless, in the mean time, it has vanished from sight entirely. this motion of the spots is indicated by the arrow in fig. . the interval between two successive appearances of the same spot on the eastern edge of the sun is about twenty-seven days. [illustration: fig. .] . _the rotation of the sun._--the spots are evidently carried around by the rotation of the sun on its axis. it is evident, from fig. , that the sun will need to make more than a complete rotation in order to bring a spot again upon the same part of the disk as seen from the earth. _s_ represents the sun, and _e_ the earth. the arrows indicate the direction of the sun's rotation. when the earth is at _e_, a spot at _a_ would be seen at the centre of the solar disk. while the sun is turning on its axis, the earth moves in its orbit from _e_ to _e'_: hence the sun must make a complete rotation, and turn from _a_ to _a'_ in addition, in order to bring the spot again to the centre of the disk. to carry the spot entirely around, and then on to _a'_, requires about twenty-seven days. from this _synodical period_ of the spot, as it might be called, it has been calculated that the sun must rotate on its axis in about twenty-five days. [illustration: fig. .] . _the inclination of the sun's axis._--the paths described by sun-spots across the solar disk vary with the position of the earth in its orbit, as shown in fig. . we therefore conclude that the sun's axis is not perpendicular to the plane of the earth's orbit. the sun rotates on its axis from west to east, and the axis leans about seven degrees from the perpendicular to the earth's orbit. . _the proper motion of the spots._--when the period of the sun's rotation is deduced from the motion of spots in different solar latitudes, there is found to be considerable variation in the results obtained. thus spots near the equator indicate that the sun rotates in about twenty-five days; while those in latitude ° indicate a period about eighteen hours longer; and those in latitude ° a period of twenty-seven days and a half. strictly speaking, the sun, as a whole, has no single period of rotation; but different portions of its surface perform their revolutions in different times. the equatorial regions not only move more rapidly in miles per hour than the rest of the solar surface, but they _complete the entire rotation in shorter time_. [illustration: fig. .] there appears to be a peculiar surface-drift in the equatorial regions of the sun, the cause of which is unknown, but which gives the spots a _proper_ motion; that is, a motion of their own, independent of the rotation of the sun. [illustration: fig. .] . _distribution of the sun-spots._--the sun-spots are not distributed uniformly over the sun's surface, but occur mainly in two zones on each side of the equator, and between the latitudes of ° and °, as shown in fig. . on and near the equator itself they are comparatively rare. there are still fewer beyond ° of latitude, and only a single spot has ever been recorded more than ° from the solar equator. fig. shows the distribution of the sun-spots observed by carrington during a period of eight years. the irregular line on the left-hand side of the figure indicates by its height the comparative frequency with which the spots occurred in different latitudes. in fig. the same thing is indicated by different degrees of darkness in the shading of the belts. [illustration: fig. .] . _the periodicity of the spots._--careful observations of the solar spots indicate a period of about eleven years in the spot-producing activity of the sun. during two or three years the spots increase in number and in size; then they begin to diminish, and reach a minimum five or six years after the maximum. another period of about six years brings the return of the maximum. the intervals are, however, somewhat irregular. [illustration: fig. .] fig. gives a graphic representation of the periodicity of the sun-spots. the height of the curve shows the frequency of the sun-spots in the years given at the bottom of the figure. it appears, from an examination of this sun-spot curve, that the average interval from a minimum to the next following maximum is only about four years and a half, while that from a maximum to the next following minimum is six years and six-tenths. the disturbance which produces the sun-spots is developed suddenly, but dies away gradually. . _connection between sun-spots and terrestrial magnetism._--the magnetic needle does not point steadily in the same direction, but is subject to various disturbances, some of which are regular, and others irregular. ( ) one of the most noticeable of the regular magnetic changes is the so-called _diurnal oscillation_. during the early part of the day the north pole of the needle moves toward the west in our latitude, returning to its mean position about ten p.m., and remaining nearly stationary during the night. the extent of this oscillation in the united states is about fifteen minutes of arc in summer, and not quite half as much in winter; but it differs very much in different localities and at different times, and the average diurnal oscillation in any locality increases and decreases pretty regularly during a period of about eleven years. the maximum and minimum of this period of magnetic disturbance are found to coincide with the maximum and minimum of the sun-spot period. this is shown in fig. , in which the dotted lines indicate the variations in the intensity of the magnetic disturbance. ( ) occasionally so-called _magnetic storms_ occur, during which the compass-needle is sometimes violently disturbed, oscillating five degrees, or even ten degrees, within an hour or two. these storms are generally accompanied by an aurora, and an aurora is _always_ accompanied by magnetic disturbance. a careful comparison of aurora observations with those of sun-spots shows an almost perfect parallelism between the curves of auroral and sun-spot frequency. ( ) a number of observations render it very probable that every intense disturbance of the solar surface is propagated to our terrestrial magnetism with the speed of light. [illustration: fig. .] fig. shows certain of the solar lines as they were observed by professor young on aug. , . the contortions of the _f_ line indicated an intense disturbance in the atmosphere of the sun. there were three especially notable paroxysms in this distortion, occurring at a quarter of nine, half-past ten, and ten minutes of twelve, a.m. [illustration: fig. .] fig. shows the curve of magnetic disturbance as traced at greenwich on the same day. it will be seen from the curve that it was a day of general magnetic disturbance. at the times of the three paroxysms, which are given at the bottom of the figure, it will be observed that there is a peculiar shivering of the magnetic curve. . _the spots are depressions in the photosphere._--this fact was first clearly brought out by dr. wilson of glasgow, in , from observations upon the penumbra of a spot in november of that year. he found, that when the spot appeared at the eastern limb, or edge of the sun, just moving into sight, the penumbra was well marked on the side of the spot nearest to the edge of the disk; while on the other edge of the spot, towards the centre of the sun, there was no penumbra visible at all, and the umbra itself was almost hidden, as if behind a bank. when the spot had moved a day's journey toward the centre of the disk, the whole of the umbra came into sight, and the penumbra on the inner edge of the spot began to be visible as a narrow line. after the spot was well advanced upon the disk, the penumbra was of the same width all around the spot. when the spot approached the sun's western limb, the same phenomena were repeated, but in the inverse order. the penumbra on the _inner_ edge of the spot narrowed much faster than that on the outer, disappeared entirely, and finally seemed to hide from sight much of the umbra nearly a whole day before the spot passed from view around the limb. this is precisely what would occur (as fig. clearly shows) if the spot were a saucer-shaped depression in the solar surface, the bottom of the saucer corresponding to the umbra, and the sloping sides to the penumbra. [illustration: fig. .] [illustration: fig. .] . _sun-spot spectrum._--when the image of a sun-spot is thrown upon the slit of the spectroscope, the spectrum is seen to be crossed longitudinally by a continuous dark band, showing an increased general absorption in the region of the sun-spot. many of the spectral lines are greatly thickened, as shown in fig. . this thickening of the lines shows that the absorption is taking place at a greater depth. new lines and shadings often appear, which indicate, that, in the cooler nucleus of the spot, certain compound vapors exist, which are dissociated elsewhere on the sun's surface. these lines and shadings are shown in fig. . [illustration: fig. .] it often happens that certain of the spectral lines are reversed in the spectrum of the spot, a thin bright line appearing over the centre of a thick dark one, as shown in fig. . these reversals are due to very bright vapors floating over the spot. [illustration: fig. .] at times, also, the spectrum of a spot indicates violent motion in the overlying gases by distortion and displacement of the lines. this phenomenon occurs oftener at points near the outer edge of the penumbra than over the centre of the spot; but occasionally the whole neighborhood is violently agitated. in such cases, lines in the spectrum side by side are often affected in entirely different ways, one being greatly displaced while its neighbor is not disturbed in the least, showing that the vapors which produce the lines are at different levels in the solar atmosphere, and moving independently of each other. [illustration: fig. .] . _the cause and nature of sun-spots._--according to professor young, the arrangement and relations of the photospheric clouds in the neighborhood of a spot are such as are represented in fig. . "over the sun's surface generally, these clouds probably have the form of vertical columns, as at _aa_. just outside the spot, the level of the photosphere is the most part, overtopped by eruptions of hydrogen and usually raised into faculæ, as at _bb_. these faculæ are, for metallic vapors, as indicated by the shaded clouds.... while the great clouds of hydrogen are found everywhere upon the sun, these spiky, vivid outbursts of metallic vapors seldom occur except just in the neighborhood of a spot, and then only during its season of rapid change. in the penumbra of the spot the photospheric filaments become more or less nearly horizontal, as at _pp_; in the umbra at _u_ it is quite uncertain what the true state of affairs may be. we have conjecturally represented the filaments there as vertical also, but depressed and carried down by a descending current. of course, the cavity is filled by the gases which overlie the photosphere; and it is easy to see, that, looked at from above, such a cavity and arrangement of the luminous filaments would present the appearances actually observed." professor young also suggests that the spots may be depressions in the photosphere caused "by the _diminution of upward pressure_ from below, in consequence of eruptions in the neighborhood; the spots thus being, so to speak, _sinks_ in the photosphere. undoubtedly the photosphere is not a strictly continuous shell or crust; but it is _heavy_ as compared with the uncondensed vapors in which it lies, just as a rain-cloud in our terrestrial atmosphere is heavier than the air; and it is probably continuous enough to have its upper level affected by any diminution of pressure below. the gaseous mass below the photosphere supports its weight and the weight of the products of condensation, which must always be descending in an inconceivable rain and snow of molten and crystallized material. to all intents and purposes, though nothing but a layer of clouds, the photosphere thus forms a constricting shell, and the gases beneath are imprisoned and compressed. moreover, at a high temperature the viscosity of gases is vastly increased, so that quite probably the matter of the solar nucleus resembles pitch or tar in its consistency more than what we usually think of as a gas. consequently, any sudden diminution of pressure would propagate itself slowly from the point where it occurred. putting these things together, it would seem, that, whenever a free outlet is obtained through the photosphere at any point, thus decreasing the inward pressure, the result would be the sinking of a portion of the photosphere somewhere in the immediate neighborhood, to restore the equilibrium; and, if the eruption were kept up for any length of time, the depression in the photosphere would continue till the eruption ceased. this depression, filled with the overlying gases, would constitute a spot. moreover, the line of fracture (if we may call it so) at the edges of the sink would be a region of weakness in the photosphere, so that we should expect a series of eruptions all around the spot. for a time the disturbance, therefore, would grow, and the spot would enlarge and deepen, until, in spite of the viscosity of the internal gases, the equilibrium of pressure was gradually restored beneath. so far as we know the spectroscopic and visual phenomena, none of them contradict this hypothesis. there is nothing in it, however, to account for the distribution of the spots in solar latitudes, nor for their periodicity." iv. the chromosphere and prominences. . _the sun's outer atmosphere._--what we see of the sun under ordinary circumstances is but a fraction of his total bulk. while by far the greater portion of the solar _mass_ is included within the photosphere, the larger portion of his _volume_ lies without, and constitutes a gaseous envelope whose diameter is at least double, and its bulk therefore sevenfold, that of the central globe. this outer envelope, though mainly gaseous, is not spherical, but has an exceedingly irregular and variable outline. it seems to be made up, not of regular strata of different density, like our atmosphere, but rather of flames, beams, and streamers, as transient and unstable as those of the aurora borealis. it is divided into two portions by a boundary as definite, though not so regular, as that which separates them both from the photosphere. the outer and far more extensive portion, which in texture and rarity seems to resemble the tails of comets, is known as the _coronal atmosphere_, since to it is chiefly due the _corona_, or glory, which surrounds the darkened sun during an eclipse. . _the chromosphere._--at the base of the coronal atmosphere, and in contact with the photosphere, is what resembles a sheet of scarlet fire. it appears as if countless jets of heated gas were issuing through vents over the whole surface, clothing it with flame, which heaves and tosses like the blaze of a conflagration. this is the _chromosphere_, or color-sphere. it owes its vivid redness to the predominance of hydrogen in the flames. the average depth of the chromosphere is not far from ten or twelve seconds, or five thousand or six thousand miles. . _the prominences._--here and there masses of this hydrogen, mixed with other substances, rise far above the general level into the coronal regions, where they float like clouds, or are torn to pieces by conflicting currents. these cloud-masses are known as solar _prominences_, or _protuberances_. . _magnitude and distribution of the prominences._--the prominences differ greatly in magnitude. of the , observed by secchi, , attained an altitude of eighteen thousand miles; , or nearly a fourth of the whole, reached a height of twenty-eight thousand miles; several exceeded eighty-four thousand miles. in rare instances they reach elevations as great as a hundred thousand miles. a few have been seen which exceeded a hundred and fifty thousand miles; and secchi has recorded one of three hundred thousand miles. [illustration: fig. .] the irregular lines on the right-hand side of fig. show the proportion of the prominences observed by secchi, that were seen in different parts of the sun's surface. the outer line shows the distribution of the smaller prominences, and the inner dotted line that of the larger prominences. by comparing these lines with those on the opposite side of the circle, which show the distribution of the spots, it will be seen, that, while the spots are confined mainly to two belts, the prominences are seen in all latitudes. . _the spectrum of the chromosphere._--the spectrum of the chromosphere is comparatively simple. there are eleven lines only which are always present; and six of these are lines of hydrogen, and the others, with a single exception, are of unknown elements. there are sixteen other lines which make their appearance very frequently. among these latter are lines of sodium, magnesium, and iron. where some special disturbance is going on, the spectrum at the base of the chromosphere is very complicated, consisting of hundreds of bright lines. "the majority of the lines, however, are seen only occasionally, for a few minutes at a time, when the gases and vapors, which generally lie low (mainly in the interstices of the clouds which constitute the photosphere), and below its upper surface, are elevated for the time being by some eruptive action. for the most part, the lines which appear only at such times are simply _reversals_ of the more prominent dark lines of the ordinary solar spectrum. but the selection of the lines seems most capricious: one is taken, and another left, though belonging to the same element, of equal intensity, and close beside the first." some of the main lines of the chromosphere and prominences are shown in fig. . [illustration: fig. .] . _method of studying the chromosphere and prominences._--until recently, the solar atmosphere could be seen only during a total eclipse of the sun; but now the spectroscope enables us to study the chromosphere and the prominences with nearly the same facility as the spots and faculæ. the protuberances are ordinarily invisible, for the same reason that the stars cannot be seen in the daytime; they are hidden by the intense light reflected from our own atmosphere. if we could only get rid of this aerial illumination, without at the same time weakening the light of the prominences, the latter would become visible. this the spectroscope enables us to accomplish. since the air-light is reflected sunshine, it of course presents the same spectrum as sunlight,--a continuous band of color crossed by dark lines. now, this sort of spectrum is weakened by increase of dispersive power ( ), because the light is spread out into a longer ribbon, and made to cover a greater area. on the other hand, the spectrum of the prominences, being composed of bright lines, undergoes no such diminution by increased dispersion. [illustration: fig. .] when the spectroscope is used as a means of examining the prominences, the slit is more or less widened. the telescope is directed so that the image of that portion of the solar limb which is to be examined shall be tangent to the opened slit, as in fig. , which represents the slit-plate of the spectroscope of its actual size, with the image of the sun in the proper position for observation. [illustration: fig. .] if, now, a prominence exists at this part of the solar limb, and if the spectroscope itself is so adjusted that the _c_ line falls in the centre of the field of view, then one will see something like fig. . "the red portion of the spectrum will stretch athwart the field of view like a scarlet ribbon with a darkish band across it; and in that band will appear the prominences, like scarlet clouds, so like our own terrestrial clouds, indeed, in form and texture, that the resemblance is quite startling. one might almost think he was looking out through a partly-opened door upon a sunset sky, except that there is no variety or contrast of color; all the cloudlets are of the same pure scarlet hue. along the edge of the opening is seen the chromosphere, more brilliant than the clouds which rise from it or float above it, and, for the most part, made up of minute tongues and filaments." . _quiescent prominences._--the prominences differ as widely in form and structure as in magnitude. the two principal classes are the _quiescent_, _cloud-formed_, or _hydrogenous_, and the _eruptive_, or _metallic_. [illustration: plate .] the _quiescent_ prominences resemble almost exactly our terrestrial clouds, and differ among themselves in the same manner. they are often of enormous dimensions, especially in horizontal extent, and are comparatively permanent, often undergoing little change for hours and days. near the poles they sometimes remain during a whole solar revolution of twenty-seven days. sometimes they appear to lie upon the limb of the sun, like a bank of clouds in the terrestrial horizon, probably because they are so far from the edge that only their upper portions are in sight. when fully seen, they are usually connected to the chromosphere by slender columns, generally smallest at the base, and often apparently made up of separate filaments closely intertwined, and expanding upward. sometimes the whole under surface is fringed with pendent filaments. sometimes they float entirely free from the chromosphere; and in most cases the larger clouds are attended by detached cloudlets. various forms of quiescent prominences are shown in plate iii. other forms are given in figs. and . [illustration: fig. .] their spectrum is usually very simple, consisting of the four lines of hydrogen and the orange _d_^ : hence the appellation _hydrogenous_. occasionally the sodium and magnesium lines also appear, even near the tops of the clouds. [illustration: fig. .] . _eruptive prominences._--the _eruptive_ prominences ordinarily consist of brilliant spikes or jets, which change very rapidly in form and brightness. as a rule, their altitude is not more than twenty thousand or thirty thousand miles; but occasionally they rise far higher than even the largest of the quiescent protuberances. their spectrum is very complicated, especially near their base, and often filled with bright lines. the most conspicuous lines are those of sodium, magnesium, barium, iron, and titanium: hence secchi calls them _metallic_ prominences. [illustration: fig. .] they usually appear in the immediate vicinity of a spot, never very near the solar poles. they change with such rapidity, that the motion can almost be seen with the eye. sometimes, in the course of fifteen or twenty minutes, a mass of these flames, fifty thousand miles high, will undergo a total transformation; and in some instances their complete development or disappearance takes no longer time. sometimes they consist of pointed rays, diverging in all directions, as represented in fig. . "sometimes they look like flames, sometimes like sheaves of grain, sometimes like whirling water-spouts capped with a great cloud; occasionally they present most exactly the appearance of jets of liquid fire, rising and falling in graceful parabolas; frequently they carry on their edges spirals like the volutes of an ionic column; and continually they detach filaments, which rise to a great elevation, gradually expanding and growing fainter as they ascend, until the eye loses them." [illustration: fig. .] . _change of form in prominences._--fig. represents a prominence as seen by professor young, sept. , . it was an immense quiescent cloud, a hundred thousand miles long and fifty-four thousand miles high. at _a_ there was a brilliant lump, somewhat in the form of a thunder-head. on returning to the spectroscope less than half an hour afterwards, he found that the cloud had been literally blown into shreds by some inconceivable uprush from beneath. the prominence then presented the form shown in fig. . the _débris_ of the cloud had already attained a height of a hundred thousand miles. while he was watching them for the next ten minutes, they rose, with a motion almost perceptible to the eye, till the uppermost reached an altitude of two hundred thousand miles. as the filaments rose, they gradually faded away like a dissolving cloud. [illustration: fig. .] meanwhile the little thunder-head had grown and developed into what appeared to be a mass of rolling and ever-changing flame. figs. and give the appearance of this portion of the prominence at intervals of fifteen minutes. other similar eruptions have been observed. [illustration: fig. .] [illustration: fig. .] v. the corona. . _general appearance of the corona._--at the time of a total eclipse of the sun, if the sky is clear, the moon appears as a huge black ball, the illumination at the edge of the disk being just sufficient to bring out its rotundity. "from behind it," to borrow professor young's vivid description, "stream out on all sides radiant filaments, beams, and sheets of pearly light, which reach to a distance sometimes of several degrees from the solar surface, forming an irregular stellate halo, with the black globe of the moon in its apparent centre. the portion nearest the sun is of dazzling brightness, but still less brilliant than the prominences which blaze through it like carbuncles. generally this inner corona has a pretty uniform height, forming a ring three or four minutes of arc in width, separated by a somewhat definite outline from the outer corona, which reaches to a much greater distance, and is far more irregular in form. usually there are several _rifts_, as they have been called, like narrow beams of darkness, extending from the very edge of the sun to the outer night, and much resembling the cloud-shadows which radiate from the sun before a thunder-shower; but the edges of these rifts are frequently curved, showing them to be something else than real shadows. sometimes there are narrow bright streamers, as long as the rifts, or longer. these are often inclined, occasionally are even nearly tangential to the solar surface, and frequently are curved. on the whole, the corona is usually less extensive and brilliant over the solar poles, and there is a recognizable tendency to accumulations above the middle latitudes, or spot-zones; so that, speaking roughly, the corona shows a disposition to assume the form of a quadrilateral or four-rayed star, though in almost every individual case this form is greatly modified by abnormal streamers at some point or other." [illustration: fig. .] . _the corona as seen at recent eclipses._--the corona can be seen only at the time of a total eclipse of the sun, and then for only a few minutes. its form varies considerably from one eclipse to another, and apparently also during the same eclipse. at least, different observers at different stations depict the same corona under very different forms. fig. represents the corona of as observed by liais. in this view the _petal-like_ forms, which have been noticed in the corona at other times, are especially prominent. [illustration: fig. .] fig. shows the corona of as it was observed by temple. [illustration: fig. .] fig. shows the corona of . this is interesting as being a corona at the time of sun-spot minimum. [illustration: fig. .] fig. represents the corona of . this is a larger and more irregular corona than usual. [illustration: fig. .] the corona of is shown in fig. . [illustration: fig. .] fig. is a view of the corona of as seen by capt. tupman. [illustration: fig. .] fig. shows the same corona as seen by foenander. [illustration: fig. .] fig. shows the same corona as photographed by davis. [illustration: fig. .] fig. shows the corona of made up from several views as combined by professor young. . _the spectrum of the corona._--the chief line in the spectrum of the corona is the one usually designated as , and now known as the _coronal_ line. it is seen as a dark line on the disk of the sun; and a spectroscope of great dispersive power shows this dark line to be closely double, the lower component being one of the iron lines, and the upper the coronal line. this dark line is shown at _x_, fig. . [illustration: fig. .] besides this bright line, the hydrogen lines appear faintly in the spectrum of the corona. the line has been sometimes traced with the spectroscope to an elevation of nearly twenty minutes above the moon's limb, and the hydrogen lines nearly as far; and the lines were just as strong _in the middle of a dark rift_ as anywhere else. the substance which produces the line is unknown as yet. it seems to be something with a vapor-density far below that of hydrogen, which is the lightest substance of which we have any knowledge. it can hardly be an "allotropic" form of any terrestrial element, as some scientists have suggested; for in the most violent disturbances in prominences and near sun-spots, when the lines of hydrogen, magnesium, and other metals, are contorted and shattered by the rush of the contending elements, this line alone remains fine, sharp, and straight, a little brightened, but not otherwise affected. for the present it remains, like a few other lines in the spectrum, an unexplained mystery. besides bright lines, the corona shows also a faint continuous spectrum, in which have been observed a few of the more prominent _dark_ lines of the solar spectrum. this shows, that, while the corona may be in the main composed of glowing gas (as indicated by the bright lines of its spectrum), it also contains considerable matter in such a state as to reflect the sunlight, probably in the form of dust or fog. v. eclipses. [illustration: fig. .] . _the shadows of the earth and moon._--the shadows cast by the earth and moon are shown in fig. . each shadow is seen to be made up of a dark portion called the _umbra_, and of a lighter portion called the _penumbra_. the light of the sun is completely excluded from the umbra, but only partially from the penumbra. the umbra is in the form of a cone, with its apex away from the sun; though in the case of the earth's shadow it tapers very slowly. the penumbra surrounds the umbra, and increases in size as we recede from the sun. the axis of the earth's shadow lies in the plane of the ecliptic, which in the figure is the surface of the page. as the moon's orbit is inclined five degrees to the plane of the ecliptic, the axis of the moon's shadow will sometimes lie above, and sometimes below, the ecliptic. it will lie on the ecliptic only when the moon is at one of her nodes. . _when there will be an eclipse of the moon._--the moon is eclipsed _whenever it passes into the umbra of the earth's shadow_. it will be seen from the figure that the moon can pass into the shadow of the earth only when she is in opposition, or _at full_. owing to the inclination of the moon's orbit to the ecliptic, the moon will pass either above or below the earth's shadow when she is at full, unless she happens to be near her node at this time: hence there is not an eclipse of the moon every month. when the moon simply passes into the penumbra of the earth's shadow, the light of the moon is somewhat dimmed, but not sufficiently to attract attention, or to be denominated an eclipse. [illustration: fig. .] . _the lunar ecliptic limits._--in fig. the line _ab_ represents the plane of the ecliptic, and the line _cd_ the moon's orbit. the large black circles on the line _ab_ represent sections of the umbra of the earth's shadow, and the smaller circles on _cd_ represent the moon at full. it will be seen, that, if the moon is full at _e_, she will just graze the umbra of the earth's shadow. in this case she will suffer no eclipse. were the moon full at any point nearer her node, as at _f_, she would pass into the umbra of the earth's shadow, and would be _partially_ eclipsed. were the moon full at _g_, she would pass through the centre of the earth's shadow, and be _totally_ eclipsed. it will be seen from the figure that full moon must occur when the moon is within a certain distance from her node, in order that there may be a lunar eclipse; and this space is called the _lunar ecliptic limits_. the farther the earth is from the sun, the less rapidly does its shadow taper, and therefore the greater its diameter at the distance of the moon; and, the nearer the moon to the earth, the greater the diameter of the earth's shadow at the distance of the moon. of course, the greater the diameter of the earth's shadow, the greater the ecliptic limits: hence the lunar ecliptic limits vary somewhat from time to time, according to the distance from the earth to the sun and from the earth to the moon. the limits within which an eclipse is inevitable under all circumstances are called the _minor ecliptic limits_; and those within which an eclipse is possible under some circumstances, the _major ecliptic limits_. [illustration: fig. .] . _lunar eclipses._--fig. shows the path of the moon through the earth's shadow in the case of a _partial eclipse_. the magnitude of such an eclipse depends upon the nearness of the moon to her nodes. the magnitude of an eclipse is usually denoted in _digits_, a digit being one-twelfth of the diameter of the moon. [illustration: fig. .] fig. shows the path of the moon through the earth's shadow in the case of a _total eclipse_. it will be seen from the figure that it is not necessary for the moon to pass through the centre of the earth's shadow in order to have a total eclipse. when the moon passes through the centre of the earth's shadow, the eclipse is both _total_ and _central_. at the time of a total eclipse, the moon is not entirely invisible, but shines with a faint copper-colored light. this light is refracted into the shadow by the earth's atmosphere, and its amount varies with the quantity of clouds and vapor in that portion of the atmosphere which the sunlight must graze in order to reach the moon. the duration of an eclipse varies between very wide limits, being, of course, greatest when the eclipse is central. a total eclipse of the moon may last nearly two hours, or, including the _partial_ portions of the eclipse, three or four hours. every eclipse of the moon, whether total or partial, is visible at the same time to the whole hemisphere of the earth which is turned towards the moon; and the eclipse will have exactly the same magnitude at every point of observation. . _when there will be an eclipse of the sun._--there will be an eclipse of the sun _whenever any portion of the moon's shadow is thrown on the earth_. it will be seen from fig. that this can occur only when the moon is in conjunction, or at _new_. it does not occur every month, because, owing to the inclination of the moon's orbit to the ecliptic, the moon's shadow is usually thrown either above or below the earth at the time of new moon. there can be an eclipse of the sun only when new moon occurs at or near one of the nodes of her orbit. . _solar ecliptic limits._--the distances from the moon's node within which a new moon would throw some portion of its shadow on the earth so as to produce an eclipse of the sun are called the _solar ecliptic limits_. as in the case of the moon, there are _major_ and _minor_ ecliptic limits; the former being the limits within which an eclipse of the sun is _possible_ under some circumstances, and the latter those under which an eclipse is _inevitable_ under all circumstances. the limits within which a solar eclipse may occur are greater than those within which a lunar eclipse may occur. this will be evident from an examination of fig. . were the moon in that figure just outside of the lines _ab_ and _cd_, it will be seen that the penumbra of her shadow would just graze the earth: hence the moon must be somewhere within the space bounded by these lines in order to cause an eclipse of the sun. now, these lines mark the prolongation to the sun of the cone of the umbra of the earth's shadow: hence, in order to produce an eclipse of the sun, new moon must occur somewhere within this prolongation of the umbra of the earth's shadow. now, it is evident that the diameter of this cone is greater on the side of the earth toward the sun than on the opposite side: hence the solar ecliptic limits are greater than the lunar ecliptic limits. . _solar eclipses._--an observer in the umbra of the moon's shadow would see a _total_ eclipse of the sun, while one in the penumbra would see only a _partial_ eclipse. the magnitude of this partial eclipse would depend upon the distance of the observer from the umbra of the moon's shadow. [illustration: fig. .] [illustration: fig. .] the umbra of the moon's shadow is just about long enough to reach the earth. sometimes the point of this shadow falls short of the earth's surface, as shown in fig. , and sometimes it falls upon the earth, as shown in fig. , according to the varying distance of the sun and moon from the earth. the diameter of the umbra at the surface of the earth is seldom more than a hundred miles: hence the belt of a total eclipse is, on the average, not more than a hundred miles wide; and a total eclipse seldom lasts more than five or six minutes, and sometimes only a few seconds. owing, however, to the rotation of the earth, the umbra of the moon's shadow may pass over a long reach of the earth's surface. fig. shows the track of the umbra of the moon's shadow over the earth in the total eclipse of . [illustration: fig. .] [illustration: fig. .] fig. shows the track of the total eclipse of across india and the adjacent seas. [illustration: fig. .] [illustration: fig. .] in a partial eclipse of the sun, more or less of one side of the sun's disk is usually concealed, as shown in fig. . occasionally, however, the centre of the sun's disk is covered, leaving a bright ring around the margin, as shown in fig. . such an eclipse is called an _annular_ eclipse. an eclipse can be annular only when the cone of the moon's shadow is too short to reach the earth, and then only to observers who are in the central portion of the penumbra. . _comparative frequency of solar and lunar eclipses._--there are more eclipses of the sun in the year than of the moon; and yet, at any one place, eclipses of the moon are more frequent than those of the sun. there are more lunar than solar eclipses, because, as we have seen, the limits within which a solar eclipse may occur are greater than those within which a lunar eclipse may occur. there are more eclipses of the moon visible at any one place than of the sun; because, as we have seen, an eclipse of the moon, whenever it does occur, is visible to a whole hemisphere at a time, while an eclipse of the sun is visible to only a portion of a hemisphere, and a total eclipse to only a very small portion of a hemisphere. a total eclipse of the sun is, therefore, a very rare occurrence at any one place. the greatest number of eclipses that can occur in a year is seven, and the least number, two. in the former case, five may be of the sun and two of the moon, or four of the sun and three of the moon. in the latter case, both must be of the sun. vi. the three groups of planets. i. general characteristics of the groups. . _the inner group._--the _inner group_ of planets is composed of _mercury_, _venus_, the _earth_, and _mars_; that is, of all the planets which lie between the asteroids and the sun. the planets of this group are comparatively small and dense. so far as known, they rotate on their axes in about twenty-four hours, and they are either entirely without moons, or are attended by comparatively few. the comparative sizes and eccentricities of the orbits of this group are shown in fig. . the dots round the orbits show the position of the planets at intervals of ten days. [illustration: fig. .] . _the outer group._--the _outer group_ of planets is composed of _jupiter_, _saturn_, _uranus_, and _neptune_. these planets are all very large and of slight density. so far as known, they rotate on their axes in about ten hours, and are accompanied with complicated systems of moons. fig. , which represents the comparative sizes of the planets, shows at a glance the immense difference between those of the inner and outer group. fig. shows the comparative sizes and eccentricities of the orbits of the outer planets. the dots round the orbits show the position of the planets at intervals of a thousand days. [illustration: fig. .] [illustration: fig. .] . _the asteroids._--between the inner and outer groups of planets there is a great number of very small planets known as the _minor planets_, or _asteroids_. over two hundred planets belonging to this group have already been discovered. their orbits are shown by the dotted lines in fig. . the sizes of the four largest of these planets, compared with the earth, are shown in fig. . [illustration: fig. .] the asteroids of this group are distinguished from the other planets, not only by their small size, but by the great eccentricities and inclinations of their orbits. if we except mercury, none of the larger planets has an eccentricity amounting to one-tenth the diameter of its orbit ( ), nor is any orbit inclined more than two or three degrees to the ecliptic; but the inclinations of many of the minor planets exceed ten degrees, and the eccentricities frequently amount to an eighth of the orbital diameter. the orbit of pallas is inclined thirty-four degrees to the ecliptic, while there are some planets of this group whose orbits nearly coincide with the plane of the ecliptic. [illustration: fig. .] fig. shows one of the most and one of the least eccentric of the orbits of this group as compared with that of the earth. [illustration: fig. .] the intricate complexity of the orbits of the asteroids is shown in fig. . ii. the inner group of planets. mercury. . _the orbit of mercury._--the orbit of mercury is more eccentric than that of any of the larger planets, and it has also a greater inclination to the ecliptic. its eccentricity ( ) is a little over a fifth, and its inclination to the ecliptic somewhat over seven degrees. the mean distance of mercury from the sun is about thirty-five million miles; but, owing to the great eccentricity of its orbit, its distance from the sun varies from about forty-three million miles at aphelion to about twenty-eight million at perihelion. [illustration: fig. .] . _distance of mercury from the earth._--it is evident, from fig. , that an inferior planet, like mercury, is the whole diameter of its orbit nearer the earth at inferior conjunction than at superior conjunction: hence mercury's distance from the earth varies considerably. owing to the great eccentricity of its orbit, its distance from the earth at inferior conjunction also varies considerably. mercury is nearest to the earth when its inferior conjunction occurs at its own aphelion and at the earth's perihelion. [illustration: fig. .] . _apparent size of mercury._--since mercury's distance from the earth is variable, the apparent size of the planet is also variable. fig. shows its apparent size at its extreme and mean distances from the earth. its apparent diameter varies from five seconds to twelve seconds. [illustration: fig. .] . _volume and density of mercury._--the real diameter of mercury is about three thousand miles. its size, compared with that of the earth, is shown in fig. . the earth is about sixteen times as large as mercury; but mercury is about one-fifth more dense than the earth. . _greatest elongation of mercury._--mercury, being an _inferior_ planet (or one within the orbit of the earth), appears to oscillate to and fro across the sun. its greatest apparent distance from the sun, or its _greatest elongation_, varies considerably. the farther mercury is from the sun, and the nearer the earth is to mercury, the greater is its angular distance from the sun at the time of its greatest elongation. under the most favorable circumstances, the greatest elongation amounts to about twenty-eight degrees, and under the least favorable to only sixteen or seventeen degrees. . _sidereal and synodical periods of mercury._--mercury accomplishes a complete revolution around the sun in about eighty-eight days; but it takes it a hundred and sixteen days to pass from its greatest elongation east to the same elongation again. the orbital motion of this planet is at the rate of nearly thirty miles a second. in fig. , _p'''_ represents elongation east of the sun, and _p'_ elongation west. it will be seen that it is much farther from _p'_ around to _p'''_ than from _p'''_ on to _p'_. mercury is only about forty-eight days in passing from greatest elongation east to greatest elongation west, while it is about sixty-eight days in passing back again. . _visibility of mercury._--mercury is too close to the sun for favorable observation. it is never seen long after sunset, or long before sunrise, and never far from the horizon. when visible at all, it must be sought for low down in the west shortly after sunset, or low in the east shortly before sunrise, according as the planet is at its east or west elongation. it is often visible to the naked eye in our latitude; but the illumination of the twilight sky, and the excess of vapor in our atmosphere near the horizon, combine to make the telescopic study of the planet difficult and unsatisfactory. [illustration: fig. .] . _the atmosphere and surface of mercury._--mercury seems to be surrounded by a dense atmosphere. one proof of the existence of such an atmosphere is furnished at the time of the planet's _transit_ across the disk of the sun, which occasionally happens. the planet is then seen surrounded by a border, as shown in fig. . a bright spot has also been observed on the dark disk of the planet during a transit, as shown in fig. . the border around the planet seems to be due to the action of the planet's atmosphere; but it is difficult to account for the bright spot. [illustration: fig. .] [illustration: fig. .] schröter, a celebrated german astronomer, at about the beginning of the present century, thought that he detected spots and shadings on the disk of the planet, which indicated both the presence of an atmosphere and of elevations. the shading along the terminator, which seemed to indicate the presence of a twilight, and therefore of an atmosphere, are shown in fig. . it also shows the blunted aspect of one of the cusps, which schröter noticed at times, and which he attributed to the shadow of a mountain, estimated to be ten or twelve miles high. fig. shows this mountain near the upper cusp, as schröter believed he saw it in the year . by watching certain marks upon the disk of mercury, schröter came to the conclusion that the planet rotates on its axis in about twenty-four hours. modern observers, with more powerful telescopes, have failed to verify schröter's observations as to the indications of an atmosphere and of elevations. nothing is known with certainty about the rotation of the planet. [illustration: fig. .] the border around mercury, and the bright spot on its disk at the time of the transit of the planet across the sun, have been seen since schröter's time, and the existence of these phenomena is now well established; but astronomers are far from being agreed as to their cause. . _intra-mercurial planets._--it has for some time been thought probable that there is a group of small planets between mercury and the sun; and at various times the discovery of such bodies has been announced. in a french observer believed that he had detected an intra-mercurial planet, to which the name of _vulcan_ was given, and for which careful search has since been made, but without success. during the total eclipse of professor watson observed two objects near the sun, which he thought to be planets; but this is still matter of controversy. venus. . _the orbit of venus._--the orbit of venus has but slight eccentricity, differing less from a circle than that of any other large planet. it is inclined to the ecliptic somewhat more than three degrees. the mean distance of the planet from the sun is about sixty-seven million miles. . _distance of venus from the earth._--the distance of venus from the earth varies within much wider limits than that of mercury. when venus is at inferior conjunction, her distance from the earth is ninety-two million miles _minus_ sixty-seven million miles, or twenty-five million miles; and when at superior conjunction it is ninety-two million miles _plus_ sixty-seven million miles, or a hundred and fifty-nine million miles. venus is considerably more than _six times_ as far off at superior conjunction as at inferior conjunction. [illustration: fig. .] . _apparent size of venus._--owing to the great variation in the distance of venus from the earth, her apparent diameter varies from about ten seconds to about sixty-six seconds. fig. shows the apparent size of venus at her extreme and mean distances from the earth. . _volume and density of venus._--the real size of venus is about the same as that of the earth, her diameter being only about three hundred miles less. the comparative sizes of the two planets are shown in fig. . the density of venus is a little less than that of the earth. [illustration: fig. .] . _the greatest elongation of venus._--venus, like mercury, appears to oscillate to and fro across the sun. the angular value of the greatest elongation of venus varies but slightly, its greatest value being about forty-seven degrees. . _sidereal and synodical periods of venus._--the _sidereal_ period of venus, or that of a complete revolution around the sun, is about two hundred and twenty-five days; her orbital motion being at the rate of nearly twenty-two miles a second. her _synodical_ period, or the time it takes her to pass around from her greatest eastern elongation to the same elongation again, is about five hundred and eighty-four days, or eighteen months. venus is a hundred and forty-six days, or nearly five months, in passing from her greatest elongation east through inferior conjunction to her greatest elongation west. . _venus as a morning and an evening star._--for a period of about nine months, while venus is passing from superior conjunction to her greatest eastern elongation, she will be east of the sun, and will therefore set after the sun. during this period she is the _evening star_, the _hesperus_ of the ancients. while passing from inferior conjunction to superior conjunction, venus is west of the sun, and therefore rises before the sun. during this period of nine months she is the _morning star_, the _phosphorus_, or _lucifer_, of the ancients. . _brilliancy of venus._--next to the sun and moon, venus is at times the most brilliant object in the heavens, being bright enough to be seen in daylight, and to cast a distinct shadow at night. her brightness, however, varies considerably, owing to her phases and to her varying distance from the earth. she does not appear brightest when at full, for she is then farthest from the earth, at superior conjunction; nor does she appear brightest when nearest the earth, at inferior conjunction, for her phase is then a thin crescent (see fig. ). she is most conspicuous while passing from her greatest eastern elongation to her greatest western elongation. after she has passed her eastern elongation, she becomes brighter and brighter till she is within about forty degrees of the sun. her phase at this point in her orbit is shown in fig. . her brilliancy then begins to wane, until she comes too near the sun to be visible. when she re-appears on the west of the sun, she again becomes more brilliant; and her brilliancy increases till she is about forty degrees from the sun, when she is again at her brightest. venus passes from her greatest brilliancy as an evening star to her greatest brilliancy as a morning star in about seventy-three days. she has the same phase, and is at the same distance from the earth, in both cases of maximum brilliancy. of course, the brilliancy of venus when at the maximum varies somewhat from time to time, owing to the eccentricities of the orbits of the earth and of venus, which cause her distance from the earth, at her phase of greatest brilliancy, to vary. she is most brilliant when the phase of her greatest brilliancy occurs when she is at her aphelion and the earth at its perihelion. [illustration: fig. .] . _the atmosphere and surface of venus._--schröter believed that he saw shadings and markings on venus similar to those on mercury, indicating the presence of an atmosphere and of elevations on the surface of the planet. fig. represents the surface of venus as it appeared to this astronomer. by watching certain markings on the disk of venus, schröter came to the conclusion that venus rotates on her axis in about twenty-four hours. [illustration: fig. .] it is now generally conceded that venus has a dense atmosphere; but schröter's observations of the spots on her disk have not been verified by modern astronomers, and we really know nothing certainly of her rotation. . _transits of venus._--when venus happens to be near one of the nodes of her orbit when she is in inferior conjunction, she makes a transit across the sun's disk. these transits occur in pairs, separated by an interval of over a hundred years. the two transits of each pair are separated by an interval of eight years, the dates of the most recent being and . venus, like mercury, appears surrounded with a border on passing across the sun's disk, as shown in fig. . [illustration: fig. .] mars. . _the orbit of mars._--the orbit of mars is more eccentric than that of any of the larger planets, except mercury; its eccentricity being about one-eleventh. the inclination of the orbit to the ecliptic is somewhat under two degrees. the mean distance of mars from the sun is about a hundred and forty million miles; but, owing to the eccentricity of his orbit, the distance varies from a hundred and fifty-three million miles to a hundred and twenty-seven million miles. [illustration: fig. .] . _distance of mars from the earth._--it will be seen, from fig. , that a _superior_ planet (or one outside the orbit of the earth), like mars, is nearer the earth, by the whole diameter of the earth's orbit, when in opposition than when in conjunction. the mean distance of mars from the earth, at the time of opposition, is a hundred and forty million miles _minus_ ninety-two million miles, or forty-eight million miles. owing to the eccentricity of the orbit of the earth and of mars, the distance of this planet when in opposition varies considerably. when the earth is in aphelion, and mars in perihelion, at the time of opposition, the distance of the planet from the earth is only about thirty-three million miles. on the other hand, when the earth is in perihelion, and mars in aphelion, at the time of opposition, the distance of the planet is over sixty-two million miles. the mean distance of mars from the earth when in conjunction is a hundred and forty million miles _plus_ ninety-two million miles, or two hundred and thirty-two million miles. it will therefore be seen that mars is nearly five times as far off at conjunction as at opposition. [illustration: fig. .] . _the apparent size of mars._--owing to the varying distance of mars from the earth, the apparent size of the planet varies almost as much as that of venus. fig. shows the apparent size of mars at its extreme and mean distances from the earth. the apparent diameter varies from about four seconds to about thirty seconds. [illustration: fig. .] . _the volume and density of mars._--among the larger planets mars is next in size to mercury. its real diameter is somewhat more than four thousand miles, and its bulk is about one-seventh of that of the earth. its size, compared with that of the earth, is shown in fig. . [illustration: plate .] the density of mars is only about three-fourths of that of the earth. . _sidereal and synodical periods of mars._--the _sidereal_ period of mars, or the time in which he makes a complete revolution around the sun, is about six hundred and eighty-seven days, or nearly twenty-three months; but he is about seven hundred and eighty days in passing from opposition to opposition again, or in performing a _synodical_ revolution. mars moves in his orbit at the rate of about fifteen miles a second. . _brilliancy of mars._--when near his opposition, mars is easily recognized with the naked eye by his fiery-red light. he is much more brilliant at some oppositions than at others, for reasons already explained ( ), but always shines brighter than an ordinary star of the first magnitude. . _telescopic appearance of mars._--when viewed with a good telescope (see plate iv.), mars is seen to be covered with dusky, dull-red patches, which are supposed to be continents, like those of our own globe. other portions, of a greenish hue, are believed to be tracts of water. the ruddy color, which overpowers the green, and makes the whole planet seem red to the naked eye, was believed by sir j. herschel to be due to an ochrey tinge in the general soil, like that of the red sandstone districts on the earth. in a telescope, mars appears less red, and the higher the power the less the intensity of the color. the disk, when well seen, is mapped out in a way which gives at once the impression of land and water. the bright part is red inclining to orange, sometimes dotted with brown and greenish points. the darker spaces, which vary greatly in depth of tone, are of a dull gray-green, having the aspect of a fluid which absorbs the solar rays. the proportion of land to water on the earth appears to be reversed on mars. on the earth every continent is an island; on mars all seas are lakes. long, narrow straits are more common than on the earth; and wide expanses of water, like our atlantic ocean, are rare. (see fig. .) [illustration: fig. .] [illustration: fig. .] fig. represents a chart of the surface of mars, which has been constructed from careful telescopic observation. the outlines, as seen in the telescope, are, however, much less distinct than they are represented here; and it is by no means certain that the light and dark portions are bodies of land and water. in the vicinity of the poles brilliant white spots may be noticed, which are considered by many astronomers to be masses of snow. this conjecture is favored by the fact that they appear to diminish under the sun's influence at the beginning of the martial summer, and to increase again on the approach of winter. . _rotation of mars._--on watching mars with a telescope, the spots on the disk are found to move (as shown in fig. ) in a manner which indicates that the planet rotates in about twenty-four hours on an axis inclined about twenty-eight degrees from a perpendicular to the plane of its orbit. the inclination of the axis is shown in fig. . it is evident from the figure that the variation in the length of day and night, and the change of seasons, are about the same on mars as on the earth. the changes will, of course, be somewhat greater, and the seasons will be about twice as long. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] . _the satellites of mars._--in professor hall of the washington observatory discovered that mars is accompanied by two small moons, whose orbits are shown in fig. . the inner satellite has been named _phobos_, and the outer one _deimos_. it is estimated that the diameter of the outer moon is from five to ten miles, and that of the inner one from ten to forty miles. phobos is remarkable for its nearness to the planet and the rapidity of its revolution, which is performed in seven hours thirty-eight minutes. its distance from the centre of the planet is about six thousand miles, and from the surface less than four thousand. astronomers on mars, with telescopes and eyes like ours, could readily find out whether this satellite is inhabited, the distance being less than one-sixtieth of that of our moon. it will be seen that phobos makes about three revolutions to one rotation of the planet. it will, of course, rise in the west; though the sun, the stars, and the other satellite rise in the east. deimos makes a complete revolution in about thirty hours. iii. the asteroids. . _bode's law of planetary distances._--there is a very remarkable law connecting the distances of the planets from the sun, which is generally known by the name of _bode's law_. attention was drawn to it in by the astronomer bode, but he was not really its author. to express this law we write the following series of numbers:-- , , , , , , ; each number, with the exception of the first, being double the one which precedes it. if we add to each of these numbers, the series becomes-- , , , , , , ; which series was known to kepler. these numbers, with the exception of , are sensibly proportional to the distances of the principal planets from the sun, the actual distances being as follows:-- mercury. venus. earth. mars. ---- jupiter. saturn. · · · · · . _the first discovery of the asteroids._--the great gap between mars and jupiter led astronomers, from the time of kepler, to suspect the existence of an unknown planet in this region; but no such planet was discovered till the beginning of the present century. _ceres_ was discovered jan. , , _pallas_ in , _juno_ in , and _vesta_ in . then followed a long interval of thirty-eight years before _astræa_, the fifth of these minor planets, was discovered in . . _olbers's hypothesis._--after the discovery of pallas, olbers suggested his celebrated hypothesis, that the two bodies might be fragments of a single planet which had been shattered by some explosion. if such were the case, the orbits of all the fragments would at first intersect each other at the point where the explosion occurred. he therefore thought it likely that other fragments would be found, especially if a search were kept up near the intersection of the orbits of ceres and pallas. professor newcomb makes the following observations concerning this hypothesis:-- "the question whether these bodies could ever have formed a single one has now become one of cosmogony rather than of astronomy. if a planet were shattered, the orbit of each fragment would at first pass through the point at which the explosion occurred, however widely they might be separated through the rest of their course; but, owing to the secular changes produced by the attractions of the other planets, this coincidence would not continue. the orbits would slowly move away, and after the lapse of a few thousand years no trace of a common intersection would be seen. it is therefore curious that olbers and his contemporaries should have expected to find such a region of intersection, as it implied that the explosion had occurred within a few thousand years. the fact that the required conditions were not fulfilled was no argument against the hypothesis, because the explosion might have occurred millions of years ago; and in the mean time the perihelion and node of each orbit would have made many entire revolutions, so that the orbits would have been completely mixed up.... a different explanation of the group is given by the nebular hypothesis; so that olbers's hypothesis is no longer considered by astronomers." . _later discoveries of asteroids._--since over two hundred asteroids have been discovered. all these are so small, that it requires a very good telescope to see them; and even in very powerful telescopes they appear as mere points of light, which can be distinguished from the stars only by their motions. to facilitate the discovery of these bodies, very accurate maps have been constructed, including all the stars down to the thirteenth magnitude in the neighborhood of the ecliptic. a reduced copy of one of these maps is shown in fig. . [illustration: fig. .] furnished with a map of this kind, and with a telescope powerful enough to show all the stars marked on it, the observer who is searching for these small planets will place in the field of view of his telescope six spider-lines at right angles to each other, and at equal distances apart, in such a manner that several small squares will be formed, embracing just as much of the heavens as do those shown in the map. he will then direct his telescope to the region of the sky he wishes to examine, represented by the map, so as to be able to compare successively each square with the corresponding portion of the sky. fig. shows at the right hand the squares in the telescopic field of view, and at the left hand the corresponding squares of the map. [illustration: fig. .] he can then assure himself if the numbers and positions of the stars mapped, and of the stars observed, are identical. if he observes in the field of view a luminous point which is not marked in the map, it is evident that either the new body is a star of variable brightness which was not visible at the time the map was made, or it is a planet, or perhaps a comet. if the new body remains fixed at the same point, it is the former; but, if it changes its position with regard to the neighboring stars, it is the latter. the motion is generally so sensible, that in the course of one evening the change of position may be detected; and it can soon be determined, by the direction and rate of the motion, whether the body is a planet or a comet. iv. outer group of planets. jupiter. . _orbit of jupiter._--the orbit of jupiter is inclined only a little over one degree to the ecliptic; and its eccentricity is only about half of that of mars, being less than one-twentieth. the mean distance of jupiter from the sun is about four hundred and eighty million miles; but, owing to the eccentricity of his orbit, his actual distance from the sun ranges from four hundred and fifty-seven to five hundred and three million miles. . _distance of jupiter from the earth._--when jupiter is in opposition, his mean distance from the earth is four hundred and eighty million miles _minus_ ninety-two million miles, or three hundred and eighty-eight million miles, and, when he is in conjunction, four hundred and eighty million miles _plus_ ninety-two million miles, or five hundred and seventy-two million miles. it will be seen that he is less than twice as far off in conjunction as in opposition, and that the ratio of his greatest to his least distance is very much less than in the case of venus and mars. this is owing to his very much greater distance from the sun. owing to the eccentricities of the orbits of the earth and of jupiter, the greatest and least distances of jupiter from the earth vary somewhat from year to year. [illustration: fig. .] . _the brightness and apparent size of jupiter._--the apparent diameter of jupiter varies from about fifty seconds to about thirty seconds. his apparent size at his extreme and mean distances from the earth is shown in fig. . jupiter shines with a brilliant white light, which exceeds that of every other planet except venus. the planet is, of course, brightest when near opposition. . _the volume and density of jupiter._--jupiter is the "giant planet" of our system, his mass largely exceeding that of all the other planets combined. his mean diameter is about eighty-five thousand miles; but the equatorial exceeds the polar diameter by five thousand miles. in volume he exceeds our earth about thirteen hundred times, but in mass only about two hundred and thirteen times. his specific gravity is, therefore, far less than that of the earth, and even less than that of water. the comparative size of jupiter and the earth is shown in fig. . [illustration: fig. .] . _the sidereal and synodical periods of jupiter._--it takes jupiter nearly twelve years to make a _sidereal_ revolution, or a complete revolution around the sun, his orbital motion being at the rate of about eight miles a second. his _synodical_ period, or the time of his passage from opposition to opposition again, is three hundred and ninety-eight days. . _the telescopic aspect of jupiter._--there are no really permanent markings on the disk of jupiter; but his surface presents a very diversified appearance. the earlier telescopic observers descried dark belts across it, one north of the equator, and the other south of it. with the increase of telescopic power, it was seen that these bands were of a more complex structure than had been supposed, and consisted of stratified, cloud-like appearances, varying greatly in form and number. these change so rapidly, that the face of the planet rarely presents the same appearance on two successive nights. they are most strongly marked at some distance on each side of the planet's equator, and thus appear as two belts under a low magnifying power. both the outlines of the belts, and the color of portions of the planet, are subject to considerable changes. the equatorial regions, and the spaces between the belts generally, are often of a rosy tinge. this color is sometimes strongly marked, while at other times hardly a trace of it can be seen. a general telescopic view of jupiter is given in plate v. [illustration: plate .] . _the physical constitution of jupiter._--from the changeability of the belts, and of nearly all the visible features of jupiter, it is clear that what we see on that planet is not the solid nucleus, but cloud-like formations, which cover the entire surface to a great depth. the planet appears to be covered with a deep and dense atmosphere, filled with thick masses of clouds and vapor. until recently this cloud-laden atmosphere was supposed to be somewhat like that of our globe; but at present the physical constitution of jupiter is believed to resemble that of the sun rather than that of the earth. like the sun, he is brighter in the centre than near the edges, as is shown in the transits of the satellites over his disk. when the satellite first enters on the disk, it commonly seems like a bright spot on a dark background; but, as it approaches the centre, it appears like a dark spot on the bright surface of the planet. the centre is probably two or three times brighter than the edges. this may be, as in the case of the sun, because the light near the edge passes through a greater depth of atmosphere, and is diminished by absorption. it has also been suspected that jupiter shines partly by his own light, and not wholly by reflected sunlight. the planet cannot, however, emit any great amount of light; for, if it did, the satellites would shine by this light when they are in the shadow of the planet, whereas they totally disappear. it is possible that the brighter portions of the surface are from time to time slightly self-luminous. [illustration: fig. .] again: the interior of jupiter seems to be the seat of an activity so enormous that it can be ascribed only to intense heat. rapid movements are always occurring on his surface, often changing its aspect in a few hours. it is therefore probable that jupiter is not yet covered by a solid crust, and that the fiery interior, whether liquid or gaseous, is surrounded by the dense vapors which cease to be luminous on rising into the higher and cooler regions of the atmosphere. figs. and show the disk of jupiter as it appeared in december, . [illustration: fig. .] . _rotation of jupiter_.--spots are sometimes visible which are much more permanent than the ordinary markings on the belts. the most remarkable of these is "the great red spot," which was first observed in july, , and is still to be seen in february, . it is shown just above the centre of the disk in fig. . by watching these spots from day to day, the time of jupiter's axial rotation has been found to be about nine hours and fifty minutes. the axis of jupiter deviates but slightly from a perpendicular to the plane of its orbit, as is shown in fig. . [illustration: fig. .] the satellites of jupiter. [illustration: fig. .] . _jupiter's four moons._--jupiter is accompanied by four moons, as shown in fig. . the diameters of these moons range from about twenty-two hundred to thirty-seven hundred miles. the second from the planet is the smallest, and the third the largest. the smallest is about the size of our moon; the largest considerably exceeds mercury, and almost rivals mars, in bulk. the sizes of these moons, compared with those of the earth and its moon, are shown in fig. . [illustration: fig. .] the names of these satellites, in the order of their distance from the planet, are _io_, _europa_, _ganymede_, and _callisto_. their times of revolution range from about a day and three-fourths up to about sixteen days and a half. their orbits are shown in fig. . [illustration: fig. .] . _the variability of jupiter's satellites._--remarkable variations in the light of these moons have led to the supposition that violent changes are taking place on their surfaces. it was formerly believed, that, like our moon, they always present the same face to the planet, and that the changes in their brilliancy are due to differences in the luminosity of parts of their surface which are successively turned towards us during a revolution; but careful measurements of their light show that this hypothesis does not account for the changes, which are sometimes very sudden. the satellites are too distant for examination of their surfaces with the telescope: hence it is impossible to give any certain explanation of these phenomena. [illustration: fig. .] . _eclipses of jupiter's satellites._--jupiter, like the earth, casts a shadow away from the sun, as shown in fig. ; and, whenever one of his moons passes into this shadow, it becomes eclipsed. on the other hand, whenever one of the moons throws its shadow on jupiter, the sun is eclipsed to that part of the planet which lies within the shadow. to the inhabitants of jupiter (if there are any, and if they can see through the clouds) these eclipses must be very familiar affairs; for in consequence of the small inclinations of the orbits of the satellites to the planet's equator, and the small inclination of the latter to the plane of jupiter's orbit, all the satellites, except the most distant one, are eclipsed in every revolution. a spectator on jupiter might therefore witness during the planetary year forty-five hundred eclipses of the moons, and about the same number of the sun. [illustration: fig. .] . _transits of jupiter's satellites._--whenever one of jupiter's moons passes in front of the planet, it is said to make a _transit_ across his disk. when a moon is making a transit, it presents its bright hemisphere towards the earth, as will be seen from fig. : hence it is usually seen as a bright spot on the planet's disk; though sometimes, on the brighter central portions of the disk, it appears dark. [illustration: fig. .] it will be seen from fig. that the shadow of a moon does not fall upon the part of the planet's disk that is covered by the moon: hence we may observe the transit of both the moon and its shadow. the shadow appears as a small black spot, which will precede or follow the moon according to the position of the earth in its orbit. fig. shows two moons of jupiter in transit. . _occultations of jupiter's satellites._--the eclipse of a moon of jupiter must be carefully distinguished from the _occultation_ of a moon by the planet. in the case of an eclipse, the moon ceases to be visible, because the mass of jupiter is interposed between the sun and the moon, which ceases to be luminous, because the sun's light is cut off; but, in the case of an occupation, the moon gets into such a position that the body of jupiter is interposed between it and the earth, thus rendering the moon invisible to us. the third satellite, _m''_ (fig. ), is invisible from the earth _e_, having become _occulted_ when it passed behind the planet's disk; but it will not be _eclipsed_ until it passes into the shadow of jupiter. . _jupiter without satellites._--it occasionally happens that every one of jupiter's satellites will disappear at the same time, either by being eclipsed or occulted, or by being in transit. in this event, jupiter will appear without satellites. this occurred on the st of august, . the position of jupiter's satellites at this time is shown in fig. . [illustration: fig. .] saturn. the planet and his moons. . _the orbit of saturn._--the orbit of saturn is rather more eccentric than that of jupiter, its eccentricity being somewhat more than one-twentieth. its inclination to the ecliptic is about two degrees and a half. the mean distance of saturn from the sun is about eight hundred and eighty million miles. it is about a hundred million miles nearer the sun at perihelion than at aphelion. . _distance of saturn from the earth._--the mean distance of saturn from the earth at opposition is eight hundred and eighty million miles _minus_ ninety-two million miles, or seven hundred and eighty-eight million; and at conjunction, eight hundred and eighty million miles _plus_ ninety-two million, or nine hundred and seventy-two million. owing to the eccentricity of the orbit of saturn, his distance from the earth at opposition and at conjunction varies by about a hundred million miles at different times; but he is so immensely far away, that this is only a small fraction of his mean distance. . _apparent size and brightness of saturn._--the apparent diameter of saturn varies from about twenty seconds to about fourteen seconds. his apparent size at his extreme and mean distances from the earth is shown in fig. . [illustration: fig. .] the planet generally shines with the brilliancy of a moderate first-magnitude star, and with a dingy, reddish light, as if seen through a smoky atmosphere. . _volume and density of saturn._--the real diameter of saturn is about seventy thousand miles, and its volume over seven hundred times that of the earth. the comparative size of the earth and saturn is shown in fig. . this planet is a little more than half as dense as jupiter. [illustration: fig. .] . _the sidereal and synodical periods of saturn._--saturn makes a complete revolution round the sun in a period of about twenty-nine years and a half, moving in his orbit at the rate of about six miles a second. the planet passes from opposition to opposition again in a period of three hundred and seventy-eight days, or thirteen days over a year. . _physical constitution of saturn._--the physical constitution of saturn seems to resemble that of jupiter; but, being twice as far away, the planet cannot be so well studied. the farther an object is from the sun, the less it is illuminated; and, the farther it is from the earth, the smaller it appears: hence there is a double difficulty in examining the more distant planets. under favorable circumstances, the surface of saturn is seen to be diversified with very faint markings; and, with high telescopic powers, two or more very faint streaks, or belts, may be discerned parallel to its equator. these belts, like those of jupiter, change their aspect from time to time; but they are so faint that the changes cannot be easily followed. it is only on rare occasions that the time of rotation can be determined from a study of the markings. . _rotation of saturn._--on the evening of dec. , , professor hall, who had been observing the satellites of saturn with the great washington telescope ( ), saw a brilliant white spot near the equator of the planet. it seemed as if an immense eruption of incandescent matter had suddenly burst up from the interior. the spot gradually spread itself out into a long light streak, of which the brightest point was near the western end. it remained visible until january, when it became faint and ill-defined, and the planet was lost in the rays of the sun. from all the observations on this spot, professor hall found the period of saturn to be ten hours fourteen minutes, reckoning by the brightest part of the streak. had the middle of the streak been taken, the time would have been less, because the bright matter seemed to be carried along in the direction of the planet's rotation. if this motion was due to a wind, the velocity of the current must have been between fifty and a hundred miles an hour. the axis of saturn is inclined twenty-seven degrees from the perpendicular to its orbit. [illustration: fig. .] . _the satellites of saturn._--saturn is accompanied by eight moons. seven of these are shown in fig. . the names of these satellites, in the order of their distances from the planet, are given in the accompanying table:-- number. name. distance sidereal discoverer. from period. planet mimas , . herschel enceladus , . herschel tethys , . cassini dione , . cassini rhea , . cassini titan , . huyghens hyperion , . bond japetus , , . cassini the apparent brightness or visibility of these satellites follows the order of their discovery. the smallest telescope will show titan, and one of very moderate size will show japetus in the western part of its orbit. an instrument of four or five inches aperture will show rhea, and perhaps tethys and dione; while seven or eight inches are required for enceladus, even at its greatest elongation from the planet. mimas can rarely be seen except at its greatest elongation, and then only with an aperture of twelve inches or more. hyperion can be detected only with the most powerful telescopes, on account of its faintness and the difficulty of distinguishing it from minute stars. _japetus_, the outermost satellite, is remarkable for the fact, that while, in one part of its orbit, it is the brightest of the satellites except titan, in the opposite part it is almost as faint as hyperion, and can be seen only in large telescopes. when west of the planet, it is bright; when east of it, faint. this peculiarity has been accounted for by supposing that the satellite, like our moon, always presents the same face to the planet, and that one side of it is white and the other intensely black; but it is doubtful whether any known substance is so black as one side of the satellite must be to account for such extraordinary changes of brilliancy. [illustration: fig. .] _titan_, the largest of these satellites, is about the size of the largest satellite of jupiter. the relative sizes of the satellites are shown in fig. , and their orbits in fig. . [illustration: fig. .] [illustration: fig. .] fig. shows the transit of one of the satellites, and of its shadow, across the disk of the planet. the rings of saturn. . _general appearance of the rings._--saturn is surrounded by a thin flat ring lying in the plane of its equator. this ring is probably less than a hundred miles thick. the part of it nearest saturn reflects little sunlight to us; so that it has a dusky appearance, and is not easily seen, although it is not quite so dark as the sky seen between it and the planet. the outer edge of this dusky portion of the ring is at a distance from saturn of between two and three times the earth's diameter. outside of this dusky part of the ring is a much brighter portion, and outside of this another, which is somewhat fainter, but still so much brighter than the dusky part as to be easily seen. the width of the brighter parts of the ring is over three times the earth's diameter. to distinguish the parts, the outer one is called ring _a_, the middle one ring _b_, and the dusky one ring _c_. between _a_ and _b_ is an apparently open space, nearly two thousand miles wide, which looks like a black line on the ring. other divisions in the ring have been noticed at times; but this is the only one always seen with good telescopes at times when either side of the ring is in view from the earth. the general telescopic appearance of the ring is shown in fig. . [illustration: fig. .] [illustration: fig. .] fig. shows the divisions of the rings as they were seen by bond. . _phases of saturn's ring._--the ring is inclined to the plane of the planet's orbit by an angle of twenty-seven degrees. the general aspect from the earth is nearly the same as from the sun. as the planet revolves around the sun, the axis and plane of the ring keep the same direction in space, just as the axis of the earth and the plane of the equator do. when the planet is in one part of its orbit, we see the upper or northern side of the ring at an inclination of twenty-seven degrees, the greatest angle at which the ring can ever be seen. this phase of the ring is shown in fig. . [illustration: fig. .] when the planet has moved through a quarter of a revolution, the edge of the ring is turned towards the sun and the earth; and, owing to its extreme thinness, it is visible only in the most powerful telescopes as a fine line of light, stretching out on each side of the planet. this phase of the ring is shown in fig. . [illustration: fig. .] all the satellites, except japetus, revolve very nearly in the plane of the ring: consequently, when the edge of the ring is turned towards the earth, the satellites seem to swing from one side of the planet to the other in a straight line, running along the thin edge of the ring like beads on a string. this phase affords the best opportunity of seeing the inner satellites, mimas and enceladus, which at other times are obscured by the brilliancy of the ring. [illustration: fig. .] fig. shows a phase of the ring intermediate between the last two. when the planet has moved ninety degrees farther, we again see the ring at an angle of twenty-seven degrees; but now it is the lower or southern side which is visible. when it has moved ninety degrees farther, the edge of the ring is again turned towards the earth and sun. [illustration: fig. .] the successive phases of saturn's ring during a complete revolution are shown in fig. . it will be seen that there are two opposite points of saturn's orbit in which the rings are turned edgewise to us, and two points half-way between the former in which the ring is seen at its maximum inclination of about twenty-seven degrees. since the planet performs a revolution in twenty-nine years and a half, these phases occur at average intervals of about seven years and four months. [illustration: fig. .] [illustration: fig. .] . _disappearance of saturn's ring._--it will be seen from fig. that the plane of the ring may not be turned towards the sun and the earth at exactly the same time, and also that the earth may sometimes come on one side of the plane of the ring while the sun is shining on the other. in the figure, _e_, _e'_, _e''_, and _e'''_ is the orbit of the earth. when saturn is at _s'_, or opposite, at _f_, the plane of the ring will pass through the sun, and then only the edge of the ring will be illumined. were saturn at _s_, and the earth at _e'_, the plane of the ring would pass through the earth. this would also be the case were the earth at _e'''_, and saturn at _s''_. were saturn at _s_ or at _s''_, and the earth farther to the left or to the right, the sun would be shining on one side of the ring while we should be looking on the other. in all these cases the ring will disappear entirely in a telescope of ordinary power. with very powerful telescopes the ring will appear, in the first two cases, as a thin line of light (fig. ). it will be seen that all these cases of disappearance must take place when saturn is in the parts of his orbit intercepted between the parallel lines _ac_ and _bd_. these lines are tangent to the earth's orbit, which they enclose, and are parallel to the plane of saturn's ring. as saturn passes away from these two lines on either side, the rings appear more and more open. when the dark side of the ring is in view, it appears as a black line crossing the planet; and on such occasions the sunlight reflected from the outer and inner edges of the rings _a_ and _b_ enables us to see traces of the ring on each side of saturn, at least in places where two such reflections come nearly together. fig. illustrates this reflection from the edges at the divisions of the rings. [illustration: fig. .] . _changes in saturn's ring._--the question whether changes are going on in the rings of saturn is still unsettled. some observers have believed that they saw additional divisions in the rings from time to time; but these may have been errors of vision, due partly to the shading which is known to exist on portions of the ring. professor newcomb says, "as seen with the great washington equatorial in the autumn of , there was no great or sudden contrast between the inner or dark edge of the bright ring and the outer edge of the dusky ring. there was some suspicion that the one shaded into the other by insensible gradations. no one could for a moment suppose, as some observers have, that there was a separation between these two rings. all these considerations give rise to the question whether the dusky ring may not be growing at the expense of the inner bright ring." struve, in , advanced the startling theory that the inner edge of the ring was gradually approaching the planet, the whole ring spreading inwards, and making the central opening smaller. the theory was based upon the descriptions and drawings of the rings by the astronomers of the seventeenth century, especially huyghens, and the measures made by later astronomers up to . this supposed change in the dimension of the ring is shown in fig. . [illustration: fig. .] . _constitution of saturn's ring._--the theory now generally held by astronomers is, that the ring is composed of a cloud of satellites too small to be separately seen in the telescope, and too close together to admit of visible intervals between them. the ring looks solid, because its parts are too small and too numerous to be seen singly. they are like the minute drops of water that make up clouds and fogs, which to our eyes seem like solid masses. in the dusky ring the particles may be so scattered that we can see through the cloud, the duskiness being due to the blending of light and darkness. some believe, however, that the duskiness is caused by the darker color of the particles rather than by their being farther apart. uranus. . _orbit and dimensions of uranus._--uranus, the smallest of the outer group of planets, has a diameter of nearly thirty-two thousand miles. it is a little less dense than jupiter, and its mean distance from the sun is about seventeen hundred and seventy millions of miles. its orbit has about the same eccentricity as that of jupiter, and is inclined less than a degree to the ecliptic. uranus makes a revolution around the sun in eighty-four years, moving at the rate of a little over four miles a second. it is visible to the naked eye as a star of the sixth magnitude. as seen in a large telescope, the planet has a decidedly sea-green color; but no markings have with certainty been detected on its disk, so that nothing is really known with regard to its rotation. fig. shows the comparative size of uranus and the earth. [illustration: fig. .] . _discovery of uranus._--this planet was discovered by sir william herschel in march, . he was engaged at the time in examining the small stars of the constellation _gemini_, or the twins. he noticed that this object which had attracted his attention had an appreciable disk, and therefore could not be a star. he also perceived by its motion that it could not be a nebula; he therefore concluded that it was a comet, and announced his discovery as such. on attempting to compute its orbit, it was soon found that its motions could be accounted for only on the supposition that it was moving in a circular orbit at about twice the distance of saturn from the sun. it was therefore recognized as a new planet, whose discovery nearly doubled the dimensions of the solar system as it was then known. . _the name of the planet._--herschel, out of compliment to his patron, george iii., proposed to call the new planet _georgium sidus_ (the georgian star); but this name found little favor. the name of _herschel_ was proposed, and continued in use in england for a time, but did not meet with general approval. various other names were suggested, and finally that of _uranus_ was adopted. [illustration: fig. .] . _the satellites of uranus._--uranus is accompanied by four satellites, whose orbits are shown in fig. . these satellites are remarkable for the great inclination of their orbits to the plane of the planet's orbit, amounting to about eighty degrees, and for their _retrograde_ motion; that is, they move _from east to west_, instead of from west to east, as in the case of all the planets and of all the satellites previously discovered. neptune. . _orbit and dimensions of neptune._--so far as known, neptune is the most remote member of the solar system, its mean distance from the sun being twenty-seven hundred and seventy-five million miles. this distance is considerably less than twice that of uranus. neptune revolves around the sun in a period of a little less than a hundred and sixty-five years. its orbit has but slight eccentricity, and is inclined less than two degrees to the ecliptic. this planet is considerably larger than uranus, its diameter being nearly thirty-five thousand miles. it is somewhat less dense than uranus. neptune is invisible to the naked eye, and no telescope has revealed any markings on its disk: hence nothing is certainly known as to its rotation. fig. shows the comparative size of neptune and the earth. [illustration: fig. .] . _the discovery of neptune._--the discovery of neptune was made in , and is justly regarded as one of the grandest triumphs of astronomy. soon after uranus was discovered, certain irregularities in its motion were observed, which could not be explained. it is well known that the planets are all the while disturbing each other's motions, so that none of them describe perfect ellipses. these mutual disturbances are called _perturbations_. in the case of uranus it was found, that, after making due allowance for the action of all the known planets, there were still certain perturbations in its course which had not been accounted for. this led astronomers to the suspicion that these might be caused by an unknown planet. leverrier in france, and adams in england, independently of each other, set themselves the difficult problem of computing the position and magnitude of a planet which would produce these perturbations. both, by a most laborious computation, showed that the perturbations were such as would be produced by a planet revolving about the sun at about twice the distance of uranus, and having a mass somewhat greater than that of this planet; and both pointed out the same part of the heavens as that in which the planet ought to be found at that time. almost immediately after they had announced the conclusion to which they had arrived, the planet was found with the telescope. the astronomer who was searching for the planet at the suggestion of leverrier was the first to recognize it: hence leverrier has obtained the chief credit of the discovery. the observed planet is proved to be nearer than the one predicted by leverrier and adams, and therefore of smaller magnitude. . _the observed planet not the predicted one._--professor peirce always maintained that the planet found by observation was not the one whose existence had been predicted by leverrier and adams, though its action would completely explain all the irregularities in the motion of uranus. his last statement on this point is as follows: "my position is, that there were _two possible planets_, either of which might have caused the observed irregular motions of uranus. each planet excluded the other; so that, if one was, the other was not. they coincided in direction from the earth at certain epochs, once in six hundred and fifty years. it was at one of these epochs that the prediction was made, and at no other time for six centuries could the prediction of the one planet have revealed the other. the observed planet was not the predicted one." . _bode's law disproved._--the following table gives the distances of the planets according to bode's law, their actual distances, and the error of the law in each case:-- planet. numbers of actual errors. bode. distances. mercury + = . . venus + = . . earth + = . . mars + = . . minor + = to planets jupiter + = . . saturn + = . . uranus + = . . neptune + = . . it will be seen, that, before the discovery of neptune, the agreement was so close as to indicate that this was an actual law of the distances; but the discovery of this planet completely disproved its existence. [illustration: fig. .] . _the satellite of neptune._--neptune is accompanied by at least one moon, whose orbit is shown in fig. . the orbit of this satellite is inclined about thirty degrees to the plane of the ecliptic, and the motion of the satellite is retrograde, or from east to west. vii. comets and meteors. i. comets. general phenomena of comets. . _general appearance of a bright comet._--comets bright enough to be seen with the naked eye are composed of three parts, which run into each other by insensible gradations. these are the _nucleus_, the _coma_, and the _tail_. the _nucleus_ is the bright centre of the comet, and appears to the eye as a star or planet. the _coma_ is a nebulous mass surrounding the nucleus on all sides. close to the nucleus it is almost as bright as the nucleus itself; but it gradually shades off in every direction. the nucleus and coma combined appear like a star shining through a small patch of fog; and these two together form what is called the _head_ of the comet. the _tail_ is a continuation of the coma, and consists of a stream of milky light, growing wider and fainter as it recedes from the head, till the eye is unable to trace it. [illustration: fig. .] the general appearance of one of the smaller of the brilliant comets is shown in fig. . [illustration: fig. .] [illustration: fig. .] . _general appearance of a telescopic comet._--the great majority of comets are too faint to be visible with the naked eye, and are called _telescopic_ comets. in these comets there seems to be a development of coma at the expense of nucleus and tail. in some cases the telescope fails to reveal any nucleus at all in one of these comets; at other times the nucleus is so faint and ill-defined as to be barely distinguishable. fig. shows a telescopic comet without any nucleus at all, and another with a slight condensation at the centre. in these comets it is generally impossible to distinguish the coma from the tail, the latter being either entirely invisible, as in fig. , or else only an elongation of the coma, as shown in fig. . many comets appear simply as patches of foggy light of more or less irregular form. [illustration: fig. .] . _the development of telescopic comets on their approach to the sun._--as a rule, all comets look nearly alike when they first come within the reach of the telescope. they appear at first as little foggy patches, without any tail, and often without any visible nucleus. as they approach the sun their peculiarities are rapidly developed. fig. shows such a comet as first seen, and the gradual development of its nucleus, head, and tail, as it approaches the sun. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] if the comet is only a small one, the tail developed is small; but these small appendages have a great variety of form in different comets. fig. shows the singular form into which _encke's_ comet was developed in . figs. and show other peculiar developments of telescopic comets. . _development of brilliant comets on their approach to the sun._--brilliant comets, as well as telescopic comets, appear nearly alike when they come into the view of the telescope; and it is only on their approach to the sun that their distinctive features are developed. not only do these comets, when they first come into view, resemble each other, but they also bear a close resemblance to telescopic comets. as the comet approaches the sun, bright vaporous jets, two or three in number, are emitted from the nucleus on the side of the sun and in the direction of the sun. these jets, though directed towards the sun, are soon more or less carried backward, as if repelled by the sun. fig. shows a succession of views of these jets as they were developed in the case of _halley's_ comet in . [illustration: fig. .] the jets in this case seemed to have an oscillatory motion. at and they seemed to be attracted towards the sun, and in to be repelled by him. in and they seemed to be again attracted, and in to be repelled, but in a reverse direction to that in . in they appeared to be again attracted. bessel likened this oscillation of the jets to the vibration of a magnetic needle when presented to the pole of a magnet. in the case of larger comets these luminous jets are surrounded by one or more envelops, which are thrown off in succession as the comet approaches the sun. the formation of these envelops was a conspicuous feature of _donati's_ comet of . a rough view of the jets and the surrounding envelops is given in fig. . fig. gives a view of the envelops without the jets. [illustration: fig. .] [illustration: fig. .] . _the tails of comets._--the _tails_ of brilliant comets are rapidly formed as the comet approaches the sun, their increase in length often being at the rate of several million miles a day. these appendages seem to be formed entirely out of the matter which is emitted from the nucleus in the luminous jets which are at first directed towards the sun. the tails of comets are, however, always directed away from the sun, as shown in fig. . [illustration: fig. .] it will be seen that the comet, as it approaches the sun, travels head foremost; but as it leaves the sun it goes tail foremost. the apparent length of the tail of a comet depends partly upon its real length, partly upon the distance of the comet, and partly upon the direction of the axis of the tail with reference to the line of vision. the longer the tail, the nearer the comet; and the more nearly at right angles to the line of vision is the axis of the tail, the greater is the apparent length of the tail. in the majority of cases the tails of comets measure only a few degrees; but, in the case of many comets recorded in history, the tail has extended half way across the heavens. the tail of a comet, when seen at all, is usually several million miles in length; and in some instances the tail is long enough to reach across the orbit of the earth, or twice as far as from the earth to the sun. the tails of comets are apparently hollow, and are sometimes a million of miles in diameter. so great, however, is the tenuity of the matter in them, that the faintest stars are seen through it without any apparent obscuration. see fig. , which is a view of the great comet of . [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] the tails of comets are sometimes straight, as in fig. , but usually more or less curved, as in fig. , which is a view of _donati's_ comet as it appeared at one time. the tail of a comet is occasionally divided into a number of streamers, as in figs. and . fig. is a view of the great comet of , and fig. of the great comet of . no. , in fig. , is a view of the comet of ; no. , of the comet of ; and no. , of the comet of . [illustration: fig. .] fig. shows some of the forms which the imagination of a superstitious age saw depicted in comets, when these heavenly visitants were thought to be the forerunners of wars, pestilence, famine, and other dire calamities. . _visibility of comets._--even the brightest comets are visible only a short time near their perihelion passage. when near the sun, they sometimes become very brilliant, and on rare occasions have been visible even at mid-day. it is seldom that a comet can be seen, even with a powerful telescope, during its perihelion passage, unless its perihelion is either inside of the earth's orbit, or but little outside of it. motion and origin of comets. . _recognition of a telescopic comet._--it is impossible to distinguish telescopic comets by their appearance from another class of heavenly bodies known as _nebulæ_. such comets can be recognized only by their motion. thus, in fig. , the upper and lower bodies look exactly alike; but the upper one is found to remain stationary, while the lower one moves across the field of view. the upper one is thus shown to be a nebula, and the lower one a comet. [illustration: fig. .] . _orbits of comets._--all comets are found to move in _very eccentric ellipses_, in _parabolas_, or in _hyperbolas_. since an ellipse is a _closed_ curve ( ), all comets that move in ellipses, no matter how eccentric, are permanent members of the solar system, and will return to the sun at intervals of greater or less length, according to the size of the ellipses and the rate of the comet's motion. parabolas and hyperbolas being _open_ curves ( ), comets that move in either of these orbits are only temporary members of our solar system. after passing the sun, they move off into space, never to return, unless deflected hither by the action of some heavenly body which they pass in their journey. [illustration: fig. .] since a comet is visible only while it is near the sun, it is impossible to tell, by the form of the portion of the orbit which it describes during the period of its visibility, whether it is a part of a very elongated ellipse, a parabola, or a hyperbola. thus in fig. are shown two orbits, one of which is a very elongated ellipse, and the other a parabola. the part _ab_, in each case, is the portion of the orbit described by the comet during its visibility. while describing the dotted portions of the orbit, the comet is invisible. now it is impossible to distinguish the form of the visible portion in the two orbits. the same would be true were one of the orbits a hyperbola. whether a comet will describe an ellipse, a parabola, or a hyperbola, can be determined only by its _velocity_, taken in connection with its _distance from the sun_. were a comet ninety-two and a half million miles from the sun, moving away from the sun at the rate of twenty-six miles a second, it would have just the velocity necessary to describe a _parabola_. were it moving with a greater velocity, it would necessarily describe a _hyperbola_, and, with a less velocity, an _ellipse_. so, at any distance from the sun, there is a certain velocity which would cause a comet to describe a parabola; while a greater velocity would cause it to describe a hyperbola, and a less velocity to describe an ellipse. if the comet is moving in an ellipse, the less its velocity, the less the eccentricity of its orbit: hence, in order to determine the form of the orbit of any comet, it is only necessary to ascertain its distance from the sun, and its velocity at any given time. comets move in every direction in their orbits, and these orbits have every conceivable inclination to the ecliptic. . _periodic comets._--there are quite a number of comets which are known to be _periodic_, returning to the sun at regular intervals in elliptic orbits. some of these have been observed at several returns, so that their period has been determined with great certainty. in the case of others the periodicity is inferred from the fact that the velocity fell so far short of the parabolic limit that the comet must move in an ellipse. the number of known periodic comets is increasing every year, three having been added to the list in . the velocity of most comets is so near the parabolic limit that it is not possible to decide, from observations, whether it falls short of it, or exceeds it. in the case of a few comets the observations indicate a minute excess of velocity; but this cannot be confidently asserted. it is not, therefore, absolutely certain that any known comet revolves in a hyperbolic orbit; and thus it is possible that all comets belong to our system, and will ultimately return to it. it is, however, certain, that, in the majority of cases, the return will be delayed for many centuries, and perhaps for many thousand years. . _origin of comets._--it is now generally believed that the original home of the comets is in the stellar spaces outside of our solar system, and that they are drawn towards the sun, one by one, in the long lapse of ages. were the sun unaccompanied by planets, or were the planets immovable, a comet thus drawn in would whirl around the sun in a parabolic orbit, and leave it again never to return, unless its path were again deflected by its approach to some star. but, when a comet is moving in a parabola, the slightest _retardation_ would change its orbit to an ellipse, and the slightest _acceleration_ into a hyperbola. owing to the motion of the several planets in their orbits, the velocity of a comet would be changed on passing each of them. whether its velocity would be accelerated or retarded, would depend upon the way in which it passed. were the comet accelerated by the action of the planets, on its passage through our system, more than it was retarded by them, it would leave the system with a more than parabolic orbit, and would therefore move in a hyperbola. were it, on the contrary, retarded more than accelerated by the action of the planets, its velocity would be reduced, so that the comet would move in a more or less elongated ellipse, and thus become a permanent member of the solar system. in the majority of cases the retardation would be so slight that it could not be detected by the most delicate observation, and the comet would return to the sun only after the expiration of tens or hundreds of thousands of years; but, were the comet to pass very near one of the larger planets, the retardation might be sufficient to cause the comet to revolve in an elliptical orbit of quite a short period. the orbit of a comet thus captured by a planet would have its aphelion point near the orbit of the planet which captured it. now, it happens that each of the larger planets has a family of comets whose aphelia are about its own distance from the sun. it is therefore probable that these comets have been captured by the action of these planets. as might be expected from the gigantic size of jupiter, the jovian family of comets is the largest. the orbits of several of the comets of this group are shown in fig. . [illustration: fig. .] . _number of comets._--the number of comets recorded as visible to the naked eye since the birth of christ is about five hundred, while about two hundred telescopic comets have been observed since the invention of the telescope. the total number of comets observed since the christian era is therefore about seven hundred. it is certain, however, that only an insignificant fraction of all existing comets have ever been observed. since they can be seen only when near their perihelion, and since it is probable that the period of most of those which have been observed is reckoned by thousands of years (if, indeed, they ever return at all), our observations must be continued for many thousand years before we have seen all which come within range of our telescopes. besides, as already stated ( ), a comet can seldom be seen unless its perihelion is either inside the orbit of the earth, or but little outside of it; and it is probable that the perihelia of the great majority of comets are beyond this limit of visibility. remarkable comets. . _the comet of ._--the great comet of , shown in fig. , is one of the most celebrated on record. it was by his study of its motions that newton proved the orbit of a comet to be one of the conic sections, and therefore that these bodies move under the influence of gravity. this comet descended almost in a direct line to the sun, passing nearer to that luminary than any comet before known. newton estimated, that, at its perihelion point, it was exposed to a temperature two thousand times that of red-hot iron. during its perihelion passage it was exceedingly brilliant. halley suspected that this comet had a period of five hundred and seventy-five years, and that its first recorded appearance was in b.c., its third in , and its fourth in . if this is its real period, it will return in . the comet of b.c. made its appearance just after the assassination of julius cæsar. the romans called it the _julian star_, and regarded it as a celestial chariot sent to convey the soul of cæsar to the skies. it was seen two or three hours before sunset, and continued visible for eight successive days. the great comet of was described as an object of terrific splendor, and was visible in close proximity to the sun. the comet of has become celebrated, not only on account of its great brilliance, and on account of newton's investigation of its orbit, but also on account of the speculation of the theologian whiston in regard to it. he accepted five hundred and seventy-five years as its period, and calculated that one of its earlier apparitions must have occurred at the date of the flood, which he supposed to have been caused by its near approach to the earth; and he imagined that the earth is doomed to be destroyed by fire on some future encounter with this comet. [illustration: fig. .] . _the comet of ._--the great comet of , a view of which is given in fig. , is, perhaps, the most remarkable comet on record. it was visible for nearly seventeen months, and was very brilliant, although at its perihelion passage it was over a hundred million miles from the sun. its tail was a hundred and twenty million miles in length, and several million miles through. it has been calculated that its aphelion point is about two hundred times as far from the sun as its perihelion point, or some seven times the distance of neptune from the sun. its period is estimated at about three thousand years. it was an object of superstitious terror, especially in the east. the russians regarded it as presaging napoleon's great and fatal war with russia. [illustration: fig. .] [illustration: fig. .] . _halley's comet._--halley's comet has become one of the most celebrated of modern times. it is the first comet whose return was both predicted and observed. it made its appearance in . halley computed its orbit, and compared it with those of previous comets, whose orbits he also computed from recorded observations. he found that it coincided so exactly with that of the comet observed by kepler in , that there could be no doubt of the identity of the two orbits. so close were they together, that, were they both drawn in the heavens, the naked eye would almost see them joined into one line. there could therefore be no doubt that the comet of was the same that had appeared in , and that it moved in an elliptic orbit, with a period of about seventy-five years. he found that this comet had previously appeared in and in ; and he predicted that it would return about . its actual return was retarded somewhat by the action of the planets on it in its passage through the solar system. it, however, appeared again in , and a third time in . its next appearance will be about . the orbit of this comet is shown in fig. . fig. shows the comet as it appeared to the naked eye, and in a telescope of moderate power, in . this comet appears to be growing less brilliant. in it appeared as a comet of great splendor; and coming as it did in a very superstitious age, soon after the fall of constantinople, and during the threatened invasion of europe by the turks, it caused great alarm. fig. shows the changes undergone by the nucleus of this comet during its perihelion passage in . [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] . _encke's comet._--this telescopic comet, two views of which are given in figs. and , appeared in . encke computed its orbit, and found it to lie wholly within the orbit of jupiter (fig. ), and the period to be about three years and a third. by comparing the intervals between the successive returns of this comet, it has been ascertained that its orbit is continually growing smaller and smaller. to account for the retardation of this comet, olbers announced his celebrated hypothesis, that the celestial spaces are filled with a subtile _resisting medium_. this hypothesis was adopted by encke, and has been accepted by certain other astronomers; but it has by no means gained universal assent. . _biela's comet._--this comet appeared in , and was found to have a period of about six years and two thirds. on its return in , it met with a singular, and as yet unexplained, accident, which has rendered the otherwise rather insignificant comet famous. in november and december of that year it was observed as usual, without any thing remarkable about it; but, in january of the following year, it was found to have been divided into two distinct parts, so as to appear as two comets instead of one. the two parts were at first of very unequal brightness; but, during the following month, the smaller of the two increased in brilliancy until it equalled its companion; it then grew fainter till it entirely disappeared, a month before its companion. the two parts were about two hundred thousand miles apart. fig. shows these two parts as they appeared on the th of february, and fig. as they appeared on the st of february. on its return in , the comets were found still to be double; but the two components were now about a million and a half miles apart. they are shown in fig. as they appeared at this time. sometimes one of the parts appeared the brighter, and sometimes the other; so that it was impossible to decide which was really the principal comet. the two portions passed out of view in september, and have not been seen since; although in the position of the comet would have been especially favorable for observation. the comet appears to have become completely broken up. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] . _the comet of ._--the great comet of , a view of which is given in fig. , was favorably situated for observation only in southern latitudes. it was exceedingly brilliant, and was easily seen in full daylight, in close proximity to the sun. the apparent length of its tail was sixty-five degrees, and its real length a hundred and fifty million miles, or nearly twice the distance from the earth to the sun. this comet is especially remarkable on account of its near approach to the sun. at the time of its perihelion passage the distance of the comet from the photosphere of the sun was less than one-fourteenth of the diameter of the sun. this distance was only one-half that of the comet of when at its perihelion. when at perihelion, this comet was plunging through the sun's outer atmosphere at the rate of one million, two hundred and eighty thousand miles an hour. it passed half way round the sun in the space of _two hours_, and its tail was whirled round through a hundred and eighty degrees in that brief time. as the tail extended almost double the earth's distance from the sun, the end of the tail must have traversed in two hours a space nearly equal to the circumference of the earth's orbit,--a distance which the earth, moving at the rate of about twenty miles a second, is a _whole year_ in passing. it is almost impossible to suppose that the matter forming this tail remained the same throughout this tremendous sweep. . _donati's comet._--the great comet of , known as _donati's_ comet, was one of the most magnificent of modern times. when at its brightest it was only about fifty million miles from the earth. its tail was then more than fifty million miles long. had the comet at this time been directly between the earth and sun, the earth must have passed through its tail; but this did not occur. the orbit of this comet was found to be decidedly elliptic, with a period of about two thousand years. this comet is especially celebrated on account of the careful telescopic observations of its nucleus and coma at the time of its perihelion passage. attention has already been called ( ) to the changes it underwent at that time. its tail was curved, and of a curious feather-like form, as shown in fig. . at times it developed lateral streamers, as shown in fig. . fig. shows the head of the comet as it was seen by bond of the harvard observatory, whose delineations of this comet have been justly celebrated. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] . _the comet of ._--the great comet of is remarkable for its great brilliancy, for its peculiar fan-shaped tail, and for the probable passage of the earth through its tail. sir john herschel declared that it far exceeded in brilliancy any comet he had ever seen, not excepting those of and . secchi found its tail to be a hundred and eighteen degrees in length, the largest but one on record. fig. shows this comet as it appeared at one time. fig. shows the position of the earth at _e_, in the tail of this comet, on the th of june, . fig. shows the probable passage of the earth through the tail of the comet on that date. as the tail of a comet doubtless consists of something much less dense than our atmosphere, it is not surprising that no noticeable effect was produced upon us by the encounter, if it occurred. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] . _coggia's comet._--this comet, which appeared in , looked very large, because it came very near the earth. it was not at all brilliant. its nucleus was carefully studied, and was found to develop a series of envelops similar to those of donati's comet. figs. and are two views of the head of this comet. fig. shows the system of envelops that were developed during its perihelion passage. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] . _the comet of june, ._--this comet, though far from being one of the largest of modern times, was still very brilliant. it will ever be memorable as the first brilliant comet which has admitted of careful examination with the spectroscope. connection between meteors and comets. . _shooting-stars._--on watching the heavens any clear night, we frequently see an appearance as of a star shooting rapidly through a short space in the sky, and then suddenly disappearing. three or four such _shooting-stars_ may, on the average, be observed in the course of an hour. they are usually seen only a second or two; but they sometimes move slowly, and are visible much longer. these stars begin to be visible at an average height of about seventy-five miles, and they disappear at an average height of about fifty miles. they are occasionally seen as high as a hundred and fifty miles, and continue to be visible till within thirty miles of the earth. their visible paths vary from ten to a hundred miles in length, though they are occasionally two hundred or three hundred miles long. their average velocity, relatively to the earth's surface, varies from ten to forty-five miles a second. the average number of shooting-stars visible to the naked eye at any one place is estimated at about _a thousand an hour_; and the average number large enough to be visible to the naked eye, that traverse the atmosphere daily, is estimated at _over eight millions_. the number of telescopic shooting-stars would of course be much greater. occasionally, shooting-stars leave behind them a trail of light which lasts for several seconds. these trails are sometimes straight, as shown in fig. , and sometimes curved, as in figs. and . they often disappear like trails of smoke, as shown in fig. . [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] shooting-stars are seen to move in all directions through the heavens. their apparent paths are, however, generally inclined downward, though sometimes upward; and after midnight they come in the greatest numbers from that quarter of the heavens toward which the earth is moving in its journey around the sun. . _meteors._--occasionally these bodies are brilliant enough to illuminate the whole heavens. they are then called _meteors_, although this term is equally applicable to ordinary shooting-stars. such a meteor is shown in fig. . [illustration: fig. .] sometimes these brilliant meteors are seen to explode, as shown in fig. ; and the explosion is accompanied with a loud detonation, like the discharge of cannon. [illustration: fig. .] ordinary shooting-stars are not accompanied by any audible sound, though they are sometimes seen to break in pieces. meteors which explode with an audible sound are called _detonating meteors_. . _aerolites._--there is no certain evidence that any deposit from ordinary shooting-stars ever reaches the surface of the earth; though a peculiar dust has been found in certain localities, which has been supposed to be of meteoric origin, and which has been called _meteoric dust_. but solid bodies occasionally descend to the earth from beyond our atmosphere. these generally penetrate a foot or more into the earth, and, if picked up soon after their fall, are found to be warm, and sometimes even hot. these bodies are called _aerolites_. when they have a stony appearance, and contain but little iron, they are called _meteoric stones_; when they have a metallic appearance, and are composed largely of iron, they are called _meteoric iron_. there are eighteen well-authenticated cases in which aerolites have fallen in the united states during the last sixty years, and their aggregate weight is twelve hundred and fifty pounds. the entire number of known aerolites the date of whose fall is well determined is two hundred and sixty-one. there are also on record seventy-four cases of which the date is more or less uncertain. there have also been found eighty-six masses, which, from their peculiar composition, are believed to be aerolites, though their fall was not seen. the weight of these masses varies from a few pounds to several tons. the entire number of aerolites of which we have any knowledge is therefore about four hundred and twenty. aerolites are composed of the same elementary substances as occur in terrestrial minerals, not a single new element having been found in their analysis. of the sixty or more elements now recognized by chemists, about twenty have been found in aerolites. while aerolites contain no new elements, their appearance is quite peculiar; and the compounds found in them are so peculiar as to enable us by chemical analysis to distinguish an aerolite from any terrestrial substance. iron ores are very abundant in nature, but iron in the metallic state is exceedingly rare. now, aerolites invariably contain metallic iron, sometimes from ninety to ninety-six per cent. this iron is malleable, and may be readily worked into cutting instruments. it always contains eight or ten per cent of nickel, together with small quantities of cobalt, copper, tin, and chromium. this composition _has never been found in any terrestrial mineral_. aerolites also contain, usually in small amount, a compound of iron, nickel, and phosphorus, which has never been found elsewhere. meteorites often present the appearance of having been fused on the surface to a slight depth, and meteoric iron is found to have a peculiar crystalline structure. the external appearance of a piece of meteoric iron found near lockport, n.y., is shown in fig. . fig. shows the peculiar internal structure of meteoric iron. [illustration: fig. .] [illustration: fig. .] . _meteoroids._--astronomers now universally hold that shooting-stars, meteors, and aerolites are all minute bodies, revolving, like the comets, about the sun. they are moving in every possible direction through the celestial spaces. they may not average more than one in a million of cubic miles, and yet their total number exceeds all calculation. of the nature of the minuter bodies of this class nothing is certainly known. the earth is continually encountering them in its journey around the sun. they are burned by passing through the upper regions of our atmosphere, and the shooting-star is simply the light of that burning. these bodies, which are invisible till they plunge into the earth's atmosphere, are called _meteoroids_. . _origin of the light of meteors._--when one of these meteoroids enters our atmosphere, the resistance of the air arrests its motion to some extent, and so converts a portion of its energy of motion into that of heat. the heat thus developed is sufficient to raise the meteoroid and the air around it to incandescence, and in most cases either to cause the meteoroid to burn up, or to dissipate it as vapor. the luminous vapor thus formed constitutes the luminous train which occasionally accompanies a meteor, and often disappears as a puff of smoke. when a meteoroid is large enough and refractory enough to resist the heat to which it is exposed, its motion is sufficiently arrested, on entering the lower layers of our atmosphere, to cause it to fall to the earth. we then have an _aerolite_. a brilliant meteor differs from a shooting-star simply in magnitude. . _the intensity of the heat to which a meteoroid is exposed._--it has been ascertained by experiment that a body moving through the atmosphere at the rate of a hundred and twenty-five feet a second raises the temperature of the air immediately in front of it one degree, and that the temperature increases as the square of the velocity of the moving body; that is to say, that, with a velocity of two hundred and fifty feet, the temperature in front of the body would be raised four degrees; with a velocity of five hundred feet, sixteen degrees; and so on. to find, therefore, the temperature to which a meteoroid would be exposed in passing through our atmosphere, we have merely to divide its velocity in feet per second by a hundred and twenty-five, and square the quotient. with a velocity of forty-four miles a second in our atmosphere, a meteoroid would therefore be exposed to a temperature of between three and four million degrees. the air acts upon the body as if it were raised to this intense heat. at such a temperature small masses of the most refractory or incombustible substances known to us would flash into vapor with the evolution of intense light and heat. if one of these meteoric bodies is large enough to pass through the atmosphere and reach the earth, without being volatilized by the heat, we have an aerolite. as it is only a few seconds in making the passage, the heat has not time to penetrate far into its interior, but is expended in melting and vaporizing the outer portions. the resistance of the denser strata of the atmosphere to the motion of the aerolite sometimes becomes so enormous that the body is suddenly rent to pieces with a loud detonation. it seems like an explosion produced by some disruptive action within the mass; but there can be little doubt that it is due to the velocity--perhaps ten, twenty, or thirty miles a second--with which the body strikes the air. if, on the other hand, the meteoroid is so small as to be burned up or volatilized in the upper regions of the atmosphere, we have a common shooting-star, or a meteor of greater or less brilliancy. [illustration: fig. .] . _meteoric showers._--on ordinary nights only four or five shooting-stars are seen in an hour, and these move in every direction. their orbits lie in all possible positions, and are seemingly scattered at random. such meteors are called _sporadic_ meteors. on occasional nights, shooting-stars are more numerous, and all move in a common direction. such a display is called a _meteoric shower_. these showers differ greatly in brilliancy; but during any one shower the meteors all appear to radiate from some one point in the heavens. if we mark on a celestial globe the apparent paths of the meteors which fall during a shower, or if we trace them back on the celestial sphere, we shall find that they all meet in the same point, as shown in fig. . this point is called the _radiant point_. it always appears in the same position, wherever the observer is situated, and does not partake of the diurnal motion of the earth. as the stars move towards the west, the radiant point moves with them. the point in question is purely an effect of perspective, being the "vanishing point" of the parallel lines in which the meteors are actually moving. these lines are seen, not in their real direction in space, but as projected on the celestial sphere. if we look upwards, and watch snow falling through a calm atmosphere, the flakes which fall directly towards us do not seem to move at all, while the surrounding flakes seem to diverge from them on all sides. so, in a meteoric shower, a meteor coming directly towards the observer does not seem to move at all, and marks the point from which all the others seem to radiate. . _the august meteors._--a meteoric shower of no great brilliancy occurs annually about the th of august. the radiant point of this shower is in the constellation _perseus_, and hence these meteors are often called the _perseids_. the orbit of these meteoroids has been pretty accurately determined, and is shown in fig. . [illustration: fig. .] it will be seen that the perihelion point of this orbit is at about the distance of the earth from the sun; so that the earth encounters the meteors once a year, and this takes place in the month of august. the orbit is a very eccentric ellipse, reaching far beyond neptune. as the meteoric display is about equally brilliant every year, it seems probable that the meteoroids form a stream quite uniformly distributed throughout the whole orbit. it probably takes one of the meteoroids about a hundred and twenty-four years to pass around this orbit. [illustration: fig. .] . _the november meteors._--a somewhat brilliant meteoric shower also occurs annually, about the th of november. the radiant point of these meteors is in the constellation _leo_, and hence they are often called the _leonids_. their orbit has been determined with great accuracy, and is shown in fig. . while the november meteors are not usually very numerous or bright, a remarkably brilliant display of them has been seen once in about thirty-three or thirty-four years: hence we infer, that, while there are some meteoroids scattered throughout the whole extent of the orbit, the great majority are massed in a group which traverses the orbit in a little over thirty-three years. a conjectural form of this condensed group is shown in fig. . the group is so large that it takes it two or three years to pass the perihelion point: hence there may be a brilliant meteoric display two or three years in succession. [illustration: fig. .] the last brilliant display of these meteors was in the years and . the display was visible in this country only a short time before sunrise, and therefore did not attract general attention. the display of was remarkably brilliant in this country, and caused great consternation among the ignorant and superstitious. [illustration: fig. .] . _connection between meteors and comets._--it has been found that a comet which appeared in , and which is designated as , i., has exactly the same orbit and period as the november meteors, and that another comet, known as the , iii., has the same orbit as the august meteors. it has also been ascertained that a third comet, , i., has the same orbit as a stream of meteors which the earth encounters in april. furthermore, it was found, in , that there was a small stream of meteors following in the train of the lost comet of biela. these various orbits of comets and meteoric streams are shown in fig. . the coincidence of the orbits of comets and of meteoric streams indicates that these two classes of bodies are very closely related. they undoubtedly have a common origin. the fact that there is a stream of meteors in the train of biela's comet has led to the supposition that comets may become gradually disintegrated into meteoroids. physical and chemical constitution of comets. . _physical constitution of telescopic comets._--we have no certain knowledge of the physical constitution of telescopic comets. they are usually tens of thousands of miles in diameter, and yet of such tenuity that the smallest stars can readily be seen through them. it would seem that they must shine in part by reflected light; yet the spectroscope shows that their spectrum is composed of bright bands, which would indicate that they are composed, in part at least, of incandescent gases. it is, however, difficult to conceive how these gases become sufficiently heated to be luminous; and at the same time such gases would reflect no sunlight. it seems probable that these comets are really made up of a combination of small, solid particles in the form of minute meteoroids, and of gases which are, perhaps, rendered luminous by electric discharges of slight intensity. . _physical constitution of large comets._--in the case of large comets the nucleus is either a dense mass of solid matter several hundred miles in diameter, or a dense group of meteoroids. professor peirce estimated that the density of the nucleus is at least equal to that of iron. as such a comet approaches the sun, the nucleus is, to a slight extent, vaporized, and out of this vapor is formed the coma and the tail. that some evaporating process is going on from the nucleus of the comet is proved by the movements of the tail. it is evident that the tail cannot be an appendage carried along with the comet, as it seems to be. it is impossible that there should be any cohesion in matter of such tenuity that the smallest stars could be seen through a million of miles of it, and which is, moreover, continually changing its form. then, again, as a comet is passing its perihelion, the tail appears to be whirled from one side of the sun to another with a rapidity which would tear it to pieces if the movement were real. the tail seems to be, not something attached to the comet, and carried along with it, but a stream of vapor issuing from it, like smoke from a chimney. the matter of which it is composed is continually streaming outwards, and continually being replaced by fresh vapor from the nucleus. the vapor, as it emanates from the nucleus, is repelled by the sun with a force often two or three times as great as the ordinary solar attraction. the most probable explanation of this phenomenon is, that it is a case of electrical repulsion, the sun and the particles of the cometary mist being similarly electrified. with reference to this electrical theory of the formation of comets' tails, professor peirce makes the following observation: "in its approach to the sun, the surface of the nucleus is rapidly heated: it is melted and vaporized, and subjected to frequent explosions. the vapor rises in its atmosphere with a well-defined upper surface, which is known to observers as an _envelop_.... the electrification of the cometary mist is analogous to that of our own thunder-clouds. any portion of the coma which has received the opposite kind of electricity to the sun and to the repelled tail will be attracted. this gives a simple explanation of the negative tails which have been sometimes seen directed towards the sun. in cases of violent explosion, the whole nucleus might be broken to pieces, and the coma dashed around so as to give varieties of tail, and even multiple tails. there seems, indeed, to be no observed phenomenon of the tail or the coma which is not consistent with a reasonable modification of the theory." professor peirce regarded comets simply as the largest of the meteoroids. they appear to shine partly by reflected sunlight, and partly by their own proper light, which seems to be that of vapor rendered luminous by an electric discharge of slight intensity. [illustration: fig. .] . _collision of a comet and the earth._--it sometimes happens that the orbit of a comet intersects that of the earth, as is shown in fig. , which shows a portion of the orbit of biela's comet, with the positions of the comet and of the earth in . of course, were a comet and the earth both to reach the intersection of their orbits at the same time, a collision of the two bodies would be inevitable. with reference to the probable effect of such a collision, professor newcomb remarks,-- "the question is frequently asked, what would be the effect if a comet should strike the earth? this would depend upon what sort of a comet it was, and what part of the comet came in contact with our planet. the latter might pass through the tail of the largest comet without the slightest effect being produced; the tail being so thin and airy that a million miles thickness of it looks only like gauze in the sunlight. it is not at all unlikely that such a thing may have happened without ever being noticed. a passage through a telescopic comet would be accompanied by a brilliant meteoric shower, probably a far more brilliant one than has ever been recorded. no more serious danger would be encountered than that arising from a possible fall of meteorites; but a collision between the nucleus of a large comet and the earth might be a serious matter. if, as professor peirce supposes, the nucleus is a solid body of metallic density, many miles in diameter, the effect where the comet struck would be terrific beyond conception. at the first contact in the upper regions of the atmosphere, the whole heavens would be illuminated with a resplendence beyond that of a thousand suns, the sky radiating a light which would blind every eye that beheld it, and a heat which would melt the hardest rocks. a few seconds of this, while the huge body was passing through the atmosphere, and the collision at the earth's surface would in an instant reduce everything there existing to fiery vapor, and bury it miles deep in the solid earth. happily, the chances of such a calamity are so minute that they need not cause the slightest uneasiness. there is hardly a possible form of death which is not a thousand times more probable than this. so small is the earth in comparison with the celestial spaces, that if one should shut his eyes, and fire a gun at random in the air, the chance of bringing down a bird would be better than that of a comet of any kind striking the earth." [illustration: fig. .] [illustration: fig. .] . _the chemical constitution of comets._--fig. shows the bands of the spectrum of a telescopic comet of , as seen by two different observers. fig. shows the spectrum of the coma and tail of the comet of ; and the spectrum of the bright comet of showed the same three bands for the coma and tail. now, these three bands are those of certain hydrocarbon vapors: hence it would seem that the coma and tails of comets are composed chiefly of such vapors ( ). ii. the zodiacal light. . _the general appearance of the zodiacal light._--the phenomenon known as the _zodiacal light_ consists of a very faint luminosity, which may be seen rising from the western horizon after twilight on any clear winter or spring evening, also from the eastern horizon just before daybreak in the summer or autumn. it extends out on each side of the sun, and lies nearly in the plane of the ecliptic. it grows fainter the farther it is from the sun, and can generally be traced to about ninety degrees from that luminary, when it gradually fades away. in a very clear, tropical atmosphere, it has been traced all the way across the heavens from east to west, thus forming a complete ring. the general appearance of this column of light, as seen in the morning, in the latitude of europe, is shown in fig. . [illustration: fig. .] taking all these appearances together, they indicate that it is due to a lens-shaped appendage surrounding the sun, and extending a little beyond the earth's orbit. it lies nearly in the plane of the ecliptic; but its exact position is not easily determined. fig. shows the general form and position of this solar appendage, as seen in the west. [illustration: fig. .] . _the visibility of the zodiacal light._--the reason why the zodiacal light is more favorably seen in the evening during the winter and spring than in the summer and fall is evident from fig. , which shows the position of the ecliptic and the zodiacal light with reference to the western horizon at the time of sunset in march and in september. it will be seen that in september the axis of the light forms a small angle with the horizon, so that the phenomenon is visible only a short time after sunset and low down where it is difficult to distinguish it from the glimmer of the twilight; while in march, its axis being nearly perpendicular to the horizon, the light may be observed for some hours after sunset and well up in the sky. fig. gives the position of the ecliptic and of the zodiacal light with reference to the eastern horizon at the time of sunrise, and shows why the zodiacal light is seen to better advantage in the morning during the summer and fall than during the winter and spring. it will be observed that here the angle made by the axis of the light with the horizon is small in march, while it is large in september; the conditions represented in the preceding figure being thus reversed. [illustration: fig. .] [illustration: fig. .] . _nature of the zodiacal light._--various attempts have been made to explain the phenomena of the zodiacal light; but the most probable theory is, that it is due to an immense number of meteors which are revolving around the sun, and which lie mostly within the earth's orbit. each of these meteors reflects a sensible portion of sunlight, but is far too small to be separately visible. all of these meteors together would, by their combined reflection, produce a kind of pale, diffused light. iii. the stellar universe. i. general aspect of the heavens. . _the magnitude of the stars._--the stars that are visible to the naked eye are divided into six classes, according to their brightness. the brightest stars are called stars of the _first magnitude_; the next brightest, those of the _second magnitude_; and so on to the _sixth magnitude_. the last magnitude includes the faintest stars that are visible to the naked eye on the most favorable night. stars which are fainter than those of the sixth magnitude can be seen only with the telescope, and are called _telescopic stars_. telescopic stars are also divided into magnitudes; the division extending to the _sixteenth_ magnitude, or the faintest stars that can be seen with the most powerful telescopes. the classification of stars according to magnitudes has reference only to their brightness, and not at all to their actual size. a sixth magnitude star may actually be larger than a first magnitude star; its want of brilliancy being due to its greater distance, or to its inferior luminosity, or to both of these causes. none of the stars present any sensible disk, even in the most powerful telescope: they all appear as mere points of light. the larger the telescope, the greater is its power of revealing faint stars; not because it makes these stars appear larger, but because of its greater light-gathering power. this power increases with the size of the object-glass of the telescope, which plays the part of a gigantic pupil of the eye. the classification of the stars into magnitudes is not made in accordance with any very accurate estimate of their brightness. the stars which are classed together in the same magnitude are far from being equally bright. the stars of each lower magnitude are about two-fifths as bright as those of the magnitude above. the ratio of diminution is about a third from the higher magnitude down to the fifth. were the ratio two-fifths exact, it would take about - / stars of the d magnitude to make one of the st. stars of the d magnitude to make one of the st. stars of the th magnitude to make one of the st. stars of the th magnitude to make one of the st. stars of the th magnitude to make one of the st. , stars of the th magnitude to make one of the st. , , stars of the th magnitude to make one of the st. . _the number of the stars._--the total number of stars in the celestial sphere visible to the average naked eye is estimated, in round numbers, at five thousand; but the number varies much with the perfection and the training of the eye and with the atmospheric conditions. for every star visible to the naked eye, there are thousands too minute to be seen without telescopic aid. fig. shows a portion of the constellation of the twins as seen with the naked eye; and fig. shows the same region as seen in a powerful telescope. [illustration: fig. .] [illustration: fig. .] struve has estimated that the total number of stars visible with herschel's twenty-foot telescope was about twenty million. the number that can be seen with the great telescopes of modern times has not been carefully estimated, but is probably somewhere between thirty million and fifty million. the number of stars between the north pole and the circle thirty-five degrees south of the equator is about as follows:-- of the st magnitude about stars. of the d magnitude about stars. of the d magnitude about stars. of the th magnitude about stars. of the th magnitude about stars. of the th magnitude about stars. ---- total visible to naked eye stars. the number of stars of the several magnitudes is approximately in inverse proportion to that of their brightness, the ratio being a little greater in the higher magnitudes, and probably a little less in the lower ones. . _the division of the stars into constellations._--a glance at the heavens is sufficient to show that the stars are not distributed uniformly over the sky. the larger ones especially are collected into more or less irregular groups. the larger groups are called _constellations_. at a very early period a mythological figure was allotted to each constellation; and these figures were drawn in such a way as to include the principal stars of each constellation. the heavens thus became covered, as it were, with immense hieroglyphics. there is no historic record of the time when these figures were formed, or of the principle in accordance with which they were constructed. it is probable that the imagination of the earlier peoples may, in many instances, have discovered some fanciful resemblance in the configuration of the stars to the forms depicted. the names are still retained, although the figures no longer serve any astronomical purpose. the constellation hercules, for instance, no longer represents the figure of a man among the stars, but a certain portion of the heavens within which the ancients placed that figure. in star-maps intended for school and popular use it is still customary to give these figures; but they are not generally found on maps designed for astronomers. . _the naming of the stars._--the brighter stars have all proper names, as _sirius_, _procyon_, _arcturus_, _capella_, _aldebaran_, etc. this method of designating the stars was adopted by the arabs. most of these names have dropped entirely out of astronomical use, though many are popularly retained. the brighter stars are now generally designated by the letters of the greek alphabet,--_alpha_, _beta_, _gamma_, etc.,--to which is appended the genitive of the name of the constellation, the first letter of the alphabet being used for the brightest star, the second for the next brightest, and so on. thus _aldebaran_ would be designated as _alpha tauri_. in speaking of the stars of any one constellation, we simply designate them by the letters of the greek alphabet, without the addition of the name of the constellation, which answers to a person's surname, while the greek letter answers to his christian name. the names of the seven stars of the "dipper" are given in fig. . when the letters of the greek alphabet are exhausted, those of the roman alphabet are employed. the fainter stars in a constellation are usually designated by some system of numbers. [illustration: fig. .] . _the milky-way, or galaxy._--the milky-way is a faint luminous band, of irregular outline, which surrounds the heavens with a great circle, as shown in fig. . through a considerable portion of its course it is divided into two branches, and there are various vacant spaces at different points in this band; but at only one point in the southern hemisphere is it entirely interrupted. [illustration: fig. .] the telescope shows that the galaxy arises from the light of countless stars too minute to be separately visible with the naked eye. the telescopic stars, instead of being uniformly distributed over the celestial sphere, are mostly condensed in the region of the galaxy. they are fewest in the regions most distant from this belt, and become thicker as we approach it. the greater the telescopic power, the more marked is the condensation. with the naked eye the condensation is hardly noticeable; but with the aid of a very small telescope, we see a decided thickening of the stars in and near the galaxy, while the most powerful telescopes show that a large majority of the stars lie actually in the galaxy. if all the stars visible with a twelve-inch telescope were blotted out, we should find that the greater part of those remaining were in the galaxy. [illustration: fig. .] the increase in the number of the stars of all magnitudes as we approach the plane of the milky-way is shown in fig. . the curve _acb_ shows by its height the distribution of the stars above the ninth magnitude, and the curve _acb_ those of all magnitudes. . _star-clusters._--besides this gradual and regular condensation towards the galaxy, occasional aggregations of stars into _clusters_ may be seen. some of these are visible to the naked eye, sometimes as separate stars, like the "seven stars," or pleiades, but more commonly as patches of diffused light, the stars being too small to be seen separately. the number visible in powerful telescopes is, however, much greater. sometimes hundreds or even thousands of stars are visible in the field of view at once, and sometimes the number is so great that they cannot be counted. . _nebulæ._--another class of objects which are found in the celestial spaces are irregular masses of soft, cloudy light, known as _nebulæ_. many objects which look like nebulæ in small telescopes are shown by more powerful instruments to be really star-clusters. but many of these objects are not composed of stars at all, being immense masses of gaseous matter. [illustration: fig. .] the general distribution of nebulæ is the reverse of that of the stars. nebulæ are thickest where stars are thinnest. while stars are most numerous in the region of the milky-way, nebulæ are most abundant about the poles of the milky-way. this condensation of nebulæ about the poles of the milky-way is shown in figs. and , in which the points represent, not stars, but nebulæ. ii. the stars. the constellations. [illustration: fig. .] [illustration: fig. .] . _the great bear._--the great bear, or _ursa major_, is one of the circumpolar constellations ( ), and contains one of the most familiar _asterisms_, or groups of stars, in our sky; namely, the _great dipper_, or _charles's wain_. the positions and names of the seven prominent stars in it are shown in fig. . the two stars alpha and beta are called the _pointers_. this asterism is sometimes called the _butcher's cleaver_. the whole constellation is shown in fig. . a rather faint star marks the nose of the bear, and three equidistant pairs of faint stars mark his feet. . _the little bear, draco, and cassiopeia._--these are all circumpolar constellations. the most important star of the little bear, or _ursa minor_, is _polaris_, or the _pole star_. this star may be found by drawing a line from beta to alpha of the dipper, and prolonging it as shown in fig. . this explains why these stars are called the _pointers_. the pole star, with the six other chief stars of the little bear, form an asterism called the _little dipper_. these six stars are joined with polaris by a dotted line in fig. . [illustration: fig. .] the stars in a serpentine line between the two dippers are the chief stars of _draco_, or the _dragon_; the trapezium marking its head. fig. shows the constellations of ursa minor and draco as usually figured. [illustration: fig. .] to find _cassiopeia_, draw a line from delta of the dipper to polaris, and prolong it about an equal distance beyond, as shown in fig. . this line will pass near alpha of cassiopeia. the five principal stars of this constellation form an irregular _w_, opening towards the pole. between cassiopeia and draco are five rather faint stars, which form an irregular _k_. these are the principal stars of the constellation _cepheus_. these two constellations are shown in fig. . [illustration: fig. .] [illustration: fig. .] . _the lion, berenice's hair, and the hunting-dogs._--a line drawn from alpha to beta of the dipper, and prolonged as shown in fig. , will pass between the two stars _denebola_ and _regulus_ of _leo_, or the _lion_. regulus forms a _sickle_ with several other faint stars, and marks the heart of the lion. denebola is at the apex of a right-angled triangle, which it forms with two other stars, and marks the end of the lion's tail. this constellation is visible in the evening from february to july, and is figured in fig. . [illustration: fig. .] in a straight line between denebola and eta, at the end of the great bear's tail, are, at about equal distances, the two small constellations of _coma berenices_, or _berenice's hair_, and _canes venatici_, or the _hunting-dogs_. these are shown in fig. . the dogs are represented as pursuing the bear, urged on by the huntsman _boötes_. [illustration: fig. .] . _boötes, hercules, and the northern crown._--_arcturus_, the principal star of _boötes_, may be found by drawing a line from zeta to eta of the dipper, and then prolonging it with a slight bend, as shown in fig. . arcturus and polaris form a large isosceles triangle with a first-magnitude star called _vega_. this triangle encloses at one corner the principal stars of boötes, and the head of the dragon near the opposite side. the side running from arcturus to vega passes through _corona borealis_, or the _northern crown_, and the body of _hercules_, which is marked by a quadrilateral of four stars. [illustration: fig. .] _boötes_, who is often represented as a husbandman, _corona borealis_, and _hercules_, are delineated in fig. . these constellations are visible in the evening from may to september. [illustration: fig. .] [illustration: fig. .] . _the lyre, the swan, the eagle, and the dolphin._--_altair_, the principal star of _aquila_, or the _eagle_, lies on the opposite side of the milky-way from vega. altair is a first-magnitude star, and has a faint star on each side of it, as shown in fig. . vega, also of the first magnitude, is the principal star of _lyra_, or the _lyre_. between these two stars, and a little farther to the north, are several stars arranged in the form of an immense cross. the bright star at the head of this cross is called _deneb_. the cross lies in the milky-way, and contains the chief stars of the constellation _cygnus_, or the _swan_. a little to the north of altair are four stars in the form of a diamond. this asterism is popularly known as _job's coffin_. these four stars are the chief stars of _delphinus_, or the _dolphin_. these four constellations are shown together in fig. . the _swan_ is visible from june to december, in the evening. [illustration: fig. .] . _virgo._--a line drawn from alpha to gamma of the dipper, and prolonged with a slight bend at gamma, will reach to a first-magnitude star called _spica_ (fig. ). this is the chief star of the constellation _virgo_, or the _virgin_, and forms a large isosceles triangle with _arcturus_ and _denebola_. [illustration: fig. .] _virgo_ is represented in fig. . to the right of this constellation, as shown in the figure, there are four stars which form a trapezium, and mark the constellation _corvus_, or the _crow_. this bird is represented as standing on the body of _hydra_, or the _water-snake_. _virgo_ is visible in the evening, from april to august. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] . _the twins._--a line drawn from delta to beta of the dipper, and prolonged as shown in fig. , passes between two bright stars called _castor_ and _pollux_. the latter of these is usually reckoned as a first-magnitude star. these are the principal stars of the constellation _gemini_, or the _twins_, which is shown in fig. . the constellation _canis minor_, or the _little dog_, is shown in the lower part of the figure. there are two conspicuous stars in this constellation, the brightest of which is of the first magnitude, and called _procyon_. the region to which we have now been brought is the richest of the northern sky, containing no less than seven first-magnitude stars. these are _sirius_, _procyon_, _pollux_, _capella_, _aldebaran_, _betelgeuse_, and _rigel_. they are shown in fig. . [illustration: fig. .] _betelgeuse_ and _rigel_ are in the constellation _orion_, being about equally distant to the north and south from the three stars forming the _belt_ of orion. betelgeuse is a red star. _sirius_ is the brightest star in the heavens, and belongs to the constellation _canis major_, or the _great dog_. it lies to the east of the belt of orion. _aldebaran_ lies at about the same distance to the west of the belt. it is a red star, and belongs to the constellation _taurus_, or the _bull_. _capella_ is in the constellation _auriga_, or the _wagoner_. these stars are visible in the evening, from about december to april. . _orion and his dogs, and taurus._--_orion_ and his _dogs_ are shown in fig. , and _orion_ and _taurus_ in fig. . _aldebaran_ marks one of the eyes of the bull, and is often called the _bull's eye_. the irregular _v_ in the face of the bull is called the _hyades_, and the cluster on the shoulder the _pleiades_. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] . _the wagoner._--the constellation _auriga_, or the _wagoner_ (sometimes called the _charioteer_), is shown in fig. . _capella_ marks the _goat_, which he is represented as carrying on his back, and the little right-angled triangle of stars near it the _kids_. the five chief stars of this constellation form a large, irregular pentagon. gamma of _auriga_ is also beta of _taurus_, and marks one of the horns of the _bull_. [illustration: fig. .] . _pegasus, andromeda, and perseus._--a line drawn from polaris near to beta of _cassiopeia_ will lead to a bright second-magnitude star at one corner of a large square (fig. ). alpha belongs both to the _square of pegasus_ and to _andromeda_. beta and gamma, which are connected with alpha in the figure by a dotted line, also belong to andromeda. _algol_, which forms, with the last-named stars and with the _square of pegasus_, an asterism similar in configuration to the _great dipper_, belongs to _perseus_. _algenib_, which is reached by bending the line at gamma in the opposite direction, is the principal star of _perseus_. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] _pegasus_ is shown in fig. , and _andromeda_ in fig. . _cetus_, the _whale_, or the _sea monster_, shown in fig. , belongs to the same mythological group of constellations. [illustration: fig. .] . _scorpio, sagittarius, and ophiuchus._--during the summer months a brilliant constellation is visible, called _scorpio_, or the _scorpion_. the configuration of the chief stars of this constellation is shown in fig. . they bear some resemblance to a boy's kite. the brightest star is of the first magnitude, and called _antares_ (from _anti_, instead of, and _ares_, the greek name of mars), because it rivals mars in redness. the stars in the tail of the scorpion are visible in our latitude only under very favorable circumstances. this constellation is shown in fig. , together with _sagittarius_ and _ophiuchus_. _sagittarius_, or the _archer_, is to the east of _scorpio_. it contains no bright stars, but is easily recognized from the fact that five of its principal stars form the outline of an inverted dipper, which, from the fact of its being partly in the milky-way, is often called the _milk dipper_. [illustration: fig. .] _ophiuchus_, or the _serpent-bearer_, is a large constellation, filling all the space between the head of _hercules_ and _scorpio_. it is difficult to trace, since it contains no very brilliant stars. this constellation and _libra_, or the _balances_, which is the zodiacal constellation to the west of scorpio, are shown in fig. . [illustration: fig. .] [illustration: fig. .] . _capricornus, aquarius, and the southern fish._--the two zodiacal constellations to the east of sagittarius are _capricornus_ and _aquarius_. _capricornus_ contains three pairs of small stars, which mark the head, the tail, and the knees of the animal. _aquarius_ is marked by no conspicuous stars. an irregular line of minute stars marks the course of the stream of water which flows from the water-bearer's urn into the mouth of the _southern fish_. this mouth is marked by the first-magnitude star _fomalhaut_. these constellations are shown in fig. . [illustration: fig. .] . _pisces and aries._--the remaining zodiacal constellations are _pisces_, or the _fishes_, _aries_, or the _ram_ (fig. ), and _cancer_, or the _crab_. the _fishes_ lie under _pegasus_ and _andromeda_, but contain no bright stars. _aries_ (between _pisces_ and _taurus_) is marked by a pair of stars on the head,--one of the second, and one of the third magnitude. _cancer_ (between _leo_ and _gemini_) has no bright stars, but contains a remarkable cluster of small stars called _præsepe_, or the _beehive_. clusters. . _the hyades._--the _hyades_ are a very open cluster in the face of _taurus_ ( ). the three brightest stars of this cluster form a letter _v_, the point of the _v_ being on the nose, and the open ends at the eyes. this cluster is shown in fig. . the name, according to the most probable etymology, means _rainy_; and they are said to have been so called because their rising was associated with wet weather. they were usually considered the daughters of atlas, and sisters of the pleiades, though sometimes referred to as the nurses of bacchus. [illustration: fig. .] . _the pleiades._--the _pleiades_ constitute a celebrated group of stars, or a miniature constellation, on the shoulder of _taurus_. hesiod mentions them as "the seven virgins of atlas born," and milton calls them "the seven atlantic sisters." they are referred to in the book of job. the spaniards term them "the little nanny-goats;" and they are sometimes called "the hen and chickens." [illustration: fig. .] [illustration: fig. .] usually only six stars in this cluster can be seen with the naked eye, and this fact has given rise to the legend of the "lost pleiad." on a clear, moonless night, however, a good eye can discern seven or eight stars, and some observers have distinguished as many as eleven. fig. shows the _pleiades_ as they appear to the naked eye under the most favorable circumstances. fig. shows this cluster as it appears in a powerful telescope. with such an instrument more than five hundred stars are visible. . _cluster in the sword-handle of perseus._--this is a somewhat dense double cluster. it is visible to the naked eye, appearing as a hazy star. a line drawn from _algenib_, or _alpha_ of _perseus_ ( ), to _delta_ of _cassiopeia_ ( ), will pass through this cluster at about two-thirds the distance from the former. this double cluster is one of the most brilliant objects in the heavens, with a telescope of moderate power. [illustration: fig. .] . _cluster of hercules._--the celebrated globular cluster of _hercules_ can be seen only with a telescope of considerable power, and to resolve it into distinct stars (as shown in fig. ) requires an instrument of the very highest class. [illustration: fig. .] . _other clusters._--fig. shows a magnificent globular cluster in the constellation _aquarius_. herschel describes it as appearing like a heap of sand, being composed of thousands of stars of the fifteenth magnitude. [illustration: fig. .] fig. shows a cluster in the constellation _toucan_, which sir john herschel describes as a most glorious globular cluster, the stars of the fourteenth magnitude being immensely numerous. there is a marked condensation of light at the centre. [illustration: fig. .] [illustration: fig. .] fig. shows a cluster in the _centaur_, which, according to the same astronomer, is beyond comparison the richest and largest object of the kind in the heavens, the stars in it being literally innumerable. fig. shows a cluster in _scorpio_, remarkable for the peculiar arrangement of its component stars. star clusters are especially abundant in the region of the milky-way, the law of their distribution being the reverse of that of the nebulæ. double and multiple stars. . _double stars._--the telescope shows that many stars which appear single to the naked eye are really _double_, or composed of a pair of stars lying side by side. there are several pairs of stars in the heavens which lie so near together that they almost seem to touch when seen with the naked eye. [illustration: fig. .] [illustration: fig. .] pairs of stars are not considered double unless the components are so near together that they both appear in the field of view when examined with a telescope. in the majority of the pairs classed as double stars the distance between the components ranges from half a second to fifteen seconds. [illustration: fig. .] _epsilon lyræ_ is a good example of a pair of stars that can barely be separated with a good eye. figs. and show this pair as it appears in telescopes magnifying respectively four and fifteen times; and fig. shows it as seen in a more powerful telescope, in which each of the two components of the pair is seen to be a truly double star. [illustration: fig. .] [illustration: fig. .] . _multiple stars._--when a star is resolved into more than two components by a telescope, it is called a _multiple_ star. fig. shows a _triple_ star in _pegasus_. fig. shows a quadruple star in _taurus_. fig. shows a _sextuple_ star, and fig. a _septuple_ star. fig. shows the celebrated septuple star in _orion_, called _theta orionis_, or the _trapezium_ of orion. . _optically double and multiple stars._--two or more stars which are really very distant from each other, and which have no physical connection whatever, may appear to be near together, because they happen to lie in the same direction, one behind the other. such accidental combinations are called _optically_ double or multiple stars. [illustration: fig. .] [illustration: fig. .] . _physically double and multiple stars._--in the majority of cases the components of double and multiple stars are in reality comparatively near together, and are bound together by gravity into a physical system. such combinations are called _physically_ double and multiple stars. the components of these systems all revolve around their common centre of gravity. in many instances their orbits and periods of revolution have been ascertained by observation and calculation. fig. shows the orbit of one of the components of a double star in the constellation _hercules_. [illustration: fig. .] . _colors of double and multiple stars._--the components of double and multiple stars are often highly colored, and frequently the components of the same system are of different colors. sometimes one star of a binary system is _white_, and the other _red_; and sometimes a _white_ star is combined with a _blue_ one. other colors found in combination in these systems are _red_ and _blue_, _orange_ and _green_, _blue_ and _green_, _yellow_ and _blue_, _yellow_ and _red_, etc. [illustration: fig. .] if these double and multiple stars are accompanied by planets, these planets will sometimes have two or more suns in the sky at once. on alternate days they may have suns of different colors, and perhaps on the same day two suns of different colors. the effect of these changing colored lights on the landscape must be very remarkable. new and variable stars. . _variable stars._--there are many stars which undergo changes of brilliancy, sometimes slight, but occasionally very marked. these changes are in some cases apparently irregular, and in others _periodic_. all such stars are said to be _variable_, though the term is applied especially to those stars whose variability is _periodic_. [illustration: fig. .] . _algol._--_algol_, a star of _perseus_, whose position is shown in fig. , is a remarkable variable star of a short period. usually it shines as a faint second-magnitude star; but at intervals of a little less than three days it fades to the fourth magnitude for a few hours, and then regains its former brightness. these changes were first noticed some two centuries ago, but it was not till that they were accurately observed. the period is now known to be two days, twenty hours, forty-nine minutes. it takes about four hours and a half to fade away, and four hours more to recover its brilliancy. near the beginning and end of the variations, the change is very slow, so that there are not more than five or six hours during which an ordinary observer would see that the star was less bright than usual. this variation of light was at first explained by supposing that a large dark planet was revolving round algol, and passed over its face at every revolution, thus cutting off a portion of its light; but there are small irregularities in the variation, which this theory does not account for. . _mira._--another remarkable variable star is _omicron ceti_, or _mira_ (that is, the _wonderful_ star). it is generally invisible to the naked eye; but at intervals of about eleven months it shines forth as a star of the second or third magnitude. it is about forty days from the time it becomes visible until it attains its greatest brightness, and is then about two months in fading to invisibility; so that its increase of brilliancy is more rapid than its waning. its period is quite irregular, ranging from ten to twelve months; so that the times of its appearance cannot be predicted with certainty. its maximum brightness is also variable, being sometimes of the second magnitude, and at others only of the third or fourth. [illustration: fig. .] . _eta argus._--perhaps the most extraordinary variable star in the heavens is _eta argus_, in the constellation _argo_, or the _ship_, in the southern hemisphere (fig. ). the first careful observations of its variability were made by sir john herschel while at the cape of good hope. he says, "it was on the th of december, , that, resuming the photometrical comparisons, my astonishment was excited by the appearance of a new candidate for distinction among the very brightest stars of the first magnitude in a part of the heavens where, being perfectly familiar with it, i was certain that no such brilliant object had before been seen. after a momentary hesitation, the natural consequence of a phenomenon so utterly unexpected, and referring to a map for its configuration with other conspicuous stars in the neighborhood, i became satisfied of its identity with my old acquaintance, _eta argus_. its light was, however, nearly tripled. while yet low, it equalled rigel, and, when it attained some altitude, was decidedly greater." it continued to increase until jan. , , then faded a little till april following, though it was still as bright as aldebaran. in and it blazed up brighter than ever, and in march of the latter year was second only to _sirius_. during the twenty-five years following it slowly but steadily diminished. in it was barely visible to the naked eye; and the next year it vanished entirely from the unassisted view, and has not yet begun to recover its brightness. the curve in fig. shows the change in brightness of this remarkable star. the numbers at the bottom show the years of the century, and those at the side the brightness of the star. [illustration: fig. .] . _new stars._--in several cases stars have suddenly appeared, and even become very brilliant; then, after a longer or shorter time, they have faded away and disappeared. such stars are called _new_ or _temporary_ stars. for a time it was supposed that such stars were actually new. they are now, however, classified by astronomers among the variable stars, their changes being of a very irregular and fitful character. there is scarcely a doubt that they were all in the heavens as very small stars before they blazed forth in so extraordinary a manner, and that they are in the same places still. there is a wide difference between these irregular variations, or the breaking-forth of light on a single occasion in the course of centuries, and the regular and periodic changes in the case of a star like _algol_; but a long series of careful observation has resulted in the discovery of stars of nearly every degree of irregularity between these two extremes. some of them change gradually from one magnitude to another, in the course of years, without seeming to follow any law whatever; while in others some slight tendency to regularity can be traced. _eta argus_ may be regarded as a connecting link between new and variable stars. . _tycho brahe's star._--an apparently new star suddenly appeared in _cassiopeia_ in . it was first seen by tycho brahe, and is therefore associated with his name. its position in the constellation is shown in fig. . it was first seen on nov. , when it had already attained the first magnitude. it became rapidly brighter, soon rivalling venus in splendor, so that good eyes could discern it in full daylight. in december it began to wane, and gradually faded until the following may, when it disappeared entirely. [illustration: fig. .] a star showed itself in the same part of the heavens in and in . if these were three appearances of the same star, it must be reckoned as a periodic star with a period of a little more than three hundred years. . _kepler's star._--in a new star was seen in the constellation _ophiuchus_. it was first noticed in october of that year, when it was of the first magnitude. in the following winter it began to fade, but remained visible during the whole year . early in it disappeared entirely. a very full history of this star was written by kepler. one of the most remarkable things about this star was its brilliant scintillation. according to kepler, it displayed all the colors of the rainbow, or of a diamond cut with multiple facets, and exposed to the rays of the sun. it is thought that this star also appeared in , , and ; if so, it is a variable star with a period of a little over four hundred years. . _new star of ._--the most striking case of this kind in recent times was in may, , when a star of the second magnitude suddenly appeared in _corona borealis_. on the th and th of that month it was observed independently by at least five observers in europe and america. the fact that none of these new stars were noticed until they had nearly or quite attained their greatest brilliancy renders it probable that they all blazed up very suddenly. . _cause of the variability of stars._--the changes in the brightness of variable and temporary stars are probably due to operations similar to those which produce the spots and prominences in our sun. we have seen ( ) that the frequency of solar spots shows a period of eleven years, during one portion of which there are few or no spots to be seen, while during another portion they are numerous. if an observer so far away as to see our sun like a star could from time to time measure its light exactly, he would find it to be a variable star with a period of eleven years, the light being least when we see most spots, and greatest when few are visible. the variation would be slight, but it would nevertheless exist. now, we have reason to believe that the physical constitution of the sun and the stars is of the same general nature. it is therefore probable, that, if we could get a nearer view of the stars, we should see spots on their disks as we do on the sun. it is also likely that the varying physical constitution of the stars might give rise to great differences in the number and size of the spots; so that the light of some of these suns might vary to a far greater degree than that of our own sun does. if the variations had a regular period, as in the case of our sun, the appearances to a distant observer would be precisely what we see in the case of a periodic variable star. the spectrum of the new star of was found to be a continuous one, crossed by bright lines, which were apparently due to glowing hydrogen. the continuous spectrum was also crossed by dark lines, indicating that the light had passed through an atmosphere of comparatively cool gas. mr. huggins infers from this that there was a sudden and extraordinary outburst of hydrogen gas from the star, which by its own light, as well as by heating up the whole surface of the star, caused the extraordinary increase of brilliancy. now, the spectroscope shows that the red flames of the solar chromosphere ( ) are largely composed of hydrogen; and it is not unlikely that the blazing-forth of this star arose from an action similar to that which produces these flames, only on an immensely larger scale. distance of the stars. . _parallax of the stars._--such is the distance of the stars, that only in a comparatively few instances has any displacement of these bodies been detected when viewed from opposite parts of the earth's orbit, that is, from points a hundred and eighty-five million miles apart; and in no case can this displacement be detected except by the most careful and delicate measurement. half of the above displacement, or the displacement of the star as seen from the earth instead of the sun, is called the _parallax_ of the star. in no case has a parallax of one second as yet been detected. . _the distance of the stars._--the distance of a star whose parallax is one second would be , times the distance of the earth from the sun, or about nineteen million million miles. it is quite certain that no star is nearer than this to the earth. light has a velocity which would carry it seven times and a half around the earth in a second; but it would take it more than three years to reach us from that distance. were all the stars blotted out of existence to-night, it would be at least three years before we should miss a single one. _alpha centauri_, the brightest star in the constellation of the _centaur_, is, so far as we know, the nearest of the fixed stars. it is estimated that it would take its light about three years and a half to reach us. it has also been estimated that it would take light over sixteen years to reach us from _sirius_, about eighteen years to reach us from _vega_, about twenty-five years from _arcturus_, and over forty years from the _pole-star_. in many instances it is believed that it would take the light of stars hundreds of years to make the journey to our earth, and in some instances even thousands of years. proper motion of the stars. . _why the stars appear fixed._--the stars seem to retain their relative positions in the heavens from year to year, and from age to age; and hence they have come universally to be denominated as _fixed_. it is, however, now well known that the stars, instead of being really stationary, are moving at the rate of many miles a second; but their distance is so enormous, that, in the majority of cases, it would be thousands of years before this rate of motion would produce a sufficient displacement to be noticeable to the unaided eye. [illustration: fig. .] . _secular displacement of the stars._--though the proper motion of the stars is apparently slight, it will, in the course of many ages, produce a marked change in the configuration of the stars. thus, in fig. , the left-hand portion shows the present configuration of the stars of the great dipper. the small arrows attached to the stars show the direction and comparative magnitudes of their motion. the right-hand portion of the figure shows these stars as they will appear thirty-six thousand years from the present time. [illustration: fig. .] fig. shows in a similar way the present configuration and proper motion of the stars of _cassiopeia_, and also these stars as they will appear thirty-six thousand years hence. [illustration: fig. .] fig. shows the same for the constellation _orion_. . _the secular motion of the sun._--the stars in all parts of the heavens are found to move in all directions and with all sorts of velocities. when, however, the motions of the stars are averaged, there is found to be an apparent proper motion common to all the stars. the stars in the neighborhood of _hercules_ appear to be approaching us, and those in the opposite part of the heavens appear to be receding from us. in other words, all the stars appear to be moving away from hercules, and towards the opposite part of the heavens. [illustration: fig. .] this apparent motion common to all the stars is held by astronomers to be due to the real motion of the sun through space. the point in the heavens towards which our sun is moving at the present time is indicated by the small circle in the constellation hercules in fig. . as the sun moves, he carries the earth and all the planets along with him. fig. shows the direction of the sun's motion with reference to the ecliptic and to the axis of the earth. fig. shows the earth's path in space; and fig. shows the paths of the earth, the moon, mercury, venus, and mars in space. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] whether the sun is actually moving in a straight line, or around some distant centre, it is impossible to determine at the present time. it is estimated that the sun is moving along his path at the rate of about a hundred and fifty million miles a year. this is about five-sixths of the diameter of the earth's orbit. . _star-drift._--in several instances, groups of stars have a common proper motion entirely different from that of the stars around and among them. such groups probably form connected systems, in the motion of which all the stars are carried along together without any great change in their relative positions. the most remarkable case of this kind occurs in the constellation _taurus_. a large majority of the brighter stars in the region between _aldebaran_ and the _pleiades_ have a common proper motion of about ten seconds per century towards the east. proctor has shown that five out of the seven stars which form the great dipper have a common proper motion, as shown in fig. (see also fig. ). he proposes for this phenomenon the name of _star-drift_. [illustration: fig. .] . _motion of stars along the line of sight._--a motion of a star in the direction of the line of sight would produce no displacement of the star that could be detected with the telescope; but it would cause a change in the brightness of the star, which would become gradually fainter if moving from us, and brighter if approaching us. motion along the line of sight has, however, been detected by the use of the tele-spectroscope ( ), owing to the fact that it causes a displacement of the spectral lines. as has already been explained ( ), a displacement of a spectral line towards the red end of the spectrum indicates a motion away from us, and a displacement towards the violet end, a motion towards us. * * * * * by means of these displacements of the spectral lines, huggins has detected motion in the case of a large number of stars, and calculated its rate:-- stars receding from us. sirius miles per second. betelgeuse miles per second. rigel miles per second. castor miles per second. regulus miles per second. stars approaching us. arcturus miles per second. vega miles per second. deneb miles per second. pollux miles per second. alpha ursæ majoris miles per second. these results are confirmed by the fact that the amount of motion indicated is about what we should expect the stars to have, from their observed proper motions, combined with their probable distances. again: the stars in the neighborhood of hercules are mostly found to be approaching the earth, and those which lie in the opposite direction to be receding from it; which is exactly the effect which would result from the sun's motion through space. the five stars in the dipper, which have a common proper motion, are also found to have a common motion in the line of sight. but the displacement of the spectral lines is so slight, and its measurement so difficult, that the velocities in the above table are to be accepted as only an approximation to the true values. chemical and physical constitution of the stars. . _the constitution of the stars similar to that of the sun._--the stellar spectra bear a general resemblance to that of the sun, with characteristic differences. these spectra all show fraunhofer's lines, which indicate that their luminous surfaces are surrounded by atmospheres containing absorbent vapors, as in the case of the sun. the positions of these lines indicate that the stellar atmospheres contain elements which are also found in the sun's, and on the earth. [illustration: fig. .] . _four types of stellar spectra._--the spectra of the stars have been carefully observed by secchi and huggins. they have found that stellar spectra may be reduced to four types, which are shown in fig. . in the spectrum of _sirius_, a representative of _type i._, very few lines are represented; but the lines are very thick. next we have the solar spectrum, which is a representative of _type ii._, one in which more lines are represented. in _type iii._ fluted spaces begin to appear, and in _type iv._, which is that of the red stars, nothing but fluted spaces is visible; and this spectrum shows that something is at work in the atmosphere of those red stars different from what there is in the simpler atmosphere of _type i._ lockyer holds that these differences of spectra are due simply to differences of temperature. according to him, the red stars, which give the fluted spectra, are of the lowest temperature; and the temperature of the stars of the different types gradually rises till we reach the first type, in which the temperature is so high that the dissociation ( ) of the elements is nearly if not quite complete. iii. nebulÆ. classification of nebulæ. . _planetary nebulæ._--many nebulæ ( ) present a well-defined circular disk, like that of a planet, and are therefore called _planetary_ nebulæ. specimens of planetary nebulæ are shown in fig. . [illustration: fig. .] . _circular and elliptical nebulæ._--while many nebulæ are circular in form, others are elliptical. the former are called _circular_ nebulæ, and the latter _elliptical_ nebulæ. elliptical nebulæ have been discovered of every degree of eccentricity. examples of various circular and elliptical nebulæ are given in fig. . [illustration: fig. .] . _annular nebulæ._--occasionally ring-shaped nebulæ have been observed, sometimes with, and sometimes without, nebulous matter within the ring. they are called _annular_ nebulæ. they are both circular and elliptical in form. several specimens of this class of nebulæ are given in fig. . [illustration: fig. .] . _nebulous stars._--sometimes one or more minute stars are enveloped in a nebulous haze, and are hence called _nebulous stars_. several of these nebulæ are shown in fig. . [illustration: fig. .] . _spiral nebulæ._--very many nebulæ disclose a more or less spiral structure, and are known as _spiral_ nebulæ. they are illustrated in fig. . there are, however, a great variety of spiral forms. we shall have occasion to speak of these nebulæ again ( - ). [illustration: fig. .] . _double and multiple nebulæ._--many _double_ and _multiple_ nebulæ have been observed, some of which are represented in fig. . [illustration: fig. .] fig. shows what appears to be a double annular nebula. fig. gives two views of a double nebula. the change of position in the components of this double nebula indicates a motion of revolution similar to that of the components of double stars. [illustration: fig. .] [illustration: fig. .] irregular nebulæ. . _irregular forms._--besides the more or less regular forms of nebulæ which have been classified as indicated above, there are many of very irregular shapes, and some of these are the most remarkable nebulæ in the heavens. fig. shows a curiously shaped nebula, seen by sir john herschel in the southern heavens; and fig. , one in _taurus_, known as the _crab_ nebula. [illustration: fig. .] [illustration: fig. .] . _the great nebula of andromeda._--this is one of the few nebulæ that are visible to the naked eye. we see at a glance that it is not a star, but a mass of diffused light. indeed, it has sometimes been very naturally mistaken for a comet. it was first described by marius in , who compared its light to that of a candle shining through horn. this gives a very good idea of the impression it produces, which is that of a translucent object illuminated by a brilliant light behind it. with a small telescope it is easy to imagine it to be a solid like horn; but with a large one the effect is more like fog or mist with a bright body in its midst. unlike most of the nebulæ, its spectrum is a continuous one, similar to that from a heated solid, indicating that the light emanates, not from a glowing gas, but from matter in the solid or liquid state. this would suggest that it is really an immense star-cluster, so distant that the highest telescopic power cannot resolve it; yet in the largest telescopes it looks less resolvable, and more like a gas, than in those of moderate size. if it is really a gas, and if the spectrum is continuous throughout the whole extent of the nebula, either it must shine by reflected light, or the gas must be subjected to a great pressure almost to its outer limit, which is hardly possible. if the light is reflected, we cannot determine whether it comes from a single bright star, or a number of small ones scattered through the nebula. with a small telescope this nebula appears elliptical, as in fig. . fig. shows it as it appeared to bond, in the cambridge refractor. [illustration: fig. .] [illustration: fig. .] . _the great nebula of orion._--the nebula which, above all others, has occupied the attention of astronomers, and excited the wonder of observers, is the _great nebula of orion_, which surrounds the middle star of the three which form the sword of orion. a good eye will perceive that this star, instead of looking like a bright point, has a hazy appearance, due to the surrounding nebula. this object was first described by huyghens in , as follows:-- "there is one phenomenon among the fixed stars worthy of mention, which, so far as i know, has hitherto been noticed by no one, and indeed cannot be well observed except with large telescopes. in the sword of orion are three stars quite close together. in , as i chanced to be viewing the middle one of these with the telescope, instead of a single star, twelve showed themselves (a not uncommon circumstance). three of these almost touched each other, and with four others shone through a nebula, so that the space around them seemed far brighter than the rest of the heavens, which was entirely clear, and appeared quite black; the effect being that of an opening in the sky, through which a brighter region was visible." [illustration: fig. .] the representation of this nebula in fig. is from a drawing made by bond. in brilliancy and variety of detail it exceeds any other nebula visible in the northern hemisphere. in its centre are four stars, easily distinguished by a small telescope with a magnifying power of forty or fifty, together with two smaller ones, requiring a nine-inch telescope to be well seen. besides these, the whole nebula is dotted with stars. in the winter of - the spectrum of this nebula was examined independently by secchi and huggins, who found that it consisted of three bright lines, and hence concluded that the nebula was composed, not of stars, but of glowing gas. the position of one of the lines was near that of a line of nitrogen, while another seemed to coincide with a hydrogen line. this would suggest that the nebula is a mixture of hydrogen and nitrogen gas; but of this we cannot be certain. [illustration: fig. .] . _the nebula in argus._--there is a nebula (fig. ) surrounding the variable star _eta argus_ ( ), which is remarkable as exhibiting variations of brightness and of outline. in many other nebulæ, changes have been suspected; but the indistinctness of outline which characterizes most of these objects, and the very different aspect they present in telescopes of different powers, render it difficult to prove a change beyond a doubt. . _the dumb-bell nebula._--this nebula was named from its peculiar shape. it is a good illustration of the change in the appearance of a nebula when viewed with different magnifying powers. fig. shows it as it appeared in herschel's telescope, and fig. as it appears in the great parsonstown reflector ( ). [illustration: fig. .] [illustration: fig. .] spiral nebulæ. . _the spiral nebula in canes venatici._--the great spiral nebula in the constellation _canes venatici_, or the _hunting-dogs_, is one of the most remarkable of its class. fig. shows this nebula as it appeared in herschel's telescope, and fig. shows it as it appears in the parsonstown reflector. [illustration: fig. .] [illustration: fig. .] . _condensation of nebulæ._--the appearance of the nebula just mentioned suggests a body rotating on its axis, and undergoing condensation at the same time. it is now a generally received theory that nebulæ are the material out of which stars are formed. according to this theory, the stars originally existed as nebulæ, and all nebulæ will ultimately become condensed into stars. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] . _other spiral nebulæ._--fig. represents a spiral nebula of the _great bear_. this nebula seems to have several centres of condensation. fig. is a view of a spiral nebula in _cepheus_, and fig. of a singular spiral nebula in the _triangle_. this also appears to have several points of condensation. figs. and represent oval and elliptical nebulæ having a spiral structure. [illustration: fig. .] [illustration: fig. .] _the magellanic clouds._ [illustration: fig. .] . _situation and general appearance of the magellanic clouds._--the _magellanic clouds_ are two nebulous-looking bodies near the southern pole of the heavens, as shown in the right-hand portion of fig. . in the appearance and brightness of their light they resemble portions of the milky-way. [illustration: fig. .] the larger of these clouds is called the _nubecula major_. it is visible to the naked eye in strong moonlight, and covers a space about two hundred times the surface of the moon. it is shown in fig. . the smaller cloud is called the _nubecula minor_. it has only about a fourth the extent of the larger cloud, and is considerably less brilliant. it is visible to the naked eye, but it disappears in full moonlight. this cloud is shown in fig. . the region around this cloud is singularly bare of stars; but the magnificent cluster of _toucan_, already described ( ), is near, and is shown a little to the right of the cloud in the figure. [illustration: fig. .] [illustration: fig. .] . _structure of the nubeculæ._--fig. shows the structure of these clouds as revealed by a powerful telescope. the general ground of both consists of large tracts and patches of nebulosity in every stage of resolution,--from that which is irresolvable with eighteen inches of reflecting aperture, up to perfectly separated stars, like the milky-way and clustering groups. there are also nebulæ in abundance, both regular and irregular, globular clusters in every state of condensation, and objects of a nebulous character quite peculiar, and unlike any thing in other regions of the heavens. in the area occupied by the _nubecula major_ two hundred and seventy-eight nebulæ and clusters have been enumerated, besides fifty or sixty outliers, which ought certainly to be reckoned as its appendages, being about six and a half per square degree; which very far exceeds the average of any other part of the nebulous heavens. in the _nubecula minor_ the concentration of such objects is less, though still very striking. the nubeculæ, then, combine, each within its own area, characters which in the rest of the heavens are no less strikingly separated; namely, those of the galactic and the nebular system. globular clusters (except in one region of small extent) and nebulæ of regular elliptic forms are comparatively rare in the milky-way, and are found congregated in the greatest abundance in a part of the heavens the most remote possible from that circle; whereas in the nubeculæ they are indiscriminately mixed with the general starry ground, and with irregular though small nebulæ. the nebular hypothesis. . _the basis of the nebular hypothesis._--we have seen that the planets all revolve around the sun from west to east in nearly the same plane, and that the sun rotates on his axis from west to east. the planets, so far as known, rotate on their axes from west to east; and all the moons, except those of uranus and neptune, revolve around their planets from west to east. these common features in the motion of the sun, moons, and planets, point to the conclusion that they are of a common origin. . _kant's hypothesis._--kant, the celebrated german philosopher, seems to have the best right to be regarded as the founder of the modern nebular hypothesis. his reasoning has been concisely stated thus: "examining the solar system, we find two remarkable features presented to our consideration. one is, that six planets and nine satellites [the entire number then known] move around the sun in circles, not only in the same direction in which the sun himself revolves on his axis, but very nearly in the same plane. this common feature of the motion of so many bodies could not by any reasonable possibility have been a result of chance: we are therefore forced to believe that it must be the result of some common cause originally acting on all the planets. "on the other hand, when we consider the spaces in which the planets move, we find them entirely void, or as good as void; for, if there is any matter in them, it is so rare as to be without effect on the planetary motions. there is, therefore, no material connection now existing between the planets through which they might have been forced to take up a common direction of motion. how, then, are we to reconcile this common motion with the absence of all material connection? the most natural way is to suppose that there was once some such connection, which brought about the uniformity of motion which we observe; that the materials of which the planets are formed once filled the whole space between them. there was no formation in this chaos, the formation of separate bodies by the mutual gravitation of parts of the mass being a later occurrence. but, naturally, some parts of the mass would be more dense than others, and would thus gather around them the rare matter which filled the intervening spaces. the larger collections thus formed would draw the smaller ones into them, and this process would continue until a few round bodies had taken the place of the original chaotic mass." kant, however, failed to account satisfactorily for the motion of the sun and planets. according to his system, all the bodies formed out of the original nebulous mass should have been drawn to a common centre so as to form one sun, instead of a system of revolving bodies like the solar system. . _herschel's hypothesis._--the idea of the gradual transmutation of nebulæ into stars seems to have been suggested to herschel, not by the study of the solar system, but by that of the nebulæ themselves. many of these bodies he believed to be immense masses of phosphorescent vapor; and he conceived that these must be gradually condensing, each around its own centre, or around the parts where it is most dense, until it should become a star, or a cluster of stars. on classifying the nebulæ, it seemed to him that he could see this process going on before his eyes. there were the large, faint, diffused nebulæ, in which the condensation had hardly begun; the smaller but brighter ones, which had become so far condensed that the central parts would soon begin to form into stars; yet others, in which stars had actually begun to form; and, finally, star-clusters in which the condensation was complete. the spectroscopic revelations of the gaseous nature of the true nebulæ tend to confirm the theory of herschel, that these masses will all, at some time, condense into stars. . _laplace's hypothesis._--laplace was led to the nebular hypothesis by considering the remarkable uniformity in the direction of the rotation of the planets. believing that this could not have been the result of chance, he sought to investigate its cause. this, he thought, could be nothing else than the atmosphere of the sun, which once extended so far out as to fill all the space now occupied by the planets. he begins with the sun, surrounded by this immense fiery atmosphere. since the sum total of rotary motion now seen in the planetary system must have been there from the beginning, he conceives the immense vaporous mass forming the sun and his atmosphere to have had a slow rotation on its axis. as the intensely hot mass gradually cooled, it would contract towards the centre. as it contracted, its velocity of rotation would, by the laws of mechanics, constantly increase; so that a time would arrive, when, at the outer boundary of the mass, the centrifugal force due to the rotation would counterbalance the attractive force of the central mass. then those outer portions would be left behind as a revolving ring, while the next inner portions would continue to contract until the centrifugal and attractive forces were again balanced, when a second ring would be left behind; and so on. thus, instead of a continuous atmosphere, the sun would be surrounded by a series of concentric revolving rings of vapor. as these rings cooled, their denser materials would condense first; and thus the ring would be composed of a mixed mass, partly solid and partly vaporous, the quantity of solid matter constantly increasing, and that of vapor diminishing. if the ring were perfectly uniform, this condensation would take place equally all around it, and the ring would thus be broken up into a group of small planets, like the asteroids. but if, as would more likely be the case, some portions of the ring were much denser than others, the denser portions would gradually attract the rarer portions, until, instead of a ring, there would be a single mass composed of a nearly solid centre, surrounded by an immense atmosphere of fiery vapor. this condensation of the ring of vapor around a single point would not change the amount of rotary motion that had existed in the ring. the planet with its atmosphere would therefore be in rotation; and would be, on a smaller scale, like the original solar mass surrounded by its atmosphere. in the same way that the latter formed itself first into rings, which afterwards condensed into planets, so the planetary atmospheres, if sufficiently extensive, would form themselves into rings, which would condense into satellites. in the case of saturn, however, one of the rings was so uniform throughout, that there was no denser portion to attract the rest around it; and thus the ring of saturn retained its annular form. [illustration: fig. .] such is the celebrated nebular hypothesis of laplace. it starts, not with a purely nebulous mass, but with the sun, surrounded by an immense atmosphere, out of which the planets were formed by gradual condensation. fig. represents the condensing mass according to this theory. . _the modern nebular hypothesis._--according to the nebular hypothesis as held at the present time, the sun, planets, and meteoroids originated from a purely nebulous mass. this nebula first condensed into a nebulous star, the star being the sun, and its surrounding nebulosity being the fiery atmosphere of laplace. the original nebula must have been put into rotation at the beginning. as it contracted and became condensed through the loss of heat by radiation into space, and under the combined attraction of gravity, cohesion, and affinity, its speed of rotation increased; and the nebulous envelop became, by the centrifugal force, flattened into a thin disk, which finally broke up into rings, out of which were formed the planets and their moons. according to laplace, the rings which were condensed into the planets were thrown off in succession from the equatorial region of the condensing nebula; and so the outer planets would be the older. according to the more modern idea, the nebulous mass was first flattened into a disk, and subsequently broken up into rings, in such a way that there would be no marked difference in the ages of the planets. the sun represents the central portion of the original nebula, and the comets and meteoroids its outlying portion. at the sun the condensation is still going on, and the meteoroids appear to be still gradually drawn in to the sun and planets. the whole store of energy with which the original solar nebula was endowed existed in it in the potential form. by the condensation and contraction this energy was gradually transformed into the kinetic energy of molar motion and of heat; and the heat became gradually dissipated by radiation into space. this transformation of potential energy into heat is still going on at the sun, the centre of the condensing mass, by the condensation of the sun itself, and by the impact of meteors as they fall into it. it has been calculated, that, by the shrinking of the sun to the density of the earth, the transformation of potential energy into heat would generate enough heat to maintain the sun's supply, at the present rate of dissipation, for seventeen million years. a shrinkage of the sun which would generate all the heat he has poured into space since the invention of the telescope could not be detected by the most powerful instruments yet constructed. the least velocity with which a meteoroid could strike the sun would be two hundred and eighty miles a second; and it is easy to calculate how much heat would be generated by the collision. it has been shown, that, were enough meteoroids to fall into the sun to develop its heat, they would not increase his mass appreciably during a period of two thousand years. the sun's heat is undoubtedly developed by contraction and the fall of meteoroids; that is to say, by the transformation of the potential energy of the original nebula into heat. it must be borne in mind that the nebular hypothesis is simply a supposition as to the way in which the present solar system may have been developed from a nebula endowed with a motion of rotation and with certain tendencies to condensation. of course nothing could have been developed out of the nebula, the germs of which had not been originally implanted in it by the creator. iv. the structure of the stellar universe. . _sir william herschel's view._--sir william herschel assumed that the stars are distributed with tolerable uniformity throughout the space occupied by our stellar system. he accounted for the increase in the number of stars in the field of view as he approached the plane of the milky-way, not by the supposition that the stars are really closer together in and about this plane, but by the supposition that our stellar system is in the form of a flat disk cloven at one side, and with our sun near its centre. a section of this disk is shown in fig. . [illustration: fig. .] an observer near _s_, with his telescope pointed in the direction of _s b_, would see comparatively few stars within the field of view, because looking through a comparatively thin stratum of stars. with his telescope pointed in the direction _s a_, he would see many more stars within his field of view, even though the stars were really no nearer together, because he would be looking through a thicker stratum of stars. as he directed his telescope more and more nearly in the direction _s f_, he would be looking through a thicker and thicker stratum of stars, and hence he would see a greater and greater number of them in the field of view, though they were everywhere in the disk distributed at uniform distances. he assumed, also, that the stars are all tolerably uniform in size, and that certain stars appear smaller than others, only because they are farther off. he supposed the faint stars of the milky-way to be merely the most distant stars of the stellar disk; that they are really as large as the other stars, but appear small owing to their great distance. the disk was assumed to be cloven on one side, to account for the division of the milky-way through nearly half of its course. this theory of the structure of the stellar universe is often referred to as the _cloven disk_ theory. [illustration: fig. .] . _the cloven ring theory._--according to mädler, the stars of the milky-way are entirely separated from the other stars of our system, belonging to an outlying ring, or system of rings. to account for the division of the milky-way, the ring is supposed to be cloven on one side: hence this theory is often referred to as the _cloven ring_ theory. according to this hypothesis, the stellar system viewed from without would present an appearance somewhat like that in fig. . the outlying ring cloven on one side would represent the stars of the milky-way; and the luminous mass at the centre, the remaining stars of the system. . _proctor's view._--according to proctor, the milky-way is composed of an irregular spiral stream of minute stars lying in and among the larger stars of our system, as represented in fig. . the spiral stream is shown in the inner circle as it really exists among the stars, and in the outer circle as it is seen projected upon the sky. according to this view, the stars of the milky-way appear faint, not because they are distant, but because they are really small. [illustration: fig. .] . _newcomb's view._--according to newcomb, the stars of our system are all situated in a comparatively thin zone lying in the plane of the milky-way, while there is a zone of nebulæ lying on each side of the stellar zone. he believes that so much is certain with reference to the structure of our stellar universe; but he considers that we are as yet comparatively ignorant of the internal structure of either the stellar or the nebular zones. the structure of the stellar universe, according to this view, is shown in fig. . [illustration: fig. .] index a. aberration of light, . aerolites, . aldebaran, star in taurus, , . algol, a variable star, , . almanac, perpetual, . alps, lunar mountains, . altair, star in aquila, . alt-azimuth instrument, . altitude, . andromeda (constellation), , . nebula in, . angström's map of spectrum, . antares, star in scorpio, . apennines, lunar mountains, , . aphelion, . apogee, . aquarius, or the water-bearer, . cluster in, . aquila, or the eagle, . arcturus, star in boötes, , , . argo, or the ship, . nebula in, . variable star in, . aries, or the ram, . asteroids, , . astræa, an asteroid, . auriga, or the wagoner, . azimuth, . b. betelgeuse, star in orion, , . berenice's hair (constellation), . bode's law, . disproved, . boötes (constellation), , . c. calendar, the, . callisto, moon of jupiter, . cancer, or the crab, . tropic of, . canes venatici, or the hunting-dogs, . canes venatici, nebula in, . canis major, or the great dog, . canis minor, or the little dog, . capella, star in auriga, , . capricorn, tropic of, . capricornus, or the goat, . cassiopeia (constellation), . new star in, . castor, star in gemini, , . caucasus, a lunar range, . centaurus, star-cluster in, . cepheus (constellation), . nebula in, . ceres, the planet, . cetus, or the whale, . variable star in, . charles's wain, . circles, great, . diurnal, . hour, . small, . vertical, . clock, astronomical, . time, . coma berenices, or berenice's hair, . comet, biela's, . and earth, collision of, . coggia's, . donati's, . encke's, . halley's, . of , . of , . of , . of , . of june, , . comets, appearance of, . and meteors, . bright, . chemical constitution of, . development of, . number of, . orbits of, . origin of, . periodic, . physical constitution of, . tails of, . telescopic, , . visibility of, . conic sections, . conjunction, . inferior, . superior, , . constellations, . zodiacal, . copernican system, the, , . copernicus, a lunar crater, , . corona borealis, or the northern crown, . corona borealis, new star in, . corvus, or the crow, . crystalline spheres, . cycles and epicycles, . cygnus, or the swan, . d. day and night, . civil, . lunar, . sidereal, . solar, . declination, . deimos, satellite of mars, . delphinus, or the dolphin, . deneb, star in cygnus, , . dione, satellite of saturn, . dipper, the great, , , , . the little, . the milk, . dissociation, . dominical letter, the, . draco, or the dragon, . e. earth, density of, . flattened at poles, . form of, . in space, . seen from moon, . size of, . weight of, . eccentric, the . eccentricity, . eclipses, . annular, . lunar, , . solar, . ecliptic, the, . obliquity of, . ellipse, the, , . elongation, of planet, . enceladus, moon of saturn, . epicycles, . epicycloid, . epsilon lyræ, a double star, . equator, the celestial, . equinoctial, the, . colure, . elevation of, . equinox, autumnal, . vernal, , . equinoxes, precession of, , . eta argus, a variable star, , . europa, moon of jupiter, . f. faculæ, solar, . fomalhaut, star in southern fish, . fraunhofer's lines, , . g. galaxy, the, . ganymede, moon of jupiter, . gemini, or the twins, . georgium sidus, . h. hercules (constellation), . cluster in, . orbit of double star in, . solar system moving towards, . herschel, the planet (see uranus). herschel's hypothesis, , . horizon, the, . hyades, the, , . hydra, or the water-snake, . hyperbola, the, . hyperion, moon of saturn, . i. io, moon of jupiter, . irradiation, , . j. japetus, moon of saturn, . job's coffin (asterism), . juno, the planet, . jupiter, apparent size of, . distance of, . great red spot of, . orbit of, . periods of, . physical constitution of, . rotation of, . satellites of, . eclipses of, . transits of, . volume of, . without satellites, . k. kant's hypothesis, . kepler, a lunar crater, . kepler's system, . laws, . star, . kirchhoff's map of spectrum, . l. laplace's hypothesis, . latitude, celestial, . leap year, . leo, or the lion, . leonids (meteors), . libra, or the balances, . libration, . longitude, celestial, . lyra, or the lyre, . double star in, . m. magellanic clouds, the, . magnetic storms, . magnetism and sun-spots, . mars, apparent size of, . brilliancy of, . distance of, . orbit of, . periods of, . rotation of, . satellites of, . volume of, . mercury, apparent size of, . atmosphere of, . distance of, . elongation of, . orbit of, . periods of, . volume of, . meridian, the, . meridian circle, . meridians, celestial, . meteoric iron, , . showers, . stones, . meteors, . august, . light of, . november, . sporadic, . meteoroids, . micrometers, , . milky-way, the, . mimas, moon of saturn, . mira, a variable star, . moon, apparent size of, , . aspects of, . atmosphere of, . chasms in, . craters in, . day of, . distance of, . eclipses of, . form of orbit, . harvest, . hunter's, . inclination of orbit, . kept in her path by gravity, . librations of, . mass of, . meridian altitude of, . mountains of, . orbital motion of, . phases of, . real size of, . rising of, . rotation of, . sidereal period of, . surface of, . synodical period of, . terminator of, . wet and dry, . n. nadir, the, . neap-tides, . nebula, in andromeda, . crab, . dumb-bell, . in argus, . in canes venatici, . in cepheus, . in orion, . in the triangle, . in ursa major, . nebulæ, , , . annular, . circular, . condensation of, . double, . elliptical, . irregular, . multiple, . spiral, , . nebular hypothesis, the, . neptune, discovery of, . orbit of, . satellite of, . new style, . newcomb's theory of the stellar universe, . newton's system, . nodes, . nubecula, major, . minor, . nutation, . o. olbers's hypothesis, . old style, . ophiuchus (constellation), . new star in, . opposition, , . orion, . nebula in, . the trapezium of, . p. pallas, the planet, . parabola, the, . parallax, . pegasus (constellation), , . triple star in, . perigee, . perihelion, . perseids (meteors), . perseus (constellation), . cluster in, . phobos, satellite of mars, . pico, a lunar mountain, . pisces, or the fishes, . piscis australis, or the southern fish, . planets, . inferior, . periods of, . phases of, . inner group of, . intra-mercurial, . minor, . outer group of, , . superior, . motion of, . periods of, . phases of, . three groups of, . pleiades, the, , , . pointers, the, . polar distance, . pole star, the, , , . poles, celestial, , . pollux, star in gemini, , . præsepe, or the beehive, . precession of equinoxes, , . prime vertical, the, . proctor's theory of the stellar universe, . procyon, star in canis minor, . ptolemaic system, the, . q. quadrature, , . r. radiant point (meteors), . radius vector, . refraction, . regulus, star in leo, , . rhea, moon of saturn, . rigel, star in orion, , . right ascension, . s. sagittarius, or the archer, . saturn, apparent size of, . distance of, . orbit of, . periods of, . physical constitution of, . ring of, . changes in, . constitution of, . phases of, . rotation of, . satellites of, . volume of, . scorpio, or the scorpion, . cluster in, . seasons, the, . sirius, the dog-star, , , , , . solar system, the, . solstices, , , . sound, effect of motion on, . spectra, bright-lined, . comparison of, . continuous, . displacement of lines in, . of comets, . reversed, . sun-spot, . types of stellar, . spectroscope, the, . diffraction, . direct-vision, . dispersion, . spectrum analysis, . solar, . sphere, defined, . the celestial, . rotation of, . spring-tides, . stars, circumpolar, . clusters of, , . color of, . constellations of, . constitution of, . distance of, . double, . drift of, . four sets of, . magnitude of, . motion of, in line of sight, . multiple, . names of, . nebulous, . new, . number of, . parallax of, . proper motion of, . secular displacement of, . temporary, . variable, . sun, atmosphere of, . brightness of, . chemical constitution of, . chromosphere of, , . corona of, , , . distance of, . faculæ of, . heat radiated by, . inclination of axis of, . mass of, . motion of, among the stars, . at surface of, . in atmosphere of, . secular, . photosphere of, , . prominences of, , . rotation of, . spectrum of, , . temperature of, . volume of, . winds on, . sun-spots, . and magnetism, . birth and decay of, . cause of, . cyclonic motion in, . distribution of, . duration of, . groups of, . periodicity of, . proper motion of, . size of, . spectrum of, . t. taurus, or the bull, . quadruple star in, . telescope, cassegrainian, . equatorial, . front-view, . gregorian, . herschelian, . lord rosse's, . melbourne, . newall, . newtonian, . paris, . reflecting, . washington, . vienna, . telespectroscope, the, . telluric lines of spectrum, . tethys, moon of saturn, . tides, . time, clock, . sun, . titan, moon of saturn, , . toucan, star cluster in, , . transit instrument, . transits of venus, . triesneker, lunar formation, . tropics, . twilight, . tycho brahe's star, . system, . tycho, a lunar crater, . u. universe, structure of the stellar, . uranus, discovery of, . name of, . orbit of, . satellites of, . ursa major, or the great bear, . nebula in, . ursa minor, or the little bear, . v. vega, star in lyra, , , . venus, apparent size of, . atmosphere of, . brilliancy of, . distance of, . elongation of, . orbit of, . periods of, . volume of, . transits of, , . vernier, the, . virgo, or the virgin, . vesta, the planet, . vulcan, the planet, . y. year, the, . anomalistic, . julian, . sidereal, . tropical, . z. zenith, the, . distance, . zodiac, the, . zodiacal constellations, . light, . zones, . * * * * * * transcriber's note: missing or obscured punctuation was corrected. typographical errors were silently corrected. astronomy for young folks [illustration: northern portion of the moon at last quarter taken with -inch hooker telescope of the mt. wilson observatory (see chapter xxi)] astronomy _for_ young folks by isabel martin lewis, a. m. (_connected with the nautical almanac office of the u. s. naval observatory_) new york duffield and company copyright, , by the century company copyright, , by duffield and company printed in u. s. a. contents chapter page preface xiii i. the constellations ii. january iii. february iv. march v. april vi. may vii. june viii. july ix. august x. september xi. october xii. november xiii. december xiv. stars of the southern hemisphere xv. the milky way or galaxy xvi. the surface of the sun xvii. the solar system xviii. the origin of the earth xix. jupiter and his nine moons xx. the rings and moons of saturn xxi. is the moon a dead world xxii. comets xxiii. meteorites xxiv. the earth as a magnet xxv. some effects of the earth's atmosphere upon sunlight xxvi. keeping track of the moon xxvii. the motions of the heavenly bodies xxviii. the evolution of the stars--from red giants to red dwarfs xxix. double and multiple stars xxx. astronomical distances xxxi. some astronomical facts worth remembering illustrations page northern portion of the moon at last quarter _frontispiece_ the great hercules cluster--a universe of suns _facing page_ a dark nebula: the dark bay or dark horse nebula in orion _facing page_ a. venus. b. mars. c. jupiter. d. saturn _facing page_ spiral nebula in canes venatici _facing page_ spiral nebula in andromeda viewed edgewise _facing page_ list of tables page i. the principal elements of the solar system ii. the satellites of the solar system iii. the twenty brightest stars in the heavens iv. a list of the principal constellations - v. pronunciations and meanings of names of stars and constellations - preface astronomy, it has been said, is the oldest and the noblest of the sciences. yet it is one of the few sciences for which most present-day educators seem to find little, if any, room in their curriculum of study for the young, in spite of its high cultural value. it is, we are told, too abstruse a subject for the youthful student. this is doubtless true of theoretical or mathematical astronomy and the practical astronomy of the navigator, surveyor and engineer, but it is not true of general, descriptive astronomy. there are many different aspects of this many-sided science, and some of the simplest and grandest truths of astronomy can be grasped by the intelligent child of twelve or fourteen years of age. merely as a branch of nature study the child should have some knowledge of the sun, moon, stars and planets, their motions and their physical features, for they are as truly a part of nature as are the birds, trees and flowers, and the man, woman or child who goes forth beneath the star-lit heavens at night absolutely blind to the wonders and beauties of the universe of which he is a part, loses as much as the one who walks through field or forest with no thought of the beauties of nature that surround him. the astronomer is the pioneer and explorer of today in realms unknown just as the pioneers and explorers of several centuries ago were to some extent astronomers as they sailed unknown seas and traversed unexplored regions. as the years pass by the astronomer extends more and more his explorations of the universe and brings back among the fruits of discovery measures of giant suns and estimates of the form and extent of the universe, views of whirling, seething nebulæ, mysterious dark clouds drifting through space, tremendous solar upheavals or glimpses of strangely marked surfaces of nearby planets. in the following pages the author has endeavored to tell in words not beyond the comprehension of the average fourteen-year-old child something of the nature of the heavenly bodies. in part i an effort is made to make the child familiar with the stars by indicating when and where they can be found in the early evening hours. in addition to identifying the principal constellations and their brightest stars by means of diagrams an attempt has been made to acquaint the child with the most interesting recent discoveries that have been made concerning the principal stars or objects in each group as well as with some of the stories and legends that have been associated with these groups of stars for centuries, and that have been handed down in the folk-lore of all nations. chapters - , inclusive, appeared originally with diagrams similar to those shown here, under the department of nature and science for young folk in _st. nicholas_ from may, , to april, , inclusive. the introductory chapter and chapters and , on the milky way and stars of the southern hemisphere, respectively, are published here for the first time, as is also the chapter in part ii on the evolution of the stars from red giants to red dwarfs, which gives the order of the evolution of the stars as now accepted as a result of the brilliant astronomical researches of dr. henry norris russell of the united states and prof. a. s. eddington, of england. the remaining chapters in part ii have been chosen from a series of articles that have appeared in _science and invention_, formerly _the electrical experimenter_, in the past four years, and have been considerably revised and in some parts rewritten to adapt them to the understanding of more youthful readers. these chapters deal with a variety of astronomical subjects of general popular interest and an effort has been made to select subjects that would cover as wide an astronomical field as possible in a limited space. the author's aim has not been to write a text-book of astronomy or to treat in detail of any one aspect of this extensive science, but simply to give the average child some general knowledge of the nature of the heavenly bodies, both those that form a part of our own solar system and those that lie in the depths of space beyond. it has been necessary to write very briefly, and we feel inadequately, of many topics of special interest such as the sun and moon. books have been written on these two subjects alone as well as upon such subjects as mars, eclipses, comets, meteors, etc., but the object has been to acquaint the child with the outstanding features of a variety of celestial objects rather than to treat of a few in detail. if the writer succeeds in arousing the child's interest in the stars so that he may look forth with intelligence at the heavens and greet the stars as friends and at the same time grasps some of the simplest and most fundamental of astronomical truths such as the distinction between stars and planets, the motions of the heavenly bodies and their relative distances from us and the place of our own planet-world in the universe, this book will have served its purpose. astronomy for young folks "the heavens declare the glory of god, and the firmament showeth his handiwork." psalm xix. i the constellations "canst thou bind the sweet influences of the pleiades or loose the bands of orion? canst thou bring forth mazzaroth in his season or canst thou guide arcturus with his sons?" --book of job. who would not like to know the stars and constellations by their names and in their seasons as we know the birds and the trees and the flowers, to recognize at their return, year by year, sirius and spica, arcturus and antares, vega and altair, to know when ursa major swings high overhead and orion sinks to rest beneath the western horizon, when leo comes into view in the east or the northern crown lies overhead? often we deprive ourselves of the pleasure of making friends with the stars and shut our eyes to the glories of the heavens above because we do not realize how simple a matter it is to become acquainted with the various groups of stars as they cross our meridian, one by one, day after day and month after month in the same orderly sequence. when the robin returns once more to nest in the same orchard in the spring time, leo and virgo may be seen rising above the eastern horizon in the early evening hours. when the first snow flies in the late fall and the birds have all gone southward the belt of orion appears in the east and cygnus dips low in the west. when we once come to know brilliant blue-white vega, ruddy arcturus, golden capella and sparkling sirius we watch for them to return each in its proper season and greet them as old friends. in the following pages we give for each month the constellations or star-groups that are nearest to our meridian, that is, that lie either due north or due south or exactly overhead in the early part of the month and the early part of the evening. we do not need to start our study of the constellations in january. we may start at any month in the year and we will find the constellations given for that month on or near the meridian at the time indicated. in using the charts or diagrams of the constellations, we should hold them in an inverted position with the top of the page toward the north or else remember that the left-hand side of the page is toward the _east_ and the right-hand side of the page toward the _west_, which is the opposite of the arrangement for charts and maps of the earth's surface. we should also bear in mind that the constellations are all continually shifting westward for the stars and the moon and the planets as well as the sun rise daily in the east and set in the west. this is due to the fact that the earth is turning in the opposite direction on its axis, that is from west to east. in twenty-four hours the earth turns completely around with respect to the heavens or through an angle of °, so in one hour it turns through an angle of ° ÷ or °. as a result the stars appear to shift westward ° every hour. this is a distance about equal in length to the handle of the big dipper, which i am sure we all know, even if we do not know another constellation in the heavens. if, then, we look at the heavens at a later hour than that for which the constellations are given we will find them farther westward and if our time of observation is earlier in the evening than the hour mentioned we will find them farther eastward. in the course of a year the earth makes one trip around the sun and faces in turn all parts of the heavens. that is, it turns through an angle of ° with respect to the heavens in a year or through an angle of ° ÷ or ° in one month. so as a result of our revolution around the sun, which is also in a west to east direction, we see that all the constellations are gradually shifting westward at the rate of ° a month. it is for this reason that we see different constellations in different months, and it is because of the turning of the earth on its axis that we see different constellations at different hours of the night. if we should sit up from sunset to sunrise and watch the stars rise in the east, pass the meridian and set in the west--as the sun does by day--we should see in turn the same constellations that are to pass across the heavens in the next six months. this is because in twelve hours' time we are carried through the same angle with respect to the heavens by the earth's rotation on its axis that we are in the next six months by the motion of the earth around the sun. let us suppose then that the time we choose for our observation of the heavens is the last of the month while our charts are given for the first of the month. we must look then farther westward for our constellations just as we must look farther westward if we chose a later hour in the evening for our observations. let us suppose that we choose for our time of observation half-past eight in the early part of december. on or close to the meridian we will find the constellations outlined in the charts for december. to the east of the meridian we will find the constellations that are given for january and february, and to the west of the meridian the constellations that are given for november and october. so if we are particularly ambitious or wish to become acquainted with the constellations more rapidly we may study at the same time the constellations for the preceding months now west of the meridian and the constellations for the following months now east of the meridian as well as the constellations for the month which will be due north or south or directly overhead as the case may be. if we were able to see the stars by day as well as by night we would observe that as the days go by the sun is apparently moving continuously eastward among certain constellations. this is a result of the earth's actual motion around the sun in the same direction. the apparent path of the sun among the stars is called the ecliptic and the belt of the heavens eight degrees wide on either side of the ecliptic is called the zodiac. the constellations that lie within this belt of the zodiac are called zodiacal constellations. the zodiac was divided by the astronomer hipparchus, who lived - b.c., into twelve signs ° wide, and the signs were named for the constellations lying at that time within each of these divisions. these zodiacal constellations are aries, taurus, gemini, cancer, leo, virgo, libra, scorpio, sagittarius, capricornus, aquarius and pisces. with the exception of libra, the scales, all of these constellations are named for people or animals and the word zodiac is derived from the greek word meaning "of animals." each month the sun moves eastward ° through one of these zodiacal constellations. in the days of hipparchus the sun was in aries at the beginning of spring, at the point where the ecliptic crosses the celestial equator--which lies directly above the earth's equator. this point where the ecliptic crosses the equator was then known as the first point in aries. the autumnal equinox was ° distant in aquarius and the two points were called the equinoxes because when the sun is at either equinox the day and night are equal in length all over the world. now for certain reasons which we will not explain here the equinoctial points are not fixed in position but shift gradually westward at the rate of ° in years. it is as if the equinoxes were advancing each year to meet the sun on its return and their westward motion is therefore called "the precession of the equinoxes." since the days of hipparchus this motion has amounted to about ° so that the constellations no longer occupy the signs of the zodiac that bear their names. the sun is now in pisces instead of aries at the beginning of spring and in virgo instead of aquarius at the beginning of fall. not only the sun but the moon and planets as well move through the zodiacal constellations. in fact a limit for the zodiac of ° on either side of the ecliptic was chosen because it marks the extent of the excursions of the moon and planets from the ecliptic. neither moon nor planets will be found at a greater distance than ° on either side of the ecliptic. for convenience in determining the positions of the heavenly bodies the astronomer assumes that they lie upon the surface of a celestial sphere that has its center at the center of the earth. the north pole of the celestial sphere lies directly above the north pole of the earth and the south pole of the celestial sphere directly above the south pole of the earth. the celestial equator is the great circle of the celestial sphere that lies midway between its north and south poles and directly above the earth's equator. the ecliptic is also a great circle of the celestial sphere and cuts the celestial equator at an angle of - / ° in the two points ° apart known as the equinoctial points, of which we have already spoken. the zodiacal constellations lie nearly overhead within the tropics and can be seen to advantage all over the world except in polar regions. for every position of the earth's surface except at the equator we have also our circumpolar constellations which are the ones that never pass below the horizon for the place of observation. in ° n. latitude the big dipper is a circumpolar constellation for it is above the horizon at all hours of the day and night and all times of the year. if our latitude is ° n., all stars within ° of the north pole of the heavens are circumpolar and never set, while stars within ° of the south pole of the heavens never rise. all other stars rise and set daily. if we were at the north pole all stars within ° of the north pole of the heavens would be circumpolar and would describe daily circuits of the pole parallel to the horizon remaining always above it. if we were at the equator all stars within _zero_ degrees of either pole would be circumpolar, that is _no_ stars would be circumpolar, all stars rising and setting daily. as a general rule, then, we may say that stars within an angular distance of the nearest pole of the heavens equal to the latitude never set and stars within an equal distance of the opposite pole never rise while all stars outside of these limits rise and set daily. the beginner who attempts to make the acquaintance of the principal stars and constellations occasionally may find a bright star in a constellation that is not noted in the diagrams. in this case he has probably happened upon one of the bright planets. it is not possible to include the planets in our diagrams for the reason that they are not fixed in position but apparently wander among the stars. the name planet is, in fact, derived from a greek word meaning "wanderer." the stars shine by their own light but the planets shine only by reflected light from the sun. of the seven planets in the solar system additional to our own planet earth, there are two, uranus and neptune that we need not consider for neptune is not visible without the aid of a telescope and uranus is fainter than any of the stars included in our diagrams. mercury will never appear except in the morning or evening twilight, when none but the very brightest stars are visible, since it never departs far from the sun. it will only be seen under certain favorable conditions, and usually it will escape our notice altogether unless we know exactly where to look for it although there are but two or three stars in the heavens that surpass it in brightness. venus, we will probably never mistake for any star in the heavens for it far surpasses all stars in brightness. it will always be seen in the west after sunset or in the east before sunrise and it is never seen more than three hours before or after the sun. this leaves us but three planets, jupiter, saturn and mars that we may mistake for bright stars. there is little chance that jupiter will be thus mistaken for it also is far brighter than all of the stars except sirius which differs greatly from jupiter in color. sirius is a brilliant white and jupiter is a golden yellow. the planets do not twinkle as the stars do and this is particularly true of jupiter which is remarkable for the quiet steadiness of its yellow light. this alone would serve to identify it. saturn is probably mistaken for a star oftener than any of the other planets. it moves so slowly among the stars that we would have to watch it for a number of successive evenings before we could discover that it is moving with respect to the stars. saturn is yellowish in color and we can probably best distinguish it by the steadiness of its light. if we find in one of the zodiacal groups of stars--for the planets appear among no other constellations--a bright yellowish star where no bright star is indicated on the diagram we may be reasonably certain that we have found the planet saturn. mars is the only planet that is reddish in color. once in fifteen or seventeen years, when it is particularly near to the earth, it surpasses even jupiter in brightness, but ordinarily it appears no more brilliant than one of the brighter stars. there are only two stars with which we are likely to confuse mars,--aldebaran and antares--which are very similar to it in color, and, at times, in brightness. moreover, both of these stars are zodiacal stars and mars frequently passes through the constellations to which they belong. there should be no trouble about identifying aldebaran and antares, however, from their distinctive positions in the diagrams so that any other reddish star appearing in any of the zodiacal groups we may feel certain is the planet mars. in the following diagrams of the constellations the brightest and most conspicuous stars, called first-magnitude stars, are represented by white stars. these are the stars we should all be able to recognize and call by name and in every instance the name of a first-magnitude star is given on the diagram. all other stars are represented by circles, and the size of the circle is an indication of the brightness of the star. stars visible without the aid of a telescope are referred to usually as "naked-eye stars." they are classed as first, second, third, four, fifth or sixth magnitude stars, according to their relative brightness. a star of the first magnitude is about two and one-half times brighter than a star of the second magnitude, which in turn is two and one-half times brighter than a star of the third magnitude and so on. a first-magnitude star is, then, one hundred times brighter than a sixth magnitude star which is the faintest star that can be seen without the aid of the telescope. this ratio between successive magnitudes continues among the telescopic stars. a star of the sixth magnitude is one hundred times brighter than a star of the eleventh magnitude which in turn is one hundred times brighter than a star of the sixteenth magnitude. the faintest stars that can be seen visually in the greatest telescopes are of the seventeenth or eighteenth magnitude, though stars two or three magnitudes fainter can be photographed. the faintest stars shown in the diagrams are fifth-magnitude stars and stars of this magnitude as well as stars of the fourth magnitude are only given when needed to fill out the distinctive outlines of the constellations which have been formed by connecting the principal stars in each group by dotted lines. all stars of first, second and third magnitude are given in the diagrams without exceptions as such stars are visible to everyone on clear nights. the constellations given in the following pages include practically all of the constellations that can be seen in ° n. latitude. a diagram is given for each constellation. in this latitude it is impossible to see the constellations of the southern hemisphere that lie within ° of the south pole of the heavens. a brief chapter with diagram treats of these constellations that are invisible in mid-latitudes of the northern hemisphere. ii january one of the most easily recognized constellations in the heavens is taurus, the bull, a zodiacal group which lies just south of the zenith in our latitudes in the early evening hours about the first of january. taurus is distinguished by the v-shaped group of the hyades, which contains the bright, red, first-magnitude star aldebaran, representing the fiery eye of the bull. it also contains the famous cluster of faint stars known as the pleiades, lying a short distance northwest of the hyades. no group of stars is more universally known than the pleiades. all tribes and nations of the world, from the remotest days of recorded history up to the present time, have sung the praises of the pleiades. they were "the many little ones" of the babylonians, "the seven sisters" of the greeks, "the seven brothers" of the american indians, "the hen and chickens" of many nations of europe, "the little eyes" of the south sea islanders. they were honored in the religious ceremonies of the aztecs, and the savage tribes of australia danced in their honor. many early tribes of men began their year with november, the pleiad month; and on november th, when the pleiades crossed the meridian at midnight, it was said that no petition was ever presented in vain to the kings of ancient persia. [illustration: january--taurus] poets of all ages have felt the charm of the pleiades. tennyson gives the following beautiful description of the pleiades in _locksley hall_: "many a night i saw the pleiades, rising through the mellow shade, glitter like a swarm of fireflies tangled in a silver braid." a well-known astronomer, not so many years ago, also felt the mysterious charm of the pleiades and seriously expressed the belief that alcyone, the brightest star of the pleiades, was a central sun about which all other suns were moving. but we know that there is no foundation whatever for such a belief. a fairly good eye, when the night is clear and dark, will make out six stars in this group arranged in the form of a small dipper. a seventh star lies close to the star at the end of the handle and is more difficult to find. it is called pleione, and is referred to in many legends as the lost pleiad. persons gifted with exceptionally fine eyesight have made out as many as eleven stars in the group; and with the aid of an ordinary opera-glass, anyone can see fully one hundred stars in this cluster. astronomers have found that the pleiades cluster contains at least two hundred and fifty stars, all drifting slowly in the same general direction through space, and that the entire group is enveloped in a fiery, nebulous mist which is most dense around the brightest stars. it is not known whether the stars are being formed from the nebula or whether the nebula is being puffed off from the stars. the brightest star, alcyone, is at least two hundred times more brilliant than our own sun, and all of the brighter stars in the group surpass the sun many times in brightness. it is believed that this cluster is so large that light takes many years to cross from one end of it to the other, and that it is so far from the earth that its light takes over three centuries to reach us, traveling at the rate of , miles a second. the hyades is a group of stars scarcely less famous than the pleiades, and the stars in the group also form a moving cluster of enormous extent at a distance of light-years from the earth. among the ancients, the hyades were called the rain-stars, and the word hyades is supposed to come from the greek word for rain. among the many superstitions of the past was the belief that the rising or setting of a group of stars with the sun had some special influence over human affairs. since the hyades set just after the sun in the showery springtime and just before sunrise in the stormy days of late fall, they were always associated with rain. in tennyson's _ulysses_ we read: "through scudding drifts the rainy hyades vex'd the dim sea." the hyades outline the forehead of taurus, while two bright stars some distance to the northeast of the v form the tips of the horns. only the head and forequarters of the bull are shown in the star-atlases that give the mythological groups, for, according to one legend, he is swimming through the sea and the rest of his body is submerged. according to another legend, taurus is charging down upon orion, the warrior, represented by the magnificent constellation just to the southeast of taurus, of which we shall have more to say next month. aldebaran is the arabic word for "the hindmost," and the star is so called because it follows the pleiades across the sky. it is one of the most beautiful of all the many brilliant stars visible at this time and we might profit by following the advice of mrs. sigourney in _the stars_: "go forth at night and talk with aldebaran, where he flames in the cold forehead of the wintry sky." next to aldebaran in the v is the interesting double star theta, which we can see as two distinct stars without a telescope. directly south of taurus is eridanus, sometimes called fluvius eridanus, or the river eridanus. starting a little to the southeast of taurus, close to the brilliant blue-white star rigel in orion, it runs to the westward for a considerable distance in a long curving line of rather faint stars, bends sharply southward for a short distance, then curves backward toward the east once more, and, after running for some distance, makes another sharp curve to the southwest and disappears below the southern horizon. its course is continued far into the southern hemisphere. its brightest star, achernar, is a star of the first magnitude, but it lies below the horizon in our latitudes. [illustration: january--eridanus] eridanus contains no star of particular interest to us. most of the numerous stars that mark its course are of the fourth and fifth magnitude. it contains but two stars of the third magnitude, one at the beginning of its course and one close to the southwestern horizon. the beautiful constellation of perseus lies just to the north of taurus and should rightfully be considered among the constellations lying nearest to the meridian in january, but we give this constellation among the star groups for december because of its close association with the nearby constellations andromeda and pegasus in legend and story. iii february across the meridian, due south, between eight and nine o'clock in the evening in the early part of february, lies orion, the warrior, generally considered to be the finest constellation in the heavens. orion is directly overhead at the equator, and so is seen to advantage from all parts of the world except the extreme northern and southern polar regions. a group of three faint stars outlines the head of orion. his right shoulder is marked by the deep-red, first-magnitude star betelgeuze (meaning armpit), and his left shoulder by the bright white star bellatrix, the amazon. orion stands facing taurus, the bull, and brandishes in his right hand a club, outlined by a number of faint stars extending from betelgeuze toward the northeast. the top of the club lies near the tips of the horns of taurus. in his left hand he holds up a lion's skin, which we can trace in another curving line of faint stars to the west and northwest of bellatrix. the brilliant, blue-white, first-magnitude star rigel lies in the left foot, and the second-magnitude star saiph, a little to the east of rigel, is in the right knee. three evenly spaced stars lying in a straight line that is exactly three degrees in length form the belt of orion, and from the belt hangs the sword of orion, outlined by three faint stars. the central star in the sword appears somewhat blurred and is the multiple star theta, in the midst of the great orion nebula, the finest object of its kind in the heavens. entangled in the meshes of this glowing nebula are a number of brilliant suns, appearing to us as faint stars because of their great distance. the star theta, in the heart of the nebula, is seen with a powerful telescope to consist of six stars; that is, it is a sextuple star. even with a small telescope, four of these stars can readily be seen, arranged in the form of a small trapezium. the lowest star in the sword is a triple star, and the entire constellation abounds in double, triple, and multiple stars. from the central portion of the nebula extend many branches and streamers of nebulous light, and it is known that the entire constellation of orion is enwrapped in the folds of this nebulosity, which forms a glowing, whirling mass of fiery gases in rapid rotation. this constellation is remarkable for the fact that all of its brighter stars, with the exception of the deep-red betelgeuze, form one enormous, connected group of stars. they are all more or less associated with the great nebula and its branches, and are all extremely hot, white or bluish-white stars, known as helium stars, because the gas helium is so conspicuous in their atmospheres. the orion stars are the hottest and brightest of all the stars. blazing rigel, bellatrix, and saiph, marking three corners of the great quadrilateral, of which betelgeuze marks the fourth corner, are all brilliant helium stars. so are the three stars in the belt and the fainter stars in the sword and the great nebula. [illustration: february--orion] it has been estimated that the great orion group of stars is over six hundred light-years from the earth, or about forty million times more distant than the sun. for more than six centuries the rays of light that now enter our eyes from these stars have been traveling through space with the speed of lightning. so we see orion not as it exists today, but as it was six centuries ago. the extent of the orion group of stars is also inconceivably great. even the central part of the great nebula, which appears to our unaided eyes only as a somewhat fuzzy star, would extend from here to the nearest star and beyond, while our entire solar system would be the merest speck in its midst. betelgeuze, the red star that marks the right shoulder of orion, is, as we have said, not a member of the orion group. it has been estimated that it is about two hundred light-years from the earth, or only about one-third as far away as the other stars of the constellation. betelgeuze very recently has attracted universal attention, and will probably be considered an object of historic interest in the future, because it is the first star to have its diameter measured with the new michelson interferometer, which is now being used so successfully to measure the diameters of the largest stars. the truly sensational discovery has been made that betelgeuze is a supergiant of the universe, with a diameter of about , , miles. our own sun, which is known as a "dwarf" star, has a diameter of , miles. that is, betelgeuze would make about thirty million suns the size of our own. if placed at the center of the solar system, it would fill all of the space within the orbit of mars; and the planets mercury, venus, and the earth would lie far beneath its surface. measurements of the diameters of other giant stars which are now being made with the interferometer give results quite as startling as have been obtained in the case of betelgeuze; and it has been found that several of these stars may even exceed betelgeuze in size. such a star is antares, the fiery-red star in the heart of scorpio, which is such a conspicuous object in the summer evening skies. all these huge stars are deep red in color, and some of them vary irregularly in brightness. betelgeuze is one of the stars that changes in brightness in a peculiar manner from time to time. when shining with its greatest brilliancy it is a brighter object than the nearby star aldebaran, in taurus; but a few months or a year later it may lose so much of its light as to be decidedly inferior to aldebaran. we may note for ourselves this remarkable change in the brightness of betelgeuze by comparing the two stars from time to time. directly south of orion lies the small constellation of lepus, the hare, which is made up of third-magnitude and fourth-magnitude stars. the four brighter stars are arranged in the form of a small, but distinct, quadrilateral, or four-sided figure, which may be easily seen in our latitudes. the small constellation of columba, the dove, which lies just south of lepus, is so close to the horizon that it can not be seen to advantage in the mid-latitudes of the northern hemisphere. neither lepus nor columba contain any object of unusual interest. [illustration: february--auriga] due north of orion, and lying in the zenith at this time, is auriga, the charioteer, represented, strange to say, with capella, a goat, in his arms. the beautiful first-magnitude star capella, golden-yellow in color, serves us in identifying the constellation. close at hand are the kids, represented by a group of three faint stars. capella is one of the most brilliant stars of the northern hemisphere. it is almost exactly equal in brightness to arcturus and vega, stars conspicuous in the summer months, and it is a shade brighter than magnificent blue-white rigel in orion. capella is about fifty light-years distant from the earth and is fully two hundred times more brilliant than our own sun. at the distance of capella, the sun would appear to be considerably fainter than any one of the three stars in the nearby group of the kids. capella is attended by a companion star so close to its brilliant ruler that it can not be seen as a separate star save with the aid of the most powerful telescopes. its distance from capella has been very accurately measured, however, by means of the interferometer, which is giving us the measurements of the diameters of the giant stars. it is known that this companion sun is closer to capella than our planet earth is to the sun. at no time of the year shall we find near the meridian so many brilliant and beautiful stars as appear in the month of february at this time in the evening. in addition to capella, which is one of the three most brilliant stars in the northern hemisphere of the heavens, we have, in orion alone, two stars of the first magnitude, betelgeuze and rigel, and five stars of the second magnitude, bellatrix and saiph and the three stars in the belt. in addition, we have not far distant in the western sky, fiery aldebaran in taurus, and close on the heel of orion in the east, sirius, the brightest star in the heavens, in the constellation of canis major, the greater dog, as well as the first-magnitude star procyon in canis minor, the lesser dog. of these two groups we shall have more to say under the constellations for march. iv march to the southeast of orion and almost due south at eight o'clock in the evening on the first of march lies the constellation of canis major, the greater dog, containing sirius, the dog-star, which far surpasses all other stars in the heavens in brilliancy. sirius lies almost in line with the three stars that form the belt of orion. we shall not have the slightest difficulty in recognizing it, owing to its surpassing brilliancy as well as to the fact that it follows so closely upon the heels of orion. sirius is the greek for "scorching" or "sparkling," and the ancients attributed the scorching heat of summer to the fact that sirius then rose with the sun. the torrid days of midsummer they called the "dog-days" for this reason, and we have retained the expression to the present time. since sirius was always associated with the discomforts of the torrid season, it did not have an enviable reputation among the greeks. we find in pope's translation of the _iliad_ this reference to sirius: "terrific glory! for his burning breath taints the red air with fever, plagues, and death." in egypt, however, many temples were dedicated to the worship of sirius, for the reason that some five thousand years ago it rose with the sun at the time of the summer solstice, which marks the beginning of summer, and heralded the approaching inundation of the nile, which was an occasion for great rejoicing among the egyptians. it was, therefore, called the nile star and regarded by them with the greatest reverence. sirius is an intensely white hydrogen star; but owing to its great brilliancy and to the fact that it does not attain a great height above the horizon in our latitudes, its rays are greatly refracted or broken up by the atmosphere, which is most dense near the horizon, and as a result, it twinkles or scintillates more noticeably than other stars and flashes the spectrum colors--chiefly red and green--like a true "diamond in the sky"--a magnificent object in the telescope. sirius is one of our nearest neighbors among the stars. only two stars are known to be nearer to the solar system. yet its light takes about eight and a half years to flash with lightning speed across the great intervening chasm. it is attended also by a very faint star that is so lost in the rays of its brilliant companion that it can only be found with the aid of a powerful telescope. the two stars are separated by a distance of , , , miles; that is they are about as far apart as neptune and the sun. they swing slowly and majestically about a common center, called their center of gravity, in a period of about forty-nine years. so faint is the companion of sirius that it is estimated that twenty thousand such stars would be needed to give forth as much light as sirius. the two stars together, sirius and its companion, give forth twenty-six times as much light as our own sun. they weigh only about three times as much, however. the companion of sirius, in spite of its extreme faintness, weighs fully half as much as the brilliant star. [illustration: march--canis major] there are a number of bright stars in the constellation of canis major. a fairly bright star a little to the west of sirius marks the uplifted paw of the dog, and to the southeast, in the tail and hind quarters, are several conspicuous stars of the second magnitude. a little to the east and much farther to the north, we find canis minor, the lesser dog, containing the beautiful first-magnitude star procyon, "precursor of the dog"--that is, of sirius. since procyon is so much farther north than sirius and very little to the east, we see its brilliant rays in the eastern sky some time before sirius appears above the southeastern horizon, hence its name. long after sirius has disappeared from view beneath the western horizon in the late spring, procyon may still be seen low in the western sky. procyon, also is one of our nearer neighbors among the stars, being only about ten light-years distant from the solar system. like sirius, it is a double star with a much fainter companion, that by its attraction sways the motion of procyon to such an extent that we should know of its existence, even if it were not visible, by the disturbances it produces in the motion of procyon. the period of revolution of procyon and its companion about a common center is about forty years, and the two stars combined weigh about a third more than our own sun and give forth six times as much light. canis minor contains only one other bright star, beta, a short distance to the northwest of procyon. originally, the name procyon was given to the entire constellation, but it was later used only with reference to the one star. procyon, sirius, and betelgeuze in orion form a huge equal-sided triangle that lies across the meridian at this time and is a most conspicuous configuration in the evening sky. [illustration: march--gemini and canis minor] directly south of the zenith we find gemini, the twins, one of the zodiacal constellations. it is in gemini that the sun is to be found at the beginning of summer. the two bright stars castor and pollux mark the heads of the twins, and the two stars in the opposite corners of the four-sided figure shown in the chart mark their feet. castor and pollux, according to the legend, were the twin brothers of helen of troy who went on the argonautic expedition. when a storm overtook the vessel on its return voyage, orpheus invoked the aid of apollo, who caused two stars to shine above the heads of the twins, and the storm immediately ceased. it was for this reason that castor and pollux became the special deities of seamen, and it was customary to place their effigies upon the prows of vessels. the "by jimini!" of today is but a corruption of the "by gemini!" heard so frequently among the sailors of the ancient world. the astronomical name for castor, the fainter star, is alpha geminorum, meaning alpha of gemini. as it was customary to call the brightest star in a constellation by the first letter in the greek alphabet, it is believed that castor has decreased considerably in brightness since the days of the ancients, for it is now decidedly inferior to pollux in brightness, which is called beta geminorum. of the two stars, castor is the more interesting because it is a double star that is readily separated into two stars with the aid of a small telescope. the two principal stars are known to be, in turn, extremely close double stars revolving almost in contact in periods of a few days. where we see but one star with the unaided eye, there is, then a system of four suns, the two close pairs revolving slowly about a common center of gravity in a period of several centuries and at a great distance apart. the star pollux, which we can easily distinguish by its superior brightness, is the more southerly of the twin stars and lies due north of procyon and about as far from procyon as procyon is from sirius. the appearance of gemini on the meridian in the early evening and of the huge triangle, with its corners marked by the brilliants, procyon, sirius, and betelgeuze, due south, with "great orion sloping slowly to the west," is as truly a sign of approaching spring as the gradual lengthening of the days, the appearance of crocuses and daffodils, and the first robin. it is only a few weeks later--as pictured by tennyson in _maud_-- "when the face of the night is fair on the dewy downs, and the shining daffodil dies, and the charioteer and starry gemini hang like glorious crowns over orion's grave low down in the west." v april in the early evening hours of april the western sky is still adorned with the brilliant jewels with which we became familiar on the clear frosty evenings of winter. orion is now sinking fast to his rest beneath the western horizon. beautiful, golden capella in auriga glows in the northwest. sirius sparkles and scintillates, a magnificent diamond of the sky, just above the southwestern horizon, while procyon in canis minor, the lesser dog, and castor and pollux, the twins, in the constellation of gemini, are still high in the western part of the heavens. in the northeast and east may be seen the constellations that will be close to the meridian at this time next month. ursa major, the greater bear, with its familiar big dipper, is now in a favorable position for observation. the sickle in leo is high in the eastern sky, and spica, the brilliant white diamond of the evening skies of spring, is low in the southeast in virgo. near the meridian this month we find between auriga and ursa major, and east of gemini, the inconspicuous constellation of lynx, which contains not a single bright star and is a modern constellation devised simply to fill the otherwise vacant space in circumpolar regions between ursa major and auriga. [illustration: april--cancer] just south of the zenith at this time, and lying between gemini and leo, is cancer, the crab, the most inconspicuous of all the zodiacal constellations. there are no bright stars in this group, and there is also nothing distinctive about the grouping of its faint stars, though we can readily find it, from its position between the two neighboring constellations of gemini and leo by reference to the chart. in the position indicated there we will see on clear evenings a faint, nebulous cloud of light, which is known as praesepe, the beehive, or as the manger, the two faint stars flanking it on either side being called aselli, the asses. this faint cloud can be easily resolved by an opera-glass into a coarse cluster of stars that lie just beyond the range of the unaided human vision. to the ancients, praesepe served as an indicator of weather conditions, and aratus, an ancient astronomer, wrote of this cluster: "a murky manger, with both stars shining unaltered, is a sign of rain. if while the northern ass is dimmed by vaporous shroud, he of the south gleam radiant, expect a south wind; the vaporous shroud and radiance exchanging stars, harbinger boreas." this was not entirely a matter of superstition, as we might possibly imagine, for the dimness of the cluster is simply an indication that vapor is gathering and condensing in the atmosphere, just as a ring around the moon is an indication of the same gathering and condensation of vapor that precedes a storm. some centuries ago the sun reached its greatest distance north of the equator--which occurs each year at the beginning of summer--at the time when it was passing through the constellation of cancer. our tropic of cancer, which marks the northern limit of the torrid zone, received its name from this fact. at the time when the sun reaches the point farthest north, its height above the horizon changes very little from day to day, and for a short time it appears to be slowly crawling sideways through the heavens, as a crab walks, and for this reason, possibly, the constellation was called cancer, the crab. at the present time the "precession of the equinoxes," or westward shifting of the vernal equinox--the point where the sun crosses the equator going north in the spring--brings the sun, when it is farthest north, in gemini instead of in cancer. at the present time, then it would be more accurate to speak of the tropic of gemini, though this in turn would be inaccurate after a lapse of centuries, as the sun passed into another constellation at the beginning of summer. the tropic of capricorn, which marks the farthest southern excursions of the sun in its yearly circuit of the heavens, should also more appropriately be called the tropic of sagittarius, as the sun is now in sagittarius instead of capricornus at the time when it is farthest south, though the point is slowly shifting westward into scorpio. mythology tells us that cancer was sent by juno to distract hercules by pinching his toes while he was contending with the many-headed serpent in the lernean swamp. hercules, the legend says, crushed the crab with a single blow, and juno by way of reward placed it in the heavens. in cancer, according to the belief of the chaldeans, was located the "gate of men," by which souls descended into human bodies, while in capricornus was the "gate of the gods," through which the freed souls of men returned to heaven. [illustration: april--hydra] hydra, the many-headed serpent with which hercules contended, is represented by a constellation of great length. it extends from a point just south of cancer, where a group of faint stars marks the heads, to the south and southeast in a long line of faint stars. it passes in its course just south of crater and corvus, the two small star-groups below leo (see constellations for may), which are sometimes called its riders, and it also stretches below the entire length of the long, straggling constellation of virgo. at this time we can trace it only to the point where it disappears below the horizon in the southeast. it contains but one bright star, alphard, or cor hydrae as it is also called, standing quite alone and almost due south at this time. hydra, as well as lynx and cancer, contains no noteworthy or remarkable object and consists chiefly of faint stars. alphard is, in fact, the only bright star that we have in the constellations for this month. it chances that these three inconspicuous star-groups, lynx, crater, and hydra, lie nearest to the meridian at this time, separating the brilliant groups of winter from those of the summer months. [illustration: april--lynx] vi may ursa major, the great bear, and ursa minor, the lesser bear, or, as they are more familiarly called, the big dipper and the little dipper, are the best known of all the constellations visible in northern latitudes. they are called circumpolar constellations, which means "around the pole." for those who live north of ° n. lat. they never set, but can be seen at all hours of the night and at all times of the year. in fall and winter evenings ursa major lies below the pole and near the horizon, and so is usually hidden more or less from view by trees or buildings. it is during the early evening hours of late spring and summer that this constellation is seen to the best advantage high in the sky above the pole. if one looks due north at the time mentioned, it will be impossible to miss either of these constellations. to complete the outline of the great bear, it is necessary to include faint stars to the east, which form the head of the bear, and other faint stars to the south, which form the feet, but these are all inconspicuous and of little general interest. the two stars in the bowl of the big dipper through which an arrow is drawn in the chart, are called the pointers, because an imaginary line drawn through these two stars and continued a distance about equal to the length of the big dipper, brings us to the star polaris, or the north star, at the end of the handle of the little dipper, which is very close to the north pole of the heavens, the direction in which the earth's axis points. the pole lies on the line connecting the star at the bend in the handle of the big dipper with polaris, and is only one degree distant from the pole-star. [illustration: may--ursa major and ursa minor] the distance between the pointers is five degrees of arc, and the distance from the more northerly of these two stars to polaris is nearly thirty degrees. we may find it useful to remember this in estimating distances between objects in the heavens, which are always given in angular measure. a small two and one-half inch telescope will separate polaris into two stars eighteen seconds of arc apart. the companion star is a faint white star of the ninth magnitude. twenty years or so ago it was discovered with the aid of the spectroscope that the brighter of the two stars was also a double star, but the two stars were so close together that they could not be seen as separate stars in any telescope. later it was found that the brighter star was in reality triple, that is, it consists of three suns close together. the faint white companion star formed with these three suns a system of four suns revolving about a common center of gravity. still more recently it has been discovered that the brightest of these four suns varies regularly in brightness in a period of a little less than four days. it belongs to the important class of stars known as cepheid variable stars, whose changes of light, it is believed, are produced by some periodic form of disturbance taking place within the stars themselves. with one exception, polaris is the nearest to the earth of all these cepheid variable stars, which are in most instances at very great distances from the solar system. the latest measurements of the distance of polaris show that its light takes about two centuries to travel to the earth, or, in other words, that it is distant two hundred light-years. like all cepheid variables, polaris is a giant star. it gives forth about five hundred and twenty-five times as much light as our own sun. if polaris and the sun were placed side by side at a distance of thirty-three light-years, the sun would appear as a star of the fifth magnitude, just well within the range of visibility of the human eye, while polaris would outshine sirius, the brightest star in the heavens. as a practical aid to navigators, polaris is unsurpassed in importance by any star of the northern hemisphere of the heavens. at the equator the pole-star lies in the horizon; at the north pole of the earth it is in the zenith or directly overhead. its altitude or height above the horizon is always equal to the latitude of the place of observation. as we travel northward from the equator toward the pole we see polaris rise higher and higher in the sky. in new york the elevation of polaris above the horizon is forty degrees, which is the latitude of the city. the pointers indicate the direction of polaris and the true north, while the height of polaris above the horizon tells us our latitude. these kindly stars direct us by night when we are uncertain of our bearings, whether we travel by land or sea or air. they are the friends and aids of explorers, navigators and aviators, who often turn to them for guidance. bryant writes thus beautifully of polaris in his _hymn to the north star_: constellations come and climb the heavens, and go. star of the pole! and thou dost see them set. alone in thy cold skies, thou keep'st thy old unmoving station yet, nor join'st the dances of that glittering train, nor dipp'st thy virgin orb in the blue western main. on thy unaltering blaze the half wrecked mariner, his compass lost, fixes his steady gaze, and steers, undoubting, to the friendly coast; and they who stray in perilous wastes by night, are glad when thou dost shine to guide their footsteps right. the star at the bend in the handle of the big dipper, called mizar, is of special interest. if one has good eyesight, he will see close to it a faint star. this is alcor, which is arabic for the test. the two stars are also called the horse and the rider. mizar forms with alcor what is known as a wide double star. it is, in fact, the widest of all double stars. many stars in the heavens that appear single to us are separated by the telescope into double or triple or multiple stars. they consist of two or more suns revolving about a common center, known as their center of gravity. sometimes the suns are so close together that even the most powerful telescope will not separate them. then a most wonderful little instrument, called the spectroscope, steps in and analyzes the light of the stars and shows which are double and which are single. a star shown to be double by the spectroscope, but not by the telescope, is called a spectroscopic binary star. mizar is of historic interest, as being the first double star to be detected with the aid of the telescope. a very small telescope will split mizar up into two stars. the brighter of the two is a spectroscopic binary star beside, so that it really consists of two suns instead of one, with the distance between the two so small that even the telescope cannot separate them. about this system of three suns which we know as the star mizar, the faint star alcor revolves at a distance equal to sixteen thousand times the distance of the earth from the sun. [illustration: may--leo] if we follow the imaginary line drawn through the pointers in a _southerly_ direction about forty-five degrees, we come to leo, the lion, one of the zodiacal constellations. there should be no difficulty in finding the constellation leo, as its peculiar sickle-shaped group of bright stars makes it distinctive from all other constellations. at the time we have mentioned, that is, the early evening hours, it will lie a little to the southwest of the zenith. leo is one of the finest constellations and is always associated with the spring months because it is then high in the sky in the evening. regulus is the beautiful white star which marks the handle of the sickle, and the heart of leo; and denebola is the second-magnitude star in the tail of leo. [illustration: may--corvus and crater] due south of denebola, about thirty degrees, we find the small star-group known as crater, the cup, which is composed of rather faint and inconspicuous stars. just east of crater is the group known as corvus, the crow, which forms a very characteristic little four-sided figure of stars differing very little from one another in brightness. these two star groups lie far to the south in our latitudes; but if we lived twenty degrees south of the equator, we would find them nearly overhead, at this time of the year. just south of corvus and crater we find hydra, one of the constellations for april which extends beneath these constellations and also beneath virgo, one of the june constellations. vii june the star-groups that occupy the center of the celestial stage in mid-latitudes of the northern hemisphere during the early evening hours of june are boötes, often called the hunter, (although the word means herdsman or shouter), which will be found overhead at this time; virgo, the maiden, largest of the zodiacal constellations, lying nearly due south; canes venatici, the hunting dogs; corona borealis, the northern crown, and coma berenices. the gorgeous orange-hued arcturus in boötes and the beautiful bluish-white spica in virgo, like a diamond in its sparkling radiance, form with denebola in leo, which we identified in may, a huge equal-sided triangle that is always associated with the spring and early summer months. to the west of boötes, below the handle of the big dipper, is a region where there are few conspicuous stars. here will be found canes venatici (the hunting dogs with which boötes is supposed to be pursuing the great bear around the north pole), and, further south, coma berenices (bernice's hair). the brighter of the two hunting dogs, which is also the brightest star in the entire region covered by these two constellations, appears as a beautiful blue-and-yellow double star in the telescope. it was named cor caroli (heart of charles) by the astronomer halley in honor of charles ii of england, at the suggestion of the court physician, who imagined it shone more brightly than usual the night before the return of charles to london. of more interest to astronomers is the magnificent spiral nebula in this constellation, known as the "whirlpool nebula," appearing as a faint, luminous patch in the sky, of which many photographs have been taken with the great telescopes. this entire region, from canes venatici to virgo, abounds in faint spiral nebulæ that for some reason not yet understood by astronomers are crowded together in this part of the heavens where stars are comparatively few. it is believed that there are between five hundred thousand and a million of these spiral nebulæ in the entire heavens, and the problem of their nature and origin and distance is one that the astronomers are very anxious to solve. many wonderful facts are now being learned concerning these faint nebulous wisps of light which, with a few exceptions, are observable only with great telescopes. they reveal their spiral structure more clearly to the photographic plate than to the human eye, and some magnificent photographs of them have been taken with powerful telescopes. coma berenices, south of canes venatici and southwest of boötes, is a constellation that consists of a great number of stars closely crowded together, and just barely visible to the unaided eye. as a result, it has the appearance of filmy threads of light, which doubtless suggested its name to the imaginative ancients, who loved to fill the heavens with fanciful creations associated with their myths and legends. these stars form a moving cluster of stars estimated to be at a distance of about light-years from the solar systems. [illustration: june--boÖtes, canes venatici and coma berenicis] this region, so lacking in interesting objects for the naked-eye observer, is a mine of riches to the fortunate possessors of telescopes; and the great telescopes of the world are frequently pointed in this direction, exploring the mysteries of space that abound here. just to the east of boötes is the exquisite little circlet of stars known as corona borealis, the northern crown. it consists of six stars arranged in a nearly perfect semicircle, and one will have no difficulty in recognizing it. its brightest star, alpha, known also by the name of alphacca, is a star of the second magnitude. boötes is one of the largest and finest of the northern constellations. it can be easily distinguished by its peculiar kite-shaped grouping of stars or by the conspicuous pentagon (five-sided figure) of stars which it contains. the most southerly star in this pentagon is known as epsilon boötes and is one of the finest double stars in the heavens. the two stars of which it consists are respectively orange and greenish-blue in color. by far the finest object in boötes, however, is the magnificent arcturus, which is the brightest star in the northern hemisphere of the heavens. this star will be conspicuous in the evening hours throughout the summer months, as will also the less brilliant spica in virgo. some recent measurements show that arcturus is one of our nearer neighbors among the stars. its distance is now estimated to be about twenty-one light-years. that is, a ray of light from this star takes twenty-one years to reach the earth, traveling at the rate of one hundred and eighty-six thousand miles per second. it would seem as if we should hardly speak of arcturus, twenty-one light-years away, as a near neighbor, yet there are millions of stars that are far more distant from the earth, and very few that are nearer to us than arcturus. the brightness of arcturus is estimated to be about forty times that of the sun. that is, if the two bodies were side by side, arcturus would give forth forty times as much light and heat as the sun. arcturus is also one of the most rapidly moving stars in the heavens. in the past sixteen centuries it has traveled so far as to have changed its position among the other stars by as much as the apparent width of the moon. most of the stars, in spite of their motions through the heavens in various directions, appear today in the same relative positions in which they were several thousand years ago. it is for this reason that the constellations of the egyptians and of the greeks and romans are the same constellations that we see in the heavens today. were all the stars as rapidly moving as arcturus, the distinctive forms of the constellations would be preserved for only a very few centuries. [illustration: june--virgo] virgo, which lies south and southwest of boötes, is a large, straggling constellation, consisting of a y-shaped configuration of rather inconspicuous stars. it lies in the path of our sun, moon and planets, and so is one of the zodiacal constellations. the cross in the diagram indicates the present position of the autumnal equinox, the point where the sun crosses the equator going south, and the position the sun now occupies at the beginning of fall. spica, the brightest star in virgo, is a bluish-white, first-magnitude star, standing very much alone in the sky. in fact, the arabs referred to this star as "the solitary one." its distance from the earth is not known, but must be very great as it cannot be found by the usual methods. the spectroscope shows that it consists of two suns very close together, revolving about a common center in a period of only four days. within the branches of the "y" in virgo, and just to the north of it, is the wonderful nebulous region of this constellation, but it takes a powerful telescope to show the faint spiral nebulæ that exist here in great profusion. all of these spirals are receding from the plane of the milky way with enormous velocities. the spiral nebulæ are, in fact, the most rapidly moving objects in the heavens. viii july due east of the little circlet of stars known as corona borealis, and almost directly overhead in our latitude ( ° n.) about nine o'clock in the evening in the early part of july, is the large constellation of hercules, named for the famous hero of grecian mythology. there are no stars of great brilliancy in this group, but it contains a large number of fairly bright stars arranged in the form outlined in the chart. the hero is standing with his head, marked by the star alpha herculis, toward the south, and his foot resting on the head of draco, the dragon, a far-northern constellation with which we become acquainted in august. [illustration: july--hercules] alpha herculis, the best known star in this constellation, is of unusual interest. not only is it a most beautiful double star, the brighter of the two stars of which it is composed being orange, and the fainter greenish-blue, but it is also a star that changes in brightness irregularly. both the orange and the blue star share in this change of brightness. there are a number of stars in the heavens that vary in brightness, some in very regular periods, and others, like alpha herculis, irregularly. these latter stars are nearly always deep orange or reddish in color. one may note this variation in the brightness of alpha herculis by comparing it from time to time with some nearby star that does not vary in brightness. [illustration: the great hercules cluster--a universe of suns taken with -inch reflector of the mt. wilson observatory] the constellation of hercules is a very rich field for the possessor of even a small telescope. here are to be found beautifully colored double stars in profusion, and, in addition, two remarkable clusters of stars. the brighter of the two is known as the great hercules cluster. its position is shown on the chart, and, under favorable conditions--that is, on a clear, dark night, when there is no moonlight--it may be seen without the aid of a telescope as a small, faint patch of light. one would never suspect from such a view what a wonderful object this cluster becomes when seen with the aid of a powerful telescope. photographs taken with the great telescopes show this faint wisp of light as a magnificent assemblage of thousands of stars, each a sun many times more brilliant than our own sun. the crowded appearance of the stars in the cluster is due partly to the fact that it is very distant from the earth, though neighboring stars in the cluster are indeed much nearer to one another than are the stars in the vicinity of our solar system. it has been found that this cluster is so far away that its light takes over thirty-six thousand years to reach the earth. at the distance of this cluster, a sun equal in brightness to our own sun would be so faint that the most powerful telescope in the world would not show it. so we know that the stars that are visible in the hercules cluster are far more brilliant than our sun. a fair-sized telescope will show about four thousand stars in this cluster, but the greatest telescopes show over one hundred thousand in it, and there are without doubt many more too faint to be seen at all. the hercules cluster is called a globular star-cluster, because the stars in it are arranged nearly in the form of a sphere. there are in the heavens about ninety such clusters whose distances have been found, and they are among the most distant of all objects. most of them are very faint, and a few are over two hundred thousand light-years distant from the earth. the hercules cluster is one of the nearest and is the most noted of all of these globular clusters. it is considered to be one of the finest objects in the heavens. the other cluster in hercules is also very fine, but not to be compared with this one. [illustration: july--ophiuchus and serpens] just to the south of hercules are two constellations, ophiuchus, the serpent-bearer, and serpens, the serpent, which are so intermingled that it is difficult to distinguish them. there are in these two constellations, as in hercules, no stars of unusual brilliancy, but a large number of fairly bright stars. the brightest star in ophiuchus is known as alpha ophiuchi and it marks the head of the serpent-bearer. the two stars, alpha ophiuchi and alpha herculis, are close together, being separated by a distance about equal to that between the pointers of the big dipper. alpha ophiuchi is the brighter of the two, and it is farther east. ophiuchus, according to one legend, was once a physician on earth, and was so successful as a healer that he could raise the dead. pluto, the god of the lower world, became alarmed for fear his kingdom would become depopulated, and persuaded jupiter to remove ophiuchus to a heavenly abode, where he would be less troublesome. the serpent is supposed to be a symbol of his healing powers. the head of serpens is marked by a group of faint stars just south of corona borealis and southwest of hercules. from here a line of fairly bright stars marks the course of serpens southward to the hand of ophiuchus. two stars close together and nearly equal in brightness mark the hand with which the hero grasps the body of the serpent. the other hand is marked by an equally bright single star some distance to the eastward where the two constellations again meet. ophiuchus is thus represented as holding the serpent with both hands. it is not an easy matter to make out the outlines of these straggling groups, but there are in them several pairs of stars nearly equal in brightness and about as evenly spaced as the two stars in the one hand of ophiuchus, and these, as well as the diagram, will be of aid in tracing the two groups. just south of serpens and ophiuchus lies one of the most beautiful and easily recognized constellations in the heavens. this is the constellation of scorpio, the scorpion, which will be found not far above the southern horizon at this time. the small constellation of libra, the scales, which lies just to the northwest of scorpio, was at one time a part of this constellation and represented the creature's claws, but some centuries ago its name was changed to libra. both scorpio and libra are numbered among the twelve zodiacal constellations--that is, they lie along the ecliptic, or apparent yearly path of the sun among the stars. scorpio is the most brilliant and interesting of all the zodiacal groups. the heart of the scorpion is marked by the magnificent first-magnitude star antares, which is of a deep reddish color. the name signifies rival of ares (mars). it is so called because it is the one star in the heavens that most closely resembles mars, and it might be mistaken for the ruddy planet if one were not familiar with the constellations. at times, when mars is at a considerable distance from the earth, it is almost equal in brightness and general appearance to this glowing red star in the heart of the scorpion. in its trips around the sun, mars passes occasionally very close to antares, and the two then present a very striking appearance. [illustration: july--libra and scorpio] with a telescope of medium size, one will find an exquisite little green companion-star close to antares. the little companion is so close to antares that it is difficult to find it in the glare of light from its more brilliant neighbor. antares is one of the giant stars of the universe. in fact it is, so far as we know, the greatest of all the giants. its diameter is more than five hundred times that of our own sun and nearly twice that of the giant star betelgeuze in orion. if placed at the center of the solar system its surface would lie far beyond the orbit of mars. both ophiuchus and scorpio are crossed by the milky way, that broad belt of numberless faint stars that encircles the heavens. some of the most wonderful and beautiful regions of the milky way are to be found in these two constellations. at various times in the past, there have suddenly flashed forth brilliant stars in the milky way which are known as "temporary stars," or "novæ." these outbursts signify that some celestial catastrophe has taken place, the nature or cause of which is not clearly understood. some of the most brilliant of these outbursts have occurred in these two constellations. the life of a nova is very short, a matter of a few months, and it rapidly sinks into oblivion, so nothing is to be seen of some of the most brilliant of all these stars that have appeared in this region in the past. a few are still faintly visible in large telescopes. ix august it was one of the twelve labors of hercules, the hero of grecian mythology, to vanquish the dragon that guarded the golden apples in the garden of the hesperides. among the constellations for july we found the large group of stars that represents the hero himself, and this month we find just to the north of hercules the head of draco, the dragon. the foot of the hero rests upon the dragon's head, which is outlined by a group of four fairly bright stars forming a quadrilateral or four-sided figure. the brightest star in this group passes in its daily circuit of the pole almost through the zenith of london. that is, as it crosses the meridian of london, it is almost exactly overhead. from the head of draco, the creature's body can be traced in a long line of stars curving first eastward, then northward, toward the pole-star to a point above hercules, where it bends sharply westward. the body of the monster lies chiefly between its head and the bowl of the little dipper. the tail extends in a long line of faint stars midway between the two dippers, or the constellations of ursa major and ursa minor, the tip of the tail lying on the line connecting the pointers of the big dipper with the pole-star polaris. draco, as well as ursa major and ursa minor, is a circumpolar constellation in our latitude; that is, it makes its circuit of the pole without at any time dipping below the horizon in latitudes north of °. it is, therefore, visible at all hours of the night in mid-latitudes of the northern hemisphere, but is seen to the best advantage during the early evening hours in the summer months. there are no remarkable stars in this constellation with the exception of alpha, which lies halfway between the bowl of the little dipper and mizar, the star at the bend in the handle of the big dipper. [illustration: august--draco and lyra] about four thousand seven hundred years ago, this star was the pole-star--lying even nearer to what was then the north pole of the heavens than polaris does to the present position of the pole. the sun and moon exert a pull on the bulging equatorial regions of the earth, which tends to draw the plane of the earth's equator down into the plane of the ecliptic. this causes the "precession of the equinoxes" and at the same time a slow revolution of the earth's axis of rotation about the pole of the ecliptic. the north pole of the heavens as a result describes a circle about the pole of the ecliptic of radius - / ° in a period of , years. each bright star that lies near the circumference of this circle becomes in turn the pole-star sometime within this period. the star alpha, in draco, had its turn at being pole-star some forty-seven centuries ago. polaris is now a little over a degree from the north pole of the heavens. during the next two centuries it will continue to approach the pole until it comes within a quarter of a degree of it, when its distance from the pole will begin to increase again. about twelve thousand years hence the magnificent vega, whose acquaintance we will now make, will be the most brilliant and beautiful of all pole-stars. vega (arabic for "falling eagle") is the resplendent, bluish-white, first-magnitude star that lies in the constellation of lyra, the lyre or harp, a small, but important, constellation just east of hercules and a little to the southeast of the head of draco. vega is almost exactly equal in brightness to arcturus, the orange-colored star in boötes, now lying west of the meridian in the early evening hours. it is also a near neighbor of the solar system, its light taking something like forty years to travel to the earth. vega is carried nearly through the zenith of washington and all places in the same latitude by the apparent daily rotation of the heavens. it is a star that we have no difficulty in recognizing, owing to the presence of two nearby stars that form, with it, a small equal-sided triangle with sides only two degrees in extent. if our own sun were at the distance of vega, it would not appear as bright as one of these faint stars, so much more brilliant is this magnificent sun than our own. the two faint stars that follow so closely after vega and form the little triangle with it are also of particular interest. epsilon lyræ, which is the northern one of these two stars, may be used as a test of keen eyesight. it is the finest example in the heavens of a quadruple star--that is, "a double-double star." a keen eye can just separate this star into two without a telescope, and with the aid of a telescope, each of the two splits up into two stars, making four stars in place of the one visible to the average eye. zeta, the other of the two stars that form the little triangle with vega, is also a fine double star. the star that lies almost in a straight line with epsilon and zeta and a short distance to the south of them is a very interesting variable star known as beta lyræ. its brightness changes very considerably in a period of twelve days and twenty-two hours. this change of brightness is due to the presence of a companion star. the two stars are in mutual revolution, and their motion is viewed at such an angle from the earth that, in each revolution, one star is eclipsed by the other, producing a variation in the amount of light that reaches our eyes. by comparing this star from day to day with the star just a short distance to the southeast of it, which does not vary in brightness, we can observe for ourselves this change in the light of beta lyræ. there are a number of stars in the heavens that vary in brightness in the same manner as beta lyræ, and they are called eclipsing-variable stars. on the line connecting beta lyræ with the star southeast of it and one-third of the distance from beta to this star, lies the noted ring nebula in lyra, which is a beautiful object even in a small telescope. it consists of a ring of luminous gas surrounding a central star. the star shines with a brilliant, bluish-white light and is visible only in powerful telescopes though it is easily photographed since it gives forth rays to which the photographic plate is particularly sensitive. in small telescopes the central part of this nebula appears dark but with a powerful telescope a faint light may be seen even in the central portion of the nebula. this is one of the most interesting and beautiful telescopic objects in the heavens. it is in the general direction of the constellation of lyra that our solar system is speeding at the rate of more than a million miles a day. this point toward which we are moving at such tremendous speed lies a little to the southwest of vega, on the border between the constellations of lyra and hercules, and is spoken of as the apex of the sun's way. [illustration: august--sagittarius] in the southern sky we have this month the constellation of sagittarius, the archer, which is just to the east of scorpio and a considerable distance south of lyra. it can be recognized by its peculiar form, which is that of a short-handled milk dipper, with the bowl turned toward the south and a trail of bright stars running from the end of the handle toward the southwest. this is one of the zodiacal groups which contain no first-magnitude stars, but a number of the second and third magnitude. it is crossed by the milky way, which is very wonderful in its structure at this point. some astronomers believe that here--among the star-clouds and mists of nebulous light which are intermingled with dark lanes and holes, in reality dark nebulæ--lies the center of the vast system of stars and nebulæ in which our entire solar system is but the merest speck. some of the grandest views through the telescope are also to be obtained in this beautiful constellation of sagittarius, which is so far south that it is seen to better advantage in the tropics than in the mid-latitudes of the northern hemispheres. x september one of the most beautiful constellations of the northern hemisphere is cygnus, the swan, which is in the zenith in mid-latitudes about nine o'clock in the evening the middle of september. it lies directly in the path of the milky way which stretches diagonally across the heavens from the northeast to the southwest at this time. in cygnus, the milky way divides into two branches, one passing through ophiuchus and serpens to scorpio, and the other through sagitta and aquila to sagittarius, to meet again in the southern constellation of ara, just south of scorpio and sagittarius. on clear, dark evenings, when there is no moonlight, this long, dark rift in the milky way can be seen very clearly. in cygnus, as in ophiuchus, scorpio, and sagittarius we find wonderful star-clouds, consisting of numberless stars so distant from us and, therefore, so faint that they do not appear as distinct points of light except in the greatest telescopes. it is the combined light from these numberless stars that cannot be seen separately that produces the impression of stars massed in clouds of nebulous light and gives to this girdle of the heavens its name of the milky way. in cygnus, as in a number of other constellations of both hemispheres, the milky way is crossed by dark rifts and bars and is very complicated in its structure. it is in cygnus, also, that one may see with the aid of powerful telescopes the vast, irregular, luminous nebulæ, that are like great clouds of fiery mist. these nebulæ are of enormous extent, for they cover space that could be occupied by hundreds of stars. [illustration: september--cygnus] cygnus is a constellation that is filled with the wonders and mysteries of space and that abounds in beautiful objects of varied kinds. it is a region one never tires of exploring with the telescope. the principal stars in cygnus form the well-known northern cross, with the beautiful, white, first-magnitude star deneb, or arided, as it is sometimes called, at the top of the cross, and albireo, the orange-and-blue double star at the foot. albireo, among all the pairs of contrasting hues, has the distinction of being considered the finest double star in the heavens for small telescopes. this star marks the head of the swan, as well as the foot of the northern cross, and deneb marks the tail of the swan. a short distance to the southeast of deneb, on the right wing of the swan, is a famous little star, cygni, barely visible to the naked eye and forming a little triangle with two brighter stars to the east. this star has the distinction of being the first one to have its distance from the solar system determined. the famous mathematician and astronomer bessel accomplished this difficult feat in the year . since that day, the distances of many stars have been found by various methods, but of all these stars only four or five are known to be nearer to us than cygni. its distance is about eight light-years, so its light takes about eight years to travel the distance that separates it from the solar system. as a result, we see it not as it is tonight, but as it was at the time when the light now entering our eyes first started on its journey eight years ago. cygni is also a double star, and the combined light of the two stars gives forth only one-tenth as much light as our own sun. most of the brilliant first-magnitude stars give forth many times as much light as the sun; but among the fainter stars, we find some that appear faint because they are very distant, and some that are faint because they are dwarf stars and have little light to give forth. to the class of nearby, feebly-shining dwarf stars cygni belongs. deneb, on the other hand, is one of the giant stars, and is at an immeasurably great distance from the solar system. just south of cygnus in the eastern branch of the milky way lie sagitta, the arrow, and aquila, the eagle. not far to the northeast of aquila is the odd little constellation of delphinus, the dolphin, popularly referred to as job's coffin. there will be no difficulty in finding this small star-group, owing to its peculiar diamond-shaped configuration. its five principal stars are of the fourth magnitude. it is in the constellation of delphinus that the most distant known object in the heavens is located. this is the globular star cluster known only by its catalogue number of n.g.c. . it is estimated to be at a distance of , light-years from the earth. sagitta, the arrow, lies midway between albireo and the brilliant altair in aquila. the point of the arrow is indicated by the star that is farthest east; and the feather, by the two faint stars to the west. like delphinus, this constellation is very small and contains no objects of particular interest. altair (flying eagle) is the brilliant white star of the first magnitude in aquila and is attended by two fainter stars, one on either side, at nearly equal distances from it. these two stars serve readily to distinguish this star, all three stars being nearly in a straight line. altair is one of the nearer stars, its distance from the earth being about sixteen light-years. it gives forth about ten times as much light as the sun. [illustration: september--delphinus, aquila and sagitta] we cannot leave the constellation of aquila without referring to the wonderful temporary star or nova, known as nova aquilæ no. (because it was the third nova to appear in this constellation), which appeared in the position indicated on the chart upon the eighth of june, . a few days previous to this date, there was in this position an extremely faint star, invisible to the naked eye and in small telescopes. this fact became known from later examinations of old photographs of this region that had been taken at the harvard college observatory, where the photographing of the heavens is carried on regularly for the purpose of having a record of celestial changes and happenings. clouds prevented the obtaining of any photographs of this part of the heavens on the four nights preceding the eighth of june, but on this evening there shone in the place of the faint telescopic star, a wonderful temporary star, or nova, which was destined on the next evening to outshine all stars in the heavens, with the exception of the brightest of all, sirius, which it closely rivaled in brilliancy at the height of its outburst. within less than a week's time, this faint star in the milky way for some mysterious reason increased in brightness many thousandfold. such outbursts have been recorded before, but on rare occasions, however. no star since the appearance of the nova known as kepler's star, in the year , which at its greatest brilliancy rivaled jupiter, shone with such splendor or attracted so much attention as nova aquilæ. in the year , there appeared in the constellation of perseus a star known as nova persei which at its brightest surpassed vega, but its splendor was not as great as that of the wonderful nova of . it speaks well for the zeal and interest of amateur astronomers, as well as for their acquaintance with the stars, that nova persei was discovered by an amateur astronomer, dr. anderson, and that among the deluge of telephone calls and telegrams received at the harvard college observatory on the night of june th, announcing independent discoveries of the "new star," were many from non-professional astronomers. like all stars of this class, nova aquilæ no. sank rapidly into oblivion. in a few weeks it was only a third-magnitude star; a few weeks more and it was invisible without a telescope. many wonderful and interesting changes have been recorded in the appearance of this star, however, even after it became visible only in the telescope. soon after its outburst it appeared to develop a nebulous envelope, as have other novas before it. it showed in addition many of the peculiarities of the nebulæ, though the central star remained visible as before the outburst. astronomers are still in doubt as to the cause of these outbursts, which certainly indicate celestial catastrophies of some form on a gigantic scale. all novas possess one characteristic in common--that of appearing exclusively in the milky way; and another characteristic is the development of a nebular envelope after the outburst of greatest brightness. in some cases temporary stars have been known to be variable in brightness for years before the great outburst. such a star was nova aquila, for the examination of photographs of this region taken some years previous showed variations in its brightness for a period of thirty years at least. up to the beginning of this century only about thirty novas had been discovered. since that date, thanks to the vigilance of the astronomers of today and to the aid of photography, more have been discovered than in all the preceding centuries. these outbursts of new stars appear to be not so rare as the earlier astronomers believed, though great outbursts as brilliant as that of nova aquila are very uncommon. xi october the constellations that will be found nearest the meridian in early october evenings are the circumpolar constellations cepheus and cassiopeia, and in the southern sky, capricornus and aquarius. cepheus, the king, and cassiopeia, his queen, of whom we shall have more to say later in connection with the constellations of andromeda and perseus, sit facing the north pole of the heavens opposite ursa major, the great bear, familiar to us under the name of the big dipper. the foot of cepheus rests upon the tail of the little bear, and the star farthest north in the diagram is in the left knee. the head is marked by a small triangle of faint stars, shown in the diagram. one of these three faint stars--the one farthest east--known as delta cephei, is a very remarkable variable star, changing periodically in brightness every five and one-third days. its name has been given to a large class of variable stars--the cepheid variables--that resemble delta cephei in being giant suns, faint only because they are at very great distances from the earth, and varying in brightness with the greatest regularity in periods that range from a few hours to several weeks. it has been found that the longer the period of light change the greater is the star in size and brightness. the cepheids of longest period are , times more brilliant than our own sun. cepheus contains no very bright or conspicuous stars. alpha cephei, the brightest star in the group, marks the king's right shoulder. it is the star farthest to the west in the diagram, and is only a third-magnitude star. [illustration: october--cassiopeia and cepheus] cassiopeia is a constellation with which every one in the northern hemisphere should be familiar, owing to its very distinctive w-shape and its far northern position, which brings it conspicuously into view throughout the clear fall and winter evenings. cassiopeia is pictured in all star atlases that show the mythological figures, with her face toward the north pole. the stars in the w outline the body. alpha, the star farthest south in the diagram marks the breast of cassiopeia. her head and uplifted hands are represented by faint stars south of alpha. this star is occasionally referred to by its arabic name of schedir. beta, the leader of all the stars in the w in their daily westward motion, is also known by an arabic name, caph. in the constellation of cassiopeia there appeared in the year a.d., a wonderful temporary star which suddenly, within a few days' time, became as brilliant as the planet venus and was clearly visible in broad daylight. this star is often referred to as tycho's star, because it was observed, and its position very accurately determined, by tycho brahe, one of the most famous of the old astronomers. this star remained visible to every one for about sixteen months, but it finally faded completely from view, and it is believed that a faint, nebulous red star, visible only in the telescope and close to the position recorded by tycho, represents the smoldering embers of the star that once struck terror to the hearts of the superstitious and ignorant among all the nations of europe, who took it to be a sign that the end of the world was at hand. both cassiopeia and cepheus lie in the path of the milky way, which reaches its farthest northern point in cassiopeia and passes from cepheus in a southerly direction into the constellation of cygnus. [illustration: october--aquarius and capricornus] turning now to southern skies, we find on and to the west of the meridian at this time the rather inconspicuous zodiacal constellation of capricornus, the goat. it contains no stars of great brightness and is chiefly remarkable for the fact that it contains one of the few double stars that can be seen without the aid of a telescope. the least distance in the heavens that the unaided human eye can separate is about four minutes of arc. the star alpha in capricornus is made up of two stars separated by a distance of six minutes of arc, so that any one can readily see that it consists of two stars very close together. this star, alpha, will be found in the extreme western part of the constellation, and can best be located in conjunction with the star beta, which is slightly brighter and lies but a short distance almost due south of alpha, the two stars standing somewhat alone in this part of the heavens. to the north and east of capricornus we find aquarius, which is also a zodiacal constellation. aquarius is the water-bearer, and the water jar which he carries is represented by a small, but distinct, y of stars from which flows a stream of faint stars toward the southeast and south. aquarius, like capricornus, is a rather uninteresting constellation, as it is made up of inconspicuous third- and fourth-magnitude stars. the entire region covered by these two groups of stars is remarkably barren, since it contains not a single first- or even second-magnitude star and little to attract the observer's eye. to relieve the barrenness of this region, there appears just to the south of aquarius and southeast of capricornus, sparkling low in the southern sky on crisp october evenings, the beautiful first-magnitude star fomalhaut in the small southern constellation of piscis australis, the southern fish. this star is the farthest south of all the brilliant first-magnitude stars that can be seen from the middle latitudes of the northern hemisphere. the constellation in which it lies is so close to the southern horizon in our latitudes that it cannot be seen to any advantage, and it is at best very inconspicuous, containing no other objects of interest. fomalhaut cannot be mistaken for any other star visible at this time of year in the evening, since it stands in such a solitary position far to the south. at the time of which we are writing it will be found a few degrees east of the meridian. xii november directly south of cassiopeia and cepheus, the circumpolar constellations with which we became acquainted last month, and almost overhead in our latitudes in the early evening hours of november, lie pegasus, the winged horse, and andromeda, the woman chained. according to the legend, cepheus was king of ethiopia, and cassiopeia was the beautiful, but vain, queen who dared to compare herself in beauty with the sea-nymphs. this so enraged the nymphs that, as a punishment for her presumption, they decided to send a terrible sea-monster to ravage the coast of the kingdom. the king and queen, upon consulting the oracle, found that the only way to avert this calamity would be to chain their daughter andromeda to the rocks and permit the monster to devour her. [illustration: november--andromeda and pegasus] as the story goes, the valiant hero, perseus, chanced to be riding through the air on his winged horse and saw, far beneath him the beautiful maiden chained to the rocks and the frightful monster approaching to devour her. he immediately went to the rescue, and, after a terrible struggle with the monster, succeeded in overpowering him and thus saved the maiden from a dreadful fate. perseus and the fair andromeda were married shortly afterward, and at the end of a happy life the pair were transferred to the heavens. cassiopeia, the vain queen, was ordered to be bound to a chair and, with the king cepheus at her side, to be swung continually around the north pole of the heavens that she might be taught a lesson in humility. the constellation cetus, representing the sea-monster, will be found to the southeast and south of pisces, the fishes, which lie south of andromeda and pegasus. the great square in pegasus is the most conspicuous configuration of stars to be seen in the heavens in autumn evenings. the star that marks the northeastern corner of the great square belongs to the constellation of andromeda and marks the head of the maiden, who is resting upon the shoulders of pegasus, the winged horse. the three bright stars nearly in a straight line outline the maiden's body, alpha, or alpheratz, as it is called, being the star in the head, beta or mirach in the waist, and gamma or almach in the left foot. the last-named star, which is farthest to the northeast in the diagram, was, in the opinion of the noted astronomer herschel, the finest double star in the heavens. the two stars into which the telescope splits it are of the beautifully contrasted shades of orange and sea green. a second most interesting object in andromeda and one of the finest in the entire heavens is the great andromeda nebula, which is faintly visible without the aid of a telescope as a hazy patch of light. it is believed that in reality this nebula is a great universe composed of many thousands of stars so distant that no telescope can show the individual members and that the light from it takes many thousands of years to span the abyss that separates it from the solar system. some magnificent photographs of the great andromeda nebula have been taken with powerful telescopes. it is through the use of photography that the nebulæ can best be studied, for a photographic plate after long exposure, reveals a wonderful detail in the structure of these objects that the human eye fails to see. on a clear, dark evening one may find the great andromeda nebula by the aid of two faint stars with which it makes a small triangle, as shown in the chart. this nebula is the only one of the spiral nebula that can be seen in these latitudes without the aid of a telescope, though there are several spiral nebulæ in the southern heavens that can be thus seen. lying to the northwest of the great square in pegasus are a number of faint stars that outline the shoulders and head of the winged steed, while the stars to the southwest of the square outline his forelegs. the creature is represented without hind quarters in all star atlases. the space within the great square contains no bright stars, and as a result, the outline of the square stands out with great distinctness. there are, in fact, no stars of the first magnitude in either pegasus or andromeda, though there are a number of the second and third magnitude which very clearly show the distinctive forms of these two star-groups. pisces, the fishes, the constellation just south of andromeda and pegasus, is the first of the twelve zodiacal constellations. it consists of the southern fish, lying in an east-to-west direction, and the northern fish, lying nearly north and south, the two touching at the southeastern extremity of the constellation. [illustration: november--pisces] there is in pisces not a single bright star, and its only point of interest is to be found in the fact that it contains the point, marked by the cross and letter v in the diagram, that is known variously as "the vernal equinox," "the equinoctial point" and "the first point in aries." this is a very important point of reference in the heavens, just as the meridian of greenwich is for the earth, and it marks the point where the sun crosses the equator going north in the spring. owing to the precession of the equinoxes, as it is called, this point is gradually shifting its position westward through the zodiacal constellations at a rate that will carry it completely around the heavens through the twelve zodiacal groups in a period of , years. since the beginning of the christian era, this point has backed from the constellation of aries, which lies just east of pisces, into pisces, though it still retains its name of "the first point in aries." xiii december the eastern half of the sky on early december evenings is adorned with some of the finest star-groups in the heavens; but as we are considering for each month only the constellations that lie on or near the meridian in the early evening hours, we must turn our eyes for the present from the sparkling brilliants in the east to the stars in the less conspicuous groups of aries, the ram, and cetus, the whale. we will also become acquainted this month with the beautiful and interesting constellation of perseus, the hero of mythical fame to whom we referred last month in connection with the legend of cepheus and cassiopeia. cetus, you will recall, represents the sea-monster sent to devour andromeda, the daughter of cepheus and cassiopeia. we have included the constellation of andromeda in our diagram for this month, since it is so closely associated in legend with the constellations of perseus and cetus, though we also showed it last month. the brightest star in perseus, known as alpha persei, is at the center of a curved line of stars that is concave or hollow toward the northeast. this line of stars is called the segment of perseus, and it lies along the path of the milky way, which passes from this point in a northwesterly direction into cassiopeia. according to the legend, perseus, in his great haste to rescue the maiden from cetus, the monster, stirred up a great dust, which is represented by the numberless faint stars in the milky way at this point. the star alpha is in the midst of one of the finest regions of the heavens for the possessor of a good field-glass or small telescope. a short distance to the southwest of alpha is one of the most interesting objects in the heavens. to the ancients, it represented the baleful, winking demon-eye in the head of the snaky-locked gorgon, medusa, whom perseus vanquished in one of his earliest exploits and whose head he carried in his hand at the time of the rescue of andromeda. to the astronomers, however, algol is known as beta persei, a star that has been found to consist of two stars revolving about each other and separated by a distance not much greater than their own diameters. one of the stars is so faint that we speak of it as a dark star, though it does emit a faint light. once in every revolution the faint star passes directly between us and the bright star and partly eclipses it, shutting off five-sixths of its light. this happens with great regularity once in a little less than three days. it is for this reason that algol varies in brightness in this period. there are a number of stars that vary in brightness in a similar manner. their periods of light-change are all very short, and the astronomers call them eclipsing variables. at its brightest, algol is slightly brighter than the star nearest to it in andromeda, which is an excellent star with which to compare it. [illustration: december--perseus, aries and cetus] perseus is another one of the constellations lying in the milky way in which temporary stars or novas have suddenly flashed forth. at the point indicated by a cross in the diagram, dr. anderson, an amateur astronomer of scotland, found on february , , a new star as brilliant as the pole-star. on the following day it became brighter than a star of the first magnitude. a day later it had lost a third of its light, and in a few weeks it was invisible without the aid of a telescope. in a year it was invisible in all except the most powerful telescopes. with such telescopes, it may still be seen as a very faint nebulous light. triangulum and aries are two rather inconspicuous constellations that lie on, or close to, the meridian at this time. there is nothing remarkable about either of these groups, except that aries is one of the twelve zodiacal constellations. some centuries ago, the sun was to be found in aries at the beginning of spring and the position it occupies in the sky at that time was called the first point in aries. as this point is slowly shifting westward, as we have explained elsewhere, the sun is now to be found in pisces, instead of aries at the beginning of spring and does not enter aries until a month later. pisces was one of the constellations for november and we showed in that constellation the present position of the sun at the beginning of spring. two stars in aries--alpha and beta--are fairly bright, alpha being fully as bright as the brightest star in andromeda. beta lies a short distance to the southwest of alpha, and a little to the southwest of beta is gamma, the three stars forming a short curved line of stars that distinguishes this constellation from other groups. the remaining stars in aries are all faint. just south of aries lies the head of cetus, the whale. this is an enormous constellation that extends far to the southwest, below a part of pisces, which runs in between andromeda and cetus. its brightest star, beta, diphda, or deneb kaitos, as it is severally called, stands quite alone not far above the southwestern horizon. it is almost due south of alpha andromedæ, the star in andromeda farthest to the west, which it exactly equals in brightness. the head of cetus is marked by a five-sided figure composed of stars that are all faint with the single exception of alpha, which is fairly bright, though inferior to beta or diphda. cetus, though made up chiefly of faint stars, and on the whole uninteresting, contains one of the most peculiar objects in the heavens, the star known as omicron ceti or mira (the wonderful). this star suddenly rises from invisibility nearly to the brightness of a first-magnitude star for a short period once every eleven months. mira was the first known variable star. its remarkable periodic change in brightness was discovered by fabricius in the year , so its peculiar behavior has been under observation for three hundred and twenty-five years. it is called a long-period variable star, because its variations of light take place in a period of months instead of a few hours or days, as is the case with stars such as algol. mira is not only a wonderful star, it is a mysterious star as well, for the cause of its light-changes are not known, as in the case of algol where the loss of light is produced by a dark star passing in front of a brighter star. mira is a deep-red star, as are all long-period variable stars that change irregularly in brightness. it is visible without a telescope for only one month or six weeks out of the eleven months. during the remainder of this time, it sometimes loses so much of its light that it cannot be found with telescopes of considerable size. its periods of light-change are quite variable as is also the amount of light it gains at different appearances. it is believed that the cause of the light-changes of mira is to be found within the star itself. it has been thought that dense clouds of vapors may surround these comparatively cool, red stars and that the imprisoned heat finally bursts through these vapors and we see for a short time the glowing gases below; then the vapors once more collect for a long period, to be followed by another sudden outburst of heat and light. it is interesting to remember in this connection that our own sun has been found to be slightly variable in the amount of light and heat that it gives forth at different times, and the cause of its changes in light and heat are believed to lie within the sun itself. xiv stars of the southern hemisphere as one travels southward from the mid-latitudes of the northern hemisphere into the tropics our familiar circumpolar constellations sink lower and lower in the northern heavens and strange and unfamiliar star-groups rise gradually above the southern horizon. if we make our southward journey in the winter months the first of the southern constellations to come fully into view is the small star-group just south of lepus known as columba (the dove), whose brightest star phact is of the second magnitude. a line drawn from procyon to sirius and extended as far again brings us to this star and a line from betelgeuze to sirius extended an equal distance brings us to zeta argus which is equal to phact in brightness. the two lines intersecting at sirius make the "egyptian x" as it is called. magnificent, blue-white canopus, the most brilliant star in the heavens next to sirius, a veritable diamond sparkling low in the southern sky, now commands our unqualified admiration. canopus lies about ° south of sirius and is invisible north of the th parallel of latitude. at nine o'clock in the evening of february th it can be seen just above the southern horizon in that latitude and is then a conspicuous object in georgia, florida and the gulf states. "the star of egypt whose proud light never hath beamed on those who rest in the white islands of the west." writes moore of canopus in "lalla rookh." along the nile canopus was an object of worship as the god of waters. at the time of their erection, b.c., a number of temples in upper egypt were oriented so as to show canopus at sunrise at the autumnal equinox, and other temples erected many centuries later were oriented in a similar manner. in china, as late as b.c., and in india also canopus was an object of worship. the astronomer tells us that canopus is immeasurably distant from the earth. it has been estimated to be forty thousand times more luminous than our sun. canopus is located in the constellation of argo navis which is the largest and most conspicuous constellation in the heavens. in addition to canopus it contains a number of second- and third-magnitude stars and is subdivided for convenience into puppis, the prow; carina, the keel; and vela, the sails. huge as it is, argo navis represents only half of a ship for the stern is lacking. according to the legend this ship was built by argos, aided by pallas athene, for jason, the leader of the expedition of the fifty argonauts who sailed from greece to colchis in search of the golden fleece. pallas athene placed in the bow of the ship a piece of timber from the speaking oak of dodona to guide the crew and warn them of dangers and after the voyage the ship was supposed to have been placed in the heavens. [illustration: southern constellations-- . in february] in argo navis is one of the finest telescopic objects in the heavens, the keyhole nebula, as it is usually called, from a peculiar-shaped dark patch in its brightest part. on the border of this nebula is the deep-red wonder star of the southern hemisphere, eta argus, which varies suddenly and unexpectedly in brightness between the seventh and first magnitudes. in it burst forth with a splendor rivaling sirius and maintained this brilliancy for nearly ten years and then slowly waned in brilliancy until it disappeared to the unaided eye in . the surrounding nebula also seems to share in its peculiar fluctuations of brightness. eta argus is now a star of the seventh magnitude and since it is still varying fitfully in brightness it is believed that the history of its light-changes is not complete. among the constellations of the southern heavens near the meridian in february we see in addition to argo navis the constellations of dorado, the goldfish; hydrus, the serpent, and tucana, the toucan. though insignificant in appearance dorado contains what was described by sir john herschel as one of the most extraordinary objects in the heavens, a worthy rival of the great orion nebula and in some respects very similar to it, the great looped nebula, "the center of a great spiral." in dorado also is located the greater magellanic cloud which looks like a detached portion of the milky way though it is far removed from it. to the naked eye it resembles a small white cloud about ° in extent. in the telescope it bears a close resemblance to a typical portion of the milky way. a similar formation known as the lesser magellanic cloud is located in hydrus. it has been estimated that the distance of the lesser cloud is , light-years and that it is receding from us. in tucana is located one of the finest globular star clusters in the heavens, known as tucanæ. this cluster and omega centauri, a globular star cluster in centaurus, are the two nearest of all the globular clusters. they are distant from the earth about , light-years and it is known that the combined light of the thousands of stars of which each cluster is composed is about one million times that of our own sun. in the western sky in the southern hemisphere in february may be seen the brilliant, white, first-magnitude star achernar in the river eridanus, the long, winding constellation that, we recall, starts near the brilliant rigel in orion and disappears from the view of northern observers below the southern horizon, extending its course far into the southern hemisphere. achernar means "the end of the river" and this is nearly its position in the constellation. though argo navis is the largest and most important constellation of the southern hemisphere, crux, the southern cross, far-famed in story and legend as well as for its historical associations, is beyond a doubt the most popular. the best time to view the southern cross is in june or july when it is near the meridian. it is not seen to advantage in the months of january or february. it then lies on its side and close to the horizon and therefore is dimmed by atmospheric haze so that it almost invariably is a disappointing object to the tourist from the north who usually views it for the first time in one of these months. the cross is viewed to advantage in the latitude of rio or valparaiso and it is best seen from the straits where it rides high overhead. it is not seen to advantage from the latitudes of cuba or jamaica. it is small, only ° in extent from north to south and less in width and it lies in the most brilliant portion of the milky way which is here a narrow stream only three or four degrees wide. directly below the cross is the noted coal sack, apparently a yawning chasm in the midst of its brilliant surroundings though probably in reality a dark nebula. viewed with the telescopes a number of stars are to be seen projected on this dark background. the southern cross is to the inhabitants of the southern hemisphere what the big dipper is to those who dwell in the northern hemisphere--an infallible timepiece. the upright of the cross points toward the south pole of the heavens which lies in a region where there is a singular dearth of bright stars, the nearest star to the south pole being a faint fifth-magnitude star called sigma octantis. when seen in the southeast or southwest the cross lies on its side, but when passing the meridian it stands nearly upright. humboldt, referring to this fact, says: "how often have we heard our guides exclaim in the savannahs of venezuela and in the desert extending from lima to truxillo, 'midnight is past, the cross begins to bend.'" by the explorers of the sixteenth century the cross was taken as a sign of heaven's approval of their attempt to establish the christian religion in the wilds of the new world. this thought is beautifully expressed in mrs. hemans' lines in "the cross of the south." "but to thee, as thy lode-stars resplendently burn in their clear depths of blue, with devotion i turn bright cross of the south! and beholding thee shine, scarce regret the loved land of the olive and vine. thou recallest the ages when first o'er the main my fathers unfolded the ensign of spain, and planted their faith in the regions that see its imperishing symbol ever blazoned in thee." alpha crucis, the brightest star in crux, is at the foot of the cross. it consists in reality of two second-magnitude stars forming a beautiful double while a third fifth-magnitude star one and one-half minutes of arc distant makes with this pair a combination similar to our mizar and alcor of the big dipper though the separation is not great enough to be visible to the naked eye. the second-magnitude star at the head of the cross is a deep orange in color and the two stars that mark the ends of the cross-arm are white third-magnitude stars. [illustration: southern constellations-- . in july] one of the finest constellations of the southern hemisphere is centaurus, the centaur, which surrounds crux on the north and is more than ° in length. its center lies about ° south of spica in virgo and below the tail of hydra. alpha centauri, its brightest star and the nearest star to the solar system, four and one-third light-years away, is a golden-yellow double star that forms with the star beta centauri on the west a configuration similar to that of castor and pollux in gemini, only one that is far more striking because of the superior brilliancy of the stars. alpha centauri lies in the milky way and transits the meridian at the same time with arcturus though it cannot be seen north of the th parallel. alpha centauri, like canopus, was an object of worship in egypt and a number of temples in northern egypt were oriented to its emergence from the sun's rays in the morning at the autumnal equinox, between and b.c. north of centaurus is the constellation lupus, the wolf, which is also crossed by the milky way. according to one myth lupus is held in the right hand of the centaur as an offering upon the altar which is represented by the constellation of ara next to centaurus on the east. ara also is crossed by the milky way. neither lupus nor ara contain any objects that are worthy of special attention. triangulum australe, the southern triangle, a little to the southeast of alpha centauri, is far more conspicuous than the triangulum of the northern hemisphere. the accompanying charts give two views of these principal southern constellations that lie within ° of the south pole of the heavens and that are below the horizon in ° north latitude. the first of these charts shows the constellations that are nearest the meridian in the early evening hours in february. canopus in argo navis and the greater magellanic cloud then lie close to the meridian. argo navis with its subdivisions puppis, vela and carina are found east of the meridian lying directly in the path of the milky way, which stretches diagonally across the sky from the northwest to the southeast. far over in the southeast appears crux, the southern cross, also directly in the path of the milky way. in the western heavens may be seen the lesser magellanic cloud in hydrus, brilliant achernar in eridanus and the inconspicuous star-group of tucana. in the early evening hours of july we find as shown on the second chart, alpha and beta centauri in centaurus close to the meridian, lupus due north of centaurus, ara and triangulum australe in the southeast and crux in fine position for observation just west of the meridian. carina of argo navis lies to the southwest of crux. the milky way now arches magnificently across the heavens from carina through crux, centaurus and lupus and ara to the zodiacal groups of scorpio and sagittarius in the northeast. in the northern part of the heavens, as seen from the southern hemisphere, appear the familiar zodiacal constellations that we of the northern hemisphere find south of the zenith, as well as the constellations of orion, lepus and canis major, hydra, corvus and crater, ophiuchus and serpens and aquila, all finely in view in their appropriate seasons. it is only our familiar circumpolar constellations of the north--the two bears, draco, cassiopeia, and cepheus, andromeda and perseus and auriga that are invisible in mid-latitudes of the southern hemisphere just as the constellations shown in the diagrams, and a few additional groups such as pavo, grus, phoenix, apus, mensa and volans which we have not shown, lie hidden from view beneath the southern horizon in mid-latitudes of the northern hemisphere. the northern visitor to the southern hemisphere familiar with the constellations of his own land is filled with a queer sensation of being in topsy-turvydom as he sees familiar orion standing on his head and all of the zodiacal constellations passing in their daily motions to the north instead of to the south of his zenith while by day the sun passes across the northern part of the heavens and culminates north instead of south of his zenith. he misses the familiar dippers of his own land and searches in vain for a pole-star in the unfamiliar circumpolar regions of the south. xv the milky way or galaxy "broad and ample road whose dust is gold, and pavement stars, as stars to thee appear seen in the galaxy, that milky way which nightly as a circling zone thou seest powder'd with stars." --milton, _paradise lost_. on clear, winter evenings one may see a portion of the zone of the milky way, which encircles the heavens, arching magnificently across the heavens as it passes from cassiopeia and cepheus in the northwest, through perseus and auriga and the eastern part of taurus, across the feet of gemini, between canis minor and orion and through the eastern part of canis major to the southern horizon. at this point it passes beyond our range of vision into the star-groups of puppis, vela and carina, subdivisions of the huge southern constellation of argo navis. it reaches its greatest distance south of the celestial equator and also attains its greatest brilliancy in crux, the far-famed southern cross. from here it turns northward once more, passing into centaurus, musca, circinus ara and lupus constellations of the southern hemisphere and comes within our range of vision again in sagittarius and scorpio. here the milky way divides into two branches, though some astronomers now believe that this apparent division into two branches is due to the presence of an enormous cloud of non-luminous matter lying along the course of the milky way at this point, similar in its nature to the dark "holes" and "caves" and streaks that appear in all portions of the milky way and most noticeably athwart its course in argo and centaurus. one of these branches of the milky way passes from sagittarius through aquila to cygnus and the other through scorpio, ophiuchus and serpens to cygnus, the two extending diagonally across the heavens in the late summer and early fall evenings from the northeast to the southwest. from cygnus, the milky way passes into cepheus and cassiopeia and thus completes its circuit of the heavens. it is not seen to advantage in spring or early summer evenings because it then rests nearly on the horizon. its plane is inclined about ° to the celestial equator and its poles lie in the constellations of coma berenicis and cetus. these are the two points that lie farthest from the milky way. the milky way has been called the groundwork of the universe. by far the greater number of all the stars are crowded towards its plane in the form of an enormous flattened disk or lens. our solar system, it has been estimated, lies close to the plane of the milky way and at a distance of some , or , light-years from its center. the diameter of the milky way as deduced from dr. harlow shapley's work on globular star clusters is about , light-years in extent, or ten times greater than the limit set some years ago. the apparent crowding together of the stars into dense clouds in the milky way is partly an effect due to our position in the milky way. when we look at the heavens in a direction at right angles to this plane we find comparatively few stars lying along our line of vision because the stars are actually fewer in number in this direction. if we look _along_ the plane of the milky way, however, we see to a greater distance through an enormous depth of stars. though individual stars may not be much closer together in the milky way than they are outside of it, there are on the whole more of them and the effect of greater density is produced. father hagen of the vatican observatory, who has for years made a study of the dark clouds of obscuring matter and dark nebulæ that abound in space, has found evidence of the existence of many vast clouds of dark obscuring matter over the entire heavens above and below the plane of the milky way as well as surrounding the milky way in its own plane. the existence of such clouds of non-luminous matter would account partly for the comparative fewness of stars in space outside of the plane of the milky way since many stars would be concealed from our eyes by these obscuring clouds. there is, however, in addition, an actual crowding of all the visible stars toward this plane. the peoples of all ages have honored the milky way in story and legend. it has been universally referred to as the sky river and the pathway of souls. to the little hiawatha, we remember, the "wrinkled old nakomis" "showed the broad white road in heaven pathway of the ghosts, the shadows, running straight across the heavens, crowded with the ghosts, the shadows. to the kingdom of ponemah to the land of the hereafter." in _the galaxy_, longfellow thus describes the milky way: "torrent of light and river of the air along whose bed the glimmering stars are seen like gold and silver sands in some ravine where mountain streams have left their channels bare!" in sweden, where the milky way arches high through the zenith in winter, it is called the winter street, and miss edith thomas writes thus beautifully of it in her poem entitled, "the winter street": "silent with star dust, yonder it lies-- the winter street, so fair and so white; winding along through the boundless skies, down heavenly vale, up heavenly height. faintly it gleams, like a summer road when the light in the west is sinking low, silent with star dust! by whose abode does the winter street in its windings go? and who are they, all unheard and unseen-- o who are they, whose blessèd feet pass over that highway smooth and sheen? what pilgrims travel the winter street? are they not those whom here we miss in the ways and the days that are vacant below? as the dust of that street their footfalls kiss does it not brighter and brighter grow?" beautiful indeed are these poetic fancies but none of them picture even remotely the awe-inspiring grandeur of the milky way as it actually exists. [illustration: a dark nebula: the dark bay or dark horse nebula in orion taken with -inch hooker telescope of the mt. wilson observatory] millions upon millions of far distant suns equal to or surpassing our own sun in brilliancy are gathered within this vast encircling zone of the heavens, their combined light giving to the naked eye the impression of a milky band of light. nine-tenths of all the stars, it has been estimated, lie close to the plane of the galaxy, as well as all the vast expanses of luminous gaseous nebulæ and clouds of dark obscuring matter all seemingly intermingled in chaotic confusion; yet law and order govern the motions of all. here also are the great moving star clusters such as the pleiades and the hyades and all of the brilliant "orion" stars. the structure of the milky way is not clearly understood but many astronomers believe there is evidence that it takes the form of a vast spiral nebula along whose arms the stars pass to and fro. beyond the milky way at enormous distances of many thousands of light-years, but apparently influenced by it, lie the globular star-clusters and the spiral nebulæ. the spirals appear to avoid the plane of the milky way for they are receding in the direction of its poles at high velocities; the globular clusters on the other hand are drawing in toward the milky way on either side, and in time will cross it. whether these objects external to the milky way form with it one enormous universe or whether the spiral nebulæ are in turn galaxies or "island universes," as the astronomer calls them, similar in form and structure to our own galaxy and at inconceivably great distances of millions of light-years from it, is still one of the riddles of the universe which the astronomers are attempting to solve. xvi the surface of the sun the visible surface of the sun is called the _photosphere_. even the smallest telescopes will show its peculiar "rice-grain" structure, consisting of intensely brilliant flecks or nodules about miles in diameter, which can be resolved by the more powerful telescopes into smaller particles about miles in diameter, against a darker background. it has been estimated that these bright nodules or rice-grains occupy only one-fifth of the total surface of the sun, yet radiate three-fourths of the total light. it is generally believed that the "rice grains" are the summits of highly heated columns of gas, arising from the sun's interior, and that the darker portions between are cooler descending currents. it is well known that the photosphere or visible surface of the sun appears to be much brighter in the center of the disk than near its circumference. this is due entirely to its gaseous nature and to the fact that it is surrounded by an atmosphere of dense enveloping cooler gases. rays from the center of the disk travel a shorter distance through this atmosphere than the rays from the rim and therefore are absorbed less by surrounding gases. we look further down into the sun's interior near the center of the disk than in the direction of its circumference and so the light appears more intense there. the photosphere is the region where sun-spots appear and they are found in zones extending from ° to ° on either side of the solar equator, never appearing exactly at the equator or near the poles. the disturbances that produce sun-spots and many allied phenomena occur cyclically in periods of eleven years on the average. the first outburst of the disturbance is manifested by the appearance of sun-spots in high solar latitudes. these break out and disappear and break out again with increased vigor, working gradually downward toward the solar equator, the maximum spottedness for a given period occurring in solar latitude about °. the disturbance finally dies out within ° or ° of the equator, but even before one cycle of disturbance has entirely passed away a new cycle has broken forth in high latitudes. so during the period of minimum spottedness there are four distinct belts, two in low latitudes, due to the dying disturbance, and two in high latitudes, due to the new disturbance. at sun-spot maximums there are two well-marked zones of great intensity, approximately ° north and south of the sun's equator. sun-spots are solar cyclones, occurring usually in groups, though large single spots appear less frequently. each spot is quite sharply divided into an umbra and a penumbra. the umbra is the darker central portion, the funnel of the whirling cyclone, and the penumbra is composed of the outspreading gases, and is less dark than the umbra. the peculiar "thatch-straw" structure of the penumbra is due, it is believed, to the fact that the columns of gases that usually rise vertically from the sun's interior and from the "rice grains" of the photosphere are drawn into a horizontal position by the whirling motion that exists in the penumbra regions of a sun-spot and therefore we get a longitudinal rather than a cross sectional view of them. the umbra of a sun-spot is anywhere from a few hundred miles to fifty thousand miles in diameter, frequently exceeding the earth in size, while the penumbra occasionally reaches a diameter of two hundred thousand miles. sun-spots of exceptional size can be seen even without the aid of a telescope. the darkness of sun-spots is only by comparison with their more brilliant background. owing to the rapid expansion and cooling of gases the temperature in sun-spot regions is far below the normal solar temperature of , ° centigrade, lying between , ° and , ° centigrade. at this temperature it is possible for the more refractory chemical compounds to form, the oxides and the hydrides, and the spectra of sun-spots reveal the presence of titanium oxide and magnesium and calcium hydride. at the higher solar temperatures that exist elsewhere in the photosphere and in its overlying gaseous envelopes all chemical elements occur in a free state, intermingling as incandescent vapors without the formation of any chemical compounds. strong magnetic fields exist in sun-spot regions and magnetic storms in our own atmosphere frequently accompany the appearance of exceptionally large sun-spots. directly above the photosphere of the sun lies the "reversing layer," which is about five hundred miles in depth and is composed of the incandescent vapors of all the chemical elements that exist on the sun, which are also the same familiar elements that exist on the earth, with the exception of coronium, the unknown element in the solar corona, there is no element in the sun that has not been found on our own planet. the "reversing layer" receives its name from the fact that it reverses the solar spectrum. it produces by its absorption of the rays of light from gases below the dark absorption lines found in the spectrum that serve to identify all the elements existing in the sun. during the time immediately preceding and following a total eclipse of the sun this reversing layer produces what is known as the flash spectrum. when the photosphere, which gives the bright continuous background of the solar spectrum, is concealed by the moon, the normally dark lines of the reversing layer--dark only by contrast with the bright background--become momentarily intensely bright lines against a dark background. the flash spectrum only lasts a second or so, as the reversing layer itself is soon covered by the moon. just above the reversing layer lies the _chromosphere_, which is between five thousand and ten thousand miles in depth. many of the gaseous vapors of the reversing layer are found in the chromosphere, thrown there continually by the vast upheavals of gases that are constantly disturbing the surface of the sun. the greater the solar activity the more is the chromosphere charged with the vapors of the lower strata of the sun's atmosphere. the gases that are most characteristic of the chromosphere, however, are the incandescent gases of hydrogen and calcium, which give it the pink or reddish tinge so noticeable during total solar eclipse. helium is also found in great abundance in the solar chromosphere. shooting upward from the photosphere with the tremendous velocity of one hundred or more miles per second, can be seen at all times, by properly screening off the light from the photosphere, the vast solar eruptions known as the _prominences_. these are composed chiefly of hydrogen and calcium gas, though other elements also appear, especially near the bases of the prominences. prominences are of two varieties, the quiescent, or cloud-like prominences, that float high above the solar surface for days at a time in some instances and resemble terrestrial clouds in form, and the eruptive, or metallic prominences, that dart up from the surface of the sun in an infinite variety of forms that may be entirely changed in the short interval of fifteen or twenty minutes. these eruptive prominences usually attain heights of thirty or forty thousand miles on the average, but _exceptional prominences reach heights of more than one hundred thousand miles and in a few rare cases have reached elevations of over five hundred thousand miles, or more than one-half of the solar diameter_. prominences are the most spectacular and beautiful of all solar phenomena, with the possible exception of the solar corona, which is the outermost of all the solar envelopes and also the most tenuous. the extent of the corona is enormous. its outer streamers extend usually to distances of several million miles from the center of the sun. measurements of the coronal light during total eclipses of the sun have shown that its intensity is only about one-half that of full moonlight, and it seems almost impossible to devise methods for detecting it, except during total eclipses, on account of the extreme faintness of its light. the sun, it is now known, is surrounded by a strong magnetic field in addition to the magnetic fields that exist in sun-spots. the cycle of sun-spot change is attended by marked changes in many forms of solar activity. the frequency of outbursts of eruptive prominences, the brightness and form of the corona, magnetic storms and weather changes on the earth are all closely associated with the sun-spot cycle. the cause of this sun-spot cycle, with all the attendant changes in the general solar activity, and the source of the apparently limitless supply of solar energy still remain the two chief unsolved secrets of the sun. xvii the solar system our sun is but a star traveling through the universe. it is accompanied in its journey to unknown parts of space, that lie in the general direction of the constellation hercules, by an extensive family of minor bodies consisting of the eight planets and their encircling moons, one thousand or more asteroids, numerous comets, and meteors without number, all moving in prescribed paths around their ruler. the most important members of the sun's family are the planets, mercury, venus, earth, mars, jupiter, saturn, uranus and neptune, named in the order of their position outward from the sun. we hear occasionally of the possibility of the existence of intra-mercurial and trans-neptunian planets and it is possible that some day an additional planet may be discovered within the orbit of mercury or beyond the orbit of neptune. the gravitational control of the sun extends far beyond the orbit of neptune and there are reasons for believing in the existence of at least one or two additional planets on the outskirts of the solar system. the existence of a planet within the orbit of mercury is now, after long continued and diligent search, believed to be very doubtful. were it possible to view the sun from the distance of the nearest star with the aid of the greatest telescope on earth all the members of his family would be hopelessly invisible. so, also, we cannot tell as we point our powerful telescopes at the stars whether these other suns are attended by planet families. we may only argue that it is very unlikely that there should be only one star among hundreds of millions that is attended by a group of comparatively small dark bodies that shine by the reflected light from the star they encircle. with the exception of the two planets, mercury and venus, which are known as the inferior planets, since their paths lie between the earth and the sun, all the planets have moons or satellites of their own that encircle the planet just as the planet encircles the sun. our planet earth has one satellite, the moon, that has the distinction of being the largest of all the moons in proportion to the size of the planet it encircles. jupiter and saturn have moons that surpass our moon in actual size; in fact, two of the moons of the outer planets are actually larger than the smallest planet mercury, but they are very small in proportion to the size of the planets around which they revolve. mars, the next planet beyond the earth, the nearest of the superior or outer planets, has two tiny moons that bear the names of deimos and phobos, respectively. they are both less than twenty miles in diameter and revolve very near to the surface of mars. they can only be seen with the aid of very powerful telescopes. the inner moon, phobos, is unique in the solar system for it makes three trips around mars while the planet is turning once on its axis. jupiter, the next planet outward from the sun, is almost a sun itself to its extensive family of nine moons. four of these moons were first seen about three hundred years ago when galileo pointed his first crude telescope at the heavens and any one can now see them with the aid of an opera glass. one of the four is equal in size to our own moon; the others surpass it in size. these moons are most interesting little bodies to observe. their eclipses in the shadow of jupiter, occultations or disappearances behind his disk, and the transits of the shadows as well as the moons themselves across the face of the planet can be easily seen even with the smallest telescope. the five remaining moons have all been discovered in modern times. they are extremely small bodies visible only in large telescopes. satellite v is the nearest of all the moons to jupiter. the other four are at great distances from the planet. the planet saturn has nine moons. titan, the largest, is nearly equal in size to jupiter's largest moon, and is larger than mercury; four of the other moons have diameters between one thousand and two thousand miles in length. since saturn is nearly twice as far from the sun as jupiter his moons are more difficult to observe, though the two largest are visible in small telescopes. saturn is unique in the solar system in possessing in addition to his nine satellites a most wonderful ring system, composed of swarms of minute moonlets, each pursuing its individual path around the planet. it is this unusual ring system that makes saturn the most interesting to observe telescopically of all the planets. the planet uranus has four satellites and neptune one. these planets and their satellites cannot be well observed on account of their great distances from the earth. the indistinctness of surface markings makes it impossible to determine the period of rotation of these two outer planets on their axes. it is believed that their rotation is very rapid, however, as is the case with the other planets jupiter and saturn. all the planets in the solar system fall naturally into two groups. jupiter, saturn, uranus, and neptune, the members of the outer group, have on the average, diameters ten times as great and, therefore, volumes one thousand times as great as mercury, venus, earth and mars, the members of the inner or terrestrial group. [illustration: a. venus. b. mars. c. jupiter. d. saturn. taken by prof. e. e. barnard with the -inch telescope of the yerkes observatory, with exception of saturn, which was taken by prof. barnard on mt. wilson.] note: the reader must bear in mind that the telescopic views of the four planets have not been reduced to the same scale and so are not to be compared in size. the terrestrial planets are the pigmies of the solar system, the outer planets are the giants. the densities of the planets mercury, venus, earth and mars are several times greater than the density of water. they are all extremely heavy bodies for their size, and probably have rigid interiors with surface crusts. the existence of life on mercury is made impossible by the absence of an atmosphere. venus and mars both have atmospheres and it is possible that both of these planets may support life. mars has probably been the most discussed of all the planets, though venus is the earth's twin planet in size, mass, density and surface gravity, just as uranus and neptune are the twins of the outer group. it is now believed that water and vegetation exist on mars. the reddish color of this planet is supposed to be due to its extensive desert tracts. the nature of certain peculiar markings on this planet, known as canals, still continues to be a matter of dispute. it is generally believed since air, water and vegetation exist on mars, that some form of animal life also exists there. the length of the day on mars is known very accurately, for the rareness of its atmosphere permits us to see readily many of its surface markings. the length of the day is about twenty-four and one-half hours, and the seasonal changes on mars strongly resemble our own, though the seasons on mars are twice as long as they are on our own planet since the martian year is twice as long as the terrestrial year. the question of life on venus depends largely upon the length of the planet's rotation period. this is still uncertain since no definite surface markings can be found on the planet by which the period of its rotation can be determined. so dense is the atmosphere of venus that its surface is, apparently, always hidden from view beneath a canopy of clouds. it is the more general belief that venus, as well as mercury, rotates on its axis in the same time that it takes to make a revolution around the sun. in this case the same side of the planet is always turned toward the sun and, as a result, the surface is divided into two hemispheres--one of perpetual day, the other of perpetual night. this peculiar form of rotation in which the period of rotation and revolution are equal is by no means unknown in the solar system. our own moon always keeps the same face turned toward the earth and there are reasons for believing that some of the satellites of jupiter and saturn rotate in the same manner. life on any one of the outer planets is impossible. the density of these planets averages about the same as the density of the sun, which is a little higher than the density of water. the density of saturn is even less than water. in other words, saturn would float in water and it is the lightest of all the planets. it is assumed from these facts that the four outer planets are largely in a gaseous condition. they all possess dense atmospheres and, in spite of their huge size, rotate on their axis with great rapidity. the two whose rotation periods are known, jupiter and saturn, turn on their axis in about ten hours. on account of this rapid rotation and their gaseous condition both jupiter and saturn are noticeably flattened at the poles. the terrestrial planets are separated from the outer group by a wide gap. within this space are to be found the asteroid or planetoid group. there are known to be over nine hundred and fifty of these minor bodies whose diameters range from five hundred miles for the largest to three or four miles for the smallest. there are only four asteroids whose diameters exceed one hundred miles and the majority have diameters of less than twenty miles. the total mass of the asteroids is much less than that of the smallest of the planets. it was believed at one time that these small bodies were fragments of a shattered planet, but this view is no longer held. the asteroids as well as the comets and meteors probably represent the material of the primitive solar nebula that was not swept up when the larger planets were formed. with few exceptions the asteroids are only to be seen in large telescopes and then only as star-like points of light. most of them are simply huge rocks and all are necessarily devoid of life since such small bodies have not sufficient gravitational force to hold an atmosphere. the revolution of the planets around the sun and of the satellites of the planets around the primary planets are performed according to known laws of motion that make it possible to foretell the positions of these bodies years in advance. asteroids and comets also obey these same laws, and after three observations of the positions of one of these bodies have been obtained its future movements can be predicted. all the planets and their satellites are nearly perfect spheres. they all, with few exceptions, rotate on their axes and revolve around the sun, or, in the case of moons, around their primaries, in the same direction, from west to east. only the two outermost satellites of jupiter, the outermost satellites of saturn and the satellites of uranus and neptune retrograde or travel in their orbits from east to west, which is opposite to the direction of motion of all the other planets and satellites. the paths of all the planets around the sun are ellipses that are nearly circular, and they all lie nearly in the same plane. the asteroids have orbits that are more flattened or elliptical and these orbits are in some instances highly inclined to the planetary orbits. the comets have orbits that are usually very elongated ellipses or parabolas. some of the comets may be only chance visitors to our solar system, though astronomers generally believe that they are all permanent members. paths of comets pass around the sun at all angles and some comets move in their orbits from west to east, while others move in the opposite direction or retrograde. the behavior of the asteroids and comets is not at all in accord with the theory that was, until recently, universally advanced to explain the origin of the solar system. some astronomers have made attempts to modify the nebular hypothesis that has held sway for so many years, in order to make it fit in with more recent discoveries, but others feel that a new theory is now required to explain the origin of the solar system. several theories have been advanced but no new theory has yet definitely replaced the famous nebular hypothesis of the noted french astronomer la place. xviii the origin of the earth it is not possible to consider the question of the origin of the earth apart from the question of the origin of the solar system. that all the planets, as well as the asteroids, originated from a common parent-mass has never been seriously questioned. all of these bodies revolve about the sun, and rotate upon their axes in the same direction--from west to east. moreover, all of the planetary orbits lie very nearly in the same plane and are nearly circular in form. the orbits of the asteroids are more elliptical and more highly inclined to one another than are the orbits of the planets, but on the average they are neither very elliptical nor very highly inclined to the planetary orbits. the sun rotates upon its axis in the same direction in which the planets rotate and perform their revolutions, and the orbits of the planets are inclined at small angles to the plane of the sun's equator. these facts are all significant and cannot be overlooked in formulating a theory to explain the origin of the planetary system in general and of the earth in particular. presumably the planets and asteroids formed at one time a part of a central body which rotated on its axis in the direction in which they now revolve about the sun. when and by the operation of what force, external or internal, they were separated from this central body is the question. in la place advanced his celebrated _nebular hypothesis_ to explain the origin of the solar system. it was received with favor both by scientists and laymen, and in a short time was almost universally accepted as closely approximating to the truth. according to the nebular hypothesis the solar nebula from which the planetary system was formed, originally extended at least as far as the orbit of neptune and rotated slowly in the direction in which the planets now revolve. as it lost heat by radiation and contracted under the gravitation of its parts its rate of rotation increased. when the centrifugal (center-fleeing) force at the equator equalled the gravitational force directed toward the center, a ring would be left behind by the contracting nebula. such a ring would not be absolutely uniform and would break at some point and gather into a planetary mass under the gravitation of its parts. this planetary mass would abandon rings in turn and these would break up to form satellites. successive rings were supposed to have been abandoned at intervals by the solar nebula at the present distances of the planets from the sun in the manner described above until the original solar nebula had contracted to its present size. the rings of saturn were supposed to be the single example remaining of this process of forming planets and satellites from a _contracting nebulous mass_. the la placian hypothesis attempted to explain why all the planets and their satellites revolve in the same direction in which the sun turns on its axis, in nearly circular orbits and nearly in the same plane. at the time it was advanced it appeared to be in accord with all the facts then known regarding the solar system. the planetoids with their interlacing and in some instances highly inclined and elliptical orbits were then undiscovered. it would have been impossible for them to have been formed by the abandonment of successive rings from a central, rotating mass. the constitution of saturn's rings was unknown at this time; also the fact that the moonlets of the inner ring revolve about saturn in _half_ the time required for the planet to turn on its axis--another impossibility under the nebular hypothesis, for, according to the assumptions of the nebular hypothesis it would be impossible for a satellite to revolve about a central body in a shorter time than that body turns on its axis. the satellites of mars were not discovered until many years later, as well as the retrograding satellites of jupiter and saturn, all presenting difficulties in the way of accepting the nebular hypothesis without radical changes. attempts, mostly unsuccessful, have been made from time to time to make these exceptional cases fit in with the requirements of the nebular hypothesis. the theory that the sun's heat was maintained by the contraction of the original solar nebula, which would cause its temperature to rise, appeared to give considerable support to the theory of la place, but the mathematicians got to work and showed that the amount of heat that would be furnished by the contraction of the sun from beyond the orbit of neptune to its present dimensions would be sufficient to supply heat to the earth at the present rate for only twenty-five million years, a period far too brief, the geologists and biologists said, to cover all the vast cyclical changes that are known to have taken place upon the surface of this planet since its surface crust was formed. evidently gravitational contraction is by no means the only or even the chief source of the sun's heat. it was also shown indisputably, that it would have been impossible for successive rings to have been abandoned at certain definite intervals by a contracting nebula, and granted a ring could have been formed it would have been impossible for it to condense into a planet, since forces residing in the sun would offset the gravitation of its parts. when la place advanced his famous theory it was, to use his own words, "with that distrust which everything ought to inspire that is not a result of observation or of calculation." were la place living today he would be, we believe, the first to abandon a theory that is now known to be in accord neither with observation nor calculation. deprived of a theory that has served to explain the outstanding features of the solar system more or less adequately for one hundred and twenty-five years, astronomers are seeking in the light of recent observations and discoveries to formulate a satisfactory theory of the origin of the solar system. in the planetesimal theory of chamberlin and moulton and the recently suggested theory of the well-known english mathematician, jeans, _a second sun passing close to our own sun is assumed to have been the cause of the origin of the planetary system_. the effect of the close approach of such a sun would be the ejection of a stream of matter from our sun, as we term it, in the direction of the passing body and also in a diametrically opposite direction. this ejection would be continuous as long as the stars remained near one another, the height attained by the ejected stream decreasing as the passing star receded. the result would be the formation of a _spiral nebula_ in which the motion of the ejected particles--planetesimals--would be across the spiral arms, toward and away from the passing star. after the sun had receded so far as to have no further effect upon these ejected particles they would revolve about the sun in more or less elliptical orbits which would gradually be reduced to nearly circular forms by repeated collisions between planetesimals. larger nuclei would be formed and these would gradually sweep up smaller fragments and become the planets of the present system. smaller nuclei in the vicinity of larger ones would become their satellites and in the course of many millions of years all of the larger fragments would be swept up by the planetary nuclei and their satellites--leaving only the asteroids, comets and meteors as survivors of the original spiral system. it must be borne in mind that a spiral nebula formed by the close approach of two suns would resemble in form only the great spiral nebulæ that are known to exist by hundreds of thousands in the heavens. these are far too extensive to form anything so small as a single solar system, but might condense into systems composed of many suns--either galaxies or star clusters. jean's suggested theory of the origin of the planetary system differs in its details from the above, though a passing sun is assumed to be the disturbing force that causes the ejection of a stream of matter which condenses to form the planets and their satellites. the origin of the inner planets is left greatly in doubt by this theory, however, and the system which interests us chiefly--the earth-moon system--is the one about which it is most difficult to arrive at any definite conclusion. our own sun, it is assumed, was dark and cold, of low density and with a diameter about equal to that of neptune's orbit at the time of the catastrophe which is placed at some , , years ago. in jean's words, "... the time for arriving at conclusions in cosmogony has not yet come--and it must be left to future investigators armed with more mathematical and observational knowledge than we at present possess to pronounce a final decision." however, since la place advanced his celebrated nebular hypothesis, great advances in astronomy have been made, and man is in a better position to theorize on this fascinating problem today than he was one hundred and twenty-five years ago. all such theories must necessarily be regarded as working hypotheses only, to be discarded or modified as our knowledge and understanding of the laws of the universe increase. no theory can ever be regarded as final or perfect. the discovery of radio-activity furnishes us with new material for new theories. the sun and the planets may be and probably are far older than we ever dreamed could be possible. it is no longer necessary or reasonable to assume that a greatly extended solar nebula once existed and supplied the planets with heat through gravitational contraction or to place a time limit upon the period required for the formation of the planets and their satellites that is not in accord with the requirements of other sciences. we know today that there exist within the sun powerful repulsive forces, which even under present conditions occasionally eject gaseous matter to heights of five hundred thousand miles or more with a velocity of over two hundred miles per second. small changes in the velocity of ejection produce great differences in the height of the ejected columns. with an initial velocity of three hundred and eighty miles per second, matter would be thrown from the solar surface to a height of fifty million miles. were the velocity of ejection three hundred and eighty-three miles per second the height of the column would be five hundred million miles, while a further increase in the initial velocity would send matter away from the sun, never to return. instead of suns and solar systems evolved from nebulæ we are now more familiar with the idea of nebulæ evolved from stars through some terrific cataclysm as in the case of novas or temporary stars. it is now known that there exist in certain parts of space a number of sharply defined stars surrounded by extensive nebulous envelopes. are these possibly suns that are going through the process of forming their planetary systems? it is now known that pressure of light and electrical repulsion are forces to be reckoned with in the evolution of stars and nebulæ as well as gravitational contraction. it has long been felt that the peculiar formations existing among the vast irregular gaseous nebulæ could not be explained as gravitational effects alone. _light-pressure and electrical repulsion_, as well as _gravitation_ are at work within the solar system and the sun is the seat of powerful disturbances which produce periodic outbursts of exceptional activity and which may have produced in the distant past more startling effects than any with which we are familiar at present. the earth and moon form a system that is in a way unique. no satellite in the solar system is so large in proportion to its primary as is our own moon. seen from the distance of venus or mars, the two bodies would apparently form a _double star_. the diameter of the moon is one-fourth that of the earth. satellite iii of jupiter far exceeds our own moon in actual size but its diameter is only about four-hundredths of the diameter of the planet about which it revolves. the diameter of titan, the largest satellite of the saturnian system, bears the same ratio to the diameter of saturn. moreover, all the nearer satellites of jupiter and saturn lie nearly in the equatorial planes of these planets, but the orbit of the moon is inclined at a high angle to the plane of the earth's equator. it is not difficult to believe that the satellites of jupiter or saturn were at some time thrown off from the equatorial belts of their primaries, just as the planets themselves may have been ejected from the equatorial belt of the sun, but we cannot so readily believe that our own satellite was formed from the earth in a similar manner. the moon's orbit lies nearly in the plane of the sun's equator, however, and it is conceivable that both earth and moon were simultaneously ejected from the equatorial zone of the sun, the two nuclei being so close together that the smaller one remained under the gravitational control of the larger. the difficulties in the way of believing that the moon once formed a part of the earth are very great. it can be shown mathematically that if the two bodies at one time formed a single mass it would have been impossible for the moon to break away from the earth, unless the force that caused the separation were sufficient to hurl the moon to a greater distance than two and a half times the earth's radius. the mathematician, roche, found out by computation that a satellite could not remain intact within this distance of the planet, but would be broken up into small fragments under the effects of the tides raised by the larger body. if, then, the moon had originally been ejected from the earth to a less distance than two and one-half radii of the earth ( . to be exact) it would have been disintegrated into small particles, or moonlets, under the tidal strains exerted upon it by the earth and would have been gradually distributed about the earth in the form of a meteoric ring which, in the course of ages, would be absorbed by the earth, just as saturn is now gradually absorbing its rings. the planets differ greatly in density. the more distant and larger planets--jupiter, saturn, uranus and neptune--have densities equal to or less than that of the sun. the densities of the inner planets--mercury, venus, earth and mars--are, relatively, extremely high, the density of the earth's core being about that of meteoric iron. the densities of mercury and venus are slightly less than that of the earth and the densities of mars and the moon about equal to that of the earth's crust. if a stream of matter were ejected from the sun under the influence of some external force, such as that exerted by a passing star, the outlying parts of the stream would consist of the lighter elements and the lower parts of the heavier elements, since the lighter solar elements lie at or near the surface of the sun and the heavier elements at greater depths. at the time of ejection the lighter elements would be thrown to great distances and would go to form the less dense outer planets; the heavier elements would go to form the inner planets of high density. it is conceivable that ejection of solar material might have taken place under the influence of certain forces at work within the sun itself, such as electrical repulsion or pressure of light which might become powerful enough under certain conditions to overcome the effect of gravitation. next to nothing is known about the physical state of matter at great solar depths, where abnormal conditions of temperature and pressure must exist, and where great physical changes and disturbances may have taken place in the past. even today solar activity goes through a cycle of change during the sun-spot period, and many millions of years ago the sun-spot cycle of solar activity may have been far different from what it is today and a far more powerful factor in producing changes in the solar system. outbursts of novas indicate that agencies making for peace and order are not the only ones at work among the stars. the cause of such outbursts has never been satisfactorily explained. the theory that they are caused by the close approach of two suns or by the encounter of a star with a dark nebula does not account for all of the circumstances of such outbursts. the nebulous matter seen about a nova after the outburst is now generally believed to have been expelled from the star itself at the time of the catastrophe and may conceivably be the stuff of which planetary systems are made. at some epoch in the past, probably at least one thousand million years ago, our own sun may have undergone some cataclysmic change and this may, conceivably, have been brought about by disturbances within the sun itself. elements may have been so formed and distributed within the interior of the sun that friction and internal instability resulted and in time produced an upheaval of solar elements with initial velocities so great that, possibly, through electrical repulsion and light-pressure, portions of the ejected streams were permanently detached from the sun and became the nuclei of future planets. in some such way, it is conceivable, our own planet earth and the other members of our solar system may have been brought into existence in the dim and distant past--many hundred million years ago. xix jupiter and his nine moons jupiter shines by reflected sunlight with a brilliancy that usually exceeds that of the brightest of the stars, sirius. when seen during the midnight hours the remarkable unflickering brightness of this largest and most distinguished member of the solar system at once serves to set it apart from the scintillating stars far beyond. there is but one planet, venus, that always surpasses jupiter in brilliancy, though mars on the occasions of its close approaches to the earth may equal or slightly surpass jupiter in brightness. as venus never departs more than forty-eight degrees from the sun, and so is never seen in the midnight hours, jupiter usually shines without a rival when visible at midnight. to one who has observed the two planets together the silvery radiance and surpassing brilliancy of venus, due not to its size, but to its comparative nearness to the earth, at once serves to distinguish it from the golden glow of jupiter. even the smallest telescopes of two- or three-inch aperture will show the four historic moons of jupiter which were the first celestial objects to be discovered when galileo turned his crude telescope to the heavens in the year . the fact that these tiny points of light were actually revolving around the great planet was soon detected by the famous astronomer and we can imagine with what breathless interest he observed these satellites of another world whose discovery dealt such a severe blow to the old ptolemaic theory that the earth was the center of the universe. it was not until the great telescopes of modern times were invented that the five additional moons of jupiter were discovered. the four satellites first observed by galileo were fancifully named io, europa, ganymede and callisto, in the order of their positions outward from the planet, but these names are rarely used now, the satellites being designated for convenience i, ii, iii and iv, respectively. the first of the new satellites to be discovered was satellite v, which is the nearest to jupiter of all the nine moons. it is an extremely small body, not more than one hundred miles in diameter, and to discover this tiny body as it skirted rapidly around the great planet within sixty-seven thousand miles of its surface, nearly lost in the glaring rays, was a difficult feat even for an experienced observer. it was accomplished, however, by prof. e. e. barnard with the great lick refractor in . satellite v is hopelessly beyond the reach of any but the greatest telescopes, as are also the four satellites discovered since that date. in fact, most of these tiny moons are observed photographically. satellites vi and vii were discovered photographically in . they are both about seven million miles from the planet and their paths loop through one another; they are, moreover, highly inclined to each other at an angle of nearly thirty degrees. when nearest together they are separated by a distance of two million miles. two more extremely small bodies, known as satellites viii and ix, have been discovered since then, one at greenwich, england, in , the other at the lick observatory in . these excessively faint bodies are the most remote satellites of jupiter and they are of particular interest because they travel around the planet in a retrograde direction, or from east to west, which is _opposite_ to the direction of revolution prevailing in the solar system. the ninth and most distant satellite of saturn also retrogrades, or revolves in a clockwise rather than a counter-clockwise direction around the planet. one explanation given for this peculiarity of the outermost satellites of jupiter and saturn is that this backward revolution around the planet is more stable when the satellites are at great distances from the primary, and the gravitational control that the planet exerts is therefore weak. the moons of the planets are, of course, subject to the attraction of the sun as well as to the attraction of the controlling planet, and the greater the distance of the satellite from the planet the stronger the pull exerted by the sun and the weaker the bonds that bind the moon to the planet. beyond a certain limit it would be impossible for the planet to hold the satellite against the sun's greater attraction and the satellite would leave the planet to revolve directly around the sun, thereby becoming a planet. it appears that as this danger limit is neared it is safer for the satellite to "back" around the planet than to follow the usual "west to east" direction of revolution. the eighth satellite of jupiter is more than fourteen million and the ninth more than fifteen million miles from the parent planet and they require about two years and three years, respectively, to complete one trip around jupiter. when we consider that satellite v darts around the planet in less than twelve hours at a distance of only sixty-seven thousand miles from its surface we realise what tremendous differences exist in the distances and periods of revolution of the nine moons. there is also great disparity in the sizes of the various moons. the five moons discovered in modern times are all excessively faint and extremely small. the diameter of the largest of these, satellite v, is less than one hundred miles. on the other hand, the four historic moons of jupiter are of planetary dimensions. the smallest, satellite ii, is slightly larger than our own moon, while the largest, satellite iii, has a diameter, according to measurements made with the -inch yerkes refractor in , of three thousand nine hundred and eight miles, which is only four hundred miles less than the diameter of mars. the periods of revolution of these four satellites range from one day and eighteen hours for the nearest, which is about two hundred and sixty-one thousand miles from the center of jupiter, to sixteen days and sixteen and one-half hours for the most distant, which is more than one million one hundred and sixty thousand miles from the planet. these four moons are so near to the great planet that they are continually dipping into his huge shadow and experiencing an eclipse of the sun which, owing to the nearness and great size of jupiter, lasts for two or three hours. at times of eclipse the moon suddenly disappears from the observer's view, though it may be considerably to one side of the planet. its reappearance later on is just as sudden, or it may pass out of the shadow while hidden from us behind the disk of the planet, in which case its reappearance is invisible from the earth. the occultations of the satellites, or, in other words, their disappearance behind the planet's disk, are also interesting phenomena to observe, as are their "transits" across the disk of the planet as the satellite passes in front of it. not only the satellite itself but its shadow as well can be seen, a small black dot passing over the surface of jupiter. the satellite is totally eclipsing the sun for this small dark portion of the planet's disk. two satellites and their shadows are frequently seen crossing the face of the planet at the same time. it is possible to observe all the phenomena of the satellite's transits and shadows, eclipses and occultations with very small telescopes. from observations of the eclipses of jupiter's satellites the important discovery of the finite velocity of light was first made as far back as the year . faint surface markings have been made out at certain times on the largest of the four satellites, satellite iii, and also on satellite i. observations of the markings on the former seem to indicate that it always keeps the same face turned toward jupiter as does our own moon toward the earth. there are also reasons for believing that the equatorial regions of satellite i are light colored and the polar regions dark. there is the possibility that forms of life may exist on these satellites of jupiter, though they are more likely barren, lifeless worlds, such as mercury and the moon. their great distance from the earth, never less than three hundred and sixty-eight million miles, makes observations of their surface markings very difficult. how beautiful beyond description must the heavens appear as viewed from the satellites of jupiter! viewed from the distance of io, or satellite i, the mighty planet jupiter presents a spectacle such as the eye of man has never been privileged to behold. the huge flattened globe, ninety thousand miles in equatorial diameter, equal in mass to _three hundred planets such as our own_ and in volume to nearly _fourteen hundred_, fills a space in the heavens nearly twenty degrees in extent as viewed from this satellite. fifteen hundred of our own full moons would hardly fill the same space. whirling on its axis with frightful speed in a period of less than ten hours, the huge ball glides rapidly but majestically onward through the sky. a far distant sun shrunk to but one-fifth the diameter of the full moon throws light and shade across the rapidly changing surface of the planet, rich in the reds, browns and yellows and all the gorgeous shades and tints of its dense, seething, gaseous envelope. the phases of the moon on a greatly enlarged scale rapidly succeed each other on jupiter as it is viewed from the satellite in all positions with reference to the sun. the cause of the belts of jupiter, that lie parallel to the planet's equator and are constantly changing in number, width and shade, as well as the nature of all the peculiar splashes of color and intensely white flecks that come and go in the dense atmosphere of the planet would not be such a mystery to us were it possible to view the great planet from the distance of satellite i, which is about as far from the surface of jupiter as the moon is from the earth. it is uncertain whether the planet is entirely gaseous throughout or has a central core of solid or liquid matter. its density is only one and one-quarter times that of water and slightly less than that of the sun, showing that it is composed largely, if not entirely, of matter in a gaseous state. jupiter is a world as different from our own as it is possible to imagine. there is no visible surface crust and there are no permanent markings. different spots on the planet's disk give different periods of rotation showing that it is atmospheric phenomena that we observe. all is constant flux and change on jupiter. dense vapors arise from a highly heated interior and spread out into belts parallel to the equator in the direction of the planet's rotation. from its nearest satellite all the interesting changes of color and form that constantly take place in the atmosphere of this great globe could be observed in great detail. the high percentage of light and heat that jupiter reflects from the sun to its nearer satellites makes it a secondary sun to them of tremendous size though feeble strength. as seen from satellite i the other three major moons of jupiter present all the phases of our own moon in rapid succession, due to their constantly changing positions with reference to the sun. the five small moons, discovered in modern times, are so minute that they are simply star-like points of light even when viewed from the other moons of jupiter. to keep track of the rapidly changing positions and various phases of the moons of jupiter as seen from any one of them, as well as the rapid _apparent_ motion of the planet through the sky due to the revolutions of the satellite around the planet, would be a troublesome task for an astronomer stationed on one of these far distant worlds. it would be a common sight to see in the sky at one time the huge planet, the far-distant, shrunken sun, and one, two or three moons. seen from the moons of jupiter the constellations would appear as they do to us on earth, for such a slight change in position as five hundred million miles, more or less, is trivial when one is looking at the stars. observations of the stars from the nearest moon of jupiter would be attended with great difficulties at times, since reflected sunlight from a body nearly twenty degrees in diameter would be extremely troublesome, especially were the phases of the planets near that of the full moon. we know how the presence of our own moon in the heavens at the full dims the brightness of the stars so that only the brightest stars are seen. even as viewed from the fourth or most distant of the major satellites the planet subtends an angle of nearly five degrees. occultations of the stars are many and frequent as the huge planet globe glides swiftly through the heavens. many a moonlight night appears almost as day owing to the presence of the enormous, brilliantly reflecting ball of light and at times two or three moons in addition. only the brightest stars could possibly be seen under such circumstances. when, however, the small worlds pass into the shadow of the great mother planet and not only the light of the sun but also the reflected light of jupiter disappears for many minutes, the stars shine forth in all their glory there as here. at such times some of the larger moons would usually be seen shining by the reflected light of the far distant sun. saturn also would be visible as a magnificent star, but beautiful venus and ruddy mars would fail to appear. tiny bodies, mere specks of light at this distance, they would be lost to view in the glare of the sun. xx the rings and moons of saturn nearly everyone has felt at some time or other a strong desire to gaze at some of the beauties and wonders of the heavens through a telescope and the one object that all of us wish to see, if, perchance, this desire is to be gratified is saturn, whose unusual ring system has, so far as we know, no counterpart in the sky. all the planets in the solar system with the exception of the two innermost, mercury and venus, are attended by satellites, but saturn, alone, has in addition to a family of nine moons, three distinct rings of great dimensions which are composed of swarms of minute particles revolving around the planet. why saturn should be the only planet to possess such a system of rings has never been explained in an entirely satisfactory manner. there is an interesting law known as "roche's law," however, named from its investigator, that states that no satellite of a planet can exist intact with . times the radius of the planet. this limit is spoken of as "roche's limit" and applying it to the planet saturn we find that the rings of saturn fall within this limit. it does not necessarily follow from this that the minute particles of which the rings are composed are the shattered remains of one small satellite, but rather that they are the material from which a satellite might have been formed were it not so close to the planet. within "roche's limit" the mutual attraction of the various particles for each other that would tend eventually to gather them into one body is overcome by tidal forces that arise from such close proximity to the huge planet. the stress and strain of such forces is so great that no grouping of particles can take place. this explains, possibly, why the rings continue to exist in their present condition. the total quantity of matter in the rings is known to be very small, for it does not disturb the motions of any of the nearer and smaller satellites, though tiny mimas, six hundred miles in diameter, is only thirty-one thousand miles beyond the outer edge of the outer ring. an interesting observation was made a few years ago of the passage of the rings of the planet between us and a star. though the light of the star was diminished to one-fourth of its normal brightness when the rings passed before it, at no time was its light entirely eclipsed by any of the particles. it was computed that if the diameters of any of the individual particles had amounted to as much as three or four miles the star would have been temporarily eclipsed. an upper limit for the size of the moonlets was thus obtained. the average diameter of the particles is probably much less than three miles. the thickness of the ring system is not over fifty or one hundred miles, but its total diameter is one hundred and seventy-two thousand miles. there are, in all, three concentric rings. the faint inner ring, known as the "crape" ring, is invisible in a telescope under four inches in aperture. the width of this inner ring is eleven thousand miles. just beyond the crape ring is the chief, bright ring, eighteen thousand miles in width. it shades gradually in brightness from its juncture with the crape ring to its most luminous portion at its outer edge, which is separated from the third or outer ring by a gap two thousand two hundred miles in width, known as cassini's division. the third or outer ring is eleven thousand miles wide and is less bright than the central ring. the inner edge of the inner ring is but six thousand miles above the surface of the planet. on account of the curvature of the planet the ring system is invisible from the north and south poles of saturn. as in the case of the satellites of a planet the inner particles of the rings revolve around the planet more rapidly than the outer particles. the innermost particles of the crape ring require but five hours for one journey around saturn while the outermost particles of the outer ring require one hundred and thirty-seven hours, or nearly six days to complete one revolution. in addition to the gap in the rings known as cassini's division several other fainter divisions exist. if a group of moonlets were to revolve around the planet in the positions marked by these gaps their periods of revolution would be commensurable with the periods of several of the satellites of saturn. as a result the attraction exerted on such particles by these satellites would gradually disturb their motion in such a way as to draw them away from these positions. it is owing, therefore, to the attraction of the satellites of saturn for the moonlets that these gaps in the rings exist. as a result of the disturbances produced in the motion of the moonlets by the satellites of saturn collisions are bound to occur occasionally among the various particles. when two particles collide the period of revolution of one or both of them is reduced and as a result collisions tend to bring the moonlets gradually closer and closer to the surface of the planet. the dusky inner ring, it is believed, may consist largely of particles whose periods have been continually shortened by collisions. saturn may, therefore, lose its ring system in the course of time through its gradually being drawn down upon the planet by collisions of the various particles until all of the material is finally swept up by the planet. such a change would probably require millions of years, however, as collisions are probably, on the whole, infrequent. it is possible that the ring system of saturn may have been much more extensive in the past than it is now and other members of our solar system may have had such appendages in the far distant past. the appearance of the rings of saturn as viewed from our planet changes periodically as a result of the revolution of the earth and saturn around the sun, which places them in constantly changing positions with reference to each other. the rings lie in the plane of saturn's equator, which is inclined twenty-seven degrees to its orbit and twenty-eight degrees to the earth's orbit. since the position of the equator remains parallel to itself while the planet is journeying around the sun it happens that half the time the earth is elevated above the plane of the rings and the remainder of the time it lies below the plane of the rings. twice in the period of saturn's revolution around the sun, which occupies nearly thirty years, the earth lies directly in the plane of the rings and at this time the rings entirely disappear from view for a short time. mid-way between the two dates of disappearance the rings are tilted at their widest angle with reference to the earth and they are then seen to the best advantage. as the date of their disappearance approaches they appear more and more like a line of light extending to either side of the planet's equator. even in the most powerful telescope the rings entirely disappear from view for a few hours at the time the earth lies exactly in the same plane. it is at this time that the ball of the planet is best seen. its flattening at the poles, which is nearly ten per cent. of its equatorial diameter then gives it a decidedly oval appearance. ordinarily one of the hemispheres of saturn is partly or entirely concealed by the rings so that the oblate form is not so noticeable. it was the change in the tilt and visibility of the rings that so perplexed galileo when he attempted to make out the nature of these appendages of saturn with his crude telescope of insufficient magnifying power. so great was his bewilderment when the rings finally disappeared that he cried out in despair that saturn must have swallowed his children, according to the legend. he finally became so exasperated with the results of his observations that he gave up observing the planet. the true nature of these appendages of saturn remained a mystery until huygens solved the problem in , some time after the death of galileo. in addition to the rings, saturn has nine satellites named, in the order of their distance outward from the planet, mimas, enceladus, tethys, dione, rhea, titan, hyperion, iapetus and phoebe. the last-mentioned satellite was discovered by w. h. pickering in . it aroused great interest at the time because it was the first satellite to be discovered with "retrograde" motion in its orbit. two satellites of jupiter since discovered revolve in the same direction around their primary. the satellites of saturn are approximate to those of jupiter in size and exactly equal them in number. the largest, titan, is three thousand miles in diameter and can be easily seen with the smallest telescopes. with a four-inch telescope five of the satellites can be readily found, though they are not as interesting to observe as the satellites of jupiter because they are far more distant from the earth. the time they require to make one journey around saturn varies from nearly twenty-three hours for mimas, the nearest, to approximately five hundred and twenty-four days for phoebe, the most distant. saturn as well as jupiter is marked by belts parallel to the equator though they appear more indistinct than those of jupiter on account of the greater distance of saturn. saturn also resembles jupiter in its physical composition which is largely, if not entirely, gaseous, and in the extremely short period of rotation on its axis which is approximately ten hours. in more ways than one saturn is a very unusual planet. in addition to possessing an enormous ring system it is the lightest of all the planets, its density being only sixty-three hundredths that of water, and it is the most oblate, its flattening at the poles amounting nearly to one-tenth of its diameter. its equator is more highly inclined to its orbit than is the case with any other planet, not even excepting the earth and mars. for this reason its seasonal changes are very great, in marked contrast to jupiter whose equator lies very nearly in the plane of its orbit. since saturn is so far away from the sun that it receives only one ninetieth as much light and heat per unit area as the earth, its outer gaseous surface must be extremely cold unless considerable heat is conveyed to the surface from within its hot interior. the late prof. lowell concluded from certain observations made at flagstaff, ariz., that saturn is composed of layers of different densities and that the inner layers are more flattened at the poles and rotate faster than the outer layers. marked variations in the color and brightness of the ball of the planet have been noted from time to time. in observers of saturn described the planet as pinkish-brown and conspicuously darker than the brighter portions of the rings. it is believed that these very noticeable changes in the color and brightness of saturn are due to slight, irregular changes in the intensity of the radiations of the sun which set up certain secondary effects in the atmosphere of the planets. similar changes in color and brightness have been observed also in the case of jupiter. xxi is the moon a dead world? it has been a generally accepted belief among astronomers for years that the moon is a dead world devoid of air and water and so, necessarily, lifeless. it is certain that the moon has no extensive atmosphere such as envelops our own planet. there is abundant proof of this fact. the edge of the lunar disk is clear-cut. whenever, as happens frequently, the moon passes between us and a star the disappearance of the star is instantaneous. there is no gradual dimming or refraction of the star's light by atmospheric vapors. moreover, lunar shadows are harsh and black. there is no evidence of diffusion of light on the moon by atmospheric gases. the absence of water or water vapor on the visible surface of the moon, at least in any appreciable quantity, is plainly evident to anyone who observes the moon through the telescope. even with small telescopes, objects five miles or so in diameter can be readily detected and clouds drifting over the surface could not possibly escape our observation if they existed. bodies of water, great or small, would be plainly visible and would besides give rise to water vapor and clouds, which we would not fail to detect. since the surface of the moon is unscreened by air and water vapor to absorb the incoming rays from the sun, and the outgoing radiations from the surface, the extremes of temperature between day and night are very great, and are augmented by the fact that the lunar day equals the lunar month in length, so that fourteen days of untempered heat are followed by fourteen days of frigid darkness. observations of the rate of radiation from the moon's surface during total eclipses of the moon indicate that the moon's radiation is very rapid, and that its temperature during the height of the lunar day probably approaches ° f., while at the lunar midnight it may have fallen to ° below zero f., or even lower. with air and water both lacking and such extremes of temperature existing why should we seriously consider the question of life on the moon? this is the point of view of the majority of astronomers and it seems well taken. yet many astronomers who have made a special study of the lunar surface for years under all conditions of illumination and phase, and have most carefully observed and mapped and photographed its characteristic markings, are agreed that there are evidences that changes are taking place on the moon, and recently prof. w. h. pickering has expressed the belief, substantiated by drawings, that there is a progressive change of color or darkening within certain lunar craters with the advance of the lunar day, indicating, in his opinion, a rapid vegetational growth that springs up in the height of the lunar day and dies out as the lunar night approaches. some years ago certain selenographers suggested that there might exist in the numberless crater-pits and craters, in the deep-lying _maria_ or "seas," and in the clefts and rills and cracks that form intricate systems all over the lunar surface, certain exhalations from the surface and heavy vapors including possibly carbon dioxide and water vapor to temper the extremes of the long lunar days and night and furnish the necessary medium for the support of certain forms of animal and vegetable life. many astronomers, including a number who are not in sympathy with the above view, believe that snow and ice exist on the moon, even though water in the form of liquid and vapor is not observable. all the extremely brilliant portions of the surface, according to some astronomers, are covered with snow and ice. certainly, some portions of the moon's surface reflect sunlight as brilliantly as if they were covered with freshly fallen snow, while other portions appear to be black by contrast. there also appears to be evidence that certain small markings, described as crater-cones and resembling our terrestrial volcanoes more than any other lunar feature, are at times temporarily obscured from view by a veil of vapors. many observers believe that these crater-cones are active volcanic vents, and that there is considerable volcanic activity still taking place upon the moon. these small crater-cones resemble, we are told, parasitic cones found on the sides of terrestrial volcanoes, and they are frequently seen on the floors of craters closely associated with light streaks. these crater-cones appear under a high sun as minute white spots and can be studied to advantage only with powerful instruments. the italian astronomer, maggini, observing the floor of the lunar crater, plato, in noted that one of the small crater-cones that exist there in great numbers, was temporarily obscured from view by a cloud of reddish vapors, and prof. w. h. pickering, at arequipa, peru, observing the same region some years ago, believed that he saw evidence of change in some of these small markings. the crater, plato, has probably been more carefully studied than any other portion of the lunar surface. it is sixty miles in diameter and may be seen even without a telescope as a dark "eye" not far from the northern edge of the moon. its floor is one of the darkest objects in the moon--a dark steel-grey in color--and there is no doubt that for some unknown reason its dark hue deepens from the time the sun has an altitude of twenty degrees until after full moon. it has a brilliant white wall rising from , to , feet above its floor, crowned with several lofty peaks and intersected by a number of valleys and passes. the spots and faint light markings on the floor have been the object of much study with small or moderate sized instruments, and at least six of them are known to be crater-cones. since they can only be studied to advantage with powerful instruments and as such instruments are rarely used for a systematic study of lunar markings, it is difficult to settle the controversy as to whether they have changed in appearance or have been at any time obscured by vapors. most lunar observing is done--necessarily--with smaller instruments because the majority of astronomers appear to have accepted the view that the moon is a dead world, and those who are engaged in astronomical work with our greatest telescopes seem to feel that other fields of research will prove more fruitful. possibly it is for this reason that we know so little about our nearest neighbor in space! there are at least as many unsolved problems confronting us on the moon as there are among the distant stars. geologists tell us that more oxygen is to be found in the first six feet of the earth's crust than in all of the atmosphere above. does oxygen not exist in the surface rocks of the moon as well? volcanic action, we are told, is primarily an escape of gases from the interior, chiefly hydrogen, nitrogen, hydrocarbons, sulphur, and various compounds, as well as vast quantities of steam. beneath the surface chemical change is continually taking place which results in the release of an enormous amount of heat. some of the gases mentioned above combine with the oxygen in the surface rocks and heat is evolved. it is a known fact that there is great inherent heat in the earth's surface crust. why not in the moon's surface crust as well? the water that would be expelled in the form of steam from volcanic vents on the moon would be transformed immediately into hoar-frost, snow and ice and would settle down upon the flanks of the crater-cones or vents. it should be borne in mind that only volcanic activity on an enormous scale would be plainly visible to us even with the powerful telescopes at our command. ordinary eruptions such as occur on our own planet would be very difficult to detect. since the escaping vapors would rapidly pass into the solid state and settle down upon the flanks of the crater-cones or vents, we would observe in general little if any change in an object unless we chanced to be looking at it at the time of the eruption, when it might appear to be temporarily obscured by a veil of vapors. what are the chances that we would be carefully observing at the precise time of an eruption, a minute marking, two or three miles in diameter, on a surface as large as all of north america, a surface that is covered with some , charted craters, numberless crater-pits, streaks, rays, spots, clefts and rills in intricate systems, mountain chains and valleys and a mass of intricate detail? if we were looking at the earth from the moon with the aid of a powerful telescope would we be apt to notice an eruption of vesuvius or katmai or mauna loa? objects four or five miles in diameter would appear as hazy spots with nothing distinctive or remarkable in their appearance. yet vapor and steam arising from terrestrial volcanoes would be carried by our atmosphere over an area of many square miles, while there is no atmosphere on the moon to spread the vapors that may arise from similar volcanic vents. it would have to be a cataclysmic change indeed to be accepted as indisputable evidence that change is taking place on the moon, and the days of gigantic upheavals are probably over on our satellite as well as on the earth. if volcanic activity is still taking place on the moon it is probably in a mild form such as a comparatively quiet emission of gases from volcanic vents and fumaroles. such forms of activity would not be plainly visible at this distance, even with the aid of powerful telescopes. the problem of detecting changes on the moon is complicated by the fact that a change of illumination greatly alters the appearance of all lunar markings. such a change is continually taking place in the course of the month. a marking that stands out in bold relief at lunar sunrise or sunset will change entirely in appearance a few days later under a high sun or even disappear from view entirely. these changes in phase or illumination have to be taken account of in the search for evidence of actual change. to decide whether or not change has actually taken place the object must be viewed under similar conditions, so far as they can be obtained. even when special care is taken in this respect the suspected evidence of change is usually "explained away" as due to differences in illumination or seeing, by those who have not observed the object themselves and are not in sympathy with the view that the moon is anything but a dead world. as regards the question of life on the moon, it is interesting to consider the facts brought out by investigations made by scientists connected with the geophysical laboratory of the carnegie institute in the valley of ten thousand smokes. the volcanic activity there takes the form of eruptions from numerous small vents or fumaroles and ninety-nine per cent. of the emanations are water vapor. it was observed that blue-green algae were living at the edge of active vents emitting ammonia compounds at a temperature of ° f. they were not found, however, near vents from which ammonia compounds were not being emitted. if life exists under such conditions it is conceivable that suitable conditions for the support of certain forms of life, animal as well as vegetable, may be found in low-lying valleys and crevices and upon the floors of craters, where certain gases essential to the support of life might be evolved from many small volcanic vents and fumaroles. many theories have been advanced to explain the origin of the lunar craters which have no counterpart on our own planet. they are saucer-like depressions in the surface of the moon, frequently of such great size that an observer standing in the center would not be able to see either side of the crater owing to the curvature of the moon's surface. craters fifty, sixty or one hundred miles in diameter are by no means uncommon, while there are thousands between five and fifty miles in diameter. a characteristic feature of many craters is a central peak, and the surrounding walls are often a mile or more high and in some instances are symmetrically terraced. new craters have been formed on the sides or floors of old craters, and these are always more clear-cut and sharper in outlines than the old formations, and generally much smaller. a number of craters are surrounded by a system of light streaks or rays of unknown origin that extend in some instances to enormous distances on all sides of the crater. the most conspicuous system is the one surrounding the lunar crater tycho near the south pole of the moon. the rays originating in this crater extend in all directions for hundreds of miles without turning aside for any obstructions, passing over mountains, craters and plains in their course in practically straight lines like spokes in a wheel. this ray system of tycho is the most noticeable marking on the moon's surface at the time of full moon. as these streaks cast no shadow they are apparently cracks in the surface that have filled up with some light-colored material from below. their origin has never been satisfactorily explained. as to the origin of the lunar craters, some believe that they were produced in past ages by a bombardment of the lunar surface by huge meteoric masses; but there are many objections to this theory that we will not take up here. it is more generally believed that the lunar craters are a result of volcanic activity on an enormous scale which took place on the moon many ages ago and which has now practically ceased, its only manifestations now taking the form of a quiet emission of gases from small volcanic vents or fumaroles which exist all over the lunar surface but which are to be found in greatest numbers on the floors and sides of craters. xxii comets the orbits of comets are inclined at all angles to each other and to the orbits of the planets which, on the other hand, lie very nearly in the same plane. the larger members of the sun's family, the planets and their satellites, revolve from west to east around the sun. comets on the contrary frequently retrograde or back around the sun in the opposite direction--from east to west. the paths that these erratic visitors follow in their journeys around the sun bear not the slightest resemblance to the paths of the planets, which are almost perfect circles. the orbits of comets are ellipses that are greatly elongated or parabolas. if the orbit is a parabola the comet makes one and only one visit to the sun, coming from interstellar space and returning thereto after a brief sojourn within our solar system. donati's comet of , one of the greatest comets of the nineteenth century, had a period of more than two thousand years and its aphelion (the point in its orbit farthest away from the sun) was five times more distant than the orbit of neptune. there is, however, a class of comets known as _periodic_ comets that have extremely short periods of revolution around the sun. to this class belongs halley's comet whose period of seventy-five years exceeds that of any other short period comet. encke's comet, on the other hand, has a period of three and a third years which is the shortest cometary period known. most of the periodic comets are inconspicuous and only visible telescopically even when comparatively near to the earth. halley's comet is the only one of this class that lays any pretensions to remarkable size or brilliancy and it also is showing the effects of disintegration resulting from too frequent visits to the sun. comets are bodies of great bulk or volume and small total mass. their tails, which only develop in the vicinity of the sun, are formed of the rarest gases, and the best vacuum that man can produce would not be in as tenuous a state as the material existing in the tails of comets. there are many proofs of the extreme tenuity of comets. the earth has on a number of occasions passed directly through the tails of comets without experiencing the slightest visible effects. stars shine undimmed in luster even through the heads of comets. if the earth should encounter a comet "head on" it is doubtful if it would experience anything more serious than a shower of meteors which would be consumed by friction with the earth's atmosphere, or a fall of meteorites over a small area of a few square miles. it is possible, however, that matter in the nucleus, the star-like condensation in the head of a comet, may consist of individual particles weighing in some instances a number of tons, surrounded by a gaseous envelope and held together by the loose bonds of their mutual attraction. if the earth should encounter the nucleus of a comet considerable damage might be done over a portion of the earth's surface, but the chances of such an occurrence are less than one in a million. since the total mass of a comet is so small, a close approach to one of the planets, especially jupiter, produces great changes in the form of the comet's orbit, though the motion of the planet is not disturbed in the slightest degree by the encounter. the majority of all the short-period comets have been "captured" by jupiter, that is, the original orbits have been so changed by the perturbations produced by close approaches to the giant planet that their aphelia, or the points in their orbits farthest from the sun, lie in the vicinity of jupiter's orbit. several of the other planets have also "captured" comets in this sense, and the fact that the aphelia of a number of comets are grouped at certain definite intervals beyond the orbit of neptune has been considered by some astronomers to be an indication that there are two or more additional planets in the solar system revolving around the sun at these distances. the most interesting feature of a comet is its characteristic tail which develops and increases in size and brilliancy as the comet approaches the sun. as the tail is always turned away from the sun it follows the comet as it draws near the sun and precedes it as it departs. its origin is due, it is believed, both to electrical repulsion and light-pressure acting upon minute particles of matter in the coma or head of the comet. the curvature of the tail depends upon the nature of the gases of which it is composed. long, straight tails consist chiefly of hydrogen, it has been found, curved tails of hydrocarbons and short, bushy tails of mixtures of iron, sodium and other metallic vapors. at times the same comet will have two or more tails of different types. since the material driven off from the nucleus or head of a comet by electrical repulsion and light-pressure is never recovered, it is evident that comets are continually disintegrating. also, comets that have passed close to the sun at perihelion have frequently been so disrupted by tidal forces that one nucleus has separated into several parts and the newly formed nuclei have pursued paths parallel to the original orbit, each nucleus developing a tail of its own. many periodic comets, it is now known, have gradually been broken up and dissipated into periodic swarms of meteors as a result of the disruptive effect produced by too frequent returns to the vicinity of the sun. these swarms of meteors continue to travel around the sun in the orbits of the former comets. the earth encounters a number of such swarms every year at certain definite times. the largest and best known of these swarms or showers are the leonids, which appear about november ; the andromedas (or bielids), which appear later in the same month and the perseids, which appear early in august. these swarms are named for the constellations in which their "radiant" lies, that is, the point in the heavens from which they appear to radiate. the position of the radiant depends upon the direction from which the swarm is coming. it is simply a matter of perspective that the individual particles appear to radiate from the one point, for they are actually travelling in parallel lines. the luminosity of these meteoric particles is caused by the friction produced by their passage through the atmosphere. they always appear noiselessly because they are mere particles of meteoric dust weighing at the most scarcely a grain. they differ greatly in this respect from their large and noisy relatives, the meteorites, bolides and fireballs. numberless small meteoric particles are entrapped by the earth's atmosphere every day. they are referred to as "shooting" stars or "falling" stars though, of course, they are not in any sense stars. it is only when these meteoric particles travel in well-defined cometary orbits and appear at certain definite times every year that they are referred to as swarms or showers of meteors. the luminosity of comets is due not only to reflected sunlight, but to certain unknown causes that produce sudden and erratic increases or decreases of brilliancy. the causes of these sudden changes in luminosity are unknown; possibly electrical discharges or chance collisions between fragments of considerable size may account for some of them. the peculiar behavior of the tails of comets at certain times has frequently been noted and suggests the existence of quantities of finely-divided meteoric or gaseous matter within the solar system that has no appreciable effect upon the huge planetary masses, but offers sensible resistance to the passage of the tenuous gases of which the tails of comets are composed. the fact that the earth daily encounters meteoric dust, meteorites and fireballs also indicates that meteoric matter exists in considerable quantities within our solar system. tails of comets appear at times to be twisted or brushed aside as if they had encountered some unknown force or some resisting medium. up to the present time several hundred comets have been discovered. nearly three-fourths of this number travel in orbits that appear to be parabolas. of the remaining number there are about forty that have been "captured" by the major planets, jupiter, saturn, uranus and neptune, though jupiter possesses the lion's share of these captured comets. scarcely a year passes by that several comets are not discovered. most of these are telescopic, however, even when they are near the sun and at their greatest brilliancy. naked-eye comets of great splendor and brilliancy are comparatively rare and there has been a particular dearth of such unusual comets during the past thirty years or so. the last spectacular comet, unless we make an exception of halley's periodic comet, which made its return according to prediction in , was the great comet of which was visible in broad daylight close to the sun and at its perihelion passage swept through the solar corona with a velocity that exceeded two hundred and fifty miles a second and carried it through one hundred and eighty degrees of its orbit in less than three hours. some comets approach much closer to the sun than others. the majority of all comets observed have come within the earth's orbit and no known comet has its perihelion beyond the orbit of jupiter. it is, of course, possible that there may be a number of comets that never come within the orbit of jupiter, but it is very unlikely that any such comet would ever be discovered. the majority of comets are simply small, fuzzy points of light that are only visible telescopically and the greater the perihelion distance of the comet the less likely is it to be seen with the naked eye. since comets as well as planets obey kepler's first law, known as the law of areas, and sweep over equal areas in equal times, it is evident that when a comet is at perihelion, or nearest to the sun, it is moving at maximum speed and when it is at aphelion, or farthest from the sun, it is moving at minimum speed. moreover, its speed at these two points in its orbit varies tremendously since the orbits of comets are ellipses of very high eccentricity. the speed with which the planets are traveling is, on the other hand, remarkably uniform since their orbits are nearly circular. the leisurely speed with which a comet travels through the frigid outer regions of the solar system is gradually accelerated as the comet draws nearer and nearer to the sun until it has acquired near the time of perihelion passage a velocity that occasionally exceeds two hundred miles a second. here, also, the great increase in light and heat and the strong magnetic field of the sun produce complex changes in the gaseous and meteoric substances of which the comet is composed until the characteristic tail and peculiar cometary features are fully developed. as the comet again recedes from the sun after perihelion passage its speed slackens once more. it soon parts with its tail and other spectacular features and fades rapidly from view even in the largest telescopes. xxiii meteorites meteorites, bolides or fireballs, as they are variously called, are stones that fall to the earth from the heavens. they furnish the one tangible evidence that we possess, aside from that furnished by the spectroscope, as to the composition of other bodies in space and it is a significant fact that no unknown elements have ever been found in meteorites, though the forms in which they appear are so characteristic that they make these stones readily distinguishable from stones of terrestrial origin. the origin of meteorites is not definitely known, but the evidence is very strong in favor of the theory that they are the larger fragments of disintegrated comets of which meteors and shooting stars are the smaller; the distinction between the two being simply that the latter class includes all bodies that are completely consumed by friction with the earth's atmosphere and, therefore, only reach the surface in the form of meteoric dust. according to other theories meteorites may be fragments of shattered worlds that have chanced to come too near to a larger body and have been disrupted, or they may possibly be the larger fragments of the disintegrated comets of which the meteoric swarms are the smaller. interplanetary space is not altogether a void. our own planet sweeps up in the course of a single day, it has been estimated, approximately twenty million shooting stars or meteors of sufficient size to be visible to the naked eye, while the estimate for the telescopic particles runs up to four hundred million. meteorites on the other hand are comparatively rare. on the average it has been estimated about one hundred meteorites strike the earth in the course of a year, of which number only two or three are actually seen. according to _bulletin , u. s. national museum_, approximately six hundred and fifty falls and finds of meteorites have been reported, representatives of which appear in museums and private collections. meteorites, as well as shooting stars and meteors, frequently appear in showers. in such instances the fall usually consists of several hundred or thousand individual stones and the area over which they fall is several square miles in extent and roughly ellipsoidal in shape. one of the most remarkable of such falls [...] hundred thousand stones, varying in weight from fifteen pounds to a small fraction of an ounce, fell near pultusk, poland. another remarkable fall of meteorites occurred at l'aigle, france, in . between two thousand and three thousand stones fell over an ellipsoidal area of six and two-tenths miles in greatest diameter, the aggregate weight of the stones being not less than seventy-five pounds. this fall of stones is of particular interest since it took place at a time when men were still very doubtful as to whether or not stones actually fell to earth from the heavens. after this fall had occurred in a most populous district of france in broad daylight and attended by violent explosions that lasted for five or six minutes and were heard for a distance of seventy-five miles, no reasonable doubt could longer be held as to the actuality of such phenomena. meteorites are without exception of an igneous nature, that is, they are rocks that have solidified from a molten condition. they can be classified into three groups, aerolites or stony meteorites, siderolites or stony-iron meteorites, and siderites or iron meteorites. more iron meteorites seem to have fallen in mexico and greenland than in any other part of the world--at least of its land surface. yet strange to say, of all the meteorites that have been seen to fall only nine belong to the group of siderites or iron meteorites, though the three largest meteorites known, peary's meteorite from cape york, greenland, weighing - / tons, the meteorite lying on the plain near bacubirito, mexico, weighing about tons, and the willamette, oregon, meteorite, weighing - / tons all belong to this group. moreover, all the canyon diablo meteorites, which are strewn concentrically around coon mountain crater in northern arizona to a distance of about five miles, are members of this same group. coon mountain or meteor crater itself is a perfectly round hole, about six hundred feet deep and over four thousand feet in diameter and was formed, it is believed, by the impact of a huge meteorite which has never been found. it is believed that the canyon diablo meteorites, of which there are nearly four hundred individuals in the u. s. national museum alone, were all members of this same fall. it is possible that these meteorites of the canyon diablo district, with the huge meteorite that produced the crater itself, formed the nucleus of a comet that struck the earth not more than five thousand years ago, according to the geological evidence. all iron meteorites or siderites (from the greek sideros, iron) are composed chiefly of alloys of nickel and iron. the percentage of nickel in these iron meteorites is very small, usually from five to ten per cent., while the iron forms about ninety or ninety-five per cent. of the whole. cobalt is also present in practically all iron meteorites in small quantities of per cent. or less. usually small quantities of iron sulphide and phosphide as well as graphite or some other form of carbon appear in the iron meteorites and in some instances black and white diamonds have been found, as in some of the canyon diablo irons. a very interesting and beautiful feature of many iron meteorites is the widmanstätten figures which appear when a section of such a stone is polished and treated by means of a weak acid. these figures are due to the unequal solubility of the three different alloys of nickel and iron of which the stones are composed. the irons giving the widmanstätten figures are known as octahedral irons. other irons known as hexahedral irons give figures of a different type known as neumann figures when the polished section is treated with weak acid, while other irons are so homogeneous in their composition that they show no figures at all. aerolites or stony meteorites occur more abundantly than iron or stony-iron types, and they are classified into many divisions and subdivisions according to their composition. in these stones appear certain compounds that are commonly met with in terrestrial igneous rocks. the mineral that is most abundant in the stony meteorites, composing sometimes nearly seventy-five per cent. of the stone, is a magnesium and iron silicate known as olivine, which is also usually present in terrestrial rocks of an igneous nature. certain compounds found in the stony meteorites are rarely if ever found in terrestrial rocks, however, and these serve to distinguish the stony meteorites readily from stones of terrestrial origin. the alloys of iron and nickel, for instance, that occur in minor quantities in the stony meteorites and make up usually about ninety-five per cent. of the mass of the iron meteorites, are never found in terrestrial rocks. although about thirty of the terrestrial elements are to be found in meteorites, the forms and compounds in which they appear are so characteristic and on the whole so different from those occurring in terrestrial rocks, that the analyst has no difficulty in distinguishing between the two. there are, for instance certain formations known as chondrules, peculiar spherical and oval shapes, varying in size from minute particles to objects the size of walnuts, appearing in many varieties of stony meteorites that are never found in terrestrial rocks, and that are one of the most puzzling features associated with the origin and nature of these stones. sometimes the chondrules are so loosely embedded in the stone that they fall away when it is broken. in some instances almost the entire stone is made up of these chondrules. according to one theory the chondrules were originally molten drops, like fiery rain, and their internal structure, which is greatly varied, depends upon their conditions of cooling. stony meteorites, in which these chondrules are to be found, are spoken of as chondrites. there are white and gray and black chondrites and crystalline and carbonaceous chondrites, according to the nature of the chondrules found in the stones. stony meteorites also contain minute quantities of iron and nickel alloys in the form of drops or stringers. upon entering the earth's atmosphere stony meteorites become coated with a thin black crust which is a glass formed by the fusion of its surface materials by the heat generated during its passage through the atmosphere. in many of the stony meteorites there also appear fine thread-like veins which are due to the fracturing of the stone prior to its entrance into the atmosphere. the material filling these veins is coal black in color, opaque and of an unknown composition. many meteorites show signs of collisions and encounters with other meteorites outside of the atmosphere as would be expected as they travel in swarms and groups. sometimes the entire meteorite is composed of fragments of two or more distinct stones cemented together. such a stone is spoken of as a _breccia_. in the third class of meteorites to which we now come, known as the stony-iron meteorites, there is a network or sponge of nickel-iron alloy, the interstices of which are filled with stony material. when this network or sponge is continuous the meteorite is spoken of as a stony-iron pallasite. when the network of metal is more or less disconnected the meteorite is a meso-siderite. if meteorites are heated in a vacuum, the conditions existing in interplanetary space being thus produced to a certain extent, they give forth their occluded gases and it has been found that these gases give spectra identical with the spectra of certain comets. meteoric irons give forth hydrogen as their characteristic gas while the gases occluded in the stony meteorites are chiefly the oxides of carbon, carbon monoxide and carbon dioxide. it has been found that the amount of gases contained in a large meteorite or shower of meteorites is sufficient to form the tail of a comet. these facts all tend to strengthen the belief that meteorites are indeed cometary fragments. in view of the fact that some geologists believe meteorites may be fragments of other worlds, it is of interest to know that so far no fossil-bearing meteorites have been found, and if meteorites are fragments of a shattered world, such worlds must have been reduced to a molten condition at the time of the catastrophe. the rapid passage of the meteorite through the air leaves a partial vacuum in its trail into which rush the molecules of air from all sides, producing the characteristic noises that accompany the passage of a meteorite, which have been variously compared to the rattle of artillery, the distant booming of cannons or the rumble of thunder. there may be, also, explosions of inflammable gases occluded in the crevices of the meteorite which will shatter it into fragments or the meteorite may be shattered by the resistance and pressure of the atmosphere or as a result of the extremes of temperature existing between the interior and its surface. many meteorites have actually been seen to burst into fragments in the air with a loud report. there is practically no foundation for the belief that germs of life have been brought to our planet on such igneous rocks. no microscopic examinations of meteorites have yielded any results that could be interpreted in favor of such a view. falls of meteorites are accompanied in nearly every instance by terrific explosions and sharp reports that can be heard for many miles around, often causing the ground to shake as in an earthquake. the meteorite itself has been described as resembling a ball of fire or the headlight of a locomotive, and is followed frequently by a trail of light or a cloud of smoke. at the time it enters our atmosphere a meteorite is moving with planetary velocity ranging from two to forty-five miles per second. its interior is intensely cold, approaching in temperature the absolute zero of interplanetary space, and it is, therefore, far more brittle than it would be at ordinary temperatures. as it ploughs its way into the earth's atmosphere its surface temperature is soon raised by friction to at least , ° or , ° c., which is sufficient to fuse all surface materials into the characteristic black crust, with which stony meteorites are coated. meteorites are usually first seen at an altitude of fifty or sixty miles. although they are moving with a velocity comparable to that of the planets, when they enter the earth's atmosphere, this velocity is so rapidly reduced by friction with the atmosphere that they usually drop to the surface of the earth with a velocity about equal to that of ordinary falling objects. the flight of a meteorite often extends over a path several hundred miles in length and the meteorite may be seen by many observers in several different states and yet finally fall in some unknown spot and never be found. the evidence gathered regarding the actual fall of meteorites is often contradictory. some stones are too hot to handle for hours after they fall, others are merely warm, while still others have been picked up cool or even intensely cold. meteorites have been seen to fall upon dried grass and upon straw without producing even charring effects. the evidence regarding the depths to which meteorites penetrate the ground is quite as conflicting. the largest of all the stony meteorites which fell at krnyahinya, hungary, weighed pounds and buried itself to a depth of eleven feet. yet peary's cape york iron meteorite, weighing - / tons, was only partially covered and showed no signs of abrasions of surface resulting from the fall. the willamette iron meteorite, weighing - / tons, lay in a forest when found and was not deeply buried. the bacubirito iron meteorite, weighing tons, lay in soft soil, barely beneath the level of the surface. on the other hand a fragment of a stony-iron meteorite, weighing pounds, that fell at estherville, iowa, buried itself eight feet in stiff clay. geologists in charge of the meteoric collections of various museums quite frequently have stones sent to them for analysis that are reputed to be of celestial origin. more often than not such stones are found to be purely terrestrial in their origin. the composition of a meteorite is so characteristic and unique that such a stone can never be mistaken. finds of bona-fide meteorites are on the whole extremely rare. it is also a peculiar fact that meteorites are usually observed in the months when ordinary meteors or periodic swarms of meteors are least prevalent, that is in the months of may, june and july. xxiv the earth as a magnet if a small, freely suspended compass needle is moved over a highly magnetized steel sphere, it will be seen that it constantly changes its position both horizontally and vertically so as to lie always along the "lines of force" of the sphere. there will be one point on the sphere which we will call the _north magnetic pole_, where the _north-seeking_ end of the needle will point vertically downward or make a "dip" of ° with the tangent plane. at the diametrically opposite point on the sphere, called the south magnetic pole, the opposite end of the compass, the south-seeking end, will point vertically downward; while at a point midway between the magnetic poles of the sphere the needle will lie parallel to the diameter connecting the two poles and there will be no dip. the total intensity of the magnetic field surrounding the sphere will be found to be greatest in the vicinity of the magnetic poles and least, midway between the poles. now, a freely suspended compass needle carried to all parts of the earth will behave very much in the same manner as the needle moved over the magnetized steel sphere. there are two points on the earth's surface, known as the north and south magnetic poles, where the needle points vertically downward and approximately midway between is the _magnetic equator_ where the compass needle places itself in a perfectly horizontal position and the "dip" of the needle is zero. in other words, the earth acts as a huge magnet and possesses a magnetic field with lines of force converging towards its poles similar to the lines of force of the steel sphere. there are, however, some very important differences between the sphere of steel and our earth. the matter of which the earth is composed is not homogeneous. it is believed to possess an iron core of considerable size, it is true, but its outer shell is composed of heterogeneous masses that in certain regions cause very appreciable local deflections of the needle. it is surrounded, moreover, by an atmosphere permeated by electrified particles of matter shot forth from the sun, which we now know is a still greater magnet surrounded by a magnetic field that is of the order of gausses at the poles and about eighty times more powerful than that of the earth. it is now a well-established fact that the sun's magnetic field exerts a powerful influence over the condition of the earth's magnetic field, and that vast solar disturbances affect very materially the direction and intensity of the lines of force. it is thus little wonder that this non-homogeneous and rapidly rotating terrestrial globe, surrounded by an electrified atmosphere and subject to the action of a still more powerful magnet, the sun, should not behave in a manner exactly analogous to a uniformly magnetized steel sphere. the earth's magnetic poles are neither symmetrically placed nor absolutely fixed in position. there is every reason to suspect that they shift about from year to year, and possibly fluctuate irregularly in position in the course of a few days or hours under the influence of disturbing forces. the position of the earth's north magnetic pole, last visited by amundsen in , now lies approximately in latitude ° n. longitude ° w. the position of the south magnetic pole, according to the latest determinations, is, in round numbers, in latitude ° s. and longitude ° e. of greenwich. it is evident, therefore, that the magnetic poles of the earth are not symmetrically placed and that they lie fully ° from the _geographical_ poles. the chord connecting the magnetic poles passes miles from the earth's center, and it is about , miles from the geographic pole to the nearest magnetic pole. there exist, moreover, in high latitudes local magnetic poles, due possibly to heavy local deposits of ore. one such pole was discovered at cape treadwell, near juneau, alaska, during dr. l. a. bauer's observations there in and . in the center of the observing tent at this point the needle pointed vertically downward and the compass _reversed_ its direction when carried from one side of the tent to the other. it is a well-known fact that there are very few points on the earth's surface where the compass needle points either to the true geographical pole or to the magnetic pole, and if it does chance to do so, it is a transient happening. the "variation of the compass" or the declination of the needle, as it is called, is the angle that the compass needle makes with the true north and south line or the meridian. it is an angle of greatest importance to navigators and explorers, for it gives them their bearings, yet it is unfortunately subject to ceaseless variations of a most complicated nature, since it depends on the constantly pulsating and never ceasing magnetic changes that sweep over the surface of the earth and through its crust. it is affected by long period or secular changes, as they are called, progressing more or less regularly in obscure cycles of unknown period. it is subject to a diurnal change that depends on the position of the sun relative to the meridian, and that varies with the seasons and with the hour of the day. it is affected by the sun spot cycle of . years which has a direct effect upon the intensity of the earth's magnetic field. the intensity of the magnetic field in sun spots is, according to abbot, sometimes as high as , gausses or , times the intensity of the earth's field. at times of maximum spottedness of the sun the intensity of the earth's magnetic field is reduced. moreover, when great and rapidly changing spots appear upon the sun, electrified particles are shot forth from the sun with great velocity and in great numbers, and are drawn in towards the magnetic poles of the earth. meeting the rarefied gases of the earth's upper atmosphere, they illuminate them as electric discharges illuminate a vacuum tube. some of these electrons are absorbed by gases at high elevations, other descend to lower levels. the most penetrating rays have been known to descend to an altitude of twenty-five miles which is about the lowest limit yet found for auroral displays. it is the passage of these rays through the atmosphere that cause the magnetic disturbances known as _magnetic storms_, that are associated with the appearance of great sun spots and auroral displays. at such times sudden changes take place in the intensity of the earth's magnetic field that cause the compass needle to shiver and tremble and temporarily lose its directive value. these _magnetic storms_ have been known to produce great temporal changes in the intensity of the earth's field. according to dr. l. a. bauer, director of the department of terrestrial magnetism of the carnegie institute of washington, the earth's intensity of magnetization was altered by about one-twentieth or one-thirtieth part by the magnetic storm of september , , which was one of the most remarkable on record, and the earth's magnetic condition was below par for fully three months afterwards as a result. in addition to these various regular and irregular changes in the variation of the compass, or declination of the needle, due to changes in the earth's magnetic field _as a whole_, there are local effects due to restricted regional disturbances of the earth's magnetic field or to local deposits of ore, or to volcanoes or other local causes. the effect of all these disturbances upon the declination of the needle must be determined by continual magnetic surveys of all portions of the earth's surface. as a whole the earth's magnetic field is more uniform over the oceans than over the land, with all its disturbing topographical features. yet this advantage is offset largely in navigation by the fact that every steel ship that sails the seas is a _magnet_, with its two magnetic poles and its neutral line where the two opposite magnetic forces are neutralized, as is the case with every magnet. the direction in which a steel ship lies with reference to the earth's magnetic field while it is being built determines the position of the magnetic poles in its hull and the position of its neutral line and this distribution of magnetism over a ship's hull must be taken account of in the installation of its standard compass. every piece of horizontal and vertical iron aboard ship has an effect upon the variation of the compass and compensation must be made for such disturbing forces. the direction of sailing, the position in which a ship lies at dock, storms encountered at sea, the firing of batteries (on warships) are some of the factors that are operative in producing changes in the variation of the magnetic compass aboard a ship. every ship must undergo at frequent intervals magnetic surveys for the purpose of determining its magnetic constants and its "table of deviations of the compass." the direction in which the compass needle points aboard ship is the _resultant_ of the effect of the earth's magnetic field and the magnetic field of the ship, and both fields are subject to continual and complicated variations from year to year, from day to day, _and even from hour to hour_! the elements of the earth's magnetic field are determined for any one epoch by long-continued magnetic surveys carried on to a greater or less extent by the various nations of the world, and the results are published in the form of _magnetic charts_ for land and sea, showing the values of the three magnetic elements, declination of the needle, dip or inclination, and horizontal intensity of the earth's field for a definite period. so rapid are even the long-period changes in the earth's magnetic field that a magnetic chart can be relied upon for only a very few years and fresh data for the construction of these charts that are so valuable to navigators and explorers must be gathered continually. the _department of terrestrial magnetism_ of the carnegie institute of washington is engaged in continual magnetic surveys of the earth by land and sea that are of the highest value not only to navigators but also to scientists interested in solving the great and mysterious problem of the underlying causes of the earth's magnetism. to give an idea of the extent and scope of the work of this department it may be mentioned that its non-magnetic ship _carnegie_ made in the period - a total run of , nautical miles, nearly nine times the earth's circumference, with an average day's run of nautical miles. magnetic observations were made practically every day at a distance of to miles apart. in this nine-year period five cruises were made. on her first cruise the _carnegie_ sailed from st. john's, newfoundland, to falmouth, england, over practically the same course followed by the famous astronomer, halley, in the _paramour pink_ two centuries earlier to determine the variation of the compass. in her fourth voyage the _carnegie_ circumnavigated the world in sub-antarctic regions in days--a record time. she has traversed all oceans from ° north to the parallel of ° south and has crossed and recrossed her own path and the path of her predecessor, the _galilee_, many times, thus making it possible to determine for the points of intersection the secular changes in the magnetic elements. after a thorough overhauling in and the installation of a four-cylinder gasoline engine, _made of bronze_ throughout, to take the place of the producer-gas engine used on earlier cruises, the _carnegie_ started on her sixth cruise with a crew of twenty-three officers and men on october , . a cruise of , miles was planned in the south atlantic, indian and pacific oceans to last approximately two years. unsurveyed regions in the south atlantic and indian ocean were to be covered and the route was planned so as to obtain a large number of observations of the progressive changes that have taken place in the magnetic elements. this is accomplished as stated above by intersecting former routes and obtaining new values of the element at the points of intersection. in addition to its ocean magnetic surveys the _department of terrestrial magnetism_ also carries on extensive land surveys in all parts of the globe. in special expeditions were sent out by the department to observe the total solar eclipse of may th at stations distributed over the entire zone of visibility of the eclipse and immediately outside. at dr. bauer's station in liberia the total phase was visible in a cloudless sky for more than six minutes, which is very close to the maximum length of phase that can possibly be observed. unmistakable evidence was gathered at all stations of an appreciable variation in the earth's magnetic field during a solar eclipse, which variation is the reverse of that causing the daylight portion of the solar diurnal variation of the needle. in addition to the magnetic survey work on land and sea which is the chief work of the _department of terrestrial magnetism_, atmospheric-electric observations are carried on continually on land and sea and experiments have been carried on at langley field, va., lately, in the development of methods and instruments for determining the geographical position of airplanes by astronomical observations. there has also been recently formed under this department a _section of terrestrial electricity_. the cause of the earth's magnetic field is still one of the greatest unsolved problems of astro-physics. the theory that has been advanced by schuster that all large rotating masses are magnets _as a result of their rotation_ has received considerable attention from astrophysicists, and attempts have been made to prove this experimentally. it has been found that iron globes spun at high velocities in the laboratory do _not_ exhibit magnetic properties. this may mean simply that the magnetic field is too weak to be detected in the case of a comparatively small iron sphere spun for a limited period under laboratory conditions. it must be remembered that the earth has been rotating rapidly on its axis for millions of years and is, compared to terrestrial objects, an extremely large mass. yet it has been shown that as a whole our earth is an extremely weak magnet, and that if it were made entirely of steel and magnetized as highly as an ordinary steel-bar magnet, the magnetic forces at its surface would be a thousand times greater than they actually are. if it is true that all rotating bodies are magnets, then all the heavenly bodies, planets, suns and nebulæ are surrounded by magnetic fields. we know nothing to the contrary. in fact, we know this to be true for the earth and sun, and strongly suspect that it is so in the case of the planets jupiter and saturn. when we understand more about the properties of matter, the nature of magnetism, as well as of gravity, may be revealed to us. xxv some effects of the earth's atmosphere upon sunlight it is impossible to exaggerate the importance of the atmosphere to all forms of life upon the surface of the earth. if there were no atmosphere there would be no life, because it is through the agency of the water-vapor, carbon-dioxide and oxygen in the atmosphere that all life-processes are maintained. if there were no atmosphere there would not only be no life upon the earth; there would be also none of the beautiful color effects produced by the passage of sunlight through the atmosphere. there would be no blue skies, no beautiful sunrise and sunset effects, no twilight, no rainbows, no halos, no auroral displays, no clouds, no rains, no rivers nor seas, no winds nor storms. the heavens would be perfectly black except in the direction of the heavenly bodies which would shine as brilliantly by day as by night. to understand how the atmosphere produces color effects such as blue skies, sunrise and sunset tints, rainbows and halos, as well as the twinkling of the stars, and numerous other phenomena, we must know something of the nature of light itself. light moves outward from any source, such as the sun, in all directions radially, or along straight lines (so long as it does not encounter a gravitational field) with the unimaginable velocity of , miles per second. as it advances it vibrates or oscillates back and forth across its path in all directions at right angles to this path, unless it is plane polarized light, in which case its vibrations are confined to one plane only. these vibrations or oscillations of light take the form of a wavelike motion, one wave-length being the distance passed over in the time of one vibration, measured from crest to crest or from trough to trough of adjacent waves. we may consider that a beam or ray of sunlight is made up of a great number of individual rays of different wave-lengths and different colors. the average wave-length of light, the wave-length of the green ray in sunlight, is about one-fifty-thousandth part of an inch, that is, it would take about fifty-thousand wave-lengths of green light to cover a space of one inch. now, since light makes one vibration in passing over a distance of one wave-length, it makes fifty thousand vibrations, while advancing one inch, and since it advances one hundred and eighty-six thousand miles in one second we can easily figure out that a ray of sunlight of average wave-length makes about six hundred trillion vibrations ( , , , , ) in a single second! the chief colors of which sunlight or white light is composed are red, orange, yellow, green, blue, indigo and violet, though there are an infinite number of gradations of color which blend into one another, gradually producing the intermediate tints and shades. the colors just mentioned are called the primary colors of the solar spectrum, which can be produced as a band of light of variegated colors, arranged in the order named by passing a ray of ordinary sunlight through a glass prism. the individual rays of different color and wave-length that make up a beam of sunlight, or white light, then separate out in the order of the wave-lengths. the red rays vibrate the most slowly and have the longest wave-length of all the rays of the visible spectrum. about four hundred trillion vibrations of red light reach the eye in one second. violet rays, on the other hand, vibrate the most rapidly of all the visible rays and have the shortest wave-length. about eight hundred trillion vibrations of violet light reach the eye every second. the wave-lengths of the intermediate colors decrease in length progressively from the red to the violet and, of course, the frequencies of their vibrations increase in the same order. all sunlight is made up of these rays of different colors and different vibration frequencies, and of other rays as well, to which the human eye is not sensitive, and which, therefore, do not appear in the visible spectrum. among these invisible rays are the infra-red rays which come just below the red of the visible spectrum, and which are of longer wave-length than the red rays, and the ultra-violet rays, which lie beyond the violet rays of the visible spectrum, and are of shorter wave-length than the violet rays. now a ray of ordinary sunlight is separated into the rays of various colors, which form the solar spectrum when it passes from a medium of one density obliquely to a medium of another density, as when it passes from air to glass, or from air to water, or from outer space into the earth's atmosphere. under such circumstances its velocity is slowed down when it passes from a rare to a denser medium, and the waves of different wave-lengths are bent from their former course, or refracted, by different amounts. the red rays, of longest wave-length, are bent from their former course the least, and the violet rays, of shortest wave-length, are bent the most upon passing from a rare to a denser medium. as a result the ray of sunlight is spread out or dispersed into its rays of different wave-length and color upon entering a medium of different density. it is this refraction and dispersion of sunlight that produces many color effects in the earth's atmosphere. the atmosphere is not of uniform density throughout. at high altitudes it is extremely rare. that is, there is little of it in a given volume. close to the earth's surface, however, it is comparatively dense. half of all the atmosphere is within three and one-half miles of the surface and half of the remainder lies within the next three and a half miles. we may consider it as made up, on the whole, of layers of different densities, strongly compressed near the surface. imagine a ray of sunlight entering the earth's atmosphere from without. if it comes from a point in the zenith its course is not changed upon entering the atmosphere, because light passing from a certain medium--as space--into a medium of different density, is not bent from its course, or refracted, provided it enters the new medium in a direction perpendicular to the surface. if it enters the atmosphere (which is the new medium of greater density) _obliquely_, refraction, or bending of the ray, takes place, and as the ray advances toward the earth, through layers of increasing densities, it is bent from its former course more and more. as the advancing rays of different colors and wave-lengths in the beam of sunlight are slowed down in the new medium, the red rays are turned from their course the least and the violet rays the most and the entire advancing wave-front of the beam of sunlight is bent down more and more toward the horizon, as it proceeds through the atmosphere. as we on the earth's surface see the ray not along its bent course through the atmosphere, but in the direction in which it finally enters our eyes, the effect of refraction upon a ray of light passing through the atmosphere is to displace the object in the direction of the zenith or increase its distance above the horizon. as a result of refraction we see the sun--or moon--above the western horizon after it has really set, and above the eastern horizon before it has really risen. the oval shape that the sun, or moon, often presents on rising or setting, is due to the fact that the light from the lower limb is passing through denser air than the light from the upper limb, and so is refracted more. as a result the lower limb is lifted proportionately more than the upper limb. this distorts the form of the solar or lunar disk, making it appear oval instead of circular. the familiar twinkling or scintillation of stars and, more rarely, of the planets, is a result of interference of light waves due to irregular and variable refraction in air that is not uniform in density, owing to the presence of constantly rising and descending atmospheric currents of different densities. this also produces the shimmering or unsteadiness of star images in the telescope, that interferes so greatly with accurate measurements of angles or observations of planetary markings. one may ask why it is, if light from an object, say a star, is bent from its course and separated into rays of various colors upon entering the earth's atmosphere, that we do not see the object drawn out into a band of spectral colors. it is because the angular separation of the various colors is so slight under ordinary circumstances that light from one point is blended with light from a neighboring point of complementary color to produce white light again. under certain circumstances, however, beautiful color effects may be seen in the earth's atmosphere as a result of the refraction of sunlight. the blue color of the sky and its brightness is caused by the scattering of the rays of shortest wave-length, the violet and blue rays, by the oxygen and nitrogen in the upper atmosphere. the molecules of these gases interfere with the passage of these rays, powerfully scattering and dispersing them, and thus increasing the length of their path through the air and diffusing their color and brightness in the upper atmosphere, while permitting rays of longer wave-length, the red and orange, to pass on practically undisturbed. when an object in the heavens lies close to the horizon, the rays of light from it have to travel a longer path through the atmosphere than when the object is overhead, and that too through the densest part of the atmosphere, which lies close to the earth's surface, and in which are floating many dust particles and impurities from the earth's surface. all these particles, as well as the increased density of the atmosphere, interfere with the free passage of the rays, especially of shorter wave-lengths. the violet and blue rays are sifted out and scattered in their long journey through the lower strata of air, far more than when they come to us from an object high in the sky. even the red and yellow rays are more or less scattered and bent aside--diffracted--by these comparatively large particles near the surface. the reddish color of the sun, moon and even of the stars and planets, when seen near the horizon, as well as the beautiful sunset tints, in which reds and pinks and yellows predominate, are due to the fact that the rays of longer wave-length are more successful in penetrating the dense, dust-laden layers of the lower atmosphere. it is to be free of the dust and impurities as well as the unsteadiness of the lower atmosphere, that observatories are built at high altitudes whenever possible. when there have been unusually violent volcanic eruptions, and great quantities of finely divided dust have been thrown into the upper atmosphere, the effect upon the blue and violet rays from the sun is very great. the volcanic dust particles are so large that instead of scattering these rays of shorter wave-length, as do the oxygen and nitrogen in our atmosphere, they reflect them back into space and so decrease the amount of light and heat received from the sun. for this reason the general temperature of the earth is lowered by violent volcanic eruptions. unusually cold periods, that lasted for months, followed the terrible eruption of krakatoa in and of katmai in . at times when much dust is present in the atmosphere, the sky is a milky white color by day as a result of the reflection of sunlight from the dust particles. sunrise and sunset colors are then particularly gorgeous, with reds predominating. at such times the blue and violet rays are almost completely shut out, and the red, orange and yellow rays are powerfully diffracted and scattered by the dust particles in the air. the twilight glow that is visible for some time before sunrise or after sunset is, of course, entirely an atmospheric effect caused by the reflection of sunlight to our eyes from the upper atmosphere, upon which the sun shines, while it is, itself, concealed from our view below the horizon. the atmosphere extends in quantities sufficient to produce twilight to an elevation of about sixty miles. when _all_ the rays of which sunlight is composed are reflected in equal proportions we get the impression of white light. dust and haze in the air reflect all rays strongly and give a whitish color to an otherwise blue sky. brilliant white clouds appear white, because they are reflecting all rays equally. clouds or portions of clouds appear black when they are in shade or, at times, by contrast with portions that are more strongly illuminated, or when they are moisture-laden and near the point of saturation, when they are absorbing more light than they reflect. at sunrise and sunset, when the light that falls upon the clouds is richest in red and orange and yellow, clouds reflect these colors to our eyes, and we see the brilliant sunset hues which are more intense the more the air is filled with dust and impurities. the familiar and beautiful phenomenon of the rainbow is produced by refraction, reflection and interference of sunlight by drops of falling water, such as rain or spray. as the ray of sunlight enters the drop of water, which acts as a tiny glass prism, it is refracted or bent from its course and spread out into its spectral colors. reflection of these rays next takes place (once or twice, as the case may be) from the inside of the drop and a second refraction of the reflected ray takes place as it leaves the drop. the smaller the drops the more brilliant is the rainbow and the richer in color. the most brilliant rainbows are produced by drops between . and . millimeters in diameter. in addition to the primary bow, which has a red outer border with a radius of °, there is the secondary bow with a radius of about ° and with colors reversed, the red being on the inner border; the supernumerary bows which are narrow bands of red, or green and red, appear parallel to the primary and secondary bows along the inner side of the primary bow and the outer side of the secondary bow. no rainbow arches ever appear between the primary and secondary bows, and it can be shown in fact, that the illumination between these two bows is at a minimum. the primary, secondary and supernumerary bows all lie opposite the sun in the direction of the observer's shadow and the observer must stand with his back to the sun in order to see them. the primary and secondary rainbow arches take the form of arcs of circles that have their common center on the line connecting the sun with the observer at a point as far below the horizon in angular distance as the sun is above the horizon. it is, therefore, never possible to see a rainbow arch of more than a semicircle in extent unless the observer is at an elevation above the surrounding country, under which circumstances it might be possible to see a complete circle formed by the rainbow. the highest and longest arch appears when the sun is on the horizon, and the greater the altitude of the sun the smaller and lower the visible arch. as the angular radius of the primary bow is ° and of the secondary bow ° and as the common center of the two circles is always as far below the horizon as the sun is above, it is never possible to see either primary or secondary rainbow when the altitude of the sun is over °, or the primary bow when the altitude is over °. for this reason rainbows are rarely seen at or near noon in mid-latitudes, since the sun is usually at an elevation of more than ° at noon, especially in the summer season, which is also the most favorable season for rainbows, owing to the great likelihood of rain and sunshine occurring at the same time. the light which comes to an observer from the primary bow is once reflected within the drop, and that which comes from the secondary bow is twice reflected within the drop. the sharper and brighter light therefore comes from the primary bow of ° radius. the space between the two bows is particularly dark, because it can be shown that the drops there do not reflect any light at all. the rainbow colors are rarely pure or arranged in spectral order, owing to interference of light waves. it is the interference of light waves from different parts of the same drop that produces the bands of alternate maximum and minimum brightness, that lie below the primary bow and beyond the secondary bow. the red or green and red bands of maximum brightness produced thus by interference, are called the supernumerary bows, and they are always found parallel to the primary and secondary bow within the former and above the latter. the distance of the rainbow from the observer is the distance of the drops that form it. a rainbow may be formed by clouds several miles distant or by the aid of the garden hose on our lawn. no two observers can see exactly the same rainbow because the rainbow arch encircles the surface of a cone whose vertex is at the observer's eye and no two such vertices can exactly coincide. two observers see rainbows formed by different drops. refraction of light by ice-crystals in clouds produces many beautiful color effects, such as halos of various types around sun or moon, vertical light pillars, circumzenithal arcs, and parhelia--"sun-dogs"--or paraselenæ--"moon-dogs"--which are luminous spots at equal altitudes with sun or moon--one to the left the other to the right, at an angular distance of °. the most usual form of halo is that of ° radius. this is a luminous ring of light surrounding sun or moon, with the inner edge red and sharply defined and the spectral colors proceeding outward in order; red is frequently the only color visible, the remainder of the ring appearing whitish. since the halo is produced by refraction of light by ice-crystals, which exist in clouds of a certain type gathering at high altitudes, it is always a very good indicator of an approaching storm. coronas are luminous rings showing the spectral colors in the reverse order, that is, with the inner edge blue instead of red. they are usually of very small radius, scarcely two degrees, closely surrounding sun or moon and are produced--not by refraction--but by _diffraction_ or a bending aside of the rays as they pass between--without entering--very small drops of water in clouds. as in the case of refraction, the red rays are turned from their course the least and the violet rays the most. many of these phenomena--halos, luminous spots, vertical pillars and arcs of light may, at times, be seen simultaneously, when clouds of ice-crystals are forming around the sun or moon. they then present a very complex and beautiful outline of luminous circles, arches and pillars that have a mysterious and almost startling appearance when the cause is not clearly understood. we have found then that sunlight is made up of rays of many different wave-lengths and colors and that the atmosphere acts upon these rays in various ways. it reflects them or turns them back on their course; it refracts them as they pass through the gases of which the atmosphere consists, or through the water-vapor and ice-crystals suspended in it, thus sifting out and dispersing the rays of different colors and wave-lengths and producing beautiful color effects; it _diffracts_ them or bends them aside as they pass between the fine dust particles and small drops of water in the air, again sifting out the rays of different colors and producing color effects similar to those produced by refraction; it also scatters and disperses, through the action of the molecules of oxygen and nitrogen in the upper strata, the blue and violet rays of shorter wave-length and thus produces the blue color and brightness of the sky; it produces beautifully colored auroral streamers and curtains and rays of light through the electrical discharges resulting when the rarefied gases in the upper air are bombarded by electrified particles shot forth from the sun. it is our atmosphere, then, that we have to thank for all these beautiful displays of color that delight our eyes and give pleasure to our existence, as well as for the very fact of our existence upon a planet that without its presence would be an uninhabitable waste, covered only with barren rocks. xxvi keeping track of the moon of all celestial objects the nearest and most familiar is our satellite, the moon. yet the mistakes and blunders that otherwise intelligent persons frequently make when they refer to the various aspects of the moon are quite unbelievable. who has not read in classics or in popular fiction of crescent moons riding high in midnight skies, of full moons rising above western cliffs or setting beyond eastern lakes? who has not seen the moon drawn in impossible positions with horns pointing toward the horizon, or a twinkling star shining through an apparently transparent moon? careful observation of the moon in all its various phases and at different seasons is the best method to be used in acquiring a knowledge of the elementary facts regarding the motion of the moon through the heavens from day to day, but that requires that one be up often after midnight and in the early hours preceding dawn and so it is that we feel so hazy in regard to what happens to the moon after it has passed the full. a few fundamental rules can be easily acquired, however, and these will enable us to locate the moon in the right quarter of the heavens at any time of the day or night when we know its phase and the approximate position of the sun at the same instant, and thus we may avoid some of the most obvious blunders that are made in dealing with the general aspect of the moon at any given time. as can be verified by direct observation, the moon is always moving continually eastward. since it makes a complete revolution around the earth from new moon back to new moon again in a little less than thirty days, it passes over about twelve degrees a day ( ° divided by ), on the average, or one-half a degree an hour, which is about the angular extent of its own diameter. therefore every hour the moon moves eastward a distance equal to its own diameter. this is of course only approximate as the moon moves more rapidly in some parts of its orbit than in others. in addition to its real eastward motion the moon shares the apparent daily westward motion of all celestial objects which is due to the daily rotation of the earth on its axis in the opposite direction. that is, the moon, as well as the sun, stars and planets, rises in the east and sets in the west daily. on account of its continuous eastward motion, however, the moon rises later every night, on the average about fifty minutes, though the amount of this daily retardation of moon-rise varies from less than half an hour to considerably over an hour at different seasons of the year and in different latitudes. in the course of a month then the moon has risen at all hours of the day and night and set at all hours of the day and night. it might seem unnecessary to emphasize the fact that the moon always rises in the east were it not that the astronomer occasionally meets the man who insists that he has at times seen the moon rise in the west. to be sure the new crescent moon first becomes visible above the western horizon shortly after sunset though it rises in the east the morning of the same day shortly after sunrise. as is also true of the sun the exact point on the horizon where the moon rises or sets varies from day to day and from season to season. in one month the moon passes over very nearly the same path through the heavens that the sun does in one year, for the moon's path is inclined only five degrees to the ecliptic or apparent path of the sun through the heavens. it can never pass more than - / ° ( - / ° + °) south of the celestial equator, nor more than - / ° north of it. it has a slightly greater range in altitude than the sun, therefore. north of - / ° north latitude it always crosses the meridian south of the zenith and below - / ° south latitude it crosses the meridian north of the zenith. in tropical regions the moon sometimes passes north of the zenith, sometimes south, or again directly through the zenith. since the full moon is always diametrically opposite to the sun it passes over nearly the same part of the heavens that the sun did six months before. in winter then when the sun is south of the equator the moon "rides high" at night north of the equator and, vice versa, in summer when the sun is north of the equator the full moon "rides low" south of the equator. in winter then we have more hours of moonlight than we have in summer. this may be of no great advantage in mid-latitudes but we may imagine what a boon it is to the inhabitants of the arctic and antarctic regions to have the friendly moon above the horizon during the long winter months when the sun is never seen for days at a time. at time of "new" moon the moon lies directly between us and the sun, but ordinarily passes just to the north or south of the sun since its orbit is inclined five degrees to the ecliptic or plane of the earth's orbit. if the moon's path lay exactly in the ecliptic we would have an eclipse of the sun every month at new moon and an eclipse of the moon two weeks later at full moon. now the moon crosses the ecliptic twice a month, the points of crossing being called the nodes of its orbit, but only twice a year is the moon nearly enough in line with the sun at the time it crosses to cause eclipses. every year, then, there are two "eclipse seasons," separated by intervals of six months, when the moon is in line with the sun at or close to the point where it crosses the ecliptic; then and only then can solar and lunar eclipses occur. the solar eclipses, of course, will occur when the moon is new, that is, when the moon passes directly between the earth and the sun and throws its shadow over the earth; and the lunar eclipses two weeks later when the earth passes between the sun and moon and throws its shadow over the face of the moon. probably there is no astronomical subject that has been more generally misunderstood than that of solar and lunar eclipses. it is well to remember that solar eclipses can only occur at time of new moon and lunar eclipses only at the time of full moon; and at the time of eclipses, whether lunar or solar, the moon is at or near its nodes, the points where its orbit crosses the ecliptic. there are always at least two solar eclipses every year and there may be as many as five. there are years when there are no lunar eclipses, though ordinarily both solar and lunar eclipses occur every year, some partial others total. the moon shines only by reflected sunlight. it is of itself a solid, dark body with its day surface intensely hot and its night surface intensely cold, a world of extreme temperatures. at new moon all of the night side of the moon is turned toward us, at full moon all of the day side. at other phases we see part of the day side and part of the night side and the illuminated side of the moon is always the side that is towards the sun. failure to observe this simple rule leads to many grievous blunders in depicting the moon. at the time of new moon the moon, moving continually eastward, passes north or south of the sun from west to east except when it passes directly in front of the sun, causing eclipses. a day or so later the waxing crescent moon or the "new moon," as it is popularly called, becomes visible low in the west immediately after sunset. the moon is now east of the sun and will remain east of the sun until the time of full moon. during the period from new moon to full moon it will, therefore, rise after the sun and set after the sun. the waxing crescent moon will not be visible in the morning hours because, inasmuch as it rises after the sun, it is lost to view in the sun's brilliant rays. nevertheless, it follows the sun across the sky and becomes visible in the west as soon as the sun has disappeared below the western horizon. the thin illuminated crescent has its horns or cusps turned _away_ from the point where the sun has set. the horns of the crescent can never point _toward_ the horizon since the illuminated side of the moon is always turned toward the sun whether the sun is above or below our horizon. as hour by hour and day by day the moon draws farther eastward and increases its angular distance from the sun, more and more of the illuminated side becomes visible; the crescent increases in width and area and the moon appears higher in the western sky each night at sunset. usually about seven and a fraction days after the date of new moon the moon completes the first quarter of its revolution around the earth. the period from one phase to the next is variable and irregular, being sometimes less than seven days and at other times more than eight days, since the moon does not move at a uniform rate in different parts of its orbit. when the moon has completed the first quarter of a revolution it is ninety degrees east of the sun and presents the phase known as "half-moon" since half of the surface that is turned toward the earth is illuminated and half is in darkness. it is said to be "at the first quarter." the illuminated half is of course the western half because the sun is to the west of the moon. the half moon is near the meridian at sunset and sets near midnight. up to the first quarter, then, the moon is a crescent in the western sky during the first part of the night and should never be represented as east of the meridian or near the meridian at midnight. after the moon has passed the first quarter and before it is full more than half of the side turned toward the earth is illuminated and it is in the "gibbous" phase. it is still the western limb that is fully illuminated. the moon is now east of the meridian at sunset and it crosses the meridian before midnight and sets before sunrise. all who are abroad during the first half of the night find this phase of the moon more favorable to them than the gibbous phase following full moon. the moon now being above the horizon at sunset is visible continuously from sunset to midnight but sets some time during the second half of the night, while the full moon shines throughout the night, rising in the east at sunset and setting in the west at sunrise. when the moon is full it is ° east, or west, of the sun and so both its eastern and western limbs are perfectly illuminated. after the full the moon goes through its phases in reverse order, being first gibbous, then a half-moon once more, and lastly a waning crescent. it is now west instead of east of the sun and so it is the eastern limb that is fully illuminated by the sun. being west of the sun it will now rise before the sun and set before the sun, the interval decreasing each day as the moon draws in toward the sun once more. the gibbous phase preceding full moon is favorable to all abroad before midnight but the gibbous phase following full moon is more favorable to those who are abroad after midnight, for from full moon to last quarter the moon is below the horizon at sunset, and of course, is rising later and later each night, while at sunrise it is still above the horizon, appearing each day higher and higher above the western horizon at sunrise as it approaches the third or last quarter. when it has reached this point it is once more a half-moon, though now it is the eastern half instead of the western half of the disk that is fully illuminated. the moon is ° west of the sun at third quarter and from this phase to the phase of new moon it is a crescent once more, but now a waning instead of a waxing crescent. it appears east of the meridian before sunrise and as the crescent grows thinner it draws nearer and nearer to the eastern horizon and the rising sun. as with the waxing crescent moon the horns are turned away from the horizon. the waning crescent moon is always to be looked for east of the meridian and to be associated with the rising sun, while the waxing crescent moon is to be looked for west of the meridian and associated with the setting sun. neither the waxing nor the waning crescent moon will be visible during the midnight hours. as the waning crescent moon grows thinner and draws in closer to the sun each successive night, its time of rising precedes that of the sun by an ever-decreasing interval until finally the crescent disappears from view in the eastern sky; the next day we see no crescent either in the eastern or western skies--the moon is once more in conjunction with the sun and "new." one revolution of the moon about the earth with respect to the sun has been completed and a day or so later we may look for a new crescent moon in the western sky after sunset. xxvii the motions of the heavenly bodies about three hundred and twenty years ago giordano bruno was burned at the stake for his audacity in believing in the existence of other worlds. a few decades later the famous astronomer galileo was forced to publicly recant his belief that the earth moved. yet the truth could not long be suppressed by such means, and since those dark days man's advance in knowledge has been so rapid that it seems to us today in this wonderful age of scientific discovery almost inconceivable that man ever believed that the earth, a tiny planet of a vast solar system, was "the hub of the universe," the fixed and immovable center about which revolved all the heavenly bodies. very reluctantly, however, and with bitter feeling, but in the light of overwhelming evidence man finally gave up his long-cherished idea of terrestrial importance, and when finally forced to move his fixed center of the universe, he moved it only so far as the comparatively nearby sun. this center he then regarded as fixed in space and also held to his belief that the stars, set in an imaginary celestial sphere, were immovable in space as well, and all at the same distance from the sun. so, scarcely two hundred years ago we find that the astronomer bradley was endeavoring to measure this common distance of the "fixed stars." though he failed in this attempt he made the important discovery that the observed positions of the stars are not their true positions, owing to the fact that the velocity of light is not infinite but takes a definite finite interval of time to travel a given distance. as a result the stars always appear displaced in the direction of the earth's motion around the sun, the amount of the displacement depending upon the velocity of the earth in its orbit and the velocity of light. this "aberration of light," as it is called, furnished additional proof that the earth revolves about the sun and was one more nail driven into the coffin of the old ptolemaic theory that the earth was the center of the universe. bradley also discovered that the positions of the stars were affected by the wabbling of the earth's axis, called its "nutation." although in the days of bradley neither the methods of observation nor the instruments were sufficiently accurate to show the minute shifts in the positions of the stars that reveal the individual motions of the stars and the distances of those nearest to us, yet the discovery of the two large displacements in the positions of all the stars, due to the aberration of light and the nodding of the earth's axis were of the greatest value, for they were a necessary step in the direction of the precise measurements of modern times. it is only through measurements of the greatest refinement and accuracy that it is possible to detect the motions and distances of the stars and to discover the wonderful truths about the nature and structure of the universe that they are revealing to us today. after unsuccessful attempts extending over several centuries the distance of one of the nearest stars, the faint cygni, as it is catalogued, was finally determined by the astronomer bessel in the year . this star is about ten light-years distant from the earth, which places it about six hundred and thirty thousand times farther away from us than the sun; that is, we would have to travel six hundred and thirty thousand times the distance from the earth to the sun to reach this very close stellar neighbor, cygni. the _nearest_ of all the stars, alpha centauri, is over two hundred and seventy thousand times the distance from the earth to the sun. it is, therefore, little wonder that the early astronomers believed that the stars were fixed in space since even the nearest is so far away that, viewed from opposite points in the earth's orbit, its apparent change in position due to our actual change in position of , , miles, amounts to only one and a half seconds of arc. two stars separated by _one hundred and sixty times_ this angular distance might possibly be glimpsed as two distinct stars by a person with good eyesight, though to most of us they would appear as one star. upon the measurement of such minute angles depended a knowledge of the distances of the nearest stars. it is to sir william herschel that we owe the discovery, more than a hundred years ago, of the motion of the sun through the universe. from the consideration of a long series of observations of the positions of the stars this famous astronomer discovered that the stars in the direction of the constellation hercules were separated by much greater angular distances than the stars diametrically opposite in the heavens. in other words, the stars were spreading apart in one portion of the heavens and crowding together in the opposite direction and he rightly interpreted this to mean that the sun was moving in the direction of the constellation of hercules. it was not until the spectroscope was applied to the study of the heavens in the latter part of the nineteenth century that the amount of this motion of the sun was found to be about twelve and a half miles per second, or four times the distance from the earth to the sun in a year. it is to sir william herschel that we owe also the discovery of binary systems of stars in which two stars swing around a point between them called their center of gravity. [illustration: spiral nebula in canes venatici taken with -inch reflector of the mt. wilson observatory] our first conception of the immensity and grandeur of the universe dates from the time of the older herschel only a century or so ago. the mysterious nebulæ and star clusters were then discovered, the wonders of the milky way were explored, and a new planet and satellites in our own solar system were discovered. it was found that the sun and the stars as well as the planets were in motion. neither sun nor earth could be regarded any longer as a fixed point in the universe. with the application of the spectroscope to the study of the heavens toward the end of the nineteenth century the key to a treasure-house of knowledge was placed in the hands of the astronomers of modern times and as a result we are now learning more, in a few decades, about the wonders and mysteries of the heavens than was granted to man to learn in centuries of earlier endeavor. yet it is the feeling of the astronomer of today that he is only standing on the threshold of knowledge and that the greatest of all discoveries, that of the nature of matter and of time and space is yet to be made. it is the spectroscope that tells us so many wonderful facts about the motions of the stars, nebulæ and star clusters. it tells us also practically all we know about the physical condition of our own sun and of the other suns of the universe, their temperature and age, and the peculiarities of their atmospheres. some of the most important astronomical discoveries that have been made in the past few years have to do with the distribution and velocities of the heavenly bodies as revealed by the spectroscope. it has been found, with the aid of the spectroscope, that the most slowly moving of all stars are the extremely hot bluish orion stars with an average velocity of eight miles per second, while the most rapidly moving stars are the deep-red stars with an average velocity of twenty-one miles per second, and there is in all cases a relationship existing between the color, or spectrum, of a star and its velocity. the reason for this connection between the two still remains undiscovered. the spectroscope has also told us some astonishing facts in recent years about the velocities of the spiral nebulæ. it is now known that these mysterious objects are moving with the tremendous average velocity of _four hundred and eighty miles per second_, which exceeds the average velocity of the stars fully twenty-five fold. they possess, moreover, internal motions of rotation that are almost as high as their velocities through space. it is now generally believed that spiral nebulæ are far distant objects of enormous size and mass, exterior to our own system of stars and similar to it in form. in place of the universe of the "fixed stars" and the immovable sun or earth of a few centuries ago we find that modern astronomical discovery is substituting a universe of inconceivable grandeur and immensity in a state of ceaseless flux and change. our earth--an atom spinning about on its axis and revolving rapidly around a huge sun that is equal in volume to more than a million earths--is carried onward with this sun through a vast universe of suns. only an average-sized star among several hundred million other stars is this huge sun of ours, moving with its planet family through the regions of the milky way, where are to be found not only moving clusters and groups of stars, speeding along their way in obedience to the laws of motion of the system to which they belong, but also strangely formed nebulæ covering vast stretches of space, whirling and seething internally and shining with mysterious light, and still other stretches of dark obscuring matter shutting off the rays of suns beyond. the extent and form of this enormous system of stars and nebulæ and the laws that govern the motions of its individual members are among the problems that the astronomers of today are attempting to solve. on both sides of these regions of the milky way, wherein lies our own solar system, lie other vast systems, such as the globular star clusters, composed of thousands, possibly hundreds of thousands, of suns; the magellanic clouds, which resemble detached portions of the milky way, and, probably, the much discussed spiral nebulæ, possible "island universes" similar to our own. we have come far in the past three hundred years from the conception of an immovable earth at the center of the universe to this awe-inspiring conception of the universe that we have today, which is based upon modern astronomical discoveries. whatever may be discovered in the future in regard to the form and extent of the universe the idea of a fixed and immovable center either within the solar system or among the stars beyond has gone from the minds of men at last. [illustration: spiral nebula in andromeda viewed edgewise taken with -inch reflector of the mt. wilson observatory] not more than a generation ago a survival of the old idea of a fixed center was seen in the belief that alcyone, in the pleiades was a "central sun" about which all the stars revolved. it is now well known that the pleiades form a moving star cluster. alcyone is therefore drifting slowly onward through the universe and the idea of a fixed and immovable center to which man may anchor his ideas is drifting away also. there are, it is true, local centers of systems, such, for instance, as the sun occupies in the solar system or some group of stars may occupy in the stellar system to which our sun belongs, yet _as a whole_ these systems move on and their centers with them. there is no evidence today that any absolutely immovable point exists in the heavens. no celestial object has been found to be without the attribute of _motion_, not only motion _onward_ through the universe, but also _rotational_ motion about an axis of the body. the planets rotate on their axes as well as revolve about the sun, and the sun also turns on its axis as it moves onward through space. this rotational motion is also found in the nebulæ and star clusters as well as in the stars and planets. no object in the heavens is known to be without it. even the slowly drifting orion nebula possesses a rapid internal velocity of rotation. there is no such thing as a body absolutely at rest in the universe. table showing the number and relative size, velocity and distribution of the various types of celestial objects. ================+====================+=====================+===========+===================== | | |velocities | object | number | diameter | miles | distribution | | | per sec. | ----------------+--------------------+---------------------+-----------+--------------------- . solar system | | | | | | | | a. planets |eight | , to , mi. | to |revolving in nearly | | | miles | circular orbits | | | per sec. | about the sun. b. sun | | , mi. | - / mi. |travelling through | | | | galactic systems of . stars | | | | stars (milky way). | | | | a. helium | | | mi. | (bluish) | | | | b. hydrogen |approx. | dwarfs | mi. |all types of stars are (white) | , , , (two | , to | | more or less crowded c. solar | thousand million) | , , mi. | - mi. | toward plane of milky (yellow) | | | | way in lens shaped d. type m |including all types | giants | mi. | formation. (milky (red) | | , , to | | way possibly a spiral | | , , mi. | | nebula.) . nebulæ | | | | | | | | a. diffuse or |numerous |very extensive, many |very low |in or close to milky gaseous | | light years. | | way. | | | | b. spiral |approx. , |size and distance |average |far external to milky | (seven hundred | doubtful but | mi. | way and numerous | thousand) | very great. | | near its poles. | | | | c. planetary |one hundred and |several times that |average |in or close to milky | fifty ( ) | of the solar system | mi. | way. | | on the average. | | | | | | . globular star|about one hundred | , - , |very high |external to milky way clusters | known | light-years. | | and spherically | | | | distributed about it. | | | | . magellanic |two (greater and |thousands of |very high |far beyond milky way. clouds | lesser) | light-years. | | ================+====================+=====================+===========+===================== xxviii the evolution of the stars--from red giants to red dwarfs the most casual of star-gazers is aware that the stars differ one from another in color and in brightness. there are red stars, yellow stars, white stars and bluish-white stars. there are the brilliant stars of first magnitude such as vega, capella and antares, and there are, on the other hand, stars so faint that they can barely be glimpsed with the most powerful telescopes. in general the most brilliant stars are the nearest and the faintest stars are the most distant, but there are many exceptions to the rule, since there are stars that appear faint even when comparatively near because they are small and shine with a feeble light. such a star is the faint, sixth-magnitude star, cygni, one of the nearest of all the stars. again, there are stars in far-distant clusters visible only in powerful telescopes that in actual brightness exceed our own sun several thousand times and in volume several million times. a star the size of the sun would be invisible in the most powerful telescope in existence if it were at the distance of many stars in the milky way or globular star clusters. stars differ in color because they differ in temperature. we are all aware of the fact that a piece of iron when heated first glows a deep red, then appears yellowish in color and finally attains to white heat. it is the same among the stars. the red stars are the coolest of all the stars and the bluish-white stars are the hottest of all the stars, while intermediate between them in temperature come the yellow and the white stars. now as the biologist and the geologist see in this world of ours signs of evolution, or gradual development and change from the simple to the more complex forms, and of growth and decay, so the astronomer sees among the stars signs of a continuous, progressive development from one type of star to another. stars share in the general evolution that is the law of the universe, and are born, reach the height of their development, decline to old age and die. within the past few years important astronomical discoveries have been made that show the true order of this evolution of the stars. it was believed not so long ago that the blue-white helium stars--the type b stars the astronomers called them, or the orion stars, since there are so many stars of this type in the constellation of orion--were not only the hottest but also the youngest of the stars and that they represented the first stage in the development of a star from a primitive gaseous nebula such as the great orion nebula. it is now known that these brilliant, hot helium stars represent the peak of development of the most massive of all the stars and not the first stages in the development. a star, it is now known, comes into existence as a giant, reddish star of enormous size and of a density only about one-thousandth that of the earth's atmosphere at sea-level. it is inconceivably tenuous or rare, and its temperature is comparatively low, about , ° centigrade or less. it is not evolved from the luminous, gaseous nebulæ because red stars are never found associated with the gaseous nebulæ, as are the blue-white stars. the origin of these red giant stars is uncertain, but it is possible that they may be gradually evolved in some manner from the dark clouds of obscuring matter or dark nebulæ that exists so abundantly in the heavens. in the next stage of its development the deep-red giant star increases in temperature as it contracts under the action of gravitation and its color gradually changes from red to yellow. its density increases slightly and its volume decreases. it is now a yellow giant star. as the evolution progresses in the course of ages the star continues to contract, its temperature increases greatly as does also its density and it continues to decrease in volume. it is now a brilliant white star, a hydrogen star, so called because its spectrum is chiefly characterized by the lines of hydrogen. as the star contracts under the gravitation of its parts and increases in temperature and density there comes more and more into play an important factor that has a great effect upon its future development. this is light-pressure or radiation pressure which acts in opposition to gravity and exerts a strong outward pressure upon matter within the depths of the star, tending to push it outward from the center where the temperature is greatest and the light is most intense. it is a most interesting fact that if the mass of a star, that is the quantity of matter that it contains, exceeds a certain value the pressure of light or radiation within it overbalances the gravitational attraction that draws matter towards its center and the star disintegrates or ceases to exist as a star. this accounts for the fact that the stars differ very little among themselves in the quantity of matter that they contain, that is, in their masses, though they may differ enormously in size. stars that exceed a certain mass will become unstable and this may account for the association of luminous nebulæ with the hottest of all stars, the nebulæ possibly being puffed off from the surfaces of these stars under the action of radiation pressure. after a star has reached the height of its development as a bluish-white helium star with a temperature of something like , ° centigrade and a density about one-tenth that of the sun, it begins to lose heat and to cool gradually though it continues to contract and increase in density. it is now on the descending scale of evolution and is to be counted among the dwarfs instead of the giants. a brilliant blue-white helium or orion star is about one hundred times more luminous than the sun, and its diameter is about ten times that of the sun. our own sun, we find, is on the descending scale of stellar evolution. it is a yellow dwarf star of temperature about , ° centigrade and density one and one-fourth that of water, which is probably about as great a density as is attained by any star since even the non-luminous planets jupiter and saturn have lower densities than the sun. the last stage in the development of a star is represented by the dwarf red star of high density and low temperature. the diameter of the dwarf red star probably averages about five hundred thousand miles and its temperature is , ° centigrade or less. after this we have the extinct stars, similar possibly to our planet jupiter, though considerably larger, with a dense gaseous atmosphere and a certain degree of internal heat. we have traced the evolution of a star from a red giant to a red dwarf through the intermediate stages from yellow giant to a giant helium star with increasing temperature and thence to yellow dwarf and red dwarf as the temperature decreases. only the most massive stars pass through this entire chain of evolution. stars of small mass never attain to the splendor of brilliant blue-white helium stars, but begin to decrease in temperature and brightness before this stage is reached. the time required for the evolution of a star from red giant to red dwarf is not known, but it must be very great. the age of the earth, which is probably equal to that of the solar system, is estimated as something like one thousand million years. it is probable that the average life of a star far exceeds this limit. xxix double and multiple stars the plan of the solar system which consists of a central sun encircled by satellites that are far inferior to their luminary in size, and that move about it in orbits that are almost perfect circles, is not the only, nor possibly, even the most general one in the universe. sweeping the heavens with powerful telescopes one is astonished to find that myriads of stars can be separated into two or more physically connected suns that are often, moreover, of exquisitely tinted and contrasting shades. green and red, orange and blue, white and golden or white and blue pairs exist in profusion, and strange to say there are well-authenticated instances of color changes taking place temporarily within the same system. a pair of white stars has been known to change within a few decades, first to golden yellow and bluish green and then to orange and green. the famous pair catalogued as " herculis" was noted to change from green and red to a palish yellow and back to the original strongly contrasting hues within the course of a single year, while at another time they appeared to be a perfectly white pair. at the present time both of these stars are decidedly yellowish in color. such changes in hue are probably due to temporary disturbances in the atmospheres of the stars, possibly of an electrical nature or to sudden or unusual outbursts of activity, concerning the origin of which we are as much in doubt as we are of the cause of the sun-spot cycle and periodic variation in the intensity of radiation of our own sun. temporary changes in the color of the components of double star systems sometimes take place when the two stars approach their "periastron" or point of nearest approach. owing to the great eccentricity of the orbits of double stars, such stars are anywhere from twice to nineteen times as near to each other at periastron as they are at "apastron," or point of greatest departure. such great changes in the relative distances of two physically connected suns would produce marked changes in the intensity of the tides raised upon each of them by their mutual gravitational attraction and unusual outbursts of gases or electrical excitement in the atmospheres of the stars might cause very noticeable changes in the color of these stars as they drew nearer to each other, which would subside as they receded toward apastron. in addition to "visual" double or multiple stars, there exists a very extensive class of stars known as "spectroscopic binaries," in which the two components are so close to each other that even the most powerful telescopes cannot divide them. it is only from the shifting of the lines of their overlapping spectra, caused by their alternate motion toward and from the earth as they revolve about their common center of gravity, that their duplex nature is revealed to us. in some instances one member of the system is so faint that its spectrum is not visible and its presence is disclosed only by the shifting of the lines of the bright star. according to doppler's law, when a star is approaching the earth the lines of its spectrum shift toward the blue end of the spectrum, and when the star is receding from the earth the lines are shifted toward the red end of the spectrum. the amount of this shift can be very accurately measured, and gives the relative velocities of the stars in their orbits directly in miles per second. knowing in addition, by observation, the period of mutual revolution of the stars, it is possible to find the dimensions of these spectroscopic binary systems compared to our own solar system, and also the masses of the stars compared to the mass of our own sun. if the spectrum of the fainter star is not visible, only the velocity of the brighter star with respect to the center of gravity of the system can be found and the mass found for the system comes out too small. in such cases we can obtain only a lower limit for the mass of the system. then, too, it must be remembered that these systems of stars lie at all angles with reference to our line of sight, and so we rarely see the orbits in their true form. the measured velocities are as a result smaller than the true velocities, and on the average amount to only sixty per cent. of the true orbital velocities. the calculated masses of spectroscopic binary stars are, therefore, in general only about sixty per cent. of the true masses. it has been found from calculating the masses of a number of binary systems, that the combined masses of the stars in these systems do not differ very greatly among themselves, nor as compared to our own sun, though in light-giving power these stars may differ hundreds, thousands, even millions of times. for instance, there are stars that give only one ten-thousandth part of the light of our own sun, and other stars that give ten thousand times as much light as the sun. moreover, there are many instances of physically connected stars differing thousands of times in luminosity, though in mass, or quantity of matter found in the stars, they differ only two or three times. why this is so remains one of the great mysteries of the heavens, and makes it extremely difficult to give any satisfactory theory of the origin of double-star systems. it has never been explained satisfactorily why of two suns physically connected and, therefore, presumably originating at the same time, one should be radiating with the greatest intensity, while the other is practically an extinct sun, in spite of the fact that the quantity of matter in the two bodies differs but slightly. in a few systems the plane in which the stars revolve passes so nearly through the earth that the two stars temporarily eclipse one another during each revolution. such systems are called _eclipsing binaries_. to such a system belongs the famous _algol_. its light waxes and wanes periodically with the greatest punctuality in a period of ^d ^h . ^m, owing to its temporary eclipse by a very large but extremely faint attendant sun. the diameter of the faint star is slightly greater than the diameter of the bright star which is about one million miles in extent. the distance between the _centers_ of the stars is only about three million miles, which brings their surfaces within two million miles of each other. the masses of the two stars are in the ratio of two to one, the brighter and more massive star being about half as heavy as our own sun, though its density is only about two-tenths that of the sun. the density of the fainter star is still less, being only about half that of the brighter star. very low density of both components and extreme faintness of one member compared to the other, appears to be a very general characteristic of closely associated eclipsing and spectroscopic binary stars. among the extremely hot and brilliant helium and hydrogen stars, spectroscopic binaries exist in great numbers. in fact, among these types there appear to be as many binary and multiple systems as there are systems of isolated suns. sometimes these close binary stars are egg-shaped or oval and revolve rapidly almost in contact about their common center of gravity. inhabitants of satellites of such a system would see in their heavens the, to us, strange and startling phenomenon of _two_ suns, each equal to our own or even greater in size, whirling rapidly about each other and separated by a space comparable in extent to their own diameters. eclipses in such a system would be of daily occurrence, and, if one star were dark, would produce for the satellite world the same effect of alternate day and night that results from axial rotation of a satellite. the two hemispheres of the faint companion sun would be very unequally illuminated owing to the fact that the side turned toward the brilliant sun would always reflect its neighbor's brightness in addition to shining with its own comparatively feeble inherent light, while the opposite hemisphere would shine only by its own dim light, and would, therefore, be in comparative darkness. the spectroscopic binaries generally revolve closely and rapidly about their common center of gravity; there are to be found, on the other hand, among the wider visual doubles, many systems wherein the components are separated by distances comparable to the distances of the outer planets, saturn, uranus and neptune, from the sun. it is evident that the individual stars of such binary systems could not possibly be encircled by any such extensive system of satellites as attends our own sun, though satellites such as our own planet earth, or the inferior planets mercury and venus, might conceivably encircle the individual components of such binary systems at distances not greater than that of the earth from the sun. no planet could safely exist at a much greater distance from one of these suns without being subject to most dangerous perturbations and disruptive tidal forces arising from the vicinity of the second sun. granted that planets might encircle one of these suns at a distance approximating that of venus or our own planet from the sun, the inhabitants of such worlds would behold the strange phenomenon of _two_ suns in the heavens, not almost in contact as in spectroscopic binary systems, but at one time comparatively near and again in opposite portions of the heavens as is the case with the sun and moon in our own heavens. as the planet advanced in its orbit about the ruling sun, the secondary sun would be visible at first by day and again by night. if the two suns were of contrasting hues, as, for instance, green and red, there might appear in the nearby heavens at a distance of one hundred million miles or so a magnificent sun of deep reddish hue, equal to or surpassing our own in splendor, while in a far distant part of the sky, at a distance as great as that which separates us from the planet saturn, might appear a rival sun of greenish hue, smaller and fainter, but nevertheless, hot and extremely brilliant and capable of exerting through its great gravitational attraction a most disturbing effect upon the motion of the planet of its neighbor. at times the rays of the two suns, red and green, would combine to produce a day characterized by terrific heat and intense illumination. again the green orb would rise in the east as the red sun set in the west and night would be turned into a weird, dimly-lighted day by the greenish rays of the secondary sun. compared to the wonders and beauties of the heavens in such a system, our own well-regulated and orderly planet family, undisturbed by the exciting proximity of a rival sun, seems to pale into insignificance. yet we have every good reason to be content with the ordering of affairs within our own solar system, and to feel relief rather than regret at the absence of a secondary sun. in a planet world revolving about one member of a double star system, we may imagine the dread rather than pleasure with which the periodic near-approach of a rival sun would be hailed, and even the possible hurried migration from exposed to sheltered portions of the planetary world to escape the rapidly increasing heat and intensity of light from the approaching sun. in such systems the coming and going of the seasons might indeed be a matter of life and death to the inhabitants of satellite worlds! within our solar system the masses of the planets are practically negligible compared to the mass of the sun, and it is for this reason that they appear to revolve about the _center_ of the sun. as a matter of fact, no body in the universe revolves about the _exact geometrical center_ of another body, but two mutually attracting bodies revolve in orbits about their common center of gravity, which always lies between the two bodies on the line connecting them and at a distance from each of them that is in inverse proportion to the mass of the body. the moon does not revolve about the _center of the earth_, but about the _center of gravity_ of the earth and moon, which lies on the line connecting the two bodies and at a distance from the earth's center that is one eighty-first of the distance from the center of the earth to the center of the moon, since this represents the ratio of the masses of the two bodies. this center of gravity of the earth and moon, lies, then, about two thousand miles from the earth's center, and about this point both earth and moon trace out orbits of revolution that are identical in form and differ only in size. in the same way each of the planets of the solar system revolves about the center of gravity of itself and the sun, but the mass of the sun is so far in excess of the combined masses of all the planets that we may consider, for all practical purposes, that the planets revolve about the sun's center, the center of gravity of the system being within the sun, just as the center of gravity of the earth and moon is within the earth. prof. t. j. j. see found from the investigation of forty binary star orbits that the average eccentricity of double star orbits is twelve times as great as the average eccentricity of a planetary orbit, and that the masses of the component suns never differ very greatly. the center of gravity of a binary system, therefore, lies at a great distance from the centers of the stars, and about this point, as a focus, the stars move in orbits that are exactly similar in form but differ in size in inverse proportion to the ratio of the masses. since the orbits of binaries are, moreover, very highly eccentric, the two suns are, as we have said, anywhere from two to nineteen times nearer to each other at periastron than they are at "apastron." we have spoken so far only of systems of two associated suns, but many systems exist in which three or more sun-like bodies are in revolution about a common center of gravity. frequently two fairly close suns are in revolution about a common center of gravity, in a period, say, of fifty or sixty years, while a third sun revolves at a comparatively great distance about the center of gravity of itself and the first pair in a period of several hundred years. or possibly the third sun also possesses a close attendant and the two pairs revolve in a period of great length about a common center of gravity. such, for instance, are the systems of _zeta cancri_ and _epsilon lyræ_. in the former system the closer components revolve rapidly about their center of gravity in a period of about sixty years, while the remote companion shows irregularities in its motion that indicate that it is revolving about a dark body in a period of seventeen and a half years, while the two together are revolving very slowly in a period of six or seven centuries, about a common center of gravity with the first pair in a retrograde direction. the wider pair of _epsilon lyræ_ is a naked-eye double for it can be seen as a double star by a keen eye, while even a three-inch telescope will separate each of the components into a double star. so extensive is this system that the periods of revolution of the closer components occupy several centuries, one pair appearing to revolve about twice as rapidly as the other, while the period of revolution of the two pairs about a common center is probably a matter of thousands of years. the gap that separates the two pairs may be so great that light requires months to cross it. these multiple systems are by no means exceptional. they are to be found in profusion among the brilliant _orion_ stars. they have been referred to as "knots" of stars and it has been suggested that they may have originated as local condensations in one vast nebulous tract. a system of only two components appears to be the exception rather than the rule, groups of several connected suns being more numerous than single pairs. in all of these double and multiple systems there exists the possibility of minute satellites, such as our own earth, in attendance upon some one component of the system. such tiny bodies shining only by reflected light from a nearby brilliant sun would be hopelessly invisible in the most powerful telescope. we can only assume that it is far more reasonable to believe in than to disprove the existence of such satellites. our own solar system, then, represents neither in its mechanical nor physical features, the only possibilities for the maintenance of life; it can neither be considered a unique form, nor even the most generally prevalent form in the universe. xxx astronomical distances the grandeur of the scale upon which the visible universe is fashioned lies almost beyond human comprehension. in measuring the vast extent of our own solar system, which is but a single unit in the system of the stars, we may have recourse to some earthly standard of measurement, such as the mile. but when we desire to express in terms of units that can be grasped by our imagination, the distances of the stars that lie far, far beyond, we find that all ordinary standards of measurement become utterly inadequate for our purpose. in the measurement of celestial distances within the solar system the unit employed is either the familiar mile or kilometer or the "astronomical unit," which is the mean distance from the earth to the sun (ninety-two million nine hundred thousand miles in round numbers). in the measurement of distances _beyond_ the solar system the unit employed is either the _light-year_ or more recently the _parsec_, which is rapidly replacing the light-year among astronomers. a "light-year" is the distance that light, with its finite but almost unimaginable velocity of one hundred and eighty-six thousand miles _per second, travels in a year_. it is equal in round numbers to sixty-three thousand times the distance from the earth to the sun or approximately six thousand billions of miles. the parsec is equal to three and twenty-six hundredths ( . ) light-years, and it is approximately two hundred thousand times the distance from the earth to the sun. it is "the distance of a star with the _parallax_ of a second," a fact which its name, parsec, conveys to us. in other words, at the distance of one parsec the distance from the earth to the sun, "the astronomical unit," would subtend an angle equal to one second of an arc. this angle is spoken of as the parallax of the star. the larger the parallax, that is, the larger the angle the astronomical unit or radius of the earth's orbit subtends, viewed from the star, the nearer the star is to us. the fact that there is no known star within one parsec, or three and twenty-six hundredths light-years, of the sun shows the immensity of the scale of the universe of stars. before considering the distances of the stars and the extent of the sidereal system of which our sun and his satellites form a part, let us undertake to express the distance of the sun, moon and planets from the earth and the extent of the solar system in terms with which we are familiar. the nearest to the earth of all celestial bodies is its satellite, the moon. so near is the moon that if we should make on some great plain a model of the solar system in which the astronomical unit, the distance from earth to sun, would be four hundred feet, the distance between the earth and moon would be only one foot. on the same scale the most distant planet neptune would be two and one-quarter miles away. granted that it were possible to escape the earth's gravitational bonds and to travel by our swiftest means of conveyance, the airplane, through interplanetary space, let us consider how long it would take us to reach the moon, sun and planets if our speed were maintained at a uniform rate of two hundred miles an hour. an airplane traveling at this rate would circumnavigate the earth in a little over five days and would reach the moon in seven weeks. a trip to the sun, however, would take fifty-three years. after traveling for fourteen and a fraction years we would pass the orbit of venus and eighteen years later the orbit of mercury. if we preferred to travel outward from the earth in the direction of mars and the outer planets instead of toward the sun, more than twenty-seven years would elapse before we would reach the orbit of mars. an airplane journey to jupiter would be a matter of more than two hundred years, to saturn four hundred and fifty years, to uranus nearly one thousand years, and to neptune, about one thousand five hundred years. to cross the solar system on the diameter of neptune's orbit in an airplane, traveling day and night without stopping at the rate of miles per hour would take more than three thousand years. the sun's attraction reaches far beyond neptune's orbit, however. there are comets belonging to the solar system compelled by the sun's attraction to accompany him on his travels through space that return periodically to the immediate vicinity of the sun from regions far beyond the orbit of neptune and there is also the possibility that one or more undiscovered planets may travel around the sun in orbits far exterior to neptune's orbit. measured in terms of familiar units, such as are employed for the measurement of distances on our own planet, the extent of the solar system is tremendously great. viewed from neptune, the sun is so far away that it presents no appreciable disk. it is in this sense star-like to the neptunians, but at the distance of neptune the stars appear no more brilliant and no nearer than they do to us. to neptune the sun, though star-like in form, supplies a very appreciable quantity of light and heat (one nine-hundredth of the amount the earth receives) while the amount of light and heat that neptune receives from the nearest stars is entirely inappreciable. when our airplane reaches neptune after a journey of one thousand five hundred years, it is, as it were, just clearing the ground for its flight to the stars. to cover the intervening space to the nearest star, traveled by light in four and a third years, an airplane would need _fourteen and a half million years_. in that time the solar system itself would be in some far distant part of the universe, since it is speeding onward through space at the rate of twelve miles a second or about four astronomical units a year. changing now our unit of measurement that we may express interstellar distances in comprehensible numbers, we prepare to travel from the earth to the stars with the velocity of light. with this velocity, one hundred and eighty-six thousand miles per second, we circumnavigate our globe in one-seventh of a second, reach the moon in one and a fourth seconds and the sun in eight minutes. in a little over four hours we pass the orbit of neptune and are started on our journey to the stars, penetrating further and further into interstellar space. for a year we travel and reach not a single star though we are speeding ever onward with the velocity of light. we have now covered the distance of one light-year, which means that the waves of light from the sun we have left behind must travel for a year before they reach us. we continue our journey and find ourselves next at a distance of one parsec from the sun. we have traveled a distance of approximately three and a quarter light-years, and were it possible to see the earth as well as the sun at this distance, the two would appear to be but one second of arc apart, a distance that requires the most careful adjustment and manipulation of the telescope to measure accurately. we are still one light-year distant from alpha centauri, the nearest of the bright stars. a few of the stars will now appear somewhat brighter than they appeared to us on earth, but the majority of the stars appear just as we see them here and the forms of the constellations remain practically unchanged in appearance, for we are only beginning our journey through the sidereal universe and our position in it has only shifted by a very slight amount. if we should continue our journey to the immediate vicinity of alpha centauri, we would find that it is not like our own sun, a single star, but is a binary star consisting of two suns in revolution around their common center of gravity. the distance of this binary system from the solar system has been measured with considerable accuracy and is known to be four and a third light-years. though there may be a few faint stars or non-luminous stars nearer to us than alpha centauri, this star has long held the distinction of being the nearest of the stars. as the sun continues his journey through the universe the two stars, alpha centauri and our sun, will finally draw away from each other after many ages have passed and some other sun of space will be our nearest star. the distances that separate the stars from each other probably average as great as the distance from the sun to alpha centauri. within a sphere whose center is at the earth and whose radius is five parsecs, or about sixteen light-years, there are only about twenty known stars. there is, therefore, small chance of collision among bodies that are so small in proportion to the tremendous intervals of space that separate them from each other. there is ample room for the individual stars to pursue their journey through space without interfering with each other's motion so long as they are as widely scattered as they appear to be in this portion of the universe. the fact that our own sun has continued its journey through the universe for some hundreds of millions of years without any catastrophe such as would result from closely approaching or colliding with another sun of space shows how enormous is the scale upon which our sidereal system is fashioned. stars that are ten, fifty or even one hundred light-years from the earth are our nearest neighbors in space. they are the stars that show a slight displacement in the heavens or measurable parallax, viewed from opposite sides of the earth's orbit. there are probably a thousand stars among the hundreds of millions of stars within reach of the greatest telescopes whose distances have been determined in light-years by direct measurement of their displacement in the heavens resulting from the change of position of the earth in its orbit. the most distant of the stars are apparently immovable in the heavens showing neither the effect of the sun's motion or their own motion through space. methods for finding the distances of many far remote stars and star-clusters have been devised, however, and some comparatively recent investigations have given results for the distances of these objects indicating that the diameter of the system of stars to which our sun belongs is approximately three hundred thousand light-years. it is difficult to grasp the full significance of this fact. it means that hundreds of millions of the suns of space throng the visible universe at distances from us and from each other running into hundreds, thousands and even hundreds of thousands of light-years. the light waves from some tiny object that we view today in one of our great reflectors may have started on their journey through space over one hundred thousand years ago when men of the old stone age inhabited our planet earth! astronomers have found as a result of their investigations that the sidereal system to which our solar system belongs is in the form of a flattened spheroid with its longest axis in the plane of the milky way. the extent of this star system composed of hundreds of millions of individual suns in addition to nebulæ and clusters is probably something like three hundred thousand light-years along its longest axis, while globular star clusters lying above and below its central plane are estimated to be at distances from it ranging from ten thousand to two hundred thousand light-years. this entire organized system is our sidereal universe. space beyond is unexplored. the globular star clusters are among the most distant celestial objects so far discovered. the spiral nebulæ may be entirely within the limits of this system or they may be even more distant than the globular clusters for their distances are not known as yet. there is a possibility that our sidereal universe, vast as it is known to be, may be but a unit in some still greater unit and that other similar systems lie beyond the reach of existing telescopes at unimaginable distances. the mind of man is overwhelmed by the thought of sidereal systems as vast as our own lying far beyond his ken. whether or not such external systems do exist and are with our own sidereal system units in some still vaster creation we cannot know. so vast, indeed, is this one visible universe of ours that the mind of man, accustomed to earthly standards, cannot comprehend its magnitude or the infinitesimal size of our whole solar system compared to it. xxxi some astronomical facts worth remembering kepler's three laws of planetary motion: i. the planets move in ellipses with the sun at one focus. ii. the radius vector of a planet (line adjoining sun and planet) sweeps over equal areas in equal times. iii. the square of the time of revolution (the year) of each planet is proportional to the cube of its mean distance from the sun. * * * * * sir isaac newton discovered that the law of gravitation extends to the stars. that is, every mass in the universe attracts every other mass with an attraction directly proportional to the product of the masses and inversely proportional to the square of the distances between them. * * * * * ocean tides are caused by the difference between the attraction of the sun and moon for the main body of the earth and their attraction for different particles of the earth's surface. the tide-raising force of the disturbing body is proportional to its mass and inversely proportional to the cube of its distance. the tides produced by the sun are, therefore, only two-fifths as great as the tides produced by the moon. * * * * * the celestial sphere is an imaginary sphere of infinite radius, with the earth at its center, upon which the celestial bodies are considered to be projected for convenience in determining their positions with respect to fixed points of reference in the heavens. the north and south poles of the heavens are the points on the celestial sphere directly above the north and south poles of the earth. the celestial equator is the great circle in which the plane of the earth's equator intersects the celestial sphere. it passes through the east and west points of the horizon and through the zenith--or point directly overhead--at the earth's equator. the ecliptic is the great circle in which the plane of the earth's orbit intersects the celestial sphere. the celestial equator and the ecliptic are inclined to each other at an angle of - / °, which is called the obliquity of the ecliptic. the two points in which the celestial equator and the ecliptic intersect are called respectively the vernal equinox and the autumnal equinox. the vernal equinox is an important point of reference on the celestial sphere. as the position of a point on the earth's surface is determined by its longitude and latitude so the position of an object on the celestial sphere--star, sun, planet--is determined by its right ascension and declination. the declination of a celestial object is its distance north or south of the celestial equator, measured in degrees, minutes and seconds of arc, on a great circle of the celestial sphere passing through the object and north and south poles of the heavens. these great circles are called hour circles and they correspond to the meridians or circles of longitude on the earth's surface. the declination of an object in the heavens corresponds to the latitude of a point on the earth's surface. the right ascension of a point on the celestial sphere corresponds to the longitude of a point on the earth's surface. it is measured--as longitude is measured--in degrees, minutes and seconds of arc or in hours, minutes and seconds of time--eastward along the celestial equator from the hour circle passing through the vernal equinox to the foot of the hour circle passing through the object. the hour circle passing through the vernal equinox is the zero meridian for the celestial sphere just as the meridian of greenwich is the zero meridian on the earth's surface. * * * * * the mean distance of the earth from the sun is , , miles and is called the astronomical unit. the sun with its satellites advances through the universe at the rate of astronomical units in a year or approximately one million miles a day. the parallax of a star is the angle at the star subtended by the radius of the earth's orbit, , , miles, or the astronomical unit. it is, in other words, the angular distance between the earth and sun as viewed from the star. the larger the parallax the nearer the star. the largest known stellar parallax is that of alpha centauri and its value is ". . the light-year is the distance that light travels in one year. it is equal to about , astronomical units or nearly six trillion ( , , , , ) miles. the velocity of light is , miles per second. the parsec is equal to . light-years. it is the distance of a star that has a parallax of one second of arc. the apparent magnitude of a star is its apparent brightness estimated on a scale in which a difference of one magnitude corresponds to a difference in brightness of . , or the fifth root of one hundred. a difference of five magnitudes corresponds to a difference one one hundredfold in brightness, of ten magnitudes to ten thousandfold in brightness. in exact measurements on this scale magnitudes are estimated to tenths. stars that are one magnitude brighter than stars of the standard first magnitude are of the zero magnitude and stars still brighter are of negative magnitudes. sirius is a star of the - . magnitude. jupiter at opposition is of - . magnitude and venus at greatest brilliancy of - . magnitude. the sun on this scale of comparative brightness is of the - . apparent magnitude. the faintest stars visible in the most powerful telescope in the world--the -inch mt. wilson hooker telescope--are of the twentieth magnitude. the _absolute_ magnitude of a star is its apparent magnitude at the standard distance of ten parsecs or . light years. the absolute magnitude of the sun is five. that is, the sun would be a fifth-magnitude star at the standard distance of . light-years. the absolute magnitudes of stars indicate how bright they would be relatively if they were all at the same standard distance. apparent magnitudes indicate how bright the stars appear to be at their true distances. * * * * * the mean distance of the moon from the earth is approximately , miles or sixty times the earth's radius. the sun is four hundred times farther away than the moon and its diameter is about four hundred times greater than the moon's diameter. the nearest star is about , times more distant than the sun, and the most distant known object, the globular star cluster, n.g.c. , is about fourteen billion times more distant than the sun. the earth is a spheroid flattened at the poles and its polar diameter is about twenty-seven miles shorter than its equatorial diameter. an object weighs less at the poles than at the equator. the earth's interior is as rigid as steel and probably consists of a core of magnetic iron surrounded by an outer stony shell. eclipses of the sun occur when the moon passes between the earth and sun. they can only occur at the time of new moon. there must be at least two solar eclipses every year separated by an interval of six months and there may be as many as five solar eclipses in a year. eclipses of the moon occur when the earth comes between the sun and moon, and the moon passes into the earth's shadow. eclipses of the moon can only occur at full moon. there may or may not be eclipses of the moon every year. the greatest number of eclipses than can occur in any one year, solar and lunar combined, is seven and the least number is two and in that case they are both solar eclipses. the sun is a yellow, dwarf star of a density of one and one-fourth that of water and with a surface temperature of about , ° f. except in sun-spot regions where the temperature is about , ° f. it is probably gaseous throughout. the sun, as well as the planets, rotates on its axis and different portions of the surface rotate at slightly different rates. the average period of the rotation of the sun on its axis is about twenty-six days. the sun is a variable star with a twofold variation. one is of long period during the eleven-year sun-spot cycle with a range of from three to five per cent. the other is a short irregular variation with a period of a few days, weeks or months and a range of from three to ten per cent. sun-spots are solar cyclones and appear black only by contrast with their hotter and brighter surroundings. they come in eleven-year cycles (approximately) with periods of maximum and minimum appearance. the brightness and blue color of the sky is due to the scattering of sunlight by the molecules of oxygen and nitrogen in the earth's upper atmosphere. if there were no atmosphere the skies would appear black except in the direction of the heavenly bodies, which would be visible by day as well as by night. the solar corona is the rare outer envelope of the sun and it is visible only during a total eclipse of the sun. it is partly of an electrical nature and it varies in form during the sun-spot cycle. it often extends to a distance of several solar diameters on either side of the sun. the warmth and the habitability of the earth's surface is due to the presence of water-vapor and carbon-dioxide in the atmosphere. without these substances in the atmosphere life on the earth's surface would be impossible. half of the earth's atmosphere and all clouds lie within seven miles of the earth's surface, and at high elevations above the earth the temperature is many degrees below zero. the temperature of space approaches the absolute zero of - ° f. the only planets in the solar system with the exception of the earth that might possibly support life are venus and mars. stars shine by their own light but planets shine only by reflected light from the sun. * * * * * if the earth were represented by a six-inch school globe the sun would be on the same scale a globe fifty-four feet in diameter. mercury would be a small ball two and a third inches in diameter. venus would be another six-inch globe. mars would be a ball about the size of a baseball, three and a fifth inches in diameter. the moon would be about the size of a golf ball, one and a half inches in diameter. the largest asteroids would be the size of marbles. average-sized asteroids would be the size of shot and the smallest would be merely grains of sand. jupiter would be a huge globe standing as tall as a man five feet six inches in height. saturn would be a smaller globe four and a half feet in diameter and its ring system would extend to a distance of five and a half feet on either side of the globe. uranus would be represented by a globe almost exactly two feet in diameter and neptune would be a slightly larger globe with a diameter of two feet two and a half inches. the satellites of the outer planets would range in size from tennis and golf balls for the largest, to marbles for the smaller and grains of sand for the smallest. on the same scale of measurement the distance of the six-inch globe of the earth from the fifty-four foot globe representing the sun would be one and one-tenth miles. the moon would be placed fifteen feet from the earth-globe and the diameter of the solar system on the same scale measured across the orbit of neptune would be sixty-six miles. the nearest star on this scale would be three hundred thousand miles away. * * * * * if the distance from the earth to the sun is taken as one inch so that the scale of the universe is reduced six trillion times, the diameter of the solar system across neptune's orbit is five feet and the distance of one light-year comes out almost exactly equal to one mile. the nearest star to the five-foot solar system would be four and a third miles away; the most distant known object would be two hundred and twenty thousand miles away, and the extent of the visible universe would be three hundred thousand miles. on the same scale the diameter of our sun would be about one hundredth of an inch and the diameters of the giant stars antares and betelgeuze would be four inches and two and three-fourth inches respectively. to see the earth we would need a microscope. tables table i the principal elements of the solar system =============+=============+========+============+========+============+=========== |mean distance from sun| |velocity| | +-------------+--------+ | in | |inclination | |relative| period | orbit |eccentricity| of planet | in miles | to | of |(miles | of | orbit to | |earth's | revolution | per | orbit | ecliptic | |distance| |second )| | ------------+-------------+--------+------------+--------+------------+----------- mercury | , , | . | . days | to | . | ° ' venus | , , | . | . days | . | . | earth | , , | . | . days | . | . | mars | , , | . | . years| . | . | asteroids[ ]| ...........| . - . | ......... | ...... | . to . | ° to ° jupiter | , , | . | . years| . | . | saturn | , , | . | . years| . | . | uranus | , , , | . | . years| . | . | neptune | , , , | . | . years| . | . | ============+=============+========+============+========+============+=========== footnote: [ ] about have been discovered up to the present time. the principal elements of the solar system (continued) =========+========+==========+==========+========+============+==========+==========+============+=========== | mean | | |density | surface |velocity |reflecting| period |inclination name |diameter| mass | volume |relative| gravity |of escape | power | of | of | in +----------+----------+to that | (relative |(miles per| in | axial | equator | miles | relative to earth's |of water| to earth's)| second) | per cent | rotation | to orbit ---------+--------+----------+----------+--------+------------+----------+----------+------------+----------- sun | , | , | , , | . | . | | ..... | d. h.| ° ' moon | , | . | . | . | . | . | | d. . h.| mercury | , | . | . | . ? | . ? | . | | d. ? | ? venus | , | . | . | . ? | . | . | | ? | ? earth | , | . | . | . | . | | | h. m. | mars | , | . | . | . | . | . | | | asteroids| - [ ]|very small|very small| . |. to . |. to . | | ..... | ..... jupiter | , | . | | . | . | | | ± | ° saturn | , | . | | . | . | | | ± | ° uranus | , | . | | . | . | | | ± | ? neptune | , | . | | . | . | | | ? | ? =========+========+==========+==========+========+============+==========+==========+============+=========== footnote: [ ] extreme values. table ii the satellites of the solar system =============+=========+=============+========+=====================+============+========= | | | | | | | |mean distance| | | | |apparent |from planet's|diameter| period of | | year of name |magnitude| center, |in miles| revolution | discoverer |discovery | | in miles | | | | -------------+---------+-------------+--------+---------------------+------------+--------- | | | | | | the earth | | | | | | moon | | , | | days, hours, | | | | | | minutes | | mars | | | | | | . phobos | | , | ? | days, hours, | asaph hall | | | | | minutes | | . deimos | | , | ? | day, hours, | asaph hall | | | | | minutes | | jupiter | | | | | | v. | | , | ? | day, hours, | barnard | | | | | minutes | | i. | . | , | | day, hours, | galileo | | | | | minutes | | ii. | . | , | | days, hours, | galileo | | | | | minutes | | iii. | | , | | days, hours, | galileo | | | | | minutes | | iv. | | , , | | days, hours, | galileo | | | | | minutes | | vi. | | , , | small | days, hours, | perrine | | | | | minutes | | vii. | | , , |very | days, hours, | perrine | | | | small | minutes | | viii. | | , , |very | days, hours, | melotte | | | | small | minutes | | ix. | | , , | ? | years | nicholson | | | | | | | saturn | | | | | | . meimas | | , | | days, hours, | herschel | | | | | minutes | | . enceladus | | , | | day, hours, | herschel | | | | | minutes | | . tethys | | , | | day, hours, | cassini | | | | | minutes | | . dione | | , | | days, hours, | cassini | | | | | minutes | | . rhea | | , | | days, hours, | cassini | | | | | minutes | | . titan | | , | | days, hours, | huygens | | | | | minutes | | . hyperion | | , | | days, hours, | bond | | | | | minutes | | . japetus | | , , | | days, hours, | cassini | | | | | minutes | | . phoebe | | , , | ? | days, hours, | w.h. | | | | | minutes | pickering | . themis | | , | ? | days, hours, | w.h. | | | | | minutes | pickering | uranus | | | | | | . ariel | | , | | days, hours, | lassell | | | | | minutes | | . umbriel | | , | | days, hours, | lassell | | | | | minutes | | . titania | | , | | days, hours, | herschel | | | | | minutes | | . oberon | | , | | days, hours, | herschel | | | | | minutes | | neptune | | | | | | . nameless | | , | | days, hours, | lassell | | | | | minutes | | =============+=========+=============+========+=====================+============+========= rings of saturn ===================+==========+===================+===================+==================== | | distance of | distance of |diameter of ring | | inner edge from | outer edge from | system from outer name | width, | surface of saturn,| surface of saturn,| edge to outer edge, | in miles | in miles | in miles | , miles. -------------------+----------+-------------------+-------------------|thickness of ring | | | | system, about one dark or crape ring | , | , | , | hundred miles. bright ring | , | , | , |size of individual cassini's division | , | , | , | moonlets, probably outer ring | , | , | , | less than three | | | | miles in diameter. ===================+==========+===================+===================+==================== table iii the twenty brightest stars in the heavens =============================+==========+============+============+===========+=========== | | | | passes | | | | on | through | distance name |magnitude | color | meridian |the zenith | in | | | p. m. |in latitude|light-years -----------------------------+----------+------------+------------+-----------+----------- sirius, alpha canis majoris | - . |white |february | s. | . canopus,[ ] alpha argus | - . |white |february | s. | ? alpha centauri[ ] | . |yellow |june | s. | . vega, alpha lyræ | . |white |august | n. | capella, alpha aurigæ | . |yellow |january | n. | arcturus, alpha boötis | . |orange |june | n. | rigel, beta orionis | . |bluish-white|january | s. | ? procyon, alpha canis minoris | . |white |february | n. | achernar,[ ] alpha eridani | . |bluish-white|december | s. | beta centauri | . |bluish-white|june | s. | betelgeuze, alpha orionis | var. |red |january | n. | - ? | . - . | | | | altair, alpha aquilæ | . |white |september | n. | alpha crucis[ ] (double star)| . - . |bluish-white|may | s. | aldebaran, alpha tauri | . |red |january | n. | pollux, beta geminorum | . |yellow |february | n. | spica, alpha virginis | . |bluish-white|may | s. | ? antares, alpha scorpii | . |red |july | s. | fomalhaut, alpha piscis | . |white |october | s. | australis | | | | | deneb, alpha cygni | . |white |september | n. | ? regulus, alpha leonis | . |white |april | n. | =============================+==========+============+============+===========+=========== footnote: [ ] invisible north of ° n. lat. (approximate). table iv a list of the principal constellations . visible in ° north latitude =============================================================== | | | passes name | chief star | on meridian | overhead | or | p. m. | in latitude[ ] | noted object | | (degrees) -----------------+---------------+-------------+--------------- andromeda | great nebula | november | n. aquarius | | october | s. aquila | altair | september | ° aries | | december | n. auriga | capella | february | n. boötes | arcturus | june | n. cancer | praesepe | march | n. canes venatici | cor caroli | june | n. canis major | sirius | march | s. canis minor | procyon | march | n. capricornus | | october | s. cassiopeia | | november | n. cepheus | | november | n. cetus | mira | december | s. columba | | february | s. coma berenices | | may | n. corona borealis | alphecca | july | n. corvus | | may | s. crater | | may | s. cygnus. | deneb | september | n. delphinus | most distant | september | n. | globular | | | cluster | | draco | alpha | august | n. eridanus | achernar | january | ° n. to ° s. gemini | pollux | march | n. hercules | great cluster | july | n. hydra | | april | s. leo | regulus | april | n. lepus | | february | s. libra | | june | s. lynx | | april | n. lyra | vega | august | n. ophiuchus | | july | s. orion | great nebula | february | ° piscis australis | fomalhaut | october | s. pegasus | | november | n. perseus | algol | january | n. pisces | | december | n. sagitta | | september | n. sagittarius | | august | s. scorpio | antares | july | s. serpens | | july | ° n. to ° s. taurus | pleiades | january | n. triangulum | | december | n. ursa major | mizar | may | n. ursa minor | polaris | | n. virgo | spica | june | ° =================+===============+=============+=============== footnote: [ ] the approximate position of the center of the constellation. . invisible in ° north latitude ===========+===============+===========+============== | | on | passes name | chief star | meridian | overhead | or | p. m. | in latitude[ ] | noted object | | (degrees) -----------+---------------+-----------+-------------- apus | | july | s. ara | | july | s. argo navis | canopus | march | s. . carina | | march | s. . puppis | | march | s. . vela | | march | s. centaurus | alpha centauri| june | s. crux | | | (southern | | | cross) | alpha crucis | june | s. dorado | gt. magellanic| | | cloud | february | s. grus | | october | s. hydrus | lesser mag. | | | cloud | | s. indus | | september | s. lupus | | june | s. musca | | june | s. octans | | | s. pavo | | october | s. phoenix | | november | s. telescopium| | july | s. triangulum | | | australe | | july | s. tucana | great cluster | november | s. volans | | march | s. ===========+===============+===========+============== table v pronunciations and meanings of names of stars and constellations . stars =============+====================+==================== name | pronunciation | meaning -------------+--------------------+-------------------- achernar | a-ke´r-när | end-of-the-river aldebaran | al-de´b-ar-an | the hindmost altair | al-ta´r | antares | an-ta´-rez | rival of ares (mars) arcturus | ärk-t´u-rus | bellatrix | bel-la´trix | the female warrior betelgeuze | be´t-el-gerz | | or be´t-el-gez | the arm-pit canopus | cän-o´-pus | capella | ca-pel-la | little she-goat deneb | de´n-eb | denebola | de-ne´b-o-la | the lion's tail fomalhaut | fo´-mal-o | the fish's mouth hyades | hi-a-dez | the rainy ones pleiades | ple´-ad-ez | pollux | po´l-lux | praesepe | pre-se´-pe | the beehive procyon | pro-si´-on | precursor of the dog regulus | reg´-u-lus | the ruler rigel | ri´-gel or ri-jel | sirius | sir´-i-us | the sparkling one spica | spi´-ka | the ear of wheat vega | ve´-ga | =============+====================+==================== . constellations ================+=======================+================== name | pronunciation | meaning ----------------+-----------------------+------------------ andromeda | an-d´rom-e-da | the woman chained aquarius | a-kwa´-ri-us | the water-bearer aquila | a´k-wi-la | the eagle ara | a´-ra | the altar argo navis | ä´r-go-n´a-vis | the ship argo aries | a´-res | the ram auriga | äw-ri´-ga | the charioteer boötes | bo-o´-tez | the herdsman cancer | ca´n-ser | the crab canes venatici | ca´-nez ven-a´t-i-si | the hunting dogs canis major | ca´-nis ma´jor | the greater dog canis minor | ca´-nis mi´nor | the lesser dog capricornus | ca´p-ri-kö´r-nus | the goat cassiopeia | ca´s-si-o-p´e-ya | centaurus | cen-tä´w-rus | the centaur cepheus | se-fe-us | cetus | s´e-tus | the whale columba | col-u´m-ba | the dove coma berenices | co´ma ber-e-ni-ses | berenice's hair corona borealis | co-ro´-na bo-re-a´-lis| the northern crown corvus | cô´r-vus | the crow crater | cr´a-ter | the cup crux | kru´x | the cross cygnus | si´g-nus | the swan delphinus | del-fi´-nus | the dolphin dorado | dôr-a´-do | the goldfish draco | dra´-co | the dragon eridanus | e-ri´d-a-nus | the river eridanus gemini | jem´-i-ni | the twins grus | gru´s | the crane hercules | her-ku-lez | hydra | hi´-dra | the water-snake hydrus | hi´-drus | the serpent indus | i´nd-us | the indian leo | le´-o | the lion lepus | le´-pus | the hare libra | li´-bra | the scales lupus | lu´-pus | the wolf lynx | | the fox lyra | li´-ra | the lyre musca | mus´-ca | the fly octans | o´ct-ans | the octant ophiuchus | o´-fi-u´-kus | the serpent-holder orion | o-ri´-on | the warrior pavo | pä´-vo | the peacock phoenix | fe´-nix | piscis australis| pi´s-sis aus-tra´-lis | the southern fish pegasus | peg´-a-sus | the winged horse perseus | pe´r-se-us or per-sus | pisces | pi´s-sez | the fishes sagitta | sa-ji´t-ta | the arrow sagittarius | sa-jit-ta´-ri-us | the archer scorpio | skô´r-pi-o | the scorpion serpens | ser-pens | the serpent taurus | täu-rus | the bull telescopium | tel-es-cop´-i-um | the telescope triangulum | tri-a´n-gu-lum | the triangle tucana | tu´c-an-a | the toucan ursa major | u´r-sa ma´-jor | the greater bear ursa minor | u´r-sa mi´-nor | the lesser bear virgo | ve´r-go | the maiden volans | vo´l-ans | the flying fish ================+=======================+================== * * * * * transcriber's note: obvious typographical errors have been repaired. _underscores_ surround italicized content. mid-paragraph illustrations were moved near to the text describing the illustrated material. redundant title--astronomy for young folks--on p. was deleted. p. : canst thou bring forth mazzaroth--"canst" is assumed in blank space. p. : brighter object than the nearby star aldebaran--"star" is assumed in blank space. p. : illustration originally stated "see note page ". that statement was removed, and the actual note from page was moved to its place with the illustration. p. : [...]--duplicate of later line "occurred at l'aigle, france, in . between two" appeared at this spot. possible missing text where the line occurred. p. : period of ^d ^h . ^m--carat (^) indicates that the letter immediately following appears as a superscript. data in tables retained as in original, but may be incorrect--for example, the escape velocity of mars, represented as . miles per second in table i, is closer to . . none the story of the herschels a family of astronomers. sir william herschel sir john herschel caroline herschel. "stars numberless, as thou seest, and how they move; each has his place appointed, each his course." milton. prefatory note from the best available sources have been gathered the following biographical particulars of a remarkable family of astronomers--the herschels. they will serve to show the young reader how great a pleasure may be found in the acquisition of knowledge, and how solid a happiness in quietly pursuing the path of duty. on the value of biography it is unnecessary to insist. it is now well understood that we may learn to make our own lives good and honest and true, by carefully and diligently following the example of the good and honest and true who have gone before us. and certain it is that the lessons taught by the lives of the herschels are such as young readers will do well to lay to heart. contents chapter i. the study of astronomy a source of intellectual pleasure--by contemplating the heavens, the mind is led to wonder and adore--a proof of the existence of a creator is afforded by creation--"we praise thee, o lord!"--the beauty of nature--intellectual curiosity--"order is heaven's first law"--value of astronomical study chapter ii. herschel's parents--the two brothers--a musical family--an inventive genius--the brothers in england--herschel as an organist--a laborious life--mechanical ingenuity of william herschel--telescope-making--a sunday misadventure--constructing a twenty-foot telescope--a domestic picture--discovery of a new planet--herschel's combined musical and astronomical pursuits--a thirty-foot telescope--casting the mirror--an explosion chapter iii. the house at datchet--housekeeping details--a devoted sister--life at datchet--herschel's astronomical observations--testing and trying "eyepieces"--the colossal telescope--miss herschel's accident--removed to slough--constructing a forty-foot telescope--brother and sister--heroic self-denial--occupations at slough--royal liberality--an astronomer's triumphs--about the nebulae--investigation of the sun's constitution--the solar spots, and their influence--physical constitution of the moon--lunar volcanoes--arago's explanation--herschel's study of the planets--satellites of saturn--discovery of uranus--and of its six satellites--study of pigott's comet and the comet of --description of the latter--an uneventful life--herschel's marriage--his honours--extracts from his sister's diary--decaying strength--herschel removes to bath--last days of an astronomer--illustration of the ruling passion--death of sir william herschel--his achievements chapter iv. birth and education of sir john herschel--honours at cambridge--first publication--continues his scientific studies--his numerous literary contributions--his devotion to his father's reputation--the forty-foot telescope--herschel's observations on the double and triple stars--on the refraction and polarization of light--catalogue of nebulae and star-clusters--voyage to cape town--letter to miss herschel--study of the southern heavens--return to england--distinctions conferred upon him--his "familiar lectures on scientific subjects"--his description of volcanoes and earthquakes--continual changes in the configuration of the earth--violent earthquakes--phenomena of volcanic eruptions--in mexico--in the island of sumbawa--herschel's theory of volcanic forces--his character chapter v. caroline herschel's devotion to her brother william--her grief and solitariness at his death--reflections on the mutability of human things--aunt and nephew--a parsimonious government--miss herschel's gold medal--south on sir william's discoveries--on miss herschel's devotion--her own astronomical discoveries--her life at hanover--her wonderful memory--anecdotes of sir john herschel--correspondence between aunt and nephew--the path of duty--sir john's visit to miss herschel--reminiscences of early years--a nonogenarian--a christmas in hanover--last days of caroline herschel--her death--her epitaph the story of the herschels. chapter i. of all the sciences, none would seem to yield a purer intellectual gratification than that of astronomy. man cannot but feel a sense of pleasure, and even of power, when, through the instruments constructed by his ingenuity, he finds himself brought within reach, as it were, of the innumerable orbs that roll through the domains of space. he cannot but feel a sense of pleasure, and even of power, when the telescope reveals to his gaze not only the worlds that constitute his own so-called solar system, but the suns that light up the borders of the universe, system upon system, sun upon sun, covering the unbounded area almost as thickly as the daisies cover a meadow in spring. he cannot but feel a sense of pleasure, and even, of power, when he tracks the course of the flashing comet, examines into the physical characteristics of the sun and moon, and records the various phases of the distant planets. but if such be his feeling, it is certainly tempered with awe and wonder as he contemplates the phenomena of the heavens,--the beauty of the stars, the immensity of their orbits, the regularity with which each bright world performs its appointed course, the simplicity of the laws which govern its motions, and the mystery which attends its far-off existence. it has been, said that "an undevout astronomer is mad;" and if astronomy, of all the sciences, be the one most calculated to gratify the intellect, surely it is the one which should most vividly awaken the religious sentiment. is it possible to look upon all those worlds within worlds, all those endless groups of mighty suns, all those strange and marvellous combinations of coloured stars, all those remote nebulous clusters,--to look upon them in their perfect order and government,--to consider their infinite number and astonishing dimensions,--without acknowledging the fulness of the power of an everlasting god, who created them, set them in their appointed places, and still controls them? is it possible to be an astronomer and an atheist? is it possible not to see in their relations to one another and to our own little planet an almighty wisdom as well as an almighty love? could any "fortuitous concourse of atoms" have strewed the depths of space with those mighty and beautiful orbs, and defined for each the exact limits of its movements? alas! to human folly and human vanity everything is possible; and men may watch the stars in their courses, and delight in the beauty of sun and moon, and perceive all the wonders of the sunrise and all the glories of the sunset, without any recognition in their hearts of him who made them--of him in whom we and they alike live and move and have our being! yet it is not the less true that only the devout and thankful heart can adequately and thoroughly sympathize with the love and wisdom and power which are written in such legible characters on the face of heaven. astronomy gives up _all_ its treasures only to him who enters upon its study in a reverent spirit. it affords the purest intellectual gratification only when its pursuits are undertaken with a humble acknowledgment of the littleness of man and the greatness of god. half the wonder, half the mystery of creation is lost, when we fail to recognize the truth that it is governed by eternal laws springing from an almighty intelligence. take the creator out of creation, and it becomes a hopeless puzzle--a dreary problem, incapable of solution. but we restore to it all its brightness, all its beauty, all its charm, when we are able to lift up our hearts with the psalmist and to say: "praise ye the lord. praise ye the lord from the heavens; praise him in the heights. praise ye him, sun and moon: praise him, all ye stars of light. let them praise the name of the lord: for his name alone is excellent: his glory is above the earth and heaven." and it is to be observed that the soul cannot be satisfied without this religious view of nature. the heavens and the earth are as nothing to man, if they do not excite his awe and call forth his thanksgiving. we might almost suppose that it is for this purpose that the sea rolls its waves on the shore, and the violet smiles by the wayside, and the moon floods the night with its silver radiance. as a recent writer has observed,[ ] the beauty of nature is necessary for the perfection of _praise_; without it the praise of the creator would be essentially weakened; our hearts must be roused and excited by what we see. "it may seem extraordinary," adds our authority, "but it is the case, that, though we certainly look at contrivance or machinery in nature with a high admiration, still, with all its countless and multitudinous uses, which we acknowledge with gratitude, there is nothing in it which raises the mind's interest in nearly the same degree that beauty does. it is an awakening sight; and one way in which it acts is by exciting a certain curiosity about the deity. in what does god possess character, feelings, relations to us?--all unanswerable questions, but the very entertainment of which is an excitement of the reason, and throws us upon the thought of what there is behind the veil. this curiosity is a strong part of worship and of praise. to think that we know everything about god, is to benumb and deaden worship; but mystical thought quickens worship, and the beauty of nature raises mystical thought. so long as a man is probing nature, and in the thick of its causes and operations, he is too busy about his own inquiries to receive this impress from her; but place the picture before him, and he becomes conscious of a veil and curtain which has the secrets of a moral existence behind it,--interest is inspired, curiosity is awakened, and worship is raised. 'surely thou art a god that hidest thyself.' but if god simply hid himself and nothing more, if we knew nothing, we should not wish to know more. but the veil suggests that it _is_ a veil, and that there is something behind it which it conceals." [footnote : professor mozley, "university sermons," pp. , .] now, this is the feeling which the study of astronomy very certainly awakens. every day the astronomer discovers something which quickens his curiosity to discover more. every day he catches new glimpses of the almighty wisdom, which stimulate his desire for a further revelation. and all he learns, and all he anticipates learning, combine to produce in him an emotion of awe. what grandeur lies before him in that endless procession of worlds--in that array of suns and stars extending beyond the limits of the most powerful telescopic vision! how marvellous it is! how beautiful! observe the combination of simplicity with power; note how a great principle of "law" underlies the apparent intricacy of eccentric and intersecting orbits. and then the field of inquiry is inexhaustible. the astronomer has no fear of feeling the satiety of an alexander, when he lamented that he had no more worlds to conquer. what newton said of himself is true of every astronomer,--he is but as a child on the sea-shore, picking up a shell here and a shell there, but unable to grasp a full conception of the mighty ocean that thunders in his ears! and, therefore, because astronomy cherishes the feelings of awe and reverence and praise, because it inspires a continual yearning after additional knowledge, because it reveals to us something of the character of god, we conceive that of all the sciences it affords the purest intellectual gratification. certainly it is one of the most absorbing. its attraction seems to be irresistible. once an astronomer, always an astronomer; the stars, we may fancy, will not relax the spell they lay upon their votary. he willingly withdraws himself from the din and gaiety of social life, to shut himself up in his chamber, and, with the magic tube due to the genius of a galileo, survey with ever-new delight the celestial wonders. so was it with tycho brahé, and copernicus, and kepler; so was it, as the following pages will show, with that remarkable family of astronomers--astronomers for three generations--the herschels. chapter ii. in the quiet city of hanover, nearly a century and a half ago, lived a professor of music, by name isaac herschel, a protestant in religion, though presumably of jewish descent. he had been left an orphan at the early age of eleven, and his friends wished him to adopt the vocation of a landscape-gardener; but being passionately fond of music, and having acquired some skill on the violin, he left dresden, his birthplace, in order to seek his fortune; wandering from place to place, until at hanover, in , he obtained an engagement in the band of the guards. soon afterwards he married; and by his wife, anna ilse moritzen, had ten children, four of whom died in infancy. of the others, two--a brother and a sister--lived to distinguish themselves by their intellectual power; and all true lovers of science will regard with reverence the memories of william and caroline herschel. frederick william herschel was born on the th of november . like his father, he displayed an innate musical ability, which was sedulously cultivated and constantly developed; while his general mental training was left to the care of the master of the garrison-school. those who are gifted with a love and a capacity for music sometimes show to little advantage in other pursuits; but such was not the case with william herschel, who progressed so rapidly in all his studies that the pupil soon outstripped the teacher. although, we are told, four years younger than his brother jacob, the two began french together, and william mastered the language in half the time occupied by his senior. his leisure time out of school, when not given up to practice on the oboe and the violin, was devoted to the acquisition, of latin and arithmetic. his father in was present at the battle of dettingen; and the exposure consequent on a night spent on the rain-soaked battle-field afflicted him with an asthmatic complaint and a partial paralysis of the limbs, which darkened for years the musician's peaceful household. he himself, however, was greatly cheered by the musical proficiency of his two sons, and the intellectual refinement of frederick william. "my brothers," says caroline herschel, "were often introduced as solo performers and assistants in the orchestra of the court; and i remember that i was frequently prevented"--she was then a child about five years old--"from going to sleep by the lively criticism on music on [their] coming from a concert, or conversations on philosophical subjects, which lasted frequently till morning, in which my father was a lively partaker, and assistant of my brother william by contriving self-made instruments." she adds that she often kept herself awake in order to listen to their animating remarks, feeling inexpressibly happy in _their_ happiness,--an indication of that devoted and unselfish affection which afterwards consecrated her whole life. but, generally, their conversation branched out into philosophical subjects; and father and son argued with so much fervour, that the fond mother's interference became necessary,--the immortal names of leibnitz, newton, and euler ringing with a clarion-like peal that boded ill for the repose of the younger members of the family. "but it seems," says caroline, "that on the brothers retiring to their own room, where they shared the same bed, my brother william had still a great deal to say; and frequently it happened that, when he stopped for an assent or a reply, he found his hearer had gone to sleep; and i suppose it was not till then that he bethought himself to do the same. the recollection of these happy scenes confirms me in the belief that, had my brother william not then been interrupted in his philosophical pursuits, we should have had much earlier proofs of his inventive genius. my father," she continues, "was a great admirer of astronomy, and had some knowledge of that science; for i remember him taking me, on a clear frosty night, into the street, to make me acquainted with several of the most beautiful constellations, after we had been gazing at a comet which was then visible. and i well remember with what delight he used to assist my brother william in his various contrivances in the pursuit of his philosophical studies; among which was a neatly-turned four-inch globe, upon which the equator and ecliptic were engraved by my brother." * * * * * in , the tranquil family circle was broken up--the hanoverian regiment in whose band william and jacob were engaged having been ordered to england. the parting was very sorrowful; for the invalid father had derived much support as well as enjoyment from the company of his sons. at first, the english experiences of the young germans were somewhat severe. they endured all the pangs of poverty; pangs endured with heroic composure, while william relaxed not a whit in his devotion to the pursuit of knowledge. happily, however, his musical proficiency attracted the attention of lord durham, who offered him the appointment of bandmaster to a militia regiment stationed in the north of england. in this position he gradually formed a connection among the wealthier families of leeds, pontefract, and doncaster, where he taught music, and conducted the public concerts and oratorios with equal zeal and success. in he paid a brief but happy visit to his family, much to the joy of his faithful sister, caroline. returning to england, for which country he cherished a strong affection, he resumed his career of patient industry, and in was appointed organist at halifax. he was now in receipt of an income which secured him due domestic comforts, and enabled him to remedy the defects of his early education. with the help of a grammar and a dictionary he mastered italian. he also studied mathematics and the scientific theory of music, losing no opportunity of adding to his stores of knowledge. in he obtained the lucrative post of organist to the octagon chapel at bath. increased emoluments, however, brought with them increased obligations. he was required to play almost incessantly, either at the oratorios or in the rooms at the baths, at the theatre, and in the public concerts. when his sister caroline joined him, in , she found him immersed in his various labours. for the choir of the octagon chapel he composed anthems, chants, and complete morning and evening services. a part of every day was occupied in giving lessons to his numerous pupils. in truth, he was one of the busiest men in england; yet in all his arrangements he was so methodical that he found time for everything--and time, more particularly, for the studies in which his soul delighted. his life furnishes an admirable example of what may be accomplished by a man with a firm will and a strong purpose, who sets before himself an end to be attained, and controls all his efforts towards its attainment. he toiled so hard as a musician, because he wanted to be something more. every spare moment of the day, and frequently many hours of the night, he gave up to the pursuits which were gradually leading him into the path best fitted for his genius. the study of mathematics proved but a preliminary to the study of optics; and an accident made him once for all an astronomer. a common two-foot telescope falling into his hands, revealed to him the wonders of the heavens. his imagination was inspired by their contemplation; with ever-increasing enthusiasm he gazed on the revolving planets, on the flashing stars; he determined to fathom more profoundly the constellated depths. a larger instrument was necessary, and herschel wrote to london for it; but the price demanded proved far beyond the resources of the sanguine organist. what should he do? he was not the man to be beaten back by a difficulty: as he could not buy a telescope, he resolved to make one; an instrument eighteen or twenty feet long, which would reveal to him the phases of the remotest planets. and straightway the musician entered on a multitude of ingenious experiments, so as to discover the particular metallic alloys that reflected light with the greatest intensity, the best means of giving the parabolic figure to the mirrors, the necessary degree of polish, and other practical details. in his eager pursuit he enlisted the services of his loving and intelligent sister. "i was much hindered in my musical practice," she writes, "by my help being continually wanted in the execution of the various contrivances; and i had to amuse myself by making the tube of pasteboard for the glasses which were to arrive from london--for at that time no optician had settled at bath. but when all was finished, no one besides my brother could get a glimpse of jupiter or saturn, for the great length of the tube would not allow it to be kept in a straight line. this difficulty, however, was soon removed, by substituting tin tubes." the work went on famously, as might be expected from so much ardour, perseverance, and ingenuity. of a quaker resident at bath, the musician-astronomer purchased a quantity of patterns, tools, hones, polishers, and unfinished mirrors. every room in the house was converted into a workshop. in a handsomely-furnished drawing-room might be seen a cabinetmaker constructing a tube and stands of all descriptions; while herschel's brother alex was engaged in a bedroom in putting up a gigantic turning-machine. meantime, the claims of music could not be ignored: there were frequent rehearsals for the public concerts; lessons to pupils; the composition of glees and catches, and the like; the superintendence of the practice of the chapel choir; and the study of sonatas and concertos for public performance. but all the leisure that could be made or stolen was occupied in labours which proved their own reward. straight from the concert-platform rushed the musician to his workshop, and many a lace ruffle was torn by nails or bespattered by molten pitch; to say nothing of the positive danger to which herschel continually exposed himself by the precipitancy of his movements. for example: one saturday evening, when the two brothers returned from a concert between eleven and twelve o'clock, william amused himself all the way home with the idea of being at liberty to spend the next day, except the few hours' duty at chapel, at the turning-bench; but recollecting that the tools wanted sharpening, they ran with them and a lantern to their landlord's grindstone in a public yard, where, very naturally, they did not wish to be seen on a sunday morning. but william was soon brought back by his brother, almost swooning with the loss of one of his finger-nails. this incident took place in the winter of , at a house situated near walcot turnpike, to which herschel had removed in the summer of the previous year. here, on a grass plot behind the house, he made active preparations for the erection of a twenty-foot telescope. so assiduous was his devotion to this work, that while he was engaged in polishing the mirror, his sister was constantly obliged to feed him by putting his victuals into his mouth. otherwise he would have reduced himself to a condition of positive emaciation! once, when finishing a seven-foot mirror, he did not take his hands from it for sixteen consecutive hours; for in these days machinery had not been devised as a substitute for manual toil. he was seldom unemployed at meals; but at such times employed himself in contriving or making drawings of whatever occurred to his fertile fancy. usually his sister caroline read to him while he was engaged at the turning-lathe, or polishing mirrors; choosing such books as "don quixote," the "arabian nights," the novels of sterne and fielding; and tea and supper were served without any interruption to the task in which herschel was absorbed. in miss herschel's charming letters we find a vivid sketch of the family avocations at this period:--- "my brother applied himself to perfect his mirrors, erecting in his garden a stand for his twenty-foot telescope: many trials were necessary before the required motions for such an unwieldy machine could be contrived. many attempts were made by way of experiment against a mirror before an intended thirty-foot telescope could be completed, for which, between whiles (not interrupting the observations with seven, ten, and twenty-foot, and writing papers for both the royal and bath philosophical societies), gauges, shapes, weights, &c, of the mirror were calculated, and trials of the composition of the metal were made. in short, i saw nothing else and heard nothing else talked of but about these things when my brothers were together. alex was always very alert, assisting when anything new was going forward; but he wanted perseverance, and never liked to confine himself at home for many hours together. and so it happened that my brother william was obliged to make trial of my abilities in copying for him catalogues, tables, &c, and sometimes whole papers which were lent [to] him for his perusal. among them was one by mr. michel, and a catalogue of christian mayer in latin, which kept me employed when my brother was at the telescope at night. when i found that a hand was sometimes wanted when any particular measures were to be made with the lamp micrometer, or a fire to be kept up, or a dish of coffee necessary during a night's long watching, i undertook with pleasure what others might have thought a hardship." the astronomer-musician's patient survey of the heavens was rewarded, on the th of march , by the discovery of a new planet, situated on the borders of our solar system. in every way this was a discovery of signal importance. it broke up the traditional conservatism of astronomers, which had almost refused to regard as possible the existence of any planets beyond the orbit of saturn, because for so many years none had revealed themselves to the watchful gaze. men's minds were widened, so to speak, at a bound; their conceptions strengthened and enlarged; for the discovery of georgium sidus--as the new planet was designated by its discoverer, in honour of george iii.--rendered possible and probable the discovery of other planets, and thus extended immeasurably the limits of the solar system. herschel, whose reputation as a musician had hitherto been local, now sprang into world-wide fame as an astronomer. george iii., who was a true lover of science, and not disinclined to bestow his patronage on men and things of hanoverian origin, summoned him to his presence; and was so much pleased with his modest and interesting account of the long labours which had led to the great result, that, after a brief interval, he bestowed upon him an annual pension of three hundred guineas, and a residence, first at clay hall, and then at slough. but before this well-deserved good fortune fell to him, herschel continued his industrious career as both musician and astronomer. during the concert season, which lasted five or six months, he had never a night disengaged, but was conducting oratorios at bath or bristol, arranging for public concerts, attending rehearsals, and superintending the performances of his choir. as soon as a lull came, the indomitable man, assisted by his faithful sister, returned to his astronomical pursuits. to gain a fuller and clearer knowledge of the starry worlds scattered over the vast fields of space, herschel from the first had seen that instruments of much greater power were necessary than any hitherto used by astronomers. he set to work, therefore, on the construction of a thirty-foot telescope; the metallic mirror of which must, of course, be of proportionate dimensions. this huge mirror was to be cast in a mould of loam prepared from horse-dung, of which an immense quantity was to be pounded in a mortar, and sifted through a fine sieve; an arduous and almost endless task, undertaken by caroline herschel and her brother alex. then a furnace was erected in a back-room on the ground-floor; and every preparation having been made, a day was set apart for the casting. the day came, and herschel and his collaborateurs looked forward to the consummation of their hopes. the metal was placed in the furnace; but, unfortunately, just when it was ready for pouring in a molten stream into the mould, it began to leak, and both the herschels, and the caster with his men, were compelled to fly from the apartment, the stone flooring exploding, and flying about in all directions, as high as the ceiling. the astronomer, exhausted with heat and exertion, fell on a heap of brickbats; exhausted, but not dismayed. the work was renewed; and a second casting being attempted, it proved entirely successful, and a very perfect metal was formed in the mould. chapter iii. in august the herschels removed to datchet. their new home was "a large neglected place; the house in a deplorably ruinous condition, the garden and grounds overgrown with weeds." nor were the domestic arrangements more favourable. for a fortnight the little family were without a female servant; and an old woman, the gardener's wife, showed miss herschel the shops, where the high prices of every article, from coals to butcher's meat, appalled her. but of these inconveniences herschel took no account. enough for him that he was released from the drudgery of teaching, and free thenceforth to devote himself to the heavens and their wonders. a man whose thoughts are always with the stars can hardly be expected to trouble himself about the price of tallow-candles! were there not capacious stables in which mirrors of any size could be ground; and a roomy laundry capable of easy conversion into a library, with one door opening on a large lawn, where the "small twenty-foot" was to take its stand? compared with advantages such as these, what mattered the scarcity of "butcher's meat"? herschel laughingly assured his sister that they could live on eggs and bacon; which, he confidently asserted, would cost next to nothing, now that they were really in the country! and so he settled down to a life of quiet, industry at datchet; his admirable sister being formally adopted as his assistant and secretary. never had master a more devoted, a more enthusiastic, or a more intelligent servant! she shared in all his night-watches, with her eye constantly on the clock, and the pencil in her hand; with unerring accuracy she made all the complex calculations so frequently required; she made three or four copies of every observation in separate registers, co-ordinating, classifying, and analyzing them. if the scientific world, says arago, saw with astonishment the unexampled rapidity with which herschel's works succeeded one another for many years, they were greatly indebted for this affluence of production to the affectionate ardour of his sister caroline. her enthusiasm never failed; her industry knew no check; and her brother's fame was dearer to her than life. in one of her letters she describes with graphic simplicity the "interior" at datchet:-- "i found that i was to be trained for an assistant-astronomer; and by way of encouragement, a telescope adapted for 'sweeping' (or rapidly surveying a wide extent of space), consisting of a tube with two glasses, was given [to] me. i was to 'sweep for comets;' and i see by my journal that i began august nd, , to write down and describe all remarkable appearances i saw in my 'sweeps.' but it was not till the last two months of the same year that i felt the least encouragement to spend the starlit nights on a grass-plot covered with dew or hoar-frost, without a human being near enough to be within call. i knew too little of the real heavens to be able to point out every object so as to find it again without losing too much time by consulting the atlas. but all these troubles were removed when i knew my brother to be at no great distance, making observations with his various instruments on double stars, planets, and the like; and i could have his assistance immediately when i found a nebula, or cluster of stars, of which i intended to give a catalogue. i had the comfort to see," she continues, "that my brother was satisfied with my endeavours to assist him when he wanted another person either to run to the clocks, write down a memorandum, fetch and carry instruments, or measure the ground with poles,--of which something of the kind every moment would occur." the conscientious care and assiduous industry with which herschel made his measurements of the diameter of the georgium sidus (now called uranus), and his interesting observations of other planets, of double stars with their coloured light, of cometary and nebulous appearances, were truly remarkable; as may be seen by the various papers which he wrote at this time for the royal society. in addition to all this labour, he perfected a twelve-inch speculum of vast magnifying power before the spring of ; and many hours were spent at the turning-bench, as not a night clear enough for observing ever passed without the devising of improvements in the mounting and motion of the various instruments then in use, or the test and trial of newly-constructed "eyepieces," most of which were executed by herschel's own hands. "wishing to save his time, he began to have some work of that kind done by a watchmaker, who had retired from business, and lived on datchet common; but the work was so bad, and the charges [were] so unreasonable, that he could not be employed. it was not till some time afterwards, in his frequent visits to the meetings of the royal society (made in moonlight nights), that he had an opportunity of looking about for mathematical workmen, opticians, and founders. but the work seldom answered expectation, and it was kept to be executed with improvements by alexander during the few months he spent with us." * * * * * in july herschel began his observations with his large twenty-foot telescope, though it was in an unfinished state; and his sister watched and waited with much apprehension when she knew him to be elevated some fifteen feet or more on a temporary crossbeam instead of a safe gallery. here it is needful to explain, perhaps, that these huge astronomical telescopes are not used like ordinary glasses, to one end of which the observer applies his eye; the objects towards which the tube is directed being thrown upon a large mirror, which is attached to it externally at some distance from the ground. the observer, therefore, needs to be mounted on an elevated platform or gallery, from which he can conveniently inspect the mirror. one night, in a very high wind, herschel had scarcely descended from his station before the whole apparatus came down; and his sister was in continual apprehension of some serious accident. one such, indeed, occurred, and to herself. the evening of the st of december had been cloudy, but as a few stars shone forth about ten o'clock, hurried preparations were made for observing. herschel, standing at the front of the telescope, directed his sister to make a certain alteration in the lateral motion, which was done by machinery, on which the point of support of the tube and mirror rested. at each end of the machine or trough was an iron hook, such as butchers use for suspending their joints of meat; and having to run in the dark across ground covered a foot deep with melting snow, miss herschel fell on one of these hooks, which entered her right leg above the knee. to her brother's injunction, "make haste!" she could answer only by a pitiful cry, "i am hooked!" he and the workmen hastened immediately to her assistance, but they could not disentangle her without leaving nearly two ounces of her flesh behind. for some weeks she was an invalid, and at one time it was feared that amputation might be necessary. * * * * * not satisfied with the magnifying power of any of the instruments he had hitherto constructed, herschel resolved, in , to attempt a forty-foot telescope. such a work, however, was far beyond his limited private resources; and he did not venture to undertake it until promised a royal bounty of £ . then he removed from datchet to clay hall, old windsor; and again, in , to slough, where he finally settled, and succeeded in erecting a commodious and well-equipped observatory. "we may confidently assert," says arago, "relative to the little house and garden of slough, that it is the spot of all the world where the greatest number of discoveries have been made. the name of that village will never perish: science will transmit it religiously to our latest posterity." at slough, as at datchet, prevailed the most enthusiastic industry; and the house was soon as full of well-ordered labour as a bee-hive. smiths were kept constantly at work on different parts of the new telescopic leviathan; and a whole troop of labourers was engaged in grinding the tools required for shaping and polishing its mirror. had not a cloudy or moonlight night sometimes intervened, herschel and his sister must have died of sheer exhaustion, for they toiled with unremitting ardour both day and night. with the morning came the workpeople, of whom no fewer than between thirty and forty were at work for upwards of three months together: some employed in felling and rooting out trees, some digging and preparing the ground for the bricklayers, who were laying the foundation for the telescope. then there were the carpenter and his men; and, meanwhile, the smith was converting a wash-house into a forge, and manufacturing complete sets of tools for his own share of the labour. in short, the place was at one time a complete workshop for the manufacture of optical instruments; and it was a pleasure to enter it for the purpose of observing the fervour of the great astronomer, and the reverent attention given to his orders. it is impossible not to refer here to the sisterly devotion of caroline herschel, who was in every respect worthy of her noble-minded, tender-hearted, and enthusiastic brother. she stood beside him to the last, sharing his labours, brightening his life. in the days, says her biographer, when herschel gave up a lucrative career that he might dedicate all his energies to astronomical pursuits, it was through her care and thriftiness that he was spared from the unrest of pecuniary anxieties. as she had been his helper and assistant during his career as a popular musician, so she became his helper and assistant when he gave himself up, like the chaldeans of old, to the study of the stars. by dint of a resolute will and a love that shrank from no sacrifice or exertion, she acquired such a knowledge of mathematics and calculations, mysterious as these generally seem to the feminine mind, that she was able to formulate with exactness the result of her brother's researches. she never failed to be his willing fellow-labourer in the workshop; she helped him to grind and polish his mirrors; she stood beside his telescope, in order to record his observations, during the dark and bitter mid-winter nights, when the very ink was frozen in the bottle. it may be said, without exaggeration, that she kept him alive by her care: thinking nothing of herself, she lived for him, and him alone. she loved him, she believed in him, she aided him with all her heart and all her strength. her mental powers were very considerable; and undoubtedly she might have attained to eminence on her own account, for she herself discovered no fewer than eight comets. but she shunned self-glorification; she desired to live in her brother's shadow; she worked for him, never for herself; and in her elevated character no feature more strongly demands our admiration than her heroic though unconscious self-denial. happy the man who has such a sister; happy the sister whose brother is worthy of so much devotion! it is pleasant to know that william herschel deserved the love so lavishly poured out at his feet; that great as were his achievements in science, lofty and broad as was his genius, they were fully sustained and ennobled by the beauty and worth of his inner life. who can contemplate their twofold career in all its singleness of purpose, its purity, its unselfishness, its sublime disregard of worldly pleasures, without emotion? the lessons told by such a life are worth all the moral treatises ever written. to miss herschel's diary we again refer, for a glimpse of the occupations of her brother and herself at slough in the first two years of their residence. these two years, to use an apt expression of her own, were spent in a perfect chaos of business. the garden and workrooms swarmed with labourers and workmen--smiths and carpenters speeding to and fro between the forge and the forty-foot machinery; and so incessant was the vigilance of herschel, that not a screw-bolt in the whole apparatus was fixed except under his eye. "i have seen him," writes his sister, "lying stretched many an hour in the burning sun, across the top beam, whilst the iron-work for the various motions [of the great telescope] was being fixed." at one time no fewer than twenty-four men, in relays of twelve each, were engaged in grinding and polishing day and night; and herschel never left them, taking his food without allowing himself time to sit down to table. "in august ," writes the diarist, "an additional man-servant was engaged, who would be wanted at the handles of the motions of the forty-foot,"--that is, to raise or lower it, or move it from side to side, as might be required,--"for which the mirror in the beginning of july was so far finished as to be used for occasional observations on trial. such a person was also necessary for showing the telescopes to the curious strangers, as by their numerous visits my brother and myself had for some time past been much incommoded. in consequence of an application made through sir j. banks to the king, my brother had in august a second sum of £ granted for completing the forty-foot, and £ yearly for the expense of repairs; such as ropes, painting, &c., and the keep and clothing of the men who attended at night. a salary of £ a year was also settled on me, as an assistant to my brother. a great uneasiness was by this means removed from my mind; for though i had generally (and especially during the last busy six years) been almost the keeper of my brother's purse, with a charge to provide for my personal wants, only annexing in my accounts the memorandum '_for car_.' to the sums so laid out. when cast up, they hardly amounted to seven or eight pounds per year since the time we had left bath. nothing but bankruptcy had all the while been running through my silly head, when looking at the sums of my weekly accounts, and knowing they could be but trifling in comparison with what had been and had yet to be paid in town. i will only add, that from this time the utmost activity prevailed to forward the completion of the forty-foot." in recognition of his scientific triumphs, the honorary degree of ll.d was conferred upon herschel, in , by the university of oxford. they were triumphs that well merited such a recognition. he had already made some important observations on the nature of double stars, on the dimensions of the telescopic planets, and had begun his famous investigations into the composition of the nebulae,--those clusters of stars and nebulous matter which had previously proved such a problem to astronomers. the remarkable phenomenon of a periodical change of intensity in certain stars, which wax and wane in radiance like a revolving light, had also excited his attention. further, he had entered upon the experiments which ultimately showed that the sun positively moves; that in this, as in other respects, the magnificent orb of day must be ranged among the stars; that the apparently inextricable irregularities of numerous sidereal proper motions arise in great part from the displacement of the solar system; that, in short, the point of space toward which earth and its sister planets are annually advancing, is situated in the constellation of hercules. "let us," says a french writer, "to these immortal labours add the ingenious ideas that we owe to herschel on the nebulae, on the constitution of the milky way, on the universe as a whole,--ideas which almost by themselves constitute the actual history of the formation of the worlds,--and we cannot but have a deep reverence for that powerful genius that scarcely ever erred, notwithstanding the ardour of its imagination." the ordinary spectator, looking upon the face of the heavens through a telescope, had, prior to herschel's time, felt his curiosity excited by the appearance here and there of filmy patches, vague in structure and irregular in shape, which, from their resemblance to clouds, received the name of _nebulae_. what these were, no astronomer had succeeded in defining. it was left for herschel, with his rare powers of patient and discriminating observation, assisted by the more powerful instruments which his ingenuity succeeded in constructing, to discern in them innumerable groups of worlds, in various stages of formation! a new light was thrown upon the history of the universe. man was able to assist, as it were, at the process of creation, and to watch the development of a mass of incoherent matter into a perfect star. this alone was a discovery which might well have immortalised the name of herschel. but we owe to him the elements of our knowledge of the sun's physical constitution. he swept aside the erroneous theories and conjectures which had previously prevailed, and guided the astronomical inquirer into the right path. he convinced himself, by long and patient researches, that the luminous envelope of the great "orb of day" was neither a liquid nor an elastic fluid; that it was in certain respects analogous to the clouds which wreathe our mountain-summits and fertilize our plains; that it floated in the solar atmosphere. thence he came to the conclusion that the sun has two atmospheres, endowed with motions quite independent of each other. an elastic fluid, now known as the _photosphere_, is in course of continual formation on the dark rugged surface of the solar mass; and rising, on account of its specific lightness, it forms the _pores_ in the stratum of reflecting clouds; then, combining with other gases, it produces the irregularities or furrows in the luminous cloud-region. when the ascending currents are powerful, they create those appearances which astronomers designate the _nuclei_, the _penumbrae_, the _faculae_. such was herschel's explanation of the mode of formation of the solar spots; and allowing it to be well-founded, we must expect to find--what is, indeed, the case--that the sun does not always and regularly pour forth equal quantities of light and heat. it is true that herschel's hypothesis has been modified by later astronomers; but his is the credit of having directed them into the right course of inquiry and observation. * * * * * the physical constitution of the moon was a subject which also engaged the attention of our indefatigable enthusiast. as early as he attempted the measurement of the lunar mountains, and came to the conclusion that few of them exceeded feet in height. later research, however, has proved these figures to be inadequate. next he addressed himself to a study of the lunar volcanoes, three of which he declared to be in a state of ignition; two of them apparently on the decline, the third still active. he was so convinced of the reality of the phenomenon, that on the th of april he wrote:--"the volcano burns with greater violence than it did last night." the real diameter of the volcanic light he estimated at , feet. its intensity he described as superior to that of the nucleus of a comet then flashing across our system. the objects situated near the crater were fully illuminated by the glare of its burning matter. it may seem strange that, after observations so exact and minute, few astronomers now admit the existence of active volcanoes in the moon. the reasons for their incredulity are thus stated:-- the various parts of the moon do not all reflect with the same intensity. here, that intensity may be dependent on the form; elsewhere, on the nature of the materials. those persons who have examined the lunar orb with telescopes, know how very considerable the difference arising from these two causes may be,--with how much keener and stronger a radiance one point of the moon will sometimes shine than those around it. well, it would seem to be obvious that the ratio of intensity between the brilliant parts and the faint parts must always be the same, whatever the origin of the illuminating light. in that portion of the lunar sphere which receives the glow and glory of the sun, we know that some points exist, the brightness of which is extraordinary compared with the feeble flickering gleam of those around them. and these same points, when seen in the dim reflection of the earth, will still predominate in intensity over the neighbouring regions. in this way arago and others explain the observations of herschel, without admitting the existence of active volcanoes in the moon. that volcanoes there are, is a familiar fact; but they would seem to have exhausted their activity in long-past ages. the lunar surface is now a dreary waste of rugged lava and ashes, covered with the matter ejected from craters once in a state of furious eruption. the moon, in fact, is a world which has burned itself out. how strange the thought that in a far-back period the inhabitants of earth, had earth then been inhabited, might have seen the glare of countless volcanoes diffused, lurid and threatening, over the face of their satellite! how strange the thought that the once active fires should all have died away, and the moon have thus been prepared for the better reception and reflection of the solar radiance in order to illuminate the nights of earth! the planets, needless to say, were the objects of herschel's assiduous attention. mercury was the one which least interested him; but he ascertained the perfect circularity of its disc. with respect to venus, he endeavoured to determine the time of its rotation from . we owe to him the discovery of the true shape of the "red planet mars,"--that, like the earth, it is an oblate spheroid, or flattened at the poles. after piazzi, olbers, and harding had discovered the small planets, ceres, pallas, juno, and vesta, he applied himself to the measurement of their angular diameters. his researches led him to the conclusion that these four new bodies could not properly be ranked with the planets, and he proposed to call them asteroids--a name now generally adopted. since herschel's time, the number of these minor planets known to astronomers has increased to upwards of one hundred. with respect to jupiter, our astronomer arrived at some important facts in connection with the duration of its rotation. he also made numerous observations on the intensities and comparative magnitudes of its satellites. we come next in order to saturn, the gloomy planet which the ancient astrologers regarded with so much dislike. here, too, we find traces of herschel's labours. not only has he enlarged our knowledge of its equatorial compression, of its physical constitution, and of the rotation of its luminous belt or ring, but he added two to the number of its satellites. five only of these were known at the close of the seventeenth century; of which cussiric discovered four, and huygens one. it was universally believed that the subject was exhausted. but, on the th of august , herschel's colossal tube revealed to his delighted gaze a satellite nearer to the saturnian ring than those previously observed. and a few days later, on the th of september, a seventh and last satellite crossed his field of vision. it was situated between the former and the ring; that is, it is the nearest to it of the seven. but the most remarkable of herschel's achievements was the discovery of the planet uranus, and the detection of its satellites. on the th of march , between ten and eleven o'clock at night, the great astronomer was engaged in examining the small stars near h in the constellation gemini, with a seven-foot telescope, bearing a magnifying power of two hundred and twenty-seven times. it appeared to him that one of these stars was of an unusual diameter; and he came to the conclusion, therefore, that it was a comet. it was under this denomination that it was discussed at the meeting of the royal society. but the researches of herschel at a later period showed that the orbit of the new body was circular, and accordingly it was elevated to the rank of a planet. as already stated, herschel named it, in compliment to george iii., the georgium sidus; in this copying the example of galileo with his "medicaean stars." afterwards, astronomers christened it herschel, and subsequently uranus, in conformity with the mythological nomenclature of the other planets. the immense distance of uranus from our earth, its small angular diameter, and the feebleness of its light, seemed to preclude the hope that, if it were attended by satellites of the same dimensions in proportion to its own magnitude as those of the satellites of jupiter and saturn in proportion to _their_ magnitude, they could be descried by any human observer. the patient, persevering, reverent temper of herschel took no account, however, of any discouraging or unpropitious circumstances. what he did was to substitute for telescopes of the ordinary construction the new and gigantic forty-foot tube already described; and, thus, with unremitting vigilance and intense zeal, he arrived at the discovery (between january , , and february , ) of the _six_ satellites of uranus; in other words, he revealed to man the completeness of a new system,--a system which will always be identified with his name. * * * * * those singular meteors, the comets, which flash through heaven with long trails of light, and of old astonished the nations as if they were harbingers of some overwhelming calamity, were also the frequent subjects of our astronomer's investigations. he brought some of his fine and powerful instruments to bear on a comet discovered by mr. pigott in , and closely and carefully investigated its physical constitution. the nucleus, or head, was circular and well determined, and evidently shone by its own light. very small stars seemed to grow pale, "to hide their diminished heads," when seen through its _coma_ or tail. it is true, however, that this faintness may have been only apparent, and due to the circumstance of the stars being projected on a luminous background. such was herschel's explanation. a gaseous medium, capable of absorbing sufficient solar radiance to efface the light of some "lesser stars," appeared to him to possess in each stratum a sensible quantity of matter. hence it would cause a real diminution of the light transmitted, though nothing would indicate the existence of such a cause.[ ] [footnote : this conclusion is disputed by many astronomers.] herschel examined the beautiful comet of with equal accuracy. "large telescopes showed him, in the midst of the gaseous head, a rather reddish body of planetary appearance, which bore strong magnifying powers, and showed no sign of _phase_ (that is, of change of aspect, as in the case of the moon). hence herschel concluded that it was self-luminous. yet, if we reflect that the planetary body under consideration was not a second in diameter, the absence of a phase," says arago, "does not appear a demonstrative argument." the same writer adds:-- "the light of the head had a bluish-green tint." was this a real tint, or did the central reddish body, only through contrast, make the surrounding vapour appear to be coloured? herschel did not examine the question from this point of view. "the head of the comet appeared to be enveloped at a certain distance, on the side towards the sun, by a brilliant narrow zone, embracing about a semicircle, and of a yellowish colour. from the two extremities of the semicircle arose, towards the region away from the sun, two long luminous streaks which limited the tail. between the brilliant circular semi-ring and the head, the cometary substance appeared to be dark, of great rarity, and very diaphanous. "the luminous self-ring floated: one day it seemed to be suspended in the diaphanous atmosphere by which the head of the comet was surrounded, at a distance of , english miles from the nucleus. "this distance was not constant. the matter of the semi-annular envelope seemed even to be precipitated by slow degrees through the diaphanous atmosphere; finally, it reached the nucleus; the earlier appearances vanished; the comet was reduced to a globular nebula. "during its period of dissolution, the ring appeared sometimes to have several branches. "the luminous shreds of the tail apparently underwent rapid, frequent, and considerable variations of length. herschel discerned symptoms of a rotatory movement both in the comet and its tail; a movement which carried unequal shreds from the centre towards the border, and the border towards the centre. on examining at intervals the same region of the tail--the border, for example--sensible changes of length must have been perceptible; which, however, had no reality in them. herschel thought that both the comet of and that of were self-luminous. the second comet of appeared to him to shine only by borrowed light. it must be acknowledged that these conjectures did not rest on anything demonstrative. "in attentively comparing the comet of with the beautiful comet of , relative to the changes of distance from the sun, and the modifications resulting thence, herschel put it beyond doubt that these modifications have something individual in them,--something relative to a special state of the nebulous matter. on one celestial body the changes of distance produce an enormous effect, on another the modifications are insignificant." we have reproduced these observations by a distinguished french astronomer, in order to show the reader what was the nature, and how great was the importance, of herschel's labours, and in how remarkable and comprehensive a manner he conducted his survey of the celestial phenomena. we now return to our brief narrative of his life. such a life, absorbed in tranquil and incessant studies, presents no curious, romantic, or surprising incidents. it was the life of a reverent, patient, gentle, and devoted man of genius, who dedicated himself to the task of making known the "wondrous works of god" to his fellow-men, and who in all his social and domestic relations was without blot or stain. in he married the widow of john pitt, esq., with whom he received a considerable fortune, and thus for the remainder of his life he was enabled to give himself up to his favourite pursuits unembarrassed by pecuniary anxieties. his marriage was in every respect a happy one, and effectually secured his domestic peace. by his wife he had an only son,--the late sir john herschel,--who worthily maintained the scientific dignity of his name. it is said, by the highest of all authority, that a prophet is not honoured in his own country. but our astronomer was not without the reward of his work, even in his lifetime. the university of oxford conferred upon him the illustrious honorary degree of d.c.l. in he received the guelphic order of knighthood; and in he was chosen the first president of the astronomical society. from his sister's diary we gather a few particulars illustrative of his mode of life. on the th of october she writes:-- "my brother came from brighton. the same night two parties from the castle [windsor] came to see the comet, and during the whole month my brother had not an evening to himself. as he was then in the midst of polishing the forty-foot mirror, rest became absolutely necessary after a day spent in that most laborious work; and it has ever been my opinion, that on the th of october his nerves received a shock of which he never got the better afterwards; for on that day (in particular) he had hardly dismissed his troop of men, when visitors assembled, and from the time it was dark till past midnight he was on the grass-plot, surrounded by between fifty and sixty persons, without having had time for putting on proper clothing, or for the least nourishment passing his lips. "_february th, _.--when i came to slough to assist my brother in polishing the forty-foot mirror, i found my nephew[ ] very ill with an inflammatory sore throat and fever. "_february th_.--still very ill; and my brother obliged to go on with the polishing of the great mirror, as every arrangement had been made for that purpose.--_mem_. i believe my brother had reasons for choosing the cold season for this laborious work, the exertion of which alone must put any man into a fever, if he were ever so strong. "_february th_.--from this day my nephew's health kept on mending. "_february th_.--my nephew mending, but my brother not well. "_february th_.--my brother so ill that i was not allowed to see him, and till march th his life was despaired of; and by march th i was permitted to see him, but only for two or three minutes, as he was not allowed to speak. "_march nd_.--he (sir william) went for the first time into his library, but could only remain for a few moments." [footnote : afterwards sir john herschel.] from this dangerous attack sir william recovered, but thenceforth it was clear to his friends that his strength gradually decreased, though not his enthusiasm or his industry. he persevered in his life-long labours with all his old intellectual force. what failed him was neither his tender affections nor his mental powers; but his body refused to answer all the demands made upon it by the resolute will,--the sword was slowly but surely wearing out the scabbard. under the date of april , , we meet with an ominous entry in his loving and faithful sister's diary:-- "my brother left slough, accompanied by lady herschel, for bath, he being very unwell; and the constant complaint of giddiness in the head so much increased, that they were obliged to be four nights on the road both going and coming. the last moments before he stepped into the carriage were spent in walking with me through his library and workrooms, pointing with anxious looks to every shelf and drawer, desiring me to examine all, and to make memorandums of them as well as i could. he was hardly able to support himself; and his spirits were so low, that i found difficulty in commanding my voice so far as to give him the assurance he should find on his return that my time had not been misspent. "when i was left alone, i found that i had no easy task to perform, for there were packets of writings to be examined which had not been looked at for the last forty years. but i did not pass a single day without working in the library as long as i could read a letter without candlelight, and taking with me papers to copy, which employed me for best part of the night; and thus i was enabled to give my brother a clear account of what had been done at his return. but (may ) he returned home much worse than he went, and for several days hardly noticed my handiwork." to this same year of decay and decline ( ) belongs a small slip of yellow paper, inscribed with the following lines in a tremulous and feeble handwriting, which is jealously preserved by the illustrious astronomer's descendants:-- "lina,--there is a great comet. i want you to assist me. come to dine, and spend the day here. if you can come soon after one o'clock, we shall have time to prepare maps and telescopes. i saw its situation last night,--it has a long tail. "_july , _." then follows:-- "i keep this as a relic! every line _now_ traced by the hand of my dear brother becomes a treasure to me. "c. herschel." we know of nothing more touching in literary history than this noble, self-sacrificing, generous affection of the sister towards her eminent brother. such instances of absolute self-denial and all-absorbing love elevate our opinion of human nature generally, and prove that something of the divine image lingers in it still. herschel was now bordering upon the ripe old age of eighty, and it is no wonder that, after a life of incessant study, his strength should daily diminish. in it became painfully evident to his attached relatives and friends that the end was not far off; and on the th of august he passed away to his rest. we owe an account of his last days to his sister, but for whose pious care, indeed, very little of his private life would have been known, and herschel could have been judged only from the recorded results of his immense labours. "_may th_.--the summer proved very hot; my brother's feeble nerves were very much affected, and there being in general much company, added to the difficulty of choosing the most airy rooms for his retirement. "_july th_.--i had a dawn of hope that my brother might regain once more a little strength, for i have a memorandum in my almanac of his walking with a firmer step than usual above three or four times the distance from the dwelling-house to the library, in order to gather and eat raspberries, in his garden, with me. but i never saw the like again. "the latter end of july i was seized by a bilious fever, and i could for several days only rise for a few hours to go to my brother about the time he was used to see me. but one day i was entirely confined to my bed, which alarmed lady herschel and the family _on my brother's account_. miss baldwin [a niece of lady herschel] called and found me in despair about my own confused affairs, which i never had had time to bring into any order. the next day she brought my nephew to me, who promised to fulfil all my wishes which i should have expressed on paper; he begged me not to exert myself, for his father's sake, of whom he believed _it would be the immediate death if anything should happen to me_." afterwards she wrote:-- "of my dear nephew's advice i could not avail myself, for i knew that at that time he had weighty concerns on his mind. and, besides, my whole life almost has passed away in the delusion that, next to my eldest brother, none but dietrich was capable of giving me advice where to leave my few relics, consisting of a few books and my sweeper [that is, the seven-foot telescope with which she was accustomed to sweep the heavens for comets]. and for the last twenty years i kept to the resolution of never opening my lips to my dear brother william about worldly concerns, let me be ever so much at a loss for knowing right from wrong." miss herschel proceeds to note that on the afternoons of the th, th, th, and th of august, she, "as usual," spent some hours with her brother. on the th she hastened to the accustomed place, where she generally found him, with the newspaper which she was to read aloud for his amusement. but, instead, she found assembled there several of his nearest friends, who informed her that her aged brother had been compelled to return to his room. she lost no time in seeking him. he was attended by lady herschel and his housekeeper, who were administering everything which was likely to keep up his failing strength. miss herschel observed that he was much irritated, with the irritation natural to old age and extreme bodily feebleness, at his inability to grant a friend's request for some token of remembrance for his father. no sooner did he see miss herschel, the loving companion and fellow-worker of so many years, than he characteristically employed her to fetch one of his last papers, and a plate (or map) of the forty-foot telescope. "but, for the universe," says miss herschel, "i could not have looked twice at what i had snatched from the shelf; and when he faintly asked if the breaking up of the milky way[ ] was in it, i said, 'yes,' and he looked content." i cannot help remembering this circumstance; it was the last time i was sent to the library on such an occasion. that the anxious care for his papers and workrooms never ended but with his life, was proved by his frequent whispered inquiries if they were locked and the key safe; of which i took care to assure him that they were, and the key in lady herschel's hands. [footnote : the _via lactea_, or "milky way," had long been supposed to consist of a nebulous, vague, luminous matter, but herschel showed that it was really made up of stars and systems of stars.] after struggling for some thirty minutes against his rapidly increasing weakness, the great astronomer, bowed by his burden of years and labours, was forced to retire to his bed, with little hope that he would ever rise from it again. for ten days and nights his wife and sister watched by his side in painful suspense, until, on the th of august, the end came. peacefully closed a life which had passed in a peace and quietness not often vouchsafed to man. * * * * * herschel, says a brother astronomer, will never cease to occupy an eminent place in the small group of our contemporary men of genius, while his name will descend to the most distant posterity. the variety and the magnificence of his labours vie with their extent. the more they are studied, the more they are admired. for it is with great men as it is with great movements in the arts and in national history,--we cannot understand them without observing them from different points of view. what a brilliant roll of achievements is recalled to the mind by the name of william herschel! the discovery of uranus, and of its satellites; of the fifth and sixth satellites of saturn; of the many spots at the poles of mars; of the rotation of saturn's ring; of the belts of saturn; of the rotation of jupiter's satellites; of the daily period of saturn and venus; and of the motions of binary sidereal systems,--added to his investigations into nebulae, the milky way, and double, triple, and multiple stars;--all this we owe to his patient, his persevering, his daring genius! he may almost be styled the father of modern astronomy. chapter iv. we now propose to furnish a brief sketch of the life of sir john frederick william herschel, the only son of sir william, and not less illustrious as a man of science. he was born at slough, in the year . evincing considerable talents at a very early age, he received a careful private education under mr. rogers, a scottish mathematician of distinguished merit; and afterwards was sent to st. john's college, cambridge, always famous as a nursery of mathematical and scientific prodigies! here he pursued his studies with remarkable success, suffering no obstacles to daunt him, and wasting no opportunities of improvement. his fellow-collegians regarded him as one who would add to the high repute of the college, and rejoiced at the brilliant ease with which he passed every examination. in he took his degree of b.a., and consummated a long series of successes by becoming "senior wrangler," and "smith's prizeman;" these being the two highest distinctions to which a cambridge scholar can attain. in the same year, when he was hardly twenty-one, he published a work entitled, "a collection of examples of the application of the calculus to finite differences." to our young readers such a title will convey no meaning; and we refer to it here only to illustrate the industry and careful thought of the young student, which had rendered possible such a result. returning to slough, he continued his studies in mathematics, chemistry, and natural philosophy, and in various publications exhibited that faculty of observation and analyzation, that intelligence and scrupulousness in collecting facts, and that boldness in deducing new inferences from them, which were characteristic of his illustrious father. the subjects he took up were so abstruse, that we could not hope to make our readers understand what he accomplished, or how far he excelled his predecessors in his grasp and comprehension of them. for instance: if we tell them that in he wrote a paper "on the theory and summation of series;" communicated to the cambridge philosophical society his discovery that the two kinds of rotatory polarization in rock crystal were related to the plagihedral faces of that mineral; and issued an able treatise "on certain remarkable instances of deviation from newton's tints in the polarized tints of uniaxal crystals,"--they will gain no very distinct idea of the significance or value of these researches. again: it will not be very intelligible to them to be informed that, in , he communicated to the royal society of edinburgh a paper "on the absorption of light by coloured media", in which he enunciated a new method of measuring the dispersion of transparent bodies by stopping the green, yellow, and most refrangible red rays, and thus rendering visible the rays situated rigorously at the end of the spectrum. but they will understand that these results could have been attained only by the most assiduous industry and the most unflinching perseverance. and it is on account of this industry and this perseverance that we recommend herschel as an example to our readers. they may not make the same progress in science, or achieve the same reputation. it is not necessary they should. humble work is not less honourable, if it be done conscientiously, and with a sincere desire to do the best that it is in our power to do. an interesting feature in the younger herschel's character was his loving care for his father's fame. he was ever most anxious that the full measure of his services to science should be recognized and appreciated. thus, in , he writes to his aunt:-- "i have been long threatening to send you a long letter, but have always been prevented by circumstances and want of leisure from executing my intention. the truth is, i have been so much occupied with astronomy of late, that i have had little time for anything else--the reduction of those double stars, and the necessity it has put me under of looking over the journals, reviews, &c, for information on what has already been done, and in many cases of re-casting up my father's measures, swallows up a great deal of time and labour. but i have the satisfaction of being able to state that our results in most instances confirm and establish my father's views in a remarkable manner. these inquiries have taken me off the republication of his printed papers for the present. "i think i shall be adding more to his fame by pursuing and verifying his observations than by reprinting them. but i have by no means abandoned the idea. meanwhile, i am not sorry to hear they are about to be translated into german.... i hope this season to commence a series of observations with the twenty-foot reflector, which is now in fine order. the forty-foot is no longer capable of being used, but i shall suffer it to stand as a monument." * * * * * in reference to this famous telescope, we may digress to state that its remains have been carefully preserved. the metal tube of the instrument, carrying at one end the recently cleaned mirror of four feet ten inches in diameter, has been placed horizontally in the meridian line, on solid piles of masonry, in the midst of the circle where the apparatus used in manoeuvring it was formerly placed. on the st of january , sir john herschel, his wife, their seven children, and some old family servants, assembled at slough. exactly at noon the party walked several times in procession round the instrument; they then entered the gigantic tube, seated themselves on benches previously prepared, and chanted a requiem with english words composed by sir john herschel himself. then issuing from the tube, they ranged themselves around it, while its opening was hermetically sealed. * * * * * in march , the younger herschel, in conjunction with sir james south, undertook a series of observations on the distances and positions of three hundred and eighty double and triple stars, by means of two splendid achromatic telescopes of five and seven focal length. these were continued during and , and have proved of great service to astronomers. having pursued with much zeal the study of optics, and experimented largely and carefully on the double refraction and polarization of light, he compiled a treatise on the subject for the "encyclopaedia metropolitana" it has been translated into french by m. quetelet; and both foreign and english men of science have been accustomed to regard it as indicating a new point of departure in the important branch of science to which it is devoted. astronomy, however, became for him, as for his father, the great pursuit of his laborious life; and having constructed telescopes of singular magnitude and power, he entered upon a study of the sidereal world. in he commenced a careful re-examination of the numerous nebulae and starry clusters which had been discovered by his father, and described in the "philosophical transactions," fixing their positions and investigating their aspects. he devoted eight years to this _magnum opus_, completing it in . the catalogue which he then contributed to the "philosophical transactions" includes nebulae and star-clusters, of which were discovered by himself. while engaged in this difficult task, herschel discovered between three and four thousand double stars, which he described in the memoirs of the astronomical society. his observations were made with an excellent newtonian telescope, twenty feet in focal length, and eighteen and a half inches in aperture; and having obtained, to use his own expression, "a sufficient mastery over the instrument," the idea occurred to him of making it available for a survey of the southern heavens. accordingly, he left england on the th of november , and arrived at cape town on the th of january . five days later he wrote to his aunt as follows:-- "here we are safely lauded and comfortably housed at the far end of africa; and having secured the landing and final storage of all the telescopes and other matters, as far as i can see, without the slightest injury, i lose no time in reporting to you our good success _so far_. m----[ ] and the children are, thank god, quite well; though, for fear you should think her too good a sailor, i ought to add that she continued sea-sick, at intervals, during the whole passage. we were nine weeks and two days at sea, during which period we experienced only one day of contrary wind. we had a brisk breeze 'right aft' all the way from the bay of biscay (which we never entered) to the 'calm latitudes;' that is to say, to the space about five or six degrees broad near the equator, where the trade-winds cease, and where it is no unusual thing for a ship to lie becalmed for a month or six weeks, frying under a vertical sun. such, however, was not our fate. we were detained only three or four days by the calms usual in that zone, but never _quite_ still, or driven out of our course; and immediately on crossing 'the line' got a good breeze (the south-east trade-wind), which carried us round trinidad; then exchanged it for a north-west wind, which, with the exception of one day's squall from the south-east, carried us straight into table bay. on the night of the th we were told to prepare to see the table mountain. next morning (_n.b._, we had not seen land before since leaving england), at dawn, the welcome word land' was heard; and there stood this magnificent hill, with all its attendant mountain-range down to the farthest point of south africa, full in view, with a clear blue ghost-like outline; and that night we cast anchor within the bay. next morning early we landed, under escort of dr. stewart, m----'s brother, and you may imagine the meeting. we took up our quarters at a most comfortable lodging-house (miss robe's), and i proceeded, without loss of time, to unship the instruments. this was no trifling operation, as they filled (with the rest of our luggage) fifteen large boats; and, owing to the difficulty of getting them up from the hold of the ship, required several days to complete the landing. during the whole time (and indeed up to this moment) not a single south-east gale, the summer torment of this harbour, has occurred. this is a thing almost unheard of here, and has indeed been most fortunate, since otherwise it is not at all unlikely that some of the boats, laden as they were to the water's edge, might have been lost, and the whole business crippled. [footnote : herschel married a miss stewart in february .] "for the last two or three days we have been looking at houses, and have all but agreed for one--a most beautiful place within four or five miles out of town, called 'the grove.' in point of situation it is a perfect paradise, in rich and magnificent mountain-scenery, and sheltered from all winds, even the fierce south-easter, by thick surrounding woods. i must reserve for my next all description of the gorgeous display of flowers which adorns this splendid country, as well as of the astonishing brilliancy of the constellations, which the calm, clear nights show off to great advantage." mr. herschel settled at feldhausen, about feet above the sea, and in long. ° ' ". e., and lat. ° ' ". s. here he entered upon his great series of observations of the southern heavens, which he continued with unwearied ardour for a period of four years. the results were afterwards published, at the cost of the duke of northumberland, in a work entitled "results of astronomical observations made in - - - - , at the cape of good hope." in this superb work, which placed its author on an equality with the most brilliant and illustrious astronomers, he defined and described of the nebulae and star-groups in the southern hemisphere, and of the double stars; besides entering into a variety of valuable particulars relative to halley's comet, the solar spots, the satellites of saturn, and the measurement of the apparent magnitude of stars. on his return to england (in ) the astronomer received a noble welcome. honours poured in upon him. the gold medal of the astronomical society was conferred upon him for a second time. william iv. had previously distinguished him with the hanoverian order of k.h.; but, on the coronation of queen victoria, he received a baronetcy; and in the university of oxford made him a d.c.l. continuing his career of scientific industry, he issued, in , his important and very valuable treatise entitled "outlines of astronomy." in , he was appointed president of the british association; and in , of the royal astronomical society. to his other honours was added that of chevalier of the prussian order, "pour la mérite," founded by frederick the great, and bestowed at all times with a discrimination which renders it a deeply-coveted distinction. of the academies and leading scientific institutions of the continent and the united states, he was also an honorary or corresponding member. besides his works on meteorology and physical geography, he published, in , an admirable little volume--"familiar lectures on scientific subjects." in this he showed that he could write with as much ease and intelligibility for the general public as for the higher order of scientific inquirers. his style in this valuable manual of information has a charm of its own, and entices the reader into the consideration of subjects apparently abstruse. he is earned on from page to page without any great mental effort, and finds himself rapidly mastering difficulties which he had been accustomed to regard as insuperable. let us take the first lecture on "volcanoes and earthquakes," and obtain a glimpse of herschel's mode of treatment. he refers to the greater and more permanent agencies which affect the configuration of our planet. everywhere, he says, and along every coast-line, we see the sea warring against the land, and overcoming it; wearing it and eating it down, and battering it to pieces; grinding those pieces to powder; carrying that powder away, and spreading it out over its own bottom, by the continued effect of the tides and currents. what a scene of continual activity is presented by the chalk-cliffs of old england! how they are worn, and broken up, and fantastically sculptured by the influence of winds and waters! precipices cut down to the sea-beach, constantly hammered by the waves, and constantly crumbling; the beach itself made of the flints outstanding after the softer chalk has been ground down and washed away; themselves grinding one another under the same ceaseless discipline--first rounded into pebbles, then worn into sand, and then carried further and further down the slope, to be replaced by fresh ones from the same source. here the likeness of an old gothic cathedral, with lofty arch, and shapely pinnacle; there the similitude of a mass of medieval fortifications, with crumbling battlements and shattered towers! the same thing, the same waste and wear, is going on everywhere, round every coast. the rivers contribute their share to the great work of change. look at the sand-banks at the mouth of the thames. what are they, says sir john herschel, but the materials of our island carried out to sea by the stream? the ganges carries away from the soil of india, and delivers into the sea, twice as much solid substance weekly as is contained in the great pyramid of egypt. the irawaddy sweeps off from burmah sixty-two cubic feet of earth in every second of time, on an average sometimes vast amount of earthy materials is transferred from one locality to another by river agency, as is the case in the deltas of the nile and the mississippi. these changes operate silently, continuously, and unperceived by the ordinary observer; but nature does not limit herself always and everywhere to such peaceful agencies. at times, and in certain places, she acts with startling abruptness and extraordinary violence. let the volcano and the earthquake attest the immensity of her power. let the earthquake tell how, within the memory of man, the whole coast-line of chili, for miles about valparaiso, with the mighty chain of the andes, was hoisted at one blow, and in a single night (november , ), from two to seven feet above its former level, leaving the beach below the old low-water mark high and dry. one of the andean peaks upheaved on this occasion was the colossal mass of aconcagua, which overlooks valparaiso, and measures nearly , feet in height. on the same occasion, at least , square miles of country were estimated as having been upheaved; and the upheaval was not confined to the land, but extended far away to sea,--which was proved by the soundings off valparaiso and along the coast having been found considerably shallower than they were before the shock. in the year , in an earthquake in india, in the district of cutch, bordering on the indus, a tract of country more than fifty miles long and sixteen miles broad was suddenly raised _ten feet_ above its former level. the raised portion still stands up above the unraised, like a long perpendicular rampart, known by the name of ullah bund, or god's wall. * * * * * with a similar fertility of illustration, herschel sets before us the phenomena of volcanic eruptions and their extraordinary effects. in a district of mexico, between the two streams of the cintimba and the san pedro, on the th of september , a whole tract of ground, from three to four miles in extent, surged up like a foam-bubble, or the swell of a wave, to a height of upwards of feet. flames, lurid and crackling, broke forth over a surface of more than half a square league; and the earth, as if softened by heat, was seen to rise and sink like the rolling tide. vast chasms opened in the earth, into which the two rivers poured their waters headlong; reappearing afterwards at no great distance from a cluster of _hornitos_, or small volcanic cones, which sprang out of the mighty mud-torrent that gradually covered the entire plain. wonderful and awful as were these phenomena, they were surpassed by the sudden opening of a chasm which vomited forth fire, and red-hot stones and ashes, until they accumulated in a range of six large mountain masses,--one of which, now known as the volcano of jorullo, attains an altitude of feet above the ancient level. in like manner sir john proceeds to describe an eruption of mount tomboro, in the island of sumbawa, the influence of which was felt to a distance of miles from its centre, in strange tremulous motions of the earth, and in the clash and clang of loud explosions. he says that he had seen it computed that the quantity of ashes and lava ejected in the course of this tremendous eruption would have formed three mountains of the size of mont blanc. as to the nature of the forces which operate to produce this astounding result, herschel puts forward a theory of singular simplicity and directness. "the origin," he says, "of such an enormous power thus occasionally exerting itself, will no doubt seem very marvellous--little short, indeed, of miraculous intervention; but the mystery, after all, is not quite so great as at first it seems. we are permitted to look a little way into these great secrets of nature; not far enough, indeed, to clear up every difficulty, but quite enough to penetrate us with admiration of that wonderful system of counterbalances and compensations, that adjustment of causes and consequences, by which, throughout all nature, evils are made to work their own cure, life to spring out of death, and renovation to tread in the steps and efface the vestiges of decay." and he finds the clew to the secret, the key of the whole matter, in the earth's vast central heat. this it is which produces the convulsions that change the terrestrial configuration, and fill the minds of men with fear and awe. conceive of "a sea of fire, on which we are all floating, land and sea,"--a boiling, seething, incandescent reservoir in the centre of our planet; and the solution of the problem will seem to you not difficult. such a sea would necessarily roll its liquid matter to and fro; and the removal of ever so small a portion from one point to another on the earth's surface would tend to disturb the equilibrium of the floating mass; just as, when a ship is launched into the river, the water it displaces is carried to the opposite bank with greater or less violence, according to the amount of displacement. it is impossible, adds herschel, but that this increase of pressure in some places and relief in others must be very unequal in their bearings. so that at some point or another our planet's floating crust must be brought into a state of strain, and if there be a weak or a soft part a crack will at last take place. this is exactly what happened in the earthquake which originated the allah bund, or god's wall, in cutch. volcanic eruptions are easily explicable on this principle,--the volcano being simply a vent for the passage of heated and molten matter, which the elevating pressure of the liquid below tends to eject. it is a well-known fact that volcanoes and earthquake-centres are nearly all situated on the borders or in the immediate neighbourhood of seas and oceans; and the reason would seem to be, that at such positions the accumulation of transported matter would necessarily attain its maximum, to whatever cause it might be due. then again, as herschel points out, the eruption of scorite and lava from the mouths of volcanoes, the result of the upward movement of the fiery liquid below, compensates in some degree for the downward transfer of material by detritus and alluvial deposits. hence it may be inferred that, on the whole, the quantity of solid matter above the ocean-level probably remains nearly always at the same amount. * * * * * it is with this ease and lucidity that sir john deals with scientific subjects of the greatest importance,--his genius resembling the elephant's trunk, which can balance a straw or rend an oak. in private life he displayed a simplicity of manner in harmony with the general unassumingness of his character. in his books as in society, in society as in his books, he was the same,--that is, free from all ostentation, free from self-pride, free from the arrogance of superior knowledge, and as ready to unbend himself to a child as to discourse with men of science. his career was a tranquil and a prosperous one, and, apart from the record of his discoveries and his honours, presents nothing of interest. he was peculiarly happy in his domestic relations; and in the wide circle of friends attracted by the mingled charm of his intellect and manners. a devout christian, a man of generosity and culture, a philosopher of great breadth of view and infinite patience of research,--we can place few better or brighter examples before our english youth than sir john herschel. chapter v. we could not conclude our notice of this remarkable family without some further allusion to its not least remarkable member--caroline lucretia herschel. to her varied accomplishments, her astronomical researches, and, above all, to her unwearied and unselfish devotion to her brother william, we have already made frequent allusion. she seemed to live for him and in him, to live for his fame and prosperity; and she poured out at his feet the treasures of an inexhaustible affection. to assist him in his labours, at whatever sacrifice, was her sole object in life; and she was certainly more careful for his reputation than was he himself. during his declining years she was his principal stay and support, and she was in daily attendance to note down or to calculate the results of his observations. his death was a severe blow to her; but, with characteristic courage, she retired to hanover, gave herself up to scientific pursuits, and in comparative solitude spent her later years. her biographer writes:-- "when all was over, her only desire seems to have been to hurry away. hardly was her brother laid in his grave than she collected the few things she cared to keep, and left for ever the country where she had spent fifty years of her life, living and toiling for him and him only. 'if i should leave off making memorandums of such events as affect or are interesting to me, i should feel like what i am,--namely, a person that has nothing more to do in this world.' mournful words! doubly mournful, when we know that the writer had nearly half an ordinary lifetime still between her and that grave which she made haste to prepare, in the hope that her course was nearly run. who can think of her, at the age of seventy-two, heart-broken and desolate, going back to the home of her youth in the fond expectation of finding consolation, without a pang of sympathetic pity? she found everything changed." _that_, indeed, is to all of us the greatest grief, when we return to the home of our youth. it is as if, during the years of our absence, we had expected everything to stand as still as in the palace of the sleeping beauty while the charm rested upon it. we are fain to see the trees in their young greenness as when they sheltered our childhood, to find the hedgerows blooming with the same violets, to hear the mill-stream murmuring with the same music. time furrows our brows with wrinkles, and streaks our hair with silver; our hearts grow colder; our minds lose their elasticity and freshness; our friends pass away from our side. but still we think to ourselves that in the old scenes all things are as they were. we say to ourselves: the bird sings as of old in the elm-trees at the garden-foot; the rose-bush blossoms as of old against our favourite window. "the varying year with blade and sheaf clothes and re-clothes the happy plains; here rests the sap within the leaf, here stays the blood along the veins. faint shadows, vapours lightly curled, faint murmurs from the meadows come, like hints and echoes of the world to spirits folded in the womb." * * * * * but we regain the old familiar places, and, alas! we find that change has been as busy with them as with us. the signs of decay are upon the trees; the brook has ceased to flow; the rose-bush has withered to the ground. there are trees as green and streams as musical and flowers as sweet as in our youth; but they are not the streams or flowers or trees which delighted us, and to us they can never be as dear. but a worse alteration has taken place than any visible in the face of nature. we discover that we have lost the old habits, the old capacity of enjoyment; and we soon discover that it was the sympathies, the hopes, the aspirations of youth which, after all, lent to these early scenes their rare and irrecoverable attraction. and thus it was that miss herschel found everything changed. a life of fifty years spent in a certain routine and upon certain objects, had unfitted her to tread in the old paths. it soon became clear to her that all her ideas and feelings had been shaped and influenced in a totally different path. more bitter still, we are told, she came to know that in her great sorrow and inextinguishable love she was all alone. and bitterest of all was the feeling that, in losing her brother she had lost the glory of her life, the source of her intellectual enjoyment. "you don't know," she wrote to a friend, "the blank of life after having lived within the radiance of genius." yet to live in this blankness, and to do the best she could with it, became the work of caroline lucretia herschel at the age of threescore years and ten,--an age when most of us have already put off our cares and anxieties, but when she began to enter on a new life, with new habits, new duties, and new associations. her interest in astronomical pursuits never slackened, and she watched with eagerness the labours and successes of her nephew. the respect paid to her in society as a "woman of science" was not unwelcome, though she affected to make light of it. "you must give me leave," she wrote to sir john, "to send you any publications you can think of, without mentioning anything about paying for them. for it is necessary i should every now and then lay out a little of my spare cash in that, for the sake of supporting the reputation of being a learned lady; (there is for you!) for i am not only looked at for such a one, but even stared at here in hanover!" it was with unaffected modesty she deprecated the honorary membership of the irish academy, conferred on one who, she said, had not for many years discovered even a comet; yet she was by no means insensible to the distinction. every man of scientific eminence who visited hanover visited this aged lady; and her presence in the theatre, even in her latest years, was a constant source of attraction. such was the simple frugality of her habits, that she experienced an actual difficulty in disposing of her income. she affirmed that the largest sum she could spend upon herself was £ a year; and the annual pension of £ , left by her brother, she refused, or else devoted the quarterly or half-yearly payment to the purchase of some handsome present for her nephew or niece. such was caroline lucretia herschel; and as such she was a remarkable proof that the rarest womanly gifts of affectionate forethought and loving devotion may exist in combination with intellectual strength and scientific enthusiasm. of the force, keenness, and permanency of her sisterly love, an illustration of a pathetic character occurs in a letter which she addressed to her nephew, february , :-- "i am grown much thinner than i was six months ago: when i look at my hands, they put me so in mind of what your dear father's were, when i saw them tremble under my eyes, as we latterly played at backgammon together." it has long been the reproach of england that she treats, or rather that her government treats, her men of science, her artists, and her litterateurs with a disgraceful parsimony. it would appear from the following letter that sir william herschel was inadequately rewarded, and that his sister felt this keenly:-- "there can be no harm," she says, "in telling my own dear nephew that i never felt satisfied with the support your father received towards his undertakings, and far less with the ungracious manner in which it was granted. for the last sum came with a message that more must never be asked for. (oh! how degraded i felt, even for myself, whenever i thought of it!) and after all it came too late, and was not sufficient; for if expenses had been out of question, there would not have been so much time, and labour, and expense, for twenty-four men were at times by turns, day and night, at work, wasted on the first mirror, which had come out too light in the casting (alex more than once would have destroyed it secretly, if i had not persuaded him against it); and without two mirrors, you know, such an instrument cannot be always ready for observing. "but what grieved me most was that to the last your poor father was struggling above his strength against difficulties which he well knew might have been removed if it had not been attended with too much expense. the last time the mirror was obliged to be taken from the polisher on account of some obstacle, i heard him say (in his usual manner of thinking aloud on such occasions), 'it is impossible to make the machine act as required without a room three times as large as this.' "i must say a few words of apology for the good king (george iii.), and ascribe the close bargains which were made between him and my brother to the _shabby, mean-spirited advisers_ who were undoubtedly consulted on such occasions; but they are dead and gone, and no more of them." in february , the great services which this high-souled woman had rendered to astronomical science were fitly rewarded by the presentation to her of the royal astronomical society's gold medal,--the greatest honour which an astronomer can receive. mr. south, himself an astronomer of deserved repute, was charged with the duty of presenting the medal; and in the course of his address he dwelt on the labours of her brother, and the share she had had in them. sir william's first catalogue of new nebulae and clusters of stars, he said, amounting in number to one thousand, was compiled with observations made from a twenty-foot reflector in the years , , and . by the same instrument he was enabled to discover the positions of a second thousand of these distant worlds in to ; while the places of five hundred others were registered on the celestial map between and . what, we may ask, were the discoveries of columbus compared with these? he revealed to europe the existence of only a single continent; herschel unfolded to man the mysteries of the depths of the heavens. but, continued mr. south, when we have thus enumerated the results obtained in the course of "sweeps" with this instrument, and taken into consideration the extent and variety of the other observations which were at the same time in progress, a most important part yet remains untold. who participated in his toils? who braved with him all the experiences of inclement weather? who shared, and consoled him in, his privations? a woman. and who was she? his sister. miss herschel it was who by night acted as his amanuensis; she it was whose pen conveyed to paper his observations as they issued from his lips; she it was who noted the various aspects and phenomena of the objects observed; she it was who, after spending the still night beside the wonder-exhibiting instrument, carried the rough, blurred manuscripts to her cottage at daybreak, and by the morning produced a clean copy and register of the night's achievements; she it was who planned the labour of each succeeding night; she it was who reduced into exact form every calculation; she it was who arranged the whole in systematic order; and she it was who largely assisted her illustrious brother to obtain his imperishable renown. miss herschel's claims to the gratitude of men of science, and to the admiration of all who can appreciate the beauty of self-sacrifice, did not end here. she was herself an astronomer, and an original observer. at times her brother was enabled to dispense with her attendance. you would suppose that such leisure nights she would gladly give up to rest. not she. her brother might, at some unforeseen moment, require her aid, and consequently she preferred to be close at hand. a seven-foot telescope planted on the lawn helped to while away the hours of waiting; and it was to the occupation of these hours that science owed the discovery of the comet of , of the comet of , of the comet of , of the comet of , and of that of , now connected with the name of encke. many, also, of the nebulae contained in sir william herschel's catalogues were detected by her keen and accurate gaze during these nights of lonely observation. indeed, as south remarked, when looking at the joint-labours of these two enthusiasts, we scarcely know whether the warmer praise should be given to the intellectual might of the brother or the ardent industry of the sister. in , continued her eulogist, she presented to the royal society a catalogue of stars, taken from flamsteed's observations, the exact positions of which had not been previously defined. soon after the death of him to whom she had given up so much of her life, her best energies, and her ripest faculties, she returned to hanover,--unwilling, however, to relinquish the astronomical researches which had been so pure and permanent a source of pleasure. she undertook and completed the laborious "reduction" or registration of the places of nebulae, down to the st of january ; thus presenting in one view the results of all the observations sir william herschel had made upon those wonderful bodies, and triumphantly bringing to a close half a century of scientific toil. * * * * * we return to miss herschel's biography, in order to gather up a few particulars of her last years, and to exhibit some of the tenderer features of her character. on the occasion of her nephew's marriage, in , she wrote to him in the following terms:-- "my dearest nephew,--i have spent four days in vain endeavours to gain composure enough to give you an idea of the joyful sensation your letter of february th has caused me. but i can at this present moment find no words which would better express my happiness than those which escaped in exclamation from my lips, according to simeon (see st. luke ii. ), 'lord, now lettest thou thy servant depart in peace.' "i have now some hopes of passing the few remainder of my days in as much comfort as the separation from the land where i spent the greatest portion of my life, and from all those which are most dear to me, can admit. for, from the description given me of the dear young lady of your choice, i am confident my dear nephew's future happiness is now established. "i beg you will give my love to your dear lady, and best regards to all your new connections where they are due, in the best terms you can think of, for i am at present too unwell for writing all i could wish to say. "i have suffered much during this severe winter, and have not been able to leave my habitation above three or four times for the last three months; and feel, moreover, much fatigued by sitting eight times within the last ten days to professor tiedemann for having my picture taken--which he did at my apartment, and now he has taken it home to finish. i must conclude, for i wish to say a few words to your dear mother. it is now between eleven and twelve, and perhaps you are at this very moment receiving the blessing of dr. jennings; in which i most fervently join by saying, 'god bless you both!'" though eighty-three years old, miss herschel retained all her old powers of memory; and in a letter to her new niece, lady herschel, written in , she narrated some amusing reminiscences of her nephew's early childhood. he was only in his sixth year, she said, when she was separated for a while from the family circle. but this did not hinder "john" and her from remaining the most affectionate friends, and many a half or whole holiday he spent with her, devoting it to chemical experiments, in which all kinds of boxes, tops of tea-canisters, pepper-cruets, tea-cups, and the like, served for the necessary vessels, and the sand-tub furnished the matter to be analysed. miss herschel's task was to prevent the introduction of water, which would have produced havoc on her carpet. for his first notion of building, "john" was indebted to the affection of his aunt, who, on his second or third birthday, lifted him in the trenches to lay the south corner-stone of the building which was added to sir william's original house at slough. on further reflection, she felt convinced that this incident occurred in the second year of her nephew's age, for she remembered being obliged to use "a deal of coaxing" to make him part with the money he was to lay on the comer-stone. about the same time, when she was sitting near him one day, listening to his prattle, her attention was drawn to his repeated and formidable hammering. on investigating into its object, she found that it was the continuation of the labour of many days, during which he had undermined the ground about the corner of the house, had entirely removed the corner-stone, and was zealously toiling to overthrow the next! his aunt gave the alarm, and old john wiltshire, a favourite carpenter, ran to the spot, exclaiming, "heaven bless the boy! if he is not going to pull the house down!" * * * * * in , sir john, as already stated, made a voyage to the cape of good hope, in order to undertake a series of observations of the southern heavens. his aunt had now reached the ripe old age of eighty-four, an age attained by few,--and when attained, bringing with it in almost every case a painful diminution of physical energy, and a corresponding decline in mental force. but such was not the case with this remarkable woman. she still continued an active correspondence with her nephew, and manifested the liveliest interest in all his movements. it is astonishing to mark the vivacity and clearness of the letters she wrote at this advanced period of her life. thus, on the st of may , she writes to sir john:-- "both yourself and my dear niece urged me to write often, and to write always twice; but, alas! i could not overcome the reluctance i felt of [at] telling you that it is over with me for getting up at eight or nine o'clock, dressing myself, eating my dinner alone without an appetite, falling asleep over a novel (i am obliged to lay down to recover the fatigue of the morning's exertions), awaking with nothing but the prospect of the trouble of getting into bed, where very seldom i get above two hours' sleep. it is enough to make a parson swear! to this i must add, i found full employment for the few moments, when i could rouse myself from a melancholy lethargy, to spend in looking over my store of astronomical and other memorandums of upwards of fifty years' collecting." later in the year she writes:-- "i know not how to thank you sufficiently for the cheering account you give of the climate agreeing so well with you and all who are so dear to me, and that you find all about you so agreeable and comfortable;... so that i have nothing left to wish for but a continuation of the same, and that i may only live to see the handwriting of your dear caroline, though i have my doubts about lasting till then, for the thermometer standing ° and ° for upwards of two mouths, day and night, in nay rooms (to which i am mostly confined), has made great havoc in my brittle constitution. i beg you will look to it that she learns to make her figures as you find them in your father's mss., such as he taught me to make. the daughter of a mathematician must write plain figures. "my little grand-nephew making alliance with your workmen shows that he is taking after his papa. i see you now in idea, running about in petticoats among your father's carpenters, working with little tools of your own; and john wiltshire (one of pitt's men, whom you may perhaps remember) crying out, 'dang the boy, if he can't drive in a nail as well as i can!' "i thank you for the astronomical portion of your letter, and for your promise of future accounts of uncommon objects. it is not _clusters of stars_ i want you to discover in the body of the scorpion [the astronomical sign, so called], or thereabout, for that does not answer my expectation, remembering having once heard your father, after a long, awful silence, exclaim, 'hier ist wahrhaftig ein loch ein himmel!' [here, indeed, is a great gap in heaven!], and, as i said before, stopping afterwards at the same spot, but leaving it unsatisfied." these extracts may seem trivial to some of our readers, but they are not so, rightly considered. they illustrate the wonderful mental vivacity of their venerable writer, and in this respect are useful; but still more useful in showing how cheerfully she bore the burden of her years, and with what intellectual serenity she looked forward to her end. we own that the lives of the herschels are what the world would call uneventful. the discovery of a new planet, or of the orbit of a star, seems less romantic to the vulgar taste than the slaughter of ten thousand men on a field of battle. it will seem to the unthinking that the victorious general or the daring seaman, the leader of a forlorn hope, or the captain who goes down with his sinking ship, affords an example worthier of imitation than the patient, watchful, enthusiastic astronomer or his devoted sister. _his_, they will say, was a noble life. be it so; but every life is noble which is spent in the path of duty. do what comes to your hand to do with all honesty and completeness, and you will make _your_ life noble. subdue your passions, master your evil thoughts, observe the laws of temperance and purity, be truthful, be firm, be honest, and keep ever before you the law of christ as the law of your daily work, and you will make _your_ life noble. we cannot all be great commanders or daring captains, we cannot all be distinguished men of science; but we can all be righteously-living men, endeavouring to raise others by our example, and it is a higher aim to live purely than to live successfully. we cannot all command the success, just as we do not all enjoy the intellectual powers, of a herschel; but we can emulate the industry and perseverance of the astronomer, we can copy the devoted affection and self-denial of his sister. the sorriest mistake of which men can be guilty,--yet it is a mistake which has clouded many lives,--is to suppose that duty is less imperative in its claims on the humble and unknown than on men raised or born to eminent position. let it be understood and remembered that each one of us can rise to a standard of true heroism, by cultivating the graces of the christian character, and doing the work which god has appointed. * * * * * sir john herschel returned to england in , and in july of the same year he and his little son paid a visit to miss herschel. it is characteristic that her intense anxiety as to the proper treatment of her little grand-nephew--his sleep, his food, his playthings--greatly disturbed her peace. "i rather suffered him," she writes, "to hunger, than would let him eat anything hurtful; indeed, i would not let him eat anything at all unless his papa was present." her biographer remarks, that great as was her joy to see once more almost the only living being upon whom she poured some of that wealth of affection with which her heart never ceased to overflow, yet it was on the disappointments and shortcomings of those few days, those precious days, that she chiefly dwelt; and the abrupt termination of her nephew's visit filled her with the deepest sorrow. with the generous, but, as it proved, mistaken intention of sparing her feelings, her nephew left without informing her beforehand of the exact time of his departure, simply bidding her good-night prior to his return to his inn. great was her distress when she found that he and his son had quitted hanover at four o'clock on the following morning. her introduction to her grand-nephew, as described by his father, sir john, was exceedingly quaint:-- "now, let me tell you how tilings fell out. dr. groskopff took willie with him to aunty, but without saying who he was. says she, 'what little boy is that?' says he, 'the son of a friend of mine. ask him his name.' however, willie would not tell his name. 'where do you come from, little fellow?' 'from the cape of good hope,' says willie. 'what is that he says?' 'he says he comes from the cape of good hope.' 'ay! and who is he? what is his name?' 'his name is herschel.' 'yes,'says willie. 'what is that he says?' 'he says he comes from the cape of good hope.' 'ay! and who is he? what is his name?' 'his name is herschel.' 'yes,' says willie, 'william james herschel.' 'ach, mem gott! das nicht möglich; ist dieser kleines neffeu's sohn?' and so it all came out; and when i came to her all was understood, and we sat down and talked as quietly as if we had parted but yesterday." * * * * * in a letter which she wrote to lady herschel in , we find some reminiscences of her early years. she says that when, at the age of twenty-two, she first visited england, there was no kind of ornamental needle-work, knitting, plaiting hair, stringing beads and bugles, and the like, of which she did not make samples by way of mastering the art. as she was the only girl, and consequently the cinderella, of the family, she could not find time, however, for much self-improvement. she was not, for instance, a skilled musician, but she was able to play the second violin part of an overture or easy quartette. and it is worth notice that the herschels were something more than astronomers only. both sir william and his son, great as they were in their special department of science, took care to cultivate their minds generally; were mathematicians, chemists, geologists, and men of letters. and here is a lesson for our younger readers. the mind should always be diverted towards one particular object; it should be the aim of everybody to attain towards supreme excellence, if possible, in some one pursuit. on the other hand, he should gather knowledge, more or less, in every field, so as to avoid narrowness of view and poverty of idea. versatility does not necessarily mean superficiality; we may know much of many things, and more of one thing. a man who is only a botanist, shuts himself out from all the truest and deepest pleasures of knowledge. it may be very clever for a violinist to play on a single string; but he must play on _all_, if he would bring out the full harmonies of his instrument, and do justice to its extraordinary powers. * * * * * miss herschel's enjoyment of life, which, when not carried to an excess, is a christian duty, continued to the very last. when she was in her ninetieth year, she rose as usual every day, dressed, ate, drank, rested on her sofa, read and conversed with her numerous visitors; still taking an interest in science and literature, even in public affairs, and still occupying herself with all that concerned the evergrowing reputation of her nephew. of course, she could not escape the infirmities of old age, but by cheerfulness and patience she did her best to alleviate them. in recalling incidents of her early life, she frequently gave evidence of her good-humoured contentment. in , writing to her niece, she refers to an incident which occurred in the early part of the forty-foot telescope's existence, when "god save the king" was sung in it by her brother and his guests, who rose from the dinner-table for the purpose, and entered the tube in procession. she adds that among the company were two misses stows, one of whom was a famous pianoforte player; some of the griesbachs (well-known musicians), who accompanied on the oboe, or any instrument they could get hold of; and herself, who was one of the nimblest and foremost to get in and out of the tube. "but now," she adds, "lack-a-day! i can hardly cross the room without help. but what of that? dorcas, in the _beggar's opera_, says, 'one cannot eat one's cake and have it too!'" she relates, in the same letter, a curious anecdote of the old and celebrated tube. before the optical apparatus was finished, many visitors took a pleasure in walking through it,--among the rest, on one occasion, king george iii. and the archbishop of canterbury. the latter following the king, and finding it difficult to proceed, his majesty turned and gave him his hand, saying, "come, my lord bishop; i will show you the way to heaven!" then, with that astonishing memory of hers, which kept its greenness until the very last, she notes that this occurred on august , , when the king and queen, the duke of york, and some of the princesses were of the company. * * * * * from another letter we take a lively little picture of a christmas in hanover:-- she had been told that keeping christmas in the german sense was coming to be very general in england; but her shrewd, practical turn of mind induced her to hope that the english would never go "such lengths in foolery." at hanover, she wrote, the tradespeople had been for many weeks in full employ, framing and mounting the embroideries of the ladies and girls of all classes; of _all_ classes, for not a folly or extravagancy existed among the great but it was imitated by the little. the shops were beautifully lighted up by gas, and the last three days before christmas all that could tempt or attract was exhibited in the market-places in booths lighted up in the evening, whither everybody hastened to gaze and to spend their money. cooks and housemaids presented one another with knitted bags and purses; the cobbler's daughter embroidered "neck-cushions" for her friend the butcher's daughter. these were made up by the upholsterer at great expense, lined with white satin; the upper part, on which the back rested, being wrought with gold, silver, and pearls. * * * * * but we must no longer delay the reader by our gossip. enough has been said to illustrate the character of a remarkable woman, and of those features of it--her cheerfulness, her patience, her industry, her devoted affection, her unselfishness--which all of us may be the better for studying and imitating. our limits compel us to draw our simple narrative to a close, and we must pass over the delight with which she received and read sir john herschel's great work, "cape observations,"--a noble monument of the perseverance and strenuous labour of genius; but of twofold interest to her, because it not only testified to the eminent qualities of her nephew, but brought to a noble conclusion the vast undertaking of that nephew's father and her own beloved brother--the survey of the nebulous heavens. a letter written by her friend miss becksdorff, on the th of january , describes caroline herschel's last days:-- "her decided objection to having her bed placed in a warmer room had brought on a cold and cough; and so firm was her determination to preserve her old customs, and not to yield to increasing infirmities, that when, upon her doctor's positive orders, i had a bed made up in her room, before she came to sit in it one day, it was not till two o'clock in the night that betty could persuade her to lie down in it. upon going to her the next morning, i had the satisfaction, however, of finding her perfectly reconciled to the arrangement; she now felt the comfort of being undisturbed, and she has kept to her bed ever since. her mental and bodily strength is gradually declining. but a few days ago she was ready for a joke. when mrs. clarke told her that general halkett sent his love, and 'hoped she would soon be so well again that he might come and give her a kiss, as he had done on her birthday,' she looked only archly at her, and said, 'tell the general that i have not tasted anything since i liked so well.' i have just left her, and upon my asking her to give me a message for her nephew, she said, 'tell them i am good for nothing,' and went to sleep again." on the th of january she breathed her last, passing away with a christian's tranquillity.[ ] [footnote : the particulars recorded in the foregoing pages are chiefly taken from mrs. john herschel's very interesting "memoir and correspondence of caroline herschel."] * * * * * her body was followed to the grave by many of her relatives and friends, the royal carriages forming part of the funeral procession. the coffin was adorned with garlands of laurel and cypress and palm branches, sent by the crown-princess from herrnhausen; and the service was conducted in that same garrison-church in which, nearly a century before, she had been christened, and afterwards confirmed. and, as proving her love and fidelity to the last, in her coffin were placed, by her express desire, "a lock of her beloved brother's hair, and an old, almost obliterated almanac that had been used by her father." * * * * * may our readers be induced, by their perusal of these pages, to emulate the herschels--brother, sister, nephew--in all the bright and lovely qualities that ennoble life; in their fixity of purpose, their elevation of thought, their purity of character, their self-denial, their industry, their hopefulness, and their faith! [the following inscription is engraved on miss herschel's tomb. it begins: "hier ruhet die irdische hülle von carolina herschel, geboren zu hannover den ten marz , gestorben, den ten januar ." but, for the convenience of our young readers, we give it in english:-- here rests the earthly case of c a r o l i n e h e r s c h e l. born at hanover, march , . died january , . "the eyes of her now glorified were, while here below, directed towards the starry heavens. her own discoveries of comets, and her share in the immortal labours of her brother, william herschel, bear witness of this to succeeding ages. "the royal irish academy of dublin, and the royal astronomical society of london, enrolled her name among their members. "at the age of years months, she fell asleep in calm rest, and in the full possession of her faculties; following into a better life her father, isaac herschel, who lived to the age of years, months, days, and has lain buried not far off since the th of march ." this epitaph was mainly written by miss herschel herself, and the allusion to her brother is characteristic.] [illustration path of biela's comet.] letters on astronomy, in which the elements of the science are familiarly explained in connection with biographical sketches of the most eminent astronomers. with numerous engravings. by denison olmsted, ll.d., professor of natural philosophy and astronomy in yale college revised edition. including the latest discoveries. new york: harper & brothers, publishers, & pearl street, franklin square. . entered according to act of congress, in the year , by marsh, capen, lyon, and webb, in the clerk's office of the district court of massachusetts. advertisement to the revised edition. since the first publication of these letters, in , the work has passed through numerous editions, and received many tokens of public favor, both as a class-book for schools and as a reading-book for the family circle. the valuable discoveries made in the science within a few years have suggested an additional letter, which is accordingly annexed to the series in the present revised form, giving a brief but comprehensive notice of all the leading contributions with which astronomy has of late been enriched. the form of _letters_ was chosen on account of the greater freedom it admits, both of matter and of style, than a dress more purely scientific. thus the technical portion of the work, it was hoped, might be relieved, and the whole rendered attractive to the youthful reader of either sex by interspersing sketches of the master-builders who, in successive ages, have reared the great temple of astronomy, composing, as they do, some of the most remarkable and interesting specimens of the human race. the work was addressed to a female friend (now no more), who was a distinguished ornament of her sex, and whose superior intellect and refined taste required that the work should be free from every thing superficial in matter or negligent in style; and it was deemed by the writer no ordinary privilege that, in the composition of the work, an image at once so exalted and so pure was continually present to his mental vision. yale college, _january_, . contents. preface, letter i. introductory observations, letter ii. doctrine of the sphere, letter iii. astronomical instruments.--telescope, letter iv. telescope continued, letter v. observatories, letter vi. time and the calendar, letter vii. figure of the earth, letter viii. diurnal revolution, letter ix. parallax and refraction, letter x. the sun, letter xi. annual revolution.--seasons, letter xii. laws of motion, letter xiii. terrestrial gravity, letter xiv. sir isaac newton.--universal gravitation.--figure of the earth's orbit.--precession of the equinoxes, letter xv. the moon, letter xvi. the moon.--phases.--harvest moon.--librations, letter xvii. moon's orbit.--her irregularities, letter xviii. eclipses, letter xix. longitude.--tides, letter xx. planets.--mercury and venus, letter xxi. superior planets: mars, jupiter, saturn, and uranus, letter xxii. copernicus.--galileo, letter xxiii. saturn.--uranus.--asteroids, letter xxiv. the planetary motions.--kepler's laws.--kepler, letter xxv. comets, letter xxvi. comets, letter xxvii. meteoric showers, letter xxviii. fixed stars, letter xxix. fixed stars, letter xxx. system of the world, letter xxxi. natural theology, letter xxxii. recent discoveries, index, letters on astronomy. letter . introductory observations. "ye sacred muses, with whose beauty fired, my soul is ravished, and my brain inspired, whose priest i am, whose holy fillets wear; would you your poet's first petition hear, give me the ways of wandering stars to know, the depths of heaven above, and earth below; teach me the various labors of the moon, and whence proceed th' eclipses of the sun; why flowing tides prevail upon the main, and in what dark recess they shrink again; what shakes the solid earth, what cause delays the summer nights, and shortens winter days." _dryden's virgil_ to mrs. c---- m----. dear madam,--in the conversation we recently held on the study of astronomy, you expressed a strong desire to become better acquainted with this noble science, but said you had always been repelled by the air of severity which it exhibits, arrayed as it is in so many technical terms, and such abstruse mathematical processes: or, if you had taken up some smaller treatise, with the hope of avoiding these perplexities, you had always found it so meager and superficial, as to afford you very little satisfaction. you asked, if a work might not be prepared, which would convey to the general reader some clear and adequate knowledge of the great discoveries in astronomy, and yet require for its perusal no greater preparation, than may be presumed of every well-educated english scholar of either sex. you were pleased to add the request, that i would write such a work,--a work which should combine, with a luminous exposition of the leading truths of the science, some account of the interesting historical facts with which it is said the records of astronomical discovery abound. having, moreover, heard much of the grand discoveries which, within the last fifty years, have been made among the _fixed stars_, you expressed a strong desire to learn more respecting these sublime researches. finally, you desired to see the argument for the existence and natural attributes of the deity, as furnished by astronomy, more fully and clearly exhibited, than is done in any work which you have hitherto perused. in the preparation of the proposed treatise, you urged me to supply, either in the text or in notes, every _elementary principle_ which would be essential to a perfect understanding of the work; for although, while at school, you had paid some attention to geometry and natural philosophy, yet so much time had since elapsed, that your memory required to be refreshed on the most simple principles of these elementary studies, and you preferred that i should consider you as altogether unacquainted with them. although, to satisfy a mind, so cultivated and inquisitive as yours, may require a greater variety of powers and attainments than i possess, yet, as you were pleased to urge me to the trial, i have resolved to make the attempt, and will see how far i may be able to lead you into the interior of this beautiful temple, without obliging you to force your way through the "jargon of the schools." astronomy, however, is a very difficult or a comparatively easy study, according to the view we take of it. the investigation of the great laws which govern the motions of the heavenly bodies has commanded the highest efforts of the human mind; but profound truths, which it required the mightiest efforts of the intellect to disclose, are often, when once discovered, simple in their complexion, and may be expressed in very simple terms. thus, the creation of that element, on whose mysterious agency depend all the forms of beauty and loveliness, is enunciated in these few monosyllables, "and god said, let there be light, and there was light;" and the doctrine of universal gravitation, which is the key that unlocks the mysteries of the universe, is simply this,--that every portion of matter in the universe tends towards every other. the three great laws of motion, also, are, when stated, so plain, that they seem hardly to assert any thing but what we knew before. that all bodies, if at rest, will continue so, as is declared by the first law of motion, until some force moves them; or, if in motion, will continue so, until some force stops them, appears so much a matter of course, that we can at first hardly see any good reason why it should be dignified with the title of the first great law of motion; and yet it contains a truth which it required profound sagacity to discover and expound. it is, therefore, a pleasing consideration to those who have not either the leisure of the ability to follow the astronomer through the intricate and laborious processes, which conducted him to his great discoveries, that they may fully avail themselves of the _results_ of this vast toil, and easily understand truths which it required ages of the severest labor to unfold. the descriptive parts of astronomy, or what may be called the natural history of the heavens, is still more easily understood than the laws of the celestial motions. the revelations of the telescope, and the wonders it has disclosed in the sun, in the moon, in the planets, and especially in the fixed stars, are facts not difficult to be understood, although they may affect the mind with astonishment. the great practical purpose of astronomy to the world is, enabling us safely to navigate the ocean. there are indeed many other benefits which it confers on man; but this is the most important. if, however, you ask, what advantages the study of astronomy promises, as a branch of education, i answer, that few subjects promise to the mind so much profit and entertainment. it is agreed by writers on the human mind, that the intellectual powers are enlarged and strengthened by the habitual contemplation of great objects, while they are contracted and weakened by being constantly employed upon little or trifling subjects. the former elevate, the latter depress, the mind, to their own level. now, every thing in astronomy is great. the magnitudes, distances, and motions, of the heavenly bodies; the amplitude of the firmament itself; and the magnificence of the orbs with which it is lighted, supply exhaustless materials for contemplation, and stimulate the mind to its noblest efforts. the emotion felt by the astronomer is not that sudden excitement or ecstasy, which wears out life, but it is a continued glow of exalted feeling, which gives the sensation of breathing in a purer atmosphere than others enjoy. we should at first imagine, that a study which calls upon its votaries for the severest efforts of the human intellect, which demands the undivided toil of years, and which robs the night of its accustomed hours of repose, would abridge the period of life; but it is a singular fact, that distinguished astronomers, as a class, have been remarkable for longevity. i know not how to account for this fact, unless we suppose that the study of astronomy itself has something inherent in it, which sustains its votaries by a peculiar aliment. it is the privilege of the student of this department of nature, that his cabinet is already collected, and is ever before him; and he is exempted from the toil of collecting his materials of study and illustration, by traversing land and sea, or by penetrating into the depths of the earth. nor are they in their nature frail and perishable. no sooner is the veil of clouds removed, that occasionally conceals the firmament by night, than his specimens are displayed to view, bright and changeless. the renewed pleasure which he feels, at every new survey of the constellations, grows into an affection for objects which have so often ministered to his happiness. his imagination aids him in giving them a personification, like that which the ancients gave to the constellations; (as is evident from the names which they have transmitted to us;) and he walks abroad, beneath the evening canopy, with the conscious satisfaction and delight of being in the presence of old friends. this emotion becomes stronger when he wanders far from home. other objects of his attachment desert him; the face of society changes; the earth presents new features; but the same sun illumines the day, the same moon adorns the night, and the same bright stars still attend him. when, moreover, the student of the heavens can command the aid of telescopes, of higher and higher powers, new acquaintances are made every evening. the sight of each new member of the starry train, that the telescope successively reveals to him, inspires a peculiar emotion of pleasure; and he at length finds himself, whenever he sweeps his telescope over the firmament, greeted by smiles, unperceived and unknown to his fellow-mortals. the same personification is given to these objects as to the constellations, and he seems to himself, at times, when he has penetrated into the remotest depths of ether, to enjoy the high prerogative of holding converse with the celestials. it is no small encouragement, to one who wishes to acquire a knowledge of the heavens, that the subject is embarrassed with far less that is technical than most other branches of natural history. having first learned a few definitions, and the principal circles into which, for convenience, the sphere is divided, and receiving the great laws of astronomy on the authority of the eminent persons who have investigated them, you will find few hard terms, or technical distinctions, to repel or perplex you; and you will, i hope, find that nothing but an intelligent mind and fixed attention are requisite for perusing the letters which i propose to address to you. i shall indeed be greatly disappointed, if the perusal does not inspire you with some portion of that pleasure, which i have described as enjoyed by the astronomer himself. the dignity of the study of the heavenly bodies, and its suitableness to the most refined and cultivated mind, has been recognised in all ages. virgil celebrates it in the beautiful strains with which i have headed this letter, and similar sentiments have ever been cherished by the greatest minds. as, in the course of these letters, i propose to trace an outline of the history of astronomy, from the earliest ages to the present time, you may think this the most suitable place for introducing it; but the successive discoveries in the science cannot be fully understood and appreciated, until after an acquaintance has been formed with the science itself. we must therefore reserve the details of this subject for a future opportunity; but it may be stated, here, that astronomy was cultivated the earliest of all the sciences; that great attention was paid to it by several very ancient nations, as the egyptians and chaldeans, and the people of india and china, before it took its rise in greece. more than six hundred years before the christian era, however, it began to be studied in this latter country. thales and pythagoras were particularly distinguished for their devotion to this science; and the celebrated school of alexandria, in egypt, which took its rise about three hundred years before the christian era, and flourished for several hundred years, numbered among its disciples a succession of eminent astronomers, among whom were hipparchus, eratosthenes, and ptolemy. the last of these composed a great work on astronomy, called the 'almagest,' in which is transmitted to us an account of all that was known of the science by the alexandrian school. the 'almagest' was the principal text-book in astronomy, for many centuries afterwards, and comparatively few improvements were made until the age of copernicus. copernicus was born at thorn, in prussia, in . previous to his time, the doctrine was held, that the earth is at rest in the centre of the universe, and that the sun, moon, and stars, revolve about it, every day, from east to west; in short, that the _apparent_ motions of the heavenly bodies are the same with their _real_ motions. but copernicus expounded what is now known to be the true theory of the celestial motions, in which the sun is placed in the centre of the solar system, and the earth and all the planets are made to revolve around him, from west to east, while the apparent diurnal motion of the heavenly bodies, from east to west, is explained by the revolution of the earth on its axis, in the same time, from west to east; a motion of which we are unconscious, and which we erroneously ascribe to external objects, as we imagine the shore is receding from us, when we are unconscious of the motion of the ship that carries us from it. although many of the appearances, presented by the motions of the heavenly bodies, may be explained on the former erroneous hypothesis, yet, like other hypotheses founded in error, it was continually leading its votaries into difficulties, and blinding their minds to the perception of truth. they had advanced nearly as far as it was practicable to go in the wrong road; and the great and sublime discoveries of modern times are owing, in no small degree, to the fact, that, since the days of copernicus, astronomers have been pursuing the plain and simple path of truth, instead of threading their way through the mazes of error. near the close of the sixteenth century, tycho brahe, a native of sweden, but a resident of denmark, carried astronomical observations (which constitute the basis of all that is valuable in astronomy) to a far greater degree of perfection than had ever been done before. kepler, a native of germany, one of the greatest geniuses the world has ever seen, was contemporary with tycho brahe, and was associated with him in a part of his labors. galileo, an italian astronomer of great eminence, flourished only a little later than tycho brahe. he invented the telescope, and, both by his discoveries and reasonings, contributed greatly to establish the true system of the world. soon after the commencement of the seventeenth century, ( ,) lord bacon, a celebrated english philosopher, pointed out the true method of conducting all inquiries into the phenomena of nature, and introduced the _inductive method of philosophizing_. according to the inductive method, we are to begin our inquiries into the causes of any events by first examining and classifying all the _facts_ that relate to it, and, from the comparison of these, to deduce our conclusions. but the greatest single discovery, that has ever been made in astronomy, was the law of universal gravitation, a discovery made by sir isaac newton, in the latter part of the seventeenth century. the discovery of this law made us acquainted with the hidden forces that move the great machinery of the universe. it furnished the key which unlocks the inner temple of nature; and from this time we may regard astronomy as fixed on a sure and immovable basis. i shall hereafter endeavor to explain to you the leading principles of universal gravitation, when we come to the proper place for inquiring into the causes of the celestial motions, as exemplified in the motion of the earth around the sun. letter ii. doctrine of the sphere. "all are but parts of one stupendous whole, whose body nature is, and god the soul."--_pope._ let us now consider what astronomy is, and into what great divisions it is distributed; and then we will take a cursory view of the doctrine of the sphere. this subject will probably be less interesting to you than many that are to follow; but still, permit me to urge upon you the necessity of studying it with attention, and reflecting upon each definition, until you fully understand it; for, unless you fully and clearly comprehend the circles of the sphere, and the use that is made of them in astronomy, a mist will hang over every subsequent portion of the science. i beg you, therefore, to pause upon every paragraph of this letter; and if there is any point in the whole which you cannot clearly understand, i would advise you to mark it, and to recur to it repeatedly; and, if you finally cannot obtain a clear idea of it yourself, i would recommend to you to apply for aid to some of your friends, who may be able to assist you. _astronomy is that science which treats of the heavenly bodies._ more particularly, its object is to teach what is known respecting the sun, moon, planets, comets, and fixed stars; and also to explain the methods by which this knowledge is acquired. astronomy is sometimes divided into descriptive, physical, and practical. descriptive astronomy respects _facts_; physical astronomy, _causes_; practical astronomy, the _means of investigating the facts_, whether by instruments or by calculation. it is the province of descriptive astronomy to observe, classify, and record, all the phenomena of the heavenly bodies, whether pertaining to those bodies individually, or resulting from their motions and mutual relations. it is the part of physical astronomy to explain the causes of these phenomena, by investigating the general laws on which they depend; especially, by tracing out all the consequences of the law of universal gravitation. practical astronomy lends its aid to both the other departments. the definitions of the different lines, points, and circles, which are used in astronomy, and the propositions founded upon them, compose the _doctrine of the sphere_. before these definitions are given, i must recall to your recollection a few particulars respecting the method of measuring angles. (see fig. , page .) a line drawn from the centre to the circumference of a circle is called a _radius_, as c d, c b, or c k. any part of the circumference of a circle is called an _arc_, as a b, or b d. an angle is measured by an arc included between two radii. thus, in fig. , the angle contained between the two radii, c a and c b, that is, the angle a c b, is measured by the arc a b. every circle, it will be recollected, is divided into three hundred and sixty equal parts, called degrees; and any arc, as a b, contains a certain number of degrees, according to its length. thus, if the arc a b contains forty degrees, then the opposite angle a c b is said to be an angle of forty degrees, and to be measured by a b. but this arc is the same part of the smaller circle that e f is of the greater. the arc a b, therefore, contains the same number of degrees as the arc e f, and either may be taken as the measure of the angle a c b. as the whole circle contains three hundred and sixty degrees, it is evident, that the quarter of a circle, or _quadrant_, contains ninety degrees, and that the semicircle a b d g contains one hundred and eighty degrees. [illustration fig. .] the _complement_ of an arc, or angle, is what it wants of ninety degrees. thus, since a d is an arc of ninety degrees, b d is the complement of a b, and a b is the complement of b d. if a b denotes a certain number of degrees of latitude, b d will be the complement of the latitude, or the colatitude, as it is commonly written. the _supplement_ of an arc, or angle, is what it wants of one hundred and eighty degrees. thus, b a is the supplement of g d b, and g d b is the supplement of b a. if b a were twenty degrees of longitude, g d b, its supplement, would be one hundred and sixty degrees. an angle is said to be _subtended_ by the side which is opposite to it. thus, in the triangle a c k, the angle at c is subtended by the side a k, the angle at a by c k, and the angle at k by c a. in like manner, a side is said to be subtended by an angle, as a k by the angle at c. let us now proceed with the doctrine of the sphere. a section of a sphere, by a plane cutting it in any manner, is a circle. _great circles_ are those which pass through the centre of the sphere, and divide it into two equal hemispheres. _small circles_ are such as do not pass through the centre, but divide the sphere into two unequal parts. the _axis_ of a circle is a straight line passing through its centre at right angles to its plane. the _pole_ of a great circle is the point on the sphere where its axis cuts through the sphere. every great circle has two poles, each of which is every where ninety degrees from the great circle. all great circles of the sphere cut each other in two points diametrically opposite, and consequently their points of section are one hundred and eighty degrees apart. a great circle, which passes through the pole of another great circle, cuts the latter at right angles. the great circle which passes through the pole of another great circle, and is at right angles to it, is called a _secondary_ to that circle. the angle made by two great circles on the surface of the sphere is measured by an arc of another great circle, of which the angular point is the pole, being the arc of that great circle intercepted between those two circles. in order to fix the position of any place, either on the surface of the earth or in the heavens, both the earth and the heavens are conceived to be divided into separate portions, by circles, which are imagined to cut through them, in various ways. the earth thus intersected is called the _terrestrial_, and the heavens the _celestial_, sphere. we must bear in mind, that these circles have no existence in nature, but are mere landmarks, artificially contrived for convenience of reference. on account of the immense distances of the heavenly bodies, they appear to us, wherever we are placed, to be fixed in the same concave surface, or celestial vault. the great circles of the globe, extended every way to meet the concave sphere of the heavens, become circles of the celestial sphere. the _horizon_ is the great circle which divides the earth into upper and lower hemispheres, and separates the visible heavens from the invisible. this is the _rational_ horizon. the _sensible_ horizon is a circle touching the earth at the place of the spectator, and is bounded by the line in which the earth and skies seem to meet. the sensible horizon is parallel to the rational, but is distant from it by the semidiameter of the earth, or nearly four thousand miles. still, so vast is the distance of the starry sphere, that both these planes appear to cut the sphere in the same line; so that we see the same hemisphere of stars that we should see, if the upper half of the earth were removed, and we stood on the rational horizon. the poles of the horizon are the zenith and nadir. the _zenith_ is the point directly over our heads; and the _nadir_, that directly under our feet. the plumb-line (such as is formed by suspending a bullet by a string) is in the axis of the horizon, and consequently directed towards its poles. every place on the surface of the earth has its own horizon; and the traveller has a new horizon at every step, always extending ninety degrees from him, in all directions. _vertical circles_ are those which pass through the poles of the horizon, (the zenith and nadir,) perpendicular to it. the _meridian_ is that vertical circle which passes through the north and south points. the _prime vertical_ is that vertical circle which passes through the east and west points. the _altitude_ of a body is its elevation above the horizon, measured on a vertical circle. the _azimuth_ of a body is its distance, measured on the horizon, from the meridian to a vertical circle passing through that body. the _amplitude_ of a body is its distance, on the horizon, from the prime vertical to a vertical circle passing through the body. azimuth is reckoned ninety degrees from either the north or south point; and amplitude ninety degrees from either the east or west point. azimuth and amplitude are mutually complements of each other, for one makes up what the other wants of ninety degrees. when a point is _on_ the horizon, it is only necessary to count the number of degrees of the horizon between that point and the meridian, in order to find its azimuth; but if the point is _above_ the horizon, then its azimuth is estimated by passing a vertical circle through it, and reckoning the azimuth from the point where this circle cuts the horizon. the _zenith distance_ of a body is measured on a vertical circle passing through that body. it is the complement of the altitude. the _axis of the earth_ is the diameter on which the earth is conceived to turn in its diurnal revolution. the same line, continued until it meets the starry concave, constitutes the _axis of the celestial sphere_. the _poles of the earth_ are the extremities of the earth's axis: the _poles of the heavens_, the extremities of the celestial axis. the _equator_ is a great circle cutting the axis of the earth at right angles. hence, the axis of the earth is the axis of the equator, and its poles are the poles of the equator. the intersection of the plane of the equator with the surface of the earth constitutes the _terrestrial_, and its intersection with the concave sphere of the heavens, the _celestial_, equator. the latter, by way of distinction, is sometimes denominated the _equinoctial_. the secondaries to the equator,--that is, the great circles passing through the poles of the equator,--are called _meridians_, because that secondary which passes through the zenith of any place is the meridian of that place, and is at right angles both to the equator and the horizon, passing, as it does, through the poles of both. these secondaries are also called _hour circles_ because the arcs of the equator intercepted between them are used as measures of time. the _latitude_ of a place on the earth is its distance from the equator north or south. the _polar distance_, or angular distance from the nearest pole, is the complement of the latitude. the _longitude_ of a place is its distance from some standard meridian, either east or west, measured on the equator. the meridian, usually taken as the standard, is that of the observatory of greenwich, in london. if a place is directly _on_ the equator, we have only to inquire, how many degrees of the equator there are between that place and the point where the meridian of greenwich cuts the equator. if the place is north or south of the equator, then its longitude is the arc of the equator intercepted between the meridian which passes through the place and the meridian of greenwich. the _ecliptic_ is a great circle, in which the earth performs its annual revolutions around the sun. it passes through the centre of the earth and the centre of the sun. it is found, by observation, that the earth does not lie with its axis at right angles to the plane of the ecliptic, so as to make the equator coincide with it, but that it is turned about twenty-three and a half degrees out of a perpendicular direction, making an angle with the plane itself of sixty-six and a half degrees. the equator, therefore, must be turned the same distance out of a coincidence with the ecliptic, the two circles making an angle with each other of twenty-three and a half degrees. it is particularly important that we should form correct ideas of the ecliptic, and of its relations to the equator, since to these two circles a great number of astronomical measurements and phenomena are referred. the _equinoctial points_, or _equinoxes_, are the intersections of the ecliptic and equator. the time when the sun crosses the equator, in going northward, is called the _vernal_, and in returning southward, the _autumnal_, equinox. the vernal equinox occurs about the twenty-first of march, and the autumnal, about the twenty-second of september. the _solstitial points_ are the two points of the ecliptic most distant from the equator. the times when the sun comes to them are called _solstices_. the summer solstice occurs about the twenty-second of june, and the winter solstice about the twenty-second of december. the ecliptic is divided into twelve equal parts, of thirty degrees each, called _signs_, which, beginning at the vernal equinox, succeed each other, in the following order: . aries, [zodiac: aries] . taurus, [zodiac: taurus] . gemini, [zodiac: gemini] . cancer, [zodiac: cancer] . leo, [zodiac: leo] . virgo, [zodiac: virgo] . libra, [zodiac: libra] . scorpio, [zodiac: scorpio] . sagittarius, [zodiac: sagittarius] . capricornus, [zodiac: capricornus] . aquarius, [zodiac: aquarius] . pisces. [zodiac: pisces] the mode of reckoning on the ecliptic is by signs, degrees, minutes, and seconds. the sign is denoted either by its name or its number. thus, one hundred degrees may be expressed either as the tenth degree of cancer, or as s °. it will be found an advantage to repeat the signs in their proper order, until they are well fixed in the memory, and to be able to recognise each sign by its appropriate character. of the various meridians, two are distinguished by the name of _colures_. the _equinoctial colure_ is the meridian which passes through the equinoctial points. from this meridian, right ascension and celestial longitude are reckoned, as longitude on the earth is reckoned from the meridian of greenwich. the _solstitial colure_ is the meridian which passes through the solstitial points. the position of a celestial body is referred to the equator by its right ascension and declination. _right ascension_ is the angular distance from the vernal equinox measured on the equator. if a star is situated _on_ the equator, then its right ascension is the number of degrees of the equator between the star and the vernal equinox. but if the star is north or south of the equator, then its right ascension is the number of degrees of the equator, intercepted between the vernal equinox and that secondary to the equator which passes through the star. _declination_ is the distance of a body from the equator measured on a secondary to the latter. therefore, right ascension and declination correspond to terrestrial longitude and latitude,--right ascension being reckoned from the equinoctial colure, in the same manner as longitude is reckoned from the meridian of greenwich. on the other hand, celestial longitude and latitude are referred, not to the equator, but to the ecliptic. _celestial longitude_ is the distance of a body from the vernal equinox measured on the ecliptic. _celestial latitude_ is the distance from the ecliptic measured on a secondary to the latter. or, more briefly, longitude is distance _on_ the ecliptic: latitude, distance _from_ the ecliptic. the _north polar distance_ of a star is the complement of its declination. _parallels of latitude_ are small circles parallel to the equator. they constantly diminish in size, as we go from the equator to the pole. the _tropics_ are the parallels of latitude which pass through the solstices. the northern tropic is called the tropic of cancer; the southern, the tropic of capricorn. the _polar circles_ are the parallels of latitude that pass through the poles of the ecliptic, at the distance of twenty-three and a half degrees from the poles of the earth. the _elevation of the pole_ of the heavens above the horizon of any place is always equal to the latitude of the place. thus, in forty degrees of north latitude we see the north star forty degrees above the northern horizon; whereas, if we should travel southward, its elevation would grow less and less, until we reached the equator, where it would appear _in_ the horizon. or, if we should travel northwards, the north star would rise continually higher and higher, until, if we could reach the pole of the earth, that star would appear directly over head. the _elevation of the equator_ above the horizon of any place is equal to the complement of the latitude. thus, at the latitude of forty degrees north, the equator is elevated fifty degrees above the southern horizon. the earth is divided into five zones. that portion of the earth which lies between the tropics is called the _torrid zone_; that between the tropics and the polar circles, the _temperate zones_; and that between the polar circles and the poles, the _frigid zones_. the _zodiac_ is the part of the celestial sphere which lies about eight degrees on each side of the ecliptic. this portion of the heavens is thus marked off by itself, because all the planets move within it. after endeavoring to form, from the definitions, as clear an idea as we can of the various circles of the sphere, we may next resort to an artificial globe, and see how they are severally represented there. i do not advise to _begin_ learning the definitions from the globe; the mind is more improved, and a power of conceiving clearly how things are in nature is more effectually acquired, by referring every thing, at first, to the grand sphere of nature itself, and afterwards resorting to artificial representations to aid our conceptions. we can get but a very imperfect idea of a man from a profile cut in paper, unless we know the original. if we are acquainted with the individual, the profile will assist us to recall his appearance more distinctly than we can do without it. in like manner, orreries, globes, and other artificial aids, will be found very useful, in assisting us to form distinct conceptions of the relations existing between the different circles of the sphere, and of the arrangements of the heavenly bodies; but, unless we have already acquired some correct ideas of these things, by contemplating them as they are in nature, artificial globes, and especially orreries, will be apt to mislead us. i trust you will be able to obtain the use of a globe,[ ] to aid you in the study of the foregoing definitions, or doctrine of the sphere; but if not, i would recommend the following easy device. to represent the earth, select a large _apple_, (a melon, when in season, will be found still better.) the eye and the stem of the apple will indicate the position of the two poles of the earth. applying the thumb and finger of the left hand to the poles, and holding the apple so that the poles may be in a north and south line, turn this globe from west to east, and its motion will correspond to the diurnal movement of the earth. pass a wire or a knitting needle through the poles, and it will represent the _axis_ of the sphere. a circle cut around the apple, half way between the poles, will be the _equator_; and several other circles cut between the equator and the poles, parallel to the equator, will represent _parallels of latitude_; of which, two, drawn twenty-three and a half degrees from the equator, will be the _tropics_, and two others, at the same distance from the poles, will be the _polar circles_. a great circle cut through the poles, in a north and south direction, will form the _meridian_, and several other great circles drawn through the poles, and of course perpendicularly to the equator, will be secondaries to the equator, constituting meridians, or _hour circles_. a great circle cut through the centre of the earth, from one tropic to the other, would represent the _plane_ of the ecliptic; and consequently a line cut round the apple where such a section meets the surface, will be the terrestrial _ecliptic_. the points where this circle meets the tropics indicate the position of the _solstices_; and its intersection with the equator, that of the _equinoctial points_. the _horizon_ is best represented by a circular piece of pasteboard, cut so as to fit closely to the apple, being movable upon it. when this horizon is passed through the poles, it becomes the horizon of the equator; when it is so placed as to coincide with the earth's equator, it becomes the horizon of the poles; and in every other situation it represents the horizon of a place on the globe ninety degrees every way from it. suppose we are in latitude forty degrees; then let us place our movable paper parallel to our own horizon, and elevate the pole forty degrees above it, as near as we can judge by the eye. if we cut a circle around the apple, passing through its highest part, and through the east and west points, it will represent the _prime vertical_. simple as the foregoing device is, if you will take the trouble to construct one for yourself, it will lead you to more correct views of the doctrine of the sphere, than you would be apt to obtain from the most expensive artificial globes, although there are many other useful purposes which such globes serve, for which the apple would be inadequate. when you have thus made a sphere for yourself, or, with an artificial globe before you, if you have access to one, proceed to point out on it the various arcs of azimuth and altitude, right ascension and declination, terrestrial and celestial latitude and longitude,--these last being referred to the equator on the earth, and to the ecliptic in the heavens. when the circles of the sphere are well learned, we may advantageously employ projections of them in various illustrations. by the _projection of the sphere_ is meant a representation of all its parts on a plane. the plane itself is called the plane of projection. let us take any circular ring, as a wire bent into a circle, and hold it in different positions before the eye. if we hold it parallel to the face, with the whole breadth opposite to the eye, we see it as an entire circle. if we turn it a little sideways, it appears oval, or as an ellipse; and, as we continue to turn it more and more round, the ellipse grows narrower and narrower, until, when the edge is presented to the eye, we see nothing but a line. now imagine the ring to be near a perpendicular wall, and the eye to be removed at such a distance from it, as not to distinguish any interval between the ring and the wall; then the several figures under which the ring is seen will appear to be inscribed on the wall, and we shall see the ring as a circle, when perpendicular to a straight line joining the centre of the ring and the eye, or as an ellipse, when oblique to this line, or as a straight line, when its edge is towards us. [illustration: fig. .] it is in this manner that the circles of the sphere are projected, as represented in the following diagram, fig. . here, various circles are represented as projected on the meridian, which is supposed to be situated directly before the eye, at some distance from it. the horizon h o, being perpendicular to the meridian, is seen edgewise, and consequently is projected into a straight line. the same is the case with the prime vertical z n, with the equator e q, and the several small circles parallel to the equator, which represent the two tropics and the two polar circles. in fact, all circles whatsoever, which are perpendicular to the plane of projection, will be represented by straight lines. but every circle which is perpendicular to the horizon, except the prime vertical, being seen obliquely, as z m n, will be projected into an ellipse, one half only of which is seen,--the other half being on the other side of the plane of projection. in the same manner, p r p, an hour circle, is represented by an ellipse on the plane of projection. footnote: [ ] a small pair of globes, that will answer every purpose required by the readers of these letters, may be had of the publishers of this work, at a price not exceeding ten dollars; or half that sum for a celestial globe, which will serve alone for studying astronomy. letter iii. astronomical instruments.----telescope. "here truths sublime, and sacred science charm, creative arts new faculties supply, mechanic powers give more than giant's arm, and piercing optics more than eagle's eye; eyes that explore creation's wondrous laws, and teach us to adore the great designing cause."--_beattie_. if, as i trust, you have gained a clear and familiar knowledge of the circles and divisions of the sphere, and of the mode of estimating the position of a heavenly body by its azimuth and altitude, or by its right ascension and declination, or by its longitude and latitude, you will now enter with advantage upon an account of those _instruments_, by means of which our knowledge of astronomy has been greatly promoted and perfected. the most ancient astronomers employed no instruments of observation, but acquired their knowledge of the heavenly bodies by long-continued and most attentive inspection with the naked eye. instruments for measuring angles were first used in the alexandrian school, about three hundred years before the christian era. wherever we are situated on the earth, we appear to be in the centre of a vast sphere, on the concave surface of which all celestial objects are inscribed. if we take any two points on the surface of the sphere, as two stars, for example, and imagine straight lines to be drawn to them from the eye, the angle included between these lines will be measured by the arc of the sky contained between the two points. thus, if d b h, fig. , page , represents the concave surface of the sphere, a, b, two points on it, as two stars, and c a, c b, straight lines drawn from the spectator to those points, then the angular distance between them is measured by the arc a b, or the angle a c b. but this angle may be measured on a much smaller circle, having the same centre, as g f k, since the arc e f will have the same number of degrees as the arc a b. the simplest mode of taking an angle between two stars is by means of an arm opening at a joint like the blade of a penknife, the end of the arm moving like c e upon the graduated circle k f g. in fact, an instrument constructed on this principle, resembling a carpenter's rule with a folding joint, with a semicircle attached, constituted the first rude apparatus for measuring the angular distance between two points on the celestial sphere. thus the sun's elevation above the horizon might be ascertained, by placing one arm of the rule on a level with the horizon, and bringing the edge of the other into a line with the sun's centre. [illustration fig. .] the common surveyor's compass affords a simple example of angular measurement. here, the needle lies in a north and south line, while the circular rim of the compass, when the instrument is level, corresponds to the horizon. hence the compass shows the azimuth of an object, or how many degrees it lies east or west of the meridian. it is obvious, that the larger the graduated circle is, the more minutely its limb may be divided. if the circle is one foot in diameter, each degree will occupy one tenth of an inch. if the circle is twenty feet in diameter, a degree will occupy the space of two inches, and could be easily divided into minutes, since each minute would cover a space one thirtieth of an inch. refined astronomical circles are now divided with very great skill and accuracy, the spaces between the divisions being, when read off, magnified by a microscope; but in former times, astronomers had no mode of measuring small angles but by employing very large circles. but the telescope and microscope enable us at present to measure celestial arcs much more accurately than was done by the older astronomers. in the best instruments, the measurements extend to a single second of space, or one thirty-six hundredth part of a degree,--a space, on a circle twelve feet in diameter, no greater than one fifty-seven hundredth part of an inch. to divide, or _graduate_, astronomical instruments, to such a degree of nicety, requires the highest efforts of mechanical skill. indeed, the whole art of instrument-making is regarded as the most difficult and refined of all the mechanical arts; and a few eminent artists, who have produced instruments of peculiar power and accuracy, take rank with astronomers of the highest celebrity. i will endeavor to make you acquainted with several of the principal instruments employed in astronomical observations, but especially with the telescope, which is the most important and interesting of them all. i think i shall consult your wishes, as well as your improvement, by giving you a clear insight into the principles of this prince of instruments, and by reciting a few particulars, at least, respecting its invention and subsequent history. the _telescope_, as its name implies, is an instrument employed for viewing distant objects.[ ] it aids the eye in two ways; first, by enlarging the visual angle under which objects are seen, and, secondly, by collecting and conveying to the eye a much larger amount of the light that emanates from the object, than would enter the naked pupil. a complete knowledge of the telescope cannot be acquired, without an acquaintance with the science of optics; but one unacquainted with that science may obtain some idea of the leading principles of this noble instrument. its main principle is as follows: _by means of the telescope, we first form an image of a distant object,--as the moon, for example,--and then magnify that image by a microscope._ [illustration fig. .] let us first see how the image is formed. this may be done either by a convex lens, or by a concave mirror. a convex lens is a flat piece of glass, having its two faces convex, or spherical, as is seen in a common sun-glass, or a pair of spectacles. every one who has seen a sun-glass, knows, that, when held towards the sun, it collects the solar rays into a small bright circle in the focus. this is in fact a small _image_ of the sun. in the same manner, the image of any distant object, as a star, may be formed, as is represented in the following diagram. let a b c d, fig. , represent the tube of the telescope. at the front end, or at the end which is directed towards the object, (which we will suppose to be the moon,) is inserted a convex lens, l, which receives the rays of light from the moon, and collects them into the focus at _a_, forming an image of the moon. this image is viewed by a magnifier attached to the end b c. the lens, l, is called the _object-glass_, and the microscope in b c, the _eyeglass_. we apply a microscope to this image just as we would to any object; and, by greatly enlarging its dimensions, we may render its various parts far more distinct than they would otherwise be; while, at the same time, the lens collects and conveys to the eye a much greater quantity of light than would proceed directly from the body under examination. a very few rays of light only, from a distant object, as a star, can enter the eye directly; but a lens one foot in diameter will collect a beam of light of the same dimensions, and convey it to the eye. by these means, many obscure celestial objects become distinctly visible, which would otherwise be either too minute, or not sufficiently luminous, to be seen by us. but the image may also be formed by means of a _concave mirror_, which, as well as the concave lens, has the property of collecting the rays of light which proceed from any luminous body, and of forming an image of that body. the image formed by a concave mirror is magnified by a microscope, in the same manner as when formed by the concave lens. when the lens is used to form an image, the instrument is called a _refracting telescope_; when a concave mirror is used, it is called a _reflecting telescope_. the office of the object-glass is simply _to collect_ the light, and to form an _image_ of the object, but not to magnify it: the magnifying power is wholly in the eyeglass. hence the principle of the telescope is as follows: _by means of the object-glass_, (in the refracting telescope,) _or by the concave mirror_, (in the reflecting telescope,) _we form an image of the object_, _and magnify that image by a microscope_. the invention of this noble instrument is generally ascribed to the great philosopher of florence, galileo. he had heard that a spectacle maker of holland had accidentally hit upon a discovery, by which distant objects might be brought apparently nearer; and, without further information, he pursued the inquiry, in order to ascertain what forms and combinations of glasses would produce such a result. by a very philosophical process of reasoning, he was led to the discovery of that peculiar form of the telescope which bears his name. although the telescopes made by galileo were no larger than a common spyglass of the kind now used on board of ships, yet, as they gave new views of the heavenly bodies, revealing the mountains and valleys of the moon, the satellites of jupiter, and multitudes of stars which are invisible to the naked eye, it was regarded with infinite delight and astonishment. _reflecting_ telescopes were first constructed by sir isaac newton, although the use of a concave reflector, instead of an object-glass, to form the image, had been previously suggested by gregory, an eminent scotch astronomer. the first telescope made by newton was only six inches long. its reflector, too, was only a little more than an inch. notwithstanding its small dimensions, it performed so well, as to encourage further efforts; and this illustrious philosopher afterwards constructed much larger instruments, one of which, made with his own hands, was presented to the royal society of london, and is now carefully preserved in their library. newton was induced to undertake the construction of reflecting telescopes, from the belief that refracting telescopes were necessarily limited to a very small size, with only moderate illuminating powers, whereas the dimensions and powers of the former admitted of being indefinitely increased. considerable _magnifying_ powers might, indeed, be obtained from refractors, by making them very long; but the _brightness_ with which telescopic objects are seen, depends greatly on the dimensions of the beam of light which is collected by the object-glass, or by the mirror, and conveyed to the eye; and therefore, small object-glasses cannot have a very high illuminating power. now, the experiments of newton on colors led him to believe, that it would be impossible to employ large lenses in the construction of telescopes, since such glasses would give to the images, they formed, the colors of the rainbow. but later opticians have found means of correcting these imperfections, so that we are now able to use object-glasses a foot or more in diameter, which give very clear and bright images. such instruments are called _achromatic_ telescopes,--a name implying the absence of prismatic or rainbow colors in the image. it is, however, far more difficult to construct large achromatic than large reflecting telescopes. very large pieces of glass can seldom be found, that are sufficiently pure for the purpose; since every inequality in the glass, such as waves, tears, threads, and the like, spoils it for optical purposes, as it distorts the light, and produces nothing but confused images. the achromatic telescope (that is, the refracting telescope, having such an object-glass as to give a colorless image) was invented by dollond, a distinguished english artist, about the year . he had in his possession a quantity of glass of a remarkably fine quality, which enabled him to carry his invention at once to a high degree of perfection. it has ever since been, with the manufacturers of telescopes, a matter of the greatest difficulty to find pieces of glass, of a suitable quality for object-glasses, more than two or three inches in diameter. hence, large achromatic telescopes are very expensive, being valued in proportion to the _cubes_ of their diameters; that is, if a telescope whose aperture (as the breadth of the object-glass is technically called) is two inches, cost one hundred dollars, one whose aperture is eight inches would cost six thousand four hundred dollars. since it is so much easier to make large reflecting than large refracting telescopes, you may ask, why the latter are ever attempted, and why reflectors are not exclusively employed? i answer, that the achromatic telescope, when large and well constructed, is a more perfect and more durable instrument than the reflecting telescope. much more of the light that falls on the mirror is absorbed than is lost in passing through the object-glass of a refractor; and hence the larger achromatic telescopes afford a stronger light than the reflecting, unless the latter are made of an enormous and unwieldy size. moreover, the mirror is very liable to tarnish, and will never retain its full lustre for many years together; and it is no easy matter to restore the lustre, when once impaired. in my next letter, i will give you an account of some of the most celebrated telescopes that have ever been constructed, and point out the method of using this excellent instrument, so as to obtain with it the finest views of the heavenly bodies. footnote: [ ] from two greek words, =têle=, (_tele_,) _far_, and =schopeô=, (_skopeo_,) _to see_. letter iv telescope continued. ----"the broad circumference hung on his shoulders like the moon, whose orb through _optic glass_ the tuscan artist views at evening, from the top of fesolé or in valdarno, to descry new lands, rivers or mountains, in her spotted globe."--_milton._ the two most celebrated telescopes, hitherto made, are herschel's _forty-feet reflector_, and the _great dorpat refractor_. herschel was a hanoverian by birth, but settled in england in the younger part of his life. as early as , he began to make telescopes for his own use; and, during his life, he made more than four hundred, of various sizes and powers. under the patronage of george the third, he completed, in , his great telescope, having a tube of iron, forty feet long, and a speculum, forty-nine and a half inches or more than four feet in diameter. let us endeavor to form a just conception of this gigantic instrument, which we can do only by dwelling on its dimensions, and comparing them with those of other objects with which we are familiar, as the length or height of a house, and the breadth of a hogshead or cistern, of known dimensions. the reflector alone weighed nearly a ton. so large and ponderous an instrument must require a vast deal of machinery to work it, and to keep it steady; and, accordingly, the framework surrounding it was formed of heavy timbers, and resembled the frame of a large building. when one of the largest of the fixed stars, as sirius, is entering the field of this telescope, its approach is announced by a bright dawn, like that which precedes the rising sun; and when the star itself enters the field, the light is insupportable to the naked eye. the planets are expanded into brilliant luminaries, like the moon; and innumerable multitudes of stars are scattered like glittering dust over the celestial vault. the great dorpat telescope is of more recent construction. it was made by fraunhofer, a german optician of the greatest eminence, at munich, in bavaria, and takes its name from its being attached to the observatory at dorpat, in russia. it is of much smaller dimensions than the great telescope of herschel. its object-glass is nine and a half inches in diameter, and its length, fourteen feet. although the price of this instrument was nearly five thousand dollars, yet it is said that this sum barely covered the actual expenses. it weighs five thousand pounds, and yet is turned with the finger. in facility of management, it has greatly the advantage of herschel's telescope. moreover, the sky of england is so much of the time unfavorable for astronomical observation, that _one hundred_ good hours (or those in which the higher powers can be used) are all that can be obtained in a whole year. on this account, and on account of the difficulty of shifting the position of the instrument, herschel estimated that it would take about six hundred years to obtain with it even a momentary glimpse of every part of the heavens. this remark shows that such great telescopes are unsuited to the common purposes of astronomical observation. indeed, most of herschel's discoveries were made with his small telescopes; and although, for certain rare purposes, powers were applied which magnified seven thousand times, yet, in most of his observations, powers magnifying only two or three hundred times were employed. the highest power of the dorpat telescope is only seven hundred, and yet the director of this instrument, professor struve, is of the opinion, that it is nearly or quite equal in quality, all things considered, to herschel's forty-feet reflector. it is not generally understood in what way greatness of size in a telescope increases its powers; and it conveys but an imperfect idea of the excellence of a telescope, to tell how much it magnifies. in the same instrument, an increase of magnifying power is always attended with a diminution of the light and of the field of view. hence, the lower powers generally afford the most agreeable views, because they give the clearest light, and take in the largest space. the several circumstances which influence the qualities of a telescope are, illuminating power, distinctness, field of view, and magnifying power. large mirrors and large object-glasses are superior to smaller ones, because they collect a larger beam of light, and transmit it to the eye. stars which are invisible to the naked eye are rendered visible by the telescope, because this instrument collects and conveys to the eye a large beam of the few rays which emanate from the stars; whereas a beam of these rays of only the diameter of the pupil of the eye, would afford too little light for distinct vision. in this particular, large telescopes have great advantages over small ones. the great mirror of herschel's forty-feet reflector collects and conveys to the eye a beam more than four feet in diameter. the dorpat telescope also transmits to the eye a beam nine and one half inches in diameter. this seems small, in comparison with the reflector; but much less of the light is lost on passing through the glass than is absorbed by the mirror, and the mirror is very liable to be clouded or tarnished; so that there is not so great a difference in the two instruments, in regard to illuminating power, as might be supposed from the difference of size. _distinctness of view_ is all-important to the performance of an instrument. the object may be sufficiently bright, yet, if the image is distorted, or ill-defined, the illumination is of little consequence. this property depends mainly on the skill with which all the imperfections of figure and color in the glass or mirror are corrected, and can exist in perfection only when the image is rendered completely achromatic, and when all the rays that proceed from each point in the object are collected into corresponding points of the image, unaccompanied by any other rays. distinctness is very much affected by the _steadiness_ of the instrument. every one knows how indistinct a page becomes, when a book is passed rapidly backwards and forwards before the eyes, and how difficult it is to read in a carriage in rapid motion on a rough road. _field of view_ is another important consideration. the finest instruments exhibit the moon, for example, not only bright and distinct, in all its parts, but they take in the whole disk at once; whereas, the inferior instruments, when the higher powers, especially, are applied, permit us to see only a small part of the moon at once. i hope, my friend, that, when you have perused these letters, or rather, while you are perusing them, you will have frequent opportunities of looking through a good telescope. i even anticipate that you will acquire such a taste for viewing the heavenly bodies with the aid of a good glass, that you will deem a telescope a most suitable appendage to your library, and as certainly not less an ornament to it than the more expensive statues with which some people of fortune adorn theirs. i will therefore, before concluding this letter, offer you a few _directions for using the telescope_. some states of weather, even when the sky is clear, are far more favorable for astronomical observation than others. after sudden changes of temperature in the atmosphere, the medium is usually very unsteady. if the sun shines out warm after a cloudy season, the ground first becomes heated, and the air that is nearest to it is expanded, and rises, while the colder air descends, and thus ascending and descending currents of air, mingling together, create a confused and wavy medium. the same cause operates when a current of hot air rises from a chimney; and hence the state of the atmosphere in cities and large towns is very unfavorable to the astronomer, on this account, as well as on account of the smoky condition in which it is usually found. after a long season of dry weather, also, the air becomes smoky, and unfit for observation. indeed, foggy, misty, or smoky, air is so prevalent in some countries, that only a very few times in the whole year can be found, which are entirely suited to observation, especially with the higher powers; for we must recollect, that these inequalities and imperfections are magnified by telescopes, as well as the objects themselves. thus, as i have already mentioned, not more than one hundred good hours in a year could be obtained for observation with herschel's great telescope. by _good_ hours, herschel means that the sky must be very clear, the moon absent, no twilight, no haziness, no violent wind, and no sudden change of temperature. as a general fact, the warmer climates enjoy a much finer sky for the astronomer than the colder, having many more clear evenings, a short twilight, and less change of temperature. the watery vapor of the atmosphere, also, is more perfectly dissolved in hot than in cold air, and the more water air contains, provided it is in a state of perfect solution, the clearer it is. a _certain preparation of the observer himself_ is also requisite for the nicest observations with the telescope. he must be free from all agitation, and the eye must not recently have been exposed to a strong light, which contracts the pupil of the eye. indeed, for delicate observations, the observer should remain for some time beforehand in a dark room, to let the pupil of the eye dilate. by this means, it will be enabled to admit a larger number of the rays of light. in ascending the stairs of an observatory, visitors frequently get out of breath, and having perhaps recently emerged from a strongly-lighted apartment, the eye is not in a favorable state for observation. under these disadvantages, they take a hasty look into the telescope, and it is no wonder that disappointment usually follows. want of steadiness is a great difficulty attending the use of the highest magnifiers; for the motions of the instrument are magnified as well as the object. hence, in the structure of observatories, the greatest pains is requisite, to avoid all tremor, and to give to the instruments all possible steadiness; and the same care is to be exercised by observers. in the more refined observations, only one or two persons ought to be near the instrument. in general, _low powers_ afford better views of the heavenly bodies than very high magnifiers. it may be thought absurd, to recommend the use of low powers, in respect to large instruments especially, since it is commonly supposed that the advantage of large instruments is, that they will bear high magnifying powers. but this is not their only, nor even their principal, advantage. a good light and large field are qualities, for most purposes, more important than great magnifying power; and it must be borne in mind, that, as we increase the magnifying power in a given instrument, we diminish both the illumination and the field of view. still, different objects require different magnifying powers; and a telescope is usually furnished with several varieties of powers, one of which is best fitted for viewing the moon, another for jupiter, and a still higher power for saturn. comets require only the lowest magnifiers; for here, our object is to command as much light, and as large a field, as possible, while it avails little to increase the dimensions of the object. on the other hand, for certain double stars, (stars which appear single to the naked eye, but double to the telescope,) we require very high magnifiers, in order to separate these minute objects so far from each other, that the interval can be distinctly seen. whenever we exhibit celestial objects to inexperienced observers, it is useful to precede the view with good _drawings_ of the objects, accompanied by an explanation of what each appearance, exhibited in the telescope, indicates. the novice is told, that mountains and valleys can be seen in the moon by the aid of the telescope; but, on looking, he sees a confused mass of light and shade, and nothing which looks to him like either mountains or valleys. had his attention been previously directed to a plain drawing of the moon, and each particular appearance interpreted to him, he would then have looked through the telescope with intelligence and satisfaction. letter v. observatories. "we, though from heaven remote, to heaven will move, with strength of mind, and tread the abyss above; and penetrate, with an interior light, those upper depths which nature hid from sight. pleased we will be, to walk along the sphere of shining stars, and travel with the year."--_ovid._ an observatory is a structure fitted up expressly for astronomical observations, and furnished with suitable instruments for that purpose. the two most celebrated observatories, hitherto built, are that of tycho brahe, and that of greenwich, near london. the observatory of tycho brahe, fig. , was constructed at the expense of the king of denmark, in a style of royal magnificence, and cost no less than two hundred thousand crowns. it was situated on the island of huenna, at the entrance of the baltic, and was called uraniburg, or the palace of the skies. before i give you an account of tycho's observatory, i will recite a few particulars respecting this great astronomer himself. tycho brahe was of swedish descent, and of noble family; but having received his education at the university of copenhagen, and spent a large part of his life in denmark, he is usually considered as a dane, and quoted as a danish astronomer. he was born in the year . when he was about fourteen years old, there happened a great eclipse of the sun, which awakened in him a high interest, especially when he saw how [illustration fig. .] accurately all the circumstances of it answered to the prediction with which he had been before made acquainted. he was immediately seized with an irresistible passion to acquire a knowledge of the science which could so successfully lift the veil of futurity. his friends had destined him for the profession of law, and, from the superior talents of which he gave early promise, and with the advantage of powerful family connexions, they had marked out for him a distinguished career in public life. they therefore endeavored to discourage him from pursuing a path which they deemed so much less glorious than that, and vainly sought, by various means, to extinguish the zeal for astronomy which was kindled in his youthful bosom. despising all the attractions of a court, he contracted an alliance with a peasant girl, and, in the peaceful retirement of domestic life, desired no happier lot than to peruse the grand volume which the nocturnal heavens displayed to his enthusiastic imagination. he soon established his fame as one of the greatest astronomers of the age, and monarchs did homage to his genius. the king of denmark became his munificent patron, and james the first, king of england, when he went to denmark to complete his marriage with a danish princess, passed eight days with tycho in his observatory, and, at his departure, addressed to the astronomer a latin ode, accompanied with a magnificent present. he gave him also his royal license to print his works in england, and added to it the following complimentary letter: "nor am i acquainted with these things on the relation of others, or from a mere perusal of your works, but i have seen them with my own eyes, and heard them with my own ears, in your residence at uraniburg, during the various learned and agreeable conversations which i there held with you, which even now affect my mind to such a degree, that it is difficult to decide, whether i recollect them with greater pleasure or admiration." admiring disciples also crowded to this sanctuary of the sciences, to acquire a knowledge of the heavens. the observatory consisted of a main building, which was square, each side being sixty feet, and of large wings in the form of round towers. the whole was executed in a style of great magnificence, and tycho, who was a nobleman by descent, gratified his taste for splendor and ornament, by giving to every part of the structure an air of the most finished elegance. nor were the instruments with which it was furnished less magnificent than the buildings. they were vastly larger than had before been employed in the survey of the heavens, and many of them were adorned with costly ornaments. the cut on page , fig. , represents one of tycho's large and splendid instruments, (an astronomical quadrant,) on one side of which was figured a representation of the astronomer and his assistants, in the midst of their instruments, and intently engaged in making and recording observations. it conveys to us a striking idea of the magnificence of his arrangements, and of the extent of his operations. here tycho sat in state, clad in the robes of nobility, and supported throughout his establishment the etiquette due to his rank. his observations were more numerous than all that had ever been made before, and they were carried to a degree of accuracy that is astonishing, when we consider that they were made without the use of the telescope, which was not yet invented. tycho carried on his observations at uraniburg for about twenty years, during which time he accumulated an immense store of accurate and valuable _facts_, which afforded the groundwork of the discovery of the great laws of the solar system established by kepler, of whom i shall tell you more hereafter. but the high marks of distinction which tycho enjoyed, not only from his own sovereign, but also from foreign potentates, provoked the envy of the courtiers of his royal patron. they did not indeed venture to make their attacks upon him while his generous patron was living; but the king was no sooner dead, and succeeded by a young monarch, who did not feel the same [illustration fig. .] interest in protecting and encouraging this great ornament of the kingdom, than his envious foes carried into execution their long-meditated plot for his ruin. they represented to the young king, that the treasury was exhausted, and that it was necessary to retrench a number of pensions, which had been granted for useless purposes, and in particular that of tycho, which, they maintained, ought to be conferred upon some person capable of rendering greater services to the state. by these means, they succeeded in depriving him of his support, and he was compelled to retreat under the hospitable mansion of a friend in germany. here he became known to the emperor, who invited him to prague, where, with an ample stipend, he resumed his labors. but, though surrounded with affectionate friends and admiring disciples, he was still an exile in a foreign land. although his country had been base in its ingratitude, it was yet the land which he loved; the scene of his earliest affection; the theatre of his scientific glory. these feelings continually preyed upon his mind, and his unsettled spirit was ever hovering among his native mountains. in this condition he was attacked by a disease of the most painful kind, and, though its agonizing paroxysms had lengthened intermissions, yet he saw that death was approaching. he implored his pupils to persevere in their scientific labors; he conversed with kepler on some of the profoundest points of astronomy; and with these secular occupations he mingled frequent acts of piety and devotion. in this happy condition he expired, without pain, at the age of fifty-five.[ ] the observatory at greenwich was not built until a hundred years after that of tycho brahe, namely, in . the great interests of the british nation, which are involved in navigation, constituted the ruling motive with the government to lend their aid in erecting and maintaining this observatory. the site of the observatory at greenwich is on a commanding eminence facing the river thames, five miles east of the central parts of london. being part of a royal park, the neighboring grounds are in no danger of being occupied by buildings, so as to obstruct the view. it is also in full view of the shipping on the thames; and, according to a standing regulation of the observatory, at the instant of one o'clock, every day, a huge ball is dropped from over the house, as a signal to the commanders of vessels for regulating their chronometers. the buildings comprise a series of rooms, of sufficient number and extent to accommodate the different instruments, the inmates of the establishment, and the library; and on the top is a celebrated camera obscura, exhibiting a most distinct and perfect picture of the grand and unrivalled scenery which this eminence commands. this establishment, by the accuracy and extent of its observations, has contributed more than all other institutions to perfect the science of astronomy. to preside over and direct this great institution, a man of the highest eminence in the science is appointed by the government, with the title of _astronomer royal_. he is paid an ample salary, with the understanding that he is to devote himself exclusively to the business of the observatory. the astronomers royal of the greenwich observatory, from the time of its first establishment, in , to the present time, have constituted a series of the proudest names of which british science can boast. a more detailed sketch of their interesting history will be given towards the close of these letters. six assistants, besides inferior laborers, are constantly in attendance; and the business of making and recording observations is conducted with the utmost system and order. the great objects to be attained in the construction of an observatory are, a commanding and unobstructed view of the heavens; freedom from causes that affect the transparency and uniform state of the atmosphere, such as fires, smoke, or marshy grounds; mechanical facilities for the management of instruments, and, especially, every precaution that is necessary to secure perfect steadiness. this last consideration is one of the greatest importance, particularly in the use of very large magnifiers; for we must recollect, that any motion in the instrument is magnified by the full power of the glass, and gives a proportional unsteadiness to the object. a situation is therefore selected as remote as possible from public roads, (for even the passing of carriages would give a tremulous motion to the ground, which would be sensible in large instruments,) and structures of solid masonry are commenced deep enough in the ground to be unaffected by frost, and built up to the height required, without any connexion with the other parts of the building. many observatories are furnished with a movable dome for a roof, capable of revolving on rollers, so that instruments penetrating through the roof may be easily brought to bear upon any point at or near the zenith. you will not perhaps desire me to go into a minute description of all the various instruments that are used in a well-constructed observatory. nor is this necessary, since a very large proportion of all astronomical observations are taken on the meridian, by means of the transit instrument and clock. when a body, in its diurnal revolution, comes to the meridian, it is at its highest point above the horizon, and is then least affected by refraction and parallax. this, then, is the most favorable position for taking observations upon it. moreover, it is peculiarly easy to take observations on a body when in this situation. hence the transit instrument and clock are the most important members of an astronomical observatory. you will, therefore, expect me to give you some account of these instruments. [illustration fig. .] the _transit instrument_ is a telescope which is fixed permanently in the meridian, and moves only in that plane. the accompanying diagram, fig. , represents a side view of a portable transit instrument, exhibiting the telescope supported on a firm horizontal axis, on which it turns in the plane of the meridian, from the south point of the horizon through the zenith to the north point. it can therefore be so directed as to observe the passage of a star across the meridian at any altitude. the accompanying graduated circle enables the observer to set the instrument at any required altitude, corresponding to the known altitude at which the body to be observed crosses the meridian. or it may be used to measure the altitude of a body, or its zenith distance, at the time of its meridian passage. near the circle may be seen a spirit-level, which serves to show when the axis is exactly on a level with the horizon. the framework is made of solid metal, (usually brass,) every thing being arranged with reference to keeping the instrument perfectly steady. it stands on screws, which not only afford a steady support, but are useful for adjusting the instrument to a perfect level. the transit instrument is sometimes fixed immovably to a solid foundation, as a pillar of stone, which is built up from a depth in the ground below the reach of frost. when enclosed in a building, as in an observatory, the stone pillar is carried up separate from the walls and floors of the building, so as to be entirely free from the agitations to which they are liable. the use of the transit instrument is to show the precise instant when a heavenly body is on the meridian, or to measure the time it occupies in crossing the meridian. the _astronomical clock_ is the constant companion of the transit instrument. this clock is so regulated as to keep exact pace with the stars, and of course with the revolution of the earth on its axis; that is, it is regulated to _sidereal_ time. it measures the progress of a star, indicating an hour for every fifteen degrees, and twenty-four hours for the whole period of the revolution of the star. sidereal time commences when the vernal equinox is on the meridian, just as solar time commences when the sun is on the meridian. hence the hour by the sidereal clock has no correspondence with the hour of the day, but simply indicates how long it is since the equinoctial point crossed the meridian. for example, the clock of an observatory points to three hours and twenty minutes; this may be in the morning, at noon, or any other time of the day,--for it merely shows that it is three hours and twenty minutes since the equinox was on the meridian. hence, when a star is on the meridian, the clock itself shows its right ascension, which you will recollect is the angular distance measured on the equinoctial, from the point of intersection of the ecliptic and equinoctial, called the vernal equinox, reckoning fifteen degrees for every hour, and a proportional number of degrees and minutes for a less period. i have before remarked, that a very large portion of all astronomical observations are taken when the bodies are on the meridian, by means of the transit instrument and clock. having now described these instruments, i will next explain the manner of using them for different observations. any thing becomes a measure of time, which divides duration equally. the equinoctial, therefore, is peculiarly adapted to this purpose, since, in the daily revolution of the heavens, equal portions of the equinoctial pass under the meridian in equal times. the only difficulty is, to ascertain the amount of these portions for given intervals. now, the clock shows us exactly this amount; for, when regulated to sidereal time, (as it easily may be,) the hour-hand keeps exact pace with the equator, revolving once on the dial-plate of the clock while the equator turns once by the revolution of the earth. the same is true, also, of all the small circles of diurnal revolution; they all turn exactly at the same rate as the equinoctial, and a star situated any where between the equator and the pole will move in its diurnal circle along with the clock, in the same manner as though it were in the equinoctial. hence, if we note the interval of time between the passage of any two stars, as shown by the clock, we have a measure of the number of degrees by which they are distant from each other in right ascension. hence we see how easy it is to take arcs of right ascension: the transit instrument shows us when a body is on the meridian; the clock indicates how long it is since the vernal equinox passed it, which is the right ascension itself; or it tells us the difference of right ascension between any two bodies, simply by indicating the difference in time between their periods of passing the meridian. again, it is easy to take the _declination_ of a body when on the meridian. by declination, you will recollect, is meant the distance of a heavenly body from the equinoctial; the same, indeed, as latitude on the earth. when a star is passing the meridian, if, on the instant of crossing the meridian wire of the telescope, we take its distance from the north pole, (which may readily be done, because the position of the pole is always known, being equal to the latitude of the place,) and subtract this distance from ninety degrees, the remainder will be the distance from the equator, which is the declination. you will ask, why we take this indirect method of finding the declination? why we do not rather take the distance of the star from the equinoctial, at once? i answer, that it is easy to point an instrument to the north pole, and to ascertain its exact position, and of course to measure any distance from it on the meridian, while, as there is nothing to mark the exact situation of the equinoctial, it is not so easy to take direct measurements from it. when we have thus determined the situation of a heavenly body, with respect to two great circles at right angles with each other, as in the present case, the distance of a body from the equator and from the equinoctial colure, or that meridian which passes though the vernal equinox, we know its relative position in the heavens; and when we have thus determined the relative positions of all the stars, we may lay them down on a map or a globe, exactly as we do places on the earth, by means of their latitude and longitude. the foregoing is only a _specimen_ of the various uses of the transit instrument, in finding the relative places of the heavenly bodies. another use of this excellent instrument is, to regulate our clocks and watches. by an observation with the transit instrument, we find when the sun's centre is on the meridian. this is the exact time of _apparent_ noon. but watches and clocks usually keep _mean_ time, and therefore, in order to set our timepiece by the transit instrument, we must apply to the apparent time of noon the equation of time, as will be explained in my next letter. a _noon-mark_ may easily be made by the aid of the transit instrument. a window sill is frequently selected as a suitable place for the mark, advantage being taken of the shadow projected upon it by the perpendicular casing of the window. let an assistant stand, with a rule laid on the line of shadow, and with a knife ready to make the mark, the instant when the observer at the transit instrument announces that the centre of the sun is on the meridian. by a concerted signal, as the stroke of a bell, the inhabitants of a town may all fix a noon-mark from the same observation. if the signal be given on one of the days when apparent time and mean time become equal to each other, as on the twenty-fourth of december, no equation of time is required. as a noon-mark is convenient for regulating timepieces, i will point out a method of making one, which may be practised without the aid of the telescope. upon a smooth, level plane, freely exposed to the sun, with a pair of compasses describe a circle. in the centre, where the leg of the compasses stood, erect a perpendicular wire of such a length, that the termination of its shadow shall fall upon the circumference of the circle at some hour before noon, as about ten o'clock. make a small dot at the point where the end of the shadow falls upon the circle, and do the same where it falls upon it again in the afternoon. take a point half-way between these two points, and from it draw a line to the centre, and it will be a true meridian line. the direction of this line would be the same, whether it were made in the summer or in the winter; but it is expedient to draw it about the fifteenth of june, for then the shadow alters its length most rapidly, and the moment of its crossing the wire will be more definite, than in the winter. at this time of year, also, the sun and clock agree, or are together, as will be more fully explained in my next letter; whereas, at other times of the year, the time of noon, as indicated by a common clock, would not agree with that indicated by the sun. if the upper end of the wire is flattened, and a small hole is made in it, through which the sun may shine, the instant when this bright spot falls upon the circle will be better defined than the termination of the shadow. another important instrument of the observatory is the _mural circle_. it is a graduated circle, usually of very large size, fixed permanently in the plane of the meridian, and attached firmly to a perpendicular wall; and on its centre is a telescope, which revolves along with it, and is easily brought to bear on any object in any point in the meridian. it is made of large size, sometimes twenty feet in diameter, in order that very small angles may be measured on its limb; for it is obvious that a small angle, as one second, will be a larger space on the limb of an instrument, in proportion as the instrument itself is larger. the vertical circle usually connected with the transit instrument, as in fig. , may indeed be employed for the same purposes as the mural circle, namely, to measure arcs of the meridian, as meridian altitudes, zenith distances, north polar distances, and declinations; but as that circle must necessarily be small, and therefore incapable of measuring very minute angles, the mural circle is particularly useful in measuring these important arcs. it is very difficult to keep so large an instrument perfectly steady; and therefore it is attached to a massive wall of solid masonry, and is hence called a _mural_ circle, from a latin word, (_murus_,) which signifies a wall. the diagram, fig. , page , represents a mural circle fixed to its wall, and ready for observations. it will be seen, that every expedient is employed to give the instrument firmness of parts and steadiness of position. the circle is of solid metal, usually of brass, and it is strengthened by numerous radii, which keep it from warping or bending; and these are made in the form of hollow cones, because that is the figure which unites in the highest degree lightness and strength. on the rim of the instrument, at a, you may observe a microscope. this is attached to a micrometer,--a delicate piece of apparatus, used for reading the minute subdivisions of angles; for, after dividing the limb of the instrument as minutely as possible, it will then be necessary to magnify those divisions with the microscope, and subdivide each of these parts with the micrometer. thus, if we have a mural circle twenty feet in diameter, and of course nearly sixty-three feet in circumference, since there are twenty-one thousand and six hundred minutes in the whole circle, we shall find, by calculation, that one minute would occupy, on the limb of such an instrument, only about one thirtieth of an inch, and a second, only one eighteen hundredth of an inch. we could not, therefore, hope to carry the actual divisions to a greater degree of minuteness than minutes; but each of these spaces may again be subdivided into seconds by the micrometer. [illustration fig. .] from these statements, you will acquire some faint idea of the extreme difficulty of making perfect astronomical instruments, especially where they are intended to measure such minute angles as one second. indeed, the art of constructing astronomical instruments is one which requires such refined mechanical genius,--so superior a mind to devise, and so delicate a hand to execute,--that the most celebrated instrument-makers take rank with the most distinguished astronomers; supplying, as they do, the means by which only the latter are enabled to make these great discoveries. astronomers have sometimes made their own telescopes; but they have seldom, if ever, possessed the refined manual skill which is requisite for graduating delicate instruments. the _sextant_ is also one of the most valuable instruments for taking celestial arcs, or the distance between any two points on the celestial sphere, being applicable to a much greater number of purposes than the instruments already described. it is particularly valuable for measuring celestial arcs at sea, because it is not, like most astronomical instruments, affected by the motion of the ship. the principle of the sextant may be briefly described, as follows: it gives the angular distance between any two bodies on the celestial sphere, by reflecting the image of one of the bodies so as to coincide with the other body, as seen directly. the arc through which the reflector is turned, to bring the reflected body to coincide with the other body, becomes a measure of the angular distance between them. by keeping this principle in view, you will be able to understand the use of the several parts of the instrument, as they are exhibited in the diagram, fig. , page . it is, you observe, of a triangular shape, and it is made strong and firm by metallic cross-bars. it has two reflectors, i and h, called, respectively, the index glass and the horizon glass, both of which are firmly fixed perpendicular to the plane of the instrument. the index glass is attached to the movable arm, id, and turns as this is moved along the graduated limb, ef. this arm also carries a _vernier_, at d, a contrivance which, like the micrometer, enables us to take off minute parts of the spaces into which the limb is divided. the horizon glass, h, consists of two parts; the upper part being transparent or open, so that the eye, looking through the telescope, t, can see through it a distant body, as a star at s, while the lower part is a reflector. [illustration fig. .] suppose it were required to measure the angular distance between the moon and a certain star,--the moon being at m, and the star at s. the instrument is held firmly in the hand, so that the eye, looking through the telescope, sees the star, s, through the transparent part of the horizon glass. then the movable arm, id, is moved from f towards e, until the image of m is reflected down to s, when the number of degrees and parts of a degree reckoned on the limb, from f to the index at d, will show the angular distance between the two bodies. footnote: [ ] brewster's life of newton letter vi. time and the calendar. "from old eternity's mysterious orb was time cut off, and cast beneath the skies."--_young._ having hitherto been conversant only with the many fine and sentimental things which the poets have sung respecting old time, perhaps you will find some difficulty in bringing down your mind to the calmer consideration of what time really is, and according to what different standards it is measured for different purposes. you will not, however, i think, find the subject even in our matter-of-fact and unpoetical way of treating it, altogether uninteresting. what, then, is time? _time is a measured portion of indefinite duration._ it consists of equal portions cut off from eternity, as a line on the surface of the earth is separated from its contiguous portions that constitute a great circle of the sphere, by applying to it a two-foot scale; or as a few yards of cloth are measured off from a piece of unknown or indefinite extent. any thing, or any event which takes place at equal intervals, may become a measure of time. thus, the pulsations of the wrist, the flowing of a given quantity of sand from one vessel to another, as in the hourglass, the beating of a pendulum, and the revolution of a star, have been severally employed as measures of time. but the great standard of time is the period of the revolution of the earth on its axis, which, by the most exact observations, is found to be always the same. i have anticipated a little of this subject, in giving an account of the transit instrument and clock, but i propose, in this letter, to enter into it more at large. the time of the earth's revolution on its axis, as already explained, is called a sidereal day, and is determined by the revolution of a star in the heavens. this interval is divided into twenty-four _sidereal_ hours. observations taken on numerous stars, in different ages of the world, show that they all perform their diurnal revolution in the same time, and that their motion, during any part of the revolution, is always uniform. here, then, we have an exact measure of time, probably more exact than any thing which can be devised by art. _solar time_ is reckoned by the apparent revolution of the sun from the meridian round to the meridian again. were the sun stationary in the heavens, like a fixed star, the time of its apparent revolution would be equal to the revolution of the earth on its axis, and the solar and the sidereal days would be equal. but, since the sun passes from west to east, through three hundred and sixty degrees, in three hundred and sixty-five and one fourth days, it moves eastward nearly one degree a day. while, therefore, the earth is turning round on its axis, the sun is moving in the same direction, so that, when we have come round under the same celestial meridian from which we started, we do not find the sun there, but he has moved eastward nearly a degree, and the earth must perform so much more than one complete revolution, before we come under the sun again. now, since we move, in the diurnal revolution, fifteen degrees in sixty minutes, we must pass over one degree in four minutes. it takes, therefore, four minutes for us to _catch up_ with the sun, after we have made one complete revolution. hence the solar day is about four minutes longer than the sidereal; and if we were to reckon the sidereal day twenty-four hours, we should reckon the solar day twenty-four hours four minutes. to suit the purposes of society at large, however, it is found more convenient to reckon the solar days twenty-four hours, and throw the fraction into the sidereal day. then, h. m. : h. :: h. : h. m. s. that is, when we reduce twenty-four hours and four minutes to twenty-four hours, the same proportion will require that we reduce the sidereal day from twenty-four hours to twenty-three hours fifty-six minutes four seconds; or, in other words, a sidereal day is such a part of a solar day. the solar days, however, do not always differ from the sidereal by precisely the same fraction, since they are not constantly of the same length. time, as measured by the sun, is called _apparent time_, and a clock so regulated as always to keep exactly with the sun, is said to keep apparent time. _mean time_ is time reckoned by the _average_ length of all the solar days throughout the year. this is the period which constitutes the _civil_ day of twenty-four hours, beginning when the sun is on the lower meridian, namely, at twelve o'clock at night, and counted by twelve hours from the lower to the upper meridian, and from the upper to the lower. the _astronomical_ day is the apparent solar day counted through the whole twenty-four hours, (instead of by periods of twelve hours each, as in the civil day,) and begins at noon. thus it is now the tenth of june, at nine o'clock, a.m., according to civil time; but we have not yet reached the tenth of june by astronomical time, nor shall we, until noon to-day; consequently, it is now june ninth, twenty-first hour of astronomical time. astronomers, since so many of their observations are taken on the meridian, are always supposed to look towards the south. geographers, having formerly been conversant only with the northern hemisphere, are always understood to be looking towards the north. hence, left and right, when applied to the astronomer, mean east and west, respectively; but to the geographer the right is east, and the left, west. clocks are usually regulated so as to indicate mean solar time; yet, as this is an artificial period not marked off, like the sidereal day, by any natural event, it is necessary to know how much is to be added to, or subtracted from, the apparent solar time, in order to give the corresponding mean time. the interval, by which apparent time differs from mean time, is called the _equation of time_. if one clock is so constructed as to keep exactly with the sun, going faster or slower, according as the lengths of the solar days vary, and another clock is regulated to mean time, then the difference of the two clocks, at any period, would be the equation of time for that moment. if the apparent clock were _faster_ than the mean, then the equation of time must be subtracted; but if the apparent clock were slower than the mean, then the equation of time must be added, to give the mean time. the two clocks would differ most about the third of november, when the apparent time is sixteen and one fourth minutes greater than the mean. but since apparent time is sometimes greater and sometimes less than mean time, the two must obviously be sometimes equal to each other. this is, in fact, the case four times a year, namely, april fifteenth, june fifteenth, september first, and december twenty-fourth. astronomical clocks are made of the best workmanship, with every advantage that can promote their regularity. although they are brought to an astonishing degree of accuracy, yet they are not as regular in their movements as the stars are, and their accuracy requires to be frequently tested. the transit instrument itself, when once accurately placed in the meridian, affords the means of testing the correctness of the clock, since one revolution of a star, from the meridian to the meridian again, ought to correspond exactly to twenty-four hours by the clock, and to continue the same, from day to day; and the right ascensions of various stars, as they cross the meridian, ought to be such by the clock, as they are given in the tables, where they are stated according to the accurate determinations of astronomers. or, by taking the difference of any two stars, on successive days, it will be seen whether the going of the clock is uniform for that part of the day; and by taking the right ascensions of different pairs of stars, we may learn the rate of the clock at various parts of the day. we thus learn, not only whether the clock accurately measures the length of the sidereal day, but also whether it goes uniformly from hour to hour. although astronomical clocks have been brought to a great degree of perfection, so as hardly to vary a second for many months, yet none are absolutely perfect, and most are so far from it, as to require to be corrected by means of the transit instrument, every few days. indeed, for the nicest observations, it is usual not to attempt to bring the clock to a state of absolute correctness, but, after bringing it as near to such a state as can conveniently be done, to ascertain how much it gains or loses in a day; that is, to ascertain the _rate_ of its going, and to make allowance accordingly. having considered the manner in which the smaller divisions of time are measured, let us now take a hasty glance at the larger periods which compose the calendar. as a _day_ is the period of the revolution of the earth on its axis, so a _year_ is the period of the revolution of the earth around the sun. this time, which constitutes the _astronomical year_, has been ascertained with great exactness, and found to be three hundred and sixty-five days five hours forty-eight minutes and fifty-one seconds. the most ancient nations determined the number of days in the year by means of the _stylus_, a perpendicular rod which casts its shadow on a smooth plane bearing a meridian line. the time when the shadow was shortest, would indicate the day of the summer solstice; and the number of days which elapsed, until the shadow returned to the same length again, would show the number of days in the year. this was found to be three hundred and sixty-five whole days, and accordingly, this period was adopted for the civil year. such a difference, however, between the civil and astronomical years, at length threw all dates into confusion. for if, at first, the summer solstice happened on the twenty-first of june, at the end of four years, the sun would not have reached the solstice until the twenty-second of june; that is, it would have been behind its time. at the end of the next four years, the solstice would fall on the twenty-third; and in process of time, it would fall successively on every day of the year. the same would be true of any other fixed date. julius cæsar, who was distinguished alike for the variety and extent of his knowledge, and his skill in arms, first attempted to make the calendar conform to the motions of the sun. "amidst the hurry of tumultuous war, the stars, the gods, the heavens, were still his care." aided by sosigenes, an egyptian astronomer, he made the first correction of the calendar, by introducing an additional day every fourth year, making february to consist of twenty-nine instead of twenty-eight days, and of course the whole year to consist of three hundred and sixty-six days. this fourth year was denominated _bissextile_, because the sixth day before the kalends of march was reckoned twice. it is also called leap year. the julian year was introduced into all the civilized nations that submitted to the roman power, and continued in general use until the year . but the true correction was not six hours, but five hours forty-nine minutes; hence the addition was too great by eleven minutes. this small fraction would amount in one hundred years to three fourths of a day, and in one thousand years to more than seven days. from the year to the year , it had, in fact, amounted to more than ten days; for it was known that, in , the vernal equinox fell on the twenty-first of march, whereas, in , it fell on the eleventh. it was ordered by the council of nice, a celebrated ecclesiastical council, held in the year , that easter should be celebrated upon the first sunday after the first full moon, next following the vernal equinox; and as certain other festivals of the romish church were appointed at particular seasons of the year, confusion would result from such a want of constancy between any fixed date and a particular season of the year. suppose, for example, a festival accompanied by numerous religious ceremonies, was decreed by the church to be held at the time when the sun crossed the equator in the spring, (an event hailed with great joy, as the harbinger of the return of summer,) and that, in the year , march twenty-first was designated as the time for holding the festival, since, at that period, it was on the twenty-first of march when the sun reached the equinox; the next year, the sun would reach the equinox a little sooner than the twenty-first of march, only eleven minutes, indeed, but still amounting in twelve hundred years to ten days; that is, in , the sun reached the equinox on the eleventh of march. if, therefore, they should continue to observe the twenty-first as a religious festival in honor of this event, they would commit the absurdity of celebrating it ten days after it had passed by. pope gregory the thirteenth, who was then at the head of the roman see, was a man of science, and undertook to reform the calendar, so that fixed dates would always correspond to the same seasons of the year. he first decreed, that the year should be brought forward ten days, by reckoning the fifth of october the fifteenth; and, in order to prevent the calendar from falling into confusion afterwards, he prescribed the following rule: _every year whose number is not divisible by four, without a remainder, consists of three hundred and sixty-five days; every year which is so divisible, but is not divisible by one hundred, of three hundred and sixty-six; every year divisible by one hundred, but not by four hundred, again, of three hundred and sixty-five; and every year divisible by four hundred, of three hundred and sixty-six._ thus the year , not being divisible by four, contains three hundred and sixty-five days, while and are leap years. yet, to make every fourth year consist of three hundred and sixty-six days would increase it too much, by about three fourths of a day in a century; therefore every hundredth year has only three hundred and sixty-five days. thus , although divisible by four, was not a leap year, but a common year. but we have allowed a _whole_ day in a hundred years, whereas we ought to have allowed only _three fourths_ of a day. hence, in four hundred years, we should allow a day too much, and therefore, we let the four hundredth remain a leap year. this rule involves an error of less than a day in four thousand two hundred and thirty-seven years. the pope, who, you will recollect, at that age assumed authority over all secular princes, issued his decree to the reigning sovereigns of christendom, commanding the observance of the calendar as reformed by him. the decree met with great opposition among the protestant states, as they recognised in it a new exercise of ecclesiastical tyranny; and some of them, when they received it, made it expressly understood, that their acquiescence should not be construed as a submission to the papal authority. in , the gregorian year, or _new style_, was established in great britain by act of parliament; and the dates of all deeds, and other legal papers, were to be made according to it. as above a century had then passed since the first introduction of the new style, eleven days were suppressed, the third of september being called the fourteenth. by the same act, the beginning of the year was changed from march twenty-fifth to january first. a few persons born previously to have come down to our day, and we frequently see inscriptions on tombstones of those whose time of birth is recorded in old style. in order to make this correspond to our present mode of reckoning, we must add eleven days to the date. thus the same event would be june twelfth of old style, or june twenty-third of new style; and if an event occurred between january first and march twenty-fifth, the date of the year would be advanced one, since february st, , o.s. would be february st, , n.s. thus, general washington was born february th, , o.s., or february d, , n.s. if we inquire how any present event may be made to correspond in date to the old style, we must subtract twelve days, and put the year back one, if the event lies between january first and march twenty-fifth. thus, june tenth, n.s. corresponds to may twenty-ninth, o.s.; and march th, , to march th, . france, being a roman catholic country, adopted the new style soon after it was decreed by the pope; but protestant countries, as we have seen, were much slower in adopting it; and russia, and the greek church generally, still adhere to the old style. in order, therefore, to make the russian dates correspond to ours, we must add to them twelve days. it may seem to you very remarkable, that so much pains should have been bestowed upon this subject; but without a correct and uniform standard of time, the dates of deeds, commissions, and all legal papers; of fasts and festivals, appointed by ecclesiastical authority; the returns of seasons, and the records of history,--must all fall into inextricable confusion. to change the observance of certain religious feasts, which have been long fixed to particular days, is looked upon as an impious innovation; and though the times of the events, upon which these ceremonies depend, are utterly unknown, it is still insisted upon by certain classes in england, that the glastenbury thorn blooms on christmas day. although the ancient grecian calendar was extremely defective, yet the common people were entirely averse to its reformation. their superstitious adherence to these errors was satirized by aristophanes, in his comedy of the clouds. an actor, who had just come from athens, recounts that he met with diana, or the moon, and found her extremely incensed, that they did not regulate her course better. she complained, that the order of nature was changed, and every thing turned topsyturvy. the gods no longer knew what belonged to them; but, after paying their visits on certain feast-days, and expecting to meet with good cheer, as usual, they were under the disagreeable necessity of returning back to heaven without their suppers. among the greeks, and other ancient nations, the length of the year was generally regulated by the course of the moon. this planet, on account of the different appearances which she exhibits at her full, change, and quarters, was considered by them as best adapted of any of the celestial bodies for this purpose. as one lunation, or revolution of the moon around the earth, was found to be completed in about twenty-nine and one half days, and twelve of these periods being supposed equal to one revolution of the sun, their months were made to consist of twenty-nine and thirty days alternately, and their year of three hundred and fifty-four days. but this disagreed with the annual revolution of the sun, which must evidently govern the seasons of the year, more than eleven days. the irregularities, which such a mode of reckoning would occasion, must have been too obvious not to have been observed. for, supposing it to have been settled, at any particular time, that the beginning of the year should be in the spring; in about sixteen years afterwards, the beginning would have been in autumn; and in thirty-three or thirty-four years, it would have gone backwards through all the seasons, to spring again. this defect they attempted to rectify, by introducing a number of days, at certain times, into the calendar, as occasion required, and putting the beginning of the year forwards, in order to make it agree with the course of the sun. but as these additions, or _intercalations_, as they were called, were generally consigned to the care of the priests, who, from motives of interest or superstition, frequently omitted them, the year was made long or short at pleasure. the _week_ is another division of time, of the highest antiquity, which, in almost all countries, has been made to consist of seven days; a period supposed by some to have been traditionally derived from the creation of the world; while others imagine it to have been regulated by the phases of the moon. the names, saturday, sunday, and monday, are obviously derived from saturn, the sun, and the moon; while tuesday, wednesday, thursday, and friday, are the days of tuisco, woden, thor, and friga, which are saxon names for mars, mercury, jupiter, and venus.[ ] the common year begins and ends on the same day of the week; but leap year ends one day later than it began. fifty-two weeks contain three hundred and sixty-four days; if, therefore, the year begins on tuesday, for example, we should complete fifty-two weeks on monday, leaving one day, (tuesday,) to complete the year, and the following year would begin on wednesday. hence, any day of the month is one day later in the week, than the corresponding day of the preceding year. thus, if the sixteenth of november, , falls on friday, the sixteenth of november, , fell on thursday, and will fall, in , on saturday. but if leap year begins on sunday, it ends on monday, and the following year begins on tuesday; while any given day of the month is two days later in the week than the corresponding date of the preceding year. footnote: [ ] bonnycastle's astronomy. letter vii. figure of the earth. "he took the golden compasses, prepared in god's eternal store, to circumscribe this universe, and all created things; one foot he centred, and the other turned round through the vast profundity obscure, and said, 'thus far extend, thus far thy bounds, this be thy just circumference, o world!'"--_milton._ in the earliest ages, the earth was regarded as one continued plane; but, at a comparatively remote period, as five hundred years before the christian era, astronomers began to entertain the opinion that the earth is round. we are able now to adduce various arguments which severally prove this truth. first, when a ship is coming in from sea, we first observe only the very highest parts of the ship, while the lower portions come successively into view. were the earth a continued plane, the lower parts of the ship would be visible as soon as the higher, as is evident from fig. , page . [illustration fig. .] [illustration fig. .] since light comes to the eye in straight lines, by which objects become visible, it is evident, that no reason exists why the parts of the ship near the water should not be seen as soon as the upper parts. but if the earth be a sphere, then the line of sight would pass above the deck of the ship, as is represented in fig. ; and as the ship drew nearer to land, the lower parts would successively rise above this line and come into view exactly in the manner known to observation. secondly, in a lunar eclipse, which is occasioned by the moon's passing through the earth's shadow, the figure of the shadow is seen to be spherical, which could not be the case unless the earth itself were round. thirdly, navigators, by steering continually in one direction, as east or west, have in fact come round to the point from which they started, and thus confirmed the fact of the earth's rotundity beyond all question. one may also reach a given place on the earth, by taking directly opposite courses. thus, he may reach canton in china, by a westerly route around cape horn, or by an easterly route around the cape of good hope. all these arguments severally prove that the earth is round. but i propose, in this letter, to give you some account of the unwearied labors which have been performed to ascertain the _exact_ figure of the earth; for although the earth is properly described in general language as round, yet it is not an exact sphere. were it so, all its diameters would be equal; but it is known that a diameter drawn through the equator exceeds one drawn from pole to pole, giving to the earth the form of a _spheroid_,--a figure resembling an orange, where the ends are flattened a little and the central parts are swelled out. although it would be a matter of very rational curiosity, to investigate the precise shape of the planet on which heaven has fixed our abode, yet the immense pains which has been bestowed on this subject has not all arisen from mere curiosity. no accurate measurements can be taken of the distances and magnitudes of the heavenly bodies, nor any exact determinations made of their motions, without a knowledge of the exact figure of the earth; and hence is derived a powerful motive for ascertaining this element with all possible precision. the first satisfactory evidence that was obtained of the exact figure of the earth was derived from reasoning on the effects of the earth's _centrifugal force_, occasioned by its rapid revolution on its own axis. when water is whirled in a pail, we see it recede from the centre and accumulate upon the sides of the vessel; and when a millstone is whirled rapidly, since the portions of the stone furthest from the centre revolve much more rapidly than those near to it, their greater tendency to recede sometimes makes them fly off with a violent explosion. a case, which comes still nearer to that of the earth, is exhibited by a mass of clay revolving on a potter's wheel, as seen in the process of making earthen vessels. the mass swells out in the middle, in consequence of the centrifugal force exerted upon it by a rapid motion. now, in the diurnal revolution, the equatorial parts of the earth move at the rate of about one thousand miles per hour, while the poles do not move at all; and since, as we take points at successive distances from the equator towards the pole, the rate at which these points move grows constantly less and less; and since, in revolving bodies, the centrifugal force is proportioned to the velocity, consequently, those parts which move with the greatest rapidity will be more affected by this force than those which move more slowly. hence, the equatorial regions must be higher from the centre than the polar regions; for, were not this the case, the waters on the surface of the earth would be thrown towards the equator, and be piled up there, just as water is accumulated on the sides of a pail when made to revolve rapidly. huyghens, an eminent astronomer of holland, who investigated the laws of centrifugal forces, was the first to infer that such must be the actual shape of the earth; but to sir isaac newton we owe the full developement of this doctrine. by combining the reasoning derived from the known laws of the centrifugal force with arguments derived from the principles of universal gravitation, he concluded that the distance through the earth, in the direction of the equator, is greater than that in the direction of the poles. he estimated the difference to be about thirty-four miles. but it was soon afterwards determined by the astronomers of france, to ascertain the figure of the earth by actual measurements, specially instituted for that purpose. let us see how this could be effected. if we set out at the equator and travel towards the pole, it is easy to see when we have advanced one degree of latitude, for this will be indicated by the rising of the north star, which appears in the horizon when the spectator stands on the equator, but rises in the same proportion as he recedes from the equator, until, on reaching the pole, the north star would be seen directly over head. now, were the earth a perfect sphere, the meridian of the earth would be a perfect circle, and the distance between any two places, differing one degree in latitude, would be exactly equal to the distance between any other two places, differing in latitude to the same amount. but if the earth be a spheroid, flattened at the poles, then a line encompassing the earth from north to south, constituting the terrestrial meridian, would not be a perfect circle, but an ellipse or oval, having its longer diameter through the equator, and its shorter through the poles. the part of this curve included between two radii, drawn from the centre of the earth to the celestial meridian, at angles one degree asunder, would be greater in the polar than in the equatorial region; that is, the degrees of the meridian would lengthen towards the poles. the french astronomers, therefore, undertook to ascertain by actual measurements of arcs of the meridian, in different latitudes, whether the degrees of the meridian are of uniform length, or, if not, in what manner they differ from each other. after several indecisive measurements of an arc of the meridian in france, it was determined to effect simultaneous measurements of arcs of the meridian near the equator, and as near as possible to the north pole, presuming that if degrees of the meridian, in different latitudes, are really of different lengths, they will differ most in points most distant from each other. accordingly, in , the french academy, aided by the government, sent out two expeditions, one to peru and the other to lapland. three distinguished mathematicians, bouguer, la condamine, and godin, were despatched to the former place, and four others, maupertius, camus, clairault, and lemonier, were sent to the part of swedish lapland which lies at the head of the gulf of tornea, the northern arm of the baltic. this commission completed its operations several years sooner than the other, which met with greater difficulties in the way of their enterprise. still, the northern detachment had great obstacles to contend with, arising particularly from the extreme length and severity of their winters. the measurements, however, were conducted with care and skill, and the result, when compared with that obtained for the length of a degree in france, plainly indicated, by its greater amount, a compression of the earth towards the poles. mean-while, bouguer and his party were prosecuting a similar work in peru, under extraordinary difficulties. these were caused, partly by the localities, and partly by the ill-will and indolence of the inhabitants. the place selected for their operations was in an elevated valley between two principal chains of the andes. the lowest point of their arc was at an elevation of a mile and a half above the level of the sea; and, in some instances, the heights of two neighboring signals differed more than a mile. encamped upon lofty mountains, they had to struggle against storms, cold, and privations of every description, while the invincible indifference of the indians, they were forced to employ, was not to be shaken by the fear of punishment or the hope of reward. yet, by patience and ingenuity, they overcame all obstacles, and executed with great accuracy one of the most important operations, of this nature, ever undertaken. to accomplish this, however, took them nine years; of which, three were occupied in determining the latitudes alone.[ ] i have recited the foregoing facts, in order to give you some idea of the unwearied pains which astronomers have taken to ascertain the exact figure of the earth. you will find, indeed, that all their labors are characterized by the same love of accuracy. years of toilsome watchings, and incredible labor of computation, have been undergone, for the sake of arriving only a few seconds nearer to the truth. the length of a degree of the meridian, as measured in peru, was less than that before determined in france, and of course less than that of lapland; so that the spheroidal figure of the earth appeared now to be ascertained by actual measurement. still, these measures were too few in number, and covered too small a portion of the whole quadrant from the equator to the pole, to enable astronomers to ascertain the exact law of curvature of the meridian, and therefore similar measurements have since been prosecuted with great zeal by different nations, particularly by the french and english. in , two english mathematicians of great eminence, mason and dixon, undertook the measurement of an arc in pennsylvania, extending more than one hundred miles. [illustration fig. .] [illustration fig. .] these operations are carried on by what is called a system of _triangulation_. without some knowledge of trigonometry, you will not be able fully to understand this process; but, as it is in its nature somewhat curious, and is applied to various other geographical measurements, as well as to the determination of arcs of the meridian, i am desirous that you should understand its general principles. let us reflect, then, that it must be a matter of the greatest difficulty, to execute with exactness the measurement of a line of any great length in one continued direction on the earth's surface. even if we select a level and open country, more or less inequalities of surface will occur; rivers must be crossed, morasses must be traversed, thickets must be penetrated, and innumerable other obstacles must be surmounted; and finally, every time we apply an artificial measure, as a rod, for example, we obtain a result not absolutely perfect. each error may indeed be very small, but small errors, often repeated, may produce a formidable aggregate. now, one unacquainted with trigonometry can easily understand the fact, that, when we know certain parts of a triangle, we can find the other parts by calculation; as, in the rule of three in arithmetic, we can obtain the fourth term of a proportion, from having the first three terms given. thus, in the triangle a b c, fig. , if we know the side a b, and the angles at a and b, we can find by computation, the other sides, a c and b c, and the remaining angle at c. suppose, then, that in measuring an arc of the meridian through any country, the line were to pass directly through a b, but the ground was so obstructed between a and b, that we could not possibly carry our measurement through it. we might then measure another line, as a c, which was accessible, and with a compass take the bearing of b from the points a and c, by which means we should learn the value of the angles at a and c. from these data we might calculate, by the rules of trigonometry, the exact length of the line a b. perhaps the ground might be so situated, that we could not reach the point b, by any route; still, if it could be seen from a and c, it would be all we should want. thus, in conducting a trigonometrical survey of any country, conspicuous signals are placed on elevated points, and the bearings of these are taken from the extremities of a known line, called the base, and thus the relative situation of various places is accurately determined. were we to undertake to run an exact north and south line through any country, as new england, we should select, near one extremity, a spot of ground favorable for actual measurement, as a level, unobstructed plain; we should provide a measure whose length in feet and inches was determined with the greatest possible precision, and should apply it with the utmost care. we should thus obtain a _base line_. from the extremities of this line, we should take (with some appropriate instrument) the bearing of some signal at a greater or less distance, and thus we should obtain one side and two angles of a triangle, from which we could find, by the rules of trigonometry, either of the unknown sides. taking this as a new base, we might take the bearing of another signal, still further on our way, and thus proceed to run the required north and south line, without actually measuring any thing more than the first, or base line. thus, in fig. , we wish to measure the distance between the two points a and o, which are both on the same meridian, as is known by their having the same longitude; but, on account of various obstacles, it would be found very inconvenient to measure this line directly, with a rod or chain, and even if we could do it, we could not by this method obtain nearly so accurate a result, as we could by a series of triangles, where, after the base line was measured, we should have nothing else to measure except angles, which can be determined, by observation, to a greater degree of exactness, than lines. we therefore, in the first place, measure the base line, a b, with the utmost precision. then, taking the bearing of some signal at c from a and b, we obtain the means of calculating the side b c, as has been already explained. taking b c as a new base, we proceed, in like manner, to determine successively the sides c d, d e, and e f, and also a c, and c e. although a c is not in the direction of the meridian, but considerably to the east of it, yet it is easy to find the corresponding distance on the meridian, a m; and in the same manner we can find the portions of the meridian m n and n o, corresponding respectively to c e and e f. adding these several parts of the meridian together, we obtain the length of the arc from a to o, in miles; and by observations on the north star, at each extremity of the arc, namely, at a and at o, we could determine the difference of latitude between these two points. suppose, for example, that the distance between a and o is exactly five degrees, and that the length of the intervening line is three hundred and forty-seven miles; then, dividing the latter by the former number, we find the length of a degree to be sixty-nine miles and four tenths. to take, however, a few of the results actually obtained, they are as follows: places of observation. latitude. length of a deg. in miles. peru, ° ' " . pennsylvania, . france, . england, - / . sweden, . this comparison shows, that the length of a degree gradually increases, as we proceed from the equator towards the pole. combining the results of various estimates, the dimensions of the terrestrial spheroid are found to be as follows: equatorial diameter, . miles. polar diameter, . " average diameter, . " the difference between the greatest and the least is about twenty-six and one half miles, which is about one two hundred and ninety-ninth part of the greatest. this fraction is denominated the _ellipticity_ of the earth,--being the excess of the equatorial over the polar diameter. the operations, undertaken for the purpose of determining the figure of the earth, have been conducted with the most refined exactness. at any stage of the process, the length of the last side, as obtained by calculation, may be actually measured in the same manner as the base from which the series of triangles commenced. when thus measured, it is called the _base of verification_. in some surveys, the base of verification, when taken at a distance of four hundred miles from the starting point, has not differed more than one foot from the same line, as determined by calculation. another method of arriving at the exact figure of the earth is, by observations with the _pendulum_. if a pendulum, like that of a clock, be suspended, and the number of its vibrations per hour be counted, they will be found to be different in different latitudes. a pendulum that vibrates thirty-six hundred times per hour, at the equator, will vibrate thirty-six hundred and five and two thirds times, at london, and a still greater number of times nearer the north pole. now, the vibrations of the pendulum are produced by the force of gravity. hence their comparative number at different places is a measure of the relative forces of gravity at those places. but when we know the relative forces of gravity at different places, we know their relative distances from the centre of the earth; because the nearer a place is to the centre of the earth, the greater is the force of gravity. suppose, for example, we should count the number of vibrations of a pendulum at the equator, and then carry it to the north pole, and count the number of vibrations made there in the same time,--we should be able, from these two observations, to estimate the relative forces of gravity at these two points; and, having the relative forces of gravity, we can thence deduce their relative distances from the centre of the earth, and thus obtain the polar and equatorial diameters. observations of this kind have been taken with the greatest accuracy, in many places on the surface of the earth, at various distances from each other, and they lead to the same conclusions respecting the figure of the earth, as those derived from measuring arcs of the meridian. it is pleasing thus to see a great truth, and one apparently beyond the pale of human investigation, reached by two routes entirely independent of each other. nor, indeed, are these the only proofs which have been discovered of the spheroidal figure of the earth. in consequence of the accumulation of matter above the equatorial regions of the earth, a body weighs less there than towards the poles, being further removed from the centre of the earth. the same accumulation of matter, by the force of attraction which it exerts, causes slight inequalities in the motions of the moon; and since the amount of these becomes a measure of the force which produces them, astronomers are able, from these inequalities, to calculate the exact quantity of the matter thus accumulated, and hence to determine the figure of the earth. the result is not essentially different from that obtained by the other methods. finally, the shape of the earth's shadow is altered, by its spheroidal figure,--a circumstance which affects the time and duration of a lunar eclipse. all these different and independent phenomena afford a pleasing example of the harmony of truth. the known effects of the centrifugal force upon a body revolving on its axis, like the earth, lead us to infer that the earth is of a spheroidal figure; but if this be the fact, the pendulum ought to vibrate faster near the pole than at the equator, because it would there be nearer the centre of the earth. on trial, such is found to be the case. if, again, there be such an accumulation of matter about the equatorial regions, its effects ought to be visible in the motions of the moon, which it would influence by its gravity; and there, also, its effects are traced. at length, we apply our measures to the surface of the earth itself, and find the same fact, which had thus been searched out among the hidden things of nature, here palpably exhibited before our eyes. finally, on estimating from these different sources, what the exact amount of the compression at the poles must be, all bring out nearly one and the same result. this truth, so harmonious in itself, takes along with it, and establishes, a thousand other truths on which it rests. footnote: [ ] library of useful knowledge: history of astronomy, page . letter viii. diurnal revolutions. "to some she taught the fabric of the sphere, the changeful moon, the circuit of the stars, the golden zones of heaven."--_akenside._ with the elementary knowledge already acquired, you will now be able to enter with pleasure and profit on the various interesting phenomena dependent on the revolution of the earth on its axis and around the sun. the apparent diurnal revolution of the heavenly bodies, from east to west, is owing to the actual revolution of the earth on its own axis, from west to east. if we conceive of a radius of the earth's equator extended until it meets the concave sphere of the heavens, then, as the earth revolves, the extremity of this line would trace out a curve on the face of the sky; namely, the celestial equator. in curves parallel to this, called the _circles of diurnal revolution_, the heavenly bodies actually _appear_ to move, every star having its own peculiar circle. after you have first rendered familiar the real motion of the earth from west to east, you may then, without danger of misapprehension, adopt the common language, that all the heavenly bodies revolve around the earth once a day, from east to west, in circles parallel to the equator and to each other. i must remind you, that the time occupied by a star, in passing from any point in the meridian until it comes round to the same point again, is called a _sidereal day_, and measures the period of the earth's revolution on its axis. if we watch the returns of the same star from day to day, we shall find the intervals exactly equal to each other; that is, _the sidereal days are all equal_. whatever star we select for the observation, the same result will be obtained. the stars, therefore, always keep the same relative position, and have a common movement round the earth,--a consequence that naturally flows from the hypothesis that their _apparent_ motion is all produced by a single _real_ motion; namely, that of the earth. the sun, moon, and planets, as well as the fixed stars, revolve in like manner; but their returns to the meridian are not, like those of the fixed stars, at exactly equal intervals. the _appearances_ of the diurnal motions of the heavenly bodies are different in different parts of the earth,--since every place has its own horizon, and different horizons are variously inclined to each other. nothing in astronomy is more apt to mislead us, than the obstinate habit of considering the horizon as a fixed and immutable plane, and of referring every thing to it. we should contemplate the earth as a huge globe, occupying a small portion of space, and encircled on all sides, at an immense distance, by the starry sphere. we should free our minds from their habitual proneness to consider one part of space as naturally _up_ and another _down_, and view ourselves as subject to a force (gravity) which binds us to the earth as truly as though we were fastened to it by some invisible cords or wires, as the needle attaches itself to all sides of a spherical loadstone. we should dwell on this point, until it appears to us as truly up, in the direction b b, c c, d d, when one is at b, c, d, respectively, as in the direction a a, when he is at a, fig. . let us now suppose the spectator viewing the diurnal revolutions from several different positions on the earth. on the _equator_, his horizon would pass through both poles; for the horizon cuts the celestial vault at ninety degrees in every direction from the zenith of the spectator; but the pole is likewise ninety degrees from his zenith, when he stands on the equator; and consequently, the pole must be in the horizon. here, also, the celestial equator would coincide with the prime vertical, being a great circle passing through the east and west points. since all the diurnal circles are parallel to the equator, consequently, they would all, like the equator be perpendicular to the horizon. such a view of the heavenly bodies is called a right sphere, which may be thus defined: _a right sphere is one in which all the daily revolutions of the stars are in circles perpendicular to the horizon_. [illustration fig. .] a right sphere is seen only at the equator. any star situated in the celestial equator would appear to rise directly in the east, at midnight to be in the zenith of the spectator, and to set directly in the west. in proportion as stars are at a greater distance from the equator towards the pole, they describe smaller and smaller circles, until, near the pole, their motion is hardly perceptible. if the spectator advances one degree from the equator towards the north pole, his horizon reaches one degree beyond the pole of the earth, and cuts the starry sphere one degree below the pole of the heavens, or below the north star, if that be taken as the place of the pole. as he moves onward towards the pole, his horizon continually reaches further and further beyond it, until, when he comes to the pole of the earth, and under the pole of the heavens, his horizon reaches on all sides to the equator, and coincides with it. moreover, since all the circles of daily motion are parallel to the equator, they become, to the spectator at the pole, parallel to the horizon. or, _a parallel sphere is that in which all the circles of daily motion are parallel to the horizon_. to render this view of the heavens familiar, i would advise you to follow round in mind a number of separate stars, in their diurnal revolution, one near the horizon, one a few degrees above it, and a third near the zenith. to one who stood upon the north pole, the stars of the northern hemisphere would all be perpetually in view when not obscured by clouds, or lost in the sun's light, and none of those of the southern hemisphere would ever be seen. the sun would be constantly above the horizon for six months in the year, and the remaining six continually out of sight. that is, at the pole, the days and nights are each six months long. the appearances at the south pole are similar to those at the north. a perfect parallel sphere can never be seen, except at one of the poles,--a point which has never been actually reached by man; yet the british discovery ships penetrated within a few degrees of the north pole, and of course enjoyed the view of a sphere nearly parallel. as the circles of daily motion are parallel to the horizon of the pole, and perpendicular to that of the equator, so at all places between the two, the diurnal motions are oblique to the horizon. this aspect of the heavens constitutes an oblique sphere, which is thus defined: _an oblique sphere is that in which the circles of daily motion are oblique to the horizon_. suppose, for example, that the spectator is at the latitude of fifty degrees. his horizon reaches fifty degrees beyond the pole of the earth, and gives the same apparent elevation to the pole of the heavens. it cuts the equator and all the circles of daily motion, at an angle of forty degrees,--being always equal to what the altitude of the pole lacks of ninety degrees: that is, it is always equal to the co-altitude of the pole. thus, let h o, fig. , represent the horizon, e q the equator, and p p´ the axis of the earth. also, _l l, m m, n n_, parallels of latitude. then the horizon of a spectator at z, in latitude fifty degrees, reaches to fifty degrees beyond the pole; and the angle e c h, which the equator makes with the horizon, is forty degrees,--the complement of the latitude. as we advance still further north, the elevation of the diurnal circle above the horizon grows less and less, and consequently, the motions of the heavenly bodies more and more oblique to the horizon, until finally, at the pole, where the latitude is ninety degrees, the angle of elevation of the equator vanishes, and the horizon and the equator coincide with each other, as before stated. [illustration fig. .] _the circle of perpetual apparition is the boundary of that space around the elevated pole, where the stars never set._ its distance from the pole is equal to the latitude of the place. for, since the altitude of the pole is equal to the latitude, a star, whose polar distance is just equal to the latitude, will, when at its lowest point, only just reach the horizon; and all the stars nearer the pole than this will evidently not descend so far as the horizon. thus _m m_, fig. , is the circle of perpetual apparition, between which and the north pole, the stars never set, and its distance from the pole, o p, is evidently equal to the elevation of the pole, and of course to the latitude. in the opposite hemisphere, a similar part of the sphere adjacent to the depressed pole never rises. hence, _the circle of perpetual occultation is the boundary of that space around the depressed pole, within which the stars never rise._ thus _m´ m´_, fig. , is the circle of perpetual occultation, between which and the south pole, the stars never rise. in an oblique sphere, the horizon cuts the circles of daily motion unequally. towards the elevated pole, more than half the circle is above the horizon, and a greater and greater portion, as the distance from the equator is increased, until finally, within the circle of perpetual apparition, the whole circle is above the horizon. just the opposite takes place in the hemisphere next the depressed pole. accordingly, when the sun is in the equator, as the equator and horizon, like all other great circles of the sphere, bisect each other, the days and nights are equal all over the globe. but when the sun is north of the equator, the days become longer than the nights, but shorter, when the sun is south of the equator. moreover, the higher the latitude, the greater is the inequality in the lengths of the days and nights. by examining fig. , you will easily see how each of these cases must hold good. most of the appearances of the diurnal revolution can be explained, either on the supposition that the celestial sphere actually turns around the earth once in twenty-four hours, or that this motion of the heavens is merely apparent, arising from the revolution of the earth on its axis, in the opposite direction,--a motion of which we are insensible, as we sometimes lose the consciousness of our own motion in a ship or steam-boat, and observe all external objects to be receding from us, with a common motion. proofs, entirely conclusive and satisfactory, establish the fact, that it is the earth, and not the celestial sphere, that turns; but these proofs are drawn from various sources, and one is not prepared to appreciate their value, or even to understand some of them, until he has made considerable proficiency in the study of astronomy, and become familiar with a great variety of astronomical phenomena. to such a period we will therefore postpone the discussion of the earth's rotation on its axis. while we retain the same place on the earth, the diurnal revolution occasions no change in our horizon, but our horizon goes round, as well as ourselves. let us first take our station on the equator, at sunrise; our horizon now passes through both the poles and through the sun, which we are to conceive of as at a great distance from the earth, and therefore as cut, not by the terrestrial, but by the celestial, horizon. as the earth turns, the horizon dips more and more below the sun, at the rate of fifteen degrees for every hour; and, as in the case of the polar star, the sun appears to rise at the same rate. in six hours, therefore, it is depressed ninety degrees below the sun, bringing us directly under the sun, which, for our present purpose, we may consider as having all the while maintained the same fixed position in space. the earth continues to turn, and in six hours more, it completely reverses the position of our horizon, so that the western part of the horizon, which at sunrise was diametrically opposite to the sun, now cuts the sun, and soon afterwards it rises above the level of the sun, and the sun sets. during the next twelve hours, the sun continues on the invisible side of the sphere, until the horizon returns to the position from which it set out, and a new day begins. let us next contemplate the similar phenomena at the _poles_. here the horizon, coinciding, as it does, with the equator, would cut the sun through its centre and the sun would appear to revolve along the surface of the sea, one half above and the other half below the horizon. this supposes the sun in its annual revolution to be at one of the equinoxes. when the sun is north of the equator, it revolves continually round in a circle, which, during a single revolution, appears parallel to the equator, and it is constantly day; and when the sun is south of the equator, it is, for the same reason, continual night. when we have gained a clear idea of the appearances of the diurnal revolutions, as exhibited to a spectator at the equator and at the pole, that is, in a right and in a parallel sphere, there will be little difficulty in imagining how they must be in the intermediate latitudes, which have an oblique sphere. the appearances of the sun and stars, presented to the inhabitants of different countries, are such as correspond to the sphere in which they live. thus, in the fervid climates of india, africa, and south america, the sun mounts up to the highest regions of the heavens, and descends directly downwards, suddenly plunging beneath the horizon. his rays, darting almost vertically upon the heads of the inhabitants, strike with a force unknown to the people of the colder climates; while in places remote from the equator, as in the north of europe, the sun, in summer, rises very far in the north, takes a long circuit towards the south, and sets as far northward in the west as the point where it rose on the other side of the meridian. as we go still further north, to the northern parts of norway and sweden, for example, to the confines of the frigid zone, the summer's sun just grazes the northern horizon, and at noon appears only twenty-three and one half degrees above the southern. on the other hand, in mid-winter, in the north of europe, as at st. petersburgh, the day dwindles almost to nothing,--lasting only while the sun describes a very short arc in the extreme south. in some parts of siberia and iceland, the only day consists of a little glimmering of the sun on the verge of the southern horizon, at noon. letter ix. parallax and refraction. "go, wondrous creature! mount where science guides, go measure earth, weigh air, and state the tides; instruct the planets in what orbs to run, correct old time, and regulate the sun."--_pope._ i think you must have felt some astonishment, that astronomers are able to calculate the exact distances and magnitudes of the sun, moon, and planets. we should, at the first thought, imagine that such knowledge as this must be beyond the reach of the human faculties, and we might be inclined to suspect that astronomers practise some deception in this matter, for the purpose of exciting the admiration of the unlearned. i will therefore, in the present letter, endeavor to give you some clear and correct views respecting the manner in which astronomers acquire this knowledge. in our childhood, we all probably adopt the notion that the sky is a real dome of definite surface, in which the heavenly bodies are fixed. when any objects are beyond a certain distance from the eye, we lose all power of distinguishing, by our sight alone, between different distances, and cannot tell whether a given object is one million or a thousand millions of miles off. although the bodies seen in the sky are in fact at distances extremely various,--some, as the clouds, only a few miles off; others, as the moon, but a few thousand miles; and others, as the fixed stars, innumerable millions of miles from us,--yet, as our eye cannot distinguish these different distances, we acquire the habit of referring all objects beyond a moderate height to one and the same surface, namely, an imaginary spherical surface, denominated the celestial vault. thus, the various objects represented in the diagram on next page, though differing very much in shape and diameter, would all be _projected_ upon the sky alike, and compose a part, indeed, of the imaginary vault itself. the place which each object occupies is determined by lines drawn from the eye of the spectator through the extremities of the body, to meet the imaginary concave sphere. thus, to a spectator at o, fig , the several lines a b, c d, and e f, would all be projected into arches on the face of the sky, and be seen as parts of the sky itself, as represented by the lines a´ b´, c´ d´, and e´ f´. and were a body actually to move in the several directions indicated by these lines, they would appear to the spectator to describe portions of the celestial vault. thus, even when moving through the crooked line, from _a_ to _b_, a body would appear to be moving along the face of the sky, and of course in a regular curve line, from _c_ to _d_. [illustration fig. .] but, although all objects, beyond a certain moderate height, are projected on the imaginary surface of the sky, yet different spectators will project the same object on _different parts_ of the sky. thus, a spectator at a, fig. , would see a body, c, at m, while a spectator at b would see the same body at n. this change of place in a body, as seen from different points, is called parallax, which is thus defined: _parallax is the apparent change of place which bodies undergo by being viewed from different points_. [illustration fig. .] the arc m n is called the _parallactic arc_, and the angle a c b, the _parallactic angle_. it is plain, from the figure, that near objects are much more affected by parallax than distant ones. thus, the body c, fig. , makes a much greater parallax than the more distant body d,--the former being measured by the arc m n, and the latter by the arc o p. we may easily imagine bodies to be so distant, that they would appear projected at very nearly the same point of the heavens, when viewed from places very remote from each other. indeed, the fixed stars, as we shall see more fully hereafter, are so distant, that spectators, a hundred millions of miles apart, see each star in one and the same place in the heavens. it is by means of parallax, that astronomers find the distances and magnitudes of the heavenly bodies. in order fully to understand this subject, one requires to know something of trigonometry, which science enables us to find certain unknown parts of a triangle from certain other parts which are known. although you may not be acquainted with the principles of trigonometry, yet you will readily understand, from your knowledge of arithmetic, that from certain things given in a problem others may be found. every triangle has of course three sides and three angles; and, if we know two of the angles and one of the sides, we can find all the other parts, namely, the remaining angle and the two unknown sides. thus, in the triangle a b c, fig. , if we know the length of the side a b, and how many degrees each of the angles a b c and b c a contains, we can find the length of the side b c, or of the side a c, and the remaining angle at a. now, let us apply these principles to the measurements of some of the heavenly bodies. [illustration fig. .] [illustration fig. .] in fig. , let a represent the earth, c h the horizon, and h z a quadrant of a great circle of the heavens, extending from the horizon to the zenith; and let e, f, g, o, be successive positions of the moon, at different elevations, from the horizon to the meridian. now, a spectator on the surface of the earth, at a, would refer the moon, when at e, to _h_, on the face of the sky, whereas, if seen from the centre of the earth, it would appear at h. so, when the moon was at f, a spectator at a would see it at _p_, while, if seen from the centre, it would have appeared at p. the parallactic arcs, h _h_, p _p_, r _r_, grow continually smaller and smaller, as a body is situated higher above the horizon; and when the body is in the zenith, then the parallax vanishes altogether, for at o the moon would be seen at z, whether viewed from a or c. since, then, a heavenly body is liable to be referred to different points on the celestial vault, when seen from different parts of the earth, and thus some confusion be occasioned in the determination of points on the celestial sphere, astronomers have agreed to consider the true place of a celestial object to be that where it would appear, if seen from the centre of the earth; and the doctrine of parallax teaches how to reduce observations made at any place on the surface of the earth, to such as they would be, if made from the centre. when the moon, or any heavenly body, is seen in the horizon, as at e, the change of place is called the horizontal parallax. thus, the angle a e c, measures the horizontal parallax of the moon. were a spectator to view the earth from the centre of the moon, he would see the semidiameter of the earth under this same angle; hence, _the horizontal parallax of any body is the angle subtended by the semidiameter of the earth, as seen from the body_. please to remember this fact. it is evident from the figure, that the effect of parallax upon the place of a celestial body is to _depress_ it. thus, in consequence of parallax, e is depressed by the arc h _h_; f, by the arc p _p_; g, by the arc r _r_; while o sustains no change. hence, in all calculations respecting the altitude of the sun, moon, or planets, the amount of parallax is to be added: the stars, as we shall see hereafter, have no sensible parallax. it is now very easy to see how, when the parallax of a body is known, we may find its distance from the centre of the earth. thus, in the triangle a c e, fig. , the side a c is known, being the semidiameter of the earth; the angle c a e, being a right angle, is also known; and the parallactic angle, a e c, is found from observation; and it is a well-known principle of trigonometry, that when we have any two angles of a triangle, we may find the remaining angle by subtracting the sum of these two from one hundred and eighty degrees. consequently, in the triangle a e c, we know all the angles and one side, namely, the side a c; hence, we have the means of finding the side c e, which is the distance from the centre of the earth to the centre of the moon. [illustration fig. .] when the distance of a heavenly body is known, and we can measure, with instruments, its angular breadth, we can easily determine its _magnitude_. thus, if we have the distance of the moon, e s, fig. , and half the breadth of its disk s c, (which is measured by the angle s e c,) we can find the length of the line, s c, in miles. twice this line is the diameter of the body; and when we know the diameter of a sphere, we can, by well-known rules, find the contents of the surface, and its solidity. you will perhaps be curious to know, _how the moon's horizontal parallax is found_; for it must have been previously ascertained, before we could apply this method to finding the distance of the moon from the earth. suppose that two astronomers take their stations on the same meridian, but one south of the equator, as at the cape of good hope, and another north of the equator, as at berlin, in prussia, which two places lie nearly on the same meridian. the observers would severally refer the moon to different points on the face of the sky,--the southern observer carrying it further north, and the northern observer further south, than its true place, as seen from the centre of the earth. this will be plain from the diagram, fig. . if a and b represent the positions of the spectators, m the moon, and c d an arc of the sky, then it is evident, that c d would be the parallactic arc. [illustration fig. .] these observations furnish materials for calculating, by the aid of trigonometry, the moon's horizontal parallax, and we have before seen how, when we know the parallax of a heavenly body, we can find both its distance from the earth and its magnitude. beside the change of place which these heavenly bodies undergo, in consequence of parallax, there is another, of an opposite kind, arising from the effect of the atmosphere, called _refraction_. refraction elevates the apparent place of a body, while parallax depresses it. it affects alike the most distant as well as nearer bodies. in order to understand the nature of refraction, we must consider, that an object always appears in the direction in which the _last_ ray of light comes to the eye. if the light which comes from a star were bent into fifty directions before it reached the eye, the star would nevertheless appear in the line described by the ray nearest the eye. the operation of this principle is seen when an oar, or any stick, is thrust into water. as the rays of light by which the oar is seen have their direction changed as they pass out of water into air, the apparent direction in which the body is seen is changed in the same degree, giving it a bent appearance,--the part below the water having apparently a different direction from the part above. thus, in fig. , page , if s _a x_ be the oar, s _a b_ will be the bent appearance, as affected by refraction. the transparent substance through which any ray of light passes is called a _medium_. it is a general fact in optics, that, when light passes out of a rarer into a denser medium, as out of air into water, or out of space into air, it is turned _towards_ a perpendicular to the surface of the medium; and when it passes out of a denser into a rarer medium, as out of water into air, it is turned _from_ the perpendicular. in the above case, the light, passing out of space into air, is turned towards the radius of the earth, this being perpendicular to the surface of the atmosphere; and it is turned more and more towards that radius the nearer it approaches to the earth, because the density of the air rapidly increases near the earth. [illustration fig. .] let us now conceive of the atmosphere as made up of a great number of parallel strata, as a a, b b, c c, and d d, increasing rapidly in density (as is known to be the fact) in approaching near to the surface of the earth. let s be a star, from which a ray of light, s _a_, enters the atmosphere at _a_, where, being much turned towards the radius of the convex surface, it would change its direction into the line _a b_, and again into _b c_, and _c_ o, reaching the eye at o. now, since an object always appears in the direction in which the light finally strikes the eye, the star would be seen in the direction o _c_, and, consequently, the star would apparently change its place, by refraction, from s to s´, being elevated out of its true position. moreover, since, on account of the continual increase of density in descending through the atmosphere, the light would be continually turned out of its course more and more, it would therefore move, not in the polygon represented in the figure, but in a corresponding curve line, whose curvature is rapidly increased near the surface of the earth. when a body is in the zenith, since a ray of light from it enters the atmosphere at right angles to the refracting medium, it suffers no refraction. consequently, the position of the heavenly bodies, when in the zenith, is not changed by refraction, while, near the horizon, where a ray of light strikes the medium very obliquely, and traverses the atmosphere through its densest part, the refraction is greatest. the whole amount of refraction, when a body is in the horizon, is thirty-four minutes; while, at only an elevation of one degree, the refraction is but twenty-four minutes; and at forty-five degrees, it is scarcely a single minute. hence it is always important to make our observations on the heavenly bodies when they are at as great an elevation as possible above the horizon, being then less affected by refraction than at lower altitudes. since the whole amount of refraction near the horizon exceeds thirty-three minutes, and the diameters of the sun and moon are severally less than this, these luminaries are in view both before they have actually risen and after they have set. the rapid increase of refraction near the horizon is strikingly evinced by the _oval_ figure which the sun assumes when near the horizon, and which is seen to the greatest advantage when light clouds enable us to view the solar disk. were all parts of the sun equally raised by refraction, there would be no change of figure; but, since the lower side is more refracted than the upper, the effect is to shorten the vertical diameter, and thus to give the disk an oval form. this effect is particularly remarkable when the sun, at his rising or setting, is observed from the top of a mountain, or at an elevation near the seashore; for in such situations, the rays of light make a greater angle than ordinary with a perpendicular to the refracting medium, and the amount of refraction is proportionally greater. in some cases of this kind, the shortening of the vertical diameter of the sun has been observed to amount to six minutes, or about one fifth of the whole. the apparent enlargement of the sun and moon, when near the horizon, arises from an optical illusion. these bodies, in fact, are not seen under so great an angle when in the horizon as when on the meridian, for they are nearer to us in the latter case than in the former. the distance of the sun, indeed, is so great, that it makes very little difference in his apparent diameter whether he is viewed in the horizon or on the meridian; but with the moon, the case is otherwise; its angular diameter, when measured with instruments, is perceptibly larger when at its culmination, or highest elevation above the horizon. why, then, do the sun and moon appear so much larger when near the horizon? it is owing to a habit of the mind, by which we judge of the magnitudes of distant objects, not merely by the angle they subtend at the eye, but also by our impressions respecting their distance, allowing, under a given angle, a greater magnitude as we imagine the distance of a body to be greater. now, on account of the numerous objects usually in sight between us and the sun, when he is near the horizon, he appears much further removed from us than when on the meridian; and we unconsciously assign to him a proportionally greater magnitude. if we view the sun, in the two positions, through a smoked glass, no such difference of size is observed; for here no objects are seen but the sun himself. _twilight_ is another phenomenon depending on the agency of the earth's atmosphere. it is that illumination of the sky which takes place just before sunrise and which continues after sunset. it is owing partly to refraction, and partly to reflection, but mostly to the latter. while the sun is within eighteen degrees of the horizon, before it rises or after it sets, some portion of its light is conveyed to us, by means of numerous reflections from the atmosphere. at the equator, where the circles of daily motion are perpendicular to the horizon, the sun descends through eighteen degrees in an hour and twelve minutes. the light of day, therefore, declines rapidly, and as rapidly advances after daybreak in the morning. at the pole, a constant twilight is enjoyed while the sun is within eighteen degrees of the horizon, occupying nearly two thirds of the half year when the direct light of the sun is withdrawn, so that the progress from continual day to constant night is exceedingly gradual. to an inhabitant of an oblique sphere, the twilight is longer in proportion as the place is nearer the elevated pole. were it not for the power the atmosphere has of dispersing the solar light, and scattering it in various directions, no objects would be visible to us out of direct sunshine; every shadow of a passing cloud would involve us in midnight darkness; the stars would be visible all day; and every apartment into which the sun had not direct admission would be involved in the obscurity of night. this scattering action of the atmosphere on the solar light is greatly increased by the irregularity of temperature caused by the sun, which throws the atmosphere into a constant state of undulation; and by thus bringing together masses of air of different temperatures, produces partial reflections and refractions at their common boundaries, by which means much light is turned aside from a direct course, and diverted to the purposes of general illumination.[ ] in the upper regions of the atmosphere, as on the tops of very high mountains, where the air is too much rarefied to reflect much light, the sky assumes a black appearance, and stars become visible in the day time. although the atmosphere is usually so transparent, that it is invisible to us, yet we as truly move and live in a fluid as fishes that swim in the sea. considered in comparison with the whole earth, the atmosphere is to be regarded as a thin layer investing the surface, like a film of water covering the surface of an orange. its actual height, however, is over a hundred miles, though we cannot assign its precise boundaries. being perfectly elastic, the lower portions, bearing as they do, the weight of all the mass above them, are greatly compressed, while the upper portions having little to oppose the natural tendency of air to expand, diffuse themselves widely. the consequence is, that the atmosphere undergoes a rapid diminution of density, as we ascend from the earth, and soon becomes exceedingly rare. at so moderate a height as seven miles, it is four times rarer than at the surface, and continues to grow rare in the same proportion, namely, being four times less for every seven miles of ascent. it is only, therefore, within a few miles of the earth, that the atmosphere is sufficiently dense to sustain clouds and vapors, which seldom rise so high as eight miles, and are usually much nearer to the earth than this. so rare does the air become on the top of mount chimborazo, in south america, that it is incompetent to support most of the birds that fly near the level of the sea. the condor, a bird which has remarkably long wings, and a light body, is the only bird seen towering above this lofty summit. the transparency of the atmosphere,--a quality so essential to fine views of the starry heavens,--is much increased by containing a large proportion of water, provided it is perfectly dissolved, or in a state of invisible vapor. a country at once hot and humid, like some portions of the torrid zone, presents a much brighter and more beautiful view of the moon and stars, than is seen in cold climates. before a copious rain, especially in hot weather, when the atmosphere is unusually humid, we sometimes observe the sky to be remarkably resplendent, even in our own latitude. accordingly, this unusual clearness of the sky, when the stars shine with unwonted brilliancy, is regarded as a sign of approaching rain; and when, after the rain is apparently over, the air is remarkably transparent, and distant objects on the earth are seen with uncommon distinctness, while the sky exhibits an unusually deep azure, we may conclude that the serenity is only temporary, and that the rain will probably soon return. footnote: [ ] sir j. herschel. letter x. the sun. "great source of day! best image here below of thy creator, ever pouring wide, from world to world, the vital ocean round, on nature write, with every beam, his praise."--_thomson._ the subjects which have occupied the preceding letters are by no means the most interesting parts of our science. they constitute, indeed, little more than an introduction to our main subject, but comprise such matters as are very necessary to be clearly understood, before one is prepared to enter with profit and delight upon the more sublime and interesting field which now opens before us. we will begin our survey of the heavenly bodies with the sun, which first claims our homage, as the natural monarch of the skies. the moon will next occupy our attention; then the other bodies which compose the solar system, namely, the planets and comets; and, finally, we shall leave behind this little province in the great empire of nature, and wing a bolder flight to the fixed stars. the _distance_ of the sun from the earth is about ninety-five millions of miles. it may perhaps seem incredible to you, that astronomers should be able to determine this fact with any degree of certainty. some, indeed, not so well informed as yourself, have looked upon the marvellous things that are told respecting the distances, magnitudes, and velocities, of the heavenly bodies, as attempts of astronomers to impose on the credulity of the world at large; but the certainty and exactness with which the predictions of astronomers are fulfilled, as an eclipse, for example, ought to inspire full confidence in their statements. i can assure you, my dear friend, that the evidence on which these statements are founded is perfectly satisfactory to those whose attainments in the sciences qualify them to understand them; and, so far are astronomers from wishing to impose on the unlearned, by announcing such wonderful discoveries as they have made among the heavenly bodies, no class of men have ever shown a stricter regard and zeal than they for the exact truth, wherever it is attainable. ninety-five millions of miles is indeed a vast distance. no human mind is adequate to comprehend it fully; but the nearest approaches we can make towards it are gained by successive efforts of the mind to conceive of great distances, beginning with such as are clearly within our grasp. let us, then, first take so small a distance as that of the breadth of the atlantic ocean, and follow, in mind, a ship, as she leaves the port of new york, and, after twenty days' steady sail, reaches liverpool. having formed the best idea we are able of this distance, we may then reflect, that it would take a ship, moving constantly at the rate of ten miles per hour, more than a thousand years to reach the sun. the diameter of the sun is towards a million of miles; or, more exactly, it is eight hundred and eighty-five thousand miles. one hundred and twelve bodies as large as the earth, lying side by side, would be required to reach across the solar disk; and our ship, sailing at the same rate as before, would be ten years in passing over the same space. immense as is the sun, we can readily understand why it appears no larger than it does, when we reflect, that its distance is still more vast. even large objects on the earth, when seen on a distant eminence, or over a wide expanse of water, dwindle almost to a point. could we approach nearer and nearer to the sun, it would constantly expand its volume, until finally it would fill the whole vault of heaven. we could, however, approach but little nearer to the sun without being consumed by the intensity of his heat. whenever we come nearer to any fire, the heat rapidly increases, being four times as great at half the distance, and one hundred times as great at one tenth the distance. this fact is expressed by saying, that the heat increases as the square of the distance decreases. our globe is situated at such a distance from the sun, as exactly suits the animal and vegetable kingdoms. were it either much nearer or much more remote, they could not exist, constituted as they are. the intensity of the solar light also follows the same law. consequently, were we nearer to the sun than we are, its blaze would be insufferable; or, were we much further off, the light would be too dim to serve all the purposes of vision. the sun is one million four hundred thousand times as large as the earth; but its matter is not more than about one fourth as dense as that of the earth, being only a little heavier than water, while the average density of the earth is more than five times that of water. still, on account of the immense magnitude of the sun, its entire quantity of matter is three hundred and fifty thousand times as great as that of the earth. now, the force of gravity in a body is greater, in proportion as its quantity of matter is greater. consequently, we might suppose, that the weight of a body (weight being nothing else than the measure of the force of gravity) would be increased in the same proportion; or, that a body, which weighs only one pound at the surface of the earth, would weigh three hundred and fifty thousand pounds at the sun. but we must consider, that the attraction exerted by any body is the same as though all the matter were concentrated in the centre. thus, the attraction exerted by the earth and by the sun is the same as though the entire matter of each body were in its centre. hence, on account of the vast dimensions of the sun, its surface is one hundred and twelve times further from its centre than the surface of the earth is from its centre; and, since the force of gravity diminishes as the square of the distance increases, that of the sun, exerted on bodies at its surface, is (so far as this cause operates) the square of one hundred and twelve, or twelve thousand five hundred and forty-four times less than that of the earth. if, therefore, we increase the weight of a body three hundred and fifty-four thousand times, in consequence of the greater amount of matter in the sun, and diminish it twelve thousand five hundred and forty-four times, in consequence of the force acting at a greater distance from the body, we shall find that the body would weigh about twenty-eight times more on the sun than on the earth. hence, a man weighing three hundred pounds would, if conveyed to the surface of the sun, weigh eight thousand four hundred pounds, or nearly three tons and three quarters. a limb of our bodies, weighing forty pounds, would require to lift it a force of one thousand one hundred and twenty pounds, which would be beyond the ordinary power of the muscles. at the surface of the earth, a body falls from rest by the force of gravity, in one second, sixteen and one twelfth feet; but at the surface of the sun, a body would, in the same time, fall through four hundred and forty-eight and seven tenths feet. the sun turns on his own axis once in a little more than twenty-five days. this fact is known by observing certain dark places seen by the telescope on the sun's disk, called _solar spots_. these are very variable in size and number. sometimes, the sun's disk, when viewed with a telescope, is quite free from spots, while at other times we may see a dozen or more distinct clusters, each containing a great number of spots, some large and some very minute. occasionally, a single spot is so large as to be visible to the naked eye, especially when the sun is near the horizon, and the glare of his light is taken off. one measured by dr. herschel was no less than fifty thousand miles in diameter. a solar spot usually consists of two parts, the _nucleus_ and the _umbra_. the nucleus is black, of a very irregular shape, and is subject to great and sudden changes, both in form and size. spots have sometimes seemed to burst asunder, and to project fragments in different directions. the umbra is a wide margin, of lighter shade, and is often of greater extent than the nucleus. the spots are usually confined to a zone extending across the central regions of the sun, not exceeding sixty degrees in breadth. fig. exhibits the appearance of the solar spots. as these spots have all a common motion from day to day, across the sun's disk; as they go off at one limb, and, after a certain interval, sometimes come on again on the opposite limb, it is inferred that this apparent motion is imparted to them by an actual revolution of the sun on his own axis. we know that the spots must be in contact, or very nearly so, at least, with the body of the sun, and cannot be bodies revolving around it, which are projected on the solar disk when they are between us and the sun; for, in that case, they would not be so long in view as out of view, as will be evident from inspecting the following diagram. let s, fig. , page , represent the sun, and _b_ a body revolving round it in the orbit _a b c_; and let e represent the earth, where, of course, the spectator is situated. the body would be seen projected on the sun only while passing from _b_ to _c_, while, throughout the remainder of its orbit, it would be out of view, whereas no such inequality exists in respect to the two periods. [illustration fig. .] [illustration fig. .] if you ask, what is the _cause_ of the solar spots, i can only tell you what different astronomers have supposed respecting them. they attracted the notice of galileo soon after the invention of the telescope, and he formed an hypothesis respecting their nature. supposing the sun to consist of a solid body embosomed in a sea of liquid fire, he believed that the spots are composed of black cinders, formed in the interior of the sun by volcanic action, which rise and float on the surface of the fiery sea. the chief objections to this hypothesis are, first, the _vast extent_ of some of the spots, covering, as they do, two thousand millions of square miles, or more,--a space which it is incredible should be filled by lava in so short a time as that in which the spots are sometimes formed; and, secondly, the _sudden disappearance_ which the spots sometimes undergo, a fact which can hardly be accounted for by supposing, as galileo did, that such a vast accumulation of matter all at once sunk beneath the fiery flood. moreover, we have many reasons for believing that the spots are _depressions_ below the general surface. la lande, an eminent french astronomer of the last century, held that the sun is a solid, opaque body, having its exterior diversified with high mountains and deep valleys, and covered all over with a burning sea of liquid matter. the spots he supposed to be produced by the flux and reflux of this fiery sea, retreating occasionally from the mountains, and exposing to view a portion of the dark body of the sun. but it is inconsistent with the nature of fluids, that a liquid, like the sea supposed, should depart so far from its equilibrium and remain so long fixed, as to lay bare the immense spaces occupied by some of the solar spots. dr. herschel's views respecting the nature and constitution of the sun, embracing an explanation of the solar spots, have, of late years, been very generally received by the astronomical world. this great astronomer, after attentively viewing the surface of the sun, for a long time, with his large telescopes, came to the following conclusions respecting the nature of this luminary. he supposes the sun to be a planetary body like our earth, diversified with mountains and valleys, to which, on account of the magnitude of the sun, he assigns a prodigious extent, some of the mountains being six hundred miles high, and the valleys proportionally deep. he employs in his explanation no volcanic fires, but supposes two separate regions of dense clouds floating in the solar atmosphere, at different distances from the sun. the exterior stratum of clouds he considers as the depository of the sun's light and heat, while the inferior stratum serves as an awning or screen to the body of the sun itself, which thus becomes fitted to be the residence of animals. the proofs offered in support of this hypothesis are chiefly the following: first, that the appearances, as presented to the telescope, are such as accord better with the idea that the fluctuations arise from the motions of clouds, than that they are owing to the agitations of a liquid, which could not depart far enough from its general level to enable us to see its waves at so great a distance, where a line forty miles in length would subtend an angle at the eye of only the tenth part of a second; secondly, that, since clouds are easily dispersed to any extent, the great dimensions which the solar spots occasionally exhibit are more consistent with this than with any other hypothesis; and, finally, that a lower stratum of clouds, similar to those of our atmosphere, was frequently seen by the doctor, far below the luminous clouds which are the fountains of light and heat. such are the views of one who had, it must be acknowledged, great powers of observation, and means of observation in higher perfection than have ever been enjoyed by any other individual; but, with all deference to such authority, i am compelled to think that the hypothesis is encumbered with very serious objections. clouds analogous to those of our atmosphere (and the doctor expressly asserts that his lower stratum of clouds are analogous to ours, and reasons respecting the upper stratum according to the same analogy) cannot exist in hot air; they are tenants only of cold regions. how can they be supposed to exist in the immediate vicinity of a fire so intense, that they are even dissipated by it at the distance of ninety-five millions of miles? much less can they be supposed to be the depositories of such devouring fire, when any thing in the form of clouds, floating in our atmosphere, is at once scattered and dissolved by the accession of only a few degrees of heat. nothing, moreover, can be imagined more unfavorable for radiating heat to such a distance, than the light, inconstant matter of which clouds are composed, floating loosely in the solar atmosphere. there is a logical difficulty in the case: it is ascribing to things properties which they are not known to possess; nay, more, which they are known not to possess. from variations of light and shade in objects seen at moderate distances on the earth, we are often deceived in regard to their appearances; and we must distrust the power of an astronomer to decide upon the nature of matter seen at the distance of ninety-five millions of miles. i think, therefore, we must confess our ignorance of the nature and constitution of the sun; nor can we, as astronomers, obtain much more satisfactory knowledge respecting it than the common apprehension, namely, that it is an immense globe of fire. we have not yet learned what causes are in operation to maintain its undecaying fires; but our confidence in the divine wisdom and goodness leads us to believe, that those causes are such as will preserve those fires from extinction, and at a nearly uniform degree of intensity. any material change in this respect would jeopardize the safety of the animal and vegetable kingdoms, which could not exist without the enlivening influence of the solar heat, nor, indeed, were that heat any more or less intense than it is at present. if we inquire whether the surface of the sun is in a state of actual combustion, like burning fuel, or merely in a state of intense ignition, like a stone heated to redness in a furnace, we shall find it most reasonable to conclude that it is in a state of ignition. if the body of the sun were composed of combustible matter and were actually on fire, the material of the sun would be continually wasting away, while the products of combustion would fill all the vast surrounding regions, and obscure the solar light. but solid bodies may attain a very intense state of ignition, and glow with the most fervent heat, while none of their material is consumed, and no clouds or fumes rise to obscure their brightness, or to impede their further emission of heat. an ignited surface, moreover, is far better adapted than flame to the radiation of heat. flame, when made to act in contact with the surfaces of solid bodies, heats them rapidly and powerfully; but it sends forth, or _radiates_, very little heat, compared with solid matter in a high state of ignition. these various considerations render it highly probable to my mind, that the body of the sun is not in a state of actual combustion, but merely in a state of high ignition. the solar beam consists of a mixture of several different sorts of rays. first, there are the _calorific_ rays, which afford heat, and are entirely distinct from those which afford light, and may be separated from them. secondly, there are the _colorific_ rays, which give light, consisting of rays of seven distinct colors, namely, violet, indigo, blue, green, yellow, orange, red. these, when separated, as they may be by a glass prism, compose the _prismatic spectrum_. they appear also in the rainbow. when united again, in due proportions, they constitute white light, as seen in the light of the sun. thirdly, there are found in the solar beam a class of rays which afford neither heat nor light, but which produce chemical changes in certain bodies exposed to their influence, and hence are called _chemical_ rays. fourthly, there is still another class, called _magnetizing_ rays, because they are capable of imparting magnetic properties to steel. these different sorts of rays are sent forth from the sun, to the remotest regions of the planetary worlds, invigorating all things by their life-giving influence, and dispelling the darkness that naturally fills all space. but it was not alone to give heat and light, that the sun was placed in the firmament. by his power of attraction, also, he serves as the great regulator of the planetary motions, bending them continually from the straight line in which they tend to move, and compelling them to circulate around him, each at nearly a uniform distance, and all in perfect harmony. i will hereafter explain to you the manner in which the gravity of the sun thus acts, to control the planetary motions. for the present, let us content ourselves with reflecting upon the wonderful force which the sun must put forth, in order to bend out of their courses, into circular orbits, such a number of planets, some of which are more than a thousand times as large as the earth. were a ship of war under full sail, and it should be required to turn her aside from her course by a rope attached to her bow, we can easily imagine that it would take a great force to do it, especially were it required that the force should remain stationary and the ship be so constantly diverted from her course, as to be made to go round the force as round a centre. somewhat similar to this is the action which the sun exerts on each of the planets by some invisible influence, called gravitation. the bodies which he thus turns out of their course, and bends into a circular orbit around himself, are, however, many millions of times as ponderous as the ship, and are moving many thousand times as swiftly. letter xi. annual revolution.--seasons "these, as they change, almighty father, these are but the varied god. the rolling year is full of thee."--_thomson._ we have seen that the apparent revolution of the heavenly bodies, from east to west, every twenty-four hours, is owing to a real revolution of the earth on its own axis, in the opposite direction. this motion is very easily understood, resembling, as it does, the spinning of a top. we must, however, conceive of the top as turning without any visible support, and not as resting in the usual manner on a plane. the annual motion of the earth around the sun, which gives rise to an apparent motion of the sun around the earth once a year, and occasions the change of seasons, is somewhat more difficult to understand; and it may cost you some reflection, before you will settle all the points respecting the changes of the seasons clearly in your mind. we sometimes see these two motions exemplified in a top. when, as the string is pulled, the top is thrown forwards on the floor, we may see it move forward (sometimes in a circle) at the same time that it spins on its axis. let a candle be placed on a table, to represent the sun, and let these two motions be imagined to be given to a top around it, and we shall have a case somewhat resembling the actual motions of the earth around the sun. when bodies are at such a distance from each other as the earth and the sun, a spectator on either would project the other body upon the concave sphere of the heavens, always seeing it on the opposite side of a great circle one hundred and eighty degrees from himself. recollect that the path in which the earth moves round the sun is called the ecliptic. we are not to conceive of this, or of any other celestial circle, as having any real, palpable existence, any more than the path of a bird through the sky. you will perhaps think it quite superfluous for me to remind you of this; but, from the habit of seeing the orbits of the heavenly bodies represented in diagrams and orreries, by palpable lines and circles, we are apt inadvertently to acquire the notion, that the orbits of the planets, and other representations of the artificial sphere, have a real, palpable existence in nature; whereas, they denote the places where mere geometrical or imaginary lines run. you might have expected to see an orrery, exhibiting a view of the sun and planets, with their various motions, particularly described in my letter on astronomical instruments and apparatus. i must acknowledge, that i entertain a very low opinion of the utility of even the best orreries, and i cannot recommend them as auxiliaries in the study of astronomy. the numerous appendages usually connected with them, some to support them in a proper position, and some to communicate to them the requisite motions, enter into the ideas which the learner forms respecting the machinery of the heavens; and it costs much labor afterwards to divest the mind of such erroneous impressions. astronomy can be exhibited much more clearly and beautifully to the mental eye than to the visual organ. it is much easier to conceive of the sun existing in boundless space, and of the earth as moving around him at a great distance, the mind having nothing in view but simply these two bodies, than it is, in an orrery, to contemplate the motion of a ball representing the earth, carried by a complicated apparatus of wheels around another ball, supported by a cylinder or wire, to represent the sun. i would advise you, whenever it is practicable, to think how things are in nature, rather than how they are represented by art. the machinery of the heavens is much simpler than that of an orrery. in endeavoring to obtain a clear idea of the revolution of the earth around the sun, imagine to yourself a plane (a geometrical plane, having merely length and breadth, but no thickness) passing through the centres of the sun and the earth, and extended far beyond the earth till it reaches the firmament of stars. although, indeed, no such dome actually exists as that under which we figure to ourselves the vault of the sky, yet, as the fixed stars appear to be set in such a dome, we may imagine that the circles of the sphere, when indefinitely enlarged, finally reach such an imaginary vault. all that is essential is, that we should imagine this to exist far beyond the bounds of the solar system, the various bodies that compose the latter being situated close around the sun, at the centre. along the line where this great circle meets the starry vault, are situated a series of constellations,--aries, taurus, gemini, &c.,--which occupy successively this portion of the heavens. when bodies are at such a distance from each other as the sun and the earth, i have said that a spectator on either would project the other body upon the concave sphere of the heavens, always seeing it on the opposite side of a great circle one hundred and eighty degrees from himself. the place of a body, when viewed from any point, is denoted by the position it occupies among the stars. thus, in the diagram, fig. , page , when the earth arrives at e, it is said to be in aries, because, if viewed from the sun, it would be projected on that part of the heavens; and, for the same reason, to a spectator at e, the sun would be in libra. when the earth shifts its position from aries to taurus, as we are unconscious of our own motion, the sun it is that appears to move from libra to scorpio, in the opposite part of the heavens. hence, as we go forward, in the order of the signs, on one side of the ecliptic, the sun seems to be moving forward at the same rate on the opposite side of the same great circle; and therefore, although we are unconscious of our own motion, we can read it, from day to day, in the motions of the sun. if we could see the stars at the same time with the sun, we could actually observe, from day to day, the sun's progress through them, as we observe the progress of the moon at night; only the sun's rate of motion would be nearly fourteen times slower than that of the moon. although we do not see the stars when the sun is present, we can observe that it makes daily progress eastward, as is apparent from the constellations of the zodiac occupying, successively, the western sky immediately after sunset, proving that either all the stars have a common motion westward, independent of their diurnal motion, or that the sun has a motion past them from west to east. we shall see, hereafter, abundant evidence to prove, that this change in the relative position of the sun and stars, is owing to a change in the apparent place of the sun, and not to any change in the stars. [illustration fig. .] to form a clear idea of the two motions of the earth, imagine yourself standing on a circular platform which turns slowly round its centre. while you are carried slowly round the entire of the circuit of the heavens, along with the platform, you may turn round upon your heel the same way three hundred and sixty-five times. the former is analogous to our annual motion with the earth around the sun; the latter, to our diurnal revolution in common with the earth around its own axis. although the apparent revolution of the sun is in a direction opposite to the real motion of the earth, as regards absolute space, yet both are nevertheless from west to east, since these terms do not refer to any directions in absolute space, but to the order in which certain constellations (the constellations of the zodiac) succeed one another. the earth itself, on opposite sides of its orbit, does in fact move towards directly opposite points of space; but it is all the while pursuing its course in the order of the signs. in the same manner, although the earth turns on its axis from west to east, yet any place on the surface of the earth is moving in a direction in space exactly opposite to its direction twelve hours before. if the sun left a visible trace on the face of the sky, the ecliptic would of course be distinctly marked on the celestial sphere, as it is on an artificial globe; and were the equator delineated in a similar manner, we should then see, at a glance, the relative position of these two circles,--the points where they intersect one another, constituting the equinoxes; the points where they are at the greatest distance asunder, that is, the solstices; and various other particulars, which, for want of such visible traces, we are now obliged to search for by indirect and circuitous methods. it will aid you, to have constantly before your mental vision an imaginary delineation of these two important circles on the face of the sky. the equator makes an angle with the ecliptic of twenty-three degrees and twenty-eight minutes. this is called the obliquity of the ecliptic. as the sun and earth are both always in the ecliptic, and as the motion of the earth in one part of it makes the sun appear to move in the opposite part, at the same rate, the sun apparently descends, in winter, twenty-three degrees and twenty-eight minutes to the south of the equator, and ascends, in summer, the same number of degrees north of it. we must keep in mind, that the celestial equator and celestial ecliptic are here understood, and we may imagine them to be two great circles delineated on the face of the sky. on comparing observations made at different periods, for more than two thousand years, it is found, that the obliquity of the ecliptic is not constant, but that it undergoes a slight diminution, from age to age, amounting to fifty-two seconds in a century, or about half a second annually. we might apprehend that, by successive approaches to each other, the equator and ecliptic would finally coincide; but astronomers have discovered, by a most profound investigation, based on the principles of universal gravitation, that this irregularity is confined within certain narrow limits; and that the obliquity, after diminishing for some thousands of years, will then increase for a similar period, and will thus vibrate forever about a mean value. as the earth traverses every part of her orbit in the course of a year, she will be once at each solstice, and once at each equinox. the best way of obtaining a correct idea of her two motions is, to conceive of her as standing still for a single day, at some point in her orbit, until she has turned once on her axis, then moving about a degree, and halting again, until another diurnal revolution is completed. let us suppose the earth at the autumnal equinox, the sun, of course, being at the vernal equinox,--for we must always think of these two bodies as diametrically opposite to each other. suppose the earth to stand still in its orbit for twenty-four hours. the revolution of the earth on its axis, from west to east, will make the sun appear to describe a great circle of the heavens from east to west, coinciding with the equator. at the end of this period, suppose the sun to move northward one degree, and to remain there for twenty-four hours; in which time, the revolution of the earth, will make the sun appear to describe another circle, from east to west, parallel to the equator, but one degree north of it. thus, we may conceive of the sun as moving one degree north, every day, for about three months, when it will reach the point of the ecliptic furthest from the equator, which point is called the _tropic_, from a greek word, signifying _to turn_; because, after the sun has passed this point, his motion in his orbit carries him continually towards the equator, and therefore he seems to turn about. the same point is also called the _solstice_, from a latin word, signifying to _stand still_; since, when the sun has reached its greatest northern or southern limit, while its declination is at the point where it ceases to increase, but begins to decrease, there the sun seems for a short time stationary, with regard to the equator, appearing for several days to describe the same parallel of latitude. when the sun is at the northern tropic, which happens about the twenty-first of june, his elevation above the southern horizon at noon is the greatest in the year; and when he is at the southern tropic, about the twenty-first of december, his elevation at noon is the least in the year. the difference between these two meridian altitudes will give the whole distance from one tropic to the other, and consequently, twice the distance from each tropic to the equator. by this means, we find how far the tropic is from the equator, and that gives us the angle which the equator and ecliptic make with each other; for the greatest distance between any two great circles on the sphere is always equal to the angle which they make with each other. thus, the ancient astronomers were able to determine the obliquity of the ecliptic with a great degree of accuracy. it was easy to find the situation of the zenith, because the direction of a plumb-line shows us where that is; and it was easy to find the distances from the zenith where the sun was at the greatest and least distances; respectively. the difference of these two arcs is the angular distance from one tropic to the other; and half this arc is the distance of either tropic from the equator, and of course, equal to the obliquity of the ecliptic. all this will be very easily understood from the annexed diagram, fig. . let z be the zenith of a spectator situated at c; z _n_ the least, and z _s_ the greatest distance of the sun from the zenith. from z _s_ subtract z _n_, and then _s n_, the difference, divided by two, will give the obliquity of the ecliptic. [illustration fig. .] the motion of the earth in its orbit is nearly seventy times as great as its greatest motion around its axis. in its revolution around the sun, the earth moves no less than one million six hundred and forty thousand miles per day, sixty-eight thousand miles per hour, eleven hundred miles per minute, and nearly nineteen miles every second; a velocity nearly sixty times as great as the greatest velocity of a cannon ball. places on the earth turn with very different degrees of velocity in different latitudes. those near the equator are carried round on the circumference of a large circle; those towards the poles, on the circumference of a small circle; while one standing on the pole itself would not turn at all. those who live on the equator are carried about one thousand miles an hour. in our latitude, (forty-one degrees and eighteen minutes,) the diurnal velocity is about seven hundred and fifty miles per hour. it would seem, at first view, quite incredible, that we should be whirled round at so rapid a rate, and yet be entirely insensible of any motion; and much more, that we could be going so swiftly through space, in our circuit around the sun, while all things, when unaffected by local causes, appear to be in such a state of quiescence. yet we have the most unquestionable evidence of the fact; nor is it difficult to account for it, in consistency with the general state of repose among bodies on the earth, when we reflect that their relative motions, with respect to each other, are not in the least disturbed by any motions which they may have in common. when we are on board a steam-boat, we move about in the same manner when the boat is in rapid motion, as when it is lying still; and such would be the case, if it moved steadily a hundred times faster than it does. were the earth, however, suddenly to stop its diurnal revolution, all movable bodies on its surface would be thrown off in tangents to the surface with velocities proportional to that of their diurnal motion; and were the earth suddenly to halt in its orbit, we should be hurled forward into space with inconceivable rapidity. i will next endeavor to explain to you the phenomena of the _seasons_. these depend on two causes; first, the inclination of the earth's axis to the plane of its orbit; and, secondly, to the circumstance, that the axis always remains parallel to itself. imagine to yourself a candle placed in the centre of a ring, to represent the sun in the centre of the earth's orbit, and an apple with a knittingneedle running through it in the direction of the stem. run a knife around the central part of the apple, to mark the situation of the equator. the circumference of the ring represents the earth's orbit in the plane of the ecliptic. place the apple so that the equator shall coincide with the wire; then the axis will lie directly across the plane of the ecliptic; that is, at right angles to it. let the apple be carried quite round the ring, constantly preserving the axis parallel to itself, and the equator all the while coinciding with the wire that represents the orbit. now, since the sun enlightens half the globe at once, so the candle, which here represents the sun, will shine on the half of the apple that is turned towards it; and the circle which divides the enlightened from the unenlightened side of the apple, called the _terminator_, will pass through both the poles. if the apple be turned slowly round on its axis, the terminator will successively pass over all places on the earth, giving the appearance of sunrise to places at which it arrives, and of sunset to places from which it departs. if, therefore, the equator had coincided with the ecliptic, as would have been the case, had the earth's axis been perpendicular to the plane of its orbit, the diurnal motion of the sun would always have been in the equator, and the days and nights would have been equal all over the globe. to the inhabitants of the equatorial parts of the earth, the sun would always have appeared to move in the prime vertical, rising directly in the east, passing through the zenith at noon, and setting in the west. in the polar regions, the sun would always have appeared to revolve in the horizon; while, at any place between the equator and the pole, the course of the sun would have been oblique to the horizon, but always oblique in the same degree. there would have been nothing of those agreeable vicissitudes of the seasons which we now enjoy; but some regions of the earth would have been crowned with perpetual spring, others would have been scorched with the unremitting fervor of a vertical sun, while extensive regions towards either pole would have been consigned to everlasting frost and sterility. to understand, then, clearly, the causes of the change of seasons, use the same apparatus as before; but, instead of placing the axis of the earth at right angles to the plane of its orbit, turn it out of a perpendicular position a little, (twenty-three degrees and twenty-eight minutes,) then the equator will be turned just the same number of degrees out of a coincidence with the ecliptic. let the apple be carried around the ring, always holding the axis inclined at the same angle to the plane of the ring, and always parallel to itself. you will find that there will be two points in the circuit where the plane of the equator, that you had marked around the centre of the apple, will pass through the centre of the sun; these will be the points where the celestial equator and the ecliptic cut one another, or the equinoxes. when the earth is at either of these points, the sun shines on both poles alike; and, if we conceive of the earth, while in this situation, as turning once round on its axis, the apparent diurnal motion of the sun will be the same as it would be, were the earth's axis perpendicular to the plane of the equator. for that day, the sun would revolve in the equator, and the days and nights would be equal all over the globe. if the apple were carried round in the manner supposed, then, at the distance of ninety degrees from the equinoxes, the same pole would be turned from the sun on one side, just as much as it was turned towards him on the other. in the former case, the sun's light would fall short of the pole twenty-three and one half degrees, and in the other case, it would reach beyond it the same number of degrees. i would recommend to you to obtain as clear an idea as you can of the cause of the change of seasons, by thinking over the foregoing illustration. you may then clear up any remaining difficulties, by studying the diagram, fig. , on page . [illustration fig. .] let a b c d represent the earth's place in different parts of its orbit, having the sun in the centre. let a, c, be the positions of the earth at the equinoxes, and b, d, its positions at the tropics,--the axis _n s_ being always parallel to itself. it is difficult to represent things of this kind correctly, all on the same plane; but you will readily see, that the figure of the earth, here, answers to the apple in the former illustration; that the hemisphere towards _n_ is above, and that towards _s_ is below, the plane of the paper. when the earth is at a and c, the vernal and autumnal equinoxes, the sun, you will perceive, shines on both the poles _n_ and _s_; and, if you conceive of the globe, while in this position, as turned round on its axis, as it is in the diurnal revolution, you will readily understand, that the sun would describe the celestial equator. this may not at first appear so obvious, by inspecting the figure; but if you consider the point _n_ as raised above the plane of the paper, and the point _s_ as depressed below it, you will readily see how the plane of the equator would pass through the centre of the sun. again, at b, when the earth is at the southern tropic, the sun shines twenty-three and a half degrees beyond the north pole, _n_, and falls the same distance short of the south pole, _s_. the case is exactly reversed when the earth is at the northern tropic, and the sun at the southern. while the earth is at one of the tropics, at b, for example, let us conceive of it as turning on its axis, and we shall readily see, that all that part of the earth which lies within the north polar circle will enjoy continual day, while that within the south polar circle will have continual night; and that all other places will have their days longer as they are nearer to the enlightened pole, and shorter as they are nearer to the unenlightened pole. this figure likewise shows the successive positions of the earth, at different periods of the year, with respect to the signs, and what months correspond to particular signs. thus, the earth enters libra, and the sun aries, on the twenty-first of march, and on the twenty-first of june, the earth is just entering capricorn, and the sun, cancer. you will call to mind what is meant by this phraseology,--that by saying the earth enters libra, we mean that a spectator placed on the sun would see the earth in that part of the celestial ecliptic, which is occupied by the sign libra; and that a spectator on the earth sees the sun at the same time projected on the opposite part of the heavens, occupied by the sign cancer. had the axis of the earth been perpendicular to the plane of the ecliptic, then the sun would always have appeared to move in the equator, the days would every where have been equal to the nights, and there could have been no change of seasons. on the other hand, had the inclination of the ecliptic to the equator been much greater than it is, the vicissitudes of the seasons would have been proportionally greater, than at present. suppose, for instance, the equator had been at right angles to the ecliptic, in which case, the poles of the earth would have been situated in the ecliptic itself; then, in different parts of the earth, the appearances would have been as follows: to a spectator on the _equator_, (where all the circles of diurnal revolution are perpendicular to the horizon,) the sun, as he left the vernal equinox, would every day perform his diurnal revolution in a smaller and smaller circle, until he reached the north pole, when he would halt for a moment, and then wheel about and return to the equator, in a reverse order. the progress of the sun through the southern signs, to the south pole, would be similar to that already described. such would be the appearances to an inhabitant of the equatorial regions. to a spectator living in an _oblique_ sphere, in our own latitude, for example, the sun, while north of the equator, would advance continually northward, making his diurnal circuit in parallels further and further distant from the equator, until he reached the circle of perpetual apparition; after which, he would climb, by a spiral course, to the north star, and then as rapidly return to the equator. by a similar progress southward, the sun would at length pass the circle of perpetual occultation, and for some time (which would be longer or shorter, according to the latitude of the place of observation) there would be continual night. to a spectator on the _pole_ of the earth and under the pole of the heaven, during the long day of six months, the sun would wind its way to a point directly over head, pouring down upon the earth beneath not merely the heat of the torrid zone, but the heat of a torrid noon, accumulating without intermission. the great vicissitudes of heat and cold, which would attend these several movements of the sun, would be wholly incompatible with the existence of either the animal or the vegetable kingdom, and all terrestrial nature would be doomed to perpetual sterility and desolation. the happy provision which the creator has made against such extreme vicissitudes, by confining the changes of the seasons within such narrow bounds, conspires with many other express arrangements in the economy of nature, to secure the safety and comfort of the human race. perhaps you have never reflected upon all the reasons, why the several changes of position, with respect to the horizon, which the sun undergoes in the course of the year, occasion such a difference in the amount of heat received from him. two causes contribute to increase the heat of summer and the cold of winter. the higher the sun ascends above the horizon, the more directly his rays fall upon the earth; and their heating power is rapidly augmented, as they approach a perpendicular direction. when the sun is nearly over head, his rays strike us with far greater force than when they meet us obliquely; and the earth absorbs a far greater number of those rays of heat which strike it perpendicularly, than of those which meet it in a slanting direction. when the sun is near the horizon, his rays merely glance along the ground, and many of them, before they reach it, are absorbed and dispersed in passing through the atmosphere. those who have felt only the oblique solar rays, as they fall upon objects in the high latitudes, have a very inadequate idea of the power of a vertical, noonday sun, as felt in the region of the equator. the increased length of the day in summer is another cause of the heat of this season of the year. this cause more sensibly affects places far removed from the equator, because at such places the days are longer and the nights shorter than in the torrid zone. by the operation of this cause, the solar heat accumulates there so much, during the longest days of summer, that the temperature rises to a higher degree than is often known in the torrid climates. but the temperature of a place is influenced very much by several other causes, as well as by the force and duration of the sun's heat. first, the _elevation_ of a country above the level of the sea has a great influence upon its climate. elevated districts of country, even in the torrid zone, often enjoy the most agreeable climate in the world. the cold of the upper regions of the atmosphere modifies and tempers the solar heat, so as to give a most delightful softness, while the uniformity of temperature excludes those sudden and excessive changes which are often experienced in less favored climes. in ascending certain high mountains situated within the torrid zone, the traveller passes, in a short time, through every variety of climate, from the most oppressive and sultry heat, to the soft and balmy air of spring, which again is succeeded by the cooler breezes of autumn, and then by the severest frosts of winter. a corresponding difference is seen in the products of the vegetable kingdom. while winter reigns on the summit of the mountain, its central regions may be encircled with the verdure of spring, and its base with the flowers and fruits of summer. secondly, the proximity of the _ocean_ also has a great effect to equalize the temperature of a place. as the ocean changes its temperature during the year much less than the land, it becomes a source of warmth to contiguous countries in winter, and a fountain of cool breezes in summer. thirdly, the relative _humidity_ or _dryness_ of the atmosphere of a place is of great importance, in regard to its effects on the animal system. a dry air of ninety degrees is not so insupportable as a humid air of eighty degrees; and it may be asserted as a general principle, that a hot and humid atmosphere is unhealthy, although a hot air, when dry, may be very salubrious. in a warm atmosphere which is dry, the evaporation of moisture from the surface of the body is rapid, and its cooling influence affords a most striking relief to an intense heat without; but when the surrounding atmosphere is already filled with moisture, no such evaporation takes place from the surface of the skin, and no such refreshing effects are experienced from this cause. moisture collects on the skin; a sultry, oppressive sensation is felt; and chills and fevers are usually in the train. letter xii. laws of motion. "what though in solemn silence, all move round this dark, terrestrial ball! in reason's ear they all rejoice, and utter forth a glorious voice; for ever singing, as they shine, 'the hand that made us is divine.'"--_addison._ however incredible it may seem, no fact is more certain, than that the earth is constantly on the wing, flying around the sun with a velocity so prodigious, that, for every breath we draw, we advance on our way forty or fifty miles. if, when passing across the waters in a steam-boat, we can wake, after a night's repose, and find ourselves conducted on our voyage a hundred miles, we exult in the triumphs of art, which could have moved so ponderous a body as a steam-ship over such a space in so short a time, and so quietly, too, as not to disturb our slumbers; but, with a motion vastly more quiet and uniform, we have, in the same interval, been carried along with the earth in its orbit more than half a million of miles. in the case of the steam-ship, however perfect the machinery may be, we still, in our waking hours at least, are made sensible of the action of the forces by which the motion is maintained,--as the roaring of the fire, the beating of the piston, and the dashing of the paddle-wheels; but in the more perfect machinery which carries the earth forward on her grander voyage, no sound is heard, nor the least intimation afforded of the stupendous forces by which this motion is achieved. to the pious observer of nature it might seem sufficient, without any inquiry into second causes, to ascribe the motions of the spheres to the direct agency of the supreme being. if, however, we can succeed in finding the secret springs and cords, by which the motions of the heavenly bodies are immediately produced and controlled, it will detract nothing from our just admiration of the great first cause of all things. we may therefore now enter upon the inquiry into the nature or laws of the forces by which the earth is made to revolve on her axis and in her orbit; and having learned what it is, that causes and maintains the motions of the earth, you will then acquire, at the same time, a knowledge of all the celestial machinery. the subject will involve an explanation of the laws of motion, and of the principles of universal gravitation. it was once supposed, that we could never reason respecting the laws that govern the heavenly bodies from what we observe in bodies around us, but that motion is one thing on the earth and quite another thing in the skies; and hence, that it is impossible for us, by any inquiries into the laws of terrestrial nature, to ascertain how things take place among the heavenly bodies. galileo and newton, however, proceeded on the contrary supposition, that nature is uniform in all her works; that the same almighty arm rules over all; and that he works by the same fixed laws through all parts of his boundless realm. the certainty with which all the predictions of astronomers, made on these suppositions, are fulfilled, attests the soundness of the hypothesis. accordingly, those laws, which all experience, endlessly multiplied and varied, proves to be the laws of terrestrial motion, are held to be the laws that govern also the motions of the most distant planets and stars, and to prevail throughout the universe of matter. let us, then, briefly review these great laws of motion, which are three in number. the first law is as follows: _every body perseveres in a state of rest, or of uniform motion in a straight line, unless compelled by some force to change its state_. by _force_ is meant any thing which produces motion. the foregoing law has been fully established by experiment, and is conformable to all experience. it embraces several particulars. first, a body, when at rest, remains so, unless some force puts it in motion; and hence it is inferred, when a body is found in motion, that some force must have been applied to it sufficient to have caused its motion. thus, the fact, that the earth is in motion around the sun and around its own axis, is to be accounted for by assigning to each of these motions a force adequate, both in quantity and direction, to produce these motions, respectively. secondly, when a body is once in motion, it will continue to move for ever, unless something stops it. when a ball is struck on the surface of the earth, the friction of the earth and the resistance of the air soon stop its motion; when struck on smooth ice, it will go much further before it comes to a state of rest, because the ice opposes much less resistance than the ground; and, were there no impediment to its motion, it would, when once set in motion, continue to move without end. the heavenly bodies are actually in this condition: they continue to move, not because any new forces are applied to them; but, having been once set in motion, they continue in motion because there is nothing to stop them. this property in bodies to persevere in the state they are actually in,--if at rest, to remain at rest, or, if in motion, to continue in motion,--is called _inertia_. the inertia of a body (which is measured by the force required to overcome it) is proportioned to the quantity of matter it contains. a steam-boat manifests its inertia, on first starting it, by the enormous expenditure of force required to bring it to a given rate of motion; and it again manifests its inertia, when in rapid motion, by the great difficulty of stopping it. the heavenly bodies, having been once put in motion, and meeting with nothing to stop them, move on by their own inertia. a top affords a beautiful illustration of inertia, continuing, as it does, to spin after the moving force is withdrawn. thirdly, the motion to which a body naturally tends is _uniform_; that is, the body moves just as far the second minute as it did the first, and as far the third as the second; and passes over equal spaces in equal times. i do not assert that the motion of all moving bodies is _in fact_ uniform, but that such is their _tendency_. if it is otherwise than uniform, there is some cause operating to disturb the uniformity to which it is naturally prone. fourthly, a body in motion will move in a _straight line_, unless diverted out of that line by some external force; and the body will resume its straight-forward motion, whenever the force that turns it aside is withdrawn. every body that is revolving in an orbit, like the moon around the earth, or the earth around the sun, _tends_ to move in a straight line which is a tangent[ ] to its orbit. thus, if a b c, fig. , represents the orbit of the moon around the earth, were it not for the constant action of some force that draws her towards the earth, she would move off in a straight line. if the force that carries her towards the earth were suspended at a, she would immediately desert the circular motion, and proceed in the direction a d. in the same manner, a boy whirls a stone around his head in a sling, and then letting go one of the strings, and releasing the force that binds it to the circle, it flies off in a straight line which is a tangent to that part of the circle where it was released. this tendency which a body revolving in an orbit exhibits, to recede from the centre and to fly off in a tangent, is called the _centrifugal force_. we see it manifested when a pail of water is whirled. the water rises on the sides of the vessel, leaving a hollow in the central parts. we see an example of the effects of centrifugal action, when a horse turns swiftly round a corner, and the rider is thrown outwards; also, when a wheel passes rapidly through a small collection of water, and portions of the water are thrown off from the top of the wheel in straight lines which are tangents to the wheel. [illustration fig. .] the centrifugal force is increased as the velocity is increased. thus, the parts of a millstone most remote from the centre sometimes acquire a centrifugal force so much greater than the central parts, which move much slower, that the stone is divided, and the exterior portions are projected with great violence. in like manner, as the equatorial parts of the earth, in the diurnal revolution, revolve much faster than the parts towards the poles, so the centrifugal force is felt most at the equator, and becomes strikingly manifest by the diminished weight of bodies, since it acts in opposition to the force of gravity. although the foregoing law of motion, when first presented to the mind, appears to convey no new truth, but only to enunciate in a formal manner what we knew before; yet a just understanding of this law, in all its bearings, leads us to a clear comprehension of no small share of all the phenomena of motion. the second and third laws may be explained in fewer terms. the second law of motion is as follows: _motion is proportioned to the force impressed, and in the direction of that force_. the meaning of this law is, that every force that is applied to a body produces its full effect, proportioned to its intensity, either in causing or in preventing motion. let there be ever so many blows applied at once to a ball, each will produce its own effect in its own direction, and the ball will move off, not indeed in the zigzag, complex lines corresponding to the directions of the several forces, but in a single line expressing the united effect of all. if you place a ball at the corner of a table, and give it an impulse, at the same instant, with the thumb and finger of each hand, one impelling it in the direction of one side of the table, and the other in the direction of the other side, the ball will move diagonally across the table. if the blows be exactly proportioned each to the length of the side of the table on which it is directed, the ball will run exactly from corner to corner, and in the same time that it would have passed over each side by the blow given in the direction of that side. this principle is expressed by saying, that a body impelled by two forces, acting respectively in the directions of the two sides of a parallelogram, and proportioned in intensity to the lengths of the sides, will describe the diagonal of the parallelogram in the same time in which it would have described the sides by the forces acting separately. the converse of this proposition is also true, namely, that any single motion may be considered as the _resultant_ of two others,--the motion itself being represented by the diagonal, while the two _components_ are represented by the sides, of a parallelogram. this reduction of a motion to the individual motions that produce it, is called the _resolution of motion_, or the _resolution of forces_. nor can a given motion be resolved into _two_ components, merely. these, again, may be resolved into others, varying indefinitely, in direction and intensity, from all which the given motion may be considered as having resulted. this composition and resolution of motion or forces is often of great use, in inquiries into the motions of the heavenly bodies. the composition often enables us to substitute a single force for a great number of others, whose individual operations would be too complicated to be followed. by this means, the investigation is greatly simplified. on the other hand, it is frequently very convenient to resolve a given motion into two or more others, some of which may be thrown out of the account, as not influencing the particular point which we are inquiring about, while others are far more easily understood and managed than the single force would have been. it is characteristic of great minds, to simplify these inquiries. they gain an insight into complicated and difficult subjects, not so much by any extraordinary faculty of seeing in the dark, as by the power of removing from the object all incidental causes of obscurity, until it shines in its own clear and simple light. if every force, when applied to a body, produces its full and legitimate effect, how many other forces soever may act upon it, impelling it different ways, then it must follow, that the smallest force ought to move the largest body; and such is in fact the case. a snap of a finger upon a seventy-four under full sail, if applied in the direction of its motion, would actually increase its speed, although the effect might be too small to be visible. still it is something, and may be truly expressed by a fraction. thus, suppose a globe, weighing a million of pounds, were suspended from the ceiling by a string, and we should apply to it the snap of a finger,--it is granted that the motion would be quite insensible. let us then divide the body into a million equal parts, each weighing one pound; then the same impulse, applied to each one separately, would produce a sensible effect, moving it, say one inch. it will be found, on trial, that the same impulse given to a mass of two pounds will move it half an inch; and hence it is inferred, that, if applied to a mass weighing a million of pounds, it would move it the millionth part of an inch. it is one of the curious results of the second law of motion, that an unlimited number of motions may exist together in the same body. thus, at the same moment, we may be walking around a post in the cabin of a steam-boat, accompanying the boat in its passage around an island, revolving with the earth on its axis, flying through space in our annual circuit around the sun, and possibly wheeling, along with the sun and his whole retinue of planets, around some centre in common with the starry worlds. the third law of motion is this: _action and reaction are equal, and in contrary directions_. whenever i give a blow, the body struck exerts an equal force on the striking body. if i strike the water with an oar, the water communicates an equal impulse to the oar, which, being communicated to the boat, drives it forward in the opposite direction. if a magnet attracts a piece of iron, the iron attracts the magnet just as much, in the opposite direction; and, in short, every portion of matter in the universe attracts and is attracted by every other, equally, in an opposite direction. this brings us to the doctrine of universal gravitation, which is the very key that unlocks all the secrets of the skies. this will form the subject of my next letter. footnote: [ ] a tangent is a straight line touching a circle, as a d, in fig. letter xiii. terrestrial gravity. "to him no high, no low, no great, no small, he fills, he bounds, connects, and equals all."--_pope._ we discover in nature a tendency of every portion of matter towards every other. this tendency is called _gravitation_. in obedience to this power, a stone falls to the ground, and a planet revolves around the sun. we may contemplate this subject as it relates either to phenomena that take place near the surface of the earth, or in the celestial regions. the former, _gravity_, is exemplified by falling bodies; the latter, _universal gravitation_, by the motions of the heavenly bodies. the laws of terrestrial gravity were first investigated by galileo; those of universal gravitation, by sir isaac newton. terrestrial gravity is only an individual example of universal gravitation; being the tendency of bodies towards the centre of the earth. we are so much accustomed, from our earliest years, to see bodies fall to the earth, that we imagine bodies must of necessity fall "downwards;" but when we reflect that the earth is round, and that bodies fall towards the centre on all sides of it, and that of course bodies on opposite sides of the earth fall in precisely opposite directions, and towards each other, we perceive that there must be some force acting to produce this effect; nor is it enough to say, as the ancients did, that bodies "naturally" fall to the earth. every motion implies some force which produces it; and the fact that bodies fall towards the earth, on all sides of it, leads us to infer that that force, whatever it is, resides in the earth itself. we therefore call it _attraction_. we do not, however, say what attraction _is_, but what it _does_. we must bear in mind, also, that, according to the third law of motion, this attraction is mutual; that when a stone falls towards the earth, it exerts the same force on the earth that the earth exerts on the stone; but the motion of the earth towards the stone is as much less than that of the stone towards the earth, as its quantity of matter is greater; and therefore its motion is quite insensible. but although we are compelled to acknowledge the _existence_ of such a force as gravity, causing a tendency in all bodies towards each other, yet we know nothing of its _nature_, nor can we conceive by what medium bodies at such a distance as the moon and the earth exercise this influence on each other. still, we may trace the modes in which this force acts; that is, its _laws_; for the laws of nature are nothing else than the modes in which the powers of nature act. we owe chiefly to the great galileo the first investigation of the laws of terrestrial gravity, as exemplified in falling bodies; and i will avail myself of this opportunity to make you better acquainted with one of the most interesting of men and greatest of philosophers. galileo was born at pisa, in italy, in the year . he was the son of a florentine nobleman, and was destined by his father for the medical profession, and to this his earlier studies were devoted. but a fondness and a genius for mechanical inventions had developed itself, at a very early age, in the construction of his toys, and a love of drawing; and as he grew older, a passion for mathematics, and for experimental research, predominated over his zeal for the study of medicine, and he fortunately abandoned that for the more congenial pursuits of natural philosophy and astronomy. in the twenty-fifth year of his age, he was appointed, by the grand duke of tuscany, professor of mathematics in the university of pisa. at that period, there prevailed in all the schools a most extraordinary reverence for the writings of aristotle, the preceptor of alexander the great,--a philosopher who flourished in greece, about three hundred years before the christian era. aristotle, by his great genius and learning, gained a wonderful ascendency over the minds of men, and became the oracle of the whole reading world for twenty centuries. it was held, on the one hand, that all truths worth knowing were contained in the writings of aristotle; and, on the other, that an assertion which contradicted any thing in aristotle could not be true. but galileo had a greatness of mind which soared above the prejudices of the age in which he lived, and dared to interrogate nature by the two great and only successful methods of discovering her secrets,--experiment and observation. galileo was indeed the first philosopher that ever fully employed experiments as the means of learning the laws of nature, by imitating on a small what she performs on a great scale, and thus detecting her modes of operation. archimedes, the great sicilian philosopher, had in ancient times introduced mathematical or geometrical reasoning into natural philosophy; but it was reserved for galileo to unite the advantages of both mathematical and experimental reasonings in the study of nature,--both sure and the only sure guides to truth, in this department of knowledge, at least. experiment and observation furnish materials upon which geometry builds her reasonings, and from which she derives many truths that either lie for ever hidden from the eye of observation, or which it would require ages to unfold. this method, of interrogating nature by experiment and observation, was matured into a system by lord bacon, a celebrated english philosopher, early in the seventeenth century,--indeed, during the life of galileo. previous to that time, the inquirers into nature did not open their eyes to see how the facts really _are_; but, by metaphysical processes, in imitation of aristotle, determined how they _ought to be_, and hastily concluded that they were so. thus, they did not study into the laws of motion, by observing how motion actually takes place, under various circumstances, but first, in their closets, constructed a definition of motion, and thence inferred all its properties. the system of reasoning respecting the phenomena of nature, introduced by lord bacon, was this: in the first place, to examine all the facts of the case, and then from these to determine the laws of nature. to derive general conclusions from the comparison of a great number of individual instances constitutes the peculiarity of the baconian philosophy. it is called the _inductive_ system, because its conclusions were built on the induction, or comparison, of a great many single facts. previous to the time of lord bacon, hardly any insight had been gained into the causes of natural phenomena, and hardly one of the laws of nature had been clearly established, because all the inquirers into nature were upon a wrong road, groping their way through the labyrinth of error. bacon pointed out to them the true path, and held before them the torch-light of experiment and observation, under whose guidance all successful students of nature have since walked, and by whose illumination they have gained so wonderful an insight into the mysteries of the natural world. it is a remarkable fact, that two such characters as bacon and galileo should appear on the stage at the same time, who, without any communication with each other, or perhaps without any personal knowledge of each other's existence, should have each developed the true method of investigating the laws of nature. galileo practised what bacon only taught; and some, therefore, with much reason, consider galileo as a greater philosopher than bacon. "bacon," says hume, "pointed out, at a great distance, the road to philosophy; galileo both pointed it out to others, and made, himself, considerable advances in it. the englishman was ignorant of geometry; the florentine revived that science, excelled in it, and was the first who applied it, together with experiment, to natural philosophy. the former rejected, with the most positive disdain, the system of copernicus; the latter fortified it with new proofs, derived both from reason and the senses." when we reflect that geometry is a science built upon self-evident truths, and that all its conclusions are the result of pure demonstration, and can admit of no controversy; when we further reflect, that experimental evidence rests on the testimony of the senses, and we infer a thing to be true because we actually see it to be so; it shows us the extreme bigotry, the darkness visible, that beclouded the human intellect, when it not only refused to admit conclusions first established by pure geometrical reasoning, and afterwards confirmed by experiments exhibited in the light of day, but instituted the most cruel persecutions against the great philosopher who first proclaimed these truths. galileo was hated and persecuted by two distinct bodies of men, both possessing great influence in their respective spheres,--the one consisting of the learned doctors of philosophy, who did nothing more, from age to age, than reiterate the doctrines of aristotle, and were consequently alarmed at the promulgation of principles subversive of those doctrines; the other consisting of the romish priesthood, comprising the terrible inquisition, who denounced the truths taught by galileo, as inconsistent with certain declarations of the holy scriptures. we shall see, as we advance, what a fearful warfare he had to wage against these combined powers of darkness. aristotle had asserted, that, if two different weights of the same material were let fall from the same height, the heavier one would reach the ground sooner than the other, in proportion as it was more weighty. for example: if a ten-pound leaden weight and a one-pound were let fall from a given height at the same instant, the former would reach the ground ten times as soon as the latter. no one thought of making the trial, but it was deemed sufficient that aristotle had said so; and accordingly this assertion had long been received as an axiom in the science of motion. galileo ventured to appeal from the authority of aristotle to that of his own senses, and maintained, that both weights would fall in the same time. the learned doctors ridiculed the idea. galileo tried the experiment in their presence, by letting fall, at the same instant, large and small weights from the top of the celebrated leaning tower of pisa. yet, with the sound of the two weights clicking upon the pavement at the same moment, they still maintained that the ten-pound weight would reach the ground in one tenth part of the time of the other, because they could quote the chapter and verse of aristotle where the fact was asserted. wearied and disgusted with the malice and folly of these aristotelian philosophers, galileo, at the age of twenty-eight, resigned his situation in the university of pisa, and removed to padua, in the university of which place he was elected professor of mathematics. up to this period, galileo had devoted himself chiefly to the studies of the laws of motion, and the other branches of mechanical philosophy. soon afterwards, he began to publish his writings, in rapid succession, and became at once among the most conspicuous of his age,--a rank which he afterwards well sustained and greatly exalted, by the invention of the telescope, and by his numerous astronomical discoveries. i will reserve an account of these great achievements until we come to that part of astronomy to which they were more immediately related, and proceed, now, to explain to you the leading principles of _terrestrial gravity_, as exemplified in falling bodies. first, _all bodies near the earth's surface fall in straight lines towards the centre of the earth_. we are not to infer from this fact, that there resides at the centre any peculiar force, as a great loadstone, for example, which attracts bodies towards itself; but bodies fall towards the centre of the sphere, because the combined attractions of all the particles of matter in the earth, each exerting its proper force upon the body, would carry it towards the centre. this may be easily illustrated by a diagram. let b, fig. , page , be the centre of the earth, and a a body without it. every portion of matter in the earth exerts some force on a, to draw it down to the earth. but since there is just as much matter on one side of the line a b, as on the other side, each half exerts an equal force to draw the body towards itself; therefore it falls in the direction of the diagonal between the two forces. thus, if we compare the effects of any two particles of matter at equal distances from the line a b, but on opposite sides of it, as _a_, _b_, while the force of the particle at _a_ would tend to draw a in the direction of a _a_, that of _b_ would draw it in the direction of a _b_, and it would fall in the line a b, half way between the two. the same would hold true of any other two corresponding particles of matter on different sides of the earth, in respect to a body situated in any place without it. [illustration fig. .] secondly, _all bodies fall towards the earth, from the same height, with equal velocities_. a musket-ball, and the finest particle of down, if let fall from a certain height towards the earth, tend to descend towards it at the same rate, and would proceed with equal speed, were it not for the resistance of the air, which retards the down more than it does the ball, and finally stops it. if, however, the air be removed out of the way, as it may be by means of the air-pump, the two bodies keep side by side in falling from the greatest height at which we can try the experiment. thirdly, _bodies, in falling towards the earth, have their rate of motion continually accelerated_. suppose we let fall a musket-ball from the top of a high tower, and watch its progress, disregarding the resistance of the air: the first second, it will pass over sixteen feet and one inch, but its speed will be constantly increased, being all the while urged onward by the same force, and retaining all that it has already acquired; so that the longer it is in falling, the swifter its motion becomes. consequently, when bodies fall from a great height, they acquire an immense velocity before they reach the earth. thus, a man falling from a balloon, or from the mast-head of a ship, is broken in pieces; and those meteoric stones, which sometimes fall from the sky, bury themselves deep in the earth. on measuring the spaces through which a body falls, it is found, that it will fall four times as far in two seconds as in one, and one hundred times as far in ten seconds as in one; and universally, the space described by a falling body is proportioned to the time multiplied into itself; that is, to the square of the time. fourthly, _gravity is proportioned to the quantity of matter_. a body which has twice as much matter as another exerts a force of attraction twice as great, and also receives twice as much from the same body as it would do, if it were only just as heavy as that body. thus the earth, containing, as it does, forty times as much matter as the moon, exerts upon the moon forty times as much force as it would do, were its mass the same with that of the moon; but it is also capable of _receiving_ forty times as much gravity from the moon as it would do, were its mass the same as the moon's; so that the power of attracting and that of being attracted are reciprocal; and it is therefore correct to say, that the moon attracts the earth _just as much_ as the earth attracts the moon; and the same may be said of any two bodies, however different in quantity of matter. fifthly, _gravity, when acting at a distance from the earth, is not as intense as it is near the earth_. at such a distance as we are accustomed to ascend above the general level of the earth, no great difference is observed. on the tops of high mountains, we find bodies falling towards the earth, with nearly the same speed as they do from the smallest elevations. it is found, nevertheless, that there is a real difference; so that, in fact, the weight of a body (which is nothing more than the measure of its force of gravity) is not quite so great on the tops of high mountains as at the general level of the sea. thus, a thousand pounds' weight, on the top of a mountain half a mile high, would weigh a quarter of a pound less than at the level of the sea; and if elevated four thousand miles above the earth,--that is, _twice_ as far from the centre of the earth as the surface is from the centre,--it would weigh only one fourth as much as before; if _three times_ as far, it would weigh only one ninth as much. so that the force of gravity decreases, as we recede from the earth, in the same proportion as the square of the distance increases. this fact is generalized by saying, that _the force of gravity, at different distances from the earth, is inversely as the square of the distance_. were a body to fall from a great distance,--suppose a thousand times that of the radius of the earth,--the force of gravity being one million times less than that at the surface of the earth, the motion of the body would be exceedingly slow, carrying it over only the sixth part of an inch in a day. it would be a long time, therefore, in making any sensible approaches towards the earth; but at length, as it drew near to the earth it would acquire a very great velocity, and would finally rush towards it with prodigious violence. falling so far, and being continually accelerated on the way, we might suppose that it would at length attain a velocity infinitely great; but it can be demonstrated, that, if a body were to fall from an infinite distance, attracted to the earth only by gravity, it could never acquire a velocity greater than about seven miles per second. this, however, is a speed inconceivably great, being about eighteen times the greatest velocity that can be given to a cannon-ball, and more than twenty-five thousand miles per hour. but the phenomena of falling bodies must have long been observed, and their laws had been fully investigated by galileo and others, before the cause of their falling was understood, or any such principle as gravity, inherent in the earth and in all bodies, was applied to them. the developement of this great principle was the work of sir isaac newton; and i will give you, in my next letter, some particulars respecting the life and discoveries of this wonderful man. letter xiv. sir isaac newton.--universal gravitation.--figure of the earth's orbit.--precession of the equinoxes. "the heavens are all his own; from the wild rule of whirling vortices, and circling spheres, to their first great simplicity restored. the schools astonished stood; but found it vain to combat long with demonstration clear, and, unawakened, dream beneath the blaze of truth. at once their pleasing visions fled, with the light shadows of the morning mixed, when newton rose, our philosophic sun."--_thomson's elegy._ sir isaac newton was born in lincolnshire, england, in , just one year after the death of galileo. his father died before he was born, and he was a helpless infant, of a diminutive size, and so feeble a frame, that his attendants hardly expected his life for a single hour. the family dwelling was of humble architecture, situated in a retired but beautiful valley, and was surrounded by a small farm, which afforded but a scanty living to the widowed mother and her precious charge. the cut on page , fig , represents the modest mansion, and the emblems of rustic life that first met the eyes of this pride of the british nation, and ornament of human nature. it will probably be found, that genius has oftener emanated from the cottage than from the palace. [illustration fig. .] the boyhood of newton was distinguished chiefly for his ingenious mechanical contrivances. among other pieces of mechanism, he constructed a windmill so curious and complete in its workmanship, as to excite universal admiration. after carrying it a while by the force of the wind, he resolved to substitute animal power, and for this purpose he inclosed in it a mouse, which he called the miller, and which kept the mill a-going by acting on a tread-wheel. the power of the mouse was brought into action by unavailing attempts to reach a portion of corn placed above the wheel. a water-clock, a four-wheeled carriage propelled by the rider himself, and kites of superior workmanship, were among the productions of the mechanical genius of this gifted boy. at a little later period, he began to turn his attention to the motions of the heavenly bodies, and constructed several sun-dials on the walls of the house where he lived. all this was before he had reached his fifteenth year. at this age, he was sent by his mother, in company with an old family servant, to a neighboring market-town, to dispose of products of their farm, and to buy articles of merchandise for their family use; but the young philosopher left all these negotiations to his worthy partner, occupying himself, mean-while, with a collection of old books, which he had found in a garret. at other times, he stopped on the road, and took shelter with his book under a hedge, until the servant returned. they endeavored to educate him as a farmer; but the perusal of a book, the construction of a water-mill, or some other mechanical or scientific amusement, absorbed all his thoughts, when the sheep were going astray, and the cattle were devouring or treading down the corn. one of his uncles having found him one day under a hedge, with a book in his hand, and entirely absorbed in meditation, took it from him, and found that it was a mathematical problem which so engrossed his attention. his friends, therefore, wisely resolved to favor the bent of his genius, and removed him from the farm to the school, to prepare for the university. in the eighteenth year of his age, newton was admitted into trinity college, cambridge. he made rapid and extraordinary advances in the mathematics, and soon afforded unequivocal presages of that greatness which afterwards placed him at the head of the human intellect. in , at the age of twenty-seven, he became professor of mathematics at cambridge, a post which he occupied for many years afterwards. during the four or five years previous to this he had, in fact, made most of those great discoveries which have immortalized his name. we are at present chiefly interested in one of these, namely, that of _universal gravitation_; and let us see by what steps he was conducted to this greatest of scientific discoveries. in the year , when newton was about twenty-four years of age, the plague was prevailing at cambridge, and he retired into the country. one day, while he sat in a garden, musing on the phenomena of nature around him, an apple chanced to fall to the ground. reflecting on the mysterious power that makes all bodies near the earth fall towards its centre, and considering that this power remains unimpaired at considerable heights above the earth, as on the tops of trees and mountains, he asked himself,--"may not the same force extend its influence to a great distance from the earth, even as far as the moon? indeed, may not this be the very reason, why the moon is drawn away continually from the straight line in which every body tends to move, and is thus made to circulate around the earth?" you will recollect that it was mentioned, in my letter which contained an account of the first law of motion, that if a body is put in motion by any force, it will always move forward in a straight line, unless some other force compels it to turn aside from such a direction; and that, when we see a body moving in a curve, as a circular orbit, we are authorized to conclude that there is some force existing within the circle, which continually draws the body away from the direction in which it tends to move. accordingly, it was a very natural suggestion, to one so well acquainted with the laws of motion as newton, that the moon should constantly bend towards the earth, from a tendency to fall towards it, as any other heavy body would do, if carried to such a distance from the earth. newton had already proved, that if such a power as gravity extends from the earth to distant bodies, it must decrease, as the square of the distance from the centre of the earth increases; that is, at double the distance, it would be four times less; at ten times the distance, one hundred times less; and so on. now, it was known that the moon is about sixty times as far from the centre of the earth as the surface of the earth is from the centre, and consequently, the force of attraction at the moon must be the square of sixty, or thirty-six hundred times less than it is at the earth; so that a body at the distance of the moon would fall towards the earth very slowly, only one thirty-six hundredth part as far in a given time, as at the earth. does the moon actually fall towards the earth at this rate; or, what is the same thing, does she depart at this rate continually from the straight line in which she tends to move, and in which she would move, if no external force diverted her from it? on making the calculation, such was found to be the fact. hence gravity, and no other force than gravity, acts upon the moon, and compels her to revolve around the earth. by reasonings equally conclusive, it was afterwards proved, that a similar force compels all the planets to circulate around the sun; and now, we may ascend from the contemplation of this force, as we have seen it exemplified in falling bodies, to that of a universal power whose influence extends to all the material creation. it is in this sense that we recognise the principle of universal gravitation, the law of which may be thus enunciated; _all bodies in the universe, whether great or small, attract each other, with forces proportioned to their respective quantities of matter, and inversely as the squares of their distances from each other_. this law asserts, first, that attraction reigns throughout the material world, affecting alike the smallest particle of matter and the greatest body; secondly, that it acts upon every mass of matter, precisely in proportion to its quantity; and, thirdly, that its intensity is diminished as the square of the distance is increased. observation has fully confirmed the prevalence of this law throughout the solar system; and recent discoveries among the fixed stars, to be more fully detailed hereafter, indicate that the same law prevails there. the law of universal gravitation is therefore held to be the grand principle which governs all the celestial motions. not only is it consistent with all the observed motions of the heavenly bodies, even the most irregular of those motions, but, when followed out into all its consequences, it would be competent to assert that such irregularities must take place, even if they had never been observed. newton first published the doctrine of universal gravitation in the 'principia,' in . the name implies that the work contains the fundamental principles of natural philosophy and astronomy. being founded upon the immutable basis of mathematics, its conclusions must of course be true and unalterable, and thenceforth we may regard the great laws of the universe as traced to their remotest principle. the greatest astronomers and mathematicians have since occupied themselves in following out the plan which newton began, by applying the principles of universal gravitation to all the subordinate as well as to the grand movements of the spheres. this great labor has been especially achieved by la place, a french mathematician of the highest eminence, in his profound work, the 'mecanique celeste.' of this work, our distinguished countryman, dr. bowditch, has given a magnificent translation, and accompanied it with a commentary, which both illustrates the original, and adds a great amount of matter hardly less profound than that. [illustration fig. .] we have thus far taken the earth's orbit around the sun as a great circle, such being its projection on the sphere constituting the celestial ecliptic. the real path of the earth around the sun is learned, as i before explained to you, by the apparent path of the sun around the earth once a year. now, when a body revolves about the earth at a great distance from us, as is the case with the sun and moon, we cannot certainly infer that it moves in a circle because it appears to describe a circle on the face of the sky, for such might be the appearance of its orbit, were it ever so irregular a curve. thus, if e, fig. , represents the earth, and acb, the irregular path of a body revolving about it, since we should refer the body continually to some place on the celestial sphere, xyz, determined by lines drawn from the eye to the concave sphere through the body, the body, while moving from a to b through c, would appear to move from x to z, through y. hence, we must determine from other circumstances than the actual appearance, what is the true figure of the orbit. [illustration fig. .] were the earth's path a circle, having the sun in the centre, the sun would always appear to be at the same distance from us; that is, the radius of the orbit, or _radius vector_, (the name given to a line drawn from the centre of the sun to the orbit of any planet,) would always be of the same length. but the earth's distance from the sun is constantly varying, which shows that its orbit is not a circle. we learn the true figure of the orbit, by ascertaining the _relative distances_ of the earth from the sun, at various periods of the year. these distances all being laid down in a diagram, according to their respective lengths, the extremities, on being connected, give us our first idea of the shape of the orbit, which appears of an oval form, and at least resembles an ellipse; and, on further trial, we find that it has the properties of an ellipse. thus, let e, fig. , be the place of the earth, and _a_, _b_, _c_, &c., successive positions of the sun; the _relative_ lengths of the lines e _a_, e _b_, &c., being known, on connecting the points _a_, _b_, _c_, &c., the resulting figure indicates the true figure of the earth's orbit. these relative distances are found in two different ways; first, _by changes in the sun's apparent diameter_, and, secondly, _by variations in his angular velocity_. the same object appears to us smaller in proportion as it is more distant; and if we see a heavenly body varying in size, at different times, we infer that it is at different distances from us; that when largest, it is nearest to us, and when smallest, furthest off. now, when the sun's diameter is accurately measured by instruments, it is found to vary from day to day; being, when greatest, more than thirty-two minutes and a half, and when smallest, only thirty-one minutes and a half,--differing, in all, about seventy-five seconds. when the diameter is greatest, which happens in january, we know that the sun is nearest to us; and when the diameter is least, which occurs in july, we infer that the sun is at the greatest distance from us. the point where the earth, or any planet, in its revolution, is nearest the sun, is called its _perihelion_; the point where it is furthest from the sun, its _aphelion_. suppose, then, that, about the first of january, when the diameter of the sun is greatest, we draw a line, e _a_, fig. , to represent it, and afterwards, every ten days, draw other lines, e _b_, e _c_, &c.; increasing in the same ratio as the apparent diameters of the sun decrease. these lines must be drawn at such a distance from each other, that the triangles, e _a b_, e _b c_, &c., shall be all equal to each other, for a reason that will be explained hereafter. on connecting the extremities of these lines, we shall obtain the figure of the earth's orbit. similar conclusions may be drawn from observations on the sun's _angular velocity_. a body appears to move most rapidly when nearest to us. indeed, the apparent velocity increases rapidly, as it approaches us, and as rapidly diminishes, when it recedes from us. if it comes twice as near as before, it appears to move not merely twice as swiftly, but four times as swiftly; if it comes ten times nearer, its apparent velocity is one hundred times as great as before. we say, therefore, that the velocity varies inversely as the square of the distance; for, as the distance is diminished ten times, the velocity is increased the square of ten; that is, one hundred times. now, by noting the time it takes the sun, from day to day, to cross the central wire of the transit-instrument, we learn the comparative velocities with which it moves at different times; and from these we derive the comparative distances of the sun at the corresponding times; and laying down these relative distances in a diagram, as before, we get our first notions of the actual figure of the earth's orbit, or the path which it describes in its annual revolution around the sun. having now learned the fact, that the earth moves around the sun, not in a circular but in an elliptical orbit, you will desire to know by what forces it is impelled, to make it describe this figure, with such uniformity and constancy, from age to age. it is commonly said, that gravity causes the earth and the planets to circulate around the sun; and it is true that it is gravity which turns them aside from the straight line in which, by the first law of motion, they tend to move, and thus causes them to revolve around the sun. but what force is that which gave to them this original impulse, and impressed upon them such a tendency to move forward in a straight line? the name _projectile_ force is given to it, because it is the same _as though_ the earth were originally projected into space, when first created; and therefore its motion is the result of two forces, the projectile force, which would cause it to move forward in a straight line which is a tangent to its orbit, and gravitation, which bends it towards the sun. but before you can clearly understand the nature of this motion, and the action of the two forces that produce it, i must explain to you a few elementary principles upon which this and all the other planetary motions depend. you have already learned, that when a body is acted on by two forces, in different directions, it moves in the direction of neither, but in some direction between them. if i throw a stone horizontally, the attraction of the earth will continually draw it downward, out of the line of direction in which it was thrown, and make it descend to the earth in a curve. the particular form of the curve will depend on the velocity with which it is thrown. it will always _begin_ to move in the line of direction in which it is projected; but it will soon be turned from that line towards the earth. it will, however, continue nearer to the line of projection in proportion as the velocity of projection is greater. thus, let a c, fig. , be perpendicular to the horizon, and a b parallel to it, and let a stone be thrown from a, in the direction of a b. it will, in every case, commence its motion in the line a b, which will therefore be a tangent to the curve it describes; but, if it is thrown with a small velocity, it will soon depart from the tangent, describing the line a d; with a greater velocity, it will describe a curve nearer the tangent, as a e; and with a still greater velocity, it will describe the curve a f. [illustration fig. .] as an example of a body revolving in an orbit under the influence of two forces, suppose a body placed at any point, p, fig. , above the surface of the earth, and let p a be the direction of the earth's centre; that is, a line perpendicular to the horizon. if the body were allowed to move, without receiving any impulse, it would descend to the earth in the direction p a with an accelerated motion. but suppose that, at the moment of its departure from p, it receives a blow in the direction p b, which would carry it to b in the time the body would fall from p to a; then, under the influence of both forces, it would descend along the curve p d. if a stronger blow were given to it in the direction p b, it would describe a larger curve, p e; or, finally, if the impulse were sufficiently strong, it would circulate quite around the earth, and return again to p, describing the circle p f g. with a velocity of projection still greater, it would describe an ellipse, p i k; and if the velocity be increased to a certain degree, the figure becomes a parabola, l p m,--a curve which never returns into itself. [illustration fig. .] in fig. , page , suppose the planet to have passed the point c, at the aphelion, with so small a velocity, that the attraction of the sun bends its path very much, and causes it immediately to begin to approach towards the sun. the sun's attraction will increase its velocity, as it moves through d, e, and f, for the sun's attractive force on the planet, when at d, is acting in the direction d s; and, on account of the small angle made between d e and d s, the force acting in the line d s helps the planet forward in the path d e, and thus increases its velocity. in like manner, the velocity of the planet will be continually increasing as it passes through d, e, and f; and though the attractive force, on account of the planet's nearness, is so much increased, and tends, therefore, to make the orbit more curved, yet the velocity is also so much increased, that the orbit is not more curved than before; for the same increase of velocity, occasioned by the planet's approach to the sun, produces a greater increase of centrifugal force, which carries it off again. we may see, also, the reason why, when the planet has reached the most distant parts of its orbit, it does not entirely fly off, and never return to the sun; for, when the planet passes along h, k, a, the sun's attraction retards the planet, just as gravity retards a ball rolled up hill; and when it has reached c, its velocity is very small, and the attraction to the centre of force causes a great deflection from the tangent, sufficient to give its orbit a great curvature, and the planet wheels about, returns to the sun, and goes over the same orbit again. as the planet recedes from the sun, its centrifugal force diminishes faster than the force of gravity, so that the latter finally preponderates. [illustration fig. .] i shall conclude what i have to say at present, respecting the motion of the earth around the sun, by adding a few words respecting the precession of the equinoxes. the _precession of the equinoxes_ is a slow but continual shifting of the equinoctial points, from east to west. suppose that we mark the exact place in the heavens where, during the present year, the sun crosses the equator, and that this point is close to a certain star; next year, the sun will cross the equator a little way westward of that star, and so every year, a little further westward, until, in a long course of ages, the place of the equinox will occupy successively every part of the ecliptic, until we come round to the same star again. as, therefore, the sun revolving from west to east, in his apparent orbit, comes round to the point where it left the equinox, it meets the equinox before it reaches that point. the appearance is as though the equinox _goes forward_ to meet the sun, and hence the phenomenon is called the _precession_ of the equinoxes; and the fact is expressed by saying, that the equinoxes retrograde on the ecliptic, until the line of the equinoxes (a straight line drawn from one equinox to the other) makes a complete revolution, from east to west. this is of course a retrograde motion, since it is contrary to the order of the signs. the equator is conceived as _sliding_ westward on the ecliptic, always preserving the same inclination to it, as a ring, placed at a small angle with another of nearly the same size which remains fixed, may be slid quite around it, giving a corresponding motion to the two points of intersection. it must be observed, however, that this mode of conceiving of the precession of the equinoxes is purely imaginary, and is employed merely for the convenience of representation. the amount of precession annually is fifty seconds and one tenth; whence, since there are thirty-six hundred seconds in a degree, and three hundred and sixty degrees in the whole circumference of the ecliptic, and consequently one million two hundred and ninety-six thousand seconds, this sum, divided by fifty seconds and one tenth, gives twenty-five thousand eight hundred and sixty-eight years for the period of a complete revolution of the equinoxes. suppose we now fix to the centre of each of the two rings, before mentioned, a wire representing its axis, one corresponding to the axis of the ecliptic, the other to that of the equator, the extremity of each being the pole of its circle. as the ring denoting the equator turns round on the ecliptic, which, with its axis, remains fixed, it is easy to conceive that the axis of the equator revolves around that of the ecliptic, and the pole of the equator around the pole of the ecliptic, and constantly at a distance equal to the inclination of the two circles. to transfer our conceptions to the celestial sphere, we may easily see that the axis of the diurnal sphere (that of the earth produced) would not have its pole constantly in the same place among the stars, but that this pole would perform a slow revolution around the pole of the ecliptic, from east to west, completing the circuit in about twenty-six thousand years. hence the star which we now call the pole-star has not always enjoyed that distinction, nor will it always enjoy it, hereafter. when the earliest catalogues of the stars were made, this star was twelve degrees from the pole. it is now one degree twenty-four minutes, and will approach still nearer; or, to speak more accurately, the pole will come still nearer to this star, after which it will leave it, and successively pass by others. in about thirteen thousand years, the bright star lyra (which lies near the circle in which the pole of the equator revolves about the pole of the ecliptic, on the side opposite to the present pole-star) will be within five degrees of the pole, and will constitute the pole-star. as lyra now passes near our zenith, you might suppose that the change of position of the pole among the stars would be attended with a change of altitude of the north pole above the horizon. this mistaken idea is one of the many misapprehensions which result from the habit of considering the horizon as a fixed circle in space. however the pole might shift its position in space, we should still be at the same distance from it, and our horizon would always reach the same distance beyond it. the time occupied by the sun, in passing from the equinoctial point round to the same point again, is called the _tropical year_. as the sun does not perform a complete revolution in this interval, but falls short of it fifty seconds and one tenth, the tropical year is shorter than the sidereal by twenty minutes and twenty seconds, in mean solar time, this being the time of describing an arc of fifty seconds and one tenth, in the annual revolution. the changes produced by the precession of the equinoxes, in the apparent places of the circumpolar stars, have led to some interesting results in _chronology_. in consequence of the retrograde motion of the equinoctial points, the _signs_ of the ecliptic do not correspond, at present, to the _constellations_ which bear the same names, but lie about one sign, or thirty degrees, westward of them. thus, that division of the ecliptic which is called the sign taurus lies in the constellation aries, and the sign gemini, in the constellation taurus. undoubtedly, however, when the ecliptic was thus first divided, and the divisions named, the several constellations lay in the respective divisions which bear their names. letter xv. the moon. "soon as the evening shades prevail the moon takes up the wondrous tale, and nightly to the listening earth repeats the story of her birth."--_addison._ having now learned so much of astronomy as relates to the earth and the sun, and the mutual relations which exist between them, you are prepared to enter with advantage upon the survey of the other bodies that compose the solar system. this being done, we shall then have still before us the boundless range of the fixed stars. the moon, which next claims our notice, has been studied by astronomers with greater attention than any other of the heavenly bodies, since her comparative nearness to the earth brings her peculiarly within the range of our telescopes, and her periodical changes and very irregular motions, afford curious subjects, both for observation and speculation. the mild light of the moon also invites our gaze, while her varying aspects serve barbarous tribes, especially, for a kind of dial-plate inscribed on the face of the sky, for weeks, and months, and times, and seasons. the moon is distant from the earth about two hundred and forty thousand miles; or, more exactly, two hundred and thirty-eight thousand five hundred and forty-five miles. her angular or apparent diameter is about half a degree, and her real diameter, two thousand one hundred and sixty miles. she is a companion, or satellite, to the earth, revolving around it every month, and accompanying us in our annual revolution around the sun. although her nearness to us makes her appear as a large and conspicuous object in the heavens, yet, in comparison with most of the other celestial bodies, she is in fact very small, being only one forty-ninth part as large as the earth, and only about one seventy millionth part as large as the sun. the moon shines by light borrowed from the sun, being itself an opaque body, like the earth. when the disk, or any portion of it, is illuminated, we can plainly discern, even with the naked eye, varieties of light and shade, indicating inequalities of surface which we imagine to be land and water. i believe it is the common impression, that the darker portions are land and the lighter portions water; but if either part is water, it must be the darker regions. a smooth polished surface, like water, would reflect the sun's light like a mirror. it would, like a convex mirror, form a diminished image of the sun, but would not itself appear luminous like an uneven surface, which multiplies the light by numerous reflections within itself. thus, from this cause, high broken mountainous districts appear more luminous than extensive plains. [illustration figures , . telescopic views of the moon.] by the aid of the telescope, we may see undoubted indications of mountains and valleys. indeed, with a good glass, we can discover the most decisive evidence that the surface of the moon is exceedingly varied,--one part ascending in lofty peaks, another clustering in huge mountain groups, or long ranges, and another bearing all the marks of deep caverns or valleys. you will not, indeed, at the first sight of the moon through a telescope, recognise all these different objects. if you look at the moon when half her disk is enlightened, (which is the best time for seeing her varieties of surface,) you will, at the first glance, observe a motley appearance, particularly along the line called the _terminator_, which separates the enlightened from the unenlightened part of the disk. (fig. .) on one side of the terminator, within the dark part of the disk, you will see illuminated points, and short, crooked lines, like rude characters marked with chalk on a black ground. on the other side of the terminator you will see a succession of little circular groups, appearing like numerous bubbles of oil on the surface of water. the further you carry your eye from the terminator, on the same side of it, the more indistinctly formed these bubbles appear, until towards the edge of the moon they assume quite a different aspect. some persons, when they look into a telescope for the first time, having heard that mountains and valleys are to be seen, and discovering nothing but these unmeaning figures, break off in disappointment, and have their faith in these things rather diminished than increased. i would advise you, therefore, before you take even your first view of the moon through a telescope, to form as clear an idea as you can, how mountains, and valleys, and caverns, situated at such a distance from the eye, ought to look, and by what marks they may be recognised. seize, if possible, the most favorable period, (about the time of the first quarter,) and previously learn from drawings and explanations, how to interpret every thing you see. what, then, ought to be the respective appearances of mountains, valleys, and deep craters, or caverns, in the moon? the sun shines on the moon in the same way as it shines on the earth; and let, us reflect, then, upon the manner in which it strikes similar objects here. one half the globe is constantly enlightened; and, by the revolution of the earth on its axis, the terminator, or the line which separates the enlightened from the unenlightened part of the earth, travels along from east to west, over different places, as we see the moon's terminator travel over her disk from new to full moon; although, in the case of the earth, the motion is more rapid, and depends on a different cause. in the morning, the sun's light first strikes upon the tops of the mountains, and, if they are very high, they may be brightly illuminated while it is yet night in the valleys below. by degrees, as the sun rises, the circle of illumination travels down the mountain, until at length it reaches the bottom of the valleys; and these in turn enjoy the full light of day. again, a mountain casts a shadow opposite to the sun, which is very long when the sun first rises, and shortens continually as the sun ascends, its length at a given time, however, being proportioned to the height of the mountain; so that, if the shadow be still very long when the sun is far above the horizon, we infer that the mountain is very lofty. we may, moreover, form some judgment of the shape of a mountain, by observing that of its shadow. now, the moon is so distant that we could not easily distinguish places simply by their elevations, since they would be projected into the same imaginary plane which constitutes the apparent disk of the moon; but the foregoing considerations would enable us to infer their existence. thus, when you view the moon at any time within her first quarter, but better near the end of that period, you will observe, on the side of the terminator within the dark part of the disk, the tops of mountains which the light of the sun is just striking, as the morning sun strikes the tops of mountains on the earth. these you will recognise by those white specks and little crooked lines, before mentioned, as is represented in fig. . these bright points and lines you will see altering their figure, every hour, as they come more and more into the sun's light; and, mean-while, other bright points, very minute at first, will start into view, which also in turn grow larger as the terminator approaches them, until they fall into the enlightened part of the disk. as they fall further and further within this part, you will have additional proofs that they are mountains, from the shadows which they cast on the plain, always in a direction opposite to the sun. the mountain itself may entirely disappear, or become confounded with the other enlightened portions of the surface; but its position and its shape may still be recognised by the dark line which it projects on the plane. this line will correspond in shape to that of the mountain, presenting at one time a long serpentine stripe of black, denoting that the mountain is a continued range; at another time exhibiting a conical figure tapering to a point, or a series of such sharp points; or a serrated, uneven termination, indicating, in each case respectively, a conical mountain, or a group of peaks, or a range with lofty cliffs. all these appearances will indeed be seen in miniature; but a little familiarity with them will enable you to give them, in imagination, their proper dimensions, as you give to the pictures of known animals their due sizes, although drawn on a scale far below that of real life. in the next place, let us see how valleys and deep craters in the moon might be expected to appear. we could not expect to see depressions any more than elevations, since both would alike be projected on the same imaginary disk. but we may recognise such depressions, from the manner in which the light of the sun shines into them. when we hold a china tea-cup at some distance from a candle, in the night, the candle being elevated but little above the level of the top of the cup, a luminous crescent will be formed on the side of the cup opposite to the candle, while the side next to the candle will be covered by a deep shadow. as we gradually elevate the candle, the crescent enlarges and travels down the side of the cup, until finally the whole interior becomes illuminated. we observe similar appearances in the moon, which we recognise as deep depressions. they are those circular spots near the terminator before spoken of, which look like bubbles of oil floating on water. they are nothing else than circular craters or deep valleys. when they are so situated that the light of the sun is just beginning to shine into them, you may see, as in the tea-cup, a luminous crescent around the side furthest from the sun, while a deep black shadow is cast on the side next to the sun. as the cavity is turned more and more towards the light, the crescent enlarges, until at length the whole interior is illuminated. if the tea-cup be placed on a table, and a candle be held at some distance from it, nearly on a level with the top, but a little above it, the cup itself will cast a shadow on the table, like any other elevated object. in like manner, many of these circular spots on the moon cast deep shadows behind them, indicating that the tops of the craters are elevated far above the general level of the moon. the regularity of some of these circular spots is very remarkable. the circle, in some instances, appears as well formed as could be described by a pair of compasses, while in the centre there not unfrequently is seen a conical mountain casting its pointed shadow on the bottom of the crater. i hope you will enjoy repeated opportunities to view the moon through a telescope. allow me to recommend to you, not to rest satisfied with a hasty or even with a single view, but to verify the preceding remarks by repeated and careful inspection of the lunar disk, at different ages of the moon. the various places on the moon's disk have received appropriate names. the dusky regions being formerly supposed to be seas, were named accordingly; and other remarkable places have each two names, one derived from some well-known spot on the earth, and the other from some distinguished personage. thus, the same bright spot on the surface of the moon is called _mount sinai_ or _tycho_, and another, _mount etna_ or _copernicus_. the names of individuals, however, are more used than the others. the diagram, fig. , (see page ,) represents rudely, the telescopic appearance of the full moon. the reality is far more beautiful. a few of the most remarkable points have the following names corresponding to the numbers and letters on the map. . tycho, . eratosthenes, . kepler, . plato, . copernicus, . archimedes, . aristarchus, . eudoxus, . helicon, . aristotle. a. mare humorum, _sea of humors_, b. mare nubium, _sea of clouds_, c. mare imbrium, _sea of rains_, d. mare nectaris, _sea of nectar_, e. mare tranquillitatis, _sea of tranquillity_, f. mare serenitatis, _sea of serenity_, g. mare fecunditatis, _sea of plenty_, h. mare crisium, _crisian sea_. the heights of the lunar mountains, and the depths of the valleys, can be estimated with a considerable degree of accuracy. some of the mountains are as high as five miles, and the valleys, in some instances, are four miles deep. hence it is inferred, that the surface of the moon is more broken and irregular than that of the earth, its mountains being higher and its valleys deeper, in proportion to its magnitude, than those of the earth. the varieties of surface in the moon, as seen by the aid of large telescopes, have been well described by dr. dick, in his 'celestial scenery,' and i cannot give you a better idea of them, than to add a few extracts from his work. the lunar mountains in general exhibit an arrangement and an aspect very different from the mountain scenery of our globe. they may be arranged under the four following varieties: first, _insulated mountains_, which rise from plains nearly level, shaped like a sugar loaf, which may be supposed to present an appearance somewhat similar to mount etna, or the peak of teneriffe. the shadows of these mountains, in certain phases of the moon, are as distinctly perceived as the shadow of an upright staff, when placed opposite to the sun; and these heights can be calculated from the length of their shadows. some of these mountains being elevated in the midst of extensive plains, would present to a spectator on their summits magnificent views of the surrounding regions. secondly, _mountain ranges_, extending in length two or three hundred miles. these ranges bear a distant resemblance to our alps, apennines, and andes; but they are much less in extent. some of them appear very rugged and precipitous; and the highest ranges are in some places more than four miles in perpendicular altitude. in some instances, they are nearly in a straight line from northeast to southwest, as in the range called the _apennines_; in other cases, they assume the form of a semicircle, or crescent. thirdly, _circular ranges_, which appear on almost every part of the moon's surface, particularly in its southern regions. this is one grand peculiarity of the lunar ranges, to which we have nothing similar on the earth. a plain, and sometimes a large cavity, is surrounded with a circular ridge of mountains, which encompasses it like a mighty rampart. these annular ridges and plains are of all dimensions, from a mile to forty or fifty miles in diameter, and are to be seen in great numbers over every region of the moon's surface; they are most conspicuous, however, near the upper and lower limbs, about the time of the half moon. the mountains which form these circular ridges are of different elevations, from one fifth of a mile to three miles and a half, and their shadows cover one half of the plain at the base. these plains are sometimes on a level with the general surface of the moon, and in other cases they are sunk a mile or more below the level of the ground which surrounds the exterior circle of the mountains. fourthly, _central mountains_, or those which are placed in the middle of circular plains. in many of the plains and cavities surrounded by circular ranges of mountains there stands a single insulated mountain, which rises from the centre of the plain, and whose shadow sometimes extends, in the form of a pyramid, half across the plain to the opposite ridges. these central mountains are generally from half a mile to a mile and a half in perpendicular altitude. in some instances, they have two, and sometimes three, different tops, whose shadows can be easily distinguished from each other. sometimes they are situated towards one side of the plain, or cavity; but in the great majority of instances their position is nearly or exactly central. the lengths of their bases vary from five to about fifteen or sixteen miles. the _lunar caverns_ form a very peculiar and prominent feature of the moon's surface, and are to be seen throughout almost every region, but are most numerous in the southwest part of the moon. nearly a hundred of them, great and small, may be distinguished in that quarter. they are all nearly of a circular shape, and appear like a very shallow egg-cup. the smaller cavities appear, within, almost like a hollow cone, with the sides tapering towards the centre; but the larger ones have, for the most part, flat bottoms, from the centre of which there frequently rises a small, steep, conical hill, which gives them a resemblance to the circular ridges and central mountains before described. in some instances, their margins are level with the general surface of the moon; but, in most cases, they are encircled with a high annular ridge of mountains, marked with lofty peaks. some of the larger of these cavities contain smaller cavities of the same kind and form, particularly in their sides. the mountainous ridges which surround these cavities reflect the greatest quantity of light; and hence that region of the moon in which they abound appears brighter than any other. from their lying in every possible direction, they appear, at and near the time of full moon, like a number of brilliant streaks, or radiations. these radiations appear to converge towards a large brilliant spot, surrounded by a faint shade, near the lower part of the moon, which is named tycho,--a spot easily distinguished even by a small telescope. the spots named kepler and copernicus are each composed of a central spot with luminous radiations.[ ] the broken surface and apparent geological structure of the moon has suggested the opinion, that the moon has been subject to powerful _volcanic_ action. this opinion receives support from certain actual appearances of volcanic fires, which have at different times been observed. in a total eclipse of the sun, the moon comes directly between us and that luminary, and presents her dark side towards us under circumstances very favorable for observation. at such times, several astronomers, at different periods, have noticed bright spots, which they took to be volcanoes. it must evidently require a large fire to be visible at all, at such a distance; and even a burning spark, or point but just visible in a large telescope, might be in fact a volcano raging like etna or vesuvius. still, as fires might be supposed to exist in the moon from different causes, we should require some marks peculiar to volcanic fires, to assure us that such was their origin in a given case. dr. herschel examined this point with great attention, and with better means of observation than any of his predecessors enjoyed, and fully embraced the opinion that what he saw were volcanoes. in april, , he records his observations as follows: "i perceive three volcanoes in different places in the dark part of the moon. two of them are already nearly extinct, or otherwise in a state of going to break out; the third shows an eruption of fire or luminous matter." on the next night, he says: "the volcano burns with greater violence than last night; its diameter cannot be less than three seconds; and hence the shining or burning matter must be above three miles in diameter. the appearance resembles a small piece of burning charcoal, when it is covered with a very thin coat of white ashes; and it has a degree of brightness about as strong as that with which such a coal would be seen to glow in faint daylight." that these were really volcanic fires, he considered further evident from the fact, that where a fire, supposed to have been volcanic, had been burning, there was seen, after its extinction, an accumulation of matter, such as would arise from the production of a great quantity of lava, sufficient to form a mountain. it is probable that the moon has an _atmosphere_, although it is difficult to obtain perfectly satisfactory evidence of its existence; for granting the existence of an atmosphere bearing the same proportion to that planet as our atmosphere bears to the earth, its dimensions and its density would be so small, that we could detect its presence only by the most refined observations. as our twilight is owing to the agency of our atmosphere, so, could we discern any appearance of twilight in the moon, we should regard that fact as indicating that she is surrounded by an atmosphere. or, when the moon covers the sun in a solar eclipse, could we see around her circumference a faint luminous ring, indicating that the sunlight shone through an aerial medium, we might likewise infer the existence of such a medium. such a faint ring of light has sometimes, as is supposed, been observed. schroeter, a german astronomer, distinguished for the acuteness of his vision and his powers of observation in general, was very confident of having obtained, from different sources, clear evidence of a lunar atmosphere. he concluded, that the inferior or more dense part of the moon's atmosphere is not more than fifteen hundred feet high, and that the entire height, at least to the limit where it would be too rare to produce any of the phenomena which are relied on as proofs of its existence, is not more than a mile. it has been a question, much agitated among astronomers, whether there is _water_ in the moon. analogy strongly inclines us to reply in the affirmative. but the analogy between the earth and the moon, as derived from all the particulars in which we can compare the two bodies, is too feeble to warrant such a conclusion, and we must have recourse to other evidence, before we can decide the point. in the first place, then, there is no positive evidence in favor of the existence of water in the moon. those extensive level regions, before spoken of, and denominated seas in the geography of this planet, have no other signs of being water, except that they are level and dark. but both these particulars would characterize an earthly plain, like the deserts of arabia and africa. in the second place, were those dark regions composed of water, the terminator would be entirely smooth where it passed over these oceans or seas. it is indeed indented by few inequalities, compared with those which it exhibits where it passes over the mountainous regions; but still, the inequalities are too considerable to permit the conclusion, that these level spots are such perfect levels as water would form. they do not appear to be more perfect levels than many plain countries on the globe. the deep caverns, moreover, seen in those dusky spots which were supposed to be seas, are unfavorable to the supposition that those regions are covered by water. in the third place, the face of the moon, when illuminated by the sun and not obscured by the state of our own atmosphere, is always serene, and therefore free from clouds. clouds are objects of great extent; they frequently intercept light, like solid bodies; and did they exist about the moon, we should certainly see them, and should lose sight of certain parts of the lunar disk which they covered. but neither position is true; we neither see any clouds about the moon, with our best telescopes, nor do we, by the intervention of clouds, ever lose sight of any portion of the moon when our own atmosphere is clear. but the want of clouds in the lunar atmosphere almost necessarily implies the absence of water in the moon. this planet is at the same distance from the sun as our own, and has, in this respect, an equal opportunity to feel the influence of his rays. its days are also twenty-seven times as long as ours, a circumstance which would augment the solar heat. when the pressure of the atmosphere is diminished on the surface of water, its tendency to pass into the state of vapor is increased. were the whole pressure of the atmosphere removed from the surface of a lake, in a summer's day, when the temperature was no higher than seventy-two degrees, the water would begin to boil. now it is well ascertained, that if there be any atmosphere about the moon, it is much lighter than ours, and presses on the surface of that body with a proportionally small force. this circumstance, therefore, would conspire with the other causes mentioned, to convert all the water of the moon into vapor, if we could suppose it to have existed at any given time. but those, who are anxious to furnish the moon and other planets with all the accommodations which they find in our own, have a subterfuge in readiness, to which they invariably resort in all cases like the foregoing. "there may be," say they, "some means, unknown to us, provided for retaining water on the surface of the moon, and for preventing its being wasted by evaporation: perhaps it remains unaltered in quantity, imparting to the lunar regions perpetual verdure and fertility." to this i reply, that the bare possibility of a thing is but slight evidence of its reality; nor is such a condition possible, except by miracle. if they grant that the laws of nature are the same in the moon as in the earth, then, according to the foregoing reasoning, there cannot be water in the moon; but if they say that the laws of nature are not the same there as here, then we cannot reason at all respecting them. one who resorts to a subterfuge of this kind ruins his own cause. he argues the existence of water in the moon, from the analogy of that planet to this. but if the laws of nature are not the same there as here, what becomes of his analogy? a liquid substance which would not evaporate by such a degree of solar heat as falls on the moon, which would not evaporate the faster, in consequence of the diminished atmospheric pressure which prevails there, could not be water, for it would not have the properties of water, and things are known by their properties. whenever we desert the cardinal principle of the newtonian philosophy,--that the laws of nature are uniform throughout all her realms,--we wander in a labyrinth; all analogies are made void; all physical reasonings cease; and imaginary possibilities or direct miracles take the place of legitimate natural causes. on the supposition that the moon is inhabited, the question has often been raised, whether we may hope that our telescopes will ever be so much improved, and our other means of observation so much augmented, that we shall be able to discover either the lunar inhabitants or any of their works. the improbability of our ever identifying _artificial structures_ in the moon may be inferred from the fact, that a space a mile in diameter is the least space that could be distinctly seen. extensive works of art, as large cities, or the clearing up of large tracts of country for settlement or tillage, might indeed afford some varieties of surface; but they would be merely varieties of light and shade, and the individual objects that occasioned them would probably never be recognised by their distinctive characters. thus, a building equal to the great pyramid of egypt, which covers a space less than the fifth of a mile in diameter, would not be distinguished by its figure; indeed, it would be a mere point. still less is it probable that we shall ever discover any inhabitants in the moon. were we to view the moon with a telescope that magnifies ten thousand times, it would bring the moon apparently ten thousand times nearer, and present it to the eye like a body twenty-four miles off. but even this is a distance too great for us to see the works of man with distinctness. moreover, from the nature of the telescope itself, we can never hope to apply a magnifying power so high as that here supposed. as i explained to you, when speaking of the telescope, whenever we increase the magnifying power of this instrument we diminish its field of view, so that with very high magnifiers we can see nothing but a point, such as a fixed star. we at the same time, also, magnify the vapors and smoke of the atmosphere, and all the imperfections of the medium, which greatly obscures the object, and prevents our seeing it distinctly. hence it is generally most satisfactory to view the moon with low powers, which afford a large field of view and give a clear light. with clark's telescope, belonging to yale college, we seldom gain any thing by applying to the moon a higher power than one hundred and eighty, although the instrument admits of magnifiers as high as four hundred and fifty. some writers, however, suppose that possibly we may trace indications of lunar inhabitants in their works, and that they may in like manner recognise the existence of the inhabitants of our planet. an author, who has reflected much on subjects of this kind, reasons as follows: "a navigator who approaches within a certain distance of a small island, although he perceives no human being upon it, can judge with certainty that it is inhabited, if he perceives human habitations, villages, corn-fields, or other traces of cultivation. in like manner, if we could perceive changes or operations in the moon, which could be traced to the agency of intelligent beings, we should then obtain satisfactory evidence that such beings exist on that planet; and it is thought possible that such operations may be traced. a telescope which magnifies twelve hundred times will enable us to perceive, as a visible point on the surface of the moon, an object whose diameter is only about three hundred feet. such an object is not larger than many of our public edifices; and therefore, were any such edifices rearing in the moon, or were a town or city extending its boundaries, or were operations of this description carrying on, in a district where no such edifices had previously been erected, such objects and operations might probably be detected by a minute inspection. were a multitude of living creatures moving from place to place, in a body, or were they even encamping in an extensive plain, like a large army, or like a tribe of arabs in the desert, and afterwards removing, it is possible such changes might be traced by the difference of shade or color, which such movements would produce. in order to detect such minute objects and operations, it would be requisite that the surface of the moon should be distributed among at least a hundred astronomers, each having a spot or two allotted to him, as the object of his more particular investigation, and that the observations be continued for a period of at least thirty or forty years, during which time certain changes would probably be perceived, arising either from physical causes, or from the operations of living agents."[ ] footnote: [ ] dick's 'celestial scenery,' chapter iv letter xvi. the moon.--phases.--harvest moon.--librations. "first to the neighboring moon this mighty key of nature he applied. behold! it turned the secret wards, it opened wide the course and various aspects of the queen of night: whether she wanes into a scanty orb, or, waxing broad, with her pale shadowy light, in a soft deluge overflows the sky."--_thomson's elegy._ let us now inquire into the revolutions of the moon around the earth, and the various changes she undergoes every month, called her _phases_, which depend on the different positions she assumes, with respect to the earth and the sun, in the course of her revolution. the moon revolves about the earth from west to east. her apparent orbit, as traced out on the face of the sky, is a great circle; but this fact would not certainly prove that the orbit is really a circle, since, if it were an ellipse, or even a more irregular curve, the projection of it on the face of the sky would be a circle, as explained to you before. (see page .) the moon is comparatively so near to the earth, that her apparent movements are very rapid, so that, by attentively watching her progress in a clear night, we may see her move from star to star, changing her place perceptibly, every few hours. the interval during which she goes through the entire circuit of the heavens, from any star until she comes round to the same star again, is called a _sidereal month_, and consists of about twenty-seven and one fourth days. the time which intervenes between one new moon and another is called a _synodical month_, and consists of nearly twenty-nine and a half days. a new moon occurs when the sun and moon meet in the same part of the heavens; but the sun as well as the moon is apparently travelling eastward, and nearly at the rate of one degree a day, and consequently, during the twenty-seven days while the moon has been going round the earth, the sun has been going forward about the same number of degrees in the same direction. hence, when the moon comes round to the part of the heavens where she passed the sun last, she does not find him there, but must go on more than two days, before she comes up with him again. the moon does not pursue precisely the same track around the earth as the sun does, in his apparent annual motion, though she never deviates far from that track. the inclination of her orbit to the ecliptic is only about five degrees, and of course the moon is never seen further from the ecliptic than about that distance, and she is commonly much nearer to the ecliptic than five degrees. we may therefore see nearly what is the situation of the ecliptic in our evening sky at any particular time of year, just by watching the path which the moon pursues, from night to night, from new to full moon. the two points where the moon's orbit crosses the ecliptic are called her _nodes_. they are the intersections of the lunar and solar orbits, as the equinoxes are the intersections of the equinoctial and ecliptic, and, like the latter, are one hundred and eighty degrees apart. the changes of the moon, commonly called her _phases_, arise from different portions of her illuminated side being turned towards the earth at different times. when the moon is first seen after the setting sun, her form is that of a bright crescent, on the side of the disk next to the sun, while the other portions of the disk shine with a feeble light, reflected to the moon from the earth. every night, we observe the moon to be further and further eastward of the sun, until, when she has reached an elongation from the sun of ninety degrees, half her visible disk is enlightened, and she is said to be in her _first quarter_. the terminator, or line which separates the illuminated from the dark part of the moon, is convex towards the sun from the new to the first quarter, and the moon is said to be _horned_. the extremities of the crescent are called _cusps_. at the first quarter, the terminator becomes a straight line, coinciding with the diameter of the disk; but after passing this point, the terminator becomes concave towards the sun, bounding that side of the moon by an elliptical curve, when the moon is said to be _gibbous_. when the moon arrives at the distance of one hundred and eighty degrees from the sun, the entire circle is illuminated, and the moon is _full_. she is then _in opposition_ to the sun, rising about the time the sun sets. for a week after the full, the moon appears gibbous again, until, having arrived within ninety degrees of the sun, she resumes the same form as at the first quarter, being then at her _third quarter_. from this time until new moon, she exhibits again the form of a crescent before the rising sun, until, approaching her _conjunction_ with the sun, her narrow thread of light is lost in the solar blaze; and finally, at the moment of passing the sun, the dark side is wholly turned towards us, and for some time we lose sight of the moon. by inspecting fig. , (where t represents the earth, a, b, c, &c., the moon in her orbit, and _a_, _b_, _c_, &c., her phases, as seen in the heavens,) we shall easily see how all these changes occur. [illustration fig. .] you have doubtless observed, that the moon appears much further in the south at one time than at another, when of the same age. this is owing to the fact that the ecliptic, and of course the moon's path, which is always very near it, is differently situated with respect to the _horizon_, at a given time of night, at different seasons of the year. this you will see at once, by turning to an artificial globe, and observing how the ecliptic stands with respect to the horizon, at different periods of the revolution. thus, if we place the two equinoctial points in the eastern and western horizon, libra being in the west, it will represent the position of the ecliptic at sunset in the month of september, when the sun is crossing the equator; and at that season of the year, the moon's path through our evening sky, one evening after another, from new to full, will be nearly along the same route, crossing the meridian nearly at right angles. but if we place the winter solstice, or first degree of capricorn, in the western horizon, and the first degree of cancer in the eastern, then the position of the ecliptic will be very oblique to the meridian, the winter solstice being very far in the southwest, and the summer solstice very far in the northeast; and the course of the moon from new to full will be nearly along this track. keeping these things in mind, we may easily see why the moon runs sometimes high and sometimes low. recollect, also, that the new moon is always in the same part of the heavens with the sun, and that the full moon is in the opposite part of the heavens from the sun. now, when the sun is at the winter solstice, it sets far in the southwest, and accordingly the new moon runs very low; but the full moon, being in the opposite tropic, which rises far in the northeast, runs very high, as is known to be the case in mid-winter. but now take the position of the ecliptic in mid-summer. then, at sunset, the tropic of cancer is in the northwest, and the tropic of capricorn in the southeast; consequently, the new moons run high and the full moons low. it is a natural consequence of this arrangement, to render the moon's light the most beneficial to us, by giving it to us in greatest abundance, when we have least of the sun's light, and giving it to us most sparingly, when the sun's light is greatest. thus, during the long nights of winter, the full moon runs high, and continues a very long time above the horizon; while in mid-summer, the full moon runs low, and is above the horizon for a much shorter period. this arrangement operates very favorably to the inhabitants of the polar regions. at the season when the sun is absent, and they have constant night, then the moon, during the second and third quarters, embracing the season of full moon, is continually above the horizon, compensating in no small degree for the absence of the sun; while, during the summer months, when the sun is constantly above the horizon, and the light of the moon is not needed, then she is above the horizon during the first and last quarters, when her light is least, affording at that time her greatest light to the inhabitants of the other hemisphere, from whom the sun is withdrawn. about the time of the autumnal equinox, the moon, when near her full, rises about sunset a number of nights in succession. this occasions a remarkable number of brilliant moonlight evenings; and as this is, in england, the period of harvest, the phenomenon is called the _harvest moon_. its return is celebrated, particularly among the peasantry, by festive dances, and kept as a festival, called the _harvest home_,--an occasion often alluded to by the british poets. thus henry kirke white: "moon of harvest, herald mild of plenty, rustic labor's child, hail, o hail! i greet thy beam, as soft it trembles o'er the stream, and gilds the straw-thatch'd hamlet wide, where innocence and peace reside; 'tis thou that glad'st with joy the rustic throng, promptest the tripping dance, th' exhilarating song." to understand the reason of the harvest moon, we will, as before, consider the moon's orbit as coinciding with the ecliptic, because we may then take the ecliptic, as it is drawn on the artificial globe, to represent that orbit. we will also bear in mind, (what has been fully illustrated under the last head,) that, since the ecliptic cuts the meridian obliquely, while all the circles of diurnal revolution cut it perpendicularly, different portions of the ecliptic will cut the horizon at different angles. thus, when the equinoxes are in the horizon, the ecliptic makes a very small angle with the horizon; whereas, when the solstitial points are in the horizon, the same angle is far greater. in the former case, a body moving eastward in the ecliptic, and being at the eastern horizon at sunset, would descend but a little way below the horizon in moving over many degrees of the ecliptic. now, this is just the case of the moon at the time of the harvest home, about the time of the autumnal equinox. the sun being then in libra, and the moon, when full, being of course opposite to the sun, or in aries; and moving eastward, in or near the ecliptic, at the rate of about thirteen degrees per day, would descend but a small distance below the horizon for five or six days in succession; that is for two or three days before, and the same number of days after, the full; and would consequently rise during all these evenings nearly at the same time, namely, a little before, or a little after, sunset, so as to afford a remarkable succession of fine moonlight evenings. the moon _turns on her axis_ in the same time in which she revolves around the earth. this is known by the moon's always keeping nearly the same face towards us, as is indicated by the telescope, which could not happen unless her revolution on her axis kept pace with her motion in her orbit. take an apple, to represent the moon; stick a knittingneedle through it, in the direction of the stem, to represent the axis, in which case the two eyes of the apple will aptly represent the poles. through the poles cut a line around the apple, dividing it into two hemispheres, and mark them, so as to be readily distinguished from each other. now place a candle on the table, to represent the earth, and holding the apple by the knittingneedle, carry it round the candle, and you will see that, unless you make the apple turn round on the axis as you carry it about the candle, it will present different sides towards the candle; and that, in order to make it always present the same side, it will be necessary to make it revolve exactly once on its axis, while it is going round the circle,--the revolution on its axis always keeping exact pace with the motion in its orbit. the same thing will be observed, if you walk around a tree, always keeping your face towards the tree. if you have your face towards the tree when you set out, and walk round without turning, when you have reached the opposite side of the tree, your back will be towards it, and you will find that, in order to keep your face constantly towards the tree, it will be necessary to turn yourself round on your heel at the same rate as you go forward. since, however, the motion of the moon on its axis is uniform, while the motion in its orbit is unequal, the moon does in fact reveal to us a little sometimes of one side and sometimes of the other. thus if, while carrying the apple round the candle, you carry it forward a little faster than the rate at which it turns on its axis, a portion of the hemisphere usually out of sight is brought into view on one side; or if the apple is moved forward slower than it is turned on its axis, a portion of the same hemisphere comes into view on the other side. these appearances are called the moon's _librations in longitude_. the moon has also a _libration in latitude_;--so called, because in one part of her revolution more of the region around one of the poles comes into view, and, in another part of the revolution, more of the region around the other pole, which gives the appearance of a tilting motion to the moon's axis. this is owing to the fact, that the moon's axis is inclined to the plane of her orbit. if, in the experiment with the apple, you hold the knittingneedle parallel to the candle, (in which case the axis will be perpendicular to the plane of revolution,) the candle will shine upon both poles during the whole circuit, and an eye situated where the candle is would constantly see both poles; but now incline the needle towards the plane of revolution, and carry it round, always keeping it parallel to itself, and you will observe that the two poles will be alternately in and out of sight. the moon exhibits another appearance of this kind, called her _diurnal libration_, depending on the daily rotation of the spectator. she turns the same face towards the _centre_ of the earth only, whereas we view her from the surface. when she is on the meridian, we view her disk nearly as though we viewed it from the centre of the earth, and hence, in this situation, it is subject to little change; but when she is near the horizon, our circle of vision takes in more of the upper limb than would be presented to a spectator at the centre of the earth. hence, from this cause, we see a portion of one limb while the moon is rising, which is gradually lost sight of, and we see a portion of the opposite limb, as the moon declines to the west. you will remark that neither of the foregoing changes implies any actual motion in the moon, but that each arises from a change of position in the spectator. since the succession of day and night depends on the revolution of a planet on its own axis, and it takes the moon twenty-nine and a half days to perform this revolution, so that the sun shall go from the meridian of any place and return to the same meridian again, of course the lunar day occupies this long period. so protracted an exposure to the sun's rays, especially in the equatorial regions of the moon, must occasion an excessive accumulation of heat; and so long an absence of the sun must occasion a corresponding degree of cold. a spectator on the side of the moon which is opposite to us would never see the earth, but one on the side next to us would see the earth constantly in his firmament, undergoing a gradual succession of changes, corresponding to those which the moon exhibits to the earth, but in the reverse order. thus, when it is full moon to us, the earth, as seen from the moon, is then in conjunction with the sun, and of course presents her dark side to the moon. soon after this, an inhabitant of the moon would see a crescent, resembling our new moon, which would in like manner increase and go through all the changes, from new to full, and from full to new, as we see them in the moon. there are, however, in the two cases, several striking points of difference. in the first place, instead of twenty-nine and a half days, all these changes occur in one lunar day and night. during the first and last quarters, the changes would occur in the day-time; but during the second and third quarters, during the night. by this arrangement, the lunarians would enjoy the greatest possible benefit from the light afforded by the earth, since in the half of her revolution where she appears to them as full, she would be present while the sun was absent, and would afford her least light while the sun was present. in the second place, the earth would appear thirteen times as large to a spectator on the moon as the moon appears to us, and would afford nearly the same proportion of light, so that their long nights must be continually cheered by an extraordinary degree of light derived from this source; and if the full moon is hailed by our poets as "refulgent lamp of night,"[ ] with how much more reason might a lunarian exult thus, in view of the splendid orb that adorns his nocturnal sky! in the third place, the earth, as viewed from any particular place on the moon, would occupy invariably the same part of the heavens. for while the rotation of the moon on her axis from west to east would appear to make the earth (as the moon does to us) revolve from east to west, the corresponding progress of the moon in her orbit would make the earth appear to revolve from west to east; and as these two motions are equal, their united effect would be to keep the moon apparently stationary in the sky. thus, a spectator at e, fig. , page , in the middle of the disk that is turned towards the earth, would have the earth constantly on his meridian, and at e, the conjunction of the earth and sun would occur at mid-day; but when the moon arrived at g, the same place would be on the margin of the circle of illumination, and will have the sun in the horizon; but the earth would still be on his meridian and in quadrature. in like manner, a place situated on the margin of the circle of illumination, when the moon is at e, would have the earth in the horizon; and the same place would always see the earth in the horizon, except the slight variations that would occur from the librations of the moon. in the fourth place, the earth would present to a spectator on the moon none of that uniformity of aspect which the moon presents to us, but would exhibit an appearance exceedingly diversified. the comparatively rapid rotation of the earth, repeated fifteen times during a lunar night, would present, in rapid succession, a view of our seas, oceans, continents, and mountains, all diversified by our clouds, storms, and volcanoes. footnotes: [ ] dick's 'celestial scenery.' [ ] "as when the moon, refulgent lamp of night, o'er heaven's clear azure sheds her sacred light, when not a breath disturbs the deep serene, and not a cloud o'ercasts the solemn scene, around her throne the vivid planets roll, and stars unnumbered gild the glowing pole; o'er the dark trees a yellower verdure shed, and tip with silver every mountain's head; then shine the vales, the rocks in prospect rise, a flood of glory bursts from all the skies; the conscious swains, rejoicing in the sight, eye the blue vault, and bless the useful light." _pope's homer._ letter xvii. moon's orbit.--her irregularities. "some say the zodiac constellations have long since left their antique stations, above a sign, and prove the same in taurus now, once in the ram; that in twelve hundred years and odd, the sun has left his ancient road, and nearer to the earth is come, 'bove fifty thousand miles from home."--_hudibras._ we have thus far contemplated the revolution of the moon around the earth as though the earth were at rest. but in order to have just ideas respecting the moon's motions, we must recollect that the moon likewise revolves along with the earth around the sun. it is sometimes said that the earth _carries_ the moon along with her, in her annual revolution. this language may convey an erroneous idea; for the moon, as well as the earth, revolves around the sun under the influence of two forces, which are independent of the earth, and would continue her motion around the sun, were the earth removed out of the way. indeed, the moon is attracted towards the sun two and one fifth times more than towards the earth, and would abandon the earth, were not the latter also carried along with her by the same forces. so far as the sun acts equally on both bodies, the motion with respect to each other would not be disturbed. because the gravity of the moon towards the sun is found to be greater, at the conjunction, than her gravity towards the earth, some have apprehended that, if the doctrine of universal gravitation is true, the moon ought necessarily to abandon the earth. in order to understand the reason why it does not do thus, we must reflect, that, when a body is revolving in its orbit under the influence of the projectile force and gravity, whatever diminishes the force of gravity, while that of projection remains the same, causes the body to approach nearer to the tangent of her orbit, and of course to recede from the centre; and whatever increases the amount of gravity, carries the body towards the centre. thus, in fig. , page , if, with a certain force of projection acting in the direction a b, and of attraction, in the direction a c, the attraction which caused a body to move in the line a d were diminished, it would move nearer to the tangent, as in a e, or a f. now, when the moon is in conjunction, her gravity towards the earth acts in opposition to that towards the sun, (see fig. , page ,) while her velocity remains too great to carry her with what force remains, in a circle about the sun, and she therefore recedes from the sun, and commences her revolution around the earth. on arriving at the opposition, the gravity of the earth conspires with that of the sun, and the moon's projectile force being less than that required to make her revolve in a circular orbit, when attracted towards the sun by the sum of these forces, she accordingly begins to approach the sun, and descends again to the conjunction. the attraction of the sun, however, being every where greater than that of the earth, the actual path of the moon around the sun is every where concave towards the latter. still, the elliptical path of the moon around the earth is to be conceived of, in the same way as though both bodies were at rest with respect to the sun. thus, while a steam-boat is passing _swiftly_ around an island, and a man is walking _slowly_ around a post in the cabin, the line which he describes in space between the forward motion of the boat and his circular motion around the post, may be every where concave towards the island, while his path around the post will still be the same as though both were at rest. a nail in the rim of a coach-wheel will turn around the axis of the wheel, when the coach has a forward motion, in the same manner as when the coach is at rest, although the line actually described by the nail will be the resultant of both motions, and very different from either. we have hitherto regarded the moon as describing a great circle on the face of the sky, such being the visible orbit, as seen by projection. but, on a more exact investigation, it is found that her orbit is not a circle, and that her motions are subject to very numerous irregularities. these will be best understood in connexion with the causes on which they depend. the law of universal gravitation has been applied with wonderful success to their developement, and its results have conspired with those of long-continued observation, to furnish the means of ascertaining with great exactness the place of the moon in the heavens, at any given instant of time, past or future, and thus to enable astronomers to determine longitudes, to calculate eclipses, and to solve other problems of the highest interest. the whole number of irregularities to which the moon is subject is not less than sixty, but the greater part are so small as to be hardly deserving of attention; but as many as thirty require to be estimated and allowed for, before we can ascertain the exact place of the moon at any given time. you will be able to understand something of the cause of these irregularities, if you first gain a distinct idea of the mutual actions of the sun, the moon, and the earth. the irregularities in the moon's motions are due chiefly to the disturbing influence of the sun, which operates in two ways; first, by acting unequally on the earth and moon; and secondly, by acting obliquely on the moon, on account of the inclination of her orbit to the ecliptic. if the sun acted equally on the earth and moon, and always in parallel lines, this action would serve only to restrain them in their annual motions around the sun, and would not affect their actions on each other, or their motions about their common centre of gravity. in that case, if they were allowed to fall towards the sun, they would fall equally, and their respective situations would not be affected by their descending equally towards it. but, because the moon is nearer the sun in one half of her orbit than the earth is, and in the other half of her orbit is at a greater distance than the earth from the sun, while the power of gravity is always greater at a less distance; it follows, that in one half of her orbit the moon is more attracted than the earth towards the sun, and, in the other half, less attracted than the earth. to see the effects of this process, let us suppose that the projectile motions of the earth and moon were destroyed, and that they were allowed to fall freely towards the sun. (see fig. , page .) if the moon was in conjunction with the sun, or in that part of her orbit which is nearest to him, the moon would be more attracted than the earth, and fall with greater velocity towards the sun; so that the distance of the moon from the earth would be increased by the fall. if the moon was in opposition, or in the part of her orbit which is furthest from the sun, she would be less attracted than the earth by the sun, and would fall with a less velocity, and be left behind; so that the distance of the moon from the earth would be increased in this case, also. if the moon was in one of the quarters, then the earth and the moon being both attracted towards the centre of the sun, they would both descend directly towards that centre, and, by approaching it, they would necessarily at the same time approach each other, and in this case their distance from each other would be diminished. now, whenever the action of the sun would increase their distance, if they were allowed to fall towards the sun, then the sun's action, by endeavoring to separate them, diminishes their gravity to each other; whenever the sun's action would diminish the distance, then it increases their mutual gravitation. hence, in the conjunction and opposition, their gravity towards each other is diminished by the action of the sun, while in the quadratures it is increased. but it must be remembered, that it is not the total action of the sun on them that disturbs their motions, but only that part of it which tends at one time to separate them, and at another time to bring them nearer together. the other and far greater part has no other effect than to retain them in their annual course around the sun. the cause of the lunar irregularities was first investigated by sir isaac newton, in conformity with his doctrine of universal gravitation, and the explanation was first published in the 'principia;' but, as it was given in a mathematical dress, there were at that age very few persons capable of reading or understanding it. several eminent individuals, therefore, undertook to give a popular explanation of these difficult points. among newton's contemporaries, the best commentator was m'laurin, a scottish astronomer, who published a large work entitled 'm'laurin's account of sir isaac newton's discoveries.' no writer of his own day, and, in my opinion, no later commentator, has equalled m'laurin, in reducing to common apprehension the leading principles of the doctrine of gravitation, and the explanation it affords of the motions of the heavenly bodies. to this writer i am indebted for the preceding easy explanation of the irregularities of the moon's motions, as well as for several other illustrations of the same sublime doctrine. the figure of the moon's orbit is an ellipse. we have before seen, that the earth's orbit around the sun is of the same figure; and we shall hereafter see this to be true of all the planetary orbits. the path of the earth, however, departs very little from a circle; that of the moon differs materially from a circle, being considerably longer one way than the other. were the orbit a circle having the earth in the centre, then the radius vector, or line drawn from the centre of the moon to the centre of the earth, would always be of the same length; but it is found that the length of the radius vector is only fifty-six times the radius of the earth when the moon is nearest to us, while it is sixty-four times that radius when the moon is furthest from us. the point in the moon's orbit nearest the earth is called her _perigee_; the point furthest from the earth, her _apogee_. we always know when the moon is at one of these points, by her apparent diameter or apparent velocity; for, when at the perigee, her diameter is greater than at any time, and her motion most rapid; and, on the other hand, her diameter is least, and her motion slowest, when she is at her apogee. the moon's nodes constantly shift their positions in the ecliptic, from east to west, at the rate of about nineteen and a half degrees every year, returning to the same points once in eighteen and a half years. in order to understand what is meant by this backward motion of the nodes, you must have very distinctly in mind the meaning of the terms themselves; and if, at any time, you should be at a loss about the signification of any word that is used in expressing an astronomical proposition, i would advise you to turn back to the previous definition of that term, and revive its meaning clearly in the mind, before you proceed any further. in the present case, you will recollect that the moon's nodes are the two points where her orbit cuts the plane of the ecliptic. suppose the great circle of the ecliptic marked out on the face of the sky in a distinct line, and let us observe, at any given time, the exact moment when the moon crosses this line, which we will suppose to be close to a certain star; then, on its next return to that part of the heavens, we shall find that it crosses the ecliptic sensibly to the westward of that star, and so on, further and further to the westward, every time it crosses the ecliptic at either node. this fact is expressed by saying that _the nodes retrograde on the ecliptic_; since any motion from east to west, being contrary to the order of the signs, is called retrograde. the line which joins these two points, or the line of the nodes, is also said to have a retrograde motion, or to revolve from east to west once in eighteen and a half years. the _line of the apsides_ of the moon's orbit revolves from west to east, through her whole course, in about nine years. you will recollect that the apsides of an elliptical orbit are the two extremities of the longer axis of the ellipse; corresponding to the perihelion and aphelion of bodies revolving about the sun, or to the perigee and apogee of a body revolving about the earth. if, in any revolution of the moon, we should accurately mark the place in the heavens where the moon is nearest the earth, (which may be known by the moon's apparent diameter being then greatest,) we should find that, at the next revolution, it would come to its perigee a little further eastward than before, and so on, at every revolution, until, after nine years, it would come to its perigee nearly at the same point as at first. this fact is expressed by saying, that the perigee, and of course the apogee, revolves, and that the line which joins these two points, or the line of the apsides, also revolves. these are only a few of the irregularities that attend the motions of the moon. these and a few others were first discovered by actual observation and have been long known; but a far greater number of lunar irregularities have been made known by following out all the consequences of the law of universal gravitation. the moon may be regarded as a body endeavoring to make its way around the earth, but as subject to be continually impeded, or diverted from its main course, by the action of the sun and of the earth; sometimes acting in concert and sometimes in opposition to each other. now, by exactly estimating the amount of these respective forces, and ascertaining their resultant or combined effect, in any given case, the direction and velocity of the moon's motion may be accurately determined. but to do this has required the highest powers of the human mind, aided by all the wonderful resources of mathematics. yet, so consistent is truth with itself, that, where some minute inequality in the moon's motions is developed at the end of a long and intricate mathematical process, it invariably happens, that, on pointing the telescope to the moon, and watching its progress through the skies, we may actually see her commit the same irregularities, unless (as is the case with many of them) they are too minute to be matters of observation, being beyond the powers of our vision, even when aided by the best telescopes. but the truth of the law of gravitation, and of the results it gives, when followed out by a chain of mathematical reasoning, is fully confirmed, even in these minutest matters, by the fact that the moon's place in the heavens, when thus determined, always corresponds, with wonderful exactness, to the place which she is actually observed to occupy at that time. the mind, that was first able to elicit from the operations of nature the law of universal gravitation, and afterwards to apply it to the complete explanation of all the irregular wanderings of the moon, must have given evidence of intellectual powers far elevated above those of the majority of the human race. we need not wonder, therefore, that such homage is now paid to the genius of newton,--an admiration which has been continually increasing, as new discoveries have been made by tracing out new consequences of the law of universal gravitation. the chief object of astronomical _tables_ is to give the amount of all the irregularities that attend the motions of the heavenly bodies, by estimating the separate value of each, under all the different circumstances in which a body can be placed. thus, with respect to the moon, before we can determine accurately the distance of the moon from the vernal equinox, that is, her longitude at any given moment, we must be able to make exact allowances for all her irregularities which would affect her longitude. these are in all no less than sixty, though most of them are so exceedingly minute, that it is not common to take into the account more than twenty-eight or thirty. the values of these are all given in the lunar tables; and in finding the moon's place, at any given time, we proceed as follows: we first find what her place would be on the supposition that she moves uniformly in a circle. this gives her _mean_ place. we next apply the various corrections for her irregular motions; that is, we apply the _equations_, subtracting some and adding others, and thus we find her _true_ place. the astronomical tables have been carried to such an astonishing degree of accuracy, that it is said, by the highest authority, that an astronomer could now predict, for a thousand years to come, the precise moment of the passage of any one of the stars over the meridian wire of the telescope of his transit-instrument, with such a degree of accuracy, that the error would not be so great as to remove the object through an angular space corresponding to the semidiameter of the finest wire that could be made; and a body which, by the tables, ought to appear in the transit-instrument in the middle of that wire, would in no case be removed to its outer edge. the astronomer, the mathematician, and the artist, have united their powers to produce this great result. the astronomer has collected the data, by long-continued and most accurate observations on the actual motions of the heavenly bodies, from night to night, and from year to year; the mathematician has taken these data, and applied to them the boundless resources of geometry and the calculus; and, finally, the instrument-maker has furnished the means, not only of verifying these conclusions, but of discovering new truths, as the foundation of future reasonings. since the points where the moon crosses the ecliptic, or the moon's nodes, constantly shift their positions about nineteen and a half degrees to the westward, every year, the sun, in his annual progress in the ecliptic, will go from the node round to the same node again in less time than a year, since the node goes to meet him nineteen and a half degrees to the west of the point where they met before. it would have taken the sun about nineteen days to have passed over this arc; and consequently, the interval between two successive conjunctions between the sun and the moon's node is about nineteen days shorter than the solar year of three hundred and sixty-five days; that is, it is about three hundred and forty-six days; or, more exactly, it is . days. the time from one new moon to another is . days. now, nineteen of the former periods are almost exactly equal to two hundred and twenty-three of the latter: for . Ã� = . days= y. d. and . Ã� = . " = " " " " hence, if the sun and moon were to leave the moon's node together, after the sun had been round to the same node nineteen times, the moon would have made very nearly two hundred and twenty-three conjunctions with the sun. if, therefore, she was in conjunction with the sun at the beginning of this period, she would be in conjunction again at the end of it; and all things relating to the sun, the moon, and the node, would be restored to the same relative situation as before, and the sun and moon would start again, to repeat the same phenomena, arising out of these relations, as occurred in the preceding period, and in the same order. now, when the sun and moon meet at the moon's node, an eclipse of the sun happens; and during the entire period of eighteen and a half years eclipses will happen, nearly in the same manner as they did at corresponding times in the preceding period. thus, if there was a great eclipse of the sun on the fifth year of one of these periods, a similar eclipse (usually differing somewhat in magnitude) might be expected on the fifth year of the next period. hence this period, consisting of about eighteen years and ten days, under the name of the _saros_, was used by the chaldeans, and other ancient nations, in predicting eclipses. it was probably by this means that thales, a grecian astronomer who flourished six hundred years before the christian era, predicted an eclipse of the sun. herodotus, the old historian of greece, relates that the day was suddenly changed into night, and that thales of miletus had foretold that a great eclipse was to happen _this year_. it was therefore, at that age, considered as a distinguished feat to predict even the year in which an eclipse was to happen. this eclipse is memorable in ancient history, from its having terminated the war between the lydians and the medes, both parties being smitten with such indications of the wrath of the gods. the _metonic cycle_ has sometimes been confounded with the saros, but it is not the same with it, nor was the period used, like the saros, for foretelling eclipses, but for ascertaining the _age_ of the moon at any given period. it consisted of nineteen tropical years, during which time there are exactly two hundred and thirty-five new moons; so that, at the end of this period, the new moons will recur at seasons of the year corresponding exactly to those of the preceding cycle. if, for example, a new moon fell at the time of the vernal equinox, in one cycle, nineteen years afterwards it would occur again at the same equinox; or, if it had happened ten days after the equinox, in one cycle, it would also happen ten days after the equinox, nineteen years afterwards. by registering, therefore, the exact days of any cycle at which the new or full moons occurred, such a calendar would show on what days these events would occur in any other cycle; and, since the regulation of games, feasts, and fasts, has been made very extensively, both in ancient and modern times, according to new or full moons, such a calendar becomes very convenient for finding the day on which the new or full moon required takes place. suppose, for example, it were decreed that a festival should be held on the day of the first full moon after the vernal equinox. then, to find on what day that would happen, in any given year, we have only to see what year it is of the lunar cycle; for the day will be the same as it was in the corresponding year of the calendar which records all the full moons of the cycle for each year, and the respective days on which they happen. the athenians adopted the metonic cycle four hundred and thirty-three years before the christian era, for the regulation of their calendars, and had it inscribed in letters of gold on the walls of the temple of minerva. hence the term _golden number_, still found in our almanacs, which denotes the year of the lunar cycle. thus, fourteen was the golden number for , being the fourteenth year of the lunar cycle. the inequalities of the moon's motions are divided into periodical and secular. _periodical_ inequalities are those which are completed in comparatively short periods. _secular_ inequalities are those which are completed only in very long periods, such as centuries or ages. hence the corresponding terms _periodical equations_ and _secular equations_. as an example of a secular inequality, we may mention the acceleration of the _moon's mean motion_. it is discovered that the moon actually revolves around the earth in a less period now than she did in ancient times. the difference, however, is exceedingly small, being only about ten seconds in a century. in a lunar eclipse, the moon's longitude differs from that of the sun, at the middle of the eclipse, by exactly one hundred and eighty degrees; and since the sun's longitude at any given time of the year is known, if we can learn the day and hour when an eclipse occurred at any period of the world, we of course know the longitude of the sun and moon at that period. now, in the year , before the christian era, ptolemy records a lunar eclipse to have happened, and to have been observed by the chaldeans. the moon's longitude, therefore, for that time, is known; and as we know the mean motions of the moon, at present, starting from that epoch, and computing, as may easily be done, the place which the moon ought to occupy at present, at any given time, she is found to be actually nearly a degree and a half in advance of that place. moreover, the same conclusion is derived from a comparison of the chaldean observations with those made by an arabian astronomer of the tenth century. this phenomenon at first led astronomers to apprehend that the moon encountered a resisting medium, which, by destroying at every revolution a small portion of her projectile force, would have the effect to bring her nearer and nearer to the earth, and thus to augment her velocity. but, in , la place demonstrated that this acceleration is one of the legitimate effects of the sun's disturbing force, and is so connected with changes in the eccentricity of the earth's orbit, that the moon will continue to be accelerated while that eccentricity diminishes; but when the eccentricity has reached its minimum, or lowest point, (as it will do, after many ages,) and begins to increase, then the moon's motions will begin to be retarded, and thus her mean motions will oscillate for ever about a mean value. letter xviii. eclipses. ----"as when the sun, new risen, looks through the horizontal misty air, shorn of his beams, or from behind the moon, in dim eclipse, disastrous twilight sheds on half the nations, and with fear of change perplexes monarchs: darkened so, yet shone, above them all, the archangel."--_milton._ having now learned various particulars respecting the earth, the sun, and the moon, you are prepared to understand the explanation of solar and lunar eclipses, which have in all ages excited a high degree of interest. indeed, what is more admirable, than that astronomers should be able to tell us, years beforehand, the exact instant of the commencement and termination of an eclipse, and describe all the attendant circumstances with the greatest fidelity. you have doubtless, my dear friend, participated in this admiration, and felt a strong desire to learn how it is that astronomers are able to look so far into futurity. i will endeavor, in this letter, to explain to you the leading principles of the calculation of eclipses, with as much plainness as possible. an _eclipse of the moon_ happens when the moon, in its revolution around the earth, falls into the earth's shadow. an _eclipse of the sun_ happens when the moon, coming between the earth and the sun, covers either a part or the whole of the solar disk. the earth and the moon being both opaque, globular bodies, exposed to the sun's light, they cast shadows opposite to the sun, like any other bodies on which the sun shines. were the sun of the same size with the earth and the moon, then the lines drawn touching the surface of the sun and the surface of the earth or moon (which lines form the boundaries of the shadow) would be parallel to each other, and the shadow would be a cylinder infinite in length; and were the sun less than the earth or the moon, the shadow would be an increasing cone, its narrower end resting on the earth; but as the sun is vastly greater than either of these bodies, the shadow of each is a cone whose base rests on the body itself, and which comes to a point, or vertex, at a certain distance behind the body. these several cases are represented in the following diagrams, figs. , , . [illustration figs. , , .] it is found, by calculation, that the length of the moon's shadow, on an average, is just about sufficient to reach to the earth; but the moon is sometimes further from the earth than at others, and when she is nearer than usual, the shadow reaches considerably beyond the surface of the earth. also, the moon, as well as the earth, is at different distances from the sun at different times, and its shadow is longest when it is furthest from the sun. now, when both these circumstances conspire, that is, when the moon is in her perigee and along with the earth in her aphelion, her shadow extends nearly fifteen thousand miles beyond the centre of the earth, and covers a space on the surface one hundred and seventy miles broad. the earth's shadow is nearly a million of miles in length, and consequently more than three and a half times as long as the distance of the earth from the moon; and it is also, at the distance of the moon, three times as broad as the moon itself. an eclipse of the sun can take place only at new moon, when the sun and moon meet in the same part of the heavens, for then only can the moon come between us and the sun; and an eclipse of the moon can occur only when the sun and moon are in opposite parts of the heavens, or at full moon; for then only can the moon fall into the shadow of the earth. [illustration fig. .] the nature of eclipses will be clearly understood from the following representation. the diagram, fig. , exhibits the relative position of the sun, the earth, and the moon, both in a solar and in a lunar eclipse. here, the moon is first represented, while revolving round the earth, as passing between the earth and the sun, and casting its shadow on the earth. as the moon is here supposed to be at her average distance from the earth, the shadow but just reaches the earth's surface. were the moon (as is sometimes the case) nearer the earth her shadow would not terminate in a point, as is represented in the figure, but at a greater or less distance nearer the base of the cone, so as to cover a considerable space, which, as i have already mentioned, sometimes extends to one hundred and seventy miles in breadth, but is commonly much less than this. on the other side of the earth, the moon is represented as traversing the earth's shadow, as is the case in a lunar eclipse. as the moon is sometimes nearer the earth and sometimes further off, it is evident that it will traverse the shadow at a broader or a narrower part, accordingly. the figure, however, represents the moon as passing the shadow further from the earth than is ever actually the case, since the distance from the earth is never so much as one third of the whole length of the shadow. it is evident from the figure, that if a spectator were situated where the moon's shadow strikes the earth, the moon would cut off from him the view of the sun, or the sun would be totally eclipsed. or, if he were within a certain distance of the shadow on either side, the moon would be partly between him and the sun, and would intercept from him more or less of the sun's light, according as he was nearer to the shadow or further from it. if he were at _c_ or _d_, he would just see the moon entering upon the sun's disk; if he were nearer the shadow than either of these points, he would have a portion of this light cut off from his view, and more, in proportion as he drew nearer the shadow; and the moment he entered the shadow, he would lose sight of the sun. to all places between _a_ or _b_ and the shadow, the sun would cast a partial shadow of the moon, growing deeper and deeper, as it approached the true shadow. this partial shadow is called the moon's _penumbra_. in like manner, as the moon approaches the earth's shadow, in a lunar eclipse, as soon as she arrives at _a_, the earth begins to intercept from her a portion of the sun's light, or she falls in the earth's penumbra. she continues to lose more and more of the sun's light, as she draws near to the shadow, and hence her disk becomes gradually obscured, until it enters the shadow, when the sun's light is entirely lost. as the sun and earth are both situated in the plane of the ecliptic, if the moon also revolved around the earth in this plane, we should have a solar eclipse at every new moon, and a lunar eclipse at every full moon; for, in the former case, the moon would come directly between us and the sun, and in the latter case, the earth would come directly between the sun and the moon. but the moon is inclined to the ecliptic about five degrees, and the centre of the moon may be all this distance from the centre of the sun at new moon, and the same distance from the centre of the earth's shadow at full moon. it is true, the moon extends across her path, one half her breadth lying on each side of it, and the sun likewise reaches from the ecliptic a distance equal to half his breadth. but these luminaries together make but little more than a degree, and consequently, their two semidiameters would occupy only about half a degree of the five degrees from one orbit to the other where they are furthest apart. also, the earth's shadow, where the moon crosses it, extends from the ecliptic less than three fourths of a degree, so that the semidiameter of the moon and of the earth's shadow would together reach but little way across the space that may, in certain cases, separate the two luminaries from each other when they are in opposition. thus, suppose we could take hold of the circle in the figure that represents the moon's orbit, (fig. , page ,) and lift the moon up five degrees above the plane of the paper, it is evident that the moon, as seen from the earth, would appear in the heavens five degrees above the sun, and of course would cut off none of his light; and it is also plain that the moon, at the full, would pass the shadow of the earth five degrees below it, and would suffer no eclipse. but in the course of the sun's apparent revolution round the earth once a year he is successively in every part of the ecliptic; consequently, the conjunctions and oppositions of the sun and moon may occur at any part of the ecliptic, and of course at the two points where the moon's orbit crosses the ecliptic,--that is, at the nodes; for the sun must necessarily come to each of these nodes once a year. if, then, the moon overtakes the sun just as she is crossing his path, she will hide more or less of his disk from us. since, also, the earth's shadow is always directly opposite to the sun, if the sun is at one of the nodes, the shadow must extend in the direction of the other node, so as to lie directly across the moon's path; and if the moon overtakes it there, she will pass through it, and be eclipsed. thus, in fig. , let bn represent the sun's path, and an, the moon's,--n being the place of the node; then it is evident, that if the two luminaries at new moon be so far from the node, that the distances between their centres is greater than their semidiameters, no eclipse can happen; but if that distance is less than this sum, as at e, f, then an eclipse will take place; but if the position be as at c, d, the two bodies will just touch one another. if a denotes the earth's shadow, instead of the sun, the same illustration will apply to an eclipse of the moon. [illustration fig. .] since bodies are defined to be in conjunction when they are in the _same_ part of the heavens, and to be in opposition when they are in _opposite_ parts of the heavens, it may not appear how the sun and moon can be in conjunction, as at a and b, when they are still at some distance from each other. but it must be recollected that bodies are in conjunction when they have the same longitude, in which case they are situated in the same great circle perpendicular to the ecliptic,--that is, in the same secondary to the ecliptic. one of these bodies may be much further from the ecliptic than the other; still, if the same secondary to the ecliptic passes through them both, they will be in conjunction or opposition. in a total eclipse of the moon, its disk is still visible, shining with a dull, red light. this light cannot be derived directly from the sun, since the view of the sun is completely hidden from the moon; nor by reflection from the earth, since the illuminated side of the earth is wholly turned from the moon; but it is owing to refraction from the earth's atmosphere, by which a few scattered rays of the sun are bent round into the earth's shadow and conveyed to the moon, sufficient in number to afford the feeble light in question. it is impossible fully to understand the _method of calculating eclipses_, without a knowledge of trigonometry; still it is not difficult to form some general notion of the process. it may be readily conceived that, by long-continued observations on the sun and moon, the laws of their revolution may be so well understood, that the exact places which they will occupy in the heavens at any future times may be foreseen and laid down in tables of the sun and moon's motions; that we may thus ascertain, by inspecting the tables, the instant when these two bodies will be together in the heavens, or be in conjunction, and when they will be one hundred and eighty degrees apart, or in opposition. moreover, since the exact place of the moon's node among the stars at any particular time is known to astronomers, it cannot be difficult to determine when the new or full moon occurs in the same part of the heavens as that where the node is projected, as seen from the earth. in short, as astronomers can easily determine what will be the relative position of the sun, the moon, and the moon's nodes, for any given time, they can tell when these luminaries will meet so near the node as to produce an eclipse of the sun, or when they will be in opposition so near the node as to produce an eclipse of the moon. a little reflection will enable you to form a clear idea of the situation of the sun, the moon, and the earth, at the time of a solar eclipse. first, suppose the conjunction to take place at the node; that is, imagine the moon to come _directly_ between the earth and the sun, as she will of course do, if she comes between the earth and the sun the moment she is crossing the ecliptic; for then the three bodies will all lie in one and the same straight line. but when the moon is in the ecliptic, her shadow, or at least the axis, or central line, of the shadow, must coincide with the line that joins the centres of the sun and earth, and reach along the plane of the ecliptic towards the earth. the moon's shadow, at her average distance from the earth, is just about long enough to reach the surface of the earth; but when the moon, at the new, is in her apogee, or at her greatest distance from the earth, the shadow is not long enough to reach the earth. on the contrary, when the moon is nearer to us than her average distance, her shadow is long enough to reach beyond the earth, extending, when the moon is in her perigee, more than fourteen thousand miles beyond the centre of the earth. now, as during the eclipse the moon moves nearly in the plane of the ecliptic, her shadow which accompanies her must also move nearly in the same plane, and must therefore traverse the earth across its central regions, along the terrestrial ecliptic, since this is nothing more than the intersection of the plane of the celestial ecliptic with the earth's surface. the motion of the earth, too, on its axis, in the same direction, will carry a place along with the shadow, though with a less velocity by more than one half; so that the actual velocity of the shadow, in respect to places over which it passes on the earth, will only equal the difference between its own rate and that of the places, as they are carried forward in the diurnal revolution. we have thus far supposed that the moon comes to her conjunction precisely at the node, or at the moment when she is crossing the ecliptic. but, secondly, suppose she is on the north side of the ecliptic at the time of conjunction, and moving towards her descending node, and that the conjunction takes place as far from the node as an eclipse can happen. the shadow will not fall in the plane of the ecliptic, but a little northward of it, so as just to graze the earth near the pole of the ecliptic. the nearer the conjunction comes to the node, the further the shadow will fall from the polar towards the equatorial regions. in a solar eclipse, the shadow of the moon travels over a portion of the earth, as the shadow of a small cloud, seen from an eminence in a clear day, rides along over hills and plains. let us imagine ourselves standing on the moon; then we shall see the earth partially eclipsed by the moon's shadow, in the same manner as we now see the moon eclipsed by the shadow of the earth; and we might calculate the various circumstances of the eclipse,--its commencement, duration, and quantity,--in the same manner as we calculate these elements in an eclipse of the moon, as seen from the earth. but although the general characters of a solar eclipse might be investigated on these principles, so far as respects the earth at large, yet, as the appearances of the same eclipse of the sun are very different at different places on the earth's surface, it is necessary to calculate its peculiar aspects for each place separately, a circumstance which makes the calculation of a solar eclipse much more complicated and tedious than that of an eclipse of the moon. the moon, when she enters the shadow of the earth, is deprived of the light of the part immersed, and the effect upon its appearance is the same as though that part were painted black, in which case it would be black alike to all places where the moon was above the horizon. but it not so with a solar eclipse. we do not see this by the shadow cast on the earth, as we should do, if we stood on the moon, but by the interposition of the moon between us and the sun; and the sun may be hidden from one observer, while he is in full view of another only a few miles distant. thus, a small insulated cloud sailing in a clear sky will, for a few moments, hide the sun from us, and from a certain space near us, while all the region around is illuminated. but although the analogy between the motions of the shadow of a small cloud and of the moon in a solar eclipse holds good in many particulars, yet the velocity of the lunar shadow is far greater than that of the cloud, being no less than two thousand two hundred and eighty miles per hour. the moon's shadow can never cover a space on the earth more than one hundred and seventy miles broad, and the space actually covered commonly falls much short of that. the portion of the earth's surface ever covered by the moon's penumbra is about four thousand three hundred and ninety-three miles. the apparent diameter of the moon varies materially at different times, being greatest when the moon is nearest to us, and least when she is furthest off; while the sun's apparent dimensions remain nearly the same. when the moon is at her average distance from the earth, she is just about large enough to cover the sun's disk; consequently, if, in a central eclipse of the sun, the moon is at her mean distance, she covers the sun but for an instant, producing only a momentary eclipse. if she is nearer than her average distance, then the eclipse may continue total some time, though never more than eight minutes, and seldom so long as that; but if she is further off than usual, or towards her apogee, then she is not large enough to cover the whole solar disk, but we see a ring of the sun encircling the moon, constituting an _annular eclipse_, as seen in fig. . even the elevation of the moon above the horizon will sometimes sensibly affect the dimensions of the eclipse. you will recollect that the moon is nearer to us when on the meridian than when in the horizon by nearly four thousand miles, or by nearly the radius of the earth; and consequently, her apparent diameter is largest when on the meridian. the difference is so considerable, that the same eclipse will appear total to a spectator who views it near his meridian, while, at the same moment, it appears annular to one who has the moon near his horizon. an annular eclipse may last, at most, twelve minutes and twenty-four seconds. [illustration fig. .] eclipses of the sun are more frequent than those of the moon. yet lunar eclipses being visible to every part of the terrestrial hemisphere opposite to the sun, while those of the sun are visible only to a small portion of the hemisphere on which the moon's shadow falls, it happens that, for any particular place on the earth, lunar eclipses are more frequently visible than solar. in any year, the number of eclipses of both luminaries cannot be less than two nor more than seven: the most usual number is four, and it is very rare to have more than six. a total eclipse of the moon frequently happens at the next full moon after an eclipse of the sun. for since, in a solar eclipse, the sun is at or near one of the moon's nodes,--that is, is projected to the place in the sky where the moon crosses the ecliptic,--the earth's shadow, which is of course directly opposite to the sun, must be at or near the other node, and may not have passed too far from the node before the moon comes round to the opposition and overtakes it. in total eclipses of the sun, there has sometimes been observed a remarkable radiation of light from the margin of the sun, which is thought to be owing to the zodiacal light, which is of such dimensions as to extend far beyond the solar orb. a striking appearance of this kind was exhibited in the total eclipse of the sun which occurred in june, . a total eclipse of the sun is one of the most sublime and impressive phenomena of nature. among barbarous tribes it is ever contemplated with fear and astonishment, and as strongly indicative of the displeasure of the gods. two ancient nations, the lydians and medes, alluded to before, who were engaged in a bloody war, about six hundred years before christ, were smitten with such awe, on the appearance of a total eclipse of the sun, just on the eve of a battle, that they threw down their arms, and made peace. when columbus first discovered america, and was in danger of hostility from the natives, he awed them into submission by telling them that the sun would be darkened on a certain day, in token of the anger of the gods at them, for their treatment of him. among cultivated nations, a total eclipse of the sun is recognised, from the exactness with which the time of occurrence and the various appearances answer to the prediction, as affording one of the proudest triumphs of astronomy. by astronomers themselves, it is of course viewed with the highest interest, not only as verifying their calculations, but as contributing to establish, beyond all doubt, the certainty of those grand laws, the truth of which is involved in the result. i had the good fortune to witness the total eclipse of the sun of june, , which was one of the most remarkable on record. to the wondering gaze of childhood it presented a spectacle that can never be forgotten. a bright and beautiful morning inspired universal joy, for the sky was entirely cloudless. every one was busily occupied in preparing smoked glass, in readiness for the great sight, which was to be first seen about ten o'clock. a thrill of mingled wonder and delight struck every mind when, at the appointed moment, a little black indentation appeared on the limb of the sun. this gradually expanded, covering more and more of the solar disk, until an increasing gloom was spread over the face of nature; and when the sun was wholly lost, near mid-day, a feeling of horror pervaded almost every beholder. the darkness was wholly unlike that of twilight or night. a thick curtain, very different from clouds, hung upon the face of the sky, producing a strange and indescribably gloomy appearance, which was reflected from all things on the earth, in hues equally strange and unnatural. some of the planets, and the largest of the fixed stars, shone out through the gloom, yet with their usual brightness. the temperature of the air rapidly declined, and so sudden a chill came over the earth, that many persons caught severe colds from their exposure. even the animal tribes exhibited tokens of fear and agitation. birds, especially, fluttered and flew swiftly about, and domestic fowls went to rest. indeed, the word _eclipse_ is derived from a greek word, (= ekleipsis=, _ekleipsis_,) which signifies to fail, to faint or swoon away; since the moon, at the period of her greatest brightness, falling into the shadow of the earth, was imagined by the ancients to sicken and swoon, as if she were going to die. by some very ancient nations she was supposed, at such times, to be in pain; and, in order to relieve her fancied distress, they lifted torches high in the atmosphere, blew horns and trumpets, beat upon brazen vessels, and even, after the eclipse was over, they offered sacrifices to the moon. the opinion also extensively prevailed, that it was in the power of witches, by their spells and charms, not only to darken the moon, but to bring her down from her orbit, and to compel her to shed her baleful influences upon the earth. in solar eclipses, also, especially when total, the sun was supposed to turn away his face in abhorrence of some atrocious crime, that either had been perpetrated or was about to be perpetrated, and to threaten mankind with everlasting night, and the destruction of the world. to such superstitions milton alludes, in the passage which i have taken for the motto of this letter. the chinese, who, from a very high period of antiquity, have been great observers of eclipses, although they did not take much notice of those of the moon, regarded eclipses of the sun in general as unfortunate, but especially such as occurred on the first day of the year. these were thought to forebode the greatest calamities to the emperor, who on such occasions did not receive the usual compliments of the season. when, from the predictions of their astronomers, an eclipse of the sun was expected, they made great preparation at court for observing it; and as soon as it commenced, a blind man beat a drum, a great concourse assembled, and the mandarins, or nobility, appeared in state. letter xix. longitude.--tides. "first in his east, the glorious lamp was seen, regent of day, and all the horizon round invested with bright rays, jocund to run his _longitude_ through heaven's high road; the gray dawn and the pleiades before him danced, shedding sweet influence."--_milton._ the ancients studied astronomy chiefly as subsidiary to astrology, with the vain hope of thus penetrating the veil of futurity, and reading their destinies among the stars. the moderns, on the other hand, have in view, as the great practical object of this study, the perfecting of the art of navigation. when we reflect on the vast interests embarked on the ocean, both of property and life, and upon the immense benefits that accrue to society from a safe and speedy intercourse between the different nations of the earth, we cannot but see that whatever tends to enable the mariner to find his way on the pathless ocean, and to secure him against its multiplied dangers, must confer a signal benefit on society. in ancient times, to venture out of sight of land was deemed an act of extreme audacity; and horace, the roman poet, pronounces him who first ventured to trust his frail bark to the stormy ocean, endued with a heart of oak, and girt with triple folds of brass. but now, the navigator who fully avails himself of all the resources of science, and especially of astronomy, may launch fearlessly on the deep, and almost bid defiance to rocks and tempests. by enabling the navigator to find his place on the ocean with almost absolute precision, however he may have been driven about by the winds, and however long he may have been out of sight of land, astronomers must be held as great benefactors to all who commit either their lives or their fortunes to the sea. nor have they secured to the art of navigation such benefits without incredible study and toil, in watching the motions of the heavenly bodies, in investigating the laws by which their movements are governed, and in reducing all their discoveries to a form easily available to the navigator, so that, by some simple observation on one or two of the heavenly bodies, with instruments which the astronomer has invented, and prepared for his use, and by looking out a few numbers in tables which have been compiled for him, with immense labor, he may ascertain the exact place he occupies on the surface of the globe, thousands of miles from land. the situation of any place is known by its latitude and longitude. as charts of every ocean and sea are furnished to the sailor, in which are laid down the latitudes and longitudes of every point of land, whether on the shores of islands or the main, he has, therefore, only to ascertain his latitude and longitude at any particular place on the ocean, in order to find where he is, with respect to the nearest point of land, although this may be, and may always have been, entirely out of sight to him. to determine the _latitude_ of a place is comparatively an easy matter, whenever we can see either the sun or the stars. the distance of the sun from the zenith, when on the meridian, on a given day of the year, (which distance we may easily take with the sextant,) enables us, with the aid of the tables, to find the latitude of the place; or, by taking the altitude of the north star, we at once obtain the latitude. the _longitude_ of a place may be found by any method, by which we may ascertain how much its time of day differs from that of greenwich at the same moment. a place that lies eastward of another comes to the meridian an hour earlier for every fifteen degrees of longitude, and of course has the hour of the day so much in advance of the other, so that it counts one o'clock when the other place counts twelve. on the other hand, a place lying westward of another comes to the meridian later by one hour for every fifteen degrees, so that it counts only eleven o'clock when the other place counts twelve. keeping these principles in view, it is easy to see that a comparison of the difference of time between two places at the same moment, allowing fifteen degrees for an hour, sixty minutes for every four minutes of time, and sixty seconds for every four seconds of time, affords us an accurate mode of finding the difference of longitude between the two places. this comparison may be made by means of a chronometer, or from solar or lunar eclipses, or by what is called the lunar method of finding the longitude. _chronometers_ are distinguished from clocks, by being regulated by means of a balance-wheel instead of a pendulum. a watch, therefore, comes under the general definition of a chronometer; but the name is more commonly applied to larger timepieces, too large to be carried about the person, and constructed with the greatest possible accuracy, with special reference to finding the longitude. suppose, then, we are furnished with a chronometer set to greenwich time. we arrive at new york, for example, and compare it with the time there. we find it is five hours in advance of the new-york time, indicating five o'clock, p.m., when it is noon at new york. hence we find that the longitude of new york is Ã� = degrees.[ ] the time at new york, or any individual place, can be known by observations with the transit-instrument, which gives us the precise moment when the sun is on the meridian. it would not be necessary to resort to greenwich, for the purpose of setting our chronometer to greenwich time, as it might be set at any place whose longitude is known, having been previously determined. thus, if we know that the longitude of a certain place is exactly sixty degrees east of greenwich, we have only to set our chronometer four hours behind the time at that place, and it will be regulated to greenwich time. hence it is a matter of the greatest importance to navigation, that the longitude of numerous ports, in different parts of the earth, should be accurately determined, so that when a ship arrives at any such port, it may have the means of setting or verifying its chronometer. this method of taking the longitude seems so easy, that you will perhaps ask, why it is not sufficient for all purposes, and accordingly, why it does not supersede the move complicated and laborious methods? why every sailor does not provide himself with a chronometer, instead of finding his longitude at sea by tedious and oft-repeated calculations, as he is in the habit of doing? i answer, it is only in a few extraordinary cases that chronometers have been constructed of such accuracy as to afford results as exact as those obtained by the other methods, to be described shortly; and instruments of such perfection are too expensive for general use among sailors. indeed, the more common chronometers cost too much to come within the means of a great majority of sea-faring men. moreover, by being transported from place to place, chronometers are liable to change their _rate_. by the rate of any timepiece is meant its deviation from perfect accuracy. thus, if a clock should gain one second per day, one day with another, and we should find it impossible to bring it nearer to the truth, we may reckon this as its rate, and allow for it in our estimate of the time of any particular observation. if the error was not uniform, but sometimes greater and sometimes less than one second per day, then the amount of such deviation is called its "variation from its mean rate." i introduce these minute statements, (which are more precise than i usually deem necessary,) to show you to what an astonishing degree of accuracy chronometers have in some instances been brought. they have been carried from london to baffin's bay, and brought back, after a three years' voyage, and found to have varied from their mean rate, during the whole time, only a second or two, while the extreme variation of several chronometers, tried at the royal observatory at greenwich, never exceeded a second and a half. could chronometers always be depended on to such a degree of accuracy as this, we should hardly desire any thing better for determining the longitude of different places on the earth. a recent determination of the longitude of the city hall in new york, by means of three chronometers, sent out from london expressly for that purpose, did not differ from the longitude as found by a solar eclipse (which is one of the best methods) but a second and a quarter. _eclipses of the sun and moon_ furnish the means of ascertaining the longitude of a place, because the entrance of the moon into the earth's shadow in a lunar eclipse, and the entrance of the moon upon the disk of the sun in a solar eclipse, are severally examples of one of those instantaneous occurrences in the heavens, which afford the means of comparing the times of different places, and of thus determining their differences of longitude. thus, if the commencement of a lunar eclipse was seen at one place an hour sooner than at another, the two places would be fifteen degrees apart, in longitude; and if the longitude of one of the places was known, that of the other would become known also. the exact instant of the moon's entering into the shadow of the earth, however, cannot be determined with very great precision, since the moon, in passing through the earth's penumbra, loses its light gradually, so that the moment when it leaves the penumbra and enters into the shadow cannot be very accurately defined. the first contact of the moon with the sun's disk, in a solar eclipse, or the moment of leaving it,--that is, the beginning and end of the eclipse,--are instants that can be determined with much precision, and accordingly they are much relied on for an accurate determination of the longitude. but, on account of the complicated and laborious nature of the calculation of the longitude from an eclipse of the sun, (since the beginning and end are not seen at different places, at the same moment,) this method of finding the longitude is not adapted to common use, nor available at sea. it is useful, however, for determining the longitude of fixed observatories. the _lunar method of finding the longitude_ is the most refined and accurate of all the modes practised at sea. the motion of the moon through the heavens is so rapid, that she perceptibly alters her distance from any star every minute; consequently, the moment when that distance is a certain number of degrees and minutes is one of those instantaneous events, which may be taken advantage of for comparing the times of different places, and thus determining their difference of longitude. now, in a work called the 'nautical almanac,' printed in london, annually, for the use of navigators, the distance of the moon from the sun by day, or from known fixed stars by night, for every day and night in the year, is calculated beforehand. if, therefore, a sailor wishes to ascertain his longitude, he may take with his sextant the distance of the moon from one of these stars at any time,--suppose nine o'clock, at night,--and then turn to the 'nautical almanac,' and see _what time it was at greenwich_ when the distance between the moon and that star was the same. let it be twelve o'clock, or three hours in advance of his time: his longitude, of course, is forty-five degrees west. this method requires more skill and accuracy than are possessed by the majority of seafaring men; but, when practised with the requisite degree of skill, its results are very satisfactory. captain basil hall, one of the most scientific commanders in the british navy, relates the following incident, to show the excellence of this method. he sailed from san blas, on the west coast of mexico, and, after a voyage of eight thousand miles, occupying eighty-nine days, arrived off rio de janeiro, having, in this interval, passed through the pacific ocean, rounded cape horn, and crossed the south atlantic, without making any land, or even seeing a single sail, with the exception of an american whaler off cape horn. when within a week's sail of rio, he set seriously about determining, by lunar observations, the precise line of the ship's course, and its situation at a determinate moment; and having ascertained this within from five to ten miles, ran the rest of the way by those more ready and compendious methods, known to navigators, which can be safely employed for short trips between one known point and another, but which cannot be trusted in long voyages, where the moon is the only sure guide. they steered towards rio janeiro for some days after taking the lunars, and, having arrived within fifteen or twenty miles of the coast, they hove to, at four in the morning, till the day should break, and then bore up, proceeding cautiously, on account of a thick fog which enveloped them. as this cleared away, they had the satisfaction of seeing the great sugar-loaf rock, which stands on one side of the harbor's mouth, so nearly right ahead, that they had not to alter their course above a point, in order to hit the entrance of the harbor. this was the first land they had seen for three months, after crossing so many seas, and being set backwards and forwards by innumerable currents and foul winds. the effect on all on board was electric; and the admiration of the sailors was unbounded. indeed, what could be more admirable than that a man on the deck of a vessel, by measuring the distance between the moon and a star, with a little instrument which he held in his hand, could determine his exact place on the earth's surface in the midst of a vast ocean, after having traversed it in all directions, for three months, crossing his track many times, and all the while out of sight of land? the lunar method of finding the longitude could never have been susceptible of sufficient accuracy, had not the motions of the moon, with all their irregularities, been studied and investigated by the most laborious and profound researches. hence newton, while wrapt in those meditations which, to superficial minds, would perhaps have appeared rather curious than useful, inasmuch as they respected distant bodies of the universe which seemed to have little connexion with the affairs of this world, was laboring night and day for the benefit of the sailor and the merchant. he was guiding the vessel of the one, and securing the merchandise of the other; and thus he contributed a large share to promote the happiness of his fellow-men, not only in exalting the powers of the human intellect, but also in preserving the lives and fortunes of those engaged in navigation and commerce. principles in science are rules in art; and the philosopher who is engaged in the investigation of these principles, although his pursuits may be thought less practically useful than those of the artisan who carries out those principles into real life, yet, without the knowledge of the principles, the rules would have never been known. studies, therefore, the most abstruse, are, when viewed as furnishing rules to act, often productive of the highest practical utility. since the _tides_ are occasioned by the influence of the sun and moon, i will conclude this letter with a few remarks on this curious phenomenon. by the tides are meant the alternate rising and falling of the waters of the ocean. its greatest and least elevations are called _high and low water_; its rising and falling are called _flood and ebb_; and the extraordinary high and low tides that occur twice every month are called _spring and neap tides_. it is high or low tide on opposite sides of the globe at the same time. if, for example, we have high water at noon, it is also high water to those who live on the meridian below us, where it is midnight. in like manner, low water occurs simultaneously on opposite sides of the meridian. the average amount of the tides for the whole globe is about two and a half feet; but their actual height at different places is very various, sometimes being scarcely perceptible, and sometimes rising to sixty or seventy feet. at the same place, also, the phenomena of the tides are very different at different times. in the bay of fundy, where the tide rises seventy feet, it comes in a mighty wave, seen thirty miles off, and roaring with a loud noise. at the mouth of the severn, in england, the flood comes up in one head about ten feet high, bringing certain destruction to any small craft that has been unfortunately left by the ebbing waters on the flats and as it passes the mouth of the avon, it sends up that small river a vast body of water, rising, at bristol, forty or fifty feet. tides are caused by the unequal attractions of the sun and moon upon different parts of the earth. suppose the projectile force by which the earth is carried forward in her orbit to be suspended, and the earth to fall towards one of these bodies,--the moon, for example,--in consequence of their mutual attraction. then, if all parts of the earth fell equally towards the moon, no derangement of its different parts would result, any more than of the particles of a drop of water, in its descent to the ground. but if one part fell faster than another, the different portions would evidently be separated from each other. now, this is precisely what takes place with respect to the earth, in its fall towards the moon. the portions of the earth in the hemisphere next to the moon, on account of being nearer to the centre of attraction, fall faster than those in the opposite hemisphere, and consequently leave them behind. the solid earth, on account of its cohesion, cannot obey this impulse, since all its different portions constitute one mass, which is acted on in the same manner as though it were all collected in the centre; but the waters on the surface, moving freely under this impulse, endeavor to desert the solid mass and fall towards the moon. for a similar reason, the waters in the opposite hemisphere, falling less towards the moon than the solid earth does, are left behind, or appear to rise. [illustration fig. .] but if the moon draws the waters of the earth into an oval form towards herself, raising them simultaneously on the opposite sides of the earth, they must obviously be drawn away from the intermediate parts of the earth, where it must at the same time be low water. thus, in fig. , the moon, m, raises the waters beneath itself at z and n, at which places it is high water, but at the same time depresses the waters at h and r, at which places it is low water. hence, the interval between the high and low tide, on successive days, is about fifty minutes, corresponding to the progress of the moon in her orbit from west to east, which causes her to come to the meridian about fifty minutes later every day. there occurs, however, an intermediate tide, when the moon is on the lower meridian, so that the interval between two high tides is about twelve hours, and twenty-five minutes. were it not for the impediments which prevent the force from producing its full effects, we might expect to see the great tide-wave, as the elevated crest is called, always directly beneath the moon, attending it regularly around the globe. but the inertia of the waters prevents their instantly obeying the moon's attraction, and the friction of the waters on the bottom of the ocean still further retards its progress. it is not, therefore, until several hours (differing at different places) after the moon has passed the meridian of a place, that it is high tide at that place. the _sun_ has an action similar to that of the moon, but only _one third_ as great. on account of the great mass of the sun, compared with that of the moon, we might suppose that his action in raising the tides would be greater than the moon's; but the nearness of the moon to the earth more than compensates for the sun's greater quantity of matter. as, however, wrong views are frequently entertained on this subject, let us endeavor to form a correct idea of the advantage which the moon derives from her proximity. it is not that her actual amount of attraction is thus rendered greater than that of the sun; but it is that her attraction for the _different parts_ of the earth is very unequal, while that of the sun is nearly uniform. it is the _inequality_ of this action, and not the absolute force, that produces the tides. the sun being ninety-five millions of miles from the earth, while the diameter of the earth is only one twelve thousandth part of this distance, the effects of the sun's attraction will be nearly the same on all parts of the earth, and therefore will not, as was explained of the moon, tend to separate the waters from the earth on the nearest side, or the earth from the waters on the remotest side, but in a degree proportionally smaller. but the diameter of the earth is one thirtieth the distance of the moon, and therefore the moon acts with considerably greater power on one part of the earth than on another. as the sun and moon both contribute to produce the tides, and as they sometimes act together and sometimes in opposition to each other, so corresponding variations occur in the height of the tide. the _spring tides_, or those which rise to an unusual height twice a month, are produced by the sun and moon's acting together; and the _neap tides_, or those which are unusually low twice a month, are produced by the sun and moon's acting in opposition to each other. the spring tides occur at the syzygies: the neap tides at the quadratures. at the time of new moon, the sun and moon both being on the same side of the earth, and acting upon it in the same line, their actions conspire, and the sun may be considered as adding so much to the force of the moon. we have already seen how the moon contributes to raise a tide on the opposite side of the earth. but the sun, as well as the moon, raises its own tide-wave, which at new moon coincides with the lunar tide-wave. this will be plain on inspecting the diagram, fig. , on page , where s represents the sun, c, the moon in conjunction, o, the moon in opposition, and z, n, the tide-wave. since the sun and moon severally raise a tide-wave, and the two here coincide, it is evident that a peculiarly high tide must occur when the two bodies are in conjunction, or at new moon. at full moon, also, the two luminaries conspire in the same way to raise the tide; for we must recollect that each body contributes to raise a tide on the opposite side. thus, when the sun is at s and the moon at o, the sun draws the waters on the side next to it away from the earth, and the moon draws the earth away from the waters on that side; their united actions, therefore, conspire, and an unusually high tide is the result. on the side next to o, the two forces likewise conspire: for while the moon draws the waters away from the earth, the sun draws the earth away from the waters. in both cases an unusually low tide is produced; for the more the water is elevated at z and n, the more it will be depressed at h and r, the places of low tide. [illustration fig. .] twice a month, also, namely, at the quadratures of the moon, the tides neither rise so high nor fall so low as at other times, because then the sun and moon act against each other. thus, in fig. , while f tends to raise the water at z, s tends to depress it, and consequently the high tide is less than usual. again, while f tends to depress the water at r, s tends to elevate it, and therefore the low tide is less than usual. hence the difference between high and low water is only half as great at neap as at spring tide. in the diagrams, the elevation and depression of the waters is represented, for the sake of illustration, as far greater than it really is; for you must recollect that the average height of the tides for the whole globe is only about two and a half feet, a quantity so small, in comparison with the diameter of the earth, that were the due proportions preserved in the figures, the effect would be wholly insensible. [illustration fig. .] the variations of distance in the sun are not great enough to influence the tides very materially, but the variations in the moon's distances have a striking effect. the tides which happen, when the moon is in perigee, are considerably greater than when she is in apogee; and if she happens to be in perigee at the time of the syzygies, the spring tides are unusually high. the motion of the tide-wave is not a _progressive_ motion, but a mere undulation, and is to be carefully distinguished from the currents to which it gives rise. if the ocean completely covered the earth, the sun and moon being in the equator, the tide-wave would travel at the same rate as the earth revolves on its axis. indeed, the correct way of conceiving of the tide-wave, is to consider the moon at rest, and the earth, in its rotation from west to east, as bringing successive portions of water under the moon, which portions being elevated successively, at the same rate as the earth revolves on its axis, have a relative motion westward, at the same rate. the tides of rivers, narrow bays, and shores far from the main body of the ocean, are not produced in those places by the direct action of the sun and moon, but are subordinate waves propagated from the great tide-wave, and are called _derivative_ tides, while those raised directly by the sun and moon are called _primitive_ tides. [illustration fig. .] the velocity with which the tide moves will depend on various circumstances, but principally on the depth, and probably on the regularity, of the channel. if the depth is nearly uniform the tides will be regular; but if some parts of the channel are deep while others are shallow, the waters will be detained by the greater friction of the shallow places, and the tides will be irregular. the direction, also, of the derivative tide may be totally different from that of the primitive. thus, in fig. , if the great tide-wave, moving from east to west, is represented by the lines , , , , the derivative tide, which is propagated up a river or bay, will be represented by the lines , , , , . advancing faster in the channel than next the bank, the tides will lag behind towards the shores, and the tide-wave will take the form of curves, as represented in the diagram. on account of the retarding influence of shoals, and an uneven, indented coast, the tide-wave travels more slowly along the shores of an island than in the neighboring sea, assuming convex figures at a little distance from the island, and on opposite sides of it. these convex lines sometimes meet, and become blended in such a way, as to create singular anomalies in a sea much broken by islands, as well as on coasts indented with numerous bays and rivers. peculiar phenomena are also produced, when the tide flows in at opposite extremities of a reef or island, as into the two opposite ends of long-island sound. in certain cases, a tide-wave is forced into a narrow arm of the sea, and produces very remarkable tides. the tides of the bay of fundy (the highest in the world) are ascribed to this cause. the tides on the coast of north america are derived from the great tide-wave of the south atlantic, which runs steadily northward along the coast to the mouth of the bay of fundy, where it meets the northern tide-wave flowing in the opposite direction. this accumulated wave being forced into the narrow space occupied by the bay, produces the remarkable tide of that place. the largest lakes and inland seas have no perceptible tides. this is asserted by all writers respecting the caspian and euxine; and the same is found to be true of the largest of the north-american lakes, lake superior. although these several tracts of water appear large, when taken by themselves, yet they occupy but small portions of the surface of the globe, as will appear evident from the delineation of them on the artificial globe. now, we must recollect that the primitive tides are produced by the _unequal_ action of the sun and moon upon the different parts of the earth; and that it is only at points whose distance from each other bears a considerable ratio to the whole distance of the sun or moon, that the inequality of action becomes manifest. the space required to make the effect sensible is larger than either of these tracts of water. it is obvious, also, that they have no opportunity to be subject to a derivative tide. although all must admit that the tides have _some connexion_ with the sun and the moon, yet there are so many seeming anomalies, which at first appear irreconcilable with the theory of gravitation, that some are unwilling to admit the explanation given by this theory. thus, the height of the tide is so various, that at some places on the earth there is scarcely any tide at all, while at other places it rises to seventy feet. the time of occurrence is also at many places wholly unconformable to the motions of the moon, as is required by the theory, being low water where it should be high water; or, instead of appearing just beneath the moon, as the theory would lead us to expect, following it at the distance of six, ten, or even fifteen, hours; and finally, the moon sometimes appears to have no part at all in producing the tide, but it happens uniformly at noon and midnight, (as is said to be the case at the society islands,) and therefore seems wholly dependent on the sun. notwithstanding these seeming inconsistencies with the law of universal gravitation, to which the explanation of the tides is referred, the correspondence of the tides to the motions of the sun and moon, in obedience to the law of attraction, is in general such as to warrant the application of that law to them, while in a great majority of the cases which appear to be exceptions to the operation of that law, local causes and impediments have been discovered, which modified or overruled the uniform operation of the law of gravitation. thus it does not disprove the reality of the existence of a force which carries bodies near the surface of the earth towards its centre, that we see them sometimes compelled, by the operation of local causes, to move in the opposite direction. a ball shot from a cannon is still subject to the law of gravitation, although, for a certain time, in obedience to the impulse given it, it may proceed in a line contrary to that in which gravity alone would carry it. the fact that water may be made to run up hill does not disprove the fact that it usually runs down hill by the force of gravity, or that it is still subject to this force, although, from the action of modifying or superior forces, it may be proceeding in a direction contrary to that given by gravity. indeed, those who have studied the doctrine of the tides most profoundly consider them as affording a striking and palpable exhibition of the truth of the doctrine of universal gravitation. footnote: [ ] the exact longitude of the city hall, in the city of new york, is h. m. . s. letter xx. planets.--mercury and venus. "first, mercury, amidst full tides of light, rolls next the sun, through his small circle bright; our earth would blaze beneath so fierce a ray, and all its marble mountains melt away. fair venus next fulfils her larger round, with softer beams, and milder glory crowned; friend to mankind, she glitters from afar, now the bright evening, now the morning, star."--_baker._ there is no study in which more is to be hoped for from a lucid arrangement, than in the study of astronomy. some subjects involved in this study appear very difficult and perplexing to the learner, before he has fully learned the doctrine of the sphere, and gained a certain familiarity with astronomical doctrines, which would seem very easy to him after he had made such attainments. such an order ought to be observed, as shall bring out the facts and doctrines of the science just in the place where the mind of the learner is prepared to receive them. some writers on astronomy introduce their readers at once to the most perplexing part of the whole subject,--the planetary motions. i have thought a different course advisable, and have therefore commenced these letters with an account of those bodies which are most familiarly known to us, the earth, the sun, and the moon. in connexion with the earth, we are able to acquire a good knowledge of the artificial divisions and points of reference that are established on the earth and in the heavens, constituting the doctrine of the sphere. you thus became familiar with many terms and definitions which are used in astronomy. these ought to be always very clearly borne in mind; and if you now meet with any term, the definition of which you have either partially or wholly forgotten, let me strongly recommend to you, to turn back and review it, until it becomes as familiar to you as household words. indeed, you will find it much to your advantage to go back frequently, and reiterate the earlier parts of the subject, before you advance to subjects of a more intricate nature. if this process should appear to you a little tedious, still you will find yourself fully compensated by the clear light in which all the succeeding subjects will appear. this clear and distinct perception of the ground we have been over shows us just where we are on our journey, and helps us to find the remainder of the way with far greater ease than we could otherwise do. i do not, however, propose by any devices to relieve you from the trouble of thinking. those who are not willing to incur this trouble can never learn much of astronomy. in introducing you to the planets, (which next claim our attention,) i will, in the first place, endeavor to convey to you some clear views of these bodies individually, and afterwards help you to form as correct a notion as possible of their motions and mutual relations. the name _planet_ is derived from a greek word, (= planêtês=, _planetes_,) which signifies a _wanderer_, and is applied to this class of bodies, because they shift their positions in the heavens, whereas the fixed stars constantly maintain the same places with respect to each other. the planets known from a high antiquity are, mercury, venus, earth, mars, jupiter, and saturn. to these, in , was added uranus, (or _herschel_, as it is sometimes called, from the name of its discoverer;) and, as late as the commencement of the present century, four more were added, namely, ceres, pallas, juno, and vesta. these bodies are designated by the following characters: . mercury, [planet: mercury] . venus, [planet: venus] . earth, [planet: earth] . mars, [planet: mars] . vesta, [planet: vesta] . juno, [planet: juno] . ceres, [planet: ceres] . pallas, [planet: pallas] . jupiter, [planet: jupiter] . saturn, [planet: saturn] . uranus, [planet: uranus] the foregoing are called the _primary_ planets. several of these have one or more attendants, or satellites, which revolve around them as they revolve around the sun. the earth has one satellite, namely, the moon; jupiter has four; saturn, seven; and uranus, six. these bodies are also planets, but, in distinction from the others, they are called _secondary_ planets. hence, the whole number of planets are twenty-nine, namely, eleven primary, and eighteen secondary, planets. you need never look for a planet either very far in the north or very far in the south, since they are always near the ecliptic. mercury, which deviates furthest from that great circle, never is seen more than seven degrees from it; and you will hardly ever see one of the planets so far from it as this, but they all pursue nearly the same great route through the skies, in their revolutions around the sun. the new planets, however, make wider excursions from the plane of the ecliptic, amounting, in the case of pallas, to thirty-four and a half degrees. mercury and venus are called _inferior_ planets, because they have their orbits nearer to the sun than that of the earth; while all the others, being more distant from the sun than the earth, are called _superior_ planets. the planets present great diversities among themselves, in respect to distance from the sun, magnitude, time of revolution, and density. they differ, also, in regard to satellites, of which, as we have seen, three have respectively four, six, and seven, while more than half have none at all. it will aid the memory, and render our view of the planetary system more clear and comprehensive, if we classify, as far as possible, the various particulars comprehended under the foregoing heads. as you have had an opportunity, in preceding letters, of learning something respecting the means which astronomers have of ascertaining the distances and magnitudes of these bodies, you will not doubt that they are really as great as they are represented; but when you attempt to conceive of spaces so vast, you will find the mind wholly inadequate to the task. it is indeed but a comparatively small space that we can fully comprehend at one grasp. still, by continual and repeated efforts, we may, from time to time, somewhat enlarge the boundaries of our mental vision. let us begin with some known and familiar space, as the distance between two places we are accustomed to traverse. suppose this to be one hundred miles. taking this as our measure, let us apply it to some greater distance, as that across the atlantic ocean,--say three thousand miles. from this step we may advance to some faint conception of the diameter of the earth; and taking that as a new measure, we may apply it to such greater spaces as the distance of the planets from the sun. i hope you will make trial of this method on the following comparative statements respecting the planets. _distances from the sun, in miles._ . mercury, , , . venus, , , . earth, , , . mars, , , . vesta, , , . juno, } . ceres, } , , . pallas, } . jupiter, , , . saturn, , , . uranus, or herschel, , , the _dimensions_ of the planetary system are seen from this table to be vast, comprehending a circular space thirty-six hundred millions of miles in diameter. a rail-way car, travelling constantly at the rate of twenty miles an hour, would require more than twenty thousand years to cross the orbit of uranus. _magnitudes._ diam. in miles. . mercury, . venus, . earth, . mars, . ceres, . jupiter, , . saturn, , . uranus, , we remark here a great diversity in regard to magnitude,--a diversity which does not appear to be subject to any definite law. while venus, an inferior planet, is nine tenths as large as the earth, mars, a superior planet, is only one seventh, while jupiter is twelve hundred and eighty-one times as large. although several of the planets, when nearest to us, appear brilliant and large, when compared with most of the fixed stars, yet the angle which they subtend is very small,--that of venus, the greatest of all, never exceeding about one minute, which is less than one thirtieth the apparent diameter of the sun or moon. jupiter, also, by his superior brightness, sometimes makes a striking figure among the stars; yet his greatest apparent diameter is less than one fortieth that of the sun. _periodic times_. mercury revolves around the sun in nearly months. venus, " " " " - / " earth, " " " " year. mars, " " " " years. ceres, " " " " - / " jupiter, " " " " " saturn, " " " " " uranus, " " " " " from this view, it appears that the planets nearest the sun move most rapidly. thus, mercury performs nearly three hundred and fifty revolutions while uranus performs one. the apparent progress of the most distant planets around the sun is exceedingly slow. uranus advances only a little more than four degrees in a whole year; so that we find this planet occupying the same sign, and of course remaining nearly in the same part of the heavens, for several years in succession. after this comparative view of the planets in general, let us now look at them individually; and first, of the inferior planets, mercury and venus. mercury and venus, having their orbits so far within that of the earth, appear to us as attendants upon the sun. mercury never appears further from the sun than twenty-nine degrees, and seldom so far; and venus, never more than about forty-seven degrees. both planets, therefore, appear either in the west soon after sunset, or in the east a little before sunrise. in high latitudes, where the twilight is long, mercury can seldom be seen with the naked eye, and then only when its angular distance from the sun is greatest. copernicus, the great prussian astronomer, (who first distinctly established the order of the solar system, as at present received,) lamented, on his death-bed, that he had never been able to obtain a sight of mercury; and delambre, a distinguished astronomer of france, saw it but twice. in our latitude, however, we may see this planet for several evenings and mornings, if we will watch the time (as usually given in the almanac) when it is at its greatest elongations from the sun. it will not, however, remain long for our gaze, but will soon run back to the sun. the reason of this will be readily understood from the following diagram, fig. . let s represent the sun, e, the earth, and m, n, mercury at its greatest elongations from the sun, and o z p, a portion of the sky. then, since we refer all distant bodies to the same concave sphere of the heavens, it is evident that we should see the sun at z, and mercury at o, when at its greatest eastern elongation, and at p, when at its greatest western elongation; and while passing from m to n through q, it would appear to describe the arc o p; and while passing from n to m through r, it would appear to run back across the sun on the same arc. it is further evident that it would be visible only when at or near one of its greatest elongations; being at all other times so near the sun as to be lost in his light. [illustration fig. .] a planet is said to be in _conjunction_ with the sun when it is seen in the same part of the heavens with the sun. mercury and venus have each two conjunctions, the inferior and the superior conjunction. the _inferior conjunction_ is its position when in conjunction on the same side of the sun with the earth, as at q, in the figure; the _superior conjunction_ is its position when on the side of the sun most distant from the earth, as at r. the time which a planet occupies in making one entire circuit of the heavens, from any star, until it comes round to the same star again, is called its _sidereal revolution_. the period occupied by a planet between two successive conjunctions with the earth is called its _synodical revolution_. both the planet and the earth being in motion, the time of the synodical revolution of mercury or venus exceeds that of the sidereal; for when the planet comes round to the place where it before overtook the earth, it does not find the earth at that point, but far in advance of it. thus, let mercury come into inferior conjunction with the earth at c, fig. . in about eighty-eight days, the planet will come round to the same point again; but, mean-while, the earth has moved forward through the arc e e´, and will continue to move while the planet is moving more rapidly to overtake her; the case being analogous to that of the hour and minute hand of a clock. [illustration fig. .] the synodical period of mercury is one hundred and sixteen days, and that of venus five hundred and eighty-four days. the former is increased twenty-eight days, and the latter, three hundred and sixty days, by the motion of the earth; so that venus, after being in conjunction with the earth, goes more than twice round the sun before she comes into conjunction again. for, since the earth is likewise in motion, and moves more than half as fast as venus, by the time the latter has gone round and returned to the place where the two bodies were together, the earth is more than half way round, and continues moving, so that it will be a long time before venus comes up with it. the motion of an inferior planet is _direct_ in passing through its superior conjunction, and _retrograde_ in passing through its inferior conjunction. you will recollect that the motion of a heavenly body is said to be direct when it is in the order of the signs from west to east, and retrograde when it is contrary to the order of the signs, or from east to west. now venus, while going from b through d to a, (fig. ,) moves from west to east, and would appear to traverse the celestial vault b´ s´ a´, from right to left; but in passing from a through c to b, her course would be retrograde, returning on the same arc from left to right. if the earth were at rest, therefore, (and the sun, of course, at rest,) the inferior planets would appear to oscillate backwards and forwards across the sun. but it must be recollected that the earth is moving in the same direction with the planet, as respects the signs, but with a slower motion. this modifies the motions of the planet, accelerating it in the superior, and retarding it in the inferior, conjunction. thus, in fig. , venus, while moving through b d a, would seem to move in the heavens from b´ to a´, were the earth at rest; but, mean-while, the earth changes its position from e to e´, on which account the planet is not seen at a´, but at a´´, being accelerated by the arc a´ a´´, in consequence of the earth's motion. on the other hand, when the planet is passing through its inferior conjunction a c b, it appears to move backwards in the heavens from a´ to b´, if the earth is at rest, but from a´ to b´´, if the earth has in the mean time moved from e to e´, being retarded by the arc b´ b´´. although the motions of the earth have the effect to accelerate the planet in the superior conjunction, and to retard it in the inferior, yet, on account of the greater distance, the apparent motion of the planet is much slower in the superior than in the inferior conjunction, venus being the whole breadth of her orbit, or one hundred and thirty-six millions of miles further from us when at her greatest, than when at her least, distance, as is evident from fig. . when passing from the superior to the inferior conjunction, or from the inferior to the superior, through the greatest elongations, the inferior planets are _stationary_. thus, (fig. ,) when the planet is at a, the earth being at e, as the planet's motion is directly towards the spectator, he would constantly project it at the same point in the heavens, namely, a´; consequently, it would appear to stand still. or, when at its greatest elongation on the other side, at b, as its motion would be directly from the spectator, it would be seen constantly at b´. if the earth were at rest, the stationary points would be at the greatest elongations, as at a and b; but the earth itself is moving nearly at right angles to the planet's motion, which makes the planet appear to move in the opposite direction. its direct motion will therefore continue longer on the one side, and its retrograde motion longer on the other side, than would be the case, were it not for the motion of the earth. mercury, whose greatest angular distance from the sun is nearly twenty-nine degrees, is stationary at an elongation of from fifteen to twenty degrees; and venus, at about twenty-nine degrees, although her greatest elongation is about forty-seven degrees. mercury and venus exhibit to the telescope _phases_ similar to those of the moon. when on the side of their inferior conjunction, as from b to c through d, fig. , less than half their enlightened disk is turned towards us, and they appear horned, like the moon in her first and last quarters; and when on the side of the superior conjunction, as from c to b through a, more than half the enlightened disk is turned towards us, and they appear gibbous. at the moment of superior conjunction, the whole enlightened orb of the planet is turned towards the earth, and the appearance would be that of the full moon; but the planet is too near the sun to be commonly visible. [illustration fig. .] we should at first thought expect, that each of these planets would be largest and brightest near their inferior conjunction, being then so much nearer to us than at other times; but we must recollect that, when in this situation, only a small part of the enlightened disk is turned toward us. still, the period of greatest brilliancy cannot be when most of the illuminated side is turned towards us, for then, being at the superior conjunction, its light will be diminished, both by its great distance, and by its being so near the sun as to be partially lost in the twilight. hence, when venus is a little within her place of greatest elongation, about forty degrees from the sun, although less than half her disk is enlightened, yet, being comparatively near to us, and shining at a considerable altitude after the evening or before the morning twilight, she then appears in greatest splendor, and presents an object admired for its beauty in all ages. thus milton, "fairest of stars, last in the train of night, if better thou belong not to the dawn, sure pledge of day that crown'st the smiling morn with thy bright circlet." mercury and venus both _revolve on their axes_ in nearly the same time with the earth. the diurnal period of mercury is a little greater, and that of venus a little less, than twenty-four hours. these revolutions have been determined by means of some spot or mark seen by the telescope, as the revolution of the sun on his axis is ascertained by means of his spots. mercury owes most of its peculiarities to its proximity to the sun. its light and heat, derived from the sun, are estimated to be neatly seven times as great as on the earth, and the apparent magnitude of the sun to a spectator on mercury would be seven times greater than to us. hence the sun would present to an inhabitant of that planet, with eyes like ours, an object of insufferable brightness; and all objects on the surface would be arrayed in a light more glorious than we can well imagine. (see fig. .) the average heat on the greater portion of this planet would exceed that of boiling water, and therefore be incompatible with the existence both of an animal and a vegetable kingdom constituted like ours. the motion of mercury, in his revolution round the sun, is swifter than that of any other planet, being more than one hundred thousand miles every hour; whereas that of the earth is less than seventy thousand. eighteen hundred miles every minute,--crossing the atlantic ocean in less than two minutes,--this is a velocity of which we can form but a very inadequate conception, although, as we shall see hereafter, it is far less than comets sometimes exhibit. venus is regarded as the most beautiful of the planets, and is well known as the _morning and evening star_. the most ancient nations, indeed, did not recognise the morning and evening star as one and the same body, but supposed they were different planets, and accordingly gave them different names, calling the morning star lucifer, and the evening star hesperus. at her period of greatest splendor, venus casts a shadow, and is sometimes visible in broad daylight. her light is then estimated as equal to that of twenty stars of the first magnitude. in the equatorial regions of the earth, where the twilight is short, and venus, at her greatest elongation, appears very high above the horizon, her splendors are said to be far more conspicuous than in our latitude. [illustration fig. . apparent magnitudes of the sun, as seen from the different planets.] [illustration figures , , . venus and mars.] every eight years, venus forms her conjunction with the sun in the same part of the heavens. whatever appearances, therefore, arise from her position with respect to the earth and the sun, they are repeated every eight years, in nearly the same form. thus, every eight years, venus is remarkably conspicuous, so as to be visible in the day-time, being then most favorably situated, on several accounts; namely, being nearest the earth, and at the point in her orbit where she gives her greatest brilliancy, that is, a little within the place of greatest elongation. this is the period for obtaining fine telescopic views of venus, when she is seen with spots on her disk. thus two figures of the annexed diagram (fig. ) represent venus as seen near her inferior conjunction, and at the period of maximum brilliancy. the former situation is favorable for viewing her inequalities of surface, as indicated by the roughness of the line which separates the enlightened from the unenlightened part, (the _terminator_.) according to schroeter, a german astronomer, venus has mountains twenty-two miles high. her mountains, however, are much more difficult to be seen than those of the moon. the sun would appear, as seen from venus, twice as large as on the earth, and its light and heat would be augmented in the same proportion. (see fig. .) in many respects, however, the phenomena of this planet are similar to those of our own; and the general likeness between venus and the earth, in regard to dimensions, revolutions, and seasons, is greater than exists between any other two bodies of the system. i will only add to the present letter a few words on the _transits_ of the inferior planets. the transit of mercury or venus is its passage across the sun's disk, as the moon passes over it in a solar eclipse. the planet is seen projected on the sun's disk in a small, black, round spot, moving slowly over the face of the sun. as the transit takes place only when the planet is in inferior conjunction, at which time her motion is retrograde, it is always from left to right; and, on account of its motion being retarded by the motion of the earth, (as was explained by fig. , page ,) it remains sometimes a long time on the solar disk. mercury, when it makes its transit across the sun's centre, may remain on the sun from five to seven hours. you may ask, why we do not observe this appearance every time one of the inferior planets comes into inferior conjunction, for then, of course, it passes between us and the sun. it must, indeed, at this time, cross the meridian at the same time with the sun; but, because its orbit is inclined to that of the sun, it may cross it (and generally does) a little above or a little below the sun. it is only when the conjunction takes place at or very near the point where the two orbits cross one another, that is, near the _node_, that a transit can occur. thus, if the orbit of mercury, n m r, fig. , (page ,) were in the same plane with the earth's orbit, (and of course with the sun's apparent orbit,) then, when the planet was at q, in its inferior conjunction, the earth being at e, it would always be projected on the sun's disk at z, on the concave sphere of the heavens, and a transit would happen at every inferior conjunction. but now let us take hold of the point r, and lift the circle which represents the orbit of mercury upwards seven degrees, letting it turn upon the diameter _d b_; then, we may easily see that a spectator at e would project the planet higher in the heavens than the sun; and such would always be the case, except when the conjunction takes place at the node. then the point of intersection of the two orbits being in one and the same plane, both bodies would be referred to the same point on the celestial sphere. as the sun, in his apparent revolution around the earth every year, passes through every point in the ecliptic, of course he must every year be at each of the points where the orbit of mercury or venus crosses the ecliptic, that is, at each of the nodes of one of these planets;[ ] and as these nodes are on opposite sides of the ecliptic, consequently, the sun will pass through them at opposite seasons of the year, as in january and july, february and august. now, should mercury or venus happen to come between us and the sun, just as the sun is passing one of the planet's nodes, a transit would happen. hence the transits of mercury take place in may and november, and those of venus, in june and december. transits of mercury occur more frequently than those of venus. the periodic times of mercury and the earth are so adjusted to each other, that mercury performs nearly twenty-nine revolutions while the earth performs seven. if, therefore, the two bodies meet at the node in any given year, seven years afterwards they will meet nearly at the same node, and a transit may take place, accordingly, at intervals of seven years. but fifty-four revolutions of mercury correspond still nearer to thirteen revolutions of the earth; and therefore a transit is still more probable after intervals of thirteen years. at intervals of thirty-three years, transits of mercury are exceedingly probable, because in that time mercury makes almost exactly one hundred and thirty-seven revolutions. intermediate transits, however, may occur at the other node. thus, transits of mercury happened at the ascending node in , and , at intervals of seven years; and at the descending node in , which will return in , after thirteen years. transits of venus are events of very unfrequent occurrence. eight revolutions of the earth are completed in nearly the same time as thirteen revolutions of venus; and hence two transits of venus may occur after an interval of eight years, as was the case at the last return of the phenomenon, one transit having occurred in , and another in . but if a transit does not happen after eight years, it will not happen at the same node, until an interval of two hundred and thirty-five years: but intermediate transits may occur at the other node. the next transit of venus will take place in , being two hundred and thirty-five years after the first that was ever _observed_, which occurred in . this was seen, for the first time by mortal eyes, by two youthful english astronomers, horrox and crabtree. horrox was a young man of extraordinary promise, and indicated early talents for practical astronomy, which augured the highest eminence; but he died in the twenty-third year of his age. he was only twenty when the transit appeared, and he had made the calculations and observations, by which he was enabled to anticipate its arrival several years before. at the approach of the desired time for observing the transit, he received the sun's image through a telescope in a dark room upon a white piece of paper, and after waiting many hours with great impatience, (as his calculation did not lead him to a knowledge of the precise time of the occurrence,) at last, on the twenty-fourth of november, , old style, at three and a quarter hours past twelve, just as he returned from church, he had the pleasure to find a large round spot near the limb of the sun's image. it moved slowly across the sun's disk, but had not entirely left it when the sun set. the great interest attached by astronomers to a transit of venus arises from its furnishing the most accurate means in our power of determining the _sun's horizontal parallax_,--an element of great importance, since it leads us to a knowledge of the distance of the earth from the sun, which again affords the means of estimating the distances of all the other planets, and possibly, of the fixed stars. hence, in , great efforts were made throughout the civilized world, under the patronage of different governments, to observe this phenomenon under circumstances the most favorable for determining the parallax of the sun. the common methods of finding the parallax of a heavenly body cannot be relied on to a greater degree of accuracy than four seconds. in the case of the moon, whose greatest parallax amounts to about one degree, this deviation from absolute accuracy is not very material; but it amounts to nearly half the entire parallax of the sun. if the sun and venus were equally distant from us, they would be equally affected by parallax, as viewed by spectators in different parts of the earth, and hence their _relative_ situation would not be altered by it; but since venus, at the inferior conjunction, is only about one third as far off as the sun, her parallax is proportionally greater, and therefore spectators at distant points will see venus projected on different parts of the solar disk, as the planet traverses the disk. astronomers avail themselves of this circumstance to ascertain the sun's horizontal parallax, which they are enabled to do by comparing it with that of venus, in a manner which, without a knowledge of trigonometry, you will not fully understand. in order to make the difference in the apparent places of venus on the sun's disk as great as possible, very distant places are selected for observation. thus, in the transits of and , several of the european governments fitted out expensive expeditions to parts of the earth remote from each other. for this purpose, the celebrated captain cook, in , went to the south pacific ocean, and observed the transit at the island of otaheite, while others went to lapland, for the same purpose, and others still, to many other parts of the globe. thus, suppose two observers took their stations on opposite sides of the earth, as at a, and b, fig. , page ; at a, the planet v would be seen on the sun's disk at _a_, while at b, it would be seen at _b_. the appearance of venus on the sun's disk being that of a well-defined black spot, and the exactness with which the moment of external or internal contact may be determined, are circumstances favorable to the exactness of the result; and astronomers repose so much confidence in the estimation of the sun's horizontal parallax, as derived from observations on the transit of , that this important element is thought to be ascertained within one tenth of a second. the general result of all these observations gives the sun's horizontal parallax eight seconds and six tenths,--a result which shows at once that the sun must be a great way off, since the semidiameter of the earth, a line nearly four thousand miles in length, would appear at the sun under an angle less than one four hundredth of a degree. during the transits of venus over the sun's disk, in and , a sort of penumbral light was observed around the planet, by several astronomers, which was thought to indicate an _atmosphere_. this appearance was particularly observable while the planet was coming on or going off the solar disk. the total immersion and emersion were not instantaneous; but as two drops of water, when about to separate, form a ligament between them, so there was a dark shade stretched out between venus and the sun; and when the ligament broke, the planet seemed to have got about an eighth part of her diameter from the limb of the sun. the various accounts of the two transits abound with remarks like these, which indicate the existence of an atmosphere about venus of nearly the density and extent of the earth's atmosphere. similar proofs of the existence of an atmosphere around this planet are derived from appearances of twilight. [illustration fig. .] the elder astronomers imagined that they had discovered a _satellite_ accompanying venus in her transit. if venus had in reality any satellite, the fact would be obvious at her transits, as, in some of them at least, it is probable that the satellite would be projected near the primary on the sun's disk; but later astronomers have searched in vain for any appearances of the kind, and the inference is, that former astronomers were deceived by some optical illusion. footnote: [ ] you will recollect that the sun is said to be at the node, when the places of the node and the sun are both projected, by a spectator on the earth, upon the same part of the heavens. letter xxi. superior planets: mars, jupiter, saturn, and uranus. "with what an awful, world-revolving power, were first the unwieldy planets launched along the illimitable void! there to remain amidst the flux of many thousand years, that oft has swept the toiling race of men, and all their labored monuments, away."--_thomson._ mercury and venus, as we have seen, are always observed near the sun, and from this circumstance, as well as from the changes of magnitude and form which they undergo, we know that they have their orbits within that of the earth, and hence we call them _inferior_ planets. on the other hand, mars, jupiter, saturn, and uranus, exhibit such appearances, at different times, as show that they revolve around the sun at a greater distance than the earth, and hence we denominate them _superior_ planets. we know that they never come between us and the sun, because they never undergo those changes which mercury and venus, as well as the moon, sustain, in consequence of their coming into such a position. they, however, wander to the greatest angular distance from the sun, being sometimes seen one hundred and eighty degrees from him, so as to rise when the sun sets. all these different appearances must naturally result from their orbits' being exterior to that of the earth, as will be evident from the following representation. let e, fig. , page , be the earth, and m, one of the superior planets, mars, for example, each body being seen in its path around the sun. at m, the planet would be in opposition to the sun, like the moon at the full; at q and q´, it would be seen ninety degrees off, or in quadrature; and at m´, in conjunction. we know, however, that this must be a superior and not an inferior conjunction, for the illuminated disk is still turned towards us; whereas, if it came between us and the sun, like mercury, or venus, in its inferior conjunction, its dark side would be presented to us. [illustration fig. .] the superior planets do not exhibit to the telescope different phases, but, with a single exception, they always present the side that is turned towards the earth fully enlightened. this is owing to their great distance from the earth; for were the spectator to stand upon the sun, he would of course always have the illuminated side of each of the planets turned towards him; but so distant are all the superior planets, except mars, that they are viewed by us very nearly, in the same manner as they would be if we actually stood on the sun. mars, however, is sufficiently near to appear somewhat gibbous when at or near one of its quadratures. thus, when the planet is at q, it is plain that, of the hemisphere that is turned towards the earth, a small part is unilluminated. mars is a small planet, his diameter being only about half that of the earth, or four thousand two hundred miles. he also, at times, comes nearer to the earth than any other planet, except venus. his _mean_ distance from the sun is one hundred and forty-two millions of miles; but his orbit is so elliptical, that his distance varies much in different parts of his revolution. mars is always very near the ecliptic, never varying from it more than two degrees. he is distinguished from all the planets by his deep red color, and fiery aspect; but his brightness and apparent magnitude vary much, at different times, being sometimes nearer to us than at others by the whole diameter of the earth's orbit; that is, by about one hundred and ninety millions of miles. when mars is on the same side of the sun with the earth, or at his opposition, he comes within forty-seven millions of miles of the earth, and, rising about the time the sun sets, surprises us by his magnitude and splendor; but when he passes to the other side of the sun, to his superior conjunction, he dwindles to the appearance of a small star, being then two hundred and thirty-seven millions of miles from us. thus, let m, fig, , represent mars in opposition, and m´, in the superior conjunction, while e represents the earth. it is obvious that, in the former situation, the planet must be nearer to the earth than in the latter, by the whole diameter of the earth's orbit. when viewed with a powerful telescope, the surface of mars appears diversified with numerous varieties of light and shade. the region around the poles is marked by white spots, (see fig. , page ,) which vary their appearances with the changes of seasons in the planet. hence dr. herschel conjectured that they were owing to ice and snow, which alternately accumulate and melt away, according as it is winter or summer, in that region. they are greatest and most conspicuous when that part of the planet has just emerged from a long winter, and they gradually waste away, as they are exposed to the solar heat. fig. , represents the planet, as exhibited, under the most favorable circumstances, to a powerful telescope, at the time when its gibbous form is strikingly obvious. it has been common to ascribe the ruddy light of mars to an extensive and dense atmosphere, which was said to be distinctly indicated by the gradual diminution of light observed in a star, as it approaches very near to the planet, in undergoing an occultation; but more recent observations afford no such evidence of an atmosphere. by observations on the spots, we learn that mars revolves on his axis in very nearly the same time with the earth, (twenty-four hours thirty-nine minutes twenty-one seconds and three tenths,) and that the inclination of his axis to that of his orbit is also nearly the same, being thirty degrees eighteen minutes ten seconds and eight tenths. hence the changes of day and night must be nearly the same there as here, and the seasons also very similar to ours. since, however, the distance of mars from the sun is one hundred and forty-two while that of the earth is only ninety-five millions of miles, the sun will appear more than twice as small on that planet as on ours, (see fig. , page ,) and its light and heat will be diminished in the same proportion. only the equatorial regions, therefore, will be suitable for the existence of animals and vegetables. the earth will be seen from mars as an inferior planet, always near the sun, presenting appearances similar, in many respects, to those which venus presents to us. it will be to that planet the evening and morning star, sung by their poets (if poets they have) with a like enthusiasm. the moon will attend the earth as a little star, being never seen further from her side than about the diameter under which we view the moon. to the telescope, the earth will exhibit phases similar to those of venus; and, finally, she will, at long intervals, make her transits over the solar disk. mean-while, venus will stand to mars in a relation similar to that of mercury [illustration figures , . jupiter and saturn.] to us, revealing herself only when at the periods of her greatest elongation, and at all other times hiding herself within the solar blaze. mercury will never be visible to an inhabitant of mars. jupiter is distinguished from all the other planets by his great _magnitude_. his diameter is eighty-nine thousand miles, and his volume one thousand two hundred and eighty times that of the earth. his figure is strikingly spheroidal, the equatorial being more than six thousand miles longer than the polar diameter. such a figure might naturally be expected from the rapidity of his diurnal rotation, which is accomplished in about ten hours. a place on the equator of jupiter must turn twenty-seven times as fast as on the terrestrial equator. the distance of jupiter from the sun is nearly four hundred and ninety millions of miles, and his revolution around the sun occupies nearly twelve years. every thing appertaining to jupiter is on a grand scale. a world in itself, equal in dimensions to twelve hundred and eighty of ours; the whole firmament rolling round it in the short space of ten hours, a movement so rapid that the eye could probably perceive the heavenly bodies to change their places every moment; its year dragging out a length of more than four thousand days, and more than ten thousand of its own days, while its nocturnal skies are lighted up with four brilliant moons;--these are some of the peculiarities which characterize this magnificent planet. the view of jupiter through a good telescope is one of the most splendid and interesting spectacles in astronomy. the disk expands into a large and bright orb, like the full moon; the spheroidal figure which theory assigns to revolving spheres, especially to those which turn with great velocity, is here palpably exhibited to the eye; across the disk, arranged in parallel stripes, are discerned several dusky bands, called _belts_; and four bright satellites, always in attendance, and ever varying their positions, compose a splendid retinue. indeed, astronomers gaze with peculiar interest on jupiter and his moons, as affording a miniature representation of the whole solar system, repeating, on a smaller scale, the same revolutions, and exemplifying more within the compass of our observation, the same laws as regulate the entire assemblage of sun and planets. figure , facing page , gives a correct view of jupiter, as exhibited to a powerful telescope in a clear evening. you will remark his flattened or spheroidal figure, the belts which appear in parallel stripes across his disk, and the four satellites, that are seen like little stars in a straight line with the equator of the planet. the _belts of jupiter_ are variable in their number and dimensions. with the smaller telescopes only one or two are seen, and those across the equatorial regions; but with more powerful instruments, the number is increased, covering a large part of the entire disk. different opinions have been entertained by astronomers respecting the cause of these belts; but they have generally been regarded as clouds formed in the atmosphere of the planet, agitated by winds, as is indicated by their frequent changes, and made to assume the form of belts parallel to the equator, like currents that circulate around our globe. sir john herschel supposes that the belts are not ranges of clouds, but portions of the planet itself, brought into view by the removal of clouds and mists, that exist in the atmosphere of the planet, through which are openings made by currents circulating around jupiter. the _satellites of jupiter_ may be seen with a telescope of very moderate powers. even a common spyglass will enable us to discern them. indeed, one or two of them have been occasionally seen with the naked eye. in the largest telescopes they severally appear as bright as sirius. with such an instrument, the view of jupiter, with his moons and belts, is truly a magnificent spectacle. as the orbits of the satellites do not deviate far from the plane of the ecliptic, and but little from the equator of the planet, they are usually seen in nearly a straight line with each other, extending across the central part of the disk. (see fig. , facing page .) jupiter and his satellites exhibit in miniature all the phenomena of the solar system. the satellites perform, around their primary, revolutions very analogous to those which the planets perform around the sun, having, in like manner, motions alternately direct, stationary, and retrograde. they are all, with one exception, a little larger than the moon; and the second satellite, which is the smallest, is nearly as large as the moon, being two thousand and sixty-eight miles in diameter. they are all very small compared with the primary, the largest being only one twenty-sixth part of the primary. the outermost satellite extends to the distance from the planet of fourteen times his diameter. the whole system, therefore, occupies a region of space more than one million miles in breadth. rapidity of motion, as well as greatness of dimensions, is characteristic of the system of jupiter. i have already mentioned that the planet itself has a motion on its own axis much swifter than that of the earth, and the motions of the satellites are also much more rapid than that of the moon. the innermost, which is a little further off than the moon is from the earth, goes round its primary in about a day and three quarters; and the outermost occupies less than seventeen days. the orbits of the satellites are nearly or quite circular, and deviate but little from the plane of the planet's equator, and of course are but slightly inclined to the plane of his orbit. they are therefore in a similar situation with respect to jupiter, as the moon would be with respect to the earth, if her orbit nearly coincided with the ecliptic, in which case, she would undergo an eclipse at every opposition. the eclipses of jupiter's satellites, in their general circumstances, are perfectly analogous to those of the moon, but in their details they differ in several particulars. owing to the much greater distance of jupiter from the sun, and its greater magnitude, the cone of its shadow is much longer and larger than that of the earth. on this account, as well as on account of the little inclination of their orbit to that of the primary, the three inner satellites of jupiter pass through his shadow, and are totally eclipsed, at every revolution. the fourth satellite, owing to the greater inclination of its orbit, sometimes, though rarely, escapes eclipse, and sometimes merely grazes the limits of the shadow, or suffers a partial eclipse. these eclipses, moreover, are not seen, as is the case with those of the moon, from the centre of their motion, but from a remote station, and one whose situation with respect to the line of the shadow is variable. this makes no difference in the _times_ of the eclipses, but it makes a very great one in their visibility, and in their apparent situations with respect to the planet at the moment of their entering or quitting the shadow. [illustration fig. .] the eclipses of jupiter's satellites present some curious phenomena, which you will easily understand by studying the following diagram. let a, b, c, d, fig. , represent the earth in different parts of its orbit; j, jupiter, in his orbit, surrounded by his four satellites, the orbits of which are marked , , , . at _a_, the first satellite enters the shadow of the planet, emerges from it at _b_, and advances to its greatest elongation at _c_. the other satellites traverse the shadow in a similar manner. the apparent place, with respect to the planet, at which these eclipses will be seen to occur, will be altered by the position the earth happens at that moment to have in its orbit; but their appearances for any given night, as exhibited at greenwich, are calculated and accurately laid down in the nautical almanac. when one of the satellites is passing between jupiter and the sun, it casts its shadow on the primary, as the moon casts its shadow on the earth in a solar eclipse. we see with the telescope the shadow traversing the disk. sometimes, the satellite itself is seen projected on the disk; but, being illuminated as well as the primary, it is not so easily distinguished as venus or mercury, when seen on the sun's disk in one of their transits, since these bodies have their dark sides turned towards us; but the satellite is illuminated by the sun, as well as the primary, and therefore is not easily distinguishable from it. the eclipses of jupiter's satellites have been studied with great attention by astronomers, on account of their affording one of the easiest methods of determining the _longitude_. on this subject, sir john herschel remarks: "the discovery of jupiter's satellites by galileo, which was one of the first fruits of the invention of the telescope, forms one of the most memorable epochs in the history of astronomy. the first astronomical solution of the problem of 'the longitude,'--the most important problem for the interests of mankind that has ever been brought under the dominion of strict scientific principles,--dates immediately from this discovery. the final and conclusive establishment of the copernican system of astronomy may also be considered as referable to the discovery and study of this exquisite miniature system, in which the laws of the planetary motions, as ascertained by kepler, and especially that which connects their periods and distances, were speedily traced, and found to be satisfactorily maintained." the entrance of one of jupiter's satellites into the shadow of the primary, being seen like the entrance of the moon into the earth's shadow at the same moment of absolute time, at all places where the planet is visible, and being wholly independent of parallax, that is, presenting the same phenomenon to places remote from each other; being, moreover, predicted beforehand, with great accuracy, for the instant of its occurrence at greenwich, and given in the nautical almanac; this would seem to be one of those events which are peculiarly adapted for finding the longitude. for you will recollect, that "any instantaneous appearance in the heavens, visible at the same moment of absolute time at any two places, may be employed for determining the difference of longitude between those places; for the difference in their local times, as indicated by clocks or chronometers, allowing fifteen degrees for every hour, will show their difference of longitude." with respect to the method by the eclipses of jupiter's satellites, it must be remarked, that the extinction of light in the satellite, at its immersion, and the recovery of its light at its emersion, are not instantaneous, but gradual; for the satellite, like the moon, occupies some time in entering into the shadow, or in emerging from it, which occasions a progressive diminution or increase of light. two observers in the same room, observing with different telescopes the same eclipse, will frequently disagree, in noting its time, to the amount of fifteen or twenty seconds. better methods, therefore, of finding the longitude, are now employed, although the facility with which the necessary observations can be made, and the little calculation required, still render this method eligible in many cases where extreme accuracy is not important. as a telescope is essential for observing an eclipse of one of the satellites, it is obvious that this method cannot be practised at sea, since the telescope cannot be used on board of ship, for want of the requisite steadiness. the grand discovery of the _progressive motion of light_ was first made by observations on the eclipses of jupiter's satellites. in the year , it was remarked by roemer, a danish astronomer, on comparing together observations of these eclipses during many successive years, that they take place sooner by about sixteen minutes, when the earth is on the same side of the sun with the planet, than when she is on the opposite side. the difference he ascribes to the progressive motion of light, which takes that time to pass through the diameter of the earth's orbit, making the velocity of light about one hundred and ninety-two thousand miles per second. so great a velocity startled astronomers at first, and produced some degree of distrust of this explanation of the phenomenon; but the subsequent discovery of what is called the aberration of light, led to an independent estimation of the velocity of light, with almost precisely the same result. few greater feats have ever been performed by the human mind, than to measure the speed of light,--a speed so great, as would carry it across the atlantic ocean in the sixty-fourth part of a second, and around the globe in less than the seventh part of a second! thus has man applied his scale to the motions of an element, that literally leaps from world to world in the twinkling of an eye. this is one example of the great power which the invention of the telescope conferred on man. could we plant ourselves on the surface of this vast planet, we should see the same starry firmament expanding over our heads as we see now; and the same would be true if we could fly from one planetary world to another, until we made the circuit of them all; but the sun and the planetary system would present themselves to us under new and strange aspects. the sun himself would dwindle to one twenty-seventh of his present surface, (fig. , facing page ,) and afford a degree of light and heat proportionally diminished; mercury, venus, and even the earth, would all disappear, being too near the sun to be visible; mars would be as seldom seen as mercury is by us, and constitute the only inferior planet. on the other hand, saturn would shine with greatly augmented size and splendor. when in opposition to the sun, (at which time it comes nearest to jupiter,) it would be a grand object, appearing larger than either venus or jupiter does to us. when, however, passing to the other side of the sun, through its superior conjunction, it would gradually diminish in size and brightness, and at length become much less than it ever appears to us, since it would then be four hundred millions of miles further from jupiter than it ever is from us. although jupiter comes four hundred millions of miles nearer to uranus than the earth does, yet it is still thirteen hundred millions of miles distant from that planet. hence the augmentation of the magnitude and light of uranus would be barely sufficient to render it distinguishable by the naked eye. it appears, therefore, that saturn is the peculiar ornament of the firmament of jupiter, and would present to the telescope most interesting and sublime phenomena. as we owe the revelation of the system of jupiter and his attendant worlds wholly to the telescope, and as the discovery and observation of them constituted a large portion of the glory of galileo, i am now forcibly reminded of his labors, and will recur to his history, and finish the sketch which i commenced in a previous letter. letter xxii. copernicus.--galileo. "they leave at length the nether gloom, and stand before the portals of a better land; to happier plains they come, and fairer groves, the seats of those whom heaven, benignant, loves; a brighter day, a bluer ether, spreads its lucid depths above their favored heads; and, purged from mists that veil our earthly skies, shine suns and stars unseen by mortal eyes."--_virgil._ in order to appreciate the value of the contributions which galileo made to astronomy, soon after the invention of the telescope, it is necessary to glance at the state of the science when he commenced his discoveries for many centuries, during the middle ages, a dark night had hung over astronomy, through which hardly a ray of light penetrated, when, in the eastern part of civilized europe, a luminary appeared, that proved the harbinger of a bright and glorious day. this was copernicus, a native of thorn, in prussia. he was born in . though destined for the profession of medicine, from his earliest years he displayed a great fondness and genius for mathematical studies, and pursued them with distinguished success in the university of cracow. at the age of twenty-five years, he resorted to italy, for the purpose of studying astronomy, where he resided a number of years. thus prepared, he returned to his native country, and, having acquired an ecclesiastical living that was adequate to his support in his frugal mode of life, he established himself at frauenberg, a small town near the mouth of the vistula, where he spent nearly forty years in observing the heavens, and meditating on the celestial motions. he occupied the upper part of a humble farm-house, through the roof of which he could find access to an unobstructed sky, and there he carried on his observations. his instruments, however, were few and imperfect, and it does not appear that he added any thing to the art of practical astronomy. this was reserved for tycho brahe, who came a half a century after him. nor did copernicus enrich the science with any important discoveries. it was not so much his genius or taste to search for new bodies, or new phenomena among the stars, as it was to explain the reasons of the most obvious and well-known appearances and motions of the heavenly bodies. with this view, he gave his mind to long-continued and profound meditation. copernicus tells us that he was first led to think that the apparent motions of the heavenly bodies, in their diurnal revolution, were owing to the real motion of the earth in the opposite direction, from observing instances of the same kind among terrestrial objects; as when the shore seems to the mariner to recede, as he rapidly sails from it; and as trees and other objects seem to glide by us, when, on riding swiftly past them, we lose the consciousness of our own motion. he was also smitten with the _simplicity_ prevalent in all the works and operations of nature, which is more and more conspicuous the more they are understood; and he hence concluded that the planets do not move in the complicated paths which most preceding astronomers assigned to them. i shall explain to you, hereafter, the details of his system. i need only at present remind you that the hypothesis which he espoused and defended, (being substantially the same as that proposed by pythagoras, five hundred years before the christian era,) supposes, first, that the apparent movements of the sun by day, and of the moon and stars by night, from east to west, result from the actual revolution of the earth on its own axis from west to east; and, secondly, that the earth and all the planets revolve about the sun in circular orbits. this hypothesis, when he first assumed it, was with him, as it had been with pythagoras, little more than mere conjecture. the arguments by which its truth was to be finally established were not yet developed, and could not be, without the aid of the telescope, which was not yet invented. upon this hypothesis, however, he set out to explain all the phenomena of the visible heavens,--as the diurnal revolutions of the sun, moon, and stars, the slow progress of the planets through the signs of the zodiac, and the numerous irregularities to which the planetary motions are subject. these last are apparently so capricious,--being for some time forward, then stationary, then backward, then stationary again, and finally direct, a second time, in the order of the signs, and constantly varying in the velocity of their movements,--that nothing but long-continued and severe meditation could have solved all these appearances, in conformity with the idea that each planet is pursuing its simple way all the while in a circle around the sun. although, therefore, pythagoras fathomed the profound doctrine that the sun is the centre around which the earth and all the planets revolve, yet we have no evidence that he ever solved the irregular motions of the planets in conformity with his hypothesis, although the explanation of the diurnal revolution of the heavens, by that hypothesis, involved no difficulty. ignorant as copernicus was of the principle of gravitation, and of most of the laws of motion, he could go but little way in following out the consequences of his own hypothesis; and all that can be claimed for him is, that he solved, by means of it, most of the common phenomena of the celestial motions. he indeed got upon the road to truth, and advanced some way in its sure path; but he was able to adduce but few independent proofs, to show that it was the truth. it was only quite at the close of his life that he published his system to the world, and that only at the urgent request of his friends; anticipating, perhaps, the opposition of a bigoted priesthood, whose fury was afterwards poured upon the head of galileo, for maintaining the same doctrines. although, therefore, the system of copernicus afforded an explanation of the celestial motions, far more simple and rational than the previous systems which made the earth the centre of those motions, yet this fact alone was not sufficient to compel the assent of astronomers; for the greater part, to say the least, of the same phenomena, could be explained on either hypothesis. with the old doctrine astronomers were already familiar, a circumstance which made it seem easier; while the new doctrines would seem more difficult, from their being imperfectly understood. accordingly, for nearly a century after the publication of the system of copernicus, he gained few disciples. tycho brahe rejected it, and proposed one of his own, of which i shall hereafter give you some account; and it would probably have fallen quite into oblivion, had not the observations of galileo, with his newly-invented telescope, brought to light innumerable proofs of its truth, far more cogent than any which copernicus himself had been able to devise. galileo no sooner had completed his telescope, and directed it to the heavens, than a world of wonders suddenly burst upon his enraptured sight. pointing it to the moon, he was presented with a sight of her mottled disk, and of her mountains and valleys. the sun exhibited his spots; venus, her phases; and jupiter, his expanded orb, and his retinue of moons. these last he named, in honor of his patron, cosmo d'medici, _medicean stars_. so great was this honor deemed of associating one's name with the stars, that express application was made to galileo, by the court of france, to award this distinction to the reigning monarch, henry the fourth, with plain intimations, that by so doing he would render himself and his family rich and powerful for ever. galileo published the result of his discoveries in a paper, denominated '_nuncius sidereus_,' the 'messenger of the stars.' in that ignorant and marvellous age, this publication produced a wonderful excitement. "many doubted, many positively refused to believe, so novel an announcement; all were struck with the greatest astonishment, according to their respective opinions, either at the new view of the universe thus offered to them, or at the high audacity of galileo, in inventing such fables." even kepler, the great german astronomer, of whom i shall tell you more by and by, wrote to galileo, and desired him to supply him with arguments, by which he might answer the objections to these pretended discoveries with which he was continually assailed. galileo answered him as follows: "in the first place, i return you my thanks that you first, and almost alone, before the question had been sifted, (such is your candor, and the loftiness of your mind,) put faith in my assertions. you tell me you have some telescopes, but not sufficiently good to magnify distant objects with clearness, and that you anxiously expect a sight of mine, which magnifies images more than a thousand times. it is mine no longer, for the grand duke of tuscany has asked it of me, and intends to lay it up in his museum, among his most rare and precious curiosities, in eternal remembrance of the invention. "you ask, my dear kepler, for other testimonies. i produce, for one, the grand duke, who, after observing the medicean planets several times with me at pisa, during the last months, made me a present, at parting, of more than a thousand florins, and has now invited me to attach myself to him, with the annual salary of one thousand florins, and with the title of 'philosopher and principal mathematician to his highness;' without the duties of any office to perform, but with the most complete leisure. i produce, for another witness, myself, who, although already endowed in this college with the noble salary of one thousand florins, such as no professor of mathematics ever before received, and which i might securely enjoy during my life, even if these planets should deceive me and should disappear, yet quit this situation, and take me where want and disgrace will be my punishment, should i prove to have been mistaken." the learned professors in the universities, who, in those days, were unaccustomed to employ their senses in inquiring into the phenomena of nature, but satisfied themselves with the authority of aristotle, on all subjects, were among the most incredulous with respect to the discoveries of galileo. "oh, my dear kepler," says galileo, "how i wish that we could have one hearty laugh together. here, at padua, is the principal professor of philosophy, whom i have repeatedly and urgently requested to look at the moon and planets through my glass, which he pertinaciously refuses to do. why are you not here? what shouts of laughter we should have at this glorious folly, and to hear the professor of philosophy at pisa laboring before the grand duke, with logical arguments, as if with magical incantations, to charm the new planets out of the sky." the following argument by sizzi, a contemporary astronomer of some note, to prove that there can be only seven planets, is a specimen of the logic with which galileo was assailed. "there are seven windows given to animals in the domicile of the head, through which the air is admitted to the tabernacle of the body, to enlighten, to warm, and to nourish it; which windows are the principal parts of the microcosm, or little world,--two nostrils, two eyes, two ears, and one mouth. so in the heavens, as in a macrocosm, or great world, there are two favorable stars, jupiter and venus; two unpropitious, mars and saturn; two luminaries, the sun and moon; and mercury alone, undecided and indifferent. from which, and from many other phenomena of nature, such as the seven metals, &c., which it were tedious to enumerate, we gather that the number of planets is necessarily seven. moreover, the satellites are invisible to the naked eye, and therefore can exercise no influence over the earth, and therefore would be useless, and therefore do not exist. besides, as well the jews and other ancient nations, as modern europeans, have adopted the division of the week into seven days, and have named them from the seven planets. now, if we increase the number of planets, this whole system falls to the ground." when, at length, the astronomers of the schools found it useless to deny the fact that jupiter is attended by smaller bodies, which revolve around him, they shifted their ground of warfare, and asserted that galileo had not told the whole truth; that there were not merely _four_ satellites, but a still greater number; one said five; another, nine; and another, twelve; but, in a little time, jupiter moved forward in his orbit, and left all behind him, save the four medicean stars. it had been objected to the copernican system, that were venus a body which revolved around the sun in an orbit interior to that of the earth, she would undergo changes similar to those of the moon. as no such changes could be detected by the naked eye, no satisfactory answer could be given to this objection; but the telescope set all right, by showing, in fact, the phases of venus. the same instrument, disclosed, also, in the system of jupiter and his moons, a miniature exhibition of the solar system itself. it showed the actual existence of the motion of a number of bodies around one central orb, exactly similar to that which was predicated of the sun and planets. every one, therefore, of these new and interesting discoveries, helped to confirm the truth of the system of copernicus. but a fearful cloud was now rising over galileo, which spread itself, and grew darker every hour. the church of rome had taken alarm at the new doctrines respecting the earth's motion, as contrary to the declarations of the bible, and a formidable difficulty presented itself, namely, how to publish and defend these doctrines, without invoking the terrible punishments inflicted by the inquisition on heretics. no work could be printed without license from the court of rome; and any opinions supposed to be held and much more known to be taught by any one, which, by an ignorant and superstitious priesthood, could be interpreted as contrary to scripture, would expose the offender to the severest punishments, even to imprisonment, scourging, and death. we, who live in an age so distinguished for freedom of thought and opinion, can form but a very inadequate conception of the bondage in which the minds of men were held by the chains of the inquisition. it was necessary, therefore, for galileo to proceed with the greatest caution in promulgating truths which his own discoveries had confirmed. he did not, like the christian martyrs, proclaim the truth in the face of persecutions and tortures; but while he sought to give currency to the copernican doctrines, he labored, at the same time, by cunning artifices, to blind the ecclesiastics to his real designs, and thus to escape the effects of their hostility. before galileo published his doctrines in form, he had expressed himself so freely, as to have excited much alarm among the ecclesiastics. one of them preached publicly against him, taking for his text, the passage, "ye men of galilee, why stand ye here gazing up into heaven?" he therefore thought it prudent to resort to rome, and confront his enemies face to face. a contemporary describes his appearance there in the following terms, in a letter addressed to a romish cardinal: "your eminence would be delighted with galileo, if you heard him holding forth, as he often does, in the midst of fifteen or twenty, all violently attacking him, sometimes in one house, sometimes in another. but he is armed after such fashion, that he laughs all of them to scorn; and even if the novelty of his opinions prevents entire persuasion, at least he convicts of emptiness most of the arguments with which his adversaries endeavor to overwhelm him." in , galileo, as he himself states, had a most gracious audience of the pope, paul the fifth, which lasted for nearly an hour, at the end of which his holiness assured him, that the congregation were no longer in a humor to listen lightly to calumnies against him, and that so long as he occupied the papal chair, galileo might think himself out of all danger. nevertheless, he was not allowed to return home, without receiving formal notice not to teach the opinions of copernicus, "that the sun is in the centre of the system, and that the earth moves about it," from that time forward, in any manner. galileo had a most sarcastic vein, and often rallied his persecutors with the keenest irony. this he exhibited, some time after quitting rome, in an epistle which he sent to the arch duke leopold, accompanying his 'theory of the tides.' "this theory," says he, "occurred to me when in rome, whilst the theologians were debating on the prohibition of copernicus's book, and of the opinion maintained in it of the motion of the earth, which i at that time believed; until it pleased those gentlemen to suspend the book, and to declare the opinion false and repugnant to the holy scriptures. now, as i know how well it becomes me to obey and believe the decisions of my superiors, which proceed out of more profound knowledge than the weakness of my intellect can attain to, this theory, which i send you, which is founded on the motion of the earth, i now look upon as a fiction and a dream, and beg your highness to receive it as such. but, as poets often learn to prize the creations of their fancy, so, in like manner, do i set some value on this absurdity of mine. it is true, that when i sketched this little work, i did hope that copernicus would not, after eighty years, be convicted of error; and i had intended to develope and amplify it further; but a voice from heaven suddenly awakened me, and at once annihilated all my confused and entangled fancies." it is difficult, however, sometimes to decide whether the language of galileo is ironical, or whether he uses it with subtlety, with the hope of evading the anathemas of the inquisition. thus he ends one of his writings with the following passage: "in conclusion, since the motion attributed to the earth, which i, as a pious and catholic person, consider most false, and not to exist, accommodates itself so well to explain so many and such different phenomena, i shall not feel sure that, false as it is, it may not just as deludingly correspond with the phenomena of comets." in the year , soon after the accession of urban the eighth to the pontifical chair, galileo went to rome again, to offer his congratulations to the new pope, as well as to propitiate his favor. he seems to have been received with unexpected cordiality; and, on his departure, the pope commended him to the good graces of ferdinand, grand duke of tuscany, in the following terms: "we find in him not only literary distinction, but also the love of piety, and he is strong in those qualities by which pontifical good-will is easily obtained. and now, when he has been brought to this city, to congratulate us on our elevation, we have lovingly embraced him; nor can we suffer him to return to the country whither your liberality recalls him, without an ample provision of pontifical love. and that you may know how dear he is to us, we have willed to give him this honorable testimonial of virtue and piety. and we further signify, that every benefit which you shall confer upon him will conduce to our gratification." in the year , galileo finished a great work, on which he had been long engaged, entitled, 'the dialogue on the ptolemaic and copernican systems.' from the notion which prevailed, that he still countenanced the copernican doctrine of the earth's motion, which had been condemned as heretical, it was some time before he could obtain permission from the inquisitors (whose license was necessary to every book) to publish it. this he was able to do, only by employing again that duplicity or artifice which would throw dust in the eyes of the vain and superstitious priesthood. in , the work appeared under the following title: 'a dialogue, by galileo galilei, extraordinary mathematician of the university of pisa, and principal philosopher and mathematician of the most serene grand duke of tuscany; in which, in a conversation of four days, are discussed the two principal systems of the world, the ptolemaic and copernican, indeterminately proposing the philosophical arguments as well on one side as on the other.' the subtle disguise which he wore, may be seen from the following extract from his 'introduction,' addressed 'to the discreet reader.' "some years ago, a salutary edict was promulgated at rome, which, in order to obviate the perilous scandals of the present age, enjoined an opportune silence on the pythagorean opinion of the earth's motion. some were not wanting, who rashly asserted that this decree originated, not in a judicious examination, but in ill-informed passion; and complaints were heard, that counsellors totally inexperienced in astronomical observations ought not, by hasty prohibitions, to clip the wings of speculative minds. my zeal could not keep silence when i heard these rash lamentations, and i thought it proper, as being fully informed with regard to that most prudent determination, to appear publicly on the theatre of the world, as a witness of the actual truth. i happened at that time to be in rome: i was admitted to the audiences, and enjoyed the approbation, of the most eminent prelates of that court; nor did the publication of that decree occur without my receiving some prior intimation of it. wherefore, it is my intention, in this present work, to show to foreign nations, that as much is known of this matter in italy, and particularly in rome, as ultramontane diligence can ever have formed any notion of, and collecting together all my own speculations on the copernican system, to give them to understand that the knowledge of all these preceded the roman censures; and that from this country proceed not only dogmas for the salvation of the soul, but also ingenious discoveries for the gratification of the understanding. with this object, i have taken up in the 'dialogue' the copernican side of the question, treating it as a pure mathematical hypothesis; and endeavoring, in every artificial manner, to represent it as having the advantage, not over the opinion of the stability of the earth absolutely, but according to the manner in which that opinion is defended by some, who indeed profess to be aristotelians, but retain only the name, and are contented, without improvement, to worship shadows, not philosophizing with their own reason, but only from the recollection of the four principles imperfectly understood." although the pope himself, as well as the inquisitors, had examined galileo's manuscript, and, not having the sagacity to detect the real motives of the author, had consented to its publication, yet, when the book was out, the enemies of galileo found means to alarm the court of rome, and galileo was summoned to appear before the inquisition. the philosopher was then seventy years old, and very infirm, and it was with great difficulty that he performed the journey. his unequalled dignity and celebrity, however, commanded the involuntary respect of the tribunal before which he was summoned, which they manifested by permitting him to reside at the palace of his friend, the tuscan ambassador; and when it became necessary, in the course of the inquiry, to examine him in person, although his removal to the holy office was then insisted upon, yet he was not, like other heretics, committed to close and solitary confinement. on the contrary, he was lodged in the apartments of the head of the inquisition, where he was allowed the attendance of his own servant, who was also permitted to sleep in an adjoining room, and to come and go at pleasure. these were deemed extraordinary indulgences in an age when the punishment of heretics usually began before their trial. about four months after galileo's arrival in rome, he was summoned to the holy office. he was detained there during the whole of that day; and on the next day was conducted, in a penitential dress, to the convent of minerva, where the cardinals and prelates, his judges, were assembled for the purpose of passing judgement upon him, by which this venerable old man was solemnly called upon to renounce and abjure, as impious and heretical, the opinions which his whole existence had been consecrated to form and strengthen. probably there is not a more curious document in the history of science, than that which contains the sentence of the inquisition on galileo, and his consequent abjuration. it teaches us so much, both of the darkness and bigotry of the terrible inquisition, and of the sufferings encountered by those early martyrs of science, that i will transcribe for your perusal, from the excellent 'life of galileo' in the 'library of useful knowledge,' (from which i have borrowed much already,) the entire record of this transaction. the sentence of the inquisition is as follows: "we, the undersigned, by the grace of god, cardinals of the holy roman church, inquisitors general throughout the whole christian republic, special deputies of the holy apostolical chair against heretical depravity: "whereas, you, galileo, son of the late vincenzo galilei of florence, aged seventy years, were denounced in , to this holy office, for holding as true a false doctrine taught by many, namely, that the sun is immovable in the centre of the world, and that the earth moves, and also with a diurnal motion; also, for having pupils which you instructed in the same opinions; also, for maintaining a correspondence on the same with some german mathematicians; also, for publishing certain letters on the solar spots, in which you developed the same doctrine as true; also, for answering the objections which were continually produced from the holy scriptures, by glozing the said scriptures, according to your own meaning; and whereas, thereupon was produced the copy of a writing, in form of a letter, professedly written by you to a person formerly your pupil, in which, following the hypothesis of copernicus, you include several propositions contrary to the true sense and authority of the holy scriptures: therefore, this holy tribunal, being desirous of providing against the disorder and mischief which was thence proceeding and increasing, to the detriment of the holy faith, by the desire of his holiness, and of the most eminent lords cardinals of this supreme and universal inquisition, the two propositions of the stability of the sun, and motion of the earth, were _qualified_ by the _theological qualifiers_, as follows: " . the proposition that the sun is in the centre of the world, and immovable from its place, is absurd, philosophically false, and formally heretical; because it is expressly contrary to the holy scriptures. " . the proposition that the earth is not the centre of the world, nor immovable, but that it moves, and also with a diurnal motion, is also absurd, philosophically false, and, theologically considered, equally erroneous in faith. "but whereas, being pleased at that time to deal mildly with you, it was decreed in the holy congregation, held before his holiness on the twenty-fifth day of february, , that his eminence the lord cardinal bellarmine should enjoin you to give up altogether the said false doctrine; if you should refuse, that you should be ordered by the commissary of the holy office to relinquish it, not to teach it to others, nor to defend it, and in default of the acquiescence, that you should be imprisoned; and in execution of this decree, on the following day, at the palace, in presence of his eminence the said lord cardinal bellarmine, after you had been mildly admonished by the said lord cardinal, you were commanded by the acting commissary of the holy office, before a notary and witnesses, to relinquish altogether the said false opinion, and in future neither to defend nor teach it in any manner, neither verbally nor in writing, and upon your promising obedience, you were dismissed. "and, in order that so pernicious a doctrine might be altogether rooted out, nor insinuate itself further to the heavy detriment of the catholic truth, a decree emanated from the holy congregation of the index, prohibiting the books which treat of this doctrine; and it was declared false, and altogether contrary to the holy and divine scripture. "and whereas, a book has since appeared, published at florence last year, the title of which showed that you were the author, which title is, '_the dialogue of galileo galilei, on the two principal systems of the world, the ptolemaic and copernican_;' and whereas, the holy congregation has heard that, in consequence of printing the said book, the false opinion of the earth's motion and stability of the sun is daily gaining ground; the said book has been taken into careful consideration, and in it has been detected a glaring violation of the said order, which had been intimated to you; inasmuch as in this book you have defended the said opinion, already, and in your presence, condemned; although in the said book you labor, with many circumlocutions, to induce the belief that it is left by you undecided, and in express terms probable; which is equally a very grave error, since an opinion can in no way be probable which has been already declared and finally determined contrary to the divine scripture. therefore, by our order, you have been cited to this holy office, where, on your examination upon oath, you have acknowledged the said book as written and printed by you. you also confessed that you began to write the said book ten or twelve years ago, after the order aforesaid had been given. also, that you demanded license to publish it, but without signifying to those who granted you this permission, that you had been commanded not to hold, defend, or teach, the said doctrine in any manner. you also confessed, that the style of said book was, in many places, so composed, that the reader might think the arguments adduced on the false side to be so worded, as more effectually to entangle the understanding than to be easily solved, alleging, in excuse, that you have thus run into an error, foreign (as you say) to your intention, from writing in the form of a dialogue, and in consequence of the natural complacency which every one feels with regard to his own subtilties, and in showing himself more skilful than the generality of mankind in contriving, even in favor of false propositions, ingenious and apparently probable arguments. "and, upon a convenient time being given you for making your defence, you produced a certificate in the handwriting of his eminence, the lord cardinal bellarmine, procured, as you said, by yourself, that you might defend yourself against the calumnies of your enemies, who reported that you had abjured your opinions, and had been punished by the holy office; in which certificate it is declared, that you had not abjured, nor had been punished, but merely that the declaration made by his holiness, and promulgated by the holy congregation of the index, had been announced to you, which declares that the opinion of the motion of the earth, and stability of the sun, is contrary to the holy scriptures, and therefore cannot be held or defended. wherefore, since no mention is there made of two articles of the order, to wit, the order 'not to teach,' and 'in any manner,' you argued that we ought to believe that, in the lapse of fourteen or sixteen years, they had escaped your memory, and that this was also the reason why you were silent as to the order, when you sought permission to publish your book, and that this is said by you, not to excuse your error, but that it may be attributed to vain-glorious ambition rather than to malice. but this very certificate, produced on your behalf, has greatly aggravated your offence, since it is therein declared, that the said opinion is contrary to the holy scriptures, and yet you have dared to treat of it, and to argue that it is probable; nor is there any extenuation in the license artfully and cunningly extorted by you, since you did not intimate the command imposed upon you. but whereas, it appeared to us that you had not disclosed the whole truth with regard to your intentions, we thought it necessary to proceed to the rigorous examination of you, in which (without any prejudice to what you had confessed, and which is above detailed against you, with regard to your said intention) you answered like a good catholic. "therefore, having seen and maturely considered the merits of your cause, with your said confessions and excuses, and every thing else which ought to be seen and considered, we have come to the underwritten final sentence against you: "invoking, therefore, the most holy name of our lord jesus christ, and of his most glorious virgin mother, mary, by this our final sentence, which, sitting in council and judgement for the tribunal of the reverend masters of sacred theology, and doctors of both laws, our assessors, we put forth in this writing touching the matters and controversies before us, between the magnificent charles sincerus, doctor of both laws, fiscal proctor of this holy office, of the one part, and you, galileo galilei, an examined and confessed criminal from this present writing now in progress, as above, of the other part, we pronounce, judge, and declare, that you, the said galileo, by reason of these things which have been detailed in the course of this writing, and which, as above, you have confessed, have rendered yourself vehemently suspected, by this holy office, of heresy; that is to say, that you believe and hold the false doctrine, and contrary to the holy and divine scriptures, namely, that the sun is the centre of the world, and that it does not move from east to west, and that the earth does move, and is not the centre of the world; also, that an opinion can be held and supported, as probable, after it has been declared and finally decreed contrary to the holy scripture, and consequently, that you have incurred all the censures and penalties enjoined and promulgated in the sacred canons, and other general and particular constitutions against delinquents of this description. from which it is our pleasure that you be absolved, provided that, with a sincere heart and unfeigned faith, in our presence, you abjure, curse, and detest, the said errors and heresies, and every other error and heresy, contrary to the catholic and apostolic church of rome, in the form now shown to you. "but that your grievous and pernicious error and transgression may not go altogether unpunished, and that you may be made more cautious in future, and may be a warning to others to abstain from delinquencies of this sort, we decree, that the book of the dialogues of galileo galilei be prohibited by a public edict, and we condemn you to the formal prison of this holy office for a period determinable at our pleasure; and, by way of salutary penance, we order you, during the next three years, to recite, once a week, the seven penitential psalms, reserving to ourselves the power of moderating, commuting, or taking off the whole or part of the said punishment, or penance. "and so we say, pronounce, and by our sentence declare, decree, and reserve, in this and in every other better form and manner, which lawfully we may and can use. so we, the subscribing cardinals, pronounce." [subscribed by seven cardinals.] in conformity with the foregoing sentence, galileo was made to kneel before the inquisition, and make the following _abjuration_: "i, galileo galilei, son of the late vincenzo galilei, of florence, aged seventy years, being brought personally to judgement, and kneeling before you, most eminent and most reverend lords cardinals, general inquisitors of the universal christian republic against heretical depravity, having before my eyes the holy gospels, which i touch with my own hands, swear, that i have always believed, and with the help of god will in future believe, every article which the holy catholic and apostolic church of rome holds, teaches, and preaches. but because i had been enjoined, by this holy office, altogether to abandon the false opinion which maintains that the sun is the centre and immovable, and forbidden to hold, defend, or teach, the said false doctrine, in any manner: and after it had been signified to me that the said doctrine is repugnant to the holy scripture, i have written and printed a book, in which i treat of the same doctrine now condemned, and adduce reasons with great force in support of the same, without giving any solution, and therefore have been judged grievously suspected of heresy; that is to say, that i held and believed that the sun is the centre of the world and immovable, and that the earth is not the centre and movable; willing, therefore, to remove from the minds of your eminences, and of every catholic christian, this vehement suspicion rightfully entertained towards me, with a sincere heart and unfeigned faith, i abjure, curse, and defeat, the said errors and heresies, and generally every other error and sect contrary to the said holy church; and i swear, that i will never more in future say or assert any thing, verbally or in writing, which may give rise to a similar suspicion of me: but if i shall know any heretic, or any one suspected of heresy, that i will denounce him to this holy office, or to the inquisitor and ordinary of the place in which i may be. i swear, moreover, and promise, that i will fulfil and observe fully, all the penances which have been or shall be laid on me by this holy office. but if it shall happen that i violate any of my said promises, oaths, and protestations, (which god avert!) i subject myself to all the pains and punishments which have been decreed and promulgated by the sacred canons, and other general and particular constitutions, against delinquents of this description. so may god help me, and his holy gospels, which i touch with my own hands. i, the above-named galileo galilei, have abjured, sworn, promised, and bound myself, as above; and in witness thereof, with my own hand have subscribed this present writing of my abjuration, which i have recited, word for word. "at rome, in the convent of minerva, twenty-second june, , i, galileo galilei, have abjured as above, with my own hand." from the court galileo was conducted to prison, to be immured for life in one of the dungeons of the inquisition. his sentence was afterwards mitigated, and he was permitted to return to florence; but the humiliation to which he had been subjected pressed heavily on his spirits, beset as he was with infirmities, and totally blind, and he never more talked or wrote on the subject of astronomy. there was enough in the character of galileo to command a high admiration. there was much, also, in his sufferings in the cause of science, to excite the deepest sympathy, and even compassion. he is moreover universally represented to have been a man of great equanimity, and of a noble and generous disposition. no scientific character of the age, or perhaps of any age, forms a structure of finer proportions, or wears in a higher degree the grace of symmetry. still, we cannot approve of his employing artifice in the promulgation of truth; and we are compelled to lament that his lofty spirit bowed in the final conflict. how far, therefore, he sinks below the dignity of the christian martyr! "at the age of seventy," says dr. brewster, in his life of sir isaac newton, "on his bended knees, and with his right hand resting on the holy evangelists, did this patriarch of science avow his present and past belief in the dogmas of the romish church, abandon as false and heretical the doctrine of the earth's motion and of the sun's immobility, and pledge himself to denounce to the inquisition any other person who was even suspected of heresy. he abjured, cursed, and detested, those eternal and immutable truths which the almighty had permitted him to be the first to establish. had galileo but added the courage of the martyr to the wisdom of the sage; had he carried the glance of his indignant eye round the circle of his judges; had he lifted his hands to heaven, and called the living god to witness the truth and immutability of his opinions; the bigotry of his enemies would have been disarmed, and science would have enjoyed a memorable triumph." letter xxiii. saturn.--uranus.--asteroids. "into the heaven of heavens i have presumed, an earthly guest, and drawn empyreal air."--_milton._ the consideration of the system of jupiter and his satellites led us to review the interesting history of galileo, who first, by means of the telescope, disclosed the knowledge of that system to the world. i will now proceed with the other superior planets. saturn, as well as jupiter, has within itself a system on a scale of great magnificence. in size it is next to jupiter the largest of the planets, being seventy-nine thousand miles in diameter, or about one thousand times as large as the earth. it has likewise belts on its surface, and is attended by seven satellites. but a still more wonderful appendage is its _ring_, a broad wheel, encompassing the planet at a great distance from it. as saturn is nine hundred millions of miles from us, we require a more powerful telescope to see his glories, in all their magnificence, than we do to enjoy a full view of the system of jupiter. when we are privileged with a view of saturn, in his most favorable positions, through a telescope of the larger class, the mechanism appears more wonderful than even that of jupiter. saturn's ring, when viewed with telescopes of a high power, is found to consist of two concentred rings, separated from each other by a dark space. although this division of the rings appears to us, on account of our immense distance, as only a fine line, yet it is, in reality, an interval of not less than eighteen hundred miles. the dimensions of the whole system are, in round numbers, as follows: miles. diameter of the planet, , from the surface of the planet to the inner ring, , breadth of the inner ring, , interval between the rings, , breadth of the outer ring, , extreme dimensions from outside to outside, , figure , facing page , represents saturn, as it appears to a powerful telescope, surrounded by its rings, and having its body striped with dark belts, somewhat similar, but broader and less strongly marked than those of jupiter. in telescopes of inferior power, but still sufficient to see the ring distinctly, we should scarcely discern the belts at all. we might, however, observe the shadow cast upon the ring by the planet, (as seen in the figure on the right, on the upper side;) and, in favorable situations of the planet, we might discern glimpses of the shadow of the ring on the body of the planet, on the lower side beneath the ring. to see the division of the ring and the satellites requires a better telescope than is in possession of most observers. with smaller telescopes, we may discover an oval figure of peculiar appearance, which it would be difficult to interpret. galileo, who first saw it in the year , recognised this peculiarity, but did not know what it meant. seeing something in the centre with two projecting arms, one on each side, he concluded that the planet was triple-shaped. this was, at the time, all he could learn respecting it, as the telescopes he possessed were very humble, compared with those now used by astronomers. the first constructed by him magnified but three times; his second, eight times; and his best, only thirty times, which is no better than a common ship-glass. it was the practice of the astronomers of those days to give the first intimation of a new discovery in a latin verse, the letters of which were transposed. this would enable them to claim priority, in case any other person should contest the honor of the discovery, and at the same time would afford opportunity to complete their observations, before they published a full account of them. accordingly, galileo announced the discovery of the singular appearance of saturn under this disguise, in a line which, when the transposed letters were restored to their proper places, signified that he had observed, "that the most distant planet is triple-formed."[ ] he shortly afterwards, at the request of his patron, the emperor rodolph, gave the solution, and added, "i have, with great admiration, observed that saturn is not a single star, but three together, which, as it were, touch each other; they have no relative motion, and are constituted of this form, ooo, the middle one being somewhat larger than the two lateral ones. if we examine them with an eyeglass which magnifies the surface less than one thousand times, the three stars do not appear very distinctly, but saturn has an oblong appearance, like that of an olive, thus, {oblong symbol}. now, i have discovered a court for jupiter, (alluding to his satellites,) and two servants for this old man, (saturn,) who aid his steps, and never quit his side." it was by this mystic light that galileo groped his way through an organization which, under the more powerful glasses of his successors, was to expand into a mighty orb, encompassed by splendid rings of vast dimensions, the whole attended by seven bright satellites. this system was first fully developed by huyghens, a dutch astronomer, about forty years afterwards.[ ] it requires a superior telescope to see it to advantage; but, when seen through such a telescope, it is one of the most charming spectacles afforded to that instrument. to give some idea of the properties of a telescope suited to such observations, i annex an extract from an account, that was published a few years since, of a telescope constructed by mr. tully, a distinguished english artist. "the length of the instrument was twelve feet, but was easily adjusted, and was perfectly steady. the magnifying powers ranged from two hundred to seven hundred and eighty times; but the great excellence of the telescope consisted more in the superior distinctness and brilliancy with which objects were seen through it, than in its magnifying powers. with a power of two hundred and forty, the light of jupiter was almost more than the eye could bear, and his satellites appeared as bright as sirius, but with a clear and steady light; and the belts and spots on the face of the planet were most distinctly defined. with a power of nearly four hundred, saturn appeared large and well defined, and was one of the most beautiful objects that can well be conceived." that the ring is a solid opaque substance, is shown by its throwing its shadow on the body of the planet on the side nearest the sun, and on the other side receiving that of the body. the ring encompasses the equatorial regions of the planet, and the planet revolves on an axis which is perpendicular to the plane of the ring in about ten and a half hours. this is known by observing the rotation of certain dusky spots, which sometimes appear on its surface. this motion is nearly the same with the diurnal motion of jupiter, subjecting places on the equator of the planet to a very swift revolution, and occasioning a high degree of compression at the poles, the equatorial being to the polar diameter in the high ratio of eleven to ten. saturn's ring, in its revolution around the sun, _always remains parallel to itself_. if we hold opposite to the eye a circular ring or disk, like a piece of coin, it will appear as a complete circle only when it is at right angles to the axis of vision. when it is oblique to that axis, it will be projected into an ellipse more and more flattened, as its obliquity is increased, until, when its plane coincides with the axis of vision, it is projected into a straight line. please to take some circle, as a flat plate, (whose rim may well represent the ring of saturn,) and hold it in these different positions before the eye. now, place on the table a lamp to represent the sun, and holding the ring at a certain distance, inclined a little towards the lamp, carry it round the lamp, always keeping it parallel to itself. during its revolution, it will twice present its edge to the lamp at opposite points; and twice, at places ninety degrees distant from those points, it will present its broadest face towards the lamp. at intermediate points, it will exhibit an ellipse more or less open, according as it is nearer one or the other of the preceding positions. it will be seen, also, that in one half of the revolution, the lamp shines on one side of the ring, and in the other half of the revolution, on the other side. such would be the successive appearances of saturn's ring to a spectator on the sun; and since the earth is, in respect to so distant a body as saturn, very near the sun, these appearances are presented to us nearly in the same manner as though we viewed them from the sun. accordingly, we sometimes see saturn's ring under the form of a broad ellipse, which grows continually more and more acute, until it passes into a line, and we either lose sight of it, altogether, or, by the aid of the most powerful telescopes, we see it as a fine thread of light drawn across the disk, and projecting out from it on each side. as the whole revolution occupies thirty years, and the edge is presented to the sun twice in the revolution, this last phenomenon, namely, the disappearance of the ring, takes place every fifteen years. [illustration fig. .] you may perhaps gain a still clearer idea of the foregoing appearances from the following diagram, fig. . let a, b, c, &c., represent successive positions of saturn and his ring, in different parts of his orbit, while _a b_ represents the orbit of the earth. please to remark, that these orbits are drawn so elliptical, not to represent the eccentricity of either the earth's or saturn's orbit, but merely as the projection of circles seen very obliquely. also, imagine one half of the body of the planet and of the ring to be above the plane of the paper, and the other half below it. were the ring, when at c and g, perpendicular to c g, it would be seen by a spectator situated at _a_ or _b_ as a perfect circle; but being inclined to the line of vision twenty-eight degrees four minutes, it is projected into an ellipse. this ellipse contracts in breadth as the ring passes towards its nodes at a and e, where it dwindles into a straight line. from e to g the ring opens again, becomes broadest at g, and again contracts, till it becomes a straight line at a, and from this point expands, till it recovers its original breadth at c. these successive appearances are all exhibited to a telescope of moderate powers. the ring is extremely _thin_, since the smallest satellite, when projected on it, more than covers it. the thickness is estimated at only one hundred miles. saturn's ring shines wholly by _reflected light_ derived from the sun. this is evident from the fact that that side only which is turned towards the sun is enlightened; and it is remarkable, that the illumination of the ring is greater than that of the planet itself, but the outer ring is less bright than the inner. although we view saturn's ring nearly as though we saw it from the sun, yet the plane of the ring produced may pass between the earth and the sun, in which case, also, the ring becomes invisible, the illuminated side being wholly turned from us. thus, when the ring is approaching its node at e, a spectator at _a_ would have the dark side of the ring presented to him. the ring was invisible in , and will be invisible again in . the northern side of the ring will be in sight until , when the southern side will come into view. it appears, therefore, that there are three causes for the disappearance of saturn's ring: first, when the edge of the ring is presented to the sun; secondly, when the edge is presented to the earth; and thirdly, when the unilluminated side is towards the earth. saturn's ring _revolves_ in its own plane in about ten and a half hours. la place inferred this from the doctrine of universal gravitation. he proved that such a rotation was necessary; otherwise, the matter of which the ring is composed would be precipitated upon its primary. he showed that, in order to sustain itself, its period of rotation must be equal to the time of revolution of a satellite, circulating around saturn at a distance from it equal to that of the middle of the ring, which period would be about ten and a half hours. by means of spots in the ring, dr. herschel followed the ring in its rotation, and actually found its period to be the same as assigned by la place,--a coincidence which beautifully exemplifies the harmony of truth. although the rings have very nearly the same centre with the planet itself, yet, recent measurements of extreme delicacy have demonstrated, that the coincidence is not mathematically exact, but that the centre of gravity of the rings describes around that of the body a very minute orbit. "this fact," says sir j. herschel, "unimportant as it may seem, is of the utmost consequence to the stability of the system of rings. supposing them mathematically perfect in their circular form, and exactly concentric with the planet, it is demonstrable that they would form (in spite of their centrifugal force) a system in a state of unstable equilibrium, which the slightest external power would subvert, not by causing a rupture in the substance of the rings, but by precipitating them unbroken upon the surface of the planet." the ring may be supposed of an unequal breadth in its different parts, and as consisting of irregular solids, whose common centre of gravity does not coincide with the centre of the figure. were it not for this distribution of matter, its equilibrium would be destroyed by the slightest force, such as the attraction of a satellite, and the ring would finally precipitate itself upon the planet. sir j. herschel further observes, that, "as the smallest difference of velocity between the planet and its rings must infallibly precipitate the rings upon the planet, never more to separate, it follows, either that their motions in their common orbit round the sun must have been adjusted to each other by an external power, with the minutest precision, or that the rings must have been formed about the planet while subject to their common orbitual motion, and under the full and free influence of all the acting forces. "the rings of saturn must present a magnificent spectacle from those regions of the planet which lie on their enlightened sides, appearing as vast arches spanning the sky from horizon to horizon, and holding an invariable situation among the stars. on the other hand, in the region beneath the dark side, a solar eclipse of fifteen years in duration, under their shadow, must afford (to our ideas) an inhospitable abode to animated beings, but ill compensated by the full light of its satellites. but we shall do wrong to judge of the fitness or unfitness of their condition, from what we see around us, when, perhaps, the very combinations which convey to our minds only images of horror, may be in reality theatres of the most striking and glorious displays of beneficent contrivance." saturn is attended by _seven satellites_. although they are bodies of considerable size, their great distance prevents their being visible to any telescope but such as afford a strong light and high magnifying powers. the outermost satellite is distant from the planet more than thirty times the planet's diameter, and is by far the largest of the whole. it exhibits, like the satellites of jupiter, periodic variations of light, which prove its revolution on its axis in the time of a sidereal revolution about saturn, as is the case with our moon, while performing its circuit about the earth. the next satellite in order, proceeding inwards, is tolerably conspicuous; the three next are very minute, and require powerful telescopes to see them; while the two interior satellites, which just skirt the edge of the ring, and move exactly in its plane, have never been discovered but with the most powerful telescopes which human art has yet constructed, and then only under peculiar circumstances. at the time of the disappearance of the rings, (to ordinary telescopes,) they were seen by sir william herschel, with his great telescope, projected along the edge of the ring, and threading, like beads, the thin fibre of light to which the ring is then reduced. owing to the obliquity of the ring, and of the orbits of the satellites, to that of their primary, there are no eclipses of the satellites, the two interior ones excepted, until near the time when the ring is seen edgewise. "the firmament of saturn will unquestionably present to view a more magnificent and diversified scene of celestial phenomena than that of any other planet in our system. it is placed nearly in the middle of that space which intervenes between the sun and the orbit of the remotest planet. including its rings and satellites, it may be considered as the largest body or system of bodies within the limits of the solar system; and it excels them all in the sublime and diversified apparatus with which it is accompanied. in these respects, saturn may justly be considered as the sovereign among the planetary hosts. the prominent parts of its celestial scenery may be considered as belonging to its own system of rings and satellites, and the views which will occasionally be opened of the firmament of the fixed stars; for few of the other planets will make their appearance in its sky. jupiter will appear alternately as a morning and an evening star, with about the same degree of brilliancy it exhibits to us; but it will seldom be conspicuous, except near the period of its greatest elongation; and it will never appear to remove from the sun further than thirty-seven degrees, and consequently will not appear so conspicuous, nor for such a length of time, as venus does to us. uranus is the only other planet which will be seen from saturn, and it will there be distinctly perceptible, like a star of the third magnitude, when near the time of its opposition to the sun. but near the time of its conjunction it will be completely invisible, being then eighteen hundred millions of miles more distant than at the opposition, and eight hundred millions of miles more distant from saturn than it ever is from the earth at any period."[ ] uranus.--uranus is the remotest planet belonging to our system, and is rarely visible, except to the telescope. although his diameter is more than four times that of the earth, being thirty-five thousand one hundred and twelve miles, yet his distance from the sun is likewise nineteen times as great as the earth's distance, or about eighteen hundred millions of miles. his revolution around the sun occupies nearly eighty-four years, so that his position in the heavens, for several years in succession, is nearly stationary. his path lies very nearly in the ecliptic, being inclined to it less than one degree. the sun himself, when seen from uranus dwindles almost to a star, subtending, as it does, an angle of only one minute and forty seconds; so that the surface of the sun would appear there four hundred times less than it does to us. this planet was discovered by sir william herschel on the thirteenth of march, . his attention was attracted to it by the largeness of its disk in the telescope; and finding that it shifted its place among the stars, he at first took it for a comet, but soon perceived that its orbit was not eccentric, like the orbits of comets, but nearly circular, like those of the planets. it was then recognised as a new member of the planetary system, a conclusion which has been justified by all succeeding observations. it was named by the discoverer the _george star_, (georgium sidus,) after his munificent patron, george the third; in the united states, and in some other countries, it was called _herschel_; but the name _uranus_, from a greek word, (= ouranos=, _ouranos_,) signifying the oldest of the gods, has finally prevailed. so distant is uranus from the sun, that light itself, which moves nearly twelve millions of miles every minute, would require more than two hours and a half to pass to it from the sun. and now, having contemplated all the planets separately, just cast your eyes on the diagram facing page , fig. , and you will see a comparative view of the various magnitudes of the sun, as seen from each of the planets. uranus is attended by _six satellites_. so minute objects are they, that they can be seen only by powerful telescopes. indeed, the existence of more than two is still considered as somewhat doubtful. these two, however, offer remarkable and indeed quite unexpected and unexampled peculiarities. contrary to the unbroken analogy of the whole planetary system, _the planes of their orbits are nearly perpendicular to the ecliptic_, and in these orbits their motions are retrograde; that is, instead of advancing from west to east around their primary, as is the case with all the other planets and satellites, they move in the opposite direction. with this exception, all the motions of the planets, whether around their own axes, or around the sun, are from west to east. the sun himself turns on his axis from west to east; all the primary planets revolve around the sun from west to east; their revolutions on their own axes are also in the same direction; all the secondaries, with the single exception above mentioned, move about their primaries from west to east; and, finally, such of the secondaries as have been discovered to have a diurnal revolution, follow the same course. such uniformity among so many motions could have resulted only from forces impressed upon them by the same omnipotent hand; and few things in the creation more distinctly proclaim that god made the world. retiring now to this furthest verge of the solar system, let us for a moment glance at the aspect of the firmament by night. notwithstanding we have taken a flight of eighteen hundred millions of miles, the same starry canopy bends over our heads; sirius still shines with exactly the same splendor as here; orion, the scorpion, the great and the little bear, all occupy the same stations; and the galaxy spans the sky with the same soft and mysterious light. the planets, however, with the exception of saturn, are all lost to the view, being too near the sun ever to be seen; and saturn himself is visible only at distant intervals, at periods of fifteen years, when at its greatest elongations from the sun, and is then too near the sun to permit a clear view of his rings, much less of the satellites that unite with the rings to compose his gorgeous retinue. comets, moving slowly as they do when so distant from the sun, will linger much longer in the firmament of uranus than in ours. although the sun sheds by day a dim and cheerless light, yet the six satellites that enlighten and diversify the nocturnal sky present interesting aspects. "let us suppose one satellite presenting a surface in the sky eight or ten times larger than our moon; a second, five or six times larger; a third, three times larger; a fourth, twice as large; a fifth, about the same size as the moon; a sixth, somewhat smaller; and, perhaps, three or four others of different apparent dimensions: let us suppose two or three of those, of different phases, moving along the concave of the sky, at one period four or five of them dispersed through the heavens, one rising above the horizon, one setting, one on the meridian, one towards the north, and another towards the south; at another period, five or six of them displaying their lustre in the form of a half moon, or a crescent, in one quarter of the heavens; and, at another time, the whole of these moons shining, with full enlightened hemispheres, in one glorious assemblage, and we shall have a faint idea of the beauty, variety, and sublimity of the firmament of uranus."[ ] _the new planets,--ceres, pallas, juno, and vesta._--the commencement of the present century was rendered memorable in the annals of astronomy, by the discovery of four new planets, occupying the long vacant tract between mars and jupiter. kepler, from some analogy which he found to subsist among the distances of the planets from the sun, had long before suspected the existence of one at this distance; and his conjecture was rendered more probable by the discovery of uranus, which follows the analogy of the other planets. so strongly, indeed, were astronomers impressed with the idea that a planet would be found between mars and jupiter, that, in the hope of discovering it, an association was formed on the continent of europe, of twenty-four observers, who divided the sky into as many zones, one of which was allotted to each member of the association. the discovery of the first of these bodies was, however, made accidentally by piazzi, an astronomer of palermo, on the first of january, . it was shortly afterwards lost sight of on account of its proximity to the sun, and was not seen again until the close of the year, when it was re-discovered in germany. piazzi called it _ceres_, in honor of the tutelary goddess of sicily, and her emblem, the sickle, ([planet: ceres]) has been adopted as its appropriate symbol. the difficulty of finding ceres induced dr. olbers, of bremen, to examine with particular care all the small stars that lie near her path, as seen from the earth; and, while prosecuting these observations, in march, , he discovered another similar body, very nearly at the same distance from the sun, and resembling the former in many other particulars. the discoverer gave to this second planet the name of _pallas_, choosing for its symbol the lance, ([planet: pallas]) the characteristic of minerva. the most surprising circumstance connected with the discovery of _pallas_ was the existence of two planets at nearly the same distance from the sun, and apparently crossing the ecliptic in the same part of the heavens, or having the same node. on account of this singularity, dr. olbers was led to conjecture that ceres and pallas are only fragments of a larger planet, which had formerly circulated at the same distance, and been shattered by some internal convulsion. the hypothesis suggested the probability that there might be other fragments, whose orbits might be expected to cross the ecliptic at a common point, or to have the same node. dr. olbers, therefore, proposed to examine carefully, every month, the two opposite parts of the heavens in which the orbits of ceres and pallas intersect one another, with a view to the discovery of other planets, which might be sought for in those parts with a greater chance of success, than in a wider zone, embracing the entire limits of these orbits. accordingly, in , near one of the nodes of ceres and pallas, a third planet was discovered. this was called _juno_, and the character ([planet: juno]) was adopted for its symbol, representing the starry sceptre of the queen of olympus. pursuing the same researches, in a fourth planet was discovered, to which was given the name of _vesta_, and for its symbol the character ([planet: vesta]) was chosen,--an altar surmounted with a censer holding the sacred fire. the _average distance_ of these bodies from the sun is two hundred and sixty-one millions of miles; and it is remarkable that their orbits are very near together. taking the distance of the earth from the sun for unity, their respective distances are . , . , . , . . their _times_ of revolution around the sun are nearly equal, averaging about four and a half years. in respect to the _inclination of their orbits_ to the ecliptic, there is also considerable diversity. the orbit of vesta is inclined only about seven degrees, while that of pallas is more than thirty-four degrees. they all, therefore, have a higher inclination than the orbits of the old planets, and of course make excursions from the ecliptic beyond the limits of the zodiac. hence they have been called the _ultra-zodiacal planets_. when first discovered, before their place in the system was fully ascertained it was also proposed to call them _asteroids_, a name implying that they were planets under the form of stars. their title, however, to take their rank among the primary planets, is now generally conceded. the _eccentricity of their orbits_ is also, in general, greater than that of the old planets. you will recollect that this language denotes that their orbits are more elliptical, or depart further from the circular form. the eccentricities of the orbits of pallas and juno exceed that of the orbit of mercury. the asteroids differ so much, however, in eccentricity, that their orbits may cross each other. the orbits of the old planets are so nearly circular, and at such a great distance apart, that there is no danger of their interfering with each other. the earth, for example, when at its nearest distance from the sun, will never come so near it as venus is when at its greatest distance, and therefore can never cross the orbit of venus. but since the average distance of ceres and pallas from the sun is about the same, while the eccentricity of the orbit of pallas is much greater than that of ceres, consequently, pallas may come so near to the sun at its perihelion, as to cross the orbit of ceres. the _small size_ of the asteroids constitutes one of their most remarkable peculiarities. the difficulty of estimating the apparent diameter of bodies at once so very small and so far off, would lead us to expect different results in the actual estimates. accordingly, while dr. herschel estimates the diameter of pallas at only eighty miles, schroeter places it as high as two thousand miles, or about the diameter of the moon. the volume of vesta is estimated at only one fifteen thousandth part of the earth's, and her surface is only about equal to that of the kingdom of spain. these little bodies are surrounded by _atmospheres_ of great extent, some of which are uncommonly luminous, and others appear to consist of nebulous matter, like that of comets. these planets shine with a more vivid light than might be expected, from their great distance and diminutive size; but a good telescope is essential for obtaining a distinct view of their phenomena. although the great chasm which occurs between mars and jupiter,--a chasm of more than three hundred millions of miles,--suggested long ago the idea of other planetary bodies occupying that part of the solar system, yet the discovery of the asteroids does not entirely satisfy expectation since they are bodies so dissimilar to the other members of the series in size, in appearance, and in the form and inclinations of their orbits. hence, dr. olbers, the discoverer of three of these bodies, held that they were fragments of a single large planet, which once occupied that place in the system, and which exploded in different directions by some internal violence. of the fragments thus projected into space, some would be propelled in such directions and with such velocities, as, under the force of projection and that of the solar attraction would make them revolve in regular orbits around the sun. others would be so projected among the other bodies in the system, as to be thrown in very irregular orbits, apparently wandering lawless through the skies. the larger fragments would receive the least impetus from the explosive force, and would therefore circulate in an orbit deviating less than any other of the fragments from the original path of the large planet; while the lesser fragments, being thrown off with greater velocity, would revolve in orbits more eccentric, and more inclined to the ecliptic. dr. brewster, editor of the 'edinburgh encyclopedia,' and the well-known author of various philosophical works, espoused this hypothesis with much zeal; and, after summing up the evidence in favor of it, he remarks as follows: "these singular resemblances in the motions of the greater fragments, and in those of the lesser fragments, and the striking coincidences between theory and observation in the eccentricity of their orbits, in their inclination to the ecliptic, in the position of their nodes, and in the places of their perihelia, are phenomena which could not possibly result from chance, and which concur to prove, with an evidence amounting almost to demonstration, that the four new planets have diverged from one common node, and have therefore composed a single planet." the same distinguished writer supposes that some of the smallest fragments might even have come within reach of the earth's attraction, and by the combined effects of their projectile forces and the attraction of the earth, be made to revolve around this body as the larger fragments are carried around the sun; and that these are in fact the bodies which afford those _meteoric stones_ which are occasionally precipitated to the earth. it is now a well-ascertained fact, a fact which has been many times verified in our own country, that large meteors, in the shape of fire-balls, traversing the atmosphere, sometimes project to the earth masses of stony or ferruginous matter. such were the meteoric stones which fell at weston, in connecticut, in , of which a full and interesting account was published, after a minute examination of the facts, by professors silliman and kingsley, of yale college. various accounts of similar occurrences may be found in different volumes of the american journal of science. it is for the production of these wonderful phenomena that dr. brewster supposes the explosion of the planet, which, according to dr. olbers, produced the asteroids, accounts. others, however, as sir john herschel, have been disposed to ascribe very little weight to the doctrine of olbers. footnotes: [ ] altissimum planetam tergeminum observavi. or, as transposed, smaismrmilme poeta leumi bvne nugttaviras. [ ] in imitation of galileo, huyghens announced his discovery in this form: a a a a a a a c c c c c d e e e e e g h i i i i i i i l l l l m m n n n n n n n n n o o o o p p q r r s t t t t t u u u u u; which he afterwards recomposed into this sentence: _annulo cingitur, tenui, plano, nusquam cohærente, ad eclipticam inclinato._ [ ] dick's 'celestial scenery.' [ ] dick's 'celestial scenery.' letter xxiv. the planetary motions.----kepler's laws.----kepler. "god of the rolling orbs above! thy name is written clearly bright in the warm day's unvarying blaze, or evening's golden shower of light; for every fire that fronts the sun, and every spark that walks alone around the utmost verge of heaven, was kindled at thy burning throne."--_peabody._ if we could stand upon the sun and view the planetary motions, they would appear to us as simple as the motions of equestrians riding with different degrees of speed around a large ring, of which we occupied the centre. we should see all the planets coursing each other from west to east, through the same great highway, (the zodiac,) no one of them, with the exception of the asteroids, deviating more than seven degrees from the path pursued by the earth. most of them, indeed, would always be seen moving much nearer than that to the ecliptic. we should see the planets moving on their way with various degrees of speed. mercury would make the entire circuit in about three months, hurrying on his course with a speed about one third as great as that by which the moon revolves around the earth. the most distant planets, on the other hand, move at so slow a pace, that we should see mercury, venus, the earth, and mars, severally overtaking them a great many times, before they had completed their revolutions. but though the movements of some were comparatively rapid, and of others extremely slow, yet they would not seem to differ materially, in other respects: each would be making a steady and nearly uniform march along the celestial vault. such would be the simple and harmonious motions of the planets, as they would be seen from the sun, the centre of their motions; and such they are, in fact. but two circumstances conspire to make them appear exceedingly different from these, and vastly more complicated; one is, that we view them out of the centre of their motions; the other, that we are ourselves in motion. i have already explained to you the effect which these two causes produce on the apparent motions of the inferior planets, mercury and venus. let us now see how they effect those of the superior planets, mars, jupiter, saturn, and uranus. orreries, or machines intended to exhibit a model of the solar system, are sometimes employed to give a popular view of the planetary motions; but they oftener mislead than give correct ideas. they may assist reflection, but they can never supply its place. the impossibility of representing things in their just proportions will be evident, when we reflect that, to do this, if in an orrery we make mercury as large as a cherry, we should have to represent the sun six feet in diameter. if we preserve the same proportions, in regard to distance, we must place mercury two hundred and fifty feet, and uranus twelve thousand five hundred feet, or more than two miles from the sun. the mind of the student of astronomy must, therefore, raise itself from such imperfect representations of celestial phenomena, as are afforded by artificial mechanism, and, transferring his contemplations to the celestial regions themselves, he must conceive of the sun and planets as bodies that bear an insignificant ratio to the immense spaces in which they circulate, resembling more a few little birds flying in the open sky, than they do the crowded machinery of an orrery. the _real_ motions of the planets, indeed, or such as orreries usually exhibit, are very easily conceived of, as before explained; but the _apparent_ motions are, for the most part, entirely different from these. the apparent motions of the inferior planets have been already explained. you will recollect that mercury and venus move backwards and forwards across the sun, the former never being seen further than twenty-nine degrees, and the latter never more than about forty-seven degrees, from that luminary; that, while passing from the greatest elongation on one side, to the greatest elongation on the other side, through the superior conjunction, the apparent motions of these planets are accelerated by the motion of the earth; but that, while moving through the inferior conjunction, at which time their motions are retrograde, they are apparently retarded by the earth's motion. let us now see what are the apparent motions of the superior planets. let a, b, c, fig. , page , represent the earth in different positions in its orbit, m, a superior planet, as mars, and n r, an arc of the concave sphere of the heavens. first, suppose the planet to remain at rest in m, and let us see what apparent motions it will receive from the real motions of the earth. when the earth is at b, it will see the planet in the heavens at n; and as the earth moves successively through c, d, e, f, the planet will appear to move through o, p, q, r. b and f are the two points of greatest elongation of the earth from the sun, as seen from the planet; hence, between these two points, while passing through its orbit most remote from the planet, (when the planet is seen in superior conjunction,) the earth, by its own motion, gives an apparent motion to the planet in the order of the signs; that is, the _apparent_ motion given by the _real_ motion of the earth is _direct_. but in passing from f to b through a, when the planet is seen in opposition, the apparent motion given to the planet by the earth's motion is from r to n, and is therefore _retrograde_. as the arc described by the earth, when the motion is direct, is much greater than when the motion is retrograde, while the apparent arc of the heavens described by the planet from n to r, in the one case, and from r to n, in the other, is the same in both cases, the retrograde motion is much swifter than the direct, being performed in much less time. [illustration fig. .] but the superior planets are not in fact at rest, as we have supposed, but are all the while moving eastward, though with a slower motion than the earth. indeed, with respect to the remotest planets, as saturn and uranus, the forward motion is so exceedingly slow, that the above representation is nearly true for a single year. still, the effect of the real motions of all the superior planets, eastward, is to increase the direct apparent motion communicated by the earth, and to diminish the retrograde motion. this will be evident from inspecting the figure; for if the planet _actually_ moves eastward while it is _apparently_ carried eastward by the earth's motion, the whole motion eastward will be equal to the sum of the two; and if, while it is really moving eastward, it is apparently carried westward still more by the earth's motion, the retrograde movement will equal the difference of the two. if mars stood still while the earth went round the sun, then a second opposition, as at a, would occur at the end of one year from the first; but, while the earth is performing this circuit, mars is also moving the same way, more than half as fast; so that, when the earth returns to a, the planet has already performed more than half the same circuit, and will have completed its whole revolution before the earth comes up with it. indeed mars, after having been seen once in opposition, does not come into opposition again until after two years and fifty days. and since the planet is then comparatively near to us, as at m, while the earth is at a, and appears very large and bright, rising unexpectedly about the time the sun sets, he surprises the world as though it were some new celestial body. but on account of the slow progress of saturn and uranus, we find, after having performed one circuit around the sun, that they are but little advanced beyond where we left them at the last opposition. the time between one opposition of saturn and another is only a year and thirteen days. it appears, therefore, that the superior planets steadily pursue their course around the sun, but that their apparent retrograde motion, when in opposition, is occasioned by our passing by them with a swifter motion, of which we are unconscious, like the apparent backward motion of a vessel, when we overtake it and pass by it rapidly in a steam-boat. such are the real and the apparent motions of the planets. let us now turn our attention to the _laws of the planetary orbits_. there are three great principles, according to which the motions of the earth and all the planets around the sun are regulated, called kepler's laws, having been first discovered by the astronomer whose name they bear. they may appear to you, at first, dry and obscure; yet they will be easily understood from the explanations which follow; and so important have they proved in astronomical inquiries, that they have acquired for their renowned discoverer the appellation of the '_legislator of the skies_.' we will consider each of these laws separately; and, for the sake of rendering the explanation clear and intelligible, i shall perhaps repeat some things that have been briefly mentioned before. [illustration fig. .] first law.--_the orbits of the earth and all the planets are ellipses, having the sun in the common focus._ in a circle, all the diameters are equal to one another; but if we take a metallic wire or hoop, and draw it out on opposite sides, we elongate it into an ellipse, of which the different diameters are very unequal. that which connects the points most distant from each other is called the _transverse_, and that which is at right angles to this is called the _conjugate_, axis. thus, a b, fig. , is the transverse axis, and c d, the conjugate of the ellipse a b c. by such a process of elongating the circle into an ellipse, the centre of the circle may be conceived of as drawn opposite ways to e and f, each of which becomes a _focus_, and both together are called the _foci_ of the ellipse. the distance g e, or g f, of the focus from the centre is called the _eccentricity_ of the ellipse; and the ellipse is said to be more or less eccentric, as the distance of the focus from the centre is greater or less. figure represents such a collection of ellipses around the common focus f, the innermost, a g d, having a small eccentricity, or varying little from a circle, while the outermost, a c b, is an eccentric ellipse. the orbits of all the bodies that revolve about the sun, both planets and comets, have, in like manner, a common focus, in which the sun is situated, but they differ in eccentricity. most of the planets have orbits of very little eccentricity, differing little from circles, but comets move in very eccentric ellipses. the earth's path around the sun varies so little from a circle, that a diagram representing it truly would scarcely be distinguished from a perfect circle; yet, when the comparative distances of the sun from the earth are taken at different seasons of the year, we find that the difference between their greatest and least distances is no less than three millions of miles. [illustration fig. .] second law.--_the radius vector of the earth, or of any planet, describes equal areas in equal times._ you will recollect that the radius vector is a line drawn from the centre of the sun to a planet revolving about the sun. this definition i have somewhere given you before, and perhaps it may appear to you like needless repetition to state it again. in a book designed for systematic instruction, where all the articles are distinctly numbered, it is commonly sufficient to make a reference back to the article where the point in question is explained; but i think, in letters like these, you will bear with a little repetition, rather than be at the trouble of turning to the index and hunting up a definition long since given. [illustration fig. . ] in figure , _e a_, _e b_, _e c_, &c., are successive representations of the radius vector. now, if a planet sets out from _a_, and travels round the sun in the direction of _a b c_, it will move faster when nearer the sun, as at _a_, than when more remote from it, as at _m_; yet, if _a b_ and _m n_ be arcs described in equal times, then, according to the foregoing law, the space _e a b_ will be equal to the space _e m n_; and the same is true of all the other spaces described in equal times. although the figure _e a b_ is much shorter than _e m n_, yet its greater breadth exactly counterbalances the greater length of those figures which are described by the radius vector where it is longer. third law.--_the squares of the periodical times are as the cubes of the mean distances from the sun._ the periodical time of a body is the time it takes to complete its orbit, in its revolution about the sun. thus the earth's periodic time is one year, and that of the planet jupiter about twelve years. as jupiter takes so much longer time to travel round the sun than the earth does, we might suspect that his orbit is larger than that of the earth, and of course, that he is at a greater distance from the sun; and our first thought might be, that he is probably twelve times as far off; but kepler discovered that the distance does not increase as fast as the times increase, but that the planets which are more distant from the sun actually move slower than those which are nearer. after trying a great many proportions, he at length found that, if we take the squares of the periodic times of two planets, the greater square contains the less, just as often as the cube of the distance of the greater contains that of the less. this fact is expressed by saying, that the squares of the periodic times are to one another as the cubes of the distances. this law is of great use in determining the distance of the planets from the sun. suppose, for example, that we wish to find the distance of jupiter. we can easily determine, from observation, what is jupiter's periodical time, for we can actually see how long it takes for jupiter, after leaving a certain part of the heavens to come round to the same part again. let this period be twelve years. the earth's period is of course one year; and the distance of the earth, as determined from the sun's horizontal parallax, as already explained, is about ninety-five millions of miles. now, we have here three terms of a proportion to find the fourth, and therefore the solution is merely a simple case of the rule of three. thus:--the square of year : square of years :: cube of , , : cube of jupiter's distance. the three first terms being known, we have only to multiply together the second and third and divide by the first, to obtain the fourth term, which will give us the cube of jupiter's distance from the sun; and by extracting the cube root of this sum, we obtain the distance itself. in the same manner we may obtain the respective distances of all the other planets. so truly is this a law of the solar system, that it holds good in respect to the new planets, which have been discovered since kepler's time, as well as in the case of the old planets. it also holds good in respect to comets, and to all bodies belonging to the solar system, which revolve around the sun as their centre of motion. hence, it is justly regarded as one of the most interesting and important principles yet developed in astronomy. but who was this kepler, that gained such an extraordinary insight into the laws of the planetary system, as to be called the 'legislator of the skies?' john kepler was one of the most remarkable of the human race, and i think i cannot gratify or instruct you more, than by occupying the remainder of this letter with some particulars of his history. kepler was a native of germany. he was born in the duchy of wurtemberg, in . as copernicus, tycho brahe, galileo, kepler, and newton, are names that are much associated in the history of astronomy, let us see how they stood related to each other in point of time. copernicus was born in ; tycho, in ; galileo, in ; kepler, in ; and newton, in . hence, copernicus was seventy-three years before tycho, and tycho ninety-six years before newton. they all lived to an advanced age, so that tycho, galileo, and kepler, were contemporary for many years; and newton, as i mentioned in the sketch i gave you of his life, was born the year that galileo died. kepler was born of parents who were then in humble circumstances, although of noble descent. their misfortunes, which had reduced them to poverty, seem to have been aggravated by their own unhappy dispositions; for his biographer informs us, that "his mother was treated with a degree of barbarity by her husband and brother-in-law, that was hardly exceeded by her own perverseness." it is fortunate, therefore, that kepler, in his childhood, was removed from the immediate society and example of his parents, and educated at a public school at the expense of the duke of wurtemberg. he early imbibed a taste for natural philosophy, but had conceived a strong prejudice against astronomy, and even a contempt for it, inspired, probably, by the arrogant and ridiculous pretensions of the astrologers, who constituted the principal astronomers of his country. a vacant post, however, of teacher of astronomy, occurred when he was of a suitable age to fill it, and he was compelled to take it by the authority of his tutors, though with many protestations, on his part, wishing to be provided for in some other more brilliant profession. happy is genius, when it lights on a profession entirely consonant to its powers, where the objects successively presented to it are so exactly suited to its nature, that it clings to them as the loadstone to its kindred metal among piles of foreign ores. nothing could have been more congenial to the very mental constitution of kepler, than the study of astronomy,--a science where the most capacious understanding may find scope in unison with the most fervid imagination. much as has been said against hypotheses in philosophy, it is nevertheless a fact, that some of the greatest truths have been discovered in the pursuit of hypotheses, in themselves entirely false; truths, moreover, far more important than those assumed by the hypotheses; as columbus, in searching for a northwest passage to india, discovered a new world. thus kepler groped his way through many false and absurd suppositions, to some of the most sublime discoveries ever made by man. the fundamental principle which guided him was not, however, either false or absurd. it was, that god, who made the world, had established, throughout all his works, fixed laws,--laws that are often so definite as to be capable of expression in exact numerical terms. in accordance with these views, he sought for numerical relations in the disposition and arrangement of the planets, in respect to their number, the times of their revolution, and their distances from one another. many, indeed, of the subordinate suppositions which he made, were extremely fanciful; but he tried his own hypotheses by a rigorous mathematical test, wherever he could apply it; and as soon as he discovered that a supposition would not abide this test, he abandoned it without the least hesitation, and adopted others, which he submitted to the same severe trial, to share, perhaps, the same fate. "after many failures," he says, "i was comforted by observing that the motions, in every case, seemed to be connected with the distances; and that, when there was a great gap between the orbits, there was the same between the motions. and i reasoned that, if god had adapted motions to the orbits in some relation to the distances, he had also arranged the distances themselves in relation to something else." in two years after he commenced the study of astronomy, he published a book, called the '_mysterium cosmographicum_,' a name which implies an explanation of the mysteries involved in the construction of the universe. this work was full of the wildest speculations and most extravagant hypotheses, the most remarkable of which was, that the distances of the planets from the sun are regulated by the relations which subsist between the five regular solids. it is well known to geometers, that there are and can be only five _regular solids_. these are, first, the _tetraedron_, a four-sided figure, all whose sides are equal and similar triangles; secondly, the _cube_, contained by six equal squares; thirdly, an _octaedron_, an eight-sided figure, consisting of two four-sided pyramids joined at their bases; fourthly, a _dodecaedron_, having twelve five-sided or pentagonal faces; and, fifthly, an _icosaedron_, contained by twenty equal and similar triangles. you will be much at a loss, i think, to imagine what relation kepler could trace between these strange figures and the distances of the several planets from the sun. he thought he discovered a connexion between those distances and the spaces which figures of this kind would occupy, if interposed in certain ways between them. thus, he says the earth is a circle, the measure of all; round it describe a dodecaedron, and the circle including this will be the orbit of mars. round this circle describe a tetraedron, and the circle including this will be the orbit of jupiter. describe a cube round this, and the circle including it will be the orbit of saturn. now, inscribe in the earth an icosaedron, and the circle included in this will give the orbit of venus. in this inscribe an octaedron, and the circle included in this will be the orbit of mercury. on this supposed discovery kepler exults in the most enthusiastic expressions. "the intense pleasure i have received from this discovery never can be told in words. i regretted no more time wasted; i tired of no labor; i shunned no toil of reckoning; days and nights i spent in calculations, until i could see whether this opinion would agree with the orbits of copernicus, or whether my joy was to vanish into air. i willingly subjoin that sentiment of archytas, as given by cicero; 'if i could mount up into heaven, and thoroughly perceive the nature of the world and the beauty of the stars, that admiration would be without a charm for me, unless i had some one like you, reader, candid, attentive, and eager for knowledge, to whom to describe it.' if you acknowledge this feeling, and are candid, you will refrain from blame, such as, not without cause, i anticipate; but if, leaving that to itself, you fear, lest these things be not ascertained, and that i have shouted triumph before victory, at least approach these pages, and learn the matter in consideration: you will not find, as just now, new and unknown planets interposed; that boldness of mine is not approved; but those old ones very little loosened, and so furnished by the interposition (however absurd you may think it) of rectilinear figures, that in future you may give a reason to the rustics, when they ask for the hooks which keep the skies from falling." when tycho brahe, who had then retired from his famous uraniburg, and was settled in prague, met with this work of kepler's, he immediately recognised under this fantastic garb the lineaments of a great astronomer. he needed such an unwearied and patient calculator as he perceived kepler to be, to aid him in his labors, in order that he might devote himself more unreservedly to the taking of observations,--an employment in which he delighted, and in which, as i mentioned, in giving you a sketch of his history, he excelled all men of that and preceding ages. kepler, therefore, at the express invitation of tycho, went to prague, and joined him in the capacity of assistant. had tycho been of a nature less truly noble, he might have looked with contempt on one who had made so few observations, and indulged so much in wild speculation; or he might have been jealous of a rising genius, in which he descried so many signs of future eminence as an astronomer; but, superior to all the baser motives, he extends to the young aspirant the hand of encouragement, in the following kind invitation: "come not as a stranger, but as a very welcome friend; come, and share in my observations, with such instruments as i have with me." several years previous to this, kepler, after one or two unsuccessful trials, had found him a wife, from whom he expected a considerable fortune; but in this he was disappointed; and so poor was he, that, when on his journey to prague, in company with his wife, being taken sick, he was unable to defray the expenses of the journey, and was forced to cast himself on the bounty of tycho. in the course of the following year, while absent from prague, he fancied that tycho had injured him, and accordingly addressed to the noble dane a letter full of insults and reproaches. a mild reply from tycho opened the eyes of kepler to his own ingratitude. his better feelings soon returned, and he sent to his great patron this humble apology: "most noble tycho! how shall i enumerate, or rightly estimate, your benefits conferred on me! for two months you have liberally and gratuitously maintained me, and my whole family; you have provided for all my wishes; you have done me every possible kindness; you have communicated to me every thing you hold most dear; no one, by word or deed, has intentionally injured me in any thing; in short, not to your own children, your wife, or yourself, have you shown more indulgence than to me. this being so, as i am anxious to put upon record, i cannot reflect, without consternation, that i should have been so given up by god to my own intemperance, as to shut my eyes on all these benefits; that, instead of modest and respectful gratitude, i should indulge for three weeks in continual moroseness towards all your family, and in headlong passion and the utmost insolence towards yourself, who possess so many claims on my veneration, from your noble family, your extraordinary learning, and distinguished reputation. whatever i have said or written against the person, the fame, the honor, and the learning, of your excellency; or whatever, in any other way, i have injuriously spoken or written, (if they admit no other more favorable interpretation,) as to my grief i have spoken and written many things, and more than i can remember; all and every thing i recant, and freely and honestly declare and profess to be groundless, false, and incapable of proof." this was ample satisfaction to the generous tycho. "to err is human: to forgive, divine." on kepler's return to prague, he was presented to the emperor by tycho, and honored with the title of imperial mathematician. this was in , when he was thirty years of age. tycho died shortly after, and kepler succeeded him as principal mathematician to the emperor; but his salary was badly paid, and he suffered much from pecuniary embarrassments. although he held the astrologers, or those who told fortunes by the stars, in great contempt, yet he entertained notions of his own, on the same subject, quite as extravagant, and practised the art of casting nativities, to eke out a support for his family. when galileo began to observe with his telescope, and announced, in rapid succession, his wonderful discoveries, kepler entered into them with his characteristic enthusiasm, although they subverted many of his favorite hypotheses. but such was his love of truth, that he was among the first to congratulate galileo, and a most engaging correspondence was carried on between these master-spirits. the first planet, which occupied the particular attention of kepler, was mars, the long and assiduous study of whose motions conducted him at length to the discovery of those great principles called 'kepler's laws.' rarely do we meet with so remarkable a union of a vivid fancy with a profound intellect. the hasty and extravagant suggestions of the former were submitted to the most laborious calculations, some of which, that were of great length, he repeated seventy times. this exuberance of fancy frequently appears in his style of writing, which occasionally assumes a tone ludicrously figurative. he seems constantly to contemplate mars as a valiant hero, who had hitherto proved invincible, and who would often elude his own efforts to conquer him, "while thus triumphing over mars, and preparing for him, as for one altogether vanquished, tabular prisons, and equated, eccentric fetters, it is buzzed here and there, that the victory is vain, and that the war is raging anew as violently as before. for the enemy, left at home a despised captive, has burst all the chains of the equation, and broken forth of the prisons of the tables. skirmishes routed my forces of physical causes, and, shaking off the yoke, regained their liberty. and now, there was little to prevent the fugitive enemy from effecting a junction with his own rebellious supporters, and reducing me to despair, had i not suddenly sent into the field a reserve of new physical reasonings, on the rout and dispersion of the veterans, and diligently followed, without allowing the slightest respite, in the direction in which he had broken out." but he pursued this warfare with the planet until he gained a full conquest, by the discovery of the first two of his laws, namely, that _he revolves in an elliptical orbit_, and that _his radius vector passes over equal spaces in equal times_. domestic troubles, however, involved him in the deepest affliction. poverty, the loss of a promising and favorite son, the death of his wife, after a long illness;--these were some of the misfortunes that clustered around him. although his first marriage had been an unhappy one, it was not consonant to his genius to surrender any thing with only a single trial. accordingly, it was not long before he endeavored to repair his loss by a second alliance. he commissioned a number of his friends to look out for him, and he soon obtained a tabular list of eleven ladies, among whom his affections wavered. the progress of his courtship is thus narrated in the interesting 'life' contained in the 'library of useful knowledge.' it furnishes so fine a specimen of his eccentricities, that i cannot deny myself the pleasure of transcribing the passage for your perusal. it is taken from an account which kepler himself gave in a letter to a friend. "the first on the list was a widow, an intimate friend of his first wife and who, on many accounts, appeared a most eligible match. at first, she seemed favorably inclined to the proposal: it is certain that she took time to consider it, but at last she very quietly excused herself. finding her afterwards less agreeable in person than he had anticipated, he considered it a fortunate escape, mentioning, among other objections, that she had two marriageable daughters, whom, by the way, he had got on his list for examination. he was much troubled to reconcile his astrology with the fact of his having taken so much pains about a negotiation not destined to succeed. he examined the case professionally. 'have the stars,' says he, 'exercised any influence here? for, just about this time, the direction of the mid-heaven is in hot opposition to mars, and the passage of saturn through the ascending point of the zodiac, in the scheme of my nativity, will happen again next november and december. but, if these are the causes, how do they act? is that explanation the true one, which i have elsewhere given? for i can never think of handing over to the stars the office of deities, to produce effects. let us, therefore, suppose it accounted for by the stars, that at this season i am violent in my temper and affections, in rashness of belief, in a show of pitiful tender-heartedness, in catching at reputation by new and paradoxical notions, and the singularity of my actions; in busily inquiring into, and weighing, and discussing, various reasons; in the uneasiness of my mind, with respect to my choice. i thank god, that that did not happen which might have happened; that this marriage did not take place. now for the others.' of these, one was too old; another, in bad health; another, too proud of her birth and quarterings; a fourth had learned nothing but showy accomplishments, not at all suitable to the kind of life she would have to lead with him. another grew impatient, and married a more decided admirer while he was hesitating. 'the mischief,' says he, 'in all these attachments was, that, whilst i was delaying, comparing, and balancing, conflicting reasons, every day saw me inflamed with a new passion.' by the time he reached no. , of his list, he found his match in this respect. 'fortune has avenged herself at length on my doubtful inclinations. at first, she was quite complying, and her friends also. presently, whether she did or did not consent, not only i, but she herself, did not know. after the lapse of a few days, came a renewed promise, which, however, had to be confirmed a third time: and, four days after that, she again repented her conformation, and begged to be excused from it. upon this, i gave her up, and this time all my counsellors were of one opinion.' this was the longest courtship in the list, having lasted three whole months; and, quite disheartened by its bad success, kepler's next attempt was of a more timid complexion. his advances to no. were made by confiding to her the whole story of his recent disappointment, prudently determining to be guided in his behavior, by observing whether the treatment he experienced met with a proper degree of sympathy. apparently, the experiment did not succeed; and, when almost reduced to despair, kepler betook himself to the advice of a friend, who had for some time past complained that she was not consulted in this difficult negotiation. when she produced no. , and the first visit was paid, the report upon her was as follows: 'she has, undoubtedly, a good fortune, is of good family, and of economical habits: but her physiognomy is most horribly ugly; she would be stared at in the streets, not to mention the striking disproportion in our figures. i am lank, lean, and spare; she is short and thick. in a family notorious for fatness, she is considered superfluously fat.' the only objection to no. seems to have been, her excessive youth; and when this treaty was broken off, on that account, kepler turned his back upon all his advisers, and chose for himself one who had figured as no. , in his list, to whom he professes to have felt attached throughout, but from whom the representations of his friends had hitherto detained him, probably on account of her humble station." having thus settled his domestic affairs, kepler now betook himself, with his usual industry, to his astronomical studies, and brought before the world the most celebrated of his publications, entitled 'harmonics.' in the fifth book of this work he announced his _third law_,--that the squares of the periodical times of the planets are as the cubes of the distances. kepler's rapture on detecting it was unbounded. "what," says he, "i prophesied two-and-twenty years ago, as soon as i discovered the five solids among the heavenly orbits; what i firmly believed long before i had seen ptolemy's harmonics; what i had promised my friends in the title of this book, which i named before i was sure of my discovery; what, sixteen years ago, i urged as a thing to be sought; that for which i joined tycho brahe, for which i settled in prague, for which i have devoted the best part of my life to astronomical contemplations;--at length i have brought to light, and have recognised its truth beyond my most sanguine expectations. it is now eighteen months since i got the first glimpse of light, three months since the dawn, very few days since the unveiled sun, most admirable to gaze on, burst out upon me. nothing holds me: i will indulge in my sacred fury; i will triumph over mankind by the honest confession, that i have stolen the golden vases of the egyptians to build up a tabernacle for my god, far from the confines of egypt. if you forgive me, i rejoice: if you are angry, i can bear it; the die is cast, the book is written, to be read either now or by posterity,--i care not which. i may well wait a century for a reader, as god has waited six thousand years for an observer." in accordance with the notion he entertained respecting the "music of the spheres," he made saturn and jupiter take the bass, mars the tenor, the earth and venus the counter, and mercury the treble. "the misery in which kepler lived," says sir david brewster, in his 'life of newton,' "forms a painful contrast with the services which he performed for science. the pension on which he subsisted was always in arrears; and though the three emperors, whose reigns he adorned, directed their ministers to be more punctual in its payment, the disobedience of their commands was a source of continual vexation to kepler. when he retired to silesia, to spend the remainder of his days, his pecuniary difficulties became still more harassing. necessity at length compelled him to apply personally for the arrears which were due; and he accordingly set out, in , when nearly sixty years of age, for ratisbon; but, in consequence of the great fatigue which so long a journey on horseback produced, he was seized with a fever, which put an end to his life." professor whewell (in his interesting work on astronomy and general physics considered with reference to natural theology) expresses the opinion that kepler, notwithstanding his constitutional oddities, was a man of strong and lively piety. his 'commentaries on the motions of mars' he opens with the following passage: "i beseech my reader, that, not unmindful of the divine goodness bestowed on man, he do with me praise and celebrate the wisdom and greatness of the creator, which i open to him from a more inward explication of the form of the world, from a searching of causes, from a detection of the errors of vision; and that thus, not only in the firmness and stability of the earth, he perceive with gratitude the preservation of all living things in nature as the gift of god, but also that in its motion, so recondite, so admirable, he acknowledge the wisdom of the creator. but him who is too dull to receive this science, or too weak to believe the copernican system without harm to his piety,--him, i say, i advise that, leaving the school of astronomy, and condemning, if he please, any doctrines of the philosophers, he follow his own path, and desist from this wandering through the universe; and, lifting up his natural eyes, with which he alone can see, pour himself out in his own heart, in praise of god the creator; being certain that he gives no less worship to god than the astronomer, to whom god has given to see more clearly with his inward eye, and who, for what he has himself discovered, both can and will glorify god." in a life of kepler, very recently published in his native country, founded on manuscripts of his which have lately been brought to light, there are given numerous other examples of a similar devotional spirit. kepler thus concludes his harmonics: "i give thee thanks, lord and creator, that thou has given me joy through thy creation; for i have been ravished with the work of thy hands. i have revealed unto mankind the glory of thy works, as far as my limited spirit could conceive their infinitude. should i have brought forward any thing that is unworthy of thee, or should i have sought my own fame, be graciously pleased to forgive me." as galileo experienced the most bitter persecutions from the church of rome, so kepler met with much violent opposition and calumny from the protestant clergy of his own country, particularly for adopting, in an almanac which, as astronomer royal, he annually published, the reformed calendar, as given by the pope of rome. his opinions respecting religious liberty, also, appear to have been greatly in advance of the times in which he lived. in answer to certain calumnies with which he was assailed, for his boldness in reasoning from the light of nature, he uttered these memorable words: "the day will soon break, when pious simplicity will be ashamed of its blind superstition; when men will recognise truth in the book of nature as well as in the holy scriptures, and rejoice in the two revelations." letter xxv. comets. ----"fancy now no more wantons on fickle pinions through the skies, but, fixed in aim, and conscious of her power, sublime from cause to cause exults to rise, creation's blended stores arranging as she flies."--_beattie._ nothing in astronomy is more truly admirable, than the knowledge which astronomers have acquired of the motions of comets, and the power they have gained of predicting their return. indeed, every thing appertaining to this class of bodies is so wonderful, as to seem rather a tale of romance than a simple recital of facts. comets are truly the knights-errant of astronomy. appearing suddenly in the nocturnal sky, and often dragging after them a train of terrific aspect, they were, in the earlier ages of the world, and indeed until a recent period, considered as peculiarly ominous of the wrath of heaven, and as harbingers of wars and famines, of the dethronement of monarchs, and the dissolution of empires. science has, it is true, disarmed them of their terrors, and demonstrated that they are under the guidance of the same hand, that directs in their courses the other members of the solar system; but she has, at the same time, arrayed them in a garb of majesty peculiarly her own. although the ancients paid little attention to the ordinary phenomena of nature, hardly deeming them worthy of a reason, yet, when a comet blazed forth, fear and astonishment conspired to make it an object of the most attentive observation. hence the aspects of remarkable comets, that have appeared at various times, have been handed down to us, often with circumstantial minuteness, by the historians of different ages. the comet which appeared in the year , before the christian era, at the birth of mithridates, is said to have had a disk equal in magnitude to that of the sun. ten years before this, one was seen, which, according to justin, occupied a fourth part of the sky, that is, extended over forty-five degrees, and surpassed the sun in splendor. in the year , one was seen which resembled a sword in shape, and extended from the zenith to the horizon. such are some of the accounts of comets of past ages; but it is probable we must allow much for the exaggerations naturally accompanying the descriptions of objects in themselves so truly wonderful. a comet, when perfectly formed, consists of three parts, the nucleus, the envelope, and the tail. the nucleus, or body of the comet, is generally distinguished by its forming a bright point in the centre of the head, conveying the idea of a solid, or at least of a very dense, portion of matter. though it is usually exceedingly small, when compared with the other parts of the comet, and is sometimes wanting altogether, yet it occasionally subtends an angle capable of being measured by the telescope. the envelope (sometimes called the _coma_, from a latin word signifying hair, in allusion to its hairy appearance) is a dense nebulous covering, which frequently renders the edge of the nucleus so indistinct, that it is extremely difficult to ascertain its diameter with any degree of precision. many comets have no nucleus, but present only a nebulous mass, exceedingly attenuated on the confines, but gradually increasing in density towards the centre. indeed, there is a regular gradation of comets, from such as are composed merely of a gaseous or vapory medium, to those which have a well-defined nucleus. in some instances on record, astronomers have detected with their telescopes small stars through the densest part of a comet. the tail is regarded as an expansion or prolongation of the coma; and presenting, as it sometimes does, a train of appalling magnitude, and of a pale, portentous light, it confers on this class of bodies their peculiar celebrity. these several parts are exhibited in fig. , which [illustration figures , . comets of and .] represents the appearance of the comet of . fig. also exhibits that of the comet of . the _number_ of comets belonging to the solar system, is probably very great. many no doubt escape observation, by being above the horizon in the day-time. seneca mentions, that during a total eclipse of the sun, which happened sixty years before the christian era, a large and splendid comet suddenly made its appearance, being very near the sun. the leading particulars of at least one hundred and thirty have been computed, and arranged in a table, for future comparison. of these, _six_ are particularly remarkable; namely, the comets of , , and ; and those which bear the names of halley, biela, and encke. the comet of was remarkable, not only for its astonishing size and splendor, and its near approach to the sun, but is celebrated for having submitted itself to the observations of sir isaac newton, and for having enjoyed the signal honor of being the first comet whose elements were determined on the sure basis of mathematics. the comet of is memorable for the changes its orbit has undergone by the action of jupiter, as i shall explain to you more particularly hereafter. the comet of was the most remarkable in its appearance of all that have been seen in the present century. it had scarcely any perceptible nucleus, but its train was very long and broad, as is represented in fig. . halley's comet (the same which reappeared in ) is distinguished as that whose return was first successfully predicted, and whose orbit is best determined; and biela's and encke's comets are well known for their short periods of revolution, which subject them frequently to the view of astronomers. in _magnitude and brightness_, comets exhibit great diversity. history informs us of comets so bright, as to be distinctly visible in the day-time, even at noon, and in the brightest sunshine. such was the comet seen at rome a little before the assassination of julius cæsar. the comet of covered an arc of the heavens of ninety-seven degrees, and its length was estimated at one hundred and twenty-three millions of miles. that of had a nucleus of only four hundred and twenty-eight miles in diameter, but a tail one hundred and thirty-two millions of miles long. had it been coiled around the earth like a serpent, it would have reached round more than five thousand times. other comets are exceedingly small, the nucleus being in one case estimated at only twenty-five miles; and some, which are destitute of any perceptible nucleus, appear to the largest telescopes, even when nearest to us, only as a small speck of fog, or as a tuft of down. the majority of comets can be seen only by the aid of the telescope. indeed, the same comet has very different aspects, at its different returns. halley's comet, in , was described by the historians of that age as the comet of terrific magnitude; (_cometa horrendæ magnitudinis_;) in its tail reached from the horizon to the zenith, and inspired such terror, that, by a decree of the pope of rome, public prayers were offered up at noonday in all the catholic churches, to deprecate the wrath of heaven; while in its tail was only thirty degrees in length; and in it was visible only to the telescope until after it had passed its perihelion. at its recent return, in , the greatest length of the tail was about twelve degrees. these changes in the appearance of the same comet are partly owing to the different positions of the earth with respect to them, being sometimes much nearer to them when they cross its track than at others; also, one spectator, so situated as to see the comet at a higher angle of elevation, or in a purer sky, than another, will see the train longer than it appears to another less favorably situated; but the extent of the changes are such as indicate also a real change in magnitude and brightness. the _periods_ of comets in their revolutions around the sun are equally various. encke's comet, which has the shortest known period, completes its revolution in three and one third years; or, more accurately, in twelve hundred and eight days; while that of is estimated to have a period of thirty-three hundred and eighty three years. the _distances_ to which different comets recede from the sun are equally various. while encke's comet performs its entire revolution within the orbit of jupiter, halley's comet recedes from the sun to twice the distance of uranus; or nearly thirty-six hundred millions of miles. some comets, indeed, are thought to go a much greater distance from the sun than this, while some are supposed to pass into curves which do not, like the ellipse, return into themselves; and in this case they never come back to the sun. (see fig. , page .) comets shine _by reflecting the light of the sun_. in one or two instances, they have been thought to exhibit distinct _phases_, like the moon, although the nebulous matter with which the nucleus is surrounded would commonly prevent such phases from being distinctly visible, even when they would otherwise be apparent. moreover, certain qualities of _polarized_ light,--an affection by which a ray of light seems to have different properties on different sides,--enable opticians to decide whether the light of a given body is direct or reflected; and m. arago, of paris, by experiments of this kind on the light of the comet of , ascertained it to be reflected light. the tail of a comet usually increases very much as it approaches the sun; and it frequently does not reach its maximum until after the perihelion passage. in receding from the sun, the tail again contracts, and nearly or quite disappears before the body of the comet is entirely out of sight. the tail is frequently divided into two portions, the central parts, in the direction of the axis, being less bright than the marginal parts. in a comet appeared which had six tails spread out like a fan. the tails of comets extend in a direct line from the sun, although more or less curved, like a long quill or feather, being convex on the side next to the direction in which they are moving,--a figure which may result from the less velocity of the portion most remote from the sun. expansions of the envelope have also been at times observed on the side next the sun; but these seldom attain any considerable length. the _quantity of matter_ in comets is exceedingly small. their tails consist of matter of such tenuity, that the smallest stars are visible through them. they can only be regarded as masses of thin vapor, susceptible of being penetrated through their whole substance by the sunbeams, and reflecting them alike from their interior parts and from their surfaces. it appears perhaps incredible, that so thin a substance should be visible by reflected light, and some astronomers have held that the matter of comets is self-luminous; but it requires but very little light to render an object visible in the night, and a light vapor may be visible when illuminated throughout an immense stratum, which could not be seen if spread over the face of the sky like a thin cloud. "the highest clouds that float in our atmosphere," says sir john herschel, "must be looked upon as dense and massive bodies, compared with the filmy and all but spiritual texture of a comet." the small quantity of matter in comets is proved by the fact, that they have at times passed very near to some of the planets, without disturbing their motions in any appreciable degree. thus the comet of , in its way to the sun, got entangled among the satellites of jupiter, and remained near them four months; yet it did not perceptibly change their motions. the same comet, also, came very near the earth; so that, had its quantity of matter been equal to that of the earth, it would, by its attraction, have caused the earth to revolve in an orbit so much larger than at present, as to have increased the length of the year two hours and forty-seven minutes. yet it produced no sensible effect on the length of the year, and therefore its mass, as is shown by la place, could not have exceeded / of that of the earth, and might have been less than this to any extent. it may indeed be asked, what proof we have that comets have any matter, and are not mere reflections of light. the answer is, that, although they are not able by their own force of attraction to disturb the motions of the planets, yet they are themselves exceedingly disturbed by the action of the planets, and in exact conformity with the laws of universal gravitation. a delicate compass may be greatly agitated by the vicinity of a mass of iron, while the iron is not sensibly affected by the attraction of the needle. by approaching very near to a large planet, a comet may have its orbit entirely changed. this fact is strikingly exemplified in the history of the comet of . at its appearance in , its orbit was found to be an ellipse, requiring for a complete revolution only five and a half years; and the wonder was, that it had not been seen before, since it was a very large and bright comet. astronomers suspected that its path had been changed, and that it had been recently compelled to move in this short ellipse, by the disturbing force of jupiter and his satellites. the french institute, therefore, offered a high prize for the most complete investigation of the elements of this comet, taking into account any circumstances which could possibly have produced an alteration in its course. by tracing back the movements of this comet, for some years previous to , it was found that, at the beginning of , it had entered considerably within the sphere of jupiter's attraction. calculating the amount of this attraction from the known proximity of the two bodies, it was found what must have been its orbit previous to the time when it became subject to the disturbing action of jupiter. it was therefore evident why, as long as it continued to circulate in an orbit so far from the centre of the system, it was never visible from the earth. in january, , jupiter and the comet happened to be very near to one another, and as both were moving in the same direction, and nearly in the same plane, they remained in the neighborhood of each other for several months, the planet being between the comet and the sun. the consequence was, that the comet's orbit was changed into a smaller ellipse, in which its revolution was accomplished in five and a half years. but as it approached the sun, in , it happened again to fall in with jupiter. it was in the month of june that the attraction of the planet began to have a sensible effect; and it was not until the month of october following, that they were finally separated. at the time of their nearest approach, in august, jupiter was distant from the comet only / of its distance from the sun, and exerted an attraction upon it two hundred and twenty-five times greater than that of the sun. by reason of this powerful attraction, jupiter being further from the sun than the comet, the latter was drawn out into a new orbit, which even at its perihelion came no nearer to the sun than the planet ceres. in this third orbit, the comet requires about twenty years to accomplish its revolution; and being at so great a distance from the earth, it is invisible, and will for ever remain so unless, in the course of ages, it may undergo new perturbations, and move again in some smaller orbit, as before. with the foregoing leading facts respecting comets in view, i will now explain to you a few things equally remarkable respecting their _motions_. the paths of the planets around the sun being nearly circular, we are able to see a planet in every part of its orbit. but the case is very different with comets. for the greater part of their course, they are wholly out of sight, and come into view only while just in the neighborhood of the sun. this you will readily see must be the case, by inspecting the frontispiece, which represents the orbit of biela's comet, in . sometimes, the orbit is so eccentric, that the place of the focus occupied by the sun appears almost at the extremity of the orbit. this was the case with the orbit of the comet of . indeed, this comet, at its perihelion, came in fact nearer to the sun than the sixth part of the sun's diameter, being only one hundred and forty-six thousand miles from the surface of the sun, which, you will remark, is only a little more than half the distance of the moon from the earth; while, at its aphelion, it was estimated to be thirteen thousand millions of miles from the sun,--more than eleven thousand millions of miles beyond the planet uranus. its _velocity_, when nearest the sun, exceeded a million of miles an hour. to describe such an orbit as was assigned to it by sir isaac newton, would require five hundred and seventy-five years. during all this period, it was entirely out of view to the inhabitants of the earth, except the few months, while it was running down to the sun from such a distance as the orbit of jupiter and back. the velocity of bodies moving in such eccentric orbits differs widely in different parts of their orbits. in the remotest parts it is so slow, that years would be required to pass over a space equal to that which it would run over in a single day, when near the sun. the appearances of the same comet at different periods of its return are so various, that we can never pronounce a given comet to be the same with one that has appeared before, from any peculiarities in its physical aspect, as from its color, magnitude, or shape; since, in all these respects, it is very different at different returns; but it is judged to be the same if its _path_ through the heavens, as traced among the stars, is the same. the comet whose history is the most interesting, and which both of us have been privileged to see, is halley's. just before its latest visit, in , its return was anticipated with so much expectation, not only by astronomers, but by all classes of the community, that a great and laudable eagerness universally prevailed, to learn the particulars of its history. the best summary of these, which i met with, was given in the edinburgh review for april, . i might content myself with barely referring you to that well-written article; but, as you may not have the work at hand, and would, moreover, probably not desire to read the whole article, i will abridge it for your perusal, interspersing some remarks of my own. i have desired to give you, in the course of these letters, some specimen of the labors of astronomers, and shall probably never be able to find a better one. it is believed that the first recorded appearance of halley's comet was that which was supposed to signalize the birth of mithridates, one hundred and thirty years before the birth of christ. it is said to have appeared for twenty-four days; its light is said to have surpassed that of the sun; its magnitude to have extended over a fourth part of the firmament; and it is stated to have occupied, consequently, about four hours in rising and setting. in the year , a comet appeared in the sign virgo. another, according to the historians of the lower empire, appeared in the year , seventy-six years after the last, at an interval corresponding to that of halley's comet. the interval between the birth of mithridates and the year was four hundred and fifty-three years, which would be equivalent to six periods of seventy-five and a half years. thus it would seem, that in the interim there were five returns of this comet unobserved, or at least unrecorded. the appearance in the year was attended with extraordinary circumstances. it was described in the old writers as a "comet of monstrous size and appalling aspect, its tail seeming to reach down to the ground." the next recorded appearance of a comet agreeing with the ascertained period marks the taking of rome, in the year ,--an interval of one hundred and fifty-one years, or two periods of seventy-five and a half years having elapsed. one unrecorded return must, therefore, have taken place in the interim. the next appearance of a comet, coinciding with the assigned period, is three hundred and eighty years afterwards; namely, in the year ,--five revolutions having been completed in the interval. the next appearance is recorded in the year , after an interval of a single period of seventy-five years. three revolutions would now seem to have passed unrecorded, when the comet again makes its appearance in . in this, as well as in former appearances, it is proper to state, that the sole test of identity of these cornets with that of halley is the coincidence of the times, as near as historical records enable us to ascertain, with the epochs at which the comet of halley might be expected to appear. that such evidence, however, is very imperfect, must be evident, if the frequency of cometary appearances be considered, and if it be remembered, that hitherto we find no recorded observations, which could enable us to trace, even with the rudest degree of approximation, the paths of those comets, the times of whose appearances raise a presumption of their identity with that of halley. we now, however, descend to times in which more satisfactory evidence may be expected. in the year , a year in which the return of halley's comet might have been expected, there is recorded a comet of remarkable character: "a comet of terrific dimensions made its appearance about the time of the feast of the passover, which was followed by a great plague." had the terrific appearance of this body alone been recorded, this description might have passed without the charge of great exaggeration; but when we find the great plague connected with it as a consequence, it is impossible not to conclude, that the comet was seen by its historians through the magnifying medium of the calamity which followed it. another appearance is recorded in the year , unaccompanied by any other circumstance than its mere date. this, however, is in strict accordance with the ascertained period of halley's comet. we now arrive at the first appearance at which observations were taken, possessing sufficient accuracy to enable subsequent investigators to determine the path of the comet; and this is accordingly the first comet the identity of which with the comet of halley can be said to be conclusively established. in the year , a comet is stated to have appeared "of unheard of magnitude;" it was accompanied by a tail of extraordinary length, which extended over sixty degrees, (a third part of the heavens,) and continued to be seen during the whole month of june. the influence which was attributed to this appearance renders it probable, that in the record there is more or less of exaggeration. it was considered as the celestial indication of the rapid success of mohammed the second, who had taken constantinople, and struck terror into the whole christian world. pope calixtus the second levelled the thunders of the church against the enemies of his faith, terrestrial and celestial; and in the same bull excommunicated the turks and the comet; and, in order that the memory of this manifestation of his power should be for ever preserved, he ordained that the bells of all the churches should be rung at mid-day,--a custom which is preserved in those countries to our times. the extraordinary length and brilliancy which was ascribed to the tail, upon this occasion, have led astronomers to investigate the circumstances under which its brightness and magnitude would be the greatest possible; and upon tracing back the motion of the comet to the year , it has been found that it was then actually in the position, with respect to the earth and sun, most favorable to magnitude and splendor. so far, therefore, the result of astronomical calculation corroborates the records of history. the next return took place in . pierre appian, who first ascertained the fact that the tails of comets are usually turned from the sun, examined this comet with a view to verify his statement, and to ascertain the true direction of its tail. he made, accordingly, numerous observations upon its position, which, although rude, compared with the present standard of accuracy, were still sufficiently exact to enable halley to identify this comet with that observed by himself. the next return took place in , when the comet was observed by kepler. this astronomer first saw it on the evening of the twenty-sixth of september, when it had the appearance of a star of the first magnitude, and, to his vision, was without a tail; but the friends who accompanied him had better sight, and distinguished the tail. before three o'clock the following morning the tail had become clearly visible, and had acquired great magnitude. two days afterwards, the comet was observed by longomontanus, a distinguished philosopher of the time. he describes its appearance, to the naked eye, to be like jupiter, but of a paler and more obscured light; that its tail was of considerable length, of a paler light than that of the head, and more dense than the tails of ordinary comets. the next appearance, and that which was observed by halley himself, took place in , a little before the publication of the '_principia_.' in the interval between and , practical astronomy had made great advances; instruments of observation had been brought to a state of comparative perfection; numerous observatories had been established, and the management of them had been confided to the most eminent men in europe. in , the scientific world was therefore prepared to examine the visitor of our system with a degree of care and accuracy before unknown. in the year , about four years afterwards, newton published his '_principia_,' in which he applied to the comet of the general principles of physical investigation first promulgated in that work. he explained the method of determining, by geometrical construction, the visible portion of the path of a body of this kind, and invited astronomers to apply these principles to the various recorded comets,--to discover whether some among them might not have appeared at different epochs, the future returns of which might consequently be predicted. such was the effect of the force of analogy upon the mind of newton, that, without awaiting the discovery of a periodic comet, he boldly assumed these bodies to be analogous to planets in their revolution round the sun. extraordinary as these conjectures must have appeared at the time, they were soon strictly realized. halley, who was then a young man, but possessed one of the best minds in england, undertook the labor of examining the circumstances attending all the comets previously recorded, with a view to discover whether any, and which of them, appeared to follow the same path. antecedently to the year , four hundred and twenty-five of these bodies had been recorded in history; but those which had appeared before the fourteenth century had not been submitted to any observations by which their paths could be ascertained,--at least, not with a sufficient degree of precision, to afford any hope of identifying them with those of other comets. subsequently to the year , however, halley found twenty-four comets on which observations had been made and recorded, with a degree of precision sufficient to enable him to calculate the actual paths which these bodies followed while they were visible. he examined, with the most elaborate care, the _courses_ of each of these twenty-four bodies; he found the exact points at which each one of them crossed the ecliptic, or their _nodes_; also the angle which the direction of their motion made with that plane,--that is, the _inclination of their orbits_; he also calculated the nearest distance at which each of them approached the sun, or their _perihelion distance_; and the exact place of the body when at that nearest point,--that is, the _longitude of the perihelion_. these particulars are called the _elements_ of a comet, because, when ascertained, they afford sufficient data for determining a comet's path. on comparing these paths, halley found that one, which had appeared in , followed nearly the same path as one which had appeared in . supposing, then, these to be two successive appearances of the same comet, it would follow, that its period would be one hundred and twenty-nine years, reckoning from . had this conjecture been well founded, the comet must have appeared about the year . no comet, however, appeared at or near that time, following a similar path. in his second conjecture, halley was more fortunate, as indeed might be expected, since it was formed upon more conclusive grounds. he found that the paths of comets which had appeared in and were nearly identical, and that they were in fact the same as the path followed by the comet observed by himself in . he suspected, therefore, that the appearances at these three epochs were produced by three successive returns of the same comet, and that, consequently, its period in its orbit must be about seventy-five and a half years. the probability of this conclusion is strikingly exhibited to the eye, by presenting the elements in a tabular form, from which it will at once be seen how nearly they correspond at these regular intervals. ===================================================================== time.|inclination of|long. of the |long. per.|per. dist. |course. |the orbit. |node. | | | ===================================================================== | ° ´ | ° ´ | ° ´ | ° ´ |retrograde. | | | | | " | | | | | " | | | | | " ===================================================================== so little was the scientific world, at this time, prepared for such an announcement, that halley himself only ventured at first to express his opinion in the form of conjecture; but, after some further investigation of the circumstances of the recorded comets, he found three which, at least in point of time, agreed with the period assigned to the comet of . collecting confidence from these circumstances, he announced his discovery as the result of observation and calculation combined, and entitled to as much confidence as any other consequence of an established physical law. there were, nevertheless, two circumstances which might be supposed to offer some difficulty. first, the intervals between the supposed successive returns were not precisely equal; and, secondly, the inclination of the comet's path to the plane of the earth's orbit was not exactly the same in each case. halley, however, with a degree of sagacity which, considering the state of knowledge at the time, cannot fail to excite unqualified admiration, observed, that it was natural to suppose that the same causes which disturbed the planetary motions must likewise act upon comets; and that their influence would be so much the more sensible upon these bodies, because of their great distances from the sun. thus, as the attraction of jupiter for saturn was known to affect the velocity of the latter planet, sometimes retarding and sometimes accelerating it, according to their relative position, so as to affect its period to the extent of thirteen days, it might well be supposed, that the comet might suffer by a similar attraction an effect sufficiently great, to account for the inequality observed in the interval between its successive returns: and also for the variation to which the direction of its path upon the plane of the ecliptic was found to be subject. he observed, in fine, that, as in the interval between and , the comet passed so near jupiter that its velocity must have been augmented, and consequently its period shortened, by the action of that planet, this period, therefore, having been only seventy-five years, he inferred that the following period would probably be seventy-six years, or upwards; and consequently, that the comet ought not to be expected to appear until the end of , or the beginning of . it is impossible to imagine any quality of mind more enviable than that which, in the existing state of mathematical physics, could have led to such a prediction. the imperfect state of mathematical science rendered it impossible for halley to offer to the world a demonstration of the event which he foretold. the theory of gravitation, which was in its infancy in the time of halley's investigations, had grown to comparative maturity before the period at which his prediction could be fulfilled. the exigencies of that theory gave birth to new and more powerful instruments of mathematical inquiry: the differential and integral calculus, or the science of fluxions, as it is sometimes called,--a branch of the mathematics, expressed by algebraic symbols, but capable of a much higher reach, as an instrument of investigation, than either algebra or geometry,--was its first and greatest offspring. this branch of science was cultivated with an ardor and success by which it was enabled to answer all the demands of physics, and it contributed largely to the advancement of mechanical science itself, building upon the laws of motion a structure which has since been denominated 'celestial mechanics.' newton's discoveries having obtained reception throughout the scientific world, his inquiries and his theories were followed up; and the consequences of the great principle of universal gravitation were rapidly developed. since, according to this doctrine, _every body in nature attracts and is attracted by every other body_, it follows, that the comet was liable to be acted on by each of the planets, as well as by the sun,--a circumstance which rendered its movements much more difficult to follow, than would be the case were it subject merely to the projectile force and to the solar attraction. to estimate the time it would take for a ship to cross the atlantic would be an easy task, were she subject to only one constant wind; but to estimate, beforehand, the exact influence which all other winds and the tides might have upon her passage, some accelerating and some retarding her course, would present a problem of the greatest difficulty. clairaut, however, a celebrated french mathematician, undertook to estimate the effects that would be produced on halley's comet by the attractions of all the planets. his aim was to investigate _general rules_, by which the computation could be made arithmetically, and hand them over to the practical calculator, to make the actual computations. lalande, a practical astronomer, no less eminent in his own department, and who indeed first urged clairaut to this inquiry, undertook the management of the astronomical and arithmetical part of the calculation. in this prodigious labor (for it was one of most appalling magnitude) he was assisted by the wife of an eminent watchmaker in paris, named lepaute, whose exertions on this occasion have deservedly registered her name in astronomical history. it is difficult to convey to one who is not conversant with such investigations, an adequate notion of the labor which such an inquiry involved. the calculation of the influence of any one _planet_ of the system upon any other is itself a problem of some complexity and difficulty; but still, one general computation, depending upon the calculation of the terms of a certain series, is sufficient for its solution. this comparative simplicity arises entirely from two circumstances which characterize the planetary orbits. these are, that, though they are ellipses, they differ very slightly from circles; and though the planets do not move in the plane of the ecliptic, yet none of them deviate considerably from that plane. but these characters do not belong to the orbits of comets, which, on the contrary, are highly eccentric, and make all possible angles with the ecliptic. the consequence of this is, that the calculation of the disturbances produced in the cometary orbits by the action of the planets must be conducted not like the planets, in one general calculation applicable to the whole orbits, but in a vast number of separate calculations; in which the orbit is considered, as it were, bit by bit, each bit requiring a calculation similar to the whole orbit of the planet. now, when it is considered that the period of halley's comet is about seventy-five years, and that every portion of its course, for two successive periods, was necessary to be calculated separately in this way, some notion may be formed of the labor encountered by lalande and madame lepaute. "during six months," says lalande, "we calculated from morning till night, sometimes even at meals; the consequence of which was, that i contracted an illness which changed my constitution for the remainder of my life. the assistance rendered by madame lepaute was such, that, without her, we never could have dared to undertake this enormous labor, in which it was necessary to calculate the distance of each of the two planets, jupiter and saturn, from the comet, and their attraction upon that body, separately, for every successive degree, and for one hundred and fifty years." the attraction of a body is proportioned to its quantity of matter. therefore, before the attraction exerted upon the comet by the several planets within whose influence it might fall, could be correctly estimated, it was necessary to know the mass of each planet; and though the planets had severally been weighed by methods supplied by newton's 'principia,' yet the estimate had not then attained the same measure of accuracy as it has now reached; nor was it certain that there was not (as it has since appeared that there actually was) one or more planets beyond saturn, whose attractions might likewise influence the motions of the comet. clairaut, making the best estimate he was able, under all these disadvantages, of the disturbing influence of the planets, fixed the return of the comet to the place of its nearest distance from the sun on the fourth of april, . in the successive appearances of the comet, subsequently to , it was found to have gradually decreased in magnitude and splendor. while in it reached across one third part of the firmament, and spread terror over europe, in , its appearance, when observed by kepler and longomontanus, was that of a star of the first magnitude; and so trifling was its tail that, kepler himself, when he first saw it, doubted whether it had any. in , it excited little attention, except among astronomers. supposing this decrease of magnitude and brilliancy to be progressive, lalande entertained serious apprehensions that on its expected return it might be so inconsiderable, as to escape the observation even of astronomers; and thus, that this splendid example of the power of science, and unanswerable proof of the principle of gravitation, would be lost to the world. it is not uninteresting to observe the misgivings of this distinguished astronomer with respect to the appearance of the body, mixed up with his unshaken faith in the result of the astronomical inquiry. "we cannot doubt," says he, "that it will return; and even if astronomers cannot see it, they will not therefore be the less convinced of its presence. they know that the faintness of its light, its great distance, and perhaps even bad weather, may keep it from our view. but the world will find it difficult to believe us; they will place this discovery, which has done so much honor to modern philosophy, among the number of chance predictions. we shall see discussions spring up again in colleges, contempt among the ignorant, terror among the people; and seventy-six years will roll away, before there will be another opportunity of removing all doubt." fortunately for science, the arrival of the expected visitor did not take place under such untoward circumstances. as the commencement of the year approached, "astronomers," says voltaire, "hardly went to bed at all." the honor, however, of the first glimpse of the stranger was not reserved for the possessors of scientific rank, nor for the members of academies or universities. on the night of christmas-day, , george palitzch, of politz, near dresden,--"a peasant," says sir john herchel, "by station, an astronomer by nature," first saw the comet. an astronomer of leipzic found it soon after; but, with the mean jealousy of a miser, he concealed his treasure, while his contemporaries throughout europe were vainly directing their anxious search after it to other quarters of the heavens. at this time, delisle, a french astronomer, and his assistant, messier, who, from his unweared assiduity in the pursuit of comets, was called the _comet-hunter_, had been constantly engaged, for eighteen months, in watching for the return of halley's comet. messier passed his life in search of comets. it is related of him, that when he was in expectation of discovering a comet, his wife was taken ill and died. while attending on her, being withdrawn from his observatory, another astronomer anticipated him in the discovery. messier was in despair. a friend, visiting him, began to offer some consolation for the recent affliction he had suffered. messier, thinking only of the comet, exclaimed, "i had discovered twelve: alas, that i should be robbed of the thirteenth by montague!"--and his eyes filled with tears. then, remembering that it was necessary to mourn for his wife, whose remains were still in the house, he exclaimed, "ah! this poor woman!" (_ah! cette pauvre femme_,) and again wept for his comet. we can easily imagine how eagerly such an enthusiast would watch for halley's comet; and we could almost wish that it had been his good fortune to be the first to announce its arrival: but, being misled by a chart which directed his attention to the wrong part of the firmament, a whole month elapsed after its discovery by palitzch, before he enjoyed the delightful spectacle. the comet arrived at its perihelion on the thirteenth of march, only twenty-three days from the time assigned by clairaut. it appeared very round, with a brilliant nucleus, well distinguished from the surrounding nebulosity. it had, however, no appearance of a tail. it became lost in the sun, as it approached its perihelion, and emerged again, on the other side of the sun, on the first of april. its exhibiting an appearance, so inferior to what it presented on some of its previous returns, is partly accounted for by its being seen by the european astronomers under peculiarly disadvantageous circumstances, being almost always within the twilight, and in the most unfavorable situations. in the southern hemisphere, however, the circumstances for observing it were more favorable, and there it exhibited a tail varying from ten to forty-seven degrees in length. in my next letter i will give you some particulars respecting the late return of halley's comet. letter xxvi. comets, continued. "incensed with indignation, satan stood unterrified, and like a comet burned, that fires the length of ophiucus huge in the arctic sky, and from his horrid train shakes pestilence and war."--_milton._ among other great results which have marked the history of halley's comet, it has itself been a criterion of the existing state of the mathematical and astronomical sciences. we have just seen how far the knowledge of the great laws of physical astronomy, and of the higher mathematics, enabled the astronomers of and , respectively, to deal with this wonderful body; and let us now see what higher advantages were possessed by the astronomers of . during this last interval of seventy-six years, the science of mathematics, in its most profound and refined branches, has made prodigious advances, more especially in its application to the laws of the celestial motions, as exemplified in the 'mecanique celeste' of la place. the methods of investigation have acquired greater simplicity, and have likewise become more general and comprehensive; and mechanical science, in the largest sense of that term, now embraces in its formularies the most complicated motions, and the most minute effects of the mutual influences of the various members of our system. you will probably find it difficult to comprehend, how such hidden facts can be disclosed by formularies, consisting of _a_'s and _b_'s, and _x_'s and _y_'s, and other algebraic symbols; nor will it be easy to give you a clear idea of this subject, without a more extensive acquaintance than you have formed with algebraic investigations; but you can easily understand that even an equation expressed in numbers may be so changed in its form, by adding, subtracting, multiplying and dividing, as to express some new truth at every transformation. some idea of this may be formed by the simplest example. take the following: + = . this equation expresses the fact, that three added to four is equal to seven. by multiplying all the terms by , we obtain a new equation, in which + = . this expresses a new truth; and by varying the form, by similar operations, an indefinite number of separate truths may be elicited from the simple fundamental expression. i will add another illustration, which involves a little more algebra, but not, i think, more than you can understand; or, if it does, you will please pass over it to the next paragraph. according to a rule of arithmetical progression, _the sum of all the terms is equal to half the sum of the extremes multiplied into the number of terms_. calling the sum of the terms _s_, the first term _a_, the last _h_, and the number of terms _n_, and we have _( / )n(a+h)=s_; or _n(a+h)= s_; or _a+h= s/n_; or _a=( s/n)-h_; or _h=( s/n)-a_. these are only a few of the changes which may be made in the original expression, still preserving the equality between the quantities on the left hand and those on the right; yet each of these transformations expresses a new truth, indicating distinct and (as might be the case) before unknown relations between the several quantities of which the whole expression is composed. the last, for example, shows us that the last term in an arithmetical series is always equal to twice the sum of the whole series divided by the number of terms and diminished by the first term. in analytical formularies, as expressions of this kind are called, the value of a single unknown quantity is sometimes given in a very complicated expression, consisting of known quantities; but before we can ascertain their united value, we must reduce them, by actually performing all the additions, subtractions, multiplications, divisions, raising to powers, and extracting roots, which are denoted by the symbols. this makes the actual calculations derived from such formularies immensely laborious. we have already had an instance of this in the calculations made by lalande and madame lepaute, from formularies furnished by clairaut. the analytical formularies, contained in such works as la place's 'mecanique celeste,' exhibit to the eye of the mathematician a record of all the evolutions of the bodies of the solar system in ages past, and of all the changes they must undergo in ages to come. such has been the result of the combination of transcendent mathematical genius and unexampled labor and perseverance, for the last century. the learned societies established in various centres of civilization have more especially directed their attention to the advancement of physical astronomy, and have stimulated the spirit of inquiry by a succession of prizes, offered for the solutions of problems arising out of the difficulties which were progressively developed by the advancement of astronomical knowledge. among these questions, the determination of the return of comets, and the disturbances which they experience in their course, by the action of the planets near which they happen to pass, hold a prominent place. in , the french institute offered a prize for the determination of the exact time of the return of halley's comet to its perihelion in . m. pontecoulant aspired to the honor. "after calculations," says he, "of which those alone who have engaged in such researches can estimate the extent and appreciate the fastidious monotony, i arrived at a result which satisfied all the conditions proposed by the institute. i determined the perturbations of halley's comet, by taking into account the simultaneous actions of jupiter, saturn, uranus, and the earth, and i then fixed its return to its perihelion for the seventh of november." subsequently to this, however, m. pontecoulant made some further researches, which led him to correct the former result; and he afterwards altered the time to november fourteenth. it actually came to its perihelion on the sixteenth, within two days of the time assigned. nothing can convince us more fully of the complete mastery which astronomers have at last acquired over these erratic bodies, than to read in the edinburgh review for april, , the paragraph containing the final results of all the labors and anticipations of astronomers, matured as they were, in readiness for the approaching visitant, and then to compare the prediction with the event, as we saw it fulfilled a few months afterwards. the paragraph was as follows: "on the whole, it may be considered as tolerably certain, that the comet will become visible in every part of europe about the latter end of august, or beginning of september, next. it will most probably be distinguishable by the naked eye, like a star of the first magnitude, but with a duller light than that of a planet, and surrounded with a pale nebulosity, which will slightly impair its splendor. on the night of the seventh of october, the comet will approach the well-known constellation of the great bear; and between that and the eleventh, it will pass directly through the seven conspicuous stars of that constellation, (the dipper.) towards the end of november, the comet will plunge among the rays of the sun, and disappear, and will not issue from them, on the other side, until the end of december." let us now see how far the actual appearances corresponded to these predictions. the comet was first discovered from the observatory at rome, on the morning of the fifth of august; by professor struve, at dorpat, on the twentieth; in england and france, on the twenty-third; and at yale college, by professor loomis and myself, on the thirty-first. on the morning of that day, between two and three o'clock, in obedience to the directions which the great minds that had marked out its path among the stars had prescribed, we directed clarke's telescope (a noble instrument, belonging to yale college) towards the northeastern quarter of the heavens, and lo! there was the wanderer so long foretold,--a dim speck of fog on the confines of creation. it came on slowly, from night to night, increasing constantly in magnitude and brightness, but did not become distinctly visible to the naked eye until the twenty-second of september. for a month, therefore, astronomers enjoyed this interesting spectacle before it exhibited itself to the world at large. from this time it moved rapidly along the northern sky, until, about the tenth of october, it traversed the constellation of the great bear, passing a little above, instead of "through" the seven conspicuous stars constituting the dipper. at this time it had a lengthened train, and became, as you doubtless remember, an object of universal interest. early in november, the comet ran down to the sun, and was lost in his beams; but on the morning of december thirty-first, i again obtained, through clarke's telescope, a distinct view of it on the other side of the sun, a moment before the morning dawn. this return of halley's comet was an astronomical event of transcendent importance. it was the chronicler of ages, and carried us, by a few steps, up to the origin of time. if a gallant ship, which has sailed round the globe, and commanded successively the admiration of many great cities, diverse in language and customs, is invested with a peculiar interest, what interest must attach to one that has made the circuit of the solar system, and fixed the gaze of successive worlds! so intimate, moreover, is the bond which binds together all truths in one indissoluble chain, that the establishment of one great truth often confirms a multitude of others, equally important. thus the return of halley's comet, in exact conformity with the predictions of astronomers, established the truth of all those principles by which those predictions were made. it afforded most triumphant proof of the doctrine of universal gravitation, and of course of the received laws of physical astronomy; it inspired new confidence in the power and accuracy of that instrument (the calculus) by means of which its elements had been investigated; and it proved that the different planets, which exerted upon it severally a disturbing force proportioned to their quantity of matter, had been correctly weighed, as in a balance. i must now leave this wonderful body to pursue its sublime march far beyond the confines of uranus, (a distance it has long since reached,) and take a hasty notice of two other comets, whose periodic returns have also been ascertained; namely, those of biela and encke. biela's comet has a period of six years and three quarters. it has its perihelion near the orbit of the earth, and its aphelion a little beyond that of jupiter. its orbit, therefore, is far less eccentric than that of halley's comet; (see frontispiece;) it neither approaches so near the sun, nor departs so far from it, as most other known comets: some, indeed, never come nearer to the sun than the orbit of jupiter, while they recede to an incomprehensible distance beyond the remotest planet. we might even imagine that they would get beyond the limits of the sun's attraction; nor is this impossible, although, according to la place, the solar attraction is sensible throughout a sphere whose radius is a hundred millions of times greater than the distance of the earth from the sun, or nearly ten thousand billions of miles. some months before the expected return of biela's comet, in , it was announced by astronomers, who had calculated its path, that it would cross the plane of the earth's orbit very near to the earth's path, so that, should the earth happen at the time to be at that point of her revolution, a collision might take place. this announcement excited so much alarm among the ignorant classes in france, that it was deemed expedient by the french academy, that one of their number should prepare and publish an article on the subject, with the express view of allaying popular apprehension. this task was executed by m. arago. he admitted that the earth would in fact pass so near the point where the comet crossed the plane of its orbit, that, should they chance to meet there, the earth would be enveloped in the nebulous atmosphere of the comet. he, however, showed that the earth would not be near that point at the same time with the comet, but fifty millions of miles from it. the comet came at the appointed time, but was so exceedingly faint and small, that it was visible only to the largest telescopes. in one respect, its diminutive size and feeble light enhanced the interest with which it was contemplated; for it was a sublime spectacle to see a body, which, as projected on the celestial vault, even when magnified a thousand times, seemed but a dim speck of fog, still pursuing its way, in obedience to the laws of universal gravitation, with the same regularity as jupiter and saturn. we are apt to imagine that a body, consisting of such light materials that it can be compared only to the thinnest fog, would be dissipated and lost in the boundless regions of space; but so far is this from the truth, that, when subjected to the action of the same forces of projection and solar attraction, it will move through the void regions of space, and will describe its own orbit about the sun with the same unerring certainty, as the densest bodies of the system. encke's comet, by its frequent returns, (once in three and a third years,) affords peculiar facilities for ascertaining the laws of its revolution; and it has kept the appointments made for it with great exactness. on its return in , it exhibited to the telescope a globular mass of nebulous matter, resembling fog, and moved towards its perihelion with great rapidity. it makes its entire excursions within the orbit of jupiter. but what has made encke's comet particularly famous, is its having first revealed to us the existence of a _resisting medium_ in the planetary spaces. it has long been a question, whether the earth and planets revolve in a perfect void, or whether a fluid of extreme rarity may not be diffused through space. a perfect vacuum was deemed most probable, because no such effects on the motions of the planets could be detected as indicated that they encountered a resisting medium. but a feather, or a lock of cotton, propelled with great velocity, might render obvious the resistance of a medium which would not be perceptible in the motions of a cannon ball. accordingly, encke's comet is thought to have plainly suffered a retardation from encountering a resisting medium in the planetary regions. the effect of this resistance, from the first discovery of the comet to the present time, has been to diminish the time of its revolution about two days. such a resistance, by destroying a part of the projectile force, would cause the comet to approach nearer to the sun, and thus to have its periodic time shortened. the ultimate effect of this cause will be to bring the comet nearer to the sun, at every revolution, until it finally falls into that luminary, although many thousand years will be required to produce this catastrophe. it is conceivable, indeed, that the effects of such a resistance may be counteracted by the attraction of one or more of the planets, near which it may pass in its successive returns to the sun. still, it is not probable that this cause will exactly counterbalance the other; so that, if there is such an elastic medium diffused through the planetary regions, it must follow that, in the lapse of ages, every comet will fall into the sun. newton conjectured that this would be the case, although he did not found his opinion upon the existence of such a resisting medium as is now detected. to such an opinion he adhered to the end of life. at the age of eighty-three, in a conversation with his nephew, he expressed himself thus: "i cannot say when the comet of will fall into the sun; possibly after five or six revolutions; but whenever that time shall arrive, the heat of the sun will be raised by it to such a point, that our globe will be burned, and all the animals upon it will perish." of the _physical nature_ of comets little is understood. the greater part of them are evidently mere masses of vapor, since they permit very small stars to be seen through them. in september, , sir john herschel, when observing biela's comet, saw that body pass directly between his eye and a small cluster of minute telescopic stars of the sixteenth or seventeenth magnitude. this little constellation occupied a space in the heavens, the breadth of which was not the twentieth part of that of the moon; yet the whole of the cluster was distinctly visible through the comet. "a more striking proof," says sir john herschel, "could not have been afforded, of the extreme transparency of the matter of which this comet consists. the most trifling fog would have entirely effaced this group of stars, yet they continued visible through a thickness of the comet which, calculating on its distance and apparent diameter, must have exceeded fifty thousand miles, at least towards its central parts." from this and similar observations, it is inferred, that the nebulous matter of comets is vastly more rare than that of the air we breathe, and hence, that, were more or less of it to be mingled with the earth's atmosphere, it would not be perceived, although it might possibly render the air unwholesome for respiration. m. arago, however, is of the opinion, that some comets, at least, have a solid nucleus. it is difficult, on any other supposition, to account for the strong light which some of them have exhibited,--a light sufficiently intense to render them visible in the day-time, during the presence of the sun. the intense heat to which comets are subject, in approaching so near the sun as some of them do, is alleged as a sufficient reason for the great expansion of the thin vapory atmospheres which form their tails; and the inconceivable cold to which they are subject, in receding to such a distance from the sun, is supposed to account for the condensation of the same matter until it returns to its original dimensions. thus the great comet of , at its perihelion, approached within one hundred and forty-six thousand miles of the surface of the sun, a distance of only one sixth part of the sun's diameter. the heat which it must have received was estimated to be equal to twenty-eight thousand times that which the earth receives in the same time, and two thousand times hotter than red-hot iron. this temperature would be sufficient to volatilize the most obdurate substances, and to expand the vapor to vast dimensions; and the opposite effects of the extreme cold to which it would be subject in the regions remote from the sun would be adequate to condense it into its former volume. this explanation, however, does not account for the direction of the tail, extending, as it usually does, only in a line opposite to the sun. some writers, therefore, suppose that the nebulous matter of the comet, after being expanded to such a volume that the particles are no longer attracted to the nucleus, unless by the slightest conceivable force, are carried off in a direction from the sun, by the impulse of the solar rays themselves. but to assign such a power to the sun's rays, while they have never been proved to have any momentum, is unphilosophical; and we are compelled to place the phenomena of comets' tails among the points of astronomy yet to be explained. since comets which approach very near the sun, like the comet of , cross the orbits of all the planets, the possibility that one of them may strike the earth has frequently been suggested. still it may quiet our apprehensions on this subject, to reflect on the vast amplitude of the planetary spaces, in which these bodies are not crowded together, as we see them erroneously represented in orreries and diagrams, but are sparsely scattered at immense distances from each other. they are like insects flying, singly, in the expanse of heaven. if a comet's tail lay with its axis in the plane of the ecliptic when it was near the sun, we can imagine that the tail might sweep over the earth; but the tail may be situated at any angle with the ecliptic, as well as in the same plane with it, and the chances that it will not be in the same plane are almost infinite. it is also extremely improbable that a comet will cross the plane of the ecliptic precisely at the earth's path in that plane, since it may as probably cross it at any other point nearer or more remote from the sun. a french writer of some eminence (du sejour) has discussed this subject with ability, and arrived at the following conclusions: that of all the comets whose paths had been ascertained, none _could pass_ nearer to the earth than about twice the moon's distance; and that none ever _did pass_ nearer to the earth than nine times the moon's distance. the comet of , already mentioned, which became entangled among the satellites of jupiter, came within this limit. some have taken alarm at the idea that a comet, by approaching very near to the earth, might raise so high a _tide_, as to endanger the safety of maritime countries especially: but this writer shows, that the comet could not possibly remain more than two hours so near the earth as a fourth part of the moon's distance; and it could not remain even so long, unless it passed the earth under very peculiar circumstances. for example, if its orbit were nearly perpendicular to that of the earth, it could not remain more than half an hour in such a position. under such circumstances, the production of a tide would be impossible. eleven hours, at least, would be necessary to enable a comet to produce an effect on the waters of the earth, from which the injurious effects so much dreaded would follow. the final conclusion at which he arrives is, that although, in strict geometrical rigor, it is not physically impossible that a comet should encounter the earth, yet the probability of such an event is absolutely nothing. m. arago, also, has investigated the probability of such a collision on the mathematical doctrine of chances, and remarks as follows: "suppose, now, a comet, of which we know nothing but that, at its perihelion, it will be nearer the sun than we are, and that its diameter is equal to one fourth that of the earth; the doctrine of chances shows that, out of two hundred and eighty-one millions of cases, there is but one against us; but one, in which the two bodies could meet." la place has assigned the consequences that would result from a direct collision between the earth and a comet. "it is easy," says he, "to represent the effects of the shock produced by the earth's encountering a comet. the axis and the motion of rotation changed; the waters abandoning their former position to precipitate themselves towards the new equator; a great part of men and animals whelmed in a universal deluge, or destroyed by the violent shock imparted to the terrestrial globe; entire species annihilated; all the monuments of human industry overthrown;--such are the disasters which the shock of a comet would necessarily produce." la place, nevertheless, expresses a decided opinion that the orbits of the planets have never yet been disturbed by the influence of comets. comets, moreover, have been, and are still to some degree, supposed to exercise much influence in the affairs of this world, affecting the weather, the crops, the public health, and a great variety of atmospheric commotions. even halley, finding that his comet must have been near the earth at the time of the deluge, suggested the possibility that the comet caused that event,--an idea which was taken up by whiston, and formed into a regular theory. in gregory's astronomy, an able work, published at oxford in , the author remarks, that among all nations and in all ages, it has been observed, that the appearance of a comet has always been followed by great calamities; and he adds, "it does not become philosophers lightly to set down these things as fables." among the various things ascribed to comets by a late english writer, are hot and cold seasons, tempests, hurricanes, violent hail-storms, great falls of snow, heavy rains, inundations, droughts, famines, thick fogs, flies, grasshoppers, plague, dysentery, contagious diseases among animals, sickness among cats, volcanic eruptions, and meteors, or shooting stars. these notions are too ridiculous to require a distinct refutation; and i will only add, that we have no evidence that comets have hitherto ever exercised the least influence upon the affairs of this world; and we still remain in darkness, with respect to their physical nature, and the purposes for which they were created. letter xxvii. meteoric showers. "oft shalt thou see, ere brooding storms arise, star after star glide headlong down the skies, and, where they shot, long trails of lingering light sweep far behind, and gild the shades of night."--_virgil._ few subjects of astronomy have excited a more general interest, for several years past, than those extraordinary exhibitions of shooting stars, which have acquired the name of meteoric showers. my reason for introducing the subject to your notice, in this place, is, that these small bodies are, as i believe, derived from nebulous or cometary bodies, which belong to the solar system, and which, therefore, ought to be considered, before we take our leave of this department of creation, and naturally come next in order to comets. the attention of astronomers was particularly directed to this subject by the extraordinary shower of meteors which occurred on the morning of the thirteenth of november, . i had the good fortune to witness these grand celestial fire-works, and felt a strong desire that a phenomenon, which, as it afterwards appeared, was confined chiefly to north america, should here command that diligent inquiry into its causes, which so sublime a spectacle might justly claim. as i think you were not so happy as to witness this magnificent display, i will endeavor to give you some faint idea of it, as it appeared to me a little before daybreak. imagine a constant succession of fire-balls, resembling sky-rockets, radiating in all directions from a point in the heavens a few degrees southeast of the zenith, and following the arch of the sky towards the horizon. they commenced their progress at different distances from the radiating point; but their directions were uniformly such, that the lines they described, if produced upwards, would all have met in the same part of the heavens. around this point, or imaginary radiant, was a circular space of several degrees, within which no meteors were observed. the balls, as they travelled down the vault, usually left after them a vivid streak of light; and, just before they disappeared, exploded, or suddenly resolved themselves into smoke. no report of any kind was observed, although we listened attentively. beside the foregoing distinct concretions, or individual bodies, the atmosphere exhibited _phosphoric lines_, following in the train of minute points, that shot off in the greatest abundance in a northwesterly direction. these did not so fully copy the figure of the sky, but moved in paths more nearly rectilinear, and appeared to be much nearer the spectator than the fire-balls. the light of their trains was also of a paler hue, not unlike that produced by writing with a stick of phosphorus on the walls of a dark room. the number of these luminous trains increased and diminished alternately, now and then crossing the field of view, like snow drifted before the wind, although, in fact, their course was towards the wind. from these two varieties, we were presented with meteors of various sizes and degrees of splendor: some were mere points, while others were larger and brighter than jupiter or venus; and one, seen by a credible witness, at an earlier hour, was judged to be nearly as large as the moon. the flashes of light, although less intense than lightning, were so bright, as to awaken people in their beds. one ball that shot off in the northwest direction, and exploded a little northward of the star capella, left, just behind the place of explosion, a phosphorescent train of peculiar beauty. this train was at first nearly straight, but it shortly began to contract in length, to dilate in breadth, and to assume the figure of a serpent drawing itself up, until it appeared like a small luminous cloud of vapor. this cloud was borne eastward, (by the wind, as was supposed, which was blowing gently in that direction,) opposite to the direction in which the meteor itself had moved, remaining in sight several minutes. the point from which the meteors seemed to radiate kept a fixed position among the stars, being constantly near a star in leo, called gamma leonis. such is a brief description of this grand and beautiful display, as i saw it at new haven. the newspapers shortly brought us intelligence of similar appearances in all parts of the united states, and many minute descriptions were published by various observers; from which it appeared, that the exhibition had been marked by very nearly the same characteristics wherever it had been seen. probably no celestial phenomenon has ever occurred in this country, since its first settlement, which was viewed with so much admiration and delight by one class of spectators, or with so much astonishment and fear by another class. it strikingly evinced the progress of knowledge and civilization, that the latter class was comparatively so small, although it afforded some few examples of the dismay with which, in barbarous ages of the world, such spectacles as this were wont to be regarded. one or two instances were reported, of persons who died with terror; many others thought the last great day had come; and the untutored black population of the south gave expression to their fears in cries and shrieks. after collecting and collating the accounts given in all the periodicals of the country, and also in numerous letters addressed either to my scientific friends or to myself, the following appeared to be the _leading facts_ attending the phenomenon. the shower pervaded nearly the whole of north america, having appeared in nearly equal splendor from the british possessions on the north to the west-india islands and mexico on the south, and from sixty-one degrees of longitude east of the american coast, quite to the pacific ocean on the west. throughout this immense region, the duration was nearly the same. the meteors began to attract attention by their unusual frequency and brilliancy, from _nine to twelve_ o'clock in the evening; were most striking in their appearance from _two to five;_ arrived at their maximum, in many places, about _four_ o'clock; and continued until rendered invisible by the light of day. the meteors moved either in right lines, or in such apparent curves, as, upon optical principles, can be resolved into right lines. their general tendency was towards the northwest, although, by the effect of perspective, they appeared to move in various directions. such were the leading phenomena of the great meteoric shower of november , . for a fuller detail of the facts, as well as of the reasonings that were built on them, i must beg leave to refer you to some papers of mine in the twenty-fifth and twenty-sixth volumes of the american journal of science. soon after this wonderful occurrence, it was ascertained that a similar meteoric shower had appeared in , and, what was remarkable, almost at exactly the same time of year, namely, on the morning of the twelfth of november; and we were again surprised as well as delighted, at receiving successive accounts from different parts of the world of the phenomenon, as having occurred on the morning of the same thirteenth of november, in , , and . hence this was evidently an event independent of the casual changes of the atmosphere; for, having a periodical return, it was undoubtedly to be referred to astronomical causes, and its recurrence, at a certain definite period of the year, plainly indicated _some_ relation to the revolution of the earth around the sun. it remained, however, to develope the nature of this relation, by investigating, if possible, the origin of the meteors. the views to which i was led on this subject suggested the probability that the same phenomenon would recur on the corresponding seasons of the year, for at least several years afterwards; and such proved to be the fact, although the appearances, at every succeeding return, were less and less striking, until , when, so far as i have heard, they ceased altogether. mean-while, two other distinct periods of meteoric showers have, as already intimated, been determined; namely, about the ninth of august, and seventh of december. the facts relative to the history of these periods have been collected with great industry by mr. edward c. herrick; and several of the most ingenious and most useful conclusions, respecting the laws that regulate these singular exhibitions, have been deduced by professor twining. several of the most distinguished astronomers of the old world, also, have engaged in these investigations with great zeal, as messrs. arago and biot, of paris; doctor olbers, of bremen; m. wartmann, of geneva; and m. quetelet, of brussels. but you will be desirous to learn what are the _conclusions_ which have been drawn respecting these new and extraordinary phenomena of the heavens. as the inferences to which i was led, as explained in the twenty-sixth volume of the 'american journal of science,' have, at least in their most important points, been sanctioned by astronomers of the highest respectability, i will venture to give you a brief abstract of them, with such modifications as the progress of investigation since that period has rendered necessary. the principal questions involved in the inquiry were the following:--was the _origin_ of the meteors within the atmosphere, or beyond it? what was the _height_ of the place above the surface of the earth? by what _force_ were the meteors drawn or impelled towards the earth? in what _directions_ did they move? with what _velocity_? what was the cause of their _light_ and _heat_? of what _size_ were the larger varieties? at what height above the earth did they _disappear_? what was the nature of the _luminous trains_ which sometimes remained behind? what _sort of bodies_ were the meteors themselves; of what _kind of matter_ constituted; and in what manner did they exist _before they fell to the earth_? finally, what _relations_ did the source from which they emanated sustain to our earth? in the first place, _the meteors had their origin beyond the limits of our atmosphere_. we know whether a given appearance in the sky is within the atmosphere or beyond it, by this circumstance: all bodies near the earth, including the atmosphere itself, have a common motion with the earth around its axis from west to east. when we see a celestial object moving regularly from west to east, at the same rate as the earth moves, leaving the stars behind, we know it is near the earth, and partakes, in common with the atmosphere, of its diurnal rotation: but when the earth leaves the object behind; or, in other words, when the object moves westward along with the stars, then we know that it is so distant as not to participate in the diurnal revolution of the earth, and of course to be beyond the atmosphere. the source from which the meteors emanated thus kept pace with the stars, and hence was beyond the atmosphere. in the second place, _the height of the place whence the meteors proceeded was very great, but it has not yet been accurately determined_. regarding the body whence the meteors emanated after the similitude of a cloud, it seemed possible to obtain its height in the same manner as we measure the height of a cloud, or indeed the height of the moon. although we could not see the body itself, yet the part of the heavens whence the meteors came would indicate its position. this point we called the _radiant_; and the question was, whether the radiant was projected by distant observers on different parts of the sky; that is, whether it had any _parallax_. i took much pains to ascertain the truth of this matter, by corresponding with various observers in different parts of the united states, who had accurately noted the position of the radiant among the fixed stars, and supposed i had obtained such materials as would enable us to determine the parallax, at least approximately; although such discordances existed in the evidence as reasonably to create some distrust of its validity. putting together, however, the best materials i could obtain, i made the height of the radiant above the surface of the earth _twenty-two hundred and thirty-eight miles_. when, however, i afterwards obtained, as i supposed, some insight into the celestial origin of the meteors, i at once saw that the meteoric body must be much further off than this distance; and my present impression is, that we have not the means of determining what its height really is. we may safely place it at many thousand miles. in the third place, with respect to the _force_ by which the meteors were _drawn_ or impelled towards the earth, my first impression was, that they fell merely by the force of _gravity_; but the velocity which, on careful investigation by professor twining and others, has been ascribed to them, is greater than can possibly result from gravity, since a body can never acquire, by gravity alone, a velocity greater than about seven miles per second. some other cause, beside gravity, must therefore act, in order to give the meteors so great an apparent velocity. in the fourth place, _the meteors fell towards the earth in straight lines, and in directions which, within considerable distances, were nearly parallel with each other_. the courses are inferred to have been in _straight lines_, because no others could have appeared to spectators in different situations to have described arcs of great circles. in order to be projected into the arc of a great circle, the line of descent must be in a plane passing through the eye of the spectator; and the intersection of such planes, passing through the eyes of different spectators, must be straight lines. the lines of direction are inferred to have been _parallel_, on account of their apparent radiation from one point, that being the vanishing point of parallel lines. this may appear to you a little paradoxical, to infer that lines are parallel, because they _diverge_ from one and the same point; but it is a well-known principle of perspective, that parallel lines, when continued to a great distance from the eye, appear to converge towards the remoter end. you may observe this in two long rows of trees, or of street lamps. [illustration fig. .] some idea of the manner in which the meteors fell, and of the reason of their apparent radiation from a common point, may be gathered from the annexed diagram. let a b c, fig. , represent the vault of the sky, the centre of which, d, being the place of the spectator. let , , , &c., represent parallel lines directed towards the earth. a luminous body descending through ' , coinciding with the line d e, coincident with the axis of vision, (or the line drawn from the meteoric body to the eye,) would appear stationary all the while at ´, because distant bodies always appear stationary when they are moving either directly towards us or directly from us. a body descending through , would seem to describe the short arc ' ', appearing to move on the concave of the sky between the lines drawn from the eye to the two extremities of its line of motion; and, for a similar reason, a body descending through , would appear to describe the larger arc ' '. hence, those meteors which fell nearer to the axis of vision, would describe shorter arcs, and move slower, while those which were further from the axis and nearer the horizon would appear to describe longer arcs, and to move with greater velocity; the meteors would all seem to radiate from a common centre, namely, the point where the axis of vision met the celestial vault; and if any meteor chanced to move directly in the line of vision, it would be seen as a luminous body, stationary, for a few seconds, at the centre of radiation. to see how exactly the facts, as observed, corresponded to these inferences, derived from the supposition that the meteors moved in _parallel lines_, take the following description, as given immediately after the occurrence, by professor twining. "in the vicinity of the radiant point, a few star-like bodies were observed, possessing very little motion, and leaving very little length of trace. further off, the motions were more rapid and the traces longer; and most rapid of all, and longest in their traces, were those which originated but a few degrees above the horizon, and descended down to it." in the fifth place, had the meteors come from a point twenty-two hundred and thirty-eight miles from the earth, and derived their apparent velocity from gravity alone, then it would be found, by a very easy calculation, that their actual velocity was about four miles per second; but, as already intimated, the velocity observed was estimated much greater than could be accounted for on these principles; not less, indeed, than fourteen miles per second, and, in some instances, much greater even than this. the motion of the earth in its orbit is about nineteen miles per second; and the most reasonable supposition we can make, at present, to account for the great velocity of the meteors, is, that they derived a relative motion from the earth's passing rapidly by them,--a supposition which is countenanced by the fact that they generally tended _westward_ contrary to the earth's motion in its orbit. in the sixth place, _the meteors consisted of combustible matter, and took fire, and were consumed, in traversing the atmosphere_. that these bodies underwent combustion, we had the direct evidence of the senses, inasmuch as we saw them burn. that they took fire in the _atmosphere_, was inferred from the fact that they were not luminous in their original situations in space, otherwise, we should have seen the body from which they emanated; and had they been luminous before reaching the atmosphere, we should have seen them for a much longer period than they were in sight, as they must have occupied a considerable time in descending towards the earth from so great a distance, even at the rapid rate at which they travelled. the immediate consequence of the prodigious velocity with which the meteors fell into the atmosphere must be a powerful condensation of the air before them, retarding their progress, and producing, by a sudden compression of the air, a great evolution of heat. there is a little instrument called the _air-match_, consisting of a piston and cylinder, like a syringe, in which we strike a light by suddenly forcing down the piston upon the air below. as the air cannot escape, it is suddenly compressed, and gives a spark sufficient to light a piece of tinder at the bottom of the cylinder. indeed, it is a well-known fact, that, whenever air is suddenly and forcibly compressed, heat is elicited; and, if by such a compression as may be given by the hand in the air-match, heat is evolved sufficient to fire tinder, what must be the heat evolved by the motion of a large body in the atmosphere, with a velocity so immense. it is common to resort to electricity as the agent which produces the heat and light of shooting stars; but even were electricity competent to produce this effect, its presence, in the case before us, is not proved; and its agency is unnecessary, since so swift a motion of the meteors themselves, suddenly condensing the air before them, is both a known and adequate cause of an intense light and heat. a combustible body falling into the atmosphere, under such circumstances, would become speedily ignited, but could not burn freely, until it became enveloped in air of greater density; but, on reaching the lower portions of the atmosphere, it would burn with great rapidity. in the seventh place, _some of the larger meteors must have been bodies of great size_. according to the testimony of various individuals, in different parts of the united states, a few fire-balls appeared as large as the full moon. dr. smith, (then of north carolina, but since surgeon-general of the texian army,) who was travelling all night on professional business, describes one which he saw in the following terms: "in size it appeared somewhat larger than the full moon rising. i was startled by the splendid light in which the surrounding scene was exhibited, rendering even small objects quite visible; but i heard no noise, although every sense seemed to be suddenly aroused, in sympathy with the violent impression on the sight." this description implies not only that the body was very large, but that it was at a considerable distance from the spectator. its actual size will depend upon the distance; for, as it appeared under the same angle as the moon, its diameter will bear the same ratio to the moon's, as its distance bears to the moon's distance. we could, therefore, easily ascertain how large it was, provided we could find how far it was from the observer. if it was one hundred and ten miles distant, its diameter was one mile, and in the same proportion for a greater or less distance; and, if only at the distance of one mile, its diameter was forty-eight feet. for a moderate estimate, we will suppose it to have been twenty-two miles off; then its diameter was eleven hundred and fifty-six feet. upon every view of the case, therefore, it must be admitted, that these were bodies of great size, compared with other objects which traverse the atmosphere. we may further infer the great magnitude of some of the meteors, from the dimensions of the trains, or clouds, which resulted from their destruction. these often extended over several degrees, and at length were borne along in the direction of the wind, exactly in the manner of a small cloud. it was an interesting problem to ascertain, if possible, the height above the earth at which these fire-balls exploded, or resolved themselves into a cloud of smoke. this would be an easy task, provided we could be certain that two or more distant observers could be sure that both saw the same meteor; for as each would refer the place of explosion, or the position of the cloud that resulted from it, to a different point of the sky, a parallax would thus be obtained, from which the height might be determined. the large meteor which is mentioned in my account of the shower, (see page ,) as having exploded near the star capella, was so peculiar in its appearance, and in the form and motions of the small cloud which resulted from its combustion, that it was noticed and distinguished by a number of observers in distant parts of the country. all described the meteor as exhibiting, substantially, the same peculiarities of appearance; all agreed very nearly in the time of its occurrence; and, on drawing lines, to represent the course and direction of the place where it exploded to the view of each of the observers respectively, these lines met in nearly one and the same point, and that was over the place where it was seen in the zenith. little doubt, therefore, could remain, that all saw the same body; and on ascertaining, from a comparison of their observations, the amount of parallax, and thence deducing its height,--a task which was ably executed by professor twining,--the following results were obtained: that this meteor, and probably all the meteors, entered the atmosphere with a velocity not less, but perhaps greater, than _fourteen miles in a second_; that they became luminous many miles from the earth,--in this case, over _eighty miles_; and became extinct high above the surface,--in this case, nearly _thirty miles_. in the eighth place, _the meteors were combustible bodies, and were constituted of light and transparent materials_. the fact that they burned is sufficient proof that they belonged to the class of _combustible_ bodies; and they must have been composed of very _light materials_, otherwise their momentum would have been sufficient to enable them to make their way through the atmosphere to the surface of the earth. to compare great things with small, we may liken them to a wad discharged from a piece of artillery, its velocity being supposed to be increased (as it may be) to such a degree, that it shall take fire as it moves through the air. although it would force its way to a great distance from the gun, yet, if not consumed too soon, it would at length be stopped by the resistance of the air. although it is supposed that the meteors did in fact slightly disturb the atmospheric equilibrium, yet, had they been constituted of dense matter, like meteoric stones, they would doubtless have disturbed it vastly more. their own momentum would be lost only as it was imparted to the air; and had such a number of bodies,--some of them quite large, perhaps a mile in diameter, and entering the atmosphere with a velocity more than forty times the greatest velocity of a cannon ball,--had they been composed of dense, ponderous matter, we should have had appalling evidence of this fact, not only in the violent winds which they would have produced in the atmosphere, but in the calamities they would have occasioned on the surface of the earth. the meteors were _transparent_ bodies; otherwise, we cannot conceive why the body from which they emanated was not distinctly visible, at least by reflecting the light of the sun. if only the meteors which were known to fall towards the earth had been collected and restored to their original connexion in space, they would have composed a body of great extent; and we cannot imagine a body of such dimensions, under such circumstances, which would not be visible, unless formed of highly transparent materials. by these unavoidable inferences respecting the kind of matter of which the meteors were composed, we are unexpectedly led to recognise a body bearing, in its constitution, a strong analogy to comets, which are also composed of exceedingly light and transparent, and, as there is much reason to believe, of combustible matter. we now arrive at the final inquiry, _what relations did the body which afforded the meteoric shower sustain to the earth_? was it of the nature of a satellite, or terrestrial comet, that revolves around the earth as its centre of motion? was it a collection of nebulous, or cometary matter, which the earth encountered in its annual progress? or was it a comet, which chanced at this time to be pursuing its path along with the earth, around their common centre of motion? it could not have been of the nature of a satellite to the earth, (or one of those bodies which are held by some to afford the meteoric stones, which sometimes fall to the earth from huge meteors that traverse the atmosphere,) because it remained so long stationary with respect to the earth. a body so near the earth as meteors of this class are known to be, could not remain apparently stationary among the stars for a moment; whereas the body in question occupied the same position, with hardly any perceptible variation, for at least two hours. nor can we suppose that the earth, in its annual progress, came into the vicinity of a _nebula_, which was either stationary, or wandering lawless through space. such a collection of matter could not remain stationary within the solar system, in an insulated state, for, if not prevented by a motion of its own, or by the attraction of some nearer body, it would have proceeded directly towards the sun; and had it been in motion in any other direction than that in which the earth was moving, it would soon have been separated from the earth; since, during the eight hours, while the meteoric shower was visible, the earth moved in its orbit through the space of nearly five hundred and fifty thousand miles. the foregoing considerations conduct us to the following train of reasoning. first, if all the meteors which fell on the morning of november , , had been collected and restored to their original connexion in space, they would of themselves have constituted a nebulous body of great extent; but we have reason to suppose that they, in fact, composed but a small part of the mass from which they emanated, since, after the loss of so much matter as proceeded from it in the great meteoric shower of , and in the several repetitions of it that preceded the year , it was still capable of affording so copious a shower on that year; and similar showers, more limited in extent, were repeated for at least five years afterwards. we are therefore to regard the part that descended only as _the extreme portions of a body or collection of meteors, of unknown extent, existing in the planetary spaces_. secondly, since the earth fell in with this body in the same part of its orbit, for several years in succession, it must either have remained there while the earth was performing its whole revolution around the sun, or it must itself have had a revolution, as well as the earth. but i have already shown that it could not have remained stationary in that part of space; therefore, _it must have had a revolution around the sun_. thirdly, its period of revolution must have either been greater than the earth's, equal to it, or less. it could not have been greater, for then the two bodies could not have been together again at the end of the year, since the meteoric body would not have completed its revolution in a year. its period might obviously be the same as the earth's, for then they might easily come together again after one revolution of each; although their orbits might differ so much in shape as to prevent their being together at any intermediate point. but the period of the body might also be less than that of the earth, provided it were some _aliquot part of a year_, so as to revolve just twice, or three times, for example, while the earth revolves once. let us suppose that the period is one third of a year. then, since we have given the periodic times of the two bodies, and the major axis of the orbit of one of them, namely, of the earth, we can, by kepler's law, find the major axis of the other orbit; for the square of the earth's periodic time ^ is to the square of the body's time ( / )^ as the cube of the major axis of the earth's orbit is to the cube of the major axis of the orbit in question. now, the three first terms of this proportion are known, and consequently, it is only to solve a case in the simple rule of three, to find the term required. on making the calculation, it is found, that the supposition of a periodic time of only one third of a year gives an orbit of insufficient length; the whole major axis would not reach from the sun to the earth; and consequently, a body revolving in it could never come near to the earth. on making trial of six months, we obtain an orbit which satisfies the conditions, being such as is represented by the diagram on page , fig. ', where the outer circle denotes the earth's orbit, the sun being in the centre, and the inner ellipse denotes the path of the meteoric body. the two bodies are together at the top of the figure, being the place of the meteoric body's aphelion on the thirteenth of november, and the figures , , &c., denote the relative positions of the earth and the body for every ten days, for a period of six months, in which time the body would have returned to its aphelion. [illustration fig. '.] such would be the relation of the body that affords the meteoric shower of november, provided its revolution is accomplished in six months; but it is still somewhat uncertain whether the period be half a year or a year; it must be one or the other. if we inquire, now, why the meteors always appear to radiate from a point in the constellation leo, recollecting that this is the point to which the body is projected among the stars, the answer is, that this is the very point towards which the earth is moving in her orbit at that time; so that if, as we have proved, the earth passed through or near a nebulous body on the thirteenth of november, that body must necessarily have been projected into the constellation leo, else it could not have lain directly in her path. i consider it therefore as established by satisfactory proof, that the meteors of november thirteenth emanate from a nebulous or cometary body, revolving around the sun, and coming so near the earth at that time that the earth passes through its _skirts_, or extreme portions, and thus attracts to itself some portions of its matter, giving to the meteors a greater velocity than could be imparted by gravity alone, in consequence of passing rapidly by them. all these conclusions were made out by a process of reasoning strictly inductive, without supposing that the meteoric body itself had ever been seen. but there are some reasons for believing that we do actually see it, and that it is no other than that mysterious appearance long known under the name of the _zodiacal light_. this is a faint light, which at certain seasons of the year appears in the west after evening twilight, and at certain other seasons appears in the east before the dawn, following or preceding the track of the sun in a triangular figure, with its broad base next to the sun, and its vertex reaching to a greater or less distance, sometimes more than ninety degrees from that luminary. you may obtain a good view of it in february or march, in the west, or in october, in the morning sky. the various changes which this light undergoes at different seasons of the year are such as to render it probable, to my mind, that this is the very body which affords the meteoric showers; its extremity coming, in november, within the sphere of the earth's attraction. but, as the arguments for the existence of a body in the planetary regions, which affords these showers, were drawn without the least reference to the zodiacal light, and are good, should it finally be proved that this light has no connexion with them, i will not occupy your attention with the discussion of this point, to the exclusion of topics which will probably interest you more. it is perhaps most probable, that the meteoric showers of august and december emanate from the same body. i know of nothing repugnant to this conclusion, although it has not yet been distinctly made out. had the periods of the earth and of the meteoric body been so adjusted to each other that the latter was contained an exact even number of times in the former; that is, had it been _exactly_ either a year or half a year; then we might expect a similar recurrence of the meteoric shower every year; but only a slight variation in such a proportion between the two periods would occasion the repetition of the shower for a few years in succession, and then an intermission of them, for an unknown length of time, until the two bodies were brought into the same relative situation as before. disturbances, also, occasioned by the action of venus and mercury, might wholly subvert this numerical relation, and increase or diminish the probability of a repetition of the phenomenon. accordingly, from the year , when the meteoric shower of november was first observed, until , there was a regular increase of the exhibition; in , it came to its maximum; and after that time it was repeated upon a constantly diminishing scale, until , since which time it has not been observed. perhaps ages may roll away before the world will be again surprised and delighted with a display of celestial fire-works equal to that of the morning of november , . letter xxviii. fixed stars. ----"o, majestic night! nature's great ancestor! day's elder born, and fated to survive the transient sun! by mortals and immortals seen with awe! a starry crown thy raven brow adorns, an azure zone thy waist; clouds, in heaven's loom wrought, through varieties of shape and shade, in ample folds of drapery divine, thy flowing mantle form; and heaven throughout voluminously pour thy pompous train."--_young._ since the solar system is but one among a myriad of worlds which astronomy unfolds, it may appear to you that i have dwelt too long on so diminutive a part of creation, and reserved too little space for the other systems of the universe. but however humble a province our sun and planets compose, in the vast empire of jehovah, yet it is that which most concerns us; and it is by the study of the laws by which this part of creation is governed, that we learn the secrets of the skies. until recently, the observation and study of the phenomena of the solar system almost exclusively occupied the labors of astronomers. but sir william herschel gave his chief attention to the _sidereal heavens_, and opened new and wonderful fields of discovery, as well as of speculation. the same subject, has been prosecuted with similar zeal and success by his son, sir john herschel, and sir james south, in england, and by professor struve, of dorpat, until more has been actually achieved than preceding astronomers had ventured to conjecture. a limited sketch of these wonderful discoveries is all that i propose to offer you. the fixed stars are so called, because, to common observation, they always maintain the same situations with respect to one another. the stars are classed by their apparent _magnitudes_. the whole number of magnitudes recorded are _sixteen_, of which the first six only are visible to the naked eye; the rest are _telescopic stars_. these magnitudes are not determined by any very definite scale, but are merely ranked according to their relative degrees of brightness, and this is left in a great measure to the decision of the eye alone. the brightest stars, to the number of fifteen or twenty, are considered as stars of the first magnitude; the fifty or sixty next brightest, of the second magnitude; the next two hundred, of the third magnitude; and thus the number of each class increases rapidly, as we descend the scale, so that no less than fifteen or twenty thousand are included within the first seven magnitudes. the stars have been grouped in _constellations_ from the most remote antiquity; a few, as orion, bootes, and ursa major, are mentioned in the most ancient writings, under the same names as they bear at present. the names of the constellations are sometimes founded on a supposed resemblance to the objects to which they belong; as the swan and the scorpion were evidently so denominated from their likeness to those animals; but in most cases, it is impossible for us to find any reason for designating a constellation by the figure of the animal or hero which is employed to represent it. these representations were probably once blended with the fables of pagan mythology. the same figures, absurd as they appear, are still retained for the convenience of reference; since it is easy to find any particular star, by specifying the part of the figure to which it belongs; as when we say, a star is in the neck of taurus, in the knee of hercules, or in the tail of the great bear. this method furnishes a general clue to its position; but the stars belonging to any constellation are distinguished according to their apparent magnitudes, as follows: first, by the greek letters, alpha, beta, gamma, &c. thus, _alpha orionis_ denotes the largest star in orion; _beta andromedæ_ the second star in andromeda; and _gamma leonis_, the third brightest star in the lion. when the number of the greek letters is insufficient to include all the stars in a constellation, recourse is had to the letters of the roman alphabet, a, b, c, &c.; and in all cases where these are exhausted the final resort is to numbers. this is evidently necessary, since the largest constellations contain many hundreds or even thousands of stars. _catalogues_ of particular stars have also been published, by different astronomers, each author numbering the individual stars embraced in his list according to the places they respectively occupy in the catalogue. these references to particular catalogues are sometimes entered on large celestial globes. thus we meet with a star marked h., meaning that this is its number in herschel's catalogue; or m., denoting the place the star occupies in the catalogue of mayer. the earliest catalogue of the stars was made by hipparchus, of the alexandrian school, about one hundred and forty years before the christian era. a new star appearing in the firmament, he was induced to count the stars, and to record their positions, in order that posterity might be able to judge of the permanency of the constellations. his catalogue contains all that were conspicuous to the naked eye in the latitude of alexandria, being one thousand and twenty-two. most persons, unacquainted with the actual number of the stars which compose the visible firmament, would suppose it to be much greater than this; but it is found that the catalogue of hipparchus embraces nearly all that can now be seen in the same latitude; and that on the equator, where the spectator has both the northern and southern hemispheres in view, the number of stars that can be counted does not exceed three thousand. a careless view of the firmament in a clear night gives us the impression of an infinite number of stars; but when we begin to count them, they appear much more sparsely distributed than we supposed, and large portions of the sky appear almost destitute of stars. by the aid of the telescope, new fields of stars present themselves, of boundless extent; the number continually augmenting, as the powers of the telescope are increased. lalande, in his 'histoire celeste,' has registered the positions of no less than fifty thousand; and the whole number visible in the largest telescopes amounts to many millions. when you look at the firmament on a clear autumnal or winter evening, it appears so thickly studded with stars, that you would perhaps imagine that the task of learning even the brightest of them would be almost hopeless. let me assure you, this is all a mistake. on the contrary, it is a very easy task to become acquainted with the names and positions of the stars of the first magnitude, and of the leading constellations. if you will give a few evenings to the study, you will be surprised to find, both how rapidly you can form these new acquaintances, and how deeply you will become interested in them. i would advise you, at first, to obtain, for an evening or two, the assistance of some friend who is familiar with the stars, just to point out a few of the most conspicuous constellations. this will put you on the track, and you will afterwards experience no difficulty in finding all the constellations and stars that are particularly worth knowing; especially if you have before you a map of the stars, or, what is much better, a celestial globe. it is a pleasant evening recreation for a small company of young astronomers to go out together, and learn one or two constellations every favorable evening, until the whole are mastered. if you have a celestial globe, _rectify_ it for the evening; that is, place it in such a position, that the constellations shall be seen on it in the same position with respect to the horizon, that they have at that moment in the sky itself. to do this, i first elevate the north pole until the number of degrees on the brass meridian from the pole to the horizon corresponds to my latitude, (forty-one degrees and eighteen minutes.) i then find the sun's place in the ecliptic, by looking for the day of the month on the broad horizon, and against it noting the corresponding sign and degree. i now find the same sign and degree on the ecliptic itself, and bring that point to the brass meridian. as that will be the position of the sun at noon, i set the hour-index at twelve, and then turn the globe westward, until the index points to the given hour of the evening. if i now inspect the figures of the constellations, and then look upward at the firmament, i shall see that the latter are spread over the sky in the same manner as the pictures of them are painted on the globe. i will point out a few marks by which the leading constellations may be recognised; this will aid you in finding them, and you can afterwards learn the individual stars of a constellation, to any extent you please, by means of the globes or maps. let us begin with the _constellations of the zodiac_, which, succeeding each other, as they do, in a known order, are most easily found. _aries_ (_the ram_) is a small constellation, known by two bright stars which form his head, _alpha_ and _beta arietis_. these two stars are about four degrees apart; and directly south of beta, at the distance of one degree, is a smaller star, _gamma arietis_. it has been already intimated that the vernal equinox probably was near the head of aries, when the signs of the zodiac received their present names. _taurus_ (_the bull_) will be readily found by the seven stars, or _pleiades_, which lie in his neck. the largest star in taurus is _aldebaran_, in the bull's eye, a star of the first magnitude, of a reddish color, somewhat resembling the planet mars. aldebaran and four other stars, close together in the face of taurus, compose the _hyades_. _gemini_ (_the twins_) is known by two very bright stars, _castor and pollux_, five degrees asunder. castor (the northern) is of the first, and pollux of the second, magnitude. _cancer_ (_the crab_.) there are no large stars in this constellation, and it is regarded as less remarkable than any other in the zodiac. it contains, however, an interesting group of small stars, called _præsepe_, or the nebula of cancer, which resembles a comet, and is often mistaken for one, by persons unacquainted with the stars. with a telescope of very moderate powers this nebula is converted into a beautiful assemblage of exceedingly bright stars. _leo_ (_the lion_) is a very large constellation, and has many interesting members. _regulus_ (_alpha leonis_) is a star of the first magnitude, which lies directly in the ecliptic, and is much used in astronomical observations. north of regulus, lies a semicircle of bright stars, forming a _sickle_, of which regulus is the handle. _denebola_, a star of the second magnitude, is in the lion's tail, twenty-five degrees northeast of regulus. _virgo_ (_the virgin_) extends a considerable way from west to east, but contains only a few bright stars. _spica_, however, is a star of the first magnitude, and lies a little east of the place of the autumnal equinox. eighteen degrees eastward of denebola, and twenty degrees north of spica, is _vindemiatrix_, in the arm of virgo, a star of the third magnitude. _libra_ (_the balance_) is distinguished by three large stars, of which the two brightest constitute the beam of the balance, and the smallest forms the top or handle. _scorpio_ (_the scorpion_) is one of the finest of the constellations. his head is formed of five bright stars, arranged in the arc of a circle, which is crossed in the centre by the ecliptic nearly at right angles, near the brightest of the five, _beta scorpionis_. nine degrees southeast of this is a remarkable star of the first magnitude, of a reddish color, called _cor scorpionis_, or _antares_. south of this, a succession of bright stars sweep round towards the east, terminating in several small stars, forming the tail of the scorpion. _sagittarius_ (_the archer_.) northeast of the tail of the scorpion are three stars in the arc of a circle, which constitute the _bow_ of the archer, the central star being the brightest, directly west of which is a bright star which forms the _arrow_. _capricornus_ (_the goat_) lies northeast of sagittarius, and is known by two bright stars, three degrees apart, which form the head. _aquarius_ (_the water-bearer_) is recognised by two stars in a line with _alpha capricorni_, forming the shoulders of the figure. these two stars are ten degrees apart; and three degrees southeast is a third star, which, together with the other two, make an acute triangle, of which the westernmost is the vertex. _pisces_ (_the fishes_) lie between aquarius and aries. they are not distinguished by any large stars, but are connected by a series of small stars, that form a crooked line between them. _piscis australia_, the southern fish, lies directly below aquarius, and is known by a single bright star far in the south, having a declination of thirty degrees. the name of this star is _fomalhaut_, and it is much used in astronomical measurements. the constellations of the zodiac, being first well learned, so as to be readily recognised, will facilitate the learning of others that lie north and south of them. let us, therefore, next review the principal _northern constellations_, beginning north of aries, and proceeding from west to east. _andromeda_ is characterized by three stars of the second magnitude, situated in a straight line, extending from west to east. the middle star is about seventeen degrees north of beta arietis. it is in the girdle of andromeda, and is named _mirach_. the other two lie at about equal distances, fourteen degrees west and east of mirach. the western star, in the head of andromeda, lies in the equinoctial colure. the eastern star, _alamak_, is situated in the foot. _perseus_ lies directly north of the pleiades, and contains several bright stars. about eighteen degrees from the pleiades is _algol_, a star of the second magnitude, in the head of medusa, which forms a part of the figure; and nine degrees northeast of algol is _algenib_, of the same magnitude, in the back of perseus. between algenib and the pleiades are three bright stars, at nearly equal intervals, which compose the right leg of perseus. _auriga_ (_the wagoner_) lies directly east of perseus, and extends nearly parallel to that constellation, from north to south. _capella_, a very white and beautiful star of the first magnitude, distinguishes this constellation. the feet of auriga are near the bull's horns. the _lynx_ comes next, but presents nothing particularly interesting, containing no stars above the fourth magnitude. _leo minor_ consists of a collection of small stars north of the sickle in leo, and south of the great bear. its largest star is only of the third magnitude. _coma berenices_ is a cluster of small stars, north of denebola, in the tail of the lion, and of the head of virgo. about twelve degrees directly north of berenice's hair, is a single bright star, called _cor caroli_, or charles's heart. _bootes_, which comes next, is easily found by means of _arcturus_, a star of the first magnitude, of a reddish color, which is situated near the knee of the figure. arcturus is accompanied by three small stars, forming a triangle a little to the southwest. two bright stars, _gamma_ and _delta bootis_, form the shoulders, and _beta_, of the third magnitude, is in the head, of the figure. _corona borealis_, (_the crown_,) which is situated east of bootes, is very easily recognised, composed as it is of a semicircle of bright stars. in the centre of the bright crown is a star of the second magnitude, called _gemma_: the remaining stars are all much smaller. _hercules_, lying between the crown on the west and the lyre on the east, is very thickly set with stars, most of which are quite small. this constellation covers a great extent of the sky, especially from north to south, the head terminating within fifteen degrees of the equator, and marked by a star of the third magnitude, called _ras algethi_, which is the largest in the constellation. _ophiucus_ is situated directly south of hercules, extending some distance on both sides of the equator, the feet resting on the scorpion. the head terminates near the head of hercules, and, like that, is marked by a bright star within five degrees of _alpha herculis_ ophiucus is represented as holding in his hands the _serpent_, the head of which, consisting of three bright stars, is situated a little south of the crown. the folds of the serpent will be easily followed by a succession of bright stars, which extend a great way to the east. _aquila_ (_the eagle_) is conspicuous for three bright stars in its neck, of which the central one, _altair_, is a very brilliant white star of the first magnitude. _antinous_ lies directly south of the eagle, and north of the head of capricornus. _delphinus_ (_the dolphin_) is a small but beautiful constellation, a few degrees east of the eagle, and is characterized by four bright stars near to one another, forming a small rhombic square. another star of the same magnitude, five degrees south, makes the tail. _pegasus_ lies between aquarius on the southwest and andromeda on the northeast. it contains but few large stars. a very regular square of bright stars is composed of _alpha andromedæ_ and the three largest stars in pegasus; namely, _scheat_, _markab_, and _algenib_. the sides composing this square are each about fifteen degrees. algenib is situated in the equinoctial colure. we may now review the _constellations which surround the north pole_, within the circle of perpetual apparition. _ursa minor_ (_the little bear_) lies nearest the pole. the pole-star, _polaris_, is in the extremity of the tail, and is of the third magnitude. three stars in a straight line, four degrees or five degrees apart, commencing with the pole-star, lead to a trapezium of four stars, and the whole seven form together a _dipper_,--the trapezium being the body and the three stars the handle. _ursa major_ (_the great bear_) is situated between the pole and the lesser lion, and is usually recognised by the figure of a larger and more perfect dipper which constitutes the hinder part of the animal. this has also seven stars, four in the body of the dipper and three in the handle. all these are stars of much celebrity. the two in the western side of the dipper, alpha and beta, are called _pointers_, on account of their always being in a right line with the pole-star, and therefore affording an easy mode of finding that. the first star in the tail, next the body, is named _alioth_, and the second, _mizar_. the head of the great bear lies far to the westward of the pointers, and is composed of numerous small stars; and the feet are severally composed of two small stars very near to each other. _draco_ (_the dragon_) winds round between the great and the little bear; and, commencing with the tail, between the pointers and the pole-star, it is easily traced by a succession of bright stars extending from west to east. passing under ursa minor, it returns westward, and terminates in a triangle which forms the head of draco, near the feet of hercules, northwest of lyra. _cepheus_ lies eastward of the breast of the dragon, but has no stars above the third magnitude. _cassiopeia_ is known by the figure of a _chair_, composed of four stars which form the legs, and two which form the back. this constellation lies between perseus and cepheus, in the milky way. _cygnus_ (_the swan_) is situated also in the milky way, some distance southwest of cassiopeia, towards the eagle. three bright stars, which lie along the milky way, form the body and neck of the swan, and two others, in a line with the middle one of the three, one above and one below, constitute the wings. this constellation is among the few that exhibit some resemblance to the animals whose names they bear. _lyra_ (_the lyre_) is directly west of the swan, and is easily distinguished by a beautiful white star of the first magnitude, _alpha lyræ_. the _southern constellations_ are comparatively few in number. i shall notice only the whale, orion, the greater and lesser dog, hydra, and the crow. _cetus_ (_the whale_) is distinguished rather for its extent than its brilliancy, reaching as it does through forty degrees of longitude, while none of its stars, except one, are above the third magnitude. _menkar_ (_alpha ceti_) in the mouth, is a star of the second magnitude; and several other bright stars, directly south of aries, make the head and neck of the whale. _mira_, (_omicron ceti_,) in the neck of the whale, is a variable star. _orion_ is one of the largest and most beautiful of the constellations, lying southeast of taurus. a cluster of small stars forms the head; two large stars, _betalgeus_ of the first and _bellatrix_ of the second magnitude, make the shoulders; three more bright stars compose the buckler, and three the sword; and _rigel_, another star of the first magnitude, makes one of the feet. in this constellation there are seventy stars plainly visible to the naked eye, including two of the first magnitude, four of the second, and three of the third. _canis major_ lies southeast of orion, and is distinguished chiefly by its containing the largest of the fixed stars, _sirius_. _canis minor_, a little north of the equator, between canis major and gemini, is a small constellation, consisting chiefly of two stars, of which, _procyon_ is of the first magnitude. _hydra_ has its head near procyon, consisting of a number of stars of ordinary brightness. about fifteen degrees southeast of the head is a star of the second magnitude, forming the heart, (_cor hydræ_;) and eastward of this is a long succession of stars of the fourth and fifth magnitudes, composing the body and tail, and reaching a few degrees south of spica virginis. _corvus_ (_the crow_) is represented as standing on the tail of hydra. it consists of small stars, only three of which are as large as the third magnitude. in assigning the places of individual stars, i have not aimed at great precision; but such a knowledge as you will acquire of the constellations and larger stars, by nothing more even than you can obtain from the foregoing sketch, will not only add greatly to the interest with which you will ever afterwards look at the starry heavens, but it will enable you to locate any phenomenon that may present itself in the nocturnal sky, and to understand the position of any object that may be described, by assigning its true place among the stars; although i hope you will go much further than this mere outline, in cultivating an actual acquaintance with the stars. leaving, now, these great divisions of the bodies of the firmament, let us ascend to the next order of stars, composing clusters. in various parts of the nocturnal heavens are seen large groups which, either by the naked eye, or by the aid of the smallest telescope, are perceived to consist of a great number of small stars. such are the pleiades, coma berenices, and præsepe, or the bee-hive, in cancer. the _pleiades_, or seven stars, as they are called, in the neck of taurus, is the most conspicuous cluster. when we look _directly_ at this group, we cannot distinguish more than six stars; but by turning the eye _sideways_ upon it, we discover that there are many more; for it is a remarkable fact that indirect vision is far more delicate than direct. thus we can see the zodiacal light or a comet's tail much more distinctly and better defined, if we fix one eye on a part of the heavens at some distance and turn the other eye obliquely upon the object, than we can by looking directly towards it. telescopes show the pleiades to contain fifty or sixty stars, crowded together, and apparently insulated from the other parts of the heavens. _coma berenices_ has fewer stars, but they are of a larger class than those which compose the pleiades. the _bee-hive_, or nebula of cancer, as it is called, is one of the finest objects of this kind for a small telescope, being by its aid converted into a rich congeries of shining points. the head of orion affords an example of another cluster, though less remarkable than those already mentioned. these clusters are pleasing objects to the telescope; and since a common spyglass will serve to give a distinct view of most of them, every one may have the power of taking the view. but we pass, now, to the third order of stars, which present themselves much more obscurely to the gaze of the astronomer, and require large instruments for the full developement of their wonderful organization. these are the nebul�. [illustration figures , , , . clusters of stars and nebul�.] nebulæ are faint misty appearances which are dimly seen among the stars, resembling comets, or a speck of fog. they are usually resolved by the telescope into myriads of small stars; though in some instances, no powers of the telescope have been found sufficient thus to resolve them. the _galaxy_ or milky way, presents a continued succession of large nebulas. the telescope reveals to us innumerable objects of this kind. sir william herschel has given catalogues of two thousand nebulæ, and has shown that the nebulous matter is distributed through the immensity of space in quantities inconceivably great, and in separate parcels, of all shapes and sizes, and of all degrees of brightness between a mere milky appearance and the condensed light of a fixed star. in fact, more distinct nebulæ have been hunted out by the aid of telescopes than the whole number of stars visible to the naked eye in a clear winter's night. their appearances are extremely diversified. in many of them we can easily distinguish the individual stars; in those apparently more remote, the interval between the stars diminishes, until it becomes quite imperceptible; and in their faintest aspect they dwindle to points so minute, as to be appropriately denominated _star-dust_. beyond this, no stars are distinctly visible, but only streaks or patches of milky light. the diagram facing page represents a magnificent nebula in the galaxy. in objects so distant as the fixed stars, any apparent interval must denote an immense space; and just imagine yourself situated any where within the grand assemblage of stars, and a firmament would expand itself over your head like that of our evening sky, only a thousand times more rich and splendid. many of the nebulæ exhibit a tendency towards a globular form, and indicate a rapid condensation towards the centre. this characteristic is exhibited in the forms represented in figs. and . we have here two specimens of nebulæ of the nearer class, where the stars are easily discriminated. in figs. and we have examples of two others of the remoter kind, one of which is of the variety called _star-dust_. these wonderful objects, however, are not confined to the spherical form, but exhibit great varieties of figure. sometimes they appear as ovals; sometimes they are shaped like a fan; and the unresolvable kind often affect the most fantastic forms. the opposite diagram, fig. , as well as the preceding, affords a specimen of these varieties, as given in professor nichols's 'architecture of the heavens,' where they are faithfully copied from the papers of herschel, in the 'philosophical transactions.' [illustration figure . various forms of nebul�.] sir john herschel has recently returned from a residence of five years at the cape of good hope, with the express view of exploring the hidden treasures of the southern hemisphere. the kinds of nebulæ are in general similar to those of the northern hemisphere, and the forms are equally various and singular. the _magellan clouds_, two remarkable objects seen among the stars of that hemisphere, and celebrated among navigators, appeared to the great telescope of herschel (as we are informed by professor nichols) no longer as simple milky spots, or permanent light flocculi of cloud, as they appear to the unassisted eye, but shone with inconceivable splendor. the _nubecula major_, as the larger object is called, is a congeries of clusters of stars, of irregular form, globular clusters and nebulæ of various magnitudes and degrees of condensation, among which is interspersed a large portion of irresolvable nebulous matter, which may be, and probably is, star-dust, but which the power of the twenty-feet telescope shows only as a general illumination of the field of view, forming a bright ground on which the other objects are scattered. the _nubecula minor_ (the lesser cloud) exhibited appearances similar, though inferior in degree. [illustration figure . a nebula in the milky way.] it is a grand idea, first conceived by sir william herschel, and generally adopted by astronomers, that the whole galaxy, or milky way, is nothing else than a nebula, and appears so extended, merely because it happens to be that particular nebula to which we belong. according to this view, our sun, with his attendant planets and comets, constitutes but a single star of the galaxy, and our firmament of stars, or visible heavens, is composed of the stars of _our_ nebula alone. an inhabitant of any of the other nebulæ would see spreading over him a firmament equally spacious, and in some cases inconceivably more brilliant. it is an exalted spectacle to travel over the galaxy in a clear night, with a powerful telescope, with the heart full of the idea that every star is a world. sir william herschel, by counting the stars in a single field of his telescope, estimated that fifty thousand had passed under his review in a zone two degrees in breadth, during a single hour's observation. notwithstanding the apparent contiguity of the stars which crowd the galaxy, it is certain that their mutual distances must be inconceivably great. it is with some reluctance that i leave, for the present, this fairy land of astronomy; but i must not omit, before bringing these letters to a conclusion, to tell you something respecting other curious and interesting objects to be found among the stars. variable stars are those which undergo a periodical change of brightness. one of the most remarkable is the star _mira_, in the whale, (_omicron ceti_.) it appears once in eleven months, remains at its greatest brightness about a fortnight, being then, on some occasions, equal to a star of the second magnitude. it then decreases about three months, until it becomes completely invisible, and remains so about five months, when it again becomes visible, and continues increasing during the remaining three months of its period. another very remarkable variable star is _algol_, (_beta persei_.) it is usually visible as a star of the second magnitude, and continues such for two days and fourteen hours, when it suddenly begins to diminish in splendor, and in about three and a half hours is reduced to the fourth magnitude. it then begins again to increase, and in three and a half hours more is restored to its usual brightness, going through all its changes in less than three days. this remarkable law of variation appears strongly to suggest the revolution round it of some opaque body, which, when interposed between us and algol, cuts off a large portion of its light. "it is," says sir j. herschel, "an indication of a high degree of activity in regions where, but for such evidences, we might conclude all lifeless. our sun requires almost nine times this period to perform a revolution on its axis. on the other hand, the periodic time of an opaque revolving body, sufficiently large, which would produce a similar temporary obscuration of the sun, seen from a fixed star, would be less than fourteen hours." the duration of these periods is extremely various. while that of beta persei, above mentioned, is less than three days, others are more than a year; and others, many years. temporary stars are new stars, which have appeared suddenly in the firmament, and, after a certain interval, as suddenly disappeared, and returned no more. it was the appearance of a new star of this kind, one hundred and twenty-five years before the christian era, that prompted hipparchus to draw up a catalogue of the stars, the first on record. such, also, was the star which suddenly shone out, a.d. , in the eagle, as bright as venus, and, after remaining three weeks, disappeared entirely. at other periods, at distant intervals, similar phenomena have presented themselves. thus the appearance of a star in was so sudden, that tycho brahe, returning home one day, was surprised to find a collection of country people gazing at a star which he was sure did not exist half an hour before. it was then as bright as sirius, and continued to increase until it surpassed jupiter when brightest, and was visible at mid-day. in a month it began to diminish; and, in three months afterwards, it had entirely disappeared. it has been supposed by some that, in a few instances, the same star has returned, constituting one of the periodical or variable stars of a long period. moreover, on a careful reexamination of the heavens, and a comparison of catalogues, many stars are now discovered to be missing. double stars are those which appear single to the naked eye, but are resolved into two by the telescope; or, if not visible to the naked eye, are seen in the telescope so close together as to be recognised as objects of this class. sometimes, three or more stars are found in this near connexion, constituting triple, or multiple stars. castor, for example, when seen by the naked eye, appears as a single star, but in a telescope even of moderate powers, it is resolved into two stars, of between the third and fourth magnitudes, within five seconds of each other. these two stars are nearly of equal size; but more commonly, one is exceedingly small in comparison with the other, resembling a satellite near its primary, although in distance, in light, and in other characteristics, each has all the attributes of a star, and the combination, therefore, cannot be that of a planet with a satellite. in most instances, also, the distance between these objects is much less than five seconds; and, in many cases, it is less than one second. the extreme closeness, together with the exceeding minuteness, of most of the double stars, requires the best telescopes united with the most acute powers of observation. indeed, certain of these objects are regarded as the severest _tests_ both of the excellence of the instruments and of the skill of the observer. the diagram on page , fig. , represents four double stars, as seen with appropriate magnifiers. no. , exhibits epsilon bootis with a power of three hundred and fifty; no. , rigel, with a power of one hundred and thirty; no. , the pole-star, with a power of one hundred; and no. , castor, with a power of three hundred. our knowledge of the double stars almost commenced with sir william herschel, about the year . at the time he began his search for them, he was acquainted with only _four_. within five years he discovered nearly _seven hundred_ double stars, and during his life, he observed no less than twenty-four hundred. in his memoirs, published in the philosophical transactions, he gave most accurate measurements of the distances between the two stars, and of the angle which a line joining the two formed with a circle parallel to the equator. these data would enable him, or at least posterity, to judge whether these minute bodies ever change their position with respect to each other. since , these researches have been prosecuted, with great zeal and industry, by sir james south and sir john herschel, in england; while professor struve, of dorpat, with the celebrated telescope of fraunhofer, has published, from his own observations, a catalogue of three thousand double stars, the determination of which involved the distinct and most minute inspection of at least one hundred and twenty thousand stars. sir john herschel, in his recent survey of the southern hemisphere, is said to have added to the catalogue of double stars nearly three thousand more. [illustration fig. .] two circumstances add a high degree of interest to the phenomena of double stars: the first is, that a few of them, at least, are found to have a revolution around each other; the second, that they are supposed to afford the means of ascertaining the parallax of the fixed stars. but i must defer these topics till my next letter. letter xxix. fixed stars continued. "o how canst thou renounce the boundless store of charms that nature to her votary yields? the warbling woodland, the resounding shore, the pomp of groves, and garniture of fields; all that the genial ray of morning yields, and all that echoes to the song of even, all that the mountain's sheltering bosom shields, and all the dread magnificence of heaven,-- o how canst thou renounce, and hope to be forgiven!"--_beattie._ in , sir william herschel first determined and announced to the world, that there exist among the stars separate systems, composed of two stars revolving about each other in regular orbits. these he denominated _binary stars_, to distinguish them from other double stars where no such motion is detected, and whose proximity to each other may possibly arise from casual juxtaposition, or from one being in the range of the other. between fifty and sixty instances of changes, to a greater or less amount, of the relative positions of double stars, are mentioned by sir william herschel; and a few of them had changed their places so much, within twenty-five years, and in such order, as to lead him to the conclusion that they performed revolutions, one around the other, in regular orbits. these conclusions have been fully confirmed by later observers; so that it is now considered as fully established, that there exist among the fixed stars binary systems, in which two stars perform to each other the office of sun and planet, and that the periods of revolution of more than one such pair have been ascertained with some degree of exactness. immersions and emersions of stars behind each other have been observed, and real motions among them detected, rapid enough to become sensible and measurable in very short intervals of time. the periods of the double stars are very various, ranging, in the case of those already ascertained, from forty-three years to one thousand. their orbits are very small ellipses, only a few seconds in the longest direction, and more eccentric than those of the planets. a double star in the northern crown (_eta coronæ_) has made a complete revolution since its first discovery, and is now far advanced in its second period; while a star in the lion (_gamma leonis_) requires twelve hundred years to complete its circuit. you may not at once see the reason why these revolutions of one member of a double star around the other, should be deemed facts of such extraordinary interest; to you they may appear rather in the light of astronomical curiosities. but remark, that the revolutions of the binary stars have assured us of this most interesting fact, that _the law of gravitation extends to the fixed stars_. before these discoveries, we could not decide, except by a feeble analogy, that this law transcended the bounds of the solar system. indeed, our belief of the fact rested more upon our idea of unity of design in the works of the creator, than upon any certain proof; but the revolution of one star around another, in obedience to forces which are proved to be similar to those which govern the solar system, establishes the grand conclusion, that the law of gravitation is truly the law of the material universe. "we have the same evidence," says sir john herschel, "of the revolutions of the binary stars about each other, that we have of those of saturn and uranus about the sun; and the correspondence between their calculated and observed places, in such elongated ellipses, must be admitted to carry with it a proof of the prevalence of the newtonian law of gravity in their systems, of the very same nature and cogency as that of the calculated and observed places of comets round the centre of our own system. but it is not with the revolution of bodies of a cometary or planetary nature round a solar centre, that we are now concerned; it is with that of sun around sun, each, perhaps, accompanied with its train of planets and their satellites, closely shrouded from our view by the splendor of their respective suns, and crowded into a space, bearing hardly a greater proportion to the enormous interval which separates them, than the distances of the satellites of our planets from their primaries bear to their distances from the sun itself." many of the double stars are of different colors; and sir john herschel is of the opinion that there exist in nature suns of different colors. "it may," says he, "be easier suggested in words than conceived in imagination, what variety of illumination two suns, a red and a green, or a yellow and a blue one, must afford to a planet circulating about either; and what charming contrasts and 'grateful vicissitudes' a red and a green day, for instance, alternating with a white one and with darkness, might arise from the presence or absence of one or other or both above the horizon. insulated stars of a red color, almost as deep as that of blood, occur in many parts of the heavens; but no green or blue star, of any decided hue, has ever been noticed unassociated with a companion brighter than itself." beside these revolutions of the binary stars, _some of the fixed stars appear to have a real motion in space_. there are several _apparent_ changes of place among the stars, arising from real changes in the earth, which, as we are not conscious of them, we refer to the stars; but there are other motions among the stars which cannot result from any changes in the earth, but must arise from changes in the stars themselves. such motions are called the _proper motions_ of the stars. nearly two thousand years ago, hipparchus and ptolemy made the most accurate determinations in their power of the relative situations of the stars, and their observations have been transmitted to us in ptolemy's 'almagest;' from which it appears that the stars retain at least _very nearly_ the same places now as they did at that period. still, the more accurate methods of modern astronomers have brought to light minute changes in the places of certain stars, which force upon us the conclusion, _either that our solar system causes an apparent displacement of certain stars, by a motion of its own in space, or that they have themselves a proper motion_. possibly, indeed, both these causes may operate. if the sun, and of course the earth which accompanies him, is actually in motion, the fact may become manifest from the apparent approach of the stars in the region which he is leaving, and the recession of those which lie in the part of the heavens towards which he is travelling. were two groves of trees situated on a plain at some distance apart, and we should go from one to the other, the trees before us would gradually appear further and further asunder, while those we left behind would appear to approach each other. some years since, sir william herschel supposed he had detected changes of this kind among two sets of stars in opposite points of the heavens, and announced that the solar system was in motion towards a point in the constellation hercules; but other astronomers have not found the changes in question such as would correspond to this motion, or to any motion of the sun; and, while it is a matter of general belief that the sun has a motion in space, the fact is not considered as yet entirely proved. in most cases, where a proper motion in certain stars has been suspected, its annual amount has been so small, that many years are required to assure us, that the effect is not owing to some other cause than a real progressive motion in the stars themselves; but in a few instances the fact is too obvious to admit of any doubt. thus, the two stars, cygni, which are nearly equal, have remained constantly at the same or nearly at the same distance of fifteen seconds, for at least fifty years past. mean-while, they have shifted their local situation in the heavens four minutes twenty-three seconds, the annual proper motion of each star being five seconds and three tenths, by which quantity this system is every year carried along in some unknown path, by a motion which for many centuries must be regarded as uniform and rectilinear. a greater proportion of the double stars than of any other indicate proper motions, especially the binary stars, or those which have a revolution around each other. among stars not double, and no way differing from the rest in any other obvious particular, a star in the constellation cassiopeia, (_mu cassiopeiæ_) has the greatest proper motion of any yet ascertained, amounting to nearly four seconds annually. you have doubtless heard much respecting the "immeasurable _distances_" of the fixed stars, and will desire to learn what is known to astronomers respecting this interesting subject. we cannot ascertain the actual distance of any of the fixed stars, but we can certainly determine that the nearest star is more than twenty millions of millions of miles from the earth, ( , , , , .) for all measurements relating to the distances of the _sun and planets_, the radius of the earth furnishes the base line. the length of this line being known, and the horizontal parallax of the sun or any planet, we have the means of calculating the distance of the body from us, by methods explained in a previous letter. but any star, viewed from the opposite sides of the earth, would appear from both stations to occupy precisely the same situation in the celestial sphere, and of course it would exhibit no horizontal parallax. but astronomers have endeavored to find a parallax in some of the fixed stars, by taking the _diameter of the earth's orbit_ as a base line. yet even a change of position amounting to one hundred and ninety millions of miles proved, until very recently, insufficient to alter the place of a single star, so far as to be capable of detection by very refined observations; from which it was concluded that the stars have not even any _annual parallax_; that is, the angle subtended by the semidiameter of the earth's orbit, at the nearest fixed star, is insensible. the errors to which instrumental measurements are subject, arising from the defects of instruments themselves, from refraction, and from various other sources of inaccuracy, are such, that the angular determinations of arcs of the heavens cannot be relied on to less than one second, and therefore cannot be appreciated by direct measurement. it follows, that, when viewed from the nearest star, the diameter of the earth's orbit would be insensible; the spider-line of the telescope would more than cover it. taking, however, the annual parallax of a fixed star at one second, it can be demonstrated, that the distance of the nearest fixed star _must exceed_ � = � , or one hundred thousand times one hundred and ninety millions of miles. of a distance so vast we can form no adequate conceptions, and even seek to measure it only by the time that light (which moves more than one hundred and ninety-two thousand miles per second, and passes from the sun to the earth in eight minutes and seven seconds) would take to traverse it, which is found to be more than three and a half years. if these conclusions are drawn with respect to the largest of the fixed stars, which we suppose to be vastly nearer to us than those of the smallest magnitude, the idea of distance swells upon us when we attempt to estimate the remoteness of the latter. as it is uncertain, however, whether the difference in the apparent magnitudes of the stars is owing to a real difference, or merely to their being at various distances from the eye, more or less uncertainty must attend all efforts to determine the relative distances of the stars; but astronomers generally believe, that the lower orders of stars are vastly more distant from us than the higher. of some stars it is said, that thousands of years would be required for their light to travel down to us. i have said that the stars have always been held, until recently, to have no annual parallax; yet it may be observed that astronomers were not exactly agreed on this point. dr. brinkley, a late eminent irish astronomer, supposed that he had detected an annual parallax in alpha lyræ, amounting to one second and thirteen hundreths, and in alpha aquilæ, of one second and forty-two hundreths. these results were controverted by mr. pond, of the royal observatory of greenwich; and mr. struve, of dorpat, has shown that, in a number of cases, the supposed parallax is in a direction opposite to that which would arise from the motion of the earth. hence it is considered doubtful whether, in all cases of an apparent parallax, the effect is not wholly due to errors of observation. but as if nothing was to be hidden from our times, the long sought for parallax among the fixed stars has at length been found, and consequently the distance of some of these bodies, at least, is no longer veiled in mystery. in the year , professor bessel, of köningsberg, announced the discovery of a parallax in one of the stars of the swan, ( _cygni_,) amounting to about _one third of a second_. this seems, indeed, so small an angle, that we might have reason to suspect the reality of the determination; but the most competent judges who have thoroughly examined the process by which the discovery was made, assent to its validity. what, then, do astronomers understand, when they say that a parallax has been discovered in one of the fixed stars, amounting to one third of a second? they mean that the star in question apparently shifts its place in the heavens, to that amount, when viewed at opposite extremities of the earth's orbit, namely, at points in space distant from each other one hundred and ninety millions of miles. on calculating the distance of the star from us from these data, it is found to be six hundred and fifty-seven thousand seven hundred times ninety-five millions of miles,--a distance which it would take light more than ten years to traverse. indirect methods have been proposed, for ascertaining the parallax of the fixed stars, by means of observations on the _double stars_. if the two stars composing a double star are at different distances from us, parallax would affect them unequally, and change their relative positions with respect to each other; and since the ordinary sources of error arising from the imperfection of instruments, from precession, and from refraction, would be avoided, (as they would affect both objects alike, and therefore would not disturb their relative positions,) measurements taken with the micrometer of changes much less than one second may be relied on. sir john herschel proposed a method, by which changes may be determined that amount to only one fortieth of a second. the immense distance of the fixed stars is inferred also from the fact, that the largest telescopes do not increase their apparent magnitude. they are still points, when viewed with glasses that magnify five thousand times. with respect to the nature of the stars, it would seem fruitless to inquire into the nature of bodies so distant, and which reveal themselves to us only as shining points in space. still, there are a few very satisfactory inferences that can be made out respecting them. first, _the fixed stars are bodies greater than our earth_. if this were not the case, they would not be visible at such an immense distance. dr. wollaston, a distinguished english philosopher, attempted to estimate the magnitudes of certain of the fixed stars from the light which they afford. by means of an accurate photometer, (an instrument for measuring the relative intensities of light,) he compared the light of sirius with that of the sun. he next inquired how far the sun must be removed from us, in order to appear no brighter than sirius. he found the distance to be one hundred and forty-one thousand times its present distance. but sirius is more than two hundred thousand times as far off as the sun; hence he inferred that, upon the lowest computation, it must actually give out twice as much light as the sun; or that, in point of splendor, sirius must be at least equal to two suns. indeed, he has rendered it probable, that its light is equal to that of fourteen suns. there is reason, however, to believe that the stars are actually of various magnitudes, and that their apparent difference is not owing merely to their different distances. bessel estimates the quantity of matter in the two members of a double star in the swan, as less than half that of the sun. secondly, _the fixed stars are suns_. we have already seen that they are large bodies; that they are immensely further off than the furthest planet; that they shine by their own light; in short, that their appearance is, in all respects, the same as the sun would exhibit if removed to the region of the stars. hence we infer that they are bodies of the same kind with the sun. we are justified, therefore, by a sound analogy, in concluding that the stars were made for the same end as the sun, namely, as the centres of attraction to other planetary worlds, to which they severally dispense light and heat. although the starry heavens present, in a clear night, a spectacle of unrivalled grandeur and beauty, yet it must be admitted that the chief purpose of the stars could not have been to adorn the night, since by far the greater part of them are invisible to the naked eye; nor as landmarks to the navigator, for only a very small proportion of them are adapted to this purpose; nor, finally, to influence the earth by their attractions, since their distance renders such an effect entirely insensible. if they are suns, and if they exert no important agencies upon our world, but are bodies evidently adapted to the same purpose as our sun, then it is as rational to suppose that they were made to give light and heat, as that the eye was made for seeing and the ear for hearing. it is obvious to inquire, next, to what they dispense these gifts, if not to planetary worlds; and why to planetary worlds, if not for the use of percipient beings? we are thus led, almost inevitably, to the idea of a _plurality of worlds_; and the conclusion is forced upon us, that the spot which the creator has assigned to us is but a humble province in his boundless empire. letter xxx. system of the world "o how unlike the complex works of man, heaven's easy, artless, unincumbered, plan."--_cowper._ having now explained to you, as far as i am able to do it in so short a space, the leading phenomena of the heavenly bodies, it only remains to inform you of the different systems of the world which have prevailed in different ages,--a subject which will necessarily involve a sketch of the history of astronomy. by a system of the world, i understand an explanation of _the arrangement of all the bodies that compose the material universe, and of their relations to each other_. it is otherwise called the 'mechanism of the heavens;' and indeed, in the system of the world, we figure to ourselves a machine, all parts of which have a mutual dependence, and conspire to one great end. "the machines that were first invented," says adam smith, "to perform any particular movement, are always the most complex; and succeeding artists generally discover that, with fewer wheels, and with fewer principles of motion, than had originally been employed, the same effects may be more easily produced. the first systems, in the same manner, are always the most complex; and a particular connecting chain or principle is generally thought necessary, to unite every two seemingly disjointed appearances; but it often happens, that _one great connecting principle_ is afterwards found to be sufficient to bind together all the discordant phenomena that occur in a whole species of things!" this remark is strikingly applicable to the origin and progress of systems of astronomy. it is a remarkable fact in the history of the human mind, that astronomy is the oldest of the sciences, having been cultivated, with no small success, long before any attention was paid to the causes of the common terrestrial phenomena. the opinion has always prevailed among those who were unenlightened by science, that very extraordinary appearances in the sky, as comets, fiery meteors, and eclipses, are omens of the wrath of heaven. they have, therefore, in all ages, been watched with the greatest attention: and their appearances have been minutely recorded by the historians of the times. the idea, moreover, that the aspects of the stars are connected with the destinies of individuals and of empires, has been remarkably prevalent from the earliest records of history down to a very late period, and, indeed, still lingers among the uneducated and credulous. this notion gave rise to astrology,--an art which professed to be able, by a knowledge of the varying aspects of the planets and stars, to penetrate the veil of futurity, and to foretel approaching irregularities of nature herself, and the fortunes of kingdoms and of individuals. that department of astrology which took cognizance of extraordinary occurrences in the natural world, as tempests, earthquakes, eclipses, and volcanoes, both to predict their approach and to interpret their meaning, was called _natural astrology_: that which related to the fortunes of men and of empires, _judicial astrology_. among many ancient nations, astrologers were held in the highest estimation, and were kept near the persons of monarchs; and the practice of the art constituted a lucrative profession throughout the middle ages. nor were the ignorant and uneducated portions of society alone the dupes of its pretensions. hippocrates, the 'father of medicine,' ranks astrology among the most important branches of knowledge to the physician; and tycho brahe, and lord bacon, were firm believers in its mysteries. astrology, fallacious as it was, must be acknowledged to have rendered the greatest services to astronomy, by leading to the accurate observation and diligent study of the stars. at a period of very remote antiquity, astronomy was cultivated in china, india, chaldea, and egypt. the chaldeans were particularly distinguished for the accuracy and extent of their astronomical observations. calisthenes, the greek philosopher who accompanied alexander the great in his eastern conquests, transmitted to aristotle a series of observations made at babylon nineteen centuries before the capture of that city by alexander; and the wise men of babylon and the chaldean astrologers are referred to in the sacred writings. they enjoyed a clear sky and a mild climate, and their pursuits as shepherds favored long-continued observations; while the admiration and respect accorded to the profession, rendered it an object of still higher ambition. in the seventh century before the christian era, astronomy began to be cultivated in greece; and there arose successively three celebrated astronomical schools,--the school of miletus, the school of crotona, and the school of alexandria. the first was established by thales, six hundred and forty years before christ; the second, by pythagoras, one hundred and forty years afterwards; and the third, by the ptolemies of egypt, about three hundred years before the christian era. as egypt and babylon were renowned among the most ancient nations, for their knowledge of the sciences, long before they were cultivated in greece, it was the practice of the greeks, when they aspired to the character of philosophers and sages, to resort to these countries to imbibe wisdom at its fountains. thales, after extensive travels in crete and egypt, returned to his native place, miletus, a town on the coast of asia minor, where he established the first school of astronomy in greece. although the minds of these ancient astronomers were beclouded with much error, yet thales taught a few truths which do honor to his sagacity. he held that the stars are formed of fire; that the moon receives her light from the sun, and is invisible at her conjunctions because she is hid in the sun's rays. he taught the sphericity of the earth, but adopted the common error of placing it in the centre of the world. he introduced the division of the sphere into five zones, and taught the obliquity of the ecliptic. he was acquainted with the saros, or sacred period of the chaldeans, (see page ,) and employed it in calculating eclipses. it was thales that predicted the famous eclipse of the sun which terminated the war between the lydians and the medes, as mentioned in a former letter. indeed, thales is universally regarded as a bright but solitary star, glimmering through mists on the distant horizon. to thales succeeded, in the school of miletus, two other astronomers of much celebrity, anaximander and anaxagoras. among many absurd things held by anaximander, he first taught the sublime doctrine that the planets are inhabited, and that the stars are suns of other systems. anaxagoras attempted to explain all the secrets of the skies by natural causes. his reasonings, indeed, were alloyed with many absurd notions; but still he alone, among the astronomers, maintained the existence of one god. his doctrines alarmed his countrymen, by their audacity and impiety to their gods, whose prerogatives he was thought to invade; and, to deprecate their wrath, sentence of death was pronounced on the philosopher and all his family,--a sentence which was commuted only for the sad alternative of perpetual banishment. the very genius of the heathen mythology was at war with the truth. false in itself, it trained the mind to the love of what was false in the interpretation of nature; it arrayed itself against the simplicity of truth, and persecuted and put to death its most ardent votaries. the religion of the bible, on the other hand, lends all its aid to truth in nature as well as in morals and religion. in its very genius it inculcates and inspires the love of truth; it suggests, by its analogies, the existence of established laws in the system of the world; and holds out the moon and the stars, which the creator has ordained, as fit objects to give us exalted views of his glory and wisdom. pythagoras was the founder of the celebrated school of crotona. he was a native of samos, an island in the �gean sea, and flourished about five hundred years before the christian era. after travelling more than thirty years in egypt and chaldea, and spending several years more at sparta, to learn the laws and institutions of lycurgus, he returned to his native island to dispense the riches he had acquired to his countrymen. but they, probably fearful of incurring the displeasure of the gods by the freedom with which he inquired into the secrets of the skies, gave him so unwelcome a reception, that he retired from them, in disgust, and established his school at crotona, on the southeastern coast of italy. hither, as to an oracle, the fame of his wisdom attracted hundreds of admiring pupils, whom he instructed in every species of knowledge. from the visionary notions which are generally understood to have been entertained on the subject of astronomy, by the ancients, we are apt to imagine that they knew less than they actually did of the truths of this science. but pythagoras was acquainted with many important facts in astronomy, and entertained many opinions respecting the system of the world, which are now held to be true. among other things well known to pythagoras, either derived from his own investigations, or received from his predecessors, were the following; and we may note them as a synopsis of the state of astronomical knowledge at that age of the world. first, the principal _constellations_. these had begun to be formed in the earliest ages of the world. several of them, bearing the same name as at present, are mentioned in the writings of hesiod and homer; and the "sweet influences of the pleiades," and the "bands of orion," are beautifully alluded to in the book of job. secondly, _eclipses_. pythagoras knew both the causes of eclipses and how to predict them; not, indeed, in the accurate manner now practised, but by means of the saros. thirdly, pythagoras had divined the true _system of the world_, holding that the sun, and not the earth, (as was generally held by the ancients, even for many ages after pythagoras,) is the centre around which all the planets revolve; and that the stars are so many suns, each the centre of a system like our own. among lesser things, he knew that the earth is round; that its surface is naturally divided into five zones; and that the ecliptic is inclined to the equator. he also held that the earth revolves daily on its axis, and yearly around the sun; that the galaxy is an assemblage of small stars; and that it is the same luminary, namely, venus, that constitutes both the morning and evening star; whereas all the ancients before him had supposed that each was a separate planet, and accordingly the morning star was called lucifer, and the evening star, hesperus. he held, also, that the planets were inhabited, and even went so far as to calculate the size of some of the animals in the moon. pythagoras was also so great an enthusiast in music, that he not only assigned to it a conspicuous place in his system of education, but even supposed that the heavenly bodies themselves were arranged at distances corresponding to the intervals of the diatonic scale, and imagined them to pursue their sublime march to notes created by their own harmonious movements, called the 'music of the spheres;' but he maintained that this celestial concert, though loud and grand, is not audible to the feeble organs of man, but only to the gods. with few exceptions, however, the opinions of pythagoras on the system of the world were founded in truth. yet they were rejected by aristotle, and by most succeeding astronomers, down to the time of copernicus; and in their place was substituted the doctrine of _crystalline spheres_, first taught by eudoxus, who lived about three hundred and seventy years before christ. according to this system, the heavenly bodies are set like gems in hollow solid orbs, composed of crystal so transparent, that no anterior orb obstructs in the least the view of any of the orbs that lie behind it. the sun and the planets have each its separate orb; but the fixed stars are all set in the same grand orb; and beyond this is another still, the _primum mobile_, which revolves daily, from east to west, and carries along with it all the other orbs. above the whole spreads the _grand empyrean_, or third heavens, the abode of perpetual serenity. to account for the planetary motions, it was supposed that each of the planetary orbs, as well as that of the sun, has a motion of its own, eastward, while it partakes of the common diurnal motion of the starry sphere. aristotle taught that these motions are effected by a tutelary genius of each planet, residing in it, and directing its motions, as the mind of man directs his movements. two hundred years after pythagoras, arose the famous school of alexandria, under the ptolemies. these were a succession of egyptian kings, and are not to be confounded with ptolemy, the astronomer. by the munificent patronage of this enlightened family, for the space of three hundred years, beginning at the death of alexander the great, from whom the eldest of the ptolemies had received his kingdom, the school of alexandria concentrated in its vast library and princely halls, erected for the accommodation of the philosophers, nearly all the science and learning of the world. in wandering over the immense territories of ignorance and barbarism which covered, at that time, almost the entire face of the earth, the eye reposes upon this little spot, as upon a verdant island in the midst of the desert. among the choice fruits that grew in this garden of astronomy were several of the most distinguished ornaments of ancient science, of whom the most eminent were hipparchus and ptolemy. hipparchus is justly considered as the newton of antiquity. he sought his knowledge of the heavenly bodies not in the illusory suggestions of a fervid imagination, but in the vigorous application of an intellect of the first order. previous to this period, celestial observations were made chiefly with the naked eye: but hipparchus was in possession of instruments for measuring angles, and knew how to resolve spherical triangles. these were great steps beyond all his predecessors. he ascertained the length of the year within six minutes of the truth. he discovered the eccentricity, or elliptical figure, of the solar orbit, although he supposed the sun actually to move uniformly in a circle, but the earth to be placed out of the centre. he also determined the positions of the points among the stars where the earth is nearest to the sun, and where it is most remote from it. he formed very accurate estimates of the obliquity of the ecliptic and of the precession of the equinoxes. he computed the exact period of the synodic revolution of the moon, and the inclination of the lunar orbit; discovered the backward motion of her node and of her line of apsides; and made the first attempts to ascertain the horizontal parallaxes of the sun and moon. upon the appearance of a new star in the firmament, he undertook, as already mentioned, to number the stars, and to assign to each its true place in the heavens, in order that posterity might have the means of judging what changes, if any, were going forward among these apparently unalterable bodies. although hipparchus is generally considered as belonging to the alexandrian school, yet he lived at rhodes, and there made his astronomical observations, about one hundred and forty years before the christian era. one of his treatises has come down to us; but his principal discoveries have been transmitted through the 'almagest' of ptolemy. ptolemy flourished at alexandria nearly three centuries after hipparchus, in the second century after christ. his great work, the 'almagest,' which has conveyed to us most that we know respecting the astronomical knowledge of the ancients, was the universal text-book of astronomers for fourteen centuries. [illustration fig. .] the name of this celebrated astronomer has also descended to us, associated with the system of the world which prevailed from ptolemy to copernicus, called the _ptolemaic system_. the doctrines of the ptolemaic system did not originate with ptolemy, but, being digested by him out of materials furnished by various hands, it has come down to us under the sanction of his name. according to this system, the earth is the centre of the universe, and all the heavenly bodies daily revolve around it, from east to west. but although this hypothesis would account for the apparent diurnal motion of the firmament, yet it would not account for the apparent annual motion of the sun, nor for the slow motions of the planets from west to east. in order to explain these phenomena, recourse was had to _deferents_ and _epicycles_,--an explanation devised by apollonius, one of the greatest geometers of antiquity. he conceived that, in the circumference of a circle, having the earth for its centre, there moves the centre of a smaller circle in the circumference of which the planet revolves. the circle surrounding the earth was called the deferent, while the smaller circle, whose centre was always in the circumference of the deferent, was called the epicycle. thus, if e, fig. , represents the earth, abc will be the deferent, and dfg, the epicycle; and it is obvious that the motion of a body from west to east, in this small circle, would be alternately direct, stationary, and retrograde, as was explained, in a previous letter, to be actually the case with the apparent motions of the planets. the hypothesis, however, is inconsistent with the _phases_ of mercury and venus, which, being between us and the sun, on both sides of the epicycle, would present their dark sides towards us at both conjunctions with the sun, whereas, at one of the conjunctions, it is known that they exhibit their disks illuminated. it is, moreover, absurd to speak of a geometrical centre, which has no bodily existence, moving round the earth on the circumference of another circle. in addition to these absurdities, the whole ptolemaic system is encumbered with the following difficulties: first, it is a mere hypothesis, having no evidence in its favor except that it explains the phenomena. this evidence is insufficient of itself, since it frequently happens that each of two hypotheses, which are directly opposite to each other, will explain all the known phenomena. but the ptolemaic system does not even do this, as it is inconsistent with the phases of mercury and venus, as already observed. secondly, now that we are acquainted with the distances of the remoter planets, and especially the fixed stars, the swiftness of motion, implied in a daily revolution of the starry firmament around the earth, renders such a motion wholly incredible. thirdly, the centrifugal force which would be generated in these bodies, especially in the sun, renders it impossible that they can continue to revolve around the earth as a centre. absurd, however, as the system of ptolemy was, for many centuries no great philosophic genius appeared to expose its fallacies, and it therefore guided the faith of astronomers of all countries down to the time of copernicus. after the age of ptolemy, the science made little progress. with the decline of grecian liberty, the arts and sciences declined also; and the romans, then masters of the world, were ever more ambitious to gain conquests over man than over matter; and they accordingly never produced a single great astronomer. during the middle ages, the arabians were almost the only astronomers, and they cultivated this noble study chiefly as subsidiary to astrology. at length, in the fifteenth century, copernicus arose, and after forty years of intense study and meditation, divined the true system of the world. you will recollect that the copernican system maintains, . that the _apparent_ diurnal motions of the heavenly bodies, from east to west, is owing to the _real_ revolution of the earth on its own axis from west to east; and, . that the sun is the centre around which the earth and planets all revolve from west to east. it rests on the following arguments: in the first place, _the earth revolves on its own axis_. first, because this supposition is vastly more _simple_. secondly, it is agreeable to _analogy_, since all the other planets that afford any means of determining the question, are seen to revolve on their axes. thirdly, the _spheroidal figure_ of the earth is the figure of equilibrium, that results from a revolution on its axis. fourthly, the _diminished weight_ of bodies at the equator indicates a centrifugal force arising from such a revolution. fifthly, bodies let fall from a high eminence, fall _eastward of their base_, indicating that when further from the centre of the earth they were subject to a greater velocity, which, in consequence of their inertia, they do not entirely lose in descending to the lower level. in the second place, _the planets, including the earth, revolve about the sun_. first, the _phases_ of mercury and venus are precisely such, as would result from their circulating around the sun in orbits within that of the earth; but they are never seen in opposition, as they would be, if they circulate around the earth. secondly, the superior planets do indeed revolve around the earth; but they also revolve around the sun, as is evident from their phases, and from the known dimensions of their orbits; and that the sun, and not the earth, is the _centre_ of their motions, is inferred from the greater symmetry of their motions, as referred to the sun, than as referred to the earth; and especially from the laws of gravitation, which forbid our supposing that bodies so much larger than the earth, as some of these bodies are, can circulate permanently around the earth, the latter remaining all the while at rest. in the third place, the annual motion of _the earth_ itself is indicated also by the most conclusive arguments. for, first, since all the planets, with their satellites and the comets, revolve about the sun, analogy leads us to infer the same respecting the earth and its satellite, as those of jupiter and saturn, and indicates that it is a law of the solar system that the smaller bodies revolve about the larger. secondly, on the supposition that the earth performs an annual revolution around the sun, it is embraced along with the planets, in kepler's law, that the squares of the times are as the cubes of the distances; otherwise, it forms an exception, and the only known exception, to this law. such are the leading arguments upon which rests the copernican system of astronomy. they were, however, only very partially known to copernicus himself, as the state both of mechanical science, and of astronomical observation, was not then sufficiently matured to show him the strength of his own doctrine, since he knew nothing of the telescope, and nothing of the principle of universal gravitation. the evidence of this beautiful system being left by copernicus in so imperfect a state, and indeed his own reasonings in support of it being tinctured with some errors, we need not so much wonder that tycho brahe, who immediately followed copernicus, did not give it his assent, but, influenced by certain passages of scripture, he still maintained, with ptolemy, that the earth is in the centre of the universe; and he accounted for the diurnal motions in the same manner as ptolemy had done, namely, by an actual revolution of the whole host of heaven around the earth every twenty-four hours. but he rejected the scheme of deferents and epicycles, and held that the moon revolves about the earth as the centre of her motions; but that the sun and not the earth is the centre of the planetary motions; and that the sun, accompanied by the planets, moves around the earth once a year, somewhat in the manner in which we now conceive of jupiter and his satellites as revolving around the sun. this system is liable to most of the objections that lie against the ptolemaic system, with the disadvantage of being more complex. kepler and galileo, however, as appeared in the sketch of their lives, embraced the theory of copernicus with great avidity, and all their labors contributed to swell the evidence of its truth. when we see with what immense labor and difficulty the disciples of ptolemy sought to reconcile every new phenomenon of the heavens with their system, and then see how easily and naturally all the successive discoveries of galileo and kepler fall in with the theory of copernicus, we feel the full force of those beautiful lines of cowper which i have chosen for the motto of this letter. newton received the torch of truth from galileo, and transmitted it to his successors, with its light enlarged and purified; and since that period, every new discovery, whether the fruit of refined instrumental observation or of profound mathematical analysis, has only added lustre to the glory of copernicus. with newton commenced a new and wonderful era in astronomy, distinguished above all others, not merely for the production of the greatest of men, but also for the establishment of those most important auxiliaries to our science, the royal society of london, the academy of sciences at paris, and the observatory of greenwich. i may add the commencement of the transactions of the royal society, and the memoirs of the academy of sciences, which have been continued to the present time,--both precious storehouses of astronomical riches. the observatory of greenwich, moreover, has been under the direction of an extraordinary succession of great astronomers. their names are flamstead, halley, bradley, maskeleyne, pond, and airy,--the last being still at his post, and worthy of continuing a line so truly illustrious. the observations accumulated at this celebrated observatory are so numerous, and so much superior to those of any other institution in the world, that it has been said that astronomy would suffer little, if all other contemporary observations of the same kind were annihilated. sir william herschel, however, labored chiefly in a different sphere. the astronomers royal devoted themselves not so much to the discovery of new objects among the heavenly bodies, as to the exact determination of the places of the bodies already known, and to the developement of new laws or facts among the celestial motions. but herschel, having constructed telescopes of far greater reach than any ever used before, employed them to sound new and untried depths in the profundities of space. we have already seen what interesting and amazing discoveries he made of double stars, clusters, and nebulæ. the english have done most for astronomy in observation and discovery; but the french and germans, in developing, by the most profound mathematical investigation, the great laws of physical astronomy. it only remains to inquire, whether the copernican system is now to be regarded as a full exposition of the 'mechanism of the heavens,' or whether there subsist higher orders of relations between the fixed stars themselves. the revolutions of the _binary stars_ afford conclusive evidence of at least subordinate systems of suns, governed by the same laws as those which regulate the motions of the solar system. the _nebulæ_ also compose peculiar systems, in which the members are evidently bound together by some common relation. in these marks of organization,--of stars associated together in clusters; of sun revolving around sun; and of nebulæ disposed in regular figures,--we recognise different members of some grand system, links in one great chain that binds together all parts of the universe; as we see jupiter and his satellites combined in one subordinate system, and saturn and his satellites in another,--each a vast kingdom, and both uniting with a number of other individual parts, to compose an empire still more vast. this fact being now established, that the stars are immense bodies, like the sun, and that they are subject to the laws of gravitation, we cannot conceive how they can be preserved from falling into final disorder and ruin, unless they move in harmonious concert, like the members of the solar system. otherwise, those that are situated on the confines of creation, being retained by no forces from without, while they are subject to the attraction of all the bodies within, must leave their stations, and move inward with accelerated velocity; and thus all the bodies in the universe would at length fall together in the common centre of gravity. the immense distance at which the stars are placed from each other would indeed delay such a catastrophe; but this must be the ultimate tendency of the material world, unless sustained in one harmonious system by nicely-adjusted motions. to leave entirely out of view our confidence in the wisdom and preserving goodness of the creator, and reasoning merely from what we know of the stability of the solar system, we should be justified in inferring, that other worlds are not subject to forces which operate only to hasten their decay, and to involve them in final ruin. we conclude, therefore, that the material universe is one great system; that the combination of planets with their satellites constitutes the first or lowest order of worlds; that next to these, planets are linked to suns; that these are bound to other suns, composing a still higher order in the scale of being; and finally, that all the different systems of worlds move around their common centre of gravity. letter xxxi. natural theology. ----"philosophy, baptized in the pure fountain of eternal love, has eyes indeed; and, viewing all she sees as meant to indicate a god to man, gives him the praise, and forfeits not her own."--_cowper._ i intended, my dear friend, to comply with your request "that i would discuss the arguments which astronomy affords to natural theology;" but these letters have been already extended so much further than i anticipated, that i shall conclude with suggesting a few of those moral and religious reflections, which ought always to follow in the train of such a survey of the heavenly bodies as we have now taken. although there is evidence enough in the structure, arrangement, and laws, which prevail among the heavenly bodies, to prove the _existence_ of god, yet i think there are many subordinate parts of his works far better adapted to this purpose than these, being more fully within our comprehension. it was intended, no doubt, that the evidence of his being should be accessible to all his creatures, and should not depend on a kind of knowledge possessed by comparatively few. the mechanism of the eye is probably not more perfect than that of the universe; but we can analyze it better, and more fully understand the design of each part. but the existence of god being once proved, and it being admitted that he is the creator and governor of the world, then the discoveries of astronomy are admirably adapted to perform just that office in relation to the great first cause, which is assigned to them in the bible, namely, "to declare the glory of god, and to show his handiwork." in other words, the discoveries of astronomy are peculiarly fitted,--more so, perhaps, than any other department of creation,--to exhibit the unity, power, and wisdom, of the creator. the most modern discoveries have multiplied the proofs of the _unity_ of god. it has usually been offered as sufficient evidence of the truth of this doctrine, that the laws of nature are found to be uniform when applied to the utmost bounds of the _solar system_; that the law of gravitation controls alike the motions of mercury, and those of uranus; and that its operation is one and the same upon the moon and upon the satellites of saturn. it was, however, impossible, until recently, to predicate the same uniformity in the great laws of the universe respecting the starry worlds, except by a feeble analogy. however improbable, it was still possible, that in these distant worlds other laws might prevail, and other lords exercise dominion. but the discovery of the revolutions of the binary stars, in exact accordance with the law of gravitation, not merely in a single instance, but in many instances, in all cases, indeed, wherever those revolutions have advanced so far as to determine their law of action, gives us demonstration, instead of analogy, of the prevalence of the same law among the other systems as that which rules in ours. the marks of a still higher organization in the structure of clusters and nebulæ, all bearing that same characteristic union of resemblance and variety which belongs to all the other works of creation that fall under our notice, speak loudly of one, and only one, grand design. every new discovery of the telescope, therefore, has added new proofs to the great truth that god is one: nor, so far as i know, has a single fact appeared, that is not entirely consonant with it. light, moreover, which brings us intelligence, and, in most cases, the only intelligence we have, of these remote orbs, testifies to the same truth, being similar in its properties and uniform in its motions, from whatever star it emanates. in displays of the _power_ of jehovah, nothing can compare with the starry heavens. the magnitudes, distances, and velocities, of the heavenly bodies are so much beyond every thing of this kind which belongs to things around us, from which we borrowed our first ideas of these qualities, that we can scarcely avoid looking with incredulity at the numerical results to which the unerring principles of mathematics have conducted us. and when we attempt to apply our measures to the fixed stars, and especially to the nebulæ, the result is absolutely overwhelming: the mind refuses its aid in our attempts to grasp the great ideas. nor less conspicuous, among the phenomena of the heavenly bodies, is the _wisdom_ of the creator. in the first place, this attribute is every where exhibited _in the happy adaptation of means to their ends_. no principle can be imagined more simple, and at the same time more effectual to answer the purposes which it serves, than gravitation. no position can be given to the sun and planets so fitted, as far as we can judge, to fulfil their mutual relations, as that which the creator has given them. i say, as far as we can judge; for we find this to be the case in respect to our own planet and its attendant satellite, and hence have reason to infer that the same is the case in the other planets, evidently holding, as they do, a similar relation to the sun. thus the position of the earth at just such a distance from the sun as suits the nature of its animal and vegetable kingdoms, and confining the range of solar heat, vast as it might easily become, within such narrow bounds; the inclination of the earth's axis to the plane of its orbit, so as to produce the agreeable vicissitudes of the seasons, and increase the varieties of animal and vegetable life, still confining the degree of inclination so exactly within the bounds of safety, that, were it much to transcend its present limits, the changes of temperature of the different seasons would be too sudden and violent for the existence of either animals or vegetables; the revolution of the earth on its axis, so happily dividing time into hours of business and of repose; the adaptation of the moon to the earth, so as to afford to us her greatest amount of light just at the times when it is needed most, and giving to the moon just such a quantity of matter, and placing her at just such a distance from the earth, as serves to raise a tide productive of every conceivable advantage, without the evils which would result from a stagnation of the waters on the one hand, or from their overflow on the other;--these are a few examples of the wisdom displayed in the mutual relations instituted between the sun, the earth, and the moon. in the second place, similar marks of wisdom are exhibited in _the many useful and important purposes_ _which the same thing is made to serve_. thus the sun is at once the great regulator of the planetary motions, and the fountain of light and heat. the moon both gives light by night and raises the tides. or, if we would follow out this principle where its operations are more within our comprehension, we may instance the _atmosphere_. when man constructs an instrument, he deems it sufficient if it fulfils one single purpose as the watch, to tell the hour of the day, or the telescope, to enable him to see distant objects; and had a being like ourselves made the atmosphere, he would have thought it enough to have created a medium so essential to animal life, that to live is to breathe, and to cease to breathe is to die. but beside this, the atmosphere has manifold uses, each entirely distinct from all the others. it conveys to plants, as well as animals, their nourishment and life; it tempers the heat of summer with its breezes; it binds down all fluids, and prevents their passing into the state of vapor; it supports the clouds, distils the dew, and waters the earth with showers; it multiplies the light of the sun, and diffuses it over earth and sky; it feeds our fires, turns our machines, wafts our ships, and conveys to the ear all the sentiments of language, and all the melodies of music. in the third place, the wisdom of the creator is strikingly manifested in the provision he has made for the _stability of the universe_. the perturbations occasioned by the motions of the planets, from their action on each other, are very numerous, since every body in the system exerts an attraction on every other, in conformity with the law of universal gravitation. venus and mercury, approaching, as they do at times, comparatively near to the earth, sensibly disturb its motions; and the satellites of the remoter planets greatly disturb each other's movements. nor was it possible to endow this principle with the properties it has, and make it operate as it does in regulating the motions of the world, without involving such an incident. on this subject, professor whewell, in his excellent work composing one of the bridgewater treatises, remarks: "the derangement which the planets produce in the motion of one of their number will be very small, in the course of one revolution; but this gives us no security that the derangement may not become very large, in the course of many revolutions. the cause acts perpetually, and it has the whole extent of time to work in. is it not easily conceivable, then, that, in the lapse of ages, the derangements of the motions of the planets may accumulate, the orbits may change their form, and their mutual distances may be much increased or diminished? is it not possible that these changes may go on without limit, and end in the complete subversion and ruin of the system? if, for instance, the result of this mutual gravitation should be to increase considerably the eccentricity of the earth's orbit, or to make the moon approach continually nearer and nearer to the earth, at every revolution, it is easy to see that, in the one case, our year would change its character, producing a far greater irregularity in the distribution of the solar heat; in the other, our satellite must fall to the earth, occasioning a dreadful catastrophe. if the positions of the planetary orbits, with respect to that of the earth, were to change much, the planets might sometimes come very near us, and thus increase the effect of their attraction beyond calculable limits. under such circumstances, 'we might have years of unequal length, and seasons of capricious temperature; planets and moons, of portentous size and aspect, glaring and disappearing at uncertain intervals; tides, like deluges, sweeping over whole continents; and perhaps the collision of two of the planets, and the consequent destruction of all organization on both of them.' the fact really is, that changes are taking place in the motions of the heavenly bodies, which have gone on progressively, from the first dawn of science. the eccentricity of the earth's orbit has been diminishing from the earliest observations to our times. the moon has been moving quicker from the time of the first recorded eclipses, and is now in advance, by about four times her own breadth, of what her own place would have been, if it had not been affected by this acceleration. the obliquity of the ecliptic, also, is in a state of diminution, and is now about two fifths of a degree less than it was in the time of aristotle." but amid so many seeming causes of irregularity and ruin, it is worthy of a grateful notice, that effectual provision is made for the _stability of the solar system_. the full confirmation of this fact is among the grand results of physical astronomy. "newton did not undertake to demonstrate either the stability or instability of the system. the decision of this point required a great number of preparatory steps and simplifications, and such progress in the invention and improvement of mathematical methods, as occupied the best mathematicians of europe for the greater part of the last century. towards the end of that time, it was shown by la grange and la place, that the arrangements of the solar system are stable; that, in the long run, the orbits and motions remain unchanged; and that the changes in the orbits, which take place in shorter periods, never transgress certain very moderate limits. each orbit undergoes deviations on this side and on that side of its average state; but these deviations are never very great, and it finally recovers from them, so that the average is preserved. the planets produce perpetual perturbations in each other's motions; but these perturbations are not indefinitely progressive, but periodical, reaching a maximum value, and then diminishing. the periods which this restoration requires are, for the most part, enormous,--not less than thousands, and in some instances, millions, of years. indeed, some of these apparent derangements have been going on in the same direction from the creation of the world. but the restoration is in the sequel as complete as the derangement; and in the mean time the disturbance never attains a sufficient amount seriously to affect the stability of the system. 'i have succeeded in demonstrating,' says la place, 'that, whatever be the masses of the planets, in consequence of the fact that they all move in the same direction, in orbits of small eccentricity, and but slightly inclined to each other, their secular irregularities are periodical, and included within narrow limits; so that the planetary system will only oscillate about a mean state, and will never deviate from it, except by a very small quantity. the ellipses of the planets have been and always will be nearly circular. the ecliptic will never coincide with the equator; and the entire extent of the variation, in its inclination, cannot exceed three degrees.'" to these observations of la place, professor whewell adds the following, on the importance, to the stability of the solar system, of the fact that those planets which have _great masses_ have orbits of _small eccentricity_. "the planets mercury and mars, which have much the largest eccentricity among the old planets, are those of which the masses are much the smallest. the mass of jupiter is more than two thousand times that of either of these planets. if the orbit of jupiter were as eccentric as that of mercury, all the security for the stability of the system, which analysis has yet pointed out, would disappear. the earth and the smaller planets might, by the near approach of jupiter at his perihelion, change their nearly circular orbits into very long ellipses, and thus might fall into the sun, or fly off into remoter space. it is further remarkable, that in the newly-discovered planets, of which the orbits are still more eccentric than that of mercury, the masses are still smaller, so that the same provision is established in this case, also." with this hasty glance at the unity, power, and wisdom, of the creator, as manifested in the greatest of his works, i close. i hope enough has been said to vindicate the sentiment that called 'devotion, daughter of astronomy!' i do not pretend that this, or any other science, is adequate of itself to purify the heart, or to raise it to its maker; but i fully believe that, when the heart is already under the power of religion, there is something in the frequent and habitual contemplation of the heavenly bodies under all the lights of modern astronomy, very favorable to devotional feelings, inspiring, as it does, humility, in unison with an exalted sentiment of grateful adoration. letter xxxii. recent discoveries. "all are but parts of one stupendous whole."--_pope._ within a few years, astronomy has been enriched with a number of valuable discoveries, of which i will endeavor to give you a summary account in this letter. the heavens have been explored with far more powerful telescopes than before; instrumental measurements have been carried to an astonishing degree of accuracy; numerous additions have been made to the list of small planets or asteroids; a comet has appeared of extraordinary splendor, remarkable, above all others, for its near approach to the sun; the distances of several of the fixed stars, an element long sought for in vain, have been determined; a large planet, composing in itself a magnificent world, has been added to the solar system, at such a distance from the central luminary as nearly to double the supposed dimensions of that system; various nebulæ, before held to be irresolvable, have been resolved into stars; and a new satellite has been added to saturn. improvements in the telescope.--herschel's forty-feet telescope, of which i gave an account in my fourth letter (see page ), remained for half a century unequalled in magnitude and power; but in , lord rosse, an irish nobleman, commenced a telescope on a scale still more gigantic. like herschel's, it was a _reflector_, the image being formed by a concave mirror. this was six feet in diameter, and weighed three tons; and the tube was fifty feet in length. the entire cost of the instrument was sixty thousand dollars. its reflecting surface is nearly twice as great as the great herschelian, and consequently it greatly exceeds all instruments hitherto constructed in the _amount of light_ which it collects and transmits to the eye; and this adapts it peculiarly to viewing those objects, as nebulæ, whose light is exceedingly faint. accordingly, it has revealed to us new wonders in this curious department of astronomy. some idea of the great dimensions of the _leviathan_ telescope (as this instrument has been called) may be formed when it is said that the dean of ely, a full-sized man, walked through the tube from one end to the other, with an umbrella over his head. but still greater advances have been made in refracting than in reflecting telescopes. such was the difficulty of obtaining large pieces of glass which are free from impurities, and such the liability of large lenses to form obscure and colored images, that it was formerly supposed impossible to make a refracting telescope larger in diameter than five or six inches; but their size has been increased from one step to another, until they are now made more than fifteen inches in diameter; and so completely have all the difficulties arising from the imperfections of glass, and from optical defects inherent in lenses, been surmounted, that the great telescopes of pulkova, at st. petersburgh, and of harvard university (the two finest refractors in the world) are considered among the most perfect productions of the arts. a lens of only inches in diameter seems, indeed, diminutive when compared with a concave reflector of six feet; but for most purposes of the astronomer, the pulkova and cambridge instruments are more useful than such great reflectors as those of herschel and rosse. if there is any particular in which these are more effective, it is in observations on the faintest nebulæ, where it is necessary to collect and convey to the eye the greatest possible beam of light. instrumental measurements.--when astronomical instruments were first employed to measure the angular distance between two points on the celestial sphere, it was not attempted to measure spaces smaller than ten minutes--a space equal to the third part of the breadth of the full moon. tycho brahe, however, carried his measures to sixty times that degree of minuteness, having devised means of determining angles no larger than ten seconds, or the one hundred and eightieth part of the breadth of the lunar disk. for many years past, astronomers have carried these measures to single seconds, or have determined spaces no greater than the eighteen hundredth part of the diameter of the moon. this is considered the smallest arc which can be accurately measured directly on the limb of an instrument; but _differences_ between spaces may be estimated to a far greater degree of accuracy than this, even to the hundredth part of a second--a space less than that intercepted by a spider's web held before the eye. discovery of new planets.--in my twenty-third letter (see page ), i gave an account of the small planets called asteroids, which lie between the orbits of mars and jupiter. when that letter was written, no longer ago than , only four of those bodies had been discovered, namely, ceres, pallas, juno, and vesta. within a few years past, nineteen more have been added, making the number of the asteroids known at present twenty-three, and every year adds one or more to the list.[ ] the idea first suggested by olbers, one of the earliest discoverers of asteroids, that they are fragments of a large single planet once revolving between mars and jupiter, has gained credit since the discovery of so many additional bodies of the same class, all, like the former, exceedingly small and irregular in their motions, although there are still great difficulties in tracing them to a common origin. great comet of .--this is the most wonderful body that has appeared in the heavens in modern times; first, on account of its appearing, when first seen, in the broad light of noonday; and, secondly, on account of its approaching so near the sun as almost to graze his surface. it was first discovered, in new england, on the th of february, a little eastward of the sun, shining like a white cloud illuminated by the solar rays. it arrested the attention of many individuals from half past seven in the morning until three o'clock in the afternoon, when the sky became obscured by clouds. in mexico, it was observed from nine in the morning until sunset. at a single station in south america, it was said to have been seen on the th of february, almost in contact with the sun. early in march, it had receded so far to the eastward of that body as to be visible in the southwest after sunset, throwing upward a long train, which increased in length from night to night until it covered a space of degrees. its position may be seen on a celestial globe adjusted to the latitude of new haven ( ° ´) for the th of march, by tracing a line, or, rather, a broad band proceeding from the place of the sun towards the bright star sirius, in the south, between the ears of the hare and the feet of orion. the comet passed its perihelion on the th of february, at which time it almost came in contact with the sun. to prevent its falling into the sun it was endued with a prodigious velocity; a velocity so great that, had it continued at the same rate as at the instant of perihelion passage, it would have whirled round the sun in two hours and a half. it did, in fact, complete more than half its revolution around the sun in that short period, and it made more than three quarters of its circuit around the sun in one day. its velocity, when nearest the sun, exceeded a million of miles per hour, and its tail, at its greatest elongation, was one hundred and eight millions of miles; a length more than sufficient to have reached from the sun to the earth. its heat was estimated to be , times greater than that received by the earth from a vertical sun, and consequently it was more intense than that produced by the most powerful blowpipes, and sufficient to melt like wax the most infusible bodies. no doubt, when in the vicinity of the sun, the solid matter of the comet was first melted and then converted into vapor, which itself became red hot, or, more properly speaking, _white hot_. much discussion has arisen among astronomers respecting the periodic time of this comet. its most probable period is about years. distances of the stars.--i have already mentioned (page ) that the distance of at least one of the fixed stars has at length been determined, although at so great a distance that its annual parallax is only about one third of a second, implying a distance from the sun of nearly sixty millions of millions of miles. of a distance so immense the mind can form no adequate conception. the most successful effort towards it is made by gradual and successive approximations. let us, therefore, take the motion of a rail-way car as the most rapid with which we are familiar, and apply it first to the planetary spaces, and then to the vast interval that separates these nether worlds from the fixed stars. a rail-way car, travelling constantly night and day at the rate of twenty miles per hour, would make miles per day. at this rate, to travel around the earth on a great circle would require about days, and days to reach the moon. if we took our departure from the sun, and journeyed night and day, we should reach mercury in a little more than years, venus in nearly , and the earth in years; but to reach neptune, the outermost planet, would require , years. great as appear the dimensions of the solar system, when we imagine ourselves thus borne along from world to world, yet this space is small compared with that which separates us from the fixed stars; for to reach cygni it would take , , years. but this is believed, for certain satisfactory reasons, to be one of the nearest of the stars. several other stars whose parallax has been determined are at a much greater distance than cygni. the pole star is five times as far off; and the greater part of the stars are at distances inconceivably more remote. such, especially, are those which compose the faintest nebulæ. discovery of the planet neptune.--from the earliest ages down to the year , the solar system was supposed to terminate with the planet saturn, at the distance of nine hundred millions of miles from the sun; but the discovery of uranus added another world, and doubled the dimensions of the solar system. it seemed improbable that any more planets should exist at a distance still more remote, since such a body could hardly receive any of the vivifying influences of the central luminary. still, certain irregularities to which the uranus was subject, led to the suspicion that there exists a planet beyond it, which, by its attractions, caused these irregularities. impressed with this belief, two young astronomers of great genius, le verrier, of france, and adams, of england, applied themselves to the task of finding the hidden planet. the direction in which the disturbed body was moved afforded some clue to the part of the heavens where the disturbing body lay concealed; the kind of action it excited at different times indicated that it was beyond uranus, and not this side of that planet; and the magnitude of the forces it exerted gave some intimation of its size and mass. the law of distances from the sun which the superior planets observe (saturn being nearly twice the distance of jupiter, and uranus twice that of saturn), led both these astronomers to assume that the body sought was nearly double the distance of uranus from the sun. with these few and imperfect data, as so many leading-strings proceeding from the planet uranus, they felt their way into the abysses of space by the aid of two sure guides--the law of gravitation and the higher geometry. both astronomers arrived at nearly the same results, although they wrought independently of each other, and each, indeed, without the knowledge of the other. le verrier was the first to make public his conclusions, which he communicated to the french academy at their sitting, august , . they saw that there existed, at nearly double the distance of uranus from the sun, a planet larger than that body; that it lay near a certain star seen at that season in the southwest, in the evening sky; that, on account of its immense distance, it was invisible to the naked eye, and could be distinctly seen with a perceptible disk only by the most powerful telescopes; being no brighter than a star of the ninth magnitude, and subtending an angle of only three seconds. le verrier communicated these results to dr. galle, of berlin, with the request that he would search for the stranger with his powerful telescope, pointing out the exact spot in the heavens where it would be found. on the same evening, dr. galle directed his instrument to that part of the heavens, and immediately the planet presented itself to view, within one degree of the very spot assigned to it by le verrier. subsequent investigations have shown that its apparent size is within half a second of that which the same sagacious mind foresaw, and that its diameter is nearly equal to that of uranus, being , , while uranus is , miles.[ ] the distance from the sun is less than was predicted, being only about , instead of millions of miles; and its periodic time is - / , instead of years, as was supposed by le verrier. one satellite only has yet been discovered, and this was first seen by professor bond with the great telescope of harvard university. recent telescopic discoveries.--the great reflecting telescope of lord rosse, and the powerful refracting telescopes of pulkova and cambridge, have opened new fields of discovery to the delighted astronomer. a new satellite has been added to saturn, first revealed to the cambridge instrument, making the entire number of moons that adorn the nocturnal sky of that remarkable planet no less than eight. still more wonderful things have been disclosed among the remotest _nebulæ_. a number of these objects before placed among the irresolvable nebulæ, and supposed to consist not of stars, but of mere nebulous matter, have been resolved into stars; others, of which we before saw only a part, have revealed themselves under new and strange forms, one resembling an animal with huge branching arms, and hence called the _crab_ nebula; another imitating a scroll or vortex, and called the _whirlpool_ nebula; and other figures, which to ordinary telescopes appear only as dim specks on the confines of creation, are presented to these wonderful instruments as glorious firmaments of stars. in the year , sir john herschel left england for the cape of good hope, furnished with powerful instruments for observing the stars and nebulæ of the southern hemisphere, which had never been examined in a manner suited to disclose their full glories. this great astronomer and benefactor to science devoted five years of the most assiduous toil in observing and delineating the astronomical objects of that portion of the heavens. he had before extended the catalogue of nebulæ begun by his illustrious father, sir william herschel, to the number of ; and beginning at that point, he swelled the number, by his labors at the cape of good hope, to . he extended also the list of double stars from to , and showed that the luminous spots near the south pole, known to sailors by the name of the "magellan clouds," consist of an assemblage of several hundred brilliant nebulæ. the united states have contributed their full share to the recent progress of astronomy. powerful telescopes have been imported, made by the first european artists, and numerous others, of scarcely inferior workmanship and power, have been produced by artists of our own. the american astronomers have also been the first to bring the electric telegraph into use in astronomical observations; electric clocks have been so constructed as to beat simultaneously at places distant many hundred miles from each other, and thus to furnish means of determining the difference of longitude between places with an astonishing degree of accuracy; and facilities for recording observations on the stars have been devised which render the work vastly more rapid as well as more accurate than before. indeed, the inventive genius for which americans have been distinguished in all the useful arts seems now destined to be equally conspicuous in promoting the researches of science. footnotes: [ ] the names of all the asteroids known at present are as follows: . ceres. . metis. . psyche. . pallas. . hygeia. . melpomene. . juno. . parthenope. . fortuna. . vesta. . victoria. . massalia. . astræa. . egeria. . lutetia. . hebe. . irene. . calliope. . iris. . eunomia. . un-named. . flora. . thetis. [ ] sir john herschel, however, states its diameter at , miles index. a. alamak, aldebaran, alexandrian school, algenib, algol, alioth, almagest, altair, altitude, amplitude, anaxagoras, anaximander, andromeda, antares, antinous, apogee, apsides, aquarius, aquila, archimedes, arcturus, aries, aristotle, astrology, astronomers royal, , astronomical clock, astronomical tables, astronomy, history of, , atmosphere, , attraction, auriga, axis of the earth, azimuth, b. bacon, , base line, base of verification, bellatrix, betalgeus, bissextile, bootes, bouguer, bowditch, brahean system, c. cæsar, julius, calendar, grecian, gregorian, cancer, canis major, canis minor, capella, capricorn, cassiopeia, catalogues of the stars, central forces, cepheus, ceres, cetus, chronology, chronometers, circles, great and small, of diurnal revolution, of perpetual apparition, of perpetual occultation, vertical, clusters, colures, coma berenices, comet, biela's, encke's, halley's, comets, brightness of, comets, distances of, light of, magnitude of, mass of, motions of, number of, periods of, perturbations of, structure of, tails of, complement, conjunction, constellations, copernican system, , copernicus, , cor caroli, cor hydræ, corona borealis, corvus, crotona, crystalline spheres, cygnus, d. day, astronomical, sidereal, solar, days of the week, declination, deferents, denebola, distances of the heavenly bodies, how measured, distances of the stars, dolphin, double stars, draco, e. earth, diameter of the, ellipticity of the, figure of the, motion of the, orbit of the, eclipses, annular, calculation of, of the moon, of the sun, ecliptic, epicycles, equation of time, equations, periodical, secular, tabular, equator, equinoxes, precession of the, eudoxus, f. fomalhaut, fraunhofer, g. galaxy, galileo, abjuration of, condemnation of, life of, persecutions of, gemini, gemma, globes, artificial, gravitation, universal, gravity, terrestrial, h. hercules, herschel, sir wm., , , hesperus, hipparchus, horizon, rational, sensible, hour-circles, huyghens, i. inductive system, inquisition, instruments, astronomical, j. juno, jupiter, belts of, diameter of, distance of, eclipses of, magnitude of, satellites of, scenery of, telescopic view of, k. kepler, kepler's laws, l. latitude, how found, laws of motion, terrestrial gravity, leap year, leo, leo minor, libra, librations of the moon, light, velocity of, how measured, longitude, celestial, terrestrial, its importance, how found, by chronometers, by eclipses, by jupiter's satellites, by lunar method, lucifer, lynx, m. magnitudes, how measured, magellan clouds, mars, changes of, distance of, revolutions of, mecanique celeste, mercury, conjunctions of, diurnal revolution of, phases of, sidereal revolut'n of, synodical revolut'n of, transits of, meridian, meteoric showers, origin of, meteoric stones, metonic cycle, miletus, school of, milky way, mira, mirach, mizar, month, sidereal, synodical, moon, atmosphere of the, cusps of the, diameter of the, distance of the, eclipses of the, harvest, irregularities of the, librations of the, light of the, mountains in the, nodes of the, phases of the, revolutions of the, - scenery of the, telescopic appearance of the, volcanoes in the, volume of the, motion, laws of, motions of the planets, mural circle, n. nadir, nature of the stars, nebulæ, new planets, distances of, origin of, periods of, size of, new style, newton, , o. oblique sphere, obliquity of the ecliptic, effect of, on the seasons, how found, observatory, greenwich, - tycho's, old style, ophiucus, opposition, orion, orreries, , p. pallas, parallactic arc, parallax, , annual, horizontal, how found, parallel sphere, parallels of latitude, pegasus, pendulum, perigee, periodical inequalities, perseus, pisces, piscis australis, planets, distances of, inferior, magnitudes of, periods, superior, pleiades, pointers, polar distance, polaris, pole, of the earth, pollux, power of the deity, præsepe, precession, prime vertical, primum mobile, principia, procyon, projection of the sphere, proper motions of the stars, ptolemaic system, ptolemy, pythagoras, q. quadrant, r. radius, refraction, regulus, resolution of motion, resultant, revolution, annual, diurnal, rigel, right ascension, right sphere, s. sagittarius, saros, saturn, diameter of, ring of, satellites of, scenery of, scorpio, seasons, secondary, secular inequalities, serpent, sextant, sidereal day, month, signs, sirius, solstices, sphere, celestial, doctrine of the, oblique, parallel, right, terrestrial, spica, spots on the sun, cause of, dimensions of, number of, stability of the universe, stars, fixed, stylus, sun, attraction of the, density of the, diameter of the, distance of the, mass of the, nature and constitution of the, revolutions of the, sun, spots on the, volume of the, supplement, system of the world, - brahean, copernican, ptolemaic, t. tangent, taurus, telescope, the, achromatic, directions for using, dorpat, herschelian, history of, reflecting, temperature, changes of, temporary stars, terminator, , thales, tides, cause of, spring and neap, time, apparent, equation of, mean, sidereal, transits, triangulation, tropic, twilight, u. unity of the deity, uranus, diameter of, distance of, history of, period of, satellites of, scenery of, ursa major, ursa minor, v. variable stars, venus, conjunctions of, mountains of, phases of, revolutions of, transits of, vesta, vindemiatrix, virgo, y. year, astronomical, tropical, z. zenith, zenith distance, zodiac, zodiacal light, zones, recent discoveries. improvements in the telescope, rosse's leviathan telescope, pulkova and cambridge telescopes, improvements in instrumental measurements, new planets and asteroids, great comet of , distances of the stars, discovery of neptune, recent telescopic discoveries, longitude by the electric telegraph, * * * * * transcriber's notes obvious punctuation and spelling errors repaired. greek transliterations are inclosed by equals signs. inconsistent hyphenation has been repaired. characters that could not be fully expressed are "unpacked" and shown within braces, e.g. {oblong symbol}. in ambiguous cases, the text has been left as it appears in the original book. in particular many mismatched quotation marks, have not been changed. page , "knittingneedle" changed to "knitting needle". page , "trignometry" changed to "trigonometry". page , "dedecaedron" changed to "dodecaedron". page , "generrally" changed to "generally". autobiography of sir george biddell airy, k.c.b., m.a., ll.d., d.c.l., f.r.s., f.r.a.s., honorary fellow of trinity college, cambridge, astronomer royal from to . edited by wilfrid airy, b.a., m.inst.c.e. preface. the life of airy was essentially that of a hard-working, business man, and differed from that of other hard-working people only in the quality and variety of his work. it was not an exciting life, but it was full of interest, and his work brought him into close relations with many scientific men, and with many men high in the state. his real business life commenced after he became astronomer royal, and from that time forward, during the years that he remained in office, he was so entirely wrapped up in the duties of his post that the history of the observatory is the history of his life. for writing his business life there is abundant material, for he preserved all his correspondence, and the chief sources of information are as follows: ( ) his autobiography. ( ) his annual reports to the board of visitors. ( ) his printed papers entitled "papers by g.b. airy." ( ) his miscellaneous private correspondence. ( ) his letters to his wife. ( ) his business correspondence. ( ) his autobiography, after the time that he became astronomer royal, is, as might be expected, mainly a record of the scientific work carried on at the greenwich observatory: but by no means exclusively so. about the time when he took charge of the observatory there was an immense development of astronomical enterprise: observatories were springing up in all directions, and the astronomer royal was expected to advise upon all of the british and colonial observatories. it was necessary also for him to keep in touch with the continental observatories and their work, and this he did very diligently and successfully, both by correspondence and personal intercourse with the foreign astronomers. there was also much work on important subjects more or less connected with his official duties--such as geodetical survey work, the establishment of time-balls at different places, longitude determinations, observation of eclipses, and the determination of the density of the earth. lastly, there was a great deal of time and work given to questions not very immediately connected with his office, but on which the government asked his assistance in the capacity of general scientific adviser: such were the correction of the compass in iron ships, the railway gauge commission, the commission for the restoration of the standards of length and weight, the maine boundary, lighthouses, the westminster clock, the london university, and many other questions. besides those above-mentioned there were a great many subjects which he took up out of sheer interest in the investigations. for it may fairly be said that every subject of a distinctly practical nature, which could be advanced by mathematical knowledge, had an interest for him: and his incessant industry enabled him to find time for many of them. amongst such subjects were tides and tidal observations, clockwork, and the strains in beams and bridges. a certain portion of his time was also given to lectures, generally on current astronomical questions, for he held it as his duty to popularize the science as far as lay in his power. and he attended the meetings of the royal astronomical society with great regularity, and took a very active part in the discussions and business of the society. he also did much work for the royal society, and (up to a certain date) for the british association. all of the foregoing matters are recorded pretty fully in his autobiography up to the year . after that date the autobiography is given in a much more abbreviated form, and might rather be regarded as a collection of notes for his biography. his private history is given very fully for the first part of his life, but is very lightly touched upon during his residence at greenwich. a great part of the autobiography is in a somewhat disjointed state, and appears to have been formed by extracts from a number of different sources, such as official journals, official correspondence, and reports. in editing the autobiography it has been thought advisable to omit a large number of short notes relating to the routine work of the observatory, to technical and scientific correspondence, to papers communicated to various societies and official business connected with them, and to miscellaneous matters of minor importance. these in the aggregate occupied a great deal of time and attention. but, from their detached nature, they would have but little general interest. at various places will be found short memoirs and other matter by the editor. ( ) all of his annual reports to the board of visitors are attached to his autobiography and were evidently intended to be read with it and to form part of it. these reports are so carefully compiled and are so copious that they form a very complete history of the greenwich observatory and of the work carried on there during the time that he was astronomer royal. the first report contained only four pages, but with the constantly increasing amount and range of work the reports constantly increased in volume till the later reports contained pages. extracts from these reports relating to matters of novelty and importance, and illustrating the principles which guided him in his conduct of the observatory, have been incorporated with the autobiography. ( ) the printed "papers by g.b. airy" are bound in large quarto volumes. there are of these papers, on a great variety of subjects: a list of them is appended to this history, as also is a list of the books that he wrote, and one or two of the papers which were separately printed. they form a very important part of his life's work, and are frequently referred to in the present history. they are almost all to be found in the transactions of societies or in newspapers, and extend over a period of years ( to ). the progress made in certain branches of science during this long period can very fairly be traced by these papers. ( ) his private correspondence was large, and like his other papers it was carefully arranged. no business letters of any kind are included under this head. in this correspondence letters are occasionally found either dealing with matters of importance or in some way characteristic, and these have been inserted in this biography. as already stated the autobiography left by airy is confined almost entirely to science and business, and touches very lightly on private matters or correspondence. ( ) the letters to his wife are very numerous. they were written during his occasional absences from home on business or for relaxation. on these occasions he rarely let a day pass without writing to his wife, and sometimes he wrote twice on the same day. they are full of energy and interest and many extracts from them are inserted in this history. a great deal of the personal history is taken from them. ( ) all correspondence in any way connected with business during the time that he was astronomer royal is to be found at the royal observatory. it is all bound and arranged in the most perfect order, and any letter throughout this time can be found with the greatest ease. it is very bulky, and much of it is, in a historical sense, very interesting. it was no doubt mainly from this correspondence that the autobiography, which so far as related to the greenwich part of it was almost entirely a business history, was compiled. the history of the early part of his life was written in great detail and contained a large quantity of family matter which was evidently not intended for publication. this part of the autobiography has been compressed. the history of the latter part of his life was not written by himself at all, and has been compiled from his journal and other sources. in both these cases, and occasionally in short paragraphs throughout the narrative, it has been found convenient to write the history in the third person. , the circus, greenwich. note. the syndics of the cambridge university press desire to express their thanks to messrs macmillan & co. for their courteous permission to use in this work the steel engraving of sir george biddell airy published in _nature_ on october , . table of contents. chapter i. personal sketch of george biddell airy chapter ii. from his birth to his taking his b.a. degree at cambridge chapter iii. at trinity college, cambridge, from his taking his b.a. degree to his taking charge of the cambridge observatory as plumian professor chapter iv. at cambridge observatory, from his taking charge of the cambridge observatory to his residence at greenwich observatory as astronomer royal chapter v. at greenwich observatory, - chapter vi. at greenwich observatory, - chapter vii. at greenwich observatory, - chapter viii. at greenwich observatory, - chapter ix. at greenwich observatory, from january st, , to his resignation of office on august th, chapter x. at the white house, greenwich, from his resignation of office on august th, , to his death on january nd, appendix. list of printed papers by g.b. airy, and list of books written by g.b. airy index. chapter i. personal sketch of george biddell airy. the history of airy's life, and especially the history of his life's work, is given in the chapters that follow. but it is felt that the present memoir would be incomplete without a reference to those personal characteristics upon which the work of his life hinged and which can only be very faintly gathered from his autobiography. he was of medium stature and not powerfully built: as he advanced in years he stooped a good deal. his hands were large-boned and well-formed. his constitution was remarkably sound. at no period in his life does he seem to have taken the least interest in athletic sports or competitions, but he was a very active pedestrian and could endure a great deal of fatigue. he was by no means wanting in physical courage, and on various occasions, especially in boating expeditions, he ran considerable risks. in debate and controversy he had great self-reliance, and was absolutely fearless. his eye-sight was peculiar, and required correction by spectacles the lenses of which were ground to peculiar curves according to formulae which he himself investigated: with these spectacles he saw extremely well, and he commonly carried three pairs, adapted to different distances: he took great interest in the changes that took place in his eye-sight, and wrote several papers on the subject. in his later years he became somewhat deaf, but not to the extent of serious personal inconvenience. the ruling feature of his character was undoubtedly order. from the time that he went up to cambridge to the end of his life his system of order was strictly maintained. he wrote his autobiography up to date soon after he had taken his degree, and made his first will as soon as he had any money to leave. his accounts were perfectly kept by double entry throughout his life, and he valued extremely the order of book-keeping: this facility of keeping accounts was very useful to him. he seems not to have destroyed a document of any kind whatever: counterfoils of old cheque-books, notes for tradesmen, circulars, bills, and correspondence of all sorts were carefully preserved in the most complete order from the time that he went to cambridge; and a huge mass they formed. to a high appreciation of order he attributed in a great degree his command of mathematics, and sometimes spoke of mathematics as nothing more than a system of order carried to a considerable extent. in everything he was methodical and orderly, and he had the greatest dread of disorder creeping into the routine work of the observatory, even in the smallest matters. as an example, he spent a whole afternoon in writing the word "empty" on large cards, to be nailed upon a great number of empty packing boxes, because he noticed a little confusion arising from their getting mixed with other boxes containing different articles; and an assistant could not be spared for this work without withdrawing him from his appointed duties. his arrangement of the observatory correspondence was excellent and elaborate: probably no papers are more easy of reference than those arranged on his system. his strict habits of order made him insist very much upon detail in his business with others, and the rigid discipline arising out of his system of order made his rule irksome to such of his subordinates as did not conform readily to it: but the efficiency of the observatory unquestionably depended mainly upon it. as his powers failed with age the ruling passion for order assumed a greater prominence; and in his last days he seemed to be more anxious to put letters which he received into their proper place for reference than even to master their contents. his nature was eminently practical, and any subject which had a distinctly practical object, and could be advanced by mathematical investigation, possessed interest for him. and his dislike of mere theoretical problems and investigations was proportionately great. he was continually at war with some of the resident cambridge mathematicians on this subject. year after year he criticised the senate house papers and the smith's prize papers question by question very severely: and conducted an interesting and acrimonious private correspondence with professor cayley on the same subject. his great mathematical powers and his command of mathematics are sufficiently evidenced by the numerous mathematical treatises of the highest order which he published, a list of which is appended to this biography. but a very important feature of his investigations was the thoroughness of them. he was never satisfied with leaving a result as a barren mathematical expression. he would reduce it, if possible, to a practical and numerical form, at any cost of labour: and would use any approximations which would conduce to this result, rather than leave the result in an unfruitful condition. he never shirked arithmetical work: the longest and most laborious reductions had no terrors for him, and he was remarkably skilful with the various mathematical expedients for shortening and facilitating arithmetical work of a complex character. this power of handling arithmetic was of great value to him in the observatory reductions and in the observatory work generally. he regarded it as a duty to finish off his work, whatever it was, and the writer well remembers his comment on the mathematics of one of his old friends, to the effect that "he was too fond of leaving a result in the form of three complex equations with three unknown quantities." to one who had known, in some degree, of the enormous quantity of arithmetical work which he had turned out, and the unsparing manner in which he had devoted himself to it, there was something very pathetic in his discovery, towards the close of his long life, "that the figures would not add up." his energy and business capacity were remarkable. he was made for work and could not long be happy without it. whatever subject he was engaged upon, he kept his object clearly in view, and made straight for it, aiming far more at clearness and directness than at elegance of periods or symmetry of arrangement. he wrote his letters with great ease and rapidity: and having written them he very rarely had occasion to re-write them, though he often added insertions and interlineations, even in the most important official letters. without this it would have been impossible for him to have turned out the enormous quantity of correspondence that he did. he never dictated letters, and only availed himself of clerical assistance in matters of the most ordinary routine. in his excursions, as in his work, he was always energetic, and could not endure inaction. whatever there was of interest in the places that he visited he examined thoroughly and without delay, and then passed on. and he thus accomplished a great deal in a short vacation. his letters written to his wife, while he was on his excursions, are very numerous and characteristic, and afford ample proofs of his incessant energy and activity both of body and mind. they are not brilliantly written, for it was not in his nature to write for effect, and he would never give himself the trouble to study the composition of his letters, but they are straight-forward, clear, and concise, and he was never at a loss for suitable language to express his ideas. he had a wonderful capacity for enjoyment: the subjects that chiefly interested him were scenery, architecture, and antiquities, but everything novel or curious had an interest for him. he made several journeys to the continent, but by far the greater number of his excursions were made in england and scotland, and there were few parts of the country which he had not visited. he was very fond of the lake district of cumberland, and visited it very frequently, and each time that he went there the same set of views had an eternal freshness for him, and he wrote long descriptions of the scenery and effects with the same raptures as if he had seen it for the first time. many of his letters were written from playford, a village in a beautiful part of suffolk, a few miles from ipswich. here he had a small property, and generally stayed there for a short time once or twice a year. he was extremely fond of this country, and was never tired of repeating his walks by the well-known lanes and footpaths. and, as in cumberland, the suffolk country had an eternal freshness and novelty for him. wherever he went he was indefatigable in keeping up his acquaintance with his numerous friends and his letters abound in social reminiscences. his memory was singularly retentive. it was much remarked at school in his early days, and in the course of his life he had stored up in his memory an incredible quantity of poetry, ballads, and miscellaneous facts and information of all sorts, which was all constantly ready and at his service. it is almost needless to add that his memory was equally accurate and extensive in matters connected with science or business. his independence of character was no doubt due to and inseparable from his great powers. the value of his scientific work greatly depended upon his self-reliance and independence of thought. and in the heavy work of remodelling the observatory it was a very valuable quality. this same self-reliance made him in his latter years apt to draw conclusions too confidently and hastily on subjects which he had taken up more as a pastime than as work. but whatever he touched he dealt with ably and in the most fearless truthseeking manner, and left original and vigorous opinions. he had a remarkably well-balanced mind, and a simplicity of nature that appeared invulnerable. no amount of hero-worship seemed to have the least effect upon him. and from a very early time he was exposed to a great deal of it. his mind was incessantly engaged on investigations of nature, and this seems to have been with him, as has been the case with others, a preserving influence. this simplicity of character he retained throughout his life. at the same time he was sensible and shrewd in his money matters and attentive to his personal interests. and his practical good sense in the general affairs of life, combined with his calm and steady consideration of points submitted to him, made his advice very valuable. this was especially recognized by his own and his wife's relations, who consulted him on many occasions and placed the fullest confidence in his absolute sense of justice as well as in his wise counsel. he was extremely liberal in proportion to his means, and gave away money to a large extent to all who had any claim upon him. but he was not in any sense reckless, and kept a most cautious eye on his expenses. he was not indifferent to the honours which he received in the scientific world, but he does not appear to have sought them in any way, and he certainly did not trouble himself about them. his courtesy was unfailing: no amount of trouble could shake it. whether it was the secretary of the admiralty, or a servant girl wanting her fortune told: whether a begging-letter for money, or miscellaneous invitations: all had their answer in the most clear and courteous language. but he would not grant personal interviews when he could avoid it: they took up too much of his time. his head was so clear that he never seemed to want for the clearest and most direct language in expressing his meaning, and his letters are models of terseness. in all his views and opinions he was strongly liberal. at cambridge at an early date he was one of the members of the senate who supported the application to permit the granting of medical degrees without requiring an expression of assent to the religious doctrines of the church of england. and in he declined to sign a petition against the abolition of religious declarations required of persons admitted to fellowships or proceeding to the degree of m.a. and he was opposed to every kind of narrowness and exclusiveness. when he was appointed to the post of astronomer royal, he stipulated that he should not be asked to vote in any political election. but all his views were in the liberal direction. he was a great reader of theology and church history, and as regarded forms of worship and the interpretation of the scriptures, he treated them with great respect, but from the point of view of a freethinking layman. in the preface to his "notes on the earlier hebrew scriptures" he says, "in regard to the general tone of these notes, i will first remark that i have nothing to say on the subject of verbal inspiration. with those who entertain that doctrine, i can have nothing in common. nor do i recognize, in the professedly historical accounts, any other inspiration which can exempt them from the severest criticism that would be applicable to so-called profane accounts, written under the same general circumstances, and in the same countries." and his treatment of the subject in the "notes" shews how entirely he took a rationalistic view of the whole question. he also strongly sided with bishop colenso in his fearless criticism of the pentateuch, though he dissented from some of his conclusions. but he was deeply imbued with the spirit of religion and reflected much upon it. his whole correspondence conveys the impression of the most sterling integrity and high-mindedness, without a trace of affectation. in no letter does there appear a shadow of wavering on matters of principle, whether in public or private matters, and he was very clear and positive in his convictions. the great secret of his long and successful official career was that he was a good servant and thoroughly understood his position. he never set himself in opposition to his masters, the admiralty. he never hesitated to ask the admiralty for what he thought right, whether in the way of money grants for various objects, or for occasional permission to give his services to scientific matters not immediately connected with the observatory. sometimes the admiralty refused his requests, and he felt this very keenly, but he was far too busy and energetic to trouble himself about such little slights, and cheerfully accepted the situation. what was refused by one administration was frequently granted by another; and in the meantime he was always ready to give his most zealous assistance in any matter that was officially brought before him. this cheerful readiness to help, combined with his great ability and punctuality in business matters, made him a very valuable servant, and speaking generally he had the confidence of the admiralty in a remarkable degree. in many of his reports to the board of visitors he speaks gratefully of the liberality of the admiralty in forwarding scientific progress and research. in matters too which are perhaps of minor importance from the high stand-point of science, but which are invaluable in the conduct of an important business office, such for example as estimates and official correspondence, he was orderly and punctual in the highest degree. and, what is by no means unimportant, he possessed an excellent official style in correspondence, combined with great clearness of expression. his entire honesty of purpose, and the high respect in which he was held both at home and abroad, gave great weight to his recommendations. with regard to his habits while he resided at the observatory, his custom was to work in his official room from to about . , though in summer he was frequently at work before breakfast. he then took a brisk walk, and dined at about . . this early hour had been prescribed and insisted upon by his physician, dr haviland of cambridge, in whom he had great confidence. he ate heartily, though simply and moderately, and slept for about an hour after dinner. he then had tea, and from about to he worked in the same room with his family. he would never retire to a private room, and regarded the society of his family as highly beneficial in "taking the edge off his work." his powers of abstraction were remarkable: nothing seemed to disturb him; neither music, singing, nor miscellaneous conversation. he would then play a game or two at cards, read a few pages of a classical or historical book, and retire at . on sundays he attended morning service at church, and in the evening read a few prayers very carefully and impressively to his whole household. he was very hospitable, and delighted to receive his friends in a simple and natural way at his house. in this he was most admirably aided by his wife, whose grace and skill made everything pleasant to their guests. but he avoided dinner-parties as much as possible--they interfered too much with his work--and with the exception of scientific and official dinners he seldom dined away from home. his tastes were entirely domestic, and he was very happy in his family. with his natural love of work, and with the incessant calls upon him, he would soon have broken down, had it not been for his system of regular relaxation. two or three times a year he took a holiday: generally a short run of a week or ten days in the spring, a trip of a month or thereabouts in the early autumn, and about three weeks at playford in the winter. these trips were always conducted in the most active manner, either in constant motion from place to place, or in daily active excursions. this system he maintained with great regularity, and from the exceeding interest and enjoyment that he took in these trips his mind was so much refreshed and steadied that he always kept himself equal to his work. airy seems to have had a strong bent in the direction of astronomy from his youth, and it is curious to note how well furnished he was, by the time that he became astronomer royal, both with astronomy in all its branches, and with the kindred sciences so necessary for the practical working and improvement of it. at the time that he went to cambridge physical astronomy was greatly studied there and formed a most important part of the university course. he eagerly availed himself of this, and mastered the physical astronomy in the most thorough manner, as was evidenced by his papers collected in his "mathematical tracts," his investigation of the long inequality of the earth and venus, and many other works. as plumian professor he had charge of the small observatory at cambridge, where he did a great deal of the observing and reduction work himself, and became thoroughly versed in the practical working of an observatory. the result of this was immediately seen in the improved methods which he introduced at greenwich, and which were speedily imitated at other observatories. optics and the undulatory theory of light had been very favourite subjects with him, and he had written and lectured frequently upon them. in the construction of the new and powerful telescopes and other optical instruments required from time to time this knowledge was very essential, for in its instrumental equipment the greenwich observatory was entirely remodelled during his tenure of office. and in many of the matters referred to him, as for instance that of the lighthouses, a thorough knowledge of optics was most valuable. he had made a great study of the theory and construction of clocks, and this knowledge was invaluable to him at greenwich in the establishment of new and more accurate astronomical clocks, and especially in the improvement of chronometers. he had carefully studied the theory of pendulums, and had learned how to use them in his experiments in the cornish mines. this knowledge he afterwards utilized very effectively at the harton pit in comparing the density of the earth's crust with its mean density; and it was very useful to him in connection with geodetic surveys and experiments on which he was consulted. and his mechanical knowledge was useful in almost everything. the subjects (outside those required for his professional work) in which he took most interest were poetry, history, theology, antiquities, architecture, and engineering. he was well acquainted with standard english poetry, and had committed large quantities to memory, which he frequently referred to as a most valuable acquisition and an ever-present relief and comfort to his mind. history and theology he had studied as opportunity offered, and without being widely read in them he was much at home with them, and his powerful memory made the most of what he did read. antiquities and architecture were very favourite subjects with him. he had visited most of the camps and castles in the united kingdom and was never tired of tracing their connection with ancient military events: and he wrote several papers on this subject, especially those relating to the roman invasions of britain. ecclesiastical architecture he was very fond of: he had visited nearly all the cathedrals and principal churches in england, and many on the continent, and was most enthusiastic on their different styles and merits: his letters abound in critical remarks on them. he was extremely well versed in mechanics, and in the principles and theory of construction, and took the greatest interest in large engineering works. this led to much communication with stephenson, brunel, and other engineers, who consulted him freely on the subject of great works on which they were engaged: in particular he rendered much assistance in connection with the construction of the britannia bridge over the menai straits. there were various other subjects which he read with much interest (geology in particular), but he made no study of natural history, and knew very little about it beyond detached facts. his industry was untiring, and in going over his books one by one it was very noticeable how large a number of them were feathered with his paper "marks," shewing how carefully he had read them and referred to them. his nature was essentially cheerful, and literature of a witty and humourous character had a great charm for him. he was very fond of music and knew a great number of songs; and he was well acquainted with the theory of music: but he was no performer. he did not sketch freehand but made excellent drawings with his camera lucida. at the time when he took his degree ( ) and for many years afterwards there was very great activity of scientific investigation and astronomical enterprise in england. and, as in the times of flamsteed and halley, the earnest zeal of men of science occasionally led to much controversy and bitterness amongst them. airy was by no means exempt from such controversies. he was a man of keen sensitiveness, though it was combined with great steadiness of temper, and he never hesitated to attack theories and methods that he considered to be scientifically wrong. this led to differences with ivory, challis, south, cayley, archibald smith, and others; but however much he might differ from them he was always personally courteous, and the disputes generally went no farther than as regarded the special matter in question. almost all these controversial discussions were carried on openly, and were published in the athenaeum, the philosophical magazine, or elsewhere; for he printed nearly everything that he wrote, and was very careful in the selection of the most suitable channels for publication. he regarded it as a duty to popularize as much as possible the work done at the observatory, and to take the public into his confidence. and this he effected by articles communicated to newspapers, lectures, numerous papers written for scientific societies, reports, debates, and critiques. his strong constitution and his regular habits, both of work and exercise, are sufficient explanation of the good health which in general he enjoyed. not but what he had sharp touches of illness from time to time. at one period he suffered a good deal from an attack of eczema, and at another from a varicose vein in his leg, and he was occasionally troubled with severe colds. but he bore these ailments with great patience and threw them off in course of time. he was happy in his marriage and in his family, and such troubles and distresses as were inevitable he accepted calmly and quietly. in his death, as in his life, he was fortunate: he had no long or painful illness, and he was spared the calamity of aberration of intellect, the saddest of all visitations. chapter ii. from his birth to his taking his b.a. degree at cambridge. from july th to january th . george biddell airy was born at alnwick in northumberland on july th . his father was william airy of luddington in lincolnshire, the descendant of a long line of airys who have been traced back with a very high degree of probability to a family of that name which was settled at kentmere in westmorland in the th century. a branch of this family migrated to pontefract in yorkshire, where they seem to have prospered for many years, but they were involved in the consequences of the civil wars, and one member of the family retired to ousefleet in yorkshire. his grandson removed to luddington in lincolnshire, where his descendants for several generations pursued the calling of small farmers. george biddell airy's mother, ann airy, was the daughter of george biddell, a well-to-do farmer in suffolk. william airy, the father of george biddell airy, was a man of great activity and strength, and of prudent and steady character. when a young man he became foreman on a farm in the neighbourhood of luddington, and laid by his earnings in summer in order to educate himself in winter. for a person in his rank, his education was unusually good, in matters of science and in english literature. but at the age of he grew tired of country labour, and obtained a post in the excise. after serving in various collections he was appointed collector of the northumberland collection on the th august , and during his service there his eldest son george biddell airy was born. the time over which his service as officer and supervisor extended was that in which smuggling rose to a very high pitch, and in which the position of excise officer was sometimes dangerous. he was remarkable for his activity and boldness in contests with smugglers, and made many seizures. ann airy, the mother of george biddell airy, was a woman of great natural abilities both speculative and practical, kind as a neighbour and as head of a family, and was deeply loved and respected. the family consisted of george biddell, elizabeth, william, and arthur who died young. william airy was appointed to hereford collection on nd october , and removed thither shortly after. he stayed at hereford till he was appointed to essex collection on th february , and during this time george biddell was educated at elementary schools in writing, arithmetic, and a little latin. he records of himself that he was not a favourite with the schoolboys, for he had very little animal vivacity and seldom joined in active play with his schoolfellows. but in the proceedings of the school he was successful, and was a favourite with his master. on the appointment of william airy to essex collection, the family removed to colchester on april th . here george biddell was first sent to a large school in sir isaac's walk, then kept by mr byatt walker, and was soon noted for his correctness in orthography, geography, and arithmetic. he evidently made rapid progress, for on one occasion mr walker said openly in the schoolroom how remarkable it was that a boy years old should be the first in the school. at this school he stayed till the end of and thoroughly learned arithmetic (from walkingame's book), book-keeping by double entry (on which knowledge throughout his life he set a special value), the use of the sliding rule (which knowledge also was specially useful to him in after life), mensuration and algebra (from bonnycastle's books). he also studied grammar in all its branches, and geography, and acquired some knowledge of english literature, beginning with that admirable book the speaker, but it does not appear that latin and greek were attended to at this school. he records that at this time he learned an infinity of snatches of songs, small romances, &c., which his powerful memory retained most accurately throughout his life. he was no hand at active play: but was notorious for his skill in constructing guns for shooting peas and arrows, and other mechanical contrivances. at home he relates that he picked up a wonderful quantity of learning from his father's books. he read and remembered much poetry from such standard authors as milton, pope, gay, gray, swift, &c., which was destined to prove in after life an invaluable relaxation for his mind. but he also studied deeply an excellent cyclopaedia called a dictionary of arts and sciences in three volumes folio, and learned from it much about ship-building, navigation, fortification, and many other subjects. during this period his valuable friendship with his uncle arthur biddell commenced. arthur biddell was a prosperous farmer and valuer at playford near ipswich. he was a well-informed and able man, of powerful and original mind, extremely kind and good-natured, and greatly respected throughout the county. in the autobiography of george biddell airy he states as follows: "i do not remember precisely when it was that i first visited my uncle arthur biddell. i think it was in a winter: certainly as early as the winter of -- . here i found a friend whose society i could enjoy, and i entirely appreciated and enjoyed the practical, mechanical, and at the same time speculative and enquiring talents of arthur biddell. he had a library which, for a person in middle life, may be called excellent, and his historical and antiquarian knowledge was not small. after spending one winter holiday with him, it easily came to pass that i spent the next summer holiday with him: and at the next winter holiday, finding that there was no precise arrangement for my movements, i secretly wrote him a letter begging him to come with a gig to fetch me home with him: he complied with my request, giving no hint to my father or mother of my letter: and from that time, one-third of every year was regularly spent with him till i went to college. how great was the influence of this on my character and education i cannot tell. it was with him that i became acquainted with the messrs ransome, w. cubitt the civil engineer (afterwards sir w. cubitt), bernard barton, thomas clarkson (the slave-trade abolitionist), and other persons whose acquaintance i have valued highly. it was also with him that i became acquainted with the works of the best modern poets, scott, byron, campbell, hogg, and others: as also with the waverley novels and other works of merit." in william airy lost his appointment of collector of excise and was in consequence very much straitened in his circumstances. but there was no relaxation in the education of his children, and at the beginning of george biddell was sent to the endowed grammar school at colchester, then kept by the rev. e. crosse, and remained there till the summer of , when he went to college. the autobiography proceeds as follows: "i became here a respectable scholar in latin and greek, to the extent of accurate translation, and composition of prose latin: in regard to latin verses i was i think more defective than most scholars who take the same pains, but i am not much ashamed of this, for i entirely despise the system of instruction in verse composition. "my father on some occasion had to go to london and brought back for me a pair of -inch globes. they were invaluable to me. the first stars which i learnt from the celestial globe were alpha lyrae, alpha aquilae, alpha cygni: and to this time i involuntarily regard these stars as the birth-stars of my astronomical knowledge. having somewhere seen a description of a gunter's quadrant, i perceived that i could construct one by means of the globe: my father procured for me a board of the proper shape with paper pasted on it, and on this i traced the lines of the quadrant. "my command of geometry was tolerably complete, and one way in which i frequently amused myself was by making paper models (most carefully drawn in outline) which were buttoned together without any cement or sewing. thus i made models, not only of regular solids, regularly irregular solids, cones cut in all directions so as to shew the conic sections, and the like, but also of six-gun batteries, intrenchments and fortresses of various kinds &c. "from various books i had learnt the construction of the steam-engine: the older forms from the dictionary of arts and sciences; newer forms from modern books. the newest form however (with the sliding steam valve) i learnt from a -horse engine at bawtrey's brewery (in which mr keeling the father of my schoolfellow had acquired a partnership). i frequently went to look at this engine, and on one occasion had the extreme felicity of examining some of its parts when it was opened for repair. "in the mean time my education was advancing at playford. the first record, i believe, which i have of my attention to mechanics there is the plan of a threshing-machine which i drew. but i was acquiring valuable information of all kinds from the encyclopaedia londinensis, a work which without being high in any respect is one of the most generally useful that i have seen. but i well remember one of the most important steps that i ever made. i had tried experiments with the object-glass of an opera-glass and was greatly astonished at the appearance of the images of objects seen through the glass under different conditions. by these things my thoughts were turned to accurate optics, and i read with care rutherford's lectures, which my uncle possessed. the acquisition of an accurate knowledge of the effect of optical constructions was one of the most charming attainments that i ever reached. long before i went to college i understood the action of the lenses of a telescope better than most opticians. i also read with great zeal nicholson's dictionary of chemistry, and occasionally made chemical experiments of an inexpensive kind: indeed i grew so fond of this subject that there was some thought of apprenticing me to a chemist. i also attended to surveying and made a tolerable survey and map of my uncle's farm. "at school i was going on successfully, and distinguished myself particularly by my memory. it was the custom for each boy once a week to repeat a number of lines of latin or greek poetry, the number depending very much on his own choice. i determined on repeating every week, and i never once fell below that number and was sometimes much above it. it was no distress to me, and great enjoyment. at michaelmas i repeated lines, probably without missing a word. i do not think that i was a favourite with mr crosse, but he certainly had a high opinion of my powers and expressed this to my father. my father entertained the idea of sending me to college, which mr crosse recommended: but he heard from some college man that the expense would be _£ _ a year, and he laid aside all thoughts of it. "the farm of playford hall was in or hired by thomas clarkson, the slave-trade abolitionist. my uncle transacted much business for him (as a neighbour and friend) in the management of the farm &c. for a time, and they became very intimate. my uncle begged him to examine me in classical knowledge, and he did so, i think, twice. he also gave some better information about the probable expenses &c. at college. the result was a strong recommendation by my uncle or through my uncle that i should be sent to cambridge, and this was adopted by my father. i think it likely that this was in . "in december , dealtry's fluxions was bought for me, and i read it and understood it well. i borrowed hutton's course of mathematics of old mr ransome, who had come to reside at greenstead near colchester, and read a good deal of it. "about ladyday i began to read mathematics with mr rogers (formerly, i think, a fellow of sidney college, and an indifferent mathematician of the cambridge school), who had succeeded a mr tweed as assistant to mr crosse in the school. i went to his house twice a week, on holiday afternoons. i do not remember how long i received lessons from him, but i think to june, . this course was extremely valuable to me, not on account of mr rogers's abilities (for i understood many things better than he did) but for its training me both in cambridge subjects and in the cambridge accurate methods of treating them. i went through euclid (as far as usually read), wood's algebra, wood's mechanics, vince's hydrostatics, wood's optics, trigonometry (in a geometrical treatise and also in woodhouse's algebraical form), fluxions to a good extent, newton's principia to the end of the th section. this was a large quantity, but i read it accurately and understood it perfectly, and could write out any one of the propositions which i had read in the most exact form. my connexion with mr rogers was terminated by _his_ giving me notice that he could not undertake to receive me any longer: in fact i was too much for him. i generally read these books in a garret in our house in george lane, which was indefinitely appropriated to my brother and myself. i find that i copied out vince's conic sections in february, . the first book that i copied was the small geometrical treatise on trigonometry, in may, : to this i was urged by old mr ransome, upon my complaining that i could not purchase the book: and it was no bad lesson of independence to me." during the same period - he was occupied at school on translations into blank verse from the aeneid and iliad, and read through the whole of sophocles very carefully. the classical knowledge which he thus gained at school and subsequently at cambridge was sound, and he took great pleasure in it: throughout his life he made a practice of keeping one or other of the classical authors at hand for occasional relaxation. he terminated his schooling in june . shortly afterwards his father left colchester and went to reside at bury st edmund's. the autobiography proceeds as follows: "mr clarkson was at one time inclined to recommend me to go to st peter's college (which had been much enriched by a bequest from a mr gisborne). but on giving some account of me to his friend mr james d. hustler, tutor of trinity college, mr hustler urged upon him that i was exactly the proper sort of person to go to trinity college. and thus it was settled (mainly by mr clarkson) that i should be entered at trinity college. i think that i was sent for purposely from colchester to playford, and on march th, , i rode in company with mr clarkson from playford to sproughton near ipswich to be examined by the rev. mr rogers, incumbent of sproughton, an old m.a. of trinity college; and was examined, and my certificate duly sent to mr hustler; and i was entered on mr hustler's side as sizar of trinity college. "in the summer of i spent some time at playford. on july th, (my birthday, years old), mr clarkson invited me to dinner, to meet mr charles musgrave, fellow of trinity college, who was residing for a short time at grundisburgh, taking the church duty there for dr ramsden, the rector. it was arranged that i should go to grundisburgh the next day (i think) to be examined in mathematics by mr musgrave. i went accordingly, and mr musgrave set before me a paper of questions in geometry, algebra, mechanics, optics, &c. ending with the first proposition of the principia. i knew nothing more about my answers at the time; but i found long after that they excited so much admiration that they were transmitted to cambridge (i forget whether to mr musgrave's brother, a fellow of trinity college and afterwards archbishop of york, or to mr peacock, afterwards dean of ely) and were long preserved. "the list of the classical subjects for the first year in trinity college was transmitted to me, as usual, by mr hustler. they were--the hippolytus of euripides, the rd book of thucydides, and the nd philippic of cicero. these i read carefully and noted before going up. mr hustler's family lived in bury; and i called on him and saw him in october, introduced by mr clarkson. on the morning of october th, , i went on the top of the coach to cambridge, knowing nobody there but mr hustler, but having letters of introduction from mr charles musgrave to professor sedgwick, mr thomas musgrave, and mr george peacock, all fellows of trinity college. "i was set down at the hoop, saw trinity college for the first time, found mr hustler, was conducted by his servant to the robe-maker's, where i was invested in the cap and blue gown, and after some further waiting was installed into lodgings in bridge street. at o'clock i went to the college hall and was introduced by mr hustler to several undergraduates, generally clever men, and in the evening i attended chapel in my surplice (it being st luke's day) and witnessed that splendid service of which the occasional exhibition well befits the place. "as soon as possible, i called on mr peacock, mr musgrave, and professor sedgwick. by all i was received with great kindness: my examination papers had been sent to them, and a considerable reputation preceded me. mr peacock at once desired that i would not consider mr c. musgrave's letter as an ordinary introduction, but that i would refer to him on all occasions. and i did so for several years, and always received from him the greatest assistance that he could give. i think that i did not become acquainted with mr whewell till the next term, when i met him at a breakfast party at mr peacock's. mr peacock at once warned me to arrange for taking regular exercise, and prescribed a walk of two hours every day before dinner: a rule to which i attended regularly, and to which i ascribe the continuance of good general health. "i shewed mr peacock a manuscript book which contained a number of original propositions which i had investigated. these much increased my reputation (i really had sense enough to set no particular value on it) and i was soon known by sight to almost everybody in the university. a ridiculous little circumstance aided in this. the former rule of the university (strictly enforced) had been that all students should wear drab knee-breeches: and i, at mr clarkson's recommendation, was so fitted up. the struggle between the old dress and the trowsers customary in society was still going on but almost terminated, and i was one of the very few freshmen who retained the old habiliments. this made me in some measure distinguishable: however at the end of my first three terms i laid these aside. "the college lectures began on oct. : mr evans at on the hippolytus, and mr peacock at on euclid (these being the assistant tutors on mr hustler's side): and then i felt myself established. "i wrote in a day or two to my uncle arthur biddell, and i received from him a letter of the utmost kindness. he entered gravely on the consideration of my prospects, my wants, &c.: and offered at all times to furnish me with money, which he thought my father's parsimonious habits might make him unwilling to do. i never had occasion to avail myself of this offer: but it was made in a way which in no small degree strengthened the kindly feelings that had long existed between us. "i carefully attended the lectures, taking notes as appeared necessary. in mathematics there were geometrical problems, algebra, trigonometry (which latter subjects the lectures did not reach till the terms of ). mr peacock gave me a copy of lacroix's differential calculus as translated by himself and herschel and babbage, and also a copy of their examples. at this time, the use of differential calculus was just prevailing over that of fluxions (which i had learnt). i betook myself to it with great industry. i also made myself master of the theories of rectangular coordinates and some of the differential processes applying to them, which only a few of the best of the university mathematicians then wholly possessed. in classical subjects i read the latin (seneca's) and english hippolytus, racine's phèdre (which my sister translated for me), and all other books to which i was referred, aristotle, longinus, horace, bentley, dawes &c., made verse translations of the greek hippolytus, and was constantly on the watch to read what might be advantageous. "early in december mr hustler sent for me to say that one of the company of fishmongers, mr r. sharp, had given to mr john h. smyth, m.p. for norwich, the presentation to a small exhibition of _£ _ a year, which mr smyth had placed in mr hustler's hands, and which mr hustler immediately conferred on me. this was my first step towards pecuniary independence. i retained this exhibition till i became a fellow of the college. "i stayed at cambridge during part of the winter vacation, and to avoid expense i quitted my lodgings and went for a time into somebody's rooms in the bishop's hostel. (it is customary for the tutors to place students in rooms when their right owners are absent.) i took with me thucydides and all relating to it, and read the book, upon which the next term's lectures were to be founded, very carefully. the latter part of the vacation i spent at bury, where i began with the assistance of my sister to pick up a little french: as i perceived that it was absolutely necessary for enabling me to read modern mathematics. "during a part of the time i employed myself in writing out a paper on the geometrical interpretation of the algebraical expression sqrt(- ). i think that the original suggestion of perpendicular line came from some book (i do not remember clearly), and i worked it out in several instances pretty well, especially in de moivre's theorem. i had spoken of it in the preceding term to mr peacock and he encouraged me to work it out. the date at the end is , january . when some time afterwards i spoke of it to mr hustler, he disapproved of my employing my time on such speculations. about the last day of january i returned to cambridge, taking up my abode in my former lodgings. i shewed my paper on sqrt(- ) to mr peacock, who was much pleased with it and shewed it to mr whewell and others. "on february i commenced two excellent customs. the first was that i always had upon my table a quire of large-sized scribbling-paper sewn together: and upon this paper everything was entered: translations into latin and out of greek, mathematical problems, memoranda of every kind (the latter transferred when necessary to the subsequent pages), and generally with the date of the day. this is a most valuable custom. the other was this: as i perceived that to write latin prose well would be useful to me, i wrote a translation of english into latin every day. however much pressed i might be with other business, i endeavoured to write at least three or four words, but if possible i wrote a good many sentences. "i may fix upon this as the time when my daily habits were settled in the form in which they continued for several years. i rose in time for the chapel service at . it was the college regulation that every student should attend chapel four mornings and four evenings (sunday being one of each) in every week: and in this i never failed. after chapel service i came to my lodgings and breakfasted. at i went to college lectures, which lasted to . most of my contemporaries, being intended for the church, attended also divinity lectures: but i never did. i then returned, put my lecture notes in order, wrote my piece of latin prose, and then employed myself on the subject which i was reading for the time: usually taking mathematics at this hour. at or a little sooner i went out for a long walk, usually or miles into the country: sometimes if i found companions i rowed on the cam (a practice acquired rather later). a little before i returned, and at went to college hall. after dinner i lounged till evening chapel time, / past , and returning about i then had tea. then i read quietly, usually a classical subject, till ; and i never, even in the times when i might seem most severely pressed, sat up later. "from this time to the close of the annual examination (beginning of june) i remained at cambridge, stopping there through the easter vacation. the subjects of the mathematical lectures were ordinary algebra and trigonometry: but mr peacock always had some private problems of a higher class for me, and saw me i believe every day. the subjects of the classical lectures were, the termination of hippolytus, the book of thucydides and the oration of cicero. in mathematics i read whewell's mechanics, then just published (the first innovation made in the cambridge system of physical sciences for many years): and i find in my scribbling-paper notes, integrals, central forces, finite differences, steam-engine constructions and powers, plans of bridges, spherical trigonometry, optical calculations relating to the achromatism of eye-pieces and achromatic object-glasses with lenses separated, mechanical problems, transit of venus, various problems in geometrical astronomy (i think it was at this time that mr peacock had given me a copy of woodhouse's astronomy st edition), the rainbow, plans for anemometer and for a wind-pumping machine, clearing lunars, &c., with a great number of geometrical problems. i remark that my ideas on the differential calculus had not acquired on some important points the severe accuracy which they acquired in a few months. in classics i read the persae of aeschylus, greek and roman history very much (mitford, hooke, ferguson) and the books of thucydides introductory to that of the lecture subject (the rd): and attended to chronology. on the scribbling-paper are verse-translations from euripides, careful prose-translations from thucydides, maps, notes on points of grammar &c. i have also little ms. books with abundant notes on all these subjects: i usually made a little book when i pursued any subject in a regular way. "on may st mr dobree, the head lecturer, sent for me to say that he appointed me head-lecturer's sizar for the next year. the stipend of this office was _£ _, a sum upon which i set considerable value in my anxiety for pecuniary independence: but it was also gratifying to me as shewing the way in which i was regarded by the college authorities. "on wednesday, may th, , the examination began. i was anxious about the result of the examination, but only in such a degree as to make my conduct perfectly steady and calm, and to prevent me from attempting any extraordinary exertion. "when the classes were published the first class of the freshman's year (alphabetically arranged, as is the custom) stood thus: airy, boileau, childers, drinkwater, field, iliff, malkin, myers, romilly, strutt, tate, winning. it was soon known however that i was first of the class. it was generally expected (and certainly by me) that, considering how great a preponderance the classics were understood, in the known system of the college, to have in determining the order of merit, field would be first. however the number of marks which field obtained was about , and that which i obtained about . no other competitor, i believe, was near us."--in a letter to airy from his college tutor, mr j. d. hustler, there is the following passage: "it is a matter of extreme satisfaction to me that in the late examination you stood not only in the first class but first of the first. i trust that your future exertions and success will be commensurate with this honourable beginning." "of the men whom i have named, drinkwater (bethune) was afterwards legal member of the supreme court of india, field was afterwards rector of reepham, romilly (afterwards lord romilly) became solicitor-general, strutt (afterwards lord belper) became m.p. for derby and first commissioner of railways, tate was afterwards master of richmond endowed school, childers was the father of childers who was subsequently first lord of the admiralty. "i returned to bury immediately. while there, some students (some of them men about to take their b.a. degree at the next january) applied to me to take them as pupils, but i declined. this year of my life enabled me to understand how i stood among men. i returned to cambridge about july th. as a general rule, undergraduates are not allowed to reside in the university during the long vacation. i believe that before i left, after the examination, i had made out that i should be permitted to reside: or i wrote to mr hustler. i applied to mr hustler to be lodged in rooms in college: and was put, first into rooms in bishop's hostel, and subsequently into rooms in the great court. "the first affair that i had in college was one of disappointment by no means deserving the importance which it assumed in my thoughts. i had been entered a sizar, but as the list of foundation sizars was full, my dinners in hall were paid for. some vacancies had arisen: and as these were to be filled up in order of merit, i expected one: and in my desire for pecuniary independence i wished for it very earnestly. however, as in theory all of the first class were equal, and as there were some sizars in it senior in entrance to me, they obtained places first: and i was not actually appointed till after the next scholarship examination (easter ). however a special arrangement was made, allowing me (i forget whether others) to sit at the foundation-sizars' table whenever any of the number was absent: and in consequence i received practically nearly the full benefits. "mr peacock, who was going out for the vacation, allowed me access to his books. i had also (by the assistance of various fellows, who all treated me with great kindness, almost to a degree of respect) command of the university library and trinity library: and spent this long vacation, like several others, very happily indeed. "the only non-mathematical subjects of the next examination were the gospel of st luke, paley's evidences, and paley's moral and political philosophy. thus my time was left more free to mathematics and to general classics than last year. i now began a custom which i maintained for some years. generally i read mathematics in the morning, and classics for lectures in the afternoon: but invariably i began at o'clock in the evening to read with the utmost severity some standard classics (unconnected with the lectures) and at precisely i left off and went to bed. i continued my daily translations into latin prose as before. "on august th, , rosser, a man of my own year, engaged me as private tutor, paying at the usual rate (_£ _ for a part of the vacation, and _£ _ for a term): and immediately afterwards his friend bedingfield did the same. this occupied two hours every day, and i felt that i was now completely earning my own living. i never received a penny from my friends after this time. "i find on my scribbling-paper various words which shew that in reading poisson i was struggling with french words. there are also finite differences and their calculus, figure of the earth (force to the center), various attractions (some evidently referring to maclaurin's), integrals, conic sections, kepler's problem, analytical geometry, d'alembert's theorem, spherical aberration, rotations round three axes (apparently i had been reading euler), floating bodies, evolute of ellipse, newton's treatment of the moon's variation. i attempted to extract something from vince's astronomy on the physical explanation of precession: but in despair of understanding it, and having made out an explanation for myself by the motion round three axes, i put together a little treatise (sept. , ) which with some corrections and additions was afterwards printed in my mathematical tracts. on sept. th i bought woodhouse's physical astronomy, and this was quite an epoch in my mathematical knowledge. first, i was compelled by the process of "changing the independent variable" to examine severely the logic of the differential calculus. secondly, i was now able to enter on the theory of perturbations, which for several years had been the desired land to me. "at the fellowship election of oct. st, sydney walker (among other persons) was elected fellow. he then quitted the rooms in which he had lived (almost the worst in the college), and i immediately took them. they suited me well and i lived very happily in them till i was elected scholar. they are small rooms above the middle staircase on the south side of neville's court. (mr peacock's rooms were on the same staircase.) i had access to the leads on the roof of the building from one of my windows. this was before the new court was built: my best window looked upon the garden of the college butler. "i had brought to cambridge the telescope which i had made at colchester, and about this time i had a stand made by a carpenter at cambridge: and i find repeated observations of jupiter and saturn made in this october term. "other mathematical subjects on my scribbling-paper are: geometrical astronomy, barometers (for elevations), maclaurin's figure of the earth, lagrange's theorem, integrals, differential equations of the second order, particular solutions. in general mathematics i had much discussion with atkinson (who was senior wrangler, january ), and in physics with rosser, who was a friend of sir richard phillips, a vain objector to gravitation. in classics i read aeschylus and herodotus. "on october th i received notice from the head lecturer to declaim in english with winning. (this exercise consists in preparing a controversial essay, learning it by heart, and speaking it in chapel after the thursday evening's service.) on october th we agreed on the subject, "is natural difference to be ascribed to moral or to physical causes?" i taking the latter side. i spoke the declamation (reciting it without missing a word) on october th. on october th i received notice of latin declamation with myers: subject agreed on, "utrum civitati plus utilitatis an incommodi afferant leges quae ad vitas privatorum hominum ordinandas pertinent"; i took the former. the declamation was recited on november , when a curious circumstance occurred. my declamation was rather long: it was the first saturday of the term on which a declamation had been spoken: and it was the day on which arrived the news of the withdrawal of the bill of pains and penalties against queen caroline. (this trial had been going on through the summer, but i knew little about it.) in consequence the impatience of the undergraduates was very great, and there was such an uproar of coughing &c. in the chapel as probably was never known. the master (dr wordsworth, appointed in the beginning of the summer on the death of dr mansell, and to whom i had been indirectly introduced by mrs clarkson) and tutors and deans tried in vain to stop the hubbub. however i went on steadily to the end, not at all frightened. on the monday the master sent for me to make a sort of apology in the name of the authorities, and letters to the tutors were read at the lectures, and on the whole the transaction was nowise disagreeable to me. "on the commemoration day, december th, i received my prize (mitford's greece) as first-class man, after dinner in the college hall. after a short vacation spent at bury and playford i returned to cambridge, walking from bury on jan. nd, . during the next term i find in mathematics partial differential equations, tides, sound, calculus of variations, composition of rotary motions, motion in resisting medium, lhuillier's theorem, brightness of an object as seen through a medium with any possible law of refraction (a good investigation), star-reductions, numerical calculations connected with them, equilibrium of chain under centripetal force (geometrically treated, as an improvement upon whewell's algebraical method), investigation of the magnitude of attractive forces of glass, &c., required to produce refraction. i forget about mathematical lectures; but i have an impression that i regularly attended mr peacock's lectures, and that he always set me some private problems. "i attended mr evans's lectures on st luke: and i find many notes about the history of the jews, cerinthus and various heresies, paley's moral philosophy, paley's evidences, and biblical maps: also speculations about ancient pronunciations. "for a week or more before the annual examination i was perfectly lazy. the classes of my year (junior sophs) were not published till june . it was soon known that i was first with marks, the next being drinkwater with marks. after a short holiday at bury and playford i returned to cambridge on july th, . my daily life went on as usual. i find that in writing latin i began cicero de senectute (retranslating melmoth's translation, and comparing). some time in the long vacation the names of the prizemen for declamations were published: i was disappointed that not one, english or latin, was assigned to me: but it was foolish, for my declamations were rather trumpery. "my former pupil, rosser, came again on august th. on august th dr blomfield (afterwards bishop of london) called, to engage me as tutor to his brother george beecher blomfield, and he commenced attendance on sept. st. with these two pupils i finished at the end of the long vacation: for the next three terms i had one pupil, gibson, a newcastle man, recommended by mr peacock, i believe, as a personal friend (mr peacock being of durham). "the only classical subject appointed for the next examination was the th, th and th books of the odyssey: the mathematical subjects all the applied mathematics and newton. there was to be however the scholarship examination (sizars being allowed to sit for scholarships only in their rd year: and the scholarship being a kind of little fellowship necessary to qualify for being a candidate for the real fellowship). "when the october term began mr hustler, who usually gave lectures in mathematics to his third-year pupils, said to me that it was not worth my while to attend his lectures, and he or mr peacock suggested that drinkwater, myers, and i should attend the questionists' examinations. the questionists are those who are to take the degree of b.a. in the next january: and it was customary, not to give them lectures, but three times a week to examine them by setting mathematical questions, as the best method of preparing for the b.a. examination. accordingly it was arranged that we should attend the said examinations: but when we went the questionists of that year refused to attend. they were reported to be a weak year, and we to be a strong one: and they were disposed to take offence at us on any occasion. from some of the scholars of our year who sat at table with scholars of that year i heard that they distinguished us as 'the impudent year,' 'the annus mirabilis' &c. on this occasion they pretended to believe that the plan of our attendance at the questionists' examinations had been suggested by an undergraduate, and no explanation was of the least use. so the tutors agreed not to press the matter on them: and instead of it, drinkwater, myers, and i went three times a week to mr peacock's rooms, and he set us questions. i think that this system was also continued during the next two terms (ending in june ) or part of them, but i am not certain. "in august i copied out a m.s. on optics, i think from mr whewell: on august th one on the figure of the earth and tides; and at some other time one on the motion of a body round two centers of force; both from mr whewell. on my scribbling paper i find--a problem on the vibrations of a gig as depending on the horse's step (like that of a pendulum whose support is disturbed), maclaurin's attractions, effect of separating the lenses of an achromatic object-glass (suggested by my old telescope), barlow's theory of numbers, and division of the circle into parts, partial differentials, theory of eye-pieces, epicycloids, figure of the earth, time of body in arc of parabola, problem of sound, tides, refraction of lens, including thickness, &c., ivory's paper on equations, achromatism of microscope, capillary attraction, motions of fluids, euler's principal axes, spherical pendulum, equation b²(d²y/dx²)=(d²y/dt²), barometer, lunar theory well worked out, ordinary differential equations, calculus of variations, interpolations like laplace's for comets, kepler's theorem. in september i had my old telescope mounted on a short tripod stand, and made experiments on its adjustments. i was possessed of white's ephemeris, and i find observations of jupiter and saturn in october. i planned an engine for describing ellipses by the polar equation a/( + e cos theta) and tried to make a micrometer with silk threads converging to a point. mr cubitt called on oct. and nov. ; he was engaged in erecting a treadmill at cambridge gaol, and had some thoughts of sending plans for the cambridge observatory, the erection of which was then proposed. on nov. i find that i had received from cubitt a nautical almanac, the first that i had. on dec. i made some experiments with drinkwater: i think it was whirling a glass containing oil on water. in classics i was chiefly engaged upon thucydides and homer. on october th i had a letter from charles musgrave, introducing challis, who succeeded me in the cambridge observatory in . "at this time my poor afflicted father was suffering much from a severe form of rheumatism or pain in the legs which sometimes prevented him from going to bed for weeks together. "on the commemoration day, dec. th, i received my prize as first-class man in hall again. the next day i walked to bury, and passed the winter vacation there and at playford. "i returned to cambridge on jan. th, . on feb. th i kept my first act, with great compliments from the moderator, and with a most unusually large attendance of auditors. these disputations on mathematics, in latin, are now discontinued. on march th i kept a first opponency against sandys. about this time i received buckle, a trinity man of my own year, who was generally supposed to come next after drinkwater, as pupil. on my sheets i find integrals and differential equations of every kind, astronomical corrections (of which i prepared a book), chances, englefield's comets, investigation of the brightness within a rainbow, proof of clairaut's theorem in one case, metacentres, change of independent variable applied to a complicated case, generating functions, principal axes. on apr. th i intended to write an account of my eye: i was then tormented with a double image, i suppose from some disease of the stomach: and on may th i find by a drawing of the appearance of a lamp that the disease of my eye continued. "on feb. th i gave mr peacock a paper on the alteration of the focal length of a telescope as directed with or against the earth's orbital motion (on the theory of emissions) which was written out for reading to the cambridge philosophical society on feb. th and th. [this society i think was then about a year old.] on feb. my ms. on precession, solar inequality, and nutation, was made complete. "the important examination for scholarships was now approaching. as i have said, this one opportunity only was given to sizars (pensioners having always two opportunities and sometimes three), and it is necessary to be a scholar in order to be competent to be a candidate for a fellowship. on apr. th i addressed my formal latin letter to the seniors. there were vacancies and candidates. the election took place on apr. th, . i was by much the first (which i hardly expected) and was complimented by the master and others. wrote the formal letter of thanks as usual. i was now entitled to claim better rooms, and i took the rooms on the ground floor on the east side of the queen's gate of the great court. even now i think of my quiet residence in the little rooms above the staircase in neville's court with great pleasure. i took possession of my new rooms on may th. "the annual examination began on may th. the classes were published on june th, when my name was separated from the rest by two lines. it was understood that the second man was drinkwater, and that my number of marks was very nearly double of his. having at this time been disappointed of a proposed walking excursion into derbyshire with a college friend, who failed me at the last moment, i walked to bury and spent a short holiday there and at playford. "i returned to cambridge on july th, . i was steadily busy during this long vacation, but by no means oppressively so: indeed my time passed very happily. the scholars' table is the only one in college at which the regular possessors of the table are sure never to see a stranger, and thus a sort of family intimacy grows up among the scholars. moreover the scholars feel themselves to be a privileged class 'on the foundation,' and this feeling gives them a sort of conceited happiness. it was the duty of scholars by turns to read grace after the fellows' dinner and supper, and at this time ( ) i know it by heart. they also read the lessons in chapel on week days: but as there was no daily chapel-service during the summer vacation, i had not much of this. in the intimacy of which i speak i became much acquainted with drinkwater, buckle, rothman, and sutcliffe: and we formed a knot at the table (first the undergraduate scholars' table, and afterwards the bachelor scholars' table) for several years. during this vacation i had for pupils buckle and gibson. "i wrote my daily latin as usual, beginning with the retranslation of cicero's epistles, but i interrupted it from sept. th to feb. th. i believe it was in this vacation, or in the october term, that i began every evening to read thucydides very carefully, as my notes are marked and . on august i find that i was reading ovid's fasti. "in mathematics i find the equation x + y = a, x^q + y^q = b, caustics, calculus of variations, partial differentials, aberration of light, motions of comets, various optical constructions computed with spherical aberrations, particular solutions, mechanics of solid bodies, attractions of shells, chances, ivory's attraction-theorem, lunar theory (algebraical), degrees across meridian, theoretical refraction, newton's rd book, investigation of the tides in a shallow equatoreal canal, from which i found that there would be low-water under the moon, metacentres, rotation of a solid body round three axes, attractions of spheroids of variable density, finite differences, and complete figure of the earth. there is also a good deal of investigation of a mathematical nature not connected with college studies, as musical chords, organ-pipes, sketch for a computing machine (suggested by the publications relating to babbage's), sketch of machine for solving equations. in august there is a plan of a ms. on the differential calculus, which it appears i wrote then: one on the figure of the earth written about august th; one on tides, sept. th; one on newton's principia with algebraical additions, nov. st. on sept. th and th there are lunar distances observed with rothman's sextant and completely worked out; for these i prepared a printed skeleton form, i believe my first. on december th there are references to books on geology (conybeare and phillips, and parkinson) which i was beginning to study. on july th, being the day on which i completed my st year, i carefully did nothing. "another subject partly occupied my thoughts, which, though not (with reference to practical science) very wise, yet gave me some cambridge celebrity. in july i had (as before mentioned) sketched a plan for constructing reflecting telescopes with silvered glass, and had shewn it afterwards to mr peacock. i now completed the theory of this construction by correcting the aberrations, spherical as well as chromatic. on july th, , i drew up a paper about it for mr peacock. he approved it much, and in some way communicated it to mr (afterwards sir john) herschel. i was soon after introduced to herschel at a breakfast with mr peacock: and he approved of the scheme generally. on august th i drew up a complete mathematical paper for the cambridge philosophical society, which i entrusted to mr peacock. the aberrations, both spherical and chromatic, are here worked out very well. on nov. th it was read at the meeting of the philosophical society, and was afterwards printed in their transactions: this was my first printed memoir. before this time however i had arranged to try the scheme practically. mr peacock had engaged to bear the expense, but i had no occasion to ask him. partly (i think) through drinkwater, i communicated with an optician named bancks, in the strand, who constructed the optical part. i subsequently tried my telescope, but it would not do. the fault, as i had not and have not the smallest doubt, depends in some way on the crystallization of the mercury silvering. it must have been about this time that i was introduced to mr (afterwards sir james) south, at a party at mr peacock's rooms. he advised me to write to tulley, a well-known practical optician, who made me some new reflectors, &c. (so that i had two specimens, one gregorian, the other cassegrainian). however the thing failed practically, and i was too busy ever after to try it again. "during the october term i had no pupils. i kept my second act on nov. (opponents hamilton, rusby, field), and an opponency against jeffries on nov. . i attended the questionists' examinations. i seem to have lived a very comfortable idle life. the commemoration day was dec. th, when i received a prize, and the next day i walked to bury. on jan. th, , i returned to cambridge, and until the b.a. examination i read novels and played cards more than at any other time in college. "on thursday, jan. th, , the preliminary classes, for arrangement of details of the b.a. examination, were published. the first class, airy, drinkwater, jeffries, mason. as far as i remember, the rule was then, that on certain days the classes were grouped (in regard to identity of questions given to each group) thus: st, { nd/ rd}, { th/ th} &c., and on certain other days thus: { st/ nd}, { rd/ th}, &c. on saturday, jan. th, i paid fees. on monday, jan. th, the proceedings of examination began by a breakfast in the combination room. after this, gibson gave me breakfast every day, and buckle gave me and some others a glass of wine after dinner. the hours were sharp, the season a cold one, and no fire was allowed in the senate house where the examination was carried on (my place was in the east gallery), and altogether it was a severe time. "the course of examination was as follows: "monday, jan. th. to , printed paper of questions by mr hind (moderator); half-past to , questions given orally; to , ditto; to , paper of problems at mr higman's rooms. "tuesday, jan. th. to , higman's paper; half-past to , questions given orally; to , ditto; to , paper of problems in sidney college hall. "wednesday, jan. th. questions given orally to and to , with paper of questions on paley and locke (one question only in each was answered). "thursday, jan. th. we went in at and , but there seems to have been little serious examination. "friday, jan. . on this day the brackets or classes as resulting from the examination were published, st bracket airy, nd bracket jeffries, rd bracket drinkwater, fisher, foley, mason, myers. "on saturday, jan. th, the degrees were conferred in the usual way. it had been arranged that my brother and sister should come to see me take my degree of b.a., and i had asked gibson to conduct them to the senate house gallery: but mr hawkes (a trinity fellow) found them and stationed them at the upper end of the senate house. after the preliminary arrangements of papers at the vice-chancellor's table, i, as senior wrangler, was led up first to receive the degree, and rarely has the senate house rung with such applause as then filled it. for many minutes, after i was brought in front of the vice-chancellor, it was impossible to proceed with the ceremony on account of the uproar. i gave notice to the smith's prize electors of my intention to 'sit' for that prize, and dined at rothman's rooms with drinkwater, buckle, and others. on monday, jan. th, i was examined by professor woodhouse, for smith's prize, from to . i think that the only competitor was jeffries. on tuesday i was examined by prof. turton, to , and on wednesday by prof. lax, to . on thursday, jan. rd, i went to bury by coach, on one of the coldest evenings that i ever felt. "mr peacock had once recommended me to sit for the chancellor's medal (classical prize). but he now seemed to be cool in his advice, and i laid aside all thought of it." * * * * * it seems not out of place to insert here a copy of some "cambridge reminiscences" written by airy, which will serve to explain the acts and opponencies referred to in the previous narrative, and other matters. the acts. the examination for b.a. degrees was preceded, in my time, by keeping two acts, in the schools under the university library: the second of them in the october term immediately before the examination; the first (i think) in the october term of the preceding year. these acts were reliques of the disputations of the middle ages, which probably held a very important place in the discipline of the university. (there seems to be something like them in some of the continental universities.) the presiding authority was one of the moderators. i apprehend that the word "moderator" signified "president," in which sense it is still used in the kirk of scotland; and that it was peculiarly applied to the presidency of the disputations, the most important educational arrangement in the university. the moderator sent a summons to the "respondent" to submit three subjects for argument, and to prepare to defend them on a given day: he also named three opponents. this and all the following proceedings were conducted in latin. for my act of , nov. , i submitted the following subjects: "recte statuit newtonus in principiis suis mathematicis, libro primo, sectione undecimâ." "recte statuit woodius de iride." "recte statuit paleius de obligationibus." the opponents named to attack these assertions were hamilton of st john's, rusby of st catharine's, field of trinity. it was customary for the opponents to meet at tea at the rooms of the senior opponent, in order to discuss and arrange their arguments; the respondent was also invited, but he was warned that he must depart as soon as tea would be finished: then the three opponents proceeded with their occupation. as i have acted in both capacities, i am able to say that the matter was transacted in an earnest and business-like way. indeed in the time preceding my own (i know not whether in my own time) the assistance of a private tutor was frequently engaged, and i remember hearing a senior m.a. remark that my college tutor (james d. hustler) was the best crammer for an act in the university. at the appointed time, the parties met in the schools: the respondent first read a latin thesis on any subject (i think i took some metaphysical subject), but nobody paid any attention to it: then the respondent read his first dogma, and the first opponent produced an argument against it, in latin. after this there were repeated replies and rejoinders, all in vivâ voce latin, the moderator sometimes interposing a remark in latin. when he considered that one argument was disposed of, he called for another by the words "probes aliter." the arguments were sometimes shaped with considerable ingenuity, and required a clear head in the respondent. when all was finished, the moderator made a complimentary remark to the respondent and one to the first opponent (i forget whether to the second and third). in my respondency of , november , the compliment was, "quaestiones tuas summo ingenio et acumine defendisti, et in rebus mathematicis scientiam planè mirabilem ostendisti." in an opponency (i forget when) the compliment was, "magno ingenio argumenta tua et construxisti et defendisti." the acts of the high men excited much interest among the students. at my acts the room was crowded with undergraduates. i imagine that, at a time somewhat distant, the maintenance of the acts was the only regulation by which the university acted on the studies of the place. when the acts had been properly kept, license was given to the father of the college to present the undergraduate to the vice-chancellor, who then solemnly admitted him "ad respondendum quaestioni." there is no appearance of collective examination before this presentation: what the "quaestio" might be, i do not know. still the undergraduate was not b.a. the quaestio however was finished and approved before the day of a certain congregation, and then the undergraduate was declared to be "actualiter in artibus baccalaureum." probably these regulations were found to be insufficient for the control of education, and the january examination was instituted. i conjecture this to have been at or shortly before the date of the earliest triposes recorded in the cambridge calendar, . the increasing importance of the january examination naturally diminished the value of the acts in the eyes of the undergraduates; and, a few years after my m.a. degree, it was found that the opponents met, not for the purpose of concealing their arguments from the respondent, but for the purpose of revealing them to him. this led to the entire suppression of the system. the most active man in this suppression was mr whewell: its date must have been near to . the shape in which the arguments were delivered by an opponent, reading from a written paper, was, "si (quoting something from the respondent's challenge), &c., &c. cadit quaestio; sed (citing something else bearing on the subject of discussion), valet consequentia; ergo (combining these to prove some inaccuracy in the respondent's challenge), valent consequentia et argumentum." nobody pretended to understand these mystical terminations. apparently the original idea was that several acts should be kept by each undergraduate; for, to keep up the number (as it seemed), each student had to gabble through a ridiculous form "si quaestiones tuae falsae sint, cadit quaestio:--sed quaestiones tuae falsae sunt, ergo valent consequentia et argumentum." i have forgotten time and place when this was uttered. the senate-house examination. the questionists, as the undergraduates preparing for b.a. were called in the october term, were considered as a separate body; collected at a separate table in hall, attending no lectures, but invited to attend a system of trial examinations conducted by one of the tutors or assistant-tutors. from the acts, from the annual college examinations, and (i suppose) from enquiries in the separate colleges, the moderators acquired a general idea of the relative merits of the candidates for honours. guided by this, the candidates were divided into six classes. the moderators and assistant examiners were provided each with a set of questions in manuscript (no printed papers were used for honours in the senate house; in regard to the [greek: hoi polloi] i cannot say). on the monday on which the examination began, the father of the college received all the questionists (i believe), at any rate all the candidates for honours, at breakfast in the combination room at o'clock, and marched them to the senate house. my place with other honour-men was in the east gallery. there one examiner took charge of the st and nd classes united, another examiner took the rd and th classes united, and a third took the th and th united. on tuesday, one examiner took the st class alone, a second took the nd and rd classes united, a third took the th and th classes united, and a fourth took the th class alone. on wednesday, thursday, and friday the changes were similar. and, in all, the questioning was thus conducted. the examiner read from his manuscript the first question. those who could answer it proceeded to write out their answers, and as soon as one had finished he gave the word "done"; then the examiner read out his second question, repeating it when necessary for the understanding by those who took it up more lately. and so on. i think that the same process was repeated in the afternoon; but i do not remember precisely. in this manner the examination was conducted through five days (monday to friday) with no interruption except on friday afternoon. it was principally, perhaps entirely, bookwork. but on two _evenings_ there were printed papers of problems: and the examination in these was conducted just as in the printed papers of the present day: but in the private college rooms of the moderators. and there, wine and other refreshments were offered to the examinees. how this singular custom began, i know not. the order of merit was worked out on friday afternoon and evening, and was in some measure known through the university late in the evening. i remember mr peacock coming to a party of examinees and giving information on several places. i do not remember his mentioning mine (though undoubtedly he did) but i distinctly remember his giving the wooden spoon. on the saturday morning at o'clock the manuscript list was nailed to the door of the senate-house. the form of further proceedings in the presentation for degree (ad respondendum quaestioni) i imagine has not been much altered. the kneeling before the vice-chancellor and placing hands in the vice-chancellor's hands were those of the old form of doing homage. the form of examination which i have described was complicated and perhaps troublesome, but i believe that it was very efficient, possibly more so than the modern form (established i suppose at the same time as the abolition of the acts). the proportion of questions now answered to the whole number set is ridiculously small, and no accurate idea of relative merit can be formed from them. the college hall. when i went up in , and for several years later, the dinner was at / past . there was no supplementary dinner for special demands. boat-clubs i think were not invented, even in a plain social way, till about or ; and not in connection with the college till some years later. some of the senior fellows spoke of the time when dinner was at , and regretted the change. there was supper in hall at o'clock: i have known it to be attended by a few undergraduates when tired by examinations or by evening walks; and there were always some seniors at the upper table: i have occasionally joined them, and have had some very interesting conversations. the supper was cold, but hot additions were made when required. one little arrangement amused me, as shewing the ecclesiastical character of the college. the fasts of the church were to be strictly kept, and there was to be no dinner in hall. it was thus arranged. the evening chapel service, which was usually at - / (i think), was held at ; and at the ordinary full meal was served in hall, but as it followed the chapel attendance it was held to be supper; and there was no subsequent meal. there were no chairs whatever in hall, except the single chair of the vice-master at the head of the table on the dais and that of the senior dean at the table next the east wall. all others sat on benches. and i have heard allusions to a ludicrous difficulty which occurred when some princesses (of the royal family) dined in the hall, and it was a great puzzle how to get them to the right side of the benches. the sizars dined after all the rest; their dinner usually began soon after . for the non-foundationists a separate dinner was provided, as for pensioners. but for the foundationists, the remains of the fellows' dinner were brought down; and i think that this provision was generally preferred to the other. the dishes at all the tables of undergraduates were of pewter, till a certain day when they were changed for porcelain. i cannot remember whether this was at the time when they became questionists (in the october term), or at the time when they were declared "actualiter esse in artibus baccalaureos" (in the lent term). up to the questionist time the undergraduate scholars had no mixture whatever; they were the only pure table in the hall: and i looked on this as a matter very valuable for the ultimate state of the college society. but in the october term, those who were to proceed to b.a. were drafted into the mixed body of questionists: and they greatly disliked the change. they continued so till the lent term, when they were formally invited by the bachelor scholars to join the upper table. mathematical subjects of study and examination. in the october term , the only books on pure mathematics were:--euclid generally, algebra by dr wood (formerly tutor, but in master, of st john's college), vince's fluxions and dealtry's fluxions, woodhouse's and other trigonometries. not a whisper passed through the university generally on the subject of differential calculus; although some papers (subsequently much valued) on that subject had been written by mr woodhouse, fellow of caius college; but their style was repulsive, and they never took hold of the university. whewell's mechanics ( ) contains a few and easy applications of the differential calculus. the books on applied mathematics were wood's mechanics, whewell's mechanics, wood's optics, vince's hydrostatics, vince's astronomy, woodhouse's plane astronomy (perhaps rather later), the first book of newton's principia: i do not remember any others. these works were undoubtedly able; and for the great proportion of university students going into active life, i do not conceal my opinion that books constructed on the principles of those which i have cited were more useful than those exclusively founded on the more modern system. for those students who aimed at the mastery of results more difficult and (in the intellectual sense) more important, the older books were quite insufficient. more aspiring students read, and generally with much care, several parts of newton's principia, book i., and also book iii. (perhaps the noblest example of geometrical form of cosmical theory that the world has seen). i remember some questions from book iii. proposed in the senate-house examination . in the october term , i went up to the university. the works of wood and vince, which i have mentioned, still occupied the lecture-rooms. but a great change was in preparation for the university course of mathematics. during the great continental war, the intercourse between men of science in england and in france had been most insignificant. but in the autumn of , three members of the senate (john herschel, george peacock, and charles babbage) had entered into the mathematical society of paris, and brought away some of the works on pure mathematics (especially those of lacroix) and on mechanics (principally poisson's). in they made a translation of lacroix's differential calculus; and they prepared a volume of examples of the differential and integral calculus. these were extensively studied: but the form of the college examinations or the university examinations was not, i think, influenced by them in the winter - or the two following terms. but in the winter - peacock was one of the moderators; and in the senate-house examination, january , he boldly proposed a paper of important questions entirely in the differential calculus. this was considered as establishing the new system in the university. in january , i think the two systems were mingled. though i was myself subject to that examination, i grieve to say that i have forgotten much of the details, except that i well remember that some of the questions referred to newton, book iii. on the lunar theory. to these i have already alluded. no other work occurs to me as worthy of mention, except woodhouse's lunar theory, entirely founded on the differential calculus. the style of this book was not attractive, and it was very little read. chapter iii. at trinity college, cambridge, from his taking his b.a. degree to his taking charge of the cambridge observatory as plumian professor. from january th, , to march th, . "on jan. th, , i returned to cambridge. i had already heard that i had gained the st smith's prize, and one of the first notifications to me on my return was that the walker's good-conduct prize of _£ _ was awarded to me. "i remember that my return was not very pleasant, for our table in hall was half occupied by a set of irregular men who had lost terms and were obliged to reside somewhat longer in order to receive the b.a. degree. but at the time of my completing the b.a. degree (which is not till some weeks after the examination and admission) i with the other complete bachelors was duly invited to the table of the b.a. scholars, and that annoyance ended. "the liberation from undergraduate study left me at liberty generally to pursue my own course (except so far as it was influenced by the preparation for fellowship examination), and also left me at liberty to earn more money, in the way usual with the graduates, by taking undergraduate pupils. mr peacock recommended me to take only four, which occupied me four hours every day, and for each of them i received guineas each term. my first pupils, for the lent and easter terms, were williamson (afterwards head master of westminster school), james parker (afterwards q.c. and vice-chancellor), bissett, and clinton of caius. to all these i had been engaged before taking my b.a. degree. "i kept up classical subjects. i have a set of notes on the [greek: ploutos] and [greek: nephelai] of aristophanes, finished on mar. th, , and i began my daily writing of latin as usual on feb. th. in mathematics i worked very hard at lunar and planetary theories. i have two ms. books of lunar theory to the th order of small quantities, which however answered no purpose except that of making me perfectly familiar with that subject. i worked well, upon my quires, the figure of saturn supposed homogeneous as affected by the attraction of his ring, and the figure of the earth as heterogeneous, and the calculus of variations. i think it was now that i wrote a ms. on constrained motion. "on mar. th, , i was elected fellow of the cambridge philosophical society. on may th a cast of my head was taken for dr elliotson, an active phrenologist, by deville, a tradesman in the strand. "i had long thought that i should like to visit scotland, and on my once saying so to my mother, she (who had a most kindly recollection of alnwick) said in a few words that she thought i could not do better. i had therefore for some time past fully determined that as soon as i had sufficient spare time and money enough i would go to scotland. the interval between the end of easter term and the usual beginning with pupils in the long vacation offered sufficient time, and i had now earned a little money, and i therefore determined to go, and invited my sister to accompany me. i had no private introductions, except one from james parker to mr reach, a writer of inverness: some which drinkwater sent being too late. on may th we went by coach to stamford; thence by pontefract and oulton to york, where i saw the cathedral, which _then_ disappointed me, but i suppose that we were tired with the night journey. then by newcastle to alnwick, where we stopped for the day to see my birthplace. on may th to edinburgh. on this journey i remember well the stone walls between the fields, the place (in yorkshire) where for the first time in my life i saw rock, the hambleton, kyloe, cheviot and pentland hills, arthur's seat, but still more strikingly the revolving inch keith light. at edinburgh i hired a horse and gig for our journey in scotland, and we drove by queensferry to kinross (where for the first time in my life i saw clouds on the hills, viz. on the lomond hills), and so to perth. thence by dunkeld and killicrankie to blair athol (the dreariness of the drumochter pass made a strong impression on me), and by aviemore (where i saw snow on the mountains) to inverness. here we received much kindness and attention from mr reach, and after visiting the falls of foyers and other sights we went to fort augustus and fort william. we ascended ben nevis, on which there was a great deal of snow, and visited the vitrified fort in glen nevis. then by inverary to tarbet, and ascended ben lomond, from whence we had a magnificent view. we then passed by loch achray to glasgow, where we found james parker's brother (his father, of the house of macinroy and parker, being a wealthy merchant of glasgow). on june th to mr parker's house at blochairn, near glasgow (on this day i heard dr chalmers preach), and on the th went with the family by steamer (the first that i had seen) to fairly, near largs. i returned the gig to edinburgh, visited arran and bute, and we then went by coach to carlisle, and by penrith to keswick (by the old road: never shall i forget the beauty of the approach to keswick). after visiting ambleside and kendal we returned to cambridge by way of leeds, and posted to bury on the th june. the expense of this expedition was about _£ _. it opened a completely new world to me. "i had little time to rest at bury. in the preceding term drinkwater, buckle, and myself, had engaged to go somewhere into the country with pupils during the long vacation (as was customary with cambridge men). buckle however changed his mind. drinkwater went to look for a place, fixed on swansea, and engaged a house (called the cambrian hotel, kept by a captain jenkins). on the morning of july nd i left bury for london and by mail coach to bristol. on the morning of july rd by steamer to swansea, and arrived late at night. i had then five pupils: parker, harman lewis (afterwards professor in king's college, london), pierce morton, gibson, and guest of caius (afterwards master of the college). drinkwater had four, viz. two malkins (from bury), elphinstone (afterwards m.p.), and farish (son of professor farish). we lived a hard-working strange life. my pupils began with me at six in the morning: i was myself reading busily. we lived completely _en famille_, with two men-servants besides the house establishment. one of our first acts was to order a four-oared boat to be built, fitted with a lug-sail: she was called the granta of swansea. in the meantime we made sea excursions with boats borrowed from ships in the port. on july rd, with a borrowed boat, we went out when the sea was high, but soon found our boat unmanageable, and at last got into a place where the sea was breaking heavily over a shoal, and the two of the crew who were nearest to me (a. malkin and lewis), one on each side, were carried out: they were good swimmers and we recovered them, though with some trouble: the breaker had passed quite over my head: we gained the shore and the boat was taken home by land. when our own boat was finished, we had some most picturesque adventures at the mumbles, aberavon, caswell bay, ilfracombe, and tenby. from all this i learnt navigation pretty well. the mixture of hard study and open-air exertion seemed to affect the health of several of us (i was one): we were covered with painful boils. "my latin-writing began again on july th: i have notes on demosthenes, lucretius, and greek history. in mathematics i find chances, figure of the earth with variable density, differential equations, partial differentials, sketch for an instrument for shewing refraction, and optical instruments with effects of chromatic aberration. in august there occurred an absurd quarrel between the fellows of trinity and the undergraduates, on the occasion of commencing the building of king's court, when the undergraduates were not invited to wine, and absented themselves from the hall. "there were vacant this year ( ) five fellowships in trinity college. in general, the b.a.'s of the first year are not allowed to sit for fellowships: but this year it was thought so probable that permission would be given, that on sept. nd mr higman, then appointed as tutor to a third 'side' of the college, wrote to me to engage me as assistant mathematical tutor in the event of my being elected a fellow on oct. st, and i provisionally engaged myself. about the same time i had written to mr peacock, who recommended me to sit, and to mr whewell, who after consultation with the master (dr wordsworth), discouraged it. as there was no absolute prohibition, i left swansea on sept. th (before my engagement to my pupils was quite finished) and returned to cambridge by gloucester, oxford, and london. i gave in my name at the butteries as candidate for fellowship, but was informed in a day or two that i should not be allowed to sit. on sept. th i walked to bury. "i walked back to cambridge on oct. th, . during this october term i had four pupils: neate, cankrein, turner (afterwards nd wrangler and treasurer of guy's hospital), and william hervey (son of the marquis of bristol). in the lent term i had four (neate, cankrein, turner, clinton). in the easter term i had three (neate, cankrein, turner). "my daily writing of latin commenced on oct. th. in november i began re-reading sophocles with my usual care. in mathematics i find investigations of motion in a resisting medium, form of saturn, draft of a paper about an instrument for exhibiting the fundamental law of refraction (read at the philosophical society by mr peacock on nov. th, ), optics, solid geometry, figure of the earth with variable density, and much about attractions. i also in this term wrote a ms. on the calculus of variations, and one on wood's algebra, nd and th parts. i have also notes of the temperature of mines in cornwall, something on the light of oil-gas, and reminiscences of swansea in a view of oswick bay. in november i attended professor sedgwick's geological lectures. "at some time in this term i had a letter from mr south (to whom i suppose i had written) regarding the difficulty of my telescope: he was intimately acquainted with tulley, and i suppose that thus the matter had become more fully known to him. he then enquired if i could visit him in the winter vacation. i accordingly went from bury, and was received by him at his house in blackman street for a week or more with great kindness. he introduced me to sir humphrey davy and many other london savans, and shewed me many london sights and the greenwich observatory. i also had a little practice with his own instruments. he was then on intimate terms with mr herschel (afterwards sir john herschel), then living in london, who came occasionally to observe double stars. this was the first time that i saw practical astronomy. it seems that i borrowed his mountain barometer. in the lent term i wrote to him regarding the deduction of the parallax of mars, from a comparison of the relative positions of mars and leonis, as observed by him and by rumker at paramatta. my working is on loose papers. i see that i have worked out perfectly the interpolations, the effects of uncertainty of longitude, &c., but i do not see whether i have a final result. "in jan. , at playford, i was working on the effects of separating the two lenses of an object-glass, and on the kind of eye-piece which would be necessary: also on spherical aberrations and saturn's figure. on my quires at cambridge i was working on the effects of separating the object-glass lenses, with the view of correcting the secondary spectrum: and on jan. st i received some numbers (indices of refraction) from mr herschel, and reference to fraunhofer's numbers. "about this time it was contemplated to add to the royal observatory of greenwich two assistants of superior education. whether this scheme was entertained by the admiralty, the board of longitude, or the royal society, i do not know. somehow (i think through mr peacock) a message from mr herschel was conveyed to me, acquainting me of this, and suggesting that i should be an excellent person for the principal place. to procure information, i went to london on saturday, feb. th, sleeping at mr south's, to be present at one of sir humphrey davy's saturday evening soirées (they were then held every saturday), and to enquire of sir h. davy and dr young. when i found that succession to the post of astronomer royal was not considered as distinctly a consequence of it, i took it coolly, and returned the next night. the whole proposal came to nothing. "at this time i was engaged upon differential equations, mountain barometer problem and determination of the height of the gogmagogs and several other points, investigations connected with laplace's calculus, spherical aberration in different planes, geology (especially regarding derbyshire, which i proposed to visit), and much of optics. i wrote a draft of my paper on the figure of saturn, and on mar. th, , it was read at the philosophical society under the title of 'on the figure assumed by a fluid homogeneous mass, whose particles are acted on by their mutual attraction, and by small extraneous forces,' and is printed in their memoirs. i also wrote a draft of my paper on achromatic eye-pieces, and on may th, , it was read at the philosophical society under the title of 'on the principles and construction of the achromatic eye-pieces of telescopes, and on the achromatism of microscopes,' including also the effects of separating the lenses of the object-glass. it is printed in their memoirs. "amongst miscellaneous matters i find that on mar. nd of this year i began regularly making extracts from the books of the book society, a practice which i continued to march . on mar. th, a very rainy day, i walked to bury to attend the funeral of my uncle william biddell, near diss, and on mar. th i walked back in rain and snow. on feb. th i dined with cubitt in cambridge. on may st i gave a certificate to rogers (the assistant in crosse's school, and my instructor in mathematics), which my mother amplified much, and which i believe procured his election as master of walsall school. on june rd i went to bury. the speeches at bury school, which i wished to attend, took place next day." at this point of his autobiography the writer continues, "now came one of the most important occurrences in my life." the important event in question was his acquaintance with richarda smith, the lady who afterwards became his wife. the courtship was a long one, and in the autobiography there are various passages relating to it, all written in the most natural and unaffected manner, but of somewhat too private a nature for publication. it will therefore be convenient to digress from the straight path of the narrative in order to insert a short memoir of the lady who was destined to influence his life and happiness in a most important degree. richarda smith was the eldest daughter of the rev. richard smith, who had been a fellow of trinity college, cambridge, but was at this time private chaplain to the duke of devonshire, and held the small living of edensor, near chatsworth, in derbyshire. he had a family of two sons and seven daughters, whom he had brought up and educated very carefully. several of his daughters were remarkable both for their beauty and accomplishments. richarda smith was now in her th year, and the writer of the autobiography records that "at matlock we received great attention from mr chenery: in speaking of mr smith i remember his saying that mr smith had a daughter whom the duke of devonshire declared to be the most beautiful girl he ever saw." this was before he had made the acquaintance of the family. airy was at this time on a walking tour in derbyshire with his brother william, and they were received at edensor by mr smith, to whom he had letters of introduction. he seems to have fallen in love with miss smith "at first sight," and within two days of first seeing her he made her an offer of marriage. neither his means nor his prospects at that time permitted the least idea of an immediate marriage, and mr smith would not hear of any engagement. but he never had the least doubt as to the wisdom of the choice that he had made: he worked steadily on, winning fame and position, and recommending his suit from time to time to miss smith as opportunity offered, and finally married her, nearly six years after his first proposal. his constancy had its reward, for he gained a most charming and affectionate wife. as he records at the time of his marriage, "my wife was aged between and , but she scarcely appeared more than or . her beauty and accomplishments, her skill and fidelity in sketching, and above all her exquisite singing of ballads, made a great sensation in cambridge." their married life lasted years, but the last six years were saddened by the partial paralysis and serious illness of lady airy. the entire correspondence between them was most carefully preserved, and is a record of a most happy union. the letters were written during his numerous journeys and excursions on business or pleasure, and it is evident that his thoughts were with her from the moment of their parting. every opportunity of writing was seized with an energy and avidity that shewed how much his heart was in the correspondence. nothing was too trivial or too important to communicate to his wife, whether relating to family or business matters. the letters on both sides are always full of affection and sympathy, and are written in that spirit of confidence which arises from a deep sense of the value and necessity of mutual support in the troubles of life. and with his active and varied employments and his numerous family there was no lack of troubles. they were both of them simple-minded, sensible, and practical people, and were very grateful for such comforts and advantages as they were able to command, but for nothing in comparison with their deep respect and affection for one another. both by natural ability and education she was well qualified to enter into the pursuits of her husband, and in many cases to assist him. she always welcomed her husband's friends, and by her skill and attractive courtesy kept them well together. she was an admirable letter-writer, and in the midst of her numerous domestic distractions always found time for the duties of correspondence. in conversation she was very attractive, not so much from the wit or brilliancy of her remarks as from the brightness and interest with which she entered into the topics under discussion, and from the unfailing grace and courtesy with which she attended to the views of others. this was especially recognized by the foreign astronomers and men of science who from time to time stayed as guests at the observatory and to whom she acted as hostess. although she was not an accomplished linguist yet she was well able to express herself in french and german, and her natural good sense and kindliness placed her guests at their ease, and made them feel themselves (as indeed they were) welcomed and at home. her father, the rev. richard smith, was a man of most cultivated mind, and of the highest principles, with a keen enjoyment of good society, which the confidence and friendship of his patron the duke of devonshire amply secured to him, both at chatsworth and in london. he had a deep attachment to his alma mater of cambridge, and though not himself a mathematician he had a great respect for the science of mathematics and for eminent mathematicians. during the long courtship already related mr smith conceived the highest respect for airy's character, as well as for his great repute and attainments, and expressed his lively satisfaction at his daughter's marriage. thus on january th, , he wrote to his intended son-in-law as follows: "i have little else to say to you than that i continue with heartfelt satisfaction to reflect on the important change about to take place in my dear daughter's situation. a father must not allow himself to dilate on such a subject: of course i feel confident that you will have no reason to repent the irrevocable step you have taken, but from the manner in which richarda has been brought up, you will find such a helpmate in her as a man of sense and affection would wish to have, and that she is well prepared to meet the duties and trials (for such must be met with) of domestic life with a firm and cultivated mind, and the warm feelings of a kind heart. her habits are such as by no means to lead her to expensive wishes, nor will you i trust ever find it necessary to neglect those studies and pursuits upon which your reputation and subsistence are chiefly founded, to seek for idle amusements for your companion. i must indulge no further in speaking of her, and have only at present to add that i commit in full confidence into your hands the guardianship of my daughter's happiness." and on april th, , shortly after their marriage, he wrote to his daughter thus: "if thinking of you could supply your place amongst us you would have been with us unceasingly, for we have all of us made you the principal object of our thoughts and our talk since you left us, and i travelled with you all your journey to your present delightful home. we had all but one feeling of the purest pleasure in the prospect of the true domestic comfort to which we fully believe you to be now gone, and we rejoice that all your endearing qualities will now be employed to promote the happiness of one whom we think so worthy of them as your dear husband, who has left us in the best opinion of his good heart, as well as his enlightened and sound understanding. his late stay with us has endeared him to us all. never did man enter into the married state from more honourable motives, or from a heart more truly seeking the genuine happiness of that state than mr airy, and he will, i trust, find his reward in you from all that a good wife can render to the best of husbands, and his happiness be reflected on yourself." it would be difficult to find letters of more genuine feeling and satisfaction, or more eloquently expressed, than these. the narrative of the autobiography will now be resumed. "i had been disappointed two years before of an expedition to derbyshire. i had wished still to make it, and my brother wished to go: and we determined to make it this year ( ). we were prepared with walking dresses and knapsacks. i had well considered every detail of our route, and was well provided with letters of introduction, including one to the rev. r. smith of edensor. on june th we started by coach to newmarket and walked through the fens by ramsay to peterborough. then by stamford and ketton quarries to leicester and derby. here we were recognized by a mr calvert, who had seen me take my degree, and he invited us to breakfast, and employed himself in shewing us several manufactories, &c. to which we had been denied access when presenting ourselves unsupported. we then went to belper with an introduction from mr calvert to jedediah strutt: saw the great cotton mills, and in the evening walked to matlock. up to this time the country of greatest interest was the region of the fens about ramsay (a most remarkable district), but now began beauty of scenery. on july th we walked by rowsley and haddon hall over the hills to edensor, where we stayed till the th with mr smith. we next visited hathersage, castleton, and marple (where i wished to see the canal aqueduct), and went by coach to manchester, and afterwards to liverpool. here dr traill recommended us to see the pontycyssylte aqueduct, and we went by chester and wrexham to rhuabon, saw the magnificent work, and proceeded to llangollen. thence by chester and northwich (where we descended a salt-mine) to macclesfield. then to the ecton mine (of which we saw but little) through dovedale to ashbourn, and by coach to derby. on july th to birmingham, where we found mr guest, lodged in his house, and were joined by my pupil guest. here we were fully employed in visiting the manufactures, and then went into the iron country, where i descended a pit in the staffordshire main. thence by coach to cambridge, where i stopped to prepare for the fellowship examination. "i had two pupils in this portion of the long vacation, turner and dobbs. on august nd my writing of latin began regularly as before. my principal mathematics on the quires are optics. on august th i made experiments on my left eye, with good measures, and on aug. th ordered a cylindrical lens of peters, a silversmith in the town, which i believe was never made. subsequently, while at playford, i ordered cylindrical lenses of an artist named fuller, living at ipswich, and these were completed in november, . "my letter to the examiners, announcing my intention of sitting for fellowship (which like all other such documents is preserved on my quires) was delivered on sept st. the examination took place on sept. nd and the two following days. on oct. st, , at the usual hour of the morning, i was elected fellow. there were elected at the same time t.b. macaulay (afterwards lord macaulay), who was a year senior to me in college, and i think field of my own year. i drew up my letter of acknowledgment to the electors. on oct. nd at in the morning i was admitted fellow with the usual ceremonies, and at i called on the electors with my letter of acknowledgment. i immediately journeyed to derbyshire, paid a visit at edensor, and returned by sheffield. "on oct. th (it having been understood with mr higman that my engagement as assistant mathematical tutor stood) the master sent for me to appoint me and to say what was expected as duty of the office. he held out to me the prospect of ultimately succeeding to the tutorship, and i told him that i hoped to be out of college before that time. "about this time the 'athenaeum,' a club of a scientific character, was established in london, and i was nominated on it, but i declined" (oct. th). in this year ( ) i commenced account with a banker by placing _£ _ in the hands of messrs mortlock and co. on oct. th i walked to bury, and after a single day's stay there returned to cambridge. "on oct. rd, , began my lectures as mathematical assistant tutor. i lectured the senior sophs and junior sophs on higman's side. the number of senior sophs was . besides this i took part in the 'examinations of the questionists,' a series of exercises for those who were to take the bachelor's degree in the next january. i examined in mechanics, newton, and optics. i had also as private pupils turner, dobbs, and cooper. i now ceased from the exercise which i had followed with such regularity for five years, namely that of daily writing latin. in its stead i engaged a french master (goussel) with whom i studied french with reasonable assiduity for the three terms to june, . "among mathematical investigations i find: theory of the moon's brightness, motion of a body in an ellipse round two centres of force, various differential equations, numerical computation of sin pi from series, numerical computation of sines of various arcs to decimals, curvature of surfaces in various directions, generating functions, problem of sound. i began in the winter a latin essay as competing for the middle bachelors' prize, but did not proceed with it. i afterwards wished that i had followed it up: but my time was fully occupied. "on jan. th, , i started for edensor, where i paid a visit, and returned on feb. nd. on feb. th i wrote to mr clarkson, asking his advice about a profession or mode of life (the cares of life were now beginning to press me heavily, and continued to do so for several years). he replied very kindly, but his answer amounted to nothing. about the same time i had some conversation of the same kind with mr peacock, which was equally fruitless. "on feb. th i have investigations of the density of light near a caustic (on the theory of emissions). on feb. th i finished a paper about the defect in my eye, which was communicated to the cambridge philosophical society on feb. st. mr peacock or mr whewell had some time previously applied to me to write a paper on trigonometry for the encyclopaedia metropolitana, and i had been collecting some materials (especially in regard to its history) at every visit to london, where i read sometimes at the british museum: also in the cambridge libraries. i began this paper (roughly) on feb. th, and finished it on mar. rd. the history of which i speak, by some odd management of the editors of the encyclopaedia, was never published. the ms. is now amongst the mss. of the royal observatory, greenwich. other subjects on my quires are: theory of musical concords, many things relating to trigonometry and trigonometrical tables, achromatic eye-pieces, equation to the surface bounding the rays that enter my left eye, experiments on percussion. also notes on cumberland and wales (i had already proposed to myself to take a party of pupils in the long vacation to keswick), and notes on history and geology. "i had been in correspondence with dr malkin (master of bury school), who on feb. th sent a certificate for my brother william, whom i entered at trinity on peacock's side. on mar. th i changed my rooms, quitting those on the ground-floor east side of queen mary's gate for first-floor rooms in neville's court, south side, the easternmost rooms. in this term my lectures lasted from apr. th to may th. apparently i had only the senior sophs, in number, and the same four pupils (turner, dobbs, cooper, hovenden) as in the preceding term. the only scientific subjects on which i find notes are, a paper on the forms of the teeth of wheels, communicated to the philosophical society on may nd; some notes about musical concords, and some examination of a strange piece of iceland spar. on apr. th i was elected to the northern institution (of inverness); the first compliment that i received from an extraneous body. "on may th i have a most careful examination of my money accounts, to see whether i can make an expedition with my sister into wales. my sister came to cambridge, and on monday, may rd, , we started for wales, equipped in the lightest way for a walking expedition. we went by birmingham to shrewsbury: then to the pontycyssylte aqueduct and by various places to bala, and thence by llanrwst to conway. here the suspension bridge was under construction: the mole was made and the piers, but nothing else. then on to bangor, where nine chains of the suspension bridge were in place, and so to holyhead. then by carnarvon to bethgelert, ascending snowdon by the way, and in succession by festiniog, dolgelly, and aberystwyth to hereford (the first time that i had visited it since my father left it). from thence we went by coach to london, and i went on to cambridge on the rd of june. "i had arranged to take a party of pupils to keswick, and to take my brother there. mr clarkson had provided me with introductions to mr southey and mr wordsworth. on wednesday, june th, , we started, and went by leicester, sheffield, leeds, and kendal, to keswick, calling at edensor on the way. my pupils were cleasby, marshman, clinton, wigram, tottenham, and m. smith. at keswick i passed three months very happily. i saw mr southey's family frequently, and mr wordsworth's occasionally. by continual excursions in the neighbourhood, and by a few excursions to places as distant as bowness, calder bridge, &c. (always climbing the intermediate mountains), i became well acquainted with almost the whole of that beautiful country, excepting some of the s. w. dales. a geological hammer and a mountain barometer were very interesting companions. i had plenty of work with my pupils: i worked a little lunar theory, a little of laplace's equations, something of the figure of the earth, and i wrote out very carefully my trigonometry for the encyclopaedia metropolitana. i read a little of machiavelli, and various books which i borrowed of mr southey. on friday, sept. th, my brother and i left for kendal, and after a stay of a few days at edensor, arrived at cambridge on oct. th. "on oct. st my lectures to the junior sophs began, names, lasting to dec. th. those to the senior sophs, names, oct th to dec. th. i also examined questionists as last year. i have notes about a paper on the connection of impact and pressure, read at the philosophical society on nov. th, but not printed, dipping-needle problems, curve described round three centres of force, barometer observations, theory of the figure of the earth with variable density, and effect on the moon, correction to the madras pendulum, wedge with friction, spots seen in my eyes, density of rays near a caustic. in this term i accomplished the preparation of a volume of mathematical tracts on subjects which, either from their absolute deficiency in the university or from the unreadable form in which they had been presented, appeared to be wanted. the subjects of my tracts were, lunar theory (begun oct. th, finished nov. st), figure of the earth ( st part finished nov. th), precession and nutation (my old ms. put in order), and the calculus of variations. i applied, as is frequently done, to the syndicate of the university press for assistance in publishing the work; and they agreed to give me paper and printing for copies. this notice was received from professor turton on nov. th, . it was probably also in this year that i drew up an imperfect 'review' of coddington's optics, a work which deserved severe censure: my review was never finished. "in the long vacation at keswick i had six pupils at _£ _ each. in the october term i had marshman and ogilby at _£ _ for three terms, and dobbs at _£ _ for three terms. i had, at mr peacock's suggestion, raised my rate from to guineas for three terms: this prevented some from applying to me, and induced some to withdraw who had been connected with me: but it did me no real hurt, for engrossment by pupils is the worst of all things that can happen to a man who hopes to distinguish himself. on dec. th i went to bury, and returned to cambridge on jan. th, . "i have the attendance-bills of my lectures to senior sophs ( ) from feb. rd to feb. rd, and to freshmen ( ) from feb. th to mar. . it would appear that i gave but one college-lecture per day (my belief was that i always had two). the tutor's stipend per term was _£ _. on my quires i find, investigations for the ellipticity of a heterogeneous spheroid when the density is expressed by sin _qc_/_qc_ (the remarkable properties of which i believe i discovered entirely myself, although they had been discovered by other persons), theoretical numbers for precession, nutation, &c., some investigations using laplace's y, hard work on the figure of the earth to the nd order,'woodhouse's remaining apparatus,' notes about lambton's and kater's errors, depolarization, notes of papers on depolarization in the phil. trans., magnetic investigations for lieut. foster, isochronous oscillations in a resisting medium, observations on a strange piece of iceland spar. on mar. th forwarded preface and title page for my mathematical tracts. "some time in this term i began to think of the possibility of observing the diminution of gravity in a deep mine, and communicated with whewell, who was disposed to join in experiments. my first notion was simply to try the rate of a clock, and the ecton mine was first thought of. i made enquiries about the ecton mine through mr smith (of edensor), and visited the mine, but in the meantime whewell had made enquiries in london and found (principally from dr paris) that the mine of dolcoath near camborne in cornwall would be a better place for the experiment. dr paris wrote to me repeatedly, and ultimately we resolved on trying it there. in my papers on mar. st are various investigations about attractions in both mines. on apr. rd i went to london, principally to arrange about dolcoath, and during april and may i was engaged in correspondence with sir h. davy (president of the royal society), mr herschel, and dr young (secretary of the board of longitude) about the loan of instruments and pendulums. on apr. rd i was practising pendulum-observations (by coincidence); and about this time repeatedly practised transits with a small instrument lent by mr sheepshanks (with whom my acquaintance must have begun no long time before) which was erected under a tent in the fellows' walks. on my quires i find various schemes for graduating thermometers for pendulum experiments. "i find also notes of examination of my brother william, who had come to college last october; and a great deal of correspondence with my mother and sister and mr case, a lawyer, about a troublesome business with mr cropley, an old friend of g. biddell, to whom my father had lent _£ _ and whose affairs were in chancery. "my lectures in this term were to the junior sophs from apr. th to may th: they were six in number and not very regular. on apr. th i sent to mawman the copy of my trigonometry for the encyclopaedia metropolitana, for which i received _£ _. i received notice from the press syndicate that the price of my mathematical tracts was fixed at _ s. d._: i sold the edition to deighton for _£ _, and it was immediately published. about this time i have letters from mr herschel and sir h. davy about a paper to be presented to the royal society--i suppose about the figure of the earth to the nd order of ellipticity, which was read to the royal society on june th. "on saturday, may th, , i went to london on the way to dolcoath, and received four chronometers from the royal observatory, greenwich. i travelled by devonport and falmouth to camborne, where i arrived on may th and dined at the count-house dinner at the mine. i was accompanied by ibbotson, who was engaged as a pupil, and intended for an engineer. on may th whewell arrived, and we took a pendulum and clock down, and on the th commenced the observation of coincidences in earnest. this work, with the changing of the pendulums, and sundry short expeditions, occupied nearly three weeks. we had continued the computation of our observations at every possible interval. it is to be understood that we had one detached pendulum swinging in front of a clock pendulum above, and another similarly mounted below; and that the clocks were compared by chronometers compared above, carried down and compared, compared before leaving, and brought up and compared. the upper and lower pendulums had been interchanged. it was found now that the reliance on the steadiness of the chronometers was too great; and a new method was devised, in which for each series the chronometers should make four journeys and have four comparisons above and two below. this arrangement commenced on the th june and continued till the th. on the th we packed the lower instruments, intending to compare the pendulum directly with the upper one, and sent them up the shaft: when an inexplicable occurrence stopped all proceedings. the basket containing all the important instruments was brought up to the surface (in my presence) on fire; some of the instruments had fallen out with their cases burning. whether a superstitious miner had intentionally fired it, or whether the snuff of a candle had been thrown into it, is not known. our labour was now rendered useless. on the th i packed up what remained of instruments, left for truro, and arrived at bury on july st. during our stay in cornwall i had attended a 'ticketing' or sale of ore at camborne, and we had made expeditions to the n.w. coast, to portreath and illogan, to marazion and st michael's mount, and to penzance and the land's end. on july rd i saw mr cropley in bury gaol, and went to cambridge. on the th i was admitted a.m., and on the th was admitted major fellow. "i had engaged with four pupils to go to orléans in this long vacation: my brother william was also to go. one of my pupils, dobbs, did not join: the other three were tinkler, ogilby, and ibbotson. we left london on july th, and travelled by brighton, dieppe, rouen, and paris to orléans. at paris i saw bouvard, pouillet, laplace and arago. i had introductions from mr peacock, mr south, mr herschel, dr young; and from professor sedgwick to an english resident, mr underwood. on the th i was established in the house of m. lagarde, protestant minister. here i received my pupils. on the th i commenced italian with an italian master: perhaps i might have done more prudently in adhering to french, for i made no great progress. on aug. nd i saw a murderer guillotined in the place martroi. the principal investigations on my quires are--investigations about pendulums, calculus of variations, notes for the figure of the earth (encyc. metrop.) and commencement of the article, steam-engine machinery, &c. i picked up various french ballads, read various books, got copies of the marseillaise (this i was obliged to obtain rather secretly, as the legitimist power under charles x. was then at its height) and other music, and particulars of farm wages for whewell and r. jones. the summer was intensely hot, and i believe that the heat and the work in dolcoath had weakened me a good deal. the family was the old clergyman, his wife, his daughter, and finally his son. we lived together very amicably. my brother lodged in a café in the place martroi; the others in different families. i left orléans on sept. th for paris. here i attended the institut, and was present at one of ampère's lectures. i arrived at cambridge on oct. th. "on oct. th whewell mentioned to me that the lucasian professorship would be immediately vacated by turton, and encouraged me to compete for it. shortly afterwards mr higman mentioned the professorship, and joshua king (of queens') spoke on the restriction which prevented college tutors or assistant tutors from holding the office. about this time mr peacock rendered me a very important service. as the emolument of the lucasian professorship was only _£ _, and that of the assistant tutorship _£ _, i had determined to withdraw from the candidature. but mr peacock represented to me the advantage of position which would be gained by obtaining the professorship (which i then instantly saw), and i continued to be a candidate. i wrote letters to the heads of colleges (the electors) and canvassed them personally. only dr davy, the master of caius college, at once promised me his vote. dr french, master of jesus college, was a candidate; and several of the heads had promised him their votes. mr babbage, the third candidate, threatened legal proceedings, and dr french withdrew. the course was now open for mr babbage and me. "in the meetings of the philosophical society a new mode of proceeding was introduced this term. to enliven the meetings, private members were requested to give oral lectures. mine was the second, i think, and i took for subject the machinery of the steam engines in the cornish mines, and especially of the pumping engines and pumps. it made an excellent lecture: the subjects were at that time undescribed in books, and unknown to engineers in general out of cornwall. "my college lectures seem to have been, oct. st to dec. th to junior sophs, dec. th to th to senior sophs. i assisted at the examinations of the questionists. i had no private pupils. on nov. th i communicated to the cambridge philosophical society a paper on the theory of pendulums, balances, and escapements: and i find applications of babbage's symbolism to an escapement which i proposed. i have various investigations about the earth, supposed to project at middle latitudes above the elliptical form. in november an account of the dolcoath failure (by whewell) was given to the royal society. "at length on dec. th, , the election to the lucasian professorship took place: i was elected (i think unanimously) and admitted. i believe that this gave great satisfaction to the university in general. my uncle, arthur biddell, was in cambridge on that evening, and was the first of my friends who heard of it. on the same page of my quires on which this is mentioned, there is a great list of apparatus to be constructed for lucasian lectures, notes of experiments with atwood's machine, &c. in december, correspondence with dollond about prisms. i immediately issued a printed notice that i would give professorial lectures in the next term. "on dec. th i have a letter from mr smith informing me of the dangerous illness (fever) which had attacked nearly every member of his family, richarda worst of all. on dec. rd i went to bury. the affairs with cropley had been settled by the sale of his property under execution, and my father did not lose much of his debt. but he had declined much in body and mind, and now had strange hallucinations. "the commencement of found me in a better position (not in money but in prospects) than i had before stood in: yet it was far from satisfactory. i had resigned my assistant tutorship of _£ _ per annum together with the prospect of succeeding to a tutorship, and gained only the lucasian professorship of _£ _ per annum. i had a great aversion to entering the church: and my lay fellowship would expire in years. my prospects in the law or other professions might have been good if i could have waited: but then i must have been in a state of starvation probably for many years, and marriage would have been out of the question: i much preferred a moderate income in no long time, and i am sure that in this i judged rightly for my happiness. i had now in some measure taken science as my line (though not irrevocably), and i thought it best to work it well, for a time at least, and wait for accidents. "the acceptance of the lucasian professorship prevented me from being pressed by sedgwick (who was proctor this year) to take the office of moderator: which was a great relief to me. as lucasian professor i was ipso facto member of the board of longitude. a stipend of _£ _ a year was attached to this, on condition of attending four meetings: but i had good reason (from intimations by south and other persons in london) for believing that this would not last long. the fortnightly notices of the meetings of the board were given on jan. th, mar. nd, may th and oct. th. "on jan. nd, , i came from london to bury. i found my father in a very declining state (the painful rheumatism of some years had changed to ulcerations of the legs, and he was otherwise helpless and had distressing hallucinations). on jan. th i walked to cambridge. at both places i was occupied in preparations for the smith's prize examination and for lectures (for the latter i obtained at bury gaol some numerical results about tread-mills). "of the smith's prize i was officially an examiner: and i determined to begin with---what had never been done before--making the examination public, by printing the papers of questions. the prize is the highest mathematical honour in the university: the competitors are incepting bachelors of arts after the examination for that degree. my day of examination (apparently) was jan. st. the candidates were turner, cankrein, cleasby, and mr gordon. the first three had been my private pupils: mr gordon was a fellow-commoner of st peter's college, and had just passed the b.a. examination as senior wrangler, turner being second. my situation as examiner was rather a delicate one, and the more so as, when i came to examine the papers of answers, turner appeared distinctly the first. late at night i carried the papers to whewell's rooms, and he on inspection agreed with me. the other examiners (professors lax and woodhouse, lowndean and plumian professors) generally supported me: and turner had the honour of first smith's prize. "on jan. th my mother wrote, asking if i could see cropley in london, where he was imprisoned for contempt of chancery. i attended the meeting of the board of longitude on feb. st, and afterwards visited cropley in the fleet prison. he died there, some time later. it was by the sale of his effects under execution that my father's debt was paid. "on feb. th i communicated to the royal society a paper on the correction of the solar tables from south's observations. i believe that i had alluded to this at the february meeting of the board of longitude, and that in consequence mr pond, the astronomer royal, had been requested to prepare the errors of the sun's place from the greenwich observations: which were supplied some months later. with the exception of south's solar errors, and some investigations about dipping-needles, i do not find anything going on but matters connected with my approaching lectures. there are bridges, trusses, and other mechanical matters, theoretical and practical, without end. several tradesmen in cambridge and london were well employed. on feb. th i have a letter from cubitt about groins: i remember studying those of the custom-house and other places. on feb. th my syllabus of lectures was finished: this in subsequent years was greatly improved. i applied to the royal society for the loan of huyghens's object-glass, but they declined to lend it. about this time i find observations of the spectrum of sirius. "there had been no lectures on experimental philosophy (mechanics, hydrostatics, optics) for many years. the university in general, i believe, looked with great satisfaction to my vigorous beginning: still there was considerable difficulty about it. there was no understood term for the lectures: no understood hour of the day: no understood lecture room. i began this year in the lent term, but in all subsequent years i took the easter term, mainly for the chance of sunlight for the optical experiments, which i soon made important. i could get no room but a private or retiring room (not a regular lecture room) in the buildings at the old botanic garden: in following years i had the room under the university library. the lectures commenced on some day in february : i think that the number who attended them was about . i remember very well that the matter which i had prepared as an introductory lecture did not last above half the time that i had expected, but i managed very well to fill up the hour. on another occasion i was so ill-prepared that i had contemplated giving notice that i was unable to complete the hour's lecture, but i saw in the front row some strangers, introduced by some of my regular attendants, very busy in taking notes, and as it was evident that a break-down now would not do, i silently exerted myself to think of something, and made a very good lecture. "on mar. st, as official examiner, i received notices from candidates for bell's scholarships, and prepared my paper of questions. i do not remember my day of examination; but i had all the answers to all the examiners' questions in my hands, when on mar. th i received notice that my father had died the preceding evening. this stopped my lectures: they were concluded in the next term. i think that i had only mechanics and imperfect optics this term, no hydrostatics; and that the resumed lectures were principally optical. they terminated about may th. "with my brother i at once went to bury to attend my father's funeral. he was buried on mar. st, , in the churchyard of little whelnetham, on the north side of the church. shortly afterwards i went to london, and on apr. th i attended a meeting of the board of longitude, at which herschel produced a paper regarding improvements of the nautical almanac. herschel and i were in fact the leaders of the reforming party in the board of longitude: dr young the secretary resisted change as much as possible. after the meeting i went to cambridge. i find then calculations of achromatic eye-pieces for a very nice model with silk threads of various colours which i made with my own hands for my optical lectures. "on apr. th herschel wrote to me that the professorship held by dr brinkley (then appointed bishop of cloyne) at dublin would be vacant, and recommended it to my notice, and sent me some introductions. i reached dublin on apr. th, where i was received with great kindness by dr brinkley and dr macdonnell (afterwards provost). i there met the then provost dr bartholomew lloyd, dr lardner, mr hamilton (afterwards sir w. r. hamilton) and others. in a few days i found that they greatly desired to appoint hamilton if possible (they did in fact overcome some difficulties and appoint him in a few months), and that they would not make such an augmentation as would induce me to offer myself as a candidate, and i withdrew. i have always remembered with gratitude dr macdonnell's conduct, in carefully putting me on a fair footing in this matter. i returned by holyhead, and arrived at birmingham on apr. rd. while waiting there and looking over some papers relating to the spherical aberration of eye-pieces, in which i had been stopped some time by a geometrical difficulty, i did in the coffee-room of a hotel overcome the difficulty; and this was the foundation of a capital paper on the spherical aberration of eye-pieces. this paper was afterwards presented to the cambridge philosophical society. "about this time a circumstance occurred of a disagreeable nature, which however did not much disconcert me. mr ivory, who had a good many years before made himself favourably known as a mathematician, especially by his acquaintance with laplace's peculiar analysis, had adopted (as not unfrequently happens) some singular hydrostatical theories. in my last paper on the figure of the earth, i had said that i could not receive one of his equations. in the philosophical magazine of may he attacked me for this with great heat. on may th i wrote an answer, and i think it soon became known that i was not to be attacked with impunity. "long before this time there had been some proposal about an excursion to the lake district with my sister, and i now arranged to carry it out. on may rd i went to bury and on to playford: while there i sketched the cumberland excursion. on june th i went to london, i believe to the visitation of the greenwich observatory to which i was invited. i also attended the meeting of the board of longitude. i think it was here that pond's errors of the sun's place in the nautical almanac from greenwich observations were produced. on june th i went by coach to rugby, where i met my sister, and we travelled to edensor. we made a number of excursions in derbyshire, and then passed on by penrith to keswick, where we arrived on june nd. from keswick we made many excursions in the lake district, visited mr southey and mr wordsworth, descended a coal mine at whitehaven, and returned to edensor by the way of ambleside, kendal, and manchester. with sundry excursions in derbyshire our trip ended, and we returned to cambridge on the st july. "during this long vacation i had one private pupil, crawford, the only pupil this year, and the last that i ever had. at this time there is on my papers an infinity of optical investigations: also a plan of an eye-piece with a concave lens to destroy certain aberrations. on aug. th i went to woodford to see messrs gilbert's optical works. from aug. th i had been preparing for the discussion of the greenwich solar errors, and i had a man at work in my rooms, engaged on the calculation of the errors. i wrote to bouvard at paris for observations of the sun, but he recommended me to wait for the tables which bessel was preparing. i was busy too about my lectures: on sept. th i have a set of plans of printing presses from hansard the printer (who in a visit to cambridge had found me making enquiries about them), and i corresponded with messrs gilbert about optical constructions, and with w. and s. jones, eastons, and others about pumps, hydraulic rams, &c. on sept. th occurred a very magnificent aurora borealis. "i do not find when the investigation of corrections of solar elements was finished, or when my extracts from burckhardt, connaissance des temps , were made. but these led me to suspect an unknown inequality in the sun's motion. on sept. th and th i find the first suspicions of an inequality depending on � mean longitude of venus-- � mean longitude of earth. the thing appeared so promising that i commenced the investigation of the perturbation related to this term, and continued it (a very laborious work) as fast as i was able, though with various interruptions, which in fact were necessary to keep up my spirits. on oct. th i went to london for the board of longitude meeting. here i exhibited the results of my sun investigations, and urged the correction of the elements used in the nautical almanac. dr young objected, and proposed that bouvard should be consulted. professor woodhouse, the plumian professor, was present, and behaved so captiously that some members met afterwards to consider how order could be maintained. i believe it was during this visit to london that i took measures of hammersmith suspension bridge for an intended lecture-model. frequently, but not always, when in london, i resided at the house of mr sheepshanks and his sister miss sheepshanks, woburn place. my quires, at this time, abound with suggestions for lectures and examinations. "on some day about the end of november or beginning of december , when i was walking with mr peacock near the outside gate of the trinity walks, on some mention of woodhouse, the plumian professor, mr peacock said that he was never likely to rise into activity again (or using some expression importing mortal illness). instantly there had passed through my mind the certainty of my succeeding him, the good position in which i stood towards the university, the probability of that position being improved by improved lectures, &c., &c., and by increased reputation from the matters in which i was now engaged, the power of thus commanding an increase of income. i should then have, independent of my fellowship, some competent income, and a house over my head. i was quite aware that some time might elapse, but now for the first time i saw my way clearly. the care of the observatory had been for two or three years attached to the plumian professorship. a grace was immediately prepared, entrusting the temporary care of the observatory to dr french, to me, mr catton, mr sheepshanks, and mr king (afterwards master of queens' college). on dec. th i have a note from mr king about going to the observatory. "on dec. th my paper on corrections of the elements of the solar tables was presented to the royal society. on dec. th, at h. m. a.m. (sunday morning), i arrived at the result of my calculations of the new inequality. i had gone through some fluctuations of feeling. usually the important part of an inequality of this kind depends entirely on the eccentricities of the orbits, but it so happened that from the positions of the axes of the orbits, &c., these terms very nearly destroyed each other. after this came the consideration of inclinations of orbits; and here were sensible terms which were not destroyed. finally i arrived at the result that the inequality would be about "; just such a magnitude as was required. i slipped this into whewell's door. this is, to the time of writing ( ), the last improvement of any importance in the solar theory. some little remaining work went on to dec. th, and then, being thoroughly tired, i laid by the work for revision at some future time. i however added a postscript to my royal society paper on solar errors, notifying this result. "on dec. th i went to bury. while there i heard from whewell that woodhouse was dead. i returned to cambridge and immediately made known that i was a candidate for the now vacant plumian professorship. of miscellaneous scientific business, i find that on oct. th professor barlow of woolwich prepared a memorial to the board of longitude concerning his fluid telescope (which i had seen at woodford), which was considered on nov. st, and i had some correspondence with him in december. in june and august my trigonometry was printing. "on jan. th, , i came from london. it seems that i had been speculating truly 'without book' on perturbations of planetary elements, for on jan. th and th i wrote a paper on a supposed error of laplace, and just at the end i discovered that he was quite right: i folded up the paper and marked it 'a lesson.' i set two papers of questions for smith's prizes (there being a deficiency of one examiner, viz. the plumian professor). "before the beginning of whewell and i had determined on repeating the dolcoath experiments. on jan. th i have a letter from davies gilbert (then president of the royal society) congratulating me upon the solar theory, and alluding to our intended summer's visit to cornwall. we had somehow applied to the board of longitude for pendulums, but dr young wished to delay them, having with capt. basil hall concocted a scheme for making lieut. foster do all the work: whewell and i were indignant at this, and no more was said about it. on jan. th dr young, in giving notice of the board of longitude meeting, informs me that the clocks and pendulums are ready. "i had made known that i was a candidate for the plumian professorship, and nobody thought it worth while to oppose me. one person at least (earnshaw) had intended to compete, but he called on me to make certain that i was a candidate, and immediately withdrew. i went on in quality of syndic for the care of the observatory, ingrafting myself into it. but meantime i told everybody that the salary (about _£ _) was not sufficient for me; and on jan. th i drafted a manifesto or application to the university for an increase of salary. the day of election to the professorship was feb. th. as i was officially (as lucasian professor) an elector, i was present, and i explained to the electors that i could not undertake the responsibility of the observatory without augmentation of income, and that i requested their express sanction to my application to the university for that purpose. they agreed to this generally, and i was elected. i went to london immediately to attend a meeting of the board of longitude and returned on feb. th. on feb. th i began my lectures (which, this year, included mechanics, optics, pneumatics, and hydrostatics) in the room below the university library. the number of names was . the lectures terminated on mar. nd. "on feb. th i received from mr pond information on the emoluments at greenwich observatory. i drew up a second manifesto, and on feb. th i wrote and signed a formal copy for the plumian electors. on feb. th i met them at caius lodge (the master, dr davy, being vice-chancellor). i read my paper, which was approved, and their sanction was given in the form of a request to the vice-chancellor to permit the paper to be printed and circulated. my paper, with this request at the head, was immediately printed, and a copy was sent to every resident m.a. (more than went out in one day). the statement and composition of the paper were generally approved, but the university had never before been taken by storm in such a manner, and there was some commotion about it. i believe that very few persons would have taken the same step. mr sheepshanks wrote to me on mar. th, intimating that it was desperate. i had no doubt of success. whewell told me that some people accused me of bad faith, in omitting allusion to the _£ _ a year received as member of the board of longitude, and to the profits of lectures. i wrote him a note, telling him that i had most certain information of the intention to dissolve the board of longitude (which was done in less than six months), and that by two years' lectures i had gained _£ _ (the expenses being _£ _, receipts _£ _). this letter was sent to the complaining people, and no more was said. by the activity of sheepshanks and the kindness of dr davy the business gradually grew into shape, and on mar. st a grace passed the senate for appointing a syndicate to consider of augmentation. sheepshanks was one of the syndicate, and was understood to represent, in some measure, my interests. the progress of the syndicate however was by no means a straightforward one. members of the senate soon began to remark that before giving anything they ought to know the amount of the university revenue, and another syndicate was then appointed to enquire and report upon it. it was more than a year before my syndicate could make their recommendation: however, in fact, i lost nothing by that delay, as i was rising in the estimation of the university. the observatory house was furnished, partly from woodhouse's sale, and partly from new furniture. my mother and sister came to live with me there. on mar. th i began the observatory journal; on mar. th i slept at the observatory for the first time, and on apr. th i came to reside there permanently, and gave up my college rooms." chapter iv. at cambridge observatory. from his taking charge of the cambridge observatory to his residence at greenwich observatory as astronomer royal. from march th to jan. st . "i attended a meeting of the board of longitude on apr. rd. and again on june th; this was the last meeting: sheepshanks had previously given me private information of the certainty of its dissolution.--on apr. th i visited mr herschel at slough, where one evening i saw saturn with his -foot telescope, the best view of it that i have ever had.--in june i attended the greenwich observatory visitation.--before my election (as plumian professor) there are various schemes on my quires for computation of transit corrections, &c. after apr. th there are corrections for deficient wires, inequality of pivots, &c. and i began a book of proposed regulations for observations. in this are plans for groups of stars for r.a. (the transit instrument being the only one finished): order of preference of classes of observations: no reductions to be made after dinner, or on sunday: no loose papers: observations to be stopped if reductions are two months in arrear: stars selected for parallax.--the reduction of transits begins on apr. th. on may th mr pond sent me some moon-transits to aid in determining my longitude.--dr young, in a letter to me of may th, enquires whether i will accept a free admission to the royal society, which i declined. on may th i was elected to the astronomical society.--towards the end of the year i observed encke's comet: and determined the latitude of the observatory with sheepshanks's repeating circle.--on my papers i find a sketch of an article on the figure of the earth for the encyclopaedia metropolitana. "as early as feb. rd i had been in correspondence with t. jones, the instrument-maker, about pendulums for a repetition of the dolcoath experiments. invitations had been received, and everything was arranged with whewell. sheepshanks, my brother, and mr jackson of ipswich (caius coll.) were to go, and we were subsequently joined by sedgwick, and lodge (magdalene coll.). on july rd sheepshanks and i started by salisbury, taking sherborne on our way to look at the church, which had alarmed the people by signs of a crack, and arrived at camborne on july th. on the th we set up the pendulums, and at once commenced observations, our plan being, to have no intermission in the pendulum observations, so that as soon as the arc became too small a fresh series was started. on july th we raised the instruments, and sheepshanks, who managed much of the upper operations, both astronomical and of pendulums, mounted the pendulums together in his observatory. we went on with our calculations, and on august th, on returning from a visit to john williams at barncoose, we heard that there was a 'run' in dolcoath, that is a sinking of the whole mass of rock where it had been set free by the mine excavations: probably only a few inches, but enough to break the rock much and to stop the pumps. on aug. th the calculations of our observations shewed that there was something wrong, and on the th i perceived an anomaly in the form of the knife edge of one pendulum, and of its agate planes, and suggested cautions for repeating the observations. we determined at once to repeat them: and as the water was rising in the mine there was no time to be lost. we again sent the instruments down, and made observations on the th, th and th. on the th i sent the instruments up, for the water was near our station, and sedgwick, whewell, and i went on a geological expedition to the lizard. on our return we met sheepshanks and the others, and found the results of the last observations unsatisfactory. the results of comparing the pendulums were discordant, and the knife edge of the faulty pendulum had very sensibly altered. we now gave up observations, with the feeling that our time had been totally lost, mainly through the fault of the maker of the pendulum (t. jones). on the th we made an expedition to penzance and other places, and arrived at cambridge on the th of september. "in the course of the work at dolcoath we made various expeditions as opportunity offered. thus we walked to carn brea and witnessed the wrestling, the common game of the country. on another occasion sedgwick, whewell, and i had a capital geological expedition to trewavas head to examine granite veins. we visited at pendarves and trevince, and made the expedition to the lizard already referred to, and saw many of the sights in the neighbourhood. after visiting penzance on the conclusion of our work we saw cape cornwall (where whewell overturned me in a gig), and returned homewards by way of truro, plymouth (where we saw the watering-place and breakwater: also the dockyard, and descended in one of the working diving-bells), exeter, salisbury, and portsmouth. in returning from camborne in i lost the principal of our papers. it was an odd thing that, in going through exeter on our way to camborne in , i found them complete at exeter, identified to the custodian by the dropping out of a letter with my address. "on my return to cambridge i was immediately immersed in the work of the observatory. the only instrument then mounted at the observatory was the transit. i had no assistant whatever.--a mr galbraith of edinburgh had questioned something in one of my papers about the figure of the earth. i drew up a rather formal answer to it: whewell saw my draft and drew up a much more pithy one, which i adopted and sent to the philosophical magazine.--for comparing our clocks at the upper and lower stations of dolcoath we had borrowed from the royal observatory, greenwich, six good pocket chronometers: they were still in the care of mr sheepshanks. i arranged with him that they should be sent backwards and forwards a few times for determining the longitude of cambridge observatory. this was done on oct. st, nd, rd: the result was ° , and this has been used to the present time ( ). it evinced an error in the trigonometrical survey, the origin of which was found, i think, afterwards (dr pearson in a letter of dec. th spoke of the mistake of a may-pole for a signal-staff). i drew up a paper on this, and gave it to the cambridge philosophical society on nov. th. (my only academical paper this year.)--i had several letters from dr young, partly supplying me with calculations that i wanted, partly on reform or extension of the nautical almanac (which dr young resisted as much as possible). he considered me very unfairly treated in the dissolution of the board of longitude: professor lax wished me to join in some effort for its restoration, but i declined. "as my reduction of observations was kept quite close, i now began to think of printing. in regard to the form i determined to adopt a plan totally different from that of any other observations which i had seen. the results were to be the important things: i was desirous of suppressing the separate wires of transits. but upon consulting herschel and other persons they would not agree to it, and i assented to keeping them. i applied to the press syndicate to print the work, and on nov. th at the request of t. musgrave (afterwards archbishop of york) i sent a specimen of my ms.: on nov. th they granted copies, and the printing soon commenced." "during a winter holiday at playford i wrote out some investigations about the orbits of comets, and on jan. rd i returned to cambridge. the smith's prize examination soon followed, in which i set a paper of questions as usual. on feb. th i made notes on liesganig's geodetic work at the british museum. "i was naturally anxious now about the settlement of my salary and of the observatory establishment. i do not know when the syndicate made their report, but it must have been in the last term of . it recommended that the salary should be annually made up (by grace) to _£ _: that an assistant should be appointed with the assent of the vice-chancellor and dismissable by the plumian professor: and that a visiting syndicate should be appointed, partly official and partly of persons to be named every year by grace. the grace for adopting this report was to be offered to the senate on feb. th. the passing of the grace was exposed to two considerable perils. first, i found out (just in time) that a senior fellow of trinity (g.a. browne) was determined to oppose the whole, on account of the insignificant clause regarding dismissal of assistants, which he regarded as tyrannical. i at once undertook that that clause should be rejected. secondly, by the absurd constitution of the 'caput' at cambridge, a single m.a. had the power of stopping any business whatever, and an m.a. actually came to the senate house with the intention of throwing out all the graces on various business that day presented to the senate. luckily he mistook the hour, and came at instead of , and found that all were dispatched. the important parts of the grace passed without any opposition: but i mustered some friends who negatived that part which had alarmed g.a. browne, and it was corrected to his satisfaction by a new grace on mar. th. i was now almost set at rest on one of the great objects of my life: but not quite. i did not regard, and i determined not to regard, the addition to my salary as absolutely certain until a payment had been actually made to me: and i carefully abstained, for the present, from taking any steps based upon it. i found for assistant at the observatory an old lieutenant of the royal navy, mr baldrey, who came on mar. . "on may th i began lectures: there were names. the lectures were improving, especially in the optical part. i do not find note of the day of termination.--i do not know the actual day of publication of my first small volume of cambridge observations, , and of circulation. the date of the preface is apr. th . i have letters of approval of it from davies gilbert, rigaud, and lax. the system which i endeavoured to introduce into printed astronomical observations was partially introduced into this volume, and was steadily improved in subsequent volumes. i think that i am justified, by letters and other remarks, in believing that this introduction of an orderly system of exhibition, not merely of observations but of the steps for bringing them to a practical result--quite a novelty in astronomical publications--had a markedly good effect on european astronomy in general.--in feb. and march i have letters from young about the nautical almanac: he was unwilling to make any great change, but glad to receive any small assistance. south, who had been keeping up a series of attacks on young, wrote to me to enquire how i stood in engagements of assistance to young: i replied that i should assist young whenever he asked me, and that i disapproved of south's course.--the date of the first visitation of the (cambridge) observatory must have been near may th: i invited south and baily to my house; south and i were very near quarrelling about the treatment of young.--in a few days after dr young died: i applied to lord melville for the superintendence of the nautical almanac: mr croker replied that it devolved legally upon the astronomer royal, and on may th pond wrote to ask my assistance when i could give any. on june th i was invited to the greenwich visitation, to which i believe i went on the th. "i had long desired to see switzerland, and i wished now to see some of the continental observatories. i was therefore glad to arrange with mr lodge, of magdalene college (perhaps years senior to myself), to make a little tour. capt. w.h. smyth and others gave me introductions. i met lodge in london, and we started for calais on july th . we visited a number of towns in belgium (at brussels i saw the beginning of the observatory with quetelet), and passed by cologne, frankfort, fribourg, and basle to zurich. thus far we had travelled by diligence or posting: we now procured a guide, and travelled generally on foot. from the th to the st august we travelled diligently through the well-known mountainous parts of switzerland and arrived at geneva on the st august. here i saw m. gautier, m. gambard, and the beginning of the observatory. mr lodge was now compelled to return to cambridge, and i proceeded alone by chambéry to turin, where i made the acquaintance of m. plana and saw the observatory. i then made a tour through north italy, looking over the observatories at milan, padua, bologna, and florence. at leghorn i took a passage for marseille in a xebeque, but after sailing for three days the weather proved very unfavourable, and i landed at spezia and proceeded by genoa and the cornici road to marseille. at marseille i saw m. gambart and the observatory, and passed by avignon, lyons, and nevers to orléans, where i visited my old host m. legarde. thence by paris, beauvais, and calais to london and cambridge, where i arrived on the th october. i had started with more than _£ _ and returned with _ s. d_. the expedition was in many ways invaluable to me. "on my return i found various letters from scientific men: some approving of my method for the mass of the moon: some approving highly of my printed observations, especially d. gilbert, who informed me that they had produced good effect (i believe at greenwich), and herschel.--on nov. th i gave the royal astronomical society a paper about deducing the mass of the moon from observations of venus: on nov. th a paper to the cambridge philosophical society on a correction to the length of a ball-pendulum: and on dec. th a paper on certain conditions under which perpetual motion is possible.--the engravings for my figure of the earth in the encyclopaedia metropolitana were dispatched at the end of the year. some of the paper (perhaps much) was written after my return from the continent.--i began, but never finished, a paper on the form of the earth supposed to be projecting at middle latitudes. in this i refer to the printed paper which nicollet gave me at paris. i believe that the investigations for my paper in the encyclopaedia metropolitana led me to think the supposition unnecessary.--on nov. th i was elected member of the geological society. "on nov. th notice was given of a grace to authorize payment to me of _£ . s. d._, in conformity with the regulations adopted on feb. th, and on nov. th the grace passed the senate. on nov. th the vice-chancellor wrote me a note enclosing the cheque. on nov. rd (practically the first day on which i could go) i went to london and travelled to edensor, where i arrived on the th. here i found richarda smith, proposed to her, and was accepted. i stayed there a few days, and returned to cambridge." "on jan. th the smith's prize paper was prepared. i was (with my assistant, mr baldrey) vigorously working the transit instrument and its reductions, and gradually forming a course of proceeding which has had a good effect on european astronomy. and i was preparing for my marriage. "on mar. th i started with my sister to london, and arrived at edensor on the afternoon of the th. on the th i started alone for manchester and liverpool. through mr mason, a cotton-spinner at calver, near edensor, i had become acquainted with mr john kennedy of manchester, and i had since been acquainted with dr traill of liverpool. amongst other things, i saw the works of the manchester and liverpool railway, then advancing and exciting great interest, and saw george stephenson and his son. on mar. th i was married to richarda smith by her father in edensor. we stopped at edensor till apr. st, and then started in chaises by way of newark and kettering (where we were in danger of being stopped by the snow), and arrived at cambridge on apr. rd. "i was now busy in preparing for lectures, especially the part of the optical lectures which related to the theory of interferences and polarization. i think it was now that my wife drew some of my lecture pictures, exhibiting interference phenomena. my lectures began on apr. th and finished on may th. the number of names was . they were considered an excellent course of lectures. "may th is the date of my preface to the observations: all was then printed. apparently i did not go to the visitation of the greenwich observatory this year.--i was at this time pressing tulley, the optician, about an object-glass for the mural circle.--a new edition of my 'tracts' was wanted, and i prepared to add a tract on the undulatory theory of light in its utmost extent. the syndicate of the university press intimated through dr turton that they could not assist me (regarding the book as a second edition). on july th i have some negociation about it with deighton the bookseller.--on may th i have a note from whewell about a number of crystals of plagiedral quartz, in which he was to observe the crystalline indication, and i the optical phenomena.--the report of the syndicate for visiting the observatory is dated june th: it is highly laudatory.--the proctor (barnard of king's college) requested me to name the moderator for the next b.a. examination: i named mr challis. "on june th my wife and i went, in company with professor and mrs henslow, to london and oxford; at oxford we were received in christchurch college by dr and mrs buckland. my wife and i then went to bedford to visit capt. and mrs smyth, and returned to cambridge on the rd. on july th we went on a visit to my mother and uncle at playford. while there i took a drive with my uncle into some parts near the valley of the gipping, in which i thought that the extent of the chalk was inadequately exhibited on greenough's map, and communicated my remarks to buckland. "i find letters from dr robinson and col. colby about determining longitudes of certain observatories by fire signals: i proposed chronometers as preferable. also from herschel, approving of my second volume of observations: and from f. baily, disclaiming the origination of the attack on the old nautical almanac (with which i suppose i had reproached him). on july th i received a summons from south to a committee for improving the nautical almanac; and subsequently a letter from baily about schumacher's taking offence at a passage of mine in the cambridge observations, on the comparative merits of ephemerides, which i afterwards explained to his satisfaction. "on aug. th my wife and i started for edensor, and after a short stay there proceeded by manchester to cumberland, where we made many excursions. we returned by edensor, and reached cambridge on oct. th, bringing my wife's sister susanna on a visit. my mother had determined, as soon as my intention of marriage was known to her, to quit the house, although always (even to her death) entertaining the most friendly feelings and fondness for my wife. it was also judged best by us all that my sister should not reside with us as a settled inhabitant of the house. they fixed themselves therefore at playford in the farm-house of the luck's farm, then in the occupation of my uncle arthur biddell. on oct. st i have a letter from my sister saying that they were comfortably settled there. "in this month of october (principally, i believe) i made some capital experiments on quartz, which were treated mathematically in a paper communicated in the next year to the cambridge philosophical society. in some of these my wife assisted me, and also drew pictures.--on nov. th the grace for paying me _£ . s. d._ to make my income up to _£ _ passed the senate.--i made three journeys to london to attend committees, one a committee on the nautical almanac, and one a royal society committee about two southern observatories.--on dec. st i have a letter from maclear (medical practitioner and astronomer at biggleswade) about occultations.--in this december i had a quartz object-glass by cauchaix mounted by dollond, and presented it to the observatory.--in this december occurred the alarm from agrarian fires. there was a very large fire at coton, about a mile from the observatory. this created the most extraordinary panic that i ever saw. i do not think it is possible, without having witnessed it, to conceive the state of men's minds. the gownsmen were all armed with bludgeons, and put under a rude discipline for a few days." "on jan. th i went with my wife, first to miss sheepshanks in london, at , woburn place, and next to the house of my wife's old friend, the rev. john courtney, at sanderstead, near croydon. i came to london on one day to attend a meeting of the new board of visitors of the greenwich observatory. formerly the board of visitors consisted of the council of the royal society with persons invited by them (in which capacity i had often attended). but a reforming party, of which south, babbage, baily and beaufort were prominent members, had induced the admiralty to constitute a new board, of which the plumian professor was a member. mr pond, the astronomer royal, was in a rather feeble state, and south seemed determined to bear him down: sheepshanks and i did our best to support him. (i have various letters from sheepshanks to this purpose.)--on jan. nd we returned to cambridge, and i set an examination paper for smith's prizes as usual.--on jan. th i have a letter from herschel about improving the arrangement of pond's observations. i believe that much of this zeal arose from the example of the cambridge observations. "on feb. st my paper 'on the nature of the light in the two rays of quartz' was communicated to the philosophical society: a capital piece of deductive optics. on mar. nd i went to london, i suppose to attend the board of visitors (which met frequently, for the proposed reform of pond's observations, &c.). as i returned on the outside of the coach there occurred to me a very remarkable deduction from my ideas about the rays of quartz, which i soon tried with success, and it is printed as an appendix to the paper above mentioned. on mar. th my son george richard was born." miscellaneous matters in the first half of this year are as follows: "faraday sends me a piece of glass for amici (he had sent me a piece before).--on apr. th i dispatched the preface of my observations: this implies that all was printed.--on apr. th i began my lectures and finished on may th. there were names. a very good series of lectures.--i think it was immediately after this, at the visitation of the cambridge observatory, that f. baily and lieut. stratford were present, and that sheepshanks went to tharfield on the royston downs to fire powder signals to be seen at biggleswade (by maclear) and at bedford (by capt. smyth) as well as by us at cambridge.--on may th i received _£ _ for my article on the figure of the earth from baldwin the publisher of the encyclopaedia metropolitana.--i attended the greenwich visitation on june rd.--on june th the observatory syndicate made their report: satisfactory. "on july th i started with my wife and infant son for edensor, and went on alone to liverpool. i left for dublin on the day on which the loss of the 'rothsay castle' was telegraphed, and had a bad voyage, which made me ill during my whole absence. after a little stay in dublin i went to armagh to visit dr robinson, and thence to coleraine and the giant's causeway, returning by belfast and dublin to edensor. we returned to cambridge on sept. th. "up to this time the observatory was furnished with only one large instrument, namely the -foot transit. on feb. th of this year i had received from thomas jones ( , charing cross) a sketch of the stone pier for mounting the equatoreal which he was commissioned to make: and the pier was prepared in the spring or summer. on sept. th part of the instrument was sent to the observatory; other parts followed, and jones himself came to mount it. on sept. th i received simms's assurance that he was hastening the mural circle.--in this autumn i seriously took up the recalculation of my long inequality of venus and the earth, and worked through it independently; thus correcting two errors. on nov. th i went to slough, to put my paper in the hands of mr herschel for communication to the royal society. the paper was read on nov. th.--this was the year of the first meeting of the british association at york. the next year's meeting was to be at oxford, and on oct. th i received from the rev. w. vernon harcourt an invitation to supply a report on astronomy, which i undertook: it employed me much of the winter, and the succeeding spring and summer.--the second edition of my tracts was ready in october. it contained, besides what was in the first edition, the planetary theory, and the undulatory theory of light. the profit was _£ _.--on nov. th i presented to the cambridge philosophical society a paper 'on a remarkable modification of newton's rings': a pretty good paper.--in november the copley medal was awarded to me by the royal society for my advances in optics.--amongst miscellaneous matters i was engaged in correspondence with col. colby and capt. portlock about the irish triangulation and its calculation. also with the admiralty on the form of publication of the greenwich and cape observations." "in january my examination paper for smith's prizes was prepared as usual.--two matters (in addition to the daily routine of observatory work) occupied me at the beginning of this year. one was the translation of encke's paper in successive numbers of the astronomische nachrichten concerning encke's comet; the university press printed this gratuitously, and i distributed copies, partly by the aid of capt. beaufort.--the other was the report on astronomy for the british association, which required much labour. my reading for it was principally in the university library (possibly some in london), but i borrowed some books from f. baily, and i wrote to capt. beaufort about the possible repetition of lacaille's meridian arc at the cape of good hope. the report appears to have been finished on may nd.--at this time the reform bill was under discussion, and one letter written by me (probably at sheepshanks's request) addressed i think to mr drummond, lord althorp's secretary, was read in the house of commons. "optics were not neglected. i have some correspondence with brewster and faraday. on mar. th i gave the cambridge philosophical society a paper 'on a new analyzer,' and on mar. th one 'on newton's rings between two substances of different refractive powers,' both papers satisfactory to myself.--on the death of mr f. fallows, astronomer at the cape of good hope observatory, the admiralty appointed mr henderson, an edinburgh lawyer, who had done some little things in astronomical calculation. on jan. th i discussed with him observations to be made, and drew up his official instructions which were sent on jan. th.--on feb. th sir james south writes that encke's comet is seen: also that with his -inch achromatic, purchased at paris, and which he was preparing to mount equatoreally, he had seen the disk of aldebaran apparently bisected by the moon's limb.--capt. beaufort and d. gilbert write in march about instructions to dunlop, the astronomer at paramatta. i sent a draft to capt. beaufort on apr. th. "the preface to my observations is dated mar. th. the distribution of the book would be a few weeks later.--on may th i began my lectures: names: i finished on may th.--the mounting of the equatoreal was finished some time before the syndicate visitation at the end of may, but jones's charge appeared to be exorbitant: i believe it was paid at last, but it was considered unfair.--on june nd i went to london: i presume to the greenwich visitation.--i went to oxford to the meeting of the british association (lodging i think with prof. rigaud at the observatory) on june th, and read part of my report on astronomy in the theatre. "on june th i started with my wife for the highlands of scotland. after a short stay at edensor, we went by carlisle to glasgow, and through the lake district to inverness. thence by auchnanault to balmacarra, where we were received by mr lillingstone. after an expedition in skye, we returned to balmacarra, and passed on to invermoriston, where we were received by grant of glenmoriston. we then went to fort william and oban, and crossed over to mull, where we were received by maclean of loch buy. we returned to oban and on to edinburgh, where we made a short stay. then to melrose, where we were received by sir d. brewster, and by edensor to cambridge, where we arrived on sept. th. "i received (at edinburgh i believe) a letter from arago, writing for the plans of our observing-room shutters.--mr vernon harcourt wrote deprecating the tone of my report on astronomy as related to english astronomers, but i refused to alter a word.--sheepshanks wrote in september in great anxiety about the cambridge circle, for which he thought the pier ought to be raised: i would have no such thing, and arranged it much more conveniently by means of a pit. on oct. th simms says that he will come with the circle immediately, and jones on sept. th says that he will make some alteration in the equatoreal: thus there was at last a prospect of furnishing the observatory properly.--on oct. th, i have encke's thanks for the translation of the comet paper.--one of the desiderata which i had pointed out in my report on astronomy was the determination of the mass of jupiter by elongations of the th satellite: and as the equatoreal of the cambridge observatory was on the point of coming into use, i determined to employ it for this purpose. it was necessary for the reduction of the observations that i should prepare tables of the motion of jupiter's th satellite in a form applicable to computations of differences of right-ascension. the date of my tables is oct. rd, .--in october the observatory syndicate made their report: quite satisfactory. "on oct. th sheepshanks wrote asking my assistance in the penny cyclopaedia: i did afterwards write 'gravitation' and 'greenwich.' --capt. beaufort wrote in november to ask my opinion on the preface to an edition of groombridge's catalogue which had been prepared by h. taylor: sheepshanks also wrote; he had objected to it. this was the beginning of an affair which afterwards gave me great labour.--vernon harcourt writes, much offended at some terms which i had used in reference to an office in the british association. "the equatoreal mounting which troughton and simms had been preparing for sir james south's large telescope had not entirely succeeded. i have various letters at this time from sheepshanks and simms, relating to the disposition which sir james south shewed to resist every claim till compelled by law to pay it.--a general election of members of parliament was now coming on: mr lubbock was candidate for the university. on nov. th i had a letter from sedgwick requesting me to write a letter in the newspapers in favour of lubbock; which i did. on dec. th i have notice of the county voting at newmarket on dec. th and th: i walked there to vote for townley; he lost the election by two or three votes in several thousands. "the mural circle was now nearly ready in all respects, and it was known that another assistant would be required. mr richardson (one of the assistants of greenwich observatory) and mr simms recommended to me mr glaisher, who was soon after appointed, and subsequently became an assistant at greenwich.--on dec. th i have a letter from bessel (the first i believe). i think that i had written to him about a general reduction of the greenwich planetary observations, using his tabulae regiomontanae as basis, and that this was his reply approving of it." "on jan. th my daughter elizabeth was born.--i prepared an examination paper for smith's prizes as usual.--on jan. th i received notice from simms that he had received payment (_£ _) for the mural circle from the vice-chancellor. about this time the circle was completely made serviceable, and i (with mr glaisher as assistant) immediately began its use. a puzzling apparent defect in the circle (exhibiting itself by the discordance of zenith points obtained by reflection observations on opposite sides of the zenith) shewed itself very early. on feb. th i have letters about it from sheepshanks and simms.--on jan. th i received notice from f. baily that the astronomical society had awarded me their medal for my long inequality of venus and the earth: on feb. th i went to london, i suppose to receive the medal.--i also inspected sir j. south's telescope, then becoming a matter of litigation, and visited mr herschel at slough: on feb. th i wrote to sir j. south about the support of the instrument, hoping to remove one of the difficulties in the litigation; but it produced no effect.--herschel wrote to me, from poisson, that pontécoulant had verified my long inequality. "mar. th is the date of the preface to my volume of observations: it was of course distributed a few weeks later.--in my report on astronomy i had indicated the mass of jupiter as a subject requiring fresh investigation. during the last winter i had well employed the equatoreal in observing elongations in r.a. of the th satellite. to make these available it was necessary to work up the theory carefully, in which i discovered some remarkable errors of laplace. some of these, for verification, i submitted to mr lubbock, who entirely agreed with me. the date of my first calculations of the mass of jupiter is mar. st: and shortly after that i gave an oral account of them to the cambridge philosophical society. the date of my paper for the astronomical society is april th. the result of my investigations (which was subsequently confirmed by bessel) entirely removed the difficulty among astronomers; and the mass which i obtained has ever since been received as the true one. "on apr. th my wife's two sisters, elizabeth and georgiana smith, came to stay with me.--on apr. nd i began lectures, and finished on may st: there were names. during the course of the lectures i communicated a paper to the philosophical society 'on the calculation of newton's experiments on diffraction.'--i went to london on the visitation of the greenwich observatory: the dinner had been much restricted, but was now made more open.--it had been arranged that the meeting of the british association was to be held this year at cambridge. i invited sir david brewster and mr herschel to lodge at the observatory. the meeting lasted from june th to th. we gave one dinner, but had a breakfast party every day. i did not enter much into the scientific business of the meeting, except that i brought before the committee the expediency of reducing the greenwich planetary observations from . they agreed to represent it to the government, and a deputation was appointed (i among them) who were received by lord althorp on july th. on aug. rd herschel announced to me that _£ _ was granted. "on aug. th i started with my wife for edensor. at leicester we met sedgwick and whewell: my wife went on to edensor, and i joined sedgwick and whewell in a geological expedition to mount sorrel and various parts of charnwood forest. we were received by mr allsop of woodlands, who proved an estimable acquaintance. this lasted four or five days, and we then went on to edensor.--on aug. th herschel wrote to me, communicating an offer of the duke of northumberland to present to the cambridge observatory an object-glass of about inches aperture by cauchaix. i wrote therefore to the duke, accepting generally. the duke wrote to me from buxton on aug. rd (his letter, such was the wretched arrangement of postage, reaching bakewell and edensor on the th) and on the th i drove before breakfast to buxton and had an interview with him. on sept. st the duke wrote, authorizing me to mount the telescope entirely, and he subsequently approved of cauchaix's terms: there was much correspondence, but on dec. th i instructed cauchaix how to send the telescope.--on our return we paid a visit to dr davy, master of caius college, at heacham, and reached cambridge on oct. th. "groombridge's catalogue, of which the editing was formally entrusted to mr henry taylor (son of taylor the first-assistant of the greenwich observatory), had been in some measure referred to sheepshanks: and he, in investigating the work, found reason for thinking the whole discreditable. about may he first wrote to me on his rising quarrel with h. taylor, but on sept. th he found things coming to a crisis, and denounced the whole. capt. beaufort the hydrographer (in whose office this matter rested) begged me with baily to decide upon it. we did not at first quite agree upon the terms of investigation &c., but after a time all was settled, and on oct. th the admiralty formally applied, and i formally accepted. little or nothing had been done by mr baily and myself, when my work was interrupted by illness. "sheepshanks had thought that something might be done to advance the interests of myself or the observatory by the favour of lord brougham (then lord chancellor), and had urged me to write an article in the penny cyclopaedia, in which lord brougham took great interest. i chose the subject 'gravitation,' and as i think wrote a good deal of it in this autumn: when it was interrupted by my illness. "on dec. th , having at first intended to attend the meeting of the philosophical society and then having changed my mind, i was engaged in the evening on the formulae for effects of small errors on the computation of the solar eclipse of . a dizziness in my head came on. i left off work, became worse, and went to bed, and in the night was in high fever with a fierce attack of scarlet fever. my wife was also attacked but very slightly. the first day of quitting my bedroom was dec. st. somewhere about the time of my illness my wife's sister, susanna smith, who was much reduced in the summer, died of consumption. "miscellaneous notes in are as follows: henderson (at the cape) could not endure it much longer, and on oct. th stratford writes that maclear had just sailed to take his place: henderson is candidate for the edinburgh observatory.--stratford writes on dec. nd that the madras observations have come to england, the first whose arrangement imitates mine.--on nov. rd herschel, just going to the cape, entrusted to me the revisal of some proof sheets, if necessary: however it was never needed.--in november i sat for my portrait to a painter named purdon (i think): he came to the house and made a good likeness. a pencil portrait was taken for a print-seller (mason) in cambridge: it was begun before my illness and finished after it.--i applied through sheepshanks for a copy of maskelyne's observations, to be used in the reduction of the planetary observations: and on dec. th (from my bedroom) i applied through prof. rigaud to the delegates of the clarendon press for a copy of bradley's observations for the same. the latter request was refused. in october i applied to the syndics of the university press for printed forms for these reductions: the syndics agreed to grant me , copies." "on jan. th i went with my wife to london for the recruiting of my strength. we stayed at the house of our friend miss sheepshanks, and returned on feb. th.--i drew up a paper of questions for smith's prizes, but left the whole trouble of examination and adjudication to professor miller, who at my request acted for me.--while i was in london i began to look at the papers relating to groombridge's catalogue: and i believe that it was while in london that i agreed with mr baily on a report condemnatory of h. taylor's edition, and sent the report to the admiralty. the admiralty asked for further advice, and on feb. th i replied, undertaking to put the catalogue in order. on mar. th capt. beaufort sent me all the papers. some time however elapsed before i could proceed with it. "there was in this spring a furious discussion about the admission of dissenters into the university: i took the liberal side. on apr. th there was a letter of mine in the cambridge newspaper.--on apr. th i began lectures, and finished on may th: there were names.--my 'gravitation' was either finished or so nearly finished that on jan. th i had some conversation with knight the publisher about printing it. it was printed in the spring, and on apr. th sheepshanks sent a copy of it to lord brougham. i received from knight _£ . s. d._ for this paper.--on may th i went to london, i believe to attend one of the soirées which the duke of sussex gave as president of the royal society. the duke invited me to breakfast privately with him the next morning. he then spoke to me, on the part of the government, about my taking the office of astronomer royal. on may th i wrote him a semi-official letter, to which reference was made in subsequent correspondence on that subject. "on may th my son arthur was born.--in june the observatory syndicate made a satisfied report.--on june th i went to the greenwich visitation, and again on june th i went to london, i believe for the purpose of trying the mounting of south's telescope, as it had been strengthened by mr simms by sheepshanks's suggestions. i was subsequently in correspondence with sheepshanks on the subject of the arbitration on south's telescope, and my giving evidence on it. on july th, as i was shortly going away, i wrote him a report on the telescope, to be used in case of my absence. the award, which was given in december, was entirely in favour of simms.--on july rd i went out, i think to my brother's marriage at ixworth in suffolk.--on aug. st i started for edensor and cumberland, with my wife, sister, and three children: georgiana smith joined us at edensor. we went by otley, harrogate, ripon, and stanmoor to keswick, from whence we made many excursions. on aug. th i went with whewell to the clouds on skiddaw, to try hygrometers. mr baily called on his way to the british association at edinburgh. on sept. th we transferred our quarters to ambleside, and after various excursions we returned to edensor by skipton and bolton. on sept. th i went to doncaster and finningley park to see mr beaumont's observatory. on sept. th we posted in one day from edensor to cambridge. "on aug. th mr spring rice (lord monteagle) wrote to me to enquire whether i would accept the office of astronomer royal if it were vacant. i replied (from keswick) on aug. th, expressing my general willingness, stipulating for my freedom of vote, &c., and referring to my letter to the duke of sussex. on oct. th lord auckland, first lord of the admiralty, wrote: and on oct. th i provisionally accepted the office. on oct. th i wrote to ask for leave to give a course of lectures at cambridge in case that my successor at cambridge should find difficulty in doing it in the first year: and to this lord auckland assented on oct. st. all this arrangement was for a time upset by the change of ministry which shortly followed. "amongst miscellaneous matters, in march i had some correspondence with the duke of northumberland about the cauchaix telescope. in august i had to announce to him that the flint-lens had been a little shattered in cauchaix's shop and required regrinding: finally on dec. th i announced its arrival at cambridge.--in the planetary reductions, i find that i employed one computer (glaisher) for weeks.--in november the lalande medal was awarded to me by the french institut, and mr pentland conveyed it to me in december.--on march th i gave the cambridge philosophical society a paper, 'continuation of researches into the value of jupiter's mass.' on apr. th, 'on the latitude of cambridge observatory.' on june th, 'on the position of the ecliptic,' and 'on the solar eclipse of ,' to the royal astronomical society. on nov. th, 'on computing the diffraction of an object glass,' to the cambridge society. and on dec. rd, 'on the calculation of perturbations,' to the nautical almanac: this paper was written at keswick between aug. nd and th.--i also furnished mr sheepshanks with investigations regarding the form of the pivots of the cape circle." "on jan. th i was elected correspondent of the french academy; and on jan. th mr pentland sent me _£ . s._, the balance of the proceeds of the lalande medal fund.--i prepared my paper for smith's prizes, and joined in the examination as usual. "there had been a very sudden change of administration, and sir r. peel was now prime minister as first lord of the treasury, and lord lyndhurst was lord chancellor. on jan. th i wrote to lord lyndhurst, asking him for a suffolk living for my brother william, which he declined to give, though he remembered my application some years later. whether my application led to the favour which i shortly received from the government, i do not know. but, in dining with the duke of sussex in the last year, i had been introduced to sir r. peel, and he had conversed with me a long time, and appeared to have heard favourably of me. on feb. th he wrote to me an autograph letter offering a pension of _£ _ per annum, with no terms of any kind, and allowing it to be settled if i should think fit on my wife. i wrote on feb. th accepting it for my wife. in a few days the matter went through the formal steps, and mr whewell and mr sheepshanks were nominated trustees for my wife. the subject came before parliament, by the whig party vindicating their own propriety in having offered me the office of astronomer royal in the preceding year; and spring rice's letter then written to me was published in the times, &c." * * * * * the correspondence relating to the pension above-mentioned is given below, and appears to be of interest, both as conveying in very felicitous terms the opinion of a very eminent statesman on the general subject of such pensions, and as a most convincing proof of the lofty position in science which the subject of this memoir had then attained. whitehall gardens, _feb. _. sir, you probably are aware that in a resolution voted by the house of commons in the last session of parliament, an opinion was expressed, that pensions on the civil list, ought not thereafter to be granted by the crown excepting for the satisfaction of certain public claims, among which those resting on scientific or literary eminence were especially mentioned. i trust that no such resolution would have been necessary to induce me as minister of the crown fully to recognize the justice of such claims, but i refer to the resolution, as removing every impediment to a communication of the nature of that which i am about to make to you. in acting upon the principle of the resolution in so far as the claims of science are concerned, my _first_ address is made to you, and made directly, and without previous communication with any other person, because it is dictated exclusively by public considerations, and because there can be no advantage in or any motive for indirect communication. i consider you to have the first claim on the royal favour which eminence in those high pursuits to which your life is devoted, can give, and i fear that the emoluments attached to your appointment in the university of cambridge are hardly sufficient to relieve you from anxiety as to the future on account of those in whose welfare you are deeply interested. the state of the civil list would enable me to advise the king to grant a pension of three hundred pounds per annum, and if the offer be acceptable to you the pension shall be granted either to mrs airy or yourself as you may prefer. i beg you distinctly to understand that your acquiescence in this proposal, will impose upon you no obligation personal or political in the slightest degree. i make it solely upon public grounds, and i ask you, by the acceptance of it, to permit the king to give some slight encouragement to science, by proving to those who may be disposed to follow your bright example, that devotion to the highest branches of mathematical and astronomical knowledge shall not necessarily involve them in constant solicitude as to the future condition of those, for whom the application of the same talents to more lucrative pursuits would have ensured an ample provision. i have the honor to be, sir, with true respect and esteem, your faithful servant, robert peel. _mr professor airy, &c., &c., cambridge_. observatory, cambridge, _ , feb. _. sir, i have the honor to acknowledge your letter of the th acquainting me with your intention of advising the king to grant a pension of _£ _ per annum from the civil list to me or mrs airy. i trust you will believe that i am sensible of the flattering terms in which this offer is made, and deeply grateful for the considerate manner in which the principal arrangement is left to my choice, as well as for the freedom from engagement in which your offer leaves me. i beg to state that i most willingly accept the offer. i should prefer that the pension be settled on mrs airy (by which i understand that in case of her surviving me the pension would be continued to her during her life, or in the contrary event would cease with her life). i wish that i may have the good fortune to prove to the world that i do not accept this offer without an implied engagement on my part. i beg leave again to thank you for your attention, and to assure you that the form in which it is conveyed makes it doubly acceptable. with sincere respect i have the honor to be, sir, your very faithful servant, g.b. airy. _the right hon. sir robert peel, bart., first lord of the treasury, &c., &c._ whitehall, _feb. th _. sir, i will give immediate directions for the preparation of the warrant settling the pension on mrs airy--the effect of which will be, as you suppose, to grant the pension to her for her life. i assure you i never gave an official order, which was accompanied with more satisfaction to myself than this. i have the honor to be, sir. your faithful servant, robert peel. _mr professor airy, &c., &c., cambridge_. * * * * * "on march th i started (meeting sheepshanks at kingstown) for ireland. we visited dublin observatory, and then went direct to markree near sligo, to see mr cooper's telescope (our principal object). we passed on our return by enniskillen and ballyjamesduff, where my former pupil p. morton was living, and returned on apr. rd.--on apr. th i was elected to the royal society, edinburgh.--apr. nd my wife wrote me from edensor that her sister florence was very ill: she died shortly after.--on may th i began lectures and finished on may th: there were names.--my former pupil guest asks my interest for the recordership of birmingham.--in june was circulated the syndicate report on the observatory.--the date of the preface to the observations is june th. "the ministry had been again changed in the spring, and the whigs were again in power. on june th lord auckland, who was again first lord of the admiralty (as last year), again wrote to me to offer me the office of astronomer royal, or to request my suggestions on the filling up of the office. on june th i wrote my first reply, and on june th wrote to accept it. on june th lord auckland acknowledges, and on june nd the king approved. lord auckland appointed to see me on friday, june rd, but i was unwell. i had various correspondence with lord auckland, principally about buildings, and had an appointment with him for august th. as lord auckland was just quitting office, to go to india, i was introduced to mr charles wood, the secretary of the admiralty, with whom principally the subsequent business was transacted. at this meeting lord auckland and mr wood expressed their feeling, that the observatory had fallen into such a state of disrepute that the whole establishment ought to be cleared out. i represented that i could make it efficient with a good first assistant; and the other assistants were kept. but the establishment was in a queer state. the royal warrant under the sign manual was sent on august th. it was understood that my occupation of office would commence on october st, but repairs and alterations of buildings would make it impossible for me to reside at greenwich before the end of the year. on oct. st i went to the observatory, and entered formally upon the office (though not residing for some time). oct th is the date of my official instructions. "i had made it a condition of accepting the office that the then first assistant should be removed, and accordingly i had the charge of seeking another. i determined to have a man who had taken a respectable cambridge degree. i made enquiry first of mr bowstead (brother to the bishop) and mr steventon: at length, consulting mr hopkins (a well-known private tutor at cambridge), he recommended to me mr robert main, of queens' college, with whom i corresponded in the month (principally) of august, and whom on august th i nominated to the admiralty. on oct. st f.w. simms, one of the assistants (who apparently had hoped for the office of first assistant, for which he was quite incompetent) resigned; and on dec. th i appointed in his place mr james glaisher, who had been at cambridge from the beginning of , and on dec. th the admiralty approved. "during this quarter of a year i was residing at cambridge observatory, visiting greenwich once a week (at least for some time), the immediate superintendence of the observatory being placed with mr main. i was however engaged in reforming the system of the greenwich observatory, and prepared and printed skeleton forms for reductions of observations and other business. on dec. th i resigned my professorship to the vice-chancellor. but i continued the reduction of the observations, so that not a single figure was left to my successor: the last observations were those of halley's comet. the preface to my cambridge observations is dated aug. nd, . "in regard to the northumberland telescope, i had for some time been speculating on plans of mounting and enclosing the instrument, and had corresponded with simms, a. biddell, cubitt, and others on the subject. on apr. th tulley the younger was endeavouring to adjust the object-glass. on may st i plainly asked the duke of northumberland whether he would defray the expense of the mounting and building. on june th he assented, and money was placed at a banker's to my order. i then proceeded in earnest: in the autumn the building was erected, and the dome was covered before the depth of winter. i continued in to superintend the mounting of the instrument. "in regard to the planetary reductions: to july th j. glaisher had been employed weeks, and from july th to jan. th, , weeks. mr spring rice, when chancellor of the exchequer, had promised money, but no official minute had been made, and no money had been granted. on aug. st i applied to mr baring (secretary of the treasury). after another letter he answered on oct. th that he found no official minute. after writing to vernon harcourt and to spring rice, the matter was arranged: my outlay was refunded, and another sum granted.--in regard to groombridge's observations, i find that on dec. th certain trial reductions had been made under my direction by j. glaisher.--i had attempted some optical experiments in the summer, especially on the polarization of sky-light; but had been too busy with the observatory to continue them. "in august my wife was in a critical state of health.--in december i received information regarding merchant ships' chronometers, for which i had applied to mr charles parker of liverpool.--on dec. th mr spring rice and lord john russell offered me knighthood, but i declined it.--on july rd i went into suffolk with my wife's sisters elizabeth and georgiana, and returned on august rd: this was all the holiday that i got in this year.--on the th of august i saw mr taylor, the admiralty civil architect in london, and the extension of buildings at greenwich observatory was arranged.--i made various journeys to greenwich, and on dec. th, having sent off our furniture, we all quitted the cambridge observatory, and stayed for some days at the house of miss sheepshanks. "thus ended a busy and anxious year." * * * * * with reference to the offer of knighthood above-mentioned, airy's reply is characteristic, and the short correspondence relating to it is therefore inserted.--the offer itself is an additional proof of the high estimation in which he stood at this time. downing street, _dec. th _. my dear sir, i have been in communication with my colleague lord john russell which has made me feel rather anxious to have the pleasure of seeing you, but on second thoughts it has occurred to me that the subject of my communication would render it more satisfactory to you to receive a letter than to pay a visit. in testimony of the respect which is felt for your character and acquirements, there would be every disposition to recommend you to his majesty to receive the distinction of knighthood. i am quite aware that to you individually this may be a matter of small concern, but to the scientific world in general it will not be indifferent, and to foreign countries it will mark the consideration felt for you personally as well as for the position which you occupy among your learned contemporaries. from a knowledge of the respect and esteem which i feel for you lord john russell has wished that the communication should be made through me rather than through any person who had not the pleasure of your acquaintance. pray let me hear from you and believe me my dear sir, with compliments to mrs airy, very truly yours, t. spring rice. p.s.--it may be right to add that when a title of honor is conferred on grounds like those which apply to your case, no fees or charges of any kind would be payable. observatory, cambridge, _ , dec. th_. my dear sir, i beg to acknowledge your letter of the th, which i have received at this place, conveying to me an intimation of the wish of his majesty's ministers to recommend me to the king for the honor of knighthood. i beg to assure you that i am most sensible to the liberality which i have experienced from the government in other as well as in pecuniary matters, and that i am very highly gratified by the consideration (undeserved by me, i fear) which they have displayed in the present instance. and if i now request permission to decline the honor offered to me, i trust i may make it fully understood that it is not because i value it lightly or because i am not anxious to receive honors from such a source. the unalterable custom of this country has attached a certain degree of light consideration to titles of honor which are not supported by considerable fortune; or at least, it calls for the display of such an establishment as may not be conveniently supported by even a comfortable income. the provision attached to my official situation, and the liberality of the king towards one of the members of my family, have placed me in a position of great comfort. these circumstances however have bound me to consider myself as the devoted servant of the country, and to debar myself from efforts to increase my fortune which might otherwise have been open to me. i do not look forward therefore to any material increase of income, and that which i enjoy at present is hardly sufficient, in my opinion, to support respectably the honor which you and lord john russell have proposed to confer upon me. for this reason only i beg leave most respectfully to decline the honor of knighthood at the present time. i have only to add that my services will always be at the command of the government in any scientific subject in which i can be of the smallest use. i am, my dear sir, your very faithful servant, g.b. airy. _the right honorable t. spring rice_. * * * * * "in brief revision of the years from to i may confine myself to the two principal subjects--my professorial lectures, and my conduct of the cambridge observatory. "the lectures as begun in included ordinary mechanics, ordinary hydrostatics and pneumatics (i think that i did not touch, or touched very lightly, on the subjects connected with the hydraulic ram), and ordinary optics (with a very few words on polarization and depolarization). in the two first were generally improved, and for the third (optics) i introduced a few words on circular polarization. i believe that it was in that i made an addition to the syllabus with a small engraving, shewing the interference of light in the best practical experiment (that of the flat prism); and i went thoroughly into the main points of the undulatory theory, interference, diffraction, &c. in i believe i went (in addition to what is mentioned above) into polarization and depolarization of all kinds. my best lecture diagrams were drawn and painted by my wife. the lectures were universally pronounced to be valuable. the subjects underwent no material change in , , , , ; and i believe it was a matter of sincere regret to many persons that my removal to greenwich terminated the series. each lecture nominally occupied an hour. but i always encouraged students to stop and talk with me; and this supplement was usually considered a valuable part of the lecture. practically the lecture, on most days, occupied two hours. i enjoyed the lectures much: yet i felt that the labour (in addition to other work) made an impression on my strength, and i became at length desirous of terminating them. "the observatory, when i took charge of it, had only one instrument--the transit-instrument the principles however which i laid down for my own direction were adapted to the expected complete equipment, planets (totally neglected at greenwich) were to be observed. observations were to be reduced completely, and the reductions were to be exhibited in an orderly way: this was a novelty in astronomy. i considered it so important that i actually proposed to omit in my publication the original observations, but was dissuaded by herschel and others. i sometimes suspended, observations for a short time, in order to obtain leisure for; the reductions. i had at first no intention of correcting the places of the fundamental stars as settled at greenwich. but i found myself compelled to do so, because they were not sufficiently accurate; and then i took the course of observing and reducing as an independent observer, without reference to any other observatory. i introduced the principle of not correcting instrumental errors, but measuring them and applying numerical corrections. i determined my longitude by chronometers, and my latitude by a repeating circle borrowed from mr sheepshanks, which i used so well that the result; was only half a second in error. the form of my reductions in the published volume for is rather irregular, but the matter is good: it soon attracted attention. in the process was much the same: i had an assistant, mr baldrey. in still the same, with the additions:--that i formally gave the corrections of relative right-ascension of fundamental stars (without alteration of equinox, which i had not the means of obtaining) to be used in the year ; and that i reduced completely the observed occultations (with a small error, subsequently corrected). in the system of correction of broken transits was improved: the errors of assumed r.a. of fundamental stars were exhibited: mean solar time was obtained from sidereal time by time of transit of [symbol: aries] (computed by myself): the method of computing occultations was improved. in the small equatoreal was erected, and was soon employed in observations of the elongation of the th satellite of jupiter for determining the mass of jupiter. the mural circle was erected at the end of the year, but not used. the calculation of r.a. of fundamental stars was made homogeneously with the others: separate results of all were included in ledgers: a star-catalogue was formed: all as to the present time ( ). with the equatoreal the difference of n.p.d. of mars and stars was observed. "with the beginning of the mural circle was established at work, a second assistant (mr glaisher) was appointed, and the observatory might be considered complete. i made experiments on the graduations of the circle. i detected and was annoyed by the r--d. i determined the latitude. i exhibited the separate results for n.p.d. of stars in ledger, and their means in catalogue. i investigated from my observations the place of equinox and the obliquity of the ecliptic. i made another series of observations of jupiter's th satellite, for the mass of jupiter. i observed the solar eclipse with the equatoreal, by a method then first introduced, which i have since used several times at cambridge and greenwich with excellent effect. the moon and the planets were usually observed till near two in the morning. correction for defective illumination applied when necessary. the volume is very complete, the only deficiency being in the observation of moon and planets through the severe morning hours. in the only novelties are--examination of the graduations of the declination circle of the equatoreal (excessively bad): observations of a spot on jupiter for rotation, and of mars and stars. in (including january ) there is a more complete examination of the equatoreal graduations: parallax and refraction for equatoreal observations: a spot on jupiter: a series of observations on jupiter's th satellite for the mass of jupiter: mars and stars: halley's comet (the best series of observations which could be made in the season): and a short series of meteorological observations, on a plan suggested by sir john herschel then at the cape of good hope. "i cannot tell precisely in which year i introduced the following useful custom. towards the end of each year i procured a pocket-book for the following year with a space for every day, and carefully examining all the sources of elements of observations, and determining the observations to be made every day, i inserted them in the pocket-book. this system gave wonderful steadiness to the plan of observations for the next year. the system has been maintained in great perfection at the observatory of greenwich. (the first of these pocket-books which prof. adams has found is that for .) printed skeleton forms were introduced for all calculations from . in the greenwich observatory library there is a collection, i believe complete, of printed papers commencing with my manifesto, and containing all syndicate reports except for (when perhaps there was none). it seems from these that my first written report on observations, &c., was on may th, . the first syndicate report is on may th, ." * * * * * a few remarks on airy's private life and friends during his residence at cambridge observatory may be here appropriately inserted. amid the laborious occupations recorded in the foregoing pages, his social life and surroundings appear to have been most pleasant and congenial. at that period there were in residence in cambridge, and particularly at trinity, a large number of very brilliant men. airy was essentially a cambridge man. he had come up poor and friendless: he had gained friends and fame at the university, and his whole work had been done there. from the frequent references in after times both by him and his wife to their life at cambridge, it is clear that they had a very pleasant recollection of it, and that the social gatherings there were remarkably attractive. he has himself recorded that with whewell and sedgwick, and his accomplished sisters-in-law, who were frequently on long visits at the observatory, they formed pretty nearly one family. his friendship with whewell was very close. although whewell was at times hasty, and rough-mannered, and even extremely rude, yet he was generous and large-minded, and thoroughly upright. [footnote: the following passage occurs in a letter from airy to his wife, dated , sept. th: "i am sorry that ---- speaks in such terms of the 'grand master,' as she used to be so proud of him: it is only those who have _well_ gone through the ordeal of quarrels with him and almost insults from him, like sheepshanks and me, that thoroughly appreciate the good that is in him: i am sure he will never want a good word from me."] in power of mind, in pursuits, and interests, airy had more in common with whewell than with any other of his friends. it was with whewell that he undertook the experiments at dolcoath: it was to whewell that he first communicated the result of his remarkable investigation of the long inequality of venus and the earth; and some of his optical researches were conducted jointly with whewell. whewell took his degree in , seven years before airy, and his reputation, both for mathematical and all-round knowledge, was extremely and deservedly great, but he was always most generous in his recognition of airy's powers. thus in a letter of mar. th, (life of william whewell by mrs stair douglas), he says, "airy is certainly a most extraordinary man, and deserves everything that can be said of him"; and again in the autumn of he writes to his aunt, "you mentioned a difficulty which had occurred to you in one of your late letters; how airy should be made professor while i was here, who, being your nephew, must of course, on that account, deserve it better than he could. now it is a thing which you will think odd, but it is nevertheless true, that airy is a better mathematician than your nephew, and has moreover been much more employed of late in such studies.... seriously speaking, airy is by very much the best person they could have chosen for the situation, and few things have given me so much pleasure as his election." how much whewell depended upon his friends at the observatory may be gathered from a letter which he wrote to his sister on dec. st, . "we have lately been in alarm here on the subject of illness. two very near friends of mine, prof. and mrs airy, have had the scarlet fever at the same time; she more slightly, he very severely. they are now, i am thankful to say, doing well and recovering rapidly. you will recollect that i was staying with them at her father's in derbyshire in the summer. they are, i think, two of the most admirable and delightful persons that the world contains." and again on dec. th, , he wrote to his sister ann, "my friends--i may almost say my dearest friends --professor airy and his family have left cambridge, he being appointed astronomer royal at greenwich--to me an irreparable loss; but i shall probably go and see how they look in their new abode." their close intercourse was naturally interrupted by airy's removal to greenwich, but their friendly feelings and mutual respect continued without material break till whewell's death. there was frequent correspondence between them, especially on matters connected with the conduct and teaching of the university, in which they both took a keen interest, and a warm welcome at trinity lodge always awaited mr and mrs airy when they visited cambridge. in a letter written to mrs stair douglas on feb. th, , enclosing some of whewell's letters, there occurs the following passage: "after the decease of mrs whewell, whewell wrote to my wife a mournful letter, telling her of his melancholy state, and asking her to visit him at the lodge for a few days. and she did go, and did the honours of the house for several days. you will gather from this the relation in which the families stood." whewell died on mar. th, , from the effects of a fall from his horse, and the following extract is from a letter written by airy to whewell's niece, mrs sumner gibson, on hearing of the death of his old friend: "the master was, i believe, my oldest surviving friend (beyond my own family), and, after an acquaintance of years, i must have been one of his oldest friends. we have during that time been connected privately and officially: we travelled together and experimented together: and as opportunity served (but i need not say in very different degrees) we both laboured for our college and university. a terrible blank is left on my mind." sedgwick was probably years older than airy: he took his degree in . but the astonishing buoyancy of spirits and bonhomie of sedgwick fitted him for all ages alike. he was undoubtedly the most popular man in cambridge in modern times. his ability, his brightness and wit, his fearless honesty and uprightness, his plain-speaking and good humour, rendered him a universal favourite. his close alliance with airy was much more social than scientific. it is true that they made some geological excursions together, but, at any rate with airy, it was far more by way of recreation than of serious study, and sedgwick's science was entirely geological. their friendship continued till sedgwick's death, though it was once or twice imperilled by sedgwick's impulsive and hasty nature. peacock took his degree in (herschel's year), and was therefore probably years older than airy. he was the earliest and staunchest friend of airy in his undergraduate years, encouraged him in every possible way, lent him books, assisted him in his studies, helped him with wise advice on many occasions, and took the greatest interest in his success. he was a good and advanced mathematician, and with a great deal of shrewdness and common-sense he united a singular kindness and gentleness of manner. it is therefore not to be wondered at that he was regarded by airy with the greatest esteem and affection, and though they were afterwards separated, by peacock becoming dean of ely and airy astronomer royal, yet their warm friendship was never broken. the following letter, written by airy to mrs peacock on receiving the news of the death of the dean, well expresses his feelings towards his old friend: trinity lodge, cambridge, _ , dec. _. my dear madam, i have desired for some time to express to you my sympathies on occasion of the sad bereavement which has come upon me perhaps as strongly as upon any one not connected by family ties with my late friend. but i can scarcely give you an idea how every disposable moment of my time has been occupied. i am now called to cambridge on business, and i seize the first free time to write to you. my late friend was the first person whom i knew in college (i had an introduction to him when i went up as freshman). from the first, he desired me to consider the introduction not as entitling me to a mere formal recognition from him, but as authorizing me at all times to call on him for any assistance which i might require. and this was fully carried out: i referred to him in every difficulty: i had the entire command of his rooms and library (a very important aid in following the new course of mathematics which he had been so instrumental in introducing into the university) in his occasional absences: and in all respects i looked to him as to a parent. all my debts to other friends in the university added together are not comparable to what i owe to the late dean. latterly i need not say that i owed much to him and that i owe much to you for your kind notice of my two sons, even since the sad event which has put it out of his power to do more. in the past summer, looking to my custom of making a visit to cambridge in some part of the october term, i had determined that a visit to ely this year should not depend on the chance of being free to leave cambridge, but that, if it should be found convenient to yourself and the dean, the first journey should be made to ely. i wish that i had formed the same resolution one or two years ago. with many thanks for your kindness, and with deep sympathy on this occasion, i am, my dear madam, yours very faithfully, g.b. airy. sheepshanks was a fellow of trinity, in orders: he was probably seven years older than airy (he took his degree in ). he was not one of airy's earliest friends, but he had a great taste and liking for astronomy, and the friendship between them when once established became very close. he was a very staunch and fearless friend, an able and incisive writer, and remarkably energetic and diligent in astronomical investigations. he, or his sister, miss sheepshanks, had a house in london, and sheepshanks was very much in london, and busied himself extremely with the work of the royal observatory, that of the board of longitude, and miscellaneous astronomical matters. he was most hospitable to his friends, and while airy resided at cambridge his house was always open to receive him on his frequent visits to town. in the various polemical discussions on scientific matters in which airy was engaged, sheepshanks was an invaluable ally, and after airy's removal to greenwich had more or less separated him from his cambridge friends, sheepshanks was still associated with him and took a keen interest in his greenwich work. and this continued till sheepshanks's death. the warmest friendship always subsisted between the family at the observatory and mr and miss sheepshanks. there were many other friends, able and talented men, but these four were the chief, and it is curious to note that they were all much older than airy. it would seem as if airy's knowledge had matured in so remarkable a manner, and the original work that he produced was so brilliant and copious, that by common consent he ranked with men who were much his seniors: and the natural gravity and decorum of his manners when quite a young man well supported the idea of an age considerably greater than was actually the case. chapter v. at greenwich observatory-- to . "through the last quarter of i had kept everything going on at the greenwich observatory in the same manner in which mr pond had carried it on. with the beginning of my new system began. i had already prepared printed skeleton forms (a system totally unknown to mr pond) which were now brought into use. and, having seen the utility of the copying press in merchants' offices, i procured one. from this time my correspondence, public and private, is exceedingly perfect. "at this time the dwelling house was still unconnected with the observatory. it had no staircase to the octagon room. four new rooms had been built for me on the western side of the dwelling house, but they were not yet habitable. the north-east dome ground floor was still a passage room. the north terrace was the official passage to the north-west dome, where there was a miserable equatoreal, and to the -foot zenith tube (in a square tower like a steeple, which connected the n.w. dome with flamsteed's house). the southern boundary of the garden ran down a hollow which divides the peninsula from the site of the present magnetic observatory, in such a manner that the principal part of the garden was fully exposed to the public. the computing room was a most pitiful little room. there was so little room for me that i transported the principal table to a room in my house, where i conducted much of my own official business. a large useless reflecting telescope (ramage's), on the plan and nearly of the size of sir w. herschel's principal telescope, encumbered the centre of the front court. "on jan. th i addressed mr buck, agent of the princess sophia of gloucester, ranger of greenwich park, for leave to enclose a portion of the ground overlooking my garden. this was soon granted, and i was partially delivered from the inconvenience of the public gaze. the liberation was not complete till the magnetic ground was enclosed in . "in the inferior departments of the admiralty, especially in the hydrographic office (then represented by captain beaufort) with which i was principally connected, the observatory was considered rather as a place for managing government chronometers than as a place of science. the preceding first assistant (taylor) had kept a book of letter references, and i found that out of letters, related to government chronometers only. on jan. th i mentally sketched my regulations for my own share in chronometer business. i had some correspondence with captain beaufort, but we could not agree, and the matter was referred to the admiralty. finally arrangements were made which put the chronometer business in proper subordination to the scientific charge of the observatory. "in my first negociations with the admiralty referring to acceptance of the office of astronomer royal, in , lord auckland being then first lord of the admiralty, i had stipulated that, as my successor at cambridge would be unprepared to carry on my lectures, i should have permission to give a final course of lectures there. at the end of lord auckland was succeeded by lord minto: i claimed the permission from him and he refused it. when this was known in cambridge a petition was presented by many cambridge residents, and lord minto yielded. on april th i went to cambridge with my wife, residing at the bull inn, and began lectures on april st: they continued (apparently) to may th. my lecture-room was crowded (the number of names was ) and the lectures gave great satisfaction. i offered to the admiralty to put all the profits in their hands, and transmitted a cheque to the accountant general of the navy: but the admiralty declined to receive them. "on june th the annual visitation of the observatory was held, mr f. baily in the chair. i presented a written report on the observatory (a custom which i had introduced at cambridge) in which i did not suppress the expression of my feelings about chronometer business. the hydrographer, captain beaufort, who was one of the official visitors, was irritated: and by his influence the report was not printed. i kept it and succeeding reports safe for three years, and then the board of visitors agreed to print them; and four reports were printed together, and bound with the greenwich observations of . "in the course of this year i completed the volume of observations made at cambridge observatory in and on nov. th the printed copies were distributed. about the end of the dome for the northumberland telescope was erected: but apparently the polar frame was not erected." the following account of an accident which occurred during the construction of the dome is extracted from a letter by airy to his wife dated jan. st. "the workmen's account of the dome blowing off is very curious: it must have been a strange gust. it started suddenly when the men were all inside and beaumont was looking up at it: the cannon balls were thrown in with great violence (one of them going between the spokes of ransomes' large casting), and instantly after the dome had started, the boards of the outside scaffolding which had been tossed up by the same gust dropped down into the gap which the dome had left. it is a wonder that none of the men were hurt and that the iron was not broken. the dome is quite covered and i think does not look so well as when the hooping was visible." "previous to i had begun to contemplate the attachment of magnetic observations to the observatory, and had corresponded with prof. christie, prof. lloyd, prof. j. d. forbes, and mr gauss on the subject. on jan. th i addressed a formal letter to the admiralty, and on jan. th received their answer that they had referred it to the board of visitors. on march th i received authority for the expenditure of _£ _, and i believe that i then ordered merz's -foot magnet. the visitors met on feb. th and after some discussion the site was chosen and the extent of ground generally defined, and on dec. nd mr spring rice (lord monteagle) as chancellor of the exchequer virtually effected the transfer of the ground. but no further steps were taken in . a letter on a systematic course of magnetic observations in various parts of the world was addressed by baron alexander humboldt to the duke of sussex, president of the royal society; and was referred to prof. christie and me. we reported on it on june th , strongly recommending the adoption of the scheme. "a plan had been proposed by the promoters of the london and gravesend railway (col. landman, engineer) for carrying a railway at high level across the bottom of the park. on jan. th i received orders from the admiralty to examine into its possible effect in producing vibrations in the observatory. after much correspondence, examination of ground, &c., i fixed upon a part of the greenwich railway (not yet opened for traffic) near the place where the croydon trunk line now joins it, as the place for trains to run upon, while i made observations with a telescope viewing a collimator by reflection in mercury at the distance of feet. the experiments were made on jan. th, and i reported on feb. th. it was shewn that there would be some danger to the observatory. on nov. nd mr james walker, engineer, brought a model of a railway to pass by tunnel under the lower part of the park: apparently this scheme was not pressed. "in addition to the routine work of the observatory, a special set of observations were made to determine the mass of jupiter.--also the solar eclipse of may th was observed at greenwich in the manner which i had introduced at cambridge.--the ordnance zenith sector, and the instruments for the st helena observatory were brought for examination.--much attention was given to chronometers, and various steps were taken for their improvement.--i had some important correspondence with mr (sir john) lubbock, upon the lunar theory generally and his proposed empirical lunar tables. this was the first germ of the great reduction of lunar observations which i subsequently carried out.--in october i was nominated on the council of the royal society, having been admitted a fellow on feb. th . i was president of the astronomical society during this and the preceding year ( and ). "my connection with groombridge's catalogue of stars began in , and the examination, in concert with mr baily, of the edition printed by mr henry taylor, resulted in its condemnation. in i volunteered to the admiralty to prepare a new edition, and received their thanks and their authority for proceeding. it required a great deal of examination of details, and much time was spent on it in : but it was not brought to the state of readiness for press. "my predecessor, mr pond, died on sept. th , and was interred in halley's tomb in lee churchyard." * * * * * the following letter was written by airy in support of the application for a pension to mrs pond, who had been left in great distress: to henry warburton, esq. "the points upon which in my opinion mr pond's claims to the gratitude of astronomers are founded, are principally the following. _first_ and chief, the accuracy which he introduced into all the principal observations. this is a thing which from its nature it is extremely difficult to estimate now, so long after the change has been made, and i can only say that so far as i can ascertain from books the change is one of very great extent: for certainty and accuracy, astronomy is quite a different thing from what it was, and this is mainly due to mr pond. the most striking exemplification of this is in his laborious working out of every conceivable cause or indication of error in the circle and the two circles: but very great praise is also due for the new system which he introduced in working the transit. in comparing mr pond's systems of observation with dr maskelyne's, no one can avoid being impressed with the inferiority of dr maskelyne's. it is very important to notice that the continental observatories which have since attracted so much attention did not at that time exist or did not exist in vigour. _secondly_, the attention bestowed by mr pond on those points (chiefly of sidereal astronomy) which he regarded as fundamental: to which such masses of observations were directed as entirely to remove the doubts from probable error of individual observations or chance circumstances which have injured many other determinations. _thirdly_, the regularity of observation. the effect of all these has been that, since the commencement of mr pond's residence at greenwich, astronomy considered as an accurate representation of the state of the heavens in the most material points has acquired a certainty and an extent which it never had before. there is no period in the history of the science so clean. on some matters (in regard to the choice of observations) i might say that my own judgment would have differed in some degree from mr pond's, but one thing could have been gained only by giving up another, and upon the general accuracy no improvement could have been made. mr pond understood nothing of physical astronomy; but neither did anybody else, in england. "the supposed decrease of general efficiency in the last few years is to be ascribed to the following causes: . mr pond's ill health. . the inefficiency of his first assistant. . the oppression of business connected with chronometers. "the last of these, as i have reason to think, operated very far. business of this nature which (necessarily) is _daily_ and _peremptory_ will always prevail over that which is _general_ and _confidential_. i will not trouble you with an account of the various ways in which the chronometer business teazed the astronomer royal (several alterations having been made at my representation), but shall merely remark that much of the business had no connection whatever with astronomy. "i beg to submit these remarks to your perusal, requesting you to point out to me _what part_ of them should be laid before any of the king's ministers, _at what time, in what shape_, and to whom addressed. i am quite sure that mrs pond's claims require nothing to ensure favourable consideration but the impression of such a feeling of mr pond's astronomical merits as must be entertained by any reasonable astronomer; and i am most anxious to assist in conveying this impression. "of private history: i went to suffolk for a week on mar. th. on sept. th my son wilfrid (my fourth child) was born. in october i made an excursion for a week round the coast of kent. in november i went to my brother's house at keysoe in bedfordshire: i was much exposed to cold on the return-journey, which probably aggravated the illness that soon followed. from nov. th i was ill; made the last journal entry of the year on dec. th; the next was on jan. th, . i find that in this year i had introduced arthur biddell to the tithe commutation office, where he was soon favourably received, and from which connection he obtained very profitable employment as a valuer." "my connection with cambridge observatory was not yet finished. i had determined that i would not leave a figure to be computed by my successor. in october i had (at my private expense) set mr glaisher to work on reducing the observations of sun, moon, and planets made in , , ; and subsequently had the calculations examined by mr hartnup. this employed me at times through . i state here, once for all, that every calculation or other work in reference to the cambridge observatory, in this and subsequent years, was done at my private expense. the work of the northumberland telescope was going on through the year: from nov. th to th i was at cambridge on these works. "an object-glass of - / inches aperture (a most unusual size at this time, when it was difficult to find a -inch or -inch glass) had been presented to the greenwich observatory by my friend mr sheepshanks, and on mar. th i received from the admiralty authority for mounting it equatoreally in the empty south dome, which had been intended for a copy of the palermo circle.--in the month of july the admiralty wished for my political assistance in a greenwich election, but i refused to give any.--on jan. rd i gave notice to the admiralty that i had finished the computations of groombridge's catalogue, and was ready to print. the printing was authorized and proceeded (the introduction was finished on nov. nd), but the book was not quite ready till the beginning of .--in connection with the cavendish experiment: on june th i wrote to spring rice (chancellor of the exchequer) for _£ _, which was soon granted: and from this time there is a great deal of correspondence (mainly with mr baily) upon the details of the experiment and the theory of the calculation.--on july th i saw the descent of the parachute by which mr cocking was killed. i attended the coroner's inquest and gave evidence a few days later. "the planetary reductions from to had been going on: the computers (glaisher, hartnup, and thomas) worked in the octagon room, and considerable advance was made.--in consequence of the agitation of the proposal by mr lubbock to form empirical tables of the moon, for which i proposed to substitute complete reduction of the observations of the moon from , the british association at york (oct. rd, ) appointed a deputation (including myself) to place the matter before the government. i wrote on the matter to mr wood (lord halifax) stating that it would be proper to raise the first assistant's salary, and to give me more indefinite power about employing computers. in all these things i received cordial assistance from mr wood. the chancellor of the exchequer (mr spring rice) received us on dec. th: statements were furnished by me, and the business was sanctioned immediately.--during this year i was very much engaged in correspondence with lubbock and others on improvements of the lunar theory. "in the operations of and a great quantity of papers had been accumulated. i had kept them in reasonably good order, tied up in bundles: but this method began to fail in convenience, as the number increased. the great lines of classification were however now well understood. i believe it was in the latter part of the year that i finally settled on the principle of arranging papers in packets and subordinate packets, every paper being flat, by the use of four punched holes in every paper. i have never seen any principle of arrangement comparable to this. it has been adopted with the greatest ease by every assistant, and is used to the present time ( ) without alteration. "on jan. rd i was informed unofficially by mr wood (admiralty secretary) that the addition of the magnetic ground was sanctioned. on feb. th mr rhodes (an officer of the department of woods and works) came to put me formally in possession of the ground. between apr. th and may th the ground was enclosed, and my garden was completely protected from the public. the plan of the building was settled, and numerous experiments were made on various kinds of concrete: at last it was decided to build with wood. "after a dinner given by lord burlington, chancellor, the first meeting of the london university was held on mar. th, and others followed. on apr. th i handed to the chancellor a written protest against a vote of a salary of _£ _ to the registrar: which salary, in fact, the government refused to sanction. dissensions on the question of religious examination were already beginning, but i took little part in them. "in mr henderson had resigned the superintendance of the cape of good hope observatory, and mr maclear was appointed. i recommended the same official instructions for him (they had included an allusion to la caille's arc of meridian) with an addition on the probability of trigonometrical survey, on aug. th, . on feb. th, , i wrote to beaufort suggesting that bradley's sector should be used for verifying the astronomical determinations, and subsequently received the approval of the admiralty. in june sir j. herschel and i had an interview with mr wood on the cape equipment generally. the sector was erected with its new mounting, careful drawings were made of every part, instructions were prepared for its use, and on aug. th it was sent to woolwich dockyard and shipped for the cape. "of private history: on aug. rd i started with my wife for an excursion in south wales, &c. on sept. th i gave a lecture in the town hall of neath. while at swansea we received news of the death of my wife's father, the rev. richard smith, and returned at once.--in this year arthur biddell bought the little eye estate for me." "cambridge observatory:--on dec. th, , i had set mr glaisher to work in collecting the annual results for star-places from the cambridge observations, to form one catalogue: i examined the calculations and the deduced catalogue, and on dec. , , presented it to the royal astronomical society, under the title of 'the first cambridge catalogue.'--for the northumberland telescope i was engaged with simms about the clockwork from time to time up to apr. th, and went to cambridge about it. the instrument was brought to a useable state, but some small parts were still wanting. "at greenwich:--in april i drew up a little history of the observatory for the penny cyclopaedia.--on june th the lords of the admiralty paid a short visit to the observatory: on this occasion mr wood suggested a passage connecting the observatory with the dwelling-house, and i subsequently prepared sketches for it; it was made in the next year.--in the course of the year the sheepshanks equatoreal was mounted, and encke's comet was observed with it from oct. th to nov. th.--on mar. st, &c. i reported to the admiralty on the selection of chronometers for purchase, from a long list: this was an important beginning of a new system.--the magnetic observatory was built, in the form originally planned for it (a four-armed cross with equal arms, one axis being in the magnetic meridian) in the beginning of this year. (no alteration has since been made in form up to the present time, , except that the north arm has been lengthened feet a few years ago.) on may st a magnet was suspended for the first time, mr baily and lieut. (afterwards sir william) denison being present.--groombridge's catalogue was finished, and on mar. rd i arranged for sending out copies.--the planetary reductions were carried on vigorously. on may st, , the treasury assented to the undertaking of the lunar reductions and allotted _£ , _ for it: preparations were made, and in the autumn computers were employed upon it. it will easily be seen that this undertaking added much to my labours and cares.--the geodetic affairs of the cape of good hope began to be actively pressed, and in february beaufort wrote to me in consequence of an application from maclear, asking about a standard of length for maclear (as foundation for a geodetic survey). i made enquiries, and on mar. th wrote to mr wood, alluding also generally to the want of a national english standard after the destruction of the houses of parliament. on apr. th the admiralty sanctioned my procuring proper standard bars.--in connection with the cavendish experiment, i have an immense quantity of correspondence with mr baily, and all the mathematics were furnished by me: the experiment was not finished at the end of the year.--the perturbations of uranus were now attracting attention. i had had some correspondence on this subject with dr hussey in , and in with eugène bouvard. on feb. th, of , i wrote to schumacher regarding the error in the tabular radius-vector of uranus, which my mode of reducing the observations enabled me to see. "the national standards of length and weight had been destroyed in the fire of the houses of parliament. on may th i received a letter from mr spring rice, requesting me to act (as chairman) with a committee consisting of f. baily, j.e. drinkwater bethune, davies gilbert, j.g.s. lefevre, j.w. lubbock, g. peacock, and r. sheepshanks, to report on the steps now to be taken. i accepted the charge, and the first meeting was held at the observatory on may nd; all subsequent meetings in london, usually in the apartments of the royal astronomical society. i acted both as chairman and as working secretary. our enquiries went into a very wide field, and i had much correspondence. "on jan. th mr wood wrote to me, mentioning that capt. johnson had made some observations on the magnetism of iron ships, and asking whether they ought to be continued; a steamer being offered at _£ _ per week. i applied to beaufort for a copy of johnson's observations, and on jan. th replied very fully, discouraging such observations; but recommending a train of observations expressly directed to theoretical points. on feb. th i reported that i had examined the deptford basin, and found that it would do fairly well for experiments. on july th, , capt. beaufort wrote to me that the admiralty wished for experiments on the ship, the 'rainbow,' then in the river, and enquired whether i would undertake them and what assistance i desired, as for instance that of christie or barlow. i replied that one person should undertake it, either christie, barlow, or myself, and that a basin was desirable. on july th and th i looked at the basins of woolwich and deptford, approving the latter. on july st the admiralty gave me full powers. from july rd i was almost entirely employed on preparations. the course of operations is described in my printed paper: the original maps, curves, and graphical projections, are in the bound mss.: 'correction of compass in iron ships--"rainbow,"' at the greenwich observatory. the angular disturbances were found on july th and th, requiring some further work on a raft, so that they were finally worked out on aug. th. i struggled hard with the numbers, but should not have succeeded if it had not occurred to me to examine the horizontal magnetic intensities. this was done on aug. th, and the explanation of the whole was suggested at once: graphical projections were made on aug. th and th for comparison of my explanation with observations, and the business was complete. on aug. th and th i measured the intensity of some magnets, to be used in the ship for correction. it is to be remarked that, besides the effect of polar magnetism, there was no doubt of the existence of an effect of induced magnetism requiring correction by other induced magnetism: and experiments for this were made in the magnetic observatory. all was ready for trial: and on aug. th i carried my magnets and iron correctors to deptford, mounted them in the proper places, tried the ship, and the compass, which had been disturbed degrees to the right and degrees to the left, was now sensibly correct. on aug. st i reported this to the admiralty, and on aug. th i tried the ship to gravesend. on aug. th i had the loan of her for an expedition with a party of friends to sheerness, and on sept. th i accompanied her to gravesend, on her first voyage to antwerp.--on oct. th application was made to me by the owner of the 'ironsides' to correct her compasses. in consequence of this i went to liverpool on oct. th, and on this occasion made a very important improvement in the practical mode of performing the correction.--on nov. th i reported to the admiralty in considerable detail. on dec. th i had an interview with lord minto (first lord of the admiralty) and mr wood. they refused to sanction any reward to me.--the following is a copy of the report of the captain of the 'rainbow' after her voyage to antwerp: 'having had the command of the rainbow steamer the two voyages between london and antwerp, i have the pleasure to inform you that i am perfectly satisfied as to the correctness of the compasses, and feel quite certain they will continue so. i took particular notice from land to land from our departure and found the bearings by compass to be exact.'"--the following extracts from letters to his wife refer to the "ironsides": on oct. th he writes, "i worked up the observations so much as to see that the compass disturbance is not so great as in the 'rainbow' ( ° instead of °), but quite enough to make the vessel worthless; and that it is quite different in direction from that in the 'rainbow'--so that if they had stolen one of the 'rainbow' correctors and put it into this ship it would have been much worse than before." and on nov. st he writes, "on wednesday i again went to the ship and tried small alterations in the correctors: i am confident now that the thing is very near, but we were most abominably baffled by the sluggishness of the compass." "the university of london:--on jan. th i attended a sub-committee meeting on the minimum of acquirements for b.a. degree, and various meetings of the senate. on july th i intimated to mr spring rice my wish to resign. i had various correspondence, especially with mr lubbock, and on dec. th i wrote to him on the necessity of stipends to members of senate. the dissensions on religious examination became very strong. i took a middle course, demanding examination in the languages and books, but absolutely refusing to claim any religious assent. i expressed this to dr jerrard, the principal representative on the religious side, by calling on him to substitute the words 'recognition of christian literature' for 'recognition of christian religion': i addressed a printed letter to lord burlington (chancellor) and the members of the senate, on this subject. "of private history: in january i made a short excursion in norfolk and suffolk, and visited prof. sedgwick at norwich. in april i paid a short visit to mr courtney at sanderstead, with my wife. on june th my son hubert was born. in september i went with my sister by cambridge, &c., to luddington, where i made much enquiry concerning my father and the family of airy who had long been settled there. we then visited various places in yorkshire, and arrived at brampton, near chesterfield, where mrs smith, my wife's mother, now resided. and returned by rugby. i had much correspondence with my brother and for him about private pupils and a better church living. i complained to the bishop of norwich about the mutilation of a celebrated monument in playford church by the incumbent and curate." the following extracts are from letters to his wife relating to the above-mentioned journeys: close, norwich. _ , jan. _. i do not know what degree of cold you may have had last night, but here it was (i believe) colder than before--thermometer close to the house at °. i have not suffered at all. however i do not intend to go to lowestoft. brampton. _ , sept. th_. we began to think that we had seen enough of scarborough, so we took a chaise in the afternoon to pickering, a small agricultural town, and lodged in a comfortable inn there. on wednesday morning at we started by the railroad for whitby, in a huge carriage denominated the lady hilda capable of containing persons or more drawn by one horse, or in the steep parts of the railway by two horses. the road goes through a set of defiles of the eastern moorlands of yorkshire which are extremely pretty: at first woody and rich, then gradually poorer, and at last opening on a black moor with higher moors in sight: descending in one part by a long crooked inclined plane, the carriage drawing up another load by its weight: through a little tunnel: and then along a valley to whitby. the rate of travelling was about miles an hour. betsy declares that it was the most agreeable travelling that she ever had. yesterday (saturday) caroline drove betsy and miss barnes drove me to clay cross to see the works at the great railroad tunnel there. coming from the north, the railroad passes up the chesterfield valley close by the town and continues up the same valley, till it is necessary for it to enter the valley which runs the opposite way towards buttersley: the tunnel passes under the high ground between these two vallies: so that it is in reality at the water-shed: it is to be i think more than a mile long, and when finished feet clear in height, so it is a grand place. we saw the preparations for a blast, and heard it fired: the ladies stopping their ears in due form. "cambridge observatory:--on mar. th i went to cambridge on the business of the northumberland telescope: i was subsequently engaged on the accounts, and on aug. th i finally resigned it to prof. challis, who accepted it on aug. th. on sept. th i communicated its completion and the settlement of accounts to the duke of northumberland. the total expense was _£ . s. d._ + francs for the object-glass. "at greenwich observatory:--on jan. rd i received the last revise of the observations, and on jan. th the first sheet for .--in july i report on selection from a long list of chronometers which had been on trial, and on sept. nd i pointed out to capt. beaufort that the system of offering only one price would be ruinous to the manufacture of chronometers, and to the character of those supplied to the admiralty: and that i would undertake any trouble of classifying the chronometers tried. this letter introduced the system still in use ( ), which has been most beneficial to the manufacture. on sept. th i proposed that all trials begin in the first week of january: this also has been in use as an established system to the present time.--it was pointed out to me that a certain chronometer was affected by external magnetic power. i remedied this by placing under it a free compass magnet: a stand was specially prepared for it. i have never found another chronometer sensibly affected by magnetism.--in november and december i tried my new double-image micrometer.--between may th and oct. th a fireproof room was constructed in the southern part of the quadrant room; and in november a small shed was erected over the entrance to the north terrace.--the position of the free meridional magnet (now mounted in the magnetic observatory) was observed at every m. through hours on feb. nd and rd, may th and th, aug. th and st, and nov. th and th. this was done in cooperation with the system of the magnetic union established by gauss in germany.--the reduction of the greenwich planetary and lunar observations, to , went on steadily. i had six and sometimes seven computers constantly at work, in the octagon room.--as in i had a great amount of correspondence with mr baily on the cavendish experiment.--i attended as regularly as i could to the business of the university of london. the religious question did not rise very prominently. i took a very active part, and have a great deal of correspondence, on the nature of the intended examinations in hydrography and civil engineering.--on the standards commission the chief work was in external enquiries.--on june th i had enquiries from john quincey adams (u.s.a.) on the expense, &c., of observatories: an observatory was contemplated in america.--i had correspondence about the proposed establishment of observatories at durham, glasgow, and liverpool. "i had in this year a great deal of troublesome and on the whole unpleasant correspondence with the admiralty about the correction of the compass in iron ships. i naturally expected some acknowledgment of an important service rendered to navigation: but the admiralty peremptorily refused it. my account of the experiments &c. for the royal society is dated april th. the general success of the undertaking soon became notorious, and (as i understood) led immediately to extensive building of iron ships: and it led also to applications to me for correction of compasses. on jan. th i was addressed in reference to the royal sovereign and royal george at liverpool; july th the orwell; may th two russian ships built on the thames; sept. th the ships of the lancaster company. "i had much work in connection with the cape of good hope observatory, chiefly relating to the instrumental equipment and to the geodetical work. as it was considered advisable that any base measured in the cape colony should be measured with compensation bars, i applied to major jervis for the loan of those belonging to the east indian survey, but he positively refused to lend them. on jan. th i applied to col. colby for the compensation bars of the british survey, and he immediately assented to lending them. col. colby had suggested to the ordnance department that capt. henderson and several sappers should be sent to use the measuring bars, and it was so arranged. it still appeared desirable to have the command of some soldiers from the garrison of cape town, and this matter was soon arranged with the military authorities by the admiralty. "the following are the principal points of my private history: it was a very sad year. on jan. th i went with my wife to norwich, on a visit to prof. sedgwick, and in june i visited sir j. herschel at slough. on june th my dear boy arthur was taken ill: his malady soon proved to be scarlet fever, of which he died on june th at in the morning. it was arranged that he should be buried in playford churchyard on the th, and on that day i proceeded to playford with my wife and my eldest son george richard. at chelmsford my son was attacked with slight sickness, and being a little unwell did not attend his brother's funeral. on july st at h. m. in the morning he also died: he had some time before suffered severely from an attack of measles, and it seemed probable that his brain had suffered. on july th he was buried by the side of his brother arthur in playford churchyard.--on july rd i went to colchester on my way to walton-on-the-naze, with my wife and all my family; all my children had been touched, though very lightly, with the scarlet fever.--it was near the end of this year that my mother quitted the house (luck's) at playford, and came to live with me at greenwich observatory, where she lived till her death; having her own attendant, and living in perfect confidence with my wife and myself, and being i trust as happy as her years and widowhood permitted. my sister also lived with me at the observatory." "in the latter part of , and through , i had much correspondence with the admiralty, in which i obtained a complete account of the transfer of the observatory from the ordnance department to the admiralty, and the transfer of the visitation of the observatory from the royal society to the present board of visitors. in i found that the papers of the board of longitude were divided between the royal society and the admiralty: i obtained the consent of both to bring them to the observatory. "in this year i began to arrange about an annual dinner to be held at the visitation.--my double-image micrometer was much used for observations of circumpolar double stars.--in magnetism and meteorology, certain quarterly observations were kept up; but in november the system of incessant eye-observations was commenced. i refused to commence this until i had secured a 'watchman's clock' for mechanical verification of the regular attendance of the assistants.--with regard to chronometers: in this year, for the first time, i took the very important step of publishing the rates obtained by comparisons at the observatory. i confined myself on this occasion to the chronometers purchased by the admiralty. in march a pigeon-house was made for exposure of chronometers to cold.--the lunar and planetary reductions were going on steadily.--i was consulted about an observatory at oxford, where i supported the introduction of the heliometer.--the stipend of the bakerian lecture was paid to me for my explanation of brewster's new prismatic fringes.--the business of the cape observatory and survey occupied much of my time.--in the rev. h. j. rose (editor of the encyclopaedia metropolitana) had proposed my writing a paper on tides, &c.; in oct. i gave him notice that i must connect tides with waves, and in that way i will take up the subject. much correspondence on tides, &c., with whewell and others followed. "with regard to the magnetical and meteorological establishment. on june th mr lubbock reported from the committee of physics of the royal society to the council in favour of a magnetic and meteorological observatory near london. after correspondence with sheepshanks, lord northampton, and herschel, i wrote to the council on july th, pointing out what the admiralty had done at greenwich, and offering to cooperate. in a letter to lord minto i stated that my estimate was _£ _, including _£ _ to the first assistant: lubbock's was _£ , _. on aug. th the treasury assented, limiting it to the duration of ross's voyage. on aug. th wheatstone looked at our buildings and was satisfied. my estimate was sent to the admiralty, viz. _£ _ outfit, _£ _ annual expense; and glaisher to be superintendent. i believe this was allowed for the present; for the following year it was placed on the estimates. most of the contemplated observations were begun before the end of : as much as possible in conformity with the royal society's plan. mr hind (subsequently the superintendent of the nautical almanac) and mr paul were the first extra assistants. "of private history. on feb. th i went to cambridge with my paper on the going fusee. on mar. th i went to visit mrs smith, my wife's mother, at brampton near chesterfield. i made a short visit to playford in april and a short expedition to winchester, portsmouth, &c., in june. from sept. th to oct. rd i was travelling in the north of england and south of scotland." [this was an extremely active and interesting journey, in the course of which a great number of places were visited by airy, especially places on the border mentioned in scott's poems, which always had a great attraction for him. he also attended a meeting of the british association at glasgow and made a statement regarding the planetary and lunar reductions: and looked at a site for the glasgow observatory.] "in november i went for a short time to cambridge and to keysoe (my brother's residence). on dec. th my daughter hilda was born (subsequently married to e.j. routh). in this year i had a loss of _£ _ by a fire on my eye estate." * * * * * the following extracts are from letters to his wife. some of them relate to matters of general interest. they are all of them characteristic, and serve to shew the keen interest which he took in matters around him, and especially in architecture and scenery. the first letter relates to his journey from chesterfield on the previous day. flamsteed house, _ , april _. i was obliged to put up with an outside place to derby yesterday, much against my will, for i was apprehensive that the cold would bring on the pain in my face. of that i had not much; but i have caught something of sore throat and catarrh. the coach came up at about minutes past . it arrived in derby at minutes or less past (same guard and coachman who brought us), and drew up in the street opposite the inn at which we got no dinner, abreast of an omnibus. i had to go to a coach office opposite the inn to pay and be booked for london, and was duly set down in a way-bill with _name_; and then entered the omnibus: was transferred to the railway station, and then received the railway ticket by shouting out my name. if you should come the same way, you would find it convenient to book your place at chesterfield to london by your name (paying for the whole, namely, coach fare, omnibus fare _-/ _, and railway fare _£ . s. d._ first class). then you will only have to step out of the coach into the omnibus, and to scream out once or twice to the guard to make sure that you are entered in the way-bill and that your luggage is put on the omnibus. * * * * * flamsteed house, greenwich, _ , april _. i forgot to tell you that at lord northampton's i saw some specimens of the daguerrotype, pictures made by the camera obscura, and they surpass in beauty of execution anything that i could have imagined. baily who has two or three has promised to lend them for your inspection when you return. also i saw some post-office stamps and stamped envelopes: i do not much admire the latter. * * * * * the following relates to the fire on his eye farm, referred to above: playford, _ , april _. on wednesday (yesterday) went with my uncle to the eye estate, to see the effects of the fire. the farming buildings of every kind are as completely cleared away as if they had been mown down: not a bit of anything but one or two short brick walls and the brick foundations of the barns and stacks. the aspect of the place is much changed, because in approaching the house you do not see it upon a back-ground of barns, &c., but standing alone. the house is in particularly neat and good order. i did not think it at all worth while to make troublesome enquiries of the people who reside there, but took mr case's account. there seems no doubt that the fire was caused by the maid-servant throwing cinders into a sort of muck-place into which they had been commonly thrown. i suppose there was after all this dry weather straw or muck drier than usual, and the cinders were hotter than usual. the whole was on fire in an exceedingly short time; and everything was down in less than an hour. two engines came from eye, and all the population of the town (as the fire began shortly after two o'clock in the afternoon). it is entirely owing to these that my house, and the farm (sewell's) on the opposite side of the road, were not burned down. at the beginning of the fire the wind was n.e. which blew directly towards the opposite farm (sewell's): although the nearest part of it (tiled dwelling house) was yards off or near it, and the great barn (thatched roof) considerably further, yet both were set on fire several times. all this while, the tail of my house was growing very hot: and shortly after the buildings fell in burning ruins, the wind changed to n.w., blowing directly to my house. if this change had happened while the buildings were standing and burning, there would have been no possibility of saving the house. as it was, the solder is melted from the window next the farm-yard, and the roof was set on fire in three or four places. one engine was kept working on my house and one on the opposite farm. a large pond was pretty nearly emptied. mr case's horses and bullocks were got out, not without great difficulty, as the progress of the fire was fearfully rapid. a sow and nine pigs were burnt, and a large hog ran out burnt so much that the people killed it immediately. * * * * * george inn, winchester, _ , june _. at winchester we established ourselves at the george and then without delay proceeded to st cross. i did not know before the nature of its hospital establishment, but i find that it is a veritable set of alms-houses. the church is a most curious specimen of the latest norman. i never saw one so well marked before--norman ornaments on pointed arches, pilasters detached with cushion capitals, and various signs: and it is clearly an instance of that state of the style when people had been forced by the difficulties and inelegancies of the round arch in groining to adopt pointed arches for groining but had not learnt to use them for windows.......this morning after breakfast went to the cathedral (looking by the way at a curious old cross in the street). i thought that its inside was wholly norman, and was most agreeably surprised by finding the whole inside groined in every part with excellent late decorated or perpendicular work. yet there are several signs about it which lead me to think that the whole inside has been norman, and even that the pilasters now worked up into the perpendicular are norman. the transepts are most massive old norman, with side-aisles running round their ends (which i never saw before). the groining of the side aisles of the nave very effective from the strength of the cross ribs. the clerestory windows of the quire very large. the organ is on one side. but the best thing about the quire is the wooden stall-work, of early decorated, very beautiful. a superb lady chapel, of early english. * * * * * portsmouth, _ , june _. we left winchester by evening train to the dolphin, southampton, and slept there. at nine in the morning we went by steamboat down the river to ryde in the isle of wight: our steamer was going on to portsmouth, but we thought it better to land at ryde and take a boat for ourselves. we then sailed out (rather a blowing day) to the vessel attending col. pasley's operations, and after a good deal of going from one boat to another (the sea being so rough that our boat could not be got up to the ships) and a good deal of waiting, we got on board the barge or lump in which col. pasley was. here we had the satisfaction of seeing the barrel of gunpowder lowered (there was more than a ton of gunpowder), and seeing the divers go down to fix it, dressed in their diving helmets and supplied with air from the great air-pump above. when all was ready and the divers had ascended again, the barge in which we were was warped away, and by a galvanic battery in another barge (which we had seen carried there, and whose connection with the barrel we had seen), upon signal given by sound of trumpet, the gunpowder was fired. the effect was most wonderful. the firing followed the signal instantaneously. we were at between and yards from the place (as i judge), and the effects were as follows. as soon as the signal was given, there was a report, louder than a musket but not so loud as a small cannon, and a severe shock was felt at our feet, just as if our barge had struck on a rock. almost immediately, a very slight swell was perceived over the place of the explosion, and the water looked rather foamy: then in about a second it began to rise, and there was the most enormous outbreak of spray that you can conceive. it rose in one column of or feet high, and broad at the base, resembling a stumpy sheaf with jagged masses of spray spreading out at the sides, and seemed to grow outwards till i almost feared that it was coming to us. it sunk, i suppose, in separate parts, for it did not make any grand squash down, and then there were seen logs of wood rising, and a dense mass of black mud, which spread gradually round till it occupied a very large space. fish were stunned by it: our boatmen picked up some. it was said by all present that this was the best explosion which had been seen: it was truly wonderful. then we sailed to portsmouth.......the explosion was a thing worth going many miles to see. there were many yachts and sailing boats out to see it (i counted before they were at the fullest), so that the scene was very gay. * * * * * here are some notes on york cathedral after the fire: red lion hotel, redcar, _ , sept. _. my first letter was closed after service at york cathedral. as soon as i had posted it, i walked sedately twice round the cathedral, and then i found the sexton at the door, who commiserating me of my former vain applications, and having the hope of lucre before his eyes, let me in. i saw the burnt part, which looks not melancholy but unfinished. every bit of wood is carried away clean, with scarcely a smoke-daub to mark where it has been: the building looks as if the walls were just prepared for a roof, but there are some deep dints in the pavement, shewing where large masses have fallen. the lower parts of some of the columns (to the height of or feet) are much scaled and cracked. the windows are scarcely touched. i also refreshed my memory of the chapter-house, which is most beautiful, and which has much of its old gilding reasonably bright, and some of its old paint quite conspicuous. and i looked again at the old crypt with its late norman work, and at the still older crypt of the pre-existing church. * * * * * "the routine work of the observatory in its several departments was carried on steadily during this year.--the camera obscura was removed from the n.w. turret of the great room, to make way for the anemometer.--in magnetism and meteorology the most important thing was the great magnetic storm of sept. th, which revealed a new class of magnetic phenomena. it was very well observed by mr glaisher, and i immediately printed and circulated an account of it.--in april i reported that the planetary reductions were completed, and furnished estimates for the printing.--in august i applied for , copies of the great skeleton form for computing lunar tabular places, which were granted.--i reported, as usual, on various papers for the royal society, and was still engaged on the cavendish experiment.--in the university of london i attended the meeting of dec. th, on the reduction of examiners' salaries, which were extravagant.--i furnished col. colby with a plan of a new sector, still used in the british survey.--i appealed to colby about the injury to the cistern on the great gable in cumberland, by the pile raised for the survey signal.--on jan. rd occurred a most remarkable tidal disturbance: the tide in the thames was feet too low. i endeavoured to trace it on the coasts, and had a vast amount of correspondence: but it elicited little. "of private history: i was a short time in suffolk in march.--on mar. st i started with my wife (whose health had suffered much) for a trip to bath, bristol, cardiff, swansea, &c. while at swansea we received news on apr. th of the deadly illness of my dear mother. we travelled by neath and cardiff to bath, where i solicited a rest for my wife from my kind friend miss sutcliffe, and returned alone to greenwich. my dear mother had died on the morning of the th. the funeral took place at little whelnetham (near bury) on may st, where my mother was buried by the side of my father. we went to cambridge, where my wife consulted dr haviland to her great advantage, and returned to greenwich on may th.--on may th to th i was at sanderstead (rev. j. courtney) with whewell as one sponsor, at the christening of my daughter hilda.--in september i went for a trip with my sister to yorkshire and cumberland, in the course of which we visited dent (sedgwick's birthplace), and paid visits to mr wordsworth, miss southey, and miss bristow, returning to greenwich on the th sept.--from june th to th i visited my brother at keysoe." the following extracts are from letters written to his wife while on the above trip in yorkshire and cumberland: red lion inn, redcar, _ , sept. _. we stopped at york: went to the tavern hotel. in the morning (friday) went into the cathedral. i think that it improves on acquaintance. the nave is now almost filled with scaffolding for the repair of the roof, so that it has not the bare unfinished appearance that it had when i was there last year. the tower in which the fire began seems to be a good deal repaired: there are new mullions in its windows, &c. we stopped to hear part of the service, which was not very effective. * * * * * here are notes of his visit to dentdale in yorkshire, the birthplace of his friend sedgwick: king's head, kendal, _ , sept. _. the day was quite fine, and the hills quite clear. the ascent out of hawes is dull; the little branch dale is simple and monotonous, and so are the hills about the great dale which are in sight. the only thing which interested us was the sort of bird's-eye view of hardraw dell, which appeared a most petty and insignificant opening in the great hill side. but when we got to the top of the pass there was a magnificent view of ingleborough. the dale which was most nearly in front of us is that which goes down to ingleton, past the side of ingleborough. the mountain was about nine miles distant. we turned to the right and immediately descended dent-dale. the three dales (to hawes, to ingleton, and to dent) lay their heads together in a most amicable way, so that, when at the top, it is equally easy to descend down either of them. we found very soon that dent-dale is much more beautiful than that by which we had ascended. the sides of the hills are steeper, and perhaps higher: the bottom is richer. the road is also better. the river is a continued succession of very pretty falls, almost all of which have scooped out the lower strata of the rock, so that the water shoots clear over. for several miles (perhaps ) it runs upon bare limestone without a particle of earth. from the head of the dale to the village of dent is eight miles. at about half-way is a new chapel, very neat, with a transept at its west end. the village of dent is one of the strangest places that i ever saw. narrow street, up and down, with no possibility of two carriages bigger than children's carts passing each other. we stopped at the head inn and enquired about the geolog: but he is not in the country. we then called on his brother, who was much surprised and pleased to see us. his wife came in soon after (his daughter having gone with a party to see some waterfall) and they urged us to stop and dine with them. so we walked about and saw every place about the house, church, and school, connected with the history of the geolog: and then dined. i promised that you should call there some time when we are in the north together and spend a day or two with them. mr sedgwick says it is reported that whewell will take sedbergh living (which is now vacant: trinity college is patron). then we had our chaise and went to sedbergh. the very mouth of dent-dale is more contracted than its higher parts. sedbergh is embosomed among lumping hills. then we had another carriage to drive to kendal. * * * * * here is a recollection of wordsworth: salutation, ambleside, _ , sept. _. we then got our dinner at lowwood, and walked straight to ambleside, changed our shoes, and walked on to rydal to catch wordsworth at tea. miss wordsworth was being drawn about in a chair just as she was seven years ago. i do not recollect her appearance then so as to say whether she is much altered, but i think not. mr wordsworth is as full of good talk as ever, and seems quite strong and well. mrs wordsworth looks older. their son william was at tea, but he had come over only for the day or evening. there was also a little girl, who i think is mrs wordsworth's niece. "in this year i commenced a troublesome work, the description of the northumberland telescope. on sept. th i wrote to the duke of northumberland suggesting this, sending him a list of plates, and submitting an estimate of expense _£ _. on sept. th i received the duke's assent. i applied to prof. challis (at the cambridge observatory) requesting him to receive the draughtsman, sly, in his house, which he kindly consented to do. "with regard to estimates. i now began to point out to the admiralty the inconvenience of furnishing separate estimates, viz. to the admiralty for the astronomical establishment, and to the treasury for the magnetical and meteorological establishment.--the great work of the lunar reductions proceeded steadily: computers were employed on them.--with regard to the magnetical and meteorological establishment: i suppose that james ross's expedition had returned: and with this, according to the terms of the original grant, the magnetical and meteorological establishments expired. there was much correspondence with the royal society and the treasury, and ultimately sir r. peel consented to the continuation of the establishments to the end of .--in this year began my correspondence with mr mitchell about the cincinnati observatory. on aug. mr mitchell settled himself at greenwich, and worked for a long time in the computing room.--and in this year mr aiken of liverpool first wrote to me about the liverpool observatory, and a great deal of correspondence followed: the plans were in fact entirely entrusted to me.--july th was the day of the total eclipse of the sun, which i observed with my wife at the superga, near turin. i wrote an account of my observations for the royal astronomical society.--on jan. th i notified to mr goulburn that our report on the restoration of the standards was ready, and on jan. th i presented it. after this followed a great deal of correspondence, principally concerning the collection of authenticated copies of the old standards from all sides.--in some discussions with capt. shirreff, then captain superintendent of the chatham dockyard, i suggested that machinery might be made which would saw ship-timbers to their proper form, and i sent him some plans on nov. th. this was the beginning of a correspondence which lasted long, but which led to nothing, as will appear hereafter.--on dec. th, being on a visit to dean peacock at ely, i examined the drainage scoop wheel at prickwillow, and made a report to him by letter, which obtained circulation and was well known.--on may th the manuscript of my article, 'tides and waves,' for the encyclopaedia metropolitana was sent to the printer. i had extensive correspondence, principally on local tides, with whewell and others. tides were observed for me by colby's officers at southampton, by myself at christchurch and poole, at ipswich by ransome's man; and a great series of observations of irish tides were made on my plan under colby's direction in june, july and august.--on sept. th mr goulburn, chancellor of the exchequer, asked my opinion on the utility of babbage's calculating machine, and the propriety of expending further sums of money on it. i replied, entering fully into the matter, and giving my opinion that it was worthless.--i was elected an honorary member of the institution of civil engineers, london. "the reduction and printing of the astronomical observations had been getting into arrear: the last revise of the observations went to press on may th, . on aug. th came into operation a new organization of assistants' hours of attendance, &c., required for bringing up reductions. i worked hard myself and my example had good effect." his reference to this subject in his report to the visitors is as follows: "i have in one of the preceding articles alluded to the backwardness of our reductions. in those which follow it i trust that i have sufficiently explained it. to say nothing of the loss, from ill health, of the services of most efficient assistants, i am certain that the quantity of current work will amply explain any backwardness. perhaps i may particularly mention that in the observations of there was an unusual quantity of equatoreal observations, and the reductions attending these occupied a very great time. but, as regards myself, there has been another cause. the reduction of the ancient lunar and planetary observations, the attention to chronometer constructions, the proposed management of the printing of papers relating to important operations at the cape of good hope; these and similar operations have taken up much of my time. i trust that i am doing well in rendering greenwich, even more distinctly than it has been heretofore, the place of reference to all the world for the important observations, and results of observations, on which the system of the universe is founded. as regards myself, i have been accustomed, in these matters, to lay aside private considerations; to consider that i am not a mere superintendent of current observations, but a trustee for the honour of greenwich observatory generally, and for its utility generally to the world; nay, to consider myself not as mere director of greenwich observatory, but (however unworthy personally) as british astronomer, required sometimes by my office to interfere (when no personal offence is given) in the concerns of other establishments of the state. if the board supports me in this view there can be little doubt that the present delay of computations, relating to current observations, will be considered by them as a very small sacrifice to the important advantage that may be gained by proper attention to the observations of other times and other places." "of private history: in february i went for a week to playford and norwich, visiting prof. sedgwick at the latter place. on mar. st my third daughter christabel was born. in march i paid a short visit to sir john herschel at hawkhurst. from june th to aug. th i was travelling with my wife on the continent, being partly occupied with the observation of the total eclipse of the sun on july th. the journey was in switzerland and north italy. in december i went to cambridge and ely, visiting dr peacock at the latter place." from feb. rd to th airy was engaged on observations of tides at southampton, christchurch, poole, and weymouth. during this expedition he wrote frequently (as he always did) to his wife on the incidents of his journey, and the following letters appear characteristic: king's arms, christchurch, or xchurch, _ , feb. _. the lower of the above descriptions of my present place of abode is the correct one, as i fearlessly assert on the authority of divers direction-posts on the roads leading to it (by the bye this supports my doctrine that x in latin was not pronounced eks but khi, because the latter is the first letter of christ, for which x is here traditionally put). finding this morning that yolland (who called on me as soon as i had closed the letter to you) was perfectly inclined to go on with the tide observations at southampton, and that his corporals of sappers were conducting them in the most exemplary manner, i determined on starting at once. however we first went to look at the new docks (mud up to the knees) and truly it is a very great work. there is to be enclosed a good number of acres of water feet deep: one dock locked in, the other a tidal dock or basin with that depth at low water. they are surrounded by brick walls eight feet thick at top, or more at bottom; and all the parts that ever can be exposed are faced with granite. the people reckon that this work when finished will attract a good deal of the london commerce, and i should not be surprised at it. for it is very much easier for ships to get into southampton than into london, and the railway carriage will make them almost one. a very large steamer is lying in southampton water: the oriental, which goes to alexandria. the lady mary wood, a large steamer for lisbon and gibraltar, was lying at the pier. the said pier is a very pleasant place of promenade, the water and banks are so pretty, and there is so much liveliness of ships about it. well i started in a gig, in a swashing rain, which continued off and on for a good while. of the miles, i should think that were across the new forest. i do not much admire it. as for norman william's destruction of houses and churches to make it hunting ground, that is utter nonsense which never could have been written by anybody that ever saw it: but as to hunting, except his horses wore something like mud-pattens or snow-shoes, it is difficult to conceive it. almost the whole forest is like a great sponge, water standing in every part. in the part nearer to xchurch forest trees, especially beeches, seem to grow well. we stopped to bait at lyndhurst, a small place high up in the forest: a good view, such as it is, from the churchyard. the hills of the isle of wight occasionally in sight. on approaching xchurch the chalk cliffs of the west end of the isle of wight (leading to the needles) were partly visible; and, as the sun was shining on them, they fairly blazed. xchurch is a small place with a magnificent-looking church (with lofty clerestory, double transept, &c., but with much irregularity) which i propose to visit to-morrow. also a ruin which looks like an abbey, but the people call it a castle. there is a good deal of low land about it, and the part between the town and the sea reminded me a good deal of the estuary above cardigan, flat ill-looking bogs (generally islands) among the water. i walked to the mouth of the river (more than two miles) passing a nice little place called sandford, with a hotel and a lot of lodgings for summer sea-people. at the entrance of the river is a coastguard station, and this i find is the place to which i must go in the morning to observe the tide. i had some talk with the coastguard people, and they assure me that the tide is really double as reported. as i came away the great full moon was rising, and i could read in her unusually broad face (indicating her nearness to the earth) that there will be a powerful tide. i came in and have had dinner and tea, and am now going to bed, endeavouring to negociate for a breakfast at six o'clock to-morrow morning. it is raining cats and dogs. * * * * * luce's hotel, weymouth, _ , feb. _. this morning when i got up i found that it was blowing fresh from s.w. and the sea was bursting over the wall of the eastern extremity of the esplanade very magnanimously. so (the swell not being favourable for tide-observations) i gave them up and determined to go to see the surf on the chesil bank. i started with my great-coat on, more for defence against the wind than against rain; but in a short time it began to rain, and just when i was approaching the bridge which connects the mainland with the point where the chesil bank ends at portland (there being an arm of the sea behind the chesil bank) it rained and blew most dreadfully. however i kept on and mounted the bank and descended a little way towards the sea, and there was the surf in all its glory. i cannot give you an idea of its majestic appearance. it was evidently very high, but that was not the most striking part of it, for there was no such thing as going within a considerable distance of it (the occasional outbreaks of the water advancing so far) so that its magnitude could not be well seen. my impression is that the height of the surf was from to feet. but the striking part was the clouds of solid spray which formed immediately and which completely concealed all the other operations of the water. they rose a good deal higher than the top of the surf, so the state of things was this. a great swell is seen coming, growing steeper and steeper; then it all turns over and you see a face just like the pictures of falls of niagara; but in a little more than one second this is totally lost and there is nothing before you but an enormous impenetrable cloud of white spray. in about another second there comes from the bottom of this cloud the foaming current of water up the bank, and it returns grating the pebbles together till their jar penetrates the very brain. i stood in the face of the wind and rain watching this a good while, and should have stood longer but that i was so miserably wet. it appeared to me that the surf was higher farther along the bank, but the air was so thickened by the rain and the spray that i could not tell. when i returned the bad weather abated. i have now borrowed somebody else's trowsers while mine are drying (having got little wet in other parts, thanks to my great-coat, which successfully brought home a hundredweight of water), and do not intend to stir out again except perhaps to post this letter. * * * * * flamsteed house, _ , may _. yesterday after posting the letter for you i went per steamboat to hungerford. i then found mr vignoles, and we trundled off together, with another engineer named smith, picking up stratford by the way, to wormwood scrubs. there was a party to see the atmospheric railway in action: including (among others) sir john burgoyne, whom i met in ireland several years ago, and mr pym, the engineer of the dublin and kingstown railway, whom i have seen several times, and who is very sanguine about this construction; and mr clegg, the proposer of the scheme (the man that invented gas in its present arrangements), and messrs samuda, two jews who are the owners of the experiment now going on; and sir james south! with the latter hero and mechanician we did not come in contact. unfortunately the stationary engine (for working the air-pump which draws the air out of the pipes and thus sucks the carriages along) broke down during the experiment, but not till we had seen the carriage have one right good run. and to be sure it is very funny to see a carriage running all alone "as if the devil drove it" without any visible cause whatever. the mechanical arrangements we were able to examine as well after the engine had broken down as at any time. and they are very simple and apparently very satisfactory, and there is no doubt of the mechanical practicability of the thing even in places where locomotives can hardly be used: whether it will pay or not is doubtful. i dare say that the commissioners' report has taken a very good line of discrimination. * * * * * "in march i wrote to dr wynter (vice-chancellor) at oxford, requesting permission to see bradley's and bliss's manuscript observations, with the view of taking a copy of them. this was granted, and the books of transits were subsequently copied under mr breen's superintendence. --the following paragraph is extracted from the report to the visitors: 'in the report of last year, i stated that our reductions had dropped considerably in arrear. i have the satisfaction now of stating that this arrear and very much more have been completely recovered, and that the reductions are now in as forward a state as at any time since my connection with the observatory.' in fact the observations of were sent to press on mar. st, .--about this year the annual dinner at the visitation began to be more important, principally under the management of capt. w.h. smyth, r.n.--in november i was enquiring about an -inch object-glass. i had already in mind the furnishing of our meridional instruments with greater optical powers.--on july th the admiralty referred to me a memorial of mr j.g. ulrich, a chronometer maker, claiming a reward for improvements in chronometers. i took a great deal of trouble in the investigation of this matter, by books, witnesses, &c., and finally reported on nov. th that there was no ground for claim.--in april i received the first application of the royal exchange committee, for assistance in the construction of the clock: this led to a great deal of correspondence, especially with dent.--the lunar reductions were going on in full vigour.--i had much work in connection with the cape observatory: partly about an equatoreal required for the observatory, but chiefly in getting maclear's work through the press.--in this year i began to think seriously of determining the longitude of valencia in ireland, as a most important basis for the scale of longitude in these latitudes, by the transmission of chronometers; and in august i went to valencia and examined the localities. in september i submitted a plan to the admiralty, but it was deferred.--the new commission for restoring the standards was appointed on june th, i being chairman. the work of collecting standards and arranging plans was going on; mr baily attending to standards of length, and prof. w.h. miller to standards of weight. we held two meetings.--a small assistance was rendered to me by mr charles may (of the firm of ransomes and may), which has contributed much to the good order of papers in the observatory. mr robert ransome had remarked my method of punching holes in the paper by a hand-punch, the places of the holes being guided by holes in a piece of card, and said that they could furnish me with something better. accordingly, on aug. th mr may sent me the punching machine, the prototype of all now used in the observatory. "on sept. th was made my proposal for an altazimuth instrument for making observations of the moon's place more frequently and through parts of her orbit where she could never be observed with meridional instruments; the most important addition to the observatory since its foundation. the board of visitors recommended it to the admiralty, and the admiralty sanctioned the construction of the instrument and the building to contain it." the following passage is quoted from the address of the astronomer royal to the board of visitors at the special meeting of nov. th, : "the most important object in the institution and maintenance of the royal observatory has always been the observations of the moon. in this term i include the determination of the places of fixed stars which are necessary for ascertaining the instrumental errors applicable to the instrumental observations of the moon. these, as regards the objects of the institution, were merely auxiliaries: the history of the circumstances which led the government of the day to supply the funds for the construction of the observatory shews that, but for the demands of accurate lunar determinations as aids to navigation, the erection of a national observatory would never have been thought of. and this object has been steadily kept in view when others (necessary as fundamental auxiliaries) were passed by. thus, during the latter part of bradley's time, and bliss's time (which two periods are the least efficient in the modern history of the observatory), and during the latter part of maskelyne's presidency (when, for years together, there is scarcely a single observation of the declination of a star), the observations of the moon were kept up with the utmost regularity. and the effect of this regularity, as regards its peculiar object, has been most honourable to the institution. the existing theories and tables of the moon are founded entirely upon the greenwich observations; the observatory of greenwich has been looked to as that from which alone adequate observations can be expected, and from which they will not be expected in vain: and it is not perhaps venturing too much to predict that, unless some gross dereliction of duty by the managers of the observatory should occur, the lunar tables will always be founded on greenwich observations. with this impression it has long been to me a matter of consideration whether means should not be taken for rendering the series of observations of the moon more complete than it can be made by the means at present recognized in our observatories."--in illustration of the foregoing remarks, the original inscription still remaining on the outside of the wall of the octagon room of the observatory may be quoted. it runs thus: 'carolus ii's rex optimus astronomiae et nauticae artis patronus maximus speculam hanc in utriusque commodum fecit anno d'ni mdclxxvi regni sui xxviii curante iona moore milite rtsg.' "the ashburton treaty had been settled with the united states, for the boundary between canada and the state of maine, and one of its conditions was, that a straight line about miles in length should be drawn through dense woods, connecting definite points. it soon appeared that this could scarcely be done except by astronomical operations. lord canning, under secretary of the foreign office, requested me to nominate two astronomers to undertake the work. i strongly recommended that military officers should carry out the work, and capt. robinson and lieut. pipon were detached for this service. on mar. st they took lodgings at greenwich, and worked at the observatory every day and night through the month. my detailed astronomical instructions to them were drawn out on mar. th. i prepared all the necessary skeleton forms, &c., and looked to their scientific equipment in every way. the result will be given in . "of private history: in january i went to dover with my wife to see the blasting of a cliff there: we also visited sir j. herschel at hawkhurst. in april i was at playford, on a visit to arthur biddell. on apr. th my daughter annot was born. from july nd to august th i was travelling in the south of ireland, chiefly to see valencia and consider the question of determining its longitude: during this journey i visited lord rosse at birr castle, and returned to weymouth, where my family were staying at the time. in october i visited cambridge, and in december i was again at playford." the journey to cambridge (oct. th to th) was apparently in order to be present on the occasion of the queen's visit there on the th: the following letter relating to it was written to his wife: sedgwick's rooms, trinity college, cambridge. _ , oct. , thursday_. i have this morning received your letter: i had no time to write yesterday. there are more things to tell of than i can possibly remember. the dean of ely yesterday was in a most ludicrous state of misery because his servant had sent his portmanteau (containing his scarlet academicals as well as everything else) to london, and it went to watford before it was recovered: but he got it in time to shew himself to-day. yesterday morning i came early to breakfast with sedgwick. then i walked about the streets to look at the flags. cambridge never had such an appearance before. in looking along trinity street or trumpington street there were arches and flags as close as they could stand, and a cord stretched from king's entrance to mr deck's or the next house with flags on all its length: a flag on st mary's, and a huge royal standard ready to hoist on trinity gateway: laurels without end. i applied at the registrar's office for a ticket which was to admit me to trinity court, the senate house, &c., and received from peacock one for king's chapel. then there was an infinity of standing about, and very much i was fatigued, till i got some luncheon at blakesley's rooms at o'clock. this was necessary because there was to be no dinner in hall on account of the address presentation. the queen was expected at , and arrived about minutes after . when she drove up to trinity gate, the vice-chancellor, masters, and beadles went to meet her, and the beadles laid down their staves, which she desired them to take again. then she came towards the lodge as far as the sundial, where whewell as master took the college keys (a bundle of rusty keys tied together by a particularly greasy strap) from the bursar martin, and handed them to the queen, who returned them. then she drove round by the turret-corner of the court to the lodge door. almost every member of the university was in the court, and there was a great hurraing except when the ceremonies were going forward. presently the queen appeared at a window and bowed, and was loudly cheered. then notice was given that the queen and prince would receive the addresses of the university in trinity hall, and a procession was formed, in which i had a good place, as i claimed rank with the professors. a throne and canopy were erected at the top of the hall, but the queen did not sit, which was her own determination, because if she had sat it would have been proper that everybody should back out before presenting the address to the prince: which operation would have suffocated at least people. the queen wore a blue gown and a brown shawl with an immense quantity of gold embroidery, and a bonnet. then it was known that the queen was going to service at king's chapel at half past three: so everybody went there. i saw the queen walk up the antechapel and she looked at nothing but the roof. i was not able to see her in chapel or to see the throne erected for her with its back to the table, which has given great offence to many people. (i should have said that before the queen came i called on dr haviland, also on scholefield, also on the master of christ's.) after this she returned to trinity, and took into her head to look at the chapel. the cloth laid on the pavement was not long enough and the undergraduates laid down their gowns. several of the undergraduate noblemen carried candles to illuminate newton's statue. after this the prince went by torchlight to the library. then i suppose came dinner, and then it was made known that at half-past nine the queen would receive some members of the university. so i rigged myself up and went to the levée at the lodge and was presented in my turn; by the vice-chancellor as "ex-professor airy, your majesty's astronomer royal." the queen and the prince stood together, and a bow was made to and received from each. the prince recognised me and said "i am glad to see you," or something like that. next to him stood goulburn, and next lord lyndhurst, who to my great surprise spoke very civilly to me (as i will tell you afterwards). the queen had her head bare and a sort of french white gown and looked very well. she had the ribbon of the garter on her breast; but like a ninny i forgot to look whether she had the garter upon her arm. the prince wore his garter. i went to bed dead tired and got up with a headache.--about the degree to the prince and the other movements i will write again. * * * * * here is a note from cubitt relating to the blasting of the round down cliff at dover referred to above: great george street, _jan. th, _. my dear sir, _thursday_ next the th at is the time fixed for the attempt to blow out the foot of the "round down" cliff near dover. the galvanic apparatus has been repeatedly tried in place--that is by exploding cartridges in the very chambers of the rock prepared for the powder--with the batteries at feet distance they are in full form and act admirably so that i see but little fear of failure on that head. they have been rehearsing the explosions on the plan i most strongly recommended, that is--to fire each chamber by an independent battery and circuit and to discharge the three batteries simultaneously by signal or word of command which answers well and "no mistake." i shall write to sir john herschel to-day, and remain my dear sir, very truly yours, w. cubitt. g.b. airy, esq. * * * * * the following extracts are from letters to his wife written in ireland when on his journey to consider the determination of the longitude of valencia. skibbereen, _ , july _. by the bye, to shew the quiet of ireland now, i saw in a newspaper at cork this account. at some place through which a repeal-association was to pass (i forget its name) the repealers of the place set up a triumphal arch. the police pulled it down, and were pelted by the repealers, and one of the policemen was much bruised. o'connell has denounced this place as a disgrace to the cause of repeal, and has moved in the full meeting that the inhabitants of this place be struck off the repeal list, with no exception but that of the parish priest who was proved to be absent. and o'connell declares that he will not pass through this place. now for my journey. it is a sort of half-mountain country all the way, with some bogs to refresh my eyes. valencia hotel, _ , august _. it seems that my coming here has caused infinite alarm. the common people do not know what to conjecture, but have some notion that the "sappers and miners" are to build a bridge to admit the charge of cavalry into the island. an attendant of mrs fitzgerald expressed how strange it was that a man looking so mild and gentle could meditate such things "but never fear, maam, those that look so mild are always the worst": then she narrated how that her husband was building some stables, but that she was demanding of him "pat, you broth of a boy, what is the use of your building stables when these people are coming to destroy everything." i suspect that the people who saw me walking up through the storm yesterday must have thought me the prince of the powers of the air at least. hibernian hotel, tralee, _ , august _. i sailed from valencia to cahersiveen town in a sail-boat up the water (not crossing at the ferry). i had accommodated my time to the wish of the boatman, who desired to be there in time for prayers: so that i had a long waiting at cahersiveen for the mail car. in walking through the little town, i passed the chapel (a convent chapel) to which the people were going: and really the scene was very curious. the chapel appeared to be overflowing full, and the court in front of it was full of people, some sitting on the ground, some kneeling, and some prostrate. there were also people in the street, kneeling with their faces towards the gate pillars, &c. it seemed to me that the priest and the chapel were of less use here than even in the continental churches, and i do not see why both parties should not have stopped at home. when the chapel broke up, it seemed as if the streets were crammed with people. the turnout that even a small village in ireland produces is perfectly amazing. "in the course of i had put in hand the engraving of the drawings of the northumberland telescope at cambridge observatory, and wrote the description for letterpress. in the course of the work was completed, and the books were bound and distributed. "the building to receive the altazimuth instrument was erected in the course of the year; during the construction a foreman fell into the foundation pit and broke his leg, of which accident he died. this is the only accident that i have known at the observatory.--the electrometer mast and sliding frame were erected near the magnetic observatory.--the six-year catalogue of stars was finished; this work had been in progress during the last few years.--in may i went to woolwich to correct the compasses of the 'dover,' a small iron steamer carrying mails between dover and ostend: this i believe was the first iron ship possessed by the admiralty.--the lunar reductions were making good progress; computers were employed upon them. i made application for printing them and the required sum (_£ _) was granted by the treasury.--in this year commenced that remarkable movement which led to the discovery of neptune. on feb. th prof. challis introduced mr adams to me by letter. on feb. th i sent my observed places of uranus, which were wanted. on june th i also sent places to mr e. bouvard.--as regards the national standards, mr baily (who undertook the comparisons relating to standards of length) died soon, and mr sheepshanks then undertook the work.--i attended the meeting of the british association held at york (principally in compliment to the president, dr peacock), and gave an oral account of my work on irish tides.--at the oxford commemoration in june, the honorary degree of d.c.l. was conferred on m. struve and on me, and then a demand was made on each of us for _£ . s._ for fees. we were much disgusted and refused to pay it, and i wrote angrily to dr wynter, the vice-chancellor. the fees were ultimately paid out of the university chest. "in this year the longitude of altona was determined by m. struve for the russian government. for this purpose it was essential that facilities should be given for landing chronometers at greenwich. but the consent of the customhouse authorities had first to be obtained, and this required a good deal of negotiation. ultimately the determination was completed in the most satisfactory manner. the chronometers, forty-two in number, crossed the german sea sixteen times. the transit observers were twice interchanged, in order to eliminate not only their personal equation, but also the gradual change of personal equation. on sept. th otto struve formally wrote his thanks for assistance rendered. "for the determination of the longitude of valencia, which was carried out in this year, various methods were discussed, but the plan of sending chronometers by mail conveyance was finally approved. from london to liverpool the chronometers were conveyed by the railways, from liverpool to kingstown by steamer, from dublin to tralee by the mail coaches, from tralee to cahersiveen by car, from cahersiveen to knightstown by boat, and from knightstown to the station on the hill the box was carried like a sedan-chair. there were numerous other arrangements, and all succeeded perfectly without a failure of any kind. thirty pocket chronometers traversed the line between greenwich and kingstown about twenty-two times, and that between kingstown and valencia twenty times. the chronometrical longitudes of liverpool observatory, kingstown station, and valencia station are m . s, m . s, m . s; the geodetic longitudes, computed from elements which i published long ago in the encyclopaedia metropolitana, are m . s, m . s, m . s. it appears from this that the elements to which i have alluded represent the form of the earth here as nearly as is possible. on the whole, i think it probable that this is the best arc of parallel that has ever been measured. "with regard to the maine boundary: on may th col. estcourt, the british commissioner, wrote to me describing the perfect success of following out my plan: the line of miles was cut by directions laid out at the two ends, and the cuttings met within feet. the country through which this line was to pass is described as surpassing in its difficulties the conception of any european. it consists of impervious forests, steep ravines, and dismal swamps. a survey for the line was impossible, and a tentative process would have broken the spirit of the best men. i therefore arranged a plan of operations founded on a determination of the absolute latitudes and the difference of longitudes of the two extremities. the difference of longitudes was determined by the transfer of chronometers by the very circuitous route from one extremity to the other; and it was necessary to divide the whole arc into four parts, and to add a small part by measure and bearing. when this was finished, the azimuths of the line for the two ends were computed, and marks were laid off for starting with the line from both ends. one party, after cutting more than forty-two miles through the woods, were agreeably surprised, on the brow of a hill, at seeing directly before them a gap in the woods on the next line of hill; it opened gradually, and proved to be the line of the opposite party. on continuing the lines till they passed abreast of each other, their distance was found to be feet. to form an estimate of the magnitude of this error, it is to be observed that it implies an error of only a quarter of a second of time in the difference of longitudes; and that it is only one-third (or nearly so) of the error which would have been committed if the spheroidal form of the earth had been neglected. i must point out the extraordinary merit of the officers who effected this operation. transits were observed and chronometers were interchanged when the temperature was lower than ° below zero: and when the native assistants, though paid highly, deserted on account of the severity of the weather, the british officers still continued the observations upon whose delicacy everything depended. "of private history: from july rd to aug. th i was in ireland with my wife. this was partly a business journey in connection with the determination of the longitude of valencia. on jan. th i asked lord lyndhurst (lord chancellor) to present my brother to the living of helmingham, which he declined to do: but on dec. th he offered binbrooke, which i accepted for my brother." "a map of the buildings and grounds of the observatory was commenced in , and was still in progress.--on mar. th i was employed on a matter which had for some time occupied my thoughts, viz., the re-arrangement of current manuscripts. i had prepared a sloping box (still in use) to hold portfolios: and at this time i arranged papers a, and went on with b, c, &c. very little change has been made in these.--in reference to the time given to the weekly report on meteorology to the registrar general, the report to the board of visitors contains the following paragraph: 'the devotion of some of my assistants' time and labour to the preparation of the meteorological report attached to the weekly report of the registrar general, is, in my opinion, justified by the bearing of the meteorological facts upon the medical facts, and by the attention which i understand that report to have excited.'--on dec. th the sleep of astronomy was broken by the announcement that a new planet, astraea, was discovered by mr hencke. i immediately circulated notices.--but in this year began a more remarkable planetary discussion. on sept. nd challis wrote to me to say that mr adams would leave with me his results on the explanation of the irregularities of uranus by the action of an exterior planet. in october adams called, in my absence. on nov. th i wrote to him, enquiring whether his theory explained the irregularity of radius-vector (as well as that of longitude). i waited for an answer, but received none. (see the papers printed in the royal astronomical society's memoirs and monthly notices).--in the royal society, the royal medal was awarded to me for my paper on the irish tides.--in the royal astronomical society i was president; and, with a speech, delivered the medal to capt. smyth for the bedford catalogue of double stars.--on jan. st i was appointed (with schumacher) one of the referees for the king of denmark's comet medal: i have the king's warrant under his sign manual.--the tidal harbour commission commenced on apr. th: on july st my report on wexford harbour (in which i think i introduced important principles) was communicated. one report was made this year to the government.--in the matter of saw mills (which had begun in ), i had prepared a second set of plans in , and in this year mr nasmyth made a very favourable report on my plan. a machinist of the chatham dock yard, sylvester, was set to work (but not under my immediate command) to make a model: and this produced so much delay as ultimately to ruin the design.--on jan. st i was engaged on my paper 'on the flexure of a uniform bar, supported by equal pressures at equidistant points.'" (this was probably in connection with the support of standards of length, for the commission. ed.).--in june i attended the meeting of the british association at cambridge, and on the th i gave a lecture on magnetism in the senate house. the following quotation relating to this lecture is taken from a letter by whewell to his wife (see life of william whewell by mrs stair douglas): "i did not go to the senate house yesterday evening. airy was the performer, and appears to have outdone himself in his art of giving clearness and simplicity to the hardest and most complex subjects. he kept the attention of his audience quite enchained for above two hours, talking about terrestrial magnetism."--on nov. th i gave evidence before a committee of the house of commons on dover harbour pier. "with respect to the magnetical and meteorological establishment, the transactions in this year were most important. it had been understood that the government establishments had been sanctioned twice for three-year periods, of which the second would expire at the end of : and it was a question with the scientific public whether they should be continued. my own opinion was in favour of stopping the observations and carefully discussing them. and i am convinced that this would have been best, except for the subsequent introduction of self-registering systems, in which i had so large a share. there was much discussion and correspondence, and on june th the board of visitors resolved that 'in the opinion of the visitors it is of the utmost importance that these observations should continue to be made on the most extensive scale which the interests of those sciences may require.' the meeting of the british association was held at cambridge in june: and one of the most important matters there was the congress of magnetic philosophers, many of them foreigners. it was resolved that the magnetic observatory at greenwich be continued permanently. at this meeting i proposed a resolution which has proved to be exceedingly important. i had remarked the distress which the continuous two-hourly observations through the night produced to my assistants, and determined if possible to remove it. i therefore proposed 'that it is highly desirable to encourage by specific pecuniary reward the improvement of self-recording magnetical and meteorological apparatus: and that the president of the british association and the president of the royal society be requested to solicit the favourable consideration of her majesty's government to this subject,' which was adopted. in october the admiralty expressed their willingness to grant a reward up to _£ _. mr charles brooke had written to me proposing a plan on sept. rd, and he sent me his first register on nov. th. on nov. st the treasury informed the admiralty that the magnetic observatories will be continued for a further period. "the railway gauge commission in this year was an important employment. the railways, which had begun with the manchester and liverpool railway (followed by the london and birmingham) had advanced over the country with some variation in their breadth of gauge. the gauge of the colchester railway had been altered to suit that of the cambridge railway. and finally there remained but two gauges: the broad gauge (principally in the system allied with the great western railway); and the narrow gauge (through the rest of england). these came in contact at gloucester, and were likely to come in contact at many other points--to the enormous inconvenience of the public. the government determined to interfere, beginning with a commission. on july rd mr laing (then on the board of trade) rode to greenwich, bearing a letter of introduction from sir john lefevre and a request from lord dalhousie (president of the board of trade) that i would act as second of a royal commission (col. sir frederick smith, airy, prof. barlow). i assented to this: and very soon began a vigorous course of business. on july rd and th i went with prof. barlow and our secretary to bristol, gloucester, and birmingham: on dec. th i went on railway experiments to didcot: and on dec. th to jan. nd i went to york, with prof. barlow and george arthur biddell, for railway experiments. on nov. st i finished a draft report of the railway gauge commission, which served in great measure as a basis for that adopted next year. "of private history: i wrote to lord lyndhurst on feb. th, requesting an exchange of the living to which he had presented my brother in dec. for that of swineshead: to which he consented.--on jan. th i went with my wife on a visit to my uncle george biddell, at bradfield st george, near bury.--on june th i went into the mining district of cornwall with george arthur biddell.--from aug. th to sept. th i was travelling in france with my sister and my wife's sister, georgiana smith. i was well introduced, and the journey was interesting.--on oct. th my son osmund was born.--mr f. baily bequeathed to me _£ _, which realized _£ _." here are some extracts from letters written to his wife relating to the visit to the cornish mines, &c.-- pearce's hotel, falmouth, _ , june th, thursday_. then we walked to the united mines in gwennap. the day was very fine and now it was perfectly broiling: and the hills here are long and steep. at the united mines we found the captain, and he invited us to join in a rough dinner, to which he and the other captains were going to sit down. then we examined one of the great pumping engines, which is considered the best in the country: and some other engines. between and there was to be a setting out of some work to the men by a sort of dutch auction (the usual way of setting out the work here): some refuse ores were to be broken up and made marketable, and the subject of competition was, for how little in the pound on the gross produce the men would work them up. while we were here a man was brought up who was hurt in blasting: a piece of rock had fallen on him. at this mine besides the ladder ways, they have buckets sliding in guides by which the men are brought up: and they are just preparing for work another apparatus which they say is tried successfully at another mine (tresavean): there are two wooden rods _a_ and _b_ reaching from the top to the bottom, moved by cranks from the same wheel, so that one goes up when the other goes down, and vice versâ: each of these rods has small stages, at such a distance that when the rod _a_ is down and the rod _b_ is up, the first stage of _a_ is level with the first stage of _b_: but when the rod _a_ is up and the rod _b_ is down, the second stage of _a_ is level with the first stage of _b_: so a man who wants to descend steps on the first stage of _a_ and waits till it goes down: then he steps sideways on the first stage of _b_ and waits till it goes down: then he steps sideways to the second stage of _a_ and waits till it goes down, and so on: or if a man is coming up he does just the same. while we were here mr r. taylor came. we walked home (a long step, perhaps seven miles) in a very hot sun. went to tea to mr alfred fox, who has a house in a beautiful position looking to the outside of falmouth harbour. * * * * * penzance, _ , june , saturday_. yesterday morning we breakfasted early at falmouth, and before started towards gwennap. i had ascertained on thursday that john williams (the senior of a very wealthy and influential family in this country) was probably returned from london. so we drove first to his house burntcoose or barncoose, and found him and his wife at home. (they are quakers, the rest of the family are not.) sedgwick, and whewell, and i, or some of our party including me, had slept once at their house. they received george and me most cordially, and pressed us to come and dine with them after our visit to tresavean mine, of which intention i spoke in my last letter: so i named o'clock as hour for dinner. after a little stay we drove to tresavean, where i found the captain of the mine prepared to send an underground captain and a pit-man to descend with us. so we changed our clothes and descended by the ladders in the pumpshaft. pretty work to descend with the huge pump-rods (garnished with large iron bolts) working violently, making strokes of feet, close to our elbows; and with a nearly bottomless pit at the foot of every ladder, where we had to turn round the foot of the ladder walking on only a narrow board. however we got down to the bottom of the mine with great safety and credit, seeing all the mighty machinery on the way, to a greater depth than i ever reached before, namely feet. from the bottom of the pump we went aside a short distance into the lowest workings where two men nearly naked were driving a level towards the lode or vein of ore. here i felt a most intolerable heat: and upon moving to get out of the place, i had a dreadful feeling of feebleness and fainting, such as i never had in my life before. the men urged me to climb the ladders to a level where the air was better, but they might as well have urged me to lift up the rock. i could do nothing but sit down and lean fainting against the rocks. this arose entirely from the badness of the air. after a time i felt a trifle better, and then i climbed one short ladder, and sat down very faint again. when i recovered, two men tied a rope round me, and went up the ladder before me, supporting a part of my weight, and in this way i ascended four or five ladders (with long rests between) till we came to a level, fathoms below the adit or nearly fathoms below the surface, where there was a tolerable current of pretty good air. here i speedily recovered, though i was a little weak for a short time afterwards. george also felt the bad air a good deal, but not so much as i. he descended to some workings equally low in another place (towards which the party that i spoke of were directing their works), but said that the air there was by no means so bad. we all met at the bottom of the man-engine fathoms below the adit. we sat still a little while, and i acquired sufficient strength and nerve, so that i did not feel the slightest alarm in the operation of ascending by the man-engine. this is the funniest operation that i ever saw: it is the only absolute novelty that i have seen since i was in the country before: it has been introduced - / years in tresavean, and one day in the united mines. in my last letter i described the principle. in the actual use there is no other motion to be made by the person who is ascending or descending than that of stepping sideways each time (there being proper hand-holds) with no exertion at all, except that of stepping exactly at the proper instant: and not the shadow of unpleasant feeling in the motion. any woman may go with the most perfect comfort, if she will but attend to the rules of stepping, and forget that there is an open pit down to the very bottom of the mine. in this way we were pumped up to the surface, and came up as cool as cucumbers, instead of being drenched with perspiration. in my description in last letter i forgot to mention that between the stages on the moving rods which i have there described there are intermediate stages on the moving rods (for which there is ample room, inasmuch as the interval between the stages on each rod used by one person is feet), and these intermediate stages are used by persons _descending_: so that there are persons _ascending_ and persons _descending_ at the same time, who never interfere with each other and never step on the same stages, but merely see each other passing on the other rods--it is a most valuable invention. we then changed our clothes and washed, and drove to barncoose, arriving in good time for the dinner. i found myself much restored by some superb sauterne with water. when we were proposing to go on to camborne, mr and mrs williams pressed us so affectionately to stop that we at length decided on stopping for the night, only bargaining for an early breakfast this morning. this morning after breakfast, we started for redruth and camborne. the population between them has increased immensely since i was here before. &c. &c. * * * * * here is a letter written to his wife while he was engaged on the business of the railway gauge commission. it contains reminiscences of some people who made a great figure in the railway world at that time, and was preceded by a letter which was playfully addressed "from the palace of king hudson, york." george inn, york, _ , dec. _. i wrote yesterday from mr hudson's in time for the late post, and hope that my letter might be posted by the servant to whom it was given. our affairs yesterday were simple: we reached euston station properly, found watson there, found a carriage reserved for us, eat pork-pie at wolverton (not so good as formerly), dined at derby, and arrived in york at . . on the way watson informed me that the government have awarded us _£ _ each. sir f. smith had talked over the matter with us, and i laid it down as a principle that we considered the business as an important one and one of very great responsibility, and that we wished either that the government should treat us handsomely or should consider us as servants of the state acting gratuitously, to which they assented. i think the government have done very well. mr hudson, as i have said, met us on the platform and pressed us to dine with him (though i had dined twice). then we found the rival parties quarrelling, and had to arrange between them. this prevented me from writing for the early post. (i forgot to mention that saunders, the great western secretary, rode with us all the way). at hudson's we had really a very pleasant dinner: i sat between vernon harcourt and mrs malcolm (his sister georgiana) and near to mr hudson. this morning we were prepared at at the station for some runs. brunel and other people had arrived in the night. and we have been to darlington and back, with a large party in our experimental train. george arthur biddell rode on the engine as representing me. but the side wind was so dreadfully heavy that, as regards the wants of the case, this day is quite thrown away. we have since been to lunch with vernon harcourt (mrs harcourt not at home) and then went with him to look at the cathedral. the chapter-house, which was a little injured, has been pretty well restored: all other things in good order. the cathedral looks smaller and lower than french cathedrals. now that we have come in, the lord mayor of york has just called to invite us to dinner to-morrow.--i propose to george arthur biddell that he go to newcastle this evening, in order to see glass works and other things there to-morrow, and to return when he can. i think that i can persuade barlow to stop to see the experiments out, and if so i shall endeavour to return as soon as possible. the earliest day would be the day after to-morrow. * * * * * the following extract is from a letter written to mr murray for insertion in his handbook of france, relating to the breakwater at cherbourg, which airy had visited during his journey in france in the autumn of this year. royal observatory, greenwich, _ , oct. th_. my opinion on the construction i need not say ought not to be quoted: but you are quite welcome to found any general statement on it; or perhaps it may guide you in further enquiries. to make it clear, i must speak rather generally upon the subject. there are three ways in which a breakwater may be constructed. . by building a strong wall with perpendicular face from the bottom of the sea. . by making a bank with nothing but slopes towards the sea. . by making a sloping bank to a certain height and then building a perpendicular wall upon it.--now if the st of these constructions could be arranged, i have no doubt that it would be the best of all, because a sea does not _break_ against a perpendicular face, but recoils in an unbroken swell, merely making a slow quiet push at the wall, and not making a violent impact. but practically it is nearly impossible. the nd construction makes the sea to break tremendously, but if the sloping surface be made of square stone put together with reasonable care there is not the smallest tendency to unseat these stones. this is the principle of construction of plymouth breakwater. in the rd construction, the slope makes the sea to break tremendously, and then it strikes the perpendicular face with the force of a battering ram: and therefore in my opinion this is the worst construction of all. a few face-stones may easily be dislodged, and then the sea entering with this enormous force will speedily destroy the whole. this is the form of the cherbourg digue. from this you will gather that i have a full belief that plymouth breakwater will last very long, and that the digue of cherbourg, at least its upper wall, will not last long. the great bank will last a good while, gradually suffering degradation, but still protecting the road pretty well. i was assured by the officers residing on the digue that the sea which on breaking is thrown vertically upwards and then falls down upon the pavement does sometimes push the stones about which are lying there and which weigh three or four tons. i saw some preparations for the foundations of the fort at the eastern extremity of the digue. one artificial stone of concrete measured ' " � ' " � ' ", and was estimated to weigh kilogrammes. chapter vi. at greenwich observatory-- to . "on nov. th i proposed a change in the form of estimates for the observatory. the original astronomical part was provided by the admiralty, and the new magnetical and meteorological part was provided by the treasury: and the whole estimates and accounts of the observatory never appeared in one public paper. i proposed that the whole should be placed on the navy estimates, but the admiralty refused. i repeated this in subsequent years, with no success. meantime i always sent to the admiralty a duplicate of my treasury estimate with the proper admiralty estimate.--stephenson's railway through the lower part of the park, in tunnel about feet from the observatory, was again brought forward. on feb. th it was put before me by the government, and on march th i made experiments at kensal green, specially on the effect of a tunnel: which i found to be considerable in suppressing the tremors. on may th i made my report, generally favourable, supposing the railway to be in tunnel. on may th i, with mr stephenson, had an interview at the admiralty with lord ellenborough and sir george cockburn. the earl appeared willing to relax in his scruples about allowing a railway through the park, when sir george cockburn made a most solemn protest against it, on the ground of danger to an institution of such importance as the observatory. i have no doubt that this protest of sir george cockburn's really determined the government. on june th i was informed that the government refused their consent. after this the south eastern railway company adopted the line through tranquil vale.--in consequence of the defective state of paramatta observatory i had written to sir robert peel on april th raising the question of a general superintending board for colonial observatories: and on june th i saw mr gladstone at the colonial office to enquire about the possibility of establishing local boards. on june th a general plan was settled, but it never came to anything.--forty volumes of the observatory mss. were bound--an important beginning.--deep-sunk thermometers were prepared by prof. forbes.--on june nd sir robert inglis procured an order of the house of commons for printing a paper of sir james south's, ostensibly on the effects of a railway passing through greenwich park, but really attacking almost everything that i did in the observatory. i replied to this on july st by a letter in the athenaeum addressed to sir robert inglis, in terms so strong and so well supported that sir james south was effectually silenced." the following extract from a letter of airy's to the earl of rosse, dated dec. th , will shew how pronounced the quarrel between airy and south had become in consequence of the above-mentioned attack and previous differences: "after the public exposure which his conduct in the last summer compelled me to make, i certainly cannot meet him on equal terms, and desire not to meet him at all." (ed.).--"in the mag. and met. department, i was constantly engaged with mr charles brooke in the preparation and mounting of the self-registering instruments, and the chemical arrangements for their use, to the end of the year. with mr ronalds i was similarly engaged: but i had the greatest difficulty in transacting business with him, from his unpractical habits.--the equipment of the liverpool observatory, under me, was still going on: i introduced the use of siemens's chronometric governor for giving horary motion to an equatoreal there. i have since introduced the same principle in the chronograph barrel and the great equatoreal at greenwich: i consider it important.--on feb. th i received the astronomical society's medal for the planetary reductions.--in the university of london: at this time seriously began the discussion whether there should be a compulsory examination in matters bearing on religious subjects. after this there was no peace.--for discovery of comets three medals were awarded by schumacher and me: one to peters, two to de vico. a comet was seen by hind, and by no other observer: after correspondence, principally in , the medal was refused to him.--with respect to the railway gauge commission: on jan. st, in our experiments near york, the engine ran off the rails. on jan. th the commissioners signed the report, and the business was concluded by the end of april. our recommendation was that the narrow gauge should be carried throughout. this was opposed most violently by partisans of the broad gauge, and they had sufficient influence in parliament to prevent our recommendation from being carried into effect. but the policy, even of the great western railway (in which the broad gauge originated), has supported our views: the narrow gauge has been gradually substituted for the broad: and the broad now ( ) scarcely exists.--on june th lord canning enquired of me about makers for the clock in the clock tower of westminster palace. i suggested vulliamy, dent, whitehurst; and made other suggestions: i had some correspondence with e. b. denison, about clocks.--i had much correspondence with stephenson about the tubular bridge over the menai straits. stephenson afterwards spoke of my assistance as having much supported him in this anxious work: on dec. th i was requested to make a report, and to charge a fee as a civil engineer; but i declined to do so. in january i went, with george arthur biddell, to portsmouth, to examine lord dundonald's rotary engine as mounted in the 'janus,' and made a report on the same to the admiralty: and i made several subsequent reports on the same matter. the scheme was abandoned in the course of next year; the real cause of failure, as i believe, was in the bad mounting in the ship. "the engrossing subject of this year was the discovery of neptune. as i have said ( ) i obtained no answer from adams to a letter of enquiry. beginning with june th of i had correspondence of a satisfactory character with le verrier, who had taken up the subject of the disturbance of uranus, and arrived at conclusions not very different from those of adams. i wrote from ely on july th to challis, begging him, as in possession of the largest telescope in england, to sweep for the planet, and suggesting a plan. i received information of its recognition by galle, when i was visiting hansen at gotha. for further official history, see my communications to the royal astronomical society, and for private history see the papers in the royal observatory. i was abused most savagely both by english and french." the report to the visitors contains an interesting account of the great lunar reductions, from which the following passage is extracted: "of the third section, containing the comparison of observed places with tabular places, three sheets are printed, from to . this comparison, it is to be observed, does not contain a simple comparison of places, but contains also the coefficients of the various changes in the moon's place depending on changes in the elements.... the process for the correction of the elements by means of these comparisons is now going on: and the extent of this work, even after so much has been prepared, almost exceeds belief. for the longitude, ten columns are added in groups, formed in thirteen different ways, each different way having on the average about nine hundred groups. for the ecliptic polar distance, five columns are added in groups, formed in seven different ways, each different way having on the average about nine hundred groups. thus it will appear that there are not fewer than , additions of columns of figures. this part of the work is not only completed but is verified, so that the books of comparison of observed and tabular places are, as regards this work, completely cleared out. the next step is to take the means of these groups, a process which is now in hand: it will be followed by the formation and solution of the equations on which the corrections of the elements depend." the following remarks, extracted from the report to the visitors, with respect to the instrumental equipment of the observatory, embody the views of the astronomer royal at this time: "the utmost change, which i contemplate as likely to occur in many years, in regard to our meridional instruments, is the substitution of instruments of the same class carrying telescopes of larger aperture. the only instrument which, as i think, may possibly be called for by the demands of the astronomer or the astronomical public, is a telescope of the largest size, for the observation of faint nebulae and minute double stars. whether the addition of such an instrument to our apparatus would be an advantage, is, in my opinion, not free from doubt. the line of conduct for the observatory is sufficiently well traced; there can be no doubt that our primary objects ought to be the accurate determination of places of the fundamental stars, the sun, the planets, and, above all, the moon. any addition whatever to our powers or our instrumental luxuries, which should tend to withdraw our energies from these objects, would be a misfortune to the observatory." of private history: "in march i visited prof. sedgwick at norwich.--on mar. th the 'sir henry pottinger' was launched from fairbairn's yard on the isle of dogs, where i was thrown down and dislocated my right thumb.--from apr. th to th i was at playford.--on june th prof. hansen arrived, and stayed with me to july th.--from july th to th i was visiting dean peacock at ely.--from july rd to th i was at playford, where for the first time i lodged in my own cottage. i had bought it some time before, and my sister had superintended alterations and the addition of a room. i was much pleased thus to be connected with the happy scenes of my youth.--from aug. th to oct. th i was with my wife and her sister elizabeth smith on the continent. we stayed for some time at wiesbaden, as my nerves were shaken by the work on the railway gauge commission, and i wanted the wiesbaden waters. we visited various places in germany, and made a -days' excursion among the swiss mountains. at gotha we lodged with prof. hansen for three days; and it was while staying here that i heard from prof. encke (on sept. th) that galle had discovered the expected planet. we visited gauss at göttingen and miss caroline herschel at hannover. we had a very bad passage from hamburgh to london, lasting five days: a crank-pin broke and had to be repaired: after four days our sea-sickness had gone off, during the gale--a valuable discovery for me, as i never afterwards feared sea-sickness.--on dec. nd i attended the celebration of the th anniversary of trinity college." * * * * * the following extracts relating to the engines of the "janus" are taken from letters to his wife dated from portsmouth, jan. th and th, : as soon as possible we repaired to the dock yard and presented ourselves to the admiral superintendant--admiral hyde parker (not sir hyde parker). found that the "janus" had not arrived: the admiral superintendant (who does not spare a hard word) expressing himself curiously thereon. but he had got the proper orders from the admiralty relating to me: so he immediately sent for mr taplin, the superintendant of machinery: and we went off to see the small engine of lord d--d's construction which is working some pumps and other machinery in the yard. it was kept at work a little longer than usual for us to see it. and i have no hesitation in saying that it was working extremely well. it had not been opened in any way for half a year, and not for repair or packing for a much longer time.... this morning we went to the dock yard, and on entering the engine house there was shirreff, and lord d--d soon appeared. the "janus" had come to anchor at spithead late last night, and had entered the harbour this morning. blowing weather on saturday night. we had the engine pretty well pulled to pieces, and sat contemplating her a long time. before this denison had come to us. we then went on board the "janus" with shirreff but not with lord d--d. the engines were still hot, and so they were turned backwards a little for my edification. (this was convenient because, the vessel being moored by her head, she could thus strain backwards without doing mischief.) the vacuum not good. then, after a luncheon on board, it was agreed to run out a little way. but the engines absolutely stuck fast, and would not stir a bit. this i considered a perfect godsend. so the paddle-wheels (at my desire) were lashed fast, and we are to see her opened to-morrow morning. this morning (jan. th) we all went off to the "janus," where we expected to find the end of the cylinder (where we believe yesterday's block to have taken place) withdrawn. but it was not near it. after a great many bolts were drawn, it was discovered that one bolt could not be drawn, and in order to get room for working at it, it was necessary to take off the end of the other cylinder. and such a job! three pulley hooks were broken in my sight, and i believe some out of my sight. however this auxiliary end was at last got off: and the people began to act on the refractory bolt. but by this time it was getting dark and the men were leaving the dockyard, so i left, arranging that what they could do in preparation for me might be done in good time to-morrow morning. "on nov. th i circulated an address, proposing to discontinue the use of the zenith tube, because it had been found by a long course of comparative trials that the zenith tube was not more accurate than the mural circle. the address stated that 'this want of superior efficiency of the zenith tube (which, considered in reference to the expectations that had been formed of its accuracy, must be estimated as a positive failure) is probably due to two circumstances. one is, the use of a plumb-line; which appears to be affected with various ill-understood causes of unsteadiness. the other is, the insuperable difficulty of ventilating the room in which the instrument is mounted.'--on december th i circulated an address, proposing a transit circle, with telescope of inches aperture. the address states as follows: 'the clear aperture of the object-glass of our transit instrument is very nearly inches, that of our mural circle is very nearly inches.'--i had been requested by the master-general of ordnance (i think) to examine candidates for a mastership in woolwich academy, and i was employed on it in february and march, in conjunction with prof. christie.--in january i applied to lord auckland for money-assistance to make an astronomical journey on the continent, but he refused.--on mar. th sir james south addressed to the admiralty a formal complaint against me for not observing with the astronomical instruments: on mar. st i was triumphantly acquitted by the admiralty.--in june i was requested by the commissioners of railways to act as president of a commission on iron bridges (suggested by the fall of the bridge at chester). lord auckland objected to it, and i was not sorry to be spared the trouble of it.--in december i was requested, and undertook to prepare the astronomical part of the scientific manual for naval officers.--on sept. th occurred a very remarkable magnetic storm, to which there had been nothing comparable before. mr glaisher had it observed by eye extremely well, and i printed and circulated a paper concerning it.--hansen, stimulated by the lunar reductions, discovered two long inequalities in the motion of the moon, produced by the action of venus. in the report to the visitors this matter is thus referred to: 'in the last summer i had the pleasure of visiting prof. hansen at gotha, and i was so fortunate as to exhibit to him the corrections of the elements from these reductions, and strongly to call his attention to their certainty, the peculiarity of their fluctuations, and the necessity of seeking for some physical explanation. i have much pleasure in indulging in the thought, that it was mainly owing to this representation that prof. hansen undertook that quest, which has terminated in the discovery of his two new lunar inequalities, the most remarkable discovery, i think, in physical astronomy.'--in discussing points relating to the discovery of neptune, i made an unfortunate blunder. in a paper hastily sent to the athenaeum (feb. th) i said that arago's conduct had been indelicate. i perceived instantly that i had used a wrong expression, and by the very next post i sent an altered expression. this altered expression was not received in time, and the original expression was printed, to my great sorrow. i could not then apologize. but at what appeared to be the first opportunity, in december, i did apologize; and my apology was accepted. but i think that arago was never again so cordial as before.--on july th hebe was discovered. after this iris and flora. now commenced that train of discoveries which has added more than planets to the solar system.--on oct. th was an annular eclipse of the sun, of which the limit of annularity passed near to greenwich. to determine the exact place, i equipped observatories at hayes, lewisham south end, lewisham village, blackwall, stratford, walthamstow, and chingford. the weather was bad and no observation was obtained.--in the royal astronomical society: in , the dispute between the partisans of adams and le verrier was so violent that no medal could be awarded to either. in i (with other fellows of the society) promoted a special meeting for considering such a modification of the bye-laws that for this occasion only it might be permissible to give two medals. after two days' stormy discussion, it was rejected.--in the university of london: at a meeting in july, where the religious question was discussed, it was proposed to receive some testimonial from affiliated bodies, or to consider that or some other plan for introducing religious literature. as the propriety of this was doubtful, there was a general feeling for taking legal advice: and it was set aside solely on purpose to raise the question about legal consultation. _that_ was negatived by vote: and i then claimed the consideration of the question which we had put aside for it. by the influence of h. warburton, m.p., this was denied. i wrote a letter to be laid before the meeting on july th, when i was necessarily absent, urging my claim: my letter was put aside. i determined never to sit with warburton again: on aug. nd i intimated to lord burlington my wish to retire, and on aug. th he transmitted to the home secretary my resignation. he (lord burlington) fully expressed his opinion that my claim ought to have been allowed.--on june th, on the occasion of prince albert's state visit to cambridge, knighthood was offered to me through his secretary, prof. sedgwick, but i declined it.--in september, the russian order of st stanislas was offered to me, mr de berg, the secretary of embassy, coming to greenwich personally to announce it: but i was compelled by our government rules to decline it.--i invited le verrier to england, and escorted him to the meeting of the british association at oxford in june.--as regards the westminster clock on the parliamentary building: in may i examined and reported on dent's and whitehurst's clock factories. vulliamy was excessively angry with me. on may st a great parliamentary paper was prepared in return to an order of the house of lords for correspondence relating to the clock.--with respect to the saw mills for ship timber: work was going on under the direction of sylvester to mar. th. it was, i believe, at that time, that the fire occurred in chatham dock yard which burnt the whole of the saw-machinery. i was tired of my machinery: and, from the extending use of iron ships, the probable value of it was much diminished; and i made no effort to restore it." of private history: "in february i went to derby to see whitehurst's clock factory; and went on with my wife to brampton near chesterfield, where her mother was living.--from apr. st to th i was at playford.--on holy thursday, i walked the parish bounds (of greenwich) with the parish officers and others. from apr. th to th i was at birmingham (on a visit to guest, my former pupil, and afterwards master of caius college) and its neighbourhood, with george arthur biddell.--from june rd to th i was at oxford and malvern: my sister was at malvern, for water-cure: the meeting of the british association was at oxford and i escorted le verrier thither.--july th to th i was at brampton.--from august th to september th i was engaged on an expedition to st petersburg, chiefly with the object of inspecting the pulkowa observatory. i went by hamburg to altona, where i met struve, and started with him in an open waggon for lübeck, where we arrived on aug. th. we proceeded by steamer to cronstadt and petersburg, and so to pulkowa, where i lodged with o. struve. i was here engaged till sept. th, in the observatory, in expeditions in the neighbourhood and at st petersburg, and at dinner-parties, &c. i met count colloredo, count ouvaroff, count stroganoff, lord bloomfield (british ambassador), and others. on sept. th i went in a small steamer to cronstadt, and then in the vladimir to swinemünde: we were then towed in a passage boat to stettin, and i proceeded by railway to berlin. on sept. th i found galle and saw the observatory. on sept. th i went to potzdam and saw humboldt. on the th i went to hamburg and lodged with schumacher: i here visited repsold and rümker. on sept. th i embarked in the john bull for london, and arrived there on the evening of the th: on the th it was blowing 'a whole gale,' reported to be the heaviest gale known for so many hours; bullocks and sheep were thrown overboard.--from dec. rd to th i was at cambridge, and from the nd to st at playford." * * * * * here is a letter to his wife written from birmingham, containing a note of the progress of the ironwork for the menai bridge: edgbaston, birmingham, _ , apr. _. yesterday morning we started between and for stourbridge, first to see some clay which is celebrated all over the world as the only clay which is fit to make pots for melting glass, &c. you know that in all these fiery regions, fire-clay is a thing of very great importance, as no furnace will stand if made of any ordinary bricks (and even with the fire-clay, the small furnaces are examined every week), but this stourbridge clay is as superior to fire-clay as fire-clay is to common brick-earth. then we went to fosters' puddling and rolling works near stourbridge. these are on a very large scale: of course much that we saw was a repetition of what we had seen before, but there were slitting mills, machines for rolling the puddled blooms instead of hammering them, &c., and we had the satisfaction of handling the puddling irons ourselves. then we went to another work of the fosters not far from dudley, where part of the work of the tube bridge for the menai is going on. the fosters are, i believe, the largest iron masters in the country, and the two principal partners, the elder mr foster and his nephew, accompanied us in all our inspections and steppings from one set of works to another. the length of tube bridge which they have in hand here is only feet, about / of the whole length: and at present they are only busy on the bottom part of it: but it is a prodigious thing. i shall be anxious about it. then we went to other works of the fosters' at king's wynford, where they have blast furnaces: and here after seeing all other usual things we saw the furnaces tapped. in this district the fosters work the -yard coal in a way different from any body else: they work out the upper half of its thickness and then leave the ground to fall in: after a year or two this ground becomes so hard as to make a good safe roof, and then they work away the other half: thus they avoid much of the danger and difficulty of working the thick bed all at once. the ventilation of these mines scarcely ever requires fires, and then only what they call "lamps," those little fire-places which are used for giving light at night. (in the northumberland and durham pits, they constantly have immense roaring fires to make a draught.) then we came home through dudley. * * * * * during his stay in russia, there was a great desire manifested by the astronomers and scientific men of russia that he should be presented to the emperor. this would no doubt have taken place had not the movements of the court and his own want of time prevented it. the following letter to the british ambassador, lord bloomfield, relates to this matter: pulkowa, _ , august th_. _wednesday evening_. my lord, i had the honour yesterday to receive your lordship's note of sunday last, which by some irregularity in the communications with this place reached me, i believe, later than it ought. from this circumstance, and also from my being made acquainted only this afternoon with some official arrangements, i am compelled to trouble you at a time which i fear is less convenient than i could have desired. the object of my present communication is, to ask whether (if the movements of the court permit it) it would be agreeable to your lordship to present me to the emperor. in explanation of this enquiry, i beg leave to state that this is an honour to which, personally, i could not think of aspiring. my presence however at pulkowa at this time is in an official character. as astronomer royal of england, i have thought it my duty to make myself perfectly acquainted with the observatory of pulkowa, and this is the sole object of my journey to russia. it is understood that the emperor takes great interest in the reputation of the observatory, and i am confident that the remarks upon it which i am able to make would be agreeable to him. i place these reasons before you, awaiting entirely your lordship's decision on the propriety of the step to which i have alluded. i am to leave st petersburg on saturday the th of september. i have the honor to be my lord, your lordship's very faithful servant, g. b. airy. _lord bloomfield, &c., &c._ * * * * * it was probably in acknowledgment of this letter that in due time he received the following letter with the offer of the russian order of st stanislas: monsieur l'astronome royal, sa majesté l'empereur en appréciant les travaux assidus qui vous ont donné une place distinguée au rang des plus illustres astronomes de l'europe, et la coopération bienveillante, que vous n'avez cessé de témoigner aux astronomes russes dans les expéditions, dont ils étaient chargés, et en dernier lieu par votre visite à l'observatoire central de poulkova, a daigné sur mon rapport, vous nommer chevalier de la seconde classe de l'ordre impérial et royal de st stanislas. je ne manquerai pas de vous faire parvenir par l'entremise de lord bloomfield les insignes et la patente de l'ordre. veuillez en attendant, monsieur, recevoir mes sincères félicitations et l'assurance de ma parfaite considération. le ministre de l'instruction publique, cte ouvaroff. st p�tersbourg, _ce_ _août_, ---------- _septbr._ _à mr g. b. airy, esq., astronome royal de s. m. britannique à greenwich_. * * * * * airy provisionally accepted the order, but wrote at once to lord john russell the following letter of enquiry: royal observatory, greenwich, _ , oct. _. my lord, in respect of the office of astronomer royal, i refer to the first lord of the treasury as official patron. in virtue of this relation i have the honour to lay before your lordship the following statement, and to solicit your instructions thereon. for conducting with efficiency and with credit to the nation the institution which is entrusted to me, i have judged it proper to cultivate intimate relations with the principal observatories of europe, and in particular with the great observatory founded by the emperor of russia at pulkowa near st petersburg. i have several times received mr struve, the director of that observatory, at greenwich: and in the past summer i made a journey to st petersburg for the purpose of seeing the observatory of pulkowa. since my return from russia, i have received a communication from count ouvaroff, minister of public instruction in the russian empire, informing me that the emperor of russia desires to confer on me the decoration of knight commander in the second rank of the order of st stanislas. and i have the honour now to enquire of your lordship whether it is permitted to me to accept from the emperor of russia this decoration. i have the honour to be, my lord, your lordship's very obedient servant, g.b. airy. _the rt honble lord john russell, &c. &c. &c. first lord of the treasury_. * * * * * the answer was as follows: downing street, _october , _. sir, i am desired by lord john russell to acknowledge the receipt of your letter, of the th inst. and to transmit to you the enclosed paper respecting foreign orders by which you will perceive that it would be contrary to the regulations to grant you the permission you desire. i am, sir, your obedient servant, c.a. grey. _g. b. airy, esq_. * * * * * the passage in the regulations referred to above is quoted in the following letter to count ouvaroff: royal observatory, greenwich, _ , oct. _. sir, referring to your excellency's letter of the august/ september, and to my answer of the th september, in which i expressed my sense of the high honor conferred on me by his majesty the emperor of russia in offering me, through your excellency, the order of st stanislas, and my pride in accepting it:--i beg leave further to acquaint you that i have thought it necessary to make enquiry of lord john russell, first lord of her majesty's treasury, as to my competency to accept this decoration from his majesty the emperor of russia: and that his lordship in reply has referred me to the following regulation of the british court; " th. that no subject of her majesty could be allowed to accept the insignia of a foreign order from any sovereign of a foreign state, except they shall be so conferred in consequence of active and distinguished services before the enemy, either at sea, or in the field; or unless he shall have been actually employed in the service of the foreign sovereign." in consequence of the stringency of this regulation, it is my duty now to state to your excellency that i am unable to accept the decoration which his majesty the emperor of russia was pleased, through your excellency, to offer to me. i beg leave to repeat the expression of my profound reverence to his majesty and of my deep sense of the honor which he has done me. i have the honor to be, sir, your excellency's very faithful and obedient servant, g.b. airy. _to his excellency count ouvaroff, &c. &c._ in the course of the following year a very handsome gold medal, specially struck, was transmitted by count ouvaroff on the part of the emperor of russia, to mr airy. "in april i received authority to purchase of simms an -inch object-glass for the new transit circle for _£ _. the glass was tested and found satisfactory. while at playford in january i drew the first plans of the transit circle: and c. may sketched some parts. definite plans were soon sent to ransomes and may, and to simms in march. the instrument and the building were proceeded with during the year. the new transit circle was to be erected in the circle room, and considerable arrangement was necessary for continuing the circle observations with the existing instruments, whilst the new instrument was under erection. when the new transit is completely mounted, the old transit instrument may be removed, and the transit room will be free for any other purpose. i propose to take it as private room for the astronomer royal.--on may th i made my first proposal of the reflex zenith tube. the principle of it is as follows: let the micrometer be placed close to the object-glass, the frame of the micrometer being firmly connected with the object-glass cell, and a reflecting eye-piece being used with no material tube passing over the object-glass: and let a basin of quicksilver be placed below the object-glass, but in no mechanical connection with it, at a distance equal to half the focal length of the object-glass. such an instrument would at least be free from all uncertainties of twist of plumb-line, viscosity of water, attachment of upper plumb-line microscope, attachment of lower plumb-line microscope, and the observations connected with them: and might be expected, as a result of this extreme simplicity, to give accurate results.--a considerable error was discovered in the graduation of troughton's circle, amounting in one part to six seconds, which is referred to as follows: 'this instance has strongly confirmed me in an opinion which i have long held--that no independent division is comparable in general accuracy to engine-division,--where the fundamental divisions of the engine have been made by troughton's method, and where in any case the determination by the astronomer of errors of a few divisions will suffice, in consequence of the uniformity of law of error, to give the errors of the intermediate divisions.'--the method of observing with the altazimuth is carefully described, and the effect of it, in increasing the number of observations of the moon, is thus given for the thirteen lunations between , may , and , may . 'number of days of complete observations with the meridional instruments, ; number of days of complete observations with altitude and azimuth instrument, . the results of the observations appear very good; perhaps a little, and but a little, inferior to those of the meridional instruments. i consider that the object for which this instrument was erected is successfully attained.'--being satisfied with the general efficiency of the system arranged by mr brooke for our photographic records (of magnetical observations) i wrote to the admiralty in his favour, and on aug. th the admiralty ordered the payment of _£ _ to him. a committee of the royal society also recommended a reward of _£ _ to mr ronalds, which i believe was paid to him.--on may st the last revise of the lunar reductions was passed, and on may th, copies were sent for binding.--in this year schumacher and i refused a medal to miss mitchell for a comet discovered, because the rules of correspondence had not been strictly followed: the king of denmark gave one by special favour.--in this year occurred the discovery of saturn's th satellite by mr lassell: upon which i have various correspondence.--on the th of december the degree of ll.d. was conferred upon me by the university of edinburgh.--the ipswich lectures: a wish had been expressed that i would give a series of astronomical lectures to the people of ipswich. i therefore arranged with great care the necessary apparatus, and lectured six evenings in a room (i forget its name--it might be temperance hall--high above st matthew's street), from mar. th to the end of the week. a shorthand writer took them down: and these formed the 'ipswich lectures,' which were afterwards published by the ipswich museum (for whose benefit the lectures were given) and by myself, in several editions, and afterwards by messrs macmillan in repeated editions under the title of 'airy's popular astronomy.'--it had been found necessary to include under one body all the unconnected commissions of sewers for the metropolis, and lord morpeth requested me to be a member. its operations began on oct. th. in constitution it was the most foolish that i ever knew: consisting of, i think, some persons, who could not possibly attend to it. it came to an end in the next year." of private history: "i was at playford from jan. st to th, and again from jan. th to th: also at playford from june st to july th.--from aug. rd to sept. th i was in ireland on a visit to lord rosse at parsonstown, chiefly engaged on trials of his large telescope. i returned by liverpool, where i inspected the liverpool equatoreal and clockwork, and examined mr lassell's telescopes and grinding apparatus.--from dec. th to th i was at edinburgh with my wife, on a visit to prof. j. d. forbes. we made various excursions, and i attended lectures by prof. wilson and sir w. hamilton: on the th i gave a lecture in prof. forbes's room. i received the honorary degree of ll.d., and made a statement on the telescopes of lord rosse and mr lassell to the royal society of edinburgh. returned to greenwich by brampton." * * * * * here is a reminiscence of the "ipswich lectures," in a letter to his wife, dated playford, mar. , "at the proper time i went to the hall: found a chairman installed (mr western): was presented to him, and by him presented to the audience: made my bow and commenced. the room was quite full: i have rarely seen such a sea of faces; about i believe. everything went off extremely well, except that the rollers of the moving piece of sky would squeak: but people did not mind it: and when first a star passed the meridian, then jupiter, then some stars, and then saturn, he was much applauded. before beginning i gave notice that i should wait to answer questions: and as soon as the lecture was finished the chairman repeated this and begged people to ask. so several people did ask very pertinent questions (from the benches) shewing that they had attended well. others came up and asked questions." * * * * * the following extracts are from letters written to his wife while on his visit to lord rosse at parsonstown in ireland. on the way he stopped at bangor and looked at the tubular bridge works, which are thus referred to: "stopped at bangor, settled _pro tem_. at the castle, and then walked past the suspension bridge towards the tube works, which are about - / mile south-west of the suspension bridge. the way was by a path through fields near the water side: and from one or two points in this, the appearance of the suspension bridge was most majestic. the tube bridge consists of four spans, two over water and two over sloping land. the parts for the double tube over the water spans (four lengths of tube) are building on a platform as at conway, to be floated by barges as there: the parts over the sloping banks are to be built in their place, on an immense scaffolding. i suspect that, in regard to these parts, stephenson is sacrificing a great deal of money to uniformity of plan: and that it would have been much cheaper to build out stone arches to the piers touching the water.... the tube works are evidently the grand promenade of the idlers about bangor: i saw many scores of ladies and gentlemen walking that way with their baskets of provision, evidently going to gipsy in the fields close by." the castle, parsonstown, _ , aug. _. after tea it was voted that the night was likely to be fine, so we all turned out. the night was uncertain: sometimes entirely clouded, sometimes partially, but objects were pretty well seen when the sky was clear: the latter part was much steadier. from the interruption by clouds, the slowness of finding with and managing a large instrument (especially as their finding apparatus is not perfectly arranged) and the desire of looking well at an object when we had got it, we did not look at many objects. the principal were, saturn and the annular nebula of lyra with the -feet; saturn, a remarkable cluster of stars, and a remarkable planetary nebula, with the -feet. with the large telescope, the evidence of the quantity of light is prodigious. and the light of an object is seen in the field without any colour or any spreading of stray light: and it is easy to see that the vision with a reflecting telescope may be much more perfect than with a refractor. with these large apertures, the rings round the stars are insensible. the planetary nebula looked a mass of living and intensely brilliant light: this is an object which i do not suppose can be seen at all in our ordinary telescopes. the definition of the stars near the zenith is extremely good: with a high power (as ) they are points or very nearly so--indeed i believe quite so--so that it is clear that the whole light from the great -feet mirror is collected into a space not bigger than the point of a needle. but in other positions of the telescope the definition is not good: and we must look to-day to see what is the cause of this fault. it is not a fault in the telescope, properly so-called, but it is either a tilt of the mirror, or an edge-pressure upon the mirror when the telescope points lower down which distorts its figure, or something of that kind. so i could not see saturn at all well, for which i was sorry, as i could so well have compared his appearance with what i have seen before. i shall be very much pleased if we can make out what is the fault of adjustment, and so correct it as to get good images everywhere. it is evident that the figuring of the mirror, the polishing, and the general arrangement, are perfectly managed. the castle, parsonstown, _ , aug. _. yesterday we were employed entirely about the great telescope, beginning rather late. the principal objects had relation to the fault of definition when the telescope is pointed low (which i had remarked on the preceding night), and were, to make ourselves acquainted with the mechanism of the mirror's mounting generally, and to measure in various ways whether the mirror actually does shift its place when the telescope is set to different angles of elevation. for the latter we found that the mirror actually does tilt / of an inch when the tube points low. this of itself will not account for the fault but it indicates that the lower part is held fast in a way that may cause a strain which would produce the fault. these operations and reasonings took a good deal of time. lord rosse is disposed to make an alteration in the mounting for the purpose of correcting this possible strain. the castle, parsonstown, _ , aug. _. the weather here is still vexatious: but not absolutely repulsive. yesterday morning lord rosse arranged a new method of suspending the great mirror, so as to take its edgewise pressure in a manner that allowed the springy supports of its flat back to act. this employed his workmen all day, so that the proposed finish of polishing the new mirror could not go on. i took one camera lucida sketch of the instrument in the morning, dodging the heavy showers as well as i could; then, as the afternoon was extremely fine, i took another, with my head almost roasted by the sun. this last view is extremely pretty and characteristic, embracing parts of the mounting not shewn well in the others, and also shewing the castle, the observatory, and the -feet telescope. the night promised exceedingly well: but when we got actually to the telescope it began to cloud and at length became hopeless. however i saw that the fault which i had remarked on the two preceding nights was gone. there is now a slight exhibition of another fault to a much smaller extent. we shall probably be looking at the telescope to-day in reference to it. the castle, parsonstown, _ , sept. _. yesterday we made some alterations in the mounting of the great mirror. we found that sundry levers were loose which ought to be firm, and we conjectured with great probability the cause of this, for correction of which a change in other parts was necessary. the mirror was then found to preserve its position much more fixedly than before.... at night, upon trying the telescope, we found it very faulty for stars near the zenith, where it had been free from fault before. the screws which we had driven hard were then loosened, and immediately it was made very good. then we tried with some lower objects, and it was good, almost equally good, there. for saturn it was very greatly superior to what it had been before. still it is not satisfactory to us, and at this time a strong chain is in preparation, to support the mirror edgeways instead of the posts that there were at first or the iron hoop which we had on it yesterday. nobody would have conceived that an edgewise gripe of such a mass of metal could derange its form in this way. last night was the finest night we have had as regards clouds, though perhaps not the best for definition of objects. the castle, parsonstown, _ , sept. _. i cannot learn that the fault in the mirror had been noticed before, but i fancy that the observations had been very much confined to the zenith and its neighbourhood. "in july the new constant-service water-pipes to the observatory were laid from blackheath. before this time the supply of water to the observatory had been made by a pipe leading up from the lower part of the park, and was not constant.--in may the new staircase from my dwelling-house to the octagon room was commenced.--in the report to the visitors there is a curious account of mr breen's (one of the assistants) personal equation, which was found to be different in quantity for observations of the moon and observations of the stars.--the most important set of observations (of planets) was a series of measures of saturn in four directions, at the time when his ring had disappeared. they appear completely to negative the idea that saturn's form differs sensibly from an ellipsoid.--among the general remarks of the report the following appears: 'another change (in prospect) will depend on the use of galvanism; and as a probable instance of the application of this agent, i may mention that, although no positive step has hitherto been taken, i fully expect in no long time to make the going of all the clocks in the observatory depend on one original regulator. the same means will probably be employed to increase the general utility of the observatory, by the extensive dissemination throughout the kingdom of accurate time-signals, moved by an original clock at the royal observatory; and i have already entered into correspondence with the authorities of the south eastern railway (whose line of galvanic communication will shortly pass within nine furlongs of the observatory) in reference to this subject.'--i agreed with schumacher in giving no medal to mr g. p. bond; his comet was found to be petersen's. five medals were awarded for comets in (hind, colla, mauvais, brorsen, schweizer).--the liverpool observatory was finished this year: and the thanks of the town council were presented to me.--respecting fallows's observations at the cape of good hope: i had received the admiralty sanction for proceeding with calculations in , and i employed computers as was convenient. on july th of this year i was ready with final results, and began to make enquiries about fallows's personal history, and the early history of the cape observatory. on oct. rd i applied for sanction for printing, which was given, and the work was soon finished off, in the astronomical society's memoirs.--in the month of march i had commenced correspondence with various persons on the imperfect state of publication of the british survey. sheets of the map were issued by scores, but not one of them had an indication of latitude or longitude engraved. i knew that great pains had been taken in giving to the principal triangulation a degree of accuracy never before reached, and in fixing the astronomical latitudes of many stations with unequalled precision. finally i prepared for the council of the royal society a very strong representation on these subjects, which was adopted and presented to the government. it was entirely successful, and the maps were in future furnished with latitude and longitude lines.--i was elected president of the royal astronomical society on feb. th.--in june i went with sheepshanks to see some of the operation of measuring a base on salisbury plain. the following extract from a letter to his wife dated , june th, relates to this expedition: 'in the morning we started before eight in an open carriage to the plain: looking into old sarum on our way. the base is measured on what i should think a most unfavourable line, its north end (from which they have begun now, in verification of the old measure) being the very highest point in the whole plain, called beacon hill. the soldiers measure only feet in a day, so it will take them a good while to measure the whole seven miles. while we were there col. hall (colby's successor) and yolland and cosset came.'" of private history: "i made short visits to playford in january, april and july. from july th to sept. th i made an expedition with my wife to orkney and shetland.--from dec. th to th i was at hawkhurst, on a visit to sir john herschel." "the report to the board of visitors opens with the following paragraph: 'in recording the proceedings at the royal observatory during the last year, i have less of novelty to communicate to the visitors than in the reports of several years past. still i trust that the present report will not be uninteresting; as exhibiting, i hope, a steady and vigorous adherence to a general plan long since matured, accompanied with a reasonable watchfulness for the introduction of new instruments and new methods when they may seem desirable.'--since the introduction of the self-registering instruments a good many experiments had been made to obtain the most suitable light, and the report states that 'no change whatever has been made in these instruments, except by the introduction of the light of coal-gas charged with the vapour of coal-naptha, for photographic self-registration both of the magnetic and of the meteorological instruments.... the chemical treatment of the paper is now so well understood by the assistants that a failure is almost unknown. and, generally speaking, the photographs are most beautiful, and give conceptions of the continual disturbances in terrestrial magnetism which it would be impossible to acquire from eye-observation.' --amongst the general remarks of the report it is stated that 'there are two points which have distinctly engaged my attention. the first of these is, the introduction of the american method of observing transits, by completing a galvanic circuit by means of a touch of the finger at the instant of appulse of the transiting body to the wire of the instrument, which circuit will then animate a magnet that will make an impression upon a moving paper. after careful consideration of this method, i am inclined to believe that, in prof. mitchell's form, it does possess the advantages which have been ascribed to it, and that it may possess peculiar advantages in this observatory, where the time-connection of transits made with two different instruments (the transit and the altazimuth) is of the highest importance.... the second point is, the connection of the observatory with the galvanic telegraph of the south eastern railway, and with other lines of galvanic wire with which that telegraph communicates. i had formerly in mind only the connection of this observatory with different parts of the great british island: but i now think it possible that our communications may be extended far beyond its shores. the promoters of the submarine telegraph are very confident of the practicability of completing a galvanic connection between england and france: and i now begin to think it more than possible that, within a few years, observations at paris and brussels may be registered on the recording surfaces at greenwich, and vice versa.'--prof. hansen was engaged in forming lunar tables from his lunar theory, but was stopped for want of money. on mar. th i represented this privately to mr baring, first lord of the admiralty; and on mar. th i wrote officially to the admiralty, soliciting _£ _ with the prospect, if necessary, of making it _£ _. on apr. th the admiralty gave their assent. the existence of hansen's lunar tables is due to this grant.--the king of denmark's medal for comets was discontinued, owing to the difficulties produced by the hostility of prussia.--on aug. st i gave to the treasury my opinion on the first proposal for a large reflector in australia: it was not strongly favourable.--in august, being (with my wife and otto struve) on a visit to lady breadalbane at taymouth castle, i examined the mountain schehallien.--as in other years, i reported on several papers for the royal society, and took part in various business for them.--in the royal astronomical society i had much official business, as president.--in march i communicated to the athenaeum my views on the exodus of the israelites: this brought me into correspondence with miss corbaux, robert stephenson, capt. vetch, and prof. j.d. forbes.--in december i went to the london custom house, to see sir t. freemantle (chairman of customs), and to see how far decimal subdivisions were used in the custom house." of private history: "from mar. th to nd i was on an expedition to folkestone, dover, dungeness, &c.--from apr. rd to th at playford, and again for short periods in june and july.--from aug. st to sept. th i was travelling in scotland with my wife and otto struve (for part of the time). at edinburgh i attended the meeting of the british association, and spoke a little in section a. i was nominated president for at ipswich. we travelled to cape wrath and returned by inverness and the caledonian canal.--i was at playford for a short time in october and december." "in this year the great shed was built (first erected on the magnetic ground, and about the year transferred to the south ground).--the chronometers were taken from the old chronometer room (a room on the upper story fronting the south, now, , called library ) and were put in the room above the computing room (where they remained for or years, i think): it had a chronometer-oven with gas-heat, erected in .--the following passage is quoted from the report to the visitors:--'as regards meridional astronomy our equipment may now be considered complete. as i have stated above, an improvement might yet be made in our transit circle; nevertheless i do not hesitate to express my belief that no other existing meridional instrument can be compared with it. this presumed excellence has not been obtained without much thought on my part and much anxiety on the part of the constructors of the instrument (messrs ransomes and may, and mr simms). but it would be very unjust to omit the further statement that the expense of the construction has considerably exceeded the original estimate, and that this excess has been most liberally defrayed by the government.'--in december sir john herschel gave his opinion (to the admiralty, i believe) in favour of procuring for the cape observatory a transit circle similar to that at greenwich.--i had much correspondence about sending pierce morton (formerly a pupil of mine at cambridge, a clever gentlemanly man, and a high wrangler, but somewhat flighty) as magnetic assistant to the cape observatory: he was with me from may to october, and arrived at the cape on nov. th.--i was much engaged with the clock with conical motion of pendulum, for uniform movement of the chronographic barrel.--regarding galvanic communications: on sept. th i had prepared a draft of agreement with the south eastern railway company, to which they agreed. in november i wrote to sir t. baring (first lord of the admiralty) and to the admiralty for sanction, which was given on dec. th. in december i had various communications about laying wires through the park, &c., &c., and correspondence about the possibility of using sympathetic clocks: in june, apparently, i had seen shepherd's sympathetic clock at the great exhibition, and had seen the system of sympathetic clocks at pawson's, st paul's churchyard.--in the last quarter of this year i was engaged in a series of calculations of chronological eclipses. on sept. th mr bosanquet wrote to me about the eclipse of thales, and i urged on the computations related to it, through mr breen. in october the eclipse of agathocles (the critical eclipse for the motion of the moon's node) was going on. in october hansteen referred me to the darkness at stiklastad.--i went to sweden to observe the total eclipse of july th, having received assistance from the admiralty for the journeys of myself, mr dunkin, mr humphreys and his friend, and capt. blackwood. i had prepared a map of its track, in which an important error of the _berliner jahrbuch_ (arising from neglect of the earth's oblateness) was corrected. i gave a lecture at the royal institution, in preparation for the eclipse, and drew up suggestions for observations, and i prepared a scheme of observations for greenwich, but the weather was bad. the official account of the observations of the eclipse, with diagrams and conclusions, is given in full in a paper published in the royal astr. society's memoirs.--this year i was president of the british association, at the ipswich meeting: it necessarily produced a great deal of business. i lectured one evening on the coming eclipse. prince albert was present, as guest of sir william middleton: i was engaged to meet him at dinner, but when i found that the dinner day was one of the principal soirée days, i broke off the engagement.--on may th i had the first letter from e. hamilton (whom i had known at cambridge) regarding the selection of professors for the university of sydney. herschel, maldon, and h. denison were named as my coadjutors. plenty of work was done, but it was not finished till .--in connection with the clock for westminster palace, in february there were considerations about providing other clocks for the various buildings; and this probably was one reason for my examining shepherd's clocks at the great exhibition and at pawson's. in november i first proposed that mr e.b. denison should be associated with me. about the end of the year, the plan of the tower was supplied to me, with reference to the suspension of the weights and other particulars.--in admiral dundas (m.p. for greenwich and one of the board of admiralty) had requested me to aid the trustees of the dee navigation against an attack; and on mar. th i went to chester to see the state of the river. on jan. st i went to give evidence at the official enquiry.--at a discussion on the construction of the great exhibition building in the institution of civil engineers, i expressed myself strongly on the faulty principles of its construction.--in this year i wrote my first paper on the landing of julius caesar in britain, and was engaged in investigations of the geography, tides, sands, &c., relating to the subject." of private history: "i was several times at playford during january, and went there again on dec. rd.--in this year a very heavy misfortune fell on us. my daughter, elizabeth, had been on a visit to lady herschel at hawkhurst, and on apr. nd sir j. herschel wrote to me, saying that she was so well in health. she returned a few days later, and from her appearance i was sure that she was suffering under deadly disease. after some time, an able physician was consulted, who at once pronounced it to be pulmonary. a sea voyage was thought desirable, and my wife took her to shetland, where there was again a kind welcome from mr edmonston. but this, and the care taken on her return, availed nothing: and it was determined to take her to madeira. my wife and daughter sailed in the brig 'eclipse' from southampton on dec. th. the termination came in .--on nov. rd i went to bradfield, near bury: my uncle, george biddell, died, and i attended the funeral on nov. th.--from july th to aug. th i was in sweden for the observation of the eclipse, and returned through holland.--in october i was about a week at ventnor and torquay, and from dec. th to th at southampton, on matters connected with my daughter's illness." the following extracts are from letters to his wife, relating to the observation of the eclipse, his interview with the king of sweden, &c., and his visit to the pumping engines at haarlem: _july , half-past , morning_. the weather is at present most perfectly doubtful. nearly the whole sky is closely covered, yet there is now and then a momentary gleam of sun. the chances are greatly against much of the eclipse being seen. all is arranged to carry off the telescope, &c., at : they can be carted to the foot of the hill, and we have made out a walking-pass then to the top. we are to dine with mr dickson afterwards. _july , at night_. well we have had a glorious day. as soon as we started, the weather began to look better. we went up the hill and planted my telescope, and the sky shewed a large proportion of blue. at first i placed the telescope on the highest rock, but the wind blew almost a gale, and shook it slightly: so i descended about feet to one side. (the power of doing this was one of the elements in my choice of this station, which made me prefer it to the high hill beyond the river.) the view of scenery was inexpressibly beautiful. the beginning of eclipse was well seen. the sky gradually thickened from that time, so that the sun was in whitish cloud at the totality, and barely visible in dense cloud at the end of the eclipse. the progress of the eclipse brought on the wonderful changes that you know: just before the totality i saw a large piece of blue sky become pitch black; the horror of totality was very great; and then flashed into existence (i do not know how) a broad irregular corona with red flames _instantly seen_ of the most fantastic kind. the darkness was such that my assistant had very great trouble in reading his box chronometer. (a free-hand explanatory diagram is here given.) some important points are made out from this. st the red flames certainly belong to the sun. nd they certainly are in some instances detached. rd they are sometimes quite crooked. th they seem to be connected with spots. the corona was brilliant white. one star brilliant: i believe venus. i had no time to make observations of polarization, &c., although prepared. when the totality was more than half over i looked to n. and n.w., and in these regions there was the fullest rosy day-break light. after the sun-light reappeared, the black shadow went travelling away to the s.e. exactly like the thunder-storm from the main. the day then grew worse, and we came home here (after dinner) in pouring rain. stockholm, _ , aug. _. i then by appointment with sir edmund lyons went with him to the minister for foreign affairs, baron stjerneld, who received me most civilly. my business was to thank him for the orders which had been given to facilitate the landing of our telescopes, &c., &c. he was quite familiar with the names of my party, humphreys milaud, &c., so that i trust they have been well received (i have had no letter). he intimated, i suppose at sir e. lyons's suggestion, that perhaps king oscar might wish to see me, but that it would not be on tuesday. so i replied that i was infinitely flattered and he said that he would send a message to sir e. lyons by tuesday evening. now all this put me in a quandary: because i wanted to see upsala, miles off: and the steamboats on the mälar only go in the morning and return in the morning: and this was irreconcileable with waiting for his majesty's appointment which might be for wednesday morning. so after consultation sir e. lyons put me in the hands of a sort of courier attached to the embassy, and he procured a calèche, and i posted to upsala yesterday afternoon (knocking the people up at at night) and posted back this afternoon. and sure enough a message has come that the king expects me at to-morrow morning. posting of course is much dearer than steam-boat travelling, but it is cheap in comparison with england: two horses cost s. for nearly miles. at upsala there is a very good old cathedral, i suppose the only one in sweden: and many things about the university which interested me. i sent my card to professor fries, and he entirely devoted himself to me: but imagine our conversation--he spoke in _latin_ and i in french: however we understood each other very well. it is on the whole a dreary country except where enlivened by lakes: some parts are pine forests and birch forests, but others are featureless ground with boulder stones, like the worst part of the highlands. _august , wednesday, o'clock_. i rigged myself in black trowsers and white waistcoat and neckcloth this morning. sir edmund lyons called. baron wrede called on me: he had observed the eclipse at calmar and brought his drawing, much like mine. he conducted me to the palace. the minister for foreign affairs came to me. in the waiting-room i was introduced to the lieutenant-governor of christianstad, who had had the charge of humphreys and milaud. he had placed a _guard of soldiers_ round them while they were observing. they saw the eclipse well. captain blackwood went to helsingborg instead of bornholm, and saw well. i am sorry to hear that it was cloudy at christiania, mr dunkin's station. i heard some days ago that hind had lost his telescope, but i now heard a very different story: that he landed at ystad, and found a very bad hotel there: that he learnt from murray that the hotels at carlscrona (or wherever he meant to go) were much worse; and so he grew faint at heart and turned back. i was summoned in to the king and presented by the minister (stjerneld), and had a long conversation with him: on the eclipse, the arc of meridian, the languages, and the universities. we spoke in french. then baron wrede went with me to the rittershus (house of lords or nobles) in session, and to the gallery of scandinavian antiquities, which is very remarkable: the collection of stone axes and chisels, bronze do., iron do., ornaments, &c. is quite amazing. i was struck with seeing specimens from a very distant age of the maid of norway's brooch: the use of which i explained to the director. i dined and drove out with sir e. lyons, and called at the houses of the baron stjerneld and of the norwegian minister baron duë, and had tea at the latter. most of these people speak english well, and they seem to live in a very domestic family style. i should soon be quite at home here: for i perceive that my reception at court, &c., make people think that i am a very proper sort of person. * * * * * the extract concerning his visit to the pumping-engines at haarlem is as follows: leyden, _ , august , wednesday_. i went to see the great north holland canal, and went a mile or two in a horse-drawn-boat upon it: a very comfortable conveyance. saw windmills used for sawing timber and other purposes, as well as some for grinding and many for draining. yesterday at half-past one i went by railway to haarlem. i did not look at anything in the town except going through it and seeing that it is a curious fantastic place, but i drove at once to the burgomaster to ask permission to visit one of the three great pumping engines for draining the immense haarlem lake, and then drove to it. imagine a round tower with a steam-cylinder in its center; and the piston which works up-and-down, instead of working one great beam as they usually do, works _eight_, poking out on different sides of the round tower, and each driving a pump feet in diameter. i am glad to have seen it. then by railway here. * * * * * "galvanic communication was now established with lewisham station (thus giving power of communicating with london, deal, &c.).--from the report to the board of visitors it appears that, in the case of the transit circle, the azimuth of the instrument as determined by opposite passages of the pole star had varied four seconds; and in the case of the altazimuth, there was a discordance in the azimuthal zeros of the instrument, as determined from observations of stars. in both cases it was concluded that the discordances arose from small movements of the ground.--under the head of 'general remarks' in the report, the following paragraph occurs: 'it will be perceived that the number of equatoreal observations made here at present is small: and that they are rarely directed to new comets and similar objects which sometimes excite considerable interest. this omission is intentional. it is not because the instrumental means are wanting (for our equatoreals, though not comparable to those of either cambridge, or of pulkowa, are fully equal to those usually directed to such objects), but it is because these observations are most abundantly supplied from other observatories, public and private, and because the gain to those observations from our taking a part in them would, probably, be far less than the loss to the important class of observations which we can otherwise follow so well. moreover, i am unwilling to take any step which could be interpreted as attempting to deprive the local and private observatories of honours which they have so nobly earned. and, finally, in this act of abstinence, i am desirous of giving an example of adhesion to one principle which, i am confident, might be extensively followed with great advantage to astronomy:--the principle of division of labour.'--discoveries of small planets were now not infrequent: but the only one of interest to me is melpomene, for the following reason. on june i lost my most dear, amiable, clever daughter elizabeth: she died at southampton, two days after landing from madeira. on that evening mr hind discovered the planet; and he requested me to give a name. i remembered horace's 'praecipe lugubres cantus, melpomene,' and cowley's 'i called the buskin'd muse melpomene and told her what sad story i would write,' and suggested melpomene, or penthos: melpomene was adopted.--the first move about the deal time ball was in a letter from commander baldock to the admiralty, suggesting that a time ball, dropped by galvanic current from greenwich, should be attached to one of the south foreland lighthouses. the admiralty sent this for my report. i went to the place, and i suggested in reply (nov. th) that a better place would be at an old signal station on the chalk downs. the decisive change from this was made in .--as the result of my examination and enquiries into the subject of sympathetic clocks, i established sympathetic clocks in the royal observatory, one of which outside the entrance gate had a large dial with shepherd's name as patentee. exception was taken to this by the solicitor of a mr bain who had busied himself about galvanic clocks. after much correspondence i agreed to remove shepherd's name till bain had legally established his claim. this however was never done: and in shepherd's name was restored.--in nov. , denison had consented to join me in the preparation of the westminster clock. in feb. we began to have little disagreements. however on apr. th i was going to madeira, and requested him to act with full powers from me.--i communicated to the royal society my paper on the eclipses of agathocles, thales, and xerxes.--in the british association, i had presided at the ipswich meeting in , and according to custom i ought to attend at the meeting (held at belfast) to resign my office. but i was broken in spirit by the death of my daughter, and the thing generally was beyond my willing enterprise. i requested sir roderick murchison to act generally for me: which he did, as i understood, very gracefully.--in this year a proposal was made by the government for shifting all the meeting rooms of the scientific societies to kensington gore, which was stoutly resisted by all, and was finally abandoned." of private history: "i was at playford in january, and went thence to chester on the enquiry about the tides of the dee; and made excursions to halton castle and to holyhead.--from apr. th to may th i was on the voyage to and from madeira, and on a short visit to my wife and daughter there.--on june rd i went to southampton to meet my wife and daughter just landed from madeira: on june th my dear daughter elizabeth died: she was buried at playford on june th.--i was at playford also in july and december.--from sept. th to th i went to cumberland, viâ fleetwood and peel." "on may rd i issued an address to the individual members of the board of visitors, proposing the extension of the lunar reductions from . from this it appears that 'through the whole period (from to ), the places of the moon, deduced from the observations, are compared with the places computed in the nautical almanac: that is, with burckhardt's tables, which have been used for many years in computing the places of the nautical almanac.......very lately, however, mr adams has shewn that burckhardt's parallax is erroneous in formula and is numerically incorrect, sometimes to the amount of seven seconds. in consequence of this, every reduction of the observations of the moon, from to the present time, is sensibly erroneous. and the error is of such a nature that it is not easy, in general, to introduce its correction by any simple process.... the number of observations to the end of (after which time the parallax will be corrected in the current reductions) is about . an expense approaching to _£ _ might be incurred in their reduction.' subsequently i made application to the admiralty, and the _£ _ was granted on dec. th.--in the report to the visitors it is stated that with regard to the transit circle, changes are under contemplation in its reflection-apparatus: one of these changes relates to the material of the trough. 'several years ago, when i was at hamburgh, my revered friend prof. schumacher exhibited to me the pacifying effect of a copper dish whose surface had been previously amalgamated with quicksilver.......the rev. charles pritchard has lately given much attention to this curious property of the metals, and has brought the practical operation of amalgamation to great perfection. still it is not without difficulty, on account of a singular crystallization of the amalgam.'--with regard to the chronograph, the report states: 'the barrel apparatus for the american method of observing transits is not yet brought into use.... i have, however, brought it to such a state that i am beginning to try whether the barrel moves with sufficient uniformity to be itself used as the transit clock. this, if perfectly secured, would be a very great convenience, but i am not very sanguine on that point.'--a change had been made in the electrometer-apparatus: 'a wire for the collection of atmospheric electricity is now stretched from a chimney on the north-west angle of the leads of the octagon room to the electrometer pole.... there appears to be no doubt that a greater amount of electricity is collected by this apparatus than by that formerly in use.'--as regards the magnetical observations: 'the visitors at their last meeting, expressed a wish that some attempt should be made to proceed further in the reduction or digest of the magnetical results, if any satisfactory plan could be devised. i cannot say that i have yet satisfied myself on the propriety of any special plan that i have examined.... i must, however, confess that, in viewing the capricious forms of the photographic curves, my mind is entirely bewildered, and i sometimes doubt the possibility of extracting from them anything whatever which can be considered trustworthy.'--great progress had been made with the distribution of time. 'the same normal clock maintains in sympathetic movement the large clock at the entrance gate, two other clocks in the observatory, and a clock at the london bridge terminus of the south-eastern railway.... it sends galvanic signals every day along all the principal railways diverging from london. it drops the greenwich ball, and the ball on the offices of the electric telegraph company in the strand;... all these various effects are produced without sensible error of time; and i cannot but feel a satisfaction in thinking that the royal observatory is thus quietly contributing to the punctuality of business through a large portion of this busy country. i have the satisfaction of stating to the visitors that the lords commissioners of the admiralty have decided on the erection of a time-signal ball at deal, for the use of the shipping in the downs, to be dropped every day by a galvanic current from the royal observatory. the construction of the apparatus is entrusted to me. probably there is no roadstead in the world in which the knowledge of true time is so important.'--the report includes an account of the determination of the longitude of cambridge observatory by means of galvanic signals, which appear to have been perfectly successful.--under the head of general remarks the following passage appears: 'the system of combining the labour of unattached computers with that of attached assistants tends materially to strengthen our powers in everything relating to computation. we find also, among the young persons who are engaged merely to serve as computers, a most laudable ambition to distinguish themselves as observers; and thus we are always prepared to undertake any observations which may be required, although necessarily by an expenditure of strength which would usually be employed on some other work.'--considerable work was undertaken in preparing a new set of maps of our buildings and grounds.--on apr. rd there was a small fire in the magnetic observatory, which did little mischief.--in december i wrote my description of the transit circle.--lieut. stratford, the editor of the nautical almanac, died, and there was some competition for the office. i was willing to take it at a low rate, for the addition to my salary: mr main--and i think mr glaisher--were desirous of exchanging to it: prof. adams was anxious for it. the admiralty made the excellent choice of mr hind.--in october faraday and i, at lothbury, witnessed some remarkable experiments by mr latimer clark on a galvanic current carried four times to and from manchester by subterranean wires (more than miles) shewing the retardation of visible currents (at their maximum effect) and the concentration of active power. i made investigations of the velocity of the galvanic current.--i was engaged on the preliminary enquiries and arrangements for the deal time ball.--with respect to the westminster clock; an angry paper was issued by mr vulliamy. in october i expostulated with denison about his conduct towards sir charles barry: on november th i resigned.--on feb. th i was elected president of the royal astronomical society.--in the royal institution i lectured on the ancient eclipses.--on dec. th i was elected to the academy of brussels.--after preliminary correspondence with sir w. molesworth (first commissioner of works, &c.) and sir charles barry (architect of the westminster palace), i wrote, on may th, to mr gladstone about depositing the four parliamentary copies of standards, at the royal observatory, the royal mint, the royal society, and within a wall of westminster palace. mr gladstone assented on june rd.--on mar. th i wrote to mr gladstone, proposing to take advantage of the new copper coinage for introducing the decimal system. i was always strenuous about preserving the pound sterling. on may th i attended the committee of the house of commons on decimal coinage: and in may and september i wrote letters to the athenaeum on decimal coinage.--i had always something on hand about tides. a special subject now was, the cry about intercepting the tidal waters of the tyne by the formation of the jarrow docks, in jarrow slake; which fear i considered to be ridiculous." of private history: "from jan. th to th i was at playford.--on mar. th i went to dover to try time-signals.--from june th to aug. th i was at little braithwaite near keswick, where i had hired a house, and made expeditions with members of my family in all directions. on july th i went, with my son wilfrid, by workington and maryport to rose castle, the residence of bishop percy (the bishop of carlisle), and on to carlisle and newcastle, looking at various works, mines, &c.--on dec. th i went to playford." the chronograph barrel-apparatus for the american method of transits had been practically brought into use: "i have only to add that this apparatus is now generally efficient. it is troublesome in use; consuming much time in the galvanic preparations, the preparation of the paper, and the translation of the puncture-indications into figures. but among the observers who use it there is but one opinion on its astronomical merits--that, in freedom from personal equation and in general accuracy, it is very far superior to the observations by eye and ear."--the printing and publication of the observations, which was always regarded by airy as a matter of the first importance, had fallen into arrear: "i stated in my last report that the printing of the observations for was scarcely commenced at the time of the last meeting of the visitors. for a long time the printing went on so slowly that i almost despaired of ever again seeing the observations in a creditable state. after a most harassing correspondence, the printers were at length persuaded to move more actively, ... but the volume is still very much behind its usual time of publication."--"the deal time-ball has now been erected by messrs maudslays and field, and is an admirable specimen of the workmanship of those celebrated engineers. the galvanic connection with the royal observatory (through the telegraph wires of the south eastern railway) is perfect. the automatic changes of wire-communications are so arranged that, when the ball at deal has dropped to its lowest point, it sends a message to greenwich to acquaint me, not with the time of the beginning of its fall (which cannot be in error) but with the fact that it has really fallen. the ball has several times been dropped experimentally with perfect success; and some small official and subsidiary arrangements alone are wanting for bringing it into constant use."--the operations for the galvanic determination of the longitude of brussels are described, with the following conclusion: "thus, about effective signals were made, but only of these were admissible for the fundamental objects of the operation. the result, i need scarcely remark, claims a degree of accuracy to which no preceding determination of longitude could ever pretend. i apprehend that the probable error in the difference of time corresponds to not more than one or two yards upon the earth's surface.--a careful scheme had been arranged for the determination of the longitude of lerwick, but 'unfortunately, the demand for chronometers caused by our large naval armament has been so considerable that i cannot reckon on having at my disposal a sufficient number to carry on this operation successfully; and i have, therefore, unwillingly deferred it to a more peaceful time.'--the covering stone of halley's tomb in lee churchyard was much shattered, and i applied to the admiralty for funds for its complete restoration: these were granted on feb. rd.--in this year, under my cognizance, _£ _ was added to the hansen grant.--i had much correspondence and work in connection with the printing of maclear's work at the cape of good hope. in june, all accounts, &c. about the transit circle were closed at the admiralty, and the instrument was completely mounted at the cape.--dr scoresby (who in his own way was very imperious) had attacked my methods of correcting the compass in iron ships: i replied in a letter to the athenaeum on oct. th.--i made enquiries about operations for determining the longitude of vienna, but was utterly repelled by the foreign telegraph offices.--in the royal astronomical society; i prepared the address on presenting the medal to rümker.--in melbourne university: the first letter received was from the chancellor of the university dated jan. th, requesting that sir john herschel, prof. malden, mr lowe (subsequently chancellor of the exchequer), and i would select professors. we had a great deal of correspondence, meetings, examination of testimonials, &c., and on august th we agreed on wilson, rowe, mccoy, and hearn.--on feb. th i received the prussian order of merit.--i had correspondence with the treasury on the scale to be adopted for the maps of the british survey. i proposed / , and for some purposes / .--i printed a paper on the deluge, in which i shewed (i believe to certainty) that the deluge of genesis was merely a destructive flood of the nile.--being well acquainted with the mountains of cumberland, i had remarked that a 'man' or cairn of stones erected by the ordnance surveyors on the great gable had covered up a curious natural stone trough, known as one of the remarkable singularities of the country. this year, without giving any notice to the ordnance surveyors, i sent two wallers from borrowdale to the mountain top, to remove the 'man' about feet and expose the trough. sir henry james afterwards approved of my act, and refunded the expense.--i investigated the optical condition of an eye with conical cornea. "the harton colliery experiment: i had long wished to repeat the experiment which i had attempted unsuccessfully in and , of determining by pendulum-vibrations the measure of gravity at the bottom of a mine. residing near keswick this summer, and having the matter in my mind, i availed myself of an introduction from dr leitch to some gentlemen at south shields, for inspection of the harton colliery. i judged that it would answer pretty well. i find that on aug. th i wrote to mr anderson (lessee of the mine), and on the same day to the admiralty requesting authority to employ a greenwich assistant, and requesting _£ _ for part payment of expenses. on august th the admiralty assent. there were many preparations to be made, both personal and instrumental. my party consisted of dunkin (superintendant), ellis, criswick, simmons, pogson, and rümker: i did not myself attend the detail of observations. the observations began on oct. nd and ended on oct. st: supplementary observations were subsequently made at greenwich for examining the coefficient of temperature-correction. on oct. th i gave a lecture at south shields on the whole operation. in 'punch' of nov. th there was an excellent semi-comic account of the experiment, which as i afterwards found was written by mr percival leigh." of private history: "on jan. th i returned from playford. from mar. th to th i was at deal, and visited sir john herschel at hawkhurst.--from june th to aug. th i was staying with my family at the grange, in borrowdale near keswick: and also made an expedition to penrith, carlisle, newcastle, jarrow, &c.; and descended the harton pit.--in september and also in october i was at south shields on the harton experiments.--from dec. th to th i was at cambridge, and on the th i went to playford." the following letter, written in answer to a lady who had asked him to procure permission from lord rosse for her to observe with his telescope, is characteristic: royal observatory, greenwich. _ , september _. dear madam, the state of things with regard to lord rosse's telescope is this. if a night is fine, it is wanted for his use or for the use of professional astronomers. if it is not fine, it is of no use to anybody. now considering this, and considering that the appropriation of the telescope on a fine night to any body but a technical astronomer is a misapplication of an enormous capital of money and intellect which is invested in this unique instrument--it is against my conscience to ask lord rosse to place it at the service of any person except an experienced astronomer. no introduction, i believe, is necessary for seeing it in the day-time. the instrument stands unenclosed in the castle demesne, to which strangers are admitted without question, i believe............... faithfully yours, g.b. airy. "on may th it was notified to me (i think through the hydrographer) that the admiralty were not unwilling to increase my salary. i made application therefore; and on jan. st sir charles wood notified to me that the admiralty consented to have it raised from _£ _ to _£ _.--in the report to the board of visitors it appears that 'at the instance of the board of trade, acting on this occasion through a committee of the royal society, a model of the transit circle (with the improvement of perforated cube, &c. introduced in the cape transit circle) has been prepared for the great exhibition at paris.'--under the head of reduction of astronomical observations it is stated that 'during the whole time of which i have spoken, the galvanic-contact method has been employed for transits, with the exception of a few days, when the galvanic apparatus was out of order. from the clock errors, i have deduced the personal equations of the observers in our usual way.... the result is that the magnitude of the personal equations in the galvanic-touch method is not above half of that in the eye and ear method.'--with regard to the reduction of the magnetical observations, 'i have not yet felt sufficiently satisfied with any proposed method of discussing the magnetic results to devote any time to their further treatment.'--'the time-signal ball at deal was brought into regular use at the beginning of the present year. in a short time, however, its action was interrupted, partly by derangement of the apparatus, and partly by the severity of the weather, which froze the sulphuric acid to the state of jelly. i sent an assistant and workman to put it in order, and since that time it has generally acted very well.--application has been made to me from one of the important offices of government (the post office) for the galvanic regulation of their clocks.--on considering the risks to which various galvanic communications are liable, and the financial necessity for occupying wires as little as possible, i perceived that it was necessary to devise constructions which should satisfy the following conditions. first, that a current sent once a day should suffice for adjusting the clock, even if it had gone ten or more seconds wrong. secondly, that an occasional failure of the current should not stop the clock. i have arranged constructions which possess these characters, and the artist (mr c. shepherd) is now engaged in preparing estimates of the expense. i think it likely that this may prove to be the beginning of a very extensive system of clock regulation."--with respect to the operations for determining the longitude of paris, it is stated that, "the whole number of days of signal transmission was eighteen, and the whole number of signals transmitted was . the number of days considered available for longitude, in consequence of transits of stars having been observed at both observatories, was twelve, and the number of signals was . very great care was taken on both sides, for the adjustments of the instruments. the resulting difference of longitude, m. . s., is probably very accurate. it is less by nearly s. of time than that determined in by rocket-signals, under the superintendance of sir john herschel and col. sabine. the time occupied by the passage of the galvanic current appears to be / th of a second."--with regard to the pendulum experiments in the harton colliery, after mentioning that personal assistance had been sought and obtained from the observatories of cambridge, oxford, durham, and red hill, the report states that "the experiments appear to have been in every point successful, shewing beyond doubt that gravity is increased at the depth of feet by / th part. i trust that this combination may prove a valuable precedent for future associations of the different observatories of the kingdom, when objects requiring extensive personal organization shall present themselves."--on oct. th the astronomer royal printed an address to the individual members of the board of visitors on the subject of a large new equatoreal for the observatory. after a brief statement of the existing equipment of the observatory in respect of equatoreal instruments, the address continues thus: "it is known to the visitors that i have uniformly objected to any luxury of extrameridional apparatus, which would materially divert us from a steady adherence to the meridional system which both reason and tradition have engrafted on this observatory. but i feel that our present instruments are insufficient even for my wishes; and i cannot overlook the consideration that due provision must be made for future interests, and that we are nearer by twenty years to the time when another judgment must decide on the direction which shall be given to the force of the observatory."--"in august i had some correspondence about the egyptian wooden astronomical tablets with mr gresswell and others: they were fully examined by mr ellis.--in this year i was much engaged on schemes for compasses, and in june i sent my paper on discussions of ships' magnetism to the royal society.--on dec. th the mast of the observatory time-ball broke, and the ball fell in the front court.--on aug. th my valued friend mr sheepshanks died; and on aug. th i went to london to see the standard bars as left by him. afterwards, on oct. th i went to reading to collect the papers about standards left by mr sheepshanks.--i made a mechanical construction for euclid i. , with which i was well satisfied.--on apr. th i joined a deputation to the chancellor of the exchequer (sir g. cornewall lewis) on decimal coinage." of private history: "i was at playford for a large part of january.--on mar, th i went to reading, to visit mr sheepshanks, and afterwards to silchester and hereford.--on june st i went with my wife and two eldest sons to edinburgh and other places in scotland, but residing principally at oban, where i hired a house. amongst other expeditions, i and my son wilfrid went with the 'pharos' (northern lights steamer) to the skerry vohr lighthouse, &c. i also visited newcastle, &c., and returned to greenwich on aug. nd.--from oct. th to th i was at cambridge.--on dec. th i went to playford." chapter vii. at greenwich observatory-- to . "in the report to the visitors there is an interesting account of the difficulties experienced with the reflex zenith tube in consequence of the tremors of the quicksilver transmitted through the ground. attempts were made to reduce the tremor by supporting the quicksilver trough on a stage founded at a depth of feet below the surface, but it was not in the smallest degree diminished, and the report states that 'the experience of this investigation justifies me in believing that no practicable depth of trench prevents the propagation of tremor when the soil is like that of greenwich hill, a gravel, in all places very hard, and in some, cemented to the consistency of rock.'--with respect to the regulation of the post office clocks, 'one of the galvanic clocks in the post office department, lombard street, is already placed in connection with the royal observatory, and is regulated at noon every day ... other clocks at the general post office are nearly prepared for the same regulation, and i expect that the complete system will soon be in action.'--under the head of general remarks a careful summary is given of the work of the observatory, and the paragraph concludes as follows: 'lastly there are employments which connect the scientific observatory with the practical world; the distribution of accurate time, the improvement of marine time-keepers, the observations and communications which tend to the advantage of geography and navigation, and the study, in a practical sense, of the modifications of magnetism; a careful attention to these is likely to prove useful to the world, and conducive to the material prosperity of the observatory: and these ought not to be banished from our system.'--in september i prepared the first specification for the building to carry the s.e. dome.--in september, learning that hansen's lunar tables were finished in manuscript, i applied to lord clarendon and they were conveyed to me through the foreign office: in october i submitted to the admiralty the proposal for printing the tables, and in november i learned that the treasury had assented to the expense.--lieut. daynou's eclipses and occultations for longitudes of points in south africa, observed in and , were calculated here in this year.--on feb. th i made my first application to sir c. wood (first lord of the admiralty) for assistance to c. piazzi smyth to carry out the teneriffe experiment: grounding it in part on the failure of attempts to see the solar prominences. he gave encouragement, and on mar. th i transmitted piazzi smyth's memorial to the admiralty: on may nd the admiralty authorized an expense of _£ _. i drew up suggestions.--the sheepshanks fund: after the death of my friend richard sheepshanks, his sister miss anne sheepshanks wished to bestow some funds in connection with the university of cambridge, trinity college, and astronomy, to which his name should be attached. there must have been some conversation with me, but the first letter is one from de morgan in august. in september i had a conversation with miss sheepshanks, and sent her my first draft of a scheme, to which she assented. on sept. th i wrote to whewell (master of trinity) who was much trusted by miss sheepshanks: he consented to take part, and made some suggestions. there was further correspondence, but the business did not get into shape in this year.--in connection with the correction of the compass in iron ships: i discussed the observations made in the voyage of the royal charter. on feb. th i proposed to the admiralty a system of mounting the compasses with adjustable magnets, and it was ordered to be tried in the trident and transit.--in february i reported to the admiralty that the deal time-ball had been successful, and i proposed time-balls at portsmouth, plymouth, and sheerness. there was much correspondence in various directions about portsmouth and devonport, and in march i went to devonport and specially examined mount wise and the devonport column.--i had correspondence with sir howard douglas about the sea breaking over the unfinished dover pier. i have an idea that this followed evidence given by me to a harbour commission, in which i expressed as a certainty that the sea will not be made to break by a vertical wall." of private history: "i returned from playford on jan. th.--from june th to august th i was, with my son wilfrid, on an expedition to south italy and sicily: on our return from sicily, we remained for three days ill at marseilles from a touch of malaria.--on dec. nd i went to playford.--in acknowledgment of the pleasure which i had derived from excursions in the cumberland passes, i made a foot-bridge over a troublesome stream on the pass of the sty head." "in the report to the visitors, when on the subject of the altazimuth, the following paragraph occurs: 'i alluded in a preceding section to the cutting away of a very small portion of one of the rays of the three-armed pier which carries the altazimuth. the quality of the brickwork is the best that i have ever seen, and not a single brick was disturbed beyond those actually removed. yet the effect was to give the altazimuth an inclination of about ". this inclination evidently depends on the elasticity of the brickwork.'--with reference to the new s.e. equatoreal the report states that 'the support of the north or upper end of the polar axis has been received, and is planted within the walls of the building in a position convenient for raising it to its ultimate destination. it is one piece of cast-iron, and weighs nearly tons.'--small changes as previously mentioned had been noticed with regard to the zero of azimuth of the transit circle, and the report states that 'in regard to the azimuth of the transit circle, and the azimuth of its collimator, mr main has brought together the results of several years, and the following law appears to hold. there is a well-marked annual periodical change in the position of the transit circle, the southerly movement of the eastern pivot having its minimum value in september, and its maximum in march, the extreme range being about seconds; and there is a similar change, but of smaller amount, in the position of the collimator. i cannot conjecture any cause for these changes, except in the motion of the ground. there is also a well-marked connection between the state of level of the axis and the temperature. the eastern pivot always rises when the temperature rises, the extreme range being about seconds. i cannot offer any explanation of this.'--under the head of extraneous works the report states that 'the british government had for some years past contributed by pecuniary grants to the preparation of prof. hansen's lunar tables. in the last winter they undertook the entire expense of printing a large impression of the tables. the reading of the proof-sheets (a very considerable labour) has been effected entirely at the observatory. i may take this opportunity of stating that the use of these tables has enabled me, as i think, incontestably to fix the capture of larissa to the date b.c. , may . this identification promises to prove valuable, not merely for its chronological utility, but also for its accurate determination of an astronomical epoch, the point eclipsed being exactly known, and the shadow having been very small.'--in april i gave a lecture to the royal astronomical society on the methods available through the next years for the determination of the sun's parallax.--dr livingstone's observations for african longitudes were computed at the observatory.--the admiralty enquire of me about the feasibility of adopting piazzi smyth's construction for steadying telescopes on board ship: i gave a report, of mixed character, on the whole discouraging.--i had correspondence with g.p. bond and others about photographing the stars and moon.--on feb. th piazzi smyth's books, &c. relating to the teneriffe experiment were sent to me: i recommended that an abridged report should be sent to the royal society.--respecting the sheepshanks fund: there was correspondence with miss sheepshanks and whewell, but nothing got into shape this year: miss sheepshanks transferred to me _£ , _ lying at overend and gurney's.--in november experiments were made for the longitude of edinburgh, which failed totally from the bad state of the telegraph wire between deptford and the admiralty.--in june the first suggestion was made to me by capt. washington for time-signals on the lizard point: which in no long time i changed for the start point.--the admiralty call for estimates for a time-ball at portsmouth: on receiving them they decline further proceeding.--i was engaged in speculations and correspondence about the atlantic submarine cable.--in the royal astronomical society, i presented memoirs and gave lectures on the three great chronological eclipses (agathocles, thales, larissa)."--on dec. th airy wrote to the vice-chancellor of the university of cambridge, objecting to the proposed changes regarding the smith's prizes--a subject in which he took much interest, and to which he ascribed great importance.--on apr. th i was in correspondence with g. herbert of the trinity house, about floating beacons.--in july i reported to the treasury on the swedish calculating engine (i think on the occasion of mr farr, of the registrar-general's office, applying for one).--in november i had correspondence about the launch of the great eastern, and the main drainage of london." of private history: "on jan. th i returned from playford.--from june th to aug. th i was travelling in scotland with my wife and two eldest sons, chiefly in the west highlands. on our return we visited mrs smith (my wife's mother) at brampton.--on dec. th i went to playford." "in the minutes of the visitors it is noted that the new queen's warrant was received. the principal change was the exclusion of the astronomer royal and the other observatory officers from the board.--in the report to the visitors it is stated that 'the papers of the board of longitude are now finally stitched into books. they will probably form one of the most curious collections of the results of scientific enterprise, both normal and abnormal, which exists.'--it appears that the galvanic communications, external to the observatory, had been in a bad state, the four wires to london bridge having probably been injured by a thunderstorm in the last autumn, and the report states that 'the state of the wires has not enabled us to drop the ball at deal. the feeble current which arrives there has been used for some months merely as giving a signal, by which an attendant is guided in dropping the ball by hand.'--regarding the new equatoreal the report states that 'for the new south-east equatoreal, the object-glass was furnished by messrs merz and son in the summer of last year, and i made various trials of it in a temporary tube carried by the temporary mounting which i had provided, and finally i was well satisfied with it. i cannot yet say that i have certainly divided the small star of gamma andromedae; but, for such a test, a combination of favourable circumstances is required. from what i have seen, i have no doubt of its proving a first-rate object-glass.'--on march th was an annular eclipse of the sun, for the observation of which i sent parties fully equipped to bedford, wellingborough, and market harborough. the observations failed totally in consequence of the bad weather: i myself went to harrowden near wellingborough.--respecting the altazimuth, the report states that with due caution as to the zero of azimuth 'the results of observation are extremely good, very nearly equal to those of the meridional instrument; perhaps i might say that three observations with the altazimuth are equivalent to two with the transit circle.'--respecting meteorological observations the report states that 'the observations of the maximum and minimum thermometers in the thames, interrupted at the date of the last report, have been resumed, and are most regularly maintained. regarding the thames as the grand climatic agent on london and its neighbourhood, i should much regret the suppression of these observations.'--after much trouble the longitude of edinburgh had been determined: 'the retard of the current is . s very nearly, and the difference of longitudes m . s, subject to personal equations.'--the report concludes thus: 'with regard to the direction of our labours, i trust that i shall always be supported by the visitors in my desire to maintain the fundamental and meridional system of the observatory absolutely intact. this, however, does not impede the extension of our system in any way whatever, provided that such means are arranged for carrying out the extension as will render unnecessary the withdrawal of strength from what are now the engrossing objects of the observatory.'--i had much correspondence on comets, of which donati's great comet was one: the tail of this comet passed over arcturus on october th.--respecting the sheepshanks fund: in september i met whewell at leeds, and we settled orally the final plan of the scheme. on oct. th i saw messrs sharp, miss sheepshanks's solicitors, and drew up a draft of the deed of gift. there was much correspondence, and on nov. th i wrote to the vice-chancellor of cambridge university. a counter-scheme was proposed by dr philpott, master of st catharine's college. by arrangement i attended the council of the university on dec. rd, and explained my views, to which the council assented. on dec. th the senate accepted the gift of miss sheepshanks.--i had much correspondence throughout this year, with the treasury, herschel, sabine, and the royal society, about the continuation of the magnetic establishments. the reductions of the magnetic observations - were commenced in february of this year, under the direction of mr lucas, a computer who had been engaged on the lunar reductions.--in this year i came to a final agreement with the south eastern railway company about defining the terms of our connection with them for the passage of time signals. i was authorized by the admiralty to sign the 'protocol' or memorandum of agreement, and it was signed by the south eastern railway directors.--on aug. th i made my first proposal to sir john packington (first lord of the admiralty) for hourly time signals on the start point, and in september i went to the start to examine localities, &c. on dec. rd the admiralty declined to sanction it.--i presented to the royal society a paper about drawing a great-circle trace on a mercator's chart.--in october i gave a lecture on astronomy in the assembly room at bury.--on jan. th i was busied with my mathematical tracts for republication."--in this year airy published in the athenaeum very careful and critical remarks on the commissioners' draft of statutes for trinity college. he was always ready to take action in the interests of his old college. this paper procured him the warmest gratitude from the fellows of the college. of private history: "on jan. rd i returned from playford. from july th to aug. th i was on an expedition in switzerland with my two eldest sons. at paris we visited le verrier, and at geneva we visited gautier, de la rive, and plantamour. we returned by brussels.--on dec. rd i went to playford."--in this year was erected in playford churchyard a granite obelisk in memory of thomas clarkson. it was built by subscription amongst a few friends of clarkson's, and the negociations and arrangements were chiefly carried out by airy, who zealously exerted himself in the work which was intended to honour the memory of his early friend. it gave him much trouble during the years to . here is a letter to the editor of the athenaeum on some other trinity matters: _ , november _. dear sir, in the athenaeum of november , page , column , paragraph , there is an account of the erection of the statue of barrow in trinity college antechapel (cambridge) conceived in a spirit hostile to the university, and written in great ignorance of the facts. on the latter i can give the writer some information. the marquis of lansdowne, who was a trinity man and whose son was of trinity, intimated to the authorities of the college that he was desirous of placing in the antechapel a statue of _milton_. this, regard being had to the customs and the college-feelings of cambridge, was totally impossible. the antechapel of every college is sacredly reserved for memorials of the men of that college only; and milton was of christ's college. the marquis of lansdowne, on hearing this objection, left the choice of the person to be commemorated, to certain persons of the college, one of whom (a literary character of the highest eminence and a profound admirer of milton) has not resided in cambridge for many years. several names were carefully considered, and particularly one (not mentioned by your correspondent) of very great literary celebrity, but in whose writings there is ingrained so much of ribaldry and licentiousness that he was at length given up. finally the choice rested on barrow, not as comparable to milton, but as a person of reputation in his day and as the best who could be found under all the circumstances. cromwell never was mentioned; he was a member of sidney college: moreover it would have been very wrong to select the exponent of an extreme political party. but cromwell has i believe many admirers in cambridge, to which list i attach myself. i had no part in the negociations above mentioned, but i saw the original letters, and i answer for the perfect correctness of what i have stated. but as i am not a principal, i decline to appear in public. it is much to be desired, both for the athenaeum and for the public, that such an erroneous statement should not remain uncorrected. and i would suggest that a correction by the editor would be just and graceful, and would tend to support the athenaeum in that high position which it has usually maintained. i am, dear sir, yours very faithfully, g.b. airy. _hepworth dixon, esq._ "the report to the visitors states that 'the lunar reductions with amended elements (especially parallax) for correction of observations from to are now completed. it is, i think, matter of congratulation to the observatory and to astronomy, that there are now exhibited the results of uninterrupted lunar observations extending through more than a century, made at the same place, reduced under the same superintendence and on the same general principles, and compared throughout with the same theoretical tables.'--after reference to the great value of the greenwich lunar observations to prof. hansen in constructing his tables, and to the liberality of the british government in their grants to hansen, the report continues thus: 'a strict comparison of hansen's tables with the greenwich observations of late years, both meridional and extra-meridional, was commenced. the same observations had, in the daily routine of the observatory, been compared with the nautical almanac or burckhardt's tables. the result for one year only ( ) has yet reached me, but it is most remarkable. the sum of squares of residual errors with hansen's tables is only one-eighth part of that with burckhardt's tables. when it is remembered that in this is included the entire effect of errors and irregularities of observation, we shall be justified in considering hansen's tables as nearly perfect. so great a step, to the best of my knowledge, has never been made in numerical physical theory. i have cited this at length, not only as interesting to the visitors from the circumstance that we have on our side contributed to this great advance, but also because an innovation, peculiar to this observatory, has in no small degree aided in giving a decisive character to the comparison. i have never concealed my opinion that the introduction and vigorous use of the altazimuth for observations of the moon is the most important addition to the system of the observatory that has been made for many years. the largest errors of burckhardt's tables were put in evidence almost always by the altazimuth observations, in portions of the moon's orbit which could not be touched by the meridional instruments; they amounted sometimes to nearly " of arc, and they naturally became the crucial errors for distinction between burckhardt's and hansen's tables. those errors are in all cases corrected with great accuracy by hansen's tables.'--the report concludes with the following paragraph: 'with the inauguration of the new equatoreal will terminate the entire change from the old state of the observatory. there is not now a single person employed or instrument used in the observatory which was there in mr pond's time, nor a single room in the observatory which is used as it was used then. in every step of change, however, except this last, the ancient and traditional responsibilities of the observatory have been most carefully considered: and, in the last, the substitution of a new instrument was so absolutely necessary, and the importance of tolerating no instrument except of a high class was so obvious, that no other course was open to us. i can only trust that, while the use of the equatoreal within legitimate limits may enlarge the utility and the reputation of the observatory, it may never be permitted to interfere with that which has always been the staple and standard work here.'--concerning the sheepshanks fund: there was much correspondence about settling the gift till about feb. st. i took part in the first examination for the scholarship in october of this year, and took my place with the trinity seniority, as one of their number on this foundation, for some general business of the fund.--with respect to the correction of the compass in iron ships: i sent mr ellis to liverpool to see some practice there in the correction of the compass. in september i urged mr rundell to make a voyage in the great eastern (just floated) for examination of her compasses, and lent him instruments: very valuable results were obtained. mr archibald smith had edited scoresby's voyage in the royal charter, with an introduction very offensive to me: i replied fully in the athenaeum of nov. th.--the sale of gas act: an act of parliament promoted by private members of the house of commons had been passed, without the knowledge or recollection of the government. it imposed on the government various duties about the preparation of standards. suddenly, at the very expiration of the time allowed this came to the knowledge of government. on oct. st lord monteagle applied to me for assistance. on oct. th and nd i wrote to mr hamilton, secretary of the treasury, and received authority to ask for the assistance of prof. w.h. miller.--i made an examination of mr ball's eyes (long-sighted and short-sighted i think).--in february i made an analysis of the cambridge tripos examination, which i communicated to some cambridge residents." in a letter on this subject to one of his cambridge friends airy gives his opinion as follows: "i have looked very carefully over the examination papers, and think them on the whole very bad. they are utterly perverted by the insane love of problems, and by the foolish importance given to wholly useless parts of algebraical geometry. for the sake of these, every physical subject and every useful application of pure mathematics are cut down or not mentioned." this led to much discussion at cambridge. in this year the smith's prizes were awarded to the th and th wranglers. of private history: "on apr. th mrs smith (my wife's mother) died at brampton.--from july th to aug. nd i was in france (auvergne and the vivarais) with my two eldest sons. maclear travelled with us to paris.--on dec. rd i went to playford."--antiquities and historical questions connected with military movements had a very great attraction for airy. on his return from the expedition in france above-mentioned, he engaged in considerable correspondence with military authorities regarding points connected with the battle of toulouse. and in this year also he had much correspondence with the duke of northumberland concerning his map of the roman wall, and the military points relating to the same. "in june mr main accepted the office of radcliffe observer at oxford (mr johnson having died) and resigned the first assistancy at greenwich: in october mr stone was appointed first assistant.--at an adjourned meeting of the visitors on june th there were very heavy discussions on hansen's merits, and about the grant to him. papers were read from sir j. lubbock, babbage, south, whewell, and me. finally it was recommended to the government to grant _£ _ to hansen, which was paid to him.--in the report to the board of visitors the following remark occurs: 'the apparent existence of a discordance between the results of direct observations and reflection observations (after the application of corrections for flexure, founded upon observations of the horizontal collimator wires) to an extent far greater than can be explained by any disturbance of the direction of gravity on the quicksilver by its distance from the vertical, or by the attraction of neighbouring masses, perplexes me much.'--with respect to the discordance of dips of the dipping-needles, which for years past had been a source of great trouble and puzzle, the report states that 'the dipping-needles are still a source of anxiety. the form which their anomalies appear to take is that of a special or peculiar value of the dip given by each separate needle. with one of the -inch needles, the result always differs about a quarter of a degree from that of the others. i can see nothing in its mechanical construction to explain this.--reference is made to the spontaneous currents through the wires of telegraph companies, which are frequently violent and always occur at the times of magnetic storms, and the report continues 'it may be worth considering whether it would ever be desirable to establish in two directions at right angles to each other (for instance, along the brighton railway and along the north kent railway) wires which would photographically register in the royal observatory the currents that pass in these directions, exhibiting their indications by photographic curves in close juxtaposition with the registers of the magnetic elements.'--in connection with the reduction of the greenwich lunar observations from to , the report states that 'the comparison of hansen's lunar tables with the greenwich observations, which at the last visitation had been completed for one year only, has now been finished for the twelve years to . the results for the whole period agree entirely, in their general spirit, with those for the year cited in the last report. the greatest difference between the merits of burckhardt's and hansen's tables appears in the meridional longitudes , when the proportion of the sum of squares of errors is as (burckhardt) to (hansen). the nearest approach is in the altazimuth latitudes , when the proportion of the sum of squares of errors is as (burckhardt) to (hansen).'--a special address to the members of the board of visitors has reference to the proposals of m. struve for (amongst other matters) the improved determination of the longitude of valencia, and the galvanic determination of the extreme eastern station of the british triangles.--on sept. th i circulated amongst the visitors my remarks on a paper entitled 'on the polar distances of the greenwich transit-circle, by a. marth,' printed in the astronomische nachrichten; the paper by mr marth was an elaborate attack on the greenwich methods of observation, and my remarks were a detailed refutation of his statements.--on oct. th i made enquiry of sabine as to the advantage of keeping up magnetic observations. on oct. nd he wrote, avoiding my question in some measure, but saying that our instruments must be changed for such as those at kew (his observatory): i replied, generally declining to act on that advice.--in march and april i was in correspondence with mr cowper (first commissioner of works, &c.) about the bells of the westminster clock; also about the smoky chimneys of the various apartments of the palace. on apr. st i made my report on the clock and bells, foolscap pages. i employed a professional musician to examine the tones of the bells.--in november i was writing my book on probable errors, &c.--i was engaged on the tides of kurrachee and bombay.--the first examination of navy telescopes was made for the admiralty. --hoch's paper on aberration appeared in the astronomische nachrichten. this (with others) led to the construction of the water-telescope several years later.--in september i wrote in the athenaeum against a notion of sir h. james on the effect of an upheaval of a mountain in changing the earth's axis. in october i had drawn up a list of days for a possible evagation of the earth's poles: but apparently nothing was done upon them. "in this year i was a good deal occupied for the lighthouse commission. on feb. st admiral hamilton (chairman) applied to me for assistance. in april i went to chance's factory in birmingham on this business. in may i made my report on the start lighthouse, after inspection with the commission. in june, with my son hubert, i visited the whitby lighthouses, and discovered a fault of a singular kind which most materially diminished their power. this discovery led to a general examination of lighthouses by the trinity board, to a modification of many, and to a general improvement of system. on june th i reported on the lights at calais, cap de valde, grisnez, south foreland, and north foreland. in august i had been to the north foreland again, and in september to calais and the cap d'ailly. in october i went with my son hubert to aberdeen to see the girdleness lighthouse. on nov. th i made a general report. "this was the year of the great total solar eclipse visible in spain. at my representation, the admiralty placed at my command the large steamship 'himalaya' to carry about astronomers, british and foreign. some were landed at santander: i with many at bilbao. the eclipse was fairly well observed: i personally did not do my part well. the most important were mr de la rue's photographic operations. at greenwich i had arranged a very careful series of observations with the great equatoreal, which were fully carried out." the eclipse expedition to spain, shortly referred to above, was most interesting, not merely from the importance of the results obtained (and some of the parties were very fortunate in the weather) but from the character of the expedition. it was a wonderful combination of the astronomers of europe, who were all received on board the 'himalaya,' and were conveyed together to the coast of spain. the polyglot of languages was most remarkable, but the utmost harmony and enthusiasm prevailed from first to last, and this had much to do with the general success of the expedition. those who landed at bilbao were received in the kindest and most hospitable manner by mr c.b. vignoles, the engineer-in-chief of the bilbao and tudela railway, which was then under construction. this gentleman made arrangements for the conveyance of parties to points in the interior of the country which were judged suitable for the observation of the eclipse, and placed all the resources of his staff at the disposal of the expedition in the most liberal manner. the universal opinion was that very great difficulty would have been experienced without the active and generous assistance of mr vignoles. it is needless to say that the vote of thanks to mr vignoles, proposed by the astronomer royal during the return voyage, was passed by acclamation and with a very sincere feeling of gratitude: it was to the effect that 'without the great and liberal aid of mr c.b. vignoles, and the disinterested love of science evinced by him on this occasion, the success of the "himalaya" eclipse expedition could not have been ensured.' there is a graphic and interesting account of the reception of the party at bilbao given in the 'life of c.b. vignoles, f.r.s., soldier and civil engineer,' by o.j. vignoles, m.a. of private history: "on may th my venerable friend arthur biddell died. he had been in many respects more than a father to me: i cannot express how much i owed to him, especially in my youth.--from june th to th i visited the whitby lighthouses with my son hubert.--from july th to th i was in spain, on the 'himalaya' expedition, to observe the total eclipse: i was accompanied by my wife, my eldest son, and my eldest daughter.--from oct. th to th i went with my son hubert to aberdeen to see the girdleness lighthouse, making lateral trips to cumberland in going and returning.--on dec. st i went to playford." "in the report to the visitors there is great complaint of want of room. 'with increase of computations, we want more room for computers; with our greatly increased business of chronometers and time-distribution, we are in want of a nearly separate series of rooms for the time-department: we want rooms for book-stores; and we require rooms for the photographic operations and the computations of the magnetic department.'--the report gives a curious history of dr bradley's observations, which in had been transferred to the university of oxford, and proceeds thus: 'more lately, i applied (in the first instance through lord wrottesley) to the vice-chancellor, dr jeune, in reference to the possibility of transferring these manuscripts to the royal observatory.... finally, a decree for the transfer of the manuscript observations to the royal observatory, without any condition, was proposed to convocation on may nd, and was passed unanimously. and on may th my assistant, mr dunkin, was sent to oxford to receive them. and thus, after a delay of very nearly a century, the great work of justice is at length completed, and the great gap in our manuscript observations is at length filled up.'--with reference to the transit circle, it had been remarked that the collimators were slightly disturbed by the proximity of the gas-flames of their illuminators, and after various experiments as to the cause of it, the report proceeds thus: 'to my great surprise, i found that the disturbance was entirely due to the radiation of the flame upon a very small corner (about square inches) of the large and massive stone on which the collimator is planted. the tin plates were subsequently shaped in such a manner as to protect the stone as well as the metal; and the disturbance has entirely ceased.' --regarding the large s.e. equatoreal, the report states that 'on the character of its object-glass i am now able to speak, first, from the examination of mr otto struve, made in a favourable state of atmosphere; secondly, from the examinations of my assistants (i have not myself obtained a sight of a test-object on a night of very good definition). it appears to be of the highest order. the small star of gamma andromedae is so far separated as to shew a broad dark space between its components. some blue colour is shewn about the bright planets.'--it is noted in the report that 'the equatoreal observations of the solar eclipse are completely reduced; and the results are valuable. it appears from them that the error in right ascension of burckhardt's lunar tables at the time of the eclipse amounted to about "; while that of hansen's (ultimately adopted by mr hind for the calculation of the eclipse) did not exceed ".'--with regard to chronometers it is stated that 'by use of the chronometer oven, to which i have formerly alluded, we have been able to give great attention to the compensation. i have reason to think that we are producing a most beneficial effect on the manufacture and adjustment of chronometers in general.'--with regard to the cape of good hope observatory and survey, the admiralty enquire of me when the survey work will be completed, and i enquire of maclear 'how is the printing of your survey work?' in i began to press it strongly, and in very strongly.--i introduced a method (constantly pursued since that time at the royal observatory) for computing interpolations without changes of sign.--i had correspondence with herschel and faraday, on the possible effect of the sun's radiant heat on the sea, as explaining the curve of diurnal magnetic inequality. (that diurnal inequality was inferred from the magnetic reductions - , which were terminated in .)--regarding the proposal of hourly time-signals on the start point, i consulted telegraph engineers upon the practical points, and on dec. st i proposed a formal scheme, in complete detail. (the matter has been repeatedly brought before the admiralty, but has been uniformly rejected.)--i was engaged on the question of the bad ocular vision of two or three persons.--the british association meeting was held at manchester: i was president of section a. i gave a lecture on the eclipse of to an enormous attendance in the free trade hall." the following record of the lecture is extracted from dr e.j. routh's obituary notice of airy written for the proceedings of the royal society. "at the meeting of the british association at manchester in , mr airy delivered a lecture on the solar eclipse of to an assembly of perhaps persons. the writer remembers the great free trade hall crowded to excess with an immense audience whose attention and interest, notwithstanding a weak voice, he was able to retain to the very end of the lecture....the charm of professor airy's lectures lay in the clearness of his explanations. the subjects also of his lectures were generally those to which his attention had been turned by other causes, so that he had much that was new to tell. his manner was slightly hesitating, and he used frequent repetitions, which perhaps were necessary from the newness of the ideas. as the lecturer proceeded, his hearers forgot these imperfections and found their whole attention rivetted to the subject matter." of private history: "on jan. nd there was a most remarkable crystallization of the ice on the flooded meadows at playford: the frost was very severe.--from june th to aug. st i was at the grange near keswick (where i hired a house) with my wife and most of my family.--from nov. th to th i was on an expedition in the south of scotland with my son wilfrid: we walked with our knapsacks by the roman road across the cheviots to jedburgh.--on dec. st i went to playford." "the report to the board of visitors states that 'a new range of wooden buildings (the magnetic offices) is in progress at the s.s.e. extremity of the magnetic ground. it will include seven rooms.'--also 'i took this opportunity (the relaying of the water-main) of establishing two powerful fire-plugs (one in the front court, and one in the magnetic ground); a stock of fire-hose adapted to the "brigade-screw" having been previously secured in the observatory.'--'two wires, intended for the examination of spontaneous earth-currents, have been carried from the magnetic observatory to the railway station in the town of greenwich. from this point one wire is to be led to a point in the neighbourhood of croydon, the other to a point in the neighbourhood of dartford. each wire is to be connected at its two extremities with the earth. the angle included between the general directions of these two lines is nearly a right angle.'--'the kew unifilar magnetometer, adapted to the determination of the horizontal part of terrestrial magnetic force in absolute measure, was mounted in the summer of ; and till february, occasional observations ( in all) were taken simultaneously with the old and with the new instrument. the comparison of results shewed a steady but very small difference, not greater probably than may correspond to the omission of the inverse seventh powers of distance in the theoretical investigation; proving that the old instrument had been quite efficient for its purpose.'--great efforts had been made to deduce a law from the diurnal inequalities in declination and horizontal force, as shewn by the magnetic observations; but without success: the report states that 'the results are most amazing, for the variation in magnitude as well as in law. what cosmical change can be indicated by them is entirely beyond my power of conjecture.'--'i have alluded, in the two last reports, to the steps necessary, on the english side, for completing the great arc of parallel from valencia to the volga. the russian portion of the work is far advanced, and will be finished (it is understood) in the coming summer. it appeared to me therefore that the repetition of the measure of astronomical longitude between greenwich and valencia could be no longer delayed. two assistants of the royal observatory (mr dunkin and mr criswick) will at once proceed to valencia, for the determination of local time and the management of galvanic signals.'--'i now ask leave to press the subject of hourly time signals at the start point on the attention of the board, and to submit the advantage of their addressing the board of admiralty upon it. the great majority of outward-bound ships pass within sight of the start, and, if an hourly signal were exhibited, would have the means of regulating their chronometers at a most critical part of their voyage. the plan of the entire system of operations is completely arranged. the estimated expense of outfit is _£ _, and the estimated annual expense is _£ _; both liable to some uncertainty, but sufficiently exact to shew that the outlay is inconsiderable in comparison with the advantages which might be expected from it. i know no direction of the powers of the observatory which would tend so energetically to carry out the great object of its establishment, viz. "the finding out the so much desired longitude at sea."'--the attention of the visitors is strongly drawn to the pressure on the strength of the observatory caused by the observation of the numerous small planets, and the paragraph concludes thus: 'i shall, however, again endeavour to effect a partition of this labour with some other observatory.'--a small fire having occurred in the magnetic observatory, a new building of zinc, for the operation of naphthalizing the illuminating gas, is in preparation, external to the observatory: and thus one of the possible sources of accidental fire will be removed.--miss sheepshanks added, through me, _£ _ to her former gift: i transferred it, i believe, to the master and seniors of trinity college."--in this year airy contributed to the royal society two papers, one "on the magnetic properties of hot-rolled and cold-rolled malleable iron," the other "on the strains in the interior of beams." he gave evidence before the select committee on weights and measures, and also before the public schools commission. in the latter part of a difference arose between airy and major-general sabine, in consequence of remarks made by the latter at a meeting of the committee of recommendations of the british association. these remarks were to the effect "that it is necessary to maintain the complete system of self-registration of magnetic phenomena at the kew observatory, because no sufficient system of magnetic record is maintained elsewhere in england"; implying pointedly that the system at the royal observatory of greenwich was insufficient. this matter was taken up very warmly by airy, and after a short and acrimonious correspondence with sabine, he issued a private address to the visitors, enclosing copies of the correspondence with his remarks, and requesting the board to take the matter of this attack into their careful consideration. this address is dated november , and it was followed by another dated january , which contains a careful reply to the various points of general sabine's attack, and concludes with a distinct statement that he (the astronomer royal) can no longer act in confidence with sabine as a member of the board of visitors. of private history: there were the usual short visits to playford at the beginning and end of the year.--from june th to aug. th he was in scotland (chiefly in the western highlands) with his wife and his sons hubert and osmund. in the course of this journey he visited the corryvreckan whirlpool near the island of scarba, and the following paragraph relating to this expedition is extracted from his journal: "landed in black mile bay, island of luing, at . . here by previous arrangement with mr a. brown, agent of the steam-boat company, a -oared boat was waiting to take us to scarba and the corryvreckan. we were pulled across to the island of lunga, and rowed along its length, till we came to the first channel opening from the main sea, which the sailors called the little gulf. here the sea was rushing inwards in a manner of which i had no conception. streams were running with raving speed, sometimes in opposite directions side by side, with high broken-headed billows. where the streams touched were sometimes great whirls (one not many yards from our boat) that looked as if they would suck anything down. sometimes among all this were great smooth parts of the sea, still in a whirling trouble, which were surrounded by the mad currents. we seemed entirely powerless among all these." in the beginning of this year ( ) the duke of manchester, in writing to the rev. w. airy, had said, "i wish your brother, the astronomer royal, could be induced to have investigations made as to whether the aspects of the planets have any effect on the weather." this enquiry produced the following reply: a subject like that of the occult influences of the planets (using the word occult in no bad sense but simply as meaning not _thoroughly_ traced) can be approached in two ways--either by the à priori probability of the existence of such influences, or by the à posteriori evidence of their effects. if the two can be combined, the subject may be considered as claiming the dignity of a science. even if the effects alone are certain, it may be considered that we have a science of inferior degree, wanting however that definiteness of law and that general plausibility which can only be given when true causes, in accordance with antecedent experience in other cases, can be suggested. now in regard to the à priori probability of the existence of planetary influences, i am far from saying that such a thing is impossible. the discoveries of modern philosophy have all tended to shew that there may be many things about us, unknown even to the scientific world, but which well-followed accidents reveal with the most positive certainty. it is known that every beam of light is accompanied by a beam of chemical agency, totally undiscoverable to the senses of light or warmth, but admitting of separation from the luminous and warm rays; and producing photogenic effects. we know that there are disturbances of magnetism going on about us, affecting whole continents at a time, unknown to men in general, but traceable with facility and certainty, and which doubtless affect even our brains and nerves (which are indisputably subject to the influence of magnetism). now in the face of these things i will not undertake to say that there is any impossibility, or even any want of plausibility in the supposition that bodies external to the earth may affect us. it may well be cited in its favour that it is certain that the sun affects our magnetism (it is doubtful whether it does so _im_mediately, or mediately by giving different degrees of warmth to different parts of the earth), and it is believed on inferior evidence that the moon also affects it. it may therefore seem not impossible or unplausible that other celestial bodies may affect perhaps others of the powers of nature about us. but there i must stop. the denial of the impossibility is no assertion of the truth or probability, and i absolutely decline to take either side--either that the influences are real, or that the influences are unreal--till i see evidence of their effects. such evidence it is extremely difficult to extract from ordinary facts of observation. i have alluded to the sun's daily disturbance of the magnet as one of the most certain of influences, yet if you were to observe the magnet for a single day or perhaps for several days, you might see no evidence of that influence, so completely is it involved with other disturbances whose causes and laws are totally unknown. i believe that, in addition to the effects ascribable to newtonian gravitation (as general motion of the earth, precession of the equinoxes, and tides), this magnetic disturbance is the only one yet established as depending on an external body. men in general, however, do not think so. it appears to be a law of the human mind, to love to trace an effect to a cause, and to be ready to assent to any specious cause. thus all practical men of the lower classes, even those whose pecuniary interests are concerned in it, believe firmly in the influence of the moon upon the winds and the weather. i believe that every careful examiner of recorded facts (among whom i place myself as regards the winds) has come to the conclusion that the influence of the moon is not discoverable. i point out these two things (magnetic disturbances and weather) as tending to shew that notoriety or the assumed consent of practical men, are of no value. the unnotorious matter may be quite certain, the notorious matter may have no foundation. everything must stand on its own evidence, as completely digested and examined. of such evidence the planetary influence has not a particle. my intended short note has, in the course of writing, grown up into a discourse of very unreasonable length; and it is possible that a large portion of it has only increased obscurity. at any rate i can add nothing, i believe, which can help to explain more fully my views on this matter. * * * * * in this year ( , june th) airy received the honorary degree of ll.d. in the university of cambridge. he was nominated by the duke of devonshire, as appears from the following letter: lismore castle, ireland, _april th, _. my dear sir, it is proposed according to usage to confer a considerable number of honorary degrees on the occasion of my first visit to cambridge as chancellor of the university. i hope that you will allow me to include your name in that portion of the list which i have been invited to draw up. the ceremony is fixed for the th of june. i am, my dear sir, yours very truly, devonshire. _the astronomer royal_. * * * * * airy's reply was as follows: royal observatory, greenwich, london, s.e. _ , april _. my lord duke, i am exceedingly gratified by your communication this day received, conveying a proposal which i doubt not is suggested by your grace's recollection of transactions now many years past. i have always been desirous of maintaining my connection with my university, and have in various ways interested myself practically in its concerns. it would give me great pleasure to have the connection strengthened in the flattering way which you propose. i had conceived that alumni of the university were not admissible to honorary degrees; but upon this point the information possessed by your grace, as chancellor of the university, cannot be disputed. i am, my lord duke, your grace's very faithful servant, g.b. airy. _his grace the duke of devonshire_. * * * * * there were in all honorary degrees of doctor of laws conferred on the th of june, including men of such eminence as armstrong, faraday, and fairbairn. in this year there were several schemes for a railway through the lower part of greenwich park, the most important being the scheme of the london, chatham and dover railway company. in reference to this scheme the report to the visitors states "i may say briefly that i believe that it would be possible to render such a railway innocuous to the observatory; it would however be under restrictions which might be felt annoying to the authorities of the railway, but whose relaxation would almost ensure ruin to the observatory."--"the meridional observations of mars in the autumn of have been compared with those made at the observatory of williamstown, near melbourne, australia, and they give for mean solar parallax the value . ", exceeding the received value by about / th part. (a value nearly identical with this . " has also been found by comparing the pulkowa and cape of good hope observations.)"--"the results of the new dip-instrument in and appear to give a firm foundation for speculations on the state and change of the dip. as a general result, i may state as probable that the value of dip in the middle of was about ° ', and in the middle of about ° '. the decrease of dip appears to be more rapid in the second half of this interval than in the first; the dip at beginning of being about ° '."--with reference to the re-determination of the longitude of valencia, it is stated that "the concluded longitude agrees almost exactly with that determined by the transmission of chronometers in ; and entitles us to believe that the longitudes of kingstown and liverpool, steps in the chronometer conveyance, were determined with equal accuracy."--"the computations, for inferring the direction and amount of movement of the solar system in space from the observed proper motions of stars, have been completed. the result is, that the sun is moving towards a point, r.a. °, n.p.d. ° (not very different from sir w. herschel's, but depending much in n.p.d. on the accuracy of bradley's quadrant observations), and that its annual motion subtends, at the distance of a star of the first magnitude, the angle . ". but the comparison, of the sum of squares of apparent proper motions uncorrected, with the sum of squares of apparent proper motions corrected for motion of sun, shews so small an advance in the explanation of the star's apparent movements as to throw great doubt on the certainty of results; the sum of squares being diminished by only / th part."--"i had been writing strongly to maclear on the delays in publishing both the geodetic work and the star catalogue at the cape of good hope: he resolves to go on with these works. in december i am still very urgent about the geodesy." of private history: there was the usual short visit to playford at the beginning and end of the year.--"from june th to august th i was travelling in the north and west of scotland with my wife, my youngest son osmund, and my daughter annot." * * * * * in this year the offer of knighthood (for the third time) was made to airy through the rt hon. sir george c. lewis, bart. the offer was accepted on feb. th, , but on the same day a second letter was written as follows: _ , feb. _. dear sir, i am extremely ignorant of all matters connected with court ceremonial, and in reference to the proposed knighthood would ask you:-- . i trust that there is no expense of fees. to persons like myself of small fortune an honour may sometimes be somewhat dear. . my highest social rank is that given by my academical degree of d.c.l. which i hold in the universities of oxford and cambridge. in regard to costume, would it be proper that i should appear in the scarlet gown of that degree? or in the ordinary court dress? i am, dear sir, yours very faithfully, g.b. airy. _the right honourable sir george c. lewis, bart., &c. &c. &c._ to this letter sir g.c. lewis replied that the fees would amount to about _£ _, an intimation which produced the following letter: royal observatory, greenwich, s.e. _ , feb. th_. dear sir, i have to acknowledge your letter of yesterday: and i advert to that part of it in which it is stated that the fees on knighthood amount to about _£ _. twenty-seven years ago the same rank was offered to me by lord john russell and mr spring rice (then ministers of the crown), with the express notice that no fees would be payable. i suppose that the usage (whatever it be) on which that notice was founded still subsists. to a person whose annual income little more than suffices to meet the annual expenses of a very moderate establishment, an unsought honour may be an incumbrance. it appears, at any rate, opposed to the spirit of such an honour, that it should be loaded with court expenses in its very creation. i hope that the principle stated in may serve as precedent on this occasion. i am, dear sir, your very faithful servant, g.b. airy. _the right honourable sir g. c. lewis, bart., &c. &c. &c._ no intimation however was received that the fees would be remitted on the present occasion, and after consideration the proposed knighthood was declined in the following letter: royal observatory, greenwich, s.e. _ , april _. dear sir, i have frequently reflected on the proposal made by you of the honour of knighthood to myself. i am very grateful to you for the favourable opinion which you entertain in regard to my supposed claims to notice, and for the kindness with which you proposed publicly to express it. but on consideration i am strongly impressed with the feeling that the conditions attached by established regulation to the conferring of such an honour would be unacceptable to me, and that the honour itself would in reality, under the circumstances of my family-establishment and in my social position, be an incumbrance to me. and finally i have thought it best most respectfully, and with a full sense of the kindness of yourself and of the queen's government towards me, to ask that the proposal might be deferred. there is another direction in which a step might be made, affecting my personal position in a smaller degree, but not tending to incommode me, which i would ask leave to submit to your consideration. it is, the definition of the rank of the astronomer royal. the singular character of the office removes it from ordinary rules of rank, and sometimes may produce a disagreeable contest of opinions. the only offices of similar character corresponding in other conditions to that of the british astronomer royal are those of the imperial astronomers at pulkowa (st petersburg) and paris. in russia, where every rank is clearly defined by that of military grade, the imperial astronomer has the rank of major-general. in france, the definition is less precise, but the present imperial astronomer has been created (as an attachment of rank to the office) a senator of the empire. i am, dear sir, your very faithful servant, g.b. airy. _the rt hon. sir george c. lewis, bart., &c. &c. &c._ sir g. c. lewis died before receiving this letter, and the letter was afterwards forwarded to lord palmerston. some correspondence followed between lord palmerston and airy on the subject of attaching a definite rank to the office of astronomer royal, as proposed in the above letter. but the home office (for various reasons set forth) stated that the suggestion could not be complied with, and the whole subject dropped. the following remarks are extracted from the report of the astronomer royal to the board of visitors.--"in a very heavy squall which occurred in the gale of december of last year, the stay of the lofty iron pillar outside of the park rails, which carried our telegraph wires, gave way, and the pillar and the whole system of wires fell."--"an important alteration has been made in the magnetic observatory. for several years past, various plans have been under consideration for preventing large changes of temperature in the room which contains the magnetic instruments. at length i determined to excavate a subterraneous room or cellar under the original room. the work was begun in the last week in january, and in all important points it is now finished."--"in the late spring, some alarm was occasioned by the discovery that the parliamentary standard of the pound weight had become coated with an extraneous substance produced by the decomposition of the lining of the case in which it was preserved. it was decided immediately to compare it with the three parliamentary copies, of which that at the observatory is one. the national standard was found to be entirely uninjured."--"on november of last year, the transit instrument narrowly escaped serious injury from an accident. the plate chain which carries the large western counterpoise broke. the counterpoise fell upon the pier, destroying the massive gun-metal wheels of the lifting machinery, but was prevented from falling further by the iron stay of the gas-burner flue."--"the prismatic spectrum-apparatus had been completed in . achromatic object-glasses are placed on both sides of the prism, so that each pencil of light through the prism consists of parallel rays; and breadth is given to the spectrum by a cylindrical lens. the spectral lines are seen straighter than before, and generally it is believed that their definition is improved."--"for observation of the small planets, a convention has been made with m. le verrier. from new moon to full moon, all the small planets visible to h are observed at the royal observatory of greenwich. from full moon to new moon, all are observed at the imperial observatory of paris. the relief gained in this way is very considerable."--"in determining the variations in the power of the horizontal-force and vertical-force magnets depending on temperature, it was found by experiment that this depended materially on whether the magnet was heated by air or by water, and 'the result of these experiments (with air) is to give a coefficient for temperature correction four or five times as great as that given by the water-heatings,'"--"with regard to the discordances of the results of observations of dip-needles, experiments had been made with needles whose breadth was in the plane passing through the axis of rotation, and it appeared that the means of extreme discordances were, for an ordinary needle ' ", and for a flat needle ' "," and the report continues thus: "after this i need not say that i consider it certain that the small probable errors which have been attributed to ordinary needles are a pure delusion."--the report states that in the various operations connected with the trials and repairs of chronometers, and the system of time-signals transmitted to various time-balls and clocks, about one-fourth of the strength of the observatory is employed, and it continues thus: "viewing the close dependence of nautical astronomy upon accurate knowledge of time, there is perhaps no department of the observatory which answers more completely to the original utilitarian intentions of the founder of the royal observatory."--"with regard to the proposal of time-signals at the start point, it appears that communications referring to this proposal had passed between the board of admiralty and the board of trade, of which the conclusion was, that the board of trade possessed no funds applicable to the defraying of the expenses attending the execution of the scheme. and the admiralty did not at present contemplate the establishment of these time-signals under their own authority."--amongst other papers in this year, airy's paper entitled "first analysis of magnetic storms," &c., was read before the royal society. of private history: "there was the usual visit to playford in the beginning of the year.--from june th to rd i made an excursion with my son hubert to the isle of man, and the lake district.--from sept. th to th i was on a trip to cornwall with my two eldest sons, chiefly in the mining district.--in august of this year my eldest (surviving) daughter, hilda, was married to mr e.j. routh, fellow of st peter's college, cambridge, at greenwich parish church. they afterwards resided at cambridge." "our telegraphic communications of every kind were again destroyed by a snow-storm and gale of wind which occurred on jan. th, and which broke down nearly all the posts between the royal observatory and the greenwich railway station.--the report to the visitors states that 'the only change of buildings which i contemplate as at present required is the erection of a fire-proof chronometer room. the pecuniary value of chronometers stored in the observatory is sometimes perhaps as much as _£ _.'--the south eastern and london chatham and dover scheme for a railway through the park was again brought forward. there was a meeting of sir j. hanmer's committee at the observatory on may th. mr stone was sent hastily to dublin to make observations on earth-disturbance by railways there. i had been before the committee on may th. on sept. st i approved of an amended plan. in reference to this matter the report states that 'it is proper to remark that the shake of the altazimuth felt in the earthquake of , oct. th, when no such shake was felt with instruments nearer to the ground (an experience which, as i have heard on private authority, is supported by observation of artificial tremors), gives reason to fear that, at distances from a railway which would sufficiently defend the lower instruments, the loftier instruments (as the altazimuth and the equatoreals) would be sensibly affected.'--some of the magnets had been suspended by steel wires, instead of silk, of no greater strength than was necessary for safety, and the report states that 'under the pressure of business, the determination of various constants of adjustment was deferred to the end of the year. the immediate results of observation, however, began to excite suspicion; and after a time it was found that, in spite of the length of the suspending wire (about feet) the torsion-coefficient was not much less than / . the wires were promptly dismounted, and silk skeins substituted for them. with these, the torsion-coefficient is about / .'--the dip-instrument, which had given great trouble by the irregularities of the dip-results, had been compared with two dip-instruments from kew observatory, which gave very good and accordant results. 'it happened that mr simms, by whom our instruments now in use were prepared, and who had personally witnessed our former difficulties, was present during some of these experiments. our own instrument being placed in his hands (nov. th to th) for another purpose, he spontaneously re-polished the apparently faultless agate-bearings. to my great astonishment, the inconsistencies of every kind have nearly or entirely vanished. on raising and lowering the needles, they return to the same readings, and the dips with the same needle appear generally consistent.' some practical details of the polishing process by which this result had been secured are then given.--after numerous delays, the apparatus for the self-registration of spontaneous earth currents was brought into a working state in the month of march. a description of the arrangement adopted is given in the report.--'all chronometers on trial are rated every day, by comparison with one of the clocks sympathetic with the motor clock. every chronometer, whether on trial or returned from a chronometer-maker as repaired, is tried at least once in the heat of the chronometer-oven, the temperature being usually limited to ° fahrenheit; and, guided by the results of very long experience, we have established it as a rule, that every trial in heat be continued through three weeks.'--'the only employment extraneous to the observatory which has occupied any of my time within the last year is the giving three lectures on the magnetism of iron ships (at the request of the lords of the committee of council on education) in the theatre of the south kensington museum. the preparations, however, for these lectures, to be given in a room ill-adapted to them, occupied a great deal of my own time, and of the time of an assistant of the observatory.'--'referring to a matter in which the interests of astronomy are deeply concerned, i think it right to report to the visitors my late representation to the government, to the effect that, in reference to possible observation of the transit of venus in , it will be necessary in no long time to examine the coasts of the great southern continent.'" of private history: "there were the usual visits to playford at the beginning and end of the year.--from june th to th i was on a trip in wales with my sons hubert and osmund.--from sept. th to oct. nd i was staying with most of my family at portinscale near keswick: we returned by barnard castle, rokeby, &c." chapter viii. at greenwich observatory-- to . in this year the cube of the transit circle was pierced, to permit reciprocal observations of the collimators without raising the instrument. this involved the construction of improved collimators, which formed the subject of a special address to the members of the board of visitors on oct. st .--from the report to the visitors it appears that "on may rd , a thunderstorm of great violence passed very close to the observatory. after one flash of lightning, i was convinced that the principal building was struck. several galvanometers in the magnetic basement were destroyed. lately it has been remarked that one of the old chimneys of the principal building had been dislocated and slightly twisted, at a place where it was surrounded by an iron stay-band led from the telegraph pole which was planted upon the leads of the octagon room."--"on consideration of the serious interruptions to which we have several times been exposed from the destruction of our open-air park-wires (through snow-storms and gales), i have made an arrangement for leading the whole of our wires in underground pipes as far as the greenwich railway station."--"the committee of the house of commons, to whom the greenwich and woolwich line of the south eastern railway was referred, finally assented to the adoption of a line which i indicated, passing between the buildings of the hospital schools and the public road to woolwich."--"the galvanic chronometer attached to the s. e. equatoreal often gave us a great deal of trouble. at last i determined, on the proposal of mr ellis, to attempt an extension of mr r. l. jones's regulating principle. it is well known that mr jones has with great success introduced the system of applying galvanic currents originating in the vibrations of a normal pendulum, not to drive the wheelwork of other clocks, but to regulate to exact agreement the rates of their pendulums which were, independently, nearly in agreement; each clock being driven by weight-power as before. the same principle is now applied to the chronometer.... the construction is perfectly successful; the chronometer remains in coincidence with the transit clock through any length of time, with a small constant error as is required by mechanical theory."--"the printed volume of observations for has two appendixes; one containing the calculations of the value of the moon's semi-diameter deduced from occultations observed at cambridge and greenwich from to , and shewing that the occultation semi-diameter is less than the telescopic semi-diameter by "; the other containing the reduction of the planetary observations made at the royal observatory in the years - ; filling up the gap, between the planetary reductions - made several years ago under my superintendence, and the reductions contained in the greenwich volumes to the present time: and conducted on the same general principles."--"some trouble had been found in regulating the temperature of the magnetic basement, but it was anticipated that in future there would be no difficulty in keeping down the annual variation within about ° and the diurnal variation within °.--longitudes in america were determined in this year by way of valencia and newfoundland: finished by nov. th." of private history: in april he made a short visit to ventnor in the isle of wight.--from june th to july rd he was on an expedition in norway with his son osmund and his nephew gorell barnes.--there was probably a short stay at playford in the winter. in this and in the previous year ( ) the free-thinking investigations of colenso, the bishop of natal, had attracted much notice, and had procured him the virulent hostility of a numerous section. his income was withheld from him, and in consequence a subscription fund was raised for his support by his admirers. airy, who always took the liberal side in such questions, was a subscriber to the fund, and wrote the following letter to the bishop: royal observatory, greenwich, s.e., _ , july _. my lord, with many thanks i have to acknowledge your kind recollection of me in sending as a presentation copy the work on joshua, judges, and especially on the divided authorship of genesis; a work whose investigations, founded in great measure on severe and extensive verbal criticism, will apparently bear comparison with your lordship's most remarkable examination of deuteronomy. i should however not do justice to my own appreciation if i did not remark that there are other points considered which have long been matters of interest to me. on several matters, some of them important, my present conclusions do not absolutely agree with your lordship's. but i am not the less grateful for the amount of erudition and thought carefully directed to definite points, and above all for the noble example of unwearied research and freedom in stating its consequences, in reference to subjects which scarcely ever occupy the attention of the clergy in our country. i am, my lord, yours very faithfully, g.b. airy. _the lord bishop of natal_. * * * * * here also is a letter on the same subject, written to professor selwyn, professor of divinity at cambridge:-- royal observatory, greenwich, london, s.e., _ , may _. my dear sir, the ms. concerning colenso duly arrived. i note your remarks on the merits of colenso. i do not write to tell you that i differ from you, but to tell you why i differ. i think that you do not make the proper distinction between a person who invents or introduces a tool, and the person who uses it. the most resolute antigravitationist that ever lived might yet acknowledge his debt to newton for the method of prime and ultimate ratios and the principles of fluxions by which newton sought to establish gravitation. so let it be with colenso. he has given me a power of tracing out truth to a certain extent which i never could have obtained without him. and for this i am very grateful. as to the further employment of this power, you know that he and i use it to totally different purposes. but not the less do i say that i owe to him a new intellectual power. i quite agree with you, that the sudden disruption of the old traditional view seems to have unhinged his mind, and to have sent him too far on the other side. i would not give a pin for his judgment. nevertheless, i wish he would go over the three remaining books of the tetrateuch. i know something of myers, but i should not have thought him likely to produce anything sound on such things as the hebrew scriptures. i never saw his "thoughts." i am, my dear sir, yours very truly, g.b. airy. _professor selwyn_. * * * * * the following letter has reference to airy's proposal to introduce certain physico-mathematical subjects into the senate-house examination for b.a. honors at cambridge. on various occasions he sharply criticized the papers set for the senate-house examination and the smith's prize examination, and greatly lamented the growing importance of pure mathematics and the comparative exclusion of physical questions in those examinations. his proposal as finally submitted in the letter that follows was somewhat modified (as regards the mode of introducing the subjects) from his original draft, in deference to the opinions of whewell, adams, routh, and other friends to whom he had submitted it. his proposal was favourably received by the mathematical board, and recommendations were made in the direction, though not to the extent, that he desired, and he subsequently submitted a memorandum on those recommendations: royal observatory, greenwich, _ , may _. my dear sir, you will perceive, from perusal of the enclosed paper, that i have acted on the permission which you kindly gave me, to transmit to you my proposal for extension of the mathematical education of the university in the physical direction. it is an unavoidable consequence of the structure of the university that studies there will have a tendency to take an unpractical form depending much on the personal tastes of special examiners. i trust that, as a person whose long separation from the daily business of the university has enabled him to see in some measure the wants of the external scientific and practical world, i may be forgiven this attempt to bring to the notice of the university my ideas on the points towards which their attention might perhaps be advantageously turned. i am, my dear sir, very faithfully yours, g.b. airy. _the rev. dr cartmell, master of christ's college and vice-chancellor._ royal observatory, greenwich, _ , may _. my dear mr vice-chancellor, about two years ago, by the kindness of the university, an opportunity was presented to me of orally stating what i conceived to be deficiencies in the educational course of the university as regards mathematical physics. since that time, the consideration of those deficiencies, which had long been present to me, has urged itself on my attention with greater force: and finally i have entertained the idea that i might without impropriety communicate to you my opinion, in a less fugitive form than on the occasion to which i have alluded: with the request that, if you should deem such a course appropriate, you would bring it before the board of mathematical studies, and perhaps ultimately make it known to the resident members of the senate. i will first give the list of subjects, which i should wish to see introduced, and to the prosecution of which the generally admirable course of the university is remarkably well adapted: and i will then, without entering into every detail, advert to the process by which i think it probable the introduction of these subjects could be effected. in the following list, the first head is purely algebraical, and the second nearly so: but they are closely related to observational science, and to the physical subjects which follow. some of the subjects which i exhibit on my list are partially, but in my opinion imperfectly, taught at present. i entirely omit from my list physical optics, geometrical astronomy, and gravitational astronomy of points: because, to the extent to which academical education ought to go, i believe that there is no teaching on these sciences comparable to that in the university of cambridge. (it is, of course, still possible that improvements may be made in the books commonly used.) it might, however, be a question, whether, as regards the time and manner of teaching them, some parts of these subjects might ultimately be associated with the other subjects included in my list. i. _list of subjects proposed for consideration_. ( ) partial differential equations to the second order, with their arbitrary functions: selected principally with reference to the physical subjects. ( ) the theory of probabilities as applied to the combination of observations. ( ) mechanics (including hydraulic powers) in the state which verges upon practical application, and especially including that part in which the abstract ideas of _power_ and _duty_ occur. ( ) attractions. this subject is recognized in the existing course of the university: but, so far as i can infer from examination-papers, it appears to be very lightly passed over. ( ) the figure of the earth, and its consequences, precession, &c. i believe that the proposal is sanctioned, of adopting some part of this theory in the ordinary course; but perhaps hardly so far as is desirable. ( ) the tides. ( ) waves of water. ( ) sound (beginning with newton's investigation); echoes; pipes and vibrating strings; acoustics; the mathematical part of music. ( ) magnetism, terrestrial and experimental, and their connection. (i omit for the present mineralogy and mathematical electricity.) this list of subjects appears formidable: but they are in reality easy, and would be mastered in a short time by the higher wranglers. ii. _mode of introducing these subjects into the university_. after much consideration, and after learning the opinions of several persons whose judgment claims my deepest respect, i propose the gradual introduction of these subjects into the examination for honors at admission to the b.a. degree, as soon as the preparation of books and the readiness of examiners shall enable the university to take that step. i conceive that, by a judicious pruning of the somewhat luxuriant growth of pure algebra, analytical geometry, and mere problems, sufficient leisure may be gained for the studies of the undergraduates, and sufficient time for the questions of the examiners. i do not contemplate that the students could advance very far into the subjects; but i know the importance of beginning them; and, judging from the train of thoughts, of reading, and of conversation, among the bachelors with whom i associated many years ago, i believe that there is quite a sufficient number who will be anxious to go deep into the subjects if they have once entered into them. if six wranglers annually would take them up, my point would be gained. the part which these gentlemen might be expected, in a short time, to take in the government of the university, would enable them soon to act steadily upon the university course: the efficiency of the university instruction would be increased; and the external character of the university would be raised. the real difficulties, and they are not light ones, would probably be found in providing examiners and books. at present, both are wanting within the university. where there is a great and well-founded objection to intrusting examinations to persons foreign to the university, and where the books have to be created with labour and with absolute outlay of money (for their sale could never be remunerative), the progress must be slow. still progress would be certain, if the authorities of the university should think the matter deserving of their hearty encouragement. requesting that you and the members of the university will accept this proposal as an indication of my deep attachment to my university, i am, my dear mr vice-chancellor, your very faithful servant, g.b. airy. _the rev. dr cartmell, &c. &c. vice-chancellor of the university of cambridge_. "in this year it was arranged that my treasury accounts were to be transferred to the admiralty, making the simplification which i had so long desired.--from the report to the visitors it appears that a relic of the geodetic operations commenced in for connecting the observatories of greenwich and paris, in the shape of an observing cabin on the roof of the octagon room, was shifted and supported in such a manner that the pressure on the flat roof was entirely avoided.--with regard to the transit circle, the new collimators with telescopes of seven inches aperture had been mounted. when the transit telescope directed vertically is interposed, the interruptions in the central cube impair the sharpness of definition, still leaving it abundantly good for general use. it had been regarded as probable that the astronomical flexure of the telescope, after cutting away small portions of the central cube, would be found sensibly changed: and this proved to be the case. the difference of flexures of the two ends has been altered more than a second of arc.--referring to a new portable altazimuth which had lately been tested, the report states as follows: 'i may mention that a study of defects in the vertical circle of a small altazimuth formerly used by me, and an inspection of the operations in the instrument-maker's work-shop, have convinced me that the principal error to be feared in instruments of this class is ovality of the graduated limb; this cannot be eliminated by two microscopes, and such an instrument should never be fitted with two only. our instrument has four.'--'in osler's anemometer, a surface of square feet is now exposed to the wind instead of one foot as formerly; and the plate is supported by weak vertical springs instead of rods running on rollers. its indications are much more delicate than formerly.'--'the meteors on nov. th were well observed. eight thousand and three hundred were registered. the variations of frequency at different times were very well noted. the points of divergence were carefully determined.'--referring to the gradual improvement in the steadiness of chronometers from to , it appears that from to the 'trial number' (which is a combination of changes of weekly rate representing the fault of the chronometer) varied from . s to . s, while from to it varied from . s to . s.--the following statement will shew the usual steadiness of the great clock on the westminster palace: on per cent. of days of observation, the clock's error was below s. on per cent, the error was between s and s. on per cent. it was between s and s. on per cent. between s and s. on per cent. between s and s.--the report contains an account of the determination of the longitude of cambridge u.s. by dr b. a. gould, by means of galvanic currents through the atlantic cable, in the spring of : and advantage was taken of this opportunity for re-determining the longitude of feagh main near valencia in ireland. the longitude of feagh main, found by different methods is as follows: by chronometers in , m . s; by galvanic communication with knight's town in , m . s; by galvanic communication with foilhommerum in , m . s. the collected results for longitude of cambridge u.s. from different sources are: by moon-culminators (walker in , and newcomb in - ), h m . s and h m . s respectively; by eclipses (walker in ), h m . s; by occultations of pleiades (peirce - , and - ), h m . s and h m . s respectively; by chronometers (w. c. bond in , and g. p. bond in ), h m . s and h m . s respectively; by atlantic cable , h m . s.--after noticing that many meteorological observatories had suddenly sprung up and had commenced printing their observations in detail, the report continues thus: 'whether the effect of this movement will be that millions of useless observations will be added to the millions that already exist, or whether something may be expected to result which will lead to a meteorological theory, i cannot hazard a conjecture. this only i believe, that it will be useless, at present, to attempt a process of mechanical theory; and that all that can be done must be, to connect phenomena by laws of induction. but the induction must be carried out by numerous and troublesome trials in different directions, the greater part of which would probably be failures.'--there was this year an annular eclipse; i made large preparations at the limits of the annularity; failed entirely from very bad weather."--in this year airy contributed a paper to the institution of civil engineers 'on the use of the suspension bridge with stiffened roadway for railway and other bridges of great span,' for which a telford medal was awarded to him by the council of the institution. and he communicated several papers to the royal society and the royal astronomical society. of private history: there was the usual visit to playford in january.--in april there was a short run to alnwick and the neighbourhood, in company with mr and mrs routh.--from june th to july th he was in wales with his two eldest sons, visiting uriconium, &c. on his return.--from august th to sept. th he spent a holiday in scotland and the lake district of cumberland with his daughter christabel, visiting the langtons at barrow house, near keswick, and isaac fletcher at tarn bank. in june of this year ( ) airy was elected an honorary fellow of his old college of trinity in company with connop thirlwall, the bishop of st david's. they were the first honorary fellows elected by the college. the announcement was made in a letter from the master of trinity (w.h. thompson), and airy's reply was as follows: royal observatory, greenwich, london, s.e. _ , june th_. my dear master, i am very much gratified by your kind note received this morning, conveying to me the notice that the master and sixteen senior fellows had elected me, under their new powers, as honorary fellow of the college. it has always been my wish to maintain a friendly connection with my college, and i am delighted to receive this response from the college. the peculiar form in which the reference to the statute enables them to put it renders it doubly pleasing. as the statute is new, i should be obliged by a copy of it. and, at any convenient time, i should be glad to know the name of the person with whom i am so honorably associated. i am, my dear master, very faithfully yours, g.b. airy. * * * * * consequent on airy's proposals in for the introduction of new physical subjects into the senate-house examination and his desire that the large number of questions set in pure mathematics, or as he termed it "useless algebra," should be curtailed, there was a smart and interesting correspondence between him and prof. cayley, who was the great exponent and advocate of pure mathematics at cambridge. both of them were men of the highest mathematical powers, but diametrically opposed in their views of the use of mathematics. airy regarded mathematics as simply a useful machine for the solution of practical problems and arriving at practical results. he had a great respect for pure mathematics and all the processes of algebra, so far as they aided him to solve his problems and to arrive at useful results; but he had a positive aversion to mathematical investigations, however skilful and elaborate, for which no immediate practical value could be claimed. cayley on the contrary regarded mathematics as a useful exercise for the mind, apart from any immediate practical object, and he considered that the general command of mathematics gained by handling abstruse mathematical investigations (though barren in themselves) would be valuable for whatever purpose mathematics might be required: he also thought it likely that his researches and advances in the field of pure mathematics might facilitate the solution of physical problems and tend to the progress of the practical sciences. their different views on this subject will be seen from the letters that follow: royal observatory, greenwich, london, s.e. _ , nov. _. my dear sir, i think it best to put in writing the purport of what i have said, or have intended to say, in reference to the mathematical studies in the university. first, i will remark on the study of partial differential equations. i do not know that one branch of pure mathematics can be considered higher than another, except in the utility of the power which it gives. measured thus, the partial differential equations are very useful and therefore stand very high, as far as the second order. they apply, to that point, in the most important way, to the great problems of nature concerning _time_, and _infinite division of matter_, and _space_: and are worthy of the most careful study. beyond that order they apply to nothing. it was for the purpose of limiting the study to the second order, and at the same time working it carefully, philosophically, and practically, up to that point, that i drew up my little work. on the general question of mathematical studies, i will first give my leading ideas on what i may call the moral part. i think that a heavy responsibility rests on the persons who influence most strongly the course of education in the university, to direct that course in the way in which it will be most useful to the students--in the two ways, of disciplining their powers and habits, and of giving them scientific knowledge of the highest and most accurate order (applying to the phenomena of nature) such as will be useful to them through life. i do not think that the mere personal taste of a teacher is sufficient justification for a special course, unless it has been adopted under a consideration of that responsibility. now i can say for myself that i have, for some years, inspected the examination papers, and have considered the bearing of the course which they imply upon the education of the student, and am firmly convinced that as regards men below the very few first--say below the ten first--there is a prodigious loss of time without any permanent good whatever. for the great majority of men, such subjects as abstract analytical geometry perish at once. with men like adams and stokes they remain, and are advantageous; but probably there is not a single man (beside them) of their respective years who remembers a bit, or who if he remembers them has the leisure and other opportunities of applying them. i believe on the other hand that a careful selection of physical subjects would enable the university to communicate to its students a vast amount of information; of accurate kind and requiring the most logical treatment; but so bearing upon the natural phenomena which are constantly before us that it would be felt by every student to possess a real value, that (from that circumstance) it would dwell in his mind, and that it would enable him to correct a great amount of flimsy education in the country, and, so far, to raise the national character. the consideration of the education of the reasoning habits suggests ideas far from favourable to the existing course. i am old enough to remember the time of mere geometrical processes, and i do not hesitate to say that for the cultivation of accurate mental discipline they were far superior to the operations in vogue at the present day. there is no subject in the world more favourable to logical habit than the differential calculus in all its branches _if logically worked in its elements_: and i think that its applications to various physical subjects, compelling from time to time an attention to the elementary grounds of the calculus, would be far more advantageous to that logical habit than the simple applications to pure equations and pure algebraical geometry now occupying so much attention. i am, my dear sir, yours very truly, g.b. airy. _professor cayley_. * * * * * dear sir, i have been intending to answer your letter of the th november. so far as it is (if at all) personal to myself, i would remark that the statutory duty of the sadlerian professor is that he shall explain and teach the principles of pure mathematics and apply himself to the advancement of the science. as to partial differential equations, they are "high" as being an inverse problem, and perhaps the most difficult inverse problem that has been dealt with. in regard to the limitation of them to the second order, whatever other reasons exist for it, there is also the reason that the theory to this order is as yet so incomplete that there is no inducement to go beyond it; there could hardly be a more valuable step than anything which would give a notion of the form of the general integral of a partial differential equation of the second order. i cannot but differ from you _in toto_ as to the educational value of analytical geometry, or i would rather say of modern geometry generally. it appears to me that in the physical sciences depending on partial differential equations, there is scarcely anything that a student can do for himself:--he finds the integral of the ordinary equation for sound--if he wishes to go a step further and integrate the non-linear equation (dy/dx)²(d²y/dt²) = a²(d²y/dx²) he is simply unable to do so; and so in other cases there is nothing that he can add to what he finds in his books. whereas geometry (of course to an intelligent student) is a real inductive and deductive science of inexhaustible extent, in which he can experiment for himself--the very tracing of a curve from its equation (and still more the consideration of the cases belonging to different values of the parameters) is the construction of a theory to bind together the facts--and the selection of a curve or surface proper for the verification of any general theorem is the selection of an experiment in proof or disproof of a theory. i do not quite understand your reference to stokes and adams, as types of the men who alone retain their abstract analytical geometry. if a man when he takes his degree drops mathematics, he drops geometry--but if not i think for the above reasons that he is more likely to go on with it than with almost any other subject--and any mathematical journal will shew that a very great amount of attention is in fact given to geometry. and the subject is in a very high degree a progressive one; quite as much as to physics, one may apply to it the lines, yet i doubt not thro' the ages one increasing purpose runs, and the thoughts of men are widened with the progress of the suns. i remain, dear sir, yours very sincerely, a. cayley. cambridge, _ dec., _. * * * * * royal observatory, greenwich, london, s.e. _ , december _. my dear sir, i have received with much pleasure your letter of december . in this university discussion, i have acted only in public, and have not made private communication to any person whatever till required to do so by private letter addressed to me. your few words in queens' hall seemed to expect a little reply. now as to the modern geometry. with your praises of this science--as to the room for extension in induction and deduction, &c.; and with your facts--as to the amount of space which it occupies in mathematical journals; i entirely agree. and if men, after leaving cambridge, were designed to shut themselves up in a cavern, they could have nothing better for their subjective amusement. they might have other things as good; enormous complication and probably beautiful investigation might be found in varying the game of billiards with novel islands on a newly shaped billiard table. but the persons who devote themselves to these subjects do thereby separate themselves from the world. they make no step towards natural science or utilitarian science, the two subjects which the world specially desires. the world could go on as well without these separatists. now if these persons lived only for themselves, no other person would have any title to question or remark on their devotion to this barren subject. but a cambridge examiner is not in that position. the university is a national body, for education of young men: and the power of a cambridge examiner is omnipotent in directing the education of the young men; and his responsibility to the cause of education is very distinct and very strong. and the question for him to consider is--in the sense in which mathematical education is desired by the best authorities in the nation, is the course taken by this national institution satisfactory to the nation? i express my belief that it is _not_ satisfactory. i believe that many of the best men of the nation consider that a great deal of time is lost on subjects which they esteem as puerile, and that much of that time might be employed on noble and useful science. you may remember that the commissions which have visited cambridge originated in a memorial addressed to the government by men of respected scientific character: sabine was one, and i may take him as the representative. he is a man of extensive knowledge of the application of mathematics as it has been employed for many years in the science of the world; but he has no profundity of science. he, as i believe, desired to find persons who could enter accurately into mathematical science, and naturally looked to the great mathematical university; but he must have been much disappointed. so much time is swallowed up by the forced study of the pure mathematics that it is not easy to find anybody who can really enter on these subjects in which men of science want assistance. and so sabine thought that the government ought to interfere, probably without any clear idea of what they could do. i am, my dear sir, yours very truly, g.b. airy. _professor cayley_. * * * * * dear sir, i have to thank you for your last letter. i do not think everything should be subordinated to the educational element: my idea of a university is that of a place for the cultivation of all science. therefore among other sciences pure mathematics; including whatever is interesting as part of this science. i am bound therefore to admit that your proposed extension of the problem of billiards, _if it_ were found susceptible of interesting mathematical developments, would be a fit subject of study. but in this case i do not think the problem could fairly be objected to as puerile--a more legitimate objection would i conceive be its extreme speciality. but this is not an objection that can be brought against modern geometry as a whole: in regard to any particular parts of it which may appear open to such an objection, the question is whether they are or are not, for their own sakes, or their bearing upon other parts of the science to which they belong, worthy of being entered upon and pursued. but admitting (as i do not) that pure mathematics are only to be studied with a view to natural and physical science, the question still arises how are they best to be studied in that view. i assume and admit that as to a large part of modern geometry and of the theory of numbers, there is no present probability that these will find any physical applications. but among the remaining parts of pure mathematics we have the theory of elliptic functions and of the jacobian and abelian functions, and the theory of differential equations, including of course partial differential equations. now taking for instance the problem of three bodies--unless this is to be gone on with by the mere improvement in detail of the present approximate methods--it is at least conceivable that the future treatment of it will be in the direction of the problem of two fixed centres, by means of elliptic functions, &c.; and that the discovery will be made not by searching for it directly with the mathematical resources now at our command, but by "prospecting" for it in the field of these functions. even improvements in the existing methods are more likely to arise from a study of differential equations in general than from a special one of the equations of the particular problem: the materials for such improvements which exist in the writings of hamilton, jacobi, bertrand, and bour, have certainly so arisen. and the like remarks would apply to the physical problems which depend on partial differential equations. i think that the course of mathematical study at the university is likely to be a better one if regulated with a view to the cultivation of science, as if for its own sake, rather than directly upon considerations of what is educationally best (i mean that the best educational course will be so obtained), and that we have thus a justification for a thorough study of pure mathematics. in my own limited experience of examinations, the fault which i find with the men is a want of analytical power, and that whatever else may have been in defect pure mathematics has certainly not been in excess. i remain, dear sir, yours sincerely, a. cayley. cambridge, _ th dec., _. * * * * * _ , december _. my dear sir, since receiving your letter of th i positively have not had time to express the single remark which i proposed to make on it. you state your idea that the educational element ought not to be the predominating element in the university. "i do not think that every thing should be subordinated to the educational element." i cannot conceal my surprise at this sentiment. assuredly the founders of the colleges intended them for education (so far as they apply to persons in statu pupillari), the statutes of the university and the colleges are framed for education, and fathers send their sons to the university for education. if i had not had your words before me, i should have said that it is impossible to doubt this. it is much to be desired that professors and others who exercise no control by force should take every method, not only of promoting science in themselves, but also of placing the promoted science before students: and it is much to be desired that students who have passed the compulsory curriculum should be encouraged to proceed into the novelties which will be most agreeable to them. but this is a totally different thing from using the compulsory force of examination to drive students in paths traced only by the taste of the examiner. for them, i conceive the obligation to the nation and the duty to follow the national sense on education (as far as it can be gathered from its best representatives) to be undoubted; and to be, in the intensity of the obligation and duty, most serious. i am, my dear sir, yours very truly, g.b. airy. _professor cayley_. * * * * * "in the south-east dome, the alteration proposed last year for rendering the building fire-proof had been completely carried out. the middle room, which was to be appropriated to chronometers, was being fitted up accordingly.--from the report it appears that 'our subterranean telegraph wires were all broken by one blow, from an accident in the metropolitan drainage works on groom's hill, but were speedily repaired.'--in my office as chairman of successive commissions on standards, i had collected a number of standards, some of great historical value (as ramsden's and roy's standards of length, kater's scale-beam for weighing great weights, and others), &c. these have been transferred to the newly-created standards department of the board of trade."--in the report is given a detailed account of the system of preserving and arranging the manuscripts and correspondence of the observatory, which was always regarded by airy as a matter of the first importance.--from a careful discussion of the results of observation mr stone had concluded that the refractions ought to be diminished. 'relying on this, we have now computed our mean refractions by diminishing those of bessel's fundamenta in the proportion of to . .'--the magnetometer-indications for the period - had been reduced and discussed, with remarkable results. it is inferred that magnetic disturbances, both solar and lunar, are produced mediately by the earth, and that the earth in periods of several years undergoes changes which fit it and unfit it for exercising a powerful mediate action.--the earth-current records had been reduced, and the magnetic effect which the currents would produce had been computed. the result was, that the agreement between the magnetic effects so computed and the magnetic disturbances really recorded by the magnetometers was such as to leave no doubt on the general validity of the explanation of the great storm-disturbances of the magnets as consequences of the galvanic currents through the earth.--referring to the difficulty experienced in making the meteorological observations practically available the report states thus: 'the want of meteorology, at the present time, is principally in suggestive theory.'--in this year airy communicated to the royal astronomical society a paper 'on the preparatory arrangements for the observation of the transits of venus and ': this subject was now well in hand.--the first report of the commissioners (of whom he was chairman) appointed to enquire into the condition of the exchequer standards was printed: this business took up much time.--he was in this year much engaged on the coinage commission. of private history: there was the usual winter visit to playford, and a short visit to cambridge in june.--from about aug. st to sept. rd he was travelling in switzerland with his youngest son and his two youngest daughters. in the course of this journey they visited zermatt. there had been much rain, the rivers were greatly flooded, and much mischief was done to the roads. during the journey from visp to zermatt, near st nicholas, in a steep part of the gorge, a large stone rolled from the cliffs and knocked their baggage horse over the lower precipice, a fall of several hundred feet. the packages were all burst, and many things were lost, but a good deal was recovered by men suspended by ropes. in this year also airy was busy with the subject of university examination, which in previous years had occupied so much of his attention, as will be seen from the following letters: royal observatory, greenwich, london, s.e. _ , march _. my dear master, i have had the pleasure of corresponding with you on matters of university examination so frequently that i at once turn to you as the proper person to whom i may address any remarks on that important subject. circumstances have enabled me lately to obtain private information of a most accurate kind on the late mathematical tripos: and among other things, i have received a statement of every individual question answered or partly answered by five honour-men. i have collected the numbers of these in a small table which i enclose. i am struck with the _almost_ nugatory character of the five days' honour examination as applied to senior optimes, and i do not doubt that it is _totally_ nugatory as applied to junior optimes. it appears to me that, for all that depends on these days, the rank of the optimes is mere matter of chance. in the examinations of the civil service, the whole number of marks is published, and also the number of marks gained by each candidate. i have none of their papers at hand, but my impression is that the lowest candidates make about in ; and the fair candidates about in , instead of in or in as our good senior optimes. i am, my dear master, very truly yours, g.b. airy. _the rev. dr cookson, master of st peters college, &c. &c._ the table referred to in the above letter is as follows: number of questions, and numbers of answers to questions as given by several wranglers and senior optimes, in the examination of mathematical tripos for honours, , january , , , , . number of questions and riders in the printed papers. questions. riders. aggregate. in the papers of the days number of questions and riders answered. questions. riders. aggregate. by a wrangler, between the st and th - / - / in . by a wrangler, between the th and nd - / - / in . by a wrangler, between the nd and nd - / - / in . by a sen. opt. between the st and th - / - / in . by a sen. opt. between the th and th - / - / in . g.b. airy. _ , march _. * * * * * st peter's college lodge, cambridge, _march th, _. my dear sir, i am much obliged by your letter and enclosed paper. anything done in the last five days by a junior optime only shews (generally) that he has been employing some of his time _mischievously_, for he must have been working at subjects which he is quite unable to master or cramming them by heart on the chance of meeting with a stray question which he may answer. the chief part of the senior optimes are in something of the same situation. i think that the proposed addition of a day to the first part of the examination, in which "easy questions in physical subjects" may be set, is, on this account, a great improvement. our new scheme comes on for discussion on friday next, march , at p.m. in the arts school. it is much opposed by private tutors, examiners and others, and may possibly be thrown out in the senate this year, though i hope that with a little patience it may be carried, in an unmutilated form, eventually. the enclosed report on the smith's prize examination will be discussed at the same time. i will consider what is best to be done on the subject to which your note refers, without delay. with many thanks, i am, very faithfully yours, h.w. cookson, _the astronomer royal._ * * * * * in this year certain members of the senate of the university of cambridge petitioned parliament against the abolition of religious declarations required of persons admitted to fellowships or proceeding to the degree of m.a. the document was sent to airy for his signature, and his reply was as follows: royal observatory, greenwich, london, s.e. _ , march _. my dear sir, though i sympathize to a great extent with the prayer of the petition to parliament which you sent to me yesterday, and assent to most of the reasons, i do not attach my signature to it, for the following considerations: . i understand, from the introductory clause, and from the unqualified character of the phrase "any such measures" in the second clause, that the petition objects to granting the m.a. degree without religious declaration. i do not see any adequate necessity for this objection, and i cannot join in it. . it appears to me that the colleges were intended for two collateral objects:--instruction by part of the fellows, on a religious basis; and support of certain fellows for scientific purposes, without the same ostentatious connection with religion. i like this spirit well, and should be glad to maintain it. . i therefore think (as i have publicly stated before) that the master of the college ought to be in holy orders; and so ought those of the fellows who may be expected to be usually resident and to take continuous part in the instruction. but there are many who, upon taking a fellowship, at once lay aside all thoughts of this: and i think that such persons ought not to be trammelled with declarations. . my modification of existing regulations, if it once got into shape, would i dare say be but a small fraction of that proposed by the "measures in contemplation." still i do not like to join in unqualified resistance to interference in the affairs of the established colleges, with that generality of opposition to interference which the petition seems to intimate. i agree with articles , , and ; and i am pleased with the graceful allusion in article to the assistance which has been rendered by the colleges, and by none perhaps so honourably as trinity, to the parishes connected with it. and i could much wish that the spirit of and could be carried out, with some concession to my ideas in _my_ paragraph , above. i am, my dear sir, yours very truly, g.b. airy. _rev. dr lightfoot._ * * * * * from the report to the board of visitors it appears that application had been made for an extension of the grounds of the observatory to a distance of feet south of the magnetic ground, and that a warrant for the annexation of this space was signed on , dec. . the new depôt for the printed productions of the observatory had been transferred to its position in the new ground, and the foundations for the great shed were completed.--"the courses of our wires for the registration of spontaneous terrestrial galvanic currents have been entirely changed. the lines to croydon and deptford are abandoned; and for these are substituted, a line from angerstein wharf to lady well station, and a line from north kent junction to morden college tunnel. at each of these points the communication with earth is made by a copper plate feet square. the straight line connecting the extreme points of the first station intersects that connecting the two points of the second station, nearly at right angles, and at little distance from the observatory.--the question of dependence of the measurable amount of sidereal aberration upon the thickness of glass or other transparent material in the telescope (a question which involves, theoretically, one of the most delicate points in the undulatory theory of light) has lately been agitated on the continent with much earnestness. i have calculated the curvatures of the lenses of crown and flint glass (the flint being exterior) for correcting spherical and chromatic aberration in a telescope whose tube is filled with water, and have instructed mr simms to proceed with the preparation of an instrument carrying such a telescope. i have not finally decided whether to rely on zenith-distances of gamma draconis or on right-ascensions of polaris. in any form the experiment will probably be troublesome.--the transit of mercury on , nov. th, was observed by six observers. the atmospheric conditions were favourable; and the singular appearances usually presented in a planetary transit were well seen.--mr stone has attached to the south-east equatoreal a thermo-multiplier, with the view of examining whether heat radiating from the principal stars can be made sensible in our instruments. the results hitherto obtained are encouraging, but they shew clearly that it is vain to attempt this enquiry except in the most superb weather; and there has not been a night deserving that epithet for some months past.--the preparations for observing the transits of venus were now begun in earnest. i had come to the conclusion, that after every reliance was placed on foreign and colonial observatories, it would be necessary for the british government to undertake the equipment of five or six temporary stations. on feb. th i sent a pamphlet on the subject to mr childers (first lord of admiralty), and in april i wrote to the secretary, asking authority for the purchase of instruments. on june nd authority is given to me for the instruments: the treasury assent to _£ , _. on august th i had purchased equatoreals.--i have given a short course of lectures in the university of cambridge on the subject of magnetism, with the view of introducing that important physical science into the studies of the university. the want of books available to students, and the novelty of the subject, made the preparation more laborious than the duration of the lectures would seem to imply."--in this year there was much work on the standards commission, chiefly regarding the suggested abolition of troy weight, and several papers on the subject were prepared by airy.--he also wrote a long and careful description of the great equatoreal at greenwich. of private history: there was the usual visit to playford in the winter. mrs airy was now becoming feebler, and did not now leave greenwich: since april of this year her letters were written in pencil, and with difficulty, but she still made great efforts to keep up the accustomed correspondence.--in april airy went to cambridge to deliver his lectures on magnetism to the undergraduates: the following passage occurs in one of his letters at this time: "i have a mighty attendance (there were names on my board yesterday), and, though the room is large with plenty of benches, i have been obliged to bring in some chairs. the men are exceedingly attentive, and when i look up i am quite struck to see the number of faces staring into mine. i go at , and find men at the room copying from my big papers: i lecture from to , and stop till after , and through the last hour some men are talking to me and others are copying from the papers; and i usually leave some men still at work. the men applaud and shew their respect very gracefully. there are present some two or three persons who attended my former lectures, and they say that i lecture exactly as i did formerly. one of my attendants is a man that they say cannot, from years and infirmity and habit, be induced to go anywhere else: dr archdall, the master of emmanuel. i find that some of my old lecturing habits come again on me. i drink a great deal of cold water, and am very glad to go to bed early."--from june th- th he was travelling in scotland, and staying at barrow house near keswick (the residence of mr langton), with his son hubert.--subsequently, from aug. th to st, he was again in the lake district, with his daughter christabel, and was joined there by his son hubert on the th. the first part of the time was spent at tarn bank, near carlisle, the residence of mr isaac fletcher, m.p. from thence he made several expeditions, especially to barrow in furness and seascale, where he witnessed with great interest the bessemer process of making steel. from barrow house he made continual excursions among the cumberland mountains, which he knew so well. "in this year mr stone, the first assistant, was appointed to the cape of good hope observatory, and resigned his post of first assistant. mr christie was appointed in his place.--from the report to the visitors it appears that 'a few months since we were annoyed by a failure in the illumination of the field of view of the transit circle. the reflector was cleaned, but in vain; at last it was discovered that one of the lenses (the convex lens) of the combination which forms the object-glass of a reversed telescope in the interior of the transit-axis, and through which all illuminating light must pass, had become so corroded as to be almost opaque.'--the south-east equatoreal has been partly occupied with the thermo-multiplier employed by mr stone for the measure of heat radiating from the principal stars. mr stone's results for the radiation from arcturus and alpha lyrae appear to be incontrovertible, and to give bases for distinct numerical estimation of the radiant heat of these stars.--in my last report i alluded to a proposed systematic reduction of the meteorological observations during the whole time of their efficient self-registration. having received from the admiralty the funds necessary for immediate operations, i have commenced with the photographic registers of the thermometers, dry-bulb and wet-bulb, from to .--our chronometer-room contains at present chronometers, including chronometers which have been placed here by chronometer-makers as competing for the honorary reputation and the pecuniary advantages to be derived from success in the half-year's trial to which they are subjected. i take this opportunity of stating that i have uniformly advocated the policy of offering good prices for the chronometers of great excellence, and that i have given much attention to the decision on their merits; and i am convinced that this system has greatly contributed to the remarkably steady improvement in the performance of chronometers. in the trial which terminated in august , the best chronometers (taking as usual the average of the first six) were superior in merit to those of any preceding year.--with the funds placed at my disposal for the transit of venus i purchased three -inch equatoreals, and have ordered two: i have also ordered altazimuths (with accurate vertical circles only), and clocks sufficient, as i expect, to equip five stations. for methods of observation, i rely generally on the simple eye-observation, possibly relieved of some of its uncertainty by the use of my colour-correcting eyepiece. but active discussion has taken place on the feasibility of using photographic and spectroscopic methods; and it will not be easy for some time to announce that the plan of observations is settled.--there can be no doubt, i imagine, that the first and necessary duty of the royal observatory is to maintain its place well as an observing establishment; and that this must be secured, at whatever sacrifice, if necessary, of other pursuits. still the question has not unfrequently presented itself to me, whether the duties to which i allude have not, by force of circumstances, become too exclusive; and whether the cause of science might not gain if, as in the imperial observatory of paris for instance, the higher branches of mathematical physics should not take their place by the side of observatory routine. i have often felt the desire practically to refresh my acquaintance with what were once favourite subjects: lunar theory and physical optics. but i do not at present clearly see how i can enter upon them with that degree of freedom of thought which is necessary for success in abstruse investigations." of private history: there was a longer visit than usual to playford, lasting till jan. th.--in april he made a short excursion (of less than a week) with his son hubert to monmouth, &c.--from june th to july nd he was staying at barrow house, near keswick, with his son hubert: during this time he was much troubled with a painful skin-irritation of his leg and back, which lasted in some degree for a long time afterwards.--from sept. th to oct. th he made an excursion with his daughter christabel to scarborough, whitby, &c., and again spent a few days at barrow house. "in april the assistants had applied for an increase of salary, a request which i had urged strongly upon the admiralty. on jan. of this year the admiralty answered that, on account of mr childers's illness, the consideration must be deferred to next year! the assistants wrote bitterly to me: and with my sanction they wrote to the first lord. on jan. st i requested an interview with mr baxter (secretary of the admiralty), and saw him on feb. rd, when i obtained his consent to an addition of _£ _. there was still a difficulty with the treasury, but on june th the liberal scale was allowed.--experiments made by mr stone shew clearly that a local elevation, like that of the royal observatory on the hill of greenwich park, has no tendency to diminish the effect of railway tremors.--the correction for level error in the transit circle having become inconveniently large, a sheet of very thin paper, / inch in thickness, was placed under the eastern y, which was raised from its bed for the purpose. the mean annual value of the level-error appears to be now sensibly zero.--as the siege and war operations in paris seriously interfered with the observations of small planets made at the paris observatory, observations of them were continued at greenwich throughout each entire lunation during the investment of the city.--the new water-telescope has been got into working order, and performs most satisfactorily. observations of gamma draconis have been made with it, when the star passed between h and h, with some observations for adjustment at a still more advanced time. as the astronomical latitude of the place of observation is not known, the bearing of these observations on the question of aberration cannot be certainly pronounced until the autumn observations shall have been made; but supposing the geodetic latitude to be accordant with the astronomical latitude, the result for aberration appears to be sensibly the same as with ordinary telescopes.--several years since, i prepared a barometer, by which the barometric fluctuations were enlarged, for the information of the public; its indications are exhibited on the wall, near to the entrance gate of the observatory. a card is now also exhibited, in a glass case near the public barometer, giving the highest and lowest readings of the thermometer in the preceding twenty-four hours.--those who have given attention to the history of terrestrial magnetism are aware that halley's magnetic chart is very frequently cited; but i could not learn that any person, at least in modern times, had seen it. at last i discovered a copy in the library of the british museum, and have been allowed to take copies by photolithography. these are appended to the magnetical and meteorological volume for .--the trials and certificates of hand-telescopes for the use of the royal navy have lately been so frequent that they almost become a regular part of the work of the observatory. i may state here that by availing myself of a theory of eyepieces which i published long since in the cambridge transactions, i have been able to effect a considerable improvement in the telescopes furnished to the admiralty.--the occurrence of the total eclipse of the sun in december last has brought much labour upon the observatory. as regards the assistants and computers, the actual observation on a complicated plan with the great equatoreal (a plan for which few equatoreals are sufficiently steady, but which when properly carried out gives a most complete solution of the geometrical problem) has required, in observation and in computation, a large expenditure of time.--my preparations for the transit of venus have respect only to eye-observation of contact of limbs. with all the liabilities and defects to which it is subject, this method possesses the inestimable advantage of placing no reliance on instrumental scales. i hope that the error of observation may not exceed four seconds of time, corresponding to about . " of arc. i shall be very glad to see, in a detailed form, a plan for making the proper measures by heliometric or photographic apparatus; and should take great interest in combining these with the eye-observations, if my selected stations can be made available. but my present impression is one of doubt on the certainty of equality of parts in the scale employed. an error depending on this cause could not be diminished by any repetition of observations."--after referring to the desirability of vigorously prosecuting the meteorological reductions (already begun) and of discussing the magnetic observations, the report concludes thus: "there is another consideration which very often presents itself to my mind; the waste of labour in the repetition of observations at different observatories..... i think that this consideration ought not to be put out of sight in planning the courses of different observatories."--in this year de launay's lunar theory was published. this valuable work was of great service to airy in the preparation of the numerical lunar theory, which he subsequently undertook.--in the latter part of this year airy was elected president of the royal society, and held the office during and . at this time he was much pressed with work, and could ill afford to take up additional duties, as the following quotation from a letter to one of his friends shews: "the election to the presidency of r.s. is flattering, and has brought to me the friendly remembrances of many persons; but in its material and laborious connections, i could well have dispensed with it, and should have done so but for the respectful way in which it was pressed on me." of private history: there was the usual winter visit to playford.--in april he made a short trip to cornwall with his daughter annot.--in june he was appointed a companion of the bath, and was presented at court on his appointment.--mrs airy was staying with her daughter, mrs routh, at hunstanton, during june, her state of health being somewhat improved.--from august st to th he was chiefly in cumberland, at barrow house, and at grange, borrowdale, where his son osmund was staying for a holiday. "from the report to the board of visitors it appears that 'the normal siderial clock for giving sidereal time by galvanic communication to the astronomical observatory was established in the magnetic basement in , june; that locality being adapted for it on account of the uniformity of temperature, the daily changed of temperature rarely exceeding ° fahrenheit. its escapement is one which i suggested many years ago in the cambridge transactions; a detached escapement, very closely analogous to the ordinary chronometer escapement, the pendulum receiving an impulse only at alternate vibrations.... the steadiness of rate is very far superior to any that we have previously attained.'--the aspect of railway enterprise is at present favourable to the park and to the observatory. the south-eastern railway company has made an arrangement with the metropolitan board of works for shifting the course of the great southern outfall sewer. this enables the company to trace a new line for the railway, passing on the north side of london street, at such a distance from the observatory as to remove all cause of alarm. i understand that the bill, which was unopposed, has passed the committee of the house of commons. i trust that the contest, which has lasted thirty-seven years, is now terminated.--the observations of draconis with the water-telescope, made in the autumn of , and the spring of , are reduced, the latter only in their first steps.... using the values of the level scales as determined by mr simms (which i have no reason to believe to be inaccurate) the spring and autumn observations of absolutely negative the idea of any effect being produced on the constant of aberration by the amount of refracting medium traversed by the light.--the great aurora of feb. was well observed. on this occasion the term borealis would have been a misnomer, for the phenomenon began in the south and was most conspicuous in the south. three times in the evening it exhibited that umbrella-like appearance which has been called (perhaps inaccurately) a corona. i have very carefully compared its momentary phenomena with the corresponding movements of the magnetometers. in some of the most critical times, the comparison fails on account of the violent movements and consequent faint traces of the magnetometers. i have not been able to connect the phases of aurora and those of magnetic disturbance very distinctly.--the report contains a detailed account of the heavy preparations for the observation of the transit of venus , including the portable buildings for the instruments, the instruments themselves (being a transit-instrument, an altazimuth, and an equatoreal, for each station), and first class and second-class clocks, all sufficient for the equipment of stations, and continues thus: i was made aware of the assent of the government to the wish of the board of visitors, as expressed at their last meeting, that provision should be made for the application of photography to the observation of the transit of venus. it is unnecessary for me to remark that our hope of success is founded entirely on our confidence in mr de la rue. under his direction, mr dallmeyer has advanced far in the preparation of five photoheliographs.... the subject is recognized by many astronomers as not wholly free from difficulties, but it is generally believed that these difficulties may be overcome, and mr de la rue is giving careful attention to the most important of them.--i take this opportunity of reporting to the board that the observatory was honoured by a visit of his majesty the emperor of brazil, who minutely examined every part."--after referring to various subjects which in his opinion might be usefully pursued systematically at the observatory, the report proceeds thus: "'the character of the observatory would be somewhat changed by this innovation, but not, as i imagine, in a direction to which any objection can be made. it would become, pro tanto, a physical observatory; and possibly in time its operations might be extended still further in a physical direction.'--the consideration of possible changes in the future of the observatory leads me to the recollection of actual changes in the past. in my annual reports to the visitors i have endeavoured to chronicle these; but still there will be many circumstances which at present are known only to myself, but which ought not to be beyond the reach of history. i have therefore lately employed some time in drawing up a series of skeleton annals of the observatory (which unavoidably partakes in some measure of the form of biography), and have carried it through the critical period, - . if i should command sufficient leisure to bring it down to , i think that i might then very well stop." (the skeleton annals here referred to are undoubtedly the manuscript notes which form the basis of the present biography. ed.)--"on feb. rd in this year i first (privately) formed the notion of preparing a numerical lunar theory by substituting delaunay's numbers in the proper equations and seeing what would come of it." of private history: there was the usual visit to playford--in this year later than usual--from feb. th to mar. th. the letters written during this visit are, as usual, full of freshness and delight at finding himself in his favourite country village.--on june th he went to barrow house, near keswick, to be present at the marriage of his second son hubert to miss s. c. langton, daughter of z. langton esq., of barrow house.--after the wedding he made a trip through the trossachs district of scotland with his daughter annot, and returned to greenwich on june th. on the th june airy was appointed a knight commander of the most honourable order of the bath: he was knighted by the queen at osborne on the th of july. in the course of his official career he had three times been offered knighthood, and had each time declined it: but it seemed now as if his scruples on the subject were removed, and it is probable that he felt gratified by the public recognition of his services. of course the occasion produced many letters of congratulation from his friends: to one of these he replied as follows: "the real charm of these public compliments seems to be, that they excite the sympathies and elicit the kind expressions of private friends or of official superiors as well as subordinates. in every way i have derived pleasure from these." from the assistants of the royal observatory he received a hearty letter of congratulation containing the following paragraph. "our position has naturally given us peculiar opportunities for perceiving the high and broad purposes which have characterized your many and great undertakings, and of witnessing the untiring zeal and self-denial with which they have been pursued." * * * * * on the th of march airy was nominated a foreign associate of the institut de france, to fill the place vacant by the death of sir john herschel. the following letter of acknowledgment shews how much he was gratified by this high scientific honour: royal observatory, greenwich, _ , march _. _Ã�_ messieurs messieurs elie de beaumont, _et_ j.b. dumas, _secrétaires perpetuels de l'académie des sciences, institut de france._ gentlemen, i am honoured with your letter of march , communicating to me my nomination by the academy of sciences to the place rendered vacant in the class of foreign associates of the academy by the decease of sir john herschel, and enclosing copy of the decree of the president of the french republic approving the election. it is almost unnecessary for me to attempt to express to you the pride and gratification with which i receive this announcement. by universal consent, the title of _associé etranger de l'académie des sciences_ is recognised as the highest distinction to which any man of science can aspire; and i can scarcely imagine that, unless by the flattering interpretation of my friends in the academy, i am entitled to bear it. but in any case, i am delighted to feel that the bands of friendship are drawn closer between myself and the distinguished body whom, partly by personal intercourse, partly by correspondence, and in every instance by reputation, i have known so long. i beg that you will convey to the academy my long-felt esteem for that body in its scientific capacity, and my deep recognition of its friendship to me and of the honor which it has conferred on me in the late election. i have the honor to be gentlemen, your very faithful servant, g.b. airy. * * * * * on the th november airy was nominated a grand cross in the imperial order of the rose of brazil: the insignia of the order were accompanied by an autograph letter from the emperor of brazil, of which the following is a transcript. monsieur, vous êtes un des doyens de la science, et le président de l'illustre société, qui a eu la bienveillance d'inscrire mon nom parmi ceux de ses associés. la manière, dont vous m'avez fait les honneurs de votre observatoire m'a imposé aussi l'agréable devoir d'indiquer votre nom à l'empereur de brésil pour un témoignage de haute estime, dont je suis fort heureux de vous faire part personellement, en vous envoyant les décorations que vous garderez, an moins, comme un souvenir de ma visite à greenwich. j'espère que vous m'informerez, quand il vous sera aisé, des travaux de votre observatoire, et surtout de ce que l'on aura fait pour l'observation du passage de vénus et la détermination exacte de la passage. j'ai reçu déjà les _proceedings de la royal society_ lesquels m'intéressent vivement. je voudrais vous écrire dans votre langue, mais, comme je n'en ai pas l'habitude, j'ai craigné de ne pas vous exprimer tout-à-fait les sentiments de votre affectionné, d. pedro d'alcantara. rio, _ octobre, _. * * * * * airy's reply was as follows: royal observatory, greenwich, _ , november _. sire, i am honoured with your imperial majesty's autograph letter of october informing me that, on considering the attention which the royal society of london had been able to offer to your majesty, as well as the explanation of the various parts of the establishment of this observatory which i had the honor and the high gratification to communicate, you had been pleased to place my name in the imperial order of the rose, and to present to me the decorations of grand cross of that order. with pride i receive this proof of your majesty's recollection of your visit to the scientific institutions of great britain. the diploma of the appointment to the order of the rose, under the imperial sign manual, together with the decorations of the order, have been transmitted to me by his excellency don pereira de andrada, your majesty's representative at the british court. your majesty has been pleased to advert to the approaching transit of venus, on the preparations for which you found me engaged. it is unfortunate that the transit of will not be visible at rio de janeiro. for that of , rio will be a favourable position, and we reckon on the observations to be made there. your majesty may be assured that i shall loyally bear in mind your desire to be informed of any remarkable enterprise of this observatory, or of any principal step in the preparations for the transit of venus and of its results. i have the honor to be sire, your imperial majesty's very faithful servant, g.b. airy. _to his majesty the emperor of brazil._ * * * * * airy's old friend, adam sedgwick, was now very aged and infirm, but his spirit was still vigorous, and he was warm-hearted as ever. the following letter from him (probably the last of their long correspondence) was written in this year, and appears characteristic: trinity college, cambridge, _may , _. my dear airy, i have received your card of invitation for the st of june, and with great joy should i count upon that day if i thought that i should be able to accept your invitation: but alas i have no hope of the kind, for that humiliating malady which now has fastened upon me for a full year and a half has not let go its hold, nor is it likely to do so. a man who is journeying in the th year of his pilgrimage is not likely to throw off such a chronic malady. indeed were i well enough to come i am deaf as a post and half blind, and if i were with you i should only be able to play dummy. several years have passed away since i was last at your visitation and i had great joy in seeing mrs airy and some lady friends at the observatory, but i could not then attend the dinner. at that meeting were many faces that i knew, but strangely altered by the rude handling of old time, and there were many new faces which i had never seen before at a royal society meeting; but worse than all, all the old faces were away. in vain i looked round for wollaston, davy, davies gilbert, barrow, troughton, &c. &c.; and the merry companion admiral smyth was also away, so that my last visit had its sorrowful side. but why should i bother you with these old man's mopings. i send an old man's blessing and an old man's love to all the members of your family; especially to mrs airy, the oldest and dearest of my lady friends. i remain, my dear airy, your true-hearted old friend, his adam x sedgwick. mark p.s. shall i ever again gaze with wonder and delight from the great window of your observatory. the body of the above letter is in the handwriting of an amanuensis, but the signature and postscript are in sedgwick's handwriting. (ed.) * * * * * "chronographic registration having been established at the paris observatory, mr hilgard, principal officer of the american coast survey, has made use of it for determining the longitude of harvard from greenwich, through paris, brest, and st pierre. for this purpose mr hilgard's transit instrument was planted in the magnetic court. i understand that the result does not sensibly differ from that obtained by mr gould, through valentia and newfoundland.--it was known to the scientific world that several of the original thermometers, constructed by mr sheepshanks (in the course of his preparation of the national standard of length) by independent calibration of the bores, and independent determination of the freezing and boiling points on arbitrary graduations, were still preserved at the royal observatory. it was lately stated to me by m. tresca, the principal officer of the international metrical commission, that, in the late unhappy war in paris, the french original thermometers were destroyed; and m. tresca requested that, if possible, some of the original thermometers made by mr sheepshanks might be appropriated to the use of the international commission. i have therefore transferred to m. tresca the three thermometers a. , s. , s. , with the documentary information relating to them, which was found in mr sheepshanks's papers; retaining six thermometers of the same class in the royal observatory.--the sidereal standard clock continues to give great satisfaction. i am considering (with the aid of mr buckney, of the firm of e. dent and co.) an arrangement for barometric correction, founded on the principle of action on the pendulum by means of a magnet which can be raised or lowered by the agency of a large barometer.--the altazimuth has received some important alterations. an examination of the results of observations had made me dissatisfied with the bearings of the horizontal pivots in their y's. mr simms, at my request, changed the bearings in y's for bearing in segments of circles, a construction which has worked admirably well in the pivots of the transit circle." (and in various other respects the instrument appears to have received a thorough overhauling. ed.)--"with the consent of the royal society and of the kew committee, the kew heliograph has been planted in the new dome looking over the south ground. it is not yet finally adjusted.--some magnetic observations in the britannia and conway tubular bridges were made last autumn. for this purpose i detached an assistant (mr carpenter), who was aided by capt. tupman, r.m.a.; in other respects the enterprise was private and at private expense.--the rates of the first six chronometers (in the annual trials) are published, in a form which appears most likely to lead to examination of the causes that influence their merits or demerits. this report is extensively distributed to british and foreign horologists and instrument-makers. all these artists appear to entertain the conviction that the careful comparisons made at this observatory, and the orderly form of their publication, have contributed powerfully to the improvement of chronometers.--very lately, application has been made to me, through the board of trade, for plans and other information regarding time-signal-balls, to assist in guiding the authorities of the german empire in the establishment of time signals at various ports of that state. in other foreign countries the system is extending, and is referred to greenwich as its origin.--the arrangements and preparations for the observation of the transit of venus occupied much attention. with regard to the photoheliographs it is proposed to make trial of a plan proposed by m. janssen, for numerous photographs of venus when very near to the sun's limb. on apr. th the engaging of photographic teachers was sanctioned. observers were selected and engaged. a working model of the transit was prepared, and the use of de la rue's scale was practised. there was some hostile criticism of the stations selected for the observation of the transit, which necessitated a formal reply.--reference is made to the increase of facilities for making magnetical and meteorological observations. the inevitable result of it is, that observations are produced in numbers so great that complete reduction becomes almost impossible. the labour of reduction is very great, and it is concluded that, of the enormous number of meteorological observations now made at numerous observatories, very few can ever possess the smallest utility.--referring to my numerical lunar theory: on june th, , a theory was formed, nearly but not perfectly complete. numerical development of powers of a÷r and r÷a. factors of corrections to delaunay first attempted, but entirely in numerical form."--in march of this year airy was consulted by mr w.h. barlow, c.e., and mr thomas bouch (the engineer of the tay bridge, which was blown down in , and of a proposed scheme for a forth bridge in ) on the subject of the wind pressure, &c., that should be allowed for in the construction of the bridge. airy's report on this question is dated , apr. th: it was subsequently much referred to at the official enquiry into the causes of the failure of the tay bridge.--at the end of this year airy resigned the presidency of the royal society. in his address to the society on dec. st he stated his reasons in full, as follows: "the severity of official duties, which seem to increase, while vigour to discharge them does not increase; and the distance of my residence.... another cause is a difficulty of hearing, which unfits me for effective action as chairman of council." of private history: there was the usual visit to playford in january: also a short visit in may: and a third visit at christmas.--there was a short run in june, of about a week, to coniston, with one of his daughters.--and there was a trip to weymouth, &c., for about days, with one of his daughters, in the beginning of august--on his return from the last-mentioned trip, airy found a letter from the secretary of the swedish legation, enclosing the warrant under the royal sign manual of his majesty (oscar), the king of sweden and norway, by which he was nominated as a first class commander of the order of the north star, and accompanying the decorations of that order. "in this year mr glaisher resigned his appointment: i placed his department (magnetical and meteorological) under mr ellis.--a balance of peculiar construction has been made by mr oertling, from my instructions, and fixed near the public barometer at the entrance gate. this instrument enables the public to test any ordinary pound weight, shewing on a scale the number of grains by which it is too heavy or too light.--fresh counterpoises have been attached to the great equatoreal to balance the additional weight of the new spectroscope, which was finally received from mr browning's hands on may nd of the present year. the spectroscope is specifically adapted to sweeping round the sun's limb, with a view to mapping out the prominences, and is also available for work on stars and nebulae, the dispersive power being very readily varied. an induction-coil, capable of giving a six-inch spark, has been made for this instrument by mr browning.--some new classes of reductions of the meteorological observations from to have been undertaken and completed in the past year. the general state of this work is as follows: the diurnal changes of the dry-bulb thermometer, as depending on the month, on the temperature waves, on the barometric waves, on the overcast and cloudless states of the sky, and on the direction of the wind, have been computed and examined for the whole period; and the exhibition of the results is ready for press. the similar reductions for the wet-bulb thermometer are rapidly approaching completion. --regarding the preparations for the transit of venus expeditions. originally five stations were selected and fully equipped with equatoreals, transits, altazimuths, photoheliographs, and clocks; but i have since thought it desirable to supplement these by two branch stations in the sandwich islands and one in kerguelen's island; and the additional instruments thus required have been borrowed from various sources, so that there is now an abundant supply of instrumental means.... there will thus be available for observation of the transit of venus telescopes, nine of which will be provided with double-image-micrometers; and five photoheliographs; and for determination of local time, and latitude and longitude, there will be nine transits and six altazimuths.... all the observers have undergone a course of training in photography; first, under a professional photographer, mr reynolds, and subsequently under capt. abney, r.e., whose new dry-plate process is to be adopted at all the british stations.... a janssen slide, capable of taking photographs of venus and the neighbouring part of the sun's limb at intervals of one second, has been made by mr dallmeyer for each of the five photoheliographs."--attached to the report to the visitors is a copy of the instructions to observers engaged in the transit of venus expeditions, prepared with great care and in remarkable detail.--"in the past spring i published in the monthly notices of the royal astronomical society a statement of the fundamental points in a new treatment of the lunar theory, by which, availing myself of all that has been done in the best algebraical investigations of that theory, i trust to be able by numerical operations only to give greater accuracy to final results. considerable progress has been made in the extensive numerical developments, the work being done, at my private expense, entirely by a junior computer; and i hope, at any rate, to put it in such a state that there will be no liability to its entire loss. when this was reported to the board of visitors, it was resolved on the motion of prof. stokes, that this work, as a public expense, ought to be borne by the government; and this was forwarded to the admiralty. on june th i wrote to the secretary of the admiralty, asking for _£ _ for the present year, which after the usual enquiries and explanations was sanctioned on aug. th." of private history: there were short visits to playford in january, june, and october, but only for a few days in each case.--in march there was a run of two or three days to newnham (on the severn) to see the bore on the severn, and to malvern.--in july he went to newcastle to observe with mr newall's great telescope, but the weather was unfavourable: he then went on to barrow house near keswick, and spent a few days there, with excursions among the mountains.--on aug. th he went with his daughter christabel to the isle of arran, and then by glasgow to the trosachs, where he made several excursions to verify the localities mentioned in the "lady of the lake."--while in scotland he heard of the death of his brother, the rev. william airy, and travelled to keysoe in bedfordshire to attend the funeral; and returned to greenwich on aug. th. "in october of this year i wrote to the admiralty that i had grounds for asking for an increase of my salary: because the pension which had been settled on my wife, and which i had practically recognized as part of my salary, had been terminated by her death; so that my salary now stood lower by _£ _ than that of the director of studies of the royal naval college. the admiralty reply favourably, and on nov. th the treasury raise my salary to _£ _, .--for the service of the clock movement of the great equatoreal, a water-cistern has been established in the highest part of the ball-turret, the necessity for which arose from the following circumstance: the water clock was supplied by a small pipe, about feet in length, connected with the -inch observatory main (which passes through the park), at a distance of about feet from any other branch pipe. in spite of this distance i have seen that, on stopping the water-tap in the battery-basement under the north-east turret, the pressure in the gauge of the water clock has been instantly increased by more than lbs. per square inch. the consequent derangement of the water clock in its now incessant daily use became intolerable. since the independent supply was provided, its performance has been most satisfactory.--with the spectroscope the solar prominences have been mapped on days only; but the weather of the past winter was exceptionally unfavourable for this class of observation. after mapping the prominences, as seen on the c line, the other lines, especially f and b, have been regularly examined, whenever practicable. great care has been taken in determining the position, angle, and heights of the prominences in all cases. the spectrum of coggia's comet was examined at every available opportunity last july, and compared directly with that of carbon dioxide, the bands of the two spectra being sensibly coincident. fifty-four measures of the displacement of lines in the spectra of stars, as compared with the corresponding lines in the spectra of terrestrial elements (chiefly hydrogen), have been made, but some of these appear to be affected by a constant error depending on faulty adjustment of the spectroscope.--photographs of the sun have been taken with the kew photoheliograph on days; and of these have been selected for preservation. the moon, jupiter, saturn, and several stars (including the pleiades and some double stars) have been photographed with the great equatoreal, with fairly satisfactory results, though further practice is required in this class of work.--i would mention a supplemental mechanism which i have myself introduced into some chronometers. i have long remarked that, in ordinary good chronometers, the freedom from irregularities depending on mechanical causes is most remarkable; but that, after all the efforts of the most judicious makers, there is in nearly every case a perceptible defect of thermal compensation. there is great difficulty in correcting the residual fault, not only because an inconceivably small movement of the weights on the balance-curve is required, but also because it endangers the equilibrium of the balance. the mechanism adopted to remedy the defect is described in a paper in the horological journal of july by mr w. ellis, and has received the approval of some able chronometer-makers.--with respect to the transit of venus expeditions: the parties from egypt and rodriguez are returned. i am in continual expectation of the arrival of the other parties. i believe the eye-observations and the ordinary photographs to be quite successful; i doubt the advantage of the janssen; one of the double-image-micrometers seems to have failed; and the zenith-telescope gives some trouble. at three stations at rodriguez, and three at kerguelen, the observations appear to have been most successful. at the sandwich islands, two of the stations appear to have been perfectly successful (except that i fear that the janssen has failed), and a rich series of lunar observations for longitude is obtained. at new zealand, i grieve to say, the observations were totally lost, entirely in consequence of bad weather. there has been little annoyance from the dreaded 'black drop.' greater inconvenience and doubt have been caused by the unexpected luminous ring round venus.--with regard to the progress of my proposed new lunar theory: three computers are now steadily employed on the work. it will be remembered that the detail and mass of this work are purely numerical; every numerical coefficient being accompanied with a symbolical correction whose value will sometimes depend on the time, but in every case is ultimately to be obtained in a numerical form. of these coefficients, extracted (for convenience) from delaunay's results, there are for parallax, for longitude, for latitude; the arguments being preserved in the usual form."--after reviewing the changes that had taken place at the observatory during the past forty years, the report to the board of visitors concludes thus: "i much desire to see the system of time-signals extended, by clocks or daily signals, to various parts of our great cities and our dockyards, and above all by hourly signals on the start point, which i believe would be the greatest of all benefits to nautical chronometry. should any extension of our scientific work ever be contemplated, i would remark that the observatory is not the place for new physical investigations. it is well adapted for following out any which, originating with private investigators, have been reduced to laws susceptible of verification by daily observation. the national observatory will, i trust, always remain on the site where it was first planted, and which early acquired the name of 'flamsteed hill.' there are some inconveniences in the position, arising principally from the limited extent of the hill, but they are, in my opinion, very far overbalanced by its advantages."--in a letter on the subject of the smith's prizes examination at cambridge, which was always a matter of the greatest interest to him, airy renewed his objections to the preponderance in the papers of a class of pure mathematics, which he considered was never likely under any circumstances to give the slightest assistance to physics. and, as before, these remarks called forth a rejoinder from prof. cayley, who was responsible for many of the questions of the class referred to.--in this year airy completed his "notes on the earlier hebrew scriptures," which were shortly afterwards published as a book by messrs longmans, green, & co. in his letter to the publishers introducing the subject, he says, "for many years past i have at times put together a few sentences explanatory as i conceive of the geographical and historical circumstances connected with the principal events recorded in the hebrew scriptures. the view which i take is free, but i trust not irreverent. they terminate with a brief review of colenso's great work. the collection now amounts to a small book." from the references already given in previous years to his papers and correspondence on the geography of exodus, his correspondence with colenso, &c. &c., it will be seen that he took a great interest in the early history of the israelites.--on august th, , airy celebrated the bicentenary of the royal observatory by a dinner in the octagon room, which was attended by the presidents of the royal society and the r. astr. society, and by a large number of scientific gentlemen interested in astronomy.--in february he was revising his treatise on "probabilities." of private history: up to jan. th airy was at playford as usual.--for about a week in april he was in the isle of man with his daughter christabel.--in june there was a short trip to salisbury, blandford, and wimborne.--on august th he started with his daughter annot for a holiday in cumberland, but on the next day he was recalled by a telegram with the intelligence that a change for the worse had come over his wife's health. lady airy died on august th, . for the last five years of her life she had been very helpless from the effects of a paralytic stroke--a very sad ending to a bright and happy life--and had been continually nursed throughout this time by her two unmarried daughters with the greatest self-denial and devotion. her husband had been unremitting in his care and attention. nothing was wanting that the most thoughtful kindness could supply. and in all his trips and excursions his constant and kind letters shewed how anxious he was that she should participate in all his interests and amusements. from the nature of the case it could hardly be said that her death was unexpected, and he received the shock with the manly steadiness which belonged to him. lady airy was buried in playford churchyard.--from sept. nd to oct. he made a short expedition to wales (capel curig, &c.).--on dec. th he attended the commemoration at trinity college, cambridge.--on dec. nd he went as usual to playford. in this year airy received the high honour of the freedom of the city of london, in the following communication: stone, mayor.--a common council holden in the chamber of the guildhall of the city of london, on thursday the th day of april . resolved unanimously that the freedom of this city in a gold box of the value of one hundred guineas be presented to sir george biddell airy, k.c.b., d.c.l., ll.d. &c., astronomer royal, as a recognition of his indefatigable labours in astronomy, and of his eminent services in the advancement of practical science, whereby he has so materially benefited the cause of commerce and civilization. monckton. this resolution was forwarded with a letter from benjamin scott, the chamberlain. airy's reply was as follows: royal observatory, greenwich, s.e. _ , may _. dear sir, i have the honour to acknowledge your letter of april , accompanied with copy of the resolution of the common council of the city of london passed at their meeting of april , under signature of the town clerk, that the freedom of the city of london in a valuable box be presented to me, in recognition of works stated in the resolution. and i am requested by you to inform you whether it is my intention to accept the compliment proposed by the corporation. in reply, i beg you to convey to the right honorable the lord mayor and the corporation that i accept with the greatest pride and pleasure the honour which they propose to offer to me. the freedom of our great city, conferred by the spontaneous act of its municipal governors, is in my estimation the highest honour which it is possible to receive; and its presentation at this time is peculiarly grateful to me. i have the honour to be, sir, your very obedient servant, g.b. airy. _benjamin scott, esq., &c. &c. &c. chamberlain of the corporation of the city of london._ as it was technically necessary that a freeman of the city of london should belong to one or other of the city companies, the worshipful company of spectacle makers through their clerk (with very great appropriateness) enquired whether it would be agreeable that that company should have the privilege of conferring their honorary freedom on him, and added: "in soliciting your acquiescence to the proposal i am directed to call attention to the fact that this guild is permitted to claim all manufacturers of mathematical and astronomical instruments within the city of london, which is now pleaded as an apology for the wish that one so distinguished as yourself in the use of such instruments should be enrolled as a member of this craft." in his reply, accepting the freedom of the company, airy wrote thus: "i shall much value the association with a body whose ostensible title bears so close a relation to the official engagements which have long occupied me. i have had extensive experience both in arranging and in using optical and mathematical instruments, and feel that my own pursuits are closely connected with the original employments of the company." the freedom of the company was duly presented, and the occasion was celebrated by a banquet at the albion tavern on tuesday, july th. the freedom of the city of london was conferred at a court of common council held at the guildhall on thursday the th of november. in presenting the gold box containing the freedom, the chamberlain, in an eloquent speech, first referred to the fact that this was the first occasion on which the freedom had been conferred on a person whose name was associated with the sciences other than those of war and statecraft. he then referred to the solid character of his work, in that, while others had turned their attention to the more attractive fields of exploration, the discovery of new worlds or of novel celestial phenomena, he had incessantly devoted himself to the less interesting, less obtrusive, but more valuable walks of practical astronomy. and he instanced as the special grounds of the honour conferred, the compilation of nautical tables of extraordinary accuracy, the improvement of chronometers, the correction of the compasses of iron ships, the restoration of the standards of length and weight, and the transit of venus expeditions. in his reply airy stated that he regarded the honour just conferred upon him as the greatest and proudest ever received by him. he referred to the fact that the same honour had been previously conferred on the valued friend of his youth, thomas clarkson, and said that the circumstance of his succeeding such a man was to himself a great honour and pleasure. he alluded to his having received a small exhibition from one of the london companies, when he was a poor undergraduate at cambridge, and acknowledged the great assistance that it had been to him. with regard to his occupation, he said that he had followed it in a great measure because of its practical use, and thought it fortunate that from the first he was connected with an institution in which utility was combined with science. the occasion of this presentation was celebrated by a banquet at the mansion house on saturday july rd, , to sir george airy (astronomer royal) and the representatives of learned societies. there is no doubt that airy was extremely gratified by the honour that he had received. it was to him the crowning honour of his life, and coming last of all it threw all his other honours into the shade. to his independent and liberal spirit there was something peculiarly touching in the unsolicited approbation and act of so powerful and disinterested a body as the corporation of the city of london. chapter ix. at greenwich observatory from january st, , to his resignation of office on august th, . "at the door from the front court to the staircase of the octagon room (the original entrance to the observatory as erected by sir christopher wren), a small porch-shelter has been often desired. i proposed to fix there a fan-roof of quadrantal form, covering the upper flat stone of the external steps.--on a critical examination of the micrometer-screws of the transit circle it was found that the corrections, which range from - ° " to + ° ", indicate considerable wear in the screws; and it was found that as much as one-hundreth part of an inch had been worn away from some of the threads. the old screws were consequently discarded, and new ones were made by mr simms.--the adjustment of the spectroscope has occupied a great deal of attention. there was astigmatism of the prisms; and false light reflected from the base of the prisms, causing loss both of light and of definition. the latter defect was corrected by altering the angles, and then astigmatism was corrected by a cylindrical lens near the slit. the definition in both planes was then found to be perfect.--the number of small planets has now become so great, and the interest of establishing the elements of all their orbits so small,--while at the same time the light of all those lately discovered is very faint, and the difficulty and doubt of observation greatly increased,--that i have begun to think seriously of limiting future observations to a small number of these objects.--all observations with the spectroscope have been completely reduced; the measures of lines in the spectra of elements being converted into corresponding wave-lengths, and the observations of displacement of lines in the spectra of stars being reduced so as to exhibit the concluded motion in miles per second, after applying a correction for the earth's motion. sixteen measures of the f line in the spectrum of the moon as compared with hydrogen give a displacement corresponding to a motion of less than two miles a second, which seems to shew that the method of comparison now adopted is free from systematic error; and this is supported by the manner in which motions of approach and recession are distributed among the stars examined on each night of observation. the results recently obtained appear to be on the whole as consistent as can be expected in such delicate observations, and they support in a remarkable manner the conclusions of dr huggins, with regard to the motions of those stars which he examined.--photographs of the sun have been taken with the photoheliograph on days. on one of the photographs, which was accidentally exposed while the drop slit was being drawn up, there appears to be a faint image of a cloud-like prominence close to the sun's limb, though the exposure probably only amounted to a fraction of a second. a prominence of unusual brilliancy was seen with the spectroscope about the same time and in the same position with reference to the sun's limb. all groups of sun-spots and faculae have been numbered, and the dates of their first and last appearances entered up to the present time. areas of spots have been measured, and the measures have been reduced to millionths of the sun's visible hemisphere.--the examination of the readings of the deep-sunk thermometers from to has exhibited some laws which had been sufficiently established before this time, and some which were less known. among the former were the successive retardations of seasons in successive descents, amounting to about four months at the depth of feet; and the successive diminutions of the annual range of temperature. among the latter is the character of the changes from year to year, which the great length of this series of observations brings well to light. it is found that from year to year the mean temperature of the surface for the year, varying by three or four degrees of fahrenheit, follows in its changes the mean temperature of the atmosphere for the year, and that the changes of annual temperature are propagated downwards, retarded in phase and diminishing in amount of change, in the same manner (though probably not following the same law) as the season changes. the inference from this is, that changes of temperature come entirely from the exterior and in no discoverable degree from the interior; an inference which may be important in regard both to solar action and to geology. --referring to the transit of venus observations: in the astronomical part of the reductions, there has been great labour and difficulty in the determination of local sidereal times; some books of observations required extensive transcription; some instrumental errors are still uncertain; the latter determinations have perplexed us so much that we are inclined to believe that, in spite of the great facilities of reduction given by the transit instrument, it would be better to rely on the altazimuth for time-determinations.... in the photographic part, i have confined my attention entirely to measures of the distance between the centres of the sun and planet, a troublesome and complex operation.--referring to the progress of the numerical lunar theory: with a repetition of grant from the treasury, i have usually maintained four junior computers on this work. the progress, though considerable, has not been so great as i had hoped, by reason of the excessive personal pressure upon me during the whole year.--i wrote a letter of congratulation to le verrier on the completion of his great work of planetary tables.--on may th the queen was at south kensington, and i attended to explain the astronomical instruments, and shewed her majesty one of the transit of venus photographs." of private history: he returned from his playford visit on the th of january.--in april there was a two-day trip to colchester.--from june th to july th he was travelling in the north of scotland and the orkneys with his daughters, staying for a short time with mr webster, m.p., at aberdeen, and with mr newall at newcastle.--in september there was a week's run to birkenhead and keswick.--in november a week's run to playford.--from the th to th of december he was at cambridge, and on the th he went to playford for the usual winter stay there. "in april of this year i was much engaged on the subject of mr gill's expedition to ascension to observe for the determination of the parallax of mars at the approaching opposition of that planet.--a large direct-vision spectroscope has been quite recently made by mr hilger under mr christie's direction on a new plan, in which either great dispersion or great purity of spectrum is obtained by the use of 'half-prisms,' according as the incident pencil falls first on the perpendicular or on the oblique face. in this spectroscope either one or two half prisms can be used at pleasure, according to the dispersion required, and there is facility for increasing the train to three or four half-prisms, though the dispersion with two only is nearly double of that given by the large ten-prism spectroscope. the definition in this form of spectroscope appears to be very fine.--at the end of may , spectroscopic determinations of the sun's rotation were made by observations of the relative displacement of the fraunhofer lines at the east and west limbs respectively. the results are in close agreement with the value of the rotation found from observations of sun-spots. a similar determination has also been made in the case of jupiter, with equally satisfactory results.--an electrometer on sir william thomson's plan, for continuous photographic registration of atmospheric electricity has been received from mr white of glasgow. it was mounted in december.--the computation of the photographic records of the barometer from to has so far advanced that we can assert positively that there is no trace of lunar tide in the atmosphere; but that there is a strongly marked semi-diurnal solar tide, accompanied with a smaller diurnal tide. we are at present engaged in comparing the barometric measures with the directions of the wind.--regarding the distribution of the printed observations: there is no extensive wish for separate magnetic observations, but general magnetic results are in great demand, especially for mining operations, and to meet this a map of magnetic declination is furnished in the newspaper called the 'colliery guardian.'--as regards the operations for the transit of venus: the computing staff has by degrees been reduced to two junior computers within the observatory; and one or two computers external to the observatory, who are employed on large groups of systematic calculations. the principal part of the calculations remaining at the date of the last report was that applying to the determination of the geographical longitudes of fundamental stations. at the moment of my writing, the last of these (the longitude of observatory bay, kerguelen) is not absolutely finished:... the method of determining the geographical longitude of the principal station in each group by vertical transits of the moon has been found very successful at honolulu and rodriguez. for stations in high south latitude, horizontal transits are preferable.--as regards the numerical lunar theory: with the view of preserving, against the ordinary chances of destruction or abandonment, a work which is already one of considerable magnitude, i have prepared and have printed as appendix to the greenwich observations (with additional copies as for a separate work) the ordinary equations of lunar disturbance, the novel theory of symbolical variations, and the numerical developments of the quantities on the first side of the equations.--at various times from february to may i was engaged on the reduction of malta tides, and on a paper concerning the same.--in july i was awarded the albert medal for my compass corrections, and received the same from the prince of wales.--in february, campbell's instrument for the registration of sunshine was introduced: it was mounted in july." of private history: "i was at playford until jan. th, in close correspondence as usual with mr christie at the observatory, and attending to my numerical lunar theory.--from mar. th to apr. nd i went on a short trip to hereford, worcester, &c.--from june th to th i was at playford.--from aug. th to sept. th airy was on an expedition in ireland, chiefly in the north and west, with his daughters. when at dublin he visited grubb's instrument factory. on the return journey he stayed for some time in the lake district of cumberland, and took soundings in the neighbourhood of the place of the 'floating island' in derwentwater." airy took the greatest interest in antiquarian matters, whether military or ecclesiastical, and his feelings on such matters is well illustrated by the following letter: royal observatory, greenwich, s.e. _ , february _. dear sir, i venture to ask if you can assist me in the following matter. in the parish church of playford, near ipswich, suffolk, was a splendid brass tombstone to sir thomas felbrigg. by an act of folly and barbarism, almost unequalled in the history of the world, the incumbent and curate nearly destroyed the brass inscription surrounding the image of the knight. this tombstone is figured in gough's sepulchral antiquities, which, i presume, is to be found in the british museum. and i take the liberty to ask if you would kindly look at the engraving, and give me any suggestion as to the way in which some copies of it could be made, in a fairly durable form. i am connected with the parish of playford, and am anxious to preserve for it this memorial of a family of high rank formerly resident there. i am, dear sir, very faithfully yours, g.b. airy. _t. winter jones, esq._ to this request mr winter jones immediately acceded, and the engraving was duly photographed, and copies were circulated with a historical notice of sir george (not sir thomas) felbrigg and a history of the monument. sir george felbrigg was esquire-at-arms to edward iii., and lord of the manor of playford: he died in , and was buried in the north wall of playford church. the report to the board of visitors has this paragraph: "i continue to remark the approaching necessity for library extension. without having absolutely decided on a site, i may suggest that i should wish to erect a brick building, about feet by , consisting of two very low stories (or rather of one story with a gallery running round its walls), so low that books can be moved by hand without necessity for a ladder.--in the month of december, , the azimuthal error of the transit circle had increased to ". a skilful workman, instructed by mr simms, easily reduced the error to about ". (which would leave its mean error nearly ), the western y being moved to the north so far as to reduce the reading of the transit micrometer, when pointed to the south, from r. to r. . the level error was not sensibly affected.--the sidereal standard clock preserves a rate approaching to perfection, so long as it is left without disturbance of the galvanic-contact springs (touched by its pendulum), which transmit signals at every second of time to sympathetic clocks and the chronograph. a readjustment of these springs usually disturbs the rate.--to facilitate the observations of stars, a new working catalogue has been prepared, in which are included all stars down to the third magnitude, stars down to the fifth magnitude which have not been observed in the last two catalogues, and a list of stars of about the sixth magnitude of which the places are required for the united states coast survey. the whole number of stars in our new working list is about . it may be here mentioned that an extensive series of observations was made, during the autumn, of about stars, at the request of mr gill, for comparison with mars, ariadne, and melpomene.--on apr. th last, a very heavy fall of rain took place. between apr. d. h. and apr. d. h., . inch. was recorded, and per cent. of this, or . inch., fell in the eight hours between - / h. and - / h.; and on may , inch of rain fell in minutes, of which / inch fell in minutes.--the supplementary compensation continues to be applied with success to government chronometers which offer facilities for its introduction, and a marked improvement in the performance of chronometers returned after repair by the makers appears to have resulted from the increased attention now given to the compensation. of the competitive chronometers, have the supplementary compensation."--with regard to the reduction of the observations of the transit of venus: after reference to the difficulties arising from the errors and the interpretation of the language used by some of the observers, the report continues thus: "finally a report was made to the government on july th, giving as the mean result for mean solar parallax ". ; the results from ingress and from egress, however, differing to the extent of ". .... after further examination and consideration, the result for parallax has been increased to ". or ". . the results from photography have disappointed me much. the failure has arisen, perhaps sometimes from irregularity of limb, or from atmospheric distortion, but more frequently from faintness and from want of clear definition. many photographs, which to the eye appeared good, lost all strength and sharpness when placed under the measuring microscope. a final result ". was obtained from mr burton's measures, and ". from capt. tupman's.--with regard to the numerical lunar theory: a cursory collection of the terms relating to the areas (in the ecliptic) led me to suppose that there might be some error in the computations of the annual equation and related terms. a most jealous re-examination has however detected nothing, and has confirmed my belief in the general accuracy of the numerical computations. i dare not yet venture to assume an error in delaunay's theory; but i remember that the annual equation gave great trouble to the late sir john lubbock, and that he more than once changed his conclusions as to its true value.--in february i was engaged on the drawings and preparations for my intended lecture at cockermouth on the probable condition of the interior of the earth. the lecture was delivered in april.--at different times in the autumn i was engaged on diagrams to illustrate the passage of rays through eye-pieces and double-image micrometers.--the miscellaneous scientific correspondence, which was always going on, was in this year unusually varied and heavy." of private history: he was at playford till jan. th.--in april he went to cockermouth to deliver his lecture above-mentioned: the journey was by birmingham, where he stayed for two days (probably with his son osmund, who resided there), to tarn bank (the residence of isaac fletcher, m.p.): the lecture was delivered on the nd: he made excursions to thirlmere and barrow, and to edward i.'s monument, and returned to greenwich on the th.--from june th to th he was at playford.--from aug. th to sept. th he was travelling in scotland, visiting the tay bridge, the loch katrine waterworks, &c., and spent the last fortnight of his trip at portinscale, near keswick. on dec. rd he went to playford. "the manuscripts of every kind, which are accumulated in the ordinary transactions of the observatory, are preserved with the same care and arranged on the same system as heretofore. the total number of bound volumes exceeds . besides these there is the great mass of transit of venus reductions and manuscripts, which when bound may be expected to form about volumes.--with regard to the numerous group of minor planets, the berlin authorities have most kindly given attention to my representation, and we have now a most admirable and comprehensive ephemeris. but the extreme faintness of the majority of these bodies places them practically beyond the reach of our meridian instrument, and the difficulty of observation is in many cases further increased by the large errors of the predicted places.--after a fine autumn, the weather in the past winter and spring has been remarkably bad. more than an entire lunation was lost with the transit circle, no observation of the moon on the meridian having been possible between january and march , a period of more than seven weeks. neither sun nor stars were visible for eleven days, during which period the clock-times were carried on entirely by the preceding rate of the clock. the accumulated error at the end of this time did not exceed s' .--some difficulty was at first experienced with the thomson electrometer, which was traced to want of insulation. this has been mastered by the use of glass supporters, which carry some sulphuric acid. the instrument is now in excellent order, and the photographic registers have been perfectly satisfactory since , february, when the new insulators were applied.--from the annual curves of diurnal inequality, deduced from the magnetic reductions, most important inferences may be drawn, as to the connection between magnetic phenomena and sun-spots. these annual curves shew a well-marked change in close correspondence with the number of sun-spots. about the epoch of maximum of sun-spots they are large and nearly circular, having the same character as the curves for the summer months; whilst about the time of sun-spot minimum they are small and lemniscate-shaped, with a striking resemblance to the curves for the winter months. the connection between changes of terrestrial magnetism and sun-spots is shewn in a still more striking manner by a comparison which mr ellis has made between the monthly means of the diurnal range of declination and horizontal force, and dr r. wolf's 'relative numbers' for frequency of sun-spots.--the records of sunshine with campbell's registering sun-dial are preserved in a form easily accessible for reference, and the results are communicated weekly to the agricultural gazette.--prof. oppolzer's results for the determination of the longitudes of vienna and berlin, made in , have now been made public. they shew a remarkable agreement of the chronometric determination formerly made with the telegraphic. it may be of interest to recall the fact that a similar agreement was found between the chronometric and telegraphic determinations of the longitude of valentia.--for observing the transit of venus of , the general impression appears to be that it will be best to confine our observations to simple telescopic observations or micrometer observations at ingress and egress, if possible at places whose longitudes are known. for the first phenomenon (accelerated ingress) the choice of stations is not good; but for the other phenomena (retarded ingress, accelerated egress, retarded egress) there appears to be no difficulty.--with regard to the numerical lunar theory: respecting the discordance of annual equation, i suspend my judgment. i have now discussed the theory completely; and in going into details of secular changes, i am at this time engaged on that which is the foundation of all, namely, the change of excentricity of the solar orbit, and its result in producing lunar acceleration. an important error in the theoretical formulae for variations of radius vector, longitude, and latitude, was discovered; some calculations depending on them are cancelled."--referring to the magnitude of the printed volume of "greenwich observations," and the practicability of reducing the extent of it, the report states thus: "the tendency of external scientific movement is to give great attention to the phenomena of the solar disc (in which this observatory ought undoubtedly to bear its part). and i personally am most unwilling to recede from the existing course of magnetical and meteorological observations....the general tendency of these considerations is to increase the annual expenses of the observatory. and so it has been, almost continuously, for the last years. the annual ordinary expenses are now between - / and times as great as in my first years at the royal observatory.--mr gill was appointed to the cape observatory, and i wrote out instructions for him in march: there was subsequently much correspondence respecting the equipment and repairs of the cape observatory."--in the monthly notices of the royal astronomical society for january an article had appeared headed "notes on the late admiral smyth's cycle of celestial objects, vol. ii." by mr herbert sadler. in this article mr sadler had criticized the work of admiral smyth in a manner which airy regarded as imputing bad faith to admiral smyth. he at once took up the defence of his old friend very warmly, and proposed certain drafts of resolutions to the council of the society. these resolutions were moved, but were amended or negatived, and airy immediately resigned his office of vice-president. there was considerable negociation on the subject, and discussion with lord lindsay, and on may th airy's resolutions were accepted by the council.--in october airy inspected the "faraday" telegraph ship, then lying in the river near messrs siemens' works, and broke his finger by a fall on board the vessel.--in this year airy wrote and circulated a letter to the members of the senate of the university of cambridge, on the subject of the papers set in the smith's prizes examination. in this letter, as on former occasions, he objected much to the large number of questions in "purely idle algebra, arbitrary combinations of symbols, applicable to no further purpose." and in particular he singled out for comment the following question, which was one of those set, "using the term circle as extending to the case where the radius is a pure imaginary, it is required to construct the common chord of two given circles." this drew forth as usual a rejoinder from prof. cayley, who wrote enclosing a solution of his problem, but not at all to airy's satisfaction, who replied as follows: "i am not so deeply plunged in the mists of impossibles as to appreciate fully your explanation in this instance, or to think that it is a good criterion for university candidates." of private history: on jan. st he returned from playford.--on march nd he attended the funeral of his sister at little welnetham near bury st edmunds: miss elizabeth airy had lived with him at the observatory from shortly after his appointment.--for about a week at the end of april he was visiting matlock, edensor, and buxton.--from june th to july th he was staying at portinscale near keswick.--he was at playford for two or three days in october, and went there again on dec. rd for his usual winter holiday. the following letter, relating to the life of thomas clarkson, was written to dr merivale, dean of ely, after reading the account in the "times" of october th of the unveiling of a statue of clarkson near ware: royal observatory, greenwich, london, s.e. _ , october _. dear sir, pardon my intrusion on you, in reference to a transaction which has greatly interested me--the honour paid by you to the memory of thomas clarkson. with very great pleasure i have heard of this step: and i have also been much satisfied with the remarks on it in the "times." i well remember, in clarkson's "history of the abolition," which i read some years ago, the account of the circumstance, now commemorated by you, which determined the action of his whole subsequent life. it is not improbable that, among those who still remember clarkson, my acquaintance with him began at the earliest time of all. i knew him, intimately, from the beginning of to his death. the family which he represented must have occupied a very good position in society. i have heard that he sold two good estates to defray the expenses which he incurred in his personal labours for abolition: and his brother was governor of sierra leone (i know not at what time appointed). thomas clarkson was at st john's college; and, as i gather from circumstances which i have heard him mention, must have been a rather gay man. he kept a horse, and at one time kept two. he took orders in the church; and on one occasion, in the course of his abolition struggle, he preached in a church. but he afterwards resolutely laid aside all pretensions to the title of minister of the church, and never would accept any title except as layman. he was, however, a very earnest reader of theology during my acquaintance with him, and appeared to be well acquainted with the early fathers. the precise words in which was announced the subject for prize essay in the university were "anne liceat invitos in servitutem trahere." after the first great victory on the slave trade question, he established himself in a house on the bank of ullswater. i have not identified the place: from a view which he once shewed me i supposed it to be near the bottom of the lake: but from an account of the storm of wind which he encountered when walking with a lady over a pass, it seemed to be in or near patterdale. when the remains of a mountaineer, who perished in helvellyn (as described in scott's well-known poem), were discovered by a shepherd, it was to mr clarkson that the intelligence was first brought. he then lived at bury st edmunds. mrs clarkson was a lady of bury. but i cannot assign conjecturally any dates to his removals or his marriage. his only son took his b.a. degree, i think, about . i think it was in that he began his occupation of playford hall--a moated mansion near ipswich, formerly of great importance --where he lived as gentleman farmer, managing a farm leased from the marquis of bristol, and occupying a good position among the gentry of the county. a relative of mine, with whom i was most intimately acquainted, lived in the same parish (where in defiance of school rules i spent nearly half my time, to my great advantage as i believe, and where i still retain a cottage for occasional residence), and i enjoyed much of mr clarkson's notice. it was by his strong advice that i was sent to cambridge, and that trinity college was selected: he rode with me to rev. mr rogers of sproughton for introductory examination; he introduced me to rev. c. musgrave (subsequently of halifax), accidentally doing duty at grundisburgh, who then introduced me to sedgwick, peacock, and t. musgrave (subsequently of york). in , when i spent the summer at keswick, he introduced me to southey and wordsworth. mr clarkson lived about thirty years at playford hall, and died there, and lies interred with his wife, son, and grandson, in playford churchyard. i joined several friends in erecting a granite obelisk to his memory in the same churchyard. his family is extinct: but a daughter of his brother is living, first married to t. clarkson's son, and now mrs dickinson, of the rectory, wolferton. i am, my dear sir, very faithfully yours, g.b. airy. _the very reverend, the dean of ely._ "the admiralty, on final consideration of the estimates, decided not to proceed with the erection of a new library near the magnetic observatory in the present year. in the mean time the space has been cleared for the erection of a building by feet.--i have removed the electrometer mast (a source of some expense and some danger), the perfect success of sir william thomson's electrometer rendering all further apparatus for the same purpose unnecessary.--many years ago a double-image micrometer, in which the images were formed by the double refraction of a sphere of quartz, was prepared by mr dollond for capt. smyth, r.n. adopting the same principle on a larger scale, i have had constructed by mr hilger a micrometer with double refraction of a sphere of iceland spar. marks have been prepared for examination of the scale, but i have not yet had opportunity of trying it.--the spectroscopic determination of star-motions has been steadily pursued. the stars are taken from a working list of stars, which may eventually be extended to include all stars down to the fourth magnitude, and it is expected that in the course of time the motions of about stars may be spectroscopically determined.--a new pressure-plate with springs has been applied by mr browning to osler's anemometer, and it is proposed to make such modification as will give a scale extending to lbs. pressure on the square foot. other parts of the instrument have also been renewed.--as regards the reduction of the magnetical results since : in the study of the forms of the individual curves; their relations to the hour, the month, the year; their connection with solar or meteorological facts; the conjectural physico-mechanical causes by which they are produced; there is much to occupy the mind. i regret that, though in contemplation of these curves i have remarked some singular (but imperfect) laws, i have not been able to pursue them.--the mean temperature of the year was . °, being . ° below the average of the preceding years. the highest temperature was . ° on july , and the lowest . ° on dec. . the mean temperature was below the average in every month of the year; the months of greatest deviation being january and december, respectively . ° and . ° below the average; the months of april, may, july, and november were each between ° and ° below the average. the number of hours of bright sunshine, recorded with campbell's sunshine instrument, during , was only .--in the summer of commander green, u.s.n., came over to this country for the purpose of determining telegraphically the longitude of lisbon, as part of a chain of longitudes extending from south america to greenwich. a successful interchange of signals was made with commander green between greenwich and porthcurno on four nights, , june to . the results communicated by commander green shew that the longitude of lisbon observatory, as adopted in the nautical almanac, requires the large correction of + . ".--with regard to the coming transit of venus in : from the facility with which the requirements for geographical position are satisfied, and from the rapid and accurate communication of time now given by electric telegraph, the observation of this transit will be comparatively easy and inexpensive. i have attached greater importance than i did formerly to the elevation of the sun.... i remark that it is highly desirable that steps be taken now for determining by telegraph the longitude of some point of australia. i have stated as the general opinion that it will be useless to repeat photographic observations. --in april mr barlow called, in reference to the enquiry on the tay bridge disaster. (the bridge had been blown down on dec. th, .) i prepared a memorandum on the subject for the tay bridge commission, and gave evidence in a committee room of the house of lords on apr. th." (much of the astronomer royal's evidence on this occasion had reference to the opinions which he had expressed concerning the wind-pressure which might be expected on the projected forth bridge, in .)--in may airy was consulted by the postmaster-general in the matter of a dispute which had arisen between the post office and the telephone companies, which latter were alleged to have infringed the monopoly of the post office in commercial telegraphs: airy made a declaration on the subject.--in july mr bakhuyzen came to england to determine the longitude of leyden, on which he was engaged till sept. th, and carried on his observations at the observatory.--in july airy was much engaged in perusing the records of mr gill's work at the cape of good hope. of private history: on jan. th he returned from playford.--from june th to july th he was again at playford.--from september st to october th he was staying at portinscale near keswick.--on dec. rd he went again to playford for his winter holiday. respecting the agitation at cambridge for granting university degrees to women, the following extract from a letter addressed to a young lady who had forwarded a memorial on the subject for his consideration, and dated nov. th, , contains airy's views on this matter. "i have not signed the memorial which you sent for my consideration: and i will endeavour to tell you why. i entirely approve of education of young women to a higher pitch than they do commonly reach. i think that they can successfully advance so far as to be able clearly to understand--with gratification to themselves and with advantage to those whose education they will superintend--much of the results of the highest class of science which have been obtained by men whose lives are in great measure devoted to it. but i do not think that their nature or their employments will permit of their mastering the _severe_ steps of beginning (and indeed all through) and the _complicated_ steps at the end. and i think it well that this their success should be well known--as it is sure to be--among their relatives, their friends, their visitors, and all in whom they are likely to take interest. their connection with such a place as girton college is i think sufficient to lead to this. but i desire above all that all this be done in entire subservience to what i regard as _infinitely_ more valuable than any amount of knowledge, namely the delicacy of woman's character. and here, i think, our views totally separate. i do not imagine that the university degree would really imply, as regards education, anything more than is known to all persons (socially concerned in the happiness of the young woman) from the less public testimonial of the able men who have the means of knowing their merits. and thus it appears to me that the admission to university degree would simply mean a more extended publication of their names. i dread this." "the new line of underground telegraph wires has been completed by the officers of the general post office. the new route is down croom's hill in greenwich, and the result of this change, at least as regards the earth-current wires, and probably as regards the other wires, has not been satisfactory. it was soon found that the indications of the earth-current wires were disturbed by a continual series of petty fluctuations which almost completely masked the proper features of earth currents.... if this fault cannot be removed, i should propose to return to our original system of independent wires (formerly to croydon and dartford).--the new azimuth-mark (for the altazimuth), upon the parapet of the naval college, is found to be perfectly satisfactory as regards both steadiness and visibility. the observations of a low star for zero of azimuth have been omitted since the beginning of ; the mark, in combination with a high star, appearing to give all that is necessary for this purpose.--all the instruments have suffered from the congealing of the oil during the severe weather of the past winter, and very thorough cleaning of all the moving parts has been necessary.--the solar eclipse of , dec. , was well observed. the first contact was observed by four observers and the last contact by two. the computations for the observations have been exceptionally heavy, from the circumstance that the sun was very low ( ° ' z.d. at the last observation) and that it has therefore been necessary to compute the refraction with great accuracy, involving the calculation of the zenith distance for every observation. and besides this, eighty-six separate computations of the tabular r.a. and n.p.d. of cusps have been required.--amongst other interesting spectroscopic observations of the sun, a remarkable spectrum of a sun-spot shewing strong black lines or bands, each as broad as b_ , in the solar spectrum, was observed on , nov. and . these bands to which there is nothing corresponding in the solar spectrum (except some very faint lines) have also been subsequently remarked in the spectrum of several spots.--the police ship 'royalist' (which was injured by a collision in and had been laid up in dock) has not been again moored in the river, and the series of observations of the temperature of the thames is thus terminated. --part of the month of january was, as regards cold, especially severe. the mean temperature of the period january to ( days) was only . °, or . ° below the average; the temperature fell below ° on days, and rose above the freezing point only on days. the highest temperature in this period was . °, the lowest . °. on january th (while staying at playford) my son hubert and i noticed an almost imperceptible movement in the upper clouds from the south-east. on that night began the terrible easterly gale, accompanied with much snow, which lasted to the night of the th. the limiting pressure of lbs. on the square foot of osler's anemometer was twice exceeded during this storm.--with respect to the diurnal inequalities of magnetic horizontal force: assuming it to be certain that they originate from the sun's power, not immediately, but mediately through his action on the earth, it appears to me (as i suggested long ago) that they are the effects of the attraction of the red end or north end of the needle by the heated portions of our globe, especially by the heated sea, whose effect appears to predominate greatly over that of the land. i do not say that everything is thus made perfectly clear, but i think that the leading phenomena may be thus explained. and this is almost necessarily the way of beginning a science.--in the first few years after the strict and systematic examination of competitive chronometers, beginning with , the accuracy of chronometers was greatly increased. for many years past it has been nearly stationary. i interpret this as shewing that the effects of bad workmanship are almost eliminated, and that future improvement must be sought in change of some points of construction.--referring to the transit of venus in , the printing of all sections of the observations, with specimens of the printed forms employed, and remarks on the photographic operations, is very nearly completed. an introduction is begun in manuscript. i am in correspondence with the commission which is entrusted with the arrangements for observation of the transit of .--the numerical lunar theory has been much interrupted by the pressure of the transit of venus work and other business."--in his report to the board of visitors (his th and last), airy remarks that it would be a fitting opportunity for the expression of his views on the general objects of the observatory, and on the duties which they impose on all who are actively concerned in its conduct. and this he proceeds to do in very considerable detail.--on may th he wrote to lord northbrook (first lord of the admiralty) and to mr gladstone to resign his post of astronomer royal. from time to time he was engaged on the subject of a house for his future residence, and finally took a lease of the white house at the top of croom's hill, just outside one of the gates of greenwich park. on the th of august he formally resigned his office to mr w.h.m. christie, who had been appointed to succeed him as astronomer royal, and removed to the white house on the next day, august th. his holiday movements in the portion of the year up to august th consisted in his winter visit to playford, from which he returned on jan. th: and a subsequent visit to playford from june th to th. * * * * * the following correspondence relating to airy's retirement from office testifies in a remarkable manner to the estimation in which his services were held, and to the good feeling which subsisted between him and his official superiors. , downing street, whitehall, _june , _. dear sir george airy, i cannot receive the announcement of your resignation, which you have just conveyed to me, without expressing my strong sense of the distinction you have conferred upon the office of astronomer royal, and of the difficulty of supplying your place with a person of equal eminence. let me add the expression of my best wishes for the full enjoyment of your retirement from responsibility. i remain, dear sir george airy, faithfully yours, w.e. gladstone. * * * * * admiralty, _june th, _. sir, i am commanded by my lords commissioners of the admiralty to acknowledge the receipt of your letter of the th instant, intimating your desire to retire on the th august next from the office of astronomer royal. . in reply i am to acquaint you that your wishes in this matter have been communicated to the prime minister, and that the further necessary official intimation will in due course be made to the treasury. . at the same time i am instructed by their lordships to convey to you the expression of their high appreciation of the remarkably able and gifted manner, combined with unwearied diligence and devotion to the public service (especially as regards the department of the state over which they preside), in which you have performed the duties of astronomer royal throughout the long period of forty-five years. . i am further to add that their lordships cannot allow the present opportunity to pass without giving expression to their sense of the loss which the public service must sustain by your retirement, and to the hope that you may long enjoy the rest to which you are so justly entitled. i am, sir, your obedient servant, robert hall. _sir g. b. airy, k.c.b. &c., &c., royal observatory, greenwich._ * * * * * admiralty, _ th june, _. sir, my lords commissioners of the admiralty have much pleasure in transmitting copy of a resolution passed by the board of visitors of the royal observatory on the th june last, bearing testimony to the valuable services you have rendered to astronomy, to navigation, and the allied sciences throughout the long period during which you have presided over the royal observatory. i am, sir, your obedient servant, robert hall. _sir george biddell airy, k.c.b. &c., &c., &c., royal observatory, greenwich._ "the astronomer royal (sir george b. airy) having announced his intention of shortly retiring from his position at the royal observatory, the following resolution proposed by professor j. c. adams, and seconded by professor g. g. stokes, was then unanimously adopted and ordered to be recorded in the minutes of the proceedings. "the board having heard from the astronomer royal that he proposes to terminate his connection with the observatory on the th of august next, desire to record in the most emphatic manner their sense of the eminent services which he has rendered to astronomy, to navigation and the allied sciences, throughout the long period of years during which he has presided over the royal observatory. "they consider that during that time he has not only maintained but has greatly extended the ancient reputation of the institution, and they believe that the astronomical and other work which has been carried on in it under his direction will form an enduring monument of his scientific insight and his powers of organization. "among his many services to science, the following are a few which they desire especially to commemorate: _(a)_ "the complete re-organization of the equipment of the observatory. _(b)_ "the designing of instruments of exceptional stability and delicacy suitable for the increased accuracy of observation demanded by the advance of astronomy. _(c)_ "the extension of the means of making observations of the moon in such portions of her orbit as are not accessible to the transit circle. _(d)_ "the investigation of the effect of the iron of ships upon compasses and the correction of the errors thence arising. _(e)_ "the establishment at the observatory and elsewhere of a system of time signals since extensively developed by the government. "the board feel it their duty to add that sir george airy has at all times devoted himself in the most unsparing manner to the business of the observatory, and has watched over its interests with an assiduity inspired by the strongest personal attachment to the institution. he has availed himself zealously of every scientific discovery and invention which was in his judgment capable of adaptation to the work of the observatory; and the long series of his annual reports to the board of visitors furnish abundant evidence, if such were needed, of the soundness of his judgment in the appreciation of suggested changes, and of his readiness to introduce improvements when the proper time arrived. while maintaining the most remarkable punctuality in the reduction and publication of the observations made under his own superintendance, he had reduced, collected, and thus rendered available for use by astronomers, the lunar and planetary observations of his predecessors. nor can it be forgotten that, notwithstanding his absorbing occupations, his advice and assistance have always been at the disposal of astronomers for any work of importance. "to refer in detail to his labours in departments of science not directly connected with the royal observatory may seem to lie beyond the province of the board. but it cannot be improper to state that its members are not unacquainted with the high estimation in which his contributions to the theory of tides, to the undulatory theory of light, and to various abstract branches of mathematics are held by men of science throughout the world. "in conclusion the board would express their earnest hope, that in his retirement sir george airy may enjoy health and strength and that leisure for which he has often expressed a desire to enable him not only to complete the numerical lunar theory on which he has been engaged for some years past, but also to advance astronomical science in other directions." * * * * * admiralty, _ th october, _. sir, i am commanded by my lords commissioners of the admiralty to transmit to you, herewith, a copy of a treasury minute, awarding you a special pension of _£ _ a year, in consideration of your long and brilliant services as astronomer royal. i am, sir, your obedient servant, robert hall. _sir g.b. airy, k.c.b., f.r.s., &c., &c. the white house, croom's hill, greenwich._ copy of treasury minute, dated th october, : my lords have before them a statement of the services of sir george biddell airy, k.c.b., f.r.s., who has resigned the appointment of astronomer royal on the ground of age. sir george airy has held his office since the year , and has also, during that period, undertaken various laborious works, demanding scientific qualifications of the highest order, and not always such as could strictly be said to be included among the duties of his office. the salary of sir g. airy as astronomer royal is _£ _ a year, in addition to which he enjoys an official residence rent free, and, under ordinary circumstances he would be entitled to a pension equal to two-thirds of his salary and emoluments. my lords, however, in order to mark their strong sense of the distinction which, during a long and brilliant career sir george airy has conferred upon his office, and of the great services which, in connection with, as well as in the discharge of, his duties, he has rendered to the crown and the public, decide to deal with his case under the ixth section of the superannuation act, , which empowers them to grant a special pension for special services. accordingly my lords are pleased to award to sir george biddell airy, k.c.b., f.r.s., a special retired allowance of _£ _ per annum. * * * * * the white house, croom's hill, greenwich, _ , october _. sir, i have the honour to acknowledge your letter of october , transmitting to me, by instruction of the lords commissioners of admiralty, copy of a treasury minute dated october , in which the lords commissioners of her majesty's treasury are pleased to award to me an annual retired allowance of _£ _ per annum. acknowledging the very liberal award of the lords commissioners of treasury, and the honourable and acceptable terms in which it is announced, i take leave at the same time to offer to their lordships of the admiralty my recognition of their lordships' kindness and courtesy in thus handing to me copy of the treasury minute. i have the honour to be, sir, your very obedient servant, g.b. airy. _the secretary of the admiralty,_ * * * * * from the assistants of the royal observatory, with whom he was in daily communication, whose faithful and laborious services he had so often thankfully recognized in his annual reports to the board of visitors, and to whom so much of the credit and success of the observatory was due, he received the following address: royal observatory, greenwich, _ , august _. dear sir, we cannot allow the official relation which has so long existed between yourself and us to terminate without expressing to you our sense of the admirable manner in which you have, in our opinion, upheld the dignity of the office of astronomer royal during the many years that you have occupied that important post. your long continued and varied scientific work has received such universal recognition from astronomers in all lands, that it is unnecessary for us to do more than assure you how heartily we join in their appreciation of your labours. we may however add that our position has given us opportunities of seeing that which others cannot equally well know, the untiring energy and great industry which have been therein displayed throughout a long and laborious career, an energy which leads you in retirement, and at fourscore years of age, to contemplate further scientific work. we would ask you to carry with you into private life the best wishes of each one of us for your future happiness, and that of your family, expressing the hope that the days of retirement may not be few, and assuring you that your name will long live in our remembrance. we are, dear sir, yours very faithfully, w.h.m. christie, edwin dunkin, william ellis, george strickland criswick, w. c. nash, a.m.w. downing, edward w. maunder, w.g. thackeray, thomas lewis. _sir g.b. airy, k.c.b., &c., &c., astronomer royal._ * * * * * royal observatory, greenwich, _ , august _. my dear mr christie, and gentlemen of the royal observatory, with very great pleasure i have received your letter of august . i thank you much for your recognition of the general success of the observatory, and of a portion of its conduct which--as you remark--can scarcely be known except to those who are every day engaged in it: but i thank you still more for the kind tone of your letter, which seems to shew that the terms on which we have met are such as leaves, after so many years' intercourse, no shadow of complaint on any side. reciprocating your wishes for a happy life, and in your case a progressive and successful one, i am, my dear mr christie and gentlemen, yours faithfully, g.b. airy. * * * * * throughout his tenure of office airy had cultivated and maintained the most friendly relations with foreign astronomers, to the great advantage of the observatory. probably all of them, at one time or another, had visited greenwich, and to most of them he was well known. on his retirement from office he received an illuminated address from his old friend otto struve and the staff of the pulkowa observatory, an illuminated address from the vorstand of the astronomische gesellschaft at berlin signed by dr auwers and the secretaries, a complimentary letter from the academy of sciences at amsterdam, and friendly letters of sympathy from dr gould, prof. newcombe, dr listing, and from many other scientific friends and societies. his replies to the russian and german addresses were as follows: royal observatory, greenwich, _ , august _. my dear sir, i received, with feelings which i will not attempt to describe, the address of yourself and the astronomers of pulkowa generally, on the occasion of my retirement from the office of astronomer royal. i can scarcely credit myself with possessing all the varied claims to your scientific regard which you detail. i must be permitted to attribute many of them to the long and warm friendship which has subsisted so long between the directors of the pulkowa observatory and myself, and which has influenced the feelings of the whole body of astronomers attached to that institution. on one point, however, i willingly accept your favourable expressions--i have not been sparing of my personal labour--and to this i must attribute partial success on some of the subjects to which you allude. in glancing over the marginal list of scientific pursuits, i remark with pleasure the reference to _optics_. i still recur with delight to the undulatory theory, once the branch of science on which i was best known to the world, and which by calculations, writings, and lectures, i supported against the laplacian school. but the close of your remarks touches me much more--the association of the name of w. struve and my own. i respected deeply the whole character of your father, and i believe that he had confidence in me. from our first meeting in (on a commission for improvement of the nautical almanac) i never ceased to regard him as superior to others. i may be permitted to add that the delivery of his authority to the hands of his son has not weakened the connection of myself with the observatory of poulkova. acknowledging gratefully your kindness, and that of all the astronomers of the observatory of poulkova, and requesting you to convey to them this expression, i am, my dear sir, yours most truly, g.b. airy. _to m. otto von struve, director of the observatory of poulkova and the astronomers of that observatory._ * * * * * royal observatory, greenwich, _ , august _. my dear sir, with very great pleasure i received the address of the astronomische gesellschaft on occasion of my intended resignation of the office of astronomer royal: dated july , and signed by yourself as president and messrs schoenfeld and winnecke as secretaries of the astronomische gesellschaft. i thank you much for the delicacy of your arrangement for the transmission of this document by the hands of our friend dr huggins. and i think you will be gratified to learn that it arrived at a moment when i was surrounded by my whole family assembled at my _jour-de-fête_, and that it added greatly to the happiness of the party. i may perhaps permit myself to accept your kind recognition of my devotion of time and thought to the interests of my science and my office. it is full reward to me that they are so recognized. as to the success or utility of these efforts, without presuming, myself, to form an opinion, i acknowledge that the connection made by the astronomische gesellschaft, between my name and the advance of modern astronomy, is most flattering, and will always be remembered by me with pride. it is true, as is suggested in your address, that one motive for my resignation of office was the desire to find myself more free for the prosecution of further astronomical investigations. should my health remain unbroken, i hope to enter shortly upon this undertaking. again acknowledging the kindness of yourself and the vorstand of the astronomische gesellschaft, and offering my best wishes for the continued success of that honourable institution, i am, my dear sir, yours very truly, g.b. airy. _to dr aimers and the vorstand of the astronomische gesellschaft._ chapter x. at the white house, greenwich. from his resignation of office on august th, , to his death on january nd, . history of his life after his resignation of office. on the th of august airy left the observatory which had been his residence for nearly years, and removed to the white house. whatever his feelings may have been at the severing of his old associations he carefully kept them to himself, and entered upon his new life with the cheerful composure and steadiness of temper which he possessed in a remarkable degree. he was now more than years old, and the cares of office had begun to weigh heavily upon him: the long-continued drag of the transit of venus work had wearied him, and he was anxious to carry on and if possible complete his numerical lunar theory, the great work which for some years had occupied much of his time and attention. his mental powers were still vigorous, and his energy but little impaired: his strong constitution, his regular habits of life, the systematic relief which he obtained by short holiday expeditions whenever he found himself worn with work, and his keen interest in history, poetry, classics, antiquities, engineering, and other subjects not immediately connected with his profession, had combined to produce this result. and in leaving office, he had no idea of leaving off work; his resignation of office merely meant for him a change of work. it is needless to say that his interest in the welfare and progress of the observatory was as keen as ever; his advice was always at the service of his successor, and his appointment as visitor a year or two after his resignation gave him an official position with regard to the observatory which he much valued. the white house, which was to be his home for the rest of his life, is just outside one of the upper gates of the park, and about a quarter of a mile from the observatory. here he resided with his two unmarried daughters. the house suited him well and he was very comfortable there: he preferred to live in the neighbourhood with which he was so familiar and in which he was so well known, rather than to remove to a distance. his daily habits of life were but little altered: he worked steadily as formerly, took his daily walk on blackheath, made frequent visits to playford, and occasional expeditions to the cumberland lakes and elsewhere. the work to which he chiefly devoted himself in his retirement was the completion of his numerical lunar theory. this was a vast work, involving the subtlest considerations of principle, very long and elaborate mathematical investigations of a high order, and an enormous amount of arithmetical computation. the issue of it was unfortunate: he concluded that there was an error in some of the early work, which vitiated the results obtained: and although the whole process was published, and was left in such a state that it would be a comparatively simple task for a future astronomer to correct and complete it, yet it was not permitted to the original author of it to do this. to avoid the necessity of frequent reference to this work in the history of airy's remaining years, it will be convenient to summarize it here. it was commenced in : "on feb. rd in this year i first (privately) formed the notion of preparing a numerical lunar theory by substituting delaunay's numbers in the proper equations and seeing what would come of it." from this time forward till his power to continue it absolutely failed, he pursued the subject with his usual tenacity of purpose. during his tenure of office every available opportunity was seized for making progress with his lunar theory, and in every report to the visitors a careful statement was inserted of the state in which it then stood. and, after his resignation of office, it formed the bulk of his occupation. in the theory was formed, and by it was so far advanced that he published in the monthly notices of the royal astronomical society a statement of the fundamental points of the theory. in , the theory having advanced to a stage where extensive arithmetical computation was required, he obtained a small grant from the government in aid of the expense of the work, and other grants were made in subsequent years. by the calculations were so far advanced that an opinion could be formed as to the probable accuracy of the theory, and the following remark is made: "a cursory collation of the terms relating to the areas (in the ecliptic) led me to suppose that there might be some error in the computations of the annual equation and related terms;" but no error could be discovered and the work proceeded. the complex character of the theory, and the extreme care required in the mathematical processes, are well illustrated by the following statement, which occurs in the report of , "an important error in the theoretical formulae for variations of radius vector, longitude, and latitude, was discovered; some calculations depending on them are cancelled." in and the work was continued, but was "sadly interrupted by the pressure of the transit of venus work and other business." after his resignation of the office of astronomer royal he had no further public assistance, and did much of the computations himself, but a sum of _£ _ was contributed by mr de la rue in furtherance of the work, and this sum was spent on computers. in his retirement the work made good progress, and on dec. st, , he made the following note: "i finished and put in general order the final tables of equations of variations. this is a definite point in the lunar theory.... i hope shortly to take up severely the numerical operations of the lunar theory from the very beginning." the work was continued steadily through , and on mar. th, , he made application through the board of visitors to the admiralty to print the work: after the usual enquiries as to the expense this was acceded to, and copy was sent to the printers as soon as it was ready. the first printed proofs were received on feb. th, , and the whole book was printed by the end of . from the frequent references in his journal to errors discovered and corrected during the progress of these calculations, it would seem likely that his powers were not what they had been, and that there was a probability that some important errors might escape correction. he was far too honest to blind himself to this possibility, and in the preface to his numerical lunar theory he says thus: "i have explained above that the principle of operations was, to arrange the fundamental mechanical equations in a form suited for the investigations of lunar theory; to substitute in the terms of these equations the numerical values furnished by delaunay's great work; and to examine whether the equations are thereby satisfied. with painful alarm, i find that they are not satisfied; and that the discordance, or failure of satisfying the equations, is large. the critical trial depends on the great mass of computations in section ii. these have been made in duplicate, with all the care for accuracy that anxiety could supply. still i cannot but fear that the error which is the source of discordance must be on my part. i cannot conjecture whether i may be able to examine sufficiently into this matter." he resolutely took in hand the revision of his work, and continued it till october . but it is clear from the entries in his journal that his powers were now unequal to the task, and although from time to time he suspected that he had discovered errors, yet it does not appear that he determined anything with certainty. he never doubted that there were important errors in the work, and later on he left the following private note on the subject: numerical lunar theory. _ , sept. _. i had made considerable advance (under official difficulties) in calculations on my favourite numerical lunar theory, when i discovered that, under the heavy pressure of unusual matters (two transits of venus and some eclipses) i had committed a grievous error in the first stage of giving numerical value to my theory. my spirit in the work was broken, and i have never heartily proceeded with it since. g.b. airy. probably the error referred to here is the suspected error mentioned above in his report of , as to which he subsequently became more certain. whatever may be the imperfections of the numerical lunar theory, it is a wonderful work to have been turned out by a man years old. in its idea and inception it embodies the experience of a long life actively spent in practical science. and it may be that it will yet fulfil the objects of its author, and that some younger astronomer may take it up, correct its errors (wherever they may be), and fit it for practical use. and then the labour bestowed upon it will not have been in vain. subject always to the absorbing occupations of the lunar theory he amused himself with reading his favourite subjects of history and antiquities. his movements during the remainder of the year were as follows: in september he paid a two days' visit to lady herschel at hawkhurst. from oct. th to th he was at the cumberland lakes and engaged in expeditions in the neighbourhood. from nov. th to th he was at cambridge, inspecting prof. stuart's workshops, and other scientific institutions. on dec. th he went to playford.--amongst miscellaneous matters: in november he wrote to mr rothery on the loss of the 'teuton' at some length, with suggestions for the safer construction of such vessels.--in october he was asked for suggestions regarding the establishment of a "standard time" applicable to the railway traffic in the united states: he replied as follows: _ , oct. _. sir, i have to acknowledge your letter of october , introducing to my notice the difficulty which appears to be arising in america regarding a "standard time," for extensive use throughout n. america "applicable to railway traffic only." the subject, as including considerations of convenience in all the matters to which it applies, is one of difficulties probably insuperable. the certainty, however, that objections may be raised to every scheme, renders me less timid in offering my own remarks; which are much at your service. i first comment upon your expression of "standard time... applicable to railway traffic only." but do you mean this as affecting the transactions between one railway and another railway, or as affecting each railway and the local interests (temporal and others) of the towns which it touches? the difference is so great that i should be disposed to adopt it as marking very strongly the difference to be made between the practices of railways among themselves and the practices of railways towards the public; and will base a system on that difference. as regards the practices of railways among themselves: if the various railways of america are joined and inosculated as they are in england, it appears to me indispensable that they have one common standard _among themselves_: say washington observatory time. but this is only needed for the office-transactions between the railways; it may be kept perfectly private; never communicated to the public at all. and i should recommend this as the first step. there will then be no difficulty in deducing, from these private washington times, the accurate local times at those stations (whose longitude is supposed to be fairly well known, as a sailor with a sextant can determine one in a few hours) which the railway authorities may deem worthy of that honour; generally the termini of railways. thus we shall have a series of bases of local time, of authoritative character, through the country. of such bases _we_ have two, greenwich and dublin: and they are separated by a sea-voyage. in the u.s. of america there must be a greater number, and probably not so well separated. still it is indispensable to adopt such a system of local centers. no people in this world can be induced to use a reckoning which does not depend clearly upon the sun. in all civilized countries it depends (approximately) on the sun's meridian passage. even the sailor on mid-ocean refers to that phenomenon. and the solar passage, with reasonable allowance, m. or m. one way or another, must be recognized in all time-arrangements as giving the fundamental time. the only practical way of doing this is, to adopt for a whole region the fundamental time of a center of that region. and to this fundamental time, the local time of the railway, as now entering into all the concerns of life, must be adapted. a solicitor has an appointment to meet a client by railway; a physician to a consultation. how is this to be kept if the railway uses one time and every other act of life another? there is one chain of circumstances which is almost peculiar--that of the line from new york to san francisco. here i would have two clocks at every station: those on the north side all shewing san francisco time, and those on the south all shewing new york time. every traveller's watch would then be available to the end of his journey. a system, fundamentally such as i have sketched, would give little trouble, and may i think be adopted with advantage. i am, sir, your faithful servant, g.b. airy. _mr edward barrington._ he returned from playford on jan. : his other movements during the year were as follows: from apr. th to may th he was at playford; and again from august st to th. from oct. th to nov. st he was travelling with his two unmarried daughters in the lake district of cumberland: the journey was by furness and coniston to portinscale near keswick; on oct. th he fell and sprained his ankle, and his excursions for the rest of the time were mainly conducted by driving. shortly after his return, on nov. th, while walking alone on blackheath, he was seized with a violent attack of illness, and lay helpless for some time before he was found and brought home: he seems however to have recovered to a great extent in the course of a day or two, and continued his lunar theory and other work as before. on june nd he made the following sad note, "this morning, died after a most painful illness my much-loved daughter-in-law, anna airy, daughter of professor listing of göttingen, wife of my eldest son wilfrid." in february he wrote out his reminiscences of the village of playford during his boyhood. in june he was much disturbed in mind on hearing of some important alterations made by the astronomer royal in the collimators of the transit circle, and some correspondence ensued on the subject.--during the year he had much correspondence on the subject of the subsidences on blackheath. the following letter was written in reply to a gentleman who had asked whether it could be ascertained by calculation how long it is since the glacial period existed: _ , july _. sir, i should have much pleasure in fully answering your questions of july if i were able to do so: but the subject really is very obscure. ( ) though it is recognized that the glacial period (or periods) is late, i do not think that any one has ventured to fix upon a rude number of years since elapsed. ( ) we have no reason to think that the mean distance of the earth from the sun has sensibly altered. there have been changes in the eccentricity of the orbit (making the earth's distance from the sun less in one month and greater in the opposite month), but i do not perceive that this would explain glaciers. ( ) i consider it to be certain that the whole surface of the earth, at a very distant period, was very hot, that it has cooled gradually, and (theoretically and imperceptibly) is cooling still. the glaciers must be later than these hot times, and later than our last consolidated strata: but this is nearly all that i can say. i am, sir, your obedient servant, g.b. airy. _james alston, esq._ from may nd to th he was at playford. from july th to th he was travelling in south wales with his daughters.--from oct. th to nov. th he was at playford.--between nov. th of this year and jan. th of the year , he sat several times to mr john collier for his portrait: the picture was exhibited in the academy of ; it is a most successful and excellent likeness. throughout the year he was very busy with the numerical lunar theory.--in march he was officially asked to accept the office of visitor of the royal observatory, which he accepted, and in this capacity attended at the annual visitation on june nd, and addressed a memorandum to the visitors on the progress of his lunar theory.--on march th he published in several newspapers a statement in opposition to the proposed braithwaite and buttermere railway, which he considered would be injurious to the lake district, in which he took so deep an interest.--in may he communicated to "the observatory" a statement of his objections to a theory advanced by mr stone (then president of the royal astronomical society) to account for the recognized inequality in the mean motion of the moon. this theory, on a subject to which airy had given his incessant attention for so many years, would naturally receive his careful attention and criticism, and it attracted much general notice at the time.--in december he wrote to the secretary of the royal astronomical society his opinion as to the award of the medal of the society. in this letter he stated the principles which guided him as follows: "i have always maintained that the award of the medal ought to be guided mainly by the originality of communications: that one advance in a new direction ought in our decision to outweigh any mass of work in a routine already established: and that, in any case, scientific utility as distinguished from mere elegance is indispensable."--in july lieut. pinheiro of the brazilian navy called with an autograph letter of introduction from the emperor of brazil. the lieutenant desired to make himself acquainted with the english system of lighthouses and meteorology, and airy took much trouble in providing him with introductions through which he received every facility for the thorough accomplishment of his object.--on oct. th he forwarded to prof. cayley proofs of euclid's propositions i. and iii. with the following remarks: "i place on the other side the propositions which may be substituted (with knowledge of euclid's vi. book) for the two celebrated propositions of the geometrical books. they leave on my mind no doubt whatever that they were invented as proofs by ratios, and that they were then violently expanded into cumbrous geometrical proofs."--on june th he declined to sign a memorial asking for the interment of mr spottiswoode in westminster abbey, stating as his reason, "i take it, that interment possessing such a public character is a public recognition of benefits, political, literary, or philosophical, whose effects will be great and durable. now i doubt whether it can be stated that mr spottiswoode had conferred such benefits on society. "but he adds at length his cordial recognition of mr spottiswoode's scientific services.--throughout his life airy was a regular attendant at church, and took much interest in the conduct of the church services. in october of this year he wrote a long letter to the vicar of greenwich on various points, in which occurs the following paragraph: "but there is one matter in the present form of the church service, on which my feeling is very strong, namely the (so-called, i believe) choral service, in the confession, the prayer, and the creed. i have long listened with veneration to our noble liturgy, and i have always been struck with the deep personally religious feeling which pervades it, especially those parts of it which are for 'the people.' and an earnest priest, earnestly pressing these parts by his vocal example on the notice of the people, can scarcely fail to excite a corresponding earnestness in them. all this is totally lost in the choral system. for a venerable persuasion there is substituted a rude irreverential confusion of voices; for an earnest acceptance of the form offered by the priest there is substituted--in my feeling at least--a weary waiting for the end of an unmeaning form." he also objected much to singing the responses to the commandments. from apr. th to may th he was at playford, concluding his journal there with the note "so ends a pleasant vacation."--on june th he went to cambridge and attended the trinity college commemoration service, and dined in hall.--from aug. th to sept. th he was at playford.--on sept. th he made an expedition to guildford and farnham.--during this year he was closely engaged on the numerical lunar theory, and for relaxation was reading theology and sundry books of the old testament. on june th he attended at the visitation of the royal observatory.--in a letter written in april to lt.-col. marindin, r.a., on the subject of wind pressure there occurs the following remark: "when the heavy gusts come on, the wind is blowing in directions changing rapidly, but limited in extent. my conclusion is that in arches of small extent (as in the tay bridge) every thing must be calculated for full pressure; but in arches of large extent (as in the forth bridge) every thing may be calculated for small pressure. and for a suspension bridge the pressure is far less dangerous than for a stiff arch."--in january he had some correspondence with professor tyndall on the theory of the "white rainbow," and stated that he thoroughly agreed with dr young's explanation of this phaenomenon. --the following is extracted from a letter on may st to his old friend otto struve: "i received from you about or weeks past a sign of your friendly remembrance, a copy of your paper on the annual parallax of aldebaran. it pleased me much. especially i was delighted with your noble retention of the one equation whose result differed so sensibly from that of the other equations. it is quite possible, even probable, that the mean result is improved by it. i have known such instances. the first, which attracted much attention, was capt. kater's attempt to establish a scale of longitude in england by reciprocal observations of azimuth between beachy head and dunnose. the result was evidently erroneous. but colonel colby, on examination of the original papers, found that some observations had been omitted, as suspicious; and that when these were included the mean agreed well with the scale of observation inferred from other methods."--in a letter to the rev. r.c.m. rouse, acknowledging the receipt of a geometrical book, there occurs the following paragraph: "i do not value euclid's elements as a super-excellent book of instruction--though some important points are better presented in it than in any other book of geometrical instruction that i have seen. but i value it as a book of strong and distinct reasoning, and of orderly succession of reasonings. i do not think that there is any book in the world which presents so distinctly the 'because...... therefore.......' and this is invaluable for the mental education of youth."--in may he was in correspondence with professor balfour stewart regarding a projected movement in terrestrial magnetism to be submitted to the british association. airy cordially approved of this movement, and supported it to the best of his ability, stating that in his opinion what was mainly wanted was the collation of existing records.--in january and february he was much pressed by prof. pritchard of oxford to give his opinion as to the incorrectness of statements made by dr kinns in his lectures on the scientific accuracy of the bible. airy refused absolutely to take part in the controversy, but he could not escape from the correspondence which the matter involved: and this led up to other points connected with the early history of the israelites, a subject in which he took much interest. from may th to june rd he was at playford.--from july nd to nd he was in the lake district. the journey was by windermere to kentmere, where he made enquiries concerning the airy family, as it had been concluded with much probability from investigations made by his nephew, the rev. basil r. airy, that the family was settled there at a very early date. some persons of the name of airy were still living there. he then went on by coniston and grasmere to portinscale, and spent the rest of his time in expeditions amongst the hills and visits to friends.--on july th he went to woodbridge in suffolk and distributed the prizes to the boys of the grammar school there.--from oct. th to nov. th he was again at playford.--throughout the year he was busily engaged on the numerical lunar theory, and found but little time for miscellaneous reading. of printed papers by airy in this year the most important was one on the "results deduced from the measures of terrestrial magnetic force in the horizontal plane," &c. this was a long paper, communicated to the royal society, and published in the phil. trans., and was the last scientific paper of any importance (except the volume of the numerical lunar theory) in the long list of "papers by g.b. airy." the preparation of this paper took much time.--of miscellaneous matters: in may a committee of the royal society had been appointed to advise the india office as to the publication of col. j. herschel's pendulum observations in india; and airy was asked to assist the committee with his advice. he gave very careful and anxious consideration to the subject, and it occupied much time.--in the early part of the year he was asked by sir william thomson to assist him with an affidavit in a lawsuit concerning an alleged infringement of one of his patents for the improvement of the compass. airy declined to make an affidavit or to take sides in the dispute, but he wrote a letter from which the following is extracted: "i cannot have the least difficulty in expressing my opinion that you have made a great advance in the application of my method of correcting the compass in iron ships, by your introduction of the use of short needles for the compass-cards. in my original investigations, when the whole subject was in darkness, i could only use existing means for experiment, namely the long-needle compasses then existing. but when i applied mechanical theory to explanation of the results, i felt grievously the deficiency of a theory and the construction which it suggested (necessarily founded on assumption that the proportion of the needle-length to the other elements of measure is small) when the length of the needles was really so great. i should possibly have used some construction like yours, but the government had not then a single iron vessel, and did not seem disposed to urge the enquiry. you, under happier auspices, have successfully carried it out, and, i fully believe, with much advantage to the science."--he wrote a paper for the athenaeum and had various correspondence on the subject of the badbury rings in dorsetshire, which he (and others) considered as identical with the "mons badonicus" of gildas, the site of an ancient british battle.--in february he was in correspondence with the astronomer royal on uniform time reckoning, and on considerations relating to it.--on june th he attended the annual visitation of the observatory, and brought before the board his investigations of the diurnal magnetic inequalities, and the revises of his lunar theory. from june th to july th he was at playford.--and again at playford from oct. th to nov. th.--on march th he had an attack of gout in his right foot, which continued through april and into may, causing him much inconvenience.--he was busy with the numerical lunar theory up to sept. th, when he was reading the last proof-sheet received from the printers: during this period his powers were evidently failing, and there are frequent references to errors discovered and corrected, and to uncertainties connected with points of the theory. but his great work on the numerical lunar theory was printed in this year: and there can be no doubt that he experienced a great feeling of relief when this was accomplished.--he was in correspondence with prof. adams as to the effect of his reduction of the coefficient of lunar acceleration on the calculation of the ancient historical eclipses.--he compiled a paper "on the establishment of the roman dominion in england," which was printed in .--he wrote a notice concerning events in the life of mr john jackson of rosthwaite near keswick, a well-known guide and much-respected authority on matters relating to the lake district.--he also wrote a short account of the connection of the history of mdlle de quéroualle with that of the royal observatory at greenwich.--on june th he attended at the annual visitation of the observatory. on may th to th he made a short visit to eastbourne and the neighbourhood.--from june th to july th he was at playford.--from aug. th to sept. th he was travelling in dorsetshire and wiltshire: he went first to weymouth, a very favourite centre for excursions with him, and afterwards visited bridport and lyme regis: then by dorchester to blandford, and visited the hod hill, badbury rings, &c.: at wimborne he was much interested in the architecture of the church: lastly he visited salisbury, old sarum, stonehenge, &c., and returned to greenwich.--from oct. th to nov. th he was at playford.--during this year he partly occupied himself with arranging his papers and drawings, and with miscellaneous reading. but he could not withdraw his thoughts from his lunar theory, and he still continued to struggle with the difficulties of the subject, and was constantly scheming improvements. his private accounts also now gave him much trouble. throughout his life he had been accustomed to keep his accounts by double entry in very perfect order. but he now began to make mistakes and to grow confused, and this distressed him greatly. it never seemed to occur to him to abandon his elaborate system of accounts, and to content himself with simple entries of receipts and expenses. this would have been utterly opposed to his sense of order, which was now more than ever the ruling principle of his mind. and so he struggled with his accounts as he did with his lunar theory till his powers absolutely failed. in his journal for this year there are various entries of mental attacks of short duration and other ailments ascribable to his advanced age. the last printed "papers by g.b. airy" belong to this year. one was the paper before referred to "on the establishment of the roman dominion in england": another was on the solution of a certain equation: and there were early reminiscences of the cambridge tripos, &c.--in february he attended a little to a new edition of his ipswich lectures, but soon handed it over to mr h.h. turner of the royal observatory.--on may rd he was drawing up suggestions for the arrangement of the seckford school, &c., at woodbridge.--on june th he attended the visitation of the royal observatory, when a resolution was passed in favour of complete photography of the star-sky. from the th to th of may he made a short expedition to bournemouth, and stopped on the way home to visit winchester cathedral.--from june th to aug. rd he was at playford; and again from oct. th to nov. th.--during the first half of the year he continued his examination of his lunar theory, but gradually dropped it. there are several references in his journal to his feelings of pain and weakness, both mental and bodily: at the end of march he had an attack of gout in the fingers of his right hand. during the latter part of the year he was troubled with his private accounts, as before.--he does not appear to have been engaged on any miscellaneous matters calling for special notice in this year. but he kept up his astronomical correspondence--with lockyer on the meteorite system of planetary formation; with pritchard on the work of the oxford university observatory; with adams on his numerical lunar theory, &c., and with others.--on june nd he attended the visitation of the royal observatory.--he amused himself occasionally with reading his favourite subjects of history and antiquities, and with looking over some of his early investigations of scientific questions. on june th he made a one-day's excursion to colchester.--from july nd to th he was in the cumberland lake district, chiefly at portinscale near keswick. while staying at portinscale he was seized with a sudden giddiness and fell upon the floor: he afterwards wrote a curious account of the visions which oppressed his brain immediately after the accident. he returned by solihull, where his son osmund was residing.--from oct. th to nov. th he was at playford. while there he drew up a short statement of his general state of health, adverting particularly to the loss of strength in his legs and failure of his walking powers.--his health seems to have failed a good deal in this year: on feb. th he had an accidental fall, and there are several entries in his journal of mental attacks, pains in his limbs, affection of his eye-sight, &c.--in the early part of the year he was much engaged on the history of the airy family, particularly on that of his father.--in this year the white house was sold by auction by its owners, and airy purchased it on may th.--he was still in difficulties with his private accounts, but was making efforts to abandon his old and elaborate system.--for his amusement he was chiefly engaged on theological notes which he was compiling: and also on early optical investigations, &c. on june st he attended the visitation of the royal observatory, and moved a resolution that a committee be appointed to consider whether any reduction can be effected in the amount of matter printed in the volume of observations of the royal observatory. during his tenure of office he had on various occasions brought this subject before the board of visitors, and with his usual tenacity of purpose he now as visitor pressed it upon their notice.--in may he zealously joined with others in an application to get for dr huggins a pension on the civil list.--in january he prepared a short paper illustrated with diagrams to exhibit the interference of solar light, as used by him in his lectures at cambridge in : but it does not appear to have been published.--in april he received a copy of a paper by mr rundell, referring to the complete adoption of his system of compass correction in iron ships, not only in the merchant service, but also in the navy. this was a matter of peculiar gratification to airy, who had always maintained that the method of tables of errors, which had been so persistently adhered to by the admiralty, was a mistake, and that sooner or later they would find it necessary to adopt his method of mechanical correction. the passage referred to is as follows: "the name of sir george airy, the father of the mechanical compensation of the compass in iron vessels, having just been mentioned, it may not be inappropriate to remind you that the present year is the fiftieth since sir george airy presented to the royal society his celebrated paper on this subject with the account of his experiments on the 'rainbow' and 'ironsides.' fifty years is a long period in one man's history, and sir george airy may well be proud in looking back over this period to see how complete has been the success of his compass investigation. his mode of compensation has been adopted by all the civilized world. sir william thomson, one of the latest and perhaps the most successful of modern compass adjusters, when he exhibited his apparatus in before a distinguished meeting in london, remarked that within the last ten years the application of sir george airy's method had become universal, not only in the merchant service, but in the navies of this and other countries, and added--the compass and the binnacles before you are designed to thoroughly carry out in practical navigation the astronomer royal's principles." from may th to th he was on an expedition to north wales, stopping at chester, conway, carnarvon, barmouth, and shrewsbury.--from june th to july th he was at playford; and again from oct. th to nov. th.--in this year his powers greatly failed, and he complained frequently of mental attacks, weakness of limbs, lassitude, and failure of sleep. he occupied himself as usual with his books, papers, and accounts; and read travels, biblical history, &c., but nothing very persistently. on june th he attended the visitation of the royal observatory.--from a letter addressed to him by mr j. hartnup, of liverpool observatory, it appears that there had grown up in the mercantile world an impression that very accurate chronometers were not needed for steam ships, because they were rarely running many days out of sight of land: and airy's opinion was requested on this matter. he replied as follows on mar. rd: "the question proposed in your letter is purely a practical one. ( ) if a ship is _likely_ ever to be two days out of sight of land, i think that she ought to be furnished with two _good_ chronometers, properly tested. ( ) for the proper testing of the rates of the chronometers, a rating of the chronometers for three or four days in a meridional observatory is necessary. a longer testing is desirable."--in march he was in correspondence, as one of the trustees of the sheepshanks fund, with the master of trinity relative to grants from the fund for cambridge observatory. from june th to july th he was at playford. and again from oct. th to dec. nd (his last visit). throughout the year his weakness, both of brain power and muscular power, had been gradually increasing, and during this stay at playford, on nov. th, he fell down in his bed-room (probably from failure of nerve action) and was much prostrated by the shock. for several days he remained in a semi-unconscious condition, and although he rallied, yet he continued very weak, and it was not until dec. nd that he could be removed to the white house. up to the time of his fall he had been able to take frequent drives and even short walks in the neighbourhood that he was so fond of, but he could take but little exercise afterwards, and on or about nov. th he made the following note: "the saddest expedition that i have ever made. we have not left home for several days." the rapid failure of his powers during this year is well exemplified by his handwriting in his journal entries, which, with occasional rallies, becomes broken and in places almost illegible. he makes frequent reference to his decline in strength and brain-power, and to his failing memory, but he continued his ordinary occupations, made frequent drives around blackheath, and amused himself with his family history researches, arrangement of papers, and miscellaneous reading: and he persisted to the last with his private accounts. his interest in matters around him was still keen. on june th he was driving along the greenwich marshes in order to track the course of the great sewer; and on august th he visited the crossness sewage works and took great interest in the details of the treatment of the sewage.--in march he contributed, with great satisfaction, to the fund for the portrait of his old friend sir g.g. stokes, with whom he had had so much scientific correspondence.--on july th an afternoon party was arranged to celebrate the th anniversary of his birthday (the actual anniversary was on july th). none of his early friends were there: he had survived them all. but invitations were sent to all his scientific and private friends who could be expected to come, and a large party assembled. the afternoon was very fine, and he sat in the garden and received his friends (many of whom had come from long distances) in good strength and spirits. it was a most successful gathering and was not without its meaning; for it was felt that, under the circumstances of his failing powers, it was in all probability a final leave-taking.--on july th he went down to the greenwich parish church at p.m., to be present at the illumination of the church clock face for the first time--a matter of local interest which had necessitated a good deal of time and money. on this occasion at the request of the company assembled in and around the vestry he spoke for about a quarter of an hour on time--the value of accurate time, the dissemination of greenwich time throughout the country by time-signals from the observatory, and the exhibition of it by time-balls, &c., &c.,--the subject to which so large a part of his life had been devoted. it was a pleasant and able speech and gave great satisfaction to the parishioners, amongst whom he had lived for so many years.--he received two illuminated addresses--one from the astronomer royal and staff of the royal observatory; the other from the vorstand of the astronomische gesellschaft at berlin--and various private letters of congratulation. the address from the staff of the observatory was worded thus: "we, the present members of the staff of the royal observatory, greenwich, beg to offer you our most sincere congratulations on the occasion of your th birthday. we cannot but feel how closely associated we are with you, in that our whole energies are directed to the maintenance and development of that practical astronomical work, of which you essentially laid the foundation. it affords us great pleasure to think that after the conclusion of your life's work, you have been spared to live so long under the shadow of the noble observatory with which your name was identified for half a century, and with which it must ever remain associated." after his return from playford he seemed to rally a little: but he soon fell ill and was found to be suffering from hernia. this necessitated a surgical operation, which was successfully performed on dec. th. this gave him effectual relief, and after recovering from the immediate effects of the operation, he lay for several days quietly and without active pain reciting the english poetry with which his memory was stored. but the shock was too great for his enfeebled condition, and he died peacefully in the presence of his six surviving children on jan. nd, . he was buried in playford churchyard on jan. th. the funeral procession was attended at greenwich by the whole staff of the royal observatory, and by other friends, and at his burial there were present two former fellows of the college to which he had been so deeply attached. appendix. list of printed papers by g.b. airy. list of books written by g.b. airy. printed papers by g.b. airy. with the instinct of order which formed one of his chief characteristics airy carefully preserved a copy of every printed paper of his own composition. these were regularly bound in large quarto volumes, and they are in themselves a striking proof of his wonderful diligence. the bound volumes are in number, and they occupy a space of ft. in. on a shelf. they contain papers, a list of which is appended, and they form such an important part of his life's work, that his biography would be very incomplete without a reference to them. he was very careful in selecting the channels for the publication of his papers. most of the early papers were published in the transactions of the cambridge philosophical society, but several of the most important, such as his paper "on an inequality of long period in the motions of the earth and venus," were published in the philosophical transactions of the royal society, and others, such as the articles on "the figure of the earth," "gravitation," "tides and waves," &c., were published in encyclopaedias. after his removal to greenwich nearly all his papers on scientific subjects (except astronomy), such as tides, magnetism, correction of the compass, &c., &c., were communicated to the royal society, and were published in the philosophical transactions. but everything astronomical was reserved for the royal astronomical society. his connection with that society was very close: he had joined it in its earliest days (the date of his election was may th, ), and regarded it as the proper medium for the discussion of current astronomical questions, and for recording astronomical progress. he was unremitting in his attendance at the monthly meetings of the society, and was several times president. in the memoirs of the society of his papers are printed, and in addition papers in the monthly notices. in fact a meeting of the society rarely passed without some communication from him, and such was his wealth of matter that sometimes he would communicate as many as papers on a single evening. for the publication of several short mathematical papers, and especially for correspondence on disputed points of mathematical investigation, he chose as his vehicle the philosophical magazine, to which he contributed papers. investigations of a more popular character he published in the athenaeum, which he also used as a vehicle for his replies to attacks on his work, or on the establishment which he conducted: in all he made communications to that newspaper. to various societies, such as the institution of civil engineers, the british association, the royal institution, &c., he presented papers or made communications on subjects specially suited to each; and in like manner to various newspapers: there were papers in this category. in so long an official life there would naturally be a great number of official reports, parliamentary returns, &c., and these, with other miscellaneous papers printed for particular objects and for a limited circulation, amounted in all to . under this head come his annual reports to the board of visitors, which in themselves contain an extremely full and accurate history of the observatory during his tenure of office. there are of these reports, and they would of themselves form a large volume of about pages. the following summary of his printed papers shews the manner in which they were distributed: summary of printed papers by g.b. airy. number of papers. in the transactions of the cambridge philosophical society in the philosophical transactions of the royal society in the proceedings of the royal society in the memoirs of the royal astronomical society in the monthly notices of the royal astronomical society in the philosophical magazine and journal in the athenaeum in encyclopedias, and in various newspapers and transactions in official reports, addresses, parliamentary returns, evidence before committees, lectures, letters, sundry treatises, and papers --- total printed papers by g.b. airy. date when read or published. title of paper. where published. nov. on the use of silvered glass for the mirrors camb. phil. soc. of reflecting telescopes. mar. on the figure assumed by a fluid homogeneous camb. phil. soc. mass, whose particles are acted on by their mutual attraction, and by small extraneous forces. may on the principles and construction of the camb. phil. soc. achromatic eye-pieces of telescopes, and on the achromatism of microscopes. trigonometry. encycl. metrop. feb. on a peculiar defect in the eye, and a camb. phil. soc. mode of correcting it. may on the forms of the teeth of wheels. camb. phil. soc. may on laplace's investigation of the attraction camb. phil. soc. of spheroids differing little from a sphere. june on the figure of the earth. phil. trans. nov. on the disturbances of pendulums and camb. phil. soc. balances, and on the theory of escapements. feb. remarks on a correction of the solar phil. trans. tables, required by mr south's observations. may on some passages in mr ivory's remarks phil. mag. on a memoir by m. poisson relating to the attraction of spheroids. may on the spherical aberration of the camb. phil. soc. may eyepieces of telescopes. dec. on the corrections in the elements of phil. trans. delambre's solar tables required by the observations made at the royal observatory, greenwich. feb. address to the members of the senate, on an improvement in the position of the plumian professor. nov. on the longitude of the cambridge observatory. camb. phil. soc. nov. on a method of determining the mass of astr. soc. the moon from transit observations of (memoirs) venus near her inferior conjunction. nov. on a correction requisite to be applied camb. phil. soc. to the length of a pendulum consisting of a ball suspended by a fine wire. dec. on certain conditions under which a camb. phil. soc. perpetual motion is possible. aug. figure of the earth. encycl. metrop. feb. on the nature of the light in the two camb. phil. soc. rays produced by the double refraction of quartz. apr. addition to the above paper. camb. phil. soc. nov. on a remarkable modification of newton's camb. phil. soc. rings. nov. on an inequality of long period in the phil. trans. motions of the earth and venus. jan. translation of encke's dissertation (on encke's comet) contained in nos. and of the astronomische nachrichten. mar. on a new analyzer, and its use in camb. phil. soc. experiments of polarization. mar. on the phenomena of newton's rings when formed between two transparent substances of different refractive powers. may report on the progress of astronomy trans brit. ass. during the present century. oct. report of the syndicate of the cambridge observatory. feb. remarks on mr potter's experiment on phil. mag. interference. apr. on the mass of jupiter, as determined r. astr. soc. from the observation of elongations of (memoirs) the fourth satellite. syllabus of a course of experimental lectures. may on the calculation of newton's camb. phil. soc. experiments on diffraction. may remarks on sir david brewster's paper phil. mag. "on the absorption of specific rays" &c. may results of the repetition of mr potter's phil. mag. experiment of interposing a prism in the path of interfering light. may on a supposed black bar formed by phil. mag. diffraction. june report on mr barlow's fluid-lens r. soc. (proc.) telescope mar. continuation of researches into the value r. astr. soc. of the mass of jupiter, by observation of (memoirs.) the elongations of the fourth satellite. apr. on the latitude of cambridge observatory camb. phil. soc. june report of the syndicate of the cambridge observatory. june on the position of the ecliptic, as inferred r. astr. soc. inferred from transit and circle (memoirs.) observations made at cambridge observatory in the year . june observations of the solar eclipse of july r. astr. soc. th, , made at cambridge observatory, (memoirs.) and calculations of the observations. nov. on the diffraction of an object-glass camb. phil. soc. with circular aperture. dec. on the calculation of the perturbations naut. alm. of the small planets and the comets of ( , app.) short period. may continuation of researches into the value r. astr, soc. of jupiter's mass. (memoirs.) june report of the syndicate of the cambridge observatory. june on the position of the ecliptic, as r. astr. soc. inferred from observations with the (memoirs.) cambridge transit and mural circle, made in the year . june on the time of rotation of jupiter. r. astr. soc. (memoirs.) feb. speech on delivering the medal of the r. astr. soc. r. astr. soc. to sir john herschel. (proc.) june report of the astronomer royal to the board of visitors. june report upon a letter (on a systematic r. soc. course of magnetic observations) addressed (proc.) by m. le baron de humboldt to his royal highness the president of the royal society (by s. hunter christie and g.b. airy). jan. continuation of researches into the value r. astr. soc. of jupiter's mass. (memoirs.) feb. speech on delivering the medal of the r. astr. soc. r. astr. soc. to professor rosenberger. (proc) mar. results of the observations of the sun, r. astr. soc. moon, and planets, made at cambridge (memoirs) observatory in the years , , and . may on the position of the ecliptic, as r. astr. soc. inferred from observations with the (memoirs) cambridge transit and mural circle, made in the year . june report of the astronomer royal to the board of visitors. sept. address delivered in the town hall of neath. nov. on the parallax of alpha lyrae. r. astr. soc. (memoirs.) feb. address to the earl of burlington on religious examination in the university of london. mar. on the intensity of light in the camb. phil. soc. neighbourhood of a caustic. june report of the astronomer royal to the board of visitors. dec. a catalogue of stars, deduced from r. astr. soc. the observations made at the cambridge (memoirs.) observatory, from to ; reduced to january , . apr. account of experiments on iron-built phil. trans. ships, instituted for the purpose of discovering a correction for the deviation of the compass produced by the iron of the ships. june report of the astronomer royal to the board of visitors. nov. on the determination of the orbits of r. astr. soc. comets, from observations. (memoirs.) article "gravitation." penny cyclop. article "greenwich observatory." penny cyclop. mar. on a new construction of the camb. phil. soc. going-fusee. mar. on the regulator of the clock-work for r. astr. soc. effecting uniform movement of equatoreals. may on the correction of the compass in un. serv. journ. iron-built ships. (proc.) results of experiments on the disturbance j. weale. of the compass in iron-built ships. june report of the astronomer royal to the board of visitors. june on the theoretical explanation of an phil. trans. apparent new polarity in light. nov. supplement to the above paper. phil. trans. dec. on the diffraction of an annular aperture. phil. mag. dec. remarks on professor challis's investigation phil. mag. of the motion of a small sphere vibrating in a resisting medium. jan. correction to the above paper "on the phil. mag. diffraction," &c. mar. remarks on professor challis's reply to phil. mag. mr airy's objections to the investigation of the resistance of the atmosphere to an oscillating sphere. june report of the astronomer royal to the board of visitors. july reply to professor challis, on the phil. mag. investigation of the resistance of the air to an oscillating sphere. oct. extraordinary disturbance of the magnets. nov. on the laws of the rise and fall of the phil. trans. tide in the river thames. dec. report of the commissioners appointed to consider the steps to be taken for restoration of the standards of weight and measure. apr. on the [greek: ichtis] of diodorus athenaeum. may account of the ordnance zenith sector. r. astr. soc. (proc.) june report of the astronomer royal to the board of visitors. nov. observations of the total solar eclipse of r. astr. soc. july . (memoirs.) dec. remarks on the present state of hatcliff's private charity (greenwich). article on tides and waves. encyc. metrop. mar. on the laws of individual tides at phil. trans. southampton and at ipswich. apr. on monetary and metrical systems. athenaeum. june report of the astronomer royal to the board of visitors. sept. address to the individual members of the board of visitors of the royal observatory (proposing the altazimuth). oct. account of the northumberland equatoreal and dome, attached to the cambridge observatory. nov. address and explanation of the proposed altitude and azimuth instrument to the board of visitors of the royal observatory. june report of the astronomer royal to the board of visitors. dec. on the laws of the tides on the coasts of phil. trans. ireland, as inferred from an extensive series of observations made in connection with the ordnance survey of ireland. jan. on the flexure of a uniform bar r. astr. soc. supported by a number of equal pressures (memoirs.) applied at equidistant points, &c. feb. speech on delivering the medal of the r. astr. soc. r. astr. soc. to capt. smyth (proc.) may on a new construction of the divided r. astr. soc. eye-glass double-image micrometer. (memoirs.) june report of the astronomer royal to the board of visitors. july on wexford harbour. report of the gauge commissioners. and letter to sir e. ryan. may on the equations applying to light under phil. mag. the action of magnetism. may remarks on dr faraday's paper on phil. mag. ray-vibrations. may on a change in the state of an eye camb. phil. soc. affected with a mal-formation. june report of the astronomer royal to the board of visitors. june account of the measurement of an arc of r. astr. soc. longitude between the royal observatory (month. not.) of greenwich and the trigonometrical station of feagh main, in the island of valentia. july letter to sir robert harry inglis, bart., athenaeum. m.p., in answer to sir james south's attack on the observations at the greenwich observatory. nov. on the bands formed by the partial phil. mag. interception of the prismatic spectrum. nov. account of some circumstances historically r. astr. soc. connected with the discovery of the (memoirs.) planet exterior to uranus. jan. reduction of the observations of halley's r. astr. soc. comet made at the cambridge observatory in (memoirs.) the years and . jan. on a proposed alteration of bessel's method r. astr. soc. for the computation of the corrections by (memoirs.) which the apparent places of stars are derived from the mean places. feb. on sir david brewster's new analysis of phil. mag. solar light. feb. on the name of the new planet. athenaeum. feb. mr adams and the new planet. athenaeum. plan of the buildings and grounds of the royal observatory, greenwich, with explanation and history. may explanation of hansen's perturbations of r. astr. soc. the moon by venus. (month. not.) june report of the astronomer royal to the board of visitors. nov. address to the individual members of the board of visitors of the royal observatory. (zenith tube.) dec. results deduced from the occultations of r. astr. soc. stars and planets by the moon, observed (memoirs.) at cambridge observatory from to . feb. abstract of struve's "Ã�tudes d'astronomie r. astr. soc. stellaire." (month. not.) mar. syllabus of lectures on astronomy to be delivered at the temperance hall, ipswich. apr. remarks on prof. challis's theoretical phil. mag. determination of the velocity of sound may supplement to a paper on the intensity of camb. phil. soc. light in the neighbourhood of a caustic. may address to individual members of the board of visitors. (new transit circle, reflex zenith tube, &c.) june report of the astronomer royal to the board of visitors. june corrections of the elements of the moon's r. astr. soc. orbit, deduced from the lunar (memoirs.) observations made at the royal observatory, of greenwich from to . aug. explanation of a proposed construction of zenith sector: addressed to the board of visitors of the royal observatory, greenwich. oct. on the construction of chinese balls athenaeum. description of the instruments of process used in the photographic self-registration of the magnetical and meteorological instruments at the royal observatory, greenwich. description of the altitude and azimuth instrument erected at the royal observatory, greenwich, in the year . astronomy. (tract written for the scientific manual.) mar. substance of the lecture delivered by the r. astr. soc. astronomer royal on the large reflecting (month. not.) telescopes of the earl of rosse and mr lassell. june on a difficulty in the problem of sound. phil. mag. june report of the astronomer royal to the board of visitors. june on instruments adapted to the measure of r. astr. soc. small meridional zenith distances. (month. not.) nov. results of the observations made by the r. astr. soc. rev. fearon fallows at the royal (memoirs.) observatory, cape of good hope, in the years , , . nov. on bell's calculating machine, and on r. astr. soc. lord rosse's telescope. (month. not.) nov. on the exodus of the israelites. athenaeum. dec. on the method of observing and recording r. astr. soc. transits, lately introduced in america, &c. (month. not.) jan. on a problem of geodesy. phil. mag. feb. address on presenting the medal of the r. astr. soc. r. astr. soc. to m. otto von struve. (month. not.) mar. on the present state and prospects of the r. inst. science of terrestrial magnetism. mar. on the exodus of the israelites athenaeum. mar. on the exodus of the israelites. athenaeum. may statement concerning assistance granted r. astr. soc. by the admiralty to hansen--also on (month. not.) henderson's numbers for the teeth of wheels. may on the weights to be given to the separate r. astr. soc. results for terrestrial longitudes, (memoirs.) determined by the observation of transits of the moon and fixed stars. june report of the astronomer royal to the board of visitors. june letter from hansen on his lunar tables.--valz r. astr. soc. on an arrangement of double-image (month. not.) micrometer.--on the computation of longitude from lunar transits dec. on a method of regulating the clock-work r. astr. soc. for equatoreals. (month. not.) dec. supplement to a paper "on the regulation r. astr. soc. of the clock-work for effecting uniform (memoirs.) movement of equatoreals." dec. on the relation of the direction of the phil. trans. wind to the age of the moon, as inferred from observations made at the royal observatory, greenwich, from nov. to dec. jan. remarks on mr wyatt's paper on the inst. c.e. construction of the building for the (minutes.) exhibition of the works of industry of all nations in . feb. address on presenting the medal of the r. astr. soc. r. astr. soc. to dr annibale de (month. not.) gasparis. mar. letter to professor challis regarding the adams prize. mar. on caesar's place of landing in britain. athenaeum. suggestions to astronomers for the brit. assoc. observation of the total eclipse of the sun on july , . apr. on the determination of the probable r. astr. soc. stability of an azimuthal circle by (month. not.) observations of star and a permanent collimator. may on the total solar eclipse of , july . r. inst. (lecture.) may on the vibration of a free pendulum in an r. astr. soc. oval differing little from a straight line (memoirs) june report of the astronomer royal to the board of visitors. july the president's address to the twenty-first athenaeum. meeting of the british association for the advancement of science, ipswich. oct. on julius caesar's expedition against naut. mag. england, in relation to his places of departure and landing. nov. account of the total eclipse of the sun on r. astr. soc. , july , as observed at göttenburg, (memoirs.) at christiania, and at christianstadt. dec. on the geography of the exodus. athenaeum. jan. on the solar eclipse of july , . r. astr. soc. (month. not.) on the place of caesar's departure from soc. of antiq. gaul for the invasion of britain, and (memoirs.) the place of his landing in britain, with an appendix on the battle of hastings. on a new method of computing the naut. alm. , perturbations of planets, by j.f. app. encke--translated and illustrated with notes by g.b. airy. june report of the astronomer royal to the board of visitors. feb. on the eclipses of agathocles, thales, phil. trans. and xerxes. feb. lecture on the results of recent r. inst. calculations on the eclipse of thales and eclipses connected with it. may address to the individual members of the board of visitors of the royal observatory, greenwich. (lunar reductions.) may on decimal coinage. athenaeum. june report of the astronomer royal to the board of visitors. june lecture on the determination of the r. astr. soc. longitude of the observatory of (month. not.) cambridge by means of galvanic signals. sept. on decimal coinage. athenaeum. dec. description of the transit circle of the royal observatory, greenwich. (app. gr. observ. .) dec. regulations of the royal observatory, greenwich. (app. gr. observ. .) jan. on the telegraphic longitude of brussels. athenaeum. feb. address on presenting the gold medal of r. astr. soc. the r. astr. soc. to mr charles rümker. (month. not.) feb. on reforms in the university of cambridge. athenaeum. apr. letters relating to "the late m. mauvais." liter. gaz. june report of the astronomer royal to the board of visitors. sept. the deluge. private. oct. on the correction of the compass in iron athenaeum. ships. (scoresby's experiments.) nov. on the difference of longitude between r. astr. soc. the observatories of brussels and greenwich, (memoirs.) as determined by galvanic signals. jan. lecture at s. shields on the pendulum experiments in the harton pit, and letter on the results. feb. lecture on the pendulum experiments r. inst. lately made in the harton colliery for ascertaining the mean density of the earth. feb. on the correction of the compass in iron athenaeum. ships. (remarks on dr scoresby's investigations.) address on presenting the medal of the r. astr. soc. r. astr. soc. to the rev. william rutter (month. not.) dawes. feb. on the computation of the effect of the phil. trans. attraction of mountain masses, as disturbing the apparent astronomical latitude of stations in geodetic surveys. june report of the astronomer royal to the board of visitors. oct. address to the individual members of the board of visitors of the royal observatory, greenwich. (equatoreal.) nov. remarks upon certain cases of personal r. astr. soc. equation which appear to have hitherto (memoirs.) escaped notice, accompanied with a table of results. nov. discussion of the observed deviations of phil. trans. the compass in several ships, wood-built and iron-built: with a general table for facilitating the examination of compass-deviations. description of the reflex zenith tube of the royal observatory, greenwich. (app. to the greenwich obs. for .) jan. on professor peirce's criterion for astr. journ. discordant observations. (cambr.) jan. account of pendulum experiments undertaken phil. trans. in the harton colliery, for the purpose of determining the mean density of the earth. june report of the astronomer royal to the board of visitors. aug. on scheutz's calculating machine. phil. mag. aug. science and the government. (reply to athenaeum. statements in the morning chronicle about the instrumental equipment of the royal observatory.) may on the means which will be available for r. astr. soc. correcting the measure of the sun's (month. not.) distance in the next twenty-five years. may knowledge expected in computers and assistants in the royal observatory. june report of the astronomer royal to the board of visitors. june on the eclipse of agathocles, the eclipse r. astr. soc. at larissa, and the eclipse of thales. (memoirs.) with an appendix on the eclipse of stiklastad. june account of the construction of the new phil. trans. national standard of length, and of its principal copies. dec. letter to the vice-chancellor of cambridge university regarding smith's prizes. dec. on the substitution of methods founded camb. phil. soc. on ordinary geometry for methods based on the general doctrine of proportions, in the treatment of some geometrical problems description of the galvanic chronographic gr. obs. , apparatus of the royal observatory, app. greenwich. mar. suggestions for observation of the annular eclipse of the sun on , march - . mar. note on oltmann's calculation of the r. astr. soc. eclipse of thales. also on a method (month. not.) of very approximately representing the projection of a great circle upon mercator's chart. may the atlantic cable problem. naut. mag. may report of the ordnance survey commission; together with minutes of evidence and appendix. june report of the astronomer royal to the board of visitors. june on the mechanical conditions of the phil. mag. deposit of a submarine cable. july instructions and chart for observations r. astr. soc. of mars in right ascension at the (special.) opposition of for obtaining the measure of the sun's distance. aug. on the advantageous employment of photog. notes. stereoscopic photographs for the representation of scenery. nov. on the "draft of proposed new statutes athenaeum. for trinity college, cambridge." nov. letter to the vice-chancellor of the university of cambridge, offering the sheepshanks endowment. dec. suggestion of a proof of the theorem camb. phil. soc. that every algebraic equation has a root. manual of astronomy--for the admiralty. parly. paper. feb. letter to lord monteagle relating to the standards of weights and measures. feb. remarks on mr cayley's trigonometrical phil. mag. theorem, and on prof. challis's proof that equations have roots. mar. on the movement of the solar system in r. astr. soc. space. (memoirs.) apr. on the apparent projection of stars upon r. astr. soc. the moon's disc in occultations. also (month. not.) comparison of the lunar tables of burckhardt and hansen with observations of the moon made at the royal observatory, greenwich. apr. on the apparent projection of stars upon r. astr. soc. the moon's disc in occultations. (memoirs.) june report of the astronomer royal to the board of visitors. june abstract of maxwell's paper "on the r. astr. soc. stability of the motion of saturn's rings." (month. not.) july corrections of the elements of the moon's r. astr. soc. orbit, deduced from the lunar observations (memoirs.) made at the royal observatory of greenwich from to . sept. on the invasion of britain by julius caesar. athenaeum. (answer to mr lewin.) nov. on iron ships--the royal charter. athenaeum. (answer to archibald smith's remarks.) nov. circular requesting observations of small planets. dec. notice of the approaching total eclipse of r. astr. soc. the sun of july , , and suggestions (month. not.) for observation. dec. supplement to a proof of the theorem camb. phil. soc. that every algebraic equation has a root. jan. description of the new equatoreal at the r. astr. soc. royal observatory, greenwich. also (month. not.) abstract of an essay by gen. t.f. de schubert on the figure of the earth. jan. on the claudian or plautian invasion of athenaeum britain. feb. examination of navy -foot telescopes at the royal observatory, greenwich, , jan. to feb. . feb. report on the instrumental equipments ho. of commons. of the exchequer office of weights and (parly. paper.) measures, as regards the means for preventing fraud in the sale of gas to the public; and on the amendments which may be required to the existing legislation on that subject. mar. address on the approaching solar eclipse r. astr. soc. of july , , &c. (month. not.) may correspondence between the lords ho. of commons. commissioners of her majesty's treasury, (parly. paper.) &c., and the astronomer royal, relating to gas measurement, and the sale of gas act. june report of the astronomer royal to the board of visitors. and address to the members of the board in reference to struve's geodetic suggestions. june correspondence regarding the grant of _£ _ to prof. hansen for his lunar tables. sept. remarks on a paper entitled "on the polar distances of the greenwich transit circle, by a. marth." addressed to the members of the board of visitors. sept. on change of climate, in answer to athenaeum. certain speculations by sir henry james. oct. circular relating to the distribution of greenwich observations and other publications of the royal observatory. nov. account of observations of the total r. astr. soc. solar eclipse of , july , made (month. not.) at hereña, near miranda de ebro; &c. &c. nov. on change of climate: further discussion. athenaeum. letters on lighthouses, to the commission on lighthouses. dec. note on the translation of a passage in a r. astr. soc. letter of hansen's relating to (month. not.) coefficients. feb. on the temperature-correction of syphon athenaeum. barometers. march results of observations of the solar r. astr. soc. eclipse of july made at the royal (month. not.) observatory, greenwich, for determination of the errors of the tabular elements of the eclipse. also suggestion of a new astronomical instrument, for which the name "orbit-sweeper" is proposed. also theory of the regulation of a clock by galvanic currents acting on the pendulum. june report of the astronomer royal to the board of visitors. june on a supposed failure of the calculus of phil. mag. variations. july report of a committee of the r. soc. on r. soc.(proc.) the advisability of re-measuring the indian arc of meridian. sept. lecture at manchester on the great solar athenaeum. eclipse of july , . sept. the same lecture. london review. oct. examination paper for the sheepshanks exhibition. nov. translation of dr lamont's paper "on the phil. mag. most advantageous form of magnets." nov. note on a letter received from hansen on r. astr. soc. the lunar theory. also discussion of (month. not.) a result deduced by mr d'abbadie from observations of the total solar eclipse of , july . nov. instructions for observing the total eclipse of the sun on december . dec. on a projection by balance of errors for phil. mag. maps. dec. on the circularity of the sun's disk. r. astr. soc. also table of comparative number of (month. not.) observations of small planets. jan. on the direction of the joints in the phil. mag. faces of oblique arches. mar. review of "an historical survey of the athenaeum. astronomy of the ancients" by the rt hon. sir g. cornewall lewis. apr. notes for the committee on weights and measures, . may on the magnetic properties of hot-rolled phil. trans. and cold-rolled malleable iron. june report of the astronomer royal to the board of visitors. june evidence given before the select committee on weights and measures. oct. biography of g.b. airy (probably in part london review. based upon data supplied by himself). oct. abstract of paper "on the strains in the athenaeum. interior of beams and tubular bridges." oct. translation of a letter from prof. lament phil. mag. on dalton's theory of vapour, &c. nov. on the strains in the interior of beams. phil. trans. nov. correspondence with sabine concerning his attack on the greenwich magnetic observations. (confidentially communicated to the board of visitors.) nov. evidence given before the public schools commission. nov. abstract of m. auwers's paper on the r. astr. soc. proper motion of procyon, and note on (month. not.) same. dec. abstract of mr safford's paper on the r. astr. soc. proper motion of sirius. also on the (month. not.) forms of lenses proper for the negative eye-pieces of telescopes. also on the measurements of the earth, and the dimensions of the solar system. also on fringes of light in solar eclipses. jan. address to the board of visitors on a further attack by sabine on the greenwich magnetic observations (confidential). jan. on the observations of saturn made at r. astr. soc. pulkowa and greenwich. (month. not.) feb. report to the board of trade on the proposed lines of railway through greenwich park. mar. determination of the longitude of valencia in ireland by galvanic signals in the summer of (app. iii. to the gr. astr. obsns. ). mar. on the movement of the solar system in r. astr. soc. space, deduced from the proper motions (memoirs.) of stars. by edwin dunkin (for g.b.a.). mar. on the visibility of stars in the pleiades r. astr. soc. to the unarmed eye. (month. not.) mar. on marriage odes. athenaeum. apr. further report as to the probable effects of the london, chatham and dover railway on the royal observatory in greenwich park. apr. determination of the sun's parallax from r. astr. soc. observations of mars during the (month. not.) opposition of . by e.j. stone (for g.b.a.). also remarks on struve's account of a local deviation in the direction of gravity, near moscow. also an account of an apparatus for the observation of the spectra of stars, and results obtained. apr. on the diurnal inequalities of phil. trans. terrestrial magnetism, as deduced from observations made at the royal observatory, greenwich, from to . may on the discordance between the results r. astr. soc. for zenith-distances obtained by direct (memoirs.) observation, and those obtained by observation by reflection from the surface of quicksilver. june report of the astronomer royal to the board of visitors. july on the amount of light given by the r. astr. soc. moon at the greatest stage in the (month. not.) excentrically-total eclipse, , june . aug. plan of the buildings and grounds of the royal observatory, greenwich, with explanation and history. sept. on the origin of the apparent luminous r. astr. soc. band which, in partial eclipses of the (month. not.) sun, has been seen to surround the visible portion of the moon's limb. sept. on the invasions of britain by julius athenaeum. oct. caesar. oct. the earthquake as observed from greenwich. athenaeum. nov. on the numerical expression of the phil. mag. destructive energy in the explosions of steam-boilers, &c. nov. convention arranged between m. le verrier r. astr. soc. and the astronomer royal for meridional (month. not.) observations of the small planets, &c. nov. translation of hansen's paper r. astr. soc. "calculation of the sun's parallax (month. not.) from the lunar theory," with notes by g.b.a. dec. first analysis of magnetic storms, phil. trans. registered by the magnetic instruments in the royal observatory, greenwich, from to . jan. pontécoulant's paper "sur le coefficiant r. astr. soc. de l'Ã�quation parallactique déduit de la (month. not.) théorie," with notes by g.b.a. jan. remarks on redman's paper on the east inst. c. e. coast (chesil bank, &c.). (minutes.) mar. note on a passage in capt. r. astr. soc. jacob's "measures of jupiter," &c. (month. not.) mar. notes for the committee on weights and ho. of comm. measures, . (parly. paper.) mar. on a method of slewing a ship without inst. nav. arch. the aid of the rudder. apr. comparison of the chinese record of solar r. astr. soc. eclipses in the chun tsew with the (month. not.) computations of modern theory. june report of the astronomer royal to the board of visitors. june on the transit of venus, , dec. . r. astr. soc. (month. not.) june on the bright band bordering the moon's r. astr. soc. limb in photographs of eclipses. (month. not.) notes on methods of reduction applicable to the indian survey. sept. a visit to the corryvreckan. athenaeum. sept. examination paper for the sheepshanks scholarship. jan. comparison of the transit-instrument in r. astr. soc. its ordinary or reversible form with the (month. not.) transit-instrument in its non-reversible form, as adopted at greenwich, the cape of good hope, and other observatories. mar. syllabus of a course of three lectures on "magnetical errors, &c., with special reference to iron ships and their compasses," delivered at the south kensington museum. apr. remarks on mr ellis's lecture on the horolog. journ. greenwich system of time signals. apr. free translation of some lines of virgil, athenaeum. "citharâ crinitus iopas," &c. june report of the astronomer royal to the board of visitors. june note on my recommendation (in ) athenaeum. of government superintendence of the compasses of iron ships. also note on the birthplace of thomas clarkson. july on hemiopsy. phil. mag. aug. on the value of the moon's semidiameter r. astr. soc. as obtained by the investigations of (month. not.) hugh breen, esq., from occultations observed at cambridge and greenwich. sept. on "the land of goshen"--reply to "a athenaeum. suffolk incumbent." oct. address of the astronomer royal to the individual members of the board of visitors. (on improved collimators.) oct. note on an error of expression in two r. astr. soc. previous memoirs. also description and (month. not.) history of a quadrant made by abraham sharp. nov. on the possible derivation of the national athenaeum. name "welsh." essays on the invasion of britain by julius private. caesar; the invasion of britain by plautius, and by claudius caesar; the early military policy of the romans in britain; the battle of hastings. (with corr.) mar. on "the compass in iron ships." objections athenaeum. to passages in a lecture by archibald smith. apr. on the supposed possible effect of r. astr. soc. friction in the tides, in influencing the (month. not.) apparent acceleration of the moon's mean motion in longitude. also on a method of computing interpolations to the second order without changes of algebraic sign. june report of the astronomer royal to the board of visitors. july papers relating to time signals on the ho. of comm. start point. (parly. paper.) sept. on the campaign of aulus plautius in athenaeum. britain. (reply to dr guest.) nov. on the continued change in an eye camb. phil. soc. affected with a peculiar malformation. dec. on the simultaneous disappearance of r. astr. soc. jupiter's satellites in the year . (month. not.) also inference from the observed movement of the meteors in the appearance of , nov. - . jan. memorandum for the consideration of the commission on standards. (policy of introducing metrical standards.) jan. on decimal weights and measures. athenaeum. feb. on the use of the suspension bridge with inst. c.e. stiffened roadway for railway and other (minutes.) bridges of great span. mar. computation of the lengths of the waves phil. trans. of light corresponding to the lines in the dispersion spectrum measured by kirchhoff. mar. corresponding numbers of elevation in r. obs. (also english feet, and of readings of aneroid meteor. soc. or corrected barometer in english apr. , .) inches. apr. remarks on sir w. denison's paper on inst. c.e. "the suez canal." (minutes.) may statement of the history and position of private. the blue-coat girls' school, greenwich. june report of the astronomer royal to the board of visitors. june on certain appearances of the telescopic r. astr. soc. images of stars described by the rev. (month. not.) w.r. dawes. dec. note on the total solar eclipse of , r. astr. soc. aug. - . (month. not.) biography of g.b. airy. (probably corrected by himself.) jan. biography (with portrait) of g.b. airy. ill. lond. news. (probably corrected by himself.) feb. comparison of magnetic disturbances phil. trans. recorded by the self-registering magnetometers at the royal observatory, greenwich, with magnetic disturbances deduced from the corresponding terrestrial galvanic currents recorded by the self-registering galvanometers of the royal observatory. mar. address of the astronomer royal to the individual members of the board of visitors. (number of copies of observations.) june report of the astronomer royal to the board of visitors. july first report of the commissioners appointed parly. paper. to enquire into the condition of the exchequer standards. sept. the inundation at visp. athenaeum. nov. on the factorial resolution of the trinomial camb. phil. soc. x^n - cos n. a. + /x^n. dec. on the diurnal and annual inequalities phil. trans. of terrestrial magnetism, as deduced from observations made at the royal observatory from to , &c. dec. on the preparatory arrangements for the r. astr. soc. observation of the transits of venus (month. not.) and . dec. on the migrations of the welsh nations. athenaeum. mar. memorandum by the chairman (on the use of the troy weight) for the consideration of the members of the standards commission. apr. second report of the commissioners appointed parly. paper. to enquire into the condition of the exchequer (now board of trade) standards.--the metric system. april syllabus of lectures on magnetism to be delivered in the university of cambridge. apr. remarks on shelford's paper "on the inst. c.e. outfall of the river humber." (minutes.) june memorandum for the consideration of the standards commission, on the state of the question now before them regarding the suggested abolition of troy weight. june report of the astronomer royal to the board of visitors. supplementary memorandum by the astronomer royal on the proposed abolition of troy weight. july correspondence between the treasury, the ho. of comm. admiralty, and the astronomer royal, (parly. paper.) respecting the arrangements to be made for observing the transits of venus, which will take place in the years and . aug. note on atmospheric chromatic dispersion r. astr. soc. as affecting telescopic observation, and (month. not.) on the mode of correcting it. oct. description of the great equatoreal of the royal observatory, greenwich. greenwich observations, . app. feb. note on an extension of the comparison phil. trans. of magnetic disturbances with magnetic effects inferred from observed terrestrial galvanic currents; &c. &c. apr. on the question of a royal commission journ. soc. arts. for science. may letters to the first lord of the admiralty enclosing application of the assistants for an increase of salaries. may on decimal and metrical systems. journ. soc. arts. june report of the astronomer royal to the board of visitors. aug. on the meaning of the word "whippultree." athenaeum. oct. on the locality of "paradise." athenaeum. nov. on the locality of the roman gesoriacum. athenaeum. nov. recommendation of prof. miller for a r. soc.(proc.) royal medal of the royal society. (quoted by the president.) revised edition of "astronomy." man. naut. sci. jan. the burial of sir john moore. athenaeum. mar. letter to the hydrographer of the admiralty on the qualifications and claims of the assistants of the royal observatory. apr. remarks on the determination of a ship's r. soc. (proc.) place at sea. may remarks on samuelson's paper "description inst. c.e. of two blast furnaces," &c. (minutes.) may note on barometric compensation of the phil. mag. pendulum. june report of the astronomer royal to the board of visitors. june remarks on mr abbott's observations on r. astr. soc. eta argûs. also on a.s. herschel's and (month. not.) j. herschel's mechanism for measuring time automatically in taking transits. erratum in results of greenwich r. astr. soc. observations of the solar eclipse of , (month. not.) july . also observations of the solar eclipse of , dec. - , made at the royal observatory, greenwich. aug. investigation of the law of the progress phil. mag. of accuracy in the usual process for forming a plane surface. nov. corrections to the computed lengths of phil. trans. waves of light for kirchhoff's spectral lines. on a supposed alteration in the amount r. soc. (proc.) of astronomical aberration of light, produced by the passage of the light through a considerable thickness of refracting medium. nov. biography of g.b. airy. (probably daily telegraph. corrected by himself.) dec. note on a special point in the r. astr. soc. determination of the elements of the (month. not.) moon's orbit from meridional observations of the moon. dec. proposed devotion of an observatory to r. astr. soc. observation of the phenomena of jupiter's (month. not.) satellites. jan. address to the council of the royal society on the propriety of continuing the grant to the kew observatory for meteorological observations. feb. experiments on the directive power of phil. trans. large steel magnets, of bars of magnetized soft iron, and of galvanic coils, in their action on external small magnets--with appendix by james stuart. feb. further observations on the state of an camb. phil. soc. eye affected with a peculiar malformation. mar. notes on scientific education, submitted to the royal commission on scientific instruction and the advancement of science. may on a supposed periodicity in the r. soc. (proc.) elements of terrestrial magnetism, with a period of - / days. nov. address (as president) delivered at the anniversary meeting of the royal society. dec. magnetical observations in the phil. trans. britannia and conway tubular iron bridges. feb. remarks on mr thornton's paper on inst. c.e. "the state railways of (minutes.) india"--chiefly in reference to the proposed break of gauge. mar. note on the want of observations of r. astr. soc. eclipses of jupiter's first satellite (month. not.) from to . mar. letter to the secretary of the r. astr. soc. admiralty on certain articles which (month. not.) had appeared in the public newspapers in regard to the approaching transit of venus. additional note to the paper on a r. soc. (proc.) supposed alteration in the amount of astronomical aberration of light produced by the passage of the light through a considerable thickness of refracting medium. apr. list of candidates for election into the royal society--classified. on the topography of the "lady of private. the lake." june report of the astronomer royal to the board of visitors. nov. on the rejection, in the lunar r. astr. soc. theory, of the term of longitude (month. not.) depending for argument on eight times the mean longitude of venus minus thirteen times the mean longitude of the earth, introduced by prof. hansen; &c. dec. address (as president) delivered at the anniversary meeting of the royal society. jan. on a proposed new method of treating r. astr. soc. the lunar theory. (month. not.) may british expeditions for the observation of the transit of venus, , december . instructions to observers. june report of the astronomer royal to the board of visitors. aug. regulations of the royal observatory, greenwich. appendix to the greenwich observations, . oct. science and art. the moon as carved athenaeum. on lee church. nov. preparations for the observation of the r. astr. soc. transit of venus , december - . (month. not.) nov. remarks on the paper "on the nagpur inst. c.e. waterworks." (minutes.) dec. telegrams relating to the observations r. astr. soc. of the transit of venus , dec. . (month. not.) feb. remarks on mr prestwich's paper on the inst. c.e. origin of the chesil bank. (minutes.) feb letter to the rev. n. m. ferrers, on the subject of the smith's prizes. mar. on the method to be used in reducing r. astr. soc. the observations of the transit of (month. not.) venus , dec. . mar. report on the progress made in the r. astr. soc. calculations for a new method of (month. not.) treating the lunar theory. june report of the astronomer royal to the board of visitors. june apparatus for final adjustment of the horolog. journ. thermal compensation of chronometers, by the astronomer royal. nov. chart of the apparent path of mars, , r. astr. soc. with neighbouring stars. also (month. not.) spectroscopic observations made at the royal observatory, greenwich. also observations of the solar eclipse of , september - , made at the royal observatory, greenwich. jan. report by the astronomer royal on the r. astr. soc. present state of the calculations in his (month. not.) new lunar theory. jan. note on a point in the life of sir william athenaeum. herschel. mar. evidence given before the government committee on the meteorological committee. may on toasting at public dinners. public opinion. june report of the astronomer royal to the board of visitors, aug. on a speech attributed to nelson. athenaeum. dec. spectroscopic results for the rotation of r. astr. soc. jupiter and of the sun, obtained at the (month. not.) royal observatory, greenwich. jan. stars to be compared in r.a. with mars, r. astr. soc. , for determination of the parallax (month. not.) of mars. mar. note by the astronomer royal on the r. astr. soc. numerical lunar theory. also remarks (month. not.) on le verrier's intra-mercurial planet. also on observations for the parallax of mars. mar. remarks on a paper on "the river inst. c.e. thames." (minutes.) apr. on observing for le verrier's intra-mercurial r. astr. soc. planet. also on the parallax of (month. not.) mars, and mr gill's proposed expedition. may on the vulgar notion that the sun or moon the observatory is smallest when overhead. (no. ). june report of the astronomer royal to the board of visitors. july report on the telescopic observations of ho. of commons the transit of venus , made in the parly. paper. expedition of the british government, and on the conclusion derived from those observations. sept. on spurious discs of stars produced by the observatory oval object-glasses. (no. ). sept. obituary notice of the work of le daily news. verrier--died sept. , . nov. on the value of the mean solar parallax the observatory &c. from the british telescopic observations (no. ). of the transit of venus . also remarks on prof. adams's lunar theory. nov. on the inferences for the value of mean r. astr. soc. solar parallax &c. from the telescopic (month. not.) observations of the transit of venus , which were made in the british expedition for the observation of that transit. numerical lunar theory: appendix to greenwich astronomical observations . dec. on the tides at malta. phil. trans. correspondence with le verrier on his the observatory planetary tables in . (no. ). on the proposal of the french committee the observatory to erect a statue to le verrier. also (no. ). on the observation of the approaching transit of mercury. mar. on the correction of the compass in phil. mag. iron ships without use of a fixed mark. mar. on the standards of length in the the times. guildhall, london. apr. report of lecture on "the probable w. cumberland condition of the interior of the times. earth." on the probable condition of the trans. of the interior of the earth--revised cumberland edition of above lecture. assoc., &c. june discussion of the observations of the observatory the transit of mercury on may . (no. ). abstract of lecture delivered at the observatory cockermouth on "the interior of the (no. ). earth." june report of the astronomer royal to the board of visitors. july remarks on the measurement of the the observatory photographs taken in the transit of (no. ). venus observations. july on the variable star r. scuti: the observatory distortion in the photo-heliograph. (no. ). remarks on mr gill's heliometric the observatory observations of mars. (no. ). dec. note on a determination of the mass r. astr. soc. of mars, and reference to his own (month. not.) determination in . also note on the conjunction of mars and saturn, , june . jan. on the remarkable conjunction of the observatory the planets mars and saturn which (no. ). will occur on , june . feb. on the names "cabul" and "malek." athenaeum feb. on faggot votes in cornwall in . athenaeum mar. letter on the examination papers for the smith's prizes. apr. drafts of resolutions proposed concerning sadler's notes on the late admiral smyth's "cycle of celestial objects." june letter to le verrier, dated , the observatory feb. , in support of the method (no. ). of least squares. june remarks in debate on sadler's the observatory "notes" above-mentioned. (no. ). june report of the astronomer royal to the board of visitors. july index to the records of occasional r. astr. soc. observations and calculations made (month. not. at the royal observatory, greenwich, supplementary.) and to other miscellaneous papers connected with that institution. biography of g. b. airy (perhaps corrected by himself) in french, published at geneva. sept. on the construction and use of a phil. mag. scale for gauging cylindrical measures of capacity. on the theoretical value of the the observatory acceleration of the moon's mean (no. ). motion. on the secular acceleration of the observatory the moon--additional note. (no. ). apr. memoranda for the commission appointed to consider the tay bridge casualty. apr. on the theoretical value of the r. astr. soc. acceleration of the moon's mean (month. not.) motion in longitude produced by the change of eccentricity of the earth's orbit. may on the preparations to be made for r. astr. soc. observation of the transit of venus (month. not.) , dec. . on the present proximity of jupiter the observatory to the earth, and on the intervals of (no. ). recurrence of the same phaenomena. june report of the astronomer royal to the board of visitors. sept. on the _e muet_ in french. athenaeum. sept. excursions in the keswick keswick district. guardian. dec. description of flamsteed's the observatory equatoreal sextant, and remarks on (no. ). graham. addition to a paper entitled "on r. astr. soc. the theoretical value of the moon's (month. not. mean motion in longitude," &c. supplementary.) mar. effect on the moon's movement in r. astr. soc. latitude, produced by the slow (month. not.) change of position of the plane of the ecliptic. june report of the astronomer royal to the board of visitors. logarithms of the values of all inst. c. e. vulgar fractions with numerator and (minutes.) denominator not exceeding : arranged in order of magnitude. july a new method of clearing the lunar distance.--admiralty. aug. on a systematic interruption in the order phil. mag. of numerical values of vulgar fractions, when arranged in a series of consecutive magnitudes. sept. monthly means of the highest and r. soc. (proc.) lowest diurnal temperatures of the water of the thames, and comparison with the corresponding temperatures of the air at the royal observatory, greenwich. oct. on the proposed forth bridge. nature. dec. on the proposed forth bridge. nature. jan. on the ossianic poems. athenaeum. mar. on the proposed braithwaite and daily news. buttermere railway. times. standard. apr. memorandum on the progress of the numerical lunar theory, addressed to the board of visitors of the royal observatory, greenwich. letter on the apparent inequality in the the observatory mean motion of the moon. (no. ). aug. on a singular morning dream. nature. sept. power of organization of the common nature. mouse. nov. on chepstow railway bridge, with general nature. remarks suggested by that structure. mar. on the erroneous usage of the term athenaeum. "arterial drainage." on the comparison of reversible and the observatory non-reversible transit instruments. (no. ). nov. on an obscure passage in the koran. nature. (?) may an incident in the history of trinity athenaeum. college, cambridge. june incident no. in the history of trinity athenaeum. college, cambridge. nov. results deduced from the measure of phil. trans. terrestrial magnetic force in the horizontal plane, at the royal observatory, greenwich, from to . apr. integer members of the first centenary nature. satisfying the equation a² = b² + c². feb. on the earlier tripos of the university of nature. (?) cambridge: in mss. apr. on the establishment of the roman dominion nature. in south-east britain. july on a special algebraic function, and its camb. phil. soc. application to the solution of (?) some equations: in mss. books written by g. b. airy. mathematical tracts on physical astronomy, the figure of the earth, precession and nutation, and the calculus of variations. this was published in . in a nd edition published in the undulatory theory of optics was added to the above list. four editions of this work have been published, the last in . the undulatory theory of optics was published separately in . gravitation: an elementary explanation of the principal perturbations in the solar system. written for the penny cyclopaedia, and published previously as a book in . there was a nd edition in . trigonometry. this was written for the encyclopaedia metropolitana about , and was published as a separate book in under the title of "a treatise on trigonometry." six lectures on astronomy delivered at the meetings of the friends of the ipswich museum at the temperance hall, ipswich, in the month of march . these lectures under the above title, and that of "popular astronomy, a series of lectures," have run through twelve editions. on the algebraical and numerical theory of errors of observations and the combination of observations, st edition in , nd in , rd in . essays on the invasion of britain by julius caesar; the invasion of britain by plautius, and by claudius caesar; the early military policy of the romans in britain; the battle of hastings, with correspondence. collected and printed for private distribution in . an elementary treatise on partial differential equations. . on sound and atmospheric vibrations, with the mathematical elements of music. the st edition in , the nd in . a treatise on magnetism, published in . notes on the earlier hebrew scriptures, published in . numerical lunar theory, published in . index. accidents (see also illnesses) accounts acts and opponencies adams, prof. j.c. adams, john quincey agrarian fires aiken airy, william, father of g.b.a. airy, ann, mother of g.b.a. airy, william, brother of g.b.a., and basil r. airy, his son airy, arthur, brother of g.b.a. airy, elizabeth, sister of g.b.a. airy, richarda, wife of g.b.a. airy, children of g.b.a. george richard elizabeth arthur wilfrid hubert hilda christabel annot osmund allsop alnwick altazimuth instrument althorp, lord american observatories american method of recording observations (see galvanic registration) ampère ancient eclipses anderson, lessee of harton colliery anemometer (see meteorology) anniversary parties antiquarian researches and notes arago architecture (see cathedrals, &c.) astronomical society (see royal astr. soc.) astronomische gesellschaft athenaeum newspaper athenaeum club atkinson, senior wrangler atlantic cable atmospheric railway (see railways) auckland, lord aurora borealis australian observatories (see also observatories) auwers, dr babbage, charles baily, francis bakhuysen, of leyden balance (public balance) baldock, commander baldrey, assistant banks, optician baring, sir t. barlow, prof. barlow, w.h. barnard, proctor barnes, miss barnes, gorell barometers barry, sir c. barton, bernard baxter, secretary to the admiralty beacons, floating beaufort, captain beaumont's observatory bedingfield, pupil bell scholarships (see examinations) bessell, astronomer biddell, arthur, uncle of g.b.a. biddell, george, uncle of g.b.a. biddell, william, uncle of g.b.a. biddell, george arthur, son of arthur biddell biographical notes bissett, pupil blackwood, captain blakesley, canon blasting bliss's observations blomfield, g.b., pupil bloomfield, lord board of longitude boileau bond, g.p. books, written by g.b.a., appendix book society, cambr. bosanquet bouch, t. civ. eng. boundary of canada (see canada) bouvard, e. bowstead bradley's observations brazil, emperor of breakwaters (see harbours) breen, assistant brewster, sir d. bridges brinkley, dr bristow, miss britannia bridge (see bridges) brooke, charles british association brougham, lord browne, g.a. brunel, civ. eng. buck buckland, dr buckle, pupil burgoyne, sir j. burlington, lord burton busts (see portraits) calculating machines calvert cambridge observatory: assistants instruments printed observations general cambridge university cambridge observatory, u.s.a. canada boundary cankrein, pupil canning, lord cape of good hope, observatory and survey carpenter, assistant cartmell, dr case catalogues of stars (see stars) cathedrals and churches catton cavendish experiment cayley, prof. challis, prof. chalmers, dr cherbourg (see harbours) chesil bank childers childers, first lord of admiralty christchurch christie, prof. christie, astronomer royal chronographic barrel (see galvanic registration) chronometers churches (see cathedrals) church service cincinnati observatory clarendon, lord clark, latimer clarkson, thomas, and mrs clarkson cleasby, pupil clegg clinton, pupil clocks cockburn, sir g. coinage (see decimal coinage) colby, col. colchester colenso, bishop college hall collorado, count colonial observatories (see observatories) comets commissions compass corrections cookson, dr cooper, pupil cooper's telescope (see telescopes) copying press corbaux, miss corryvreckan whirlpool courtney, rev. j. cowper, first commissioner of works crawford, pupil criswick, assistant cropley, crosse, rev. e. cubitt, sir w. daguerrotypes dalhousie, lord davy, sir humphrey davy, dr daynou, lieut. deal time ball de berg decimal coinage and decimal subdividing dee navigation (see rivers) degrees (see also orders and elections to societies) deighton, publisher de la rive de la rue de launay deluge, the de morgan, a. denison, e.b. denison, sir w. denison, h. denmark, king of dent, clockmaker dent-dale devonshire, duke of dobbs, pupil dobree, lecturer docks (see harbours) dolcoath experiments dollond, instrument maker drainage drinkwater, bethune double-image micrometer douglas, sir h. dover (see harbours) dublin professorship (see professorships) dublin observatory (see observatories) duë, baron dundas, admiral dundonald, lord dunkin, assistant dunlop, astronomer durham observatory earnshaw earth currents eastons, manufacturers eclipses (see also ancient eclipses) edinburgh observatory edmonston, dr education (see university education) egyptian astronomical tablets elections to societies, &c. (see also degrees and orders) electricity, atmospheric ellenborough, lord ellis, w., assistant elphinstone encke and encke's comet encyclopaedia metropolitana engines (see steam-engines) equatoreal, large estcourt, col. evans, lecturer examinations exhibitions and prizes exodus of the israelites eye, defects of eye, estate at fallows, astronomer faraday farish farr fellowship field fisher fishmongers' company fletcher, isaac, m.p. floating island, derwentwater fluid telescope, barlow's foley forbes, prof. j.d. foster, messrs fox, alfred freedom of the city of london freemantle, sir t. french, dr friends, personal friends at cambridge fries, prof. galbraith galle galvanic communication, time-signals, clocks, and registration (see also earth currents) gambard gas act gauss gautier geodesy geology geological society germany gibson, pupil gilbert, messrs gilbert, davies gill, astronomer gladstone, w.e. glaisher, assistant glasgow observatory gordon gosset goulburn, chancellor of the exchequer gould, dr b.a. goussel graduation of circles grant, of glenmoriston great circle sailing (see navigation) great eastern (see ships) great exhibition great gable green, commander u.s.n. greenwich greenwich observatory, before his appointment as astronomer royal greenwich observatory: appointment as astronomer royal, and subsequently as visitor buildings and grounds in, instruments assistants computations papers and manuscripts (arrangement of) estimates printed observations visitations and reports general gresswell groombridge's catalogue (see stars) guest, caius college haarlem hall, col. halley and halley's comet hamilton hamilton, sir w.r. hamilton, admiral hansard hansen, prof. hansteen harbours harcourt, rev. w. vernon hartnup, astronomer harton colliery experiments haviland, dr hawkes, trinity college hebrew scriptures heliograph hencke henderson, astronomer henslow, prof. herbert, g. hereford herschel, sir john herschel, miss caroline herschel, col. j. hervey, pupil higman, tutor, trinity college hilgard, u.s.a. himalaya expedition hind, moderator hind, superintendent nautical almanac hopkins hovenden, pupil hudson huggins, dr humboldt, baron a. humphreys hussey, dr hustler, tutor, trinity college hyde parker, admiral hygrometers ibbotson, pupil iliff illnesses inequality, venus and earth inglis, sir r. institut de france institution of civil engineers inverness, northern institution of ipswich lectures ireland, notes of ivory jackson jackson, john james, sir h. janus (see steam-engines) jarrow (see harbours) jeffries jerrard, dr jervis, major jeune, dr, v.c. of oxford johnson, capt. johnson, astronomer jones, instrument-makers jones, r. journeys: scotland and cumberland; swansea; derbyshire, &c.; wales; keswick, &c.; cornwall, &c.; orléans; lake district, &c.; continent, observatories, &c.; cornwall, &c.; derbyshire; oxford &c.; cumberland; ireland; scotland; derbyshire, &c.; cumberland, &c.; ireland; kent; s. wales; luddington and yorkshire; border of scotland; s. wales; cumberland and yorkshire; south of ireland; ireland; france; cornwall; germany; petersburg, &c.; ireland; shetland; scotland; sweden; madeira; cumberland; cumberland; oban, &c.; italy and sicily; west highlands; switzerland; central france; spain (eclipse); cumberland; west highlands; west highlands; cumberland; norway; cumberland; switzerland; cumberland; cumberland; cumberland; scotland; scotland; n. of scotland; ireland; scotland, &c.; cumberland; cumberland; cumberland; cumberland; s. wales; cumberland ; cumberland julius caesar, landing of jupiter (see planets) keeling kennedy king, joshua kingstown knight, publisher knighthood, offers of lagarde laing landman, engineer langton lardner, dr lassell, and lassell's telescope latitude determinations lax, prof. lectures: college professorial miscellaneous lefevre, j.g.s. leitch, dr le verrier lewis, h. lewis, sir g.c. lightfoot, rev. dr lighthouses lightning lillingstone lindsay, lord listing, prof. liverpool observatory livingstone, dr lloyd, dr lloyd, prof. lockyer lodge london university london, freedom of the city long vacations, with pupils longitude determinations longitude, board of (see board of longitude) lowe, chancellor of the exchequer lubbock, sir john lucas (computer) lucasian professorship (see professorships) lunar reductions lunar theory and tables (see also numerical lunar theory) lyndhurst, lord lyons, sir e. macaulay, t.b. macdonnell, dr maclean, of loch buy maclear, astronomer madras observatory magnetic observatory and magnetism (see also meteorology, compass corrections, and earth currents) main, robert maine boundary (see canada) maiden, prof. malkin malta man-engines (see mines) manuscripts (see papers) mars (see planets) marshman, pupil marth, a. martin, trin. coll. maskelyne, astronomer mason mathematical investigations (see also appendix "printed papers") mathematical tracts mathematical subjects in maudslays and field may, ransomes and may medals melbourne university melville, lord mercury (see planets) merivale, dr meteorology meteors middleton, sir w. milaud military researches miller, prof. mines minto, lord mitchell, astronomer mitchell miss molesworth, sir w. monteagle, lord monument in playford church moon: observations of theory and tables of (see lunar theory and tables) reductions of observations of (see lunar reductions) mass of morpeth, lord morton, pierce, pupil murchison, sir r. murray, publisher musgrave, charles musgrave, t. archbishop myers nasmyth nautical almanac navigation neate, pupil neptune and uranus newall newcombe, prof. new forest northampton, lord northumberland telescope numerical lunar theory observatories: see american, australian, beaumont's, cambridge, cambridge u.s.a., cape of good hope, cincinnati, colonial, dublin, durham, edinburgh, glasgow, greenwich, liverpool, madras, oxford, paris, paramatta, pulkowa, st helena, williamstown occultations o'connell ogilby, pupil oppolzer, prof. opponencies (see acts and opponencies) optics orders (see also degrees and elections to societies) ouvaroff, count oxford observatory oxford, miscellaneous packington, sir j. palmerston, lord papers (see appendix "printed papers") papers, arrangement of parachute, fall of parallax (see sun) paramatta observatory parker, charles parker, vice-chancellor paris, dr paris observatory paris exhibition parliamentary elections pasley paul peacock, george pearson, dr peel, sir robert pendulum investigations and experiments penny cyclopaedia pension pentland percy, bishop personal sketch philosophical society, cambridge philpott, dr photography piers (see harbours) pinheiro, lieut. pipon, lieut. plana, astronomer planetary influences planetary reductions planets (see also transits of venus) plantamour playford plumian professorship (see professorships) pocket-books for observations pogson, astronomer pond, astronomer portlock, capt. portraits, busts, &c. post office, (clocks, &c.) post office, stamps and envelopes pouillet prince albert pritchard, rev. c. prizes (see exhibitions) probable errors professorships: dublin; lucasian; plumian public schools commission pulkowa observatory pupils: bedingfield; bissett; blomfield; buckle; cankrein; cleasby; clinton; cooper; crawford; dobbs; gibson; guest; hervey; hovenden; ibbotson; lewis; marshman; morton; neate; ogilby; parker; rosser; smith; tinkler; tottenham; turner; wigram; williamson pym, engineer queen, h.m. the queen, quéroualle, mdlle de quetelet railways, near observatory railway gauge commission railways, miscellaneous rain (see meteorology) rainbows ransomes, also ransomes and may , reach reflex zenith tube religious tests and views repsold rhodes richardson, assistant rigaud, prof. rivers robinson, dr robinson, capt. rogers, rev. rogers, school assistant romilly, lord ronalds rose, rev. h.j. rosse, lord, and rosse's telescope rosser, pupil rothery rothman round down cliff, blasting of rouse, rev. r.c. m. routh, dr e.j. royal astronomical society (see also appendix "printed papers") royal exchange clock royal institution royal society (see also appendix "printed papers") royal society of edinburgh rüncker, paramatta rüncker rundell rusby russell, lord john sabine, col. sadler, h. saint helena observatory samuda saturn (see planets) saunders, g.w. by saw-mills (see ship timbers) schehallien, mountain scholarship scholefield schumacher scientific manual scoop-wheels scoresby, dr scriptural researches (see hebrew scriptures) sedgwick, adam selwyn, prof. senate house examination (see also university education) sewers commission sheepshanks, rev. richard, and miss sheepshanks sheepshanks fund and scholarship shepherd, clock-maker ship-timbers, machinery for sawing, shirreff, capt. simmons simms, f.w. simms (see troughton and simms) skeleton forms sly, draughtsman smith, rev. r. smith, father-in-law of g.b.a., and mrs smith, smith, the misses smith, sisters of richarda airy, susanna; elizabeth; georgiana; florence; caroline smith, archibald smith, m., pupil smith's prizes smyth, capt. w.h. smyth, piazzi societies, &c., elections to (see elections) solar eclipses (see eclipses) solar inequality (see sun) solar system (see sun) solar tables (see sun) south, sir james south's telescope south-eastern railway southampton southey (poet) spectroscopy spottiswoode spring-rice, lord monteagle standards of length and weight, and standards commission stars start point steam-engines stephenson, george stephenson, robert steventon stewart, prof. balfour stjerneld, baron stokes, prof. stone, astronomer stratford, lieut. stroganoff, count strutt, lord belper strutt, jedediah struve, otto stuart, prof. j. sun: miscellaneous parallax of (see also transits of venus) eclipses of (see eclipses) inequality, venus and earth tables of surveys (see trigonometrical surveys) sussex, duke of, sutcliffe sutcliffe, miss sydney university sylvester sweden, king of tate taylor, architect taylor, first assistant to pond, taylor, h. telegraphs (see galvanic communications) telescopes (see also cambridge observatory instruments, and greenwich observatory instruments) teneriffe experiment thames, the river, theology (see also hebrew scriptures and colenso) thermometers thermo-multiplier thirlwall, bishop thomas, assistant thompson, master trin. coll. thomson, sir w. tidal harbour commission tides, time-signals and time (see also galvanic communication, &c.) time balls (see time signals) tinkler, pupil tottenham, pupil traill, dr transit circle, transits of venus trigonometrical survey trinity college, cambridge trinity house tripos examination (see senate-house examination) troughton and simms tulley, optician tupman, capt turner, pupil turton, prof. tutorship ulrich, j.g. universities (see cambridge, dublin, edinburgh, london, melbourne, oxford, sydney) university education (see also smith's prizes and senate-house examination) university press, uranus (see neptune) valencia (see also longitude determinations) venus (see planets, and transits of venus) venus and earth inequality (see inequality) vernon harcourt (see harcourt) vetch, capt. vibrations of ground vignoles, c.b., engineer vulliamy, clockmaker wales, prince of walker, byatt walker, james, engineer walker, sydney, warburton, h. washington, capt. water telescope (see also fluid telescope) watson waves (see tides) webster, m.p. for aberdeen western westminster clock (see also clocks) wexford harbour (see harbours) wheatstone whewell, william white house, the, wigram, pupil williams, john williamson, pupil williamstown observatory wilson, prof. winchester winds (see meteorology) winning wood, sir charles wood, dr woodbridge, suffolk woodhouse, prof. woolwich academy (see examinations) wordsworth, dr, master of trin. coll. wordsworth, poet wrede, baron wynter, vice-chancellor, oxford yolland, col. york cathedral young, dr [illustration: pike's peak, colorado] earth and sky every child should know easy studies of the earth and the stars for any time and place by julia ellen rogers author of "the tree book," "the shell book," "key to the nature library," "trees every child should know." illustrated by thirty-one pages of photographs and drawings [illustration] new york grosset & dunlap publishers copyright, , by doubleday, page & company published, october, all rights reserved, including that of translation into foreign languages, including the scandinavian acknowledgments a number of the photographs in this volume are used by permission of the american museum of natural history. the star maps and drawings of the constellations are by mrs. jerome b. thomas. the poem by longfellow, quoted in part, is with the permission of the publishers, houghton, mifflin & co. * * * * * contents _part i. the earth_ page the great stone book the fossil fish the crust of the earth what is the earth made of? the first dry land a study of granite metamorphic rocks the air in motion the work of the wind rain in summer, _by henry w. longfellow_ what becomes of the rain? the soil in fields and gardens the work of earthworms quiet forces that destroy rocks how rocks are made getting acquainted with a river the ways of rivers the story of a pond the riddle of the lost rocks the question answered glaciers among the alps the great ice sheet following some lost rivers the mammoth cave of kentucky land building by rivers the making of mountains the lava flood of the northwest the first living things an ancient beach at ebb tide the lime rocks the age of fishes king coal how coal was made the most useful metal the age of reptiles the age of mammals the horse and his ancestors the age of man _part ii. the sky_ every family a "star club" the dippers and the pole star constellations you can always see winter constellations orion, his dogs, and the bull seven famous constellations the twenty brightest stars how to learn more illustrations pike's peak _frontispiece_ facing page sand dunes in arizona grand cañon of the colorado castles carved by rain and wind where all the water comes from the richest gold and silver mines rocks being ground to flour a pond made by a glacier the struggle between a stream and its banks ripple marks and glacial striæ glacial grooves and markings crinoid and ammonite fossil corals, coquina, hippurite limestone fossil fish meteorite eocene fish and trilobite how coal was made banded sandstone. opalized wood allosaurus a three-horned dinosaur remains of brontosaurus restoration of brontosaurus ornitholestes, a small dinosaur a mammoth an ancestor of the horse orion, his dogs, and the bull other fanciful sketches of constellations the sky in winter the sky in spring the sky in summer the sky in autumn part i the earth * * * * * the great stone book "the crust of our earth is a great cemetery where the rocks are tombstones on which the buried dead have written their own epitaphs. they tell us who they were, and when and where they lived."--_louis agassiz._ deep in the ground, and high and dry on the sides of mountains, belts of limestone and sandstone and slate lie on the ancient granite ribs of the earth. they are the deposits of sand and mud that formed the shores of ancient seas. the limestone is formed of the decayed shells of animal forms that flourished in shallow bays along those shores. and all we know about the life of these early days is read in the epitaphs written on these stone tables. under the stratified rocks, the granite foundations tell nothing of life on the earth. but the sea rolled over them, and in it lived a great variety of shellfish. evidently the earliest fossil-bearing rocks were worn away, for the rocks that now lie on the granite show not the beginnings, but the high tide of life. the "lost interval" of which geologists speak was a time when living forms were few in the sea. in the muddy bottoms of shallow, quiet bays lie the shells and skeletons of the creatures that live their lives in those waters and die when they grow old and feeble. we have seen the fiddler crabs by thousands on such shores, young and old, lusty and feeble. we have seen the rocks along another coast almost covered by the coiled shells of little gray periwinkles, and big clumps of black mussels hanging on the piers and wharfs. all these creatures die, at length, and their shells accumulate on the shallow sea bottom. who has not spent hours gathering dead shells which the tide has thrown up on the beach? who has not cut his foot on the broken shells that lie in the sandy bottom we walk on whenever we go into the surf to swim or bathe? read downward from the surface toward the earth's centre-- table of contents ------+----------------------+-----------+----------------------- part | _rock systems_ | _dominant | _dominant plants_ | | animals_ | ------+----------------------+-----------+----------------------- vii. | recent | man | flowering kinds |{ quaternary | | vi. | { pliocene | mammals | early flowering |{ tertiary { miocene | | | { eocene | | v. | mesozoic | reptiles | cycads iv. | carboniferous | amphibians| ferns and conifers iii. | devonian | fishes | ferns ii. | silurian | molluscs | seaweeds i. | fire-formed | no life | no life ------+----------------------+-----------+----------------------- it is by dying that the creatures of the sea write their epitaphs. the mud or sand swallows them up. in time these submerged banks may be left dry, and become beds of stone. then some of the skeletons and shells may be revealed in blocks of quarried stone, still perfect in form after lying buried for thousands of years. the leaves of this great stone book are the layers of rock, laid down under water. between the leaves are pressed specimens--fossils of animals and plants that have lived on the earth. the fossil fish i remember seeing a flat piece of stone on a library table, with the skeleton of a fish distinctly raised on one surface. the friend who owned this strange-looking specimen told me that she found it in a stone quarry. she brought home a large piece of the slate, and a stone-mason cut out the block with the fish in it, and her souvenir made a useful and interesting paper-weight. the story of that fish i heard with wonder, and have never forgotten. i had never heard of fossil animals or plants until my good neighbour talked about them. she showed me bits of stone with fern leaves pressed into them. one piece of hard limestone was as full of little sea-shells as it could possibly be. one ball of marble was a honeycombed pattern, and called "fossil coral." the fossil fish was once alive, swimming in the sea, and feeding on the things it liked to eat, as all happy fishes do. near shore a river poured its muddy water into the sea, and the sandy bottom was covered with the mud that settled on it. at last the fish grew old, and perhaps a trifle stupid about catching minnows. it died, and sank to the muddy floor of the sea. its horny bones were not dissolved by the water. they remained, and the mud filtered in and filled all the spaces. soon the fish was buried completely by the sediment the river brought. years, thousands of them, went by, and the layer of mud was so thick and heavy above the skeleton of the fish that it bore a weight of tons there, under the water. the close-packed mud became a stiff clay. after more thousands of years, the sea no longer came so far ashore, for the river had built up a great delta of land out of mud. the clay in which the fish was hidden hardened into slate. water crept down in the loose upper layers, dissolving out salt and other minerals, and having harder work to soak through, the lower it went. the water left some of the minerals it had accumulated, calcium and silica and iron, in the lower rock beds, making them harder than they were before, and heavier and less porous. when the river gorge was cut through these layers of rock, the colour and thickness of each kind were laid bare. centuries after, perhaps thousands of years, indeed, the quarrymen cut out the layers fit for building stones, flags for walks and slates for roofing. in the splitting of a flagstone, the long-buried skeleton of the fish came to light. under our feet the earth lies in layers. under the soil lie loose beds of clay and sand and gravel, and under these loose kinds of earth are close-packed clays, sandstones, limestones, shales, often strangely tilted away from the horizontal line, but variously fitted, one layer to another. under these rocks lie the foundations of the earth--the fire-formed rocks, like granite. the depth of this original rock is unknown. it is the substance out of which the earth is made, we think. all the layered rocks are made of particles of the older ones, stolen by wind and water, and finally deposited on the borders of lakes and seas. so our rivers are doing to-day what they have always done--they are tearing down rocks, grinding and sifting the fragments, and letting them fall where the current of fresh water meets a great body of water that is still, or has currents contrary to that of the river. do you see a little dead fish in the water? it is on the way to become a fossil, and the mud that sifts over it, to become a layer of slate. every seashore buries its dead in layers of sand and mud. the crust of the earth it is hard to believe that our solid earth was once a ball of seething liquid, like the red-hot iron that is poured out of the big clay cups into the sand moulds at an iron foundry. but when a mountain like vesuvius sets up a mighty rumbling, and finally a mass of white-hot lava bursts from the centre and streams down the sides, covering the vineyards and olive orchards, and driving the people out of their homes in terror, it seems as if the earth's crust must be but a thin and frail affair, covering a fiery interior, which might at any time break out. the people who live near volcanoes might easily get this idea. but they do not. they go back as soon as the lava streams are cooled, and rebuild their homes, and plant more orchards and vineyards. "it is _so_ many years," say they to one another, "since the last bad eruption. vesuvius will probably sleep now till we are dead and gone." this is good reasoning. there are few active volcanoes left on the earth, compared with the number that were once active, and long ago became extinct. and the time between eruptions of the active ones grows longer; the eruptions less violent. terrible as were the recent earthquakes of san francisco and messina, this form of disturbance of the earth's crust is growing constantly less frequent. the earth is growing cooler as it grows older; the crust thickens and grows stronger as centuries pass. we have been studying the earth only a few hundred years. the crust has been cooling for millions of years, and mountain-making was the result of the shrinking of the crust. that formed folds and clefts, and let masses of the heated substance pour out on the surface. my first geography lesson i shall never forget. the new teacher had very bright eyes and _such_ pretty hands! she held up a red apple, and told us that the earth's substance was melted and burning, inside its crust, which was about as thick, in proportion to the size of the globe, as the skin of the apple. i was filled with wonder and fear. what if we children jumped the rope so hard as to break through the fragile shell, and drop out of sight in a sea of fiery metal, like melted iron? some of the boys didn't believe it, but they were impressed, nevertheless. the theory of the heated interior of the earth is still believed, but the idea that flames and bubbling metals are enclosed in the outer layer of solid matter has generally been abandoned. the power that draws all of its particles toward the earth's centre is stated by the laws of gravitation. the amount of "pull" is the measure of the weight of any substance. lift a stone, and then a feather pillow, much larger than the stone. one is strongly drawn to the earth; the other not. one is _heavy_, we say, the other _light_. if a stone you can pick up is heavy, how much heavier is a great boulder that it takes a four-horse team to haul. what tremendous weight there is in all the boulders scattered on a hillside! the hill itself could not be made level without digging away thousands of tons of earth. the earth's outer crust, with its miles in depth of mountains and level ground, is a crushing weight lying on the heated under-substance. every foot of depth adds greatly to the pressure exerted upon the mass, for the attraction of gravitation increases amazingly as the centre of the earth is approached. it is now believed that the earth is solid to its centre, though heated to a high degree. terrific pressure, which causes this heat, is exerted by the weight of the crust. a crack in the crust may relieve this pressure at some point, and a mass of substance may be forced out and burst into a flaming stream of lava. such an eruption is familiar in volcanic regions. the fact that red-hot lava streams from the crater of vesuvius is no proof that it was seething and bubbling while far below the surface. volcanoes, geysers, and hot springs prove that the earth's interior is hot. the crust is frozen the year around in the polar regions, and never between the tropics of cancer and capricorn. the sun's rays produce our different climates, but they affect only the surface. underground, there is a rise of a degree of temperature for every fifty feet one goes down. the lowest mine shaft is about a mile deep. that is only one four-thousandth of the distance to the earth's centre. by an easy computation we could locate the known melting-point for metals and other rock materials. but one degree for each fifty feet of depth below the surface may not be correct for the second mile, as it is for the first. again, the melting-point is probably a great deal higher for substances under great pressure. the weight of the crust is a burden the under-rocks bear. probably the pressure on every square inch reaches thousands of tons. could any substance become liquid with such a weight upon it, whatever heat it attained? nobody can answer this question. the theory that volcanoes are chimneys connecting lakes of burning lava with the surface of the earth is discredited by geologists. the weight of the overlying crust would, they think, close such chambers, and reduce liquids to a solid condition. since the first land rose above the sea, the crust of the earth has gradually become more stable, but even now there is scarcely a day when the instruments called seismographs do not record earthquake shocks in some part of the earth; and the outbreaks of vesuvius and Ætna, the constant boiling of lava in the craters of the hawaiian islands and other volcanic centres, prove that even now the earth's crust is very thin and unstable. the further back in time we go, the thinner was the crust, the more frequent the outbursts of volcanic activity, the more readily did wrinkles form. the shores of new jersey and of greenland are gradually sinking, and the sea coming up over the land. certain parts of the world are gradually rising out of the sea. in earlier times the rising or the sinking of land over large areas happened much more frequently than now. what is the earth made of? "baking day" is a great institution in the comfortable farm life of the american people. the big range oven is not allowed to grow cold until rows of pies adorn the pantry shelves, and cakes, tarts, and generous loaves of bread are added to the store. cookies, perhaps, and a big pan full of crisp, brown doughnuts often crown the day's work. no gallery of art treasures will ever charm the grown-up boys and girls as those pantry shelves charmed the bright-eyed, hungry children, who were allowed to survey the treasure-house, and sample its good things while they were still warm. you could count a dozen different kinds of cakes and pies, rolls and cookies on those pantry shelves, yet several of them were made out of the same dough. instead of a loaf of bread, mother could make two or three kinds of coffee cake, or cinnamon rolls, or currant buns, or parker-house rolls. even the pastry, which made the pies and tarts, was not so different from the bread dough, for each was made of flour, and contained, besides the salt, "shortening," which was butter or lard. sugar was used in everything, from the bread, which had a table-spoonful, to the cookies, which were finished with a sifting of sugar on top. how much of the food we eat is made of a very few staple foodstuffs,--starch, sugar, fats! so in the wonderful earth and all that grows out of it and lives upon it. only seventy different elements have been discovered, counting, besides the earth, the water and the air, and even the strange wandering bodies, called meteorites, that fall upon the earth out of the sky. like the flour in the different cakes and pies, the element carbon is found in abundance and in strangely different combinations. as a gas, in combination with oxygen, it is breathed out of our lungs, and out of chimneys where coal and wood are burned. it forms a large part of the framework of trees and other plants, and remains as charcoal when the wood is slowly burned under a close covering. there is a good proportion of carbon in animal bodies, in the bones as well as the soft parts, and carbon is plentiful in the mineral substances of the earth. the chemist is the man who has determined for us the existence and the distribution of the seventy elements. he finds them in the solid substances of the globe and in the water that covers four-fifths of its surface; in the atmosphere that covers sea and land, and in all the living forms of plants and animals that live in the seas and on the land. by means of an instrument called the spectroscope, the heavenly bodies are proved to be made of the same substances that are found in the rocks. the sun tells what it is made of, and one proof that the earth is a child of the sun is in the fact that the same elements are found in the substance of both. of the seventy elements, the most important are these: oxygen, silicon, aluminum, iron, manganese, calcium, magnesium, potassium, sodium, carbon, hydrogen, phosphorus, sulphur, chlorine, nitrogen. _oxygen_ is the most plentiful and the most important element. one-fifth of the air we breathe is oxygen; one-third of the water we drink. the rock foundations of the earth are nearly one-half oxygen. no fire can burn, no plant or animal can grow, or even decay after it dies, unless oxygen is present and takes an active part in each process. strangely enough, this wonderful element is invisible. we open a window, and pure air, rich in oxygen, comes in and takes the place of the bad air but we cannot see the change. water we see, but if the oxygen and the hydrogen which compose the colourless liquid were separated, each would become at once an invisible gas. the oxygen of solid rocks exists only in combination with other elements. _silicon_ is the element which, united with oxygen, makes the rock called quartz. on the seashore the children are busy with their pails and shovels digging in the white, clean sand. these grains are of quartz,--fine crystals of a rock which forms nearly three-quarters of the solid earth's substance. not only in rocks, but out here in the garden, the soil is full of particles of sand. you cannot get away from it. _aluminum_ is a light, bluish-white metal which we know best in expensive cooking utensils. it is more abundant even than iron, but processes of extracting it from the clay are still expensive. it is oftenest found in combination with oxygen and silicon. while nearly one-tenth of the earth's crust is composed of the metal aluminum, four-fifths and more is composed of the minerals called silicates of aluminum--oxygen, silicon, and aluminum in various combinations. it is more plentiful than any other substance in rocks and in the clays and ordinary soils, which are the finely ground particles of rock material. _iron_ is one of the commonest of elements. we know it by its red colour. a rusty nail is covered with oxide of iron, a combination which is readily formed wherever iron is exposed to the action of water or air. you have seen yellowish or red streaks in clefts of the rocks. this shows where water has dissolved out the iron and formed the oxide. the red colour of new jersey soil is due to the iron it contains. indeed, the whole earth's crust is rich in iron which the water easily dissolves. the roots of plants take up quantities of iron in solution and this mounts to the blossoms, leaves, and fruit. the red or yellow colour of autumn leaves, of apples, of strawberries, of tulips, and of roses, is produced by iron. the rosy cheeks of children are due to iron in the food they eat and in the water they drink. the doctor but follows the suggestion of nature when he gives a pale and listless person a tonic of iron to make his blood red. iron is rarely found free, but it forms about five per cent. of the crust of the earth, and it is believed to form at least one-fifth of the unknown centre of the earth, the bulk of the globe, the weight of which we know, but concerning the substance of which we can say little that is positive. _manganese_ is not a conspicuous element, but is found united with oxygen in purplish or black streaks on the sides of rocks. it is somewhat like iron, but much less common. _calcium_ is the element that is the foundation of limestones. the skeletons and shells of animals are made of calcite, a common mineral formed by the uniting of carbon, oxygen, and calcium. marbles are, perhaps, the most permanent form of the limestone rocks. "hard" water has filtered through rocks containing calcite, and absorbed particles of this mineral. from water thus impregnated, all animal life on the earth obtains its bone-building and shell-building materials. _carbon_ forms a large part of the tissues of plants and animals, and in the remains of these it is chiefly found in the earth's crust. when these burn or decay, the carbon remains as charcoal or escapes to the air in union with oxygen as the well known carbonic acid gas. this is one of the most important foods of plants. joined with calcium it forms the mineral calcite, or carbonate of lime. _hydrogen_ is one of the two gases that unite to form water. oxygen is the other. many kinds of rock contain a considerable amount of water. surface water sinks into porous soils and rocks, and accumulates in pockets and veins which feed springs, and are the reserve water supply that keeps our rivers flowing, even through dry weather. more water is held by absorption in the earth's solid crust than in all the oceans and seas and great lakes. hydrogen, combined with carbon, occurs in solid rocks where the remains of plants and animals have slowly decayed. from such processes the so-called hydrocarbons, rock oil and natural gas, have accumulated. when such decay goes on above ground, these valuable products escape into the air. marsh gas, whose feeble flame above decaying vegetation is the will-o'-the-wisp of swamps, is an example. _magnesium_, _potassium_, and _sodium_ are found in equal quantities in the earth's crust, but never free. in union with chlorine, each forms a soluble salt, and is thus found in water. common salt, chloride of sodium, is the most abundant of these. water dissolves salt out of the rocks, and carries it into the sea. clouds that rise by the evaporation of ocean water leave the salt behind, hence the seas are becoming more and more salty, for the rivers carry salt to the oceans, which hold fast all they get. _phosphorus_ is an element found united with oxygen in the tissues of both plants and animals. it is most abundant in bones. rocks containing fossil bones are rich in lime phosphates, which are important commercial fertilizers for enriching the soil. beds of these rocks are found and mined in south carolina and elsewhere. _sulphur_ is well known as a yellow powder found most plentifully in rocks that are near volcanoes. it is a needed element in plant and animal bodies. it occurs in rocks, united with many different elements. in union with oxygen and a metal it forms the group of minerals called sulphates. in union with iron it forms sulphide of iron. the "fool's gold" which captain john smith's colonists found in the sand at jamestown, was this worthless iron pyrites. _chlorine_ is a greenish, yellow gas, very heavy, and dangerous to inhale. if it gets into the lungs, it settles into the lowest levels, and one must stand on one's head to get it out. as an element of the earth's crust it is not very plentiful, but it is a part of all the chlorides of sodium, magnesium, and potassium. in salt, it forms two per cent. of the sea water. it is much less abundant in the rocks. to these elements we might add _nitrogen_, that invisible gas which forms nearly four-fifths of our atmosphere, and is a most important element of plant food in the soil. most of the seventy elements are very rare. many are metals, like gold and iron and silver. some are not metals. some are solid. a few are liquid, like the metal mercury, and several are gaseous. some are free and pure, and show no disposition to unite with others. nuggets of gold are examples of this. some exist only in union with other elements. this is the common rule among the elements. changes are constantly going on. the elements are constantly abandoning old partnerships and forming new ones. growth and decay of plant and animal life are but parts of the great programme of constant change which is going on and has been in progress since the world began. the first dry land when the earth's crust first formed it was still hot, though not so hot as when it was a mass of melted, glowing substance. as it moved through the cold spaces of the sky, it lost more heat and its crust became thicker. at length the cloud masses became condensed enough to fall in torrents of water, and a great sea covered all the land. this was before any living thing, plant or animal, existed on our planet. can you imagine the continents and islands that form the land part of a map or globe suddenly overwhelmed by the oceans, the names and boundaries of which you have taken such pains to learn in the study of geography? the globe would be one blank of blue water, and geography would be abolished--and there would be nobody to study it. possibly the fishes in the sea might not notice any change in the course of their lives, except when they swam among the ruins of buried cities and peered into the windows of high buildings, or wondered what new kind of seaweed it was when they came upon a submerged forest. in that old time of the great sea that covered the globe, we are told that there was a dense atmosphere over the face of the deep. so things were shaping themselves for the far-off time when life should exist, not only in the sea, where the first life did appear, but on land. but it took millions of years to fit the earth for living things. the cooling of the earth made it shrink, and the crust began to be folded into gentle curves, as the skin of a shrunken apple becomes wrinkled on the flesh. some of these creases merely changed the depth of water on the sea bottom; but one ridge was lifted above the water. the water parted and streamed down its sloping sides, and a granite reef, which shone in the sunshine, became the first dry land. it lay east and west, and stretched for many miles. it is still dry land and is a part of our own continent. now it is but a small part of the country, but it is known by geologists, who can tell its boundaries, though newer land joins it on every side. it is named the laurentian hills, on geological maps. its southern border reaches along the northern boundary of the great lakes to the head-waters of the mississippi river. from this base, two ridges are lifted, forming a colossal v. one extends northeast to nova scotia; the other northwest to the arctic seas. the v encloses hudson bay. besides this first elongated island of bare rocks, land appeared in a strip where now the blue ridge mountains stretch from new england to georgia. the other side of the continent lifted up two folds of the crust above sea level. they are the main ridges of the colorado and the wasatch mountains. possibly the main ridge of the sierra nevada rose also at this time. the ozark group of mountains, too, showed as a few island peaks above the sea. these first rocks were rapidly eaten away, for the atmosphere was not like ours, but heavily charged with destructive gases, which did more, we believe, to disintegrate the exposed rock surfaces than did the two other forces, wind and water, combined. the sediment washed down to the sea by rains, accumulated along the shores, filling the shallows and thus adding to the width of the land areas. the ancient granite ridge of the laurentian hills is now low, and slopes gently. this is true of all very old mountains. the newer ones are high and steep. it takes time to grind down the peaks and carry off the waste material loosened by erosion. far more material than could have been washed down the slopes of the first land ridges came directly from the interior of the earth, and spread out in vast, submarine layers upon the early crust. volcanic craters opened under water, and poured out liquid mineral matter, that flowed over the sea bottom before it cooled. imagine the commotion that agitated the water as these submerged chimneys blew off their lids, and discharged their fiery contents! it was long before the sea was cool enough to be the home of living things. the layers of rock that formed under the sea during this period of the earth's history are of enormous thickness. they were four or five miles deep along the laurentian hills. they broadened the original granite ridge by filling the sea bottom along the shores. the backbones of the appalachian system and the cordilleras were built up in the same way--the oldest rocks were worn away, and their débris built up newer ones in strata. when these layers of rock became dry land, the earth's crust was much more stable and cool than it had ever been before. the vast rock-building of that era equals all that has been done since. the layers of rocks formed since then do not equal the total thickness of these first strata. so we believe that the time required to build those archæan rock foundations equals or surpasses the vast period that has elapsed since the archæan strata were formed. the northern part of north america has grown around those old granite ridges by the gradual rising of the shores. the geologist may walk along the laurentian hills, that parted the waters into a northern and a southern ocean. he crosses the rocky beds deposited upon the granite; then the successive beds formed as the land rose and the ocean receded. age after age is recorded in the rocks. gradually the sea is crowded back, and the land masses, east, west, and north, meet to form the continent. nowhere on the earth are the steps of continental growth shown in unbroken sequence as they are in north america. how long ago did those first islands appear above the sea? nobody ventures a definite answer to this question. no one has the means of knowing. but those who know most about it estimate that at the least one hundred million years have passed since then--one hundred thousand thousand years! a study of granite in every village cemetery it is easy to find shafts of gray or speckled granite, the polished surfaces of which show that the granite is made of small bits of different coloured minerals, cemented together into solid rock. outside the gate you will usually find a place where monuments and gravestones may be bought. here there is usually a stonecutter chipping away on a block with his graving tools. he is a man worth knowing, and because his work is rather monotonous he will probably be glad to talk to a chance visitor and answer questions about the different kinds of stone on which he works. there are bits of granite lying about on the ground. if you have a hand-glass of low power, such as the botany class uses to examine the parts of flowers, it will be interesting to look through it and see the magnified surface of a flake of broken granite. here are bits of glassy quartz, clear and sparkling in the sun. black and white may be all the colours you make out in this specimen, or it may be that you see specks of pink, dark green, gray, and smoky brown, all cemented together with no spaces that are not filled. the particles of quartz are of various colours, and are very hard. they scratch glass, and you cannot scratch them with the steel point of your knife, as you can scratch the other minerals associated with the grains of quartz. granite is made of quartz, feldspar, and mica, sometimes with added particles of hornblende. feldspar particles have as wide a range of colour as quartz, but it is easy to tell the two apart. a knife will scratch feldspar, as it is not so hard as quartz. the crystals of feldspar have smooth faces, while quartz breaks with a rough surface as glass does. feldspar loses its glassy lustre when exposed to the weather, and becomes dull, with the soft lustre of pearl. mica may be clear and glassy, and it ranges in colour from transparency through various shades of brown to black. it has the peculiarity of splitting into thin, leaf-like, flexible sheets, so it is easy to find out which particles in a piece of granite are mica. one has only to use one's pocket knife with a little care. hornblende is a dark mineral which contains considerable iron. it is found in lavas and granites, where it easily decays by the rusting of the iron. it is not unusual to see a rough granite boulder streaked with dark red rust from this cause. the crumbling of granite is constantly going on as a result of the exposure of its four mineral elements to the air. quartz is the most stable and resistant to weathering. soil water trickling over a granite cliff has little effect on the quartz particles; but it dissolves out some of the silicon. the bits of feldspar are even more resistant to water than quartz is, but the air causes them to decay rapidly, and finally to fall away in a sort of mealy clay. mica, like feldspar, decays easily. its substance is dissolved by water and carried away to become a kind of clay. the hornblende rusts away chiefly under the influence of moist air and trickling water. we think of granite as a firm, imperishable kind of rock, and use it in great buildings like churches and cathedrals that are to stand for centuries. but the faces that are exposed to the air suffer, especially in regions having a moist climate. the signs of decay are plainly visible on the outer surfaces of these stones. fortunate it is that the weathering process cannot go very deep. the glassy polish on a smooth granite shaft is the silicon which acts as a cement to bind all the particles together. it is resistant to the weather. a polished shaft will last longer than an unpolished one. granites differ in colouring because the minerals that compose them, the feldspars, quartzes, micas, and hornblendes, have each so wide a range of colour. again, the proportions of the different mineral elements vary greatly in different granites. a banded granite the colours of which give it a stratified appearance is called a gneiss. we have spoken before of the seventy elements found in the earth's crust. a mineral is a union of two or more of these different elements; and we have found four minerals composing our granite rock. it may be interesting to go back and inquire what elements compose these four minerals. quartz is made of silicon and oxygen. feldspar is made of silicon, oxygen, and aluminum. mica is made of silicon, oxygen, and carbon, with some mingling of potassium and iron and other elements in differing proportions. hornblende is made of silicon, oxygen, carbon, and iron. the crumbling of a granite rock separates the minerals that compose it, reducing some to the condition of clay, others to grains of sand. some of the elements let go their union and become free to form new unions. water and wind gather up the fragments of crumbling granite and carry them away. the feldspar and mica fragments form clay; the quartz fragments, sand. all of the sandstones and slates, the sand-banks and sand beaches, are made out of crumbled granite, the rocky foundations of the earth. metamorphic rocks in the dawn of life on the earth, soft-bodied creatures, lowest in the scale of being, inhabited the sea. the ancient volcanoes the subterranean eruptions of which had spread layers of mineral substance on the ocean floor, and heated the water to a high degree, had subsided. the ocean was sufficiently cool to maintain life. the land was being worn down, and its débris washed into the ocean. the first sand-banks were accumulating along sandy shores. the finer sediment was carried farther out and deposited as mud-banks. these were buried under later deposits, and finally, by the rising of the earth's crust, they became dry land. time and pressure converted the sand-banks into sandstones; the mud-banks into clay. the remains of living creatures utterly disappeared, for they had no hard parts to be preserved as fossils. the shrinking of the earth's crust had crumpled into folds of the utmost complexity those horizontal layers of lava rock poured out on the ocean floor. next, the same forces attacked the thick rock layers formed out of sediment--the aqueous or water-formed sandstones and clays. the core of the globe contracts, and the force that crumples the crust to fit the core generates heat. the alkaline water in the rocks joins with the heat produced by the crumpling and crushing forces, acting downward, and from the sides, to transform pure sandstone into glassy quartzite, and clay into slate. in other words, water-formed rocks are baked until they become fire-formed rocks. they are what the geologist calls _metamorphic_, which means _changed_. in many mountainous regions there are breaks through the strata of sandstone and slates and limestones, through which streams of lava have poured forth from the heated interior. along the sides of these fissures the hot lava has changed all the rocks it touched. the heat of the volcanic rock matter has melted the silica in the sand, which has hardened again into a crystalline substance like glass. have you ever visited a brick-yard? here men are sifting clay dug out of a pit or the side of a hill, adding sand from a sand-bank, and in a big mixing box, stirring these two "dry ingredients" with water into a thick paste. this dough is moulded into bricks, sun-dried, and then baked in kilns themselves built of bricks. at the end of the baking, the soft, doughy clay block is transformed into a hard, glassy, or dull brick. from aqueous rock materials, fire has produced a metamorphic rock. volcanic action is imitated in this common, simple process of brickmaking. milwaukee brick is made of clay that has no iron in it. for this reason the bricks are yellow after baking. most bricks are red, on account of the iron in the clay, which is converted into a red oxide, or rust, by water and heat. common flower pots and the tiles used in draining wet land are not glazed, as hard-burned bricks are. the baking of these clay things is done with much less heat. they are left somewhat porous. but the tiles of roofs are baked harder, and get a surface glaze by the melting of the glassy particles of the sand. as bricks vary in colour and quality according to the materials that compose them, so the metamorphic rocks differ. the white sand one sees on many beaches is largely quartz. this is the substance of pure, white sandstone. metamorphism melts the silica into a glassy liquid cement; the particles are bound close together on cooling. the rock becomes a white, granular quartzite, that looks like loaf sugar. if banded, it is called gneiss. such rocks take a fine polish. pure limestone is also white and granular. when metamorphosed by heat, it becomes white marble. the glassy cement that holds the particles of lime carbonate shows as the glaze of the polished surface. it is silica. one sees the same mineral on the face of polished granite. clays are rarely pure. kaolin is a white clay which, when baked, becomes porcelain. china-ware is artificially metamorphosed kaolin. in the early rocks the clay beds were transformed by heat into jasper and slates. in beds where clay mingled with sand, in layers, gneiss was formed. if mica is a prominent element, the metamorphic rock is easily parted into overlapping, scaly layers. it is a mica schist. if hornblende is the most abundant mineral, the same scaly structure shows in a dark rock called hornblende schist, rich in iron. a schist containing much magnesia is called serpentine. the bricks of the wall, the tiles on the roof, the flower pots on the window sill, and the dishes on the breakfast table, are examples of metamorphic rocks made by man's skill, by the use of fire and water acting on sand and clay. pottery has preserved the record of civilization, from the making of the first crude utensils by cave men to the finest expression of decorative art in glass and porcelain. the choicest material of the builder and the sculptor is limestone baked by the fires under the earth's crust into marble. the most enduring of all the rocks are the foundation granite, and the metamorphic rocks that lie next to them. over these lie thick layers of sedimentary rocks laid down by water. in them the record of life on the earth is written in fossils. the air in motion most of the beautiful things that surround us and make our lives full of happiness appeal to one or more of our five senses. the green trees we can see, the bird songs we hear, the perfume of honey-laden flowers we smell, the velvety smoothness of a peach we feel, and its rich pulp we taste. but over all and through all the things we see and feel and hear and taste and smell, is the life-giving air, that lies like a blanket, miles in depth, upon the earth. the substance which makes the life of plants and animals possible is, when motionless, an invisible, tasteless, odourless substance, which makes no sound and is not perceptible to the touch. air fills the porous substance of the earth's crust for a considerable distance, and even the water has so much air in it that fishes are able to breathe without coming to the surface. it is not a simple element, like gold, or carbon, or calcium, but is made up of several elements, chief among which are nitrogen and oxygen. four-fifths of its bulk is nitrogen and one-fifth oxygen. there is present in air more or less of watery vapour and of carbon dioxide, the gas which results from the burning or decay of any substance. although no more than one per cent. of the air that surrounds us is water, yet this is a most important element. it forms the clouds that bear water back from the ocean and scatter it in rain upon the thirsty land. solid matter in the form of dust, and soot from chimneys, accumulates in the clouds and does a good work in condensing the moisture and causing it to fall. it is believed that the air reaches to a height of one hundred to two hundred miles above the earth's surface. if a globe six feet in diameter were furnished with an atmosphere proportionately as deep as ours, it would be about an inch in depth. at the level of the sea the air reaches its greatest density. two miles above sea-level it is only two-thirds as dense. on the tops of high mountains, four or five miles above sea level, the air is so rarefied as to cause the blood to start from the nostrils and eyelids of explorers. the walls of the little blood-vessels are broken by the expansion of the air that is inside. at the sea-level air presses at the rate of fifteen pounds per square foot in all directions. as one ascends to higher levels, the air pressure becomes less and less. the barometer is the instrument by which the pressure of air is measured. a glass tube, closed at one end, and filled with mercury, the liquid metal often called quicksilver, is inverted in a cup of the same metal, and so supported that the metal is free to flow between the two vessels. the pressure of air on the surface of the mercury in the cup is sufficient at the sea-level to sustain a column of mercury thirty inches high in the tube. as the instrument is carried up the side of a mountain the mercury falls in the tube. this is because the air pressure decreases the higher up we go. if we should descend into the shaft of the deepest mine that reaches below the sea level, the column of air supported by the mercury in the cup would be a mile higher, and for this reason its weight would be correspondingly greater. the mercury would thus be forced higher in the tube than the thirty-inch mark, which indicates sea-level. another form of barometer often seen is a tube, the lower and open end of which forms a u-shaped curve. in this open end the downward pressure of the air rests upon the mercury and holds it up in the closed end, forcing it higher as the instrument is carried to loftier altitudes. at sea level a change of feet in altitude makes a change of an inch in the height of the mercury in the column. the glass tube is marked with the fractions of inches, or of the metre if the metric system of measurements is used. it is a peculiarity of air to become heated when it is compressed, and cooled when it is allowed to expand again. it is also true that when the sun rises, the atmosphere is warmed by its rays. this is why the hottest part of the day is near noon when the sun's rays fall vertically. the earth absorbs a great deal of the sun's heat in the daytime and through the summer season. when it cools this heat is given off, thus warming the surrounding atmosphere. in the polar regions, north and south, the air is far below freezing point the year round. in the region of the equator it rarely falls below degrees, a temperature which we find very uncomfortable, especially when there is a good deal of moisture in the air. if we climb a mountain in mexico, we leave the sultry valley, where the heat is almost unbearable, and very soon notice a change. for every three hundred feet of altitude we gain there is a fall of one degree in the temperature. before we are half way up the slope we have left behind the tropical vegetation, and come into a temperate zone, where the plants are entirely different from those in the lower valley. as we climb, the vegetation becomes stunted, and the thermometer drops still lower. at last we come to the region of perpetual snow, where the climate is like that of the frozen north. so we see that the air becomes gradually colder as we go north or south from the equator, and the same change is met as we rise higher and higher from the level of the sea. it is only when air is in motion that we can feel and hear it, and there are very few moments of the day, and days of the year, when there is not a breeze. on a still day fanning sets the air in motion, and creates a miniature breeze, the sound of which we hear in the swishing of the fan. the great blanket of air that covers the earth is in a state of almost constant disturbance, because of the lightness of warm air and the heaviness of cold air. these two different bodies are constantly changing places. for instance, the heated air at the equator is constantly being crowded upward by cold air which settles to the level of the earth. cold streams of air flow to the tropics from north and south of the equator, and push upward the air heated by the sun. this constant inrush of air from north and south forms a double belt of constant winds. if the earth stood still, no doubt the direction would be due north and due south for these winds; but the earth rotates rapidly from west to east upon its axis, carrying with it everything that is securely fastened to the surface: the trees, the houses, etc. but the air is not a part of the earth, not even so much as the seas, the waters of which must stay in their proper basins, and be whirled around with other fixed objects. the earth whirls so rapidly that the winds from north and south of the equator lag behind, and thus take a constantly diagonal direction. instead of due south the northern belt of cold air drifts south-west and the southern belt drifts northwest. these are called the trade winds. near the equator they are practically east winds. the belt of trade winds is about fifty degrees wide. it swings northward in our summer and southward in our winter, its centre following the vertical position of the sun. near the centre of the course which marks the meeting of the northern with the southern winds is a "belt of calms" where the air draws upward in a strong draught. the colder air of the trade winds is pushing up the columns of light, heated air. this strip is known by sailors as "the doldrums," or "the region of equatorial calms." though never wider than two or three hundred miles, this is a region dreaded by captains of sailing-vessels, for they often lie becalmed for weeks in an effort to reach the friendly trade winds that help them to their desired ports. vessels becalmed are at the mercy of sudden tempests which come suddenly like thunder-storms, and sometimes do great damage to vessels because they take the sailors unawares and allow no time to shorten sail. until late years the routes of vessels were charted so that sailors could take advantage of the trade winds in their long voyages. it was necessary in the days of sailing-vessels for the captain to understand the movements of winds which furnished the motive power that carried his vessel. fortunate it was for him that there were steady winds in the temperate zones that he could take advantage of in latitudes north of the tropic of cancer and south of the tropic of capricorn. what becomes of the hot air that rises in a constant stream above the "doldrums," pushed up by the cooler trade winds that blow in from north and south? naturally this air cannot ascend very high, for it soon reaches an altitude in which its heat is rapidly lost, and it would sink if it were not constantly being pushed by the rising column of warm air under it. so it turns and flows north and south at a level above the trade winds. not far north of the tropic of cancer it sinks to the level of sea and land, and forms a belt of winds that blows ships in a northeasterly direction. between trades and anti-trades is another zone of calms,--near the tropics of cancer and of capricorn. the land masses of the continents with their high mountain ranges interfere with these winds, especially in the northern hemisphere, but in the southern pacific and on the opposite side of the globe the "roaring forties," as these prevailing westerly winds are known by the sailors, have an almost unbroken waste of seas over which they blow. in the long voyages between england and australia, and in the indian trade, the ships of england set their sails to catch the roaring forties both going and coming. they accomplish this by sailing past the cape of good hope on the outward voyage and coming home by way of cape horn, thus circling the globe with every trip. in the north atlantic, traffic is now mostly carried on in vessels driven by engines, not by sails. yet the westerly winds that blow from the west indies diagonally across the atlantic are still useful to all sailing craft that are making for british ports. from the north and from the south cold air flows down into the regions of warmer climate. these polar winds are not so important to sea commerce, but they do a great work in tempering the heat in the equatorial regions. we cannot know how much our summers are tempered by the cool breath of winds that blow over polar ice-fields. and the cold regions of the earth, in their brief summer, enjoy the benefits of the warm breezes that flow north and south from the heated equatorial regions. the land, north and south, is made habitable by the clouds. they gather their burdens of vapour from the warm seas, the wind drifts them north and south, where they let it fall in rains that make and keep the earth green and beautiful. from the clouds the earth gathers, like a great sponge, the water that stores the springs and feeds rivers and lakes. how necessary are the winds that transport the cloud masses! the air is the breath of life to all living things on our planet. mars is one of the sun's family so provided. plants or animals could probably live on the planet mars. do we think often enough of this invisible, life-giving element upon which we depend so constantly? the open air which the wind purifies by keeping it in motion is the best place in which to work, to play, and to sleep, when work and play are done and we rest until another day comes. indoors we need all the air we can coax to come in through windows and doors. fresh air purifies air that is stale and unwholesome from being shut up. nobody is afraid, nowadays, to breathe night air! what a foolish notion it was that led people to close their bedroom windows at night. clean air, in plenty, day and night, we need. air and sunshine are the two best gifts of god. the work of the wind when the march wind comes blustering down the street, rudely dashing a cloud of dust in our faces, we are uncomfortable and out of patience. we duck our heads and cover our faces, but even then we are likely to get a cinder in one eye, to swallow germs by the dozens, and to get a gray coating of plain, harmless dust. we welcome the rain that lays the dust, or its feeble imitation, the water sprinkler, that brings us temporary relief. on the quietest day, even after a thorough sweeping and dusting of the library, you are able to write your name plainly on the film of dust that lies on the polished table. take a book from the open shelves, and blow into the trough of its top. this is always dusty. where does the dust come from? this is the house-keeper's riddle. the answer is not a hard one. i look out of my window on a street which is famous as the road washington took on his retreat from white plains to trenton. it has always been the main thoroughfare between new york and philadelphia, and now is the route that automobiles follow. a constant procession of vehicles passes my house, and to-day each one approaches in a cloud of dust. the air is gray with suspended particles of dirt. the wind carries the successive clouds, and they roll up against the houses like breakers on the beach. windows and doors are loose enough to let dust sift in. when a door opens, the cloud enters and lights on rugs and carpets and curtains. any ledge collects its share of dust. the beating of carpets and rugs disturbs the accumulated dust of many months. [illustration: in this lonely arizona desert the wind drifts the sand into dunes, just as it does on the toe of cape cod] [illustration: the grand canyon of the colorado shows on a magnificent scale the work of water in cutting away rock walls] the wind sweeps the ploughed field, and takes all the dust it can carry. it blows the finest top soil from our gardens into the street. it blows soil from other fields and gardens into ours, so the level of our land is not noticeably lowered. the wind strips the high land and drops its burden on lower levels. this is one of the big jobs the water has to do, and the wind is a valuable helper. to tear down the mountains and fill in the valleys is the great work of the two partners, wind and water. dead, still air holds the finest dust, without letting it fall. the buoyancy of the particles overcomes their weight. we see them in a sunbeam, like shining points of precious metal, and watch them. a light breeze picks up bits of soil and litter, from the smallest up to a certain size and weight. if the velocity of the wind increases, its carrying power increases. it is able to carry bits that are larger and heavier. the following table is exact and interesting: _velocity_ _pressure_ _in miles_ _in pounds_ _per hour_ _per sq. ft._ light breeze strong breeze strong gale hurricane the terrible paths of hurricanes are seen in forest countries. the trees are uprooted, as if a great roller had crushed them, throwing the tops all in one direction, and leaving the roots uncovered, and a sunken pocket where each tree stood. on a steep, rocky slope, the uprooting of scattered trees often loosens tons of rock, and sends the mass thundering down the mountain-side. much more destruction may be accomplished by one brief tornado than by years of wear by ordinary breezes. the wind does much to help the waves in their patient beating on rocky shores. if the wind blows from the ocean and the tide is landward, the two forces combine, and the loose rocks are thrown against the solid beach with astonishing force. even the gravel and the sharp sand are tools of great usefulness to the waves in grinding down the resisting shore. up and back they are swept by the water, and going and coming they have their chance to scratch or strike a blow. boulders on the beach become pockmarked by the constant sand-blast that plays upon them. the lower windows of exposed seaside houses are dimmed by the sand that picks away the smooth surface outside, making it ground glass by the same process used in the factory. lighthouses have this difficulty in keeping their windows clear. the "lantern" itself is sometimes reached by the sand grains. that is the cupola in which burns the great light that warns vessels away from the rocks and tells the captain where he is. in the far western states the telegraph poles and fence posts are soon cut off at the ground by the flinty knives the wind carries. these are the grains of sand that are blown along just above the ground. the trees are killed by having their bark girdled in this way. the sand-storms which in the orange and lemon region of california are called "santa anas" sometimes last two or three days, and damage the trees by piercing the tender bark with the needle-pointed sand. wind-driven soil, gathered from the sides of bare hills and mountains, fills many valleys of china with a fine, hard-packed material called "loess." in some places it is hundreds of feet deep. the people dig into the side of a hill of this loess and carry out the diggings, making themselves homes, of many rooms, with windows, doors, and solid walls and floors, all in one solid piece, like the chambered house a mole makes underground in the middle of a field. so compact is the loess that there is no danger of a cave-in. the hills of sand piled up on the southern shore of lake michigan, and at provincetown, at the toe of cape cod, are the work of the wind. on almost any sandy shore these "dunes" are common. the long slope is toward the beach that furnishes the sand. the wind does the building. up the slope it climbs, then drops its burden, which slides to the bottom of an abrupt landward steep. there is a gradual movement inland if the strongest winds come from the water. the shifting of the dunes threatens to cover fertile land near them. in the desert regions, the border-land is always in danger of being taken back again, even though it has been reclaimed from the desert and cultivated for long years. besides tearing down, carrying away, and building up again the fragments of the earth's crust, the wind does much that makes the earth a pleasant planet to live on. it drives the clouds over the land, bringing rains and snows and scattering them where they will bless the thirsty ground and feed the springs and brooks and rivers. it scatters the seeds of plants, and thus plants forests and prairies and lovely mountain slopes, making the wonderful wild gardens that men find when they first enter and explore a new region. the trade winds blow the warm air of the tropics north and south, making the climate of the northern countries milder than it would otherwise be. sea winds blow coolness over the land in summer, and cool lake breezes temper the inland regions. from the snow-capped mountains come the winds that refresh the hot, tired worker in the valleys. everywhere the wind blows, the life-giving oxygen is carried. this is what we mean when we speak of fresh air. stagnant air is as unwholesome as stagnant water. constant moving purifies both. so we must give the wind credit for some of the greatest blessings that come into our lives. light and warmth come from the sun. pure water and pure air are gifts the bountiful earth provides. without them there would be no life on the earth. rain in summer how beautiful is the rain! after the dust and heat, in the broad and fiery street, in the narrow lane, how beautiful is the rain! how it clatters along roofs, like the tramp of hoofs! how it gushes and struggles out from the throat of the overflowing spout! across the window-pane it pours and pours; and swift and wide, with a muddy tide, like a river down the gutter roars the rain, the welcome rain! the sick man from his chamber looks at the twisted brooks; he can feel the cool breath of each little pool; his fevered brain grows calm again, and he breathes a blessing on the rain. --henry w. longfellow. what becomes of the rain? the clouds that sail overhead are made of watery vapour. sometimes they look like great masses of cotton-wool against the intense blue of the sky. sometimes they are set like fleecy plumes high above the earth. sometimes they hang like a sullen blanket of gray smoke, so low they almost touch the roofs of the houses. indeed, they often rest on the ground and then we walk through a dense fog. in their various forms, clouds are like wet sponges, and when they are wrung dry they disappear--all their moisture falls upon the earth. when the air is warm, the water comes in the form of rain. if it is cold, the drops are frozen into hail, sleet, or snow. all of the water in the oceans, in the lakes and rivers, great and small, all over the earth, comes from one source, the clouds. in the course of a year enough rain and snow fall to cover the entire surface of the globe to a depth of forty inches. this quantity of water amounts to , barrels on every acre. what becomes of it all? we can easily understand that all the seas and the other bodies of water would simply add forty inches to their depth, and many would become larger, because the water would creep up on their gradually sloping shores. we have to account for the rain and the snow that fall upon the dry land and disappear. go out after a drenching rainstorm and look for the answer to this question. the gullies along the street are full of muddy, running water. there are pools of standing water on level places, but on every slope the water is hurrying away. the ground is so sticky that wagons on country roads may mire to the hubs in the pasty earth. there is no use in trying to work in the garden or to mow the lawn. the sod is soft as a cushion, and the garden soil is water-soaked below the depth of a spading-fork. the sun comes out, warm and bright, and the flagstones of the sidewalk soon begin to steam like the wooden planks of the board walk. the sun is changing the surface water into steam which rises into the sky to form a part of another bank of clouds. the earth has soaked up quantities of the water that fell. if we followed the racing currents in the gullies we should find them pouring into sewer mains at various points, and from these underground pipes the water is conducted to some outlet like a river. all of the streams are swollen by the hundreds of brooks and rivulets that are carrying the surface water to the lowest level. [illustration: rain and wind are the sculptors that have carved these strange castles out of a rocky table] [illustration: all the water in the seas, lakes, rivers, and springs came out of the clouds] so we can see some of the rainfall going back to the sky, some running off through rivulets to the sea, and some soaking into the ground. it will be interesting to follow this last portion as it gradually settles into the earth. the soil will hold a certain quantity, for it is made up of fine particles, all separated by air spaces, and it acts like a sponge. in seasons of drought and great heat the sun will draw this soil water back to the surface, by forming cracks in the earth, and fine, hair-like tubes, through which the vapour may easily rise. the gardener has to rake the surface of the beds frequently to stop up these channels by which the sun is stealing the precious moisture. the water that the surface soil cannot absorb sinks lower and lower into the ground. it finds no trouble to settle through layers of sand, for the particles do not fit closely together. it may come to a bed of clay which is far closer. here progress is retarded. the water may accumulate, but finally it will get through, if the clay is not too closely packed. again it may sink rapidly through thick beds of gravel or sand. reaching another bed of clay which is stiffer by reason of the weight of the earth above it, the water may find that it cannot soak through. the only way to pass this clay barrier is to fill the basin, and to trickle over the edge, unless a place is found in the bottom where some looser substance offers a passage. let us suppose that a concave clay basin of considerable depth is filled with water-soaked sand. at the very lowest point on the edge of this basin a stream will slowly trickle out, and will continue to flow, as long as water from above keeps the bowl full. it is not uncommon to find on hillsides, in many regions, little brooks whose beginnings are traceable to springs that gush out of the ground. the spring fills a little basin, the overflow of which is the brook. if the source of this spring could be traced underground, we might easily follow it along some loose rock formation until we come to a clay basin like the one described above. we might have to go down quite a distance and then up again to reach the level of this supply, but the level of the water at the mouth of the spring can never be higher than the level of the water in the underground supply basin. often in hot summers springs "go dry." the level of water in the supply basin has fallen below the level of the spring. we must wait until rainfall has added to the depth of water in the basin before we can expect any flow into the pool which marks the place where the brook begins. suppose we had no beds of clay, but only sand and gravel under the surface soil. we should then expect the water to sink through this loose material without hindrance, and, finding its way out of the ground, to flow directly into the various branches of the main river system of our region. after a long rain we should have the streams flooded for a few days, then dry weather and the streams all low, many of them entirely dry until the next rainstorm. instead of this, the soil to a great depth is stored with water which cannot get away, except by the slow process by which the springs draw it off. this explains the steady flow of rivers. what should we do for wells if it were not for the water basins that lie below the surface? a shallow well may go dry. its owner digs deeper, and strikes a lower "vein" of water that gives a more generous supply. in the regions of the country where the drift soil, left by the great ice-sheet, lies deepest, the glacial boulder clay is very far down. the surface water, settling from one level to another, finally reaches the bottom of the drift. wells have to be deep that reach this water bed. the water follows the slope of this bed and is drained into the ocean, sometimes by subterranean channels, because the bed of the nearest river is on a much higher level. so we must not think that the springs contain only the water that feeds the rivers. they contain more. the layers of clay at different levels, from the surface down to the bottom of the drift, form water basins and make it possible for people to obtain a water supply without the expense of digging deep wells. the clayey subsoil, only a few feet below the surface, checks the downward course of the water, so that the sun can gradually draw it back, and keep a supply where plant roots can get it. the vapour rising keeps the air humid, and furnishes the dew that keeps all plant life comfortable and happy even through the hot summer months. under the drift lie layers of stratified rock, and under these are the granites and other fire-formed rocks, the beginning of those rock masses which form the solid bulk of the globe. we know little about the core of the earth, but the granites that are exposed in mountain ridges are found to have a great capacity for absorbing water, so it is not unlikely that much surface water soaks into the rock foundations and is never drained away into the sea. the water in our wells is often hard. it becomes so by passing through strata of soil and rock made, in part, at least, of limestone, which is readily dissolved by water which contains some acid. soil water absorbs acids from the decaying vegetation,--the dead leaves and roots of plants. rain water is soft, and so is the water in ponds that have muddy basins, destitute of lime. water in the springs and wells of the mid-western states is "hard" because it percolates through limestone material. in many parts of this country the well water is "soft," because of the scarcity of limestone in the soil. i have seen springs around which the plants and the pebbles were coated with an incrustation of lime. "petrified moss" is the name given to the plants thus turned to stone. the reason for this deposit is clear. underground water is often subjected to great pressure, and at this time it is able to dissolve much more of any mineral substance than under ordinary conditions. when the pressure is released, the water is unable to hold in solution the quantity of mineral it contains; therefore, as it flows out through the mouth of the spring, the burden of mineral is laid down. the plants coated with the lime gradually decay, but their forms are preserved. there are springs the water of which comes out burdened with iron, which is deposited as a yellowish or red mineral on objects over which it flows. ponds fed by these springs accumulate deposits of the mineral in the muddy bottoms. some of the most valuable deposits of iron ore have accumulated in bogs fed by iron-impregnated spring water. in a similar way lime deposits called marl or chalk are made. the soil in fields and gardens city and country teachers are expected to teach classes about the formation and cultivation of soil. it is surprising how much of the needed materials can be brought in by the children, even in the cities. the beginning is a flowering plant growing in a pot. a window box is a small garden. a garden plot is a miniature farm. _materials to collect for study indoors._ a few pieces of different kinds of rock: granite, sandstone, slate; gravelly fragments of each, and finer sand. pebbles from brooks and seashore. samples of clays of different colors, and sands. samples of sandy and clay soils, black pond muck, peat and coal. rock fossils. a box of moist earth with earthworms in it. _keep it moist._ a piece of sod, and a red clover plant with the soil clinging to its roots. _what is soil?_ it is the surface layer of the earth's crust, sometimes too shallow on the rocks to plough, sometimes much deeper. under deep soil lies the "subsoil," usually hard and rarely ploughed. _what is soil made of?_ ground rock materials and decayed remains of animal and plant life. by slow decay the soil becomes rich food for the growing of new plants. wild land grows up to weeds and finally to forests. the soil in fields and gardens is cultivated to make it fertile. plants take fertility from the soil. to maintain the same richness, plant food must be put back into the soil. this is done by deep tillage, and by mixing in with the soil manures, green crops, like clover, and commercial fertilizers. _plants must be made comfortable, and must be fed._ few plants are comfortable in sand. it gets hot, it lets water through, and it shifts in wind and is a poor anchor for roots. clay is so stiff that water cannot easily permeate it; roots have the same trouble to penetrate it and get at the food it is rich in. air cannot get in. sand mixed with clay makes a mellow soil, which lets water and air pass freely through. the roots are more comfortable, and the tiny root hairs can reach the particles of both kinds of mineral food. but the needful third element is decaying plant and animal substances, called "humus." these enrich the soil, but they do a more important thing: their decay hastens the release of plant food from the earthy part of the soil, and they add to it a sticky element which has a wonderful power to attract and hold the water that soaks into the earth. _what is the best garden soil?_ a mixture of sand, clay, and humus is called "loam." if sand predominates, it is a sandy loam--warm, mellow soil. if clay predominates, we have a clay loam--a heavy, rich, but cool soil. all gradations between the two extremes are suited to the needs of crops, from the melons on sandy soil, to celery that prefers deep, cool soil, and cranberries that demand muck--just old humus. _how do plant roots feed in soil?_ by means of delicate root hairs which come into contact with particles of soil around which a film of soil water clings. this fluid dissolves the food, and the root absorbs the fluid. plants can take no food in solid form. hence it is of the greatest importance to have the soil pulverized and spongy, able to absorb and hold the greatest amount of water. the moisture-coated soil particles must have air-spaces between them. air is as necessary to the roots as to the tops of growing plants. _why does the farmer plough and harrow and roll the land?_ to pulverize the soil; to mellow and lighten it; to mix in thoroughly the manure he has spread on it, and to reach, if he can, the deeper layers that have plant food which the roots of his crops have not yet touched. killing weeds is but a minor business, compared with tillage. later, ploughing or cultivating the surface lightly not only destroys the weeds, but it checks the loss of water by evaporation from the cracks that form in dry weather. raking the garden once a day in dry weather does more good than watering it. the "dust mulch" acts as a cool sunguard over the roots. _the process of soil-making._ if the man chopping wood in the yosemite valley looks about him he can see the soil-making forces at work on a grand scale. the bald, steep front of el capitan is of the hardest granite, but it is slowly crumbling, and its fragments are accumulating at the bottom of the long slope. rain and snow fill all crevices in the rocks. frost is a wonderful force in widening these cracks, for water expands when it freezes. the loosened rock masses plough their way down the steep, gathering, as they go, increasing power to tear away any rocks in their path. wind blows finer rock fragments along, and they lodge in cracks. fine dust and the seeds of plants are lodged there. the rocky slopes of the yosemite valley are all more or less covered with trees and shrubs that have come from wind-sown seeds. these plants thrust their roots deeper each year into the rock crevices. the feeding tips of roots secrete acids that eat away lime and other substances that occur in rocks. dead leaves and other discarded portions of the trees rot about their roots, and form soil of increasing depth. the largest trees grow on the rocky soil deposited at the base of the slope. the tree's roots prevent the river from carrying it off. when granite crumbles, its different mineral elements are separated. clear, glassy particles of quartz we call sand. dark particles of feldspar become clay, and may harden into slate. sand may become sandstone. exposed slate and sandstone are crumbled by exposure to wind and frost and moving water, and are deposited again as sand-bars and beds of clay. the most interesting phase of soil study is the discovery of what a work the humble earthworm does in mellowing and enriching the soil. the work of earthworms the farmer and the gardener should expect very poor crops if they planted seed without first ploughing or spading the soil. next, its fine particles must be separated by the breaking of the hard clods. a wise man ploughs heavy soil in the fall. it is caked into great clods which crumble before planting time. the water in the clods freezes in winter. the expansion due to freezing makes this soil water a force that separates the fine particles. so the frost works for the farmer. just under the surface of the soil lives a host of workers which are our patient friends. they work for their living, and are perhaps unconscious of the fact that they are constantly increasing the fertility of the soil. they are the earthworms, also called fishworms, which are distributed all over the world. they are not generally known to farmers and gardeners as friendly, useful creatures, and their services are rarely noticed. we see robins pulling them out of the ground, and we are likely to think the birds are ridding us of a garden pest. what we need is to use our eyes, and to read the wonderful discoveries recorded in a book called "vegetable mould and earthworms," written by charles darwin. the benefits of ploughing and spading are the loosening and pulverizing of the packed earth; the mixing of dead leaves and other vegetation on and near the surface with the more solid earth farther down; the letting in of water and air; and the checking of loss of water through cracks the sun forms by baking the soil dry. the earthworm is a creature of the dark. it cannot see, but it is sufficiently sensitive to light to avoid the sun, the rays of which would shrivel up its moist skin. having no lungs or gills, the worm uses the skin as the breathing organ; and it must be kept moist in order to serve its important use. this is why earthworms are never seen above ground except on rainy days, and never in the top soil if it has become dry. in seasons of little rain, they go down where the earth is moist, and venture to the surface only at night, when dew makes their coming up possible. earthworms have no teeth, but they have a long snout that protrudes beyond the mouth. their food is found on and in the surface soil. they will eat scraps of meat by sucking the juices, and scrape off the pulp of leaves and root vegetables in much the same way. much of their subsistence is upon organic matter that can be extracted from the soil. quantities of earth are swallowed. it is rare that an earthworm is dug up that does not show earth pellets somewhere on their way through the long digestive canal. the rich juices of plant substance are absorbed from these pellets as they pass through the body. earthworms explore the surface of the soil by night, and pick up what they can find of fresh food. nowhere have i heard of them as a nuisance in gardens, but they eagerly feed on bits of meat, especially fat, and on fresh leaves. they drag all such victuals into their burrows, and begin the digestion of the food by pouring on it from their mouths a secretion somewhat like pancreatic juice. the worms honeycomb the earth with their burrows, which are long, winding tubes. in dry or cold weather these burrows may reach eight feet under ground. they run obliquely, as a rule, from the surface, and are lined with a layer of the smooth soil, like soft paste, cast from the body. the lining being spread, the burrow fits the worm's body closely. this enables it to pass quickly from one end to the other, though it must wriggle backward or forward without turning around. at the lower end of the burrow, an enlarged chamber is found, where hibernating worms coil and sleep together in winter. at the top, a lining of dead leaves extends downward for a few inches, and in day time a plug of the same material is the outside door. at night the worm comes to the surface, and casts out the pellets of earth swallowed. the burrow grows in length by the amount of earth scraped off by the long snout and swallowed. the daily amount of excavation done is fairly estimated by the castings observed each morning on the surface. one earthworm's work for the farmer is not very much, but consider how many are at work, and what each one is doing. it is boring holes through the solid earth, and letting in the surface water and the air. it is carrying the lower soil up to the surface, often the stubborn subsoil, that no plough could reach. it is burying and thus hastening the decay of plant fibre, which lightens heavy soil and makes it rich because it is porous. moreover, the earthworms are doing over and over again this work of fining and turning over the soil, which the plough does but seldom. by the continuous carrying up of their castings, the earthworms gradually bury manures spread on the surface. the collapse of their burrows and the making of new ones keep the soil constantly in motion. the particles are being loosened and brought into contact with the soil water, that dissolves, and thus frees for the use of feeding roots, the plant food stored in the rock particles that compose the mineral part of the soil. the weight of earth brought to the surface by worms in the course of a year has been carefully estimated. darwin gives seven to eighteen tons per acre as the lowest and highest reports, based on careful collecting of castings by four observers, working on small areas of totally different soils. in england, earthworms have done a great deal more toward burying boulders and ancient ruins than any other agency. they eagerly burrow under heavy objects, the weight of which causes them to crush the honeycombed earth. undiscouraged, the earthworms repeat their work. "long before man existed, the land was regularly ploughed, and continues still to be ploughed by earthworms. it may be doubted whether there are many other animals which have played so important a part in the history of the world as have these lowly organized creatures." after years of study, charles darwin came to this conclusion. the more we study the lives of these earth-consuming creatures, the more fully do we believe what the great nature student said. the fertile soil is made of rock meal and decayed leaves and roots. only recently have ploughs been invented. but the great forest crops have grown in soil made mellow by the earthworm's ploughing. quiet forces that destroy rocks wind and water are the blustering active agents we see at work tearing down rocks and carrying away their particles. they do the most of this work of levelling the land; but there are quiet forces at work which might not attract our attention at all, and yet, without their help, wind and running water would not accomplish half the work for which they take the credit. the air contains certain destructive gases which by their chemical action separate the particles of the hardest rocks, causing them to crumble. now the wind blows away these crumbling particles, and the solid unchanged rock beneath is again exposed to the crumbling agencies. the changes in temperature between day and night cause rocks to contract and expand, and these changes put a strain upon the mineral particles that compose them. much scaling of rock surfaces is due to these causes. building a fire on top of a rock, and then dashing water upon the heated mass, shatters it in many directions. this process merely intensifies the effect produced by the mild changes of winter and summer. water is present in most rocks, in surprising quantities, often filling the spaces in porous rocks like sandstones. when winter brings the temperature down to the freezing point, the water near the surface of the rock first feels it. ice forms, and every particle of water is swollen by the change. a strain is put upon the mineral particles against which the particles of ice crowd for more room. frost is a very powerful agent in the crumbling of rocks, as well as of stubborn clods of earth. in warm climates, and in desert regions where there is little moisture in the rocks, this destructive action of freezing water is not known. in cold countries, and in high altitudes, where the air is heavy with moisture, its greatest work is done. some kinds of rock decay when they become dry, and resist crumbling better when they absorb a certain amount of moisture. alternate wetting and drying is destructive to certain rocks. one of the unnoticed agents of rock decay is the action of lowly plants. mosses grow upon the faces of rocks, thrusting their tiny root processes into pits they dig deeper by means of acids secreted by the delicate tips. you have seen shaded green patches of lichens, like little rugs, of different shapes, spread on the surface of rocks. but you cannot see so well the work these growths are doing in etching away the surface, and feeding upon the decaying mineral substance. mosses and lichens do a mighty work, with the help of water, in reducing rocks to their original elements, and thus forming soil. no plants but lichens and mosses can grow on the bare faces of rocks. as their root-like processes lengthen and go deeper into the rock face, particles are pried off, and the under-substance is attacked. higher plants then find a footing. have you not seen little trees growing on a patch of moss which gets its food from the air and the rock to which it clings? the spongy moss cushion soaks up the rain and holds it against the rock face. a streak of iron in the rock may cause the water to follow and rust it out, leaving a distinct crevice. now the roots of any plant that happens to be growing on the moss may find a foot-hold in the crack. streaks of lime in a rock readily absorb water, which gradually dissolves and absorbs its particles, inviting the roots to enter these new passages and feed upon the disintegrating minerals. dead leaves decay, and the acids the trickling water absorbs from them are especially active in disintegrating lime rocks. from such small beginnings has resulted the shattering of great rock masses by the growth of plants upon them. tree roots that grow in rock crevices exert a power that is irresistible. the roots of smaller plants do the same great work in a quieter way. when a hurricane or a flood tears down the mountain-side, sweeping everything before it, trees, torn out by the roots, drag great masses of rock and soil into the air, and fling them down the slope. wind and water thus finish the destruction which the humble mosses and lichens began. what seemed an impregnable fortress of granite has crumbled into fragments. its particles are reduced to dust, or are on the way to this condition. the plant food locked up in granite boulders becomes available to hungry roots. forests, grain-fields, and meadows cover the work of destructive agencies with a mantle of green. how rocks are made the granite shaft is made out of the original substance of the earth's crust. its minerals are the elements out of which all of the rock masses of the earth are formed, no matter how different they look from granite. sandstone is made of particles of quartz. clay and slate are made out of feldspar and mica. iron ore comes from the hornblende in granite. the mineral particles, reassembled in different proportions, form all of the different rocks that are known. here in my hand is a piece of pudding-stone. it is made of pebbles of different sizes, each made of different coloured minerals. the pebbles are cemented together with a paste that has hardened into stone. this kind of rock the geologists call _conglomerate_. pudding-stone is the common name, for the pebbles in the pasty matrix certainly do suggest the currants and the raisins that are sprinkled through a christmas pudding. under the seashores there are forming to-day thick beds of sand. the rivers bring the rock material down from the hills, and it is sorted and laid down. the moving water drops the heaviest particles near shore, and carries the finer ones farther out before letting them fall. [illustration: the town of cripple creek, colorado, which has grown up like magic since , covers the richest gold and silver mines in the world] [illustration: the level valley is filled up with fine rock flour washed from the sides of the neighboring mountains] the hard water, that comes through limestone rocks, adds lime in solution to the ocean water. all the shellfish of the sea, and the creatures with bony skeletons, take in the bone-building, shell-making lime with their food. generations of these inhabitants of the sea have died, and their shells and bones have accumulated and been transformed into thick beds of limestone on the ocean floor. this is going on to-day; but the limestone does not accumulate as rapidly as when the ocean teemed with shell-bearing creatures of gigantic size. of these we shall speak in another chapter. the fine dust that is blown into the ocean from the land, and that makes river water muddy, accumulates on the sea bottom as banks of mud, which by the burden of later deposits is converted into clay. sandstone is but the compressed sand-bank. in the study of mountains, geologists have discovered that old seashores were thrown up into the first great ridges that form the backbone of a mountain system. the rocky mountains, and the appalachian system on the east, were made out of thick strata of rocks that had been formed by accumulations of mud and sand--the washings of the land--on the opposite shores of a great mid-continental sea, that stretched from the crest of one great mountain system across to the other, and north and south from the laurentian hills to the gulf of mexico. the great weight of the accumulating layers of rock materials on one side, and the wasted land surfaces on the other, made the sea border a line of greatest weakness in the crust of the earth. the shrinking of the globe underneath caused the break; mashing and folding followed, throwing the ridge above sea-level, and making dry land out of rock waste which had been accumulating, perhaps for millions of years, under the sea. the wrinkling of the earth's crust was the result of crushing forces which produced tremendous heat. streams of lava sprang out through the fissures and poured streams of melted rock down the sides of the fold, quite burying, in many places, the layers of limestone, sandstone, and clay. between the strata of water-formed rocks there were often created chimney-like openings, into which molten rock from below was forced, forming, when cool, veins and dikes of rock material, specimens of the substance of the earth's interior. tremendous pressure and heat, acting upon stratified rocks saturated with water transform them into very different kinds of rock. limestone, subjected to these forces, is changed into marble. clays are transformed into slates. sandstone is changed into quartzite, the sand grains being melted so as to become no longer visible to the naked eye. the anthracite coal of the pennsylvania mountains is the result of heat and pressure acting upon soft coal. associated with these beds of hard coal are beds of black lead, or graphite, the substance used in making "lead" pencils. we believe that the same forces that operated to transform clay rocks into slate, and limestone into marble, transformed soft coal into hard, and hard coal into graphite, in the days when the earth was young. the word _sedimentary_ is applied to rocks which were originally laid down under water, as sediment, brought by running water, or by wind, or by the decay of organic substances. _stratified_ rocks are those which are arranged in layers. sedimentary rocks will fall into this class. _aqueous_ rocks are those which are formed under water. most of the stratified and sedimentary rocks, but not all, may be included under this term. rocks that are made out of fragments of other rocks torn down by the agencies of erosion are called _fragmental_. wind, water, and ice are the three great agencies that wear away the land, bring rock fragments long distances, and deposit them where aqueous rocks are being formed. volcanic eruptions bring material from the earth's interior. this material ranges all the way from huge boulders to the finest impalpable dust, called volcanic ashes. rivers of ice called glaciers crowd against their banks, loosening rock masses and carrying away fragments of all sizes, in their progress down the valley. brooks and rivers carry the pebbles and the larger rock masses they are able to loosen from their walls and beds, and grind them smooth as they move along toward lower levels. the air itself causes rocks to crumble; percolating water robs them of their soluble salts, reducing even solid granite to a loose mass of quartz grains and clay. plants and animals absorb as food the mineral substances of rocks, when they are dissolved in water. they transform these food elements into their own body substance, and finally give back their dead bodies, the mineral substances of which are freed by decay to return to the earth, and become elements of rock again. the decay of rock is well shown by the materials that accumulate at the base of a cliff. angular fragments of all sizes, but all more or less flattened, come from strata of shaly rock, that can be seen jutting out far above. a great deal of this sort of material is found mingled with the soil of the northeastern states. round pebbles in pudding-stone have been formed in brook beds and deposited on beaches where they have become caked in mud and finally consolidated into rock. if the beach chanced to be sandy instead of muddy, a matrix of sandy paste holds the larger pebbles in place. limestone paste cements together the pebbles of limestone conglomerates. in st. augustine many of the houses are built of coquina rock, a mass of broken shells which have become cemented together by lime mud, derived from their own decay. on the slopes of volcanoes, rock fragments of all kinds are cemented together by the flowing lava. so we see that there are pudding-stones of many kinds to be found. if some solvent acid is present in the water that percolates through these rocks it may soften the cement and thus free the pebbles, reducing the conglomerate again to a mere heap of shell fragments, or gravel, or rounded pebbles. the story of rock formation tells how fire and water, and the two combined, have made, and made over, again and again, the substance of the earth's crust. chemical and physical changes constantly tear down some portions of the earth to build up others. the constant, combined effort of wind and water is to level the earth and fill up the ocean bed. rocks are constantly being formed; the changes that have been going on since the world began are still in progress. we can see them all about us on any and every day of our lives. getting acquainted with a river i have two friends whose childhood was spent in a home on the banks of a noble eastern river. their father taught the boy and the girl to row a boat, and later each learned the more difficult art of managing a canoe. on holidays they enjoyed no pleasure so much as a picnic on the river-bank at some point that could be reached by rowing. as they grew older, longer trips were planned, and the river was explored as far as it was navigable by boat or canoe. last summer when vacation came, these two carried out a long-cherished plan to find the beginning of the river--to follow it to its source. so they left home, and canoed up-stream, until the stream became a brook, so shallow they could go no farther. then they followed it on foot--wading, climbing, making little détours, but never losing the little river. at last they came to the beginning of it--a tiny rivulet trickled out of the side of a hill, filling a wooden keg that formed a basin, where thirsty passers-by could stoop and drink. they decided to mark the spring, so that people who found it later, and were refreshed by its clear water, might know that here was born the greatest river of a great state. but they were not the original discoverers. above the spring, a board was nailed to a tree, saying that this is the headwater of the river with the beautiful indian name, susquehanna. it was a dry summer, and the overflow of the basin was almost all drunk up by the thirsty ground. they could scarcely follow it, except by the groove cut by the rivulet in seasons when the flow was greater. they followed the runaway brook, through the grass roots, that almost hid it. as the ground grew steeper, it hurried faster. soon it gathered the water of other springs, which hurried toward it in small rivulets, because its level was lower. water always seeks the lowest level it can find. sometimes marshy spots were reached where water stood in the holes made by the feet of cattle that came there to drink. the water was muddy, and seemed to stand still. but it was settling steadily, and at one side the little river was found, flowing away with the water it drew from the swampy, springy ground. all the mud was gone, now; the water was clear. it flowed in a bed with a stony floor, and there were rough steps where the water fell down in little sheets, forming a waterfall, the first of many that make this river beautiful in the upper half of its course. to get from the high level of that hillside spring to the low level of the sea, the water has to make a fall of twenty-three hundred feet, but it makes the descent gradually. it could not climb over anything, but always found a way to get around the rocks and hills that stood in its way. when the flat marsh land interfered, the water poured in and overflowed the basin at the lowest margin. in the rocky ground the two explorers found that the stream had widened its channel by entering a narrow crevice and wearing away its walls. the continual washing of the water wears away stone. rocks are softened by being wet. streaks of iron in the hardest granite will rust out and let the water in. then the lime in rocks is easily dissolved. every dead leaf the river carried along added an acid to the water, and this made easier the process of dissolving the limestone. every crumbling rock gives the river tools that it uses like hammer and chisel and sandpaper to smooth all the uneven surfaces in its bed, to move stumbling blocks, and to dig the bed deeper and wider. the steeper the slope is, the faster the stream flows, and the larger the rocks it can carry. rocks loosened from the stream bed are rolled along by the current. then bang! against the rocks that are not loose, and often they are able to break them loose. the fine sand is swept along, and its sharp points strike like steel needles, and do a great work in polishing roughness and loosening small particles from the stream bed. the bigger pebbles of the stream have banged against the rock walls, with the same effect, smoothing away unevenness and pounding fragments loose, rolling against one another, and getting their own rough corners worn away. the makers of stone marbles learned their business from a brook. they cut the stone into cubical blocks, and throw them into troughs, into which is poured a stream of running water. the blocks are kept in motion, and the grinding makes each block help the rest to grind off the eight corners and the twelve ridges of each one. the water becomes muddy with the fine particles, just as the drip from a grindstone becomes unclean when an axe is ground. pretty soon all the blocks in the trough are changed into globes--the marbles that children buy at the shops when marble season comes around. i suppose if the troughs are not watched and emptied in time, the marbles would gradually be ground down to the size of peas, then to the size of small bird shot, and finally they would escape as muddy water and fine sand grains. sure it is that the sandy shores that line most rivers are the remnants of hard rocks that have been torn out and ground up by the action of the current. not very many miles from its first waterfall the stream had grown so large that my two friends knew that they would soon find their canoes. the plan now was to float down the curious, winding river and to learn, if the river and the banks could tell them, just why the course was so crooked on the map. they came into a broad, level valley where streams met them, coming out of deep clefts between the hills they were leaving behind them. the banks were pebbly, but blackened with slimy mud that made the water murky. the current swerved from one side to the other, sometimes quite close to the bank, where the river turned and formed a deep bend. on this side the bank was steep, the roots of plants and trees exposed. on the opposite side a muddy bank sloped gently out into the stream. here building up was going on, to offset the tearing down. the sharp bends are made sharper, once the current is deflected from the middle of the stream to one side. at length the loops bend on each other and come so near together that the current breaks through, leaving a semicircular bayou of still water, and the river's course straightened at that place. it must have been in a spring flood that this cut-off was made, and, the break once made was easily widened, for the soil is fine mud which, when soaked, crumbles and dissolves into muddy water. stately and slow that river moves down to the bay, into which it empties its load. the rain that falls on hundreds of square miles of territory flows into the streams that feed this trunk. the little spring that is the headwater of the system is but one of many pockets in the hillsides that hold the water that soaks into the ground and give it out by slow degrees. surface water after a rain flows quickly into the streams. it is the springs that hold back their supply and keep the rivers from running dry in hot weather. do they feel now that they know their river? are they ready to leave it, and explore some other? indeed, no. they are barely introduced to it. all kinds of rivers are shown by the different parts of this one. it is a river of the mountains and of the lowland. it flows through woods and prairies, through rocky passes and reedy flats. it races impetuously in its youth, and plods sedately in later life. the trees and the other plants that shadow this stream, and live by its bounty, are very different in the upland and in the lowland. the scenery along this stream shows endless variety. up yonder all is wild. down here great bridges span the flood, boats of all kinds carry on the commerce between two neighbour cities. a great park comes down to the river-bank on one side. canoes are thick as they can paddle on late summer afternoons. no one can ever really know a river well enough to feel that it is an old story. there is always something new it has to tell its friends. so my two explorers say, and they know far more about their friendly river than i do. the ways of rivers a canal is an artificial river, built to carry boats from one place to another. its course is, as nearly as possible, a straight line between two points. a river, we all agree, is more beautiful than a canal, for it winds in graceful curves, in and out among the hills, its waters seeking the lowest level, always. no artist could lay out curves more beautiful than the river forms. these curves change from year to year, some slowly, some more rapidly. it is not hard to understand just why these changes take place. some rivers are dangerous for boating at certain points. the current is strong, and there are eddies and whirlpools that have to be avoided, or the boat becomes unmanageable. people are drowned each season by trusting themselves to rivers the dangerous tricks of which they do not know. deep holes are washed out of the bed of the stream by whirling eddies. the pot-holes of which people talk are deep, rounded cavities, ground out of the rocky stream-bed by the scouring of sand and loose stones driven by whirling eddies in shallow basins. every year deepens each pot-hole until some change in the stream-bed shifts the eddy to another place. no stream finds its channel ready-made; it makes its own, and constantly changes it. the current swings to one side of the channel, lifting the loose sediment and grinding deeper the bed of the stream. the water lags on the opposite side, and sediment falls to the bottom. so the building-up of one side is going on at the same time that the tearing-down process is being carried on on the other. with the lowering of the bed the river swerves toward one bank, and a hollow is worn by slow degrees. the current swings into this hollow, and in passing out is thrown across the stream to the opposite bank. here its force wears away another hollow; and so it zigzags down-stream. the deeper the hollows, the more curved becomes the course, if the general fall is but moderate. it is toward the lower courses of the stream that the winding becomes more noticeable. the sediment that is carried is deposited at the point where the current is least strong, so that while the outcurves become sharper by the tearing away of the stream's bank, the incurves become sharper by the building up of this bank. the mississippi below memphis is thrown into a wonderful series of curves by the erosion and the deposit caused by the current zigzagging back and forth from one bank to the other. gradually the curves become loops. the river's current finally jumps across the meeting of the curves, and abandons the circular bend. it becomes a bayou or lagoon of still water, while the current flows on in the straightened channel. all rivers that flow through flat, swampy land show these intricate winding channels and many lagoons that have once been curves of the river. no one would ever mistake a river for a lake or any other body of water, yet rivers differ greatly in character. one tears its way along down its steep, rock-encumbered channel between walls that rise as vertical precipices on both sides. the roaming, angry waters are drawn into whirlpools in one place. they lie stagnant as if sulking in another, then leap boisterously over ledges of rock and are churned into creamy foam at the bottom. outside the mountainous part of its course this same river flows broad and calm through a mud-banked channel, cut by tributary streams that draw in the water of low, sloping hills. the missouri is such a wild mountain stream at its headwaters. we who have seen its muddy waters from sioux city to st. louis would hardly believe that its impetuous and picturesque youth could merge into an old age so comfortable and placid and commonplace. this thing is true of all rivers. they flow, gradually or suddenly, from higher to lower levels. to reach the lowest level as soon as possible is the end each river is striving toward. if it could, each river would cut its bed to this depth at the first stage of its course. its tools are the rocks it carries, great and small. the force that uses these tools is the power of falling water, represented by the current of the stream. the upper part of a river such as the missouri or mississippi engages in a campaign of widening and deepening its channel, and carrying away quantities of sediment. the lower reaches of the stream flow through more level country; the current is checked, and a vast burden of sediment is laid down. instead of tearing away its banks and bottom, the river fills up gradually with mud. the current meanders between banks of sediment over a bottom which becomes shallower year by year. the rocky mountains are being carried to the gulf of mexico. the commerce of the river is impeded by mountain débris deposited as mud-banks along the river's lower course. many rivers are quiet and commonplace throughout their length. they flow between low, rounded hills, and are joined by quiet streams, that occupy the separating grooves between the hills. this is the oldest type of river. it has done its work. rainfall and stream-flow have brought the level of the land nearly to the level of the stream. very little more is left to be ground down and carried away. the landscape is beautiful, but it is no longer picturesque. wind and water have smoothed away unevennesses. trees and grass and other vegetation check erosion, and the river has little to do but to carry away the surface water that falls as rain. but suppose our river, flowing gently between its grassy banks, should feel some mighty power lifting it up, with all its neighbour hills and valleys, to form a wrinkle in the still unstable crust of the earth. away off at the river's mouth the level may not have changed, or that region may have been depressed instead of elevated by the shrinking process. suppose the great upheaval has not severed the upper from the lower courses of the stream. with tremendous force and speed, the current flows from the higher levels to the lower. the river in the highlands strikes hard to reach the level of its mouth. it grinds with all its might, and all its rocky tools, upon its bed. all the mud is scoured out, and then the underlying rocks are attacked. if these rocks are soft and easily worn away, the channel deepens rapidly. one after another the alternating layers are excavated, and the river flows in a canyon which deepens more and more. as the level is lowered, the current of the stream becomes slower and the cutting away of its bed less rapid. the stream is content to flow gently, for it has almost reached the old level, on which it flowed before the valley became a ridge or table-land. the rivers that flow in canyons have been thousands of years in carving out their channels, yet they are newer, geologically speaking, than the streams that drain the level prairie country. the earth has risen, and the canyons have been carved since the prairies became rolling, level ground. [illustration: this little pond is a basin hollowed by the same glacier that scattered the stones and rounded the hills] [illustration: every stream is wearing away its banks, while trees and grass blades are holding on to the soil with all their roots] the colorado river flows through a canyon with walls that in places present sheer vertical faces a mile in depth, and so smooth that no trail can be found by which to reach from top to bottom. the region has but slight erosion by wind, and practically none by rain. the local rainfall is very slight. so the river is the one force that has acted to cut down the rocks, and its force is all expended in the narrow area of its own bed. had frequent rains been the rule on the colorado plateau, the angles of the mesas would have been rounded into hills of the familiar kind so constantly a part of the landscape in the eastern half of the continent. the colorado is an ancient river which has to carry away the store of moisture that comes from the pacific ocean and falls as snow on the high peaks of the rocky mountains. similar river gorges with similar stories to tell are the arkansas, the platte, and the yellowstone. all cut their channels unaided through regions of little rain. when the earth's crust is thrown up in mountain folds, and between them valleys are formed, the level of rivers is sometimes lowered and the rapidity of their flow is checked. a stream which has torn down its walls at a rapid rate becomes a sluggish water-course, its current clogged with sediment, which it has no power to carry farther. when such a river begins to build and obstruct its own waters it bars its progress and may form a lake as the outlet of its tributary streams. many ancient rivers have been utterly changed and some obliterated by general movements of the earth's crust. the story of a pond look out of the car window as you cross a flat stretch of new prairie country, and you see a great many little ponds of water dotting the green landscape. forty years ago iowa was a good place to see ponds of all shapes and sizes. the copious rainfall of the early spring gathered in the hollows of the land, and the stiff clay subsoil prevented the water from soaking quickly into the ground. the ponds might dry away during the hot, dry summer, leaving a baked clay basin, checked with an intricate system of cracks. or if rains were frequent and heavy, they might keep full to the brim throughout the season. tall bulrushes stood around the margins of the largest ponds, and water-lilies blossomed on the surface during the summer. the bass and the treble of the spring chorus were made by frogs and toads and little hylas, all of which resorted to the ponds to lay their eggs, in coiled ropes or spongy masses, according to their various family traditions. on many a spring night my zoölogy class and i have visited the squashy margins of these ponds, and, by the light of a lantern, seen singing toads and frogs sitting on bare hummocks of grass roots that stood above the water-line. the throat of each musician was puffed out into a bag about the size and shape of a small hen's egg; and all were singing for dear life, and making a din that was almost ear-splitting at close range. so great was the self-absorption of these singers that we could approach them, daze them with the light of the lantern, and capture any number of them with our long-handled nets before they noticed us. but it was not easy to persuade them to sing in captivity, no matter how many of the comforts of home we provided in the school aquariums. so, after some very interesting nature studies, we always carried them back and liberated them, where they could rejoin their kinsfolk and neighbours. it was when we were scraping the mud from our rubber boots that we realized the character of the bottoms of our prairie ponds. the slimy black deposit was made partly of the clay bottom, but largely of decaying roots and tops of water plants of various kinds. whenever it rained or the wind blew hard, the bottom was stirred enough to make the water muddy; and on the quietest days a pail of pond water had a tinge of brown because there were always decaying leaves and other rubbish to stain its purity. the farmers drained the ponds as fast as they were able, carrying the water, by open ditches first, and later by underground tile drains, to lower levels. finally these trunk drain pipes discharged the water into streams or lakes. to-day a large proportion of the pond areas of iowa has disappeared; the hollow tile of terra-cotta has been the most efficient means of converting the waste land, covered by ponds, into fertile fields. but the ponds that have not been drained are smaller than they used to be, and are on the straight road to extinction. this process one can see at any time by visiting a pond. every year a crop of reeds and a dozen other species of vigorous water plants dies at the top and adds the substance of their summer growth to the dust and other refuse that gathers in the bottom of the pond. each spring roots and seeds send up another crop, if possible more vigorous than the last, and this top growth in turn dies and lies upon the bottom. the pond level varies with the rainfall of the years, but it averages a certain depth, from which something is each year subtracted by the accumulations of rotting vegetable matter in the bottom. evaporation lowers the water-level, especially in hot, dry summers. from year to year the water plants draw in to form a smaller circle, the grassy meadow land encroaches on all sides. the end of the story is the filling up of the pond basin with the rotting substance of its own vegetation. this is what is happening to ponds and inland marshes by slow degrees. the tile drain pipes obliterate the pond in a single season. nature is more deliberate. she may require a hundred years to fill up a single pond which the farmer can rid himself of by a few days of work and a few rods of tiling. the riddle of the lost rocks outside of my window two robins are building a nest in the crotch of a blossoming red maple tree. and just across the hedge, men are digging a big square hole in the ground--the cellar of our neighbour's new house. it looks now as if the robins would get their house built first, for they need but one room, and they do not trouble about a cellar. i shall watch both houses as they grow through the breezy march days. the brown sod was first torn up by a plough, which uncovered the red new jersey soil. two men, with a team hitched to a scraper, have carried load after load of the loose earth to a heap on the back of the lot, while two other men with pickaxes dug into the hard subsoil, loosening it, so that the scraper could scoop it up. this subsoil is heavy, like clay, and it breaks apart into hard clods. at the surface the men found a network of tree roots, about which the soil easily crumbled. often i hear a sharp, metallic stroke, unlike the dull sound of the picks striking into the earth. the digger has struck a stone, and he must work around it, pry it up and lift it out of the way. a row of these stones is seen at one side of the cellar hole, ranged along the bank. they are all different in size and shape, and red with clay, so i can't tell what they are made of. but from this distance i see plainly that they are irregular in form and have no sharp corners. the soil strewn along the lot by the scraper is full of stones, mostly irregular, but some rounded; some are as big as your head, others grade down to the sizes of marbles. when i went down and examined this red earth, i found pebbles of all shapes and sizes, gravel in with the clay, and grains of sand. this rock-sprinkled soil in new jersey is very much like soil which i know very well in iowa; it looks different in colour, but those pebbles and rock fragments must be explained in the same way here as there. these are not native stones, the outcrop of near-by hillsides, but strangers in this region. the stones in iowa soil are also imported. the prairie land of iowa has not many big rocks on the surface, yet enough of them to make trouble. the man who was ploughing kept a sharp lookout, and swung his plough point away from a buried rock that showed above ground, lest it should break the steel blade. one of the farmer's jobs for the less busy season was to go out with sledge and dynamite sticks, and blast into fragments the buried boulders too large to move. sometimes building a hot fire on the top of it, and throwing on water, would crack the stubborn "dornick" into pieces small enough to be loaded on stone-boats. i remember when the last giant boulder whose buried bulk scarcely showed at the surface, was fractured by dynamite. its total weight proved to be many tons. we hauled the pieces to the great stone pile which furnished materials for walling the sides of a deep well and for laying the foundation of the new house. yet for years stones have been accumulating, all of them turned out of the same farm, when pastures and swampy land came under the plough. draw a line on the map from new york to st. louis, and then turn northward a little and extend it to the yellowstone park. the boulder-strewn states lie north of this line, and are not found south of it, anywhere. canada has boulders just like those of our northern states. the same power scattered them over all of the vast northern half of north america and a large part of europe. what explanation is there for this extensive distribution of unsorted débris? the question answered the rocks tell their own story, partly, but not wholly. they told just enough to keep the early geologists guessing; and only very recently has the guessing come upon the truth. these things the rocks told: . we have come from a distance. . we have had our sharp corners worn off. . many of us have deep scratches on our sides. . at various places we have been dumped in long ridges, mixed with much earth. . a big boulder is often balanced on another one. the first thing the geologist noted was the fact that these boulders are strangers--that is, they are not the native rocks that outcrop on hillsides and on mountain slopes near where they are found. far to the north are beds of rock from which this débris undoubtedly came. could a flood have scattered them as they are found? no, for water sorts the rock débris it deposits, and it rounds and polishes rock fragments, instead of scratching and grooving them and leaving them angular, as these are. professor agassiz went to switzerland and studied the glaciers. he found unsorted rock fragments where the glacier's nose melted, and let them fall. they were worn and scratched and grooved, by being frozen into the ice, and dragged over the rocky bed of the stream. the rocky walls of the valley were scored by the glacier's tools. rounded domes of rock jutted out of the ground, in the paths of the ice streams, just like the granite outcrop in central park in new york, and many others in the region of scattered boulders. after long studies in europe and in north america, professor agassiz declared his belief that a great ice-sheet once covered the northern half of both countries, rounding the hills, scooping out the valleys and lake basins, and scattering the boulders, gravel, and clay, as it gradually melted away. the belief of professor agassiz was not accepted at once, but further studies prove that he guessed the riddle of the boulders. the rich soil of the northern states is the glacial drift--the mixture of rock fragments of all sizes with fine boulder clay, left by the gradual melting of the great ice-sheet as it retreated northward at the end of the "glacial epoch." glaciers among the alps switzerland is a little country without any seacoast, mountainous, with steep, lofty peaks, and narrow valleys. the climate is cool and moist, and snow falls the year round on the mountain slopes. a snow-cap covers the lower peaks and ridges. above the level of nine thousand feet the bare peaks rise into a dry atmosphere; but below this altitude, and above the six thousand-foot mark, lies the belt of greatest snowfall. peaks between six and nine thousand feet high are buried under the alpine snow-field, which adds thickness with each storm, and is drained away to feed the rushing mountain streams in the lower valleys. the snow that falls on the steep, smooth slope clings at first; but as the thickness and the weight of these snow banks increase, their hold on the slope weakens. they may slip off, at any moment. the village at the foot of the slope is in danger of being buried under a snow-slide, which people call an avalanche. "challanche" is another name for it. the hunter on the snow-clad mountains dares not shout for fear that his voice, reëchoing among the silent mountains, may start an avalanche on its deadly plunge into the valley. on the surface of the snow-field, light snow-flakes rest. under them the snow is packed closer. deeper down, the snow is granular, like pellets of ice; and still under this is ice, made of snow under pressure. the weight of the accumulated snow presses the underlying ice out into the valleys. these streams are the glaciers--rivers of ice. the glaciers of the alps vary in length from five to fifteen miles, from one to three miles in width, and from two hundred to six hundred feet in thickness. they flow at the rate of from one to three feet a day, going faster on the steeper slopes. it is hard to believe that any substance as solid and brittle as ice can flow. its movement is like that of stiff molasses, or wax, or pitch. the tremendous pressure of the snow-field pushes the mass of ice out into the valleys, and its own weight, combined with the constant pressure from behind, keeps it moving. the glacier's progress is hindered by the uneven walls and bed of the valley, and by any decrease in the slope of the bed. when a flat, broad area is reached, a lake of ice may be formed. these are not frequent in the alps. the water near the banks and at the bottom of a river does not flow as swiftly as in the middle and at the surface of the stream. the flow of ice in a glacier is just so. friction with the banks and bottom retards the ice while the middle parts go forward, melting under the strain, and freezing again. there is a constant readjusting of particles, which does not affect the solidity of the mass. the ice moulds itself over any unevenness in its bed if it cannot remove the obstruction. the drop which would cause a small waterfall in a river, makes a bend in the thick body of the ice river. great cracks, called _crevasses_, are made at the surface, along the line of the bend. the width of the v-shaped openings depends upon the depth of the glacier and the sharpness of the bend that causes the breaks. rocky ridges in the bed of the ice-stream may cause crevasses that run lengthwise of the glacier. snow may fill these chasms or bridge them over. the hunter or the tourist who ventures on the glacier is in constant danger, unless he sees solid ice under him. men rope themselves together in climbing over perilous places, so that if one slips into a crevasse his mates can save him. a glacier tears away and carries away quantities of rock and earth that form the walls of its bed. as the valley narrows, tremendous pressure crowds the ice against the sides, tearing trees out by the roots and causing rock masses to fall on the top of the glacier, or to be dragged along frozen solidly into its sides. the weight of the ice bears on the bed of the glacier, and its progress crowds irresistibly against all loose rock material. the glacier's tools are the rocks it carries frozen into its icy walls and bottom. these rocks rub against the walls, grinding off débris which is pushed or carried along. no matter how heavy the boulders are that fall in the way of the ice river, the ice carries them along. it cannot drop them as a river of water would do. slowly they travel, and finally stop where the nose of the glacier melts and leaves all débris that the mountain stream, fed by the melting of the ice, cannot carry away. the bedrock under a glacier is scraped and ground and scored by the glacier's tools--the rock fragments frozen into the bottom of the ice. these rocks are worn away by constant grinding, just as a steel knife becomes thin and narrow by use. scratches and scorings and polished surfaces are found in all rocks that pass one another in close contact. its worn-out tools the glacier drops at the point where its ice melts. this great, unsorted mass of rock meal and coarser débris the stream is gradually scattering down the valley. the name "moraine" has been given to the earth rubbish a glacier collects and finally dumps. the _top moraine_ is at the surface of the ice. the _lateral moraines_, one at each side, are the débris gathered from the sides of the valley. the _ground moraine_ is what débris the ice pushes and drags along on the bottom. the _terminal moraine_ is the dumping-ground of this mass of material, where the ice river melts. glaciers, like other rivers, often have tributary streams. a _median moraine_, seen as a dark streak running lengthwise on the surface of a glacier, means that two branch glaciers have united to form this one. go back far enough and you will reach the place where the two streams come together. the two lateral moraines that join form the middle line of débris, the median moraine. three ice-streams joined produce two top moraines. they locate the lateral moraines of the middle glacier. the surface of a glacier is often a mass of broken and rough ice, forming a series of pits and pinnacles that make crossing impossible. the sun melts the surface, forming pools and percolating streams of water, that honeycomb the mass. underneath, the ice is tunnelled, and a rushing stream flows out under the end of the glacier. it is not clear, but black with mud, called _boulder clay_, or _till_, made of ground rock, and mixed with fragments of all shapes and sizes. this is the meal from the glacier's mill, dumped where the water can sift it. "balanced rocks" are boulders, one upon another, that once lay on a glacier, and were left in this strange, unstable position when the supporting ice walls melted away from them. in bronx park in new york the "rocking stone" always attracts attention. the glacier that lodged it there, also rounded the granite dome in central park and scattered the rock-strewn boulder clay on long island. doubtless in an earlier day the edges of this glacier were thrust out into the atlantic, not far from the great south bay, and icebergs broke off and floated away. [illustration: potsdam sandstone showing ripple marks] [illustration: _by permission of the american museum of natural history_ glacial striæ on lower helderberg limestone] [illustration: glacial grooves in the south meadow, central park, new york] [illustration: _by permission of the american museum of natural history_ mt. tom, west d st., new york] glaciers are small to-day compared with what they were long ago, in europe and in america. the climate became warmer, and the ice-cap retreated. old moraines show that the ice rivers of the alps once came much farther down the valleys than they do now. smooth, deeply scored domes of rock, the one in central park and the bald head of mount tom, are just like those that lie in alpine valleys from which the glaciers have long ago retreated. there are old moraines far up the sides of valleys, showing that once the glaciers were far deeper than now. no other power could have brought rocks from strata higher up the mountains, and lodged them thus. nearer home, mt. shasta and mt. rainier still have glaciers that have dwindled in size, until they bear little comparison to the gigantic ice-streams that once filled the smooth beds their puny successors flow into. remnants of glaciers lie in the hollows of the sierras. we must go north to find the snow-fields of alaska and glaciers worthy to be compared with those ancient ice rivers whose work is plainly to be seen, though they are gone. the great ice-sheet greenland is green only along its southern edge, and only in summer, so its name is misleading. it is a frozen continent lying under a great ice-cap, which covers , square miles and is several thousand feet in thickness. the top of this icy table-land rises from five thousand to ten thousand feet above the sea-level. the long, cold winters are marked by great snowfall, and the drifts do not have time to melt during the short summer; and so they keep getting deeper and deeper. streams of ice flow down the steeps into the sea, and break off by their weight when they are pushed out into the water. these are the icebergs which float off into the north atlantic, and are often seen by passengers on transatlantic steamers. long ago greenland better deserved its name. explorers who have climbed the mountain steeps that guard the unknown ice-fields of the interior have discovered, a thousand feet above the sea-level, an ancient beach, strewn with shells of molluscs like those which now inhabit salt water, and skeletons of fishes lie buried in the sand. it is impossible to think that the ocean has subsided. the only explanation that accounts for the ancient beach, high and dry on the side of greenland's icy mountain is that the continent has been lifted a thousand feet above its former level. this is an accepted fact. we know that climate changes with changed altitude as well as latitude. going up the side of a mountain, even in tropical regions, we may reach the snow-line in the middle of summer. magnolia trees and tree ferns once grew luxuriantly in greenland forests. their fossil remains have been found in the rocks. this was long before the continent was lifted into the altitude of ice and snow. and it is believed that the climate of northern latitudes has become more severe than formerly from other causes. it is possible that the earth's orbit has gradually changed in form and position. if greenland should ever subside until the ancient beach rests again at sea-level, the secrets of that unknown land would be revealed by the melting of the glacial sheet that overspreads it. possibly it would turn out to be a mere flock of islands. we can only guess. north america had, not so long ago, two-thirds of its area covered with an ice-sheet like that of greenland, and a climate as cold as greenland's. at this time the land was lifted two to three thousand feet higher than its present level. all of the rain fell as snow, and the ice accumulated and became thicker year by year. instead of glaciers filling the gorges, a great ice flood covered all the land, and pushed southward as far as the ohio river on the east and yellowstone park in the west. the rocky mountains and some parts of the appalachian system accumulated snow and formed local glaciers, separated from the vast ice-sheet. the unstable crust of the earth began to sink at length, and gradually the ice-sheet's progress southward was checked, and it began to recede by melting. all along the borders of this great fan-shaped ice-field water accumulated from the melting, and flooded the streams which drained it to the atlantic and the gulf. icebergs broken off of the edge of the retiring ice-sheet floated in a great inland sea. the land sank lower and lower until the general level was five hundred to one thousand feet lower than it now is. the climate became correspondingly warm, and the icebergs melted away. then the land rose again, and in time the inland sea was drained away into the ocean, except for the waters that remained in thousands of lakes great and small that now occupy the region covered by the ice. ancient sea beaches mark the level of high water at the time that the flood followed the melting ice. on the shores of lake champlain, but nearly five hundred feet higher than the present level of the lake, curious geologists have found many kinds of marine shells on a well-marked old sea beach. the members of one exploring party in the same region were surprised and delighted to come by digging upon the skeleton of a whale that had drifted ashore in the ancient days when the inland sea joined the atlantic. lake ontario's ancient beach is five hundred feet above the present water-level; lake erie's is two hundred fifty feet above it; lake superior's three hundred thirty feet higher than the present beach. no doubt when the water stood at the highest level, the great lakes formed one single sheet of water which settled to a lower level as the rivers flowing south cut their channels deep enough to draw off the water toward the gulf. lake winnipeg is now the small remnant of a vast lake the shores of which have been traced. the minnesota river finally made its way into the mississippi and drained this great area the stranded beaches of which still remain. the name of agassiz has been given to the ancient lake formed by the glacial flood and drained away thousands of years ago but not until it had built the terraced beach which locates it on the geological map of the region. when the ice-sheet came down from the north it dragged along all of the soil and loose rock material that lay in its path. with the boulders frozen into its lower surface it scratched and grooved the firm bedrock over which it slid, and rounded it to a smooth and billowy surface. the progress of the ice-sheet was southward, but it spread like a fan so that its widening border turned to east and west. when it reached its southernmost limit and began to melt, it laid down a great ridge of unsorted rock material, remnants of which remain to this day,--the terminal moraine of the ancient ice-sheet. the line of this ancient deposit starts on long island, crosses new jersey and pennsylvania, then dips southward, following the general course of the ohio river to its mouth, forming bluffs in southern ohio, indiana, and illinois. the line bends upward as it crosses central missouri, a corner of kansas, and eastern nebraska, parallel with the course of the missouri. as the ice-sheet melted, boulders were dropped all over the northern states and canada. these were both angular and rounded. in some places they are scattered thickly over the surface and are so numerous as to be a great hindrance to agriculture. in many places great boulders of thousands of tons weight are perched on very slight foundations, just where they lodged when the ice went off and left them, after carrying them hundreds of miles. around them are scattered quantities of loose rock material, not scored or ground as are those which were carried on the under-surface of the glacial ice. these unscarred fragments rode on the top of the ice. they were a part of the top moraine of the glacial sheet. the finest material deposited is rock meal, ground by the great glacial mill, and called "boulder clay." it is a stiff, dense, stony paste in which boulders of all sizes, gravel, pebbles, and cobblestones are cemented. the "drift" of the ice-sheet is the rubbish, coarse and fine, it left behind as it retreated. below the ohio river there is a deep soil produced by the decay of rocks that lie under it. north of ohio is spread that peculiar mixture of earth and rock fragments which was transported from the north and spread over the land which the ice-sheet swept bare and ground smooth and polished. the drift has been washed away in places by the floods that followed the ice. granite domes are thus exposed, the grooves and scratches of which tell in what direction the ice flood was travelling. miles away from that scored granite, but in the same direction as the scratches, scattered fragments of the same foundation rock cover fields and meadows. thus, much of the drift material can be traced to its original home, and the course of the ice-sheet can be determined. many immense boulders the home of which was in the northern highlands of canada rode southward, frozen into icebergs that floated in the great inland sea. great quantities of débris were added to the original glacial drift through the agency of these floating ice masses, which melted by slow degrees. following some lost rivers what would you think if the boat in which you were floating down a pleasant river should suddenly grate upon sand, and you should look over the gunwale and find that here the waters sank out of sight, the river ended? i believe you would rub your eyes, and feel sure that you were dreaming. do not all rivers flow along their beds, growing larger with every mile, and finally empty their waters into a sea, or bay, or lake, or flow into some larger stream? this is the way of most rivers, but there are exceptions. in the far west there are some great rivers that absolutely disappear before they reach a larger body of water. they simply sink away into the sand, and sometimes reappear to finish their courses after flowing underground for miles. do you know the name of one great western river of which i am thinking? is there any stream in your neighbourhood which has such peculiar ways? down in kentucky there is a region where, it is said, one may walk fifty miles without crossing running water. in the middle of our country, in the region of plentiful rainfall, and in a state covered with beautiful woodlands and famous for blue grass and other grain crops, it is amazing that, over a large area, brooks and larger streams are lacking. in most of the state there is plenty of water flowing in streams like those in other parts of the eastern half of the united states. in the near neighbourhood of this peculiar section of the state the streams come to an end suddenly, pouring their water into funnel-shaped depressions of the ground called sink-holes. after a heavy rain the surface water, accumulating in rivulets, may also be traced to small depressions which seem like leaks in the earth's crust, into which the water trickles and disappears. it must have been noticed by the early settlers who came over the mountains from the eastern colonies, and settled in the new, wild, hilly country, which they called kentucky. the first settlers built their log cabins along the streams they found, and shot deer and wild turkey and other game that was plentiful in the woods. the deer showed them where salt was to be found in earthy deposits near the streams; for salt is necessary to every creature. deer trails led from many directions to the "salt licks" which the wild animals visited frequently. perhaps the same pioneers who dug the salt out of the earth found likewise deposits of _nitre_, called also _saltpetre_, a very precious mineral, for it is one of the elements necessary in the manufacture of gunpowder. with the indians all about him, and often showing themselves unfriendly, the pioneer counted gunpowder a necessity of life. he relied on his gun to defend and to feed his family. there were men among those first settlers who knew how to make gunpowder, and saltpetre was one of the things that had to be carried across the mountains into kentucky, until they found it in the hills. no wonder that prospectors went about looking for nitre beds in the overhanging ledges of rocks along stream-beds. in such situations the deposits of nitre were found. the earth was washed in troughs of running water to remove the clayey impurity. after a filtering through wood-ashes, the water which held the nitre in solution was boiled down, and left to evaporate, after which the crystals of saltpetre remained. solid masses of saltpetre weighing hundreds of pounds were sometimes found in protected corners under shelving rocks. it was no doubt in the fascinating hunt for lumps of this pure nitre that the early prospectors discovered that the streams which disappeared into the sink-holes made their way into caverns underground. digging in the sides of ravines often made the earthy wall cave in, and the surprised prospector stood at the door of a cavern. the discoverer of a cave had hopes that by entering he might find nitre beds richer than those he could uncover on the surface, and this often turned out to be true. the hope of finding precious metals and beds of iron ore also encouraged the exploration of these caves. by the time the war of was declared, the mining of saltpetre was a good-sized industry in kentucky. most of the mineral was taken out of small caves, and shipped, when purified, over the mountains, on mule-back by trails, and in carts over good roads that were built on purpose to bring this mineral product to market. as long as war threatened the country, the government was ready to buy all the saltpetre the kentucky frontiersmen could produce. and the miners were constantly in search of richer beds that promised better returns for their labour. it was this search that led to the exploration of the caves discovered, although the explorer took his life in his hands when he left the daylight behind him and plunged into the under-world. not all lost rivers tell as interesting stories or reveal as valuable secrets as did those the neighbours of daniel boone traced along their dark passages underground, and finally saw emerge as hillside springs, in many cases, to feed kentucky rivers. but it is plain that no river sinks from sight unless it finds porous or honeycombed rocks that let it through. the water seeks the nearest and easiest route to the sea. its weight presses toward the lowest level, always. the more water absorbs of acid, the more powerfully does it attack and carry away the substance of lime rocks through which it passes. the mammoth cave of kentucky there is no more fertile soil in the country than that of the famous blue grass region of kentucky. the surface soil rests upon a deep foundation of limestone rocks, and very gradually the plant food locked up in these underlying strata is pulled up to the surface by the soil water, and greedily appropriated by the roots of the plants. part of the water of the abundant rainfall of this region soaks into the layers of the lime rock, carrying various acids in solution which give it power to dissolve the limestone particles, and thus to make its way easily through comparatively porous rock to the very depths of the earth. so it has come about that the surface of the earth is undermined. vast empty chambers have been carved by the patient work of trickling water, which has carried away the lime that once formed solid and continuous layers of the earth's crust. we must believe that the work has taken thousands of years, at least, for no perceptible change has come to these wonderful caves since the discovery and exploration of them a century and more ago. the streams that flow into the region of these caves disappear suddenly into sink-holes and flow through caverns. after wearing away their subterranean channels, leaping down from one level to another, forming waterfalls and lakes, some emerge finally through hillsides in the form of springs. the cavern region of kentucky covers eight thousand square miles. the underground chambers found there are in the limestone rock which varies from ten to four hundred feet in thickness, and averages a little less than two hundred feet. over this territory the number of sink-holes average one hundred to the square mile; and the streams that have poured their water into these basins have made a network of open caverns one hundred thousand miles in length. a great many small caverns have been thoroughly explored and are famous for their beauty. the diamond cave is one of the most splendid, for it is lined with walls and pillars of alabaster that sparkle in the torchlight with crystals that look like veritable diamonds. beautiful springs and waterfalls are found in many caves, but the grandest of all is the mammoth cave, beside which no other is counted worthy to be compared. great tales the miners told of the wonder and the beauty of these caverns, the walls of which were supported by arching alabaster columns and wonderful domes, of indescribable beauty of form and colouring. in , the year that washington died, a pioneer discovered the entrance to a cave, the size and beauty of which surpassed anything he had seen before. after exploring it for a short distance he returned home and took his whole family with him to enjoy the first view of the wonderful cavern he had discovered. they carried pine knots and a lighted torch, by which they made their way for some distance, but the torch was accidentally extinguished and they groped their way in darkness and missed the entrance. without anything to guide them, they wandered in darkness for three days, and were almost dead when at last they stumbled upon the exit. this is the doorway of the mammoth cave of kentucky, one of the wonders of the world. this was a terrible experience. the next persons who attempted to explore the new cave were better provisioned against the chance of spending some time underground. the pioneers found rich deposits of nitre in the "great cave," as they called it. scientists visited it and explored many of its chambers. the reputation of this cavern has been spread by thousands of visitors who have come from all over the world to see it. the cave has not yet been completely explored. the regular tours, on which the guides conduct visitors, cover but a small part of the one hundred and fifty miles measured by the two hundred or more avenues. the passages wind in and out, crossing each other, sometimes at different levels, and forming a network of avenues in which the unaccustomed traveller would surely be lost. the old guides know every inch of their regular course, and their quaint and edifying talk adds greatly to the pleasure of the visitors. from the hotel, parties are organized for ten o'clock in the morning and seven o'clock in the evening. each visitor is provided with a lard-oil lamp. the guide carries a flask of oil and plenty of matches. no special garb is necessary, though people usually dress for comfort, and wear easy shoes. the temperature of the cave is uniform winter and summer, varying between fifty-three and fifty-four degrees fahrenheit. the cave entrance is an arch of seventy-foot span in the hillside. a winding flight of seventy stone steps leads the party around a waterfall, into a great chamber under the rocks. then the way goes through a narrow passage, where the guide unlocks an iron gate to let them in. the visitors now leave all thoughts of daylight behind, for the breeze that put out their lights as they entered the cave is past, and they stand in the rotunda, a vast high-ceilinged chamber, silent and impressive, with walls of creamy limestone, encrusted with gypsum, which has been stained black by manganese. from the vestibule on, each passage and each room has a name, based upon some historic event or some fancied resemblance. the giant's coffin is a great kite-shaped rock lying in one of the rooms of the cave. the star chamber has a wonderful crystal-studded dome in which the guide produces the effect of a sunrise by burning coloured lights. bonfires built at suitable points produce wonderful shadow effects, which are like nothing else in the world. the old saltpetre vats which the visitors pass in taking the "long route" through the cave, point them back to the days during the war of , when this valuable mineral was extracted from the earth in the floor of the cave. the industry greatly enriched the thrifty owners of the cave, but the works were abandoned after peace was declared. it must be a wonderful experience to walk steadily for nine hours over the long route, for so pure is the air and so wonderful is the scenery that people rarely complain of fatigue when the experience is over. there is no dust on the floors of these subterranean chambers, and they are not damp except near places where water trickles, here and there, in rivulets and cascades. pools of water at the bottoms of pits so deep that a lighted torch requires several seconds to reach the bottom, and rivers and lakes of considerable size, show where some of the surface water goes to. a strange underground suction creates whirlpools in some of these streams. people go in boats holding twenty passengers for a row on echo river, and the guide dips up with a net the blind fish and crayfish and cave lizards which inhabit these subterranean waters. the echoes in various chambers of the mammoth cave are remarkable. in some of them a song by a single voice comes back with full chords, as if several voices carried the different parts. the single notes of flute and cornet are returned with the same beautiful harmonies. a pistol shot is given back a dozen times, the sound rebounding like a ball from rock to rock of the arching walls. the vibrations of the water made by the rower's paddles reëcho in sounds like bell notes, and they are multiplied into harmonies that suggest the chimes in the belfry of a cathedral. the walls of various chambers differ from each other according to the minerals that compose them. some are creamy white limestone arches, some are walled with black gypsum, some are hung with great curtains of stalagmites, solid but suggesting the lightness and grace of folds of crêpe. under such hangings the floor is built up in stalactites. the mineral-laden water, the constant drip of which has produced a hanging, icicle-like stalagmite, has built up the stalactite to meet it. probably nothing is more beautiful than the flower-like crystals that bloom all over the walls of a chamber called "mary's bower." the floor, even, sparkles with jewels that have fallen from the wonderful and delicate flower clusters built from deposits of the lime-laden water which goes on building and replacing the bits that fall. "martha's vineyard" is decorated with nodules, like bunches of grapes, that glisten as if the dew were on them. the white gypsum in some caves makes the walls look as if they were carved out of snow. still others have clear, transparent crystals that make them gleam in the torches' light as if the walls were encrusted with diamonds. the cave region of indiana is also famous. the great wyandotte cave in crawford county is the most noted of many similar caverns. in some of the chambers, bats are found clinging to the ceiling, heads downward, like swarms of bees. the caverns of luray, in virginia, are complex and wonderful in their structure, and famous for the beautiful stalactites and stalagmites they contain. but there is no cave in this country so wonderful and so grand in its dimensions as the mammoth cave in kentucky. land-building by rivers once a year, when the rainy season comes in the mountainous country south of egypt, the old nile floods its banks and spreads its slimy waters over the land, covering the low plains to the very edge of the sahara desert. the people know it is coming, and are prepared for this flood. we should think such an overflow of our nearest river a monstrous calamity, but the egyptians bless the river which blesses them. they know that without the nile's overflow their country would be added to the desert of sahara. in a short time after the overflow, the river reaches its highest point and begins to ebb. canals lying parallel to its course are filled with water which is saved for use in the hot, dry summer. as the flood goes down, a deposit of slimy mud lies as a rich fertilizer on the land. it is this and the water which the earth has absorbed that make egypt one of the most fertile agricultural countries in the world. the region covered by the nile's overflow is the flood plain of this river. on this plain the pyramids, the sphinx, and other famous monuments of egypt stand. the statue of rameses ii. built , years ago, has its base buried nine feet deep in the rich soil made of nile sediment. a well dug in this region goes through forty feet of this soil before striking the underlying sand. how many years ago did the first nile overflow take place? we may begin our calculation by finding out the average yearly deposit. it is a slow process that accumulates but nine feet in , years. if you were in egypt when the nile went back into its banks, you would see that the scum it leaves in a single overflow adds not a great deal to the thickness of the soil. possibly floods have varied in their deposits from year to year, so that any calculation of the time it took to build that forty feet of surface soil must be but a rough estimate. this much we know: it has been an uninterrupted process which has taken place within the present geological epoch, "the age of man." not all the rich sediment the nile brings down is left on the level flood plain along its course. a vast quantity is dumped at the river's mouth, where the tides of the mediterranean check the river's current. thus the great delta is formed. the broad river splits into many mouths that spread out like a fan and build higher and broader each year the mud-banks between the streams. upper egypt consists of river swamps. lower egypt, from cairo to the sea, is the delta built by the river itself on sea bottom. from the head of the delta, where the river commences to divide, to the sea, is an area of , square miles made out of material contributed by upper egypt, and built by the river. layer upon layer, it is constantly forming, but most rapidly during the season of floods. coming closer home, let us look at the map of the mississippi valley. begin as far north as st. louis. for the rest of its course the mississippi river flows through a widening plain of swamp land, flooded in rainy seasons. through this swampy flood plain the river meanders; its current, heavily loaded with sediment, swings from one side to the other of the channel, building up here, wearing away there, and straightening its course when the curves become so sharp that their sides meet. then the current breaks through the thin wall, and a bayou of still water is left behind. below baton rouge the mississippi breaks into many mouths, that spread and carry the water of the great river into the gulf of mexico. the nile delta is triangular, like delta, [greek: d], the fourth letter of the greek alphabet; but the mississippi's delta is very irregular. the main mouth of the river flows fifty miles out into the gulf between mud-banks, narrow and low. at the tip it branches into several streams. from the mouth of the ohio to the gulf, the mississippi flood plain covers , square miles. over this area, sediment to an average depth of fifty feet has been laid down. in earlier times the river flooded this whole area, when freshets swelled its tributaries in the spring. the flood plain then became a sea, in the middle of which the river's current flowed swiftly. the slow-flowing water on each side of the main current let go of its burden of sediment and formed a double ridge. between these two natural walls the main river flowed. when its level fell, two side streams, running parallel with the main river drained the flood plains on each side into the main tributaries to right and left. these natural walls deposited when the river was in flood are called _levees_. each heavy flood builds them higher, and the bed of the stream rises by deposits of sediment. so it happens that the level of the river bed is higher than the level of its flood plain. this is an interesting fact in geology. but the people who have taken possession of the rich flood plain of the mississippi river, who have built their homes there, drained and cultivated the land, and built cities and towns on the areas reclaimed from swamps, recognize the elevation of the river bed as the greatest danger that threatens them. suppose a flood should come. even if it does not overflow the levees, it may break through the natural banks and thus overflow the cities and the farm lands to left and right. instead of living in constant fear of such a calamity, the people of the mississippi flood plain have sought safety by making artificial levees, to make floods impossible. these are built upon the natural levees. as the river bed rises by the deposit of mud, the levees are built higher to contain the rising waters. no longer does the rich soil of the mississippi flood plain receive layers of sediment from the river's overflow. the river very rarely breaks through a levee. the united states government has spent great sums in walling in the river, and each state along its banks does its share toward paying for this self-protection. by means of _jetties_ the river's current is directed into a straightened course, and its power is expended upon the work of deepening its own channel and carrying its sediment to the gulf. much as the river has been forced to do in cleaning its own main channel, dredging is needed at various harbours to keep the river deep enough for navigation. the forests of the mountain slopes in colorado are being slaughtered, and the headwaters of the missouri are carrying more and more rocky débris to choke the current of the mississippi. colorado soil is stolen to build land in the vast delta, which is pushing out into the gulf at the rate of six miles in a century--a mile in every sixteen years. the mississippi delta measures , square miles. with the continued denuding of mountain slopes, we shall expect the rate of delta growth to be greatly increased, until reforesting checks the destructive work of wind and water. the making of mountains the gradual thickening and shrinking of the earth's crust as it cools have made the wrinkles we call mountain systems. through millions of years the globe has been giving off heat to the cold sky spaces through which it swings in its orbit around the sun. the cooling caused the contraction of the outer layer to fit the shrinking of the mass. when a plump peach dries on its pit, the skin wrinkles down to fit the dried flesh. the fruit shrinks by loss of water, just as the face of an old person shrinks by loss of fat. the skin becomes wrinkled in both cases. the weakest places in the earth's crust were the places to crumple, because they could not resist the lateral pressure that was exerted by the shrinking process. along the shores of the ancient seas the rivers piled great burdens of sediment. this caused the thin crust to sink and to become a basin alongside of a ridge. the wearing away of the land in certain places lightened and weakened the crust at these places, so that it bent upward in a ridge. perhaps the first wrinkles were not very high and deep. the gradual cooling must have exerted continued pressure, and the wrinkles have become larger. it is not likely that new wrinkles would be formed as long as the old ones would crumple and draw up into narrower, steeper slopes, in response to the lateral crushing. we can imagine those first mountains rising as folds under the sea. gradually their bases were narrowed, and their crests lifted out of the water. they rose as long, narrow islands, and grew in size as time went on. why is the trend of the great mountain systems almost always north and south? study the map of the continents and see how few cross ranges are shown, and how short they are, compared with the others. the molten globe bulged at its equator, as it rotated on its axis. the moon added its strong pulling force to make it bulge still more. as the crust thickened, it became less responsive to the two forces that caused it to bulge. the shrinkage was greatest where the globe had been most pulled out of shape. the rate of the earth's rotation is believed to have diminished. every change tended to let the earth draw in its (imaginary) belt, a notch at a time. the forces of contraction acted along the line of the equator, and formed folds running toward the poles. in this early time the great mountain systems were born, and they grew in size gradually, from small beginnings. these mountains of upheaval, made by the bending of the earth's crust, and the formation of alternating ridges and depressed valleys, are many. the earth is old and much wrinkled. other mountains have been formed by forces quite different. volcanic mountains have been far more numerous in ages gone than they are now. mt. hood and mt. rainier are peaks built up by the materials thrown out of the craters of volcanoes dead these thousands of years. vesuvius is at present showing us how volcanic mountains are made. each eruption builds larger the cone--that is, the chimney through which the molten rocks, the ashes, and the steam are ejected. side craters may open, the main cone be broken and its form changed, but the mass of lava and stones and ashes grows with each eruption. the mountain grows by the additions it receives. Ætna is a mountain built of lava. a third mountain system grew, not by addition, but by subtraction. the catskills illustrate this type. this group of mountains is the remnant of a table-land made of level layers of red sandstone. the rest of the high plain has been cut down and carried away, leaving these picturesque hills, the survival of which is as much a mystery as the disappearance of the balance of the plateau of which they were once a part. the fold that forms a typical mountain ridge has a cone of granite, the original rock foundation of the earth, and on this are layers of stratified rock, ancient deposits of sediment carried to the sea by streams. when exposed to wind and rain, the ridge is gradually worn down. in some places the water cuts away the soft rock and forms a stream-bed, that cuts deeper and deeper, using the rock fragments as its tools. often the layers of aqueous rocks are cut through, and the granite exposed. sometimes the hardest stratified rock-beds resist the water and the wind and are left as a series of ridges along the sides of the main range. the crumpling forces may crack the ridge open for its whole length, and one side of the chasm may slip down and the other go up. the result is a sheer wall of exposed rock strata, layers of which correspond with those that lie far below the top of the portion that slid down in the great upheaval and subsidence that parted them. these slips are known as _faults_. the lava flood of the northwest we know little about the substance that occupies the four thousand miles of distance between the surface and the centre of our earth. we know that the terrible weight borne by the central mass compresses it, so that the interior must grow denser as the core is approached. scientists have weighed the earth, and tell us that the crust is lighter than the rest. the supposition is that there is a great deal of iron in the interior, and possibly precious metals, too. our deepest wells and mines go down about a mile, then digging stops, on account of the excessive heat. but the crumpling of the crust, and the wearing away of the folded strata by wind and running water, have laid bare rocks several miles in thickness on the slopes of mountains, and exposed the underlying granite, on which the first sedimentary rocks were deposited. on this granite lie stratified rocks, which are crystalline in texture. these are the beds, sometimes miles in depth, called _metamorphic_ rocks, formed by water, then transformed by heat. the wearing away of rocks by wind and water has furnished the materials out of which the aqueous rocks have been made. layers upon layers of sandstone, shales, limestone, and the like, are exposed when a river cuts a canyon through a plateau. the layered deposits of débris at the mouth of the river make new aqueous rocks out of old. every sandy beach is sandstone in the making. this work is never ended. in the early days the earth's crust often gaped open in a mighty crater and let a flood of lava overspread the surface. the ocean floor often received this flood of melted rock. in many places the same chimney opened again and again, each time spreading a new layer of lava on top of the old, so that the surface has several lava sheets overlying the aqueous strata. if the hardened lava sheet proved a barrier to the rising tide of molten lava in the chimney it was often forced out in sheets between the layers of aqueous rocks. wherever the heated material came into contact with aqueous rocks it transformed them, for a foot or more, into crystalline, metamorphic rocks. a chimney of lava is called a _dike_. in mountainous countries dikes are common. sometimes small, they may also be hundreds of feet across, often standing high above the softer strata, which rains have worn away. dikes often look like ruined walls, and may be traced for miles where they have been overturned in the mountain-making process. the great lava flood of the northwest happened when the coast range was born. along the border of the pacific ocean vast sedimentary deposits had accumulated during the cretaceous and tertiary periods. then the mighty upheaval came, the mountain ridge rose at the end of the miocene epoch and stretched itself for hundreds of miles through the region which is now the coast of california and oregon. great fissures opened in the folded crust, and floods of lava overspread an area of , square miles. a dozen dead craters show to-day where those immense volcanic chimneys were. the depth of the lava-beds is well shown where the columbia river has worn its channel through. walls of lava three thousand feet in thickness rise on each side of the river. they are made of columns of basalt, fitted together, like cells of a honeycomb, and jointed, forming stone blocks laid one upon another. the lava shrinks on cooling and forms prisms. in ireland, the giants' causeway is a famous example of basaltic formation. in oregon, the walls of the des chutes river show thirty lava layers, each made of vertical basalt columns. the palisades of the hudson, mt. tom, and mt. holyoke are examples on the eastern side of the continent of basaltic rocks made by lava floods. northern california, northwestern nevada, and large part of idaho, montana, oregon, and washington are included in the basin filled with lava at the time of the great overflow, which extended far into british columbia. it is probable that certain chimneys continued to discharge until comparatively recent times. mt. rainier, mt. shasta, and mt. hood are among dead volcanoes. quite a different history has the great deccan lava-field of india, which covers a larger area than the basin of our northwest, and is in places more than a mile in depth. it has no volcanoes, nor signs of any ever having existed. the floods alone overspread the region, which shows no puny "follow-up system" of scattered craters, intermittently in eruption. the first living things strange days and nights those must have been on the earth when the great sea was still too hot for living things to exist in it. the land above the water-line was bare rocks. these were rapidly being crumbled by the action of the air, which was not the mild, pleasant air we know, but was full of destructive gases, breathed out through cracks in the thin crust of the earth from the heated mass below. if you stand on the edge of a lava lake, like one of those on the islands of the hawaiian group, the stifling fumes that rise might make you feel as if you were back at the beginning of the earth's history, when the solid crust was just a thin film on an unstable sea of molten rock, and this volcano but one of the vast number of openings by which the boiling lava and the condensed gases found their way to the surface. then the rivers ran black with the waste of the rocky earth they furrowed, and there was no vegetation to soften the bleakness of the landscape. the beginnings of life on the earth are a mystery. nobody can guess the riddle. the earliest rocks were subjected to great heat. it is not possible that life could have existed in the heated ocean or on the land. gradually the shores of the seas became filled up with sediment washed down by the rivers. layer on layer of this sediment accumulated, and it was crumpled by pressure, and changed by heat, so that if any plants or animals had lived along those old shores their remains would have been utterly destroyed. rocks that lie in layers on top of these oldest, fire-scarred foundations of the earth show the first faint traces of living things. limestone and beds of iron ore are signs of the presence of life. the first animals and plants lived in the ancient seas. from the traces that are left, we judge that the earliest life forms were of the simplest kind, like some plants and animals that swim in a drop of water. have you ever seen a drop of pond water under a compound microscope? it is a wonder world you look into, and you forget all the world besides. you are one of the wonderful higher animals, the highest on the earth. you focus on a shapeless creature that moves about and feels and breathes, but hasn't any eyes or mouth or stomach--in fact, it is the lowest form of animal life, and one of the smallest. it is but one of many animal forms, all simple in structure, but able to feed and grow and reproduce their kind. gaze out of the window on the garden, now. the flowering plants, the green grass, and the trees are among the highest forms of plants. in the drop of water under the microscope tiny specks of green are floating. they belong to the lowest order of plants. among the plant and the animal forms that have been studied and named, are a few living things the places of which in the scale are not agreed upon. some say they are animals; some believe they are plants. they are like both in some respects. it is probable that the first living things were like these confusing, minute things--not distinctly plants or animals, but the parent forms from which, later on, both plants and animals sprang. the lowest forms of life, plant and animal, live in water to-day. they are tiny and their bodies are made of a soft substance like the white of an egg. if these are at all like the living creatures that swarmed in the early seas, no wonder they left no traces in the rocks of the early part of the age when life is first recorded by fossils. soft-bodied creatures never do. some of the animals and the plants in the drop of water under the microscope have body walls of definite shapes, made of lime, or of a glassy substance called silica. when they die, these "skeletons" lie at the bottom of the water, and do not decay, as the living part of the body does, because they are mineral. gradually a number of these shells, or hard skeletons, accumulate. in a glass of pond water they are found at the bottom, amongst the sediment. in a pond how many thousands of these creatures must live and their shells fall to the bottom at last, buried in the mud! so it is easy to understand why the first creatures on earth left no trace. the first real fossils found in the rocks are the hard shells or skeletons of the first plants and animals that had hard parts. an ancient beach at ebb tide when the tide is out, the rocks on the maine coast have plenty of living creatures to prove this northern shore inhabited. starfishes lurk in the hollows, and the tent-shaped shells of the little periwinkle encrust the wet rocks. mussels cling to the rocks in clumps, fastened to each other by their ropes of coarse black hair. the furry coating of sea mosses that encrust the rocks is a hiding-place for many kinds of living things, some soft-bodied, some protected by shells. the shallow water is the home of plants and animals of many different kinds. as proof of this one finds dead shells and fragments of seaweeds strewn on the shore after a storm. along the outer shores of the cape cod peninsula and down the jersey coast, the sober colouring of the shells of the north gives way to a brighter colour scheme. in the warmer waters, life becomes gayer, if we may judge by the rich tints that ornament the shells. the kinds of living creatures change. they are larger and more abundant. the seaweeds are more varied and more luxuriant in growth. when we reach the shores of the west indian islands and the keys of florida the greatest abundance and variety of living forms are found. the submerged rocks blossom with flower-like sea anemones of every colour. corals, branching like trees and bushes on the sea floor, form groves under water. among them brilliant-hued fishes swim, and highly ornamented shells glide, as people know who have gazed through the glass bottoms of the boats built especially to show visitors the wonderful sea gardens at nassau, bahama islands, and at santa catalina island, southern california. on every beach the skeletons of animals which die help to build up the land; though the process is not so rapid in the north as on the shores that approach the tropics. the coast of florida has a rim of island reefs around it built out of coral limestone. indeed, the peninsula was built by coral polyps. houses in st. augustine are built of coquina rock, which is simply a mass of broken shells held together by a lime cement. every sea beach is packed with shells and other remnants of animals and plants that live in the shallow waters. deeper and deeper year by year the sand buries these skeletons, and many of them are preserved for all time. thus what is sandy beach to-day may, a few million years from now, be uncovered as a ledge of sandstone with the prints of waves distinctly shown, and fossil shells of molluscs, skeletons of fishes, and branches of seaweed--all of them different from those then growing upon the earth. in the neighbourhood of cincinnati there have been uncovered banks of stone accumulated along the border of an ancient sea. from the sides of granite highlands streams brought down the sand built into these oldest sandstone rocks. the fine mud which now appears as beds of slate was the decay of feldspar and hornblende in the same granite. limestone beds are full of the fossil shells of creatures that lived in the shallow seas. their skeletons, accumulating on the bottom, formed deep layers of limestone mud. these rocks preserve, by the fossils they contain, a great variety of shellfish which had limy skeletons. the sea fairly swarmed along its shallow margin with these creatures. we might not recognize many of the shells and other curious fossils we find in the rock uncovered by the workmen who are cutting a railroad embankment. they are not exactly like the living forms that grow along our beaches to-day, but they are enough like them for us to know that they lived along the seashore, and if we had time to examine all the rocks of this kind preserved in a museum we should decide that seashore life was quite as abundant then as it is now. the pressed specimens of plants of those earliest seashores are mere imprints showing that they were pulpy and therefore gradually decayed. only their shape is recorded by dark stains made by each branching part. the decay of the vegetable tissue painted the outline on the rock which when split apart shows us what those ancient seaweeds looked like. they belonged to the group of plants we call kelp, or tangle, which are still common enough in the sea, though the forms we now have are not exactly like the old ones. seaweeds belong to the very lowest forms of plants. [illustration: crinoid from indiana] [illustration: _by permission of the american museum of natural history_ ammonite from jurassic of england] [illustration: _by permission of the american museum of natural history_ fossil corals coquina, hippurite limestone] the limestones are full of fossils of corals. indeed, there must have been reefs like those that skirt florida to-day built by these lime-building polyps. their forms are so well preserved in the rocks that it is possible to know just how they looked when they grew in the shallows. one very common kind is called a cup coral, because the polyp formed a skeleton shaped like a cup. the body wall surrounded the skeleton, and the arms or tentacles rose from the centre of the funnel-like depression in the top. little cups budded off from their parents, but remained attached, and at length the skeletons of all formed great masses of limy rock. some cup corals grew in a solid mass, the new generation forming an outer layer, thus burying the parent cups. a second type of corals in these oldest limestones is the honeycomb group. the colonies of polyps lived in tubes which lengthened gradually, forming compact, limy cylinders like organ pipes, fitted close together. the living generation always inhabited the upper chambers of the tubes. a third type is the chain coral, made of tubes joined in rows, single file like pickets of a fence. but these walls bend into curious patterns, so that the cross-section of a mass of them looks like a complex pattern of crochet-work, the irregular spaces fenced with chain stitches. each open link is a pit in which a polyp lived. among the corals are sprays of a fine feathery growth embedded in the limestone. fine, straight, splinter-like branches are saw-toothed on one or both edges. these limy fossils might not be seen at all, were they not bedded in shales, which are very fine-grained. here again are the skeletons of animals. each notch on each thread-like branch was the home of a tiny animal, not unlike a sea anemone and a coral polyp. to believe this story it is necessary only to pick up a bit of dead shell or floating driftwood on which a feathery growth is found. these plumes, like "sea mosses," as they are called, are not plants at all, but colonies of polyps. each one lived in a tiny pit, and these pits range one above the other, so as to look like notches on the thread-like divisions of the stem. put a piece of this so-called "sea moss" in a glass of sea water, and in a few moments of quiet you will see, by the use of a magnifying glass, the spreading arms of the polyp thrust out of each pit. the ancient seas swarmed with these living hydrozoans, and their remains are found preserved as fossils in the shales which once were beds of soft mud. the hard shells of sea urchins and starfishes are made of lime. in the ancient seas, starfishes were rare and sea urchins did not exist, but all over the sea bottom grew creatures called crinoids, the soft parts of which were enclosed in limy protective cases and attached to rocks on the sea bottom by means of jointed stems. no fossils are more plentiful in the early limestones than these wonderful "stone lilies." indeed, the crinoidal limestone seemed to be built of the skeletons of these animals. the lily-like body was flung open, as a lily opens its calyx, when the creature was feeding. but any alarm caused the tentacles to be drawn in, and the petal-like divisions of the body wall to close tightly together, till that wall looked like an unopened bud. on the bottom of the atlantic, near the bahama islands, these stone lilies are still found growing. their jointed stems and body parts are as graceful in form and motion as any lily. the creature's mouth is in the centre of the flower-like top, and it feeds like the sea urchin, on particles obtained in the sea water. the old limestones contain great quantities of "lamp shells," which are old-fashioned bivalves. their shells remind us of our bivalve clams and scallops, but the internal parts were very different. the gills of clams and oysters are soft parts. inside of the lamp shells are coiled, bony arms, supporting the fringed gills. it is fortunate for us that a few lamp shells still live in the seas. by studying the soft parts of these living remnants of a very old race we can know the secrets of the lives of those ancient lamp shells, the soft parts of which were all washed away, and the fossil shells of which are preserved. gradually the lamp shells died out, and the modern bivalves have come to take their places. just so, the ancient crinoids are now almost extinct; the sea urchins and the starfishes have succeeded them. the chambered nautilus has its shell divided by partitions and it lives in the outer chamber, a many-tentacled creature, that is a close relative of the soft-bodied squid. in the ancient seas the same family was represented by huge creatures the shells of which were chambered, but not coiled. their abundance and great size are proved by the rocks in which their fossils are preserved. some of them must have been the rulers of the sea, as sharks and whales are to-day. fossil specimens have been found more than fifteen feet long and ten inches in diameter in the ancient rocks of some of the western states. it is possible to read from the lowest rock formations upward, the rise of these sea giants and their gradual decline. certain strata of limestone contain the last relics of this race, after which they became extinct. as the straight-chambered forms diminished, great coiled forms became more abundant, but all died out. one of the most abundant fossil animals in ancient rocks is called a trilobite. its body is divided by two grooves into three parts, a central ridge extending the whole length of the body and two side ridges. the front portion of the shell formed the head shield, and behind the main body part was a little tail shield. the skeleton was formed of many movable jointed plates, and the creature had eyes set in the head shield just as the king crab's are set. jointed legs in pairs fringed each side of the body. each leg had two branches, one for walking, the other for swimming. a pair of feelers rose from the head. the body could be rolled into a ball when danger threatened, by bringing head and tail together. these remarkable, extinct trilobites were the first crustaceans. their nearest living relative to-day is the horseshoe crab. the fresh-water crayfish and the lobster are more distant relatives: so are the shrimps and the prawns. no such abundance of these creatures exists to-day as existed when the trilobites thronged the shallows. so well preserved are these skeletons that, although there are no living trilobites for comparison, it is possible to find out from the fossil enough about their structure to know how they fed and lived their lives along with the straight-horns which were the scavengers of those early seas and the terror of smaller creatures. the trilobites throve, and, dying, left their record in the rocks; then disappeared entirely. we find their fossils in a great variety of forms, shapes, and sizes. the smallest is but a fraction of an inch long, the largest twenty inches long. the ancient rocks, in which these lower forms of life have left their fossils, are known as the silurian system. the time in which these rocks were accumulating under the seas covers a vast period. we call it the age of invertebrates, because these soft-bodied, hard-shelled animals, the crinoids, the molluscs, and the trilobites, with bony external skeletons and no backbones, were the most abundant. they overshadowed all other forms of life. the rocks of this wonderful series were formed on the shores of a great inland sea that covered the central portion of north america. in the ages that followed, these rocks were covered deeply with later sediments. but the upheavals of the crust have broken open and erosion has uncovered these strata in different regions. geologists have found written there, page upon page, the record of life as it existed in the early seas. the lime rocks "hard" water and "soft" water are very different. the rain that falls and fills our cisterns is not softer or more delightful to use than the well water in some favoured regions. in it, soap makes beautiful, creamy suds, and it is a real pleasure to put one's hands into it. but in hard water soap seems to curdle, and some softening agent like borax has to be added or the water will chap the hands. there is little satisfaction in using water of this kind for any purpose. hard water was as soft as any when it fell from the sky; but the rain water trickled into the ground and passed through rocks containing lime. some of this mineral was absorbed, for lime is readily soluble in water. clear though it may be, water that has lime in it has quite a different feeling from rain water. blow the breath into a basin of hard water, and a milky appearance will be noted. the carbonic acid gas exhaled from the lungs unites with the invisible lime, causing it to become visible particles of carbonate of lime, which fall to the bottom of the basin. nearly all well water is hard. so is the water of lakes and rivers and the ocean, for limestone is one of the most widely distributed rocks in the surface of the earth. rain water makes its way into the earth's crust, absorbs mineral substances, and collects in springs which feed brooks and rivers and lakes. wells are holes in the ground which bore into water-soaked strata of sand. we gain something from the lime dissolved in hard water, for it is an essential part of our food. we must drink a certain amount of water each day to keep the body in perfect health. the lime in this water goes chiefly to the building of our bones. plant roots take up lime in the water that mounts as sap through the plant bodies. we get some of the lime we need in vegetable foods we eat. all of the kingdom of vertebrate animals, from the lowest forms to the highest, all of the shell-bearing animals of sea and land, require lime. many of the lower creatures especially these in the sea, such as corals and their near relatives, encase themselves in body walls of lime. they absorb the lime from the sea water, and deposit it as unconsciously as we build the bony framework of our bodies. all the bone and shell-bearing creatures that die on the earth and in the sea restore to the land and to the water the lime taken by the creatures while they lived. carbonic acid gas in the water greatly hastens the dissolving of dead shells. carbonic acid gas, whether free in the air, or absorbed by percolating water, hastens the dissolving of skeletons of creatures that die upon land. then the raw materials are built again into lime rocks underground. the lime rocks are the most important group in the list of rocks that form the crust of the earth. they are made of the elements calcium, carbon, and oxygen, yet the different members of this calcite group differ widely in composition and appearance. so do oyster shells and beef bones, though both contain quantities of carbonate of lime. calcite is a soft mineral, light in weight, sometimes white, but oftener of some other colour. it may be found crystallized or not. whenever a drop of acid touches it, a frothy effervescence occurs. the drop of acid boils up and gives off the pungent odour of carbonic acid gas. the reason that calcite is hard to find in rocks is that percolating water, charged with acids, is constantly stealing it, and carrying it away into the ocean. the rocks that contain it crumble because the limy portions have been dissolved out. some limestones resist the destructive action of water. when they are impregnated with silica they become transformed into marble, which takes a high polish like granite. acids must be strong to make any impression on marble. the thick beds of pure limestone that underlie the surface soil in kentucky and in parts of virginia sometimes measure several hundred feet in thickness, a single stratum often being twenty feet thick. they are all horizontal, for they were formed on sea bottom, and have not been crumpled in later time. the dead bodies of sea creatures contributed their shells and skeletons to the lime deposit on the sea bottom. who can estimate the time it took to form those thick, solid layers of lime rock? the animals were corals, crinoids, and molluscs. little sand and clay show in the lime rock of this period, before the marshes of the carboniferous age took the place of the ancient inland sea of the subcarboniferous period, the sedimentary accumulations of which we are now talking about. the living corals one sees in the shallow water of the florida coast to-day are building land by building up their limy skeletons. the reefs are the dead skeletons of past generations of these tiny living things. they take in lime from the water, and use it as we use lime in building our bones. in each case it is an unconscious process of animal growth--not a "building process" like a mason's building of a wall. many people think that the coral polyp builds in this way. they give it credit for patience in a great undertaking. all the polyp does is to feed on whatever the water supplies that its digestive organs can use. it is like a sea anemone in appearance and in habits of life. it is not at all like an insect. yet it is common to hear people speak of the "coral insect"! do not let any one ever hear you repeat such a mistake. southern florida is made out of coral rock, but thinly covered with soil. it was made by the growth of reef after reef, and it is still growing. the cretaceous period of the earth's eventful history is named for the lime rock which we know as chalk. beds of this recent kind of limestone are found in england and in france, pure white, made of the skeletons of the smallest of lime-consuming creatures, foraminifera. they swarmed in deep water, and so did minute sponge animalcules and plant forms called diatoms that took silica from the water, and formed their hard parts of this glassy substance. the result is seen in the nodules of flint found in the soft, snow-white chalk. did you ever use a piece of chalk that scratched the black-board? the flint did it. have you ever seen the chalk cliffs of dover? when you do see them, notice how they gleam white in the sun. see how the rains have sculptured those cliffs. the prominences left standing out are strengthened by the flint they contain. chalk beds occur in texas and under our great plains; but the principal rocks of the age in america were sandstones and clays. the age of fishes the first animal with a backbone recorded its existence among the fossils found in rocks of the upper silurian strata. it is a fish; but the earliest fossils are very incomplete specimens. we know that these old-fashioned fishes were somewhat like the sturgeons of our rivers. their bodies were encased in bony armour of hard scales, coated with enamel. the bones of the spine were connected by ball and socket joints, and the heads were movable. in these two particulars the fishes resembled reptiles. the modern gar-pike has a number of the same characteristics. another backboned creature of the ancient seas was the ancestral type of the shark family. in some points this old-fashioned shark reminds us of birds and turtles. these early fishes foreshadowed all later vertebrates, not yet on the earth. after them came the amphibians, then the reptiles, then the birds, and latest the mammals. the race of fishes began, no doubt, with forms so soft-boned that no fossil traces are preserved in the rocks. when those with harder bones appeared, the fossil record began, and it tells the story of the passing of the early, unfish-like forms, and the coming of new kinds, great in size and in numbers, that swarmed in the seas, and were tyrants over all other living things. they conquered the giant straight-horns and trilobites, former rulers of the seas. [illustration: _by permission of the american museum of natural history_ a sixteen-foot fossil fish from cretaceous of kansas, with a modern tarpon] [illustration: _by permission of the american museum of natural history_ cañon diablo meteorite from arizona] one of these giant fishes fifteen to twenty feet long, three feet wide, had jaws two feet long, set with blade-like teeth. devonian rocks in ohio have yielded fine fossils of gigantic fishes and sharks. devonian fishes were unlike modern kinds in these particulars, the spinal column extended to the end of the tail, whether the fins were arranged equally or unequally on the sides; the paired side fins look like limbs fringed with fins. every devonian fish of the gar type seems to have had a lung to help out its gill-breathing. in these traits the first fishes were much like the amphibians. they were the parent stock from which branched later the true fishes and the amphibians, as a single trunk parts into two main boughs. the trunk is the connecting link. the sea bottom was still thronged with crinoids, and lamp shells, and cup corals. shells of both clam and snail shapes are plentiful. the chambered straight-horns are fewer and smaller, and coiled forms of this type of shell are found. trilobite forms are smaller, and their numbers decrease. the first land plants appeared during this age. ferns and giant club mosses and cycads grew in swampy ground. this was the beginning of the wonderful fern forests that marked the next age, when coal was formed. the rocks that bear the record of these living things in their fossils, form strata of great thickness that overlie the silurian deposits. there is no break between them. so we understand that the sea changed its shore-line only when the silurian deposits rose to the water-level. the devonian sea was smaller than the silurian. a great tract of devonian deposits occupies the lower half of the state of new york, canada between lakes erie and huron, and the northern portions of indiana and illinois. these older layers of the stratified rock are covered with the deposits of later periods. rivers that cut deep channels reveal the earlier rocks as outcrops along their canyon walk. the record of the age of fishes is, for the most part, still an unopened book. the pages are sealed, waiting for the geologist with his hammer to disclose the mysteries. king coal in this country, and in this age, who can doubt that coal is king? it is one of the few necessities of life. in various sections of the country, layers of coal have been discovered--some near the surface, others deep underground. these are the storehouses of fuel which the coal miners dig out and bring to the surface, and the railroads distribute. from pennsylvania and ohio to alabama stretches the richest coal-basin. illinois and indiana contain another. from iowa southward to texas another broad basin lies. central michigan and nova scotia each has isolated coal-basins. all these have been discovered and mined, for they lie in the oldest part of the country. in the west, coal-beds have been discovered in several states, but many regions have not yet been explored. vast coal-fields, still untouched, have been located in alaska. the government is trying to save this fuel supply for coming generations. many of the richest coal-beds from nova scotia southward dip under the ocean. they have been robbed by the erosive action of waves and running water. glaciers have ground away their substance, and given it to the sea. much that remains intact must be left by miners on account of the difficulties of getting out coal from tilted and contorted strata. as a rule, the first-formed coal is the best. the western coal-fields belong to the period following the carboniferous age. although conditions were favourable to abundant coal formation, western coal is not equal to the older, eastern coal. it is often called _lignite_, a word that designates its immaturity compared with anthracite. coal formed in the triassic period is found in a basin near richmond, virginia. there is an abundance of this coal, but it has been subjected to mountain-making pressure and heat, and is extremely inflammable. the miners are in constant danger on account of coal gas, which becomes explosive when the air of the shaft reaches and mingles with it. this the miner calls "fire damp." north carolina has coal of the same formation, that is also dangerous to mine, and very awkward to reach, on account of the crumpling of the strata. there are beds of coal so pure that very little ash remains after the burning. five per cent, of ash may be reasonably expected in pure coal, unmixed with sedimentary deposits. such coal was formed in that part of the swamp which was not stirred by the inflow of a river. wherever muddy water flowed in among the fallen stems of plants, or sand drifted over the accumulated peat, these deposits remained, to appear later and bother those who attempt to burn the coal. [illustration: eocene fish] [illustration: _by permission of the american museum of natural history_ trilobite from the niagara limestone, upper silurian, of western new york] [illustration: sigillaria, stigmaria and lepidodendron] [illustration: _by permission of the american museum of natural history_ coal fern] you know pure coal, that burns with great heat and leaves but little ashes. you know also the other kind, that ignites with difficulty, burns with little flame, gives out little heat, and dying leaves the furnace full of ashes. you are trying to burn ancient mud that has but a small proportion of coal mixed with it. the miners know good coal from poor, and so do the coal dealers. it is not profitable to mine the impure part of the vein. it costs as much to mine and ship as the best quality, and it brings a much lower price. the deeper beds of coal are better than those formed in comparatively recent time and found lying nearer the surface. in many bogs a layer of embedded root fibres, called peat, is cut into bricks and dried for burning. deeper than peat-beds lie the _lignites_, which are old beds of peat, on the way to become coal. _soft coal_ is older than lignite. it contains thirty to fifty per cent. of volatile matter, and burns readily, with a bright blaze. the richest of this bituminous coal is called _fat_, or _fusing coal_. the bitumen oozes out, and the coal cakes in burning. ordinary soft coal contains less, but still we can see the resinous bitumen frying out of it as it burns. there is more heat and less volatile matter in _steam coal_, so-called because it is the fuel that most quickly forms steam in an engine. _hard coal_ contains but five to ten per cent. of volatile matter. it is slow to ignite and burns with a small blue blaze. from peat to anthracite coal i have named the series which increases gradually in the amount of heat it gives out, and increases and then decreases in its readiness to burn and in the brightness of its flame. anthracite coal has the highest amount of fixed carbon. this is the reason why it makes the best fuel, for fixed carbon is the substance which holds the store of imprisoned sunlight, liberated as heat when the coal burns. tremendous pressure and heat due to shrinking of the earth's crust have crumpled and twisted the strata containing coal in eastern pennsylvania, and thus changed bituminous coal into anthracite. ohio beds, formed at the same time, but undisturbed by heat and pressure, are bituminous yet. the coal-beds of rhode island are anthracite, but the coal is so hard that it will not burn in an open fire. the terrible, mountain-making forces that crumpled these strata and robbed the coal of its volatile matter, left so little of the gas-forming element, that a very special treatment is necessary to make the carbon burn. it is used successfully in furnaces built for the smelting of ores. the last stage in the coal series is a black substance which we know as black lead, or graphite. we write with it when we use a "lead" pencil. this is anthracite coal after all of the volatile matter has been driven out of it. it cannot burn, although it is solid carbon. the beds of graphite have been formed out of coal by the same changes in the earth's crust which have converted soft coal into anthracite. the tremendous pressure that bears on the coal measures has changed a part of the carbon into liquid and gaseous form. lakes of oil have been tapped from which jets of great force have spouted out. such accumulations of oil usually fill porous layers of sandstone and are confined by overlying and underlying beds of impervious clay. gas may be similarly confined until a well is drilled, relieving the pressure, and furnishing abundant or scanty supply of this valuable fuel. western pennsylvania coal-fields have beds of gas and oil. if mountain-making forces had broken the strata, as in eastern pennsylvania, the gas and the oil would have been lost by evaporation. this is what happened in the anthracite coal-belt. how coal was made the broad, rounded dome of a maple tree shades my windows from the intense heat of this august day. the air is hot, and every leaf of the tree's thatched roof is spread to catch the sunlight. the carbon in the air is breathed in through openings on the under side of each leaf. the sap in the leaf pulp uses the carbon in making starch. the sun's heat is absorbed. it is the energy that enables the leaf-green to produce a wonderful chemical change. out of soil water, brought up from the roots, and the carbonic acid gas, taken in from the air, rich, sugary starch is manufactured in the leaf laboratory. this plant food returns in a slow current, feeding the growing cells under the bark, from leaf tip to root tip, throughout the growing tree. the sap builds solid wood. the maple tree has been built out of muddy water and carbon gas. it stands a miracle before our eyes. in its tough wood fibres is locked up all the heat its leaves absorbed from the sun, since the day the maple seed sprouted and the first pair of leaves lifted their palms above the ground. if my maple tree should die, and fall, and lie undisturbed on the ground, it would slowly decay. the carbon of its solid frame would pass back into the air, as gas, and the heat would escape so gradually that i could not notice it at all, unless i thrust my hand into the warm, crumbling mass. if my tree should be cut down to-day and chopped into stove wood, it would keep a fire in my grate for many months. burning destroys wood substance a great deal faster than decay in the open air does, but the processes of rotting and burning are alike in this: each process releases the carbon, and gives it back to the air. it gives back also the sun's heat, stored while the tree was growing. there is left on the ground, and in the ashes on the hearth, only the mineral substance taken up in the water the roots gathered underground. if my tree stood in swampy ground and fell over under a high wind, the water that covered it and saturated its substance would prevent decay. the carbon would not be allowed to escape as a gas to the air; the woody substance would become gradually changed into _peat_. in this form it might remain for thousands of years, and finally be changed into coal. whether it was burned while yet in the condition of peat, or millions of years later, when it was transformed into coal, the heat stored in its substance was liberated by the burning. the carbon and the heat went back to the air. every green plant we see spreads its leaves to the sun. every stick of wood we burn, and every lump of coal, is giving back, in the form of light and heat, the energy that came from sunshine and was captured by the green leaves. how long the wood has held this store of heat we may easily compute, for we can read the age of a tree. but the age of coal we cannot accurately state. the years probably should be counted by millions, instead of thousands. the great inland sea that covered the middle portion of the continent during the silurian and the devonian periods, became shallow by the deposit of vast quantities of sediment. along the lines of the deposits of greatest thickness, a crumpling of the earth's crust lifted the first fold of the alleghany mountains as a great sea wall on the east, and on the western shore another formed the beginning of the ozark mountain system in missouri. an island was lifted out of the sea, forming the elevated ground on which the city of cincinnati now stands. various other ridges and islands divided the ancient sea into much smaller bodies of water. hemmed in by land these shallow sea-basins gradually changed into fresh-water lakes, for they no longer had connection with the ocean, and all the water they received came from rain. after centuries of freshets, and of filling in with the rock débris brought by the streams, they became great marshes, in which grew water-loving plants. generation after generation of these plants died, and their remains, submerged by the water, were converted into peat. in the course of ages this peat became coal. this is the history of the coal measures. there is no guesswork here. the stems of plants do not lose their microscopic structure in all the ages it has taken to transform them to coal. a thin section of coal shows under a magnifier the structure of the stems of the coal-forming plants. moreover, the veins of coal preserve above or below them, in shales that were once deposits of mud, the branching trunks of trees, perfectly fossilized. there are no better proofs of the vegetable origin of coal than the lumps themselves. but they are plain to the naked eye, while the coal tells its story to the man with the microscope. the fossil remains of the plants that flourished when coal was forming are gigantic, compared with plants of the same families now living. we must conclude that the climate was tropical, the air very heavy with moisture, and charged much more heavily than it is now with carbonic acid gas. these conditions produced, in rapid succession, forests of tree ferns and horsetails and giant club mosses. these are the three types of plants out of which the coal was made. they were all rich in resin, which makes the coal burn readily. the ferns had stems as large as tree trunks. some have been found that are eighteen inches in diameter. we know they are ferns, because the leaves are found with their fruits attached to them in the manner of present-day ferns. the stems show the well known scar by which fern leaves are joined. and the wood of these fossil fern stems is tubular in structure, just as the wood of living ferns is to-day. among the ferns which dominated these old marsh forests grew one kind, the scaly leaves of which covered the stems and bore their fruits on the branching tips. these giants, some of them with trunks four feet in diameter, belong to the same group of plants as our creeping club mosses, but in the ancient days they stood up among the other ferns as trees forty or fifty feet high. the giant scouring rushes, or horsetails, had the same general characteristics as the little reed-like plants we know by those names to-day. the highest plants of the coal period were leafy trees with nut-like fruits, that resemble the yew trees of the present. these gigantic trees were the first conifers upon the earth. they foreshadowed the pines and the other cone-bearing evergreens. their leaves were broad and their fruits nut-like. the japanese ginkgo, or maidenhair fern tree, is an old-fashioned conifer somewhat like those first examples of this family. trunks sixty to seventy feet long, crowned with broad leaves and a spike of fruit, have been found lying in the upper layers of the coal-seams, and in sandstone strata that lie between the strata of coal. peculiar circular discs, which the microscope reveals along the sides of the wood fibres of these fossil trees, prove the wood structure to be like that of modern conifers. generation after generation of forests lived and died in the vast spreading swamps of this era. the land sank, and freshets came here and there, drowning out all plant life, and covering the layers of peat with beds of sand or mud. when the water went down, other forests took possession, and a new coal-bed was started. it is plainly seen that flooding often put an end to coal formation. fifteen seams of coal, one above another, is the greatest number that have been found. the veins vary from one inch to forty feet in thickness. these are separated by layers of sandstone or shale, which accumulated as sediment, covering the stumps of dead tree ferns and other growths, and preserving them as fossils to tell the story of those bygone ages as plainly as any other record could have done. fresh-water animals succeeded those of salt water in the swamps that formed the coal measures. overhead, the first insects flitted among the branches of the tree ferns. dragon-flies darted above the surface and dipped in water as they do to-day. spiders, scorpions, and cockroaches, all air-breathing insects, were represented, but none of the higher, nectar-loving insects, like flies and bees and butterflies, were there. flowering plants had not yet appeared on the earth. snakelike amphibians, some fishlike, some lizard-like, and huge crocodilian forms appeared for the first time. these air-breathing swamp-dwellers could not have lived in salt water. fresh-water molluscs and land shells appear for the first time as fossils in the rocks of the coal measures. on the shores of the ocean, the rocks of this period show that trilobites, horseshoe crabs, and fishes still lived in vast numbers, and corals continued to form limestone. the old types of marine animals changed gradually, but the coal measures show strikingly different fossils. these rocks bear the first record of fresh-water and land animals. the most useful metal it is fortunate for us all that, out of the half-dozen so-called useful metals, iron, which is the most useful of them all to the human race, should be also the most plentiful and the cheapest. aluminum is abundant in the common clay and soil under our feet. but separating it is still an expensive process; so that this metal is not commercially so plentiful as iron is, nor is it cheap. all we know of the earth's substance is based on studies of the superficial part of its crust, a mere film compared with the eight thousand miles of its diameter. nobody knows what the core of the earth--the great globe under this surface film--is made of; but we know that it is of heavier material than the surface layer; and geologists believe that iron is an important element in the central mass of the globe. one thing that makes this guess seem reasonable is the great abundance of iron in the earth's crust. another thing is that meteors which fall on the earth out of the sky prove to be chiefly composed of iron. all of their other elements are ones which are found in our own rocks. if we believe that the earth itself is a fragment of the sun, thrown off in a heated condition and cooling as it flew through space, we may consider it a giant meteor, made of the substances we find in the chance meteor that strikes the earth. iron is found, not only in the soil, but in all plant and animal bodies that take their food from the soil. the red colour in fruits and flowers, and in the blood of the higher animals, is a form in which iron is familiar to us. it does more, perhaps, to make the world beautiful than any other mineral element known. but long before these benefits were understood, iron was the backbone of civilization. it is so to-day. iron, transformed by a simple process into steel, sustains the commercial supremacy of the great civilized nations of the world. the railroad train, the steel-armoured battleship, the great bridge, the towering sky-scraper, the keen-edged tool, the delicate mechanism of watches and a thousand other scientific instruments--all these things are possible to-day because iron was discovered and has been put to use. it was probably one of the cave men, poking about in his fire among the rocks, who discovered a lump of molten metal which the heat had separated from the rest of the rocks. he examined this "clinker" after it cooled, and it interested him. it was a new discovery. it may have been he, or possibly his descendants, who learned that this metal could be pounded into other shapes, and freed by pounding from the pebbles and other impurities that clung to it when it cooled. the relics of iron-tipped spears and arrows show the skill and ingenuity of our early ancestors in making use of iron as a means of killing their prey. the earliest remains of this kind have probably been lost because the iron rusted away. man was pretty well along on the road to civilization before he learned where iron could be found in beds, and how it could be purified for his use. we now know that certain very minute plants, which live in quiet water, cause iron brought into that water to be precipitated, and to accumulate in the bottom of these boggy pools. in ancient days these bog deposits of iron often alternated with coal layers. millions of years have passed since these two useful substances were laid down. to-day the coal is dug, along with the bog iron. the coal is burned to melt the iron ore and prepare it for use. it is a fortunate region that produces both coal and iron. bituminous coal is plentiful, and scattered all over the country, while anthracite is scarce. the cheapest iron is made in alabama, which has its ore in rich deposits in hillsides, and coal measures close by, furnishing the raw material for coke. the result is that the region of birmingham has become the centre of great wealth through the development of iron and coal mines. where water flows over limestone rock, and percolates through layers of this very common mineral, it causes the iron, gathered in these rock masses, to be deposited in pockets. all along the appalachian mountains the iron has been gathered in beds which are being mined. these beds of ore are usually mixed with clay and other earthy substances from which the metal can be separated only by melting. the ore is thrown into a furnace where the metal melts and trickles down, leaving behind the non-metallic impurities. it is drawn off and run into moulds, where it cools in the form of "pig" iron. the first fuel used in the making of pig iron from the ore was charcoal. in america the early settlers had no difficulty in finding plenty of wood. indeed, the forests were weeds that had to be cut down and burned to make room for fields of grain. the finding of iron ore always started a small industry in a colony. if there was a blacksmith, or any one else among the small company who understood working in iron, he was put in charge. to make the charcoal, wood was cut and piled closely in a dome-shaped heap, which was tightly covered with sods, except for a small opening near the ground. in this a fire was built, and smothered, but kept going until all the wood within the oven was charred. this fuel burned readily, with an intense heat, and without ashes. sticks of charcoal have the form of the wood, and they are stiff enough to hold up the ore of iron so that it cannot crush out the fire. for a long time american iron was supplied by little smelters, scattered here and there. the workmen beat the melted metal on the forge, freeing it from impurities, and shaping the pure metal into useful articles. sometimes they made it into steel, by a process learned in the old world. the american iron industry, which now is one of the greatest in the world, centres in pittsburg, near which great deposits of iron and coal lie close together. the making of coke from coal has replaced the burning of charcoal for fuel. when the forests were cut away by lumbermen, the supply of charcoal threatened to give out, and experiments were made in charring coal, which resulted in the successful making of coke, a fuel made from coal by a process similar to the making of charcoal from wood. the story of the making of coke out of hard and soft coal is a long one, for it began as far back as the beginning of the nineteenth century. in the first boat-load of anthracite coal was sent to philadelphia from a little settlement along the lehigh river. a mine had been opened, the owner of which believed that the black, shiny "rocks" would burn. his neighbours laughed at him, for they had tried building fires with them, and concluded that it could not be done. in philadelphia, the owners of some coke furnaces gave the new fuel a trial, in spite of the disgust of the stokers, who thought they were putting out their fires with a pile of stones. after a little, however, the new fuel began to burn with the peculiar pale flame and intense heat that we know so well, and the stokers were convinced that here was a new fuel, with possibilities in it. but it was hard for people to be patient with the slow starting of this hard coal. not until did it come into general favour, following the discovery that if hot air was supplied at the draught, instead of cold, anthracite coal became a perfect fuel. at connellsville, pennsylvania, a vein of coal was discovered which made coke of the very finest quality. around this remarkable centre, coke ovens were built, and iron ore was shipped in, even from the rich beds of the lake superior country. but it was plain to see that connellsville coal would become exhausted; and so experiments in coke-making from other coals were still made. when soft coal burns, a waxy tar oozes out of it, which tends to smother the fire. early experiments with coal in melting iron ore indicated that soft coal was useless as a substitute for charcoal and coke; but later experiments proved that coke of fine quality can be made out of this bituminous soft coal, by drawing off the tar which makes the trouble. new processes were invented by which valuable gas and coal tar are taken out of bituminous coal, leaving, as a residue, coke that is equal in quality to that made from the connellsville coal. fortunes have been made out of the separation of the elements of the once despised soft coal. for the coke and each of its by-products, coal tar and coal gas, are commercial necessities of life. the impurities absorbed by the melting iron ore include carbon, phosphorus, and silicon. carbon is the chief cause of the brittleness of cast iron. the puddling furnace was invented to remove this trouble. the melted ore was stirred on a broad, basin-like hearth, with a long-handled puddling rake. the flames swept over the surface, burning the carbon liberated by the stirring. it was a hard, hot job for the man at the rake, but it produced forge iron, that could be shaped, hot or cold, on the anvil. the next improvement was the process of pressing the hot iron between grooved rollers to rid it of slag and other foreign matters collected in the furnace. the old way was to hammer the metal free from such impurities. this was slow and hard work. iron was an expensive and scarce metal until the hot blast-furnace cheapened the process of smelting the ore. the puddling furnace and the grooved rollers did still more to bring it into general use. the railroads developed with the iron industry. soon they required a metal stronger than iron. steel was far too expensive, though it was just what was needed. efforts were made to find a cheap way to change iron into steel. sir henry bessemer solved the problem by inventing the bessemer converter. it is a great closed retort, which is filled with melted pig iron. a draught admits air, and the carbon is all burned out. then a definite amount of carbon, just the amount required to change iron into steel, is added, by throwing in bars of an alloy of carbon and manganese. the latter gives steel its toughness, and enables it to resist greater heat without crystallizing and thus losing its temper. when the carbon has been put in, the retort is closed. the molten metal absorbs the alloy, and the product is bessemer steel. in fifteen minutes pig iron can be transformed into ingot steel. the invention made possible the use of steel in the construction of bridges, high buildings, and ships. it made this age of the world the age of steel. the age of reptiles two big and interesting reptiles we see in the zoo, the crocodile and its cousin, the alligator. in the everglades of florida both are found. the crocodile of the nile is protected by popular superstition, so it is in better luck than ours. the alligators have been killed off for their skins, and it is only a matter of time till these lumbering creatures will be found only in places where they are protected as the remnants of a vanished race. giant reptiles of other kinds are few upon the earth now. the _boa constrictor_ is the giant among snakes. the great tropical turtles represent an allied group. most of the turtles, lizards, and snakes are small, and in no sense dominant over other creatures. the rocks that lie among the coal measures contain fossils of huge animals that lived in fresh water and on land, the ancestors of our frogs, toads, and salamanders, a group we call amphibians. some of these animals had the form of snakes; some were fishlike, with scaly bodies; others were lizard-like or like huge crocodiles. these were the ancestors of the reptiles, which became the rulers of land and sea during the mesozoic era. the rocks that overlie the coal measures contain fossils of these gigantic animals. strange crocodile-like reptiles, with turtle-like beaks and tusks, but no teeth, left their skeletons in the mud of the shores they frequented. and others had teeth in groups--grinders, tearers, and cutters--like mammals. these had other traits like the old-fashioned, egg-laying mammals, the duck-billed platypus, for example, that is still found in australia. along with the remains of these creatures are found small pouched mammals, of the kangaroo kind, in the rocks of europe and america. these land animals saw squatty cycads, and cone-bearing trees, the ancestors of our evergreens, growing in forests, and marshes covered with luxuriant growths of tree ferns and horsetails, the fallen bodies of which formed the recent coal that is now dug in virginia and north carolina. ammonites, giant sea snails, with chambered shells that reached a yard and more in diameter, and gigantic squids, swam the seas. sea urchins, starfish, and oysters were abundant. insects flitted through the air, but no birds appeared among the trees or beasts in the jungles. over all forms of living creatures reptiles ruled. they were remarkable in size and numbers. there were swimming, running, and flying forms. [illustration: banded sandstone from calico cañon, south dakota] [illustration: _by permission of the american museum of natural history_ opalized wood from utah] [illustration: _by permission of the american museum of natural history_ restoration of a carnivorous dinosaur, allosaurus, from the upper jurassic and lower cretaceous of wyoming. when erect the animal was about feet high] the fish-reptile, _ichthyosaurus_, was a hump-backed creature, thirty to forty feet long, with short neck, very large head, and long jaw, set with hundreds of pointed teeth. its eye sockets were a foot across. the four short limbs were strong paddles, used for swimming. the long, slender tail ended in a flat fin. perfect skeletons of this creature have been found. its rival in the sea was the lizard-like _plesiosaurus_, the small head of which was mounted on a long neck. the tail was short, but the paddles were long and powerful. no doubt this agile creature held its own, though somewhat smaller than the more massively built ichthyosaurus. the land reptiles called _dinosaurs_ were the largest creatures that have ever walked the earth. in the american museum of natural history, in new york, the mounted skeleton of the giant dinosaur fairly takes one's breath away. it is sixty-six feet long, and correspondingly large in every part, except its head. this massive creature was remarkably short of brains. the strangest thing about the land reptiles is the fact that certain of them walked on their hind legs, like birds, and made three-toed tracks in the mud. indeed, these fossil tracks, found in slate, were called bird tracks, until the bones of the reptile skeleton with the bird-like foot were discovered. certain long grooves in the slate, hitherto unexplained, were made by the long tail that dragged in the mud. when the mud dried, and was later covered with sediment of another kind, these prints were preserved, and when the bed of rock was discovered by quarrymen, the two kinds split apart, showing the record of the stroll of a giant along the river bank in bygone days. the flying reptiles were still more bird-like in structure, though gigantic in size. imagine the appearance of a great lizard with bat-like, webbed wings and bat-like, toothed jaws! the first feathered fossil bird was discovered in the limestone rock of bavaria. it was a wonderfully preserved fossil, showing the feathers perfectly. three fingers of each "hand" were free and clawed, so that the creature could seize its prey, and yet use its feathered wings in flight. the small head had jaws set with socketed teeth, like a reptile's, and the long, lizard tail of twenty-one bones had a pair of side feathers at each joint. this _archeopteryx_ is the reptilian ancestor of birds. during this age of the world, one branch of the reptile group established the family line of birds. the bird-like reptiles are the connecting link between the two races. how much both birds and reptiles have changed from that ancient type, their common ancestor! i have mentioned but a few of the types of animals that make the reptilian age the wonder of all time. one after another skeletons are unearthed and new species are found. the connecticut river valley, with its red sandstones and shales of the mesozoic era, is famous among geologists, because it preserves the tracks of reptiles, insects, and crustaceans. these signs tell much of the life that existed when these flakes of stone were sandy and muddy stretches not many bones have been found, however. the thickness of these rocks is between one and two miles. the time required to accumulate so much sediment must have been very great. [illustration: _by permission of the american museum of natural history_ model of a three-horned dinosaur, _triceratops_, from cretaceous of montana. animal in life about feet long] [illustration: _by permission of the american museum of natural history_ mounting the forelegs of _brontosaurus_, the aquatic dinosaur] it is not clear just what caused the race of giant reptiles to decline and pass away. the climate did not materially change. perhaps races grow old, and ripe for death, after living long on the earth. it seems as if their time was up; and the clumsy giants abdicated their reign, leaving dominion over the sea, the air, and the land to those animals adapted to take the places they were obliged to vacate. the age of mammals the warm-blooded birds and mammals followed the reptiles. this does not mean that all reptiles died, after having ruled the earth for thousands of years. it means that changes in climate and other life conditions were unfavourable to the giants of the cold-blooded races, and gradually they passed away. they are represented now on the earth by lesser reptiles, which live comfortably with the wild creatures of other tribes, but which in no sense rule in the brute creation. they live rather a lurking, cautious life, and have to hide from enemies, except a few more able kinds, provided with means of defense. there were mammals on the earth in the days of reptilian supremacy, but they were small in size and numbers, and had to avoid any open conflict with the giant reptiles, or be worsted in a fight. now the time came when the ruling power changed hands. the mammals had their turn at ruling the lower animals. it was the beginning of things as they are to-day, for mammals still rule. but many millions of years have probably stood between the age when this group of animals first began to swarm over the earth, and the time when man came to be ruler over all created things. among the reptiles of the period when the sea, the land, and the air were swarming with these great creatures were certain kinds that had traits of mammals. others were bird-like. from these reptilian ancestors birds and mammals have sprung. no one doubts this. the fossils prove it, step by step. yet the rocks surprise the geologist with the suddenness with which many new kinds of mammals appeared on the earth. possibly the rocks containing the bones of so many kinds were fortunately located. the spots may have been morasses where migrating mammals were overwhelmed while passing. possibly conditions favored the rapid development of new kinds, and the multiplication of their numbers. warm, moist climate furnished abundant succulent plant food for the herbivors, and these in turn furnished prey for the carnivors. the coal formed during the tertiary period gives added proof that the plant life was luxuriant. the kinds of trees that grew far north of our present warm zones have left in the rocks evidence in the form of perfect leaves and cones and other fruits. for instance, magnolias grew in greenland, and palm trees in dakota. the temperature of greenland was thirty degrees warmer than it is now. our northern states lie in a belt that must have had a climate much like that of florida now. europe was correspondingly mild. a special chapter tells of the gradual development of the horse. one hundred different kinds of mammals have been found in the eocene rocks, many of which have representative species at the same time in europe and america. the rocks of asia probably have similar records. the eocene rocks, lowest of the tertiary strata, contain remains of animals the families of which are now extinct. next overlying the eocene, the miocene rocks have fossils of animals belonging to modern families--rhinoceroses, camels, deer, dogs, cats, horses--but the genera of which are now extinct. the pliocene strata (above the miocene) contains fossils of animals so closely related to the wild animals now on the earth as to belong to the same genera. they differ from modern kinds only in the species, as the red squirrel is a different species from the gray. so the record in the rocks shows a gradual approach of the mammals to the kinds we know, a gradual passing of the mighty forms that ruled by size and strength, and the coming of forms with greater intelligence, adapted to the change to a colder climate. it sometimes happens that a farmer, digging a well on the prairie, strikes the skeleton of a monster mammal, called the _mastodon_. this very thing happened on a neighbour's farm when i was a girl, in iowa. everybody was excited. the owner of the land dug out every bone, careful that the whole skeleton be found. as he expected, the director of a museum was glad to pay a high price for the bones. [illustration: _by permission of the american museum of natural history_ restoration of an aquatic dinosaur, _brontosaurus excelsus_, from the upper jurassic and lower cretaceous of wyoming. the animal in life was over feet long] [illustration: _by permission of the american museum of natural history_ restoration of the small carnivorous dinosaur, _ornitholestes hermanui_, catching a primitive bird _archæopteryx_. upper jurassic and lower cretaceous] the mastodon was about the size of an elephant, with massive limbs, and large, heavy head that bore two stout, up-curved tusks of ivory. the creature moved in herds like the buffalo from swamp to swamp; and old age coming on, the individual, unable to keep up with the herd, sank to his death in the boggy ground. the peat accumulated over his bones, undisturbed until thousands of years elapse, and the chance digging of a well discovers his skeleton. frozen in the ice of northern siberia, near the mouths of rivers, a number of mammoths have been found. these are creatures of the elephant family, and belonging to the extinct race that lived in the quaternary period, just succeeding the tertiary. the ice overtook the specimens, and they have been in cold storage ever since. for this reason, both flesh and bones are preserved, a rare thing to happen, and rarer still to be seen by a scientist. the ignorant natives made a business of watching the ice masses at the river mouth for dark spots that showed where a mammoth was encased in the ice. if an iceberg broke off near such a place, the sun might thaw the ice front of the glacier, until the hairy monster could at length be reached. his long hair served for many uses, and the wool that grew under the hair was used as a protection from the arctic winter. the frozen flesh was eaten; the bones carved into useful tools; but the chief value of the find was in the great tusks of ivory, that curved forward and pointed over the huge shoulders. it was worth a fortune to get a pair and sell them to a buyer from st. petersburg. one of the finest museum specimens of the mammoth was secured by buying the tusks of the dealer, and by his aid tracing the location of the carcass, which was found still intact, except that dogs had eaten away part of one foreleg, bone and all. from this carefully preserved specimen, models have been made, exactly copying the shape and the size of the animal, its skin, hair, and other details. the sabre-toothed tiger, the sharp tusks of which, six to eight inches long, made it a far more ferocious beast than any modern tiger of tropical jungles, was a quaternary inhabitant of europe and america. so was a smaller tiger, and a lion. the irish elk, which stood eleven feet high, with antlers that spread ten feet apart at the tips, was monarch in the deer family, which had several different species on both continents. wild horses and wild cattle, one or two of great size, roamed the woods, while rhinos and the hippopotamus kept near the water-courses. hyenas skulked in the shadows, and acted as scavengers where the great beasts of prey had feasted. sloths and cuirassed animals, like giant armadillos, lived in america. among bears was one, the cave bear, larger than the grizzly. true monkeys climbed the trees. flamingo, parrots, and tall secretary birds followed the giant _gastornis_, the ancestor of wading birds and ostriches, which stood ten feet high, but had wings as small and useless as the auk of later times. with the entrance of the modern types of trees, came other flowering plants, and with them the insects that live on the nectar of flowers. through a long line of primitive forms, now extinct, flowering plants and their insect friends conform to modern types. the record is written in the great stone book. the age of mammals in america and europe ended with the gradual rise of the continental areas, and a fall of temperature that ushered in the ice age. with the death of tropical vegetation, the giant mammals passed away. the horse and his ancestors every city has a horse market, where you may look over hundreds of animals and select one of any colour, size, or kind. the least in size and weight is the shetland pony, which one man buys for his children to drive or ride. another man wants a long-legged, deep-chested hunter. another wants heavy draught-horses, with legs like great pillars under them, and thick, muscular necks--horses weighing nearly a ton apiece and able to draw the heaviest trucks. what a contrast between these slow but powerful animals and the graceful, prancing racer with legs like pipe-stems--fleet and agile, but not strong enough to draw a heavy load! all these different breeds of horses have been developed since man succeeded in capturing the wild horse and making it help him. man himself was still a savage, and he had to fight with wild beasts, as if he were one of them, until he discovered that he could conquer them by some power higher than physical strength. from this point on, human intelligence has been the power that rules the lower animals. its gradual development is the story of the advance of civilization on the earth. through unknown thousands of years it has gone on, and it is not yet finished. [illustration: _by permission of the american museum of natural history_ restoration of a siberian mammoth, _elephas primogenius_, pursued by men of the old stone age of europe. late pleistocene epoch] [illustration: _by permission of the american museum of natural history_ restoration of a small four-toed ancestor of the horse family, _eohippus venticolus_. lower eocene of wyoming] just when and where and how our savage ancestors succeeded in taming the wild horse of the plains and the forests of europe or asia is unknown. man first made friends with the wild sheep, which were probably more docile than wild oxen and horses. we can imagine cold and hungry men seeking shelter from storms in rocky hollows, where sheep were huddled. how warm the woolly coats of these animals felt to their human fellow-creatures crowded in with them in the dark! it is believed that the primitive men who used stone axes as implements and weapons, learned to use horses to aid them in their hunting, and in their warfare with beasts and other men. gradually these useful animals were adapted to different uses; and at length different breeds were evolved. climate and food supply had much to do with the size and the character of the breeds. in the shetland islands the animals are naturally dwarfed by the cold, bleak winters, and the scant vegetation on which they subsist. in middle europe, where the summers are long and the winters mild, vegetation is luxurious, and the early horses developed large frames and heavy muscles. the shetland pony and the percheron draught-horse are the two extremes of size. what man has done in changing the types of horses is to emphasize natural differences. the offspring of the early heavy horses became heavier than their parents. the present draught-horse was produced, after many generations, all of which gradually approached the type desired. the slender racehorses, bred for speed and endurance rather than strength, are the offspring of generations of parents that had these qualities strongly marked. hence came the english thoroughbred and the american trotter. we can read in books the history of breeds of horses. our knowledge of what horses were like in prehistoric times is scant. it is written in layers of rock that are not very deep, but are uncovered only here and there, and only now and then seen by eyes that can read the story told by fossil skeletons of horses of the ages long past. geologists have unearthed from time to time skeletons of horses. it was professor marsh who spent so much time in studying the wonderful beds of fossil mammals in the western part of this country, and found among them the skeletons of many species of horses that lived here with camels and elephants and rhinoceroses and tigers, long before the time of man's coming. how can any one know that these bones belonged to a horse's skeleton? because some of them are like the bones of a modern horse. it is an easy matter for a student of animal anatomy to distinguish a horse from a cow by its bones. the teeth and the foot are enough. these are important and distinguishing characters. it is by peculiarities in the formation of the bones of the foot that the different species of extinct horses are recognized by geologists. wild horses still exist in the wilds of russia. remains of the same species have been dug out of the soil and found in caves in rocky regions. deeper in the earth are found the bones of horses differing from those now living. the bones of the foot indicate a different kind of horse--an unknown species. but in the main features, the skeleton is distinctly horse-like. in rocks of deeper strata the fossil bones of other horses are found. they differ somewhat from those found in rocks nearer the surface of the earth, and still more from those of the modern horse. the older the rocks, the more the fossil horse differs from the modern. could you think of a more interesting adventure than to find the oldest rocks that show the skeletons of horses? the foot of a horse is a long one, though we think of it as merely the part he walks on. a horse walks on the end of his one toe. the nail of the toe we call the "hoof." the true heel is the hock, a sharp joint like an elbow nearly half way up the leg. along each side of the cannon, the long bone of this foot, lies a splint of bone, which is the remnant of a toe, that is gradually being obliterated from the skeleton. these two splints in the modern horse's foot tell the last chapter of an interesting story. the earliest american horse, the existence of which is proved by fossil bones, tells the first chapter. the story has been read backward by geologists. it is told by a series of skeletons, found in successive strata of rock. the "bad lands" of the arid western states are rich in fossil remains of horses. below the surface soil lie the rocks of the quaternary period, which included the drift laid down by the receding glaciers and the floods that followed the melting of the ice-sheet. under the quaternary lie the tertiary rocks. these comprise three series, called the eocene, miocene, and pliocene, the eocene being the oldest. in the middle region of north america, ponds and marshy tracts were filled in during the tertiary period, by sediment from rivers; and in these beds of clay and other rock débris the remains of fresh-water and land animals are preserved. raised out of water, and exposed to erosive action of wind and water, these deposits are easily worn away, for they have not the solidity of older rocks. they are the crumbly bad lands of the west, cut through by rivers, and strangely sculptured by wind and rain. here the fossil horses have been found. _eohippus_, the dawn horse, is the name given a skeleton found in in the lower eocene strata in wyoming. this specimen lay buried in a rock formation ages older than that in which the oldest known skeleton of this family had been found. its discovery made a great sensation among scientists. this little animal, the skeleton of which is no larger than that of a fox, had four perfect toes, and a fifth splint on the forefoot, and three toes on the hind foot. the teeth are herbivorous. _orohippus_, with a larger skeleton, was found in the middle eocene strata of wyoming. its feet are like those of its predecessor, except that the splint is gone. the teeth as well as the feet are more like those of the modern horse. _mesohippus_, the three-toed horse, found in the miocene, shows the fourth toe reduced to splints, and the skeleton as big as that of a sheep. in this the horse family becomes fairly established. _hypohippus_, the three-toed forest horse, found in the middle miocene strata of colorado, is a related species, but not a direct ancestor of the modern horse. _neohipparion_, the three-toed desert horse, from the upper miocene strata, shows the three toes still present. but the pliocene rocks contain fossils showing gradual reduction of the two side toes, modification of the teeth, and increase in size of the skeleton. _protohippus_ and _pliohippus_, the one-toed species from the pliocene strata, illustrate these changes. they were about the size of small ponies. _equus_, the modern horse, was represented in the pliocene strata by a species, now extinct, called _equus scotti_. this we may regard as the true wild horse of america, for it was as large as the domesticated horse, and much like it, though more like a zebra in some respects. no one can tell why these animals, once abundant in this country, became extinct at the end of the tertiary period. but this is undoubtedly true. the types described form a series showing how the ancestors of the modern horse, grazing on the marshy borders of ancient ponds, lived and died, generation after generation, through a period covering thousands, possibly millions, of years. along the sides of the crumbling buttes these ancient burying-grounds are being uncovered. within a dozen years several expeditions, fitted out by the american museum of natural history, have searched the out-cropping strata in dakota and wyoming for bones of mammals known to have lived at the time the strata were forming in the muddy shallows along the margins of lake and marsh. duplicate skeletons of the primitive horse types above have been found, and vast numbers of their scattered bones. each summer geological excursions will add to the wealth of fossils of this family collected in museums. the tertiary rocks in europe yield the same kind of secrets. the region of paris overlies the estuary of an ancient river. when the strata are laid bare by the digging of foundations for buildings, bones are found in abundance. cuvier was a famous french geologist who made extensive studies of the remains of the prehistoric animals found in this old burial-place called by scientists the paris basin. he believed that the dead bodies floated down-stream and accumulated in the mud of the delta, where the tide checked the river's current. skeletons of the hipparion, a graceful, three-toed horse, were found in numbers in the strata of the miocene time. this animal lived in europe while the pliohippus and the protohippus were flourishing in america. a great number of species of tapir-like animals left their bones in the paris basin, among them a three-hoofed animal which may have been the connecting link between the horse and the tapir families. cuvier found the connecting link between tapirs and cud-chewing mammals. the age of man the hairy, woolly mammoth was one of the giant mammals that withstood the cold of the great ice flood, when the less hardy kinds were cut off by the changing climate of the northern half of europe and america. in caves where the wild animals took refuge from their enemies, skeletons of men have been found with those of the beasts. with these chance skeletons have been found rude, chipped stone spear-heads, hammers, and other tools. with these the savage ancestors of our race defended themselves, and preyed on such animals as they could use for food. they hunted the clumsy mammoth successfully, and shared the caverns in the rocks with animals like the hyena, the sabre-toothed tiger, and the cave bear, which made these places their homes. in california a human skull was found in the bed of an ancient river, which was buried by a lava flow from craters long ago extinct. with this buried skull a few well-shaped but rough stone tools were found. this man must have lived when the great ice flood was at its height. in southern france, caves have been opened that contained bones and implements of men who evidently lived by fishing and hunting. bone fish-hooks showed skill in carving with the sharp edges of flint flakes. a spirited drawing of a mammoth, made on a flat, stone surface, is a proof that savage instincts were less prominent in these cave men than in those who fought the great reindeer and the mammoth farther north. in later times men of higher intelligence formed tribes, tamed the wild horse, the ox, and the sheep, and made friends with the dog. great heaps of shells along the shores show where the tribes assembled at certain times to feast on oysters and clams. bones of animals used as food, and tools, are found in these heaps, called "kitchen-middens." these are especially numerous in northern europe. the stone implements used by these tribes were smoothly polished. a higher intelligence expressed itself also by the making of utensils out of clay. this pottery has been found in shell heaps. so the rude cave man, who was scarcely less a wild beast than the animals which competed with him for a living and a shelter from storms and cold, was succeeded by a higher man who brought the brutes into subjection by force of will and not by physical strength. the lake-dwellers, men of the bronze age, built houses on piles in the lakes of central europe. about sixty years ago the water was low, and these relics of a vanished race were first discovered. the lake bottoms were scraped for further evidences of their life. tools of polished stone and of bronze were taken up in considerable numbers. stored grains and dried fruits of several kinds were found. ornamental trinkets, weapons of hunters and warriors, and agricultural tools tell how the people lived. their houses were probably built over the water as a means of safety from attack of beasts or hostile men. in our country the mound-builders have left the story of their manners of life in the spacious, many-roomed tribal houses, built underground, and left with a great variety of relics to the explorers of modern times. these people worked the copper mines, and hammered and polished lumps of pure metal into implements for many uses. with these are tools of polished stone. stores of corn were found in many mounds scattered in the mississippi valley. the cliff-dwellers of the mesas of arizona and new mexico had habits like those of the mound-builders, and the aztecs, a vanished race in the southwest, at whose wealth and high civilization the invading spaniards under cortez marvelled. the plastered stone houses of the cliff-dwelling indians had many stories and rooms, each built to house a tribe, not merely a family. the pueblo, the moqui, and the zuni indians build similar dwellings to-day, isolated on the tops of almost inaccessible mesas. millions of years have passed since life appeared on the earth. gradually higher forms have followed lower ones in the sea and on the land. but not all of the lower forms have gone. all grades of plants and animals still flourish, but the dominant class in each age is more highly organized than the class that ruled the preceding age. to discover the earth's treasure, and to turn it to use; to tame wild animals and wild plants, and make them serve him; to create ever more beautiful and more useful forms in domestication; to find out the earth's life story, by reading the pages of the great stone book--these are undertakings that waited for man's coming. * * * * * part ii the sky * * * * * every family a "star club" the best family hobby we have ever had is the stars. we have a star club with no dues to pay, no officers to boss us, and only three rules: . we shall have nothing but "fun" in this club--no hard work. therefore no mathematics for us! . we can't afford a telescope. therefore we must be satisfied with what bright eyes can see. . no second-hand wonders for us! we want to see the things ourselves, instead of depending on books. you can't imagine what pleasure we have had in one short year! the baby, of course, was too young to learn anything, and besides he was in bed long before the stars came out. but ruth, our seven-year-old, knows ten of the fifteen brightest stars; and she can pick out twelve of the most beautiful groups or constellations. we grown-ups know all of the brightest stars, and all forty-eight of the most famous constellations. and the whole time we have given to it would not exceed ten minutes a day! and the best part is the _way_ we know the stars. the sky is no longer bewildering to us. the stars are not cold, strange, mysterious. they are friends. we know their faces just as easily as you know your playmates. for instance, we know sirius, because he is the brightest. we know castor and pollux, because they are twins. we know regulus, because he is in the handle of the sickle. and some we know by their colours. they are just as different as president taft, "ty" cobb, horace fletcher and maude adams. and quite as interesting! what's more, none of us can ever get lost again. no matter what strange woods or city we go to, we never get "turned around." or if we do, we quickly find the right way by means of the sun or the stars. then, too, our star club gives us all a little exercise when we need it most. winter is the time when we all work hardest and have the fewest outdoor games. winter is also the best time for young children to enjoy the stars, because it gets dark earlier in winter--by five o'clock, or long before children go to bed. it is pleasant to go out doors for half an hour before supper and learn one new star or constellation. again, it is always entertaining because every night you find the old friends in new places. no two nights are just the same. the changes of the moon make a great difference. some nights you enjoy the moonlight; other nights you wish there were no moon, because it keeps you from spying out some new star. we have a little magazine that tells us all the news of the stars and the planets and the comets _before_ the things happen! we pay a dollar a year for it. it is called the _monthly evening sky map_. when we first became enthusiastic about stars, the father of our family said: "well, i think our star club will last about two years. i judge it will cost us about two dollars and we shall get about twenty dollars worth of fun out of it." but in all three respects father was mistaken. part of the two dollars father spoke of went for a book called "the friendly stars," and seventy-five cents we spent for the most entertaining thing our family ever bought--a planisphere. this is a device which enables us to tell just where any star is, at any time, day or night, the whole year. it has a disc which revolves. all we have to do is to move it until the month and the day come right opposite the very hour we are looking at it, and then we can tell in a moment which stars can be seen at that time. then we go down the street where there is a good electric light at the corner and we hold our planisphere up, almost straight overhead. the light shines through, so that we can read it, and it is just as if we had a map of the heavens. we can pick out all the interesting constellations and name them just as easily as we could find the great lakes or rocky mountains in our geography. we became so eager not to miss any good thing that father got another book. every birthday in our family brought a new star book, until now we have about a dozen--all of them interesting and not one of them having mathematics that children cannot understand. so i think we have spent on stars fifteen dollars more than we needed to spend (but i'm glad we did it), and i think we have had about two hundred dollars worth of fun! yes, when i think what young people spend on ball games, fishing, tennis, skating, and all the other things that children love, i am sure our family has had about two hundred dollars worth of fun out of stars. and there is more to come! you would laugh to know why i enjoy stars so much. i have always studied birds and flowers and trees and rocks and shells so much that i was afraid to get interested in stars. i thought it wouldn't rest me. but it's a totally different kind of science from any i ever studied! there are no families, genera, and species among the stars, thank heaven! that's one reason they refresh me. another is that no one can press them and put them in a herbarium, or shoot them and put them in a museum. and another thing about them that brings balm to my spirit is that no human being can destroy their beauty. no one can "sub-divide" capella and fill it with tenements. no one can use vega for a bill-board. ah, well! we must not be disturbed if every member of our family has a different point of view toward the stars; we can all enjoy and love them in our own ways. how would you like to start a star club like ours? you ought to be able to persuade your family to form one, because it need not cost a cent. perhaps this book will interest them all, but the better way is for you to read about one constellation and then go out with some of the family and find it. this book does not tell about wonderful things you can never see; it tells about the wonderful things all of us can see. i wish you success with your star club. perhaps your uncles and aunts will start clubs, too. we have three star clubs in our family--one in new york, one in michigan, and one in colorado. last winter the "colorado star gazers" sent this challenge to the "new jersey night-owls:" "_we bet you can't see venus by daylight!_" that seemed possible, because during that week the "evening star" was by far the brightest object in the sky. but father and daughter searched the sky before sunset in vain, and finally we had to ask the "moonstruck michiganders" how to see venus while the sun was shining. back came these directions on a postal-card: "wait until it is dark and any one can see venus. then find some tree, or other object, which is in line with venus and over which you can just see her. put a stake where you stand. next day go there half an hour before sunset, and stand a little to the west. you will see venus as big as life. the next afternoon you can find her by four o'clock. and if you keep on you will see her day before yesterday!" that was a great "stunt." we did it; and there are dozens like it you can do. and that reminds me that father was mistaken about our interest lasting only two years. we know that it will not die till we do. for, even if we never get a telescope, there will always be new things to see. our club has still to catch algol, the "demon's eye," which goes out and gleams forth every three days, because it is obscured by some dark planet we can never see. and we have never yet seen mira the wonderful, which for some mysterious reason dies down to ninth magnitude and then blazes up to second magnitude every eleventh month. ah, yes, the wonders and the beauties of astronomy ever deepen and widen. better make friends with the stars now. for when you are old there are no friends like old friends. the dippers and the pole star i never heard of any boy or girl who didn't know the big dipper. but there is one very pleasant thing about the dipper which children never seem to know. with the aid of these seven magnificent stars you can find all the other interesting stars and constellations. so true is this that a book has been written called "the stars through a dipper." to illustrate, do you know the _pointers_? i mean the two stars on the front side of the dipper. they point almost directly toward the pole star, or north star, the correct name of which is polaris. most children can see the pole star at once because it is the only bright star in that part of the heavens. but if you can't be sure you see the right one, a funny thing happens. your friend will try to show you by pointing, but even if you look straight along his arm you can't always be sure. and then, if he tries to tell you how far one star is from another, he will try to show you by holding his arms apart. but that fails also. and so, we all soon learn the easiest and surest way to point out stars and measure distances. the easiest way to tell any one how to find a star is to get three stars in a straight line, or else at right angles. the surest way to tell any one how far one star is from another is by "degrees." you know what degrees are, because every circle is divided into of them. and if you will think a moment, you will understand why we can see only half the sky at any one time, or degrees, because the other half of the sky is on the other side of the earth. therefore, if you draw a straight line from one horizon, clear up to the top of the sky and down to the opposite horizon, it is degrees long. and, of course, it is only half that distance, or degrees, from horizon to zenith. (horizon is the point where earth and sky seem to meet, and zenith is the point straight over your head.) now ninety degrees is a mighty big distance in the sky. the pole star is nothing like ninety degrees from the dipper. it is only twenty-five degrees, or about five times the distance between the pointers. and now comes the only thing i will ask you to remember. look well at the two pointers, because the distance between them, five degrees, is the most convenient "foot rule" for the sky that you will ever find. most of the stars you will want to talk about are from two to five times that distance from some other star that you and your friends are sure of. perhaps this is a little hard to understand. if so, read it over several times, or get some one to explain it to you, for when you grasp it, it will unlock almost as many pleasures as a key to the store you like the best. now, let's try our new-found ruler. let us see if it will help us find the eighth star in the dipper. that's a famous test of sharp eyes. i don't want to spoil your pleasure by telling you too soon where it is. perhaps you would rather see how sharp your eyes are before reading any further. but if you can't find the eighth star, i will tell you where to look. look at the second star in the dipper, counting from the end of the handle. that is a famous star called mizar. now look all around mizar, and then, if you can't see a little one near it, try to measure off one degree. to do this, look at the pointers and try to measure off about a fifth of the distance between them. then look about one degree (or less) from mizar, and i am sure you will see the little beauty--its name is alcor, which means "the cavalier" or companion. the two are sometimes called "_the horse and rider_"; another name for alcor is saidak, which means "the test." i shall be very much disappointed if you cannot see saidak, because it is not considered a hard test nowadays for sharp eyes. aren't these interesting names? mizar, alcor, saidak. they sound so arabian, and remind one of the "arabian nights." at first, some of them will seem hard, but you will come to love these old names. i dare say many of these star names are , years old. shepherds and sailors were the first astronomers. the sailors had to steer by the stars, and the shepherds could lie on the ground and enjoy them without having to twist their necks. they saw and named alcor, thousands of years before telescopes were invented, and long before there were any books to help them. they saw the demon star, too, which i have never seen. it needs patience to see those things; sharp eyes are nothing to be proud of, because they are given to us. but patience is something to be eager about, because it costs us a lot of trouble to get it. let's try for it. we've had a test of sight. now let's have a test of patience. it takes more patience than sharpness of sight to trace the outline of the little dipper. it has seven stars, too, and the pole star is in the end of the handle. do you see two rather bright stars about twenty-five degrees from the pole? i hope so, for they are the only brightish stars anywhere near polaris. well, those two stars are in the outer rim of the little dipper. now, i think you can trace it all; but to make sure you see the real thing, i will tell you the last secret. the handle of the big dipper is bent _back_; the handle of the little dipper is bent _in_. now, if you have done all this faithfully, you have worked hard enough, and i will reward you with a story. once upon a time there was a princess named callisto, and the great god jupiter fell in love with her. naturally, jupiter's wife, juno, wasn't pleased, so she changed the princess into a bear. but before this happened, callisto became the mother of a little boy named arcas, who grew up to be a mighty hunter. one day he saw a bear and he was going to kill it, not knowing that the bear was really his own mother. luckily jupiter interfered and saved their lives. he changed arcas into a bear and put both bears into the sky. callisto is the big bear, and arcas is the little bear. but juno was angry at that, and so she went to the wife of the ocean and said, "please, never let these bears come to your home." so the wife of the ocean said, "i will never let them sink beneath the waves." and that is why the big and the little dipper never set. they always whirl around the pole star. and that is why you can always see them, though some nights you would have to sit up very late. is that a true story? no. but, i can tell you a true one that is even more wonderful. once upon a time, before the bear story was invented and before people had tin dippers, they used to think of the little dipper as a little dog. and so they gave a funny name to the pole star. they called it cynosura, which means "the dog's tail." we sometimes say of a great man, "he was the cynosure of all eyes," meaning that everybody looked at him. but the original cynosure was and is the pole star, because all the stars in the sky seem to revolve around it. the two dippers chase round it once every twenty-four hours, as you can convince yourself some night when you stay up late. so that's all for to-night. what! you want another true story? well, just one more. once upon a time the big dipper was a perfect cross. that was about , years ago. fifty thousand years from now the big dipper will look like a steamer chair. how do i know that? because, the two stars at opposite ends of the dipper are going in a direction different from the other five stars. how do i know that? why, i don't know it. i just believe it. there are lots of things i don't know, and i'm not afraid to say so. i hope you will learn how to say "i don't know." it's infinitely better than guessing; it saves trouble, and people like you better, because they see you are honest. i don't know how the stars in the big dipper are moving, but the men who look through telescopes and study mathematics say the end stars do move in a direction opposite to the others, and they say the dipper _must_ have looked like a cross, and will look like a dipper long, long after we are dead. and i believe them. constellations you can always see there are forty-eight well known constellations, but of these only about a dozen are easy to know. i think a dozen is quite enough for children to learn. and therefore, i shall tell you how to find only the showiest and most interesting. the best way to begin is to describe the ones that you can see almost every night in the year, because you may want to begin any month in the year, and you might be discouraged if i talked about things nobody could see in that month. there are five constellations you can nearly always see, and these are all near the pole star. doubtless you think you know two of them already--the big and the little dipper. ah, i forgot to tell you that these dippers are not the real thing. they are merely parts of bigger constellations and their real names are great bear and little bear. the oldest names are the right ones. thousands of years ago, when the greeks named these groups of stars, they thought they looked like two bears. i can't see the resemblance. but for that matter all the figures in the sky are disappointing. the people who named the constellations called them lions, and fishes, and horses, and hunters, and they thought they could see a dolphin, a snake, a dragon, a crow, a crab, a bull, a ram, a swan, and other things. but nowadays we cannot see those creatures. we can see the stars plainly enough, and they do make groups, but they do not look like animals. i was greatly disappointed when i was told this; but i soon got over it, because new wonders are always coming on. i think the only honest thing to do is to tell you right at the start that you cannot see these creatures very well. you will spoil your pleasure unless you take these resemblances good-naturedly and with a light heart. and you will also spoil your pleasure if you scold the ancients for naming the constellations badly. nobody in the world would change those old names now. there is too much pleasure in them. besides, i doubt if we could do much better. i believe those old folks were better observers than we. and i believe they had a lighter fancy. let us, too, be fanciful for once. i have asked my friend, mrs. thomas, to draw her notion of some of these famous creatures of the sky. you can draw your idea of them too, and it is pleasant to compare drawings with friends. there is only one way to see anything like a great bear. you have to imagine the dipper upside down and make the handle of the dipper serve for the bear's tail. what a funny bear to drag a long tail on the ground! miss martin says he looks more like a chubby hobby-horse. you will have to make the bowl of the dipper into hind legs and use all the other stars, somehow, to make a big, clumsy, four-legged animal. and what a monster he is! he measures twenty-five degrees from the tip of his nose to the root of his tail. yes, all those miscellaneous faint stars you see near the big dipper belong to the great bear. [illustration: orion fighting the bull. above are orion's two dogs] [illustration: the little bear, the queen in her chair, the twins and the archer] how the great bear looked to the people who named it thousands of years ago, we probably shall never know. they left no books or drawings, so far as i know. but in every dictionary and book on astronomy you can find these bears and other animals drawn so carefully and beautifully that it seems as if they _must_ be in the sky, and we must be too dull to see them. it is not so. look at the pictures in this book and, you will see that the stars do not outline the animals. many of them come at the wrong places. and so it is with all the costly books and charts and planispheres. it is all very interesting, but it isn't true. it's just fancy. and when you once understand that it isn't true, you will begin to enjoy the fancy. many a smile you will have, and sometimes a good laugh. for instance, the english children call the dipper "charles's wain" or "the wagon." and the romans called it "the plough." they thought of those seven stars as oxen drawing a plough. well, that's enough about the two bears. i want to tell you about the other three constellations you can nearly always see. these are the chair, the charioteer, and perseus (pronounced _per'soos_). the chair is the easiest to find, because it is like a very bad w, and it is always directly opposite the big dipper, with the pole star half way between the two constellations. there are five stars in the w, and to make the w into a chair you must add a fainter star which helps to make the square bottom of the chair. but what a crazy piece of furniture! i have seen several ways of drawing it, but none of them makes a comfortable chair. i should either fall over backward, or else the back of the chair would prod me in the small of my back. the correct name of this constellation is cassiopeia's chair. i think this is enough to see and enjoy in one night. to-morrow night let us look for the charioteer. i love the charioteer for several reasons. one is that it makes a beautiful pentagon, or five-sided figure, with its five brightest stars. another is that it contains the second-brightest star in the northern part of the heavens, capella. the only star in the north that is brighter is vega, but vega is bluish white or creamy. if you haven't already found the five-sided figure, i will tell you how to find capella. suppose you had a gun that would shoot anything as far as you wish. shoot a white string right through the pointers and hit the pole star. then place your gun at the pole star and turn it till it is at right angles to that string you shot. aim away from the big dipper, shoot a bullet forty-five degrees and it will hit capella. if that plan doesn't work, try this. start with the star that is at the bottom of the dipper and nearest the handle. draw a line half-way between the two pointers and keep on till you come to the first bright star. this is capella, and the distance is about fifty degrees. capella means "a kid," or "little goat," and that reminds me of the third reason why i enjoy so much the constellation of which capella is the brightest star. in the old times they sometimes called this five-sided figure "the goat-carrier." and the shepherds thought they could see a man carrying a little goat in his left hand. i am sure you can see the kid they meant. it is a triangle of faint stars which you see near capella. that's enough for to-night. to-morrow night let us look for perseus. i dare say you know that old tale about perseus rescuing the princess who was chained upon a rock. (he cut off the snaky head of medusa and showed it to the dragon that was going to devour the princess, and it turned the monster to stone. remember?) well, there are constellations named after all the people in that story, but although they contain many showy stars, i could never make them look like a hero, a princess, a king, and a queen. i do not even try to trace out all of perseus. for i am satisfied to enjoy a very beautiful part of it which is called the "arc of perseus." an arc, you know, is a portion of a circle. and the way to find this arc is to draw a curve from capella to cassiopeia. on nights that are not very clear i can see about seven stars in this arc of perseus. and the reason i love it so much is that it is the most beautiful thing, when seen through an opera-glass, that i know. you could never imagine that a mere opera-glass would make such a difference. the moment i put it to my eyes about a dozen more stars suddenly leap into my sight in and near the arc of perseus. that's enough. no more stories to-night. winter constellations by winter constellations i mean those you can see in winter at the pleasantest time--the early evening. and i want you to begin with the northern cross. i hope you can see this before christmas, for, after that, it may be hidden by trees or buildings in the west and you may not see it again for a long while. this is because the stars seem to rise in the east and set in the west. to prove this, choose some brilliant star you can see at five or six o'clock; get it in line with some bush or other object over which you can just see it. put a stake where you stand, and then go to the same spot about eight o'clock or just before you go to bed. you can tell at once how much the star seems to have moved westward. another thing, every star rises four minutes later every night, and therefore the sky looks a little different at the same hour every evening. the stars in the north set for a short time only, but when those toward the south set they are gone a long time. for instance, the brightest star of all is sirius, the dog star, which really belongs to the southern hemisphere. there are only about three months in the year when children who go to bed by seven o'clock can see it--january, february, and march. so now you understand why i am so eager that you should not miss the pleasure of seeing the famous northern cross. but although it is a big cross, and easy to find, after you know it, i have never yet known a boy who could show it to another boy simply by pointing at it. the surest and best way to find it is learn three bright stars first--altair, vega, and deneb. altair is the brightest star in the milky way. it is just at the edge of the milky way, and you are to look for three stars in a straight line, with the middle one brightest. those three stars make the constellation called "the eagle." the body of the eagle is altair, and the other two stars are the wings. i should say that altair is about five degrees from each of his companions. it is worth half an hour's patient search to find the eagle. now these three stars in the eagle point straight toward the brightest star in the northern part of the sky--vega. to make sure of it, notice four fainter stars near it which make a parallelogram--a sort of diamond. these stars are all part of a constellation called "the lyre." if you try to trace out the old musical instrument, you will be disappointed; but here is a game worth while. can you see a small triangle made by three stars, of which vega is one? well, one of those stars is double, and with an opera-glass you can see which it is. on very clear nights some people with very sharp eyes can see them lying close together, but i never could. at last we are ready to find the celebrated northern cross. first draw a line from altair to vega. then draw a line at right angles to this, until you come to another bright star--deneb--which is about as far from vega as vega is from altair. now this beautiful star, deneb, is the top of the northern cross. i can't tell you whether the cross will be right or wrong side up when you see it, or on its side. for every constellation is likely to change its position during the night, as you know from watching the dipper. but you can tell the cross by these things. there are six stars in it. it is like a kite made of two sticks. there are three stars in the crosspiece and four in the long piece. deneb, the brightest star in the cross, is at the top of the long stick. but you mustn't expect to see a perfect cross. there is one star that is a little out of place, and sometimes my fingers fairly "itch to put it where it belongs." it is the one that ought to be where the long stick of your kite is tacked to the crosspiece. and one of the stars is provokingly faint, but you can see it. counting straight down the long piece, it is the third one from deneb that is faint. it is where it ought to be, but i should like to make it brighter. have you the cross now? if not, have patience. you can't be a "true sport" unless you are patient. you can't be a great ball-player, or hunter, or any thing else, without resisting, every day, that sudden impulse to "quit the game" when you lose. be a "good loser," smile and try again. that is better than to give up, or to win by cheating or sharp practice. this is the last thing i want you to see in the northern part of the sky; and if you have done a good job, let us celebrate by having a story. once upon a time a cross didn't mean so much to the world as it does now. that was before christ was born. in those old times people did not think of the northern cross as a cross. they thought of it as a swan, and you can see the swan if you turn the cross upside down. deneb will then be in the tail of the swan, and the two stars which used to be at the tips of the crosspiece now become the wings. is that a true story? yes. if we lived in arabia the children there could tell us what deneb means. it means "the tail." another story? well, do you see the star in the beak of the swan, or foot of the cross? what color is it? white? well, they say this white star is really made up of two stars--one yellow and the other blue. that is one reason i want to buy a telescope when i can afford it, for even the smallest telescope will show that. and mr. serviss says that even a strong field-glass will help any one see this wonder. i can't tell you about all the winter constellations in one chapter. we have made friends of the northern ones. now let's see the famous southern ones. and let's start a new chapter. orion, his dogs, and the bull the most gorgeous constellation in the whole sky is orion. i really pity any one who does not know it, because it has more bright stars in it than any other group. besides, it doesn't take much imagination to see this mighty hunter fighting the great bull. i dare say half the people in the united states know orion and can tell him as quick as they see him by the famous "belt of orion." this belt is made of three stars, each of which is just one degree from the next. that is why the english people call these three stars "the ell and yard." another name for them is "the three kings." you can see the "sword of orion" hanging down from his belt. as soon as you see these things you will see the four bright stars that outline the figure of the great hunter, but only two of them are of the first magnitude. the red one has a hard name--betelgeuse (pronounced _bet-el-guz´_). that is a frenchified word from the arabic, meaning "armpit," because this star marks the right shoulder of orion. the other first-magnitude star is the big white one in the left foot. its name is rigel (pronounced _re´-jel_) from an arabian word meaning "the foot." you can see the giant now, i am sure. over his left arm hangs a lion's skin which he holds out to shield him from the bull's horns. see the shield--about four rather faint stars in a pretty good curve? now look for his club which he holds up with his right hand so as to smite the bull. see the arm and the club--about seven stars in a rather poor curve--beyond the red star betelgeuse? now you have him, and isn't he a wonder! it is even easier to see the bull which is trying to gore orion. look where orion is threatening to strike, and you will see a v. how many stars in that v? five. and which is the brightest? that red one at the top of the left branch of the v? yes. that v is the face of the bull and that red star is the baleful eye of the angry bull which is lowering his head and trying to toss orion. the name of that red eye is aldebaran (pronounced _al-deb´-ar-an_). i wish aldebaran meant "red eye," but it doesn't. it is an old arabian word meaning the "hindmost," or the "follower," because every evening it comes into view about an hour after you can see the famous group of stars called the pleiades, which are in the shoulder of the bull. i do not care to trace the outline of this enormous bull, but his horns are a great deal longer than you think at first. if you will extend the two arms of that v a long way you will see two stars which may be called the tips of his horns. one of these stars really belongs in another constellation--our old friend the charioteer, the one including capella. wow! what a pair of horns! but now we come to the daintiest of all constellations--the seven sisters, or pleiades (pronounced _plee´-a-deez_). i can see only six of them, and there is a famous old tale about the "lost pleiad." but i needn't describe them. every child finds them by instinct. some compare them to a swarm of bees; some to a rosette of diamonds; some to dewdrops. but i would not compare them to a dipper as some do, because the real little dipper is very different. the light that seems to drip from the pleiades is quivering, misty, romantic, magical. no wonder many children love the pleiades best of all the constellations. no wonder the poets have praised them for thousands of years. the oldest piece of poetry about them that i know of was written about , years before christ. you can find it in the book of job. but the most poetic description of the pleiades that i have ever read is in tennyson's poem "locksley hall," in which he says they "glitter like a swarm of fireflies tangled in a silver braid." there are a great many old tales about the lost pleiad. one is that she veiled her face because the ancient city of troy was burned. another story says she ceased to be a goddess when she married a man and became mortal. some people think she was struck by lightning. others believe the big star, canopus, came by and ran away with her. still others declare she was a new star that appeared suddenly once upon a time, and after a while faded away. for myself, i do not believe any of these stories. one reason why i don't is that a seventh star is really there, and many people can really see it. indeed, there are some people so sharp-eyed that on clear nights they can see anywhere from eight to eleven. and, what is more, they can draw a map or chart showing just where each star seems to them to be. but the most wonderful stories about the pleiades are the true stories. one is that there are really more than , stars among the pleiades. some of them can be seen only with the biggest telescopes. others are revealed only by the spectroscope. and some can be found only by means of photography. but the most amazing thing about the pleiades is the distances between them. they look so close together that you would probably say "the moon seems bigger than all of them put together." sometimes the moon comes near the pleiades, and you expect that the moon will blot them all out. but the astronomers say the full moon sails through the pleiades and covers only one of them at a time, as a rule. they even say it is possible for the moon to pass through the pleiades without touching one of them! i should like to see that. if anything like it is going to occur, the magazine i spoke of in the first chapter will tell me about it. and you'd better believe i will stay up to see that, if it takes all night! there are two more constellations in the southern part of the sky that ought to be interesting, because they are the two hunting dogs that help orion fight the bull. but i can't trace these animals, and i don't believe it is worth while. the brightest stars in them everybody can see and admire--sirius, the bigger dog, and procyon, the smaller dog. every one ought to know sirius, because he is the brightest star of all. (of course, he is not so bright as venus and jupiter, but they are planets.) to find him, draw a line from the eye of the bull through the belt of orion and extend it toward the southeast about twenty degrees. they call him the dog star because he follows the heels of orion. and people still call the hottest days of summer "dog days" because years before christ the romans noticed that the dog star rose just before the sun at that time. the romans thought he chased the sun across the sky all day and therefore was responsible for the great heat. but that was a foolish explanation. and so is the old notion that dogs are likely to go mad during the dog days "because the dog star is in the ascendant." so is the idea that sirius is an unlucky star. there are no lucky or unlucky stars. these are all superstitions, and we ought to be ashamed to believe any superstition. yet for thousands of years before we had public schools and learned to know better, people believed that every one was born under a lucky star or an unlucky one, and they believe that farmers ought to plant or not plant, according to the size of the moon. now we know that is all bosh. those old superstitions have done more harm than good. one of the most harmful was the belief in witches. let us resolve never to be afraid of these old tales, but laugh at them. why should anybody be afraid of anything so lovely as sirius? i used to think sirius twinkled more than any other star. but that was bad reasoning on my part. i might have noticed that every star twinkles more near the horizon than toward the zenith. i might have noticed that stars twinkle more on clear, frosty nights than when there is a little uniform haze. and putting those two facts together i might have reasoned that the stars never really twinkle at all; they only seem to. i might have concluded that the twinkling is all due to the atmosphere--that blanket of air which wraps the earth around. the nearer the earth, the thicker the air, and the more it interferes with the light that comes to us from the stars. they say that sirius never looks exactly alike on two successive nights. "it has a hundred moods," says mr. serviss, "according to the state of the atmosphere. by turns it flames, it sparkles, it glows, it blazes, it flares, it flashes, it contracts to a point, and sometimes when the air is still, it burns with a steady white light." (quotation somewhat altered and condensed.) it is a pity that so fine a star as procyon should be called the "smaller dog," because it suffers unjustly by comparison with sirius. if it were in some other part of the sky we might appreciate it more, because it is brighter than most of the fifteen first-magnitude stars we can see. my brother william has grown to love it, but perhaps that is because he always "sympathizes with the under dog." he was the youngest brother and knows. and curiously enough he was nicknamed "the dog"--just why, i don't know. to find procyon, drawn a line from sirius northeast about twenty degrees. and to make sure, draw one east from betelgeuse about the same distance. these three stars make a triangle of which the sides are almost equal. the name procyon means "before the dog" referring to the fact that you can see him fifteen or twenty minutes earlier every night than you can see sirius. the only kind word about procyon i have heard in recent years was in connection with that miserable business of dr. cook and the north pole. a captain somebody-or-other was making observations for dr. cook, and he wanted to know what time it was. he had no watch and didn't want to disturb any one. so he looked out of the window and saw by the star procyon that it was eleven o'clock. that sounds mysterious, but it is easy if you have a planisphere like ours. last winter when we were all enjoying orion, the bull, and the two dogs, i used to whirl the planisphere around to see where they would be at six o'clock at night, at eight, at ten, at midnight, and even at six o'clock in the morning. and so, if i waked up in the night i could tell what time it was without even turning my head. sometimes i looked out of my window, saw orion nearly overhead and knew it must be midnight. and sometimes i woke up just before daybreak and saw the great bull backing down out of sight in the west, the mighty hunter still brandishing his club, and his faithful dogs following at his heels. seven famous constellations there are only seven more constellations that seem to me interesting enough for every one to know and love all his life. these are: the lion (spring) the twins (spring) the virgin (summer) the herdsman (summer) the northern crown (summer) the scorpion (summer) southern fish (autumn) i have named the seasons when, according to some people, these constellations are most enjoyable. but these are not the only times when you can see them. (if you had that seventy-five-cent planisphere, now, you could always tell which constellations are visible and just where to find them.) no matter what time of year you read this chapter, it is worth while to go out and look for these marvels. you can't possibly miss them all. have you ever seen a sickle in the sky? it's a beauty, and whenever i have seen it it has been turned very conveniently for me, because i am left-handed. it is so easy to find that i am almost ashamed to tell. but if you need help, draw a line through the pointers backward, away from the pole star, about forty degrees, and it will come a little west of the sickle. the sickle is only part of the lion--the head and the forequarters. only fanciful map-makers can trace the rest of the lion. the bright star at the end of the handle is regulus, which means "king," from the stupid old notion that this star ruled the lives of men. to this day people speak of the "royal star," meaning regulus. and at the end of this chapter i will tell you about three other stars which the persians called "royal stars." another constellation which children particularly love is the twins--castor and pollux. but the sailors got there first! for thousands of years the twins have been supposed to bring good luck to sailors. i don't believe a word of it. but i do know that sailors gloat over castor and pollux, and like them better than any other stars. the whole constellation includes all the stars east of the bull and between the charioteer and procyon. but another way to outline the twins is to look northeast of orion where you will see two rows of stars that run nearly parallel. to me the brothers seem to be standing, but all the old picture-makers show them sitting with their arms around each other, the two brightest stars being their eyes. the eyes are about five degrees apart--the same as the pointers. pollux is now brighter than castor, but for thousands of years it was just the other way. it is only within three hundred years that this change has taken place. whether castor has faded or pollux brightened, or both, i do not know. anyhow, castor is not quite bright enough to be a first magnitude star. three hundred years is a short time in the history of man, and only a speck in the history of the stars. three hundred years ago they killed people in europe just because of the church they went to. that was why the pilgrim fathers sailed from england in , and made the first permanent settlement in america, except, of course, jamestown, va., in . there are plenty of stories about old castor and pollux, and, like all the other myths, they conflict, more or less. but all agree that these two brothers went with jason in the ship argo, shared his adventures and helped him get the golden fleece. and all agree that castor and pollux were "born fighters." and that is why the roman soldiers looked up to these stars and prayed to them to help them win their battles. now for the four summer constellations every one ought to know. the first thing to look for is two famous red or reddish stars--arcturus and antares. the way you find arcturus is amusing. look for the big dipper and find the star at the bottom of the dipper nearest the handle. got it? now draw a curve that will connect it with all the stars in the handle, and when you come to the end of the handle keep on till you come to the first very bright star--about twenty-five degrees. that is the monstrous star arcturus, probably the biggest and swiftest star we can ever see with the naked eye in the northern hemisphere. he is several times as big as our sun, and his diameter is supposed to be several million miles. he is called a "runaway sun," because he is rushing through space at the rate of between two hundred and three hundred miles a second. that means between seventeen and thirty-four million miles a day! he is coming toward us, too! at such a rate you might think that arcturus would have smashed the earth to pieces long ago. but he is still very far away, and there is no danger. some people say that if job were to come to life, the sky would seem just the same to him as it did , years ago. the only difference he might notice would be in arcturus. that would seem to him out of place by a distance about three times the apparent diameter of the moon. some people believe this because job said, "canst thou guide arcturus with his sons?" and therefore they imagine that he meant this red star. but i believe he meant the big dipper. for in king james's time, when the bible was translated into english, the word "arcturus" meant the big dipper or rather the great bear. and for centuries before it meant the great bear. one proof of it is that "arcturus" comes from an old greek word meaning "bear"--the same word from which we get arctic. it is only within a few hundred years that astronomers have agreed to call the great bear "ursa major," and this red star arcturus. so i think all the books which say job mentioned this red star are mistaken. i believe webster's dictionary is correct in this matter, and i believe the revised version translates job's hebrew phrase more correctly when it says, "canst thou guide the bear with her train?" anyhow, arcturus is a splendid star--the brightest in the constellation called the "herdsman" or boötes. it is not worth while to trace the herdsman, but here is an interesting question. is arcturus really red? the books mostly say he is yellow. they say he looks red when he is low in the sky, and yellow when he is high. how does he look to you? more yellow than red? well, there's no doubt about antares being red. to find him, draw a long line from regulus through arcturus to antares, arcturus being more than half way between the other two. but if regulus and the sickle are not visible, draw a line from altair, at right angles to the eagle, until you come to a bright star about sixty degrees away. you can't miss antares, for he is the only red star in that part of the sky. antares belongs to a showy constellation called the scorpion. i cannot trace all the outline of a spider-like creature, but his poisonous tail or "stinger" is made by a curved line of stars south and east of antares. and you can make a pretty fan by joining antares to several stars in a curve which are west of antares and a little north. there is an old tale that this scorpion is the one that stung orion to death when he began to "show off" and boast that there was no animal in the world that could kill him. another very bright star in the southern part of the sky is spica. to find it, start with the handle of the dipper, and making the same backward curve which helped you to find arcturus, keep on till you come to the white star spica--say thirty degrees beyond arcturus. this is the brightest star in the constellation called "the virgin." it is not worth while trying to trace her among nearly forty faint stars in this neighbourhood. but she is supposed to be a winged goddess who holds up in her right hand an _ear of wheat_, and that is what spica means. now for an autumn constellation--the southern fish. i don't care if you fail to outline a fish, but i do want you to see the bright star that is supposed to be in the fish's mouth. and i don't want you to balk at its hard name--fomalhaut (pronounced _fo´-mal-o_). it is worth a lot of trouble to know it as a friend. to find it, you have to draw an exceedingly long line two-thirds of the way across the whole sky. start with the pointers. draw a line through them and the pole star and keep clear on until you come to a solitary bright star rather low down in the south. that is fomalhaut. it looks lonely and is lonely, even when you look at it through a telescope. and now for the last story. once upon a time the persians thought there must be four stars that rule the lives of men. so they picked out one in the north and one in the south and one in the east and one in the west, just as if they were looking for four bright stars to mark the points of the compass. if you want to find them yourself without my help don't read the next sentence, but shut this book and go out and see. then write down on a piece of paper the stars you have selected and compare them with the list i am about to give. here are the four royal stars of the persians: fomalhaut for the north, regulus for the south, aldebaran for the east, and antares for the west. why doesn't this list agree with yours? because persia is so far south of where we live. ah, there are very few things that are absolutely true. let's remember that and not be too sure: for everything depends upon the point of view! i hope you will see fomalhaut before christmas, before he disappears in the west. he is with us only five months and is always low--near the horizon. but the other seven months in the year he gladdens the children of south america and the rest of the southern hemisphere, for they see him sweeping high and lonely far up into their sky and down again. but the loveliest of all the constellations described in this chapter is the northern crown. it is not a perfect crown--only about half a circle--but enough to suggest a complete ring. look for it east of arcturus. i can see seven or eight stars in the half-circle, one of which is brighter than all the others. that one is called "the pearl." the whole constellation is only fifteen degrees long, but "fine things come in small packages"; and children grow to love the northern crown almost as much as they love the pleiades. the twenty brightest stars if you have seen everything i have described so far, you have reason to be happy. for now you know sixteen of the most famous constellations and fifteen of the twenty brightest stars. there are only twenty stars of the first magnitude. "magnitude" ought to mean size, but it doesn't. it means brightness--or rather the apparent brightness--of the stars when seen by us. the word magnitude was used in the old days before telescopes, when people thought the brighter a star is the bigger it must be. now we know that the nearer a star is to us the brighter it is, and the farther away the fainter. some of the bright stars are comparatively near us, some are very far. deneb and canopus are so far away that it takes over three hundred years for their light to reach us. what whoppers they must be--many times as big as our sun. here is a full list of the twenty stars of the first magnitude arranged in the order of their brightness. you will find this table very useful. ----------------+---------------+-------------+-------------------------- stars | pronounced |constellation| interesting facts ----------------+---------------+-------------+-------------------------- sirius | _sir´i-us_ | big dog | brightest star. nearest | | | star visible in northern | | | hemisphere canopus* | _ca-no´pus_ | ship argo | perhaps the largest body | | | in universe alpha centauri* | _al´fa | | | sen-taw´re_ | centaur | nearest star. light four | | | years away vega | _ve´ga_ | lyre | brightest star in the | | | northern sky. bluish capella | _ca-pell´a_ | charioteer | rivals vega, but opposite | | | the pole. yellowish arcturus | _ark-tu´rus_ | herdsman | swiftest of the bright | | | stars. miles a second rigel | _re´jel_ | orion | brightest star in orion. | | | white star in left foot procyon | _pro´si-on_ | little dog | before the dog. rises a | | | little before sirius achernar* | _a-ker´nar_ | river po | means the end of the river beta centauri* | _ba´ta | | | sen-taw´re_ | centaur | this and its mate point to | | | the southern cross altair | _al-tare´_ | eagle | helps you find vega and | | | northern cross betelgeuse | _bet-el-guz´_ | orion | means "armpit." the red | | | star in the right shoulder alpha crucis* | _al´fa | southern | | cru´sis_ | cross | at the base of the most | | | famous southern | | | constellation aldebaran | _al-deb´a-ran_| bull | the red eye in the v pollux | _pol´lux_ | twins | brighter than castor spica | _spi´ca_ | virgin | means ear of wheat antares | _an-ta´rez_ | scorpion | red star. name means | | | "looks like mars" fomalhaut | _fo´mal-o_ | southern | | | fish | the lonely star in the | | | southern sky deneb | _den´eb_ | swan | top of northern cross, | | | or tail of swan regulus | _reg´u-lus_ | lion | the end of the handle | | | of the sickle ----------------+---------------+-------------+-------------------------- the five stars marked * belong to the southern hemisphere, and we can never see them unless we travel far south. last winter i went to florida and saw canopus, but to see the southern cross you should cross the tropic of cancer. how to learn more all i can hope to do in this book is to get you enthusiastic about astronomy. i don't mean "gushy." look in the dictionary and you will find that the enthusiast is not the faddist. he is the one who sticks to a subject for a lifetime. nor do i care a rap whether you become an astronomer--or even buy a telescope. there will be always astronomers coming on, but there are too few people who know and love even a few of the stars. i want you to make popular astronomy a life-long hobby. perhaps you may have to drop it for ten or fifteen years. never mind, you will take up the study again. i can't expect you to read a book on stars if you are fighting to make a living or support a family, unless it really rests you to read about the stars. it does rest me. when things go wrong at the office or at home, i can generally find rest and comfort from music. and if the sky is clear, i can look at the stars, and my cares suddenly seem small and drop away. let me tell you why and how you can get the very best the stars have to teach you, without mathematics or telescope. follow this programme and you need never be afraid of hard work, or of exhausting the pleasures of the subject. go to your public library and get one of the books i recommend in this chapter, and read whatever interests you. i don't care whether you take up planets before comets or comets before planets, but whatever you do do it well. soak the interesting facts right in. nail them down. see everything the book talks about. make notes of things to watch for. get a little blank book and write down the date you first saw each thing of interest. write down the names of the constellations you love most. before you lay down any star book you are reading, jot down the most wonderful and inspiring thing you have read--even if you have only time to write a single word that may recall it all to you. treasure that little note book as long as you live. every year it will get more precious to you. now for the books: . _martin._ _the friendly stars._ harper & brothers, new york, . this book teaches you first the twenty brightest stars and then the constellations. i cannot say that this, or any other, is the "best book," but it has helped me most, and i suppose it is only natural that we should love best the first book that introduces us to a delightful subject. . _serviss._ _astronomy with the naked eye._ harper & brothers, new york, . this teaches you the constellations first and the brightest stars incidentally. also it gives the old myths. . _serviss._ _astronomy with an opera-glass._ d. appleton & co., new york, . . _serviss._ _pleasures of the telescope._ d. appleton & co., new york, . . _milham._ _how to identify the stars._ the macmillan co., new york, . this gives a list of eighty-eight constellations, including thirty-six southern ones, and has tracings of twenty-eight. . _elson._ _star gazer's handbook._ sturgis & walton co., new york, . about the briefest and cheapest. has good charts and makes a specialty of the myths. . _serviss._ _curiosities of the sky._ harper & brothers, new york. tells about comets, asteroids, shooting stars, life on mars, nebulæ, temporary stars, coal-sacks, milky way, and other wonders. . _ball._ _starland._ ginn & co., boston, new york, etc., . this tells about a great many interesting experiments in astronomy that children can make. * * * * * if i had only a dollar or less to spend on astronomy i should buy a planisphere. i got mine from thomas whittaker, no. bible house, new york. it cost seventy-five cents, and will tell you where to find any star at any time in the year. it does not show the planets, however. a planisphere that will show the planets costs about five dollars. however, there are only two very showy planets, viz., venus and jupiter. any almanac will tell you (for nothing) when each of these is morning star, and when each of them is evening star. the best newspaper about stars, as far as i know, is a magazine called _the monthly evening sky map_, published by leon barritt, nassau st., new york. it costs a dollar a year. it gives a chart every month, showing all the planets, and all the constellations. also it tells you about the interesting things, like comets, before they come. good-bye. i hope you will never cease to learn about and love the earth and the sky. perhaps you think you have learned a great deal already. but your pleasures have only begun. wait till you learn about how the world began, the sun and all his planets, the distances between the stars, and the millions of blazing suns amid the milky way! the end [illustration: the sky in winter] note.--these simplified star maps are not as accurate as a planisphere, but they may be easier for children. all star maps are like ordinary maps, except that east and west are transposed. the reason for this is that you can hold a star map over your head, with the pole star toward the north, and the map will then match the sky. these maps contain some constellations that are only for grown-ups to study. the winter constellations every child should know are: auriga, the charioteer canis major, the big dog canis minor, the little dog cassiopeia, the queen in her chair cygnus, the swan leo, the lion orion, the hunter perseus, which has the arc taurus, the bull ursa major, the great bear ursa minor, the little bear [illustration: the sky in spring] note.--once upon a time all the educated people spoke latin. it was the nearest approach to a universal language. so most of the constellations have latin names. the english, french and german names are all different, but if all children would learn the latin names they could understand one another. the spring constellations every child should know are: leo, the lion lyra, the lyre cassiopeia, the queen in her chair scorpio, the scorpion ursa major, the great bear ursa minor, the little bear virgo, the virgin [illustration: the sky in summer] note.--every sky map is good for three months, in this way: if this is correct on june st at p.m., it will be correct july st at p.m., and august st at p.m. this is because the stars rise four minutes earlier every night. thus, after thirty days, any star will rise thirty times four minutes earlier, or minutes, or two hours. children need not learn all the summer constellations. the most interesting are: auriga, the charioteer canis major, the big dog cygnus, the swan lyra, the lyre scorpio, the scorpion [illustration: the sky in autumn] note.--this book tells how to find all the most interesting stars and constellations without maps, but many people prefer them. how to use star maps is explained under "the sky in winter." the autumn constellations most interesting to children are: aquila, the eagle auriga, the charioteer cassiopeia, the queen in her chair cygnus, the swan lyra, the lyre perseus, which has the arc taurus, the bull ursa major, the great bear ursa minor, the little bear transcriber's notes page "streams, runing" corrected to "streams, running" page "where he globe" corrected to "where the globe" page "ceatures to prove" corrected to "creatures to prove" page "this consellation is" corrected to "this constellation is" page "everybirth day" corrected to "every birthday" none transciber's note supercripts are denoted with a carat (^). whole and fractional parts are displayed as - / . italic text is displayed as _text_. new theories in astronomy by william stirling civil engineer [illustration] london: e. & f. n. spon, limited, haymarket new york: spon & chamberlain, liberty street to the reader. mr. william stirling, civil engineer, who devoted the last years of his life to writing this work, was born in kilmarnock, scotland, his father being the rev. robert stirling, d.d., of that city, and his brothers, the late mr. patrick stirling and mr. james stirling, the well known engineers and designers of locomotive engines for the great northern and south eastern railways respectively. after completing his studies in scotland he settled in south america, and was engaged as manager and constructing engineer in important railway enterprises on the west coast, besides other concerns both in peru and chile; his last work being the designing and construction of the railway from the port of tocopilla on the pacific ocean to the nitrate fields of toco in the interior, the property of the anglo-chilian and nitrate railway company. he died in lima, peru, on the th october, , much esteemed and respected, leaving the ms. of the present work behind him, which is now published as a tribute to his memory, and wish to put before those who are interested in the science of astronomy his theories to which he devoted so much thought. contents. page introduction. chapter i. the bases of modern astronomy. their late formation instruments and measures used by ancient astronomers weights and measures sought out by modern astronomers means employed to discover the density of the earth. measuring by means of plummets not sufficiently exact measurements with torsion and chemical balances more accurate sir george b. airy's theory, and experiments at the harton colliery results of experiments not reliable. theory contrary to the law of attraction proof by arithmetical calculation of its error difficulties in comparing beats of pendulums at top and bottom of a mine the theory upheld by text-books without proper examination of a particle of matter within the shell of a hollow sphere. not exempt from the law of attraction a particle so situated confronted with the law of the inverse square ofdistance from an attracting body. remarks thereon it is not true that the attraction of a spherical shell is "zero" for a particle of matter within it chapter ii. the moon cannot have even an imaginary rotation on its axis, but is generally believed to have. quotations to prove this proofs that there can be no rotation. the most confused assertion that there is rotation shown to be without foundations a gin horse does not rotate on its axis in its revolution a gin horse, or a substitute, driven instead of being a driver results of the wooden horse being driven by the mill the same results produced by the revolution of the moon. centrifugal force sufficient to drive air and water away from our side of the moon that force not sufficient to drive them away from its other side no one seems ever to have thought of centrifugal force in connection with air and water on the moon near approach made by hansen to this notion far-fetched reasons given for the non-appearance of air and water the moon must have both on the far-off hemisphere proofs of this deduced from its appearance at change where the evidences of this may be seen if looked for at the right place. the centrifugal force shown to be insufficient to drive off even air, and less water, altogether from the moon the moon must have rotated on its axis at one period of its existence the want of polar compression no proof to the contrary want of proper study gives rise to extravagant conceptions, jumping at conclusions, and formation of "curious theories" chapter iii. remarks on some of the principal cosmogonies. ancient notions the nebular hypothesis of laplace. early opinions on it. received into favour. again condemned as erroneous defects attributed to it as fatal. new cosmogonies advanced dr. croll's collision, or impact, theory discussed dr. braun's cosmogony examined m. faye's "origine du monde" defined shown to be without proper foundation, confused, and in some parts contradictory reference to other hypotheses not noticed. all more or less only variations on the nebular hypothesis necessity for more particular examination into it chapter iv. preliminaries to analysis of the nebular hypothesis definition of the hypothesis elements of solar system. tables of dimensions and masses explanation of tables and density of saturn volume, density and mass of saturn's rings, general remarks about them, and satellites to be made from them future of saturn's rings notions about saturn's satellites and their masses nature of rings seemingly not well understood masses given to the satellites of uranus and neptune. explanations of volumes of the members of the solar system at density of water chapter v. analysis of the nebular hypothesis. separation from the nebula of the rings for the separate planets, etc. excessive heat attributed to the nebula erroneous and impossible centigrade thermometer to be used for temperatures temperature of the nebula not far from absolute zero erroneous ideas about glowing gases produced by collisions of their atoms, or particles of cosmic matter in the form of vapours separation of ring for neptune. it could not have been thrown off in one mass, but in a sheet of cosmic matter thickness and dimensions of the ring uranian ring abandoned, and its dimensions saturnian ring do. do. jovian ring do. do. asteroidal ring do. do. martian ring do. do. earth ring do. do. venus ring do. do. mercurian ring do. do. residual mass. condensation of solar nebula to various diameters, and relative temperatures and densities unaccountable confusion in the mode of counting absolute temperature examined and explained. negative degrees of heat only equal degrees of absolute temperature the centigrade thermometric scale no better than any other, and cannot be made decimal the sun's account current with the nebula drawn up and represented by table iii. chapter vi. analysis continued. excessive heat of nebula involved condensation only at the surface. proof that this was laplace's idea noteworthy that some astronomers still believe in excessive heat interdependence of temperature and pressure in gases and vapours. collisions of atoms the source of heat conditions on which a nebula can be incandescent. sir robert ball no proper explanation yet given of incandescent or glowing gas how matter was thrown off, or abandoned by the jovian nebula division into rings of matter thrown off determined during contraction how direct rotary motion was determined by friction and collisions of particles saturn's rings going through the same process. left to show process form gradually assumed by nebulæ. cause of saturn's square-shouldered appearance a lens-shaped nebula could not be formed by surface condensation retrograde rotary motion of neptune and uranus, and revolution of their satellites recognised by laplace as possible satellites of mars. rapid revolution of inner one may be accounted for laplace's proportion of millions not reduced but enormously increased by discoveries of this century chapter vii. analysis continued. no contingent of heat could be imparted to any planet by the parent nebula only one degree of heat added to the nebula from the beginning till it had contracted to the density of / th of an atmosphere increase in temperature from ° to possible average of ° when condensed to , , miles in diameter time when the sun could begin to act as sustainer of life and light anywhere. temperature of space the ether devised as carrier of light, heat, etc. what effect it might have on the nebula first measure of its density, as far as we know the estimate _too_ high. may be many times less return to the solar nebula at , , miles in diameter plausible reason for the position of neptune not conforming to bode's law. the ring being very wide had separated into two rings bode's law reversed. ideas suggested by it rates of acceleration of revolution from one planet to another little possibility of there being a planet in the position assigned to vulcan densities of planets compared. seem to point to differences in the mass of matter abandoned by the nebula at different periods giving rise to the continuous sheet of matter separating into different masses. probably the rings had to arrive at a certain stage of density before contracting circumferentially possible average temperature of the sun at the present day. central heat probably very much greater churning of matter going on in the interior of the sun, caused by unequal rotation between the equator and the poles chapter viii. inquiry into the interior construction of the earth. what is really known of the exterior or surface what is known of the interior little to be learned from geology, which reaches very few miles down various notions of the interior what is learnt from earthquake and volcanoes. igno-aqueous fusion, liquid magma. generally believed that the earth consists of solid matter to the centre. mean density. surface density more detailed estimate of densities near the surface causes of increased surface density after the crust was formed calculations of densities for miles deep, and from there to the centre forming table iv. reflections on the results of the calculations notion that the centre is composed of the heaviest metals. "sorting-out" theory absurd considerations as to how solid matter got to the centre gravitation might carry it there, but attraction could not how the earth could be made out of cosmic matter, meteorites or meteors chapter ix. inquiry into the interior construction of the earth--_continued_ the earth gasiform at one period. density including the moon may have been / , th that of air. must have been a hollow body. proofs given division of the mass of the earth alone into two parts division of the two masses at miles from surface reasons why the earth cannot be solid to the centre gasiform matter condensing in a cone leaves apex empty proportions of the matter in a cone calculations of the densities of the outer half of the hollow shell of the earth. remarks upon the condensation calculations of inner half of the hollow shell remarks upon position of inner surface of the shell calculations of the same chapter x. inquiry into the interior construction of the earth--_continued_ density of · times that of water still too high for the possible compression of the component matter of the earth as known to us reasons for this conclusion drawn from crushing strains of materials a limit to density shown thereby the greatest density need not exceed · of water gases shut up in the hollow centre. their weight must so far diminish the conceded maximum of · density of inner half of earth at miles diameter. greatest density may be less than · of water supposed pressure of inclosed gases very moderate meaning of heat limit to density. temperature of interior half of shell and inclosed gases must be equal state of the hollow interior results of the whole inquiry chapter xi. the earth. the idea entertained by some celebrated men, and others difficulties of forming a sphere out of a lens-shaped nebula various studies of the earth's interior made for special purposes. difficulty some people find in conceiving how the average density of little over · can be possible, the earth being a hollow sphere what is gained by its being a hollow shell geological theories of the interior discussed. volcanoes and earthquakes in relation to the interior liquid matter on the interior surface of the shell, and gases in the hollow, better means for eruptions than magma layers focal depths of earthquakes within reach of water, but not of lavas minute vesicles in granite filled with gases, oxygen and hydrogen, but not water the moon. a small edition of the earth rotation stopped. convulsions and cataclysms caused thereby. air, water, vapour driven off thereby to far-off hemisphere. liquid matter in hollow interior would gravitate to the inside of the nearest hemisphere form and dimensions during rotation. altered form after it stopped agreeing very closely with hansen's "curious theory" chapter xii. some of the results arising from the sun's being a hollow sphere repetition of the effects of condensation on the temperature of the nebula ideas called up by the apparently anomalous increase of temperature how heat is carried from the sun to the earth the sun supposed to radiate heat only to bodies that can receive and hold it, and not to all space. the heat of the sun accumulated in a hot box to considerably beyond the boiling point of water the heat accumulated in this way supposed to be due to a peculiar function of the ether, as it is a fact that heat can be radiated from a cold to a hot body the sun must be gaseous, or rather gasiform, throughout. no matter in it solid or even liquid. divisions and densities of shell the hollow centre filled with gases, whose mass naturally diminishes the mean density of the whole body the amount of this reduction so far defined. the presence of gases or vapours in the hollow a natural result of condensation the hollow centre filled with gases not incompatible with the sun's being a hollow sphere. the temperature at the centre may be anything, not depending on any law of gases further exposition of hollow-sphere theory put off till after further development of the construction of the sun chapter xiii. the ether. its nature considered. behaves like a gas can be pumped out of a receive light and heat do not pass through a tube _in vacuo_. laboratory experiments examined light and darkness in a partial vacuum, though high electricity not a carrying agent why there are light and dark strata in a high vacuum the real carrying agent through a high vacuum is the residue of ether left in it. digression to consider the aurora how air may be carried to extraordinary heights. zones of air carried up are made luminous by electricity comparison of this method with experiments quoted experiment suggested to prove whether light passes freely through a vacuum tube the ether does not pervade all bodies freely it must be renounced altogether or acknowledged to be a material body, subject to expansion, condensation, heating or cooling how light and heat pass through glass temperature of the ether variable. zodiacal light, cause of chapter xiv. the ether considered and its nature explained. further proofs given by dr. crookes's work, of its material substance highest vacuum yet produced. absorbents cannot absorb the ether dr. crookes's definition of a gas. not satisfactory. why a fluid required to pump matter out of a vessel gas as described by dr. crookes would not suit the ether the only elastic fluid we have. the only real gas, if it is a gas a possible measure of the density of the ether causes of dark and light zones in high vacua the real conductor of light in a high vacuum how a vacuum tube glows, when electricity passes through it conclusions arrived at through foregoing discussions some exhibitions of light explained gases can be put in motion, but cannot move even themselves the ether shown to be attraction. and primitive matter also all chemical elements evolved from it. its nature stated action at a distance explained by the ether and attraction being one and the same chapter xv. construction of the solar system. matter out of which it was formed domains of the sun out of which the matter was collected stars nearest to the sun. table vii. showing distances remarks on binary stars. table viii. showing spheres of attraction between the sun and a very few sirius actually our nearest neighbour. form of the sun's domains of a very jagged nature creation of matter for the nebulæ, out of which the whole universe was elaborated. beginning of construction the law of attraction begins to operate through the agency of evolution form of the primitive solar nebula. the jagged peaks probably soon left behind in contraction how the nebula contracted. two views of the form it might take. comparison of the two forms, solid or hollow the hollow centre form adopted. the jagged peaks left behind the nebula assuming a spherical form. shreds, masses, crescents separated from one side probable form of interior of nebula. compared with envelopes in heads of some comets reflections on the nebula being hollow. opinions of others quoted the matter of a sphere solid to the centre must be inert there further proofs of the nebula being hollow how rotary motion was instituted such a nebula might take one of two forms the form depending on the class of nebula. planetary in the case of the solar system. a similar conception of how rotary motion could be instituted chapter xvi. the sun's neighbours still exercise their attraction over him regions of greatest density in the nebulæ dealt with; compared with the orbits of the planets made from them results of comparison favourable to the theory differences of size in the planets have arisen from variations in the quantity of matter accumulating on the nebulæ causes of the retrograde motions in neptune, uranus, and their satellites probable causes of the anomalous position of neptune rises and falls in the densities and dimensions of the planets explained the form of the nebulæ must have resembled a dumb-bell more about rises and falls in densities reason why the asteroid nebula was the least dense of the system not necessary to revise the dimensions given to the nebulæ causes of the anomalies in the dimensions, densities, etc., of the earth and venus the strictly spherical form of the sun accounted for. but it may yet be varied repetition that a spherical body could not be made from a lens-shaped nebula by attraction and condensation chapter xvii. former compromises taken up and begun to be fulfilled estimates of the heat-power of the sun made only from gravitation hitherto contraction and condensation of a nebula solid to the centre. heat produced from attraction as well as by gravitation what quantity of heat is produced by a stone falling upon the earth showing again that there is a difference between attraction and gravitation contraction and condensation of a hollow-sphere nebula, in the same manner as the solid one differences of rotation would be greater in a hollow nebula; because a great deal of the matter would be almost motionless in a solid sphere in neither case could matter be brought to rest, but only retarded in motion. different periods of rotation accounted for table of different rates explained heat produced by gravitation, attraction and churning, not all constituents of the heat-power of the sun there can be no matter in the sun so dense as water the hollow part of the sun acting as a reservoir of gases, heat and pressure the behaviour of heat produced in the nebula, and its power how sun-spots are produced cyclonic motions observed in sun-spots. why not all in certain directions, and why only observed in a very few cyclonic motions in prominences treated of many other things might be explained, on some of which we do not dare to venture. concluding observations chapter xviii. return to the peaks abandoned by the original nebula. an idea of their number example of their dimensions. what was made out of them what could be made from one of them how it could be divided into comets and meteor swarms an example given. how a comet may rotate on its axis. and what might be explained thereby. orbits and periods of revolution not ejected from planets. their true origin study of the velocities in orbit of comets, and results thereof how far comets may wander from the sun and return again no reason why comets should wander from one sun to another. confirmatory of the description, in chapter xv. of the sun's domains of the eternal evolution and involution of matter. the atmosphere and corona of the sun partial analogy between it and the earth's atmosphere the density of it near the sun's surface cannot be normally less than atmospheres, but might be so partially and accidentally probable causes of the enormous height of its atmosphere the mass taken into account, but cannot be valued most probably no matter in the sun exceeds half the density of water. the unknown line in the spectrum of the corona belongs to the ether new theories in astronomy. introduction. that a little knowledge is a dangerous thing to the possessor, has been pointed out often enough, probably with the idea of keeping him quiet, but it is very certain that the warning has not always had the desired effect; and in some respects it is perhaps much better that it has not, for it is sometimes the case that a little knowledge exhibited on an inappropriate occasion, or even wrongly applied, throws light upon some subject that was previously not very well understood. it sometimes happens that unconscious error leads to the discovery of what is right. the fact is, all knowledge is at first little, so that if the first possessor of it is kept quiet there is little chance of its ever increasing. on the other hand, much knowledge seems to be quite as ready to become dangerous on occasion, for it has sometimes led its possessor to fall into errors that can be easily pointed out, even by the possessor of little, if it is combined with ordinary intelligence. the possessor of much knowledge is apt to forget, in his keen desire to acquire more, that he has not examined with sufficient care all the steps by which he has attained to what he has got, and that by placing reliance on one false step he has erected for himself a structure that cannot stand; or, what is worse perhaps, has prevented those who have followed him in implicit dependence on his attainments and fame from finding out the truth. if, then, both of these classes are liable to fall into error, there appears to be no good reason why one belonging to the first mentioned of them should absolutely refrain from making his ideas known, especially as he may thus induce someone of the second to re-examine the foundations on which he has built up his knowledge. these reflections are in greater or lesser degree applicable to all knowledge and science of all kinds, even theological, in all their individual branches, and can be very easily shown to be both reasonable and true. and it may be added, or rather it is necessary to add, that every one of all the branches of all of them has a very manifest tendency towards despotism; to impose its sway and way of thinking upon the whole world. at various intervals during the present century speculation has been indulged in, and more or less lively discussion has taken place about the great benefit it would confer on universal humanity, were all the weights and measures of the whole earth arranged on the same standard. the universal standard proposed has been, of course, the metrical system, which had been elaborated by french _savants_ who most probably thought they had arrived at such a state of knowledge that they were able to establish the foundations of all science of all kinds and for all time, upon the most sure and most durable principles. these periods of metrical fever, so to speak, seem to come on without any apparent immediately exciting cause, and some people succumb to the disease, others do not, just the same as in the cases of cholera, influenza, plague, etc. whether some species of inoculation for it may be discovered, or whether it will be found that an unlimited attack is really perfect health, will most probably be found out in the course of time, although it may be some centuries hence. what is of interest to understand at the present time is, what are the benefits to be derived from the proposed universal standard of weights and measures, and how they are to be attained. the principal and most imposing reason for its adoption is that it would be of immense service to scientific men all over the world, who would thus be able to understand the discourses, writings, discoveries, etc. of each other without the necessity of having to enter into calculations of any kind in order to be able to comprehend the arithmetical part of what they have listened to or read. another argument brought forward in favour is, that it would greatly facilitate commercial transactions with foreign countries; and it has been lately advanced that great loss is suffered by one country selling its goods, manufactured according to its own measures, in countries where the metrical system has been adopted. yet another advantage held out is the convenience it would be to travellers in money matters; but as this argument cannot be admitted without taking into consideration the necessity for one universal language all over the world, it has practically no place in any discussion on the subject, until the evil caused by the building of the tower of babel has been remedied. not long after one of the periodical attacks of metric fever we came upon an essay written by j. j. jeans on "england's supremacy," and published in new york by harper and brothers, in , in which we found the following:-- numerical relation of occupations in england and wales in : professional · per cent. domestic · " agricultural · " commercial · " industrial · " in all · " this statement shows that per cent. of the whole population are occupied in some business or work of some kind, and leads us reasonably to suppose that the remaining per cent. consist of women, children, and people who--to put it short--are non-producers; the whole of whom can hardly be considered as much interested in the making of any alterations in the weights and measures of their country, rather the contrary, for they cannot expect to be much benefited by any change. the professional class naturally comprehends theology, law, medicine, and science generally, so that the · per cent. ascribed to it would be seriously reduced, if the advantage derived from the desired change were reckoned by the number really benefited by it. a similar reduction would have to be made on the · per cent. stated to be occupied in commerce, as it is not to be supposed that the whole of the number are engaged in foreign trade. thus the number of people in these two classes who might really reap some advantage from the change, may be reduced by at least one half; and if we consider that one person in ten of those occupied in the agricultural and industrial classes is a scientist--we may pardon the domestic class--a very liberal allowance indeed, we arrive at the conclusion that per cent. of the whole population might find, some more, some less, interest in the introduction into our country of the french metric system. the above statement refers only to england and wales, but if scotland and ireland are added to them, the per cent. proportion could not be very greatly altered: perhaps it would be less favourable to the change. thus per cent., or something like millions, of the whole population of the united kingdom would be called upon to change their whole system of weights and measures, in order that per cent., or somewhere between and - / millions, should find some little alleviation in a part of their labours; and surely to - / millions of scientists and merchants engaged in foreign trade is a very liberal allowance for the population of our country. if this does not show a tendency towards despotism, it would be hard to tell what it does show. of course, it would not be fair to assume that the whole of the per cent. would desire to see the proposed change carried into effect. in all likelihood, a very considerable portion of the number would be disposed to count the cost of erecting such a structure before actually laying its foundations, and would refrain from beginning the work on considering by what means it was to be brought to a conclusion; even without going so far as to find out that per cent. of it at least would have to be done by forced labour. they might even go the length of speculating on how long it would take to coerce the per cent. into furnishing the forced labour, and on the hopelessness of the task. on the other hand, they might think it more natural to lay hold of the alternative of adopting a special system of weights and measures for the use of science and foreign commerce alone, and leave the per cent. to follow their own national and natural customs, which they would be very likely to do whatever might be determined, if we may judge by the progress made in france a century after the system was thought to be established. very little opposition could be made to such a course, and if the best possible system were not adopted, the scientists would be the only parties put to inconvenience. they could improve and reform it, should they find it not to be perfect, without the necessity of coercing the per cent. into furnishing another contingent of forced labour. but little is to be gained by saying any more about it. should the metrical system be adopted some day by act of parliament, science will have obtained what it has so long coveted, will be quite satisfied, and will trouble itself very little about how it affects the rest of the population. it will perhaps never even think of how india will be brought to buy and sell through the medium of the french metrical system. and now we have only one step to take on this subject. we may say that the project of establishing one standard of weights and measures for the whole world has a most unpleasant resemblance to the object proposed by the builders of the tower of babel; the only thing that can be said in its favour being that it points towards an endeavour to do away with the bad results produced by that enterprise and to bring matters back to the state the world was in before the foundations of that celebrated edifice were laid. the foregoing is only one instance of the many that could be cited where science has schemed projects for universal progress without due thought, and has come to the conclusion that they could be easily carried out. there are as many examples of this jumping at conclusions as would fill many books, which of course it is not our purpose to do; but there is one that it is necessary to have brought forward for examination, because of its having, through a most incomprehensible want of thought, a tendency to establish natural religion on the very bases upon which the christian religion is established. the one referred to is that by which some of the most eminent scientists of the present century, following up what was done in former times, have been able by deep study and experiment, unfortunately coupled with unaccountable blindness or preconceived erroneous ideas, to formulate processes by which the whole universe may have elaborated itself from protyle and protoplasm, or some such substances which, without any foundation to build upon, they suppose to have existed from all eternity. this advance in science has been called the theory of evolution, and has been very generally considered to be new, or of comparatively very recent conception; but it is only a piece of the evidence of a very general propensity in those who come to acquire a little more knowledge, to flatter themselves that they have power to seize hold of the unknown. the theory may be _new_, but evolution most assuredly is not, as any one may convince himself who will take the trouble to read the first chapter of the book of genesis _and to think_. there he will find it stated that the earth and all things in it and on it were created and made in six days, or periods of time, showing him distinctly, if he does not shut his eyes wilfully, that two operations were employed in the process, one of creation and the other of making, which last can mean nothing but _evolution_. it does not matter a straw whether the latter operation was carried on personally by the creator and maker, or under the power of laws ordained by him for the purpose; it was evolution all the same, and just the kind of evolution the scientists above alluded to would have us believe to be new, not far from years after the account of the creation and making of the world was written by moses. it will do no harm to take special notice of the work that was done in each of the six periods, as it will help to fix attention on the subject during examination and judgment; and may even tend to open the eyes of any one who had made up his mind to keep them shut. in the first period the heavens and the earth were created, but the earth was without form and void, _inanis et vacuus_, according to _the vulgate_--(does that mean empty and hollow?)--and darkness was upon the face of the deep; but light was _let_ shine upon the earth to alternate with darkness, and between the two to establish day and night. it is therefore evident that after the earth was created it had to be reduced to something like its present form, a globe of some kind, and to rotate on an axis, otherwise there could have been no alternations of light and darkness, of day and night. where did the light come from? some people seem to think that moses should have included a treatise on the creation and evolution of the universe, in his account of the work done in the first period of creation. for all that can be truly said to the contrary, he seems to have been quite as able to do so as any scientist of the present day; but it is evident he thought it best to limit himself to writing only of the earth, as being of most interest to its inhabitants, and enough for them as a first lesson. the literature of science, however, of the present day, will tell them that long ages after the earth was _evolved_ into a globe, it must have been in a molten, liquid state, surrounded by an atmosphere of vapours of some of the chemical elements so dense that no light from without could shine through it, and could only be penetrated by light after the cooling of the earth had dispelled a sufficient portion of that dense atmosphere. with this explanation, which they had at hand for the looking for, they might have been so far satisfied, and have left moses to tell his story in his own way. in passing, it may not be out of place to say that, after the cooling of the earth had proceeded so far that the vapours of matter had been condensed and precipitated on its surface, all boiling of water whether in the seas or on its surface must soon have ceased, so that no inconceivably enormous volumes of steam could be thrown upwards to maintain an atmosphere impenetrable to light; and that when dense volumes of steam ceased to be thrown up, the condensation of what was already in the atmosphere would be so rapid, and its density so soon reduced sufficiently to admit of the passage of light through it, that one can almost fancy himself present on the occasion and appreciate the sublimity of the language. "and god said, let there be light, and there was light"; more especially if he had ever stood by the side of the cylinder of a large steam engine, and understood what he heard when the steam rushed from it into the condenser, and noted how instantaneous it seemed to be. any one who has watched a pot of water boiling on the fire and emitting clouds of steam, will have noticed how immediately the boiling ceased whenever the pot was removed from the fire; but he will also have noticed that the water still continued to emit a considerable quantity of vapour, and will be able to understand how it was that the cloudy atmosphere of the earth, at the time we are dealing with, could allow light to pass through it but still keep the source of light from being visible. he experiences daily how thin a cloud will hide the sun from his sight. but there is more to be said about this when the time comes for taking note of the actual appearance on the scene of the sun, moon, and stars. to obtain some rude idea of the time to be disposed of for evolution during the first period, let it be supposed that the whole of the time consumed in the creation and development of the earth was million years, as demanded by some geologists, the first period of the six would naturally be somewhere about millions of years, a period which would allow, probably, very liberal time for evolution, but could never have been consumed in creation, seeing that creation has always been looked upon as an almost instantaneous act. and if anyone is still capable of exacting that the period was a day of twenty-four hours, he has to acknowledge that at least twenty-three of them were dedicated to the work of evolution. the second period was evidently one solely of evolution, as all that was done during it was confined to _making_ the firmament which divides the waters from the waters; an operation which could never be confounded with creation, being probably brought about solely by the cooling of the earth, which was the only means by which a separation between the waters covering the earth, and those held in suspension above it by the atmosphere, could be brought about, and must have been purely the work of evolution. the third period was begun by collecting the waters under the firmament into one place and letting the dry land appear; which, it may be well to note, gives it to be understood that the surface of the solid part of the earth had come to be uneven either by the elevation or depression, perhaps both, of some parts of it, and next the earth was _let_ bring forth grass and trees, and in general vegetation of all kinds. these cannot be considered otherwise than as operations of evolution: there was no creation going on beyond what may have been necessary to help evolution, and of that not a word is said. here it is well to notice that until the waters were gathered together into one place and the dry land appeared there could be no alluvial deposits made in the sea, and that till well on into this third period, that is well on for million years from the beginning, there could be no geological strata deposited in it containing vegetable matter, for the very good reason that although rains and rivers may have swept earthy matter into the sea, the rivers could not carry along in their flow any vegetable matter until it had time to grow. should evolutionists think they have discovered something new in spontaneous generation, we refer them to the th verse of the chapter, where they will see--"and god said, let the earth bring forth grass, the herb yielding seed, and the fruit-tree yielding fruit after his kind, whose seed is in itself, upon the earth." the conclusion of this passage asserts plainly that the seed was already in the earth, somehow or other, ready to germinate and sprout when the necessary accompanying conditions were prepared. the words are very few, and they can have no other meaning. in the first period "god made two great lights: the greater light to rule the day and the lesser light to rule the night; he made the stars also." this passage has been "a stumbling block and rock of offence" to some people possessed of much knowledge and to some possessed of little; the one party professing to disbelieve all because the sun was _made_ four days after there was light, and the other party, supposing that there might have been light proceeding from some other source during the first four days. both parties seem to have forgotten that the earth was created without form and void, and that being so the same would naturally be the case with the sun and the moon; all of them had to be made into form after their creation. by what means? by evolution, of course, or whatever else anyone chooses to call it; that will make no difference. as far as it can penetrate into the mysteries of creation, physical astronomy has endeavoured to show how the solar system may have been formed out of a mass of nebulous matter. furthermore, as has already been adduced in evidence, that at one time the earth must have been a molten, liquid globe surrounded by vapours of metals, metalloids, gases, and finally by water; and even goes the length of supposing that the planets were evolved to something approaching their present state, long before the sun attained its present form. following up this hypothesis, it is more than probable that the sun had not attained that form when this fourth period began, and, although capable of emitting light early in the first period, still required a vast amount of evolution to reduce it to the brilliant globe now seen in the heavens. everybody knows that plants grow without sunshine, and it is generally believed that the primary forests of the earth grew most rapidly in a moist, stifling atmosphere, which neither admitted of animal life, nor could be penetrated by sunshine. thus physical astronomy cannot say that the sun could not have been made into its present state until near the end of this fourth period. it _may_ have been as bright as it is now, though very probably not, as we shall see in due time; but it could not _shine_ upon the earth, neither could the earth, nor anything thereon, see it. it is not necessary to say anything about the moon, as it only reflects sunlight, and the reflection could not reach the earth if the light could not. in the fifth period the waters were _let_ "bring forth the moving creature that hath life, and fowl that may fly above the earth in the open firmament of heaven." here again spontaneous generation may have been provided for beforehand, the same as in the case of vegetation. also it is said "god created great whales," and it is to be observed that this is only the second time that creation has been mentioned in the book, and would seem to teach that _making_, or evolution, was the most active agent at work in the construction of the earth--and, we may add, of the universe. the sixth period was one almost exclusively of evolution, unless it should be considered that spontaneous generation is a different, and newly discovered process. in it god _made_ the beast of the earth, cattle, and everything that creepeth upon the earth, after his kind. last of all: "god said, let us make man in our image, after our likeness." thus it appears that the only work of creation done in this period was that of creating man, and even that _after_ some length of time and work had been expended in _making_ or _evolution_, which may have extended over a very considerable portion of the fifty millions of years corresponding to it. we have supposed the work of creation to have extended over three hundred million years to satisfy some geologists, but our arguments would not be affected in any way by the time being reduced to the limit given by lord kelvin to the heat-giving power of the sun in the past, which he has made out to be between fifteen and twenty million years. that would only limit our periods of evolution to two and a half or three million years each; each of them quite long enough to be totally inconsistent with our ideas of creation, which conceive of this as an instantaneous act. but although lord kelvin has in rather strong terms placed this limit, he at the same time says that it could by no means exceed four hundred million years, which is one-third more than we have calculated upon. neither can our arguments be affected in any serious way by our dividing the periods into fifty million years each; these may have varied much in length, but whatever was taken from one would have to be added to the others. furthermore, we may be allowed to say that fifteen to twenty millions of years of the sun's heat at the rate it is now being expended, can be no reliable measure of the time required for the operations of geology, for the reason that its heat must have been emitted in proportion to the quantity it possessed at any time. when it was created without form and void as no doubt it was, the same as the earth, it would have no heat to emit, but that does not mean that it possessed no heat until it was formed into the brilliant globe that we cannot now bear to turn our eyes upon. even when it became hot enough to show light sufficient to penetrate the "darkness that was upon the face of the deep," it may still have been an almost shapeless mass, and have continued more or less so until it was formed into the body of the fourth period, which may even then have been very different from what it is now. thus geology would have not far from one hundred and fifty million years in which a very small fractional part of the sun's emission of heat would suffice for its operations. but we shall have more to say on this subject when the time comes. it being, therefore, a matter beyond all question--to people possessed of the faculty of thinking, and of candour to confess that they cannot help seeing what has been set plainly before their sight and understanding--that the opening chapter of the book of genesis plainly teaches that making--evolution--had a very large and active part to perform in the creation of the universe and--much more within our grasp--of the earth; we can come to the conclusion that the theory of evolution, instead of being new and wonderful, comes to be almost infinitely older than the everlasting hills, without losing any of its power of inspiring inexpressible wonder. looking back over the examination into the first chapter of the book of genesis we have just concluded, we cannot conceive how it could ever have entered into the thoughts of man, that the state of vegetable and animal life on the earth, at the present day, must have been brought about by continual and unceasing acts of creation, when creation has been mentioned only on three occasions during the whole process described in the chapter we have analysed, that is, out of verses; and while the other processes which we have brought forward--making and spontaneous generation--have never been alluded to, perhaps not even thought of. we have no desire, neither are we qualified, to follow up this subject any further, but we have still one or two things to bring into remembrance. one of the most illustrious of the founders of the theory of evolution has based his dissertations on the descent of man, on the variation of animals and plants under domestication, and on their _wonderful plasticity under the care of man_. here there is an explicit acknowledgment of the necessity for the direction of an intelligent guiding power to produce such variations; these never having any useful or progressive results except under such care. if, then, there is a necessity of such directing and guiding power in the case of variations of such inferior importance, the superintendence of some similar power must have assuredly been much more necessary for the creation and evolution of matter, of life, and of man himself. this is what, one would think, common sense and reason would point, and what the theory of evolution seems to think--evidently without studying the subject far enough; but all that it has been able to do has been to substitute nature for the creator to whom moses has ascribed not only _creation_ but the _making--evolution_--of the universe. this naturally leads us to speculate on what evolutionists consider nature to be, and as none of them--nor anyone else--as far as we know, has ever thought it necessary to define nature, we have to endeavour to draw from their writings what, in some measure and some way, they would like us to believe it to be. we find, then, that the base of their operations seems to be natural selection, which can hardly be interpreted in any other way than by calling it the selection of nature. thus, then, they apparently want us to look upon nature as the _first cause_. but, if nature can select, it must be a being, an entity, a something, that can distinguish one particle of matter from another, and be able to choose such pieces of it, be they protyle or protoplasm, and to make them unite, so as to form some special body, organic or inorganic. it is plain, also, that selection can only be performed by such a being, or something, such as just so far described, that can distinguish, choose, and arrange the particles of matter destined to form the very smallest body or the universe. thus we see that in whatever way the basis of the theory of evolution is looked upon--_even for its own evolution_--there is required a being of some kind that has knowledge and power to evolve or make all things that are "in heaven above, or in the earth beneath, or in the waters under the earth." so we see that, if the theory of evolution dethrones the creator and evolver of the first chapter of genesis, it has to enthrone another god which it calls nature; and has to get rid of that god, and any number of others, before it can be what it pretends to be. we are all very voluble in talking of nature, and enthusiastic in admiring its beauties, wonders, and wisdom, but it seldom occurs to us that we are really doing so without thinking of whence come the beauty, wonders, and wisdom. we must, therefore, not be too hard on evolutionists, as they have only done what we all do every day of our lives; but if the theory of evolution is to be looked upon as a branch of science, we would recommend its students to open their eyes and think of it as a process which has been in existence from the beginning of things at least, and not as one of their invention or discovery. they may be able some day, through more accurate study and more convincing argumentation than they generally use, to lay claim to having discovered, as far as it is possible for man to do, the _modus operandi_ of evolution, but that is all, and we would also warn some of them to think that, when we see them in their highest flights of science, genius, and self-sufficiency, we can "conceive the bard the hero of the story." we have read a good deal of what has been called the war of science, without having been able to see that there ever was any cause for such a war, with the exception of ignorance. if theology had been able, or rather had taken the trouble, to study thoroughly the first chapter of genesis, and thus to comprehend that, if the earth was created without form and void, a great deal of work had to be done, after creation, in forming it into its present condition, there was no call upon it to find fault with copernicus or persecute galileus, because they said the earth revolved round the sun; more especially as they do not appear to have ever said anything against religion or revelation. neither was there any necessity for opposing the so-called new science of evolution, because it (theology) ought to have seen that the work expended in reducing the earth into form could hardly be conceived of otherwise than as a process of evolution; and would thus have been in a position to tell the authors of the _new_ science that they had only discovered what had existed before the beginning of time. on the other hand, there was no occasion for science to take up the war. if it, in its turn, had taken the trouble to study and understand the first chapter of genesis, it could have shown theology that _it_ did not comprehend, and could not give a true account of what religion and revelation are; whereas it (science) seems to have had a strong tendency to demonstrate that religion and revelation are altogether false, and that the great work it has to perform is to dethrone theology, and set itself up it in its stead. it is not worth while even to think of who or which was the aggressor, seeing that the war originated from ignorance caused by want of thought and study on both sides. all that has to be said on the subject reduces itself to the fact that both religion and science have been coming, and are at present going, through the process of evolution. can anyone say that science has been truly scientific, without ever incurring in error, from the beginning of history up to the present day? will any one venture to maintain that there has been no evolution, no progress, no softening of the spirit of religion, from the institution of christianity up to the end of the nineteenth century? if such there be, let the one look back to the time of aristotle, and the other to the establishment of the church under constantine. there has been for long an opinion, which goes on increasing in strength, that science will ultimately reform theology and put religion in its right place; but if such is to be the case, science has to begin by reforming itself and putting an end to error it has been, in many cases, teaching for generations; and by ceasing to formulate new theories, or bases of progress, which can be in many cases exploded by suppressing some of the error just alluded to. little advance is made in science by forming hypotheses and theories, however brilliant they may appear, unless they are carefully studied and thought out to the very uttermost; because, if published abroad on the authority of some celebrated or even well-known name, they have a tendency to stop further investigation, and prevent students from exercising their own judgment and perhaps discovering what they might possibly find out were they to study them to the very end for their own satisfaction. this is in some measure the case even with respect to the solar system. we believe it can be shown that a more complete knowledge and comprehension of it, and even of the universe, has been kept back by the unquestioning acceptation by successive astronomers of the ideas and conceptions of their predecessors. we have to acknowledge, at the same time, that astronomy could not start into perfection at once, any more than any other science, and it is not to be wondered at that in times past ideas relating to it should have been formed without being properly thought out; even ideas that could not be properly thought out to the end for want of the requisite knowledge. but it is much to be regretted that such ideas should continue to be published at the present day as trustworthy instruction for readers who may look upon it as strictly correct. among those who read text-books even on astronomy, there must be a very considerable number who are rather surprised when they see statements made which do not agree with what they were taught at school, or with what they have practised in other sciences in their own professions or trades. it may be said that any person of ordinary intelligence will easily be able to correct such errors, but the evil does not stop here. if he can really correct them he will most probably find as well, that his instructors have been led into more serious errors, perhaps in more important matters, founded on the ideas which they had not fully studied out before giving them a place in their books. he may also find sometimes, in his reading, such ideas brought forward to substantiate some theory, just as far as they are required and then dropped, while a step or two further forward in the examination of these same ideas, would have exploded the theory altogether; because, although founded to a certain extent on one law of nature, they are in contradiction with what is laid down in some other law. the above will be looked upon as an unwarrantably bold assertion; but a careful study of, or attention to, what is taught in the most advanced works on the solar system, even in science generally, will show it to be perfectly true. it is not only true, but the consequences of its being true have been much more serious than will be readily believed. in our own endeavours to understand what we had been reading, we have seen that some of the notions presented to us were only half formed, and that they have led to theories being founded which could never have been entertained at all had they been thoroughly studied out. more than that, they have prevented the truth from being arrived at in the fundamental conceptions of the construction of the earth, and, as a natural consequence, of the whole solar system, perhaps even of the whole universe. there are probably many, even a great many, people who have arrived at the same conclusions as we have, but as far as it has been in our power to search into the matter, we have met with no attempt from any quarter to put an end to this defect in the literature of science; perhaps because the work has the appearance of being too great to be readily undertaken, and also because it may be thought that there is little to be gained by it--as all is sure to be set right through time. but, as we believe that it will be beneficial immediately, in the case of the earth and solar system at least, we shall first attempt to show what are some of the defects alluded to, and then what knowledge may be acquired through their removal. chapter i. page the bases of modern astronomy. their late formation instruments and measures used by ancient astronomers weights and measures sought out by modern astronomers means employed to discover the density of the earth. measuring by means of plummets not sufficiently exact measurements with torsion and chemical balances more accurate sir george b. airy's theory, and experiments at the harton colliery results of experiments not reliable. theory contrary to the law of attraction proof by arithmetical calculation of its error difficulties in comparing beats of pendulums at top and bottom of a mine the theory upheld by text-books without proper examination of a particle of matter within the shell of a hollow sphere. not exempt from the law of attraction a particle so situated confronted with the law of the inverse square ofdistance from an attracting body. remarks thereon it is not true that the attraction of a spherical shell is "zero" for a particle of matter within it before astronomers could begin to determine the relative distances from each other, and the relative dimensions and masses of the various members of the solar system, they had to establish scales of measurements appropriate to their undertaking. this entailed upon them, of course, the necessity of determining the form, the different circumferences and diameters, and the weight of the whole earth, as any other scales derived from the only available source, the earth, would have been too small to give even an approximate value of the measures and masses to be sought for. history tells us that at least one attempt had been made, over two thousand years ago, to find the circumference and necessarily the diameter of the earth, but it says nothing of any to ascertain its weight. there may have been many to determine both diameter and mass, but we know nothing of them; and when we think seriously about this, we cannot help feeling somewhat surprised that no attempt had been made to find out the density and mass till more than a century after sir isaac newton's discovery of the law of attraction, or gravitation, as it is more usually called. but perhaps this is an idea that could only occur to one who has been _spoilt_ by witnessing, in great measure, the immense strides in advance that have been made during the nineteenth century in science of all kinds, and does not duly take into account the immense labour, and the incessant meeting with almost insurmountable difficulties, that astronomers have had to encounter and overcome between the birth of modern astronomy and the end of the eighteenth century. indeed, the difficulties can hardly be looked upon as altogether overcome even yet, as efforts are still being made to find out the exact distance of the sun, and it is not impossible that some small difference may be found, plus or minus, in the density at present adopted for the earth of · times the weight of water. the geometer who, more than two thousand years ago, set himself the task of measuring the circumference of the earth, is supposed to have made use of very much the same kind of implements as those employed by modern astronomers. he must have had a very fair instrument for measuring angles, and have known very well how to use it, seeing he was able to determine a value for the obliquity of the ecliptic which agrees so well with that established by modern science, its variations being, for what we know, taken into account; and for length or distance he would doubtless have some implement analogous to the metre, chain, foot-rule, or something called by other name that would, in those days, present facilities for selling a yard of calico. his operations would probably be as plain and simple as those applied to the measuring of a village green--for we are not told that he had any idea of there being any difference between the length of a degree of the meridian at the equator and one nearer either of the poles--and involved no hypotheses or theories, any more than modern operations have done. when the time came for making efforts to ascertain the density of the earth, science seems to have employed the very simplest means it had at its disposal for attaining its object, and to have gone on refining its implements and operations in conformity with the lessons it went on learning while pursuing its self-imposed task. every one who, even for recreation, has read a fair amount of the multitude of works and writings that have been published on popular astronomy--not to speak of text-books--knows that the first attempts were made by measuring the attraction of steep, or precipitous, mountains for plummets suspended in appropriate positions in their neighbourhood; then--evidently from knowledge acquired during these operations--by the attraction for each other of large and small leaden balls suspended on frames and torsion balances, which go under the name of the cavendish experiment; and afterwards by a refinement on this in using the chemical balance, where only one large and one small ball of metal are required. all these operations and their results are to be found described in works of various kinds, and are generally reduced to something like the following tubular form, which we reproduce in order to make more intelligible what we have just said, and that we may make a few remarks upon them. there is no hypothesis, no theory, connected with any of the operations, unless it was the supposition that a plummet--which was naturally believed to point to the centre of the earth--should be pulled to one side by the attraction for it of a mountain in its neighbourhood, and that was found to be a fact. methods employed for finding the density of the earth, and their results. ( ) _deviation of plummet by the attraction of mountains_:-- experiments made. by whom, and date. mean density found. at schiehallien maskelyne · at arthur's seat sir h. james · ( ) _torsion balance experiments_:-- cavendish · at freyberg, saxony reich · at manchester francis baily - · ( ) _chemical balance experiments_:-- j. h. pointing · in the case of the plummet deviating from its absolutely straight direction towards the centre of the earth, caused by their attraction, not only the mountains themselves had to be measured and virtually weighed as far as they were measurable, but the weight of the wedge or pyramid between that measurable point, in each case, and the centre of the earth had to be estimated in some way; then the centre of gravity of the whole of this mass had to be ascertained, as well as the respective distances from the centre of the earth of this centre of gravity and that of the plummet, and only after all this and a deep study of the mutual attractions of this mass and the plummet could an estimate be formed of the mass of the earth. it will thus be seen that such measurements and estimates could never be looked upon as very exact and reliable; and nevertheless they have come very near the density of · finally adopted for the earth. in the case of the torsion balance experiments a very considerable advance was made in consequence, most undoubtedly, of the knowledge acquired from what had been done by maskelyne. when it was found that the attraction of schiehallien for the plummets was such a measurable quantity, cavendish evidently saw that the attraction of manageable leaden balls for each other would be measurable also, and that as no calculations of any kind whatever were necessary to find the masses of the balls, the mutual attraction of large and small balls would furnish a more exact means of measuring the density of the earth, than the roundabout way of having to calculate the weight of a mountain as a beginning; and with the requisite ingenuity, invention, and labour, he found the means of applying the torsion balance, to make the experiments. after these experiments were revised by reich and baily--and the density of · adopted, we believe--still another set were undertaken by j. h. pointing, with the chemical balance, in which only two metal balls, one large and one small were required, which gave a density of · as shown opposite, and from its extreme simplicity may perhaps have been the most exact of all. we have said, we think with truth, that there is no hypothesis or theory involved in any of these experiments, but only the simplest form of--we might almost say--arithmetical calculation. but there is a theory built up on hypothesis which has no foundation whatever, and about which most people, who take the trouble to study it out to the very end, will come to the conclusion that "the less said the better." this, at all events, is our opinion, and we would not have taken any notice whatever of it had it not been that up to the present day, it is published in many works on popular astronomy, and even in some text-books, and is looked upon in them, apparently, as an example of the transcendent height to which human science can reach. we allude, of course, to the theory that the deeper we go down into the earth--at least to an undefined and undefinable depth--the greater is its attraction for the bob of a pendulum at that depth, and the greater the number of vibrations the pendulum is caused to make in a given time. the explanation of the theory is, that were the earth homogeneous throughout its whole volume, the pendulum ought to make the fewer vibrations, the deeper down in the earth it is placed; but as the earth is not homogeneous, it actually makes a greater number of vibrations in a given time, because the attractive force of the earth increases--up to the undefined and undefinable depth--on account of the denser matter beneath the pendulum bob more than overbalancing the loss of attraction from the lighter matter left above it. the author of the theory was the late astronomer royal, sir george b. airy, who from it endeavoured to calculate the mean density of the earth, and with that view made two experiments which are thus described by professor c. piazzi smythe in his work on the great pyramid:-- "another species of experiment... was tried in by mr. (now sir) george b. airy, astronomer royal, dr. whewell, and the rev. richard sheepshanks, by means of pendulum observations at the top and bottom of a deep mine in cornwall; but the proceedings at that time failed. subsequently, in , the case was taken up again by sir george b. airy and his greenwich assistants, in a mine near newcastle. they were reinforced by the new invention of sympathetic electric control between clocks at the top and bottom of a mine, and had much better, though still unexpectedly large results--the mean density of the earth coming out, for them, · ." from other sources we have also found that the pit, or mine, was at the harton colliery and feet deep, that the pendulum at the bottom of it gained - / seconds on the similar one at the top, in hours; and that the surrounding country had to be extensively surveyed, the strata had to be studied, and their specific gravities ascertained. a little unbiassed thought bestowed on this theory will at once show that it begins by violating the law of attraction discovered by newton, when he showed _that the mutually attractive forces of several bodies are the same as if they were resident in the centres of gravity of the bodies_. in the case in point this means, that the attraction of the earth for the bob of the pendulum at the top of the mine was the same as if all its force was collected at its (the earth's) centre. in that position the force of the earth's attraction comprehended, most undeniably, the whole of its attractive power, including whatever might be imagined to be derived from the non-homogeneity of the earth, due to its density increasing towards the centre; and we are called upon to believe that when, virtually, the same pendulum was removed to the bottom of the mine, and a segment feet thick, at the centre as good as cut off from the earth and--as far as the pendulum was concerned--hung up on a peg in a laboratory, the diminished quantity of its matter had a greater attractive force, a very little beyond the centre--non-homogeneity again included--than the whole when the sphere was intact. this we cannot do, because all that we can see in the placing of the pendulum at the bottom of the mine, is that the position of the bob has divided the earth into two sections, one of which has a tendency to pull it up towards the surface, and the other to pull it down towards its centre of gravity; and because the mass of the smaller segment is so insignificant that its entire removal to the laboratory peg, not only could not produce the reverse action, on which the theory is based, but could not be measured by any stretch of human invention or ingenuity; it is far beyond the reach of mathematics and human comprehension of quantity. the difficulty of belief is increased when we reflect that, were the pendulum taken down towards the centre of the earth, the number of its vibrations in a given time ought gradually to decrease as it approached the centre, and would cease altogether when that point was reached. and we feel confident that no mathematician could calculate where the theoretical acceleration of the vibrations would cease, and the inevitable retardation commence; where the theory would come to an end and the law of attraction begin to assert its rights, simply because he does not know how the non-homogeneity is distributed in the earth. no man can tell, even yet, how the mean density of · is made up throughout the earth, and without that any theory founded on its non-homogeneity is out of place. but to follow up our assertion of non-commensurability. taking the diameter of the earth at miles, and its mean specific gravity at · , its mass would be represented by , , , , cubic miles of water. on the other hand, supposing the earth to be a true sphere, the volume of a segment of it cut off from one side, at one quarter of a mile deep--not , but feet--would be · cubic miles in volume, and if we suppose its specific gravity to be · --greater most probably than the average of all the strata in the neighbourhood of the harton colliery--its mass would be represented by · cubic miles of water. then, if we divide the mass of the section below the pendulum, that is, , , , , minus the mass of the one above it, · , viz. , , , , · by the mass of · just mentioned, we find that the proportion they bear to each other is as to , , . this being so, we are asked to believe that by removing / , , th part of the mass of the earth from one side of it, its force of attraction at the centre will not only not be decreased, but will be so increased that it will cause a pendulum, suspended at the centre of the flat left by the removal of the segment, to vibrate , · times in twenty-four hours instead of , times as it did when suspended at the surface before the segment was removed; that is, that the vibrations will be increased by / , th part. again we cannot do so. had we been asked to believe that the removal of so small a fraction as / , , th had decreased the earth's attraction at its centre, so much as to produce a diminution of / , th part in the number of vibrations of the pendulum, we could not have done so; how much less then can we believe that the central attractive force had increased so much as to produce an augmentation of the vibrations in the same proportions? but more in this strain presently. we have no doubt whatever that sir george b. airy and his assistants satisfied themselves that the pendulum at the bottom of the mine gained - / seconds in twenty-four hours over the one at the top, but they may have been deceived by their over-enthusiastic adoption of what seemed to be a very grandly scientific theory, or by some unperceived changes in the temperature in the pendulums, caused by varying ventilation in the mine or the varying weather outside of it, or by the insidious manifestations of the "sympathetic electric control between clocks at the top and bottom of a mine," called in to assist at the experiments. an error of / , th part of the time the sympathetic electricity would take to travel from the top to the bottom of the shaft would be sufficient to make the experiments of no value whatever; not to speak of the small errors that may have been made in surveying the surrounding country, calculating the specific gravities of the strata--for we are told that all this had to be done-and applying the elements thus obtained to the solution of the problem they had in hand. we have read of the difficulties met with by mr. francis baily when he began to revise the cavendish experiment--some twelve or fifteen years before the final harton colliery experiments were made, and suppose it possible that they met with similar difficulties without being aware of it. and / , th part is such a very small fractional difference in the vibrations in twenty-four hours, of the pendulums of the two separate clocks, that--taking into consideration the circumstances under which it was found--it would hardly be looked upon as reliable at the present day, when the clocks of astronomical observatories are placed in the deepest cellars or even caves available, so as to free them as much as possible from variations of temperature. having referred to the difficulties met with by mr. baily, we believe it worth while to transcribe professor c. piazzi smythe's account of them, given in his work already referred to at page ; because it not only has a very direct bearing on what we have been saying of changes of temperature, but is exceedingly interesting, and probably very rarely to be met with in other works. it is as follows:-- "nearly forty years after cavendish's great work, his experiment was repeated by professor reich of freyberg, in saxony, with a result of · ; and then came the grander repetition of the late mr. francis baily, representing therein the royal astronomical society, and, in fact, the british government and the british nation. "with exquisite care did that well-versed and methodical observer proceed to his task, and yet his observations did not prosper. "week after week, and month after month, unceasing measures were recorded; but only to show that some disturbing element was at work, overpowering the attraction of the larger on the smaller balls. "what could it be? "professor reich was applied to, and requested to state how he had continued to get the much greater degree of accordance with each other, that his published observations showed. "'ah!' he explained, 'he had to reject all his earlier observations until he had guarded against variations of _temperature_ by putting the whole apparatus into a cellar, and only looking at it with a telescope through a small hole in the door.' "then it was remembered that a very similar plan had been adopted by cavendish, who had furthermore left this note behind him for his successor's attention--'that even still or after all the precautions which he did take, minute variations and small changes of _temperature_ between the large and small balls were the chief obstacles to full accuracy.' "mr. baily therefore adopted yet further, and very peculiar, means to prevent sudden changes of temperature in his observing room, and then only did the anomalies vanish and the real observations begin. "the full history of them, and all the particulars of every numerical entry, and the whole of the steps of calculation, are to be found in the memoirs of the royal astronomical society, and constitute one of the most interesting volumes (the fourteenth) of that important series; and its final result for the earth's mean density was announced as · , probable error ± · ." after reading this story of baily's experiments with care, one cannot help feeling something stronger than want of confidence in those made at the harton colliery, especially after what has been shown of the smallness of the fraction of the earth that was dealt with, and due consideration is given to the insignificant difference of effect that the non-homogeneity of the earth could produce on the remainder after the supposed removal of such a small fraction; and here we might let the theory drop. perhaps it may be thought that now there is nothing to be gained by spending time and work in showing it to be more truly erroneous than we have yet made it out to be; but if there is error, it cannot be too clearly exposed, and the sooner it is put an end to, the better; more especially as it has been accepted as true by some authors of text-books, and by some competent astronomers who, in trying to explain the anomaly of the increase instead of decrease in the force of attraction at the bottom of a mine compared with the top, have used arguments which are not consistent with the law of gravitation, or rather attraction. messrs. newcomb and holden in their work, entitled "astronomy for high schools and colleges," sixth edition, , apparently accept the theory, and proceed to explain and support it by showing what would be the action of a hollow spherical shell of any substance on a particle of it, say the bob of a pendulum, placed on the outside and also on the inside of the shell; and give us two theorems which are supposed to comprehend both cases. these are:-- ( ) "if the particle be outside of the shell, it will be attracted as if the whole mass of the shell were concentrated at its centre." ( ) "if it be inside the shell, the opposite attractions in every direction will neutralise each other, no matter whereabouts in the interior the particles may be, and the resultant attraction of the shell will therefore be zero." to the first theorem no objection can be made: the particle on the outside of the shell will undoubtedly be attracted by every particle in the shell, with the same force as if the attractive power of all the particles composing it were concentrated in the centre. not so with the second theorem: for it can be objected that it altogether ignores the law of attraction laid down by sir isaac newton, where it asserts that the resultant attraction of the shell for the particle will be zero, when it is placed anywhere on the inside. in fact the theorem supposes a case impossible for the harton colliery experiments, in order to demonstrate their accuracy; for it makes use of the bob of the pendulum--a particle of matter--as if it were transferable to any part of the interior of the earth instead of being confined within the bounds of its swing. that the attraction of the shell-- feet thick all round the earth--on the pendulum bob inside of it continues in all its force, and is only divided into two opposing parts, is made plain by fig. . supposing o to represent the bob of the pendulum at the bottom of the mine, and the space between the two circles the shell of the earth. then the line b c will show where the attraction of the shell for the bob is divided into two parts acting in opposite directions. supposing these two parts to be separated from each other, only far enough to admit the bob--a particle to all intents and purposes--between them; the part b a c will attract the bob as if its whole attractive force were collected at its centre of gravity, and the part b d c as if the whole of its attractive force were collected, not at the centre b of the shell, but at its centre of gravity, a very little distance from b in the direction towards d. this is an incontrovertible fact, because it is in strict accordance with newton's law of attraction, which is: _every particle of matter in the universe attracts every other particle with a force directly as their masses, and inversely as the square of the distance which separates them._ [illustration: fig. .] if we now suppose the interior of the shell to be filled up solid, that will make no difference, because the mass of the part b d c will only be increased vastly thereby, while the mass of a b c will remain the same; the two parts only increasing their proportion to each other, and thus coming to be for the earth--in the harton colliery experiments--what we represented them to be at page ; and we can now proceed to find the attractive force of each of the two masses for the bob of the pendulum which is as the inverse square of their distances from it. these distances may be taken, without any very great stretch of conscience, as one-tenth of a mile and · miles; because the centre of gravity of the segment a b c will be about that distance from o, and that of b d c cannot be adequately represented by a greater sum than · , always supposing the diameter of the earth to be miles. thus the squares of these two distances will be · and , , miles respectively, and the relative force of attraction for the pendulum of the two segments a b c and b d c will be as × · and , , , and , , × , , ; that is as is to , , , , , , . here then we get confirmed the unbelief in the theory we expressed at pages and . surely no one will be bold enough to assert that by decreasing the total attractive force of the earth by a little less than a - / trillionth part cut off from one side of it, the want of homogeneity in what remains will not only not decrease its attractive force at the centre, but increase it so as to make a pendulum be lessened by / , th part of its time in beating one second. this fraction of time is quite small enough to inspire doubt of any theory founded upon it; and if there ever is a quantity in mathematics that can be called negligible, the fraction of attractive force found above ought to be included in the same category. we may therefore assert that no human measurements could find a true difference between the beats of a seconds pendulum at the top and bottom of the pit at the harton colliery. if all the people who have puzzled themselves with this theory had spent an hour or two in making the above calculations before they began them, there would have been no experiments made, and the theory would have died almost ere it was born. those who believed in it may have looked upon a particle as a negligible quantity, but as the whole earth is made up of particles a little thought would have put an end to such a notion. what puzzles us is how such a theory could be formed by people who knew nothing whatever of the nature of the interior of the earth at a depth of even one mile, and how they could speculate on its want of homogeneity without knowing anything of how the density of · is made up in it? to suppose that the earth is made up of strata of different densities, and that each is in some degree elliptical--the ellipticity of one stratum being different from another, as the french mathematician clairaut did--is all very allowable; but to build up any theory on any such suppositions is to build upon shifting sands without examining the foundations. for anything that is known up to the present time, the density of the earth may go on increasing gradually from the surface to the centre, or it may attain nearly its greatest density at a few miles from the surface, and continue homogeneous or nearly so from there to the centre. to go further now: it is not true that the attraction of a hollow shell of a sphere for any particle within it, is the same "no matter whereabouts in the interior the particle may be." the only place where the attraction will be the same is when the particle is at the centre. in that position a particle would be in a state of very unstable equilibrium, and a little greater thickness of the shell on one side than the others, would pull it a little, perhaps a great, distance from the centre towards that side; and if we extend our ideas to a plurality of particles within the shell of a sphere, we are led to speculate on how they would be distributed, and to see the possibility of there not being any at all at the centre. this is a point which has never been mooted, as far as we have been able to learn, and we shall have to return to it when the proper time comes. it is difficult to understand how any man could conceive the notion that a shell of a sphere, such as that shown at fig. , could have no attraction for each separate one of all the particles which make up the mass of the whole solid sphere within it; for that is the truth of the matter if properly looked into, when it is asserted, as has been done by messrs. newcomb and holden, that "the resultant attraction of the shell will therefore be zero." if such a notion could be carried out in a supposed formation of the earth, an infinity of particles would carry off the whole of the interior, and leave the earth as only a shell of feet thick, as per the hartley colliery experiment; only we are told, or left to understand, that that process could not go on for ever, but would have to come to an end somehow and somewhere; and then we are left to speculate on how the unattracted particles could come back to take part in the composition of the earth. left to ourselves we can only liken the process to that followed by a man who peels off the outer layer of an onion, eats the interior part, and when he is satisfied throws down the outer layer and thinks no more of it; not even that he might be asked what had become of the interior part. curiously enough, there is a way of explaining how, or rather why, the notion was formed--not unlike the one just given--to be found in the third of sir george b. airy's lectures on popular astronomy, delivered at ipswich several years before the final experiments were made at the harton colliery. in that lecture, while describing how the greek astronomers accounted for the motions of the sun and planets round the stationary earth, he says, "it does appear strange that any reasonable man could entertain such a theory as this. it is, however, certain that they did entertain such a notion; and there is one thing which seems to me to give something of a clue to it. in speaking to-day and yesterday of the faults of education, i said that we take things for granted without evidence; mankind in general adopts things instilled into them in early youth as truths, without sufficient examination; and i now add that philosophers are much influenced by the common belief of the common people." we can agree with sir george b. airy in his ideas about education, and now conclude by saying that he has given us a very clear and notable example of a theory being accepted very generally, without being thoroughly examined to the very end, and of how easy it is for such theories to be handed down to future generations for their admiration. chapter ii. page the moon cannot have even an imaginary rotation on its axis, but is generally believed to have. quotations to prove this. proofs that there can be no rotation. the most confused assertion that there is rotation shown to be without foundations. a gin horse does not rotate on its axis in its revolution. a gin horse, or a substitute, driven instead of being a driver. results of the wooden horse being driven by the mill. the same results produced by the revolution of the moon. centrifugal force sufficient to drive air and water away from our side of the moon. that force not sufficient to drive them away from its other side. no one seems ever to have thought of centrifugal force in connection with air and water on the moon. near approach made by hansen to this notion. far-fetched reasons given for the non-appearance of air and water. the moon must have both on the far-off hemisphere. proofs of this deduced from its appearance at change. where the evidences of this may be seen if looked for at the right place. the centrifugal force shown to be insufficient to drive off even air, and less water, altogether from the moon. the moon must have rotated on its axis at one period of its existence. the want of polar compression no proof to the contrary. want of proper study gives rise to extravagant conceptions, jumping at conclusions, and formation of "curious theories." a good deal of theorising has been expended in accounting for the absence of all but traces of an atmosphere and water on the moon, which might have been avoided had astronomers not caught up the notion, and stuck to it, that it rotates on its axis once for every revolution that it makes round the earth. it might be difficult to find out with whom the notion originated; but perhaps it was first conceived to be the case by some celebrated astronomer, and has been accepted by almost all his successors without being properly looked into. any one who chose to take the trouble to study the matter thoroughly, would have easily discovered that the moon can have no rotation of any kind on its axis, and immediately afterwards have found out the reason why nothing beyond traces of air and water were to be seen on the side of it constantly turned towards the earth. this is another example we can give of erroneous ideas leading to erroneous and impossible conclusions, and preventing the truth from being discovered. that the rotation of the moon on its axis is stated to be a fact, by recognised and celebrated astronomers, will be seen from the following quotations. ( ) sir john herschel, in his "treatise on astronomy," new edition of , says at page : "the lunar summer and winter arise, in fact, from the rotation of the moon on its own axis, the period of which rotation is exactly equal to its sidereal revolution about the earth, and is performed in a plane ° ´ ´´ inclined to the ecliptic, and therefore nearly coincident with her own orbit. this is the cause why we always see the same face of the moon, and have no knowledge of the other side." ( ) in his "poetry of astronomy," page , mr. proctor says: "for my own part, though i cannot doubt that the substance of the moon once formed a ring around the earth, i think there is good reason for believing that when the earth's vaporous mass, receding, left the moon's mass behind, this mass must have been already gathered up into a single vaporous globe. my chief reason for thinking this is, that i cannot on any other supposition find a sufficient explanation of one of the most singular characteristics of our satellite--her revolution on her axis in the same mean time, exactly, as she circuits around the earth." ( ) professor newcomb, in his "popular astronomy," th edition, , at page , has what follows: "the most remarkable feature in the motion of the moon is, that she makes one revolution on her axis in the same time that she revolves around the earth, and so always presents the same face to us. in consequence, the other side of the moon must remain for ever invisible to human eyes. the reason for this peculiarity is to be found in the ellipticity of her globe." then he enlarges upon and confirms the fact of her rotation. ( ) mr. george f. chambers, in his "handbook of astronomy," th edition, , says at page , vol. i.: "in order that the same hemisphere should be continually turned towards us, it would be necessary not only that the time of the moon's rotation on its axis should be precisely equal to the time of the revolution in its orbit, but that the angular velocity in its orbit should, in every part of its course, exactly equal its angular velocity on its axis." it may be necessary, to avoid misconception, to note that angular velocity on its axis confirms rotation; and what is more extraordinary, that chambers must have thought that its angular velocity on its axis must have increased and diminished in order to agree with its increased and diminished velocities in its elliptic orbit at its perigee, apogee, and quadratures. a rather strange notion in mechanics where there is no provision made for acceleration or retardation of rotation. ( ) dr. samuel kinns, in "moses and geology," twelfth thousand, , says at page , "the same side of its (the moon's) sphere is always towards us. this could only happen by its having an axial rotation equal in period to its orbital revolution, which is d. h. m. s." ( ) in the "story of the heavens," sir robert s. ball informs us, in the fifteenth thousand, , page , "that the moon should bend the same face to the earth depends immediately on the condition that the moon should rotate on its axis in precisely the same period as that which it requires to revolve around the earth. the tides are a regulating power of the most unremitting efficiency to ensure that this condition should be observed." ( ) and finally we have what follows from messrs. newcomb and holden, at page of their work already referred to at page , "the moon rotates on her axis in the same time and in the same direction in which she moves around the earth. in consequence, she always presents very nearly the same face to the earth." and in a footnote to this consequence, add: "this conclusion is often a _pons asinorum_ to some who conceive that, if the same face of the moon is always presented to the earth, she cannot rotate at all. the difficulty arises from a misunderstanding of the difference between a relative and an absolute rotation. it is true that she does not rotate relatively to a line drawn from the earth to her centre, but she must rotate relative to a fixed line, or a line drawn to a fixed star." in six of the above cases it is distinctly maintained that the moon rotates once on its axis in the same time that it makes one revolution round the earth, and that it is in consequence of this rotation that it always presents the same side to the earth. thus we feel authorised to conclude that their authors did either believe that it does so rotate, or that they entertained some confused idea on the subject, which they did not take the trouble to examine properly, but accepted as a dogma, because some predecessor, with a great name, had stated that such rotation was necessary in order that its same side should be always turned towards the earth. in the seventh case the authors, while actually making the same assertion, try to persuade those who they acknowledge can see that the moon does not rotate on its axis in any sense, that their difficulty in comprehending what is meant by rotation, arises from the misunderstanding of the difference between an absolute rotation and one relative to a line drawn to a fixed star. but they do not attempt to show how this relative rotation has anything to do with or has any effect in causing the moon to present always the same side to the earth; and leave the story in the same confused state, out of which nobody can draw any satisfactory conclusion. also, though they distinctly recognise that it does not rotate relatively to a line drawn from the surface of the earth to its centre, they do not include in their general description of the moon anything in any way connected with what would be the consequences of its not really rotating on its axis relatively to the earth. so they leave us the problem in much the same state as they found it, and it is still necessary to show that there can be no actual rotation of any kind on its axis; and the worst of it is that it is a thing that will have to be done in such very plain language that it will compel people to think of the absurdity of the idea so generally accepted. to begin, it is very difficult to comprehend what the authors, above alluded to, meant by saying that the moon "must rotate relative to a fixed line, or a line drawn to a fixed star." it may mean relative to the line itself or to the star to which it is drawn. if it is to the line itself we cannot form any notion of what direction the rotation will have, direct, retrograde, or otherwise; and if it is relative to the star itself, then we can see that the relative rotation must depend on what is the position of the star. should it be placed in the "milky way," we can understand how the moon could show every side it has--almost, not quite--to the star during every revolution it makes round the earth, and how they may look upon it as a relative rotation. but if we draw the line to the pole star we cannot see how the moon can show every side it has to it in every revolution round the earth, so there can be no relative rotation in that case--and the "almost, not quite," applies to every star between the pole and the ecliptic. the moon shows only the northern hemisphere, or a little more due to libration of its own kind, to that star, and would have to remove its poles to the equator, and make a new departure, in order to show the whole of its surface to that star in every revolution round the earth. thus it is clear that the explanation given us of the relative rotation, is evidently one of the kind not properly thought out to the end. no one has ever said, or perhaps even thought, that a gin-horse makes one rotation on his vertical axis, in the same time as he makes a circuit round his ring, but, all the same, he keeps his same side always towards the gin, or mill, he is giving motion to. the proof that he does not make any such rotation is easy--no proof is really required. but, suppose he is giving motion to a whim for raising ores from a mine, and that his motion is what is called direct. when the cage containing the ore is brought to bank, is emptied, and has to be lowered into the mine again, the horse has then to reverse his motion to retrograde, in doing which he has to make a half rotation on his vertical axis, and turn his other side to the whim. when again the cage has to be raised to bank, he has to resume his direct motion, for which he has to make another half rotation on his vertical axis, but it is this time in the opposite direction. thus it is shown that he can only make half rotations, under any circumstances, on his axis, and these in opposite directions, when he changes his motion from direct to retrograde, or _vice versâ_; and that, when he moves in only one direction he cannot make even one rotation on his vertical axis, however long he may travel round the mill. in the same manner the moon which never turns back in its orbit can never make even one half rotation on its axis, which is all that we have had to prove. it is hardly necessary to observe that its axis is nearly parallel to the earth's, just the same as the horse's is to that of the whim. neither could any one say that the relative rotation of the horse to a star, or tower, or, say, a bridge, outside of his ring, could have any effect on his revolution round the mill, or his always keeping his same side to it, there being no mechanical connection between them, nor any law of attraction; and the same is the case between the moon and a fixed star. now, we may begin to consider what effects must be produced by the moon not rotating on its axis, and we can do so most easily by continuing to work with our gin horse, or some equivalent substitute. it would not cost a great deal of ingenuity to plant a steam engine in the centre of the mill he is supposed to be driving, and to drive with it not only the mill but the horse also at the end of his lever. there might be some dissipation--professor tate would call it degradation--of energy in such an experiment, but we could get over that by making _divina palladis arte_ a wooden horse. we might arrange the steam-engine so as to cause the mill to make - / revolutions for one made by our wooden horse, and so have a sort of a model of the earth and moon performing their most important relative motions. then, having got our model ready for action, instead of filling it _armato milite_ we might fill it half full of water. we fill it only half full, because the armed soldiers could not lie on the top of each other in the _other_ horse, and there would be a vacant space above them for air, thus making the resemblance between the two the more similar; and also because it suits our purpose better, as will soon be seen. we have still to propose that a lot of holes should be supposed to be made in the sides of _our_ horse all round, just a little higher than between wind and water. _pallas_ did not order any holes to be made in _hers_ as far as we know, even for ventilation, though we think it would have been an advantage; but that will not spoil the experiment we are now prepared for. let the steam-engine be started now and we shall soon see what will happen to the water. as the speed increases it will not be long till it begins to be thrown out, not from the side turned towards the mill but from the one furthest from it; and if it is increased sufficiently the whole of it will be very soon thrown out. if we could now close up the holes on the side of the horse turned towards the mill, it would so happen that a good deal of the air would be expelled also; and if the speed of the horse were brought up so as to equal that of the moon in its orbit, there would be nothing more, at the most, than traces of air left even in it. the expelling agent in this experiment would, of course, be centrifugal force, and we do not need to exercise our mental faculties very greatly, to comprehend that it is the same force that has driven both air and water away from the side of the moon always turned towards the earth. all the difficulty we have to contend with will be to make sure that the orbital velocity of the moon is sufficient to produce the force required. that the force is exceedingly greater than what is required is proved by the fact, that the velocity with which the moon travels in its orbit is a little more than miles per minute, whereas the velocity of the circumference of a centrifugal machine, used for clarifying sugar, drying clothes, or any other similar industrial purpose, does not require a greater velocity than about _one_ mile per minute, in order to throw everything in the form of water out of the material to be dried, and out of the centrifugal machine itself; and we know that air would be expelled more easily than water, were none re-admitted to supply the place of what was expelled. here the idea very naturally occurs to any one, that so great a velocity would drive both air and water away, even from the far off side of the moon, into space, but in order to do so the velocity would have to be , not , miles per minute. our authority for this statement will be found in "the nineteenth century," for august , in an article written by prince kropotkin, in which he says: "but it appears from dr. johnstone stoney's investigations that even if the moon was surrounded at some time of its existence with a gaseous envelope consisting of oxygen, nitrogen and water vapour, it would not have retained much of it. the gases, as is known, consist of molecules rushing in all directions at immense speeds; and the moment that the speed of a molecule which moves near the outer boundary of the atmosphere exceeds a certain limit (which would be about , feet in a second for the moon) it can escape from the sphere of attraction of the planet. molecule by molecule the gas must wander off into interplanetary space; and the smaller the mass of the molecule of a given gas, the feebler the planet's attraction, and this is why no free hydrogen could be retained in the earth's atmosphere, and why the moon could retain no air or water vapour." a velocity of , feet per second is as near miles per minute as there is any use for, which is more than three times as great as the velocity of the moon in its orbit, so there is no possibility whatever of air and water having been swept away from the far off side of it by centrifugal force; more especially as it ought to be well known that that force is always counteracted by the attractive force of the satellite for these or any other elements. we do not want to discuss the point of whether the mutual collisions of the molecules of a gas could get up such a velocity as would enable them to free themselves from the attraction of the moon, for it looks to us too much like one of those notions that are got up to account for something that does not exist; but we do want to state our dissent to the conclusion--evidently jumped at--that because there are hardly any signs of there being air or water on our side of the moon, there can be none on the other. no astronomer, physicist, scientist of any kind, can prove that there is none, simply because he has never been round there to see or make experiments to prove it; and if there is any one bold enough to make such an assertion, it is only an example of how stupendous a jump to a conclusion can be made. when we first read, many years ago, some of the reasons given for there being no water visible on the side of the moon constantly turned to the earth, one of which was that if there ever had been any it must have been absorbed into its body during the process of cooling and consolidation; and when we had convinced ourselves, by placing two oranges on two ends of a wire and revolving the one round the other, that the moon did not rotate on its axis in any sense whatever, we came to the conclusion that both water and air could be removed to the far off hemisphere by centrifugal force. we thought this so simple, so self-evident, and so indisputable an explanation, that every one who had read what we had read must have come to the same conclusion; so that we were not a little surprised when we saw it stated by "the times" of september , , in its first report of the meeting of the british association for that year, that sir robert ball had suggested, some time previously, that the "absence of any atmosphere investing the moon is a simple and necessary consequence of the kinetic theory of gases." this at once made us suspect that the theory--our theory--must have been new, but we could not altogether believe it. it seemed to us passing strange that it should not have occurred to astronomers, from the moment they discovered that they could not find any, or hardly any, traces of air or water on the only hemisphere they could examine; but it would appear from sir robert ball's suggestion, being even discussed at that meeting, that the notion of their having been removed simply by centrifugal force to the unseen hemisphere, had never been entertained by, to say the least, any one who was present at that discussion. not satisfied with this conclusion, we proceeded to examine all the books, journals, magazines, and _papers_ we could get hold of, to see whether we could find any indication of such a conception having been published previously, and the nearest approach to anything of the kind having been conceived of by anyone, we found in chambers's work--already referred to--at page , vol. i., where we read, "professor hansen has recently started a curious theory from which he concludes that the hemisphere of the moon which is turned away from the earth may possess an atmosphere. having discovered certain irregularities in the moon's motion, which he was unable to reconcile with theory, he was led to suspect that they might arise from the centre of gravity of the moon not coinciding with the centre of figure. pursuing this idea, he found upon actual investigation that the irregularities could be almost wholly accounted for by supposing the centre of gravity to be at a distance of - / miles _beyond_ the centre of figure. assuming this hypothesis to be well founded, professor hansen remarks that the hemisphere of the moon, which is turned towards the earth, is in the condition of a high mountain, and that consequently we need not be surprised that (little or) no trace of an atmosphere exists; but that on the opposite hemisphere, the surface of which is situated _beneath_ the mean level, we have no reason to suppose that there may not exist an atmosphere and consequently both animal and vegetable life. professor newcomb has disputed these conclusions of hansen, which it is obvious must be very difficult of either proof or disproof." what professor newcomb's objections to the conclusions of hansen were we do not know, but we do know that mr. proctor also objected to the "curious theory," as it is called by mr. chambers. in his "poetry on astronomy," he discusses pretty fully the withdrawal of water from the surface of the moon during the process of cooling and condensation, ascribing the conception of it to four independent authors, namely, seeman, a german geologist, frankland in england, stanislas mennier in france, and sterry hunt in america; and in a footnote, at page , says of hansen's theory: "the idea was that the moon, though nearly spherical, is sometimes egg-shaped, the smaller end of the egg-shaped figure being directed towards the earth. now, while it is perfectly clear that on this supposition the greater part of the moon's visible half would be of the nature of a gigantic elevation above the mean level, and would, therefore, be denuded (or might be denuded) of its seas and denser parts of the air covering it, yet it is equally clear that all around the base of this monstrous lunar elevation, the seas would be gathered together, and the air would be at its densest. but it is precisely round the base of this part of the moon or, in other words, round the border of the lunar hemisphere, that we should have the best chance of perceiving the effects of air and seas, if any really existed; and it is because of the absolute absence of all evidence of the kind, that astronomers regard the moon as having no seas and very little air." had the idea of centrifugal force ever occurred to mr. proctor, he could not have written this last sentence; for he could not have failed to see that "the border of the visible lunar hemisphere" would be the very place, from which it could most easily remove air and water, after they had got so far down the monstrous elevation; because there it--the centrifugal force--would be acting at right angles to the moon's attraction, instead of having to contend against it, as it would have to do in a constantly increasing degree until it arrived at its maximum, just in proportion to the distance the air and water got down to the similar monstrous _depression_ on the other hemisphere, down which the gradient would start off under the most favourable circumstances possible. from what has been said, it is very evident that neither hansen, chambers, proctor, nor any of those whose names have been mentioned by the last, in connexion with the withdrawal of water into the body of the moon by absorption, while cooling and condensing, had ever thought of the possibility of air and water having been removed by centrifugal force from the side of the moon turned towards the earth. that it should not have occurred to hansen seems passing strange, seeing that he had conceived the idea of their possible existence on the hemisphere turned away from the earth, which could hardly fail to make him think of how they got there, and could exist only there; and the only explanation of his not having perceived the true cause seems to be, that his thoughts were hampered by a sort of confused notion that the moon actually rotates on its axis once for every revolution it makes around the earth, that being, as it were, one of the dogmas of astronomic belief, handed down from some great authority of times past, and never properly inquired into. we do not want to question the suggestion, that the absence of any atmosphere investing the moon is a simple and necessary consequence of the kinetic theory of gases--though we see that a good deal could be argued against it--as we do not consider it to be necessary--neither the questioning nor the theory. we have demonstrated clearly, how both air and water could be removed from the side of the moon constantly shown to us, and that is sufficient for our purpose both now and later on; besides it would appear that the moon really has some sort of an atmosphere somewhere. following up the quotation, made at page , from prince kropotkin's article in the "nineteenth century" as being the latest information we have on the subject, we are told that "a feeble twilight is seen on our satellite, and twilight is due, as is known, to the reflection of light within the gaseous envelope; besides it has been remarked long since at greenwich that the stars which are covered by the moon during its movements in its orbit remain visible for a couple of seconds longer than they ought to be visible if their rays were not slightly broken as they pass near the moon's surface. consequently it was concluded that the moon must have an atmosphere" ... and: "the observations made at lick, paris, and arequipa, fully confirm this view. a twilight is decidedly visible at the cusps of the crescent-moon, especially near the first and last quarters. it prolongs the cusps as a faint glow over the dark shadowed part, for a distance of about miles ( "), and this indicates the existence of an atmosphere having on the surface of the moon the same density as our atmosphere has at a height of about forty miles." what is of interest for us to know is where that "feeble twilight," or, "reflection of light within the gaseous envelope," is seen. whether it is at what mr. proctor calls "the border of the visible lunar hemisphere," on this side of it, or beyond it. it cannot be a difficult matter to decide. it must be beyond it, for the following reasons: if the atmosphere has been driven away to the far-off hemisphere of the moon by centrifugal force, its natural tendency would be to spread out immediately after it had passed the visible border where we have said the centrifugal force would be acting most effectively. also, if all the air at one time belonging to our side of the moon has been driven away to the other, that side must have a double allowance of atmosphere, which, though it does not increase its density at the surface, on account of the centrifugal force, will double its volume, and enable it to extend to a greater proportionate distance in all directions from the border and from the far-off hemisphere. in this way there must be a considerable wedge of atmosphere illuminated by the sun, and visible past the edge of the moon's disc, to reflect a feeble twilight--perhaps something stronger--towards the earth, and to intercept the light of a star before its edge and that of the moon come into actual apparent contact. but before the wedge becomes thick enough to reflect that light, the reflecting part must be far beyond the edge of the moon's disc. perhaps the feeble light might be seen more clearly when looked for in the proper place; quite possibly hundreds of miles beyond the disc. in order to make more clear the truth of what we have said about water and air--and more especially the latter--being thrown away to the far-off side of the moon by centrifugal force, we may add the following details: if the force of gravity at its surface is one-sixth part of what it is at the surface of the earth, the pressure of an atmosphere there would be · lb. per square inch, if it rotated on its axis; but as it does not so rotate and is subjected to centrifugal force, the pressure of an atmosphere will vary according to the part of it over which it exists. on the nearest part of the side turned towards the earth, gravity, which we have just seen must be equal to · lb., would be acting in the same direction as centrifugal force, which in its turn is equal to · lb. or thereby, and the whole would be · lb. per square inch tending to drive off air and water to the far-off hemisphere. but from that place, gravity would gradually diminish its aid till it came to be nil at the disc separating the two hemispheres, where it would have no effect whatever as it would be acting at right angles to centrifugal force, and this would be reduced to · lb. per square inch. then, from the edges of the disc forward, on the far-off hemisphere, gravity would begin to act against centrifugal force, or rather _vice versâ_, until it, gravity, got reduced to · lb. per square inch. also, as that hemisphere must have a double portion of air or atmosphere on it, and as its pressure on any part of it cannot be greater than the · lb. just mentioned, we can imagine that the double quantity will hang closer to the surface than if there was only one portion. such being the case the atmosphere would spread out much more rapidly than would be represented by the extension of a triangle starting from the earth and reaching beyond the moon's disc to the farthest limit of the atmosphere; and thus the wedge, which we have supposed to be visible beyond the edges of the disc may come to have a very considerable thickness. what that thickness may be, and up to what distance beyond the disc the density of the wedge would be sufficient to reflect the light of the sun, it would be very difficult to calculate, but we think it might possibly extend even as far as one-fourth of the radius of the moon--because at that point the force of gravity pulling it towards the centre, or the axis, would be very small, and its distance from the axis would be little less than the radius, not over miles--and cause it to project over the edges as far, to appearance, as the miles ( ") that have been observed at greenwich. this reflected light must be all round the moon--not at the cusps only of the crescent-moon--and it has occurred to us that it may, most probably does, account for the appearance of what we call "the old moon in the young moon's arms." we know what effect the "earth-shine" has upon the moon at its change, and the brighter _ring-shine_ just outside of it, may very well be caused by the sunlight reflected from the atmosphere far beyond the visible limit of the hemisphere turned to us. in support of this suggestion we may refer to professor c. a. young's description, in his "sun," p. , of one particular feature observed at the time of a total eclipse of the sun. he says:--"on such an occasion, if the sky is clear, the moon appears of almost inky darkness, with just a sufficient illumination at the edge of the disc to bring out its rotundity in a striking manner. it looks not like a flat screen, but like a huge black ball, as it really is. from behind it stream out on all sides radiant filaments, beams, and sheets of pearly light, which reach to a distance sometimes of several degrees from the solar surface, forming an irregular stellate halo, with the black globe of the moon in its apparent centre." there can be little doubt, we think, from what is said here, that professor young looks upon this "illumination of the edge of the disc" as pertaining to the moon, and upon the "radiant filaments, beams," etc. behind it as belonging to the sun. and in that case the illumination can only be caused by the light of the sun, refracted by the atmosphere belonging to the hemisphere of the moon that is never seen from the earth. we have taken it for granted in what we have been doing, that the moon has really rotated on its axis, and to some purpose, at some former period of its existence. some people think otherwise, or that there is at least a doubt about it; we cannot see even the shadow of a doubt. all that we need to say in support of our opinion is, that there is no other conceivable way of accounting for its perfectly circular form. all the planets are circular, or spheroidal--to speak more correctly--in form, admittedly in consequence of rotation on their axes; and if one or two of jupiter's satellites are not completely circular or spheroidal, it does not stretch our conscience very much to suppose that it is because they have not yet been rotated into form. saturn apparently has satellites still in the form of rings, and there can be nothing out of the way in supposing that all of jupiter's are not yet licked into shape. the fact that there is no appearance of compression on the moon makes us think of why there is none, and the only explanation that occurs to us is, that, as its rotation must have come to an end gradually, the compression it must have had when rotating must have disappeared gradually also, by reason of the differences of force in the equatorial and polar attractions, drawing in the bulged out, and thus forcing out the compressed parts. this is a notion that will be scoffed at by those who have always thought, and maintained, that the earth acquired its present form when in a liquid state; but they have not thought this supposition--for it is nothing else--out to the very end. several reasons could easily be given against their opinion, among others the variations in rate of rotation we so frequently see used in favour of other notions; but we shall content ourselves with the best one of all, which is this: the pressures in the interior of the earth must be so enormous that they are quite sufficient to compress steel, or adamant if that is supposed to be more resistant, into any shape whatever, almost as if it were dough, and there can be no doubt--mathematics notwithstanding--that the earth has the form, to-day, due to its present rate of rotation. we shall have to return to this subject some time hence, if we live to complete what we have taken in hand. how many things there are, in what is considered to be astronomical science, that have not been properly thought out to the end, and to what strange notions they have given rise! this one of the rotation of the moon which we have been discussing, has evidently given occasion for the conception of the theory that the absence of atmosphere and seas from the moon is the natural consequence of the kinetic theory of gases; and the author of the theory, and its supporters, have never, apparently, taken the trouble to think whether their absence from the near hemisphere is a satisfactory and convincing proof of there not being any air or water on the far-off one. in what we have proposed to write many similar examples of want of study will be met with, but we do not intend to call special attention to them, unless it be in cases where we consider it to be of some importance to do so. in fact we have already been working on that plan. chapter iii. page remarks on some of the principal cosmogonies. ancient notions. the nebular hypothesis of laplace. early opinions on it. received into favour. again condemned as erroneous. defects attributed to it as fatal. new cosmogonies advanced. dr. croll's collision, or impact, theory discussed. dr. braun's cosmogony examined. m. faye's "origine du monde" defined. shown to be without proper foundation, confused, and in some parts contradictory. reference to other hypotheses not noticed. all more or less only variations on the nebular hypothesis. necessity for more particular examination into it. we have thought it worth while to dedicate this chapter to some remarks on cosmogonies in general, and examination into a very few conceived by eminent men; these forming in our opinion the most attractive matter for those readers who do not pretend to make a study of astronomy, but are very desirous to have some knowledge of the most plausible ideas which have been conceived by astronomers, of how the universe, and more particularly the solar system, were brought into existence; while, at the same time, they are the subjects on which more crude conceptions, more limited study, and more fanciful unexamined thought have been expended, than any others we have met with. some readers will, no doubt, be able to reject what is erroneous, to speak mildly, but there will be, equally surely, some who cannot do so; and it must be confessed there are a good many to whom the most complicated conceptions, and the most difficult of comprehension, are the most attractive. a great many centuries ago, astronomers and philosophers had already conceived the idea that the sun and stars had been formed into spherical bodies by the condensation of celestial vapours; but when the telescope was invented, and the nature of nebulæ in some measure understood, it was not long till it came to be thought that the matter, out of which the sun and stars were formed, must have been much more substantial in its nature than celestial vapours. being visible, they were naturally considered to be self-luminous, and consequently endowed with great heat, because the self-luminous sun was felt to be so endowed, though perhaps not with the same degree. accordingly, astronomers began to form theories, or hypotheses, on the construction of the solar system out of a nebula, which, like everything else, went on each one improving on its predecessor as, through continued observation and study, more knowledge was acquired of the nature of nebulæ. the most notable of these cosmogonists were descartes, newton, kant, and laplace, each of whom contributed valuable contingents to the general work; which may be said to have culminated about a century ago in the nebular hypothesis of the last-named; for the many attempts that have been made to improve upon it, or to supplant it altogether, have been very far from successful. the hypothesis is about a century old, as we have said, and there may still be many people who can remember having heard it denounced as a profane, impious, atheistic speculation, for it is not over half a century since the ban begun to be taken off it. sir david brewster, in his "life of newton," said of it, "that the nebular hypothesis, that dull and dangerous heresy of the age, is incompatible with the established laws of the material universe, and that an omnipotent arm was required to give the planets their positions and motions in space, and a presiding intelligence to assign to them the different functions they had to perform." with others, its chief defect was that the time required to form even the earth in the manner prescribed by it, must have been infinitely greater than six days of twenty-four hours each. in the meantime, geologists had also discovered that, for the formation of the strata of the earth, which they had been examining and studying, the time required for their being deposited must have been, not days of twenty-four hours, but periods of many millions of years each; and the evidence adduced by them that such must have been the case was so overwhelming, that theology had to acknowledge its force, and gradually to recognise that the days must have been periods of undefinable length. thus relieved from the charge of heresy, the hypothesis rose rapidly into favour, and came to be generally accepted by the most eminent astronomers, subject always to certain modifications, which modifications have never been clearly defined, if at all. it was not, however, allowed to enjoy long the exalted station to which it had attained. astronomers had begun to consider from whence the sun had acquired the enormous quantity of heat it had been expending ever since the world began, and, after long discussion, had come to the conclusion that by far the greatest source must have been the condensation from the nebulous state of the matter of which it is composed. having settled this point, it was calculated that the amount of heat derived from that and all other sources could not have kept up its expenditure, at the present rate of consumption, for more than twenty million years, and could not maintain it for more than from six to eight million years in the time to come. owing in good part to this great difference between the calculations of astronomers and geologists about the age of the earth, the hypothesis began again to suffer in repute, and then all its faults and shortcomings were sought out and arrayed against it. the chief defects attributed to it were: the retrograde motion of rotation of uranus and neptune and revolution of their satellites--that fault in the former having been noted by sir john herschel, in his treatise on astronomy already cited; the discovery of the satellites of mars which exposed the facts, that the inner one revolves round the planet in less than one-third of the time that it ought to, and that the outer one is too small to have been thrown off by mars, in accordance with the terms of the hypothesis; the exclusion from it of comets, some of which at least have been proved, in the most irrefutable manner, to form part of the solar system; and what can only be called _speculations_, on the formation of a lens-shaped nebula brought about by the acceleration of rotation--caused by condensation according to the areolar theory--which it is supposed would be enormously in excess of the actual revolution of the inner planets, and of the rotation of the sun. here we must protest against retrograde motion of rotation in any of the members of the solar system being considered as militating against the theory, because laplace states distinctly, while explaining his hypothesis, that the rotation of the earth might just as well have been retrograde as direct: a fact that some eminent astronomers have not noticed, simply because they have not paid proper attention to what they were reading. we shall have to return to this statement again, and to present the proof of its being true. an idea of how far the hypothesis had fallen into disrepute may be formed from the following extract, from "nature" of august , , of a review of a "new cosmogony," by a. m. clerke, in which it is said: "but now the reiterated blows of objectors may fairly be said to have shattered the symmetrical mould in which laplace cast his ideas. what remains of it is summed up in the statement that the solar system did originate somehow, by the condensation of a primitive nebula. the rest is irrecoverably gone, and the field is open for ingenious theorising. it has not been wanting.... the newer cosmogonists are divided into two schools by the more or less radical tendencies of the reforms they propose. some seek wholly to abolish, others merely to renovate the kant laplace scheme. the first class is best represented by m. faye, the second by mr. wolfe and dr. braun"--the author of the "new cosmogony." we cannot pass this quotation without remarking "how glibly some people can write!" more we do not want to say about it, except that it gave us the notion to examine closely some of the new cosmogonies, _which have not been wanting_, to see whether they are better than laplace's. we have not had the opportunity of knowing what are mr. wolfe's amendments, but the review, just cited, gives us a pretty good notion of those of dr. braun, and we have been able to study carefully m. faye's "origine du monde," in which he considers the solar system to have been evolved from cosmic matter partially endowed with motion in the form of eddies, whirlwinds, vortices, or _tourbillons_, which last may comprehend all of them, and even more. we have also studied, with some surprise, in "climate and cosmology" dr. croll's impact, or collision, theory, and will confine our examination to the three of which we know something, beginning with dr. croll's, which we believe to be the oldest of the three. we understand that dr. croll accepts the nebular hypothesis in all its main features, including the intense heat in which the original nebula is supposed to have existed from the beginning; and has only invented the collision theory in order to increase its quantity, to suit the demands of geologists for unlimited time, by showing how an unlimited supply of both heat and time may be obtained. but he has incurred an oversight in not taking into consideration the kind of matter in which that unlimited supply of heat was to be stored up--whether it would hold it. he wrote in times when something was really known about heat, and we cannot suppose him to have believed that heat could exist independent of matter, or that a gas or vapour could be heated to a high temperature except under corresponding pressure; but he has evidently overlooked this point, his thoughts recurring to old notions; and he has fallen, probably for the same reason, into other oversights equally as grave. when showing how a supply of fifty millions of years of sun-heat could be produced from the collision of two half-suns colliding with velocities of miles per second, dr. croll says in his "discussions on climate and cosmology," of , at page : "the whole mass would be converted into an incandescent gas" (the handmaid of the period), "with a temperature of which we can have no adequate conception. if we assume the specific heat of the gaseous mass to be equal to that of air (viz. · ), the mass would have a temperature of about , , ° c., or more than , times that of the voltaic arc." now, let us suppose the whole mass of the whole solar system to be converted into a gas, or vapour, at the pressure of our atmosphere, and temperature of ° c., its volume would be equal to that of a sphere of not quite , , miles in diameter. suppose, then, this volume to be heated to , , ° c. in a close vessel, as would necessarily have to be the case, the pressure corresponding to that temperature would be , , atmospheres, according to the theory on which the absolute zero of temperature is founded. without stopping to consider whether air or any gas could be heated to the temperature mentioned; or the strength of the vessel , , miles in diameter required to retain it at the equivalent pressure; if we increase the diameter of the containing sphere to a little more than that of the orbit of neptune, or, say , , , miles, and allow the air or gas or vapour to expand into it; then, as the volume of the new sphere will be greater than the former one in the proportion of , , cubed to , , , cubed, or as is to , , , the pressure of the gas will be reduced to , , divided by , , , that is just over the th part of an atmosphere; which, in its turn would correspond to a temperature of a very little more than - °, or what is considered to be[a] ° c. above absolute zero of temperature; or, at all events, to the temperature of space, whatever that may be. [a] this temperature is altogether erroneous, as we shall show in due time; at present our proof would not be accepted without a demonstration, for which we have not sufficient data. dr. croll goes on to say at page : "it may be objected that enormous as would be such a temperature, it would nevertheless be insufficient to expand the mass against gravity so as to occupy the entire space included within the orbit of neptune. to this objection it might be replied, that if the temperature in question were not sufficient to produce the required expansion, it might readily have been so if the two bodies before encounter be assumed to possess a higher velocity, which of course might have been the case. but without making any such assumption, the necessary expansion of the mass can be accounted for on very simple principles. it follows in fact from the theory, that the expansion of the gaseous mass must have been far greater than could have resulted simply from the temperature produced by the concussion. this will be obvious by considering what must take place immediately after the encounter of the two bodies, and before the mass has had sufficient time to pass completely into the gaseous condition. the two bodies coming into collision with such enormous velocities would not rebound like two elastic balls, neither would they instantly be converted into vapour by the encounter. the first effect of the blow would be to shiver them into fragments, small indeed as compared with the size of the bodies themselves, but still into what might be called in ordinary language immense blocks. before the motion of the two bodies could be stopped, they would undoubtedly interpenetrate each other; and this of course would break them up into fragments. but this would only be the work of a few minutes. here then we should have all the energy of the lost motion existing in the blocks as heat (molecular motion), while they were still in the solid state; for as yet they would not have had time to assume the gaseous condition. it is obvious, however, that the greater part of the heat would exist on the surface of the blocks (the place receiving the greatest concussion), and would continue there while the blocks retained their solid condition. it is difficult in imagination to realize what the temperature of the surfaces would be at this moment. for supposing the heat were uniformly distributed through the entire mass, each pound, as we have already seen, would possess , , , foot-pounds of heat. but, as the greater part of the heat would at this instant be concentrated on the outer layers of the blocks, these layers would be at once transformed into the gaseous condition, thus enveloping the blocks and filling up the interstices. the temperature of the incandescent gas, owing to this enormous concentration of heat, would be excessive, and its expansive force inconceivably great. as a consequence the blocks would be separated from each other, and driven in all directions with a velocity far more than sufficient to carry them to an infinite distance against the force of gravity were no opposing obstacle in the way. the blocks, by their mutual impact, would be shivered into small fragments, each of which would consequently become enveloped in incandescent gas. these smaller fragments would in a similar manner break up into smaller pieces, and so on until the whole came to assume the gaseous state. the general effect of the explosion would be to disperse the blocks in all directions, radiating from the centre of the mass. those towards the circumference of the mass, meeting with little or no obstruction to their outward progress, would pass outwards into space to indefinite distances, leaving in this manner a free path for the layers of blocks behind them to follow in their track. thus eventually a space, perhaps twice or even thrice that included within the orbit of neptune, might be filled with fragments by the time the whole had assumed the gaseous condition. "it would be the suddenness and almost instantaneity with which the mass would receive the entire store of energy before it had time even to assume the molten, far less the gaseous condition, which would lead to such fearful explosions and dispersion of the materials. if the heat had been gradually applied, no explosions, and consequently no dispersion of the materials would have taken place. there would first have been a gradual melting; and then the mass would pass by slow degrees in vapour, after which the vapour would rise in temperature as the heat continued, until it became possessed of the entire amount. but the space thus occupied by the gaseous mass would necessarily be very much smaller than in the case we have been considering, where the shattered materials were first dispersed in space before the gaseous condition could be assumed." we have made this very long quotation; first, because we have not been able to condense it without running the risk of not placing sufficiently clearly the whole of the argumentations employed in it; secondly, because the purport of the whole explanation set forth is evidently to demonstrate that, by means of the explosions of gases produced by the collision, the matter of the whole mass would be more extensively distributed into space--bearing heat along with it--than were it gradually melted and converted into vapour; and thirdly, because every argument advanced in favour of the theory of explosions, if carefully looked into, brings along with it its testimony that it has not been studied thoroughly out to the end. thus the quotation in a great measure saves us that labour. dr. croll seems sometimes to demand more from the laws of nature than they can give. he says, at p. of the work cited, that the expansion of the gaseous mass, produced by the collision of the two bodies, must have been far greater than could have resulted simply from the temperature produced by the concussion; and goes on to show how it--the expansion--might be caused by explosions of gases blowing out blocks of matter in all directions to indefinite distances. but he forgets that these explosions of gases would consume a great part of the heat they contained, that is, turn it into motion of the blocks, and so diminish the quantity produced by the collision, just in proportion to the velocities given to the masses of all the blocks blown out; so that what was gained in expansion would be lost in heat, and the object aimed at--of producing heat for the expenditure of the sun--so far lost. also, that, were the thing feasible, the blocks could not carry with them any of the heat of the exploded gases that might not be used up, and that the heat contained in them derived from the concussion would have time in their flight--about two hours at miles per second--to melt the matter composing them and turn it into vapour, long before even the orbit of neptune was reached. the heat produced by the explosion of powder in a cannon gives the projectile all the impulse it can, and disappears; it is converted into motion. it does not cluster round the projectile, nor follow it up in its flight, nor push it through an armour plate when it pierces one. we cannot admit--for this reason--the possibility of a block of matter flying off into space, with a mass of heat clustering round it, like bees when swarming round a branch of a tree. thermodynamics does not teach us anything about a mass of heat sticking to the surface of a block of matter of any kind. if the heat were, at a given moment--that is, when motion was stopped--brought into existence uniformly throughout the entire mass, which, according to the law of conversion of motion into heat and _vice versâ_, would most assuredly be the case, and each pound of the mass possessed , , , foot-pounds of heat, it could not be heaped up on the outer layers of the blocks--it matters not whether this means the layers of the outside of the whole mass, or at the outsides of the blocks--for the energy of lost motion, converted into heat, must have existed at the centres of the blocks or masses just in as great force as it did at the surfaces when motion was stopped. if each pound of matter carried along with it , , , foot-pounds of heat, that given out by one pound at the centre of a block would be as great as that given out by one pound at its surface; and the pounds at the surface could not acquire any greater heat from a neighbouring pound, because its neighbour could have no greater quantity to give it. pounds of matter would be melted and vaporized, or converted into gas, just as readily at the centre of the mass or block as at its surface; and storing up of heat in the interstices of the blocks is rather a strange notion, because we are not at liberty to stow away heat in a vacuum. besides, it is impossible to conceive how anything in the shape of a block could exist in any part of the whole mass, long enough for it to be blown out into space as a block. but supposing that a block could exist, it would most notoriously be in a state of _unstable equilibrium_; and were it then to receive from an explosion of gas, an impulse sufficient to drive it off to the verge of the sun's power of attraction--or rather to a distance equal to what that is--which would imply a velocity of not less than miles per second, the shock would be quite sufficient to blow it into its constituent atoms. moreover, as already stated, the heat of the explosion of the gas required to give the impulse would be immediately converted into motion, and disappear; so that out of the heat produced by the stoppage of a motion of miles per second, that required to produce a motion of miles per second, in each one of the blocks blown out to the distance above mentioned, would be entirely lost to the stock of heat schemed for so boldly. of course, the less the distance from the centre the blocks were blown the less would be the loss, but the fact remains that there would be a loss instead of a gain of heat, in dispersing the matter of two half suns into space by explosions of gas. in fine, a given amount of heat will raise the temperature of a given amount of matter to an easily calculable degree, and no more; and if part of that is expended in expanding the volume of the matter, the whole stock of heat will be diminished by exactly the quantity required to produce the expansions. so that we come back to what we have said at page , viz., that when the matter and the heat of the collision of the two half suns were dispersed, under the most favourable circumstances, into a sphere of , , , miles in diameter, the mean density of the matter would be equal to about / th part of an atmosphere, and its temperature--what is called-- ° c. of absolute temperature, always considering the quantity of the heat to have been , , ° c. dr. croll says that if a velocity of miles per second were not sufficient to produce the quantity of heat required, any other necessary velocity might be supposed, but when we consider that his supply of , , ° c. would have to be increased to , , , ° c., in order to add ° c. of heat to the matter dispersed through a sphere of , , , miles in diameter, it seems unnecessary to pursue the subject any farther. we may now take a look at dr. braun's impact cosmogony, of which we know nothing beyond what is set forth in the review in "nature" already alluded to, but that is enough for our purpose. we understand that he extends his operations to the whole universe, which he conceives to have been formed out of almost unlimited, and almost imponderable, nebulous matter, not homogeneous, but with local irregularities in it, which "would lead to the breaking up of the nebula into a vast number of separate fragments." out of one of these fragments he supposes the solar system to have been formed. this fragment would contain local irregularities also, which through condensation would lead to the formation of separate bodies, and these bodies are supposed to have been driven into their present forms, and gyrating movements of all kinds, by centric and eccentric collisions among themselves, caused by their mutual attractions. of course anything can be supposed, but in a construction of this kind the idea is forced upon us of the necessity of the active superintendence of the creator, to create in the proper places and bring in the matter at the exact moment required, and to see that the collisions were directed with the proper degree of energy and eccentricity, to construct the kind of machine that was proposed. to this idea we have no objections whatever, but we would like to see the necessity for it acknowledged. perhaps dr. braun does acknowledge it, but the cosmogony is given to us, it would seem, to show what most probably was the original scheme of construction, and implying that no continual supervision and direction were required during the process. if dr. braun could show us some method of attraction, and suspension and variation of attraction, by which some of the separate bodies could be drawn towards each other so as to form a central mass, nebula, or sun, and to give it, by their impacts of collision, a rotary motion; and how others of the separate bodies could be formed and held in appropriate places, so as to be set in motion at the right moment; and how they were to be so set in motion without the direct action of the constructor, to revolve as planets around the central mass, we might be able to recognise that a mechanism such as that of the solar system might be brought into existence; but when we are left to discover all these requisites, and their _modus operandi_, we find that we might be as well employed in designing a cosmogony of our own. dr. braun indulges in somewhat startling numbers in temperature and pressure. he considers that the temperature of the sun, at the surface, may be from , ° to , ° c., and that it may reach to from ten to thirty million degrees at the centre. in this he may be right for anything we know to the contrary. when riding over a sandy desert, under an unclouded vertical sun, we could easily have believed anything of the central heat of such a fire, especially when we considered that it was at a distance of ninety-three millions of miles from us. but when he tells us that in the depths of the sun's interior the pressure reaches a maximum of two thousand millions of atmospheres, we "pull in resolution and begin to doubt." air at that pressure would have a density , , times that of water, or , times the mean density of the earth, and we should have a species of matter to ponder over, of which no physicist has ever as yet dreamt. we have been able to study m. faye's cosmogony in his work on "l'origine du monde," second edition of , and can give a better account of it than of dr. braun's. ( ) he repudiates almost all existence of heat in the cosmic matter he is about to deal with, recognising that its temperature must have been very near the point of absolute zero, and also that its tenuity must have been almost inconceivable; so tenuous that a cubic miriamètre of it would not contain more perhaps than · grammes in weight. and very properly, we think, he looks upon the solar systems as having, at one time, formed a part of the whole universe, all of which was brought into existence, created, more or less, about the same time. in this universe, he considers that the stars have been formed, as well as the sun, by the progressive concentration of primitive materials disseminated in space, which conception gives rise to a totally new notion of the most positive character: viz. that each star owes to its mode of formation a provision of heat essentially limited; that it is not permissible, as laplace thought he could do, to endow a sun with an indefinite amount of heat; and that what it has expended and what it still possesses, depend upon its volume and actual mass. and also that the primitive materials of the solar system were, at the beginning, part of a universal chaos from which they were afterwards separated, in virtue of movements previously impressed on the whole of the matter; and sums up his first ideas in the following manner or theorem: _"at the beginning the universe consisted of a general chaos, of extreme tenuity, formed of all the elements of chemistry more or less mixed and confounded together. these materials under the force of their mutual attractions were, from the beginning, endowed with diverse movements which brought about their separation into masses or clouds. these still retained their movements of rapid translation, and very gentle interior gyrations. these myriads of chaotic fragments have given birth, by means of progressive condensations, to the diverse worlds of the universe."_ ( ) so much for the formation of the universe, including, of course, the solar system, for which he acknowledges the necessity for the intervention of a creating power, because it is impossible to account for it simply by the laws of nature; and adds: it is unnecessary to say that the universe is an indefinite series of transformations, that what we see results logically from a previous condition, and thus necessary in the past as in the future; we cannot see how a previous condition could tend towards the immense diffusion of matter, to the chaos out of which the actual condition has arisen; and that it is, therefore, necessary to begin with a hypothesis, and postulate of god, as descartes did, the disseminated matter and the forces which govern it. ( ) from dealing with the universe, m. faye comes to the formation of an isolated star, and begins with an entirely ideal case, that of a spherical homogeneous mass, without interior movement of any kind, and concludes that the molecules would fall in straight lines towards the centre; that the mass would condense regularly without losing its homogeneity, and would end in producing an incandescent sphere perfectly immovable; and that that would be a star, but a star without satellites, without rotation, without proper movement. this not being what was wanted, he goes on to show how, previous to its separation and complete isolation from the universal chaos, such a mass would possess, and carry with it when separated, a considerable velocity of rotation, and would still retain the internal movements it had acquired from the attraction of the other masses with which it had been previously in contact; and how the molecules, drawn towards the centre in obedience to gravitation, would not fall in straight lines but in concentric ellipses. ( ) from this state of affairs, two very different results might arise. one, that the molecules might resolve themselves into a multitude of small masses without the centre acquiring a preponderating increase. the other, that the central condensation might greatly exceed the others, and there would be formed a central star accompanied by a crowd of small dark bodies. m. faye accepts the second result, in which case the ellipses described by the small bodies, now become satellites, would, as the central mass increased in preponderance, have one of their centres at the centre of the preponderating mass, and their times of revolution would vary from one to another in conformity to the third law of kepler. ( ) for the formation of the solar system m. faye finds that it is of little importance whether the movements of bodies around the sun be very eccentric or almost circular; the first cause is always the same. they arise from the eddies, _tourbillonnements_, they have brought with them from their rectilinear movements in the primitive chaos. but the circle is such a particular case of the ellipse, that we ought not to expect to see it realized in any system. it is therefore necessary that, among the initial conditions of the chaotic mass, one should be found which would prevent the gyrations, eddies, from degenerating into elliptical movements, and which has at first made right, and afterwards firmly preserved, the form, more or less circular, in all its changes. ( ) for the formation of circular rings he gives us the following conceptions: in order that a star should have companions, great or small, circulating round the centre of gravity of the system, it is necessary that the partial chaos from whence it proceeded should have possessed, from the beginning, a gentle eddying movement affecting a part of its materials. besides, if the partial chaos has been really round and homogeneous, we shall see that these gyrations must have taken up, and to some extent preserved, the circular form. he then requests the reader not to lose sight of the feeble density of the medium, in which a succession of mechanical changes are to be brought about; and not to conclude that that density was such that a cubic miriamètre of the space occupied by it might not contain grammes of matter, as he stated in the preceding chapter (we think he said grammes), but that it might contain only grammes or even less. and adds that in such a medium, the small agglomerations of matter which would be formed all through it, would move as if they were in an absolute vacuum, and any changes in them would be produced extremely slowly. ( ) then he goes on to say that the gyrating movements belonging to the chaotic mass, would have very little difficulty in transforming a part of a motion of that kind into a veritable rotation, if this last were compatible with the law of the internal gravitation; that it is the nature of that kind of masses to only permit, to the bodies moving in them, revolutions, elliptic or circular, concentric and of the same duration; that therefore notable portions of the gyrating matter could take the form and movements of a flat ring, turning around the centre with the same angular velocity, exactly as if this nebulous ring were a solid body; that all the particles which have the proper velocity in the plane of the gyrations, will arrange themselves under the influence of gravitation in a flat ring with a veritable rotation around the centre; that any other parts having velocities too great or too small, will move in the same plane, describing ellipses concentric to the ring; that if the ellipses are very elongated the materials composing them will approach the centre, where they will produce a progressive condensation, communicating to the central globe formed there a rotation in the same plane with the primitive gyrations; and finishes off the whole scheme by specifying the first results to be: ( ) the formation of concentric rings turning in one piece, in the manner of a solid body, around a centre almost empty (_d'abord vide_); and ( ) a rotation in the same direction, communicated to the condensation which would be produced, little by little, by means of matter coming in, partly, from regions affected by the internal eddyings (_tourbillonnements_). ( ) it is unnecessary to go any farther, and take note of his method of the formation of planets and satellites from rings, as it is much the same as what we have seen described by others who have written on the same subject; only interpreted by him in a way to suit his own purposes, and in which interpretation he does not do full justice to laplace, through not having paid sufficient attention to his explanation of how planets could be formed out of rings. except in so far as to note that all along he has considered that rings were formed, and even those nearest to the centre condensed into globes, long before the central condensation had attained any magnitude of importance, or assumed any distinctive shape, and that afterwards all the disposable matter of the rings and also all the exterior matter that had not formed part of what was separated from the original universal chaos, had fallen in towards the small central mass, and so completed the formation of the sun last of all. we shall now proceed to make a few remarks with respect to this condensation of m. faye's cosmogony, which we think we have made without adding to or omitting anything of importance that we have met with in his work, for which purpose we have numbered the paragraphs containing it, in the last six pages, in order to do away with the necessity of repeating the parts to which we refer. no. . all those who believe that "the solar system did originate somehow, by the condensation of a primitive nebula," agree with m. faye in considering that the density of the nebulous matter must have been extremely low, and some of them seem almost to vie with each other in showing how great must have been the degree of its tenuity; but m. faye is one of the few who, paying due respect to the law of the interdependence of temperature and pressure in a gas or vapour, maintain that it must have been almost devoid of temperature, and we have to acknowledge that he is in the right. then we believe that his assumption, that the whole universe of stars, including the sun, was created, humanly speaking, about the same time, is shared by the great majority of those who have thought at all seriously on the subject. also, we agree with him firmly in his statement that each star--and we add planet, satellite, etc.--was originally supplied with an extremely limited quantity of heat, and that what it has expended and what it still retains has been derived entirely from the condensation of the original cosmic matter out of which it was made. with regard to his theorem: we cannot follow him in his statement that the diverse movements caused by the mutual attractions of parts of the original universal mass of cosmic matter, have brought about its separation into myriads of fragments; nor how these fragments could carry with them a rapid movement of translation, unless the whole universal mass was endowed with a rapid movement of translation through space, in which case we think that such a motion would have had no greater particular effect in producing new forms of motion in the fragments, than if the whole had been created in a state of rest. stray movements of translation might give rise to collisions among the multitude of fragments, and perhaps that was one of the modes of formation into suns through which they had to pass; but we cannot follow it out. neither can we see clearly how translation could be effected of one mass into the space occupied by another mass--unless empty spaces were reserved for that purpose from the beginning. without that, translation could not exist: it would be collision. no. . we have nothing to object to what is said in this paragraph; except that a rotating sphere might have been postulated at once, in imitation of laplace, instead of trying like descartes to join fragments together, endowed with movements so adjusted that, among the whole of them, they would produce in the whole mass, when united, the kind of movement that was wanted. no. . to the ideal case of the formation of an isolated sun from a homogeneous mass without interior movement of any kind, we cannot agree in any way. the molecules of matter would not, could not, fall in towards the centre in straight lines. their mutual collisions would drive them generally in curved lines in all directions as they fell in, which would create new internal movements; and these movements would prevent the possibility of the formation of an immovable incandescent sphere such as is described. there could be no immobility in the interior of a sun, as long as its temperature was sufficient to keep the surface incandescent. but we cannot give our reasons here for this assertion--to most people they will, we think, occur at once--because we have a long road to travel before we can do so. when m. faye abandons the isolated case, he leaves us without giving us any help, to conceive for ourselves how the mass would possess and carry with it a considerable velocity of rotation, and still retain the internal movements it had acquired from the attraction of the other masses--of the universal chaos--with which it had been in contact; and also how the molecules drawn towards the centre would not fall in straight lines but in concentric ellipses. and this last we have to do without his giving us any reason why the molecules should fall in towards the centre at all; or rather in spite of the fact that one of his principal ideas would lead us to expect exactly the contrary, as we shall see presently. no. . here he places before us again, two cases in one of which the molecules might resolve themselves into a multitude of small masses, without the centre acquiring any preponderating increase; and the other where the central condensation might greatly exceed the others, and there would be formed a central star accompanied by a crowd of small dark bodies, now become satellites, describing ellipses around the central preponderating mass. this second case he seems, for the time being, to accept as the most probable; but it is strangely at variance with what he sets forth afterwards. he does not give us the least hint as to why or how the satellites acquired their various times of revolution, but only assumes that they did so; and we are very sure that it was not the third law of kepler that was the agent in the case, however much it might suit his purpose. no. . although this part of his exposition is dedicated to the formation of the solar system, all that m. faye says is that it is of little importance whether the movements of bodies around the sun be very eccentric or almost circular; and that among the initial conditions of the chaotic mass, all that we require is that one should be found which would prevent the gyrations from degenerating into elliptic movements, and which had first put right and afterwards firmly preserved the form, more or less circular, in all its changes. but he does not make any attempt to show what that one condition is, and allows us to find it out for ourselves. no. . what m. faye says about the formation of circular rings is more or less a repetition of what he has adduced, to explain all the other movements which he has derived from the universal chaos; and which he seems to think sufficient to account for such movements being nearly circular. for our part we do not think they are sufficient, and he does not show us how they influence each other to bring about the final movements he wants to present to us. we duly take note of the tenuity of the cosmic matter on which he operates, which at grammes in weight to cubic miriamètre would correspond to one grain in weight to , , , cubic feet of space, or grain to a cube of feet--more than yards--to the side. we do this in order to remind him of what he says at page of his work, when dealing with the rotation of the kant-laplace nebula--namely, that it is impossible to comprehend how an immense chaos, of almost inconceivable tenuity, could possess such a rotation from the beginning, and that for want of that inadmissible supposition nothing remains to fall back upon but the _mouvements tourbillonnaires_ of descartes. thus he wants us to believe that his _tourbillons_ could move in straight or curved lines, have motions of translation, could attract, restrain, and drive each other into all sorts of movements with the tenuity he has indicated; but that laplace's nebula, with a density of grain to a cube of feet--or at most feet--to the side, could not be conceived to have the single movement of rotation. and lastly, we repeat that if the centre of the chaos was almost empty, we do not see what induced the cosmic matter to fall into it in elliptic orbits. nos. and . in these paragraphs, the main features are repetitions of the simple assertions made in all the others, that certain movements possessed by matter in one state would produce other movements in another state, without attempting to show how they all came to so far coincide with each other and form one harmonious whole, with movements in almost one single direction. it is clear that one side of the separated chaos might have acquired motion in one direction from the universal chaos with which it had been in contact, and that the opposite side might have acquired motion in exactly the opposite direction from the original chaos with which it had been in contact; and we are left to find out how these came to agree with each other in the end. and, going back to the beginning, we are left to find out where the mass, out of which he constructs his solar system, was stowed away, after it was separated from the original universal chaos. we can conceive of its being separated by condensation, in obedience to the law of attraction, from the surrounding chaos, in which case it might fall towards a centre, or that some parts of it might come to revolve round each other, and that finally the whole of these parts might come to rotate about a common centre; but that is evidently very different from the mode of formation of the solar system which m. faye has advocated. it comes to be by far too like the nebula which laplace supposed to be endowed with rotary motion from the beginning, probably because he did not see, or did not take the trouble to see, how such a motion could be produced. in any case, laplace did not consider that the primary motion of rotation was the most important part of his hypothesis; neither was it, as it seems to have been in the case we have been considering. and he did not go much further than m. faye in postulating primary motion, only he did it in a more effectual and business-like manner. he drew on the bank at once for all the funds he required, instead of having to draw afresh every time he found himself in difficulties, as has been the lot of his critic and successor. finally, m. faye tries to show that after all his rings, flat or otherwise, converted or not converted into globes, had been formed according to his ideas, the greater mass by far of the chaos had fallen into the centre, and had formed the sun there last of all. now, if the preponderating mass of the chaos had been outside of the field of his operations, up to the period when all his planets, satellites, etc. were formed, or at least laid out, it is more natural to suppose that the matter inside of his structure, if there was any, would be drawn outwards by the attraction of the greatly preponderating mass outside, than that any portion of it should have fallen in, in elongated ellipses, towards the insignificant mass that he supposes to have been inside his structure. this, of course, would be nearly exactly the reverse of the mode of formation he was trying to demonstrate, and clearly shows that he was working on unsound principles from the beginning to the end of his cosmogony. it had never occurred to him that matter could be attracted outwards as well as inwards, most probably because it would seem to him ridiculous to imagine that anything in the universe could _gravitate_ upwards. there are other theories of the formation of the solar system from meteorites and meteors, giving us the idea of its being made out of manufactured articles instead of originally created raw material, which does not in any way simplify the process. in some of them, the inrush of meteor swarms is invoked as the cause of gyratory motion, which places them in much the same category as impact theories. we know that broadcloth is made out of woollen yarn, but we also know how the yarn is made out of wool, and how it is woven into the cloth, whereas we are not told by what process, or even out of what the meteors and meteorites are made, although some of them are said to have thumb-marks upon them. all these theories and cosmogonies may be very appropriately classified as variations of the nebula hypothesis, and like variations in another science, may be very brilliant, scientific, imaginative, grand, but after all the flights of fancy exhibited by them are set before us, we feel in a measure relieved when a return is made to the original air. they all assume original motion, varied, accidental, opportune, more dependent upon the will of the cosmogonist than on the laws of nature, which tend to confound rather than enlighten any one who tries to understand and bring them, mentally, into actual operation. laplace assumed rotary motion for the whole of his nebula, and was thus able to account at once for the relation which exists among the planets in respect of distance from, and period of revolution around the sun--arising from the original rotation of the whole mass in one piece--a result which, in any impact theory, has to be accounted for separately, and, in plain truth, empirically in each case, and at each step. seeing, then, that we have not been able to find any cosmogony, or speculation, that gives us a more plausible idea of how the solar system has been formed, we shall try whether from the original nebula as imagined by laplace, it is possible to separate the various members, and form the system in the manner described in his celebrated hypothesis. in other words, we shall endeavour to analyse the hypothesis. chapter iv. page preliminaries to analysis of the nebular hypothesis. definition of the hypothesis. elements of solar system. tables of dimensions and masses. explanation of tables and density of saturn. volume, density and mass of saturn's rings, general remarks about them, and satellites to be made from them. future of saturn's rings. notions about saturn's satellites and their masses. nature of rings seemingly not well understood. masses given to the satellites of uranus and neptune, explanations of. volumes of the members of the solar system at density of water. preliminaries to analysis of the nebular hypothesis. it may be thought that there is little benefit to be derived from analysing an hypothesis which has been declared, by very eminent authorities in the matter treated of, to be erroneous in some points of very serious importance; but hypotheses are somewhat of the nature of inventions, and we know that it has often happened that many parties, aiming at the same invention, have altogether failed, while some other person using almost exactly the same means as his predecessors, has been entirely successful in his pursuit. how many times has it been pointed out to us, that if such a person had only gone one step further in the process he was following, or had only studied more deeply the matter he had in hand, he would have anticipated by many years one of the greatest discoveries of the age! in some cases the failure to take that one step was occasioned through want of knowledge acquired long years afterwards; whereas we think that in the case we have in hand, it can be shown that the want of knowledge acquired many years after he had formulated his hypothesis, or if otherwise, the want of faith in what he knew, enabled laplace to construct an edifice which otherwise he could hardly have convinced himself could be built up in a practical form. we think also that if he had made the proper use of the knowledge he must have had of the law of attraction, he would have seen that no nebula could ever have existed such as the one he assumed, extending far beyond the orbit of the remotest planet. furthermore, we think it can be shown that if he had thoroughly considered what must have been the interior construction of his nebula, he would have found one that would have suited his hypothesis in the main point, viz. condensation at the surface, at least equally as well as endowing it with excessive heat. but to be able to show these things our first step must be to analyse the hypothesis, to examine into it as minutely and deeply as lies in our power. for this purpose it will be necessary to define what the hypothesis is. many definitions have been given, more or less clear, and it would be only a waste of time to try to set forth laplace's own exposition of it, with all its details, which he had no doubt studied very carefully. but in those definitions that have come under our observation, several of the conditions he has specified are wanting, or not made sufficiently prominent; so instead of adopting any one of them we will make a sort of condensation of the whole, adding the conditions that have been left out; because the want of them, has been the cause of mistaken conceptions of the evolution of the system having been formed by very eminent astronomers. our definition will therefore be as follows:-- i. it is supposed that before the solar system was formed the portion of space in which its planets and other bodies now perform their revolutions and other movements, was occupied by an immense nebula of cosmic matter in its most simple condition--of molecules or atoms--somewhat of a spherical form, extending far beyond its present utmost limits, and that it was endowed with excessive heat and a slow rotary motion round its centre; which means that while it made one revolution at the circumference it also made one at the centre. the excessive heat, by counteracting in a certain measure the force of gravitation, kept the molecules of matter apart from each other; but as the heat was gradually radiated into space, gravitation became more effective, and then began to condense and contract more rapidly, by which process its rotary motion was, in accordance with the areolar law, gradually increased at the surface, _in the atmosphere of the sun_, where the cooling took place, and condensation was most active; and the increase of rotation was propagated from there towards the centre. ( ) as the contraction and rotation increased a time or times arrived, when the centrifugal force produced by the rotation came to balance the force of gravitation, and a series of zones or rings were separated from the nebula, each one of them continuing to rotate--revolve now--around the central mass, with the same velocities they had at the times of their separation; until at last the nebula became so contracted that it could not abandon any more rings, and what of it remained condensed and contracted into a central mass which ultimately assumed the form of the actual sun. ( ) in the meantime, or following afterwards, each one of the rings which were abandoned by the nebula, acquired, through the friction of its molecules with each other, an equal movement of revolution throughout its entire mass, so that the real velocities of the molecules furthest removed from the centre of the nebula were greater than those of the molecules nearest to its centre, and the ring revolved as if it were in one solid piece. arrived at this stage the rings broke up and formed themselves into smaller nebulæ, each of which condensed into a globe or planet, and continued to revolve around the central mass in the same time as its mass had done when in the form of a ring. and some of these sub-nebulæ, imitating the example of their common parent more perfectly than others, abandoned in space in their turn smaller rings which in the same manner condensed, broke up, and formed themselves into smaller globes or satellites; all, as far as we know, except the rings of saturn, which have not as yet been converted into satellites. table i. elements and other data of the solar system employed in this analysis. part i.--sun and planets. --------+-------------+----------+-----------------------+-----------+ |mean distance|equatorial|volume in cubic miles. | density. | name. | from sun |diameter | |(water = )| | in miles. |in miles. | | | --------+-------------+----------+-----------------------+-----------+ sun | -- | , | , , , , , | · | mercury | , , | , | , , , | · | venus | , , | , | , , , | · | earth | , , | , | , , , | · | mars | , , | , | , , , | · | supposed| | | | | planet| , , | -- | -- | -- | jupiter | , , | , | , , , , | · | saturn | , , | , | , , , , | · | uranus | , , , | , | , , , , | · | neptune | , , , | , | , , , , | · | --------+-------------+----------+-----------------------+-----------+ | volume at density of | time of revolution | name. | water in cubic miles. | round sun in days. | | | | --------+------------------------+-----------------------+ sun | , , , , , | -- | mercury | , , , | · | venus | , , , , | · | earth | , , , , | · | mars | , , , | · | supposed| | | planet| , , , | , · | jupiter | , , , , | , · | saturn | , , , , | , · | uranus | , , , , | , · | neptune | , , , , | , · | --------+------------------------+-----------------------+ part ii.--satellites of planets. ----------+--------------+-------------+-----------------+ | mean distance| equatorial | volume | names. | from primary | diameter | in cubic miles. | | in miles. | in miles. | | ----------+--------------+-------------+-----------------+ | _of the earth._ | moon | , , , , | | | | _of jupiter._ | jo | , , , , | europa | , , , , | ganymede | , , , , | callisto | , , , , , | | | | _of saturn._ | mimas | , , , | enceladus | , ? , , | tethys | , , , | dione | , , , | rhea | , , , | titan | , , , , | hyperon | , , ? , , , | | | japetus | , , , , , | | | | _of uranus._ | ariel | , } total mass taken at | umbriel | , } / , th of primary | titania | , } , , , | oberon | , } | | | | _of neptune._ | ----- | , } mass taken at | | } / , th of primary | | , , | ----------+----------+-----------------+-----------------+ | density. |volume at density| total volume at | names. |(water= .)| of water in |density of water | | | cubic miles. | in cubic miles.| ----------+----------+-----------------+-----------------+ | | | _of the earth._ | moon | · .. , , , | | | | _of jupiter._ | jo | · , , , | europa | · , , , | ganymede | · , , , | callisto | · , , , | | ----------------- | | , , , | | | | _of saturn._ | mimas | ... | enceladus | ... | tethys | ... | dione | ... | rhea | ... | titan | ... total volume | hyperon | ... in cubic miles. | | | japetus | · , , , , , , | -----------+----------------------------------------------+ part iii.--rings of saturn. ---------+-----------------------------+--------------------------+ | | | rings. | diameters of rings | areas of rings | | in miles. | in square miles. | ---------+-----------------------------+--------------------------+ outer { | outer | , | } , , , | { | inner | , | } | | | | | middle { | outer | , | } , , , | { | inner | , | } | | | | | dark { | outer | , | } , , , | { | inner | , | } | | | | | | | total | , , , | ---------+----------+------------------+---------+----------------+ |thickness | | | volume at | rings. | of rings | volume of rings |density. |density of water| | in miles.| in cubic miles. |(water= )|in cubic miles. | ---------+----------+------------------+---------+----------------+ outer { | | | | | { | | | | | | | | | | middle { | | | | | { | | | | | | | | | | dark { | | | | | { | | | | | | | | | | | | , , , , | · | , , | ---------+----------+------------------+---------+----------------+ ( ) all of these bodies, planets, satellites, and rings were supposed to revolve around their primaries, and to rotate on their axes, in the same direction viz., from right to left, in the opposite direction to the hands of a watch. in addition to the above definition it is necessary to give some sort of description of the various parts of the machine or system which has to be made out of the nebula, with their positions, dimensions, and details. this we believe will be made plain enough, in the simplest manner, by table no. i., taken and calculated from the elements of the solar system given in almost all astronomical works, from which we have selected what we believe to be the most modern data. the construction of this table requires some explanation on account of its being made to show complete results from incomplete data. there has been no difficulty with the sun, the major planets, and the satellites of the earth and jupiter, but for the minor planets, the satellites of the three outer planets, and the rings of saturn, we have been obliged to exercise our judgment as best we could. there being almost no data whatever of the dimensions and densities of the minor planets, to be found, we have been driven in order to assign some mass to them, to imagine the existence of one planet to represent the whole of them (in fact olbers's planet before it exploded), which we have supposed to be placed at the mean distance of , , miles from the centre of the sun; and we have given to it a mass equal to one-fourth of the mass of the earth, that being, in the opinion of some astronomers, the greatest mass which the whole of them put together could have. this assumption we shall explain more fully at a more suitable time. in the case of saturn the diameters of two of the satellites are wanting which we have assumed to be the same as those of the smallest of those nearest to them, and thus have been able to compute the volumes of the whole of them; but we have not been able to find any statement anywhere of their densities, and to get over this difficulty we have reasoned in the following manner. the density of the moon is very little over two-thirds of that of the earth, while that of the satellites of jupiter varies from a little more than the same to a little more than twice as much as the density of their primary. why this difference? to account for it we appeal to the very general opinion of astronomers, that the four inner planets are in a more advanced stage of their development, or existence, than the four outer ones. in this way it is easy to conceive that the earth has arrived at the stage of being more dense than its satellite; while in the case of jupiter, his satellites being of so very much less volume than their primary, have already arrived at a higher degree of development. carrying this motion forward to saturn, we have supposed that from his being considerably less dense than any other of the outer planets--quite possibly from having been formed out of material comparatively (perhaps not actually) less dense than the others--his satellites may not have condensed to a greater degree than his own mass, and we have, therefore assumed their density, that is the density of the volume of the whole of them, to be the same as that of their primary. to determine some mass for the rings of saturn, is a much more intricate matter than for his satellites, and presents to us some ideas--facts rather--which had never before crossed our imagination. the most natural way to look upon these rings is to suppose that they are destined to become satellites at some future time. all the modern cosmogonies that have come under our notice are founded upon the idea that rings are the seed, as it were, of planets and satellites, and if those of saturn have been left, as it has been said, to show how the solar system has been evolved, it cannot be said that the supposition is not well founded. in this way we are led to speculate upon how many satellites are to be made out of the rings before us. considering, then, that the nearest satellite is , miles from the centre of saturn, leaving only , miles between his surface and that of mimas, and also that the distances between satellites diminish rapidly as they come to be nearer to their primaries, there is not room to stow away a great number of satellites. on the other hand, seeing that there are at least three distinct rings, we cannot reasonably do less than conclude that three satellites are intended to be made out of them. but let the number be what it may, all that we have to do with them for our present purpose is to assign some mass to them. with this view, we have given, arbitrarily, to each one of the three we have supposed, a volume equal to that of one of the satellites of miles in diameter, that is, about , , cubic miles, and we have supposed their density to be the same as that of water, instead of that of the planet. thus, in the table, we have assigned to the three a mass of , , cubic miles at density of water, which would be more than sufficient to make four other satellites for the system of miles in diameter each, and of the same density as the planet. for the table referred to we have calculated the areas of the three rings to be , , , square miles, and we have assumed the thickness as miles, that is about two-thirds of that estimated by chambers in his handbook of astronomy, but almost the same as that given by edmund dubois; nevertheless their total volume comes up to , , , , cubic miles, which reduces their average density to · that of water, to make up the mass of , , cubic miles at the density of water, which we have adopted for the three. this density corresponds to very nearly one-tenth of that of air, which, however strange it may appear to us, may be considered to be a very full allowance, seeing that we shall find, later on, that the planet itself was formed out of matter whose density could not have been more than one twenty-six millionth part of that of air. all the same, it is hardly matter that we could liken to brickbats. after being driven to this low estimate of density, which startled us, we referred to an article in "nature" of nov. , , on ten years' progress in astronomy, where we find what follows:--"he (newcomb) finds the mass of titan to be about / , that of saturn. it may be noted, too, that hall's observations of the motions of mimas and enceladus indicate for the rings a mass less than / that deduced by bessel; instead of being / as large as the planet, they cannot be more than / , and are probably less than / , ." (we make them / ). thinking over the numbers herein given we cannot help being surprised by them. if titan be / of the mass of saturn, we cannot conceive how the mass of his rings can be so much greater than that of titan. we cannot pretend to fit even one satellite of that size, mechanically, into a space of , miles wide, while titan revels in an ample domain with a width of , miles. but we shall not pursue this part of our speculations any further. astronomers may be able to demonstrate that the rings are of a totally different nature to those out of which the planets and their satellites are supposed to have been made, or that the nebular hypothesis or anything resembling it is no better than a foolish dream. all that we have pretended to do has been to give them their due place in the hypothesis we are attempting to analyze, and to look upon them in a practical and mechanical light, as an unfinished part of the solar system. to determine masses for the satellites of the two outer planets, we have to be more empirical even than we have yet been. a little trouble will show that the whole mass of all the satellites and rings of saturn put together is about / th of the mass of the planet, and we shall avail ourselves of this proportion to assign masses for the satellites of the remaining planets, the numbers and names of which are the only data we have been able to find. considering then, that uranus has only four satellites and no rings, we think if we give them / , th of the mass of their primary, it will be a very fair allowance; and with the same empiricism we have adopted for the solitary satellite of neptune / , th of the mass of its primary. however rude and crude these approximations may be, we have the satisfaction of thinking that the masses obtained by their means, can have no appreciable effect upon the operations into which they are to be introduced, whilst they enable us to deal with a complete system or machine. but for these we have another table no. ii. to present, a _résumé_ of the foregoing one, for greater facility of reference. table ii.--volumes of the various members of the solar system at the density of water. ---------+------------+-------------------+-----------------------+ | | volume in | total volume in | name. |designation.| cubic miles at | cubic miles at | | | density of water. | density of water. | ---------+------------+-------------------+-----------------------+ sun | | | , , , , , | | | |-----------------------| mercury | planet | | , , , | venus | " | | , , , , | earth | " | , , , , | | moon | satellite | , , , | , , , , | | |-------------------| | mars | planet | | , , , | ---- | asteroids |one fourth of earth| , , , | jupiter | planet | , , , , | | " | satellites| , , , | , , , , | | |-------------------| | saturn | planet | , , , , | | " | satellites| , , , | | " | rings | , , | , , , , | | |-------------------| | uranus | planet | , , , , | | " | satellites| , , , | , , , , | | |-------------------| | neptune | planet | , , , , | | " | satellite | , , | , , , , | ---------+------------+-------------------+-----------------------+ total of planets, satellites and rings | , , , , | ------------------------------------------+-----------------------+ dividing , , , , , by , , , , makes the mass ofthe whole of the members to be / · th part of the mass of the sun, instead of / th as generally stated by astronomers. chapter v. page analysis of the nebular hypothesis. separation from the nebula of the rings for the separate planets, etc. excessive heat attributed to the nebula erroneous and impossible. centigrade thermometer to be used for temperatures. temperature of the nebula not far from absolute zero. erroneous ideas about glowing gases produced by collisions of their atoms, or particles of cosmic matter in the form of vapours. separation of ring for neptune. it could not have been thrown off in one mass, but in a sheet of cosmic matter. thickness and dimensions of the ring. uranian ring abandoned, and its dimensions. saturnian ring do. do. jovian ring do. do. asteroidal ring do. do. martian ring do. do. earth ring do. do. venus ring do. do. mercurian ring do. do. residual mass. condensation of solar nebula to various diameters, and relative temperatures and densities. unaccountable confusion in the mode of counting absolute temperature examined and explained. negative degrees of heat only equal degrees of absolute temperature. the centigrade thermometric scale no better than any other, and cannot be made decimal. the sun's account current with the nebula drawn up and represented by table iii. analysis of the nebular hypothesis. we may now proceed to take the original nebula to pieces, by separating from it all the members of the solar system, in performing which operation we shall suppose the divisions between the nebula and each successive ring to have taken place at a little more or less than the half distances between the orbits of two neighbouring planets, because we have no other data to guide us in determining the proper places. these divisions have manifestly been brought about in obedience to some law, as is proved in great measure by what is called bode's law; although no one has as yet been able to explain the action of that law. it is no doubt certain that a division must have taken place much nearer to the outer than the inner planet in each case, if we think of what would be the limit to the sphere of attraction between the nebula and a ring just detached from it--for the attraction of the abandoned ring, and even of all those that were outside of it, would have very little influence in determining the line where gravitation and centrifugal force came to balance each other--but the data necessary for calculating what these would be are wanting. even if they existed the calculations would become too complicated for our powers as the number of rings increased; and for our purpose it is really of very little importance where the divisions took place. the breadths of the rings would be practically the same, whether they were divided at the half distances between, or much nearer to, the outermost of two neighbouring planets; and although the extreme diameters of the consecutive residuary nebulæ would be somewhat greater, their densities and temperatures would not materially differ from those we shall find for them as we proceed in our operations. their masses would be the same in all cases, which is the principal thing in which we are interested. this premised, we shall first examine into the excessive heat attributed to the nebula, that being the first condition mentioned in our definition of the hypothesis. the diameter of the sun being , miles, his volume is , , , , , cubic miles, and his density being · times that of water, his volume reduced to the density of water would be , , , , , cubic miles. now, astronomers tell us that the whole of the planets, with their satellites and rings, do not form a mass of more than / th part of the mass of the sun. if, then, we add / th part to the above volume, we get a total volume, for the whole of the system, of , , , , , cubic miles at the density of water, which corresponds to a sphere of about , miles in diameter. on the other hand, the diameter of the orbit of neptune being , , , miles, if we increase that diameter to , , , miles, so that the extreme boundary of the supposed nebula may be as far beyond his orbit, as half the distance between him and uranus is within it, we shall still be far within the limit at which the process of separation from the nebula, of the matter out of which neptune was made, must have begun. from these data we can form a very correct calculation of what the density--tenuity rather--of the nebula must have been. for, as the volumes of spheres are to each other as the cubes of their diameters, the cube , is easily found to be to the cube of , , , , as is to , , , , or in other words, the density of the nebula turns out to have been / , , , th part of density of the whole solar system reduced to that of water. carrying the comparison a little further, we find that as water is · times more dense than air, and , · times more dense than hydrogen, the density of the nebula could not have been more than / , , th part that of air, and / , , th that of hydrogen. but, confining the comparison to air, as it suits our purpose better, we see that it would take , , cubic feet of the nebula to be equal in mass to cubic foot of air at atmospheric pressure; and that were we to expand this cubic foot of air to this number of times its volume, the space occupied by it would be as nearly in the state of absolute vacuum as could be imagined, far beyond what could be produced by any human means. now, were heat a material, imponderable substance, as it was at one time supposed to be, we could conceive of its being piled up in any place in space in any desired quantity; but it has been demonstrated not only not to be a substance at all, but that its very existence cannot be detected or made manifest, unless it is introduced by some known means--friction, hammering, combustion--into a real material substance. therefore, we must conclude that if it existed at all in the nebula, it must have been in a degree corresponding to the tenuity of the medium, and the air thermometer will tell us what the temperature must have been if we only choose to apply it. applying, then, this theory of the air thermometer, if we divide[b] ° by , , --the number of times the density of the nebula was less than that of air--we get · °, as the absolute temperature of the nebula, something very different to excessive heat, incandescence, firemist, or any other name that has been given to its supposed state. furthermore, as a cubic foot of air weighs · grains, , , divided by · , which is equal to , , would be the number of cubic feet of the space occupied by the nebula, corresponding to each grain of matter in the whole solar system, which would be equal to a cube of very nearly feet to the side. and as the only means by which the nebula could acquire heat would be by collision with each other of the particles of matter of which it was composed; to conceive that two particles weighing grain each, butting each other from an average distance of feet, could not only bring themselves, but all the space corresponding to both of them--which would be , , cubic feet, _of what_?--up to the heat of incandescence, or excessive heat of any kind, is a thing which passes the wit of man. consequently, neither by primitive piling up, nor by collisions among the particles, could there be any heat in the nebula at the dimensions we have specified, beyond what we have measured above. [b] here we beg to state that in all our coming operations, we will use the centigrade scale for temperatures without adding c to each number specified, unless a different scale has to be referred to, in which case the distinctive of the scale shall be given in the usual way. this we do because it is the fashion, not because we think it possesses any advantage over any other scale, but rather the contrary. perhaps we may have something more to say about scales after we have handled the centigrade a little more than it has been our lot to do hitherto. some people believe, at least they seem to say so, that meteors or meteorites colliding would knock gas out of each other, sufficient to fill up the empty space around them, and become incandescent, and so pile up heat in nebulæ sufficient to supply suns for any number of millions of years of expenditure. but they forget that gas is not a _nothing_. it possesses substance, matter, of some kind, however tenuous. therefore, if the meteors knock matter out of each other in the form of gas, they must end by becoming gas themselves, and we come back to what we have said above; we have two grains, in weight, of gas abutting each other at an average distance of yards, instead of two grains of granite or anything else, and things are not much improved thereby. and if we compare yards with m. faye's , where are we? the next thing to deal with is the formation of the planets. separation of ring for neptune. when the nebula was , , , miles in diameter its volume would be , , , ^{ }[c] cubic miles, and we have just seen that its density must have been , , , times less than that of water, or , , less than air, and its temperature · ° above absolute zero. on the other hand, we find from table ii. that the volume of neptune and his satellite is , , , , cubic miles at the density of water. multiplying, therefore, this volume by , , , we get , , ^{ } cubic miles as the volume of the ring for the formation of neptune's system at the same density as the nebula. then, subtracting this volume from , , , ^{ }, there remain , , , ^{ } cubic miles as the volume to which the nebula was reduced by the abandonment of the ring out of which neptune and his satellite were formed. [c] the exponent in , , , ^{ } means that cyphers have to be added to complete the number. the same is the case with any other number and exponent of large quantities. then the mean diameter of the orbit of neptune being , , , miles, its circumference or length will be , , , miles, and if we divide the volume of his system as stated above, by this length, we get , , , , square miles as the area of the cross section of the ring, which is equal to the area of a square of , , miles to the side. again, if we divide the circumference of the orbit by this length of side, we find that it is / · th part of it, and therefore about minutes of arc. also if we divide the diameter of the orbit by an arc of , , miles in length, we find that it bears the proportion of to to the diameter of the orbit. thus the cross section of the ring would bear the same ratio to its diameter that a ring of foot square would bear to a globe of feet in diameter. here we find it difficult to believe that by rotating a ball of feet in diameter of cosmic matter, meteorites, or brickbats, we could detach from it, mechanically, by centrifugal force a ring of foot square, and the same difficulty presents itself to us with respect to the nebula. we cannot conceive how a ring of that form could be separated by centrifugal force from a rotating nebula, and have therefore to suppose it to have had some different form, and to apply for that to the example of saturn's rings--just the same as laplace no doubt did. we cannot tell how the idea originated that the ring should be of the form we were looking for--perhaps it was naturally--but it seems to have been very general, and in some cases to have led to misconceptions. it is not difficult to show how a saturnian or flat ring could be formed, but we shall have a better opportunity hereafter of doing so. we must try, nevertheless, to form some notion, however crude it may be, of what might be the thickness of a flat ring of the cross section and volume we have found for neptune. let us suppose that the final separation of the ring took place somewhere near the half-distance between his orbit and that of uranus, say, , , , miles from the centre of the nebula, the breadth of the ring would be the difference between the radius of the original nebula, i.e. , , , miles and the above sum, which is , , , miles. then if we divide the area of the cross section of the ring by this breadth, that is, , , , , by , , , , we find that the thickness would be , miles; provided the ring did not contract from its outer edge inwards during the process of separation. this could not, of course, be the case, but, as we have no means of finding how much it would contract in that direction, we cannot assign any other breadth for it; and we shall proceed in the same manner in calculating the thicknesses of the rings for all the other planets as we go along. we can, however, make one small approach to greater accuracy. we shall see presently that the density of the ring would be increased threefold at its inner edge as compared with the outer during the process of separation, which would reduce its average thickness to somewhere about , miles at density of water, of course. the nebula remaining after neptune's ring we may now call the uranian nebula. the volume of the nebula after abandoning the ring for the system of neptune was found to be , , , ^{ } cubic miles at its original density, but during the separation it has been condensed into a sphere of , , , miles in diameter, whose volume would be , , , ^{ } cubic miles; so that if we divide the larger of these two volumes by the smaller, we find that the density of the uranian nebula would be increased · times, and therefore it would then be , , , divided by · , equal to , , , times less dense than water. furthermore, if we compare it to the density of air, which we can do by dividing this last quantity by · , we find it to have been , , times less than that density; and if we apply the air thermometer to it, we shall find that its absolute temperature must have been divided by , , = · ° or - · .° we can now separate the ring for the system of uranus from the uranian nebula, reduced as we have seen to , , , miles in diameter, volume of , , , ^{ } cubic miles, and density of , , , times less than water. referring to table ii., we find the volume of the whole system of uranus to have been , , , , cubic miles at the density of water, but we have to multiply this volume by the new density of , , , times less than water in order to bring it to the same density as the nebula, which will make the volume of his system to be , , , ^{ } cubic miles at that density. then, subtracting this volume from , , , ^{ }, we find that the nebula has been reduced to , , , , ^{ } cubic miles in volume. then the diameter of the orbit of uranus being , , , miles, its circumference will be , , , miles, so that dividing the volume , , , ^{ } of his system by this length of circumference, the area of the cross section of the ring would be , , , , square miles. if we now suppose the diameter of the nebula, after abandoning the ring for the whole system of uranus, to have been , , , miles--dimension derived from nearly the half-distance between the orbits of uranus and saturn--we find that the breadth of the ring would be , , miles, which would be the difference between the radii of the uranian and saturnian nebulæ, respectively , , , miles, and , , , miles; so that if we divide the area of cross section of uranus' ring or , , , , square miles by this breadth we find the thickness of the ring to have been , miles. but the density of the inner edge of the ring would be · times more dense than the outer edge, for the same reason as in the case of the neptunian ring, which would make the average thickness to have been about , miles. saturnian nebula. we have seen that the volume of the nebula after the separation of the ring for uranus' system would be , , , , ^{ } cubic miles, but as we have reduced the diameter of the saturnian nebula to , , , miles, its volume would also be reduced, or condensed to , , ^{ } cubic miles, so that dividing the larger volume by the smaller we find that its density must have been increased · fold. then dividing , , , by · , we see that the density would be reduced, or increased rather, to , , , times less than that of water. this can be easily found to be , , times less than the density of air, and the air-thermometer would show that the absolute temperature of the saturnian nebula must have been · ° or - · °. we have just seen that the saturnian nebula has been condensed to , , , miles in diameter, to volume of , , ^{ } cubic miles, and density of , , , times less than that of water. then from table ii. we get the volume of the whole of the system of saturn as , , , , cubic miles at the density of water, and multiplying this by , , , will give , , , ^{ } as its volume at the same density as the nebula; and subtracting this from , , ^{ } we find that the volume of the nebula had been reduced to , , , , ^{ } cubic miles. then the diameter of the orbit of saturn being , , , miles its circumference would be , , , miles in length, and if we divide the volume of his system, viz. , , , ^{ } cubic miles, by this length, we find the area of the cross section of the ring to have been , , , , square miles. now, supposing the diameter of the nebula, after abandoning the ring, to have contracted to , , , miles and radius consequently of , , miles, the breadth of the ring would be , , , less , , or , , miles; and if we divide the area of the cross section of the ring, that is, , , , , square miles, by this breadth, we get , miles for its thickness. but in the same way as before, the inner edge of the ring would be · times more dense than the outer edge, which would reduce its average thickness to , miles. jovian nebula. the volume of the nebula after separation of the ring for saturn's system having been , , , , ^{ } cubic miles, this volume has to be condensed into the volume of the jovian nebula of , , , miles in diameter, which would be , , , , ^{ } cubic miles. then if we divide the first of these two volumes by the second, we find the density of the jovian nebula to have been increased · fold over the previous one. but the density of the saturnian nebula was , , , times less than water, dividing which by · makes the jovian nebula to have been , , , times less dense than water. dividing this by · we get a density for it of , , times less than that of air, which corresponds to the absolute temperature of · ° or - · °. from the jovian nebula of , , , miles in diameter, volume of , , , , ^{ } cubic miles, and density of , , , times less than water, we have now to deduct the whole of the system of jupiter, which, by table no. ii., is , , , , cubic miles at density of water. multiplying this by , , , we get the volume of , , , ^{ } cubic miles for his system at the same density as the nebula; therefore, substracting this amount from , , , , ^{ } we get , , , , ^{ } cubic miles as the volume to be condensed into the succeeding nebula which we shall call asteroidal, the dimensions of which we can determine in the following manner, although only very approximately. according to the nebula hypothesis, there must have been a ring detached from the nebula for the formation of the asteroids, as well as the formation of the other planets. so, in order to be able to assign elements for that ring, corresponding to those we have found for the others, we shall suppose the whole of them to have been collected into one representative planet, at the mean distance from the centre of the nebula of , , miles, more or less in the position denoted by the number in bode's law; also its mass to have been one-fourth of that of the earth, or , , , cubic miles at density of water, which, in the opinion of probably most astronomers, is a considerably greater mass than would be made up by the whole of them put together--discovered and not yet discovered. with the above distance from the centre of the nebula, the divisionary line between the jovian and the asteroidal nebulæ would be , , miles from the said centre, and the diameter of the latter , , miles in consequence. we know that some of the asteroids move in their orbits beyond this supposed divisionary line, and it may be that when we come to determine the divisionary line between the supposed asteroidal and the martian nebulæ, some of them may revolve in their orbits nearer to mars than that line, but that will not interfere in any way with our operations, because we are only dealing with the whole of them collected into one representative. for finding the dimensions of the ring for jupiter's system, we have the mean diameter of his orbit as , , miles, which makes its circumference to be , , , miles in length. therefore, dividing the volume of the ring as found above, viz. , , , ^{ } cubic miles by this length, the area of its cross-section comes to be , , , , square miles, which divided in turn by the breadth of , , --the difference between the radii of the jovian and asteroidal nebulæ, or , , less , , --makes the thickness of the ring to have been , , miles. but, as before, the inner edge of the ring had become · times more dense than the outer edge, so that the average thickness would be only , miles. asteroidal nebula. the volume of the nebula after the separation of the ring for the system of jupiter having been , , , , ^{ } cubic miles, this volume has to be condensed into the volume of the asteroidal nebula of , , miles in diameter and consequently of volume of , , , , , ^{ } cubic miles. then if we divide the first of these volumes by the second, we find the density to have been increased · fold, as used above for the average thickness of jupiter's ring. but the density of the jovian nebula was , , , times less than water, dividing which by · makes the asteroidal nebula to have been , , times less dense than water. this again divided by · makes it , times less dense than air, which will give us · ° as its absolute temperature--or the same as - · °. next, from the asteroidal nebula , , miles in diameter, volume of , , , , , ^{ } cubic miles, and density , , times less than water, we have to deduct the volume of the whole of the system which in table no. ii. we have supposed to have been , , , cubic miles at density of water. multiplying this by , , we get the volume to have been , , , ^{ } cubic miles for the ring at the same density as the nebula; so, deducting this quantity from , , , , ^{ }, we get , , , , , ^{ } cubic miles as the volume to which the nebula had been reduced by the separation of the ring. for the dimensions of the ring we have the mean diameter of the orbit of the representative asteroid as , , miles, that is twice its distance from the centre of the nebula, which makes its circumference to be , , , miles in length. dividing then the volume of the ring, which we found to have been , , , ^ cubic miles by this length, the area of its cross-section must have been , , , square miles, which divided by the breadth of , , miles--the difference between the radii of the asteroidal and martian nebula, namely , , less , , --makes the thickness of the ring to have been miles. but the inner having been · times more than the outer edge, as we shall see presently, the average thickness would be miles. martian nebula. the volume of the last nebula after the separation of the ring for the asteroids was found to have been , , , , , ^{ } cubic miles, which had to be condensed into the volume of the martian nebula of , , miles in diameter, which would give a volume of , , , , , ^{ } cubic miles. dividing then, the larger of these volumes by the smaller, we find that the density of the martian nebula had been increased · times by the condensation. but we found the density of the asteroidal nebula to have been , , times less dense than water, dividing which by · makes the martian nebula to have been , , times less dense than water. this divided again by · makes it , times less dense than air, and consequently its absolute temperature to have been · ° or - · °. from the martian nebula of , , miles in diameter, volume , , , , , ^{ } cubic miles, and density , , times less than water, we have to deduct the volume of his ring, which by table ii., was estimated at , , , cubic miles at density of water. multiplying this by , , we find its volume to be , , , ^{ } cubic miles at the same density as the nebula, deducting which from its whole volume we get , , , , , ^{ } cubic miles as the volume after the separation of the ring. for finding the dimensions of the ring we have , , miles as the mean diameter of the orbit of mars, which makes its circumference , , miles in length. then dividing the volume of the ring , , , ^{ } cubic miles by this length, the area of its cross-section comes to be , , , square miles, which, divided by the breadth of , , miles--that is one-half of the difference between the diameters of the martian and earth nebula, respectively , , and , , miles--makes the thickness of the ring to have been miles. but as before, the inner having become through condensation, · times more dense than the outer edge, the average thickness would be miles. earth nebula. as the volume of the nebula was , , , , , ^{ } cubic miles after the separation of the ring for mars, we have to condense it into the volume of the earth nebula, which at , , miles in diameter would be , , , , , ^{ } cubic miles. dividing the larger of these volumes by the smaller we find that the density of the nebula has been increased · times, as employed above. but we found the density of the martian nebula to have been , , times less than that of water, dividing which by · makes the earth nebula to have been , , times less dense than water. dividing this again by · we find it to have been , times less dense than air, and ° divided by this density of air--the same as in all the respective cases--gives · ° as the absolute temperature of the nebula and corresponds to - · °. from the earth nebula , , miles in diameter, , , , , , ^{ } cubic miles in volume, and , , times less dense than water, we have to subtract the volume of the ring of the earth's system, which, in table ii., appears as , , , , cubic miles at density of water. multiplying this by , , we find it to have been , , , ^{ } cubic miles at the same density as the nebula. and subtracting this quantity from , , , , , ^ , we get , , , , , ^ cubic miles for the volume of the previous nebula after the separation of the ring for the system of the earth. for finding the dimensions of the ring we have , , miles for the mean diameter of the earth's orbit, which makes the circumference , , miles in length, and dividing the volume of the ring for the system, which was found to be , , , ^ cubic miles, by this length, the area of its cross section comes to be , , , square miles, which divided by the breadth of , , miles--that is one-half of the difference between the diameters of the earth and venus nebulæ, respectively , , and , , miles--makes the thickness of the ring to have been miles. but the inner will presently be seen to have been · times more dense than the outer edge when its separation was completed, so that the average thickness would be miles. venus nebula. as the volume of the nebula was , , , , , ^ cubic miles after the separation of the ring for the system of the earth, we have to condense it into the volume of the venus nebula, which at , , miles in diameter would be , , , , , ^ cubic miles. then dividing the larger of these two volumes by the smaller, we find that the density of the venus nebula had been increased to · times what that of the earth nebula was. but we found the density of that nebula to have been , , times less than that of water, dividing which by · makes the venus nebula to have been , , times less dense than water. dividing this again by · we find it to have been , times less dense than air, which would make its absolute temperature to have been · °, which corresponds to - · °. from the venus nebula of , , miles in diameter, volume , , , , , , ^{ } cubic miles, and density , , times less than that of water, we have now to deduct the volume of her ring, which by table ii. is , , , , cubic miles at the density of water. multiplying this volume by , , we find the volume of the ring to have been , , , , ^{ } cubic miles at the same density as the nebula, and subtracting this amount from , , , , , , ^{ } we get , , , , , ^{ } cubic miles for the volume to be condensed into the nebula following. to find the dimensions of the ring we have , , miles for the diameter of the orbit of venus, which makes its circumference , , miles in length. then dividing the volume of the ring, i.e. , , , , ^{ } cubic miles by this length, the area of its cross-section comes to be , , , square miles, which, divided by the breadth of , , miles--that is one-half of the difference between the diameters of the venus and mercurian nebulæ, respectively , , and , , miles--makes the thickness of the ring to have been miles. but the inner edge having become, in the process of separation, · times more dense than the outer one (see below) the average thickness would be reduced to miles. mercurian nebula. as the volume of the nebula was , , , , , , ^{ } cubic miles after the separation of the ring for venus, we have to condense it into the volume of the mercurian nebula, which at , , miles in diameter would be , , , , , ^{ } cubic miles. then, dividing the larger of these two volumes by the smaller, we find that the density of the mercurian nebula must have been increased · fold over that of its predecessor. but we find the density of the venus nebula to have been , , times less than water, dividing which by · makes the mercurian nebula to have been , , times less dense than water. dividing again this density by · we find it to have been times less than air, and ° divided by this air density gives · ° as its absolute temperature, which corresponds to - · °. from the mercurian nebula , , miles in diameter, volume of , , , , , ^{ } cubic miles, and density of , , times less than water, we have to deduct the volume of his ring, which by table ii. is , , , cubic miles at density of water. multiplying this volume by , , makes the ring to have been , , , ^{ } cubic miles in volume at the density of the nebula, and subtracting this amount from , , , , , ^{ }, we get , , , , , ^{ } cubic miles for the volume to be condensed into the nebula following. to find the dimensions of the ring we have , , miles for the mean diameter of the orbit of mercury, which makes its circumference , , miles in length. then dividing the volume of his ring, i.e. , , , ^{ } cubic miles, as above, by this length, the area of its cross-section comes to be , , square miles. here we have to determine the breadth of the ring in a new way, that is empirically. seeing that the breadth of the ring for the earth's system was , , and of that for venus , , miles, we shall assume , , miles for the breadth of the ring for mercury. this will make the residuary, now the solar nebula, to have been , , miles in radius and , , miles in diameter. returning now to the area of the cross-section of the ring, that is, , , square miles, and dividing it by the assumed breadth , , miles, makes the thickness of the ring to have been miles. but, as before, its inner edge having become · times more dense than the outer one during the process of separation (see below) the average thickness must have been only miles. solar nebula. lastly, as the volume of the nebula was , , , , , ^{ } cubic miles after the separation of the ring for mercury, we have to condense it into the volume of the solar nebula, which at , , miles in diameter would be , , , , , ^ cubic miles. then dividing the first of these two volumes by the second, we find that its density must have been increased · fold. but we found that the density of the mercurian nebula was , , times less than that of water, dividing which by · makes the solar nebula to have been , times less dense than water. dividing this in turn by · shows it to have been times less dense than air, and, still further, dividing ° by this air density makes its absolute temperature to have been · ° equal to - · °. we might conclude our analysis here, but it will be more convenient to carry our calculations a few steps further, to save the additional trouble that might be occasioned by having to return to them later on. first we shall condense the solar nebula to , times less dense than water, and therefore times less dense than air, which we may note will increase its density · times. this supposed to be done, its diameter would be , , miles, its volume , , , ^{ } cubic miles, and its density / th of an atmosphere--about _one-ninth_ inch of mercury--which would, in consequence, make its absolute mean heat equal to _one degree_ of the ordinary centigrade scale, or, in another way of expressing it, equal to - °. second. let us condense this same nebula to · times less dense than water, and consequently to the density of air at atmospheric pressure, then its diameter will be , , miles, volume , , , ^ cubic miles, and the mean heat °, or the heat of freezing water--which by some unexplained process of thought has hitherto been considered to be ° of absolute _temperature_. third. by again condensing the solar nebula to the density of water, corresponding to a pressure of more than atmospheres, its diameter becomes , miles, its volume , ^{ } cubic miles, and mean heat °, including the ° acquired in condensing it to the pressure of atmosphere, as is plainly shown in table iii. before going any further we must enter into a digression to examine into the process of thought by which the absolute zero of heat has come to be called the absolute zero of temperature, and absolute temperature to be so many degrees of negative--less than ° or nothing--heat counted from the lower or wrong end, to be called positive absolute temperature; thus making heat and temperature appear to be two very different things, without giving any explanation of what is the difference between them. science has, as it were, gone down a stair of steps carrying along with it the laws of gases, and has found, most legitimately, with their assistance the total absence of even negative heat at the bottom of it; and, leaving these laws there, has jumped up to the top of the stair, thinking that it carried along with it ° of absolute heat, which it now calls temperature; instead of bringing the said laws up with it and verifying, if not at every step at least at intervals, how much it brought up with it of what it had taken down. had it done so it would have found that at the top of the stair it had got what was equal to only ° of positive heat as measured by the centigrade scale, as has been shown above, which might be called temperature, but that would not mend matters. science seems to have forgotten, for the time being at least, all about the laws of gases; it had got something which it thought would enable it to mount much higher, and was satisfied. it will not be difficult to do away with the confusion of thought that is thus shown to have occurred. the laws of gases are founded upon the fact that in gases there is a necessary interdependence between heat and pressure, and the starting points adopted by science for calculating this interdependence in them are ° of heat and atmosphere of pressure at ° of heat. obeying these laws, we have argued, from the beginning of our operations, that heat requires something to hold it in, and that the nebula from which the solar system was formed--if it was so formed--could only contain heat in proportion to its density; that is being a gas, or vapour in the form of a gas, it could not contain, i.e. hold in it, more than ° of positive heat when its density was equal to the pressure belonging to atmosphere of a gas; all as shown in the most irrefragable manner in this chapter and in the accompanying table iii. a gas can be easily compressed in a close vessel to a pressure of atmospheres, which would enable it to hold ° of heat due to that compression; in fact, were it compressed to that degree by a piston in a cylinder, without any loss of heat, it would be raised to that heat by that act alone, but that would raise it to only ° instead of ° of what is called absolute temperature according to present usage; because as a gas it could not hold any more heat at that pressure. it is, therefore, evident that this _usage_ has not been derived from the laws of gases. neither has it been derived from the other two states of liquid and solid to which all gases can be reduced, as can be very easily demonstrated. to cool steam at atmospheric pressure from its gaseous to its liquid state ° of heat of one kind and another--as measured by the centigrade thermometer--have to be abstracted from it, which leaves the liquid at its boiling point of °--a quantity that has been arbitrarily adopted to mark the difference between the freezing and boiling points of this liquid. in order, after this, to reduce the liquid, now water, to the freezing, or what is called ° of heat, these degrees of heat have to be extracted from it, which is not very difficult to do because the heat put into it arbitrarily can be extracted from it; but if it is now wanted to change the steam from its liquid to its solid state, the work, or operation assumes a very different character, because heat cannot be extracted from a substance which contains none at all. it is well known that ° of heat are required to change one pound of ice at ° into a pound of water also at ° of heat; but it is equally well known that ° of heat cannot be taken out of the pound of water which has none in it; how then, is the water to be changed into ice? even in cooling water to ° it has to be put into a bath of some kind, either of cold water or some cold mixture of other substances at least as cold; because, otherwise, extraneous heat from any source might find its way into it, and prevent it from cooling down to zero of heat. in the same manner, to change the water into its solid state of ice it has to be put into a similar bath, not to extract heat from it, because it has not any to extract, but to prevent extraneous heat from getting into it. this being the case, it is evident that if water is put into a bath at what is called - ° of heat, or even a fraction of that amount, it will be converted into ice though very gradually, by keeping extraneous heat from getting to it to sustain the collisions, or vibrations, of its constituent atoms necessary to maintain it in its liquid state. all for the very same reason why a stone, a piece of metal, or of anything assumes the same degree of heat, or absence of heat, as the medium by which it is surrounded; be it derived from sun-heat, earth-heat, or heat produced chemically or mechanically, and is not cooled down to a lower degree than the surrounding bath, be it what it may. the heat required to change a solid into a liquid is called _latent heat_, which in the case of ice and water may be a fraction of - ° or - °, or _minus_ almost anything according to the time it is necessary for it to act; so that no quantity of what is called absolute temperature can be ascribed to ice without the element time being involved in it. the absolute temperature of water and ice, just changing from freezing to frozen, might be counted as the same, seeing that a fraction of a degree of heat may make all the difference between them; but no fixed absolute temperature can be applied to ice, as it, in conjunction with all solid bodies, may have any degree of absolute temperature between its melting point and the absolute zero of heat, as far as is at present known. the same, of course, must be the case with any gas or vapour, or nebulous matter changed into its liquid and then solid state; and this fact enables us to go a little further. we have seen that what, according to present usage, is called the absolute temperature of solid hydrogen may be anything between - ° and - ° of heat, that is, between the absolute temperature of ° and °, which, of course, is no measure at all; and, therefore, absolute temperature can only be looked upon as a conventional term, which, when added to positive centigrade, or other, heat, conveys no clear idea to the mind, as it must always be mixed up with the concomitant idea of latent heat and its time of action. this leads us to think of what remains in the vessel, in which pure hydrogen has been changed into its liquid and then solid state, after these operations have been performed; and our first conclusion comes to be that there can be nothing in it but a small piece of solid hydrogen; but from the limited accounts we have seen of these operations, there does appear to be something remaining, because it seems that by it the degree of negative heat in the vessel can be measured. what that remaining something may be can hardly be anything but a matter of conjecture. the first and most probable idea that occurs is that it may be some lighter gas mixed with the pure (?) hydrogen that was put into the vessel; the next is that it may be the vapour of solid hydrogen; and the last refuge for speculation is that it may be radiant matter, whatever that may turn out to be. at one time it was supposed to be impurities mixed with the gases operated upon, which in the case of common air, were found to be removed to a certain extent by means of absorbents; but the numerous components of common air discovered since that time, have gone far to throw light upon that supposition, and we are thus led to think of what a true gas really is. but we are not yet prepared to follow up this thought. this is not an inappropriate place to say that when we adopted the centigrade scale for our work, we thought that a special thermometer, decimal throughout and consequently more handy, might be arranged for science alone, leaving every man the free use of whatever scale he liked best; but our experience acquired in this chapter put an end to that thought, and has left us totally unable to see how any decimal scale can be contrived, which will start from absolute zero of heat and will admit of any combination with any existing scale, or will assist humanity in any of its operations in connection with heat and temperature, whichever science may choose to call it. we therefore see that no known thermometer scale is superior to another, and end where we began by saying that the centigrade is the fashionable one at the present time. it is decimal as far as boiling water and resulting steam are concerned, but all the world is not boiling water; even steam has to be complicated with latent heat. table iii.--abstract of measurements, etc., resulting from the calculations made in chapter v. -------------+--------------+-------------------+---------------+----- | | | | | | volume of the | |incr. nebulæ | | mass of each | times less | of ---- |explanations. | separate system at| dense than |dens- diameter | | density of water | water. | ity in miles. | | in cubic miles. | | in | | | |times -------------+--------------+-------------------+---------------+----- original or | | | | neptunian | | | | , , , | volume of | | , , , | | neptune's | | | | ring | , , , , | , , , | | | | | | volume of | | | | nebula | | | | less ring | | | -------------+--------------+-------------------+---------------+----- uranian |condensed from| | | , , , | neptunian | | | . | nebula | | | | | | | | volume of | | | | uranus' ring | , , , , | , , , | | | | | | volume of | | | | nebula | | | | less ring | | | -------------+--------------+-------------------+---------------+------ saturnian |condensed from| | | , , , | uranian | | | . | nebula | | | | | | | | volume of | | | |saturn's ring | , , , , | , , , | | | | | | volume of | | | | nebula | | | | less ring | | | -------------+--------------+-------------------+---------------+------ jovian |condensed from| | | , , , | saturnian | | | . | nebula | | | | | | | | volume of | | | |jupiter's ring| , , , , | , , , | | | | | | volume of | | | | nebula | | | | less ring | | | -------------+--------------+-------------------+---------------+------ asteroidal |condensed from| | | , , | jovian nebula| | | . | | | | | volume of | | | | asteroidal | | | | ring | , , , | , , | | | | | | volume of | | | | nebula | | | | less ring | | | -------------+--------------+-------------------+---------------+------ martian |condensed from| | | , , | asteroid | | | . | nebula | | | | | | | | volume of | | | | martian ring | , , , | , , | | | | | | volume of | | | | nebula | | | | less ring | | | -------------+--------------+-------------------+---------------+------ earth |condensed from| | | , , |martian nebula| | | . | | | | | volume of | | | | earth ring | , , , , | , , | | | | | | volume of | | | | nebula | | | | less ring | | | -------------+--------------+-------------------+---------------+------ venus |condensed from| | | , , | earth nebula | | | . | | | | | volume of | | | | venus ring | , , , , | , , | | | | | | volume of | | | | nebula | | | | less ring | | | -------------+--------------+-------------------+---------------+------ mercurian |condensed from| | | , , | venus nebula | | | . | | | | | volume of | | | |mercurian ring| , , , | , , | | | | | | volume of | | | | nebula | | | | less ring | | | -------------+--------------+-------------------+---------------+------ solar |condensed from| | | , , | mercurian | | | | nebula | | , | . | | | | |volume @ / | | | , , | of atm. | | , | . | | | | |vol. @ density| | | , , | of atm. | | | . | | | | |vol. @ density| | | , | of water | | | . -------------+--------------+-------------------+---------------+------ -------+---------------------------------------+-----------+----------- | | | | | | | | | | volumes at densities | times | absolute | of respective nebulæ in | less dense| tempera- | cubic miles. | than air. | ture. | | | | | | (degrees.) | | | | | | -------+---------------------------------------+-----------+----------- | | | neptun-| , , , , , , , , , | , , | · ian | | | | , , , , , , , , | .. | .. -------+---------------------------------------| | | , , , , , , , , , | | uranian| | | | , , , , , , , , , | | | | | | , , , , , , , , | , , | · -------+---------------------------------------| | | , , , , , , , , , | | saturn-| | | ian | , , , , , , , , , | | | | | | , , , , , , , , | , , | · -------+---------------------------------------| | | , , , , , , , , , | | jovian| | | | , , , , , , , , , | | | | | | , , , , , , , , | , , | · -------+---------------------------------------| | | , , , , , , , , , | | aster- | | | oidal | , , , , , , , , | | | | | | , , , , , , | , | · -------+---------------------------------------| | | , , , , , , , , | | martian| | | | , , , , , , , , | | | | | | , , , , , , | , | · -------+---------------------------------------| | | , , , , , , , , | | earth | | | | , , , , , , , , | | | | | | , , , , , , | , | · -------+---------------------------------------| | | , , , , , , , , | | venus | | | | , , , , , , , , | | | | | | , , , , , , | , | · -------+---------------------------------------| | | , , , , , , , , | | mercur-| | | ian | , , , , , , , | | | | | | , , , , , | , | · -------+---------------------------------------| | | , , , , , , , | | solar | | | | , , , , , , , | | · | | | | , , , , , , , | | · | | | | , , , , , , | | · -------+-+---------------------------------+---+-----------+------+--- | at density of water. | at air density. | +---------------------------------+---------------+------+ | dimensions of rings. |space to grain | | | | of matter. | | +--------------+------------------+-------+-------+ i | | | | aver- | | side | n | | | thick- | age | | of | c | | breadth | ness |thick- | cubic | cube | h | | in miles. |in miles.| ness | feet. | in | e | | | |in miles| | feet. | s | ---------+--------------+---------+--------+-------+-------+------| | | | | | | | neptunian| ... | ... | ... | , | · | | | | | | | | | | | | | | | | | , , , | , | , | | | | ---------+--------------+---------+--------+-------+-------+------+ | | | | | | | uranian | | | | | | | | | | | | | | | | | | | | | | , , | , | , | , | · | | ---------+--------------+---------+--------+-------+-------+------+ | | | | | | | saturnian| | | | | | | | | | | | | | | | | | | | | | , , | , | , | , | · | | ---------+--------------+---------+--------+-------+-------+------+ | | | | | | | jovian | | | | | | | | | | | | | | | | | | | | | | , , | , , | , | , | · | | ---------+--------------+---------+--------+-------+-------+------+ | | | | | | | aster- | | | | | | | oidal | | | | | | | | | | | | | | | , , | | | , | · | | ---------+--------------+---------+--------+-------+-------+------+ | | | | | | | martian | | | | | | | | | | | | | | | | | | | | | | , , | | | | · | | ---------+--------------+---------+--------+-------+-------+------+ | | | | | | | earth | | | | | | | | | | | | | | | | | | | | | | , , | | | | · | | ---------+--------------+---------+--------+-------+-------+------+ | | | | | | | venus | | | | | | | | | | | | | | | | | | | | | | , , | | | · | · | | ---------+--------------+---------+--------+-------+-------+------+ | | | | | | | mercurian| | | | | | | | | | | | | | | | | | | | | | , , | | | · | · | | ---------+--------------+---------+--------+-------+-------+------+ | | | | | | | solar | | | | | | | | ... | ... | ... | · | · | · | | | | | | | | | ... | ... | ... | · | · | · | | | | | | | | | ... | ... | ... | · | · | · | ---------+--------------+---------+--------+-------+-------+------+ returning now to page , we see that the volume of the sun alone was considered to be , ^{ } cubic miles, which corresponds to a diameter of , miles. comparing this with the volume , ^{ } cubic miles, see page , left after all the members of the solar system have been separated from the original nebula, we find that there is a remainder of , , , , cubic miles _less_ than we ought to have. but it will be remembered that we added only / th part to the mass of the sun for the mass of the whole solar system, whereas it will be seen, by referring to table ii., that we ought to have added / · th part. had we done so the sphere containing the whole solar system at the density of water would have been , · miles in diameter with volume of , , ^{ } cubic miles, which would have added , , , , cubic miles to the volume we started with, and would have left us with , , , , cubic miles _more_ than we ought to have had. besides, for the sake of round numbers, we made the diameter of the nebula containing the whole solar system, at the density of water, to be , instead of , · miles, and thereby really added more to the original volume than we should have; so that the defects in accuracy at the beginning of our work partially counterbalanced each other, which accounts so far for the difference noted at the end not being much more than half of what it should have been. taking all this into consideration, and the really insignificant magnitudes of the differences that would result from the corrections that could be made, we have not thought it necessary to reform the whole of our calculations. besides, the data we have been working upon are not so absolutely exact as to insure us that we should get nearer to the truth by making the revision. the whole error would be much more than obliterated were we to apply · instead of · for the mean density of the earth to the debit side of the sun's account. to simply describe arithmetical operations conveys no really satisfactory meaning to the mind; of working them out in full there is no end; and to partially represent them as we have done in these pages, although showing how the results are arrived at, still leaves them so mixed up together that it is difficult to compare them with each other, and to note the sequences from the beginning to the end of the whole operation. for these reasons we have compiled table iii., where the whole of the principal and most important data, and results from them, may be followed out and examined. we may now say that we have taken our nebula to pieces, with the exception of the parts belonging to the satellites of those planets which have them; which would only be a tiresome repetition of what we have done for each principal member of the system, provided we had the necessary data, which we have not; and have thus acquired a certain amount of knowledge of the primitive conditions of each one of them. but we have still to examine into and draw conclusions from what we have seen and learned during the operation; which in some points, differ very much from our notions, formed from what we had previously read on the subject. chapter vi. page analysis continued. excessive heat of nebula involved condensation only at the surface. proof that this was laplace's idea. noteworthy that some astronomers still believe in excessive heat. interdependence of temperature and pressure in gases and vapours. collisions of atoms the source of heat. conditions on which a nebula can be incandescent. sir robert ball. no proper explanation yet given of incandescent or glowing gas. how matter was thrown off, or abandoned by the jovian nebula. division into rings of matter thrown off determined during contraction. how direct rotary motion was determined by friction and collisions of particles. saturn's rings going through the same process. left to show process. form gradually assumed by nebulæ. cause of saturn's square-shouldered appearance. a lens-shaped nebula could not be formed by surface condensation. retrograde rotary motion of neptune and uranus, and revolution of their satellites recognised by laplace as possible. satellites of mars. rapid revolution of inner one may be accounted for. laplace's proportion of millions not reduced but enormously increased by discoveries of this century. analysis of the nebular hypothesis--_continued_. when laplace elaborated his hypothesis, heat was considered to be an imponderable material substance, and continued to be thought of as such--though perhaps not altogether believed to be so--for somewhere about half a century afterwards; so that it cannot be wondered at that he thought the nebula could have been endowed with excessive heat, more especially as it was looked upon as imponderable, and could in no way have any effect on the mass of the nebula. he only accepted the idea that was common to almost all astronomers of his time, that nebulæ were masses of cosmic matter of extreme tenuity but self-luminous, and consequently possessed of intense heat; they saw the sun gave light and felt its heat, and very naturally thought the nebula must be hot also. without this idea he could not have formed the hypothesis at all, because he could not have conceived that the condensation of the nebula could only take place at its surface, or, as he terms it, "in the atmosphere of the sun," as most assuredly would be the case with an excessively hot body. and in order that there may be no doubt about this being his idea, we quote his own words as guaranteed by m. faye in "l'origine du monde": "la considération des mouvements planétaires nous conduit donc à penser qu'en vertu d'une chaleur excessive l'atmosphère du soleil s'est primitivement étendu au delà des orbes de toutes les planètes, et qu'elle s'est reserrée successivement jusqu'à ses limites actuelles." and again: "mais comment l'atmosphère solaire a-t-elle déterminé les mouvements de rotation et de révolution des planètes et des satellites? si ces corps avaient pénétré profondément dans cette atmosphère, sa résistance les aurait fait tomber sur le soleil. on peut donc conjecturer que les planètes ont été formées à ses limites successives par la condensation des zones de vapeurs qu'elle à dû, en se refroidissant, abandonner dans le plan de son équateur." proceeding on these ideas laplace was quite in order and logical in conceiving that successive rings could be abandoned by the hot nebula, through the centrifugal force of rotation, for the formation of planets, more or less just in the way we have separated them. having obtained his end quite legitimately, as he thought, in this way, he had no occasion to look any deeper into the affair, and consequently was not under the necessity of taking any thought of what the interior construction of the nebula might be, any more than so many others have not done since his day. that he should have conceived the nebula to have been endowed with intense heat was, as we have already said, a natural consequence of the mistaken notions of the nature of heat at that period; but that so many astronomers should, up to the present day, think that the nebula must have been intensely hot, even to the degree required to dissociate the meteorites of which they conceive it to have consisted, seems to us to be almost inconceivable. we believe we have shown abundantly plainly, that there could have been almost no heat in the primitive nebula, because there was hardly any cosmic matter to hold it in. we have given as proof of this the laws of gases recognised and accepted by every scientist, according to which a gas cannot contain a stated amount of heat except it be at a pressure corresponding to that temperature, that is, unless it is subjected to conditions foreign to its natural state. therefore we must either persist in maintaining that there was almost no heat in the original nebula, or we must throw the laws of gases to the winds, for they all depend one upon another. there may be nebulæ possessed of very high temperature, that of incandescence for example, but certainly the nebula out of which the solar system was made, could not have contained more heat than what we have shown it had at the various stages through which we have carried it. if there be nebulæ at the temperature of incandescence, they must be possessed of densities, or pressures, corresponding to that temperature. a few pages back we have spoken of the impossibility of two grains of matter feet apart, raising, by mutual collisions, their temperature and that of the space occupied by each to the temperature of incandescence, and if we now substitute for them meteorites of a pound weight each, the space occupied by each of them will be a cube of feet to the side, which does not help us in any way to believe that the spaces occupied by them could be heated up by their collisions, so as to shine with the temperature of incandescence. so we get no help from meteorites. some people evidently seem to think that nebulæ can be incandescent and give the spectrum of incandescent gas, without their density or pressure being increased to the corresponding degree. sir robert ball seems to be one of them, though at the same time he appears to be not altogether sure of it. when discussing the self-luminosity of the nebula in orion, in his "story of the heavens," ed. , p. , he says: "we have, fortunately, one or two very interesting observations on this point. on a particularly fine night, when the speculum of the great six-foot telescope of parsonstown was in its finest order, the skilled eye of the late earl of rosse and of his assistant, mr. stoney, detected in the densest part of the nebula myriads of minute stars, which had never before been recognised by human eye. unquestionably the commingled rays of these stars contribute not a little to the brilliancy of the nebula, but there still remains the question as to whether the entire luminosity of the great nebula can be explained, or whether the light thereof may not partly arise from some other source. the question is one which must necessarily be forced on the attention of any observer who has ever enjoyed the privilege of viewing the great nebula through a telescope of power really adequate to render justice to its beauty. it seems impossible to believe that the bluish light of such delicately graduated shades has really arisen merely from stellar points. the object is so soft and so continuous--might we not almost say ghost-like?--that it is impossible not to believe that we are really looking at some gaseous matter." here we see that his own belief about the matter is not very firm. he admits that the stars contribute not a little to the brilliancy of the nebula, and the most he can say in favour of its shining with its own light is, that it seems impossible to believe that the light has arisen merely from stellar points. he then goes on to show how the self-luminosity may be explained, as follows:-- "but here a difficulty may be suggested. the nebula is a luminous body, but ordinary gas is invisible. we do not see the gases which surround us and form the atmosphere in which we live. how, then, if the nebula consisted merely of gaseous matter, would we see it shining on the far distant heavens? a well-known experiment will at once explain this difficulty. we take a tube containing a very small quantity of some gas: for example hydrogen; this gas is usually invisible; no one could tell that there is any gas in the tube, or still less could the kind of gas be known; but pour a stream of electricity through the tube, and instantly the gas begins to glow with a violet light. what has the electricity done for us in this experiment? its sole effect has been to heat the gas. it is, indeed, merely a convenient means of heating the gas and making it glow. it is not the electricity which we see, it is rather the gas heated by the electricity. we infer, then, that if the gas be heated it becomes luminous. the gas does not burn in the ordinary sense of the word; no chemical change has taken place. the tube contains exactly the same amount of hydrogen after the experiment that it did before. it glows with the heat just as red-hot iron glows. if, then, we could believe that in the great nebula of orion there were vast volumes of rarefied gas in the same physical condition as the gas in the tube while the electricity was passing, then we should expect to find that this gas would actually glow." there is a great deal to be said about this explanation. we presume that a very small quantity of hydrogen gas means that it was considerably below atmospheric pressure. even so we admit that by introducing sufficient heat into the tube by means of electricity or otherwise, the gas could be raised to the temperature of incandescence, but its pressure would, at the same time, be increased to the corresponding force measured in atmospheres; and we also admit that when the gas was allowed to cool down to its original temperature, the same quantity of hydrogen would be found in the tube; but how about the tube? when the gas came to be at the temperature of incandescence the tube would be the same, or very soon raised to it, and being made of glass would be sufficiently plastic to be distorted, or even burst by the pressure within, probably even before the gas reached the temperature of incandescence. we must not forget that the first appearance of incandescence begins with red heat whose temperature is not far from ° in daylight, and that white heat rises to above °. if the experiment was made in an almost capillary tube, sufficiently thick to prevent accidents, then it might appear to prove a foregone conclusion, but nothing else; it might keep the idea of pressure out of sight, but it could not prove that the gas inside was in a rarefied state when incandescent. that the gas glowed the same as a red-hot bar of iron has not been shown. the gas had to be shut up in a tube to make it glow, but the bar of iron could glow outside of the tube. could a streak of hydrogen be put into a furnace along with a bar of iron and heated to incandescence by its side, there might be some fair comparison between them, as long as they were in the furnace together, but the moment they were taken out the glow would disappear from the gas, whereas the iron would glow for some time. on the other hand we might _say_ that a stream of incandescent gas might be made to heat a bar of iron in an oven to its own temperature, but the moment the stream of gas and the iron bar were removed from the oven, the former would disappear at once and the latter would continue to glow, simply because it was dense enough to contain a very considerable supply of heat compared to what the gas could, or rather, because the pressure of the gas, even did it correspond to the temperature, would disappear at once and the heat with it. so it is not always safe to _say_ things. but it is quite safe to say that no gas--or substance such as we are accustomed to look upon as gas--can abide in a state of incandescence, and merely glow, unless its pressure, or density, corresponds to the temperature of incandescence; which for red heat (in the dark) would be ° = · atmospheres, and for white heat at ° = · atmospheres, above absolute zero of pressure in both cases. and also, that if the self-luminosity of a nebula arises from incandescent gas, the pressure in the gas of that nebula must be somewhere between and atmospheres above absolute zero of pressure. now we have shown, at page , that the density and pressure in the solar nebula, at the stage there specified, could not have been more than the millionth part of those of our atmosphere, and consequently were justified in asserting that in it there could be almost no heat whatever. we have just been speaking of a streak of gas and a bar of iron being heated in an oven to a red or white heat side by side, but everybody knows that this could not be done; but everybody has not thought of why it could not be done, otherwise sir robert ball would not have favoured us with his laboratory experiment of a streak, or remnant, of hydrogen in a glass tube. we know that a plate, or bar, of iron can be heated up to the temperature of incandescence in an oven, but it has never occurred to anyone, who has seen the thing done, that the gas, air, or vapour which heats them must be at a pressure corresponding to that temperature. multitudes of people may have thought of how the thing is done, but apparently very few have thought that it is not the gaseous part of the current of heated matter introduced into the oven, that heats it and the metal in it, but the solid part which is the distinctive and most important part of the constituents of the current. the solid part of the matter--let it be gas or any other element--is heated to incandescence in some furnace and carried along by the gaseous part--that is the _stuff_ that fills the empty spaces between the solid molecules--to give it out to the oven and iron. we are not sure that the gaseous part even glows. we see plainly enough that the walls of the oven glow, but with respect to the gas, or carrying agent, we are inclined to think that it rather dims the glow of the oven and iron than otherwise. in passing, we say it is not unreasonable to suppose that the solid matter which contained the heat till it was given out, consisted of the elements which were put into the furnace to raise the heat, and of those which were drawn in by the draught--in a word, the elements of combustion--but about the carrying constituent there is a great deal to be said after we know more about it. it seems to us from all this that the hydrogen gas in sir robert ball's tube was not made to glow by heating up to the temperature of incandescence, but somehow by the electricity passing through it, if it did pass. we, therefore, come to the conclusion that the light of nebulæ does not come from gas--or what we call gas--heated up to be incandescent merely to make it glow, and that it might be as cold as the light that comes from the aurora, or as that of a glow-worm. sir robert ball refers to stellar points seen through the nebula, and acknowledges that part of the glow may be due to them, which shows that the nebula must have been excessively tenuous; for we know how thin a cloud will hide sirius from us, and we think that nobody will assert that two grains of matter dispersed in , , cubic feet of space, as we have seen at page , would hide sirius from us. therefore, we must acknowledge that the glow of nebula in orion, observed by sir robert ball, was caused either by the stellar points, or by some other thing that most assuredly could not be gas heated to the temperature of incandescence, or in part from both. for we believe that the glowing of nebulæ, fluorescence, phosphorescence, will-o'-the-wisp, auroras, fire-flies, fire-on-the-wave, etc., etc., all, all proceed from the same cause. we may now proceed to say a few words about the separation of the rings for the planets, brought about by the rotation of the nebula on its axis, and the centrifugal force produced throughout it thereby. we have shown, at page , that a ring could not be detached from the nebula at once in one large annular mass, as it seems to have been the common notion was the mode of separation; and we shall now try to show with some detail what the process must have been, notwithstanding that it has been in a general way described by others; because, like everything else, there is something to be learnt from it. for this purpose we shall select what we have called the jovian nebula, because we can suppose, for the present, it must have been more nearly in the form of a sphere than either the original or any of the exterior nebulæ, which may not have been properly licked into shape, as it were; and also because we have found that the thickness and mass of the ring for his, jupiter's, system were vastly greater than those for any other one of the planets. we have made the jovian nebula to have been , , , miles in diameter, and the greatest thickness of the ring detatched from it to have been , , miles. now in a circle of that diameter, a chord of the length of that thickness would subtend an arc of very little more than minutes, one half of which we shall suppose to be measured on each side of the equatorial diameter of the nebula at right angles to the diameter; then, the middle ordinate of a chord of , , miles long, would be miles long. this length would be a very small fraction of the radius of the circle which would be , , miles long, but in a rotating sphere of the same dimension, we must acknowledge that the centrifugal force at the middle of the arc would be greater--however small the difference--than at its ends, and would sooner come to balance the force of gravitation; therefore we must admit that the process of separation would begin there by abandoning a thin layer of matter, convex on the outer side and in a measure concave on the inner side, for the reason just given, much the same as a layer that could be peeled off from the equator of an orange--the poles and equator of an orange are easily distinguished. as the velocity of rotation increased another layer would be abandoned following the first, so far curved on both sides, i.e. convex and concave, and the same process would continue on and on, according as the centrifugal force continued to balance that of gravitation, till the whole of the matter for all the attendants of the sun was abandoned; so that in the process itself no such division of rings as we have been following could have taken place, but one continuous sheet, as it were, would be formed from first to last. whether the thickness of the ring for jupiter's system, or any other system or planet, was limited to the length of the chord we have been dealing with, or came to be many times greater or even less, makes no difference on our explanation. after being abandoned in a sheet, as we have shown it would be, the centrifugal force they had acquired would, for a time at least, keep the particles of the sheet near the radial positions they then occupied, and their mutual attraction would go on diminishing its thickness, till finally the radial attractions among the particles divided the sheet into entirely separate rings after the manner of those of saturn; which would in due course break up and form themselves into the smaller nebulæ from which the planets were supposed to have been made. m. faye has made it a great point against the nebula hypothesis that when these rings broke up, the rotary motions of the planets resulting from them would be retrograde, because the outer parts of them would be travelling at a slower rate than the inner ones, and has taken the trouble to construct a diagram to show how this would be the case; but he himself has told us, in "l'origine du monde," that laplace had duly considered this point, and had shown how the friction of the particles of the flat rings among themselves would, through course of time, retard and accelerate each other, so that a ring would come to revolve as if it were one solid piece, and consequently that the outer edge of the ring would come to be travelling faster than the inner one, which according to his (m. faye's) own showing would produce, on breaking up, a planet with direct motion of rotation. laplace's words, as cited by him, are:--"le frottement mutuel des molécules de chaque anneau a dû accélérer les unes et retarder les autres jusqu'à ce qu'elles aient acquis une même mouvement angulaire. ainsi les vitesses réelles des molécules éloignées du centre de l'astre out été plus grandes. la cause suivante a dû contribuer encore à cette différence de vitesse: les molécules les plus distantes du soleil et qui, par les effets du refroidissement et de la condensation, s'en sont rapprochées pour former la partie supérieure de l'anneau out toujours décrit les aires proportionnelles aux temps, puisque la force centrale dont elles étaient animées a été constamment dirigée vers cet astre; or cette constance des airs exige un accroissement de vitesse à mesure qu'elles s'en sont rapprochées. on voit que la même cause a dû diminuer la vitesse des molécules qui se sont élevées vers l'anneau pour former sa partie inférieure." in his method of bringing all the molecules of matter in a ring, to revolve round the centre as if they formed one sole piece, laplace does not appeal to any accommodating force among them except friction, while he might have called in that of the collisions of the molecules amongst themselves. it is not to be supposed that each molecule would remain fixed in the position it occupied when separated from the nebulæ, and only went on rubbing against--and creating friction with--its neighbours, and only creeping closer to the centre or farther from it, as it was acted upon by the attraction of the other parts of the ring. the molecules would be rushing against each other in all directions, in spite of, although in the main obedient to, the law of attraction; and we could conceive the possibility of molecules gradually working their way from the extreme outer edge to the extreme inner edge of a ring, or _vice versâ_, which would be a much more effectual means of bringing about one period of revolution throughout the whole ring, than the simple force of rubbing against each other. when physicists get a gas shut up in a close vessel, they grant to its molecules the power of committing exactly the same kind of freaks; and a planetary ring is, to all intents and purposes, a closed vessel to our molecules; because they have been placed in it by the laws of attraction and centrifugal force, and there is no other force acting upon them sufficiently powerful to liberate them from it. therefore there is no reason why a molecule in a ring should be always wedged up in one place, especially after we have shown that each molecule of matter, in any of the rings we have been dealing with, must have had a much greater free path to move about in, than a molecule of gas shut up in any of the vessels used by physicists. we have no reason to look upon the rings of saturn otherwise than as in process of being converted into one or more satellites, most probably more than one; because if the matter they are composed of has been separated from the planet in the form of a sheet, the same as we have seen must have been the case with the matter separated from the original nebula for the planets, the sheet has been already divided into at least three distinct parts, and surely that cannot have been done without some object. if these rings have been left, as has been said, in order to show us how the solar system has been formed, that does not authorise us to conclude that they will always remain in the form they have. there is no reason why the lesson should not be carried out to the very end, through the breaking up of the rings, formation of spherical nebiculæ, and finally satellites. it would be rash to assert that the matter of which any one of them is composed--be it atoms, molecules, meteorites, or brickbats--cannot, through friction and collisions of its particles among themselves, come to revolve around saturn as if it were one solid piece. but should anyone do so, and adopt m. faye's condemnation of laplace's mode of forming rings, he must confess that when saturn's rings are converted into satellites, their rotations must be retrograde; and it might be, for him, an interesting inquiry to find out whether the rotations of the existing satellites are direct or retrograde. astronomers have learnt the lesson as far as it has gone, have noted and registered the state of affairs as it is at present, and their successors will no doubt do the same as changes succeed each other. the day may be inconceivably remote, but it will inevitably come for the rings to be changed into satellites, unless they are disposed of in some other way. it has been said that were the rings to break up, in consequence of their being in a state of unstable equilibrium, they would fall back upon the planet, but that would depend on circumstances. if the motion of their revolution were stopped altogether, they would certainly fall back upon the planet; but if it were not stopped then each molecule would retain its centrifugal force, and would revolve around the primary on its own account, just as, according to very general opinion, it does at present. we do not see why, or for what purpose, these rings could have been separated from saturn merely to fall back upon him again. it would be rather a strange way of giving a lesson if it were stopped, by a cataclysm of some kind, just when the most interesting part of it was in a fair way of being exhibited. such a proceeding would assuredly not suit the ideas of those who believe that the solar system has been self-formed by a simple process of evolution. during the whole process of separation of rings from the original nebula, the nebulous matter would be abandoned in what we may call the form of thin hoop-shaped rings, so that the equatorial region of the nebula would be flat--as we have shown at p. --and when the nebula came to be so much reduced that it could abandon no more matter through centrifugal force, its form would be, in some measure, like that of a rotating cylinder terminating at each end in a cap in the form of a segment of a sphere. when explaining the formation of planetary rings, we have seen that in the jovian nebula the length of the flat part would have come very soon to be nearly , , miles, and that it would increase rapidly. but, remembering that the flattening of the equatorial part must have begun on the original nebula, we see that the flat part must have increased vastly in length before it reached jupiter, and that by the time the residuary, or solar, nebula was reached--which we made to be only a little over , , miles in diameter--the cylindrical part of it would bear no small proportion to that diameter. taking this form of the nebula into consideration, and also the fact that the separation of matter from it by centrifugal force could not always be absolutely equal all around it, the swaying in its rotary motion produced by the all but inevitable inequality of mass, at the two ends of the cylindrical part, and at the sides of the segmental caps, may have been the cause of the differences in the inclinations of the orbits of the planets to the ecliptic; and especially of why the difference came to be so much greater in the case of mercury than in any of the others. in connection with this very reasonable conclusion as to the form of the nebula almost from the beginning, we may add that, when it ceased to throw off rings, it would be very much in the same condition as saturn is at the present day. therefore we may conclude with very great safety, that the present form of saturn is that of a cylinder with segments of spheres forming the ends; and in this manner can account for his square-shouldered appearance, which has puzzled more than one astronomer. the idea has been very general that in condensing and contracting, the nebula would gradually come to assume the form of a lens of a very pronounced character, from the circumference of which the rings would be abandoned one after the other; but when thoroughly looked into, it is difficult to see how this could be the case. in a sphere of cosmic matter contracting equally all round towards the centre through the force of attraction, it is more natural to suppose that the separation of matter from its equator through centrifugal force, would have a tendency to diminish the equatorial more rapidly than the polar diameter, as we have been trying to show above, more especially as the attraction of the matter in the rings as they were abandoned one after the other would, in a constantly increasing degree, assist the centrifugal force in facilitating the separation by drawing the matter outwards. matter falling in from the polar regions would afterwards require to have its motion turned off at right angles before it could be sent off by centrifugal force to the equator, an operation which would be more easily effected in the equatorial regions, where the gravitating motion had only to be retarded; and as very unequal amounts of density could not be created in the interior parts of such a sphere by gravitation, so as to cause pressure outwards, it is difficult to show how the polar diameter could be more rapidly reduced than the equatorial diameter, which was being continually shorn of its length. it may be said that all that we have been writing in the last few pages is absurd, because we have been proceeding on the supposition that the condensation of the nebula was effected at or near its surface. laplace procured this condition by piling up imponderable heat in his nebula, but he might have got it otherwise. given a nebula such as the one we are dealing with of , , , miles in diameter, where would condensation be most active? most undoubtedly where there was the greatest mass of matter. compare, then, the mass of , , miles in diameter at the surface with the mass of the same diameter at the centre, and we cannot hesitate for a moment in concluding that the most active condensation would not be very far from the surface. not only so, but the same would continue to be the case, at least until the last ring was abandoned. thus by working upon what may have appeared to be an absurd foundation, i.e. condensation at the surface due to the intense heat of the nebula, we have been able to acquire more correct ideas than we had before, of how the solar system was elaborated. but we shall have much more to say on the same subject hereafter. there has been a great outcry raised about the rotation of the planets neptune and uranus being retrograde, as is correctly concluded to be the case from the revolution of their satellites being retrograde, but we do not see that there has been any good reason for it. laplace, no doubt, concluded, wrongly, that the motions of all the bodies of the solar system--as known to him--were direct, and therefore used that conclusion in showing that there were milliards against in favour of his hypothesis being right; but at the same time it cannot be concluded that he thought that it would be destroyed by the motion of rotation of one or even several of the forty-three bodies turning out to be retrograde; because, when discussing the hypothesis of buffon, he states, most distinctly, that it is not necessary that the rotation of a planet should be in the same sense as that of its revolution, and that the earth might revolve from east to west, and at the same time the absolute movement of each of its molecules might be directed from west to east. his words as cited by m. faye in "l'origine du monde," at page , are: "a la verité, le mouvement absolu des molécules d'une planète doit être alors dirigé dans le sens du mouvement de son centre de gravité, mais il ne s'ensuit point que le mouvement de rotation de la planète soit dirigé dans le même sens; ainsi la terre pouvait tourner d'orient en occident, et cependant le mouvement absolu de chacune de ses molécules serait dirigé d'occident en orient, ce qui doit s'appliquer au mouvement de révolution des satellites, dont la direction, dans l'hypothèse dont il s'agit, n'est pas nécessairement la même que celle de la projection des planètes." he seems to say, "this would suit buffon's hypothesis, but i do not require it for mine." even were this not so, it would not be very difficult to account for the retrograde rotation of these two planets, but we are not yet prepared to show, in a convincing manner, how these motions were produced. we have to show first how the nebula itself was brought to the dimensions at which we took it up, and there is a great deal to be done before we can show that. should our belief in being able to explain how the retrograde rotations of uranus and neptune were brought about turn out to be unacceptable, we would not condemn the nebular hypothesis, because, as m. faye himself says, if we add the asteroids to laplace's we should have somewhere about bodies, all with direct motion, agreeing with the hypothesis, against that do not, that is about to instead of to , which was all laplace could claim. moreover, we have not been able to see that m. faye's objections to it are well founded, rather the contrary; nor can we agree with him when he says that when one point in a hypothesis is found to be erroneous it ought to be abandoned altogether, and something better sought for. is his something any better? all acquired knowledge has been built up from ideas collected from all sides, and from errors reformed. what would a grammarian say were we to return to him his grammar as useless, because we had found one exception to one of his rules against cases in which we had found it to be right? perhaps it would put him in mind of the name of a tree. and grammar is not the only case in which we say that the exception confirms the rule. in taking the nebula to pieces, we have taken no notice of the satellites of mars, not only because they are so small that they would have had no sensible effect on our calculations, but because we cannot conceive that they could have been abandoned by the planet, when in a nebulous state, in the same manner as the planetary rings are supposed to have been by the parent nebula; and we might simply refer to the dimensions, especially the thinness, we have found for the ring out of which mercury was formed, for proof of our assertion; but for more satisfactory corroboration, we will go a little deeper into the affair. let us take the diameters of mars and of the orbits of his satellites, as they are stated in text-books of astronomy; that is , , and , miles respectively, and suppose the diameters of what--in the method we have applied to the planets--we would call the deimos and phobos nebulæ to have been , and , miles also, respectively; then these two diameters would make the breadth of the ring for the formation of deimos to have been miles. with these data, if we go through a series of calculations with respect to this outer satellite, in all respects similar to those we have made for each of the rings of the planets, we shall find that the ring for deimos would have been only · _inches_ thick, without taking into account its condensation during the process of separation. this, of course, points out at once the impossibility of any such operation going on in nature. we can imagine the possibility of a ring of even millions of miles broad, and of very great tenuity, holding together provided it be hundreds of thousands of _miles_ thick, but to think of one , miles broad and less than _inches_ thick holding together is another affair altogether. with respect to phobos, it is only necessary to say that he revolves round mars in considerably less than one-third of the time that he ought to, and is therefore not a legitimate production of the nebular hypothesis any more than deimos can be. here, then, we have come upon two bodies, one of which has not been formed in the way, and the other has not the proper motion, prescribed in the hypothesis; but we do not think ourselves justified in declaring it to be worthy of condemnation on that account, seeing that we have found no other difficulty in working out the solar system from it. moreover, it is not impossible, nor do we think it at all improbable, that through the course of time astronomers may discover that phobos is a captured asteroid--perhaps deimos also--gradually working its way into final annexation. and who can tell how many of these erratic bodies jupiter and mars may have captured already? in the dark as it were, for they may have been too small to be noticed when they were being run in. neither of these two worthies has ever been very much celebrated in song or history for respect for his neighbour's property. jupiter is credited with sorting out the asteroids and arranging them in bands, and perhaps he has been human enough to exact a commission for his labour; and it might be more in his line, and certainly much more easy for mars, to take forcible possession of as many of them as came within his reach. chapter vii. page analysis continued. no contingent of heat could be imparted to any planet by the parent nebula. only one degree of heat added to the nebula from the beginning till it had contracted to the density of / th of an atmosphere. increase in temperature from ° to possible average of ° when condensed to , , miles in diameter. time when the sun could begin to act as sustainer of life and light anywhere. temperature of space. the ether devised as carrier of light, heat, etc. etc. what effect it might have on the nebula. first measure of its density, as far as we know. the estimate _too_ high. may be many times less. return to the solar nebula at , , miles in diameter. plausible reason for the position of neptune not conforming to bode's law. the ring being very wide had separated into two rings. bode's law reversed. ideas suggested by it. rates of acceleration of revolution from one planet to another. little possibility of there being a planet in the position assigned to vulcan. densities of planets compared. seem to point to differences in the mass of matter abandoned by the nebula at different periods. giving rise to the continuous sheet of matter separating into different masses. probably the rings had to arrive at a certain stage of density before contracting circumferentially. possible average temperature of the sun at the present day. central heat probably very much greater. churning of matter going on in the interior of the sun, caused by unequal rotation between the equator and the poles. coming back to the period when we reduced the residuary nebula to the density of our atmosphere with temperature of °, or freezing water, we can with confidence affirm that none of the rings abandoned by it for the formation of planets, could have carried with them any contingent of heat to help them in their formation--any beyond the temperature of space--for even if they did it would very soon be reduced to that. each one of them in condensing, breaking up, rejoining the broken fragments, converting itself into a minor nebula, and finally constituting itself as a planet, must have accumulated in the process its own heat requisite to convert it into a molten liquid globe--a stage of existence through which they are all, that is, the major planets, acknowledged to have passed, or have to pass. during that process its primitive annular form, and the multitude of fragments into which each one of them broke up, would present sufficient radiating surface, not only to dispose of all the heat it could have brought with it from the nebula, but a considerable part of the little it could create for itself while contracting and condensing. we may even go farther and assert that no one of them would have any necessity for being supplied with extraneous heat until it had, in a great measure, exhausted the stock it had produced for itself, or so far as to cool down from the molten liquid to the solid state, and to the stage when vegetable and animal life could exist upon its surface. we have no reason for supposing that an enormous supply of extraneous heat was crammed into each nebula, merely to be radiated into space before condensation could take place, and thus retard the execution of the work in hand. if there are astronomers or physicists who believe that the sun could not acquire by gravitation, all the heat he must have expended during geological time, they must look for it in some other source than that of useless and impossible cramming. hitherto we have said nothing of heat being radiated into space by the nebula during our operations, because there could be almost absolutely none to radiate from it at ° of temperature. no doubt there is a large range between this and the absolute zero of temperature which is - °; but we have seen, at page , that when the nebula was condensed from , , to times less dense than air, only _one degree_ was added to its temperature--that is, it was raised from - ° to - °--and that these - ° of absolute temperature were added to it in its condensation from being only times less dense than air to atmospheric pressure, when its temperature became ° of the ordinary centigrade scale. therefore the only period when there could be any measurable radiation of heat into space would be between the times when the diameter of the nebula was (see table iii.) between , , miles and , , miles. even when the end of this period came, the temperature, after a contraction of , , miles in diameter, would be only - ° raised to °--in other words - ° raised to °--and that would not furnish much positive heat--heat such as we are accustomed to deal with--to be radiated into space, whose temperature is without doubt somewhat warmer, so to speak, than - °. and let us repeat, and fix it in our memory, that this - ° was equal to only ° of positive heat. if we now suppose the nebula to be condensed to one-tenth of its volume, with consequent density of atmospheres, and corresponding diameter of about , , miles, its temperature would be ° of the ordinary centigrade scale--according to our mode of calculating hitherto--provided no heat had been radiated from it into space in the meantime. of course this could not be the case, but we have no means of calculating what the amount of radiation would be, and it will not make much difference on our operations to take no notice of it. however, it is here necessary to take into consideration that ° would be the average temperature of the nebula; consequently, if condensation was most active where the greatest mass was, which certainly could not be at the centre or even near it, there also heat would be produced most rapidly, from whence it would spread towards the centre and surface. from the centre it would have no outlet, and would accumulate there as condensation advanced; whereas from the surface it would be radiated into space, and would tend to decrease in amount, so that we may conclude that the surface must have been considerably colder than the centre. if to this we add the fact that, in order to get to the surface, heat would have to be conducted, or conveyed by currents; over from one to two millions of miles, it becomes all the more certain that the central heat would be very much greater than that of the surface. how much less it would be at the surface we cannot pretend to calculate, but we may suppose it to have been from one-fifth to one-third of the average, or rather, somewhere between ° and °, which we have taken, at page , to be the temperatures of red-heat and white-heat. and thus we come to find that the nebula, which was supposed to be endowed with excessive heat when it extended far beyond the orbit of neptune, could not have radiated either heat or light into space to much purpose, until it had been condensed into not much more than , , miles in diameter. this then we must acknowledge to be the earliest period at which the sun began to act as the life sustainer of his system; because, even were it to be found that there are other planets revolving within the orbit of mercury, which we do not think very probable, we have seen that he could have no light or heat with sufficient vivifying power to radiate to them, till his diameter was reduced to not far from what we have shown above. even then the sun would most likely be very much less brilliant than he is now, but the light may have been sufficient to promote vegetation on mars--or the earth, if it was sufficiently cooled down from its molten state--and not much heat would be required by him, as there would probably be a remnant of his own interior heat, still sensible at the surface, sufficient for vegetation at least. we have had occasion to refer several times to the temperature of space, and, though we cannot pretend to determine what it is, our operations enable us to show that it must be very much less than any estimate of it that has ever come under our notice. the nearest approach made to absolute zero by m. olzewski, in his experiments on the liquefaction of gases, as reported in the "scientific american" of june , , was - °, or so-called ° of absolute temperature,[d] which would correspond to a density of · of an atmosphere. this could not be the density of space, because it can be easily shown that our nebula, when at the same density, must have had a diameter of about , , miles, and we must admit that were a globe of this diameter rotating in a medium of its own density, the friction between the two would have been so great as to put a stop to the rotation before very long. we may even say that distinct rotation could never have been imparted to it. following the same reasoning, we must acknowledge that the density of space must be much lower than that of our original nebula, if that could be, and therefore we can assert with confidence that the temperature of space must be far below - °. [d] from the same source, date june , , we learn that the greatest cold probably ever reached was - · ° or · ° of so-called absolute temperature, but that will have very little effect on our calculations, and so it is not worth while altering them all to suit. here our operations put us in mind that we have said nothing yet about the ether, or what effect it might have on our nebula and the bodies formed out of it. we have not done so for the simple reason that, with one exception, it has never been taken into account in any scientific work that has come into our hands, except so far as its being called upon to perform the offices of a dog that has been taught to carry and fetch, and we have not known how to deal with it. but as we have come along, we have seen that it must have had something to do with the density, and consequent temperature, of all the bodies we have been dealing with, and that, if properly studied, it may enable us to account for some things that we have never seen, to our mind, properly explained. we know that it was devised, or conceived of--somewhere between thousands of years ago and the birth of modern astronomy--as a medium for carrying light, heat, and anything that was hard to move, through space, or to where it was wanted to be moved, by its vibrations or undulations, in the same way that sound is conveyed by wave motion, or vibration, through air, water, and a multitude of bodies; and we understand that some time during that long period it began to be looked upon as a material substance. we are told that it is supposed to pervade all bodies of all classes, but we think this idea must be taken in a limited sense, because, whether it is combined with electricity, as some suppose, or is only a carrier of electricity, a good conductor must have a larger supply of it than a bad one, and an absolute non-conductor, if there be such a substance, must contain none at all, always provided the ether is the conducting or carrying power. we are told also, that it is neither of the nature of a gas nor a liquid, but may be of the nature of a jelly, and of its nature we shall have more to say hereafter. it was natural that it should be conceived to be a material substance, because if light and heat were to be carried from one place to another by wave motion, as sound is by water and air, then the medium for carrying it must be of the same nature as air and water--or any other carrier of sound--that is, it must be a material substance and, in consequence, possessed of some density or specific gravity. the only place where we have seen any density assigned to it has been, in a series of articles on the "origin of motion," published in "engineering" of , where it is estimated to be / , , th[e] of the density of air. how this estimate was formed is explained in the number for december , , page , from which we make the following very long quotation, because we look upon it as of great importance. [e] years after this was written we have seen it stated that the density of the ether has been calculated from the energy with which light from the sun strikes the earth, and that to represent it there are twenty-seven cyphers after the decimal point before the figures begin. but as this gives something like one thousand quadrillionth part of the density of water, we refuse to accept or even think of it. "steel of the best quality in the form of fine wire has been known to bear a tensile strain represented by not less than tons per square inch before breaking, and even this cannot be said to be the limit to the tensile strength of steel, since the tenacity increases as the diameter of the wire is reduced. rejecting 'action at a distance,' therefore, these molecules of the wire must be controlled by some external agent, and therefore, the pressure of the external agent must _at least_ equal the static value of the strain. the pressure of the ether therefore cannot be less than tons per square inch. now, since it is a known fact that the strain required to separate molecules in 'chemical union' would be very much greater than in a mere case of 'cohesion,' it follows that the ether pressure must be greater than the above figure. if we suppose the strain required to separate the molecules of oxygen and hydrogen combined in the state of water (one of the most powerful cases of chemical union) to be only three times greater than in the case of the molecules of steel, then this would give tons per square inch as the effective ether pressure. it may be taken as certain that the strain required would be greater than this, as it has not been found possible by any ordinary mechanical means to separate molecules in chemical union. however, as it is only our object to fix a limiting value for the ether pressure, or a value that is less than the actual fact, we will therefore take in round numbers tons per square inch as the total ether pressure, having thus valid grounds for inferring that this estimate is within the facts as they actually exist. the existence of such a pressure as this might well be sufficient to strike one with astonishment and legitimately excite incredulity, if it were not kept in mind that this pressure is exercised against _molecules_ of matter, a perfect equilibrium of pressure existing, so that it may be deduced with certainty beforehand, that, however great this pressure might be, it could not make itself apparent to the senses. the air exercises a pressure of some tons on the human body without such pressure being detected, how much more cause, therefore, is there for the perfect concealment of the ether pressure, which is exercised against the molecules of matter themselves. this great pressure is the absolutely essential mechanical condition to enable the ether to control forcibly the molecules of matter in stable equilibrium, and to produce forcible molecular movements when the equilibrium of pressure is disturbed (as exemplified in the molecular movements of 'chemical action,' etc.). "it is generally admitted that the ether must have a very low density, one reason being the almost imperceptible resistance opposed by it to the passage of cosmical bodies (the planets, etc.) at high speed through its substance. the pressure of an aëriform body constituted according to the theory of joule and clausius, being less as its density is less, it will therefore be necessary to show that the ether can exert so great a pressure as the above, consistent with a very low density. from the known principles belonging to gases, the pressure exerted by an aëriform medium is as the _square_ of the velocity of its component particles, and as the density. we will, in the first place, consider what the density of the ether would be, if it only gave a pressure equal to that of the atmosphere ( lb. per square inch). from the above principles, therefore, it follows that for the ether to give a pressure equal to that of the atmosphere, the ether density will require to be as much less than that of the atmosphere, as the _square_ of the velocity of the other particles is greater than the square of the velocity of the air molecules. the velocity of the air molecules giving a measure of lb. per square inch is known to amount to feet per second. taking, therefore, the square of the velocity of the ether particles in feet per second, and the square of the velocity of the air molecules and dividing the one by the other, we have the number of times the ether density must be less than that of the atmosphere, in order for the ether to give a pressure of lb. per square inch, or we have ( , × )^{ }/ = , , , . this result shows therefore that the density of ether, if it only gave a pressure equal to that of the atmosphere, would be upwards of , , , times less than the density of the atmosphere. this result expresses such an infinitesimal amount of almost vanishing quantity, that the ether density might be well much greater than this. we will now, therefore, consider what the ether density would be to give a pressure of tons per square inch. pressure and density being proportional to each other, it follows that for the ether to give a pressure of tons per square inch, the ether density would require to be as much greater than the above value, as tons is greater than lb. multiplying, therefore, the above value for the density by this ratio, we have / , , , × ( × )/ = / , , ; or this shows that the density of the ether to give a pressure of tons per square inch would be only / , , th of the density of the atmosphere. this value representing a density less than that of the best gaseous _vacua_ is therefore quite consistent with the known fact of the extremely low density of the ether. it follows, therefore, as a mathematical certainty dependent on the recognised principles belonging to gaseous bodies, that the ether could exert a pressure of not less than tons per square inch consistent with such an extremely low density as to harmonize with observation." if the ether is possessed of a density equal to that shown above, then the density of our original nebula must have been greater than what we have shown it to be. the density we found for it was / , , th that of air, or · of an atmosphere, and / , , th is equal to · of an atmosphere; if then we add these two together we get · of an atmosphere as the density of our nebula. this comes to be very slightly greater than the density of the ether, and shows that the estimate in the foregoing quotation is too high; unless it is asserted that the ether can exert no frictional action at all, which, we believe, no one has ever done; while the absolute temperature of the nebula at the new density would be · °, which would be a very small addition indeed to the · °, we found for it at first. on the other hand, when the nebula was reduced to , , miles in diameter the density of the ether would have increased its density from · , which we showed it then to have, only to · of an atmosphere, which would make no appreciable difference on its temperature, and would be so immensely greater than the · of an atmosphere of the ether that it could hardly be supposed to have any effect in retarding the rotation of so much heavier a body. and should it be found that the density of the ether is / , / , or / less, or even a great deal more, than that shown in the above quotation, it would only have proportionately less effect on our nebula, in every sense, than what we have just shown. we may, therefore, conclude that the introduction of the element ether has not vitiated our operations in any way up till now, and we shall leave it until we have acquired more knowledge of its nature and effects. although we have already condensed our nebula to somewhere about , , miles in diameter, where we have shown it might begin to radiate light--radiation of heat may have begun when the diameter was ten times as great, or even before that--we propose to return to the period when it had just abandoned the ring for the formation of mercury and was , , miles in diameter, and became what we have called the solar nebula; because there is a good deal to be learned from a careful study of our operations up to that period, and of what must have taken place during further condensation up till the final establishment of the sun such as it is at the present time. when the planet neptune was discovered, bode's law fell into disrepute for a time, because the new planet was found to be much nearer to the sun than, according to it, it should have been. all the other planets occupied the places assigned to them within per cent. of the exact appointed distance from the sun, but neptune turned out to be · per cent. out of his exact place, and hence the discredit thrown upon the law. it was hard treatment for a servant that had helped so unmistakably--as we know to have been the case--to the discovery of the first four asteroids, which has afterwards been followed by the discovery of a whole host of them, and that had been pressed into the service for the discovery of the very planet which was the cause of its discredit--but such is the world. however, first offences against the law are generally looked upon with merciful eyes, and the series of titius seems to have been so far received into favour again that, some astronomers are said to have been looking out for another planet farther off than neptune, being convinced that there must be some reason why a law that has shown itself to be right in eight cases should be altogether wrong in the ninth. here, we think that the most likely explanation that can be given is, that the ring out of which neptune was formed divided itself, after breaking up, into two planets instead of one, and that this is the reason why, bode's law could not point out the true position of either of them. it is hard enough to believe that the ring out of which uranus was made--which we have seen may have been , , miles broad, and over , , , miles in extreme diameter--could have united its fragments, after breaking up, into one planet, and the difficulty of belief becomes greater the greater the diameter comes to be. we have, in our work, considered the breadth of neptune's ring to have been , , , miles, but then we limited the diameter of the nebula to , , , miles--we had to draw the line somewhere--whereas it may have been a thousand million miles greater, which would very greatly increase the probability of two planets, perhaps even more, having been formed out of the ring. if it has been so, the law could not apply to the case. a new act was required. besides, it is not a law, never has been, but only a register of facts; and we know that truths are often discovered from similar registers. it registers, and at the same time shows, that there is a nearly fixed inter-relation, even proportion, in the distances of the planets from the centre of the sun as far out as uranus; and were we to make a similar register, beginning at the (present) outside of the planetary system, and registering the number of revolutions, beginning with for neptune, rates of acceleration of revolution in number of days, and densities of the planets, we may draw from it some useful knowledge. but we shall first extend bode's law to embrace neptune, and show the discrepancies between the actual positions of the planets and those pointed out by the law. here we see that, with the exception of the first step from neptune to uranus which is only · , we have an average gradation of acceleration of · times, from one planet to another, from the outermost as far in as mars; and that had neptune had the period of revolution sought for by leverrier in his discovery of that planet, viz. · years, or , · days, the average rate of acceleration would have been · times, from planet to planet, as far in as mars. this, we think, is pretty strong evidence that one law of acceleration was in force from the beginning of the separation of rings from the nebula up to the time when the ring for mars was separated--the departure from it in the case of neptune, notwithstanding--and goes far to prove that part of the nebular hypothesis which implies that each of the planets is now revolving round the sun in the orbit, and with the velocity, belonging to the centre of gyration of the ring out of which it was formed. from mars to venus the law--the areolar law, of course--had changed to a variable decreasing law, as seen from the foregoing register, which then again changed into an increasing one, till at mercury the rate of acceleration rose again to · times from venus, or very nearly the same rate of increase that existed from uranus to mars. the causes of these changes may or may not be able to be accounted for--we shall have to return to them hereafter, in the cases of neptune, the earth and venus--but there is one thing of some importance that is deducible from the register, which we shall endeavour to make clear. bode's law extended. --------------+----------+----------+----------+-----------+----------- ---- | mercury. | venus. | earth. | mars. | asteroids. --------------+----------+----------+----------+-----------+----------- numbers in }| | | | | progression }| | | | | | | | | | add to each}| | | | | for distance }| | | | | from sun, }| | | | | earth's = }| | | | | | | | | | distance from}| | | | | sun according}| , , | , , | , , | , , | , , to the law, }| | | | | (miles) }| | | | | | | | | | actual | , , | , , | , , | , , | .. distance | | | | | | | | | | percentage }| | | | | of distance }| .. | . | .. | .. | .. beyond law }| | | | | | | | | | percentage }| | | | | of distance }| . | .. | .. | . | .. within law }| | | | | --------------+------------------------------------------------------- ---- | jupiter. | saturn. | uranus. | neptune. --------------+-----------+-----------+-------------+----------------- numbers in }| | | | progression }| | | | | | | | add to each}| | | | for distance }| | | | from sun, }| | | | earth's = }| | | | | | | | distance from}| | | | sun according}| , , | , , | , , , | , , , to the law, }| | | | (miles) }| | | | | | | | actual | , , | , , | , , , | , , , distance | | | | | | | | percentage of}| | | | distance }| . | .. | . | .. beyond law }| | | | | | | | percentage of}| | | | distance }| .. | . | .. | . within law }| | | | --------------+-----------+-----------+-------------+----------------- our register as specified above will be the following:-- -----------+--------+-------+---------------------+-----------------+ planet. | rev. of planet,|accel. of revolution,|density of planet| |(in solar days) |(neptune taken as ) | | -----------+----------------+---------------------+-----------------+ neptune. | , . | . | . | uranus. | , . | . | . | saturn. | , . | . | . | jupiter. | , . | . | . | asteroids.| , . | . | ... | mars. | . | . | . | earth. | . | . | . | venus. | . | . | . | mercury. | . | . | . | -----------+----------------+---------------------+-----------------+ a good deal has been written about planets or other bodies existing between mercury and the sun, especially about vulcan whose existence seemed to be so certain, that his distance from the sun and period of revolution were calculated to be about , , miles and days respectively. now, with what we have seen about the rate of acceleration of planets as their orbits approach the sun, we may endeavour to form some notion of where any within the orbit of mercury may be found. if we take the same rate of acceleration we have found between venus and mercury--that is · , which may be looked upon as almost the general rate for all the planets--we find that there might be a planet revolving round the sun in · days; but here we must stop, because, though we could make no objection to the existence of a planet with the period of revolution just shown, were we to take another equal step towards the centre of the nebula, the same acceleration of rotation would give us a planet, or ring for a planet, revolving round the sun in · days; not much more than one-half the average of his rotation round his axis at the present day, which would knock on the head most completely the theory that each planet was detached from the nebula at the time that it was rotating with the velocity of the planet's orbit, or we should have to conclude that the nebula had passed, by a long way, its power to abandon matter through centrifugal force. no one could suppose that a ring for a planet could be formed within the body of the nebula and abandoned, or thrown out, afterwards, because centrifugal force could not throw out the ring and at the same time retain the surrounding matter. turning our thoughts now to the supposed planet vulcan, which was calculated to revolve round the sun in about days, we have either to conclude that it was formed in the body of the nebula and come to the same breakdown of the nebular hypothesis, or we have to acknowledge that the sun is now rotating much more slowly on its axis than the nebula did at the time the ring for vulcan was abandoned. if we now direct our attention to the densities of the several planets, we shall find some suggestive matter in their study. a general look shows us at once that there are four periods of rise and fall in their densities. there is one rise and fall (referring to our register) from neptune to uranus and on to saturn; then another rise to jupiter and fall to, we suppose, the asteroids, because we are told that the quantity of matter in the region where the asteroids travel is less than in any other zone of the solar system, and the general density must in consequence have been less there than anywhere else; still another rise from the asteroids to the earth, and fall to venus; and then a final rise to mercury accompanied, without doubt, by a fall after the planet was abandoned, because the centrifugal force of the rotating nebula must have been decreasing, at the least, preparatory to its ceasing to have the power to throw off more matter. the first rise and fall would seem to indicate that there had been a much closer mutual relation in the births of neptune, uranus and saturn than is indicated in any way in the nebular hypothesis. we could imagine that at one time they formed one flat ring, which afterwards divided itself into three, following the same law as we see dividing the rings of saturn at the present day. with respect to jupiter, his enormous size is sufficient to entitle us to believe that his ring was separated from the nebula independently of any of the others, and to account for there having been the rise and fall in the density that we have noted between saturn and the asteroids. then the rise and fall from mars to venus, or further on towards mercury as it would be, may indicate one ring divided into three in the same manner as we have supposed for the three outer planets. and the final rise to mercury and subsequent fall to the sun or to the solar nebula might be either due to one operation or to complication with other unknown bodies that may be travelling between mercury and the sun. in support of the foregoing ideas, we may also refer to our having said on a previous occasion, that the whole of the matter separated from the nebula in the form of thin hoop-shaped rings, would condense into one continuous sheet, perhaps even up to the time when centrifugal force could not throw off any more matter against the force of gravitation. in that case we can conceive that the radial attraction, outwards and inwards, of the particles of the matter forming the sheet would gradually establish lines of separation, dividing off the matter into distinctly separate rings, preparatory to their transformation into planets; but we cannot explain how these separate rings came to be more dense in one place than another. we must leave that for future discovery. meanwhile the idea of one continuous sheet of matter extending from the sun out to neptune, suggests the possibility of all the rings having been in existence as rings, more or less advanced in their evolution, at the same time; and if not so much as that, makes it more easy for us to see how the four inner planets, being made out of more condensed cosmic matter, and being of so much smaller volume, have arrived at a much more advanced stage of their being than the four outer ones. going a little further, we can see how the cosmic matter of the rings condensing from both sides in the direction of their thickness, and falling in impeded, so to speak, the tendency to contract in length, or circularly, until they arrived at a certain stage of density, when they began to contract in their orbital direction, to break up into pieces, each one of which would form itself into a small, probably shapeless, nebula with a tendency to direct rotation, as explained and shown by m. faye in "l'origine du monde," chapter xiii., page , entitled "formation de l'universe et du monde solaire"--an explanation which must have occurred to everyone who has taken the trouble to think seriously, of how nebulous spheres could be formed out of a flat nebulous ring endowed with a motion of revolution. we have seen at page that when the nebula was condensed to a little over , , miles in diameter, its average temperature might have been °, provided no heat had been radiated into space. in like manner, we can see that the sun being now condensed to · times the density of water, or times the density of air, in other words, that number of atmospheres, its present average temperature might be about , °--as each atmosphere corresponds to °--provided no radiation of heat into space had been going on. but this way of estimating could not in any way apply to the nebula after it had ceased to throw off planetary matter; because from that time, or at all events from the time when it came to be of a density equal to one atmosphere and temperature of °, or freezing point of water, that would be accumulated within it, owing to the difficulty of carrying to the surface, to be radiated into space, what was produced by condensation in the interior, as we have shown before. both heat and pressure would increase from the surface towards the centre, the former rising, in spite of surface radiation, to something far beyond what we have stated above that it might be, aided by the increase of pressure which near the centre must be enormously greater than the average of atmospheres, seeing that the pressure at the surface of the sun is estimated to be not far from atmospheres. the first cause of the increase of pressure would be the condensation produced by gravitation, which according to the areolar law would increase the rotary velocity of the nebula in proportion as the centre was approached; and as this would begin long before it had given up abandoning rings, or rather from the very beginning of its rotation; from that time, there would be different rates of rotation at different distances between the surface and the centre, which would cause friction among the particles of its matter, in other words a churning of the matter shut up in the interior of the nebula, and thus produce heat over and above that produced by the condensation of gravitation alone. if two particles of matter would produce a given quantity of heat, in falling from the surface of the nebula to any point nearer to the centre, they would surely produce more if they were rubbed against each other by churning action during their fall. reflecting on what we have written up till now, we see that the analysis of the nebular hypothesis we have made, which at first may have appeared to be unnecessary or even useless, has shown us and made us think over many details, of which we had only a vague notion previously. it has shown us that without condensation at or near the surface of the nebula--which we have pointed out must have been caused by its greatest mass being near that region, and which laplace procured by endowing it with excessive heat--the various members of the solar system could not have been evolved from it in terms of the hypothesis. from it we have been able to learn, by means of the register of the acceleration of revolution from one planet to another, when, and for what reason, the nebula ceased to be able to throw off any planet nearer to the sun than the supposed vulcan, or almost even so near. finally, and not to go into greater detail, it has so far given us some ideas, that we had not before, of the internal structure of the sun, and has made us believe that a great deal may be learnt by attempting to find out what that structure really is. for this purpose, it appears to us that a careful examination into, and study of, the interior of the earth might be a great help, and to this we shall appeal, as we cannot think of any other process by which our object can be attained. this, therefore, we shall endeavour to do in the following chapters. chapter viii. page inquiry into the interior construction of the earth. what is really known of the exterior or surface. what is known of the interior. little to be learned from geology, which reaches very few miles down. various notions of the interior. what is learnt from earthquake and volcanoes. igno-aqueous fusion, liquid magma. generally believed that the earth consists of solid matter to the centre. mean density. surface density. more detailed estimate of densities near the surface. causes of increased surface density after the crust was formed. calculations of densities for miles deep, and from there to the centre forming table iv. reflections on the results of the calculations. notion that the centre is composed of the heaviest metals. "sorting-out" theory absurd. considerations as to how solid matter got to the centre. gravitation might carry it there, but attraction could not. how the earth could be made out of cosmic matter, meteorites or meteors. the interior of the earth and its density. before attempting to inquire into the nature and structure of the interior of the earth, it will be convenient to specify the bases on which the inquiry is to be made, in other words, the data we have to proceed with; which data should be denuded of everything whatever having the semblance of a hypothesis or theory, and should consist of simple facts. anything founded upon theory must come to an end should the theory be afterwards found to be erroneous, and all the labour would be lost. what we really know of the earth in this way may be stated as follows:-- of the exterior or surface we know that it is of a spherical form, surrounded by an atmosphere of probably miles or even more, in height, consisting of common air mixed with vapour of water in more or less degree; that, of its surface, nearly three-fourths are covered by water, and the remaining fourth consists of dry land, intersected in all directions by rivers; that on the dry land there are elevated tablelands and ranges of mountains from two to three miles high, with occasional ridges and peaks rising up to altitudes of from five to near six miles, and that in the part covered by water or sea, there are depressions or furrows with depths in them probably exceeding the heights of the highest mountains; that the sea does not remain constantly at the same level but rises and falls twice in every twenty-four hours, or thereby, in obedience to the attraction of the moon and sun, forming what are called tides; and that its polar regions are enveloped in dense masses of snow and ice, which the persevering energy of man has not been able to penetrate in centuries of continued and determined effort. what we know of the interior of the earth is found in great measure from the exterior, that is, from the construction of the rocks as seen in deep ravines, in precipices, and on the sides of hills or mountains; and also from what we have been able to learn from the exploration of mines and from deep wells, the deepest of which have penetrated it very little beyond one mile in depth; all of which knowledge may be summarised as follows: that the substances which compose the earth are manifold and of manifold nature--or, more appropriately speaking, simply the elements of chemistry--varying in density, or specific gravity, from the same as that of water, or in some cases much less, to three or four times as much in some kinds of rock and earths (disintegrated rock), to more than twenty times in the heaviest metals; that from a depth great enough not to be affected by the changes of seasons, the heat of the earth increases in descending towards the centre, by one degree of fahrenheit's thermometer for every fifty to sixty feet in depth--that is about thirty metres for each degree of the centigrade scale--as far down as we have been able to penetrate; that at the greatest of these depths abundant supplies of water are found, which shows that it must exist at much greater depths than any that have yet been reached; and that at unknown depths, as shown by the eruptions of volcanoes, there are masses of matter in a molten liquid state, or that, owing to their great heat, can be suddenly liquefied by diminution of pressure. over and above what has been stated, little can be learnt from geology, because the earth must have been formed and fashioned almost to its present condition before geology could begin to exist, and all its teachings are confined to a very few miles from its surface. its first lesson could only begin when the earth was so far cooled down that a crust could be formed on its surface, and that crust could be deluged by copious falls of rain on it. some help or guidance may be obtained however, from the ideas which astronomers and physicists have formed on its interior, and it may be useful to have the principal of these ideas specified, as they may help to strengthen arguments that may be advanced, or conclusions that may be drawn. when it was discovered that the temperature of the earth increases, as we go downwards, at what may be considered a rapid rate, it was calculated that at a depth of from twenty-five to thirty miles, the heat would be great enough to melt any substances that have been found near the surface; and it was immediately concluded that from that depth to the centre the whole of the interior was a molten liquid mass, whose temperature far exceeded any heat that could be produced upon the surface. even up to the present day, the belief in a liquid interior has not disappeared. many years afterwards, the supposed liquid state of the interior of the earth was taken advantage of, to frame a theory that earthquakes and eruptions of volcanoes are caused by the attraction of the moon on the liquid interior producing tides, in the same manner as it produces tides in the sea, which in their turn act upon the crust, cracking and rending it to produce the one, and forcing the liquid matter out through the rents, or up through the vents of volcanoes to produce the other, in some way that it is more easy to imagine than to explain mechanically. also when the effect of the attraction of the moon on the liquid internal matter came to be duly considered, it was concluded that the crust, with only to miles in thickness, could not be rigid enough to resist the pressure brought upon it by the movements of the interior tides; and it began to be thought that, owing to the pressure of the superincumbent strata, the density of the matter at that depth might be so great that it would become solid at a much higher temperature than it does at the surface; and some physicists went the length of supposing that the earth has a solid crust and solid nucleus with liquid matter between them. on the other hand sir william thomson, lord kelvin, looking more as it would appear to the effects of the moon's attraction on the crust than on the liquid interior, concluded that the earth must be a solid globe, contracting through gravitation in the interior, and cooling at the surface, because a crust so thin as to miles, or even miles, would be continually rent and broken up by the tidal action of the moon; but professor clerk maxwell and others have thought that the elasticity of the crust would be great enough to admit of its accommodating itself to all the changes of form that would be caused by the action of those tides. notwithstanding that they agree with lord kelvin in the main, in his objections to the existence of a liquid interior, many scientific men suppose that, through the effects of pressure, the liquid interior of the earth may have been changed into a viscous state, as it went on contracting through gravitation, which would, according to the degree of viscosity, either annul, or almost annul, the tidal action on it of the moon. to which it may be added that that action would not raise such high waves in even perfectly liquid molten matter as it would upon water; because it would be easier for the moon to lift a cubic mile of water three or four feet high, than to lift a cubic mile of melted rock or metal to the same height. other parties look upon the earth as mainly solid to the centre, but with large reservoirs of liquid matter in various parts of it near the surface, which furnish all the material for volcanic eruptions and are the causes of earthquakes. there are others also who, believing the earth to be altogether solid, consider that when any part of the intensely heated and dense interior is relieved suddenly from pressure, as, for example, by the convulsive action of an earthquake, it will immediately assume the liquid state and become material for volcanic eruptions; a theory which they consider to be substantiated by the fact of these two phenomena generally accompanying each other. and mr. mallet seems to have demonstrated that earthquake-shocks proceed from centres not far from the surface, which would seem to point out that if a liquid interior did exist at to miles from the surface, it could have no part in causing earthquakes. there are others still who consider earthquakes and volcanic eruptions to be caused by water penetrating deeply into the interior, but it is difficult to understand how water could penetrate into the interior to a greater depth than where it would be converted into steam, that is to a greater depth than from three to four miles. many other notions about the interior state and conditions of the earth have been formed, more or less entertainable, more or less fanciful, to provide liquid matter for volcanic eruptions. one of these, referred to in "nature" of december , , takes for granted "that granite has consolidated from a state of igneo-aqueous fusion, and that the liquid magma from which all granitic intrusions have proceeded contains water-substance," and proceeds, "it is, therefore, only a further step to assume that this water-substance is an essential constituent of the liquid substratum (assumed by the author), and to suppose that it has been there since the consolidation of the earth." this mixture of water, fire, and molten granite is one that does not agree with what we have been taught of the nature of any of the three components, and we cannot accept it. why we refer to it more particularly than to the other ideas we have cited, is because it so far comprehends some of them, and that we shall have to return to it hereafter, when we think it will be seen that it has not been properly thought out. bearing in mind all these ideas we have cited, and working with the data we have considered as actual facts, we may now proceed with our inquiry. the belief that the earth is a mass of matter increasing, whether liquid or solid, or part of both, in density from the surface to the centre is so general that we shall look at it in that light first, and endeavour to find out what must be its density at any place between its surface and its centre. astronomers and geologists concur in telling us that the mean density of the earth is very near to · times that of water: knowledge that has been acquired by measuring the attraction of high and precipitous mountains for plummets; by the attraction of masses of metals for each other, measured by the torsion balance; and by the acceleration or retardation of the vibrations of pendulums, as observed in the depths of mines and on the tops of mountains, compared with each other. they also tell us that the average density of the matter and rocks of which the crust is composed is about - / times that of water; and then, in a general way, that the average density of the crust, taking into consideration that so much of its surface is covered by the sea, is not much more than - / times that of water. this estimate is manifestly incorrect, for it implies that the whole of the crust of twenty-five to thirty miles is affected by the presence of water, when we know that the depth of the sea at any place does not exceed one-fourth of that thickness. therefore, we shall endeavour to obtain some more accurate computation, as it is the only datum we have to go upon, and has a greater effect upon the result, and upon all things relating to the interior, than might at first sight be supposed. we find in "nature," of january , , that mr. john murray has calculated that if the whole solid land of the earth were reduced to one level under the sea, its surface would be covered by an ocean with a uniform depth of about miles. here we have a very good beginning for our calculations. without taking into consideration the increase of density in water at miles deep, at that depth we may suppose we have come to solid matter, the specific gravity of which could not be less than twice that of water, on account of the pressure of that depth of water upon it. if we now go down - / miles further we shall have the solid matter subjected to a pressure proportioned to that depth; and if we take its weight per cubic foot at an average between granite (at lb.) and earth (at lb.), or lb., the pressure at - / miles deep of solid matter alone will be about tons per square foot, or just about the crushing strain of our strongest granites, and therefore, the density of the matter under it must be equal to that of granite, or · times that of water. we do not add the pressure of the water, at present, because that may be looked upon by some people as of the same nature as of the atmosphere upon a human body, which neither increases the pressure upon it nor adds to its weight; but we see that at that depth the solid matter must have a density equal to the average between water on its surface and · --that of granite; and if we choose to take the average between miles of water and - / miles of solid matter, we shall have · as the average density of the outer - / miles in thickness of the crust of the earth. for our purposes, however, and for obvious reasons, we shall consider the average density of the - / miles alone of solid matter to be · times that of water. we shall now go down to miles deep, because the diameter of miles we have adopted for the earth will there be reduced to miles, which will be convenient for our further operations. at that depth we shall have a superincumbent pressure at the very least as follows:-- tons. at miles deep, miles of sea at tons per mile - / " - / " solid matter at spec. grav. · equal to · tons per mile " - / " of rock at lb. per cubic foot ---- total pressure per square foot or just about times the crushing strain of our best granites. then, as when crushing takes place compression begins, it will, we believe, be far below the mark to estimate the general specific gravity of the earth at miles deep to be times that of water. we have now added the pressure of the miles of water, because there could be no water at the depth of miles; for the critical temperature of water is known to be °, beyond which temperature water cannot be maintained in its liquid state by any amount of pressure, however great; and miles would give ° temperature at ° for each metres. at that depth there might be steam, although it is difficult to see how it could penetrate so far, because the only force to help it to penetrate would be gravitation, and that would have to act against the increasing repulsion of heat. there is another circumstance to be considered which would tend to increase the density of the outer portion of the crust, if there be a crust, and if not, of the outer portion of the earth itself. when the earth was in the molten liquid state, it is generally supposed to have been surrounded by vapours of a great proportion of the metals and of some of the metalloids, in addition to the vapour of water, air, and other gases, which floated above them higher up in the atmosphere. in that case when the crust began to be formed through cooling, these vapours would be precipitated on the surface and mixed with the half-liquid half-solid matter there, but the proportion of condensed vapours would be very small compared with what they fell upon, and the specific gravity of the mixture would not be great enough to cause it to sink much below the surface, because it would soon meet with matter as dense as itself; consequently we must consider that all these metals would remain near the surface--most likely much nearer to it than the miles which we have as yet descended to--and whatever may have been the proportion of their density it ought to be added to the weights and pressures that have been taken into account above. we believe that it will be shown later on that this estimate of a density of three times that of water at miles deep in the earth is very much lower than it should be; because, when the pressure upon the matter there came to be greater than its crushing strain, compression would go on more rapidly than shortly afterwards, and it might so be that with a strain of very much less than four times that of crushing, compression would be reduced to its utmost limit. but more of this hereafter. having determined densities for the matter composing the earth at , - / , and miles below the surface, that is, to where the mean diameter comes to be miles, if we divide that diameter into layers of miles each in thickness, compute the volume of each layer or shell, increase the density of each layer as we descend in direct proportion from --the density we have fixed for miles deep--to · times the density of water, at the centre, and multiply the volume of each layer from the surface downwards by its respective average density, we shall find a mass nearly equal to the mass of the earth at the density of water--always taking its mean diameter at miles, and mean density at · times that of water, as already premised. these calculations have been carefully carried out, and are represented in detail in table iv. for future reference. they terminate in a deficiency of over , , of cubic miles, a deficiency which would be more than made up by making the central density · instead of · . thus we see that if the density of the earth increases regularly from the surface to the centre, and if the densities we have given to the layers between the surface and miles in depth are not greater than those adopted, the central density must be exceedingly near - / times that of water. of course, if the three surface densities are in reality _less_ than those we have adopted, the central density must be greater than - / times that of water. the whole being a result to our calculations which leads us to speculate on what kind of matter there is at the centre of the earth. we are acquainted with various kinds of rocks, stones and other solid matter that have densities (specific gravities) of - / to times that of water, and we have to conceive that a cubic foot of one of these would have to be compressed into a height of - / or - / inches in order to have the density of - / required at the centre, a result which presents us with a substance which it is difficult to imagine or to believe to exist. it may be that the centre of the earth is occupied by the heaviest metals we know, arranged in layers proportioned in thickness to the masses required of them, and that they are laid one over the other according to their densities, or mixed together until a distance from the centre is attained, at which ordinary rocks compressed as highly as their nature would admit of, may exist; but we do not derive much knowledge or satisfaction from such a supposition. an examination of our table of calculations will show that miles in diameter of the central part might be filled up with platinum, the few other rarer and heavier metals, and gold amalgamated with mercury in due proportions. then there might be a mixture of mercury and lead to miles in diameter, followed by a mixture of lead and silver to miles. after that might come a compound of silver, copper, tin, and zinc to miles, and some compounds of iron might finish the filling process up to miles in diameter, or thereby; where the known rocks, compressed to half their volume at first, but gradually allowed to expand, might complete the whole mass of the earth. it will be seen, also, that by the time compressed rocks could be used for this filling process, more than per cent. of the whole volume of the earth would be occupied exclusively by pure metals mixed by rule and measure. it would appear then that the "sorting-out theory"--about which a good deal has been written--whereby, in suns and planets, the metals on account of being heavier fall more rapidly to the centre, and the lighter metalloids remain near the surface--a theory probably got up to get over the difficulty we are in--is not a very happy one, as too much metal would be required for the process, at least for the earth. no doubt it might be applied differently to what we have done by mixing metals with rocks, stones, earth, etc., forming metallic ores--very rich they would doubtless have to be--from the centre outwards; but however disposed it would seem that very much the same quantity would be required to furnish the desired densities up to miles in diameter, where we have supposed compressed granites, etc., might come into play. besides, such an arrangement would do away with the whole beauty of the theory; there would be no law to invoke; it would be all pick-and-shovel work. the sorting-out theory is one of these notions that occur to humanity and are accepted at once, without consideration of what the consequences may be. if it is made to account for the four inferior planets being so much more dense, and of coming so much sooner to maturity--so to speak--than the four superior ones, it is hard to understand why the sun up to the present day almost ranks in low density with the large planets. if that theory holds good, it would be most natural to suppose that the mean density of the sun should be very much greater than that of mercury. but it appears to be only carried as far as it suits the theorist, and to be there dropped, or rather ignored. having been defeated in our attempt to build up or construct an earth solid to the centre by appealing to the metals to make up the weight or density required for the foundation layers, and that even to somewhere about three-fourths of the diameter of the whole structure, we are forced to fall back upon our known rocks, earths, etc., in order to compound out of them the dense material we require, and of course we feel that we have in hand a more hopeless task than we had with the metals. how are we to compress the everlasting hills into one-fourth or one-fifth of their volume? some solution of the difficulty, or mystery, must be found somewhere; but at the same time the mountains of gold, silver, and less precious metals required have shown us how absurd, even laughable, it is to appeal to them. let us suppose that we have a cubic foot of matter of any kind of - / times the density of water, and that we place it in one of the scales of a balance at the centre of the earth; we shall find that it does not depress the scale one hair-breadth, for the very good reason that it has nowhere to depress it to; it would be already at what may be called the end of gravitation or tendency to fall lower. as it could not get any lower it would have a tendency to fly off anywhere--provided it was free to do so--and drag the scale and balance along with it, in obedience to its own attractive power and the attraction of all the matter of the earth surrounding it, except that the attraction might be so equally distributed all around it that it would not move in any direction. it would, however, be in a state of very unstable equilibrium, and if by some means the attraction were increased a little on one side more than the others, and it were at liberty to do so, it would abandon the centre and fly off in that direction never to return. now, this being the case, we are forced to consider how a cubic foot of matter, such as the one we are dealing with, could ever have found its way to the centre of the earth; and the law of gravitation, or rather of attraction, does not in any way help us out of the difficulty. we know that we put our cubic foot of extremely dense matter there for an experiment, but we do not know of any process of nature that could place there any equal mass of matter of that density. gravitation and attraction are generally used as synonymous terms, more especially gravitation--somewhat after the manner of the likeness between the two negroes, cæsar and pompey, the latter being most especial in the likeness--but there is a very appreciable distinction between them, if we want to use each of them in its proper and strict sense. gravitation implies the conception of a weight of some kind falling to a fixed centre, while attraction gives the idea of two weights, or masses, drawing each other to a common centre, which when properly looked at is a different thing; because the centre may be anywhere between the two, depending entirely on the difference, if any, in the weights of the masses. the confounding of the two, or rather the almost universal adoption of the less correct term, name, expression--whichever it may be called--has been the cause of wrong conceptions being formed of the construction of almost all--probably all--celestial bodies, and of that most absurd expression, _attraction of gravitation_, used by all our most eminent physicists. the _gravitation_ of _attraction_ might be excused, but putting cause for effect is hardly scientific. a name is nothing as long as what is meant by it is understood and taken into consideration, but that is not always the case, as we shall proceed to show. the term gravitation may be applied with almost, but not absolutely, perfect strictness to the attraction between the sun and the planets, because the common centres of their attractions and the centre of the sun are so near each other that they may be looked upon as one and the same thing, or point; but it is not so with the attractions of the planets for each other where there is no common fixed centre, or if there is something approaching to it in a far off way, it is constantly varying, so that the term gravitation cannot be strictly applied to them, nor even to the sun, to speak truly. planets sometimes _gravitate away_ from each other and from the sun, otherwise adams and leverrier could not have discovered neptune from the perturbations of uranus. neither can it be properly applied to the different masses of matter in the sun or in the earth--although it was no doubt notions connected with the earth that gave rise to the term, from all ponderable matter falling upon it--because _per se_ they could have no tendency to fall to the centre, for _at the centre_ there is no sufficient attractive force to draw them towards it. gravitation was a known term long before the days of newton, who had the glory of enlightening the world by showing that attraction was the cause of it; and, perhaps unfortunately, the name was continued to represent what it in reality does not. let us suppose that we have an empty earth to fill up; if we place one mass of matter at london and another at calcutta, they could have no tendency of themselves to fall to the centre, but if left alone would go for each other in a straight line and meet half-way between the two, provided they were equal in mass, and attraction, not gravitation, would be the proper term to apply to them. but supposing that two equal masses were placed at their antipodes and the four were left to themselves, they would gravitate towards and meet at the centre in the usual meaning of the word, but the force that drew them there would be really that of attraction. we could, however, place four similar and equal masses at the centre, and give the outer ones just and good reason for gravitating or falling down to it, because those at the centre being equally attracted in the four directions might remain stationary there, but would be in a state of unstable equilibrium. we may now suppose that when the masses had just left london and calcutta to meet the others, a goodly number of other equal masses were added to those at these two places and began to attract the two bound towards the centre, they would prevent the two from proceeding, or at least retard them on their journey inwards. moreover, the larger numbers at these two places would attract the four masses at the centre with more force than would the two at the antipodes, and would draw the whole of the four away from the centre and outwards towards themselves; but we might also suppose that at the same moment an equal number of equal masses were added to those at the antipodes, which would again equalize the attractions at the four outer posts, and things would continue as they were at the first; with this difference, that the four at the centre would not be able to balance the attractions at the four outer posts, and the consequence would be--seeing that the forces at the four outer stations were equal to each other, and far superior to the four at the centre--that each one of the four at the centre would be drawn away from it towards one of the outer stations--provided the law of attraction acted impartially--and so the centre would be left without any of the masses at it, that is empty. no doubt when the four outgoing masses met the larger ones coming in, they would all then move towards the centre; but the four places where they met would be immensely nearer the places occupied at first by the outer masses than half-way between them and the centre--proportioned, in exact conformance with the law of attraction, to the excess of the numbers of the masses at the outer stations over those at the centre--and they would be moving, all of them together, to a remote and void space. we may now increase the four outer stations to thousands or millions, with the security that the mode of proceeding would be the same with the whole of them; that is, that the first tendency of the masses at each one of the millions of stations would be to draw away the filling we were pouring into the hollow earth--provided we did it equally and impartially all over the hollow--from the centre, and to leave a void there. we are accustomed to look upon the earth as a solid body in which there are no acting and counteracting forces, no movements of matter from one place to another, similar to those we have been calling into play, and as if there was only one force acting upon its whole mass and driving it to the centre; we have, in our ideas, got the whole mass so compressed and wedged in that it cannot move, and never has been able to move in any direction except towards the centre, and this is no doubt the case at the present day. we never stop to think with sufficient care how this compression and wedging-in were brought about, and we only accept what we have been accustomed to believe to be facts, and trouble ourselves no more about it; but there must have been a time, according to any cosmogony we may choose to adopt--even to the vague one that the solar system was somehow made out of a nebula of some kind--when the matter of the earth was neither compressed nor wedged in, nor prevented from moving in any direction towards which it was most powerfully attracted--before superincumbent matter came, so to speak, to have any wedging-in force--and we must go back to that period and study it deeply, if we want to acquire an accurate knowledge of the construction of the earth. table iv.--calculations of the volumes and densities of the earth between the diameter specified, reduced to the density of water. -----+------+---------------+-------+-----------------+-------------+ diam.|densi-| volumes | avgs. | volumes at |observations.| in | ties.| in cubic | of |density of water | | miles| | miles. |density| in cubic miles. | | -----+------+---------------+-------+-----------------+-------------+ | | | | |{total volume| | | , , , | · | , , , , |{of the earth| | |---------------+-------+-----------------|{at density | | | | | |{ of water. | | | | | | | | | | | |{density at | | | | | |{ miles | | · | , , | · | , , |{in diameter.| | | | | |{the miles | | · | , , | · | , , , |{above being | | | | | |{at density | | | | | |{of water. | | · | , , | · | , , , | | | |---------------+-------+-----------------|{volume to | | | , , , | | , , , |{ miles deep| | | | | |{at density | | | | | |{of water. | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | |---------------| |-----------------| | | | , , , | | , , , | | -----+------+---------------+-------+-----------------+-------------+ -----+------+---------------+-------+----------------+-------------+ diam.|densi-| volumes | avgs. | volumes at |observations.| in | ties.| in cubic | of |density of water| | miles| | miles. |density| in cubic miles.| | -----+------+---------------+-------+----------------+-------------+ | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | |---------------| |----------------| | half volume}| | | |{ · of | of earth }| , , , | | , , , |{whole volume| | | | | |{of the earth| | | | | |{at density | | | | | |{of water. | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | |---------------| |----------------| | | | , , , | | , , , | | -----+------+---------------+-------+----------------+-------------+ -----+------+---------------+-------+----------------+-------------+ diam.|densi-| volumes | avgs. | volumes at |observations.| in | ties.| in cubic | of |density of water| | miles| | miles. |density| in cubic miles.| | -----+------+---------------+-------+----------------+-------------+ | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | - / | , , | · | , , , | | | |---------------| |----------------| | · } | | | |{half mass of| of whole } | , , , | | , , , |{whole earth | volume of} | | | |{at density | the earth} | | | |{of water | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | |---------------| |----------------| | | | , , , | | , , , | | -----+------+---------------+-------+----------------+-------------+ -----+------+---------------+-------+-----------------+------------+ diam.|densi-| volumes | avgs. | volumes at |observations| in | ties.| in cubic | of |density of water | | miles| | miles. |density| in cubic miles. | | -----+------+---------------+-------+-----------------+------------+ | | | | | about | | · | , , , | · | , , , | density | | | | | | of iron | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | . | , , , | . | , , , | | | |---------------| |-----------------| | | | , , , | | , , , , | | -----+------+---------------+-------+-----------------+------------+ -----+------+---------------+-------+-----------------+------------+ diam.|densi-| volumes | avgs. | volumes at |observations| in | ties.| in cubic | of |density of water | | miles| | miles. |density| in cubic miles. | | -----+------+---------------+-------+-----------------+------------+ | | | | | about | | · | , , , | · | , , , , | density | | | | | | of copper | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | |---------------| |-----------------| | | | , , , | | , , , , | | -----+------+---------------+-------+-----------------+------------+ -----+------+----------------+-------+-----------------+------------+ diam.|densi-| volumes | avgs. | volumes at |observations| in | ties.| in cubic | of |density of water | | miles| | miles. |density| in cubic miles. | | -----+------+----------------+-------+-----------------+------------+ | | | | | | | · | , , , | · | , , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | |{ about | | · | , , | · | , , , |{ density | | | | | |{of silver. | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | |---------------| |-----------------| | | | , , , | | , , , , | | -----+-------+---------------+-------+-----------------+------------+ -----+-------+---------------+-------+-----------------+------------+ diam.|densi- | volumes | avgs. | volumes at |observations| in | ties. | in cubic | of |density of water | | miles| | miles. |density| in cubic miles. | | -----+-------+---------------+-------+-----------------+------------+ | | | | | | | · | , , , | · | , , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , |} about | | | | | |} density | | · | , , | · | , , , |} of lead | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | |---------------| |-----------------| | | | , , , | | , , , , | | -----+-------+---------------+-------+-----------------+------------+ -----+-------+---------------+-------+-----------------+------------+ diam.|densi- | volumes | avgs. | volumes at |observations| in | ties. | in cubic | of |density of water | | miles| | miles. |density| in cubic miles. | | -----+-------+---------------+-------+-----------------+------------+ | | | | | | | · | , , , | · | , , , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | | | | · | , , | · | , , | | | | | | |{ about | | . | , | . | , , |{ density | | | | | |{of mercury.| | · | , | · | , | | | |---------------| |-----------------| | | | , , , | | , , , , | | | | | | | | |true volume at density of water| , , , , | | | |-----------------|{ about | | deficiency | , , |{ / , th | | | |{ part. | -----+-------------------------------+-----------------+------------+ chapter ix. page inquiry into the interior construction of the earth--_continued_. the earth gasiform at one period. density including the moon may have been / , th that of air. must have been a hollow body. proofs given. division of the mass of the earth alone into two parts. division of the two masses at miles from surface. reasons why the earth cannot be solid to the centre. gasiform matter condensing in a cone leaves apex empty. proportions of the matter in a cone. calculations of the densities of the outer half of the hollow shell of the earth. remarks upon the condensation. calculations of inner half of the hollow shell. remarks upon position of inner surface of the shell. calculations of the same. the interior of the earth and its density--_continued._ when, according to the nebular hypothesis, the ring for the formation of the earth and moon had been thrown off by the nebula, and had broken up and formed itself into one isolated mass--rotating or not on an axis, as the case may have been--it must have been in a gasiform state. what was its density, more or less, may be so far deduced from table iii., where it will be seen that when it had condensed to about one-half of its volume, it must have had a density of only / th part of our atmosphere, and in which each grain of matter would have for its habitat cubic feet of space, or a cube of · feet to the side. so that, with an average distance from its neighbours of - / feet, a grain of matter could not be looked upon as wedged-in in any way, and would be free to move anywhere. now, supposing this earth-moon nebula to have been in the form of even an almost shapeless mass, and that it was nearly homogeneous--as it could hardly be otherwise after the tumbling about it had in condensing from a flat ring--its molecules would attract each other in all directions, and as the mass--without having arrived perhaps at the stage of having any well defined centre--would have an exterior as well as an interior, the individual molecules at the exterior would draw those of the interior out towards them, just as much as those at the interior would attract those of the exterior in towards them; but as the number of those at the exterior would--owing to the much greater space there, being able to contain an immensely greater number--be almost infinitely greater than of those nearer to the central part, the latter would be more effectually attracted, or drawn, outwards than the former would be inwards, and there would be none left at the interior after condensation had fairly begun. the mass would speedily become a hollow body, the hollow part gradually increasing in diameter. but let us go deeper into the matter. let us suppose that the whole mass had assumed nearly the form of a sphere. we have already shown that, although the general force of attraction would cause all the component particles of the sphere to mutually draw each other in towards the centre, yet the more powerful tendency of the particles at the exterior--due to their greatly superior number--would at first be to draw the particles near the centre outwards towards them, and that there would consequently be a void at the centre, for a time at least. of course it is to be understood that each part of the exterior surface would draw out to it the particles on its own side of the centre, just in the same manner as the four masses we placed at the centre were shown to be drawn out by those at london, calcutta, and their antipodes. now we must try to find out what would be the ultimate result of this action; whether it would be to form a sphere solid to the centre, or whether the void at first established there would be permanent. in order to show how the heat of the sun is maintained by the condensation and contraction of that luminary, lord kelvin--in his lecture delivered at the royal institution, on friday, january , --described an ideal churn which he supposed to be placed in a pit excavated in the body of the sun, with the dimension of one metre square at the surface, and tapering inwards to nothing at the centre. in imitation of him, we shall suppose a similar pit of the same dimensions to be dug in the spherical mass, out of which we have supposed the earth to have been formed; only we shall call it a pyramid instead of a pit. this we shall suppose to be filled with cosmic matter, and try to determine what form it would assume were it condensed into solid matter, in conformity with the law of attraction. the apex of our imaginary pyramid would, mathematically speaking, have no dimension at all, but we shall assume that it had space enough to contain one molecule of the cosmic matter of which the sphere was formed. this being so arranged, we have to imagine how many similar molecules would be contained in one layer at the base of the pyramid at the surface of the sphere, and we may be sure that when brought under the influence of attraction, the great multitude of them would have far more power to draw away the solitary molecule from the apex, than the single one there would have to draw the whole of those in the layer at the base in to the centre of the sphere. a molecule of the size of a cubic millimetre would be an enormously large one, nevertheless one of that size placed at the apex of the pyramid would give us one million for the first layer at the base, and shows us what chance there would be of the solitary one maintaining its place at the apex. at the distance of one-twentieth of the radius of the sphere from the centre, the dimension of the base of the pyramid would be one-twentieth of a square metre, and the proportion of preponderance of a layer of molecules there would be as to , so that the molecule at the centre would be drawn out almost to touch those of that layer; at one-tenth of the radius from the centre, the preponderance of a layer over the solitary central molecule would be as , to ; and so on progressively to , , to , as we have already said. following up this fact, if we divide the pyramid into any number of frusta, the action of attraction will be the same in each of them; the molecules in the larger end of each will have more power to draw outwards those of the small end, than they will have to draw inwards those of the larger end; and then the condensed frusta will act upon each other in the same manner as the molecules did, the greater mass of those at the larger end, or base, drawing down, or out--whichever way it may seem best to express it--a greater number of the frusta at the smaller end of the pyramid, until, in the whole of it, a point would be reached where the number of molecules in the various frusta drawn down from the apex would be equal to those drawn up from the base, leaving a part of the pyramid void at each end, because we are dealing with attraction, not gravitation, and there would be no falling to the base or apex, but concurrence to the point, just hinted at, where the outwards and inwards attractions of the masses would balance each other. this point of meeting of the two equal portions of cosmic matter may be called the plane of attraction in the pyramid. the whole pyramid would thus be reduced to the frustum of a pyramid, whose height would be as much more than double the distance from the plane of attraction to its base, as would be required to make the upper part above the plane of attraction equal in volume, or rather in number of molecules, to the lower part. it would be impossible for us to explain how, in a pyramid such as the one we have before us, the action of attraction could condense, and at the same time cram, the whole of the molecules contained in it into the apex end. we must not, however, forget that there are two sides to a sphere, as well as to a question, and that we must place on the opposite side to the one we are dealing with, another equal pyramid with apex at the centre and base at the surface, at a place diametrically opposite to the first one, and that the tendency of the whole of this new pyramid would be to draw the whole of the first one in towards the centre of the sphere. but in the second, the law of attraction would have the same action as in the first; the molecules of the matter contained in it near the base would far exceed, in attractive force, those near the apex, and would draw them outwards till the whole were concentrated in a frustum of a pyramid, exactly the same as the one in the first pyramid. and while the whole masses of matter in the two pyramids were attracting each other at an average distance, say, for simplicity's sake, of one-half the diameter of the sphere, the molecules in each of them would be attracting each other from an average distance of one-quarter the diameter of the sphere; their action would consequently be four times more active, and they would concentrate into the frusta as we have shown, before the two pyramids had time to draw each other in to the centre. there would be then two frusta of pyramids attracting each other _towards_ the centre with an empty space between them. here then we have two elements of a hollow sphere, one on each side of the centre, and if we suppose the whole sphere to have been composed of the requisite number of similar pyramids, set in pairs diametrically opposite to each other, we see that the whole mass of the matter out of which the earth was formed must have--by the mutual attractions of its molecules--formed itself into a hollow sphere. all that has been said must apply equally well whether we consider the earth to have been in a gasiform state, or when by condensation and consequent increase of temperature it had been brought into a molten liquid condition. for up to that time it must have been a hollow sphere, and we must either consider it to be so still, or conceive that the opposite sides have continued to draw each other inwards till the hollow was closed up; in which case, the greatest density would not be at the centre, but at a distance therefrom corresponding to what has been called the plane of attraction of the pyramid. that the opposite sides have not yet met will be abundantly demonstrated by facts that will meet us, if we try to find out what is the greatest density of the earth at the region of greatest mass or attraction, wherever that may be. seeing that the foregoing reasoning forces us to look upon the earth as a hollow sphere, or shell, in which the whole of the matter composing it is divided into two equal parts, attracted outwards and inwards by each other to a common plane, or region of meeting, we shall divide its whole volume into two equal parts radially, that is, one comprising a half from the surface inwards, and the other a half from the centre outwards--that is to say, each one containing one-half of the whole volume of the earth. referring now to our calculations, table iv., we find that the actual half volume of the earth is comprised in very nearly miles from the surface, where the diameter is miles, because the total volume at miles in diameter is , , , cubic miles. this being the case, we cannot avoid coming to the conclusion, after what has just been demonstrated by the pyramids that if one-half of the whole volume is comprehended in that distance from the surface, so also must be one-half of the mass. but for further substantiation of this conclusion let us return to the table of calculations. there we find that from the surface to the depth of miles--where the diameter would be miles--which comprehends one-half of the volume--the mass at the density of water is shown to be only , , , miles instead of , , , cubic miles, which is the half of the whole mass of the earth reduced to the density of water. that is, the outer half of the volume gives only · per cent. of half the mass, while the inner half of the volume gives not only one-half of the mass but · per cent. more; or, to put it more clearly, the mass of the inner half-volume is · times, nearly twice as great, as the mass of the outer half-volume. on the other hand, we have to notice that the line of division of the mass into two halves falls at · miles from the surface, where the diameter is · miles; so that on the outer half of the earth, measured by mass, · per cent. of the whole volume of the earth contains only one-half of the mass, whereas on the inner portion, measured in the same way, · per cent. of the same whole contains the other half. all these results must be looked upon as unsatisfactory, or we must believe that two volumes of cosmic matter which at one time were not far from equal, had been so acted upon by their mutual attractions that the one has come to be not far from double the mass of the other; that the vastly greater amount of cosmic matter at the outer part of a nebula has only one-half of the attractive force of the vastly inferior quantity at the centre. this we cannot believe if the original cosmic, or nebulous, matter was homogeneous; and if it was not homogeneous we have, in order to bring about such result, to conceive that the earth was built up, like any other mound of matter, under the direction of some superintendent who pointed out where the heavier and where the lighter matter was to be placed. we shall now proceed to find out what would be the internal form, and greatest density of the earth, under the supposition that it is a hollow sphere divided into two equal volumes and masses--exterior and interior--meeting at miles from the surface; but before entering upon this subject we have something to say about the notion of the earth being solid to the centre. we are forced to believe that, according to the theory of a nucleus being formed at the centre as the first act, the matter collected there must have remained stationary ever since, because we cannot see what force there would be to uniform the nucleus just formed; gravitation, weight falling to a centre, would only tend to increase, condense, and wedge in the nucleus more thoroughly. attraction, as we have shown, would not allow the matter to get to the centre at all. convection currents, or currents of any kind, could not be established in matter that was being wedged in constantly. moreover, when in a gasiform state, it would be colder than when condensed by gravitation to, or nearly to, a liquid or solid state, and heat would be produced in it in proportion to its condensation, that is, gradually increasing from the surface to the centre in the same manner as density, which, when the cooling stage came, would be conducted back to the surface to be radiated into space, but could not be carried--by convection currents--because the matter being heavier there than any placed above it, and being acted upon by gravitation all the time, would have no force tending to move it upwards; and above all, when solidification began at the surface, it is absurd to suppose that the first formed pieces of crust could sink down to the centre through matter more dense than themselves; unless it was that by solidification they were at once converted into matter of the specific gravity of · . even so the solid matter would not be very long in being made liquid again by meeting with matter not only hotter than itself, but constantly increasing in heat through continual condensation, which would act very effectively in preventing any convection current being formed to any appreciable depth, certainly never to any depth nearly approaching to the centre. if solidification began first at the centre--as some parties have thought might be the case--owing to the enormous pressure it would be subjected to there, before it began at the surface, then, without doubt, the central matter must have remained where it was placed at first, up to the present day. this would suit the sorting-out theory very well, as all the metals would find their way to the centre and there remain; but judged under a human point of view, it would be considered very bad engineering on the part of the supreme architect to bury all the most valuable part of his structure where they could never be availed of; or that he was not sufficiently fertile in resources to be able to construct his edifice in a way that did not involve the sacrifice of all the most precious materials in it. man uses granite for foundations--following the good example he has actually given we believe, and are trying to show--and employs the metals in superstructures; but some people may also think that it was better to keep the root of all evil as far out of man's reach as possible. what a grand prospectus for a joint stock company might be drawn up, on the basis of a sphere of a couple of thousand miles in diameter of the most precious metals, could only some inventive genius discover a way to get at them! returning to our pyramids. we know that the centre of gravity of a pyramid is at one-fourth of its height, or distance from the base, and if we lay one of miles long (the radius of the earth) over a fulcrum, so that - / miles of its length be on one side of it and - / miles on the other, it will be in a state of equilibrium. this does not mean, however, that there are equal masses of matter on each side of the fulcrum, for we know that the mass of the base part must be considerably greater than that of the apex part, and that it must be counterbalanced by the greater leverage of the apex part, due to its greater distance from the point of support. this being so, in the case of a pyramid consisting of gasiform, liquid, or solid matter, the attractive power of the - / miles of the base part would be greater than that of the - / miles of the apex part, and the plane of equal attraction of the two parts would be less than miles from the base of the pyramid. this is virtually the same argument we have used before repeated, but it is placed in a simpler and more practical light, and shows that the plane of attraction in a pyramid will not be at its centre of gravity but nearer to its base, and that it must be at or near its centre of volume. thus the plane of attraction in one of the pyramids we have been considering of miles in length, and consequently the radial distance of the region of maximum attraction of the earth, would not be at miles from the base or surface, but at some lesser distance. now, if we take a pyramid, such as those we have been dealing with, whose base is square and height , its volume would be the square of the base multiplied by one-third of the height, that is ^{ } × / = · , the half of which is · . again, if we take the plane of division of the volume of the pyramid into two equal parts to be · in length on each side, and consequently (from equal triangles) the distance from the plane to the apex to be · the total height of , which is · ; then, as we have divided it into a frustum and a now smaller pyramid, if we multiply the square of the base of this new pyramid by one-third of the height we have · ^{ } × · / , or · × · = · , which is equal to the half-volume of the whole pyramid as shown above. thus we get less · = · miles as the distance from the base of the plane of division of the pyramid into two equal parts, which naturally agrees with the division of the earth into the two equal volumes that we have extracted from the table of calculations, where we have supposed the earth to be made up of the requisite number of such pyramids. so that it would seem that we are justified in considering that the greatest density of the earth must be at the meeting of the two half-volumes, outer and inner, into which we have divided it. considering, then, that one-half of the volume and mass of the earth is contained within miles in depth from the surface, this half must have an average density of · times that of water, the same as the whole is estimated to have. also, as we have seen already, that, taking its mean diameter at miles, its mass will be equivalent to , , , , cubic miles, one-half of this quantity, or , , , cubic miles will represent the half-volume of the earth reduced to the density of water. with these data let us find out what must be the greatest density where the two half-volumes meet, supposing the densities at the surface and for miles down to remain the same as in the calculations we have already made, ending with specific gravity of at miles in diameter. following the same system as before when treating of the earth as solid to the centre, and using the same table of calculations for the volumes of the layers: if we adopt a direct proportional increase between densities at miles and · at · miles in diameter, multiply the volumes by their respective densities, and add about per cent. of the following layer, taken at the same density as the previous or last one of the number, we shall find a mass (see table v.) of , , , cubic miles at the density of water, which is as near the half mass , , , cubic miles as is necessary for our purpose. it would thus appear that if the earth is a hollow sphere, its greatest density in any part need not be more than · times that of water, instead of · times, if we consider it to be solid to the centre. let us now try to find out something about the inner half-mass of the earth, and the first thing we have got to bear in mind is, that where it comes in contact with it, its density must be the same as that of the outer half-mass at the same place, and continue to be so for a considerable distance, varying much the same as the other varies in receding from that place, and diminishing at the same rate as it diminishes. this being the case--and we cannot see how it can be otherwise--if we attempt to distribute the inner half-mass over the whole of the inner half-volume, and suppose that its density decreases from its contact with the outer half--where it was found to be · times that of water--to zero at the centre, in direct proportion to the distance; then, it is clear that at half the distance between that place and the centre, the density must be just · times that of water. now, if we divide the outer moiety of the inner half-mass of the earth--that is, the distance between the diameters of · miles and · miles--into layers of miles thick each, take their volumes from table iv., and multiply each of them by a corresponding density, decreasing from · to · , we shall obtain a mass far in excess of the whole mass corresponding to the inner half of the earth. this shows that a region of no density would not be at the centre but would begin at a distance very considerably removed from it. it is another notice to us that the earth must be a hollow sphere. but why should there be a zero point or place of no density? and what would a zero of no density be? it would represent something less than the density of the nebulous matter out of which the earth was formed; and all that we have contended for, as yet, is that there is a space at the centre where there is no greater density than that corresponding to the earth nebula; but we must now go farther. if the earth is a hollow sphere, it must have an internal as well as an external surface. but how are we to find out what is the distance between these two surfaces? let us, to begin, take a look at the hollow part of the sphere. from the time of arago it began to be supposed that there is a continual deposit of cosmic matter upon the earth going on, and since then it has been proved that there is a constant and enormous shower of meteors and meteorites falling upon it. but although this is the case on the exterior surface, it may be safely asserted that on the interior surface, where the supply of cosmic matter must have been limited from the beginning, there can be no continual deposit of such matter going on now; nor can there have been from, at least, the time when the earth changed from the form of vapour to a liquid state. we may, therefore, be sure that there is no undeposited _cosmic_ matter of any kind in the hollow of the sphere, and that, as far as it is concerned, there is an absolute vacuum. table v.--calculations of the volumes and densities of the outer half of the earth--taken as a hollow sphere--at the diameters specified, and reduced to the density of water. with mean diameter of miles. diameter of half-volume at · miles, and density there of · times that of water. -----+-------+---------------+-------+----------------+--------------+ diam.|densi- | volumes | avgs. | volumes at | observations.| in | ties. | in cubic | of |density of water| | miles| | miles. |density| in cubic miles.| | -----+-------+---------------+-------+----------------+--------------+ | | | | |{ half-volumes| | | | | |{ of the earth| | ... | , , , | ... | , , , |{ actual and | | |---------------| |----------------|{ at density | | | | | |{ of water. | | | | | | | | | | | |{ density at | | | | | |{ miles | | | | | |{ in diameter.| | · | , , | · | , , |{ the miles | | | | | |{ above being | | | | | |{ at density | | | | | |{ of water. | | · | , , | · | , , , | | | | | | | | | · | , , | · | , , , | | | |---------------| |----------------| | | | | | |{ volume to | | | , , , | | , , , |{ miles deep| | | | | |{ at density | | | | | |{ of water. | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | | · | , , , | · | , , , | | | | | | | | ½| · | , , | · | , , , | | | |---------------| |----------------| | | | , , , | | , , , | | | | | | | | true half-volume at density of water | , , , | | -------------------------------------+----------------+--------------| deficiency | , , | | -------------------------------------+----------------+--------------+ as to how far the internal surface is from the centre, it may be possible to designate a position, or region, from which it cannot be very far distant, although we can never expect to be able to point out exactly where it is. going back to the time when the whole earth was in a molten liquid state, and just before the outer surface began to become solid, it is certain that the interior surface must have been in the same liquid condition, whatever may have been the condition of the mass of matter between the two surfaces, owing to the pressure of superincumbent matter; nay, we may be sure that whatever may be its state now, it continued liquid long after the other became solid, because it had no outlet by which to get rid of its melting heat by radiation, nor weight of superincumbent matter to consolidate it; and it would always be much hotter than the outer surface. at that time we have every reason to believe that the outer surface was at least as dense as it is now, there being no water upon it to lower its average density, as is the case at the present day; and we have equal reason to consider that the density at the inner surface, whether liquid or solid, is now at least equal to what the outer surface was then. duly considering, therefore, the absence of water from the interior surface, we shall suppose that the first layer of miles thick upon it will have an average density of - / times that of water, terminating at times, which is the density we have taken for the outer surface at miles deep. but there is another contingency, which it will be necessary to take into consideration before going any farther. it has been understood--as it is certainly the truth--in the calculations made with respect to the outer half of the mass of the earth, that the increase of density in descending was due to the pressure of the superincumbent matter, caused by the attraction for it of the inner half, as well as that of the whole of both the outer and inner halves on the other side of the hollow interior. in the case of the inner half we have now to consider that the attraction of the outer half alone would be the effective agent, and that the superincumbent pressure--that is, of course, the pressure acting from the centre outwards--would be interfered with, or perturbed, by the attraction of the mass on the other side of the hollow interior, so that it would not exert its full power in that direction. but that does not mean that the density would be in any way diminished. the attractions of the planets for each other perturb them in their revolutions around the sun, accelerating or retarding each other, but do not increase or diminish their density or mass; only it will lead us to expect that the same depth of miles will not produce the same amount of pressure outwards at the meeting of the two halves as it does inwards, and that to obtain an equal pressure a greater depth will be required. we believe that an expert mathematician, taking as bases two opposite pyramids in a sphere, similar to those we have used in a former part of our work, could point out, with very approximate accuracy, what ought to be the distance of the inner surface of the shell from the centre--provided a maximum density were determined for the earth--but that goes beyond our powers, and we shall limit ourselves to the use of our own implements; which will cause us to depart from the statement we have made, that the density of the inner half must decrease from the place of meeting of the two halves, at the same rate as the outer half had increased. it must decrease much more rapidly than the other increased. all this premised, and having established a density of for the interior surface, we may proceed to calculate where that surface ought to be, so as to give for the interior half of the earth a mass equal to , , , cubic miles of water. if we begin our operations with a density of · times that of water at the meeting of the two halves of the shell, and diminish it for any considerable distance at the same rate as it increased when we were finding the mass of the outer half, that is · for each layer, we soon find that before we could make up the whole mass of the inner half of the shell, the density would be decreased to at least that of water, which cannot be, as there can be no liquid or solid matter of any kind of so low density anywhere in the interior half of the shell. furthermore, if we decrease it at the same rate as the volumes of the different layers of the earth decrease as they approach the centre, it involves a mass of calculation that serves no useful purpose, as such calculations bring no contingent of satisfaction with them; because all the densities with which we are dealing have to be brought to a rational form before we can frame a proper approximate idea of what the interior construction of the earth is, as will be seen hereafter; and because it takes no account of the perturbation--above alluded to--produced by the attraction of the matter on the opposite side of the hollow. but, in order to get such a result as we can with our limited powers, if we begin with a density of · at the diameter of · miles and fix the density of --which we have adopted above--at the diameter of miles, we shall get a mass somewhat less than one-half of the earth; and with a density of · at miles diameter we get a mass of , , , cubic miles of water, which is rather greater than one-half of the mass required (see operations of table v.). this density of · reduced to · , as we mentioned, might be done when we were fixing the number , would make very little difference on the resulting mass, compared with what we have been in quest of. here we may state that we found that, had the calculations been made with documents of density proportioned to the decrease of the volumes of the layers of the earth as they approached the centre, the density would have been reduced to · at miles in diameter; which tends to show that should that process be considered to be more accurate, it would not have made any great difference on the result. with all, we may consider that it has been demonstrated, that the greatest density of the earth is not necessarily greater at any part of its interior than · times that of water. table vi.--calculations of the volumes and densities of the inner half of the earth, on the same data as those for the outer half. ------+---------------+---------+----------------+--------------+ diam. | volumes |densities| volumes at | observations.| in | in cubic | |density of water| | miles | miles. | | in cubic miles.| | ------+---------------+---------+----------------+--------------+ ½| , , , | | , , , |}half-volumes | |---------------| |----------------|}of the earth.| | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | |---------------| |----------------| | | , , , | | , , , | | -------+---------------+---------+----------------+--------------+ | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | |---------------| |----------------| | | , , , | | , , , | | -------+---------------+---------+----------------+--------------+ | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , , | · | , , , | | | | | | | | , , | · | , , , | | | | | | | | , , | · | , , , | | | | | | | | , , | · | , , , | | | | | | | | , , | · | , , , | | | | | | | | , , | · | , , , | | | | | | | | , , | · | , , , | | | | | | | | , , | · | , , , | | | | | | | | , , | · | , , , | | |---------------| |----------------| | | , , , | | , , , | | | | | |{ true | | | | , , , |{ half-volume | | | |----------------| | | | | , , | excess | -------+---------------+---------+----------------+--------------+ chapter x. page inquiry into the interior construction of the earth--_continued_. density of · times that of water still too high for the possible compression of the component matter of the earth as known to us. reasons for this conclusion drawn from crushing strains of materials. a limit to density shown thereby. the greatest density need not exceed · of water. gases shut up in the hollow centre. their weight must so far diminish the conceded maximum of · . density of inner half of earth at miles diameter. greatest density may be less than · of water. supposed pressure of inclosed gases very moderate. meaning of heat limit to density. temperature of interior half of shell and inclosed gases must be equal. state of the hollow interior. results of the whole inquiry. inquiry into the interior construction of the earth--_continued_. it may be well to revert here to the experiment we made of putting a cubic foot of rock, of specific gravity · in the scale of a balance at the centre of the earth, where we saw that it could not depress the scale one hair-breadth, and make the same experiment by placing a cubic foot of rock of · specific gravity in the same scale, at what we have called the region of greatest density of the earth, that is, at miles from its surface. here, also, we shall find that the scale is not depressed for the very same reason as in the former case, that is because it had nowhere to be depressed to; and it might be argued that for the same reasons advanced formerly there can be no matter at that place, but the cases are entirely different. in the first case, there is nearly the whole mass of the earth drawing the matter away from the centre were it at liberty to move; whereas, in the second case, the meeting of the two halves of the shell, at the region where there is the greatest mass of matter, is also the meeting place of the action of attraction in its greatest force; the place to which matter is attracted from all sides, remains stationary, and it is held there both by attraction and weight of superincumbent matter or gravitation. the attraction of the whole earth acts as if it were concentrated at its centre, but that is for external bodies. that kind of attraction on the inner half of the shell would be far inferior to that outwards of the outer half, owing to its greater distance and conflicting nature, and would perturb, as we have said, but not do away with it. the same could not occur at the centre, because it is not the centre of the mass, that is, it is not the place where the greatest quantity of matter existed originally, or is now to be found, and consequently never was, nor can now ever be, the actual centre of interior attraction. it has been said when treating of the earth as being solid to the centre, that it is not easy to comprehend what may be the nature of the rocks we are acquainted with, when compressed to one-fourth or one-fifth of their volume, and we do not find ourselves much better off when we contemplate them as reduced to one-third or one-fourth of their bulk, that is, when a cube of one foot is reduced to three or four inches in height, as would be the case with it at a maximum density of · times that of water when placed at a depth of miles from the surface of the earth. we find, therefore, the idea thrust upon us that there may be a limit to density, perhaps not an absolute limit, but a practical one; in which case, the greatest density of the earth may not greatly exceed · times that of water. for, if we conceive that it increases to its maximum at miles from the surface, and continues nearly uniform thereafter, a little calculation will show that the greatest density of the outer half of the shell need not much exceed times that of water; and, of course, the same will be the case with the inner half should its density be almost uniform till miles from the inner surface is reached. it might even so happen that at a depth of to miles the practical limit might be reached; for a column of granite of one foot square and miles high would weigh, and exert a pressure upon its base of , tons, a pressure equal to nearly fifteen times what would be sufficient to crush it into powder; in which case the greatest density of the earth might not much exceed the · that we are accustomed to think of--without thinking. it may be deemed absurd to think that there is even a practical limit to the density of matter, but on the other hand it is much more absurd to suppose that there is not an absolute limit to it. we cannot conceive of density being other than the result of compression, and we cannot believe that matter can be compressed more and more continually for ever. there must be some end to compression. perhaps it was the difficulty in conceiving of rock being compressed to so small a fraction of its volume as would enable it to take its place at the centre of the earth--where it has been said that, "it must weigh like lead"--that originated the idea of its centre being occupied by the metals, arranged as they would be in a rack in a store, the heaviest pieces at the bottom of the rack, and the lighter ones higher up. when fairly looked at, density would really seem to have a limit, except in so far as it may be combined with heat. we know that water is compressed · part of its volume for every atmosphere of pressure to which it is subjected. but · for round numbers, is in fractional numbers / , ; therefore a pressure of , atmospheres would compress a cubic foot of water into / , of a foot in height, or practically into nothing. we know, also, that as a column of water · feet high balances one atmosphere, one mile in height will be equal to · atmospheres, and , atmospheres will produce a pressure equal to a column of water miles high; therefore, a cubic foot of water, subjected to such a pressure, would be compressed into virtually nothing. again, supposing that we have a column of liquid rock, of - / times the density of water, of the same height of miles, we should have a pressure of - / times that of the column of water; and as we have no reason to believe that granite in a liquid state has to obey a different law of compression to the one obeyed by liquid ice; then a column of granite miles high would be sufficient to squeeze its own base, not only off the face of the earth but out of the bowels thereof. it will be seen, therefore, that at miles deep from the surface, the density of the earth might well be equal to not only · times the density of water but to a great deal more; and that our estimate of times the density of water, at miles deep, was far within the mark. the authors of text-books on the strength of materials tell us that "the modulus of elasticity of any material, is the force that would lengthen a bar of that material of inch square to double its length, or compress it till its length became zero; supposing it possible to stretch or compress the bar to this extent before breaking." this is neither more nor less than a counterpart of the law of gases, upon which the air thermometer is constructed, applied to solid matter, and may be used in the same manner. but we can never produce a perfect vacuum, and so annihilate a gas and temperature; neither can we annihilate matter, nor easily reduce it to one half of its volume. now, we have seen, a little way back, that a column of granite miles high would exert a pressure at its base times as great as would crush it to pieces; so that a column of ÷ , or · miles high would destroy the elasticity of the material, because, when crushing takes place, all elasticity is gone. we cannot, therefore, get much satisfaction out of any calculations made upon the theory of the strength of materials; still, by them, we can make more plain the absurdity of any notion of the indefinite compressibility of matter. but if, in the face of contravening its conditions, we follow the reasoning used for the formation of the theory, and take the modulus of elasticity for granite as , , feet, then the same modulus would compress a bar of granite of inch square in section till its height became zero. and as that length is equal to miles, at that depth from the surface of the earth, granite or any other rock or stone of a similar nature would be compressed out of existence by the weight of the superincumbent matter. thus we have arrived at two measures of force which would compress to zero the rocks that are known upon the earth. one where rocks are looked upon as in a molten, liquid state, and analogous to water, where the force is equal to that exerted by a column of the material miles high; and the other where the column requires to be miles high. in either case the same method of calculation will show that columns one-half of these heights, will compress the material into at least one-half of its volume--that is half-way between what it is at the surface and would be at the specified depths--and consequently into double its density. so we find in the one case that the density of the earth ought to be about · times that of water at a depth of - / miles; and, in the other, at somewhere less than miles deep. but, before proceeding to use and reason upon these depths, we must recall to mind that the calculations from which we have derived them, in the second case, have been made in violation of the theory that was adduced for the purpose, and that in consequence the latter depth must be excessive. for, were we to erect a structure of any kind, calculating the stresses it would have to bear, under the same violation of the theory, we should inevitably find that the structure would give way under the strains that would be brought upon it; that is the columns - / and miles high would compress the same kind of matter composing them into very far below one-half of its volume. this premised, let us go back to our layers of miles thick with their respective volumes. nine of them counted from the diameter of miles inwards, will be equal to miles and will bring us to miles deep, which at the same time that it leaves us the same volume and mass that we have always retained for the first miles in depth, will facilitate our calculations considerably without making any appreciable difference in them. we shall then have to find for the layers corresponding densities increasing from to · , and if we multiply these together respectively, and add the numbers of the volumes and masses of the outer miles in depth, we shall get, at the diameter of miles, a simple volume of , , , cubic miles, and mass volume of , , , cubic miles. deducting this latter sum from , , , cubic miles, which represents the half mass of the earth at the density of water, we have a remainder of , , , cubic miles. on the other hand we find that the simple volume of the earth comprehended between the diameters of and · miles is , , , cubic miles; so that if we divide , , , by this sum, we find that a density of · times that of water over the whole intervening space--between the two diameters just cited--will make up the whole half-volume, at the density of water, from the surface of the earth to the diameter of · miles. then, for the inner half-mass:--if we multiply the simple volume between the diameters of · miles, and miles, which is , , , cubic miles by · , we get , , , cubic miles at density of water; and if from there we run down the density to at miles in diameter we get , , , cubic miles, which added to the last mentioned amount gives , , , cubic miles, somewhat in excess of the inner half-mass of the earth at density of water. thus we see that in order that the average density of the earth of · may be made up, there is no necessity for appealing to matter of any kind with a density of more than · times of water. and there is still something else of importance to be taken into consideration before we can bind ourselves to a density even so great as that. we have said, a few pages back, that there can now be no undeposited cosmic matter in the interior of the hollow earth, and that as far as such matter is concerned the hollow part may be a perfect vacuum. this is not absolutely true, for gases may be cosmic matter, just the same as any others of the elements out of which the earth is formed, but what is generally meant by cosmic matter is solid--at least, we have always looked upon it in that light--and all solid matter must have been deposited upon the interior surface at an immeasurably long period of time before the nebula forming the earth came to have even the density of water; certainly before it came to be in a molten liquid state; and we did not want to introduce any posterior evolutions in order not to complicate our calculations, and also to obtain some tangible bases to which the consequences of these evolutions might be applied. but as we have now both form and density to work upon we may take them into account, and it will be found that neither of these two bases will be very materially altered by them. when the earth was in a molten liquid state, it is believed--as we have said on a former occasion--to have been surrounded by a dense atmosphere, composed of gases and vapours of metals, metalloids, and water, and we have no reason to doubt that the hollow of the sphere was filled with a similar atmosphere, only the vapour of water would, most probably, be dissociated into its elements of oxygen and hydrogen. also we have every reason to believe that even at the present day gases are being produced in the interior, one part of which find their way to the surface and are dissipated into the atmosphere in the same manner as the gases from the chimney of a furnace; and another part into the interior, where they could not escape but would be stored up in the hollow. thus at the present day there may be an atmosphere there, composed near the surface of vapours of the elements with gases above them, so to speak, at a very high degree of pressure. these gases could not have gone on accumulating always, but must have found an exit in some particular place, or places, when the pressure exceeded the resistance, or when this was diminished by some convulsion such as an earthquake; but we do not want to define too much, or make more suppositions on this point than what present themselves to us in a reasonable way. all that we need say is, that the resisting power of some thousands of miles of solid, or even viscous, matter must be enormous, and the pressure necessary to force its way through it must have been equal to many thousands of atmospheres. we know that a pressure of · atmospheres condenses air to the density of water, and it must be the same with any similar gas; so we have only to suppose that the pressure is atmospheres--which is equal to · multiplied by · --in order to bring the whole of the gases, and vapours of elements, in the hollow to the same density of · times that of water, which we have shown need not be exceeded in any part of the earth. and such being the case, we can place the division between solid and gasiform matter in any point of the radius that may seem to us reasonable, only we must always have as much solid matter in the inner as in the outer half-mass of the earth. following nearly the result we have obtained in another way, by placing the division of the hollow part at miles in diameter, the volume of which is , , , cubic miles, and multiplying this by · , we get a mass equal to , , , cubic miles at density of water, composed of vaporous and gaseous matter in the hollow centre, and consequently much greater than is required to make up the total mass of the earth at the density of water; which shows that the density of the mass between the diameters of and miles must be less than · times that of water. how much less is very easily found, by dividing the surplus of , , , cubic miles over the whole volume between miles in diameter and the centre, because in this way we shall include the whole mass arising from both solid and gasiform matter. this whole volume--that of a globe miles in diameter--is , , , cubic miles, which, divided by the surplus gives the amount · as the density to be deducted from · on its account, and therefore the greatest density of any part of the earth need not be over · times that of water. this result derived from our operations will be acknowledged, we doubt not, to be much more satisfactory, we might say, more comprehensible, than to have to believe that our known rocks and stones could be compressed till they were · or even · times heavier than water. at first sight , say , atmospheres or , lb. on the square inch, appears to be an enormous pressure, but it is nearly almost as nothing compared to the pressures we have been dealing with. a column of granite mile high would exert a pressure upon its base of lb. per square inch, and one of miles high of , lb., or double the number of atmospheres we have applied to the gases in the hollow of the earth. if we take a column miles high, such as we considered to be the least that would be necessary to compress granite into one-half of its volume, we get , , lb. per square inch, or over , atmospheres of pressure; and if we go into thinking of columns of and miles--this last being the depth from the surface of the division of the matter of the earth into two equal portions--we could have gases compressed to , and , atmospheres or, dividing the numbers by · , and times the density of water; so there is no cause to stumble over high pressure. with even , atmospheres, more than double the number assumed, we should have gases as heavy as the material we found at the centre of the earth, when we were looking upon it as solid to the centre--which was · times the density of water--and so get rid of burying the precious metals where they would be "matter in the wrong place," and according to d'israeli's definition, justly entitled to the epithet applied to them, sometimes, by people who have never been blessed with a superabundant supply of them. at the same time, we find out what we knew before, viz. that we may have gases heavier than the heaviest metals and as rigid as steel, if we can only find a vessel strong enough to compress them in, along with the means of doing it; and also that the thousands of miles of highly compressed matter, between the hollow centre and the surface of the earth, are far more than sufficient to imprison gases of far, very far, greater elasticity than our modest measure of atmospheres. and we hope to be able to show presently good reason for believing that the gases compressed in the hollow, at what may really be considered as very high pressures, have had, and may probably still have, a very important part to play in the evolution of the earth. we have just seen that the pressure produced by a column of granite mile high would be lb. per square inch, consequently one of double the height, or miles, would exert a pressure of , lb. per square inch at its base, equal to the crushing strain of the very strongest granite we know, while at the same time that strain would not amount to one-sixth of atmospheres; so that if the gases in the hollow of the earth were at a pressure of only atmospheres, their pressures would be able to crush granite of that class to pieces, and therefore the estimate of specific gravity of for the density of the interior surface--which we made at the beginning of our calculations for the hollow sphere--cannot be looked upon as by any means exaggerated. we might now reform our calculations of the two halves of the interior of the earth, giving a more rational and curve-like form to the densities, under the supposition that at much less distance than miles from the surface, matter might be compressed to its utmost limit; but as, according to our demonstration, the solid matter of the earth must have been divided into two equal parts at the place where the greatest mass was, long before it could have been condensed into a state to compress gases; and as the total mass of solid matter must, in order to make up the total mass of the earth, depend to some extent on the mass of imprisoned gases; we are unable to make any reform much different to what our calculations show. besides, as the difference between average densities of · and · makes a difference of , , , cubic miles on the mass of the earth reduced to the density of water, very approximate accuracy cannot be attained in any calculations. what is meant by a limit to density except in so far as it is combined with heat, is that whatever density may be given to matter by compression when it is in a heated state, a greater density will be found in it when it is deprived of that heat; that whatever may be the density of any part of the interior of the earth in its present state, that density will be increased when the earth becomes cooled down to the temperature derived from the heat of the sun, or to absolute zero of temperature, if such there be, on account of shrinking in cooling; and that therefore there can be no absolute limit to density as long as there is any heat in matter. it may not be unnecessary for us to recognise now that the weight of a column of granite would decrease as the depth increased, for the force of gravitation would be diminished by having a part of the attraction of the earth above instead of below it; but at miles in depth the diminution would be only about one-eighth--if distance is taken into account--of the miles down to the plane of greatest density, and / th part if the mass left above is considered; differences that would make extremely little alteration on our calculations. it will not be out of place either to take a look at what may be the temperature of the interior of the shell, and of the gases shut up in the hollow part of the earth; and we have not much to say on the subject, because we shall not depart from the system we have followed up till now, with considerable strictness, of not theorising or speculating on what may be; but will restrict our observations to theories that have been very generally adopted by astronomers, geologists, and scientists in general. the air thermometer will be of no use to us, for whatever may have been the temperature when the earth was in the process of formation, it must have diminished very greatly during the cooling process it has undergone since, and we know that gases heated in a closed vessel in such manner that pressure and temperature will agree to the theory on which the air thermometer is constructed, may be cooled down afterwards to almost any degree required, and the relation between temperature and pressure destroyed thereby. at one time it was thought that the earth had only a solid crust, and that, under it, the whole of the interior was in a molten liquid state. then some physicists thought that, through pressure of superincumbent matter, solidification must have begun at the centre; others that it began almost simultaneously at the surface and centre, and that there may still be a liquid mass between the two solidifications--this is repeating what we have said before, but it is done only to bring it to mind. we, at present at least, do not want to have anything to do with any of these theories, only we believe that we have shown in an indisputable manner that there could be no solidification at the centre, because there could be no matter there capable of being solidified--gases could not be solidified under such pressure, and at all events heat, as there must have been there. we believe at the same time that no one will deny that the heat of the earth increases as the centre is approached, and that the temperature of the interior may be very great. the crust of the earth was at one time supposed to be only to miles thick, because the increase of heat at that depth would be sufficient to melt any of the substances we are acquainted with on the surface--repetition again; but for many years past it has been deemed necessary to increase the thickness to even hundreds of miles, for reasons some of which will be alluded to in due time; and if, even at these depths, the increase of heat were only sufficient to fuse all the substances we know, it is very certain that at the interior surface of the shell it must be very much greater, as heat from there could only be _conducted_ outwards, and the difference required to cause conduction, of any considerable degree of activity, through more than miles must be enormous, according to the experiments made by various physicists upon metals, which have a very much higher conducting power than rocks, and especially strata, of any kind. therefore there can be no doubt, we think, that the inner surface of the shell must be at a very much higher temperature than what would preserve it in its liquid state, and that the matter composing it is liquid to a depth where it might be solidified by the pressure of superincumbent matter. we do not see how convection currents could be instituted, much less kept up, in melted matter, under the viscosity, and, at least quasi-solidity, sure to be produced by pressure of tens of thousands of pounds on the square inch, and therefore we do not take them into account. any way, whatever may be the temperature of the interior surface of the shell, the same must be that of the imprisoned gases, because there convection currents could and must exist--were they even only created by the rotation of the earth and attraction of the moon--and cannot fail to keep the whole of the hollow part at the same temperature. it would be absurd to suppose that these gases could be at a lower temperature than the upper layers, counted from the region of greatest density, of the interior surface of the shell. this section of our work may now be brought to a close by stating the conclusions at which we have arrived, leaving the results involved by them to be discussed separately, which we shall proceed to do immediately without binding ourselves so strictly, as we have done hitherto, to the avoidance of anything that may be looked upon as theorising or speculating. we believe we have conducted our operations in the most strict conformity to the law of attraction, and have no doubts whatever about the form of the interior of the earth resulting from them; but there may be some room for small variations in the details of the various densities, and the position of the interior surface of the shell, arising from the pressure of the gases in the hollow centre, and the weight they will, in consequence, add to the general mass of the earth. the conclusions are as follows:-- ( ) that the earth is not solid to the centre, nor is it possible that it could be, according to the law of attraction, but is a hollow sphere. ( ) that its greatest density must be at the region where the greatest mass of matter is to be found--as must have been always the case from the time it was a globe revolving on its axis, whether gasiform, liquid, or solid--which is now at miles deep from the surface; and that the greatest density may not be much more than the mean of · times that of water ascribed to it by astronomers. ( ) that the inner surface of the shell of the hollow globe cannot be much over or under to miles from the outer surface. ( ) that the hollow part of the globe must be filled by an atmosphere consisting possibly in part of vapours of the chemical elements, and by gases at a very high degree of pressure. ( ) that the region of greatest density, and the position of the interior surface of the shell, may be expressed with very approximate accuracy as follows:--the former must be at · of the mean radius of the earth, and the latter at · of the same; both counted from the centre. ( ) that if the earth is a hollow sphere, the same must be the case with all the major planets and their satellites, the sun, and all the suns, or stars, that are seen in the heavens; and that their interior proportions and form must be in much the same ratios to their radii as those we have found for the earth. chapter xi. page the earth. the idea entertained by some celebrated men, and others. difficulties of forming a sphere out of a lens-shaped nebula. various studies of the earth's interior made for special purposes. difficulty some people find in conceiving how the average density of little over · can be possible, the earth being a hollow sphere. what is gained by its being a hollow shell. geological theories of the interior discussed. volcanoes and earthquakes in relation to the interior. liquid matter on the interior surface of the shell, and gases in the hollow, better means for eruptions than magma layers. focal depths of earthquakes within reach of water, but not of lavas. minute vesicles in granite filled with gases, oxygen and hydrogen, but not water. the moon. a small edition of the earth. rotation stopped. convulsions and cataclysms caused thereby. air, water, vapour driven off thereby to far off hemisphere. liquid matter in hollow interior would gravitate to the inside of the nearest hemisphere. form and dimensions during rotation. altered form after it stopped. agreeing very closely with hansen's "curious theory." consequences of the earth and moon being hollow bodies. _the earth._--the idea that bodies such as those of the solar system, even of the whole universe, have their greatest density where the greatest mass is and are hollow spheres, is so natural and logical, more especially if it is supposed that they have all been formed out of some kind of nebulæ, that it seems strange it has never been brought forward prominently before. we say prominently because we know that the earth has been considered to be a hollow sphere by very eminent men, such as kepler, halley, sir john leslie, and by others of less name long after them. in support of this last remark, we shall make a few extracts--with comment on them--from an article on the "interior of the earth" in "chambers's journal" for february , which have some interest in connection with our work. . "the great astronomer kepler, for instance, in seeking to account for the ebb and flow of the ocean tides, depicted the earth as a living monster, the _earth animal_, whose whalelike mode of breathing occasioned the rise and fall of the ocean in recurring periods of sleeping and waking, dependent on solar time. he even, in his flights of fancy, attributed to the earth animal the possession of a soul having the faculties of memory and imagination." if it could be believed that kepler had any idea of the earth being formed out of a nebula, whether hollow, or solid to the centre, the idea of a breathing animal was almost a consequence, because the attraction--a thing he is supposed to have known nothing about--of the original nebula for the earth one, on matter so light as nebulous matter, would raise enormous tides and make the earth, in its then state, not far from like an enormous primitive bellows made out of goatskins. no one knows what dreams may have passed through his brain. the last part of his notion was altogether fanciful. . "halley was opposed to the idea of the globe being solid, 'regarding it as more worthy of the creator that the earth, like a house of several storeys, should be inhabited both without and within.' for light, too, in the hollow sphere, he thought provision might in some measure be contrived." this notion appears to be altogether fanciful, the fruit of an enthusiastic, exuberant imagination, leaving no trace of scientific thought upon the subject. . "sir john leslie, like halley, conceived the nucleus of the world to be a hollow sphere, but thought it filled, not with inhabitants, but with an assumed 'imponderable matter having an enormous force of expansion.'" it would be interesting to know on what bases he formed his ideas, as the filling of the hollow with imponderable matter seems to show more method than the former cases, but we have never seen any allusion made to his theory anywhere, except in the article we are quoting from. there may have been some reasons given for such a supposition in his "natural philosophy," but when we began to read that work in times long past, a more modern one was recommended to us, and we lost the chance, never to return. there are other theories referred to in the article, but we shall take notice of one more only. . "a certain captain symmes, who lived in the present century, was strongly convinced of the truth of leslie's theory. he held that near the north pole, whence the polar light emanates, was an enormous opening, through which a descent might be made into the hollow sphere, and sent frequent and pressing invitations to a. von humboldt and sir humphrey davy to undertake this subterranean expedition! but these imaginative conceptions must one and all be set aside, and the subject treated on more prosaic, though not less interesting, lines." this conception of captain symmes will probably be looked upon as the most absurd of the whole lot, but to us it seems to give evidence of more thought than any one of them. one would think that he must have formed some notion of how a hollow sphere, with an opening out to the surface at each one of its two poles, could be formed. we must note that he lived in, possibly after, the time of laplace. we doubt whether anyone has ever studied out thoroughly how even a solid sphere could be ultimately elaborated from a nebula. it has always been a very general idea that a condensing and contracting nebula would, under the areolar law, assume the form of a lens rather than of a sphere. if this be so in reality, we may ask: how can the law of attraction produce a sphere out of a lens-shaped mass of rotating vaporous or liquid matter? it seems evident that to bring about such a result attraction must cease to act altogether in the polar directions, and only continue to draw in the matter from the equatorial directions of the lens, till the desired sphere was formed; and, how were the action and inaction of the law of attraction to be regulated meanwhile? or, when the time came that a sphere of a pre-arranged diameter could be formed, a goodly part of the lens must have been cut off and abandoned; in which case we have again to ask: what was done with the surplus, the cuttings? no doubt they could be used up in meteor swarms, comets, or something; but captain symmes's theory has opened up a field for a good deal of thought, and our present knowledge of polar matters prevents us from being sure that strange discoveries may not be made as to the condition of the earth at the poles, although there may not actually be holes into the hollow interior. with regard to the last sentence of the quotation, we fully agree and are doing our best to comply with it. and in so doing, we shall have to return to the formation of globes out of nebulæ, elaborated into something more advanced than even lens-shaped discs. there is no doubt that the reasons assigned by most, if not all, of the authors of the notions above cited are very fanciful, but one can hardly believe that the true reason--why the earth must be hollow--has not occurred to some of them; and that they did not follow it out because it involved too much work, and they did not feel inclined to undertake it, or had not time. on the other hand, modern astronomers and physicists have been so fascinated by the discoveries they have made, and in following them up, that the temptation to go on in the same course has been too great to allow them to spend time on the investigation of sublunary and subterranean affairs. some of them have indeed studied the interior of the earth for special purposes, such as the thickness of the crust, solidity or liquidity, stability, precession of the equinoxes, the action of volcanoes, etc., etc.; but they never, apparently, examined into any of these features to the very end, otherwise, we believe, they would have come long ago to the same conclusion as we have. and withal it seems wonderful how near some of them have come to it. to most people it would appear absurd to think that any part of the earth of any great magnitude can be hollow, if in order to make up its mass its average specific gravity must be · --more especially, if we tell them that the greatest specific gravity at any place need hardly exceed · --forgetting that weight or mass can be taken from the interior where the volume per mile in diameter is small, and be distributed near the exterior where the volume per mile in diameter is comparatively immensely greater. but in whatever light we look upon the conclusions we have arrived at, a change in the construction of the bodies in space from solid to hollow spheres must produce changes in our ideas of them, and have consequences of great importance, too numerous to be all taken account of; we shall, therefore, only take notice of the most prominent. looking at the earth as a hollow sphere, we get rid of the difficulty of conceiving that matter can be compressed to three or four times less than the volume it has as known to us; and also of the misplacement of metals to the incredible degree we have shown to be necessary to make up its whole mass according to the sorting-out theory. and if we can only be bold enough to look upon gases as ponderable matter that can be compressed to great density, and so added to the weight of the whole mass, we may not be under the necessity of compressing the known matter composing it to even the half of its volume. somewhere in the first quarter of this century (see "edinburgh review," january ) mr. hopkins argued that the solid crust of the earth must be at least to miles thick, in order to account for the precession of the equinoxes and nutation, but about a quarter of a century afterwards m. delaunay demonstrated before the french academy by actual experiment that the thickness of the crust had no bearing whatever on the problem. and about the same time lord kelvin inferred from the same thickness of crust that "no continuous liquid vesicle at all approaching to the dimensions of a spheroid miles in diameter could possibly exist in the earth's interior without rendering the phenomena of precession and nutation sensibly different from what they are"; and that the earth, as a whole, must be far more rigid than glass and probably more rigid than steel, "while the interior must be on the whole more rigid, probably many times more rigid, than the upper crust." with the theory of a hollow shell, a better foundation is given for mr. hopkins's argument than a solid crust at about the same depth as he assumed, while at the same time the liquid vesicle of miles in diameter is removed, which lord kelvin showed would change the phenomena of precession and nutation. we have seen that imprisoned gases may have a high degree of density, and consequently rigidity, and may in some measure supply what was required by lord kelvin, who knows, also, very well that a structure with some degree of elasticity in it is stronger than one that is absolutely rigid. moreover, the shell of the earth, composed of solid materials at a very high temperature, and consequently so far plastic, could not fail to accommodate itself to any variation of centrifugal force that could take place. variations in rotation of the earth could only have come on extremely slowly, and even the most rigid matter we know will gradually yield to extreme pressure long continued. but this subject of the plasticity of the most solid part of the interior was discussed and, it may be said, demonstrated during the meeting of the british association of , as reported in "nature" from july to september of that year. any way, the possibility of plasticity is most patently shown by the hollow-sphere construction of the earth. we do not know what were m. delaunay's proofs that the thickness of the crust has no bearing whatever on precession and nutation, but if they were complicated with the fluidity, or even viscosity, of a liquid interior beyond a depth of to miles, they must be entirely changed under the notion of a hollow sphere where there could be no really liquid molten matter, except near the inner surface. one thing we may be certain of, and that is, there must be something to account for precession and nutation, and we believe that the hollow shell, with the greatest density where the mass is greatest, is a much more rational cause for these phenomena than the bulging out of the earth to the extent of miles or so at the equator. it is very difficult to find out what geologists consider to be the nature of the interior of the earth in its details, but for our purpose no particular knowledge is required. however, it is necessary to allude to the principal features of their theories in order to note and remark how far they will agree to, or be facilitated, or the reverse, when applied to a hollow sphere. it would seem that almost all geologists are agreed that the central part is solid, and possibly extremely rigid owing to the enormous pressure of superincumbent matter; that it has a solid crust of several hundreds of miles in thickness; and that under this there is a sub-crust divided into two or more layers of different densities, partially liquid or at all events plastic, extending all over the solid interior matter; the chief purpose for which it is required being apparently to supply matter for volcanic action and surface movements. under the theory we are advocating, the place of greatest density of the interior is calculated to be at miles from the surface, and its greatest approach to solidity will be there also; consequently, if geologists consider that it will have sufficient plasticity there to provide matter for volcanic eruptions, they will be at one with us so far. but should they consider that they require, for volcanoes, matter more liquid than is likely to be found at that depth, they will have to place their magma layers either much deeper or somewhere between that depth and the surface, in which case they will encroach on the requirements of astronomers, without liberating themselves from a difficulty in which they must find themselves involved under their present ideas. they say that these plastic layers exist under the solid crust all round the interior of the earth, so that if one of the duties they have to perform is to keep the various chains of volcanoes in communication with each other, their lateral movements must extend to some hundreds of miles in the cases of the enormous volumes of matter that are sometimes thrown out in even modern eruptions, and they have to provide the means for procuring that lateral motion. shrinkage from cooling, or falling in of part of the solid crust, might bring about these enormous outbursts of lava, but they would be more likely to produce simple overflows than the explosive ejection of such masses as are now being recorded from time to time. we have brought into remembrance, page , that water cannot penetrate into the interior of the earth to a greater depth than miles, more or less, as water, and that beyond that depth it can only exist in the form of steam, or dissociated into its elements of hydrogen and oxygen. as long as it continued in the form of water it could be suddenly flashed into steam, of not far from two thousand times its volume, by relief from pressure or sudden application of heat, and thus be converted into a violent explosive almost instantaneously; but when it came to have the form of a gas, it could only be heated gradually the same as any other gas. it is clear, therefore, that water cannot be looked to for producing the force, explosive or otherwise, that is required to raise even molten matter from depths of hundreds of miles to overflow from the summits or outlets of volcanoes. a pressure of atmospheres would be required to balance a column of average rock of _one mile_ high. a mass of water, through shrinkage of the crust, might get introduced to the vent of a volcano, or some cavity connected with it, a few miles under the surface of the earth and cause an earthquake--it might be introduced by an earthquake--or eruption or both, abundantly formidable and destructive, no doubt, but only comparatively superficial, such as those of naples and charleston, where the extreme depth was calculated to be only a few miles; but it seems to us to be totally inadequate to produce those outpours that last for days and weeks, covering leagues of land, and filling up bays of the sea, with floods of lavas. it may be the principal agent or ally in producing the horrors and devastation of a grand eruption that has invaded the regions of water, but it is not to be conceived as possible that it can be the prime cause. the volumes of steam, water, and mud thrown out on such occasions, only tend to distract our attention from looking deeper for the true cause of the eruption. geologists are therefore thrown back upon their magma layers to look for the motive power for producing these grand eruptions, and they cannot get water down deep enough to do it. tides produced by the sun and moon cannot be appealed to, otherwise the eruptions would be more or less uniform in their periods of occurrence. sudden evolution of gases in the magma layers could not be accounted for in any way known to us, and accumulation of gases would involve the idea of immense cavities, to serve as reservoirs to be gradually filled till the pressure was sufficient to force a way out, and would imply a formation of the interior in compartments specially adapted for particular purposes, and altogether too fanciful to be entertained. where could such enormous masses of matter, as those thrown out, come from at only a few miles from the surface? the great eruption at the sandwich islands, of about a century ago, after flowing over a distance of many miles of land, on which it left enormous quantities of lava, filled up a bay of the sea twenty miles long, and ran out a promontory of three or four miles into the sea; and we cannot conceive it to be possible that such a quantity of matter could be blown out from something less than miles deep by water suddenly flashed into steam. the critical temperature of water--that temperature at which it changes into steam under any pressure however great--being °, its pressure in the state of steam will be somewhere about lb. per square inch, let us say atmospheres; then, if atmospheres are required to balance mile in depth of average rock, as we have stated above, the pressure of steam just cited would balance only - / miles of rock. we can, therefore, see how inadequate it would be to force a column of lava up from even the depth of miles. at that depth atmospheres of pressure are required to balance a column of lava, and there are only available. it has been said that the downward pressure of steam would force up the lava through the vent of a volcano, but an arrangement of that kind would require a downcast shaft as well as the upcast one of the vent like as there are in collieries; but the downcast would have to go very deep to compress the steam--a gas now--to the required number of atmospheres. far more likely that the steam itself would put an end to any increase of water, by driving it back through the channels by which it was descending; for if they are supposed to exist under a solid crust of miles thick the pressure required would be , atmospheres, and with a crust of only miles thick , would be required. the only way, therefore, in which volcanic eruptions can be produced in the earth, if solid or liquid, or partially solid and partially liquid, to the centre--in other words, from magma layers--is by the shrinking of the crust squeezing out the lava. with a hollow earth and shell of more or less miles in thickness, liquid to some depth on the interior surface the difficulty becomes very much less. the communication between the vents of volcanoes would be complete and simple, without any lateral forcing of the lava through magma layers made expressly for the purpose; it would be an open and natural flow from one place to another. that there are such volcano vents connected with each other has been very generally believed, and even almost proved by observation of eruptions taking place in two or more almost simultaneously, or at the least showing signs of violent agitation, the motive forces for which would be the gases which we have concluded must be imprisoned in the hollow centre. when their pressure came to be sufficient to blow or force out the liquid, or semiliquid matter, bubbling and boiling in the vents in constant activity, there would be an eruption, during and after which the gases would escape till their pressure was greatly reduced, when the volcanoes would return to their semi-active state. the gases would naturally be those of the many kinds that are found in eruptions, by reason of their being generated in the earth, mixed with steam and water in the manner we have already shown. let it not be supposed that the gases would require to have force enough to raise lavas from depths of over miles from the surface. according to our arguments for a hollow earth, at miles from the surface the two halves--outer and inner--of the matter composing it meet and balance each other, so that all the pressure required would be what is necessary to overcome the inertia, viscosity, or cohesion of the matter in the vents. what that would be we do not pretend to be able to calculate, but we believe that it would be very much inferior to that required to balance a column of lava of even miles high. we have seen that gas compressed to atmospheres would be - / times more dense than water, and of equal specific gravity to the heaviest matter required in any part of the earth to make up its average density to · that of water, and we cannot assume any greater pressure than this, without diminishing that maximum. if that, or any lesser degree of compression, would supply the necessary force, then all difficulty is removed further than pointing out the means of keeping the volcano vents open or openable; and the quality of openable may be facilitated by the contraction of the interior from cooling. if a greater pressure be necessary, we need not be afraid of greatly increasing it, for the only consequence would be to diminish the maximum density of solid matter required in any part of the earth, to make up the general average to · , which means less compression of the matter. if the idea of the accumulation of gases in the hollow centre, or of the hollow centre itself, is inadmissible, then scientists in general can continue as before with their magma layers--aqueo-igneous if they like--but they must abandon the notion of lavas being expelled from them by steam pressure. we repeat that steam could never get down in the form of steam to the depths they require. the temperature there would be more than sufficient to resolve it into its elements of oxygen and hydrogen, and it would behave very much like the gases we have supposed to be in the hollow; there might be accumulation, but there could be no sudden flashing into existence like steam from water. in support of our observation--if it needs support--that water as water cannot penetrate into the earth to a greater depth than where it meets a temperature of ° we may refer to reports on earthquakes of comparatively recent occurrence. we learn from the "london quarterly review" of january , that in the neapolitan earthquake of , mr. mallett found the greatest focal depth to have been - / geographical, or · statute miles, which agrees very well with the depth to which water could penetrate and be suddenly flashed into steam. (we say nothing, for the present at least, about how the water and the heat managed to meet so instantaneously.) the shock of the instantaneous generation of steam might be felt much lower, but it would tend to interrupt, not to produce, the eruption of lavas. in speaking of the pressure on the walls of the cavity, where the shock was produced, being , millions of tons, the reviewer says, "it may have been greater because the steam might be supposed to have acquired the temperature of the lava," and that is °f.; but that could not well be. in order to meet lava of that temperature the steam would have to descend to from to miles deep; on the other hand, if the lava is assumed to have entered the cavity, it could only do so at a comparatively low velocity and would not reach more than a fraction of the steam at a time, and even for that reason there could be no flashing, as steam is only a gas, and cannot be heated otherwise than as a gas. here the spirit of facilitating the meeting of the lava and the steam, is as apparent as in bringing about the meeting of the water and the lava noticed above. on the whole, therefore, we think that we were right in saying that steam or water cannot be the cause of volcanic eruptions, but that the invasion of the domains of water by the lavas may be the cause, in the main, of the explosive part of eruptions, and of the most disastrous effects of earthquakes. moreover, the focus of the neapolitan earthquake was miles distant from vesuvius, and therefore far removed from anything like direct connection with the vent of the volcano, so that water from it in any form could have no effect upon the magmas of scientists. "the scientific american" of july , , tells us that captain e. d. dalton has calculated that the depth of the charleston earthquake was miles; statute miles, it is to be supposed, as nothing is said to the contrary. to reach the temperature of ° this would give an increase of ° in metres in depth, which is a considerably greater depth than what we have estimated, but does not invalidate our reasoning, as it has always been known that the gradient of increase of heat varies considerably from one place to another. besides, and more especially, charleston being a seaport, and, consequently, not far from the level of the sea, it is to be supposed that, owing to the presence of water, the cooling of the earth has penetrated to a greater depth there than in the heart of italy. the same authority states that in the formidable yokohama earthquake of , the mean depth was only - / miles. the mention of mean depth here makes us notice that the miles may have been the extreme depth to which the earthquake, or shock, was felt at charleston, and that the focal depth may have been considerably higher up than that. be that as it may, there is no proof existing that water or even steam can penetrate into the earth more than a very few miles, much less to hundreds of miles. having referred pretty freely to the aqueo-igneous magmas, supposed by some scientists to exist deep down in the interior of the earth, it is but fair to give our reasons for refusing to believe that there can be any such mixture in any part of it, or anywhere else. in order to do so, we shall first cite some of the bases upon which such ideas have been founded. in "nature" of december , , we find what follows:-- "let us now consider the alternative theory suggested by mr. fisher. he claims that geologists furnish him with a certain amount of positive evidence for the idea that water is an essential constituent of the liquid magma from which the igneous rocks have been derived. passing over the proofs of the existence of water in the crystals of volcanic rocks, and in the materials of deep-seated dykes, let us come at once to the granite, a rock which can only have been formed at great depths and under great pressures, and which often forms large tracts that are supposed to have been subterranean lakes or cisterns of liquid matter in direct communication with still deeper reservoirs. now, all granites contain crystals of quartz, and these crystals include numerous minute cavities which contain water and other liquids; and the quartz of some granites is so full of water-vesicles that mr. clifton ward has said: 'a thousand millions might easily be contained within a cubic inch of quartz, and sometimes the contained water must make up at least per cent. of the whole volume of the containing quartz.' this amount only represents the water that has been as it were, accidentally shut up in the granite, for some was doubtlessly given off in the form of steam which made its way through the surrounding rocks." we cannot follow mr. fisher in "passing over the proofs of the existence of water in the crystals of volcanic rocks and in the materials of deep-seated dykes"; because the presence of water in these crystals when examined in a laboratory is no proof that water was present in them when they were liquid, and before they put on the form of crystals. there is no analogy between them and general wade's read. any crystals that a man can pick up anywhere, even from the mouth of a volcano, are quite capable of absorbing vapour of water from the atmosphere before he can carry them to his laboratory. all matter is supposed to be pervaded, more or less, by the ether, and there is always an open road for it, i.e. the vapour of water to enter by. nature dives more rapidly into a piece of rock than a man can walk or drive down from the summit of a volcano, so that getting water out of it when he is in his laboratory, is no proof that the water was there when the piece of rock was at the bottom, not the mouth, of the volcano. the minute so-called water-vesicles in granite have only served the purpose of a snare to facilitate his deceiving himself, by the help of mr. clifton ward, to further his speculations. for we think it would have been far more natural for him to have supposed that these vesicles were originally filled with the all-pervading ether. or, are we to prohibit the ether from being present anywhere, except where it suits us? even the dimensions given to the vesicles of a thousand millions of them being contained in a cubic inch makes us at once think of something more ethereal than water. and the whole object of mr. fisher's argument is to show how the depth of the ocean may be increased by water expelled from such magmas. a hollow planet, with compressed gases in the centre, raises the idea of the possibility of explosion. it would have furnished olbers, or any follower of his, with the bursting force to shatter into fragments the planet, out of which he supposed the asteroids to have been made. it need not cause any alarm with respect to the earth, whose shell is very much thicker than that of the exploded planet, seeing that its whole mass has been estimated not to have exceeded one-fourth of that of the earth (see table i.). the atmospheres of pressure we have spoken of could have no such effect on so thick a shell as the earth's; and we cannot increase the number without diminishing its average density, as we have shown. when we see mars blown up, whose diameter, and consequent thickness of shell, are not much more than half those of the earth, we may begin to think of getting out of the way. the moon. this satellite is supposed, according to the nebular hypothesis, to have been at one time neither more nor less than a smaller edition of the earth itself, endowed with atmosphere, plains, mountains, volcanoes, rivers, seas, rotary motion, etc.; previous to which it had passed through the same stages of gasiform, molten-liquid, and solid as its parent had done. one would think that its almost perfectly round form proves to demonstration that it must have rotated rapidly on its, or an, axis at one time; but there are some astronomers who think that it has never rotated at all, an opinion in which we cannot concur by any means. when it arrived at the stage of having seas, the tides raised in them by the attraction of the earth must have acted like a brake on its rotation--in the same manner as its attraction is supposed to be now doing on the earth--and gradually reduced it until it ceased altogether; from which time forward it must have always presented the same side to the earth. it has been thought that the tides raised in it by the earth would be so tremendous that they would prevent anything like rotation having ever existed; but everything requires to be accounted for, and the only way to account for its perfectly circular form is by its having rotated. considering, then, the moon as having been dispossessed, absolutely, of rotation and reduced to the single motion of revolution round the earth--as far as we are at present concerned, at least--we can go back to the period when this change came over it, and consider what would happen about the time, and immediately after the rotation came to an end. when a fly-wheel is made to revolve rapidly and is then allowed to run until it stops, it very seldom comes to rest all at once, and generally swings backwards and forwards something like a pendulum, until it finally stops; because it is always a little heavier on one side than the opposite, even should the difference of weight be only that of the handle by which it was set in motion; so we may suppose it would be with the moon when at last it failed to turn the centre, as it is called--the tides, the retarding cause, giving origin to the difference of weight on opposite sides--and we can conceive what commotions would be created on its surface by the wobbles it would make. we can imagine how the seas would rush backwards and forwards over the lower land and hills, levelling them down to the flat plains that are seen spread abroad among the innumerable volcanoes which cover the side turned towards the earth, until it finally came to rest. when the commotions ceased and the centrifugal force of the moon's revolutionary motion round the earth--which is over miles per minute-came to act freely, we know that the atmosphere and seas, being the mobile parts of it, would be pretty nearly all driven off very quickly to the side farthest from the earth, perhaps even before it came to the final state of comparative rest, whose translation would involve mighty rushings of waters there as well. also, that all the liquid matter in its interior, being so much heavier and more difficult to be moved by centrifugal force, would gravitate towards the side nearest the earth, whose attractive force would soon put an end to anything in the form of interior tides of molten matter, which very probably existed up till that period. if the moon came to a stop without any wobbling, then the transference of atmosphere and seas to the farthest off hemisphere, and the gravitation of the liquid matter of the interior to the side nearest to us, might be more gradual but would finally and certainly come to pass. and here we must specially note that if it made one rotation for each revolution, or one rotation in any length of time or under any circumstances whatever, these transferences of matter from one hemisphere to the other could not have taken place, because there would be no stationary region to which they could be transferred by centrifugal force, as each part of its circumference would in its turn occupy that region. and above all--be it specially marked--because the moon would not, in that case, always present its same side to the earth. looking upon the moon as a hollow sphere of somewhat the same proportions as we have made out for the earth, the region of greatest density would be at about miles deep from the outer surface, the interior surface of the shell at the depth of miles, and the hollow centre miles in diameter, as long as it continued to rotate upon its axis. when that motion ceased and the seas were transferred to the hemisphere farthest off from the earth, and the liquid matter in the interior had gravitated towards the nearest, as we have just said above, its conditions would be very materially altered. lest it should be supposed that with a very thin crust, nearly its whole mass would gravitate to the side nearest to the earth, let us always bear in mind that the moon would be virtually solid to not far from the inner surface of the shell, through the pressure of superincumbent matter, both from without and from within, in the same manner as we have considered the earth to be. whatever water had been absorbed by the crust when it was still rotating on its axis--which, at most, could have penetrated only a few miles--and even whatever lakes or inland seas might have been left on the surface always seen by us, would be soon evaporated by the internal heat, and the heat radiated by the sun--which sir john herschel has calculated to be greater than boiling water--and driven off in the shape of vapour in the same manner as the atmosphere had been. these transferences would lead to two consequences, each one of its own nature, which we must not fail to notice particularly, as in great measure they explain to us the constitution, or rather the construction, of the moon. ( ) all air and vaporous matter being translated to the unseen hemisphere would tend to cool it more rapidly and deeply than the other, not only on account of the cooling powers of the water, but from the atmosphere and vapours preventing the heat of the sun from acting so powerfully upon it. ( ) on the other hand, owing to the accumulation of melted, or liquid, matter in the interior of the side now turned permanently towards the earth, the formerly solid part of that side would tend to increase in temperature, which, joined to the heat from the sun not intercepted by any atmosphere, and continuing without interruption for a fortnight at a time, would produce a great difference in the temperatures of the two hemispheres. thus it is natural to suppose that the thicker and cooler solid shell on the one side would tend to weaken and drive down the volcanic forces to a greater depth; while the greater temperature and thinner solid shell on the other, the down side--the one next to the earth--would have an exactly opposite tendency and would bring them nearer to the surface. in this manner we seem to find a very plausible reason for the great exuberance of the volcanic forces displayed on the surface of the moon always presented to us. both the interior construction and exterior form of the moon, as modified by losing its rotary motion, would no doubt be very different to that of a hollow sphere rotating on its axis; but hansen's "curious theory" has prepared us for this, by showing that some anomaly in its construction had been noted and commented upon, although the existence of the anomaly was not attributed to the atmosphere on its having been driven away to the far-off hemisphere. but with this subject we have dealt pretty fully already in chapter ii., which may be referred to for further explanation if required. chapter xii. page some of the results arising from the sun's being a hollow sphere. repetition of the effects of condensation on the temperature of the nebula. ideas called up by the apparently anomalous increase of temperature. how heat is carried from the sun to the earth. the sun supposed to radiate heat only to bodies that can receive and hold it, and not to all space. the heat of the sun accumulated in a hot box to considerably beyond the boiling point of water. the heat accumulated in this way supposed to be due to a peculiar function of the ether, as it is a fact that heat can be radiated from a cold to a hot body. the sun must be gaseous, or rather gasiform, throughout. no matter in it solid or even liquid. divisions and densities of shell. the hollow centre filled with gases, whose mass naturally diminishes the mean density of the whole body. the amount of this reduction so far defined. the presence of gases or vapours in the hollow a natural result of condensation. the hollow centre filled with gases not incompatible with the sun's being a hollow sphere. the temperature at the centre may be anything, not depending on any law of gases. further exposition of hollow-sphere theory put off till after further development of the construction of the sun. in the last chapter we have endeavoured to point out how much our knowledge of the interior construction of the earth and moon has been increased, and how many difficulties in the comprehension of their construction are overcome by the fact demonstrated in previous parts of our work that they are hollow bodies; and we now proceed to show some part of what may be learned from studying the sun under the same conception of its being a hollow body. we say part of what may be learned, because the whole seems to us to be so great that it would take much more time and space, not to speak of knowledge, than we can devote to the subject to make even a proper beginning to such a study. to our sight it takes away the necessity for guessing in the dark at what the construction may be, which is all that has hitherto been done; and furnishes the means of discovering, with intelligent study and investigation, what most probably is the actual constitution of the sun. in chapters v. and vii. we have followed up the contraction and condensation of the residue of the original nebula, after it had thrown off all the known planets; first, to the diameter of , , miles, with density of / th of an atmosphere and temperature of - °, or _one degree_ of absolute temperature; second, to about , , miles diameter, with density equal to air at atmospheric pressure, and temperature represented by zero of the centigrade scale, or what has been hitherto called ° of absolute temperature; third, to , , miles diameter, with density equal to ten atmospheres and temperature of ° of actual, or ° of absolute temperature; and fourth, to , miles diameter, with density equal to water and temperature which we do not venture to express. all these stated densities and temperatures are understood to be average, the temperatures being those the various stages would have had, had no heat been radiated into space by them. here, then, we might go on to set forth what might be the interior dimensions, various densities, and conditions of each one of the four stages, under the conception of their being all hollow spheres, and afterwards carry on a _résumé_ of the whole of them and apply it to the sun as it is at the present day; but this, in addition to involving an immense deal of difficult work, subject to errors and omissions in operation, would not do much towards enabling us to explain in a more simple way what may be, most probably is, its interior construction. we shall, therefore, look upon the four stages as represented by a model having the diameter and other known measurements of the sun in its present state. to begin what we propose to do we believe it is necessary to repeat, as a thing that has to be borne in mind, that when we had contracted the original nebula from , , , miles in diameter to , , miles, its density was only equal to a barometric pressure of _one-ninth_ of an inch of mercury, and its mean temperature had been increased only _one degree_, that is, from - ° to - °; and we can add that, although we had given the original nebula ten times that diameter, the result both in density and temperature would have been the same when it was condensed to , , miles in diameter. then, again, we believe it necessary to repeat that by contracting the nebula from , , to , , miles in diameter its mean density was raised from / th to full atmospheric pressure, and its mean temperature from - ° to zero of the ordinary centigrade scale, i.e. to the temperature of freezing water. these two results strike us, at first sight, as somewhat remarkable, seeing that what looks like almost unlimited condensation to , , miles diameter produced only one degree of temperature, while the comparatively insignificant condensation of from , , to , , miles in diameter produced ° of heat, _in the way we are accustomed to measure heat_. following up these two facts gives rise to ideas that have been borne in upon us ever since we stumbled upon them when making the analysis of the nebular hypothesis. one of these notions was that, were it practicable, the most effectual mode of liquefying gases would be by putting any one of them into a sealed vessel, and confining it in another vessel in which a vacuum of / th part of an atmosphere could be produced; no difficult matter as far as the vacuum is concerned, for a good exhausting air-pump would be all that is required. but the practicability? the vessel in which the vacuum is produced would have to be protected so that no extraneous heat could be conveyed or conducted into it in any way whatever. how this could be, or is, done without cutting off every possibility of manipulating the enclosed vessel, we do not see; but it seems evident that some method is available because something presenting the same difficulties has been actually done, as everybody knows. the only degree of vacuum of any use in the exterior vessel would be about one-ninth of an inch of mercury, because that would as we have just said, furnish a temperature of - °. there would be no necessity for applying pressure to the gas experimented upon. in fact pressure would be an obstacle to the experiment, according to the theory of the air thermometer; and could only be of use by furnishing a larger quantity of liquid to be handled and examined. another idea is that there can be no such condition as absolute zero of temperature of what we are accustomed to think of as a gas, as far as science is concerned; as on arriving at that condition, perhaps long before, any gas would slip out of its hands altogether. but there is a much more rational reason than this, which we have brought forward on a former occasion. we are taught that heat is a mode of motion, which means that as long as there is heat there will be motion to account for it, so that motion would have to be annihilated on the earth before absolute zero of temperature could be reached. we have, then, to come back to what we said when treating of the heat of space, and look upon the temperature of the vibration of the ether as being the lowest that can be measured by science. we said then that it must be far below - °. since then a temperature has been reached of within ° or ° of absolute zero, according as that condition is measured by or . this, of course, leads us to think of the ether as a carrier of light, heat, etc., and of how it can carry heat to the earth without becoming heated itself, as there can be no doubt about its being a material substance. how it can bring what may be called considerable heat to the earth and still have little or no heat in itself; even should it turn out, which we do not believe possible, that the estimates of the heat of space of - ° and - °, made about the beginning of this century by sir john herschel and pouillet, turn out to be near the truth. we have seen, in "nature" of july , , a monograph by captain ericsson, in which he shows that the heat radiated by the sun to where his rays strike our atmosphere is somewhere about °f., and it is not easy to see how radiated heat can be transmitted through million miles of space at a temperature of much lower than - °, and reach the confines of our atmosphere with the heat of °f. there is one supposition that occurs to us under which this can happen, and that is, that the sun only radiates heat to bodies which can receive it, and does not radiate it into all space where there is nothing but the ether to hold it. this, of course, implies that the ether acts the same part--the part for which it was really invented--with respect to heat that a telegraph wire does with respect to electricity; in which case, we could imagine that it starts from the sun with the maximum heat radiated by him, and that this goes on decreasing in the ratio of the square of the distance it travels through, the same as is understood to be the case with all radiated heat; and that the part of space not occupied, for the time necessary, by these connexions might be supposed to form the return current which we believe must exist, just the same as the earth does for electricity. for that there is a return current is demonstrated by the fact that the earth radiates heat into space when the sun is not shining upon it. again, even in this case, we have another difficulty thrown upon us, over and above that cited by captain ericsson, of the heat delivered at the bounds of our atmosphere being about °f., by our being informed in "engineering" of december , , that: "a hot box, contrived to observe the temperature which could be attained by the unconcentrated solar rays, was used on mount whitney, , feet above the sea"--well within the limits of our atmosphere--"and that the enclosed thermometer rose to · °f. on september , p.m., , the shade thermometer then reading · °f." how are we to comprehend these two facts? we have seen a way of getting over part of the first fact as far as to the boundary of our atmosphere, but from there we have to carry °f. to the top of mount whitney, through the atmosphere there and present it along with the other lot in the hot box at · °f. we may get the beginning of what may be an explanation of all the facts from another part of captain ericsson's monograph, where he says: "engineers of great experience in the application of heat for the production of motive power and other purposes deny that the temperature of a body can be increased by the application of heat of a lower degree than that of the body whose temperature we desire to augment." the soundness of their reasoning is apparently incontrovertible, yet the temperature of the mercury in the instrument just described raised to °f. by means of the parabolic reflector, increases at once when solar heat is admitted through the circular apertures, although the sun's radiant intensity at the time may not reach one-tenth of the stated temperature. it should be mentioned that the trial of this new pyrheliometer has not been concluded, owing to very unfavourable atmospheric conditions since its completion. for our present purpose the great fact established by the illustrated instrument is sufficient, namely, that the previous temperature of a body exposed to the sun's radiant heat is immaterial. the augmentation of temperature resulting from exposure to the sun, the pyrheliometer shows, depends upon the intensity of the sun's rays. a little study shows us that the steam engineers are perfectly right in their doctrine. the heat of steam can only be called a variety of the temperature of water. at lb. pressure per square inch the heat of steam is · °f., while at lb. pressure it is only · °f., and therefore the steam engineer has good reason to say that steam at the lower pressure--or derived from heat that can only produce that pressure--can add no heat to the higher; on the contrary, the only possible means of applying the heat of the lower to that of the other would be by mixing them, and we know what the result of that would be. this brings before us the fact that the steam engineer's heat is very limited, and can only be communicated in certain ways, while the sun's heat is comparatively unlimited, and can only be communicated to anything through the medium of the ether. but it probably teaches more than that. were the engineer's heat unlimited in quantity at low pressure it can easily be believed that it could be transmitted to another body at any temperature by radiation, the same as it is radiated from the sun to a hot box; but it is not, and we thus seem to find that radiation is a mode, possessed by the ether alone, of conveying heat from one body to another. it has nothing whatever to do with mixing, conduction, convection, or anything, except in so far as the ether is mixed in a more or less limited quantity with all matter. in support of this idea we can refer to professor tait's treatise on heat, where we find it stated that "heat does pass (though on an infinitesimal scale) from colder to hotter bodies"; and we can easily understand that the infinitesimal quantity so passed is due to the comparatively infinitesimal quantity of ether there is in either of the two bodies to perform the work of transference. professor tait has not told us how heat is carried from a cold to a hot body, but there can be no doubt about its being a function of the ether which can only be found out by a careful and analytical study of that agent. such a study we propose to undertake presently without much expectation of being successful, but still with the hope of helping in some measure to find out how the ether operates. meanwhile we shall return to what we had begun to say about the sun being a hollow sphere, and to our proposal to treat of the nebula contracted from million miles to its present diameter, as if it were a model representing a _résumé_ of all the effects produced on the nebula by that amount of condensation. we know from all our work that the sun must be a gasiform body, which means that all the cosmic matter contained in it must be in the form of vapour, even although its consistence should outrival a london fog--notwithstanding that some physicists have supposed that it may be solid at the centre through extreme pressure--and it is not altogether correct to compare its construction to that of a solid body such as the earth; but as we have no other we shall begin to make a comparison with it, which, it will be found, can lead us into no appreciable error. considering then the sun to be , miles in diameter, with mean density of · that of water, the hollow part being still completely empty, and applying to it the same proportion we have deduced for the earth, we find that the region of greatest density would be at · of the radius of the sphere--a proportion really derived from the line of division into two equal parts of the volume of a sphere--from the centre, or , miles from the surface; and the inner surface of the shell at · --a proportion derived from our calculations for the earth--of the radius of , miles, or , miles from the centre; which in turn makes the shell to be , miles thick, and the hollow centre to be , miles in diameter. on the other hand, still following the proportions derived from the earth, we find that the density at the surface might be one-third of the mean density or · ; that it might be one-fifth greater than the mean, or · at the region of greatest density and one-half, or · at the inner surface of the shell--all of these three densities being in terms of water. now, the hollow centre of , miles in diameter would have a volume of one-sixth of the whole volume of the sun, which, filled with gases, would diminish all these densities just in proportion to what may be considered the degree of compression and condensation the gases might be subjected to. that there should be gases in the interior hardly requires to be more than stated, as there can be no doubt that the degree of heat to which the shell had arrived by the time it came to have the dimensions above mentioned, would be amply sufficient to excite chemical action among the elements of which the sun is composed; and the gases or vapours produced by that action would flow as naturally towards the interior of the hollow centre as towards the space beyond the outer surface of the shell, until they were stopped by increase of pressure, which of course would mean increase of density in this case. we see then that if the hollow centre has a volume of one-sixth of the whole volume of the sun and we multiply this volume by , we have a mass equal to the whole mass of the sun, were its mean density only the same as that of water. consequently, if we multiply the said volume by and by · , that is by · , we get a mass equal to the whole mass of the sun at its known mean density. again, were we to suppose the hollow centre to be filled with gases of the same specific gravity of air, condensed to a pressure of atmospheres--which would correspond in density to · times the density of water--we should have in the hollow centre alone a mass equal to another sun, in addition to the one made up by the dimensions and densities stated above. we see then that if we fill the hollow centre with gases at the pressure, and with the density just stated, we have a sun of twice the mass it should be. but if we leave the specified gases in the hollow with one-half of the above density, and deduct the equivalent mass of the other half density of the gases from the shell, as estimated for the hollow centre, we should have a sun of the mass required by astronomy. in this way we should have the three specified densities reduced from · , · and · to · , · and · , for the outer surface, the region of greatest density, and the inner surface of the shell, respectively; and the pressure and density of the gases in the hollow centre reduced to atmospheres. thus, from what has just been shown, which at first sight may be thought very irrelevant matter, we discover that it is not necessary that there should be any matter in the sun even so dense as water. and still we have to think of what an insignificant pressure three or four thousand atmospheres would be in the centre of the sun. no one will pretend to allege that no gases can be produced in the shell of the sun, or to say anything against those formed in the inner half of it finding their way to the hollow centre, and going on increasing there till they were able to force their way out through the shell; that is, until their pressure was equal to the resistance offered by the gaseous body of the sun, or against their temperature increasing until it came to correspond to their density and most probably rising to a much higher degree. such, then, must even now be the construction of the sun, as reduced to its present diameter and density. that is, a hollow sphere consisting of cosmic matter combined with gases and having a hollow centre filled with chemically formed gases or vapours. here it may be argued that the sun ceases to be a hollow sphere, but that is not so. the most that can be said about it is that it is a hollow sphere with the empty part filled up. it would only be in much the same condition as a hollow globe of iron filled with melted antimony or bismuth. its construction would be in no way changed by the empty hollow being filled up, so long as its condition remained gaseous--not changed to liquid or solid. the only difference in our sphere would be that its density would virtually be the same from what we have called the region of greatest density to the centre, which would not only involve a greater distance of that region from the surface of the sphere, but another reduction of the above mentioned densities of the sun; for we cannot in any way imagine that the pressure in its interior can be less than many thousands of atmospheres. whatever may be the relative densities of the shell and the gases in the hollow, they will have no necessary effect upon the temperature of the latter, because, let the densities be what they may, the gases might be cooled down to absolute zero of temperature, or raised to any imaginary degree without any change being made in their weight as long as their volume was maintained the same. this has been proved by laboratory experiments almost as far as possible. gases at very high degrees of pressure and consequent densities have been cooled down to not far from the absolute zero of temperature, while others under very low pressures have been heated up to nearly as great heat as the enclosing vessel would bear, without their weight being altered in either case; but in the sun there is a larger laboratory in which we can place no limit to pressure or temperature. we know, however, that pressures are required sufficiently great to blow out jet prominences with velocities of , miles per second or more, to heights , and even , miles above the photosphere; and if we knew what these pressures are we might be able to learn something about the minimum temperatures of the gases. to obtain these pressures we have--in the construction we are advocating--a real containing receptacle with sides , miles thick, in the outer half of which we have the compressing force, due to the gravitation of the whole mass of the sun acting at the centre, and over and above, both in it and the inner half, we have the cohesive force of the matter of which it is composed. in fact we have a sun whose construction we can understand, in which we have gases shut up without their expansive forces being impaired in any way, ready to be exerted with full energy whenever they are relieved from compression by any commotions in any part of the whole body, and taking their part in keeping the whole of the matter composing it in constant motion. how these commotions are produced it is not difficult to explain to a very considerable extent at least, but this we must leave over until we have reconstructed the original nebula, and shown how the solar system could be elaborated from it, almost exactly in the way conceived by laplace in his nebular hypothesis. we shall then also be able to extend our exposition of what is to be learnt from our mode of construction, and to still further reduce our estimate of the mean density of the sun. meanwhile we have to go into another long digression, with the view of trying to find out something about what the nature of the ether is or may be, which we think to be quite necessary before we go any farther. chapter xiii. page the ether. its nature considered. behaves like a gas. can be pumped out of a receiver. light and heat do not pass through a tube _in vacuo_. laboratory experiments examined. light and darkness in a partial vacuum, though high. electricity not a carrying agent. why there are light and dark strata in a high vacuum. the real carrying agent through a high vacuum is the residue of ether left in it. digression to consider the aurora. how air may be carried to extraordinary heights. zones of air carried up are made luminous by electricity. comparison of this method with experiments quoted. experiment suggested to prove whether light passes freely through a vacuum tube. the ether does not pervade all bodies freely. it must be renounced altogether or acknowledged to be a material body, subject to expansion, condensation, heating or cooling. how light and heat pass through glass. temperature of the ether variable. zodiacal light, cause of. the ether a material substance, proved by its behaviour. we have said in a former part of this work, pages and following, that if the ether is capable of performing all the functions that are attributed to it, it must have some consistence or substance of some kind; that it must be matter of some kind in some form, and consequently must have density in some degree however low; and we might, for the same reasons, suppose that it must have some temperature; but as long as we believe that without motion there can be no heat, we cannot conceive it to have any temperature. no doubt we might suppose it to be in a constant state of vibration, and to have the temperature corresponding to that state, whatever that may be; but this, in addition to leaving us just where we were, would only entail upon us the task of supplying temperature as well as density to a body of whose existence no positive proof has hitherto been given, whatever we may believe about it. at the same time, the evident necessity of taking its temperature into consideration, seems to supply another reason for concluding that it is a material substance, over and above those we have cited now and before. the general belief regarding the ether has been, ever since it was invented, that it is a substance of some kind (imponderable and impalpable?) which fills and pervades all space and matter; but a little consideration will show that this belief requires to be modified. the ether is supposed to be the connecting link of the universe, and the agent for carrying light, heat, electricity, and magnetism from the sun to the earth and planets, and all over space; but it has been found that electricity will not pass through a vacuum, such as has been produced by experimenters, unless it be with a very powerful current. this, of course, would seem to prove that there must be almost no ether in such a vacuum; because if there was ether in it, of the same density as there is in space, electricity would pass through it with the same ease as it does from one body to another on the earth or in space; it would seem, also, to justify us in inferring that electricity would not pass through an absolute vacuum at all, however powerful the current might be, because there would be absolutely no ether to carry it; and, likewise, that the quantity of ether remaining in the experimenter's receiver had as much to do with the passing of a very powerful current of electricity through it--perhaps a great deal more--as the small quantity of air, or gas, or dust not altogether exhausted from it, to which the experimenters attribute its passage. moreover, it would appear that when air or any gas is pumped out of a receiver, the ether mixed with it is pumped out along with it; consequently it must be a material, tangible substance, possessing density in some degree, however low it may be. here, then, we have, it would appear, proof positive that there is such a carrying substance as the ether has been supposed to be. it is a thing which we have not to conceive of, fabricate, or build up in our minds. it is a thing we can pump out of a tube, and is as much a material substance, in that respect, as air or any other gas that is as invisible as itself--yet nevertheless in the tube until it is pumped out. against this idea of the nature of the ether, and what may be done with it, it may be argued that light and heat pass freely through a tube or receiver _in vacuo_, when electricity refuses to pass; but are we sure that they do pass? it would be a much more difficult matter to prove that they do, than to prove that electricity does not, because our eyesight gives us evidence in the latter case. besides, there are facts which, when thoroughly looked into, induce us to believe that light actually does disappear gradually from a vacuum as it is being formed. in an article on "the northern lights," in "science for all," vol. ii., reference is made to a well-known laboratory experiment in the following words: "we take a glass cylinder, covered at the ends with brass caps, one of which is fitted with a stop-cock, which we can screw to the plate of an air-pump. to the brass caps we now attach the terminals of a powerful induction coil, but as yet we perceive no result. we now begin to exhaust the air from the cylinder, and as the exhaustion goes on we soon see a soft tremulous light beginning to play about the ends of the cylinder; and this, when the air is sufficiently rarefied, gradually extends right through the cylinder. as we continue the exhaustion these phenomena will be reversed, the light gradually dying away as the exhaustion increases. we shall at once perceive how very much this resembles an aurora on a small scale, and so we have electricity suggested to us as the agent which produces the aurora." farther on in the same article we find that: "aurora displays usually take place at a great height--sometimes so high as miles--while their average height is over miles. at such heights the air must be extremely rarefied, and we should be disposed to expect that the electric discharge could not take place through it." now, at the beginning of this experiment, it must be granted that light was passing freely through the glass cylinder from side to side, and also that, when the electric current was turned on, the electricity was passing freely through the air in the cylinder though it was not visible. it could not pass through the glass on account of its being a non-conductor. then, when the air had been partially exhausted from the cylinder, and the "soft tremulous light" began to appear about its ends, it is clear that some interference with, or change in, the free passage of light through it must have been produced, both transversely and longitudinally, which occasioned the difference in the appearance of the light and caused its tremulous motion. and as change in the appearance of the light extended through the length of the cylinder as the exhaustion increased, and finally died away--light, change and all--when it approached more nearly that of an absolute vacuum, we cannot help concluding that the light disappeared because there was no medium left in the cylinder, of sufficient density at least, through which it could pass; which, of course, means that light cannot pass through a vacuum any more than electricity can. the experiment we have cited above may be considered antiquated, but similar results are presented to us in professor balfour stewart's "elementary physics," where he says at page of the reprint of : "another peculiarity of the current is the stratification of the light which is given out when it traverses a gas or vapour of very small pressure. we have a series of zones alternately light and dark, which occasionally present a display of colours. these stratifications have been much studied by gassiot and others, and are found to depend upon the nature of the substance in the tube." [the ether?] "if, however, the vacuum be a perfect one, gassiot has found that the most powerful current is unable to pass through any considerable length of such a tube." [in passing, we take the opportunity to assert, with confidence, that there can be no perfect vacuum on the earth.] here we see the gas or vapour in the tube divided into zones alternately light and dark, which occasionally present a display of colours, and are led to infer, from the colours depending upon the nature of the substance in the tube, that they disappear altogether when the exhaustion is sufficiently great; and are finally told that the most powerful current is unable to pass through such a tube of any considerable length. in this case also, we can say with perfect confidence that there can be no ether left in the tube, in sufficient quantity, or else it would be able to carry the electricity through it much more easily than from the sun to the earth, or from one part of the earth to another. if we refuse to acknowledge that the ether has been removed from the tube or cylinder, we are forced to conclude that it is not the carrying agent, for which alone it has been called into existence by the imagination of scientists; and we have to invent new theories, new methods for explaining what we have been accustomed to think we thoroughly understood. we have to look for a new dog to carry and fetch. furthermore, all that has been said about electricity is equally applicable to light, whether we can prove it or not. if light could pass freely through the experimental cylinder from side to side, as it was certainly doing before the exhaustion was begun, we cannot understand why there should be, first tremulous light which finally disappeared, and why dark strata were displayed in it by the forced passage of electricity; unless it was that the carrier of the light was removed, and then we naturally think of why there should be dark strata in the tube. we can understand electricity lighting up darkness, but not its darkening light--it lightens up midday--and we must conclude that both the one and the other were driven through the cylinder, or similarly conducted through it, by the same force, or were left behind. following up the quotations we have already made from "science for all," vol. ii., we now add another for further illustration of what we have been saying, to wit: "let us now return to the laboratory, and see whether we can make any experiment which will throw light upon this difficulty. if we send the electric discharge through one of the so-called vacuum tubes--choosing one which consists, through part of its length, of tube which is much narrower than the main portion--we find that when the discharge is passing the pressure is greater in the narrow part of the tube, showing that in some way gas is being carried along by means of the current, and professor a. s. herschel suggests that in some similar way air may be electrically carried up to these great heights." this quotation, of course, refers to the northern lights, but it serves to illustrate what we are seeking to show with respect to the ether. in this experiment, the explanation of the pressure being greater in the narrow part of the tube, is exactly the same as that for water passing through a conduit which is narrower at one place than another. the same quantity of water has to pass through the narrow as through the wide part, consequently the velocity and pressure (head) have to be greater than in the wide part--the water arranges that for itself; and the seeming difficulty of explanation arose from the idea "that in some way gas is (was) being carried along by the current," when it was only the gas that was being lighted up more vividly by the electricity passing through it, because the same amount of electricity had to be carried through the narrow part as the wide one. no portion of the gas could be carried along with the electricity, else it would very soon have been all accumulated at one end of the tube, or a reverse current must have been set up to restore the balance, which would speedily have shown itself. had the said tube been filled with copper instead of gas, the experimenter must have known that the electricity, in passing through it, would have spread itself all through the wide part, and contracted itself to pass through the narrow part, spreading itself out again through the other wide part, thus giving rise to differences of pressures and velocities at the different widths of the tube; but, of course, he would not have been able to see this, because the electricity could hardly be in sufficient quantity to light up the copper, or to impart to it sufficient heat to make it visible. neither would the electricity carry with it part, or the whole, of the copper when passing through the narrow part. it would be the gas lighted up more vividly, not set in motion, by the electricity that the operator saw in the experiment under discussion, and, no doubt, if the tube had been sufficiently exhausted of gas, the light would have disappeared the same as in the first quoted experiment, and the electricity would have ceased to pass because there was nothing, in sufficient quantity at least, to carry it along, not even the universally commissioned monopolist the ether. let us ask here: does not all this seem to prove that electricity is a carried, not a carrying, agent? in the quotation made, at page , from "elementary physics," we are told that when electricity passes through a gas or vapour of very small pressure, "we have a series of zones alternately light and dark." now we ask, why should part of these zones be dark? and the only answer to be given is--simply because there is no light in them, nothing in them to carry or hold light. otherwise, we cannot understand why they should appear to be dark. we cannot imagine a glass tube with light and dark zones in it longitudinally--we have understood the zone to be longitudinal; transverse sections would not be zones--at the same time that light is passing freely through it transversely, i.e. from side to side, unless it is that in the dark zones there is nothing, not even the all-pervading ether, to carry or hold light in; therefore, we conclude again that there is no light where there is no ether. for an explanation of the existence of light and dark zones in the almost exhausted cylinder or tube, we refer to professor tait's treatise on "heat," where he says, in section , "what happens at exceedingly small pressures is not certainly known. in fact, if the kinetic gas theory be true, a gas whose volume is immensely increased, cannot in any strict sense be said to have one definite pressure throughout. at any instant there would be here and there isolated impacts on widely different portions of the walls of the containing vessel, instead of that close and continuous bombardment which (to our coarse senses) appears as uniform and constant pressure." admitting the truth of the kinetic theory of gases, we can see that in a vacuum so rare that only electricity at a very high pressure could be forced (carried?) through it, we have the prescribed conditions in which there cannot be "one definite pressure throughout" the whole tube; in other words, we shall have some places in a vacuum tube where there is no gas at all, or perhaps even ether, and others where the gas is so rare that it takes a powerful stream of electricity to light it up in passing through, whether the lighted-up zones be composed of gas, or of ether, or part of both. if it did not pass, there would be no light-streak even. and further, we have to notice that the light and dark streaks would be changing places constantly, owing to the collisions of the small number of atoms or molecules of the gas, still not exhausted from the tube, driving each other from place to place. all this makes us think of what is the real carrier of electricity through a partial vacuum, through a gas, or through a substance of any kind whatever, and we can only imagine it to be the ether. in that case the conductivity of any substance would depend upon the quantity of ether contained in it, and we can give no other reason for there being conductors and non-conductors of electricity. all matter has been thought to be pervaded by the ether, but we have said before that this must be the case in a limited sense only. it can be shown that glass is permeable to ether, and is therefore not an absolute non-conductor. metals are supposed to consist of atoms bombarding and revolving around each other under the control of ether. intermediate conductors may have the quantity in them of ether corresponding to their conductivity; and the compressibility of water, or any liquid, may depend upon the quantity of the ether mixed with its ultimate atoms. although we consider it to be going rather beyond the course we had laid out for ourselves, we cannot help returning to the article on the "northern lights" in "science for all," quoted above in connection with electricity in the presence of a vacuum; because it helps to illustrate the subject we are dealing with. in the regions where these lights are seen, we know that there can be no want of ether, because it is supposed to pervade all space; but we know that there must be a very great want of air, or vapour of any kind, due to the height above the earth at which they are seen. here, then, we have a great field for differences of pressures being caused all through it, by the collisions among themselves of the molecules or atoms of the extremely attenuated air; we have the higher or lower pressed zones of the laboratory experiment spread out before us, and if we suppose currents of electricity to be passed through them, we have an aurora in the high heavens, a counterpart of what was seen in the vacuum tube. the bombardment of the molecules continually shifting their positions and creating zones of different pressures, when lighted up by electricity, would easily account for the flashes, coruscations, and changes of the aurora; but, how does the air get up so high as is stated in the quotation at page ? we cannot accept the supposition of professor a. s. herschel that the air is carried up to the height of from to miles by electricity. we must believe, till evidence is given to the contrary, that electricity is a carried, not a carrying, power. conductors of sound are all material substances; sound is not. it seems logical, therefore, to conclude that the ether is a material substance, because it conducts light, heat, etc. etc., which are not material substances. proof is therefore required that electricity is a material substance, before it can be called a carrier. that air does somehow get up so high there can be little doubt, as is satisfactorily proved by the burning of meteorites when they come into our atmosphere at heights said to be more than miles. how it does mount up so high is not so wonderful as it seems, when we take into consideration the causes of the trade winds, which are: the upward currents of the air created by the heat of the sun; the centrifugal force inherent in it at the time of leaving the earth; and its angular motion, which may be, at a guess, from to miles per minute, seeing that the equator has an angular velocity of over miles per hour. then, from the time it leaves the earth, the air must begin to lose its angular velocity, the impelling power being cut off, and form a bank higher up, opposing the motion forward of all the air following it, so that immediately above the tropics there must be forward motion and obstruction, producing whirlwinds of which we can see or know really nothing, though they must exist, and which may carry air or vapours up to very great heights, carrying with them densities far beyond what would correspond to the simple attraction of the earth. at these heights this attraction would be very much diminished, and almost the only way in which the density of the whirlwinds could be diminished would be by expansion, which would not be very active in bodies already very considerably attenuated, as the whirlwinds would naturally be. their movement towards the poles would be the same as that of the trade winds has always been supposed to be; and we can now see how there can be air at great heights in the aurora regions, not carried up by electricity. in fact, the air may, or rather must, have carried the electricity up with it, as we shall, we believe, presently see. we have not supposed that all the air, raised from the earth by the heat of the sun, is carried up to such altitudes and to its polar regions, but only a very small part of it; and we have to add that there is perhaps not always electricity present in sufficient quantity to illuminate the air when it is carried up, which would, from the nature of its ascent, be undoubtedly divided into zones, streams, or belts at different degrees of tenuity. we do not doubt, or rather we believe, that electricity is always present in the atmosphere; but we are not sure that it is always so in sufficient force to make itself manifest. a very homely example of this is: stroke a cat's back in ordinary circumstances, and it will only arch it up in recognition of the caress; but stroke it on a frosty night and it will emit sparks of electricity. the cat's hair does not shine--perhaps fortunately for the cat--because the electricity in it is not present in sufficient force, and only shows itself when the hand acting like a brush collects it into sparks. this shows not only that electricity is more abundant in the air at one time than at another, but that it is more so in cold and dry than in warm and moist air. it also shows one of the reasons why auroras of great brilliancy and extent are not continually in play in their own special regions, which is the want of a sufficient supply of electricity; another reason being, the absence of the requisite zones, or masses of air in cyclonic motion at different pressure and in sufficient quantity. we understand from what we have read that the glow of the aurora is seldom awanting in clear weather in the far north, and can imagine that there is always a sufficient supply of electricity and attenuated air to maintain the glow constantly; and also that the brilliant displays are only made when there is a sufficient influx of whirlwinds of air at low and varying pressures, and of electricity in sufficient force to light them up. we should suppose that the bright flashes would take place where the pressure was greatest, and the illuminated darkness, so to speak, where it was least. electricity does not carry up air to these heights, neither does magnetism bring it down from the sun; still a magnetic storm produces brilliant auroras. confronting these reflections with the laboratory experiment we have cited at page , we see that they are very fully confirmed by it; perhaps it would be more true to say that they were originated by it. when the current of electricity was first turned into the glass cylinder, no result was perceived. this must undoubtedly be construed into showing that the light in the cylinder, passing through it from side to side, was more powerful than the diffused light of the electricity passing through it from end to end; which was the reason why there was no result. by diffused, we mean that the electricity, turned into the cylinder through a thin wire, would immediately spread out over the whole of its width (or cross section) and thus very much weaken its light-giving power. when exhaustion had proceeded to a sufficient extent to produce the soft tremulous light, we can only conceive that the transverse light had decreased so far that the diffused light of the electricity, passing longitudinally, had begun to balance it, which caused the tremulous appearance on account of the one beginning to disappear and the other to take its place. and when the light extended through the whole length of the cylinder and the phenomena were reversed; and when the light died away altogether, when the vacuum became sufficiently pronounced; we can only believe that there was no light at all in it; neither natural light passing through it transversely, nor light of electricity passing longitudinally. should any one object to this demonstration, as we may call it, we refer him to the quotation, made at page , from professor balfour stewart's "elementary physics," and ask him, how could there be dark zones in a tube, through which light ought to pass freely from side to side? the thing appears to be tremendously absurd. there were dark streaks in the tube and other streaks of gas, or vapour of some kind at very low pressures (see also quotation from professor tait at page ) that were lighted up to some extent by the current of electricity, but even these died away. we do not pretend to impugn the idea that the stratification of light and dark zones depended upon the nature of the substances in the tube, we only want to insist that the substances left in it were so extremely rare that electricity could not pass freely through it longitudinally, nor daylight transversely, else there could have been no dark zones in it; and that even the ether was in such small force that it could not perform the carrying duties assigned to it. we have often wondered whether any experiments have ever been made to ascertain whether any changes, as far as the presence of light is concerned alone, have been brought about by producing a vacuum in a tube. the gradual dying away of light, and its final disappearance, are certainly suggestive of changes, and may have excited curiosity to know what actually happens. that there are changes cannot be denied, and it would be satisfactory to know what they are. it appears to us that one simple and easily made experiment would give a good deal of information on the subject. let a glass tube of cylindrical form--one of those prepared for vacuum experiments--be placed in a slit in the window-shutter of a dark room, so that absolutely no light can pass into the room except through the hollow part of the tube; which might be effectually managed by burying two opposite sixth parts of its circumference in the wood of the shutter, and there would still be left one-third of its diameter for the free passage of light from side to side. when so arranged, and when still full of air, let a spectrum be taken of sunlight passing through it, to serve for comparison. then let a high vacuum be produced in the tube, and another spectrum taken and compared with the first. this will at once show whether any change has been produced or not. should the difference we expect be found, the experiment might be extended by spectra being taken at different degrees of exhaustion, from which some useful information might be derived. we have said, at page , that the ether does not pervade all bodies of all classes, and such must be the case in some measure at least, otherwise there would be no non-conductors of electricity, no insulators for our electric telegraphs and deep sea cables. were glass, for instance, pervaded freely by the ether, and the ether is in reality the carrier of electricity, then electricity could pass freely through glass, but it does not; therefore, there can be no, or at all events very little, ether in glass or any other insulator. we can see, then, the possibility of the ether being removed from a glass tube, provided it is a material substance, by shutting up one end of it with a stopper of glass and passing a perfectly-fitting glass piston through it to the other end. suppose this done, it would be quite safe to say that electricity could not pass through the tube, because there would be nothing--absolutely nothing--to carry it, not even the piston-rod, for we could have that not only made of glass but on the outside of the piston. in this case the result would be exactly the same as when the contents of the tube were pumped out of it, and the residue left, if any, would be the same, that is, an immeasurably small quantity of the ether which had filtered through the glass. it may be argued that it would be impossible to make such an experiment as we have proposed, but that does not damage in the slightest degree the correctness of the consequences deduced from it; any more than the impossibility of constructing a perfect heat engine destroys the deductions drawn by sadi carnot, from the study of such an ideal machine. we can grant that glass being not an absolute non-conductor, the ether might, in course of time, ooze through it and fill the tube again, while gas, air, or dust could not so ooze through it, and thus re-establish the current of electricity that was stopped for want of it; but we cannot grant that there was any very perceptible quantity of ether in the tube, when the electric current could not pass through it without dismissing the ether altogether, and dropping back into the difficulties out of which it has in many cases lifted us. the evident fact that the ether cannot pass through glass freely, and therefore cannot carry electricity with it, may be disputed by referring to the free passage of light, and also of heat, through glass and other substances, in virtue of transparency and diathermancy, two terms that have the same meaning, at least, as nearly as that light and heat mean the same thing; but we believe that this free passage, instead of invalidating our reasoning, only tends to prove that the ether is a material substance; because, if it is not, it might pass through transparent bodies just as easily as light and heat do. of course, this belief obliges us to show how light and heat do pass through a transparent body such as glass, and the mode is exactly the same as of heat passing through any other body that is a conductor of heat. glass is a substance that is known to be a bad conductor, but it is also known that it is not an absolute non-conductor of heat; therefore, there is no difficulty in supposing that it, and its companion light, can be conducted through glass with velocity proportioned to its thickness. we know that in the case of a pane of glass in a window it is practically instantaneous, but that does not mean that it is absolutely so. we know also, that in passing through, both are refracted, and that comparatively little heat is imparted to the glass, even under bright sunshine, which may be very well accounted for by the ether on the other side of the window pane carrying them (light and heat) off, in the same direction they were going, quite as fast as they could be conducted through the glass. but, supposing there was no ether in the room to which the window gave light, or gas, or elementary matter of any kind--a condition which could be obtained by making the room of glass and pumping out its contents as was done with the vacuum tube--what would be the result? there would be no wave motion to carry on light and heat into the room, and it would be in the same state as the exhausted tube, except that there would be no electricity in the room--no current being passed through it--nor anything in sufficient quantity to be lighted up if there was; the light would be stopped and reflected back from the glass, and nothing inside the room could be seen; not even that it was dark, because there would be no electricity to make dark zones visible. the window, or rather the whole room, would become a many-sided mirror, for reasons almost identical with those that account for a sheet of glass being made into a mirror. we confess that all these deductions have startled us, but we can see no flaw in the reasonings which have led to them. if it is not for want of ether--in sufficient quantity at least--and the admission of variable quantity is to admit that it is a material substance, that electricity will not pass through a highly exhausted tube, we cannot imagine what can be the reason why it does not; simply accepting it as a fact is by no means satisfactory. in the dilemma between renouncing the ether altogether or acknowledging its disappearance--effective at least--it occurred to us that it might be for want of heat, and that in terms of the inter-dependency of temperature and pressure in a gas, heat disappeared in proportion to the decrease of pressure in the air or gas that was being exhausted from the tube, or from cold being applied to it from without; but that notion has already been disposed of by our own work, when we have seen that a gas in a close vessel can be heated or cooled to any degree, altogether independently of pressure. when, acknowledging that the ether ought to have some temperature as well as density, we have said that it might have the temperature of vibration whatever that might be, thereby admitting that we could not pretend to determine what it is; nevertheless, we may take a look at it from a distance, and at least see what it cannot be, anywhere within the limits of our system. we have shown, at page , that when the original nebula was about , , miles in diameter, its density must have been · that of air at atmospheric pressure, and its temperature - °, and that these could be neither the density nor temperature of space. with this temperature, then, it is evident that there was still heat enough and to spare in the ether--considering it to be a material substance--to cause it to vibrate and perform its assigned offices; and, therefore, it could not be for want of heat that neither it, nor light, nor electricity could be carried through the vacuum tube, but for want of the ether in due quantity; consequently, the temperature of vibration cannot be so great as - °. turning back now to page , we find the density of the ether estimated at / , , th of an atmosphere, which corresponds to an absolute temperature of · ° or - · °; but on the following page we expressed our opinion--well founded, we believe--that the estimate was too high, i.e. too dense, and that it might be , , or times, or more, too great. be this as it may, we can see that if the ether alone occupies space--beyond a comparatively very limited distance from any body belonging to the solar system--it must be almost absolutely free from temperature of any degree, for the difference between - · ° and - ° is virtually nothing; or it must have a special temperature derived from the collisions of its own atoms, or from the sun. we have said more than once that the temperature of space cannot be so high as _minus_ °, and now we cannot believe that it can be so low as absolute zero, because the ether in it is credited with the motion of vibration, which must be either the cause or effect of heat. what then shall we say? we can only speculate. we can suppose that when the chemical elements were created, or evolved by some process, and began to attract each other, they had the ether to carry them into collision and produce heat; and that it, being also a material substance, became heated to the same degree as the other matter, always increasing in proportion to its state of condensation, the ether mixed with the other matter being also, of course, condensed. then, following up this supposition, we can see that when the sun came to be condensed to its present state, the ether must have had the same degree of heat as itself at its surface, and be of the same density as it would in our air at the earth's surface condensed to the pressure of nearly atmospheres; knowing as we do that the attraction of the sun at the surface of its photosphere is almost times greater than that of the earth at its surface. under this supposition, therefore, the ether might emit light just as surely as any other matter that may exist, or can be seen, in the corona or atmosphere of the sun, and might be the cause of the zodiacal light, probably more naturally than any other cause that has been imagined for it. mr. proctor, in his "sun," has given us a most elaborate description of how the zodiacal light could be produced by the swarms of meteorites and meteors, that are generally supposed to be floating around the sun and continually showering in upon it, and we confess that his reasoning is very plausible; but it, along with other similar hypotheses, has one very serious defect which it is hard to get over, under our existing ideas about matter and its origin. if there is a constant rain of meteorites and meteors falling into the sun now, and the same has been going on during the multitude of millions of years that it is supposed to have existed, we have to acknowledge that it must either come to an end some day, or that there is going on a constant creation or evolution of matter to keep up the supply. it will not suffice to accept the hypothesis that the supply comes from other suns, or any idea of that kind, because each one of them would finally find itself alone with its planets, etc., if it has any, in its domains the same as our sun. neither would it suit the ideas of those who consider that matter has existed from all eternity and has _made itself_ into all sorts of bodies or systems to suit them. without continued creation, or evolution, matter must end in condensation into one mass. there can be no self-evolution to keep up the supply of matter. it would require another and exactly opposite power to unmake the final mass, and another change to original matter to start anew on the old course. but we are speculating too soon. it may be said that if the zodiacal light is caused by the ether, and if the ether is a material substance, it must be exhausted sooner or later, just the same as all other matter and the whole universe to one mass the same as before; and also that we have no authority for supposing that the ether can be heated and cooled or condensed and expanded. but we think that with what we have done in this chapter, and what we will be able to show in the following one, we shall be able to get over all these difficulties, and also show how the universe might be dissolved and renewed by the ordained process of evolution. chapter xiv. page the ether considered and its nature explained. further proofs given by dr. crookes's work, of its material substance. highest vacuum yet produced. absorbents cannot absorb the ether. dr. crookes's definition of a gas. not satisfactory. why. a fluid required to pump matter out of a vessel. gas as described by dr. crookes would not suit. the ether the only elastic fluid we have. the only real gas, if it is a gas. a possible measure of the density of the ether. causes of dark and light zones in high vacua. the real conductor of light in a high vacuum. how a vacuum tube glows, when electricity passes through it. conclusions arrived at through foregoing discussions. some exhibitions of light explained. gases can be put in motion, but cannot move even themselves. the ether shown to be attraction. and primitive matter also. all chemical elements evolved from it. its nature stated. action at a distance explained by the ether and attraction being one and the same. the idea that the ether can be pumped out of a tube of any kind, along with the air or gas that has been shut up with it therein, will very probably be declared to be absurd, by reference to dr. crookes's experiments with his radiometer, and investigations into the nature of radiant matter; but when duly considered his work seems to confirm it, and our reasonings in support of it, in a very convincing manner. radiant heat, or light, is shown, no doubt, to penetrate into an exhausted bulb and to cause a radiometer to revolve, but we have to consider what is the state of exhaustion at which its force is shown to be greatest, and why that force decreases rapidly when the exhaustion is progressively increased beyond a certain point; for a certain amount of exhaustion is required first of all to diminish the resistance of the air or gas to the vanes of the radiometer, before the radiant heat gathers force enough to make them revolve at all. its greatest power to produce revolution is shown to be when the exhaustion is at from to millionths of an atmosphere, according to the gas or medium in the bulb--see "engineering," vol. xxv., page --and decreases from that point, often rapidly, as the exhaustion is increased, till at last it ceases altogether. everybody who has taken any interest in the subject, knows that dr. crookes has exhausted radiometers to such a degree that they could not be influenced by the radiation of a candle placed a few inches from the bulb. we are not told at what degree of exhaustion this took place, nor at what degree repulsion, by radiation of heat, is supposed to have ceased altogether, but that does not matter, even though it should only cease when the vacuum comes to be absolute--most probably a stage to which it is impossible to attain. what concerns us is the fact that repulsion by radiation does reach a maximum at a certain degree of exhaustion, and then falls off as the exhaustion is increased; and what we have got to consider is what is the cause of the falling off. we are told it is caused by the attenuation of the matter, gaseous or material, contained in the bulb, and we are satisfied with the explanation. but in order to be thoroughly so, we must insist on believing that it is part of the whole of the matter that has been operated on; not only of the gas and other matter to the exclusion of the ether, but of the whole, ether and all. if the ether is left behind intact, it must perform the offices it was created for by the imagination of man, or man must discard it altogether. if it ceases to carry light and heat through a vacuum, it is of no more use than we found it to be in the case of electricity, and man is bound to dismiss it as a useless operative, who will strike work for no reason whatever. some people have supposed the ether to be an absolute non-conductor of electricity, because it does not convey that agent through a vacuum. will they also declare it to be a non-conductor of light and heat? if they will not, then they--and we presume everyone else--must admit that it can be pumped out of a bulb, in the same way as a gas or any other fluid matter. here we are led into another consideration, viz., whether the ether is exhausted from a receiver by pumping alone, or by the help of absorption. in his lecture, "on radiant matter," delivered at the british association, at sheffield, august , , dr. crookes said: "by introducing into the tubes appropriate absorbents of residual gas, i can see that the chemical attraction goes on long after the attenuation has reached the best stage for showing the phenomena now under illustration, and i am able by this means to carry the exhaustion to much higher degrees than i get by mere pumping;" and that when working with absorbents: "the highest vacuum i have succeeded in obtaining has been / , , th of an atmosphere, a degree which may be better understood if i say that it corresponds to about the hundredth of an inch in a barometer column three miles high." (we quote from "engineering," vol. xxviii., page .) now, what are we to think? are we to suppose that the ether was in part removed by the absorbents? we think we are justified in saying that the absorbents had not anything to do with the exhaustion of the ether, because dr. crookes used different kinds of absorbents for the different kinds of gases he dealt with, and it is hard to believe that all the _media_ he used were equally effective in absorbing the ether as they were with the gases. on the other hand, if we consider that the pumping was the only agent in removing the ether, we ought to acknowledge that it must have been more effective with regard to it than to the gases before absorption was resorted to with them; or that a stage had been reached at which the pump could not extract any more ether from the bulb. we shall have more to say of this presently. it is a difficult matter to determine, but there is one thing we can see clearly; when the exhaustion of the bulb was raised to / , , th of an atmosphere, the density of the ether--of itself--must have been at a lower degree than that. consequently if we assume its normal density to be / , , th of an atmosphere, in terms of the estimate we quoted from "engineering," it must have been diminished to less than one-fourth of that when the above high vacuum was obtained; because it must have been the density of the residual gas, or matter, and of the ether, added together which amounted to / , , th; the same as we have argued with regard to the solar nebula when at , , , and , , miles in diameter. one thing leads to another, and we have again to repeat our question--what is a gas? and all the answers we have been able to get to it hitherto have been far from satisfactory. a little earlier in the same lecture, referred to a few pages back, dr. crookes, after telling us, very elaborately, what would have been the definition of a gas at the beginning of this century, goes on to say: "modern research, however, has greatly enlarged and modified our views on the construction of these elastic fluids. gases are now considered to be composed of an almost infinite number of small particles or molecules, which are constantly moving in every direction with velocities of all conceivable magnitudes. as these molecules are exceedingly numerous, it follows that no molecule can move far in any direction without coming in contact with some other molecule. but if we exhaust the air or gas contained in a close vessel, the number of molecules becomes diminished, and the distance through which any one of them can move without coming in contact with another is increased, the length of the mean free path being inversely proportional to the number of molecules present. the farther this process is carried, the longer becomes the average distance a molecule can travel before entering into collision; or, in other words, the longer its mean free path, the more the physical properties of the gas or air are modified." of course, what we have looked upon as dr. crookes's definition of a gas, ends with the second sentence of the above quotation, and is far from being sufficiently complete to be satisfactory; but we have continued to quote from the lecture, because it contains matter which demands consideration, and helps very powerfully to support the conclusions we have been arriving at. why the definition is not satisfactory, is that it does not tell us what there is in the spaces between the molecules of what is called the gas. if there is room for them to move in every direction there must be spaces between them, and these spaces must either be absolutely empty, or filled with something. if they are supposed to be empty, then the molecules being actually small pellets, like diminutive marbles, or snipe-shot, we immediately begin to think why gravitation does not make them, being ponderable bodies, fall down to the bottom of the bulb; and seeing that, by the definition, they are evidently considered not to do so, we think of what can keep them from falling, and of how they can be pumped out of a bulb or any sort of vessel. if we fill a vessel with marbles, snipe-shot, wheat flour, or dust, and set a pump to work on it, we shall find that we make very little progress in pumping them out of it. at first we might extract a puff or two of flour or dust--marbles or snipe-shot by no means--carried into the pump by any air there might be mixed with them, but that would very soon come to an end; besides, there would be air, gas, something, in the interstices--if any--of the flour or dust to drive them into the pump when a vacuum was formed in it, and the puffs would cease when the air, which would be in exceedingly small quantity, was all extracted. but independently of all this, we have supposed the spaces between the gas pellets or molecules to be absolutely empty, and there would be nothing to push them into the vacuum created in the pump. there is no possibility of pumping marbles, sand, flour, or dust out of a vessel without the assistance of a fluid agent of some kind, water, gas, or air; and even then it would be done with much difficulty. let us, then, suppose there is some such agent filling the spaces between the atoms of the gas and think of what it must be. were we to ask the question we have a strong suspicion the first impulse of many people would be to reply--with gas of course. but this reply could not satisfy us. we should immediately be led to think of that gas also consisting of atoms with vacant spaces between them filled with something--some more gas; and were we to follow up that thought through a sufficient number of stages, it is easy to see that in the end the whole space occupied by any gas would come to be filled up with its own solid atoms, without any empty spaces between them through which they could move; and so rendered quite incapable of pushing each other into a vacuum formed by any pump that might be applied to extract them from any vessel of any kind; or we must suppose that each particle would fly of its own good will into the vacuum made by the pump--as it were on the wings of the morning. but we recall to memory that the wings of the morning do not always carry us to rest, and we see that filling the spaces with gas would only end in choking up the vessel altogether. it might be said: nobody imagines that the molecules of the gas in the spaces would be sufficient to fill them up altogether; and then we have only to ask, what then would there be in the spaces between the molecules of these successive gases to prevent the whole of them from gravitating to the bottom of the vessel? and to add that there would still be empty spaces left, absolutely empty, that would have nothing in them to help in any way to force the molecules or atoms of any gas or vapour into a vacuum anywhere. it is clear then that a gas, such as dr. crookes has described a gas to be, could only end in filling the spaces left between its molecules or atoms. it would be an obstruction to their collisions and bombardments which form an essential part of the description or definition. we must, therefore, have recourse to something else for filling up the spaces between the molecules of a gas, and the only thing we can lay hold of is our _limited liability_ agent the ether, which we allow to do all we want it to do and nothing more. vapours of solid or liquid matter would be of no use, for they would only condense into solid or liquid matter; unless always maintained at their temperatures of evaporation or ebullition, and that would at the best be only another form of a gas--nobody would use a liquid to assist in pumping air out of a vessel--and, besides, we should still have to show what keeps their particles apart, what fills the spaces between them, which would force us to appeal to the ether as the only source, just as before. if there are no spaces between the particles there can be no vapours. if by pumping air out of a close vessel the number of its particles is diminished, and we acknowledge that the ether pervades all space and matter, in a greater or smaller degree, then we must either recognise that a pump is able to separate the particles of the air from the ether which pervades it in the vessel, and extract them alone; or we must acknowledge that along with the particles of the air, the pump extracts a corresponding portion of the ether. which of these two consequences of the pumping we have to choose cannot for a moment be doubtful. it would be as reasonable to suppose that we could pump the colouring matter out of a pond of muddy water, or the mud itself, and leave the clear water behind, as to suppose that the molecules of air, or of a gas, could be extracted from a close vessel, by a pump, and the ether left behind in it. we have called attention two or three pages back to the fact that a fluid agent of some kind is required, in order to be able to pump matter of any description out of any kind of vessel. for solid matter a non-elastic fluid will suit, but for gaseous or vaporous bodies an elastic fluid is required; but we have just seen that what have hitherto been considered to be elastic fluids, that is, gases and vapours, have no elasticity whatever of their own, but are undoubtedly and in reality solid matter; and that in order to become elastic fluids they have to be mixed with the ether, or something that has yet to be discovered, invented, or imagined. if, then, until such a body is found we take the ether as a substitute, we have to acknowledge that it must be not only an elastic fluid but a material substance, capable of being compressed and expanded, and heated and cooled; for nobody could conceive clearly the existence of an elastic fluid that is not subject to these conditions. he could not understand how the molecules of a gas could be contracted, expanded, heated and cooled in a vessel, while the elastic fluid which gave them liberty to move or to be moved, remained constantly at one density and temperature. furthermore, until such a substitute is found, we have to acknowledge that it is the only thing we have any idea of corresponding to a gas as described by dr. crookes; that is, a multitude of molecules colliding with and bombarding each other or their prison walls. but even beyond this we can uphold it to be the only real, independent gas there is; because, being an elastic fluid, there is no necessity for there being empty spaces between its molecules, or even having molecules in the common acceptation of the term. we have no reason to think that there are empty spaces between the molecules or particles of indiarubber; and if there are, the ether is the only substance we can properly conceive them to be filled with. the law of avogadro is, that "equal volumes of gases and vapours contain the same numbers of molecules, and consequently that the relative weights of these molecules are proportioned to the densities." therefore we must always bear in mind that it is the _weights_, not the _volumes_, which are equal, and that the volumes may be very different. on this earth of ours, then, we may say with certainty that an atmosphere of gas is composed of a definite number of its special kind of molecules, mixed with a definite quantity of the ether, in such proportion that the sum of their densities shall be equal to the density of the air, at atmospheric pressure at sea level, and at ° of temperature. holding this belief, we can see that each molecule, or rather atom, of each gas must have its own amount of displacement to enable it to float in the ether with which it is mixed. this would account in the most satisfactory manner for the diffusion of gases, whereby any molecule, or atom, may float wherever it is driven by collisions with its neighbours, be it above, or below, or on a level with, a molecule of a lighter or heavier gas. therefore, were it possible to determine with sufficient accuracy the dimensions of the atoms of all gases, perhaps even of a limited number of them, it would be possible to calculate the real density, or specific gravity, of the ether. we have not forgotten that when, by pumping, the ether was reduced to at least one-fourth of its normal density, its buoyant power would be reduced in the same proportion, nor that, when in a state of rest, the displacement of a molecule, which enabled it to float in the ether, would not be sufficient to make it float at one-fourth of that density; but it might be supposed that when so far relieved from pressure, the molecule could expand in proportion to the relief, especially if its form were that of a vortex ring, or of a hollow sphere. however, should this supposition not be admissible, we shall see presently that it is not necessary. we know that as long as any degree of heat remains in a gas collisions of its molecules will continue, dependent on their attraction for each other, which may drive them to any part of the containing vessel; and that it can only be when they are cooled down to the absolute zero of temperature that they can come to be at rest. but as we believe that the ether can never be reduced to this absolute absence of temperature, nor completely extracted from any vessel, we cannot acknowledge that the molecules of any gas, left along with it in the vessel, could ever come to be absolutely at rest, even although the molecules did not increase in volume with the diminution of pressure. and we think this conclusion will agree with the opinion of professor tait, expressed in the quotation, made at page , from his work on "heat," where he says: "in fact, if the kinetic gas theory be true, a gas whose volume is immensely increased, cannot in any strict sense be said to have one definite pressure throughout." this, of course, is tantamount to saying that the diffusion of gases cannot continue to be always exactly regular at extremely low pressures, and must vary as the vacuum is increased; so that the volumes of the atoms and consequent displacements may continue always the same under all pressures. we see, then, from this quotation, that in all probability the molecules of a gas are not always equally buoyed up by the ether in a high vacuum; which very likely is the reason why there are dark streaks in it; streaks without any visible molecules of gas in them, because the ether was not dense enough to keep them afloat. we have still something to add in support of what we said, at page , of glass not being pervaded by the ether, in the common acceptation of the word, and of our acknowledging that the ether might, in the course of time, ooze through it and fill up the bulb again, while air, gas and dust could not so ooze through it--nor even the larger particles of the ether; should we be forced to acknowledge that it consists of particles. in one of a series of articles in "engineering," vol. xxv., on repulsion from radiation, we find, at page , what follows: "with the same apparatus, mr. crookes conducted a long series of experiments for determining the conductivity of the residual gas to a spark from the induction coil. in air he found, at a pressure of millionths ( / , th) of an atmosphere, which will be seen from the diagram, is the pressure at which the force of repulsion is at a maximum, that a spark whose striking distance at the normal pressure of the atmosphere is half an inch will illuminate a tube whose terminals are millimetres apart. by pushing the exhaustion farther, the half-inch spark ceases to pass, but a one-inch spark will illuminate the tube, and as a vacuum is approached more electromotive-force is required to force the spark to cross the space separating the terminals within the tube, until at still higher exhaustion a coil capable of giving a -inch spark in air at the pressure of the atmosphere is required to show any indication of conductivity in the residual air. it was found, however, in experimenting with so powerful a spark that occasionally the glass was perforated by the discharge taking place through the bulb; but it is a remarkable fact that the perforation in such cases was so excessively small that several days were occupied before equilibrium of pressure was established between the inside and outside of the bulb." here we notice first--and it was the reason why we have made the first and longest part of the quotation--that the spark whose striking distance was half an inch at the normal pressure of the atmosphere, fell to under one-fourth of its power in a vacuum of only / , th of that pressure; that when a one-inch spark was required to illuminate the tube, it must have decreased to one-eighth in a vacuum of / , th; and, if it be admissible to follow the same proportion, the -inch spark must have been exhibited in a vacuum of / , th an atmosphere at least. perhaps all this experiment was carried on in _vacua_ produced by pumping alone, and the final vacuum may have reached a greater height than that which we have just mentioned; but the most interesting part of it is the perforation of the bulb by the -inch spark. in it we have to consider what was the conveyer which carried the electric spark through the glass of the bulb, instead of to the other terminal of the coil so close at hand, and it is a very difficult problem to solve. we naturally recur for some solution to the stratification of light given out when an electric current traverses a gas at very low pressure and gives rise to zones alternately light and dark as noted in the reference we made, at page , to professor balfour stewart's experiments. we cannot think it unreasonable to suppose that the dark zones contained no matter at all that could be lighted up, and that it was the lighted zones alone which contained carrying matter for the electricity. if so, we can easily imagine one of these zones or strata carrying the perforating spark from the induction terminal to the nearest part of the glass of the bulb, for it was as possible for it to lie in that direction as in the direction of the other terminal, and the difference of distance between the first terminal and the glass, and between the two terminals, would not be so great as it appears to be on simply reading the accounts of the experiments; but we have still to think of how it managed to force itself through the glass of the bulb. to get over this difficulty, we can refer to what we have said, that is, that glass may be thoroughly pervaded by the ether in an almost infinitesimal degree, and suppose that the electricity may have discovered, or rather been led to, the ether contained in the glass tube or bulb, and so found its way to one of the oozing holes we have said might exist in the glass; even the oozing hole may not have passed quite through the glass, and there might remain a very thin film to be burst open before perforation was complete. also we may note that the zone which performed the office of carrier to the side of the bulb was much more probably composed of residual ether than residual air or gas, or at the least formed a preponderating part of the carrying element. the fact of the hole being so minute "that several days were occupied before equilibrium of pressure was established between the inside and outside of the bulb" on such occasions, goes far to prove that the carrying agent through the glass must have been the natural carrier of electricity, light, and heat. we cannot conceive that an eruptive force could open such a small passage through the glass of the bulb, but we can conceive that it should be able to force itself through a very minute passage already open, and even join two or more such passages into one. this conception makes us think of the many oozing passages there may be through a glass bulb; passages so minute that the ether might pass through them, but nothing so gross as any of our known gases; in fine, so minute that glass, for all the compact look it presents to us, may be only as a very fine sponge in respect to the ether. however, that the perforations related in the above quotation were large enough for air to pass through them there can be no doubt, otherwise the equilibrium between the pressures on the inside and outside of the bulb could not have been re-established even after many days; for there still remains the idea that the oozing holes might be so small that nothing but the ether could pass through them. should the glass of a vacuum tube or bulb be pervaded by the ether in the manner we have supposed it to be, and we believe there can be no doubt that it is so, it is obvious that its glowing when a current of electricity is passed through it must be caused by the electricity and consequently of its light, being carried into the body of the glass by means of the ether imbedded in, and forming a constituent part of, it. in connection with this we have to remember that the air in the tube does not glow when it is at full atmospheric pressure, but only when a certain degree of vacuum has been produced in it; and therefore it is equally obvious that it is only when the ether enclosed in the tube is reduced to the same degree of tenuity as that imbedded in the glass forming the tube, that the light of the electricity can be carried by it into the glass and make it glow. but to show this more clearly, it is necessary to refer to the steps by which we believe we have made very plain what must undoubtedly be the nature of the ether. ( ) first of all we have shown that, if there be such a thing as the ether, it can be pumped out of a close vessel of any kind; which proves that it must be a material substance, and in consequence can be expanded, or rarefied, and compressed the same as any other material substance; and that if there is no such thing, something else, having these qualities, has to be invented to take its place. ( ) in showing this it has been made abundantly clear by the example of the hair of a cat in variable weather, to which we may add the exhibition of lightning in daylight, that it cannot make electricity visible, or illuminate any matter, unless the quantity of electricity it has to carry bears some certain proportion to the density of the ether in the matter that is illuminated. ( ) in proof of this we have shown how, through its carrying power it can convey electricity of adequate force up to very great heights, so as to illuminate very rarefied air and cause auroras; the conveying being done either directly from the earth or by means of the ether mixed with the air carried up by whirlwinds to those great heights; and ( ) how electricity is carried into the body of a tube of glass and makes it glow. with these examples we can extend our ideas to other exhibitions of light, which, otherwise, we could hardly avoid looking upon as mysterious. we can see how marsh gas, rising up from boggy ground, becomes mixed with common air till it reaches a certain density, and forms the will-o'-the-wisp when there is sufficient electricity in the air to make the diffused marsh gas visible, through the medium of the ether always mixed with it; or, perhaps, rather when the density of the diffused gas corresponds to the density of the ether. then we have the phenomena of films of matter on the surfaces of certain liquids glowing with appropriate colours; which films must be pervaded by the ether in proportion to their conducting powers, the same as we have seen must be the case with all kinds of matter, the light given off corresponding as is natural to the composition of the films; and of course this same reasoning, or exposition, applies to the films formed on, or near, the surface of the sea which produce what sailors call "fire-on-the-wave." lastly, and akin to the glowing caused in a vacuum tube, we cite the case of the glow-worm, the radiation from which must of necessity contain a certain amount of the ether in it, and may either glow constantly or intermittently according to its capacity for carrying electricity or light of any kind, constant or inconstant. or if there is no radiation from it, its skin may possess the properties of a film on the surface of a liquid. we have seen in the "times" of september , , in its report for that day of the meeting of the british association at liverpool, that in experimenting with glow-worms dr. dawson turner had found some difficulty in getting them to glow when he wanted, but found they gave off the radiation whether glowing or not. perhaps his interference with them destroyed the balance of force between the electricity present and the density of the ether in it without stopping the radiation. hitherto the light given out by a nebula, and any light of the kind not easily accounted for, has been attributed to incandescent gas not burning or being consumed, but only glowing. now it is time to look upon it as belonging, at least in part, to the ether, and to look upon the bright line in the spectrum of a nebula as the _ether line_. we shall have to return to this later on. we said, at page , that a fluid of some kind, elastic or not elastic, is necessary to enable us to pump solid matter out of a vessel of any kind, and went on to show that a gas as described by dr. crookes, or that can be described, in its own independent state of existence, by anybody, could not supply the want; because it consists of particles, molecules, atoms--any name that can be given to them--which have no power in themselves to move or to give motion to anything; they can be moved but cannot impart motion to anything, even to one another, until they are first set in motion by attraction. this in its turn led us to see that the only elastic fluid we have is the ether, and our work since then has taught us that we were wrong in saying at page that a non-elastic fluid would suit for pumping solid matter out of a vessel; for we now see that what we have been in the habit of looking upon as non-elastic fluids, must owe their fluidity, such as it is, to the ether, which, in proportionate degree, pervades them the same as it does all other matter. in this way we are run down to the only conclusion we can come to, namely, that the ether is the only connecting medium and carrying agent of matter that we have, or even initiator of motion, except attraction; and being matter of the nature of an elastic fluid, there is no reason why we should not at once consider it to be attraction itself. it has been looked upon, for no one can exactly tell how long, as the connecting mechanism of the universe, thus having, in reality, assigned to it the attributes of the law of attraction, and all that we have to do is to put it in its right place. we are, in a manner, taught to look with suspicion on two agents being required to do one kind of work, or even two kinds of work that are so closely allied that we cannot separate them in a way that satisfies us; and this is precisely a case in which we can have one agent that can connect matter, and at the same time carry immaterial elements from one place to another. having got this length we have still to go one step farther. we cannot now doubt that the ether is a material substance, and if it is, there is nothing to prevent us from considering it to be the primitive matter; in fact it would be absurd to look upon it in any other light. we cannot conceive of anything having been created before the ether, or ordained before the law of attraction, and thus we have the two coeval and one. it is long years since physicists, chemists especially perhaps, began to think that the great number of chemical elements cannot all have existed from the beginning of things, and that it is far more probable that they have all been evolved from one primitive substance, and this idea must now be gathering more strength from day to day in view of the new elements that are being constantly discovered; the unknown is being made known, and the air we breathe instead of being one in four elements, as in former times it was considered to be, is now not far from double that number in one. adopting this notion, then, the ether is much more likely to have been the primitive element than any other material substance that can be thought of. if it has never been thought of in this light, it has come to be very remarkably near it, as may be seen by referring to the long quotation we made in chapter vii., beginning at page , where the idea of the ether being the connecting _medium_ of matter is made use of to compute its density. little thought we of this when we made the quotation, but there was the idea whether the author saw or not all that was implied in it. having broached the notion of the ether being the primitive element of the universe, or at all events, of the solar system, we might be expected to show how all the other elements were formed from it; but that has been done for us in a very much more able manner than we could have done it. anyone who chooses to refer to "nature" of september , , will find--in dr. crookes's opening address, on chemical science in section b, at the meeting of the british association for that year--a very detailed explanation of how all the chemical elements might have been elaborated from one that he called protyle; in which explanation he will only have to change this word into ether to comprehend the process much more easily than by any exposition we could pretend to draw up. to quote the whole address would be altogether out of place, and besides, our notes of it are only fragmentary. but for present satisfaction of those who cannot immediately refer to "nature," we may say that in the same report it is clearly stated that sir george b. airy was of opinion that all bodies may not be subject to the law of gravitation; and have no cause to think it strange we do not see that, were the ether and attraction one and the same, the whole universe would be finally collected into one mass, itself included. they will have better authority than ours for believing that the ether may connect matter evolved from itself, without being materially confounded with it. at the same time we acknowledge the necessity for expressing our idea of what we consider to be its nature, and in compliance with this obligation we say we have conceived it to be of the nature of indiarubber, not an elastic fluid as we have called it before, but rather an elastic substance like a jelly, as some people have conceived it to be; not a gas, because it does not require any medium to connect its particles. looking upon it in this light, action at a distance can be accounted for in a very natural manner. when a stretched indiarubber band is relieved from strain, the relief must be felt instantly throughout every part of its length; for, although the band may take time to contract, no time is required for the relief from strain being felt. in like manner an alteration in strain between the sun and the earth--and these alterations of strain are taking place every instant--connected by an indiarubber ether will be felt instantly in both bodies; and should anyone stand out for time being required to convey the attraction, let him remember that the difference of its power would be felt first at the two ends of the connecting medium, for the very good reason that even attraction itself could not prefer one extreme to the other. and that is all that is meant by action at a distance. here are some other things that could be explained more easily than they can be at present, through the ether and attraction being considered to be one and the same, than under any other conception we can form; but although we have a dim vision of such explanations in some cases, our knowledge of the sciences involved in them is not sufficient to warrant us in letting our dim conceptions see the light. therefore all that remains for us to add is, that some things we have said of the ether may have to be so far modified now, but as they have had their part in leading us to the conclusions we have arrived at, they cannot be altogether suppressed. chapter xv. page construction of the solar system. matter out of which it was formed. domains of the sun out of which the matter was collected. stars nearest to the sun. table vii. showing distances. remarks on binary stars. table viii. showing spheres of attraction between the sun and a very few. sirius actually our nearest neighbour. form of the sun's domains of a very jagged nature. creation of matter for the nebulæ, out of which the whole universe was elaborated. beginning of construction. the law of attraction begins to operate through the agency of evolution. form of the primitive solar nebula. the jagged peaks probably soon left behind in contraction. how the nebula contracted. two views of the form it might take. comparison of the two forms, solid or hollow. the hollow centre form adopted. the jagged peaks left behind. the nebula assuming a spherical form. shreds, masses, crescents separated from one side. probable form of interior of nebula. compared with envelopes in heads of some comets. reflections on the nebula being hollow. opinions of others quoted. the matter of a sphere solid to the centre must be inert there. further proofs of the nebula being hollow. how rotary motion was instituted. such a nebula might take one of two forms. the form depending on the class of nebula. planetary in the case of the solar system. a similar conception of how rotary motion could be instituted. in this chapter we proceed to consider how the original nebula was formed, and whether the solar system could be evolved therefrom in the manner shown in the analysis of chapter v. the usual way of treating the solar system has been to suppose it to have been formed out of a nebula extending far beyond the planet neptune, generally in a vague way; although some writers have specified a limit to the distance, in order to give some definite idea of what must have been the density of the nebula at some particular period of its existence. in the first part of our work we have adopted the same plan and we mean to follow it out, because it gives us a greater degree of facility for expressing our ideas, and making them more intelligible, than by adopting a new method. but we shall previously endeavour to show where the nebula itself came from and how it was formed, which seems to us to be as necessary as to show how it was transformed into the solar system. we understand laplace to have supposed the nebula to have been formed out of cosmic matter in its simplest condition, and in its most primitive atomic state, collected from enormously distant regions of space by the power or law of attraction. in this we shall follow him, because we do not see the necessity for matter having to be created in the form of meteorites or meteors, or any other form, to be afterwards dissociated and reduced to the atomic state, by heat produced by collisions amongst the dissociated atoms. surely it would show more prescience, more simplicity of work, and economy of labour, to create matter in this primitive state, than in one which required it to be passed through a mill of some kind, as it were, before it was manufactured into nebulous matter; in fact, to make brickbats in order that they should be afterwards ground down--dissociated--into impalpable powder, to render them fit to be worked up into bricks. but our first effort will be to attempt to define the collecting grounds of this cosmic matter, somewhat more particularly than has been done hitherto, as we believe that even a superficial study of them will assist us greatly in forming a more comprehensive idea of the whole solar system than anything we have met with in any of the books which we have had the opportunity of applying to for information. the collecting grounds, then, are clearly the whole region of space to which the attractive power of the sun extends, or what astronomers would call within the sphere of his attraction. these domains, like those of any other proprietor, are limited by the domains of his neighbours. at first sight, it would seem that his neighbours are infinite in number, but a little thought will show that the number may be very limited indeed. on this small earth of ours, it is a very common thing for a landed proprietor to be able to look over the domains of his neighbours, and see those of proprietors more remote; even to look over the domains of his neighbours' neighbours, and see properties so remote that he does not even know to whom they belong nor how they are named. with much more reason, the same must be the case with the sun, more especially as he, from his own mansion-house, sees nothing of the domains, but only the mansion-houses of others, there being no landmarks, hills, fences or woods to cut off his view, as there are upon the earth; the only interruption possible to his view being that another mansion-house should come to be exactly between his and that of a farther-off neighbour. for our purposes, we will assume that his nearest neighbours are those the distances of whose mansion-houses have been measured, and will adopt the following list of them, taken from mr. george chambers's "hand-book of astronomy," part , page , th edition, , and forming table vii. all that we can learn from this table is that the boundary between the sun and any one of the stars mentioned in it must be somewhere on a straight line connecting the two, but that does not furnish us with any information as to the extent of the sun's domains, although it does help to give us some idea of their form. for some knowledge of their extent, we require to know how far the lordship of each one of the proprietors extends from his mansion-house; which, very much the same as it does upon the earth, depends upon the power he has to take and keep it; it depends on the mass of each neighbour who actually marches with the sun when compared with his own mass. the list referred to does not help us in any way to determine this, as we have just said, but we have found in professor charles a. young's "lessons in astronomy," of , page , the masses of six binary stars whose distances, calculated from the parallaxes given in it, furnish us with data from which we can calculate the distance from the sun of the boundary between him and any one of them. the number is very small, but still from them we can gain some notion of what was the form of the domains from which the original nebula was collected; that is, always under the supposition that the sun and his system were evolved from a nebula. from these data, table viii. has been drawn up, which shows the distances of the six stars from the sun, and the limits of his sphere of attraction in relation to them expressed in terms of radii of the earth's orbit, and also in radii of neptune's orbit, which gives numbers more easily comprehended by us. table vii.--list of stars whose distances from the sun have been measured, and which are assumed to be his nearest neighbours. --------------------+-----+------+----+------------------+-----------+ | m | p | | distance. | | | a | r | p |---------+--------| | star. | g | o | a | sun's |time of | observers | | n | p m | r |distance | its | | | i | e o | a | = . | light | | | t | r t | l | |reaching| | | u | i | l | | earth. | | | d | o | a | | | | | e | n | x | | | | | | (") | (")| |(years) | | --------------------+-----+------+----+---------+--------+-----------+ [greek: a] centauri | | · | · | , | · |gill. | | | | | | | | cygni | | · | · | , | · |o. struve. | | | | | | | | lalande | - / | · | · | , | · |winnecke. | | | | | | | | sirius | | · | · | , | · |gill. | | | | | | | | [greek: m] cassiopeiæ| | | · | , | · |o. struve. | | | | | | | | groombridge | | · | · | , | · |auwers. | | | | | | | | lacaille | - / | · | · | , | · |gill. | | | | | | | | lalande | - / | · | · | , | · |krüger. | | | | | | | | Ö arg. | | · | · | , | · |krüger. | | | | | | | | [greek: s] draconis | | · | · | , | · |brunnow. | | | | | | | | [greek: e] indi | - / | · | · | , | · |gill. | | | | | | | | [greek: a] lyræ | | · | · | , , | · | | | | | | | | | o^ eridani | - / | · | · | , , | · |gill. | | | | | | | | [greek: r] ophiuchi | - / | · | · | , , | · |krüger. | | | | | | | | [greek: e] eridani | - / | · | · | , , | · |elkin. | | | | | | | | [greek: i] ursæ majoris | · | · | , , | · |c.a.f. | | | | | | | peters.| [greek: a] boötis | | · | · | , , | · |c.a.f. | | | | | | | peters.| [greek: g] draconis | | · | · | , , | · | | | | | | | | | groombridge | | · | · | , , | · |brunnow. | | | | | | | | polaris | | | · | , , | · |c.a.f. | | | | | | | peters.| bradley | | · | · | , , | · |brunnow. | | | | | | | | [greek: s] foucani | | · | · | , , | · |elkin. | | | | | | | | pegasi | | · | · | , , | · |brunnow. | | | | | | | | [greek: a] aurigæ | | · | · | , , | · |c.a.f. | | | | | | | peters.| canopus | | | · | , , | · |elkin. | --------------------+-----+------+----+---------+--------+-----------+ table viii.--masses of a few binary stars showing the limit of the sun's sphere of attraction with respect to them, in radii of the earth's orbit, and distances of their boundaries with the sun in the same measure, and also in neptune distances. -----------+-----+------+-------------+------------+---------------+ | p | |distance of |distance of | | | a | mass |star from sun| limit of | distance of | | r | |in radii of |sun's sphere|limit in radii | name of | a |sun's |the earth's |of attract'n| of neptune's | star. | l | mass | orbit |in radii of | orbit | | l | = . | | earth's | | | a | |( , , | miles. | | | x | | miles.) | |= , , , | | (") | | | orbit = . | | -----------+-----+------+-------------+------------+---------------+ [greek: a] | | | | | | centauri | · | · | , | , | , | | | | | | | cygni | · | · | , | , | , | | | | | | | sirius | · | · | , | , | , | | | | | | | [greek: a] | | | | | | geminorum | · | · | , , | , | , | | | | | | | ophiuchi | · | · | , , | , | , | | | | | | | [greek: ê] | | | | | | cassiopeiæ | · | · | , , | , | , | ------------+-----+------+-------------+------------+---------------+ but there is still something to be said with respect to the binary stars of table viii., and any others whose masses may be met with later on. if those forming a pair revolve around each other, or a common centre, in orbits, it must happen that they will be sometimes more or less in conjunction, opposition, and quadrature with regard to the sun; also the angles of the planes of their orbits to direct lines between them and the sun, whatever these angles may be, will cause variations in the separate and combined forces of attraction they exercise in the domains of the sun, at different periods of their revolutions; so that these powers of attraction will be constantly increasing and diminishing, and causing the boundaries of their domains to approach and recede from the sun; thus introducing between their domains and those of the sun a debatable land, which will reduce celestial to be very much like terrestrial affairs, where each proprietor, or power, takes the pull when an opportunity presents itself. no doubt all such invasions, or claims, between proprietors will be settled by the law of attraction, without lawsuit, arbitration or conflict; but as law gives right, and might is right--most emphatically in this case--we come back to the old seesaw of earthly matters. well, therefore, many astronomers teach that the whole universe is formed out of the same kind of materials, and governed by the same laws that we are having good reason to know something about on this earth of ours. accustomed to look upon [greek: a] centauri as the star nearest to us, on account of its light-distance being so much smaller than any other noted in our text-books, we were not prepared to find that, when measured by his sphere-of-attraction distance, sirius is actually a rather nearer neighbour to the sun than it; nor that his, apparently, next nearest neighbour, when measured in the same way, is twice as far away as either of them; and thus we have the conviction thrust upon us that they must have made deep hollows in the solar nebula when it was being formed. on the other hand, when we think of three of the other stars mentioned in the list of six, being practically from three to six times farther off than either of them, we come to the conclusion that the form of the nebula, when in its most primitive state, must have been of a very jagged character; a conclusion which is very considerably strengthened when we look at table vii., and see that the stars noted in it run up to from twice to not far from thirty times more distant from the sun than [greek: a] centauri. and now, having got a somewhat definite idea of the form of the sun's domains, we may attempt the construction in them, first of a nebula and afterwards of a solar system, such as our text-books describe to us; introducing into the construction, as a matter of course, the variations from existing theories which, we believe, we have demonstrated to be necessary. perhaps we ought to confine our operations to these domains, and so we will almost exclusively; but the sun has been so long considered as one of many millions of stars, and as part of what is now looked upon as our universe, that we cannot help looking upon the whole as having been the result of one act of creation; more especially as we have no reason whatever for supposing it to have been built up piece by piece; and whatever ideas we may form of our own little part of it, we are bound to apply them to the whole. we may, therefore, lay the foundations of our undertaking in the following manner. by creation we mean only creation of nebulæ. we shall suppose all space--if we can comprehend what that means--to have been filled with the ether, and the law of attraction to have been in force previous to the time when our operations are supposed to have commenced. these we may consider to have been the first acts of creation, or to have existed from all eternity. then, in that part of space occupied by our universe--even though it should extend infinitely beyond the reach of our most powerful telescopes--we shall suppose the work of creation to have begun by filling the whole of that space with what are known as the chemical elements, reduced to their atomic state. we do not want to have molecules or particles of matter, or meteorites or meteors; because they involve the idea of previous manipulation or agglomeration, but matter in its very simplest form, if there is any more simple than the atomic. at this stage the most natural idea is to suppose that the whole of this matter was at rest, without motion of any kind, because we cannot understand how motion could be an object of creation, but can very easily see how it might be of evolution; and because, under the law of attraction, matter had the elements of motion in itself. under that law it is quite possible for us to comprehend that all the suns of our universe could have been formed just as they are, with all their movements of rotation, revolution in the cases of multiple stars, and translation or what is called proper motion. and it is within the bounds of possibility that future astronomers may be able to show how these movements have been brought about, should it ever be possible for them to find out and define with sufficient accuracy what the translatory, or proper, motions are. then, as for the temperature of this newly created matter, we have no resource left but to suppose that it must have been that of space, whatever that may have been then, even as we have been obliged to say before. once created, the atoms of the cosmic matter would immediately begin to attract each other in all directions, and form themselves into groups. at first thought it might be supposed that these groups, and suns formed from them, ought to have been all of the same size, being formed from the same material under the same conditions, but nature, or evolution, seems never to be disposed to produce the same results in its manipulations of matter, whatever they may be. when the water is drawn off from a pond, and the mud left in the bottom of it allowed to dry in the sun, it breaks up into cakes of very various shapes and sizes. no doubt there are physical causes for this being the case, but, though perhaps not altogether impossible, it would be a hard task to find them out. much more so would it be with originally created matter, and we have only to accept the fact. moreover, there can be little doubt but that the universe was formed, evolved, according to some design--not at hap-hazard--and that the cosmic matter was created with the distribution necessary to carry out the plan. that the stars differ from each other in magnitude is the best proof of design; for no one can believe that inert matter could determine into what shapes and sizes it could arrange itself. but we have now nothing more to do with the universe, and will confine our operations to the domains of the sun. notwithstanding the vagueness and dimness of the description we have been able to give of the part of space to which our work is now to be confined, we can conceive it to resemble in some degree--not a comparatively flat but--a round starfish, with arms more unequal in length, and irregular in position than the kind we are accustomed to see. in such an allotment of space we can easily conceive that the work of attraction and condensation, of the newly created cosmic matter, in forming itself into a nebula, would be most active in the main body; that in the arms, or projecting peaks as they may be called, it would go on more slowly in the direction towards the centre, the quantity being smaller; and that on account of the greater distance in each from the centre of attraction, and of its being more under the influence of the still existing counter-attraction of the matter in the domains of the sun's neighbours, they might become almost, or rather altogether, detached from the more rapidly contracting main body. we shall, then, suppose that all this has taken place in our incipient nebula. the centre of attraction would at first be the centre of gravity of the whole region occupied by the cosmic matter, which would be ruled in due measure by the projecting peaks, and the indentations or hollows produced in it by the attractive force of the most powerful neighbours; which hollows would gradually disappear as the process of condensation went on, and the main mass would assume the figure of a nebula of some shape. from this stage we may reasonably conclude that, as it was contracting towards the common centre of gravity of all its parts, it would gradually assume a somewhat globular form, and we may now suppose it to have contracted to, say three times the diameter of the orbit of neptune. here, then, we may take into consideration what was the interior construction of the main mass which we may now look upon as a nebula; and we have only two states in which we can conceive it to have been. either that the whole was condensing to the common centre of gravity, in which case its greatest density would be at the centre; or that it was condensing towards the region of greatest mass, in which case its greatest density would be at that region, and its least density at the exterior of the nebula, and also at, or at some distance from, its centre; that is, that the nebula was hollow and without any cosmic matter at all at its centre. in the first case we must recognise that, from that period of time at least, the cosmic matter that was at, or even near, the centre of gravity then, must be there still all but inert, and being gradually compressed to a greater and greater degree of density. there would, no doubt, be attraction and collisions going on amongst the particles, with condensation towards the centre and production of heat--as long as the particles retained the gasiform condition--which might be increased in activity by the pressure, or superincumbent weight, of the whole exterior mass, but there would be no tendency in them to move outwards--provided their gravitation was always towards the centre; and any motion amongst them would be of the same kind as the vibration of the particles of air shut up in a cylinder and gradually compressed by a piston forced in upon them, and not allowed to escape owing to the sides of the cylinder exerting upon them a pressure increasing exactly in the same proportion as the pressure on the piston was increased. and if this was the case with the matter at or near the centre, it would be the same with that of the whole mass, with the exception, perhaps, of the outer layer, which might act the part of the piston in the cylinder. there could be no motions among the particles, except those of collisions and of falling down towards the centre. the outward impacts of collisions would be less strong than those inwards, on account of gravitation acting against them, and the general tendency of all matter would be to move towards the centre. even were we to assume that the whole mass was endowed with a rotary motion, the result would be much the same, that is, increasing stagnation of the matter as it approached to the centre. the areolar law teaches us, however, that the increase of condensation at the centre would increase the rotation there; but in that case we have to acknowledge that this increase of rotation would have to be propagated from near the centre to the circumference, which would be by far the most difficult mode of propagation, and we are forced to think of what would be the rate of rotation at the centre, of a nebulous globe, of some sixteen thousand million miles in diameter, required to produce a rotation at the circumference of even once in four or five hundred years; and from that to think of what must be the speed of rotation at the centre of the sun, at the present day, to produce one rotation at the circumference of twenty-five to twenty-seven days. we should also have to think seriously of how the rotary motion was instituted, and we could only appeal either to simple assumption, or to the impact theory, which, applied to a mass of the dimensions of the one we are dealing with, would require more explanation than the whole formation of the nebula itself. in the second case, that is, looking upon the nebula as a hollow sphere--when it was of the dimensions we have just supposed it to be--we get rid of all the difficulties, and we may add impossibilities, that we encountered in the first case. in such a formation there could be no particle of matter in a state approaching to inertness, not one that could not work its way, through force of attraction and collisions, from the outer to the inner surface of the hollow shell, or _vice versâ_, or all through and round it and from pole to pole--if it had poles then; it might increase or decrease in density, according to the density of the particles with which it came into collision, as it moved from one place to another, but it would find no spot where it could stand still or be imprisoned. even arrived at the region of greatest density, it could change places with its neighbours and move all over that region, if it were condemned to remain with one density once it had acquired it; if not, by acquiring or loosing a little density--_i.e._ by being compressed or allowed to expand a little--it could work its way outwards or inwards, as we have just said, and be as free as the law of attraction would admit of, and as active as that law would oblige it to be. it must be borne in mind that gravitation would act in two opposite directions depending on whether it was acting on the outside or inside of the region of greatest density. we do not go the length of supposing that it could escape altogether from the nebula were its progress outwards; because, as it approached the border, it would meet with plenty of other particles coming in, which would reduce its velocity and prevent its escape. besides, the law of attraction would take good care to prevent it from passing over to a neighbour nebula or sun. it may be argued that in the first case--_i.e._ condensation to the centre--the particles would have the same facilities for changing place, in so far as moving all round the interior of the nebula, or across it, on their way to quasi stagnation, as their densities and the superincumbent weight concentrated and increased; but there could be no motion outwards because the _attraction of gravitation_ would not permit it; nothing could _fall upwards_, all must _gravitate_ to the centre. thus the power of motion in the particles would be limited to very much less than half what they would have in the case of the hollow sphere. it will not do to argue that the increasing heat at the centre would create an upward current. it might create repulsion and prevent the farther-out particles from so soon reaching their final resting or vibrating place, but it could not create an upward convection current of any magnitude; because the colder particles falling down to replace those rising up--that is, if the warmer ones did rise up--being greater in number because occupying greater space, would soon cool down the centre and put an end to the upward current, that is, if it ever came to be set in motion. the greater weight of the greater number would be sure to keep the lesser number in their prison. if any one should say that those nearest the centre would be the heaviest, let him remember that the heaviest liquid or fluid does not rise to the surface. there could be no furnace at the centre to heat the cold particles as they came down to replace those that had just risen up; and if there was, it would be gradually cooled and extinguished. in fact, the centre region would become colder than that immediately outside of it, and so on until the greatest heat would be at the surface of the nebula. should it be argued that the vastly greater number of particles in the outer regions would help those at the centre to rise up, we agree; but it would be because the attraction would be greater outwards than inwards, as we have shown all along, and not because the pressure forced them out--against itself. but, it must be added, this means that if there was still a plenum at the centre the particles that had once left the centre could never come back again, nor any others to replace them, and that no convection current could ever be formed for carrying heat or matter from the centre, or its immediate neighbourhood, outwards. in view of the above comparison of the two cases--added as a complement to what we believe we have demonstrated in a former part of our work--we shall adopt the second as being most in harmony with the laws of attraction, and of nature in general, and shall endeavour to describe in some detail, the construction of the nebula out of the matter belonging to the domains of the sun, as we have marked them out. we have already said that on account of being at the greatest distance from the main body, and at the same time nearer than all other parts of it, to the attractive force in the domains of the neighbouring stars or nebulæ--which attraction continues to be exerted upon the solar system up to the present day--the matter in the high peaks which we have shown would form part of the sun's domains, would come to be completely separated from the rest of the nebulous matter. we shall now assume this to have come about, the detached pieces, somewhat in the shape of cones, occupying positions distant from the main body, in some sort of proportion to their altitudes and masses. this separation would naturally make some alteration on the centre of gravity of the remaining mass. it would come to be nearer to the deep hollows, made in the mass by the attraction of the most powerful of the nearest neighbouring stars; and as we have seen that the hollows made by sirius and [greek: a] centauri would be the deepest, and also for greater simplicity in description, we shall suppose that the centre of gravity would come to be nearer to these hollows than it had been before. then, as the condensation and contraction proceeded, the tendency would be to fill up these hollows, and, as a consequence, the matter at the opposite side of the nebula would at the same time tend to lag behind in approaching the centre--for the same reasons we have given in the case of the peaks--and might easily come to be detached from the main body altogether, first in the form of shreds, then in larger masses, and afterwards in concave segments of hollow spheres, as contraction advanced; and the whole seen from a sufficient distance, would have the appearance of a nebula with crescents, perhaps almost rings, of nebulous matter and detached masses on one side of it; all very much like what we know to be the figures presented by some nebulæ. when contraction had continued till the hollows caused by sirius and [greek: a] centauri were filled up, we might suppose that the nebula had come to be somewhat of a spherical form, although far from being very pronounced, and we have now to consider what its internal structure might be and most probably was. in describing the construction of the earth-nebula we showed that particles of matter placed at different parts of its interior, even not very far from the surface, would be drawn out, in the first place by the greater number coming in from a greater distance from the centre, and that when they met they would all be drawn in towards the centre by the conjoint attraction of the whole mass; and now we can apply this fact to the larger solar nebula, and consider what might be the result. let us fix upon a certain number of equidistant zones in a sphere of cosmic matter, extending from the centre at _a_ to _b_, _c_, _d_ and _e_, at the surface. we know that, according to our former reasoning on particles, and the law of attraction, part of the matter of the zone at _a_ will be drawn outwards by that at _b_, while part of that at _b_ will be drawn inwards by that at _a_, and that the same will take place with all the other zones out to the surface at _e_; and thus there might come to be congested layers between these equidistant places, and there might even be formed hollow spheres within hollow spheres, independent of each other, all through the nebula from near the centre to the surface. this idea is by no means fanciful, as is witnessed by the accounts given in chambers's "handbook of astronomy," already referred to, vol. i., and the figs. , and , showing the form and appearance of the remarkable comets of and . if different, almost concentric, zones or layers of cosmic matter can be constituted in the hemisphere forming the head of a comet, there is no reason why concentric layers of the same matter should not be formed in a nearly spherical nebula. in fact, we can appeal to what is seen in the heads of the two comets cited, donati's also represented in the same work, figs. - , as convincing proof of the correctness of our contention and demonstration that all satellites, planets, suns, and stars are hollow bodies. even the tails of comets, at least of the larger ones, are acknowledged to be hollow bodies. when steadily looked into we find the notion that all fluid bodies are hollow to be much more common than is perhaps generally believed. beginning with the smallest, we find what follows in the rev. dr. samuel kinn's work, entitled "moses and geology," edition , page : "a mist, whether in the form of a cloud or fog, is composed of small bodies of water obeying the laws of universal gravitation by forming themselves into spherules, which halley and other eminent philosophers thought to be hollow. as water is heavier than air, scientists were for a long time seeking for a good reason to account for clouds floating. it may be that kratzenstein has somewhat solved the problem. he was examining in the sunshine some of the vesicles of steam through a magnifying glass when he observed upon their surface coloured rings like those of soap-bubbles, and some of the rays of light were reflected by the outside surface, others penetrated through and were reflected by the inner surface; he concluded, therefore, that the envelope of the sphere must be excessively thin to admit of this taking place. we may, therefore, suppose that these vesicles are filled in some way with rarefied air, and are so many little balloons whose height in the atmosphere varies in proportion to the density of the air they contain. how this enclosed air should become rarefied on the formation of the tiny globule is a problem still to be solved." dr. kinn says nothing of _how_ the spherules of cloud or fog were formed by the laws of universal gravitation, nor _why_ halley and the other eminent philosophers thought them to be hollow, and only states the fact that kratzenstein found the vesicles of steam to be hollow; and only one cause can be assigned for such being the case, namely, the manner in which we have shown how hollow spheres can alone be formed. that the vesicles of steam examined in the sunshine were hollow it would seem there can be no doubt; and if so, there can be as little that halley and the others were right in thinking the spherules of clouds to be hollow. the steam vesicles could not come into existence at once in the air, in form large enough to be examined through a magnifying glass, but must have been built up out of a multitude of the very smallest atoms of water turned into vapour; and would follow the same law as the atoms of cosmic matter and so form the little balloons. in their formation the hollow space would be filled with air, which would expand when heated and contract when cooled, and so regulate their height in the atmosphere. and thus the problem of the last sentence of the quotation is solved. we shall now go to the opposite extreme of matter, and see what mr. proctor says when treating of the formation of a stellar system; but we must state that it is not very clear to us, whether he is exposing mädler's ideas or his own, although we think they are his own or, at least, adopted. he says in "the universe of stars" at page : "he (mädler) argues that if a galaxy has a centre within the range of the visible stars, a certain peculiarity must mark the motions of the stars which lie nearer to the centre than our sun does. as has already been mentioned, the neighbourhood of the centre of a stellar system is a scene of comparative rest. in the solar system we see the planets travelling faster and faster, the nearer they are to the great ruling centre of the scheme; and the reason is obvious. _a._ the nearer a body is to a great centre of attraction like the sun, the greater is the attraction to which it is subject, and the more rapid must its motion be to enable it to maintain itself, so to speak, against the increased attraction; but in a vast scheme of stars tolerably uniform in magnitude and distribution, _the outside of the scheme is the region of greatest attraction, for there the mass of all the stars is operative in one general direction_. (the italics are ours.) as we leave the outskirts of the scheme, the attraction towards the centre becomes counterbalanced by the attractions towards the circumference; and at the centre there is a perfect balance of force, so that a body placed there would remain in absolute rest. it is clear, then, that the nearer a body is to the centre, the more slowly will it move." (compare this last sentence with the one beginning at _a_ above.) here we have recognised, the principle that in a star system the immensely greater number of stars at the outside of the scheme would produce a perfect balance of force, and that a body placed at the centre would remain in absolute rest. this agrees wonderfully well with what we have been arguing, a few pages back, with respect to a sun solid to the centre. matter at the centre would be at absolute rest, _dead_, that nearest to it would be nearest to dead, and so on through a sun or planet, gradually coming to life as it came nearer to the surface; exactly as we have shown it would be, having in it little more than rotary motion. when once acknowledging the immense superiority of attractive force of the stars at the outskirts of the system, over the very few there could be at its centre, mr. proctor seems to have stopped short with the idea and to have contented himself with one body at the centre in absolute rest. had he gone one step further he must have seen that one, or even a very few, could not maintain themselves near the centre with such an immense number pulling them away in every direction. there could be no perfect balance of force. and had he applied the same idea to the earth, and followed it out to the end, he could not have written as he has done, in "the poetry of astronomy," at page , "that the frame of the earth is demonstrably not the hollow shell formerly imagined, but even denser at its core than near the surface." he would have found some difficulty in fixing his first dead particle at the centre, when there were such infinite hosts of near and far-off neighbours endeavouring to annex it. he would have found that the absolute rest was neither more nor less than absolute vacuum. it is utterly impossible to show how any body could be built up out of a nebula of cosmic matter, or even meteorites, from a solid centre, under the law of attraction. we repeat that any foundation laid there would be in a state of unstable equilibrium, and would be hauled away out of its place never to return; unless the cosmic matter around it were so perfectly arranged on all sides that its attraction on the foundation would be absolutely equal in all directions; a condition which cannot be imagined by any one who takes the trouble to think of it. and we think we may add, that no body could be established at the centre of a system of any kind unless it were of sufficient magnitude to control the whole matter within range of it, exactly as we see in the solar system; and that the central body could be no other than a hollow sphere. thus we have either to look upon the sun with his planets and their satellites as hollow bodies or to conclude that the solar system was not formed out of a nebula. coming back to our nebula after the hollows in it, caused by the attraction of sirius and [greek: a] centauri, were filled up, and when we showed that it might have had the interior form of a series of hollow spheres one within the other, and also might be accompanied by crescents and shreds of cosmic matter on the opposite side to the hollows--a supposition we put forward more in explanation of what is to be seen in some nebulæ and comets, than as in any way necessary for our purposes--then, even although it had been separated interiorly into different layers or concentric shells of spheres, these layers continuing to attract each other, would finally come to form one hollow sphere with its greatest density at the region where the inwards and outwards attractions came to balance each other. long previous to this stage--even from the very beginning--the atoms gradually coalescing into larger bodies, would be attracting, colliding with, repelling and revolving around each other, sometimes increasing in dimensions, at others knocking each other to atoms again; but there would be a tendency in them to combine into larger masses as they approached the region of greater density, where the attraction was greatest. now, if the collisions and encounters amongst the masses, great and small, always exactly balanced each other, the whole mass of the nebula would gradually contract towards the region of greatest density, and the whole would ever remain without any other kind of motion in it than what can be seen in a mity cheese--a kind of congeries of particles heaving in every, and at the same time in no, direction. but as an absolute balance of collisions could not be maintained for ever, especially where they would be constantly varying in force and direction, a time would come when movements of translation, as well as of collision, would be instituted on a large scale, in many directions, which, if they also did not manage to balance each other--an affair equally as impossible as in the other case--would ultimately resolve themselves into motion in one predominating direction through the whole nebula. we do not forget that we are dealing with the shell of a hollow sphere, not with a ring, or section of a cylinder, and we can conceive that there would be, from the first, partial motions of translation in multitudes of directions, radial, angular, transverse, etc. etc., constantly changing, even being sometimes reversed, but also constantly combining with each other, and gradually leading on to decided, though partial, uniformity in one direction. as a matter of course this motion of translation would be controlled by its own constituent parts attracting each other to some extent, and thus a rotary motion would be established in the interior of the nebula in the region of greatest density. we can also conceive that when the motions of translation had become nearly uniform, the plane of that uniform motion might be in any direction through the whole mass of the nebula, and might be continually varying until final uniformity was attained, when the greater part of the mass was moving in combination, and the rotation was thereby firmly established in one direction, though still not embracing the whole. we have to take into account also that when the rotary movement had settled down into one plane, it would be most active at the distance of the region of greatest density of the nebula from its centre; in fact it would be instituted at that region and be, therefore, most active there; and then the most active part of the matter would be in the form of a rotating ring, still surrounded by an immense mass of nebulous matter, both inwards and outwards, to which it would gradually communicate its own motion, until the whole mass would rotate, in one direction, on an axis. but it is evident that in the whole rotating mass there would be different degrees of velocity of rotation at different places, decreasing from the supposed ring inwards towards the centre, and outwards to the surface at what would thus become the equatorial region; and also decreasing from the equatorial plane to the poles. following up this idea, we have a more reasonable manner of accounting for the different velocities of rotation observed on the surface of the sun, between the equator and the poles, than we have seen suggested in any speculations on the cause that have come under our observation. until rotation was fully instituted, the areolar law could have no power over the multitudinous movements going on in the nebula, but from that time it would begin to act, and condensation would increase it at the region where it began; and as all increase had to be propagated from there, inwards, outwards, and in all directions, the differences in velocity of rotation throughout the sun must endure as long as he continues to contract. in this we find an immense field for producing heat in the sun, from the eternal churning which must be going on in the interior. a rotary motion produced in this way might have two different results: in one case the rotation might be continued until the matter at the polar regions had all fallen in towards the centre, and had been thrown out afterwards by centrifugal force and the whole mass converted into a nebular ring, in the form of the annular nebula in lyra. in the other case we could conceive that, in a smaller nebula, the centrifugal force of rotation caused zones to be abandoned at the equatorial surface, in the manner set forth by laplace in his hypothesis, and that the matter from the polar regions fell in more or less rapidly for the formation of the different members of a system like the sun's; and that the dimensions of the planets would be determined by the rapidity with which the matter fell in as the process went on. such a conception would help to account for the outer planets of the solar system being so much larger than the inner ones, because there would be more matter falling in; and make us think that the nebula in lyra is destined to form a system of multiple stars. some years after this mode of instituting rotary motion in a nebula was thought and written out, and also an extension of it to which we may refer later on, we came upon a kind of confirmation of the correctness of our views in an article in "science gossip" of january , on the nebular hypothesis, where it is said: "we have established, then, the existence of irregular nebulæ which are variable--that is, the various parts of which are in motion.... now, with the parts of the nebula in motion, whether the motion is in the form of currents determined hither and thither according to local circumstances, or in any other conceivable way, such motions bearing some reference to a common centre, unless the currents exactly balanced each other--a supposition against which the chances are as infinity to one--one set must eventually prevail over the other, and the mass must at last inevitably assume the form peculiar to rotating bodies in which the particles move freely upon each other. it must have become an oblate spheroid flattened at the poles and bulging at the equator, rotating faster and faster as it contracted. in some such manner has our solar system acquired its definite rotation from west to east." the writer in "science gossip" has taken the irregular motions in the nebula as made to his hand, and has come to the same conclusion as we have, namely, that they would all resolve themselves into motion in one direction only, always subject to the general attraction towards the centre of gravity of the nebula, which means motion round a centre, perhaps not necessarily rotary motion. however, the only difference between his ideas and ours is that we deal with a hollow nebular shell, in which, it will be acknowledged, it would be much more easy for the law of attraction to produce marked and distinct motions of any kind, and which would lead to one motion in one direction throughout, than in a nebula homogeneous, or nearly so, from the surface to the centre. whether it would lead to the formation of an oblate spheroid is another question, as that might depend on a variety of circumstances, one or more of which we shall have to touch later on; in fact, we have already shown how the very reverse might be the case. chapter xvi. page the sun's neighbours still exercise their attraction over him. regions of greatest density in the nebulæ dealt with; compared with the orbits of the planets made from them. results of comparison favourable to the theory. differences of size in the planets have arisen from variations in the quantity of matter accumulating on the nebulæ. causes of the retrograde motions in neptune, uranus, and their satellites. probable causes of the anomalous position of neptune. rises and falls in the densities and dimensions of the planets explained. the form of the nebulæ must have resembled a dumb-bell. more about rises and falls in densities. reason why the asteroid nebula was the least dense of the system. not necessary to revise the dimensions given to the nebulæ. causes of the anomalies in the dimensions, densities, etc., of the earth and venus. the strictly spherical form of the sun accounted for. but it may yet be varied. repetition that a spherical body could not be made from a lens-shaped nebula by attraction and condensation. testing the practicability of the hollow sphere theory. retrograde motions, positions, densities, masses, etc. etc., considered. before going any farther it will be convenient to try to find out whether the solar system could have been constructed from a hollow nebula such as we have been describing gradually contracting as the matter for the formation of one planet after another was abandoned until--as we have put it--the nebula could abandon no more matter, and finally resolved itself into the sun. for this purpose we may suppose it to have been condensed and contracted until its extreme diameter was , , , miles; the same as we supposed it to have been, when we began the analysis of the nebular hypothesis. we will not now, however, suppose it then to have contained the whole of the cosmic matter out of which the system was formed, as we did before; because we have seen as we have come along that a very considerable part of that matter must have been left behind, almost from the moment that contraction commenced. we have already given the reasons for this in describing the domains of the sun; and, leaving the peaks out of account altogether for the present, we will only deal with the regions of what we have called the main body. although we have fixed a limit beyond which the neighbouring stars could not draw off any cosmic matter from the domains of the sun, that does not mean to say that their attractive powers would cease at that limit; because we have had to acknowledge that each one of them continues, even now, to exert its attractive power up to the very centre of the sun. they would still have power to counteract, in some measure, the sun's attraction of the matter of the nebula towards his centre, and the result would follow that there would be one or more, even many, fragments of the main body which would be left more or less behind, and in varied forms, when the more central part had contracted to the dimensions to which we have now reduced the nebula--all much the same as we have already said a few pages back. when the nebula was , , , miles in diameter its volume would be , ^{ } cubic miles--as we have seen at page --the half of which is , ^{ } cubic miles, corresponding to a diameter of , , , miles, or radius of , , , miles. now, according to our theory, it would be at this distance from the centre that the greatest density and activity of the nebulous matter would be, where we have just been showing how a movement of rotation could be generated, and where, in consequence, its motive power, so to speak, originated and existed. here we find by dividing , , , by , , , that the region of greatest density in such a nebula would be at · of its diameter. in our calculations about the earth, as it is, the proportion was found to be · , but the densities of the outer layers were empirically arranged by us; and, besides, almost the whole of the mass was supposed to be solid matter, so that no accurate result could be expected from that operation. there also we found that the inner surface of the hollow shell was at · of the whole diameter, which we may adopt for the nebula we are about to deal with, as that dimension may be varied considerably--so may the other also--without in any way vitiating our theory. having found these proportions, which can only be considered as distantly approximate, let us go back to the nebulæ--excluding the final solar one--into which we supposed the original nebula to have been divided--in the analysis just alluded to--and see how the regions of greatest density in them would correspond to the orbits of the planets formed out of them. this examination requires a good deal of calculation and accompanying description, which it might be found tiresome to follow, and would really answer no good end were it written out; so we shall suppose it to be made and the results obtained from the calculations to be represented in the form of table ix., where they can be seen at a glance almost, and compared without much trouble. this arrangement will also furnish a readier means of reference for the remarks we shall have to make on, and the information obtained from, the examination. and we have still to add that the extreme diameters of the nebulæ are the same as those we used for the analysis; as also, that we make use of only the first of the proportions just cited, viz., · , it being the only one required for determining the positions of the regions of greatest density in the nebulæ. table ix.--dimensions of the nine nebulÆ, with their diameters and regions of greatest density compared with the diameters of the orbits of the planets formed from them. ----------+--------------+---------------------------+ | nebula. |region of greatest density.| | | | name of |--------------+-------------+-------------+ planet. |outer diameter| diameter | radius | | in miles. | in miles. | in miles. | ----------+--------------+-------------+-------------+ | | | | neptune | , , , | , , , | , , , | | | | | uranus | , , , | , , , | , , , | | | | | saturn | , , , | , , , | , , , | | | | | jupiter | , , , | , , , | , , | | | | | asteroids | , , | , , | , , | | | | | mars | , , | , , | , , | | | | | earth | , , | , , | , , | | | | | venus | , , | , , | , , | | | | | mercury | , , | , , | , , | +------------------------------------------+ | had the position of neptune been normal, | | the above data for him and uranus would | | have been as under. more or less. | +------------------------------------------+ neptune | , , , | , , , | , , , | | | | | uranus | , , , | , , , | , , , | ----------+--------------+-------------+-------------+ ---------+---------------------------+------------------------------+ | orbit of planet. | region of greatest density | | | compared with orbit. | name of |-------------+-------------+-----------+-----------+------+ planet. | diameter | radius | within, | without, | per | | in miles. | in miles. | (miles.) | (miles.) | cent.| ---------+-------------+-------------+-----------+-----------+------+ | | | | | | neptune | , , , | , , , | , , | | · | | | | | | | uranus | , , , | , , , | | , , | · | | | | | | | saturn | , , , | , , | | , , | · | | | | | | | jupiter | , , | , , | | , , | · | | | | | | | asteroids| , , | , , | | , , | · | | | | | | | mars | , , | , , | | , , | · | | | | | | | earth | , , | , , | | , | · | | | | | | | venus | , , | , , | , , | | · | | | | | | | mercury | , , | , , | | , , | · | +----------------------------------------------------------+ |had the position of neptune been normal, the above data | |for him and uranus would have been as under. more or less.| +----------------------------------------------------------+ neptune | , , , | , , , | | , , | · | | | | | | | uranus | , , , | , , , | | , , | · | ---------+-------------+-------------+-----------+-----------+------+ from the table we see that the region of greatest density of our original nebula was at · per cent. _within_ the distance of neptune's orbit from the sun, a state of matters which precludes the idea of condensation during, at least, a great part of the act of abandoning the ring for the formation of that planet. but it will be remembered that we gave it the diameter of , , , miles without assigning any adequate reason for doing so, and, we can say with truth, with the idea, more than anything else, of not increasing the almost unimaginable tenuity of the matter composing the nebula; and the position of neptune in the system is so peculiar compared with the other planets, that it cannot be properly used as a standard for any kind of inquiry. the result obtained above can therefore be of no use for the investigation we have undertaken. not only so, but the almost similar result in the case of uranus is also rendered useless from the same cause, in which we find that the region of greatest density of the nebula is only · per cent. beyond the orbit of the planet. if the mean distance from the sun of neptune's orbit had been what was used by leverrier in the calculations which led to his discovery, namely, · radii of the earth's orbit, the region of greatest density of the uranian nebula would have been · per cent. beyond his orbit, as may be seen from the addition to table ix., in finding which we have used the same system as in all our work. in the next four nebulæ of the table--including the one we introduced to represent the asteroids--we see that their regions of greatest density are respectively · , · , · and · per cent. farther out from the centre of the sun than the orbits of the planets formed from them. here, then, we see a very apparent approach of uniformity, and can say with much reason that planets could certainly be formed out of the matter abandoned, through centrifugal force, by hollow nebulæ similar in construction to what we have demonstrated that of the original nebula to have been; each of them occupying the position corresponding to its orbit. following these come the earth and venus nebulæ. in the former, the region of greatest density almost coincides with the orbit of the planet, being only · per cent. beyond it, instead of something like per cent. as it ought to be to conform with the four preceding cases; and in the latter it is · per cent. within the orbit of the planet to be made from it. but in this case we have to note that the orbit of venus is · per cent. beyond the position pointed out for it by bode's law, and that it is the only one of the whole number of planets whose orbit is farther removed from the sun than the distance assigned to it by that law. also we see from our reversal of bode's law, that the rates of acceleration of rotation for these two planets are · for the earth and · for venus, instead of the average of · of the four preceding planets; that the density of venus is less than that of the earth, instead of being greater as it is successively in all the other planets from saturn inwards; and we may add that the diameters are nearly equal. all showing that influences had been at work in the formation of these two planets, different to those in the preceding four; and that until we know what these influences have been, we cannot account for any anomalies produced by them. neither are we called upon to consider that our theory is destroyed by these anomalies, any more than it can be by the anomaly in the case of neptune's position. lastly, we have in mercury the region of greatest density of his nebula at · per cent. beyond his orbit, and the rate of acceleration of revolution over venus · times, both of which conform fairly well with the same noted facts; in relation to mars, the asteroids, jupiter, saturn, and, we may add, uranus. but, in justice, we must not omit to add that there may be some error in the excess of · per cent. in the distance from the sun beyond his (mercury's) orbit, arising from the fact that there may have been some difference from what we made it to be, in the line of separation between his nebula and that of venus; and also that we had to guess at the line of separation between his and the residuary nebula. moreover, it has to be taken into account that his orbit is · per cent. within the position assigned to it by bode's law. from the table ix., and an examination of it, we learn that out of the nebulæ into which we divided the original one, in the analysis of the nebular hypothesis, we have five--four of which are consecutive--which may have been almost of the same construction, and not far from the same proportions; that the original nebula cannot, for reasons assigned, be looked upon as either similar, or the reverse, to the five just classed; that one, the uranian, is practically similar to the five, and might be exactly similar could the anomaly in the position of neptune be explained; and that the remaining two, the earth and venus nebulæ, seem to show that they have been abandoned in a manner different from the others. perhaps we may be able, later on, and in a different way, to give a reasonable explanation of the anomalies in the positions occupied by neptune, the earth, and venus, and also of the peculiarities of their dimensions. so far, we believe we are justified in concluding that out of the nebulæ, may really be considered as supporting our theory, and the remaining as, in all probability, capable of being shown to be, at least, not opposed to it. to this we may add that on several occasions we have stated our opinion, that the divisions between the nebulæ we have established, could not have taken place at the half-distance between the orbits of any two planets, but much nearer to the outer one. it is evident, then, that if we had made the divisions at any distance farther out, say at three-fourths of that distance from the inner orbit, the extreme diameter of each one of the nebulæ would have been just so much greater, the region of greatest density farther out from the centre of the sun, and even that of neptune would have been beyond his orbit. all this could be done, yet but it would serve no good purpose, as will be seen presently; and we might be accused of cooking our data in order to produce a result favourable to our theory. we have made the foregoing examination because, when we began our work, the general idea was that, according to the nebular hypothesis, the material for the formation of each planet was abandoned by the ideal nebula in a distinct and separate mass from any other--we are not at all sure, however, that this was laplace's idea. this, we found out, could not be the case when we attempted to give some sort of separate or distinct form to the matter out of which neptune was supposed to have been formed; and when we became convinced that all the matter abandoned by the nebula, from first to last, must have been thrown off in one continuous and, most probably, uninterrupted sheet. this, of course, makes us think of how the division of the sheet into separate rings was brought about, for there must have been absolute separation between them, otherwise separate planets could not have been made out of the sheet; and the only explanation that can be given is, that it must have depended on the quantity of matter that was abandoned, in nearly equal times, at different periods of the operation; for the areolar law precludes the idea of there having been very rapid changes in the rate of rotation of the nebula, and certainly of its decrease at any period as long as condensation and contraction went on. whereas, although the sheet thrown off may have been continuous, we have no reason to suppose that it was of constant volume or density from beginning to end of the operation; in fact, we have already seen that its density was constantly increasing, and have suggested, in the reversal of bode's law, that the differences in dimensions and densities of the planets have arisen, from irregularity in the quantities of matter abandoned from time to time. this irregularity could only arise from the mode of construction of the nebula, and from the forms it assumed during condensation, as we shall attempt to show in due time. meanwhile we can conclude that the region of greatest density in any of our nebulæ had no influence whatever on the position of the orbit of the planet that was formed out of it. we have shown, very clearly we believe, at page , from quotations--at second hand--from his own exposition of his hypothesis, that laplace considered that condensation could only take place at the surface, or in the atmosphere as he called it, of his nebula, on account of its being possible only after radiation into space of part of its excessive heat; and that consequently there could be no acceleration of rotation in the nebula, due to the areolar law, except where there was condensation. on the other hand, in our cold hollow-sphere nebula, condensation could only take place at the region of greatest density, or greatest mass, which must be always very much nearer to the surface than to the centre; so that in both cases, equally, the abandoning of matter under the influence of centrifugal force would be virtually the same, and no further remarks are called for, on our part, on that head. neither is it necessary for us to show how planets could be formed out of the rings abandoned by their respective nebulæ, for everybody seems to agree that when they broke up, the fragments could not do otherwise than form themselves into small nebulæ, which in the course of time condensed into planets. m. faye's explanations are good for that. with respect to their motions of rotation being direct or retrograde, we have seen, at page , and following, that laplace's description of how the former motion could be brought about is mechanically correct; and, at page , that he did not consider that the direction of revolution of a ring necessarily demands that the rotation of a planet formed from it should be in the same direction. as already said, he has shown how direct rotation could be produced, and we have no doubt that he could have shown how retrograde rotation could also be produced, had he found it to be at all necessary. be that as it may, however, it is a very simple matter to show how, following our method of construction of the primitive nebula, the retrograde rotation of uranus and neptune could, or rather must, have been determined. it will be remembered that when we were "getting up" the original nebula in the domains of the sun, whose form we described as well as our limited means would admit of, we said that when the cosmic matter contained in them began to contract, not only the parts contained in the peaks and promontories would soon be left behind, and come in at a slower rate, but also large masses of the outer part of the main body, especially of what was on the sides opposite to the deep hollows made in the domains by the most powerful of the sun's neighbours, in the form of fragments, crescents, and parts of hollow segments. let us now, then, suppose the operation of planet-making to have advanced so far that the whole nebula was rotating on its axis, and abandoning matter through centrifugal force, from its equatorial regions in a continuous sheet, as we have said several times that it must have done, and that the matter destined for neptune and uranus has not only been abandoned, but divided into two distinct rings--a supposition made in this case only for facility of description. then some of the matter which had been left behind, but still being gradually drawn in, would be almost totally intercepted in the equatorial regions of the nebula by these two rings, and would fall in greater quantity upon their outer edges than anywhere else, more especially in the case of the outer one. these adventitious additions would come in without any angular, or tangential, movement whatever, because rotary motion was not yet established in them, and would retard the revolutionary movement of the rings--in decreasing degree from their outer to their inner edges--while acquiring angular motion themselves; and would also intensify the original difference in revolutionary motion already existing at these edges. at the same time these additions of extraneous matter would seriously impede the contraction of the rings in the radial direction on account of their volume, but would have little or no effect on contraction in the circumferential direction; the consequence of which would be that they would break up before friction, and the mutual collisions of their particles, had time to produce a uniform revolving motion throughout their whole breadth; that is, while their inner edges would be still revolving with more rapid velocities than the outer ones; and the rotary motions of the planets derived from them would be retrograde, according to m. faye's demonstration--or that of any other who has taken the trouble to think over the matter. and we may add that this mode of reasoning, applied with a little more detail, will very fully account for the rotation of neptune being more decidedly retrograde than that of uranus, because the quantity of matter so deposited on the outer flat ring in this process would unquestionably be greater than on the inner one, and consequently the difference of velocity between the outer and inner edges of the two rings also greatest on the outer one. we take it to be unnecessary even to say that, the revolution of the satellites of these two planets being retrograde and anomalous, the rotation of their principals must be retrograde and anomalous also. before going any farther we have something to say about the anomalous position of the orbit of neptune, which is certainly not the position sought for by m. leverrier; in fact, the elements employed by him in his calculations to discover a perturbing planet--whose existence may be said to have been known--are so different from the elements of the one actually discovered, that there would be nothing out of reason in saying that neptune is not the perturber that was sought for, but only an instalment of the perturbing force. it may raise a storm in some quarters to say so, but the fact remains the same, or it must be confessed that mathematics is a more elastic science than it professes to be. he has not the power of attraction required to produce the perturbations in the movements of uranus which gave rise to the search for an outer planet. m. leverrier made his calculations under the belief that a planet of / th part of the mass of the sun was required to produce the perturbations that had been observed in the orbital motion of uranus; whereas the planet discovered has only / , th of that mass--not one-half of what was required. on the other hand, the semi-axis major of the orbit of the planet discovered is found to be · instead of · (bode's law measures) used for the search; which greater proximity to the sun, it is true, increases its power of attraction · times, but as its mass is only · per cent. of what was expected, the attractive force would amount to less than · per cent. of what was required. then the question comes to be, where did the wanting · per cent. of attractive force come from? and the answer is that some astronomers have been searching for another planet to make up the weight, with more or less diligence, ever since the deficiency came to be recognised. but all that we want to have to do with the question is to suggest a very plausible reason for the anomalous position of the orbit of neptune. if there is another planet beyond neptune, the ring (perhaps the rings) out of which he and the others were made, must have been much greater in breadth than what we have assigned to it at page , viz. , , , miles; perhaps even one-half more, as may be deduced from the addition made to table ix., and what we have said in connection with the semi-axis major adopted for the sought-for planet, by m. leverrier in his calculations. now, that a ring of such enormous breadth should have held together in one piece, until it finally broke up through condensation and contraction, requires an extraordinary effort of imagination, after seeing what has taken place with the rings of saturn; even the breadth of , , miles appropriated to the uranian ring (see page ) demands an elastic imagination to conceive its holding together; so that the outer ring of the system may very well have been divided into two, as we have said at page , and two not very unequal planets made out of it--one into neptune, and the other into one as far beyond m. leverrier's adopted distance of · , and of such mass as would make up the missing · per cent. of deficient attractive power. no doubt the outer ring may have broken up into several planets, or even into a swarm of asteroids, but we prefer to think of only two planets; because it seems to us that to draw uranus into the position he occupied when neptune was discovered, the two planets must have been operating in conjunction; an idea that is not so easily entertained when there are several planets, or a host of asteroids, to be taken into account. we have already discussed, at page , the mode of formation of the sheet of matter abandoned by the nebula, its posterior division into separate rings, and how the part of these rings from saturn inwards could revolve themselves into planets having direct motion, so it is not necessary to go over the same ground again, merely because we are dealing with a hollow nebula instead of one full of cosmic matter to the centre. we have also shown, at page , that the nebula must have been somewhat in the form of a cylinder terminated at each end by what may be looked upon as a segment of a sphere, although it would more probably be an almost shapeless mass of cosmic matter, because the greater part of it would be very slowly brought under the influence of centrifugal force as it fell in from the polar directions; and again, a few pages back, that almost all the matter coming in from its equatorial regions--even what might be called its tropical regions--would be intercepted before it could reach the saturnian nebula. likewise, at page , when examining bode's law reversed, we have seen a limit set to the acceleration of the movement of revolution in the planets of the system as they approached the centre, because any acceleration beyond a certain limit, clearly marked out, would of necessity be within the nebula itself, and the rate of revolution would be less than that of the sun on its axis at the present day. this may be used as an argument against the nebular hypothesis, but we think we have shown in the same chapter vii. that this is not the case. but we have still to try to account for the repeated rises and falls in density in the planets from neptune to mercury, or even farther; which operation causes us to bring forward, first of all, a new idea as to what the form of the nebula would come to be. [illustration: fig. .] the accompanying rough sketch (fig. ), drawn to a scale of one-quarter inch to , , , miles shows that, supposing the saturnian nebula to have been a perfect sphere, and to have abandoned matter till the velocity of rotation came to be equal in a region corresponding to the tropical region of the earth, the cylindrical part of it would present a straight side of more than , , , miles in length; provided always that the general diameter of the nebula did not decrease through condensation and contraction during the operation; but as this could not be the case the length of the cylindrical part would be considerably less than that. how much less we have no means of calculating. on the other hand we have seen, when discussing, in the case of jupiter, how matter must have been abandoned by any nebula, that from the time the original nebula began to abandon matter through centrifugal force, it must have gone on acquiring a constantly increasing length of straight side as it contracted. thus the saturnian nebula would begin work with the accumulated cylindrical length it had inherited from neptune and uranus, so that the straight side may have been very much longer than that shown by the sketch; a simple look at it is enough to make one believe that this would be the case. but this idea naturally leads us to another digression. looking again at fig. , we see that acceleration of rotation in the nebula would originate where condensation was greatest, that is at the region of greatest density, and have to be propagated from there to its periphery so that it would reach the middle of the cylindrical part sooner than the ends; and as the nebulous matter at the ends of the cylindrical part could not be abandoned until it had acquired the centrifugal force necessary to overcome gravitation, it would lag behind and overhang, as it were, the middle of the cylindrical part; which means that instead of continuing to be straight, the line of separation between the nebula and the abandoned matter would come to be concave; and in this manner the nebula would soon assume the form of a dumb-bell, gradually becoming more and more pronounced as condensation proceeded. one can hardly help concluding that this must have been the way in which the dumb-bell nebula near star vulpeculæ was formed. the representations of it given by chambers, vol. iii., page , figs. and , as seen by smyth and sir john herschel are most confirming of this idea; notwithstanding the changes of appearance shown by lord rosse's reflectors of feet and feet diameter, figs. and , which are not difficult to account for. it is easy to imagine how fig. could be converted into fig. when observed with a much more powerful telescope. we can conceive the roundest end of it being reduced into the sort of compact segmental form on the left hand side of the figure, and the spread-out part of it into the more diffused segment on the other side; but the form of the whole figure forces us into another conception. mr. chambers says the general outline resembles a chemical retort, but to our eyes it is infinitely more like one half of a dumb-bell broken off from the other. so like it that we feel inclined to ask what has become of the other half. this again makes us think of an enormous dumb-bell nebula dividing itself into two parts, one of which has disappeared or broken up in some manner without leaving any distinguishable traces of its existence, and the other, either forming itself into a double star, assuming in the process the form of a dumb-bell, or actually of one rotating in a direction almost at right angles to that of the original one; more probably the former of the two. perhaps we have allowed our ideas, or fancy, to run on too far; nevertheless, looking over the forms of nebulæ represented in chambers's classical work, and duly considering how inconceivably strange some of them are, there is nothing impossible in all we have said. returning to the repeated changes of density in the solar planets, we know that the matter first abandoned by the original nebula, through centrifugal force, would be at the lowest stage of density, and that what followed would go on gradually increasing in density as it contracted to the saturnian nebula. but, as we have shown that immense quantities of matter belonging, so to speak, to the sun, though actually separated from the original nebula, must have fallen in upon the sheet after being abandoned, it is not difficult to see that the part of the sheet out of which neptune and uranus were made, might be more dense than the saturnian nebula, on account of this matter being added to it; and that, as the greater portion of it must, at the more advanced stage of the process of condensation, have fallen upon the uranian part of the ring, because the space from which it fell would be higher, the density of that would be greater than the neptunian part of the sheet; both of them exceeding the density of the saturnian nebula. again, we have supposed, very naturally we think, that all extraneous matter coming in from the equatorial direction would be intercepted by the rings destined for neptune and uranus, so that the density of the ring for saturn would be only what had been acquired through condensation, and the planet formed out of it would be less dense than those made out of matter accumulated in a different way. it may be argued against this deduction, that density would depend on the degree of contraction, but it is natural to think that lighter would take longer time than heavier matter to condense to the same degree; besides saturn is of necessity the youngest of the three planets, and may in due time come to be as dense as either of the other two, but his diameter will decrease proportionately. coming now to the jovian nebula, whose diameter we have made to be , , , miles, we have seen, at page , that--had it been a perfect sphere--by the time it had contracted one thousand miles in diameter, it must have had a flat side of more than , , miles in length? then if we add to that length all that the nebula had inherited from neptune, uranus, and saturn, the cylindrical part of it must have been many millions of miles in length, and the polar very much greater than the equatorial diameter of the nebula. in other words we have to deal with a body having the form of a very long cylinder terminating in spherical caps. to this we have to add that the density of the jovian was more than times greater than that of the original nebula. still farther we have to take into account that the whole of the matter abandoned by that nebula must have been thrown off in less than one-half of the space in which the ring for even saturn had been abandoned, the breadth of the two rings, as shown by us, see table iii., having been , , , and , , miles respectively. all these things considered, it is clear that the thickness of the ring for jupiter's system must have been very much greater than what we have given it in the table; which, coupled with its matter being over six times more dense than that of the preceding ring, is sufficient to account for the rise in density, the immense size, and mass of jupiter. next, we have the means of accounting for the fact that, the space occupied by the asteroids is, and has always been, the least dense of any portion of space occupied by the solar system. it is easy to understand that the enormous mass of matter abandoned by the nebula for the formation of the jovian ring--more especially towards the end of the process--would have a very appreciable effect, by its attractive power, in helping centrifugal force in freeing matter from the power of gravitation; the consequence of which would be, that the matter thrown off for the formation of the asteroidal ring would be considerably less dense than it would otherwise have been. in this way, then, we have the decrease of density, as well as the quantity of matter, in that space very plausibly accounted for. then, as the nebula continued to contract, the attractive power of jupiter's ring would decrease proportionally to the square of the distance of the receding mass, ceasing in doing so to lend so great assistance to centrifugal force in the nebula, and so letting it subside into its normal state; so that the matter abandoned would increase in density in comparison to the space over which it was distributed, thus accounting for the rise in density towards mars and the earth. with regard to the fall towards venus and final rise towards mercury, we have to take into consideration the anomalies--already taken notice of--in the dimensions, densities, etc. etc., of the two planets earth and venus; it being, we may confidently say, certain that the whole of them have arisen from the same causes. following up the idea of a dumb-bell nebula--as we might have done in the case of jupiter also--as the breadth of space for receiving matter abandoned by the nebula went on rapidly decreasing, the thickness of the ring left behind would go on increasing, and the overhanging matter of the dumb-bell would be deposited always in greater quantity on the outer than the inner part of the ring as it broadened; we can conceive that the whole extent of the sheet of matter allotted to the earth and venus would be thicker at the outer than the inner part. hence, when this part of the sheet came to be divided into two parts for the formation of two planets, the outer would naturally be the greater and denser of the two, and thus occasion the rise in density from mars to the earth, and the fall to venus. finally the rise in density to mercury would be only the beginning of the gradual, and final, rise to the sun as it is at present. if the idea of a nebula in the form of a cylinder with hemispherical ends is admitted as possible, or somewhat like a dumb-bell, the extreme diameters of the successive nebulæ we have dealt with would be considerably different in their equatorial directions to what we have given them, although their polar diameters might continue to be not far from the same; but that would have very little effect on the operations we have gone through, seeing we have shown that there could be no actual divisions between them such as we have adopted; and that the division of the sheet of matter abandoned into separate rings must have been brought about by some means which we cannot explain; a process, nevertheless, which has been subject to some law, or laws, operating evidently in a regular and steady manner throughout the whole time, during which the matter was being abandoned, as is proved by the general uniformity, or harmony, in the distances of the planets from the sun. should anyone come to be able to account for the division of this sheet of matter into distinct and separate rings, he will also be able to account for the acceleration of rate of revolution from one planet to another, and for the anomalous rates in the cases of the earth and venus. in a former part of our work we have followed up, at different stages, the condensation of the original nebula until it attained the dimensions, appearance, and some of the features of the sun as it is, but we have still something to add as to how the condensation could produce a body so strictly spherical as the sun is represented to be. all the other bodies of the solar system, as far as astronomers have been able to measure them, are spheroids more or less oblate, and it seems strange that the principal should be the only one that does not conform to the general figure. it is rather hard on the notion that the original nebula gradually assumed the form of a lens, for it would require a special mode of manipulation of a very mechanical kind, rather than the steady, imperceptible self-action of the law of attraction, to transform a lens into even an oblate spheroid; to transform it into a perfect sphere would be absolutely impossible. for, if at the end of the process it was found that there was too much material to form a sphere, it would be hard to get rid of the superabundance, unless it was converted into meteorites--evidently another hand process. on the other hand, should a hole remain to be filled up, the material would have to be lugged in somehow from some of the errant masses, or lambeaux, which impact-theorists find it so easy to have at hand when required. let us then think of why and how it came to pass that the sun is an almost perfect sphere. if we suppose that, when cosmic matter ceased to be thrown off by it, the form of the nebula was that of a cylinder terminating in semi-spherical caps at the ends, it requires no great stretch of imagination to conceive that, between attraction and centrifugal force, the whole mass should be converted through time, first into a prolate spheroid, and then into a perfect sphere. and very possibly time only is required for the sun to become an oblate spheroid, the same as his dependent planets. should this form of nebula not be admissible--and we can see no mechanical reason why it should not--and we are thrown back on a lens-shaped nebula, the only resource left us is to suppose that through continued action of attraction, and of centrifugal force, or rather revolution constantly increasing, the latter gaining the victory over attraction, finally converted the lens into an actual ring, something of the nature of the ring in lyra; and that that ring, no longer increasing in revolution, would have to yield to the law of attraction, and would condense and contract and close up into an oblate spheroid, and then into a sphere. it is a roundabout, rather fanciful, process, but any other way of converting a lens-shaped nebula into a sphere, under the law of attraction, is absolutely impossible. chapter xvii. page former compromises taken up and begun to be fulfilled. estimates of the heat-power of the sun made only from gravitation hitherto. contraction and condensation of a nebula solid to the centre. heat produced from attraction as well as by gravitation. what quantity of heat is produced by a stone falling upon the earth. showing again that there is a difference between attraction and gravitation. contraction and condensation of a hollow-sphere nebula, in the same manner as the solid one. differences of rotation would be greater in a hollow nebula; because a great deal of the matter would be almost motionless in a solid sphere. in neither case could matter be brought to rest, but only retarded in motion. different periods of rotation accounted for. table of different rates explained. heat produced by gravitation, attraction, and churning, not all constituents of the heat-power of the sun. there can be no matter in the sun so dense as water. the hollow part of the sun acting as a reservoir of gases, heat and pressure. the behaviour of heat produced in the nebula, and its power. how sun-spots are produced. cyclonic motions observed in sun-spots. why not all in certain directions, and why only observed in a very few. cyclonic motions in prominences treated of. many other things might be explained, on some of which we do not dare to venture. concluding observations. at the end of chapter vii., when making some remarks on the heat of the sun produced by gravitation, we said that according to the areolar law the condensation produced thereby would originate difference of rates of rotation in the nebula--provided it did rotate as laplace assumed--depending on its degree of contraction and consequent density increasing as the centre was approached; and that these differences of velocity of rotation would give rise to a churning action in its interior which, owing to the friction caused thereby amongst the particles of its matter, would produce heat over and above what was produced by gravitation alone. again, at the end of chapter xii., we said it would not be difficult to show what tremendous commotions throughout the whole nebula would be produced by these differences of rotation; but that this could not be properly done until we had reconstructed the original nebula, and had shown how from it the solar system might be constructed. now, therefore, that we have set forth, as fully as we can, our ideas of the formation of a hollow nebula and the construction from it of the solar system, we shall proceed to show how heat was, and must still be, produced by the churning action, over and above the definite quantity that could possibly be produced by simple gravitation. and also to show how our notions of the interior of the nebula first, and afterwards of the sun, are simplified and made more natural by looking upon it as a hollow sphere. we will begin by considering, first, what would take place during the contraction and condensation of a rotating nebula solid to the centre--i.e. filled with cosmic matter to the centre--as that is the condition under which such a body has been studied hitherto--as far as we know at least.... not to weary humanity--our own included--by repeating, what almost every one knows, who the parties were and how they came to the conclusion, that by far the greatest part--almost the whole--of the heat expended by the sun, ever since it had any to expend, has been produced by condensation caused by gravitation; we shall for the time being accept this as the general, almost universal, opinion at the present day. if any proof of this being the case is considered necessary, we have only to appeal to sir william thomson's lecture, delivered at the royal institution on january , , in which he showed how a cone of matter, similar to that of which the sun is made, with base at the surface and apex near the centre, falling into a similar hollow cone excavated in his body, would, in descending a certain distance, generate as much heat as would maintain a proportional part of his expenditure for a year; and in which, beyond stating that a very small part might be produced by the fall of meteoric matter on his surface, he makes no mention whatever of any heat-producing power except gravitation pure and simple. the weight of the cone falling into the conical pit alone, produced almost the whole of the desired supply. that this manner of calculation is one of those modes which, as we have said from the very beginning of our work, could never have been adopted had a little more thought been expended on them, can be easily demonstrated even in the case we are now considering. this we say with all due deference to so great an authority; more especially as we know how difficult it is, how seeming unnecessarily laborious, to examine everything to the very bottom; and how pleasant and satisfying it is to feel contented, when we have obtained what suits our purpose. when we began to consider, in chapter xv., what would be the interior construction of the nebula, we supposed, at page , that it had assumed a somewhat globular form when its diameter came to be three times that of the orbit of neptune, which would be , , , miles; and we will return to that supposition to set forth our conception of how heat would be produced in a nebula of that diameter solid to the centre--that is full to the centre of cosmic matter. in that case a particle of matter starting from the surface, under the power of gravitation, would have to travel , , , miles before it reached the centre, and would carry with it a constantly increasing power of producing heat, derived solely from the action of gravitation. next, we have to consider what would stop it when it reached the centre and enable it to give out its heat--for until it was stopped it could give out no heat at all--and the most easily conceived means of stoppage would be to suppose that an equal and similar particle coming in from exactly the opposite side of the nebula met it there. if it was not that it would be something equivalent and much more difficult to describe, while the result would be the same. the result would be that, as each particle came in with equal power of producing heat, the the amount produced when the two met and stopped each other would be just double what each of them brought with it; that is our way of looking at it at least, considering that the velocity with which they met would be just double what each brought with it, and the force of the shock would be double what it would have been had only one of them been stopped in some other way; that other way would have had to give or furnish its half of the shock, and would therefore be able to give out as much heat as the stopped particle. whether two of sir william thomson's cones meeting at the bottom of his pit, from exactly opposite sides of the sun, would have the same effect as we have found for the two particles, may perhaps give rise to the discussion; but we do not see why the result should be in any way different. when a stone falls from a height upon the earth it gives out, in the form of heat, all the heat-producing power it had accumulated in its fall, but we are apt to forget, perhaps have never thought at all of, the why and the how it gives it out, especially of the latter. the why is because it is stopped, and the how is by the earth coming to meet it, and these two ways have an inseparable relation to each other. and if the earth comes to meet it, which it most undoubtedly does, though we cannot measure how far it travels, it must bring along with it an amount of heat-producing power equal to that possessed by the stone, when it in its turn is stopped by the stone; thus the amount of heat arising from the fall of a stone to the earth is, apparently, just double what it is usually estimated to be. this fact comes under the category of splitting hairs or, more truly speaking, of negligible quantities; but the whole mass of the sun falling to the centre cannot enter into that category, and whether we will or no we have to take it all into account. we have conducted two particles of matter from exactly opposite points of the surface of the nebula to its centre, and shown that by simple gravitation a certain amount of heat would be produced by them when they met there and stopped each other; now, we propose to conduct two particles, not far from each other, from one side only of the nebula to the centre, and point out what would happen to them on their voyage thither. the road is long, as we have seen, and during their voyage there would be time enough for a good many things to happen, but we shall only take notice of two for the present, namely, gravitation--of which we have already almost disposed--and attraction; for as far as their journey is concerned there is a very marked difference in the meaning of the two words. gravitation--that is, the action of a ponderable body falling--acts only in a straight line from any point to a centre of attraction, while attraction acts in every possible or imaginable direction. we have already seen what happened to the first particle despatched to the centre under the power of gravitation alone, and have only to say, that the same would happen, under that power, to the two we have now in hand; but attraction--actually the father or mother of gravitation--would have a good deal to do with their journey. from the moment they started--very likely they were practising before they left--they would rush at and continue to bombard each other during the whole voyage. at each encounter or collision, however caused, a certain amount of heat would be produced in each of them which they would carry along with them, and would augment the gravitational quantity they would have to give out when stopped in their fall, in the way we have pointed out would be the only one that could bring them to rest. it may be said that that heat would be left behind in space on the way but space cannot absorb heat unless it contains something to hold it in, and that something could only be similar particles of matter on the same voyage, also creating heat and having as much to dispose of, no doubt, as the two we are conducting. this lateral attraction, so to speak, is really what instituted rotary motion in the nebula, and produced the differences of rotation and the churning action in it with which we shall have to deal presently. having passed under examination the quantity of heat produced by the contraction and condensation of the solar nebula into a globe solid to the centre, we have now to do the same for the case of its being a hollow sphere, and we may say that our work has already almost come to an end; for we have only to vary to a small extent what we have just set forth. beginning then as before, with one particle of matter falling, or rather being attracted, from the surface of a hollow-sphere nebula, we find that it would not reach the centre at all, but would be stopped by another drawn out from the centre by its own attraction, which would meet it--say for brevity--half-way between the starting points of the two, each bringing along with it its own heat-producing power and giving it out to its opponent, there being nothing else to give it to; so that if each brought with it _x_ heat-power they would have _x_ heat-power between them, just as we have said would happen in the first case, and the heat of each one of them would consequently be doubled. in this case we have to observe, though it is really unnecessary, that as yet we have spoken of attraction as acting in one direction only, that is, in doing only the work of gravitation; so we have still to consider the voyage of two particles of matter proceeding from the surface and meeting two coming from the centre, and have only to say that their mutual collisions caused by lateral attraction on the way, would enable them to bring along with them certain quantities of heat produced by these collisions, which would be over and above what they acquired in their straight-line imaginary voyage. if any one doubts that additional heat would be produced by this lateral attraction and bombarding, let him take two hammers and strike the one against the other as rapidly as he can for some time, and he will be able, by touch, to convince himself that heat can be produced by this lateral attraction as well as by the _attraction of gravitation_; and, if he could measure it afterwards, he would find that if he dropped the hammers on the ground, they would not give out any of that heat but only what they had derived from gravitation in falling from his hands to the ground, unless the ground was colder than they, and if the ground was not colder, the heat it had would be augmented from this source also. if the heat produced in both of the cases we have been examining caused differences of rotation in the nebula--as we have said on a former occasion--increasing in velocity as the region was approached where the stopping process came into action, it is clear that these differences would be greater near that region in a hollow-sphere nebula than near the centre of a solid sphere; for the reason that the particles of matter would there have more freedom, that is, more room to act in. we have shown that in the solid sphere the particles would come to be more or less inert, in proportion as they approached the centre; and also that in a hollow-sphere nebula no particle could ever come to be near to a state of rest, but that each could be freely driven by the collisions produced by lateral, angular, universal attraction over every part of the hollow shell--an effect that could by no means be produced in a nebula solid to the centre. we, therefore, think that there would be more life-power in a hollow-sphere sun than in the kind of sun from which all calculations of length of life have hitherto been made--at least, as far as we know. it will be understood that we have spoken of particles of matter being stopped, or stopping each other, before they could give out their heat, only for facility of explanation; for no particle of matter can ever be brought to absolute rest, until all its heat and heat-producing power, _i.e._ motion, could be taken out of it, and that can only be when it is reduced to absolute zero of temperature. cosmic matter could be reduced to the state of rock or steel, but its particles would not be at rest then, or else our ideas of the nature and construction of rock and steel are very erroneous; but it must be acknowledged that it would be much more easily reduced to the state of rock in a body solid to the centre than in the shell of a hollow sphere. in fact it is difficult to conceive how matter could exist at the centre of the sun at the present day without being as solid as rock, considering the enormous pressure it must be subjected to there, if its whole mass is condensing to the centre. but although the particles of the nebula could not be absolutely stopped, they might be so far retarded in their velocities derived from attraction that they would give out heat to each other, and wherever a collision took place there heat would be made evident, and condensation might take place. particles of matter would not have to fall to a centre, but only to a a meeting place, in order to condense and create heat, and might form layers of condensation anywhere between the centre and the surface, either in a solid or hollow sphere, which would ultimately, even in the former case, form a hollow shell, as we have supposed, at page , might be the case. for even a small sphere formed around the centre in that way would be hollow, and would be undone when the different concentric layers approached each other, under proportionate forces of attraction, and formed into one hollow sphere. thus we again come to the conclusion that the formation out of cosmic matter acted upon by the law of attraction, of a sphere full of that matter to the centre would be a mechanical impossibility. in either case the total quantity of heat produced by the contraction and condensation of the nebula would include, not only what has hitherto been looked upon as belonging to gravitation alone, but that other part derived from attraction in all other directions. so the age and duration of the sun still remain to be estimated. we have not said, but we have not forgotten that it may be said that, if in lord kelvin's estimate of the sun's heat, a cone of matter falling in from one side of it was stopped by a similar cone falling in from the exactly opposite side, one half of the sun's mass stopping the other could only produce the amount of heat calculated by him. neither do we deny that the same may be said of the two half-volumes of the sun meeting at the region of greatest density in a hollow sphere, and that the amount of heat produced by gravitation alone would be the same in both cases. all that we have wanted to show is that, in addition to the quantity so produced, the quantity produced by lateral attraction, so to speak, has to be taken into account, in order to estimate the total quantity ever possessed by the sun. referring now to what we have said towards the end of chapter xv., of rotary motion being instituted at the region of greatest density of the nebula, and being propagated from there to all parts both outwards and inwards, we can at once account for the different periods of rotation observed on different parts of the surface of the sun; and not only that, we can assert that these differences of rotation must exist throughout the whole volume and mass of its body up to the present day. we have no need to appeal for producing them to showers of meteors falling on its equatorial regions; neither do we pretend to say that such showers have no part in producing them; but we do say that the part they play in the affair, and the depth to which they can penetrate into the sun's body, must be altogether insignificant compared to what we have pointed out as the true and indisputable cause. we may now proceed to consider what would result from the commotions produced by these differences of rotation in the interior of the sun, and we shall begin by observing that an enormous amount of heat would be produced thereby. the churning action, as we have called it, must be of a very formidable character, for, supposing the whole of the interior to be in a gaseous or gasiform state, it must be effected under a pressure of not less than atmospheres at the surface, and at what pressures as the centre is approached no one can tell; and if the matter in the interior is in a viscous condition, the friction caused by the churning will only be the greater. but let us try to form an idea of what the force, or rather violence, of that churning action must be in the sun if constructed in the manner we are advocating; for which purpose we have to form some definite notion of what is the difference of velocity of rotation at different parts of its circumference, which can hardly be better shown than by table x., in as far as these rotations have been approximately measured. the first thing to be observed in the table is that the rate of rotation at the equator is · miles per minute, and that at lat. ° it is only · miles, giving a difference of · miles per minute in one-fourth part of the sun's circumference, which is a velocity times greater than our fastest express trains. and the next is to note, in the last column, how these · miles of difference, when divided over spaces of ° each, show decreases in velocity of from · at lat. ° to · miles between degrees and . a little thought bestowed on these two points will show what commotions must be produced at the surface by this enormous variation of rotation and make us speculate on how much greater it must be near the poles than at the half distance from the equator. then, if we look upon the sun as a hollow sphere we have to consider that, according to the theory that the condensation of a nebula increases its rotation in proportion to its approach to the region of greatest density, of the velocities of all the rotations expressed in the table, the greatest must be at that region, the others diminishing from there outwards to those of the surface, and inwards to almost nothing at the centre; for we have seen that there must be gases enclosed in the hollow, and that motion must be communicated to them, through friction, down to the very centre. taking all these things into consideration, it is certain that the churning must be very much greater than anything we have thought of up to the present moment, the commotions created more tumultuous, and the heat produced by friction incalculable. table x.--showing the differences in velocity of rotation of the surface of the sun, at distances of ° from each other, from the equator to ° of latitude. -------+----------+--------+--------+--------+--------+------------+ |circumfr. | | | | |retardation | lati- |at each °|time of |rotation|rotation|rotation|in miles per| tude |latitude |rotation|per day |per hour|per min.| minute for | degrees| from | (days) | (miles)| (miles)| (miles)| each | | ° to °.| | | | | degrees. | -------+----------+--------+--------+--------+--------+------------+ | | | | | | | | , , | · | , | | · | ... | | | | | | | | | , , | · | , | | · | · | | | | | | | | | , , | · | , | | · | · | | | | | | | | | , , | · | , | | · | · | | | | | | | | | , , | · | , | | · | · | | | | | | | | | , , | · | , | | · | · | | | | | | | | | , , | · | , | | · | · | | | | | | | | | , , | · | , | | · | · | | | | | | | | | , , | · | , | | · | · | | | | | | | | | , , | · | , | | · | · | -------+----------+--------+--------+--------+--------+------------+ note.--the times of rotation are taken from messrs. newcomb and holden's "astronomy," p. . lest we should have been misunderstood in what we have said a few pages back, and it be thought we consider that all the heat produced by this churning action ought to be added to that produced by gravitation alone, when attempts have been made to compute the total quantity ever possibly possessed by the sun, we have to insist that the idea of gravitation in itself--that is, of matter falling to a centre--is altogether erroneous in connection with the construction of the sun from a nebula, and that it is in truth utterly misleading. we know perfectly well that in the construction of the sun, heat could only be produced, in the main, by bodies colliding with, or rubbing against, each other, and that a large part of that produced by universal attraction must have been expended in producing rotary motion; but we also know that in its construction no particle of matter can ever, as yet, have been brought to the state of rest of solid matter even, that it has still the power of colliding with its neighbours and of producing heat, and that it will continue to preserve that power until it is bound up into a solid state along with its neighbours. even then it will not be absolutely at rest, but will have lost its heat-producing power, and will begin to lose the quantity it then possesses when it gets permission from its neighbours. it is a fallacy, therefore, to suppose that the matter of which the sun is composed has no other heat-producing power than what is derived from its fall, through gravitation alone, from the potential position it held to the centre of the incipient nebula. the only end to heat-producing power is fixed position. if science chooses to fix that position at the centre of the sun, or as near to it as successive particles can reach, there must be any quantity of it in a solid state even now in that neighbourhood, if due consideration is given to the pressure it must be subjected to there. if it chooses to entertain the idea of the sun's being a hollow sphere, somewhat in the form we have described, there can be nothing in its whole body so dense as even water up to the present time. in the first case it has to remember what we have done our best to prove: that _gravitation_ ceases to act when a body falls to a fixed centre or position and can fall no farther. from there it cannot rise except through upper or exterior attraction, and in that case it would leave a hollow space in the place it had occupied. it is altogether illusory to dream of convection currents where no means or force of any other kind than attraction could give rise to them, in which case we should have attraction and gravitation working against each other, two things that have been confounded into one turning out to be antagonistic, as no doubt they sometimes actually are--as we have shown when treating of the discovery of neptune--but when they are so, they never can produce convection currents. in the second case in which, as we have seen, there can be no matter at all near to the solid state or fixed position up to the present day, we can conclude that the life of the sun, measured by heat-producing power, must be very much longer than in the first case, in which a very large part of the matter of which it is composed must have lost that power ages ago. we have still to bring to mind what we have said in chapter xv. of the region of greatest density of the nebula being the region of greatest activity and greatest heat; and to add now, that the whole space between that region and the centre must have been acting as a reservoir--partly material, partly gasiform--of heat, ever since the nebula began to contract and condense, quite independently of its carrying before it the minus or plus sign. from that time that region would be the regulator of the radiation of heat into space, or to wherever it was radiated; because no heat produced on the inner side could escape into space without passing through and acquiring the temperature of that region, or first giving out to the outer side any greater heat that it might have produced and accumulated; facts which involve the necessity of the whole of the interior space, or volume being heated up or lowered down to the same degree before any of it could be transmitted outwards. thus, in addition to all we have said of the means of lengthening the sun's life, we have to take into consideration that all heat radiated from the surface must be conducted, or carried somehow, through a distance somewhere between about , , and , miles, before it could escape into space or elsewhere, according to when it began to be radiated at all. and we have also to take into consideration the probability that the heat produced and accumulated in the inner half of the volume would, by its repulsive force, retard the condensation of the nebula, and thus prolong its heat-giving life. looking back on our description of the construction of the sun, how rotary motion was established in it, and how that motion has produced the different velocities of rotation, not only on the surface where they have been observed and measured, but which must penetrate to the very centre; we may now proceed at the expense of some repetition--in which we have already somewhat indulged--to show how our mode of construction and development enables us to understand a great many things that have been observed in it, much better than we have been able to do from any explanations that have hitherto been available. it gives the most satisfactory reason possible for the sun-spots occupying principally two zones at marked distances from the equator. there is one belt round the equator of ° to ° wide on which we know, from table x., that the differences of velocity of its edges and of those of the contiguous zones, one on either side, hardly exceed mile per minute. towards the poles there are two segments measuring from ° to ° broad, at the borders of which the rotary velocity is slower by · miles per minute than it is at the equator, and · miles per minute slower than at ° less latitude, as also shown by the table. and between the central belt and these segments there are two belts or zones, each ° to ° wide, in which sun-spots are almost only to be found. in these two zones the churning of the interior would be in all its vigour, most probably more active at their centres than where they meet the central belt and the polar segments; where our knowledge of the diminished velocity ceases, but where we have no reason to suppose that it actually stops. were the period of rotation the same throughout the whole body of the sun--with the exception of what has hitherto been considered to be a mere surface difference produced by external causes--we could conceive that the heat produced solely by condensation would find its way to the surface equally in all directions, even bubble up all round like steam rising from the surface of the water in a boiler, in this way forming what is called the sierra; and that there would be neither sun-spots nor eruptive prominences, hardly any of the violent movements recorded in works on astronomy. but the churning action we have been exhibiting, extending to the deepest recesses of the sun, must produce commotions quite adequate to give birth to the most violent phenomena that have been recorded. viscous gases and vapours, gasiform vapours, ground against each other at depths of hundreds of thousands of miles, under pressures of hundreds, much more likely of many thousands, of atmospheres, and confined by superincumbent strata, so to speak, would acquire a dynamitical explosive force that could be conceived to be powerful enough to rend the sun into fragments, were it composed of anything comparable to solid matter. on the other hand, the friction of the solar matter operated under the pressure of atmospheres at the surface, and up to the unknowable number at the greatest depth, converted into heat, would have explosive energy enough to give rise to all the phenomena that have been observed; from the veiled spot to professor young's prominence, which was thrown up to the height of , miles above the photosphere. a veiled spot seems to be one that has broken through the photosphere, perhaps not even entirely, but not through the light or white clouds which float immediately over it; which, in consequence, goes a long way to prove that sun-spots have their origin in up-rushes of heated vapours from beneath; for a downfall of cooled metallic or other vapours would break through the light clouds first of all; and which is confirmed, as far as anything in solar physics can be confirmed, by what we are exposing. that there is a down-rush also, goes without saying, because there is no other way of giving account of what becomes of the vapours of metals and other elementary substances brought up by the outpours of heat, after they are cooled in the solar atmosphere. that they should fall down into the same opening they had made in rising up, is the most natural supposition that can be made; for, otherwise, they would have to be carried beyond, or outside of, the spot before falling. moreover, a sun-spot is said to be generally surrounded by prominences which bring up vapours of elementary substances, that we must believe to be much heavier than those from eruptions of sun-spots, because they issue much more violently, showing that they must have been expelled by much greater force, which must form a sort of wall all round the spot through which the matter, thrown out by it, would have to be carried before it could be deposited; and outside these walls there are no visible signs of where it falls, so that we are forced to believe that all the substances, those from prominences as well as those from sun-spots, fall into the same general receptacle. surely it could not be argued that there can be no eruptions from a sun-spot, seeing that the force required to drive matter through it must be less than when it is expelled from depths very much greater than the depths of the spots. thus we have both up-rush and down-rush in sun-spots accounted for very plainly; and they are always large enough for both operations being carried on at the same time. besides, they have been credited by eminent astronomers with the faculty of sucking in the cooled vapours from the surrounding prominences into the common pit. in some sun-spots, said to be about per cent. of those observed, cyclonic motions have been observed in the umbræ and penumbræ, which under the churning process might be expected to be universal in all of them, but it is not necessarily so; even leaving out the consideration of the difficulty of detecting them. we see in a deep smooth-flowing river eddies revolving in all directions, caused by currents of different velocities approaching each other, quite independent of the form of the banks of the river or obstructions in the places where we see them, but without doubt derived from sources of that kind higher up in the river; and so it may be with cyclonic motions in the sun-spots. the velocity and direction might be given to the vaporous matter by the churning action before issuing into the spot, which would cause eddies in it in all directions, the same as those in the water of the river. it would be absurd to think that in a space so immense as the bottom of a sun-spot, there should be only one orifice of emission of vaporous matter: there might be any number; consequently, there may be times when the out-flowing currents annul each other and none at all are seen, or when there are partial currents in any direction; others when they may be all so uniform as to produce a cyclonic motion all round a spot, or nearly all round it, or two or more in opposite directions, all as has been recorded on more than one occasion. neither could it be supposed that any cyclonic motion, caused by the churning, could depend on which side of the equator the spot was formed in. there must be little churning going on under the surface at the equatorial belt, hence the paucity of spots there; but between the surface and the centre there must be some point of meeting of the motions that are produced on each side of the equator which, even were there no special reason for it, would destroy all chance of uniformity, or distinctive direction, in the upheaved matter when it arrived at the surface, let it reach that place on whichever side of the equator it might. the original salient motion at the bottom of a sun-spot might be to right or left, or according as the material from which it proceeded had been tumbled about, and the issuing motion might also be controlled greatly by the form and position of the orifice, or rather tunnel, through which it escaped. common churning, we know, could not drive all the milk in one direction, even were the paddles of the churn solid; and in our case, the paddles have to be looked upon as even more divided, magnitude for magnitude, than they are in an ordinary churn, for the matter itself forms the paddles. the cyclonic motions observed in prominences must come from the same causes, and ought to be more general in them, seeing that they must proceed from apertures much fewer in number than in the sun-spots, and very probably from one orifice in the case of jet prominences. one would expect also that these cyclonic motions would be more regular in the prominences, from being generated deeper down in the interior than those of the sun-spots, and less affected by the motions they encountered on their way out, owing to the great original energy required to force them through the superincumbent mass of matter, and might even have--in jet prominences especially--the motion to be expected according to the hemisphere from which they proceeded. but we have already said that, deep in the interior, the churning motion may be in any direction whatever. it is natural to suppose that the highest prominences are ejected from the greatest depths, because they require the greatest ejective force to throw them to such immense heights, and because the greatest ejective force must be where the heat and pressure are greatest, that is, at the densest and most active depths. and probably the reason why prominences generally surround sun-spots is that they have had their exits facilitated by the relief from pressure, brought about by the discharge of churned matter into them (the sun-spots), and thus, as it were, attracting the eruptions of the prominences towards them. we had almost omitted to say that the churning theory would very well account for almost every sun-spot having more or less proper motion of its own independent of all others, and for all of them drifting towards the central belt, or towards the polar segments when they begin to dissolve and disappear. there are many other things in connection with the sun that could be explained through our mode of construction, some of which are so evident that they will occur to anyone, and others that lead into depths too profound for us to enter. to conclude. the construction of the sun we have set forth would be of great service towards the completion of either of what professor a. c. young calls the competing theories of m. faye and fr. secchi, in which the former would find the origin of the solar storms, to which he appeals for producing sun-spots in particular zones, and a better way of accounting for the differences in velocity of rotation between the equator and the poles than in the depths of the strata between these regions; and the latter the means of forming the dense clouds of eruption which he assumes to form sun-spots by settling down into the photosphere. but theorists seem to be partially right by a divination, and to have only failed through their not having found out the sources of the powers they called into existence, in order to have some foundation to build their theories upon. chapter xviii. page return to the peaks abandoned by the original nebula. an idea of their number. example of their dimensions. what was made out of them. what could be made from one of them. how it could be divided into comets and meteor swarms. an example given. how a comet may rotate on its axis. and what might be explained thereby. orbits and periods of revolution. not ejected from planets. their true origin. study of the velocities in orbit of comets, and results thereof. how far comets may wander from the sun and return again. no reason why comets should wander from one sun to another. confirmatory of the description, in chapter xv. of the sun's domains. of the eternal evolution and involution of matter. the atmosphere and corona of the sun. partial analogy between it and the earth's atmosphere. the density of it near the sun's surface cannot be normally less than atmospheres, but might be so partially and accidentally. probable causes of the enormous height of its atmosphere. the mass taken into account, but cannot be valued. most probably no matter in the sun exceeds half the density of water. the unknown line in the spectrum of the corona belongs to the ether. when we were attempting to describe in some measure the region of space from which the sun obtained the nebulous matter out of which it was formed, we found that it would produce a nebula somewhat resembling a most gigantic starfish, with arms or legs stretching out from it in every direction, which might be likened to mountain-peaks rising from a tableland or range of mountains; and when we began to condense the nebula we concluded that these peaks would very soon, comparatively, be left behind the main condensation, owing to their being more under the influence of the attraction of surrounding suns. and we might then have added less under the attraction of the main body, on account of its gradually increasing distance arising from its greater rapidity of contraction. now, we propose to return to these portions of the sun's property so long left out in the cold, to think of what in all probability became of them, seeing that they must all have had somehow a part of some kind to take in the formation of the solar system. first of all, we have to form some idea, however vague, of their number, which may be divined to a very limited extent from the following considerations: we see, from table viii., that the sun's sphere of attraction extends to more than neptune distances in the direction of [greek: a] centauri, the star nearest to the earth, which corresponds to billions of miles. then, although we have said, in chapter xv., that instead of there being a peak on the nebula in that direction there would be a deep hollow in it, we shall proceed to find out what might be the diameter of the base of a peak at that distance supposing it to be somewhat in the form of a cone. we know that the moon does more than eclipse the sun, which is , miles in diameter; so, for facility of calculation, we may suppose that it eclipses a portion of space at its distance of , , miles in diameter. consequently, the base of a peak such as we are measuring would be eclipsed were it , millions of miles in diameter, and then only. moreover, we have deduced the diameter of the base of such a peak from _one_ diameter of the moon; so that wherever we see two stars only one breadth of the moon from each other, there we have room for at least one peak with a base of the above diameter. last of all, when we come to think that there are as many as six to seven thousand stars visible to the naked eye, and of the intervening spaces between them, we have to conclude that the number of peaks surrounding the original nebula before they began to be left behind, or cut off, must have been almost beyond our conception; more especially if we look at table vii., where we see that the star canopus is times farther from the sun than [greek: a] centauri. we are accustomed to look with wonder on the volcanic peaks of the moon, but they can do nothing more than give us an exceedingly faint representation of the original nebula seen from an appropriate distance outside, when it had begun to contract more rapidly than the peaks could follow it; seeing that we are comparing a diameter of , miles with one really almost infinitely greater. finding ourselves, then, with an innumerable host of peaks, or cones, of cosmic matter on our hands, we have to think of what can be done with them, and we begin by saying that the use to be made of them was suggested to us when we discovered the jagged nature of the domains of the sun. some of them have been most probably swallowed up in the formation of the sun, and could we believe in the plenum of meteorites in all space, that has been fancied to exist by some physicists, we might derive its origin from a part of these peaks; but if there can be such a plenum in space, its origin might be much more naturally derived from a suggestion made in a former chapter, at page , to which we shall refer presently. in the meantime, looking upon the multitude of comets, meteor-swarms, etc., which revolve around the sun, or are supposed to exist somehow in its neighbourhood, it is very natural to entertain the belief that they have been made out of some of the most important peaks--or the refuse from them--that must have formed part of the original nebula. to deal with all of them when we cannot number them, or even with the six of table viii., about which we actually know something, is out of the question, so we shall only try to show what could be made out of one of them. confining ourselves, then, to the peak of [greek: a] geminorum, whose collecting ground had originally reached to , neptune distances, or billions of miles--this being the point of space where the attractions of the sun and that star balance each other--if we suppose it to have been contracted till its base was of the same diameter, and its distance the same from the sun, as that of the base of the peak we measured not many minutes ago, , million miles, and billions of miles, respectively, we can easily conceive that its height may have been times as great as the diameter of the base, or more than - / billions of miles. here then we have in the direction of only one star a mass of cosmic matter out of which something more than a comet, even of the grandest known to modern astronomy, could be made. of its tenuity, all that we have any necessity to think is, that it would be much less--i.e. more dense--than that of the original nebula. beginning then with the dimensions we have just stated, we know that the attraction of the nebula would draw the matter of the base-end of the peak more rapidly towards itself than that of the apex-end; we know also that there would be different rates of contraction going on in different parts of the length of the peak--for the same reason we have given for the peaks being cut off from the nebula; so that the condensation throughout its whole height, or length, would be proceeding at different rates at different places, which would certainly divide the peak into several parts, perhaps into many. if now we suppose that the leading part of it--the one nearest to the nebula or sun--or even the whole of it, formed itself into a comet, it is not difficult to see that it might have a tail infinitely longer than any comet the length of whose tail has been measured. there can be no doubt that in the whole length of the peak the action of attraction would be exactly the same as we have found it to be in the nebula itself; that is to say there is no reason why it should not come to be a hollow cone--comets are reported to be hollow in most cases--condensed into layers, and to revolve on their axes throughout a great part at least of where their diameters are greatest. this mode of formation seems to throw light on some of the phenomena that have been observed in comets. we have just said that our peak would be divided into several parts, so if we suppose the leading part of it to have been made into a comet, we can see why its tail should have the appearance of a hollow cylinder; and there might be no reason why the second division, or even the third, should not become a comet also. then for further divisions, where the diameter came to be too small to make a comet, its matter might have formed itself into a meteor-swarm, and account for the fact of some comets and meteor-swarms revolving round the sun in the same orbits; perhaps even for some of the observed meteor-swarms being denser at one part than another, owing to two or more of the sections of the peak following each other at some distance. we have to notice, after what we have just said, that it is quite possible that if the different sections of our peak did come to revolve round the sun, their perihelion distances might be so different that it would be impossible to trace any connection between them and the peak from which they were derived. but if we were to attempt to set forth all the explanations of the phenomena of comets and meteor-swarms that have occurred to us, there would be no end to our labour. passing now from one to the whole host of peaks, we have seen that at one time they projected from all sides of the nebula; it is clear, therefore, that the bodies formed from them must have fallen in towards the sun from all directions, which is exactly what they have been found to do. then, if we think of the multitude of them there would be, we have also to think that there would most certainly be collisions among them, which would smash them to atoms, and thus help to make the plenum, or host of independent meteorites that are supposed to exist, or would be swallowed up by the sun in mouthfuls. others might coalesce, which they could only do through coming in from slightly different directions and with nearly similar velocities; and they would thus account to us for comets with a plurality of tails. again, looking back to what we have just said of the form that might be assumed by the leading end of the peak [greek: a] geminorum, which was suggested by donati's comet, we could imagine another, the same in almost all respects, coalescing with it, and between the two showing us how coggia's comet was formed. furthermore, with respect to one of the gigantic comets with endless tails: if we suppose it to rotate on its axis, and to be not so smooth on its outside as a cone formed in a turning lathe, we could account for the light from the sun reflected from it having an appearance of flickering; and, were the outside very rough, for the reflected light flashing from millions of miles of its length in a few seconds. all this about nebular peaks, comets, etc. formed from them, will, far more than likely, be looked upon as imagination or speculation run mad; but if it is looked into properly, it will be found that no part of it is based on assumption; farther than that, the universe has been formed out of cosmic matter of some kind. there is no step in the whole process, from cosmic matter to the sun--even myriads of suns--that does not conform to what are generally called the laws of nature; whereas it is not difficult to show that some other speculations on the same subject have never been carried beyond the stage of conception. when thinking of how comets might be formed, we could not help thinking of their orbits and periods of revolution. it was easy to see that their orbits depended on where, and how far, they came from; that the where might be from any and every direction, and that the how far would be the principal element in their greater or lesser ellipticity, which could only be determined by measurement; but their periods of revolution, as far as we can see, could only be determined by observation, which would involve the study of several revolutions. on these points the data we have been able to collect are not very satisfying, neither are they given to us as very reliable, except as to those whose orbits have been often observed and measured; and even among these the orbits are said to vary, and some of the comets to disappear altogether. again, some of them are said to have a disposition to become associated with particular planets; and yet again, some people have gone the length of supposing that they have been ejected from some of the planets. to us it seems much more rational to suppose that the known periodical comets have been made out of part of the multitude of peaks which must have surrounded the nebula at one time, if the sun was formed out of nebulous matter, subject to the attraction of similar matter surrounding it on all sides. it seems to be only a way of getting out of a difficulty to suppose that matter ejected from, say the earth, with a velocity of miles per second would be freed from its attraction, that it would be involved somehow in the sun's attraction, and that it would revolve thenceforth round the sun like any other wanderer; because we cannot see what would stop its progress upwards, so to speak, from the earth after getting beyond its control, or communicate to it at the right height and time, the exact velocity required to make it revolve for ever afterwards round the sun; nor, supposing the sun would have nothing to do with it, where it would go. when it left the earth, it might have a direct motion of near one-third of a mile per second derived from its rotation, and also one of miles per second due to the revolution of the earth round the sun. it might also be ejected in a direction exactly away from, or directly towards the sun; so we should have two very different cases to reconcile in order to set up the theory of ejection of comets from planets, and of their being involved somehow in the sun's attraction. it presents us with a very strong case for calling for either the immediate intervention of some power other than what we conceive attraction to be, and of which we know nothing physically, or we have to trust in _man_ipulation of which we have no very exalted idea. we prefer to look upon the formation of all comets as derived from the peaks we have been treating of, or, if that is inadmissible, from shreds and patches of the original nebula; where no immediate intervention, or instant application, of supernatural power is required, but only the even and tranquil operation of original design. for comets larger, and which travel to greater distances, than those alluded to above, it is very difficult to get data on which we can form satisfactory calculations of the lengths of their orbits and mean velocities of revolution, for there is almost always awanting some one or more of their elements, or totally different statements given of their value; but we think we have found a few from which we can collect data sufficiently accurate to enable us to show that there is no necessity for going beyond the domains of the sun, as described by us in a former chapter, to account for any one of the comets which have been taken notice of in astronomical history; and still less necessity to suppose that any of them have wandered, or been shot forth, from some neighbouring star into the solar system. from the data we have been able to collect it would appear that when a comet comes to have a period of over years, it is either too far removed from the sun at its aphelion passage, or its mass is too great for it to be perturbed by the attraction of any of the planets. for instance, we have halley's comet, which has been observed for not far from years, whose period has averaged very close upon years during the whole of that time, showing that it has not been perturbed to any appreciable extent when near its perihelion passage. no doubt years is a very small period of time to judge from, and its aphelion distance being only , , , miles, it might be influenced to some extent by some planet, so we can hardly count upon its being permanently exempt from perturbation. indeed, halley himself supposed that its velocity of revolution had been considerably increased when it was in the neighbourhood of jupiter in the interval between and ; but if it was so, there must be some counter-perturbation which restores the balance so as to make the average period of years. looking over the register of its appearances, we find that in its re-appearances of the years and , the period was about years, and that in those of and it was years; so that if there are perturbations, we must claim that there are also compensations. seeing, then, that we can find no evidence to the contrary, we may suppose that when the periods of comets, and, perhaps more especially, when their aphelion distances reach to beyond--and the farther the more so--the orbit of the most distant planet, they may be looked upon as not being liable to be seriously perturbed by any of the members of the solar system, until something to the contrary had been proved. following this idea, then, it occurs to us that something may be learnt from their mean velocities in their orbits, as will be seen from the following very small list of those we have been able to submit to calculation, which form the accompanying table xi.--showing the mean velocities in orbit of several comets. ----------------------+-----------------+----------+---------------+ | | period of| mean velocity | designation of comet. |aphelion distance|revolution| in orbit. | | (miles) | (years) | (miles/sec) | ----------------------+-----------------+----------+---------------+ halley's comet | , , , | | · | | | | | comet of & | , , , | | · | | | | | donati's comet | , , , | , | · | | | | | comet of | , , , | , | · | | | | | comet of | , , , | , | · | ----------------------+-----------------+----------+---------------+ these orbital mean velocities per second have been calculated from aphelion distances as diameters and from circular orbits, which probably give results rather lower than would be derived from elliptical orbits--were they known--but on the other hand, the perihelion distances have not been taken into account in fixing the diameters--because they were unknown--so the error will be so far compensated, if not altogether. we know that the mean velocities in orbit of the planets decrease as their distances from the sun increase, and our table, as far as it goes, leads us to believe that the same holds good with comets whose aphelion distances are comparable to those of the planets, in being measured by hundreds of years or less of revolution; but with those whose periods are measured by thousands of years, the same rule seems to fail. one thing, however, that we seem entitled to believe is that, generally speaking, the greater the period of revolution of a comet is, the less will be its mean velocity per second in its orbit. it will be observed that the average mean velocity of the three remote comets in the table is only · mile per second, and it is by no means unreasonable to suppose that the average mean velocity per second of any number of comets whose aphelion distances are greater than the highest of those in the table, is not likely to be so great as the average of the three; on this understanding, then, let us take, or suppose, one whose mean velocity in orbit per second is only one mile, and look into what may be learnt from it. going back to the peak of [greek: a] geminorum which we supposed, at page , to be condensed to , million miles in diameter of base, its height - / billion miles, and distance from the sun billion miles, we may take a comet formed from it as an example. if, then, we suppose the leading part of it to have been formed into a comet with that aphelion distance-- billion miles--and other dimensions suitable to its new condition; taking its mean velocity in orbit at mile per second, we find that its period of revolution might be , , years, or three times greater than that of the comet of , namely , years, mentioned by mr. chambers as being not very reliable, probably because its angles in orbit could not be measured with sufficient accuracy. then, when we think that the sphere of the sun's attraction in that direction--of [greek: a] geminorum--extends to billions of miles, and that there are stars more than times farther off, e.g. canopus, see table vii., we see that a supposed comet might have an aphelion distance equal to that; and were we further to consider that were its major axis billion miles long, including aphelion and perihelion distances, and that it went straight from the one end of it to the other and back again, its period of revolution, if it could be so called, would be , , years; that is times greater than mr. chambers's doubtful , years for the comet of . there seems, therefore, to be no necessity for the solar system sending its cometary produce to a foreign market; and our mechanical imagination is not sufficiently vivid to allow us to conceive what kind of potential energy even jupiter can have to give an impetus to a comet, great enough to send it flying to so great a distance. what velocity would it have when it left the sun? and what would remain in it to carry it over the debatable land between the sun and a distant neighbour? or are we to believe that all the solar system's produce of that kind is only sent over the channel, as it were, to our nearest neighbour, [greek: a] centauri? conceptions of that kind are too elevated for us, and we must leave them alone. mr. chambers expresses doubts as to the determination of whether the orbit of a comet is elliptical or parabolic when its period of revolution is measured by hundreds of thousands of years, and we think we are safe in following him until actual proofs are presented. if the comet of never comes back, we may then believe it has gone elsewhere. having used up all the nebulous matter in the sun's domains, as described at the beginning of chapter xv., or at least shown how it may have been, or may yet be, used up, we have now only to make a few remarks to prove that our description of the said domains is not by any means fanciful. it matters very little whether the solar system was begun to be brought into existence at the same time as the surrounding systems or before or after them. what is certain is that the sun's sphere of attraction among its neighbours is bounded, at the present time, just in the way we have taken to describe its domains. how they were filled with cosmic matter may be disputed, but filled they must have been somehow, if the solar system was formed out of a nebula; and the way adopted by us was the only one that occurred to us when we began to reconstruct the original nebula. since then we have had time to reflect on our work, and to see how it points out the simplest way that can be conceived, which may be expressed in the few following words. we may suppose that the ether was the primitive matter, as we have done at page , and that the whole material universe has been formed from it and through it. this idea will assist physicists in forming their theory of a plenum of meteorites or meteoric matter, if such they choose to call it. it will also enable us to complete the circle of our notions with respect to matter. we believe that we can neither destroy nor produce the smallest portion of it, although we can change its form. thus, looking upon the ether as primitive matter, we can understand how the solar system could be elaborated from it; and how, after having accomplished the purposes for which it was brought into existence, it may again be resolved into the primitive element out of which it was made, ready to take its part in the evolution of some other system with, perhaps, a new earth "without form and void." we have now to direct our thoughts, as far as we can, to the mass, which furnishes the really effective power of the sun as the ruler of the system; and, first of all, we have to think of what are the real active elements which form that mass. hitherto we have looked upon them as all included within a diameter of , miles, but now we have to take notice of the clouds of meteoric matter which have been supposed by some astronomers and physicists to be revolving round the sun and continually raining into it; and of the enormous atmosphere which surrounds it. with regard to the former of these two elements, we shall compound our ignorance by looking upon it as a merchant does on his account of bills receivable, as not being available in the case of a sudden demand for cash, and therefore as not forming a part of the mass, any more than as the attraction of the earth aids the sun in its management of the planet neptune; the same as the bills receivable strengthen the credit of the merchant. but with regard to the second element of the two, we must recognise that it forms part of the mass and power over the whole of the system, and from all that is known about it we are not authorised to look upon it as a negligible quantity. it so happens that the only thing we have to which we can compare it is the atmosphere of the earth, and we immediately find that there is absolutely nothing to be learnt from such a comparison. we know that one-half of the weight or mass of the earth's atmosphere is contained in a belt of - / miles high above its surface, so that double the volume of that belt estimated at atmospheric pressure gives us the true measure of its mass. this mass, when reduced to the density of water, and compared to that of the earth as we have dealt with it all along, turns out to be about / , th part of it; and were we now to add that to the earth's mass we have been using, its mean density would be · instead · times that of water. now, let us suppose the sun to have an atmosphere of the same kind as the earth's: seeing that the force of gravity at its surface is about times greater than it is at the surface of the earth, a belt around it which would contain one-half of its mass would be × - / = miles, or say miles thick. dealing then with this dimension in the same manner as we have done in the case of the earth, we find that its supposed atmosphere would be / , th part of its mass, which, if added to the mass we have used for it, would make its mean density · instead of · times that of water. then again, if we suppose the earth's atmosphere to extend to or miles above its surface, the supposed atmosphere of the sun would extend to or miles above its surface, according to which of the above heights on the earth is adopted; whereas the highest of our authorities say that the corona, or apparent atmosphere, extends to at least , miles from its surface. it would appear then that there is no analogy whatever between the atmospheres of the sun and the earth; but there must be some analogy, because the law of attraction cannot be suppressed at the surface of the sun; neither can any vaporous matter near it cease to be attracted in the same proportion as it is at the surface. our atmosphere causes a pressure of - / inches of mercury at the earth's surface, and the attraction of the sun at its surface must cause a pressure equal to nearly times that without fail, i.e. lb. per square inch instead of the lb. of the earth. we know that some spectroscopists believe that the pressure at the surface of the sun is sometimes as low as it is at the surface of the earth, even lower; but we require an explanation of why it is so. at the surface of the sun one second of arc corresponds to a height of miles above its surface, and mr. proctor states in his "sun," page , that if even "two or three hundred miles separated the lower limit of chromatosphere from the photosphere, no telescopes we possess could suffice (when supplied with suitable spectroscopic appliances) to reveal any trace of this space. a width of two hundred miles at the sun's distance subtends an arc of less than half a second; and telescopists, who know the difficulty of separating a double star whose components lie so close as this, will readily understand that a corresponding arc upon the sun would be altogether unrecognisable." we can understand this, and perhaps find an explanation for ourselves. according to our supposition that the sun may have an atmosphere similar to the earth's, at one hundred miles in height it would be reduced in pressure to atmospheres, and, extending the analogy, at miles high the pressure would still be equal to one-eighth of atmospheres, or equal to something less than lb. per square inch at the surface of the earth; so that if spectroscopists have measured the sun's atmosphere at the disk, and found it to be lower than the earth's at its surface, their results must have been caused by some fortuitous circumstance which they did not notice at the time; because the force of attraction at the surface of the sun can never be overcome except by some counteracting force, which, if in the form of a vapour, or what we call a gas, issuing from its interior, would increase rather than diminish the pressure. we know that in the heart of a cyclone on the earth there is sometimes a vacuum sufficient to explode (pull out the walls of) houses near which it passes; and, at the same time, we know, more or less, what heat the sun sheds upon the outer atmosphere of the earth, and also the rate of rotation of the earth in the regions where the fiercest of these cyclones occur, the only two causes which can produce them. now, if we compare these causes in the two bodies, that is, the earth's rotation of about miles per minute and the sun's of, say, to miles per minute, and the temperatures of the sun and the earth at their respective surfaces, we can imagine that in the heart of a cyclone on the sun there may be a vacuum much nearer absolute zero than there can be in any one on the surface of the earth. if then the spectroscopists, without knowing it, have caught the spectra of the hearts of cyclones, we can conceive them to be right, otherwise no. again, we know that when big guns are fired off partial vacuums are formed near them, sufficient to cause disaster to windows, doors, and even walls of houses too near them, but whatever we may have said of force sufficient to produce explosions in the sun, we have never believed that matter is ejected from the sun by explosions. we have supposed the sierra, or chromosphere, to have oozed out through its pores, sometimes to less, sometimes to greater heights, like steam from an open boiler, and the prominences to be eruptive, neither of which modes could produce anything approaching to vacua in their neighbourhoods. there can be no resemblance between the ejection of matter or gas from the sun and from a cannon, but there is between the ejection of vapours and the escape of steam from the safety-valve of a closed steam boiler; both of them continue to pour out their vapours till the pressure within falls down till it is equal to the resistance to their escape; there is no explosion, therefore no vacuum, appreciable at least, in the neighbourhood. there may be surrounding matter drawn up by the velocity of the outward current, but that is all. notwithstanding all this, we see no reason why the sun should not have an atmosphere of exactly the same kind as the earth's, composed of exactly the same kinds of gases, including vapour of water in some part of it, though, perhaps, far removed from the photosphere. every other element found on the earth can be found in the sun, and so it is not unreasonable to suppose that the same kind of atmosphere may exist upon it; we have only to acknowledge that its conditions must be somewhat varied, all the difference being that the atmosphere of the sun must be heated up to the temperature of the photosphere where it comes in contact with it, while that of the earth is only of the temperature of the earth at its surface. in the case of the earth, if this were at a white heat, one-half of the weight of its atmosphere would not be comprehended in a belt around it of - / miles thick. that balance of mass might take place at a height of even hundreds of miles--we have no means of calculating how high--and still its pressure at the surface would be the same as now, as long as the earth's attraction remained the same; so must it be with the sun. instead of limiting its height to miles at the utmost as we have done above, it would be no stretch of imagination to suppose that it might extend to ten, twenty, or more times that height. in addition to this we have to take into consideration that the sun's atmosphere must be swept up to something far beyond miles high by the whirlwinds created by the velocity of rotation at its surface, the same as we saw the earth's might be when we were explaining how an aurora could be made to glow at heights far beyond what we were accustomed to believe its atmosphere could reach. adding, then, together these two motive forces for elevating the atmosphere of the sun, it would be a bold assertion to say that it cannot have one exactly similar to the earth's, reaching up to the height of , miles mentioned a few pages back. and now, having got this length, we may venture to assert that the corona of the sun is made up of this atmosphere, and of the vapours of the elements thrown out from its interior, somewhat in the manner we have described in last chapter; to which we have only to add that the bubbling up of vapours all around the sun, which produces the sierra or chromosphere, would not be interfered with in any way by the tremendous commotions which we have shown must be produced between the surfaces of the sun-spot zones and the centre; and that the projection of the high prominences would assist in elevating the aeriform atmosphere. if then the sun has a compound atmosphere of this kind, it must be considerably more dense, proportionately, than that of the earth, and will consequently form a greater addition to its mass than we have found would be made by its airlike atmosphere. but, whatever density has to be added to it on that account has to be subtracted from the interior having been ejected from thence; because, in whatever manner its mass has been calculated in respect of the other members of the system, the total amount must turn out to be always the same. we have always estimated its mass from a diameter of , miles, which gave us a volume of , , ^{ } cubic miles, so that if we now include in the diameter the , miles height of the atmosphere, we get a volume of , , ^{ } cubic miles, which is as near as possible six times the volume in which we had to distribute the volume of the sun. how to do this, we know not. we cannot fix the region of greatest density in the same manner we have done at page , but we know that it must be considerably nearer to the surface of the photosphere than we have there placed it; and of one thing we are sure, and that is, that the densities we have named for that region and the outer and inner surfaces of the shell, at page , must be less than those there expressed; how much we cannot calculate, but we have certainly found that the limits must be lower, and that most probably there is no matter in the sun exceeding the half of the density of water. whatever the composition of the sun's atmosphere, or corona if that name be preferred, may be, spectroscopists have found in it a _spectral_ line derived from some substance totally unknown to science. now, looking back on our work from almost the very beginning, it seems to have been gradually borne in upon us that this unknown substance is the ether. that it is a material substance we were hardly ever in doubt, and our studies of it have substantiated and confirmed our belief. in our analysis of the nebular hypothesis in chapter vi., after combating the notion that the light of nebulæ is occasioned by incandescent gas, we showed, by the example of an air furnace, that an incandescent gas is composed of two elements, one consisting of solid matter which takes up and gives out heat and has all the properties of a heated solid or liquid substance, and the other of gaseous matter which, being the element that fills up the empty spaces between the solid atoms of a gas or vapour, only performs the office of carrying the solid part into the furnace. this forced upon us the idea of the gaseous part being a carrying agent, and very naturally to think of its being really the ether, that being the only acknowledged agent for the carriage of light, heat, and electricity, two of which are easily seen and felt, and the third cannot be awanting, in an air furnace. again, when treating in chapter vii. of what effect the ether might have on the density of the original nebula, we concluded that its density must be much lower than what we then knew it had been estimated to be, and also that its temperature in space must be lower than - °; which two circumstances combined showed us that if it is a gaseous substance it must be very different to any gas that had been liquefied up to that time. this we repeated in great part in chapter xii., calling attention to the peculiarity of its being able to carry a higher temperature than its own--to all appearance--into a "hot box." then we have dedicated two chapters, xiii. and xiv., almost exclusively to the study of the ether, and have been led from one stage to another to look upon it as the only substance that agrees with the definition of a gas as given by science; true gas there is; as the primitive and sole element in the formation of all matter and in the evolution of the universe; and what is something more than an unfounded guess, as the mysterious and incomprehensible agent attraction, unfortunately almost universally spoken of as gravitation. and now to conclude: from what we have been able to learn, very slight differences have been found in various spectra of the position of the line representing the unknown substance, but this can cause very little doubt of its always being the same, as spectra often contain several lines of hydrogen, owing most probably to combinations with other substances; and if the ether is the primitive chemical element, there may be slight differences in the position of its line, as shown in all the phases in which we seem to have found it, but they must be slight as compared with the hydrogen lines, because even these must be in some measure, perhaps even great, influenced by the unfailing and inevitable mixture of the ether in their composition. london: printed by william clowes and sons, limited, great windmill street, w., and duke street, stamford street, s.e. transcriber note several tables were reformatted to fit a maximum -column width. a plan for securing observations of the variable stars. by edward c. pickering, director of the harvard college observatory. cambridge: john wilson and son. university press . a plan for securing observations of the variable stars. ________ for several reasons the investigations here proposed are especially suited to observers under very various conditions. the work is capable of indefinite sub-division. small as well as large telescopes may be employed and many observations are needed which can best be made with an opera-glass or field-glass, or even with the naked eye. no attachment is needed to an ordinary telescope, so that no additional expense on this account is required. useful observations may be made by an unskilled observer provided that he is capable of identifying a star with certainty. the work is quantitative, and the observer has, therefore, a continual test of the increased accuracy he has acquired by practice. as a portion of the investigation will probably lead to the discovery of interesting objects, the observations will possess an interest often wanting in quantitative research. the aid of the professional astronomer is earnestly requested for this scheme. suggestions by which it may be modified and improved will be gratefully received. the professional astronomer, in consequence of his greater skill, instrumental appliances, and command of his own time, could fill gaps in the work, and thus greatly increase its value as a whole. such observations could often be made in the intervals of other work or at times unsuitable for the observations to which he was especially devoting himself. it should be added that especial care will be taken not to interfere with observations of variable stars now in progress. observers of these objects are particularly requested to notify the writer what work they propose to carry out, so that a needless repetition of it may be avoided. it is on the amateur and student of astronomy that we must depend largely for the success of the plan here proposed. many such persons spend evening after evening at their telescopes without obtaining results of any permanent value. either no publication is made and the results are therefore valueless, or time is spent on objects that can be much more usefully examined with a larger instrument. most commonly the observer has no special plan and spends many hours without result, while the same time might have been employed with equal pleasure to himself and results of great value collected. those who have not tried it do not realize the growing interest in a systematic research and the satisfaction in feeling that by one's own labors the sum of human knowledge has been increased. much valuable assistance might be rendered by a class whose aid in such work has usually been overlooked. many ladies are interested in astronomy and own telescopes, but with two or three noteworthy exceptions their contributions to the science have been almost nothing. many of them have the time and inclination for such work, and especially among the graduates of women's colleges are many who have had abundant training to make excellent observers. as the work may be done at home, even from an open window, provided the room has the temperature of the outer air, there seems to be no reason why they should not thus make an advantageous use of their skill. it is believed that it is only necessary to point the way to secure most valuable assistance. the criticism is often made by the opponents of the higher education of women that, while they are capable of following others as far as men can, they originate almost nothing, so that human knowledge is not advanced by their work. this reproach would be well answered could we point to a long series of such observations as are detailed below, made by women observers. variable stars may be defined as those which exhibit a varying degree of brightness at different times. the following classification of them is believed to be a natural one. (proc. amer. acad. xvi, , .) i. temporary stars, or those which shine out suddenly, sometimes with great brilliancy, and gradually fade away. examples, tycho brahe's star of , new star in corona, . ii. long period variables, or those undergoing great variations of light, the changes recurring in periods of several months. examples _omicron ceti_ and _chi ceti_. iii. stars undergoing slight changes according to laws as yet unknown. examples, _alpha orionis_ and _alpha cassiopeiae_. iv. short period variables, or stars whose light is continually varying, but the changes are repeated with great regularity in a period not exceeding a few days. examples, _beta lyrae_ and _delta cephei_. v. algol stars, or stars which for the greater portion of the time undergo no change in light, but every few days suffer a remarkable diminution in light for a few hours. this phenomenon recurs with such regularity that the interval between successive minima may be determined in some cases within a fraction of a second. examples _beta persei_ (algol) and _s cancri_. stars belonging to the first of these classes are seen so rarely that the apparent discovery of one is to be received with the utmost caution. on the other hand, the importance of early observations of such an object is so great that no pains should be spared to secure an early announcement if one is really found. on the best star charts many stars are omitted of the brightness of the faintest objects given. but any star much brighter than these should be measured by the method given below, and a watch kept to see if any change takes place. if it proves to be a temporary star an immediate announcement should be made. if a telegram is sent to this observatory the object will be at once examined, and, if verified, notification will be made in this country and in europe with the name of the discoverer or sender of the telegram. a similar notification may be sent of any suspected objects, which will be examined in the same way, and announced at once if they prove to be of interest. it is essential that the position of the object should be given with all the precision practicable, and that a letter should be sent by the next mail giving the observations in detail. this often proves of the greatest value in case the object is not readily found. it also serves to establish the claims of the first discoverer. nearly three quarters of the known variables belong to the second class. most of them undergo very large changes of light, and may therefore be observed with comparative ease. our knowledge of their variations is however very defective. hitherto the attention of observers has been directed principally to determining the times at which they attain their maximum light, while their light at intermediate times has been neglected. it is now proposed to secure observations of these objects once or twice in every month, so that their light curves or variations throughout their entire periods may be determined. again, many observers are accustomed to state their brightness in magnitudes without giving any clue to the scale which they employ. in most cases such observations have little value owing to the uncertainty of the scale of the fainter magnitudes. according to dr. gould and some other observers most of the visible stars undergo slight changes of light and should therefore be assigned to the third class of variables. it is probable that our sun also belongs to this class, as it is not likely that its light is the same during the maximum and minimum of the sun spot period. at present we are unable to tell in which case the light would be greatest. it by no means follows that when the spots are most abundant the sun's total light is least, for the remaining portions of the sun may then have an increased brightness more than compensating for their diminished area. as long as the suspected variations in light of the stars are small, not exceeding half a magnitude for instance, they seem in the present state of science to have comparatively little interest. they are so liable to be affected, or even caused, by errors of observation, that the observation of such objects does not seem now to be advisable. doubtless many such so-called variables are really due to errors caused by moonlight, the proximity of brighter stars, varying position of the images on the retina of the observer, and other similar causes. they will not therefore be considered further in this paper. the stars of the fourth class as compared with the second are relatively few in number, and the changes in light small. while many of them need observation, especially to determine their light curves more precisely, it is advised that this work be left to those who have acquired a high degree of skill in these observations. that the work may be of value it is essential that the errors should be extremely small. as, however, nearly all are visible in an opera-glass, a skilful observer unprovided with a telescope may secure valuable results by their observation. this remark applies with especial force to many of those discovered in the southern heavens by dr. gould. the phenomena of the algol stars are in many respects the most striking of any. the rapidity of the changes, their surprising regularity, and the comparative rarity of these objects, combine to render the discovery of each new one a matter of unusual interest. as in the case of stars of the fourth class, however, the study of their light curves should be left to those who have acquired especial skill in this work. this is particularly desirable, when, as in this case, the unaided eye enters into competition with photometric apparatus, by which, as some think, it should properly be altogether replaced. an elaborate bibliographical work on the variable stars has been undertaken at this observatory by mr. chandler. it will include the collection of all available published observations of known or suspected variables. a catalogue of suspected variables has thus been prepared, doubtless containing many stars which are really important variables. but it is also likely that many objects have been introduced in the list by errors in the original observations. such stars often appear in one catalogue after another of suspected variables, and it is difficult to prevent the continued circulation of such an error. of course if an experienced observer at any time estimates a star as above or below its normal brightness, it is impossible to prove that the observation was not correct, and the star really variable. no amount of subsequent observing could prove that it had not then, and then only, an abnormal brightness. we can, however, prove that in all probability it does not belong to one or more of the above classes, and thus make it more and more probable that the observation is due to an error. if the star varies in light by one magnitude, what will be the chances that we shall get a series of observations having a range of variation of one fifth of a magnitude? evidently on the average, there will be only one chance out of five that any observation shall fall in the same fifth of a magnitude as another. the chances for three such observations will be only / and for four / , etc. these ratios expressed decimally are . , . , . , , . , etc. since the separate determinations of the light of a constant star by the method given below should not differ more than two or three tenths of a magnitude, it is obvious that if the variations of the star are large, a few observations would generally establish this fact. if the star belongs to class four, observations on half a dozen evenings would hardly fail to show the variation. conversely, if no such variation is detected we may be almost certain that the star is not a variable of that class, or at least that the variation, if any, is not large. if the star belongs to class two, it will change so slowly when near its maximum or minimum that a variation might not be noted if the observations are near together. an interval of several months should therefore be allowed to take place, or perhaps it would be better to wait until the star is again visible the following year. the total variation in light is usually so great in these stars that the change will often be visible at the first glance. to prove that a star does not belong to the fifth class is a matter of much greater difficulty. in fact it is almost impossible to prove that it may not be an algol star with a long period between the minima. since these stars may have their full brightness for nine tenths of the time, it is obvious that they may be examined again and again without happening to be seen at the time of a minimum. on the other hand, during a considerable portion of the time when it is varying, the light will be so much less than usual that a careful measurement is not needed to detect the change. moreover, it will be useless to look for an increase of light, and the observation may be so planned as to detect a diminution only. if we assume that only during one tenth of the time the change in light will be sufficient to be perceptible, the chance on any given evening will be out of or / that the star will have its full brightness. for two evenings the chance will be ( / )^ for three ( / )^ , etc. these quantities expressed decimally are . , . , . , . , . , . , . , etc. even after seven nights' observations, on which no change is noted, it will only be about an even chance that the star may not still be of the algol type. a different method of observing is therefore recommended when the star is supposed to belong to this class. select for comparison a star slightly fainter, so that a moment's glance will satisfy the observer that the suspected variable is the brighter. it is only necessary to repeat this observation night after night. if the star is bright enough to be visible with a field glass, a few seconds will be sufficient for this observation after the observer has become familiar with the vicinity. the fact that the light is normal, and the time to the nearest minute, should be recorded after each observation. when convenient, it is well to repeat the inspection two or more times during the night, as in determining the period all the observations will have a value, provided that they are separated by intervals of more than two or three hours. if the star is ever found below its normal brightness, comparisons should be made with the adjacent stars, and continued as long as possible, or until it has regained its usual brightness. the most complete proof that a star was not of the algol type would be for observers in the polar regions to examine it at intervals of a few hours for several days, or for observers in different longitudes to make the same observations. if it could thus be watched for a week or fortnight by enough observers to avoid interference by clouds, it would be nearly certain that it is not an algol star unless its period is greater than that of any such object as yet discovered. the problems to be undertaken may be defined as follows:-- . to observe all the long period variables once or twice every month throughout their variations according to such a system that all the observations may be reduced to the same absolute scale of magnitudes. . to observe the stars whose variability is suspected and prove either that they are really variable, or that in all probability they do not belong to the first, second, or fourth class. if any are thought to belong to the fifth class, to watch them until such a variation is proved, or is shown to be improbable. all of this work will depend on the possibility of readily determining the brightness of a star according to such a method that all the observations can ultimately be reduced to the same system. herschel and argelander have independently invented what appears to be the true method to be followed. if a star is seen to be very nearly equal to several others, from their light we can at any time define its brightness. it is essential that at least one of the stars selected should be a little brighter, another a little fainter, than the star to be observed. the range within which its light is known is thus also defined. such observations will far exceed in value any direct estimate of magnitude. when stars are to be compared many times, it is convenient to designate them by letters for brevity. let _v_ represent a star which is suspected to be variable, and _a_ an adjacent star of nearly equal brightness. owing to fluctuations in the atmosphere, each star will appear to be constantly varying in brightness. if the stars appear equal after a careful examination, or if one appears brighter as often as it appears fainter than the other, we may denote this equality by _av_ or _va_, these terms having precisely the same meaning. if one of the stars is suspected to be brighter, that is, if it appears sometimes brighter and sometimes fainter, but more frequently brighter, the interval may be designated as one grade. the observation may be written _a_ _v_ or _v_ _a_, the brightest star being named first. if one star is certainly brighter than the other, the difference, however, being very small, so that they sometimes appear equal, the difference will be two grades, and may be written _a_ _v_ or _v_ _a_. greater intervals may be estimated as three or four grades, but such observations have much less value. it is found in practice that a grade thus estimated will slightly exceed a tenth of a magnitude. a useful exercise for an observer is to select two stars of known magnitude and several others of intermediate brightness. arrange them in a series in the order of brightness, and estimate the intervals in grades. the difference in magnitude of the first stars divided by the total number of grades gives the value of one grade. by using different intermediate stars, the same standard stars may be employed repeatedly. the following well-known polar stars will be convenient, since they are always visible:-- _a alpha ursae minoris_, . magn.; gamma _ursae minoris_, . magn.; delta _ursae minoris_, . magn.; _cephi_, . magn.; lambda _ursae minoris_, . magn. the above method is essentially that of argelander. sir william herschel had already employed a method which differed mainly in his notation, a . , and -- being equivalent to one, two, or three grades. in all work of this kind the observer must look directly at the star he is observing at the moment, and never try to compare two stars by a simultaneous inspection of both. after examining one star until he has a distinct impression of its average brightness, freed from the momentary changes due to atmospheric disturbance, he should observe the other in the same manner. alternate observations of the two stars, each observation lasting for a few seconds, will give a truer impression than can be derived from a simultaneous observation in which the two images must be differently placed on the retina. the principal objection to this method is the difficulty of determining the value of a grade, as it is liable to vary with the observer, the time, the condition of the air, and the brightness of the stars. these difficulties are avoided by the following method. select two stars for comparison; one, _a_, slightly brighter than the star to be measured, _v_, the other, _b_, slightly fainter. the interval between _a_ and _b_ should never exceed one magnitude. estimate the brightness of _v_ in tenths of the interval from _a_ to _b_. thus, if _v_ is midway between _a_ and _b_ the interval will be five tenths, and we may write _a_ _b_. if _v_ is nearly as bright as _a_, we may have _a_ _b_ or _a_ _b_; if _v_ is not much brighter than _b_, we may have _a_ _b_ or _a_ _b_. an advantage of this method is that larger intervals in brightness may be used between the comparison stars, and accordingly less distant stars employed. an increase in distance of the stars always renders the comparison more difficult. we can also obtain many independent comparisons by using several comparison stars. if we have _m_ stars brighter and _n_ fainter, we shall only have _m_ + _n_ independent measures by the method of grades, while we may have _m n_ comparisons by estimating tenths, since estimates may be made in terms of the intervals between each brighter and each fainter star. on the other hand, especially when observing stars not very near together, it is a decided advantage to have to compare two stars rather than three. each method has its advantages, and that to be used should doubtless depend on the temperament of the observer. several precautions are needed to secure the best results. no observations should be made near the horizon; and, when the objects examined are at any considerable zenith distance, stars differing several degrees in altitude should be avoided. if the stars are bright and there is no choice, a correction may be made for the error due to the varying absorption at these different altitudes if the time of observation has been noted. when using a telescope or opera-glass, the stars should be brought in turn to the centre of the field, as when near the edge they will not appear of their true brightness. this is found to be better than placing them at equal distances from the centre. in selecting comparison stars, the proximity of a brighter star is very objectionable, causing a large error, which varies with the magnifying power used. double stars should be avoided if the power used is sufficient to show the companion. comparing stars of different colors is also objectionable. any persons who desire to take part in these observations are requested to communicate with the writer, and send answers to the questions given below. . what is the location of your point of observation? in the city or in the country, on the ground, from a roof, or from a window? is any part of your horizon obstructed, or can you observe in all parts of the sky? . what is the aperture, focal length, and name of maker of your telescope? also the lowest magnifying power and largest field of view you can obtain with it? have you a field-glass or opera-glass? . can you identify bright and faint stars from their designations or right ascensions and declinations? have you heis' atlas coelestis novus, the uranometria argentina, the durchmusterung, or other maps and catalogues of the stars? . would you prefer to observe the known or the suspected variables, or to divide your time between them? for convenience in making the reductions and for future reference, it is essential that all the observations should be made according to the same system. observers are accordingly requested to adopt the following form. use half-sheets of letter paper (eight inches by ten), writing only on one side and leaving a margin of half an inch for binding. begin with a new sheet every evening, and write the date and location (township and state) on the first line. each sheet when completed should be signed, and all should be numbered consecutively. when several sheets are used on the same night, the date should be entered on each. the record should be made in pencil, and all subsequent remarks or corrections added or interlined with ink, taking especial care not to obliterate or render illegible the original record. a general statement should be made each evening of the condition of the sky, as "clear," "hazy," "passing clouds," etc. the time of beginning and ending work should also be noted. one line should be assigned to each comparison. the hour and minute should be written to the left, and the comparison next to it. the right-hand half of the line will be left blank for reducing the observation. certain evenings or portions of evenings must also be devoted to the selection of the comparison stars of suspected variables. if they are contained in maps which are available, the letters assigned to each star may be marked on the maps and lines drawn to show with what suspected variable star they are associated. if preferred, a sketch may be made of the neighboring stars and the letters entered on them. this sketch with a proper description should be entered on the observing sheets described above, and a copy should be retained for reference. every month the observations will be interrupted by moonlight, and accordingly, three or four days before the full moon, all the sheets that have accumulated should be mailed, addressed harvard college observatory, cambridge, mass. an acknowledgment will be sent at once, so that if this is not received a second notification should be sent. to attain success it is particularly important that the plan should not be local or national. observers in the southern hemisphere are much needed, and for some purposes those in various longitudes. it is hoped that among the many amateurs of europe, and especially of england, may be found some ready to participate in this work. no restriction regarding the observations or publication is intended; but it is hoped that a large addition to our present knowledge of the variable stars may be secured, without interfering with what would otherwise be obtained. copies of this pamphlet and further information will be furnished on application. any persons desiring to participate are requested to address the writer, sending answers to the questions given above. the details will differ with each observer, and will be arranged by correspondence. apart from the value of the results attained, it is believed that many amateurs will find it a benefit to accustom themselves to work in a systematic manner, and that they will thus receive a training in their work not otherwise easily obtained outside of a large observatory. the lesson should be taught that time spent at a telescope is nearly wasted, unless results are secured worthy of publication and having a permanent value. those who have once accomplished such work are likely in the future to appreciate its value, and will often continue to do useful work in some other department of practical astronomy, if not in that of variable stars. the education of a class of skilled observers would be a work of no less value than the results anticipated from the observation of the variable stars. edward c. pickering. harvard college observatory, cambridge, mass.